Molecular structure and vibrational spectra of spin-crossover complexes in solution and colloidal media: Resonance Raman and time-resolved resonance Raman studies

The spin-crossover system [Fe(btpa)](PF₆)₂ (btpa = N, N,N′,N′-tetrakis(2-pyridylmethyl)-6,6′-bis(aminomethyl)-2, 2′-bipyridine) and the predominantly low-spin species [Fe(b(bdpa))] (PF₆)₂ ((b(bdpa) = N, N′-bis(benzyl)-N, N′-bis(2-pyridylmethyl)-6,6′-bis(aminomethyl)-2,2′-bipyridine) have been characterized by means of X-ray diffraction. The unit cell of [Fe(btpa)](PF₆)₂ contains two crystallographically independent molecules revealing octahedral low-spin and quasi-seven-coordinated high-spin structures. The unit cell of [Fe(b(bdpa))](PF₆)₂ contains two crystallographically independent molecules one of which corresponds to a low-spin structure, while the other reveals a disordering. On the basis of magnetic susceptibility and Mössbauer measurements, it has been proposed that this disorder involves low-spin and high-spin six-coordinated molecules. The structures of [Zn(btpa)](PF₆)₂ and [Ru(btpa)](PF₆)₂ have been determined also. Pulsed laser photoperturbation, coupled here with time-resolved resonance Raman spectroscopy (TR³), has been used to investigate, for the first time by this technique, the relaxation dynamics in solution on nanosecond and picosecond time scales of low-spin, LS (¹A) → high-spin, HS (⁵T) electronic spin-state crossover in these Fe(II) complexes. For the nanosecond experiments, use of a probe wavelength at 321 nm, falling within the π-π* transition of the polypyridyl backbone of the ligands, enabled the investigation of vibrational modes of both LS and HS isomers, through coupling to spin-state-dependent angle changes of the backbone. Supplementary investigations of the spin-crossover (SCO) equilibrium in homogeneous solution and in colloidal media assisted the assignment of prominent features in the Raman spectra of the LS and HS isomers. The relaxation data from the nanosecond studies confirm and extend earlier spectrophotometric findings, (Schenker, S.; Stein, P. C.; Wolny, J. A.; Brady, C.; McGarvey, J. J.; Toftlund, H.; Hauser, A. Inorg. Chem. 2001, 40, 134), pointing to biphasic spin-state relaxation in the case of [Fe(btpa)](PF₆)₂ but monophasic in the case of [Fe(b(bdpa))](PF₆)₂. The picosecond results suggest an early process complete in 20 ps or less, which is common to both complexes and possibly includes vibrational relaxation in the initially formed ⁵T₂ state.