Dithiocarbamate Complexes of Platinum Group Metals: Structural Aspects and Applications

: The incorporation of dithiocarbamate ligands in the preparation of metal complexes is largely prompted by the versatility of this molecule. Fascinating coordination chemistry can be obtained from the study of such metal complexes ranging from their preparation, the solid-state properties, solution behavior as well as their applications as bioactive materials and luminescent compounds, to name a few. In this overview, the dithiocarbamate complexes of platinum-group elements form the focus of the discussion. The structural aspects of these complexes will be discussed based upon the intriguing ﬁndings obtained from their solid-(crystallographic) and solution-state (NMR) studies. At the end of this review, the applications of platinum-group metal complexes will be discussed.


Introduction
Dithiocarbamates (RR NCS 2 − ; Figure 1) are highly versatile ligands that have attracted the attention of coordination chemists for decades [1][2][3][4][5].These, usually, mono-anionic species normally coordinate in an essentially symmetric fashion via the two sulfur atoms and are able to form stable complexes with a remarkable range of different metals in a variety of different oxidation states, including those that are higher than normal.This being stated, bridging modes of coordination are sometimes observed in the structural chemistry of the main group elements [6][7][8][9].It is also noted that polyfunctional dithiocarbamate ligands, that is, bearing more than one CS 2 − residue, are known although comparatively rare [10] as are multifunctional ligands incorporating CS 2 − functionality as well as other coordinating sites, for example, pyridyl donors [11].The latter two classes of ligand are invariably bridging and lead to various multi-metallic aggregates.The versatility of dithiocarbamate ligands stems from the near equal contributions of the two resonance forms, I and III (2 × 30%) and II (40%) (Figure 1).This is also significant in terms of supramolecular chemistry as the resulting MS 2 C chelate ring has significant π-character, metallo-aromaticity [12], and can participate as an acceptor in C-H• • • π(chelate ring) interactions in the same way as C-H• • • π(arene) interactions [13,14] but impart greater energies of stabilization [15][16][17].
From a structural point of view, dithiocarbamate complexes are of interest precisely because of the range of different heavy element ions to which they can coordinate and the resulting structural motifs.In summary, recent work has uncovered an even more diverse molecular and supramolecular chemistry with a wide variety of structural arrangements.
However, dithiocarbamate complexes are not just of intrinsic interest.Dithiocarbamates themselves are biologically active and when coupled with different metals, transition metals and main group elements, which often show enhanced activity, this activity can be modified, fine-tuned, and amplified [18][19][20][21][22]. Dithiocarbamate complexes also show utility as single-source precursors to metal sulfide nanoparticles [23,24].It is thus unsurprising there has been no let-up in recent years in the number of dithiocarbamate complexes reported in the literature.In this review, we explore recent advances in the molecular chemistry of platinum-group metal dithiocarbamate complexes, focusing on structural data acquired via X-ray crystallography and multi-NMR spectroscopy as well as an overview of potential applications.In many of these complexes, the dithiocarbamate ligands have been incorporated because of their ability to stabilize the metal moiety, allowing other, active functional groups to be present.In other cases, it is the dithiocarbamates themselves that are the primary focus of interest.In either case, without the presence of such a stable and tolerant ligand as dithiocarbamate, the potential utility of the resultant complexes is likely to be diminished.
Homoleptic dithiocarbamate complexes are typically prepared in a single-pot synthesis from the reaction of a secondary (or, less commonly, primary) amine with carbon disulfide under basic conditions and in the presence of the appropriate metal ion synthon (Equation ( 1)) [25].Alternative routes, more applicable to heteroleptic complexes include the reaction of a suitable dithiocarbamate salt (typically the ammonium [26], Group 1 [27] salt) with a metal halide (Equation (2)), or insertion of CS 2 into the metal-nitrogen bond of the corresponding amide complex (Equation ( 3)) [28].

Methods
A systematic search of the Cambridge Structural Database (CSD, version 5.42) [29] employing ConQuest (version 2.0.4) [30] was performed for the present bibliographic review of the crystallographic literature where the search evaluated all platinum-group metals (Ru, Rh, Pd, Os, Ir, and Pt) structures bearing a NCS 2 moiety.The search was restricted to structures deposited from 2004 onwards.No other restrictions were applied.All crystallographic diagrams were generated employing DIAMOND [31] using the Crystallographic Information Files deposited for each structure.

Results
The solid-state structures revealed by X-ray crystallographic methods will be discussed initially.This will be followed by an evaluation of solution-state structures/chemistry established by multi-NMR studies as well as an overview of complementary solid-state NMR studies.Lastly, comments will be made on the potential applications of the platinum group metal dithiocarbamate complexes.

Solid-State Structures
The discussion of the platinum group metal dithiocarbamate structures is made in the order following the Group of the Periodic Table, followed by the classification of the complexes such as binary, ternary, etc., and lastly, as organometallic species.A general overview of molecular structures will be provided and molecular packing only described for examples showing intriguing interactions.The dithiocarbamate ligands employed in this review are shown in Table 1 and Figure 2. The full chemical composition for the crystals discussed is included in the respective table under each subsection.
Table 1.List of dithiocarbamates L 1 to L 56 .

