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28 February 2023

Au(III) Cyclometallated Compounds with 2-Arylpyridines and Their Derivatives or Analogues: 34 Years (1989–2022) of NMR and Single Crystal X-ray Studies

and
1
Faculty of Chemistry, Nicolaus Copernicus University, 7 Gagarina, 87100 Toruń, Poland
2
Nikolaev Institute of Inorganic Chemistry of the Russian Academy of Sciences, 3 Acad. Lavrentiev Ave., 630090 Novosibirsk, Russia
*
Author to whom correspondence should be addressed.
Inorganics2023, 11(3), 100;https://doi.org/10.3390/inorganics11030100
This article belongs to the Special Issue Metal Complexes with N-donor Ligands
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Abstract

A review paper on Au(III) cyclometallated compounds with 2-arylpyridines (2-phenylpyridine, 2-benzylpyridine, 2-benzoylpyridine, 2-phenoxypyridine, 2-phenylsulfanylpyridine, 2-anilinopyridine, 2-(naphth-2-yl)pyridine, 2-(9,9-dialkylfluoren-2-yl)pyridines, 2-(dibenzofuran-4-yl)pyridine, and their derivatives) and their analogues (2-arylquinolines, 1- and 3-arylisoquinolines, 7,8-benzoquinoline), with 113 references. A total of 554 species, containing κ2-N(1),C(6′)*-Au(III), or analogous moiety (i.e., chelated by nitrogen of the pyridine-like ring and the deprotonated ortho- carbon of the phenyl-like ring) and, thus, possessing a character intermediate between metal complexes and organometallics, studied in the years 1989–2022 by NMR spectroscopy and/or single crystal X-ray diffraction (207 X-ray structures), are described. The compounds for which biological or catalytic activity and the luminescence properties were studied are also quoted.

1. Introduction

The compound 2-Phenylpyridine (2ppy) is aza aromatic, which is known to coordinate transition metal ions in two alternative ways: as a monodentate N(1)-donor or a bidentate N(1),C(6′)*-chelating agent 2ppy* (2ppy* = monoanionic form of 2ppy, deprotonated in the phenyl side group at the ortho-carbon C(6′)*). In the latter case, it may form Au(III)-2ppy* cycloaurated compounds, which can be regarded as either complexes or organometallics (due to the presence of both gold–nitrogen and gold–carbon bonds), usually upon the presence of some other auxiliary ligands (inorganic and/or organic), which complete the square-planar (d8) coordination sphere. The same κ2-N(1),C(6′) coordination mode is also observed for 2ppy* derivatives containing substituents in the pyridine and/or in the phenyl ring (denoted as 2PPY*) and results in a large variety of Au(III)-2PPY* species. They are presented, together with the numbering of both aromatic rings, in a general form in Scheme 1 (L1, L2, and L1L2 denote mono- and bidentate auxiliary ligands, respectively).
Scheme 1. Au(III)-2PPY* compounds (R1, R2—any substituents) with L1, L2 monodentate ligands (in particular—with X, Y halides) or L1L2 bidentate ligand.
The same κ2-N(1),C(6′) coordination of Au(III) is observed for such analogues of 2ppy* as 2-benzylpyridine*, 2-benzoylpyridine*, 2-phenoxypyridine*, 2-phenylsulfanylpyridine*, and 2-anilinopyridine* (Scheme 2), while the similar κ2-N(1),C(3′) one for 2-(naphth-2-yl)pyridine*, 2-(9,9-dialkylfluoren-2-yl)pyridines* and 2-(dibenzofuran-4-yl)pyridine* (Scheme 3), including their derivatives, substituted in any places of the parent heterocycles. These analogues of 2PPY*, generally named here as 2-arylpyridines* (denoted as 2ArPY*), yield many Au(III)-2ArPY* species.
Scheme 2. Au(III)-2ArPY* compounds with 2ArPY* of the type A (Z = CH2 in 2-benzylpyridine*, CO in 2-benzoylpyridine*, O in 2-phenoxypyridine*, S in 2-phenylsulfanylpyridine*, NH in 2-anilinopyridine*; R1, R2—any substituents) with L1, L2 monodentate ligands (in particular—with X halides) or L1L2 bidentate ligand.
Scheme 3. Au(III)-2ArPY* compounds with 2ArPY* of the type B (2-(naphth-2-yl)pyridine*—(left), 2-(9,9-dialkylfluoren-2-yl)pyridines*—(middle), 2-(dibenzofuran-4-yl)pyridine*—(right) with L1, L2 monodentate ligands (in particular—with X halides) or L1L2 bidentate ligand.
Analogous Au(III) chelation is also known for analogues of 2ArPY* containing pyridine-like ring (PY#, e.g., quinoline or isoquinoline) and aryl ring (Ar), linked by a single bond (in 2-arylquinolines* and 1- or 3-arylisoquinolines*) or fused (in 7,8-benzoquinoline*), together with their derivatives (denoted as ArPY#*). The coordination mode is generally similar, although the numbering of atoms may differ for various ring systems (usually nitrogen N(1) or N(2) in the PY# moiety and the deprotonated ortho-carbon C(6′) or C(10) in the Ar moiety; Scheme 4)—resulting in some Au(III)-ArPY#* species.
Scheme 4. Au(III)-ArPY#* compounds with 2-arylpyridines analogues: 2-phenylquinoline* (top left), 1-phenylisoquinoline* (top right), 3-phenylisoquinoline* (bottom left), 7,8-benzoquinoline* (bottom right) with L1, L2 monodentate ligands (in particular—with X halides) or L1L2 bidentate ligand (the latter case is omitted for clarity).
It must be underlined that the concerned bidentate coordination mode of 2ppy*, 2PPY*, 2ArPY*, and ArPY#* is also widely observed for many other transition metal ions; however, this review is focused on the Au(III) compounds only.
Its main aim is the summary of NMR and X-ray structural data. However, many concerned species exhibit biological (anti-tumour and/or anti-microbial) and catalytic activity, as well as luminescence (both fluorescence and phosphorescence), which allows for some practical applications; these properties were also described, at least in the most important cases.
The increasing interest in this class of Au(III) compounds can be illustrated by the numbers of molecules for which the single crystal X-ray structures were published (totally 207) every year, as presented in Scheme 5:
Scheme 5. Annual numbers of Au(III) compounds with 2ppy*, 2PPY*, 2ArPY*, and ArPY#*, for which the single crystal X-ray structures were published in the years 1995–2022.
The year-to-year fluctuations can be eliminated by calculating the numbers of molecules with the published X-ray structures for each 5-year period, as follows: 1995–1999: 8, 2000–2004: 14, 2005–2009: 8, 2010–2014: 45, 2015–2020: 79, from 2020: already 53 (in three years). This tendency clearly exhibits the growth of research intensity on the concerned compounds.
Generally, the data contained in this paper can be found in some databases, such as Reaxys or CCDC. However, their collection in one reviewing article allows the researchers to compare them and to deduce some general conclusions. In fact, it was the principal purpose of this review.

2. Reviewed Data

2.1. Au(III)-2PPY* Compounds

2.1.1. Au(III)-2PPY* Dihalides

The simplest representative of this class of chemicals is [Au(2-phenylpyridine*)Cl2] (i.e., [Au(2ppy*)Cl2]), described for the first time in 1989 by Constable et al. [1]. It is widely used as a precursor for the synthesis of some other Au(III)-2ppy* compounds; thus, the number of articles where it appears is really large, and the most noteworthy papers are those in which its NMR characterization was given [1,2,3,4,5,6,7,8,9,10], together with the single crystal X-ray structure (IJAQEP) [3]. Surprisingly, despite numerous reports about this dichloride [Au(2ppy*)Cl2] species, there are no literature data on its analogues with some other halogens (F, Br, I)—although they are available for similar Au(III)-2PPY* (2PPY* ≠ 2ppy*) dihalides.
Among the dihalides having the general formula [Au(2PPY*)XY] (X, Y = F, Cl, Br, I), including [Au(2PPY*)X2], and particularly, the most popular [Au(2PPY*)Cl2] one, 43 (not counting [Au(2ppy*)Cl2]) were reported and characterised by NMR spectroscopy and/or by single crystal X-ray diffraction [2,5,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24].
A total of 3 of them contained 2ppy* derivatives, substituted only in the pyridine ring (R1 = 3-methyl-, 4-n-propyl-, 5-n-butyl-) [2,10,11], while 25 had 2ppy* derivatives with substituent(s) exclusively in the phenyl ring (R2 = 2-, 3- and 4-methyl-; 3-n-butyl; 4-tert-butyl-; 2- and 4-fluoro-; 2,4-difluoro-; 4-chloro-; 3- and 4- trifluoromethyl-; 3- and 4-methoxy-; 4-n-butoxy-; 3,5-dimethoxy-; 2- and 4-trifluoromethoxy-; 4-formyl-; 2-, 3- and 4-phenyl; 4-(9-bromo)-n-nonoxy)-; 4-(9-trimethylammonium-n-nonoxy)-; 4-(9-(4-methylphenylsulfonoxy)-n-nonoxy-) [5,8,9,10,11,12,13,14,15,16,17,18,19,20]. Then, 15 possessed 2ppy* derivatives substituted in both the pyridine and the phenyl ring (3-methyl-2-(2-fluorophenyl)pyridine*, 3-methyl-2-(3,4,5-trimethoxyphenyl)pyridine*, 5-carboxy-2-(4-carboxyphenyl)pyridine*, 5-ethoxycarbonyl-2-(4-ethoxycarbonylphenyl)pyridine*, 4-dimethylamino-2-(2,3,4-trifluorophenyl)pyridine*, 4-dimethylamino-2-(3-trifluoromethylphenyl)pyridine*, 4-dimethylamino-2-(4-trifluoromethoxyphenyl)pyridine*) [21,22,23,24].
All these [Au(2PPY*)XY] dihalides (Scheme 1 left, for L1 = X and L2 = Y) are listed in Table 1 (the 2PPY* ligands are presented as 2ppy* derivatives, variously substituted in the pyridine ring (by R1) and/or in the phenyl ring (by R2), so having the general formula/name of a-R1-2-(b-R2-phenyl)pyridine* (a = 3–6, b = 2–5)), together with the main solvents used upon the NMR studies and the CCDC reference codes for the respective single crystal X-ray structures; moreover, the biological (BIO) and catalytic (CAT) activity, as well as luminescence properties (LUM), are indicated. The same notations will be used in all other tables.
Table 1. NMR and/or X-ray studied [Au(2PPY*)XY] (in particular, [Au(2PPY*)X2]) dihalides (2PPY* = a-R1-2-(b-R2-phenyl)pyridine*, where R1 and R2 are substituents in the pyridine ring and the phenyl ring, respectively, a = 3–6, b = 2–5; X, Y = F, Cl, Br, I).
Among these [Au(2PPY*)XY] compounds, [Au(2-(4-tert-butylphenyl)pyridine*)Cl2] is biologically active, revealing anti-tumour properties (against breast or lung cancer and leukemia) [25,26]. Some other [Au(2PPY*)Cl2] dichloride species have catalytic properties (in reactions between alkynes, carbonyl compounds, and amines or imines—yielding amines, allenes, or oxazoles [16,17]—as well as between propargyl esters and styrene—yielding cyclopropane derivatives [22]).

2.1.2. Au(III)-2ppy* Compounds with Auxiliary Ligands Other Than Halides

In addition to [Au(2ppy*)Cl2], 92 Au(III)-2ppy* compounds with various auxiliary ligands (both organic and inorganic, but not halides), having the general formula [Au(2ppy*)L1L2] (in case of L1 = L2, i.e., identical L ligands: [Au(2ppy*)L2]) or [Au(2ppy*)(L1L2)] (in case of symmetrical LL ligands: [Au(2ppy*)(LL)]), as shown in Scheme 1 (left or right, respectively; for R1 = R2 = H), were reported and characterised by NMR spectroscopy and/or by single crystal X-ray diffraction [2,3,5,6,7,8,11,15,19,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50]. They are listed (except for [Au(2ppy*)Cl2], shown in Table 1) in Table 2. In case when the sum of electric charges at auxiliary ligand(s) was different from −2 (0 or −1), the concerned Au(III)-2ppy* compound was cationic (+2 or +1 charge), and the relevant anion presented in a separate column Counterion; otherwise (the sum of electric charges at auxiliary ligand(s) being −2), the Au(III)-2ppy* molecule was electrically neutral.
Table 2. NMR and/or X-ray studied [Au(2ppy*)L1L2] (in particular, [Au(2ppy*)L2]) and [Au(2ppy*)(L1L2)] (in particular, [Au(2ppy*)(LL)]) compounds (L1, L2, L—monodentate ligands other than F, Cl, Br, I; L1L2, LL—bidentate ligands).
Many these Au(III)-2ppy* compounds are biologically active, revealing anti-tumour properties (against various breast, cervix, colon, liver, lung, and ovarian cancers, as well as glioblastoma, leukemia, and melanoma) [3,8,9,11,28,31,33,43,51]. Some others have catalytic properties (in the hydration of alkynes to enoles [7] and photo-oxidation of benzylic amines to imines [42]). Then, a large number reveals luminescence, with lifetimes of either >10 µs [15,19,37,39,42] or <10 µs [15,27,38,41].

