Charge distribution dependent on the chelating ratio. [Co(mu-bpym)(dbbq)(2)] vs [{Co(dbbq)(2)}(2)(mu-bpym)] (bpym=2,2'-bipyrimidine; dbbq=3,6-di-t-butyl-1,2-benzoquinone)

Abstract

The preparation and characterization of [Co(mu-bpym)(dbbq)(2)] and [{Co(dbbq)(2)}(2)(mu-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(mu-bpym)(dbbq)(2)] has a charge distribution of [Co-III(mu-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 [Co-II(mu-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)(mu-bpym)] is locked in a charge distribution of [{Co-II(dbsq)(2)}(2)(mu-bpym)] when the temperature is 100-400 K.