Bisamidate and Mixed Amine/Amidate NiN2S2 Complexes as Models for Nickel-Containing Acetyl Coenzyme A Synthase and Superoxide Dismutase: An Experimental and Computational Study
The distal nickel site of acetyl-CoA synthase (Ni{sub d}-ACS) and reduced nickel superoxide dismutase (Ni-SOD) display similar square-planar Ni{sup II}N{sub 2}S{sub 2} coordination environments. One difference between these two sites, however, is that the nickel ion in Ni-SOD contains a mixed amine/amidate coordination motif while the Ni{sub d} site in Ni-ACS contains a bisamidate coordination motif. To provide insight into the consequences of the different coordination environments on the properties of the Ni ions, we systematically examined two square-planar Ni{sup II}N{sub 2}S{sub 2} complexes, one with bisthiolate-bisamidate ligation (Et{sub 4}N){sub 2}(Ni(L1)){center_dot}2H{sub 2}O (2) [H{sub 4}L1 = N-(2-mercaptoacetyl)-N{prime}-(2-mercaptoethyl)glycinamide] and another with bisthiolate-amine/amidate ligation K(Ni(HL2)) (3) [H{sub 4}L2 = N-(2{double_prime}-mercaptoethyl)-2-((2{prime}-mercaptoethyl)amino)acetamide]. Although these two complexes differ only by a single amine versus amidate ligand, their chemical properties are quite different. The stronger in-plane ligand field in the bisamidate complex (Ni{sup II}(L1)){sup 2-} (2) results in an increase in the energies of the d {yields} d transitions and a considerably more negative oxidation potential. Furthermore, while the bisamidate complex (Ni{sup II}(L1)){sup 2-} (2) readily forms a trinuclear species (Et{sub 4}N){sub 2}({l_brace}Ni(L1){r_brace}{sub 2}Ni){center_dot}H{sub 2}O (1) and reacts rapidly with O{sub 2}, presumably via sulfoxidation, the mixed amine/amidate complex (Ni{sup II}(HL2)){sup -} (3) remains monomeric and is stable for days in air. Interestingly, the Ni{sup III} species of the bisamidate complex formed by chemical oxidation with I{sub 2} can be detected by electron paramagnetic resonance (EPR) spectroscopy while the mixed amine/amidate complex immediately decomposes upon oxidation. To explain these experimentally observed properties, we performed S K-edge X-ray absorption spectroscopy and low-temperature (77 K) electronic absorption measurements as well as both hybrid density functional theory (hybrid-DFT) and spectroscopy oriented configuration interaction (SORCI) calculations. These studies demonstrate that the highest occupied molecular orbital (HOMO) of the bisamidate complex (Ni{sup II}(L1)){sup 2-} (2) has more Ni character and is significantly destabilized relative to the mixed amine/amidate complex (Ni{sup II}(HL2)){sup -} (3) by 6.2 kcal mol{sup -1}. The consequence of this destabilization is manifested in the nucleophilic activation of the doubly filled HOMO, which makes (Ni{sup II}(L1)){sup 2-} (2) significantly more reactive toward electrophiles such as O{sub 2}.
- Research Organization:
- Brookhaven National Lab. (BNL), Upton, NY (United States). National Synchrotron Light Source
- Sponsoring Organization:
- DOE - OFFICE OF SCIENCE
- DOE Contract Number:
- DE-AC02-98CH10886
- OSTI ID:
- 1019910
- Report Number(s):
- BNL-95756-2011-JA; INOCAJ; TRN: US1103692
- Journal Information:
- Inorganic Chemistry, Vol. 49, Issue 12; ISSN 0020-1669
- Country of Publication:
- United States
- Language:
- English
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