2.1.3. Au(III)-2PPY* Compounds with Auxiliary Ligands Other Than Halides

In addition to [Au(2PPY*)XY] (including [Au(2PPY*)X2]) and [Au(2ppy*)L1L2], including [Au(2ppy*)L2]) or [Au(2ppy*)(L1L2)], including [Au(2ppy*)(LL)]) compounds, 209 Au(III)-2PPY* species with various auxiliary ligands (other than halides)), having the general formulae [Au(2PPY*)L1L2] (in particular, [Au(2PPY*)L2]; L1, L2, L ≠ F, Cl, Br, I) or [Au(2PPY*)(L1L2)] (in particular, [Au(2PPY*)(LL)]) were reported and characterised by NMR spectroscopy and/or by single crystal X-ray diffraction [2,5,8,11,12,14,15,18,19,21,22,23,24,25,26,27,31,37,47,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76]; this is the most general case presented in Scheme 1 (for 2PPY* ≠ 2ppy* and L1, L2 ≠ F, Cl, Br, I). They are listed (except for [Au(2PPY*)XY], [Au(2ppy*)L1L2] and [Au(2ppy*)(L1L2)], already shown in Table 1 and Table 2) in Table 3.
A total of 5 of them contained 2ppy* derivatives, substituted only in the pyridine ring (R1 = 3-methyl-, 5-n-butyl-, 4-tert-butyl-, 3,5-dimethyl-) [2,11], while 137—only in the phenyl ring (2- and 4-methyl-; 3-ethyl-; 3- and 4-n-butyl; 4-tert-butyl-; 3,5-dimethyl-; 4-fluoro-; 2,4- and 3,5-difluoro-; 3-, 4- and 5-trifluoromethyl; 4-methoxy-; 4-n-butoxy-; 2- and 4-trifluoromethoxy-; 4-formyl; 4-nitro-; 4-phenyl-; 3,5-bis(pentafluorophenyl)-), with a predominance of the 2-(4-methylphenyl)pyridine* ligand (95 species) [5,8,11,12,14,15,18,19,25,26,27,31,37,47,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66].
Then, 67 had 2ppy* derivatives with substituents in both the pyridine and the phenyl ring (2,6-bis(4-tert-butylphenyl)pyridine*; 3-methyl-2-(2-fluorophenyl)pyridine*; 4- and 5-methyl-2-(4-methoxyphenyl)pyridine*; 6-methyl-2-(4-methylphenyl)pyridine*; 5-tert-butyl-2-(4-tert-butylphenyl)pyridine*; 4-trifluoromethyl-2-(4-methylphenyl)pyridine*; 5-trifluoromethyl-2-(4-methoxyphenyl)pyridine*; 5-trifluoromethyl-2-(2-diphenylaminophenyl)pyridine*; 3-, 4-, and 6-methoxy-2-(4-methylphenyl)pyridine*; 5-carboxy-2-(4-carboxyphenyl)pyridine*; 5-ethoxycarbonyl-(2-(4-ethoxycarbonylphenyl)pyridine*; 4-dimethylamino-2-(2,3,4-trifluorophenyl)pyridine*; 4-dimethylamino-2-(3-trifluoromethylphenyl)pyridine*; 4-dimethylamino-2-(4-trifluoromethoxyphenyl)pyridine*; 3-nitro-2-(4-methylphenyl)pyridine*) [21,22,23,24,37,53,67,68,69,70,71,72,73,74,75,76]. Among these ligands, 2,6-bis(4-tert-butylphenyl)pyridine* is especially interesting because 2,6-bis(4-tert-butylphenyl)pyridine can chelate transition metal ions, not only in the bidentate way (κ2-N(1),C(6′)*), but also in the tridentate mode (κ3-N(1),C(6′)*,C(6”)*)—forming Au(III)-(2,6-bis(4-tert-butylphenyl)pyridine**) pincer compounds (2,6-bis(4-tert-butylphenyl)pyridine** = dianionic form of 2,6-bis(4-tert-butylphenyl)pyridine, deprotonated in both phenyl groups at the ortho- carbons C(6′)* and C(6”)*); however, such molecules were not included in this review.
Table 3. NMR and/or X-ray studied [Au(2PPY*)L1L2] (in particular, [Au(2PPY*)L2]) or [Au(2PPY*)(L1L2)] (in particular, [Au(2PPY*)(LL)]) compounds (2PPY* = a-R1-2-(b-R2-phenyl)pyridine*, other than 2ppy*, where R1 and R2 are substituents in the pyridine ring and the phenyl ring, respectively, a = 3–6, b = 2–5; L1, L2, L—monodentate ligands other than F, Cl, Br, I; L1L2, LL—bidentate ligands).
Table 3. NMR and/or X-ray studied [Au(2PPY*)L1L2] (in particular, [Au(2PPY*)L2]) or [Au(2PPY*)(L1L2)] (in particular, [Au(2PPY*)(LL)]) compounds (2PPY* = a-R1-2-(b-R2-phenyl)pyridine*, other than 2ppy*, where R1 and R2 are substituents in the pyridine ring and the phenyl ring, respectively, a = 3–6, b = 2–5; L1, L2, L—monodentate ligands other than F, Cl, Br, I; L1L2, LL—bidentate ligands).
R1R2L1L2L1L2CounterionNMR SolventX-ray (CCDC)GeometryActivity $
3-methylH 3-carboxy-2-thiolatepropionate DMSO-d6 [2]
3-methylHpyridineacetate ClO4− MAXQEH [2]trans(N,N)
5-n-butylH diethylaminocarbodithioatePF6−DMSO-d6 [11] BIO1 [11]
4-tert-butylHacetateacetate CDCl3
DMSO-d6 [2]
3,5-dimethylHacetateacetate CDCl3 [2]
H2-methylphenylBr JOTROB [12]trans(C,N)
H4-methylCNCN DMSO-d6 [27] LUM2 [27]
H4-methylmethylmethyl CDCl3
CD2Cl2 [52,53]		QICNUN [52]
H4-methylmethylCl CD2Cl2 [54]
H4-methylmethylBr CD2Cl2 [52,53]
H4-methylethylBr CD2Cl2 [52]
H4-methyl allyl-η3(CF3SO2)2N−CD2Cl2 [55]ROVXUY [55]
H4-methylallylBr CD2Cl2 [53,55]ROVYAF [55]trans(C,N)
H4-methylallyltrideuteroacetonitrile (CF3SO2)2N−CD3CN [55]
H4-methylethynylethynyl CD2Cl2 [56]LUWJUL [56]
H4-methyln-butylethynyln-butylethynyl CD2Cl2 [56]
H4-methyln-butylethynylBr CD2Cl2 [56]
H4-methyl n-hexane-1,6-diethynyl CD2Cl2 [56]
H4-methylthiacyclopentanemethyl CF3SO3−CD2Cl2 [54]
H4-methyl 1,4,7-trithiacyclononane-κ3-S,S,S2PF6−CD3NO2 [5]MOCFIV [5]
H4-methylacetateacetate CDCl3
CD2Cl2 [31,53]		 BIO1 [31]
H4-methylacetatemethyl CD2Cl2 [54]IDAJII [54] atrans(C,N)
H4-methyltrifluoroacetatetrifluoroacetate CD2Cl2
CD3CN
C6D6
CD3COOD
CF3COOD [52,53,57]		QICPAV [52]
H4-methyltrifluoroacetatehydroxymethyl CD2Cl2 [58]
H4-methyltrifluoroacetatemethoxyl CD2Cl2
CD3OD [58]
H4-methyltrifluoroacetatemethoxyethyl CD2Cl2
CD3OD [58]		YIDHIF [58]trans(C,N)
H4-methyltrifluoroacetateformylmethyl CD2Cl2 [59]QEFVUV [59]trans(C,N)
H4-methyltrifluoroacetateacetate CD2Cl2 [58]
H4-methyltrifluoroacetateCH3OCH(CH(CH3)2)CH2− CD2Cl2 [58]YIDGIE [58]trans(C,N)
H4-methyltrifluoroacetateCH3OC(CH3)(CH2CH3)CH2− CD2Cl2 [58]
H4-methyltrifluoroacetateCH3OC((CH3)2)CH(CH3)− CD2Cl2 [58]YIDHEB [58]trans(C,N)
H4-methyltrifluoroacetateCH3OCH(CH2CH2CH2CH3)CH2− CD2Cl2 [58]YIDHAX [58]trans(C,N)
H4-methyltrifluoroacetateCH3OCH(CH2CH2CH3)CH(CH3)− CD2Cl2 [58]YIDGOK [58]trans(C,N)
H4-methyltrifluoroacetateCH3OCH(CH3)CH(CH2CH2CH3)− CD2Cl2 [58]YIDGAW [58]trans(C,N)
H4-methyltrifluoroacetateCH3OCH(C6H5)CH2− CD2Cl2 [58]YIDGUQ [58] atrans(C,N)
H4-methyltrifluoroacetateCH3CH2OCH2CH2− CD2Cl2
CD3CD2OD [58]
H4-methyltrifluoroacetate(CH3)2CHOCH2CH2− CD2Cl2 [58]
H4-methyltrifluoroacetate(CH3)3COCH2CH2− CD2Cl2 [58]
H4-methyltrifluoroacetateCH3COOCH2CH2− CD2Cl2
CD3COOD [58]		YIDGEA [58]trans(C,N)
H4-methyltrifluoroacetateCF3COOCH2CH2− CD2Cl2
CF3COOD [57]		FUWXIG [57]trans(C,N)
H4-methyltrifluoroacetateCF3CH2OCH2CH2− CD2Cl2
CF3CD2OD [57]		FUWXOM [57]trans(C,N)
H4-methyltrifluoroacetateCF3COOCH(CH(CH3)2)CH2− CD2Cl2 [58]YIDFUP [58] btrans(C,N)
H4-methyltrifluoroacetateCF3COOCH(CH2CH2CH2CH3)CH2− CD2Cl2
CF3COOD [58]
H4-methyltrifluoroacetateCF3COOCH=CH− CD2Cl2 [59]QEFWAC [59] ctrans(C,N)
H4-methyltrifluorosulfonatemethyl CD2Cl2 [54]IDAJOO [54]trans(C,N)
H4-methyl HNC(CH3)OCH2CH2− 1CF3COO−CD2Cl2
CD3CN [60]		IPISEH [60]trans(C,N)
H4-methyl 1,1-dimethylbiguanidateCl−DMSO-d6 [8] BIO1 [8]
H4-methylacetonitrilemethyl BF4−CD3CN [60]IPISAD [60]trans(C,N)
H4-methyltrimethylsilylethynyltrimethylsilylethynyl CD2Cl2 [37,56]LUWJEV [56] a LUM1 [37]
H4-methyltrimethylsilylethynylBr CD2Cl2 [56]
H4-methyltrimethylsilylethynylethynyl CD2Cl2 [56]LUWJIZ [56]trans(HCC,N)
H4-methyl oxybis(n-propylborinate) CDCl3 [47]
