X-ray Absorption Spectroscopic Study of the Temperature and Pressure Dependence of the Electronic Spin States in Several Iron(II) and Cobalt(II) Tris(pyrazolyl)borate Complexes

An iron and cobalt K-edge X-ray absorption study has been undertaken, at 295 and 77 K, to investigate the electronic spin states of Fe[HB(pz)₃]₂, 1, Fe[HB(3,5-(CH₃)₂pz)₃]₂, 2, Fe[HB(3,4,5-(CH₃)₃pz)₃]₂, 3, Co[HB(pz)₃]₂, 4, Co[HB(3,5-(CH₃)₂pz)₃]₂, 5, and Co[HB(3,4,5-(CH₃)₃pz)₃]₂, 6, where pz is the 1-pyrazolyl moiety. Between 295 and 77 K complex 2 shows a spin-state crossover whereas 1 and 3-6 remain low-spin and high-spin, respectively. The spectra show a clear difference, in both the relative intensities and the relative energies, of the metal Is to 4p electronic transition in the high-spin and the low-spin states. An analogous study of Fe[HB(pz)₃]₂, 1, between 295 and 450 K reveals that it undergoes a gradual reversible spin-state crossover to the high-spin state above ca. 360 K. The high-pressure room-temperature XANES spectra of 1 indicate that it remains low-spin between ambient pressure and 90 kbar, whereas complex 2 shows the expected spin-state crossover between zero and ca. 30 kbar and is low-spin between 40 and 90 kbar. The three cobalt complexes are gradually converted from the high-spin to the low-spin state with increasing pressure, complex 4 showing the least spin-state change and complex 6 showing the most change. The energies of the metal 4p virtual orbitals are found to be very sensitive to pressure and to the electronic spin state of the metal. An EXAFS analysis for the cobalt complexes indicates both that they are all structurally very similar, with the expected high-spin cobalt to nitrogen bond lengths, and that they remain high-spin upon cooling from 295 to 77 K.