Charge Distribution Dependent on the Chelating Ratio. [Co(<i>μ</i>-bpym)(dbbq)2] vs. [{Co(dbbq)2}2(<i>μ</i>-bpym)] (bpym = 2,2′-bipyrimidine; dbbq = 3,6-di-<i>t</i>-butyl-1,2-benzoquinone)

Abstract

Abstract The preparation and characterization of [Co(μ-bpym)(dbbq)2] and [{Co(dbbq)2}2(μ-bpym)] (bpym = 2,2′-bipyrimidine; dbbq = 3,6-di-t-butyl-1,2-benzoquinone) have been established. The temperature-dependent magnetic moments (100—400 K) and variable-temperature near-infrared (NIR) absorption spectra are presented to show that the title complexes exhibit an equilibrium via a catechol to a cobalt intramolecular electron transfer. At temperatures below 200 K the mononuclear complex [Co(μ-bpym)(dbbq)2] has a charge distribution of [CoIII(μ-bpym)(dbcat)(dbsq)] (dbsq = 3,6-di-t-butyl-1,2-semiquinonato; dbcat = 3,6-di-t-butylcatecholato), whereas at temperatures beyond 290 K the mononuclear complex is predominantly in the [CoII(μ-bpym)(dbsq)2] form. From 200—290 K a mixture of Co(III) and Co(II) redox isomers exists at equilibrium in the solid state. In particular, a thermal analysis of a solid sample of the mononuclear complex discloses that the transition for the Co(III)/Co(II) is accompanied by a change in the heat content of 36 kJ mol−1, which may be closely related to the band at 3878 cm−1, assigned as a dbcat to Co(III) charge transfer. In striking contrast, the dinuclear complex [{Co(dbbq)2}2(μ-bpym)] is locked in a charge distribution of [{CoII(dbsq)2}2(μ-bpym)] when the temperature is 100—400 K.