Dinuclear Lanthanide(III) Complexes from the Use of Methyl 2-Pyridyl Ketoxime: Synthetic, Structural, and Physical Studies

The first use of methyl 2-pyridyl ketoxime (mepaoH) in homometallic lanthanide(III) [Ln(III)] chemistry is described. The 1:2 reactions of Ln(NO3)3·nH2O (Ln = Nd, Eu, Gd, Tb, Dy; n = 5, 6) and mepaoH in MeCN have provided access to complexes [Ln2(O2CMe)4(NO3)2(mepaoH)2] (Ln = Nd, 1; Ln = Eu, 2; Ln = Gd, 3; Ln = Tb, 4; Ln = Dy, 5); the acetato ligands derive from the LnIII—mediated hydrolysis of MeCN. The 1:1 and 1:2 reactions between Dy(O2CMe)3·4H2O and mepaoH in MeOH/MeCN led to the all-acetato complex [Dy2(O2CMe)6(mepaoH)2] (6). Treatment of 6 with one equivalent of HNO3 gave 5. The structures of 1, 5, and 6 were solved by single-crystal X-ray crystallography. Elemental analyses and IR spectroscopy provide strong evidence that 2–4 display similar structural characteristics with 1 and 5. The structures of 1–5 consist of dinuclear molecules in which the two LnIII centers are bridged by two bidentate bridging (η1:η1:μ2) and two chelating-bridging (η1:η2:μ2) acetate groups. The LnIII atoms are each chelated by a N,N’-bidentate mepaoH ligand and a near-symmetrical bidentate nitrato group. The molecular structure of 6 is similar to that of 5, the main difference being the presence of two chelating acetato groups in the former instead of the two chelating nitrato groups in the latter. The geometry of the 9-coordinate LnIII centers in 1, 5 and 6 can be best described as a muffin-type (MFF-9). The 3D lattices of the isomorphous 1 and 5 are built through H-bonding, π⋯π stacking and C-H⋯π interactions, while the 3D architecture of 6 is stabilized by H bonds. The IR spectra of the complexes are discussed in terms of the coordination modes of the organic and inorganic ligands involved. The Eu(III) complex 2 displays a red, metal-ion centered emission in the solid state; the TbIII atom in solid 4 emits light in the same region with the ligand. Magnetic susceptibility studies in the 2.0–300 K range reveal weak antiferromagnetic intramolecular GdIII…GdIII exchange interactions in 3; the J value is −0.09(1) cm−1 based on the spin Hamiltonian Ĥ = −J(ŜGd1·ŜGd2).


Introduction
Oximes are versatile organic molecules containing the >C=N-OH group, which have been extensively used as efficient reagents in analytical chemistry and as excellent ligands in inorganic chemistry. There are several types of oximes: aliphatic and aromatic monoximes derived from ketones r aldehydes, α-dioximes, and recently new families of amino/imido oximes and cyanoximes have attracted intense attention [1]. Although the nitroso-oxime tautomerism is known in solution, most solid compounds exist in their oxime form. One of the first uses of oximes in metal chemistry was the gravimetric determination of Ni(II) in the form of the red bis(dimethylglyoximato)nickel(II) [2], an experiment performed by undergraduate Chemistry students in most universities. Today, oxime and oximato metal complexes are central "players" in several aspects of coordination [3][4][5] and bioinorganic [6,7] chemistry, molecular magnetism [8][9][10][11], catalysis [12,13], and in the area of the reactivity of coordinated ligands [14][15][16].
A central theme in our research groups is the study of the coordination chemistry of 2-pyridyl oximes in which the substituent R at the oxime carbon is either a non-donor group ( Figure 1) or a donor group (e.g., R = CN, NH2, 2-pyridyl, etc.) [17]. Restricting further discussion to the former family of ligands, the emphasis of our efforts (only selected references from our work are given below) has been on (a) the optical and magnetic properties of the resulting homo-and heterometallic dinuclear [18][19][20][21] and polynuclear [22][23][24][25][26][27] complexes, (b) the metal-mediated/promoted/assisted reactivity of these ligands [28][29][30], and (c) the modeling of the solvent extraction of toxic metal ions from aqueous solutions by 2-pyridyl ketoximes [31,32]. A literature survey reveals that a plethora of homometallic complexes of paoH, mepaoH, phpaoH, and ClpaoH ( Figure 1), in their neutral or anionic forms, have been prepared and studied with transition [22][23][24][25][27][28][29]31,32] and actinide [21] metal ions. In contrast, with the exception of [LnCl3(phpaoH)3] (Ln = Nd [33], Sm [34]), such complexes are not known in lanthanide (Ln) chemistry. We have embarked on a new program aiming to fill the above-mentioned literature gap and prepare 4f-metal complexes with the ligands (neutral or deprotonated) shown in Figure 1. In this work, which is the first of a series of papers on this topic, we report our preliminary results on the reactions of Ln(III) sources and methyl 2-pyridyl ketoxime (mepaoH; R = Me in Figure 1) in the absence of an external base. Our efforts have led to dinuclear Ln(III) complexes with interesting structural features and properties; a point of interest is that the bridging acetato ligands in most of the complexes have