Non-innocent Behaviour of Dithiocarboxylate Ligands Based on N-Heterocyclic Carbenes

Homoleptic ruthenium(II) complexes bearing imidazol(in)ium-2-dithiocarboxylate ligands: synthesis, characterization, and redox properties

Five cationic ruthenium(II) chelates with the generic formula [Ru(S2C·NHC)3](PF6)2 were readily obtained upon cleavage of the [RuCl2(p-cymene)]2 dimer with representative imidazol(in)ium-2-dithiocarboxylate zwitterions (NHC·CS2) in the presence of KPF6. The homoleptic complexes were fully characterized by various analytical techniques and the molecular structure of one of them was determined by single-crystal X-ray diffraction analysis. As expected, it featured an octahedral RuS6 core surrounded by three imidazol(in)ium rings and their nitrogen substituents. The robustness of the Ru-S bonds combined with the chelate effect of the κ2-S,S'-dithiocarboxylate units and the steric protection imparted by bulky 1,3-diarylimidazol(in)ium groups most likely accounted for the outstanding stability of these species in solution. Cyclic voltammetry showed that the five homoleptic complexes featured characteristic waves for a monoelectronic redox process corresponding to the RuIII/RuII couple with E1/2 values ranging between 0.97 and 1.27 V vs. Ag/AgCl. This half-wave potential was clearly dependent on the nature of their ancillary ligands as the evolution of the Ep,ox values roughly paralleled the basicity sequence of the NHCs used to prepare them, in line with the trends observed when monitoring the chemical shifts of the CS2- unit on 13C NMR spectroscopy and the CS2- asymmetric stretching vibration wavenumbers on IR spectroscopy.

Aldiminium and 1,2,3-triazolium dithiocarboxylate zwitterions derived from cyclic (alkyl)(amino) and mesoionic carbenes

The synthesis of zwitterionic dithiocarboxylate adducts was achieved by deprotonating various aldiminium or 1,2,3-triazolium salts with a strong base, followed by the nucleophilic addition of the in situ-generated cyclic (alkyl)(amino) or mesoionic carbenes (CAACs or MICs) onto carbon disulfide. Nine novel compounds were isolated and fully characterized by 1H and 13C NMR, FTIR, and HRMS techniques. Moreover, the molecular structures of two CAAC·CS2 and two MIC·CS2 betaines were determined by X-ray diffraction analysis. The analytical data recorded for all these adducts were compared with those reported previously for related NHC·CS2 betaines derived from imidazolinium or (benz)imidazolium salts. Due to the absence of electronic communication between the CS2 unit and the orthogonal heterocycle, all the CAAC·CS2, MIC·CS2, and NHC·CS2 zwitterions displayed similar electronic properties and featured the same bite angle. Yet, their steric properties are liable to ample modifications by varying the exact nature of their cationic heterocycle and its substituents.

Synthesis, Characterization, and Biological Activity of Water-Soluble, Dual Anionic and Cationic Ruthenium-Arene Complexes Bearing Imidazol(in)ium-2-dithiocarboxylate Ligands

An efficient synthetic protocol was devised for the preparation of five cationic ruthenium-arene complexes bearing imidazol(in)ium-2-dithiocarboxylate ligands from the [RuCl₂(p-cymene)]₂ dimer and 2 equiv of an NHC·CS₂ zwitterion. The reactions proceeded cleanly and swiftly in dichloromethane at room temperature to afford the expected [RuCl(p-cymene)(S₂C·NHC)]Cl products in quantitative yields. When the [RuCl₂(p-cymene)]₂ dimer was reacted with only 1 equiv of a dithiolate betaine under the same experimental conditions, a set of five bimetallic compounds with the generic formula [RuCl(p-cymene)(S₂C·NHC)][RuCl₃(p-cymene)] was obtained in quantitative yields. These novel, dual anionic and cationic ruthenium-arene complexes were fully characterized by various analytical techniques. NMR titrations showed that the chelation of the dithiocarboxylate ligands to afford [RuCl(p-cymene)(S₂C·NHC)]⁺ cations was quantitative and irreversible. Conversely, the formation of the [RuCl₃(p-cymene)]^- anion was limited by an equilibrium, and this species readily dissociated into Cl^- anions and the [RuCl₂(p-cymene)]₂ dimer. The position of the equilibrium was strongly influenced by the nature of the solvent and was rather insensitive to the temperature. Two monometallic and two bimetallic complexes cocrystallized with water, and their molecular structures were solved by X-ray diffraction analysis. Crystallography revealed the existence of strong interactions between the azolium ring protons of the cationic complexes and neighboring donor groups from the anions or the solvent. The various compounds under investigation were highly soluble in water. They were all strongly cytotoxic against K562 cancer cells. Furthermore, with a selectivity index of 32.1, the [RuCl(p-cymene)(S₂C·SIDip)]Cl complex remarkably targeted the erythroleukemic cells vs mouse splenocytes.