Ruthenium
The CSD search for ruthenium complexes bearing a dithiocarbamate ligand consistent with the search parameters described in Section 2 resulted in 72 different crystal structures: these are listed in Table 2.The most common oxidation states of ruthenium complexes are +II and +III with ruthenium(II) complexes being the most predominant.Generally, the ruthenium complexes adopt octahedral coordination geometries with the common feature of the dithiocarbamate acting as a κ 2 ligand.Binary complexes 1-5 feature ruthenium(III) centers while ternary (6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17) and quaternary (21) complexes constitute a ruthenium(II) metal center whereas crystals of 18-20 have ruthenium(III) centers.The organization of the remaining structures, namely 22-72, is problematic.The common feature of 22-49 is the presence of at least one carbonyl ligand, 50-55, a cyclopentadienyl (Cp) ligand and 56-66, a pentamethylcyclopentadienyl (Cp*) ligand.It is acknowledged this categorization is arbitrary.For example, the complex in crystal 55 contains both CO and Cp while crystal 66 contains both CO and Cp*.The last set of organometallic complexes 67-72, containing neither CO, Cp nor Cp*, are discussed in more detail below.[67] Crystals 1-3 [25] feature binary tris dithiocarbamate complexes.The presence of three symmetrically chelating ligands defines an octahedral geometry and enables the formation of enantiomers, i.e., ∆ and Λ.A representative example is shown in Figure 3a being the ∆-enantiomer of 1.The Ru-S bond lengths are equivalent and the main distortions from the ideal geometry arise as a consequence of the small bite angle of the chelating dithiocarbamate ligands.Two crystals have dinuclear [Ru 2 (L 8 ) 5 ] + cations.Despite the common formulation, two distinct patterns of coordination are noted.In the crystal of 4 [32], Figure 3b, two dithiocarbamate ligands bridge the two ruthenium atoms, chelating one and simultaneously bridging the other via one sulfur atom only.The third ligand is also bridging but forms only one bond to each ruthenium atom; the remaining dithiocarbamate ligands are chelating.As shown in Figure 3c, in 5 [33], two dithiocarbamate ligands bridge the two ruthenium atoms by chelating one and simultaneously bridging the other via one sulfur atom only.The bridging dithiocarbamate ligands are connected to the same ruthenium atom.The S 6 -octahedral geometry for each metal center is completed by one and two chelating ligands, respectively.The presence of different counter ions and solvates in the crystal may be responsible for the observed behavior.Color code in this and subsequent crystallographic images: platinum-group element, orange; other heavy element, brown; chloride, cyan; sulfur, yellow; phosphorus, purple; oxygen, red; nitrogen, blue; carbon, grey; hydrogen, green.Non-participating H atoms are omitted.
Attention now turns to ternary complexes.In common with 1-5, mononuclear complexes in 6 [32], Figure 3d, and 7 [32] feature octahedral geometries defined by S 6 donor sets.In the latter, the S-bound DMSO molecules occupy cis-positions with the dithiocarbamate ligands chelating in a slightly asymmetric mode with the longer Ru-S(dithiocarbamate) bonds occupying positions trans to the DMSO-S atoms.In crystals, 8 [34], 9 [35], and 10 [34], Figure 3e, the DMSO molecules of 6 and 7 have been substituted by a chelating dppb ligand leading to cis-P 2 S 4 donor sets.The number of dithiocarbamate ligands in the remaining structures, 11 to 21 is reduced to one.In the cationic salts 11 [36], Figure 3f, and 12 [37], the 2,2 -bipyridyl ligands are each chelating so the donor set is cis-N 4 S 2 .A similar geometry is found for the cation in 13 [38] where the four nitrogen atoms of the former examples are now replaced by the amine-nitrogen atoms of the cyclam ligand.The ruthenium center in the complexes of crystals 14 [39], 15 [40], 16 [41], and 17 [42] have the common feature of bearing two chelating dppm ligands and in a sense, the resulting cis-P 4 S 2 coordination geometries are the inverse to those seen in the crystals of 8 to 10.The heterometallic dinuclear complex in 17, Figure 3g, is worthy of special mention in that the ruthenium and square planar palladium atoms are connected by a multifunctional dithiocarbamate dianion, L 74 .The neutral complexes in quaternary crystals of 18 [43], 19 [44], and 20 [44] have trans PPh 3 ligands with the chloride atoms occupying positions trans to the sulfur atoms of the chelating dithiocarbamate ligands; a representative molecule, i.e., 18 is illustrated in Figure 3h.The final non-organometallic molecule to be described in this section is the cation in 21 [45].Here, the nitrogen atoms of the tridentate tptz ligand occupy mer-positions in the octahedral geometry defined by the N 3 PS 2 donor set, Figure 3i.
The common feature of organometallic species 22-49 is the presence of at least one carbonyl ligand: indeed, all but 48 (two) and 49 (three) have one terminally bound carbonyl group.In each of 22 [46] and 23 [47], there are two dithiocarbamate ligands while the remaining structures have one dithiocarbamate each.In 22, Figure 4a, the ruthenium(II) center is bis chelated by two dithiocarbamate ligands with the distorted octahedral geometry completed by the phosphane and carbonyl ligands.In 23, Figure 4b, one of the dithiocarbamate ligands of 22 now bridges an already coordinated germanium atom, forming one bond to each atom with the resultant octahedral coordination geometry based on a CGePS 3 donor set.The common feature of 24-26 [48] is a CNPS 2 Si donor set as one of the chelating dithiocarbamate ligands of 22 is replaced by a Si∩N chelating ligand with the difference in structures related to the backbone of the Si∩N chelating ligand; the molecule in 24 is shown in Figure 4c.In 27 [49], the dithiocarbamate and phosphane ligands of 22 are replaced by a tridentate ligand where coordination occurs from the ring-boron atom and two pendant phosphane ligands, Figure 4d.Thus, this is the first example where ruthenium is coordinated by two phosphane ligands representing a key structural difference from 22-26.The common feature of the complexes in the crystals of 28-47 is that they conform to the general formula (PPh 3 ) 2 Ru(L n )(CO)Y.For 28 [50], 29 [48], Figure 4e, and 30 [51], Y is a hydride.In common with all structures up to an including 43, the phosphane ligands are mutually trans.Similar arrangements are found in 31 [48] and 32 [47] where the hydride has been substituted for Si(Cl)Ph 2 and Ge(p-tolyl) 3 groups, respectively.
The (PPh 3 ) 2 Ru(L n )(CO) framework figures prominently in the stabilization of a variety of ruthenium-bound alkenes and alkynes.Alkenes of the type C(H)=C(H)R feature in the structures of 33 [52], Figure 4f, 34 [39], 35 [40], 36 [53], and 37 [51], whereas those of the type C(C≡R'))=C(H)R are seen in 38 [51], Figure 4g, 39 [54], and salt 40 [55].Alkynes are observed in 41 [52] and 42 [52].In 43 [56], Figure 4h, there is a Ru-C(phenyl) bond.The introduction of chloride to replace the alkene, alkyne, and phenyl ligands of 33-43 results in a major readjustment of the coordination geometry in that the phosphane ligands are now cis.The crystals of 44-47 [52] contain the same complex species but differ in the nature of the crystalline solvent; the molecule of 44, being representative, is shown in Figure 4i.The complex molecule in 48 [47] is different from the previous carbonyl compounds in that there are two carbonyl groups, occupying positions trans to the dithiocarbamate-sulfur atoms; the phosphane and Ge(p-tolyl) 3 ligands are trans within the octahedral geometry defined by a C 2 GePS 2 donor set, Figure 4j.In 49 [57], a tri-carbonyl species lacking a phosphane ligand, the carbonyls occupy one octahedral face with the remaining sites defined by dithiocarbamate-sulfur and chloride atoms, Figure 4k.
The other common ligands featuring in the ruthenium dithiocarbamates are those based on cyclopentadienyl, namely the parent Cp and permethylated Cp* ligands.Molecules in the crystals 50-55 feature Cp and three district structural motifs are evident.The molecules in 50 [58], 51 [59], 52 [58], and 53 [60] conform to the same motif shown in Figure 5a for 50, notable for having a dissymmetric dithiocarbamate ligand.The ruthenium is coordinated by a chelating dithiocarbamate ligand, a phosphane and the three remaining positions in the octahedral geometry defined by the Cp ligand.A similar coordination geometry pertains in the dinuclear species 54 [61] where two ruthenium atoms are linked by a dppf molecule, Figure 5b.In 55 [62], a mixed carbonyl/cyclopentadienyl compound, the former effectively substitutes for a phosphane ligand leading to an octahedral geometry-based, formally, on a C 4 S 2 donor set, Figure 5c.Among the Cp* structures, a new motif is noted for 56 [63] and 57 [64], with that of cationic 57, with a dissymmetric dithiocarbamate ligand, illustrated in Figure 5d.Here, the ruthenium is chelated by two dithiocarbamates and is capped by a Cp* resulting in, formally, a seven coordinate geometry based on a piano stool.A similar geometry is formed in neutral 58 [65], Figure 5e, and 59 [64] where one dithiocarbamate ligand has been replaced by two chlorides.Similar neutral species are seen in 60 [64] and 61 [64], Figure 5f, where the one dithiocarbamate ligand of 56 has been substituted by dinegative dithiocarbonate.The geometries of the cations in 62 [64] and 63 [64], resembling that seen in Figure 5c for 55 whereby the carbonyl has been substituted for a chloride.A variation is observed in 64 [65] as dimerization has occurred leading to the dicationic species shown in Figure 5g and a coordination geometry based on a piano stool.A piano stool geometry is also noted for dicationic 65 [65].The molecule in 66 [64] is the Cp* analogue of 55 shown in Figure 5c.The remaining structures to be described may be considered miscellaneous organometallic species as they contain neither carbonyl nor cyclopentadienyl type ligands.For the cations in 67 [43], Figure 5h, and 68 [43], the ruthenium is chelated by a dithiocarbamate as well as C,N-chelated by the C(NMe 2 )XC(NMe 2 )=S, X = O (67), S (68), ligand with the octahedral geometry completed by two trans oriented phosphane ligands.Cationic structure 68 was derived from 18 whereby 18 was treated with one molar equivalent of [M(MeCN) 4 ][ClO 4 ] (M = Cu, Ag) which resulted in C-S bond cleavage of L 8 and reassembly with another L 8 anion to generate the C,S chelating ligand.Prolonged standing resulted in hydrolysis and the ligand shown in Figure 5h.The common feature of 69 [66], Figure 5i, 70 [66], and 71 [66] is the presence of the di-allyl type ligand, (η 3 , η 3-2,7-dimethylocta-1,6-dien-3,8-diyl), which is assumed.Finally, the molecule in 72 [67], with a p-cymene ligand, assumes a coordination geometry resembling that of in 55 in Figure 5c. As

