Vibrational probes and determinants of the S = 0 ⇌ S = 2 spin crossover in five-coordinate [Fe(TPP)(CN)]-.
The low-frequency vibrational characterization of the spin-crossover complex, five-coordinate cyano(tetraphenylporphyrinato)iron(II), [Fe(TPP)(CN)](-), is reported. Nuclear resonance vibrational spectroscopy has been used to measure all low-frequency vibrations involving iron at several temperatures; this yields vibrational spectra of both the low- (S = 0) and high-spin (S = 2) states. Multitemperature oriented single-crystal measurements facilitate assignments of the vibrational character of all modes and are consistent with the DFT-predicted spectra. The availability of the entire iron vibrational spectrum allows for the complete correlation of the modes between the two spin states. These data demonstrate that not only do the frequencies of the vibrations shift to lower values for the high-spin species as would be expected owing to the weaker bonds in the high-spin state, but also the mixing of iron modes with ligand modes changes substantially. Diagrams illustrating the changing character of the modes and their correlation are given. The reduced iron-ligand frequencies are the primary factor in the entropic stabilization of the high-spin state responsible for the spin crossover.
College of Materials Science and Opto-electronic Technology, University of Chinese Academy of Sciences, 19A Yuquan Road, Beijing, P.R. China 100049. jfli@ucas.ac.cn
Jianfeng
J
Li
Qian
Q
Peng
Alexander
A
Barabanschikov
Jeffrey W
JW
Pavlik
E Ercan
EE
Alp
Wolfgang
W
Sturhahn
Jiyong
J
Zhao
J Timothy
JT
Sage
W Robert
WR
Scheidt
D003486:Cyanides / Q000737:chemistry*; D005296:Ferrous Compounds / Q000737:chemistry*; D008665:Metalloporphyrins / Q000737:chemistry*; D008958:Models, Molecular; D011789:Quantum Theory; D013055:Spectrophotometry, Infrared; D013059:Spectrum Analysis, Raman
D003486:Cyanides; D005296:Ferrous Compounds; D008665:Metalloporphyrins