H4-methylphenylphenyl CD2Cl2 [14,52]
H4-methylphenylBr CD2Cl2 [52,53]
H4-methyl2-methylphenyl2-methylphenyl CD2Cl2 [14]FONRIL [14]
H4-methyl3-methylphenyl3-methylphenyl CD2Cl2 [14]
H4-methylphenylethynylphenylethynyl CD2Cl2 [56]LUWJOF [56]
H4-methylphenylethynylBr CD2Cl2 [56]LUWKAS [56]trans(C,N)
H4-methylphenylethynylethynyl CD2Cl2 [56]LUWKIA [56]trans(C6H5CC,N)
H4-methylphenylethynyltrifluoroacetate CD2Cl2 [56]
H4-methylphenylethynyltrimethylsilylethynyl CD2Cl2 [56]
H4-methyl4-n-butylphenylethynyl4-n-butylphenylethynyl CD2Cl2 [37] LUM1 [37]
H4-methylnaphth-2-ylnaphth-2-yl CD2Cl2 [14]
H4-methylnaphth-1-ylCl CD2Cl2 [14]FONSAE [14]trans(C,N)
H4-methylanthracen-2-ylanthracen-2-yl C6D6 [14]
H4-methyl4-fluorophenyl4-fluorophenyl CD2Cl2 [14]FOPCAQ [14]
H4-methyl4-fluorophenylCl CDCl3 [14]FONRUX [14]trans(C,N)
H4-methyl2,4-difluorophenyl2,4-difluorophenyl CD2Cl2 [14]
H4-methyl2,5-difluoro-3,6-dibromophenyl2,5-difluoro-3,6-dibromophenyl CD2Cl2 [61]
H4-methyl4-trifluoromethylphenyl4-trifluoromethylphenyl CD2Cl2 [14]FOPCEU [14] a
H4-methyl4-chlorophenylethynyl4-chlorophenylethynyl CD2Cl2 [61]
H4-methyl4-methoxyphenyl4-methoxyphenyl CD2Cl2 [14]
H4-methyl4-methoxyphenylethynyl4-methoxyphenylethynyl CDCl3 [37]AJOXAZ [37] LUM1 [37]
H4-methyl4-isopropoxyphenyl4-isopropoxyphenyl C6D6 [14]FONROR [14]
H4-methyl 4-formylbenzene-1,2-diolate DMSO-d6 [62]
H4-methyl4-benzoxyphenyl4-benzoxyphenyl CD2Cl2 [14]
H4-methyl4-acetylphenyl4-acetylphenyl CD2Cl2 [14]
H4-methyl acetophen-2,8-diyl CD2Cl2 [14]FONSEI [14] atrans(CCH2,N)
H4-methyl anthracene-9,10-dione-1,2-diolate DMSO-d6 solid phase [62]
H4-methyl3-ethoksycarbonylphenyl3-ethoksycarbonylphenyl CD2Cl2 [14]
H4-methyl 2-thiolatebenzoate ICUMEY [51]trans(S,N)BIO1 [51]
H4-methyl 1-phenylbiguanidateCl−DMSO-d6 [8] BIO1 [8]
H4-methyl L-phenylalaninateCl−CDCl3 [31] BIO1 [31]
H4-methyl L-methioninateCl−CDCl3 [31] BIO1 [31]
H4-methyl3-nitrophenyl3-nitrophenyl CD2Cl2 [14]FOPCIY [14]
H4-methyl ethylene-1,2-bis(4-methylphenylsulfonylazanide) unknown [63] BIO1, BIO2 [63]
H4-methyl benzene-1,2-bis(acetylazanide) unknown [63] BIO1, BIO2 [63]
H4-methyl benzene-1,2-bis(4-methylphenylsulfonylazanide) unknown [63] BIO1, BIO2 [63]
H4-methyl sulfonebis(cyanomethyl) BIO1 [64]
H4-methyl sulfonebis(benzoylmethyl) unknown [64] BIO1 [64]
H4-methyl1-benzothiophen-2-yl1-benzothiophen-2-yl CD2Cl2 [14]FONREH [14]
H4-methyltriphenylphosphinemethyl CF3SO3−CD2Cl2 [54]IDAJUU [54]trans(C,N)
H
4-methyl 1,2-bis(diphenylphosphino)benzene 2A2Cl−CDCl3 [9] BIO1 [9]
H4-methyl oxybis(phenylborinate) CDCl3 [47,65] CAT [65]
H4-methyl oxybis(naphth-1-ylborinate) CDCl3 [47]
H4-methyl oxybis(4-tert-butylphenylborinate) CDCl3 [47]
H4-methyl oxybis(4-vinylphenylborinate) CDCl3 [47]FUXGIQ [47] a
H4-methyl oxybis(4-isopropoxyphenylborinate) CDCl3 [47]FUXGAI [47]
H4-methyl oxybis(4-acetylphenylborinate) CDCl3 [47]FUXGOW [47] a
H4-methyl oxybis(thiophen-3-ylborinate) CDCl3 [47]
H4-methyl oxybis(ferrocenylborinate) CDCl3 [47]FUXGEM [47] d
H3-ethyltrifluoroacetatetrifluoroacetate CD2Cl2 [53]
H3-n-butyl diethylaminocarbodithioatePF6−DMSO-d6 [11] BIO1 [11]
H4-n-butyl diethylaminocarbodithioatePF6−DMSO-d6 [11] BIO1 [11]
H4-n-butyl di(n-butyl)aminocarbodithioatePF6−DMSO-d6 [11] BIO1 [11]
H4-tert-butyl dimethylaminocarbodithioatePF6−DMSO-d6 [26] BIO1 [26]
H4-tert-butyl diethylaminocarbodithioatePF6−DMSO-d6 [26]NIKHIB [26] e BIO1 [26]
H4-tert-butyl di(n-butyl)aminocarbodithioateSbF6−CD3CN [26] BIO1 [26]
H4-tert-butyl (n-but-1,4-diyl)aminocarbodithioatePF6−CD2Cl2 [26] BIO1 [26]
H4-tert-butyl N-(ethoxycarbonylmethyl)methylaminocarbodithioatePF6−DMSO-d6 [26] BIO1 [26]
H4-tert-butyl4-methoxyphenylethynyl4-methoxyphenylethynyl CD2Cl2 [37] LUM1 [37]
H4-tert-butyl benzene-1-olate-2-acetylazanide CD2Cl2 [25] BIO1 [25]
H4-tert-butyl benzene-1,2-bis(acetylazanide) CD2Cl2 [25] BIO1 [25]
H4-tert-butyl benzene-1-olate-2-(N-(acridin-9-yl))azanide CD2Cl2 [25]KIGPEY [25] atrans(N,N)BIO1 [25]
H4-tert-butyl benzene-1-azanide-2-(N-(acridin-9-yl)azanide CD2Cl2 [25]KIGPIC [25] atrans(NH,N)BIO1 [25]
H4-tert-butyl ortho-bis(dicarboranyl) CD2Cl2 [66]WUCKUD [66] a
H3,5-dimethyltrifluoroacetatetrifluoroacetate CD2Cl2 [53]JAZJED [53]
H4-fluoro4-fluorophenyl4-fluorophenyl CDCl3 [14]
H4-fluoro3-ethoksycarbonylphenyl3-ethoksycarbonylphenyl CDCl3 [14]
H2,4-difluorophenylethynylphenylethynyl CD2Cl2 [15] LUM2 [15]
H2,4-difluoro methylenebis(3-n-butyl-1H-imidazol-1-yl-2-ylidene)2PF6−CD3CN [19] LUM1 [19]
H2,4-difluoro methylenebis(3-(3-sulfonate-n-propyl)-1H-imidazol-1-yl-2-ylidene) D2O [19] LUM1 [19]
H3,5-difluorotrifluoroacetatetrifluoroacetate CD2Cl2 [53]JAYZAO [53]
H3-trifluoromethylpentafluorophenylpentafluorophenyl CD2Cl2 [18]MIZHIP [18] LUM2 [18]
H3-trifluoromethylpentafluorophenylCl CD2Cl2 [18]MIZHEL [18]trans(Cl,N)LUM2 [18]
H3-trifluoromethylpentafluorophenyl4-fluorophenylethynyl DMF-d7 [18]MIZJAJ [18]trans(CC,N)LUM2 [18]
H3-trifluoromethyltrifluoroacetatetrifluoroacetate CD2Cl2 [53]
H3-trifluoromethyl4-bis(2,4,6-trimethylphenyl)boranylphenylethynylpentafluorophenyl DMSO-d6 [18]MIYYAX [18]trans(CC,N)LUM2 [18]
H4-trifluoromethyltrifluoroacetatetrifluoroacetate CD2Cl2 [53]JAZJAZ [53]
H4-trifluoromethyl diethylaminocarbodithioatePF6−DMSO-d6 [11] BIO1 [11]
H5-trifluoromethyltrifluoroacetatetrifluoroacetate CD2Cl2 [53]
H4-methoxy4-methoxyphenylethynyl4-methoxyphenylethynyl CD2Cl2 [37] LUM1 [37]
H4-methoxytrifluoroacetatetrifluoroacetate CD2Cl2 [53]
H4-n-butoxy diethylaminocarbodithioatePF6−DMSO-d6 [11] BIO1 [11]
H2-trifluoromethoxypentafluorophenylpentafluorophenyl CD2Cl2 [18]MIYYEB [18] a LUM2 [18]
H2-trifluoromethoxypentafluorophenylCl CD2Cl2 [18]MIYXOK [18]trans(Cl,N)LUM2 [18]
H4-trifluoromethoxypentafluorophenylpentafluorophenyl CD2Cl2 [18]MIZHOV [18] a LUM2 [18]
H4-trifluoromethoxypentafluorophenylCl CD2Cl2 [18]MIYXUQ [18]trans(Cl,N)LUM2 [18]
H4-formyl 1,2-bis(diphenylphosphino)benzene 2B2Cl−CDCl3 [9] BIO1 [9]
H4-nitrotrifluoroacetatetrifluoroacetate CD2Cl2 [53]JAZJON [53]
H4-phenyl methylenebis(3-n-butyl-1H-imidazol-1-yl-2-ylidene)2PF6−CD3CN [19] LUM1 [19]
H3,5-bis(pentafluorophenyl)trifluoroacetatetrifluoroacetate CD2Cl2 [53]JAZCOG [53]
5-tert-butyl4-tert-butyl diethylaminocarbodithioate BIO1 [26]
6-(4-tert-butylphenyl)4-tert-butyl norbornane-2-ol-3-ylSbF6−CD2Cl2 [67]PEZQUI [67]trans(C,N)
6-(4-tert-butylphenyl)4-tert-butyldiethyl etherCl H2N(B(C6F5)3)2−CD2Cl2 [68]
6-(4-tert-butylphenyl)4-tert-butyldiethyl ethermethyl H2N(B(C6F5)3)2−CD2Cl2 [69]
6-(4-tert-butylphenyl)4-tert-butyldiethyl ether2-methylallyl H2N(B(C6F5)3)2−CD2Cl2 [69]
6-(4-tert-butylphenyl)4-tert-butyldiethyl etherphenyl H2N(B(C6F5)3)2−CD2Cl2 [68]
6-(4-tert-butylphenyl)4-tert-butyldiethyl ether4-tert-butylphenyl H2N(B(C6F5)3)2−CD2Cl2 [69]
6-(4-tert-butylphenyl)4-tert-butyldiethyl ether4-fluorophenyl H2N(B(C6F5)3)2−CD2Cl2 [68]
6-(4-tert-butylphenyl)4-tert-butyldiethyl etherpentafluorophenyl H2N(B(C6F5)3)2−CD2Cl2 [68]
6-(4-tert-butylphenyl)4-tert-butyldiethyl ether4-trifluoromethylphenyl H2N(B(C6F5)3)2−CD2Cl2 [69]