Rhodium
As summarized in Table 3, there are five known rhodium dithiocarbamate structures reported in the timeframe of this review.In keeping with the predominance of octahedral coordination geometries about the rhodium dithiocarbamate structures in 73-77, in 73 [68], one of the dithiocarbamate ligands adopts a relatively rare monodentate mode of coordination while the other retains its usual chelating mode.The three remaining positions in the octahedral geometry are occupied by the nitrogen atoms of the tripodal Tp* ligand, Figure 7a. Figure 7b gives a representation of the hydride in 74 [69].Stabilization to the molecule is provided by the tripodal ligand which coordinates by the central silicon atom and the two phosphorus atoms leading to a HP 2 S 2 Si donor set as the dithiocarbamate ligand is coordinating in the anticipated chelating mode.The molecules in 75 [70], Figure 7c, and 76 [71], are closely related with the rhodium atom being coordinated by a chelating dithiocarbamate ligand and two C,Nchelating 2-(pyridin-2-yl)phenyl ligands with the nitrogen atoms being mutually trans.Finally, in 77 [72], Figure 7d, a C 2 P 2 S 2 donor set is apparent owing to the presence of a chelating 1,2-bis(2-phenylethynylmethylene)cyclohexane ligand, the phosphorus atoms are mutually trans.