Heteromultimetallic compounds based on polyfunctional carboxylate linkers

Polyfunctional linkers bearing carboxylate, bipyridine and alkyne functionalities allow the stepwise construction of multimetallic assemblies incorporating redox and photophysical properties.

Mono- and bimetallic manganese-carbonyl complexes and clusters bearing imidazol(in)ium-2-dithiocarboxylate ligands

Five complexes with the generic formula fac-[MnBr(CO)₃(S₂C·NHC)] were obtained by reacting [MnBr(CO)₅] with a set of representative imidazol(in)ium-2-dithiocarboxylate zwitterions. These ligands are the adducts of N-heterocyclic carbenes (NHCs) and carbon disulfide. The mononuclear Mn(i) derivatives were coupled with Na[Mn(CO)₅] to afford bimetallic [Mn₂(CO)₆(S₂C·NHC)] clusters. Yet, the most convenient strategy to access these dinuclear Mn(0) products implied a direct carbonyl substitution from the [Mn₂(CO)₁₀] dimer. The molecular structures of three monometallic and four bimetallic compounds were elucidated by single crystal X-ray diffraction analysis. In the monometallic complexes, the NHC·CS₂ ligands exhibited a bidentate κ²-S,S' coordination mode with an S-C-S bite angle of about 116°. In the dinuclear clusters, the CS₂^- unit acted as a chelate toward one manganese center and as a pseudoallylic ligand toward the other one. The S-C-S bite angle was reduced to ca. 104°. Thus, the zwitterions displayed a remarkable flexibility, which also permitted a staggered arrangement of the carbonyl groups in the bimetallic systems. Examination of the [small nu, Greek, macron]_CO absorption bands on IR spectroscopy helped identify the presence of fac-Mn(CO)₃ or Mn₂(CO)₆ motifs, while the ¹³C NMR chemical shift of the CS₂^- moiety was a reliable indicator for monitoring its hapticity. Whereas the dinuclear clusters were air- and moisture-stable crystalline solids, mononuclear halido derivatives displayed only a limited stability under aerobic conditions. Both types of compounds underwent rather unselective, extensive fragmentations in the gas phase, in sharp contrast with the analogous rhenium derivatives that led to clean sequential decarbonylation processes upon collision-induced dissociation.

Small iron-carbonyl clusters bearing imidazolium-2-trithioperoxycarboxylate ligands

The reaction of [Fe₂(CO)₉] with two representative imidazolium-2-dithiocarboxylate zwitterions derived from common N-heterocyclic carbenes (NHCs) bearing mesityl (IMes) or 2,6-diisopropylphenyl substituents on their nitrogen atoms (IDip) unexpectedly afforded two small bimetallic iron-carbonyl clusters with the generic formula [Fe₂(CO)₆(μ-κ²-S,S'-κ²-S,S'-S₃C·NHC)]. After a brief optimization of the reaction conditions, these two "sulfur-enriched" products were isolated in low yields. They were fully characterized by IR, NMR, UV/Visible, and ESI-MS techniques, and their molecular structures were determined by single crystal X-ray diffraction analysis. The two compounds adopted a butterfly-type disposition in the solid state, with an [Fe₂(CO)₆] core bridged by the trithioperoxycarboxylate moiety of the in situ generated NHC·CS₃ ligands. Bond lengths recorded for the CS₃^- unit revealed that its negative charge was mostly located on the remote sulfur atom.