resulted from the Ln III -assisted hydrolysis of the MeCN (solvent). This paper can also be considered as a continuation of the interest of our groups in the chemistry and properties (photophysical, magnetic, catalytic) of the Ln III 2 complexes [35][36][37][38][39][40][41][42][43][44]; such complexes can potentially combine both optical and magnetic properties within the same molecule leading to luminescent SMMs [45,46]. Ln III 2 compounds continue to attract the intense attention of many inorganic chemistry groups around the world because they are ideal model systems for providing answers to fundamental questions regarding the origin of a certain physical, spectroscopic, or catalytic property, i.e., if such properties arise from the whole molecule (two Ln III centers) or half of it (a single Ln III ion) [47]. We have embarked on a new program aiming to fill the above-mentioned literature gap and prepare 4f-metal complexes with the ligands (neutral or deprotonated) shown in Figure 1. In this work, which is the first of a series of papers on this topic, we report our preliminary results on the reactions of Ln(III) sources and methyl 2-pyridyl ketoxime (mepaoH; R = Me in Figure 1) in the absence of an external base. Our efforts have led to dinuclear Ln(III) complexes with interesting structural features and properties; a point of interest is that the bridging acetato ligands in most of the complexes have resulted from the Ln III -assisted hydrolysis of the MeCN (solvent). This paper can also be considered as a continuation of the interest of our groups in the chemistry and properties (photophysical, magnetic, catalytic) of the Ln III 2 complexes [35][36][37][38][39][40][41][42][43][44]; such complexes can potentially combine both optical and magnetic properties within the same molecule leading to luminescent SMMs [45,46]. Ln III 2 compounds continue to attract the intense attention of many inorganic chemistry groups around the world because they are ideal model systems for providing answers to fundamental questions regarding the origin of a certain physical, spectroscopic, or catalytic property, i.e., if such properties arise from the whole molecule (two Ln III centers) or half of it (a single Ln III ion) [47].

Synthetic Comments
A variety of Ln(III)/X − /mepaoH reaction systems involving various metal ions and X − (X − = Cl − , NO 3 − , MeCO 2 − ), and different solvent media, reagent ratios, and crystallization methods were systematically employed before arriving at the optimized procedures reported in Section 3. Since we were interested in obtaining products with the neutral oxime ligand, we avoided the addition of an external base, e.g., Et 3 N, LiOH·H 2 O, and Me 4 NOH·5H 2 O. Crystalline products were obtained only with X − = NO 3 − , MeCO 2 − and representative complexes are described in this work. In the case of X − = Cl − , powders with the empirical formulae (evidence from microanalyses) {LnCl 3 (mepaoH) 3 ·nEtOH} (n = 1-2) were obtained from the reactions of hydrated lanthanoid chlorides and mepaoH in 1:3 reaction ratios in refluxing EtOH. We could not obtain single crystals for X-ray structural determination and so we did not pursue this chemistry further. The 1:2 reactions between Ln(NO 3 ) 3 ·nH 2 O (n = 5 or 6) and mepaoH in MeCN gave almost colorless pale-colored (with the color of Ln III ) or colorless solutions from which crystals (in the case of Nd III and Dy III ) or crystalline solids (in the case of Eu III , Gd III , and Tb III ) of [Ln 2 (O 2 CMe) 4 (NO 3 ) 2 (mepaoH) 2 ] (Ln = Nd, 1; Ln = Eu, 2; Ln = Gd, 3; Ln = Tb, 4; Ln = Dy, 5) were subsequently isolated in moderate yields (30-40% based on the Ln III used). The stoichiometric 1:1 reactions gave again 1-5, albeit in lower yields (~25%). Given the absence of acetate ions from the reaction mixtures, the presence of acetato ligands in the products was surprising to us. The only source of coordinated MeCO 2 − groups in 1-5 could be the solvent used (MeCN), which contains 1-2% of H 2 O. Assuming that the complexes are the only products from their respective reaction systems, and that the H 2 O responsible for the generation of acetates comes from the hydrated lanthanide(III) nitrates, the formation of the products can be summarized by the simplified Equation (1); n = 6 for Nd(III), Eu(III), Gd(III), Tb(III), and 5 for Dy(III). As mentioned above, the H 2 O present in the solvent can also contribute to the formation of the MeCO 2 − ligands: In accordance with our belief that Equation (1) represents the real chemistry of the reactions: (i) using more concentrated reaction solutions to increase the yields, the products were contaminated (IR and microanalyses evidences) by variable amounts of precipitated NH 4 (NO 3 ); the H 2 O present in the mixtures of the optimized reactions (Section 3) is enough to keep the NH 4 (NO 3 ) byproduct soluble, and (ii) employment of distilled MeCN as solvent leads to lower yields and serious contamination of the products by ammonium nitrate.