Iridium
Iridium dithiocarbamates are commonly seen as the organometallic complexes as presented in Table 6.The iridium complexes are characterized by coordination number six within a distorted octahedral geometry.As with previous precious group elements, a relatively high degree of commonality is noted in the molecular structures, with structural precedents also in existence.The most presented structural motif among the iridium dithiocarbamates is illustrated in Figure 11a for 177 [133].The iridium atom is chelated by the dithiocarbamate ligand, N,C-chelated by each of two 2-(2-pyridyl)phenyl anions, leading to a C 2 N 2 S 2 donor set, with trans-nitrogen atoms, which defines an octahedral geometry.Similar structures but with varying substitution patterns in the dithiocarbamate and N,C-chelating ligands are found in the crystals of 178 [134], 179 [135], 180 [136], 181 [136], 182 [136], 183 [137], 184 [137], 185 [137], 186 [27], 187 [138], 188 [139], 189 [139], 190 [139], 191 [140], 192 [138], 193 [140], 194 [141], 195 [142], 196 [143,144], 197 [145], and 198 [146,147].In crystals of 199 [148], 200 [148], 201 [148], and 202 [148] a C 2 P 2 S 2 donor set is featured.In neutral 199, Figure 11b, the iridium center is chelated by dithiocarbamate and C,C-chelating (1-(methylsulfanyl)penta-1,4-diene-1,5-diyl ligands with the phosphane molecules occupying mutually trans positions.The complex molecules in 200-202 are cationic and adopt the same structural motif.The remaining iridium dithiocarbamate structures feature Cp* ligands.The cation in 203 [149] has a geometry reminiscent of the ruthenium analog shown in Figure 5c.A new structural motif is noted for the dication in 204 [150].Each dithiocarbamate ligand chelates one iridium atom and uses one of the sulfur atoms to bridge the adjacent iridium atom.While not crystallographically imposed, the molecule has approximately 2-fold symmetry so the dithiocarbamate ligands lie to the same side of the central Ir 2 S 2 rectangle.In 205 [151], the carbonyl of 203 has been replaced by a 5-methyltetrazolate to give a neutral complex.As noted from Figure 11d, the anion is hydrogen bonded to 5-methyltetrazole in the crystal.In the cation of 206 [151], the hydrogen bonded molecule of 205 is replaced by a [Cp*Ir(L 8 )] to give the dinuclear molecule shown in Figure 11e.The common feature of 207 [152] and 208 [152] is the coordination by a monodentate dithiocarbamate ligand, rather unusual in the context of the present survey.In 207, Figure 11f, the iridium atom has a C 3 PS 2 donor set with the second sulfur atom derived from a hydrosulfide anion.An intramolecular hydrogen bond is noted between the dithiocarbamate-N-H . . .S(hydrosulfide) atoms.In 208, another rare example where the dithiocarbamate anion is derived from a primary amine, the hydrosulfide anion of 207 has been replaced by a hydride, Figure 11g.The last iridium structure comprises three independent [Cp*Ir(PPh 3 )(L 25 )(NCH 3 )] cation, and four iodide counterions in the crystal of 209 [153].The dithiocarbamate cation adopts the structural motif illustrated in Figure 5a.

Platinum
As for the palladium dithiocarbamates, binary structures predominate among the platinum dithiocarbamates which have been characterized by X-ray crystallography, see Table 7 for a listing.Thus, square planar geometries based on S 4 donor sets provided by two chelating dithiocarbamates ligands are found in 210 [73], 211 [73], 212 [75,76], 213 [80], 214 [154], 215 [155], 216 [156], 217 [84], 218 [155,157], 219 [157], 220 [158], 221 [159], 222 [160], 223 [161], 224 [86], 225 [162], 226 [163], and 227 [164].An exemplar molecule is shown for 222 in Figure 12a.This is particularly notable for having additional functionality on the dithiocarbamate ligands giving rise to the possibility of further complexation to other metals.The only binary dithiocarbamate complex that does not conform to the common structural motif is that of 228 [165].As shown in Figure 12b, the dithiocarbamate ligand carries a pendant pyrazolyl ring and in the case of one dithiocarbamate ligand, a S,N-chelating mode occurs rather than the normal S,S-chelating mode so the resulting donor set is NS 3 .As alluded to above, the pendent carboxylic acid residues in 228 can be employed in further coordination to generate higher-dimensional aggregation patterns.This possibility has been realized in four crystals, namely 229 [161], 230 [161], 231 [161], and 232 [161].A one-dimensional coordination polymer results as each L 17 ligand is deprotonated at one of the carboxylic acid residues and complexation occurs with one equivalent of Zn(OH 2 ) 4 .The polymer 229, as in Figure 12c, representative of those in 230 and 231, has a step-like topology.As for 229-231, a square planar geometry based on the S 4 donor set is evident for the platinum atom in 232.Here, both carboxylic acid residues of each L 52 anion are deprotonated, giving L 51 .When complexed with two equivalents of Zn(OH 2 ) 4 , a two-dimensional grid with a flat topology ensues, as illustrated in the plan view of Figure 12d.In all four coordination polymers, the carboxylate ligand is functioning in the monodentate mode.[172] * CSD depositions are available for repeated measurements.
An unusual pentanuclear structure is found in the crystal of 233 [166].As indicated in Figure 13a, each dithiocarbamate ligand chelates a platinum atom and simultaneously links to a second platinum via one sulfur atom only.The square planar geometry is completed by a chloride atom.The addition of a phosphane ligand to a binary dithiocarbamate results in one of the dithiocarbamate ligands coordinating in a monodentate mode, as in 234 [26], illustrated in Figure 13b.The bis dithiocarbamate ligand L 74 bridges two (PEt 3 ) 2 Pt entities in dicationic 235 [101], Figure 13c, and a similar structural motif is noted for 236 [101].In the quaternary complex 237 [167], a ClS 3 donor set is defined by the chelating dithiocarbamate ligand, a chloride and a S-bound DMSO molecule, Figure 13d.The complex molecules in 238 [168], 239 [166], 240 [169], 241 [169], 242 [169], and 243 [170], conform to the same structural motif as illustrated for 238 in Figure 13e.
The next series of structures are organoplatinum derivatives starting with the 244 [171] which is readily related to the previous series in that the chloride is now an iodide and the phosphane is replaced by the N-heterocyclic carbene, Figure 14a.The molecules in 245 [172], Figure 14b, 246 [172], and 247 [173] contain C,N-chelating orthometallated species as well as dithiocarbamate.In a variation, doubly orthometallated 2,2 -bipyridyl links two platinum centers in 248 [174], Figure 14c.Figure 14d represents 249 [175] and 250 [176] with CPS 2 donor sets.In the final molecular structure to be described, the platinum atom of Figure 14b is linked to Pt(C 6 F 5 ) 2 via two diphenylphosphinato ligands resulting in a rare example of a CNP 2 S 2 donor set in 251 [177], Figure 14e.The last remaining structure to be described is a coordination polymer, i.e., in the crystal of 252 [172].Here, the platinum atom is C,N-chelated by a benzo[h]quinolin-10-yl ligand with the square planar geometry completed by a chelating dithiocarbamate ligand.The latter is also connected to a silver cation which also forms connections to two platinum atoms to form a one-dimensional coordination polymer.

