Models for aerobic carbon monoxide dehydrogenase: synthesis, characterization and reactivity of paramagnetic...

Reaction of [CoCp₂][Tp^iPrMoOS(OAr)] [Cp = η⁵-cyclopentadienyl; Tp^iPr = hydrotris(3-isopropylpyrazol-1-yl)borate; OAr = phenolate or derivative thereof] with [Cu(NCMe)(Me₃tcn)]BF₄ (Me₃tcn = 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, Tp^iPrMoO(OAr)(μ-S)Cu(Me₃tcn). 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 g _iso ∼ 1.937, hyperfine coupling to ^95,97Mo (a _iso = 39-42 × 10^-4 cm^-1) and strong superhyperfine coupling to ^63,65Cu (a _iso = 34-63 × 10^-4 cm^-1). Anisotropic spectra exhibit monoclinic symmetry with g ₁ ∼ 1.996, g ₂ ∼ 1.944 and g ₃ ∼ 1.882, and nearly isotropic A _Cu values (75-90 × 10^-4 cm^-1). The X-ray structures of four derivatives (Ar = Ph, C₆H₄ Bu-2, C₆H₄ ^sBu-2, C₆H₄Ph-4) are reported and discussed along with that of the Ar = C₆H₃ Bu₂-3,5 derivative (communicated in C. Gourlay, D. J. Nielsen, J. M. White, S. Z. Knottenbelt, M. L. Kirk and C. G. Young, J. Am. Chem. Soc., 2006, 128, 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 t-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 vs. 3.806(7)-4.040(2) Å]. As well, the a _Cu value of this complex (34.3 × 10^-4 cm^-1) is much lower than the values observed for other members of the series (55-63 × 10^-4 cm^-1), 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.