It is well known in organic chemistry that nitriles can be hydrolyzed to give either amides or carboxylic acids [48,49]. The initially produced hydroxylimine may tautomerize to an amide, which in turn generates a carboxylic acid upon further hydrolysis [48,50]. For MeCN, the two hydrolysis steps are illustrated in Figure 2. Several metal ion-mediated processes for the hydration of nitriles with selective formation of metal-bound carboxamides are known [27,[51][52][53], but most of the systems are not catalytic and only a few of them are able to hydrate R'-C≡N under homogeneous catalytic conditions [13,53], usually exhibiting a low activity [53]. The second step of the metal-ion mediated hydrolysis, i.e., conversion to carboxylic acids and ammonia, has been observed only in few cases involving Pt(IV), Os(IV), and Nb(V) systems [53]. To the best of our knowledge, the present work represents the second case in which an Ln(III)-mediated hydrolysis of MeCN is experimentally observed. The first case involved the reactions of hydrated lanthanide(III) nitrates with 2-(4-carboxyphenyl)imidazo [4,5- 6 (pip) 2 ], respectively [54]. In the present case, the produced MeCO 2 H is neutralized by the also generated ammonia giving the acetato ligands and providing ammonium cations that provide charge balance for the released nitrate ions, Equation (1). case, the produced MeCO2H is neutralized by the also generated ammonia giving the acetato ligands and providing ammonium cations that provide charge balance for the released nitrate ions, Equation (1). Having established the composition of 1-5, we wondered if we could obtain dinuclear Ln(III)/mepaoH complexes with six acetato ligands, i.e., without nitrato groups. Two procedures were investigated: (i) the reaction mixtures that lead to 1-5 were refluxed overnight, and (b) complexes 1-5 were treated with an excess (3-5 equivs per equiv of dinuclear complex) of an equimolar mixture of MeCO2H and Et3N in MeCN; in both cases, complexes [Ln2(O2CMe)4(NO3)2(mepaoH)2] were again isolated, i.e., no reaction occurred. In a modification of the second route, the 1:6:6:2 Ln(NO3)3 . nH2O/MeCO2H/Et3N/mepaoH reaction mixtures in MeCN gave again complexes 1-5 in moderate yields (25-35% based on the Ln III available), Equation (2). This is strong evidence that the mixed acetato/nitrato complexes are the thermodynamically preferred products from the Ln III /NO3 − /MeCO2 − /mepaoH reaction systems in MeCN: The obvious route to prepare all-acetato dinuclear Ln(III)/mepaoH complexes was to avoid the presence of NO3 − ions in the reaction systems. This was examined in the case of Dy(III). Thus, the refluxing 1:1 or 1:2 reactions between Dy(O2CMe)3 . 4H2O and mepaoH in a solvent mixture of MeOH/MeCN (1:2 v/v) led to a slurry, the filtration of which gave an almost colorless solution from which colorless crystals of [Dy2(O2CMe)6(mepaoH)2] (6) were subsequently isolated in good yield (50-60% based on Dy III ), Equation (3). MeOH was necessary to keep an amount of Dy(O2CMe)3 . 4H2O (insoluble in MeCN) soluble in order to react with the ligand; the reaction in MeOH and subsequent precipitation with Et2O gave a powder, most probably (IR evidence) a mixture of 6 and Dy(O2CMe)3 . 4H2O. Treatment of 6 with 2-2.5 equivs of HNO3 (from a 2N solution) in MeCN gave complex 5, Equation (4), emphasizing again the thermodynamic stability of the mixed acetato/nitrato complexes. Excess of 2N HNO3 leads to the decomposition of the product:

Description of Structures
The structures of 1, 5, and 6 were determined by single-crystal X-ray crystallography. Crystallographic data are listed in the Supplementary Table S1. Various structural plots are shown in Figures 3-7 and Supplementary Figures S1-S3. Selected interatomic distances and bond angles are given in Tables 1 and 2. CShM values for the potential coordination polyhedra of the Ln III centers in 1, 5, and 6 are summarized in Table S2, while Hbonding interactions for the three structures are listed in Tables S3 and S4. Having established the composition of 1-5, we wondered if we could obtain dinuclear Ln(III)/mepaoH complexes with six acetato ligands, i.e., without nitrato groups. Two procedures were investigated: (i) the reaction mixtures that lead to 1-5 were refluxed overnight, and (b) complexes 1-5 were treated with an excess (3-5 equivs per equiv of dinuclear complex) of an equimolar mixture of MeCO 2 H and Et 3 N in MeCN; in both cases, complexes [Ln 2 (O 2 CMe) 4 (NO 3 ) 2 (mepaoH) 2 ] were again isolated, i.e., no reaction occurred. In a modification of the second route, the 1:6:6:2 Ln(NO 3 ) 3 ·nH 2 O/MeCO 2 H/Et 3 N/mepaoH reaction mixtures in MeCN gave again complexes 1-5 in moderate yields (25-35% based on the Ln III available), Equation (2). This is strong evidence that the mixed acetato/nitrato complexes are the thermodynamically preferred products from the Ln III /NO 3 − /MeCO 2 − /mepaoH reaction systems in MeCN: The obvious route to prepare all-acetato dinuclear Ln(III)/mepaoH complexes was to avoid the presence of NO 3 − ions in the reaction systems. This was examined in the case of Dy(III). Thus, the