1 H NMR
Solution 1 H NMR data has been reported for dithiocarbamate complexes for all platinum-group metals, with data for palladium(II) and ruthenium(II) complexes predominating.As a consequence of restricted rotation around the carbamoyl C-N bond (see below), it is often the case that the dithiocarbamate ligand substituents give rise to two or, depending on the number of dithiocarbamate ligands and the local symmetry, occasionally more sets of resonances, which can complicate unambiguous assignment of signals, but conversely can also provide a wealth of useful information on the solution-state structures.The α-hydrogen nuclei provide the most characteristic probe.The chemical shifts vary quite significantly and typically range between about 1.80 and 4.5 ppm, depending on both the dithiocarbamate itself and the metal moiety to which it is coordinated.Table 8 shows how the mean chemical shifts and the standard deviation of these shifts varies with metal ion; Figure 15 shows the mean and 95% range (two standard deviations) for the shifts.Note that the data given in Table 8 and Figure 15 for ruthenium(II) are for dimeric complexes which are diamagnetic by virtue of spin pairing between the two ruthenium(II) centers.Although there is much overlap, Figure 15 reveals a clear trend in the mean shifts: (i) Decrease down a Group, (ii) Increase across a Period, (iii) Increase with the oxidation state of the metal ion.
These trends are statistically significant (based on standard two-tailed t-tests).The 1 H NMR chemical shifts of the α-hydrogens are also sensitive to the R-group of R 2 NCS 2 − , with those of benzyl groups and alicyclic groups, typically cyclohexyl, resonating at the higher end of the range and those of simply alkyl groups, such as R = Me and Et at the lower end of the range.Thus, for example, in the complexes Pd(S 2 CNR 2 )(η 3-2-methylallyl), NR 2 = NMe 2 (L 8 ), N(CH 2 Ph) 2 (L 39 ), and N(Me)Cy (L 15 ), the signals due to α-hydrogens of L 39 and L 15 are found at 4.92 and 3.7 ppm, respectively, while the NCH 3 signals of L 8  and L 15 lie at 3.33 and 2.08 ppm, respectively [77].Another comparative example may be found in reference [79].The α-hydrogen signals of piperazinedithiocarbamate ligands, − S 2 CN(CH 2 CH 2 ) 2 NR, are also generally found in the mid to upper end of the range; as, for example in [(p-tolyl) 3 PPd(L 69 )Cl]: 3.60 ppm [110].Although less common, complexes of dithiocarbamates derived from primary amines have also been studied.Typical examples include Pd(L 2 ) 2 [73], Pd(L 4 ) 2 [73] and {Me 3 Pt(L 2 )} 2 [178].The NH hydrogen signal is found in the range 6.82 to 10.95 ppm.Limited data frustrate attempts to identify any trends in the shifts.
Routinely, NMR spectroscopy is generally restricted to diamagnetic compounds because the presence of unpaired electrons can substantially reduce relaxation times, which can lead to very broad and often featureless spectra.However, paramagnetism also extends the normal chemical shift range, and in so doing can provide useful additional structural information.The rate of relaxation and the chemical shift of a nuclide in the presence of a paramagnetic center are related to the distance from that paramagnetic center.At short separations, the high rates of relaxation broaden the signals to such an extent that they are not detectable, whilst at longer ranges, the effects may be too small to give appreciable effects.Between these limits-the Goldilocks zone-observable, but widely dispersed signals are seen.These signals can be used to infer more detailed information about the structure.Dithiocarbamate complexes of ruthenium(II) are a good example, with the α-hydrogens of the dithiocarbamate, often lying in the Goldilocks zone. 1 H NMR data for a series of ruthenium(III) dithiocarbamate complexes are given in Table 9.In the case of Ru(L 8 ) 3 , the magnetic moment was calculated from the 1 H NMR shifts, using the Evans' Method [179].The effective magnetic moment, µ eff , was found to be 1.92 µ B , at room temperature, consistent with a low-spin d 5 electronic configuration: i.e., with a single unpaired electron [25].For complexes of general formula M(S-S) 3 for S-S = dithiocarbamate, there exists the possibility of two isomeric forms, namely L-M(L-L) 3 and D-M(S-S) 3 , Figure 16.The two isomers are exchanged by inversion at the metal center.In the case of the Ru(III) complexes, both the barrier to inversion and the barrier to rotation about the dithiocarbamate C-N bond (see below) are such that exchange is slow on the NMR time scale at room temperature.Consequently, the NMR spectra in complexes of unsymmetrical dithiocarbamate ligands can be quite complex, with different (C-N) rotational isomers observed for each of the L-and the D-configurations.In the case of Ru(L 61 ) 3 , for example [180], 12 sets of signals due to the α-hydrogen nuclei are observed in the 1 H NMR spectrum in the region 50-20 ppm.
In the majority of cases, the dithiocarbamate ligand is coordinated to the metal center via both sulfur atoms in an essentially symmetric fashion.However, there are examples of where the dithiocarbamate is coordinated via one sulfur atom only.For example, in [Tp*Rh(η 2 -L 24 )(η 1 -L 24 )] (Tp* = HB(3,5-pyrazol-1-yl)) [68], and [Cp*Rh(η 2 -L 24 )(η 1 -L 24 )] (Cp* = pentamethylcyclopentadienyl) [181], one of the two diethyldithiocarbamate ligands is symmetrically bound via both S atoms, while the other acts as a monodentate ligand.The different coordination modes are readily distinguished because, in the case of the bidentate bound ligand, the lack of symmetry along the S 2 C-N bond results in two sets of signals being observed for the ethyl group in the symmetrically bound ligand, but one set for the monodentate bound ligand: the methylene (NCH 2 ) hydrogens appear as a doublet of quartets in the κ 2 -bound ligand and as a simple quartet in the κ 1 -bound ligand.In the case of [Cp*Rh(κ 2 -L 24 )(κ 1 -L 24 )], coalescence of the signals is observed above ca 350 K indicating exchange between the κ 2 -and κ 1 -bound ligands [181]: such exchange was not observed in [Tp*Rh(κ 2 -L 24 )(κ 1 -L 24 )].One final point to note is that the 1 H signals of the dithiocarbamate ligands such as piperazinedithiocarbamate or mono-or dicyclohexyl-dithiocarbmate may also be affected by dynamic phenomena of the ring.Thus, for example, it is not untypical for the piperazine (S 2 C)NCH 2 -signals to be broadened by ring flexing [55].
Although agostic interactions between the α-hydrogens of dithiocarbamate ligands, S 2 CN(R)CH(R ), and the metal center may be observed in the solid-state, such interactions are not generally evident in solution, at least under ambient conditions.Attempts to establish the presence of agostic interactions at lower temperatures have met with mixed success.Thus, for example, 1 H NMR of a solution Ni(L 40 ) 2 , cooled to liquid nitrogen temperatures failed to reveal any evidence of the agostic interactions observed in the crystal structure of the complex [86].Whereas, for the related palladium(II) complex, Pd(L 41 ) 2 for [86], the singlet due to the α-hydrogen nuclei of the CH 2 C 5 H 4 N-3, split into two equally intense signals at 233 K, consistent with one of the methylene hydrogen atoms being involved in an agostic interaction with the metal center.The 1 H NMR sub-spectra due to the dithiocarbamate ligand in these complexes can, overall, thus provide a wealth of structural information, and hence be of intrinsic value beyond just routine characterization.

