Golam Moula

A nickel(II)-sulfur-based radical-ligand complex as a functional model of hydrogenase.

A nickel(II) dithiolene complex [Ni II (L 2− )(L − .)][PPh 4 ] (1; see figure; L=1,2-dicarbomethoxyethylene dithiolate) electrocatalyzes hydrogen evolution at the lowest achievable reduction potential (−0.69 V) in CH3CN and also in aqueous medium (−0.71 V) to date. Compound 1 shows strikingly similar EPR and reduction potential values to those observed with native Ni-containing hydrogenases.

Replica of a Fishy Enzyme: Structure–Function Analogue of Trimethylamine-N-Oxide Reductase

Three new complexes, [Mo(IV)O(mnt)(SS)](2-) (SS = dimethylethylenedicarboxylate (DMED), toluenedithiolate (tdt), benzenedithiolate (bdt); mnt = maleonitriledithiolate), each possessing two different dithiolene ligands, are synthesized as model of trimethylamine-N-oxide reductase. The asymmetric dithiolene ligands present in these complexes simulate the two different (P and Q) pterin coordinations in the family of DMSO reductase. These complexes reduce trimethylamine-N-oxide ((CH3)3N(+)-O(-) or TMANO), the biological substrate of trimethylamine-N-oxide reductase, to trimethylamine ((CH3)3N), responsible for the fishy smell of dead aquatic animals. The reaction kinetics of trimethylamine-N-oxide reduction by these complexes follow the Michaelis-Menten saturation kinetics. These experimental findings have been rationalized by DFT, TD-DFT level of calculations.

Dangling thiyl radical: stabilized in [PPh4]2[(bdt)W(VI)(O)(μ-S)2Cu(I)(SC6H4S•)].

The synthesis, crystal structure, and spectroscopic characterization of [PPh(4)](2)[(bdt)W(O)(S(2))Cu(SC(6)H(4)S(•))] (3; bdt = benzenedithiolate) relevant to the active site of carbon monoxide dehydrogenase are presented. Curiously, in 3, the copper(I) benzenemonothiolate subcenter possesses a dangling thiyl radical that is stabilized by a disulfido-bridged oxo tungsten dithiolene core. The benzenedithiolate ligand, which is generally bidentate in nature, acts as a bidentate and also as a monodentate in 3. The formation of an unusual dangling thiyl radical has been magnetically and spectroscopically identified and has been supported by the density functional theory level of calculation.

Mono(maleonitriledithiolene)molybdenum(IV) and Bis(μ‐sulfido)‐Bridged Dimolybdenum(V) Complexes with MoS Moiety

Mono(maleonitriledithiolene)sulfidomolybdenum(IV) complex, [MoS(S4)(mnt)]2− (2; mnt=maleonitriledithiolene) was synthesized by the substitution reaction of a tetrasulfido ligand of the known [MoS(S4)2]2− (1) upon reaction with one or even excess equivalent of Na2(mnt) in aqueous MeCN solution in air. Surprisingly, 2 undergoes dimerization on treatment with alkyl halide such as MeI and PhCH2Br to form bis(μ‐sulfido)dimolybdenum(V) species, [{MoS(mnt)}2(μ‐S)2]2− (3). These complexes have been characterized by IR, UV/VIS spectroscopy, cyclic voltammetry, elemental analysis, and by X‐ray crystal‐structure analysis. Differences in the relative stability and electrochemical behavior of 1, 2, and 3 have been correlated with theoretical calculations at DFT level.

Oxomolybdenum monodithiolene complexes linked with sulfur bridged iron: antiferromagnetically coupled Fe(III)Mo(V) systems

Mo-Fe heterometallic complexes with Fe(X)(2) (X = Cl, SPh) moiety attached to monodithiolene oxomolybdenum via sulfur bridge, viz., [Ph(4)P](2)[Cl(2)FeS(2)MoOS(2)(DMED)] (2) (DMED, dimethylethylenedicarboxylate), [Ph(4)P](2)[Cl(2)FeS(2)MoO(tdt)] (3) (tdt, toluenedithiolate) and [Ph(4)P](2)[(SPh)(2)FeS(2)MoO(tdt)] (4) are reported. Mossbauer spectroscopy, magnetism, EPR, electrochemistry and electronic structure based on DFT and TD-DFT calculation show the transfer of electron from iron to molybdenum centre resulting antiferromagnetically coupled Fe(III)Mo(V) unit from the starting Fe(II) and Mo(VI) compounds. A net spin of S = 2 ground state arising from antiferromagnetically coupled Fe(III) and Mo(V) shows a rare X-band EPR in normal mode at g ~ 12 in the solid state. In addition, Mossbauer studies show that electron drifting is more pronounced upon substitution of the chloride ligand by thiophenolate. The changes in dithiolene periphery electronically affect the charge distribution between Mo-Fe in {OMo(μS)(2)Fe} core. DFT calculations indicate that the increasing stability of dative Fe → Mo hetero metal-metal bond in these complexes from 3 to 2 to 4 is related to the extent of electron transfer from the iron to molybdenum centre.

