Strong exchange coupling in lanthanide bis(phthalocyaninato) sandwich compounds

Bis(phthalocyaninato)lanthanide sandwich compounds, which have the formula Pc2-LnPc1-, have been synthesized for Ln=terbium, holmium, and lutecium. Low-temperature magnetic susceptibility data for Ho(Pc)2 and Tb(Pc)2 show a reduction in moment from that which is expected for the lanthanide free ion value, which correlates with an antiferromagnetic exchange between the lanthanide f electrons and the phthalocyaninato ligand radical electron. The g factors determined from least-squares fitting of the Curie–Weiss law to the magnetic data show excellent agreement to the g factors calculated for the strongly coupled state. Magnetic moments calculated from the experimentally determined Curie constants are also in good agreement with those expected for the strongly coupled state. Low-temperature magnetic susceptibility data for Lu(Pc)2 follows the Curie–Weiss law with a limiting moment of 1.8 μB at 100 K. Since lutecium has a 1S0 ground state, only the phthalocyanine(1-) ligand would be expected to contribute to the magnetic susceptibility in this system. Room-temperature electron paramagnetic resonance measurements show no transitions at or near g=2, indicative of a free radical, due to the strong antiferromagnetic coupling of the ligand radical electron with the lanthanide f electrons. In the case of LuPc2, where no unpaired f electrons exist, a transition at g=2.00 is present.