Activation of H 2 and CO by Sulfur‐Rich Nickel Model Complexes for [NiFe] Hydrogenases and CO Dehydrogenases

Reactions of the trinuclear complexes [Ni( R S 3 )] 3 [ H S 3 2− = bis(2‐mercaptophenyl)sulfide(2−) ( 1a ) or si S 3 2− = bis(2‐mercapto‐3‐trimethylsilylphenyl)sulfide(2−) ( 1b )] with nucleophiles L (L = NHP n Pr 3 , NHPCy 3 , NHSPh 2 , P n Pr 3 ) afforded the corresponding mononuclear complexes [Ni(L)( R S 3 )] [R = si = SiMe 3 ; L = NHP n Pr 3 ( 2b ); L = NHPCy 3 ( 3a , b ); L = NHSPh 2 ( 4a , b ); L = P n Pr 3 ( 5a )]. X‐ray structural determinations showed that 2b , 3a , 3b , 4a , and 5a exhibit tetrahedrally distorted planar [Ni(L)( R S 3 )] fragments. Complex 2b dimerizes through intermolecular N−H ··· N hydrogen bonding. In contrast to 2b , complexes 3a and 4a exhibit intramolecular hydrogen bonds between thiol groups and NH protons. Complexes 2 − 4 possess weakly acidic NH protons and undergo D + /NH exchange reactions with D 2 O or CD 3 OD. Complexes 2 − 4 and [Ni(S t Bu)( R S 3 )] − ( 9a , b ) also catalyze D 2 /H + exchange in [D 8 ]THF/H 2 O under an elevated pressure of D 2 (18 bar), as confirmed by 1 H NMR spectroscopy. It is proposed that D 2 heterolysis is achieved through attack of the Lewis‐acidic nickel centers and the Brönsted‐basic sulfur atoms at an η 2 ‐D 2 ligand. Complexes 9a and 9b are the first sulfur‐only nickel complexes that enable the modeling of the [NiFe] hydrogenase catalyzed D 2 /H + exchange reaction. Evidence for labile five‐coordinate [Ni(CO)(L)( R S 3 )] has been found in the reaction between [Ni(L)( R S 3 )] complexes and CO. The CO adducts of complexes with nitrogenous ligands L such as N 3 − , NHPR 3 (R = n Pr 3 , Cy 3 ), or NHSPh 2 showed rapid consecutive reactions. The reaction between Et 4 N[Ni(N 3 )( si S 3 )] ( 8b ) and CO gave Et 4 N[Ni(NCO)( si S 3 )] ( 10b ), whereas reactions between 2 − 4 and CO afforded only 1a or 1b . Mechanisms are suggested which have the formation of reactive five‐coordinated [Ni(CO)(L)( R S 3 )] intermediates in common. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)