A new type of organocobalt complex with a buckled equatorial ligand

Abstract

Weakening of the CoC bond in coenzyme B 12 has been attributed to steric influences, especially to flexing or folding of the equatorial corrin ring. With this in mind we have synthesized four new cobalt complexes with unusual steric properties as model compounds: Co((DO)(DOH)bn)Cl 2 ( I), where (DO)- (DOH)bn is the equatorial ligand N 2 ,N 2′ -butanediylbis (2,3-butanedione 2-imine 3-oxime), and organocobalt complexes [LCo((DO)(DOH)bn)R] +, where R = methyl, ethyl ( II), or methylpropyl and L = OH 2. The NMR spectra and crystal structures of Co((DO)- (DOH)bn)Cl 2 ( I) and [OH 2Co((DO)(DOH)bn)CH 2- CH 3]ClO 4 ( II) are reported. ▪ The respective crystal systems, space groups, lattice constants and final R factors for these two compounds are as follows: I orthorhombic, P2 12 12 1, a = 14.017(2), b = 29.014(4), c = 8.005(2) Å, Z = 8, R = 0.061, R w = 0.076; II monoclinic, C2/ c, a = 21.820(3), b = 7.386(2), c = 25.652(4) Å, β = 101.22(1)°, Z = 8, R = 0.077, R w = 0.075. These are the first cobalt complexes reported with the (DO)- (DOH)bn equatorial ligand; in both I and II the equatorial nitrogen donor atoms form a planar array around the Co atom, in contrast to the non-planar coordination geometry in [Cu((DO)(DOH)bn)]- (ClO 4). The (DO)(DOH)bn ligand folds toward the two central carbon atoms of the four-carbon bridge in both complexes. In complex II the equatorial ligand folds toward the water ligand at an angle of 6.0°; it is also folded (3.8, 12.0°) in both independent molecules of I, where the two axial Cl − ligands are identical. The new ethylcobalt complex ( II) is similar to coenzyme B 12 in that there is an equatorial ligand with a ‘built-in’ fold, and in the steric adjustments produced by the folded equatorial ligand. The CoC and CO bonds are lengthened to 2.012(6) and 2.119(3) Å and the CoCC bond angle of the alkyl ligand is widened to 119.3(5)°.