Organocobalt complexes with exceptionally poor electron-donating alkyl groups. Extension and quantitation of the relationship between structure and NMR spectra in B12 models

Article abstract of DOI:10.1021/om00141a002

The crystal and molecular structures of the complex trans-bis(dimethylglyoximato)(chlorocyanomethyl)(pyridine)cobalt(III), (py)Co(DH)₂CH(CN)Cl (I), are reported. I crystallizes in space group P2₁/n of the monoclinic system with a = 15.688 (4) ?, b = 12.714 (2) ?, c = 9.513 (3) ?, β = 93.12 (4)°, V = 1894.6 ?³, Z = 4, D_measd = 1.53 g cm^-3, and D_calcd = 1.55 g cm^-3; R = 0.040 for 3007 independent reflections. The Co-N bond length in I of 2.017 (2) ? is the shortest such bond found thus far for a (py)Co(DH)₂R compound. The Co-C bond length of 2.015 (3) ? is the shortest bond to Co from an α-C-disubstituted alkyl group found thus far in a B₁₂ model. Other compounds, LCo(DH)₂R with R = cyano-substituted alkyl groups and L = py, 4-CNpy, PhNH₂, and P(OMe)₃, were also prepared. The crystal and molecular structures of (Py)Co(DH)₂CH₂CH₂CN (II) were determined. II crystallizes in space group P2₁2₁2₁ of the orthorhombic system with a = 9.546 (3) ?, b = 12.615 (3) ?, c = 16.082 (3) ?, V = 1936.8 ?³, Z = 4, D_measd = 1.45 g cm^-3, and D_calcd = 1.45 g cm^-3; R = 0.042 for 1606 independent reflections. The rate constants, k₁, for dissociation of 4-CNpy from (4-CNpy)Co(DH)₂R with R = CH(CN)Cl, CH(CN)CH₂CN, CH(Me)CN, and CH₂CH₂CN were determined. Trends in ¹H, ¹³C, and ³¹P NMR spectra were examined for several series of LCo(DH)₂R complexes and compared with the large body of rate, structural, and spectroscopic data available for related LCo(DH)₂R compounds. On the basis of the ¹³C NMR data for the γ-C in pyCo(DH)₂R compounds, we derived a scale of substituent constants, Δ(Y_i), for all substituents Y_i on the α-C of R. Good correlations were found between Σ_i=1³Δ(Y_i) and Co-N bond length, log k₁, ¹³C NMR shifts, and ³¹P NMR shifts. It was concluded that the influence of Y_i on the properties evaluated is mainly electronic (electronic trans influence). The steric effect of Y_i affects primarily the Co-C bond length (steric cis effect). The series of Δ(Y_i) values in the order of increasing electron donation was NO₂, CN, CF₃, I ≈ CO₂Me, Br, Cl ≈ CH₂CN, C(Me)(CO₂Et)₂, H, Si(CH₃)₃, Ph, Me, Et and larger hydrocarbon groups, OMe.