refluxing 1:1 or 1:2 reactions between Dy(O 2 CMe) 3 ·4H 2 O and mepaoH in a solvent mixture of MeOH/MeCN (1:2 v/v) led to a slurry, the filtration of which gave an almost colorless solution from which colorless crystals of [Dy 2 (O 2 CMe) 6 (mepaoH) 2 ] (6) were subsequently isolated in good yield (50-60% based on Dy III ), Equation (3). MeOH was necessary to keep an amount of Dy(O 2 CMe) 3 ·4H 2 O (insoluble in MeCN) soluble in order to react with the ligand; the reaction in MeOH and subsequent precipitation with Et 2 O gave a powder, most probably (IR evidence) a mixture of 6 and Dy(O 2 CMe) 3 ·4H 2 O. Treatment of 6 with 2-2.5 equivs of HNO 3 (from a 2N solution) in MeCN gave complex 5, Equation (4), emphasizing again the thermodynamic stability of the mixed acetato/nitrato complexes. Excess of 2N HNO 3 leads to the decomposition of the product:

Description of Structures
The structures of 1, 5, and 6 were determined by single-crystal X-ray crystallography. Crystallographic data are listed in the Supplementary Table S1. Various structural plots are shown in Figures 3-7 and Supplementary Figures S1-S3. Selected interatomic distances and bond angles are given in Tables 1 and 2. CShM values for the potential coordination polyhedra of the Ln III centers in 1, 5, and 6 are summarized in Table S2, while H-bonding  interactions for the three structures are listed in Tables S3 and S4.         Complex 6 crystallizes in the monoclinic space group P21/c. Although its composition is different compared with that of 5 (and 1), the molecular structures are similar. The only significant difference is the presence of the two chelating acetato groups (η 2 or 1.11) in 6 instead of the two chelating nitrato groups (η 2 or 1.110) in 5. Again, the two Dy III centers are linked by four MeCO2 − ligands. Two of them bridge in the classical syn, syn η 1 :η 1 :μ2 (or 2.11) fashion, and the other two bridge in the less common η 1 :η 2 :μ2 (2.12) mode; the latter have an oxygen atom (O7, O7′) bound terminally to one metal ion, and the oxygen atom N2C3C4C5C6C7 and N2*C3*C4*C5*C6*C7* [(*) = −x+2, −y, −z+1] is 3.74(3) Å for 1 and 3.73(5) Å for 5. The C-H … π interactions are evaluated using the Hc(C1) … Cg1** distance, which is 3.068(1) Å for 1 and 2.910(1) Å for 5 [(**) = = −x+2, −y+1, −z+1]; Cg1 is the centroid of the pyridyl ring of mepaoH, and Hc(C1) is one of the methyl hydrogen atoms of mepaoH. Thus, layers of molecules are formed parallel to the (100) plane (Figure 6a). The layers interact through H bonds and are stacked along the a crystallographic axis, thus building the 3D architectures (Figure 6b).  Table S3.
The crystal structure of 6 is built through H bonds (Table S4). The dinuclear molecules form layers parallel to the (100) plane (Figure 7a), which interact further to create the 3D architecture of the structure (Figure 7b).

Characterization of Selected Complexes
In the solid-state (KBr) IR spectra of 1-6, the presence of a medium-intensity band at ~3250 cm −1 is assigned to the v(OH) vibration of the mepaoH ligand [18,21]; the broadness and the relatively low wavenumber of the band are both indicative of H-bonding interactions established by crystallography (vide supra). The medium-intensity band at 1116 cm −1 in the spectrum of free mepaoH has been assigned to the v(OH)oxime mode [21]; this band has been shifted to lower wavenumber (~1090 cm −1 ) in the spectra of the six complexes  Table S4.  Complexes 1 and 5 crystallize in the triclinic space group Pī and are isomorphous; thus, a general description will be given (Ln = Nd, Dy). The structures consist of dinuclear molecules [Ln 2 (O 2 CMe) 4 (NO 3 ) 2 (mepaoH) 2 ]. There is a crystallographically imposed inversion center at the midpoint of the Ln· · · Ln1 vector. The Ln III atoms are each chelated by a N,N'-bidentate mepaoH ligand (η 2 or 1.011 using Harris notation [55]) and an almost symmetrical bidentate nitrato group (η 2 or 1.110). The metal centers are bridged by two pairs of acetato ligands. One of them consists of symmetrical syn, syn η 1 :η 1 :µ 2 (or 2.11) groups, the Ln1-O5 and Ln1-O6 bond lengths being 2.426(5) and 2.404(4) Å, respectively, for 1, and 2.343(7) and 2.312(8) Å, for 5, respectively. The second pair of bridging acetato groups  (Table 1) due to lanthanide contraction; this trend is also observed for the Ln III . . . Ln III distances. The Ln-O/N bond lengths in 1 and 5 are typical for 9-coordinate Nd(III) [33,56,57] and Dy(III) [35,38,41,54,[58][59][60][61][62]. The Ln1/Ln1 coordination geometry was evaluated using the program SHAPE [61,63] and is best described as muffin-type ( Figure S1, Table S2); an alternative description of the polyhedron could be that of a spherical-capped square antiprism.
There are two crystallographically independent H bonds within the dinuclear molecules (intramolecular H bonds, Figure S2 and Table S3). The donors are the oxime oxygen atoms (O1/O1 ) and the aromatic carbon atoms C7/C7 , and the acceptors are the coordinated acetato oxygen atoms O5/O5 and O6 /O6, respectively. Given the rather small donor-H· · · acceptors angles, these bonds can be considered as weak.
