Vitamin B12 model compounds: influence of neutral ligand orientation on the CoN axial bond length

Abstract The crystal structures of two complexes of the type [L(Co(DO)(DOH)pn)Me]PF 6 where (DO)- (DOH)pn = N 2 , N 2 -propanediylbis(2,3-butanedione 2-imine 3-oxime) and L = 1-methylimidazole, 1-MeImd ( 1 ) and 1,2-dimethylimidazole, 1,2-Me 2 Imd ( 2 ) have been determined. Both compounds crystallize in the space group P 2 1 / c with cell parameters a =7.104(2), b =23.110(5), c =15.224(2) A, β=99.29(2)° for 1 and a =8.072(2), b =12.290(2), c =23.704(4) A, β=95.41(2)° for 2 . The two structures were solved by Patterson and Fourier methods and refined by least-squares methods to final R values of 0.053 ( 1 ) and 0.043 ( 2 ) using 4190 ( 1 ) and 2782 ( 2 ) independent reflections. The axial NCoC fragment is characterized by CoN and CoC distances of 2.042(2) and 2.001(3) A in 1 and 2.100(4) and 2.003(5) A in 2 , respectively. [p ]A comparison with the structure of the analogous cobaloximes shows that the nature of the equatorial ligand affects significantly the orientation of py with respect to the equatorial moiety as well as the CoN-(axial) distance. No significant variation is observed in the CoC bond length. The relationship between the CoN axial bond lengths and the rate constants for the reaction of displacement of the planar neutral ligand, L, is discussed.