PROBING THE INFLUENCE OF LOCAL COORDINATION ENVIRONMENT LIGAND-BINDING IN NICKEL HYDROGENASE MODEL COMPLEXES

We report the synthesis and structure of three structurally-related mononuclear S-ligated Ni(II) complexes, 6-coordinate [Ni(L(S3N2))(MeCN)](2+) (1;L(S3N2)=(H2NC2H4SC2H4)(2)S), 6-coordinate [Ni(BATES)C1](+)(2; BATES=(H2N-o-C6H4SC2H4)(2)S), and 5-coordinate Ni(L(S5)) (3;L(S5)=(S-o-C6H4SC2H4)(2)S), which contain Ni in a S-rich environment possibly resembling that of the active form of nickel hydrogenases. Complex 1 crystallizes in the monoclinic space group P2(1)/n with a=19.036(2)Angstrom, b=12.022(8)Angstrom, c=24.953(4)Angstrom, beta=107.78(1)degrees, and Z = 4. Complex 2 crystallizes in the space group P (1) over bar with a=11.213(1)Angstrom, b=13.482(2)Angstrom, C=14.414(2)Angstrom, alpha=72.82(1)degrees, beta=89.862(9)degrees, gamma=69.396(9)degrees, and Z = 2. Complex 3 crystallizes in the monoclinic space group P2(1)/c with a=10.759(2)Angstrom, b=11.251(3)Angstrom, c=14.198(1)Angstrom, beta=90.82(1)degrees, and Z = 4. Structures 13 share a common trithioether (S-apical(C(2)H(4)SR(eq))(2); R(eq)=C2H4NH2(1), C6H4NH2(2), or C6H4S-(3)) ''backbone'' and possess either a ''vacant'' or a labile coordination site. These structures differ in terms of their molecular charge (2+ (1), 1+ (2), 0 (3)) and terminal ligating atoms. The amine-ligated complexes 1 and 2 have; a higher affinity for axial ligands and are more reactive than the thiolate-ligated complex 3. The Ni-bound MeCN solvent molecule of 1 is exchangeable and readily displaced by weaker coordinating solvents, as well as anionic ligands. X-ray structures of the most stable derivatives of 1, [Ni(L(S3N2))(py)](2+)(7), [Ni(L(S3N2))(N-3)](+) (8), and [Ni(L(S3N2))(Cl)](+) (9), are presented, and structural parameters are compared for [Ni(L(S3N2))(L)](n+) (n=1, 2; L=MeCN (1), py (7), N-3(-) (8), Cl- (9), and Br- (4)). Reaction of 2 with NaBH4 at -42 degrees C affords a thermally unstable species 10, which behaves like a Ni-H in terms ofits reactivity. Hydride derivative 10 will hydrogenate olefins (styrene and cyclohexene) and convert CDCl3 to CHDCl2 In contrast to the amine-ligated complexes, thiolate-ligated 3 is fairly unreactive with respect to binding axial ligands. The only ligand examined in this study which was strong enough to induce the spin-state change required in converting 3 to a 6-coordinate structure was'pyridine. Implications that this observed decrease in reactivity, upon substitution of terminal amines with aromatic thiolates, has:withrespect to the probable structure of the Ni-binding site of hydrogenase is also discussed.