Complex 6 crystallizes in the monoclinic space group P2 1 /c. Although its composition is different compared with that of 5 (and 1), the molecular structures are similar. The only significant difference is the presence of the two chelating acetato groups (η 2 or 1.11) in 6 instead of the two chelating nitrato groups (η 2 or 1.110) in 5. Again, the two Dy III centers are linked by four MeCO 2 − ligands. Two of them bridge in the classical syn, syn η 1 :η 1 :µ 2 (or 2.11) fashion, and the other two bridge in the less common η 1 :η 2 :µ 2 (2.12) mode; the latter have an oxygen atom (O7, O7 ) bound terminally to one metal ion, and the oxygen atom (O8, O8 ) bound in a µ 2 manner to both Dy III atoms, forming a monoatomic bridge. One chelating 1.011 mepaoH molecule and one chelating (1.11) MeCO 2 − ion complete 9-coordination at each metal center. Due to the inversion center, the Dy1O8Dy1 O8 core is perfectly planar. The coordination polyhedron of Dy1/Dy1 is best described as muffin ( Figure S1, Table S2). The interatomic distances and bond angles in 6 and 5 are similar; a notable difference is that the Dy-N oxime bond length [2.593(8) Å] is larger than that of the Dy-N pyridyl bond length [2.525(7) Å] in 6, while the opposite trend [2.510(11) vs. 2.535(9) Å, respectively] is observed in 5.
There are two crystallographically independent, weak intramolecular H bonds in 6 ( Figure S3 and Table S4). The donors are the oxime oxygen atoms (O1/O1 ) and the aromatic carbon atoms C7/C7 , and the acceptors are the coordinated acetato oxygen atoms O6 /O6 and O5/O5 , respectively.
The crystal structure of 6 is built through H bonds (Table S4). The dinuclear molecules form layers parallel to the (100) plane (Figure 7a), which interact further to create the 3D architecture of the structure (Figure 7b).

Characterization of Selected Complexes
In the solid-state (KBr) IR spectra of 1-6, the presence of a medium-intensity band at 3250 cm −1 is assigned to the v(OH) vibration of the mepaoH ligand [18,21]; the broadness and the relatively low wavenumber of the band are both indicative of H-bonding interactions established by crystallography (vide supra). The medium-intensity band at 1116 cm −1 in the spectrum of free mepaoH has been assigned to the v(OH) oxime mode [21]; this band has been shifted to lower wavenumber (~1090 cm −1 ) in the spectra of the six complexes due to coordination of the oxime nitrogen [21]. The in-plane pyridyl deformation of free mepaoH at 632 cm −1 shifts upwards in the spectra of the complexes (~685 cm −1 ) confirming the involvement of the ring N-atom in coordination [18,24,32].  [67]; the separation of the two highest-wavenumber stretching bands is large (~200 cm −1 ) and typical for bidentate chelating (C 2v ) nitrato groups [67]. These three bands do not appear in the spectrum of the nitrate-free compound 6.
The luminescence of Ln III ions results from electronic transitions within the partially filled 4f n orbitals. Due to the intrinsically low probability of such electronic 4f-4f transitions, the photon absorption cross section and the emission brightness are small. A common approach to overcome this disadvantage is to take advantage of a sensitizing entity (usually an organic ligand) that can effectively harvest the excitation energy and transfer it to the Ln III ion. An ideal organic ligand for this purpose should: (i) feature a large photon absorption cross section; and (ii) exhibit an energy level scheme that allows for an effective transfer of the absorbed energy to the Ln III center [68][69][70][71]. Since Eu(III) and Tb(III) complexes often emit light in the visible region of the electromagnetic spectrum, we investigated the photoluminescence properties of complexes 2 and 4 (Figures 8 and 9, respectively).
