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 Mp−Nid 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)]·0.5Et2O·0.5H2O (3·0.5Et2O·0.5H2O), (Et4N)[Cu(neo)Ni(PhPepS)]·H2O (4·H2O), (Et4N)2[Ni{Ni(NpPepS)}2]·DMF (5·DMF), (Et4N)2[Ni(DMF)2{Ni(NpPepS)}2]·3DMF
(6·3DMF), (Et4N)2[Ni(DMF)2{Ni(PhPepS)}2] (8), and [Ni(dppe)Ni(PhPepS)]·CH2Cl2 (10·CH2Cl2) have been determined
by crystallography. The Nid mimics 1 and 2 resist reduction and exhibit no affinity toward CO. In contrast, the
sulfur-bridged Ni center (designated NiC) in the trinuclear models 5−8 are amenable to reduction and binds CO in
the Ni(I) 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
Nip−Nid subsite of the A-cluster more closely, and only the modeled Nip site of the dimers can be reduced. The
Ni(I)−Ni(II) species display EPR spectra typical of a Ni(I) center in distorted trigonal bipyramidal and distorted
tetrahedral geometries for 9red and 10red, respectively. Both species bind CO, and the CO-adducts 9red-CO and
10red-CO display strong νco at 2044 and 1997 cm-1, respectively. The reduction of 10 is reversible. The CO-affinity
of 10 in the reduced state and the νco value of 10red-CO closely resemble the CO-bound reduced A-cluster (νco =
1996 cm-1).