Solution and Solid State Properties for Low-Spin Cobalt(II) Dibenzotetramethyltetraaza[14]annulene [(tmtaa)Co^II] and the Monopyridine Complex

Abstract

The single-crystal X-ray structure of solvent-free (tmtaa)­CoII reveals three different π–π intermacrocyclic interactions between tmtaa units (tmtaa = dibenzotetramethyltetraaza[14]­annulene). Pairs of inequivalent (tmtaa)­CoII units in the unit cell link into a one-dimensional π–π stacked array in the solid state. Magnetic susceptibility (χ) studies from 300 to 2 K reveal the effects of intermolecular interactions between (tmtaa)­CoII units in the solid state. The effective magnetic moment per CoII center is constant at 2.83 μB from 300 to 100 K and begins to significantly decrease at lower temperatures. The magnetic data are fit to a singlet (S = 0) ground state with a triplet (S = 1) excited state that is 13 cm–1 higher in energy (−2J = 13 cm–1). Toluene solutions of (tmtaa)­CoII have 1H nuclear magnetic resonance (NMR) paramagnetic shifts, a solution-phase magnetic moment μeff (295 K) of 2.1 μB, and toluene glass electron paramagnetic resonance spectra that are most consistent with a low-spin (S = 1/2) CoII with the unpaired electron located in the dyz orbital. Pyridine interacts with (tmtaa)­CoII to form a five-coordinate monopyridine complex in which the unpaired electron is in the dz2 orbital. The five-coordinate complex has been structurally characterized by single-crystal X-ray diffraction, and the equilibrium constant for pyridine binding at 295 K has been evaluated by both electronic and 1H NMR spectra. Density functional theory computation using the UB3LYP hybrid functional places the unpaired electron for (tmtaa)­CoII in the dyz orbital and that for the monopyridine complex in the dz2 orbital, consistent with spectroscopic observations