The free mepaoH ligand emits with a maximum at~540 nm, upon maximum excitation at 397 nm. Upon the same maximum excitation (397 nm), the solid complex [Eu 2 (O 2 CMe) 4 (NO 3 ) 2 (mepaoH) 2 ] (2) displays emission peaks at 593, 618, 653, and 695 nm due to Eu III [41,68,69]. These peaks are assigned to the characteristic 5 D 0 → 7 F j (j = 0-4) transitions of this metal ion. Specific assignments [41,43] are as follows: 5 D 0 → 7 F 0,1 (593 nm); 5 D 0 → 7 F 2 (618 nm); 5 D 0 → 7 F 3 (653 nm); 5 D 0 → 7 F 4 (695 nm). The dominant peak is due to the hypersensitive 5 D 0 → 7 F 2 transition. The higher intensity of this transition compared with that of the magnetic-dipole allowing 5 D 0 → 7 F 1 transition indicates that this complex has a structure with no imposed symmetry at Eu III (the molecules of 2, which are proposed to be isostructural with those of the crystallographically characterized 1 and 5, have an inversion center at the midpoint of the Ln III . . . Ln III vector). Generally, complexes with a centrosymmetric coordination sphere of Eu III (which is not the case in 2) have a ( 5 D 0 → 7 F 2 )/( 5 D 0 → 7 F 1 ) intensity ratio lower than 0.7 [41]. The lifetime of the excited state of Eu(III) was found to be 0.68 ms, a value typical for Ln(III) complexes [43,71]. absorption cross section; and (ii) exhibit an energy level scheme that allows for an effective transfer of the absorbed energy to the Ln III center [68][69][70][71]. Since Eu(III) and Tb(III) complexes often emit light in the visible region of the electromagnetic spectrum, we investigated the photoluminescence properties of complexes 2 and 4 (Figures 8 and 9, respectively). The free mepaoH ligand emits with a maximum at ~540 nm, upon maximum excitation at 397 nm. Upon the same maximum excitation (397 nm), the solid complex [Eu2(O2CMe)4(NO3)2(mepaoH)2] (2) displays emission peaks at 593, 618, 653, and 695 nm due to Eu III [41,68,69]. These peaks are assigned to the characteristic 5 D0→ 7 Fj (j = 0-4) transitions of this metal ion. Specific assignments [41,43] are as follows: 5 D0→ 7 F0,1 (593 nm); 5 D0→ 7 F2 (618 nm); 5 D0→ 7 F3 (653 nm); 5 D0→ 7 F4 (695 nm). The dominant peak is due to the hypersensitive 5 D0→ 7 F2 transition. The higher intensity of this transition compared with that of the magnetic-dipole allowing 5 D0→ 7 F1 transition indicates that this complex has a structure with no imposed symmetry at Eu III (the molecules of 2, which are proposed to be isostructural with those of the crystallographically characterized 1 and 5, have an inversion center at the midpoint of the Ln III… Ln III vector). Generally, complexes with a centrosymmetric coordination sphere of Eu III (which is not the case in 2) have a ( 5 D0→ 7 F2)/( 5 D0→ 7 F1) intensity ratio lower than 0.7 [41]. The lifetime of the excited state of Eu(III) was found to be 0.68 ms, a value typical for Ln(III) complexes [43,71].
The emission peaks at ~545 and ~490 nm arise from the coordinated mepaoH ligand, since the free ligand (black curve in Figure 8) emits in this area with maxima at approximately these wavelengths (the slight shifts are due to coordination) under identical excitation conditions. In the case of solid 4, the detected emission spectrum seems to arise mainly from the mepaoH ligand and not from Tb III ; this was rather expected since Tb III ions' characteristic emission peaks are in the same spectral area [41,72].
Complexes of some Ln III ions are currently protagonists in several areas of molecular magnetism [71,[73][74][75][76][77][78][79][80][81][82][83]. The paramagnetic nature of most Ln III ions arises from their large number of unpaired 4f electrons (large ground-state spin). This property combined with the magnetic anisotropy of some complexes can lead to Single-Molecule Magnetism (SMM) behavior [76][77][78][79][80][81][82][83]. Single-Molecule Magnets (SMMs) are today at the forefront of new technological advances, e.g., in quantum information processing and spintronics [71]. The goal of this work is not associated with the study of the magnetic properties of Ln(III)/mepaoH complexes. A full magnetic study of ~20 carboxylato-bridged dinuclear Tb(III), Dy(III), Ho(III), Er(III), and Yb(III) complexes with the four capping ligands shown in Figure 1, including the field-induced SMM behavior of some of them, will be reported in a separate work (currently in preparation). Here, we simply describe the magnetic properties of 3, which contains Gd III centers that present no spin-orbit coupling at the first order.
Direct-current (dc) molar magnetic susceptibility (χΜ) data on a polycrystalline sample of 3 were collected in the 2.0-300 K range using an applied field of 0.1 T; the data are presented in the χΜΤ vs. T plot in Figure 10, where T is the absolute temperature. At room temperature, the χΜΤ value of 3 is 16.02 cm 3. K . mol −1 , in very good agreement with the 3. . −1 III Figure 9. Solid-state, room-temperature excitation (left, emission, at 545) and emission (right, excitation at 397 nm) of mepaoH (black curves) and complex 4 (green curves).
The emission peaks at~545 and~490 nm arise from the coordinated mepaoH ligand, since the free ligand (black curve in Figure 8) emits in this area with maxima at approximately these wavelengths (the slight shifts are due to coordination) under identical excitation conditions.
In the case of solid 4, the detected emission spectrum seems to arise mainly from the mepaoH ligand and not from Tb III ; this was rather expected since Tb III ions' characteristic emission peaks are in the same spectral area [41,72].
Complexes of some Ln III ions are currently protagonists in several areas of molecular magnetism [71,[73][74][75][76][77][78][79][80][81][82][83]. The paramagnetic nature of most Ln III ions arises from their large number of unpaired 4f electrons (large ground-state spin). This property combined with the magnetic anisotropy of some complexes can lead to Single-Molecule Magnetism (SMM) behavior [76][77][78][79][80][81][82][83]. Single-Molecule Magnets (SMMs) are today at the forefront of new technological advances, e.g., in quantum information processing and spintronics [71]. The goal of this work is not associated with the study of the magnetic properties of Ln(III)/mepaoH complexes. A full magnetic study of~20 carboxylato-bridged dinuclear Tb(III), Dy(III), Ho(III), Er(III), and Yb(III) complexes with the four capping ligands shown in Figure 1, including the field-induced SMM behavior of some of them, will be reported in a separate work (currently in preparation). Here, we simply describe the magnetic properties of 3, which contains Gd III centers that present no spin-orbit coupling at the first order.