Synthesis and complexation of superbulky imidazolium-2-dithiocarboxylate ligands

The superbulky N-heterocyclic carbenes (NHCs) 2,6-bis(diphenylmethyl)-4-methylimidazol-2-ylidene (IDip*Me) and its 4-methoxy analogue (IDip*OMe) reacted instantaneously with carbon disulfide to afford the corresponding imidazolium-2-dithiocarboxylate zwitterions in high yields. These new dithiolate ligands were fully characterized and their coordination chemistry toward common Re(i) and Ru(ii) metal sources was thoroughly investigated. Neutral [ReBr(CO)₃(S₂C·NHC)] chelates featured three facially-arranged carbonyl groups on a distorted octahedron, whereas cationic [RuCl(p-cymene)(S₂C·NHC)]PF₆ complexes displayed a piano-stool geometry. The molecular structures of the six new compounds revealed that the NHC·CS₂ inner salts were highly flexible. Indeed, the torsion angle between their anionic and cationic moieties varied between ca. 63° in the free ligands and 3° in the ruthenium complexes. Concomitantly, the S-C-S bite angle underwent a contraction from 131° to 110-113° upon chelation. Computation of the %V_Bur parameter showed that the dithiocarboxylate unit of the NHC·CS₂ betaines chiefly determined the steric requirements of the imidazolium moieties, irrespective of the metal center involved in the complexation. The replacement of the p-methyl substituents of IDip*Me with p-methoxy groups in IDip*OMe did not significantly affect the ligand bulkiness. The more electron-donating methoxy group led, however, to small changes in various IR wavenumbers used to probe the electron donor properties of the carbene moiety.

The stepwise generation of multimetallic complexes based on a vinylbipyridine linkage and their photophysical properties

The versatile rhenium complex [ReCl(CO)₃(bpyC[triple bond, length as m-dash]CH)] (HC[triple bond, length as m-dash]Cbpy = 5-ethynyl-2,2'-bipyridine) is used to generate a series of bimetallic complexes through the hydrometallation of [MHCl(CO)(BTD)(PPh₃)₂] (M = Ru, Os; BTD = 2,1,3-benzothiadiazole). The ruthenium complex [Ru{CH[double bond, length as m-dash]CH-bpyReCl(CO)₃}Cl(BTD)(CO)(PPh₃)₂] was characterised structurally. Ligand exchange reactions with bifunctional linkers bearing oxygen and sulfur donors provide access to tetra- and pentametallic complexes such as [{M{CH[double bond, length as m-dash]CH-bpyReCl(CO)₃}(CO)(PPh₃)₂}₂(S₂CNC₄H₈NCS₂)] and Fe[C₅H₄CO₂M{CH[double bond, length as m-dash]CH-bpyReCl(CO)₃}(CO)(PPh₃)₂]₂. The effect of the group 8 metal on the photophysical properties of the rhenium centre was investigated using the complexes [Ru{CH[double bond, length as m-dash]CH-bpyReCl(CO)₃}Cl(BTD)(CO)(PPh₃)₂] and [M{CH[double bond, length as m-dash]CH-bpyReCl(CO)₃}{S₂P(OEt)₂}(CO)(PPh₃)₂] (M = Ru, Os). This revealed the quenching of the rhenium-based emission in favour of weak radiative processes based on the Ru and Os centres. The potential for exploiting this effect is illustrated by the reaction of [Ru{CH[double bond, length as m-dash]CH-bpyReCl(CO)₃}Cl(CO)(BTD)(PPh₃)₂] with carbon monoxide, which results in a 5-fold fluorescence enhancement in the dicarbonyl product, [Ru{CH[double bond, length as m-dash]CH-bpyReCl(CO)₃}Cl(CO)₂(PPh₃)₂], as the quenching effect is disrupted.

Optical and XPS evidence for the electrochemical generation of an N-heterocyclic carbene and its CS2 adduct from the ionic liquid [bmim][PF6]

Electrochemical generation of an N-heterocyclic carbene–CS₂ adduct in air-ambient and under vacuum, and its confirmation by XPS.

Synthesis, characterization, and gas-phase fragmentation of rhenium-carbonyl complexes bearing imidazol(in)ium-2-dithiocarboxylate ligands

Five complexes with the generic formula [ReBr(CO)₃(κ²-S,S'-S₂C·NHC)] were obtained by reacting [ReBr(CO)₅] with a set of representative imidazol(in)ium-2-dithiocarboxylate zwitterions. These ligands are the adducts of N-heterocyclic carbenes (NHCs) and carbon disulfide. The monometallic Re(i) compounds were further coupled with Na[Re(CO)₅] to afford bimetallic Re(0) species. Depending on the experimental conditions, either octacarbonyl dimers [Re₂(CO)₈(μ₂-κ¹-S,κ¹-S'-S₂C·NHC)] or hexacarbonyl clusters [Re₂(CO)₆(κ²-S,S'-κ³-S,C,S'-S₂C·NHC)] were isolated. All the products were fully characterized using various analytical techniques. Single crystal XRD analysis helped establish with certainty the various binding modes exhibited by the NHC·CS₂ ligands. With bite angles ranging from ca. 104 to 130°, these zwitterions displayed a remarkable flexibility, which also permitted significant twists of the thiometallated rings to preserve a staggered arrangement of the carbonyl groups in the bimetallic systems. Monitoring the chemical shift of the CS₂^- moiety by ¹³C NMR spectroscopy was most useful to detect its change of hapticity upon decarbonylation of the octacarbonyl compounds into hexacarbonyl derivatives. IR spectroscopy was another very convenient tool to identify the type of complex formed in a reaction, based on the pattern of its carbonyl vibration bands. Advanced mass spectrometry techniques showed that all the compounds underwent partial or total decarbonylation in the gas phase with no concomitant fragmentation of the bimetallic assemblies into monometallic ions.

