Molecular Structure and Vibrational Spectra of Spin-Crossover Complexes in Solution and Colloidal Media: Resonance Raman and Time-Resolved Resonance Raman Studies

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• 作者：Clare Brady ; Philip L. Callaghan ; Zbigniew Ciunik ; Colin G. Coates ; Anders Dg src="http ; //pubs.acs.org/images/gifchars/oslash.gif" border="0">ssing ; Alan Hazell ; John J. McGarvey ; Sabine Schenker
• 刊名：Inorganic Chemistry
• 出版年：2004
• 出版时间：July 12, 2004
• 年：2004
• 卷：43
• 期：14
• 页码：4289 - 4299
• 全文大小：244K
• 年卷期：v.43,no.14(July 12, 2004)
• ISSN：1520-510X

文摘

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. Theunit cell of [Fe(btpa)](PF₆)₂ contains two crystallographically independent molecules revealing octahedral low-spinand quasi-seven-coordinated high-spin structures. The unit cell of [Fe(b(bdpa))](PF₆)₂ contains two crystallographicallyindependent molecules one of which corresponds to a low-spin structure, while the other reveals a disordering. Onthe basis of magnetic susceptibility and Mössbauer measurements, it has been proposed that this disorder involveslow-spin and high-spin six-coordinated molecules. The structures of [Zn(btpa)](PF₆)₂ and [Ru(btpa)](PF₆)₂ havebeen determined also. Pulsed laser photoperturbation, coupled here with time-resolved resonance Ramanspectroscopy (TR³), has been used to investigate, for the first time by this technique, the relaxation dynamics insolution on nanosecond and picosecond time scales of low-spin, LS (¹A) ges/entities/rarr.gif"> high-spin, HS (⁵T) electronic spin-statecrossover in these Fe(II) complexes. For the nanosecond experiments, use of a probe wavelength at 321 nm,falling within the ges/gifchars/pi.gif" BORDER=0 >-ges/gifchars/pi.gif" BORDER=0 >* transition of the polypyridyl backbone of the ligands, enabled the investigation of vibrationalmodes 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 colloidalmedia assisted the assignment of prominent features in the Raman spectra of the LS and HS isomers. The relaxationdata 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 tobiphasic 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 complexesand possibly includes vibrational relaxation in the initially formed ⁵T₂ state.