Direct-current (dc) molar magnetic susceptibility (χ M ) data on a polycrystalline sample of 3 were collected in the 2.0-300 K range using an applied field of 0.1 T; the data are presented in the χ M T vs. T plot in Figure 10, where T is the absolute temperature. At room temperature, the χ M T value of 3 is 16.02 cm 3 ·K·mol −1 , in very good agreement with the expected theoretical value of 15.75 cm 3 ·K·mol −1 for two noninteracting Gd III centers (S = 7/2, L = 0, g = 2.00). The value of χ M T remains almost constant in the 50-300 K range and then decreases rapidly with decreasing T to reach a value of 11.05 cm 3 ·K·mol −1 at 2.0 K; this decrease directly reveals the presence of weak antiferromagnetic, intramolecular Gd III . . . Gd III exchange interactions. The experimental data were fitted to the van Vleck analytical expression of the susceptibility derived from the isotropic Heisenberg spin Hamiltonian of Equation (5). The best-fit parameters obtained are J = −0.09 (1) cm −1 and g = 2.02. Because of the little overlap between the shielded 4f orbitals and the orbitals of the bridging oxygen atoms, the observed antiferromagnetic exchange interaction is very weak. Such small values have been observed in systems containing symmetrically bridged {Gd III 2 (µ 2 -OR") 2 } cores (R" = various groups) [38,40,42,[84][85][86].

Materials, Physical and Spectroscopic Measurements
All manipulations were performed under aerobic conditions. Reagents and solvents were purchased from Alfa Aesar (Karlsruhe, Germany) and Sigma-Aldrich (Tanfrichen, Germany), and used as received. The free ligand methyl 2-pyridyl ketoxime (mepaoH; R = Me in Figure 1) was synthesized as described in the literature [87] in >70% yields; its purity was checked by 1 H NMR spectroscopy and the determination of its melting point (found, 118-119 °C; reported, 121 °C).
Elemental analyses (C, H, N) were performed by the University of Patras Center of Instrumental Analysis. FT-IR spectra (4000-450 cm −1 ) were recorded using a PerkinElmer 16PC spectrometer (PerkinElmer, Watham, MA, USA) with samples prepared as KBr pellets under pressure. Solid-state emission and excitation spectra were recorded at room temperature using a Cary Eclipse fluorescence spectrometer (Varian, Palo Alto, CA, USA). Variable-temperature, solid-state direct current (dc) magnetic susceptibility data in the 2.0-300 K range were collected on a Quantum Design MPMS-XL SQUID magnetometer

Materials, Physical and Spectroscopic Measurements
All manipulations were performed under aerobic conditions. Reagents and solvents were purchased from Alfa Aesar (Karlsruhe, Germany) and Sigma-Aldrich (Tanfrichen, Germany), and used as received. The free ligand methyl 2-pyridyl ketoxime (mepaoH; R = Me in Figure 1) was synthesized as described in the literature [87] in >70% yields; its purity was checked by 1 H NMR spectroscopy and the determination of its melting point (found, 118-119 • C; reported, 121 • C).
Elemental analyses (C, H, N) were performed by the University of Patras Center of Instrumental Analysis. FT-IR spectra (4000-450 cm −1 ) were recorded using a PerkinElmer 16PC spectrometer (PerkinElmer, Watham, MA, USA) with samples prepared as KBr pellets under pressure. Solid-state emission and excitation spectra were recorded at room temperature using a Cary Eclipse fluorescence spectrometer (Varian, Palo Alto, CA, USA). Variable-temperature, solid-state direct current (dc) magnetic susceptibility data in the 2.0-300 K range were collected on a Quantum Design MPMS-XL SQUID magnetometer (Quantum Design, San Diego, CA, USA) operating at 0.1 T housed at the University of Crete. The magnetic data were corrected for the intrinsic diamagnetic contributions using Pascal's constants [88].  These compounds were prepared in an identical manner with 1, by using both of the above-mentioned methods, by simply replacing Nd(NO 3 ) 3 ·6H 2 O with the corresponding hydrated nitrate salts of Eu(III), Gd(III), Tb(III) and Dy(III). Typical yields were~35% using method (a) and~25% using method (b To a stirred colorless solution of mepaoH (0.136 g, 1.00 mmol) in a solvent mixture comprising MeOH (5 mL) and MeCN (10 mL), solid Dy(O 2 CMe) 3 ·4H 2 O (0.215 g, 0.50 mmol) was added. The reaction mixture was stirred under reflux overnight and the obtained slurry was filtered to remove a small quantity of solid. The filtrate was kept in a closed vial at room temperature. X-ray quality, colorless crystals of the product were precipitated within a period of 24 h. The crystals were collected by filtration, washed with cold MeOH (1 mL), MeCN (1 mL) and Et 2 O (3 × 1 mL), and dried in air. The yield was 55% (based on the available Dy III ). Anal. Calcd. (%) for C 26

Single-Crystal X-ray Crystallography
Colorless crystals of 1 (0.08 × 0.09 × 0.30 mm), 5 (0.10 × 0.16 × 0.18 mm) and 6 (0.10 × 0.13 × 0.15 mm) were taken from the mother liquor and immediately cooled to −113 • C (160 K). Diffraction data were collected on a Rigaku R-AXIS SPIDER Image Plate diffractometer (Rigaku Americas Corporation, The Woodlands, TX, USA; European Department at Karlsruhe, Germany) using graphite-monochromated Cu Kα radiation. Data collection (ω-scans) and processing (cell refinement, data reduction, and empirical/numerical absorption correction) were performed using the CrystalClear program package [89]. The structures were solved by direct methods using SHELXS, ver. 2013/1 [90] and refined by full-matrix least-squares techniques of F 2 with SHELXL, ver. 2014/6 [91]. All non-H atoms were refined anisotropically. All H atoms were introduced at calculated positions as riding on their corresponding bonded atoms. Structural plots were drawn using the Diamond 3 program package [92]. Crystallographic data are summarized in Table S1.