Zwitterionic dithiocarboxylates derived from N-heterocyclic carbenes: coordination to gold surfaces

The zwitterionic dithiocarboxylates 1(+)-CS(2)(-)-4(+)-CS(2)(-) were prepared by reacting the corresponding N-heterocyclic carbenes 1,3-bis(2,6-diisoproylphenyl)imidazol-2-ylidene (1), 1,3-diisopropylimidazol-2-ylidene (2), 1,3-dibenzylimidazol-2-ylidene (3) and 1,3-diethylbenzimidazol-2-ylidene (4) with CS(2). In the latter two cases, the corresponding N-heterocylic carbene was generated in situ. Compounds 2(+)-CS(2)(-)-4(+)-CS(2)(-) were structurally characterised by single-crystal X-ray diffraction studies. The chemisorption of these zwitterionic dithiocarboxylates on solid gold substrates was investigated in situ and in real time by optical second harmonic generation (SHG). The resulting thin films were exemplarily characterised by near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and X-ray photoelectron spectroscopy (XPS) in the case of 1(+)-CS(2)(-) and 2(+)-CS(2)(-), revealing the formation of almost contamination-free self-assembled monolayers, which exhibit a remarkable degree of orientational order.

Gold(I) complexes bearing mixed-donor ligands derived from N-heterocyclic carbenes

The new 2-phenylthiocarbamoyl-1,3-dimesitylimidazolium inner salt (IMes·CSNPh) reacts with [AuCl(L)] in the presence of NH(4)PF(6) to yield [(L)Au(SCNPh·IMes)](+) (L = PMe(3), PPh(3), PCy(3), CNBu(t)). The carbene-containing precursor [(IDip)AuCl] reacts with IMes·CSNPh under the same conditions to afford the complex [(IDip)Au(SCNPh·IMes)](+) (IDip = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene). Treatment of the diphosphine complex [(dppm)(AuCl)(2)] with one equivalent of IMes·CSNPh yields the digold metallacycle, [(dppm)Au(2)(SCNPh·IMes)](2+), while reaction of [L(2)(AuCl)(2)] with two equivalents of IMes·CSNPh results in [(L(2)){Au(SCNPh·IMes)}(2)](2+) (L(2) = dppb, dppf, or dppa; dppb = 1,4-bis(diphenylphosphino)butane, dppf = 1,1'-bis(diphenylphosphino)ferrocene, dppa = 1,4-bis(diphenylphosphino)acetylene). The homoleptic complex [Au(SCNPh·IMes)(2)](+) is formed on reaction of [AuCl(tht)] (tht = tetrahydrothiophene) with two equivalents of the imidazolium-2-phenylthiocarbamoyl ligand. This product reacts with AgOTf to yield the mixed metal compound [AuAg(SCNPh·IMes)(2)](2+). Over time, the unusual trimetallic complex [Au(AgOTf)(2)(SCNPh·IMes)(2)](+) is formed. The sulfur-oxygen mixed-donor ligands IMes·COS and SIMes·COS (SIMes = 1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene) were used to prepare [(L)Au(SOC·IMes)](+) and [(L)Au(SOC·SIMes)](+) from [(L)AuCl] (L = PPh(3), CN(t)Bu). The bimetallic examples [(dppf){Au(SOC·IMes)}(2)](2+) and [(dppf){Au(SOC·SIMes)}(2)](2+) were synthesized from the reaction of [(dppf)(AuCl)(2)] with the appropriate ligand. Reaction of [(tht)AuCl] with one equivalent of IMes·COS or SIMes·COS yields [Au(SOC·IMes)(2)](+) and [Au(SOC·SIMes)(2)](+), respectively. The compounds [(Ph(3)P)Au(SCNPh·IMes)]PF(6), [(Cy(3)P)Au(SCNPh·IMes)]PF(6) and [Au(AgOTf)(2)(SCNPh·IMes)(2)]OTf were characterized crystallographically.