Barrier to C-N Bond Rotation
The significant contribution of resonance form (II) (Figure 1) lends substantial double bond character to the S 2 C-N bond, restricting rotation to an extent that, where symmetry allows, different rotomeric species are often visible in the NMR spectra at ambient temperature.Thus, for example, in cis-[Os(CO) 2 (L 8 ) 2 ], although the two dithiocarbamate ligands are equivalent, there is no plane of symmetry aligned along the C-N bond vector, rendering the methyl groups in each ligand inequivalent.Two equally intense signals, each of which integrates to six hydrogens, are observed for the NCH 3 groups at ambient temperature.These signals broaden on warming and eventually coalesce at ca 90 • C [130].Similarly, the methylene protons of {Pd(L 13 ) 2 } [78] are also inequivalent and, due to the restricted rotation about the C-N bonds are distinguishable in the NMR spectrum.Two doublets, each of integral 2, are observed at 4.75 and 5.21 ppm ( 2 J HH = 7 Hz).Well known as the barrier to rotation is, there remain relatively few studies on the magnitude of that barrier: contemporary data for platinum-group metal complexes are collected in Table 10.Table 10 also includes a comparison of the C-N stretching frequencies, where available, with the barrier to rotation.This relationship is depicted graphically in Figure 17. Figure 17 shows that the C-N stretching frequencies are very loosely, but never-the-less negatively correlated with higher barriers to C-N bond rotation: higher barriers are more associated with lower C-N stretching frequencies.This is contrary to expectation given that higher barriers should be associated with an increased double bond character, and thence would be expected to correlate with higher C-N stretching frequencies.However, with the very limited data available more inquiry into this relationship would be of value.3.2.2. 13 C NMR 13 C NMR data are, unsurprisingly, more sparse than 1 H NMR data.The quaternary carbamoyl carbon atom, S 2 CNR 2 , is most characteristic, although it is not always observed because of its long relaxation times.For the compounds discussed in this review, shifts typically range between ca 200 and 215 ppm (mean ca 208 ppm) and, compared to the corresponding signal in the free ligand, is typically shifted to a lower frequency by around 5 ppm, due to a reduction in the delocalization in the S 2 C-N system (i.e., increases the contribution of structures I and III, Figure 1).As would logically be expected, since coordination reduces delocalization, the ν C-N bond stretching in the infrared, which is normally in the range 1320-1600 cm −1 , generally increases on coordination [183].Whilst the carbamoyl moiety provides the most-ready probe for confirming metal complexation, the wider dispersion of 13 C chemical shifts, as compared to those of 1 H, mean that the 13 C signals of the nitrogen substituents, S 2 CNR 2 , are also of utility.The 13 C NMR shifts of the N-alkyl substituent groups are to be found in the region 12-55 ppm, with those attached directly to the N-atom (α-C) found at the high-frequency end of this range: 45-55 ppm; while βand γ-carbon nuclides are typically in the ranges 20-30 and 12-20 ppm, respectively.These distinct chemical shift ranges make the assignment of the 13 C NMR spectra of alkyl-substituted dithiocarbonate complexes a relatively straightforward and unambiguous process.Solid-state NMR (SSNMR) studies, usually 13 C CP-MAS, have been reported for a number of complexes [98,164,175].Unsurprisingly, the SSNMR spectra are generally more complex than the corresponding solution NMR spectra because the three-dimensional crystalline environment effectively reduces the symmetry of the molecules.Thus, for example, the 13 C CP-MAS SSNMR spectrum of Pt(L 80 ) 2 reveals the dithiocarbamate ligands to be inequivalent in the solid-state; the carbamoyl carbon atoms (S 2 CN<) give rise to a 1:5:5:1 pseudo-quartet at ca 209.6 ppm, resulting from overlapping 1:4:1 triplets (the triplet fine structure arises from coupling to 195 Pt: I = 1 /2; 33.8% abundance), whereas in solution a single signal is observed [164].The same patterns are observed in the solid-state 13 C spectra of Pt(L 29 ) 2 and Pt(L 34 ) 2 [155].Although the symmetry is lower in the solid-state, it is clear that the basic molecular structure is the same in the solid-state and solution in these complexes.Likewise, the structure of silica-supported [Pd(Ph 2 P(CH 2 ) 3 PPh 2 )(L 24 )]BF 4 has been shown by SSNMR to be largely unaffected, compared to the solution structure [98].
3.2.3. 195Pt NMR 195 Pt NMR data are available for some platinum(II) and platinum(IV) dithiocarbamate complexes.Data are collated in Table 11.
Table 11. 195Pt NMR data reported for selected Pt(II) and Pt(IV) dithiocarbamate complexes.
[Pt(pbt)(L For the platinum(II) complexes, the chemical shifts range from 603 to 810 ppm, as compared to 1416 to 1632 ppm for the platinum(IV) complexes.The higher chemical shifts of the platinum(IV) complexes are consistent with the general observation that the resonance frequency increases with the oxidation state [184].Similarly, the trend to a higher frequency in the platinum(II) complexes on ionization (removal of H + ) is consistent with the higher charge at the metal center.

