Stereochemistry and properties of the M(II)N(py) coordination bond in the low-spin dipyridinated iron(II) and cobalt(II) phthalocyanines

Abstract

A comparison between the coordination bond M(II)N(py) and vibrational properties and stereochemistry of the dipyridinated iron(II) and cobalt(II) phthalocyaninato complexes, FePc(py) 2 and CoPc(py) 2 (Pc=C 32H 16N 8, py=C 5H 5N), is carried out using the single crystal X-ray diffraction and the vibrational spectroscopy. Both dipyridinated complexes have been synthesized in crystalline form by heating of the β-FePc or β-CoPc in pyridine at 160 °C. The crystals are formed during the slowly cooling process. The crystal of FePc(py) 2 and CoPc(py) 2 are isostructural. They crystallize in the space group P2 1/ c of the monoclinic system with two molecules per unit cell. The structural results and coordination bond properties are strictly related in these complexes. The Fe 2+ and Co 2+ cations are coordinated by four N-isoindole atoms of the phthalocyaninato(2-) macrocycle and axially by two nitrogen atoms of pyridine molecules to form a tetragonal bypyramid. The vibrational MN isoindole parameters are much more affected than the corresponding structural parameters by the difference in the electronic structure of the Co and Fe. The axial M(II)N(py) bond length depends strongly on the electron configuration of the central metal. The value of the M(II)N(py) bond length of 2.039(2) Å in FePc(py) 2 and 2.340(2) Å in CoPc(py) 2 clearly evidences on the localization of the unpaired electron on the d z 2 orbital of the Co in the cobalt complex. The electron paramagnetic resonance (EPR) spectroscopy and magnetic susceptibility measurements have also detected the unpaired electron in the molecule of CoPc(py) 2. EPR and magnetic susceptibility measurements performed on a solid sample of FePc(py) 2 shown on its diamagnetic character. The importance of the d(π)→π*(Pc) back donation is manifested in the difference between the values of the CN isoindole and CN azamethine bond lengths of the Pc macrocycle.