X-ray structure and redox chemistry of the heptacobalt cluster Co 3 (CO) 9 [μ 3 -CCO 2 CH 2 CCH{Co 4 (CO) 10 }

Abstract

The tetrahedrane cluster $${\text{Co}}_3 ({\text{CO}})_9 (\mu _3 {\text{ - CCO}}_{\text{2}} {\text{CH}}_{\text{2}} {\text{C}} \equiv {\text{CH)}}$$ reacts with Co4(CO)12 to furnish the heptacobalt compound Co3(CO)9[μ3-CCO2CH2CCH{Co4(CO)10}] in high yield. Substitution of the pendant alkyne group by the Co4(CO)10 moiety was ascertained by IR and 1H NMR spectroscopies, and the solid-state structure of Co3(CO)9[μ3-CCO2CH2CCH{Co4(CO)10}] was unequivocally determined by X-ray crystallography. Co3(CO)9[μ3-CCO3CH3CCH{Co4(CO)10}] crystallizes in the monoclinic space group P2 1/n, a = 12.895(13) A, b = 18.803(18) A, c = 13.748(13) A, β = 97.27(2)°, V = 3307(6) A3, Z = 4, d calc = 2.087 mg/m3; R = 0.0493, R w = 0.0989 for 4310 observed reflections with I > 2σ(I). The X-ray structure confirms the presence of an intact tetrahedral Co3 moiety and an alkyne-tethered Co4 butterfly cluster moiety. The cyclic voltammetric properties of Co3(CO)9[μ3-CCO2CH2CCH{Co4(CO)10}] were examined and three reduction waves were found. The first two reduction waves correspond to the regionally localized 0/1− redox couples on the tetra- and tricobalt moieties, respectively, while the third redox process is assigned to the 1−/2− reduction associated with the tetracobalt residue. Both 0/1− redox couples are reversible, while the 1−/2− reduction exhibits only quasi-reversible behavior. No evidence for electronic communication between the Co3 and Co4 portions of the complex was observed. Extended Huckel MO calculations support the site of the first reduction occurring solely on the tetracobalt moiety of this Co7 cluster.