Biological Applications
Perhaps unsurprisingly, one of the most common areas of the potential application of dithiocarbamate complexes to be explored is that related to their anti-cancer properties.Ever since cisplatin, cis-[PtCl 2 (NH 3 ) 2 ], was first approved for use in the treatment of testicular and ovarian cancers by the US food and Drug Administration in 1978 [185], the chemotherapeutic applications of platinum and, more generally, platinum group metal compounds have been the subject of ongoing investigation.Given that organic derivatives of dithiocarbamates, RSC(S)NR' 2 , also display antitumor activity [186], the exploration of anti-cancer properties of platinum-group metal dithiocarbamate complexes is an obvious avenue for research [187].
The half-maximal inhibitory concentration, IC 50 , is a commonly used measure of the effectiveness of a compound at inhibiting a particular biological function, such as cell proliferation.The in vitro cytotoxicities of those platinum-group metal complexes described in this review against various human cancer cell lines are given in Table 12.Given that the effectiveness of complexes has been investigated against a variety of different cell lines and with many experiments conducted under different laboratory conditions, it is hard to draw general conclusions from the data.Where a common cell line has been used (HeLa; cervical cancer cell line), data indicate that the ruthenium complexes are significantly more active (lower IC 50 ) than either palladium or platinum.The sparse data and diversity of cell lines used in reported cases suggest that some more systematic studies on the antitumor activity of platinum group metal dithiocarbamate complexes would add significantly to the body of literature.
DNA binding studies have also been carried out to explore the mechanism of the antitumor activity.The DNA binding of complexes of general formula (PR 3 )Pd(S 2 CNR 2 )Cl, PR 3 = PPh 3 , PPh 2 Cl, PPh 2 (tert-Bu), PPh 2 (CH 2 Ph), PPh 2 (o-tolyl), PPh 2 (p-tolyl), PPh 2 (C 6 H 4 OMe-o); R = Me (L 8 ), Et (L 24 ), n-Pr (L 28 ), n-Bu (L 31 ), 2-methoxyethyl (L 26 ), have been probed by cyclic voltammetry [103,115].The cyclic voltammograms of the complexes display a single cathodic peak due to an irreversible one-electron reduction of the complexes.The addition of DNA more normally causes a diminution in the cathodic peak height and may also induce a shift in the position of the peak, consistent with DNA binding.In some complexes, DNA binding has been shown to lead to increased cathodic peak height, indicating DNA may also catalyze the electron transfer process [103].Binding constants [103] were found to be in the range 6.16 × 10 2 -7.05 × 10 6 M −1 , with the corresponding free energies of activation being in the range −16 to −39 kJ mol −1 .The greatest binding affinity was found for [PPh 2 (p-tolyl)Pd(L 31 ) 2 }(Cl)] and was attributed to the presence of the hydrophobic butyl groups on the dithiocarbamate.
UV-Vis spectroscopy also provides a readily accessible probe for DNA binding studies.The UV-vis spectra typically display bands in the region 220-460 nm, attributable to both charge transfer (CT: metal-to-ligand and ligand-to-metal), intraligand π→ π* and metal centered d-d transitions.The CT bands are distinguishable because of their high molar extinction coefficients [25,180].The addition of increasing concentrations of DNA leads to a decrease in absorption peak intensity, from which the free energies of interaction can be calculated, with complex-specific negative values indicating spontaneous interaction between the DNA and the complexes [109,188,189].In general, the DNA-complex appears to be stable over a prolonged period (at least 48 h).For further information, the interested reader is referred to [190][191][192][193][194] being reviews of platinum group dithiocarbamates and anti-cancer potential.
Although less well studied than their anti-cancer properties, platinum group metal dithiocarbamate complexes are also known to exhibit antifungal properties.The antifungal properties of the palladium(II) and platinum(II) complexes M[S 2 CN(Me)R] 2 , where R = 2,3-dimethoxyethyl (L 16 ) or 1,3-doxolane-2-methyl (L 23 ), have been reported [80].All four complexes were found to be active against A. flavus, A. niger, and A. parasiticus: the complexes are more active than the drug Nyastin, but less active than miconazole nitrate, which is known to display extensive biocide action.In general, the palladium complexes displayed similar or greater activity.In the case of the palladium complexes, those of L 23 were generally more active, whereas in the platinum complexes, those of L 16  were more active [80].These preliminary studies suggest that the antifungal behavior of platinum-group metal dithiocarbamate complexes would warrant further study.
Another potential application offered by dithiocarbamates being their use as an antibacterial agent, as shown by the inhibitory effect of pyrrolidine dithiocarbamate against bacterial strains P. gingivalis, A. actinomycetemcomitans, S. aureus, and E. coli and the enhanced activities resulted from the coadministration of ZnCl 2 [195,196].Significantly more investigations have been performed on the antibacterial potential of heavy element dithiocarbamate compounds with the area being reviewed very recently [197].Among which, a series of six heteroleptic palladium(II) dithiocarbamates of general formula (PR 3 )Pd(S 2 CNR 2 )Cl, PR 3 = PCy

