Electronic Structure, Vibrational Spectra, and Spin-Crossover Properties of Vacuum-Evaporable Iron(II) Bis(dihydrobis(pyrazolyl)borate) Complexes with Diimine Coligands. Origin of Giant Raman Features.

The vibrational properties of spin-crossover complexes [Fe(H2B(pz)2)2(L)] (pz = pyrazole) containing L = 2,2'-bipyridine (bipy) and 1,10-phenanthroline (phen) ligands are investigated by temperature-dependent infrared and Raman spectroscopy. For comparison, the analogous cobalt(II) complexes [Co(H2B(pz)2)2(L)] (L = bipy and phen) and iron(II) compounds with L = 4,4'-dimethyl-2,2'-bipyridine and 4,7-dimethyl-1,10-phenanthroline coligands are studied. Highly intense, structured bands (giant Raman features, GRFs) are observed in the resonance Raman spectra of all Fe(II) complexes between 400 and 500 cm-1 at low temperatures in the HS state which, for the SCO complexes, is excited by the Raman laser. On the basis of magnetic field Mössbauer and saturation magnetization data electronic Raman effects are excluded to account for these features. Furthermore, detailed vibrational analysis also allows excluding a vibrational resonance Raman effect involving one of the modes of the individual complexes as a possible origin of the GRFs. Consequently, these features are attributed to coherent two-phonon excitation of metal-ligand stretching vibrations in molecular dimers coupled by π-π stacking interactions.