Models for aerobic carbon monoxide dehydrogenase: synthesis, characterization and reactivity of paramagnetic MoO(μ-S)Cu complexes.

Reaction of [CoCp][TpMoOS(OAr)] [Cp = η-cyclopentadienyl; Tp = hydrotris(3-isopropylpyrazol-1-yl)borate; OAr = phenolate or derivative thereof] with [Cu(NCMe)(Metcn)]BF (Metcn = 1,4,7-trimethyl-1,4,7-triazacyclononane) in MeCN at -30 °C results in the formation of red-brown/black, paramagnetic, μ-sulfido-Mo(v)/Cu(i) complexes, TpMoO(OAr)(μ-S)Cu(Metcn). The complexes possess the MoO(μ-S)Cu core found in aerobic carbon monoxide dehydrogenases (CODHs) and exhibit X-band EPR spectra closely related to those of semi-reduced CODH, with ∼ 1.937, hyperfine coupling to Mo ( = 39-42 × 10 cm) and strong superhyperfine coupling to Cu ( = 34-63 × 10 cm). Anisotropic spectra exhibit monoclinic symmetry with ∼ 1.996, ∼ 1.944 and ∼ 1.882, and nearly isotropic values (75-90 × 10 cm). The X-ray structures of four derivatives (Ar = Ph, CH Bu-2, CH Bu-2, CHPh-4) are reported and discussed along with that of the Ar = CH Bu-3,5 derivative (communicated in C. Gourlay, D. J. Nielsen, J. M. White, S. Z. Knottenbelt, M. L. Kirk and C. G. Young, , 2006, , 2164). The complexes exhibit distorted octahedral oxo-Mo(v) and distorted tetrahedral Cu(i) centres bridged by a single bent μ-sulfido ligand, with Mo-S and Cu-S distances and Mo-S-Cu angles in the ranges 2.262-2.300 Å, 2.111-2.134 Å and 115.87-134.27°, respectively. The 2 -butyl derivative adopts a unique phenolate conformation with O[double bond, length as m-dash]Mo-O-Cα and O[double bond, length as m-dash]Mo-S-Cu torsion angles of 92.7 and 21.1°, respectively, very different from those of the other structurally characterized derivatives (31-47 and 33-45°, respectively) and exhibits a relatively short Mo···Cu distance [3.752(2) Å 3.806(7)-4.040(2) Å]. As well, the value of this complex (34.3 × 10 cm) is much lower than the values observed for other members of the series (55-63 × 10 cm), supporting the hypothesis that the electronic structure of the MoO(μ-S)Cu core unit and the degree of intermetallic communication are strongly dependent on the geometry of the MoO(OR)(μ-S)Cu unit. The complexes participate in an electrochemically reversible Mo(vi)/Mo(v) redox couple and react with cyanide undergoing decupration and desulfurization reactions of the type observed for CODH.