Investigation of the low-spin to high-spin transition in a novel [Fe(pmea)(NCS)2] complex by IR and Raman spectroscopy and DFT calculations

The X-ray structures of the novel spin crossover complex [Fe(pmea)(NCS)(2)] with pmea = (bis[(2-pyridyl)methyl]-2-(2-pyridyl)ethylamin) were determined in its low-spin (singlet, LS) and high-spin state (quintet, HS) and compared with the optimized geometries produced by BP86 calculations. IR and Raman (lambda(ex) = 514.5, 785 and 1064 nm) spectra were recorded at different temperatures between T = 298 K and T = 30 K. Most of the experimentally observed bands could be assigned unambiguously and discussed by comparison with the normal modes calculated for both spin states. The detailed analysis of the temperature dependence of the band contours of specific modes provides strong evidence that significant changes of the inter-molecular distances occur for all molecules when a certain fraction of the individual molecules has undergone the low-spin (LS) to high-spin (HS) transition. The partition function for intra-molecular vibrations was calculated for both spin states by considering all vibrational wavenumbers that were calculated. The vibration-related entropy change connected with the LS to HS transition is determined via well-established thermodynamic relations. For the title compound we found Delta S-vib approximate to 40.3 J/(mol.K), which is about twice that determined for [Fe(phen)(2)(NCS)(2)]. Consequently, the sum of the purely electronic contribution (similar to 13.4 J/(mol.K)) and that of the intramolecular vibrations equals the average entropic driving force determined for a series of spin-crossover complexes (similar to 50 J/(mol.K)). It is therefore most interesting to experimentally determine the entropy change for the complex investigated here and to find out whether this complex exhibits an unusually large entropy change upon the LS to HS transition. Copyright (C) 2006 John Wiley & Sons, Ltd.