A Cyanide-Bridged Molybdenum Bis(maleonitriledithiolate) Square

[Et4N]2[Mo(IV)O(mnt)2] (mnt = maleonitriledithiolate) reacts, as a synthon, with Me3SiCN under an acidic medium to produce the square complex [Et4N]4[Mo4(μ-CN)4(mnt)8] (1) in high yield. Complex 1 shows strong antiferromagnetic interactions between adjacent Mo atoms in the cluster. The presence of redox-active mnt as a capping ligand strongly influences the magnetic property of 1. The physicochemical properties of 1 have been rationalized by density functional theory level of calculations.

Synthesis and characterization of cyano and isocyano complexes of bis(dithiolato) molybdenum using Me3SiCN: a route to a cyanide-bridged multimer to a monomer

Cyanide- and isocyanide-bound molybdenum complexes containing maleonitriledithiolate (mnt = 1,2-dicyanoethylenedithiolate = S2C2(CN)2) as coligands were prepared from the synthon [Et4N]2[MoIVO(mnt)2] (1) as [Et4N][(PPh3)(mnt)23−˙MoIII(μ-CN)MoIII(PPh3)(mnt)23−˙] (3), [Et4N][MoIII(PPh3)(CN)(mnt)23−˙]·CH2Cl2 (4), and [MoIII(CNSiMe3)2(mnt)23−˙](5) using trimethylsilylcyanide (Me3SiCN). Triphenylphosphine (PPh3) was used as a blocking group and also as a competing ligand to cyanide to control the geometry and reactivity for forming the cyanide-bridged molybdenum dimer 3 or with terminal cyanide for the monomer 4. The known tetramer [Et4N]4[MoIII4(μ-CN)4(mnt)8] (2) formed in the absence of PPh3 showed a remarkable reaction with the excess Me3SiCN, where the monomeric species [MoIII(CNSiMe3)2(mnt)2] (5) was formed with Me3SiNC coordination. Overall, the Mo(IV) bis (dithiolate) complex showed diverse reactions with Me3SiCN in the presence and absence of the coligand PPh3 and under protic and aprotic media. The precursor molybdenum complex in the IV oxidation state was reduced to the III state in the complexes 3, 4, and 5, where one of the coordinating chelating ligand mnt2− was oxidized to mnt1−˙. This chemistry was supported by the EPR, electrochemical studies and X-ray crystallography results and corroborated by the DFT level of calculations. The complexes 3 and 4 were blue-emitting materials with a long lifetime in the millisecond range. Further DFT and TD-DFT calculations were carried out to understand the electronic states and the origin of the electronic absorptions.

Fe 4 S 4 Cubane Type Cluster Immobilized on a Graphene Support: A High Performance H 2 Evolution Catalysis in Acidic Water

The development of alternate catalysts that utilize non-precious metal based electrode materials such as the first row transition metal complexes is an important goal for economic fuel cell design. In this direction, a new Fe4S4 cubane type cluster, [PPh4]2[Fe4S4(DMET)4] (1) (DMET = cis-1,2-dicarbomethoxyethylene dithiolate) and its composite with functionalized graphene, (1@graphene) have been synthesized and characterized. The presence of nanocrystalline structures on graphene matrix in TEM and SEM images of 1@graphene indicate that the cluster (1) has been immobilized. The composite, 1@graphene evolves H2 gas from p-toluene sulfonic acid (TsOH) in a mixture of H2O and CH3CN under ambient conditions with a significant turnover number of 3200. 1@graphene electro-catalyzes H2 evolution at Ep, −1.2 V with remarkable throughput, catalytic efficiency and stability in only H2O or in only CH3CN. The Fe4S4 cluster (1) alone electro-catalyzes hydrogen evolution at Ep, −0.75 V from TsOH in CH3CN. The X-ray crystal structure of the Fe4S4 cluster (1) (λmax, CH2Cl2, 823 nm; ε, 2200 mol−1 cm−1) shows that it is dianionic with a cumulative oxidation state of +2.5 for the iron centers and short C-S bond distances (ca., 1.712 Å & 1.727 Å) indicating the presence of sulfur based radicals.

Electronic Structure of Monodithiolated IronOxotungsten Heterometallic Complexes: Integer-Spin FeW Assembly

Fe-W heterometallic complexes, in which an FeX2 (X = Cl, SPh) moiety is attached to monodithiolene oxotungsten through a sulfide bridge, that is, [Ph4P]2[Cl2Fe(S)2WOS2] (1), [Ph4P]2[Cl2Fe(S)2WOS2(DMED)] (2, DMED = dimethylethylenedicarboxylate), [Ph4P]2[Cl2Fe(S)2WO(tdt)] (3, tdt = toluenedithiolate), [Ph4P]2[(SPh)2Fe(S)2WO(tdt)] (4), and [Ph4P]2[Cl2Fe(S)2WO(edt)] (5, edt = ethanedithiolate), are reported. Mössbauer and EPR spectroscopy, magnetism, electrochemistry, and electronic structural analysis based on DFT and TD-DFT calculations show the transfer of electron from the iron center to the tungsten center, thus resulting in a ferromagnetically coupled Fe(III) W(V) unit, along with antiferromagnetic intermolecular interactions, from the starting Fe(II) and W(VI) compounds. A net spin of a S = 3 ground state, which arises from ferromagnetically coupled Fe(III) and W(V) atoms, displays a rare X-band EPR in normal mode at g ≈ 7 in the solid state.