The most salient chemical features of the present work are: (a) the acetato ligands in 1-5 arise from the rare Ln III -mediated hydrolysis of the solvent used in the reactions (MeCN); and (b) the mixed acetato/nitrato complexes 1-5 are the thermodynamically preferred products from the Ln III /NO 3 − /MeCO 2 − /mepaoH reaction systems and that the all-acetato complex [Dy 2 (O 2 CMe) 6 (mepaoH) 2 ] (6) can be obtained only in the absence of nitrates. We have also demonstrated that the coordination of mepaoH to Eu III leads to a complex that emits red light coming from the enhanced emission of Eu(III). This can be attributed mainly to the shielding of the metal ion from H 2 O ligands (the presence of aqua ligands quenches the emission significantly). The Tb III center in 4 emits in the same region with the ligand and thus the characteristic 5 D 4 → 7 F j (j = 3-6) green emission pattern of this metal ion was not observed. In addition, the magnetic study of 3 suggests weak antiferromagnetic intramolecular Gd III . . . Gd III exchange interaction.
We do believe that the research theme presented here is not exhausted of new interesting results. The terminal nitrato (1)(2)(3)(4)(5) or acetato (6) groups might have utility as a means to obtain higher-nuclearity neutral or ionic Ln III /MeCO 2 − /mepaoH clusters and/or polymeric species, by replacing them with bis(bidentate) bridging ligands, e.g., aromatic heterocycles or dicarboxylates; such reactivity could alter the photophysical properties of the resulting complexes. The ability of mepaoH to act as an anionic ligand (mepao − ) bridging two or three metal ions gives us the stimulus to try to prepare polynuclear Ln III /mepao − /clusters. In addition, we have been working with the analogues of mepaoH (R = H, Ph, Cl in Figure 1) to study the influence of R on the identity and properties of the products. Last, but not least, inspired by the recent advances in the field of opto-magnetic materials and especially in that of optical thermometers (Single-Molecule Magnets which display thermally controlled luminescence) [93,94], we are investigating the optical/magnetic properties of 4-6 and analogous dinuclear Ln(III) complexes with a variety of other neutral 2-pyridyl oximes as capping ligands. Some of the above-mentioned efforts are already well-advanced proving that we have only scratched the surface of the lanthanide(III)/2-pyridyl oxime chemistry, and our results will be reported soon.
Supplementary Materials: The following are available online, Figure S1: Coordination polyhedra of lanthanide(III) centers in complexes 1, 5, and 6. Figure S2: ORTEP-type plots of the dinuclear molecules in 1 and 5 illustrating the intramolecular H bonds. Figure S3: ORTEP-type plot of the dinuclear molecule in 6 illustrating the intramolecular H bonds. Table S1: Crystallographic data for complexes 1, 5, and 6. Table S2: CShM values for the potential polyhedra of the lanthanide(III) centers in 1, 5, and 6. Table S3: Distances and angles of the intra-and intermolecular H-bonding interactions in the crystal structures of complexes 1 and 5. Table S4: Distances and angles of the intraand intermolecular H-bonding interactions in the crystal structure of complex 6.
Author Contributions: C.D.P. and H.N. contributed towards the syntheses, crystallization of the complexes, as well as the interpretation of the relevant results; C.P.R. and V.P. collected single-crystal X-ray diffraction data, solved the structures, and performed their refinements; the latter also studied in depth the supramolecular features of the crystal structures and wrote the relevant part of the article. V.B. and K.F.K. recorded and interpreted the solid-state, room-temperature excitation and emission spectra of the Eu(III) and Tb(III) complexes. S.P.P. coordinated the research and wrote the paper based on the reports of his collaborators. All authors exchanged opinions concerning the progress of the project and commented on the preparation of the manuscript at all steps. All authors have read and agreed to the published version of this manuscript.
Funding: S.P.P. thanks the COST Action CA15128 -Molecular Spintronics (MOLSPIN) for travel grants and for encouraging research activities in Patras, Greece.
Institutional Review Board Statement: Not applicable.

Informed Consent Statement: Not applicable.
Data Availability Statement: Crystallographic information files are available through the CCDC following the assigned numbers. The data presented here are available on request to the corresponding authors.