Synthesis and characterization of nickel-thiolate complexes as a model of the active site of (Ni-Fe) hydrogenase enzymes

The main objective of this research is to synthesize and study new nickel-thiolate compounds as models for the active site of the nickel-containing hydrogenase enzymes, (Ni-Fe) H{dollar}sb2{dollar}ase.; We have designed and synthesized a number of ligands that are derivatives of the parent ligand, (PS3) H{dollar}sb3,{dollar} where (PS3) is the tris-deprotonated thiolate ligand: (Phosphine(o-benzenethiolate){dollar}sb3rbracksp{lcub}3-{rcub},{dollar} (P(o-C{dollar}rmsb6Hsb4S)sb3rbracksp{lcub}3-{rcub}.{dollar} We have been able to incorporate a number of substituents into the aromatic ring in the position ortho to the sulfur atom, including a phenyl group (abbreviated as (PS3-Ph) or (PS3*)); an amide group (abbreviated as (PS3-CONH-R)); and, an alcohol group (abbreviated as (PS3-CH(OH)R{dollar}sb2{dollar})).; The reaction of the tripodal ligand (PS3) with excess n-butyllithium and TMEDA produces the corresponding hexa-lithiated salt, (PS3) Li{dollar}sb6.{dollar} Reaction of (PS3) Li{dollar}sb6{dollar} with electrophiles permits the synthesis of derivatives of (PS3) containing diverse functional groups.; These ligands react with metal complexes to form compounds that exhibit characteristics that provide information pertaining to the nature of the nickel centers in hydrogenase enzymes.; Herein we report the synthesis and characterization of the first examples of structurally characterized Ni(III)-thiolate complexes. The Ni(III) compounds synthesized are formulated as: {dollar}rm{lcub}Nisp{lcub}III{rcub}lbrack PS3sp{lcub}*{rcub}rbrack(L){rcub}{dollar} where L = N-methylimidazole, p-(t-butyl)pyridine, pyridine, triethylphosphine, triphenylphosphine, t-butylisocyanide and 2,6-dimethylphenylisocyanide. The Ni(III)/Ni(II) redox couple for these Ni(III) compounds occur in the range of {dollar}-{dollar}0.12 to {dollar}-{dollar}0.44 V (vs SCE). The structure of the {dollar}rm{lcub}Nisp{lcub}III{rcub}lbrack PS3sp{lcub}*{rcub}rbrack(L){rcub}{dollar} compounds is that of a distorted trigonal bipyramid with the ligand (L) trans to the phosphorus atom.; In (Ni-Fe) H{dollar}sb2{dollar}ase, the Ni(III) is reduced to Ni(II); the Ni(II) is involved in the catalytic cycle of H{dollar}rmsb2/Hsp+{dollar} activation. Carbon monoxide (CO) inhibits the Ni(II) center in (Ni-Fe) H{dollar}sb2{dollar}ase. We have been able to synthesize and characterize a variety of Ni(II) compound of the type (Ni{dollar}rmsp{lcub}II{rcub}{dollar}(PS3-substituent)(L)) {dollar}sp{lcub}1-{rcub}.{dollar} Significantly, the first structurally characterized thiolate-Ni(II)-CO has been acquired.; A number of other Ni(II) complexes have been prepared predominately with isocyanide ligands, L = CNR{dollar}spprime.{dollar} Most notable are the complexes: (Ni{dollar}rmsp{lcub}II{rcub}{dollar}(PS3-CONH-R)(CNR{dollar}spprime)rbracksp{lcub}1-{rcub}{dollar} and (Ni{dollar}rmsp{lcub}II{rcub}{dollar}(PS3-CH(OH)-R)(CNR{dollar}spprime)rbracksp{lcub}1-{rcub}{dollar}; these ligands are capable of hydrogen bonding to the sulfur atom of the metal-thiolate center.; There exist a class of hydrogenase enzymes, (Ni-Fe-Se) H{dollar}sb2{dollar}ase, in which one of the four sulfur atoms coordinated to the Ni center is substituted by a selenium atom. To investigate the effect of the selenium upon the chemistry of the metal center, selenium-containing ligands were prepared: (PSe2*) H{dollar}sb2{dollar} and (PSe) H. Several metal complexes were made from these ligands. The most relevant is the Ni(IV) complex, (Ni{dollar}rmsp{lcub}IV{rcub}(PSe2sp{lcub}*{rcub})sb2rbrack.{dollar}; It is thought that in the hydrogenase enzymes, H{dollar}sb2{dollar} is split at the metal center via a heterolytic cleavage process to give a metal-hydride and a protonated base: (M-SR) + H{dollar}sb2 to{dollar} M(H{dollar}rmsp-)+(Hsp+)SR.{dollar} We sought to prepare compounds that incorporate both a M-(H{dollar}sp-){dollar} and a M-SR or a (H{dollar}sp+){dollar}SR...