6-(4-tert-butylphenyl)4-tert-butyldiethyl ether4-chlorophenyl H2N(B(C6F5)3)2−CD2Cl2 [69]
6-(4-tert-butylphenyl)4-tert-butyldiethyl ether4-methoxyphenyl H2N(B(C6F5)3)2−CD2Cl2 [69]
6-(4-tert-butylphenyl)4-tert-butyldiethyl ether4-nitrophenyl H2N(B(C6F5)3)2−CD2Cl2 [69]
6-(4-tert-butylphenyl)4-tert-butyldimethyl thioetherphenyl H2N(B(C6F5)3)2−CD2Cl2 [69]
6-(4-tert-butylphenyl)4-tert-butyldimethyl thioether4-tert-butylphenyl H2N(B(C6F5)3)2−CD2Cl2 [69]
6-(4-tert-butylphenyl)4-tert-butyldimethyl thioether4-fluorophenyl H2N(B(C6F5)3)2−CD2Cl2 [69]
6-(4-tert-butylphenyl)4-tert-butyldimethyl thioetherpentafluorophenyl H2N(B(C6F5)3)2−CD2Cl2 [69]
6-(4-tert-butylphenyl)4-tert-butyldimethyl thioether4-trifluoromethylphenyl H2N(B(C6F5)3)2−CD2Cl2 [69]
6-(4-tert-butylphenyl)4-tert-butyldimethyl thioether4-chlorophenyl H2N(B(C6F5)3)2−CD2Cl2 [69]
6-(4-tert-butylphenyl)4-tert-butyldimethyl thioether4-methoxyphenyl H2N(B(C6F5)3)2−CD2Cl2 [69]
6-(4-tert-butylphenyl)4-tert-butyldimethyl thioether4-nitrophenyl H2N(B(C6F5)3)2−CD2Cl2 [69]
6-(4-tert-butylphenyl)4-tert-butylmethylµ-H 3 H2N(B(C6F5)3)2−CD2Cl2 [70]
6-(4-tert-butylphenyl)4-tert-butylpentafluorophenylµ-H 3 H2N(B(C6F5)3)2−CD2Cl2 [70]
6-(4-tert-butylphenyl)4-tert-butylHµ-H 4 H2N(B(C6F5)3)2−CD2Cl2 [70]
6-(4-tert-butylphenyl)4-tert-butylmethylµ-H 4 H2N(B(C6F5)3)2−CD2Cl2 [70]
6-(4-tert-butylphenyl)4-tert-butylpentafluorophenylH CD2Cl2 [71]
6-(4-tert-butylphenyl)4-tert-butylpentafluorophenyltriethylsilane H2N(B(C6F5)3)2−CD2Cl2 [71]
6-(4-tert-butylphenyl)4-tert-butylpentafluorophenyl4,4,5,5-tetramethyl-1,3,2-dioxaborolane H2N(B(C6F5)3)2−CD2Cl2 [71]
6-(4-tert-butylphenyl)4-tert-butyladamantane-1-µ-thiolate 5adamantane-1-µ-thiolate 5 H2N(B(C6F5)3)2−CD2Cl2 [72]FIDBUS [72] f
6-(4-tert-butylphenyl)4-tert-butyl (CH3)3CCOCHCH3−SbF6−CD2Cl2 [73]KEKGEP [73] atrans(C,N)
6-(4-tert-butylphenyl)4-tert-butyl CH3COCHC(CH3)3−SbF6−CD2Cl2 [73]
6-(4-tert-butylphenyl)4-tert-butyltrifluoroacetatemethyl CD2Cl2 [74]
6-(4-tert-butylphenyl)4-tert-butyltrifluoroacetateethyl CD2Cl2 [74]
6-(4-tert-butylphenyl)4-tert-butyl4-fluorophenyltrifluoroacetate CD2Cl2 [74]QEZYAX [74]trans(C,N)
6-(4-tert-butylphenyl)4-tert-butylpentafluorophenyltrifluoroacetate CD2Cl2 [74]
6-(4-tert-butylphenyl)4-tert-butylthiophen-2-yltrifluoroacetate CD2Cl2 [74]
6-(4-tert-butylphenyl)4-tert-butyldibenzopyrrolatediethyl ether H2N(B(C6F5)3)2−CD2Cl2 [72]
3-methyl2-fluoromethylF CD2Cl2 [21]DAJRUE [21]trans(C,N)
3-methyl2-fluoromethylBr CD2Cl2 [21]
3-methyl2-fluorotrifluoroacetatetrifluoroacetate CD2Cl2 [21]
3-methyl2-fluoropentafluorophenylpentafluorophenyl CD2Cl2 [21]DAJSAL [21] g
3-methyl2-fluoro3,5-bis(trifluoromethyl)phenylF CD2Cl2 [21]DAJROY [21]trans(C,N)
3-methyl2-fluoro3,5-bis(trifluoromethyl)phenylBr CD2Cl2 [21]DAJRIS [21]trans(C,N)
3-methyl2-fluoro3,5-bis(trifluoromethyl)phenyl2,4,6-trifluorophenyl CD2Cl2 [21]DAJSEP [21]trans(C3,5-bis(trifluoromethyl)phenyl,N)
4-methyl4-methoxy4-methoxyphenylethynyl4-methoxyphenylethynyl CD2Cl2 [37]AJOXIH [37] a LUM1 [37]
5-methyl4-methoxy4-methoxyphenylethynyl4-methoxyphenylethynyl CD2Cl2 [37]AJOXED [37] a LUM1 [37]
6-methyl4-methyltrifluoroacetatetrifluoroacetate CD2Cl2 [53]JAZBOF [53]
4-trifluoromethyl4-methyltrifluoroacetatetrifluoroacetate CD2Cl2 [53]JAYZOC [53] a
5-trifluoromethyl4-methoxy4-methoxyphenylethynyl4-methoxyphenylethynyl CD2Cl2 [37] LUM1 [37]
5-trifluoromethyl2-diphenylaminophenylethynylphenylethynyl CDCl3 [75]WECYUB [75] LUM2 [75]
3-methoxy4-methyltrifluoroacetatetrifluoroacetate CD2Cl2 [53]JAZKAA [53]
4-methoxy4-methyltrifluoroacetatetrifluoroacetate CD2Cl2 [53]JAYZIW [53]
6-methoxy4-methyltrifluoroacetatetrifluoroacetate CD2Cl2 [53]JAYYOB [53] a
5-carboxy4-carboxytrifluoroacetatetrifluoroacetate CD3CN
DMSO-d6 [22]		GALBAA [22] h CAT [22]
5-carboxy4-carboxytrifluorocarboxylethylCl DMSO-d6 [23]
5-carboxy4-carboxytrifluorocarboxylethylBr DMSO-d6 [23]
5-carboxy4-carboxytrifluorocarboxylethyltrifluoroacetate DMSO-d6 [23]
5-ethoxycarbonyl4-ethoxycarbonylN3N3 PUXGUN [76]
5-ethoxycarbonyl4-ethoxycarbonyltrifluoroacetatetrifluoroacetate CD2Cl2
DMSO-d6 [22]		GALBEE [22] CAT [22]
5-ethoxycarbonyl4-ethoxycarbonyltrifluorocarboxylethylCl CD2Cl2
DMSO-d6 [23]
5-ethoxycarbonyl4-ethoxycarbonyltrifluorocarboxylethylBr CD2Cl2
DMSO-d6 [23]
5-ethoxycarbonyl4-ethoxycarbonyltrifluorocarboxylethyltrifluoroacetate DMSO-d6 [23]
4-dimethylamino2,3,4-trifluoropentafluorophenylpentafluorophenyl CD2Cl2 [24]WASQOZ [24] LUM2 [24]
4-dimethylamino3-trifluoromethylpentafluorophenylpentafluorophenyl CD2Cl2 [24] LUM2 [24]
4-dimethylamino3-trifluoromethyl4-fluorophenylethynyl4-fluorophenylethy+nyl CD2Cl2 [24] LUM2 [24]
4-dimethylamino4-trifluoromethoxypentafluorophenylpentafluorophenyl CD2Cl2 [24] LUM2 [24]
3-nitro4-methyltrifluoroacetatetrifluoroacetate CD2Cl2 [53]JAZBIZ [53] a
$ Types of activity: BIO—biological (BIO1—anti-tumour, BIO2—anti-microbial, i.e., anti-bacterial and/or anti-fungal); CAT—catalytic; LUM—luminescence (LUM1—with t > 10 µs; LUM2—with t < 10 µs). 1 This monoanionic HNC(CH3)OCH2CH2− ligand chelates Au(III) by imine =NH nitrogen and terminal CH2 carbon. 2A–2B It is unclear whether these compounds should be regarded as [Au(2PPY*)(1,2-bis(diphenylphosphino)benzene)]Cl2 (2PPY = 2-(4-methylphenyl)pyridine for 2A, 2-(4-formylphenyl)pyridine for 2B), or rather [Au(2PPY$)(1,2-bis(diphenylphosphino)benzene)Cl]Cl (2PPY$ = monoanionic form of 2PPY, deprotonated and binding monodentately by C(2′)). 3 These are dimeric compounds, in which both [Au(2,6-bis(4-tert-butylphenyl)pyridine*)R] (R = CH3, C6F5) moieties are linked by a hydride ligand, with formation of the Au-H-Au bridge. 4 These are dimeric compounds, in which the [Au(2,6-bis(4-tert-butylphenyl)pyridine*)R] (R = H, CH3) and [Au(2,6-bis(4-tert-butylphenyl)pyridine**)] moieties are linked by a hydride ligand, with formation of the Au-H-Au bridge. 5 This is a dimeric compound, in which both [Au(2,6-bis(4-tert-butylphenyl)pyridine*)] moieties are linked by two adamantane-1-thiolate ligands, with formation of two Au-S-Au bridges. a dichloromethane solvate. b chloroform solvate. c dichloromethane n-pentane solvate. d toluene solvate. e acetonitrile solvate. f dihydrate. g trihydrate. h trifluoroacetic acid solvate.
Many these Au(III)-2PPY* compounds are biologically active, revealing anti-tumour (against various breast, cervix, colon, liver, lung, mammary, and ovarian cancers, as well as glioblastoma, leukemia, and melanoma) [8,9,11,25,26,31,51,63,64], as well as anti-bacterial (against Escherichia coli, Bacillus subtilis, and Pseudomonas aeruginosa) and anti-fungal (against Candida albicans, Trichophyton mentagrophytes, and Cladosporium resinae) [63] properties. Some others have catalytic activity (in reactions between propargyl esters and styrene—yielding cyclopropane derivatives [22] and upon CO oxidation by air to CO2 [65]). Then, a number of these species exhibits luminescence, with lifetimes of either >10 µs [19,37] or <10 µs [15,18,24,27,75].

