Synthetic analogues of the active site of the A-cluster of acetyl coenzyme A synthase/CO dehydrogenase: Syntheses, structures, and reactions with CO

Two metallosynthons, namely (Et4N)(2)[Ni(NpPepS)] (1) and (Et4N)(2)[Ni(PhPepS)] (2) containing carboxamido-N and thiolato-S as donors have been used to model the bimetallic M-p-Ni-d subsite of the A-cluster of the enzyme acetyl coenzyme A synthase/CO dehydrogenase. A series of sulfur-bridged Ni/Cu dinuclear and trinuclear complexes (3-10) have been synthesized to explore their redox properties and affinity of the metal centers toward CO. The structures of (Et4N)(2)[Ni(PhPepS)] (2), (Et4N)[Cu(neo)Ni(NpPepS)](.)0.5Et(2)O(.)0.5H(2)O (3(.)0.5Et(2)O(.)0.5H(2)O), (Et4N)[Cu-(neo)Ni(PhPepS)](H2O)-H-. (4(.)H(2)O), (Et4N)(2)[Ni{Ni(NpPepS)}2](DMF)-D-. (5-DMF) (Et4N)(2)[Ni(DMF)(2){Ni(NpPepS)}(2)](DMF)-D-. (6-31)DMF), (Et4N)(2)[Ni(DMF)(2){Ni(PhPepS)}2](8), and [Ni(dppe)Ni(PhPepS)](CH2Cl2)-C-. (10-CH2Cl2) have been determined by crystallography. The Ni-d mimics 1 and 2 resist reduction and exhibit no affinity toward CO. In contrast, the sulfur-bridged Ni center (designated Ni-C) in the trinuclear models 5-8 are amenable to reduction and binds CO in the Ni(l) state. Also, the sulfur-bridged Nic center can be removed from the trimers (5-8) by treatment with 1,10-phenanthroline much like the labile Ni from the enzyme. The dinuclear Ni-Ni models 9 and 10 resemble the Ni-p-Ni-d subsite of the A-cluster more closely, and only the modeled Nip site of the dimers can be reduced. The Ni(l)-Ni(II) species display EPR spectra typical of a Ni(l) center in distorted trigonal bipyramidal and distorted tetrahedral geometries for 9(red) and 10(red), respectively. Both species bind CO, and the CO-adducts 9(red)-CO and 10(red)-CO display strong nu(co) at 2044 and 1997 cm(-1), respectively. The reduction of 10 is reversible. The CO-affinity of 10 in the reduced state and the nu(co) value of 10(red)-CO closely resemble the CO-bound reduced A-cluster (v(co) (=) 1996 cm(-1)).