Rare Examples of Structurally Characterized Five-Coordinate Organocobalt Complexes. Novel Dynamic NMR Evidence for Synergistic Enhancement of Cis and Trans Effects in B12 Models

Article abstract of DOI:10.1021/ja00310a023

A major factor influencing Co-C bond stability both in B12 compounds and organocobalt B12 models appears to be the coordination number.Five-coordinate complexes are unusually stable to Co-C bond thermolysis, but structurally characterized examples of five-coordinate organocobalt complexes are rare.None are known for cases where R is a bulky group, such as i-C3H7.We report two definitive examples of five-coordinate complexes of the type Co(saloph)R, where saloph = dianion of disalicylidene-o-phenylenediamine.The structure of one species (R = i-C3H7) permits the first assessment of the effects of R bulk on the structure and also provides insight into the structural basis for the thermal stability of the species of reduced coordination number.We have also characterized the third example of a six-coordinate organocobalt saloph compound.Specifically, the crystal and molecular structures of Co(saloph)-i-C3H7*1.5H2O (I), Co(saloph)CH3*0.5H2O (II), and pyCo(saloph)CH2CF3 (py = pyridine) (III) are described.Compounds I and III are pentacoordinate and hexacoordinate, respectively, while crystals of II are comprised of both pentacoordinate and dimeric molecules.Crystallographic details follow.I: C23H21CoN2O2*1.5H2O, space group Pbcn, a = 26.075(6) Angstroem, b = 9.617(3) Angstroem, c = 16.109(3) Angstroem, D(calcd) = 1.46 g/ml, Z = 8, R = 0.057 for 5437 measured reflections.II: C21H17CoN2O2*0.5H2O, space group P1, a = 12.100(8) Angstroem, b = 20.392(9) Angstroem, c = 11.606(8) Angstroem, α = 53.3(1) deg, β = 127.6(1) deg, γ = 121.7(1) deg, D(calcd) = 1.50 g/ml, Z = 4, R = 0.041 for 7360 measured reflections.III: C27H21CoN3O2F2*0.5H2O*0.5C5H5N, space group P2₁/c, a = 14.255(8) Angstroem, b = 19.318(8) Angstroem, c = 19.434(8) Angstroem, β = 93.7(1) deg, D(calcd) = 1.45 g/cm³, Z = 8, R = 0.055 for 13588 measured reflections.Several interesting features emerge from these structures.First, the Co atom in I and II is displaced toward R.Second, the Co-C bond is shorter than in hexacoordinate compounds, but steric strain remains for i-C3H7 even in the pentacoordinate species.Third, in the unusual example of both a dimeric and monomeric molecule in the same crystal in compound II, the monomeric form seems to be stabilized by the water of crystallization.This water is hydrogen-bonded to the saloph ligand and not coordinated to Co.Dynamic 1H NMR measurements of 3,5-lut exchange in 3,5-lutCo(saloph)R (where 3,5-lut = 3,5-lutidine, R = CH2CN, CH2CF3, or CH3) reveal that the rate of exchange increases more rapidly with increasing electron donation by R in the series CH2CN to CH2CF3 to CH3 than in other classes of B12 compounds.The saloph ligand has recently been shown to exhibit a large structural cis influence and a large kinetic cis effect compared to the other classes of models.It was also recenty recognized that the trans and cis influences are synergistic.Thus, the dynamic NMR measurements provide evidence that the cis and trans kinetic effects are also synergistic.When R = CH3, for example, exchange of the neutral axial ...