2.2. Au(III)-2ArPY* Compounds

2.2.1. Au(III)-2ArPY* Dihalides

A total of 68 Au(III) dihalides with 2-arylpyridines*, other than 2PPY* (denoted as 2ArPY*), having the general formula [Au(2ArPY*)X2] (X = F, Cl, Br, I), were reported and characterised by NMR spectroscopy and/or by single crystal X-ray diffraction [6,9,13,16,17,18,19,30,33,40,62,77,78,79,80,81,82,83,84,85,86,87,88,89,90]. They are listed in Table 4 (by definition, it does not include [Au(2PPY*)X2] species, shown in Table 1).
The contained 2ArPY* ligands are of two principal types: (A) containing a bridge (denoted as Z) between the pyridine and the phenyl ring (–CH2– in 2-benzylpyridine*, –CO– in 2-benzoylpyridine*, –O– in 2-phenoxypyridine*, –S– in 2-phenylsulfanylpyridine*, –NH– in 2-anilinopyridine*; Scheme 2) and (B) having the pyridine ring linked to any aryl (but not phenyl) ring system (naphth-2-yl, 9,9-dialkylfluoren-2-yl, dibenzofuran-4-yl; Scheme 3).
There are 61 [Au(2ArPY*)X2] molecules with 2ArPY* ligands of type A [9,13,16,17,18,30,33,40,62,77,78,79,80,81,82,83,84,85,86,87,88,89] (including 8 compounds having one or two substituents at the Z bridge, with this position being numbered as 1 of the aryl moiety: –CH2– (6 species with 2ArPY* = 2-(1-methylbenzyl)pyridine*, 2-(1,1-dimethylbenzyl)pyridine*, 2-(1-methoxybenzyl)pyridine*, 2-(1-phenylbenzyl)pyridine*, 2-(1-carboxymethoxyiminobenzyl)pyridine* and 2-(1-benzoxyiminobenzyl)pyridine*) [17,77,83,84], or –NH– (2 species with 2ArPY* = 2-(1-methylanilino)pyridine* and 2-(1-propionylanilino)pyridine*) [40,83,89]), as well as 7 [Au(2ArPY*)X2] molecules with 2ArPY* ligands of the type B [6,19,90].
Then, the [Au(2ArPY*)X2] compounds with 2ArPY* of type A can be divided for those containing unsubstituted pyridine and phenyl rings (16 species) [9,13,16,17,18,30,33,40,62,77,78,79,80,81,82,83,84,85,86,87,89], substituted pyridine rings and unsubstituted phenyl rings (32 species) [83,88], as well as unsubstituted pyridine rings and substituted phenyl rings (13 species) [17,83,89], whereas those with simultaneously substituted pyridine and phenyl rings are absent. Similarly, the [Au(2ArPY*)X2] compounds with 2ArPY* of the type B can be divided for these having no substituents in the pyridine and aryl ring (four species) [6,19,90], having substituents in the pyridine ring only (two species) [6,90] or in the aryl ring only (one species) [90], but not in both rings. Furthermore, if treating [Au(2ArPY*)X2] molecules with 2ArPY* of the types A and B together, 20 has no substituents in both ring systems [6,9,13,16,17,18,19,30,33,40,62,77,78,79,80,81,82,83,84,85,86,87,89,90], while 48 is substituted either in the pyridine ring (34 examples) [6,83,88,90] or in the aryl ring (14 examples) [17,83,89,90].
Table 4. NMR and/or X-ray studied [Au(2ArPY*)X2] compounds (2ArPY* ≠ 2PPY*; X = F, Cl, Br, I); R1 and R2 are substituents in the pyridine ring and the aryl ring, respectively.
Table 4. NMR and/or X-ray studied [Au(2ArPY*)X2] compounds (2ArPY* ≠ 2PPY*; X = F, Cl, Br, I); R1 and R2 are substituents in the pyridine ring and the aryl ring, respectively.
Parent 2ArPY* Ring SystemR1R2XNMR SolventX-ray (CCDC)Activity $
2ArPy* ligands of type A (Scheme 2)
2-benzylpyridine*HHClCDCl3
CD2Cl2
CD3CN
CD3COCD3 DMSO-d6 [9,13,18,33,62,77,78,79,80,81,82]		HOSHEE [16]BIO1 [25,79,83]
CAT [16,17]
3-methylHClDMSO-d6 [83]
4-methylHClDMSO-d6 [83]
5-methylHClCD2Cl2 [83]
6-methylHClDMSO-d6 [83]
3-methoxyHClDMSO-d6 [83]
6-methoxyHClCD2Cl2 [83]
3-isopropoxyHClCD2Cl2 [83]
3-n-butoxyHClCD2Cl2 [83]
3-acetomethoxyHClCD2Cl2 [83]
H2-methylClCD2Cl2 [83]
H3-methylClDMSO-d6 [83]
H4-methylClDMSO-d6 [83]
H4-ethylClDMSO-d6 [83]
H4-n-butylClCD2Cl2 [83]
H4-tert-butylClCDCl3 [17] CAT [17]
H4-n-hexylClCD2Cl2 [83]
H4-chloroClDMSO-d6 [83]
H2-methoxyClDMSO-d6 [83]
H4-phenylClCD2Cl2 [83]
H4-benzylClCD2Cl2 [83]
2-(1-methylbenzyl)pyridine*HHClCDCl3
DMSO-d6 [77]		R/S 1CAT [17]
2-(1,1-dimethylbenzyl)pyridine*HHClCD2Cl2 [77]ZETYAY [77]
2-(1-methoxybenzyl)pyridine*HHClDMSO-d6 [17] CAT [17]
2-(1-phenylbenzyl)pyridine*HHClDMSO-d6 [83]
2-(1-carboxymethoxyiminobenzyl)pyridine*HHClDMSO-d6 [84]
2-(1-benzoxyiminobenzyl)pyridine*HHClDMSO-d6 [17] CAT [17]
2-benzoylpyridine*HHClCD2Cl2
CD3CN
DMSO-d6 [9,17,18,33,81,83,84,85,86]		PUKYAV [85]BIO1 [30]
CAT [17]
HHBrDMSO-d6 [30]AZOKOS [30]
HHIDMSO-d6 [30]
2-phenoxypyridine*HHClDMSO-d6 [9,87]FIJZIH [87]BIO1 [83]
CAT [17]
4-methylHClDMSO-d6 [88]
5-methylHClDMSO-d6 [88]
6-methylHClCD2Cl2 [88]
5-ethylHClDMSO-d6 [88]
5-n-propylHClDMSO-d6 [88]
5-n-butylHClCD2Cl2 [88]
5-fluoroHClDMSO-d6 [88]
5-chloroHClDMSO-d6 [88]
5-bromoHClDMSO-d6 [88]
5-iodoHClDMSO-d6 [88]
5-methoxyHClCD2Cl2 [83]
5-methoxycarbonylHClCD2Cl2 [88]
5-(2-methoxycarbonylethyl)HClCDCl3 [88]
5-(3-acetoxy-n-propyl)HClCD2Cl2 [88]
5-aminocarbonylHClDMSO-d6 [88]
5-methylaminocarbonylHClDMSO-d6 [88]
5-dimethylaminocarbonylHClDMSO-d6 [88]
5-cyclopentylaminocarbonylHClDMSO-d6 [88]KAGYIB [88]
5-acetamidoHClDMSO-d6 [88]
5-(3-cyano-n-propyl)HClCDCl3 [88]
5-phenylHClDMSO-d6 [88]
5-benzylHClCD2Cl2 [83]
5-benzamidoHClDMSO-d6 [88]
H4-tert-butylClCDCl3 [17]IFUQUX [17]
2-phenylsulfanylpyridine*HHClDMSO-d6 [87]FIKNOC [87]BIO1 [83]
2-anilinopyridine*HHClDMSO-d6 [9,18,87,89]PUGMAF [89]BIO1 [83]
HHBrDMSO-d6 [89]
H4-methylClDMSO-d6 [89]
2-(1-methylanilino)pyridine*HHClDMSO-d6 [83,89]
2-(1-propionylanilino)pyridine*HHClCDCl3 [40]
2ArPy* ligands of type B (Scheme 3)
2-(naphth-2-yl)pyridine*HHClDMSO-d6 [19]
2-(9,9-dimethylfluoren-2-yl)pyridine*HHClDMSO-d6 [6,90]
4-methylHClCD2Cl2 [90]
4-dimethylaminoHClCD2Cl2 [6,90]
H7-trifluoromethylClCD2Cl2 [90]
2-(9,9-di(n-butyl)fluoren-2-yl)pyridine*HHClDMSO-d6 [19]
2-(dibenzofuran-4-yl)pyridine*HHClDMSO-d6 [6]
$ Types of activity: BIO—biological (BIO1—anti-tumour, BIO2—anti-microbial, i.e., anti-bacterial and/or anti-fungal); CAT—catalytic; LUM—luminescence (LUM1—with t > 10 µs, LUM2—with t < 10 µs). 1 This compound appears in two isomeric forms, i.e., enantiomers R/S.
A few [Au(2ArPY*)Cl2] compounds are biologically active, revealing anti-tumour properties (against various breast, colon, kidney, lung, mammary, ovarian, pancreas, and prostate and uterus cancers, as well as leukemia) [25,30,79,83]. Some others exhibit catalytic activity (in reactions between alkynes, carbonyl compounds, and amines or imines—yielding amines, allenes, or oxazoles) [16,17].

