X-ray Structures and Homolysis of Some Alkylcobalt(III) Phthalocyanine Complexes

The first crystallographic data for σ-bonded alkylcobalt(III) phthalocyanine complexes are reported. A single-crystal
X-ray structure of CH3CH2CoIIIPc (Pc = dianion of phthalocyanine) reveals that the solid consists of centrosymmetric
face-to-face dimers in which the CH3CH2CoIIIPc units retain their square pyramidal geometry. The structure appears
to be the first one reported for a five-coordinate RCo(III)(chelate) complex with an electron-deficient equatorial
system. The Co−C bond in CH3CH2CoIIIPc (2.031(5) Å) is the longest found in five-coordinate RCoIII(chel) complexes
(R = simple primary alkyl group). Another X-ray study demonstrates that CH3CoIIIPc(py) has a distorted octahedral
geometry with axial bonds of very similar length to those in methylcobalamin. The axial bonds are shorter than
those in its octaethylporphyrin analogue, in accordance with a weaker trans axial influence in six-coordinate complexes
containing an electron-deficient phthalocyanine equatorial ligand. A different trend has been observed for five-coordinate RCoIII(chel) complexes:  electron-rich equatorial systems seem to make the Co−C axial bond shorter.
Kinetic data for the homolysis of RCoIIIPc complexes (R = Me, Et) in dimethylacetamide are also reported. Homolysis
of ethyl derivatives is faster. The Co−C bond dissociation energies (BDEs) for the pyridine adducts of the methyl
and the ethyl derivative are 30 ± 1 and 29 ± 1 kcal/mol, respectively. The BDE for CH3CoPc(py) is considerably
lower than that for MeCbl despite the very similar lengths of the axial bonds in the two complexes. The results of
this work do not support any correlation between the Co−C bond length and the bond strength as defined by BDE.