PYRAZINE AND 1,4-DIAZABICYCLO[2.2.2]OCTANE AS MAGNETIC EXCHANGE PROPAGATING BRIDGES IN BINUCLEAR COPPER(II) AND VANADYL COMPLEXES

The preparation and magnetic properties are reported for [Cu2(tren)2(bridge)](ClO4)4, where the bridge is either pyrazine (pyz) or 1, 4-diazabicyclo[2.2.2]octane (Dabco), and [Cu2(tren)2(bridge)](BPh4)4, where the bridge is either 4, 4ʹ- bipyridine (4, 4ʹ-bpy) or 1, 2-bis(4-pyridyl)ethylene (BPE). Only the pyrazine complex shows signs of an antiferromagnetic exchange interaction in magnetic susceptibility data taken down to 4.2 K. In this complex, the interaction is weak with J = -3.2 cm-1, in spite of the fact that the two Cu(tren)2+units have their dz2 orbitals in which the unpaired electrons reside pointing directly into the nitrogen lone-pair orbitals of the pyrazine. The Cu-tren-BPE complex does show evidence of a very weak (0.02 cm-1< |J| < ~0.5 cm-1) exchange interaction with the appearance of a seven-line copper hyperfine pattern in its Q-band EPR spectrum. The magnetic susceptibility vs. temperature curve for the potentially polymeric compound [Cu(pyz)2]-(ClO4)2shows a maximum at 12.1 K. The complex [VO(hfac)2]2B with B = Dabco does not show any signs of an exchange interaction in magnetism data taken to 4.2 K, but when B is either pyz, 2-Me-pyz, or 2, 5-Me2-pyz, there is a maximum in the susceptibility vs. temperature curves at 31.5, 12.1, and 7.0 K, respectively. The data for these last three binuclear vanadyl complexes were fit to give J = -19, -6.7, and -4.0 cm-1. It is suggested that the pyrazine bridges in these vanadyl complexes are bridging between equatorial sites. From the work on the copper(II) and vanadyl complexes, it is concluded that pyrazine (or Dabco) is not a very good bridge for propagating an exchange interaction between two metal ions with a ground states, or, for that matter, between two metal ions with π ground states. The formation constants K1and K2for binding to pyrazine one or two VO(hfac)2complexes to form VO(hfac)•pyz or [VO(hfac)]2•pyz were determined to be K1≃ 1.5 X 106M-1and K2≃ 2.0 X 103M-1, respectively. © 1979, American Chemical Society. All rights reserved.