2.2.2. Au(III)-2ArPY* Compounds with Auxiliary Ligands Other Than Halides

In addition to [Au(2ArPY*)X2] dihalides, 108 Au(III)-2ArPY* compounds (2ArPY* ≠ 2PPY*) with various auxiliary ligands (other than halides), having the general formula [Au(2ArPY*)L1L2] (particularly [Au(2ArPY*)L2]) or [Au(2ArPY*)(L1L2)]) (particularly [Au(2ArPY*)(LL)]) were reported and characterised by NMR spectroscopy and/or by single crystal X-ray diffraction [6,9,14,18,19,25,30,31,33,39,40,45,51,62,63,64,75,77,79,82,85,87,89,90,91,92,93,94,95,96,97,98,99,100,101,102]. These species correspond to the most general case presented in Scheme 2 and Scheme 3 (for L1, L2 ≠ F, Cl, Br, I), being listed in Table 5 (by definition, it does not include the Au(III)-2ppy* and Au(III)-2PPY* molecules, shown in Table 2 and Table 3). A total of 88 molecules have 2ArPY* ligands of the type A [9,14,18,25,30,31,33,40,45,51,62,63,64,77,79,82,85,87,89,91,92,93,94,95,96,97,98,99,100,101], while 20 have 2ArPY* ligands of the type B [6,19,39,75,90,102].
As many as 97 Au(III)-2ArPY* compounds contain unsubstituted (or substituted only at the Z bridge) 2ArPY* ligands [6,9,14,18,19,25,30,31,33,39,40,45,51,62,63,64,77,79,82,85,87,89,90,91,92,93,94,95,96,97,98,99,100,101,102] (all 88 molecules with 2ArPY* of the type A [9,14,18,25,30,31,33,40,45,51,62,63,64,77,79,82,85,87,89,91,92,93,94,95,96,97,98,99,100,101], including the predominant 2-benzylpyridine*—52 species and 9 molecules of the type B [6,19,39,90,102]).
In contrast, there are only 11 Au(III)-2ArPY* compounds with substituent(s) in the 2ArPY* ring system (except for those at the Z bridge). All these molecules are with 2ArPY* ligands of the type B [6,75,90], and eight are substituted in the pyridine ring only [6,75,90]; 1—in the aryl ring only [90], and 2—in both rings [75].
Table 5. NMR and/or X-ray studied [Au(2ArPY*)L1L2] (in particular, [Au(2ArPY*)L2]) or [Au(2ArPY*)(L1L2)]) (in particular, [Au(2ArPY*)(LL)]) compounds (2ArPY* ≠ 2PPY*; L1, L2, L—monodentate ligands other than F, Cl, Br, I; L1L2, LL—bidentate ligands); R1 and R2 are substituents in the pyridine ring and the aryl ring, respectively.
Table 5. NMR and/or X-ray studied [Au(2ArPY*)L1L2] (in particular, [Au(2ArPY*)L2]) or [Au(2ArPY*)(L1L2)]) (in particular, [Au(2ArPY*)(LL)]) compounds (2ArPY* ≠ 2PPY*; L1, L2, L—monodentate ligands other than F, Cl, Br, I; L1L2, LL—bidentate ligands); R1 and R2 are substituents in the pyridine ring and the aryl ring, respectively.
Parent 2ArPY* Ring SystemR1R2L1L2L1L2CounterionNMR SolventX-ray (CCDC)GeometryActivity $
2ArPy* ligands of type A (Scheme 2)
2-benzylpyridine*HHacetateacetate CDCl3 [31,91] BIO1 [31]
HH 1R,2R-cyclohexane-1,2-diamine2Cl−
2ClO4−
2BF4−		DMSO-d6 [92]
DMSO-d6 [92]
DMSO-d6 [92]		TOFDUQ [92]
TOFFAY [92] a		 BIO1 [92]
HH dimethylaminocarbodithioatePF6−CD3CN [93]RADKOA [93] BIO1 [93]
HH diethylaminocarbodithioatePF6−CD3CN [93]RADLER [93] BIO1 [93]
HH azacyclohexane-1-carbodithioatePF6−CD3CN [93]RADLAN [93] BIO1 [93]
HH 4-(4-bromophenyl)-1,4-diazacyclohexane-1-carbodithioatePF6−CD3CN [93] BIO1 [93]
HH 4-(4-methoxyphenyl)-1,4-diazacyclohexane-1-carbodithioatePF6−CD3CN [93]RADKUG [93] b BIO1 [93]
HH 4-ethoxycarbonylazacyclohexane-1-carbodithioatePF6−DMSO-d6 [82]OVIKOW [82] BIO1 [82]
HH 4-aminocarbonylazacyclohexane-1-carbodithioatePF6−DMSO-d6 [82]OVILAJ [82] OVIKUC [82] c BIO1 [82]
HH 1,1-dimethylbiguanidatePF6−DMSO-d6 [33]CEWGUK [33] BIO1 [33]
HH 1,2-bis(ethylimine)ethane-1,2-dithiolate CDCl3 [94]
HHphenylethynylphenylethynyl CD3COCD3 [95]
HHphenylethynylCl CD3COCD3 [95]ECEGOM [95] ECEGOM 01 [95]trans(C,N)
HHpentafluorophenylpentafluorophenyl CD2Cl2 [18]MIYXIE [18] LUM2 [18]
HH4-trifluoromethylphenyl4-trifluoromethylphenyl CDCl3 [14]
HH benzene-1,2-diolate CDCl3 [96] BIO1, BIO2 [96]
HH anthracene-9,10-dione-1,2-diolate DMSO-d6
solid phase [62]
HH 1,1′-binaphthyl-2-one-2′-olate-1-yl CDCl3 [45] R/S 1
HH 2-thiolatebenzoate CDCl3 [51] BIO1 [51]
HH3-ethoksycarbonylphenyl3-ethoksycarbonylphenyl CDCl3 [14]
HH benzene-1-olate-2-acetylazanide CDCl3 [96]BAZSEB [96]trans(N,N)BIO1, BIO2 [25,96]
HH ethylene-1,2-bis(4-methylphenylsulfonylazanide) unknown [63] BIO1, BIO2 [63]
HH benzene-1,2-bis(acetylazanide) unknown [63]XEWBOR [63] BIO1, BIO2 [63]
HH benzene-1,2-bis(4-methylphenylsulfonylazanide) unknown [63] BIO1, BIO2 [63]
HH1-acetamidobenzene-2-acetylazanideCl XEWBUX [63]trans(N,N)
HH benzene-1-olate-2-(N-(acridin-9-yl))azanide CD2Cl2 [25] BIO1 [25]
HHisatinateisatinate CDCl3 [97] BIO1, BIO2 [97]
HHphtalimidatephtalimidate CDCl3 [97]SEVDON [97] d BIO1, BIO2 [97]
HHsaccharinatesaccharinate CDCl3 [97]SEVDUT [97] e BIO1, BIO2 [97]
HHL-phenylalaninate Cl−CDCl3 [31] BIO1 [31]
HH sulfonebis(cyanomethyl) BIO1 [64]
HH sulfonebis(benzoylmethyl) unknown [64] BIO1 [64]
HH benzoylmethyl benzoyl(2-(tert-butylamino)ethyl) sulfone unknown [64]DEFGUR [64] BIO1 [64]
HH 4-methylphenylsulfonyliminocarbo(phenylazanide)thiolate CDCl3 [98]
HH4-nitrophenylazanideacetate CDCl3 [91]
HHthiophen-2-ylthiophen-2-yl CD2Cl2 [95]ECEGAY [95]
HH 2,5-bis(ethoxycarbonyl)thiophen-3,4-diolate CDCl3 [96] BIO1, BIO2 [96]
HH phenyliminocarbo(pyrrol-5-yl-1-ate)thiolate CDCl3 [99]
HH phenyliminocarbo(2,4-dimethylpyrrol-5-yl-1-ate)thiolate CDCl3 [99]FUJHUQ [99] dtrans(S,N)
HH phenyliminocarbo(2,4-dimethyl-3-ethylpyrrol-5-yl-1-ate)thiolate CDCl3 [99]
HH 4-nitrophenyliminocarbo(pyrrol-5-yl-1-ate)thiolate CDCl3 [99] BIO1 [99]
HH 4-nitrophenyliminocarbo(2,4-dimethylpyrrol-5-yl-1-ate)thiolate CDCl3 [99]
HH 4-nitrophenyliminocarbo(2,4-dimethyl-3-ethylpyrrol-5-yl-1-ate)thiolate CDCl3 [99]
HH 2-(4-aminophenylsulfonamido)thiazol-3-yl DMSO-d6 [97]
HH 2,2′-bipyridine CD3CN [100]
HH 1,10-phenanthroline CD3CN [100]
HHtriphenylphosphineCl BF4−
Cl−		CDCl3
CD2Cl2
DMSO-d6 [77,85]		PUKYEZ [85] dtrans(P,N)
HH 1,2-bis(diphenylphosphino)ethane2BF4−CD2Cl2 [77]
HH 1,2-bis(diphenylphosphino)benzene 2A2Cl−CDCl3 [9]USEDOO [9] 3A,f BIO1 [9]
HH1,3,5-triazaphosphaadamantaneCl PF6−CD3COCD3 [79]QUMZIJ [79] gtrans(P,N)BIO1 [79]
HH1,2,3,4-tetraacetyl-6-thioglucose-6-ylCl CD3COCD3 [79] BIO1 [79]
HH1,2,3,4-tetraacetyl-6-thioglucose-6-yl1,2,3,4-tetraacetyl-6-thioglucose-6-yl CD3COCD3 [79] BIO1 [79]
2-(1-methylbenzyl)pyridine*
HHtriphenylph osphineCl BF4−CDCl3 [77] R/S 1
HH 1,2-bis(diphenylphosphino)ethane2BF4−CD2Cl2 [77] R/S 1
2-(1,1-dimethylbenzyl)pyridine*HHtriphenylphosphineCl BF4−CDCl3 [77]
2-benzoylpyridine*HH 2-benzoylpyridine*BF4−DMSO-d6 [85]
HHSCNSCN DMSO-d6 [30]
HHN3N3 DMSO-d6 [30]AZOLAF [30]
HHn-pentane-2,4-dione-3-ylCl CDCl3 [30]
HH dimethylaminocarbodithioatePF6−CD3CN [93] BIO1 [93]
HH diethylaminocarbodithioatePF6−CD3CN [93] BIO1 [93]
HH azacyclohexane-1-carbodithioatePF6−CD3CN [93]RADLIV [93] BIO1 [93]
HH 4-(4-bromophenyl)-1,4-diazacyclohexane-1-carbodithioatePF6−CD3CN [93] BIO1 [93]
HH 4-(4-methoxyphenyl)-1,4-diazacyclohexane-1-carbodithioatePF6−CD3CN [93] BIO1 [93]
HH 1,1-dimethylbiguanidatePF6−DMSO-d6 [33] BIO1 [33]
HHpentafluorophenylpentafluorophenyl CD2Cl2 [18]MIYXEA [18] LUM2 [18]
HH 1,1′-binaphthyl-2-one-2′-olate-1-yl CDCl3 [45] R/S 1
HH 1,2-bis(diphenylphosphino)benzene 2B2Cl−CDCl3 [9]USEDII
[9] 3B		 BIO1 [9]
2-phenoxypyridine*HH4-fluorophenylethynyl4-fluorophenylethynyl CDCl3
CD3CN [40]
HH 1,1′-binaphthyl-2-one-2′-olate-1-yl CDCl3 [45] R/S 1
HH 1,2-bis(diphenylphosphino)benzene 2C2Cl−CDCl3 [9]USEDEE [9] 3C,h BIO1 [9]
2-anilinopyridine*HHpentafluorophenylpentafluorophenyl CD2Cl2 [18]MIZHUB [18] d LUM2 [18]
HH benzene-1,2-diolate CDCl3 [96] BIO1, BIO2 [96]
HH 3,5-di(tert-butyl)benzene-1,2-diolate BIO [96]
HH 2-thiolatebenzoate CDCl3 [51] BIO1 [51]
HH3-ethoksycarbonylphenyl3-ethoksycarbonylphenyl CDCl3 [14]
HH anthracene-9,10-dione-1,2-diolate DMSO-d6
solid phase [62]
HH 1,1′-binaphthyl-2-one-2′-olate-1-yl CDCl3 [45] R/S 1
HH sulfonebis(benzoylmethyl) unknown [64] BIO1 [64]
HH ethylene-1,2-bis(4-methylphenylsulfonylazanide) unknown [63] BIO1, BIO2 [63]
HH benzene-1,2-bis(acetylazanide) BIO1, BIO2 [63]
HH benzene-1,2-bis(4-methylphenylsulfonylazanide) unknown [63] BIO1, BIO2 [63]
HH dimethyliminocarbo(phenylazanide)thiolateB(C6H5)4−DMSO-d6 [101]MIRLIK [101]trans(S,N)
HH dicyclohexyliminocarbo(phenylazanide)thiolateB(C6H5)4−DMSO-d6 [101]MIRLOQ [101]trans(S,N)
HH oxydiethyliminocarbo(phenylazanide)thiolateB(C6H5)4−DMSO-d6 [101]
HHtriphenylphosphineCl BF4−DMSO-d6 [87]FIKQAR [87]trans(P,N)
HHtris(4-methylphenyl)phosphineCl Cl−CDCl3 [89]
HH 1,2-bis(diphenylphosphino)benzene 2D2Cl−CDCl3 [9]USEDUU [9] 3D BIO1 [9]
2ArPy* ligands of type B (Scheme 3)
2-(naphth-2-yl)pyridine*HH methylenebis(3-n-butyl-1H-imidazol-1-yl-2-ylidene)2PF6−CD3CN [19] LUM1 [19]
2-(9,9-dimethylfluoren-2-yl)pyridine*HH bis(2-acetylidephenyl)acetylene CD2Cl2 [39] LUM1 [39]
HHpentafluorophenylpentafluorophenyl CD2Cl2 [90]MEDPOD [90] i LUM1 [90]
HH 3,3′-bis(trifluoromethyl)-5,5′-bipyrazolate DMSO-d6 [6]
HH 1,1-dimethylmethylenebis(3-trifluoromethylpyrazol-5-ylate) CD2Cl2 [6]QOCBES [6] j LUM1 [6]
4-methylHpentafluorophenylpentafluorophenyl CD2Cl2 [90]MEDPIX [90] k LUM1 [90]
4-dimethylaminoHpentafluorophenylpentafluorophenyl CD2Cl2 [90]MEDPAP [90] LUM1 [90]
4-dimethylaminoH 1,1-dimethylmethylenebis(3-trifluoromethylpyrazol-5-ylate) CD2Cl2 [6]QOCBUI [6] LUM1 [6]
H7-trifluoromethylpentafluorophenylpentafluorophenyl CD2Cl2 [90]MEDPET [90] LUM1 [90]
2-(9,9-di(n-butyl)fluoren-2-yl)pyridine*HH methylenebis(3-n-butyl-1H-imidazol-1-yl-2-ylidene)2PF6−CD3CN [19] LUM1 [19]
2-(9,9-di(n-hexyl)fluoren-2-yl)pyridine*5-methylHphenylethynylphenylethynyl CDCl3 [75] LUM1 [75]
5-methylH3,5-bis(trifluoro)phenylethynyl3,5-bis(trifluoro)phenylethynyl CDCl3 [75] LUM1 [75]
5-trifluoromethylH3,5-bis(trifluoro)phenylethynyl3,5-bis(trifluoro)phenylethynyl CDCl3 [75] LUM1 [75]
5-methyl7-(4-diphenylaminophenyl)phenylethynylphenylethynyl CDCl3 [75] LUM1 [75]
5-trifluoromethyl7-(4-diphenylaminophenyl)phenylethynylphenylethynyl CDCl3 [75] LUM1 [75]
2-(9,9-(1,1′-biphenyl-2,2′-diyl)fluoren-2-yl)pyridine*5-trifluoromethylH3,5-bis(trifluoro)phenylethynyl3,5-bis(trifluoro)phenylethynyl CDCl3 [75] LUM1 [75]
2-(9,9-bis(2-hydroxyethyl)fluoren-2-yl)pyridine*HHphenylethynylphenylethynyl DMSO-d6 [102]
2-(dibenzofuran-4-yl)pyridine*HH bis(2-acetylidephenyl)acetylene CD2Cl2 [39] LUM1 [39]
HH 1,1-dimethylmethylenebis(3-trifluoromethylpyrazol-5-ylate) DMSO-d6 [6]QOCBOC [6] LUM1 [6]
4-dimethylaminoH 1,1-dimethylmethylenebis(3-trifluoromethylpyrazol-5-ylate) DMSO-d6 [6] LUM1 [6]
$ Types of activity: BIO—biological (BIO1—anti-tumour, BIO2—anti-microbial, i.e., anti-bacterial and/or anti-fungal); CAT—catalytic; LUM—luminescence (LUM1—with t > 10 µs, LUM2—with t < 10 µs). 1 This compound appears in two isomeric forms, i.e., enantiomers R/S. 2A–2D It is unclear whether in the solution these compounds should be regarded as [Au(2ArPY*)(1,2-bis(diphenylphosphino)benzene)]Cl2 (2ArPY = 2-benzylpyridine for 2A, 2-benzoylpyridine for 2B, 2-phenoxypyridine for 2C, 2-anilinopyridine for 2D) or rather [Au(2ArPY$)(1,2-bis(diphenylphosphino)benzene)Cl]Cl (2ArPY$ = monoanionic form of 2ArPY, deprotonated and binding monodentately by C(2′)). 3A–3D In the solid phase, the N(1),C(2′)-chelation is observed for 2A (USEDOO), while the monodentate C(2′) coordination for 2B-2D (USEDII, USEDEE, USEDUU). a 1/2 chloride, 1/2 tetrafluoroborate salt. b diethyl ether solvate. c acetonitrile solvate. d dichloromethane solvate. e dichloromethane diethyl ether solvate. f methanol solvate. g acetone solvate. h dimethylformamide solvate. i diethyl ether n-hexane solvate. j n-pentane solvate. k ethyl acetate solvate.
Many above Au(III)-2ArPY* compounds are biologically active, revealing anti-tumour (against various bowel, breast, colon, lung, and mammary and ovarian cancers, as well as leukemia) [9,25,31,33,51,63,64,79,82,92,93,96,97,99], as well as anti-bacterial (against Escherichia coli, Bacillus subtilis, and Pseudomonas aeruginosa) and anti-fungal (against Candida albicans, Trichophyton mentagrophytes, and Cladosporium resinae) [63,96,97] properties. To the best of our knowledge, the catalytic activity of these species seems to be unknown, while some of them reveal luminescence, with lifetimes of either >10 µs [6,19,39,75,90] or <10 µs [18].

