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 1s 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.