Other Applications
Besides their biological behavior, platinum-group dithiocarbamate complexes have potential utility as metal ion sensors [142,190], as single-source precursors for MOCVD [77], and as catalysts.Cyclometalated iridium(III) dithiocarbamate complexes of the type [Ir(dpci) 2 (S 2 CNR 2 )], where dpci = 3,4-diphenylcinnoline; NR 2 = NEt 2 (L 24 ) and N-carbazolyl (L 82 ) are both highly selective and highly sensitive mercury chemosensors [142,190].The sensors work by having a photoluminescent complex, with a relatively labile dithiocarbmate ligand, which can be scavenged by Hg 2+ ions; the ideal scenario is that the presence of the Hg 2+ ions act as a clear "on" switch, and these complexes display just such behavior.Furthermore, the complex [Ir(dpci) 2 (L 82 )] can operate as an AND logic gate, both changing color and displaying significant photophorescence in the presence of acetonitrile (which causes the color change) and Hg 2+ ions [142].

Conclusions
In summary, dithiocarbamates are highly versatile ligands, which are found in a wide variety of platinum group (and other) metal complexes.The resultant metal dithiocarbamate moiety may itself be the locus chemical or structural interest, or it may be that it is present in an ancillary capacity.The pendant organic groups on the dithiocarbamate nitrogen atom can be as varied as those on the parent amines, from which they are derived, allowing fine-tuning of both the electronic and steric environment around the ligand and hence of the structural and electronic properties of the complexes.The resultant platinum group metal complexes have been shown to have a variety of applications, with their anticancer properties being the most well studied.Comparison of X-ray structural data with NMR for these complexes indicates that the molecular structures are essentially the same in both the solid-state and in solution.Solution NMR, in particular, provides an excellent probe for metal-dithiocarbamate coordination, with the carbamoyl C atom and the α-hydrogen atoms of the N substituents resonating at characteristic frequencies.Given their facile preparation, considerable stability and versatility in coordination chemistry, it is highly likely that the interest in these compounds will continue to grow.

Conflicts of Interest:
The authors declare no conflict of interest.
mentioned in the Introduction, crystals of metal dithiocarbamates are prone to display assemblies featuring C-H• • • π(metal-S 2 C) interactions [15-17].While the focus of the present overview is upon molecular geometries, it is worth highlighting some examples drawn from the ruthenium dithiocarbamates 1-72 where C-H• • • π(RuS 2 C) interactions are evident in their crystals.In Figure 6a, a supramolecular dimer is formed in the crystal of 3 through intermolecular C-H• • • π(RuS 2 C) interactions occurring between centrosymmetrically related molecules.A similar aggregate is formed in the crystal of 8 despite the presence of bulky phosphane ligands.Supramolecular chains are also evident, for example in the crystal of 1 where a zigzag chain is evident with each independent molecule forming on average two C-H• • • π(RuS 2 C) interactions.In the crystal of 58, a helical chain features C-H• • • π(RuS 2 C) interactions.

Figure 6 .
Figure 6.Supramolecular aggregation between molecules in the crystals of (a) 3, (b) 8, (c) 1, and (d) 58.The C-H• • • π(RuS 2 C) interactions are represented by dashed purple lines, and the distances and angles associated with the C-H• • • π(RuS 2 C) interactions are given on each image.

Figure 15 .
Figure 15.The mean and 95% range (two standard deviations) for the 1 H chemical shifts reported for α-hydrogen nuclei.

Figure 17 .
Figure 17.Correlation of ν C-N with the barrier to C-N bond rotation.
The 13 C NMR shifts of N-aryl substituent groups are, as would be expected, less well separated and typically observed in the region 120-150 ppm: C ipso 130-160 ppm; C ortho 122-132 ppm; C meta 125-130 ppm; C para 120-135 ppm.

Author Contributions:
Writing-original draft preparation, review, and editing, Y.S.T.; Writingoriginal draft preparation, review, and editing, C.I.Y.; Writing-original draft preparation, review, and editing, E.R.T.T.; Conceptualization, writing-original draft preparation, review, and editing, P.J.H.All authors have read and agreed to the published version of the manuscript.Funding: Research in dithiocarbamate chemistry at Sunway University is funded by Sunway University Sdn Bhd, grant number GRTIN-IRG-01-2021.The APC was funded by Sunway University Sdn Bhd.

Table 2 .
Summary of ruthenium dithiocarbamate structures established by X-ray crystallography.See Table1, Figure2, and the Abbreviations section.

Table 3 .
Summary of rhodium dithiocarbamate structures established by X-ray crystallography.See Table1, Figure2, and the Abbreviations section.

Table 4 .
Summary of palladium dithiocarbamate structures established by X-ray crystallography.See Table1, Figure2, and the Abbreviations section.
* CSD depositions are available for repeated measurements.

Table 5 .
Summary of osmium dithiocarbamate structures established by X-ray crystallography.See Table1, Figure2, and the Abbreviations section.

Table 6 .
Summary of iridium dithiocarbamate structures established by X-ray crystallography.See Table1, Figure2, and the Abbreviations section.
* CSD depositions are available for repeated measurements.

Table 7 .
Summary of platinum dithiocarbamate structures established by X-ray crystallography.See Table1, Figure2, and the Abbreviations section.

Table 8 .
The mean 1 H chemical shifts reported for α-hydrogen nuclei and the standard deviation of these shifts as a function of the metal ion.

Table 9 .
1H Chemical shifts of α-hydrogen nuclei for a series of ruthenium(III) dithiocarbamate complexes.

Table 10 .
Contemporary data for the barrier to rotation about the C-N bonds of platinum-group metal complexes.

Table 12 .
In vitro cytotoxicity of platinum-group metal dithiocarbamate complexes.