2.3. Au(III)-ArPY#* Compounds

A total of 33 Au(III) compounds with analogues of 2-arylpyridines* (e.g., 2-phenylquinoline*, 1- or 3-phenylisoquinoline* and 7,8-benzoquinoline*) and their derivatives (generally denoted as ArPY#*), with various auxiliary ligands (including halides), having the general formula [Au(ArPY#*)L1L2] (particularly [Au(ArPY#*)L2], including [Au(ArPY#*)X2]) or [Au(ArPY#*)(L1L2)]) (particularly [Au(ArPY#*)(LL)]) were reported and characterised by NMR spectroscopy and/or by single crystal X-ray diffraction [2,5,9,10,15,19,27,37,39,41,45,75,102,103,104,105,106,107,108]. These species are presented in Scheme 4, being listed in Table 6 (by definition, it does not include the Au(III)-2ppy*, Au(III)-2PPY*, and Au(III)-ArPY#* molecules shown in Table 1, Table 2, Table 3, Table 4 and Table 5).
A total of 5 compounds are [Au(ArPY#*)X2] dihalides [2,5,9,10,15,19,104,105,106], while 28 molecules contain some other monodentate or bidentate ligands, revealing the general formula [Au(ArPY#*)L1L2] (including [Au(ArPY#*)L2]) or [Au(ArPY#*)(L1L2)]) (including [Au(ArPY#*)(LL)]) [2,9,15,19,27,37,39,41,45,75,102,103,105,106,107,108].
Taking into account the type of ArPy#* molecule, these are Au(III) species (their respective numbers in parentheses) with heterocycles based on 2-phenylquinoline* (4) [2,103], 1-phenyl-, 1-(naphth-2-yl)- or 1-(9,9-di(n-hexyl)fluoren-2-yl)isoquinoline* (9) [10,19,27,37,39,75], 3-phenyl- or 3-(9,9-bis(2-hydroxyethyl)fluoren-2-yl)isoquinoline* (6) [45,102,103], and 7,8-benzoquinoline* (14) [5,9,10,15,41,45,103,105,106,107,108] ring systems.
Table 6. NMR and/or X-ray studied [Au(ArPY#*)L1L2] (in particular, [Au(ArPY#*)L2]) or [Au(ArPY#*)(L1L2)]) (in particular, [Au(ArPY#*)(LL)]) compounds (ArPY#* ≠ 2PPY*, 2ArPY*; L1, L2, L—monodentate ligands (including F, Cl, Br, I); LL—bidentate ligands); R1 and R2 are substituents in the pyridine-like ring and the aryl ring, respectively.
Table 6. NMR and/or X-ray studied [Au(ArPY#*)L1L2] (in particular, [Au(ArPY#*)L2]) or [Au(ArPY#*)(L1L2)]) (in particular, [Au(ArPY#*)(LL)]) compounds (ArPY#* ≠ 2PPY*, 2ArPY*; L1, L2, L—monodentate ligands (including F, Cl, Br, I); LL—bidentate ligands); R1 and R2 are substituents in the pyridine-like ring and the aryl ring, respectively.
Parent ArPY#*
Ring System		R1R2L1L2L1L2CounterionNMR SolventX-ray (CCDC)GeometryActivity $
2-phenylquinoline*HH 2-phenylquinoline*BF4−DMSO-d6 [103]ZINHUB [103]trans(C,N)CAT [103]
4-methoxycarbonylHClCl CDCl3 [2]
4-methoxycarbonylH dimethylaminocarbodithioatePF6−DMSO-d6 [2]
4-methoxycarbonylH2-(4-methoxycarbonylquinolin-2-yl)phenylCl MAXQUX [2]trans(C,N)
1-phenylisoquinoline*HHClCl DMSO-d6 [10]
HHCNCN CD2Cl2 [27]DUZDAF [27] DUZDAF 01 [27] DUZDAF 02 [27] LUM2 [27]
HH bis(2-acetylidephenyl)acetylene CD2Cl2 [39] LUM1 [39]
HH4-methoxyphenylethynyl4-methoxyphenylethynyl CD2Cl2
[37]		 LUM1 [37]
H4-methoxy4-methoxyphenylethynyl4-methoxyphenylethynyl CD2Cl2
[37]		AJOXON [37] a LUM1 [37]
1-(naphth-2-yl)isoquinoline*HHClCl DMSO-d6 [19]
HH methylenebis(3-n-butyl-1H-imidazol-1-yl-2-ylidene)2PF6−CD3CN [19] LUM1 [19]
HH methylenebis(3-(3-sulfonate-n-propyl)-1H-imidazol-1-yl-2-ylidene) D2O [19] LUM1 [19]
1-(9,9-di(n-hexyl)fluoren-2-yl)isoquinoline*H7-(4-diphenylaminophenyl)phenylethynylphenylethynyl CDCl3 [75] LUM1 [75]
3-phenylisoquinoline*HH 3-phenylisoquinoline*BF4−DMSO-d6 [103] CAT [103]
HH 1,1′-binaphthyl-2-one-2′-olate-1-yl CDCl3
[45]		 R/S 1
H4-tert-butylphenylethynylphenylethynyl CDCl3
[102]
3-(9,9-bis(2-hydroxyethyl)fluoren-2-yl)isoquinoline*HHphenylethynylphenylethynyl DMSO-d6
[102]		 BIO1 [102]
HH4-methylphenylethynyl4-methylphenylethynyl DMSO-d6
[102]
HH4-fluorophenylethynyl4-fluorophenylethynyl DMSO-d6
[102]
7,8-benzoquinoline*HHClCl DMSO-d6 [5,9,10,15,104]
HHII CDCl3
[105,106]		EWIWUE [105]
HHn-butylCl CDCl3
[106]
HHn-butylH2O SbF6−CD2Cl2 [106]
HHtrifluoroacetatetrifluoroacetate CD3COCD3 [106,107] CAT [107]
HHphenylBr CDCl3
[106]
HH4-phenyl-n-butylCl CDCl3
[106]		FIBRUG [106] btrans(C,N)
HH4-phenyl-n-butylH2O SbF6− FIBROA [106]trans(C,N)
HHphenylethynylphenylethynyl CD2Cl2 [15]IZOTAT [15] LUM1 [15]
HH2,4,6-tris(trifluoromethyl)phenylCl CD2Cl2 [41]GIVROU [41]trans(C,N)LUM2 [41]
HH 1,1′-binaphthyl-2-one-2′-olate-1-yl CDCl3
[45]		 R/S 1
HHpyridineI SbF6−CD3CN [105]EWIXAL [105]trans(I,N)
HH 1,2-(bis(pyridin-4-yl)sulfanyl)ethene-1,2-dithiolate CD2Cl2 [108]QUVXIR [108]
HH 1,2-bis(diphenylphosphino)benzene 22Cl−CDCl3 [9] BIO1 [9]
$ Types of activity: BIO—biological (BIO1—anti-tumour, BIO2—anti-microbial, i.e., anti-bacterial and/or anti-fungal); CAT—catalytic; LUM—luminescence (LUM1—with t > 10 µs, LUM2—with t < 10 µs). 1 This compound appears in two isomeric forms, i.e., enantiomers R/S. 2 It is unclear whether this compound should be regarded as [Au(7,8-benzoquinoline*)(1,2-bis(diphenylphosphino)benzene)Cl]Cl (7,8-benzoquinoline$ = monoanionic form of 7,8-benzoquinoline, deprotonated and bound monodentately by C(10)). a dichloromethane solvate. b chloroform solvate.
Two of the above Au(III)-ArPY#* compounds are biologically active, revealing anti-tumour properties (against various breast, colon, liver, and lung and ovarian cancers, as well as melanoma) [9,102]. Some of the others have catalytic properties (in reactions between benzaldehyde, piperidine, and phenylacetylenes—yielding propargylamines—and between alkynyl alcohols and 1-methylindol—yielding substituted indols [103]—as well as upon hydroarylation reactions between diphenylacetylene and 1,3,5-trimethoxybenzene—yielding styrene derivatives [107]). Then, a number of these species exhibits luminescence, with lifetimes of either >10 µs [15,19,37,39,75] or <10 µs [27,41].

2.4. Discussion of Single Crystal X-ray Structures

Nearly all Au(III)-2PPY* (including Au(III)-2ppy*), Au(III)-2ArPY*, and Au(III)-ArPY#* compounds have coordination number 4 and square-planar geometry (the only exclusions are [Au(2-phenylpyridine*)(1,4,7-trithiacyclononane-κ3-S,S,S)]2+ and [Au(2-(4-methylphenyl)pyridine*)(1,4,7-trithiacyclononane-κ3-S,S,S)]2+ in their hexafluorophosphate salts—MOCFOB, MOCFIV [5]—which exhibit coordination number 5). Thus, in cases of L1 ≠ L2 or unsymmetrical L1L2 ligands, two geometric isomers are possible—differing in the position of both donor atoms versus the nitrogen of the pyridine (or pyridine-like) ring and the metallated carbon of the phenyl (or, more generally, aryl) ring.
In Table 1, Table 2, Table 3, Table 4, Table 5 and Table 6, it was indicated, for molecules with the known X-ray structures, which donor atom (or the whole donor moiety) of the auxiliary ligand(s) is trans to the nitrogen of the pyridine (or pyridine-like) ring (column Geometry).
The comparison of these X-ray structures exhibits that the molecules having various elements, as donor atoms of the auxiliary ligand(s) usually adopt the following geometries:
trans(N,N), instead of trans(O,N) (MAXQEH [2], KIGPEY [25], BAZSEB [96]) or trans(Cl,N) (XEWBUX [63]);
trans(S,N), instead of trans(O,N) (ILETIC [28], ICUMEY [51]), trans(Cl,N) (AZOKUY [30]) or trans(N,N) (ILETOI, ILETEY [28], LORCOM, LORCEC, LORCIG [34], FUJHUQ [99], MIRLIK, MIRLOQ [101]);
trans(I,N), instead of trans(Cl,N) (VUVKOP [23]), trans(N,N) (EWIXAL [105]) or trans(Br,N) (VUVLEG [23]);
trans(C,N), instead of trans(F,N) (DAJRUE, DAJROY [21]), trans(O,N) (BIGREP [29], XOLCEI [42], FONDIX, FONCIW [43], IDAJII, IDAJOO [54], FUWXIG, FUWXOM [57], YIDHIF, YIDGIE, YIDHEB, YIDHAX, YIDGOK, YIDGAW, YIDGUQ, YIDGEA, YIDFUP [58], QEFVUV, QEFWAC [59], PEZQUI [67], KEKGEP [73], QEZYAX [74], FIBROA [106]), trans(Cl,N) (MAXQUX [2], FONSAE, FONRUX [14], BIGRAL [29], IVAZAI [36], GIVRIO, GIVROU [41], ECEGOM, ECEGOM 01 [95], FIBRUG [106]), trans(N,N) (XOLCUY, XOLCIM, XOLCOS [42], IPISEH, IPISAD [60], ZINHUB [103]), trans(Br,N) (DAJRIS [21], JOTROB [12], ROVYAF [55], LUWKAS [56]), trans(I,N) (GIVSOV [41]), and trans(P,N) (XOLDAF, XOLDEJ [42], IDAJUU [54]);
trans(P,N), instead of trans(F,N) (IVAZEM [36]), trans(Cl,N) (QUNSIE [35], IVAYUB, IVAYOV [36], QUMZIJ [79], PUKYEZ [85], FIKQAR [87]), or trans(S,N) (MAXQIL [2]).
Hence, generally, less electronegative (less electron-acceptor) elements are preferred to be positioned trans to the pyridine (or pyridine-like) nitrogen. The exception is the pair of trans(C,N) and trans(P,N), as in the X-ray structures XOLDAF, XOLDEJ, and IDAJUU, and the former geometry is preferred [42,54], although carbon is more electronegative than phosphorus.
Even more important exclusions are the X-ray structures MIZHEL, MIYXOK, and MIYXUQ, where the trans(Cl,N) geometry was observed, instead of the seemingly more expected trans(C,N) one, upon the presence of the C6F5− and Cl− ligands [18]. However, it can be explained by the fact that, despite a large difference in the electronegativity of carbon and chlorine, in these molecules, there is a competition (in occupying trans to nitrogen position) of the pentafluorophenyl anion with the chloride one—while the former has extremely strong electron-acceptor properties, due to the presence of five fluorine atoms in the phenyl ring.
The most important structural parameters in the discussed Au(III) compounds are the Au–N and Au–C bond lengths, as well as the N–Au–C bond angles. They were usually given in original papers, but can also be deduced from the respective CIF files, which have been used as a primary source for this review. All these values, associated to the CCDC reference codes given in Table 1, Table 2, Table 3, Table 4, Table 5 and Table 6, together with indication which elements (donor atoms) of the auxiliary ligands, are in the trans position, with respect to the metallated N and C atoms, which are listed in Table 7.
Table 7. Au–N and Au–C bond lengths [Å], as well as N–Au–C bond angles [°] in Au(III)-2PPY* (including Au(III)-2ppy*), Au(III)-2ArPY*, and Au(III)-ArPY#* compounds.
In the majority of cases, the Au–N bonds are longer than those of Au–C, which is well-reflected by comparison of their mean bond lengths, averaged for 206 X-ray structures (among all 207; in case of FONREH the interatomic distances could not be deduced, due to the bad quality of data), after preliminary averaging of these parameters for each Au(III) species (when two or more slightly differing, crystallographically inequivalent molecules are present in the crystal lattice): 2.072 Å versus 2.028 Å. Similarly, the range of Au–N bond lengths (1.975–2.283 Å) also corresponds to higher values than that for Au–C (1.845–2.100 Å), despite their partial overlapping.
The N–Au–C bond angles vary within a 79.2–91.0° range, with a mean value of 82.5°.
It is interesting to compare the X-ray structures of the cyclometallated [Au(2PPY*)Cl2] and [Au(2ArPY*)Cl2] dichlorides with the respective [Au(2PPY)Cl3] and [Au(2ArPY)Cl3] trichloride complexes. Such pairs of X-ray structural data are available for Au(III) compounds with 2-phenylpyridine [3,109], 2-(2,4-difluorophenyl)pyridine [16], 2-(2-trifluoromethoxyphenyl)pyridine [18], 2-benzylpyridine [16], 2-benzoylpyridine [18,85], and 2-phenylsulfanylpyridine [87,110]. In this case, the comparable parameter is the Au–N bond length, with its values being listed in Table 8.
Table 8. Au–N bond lengths [Å] in analogous [Au(2PPY*)Cl2] or [Au(2ArPY*)Cl2] and [Au(2PPY)Cl3] or [Au(2ArPY)Cl3] compounds.
This comparison, however, does not reveal any clear relationship between the Au–N bond lengths in the respective dichloride and trichloride species. Their differences for the corresponding Au(III) compounds are of variable sign and small absolute magnitude, being statistically not significant. This is also exhibited by the overlapping of both ranges of this parameter: 2.01–2.06 Å for [Au(2PPY*)Cl2] and [Au(2ArPY*)Cl2] versus 2.03–2.06 Å for [Au(2PPY)Cl3] and [Au(2ArPY)Cl3], as well as by nearly the same mean values: 2.039 Å for [Au(2PPY*)Cl2] and [Au(2ArPY*)Cl2] versus 2.047 Å for [Au(2PPY)Cl3] and [Au(2ArPY)Cl3].

