Lariat-type B12 model complexes. 1. New pendant methylpyridyl costa-type organocobalt complexes provide insight into the role of the axial ligand in butterfly bending and redox properties

Article abstract of DOI:10.1021/ja00043a015

A new B₁₂ (cobalamin, Cbl) model system with the potentially quinquedentate ligand, 2,3,9,10-tetramethyl-6,2-pyridylmethyl-1,4,8,11-tetraazaundeca-1,3,8,10-tetraen- 1,11-diolo (C₁py), has unique properties attributable to the coordinated pendant pyridine, which is attached covalently at the 2-position by a one-methylene link to the central C of the propylene chain of (DO)(DOH)pn (N²N^2′-propanediylbis(2,3-butanedione 2-imine 3-oxime)). Treatment of C₁py, constructed from the new diamine, 2-(2-pyridylmethyl)-1,3-propanediamine, with hydrated CoBr₂ afforded [BrCo(C₁py)]ClO₄ (5). From 5, salts of [XCo(C₁py)]⁺ (X = CN, N₃, Cl) and of [RCo(C₁py)]⁺ (R= CH₃, i-C₃H₇, neo-C₅H₁₁ CH₂CF₃) were synthesized by substitution and by oxidative addition of RX to in situ generated Co^-(C₁py), respectively. In contrast to previous model systems, the C₁py model behaves more like Cbls; for example, no dicyano complex was obtained. Coordination of the pyridyl moiety in the C₁py derivatives is confirmed by X-ray structural analysis of [ClCo(C₁py)]PF₆ (7) and [CH₃Co(C₁py)](ClO₄/BF₄) (9a). The pendant arm forces the pyridine to adopt an orientation 90° different from that in the [pyCo((DO)(DOH)pn)Cl(or R)]PF₆ derivatives. This new orientation greatly decreases butterfly bending, a structural feature relevant to triggering of Co-C bond homolysis by B₁₂ enzymes. This result is strong evidence that a planar N-donor ligand can induce butterfly bending. In 7 the axial Co-N distance is 1.959 (4) A?, shorter than this distance (2.06 (1) and 2.07 (1) A?, for two independent molecules) in 9a and similar to that in [pyCo((DO)(DOH)pn)Cl]PF₆. Although electrochemical reduction of the latter was not reversible, the reversible formation of Co(II) and Co(I) species from 7 (and 5) can be attributed to the pendant nature of the axial pyridine. The Co(III)/Co(II) redox couple was more cathodic for 7, consistent with better electron donation by the pendant pyridine. The Co(II)/Co(I) redox potential (-1.0 V vs 0.01 M Ag⁺/Ag) of 7 and 5 was similar to that for nonalkyl (DO)(DOH)pn derivatives, which lack a pendant base; this suggests that Co(II)/Co(I) reduction in nonalkyl C₁py derivatives proceeds through a base-off form in which the Co-pyridine interaction is weak, at best. Such a mechanism was proposed previously for Cbls. For both [CNCo(C₁py)]ClO₄ and cyanoCbl (vitamin B₁₂), a single two-electron wave was observed upon reduction. For alkylC₁py derivatives, only one cathodic wave was observed upon reduction. Just one prominent cathodic peak has been reported for alkylCbls. For [CH₂Co(C₁py)]ClO₄ (9), reduction formed a dealkylated Co(I) species, which was oxidized to Co(II) on the reverse anodic scan. Similarly, dealkylated Co(I) species formed after one-electron reduction of alkylCbls, including methylCbl, are oxidized to Cbl(II) in the return anodic scan. The Co(III)/Co(II) reduction potentials of C₁py and Cbl derivatives were influenced nearly identically by changes in the axial ligand; for R(or X) = solvent, Cl, CN, neo-C₅H₁₁, CH₃, an excellent correlation with a slope of unity was obtained. The simpler chemistry of the C₁py derivatives, as illustrated here by electrochemical studies, makes them excellent models to study fundamental chemistry relevant to Cbls.