X-ray structures and homolysis of some alkylcobalt(III) phthalocyanine complexes

Authors:

• Włodzimierz Gałęzowski,
• Maciej Kubicki

Abstract

The first crystallographic data for σ-bonded alkylcobalt(III) phthalocyanine complexes are reported. A single-crystal X-ray structure of CH₃CH₂Co^IIIPc (Pc = dianion of phthalocyanine) reveals that the solid consists of centrosymmetric face-to-face dimers in which the CH₃CH₂Co^IIIPc 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 CH₃CH₂Co^IIIPc (2.031(5) Å) is the longest found in five-coordinate RCo ^III(chel) complexes (R = simple primary alkyl group). Another X-ray study demonstrates that CH₃Co^IIIPc(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 RCo^III(chel) complexes: electron-rich equatorial systems seem to make the Co-C axial bond shorter. Kinetic data for the homolysis of RCo ^IIIPc 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 CH ₃CoPc(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. © 2005 American Chemical Society.