2.5. Discussion of 15N NMR Spectra

In addition to the routine 1H and/or 13C (and, optionally, 19F or 31P) NMR spectra, some Au(III)-2PPY* (including Au(III)-2ppy*), Au(III)-2ArPY*, and Au(III)-ArPY#* compounds were studied by 15N NMR [4,7,53,60,78,80,86,104]. Their 15N chemical shifts (with respect to neat nitromethane), together with 15N coordination shifts (i.e., differences from 15N chemical shifts for free ligands), are listed in Table 9.
Table 9. 15N chemical shifts (with respect to CH3NO2, in ppm—δ15N) and 15N coordination shifts (Δ15Ncoord) for [Au(2PPY*)L1L2], [Au(2PPY*)(L1L2)], [Au(2ArPY*)L1L2], [Au(2ArPY*)(L1L2)], [Au(ArPY#*)L1L2], [Au(ArPY#*)(L1L2)] compounds (L1, L2, L—monodentate ligands (including F, Cl, Br, I), LL—bidentate ligands; R1 and R2 are substituents in the pyridine (or pyridine-like) ring and the phenyl (aryl) ring, respectively.
In all cases, the Au(III) coordination of 2PPY* (including 2ppy*), 2ArPY*, or ArPY#* leads to a large decrease of the 15N NMR chemical shift of the metallated nitrogen (comparing to the parent heterocycle, measured preferably in the same solvent), reflecting a strong 15N shielding phenomenon and resulting in a significant low-frequency (i.e., upfield) shift of the 15N signal (thus, the Δ15Ncoord values are negative). The absolute magnitude of this effect is ca. 45–105 ppm.
In two reviews by Pazderski [112,113], the dependency was identified in that of square-planar Au(III) complexes or organometallics with aza aromatic ligands (such as azines, e.g., pyridine derivatives, etc.), and the absolute magnitude of the 15N NMR coordination shift (|Δ15Ncoord|) mainly reflected the type of a donor atom in the trans position, with respect to the Au(III)-bonded nitrogen. For example, in the two pairs of [AuIIILCl3] and trans-[AuIIIL2Cl2]+ species, the |Δ15Ncoord| parameter for the former compound (nitrogen trans to Cl) was smaller than for the latter one (nitrogen trans to N): 84.8 ppm versus 87.2 ppm for L = pyridine, and 78.1 ppm versus 91.0 ppm for L = 4-methylpyridine [112,113].
Such observations can also be performed for some of the presently reviewed Au(III) species, when compared to the compounds containing the same cycloaurated ligand. Based on the data collected in Table 9, such a comparison is possible for the series of [Au(2-(4-methylphenyl)pyridine*)L1L2] molecules with various L1 and L2 ligands (methyl, allyl, phenyl, acetate, trifluoroacetate, and bromide anions). Thus, for [Au(2-(4-methylphenyl)pyridine*)(acetate)2] and [Au(2-(4-methylphenyl)pyridine*)(trifluoroacetate)2] (nitrogen trans to O), the |Δ15Ncoord| parameter is much larger than for [Au(2-(4-methylphenyl)pyridine*)(methyl)2] (nitrogen trans to C): 89.5–90.7 ppm versus 56.1 ppm [53]. In this way, based on the 15N NMR spectra only, one could assume that, in all other “unsymmetrical” [Au(2-(4-methylphenyl)pyridine*)LBr] (L = methyl, allyl, phenyl) compounds, the nitrogen atoms are positioned trans to C, rather than trans to Br, because their |Δ15Ncoord| values (46.7–53.0 ppm) are rather small and close to that of [Au(2-(4-methylphenyl)pyridine*)(methyl)2] (56.1 ppm). In fact, the proposed trans(C,N) geometry for these three molecules was actually confirmed by the X-ray structure of [Au(2-(4-methylphenyl)pyridine*)(allyl)Br] (ROVYAF [55]), in accordance with the already mentioned preference to form trans(C,N), instead of trans(C,Br) isomers.
A more detailed discussion of this issue is difficult, due to the small number of X-ray structures, for which, the 15N NMR data were also reported. They are available only for the pair of [Au(2-(4-methylphenyl)pyridine*)L2] molecules (QICNUN for L = methyl and QICPAV for L = trifluoroacetate) [52], where the increase of |Δ15Ncoord| upon the CH3 → CF3COO ligand transition can be related to the shortening of the Au–N bond (2.130(3) Å → 1.991(6) Å; see Table 7). However, this is only one example, not allowing for general conclusions.
The analysis of the other 15N NMR data exhibits that relatively large |Δ15Ncoord| parameters are observed for all [Au(2PPY*)(CF3COO)2] (ca. 66–105 ppm; nitrogens trans to O) and [Au(ArPY#*)Cl2] (ca. 69–105 ppm; nitrogens trans to Cl) species, with no significant differences between both classes of molecules.

3. Applications

3.1. Biological Activity

A total of 105 reviewed compounds were studied, with respect to their biological activity [3,8,9,11,25,26,28,30,31,33,43,51,63,64,79,82,83,92,93,96,97,99,102]. Generally, all these reports concerned anti-tumour properties, but some also described anti-microbial (anti-bacterial and anti-fungal) action [63,96,97].
The anti-tumour properties of these Au(III) species against various types of cancer cells (usually of carcinoma or adenocarcinoma types, but also glioblastoma, leukemia, and melanoma), in a few cases with cytotoxic activity against some non-cancerous cells (analyzed for comparison), as well as their anti-microbial properties (indicated by footnote a), are summarized, in the order of references, in Table 10.
Table 10. Summary of biological activity studies for the reviewed Au(III) compounds.
On the basis of their biological activity, the concerned Au(III) compounds can be applied in medicine, as they are potential anti-cancer, anti-bacterial, and anti-fungal drugs.

3.2. Catalytic Activity

A total of 30 reviewed compounds were exhibited to have catalytic activity [7,16,17,22,42,65,103,107]. Their application as catalysts is summarized, in the order of references, in Table 11.
Table 11. Summary of catalytic activity studies for the reviewed Au(III) compounds.
On the basis of the above catalytic activity, the concerned Au(III) compounds are applied in organic syntheses.

3.3. Luminescence

A total of 88 reviewed compounds were studied in detail, with respect to their luminescence [6,15,18,19,24,27,37,38,39,41,42,75,90].
An important parameter for luminescence differentiation is the lifetime, i.e., the average time that a molecule remains in an excited state prior to returning to the ground state by emitting a photon. In this review, we have arbitrarily chosen the lifetime of 10 µs as a border between long- and short-living excited forms; the former corresponds to phosphorescence, while the latter corresponds to either phosphorescence or fluorescence (it is often difficult to distinguish both phenomena: although they have different mechanisms, their identification may be ambiguous, even taking into account the fact that phosphorescence lifetimes are principally longer than the fluorescence ones).
A total of 54 reviewed compounds exhibited luminescence with lifetimes above 10 µs [6,15,19,37,39,42,75,90], while 34 exhibited luminescence with lifetimes below 10 µs [15,18,24,27,38,41,75]. The detailed data describing their chemical character, together with the maximal quantum yields (for a given group of compounds, rounded to 1%), are summarized, in the order of references, in Table 12.
Table 12. Summary of luminescence studies for the reviewed Au(III) compounds.
On the basis of the above luminescence properties, these Au(III) compounds can be applied in the production of organic light-emitting diodes (OLEDs).

4. Conclusions

A large numbers of reports (>100) described molecules (>500) and single crystal X-ray structures (>200) and indicated that the Au(III) compounds with 2-arylpyridines* and their derivatives or analogues are interesting from the chemical, spectroscopic, and structural viewpoints. The most popular Au(III)-2PPY* species are those with 2-phenylpyridine* and 2-(4-methylphenyl)pyridine*, while among Au(III)-2ArPY* molecules—those containing 2-benzylpyridine* ring system are the most popular.
All Au(III)-2PPY*, Au(III)-2ArPY*, and Au(III)-ArPY#* compounds exhibit a specific (N,C) coordination mode, alternative to classical monodentate complexation by nitrogen. Thus, these molecules contain both gold–nitrogen and gold–carbon bonds and can be regarded as either complexes or organometallics.
The coordination number 4 and the square-planar geometry are typical for Au(III) chemistry and allow, upon the presence of different monodentate (L1 ≠ L2) or unsymmetrical bidentate (L1L2) auxiliary ligands, for the appearance of geometric isomers differing in the position of various donor atoms versus the nitrogen of the pyridine (or pyridine-like) ring or the metallated carbon of the phenyl (or aryl) ring. There is an evident preference (with a few exceptions) to form such stereomers, in which the auxiliary ligands having less electron-acceptor properties (thus, usually containing less electronegative elements as donor atoms) are trans-positioned to the pyridine (or pyridine-like) nitrogen.
A total of 20 Au(III)-2PPY*, Au(III)-2ArPY*, and Au(III)-ArPY#* compounds were studied by 15N NMR. The comparison to analogous measurements for the corresponding 2PPY, 2ArPY, or ArPY# reveals a ca. 45–105 ppm decrease of the chemical shift of the pyridine nitrogen. This phenomenon reflects a strong 15N shielding effect at the Au(III)-bonded N atom.
About 200 Au(III)-2PPY*, Au(III)-2ArPY*, and Au(III)-ArPY#* compounds exhibit some specific activities, especially biological and/or catalytic, as well as luminescence properties. It opens the way for their application as anti-tumour or anti-microbial (anti-bacterial, anti-fungal) drugs and catalysts in organic syntheses, as well as materials for the production of organic light-emitting diodes. So, the wide application of these species is the reason for their intensive studies, as noted during the last few decades, as well as to write the present review.

Author Contributions

Conceptualization, L.P.; methodology, L.P.; software, L.P.; validation, L.P. and P.A.A.; formal analysis, L.P. and P.A.A.; investigation, L.P. and P.A.A.; resources, L.P. and P.A.A.; data curation, L.P. and P.A.A.; writing—original draft preparation, L.P.; writing—review and editing, L.P. and P.A.A.; visualization, L.P.; supervision, L.P.; project administration, L.P. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The data are available in the quoted papers.

Conflicts of Interest

The authors declare no conflict of interest.

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