Extension of the Experimental Electron Density Analysis to Metastable States:  A Case Example of the Spin Crossover Complex Fe(btr)₂(NCS)₂·H₂O

Abstract

An experimental electron density (ED) analysis of the spin crossover coordination complex Fe(btr)(2)(NCS)(2).H(2)O has been performed in the ground low-spin (LS) state and in the metastable thermally quenched high-spin (HS) state at 15 K by fitting a multipolar model to high-resolution X-ray diffraction measurements. The ED has been quantitatively analyzed using the quantum theory of atoms in molecules. This is the first time the ED distribution of a molecular metastable state has been experimentally investigated. The electron deformation densities and derived Fe 3d orbital populations are characteristic of LS (t(2g)(6) e(g)(0)) and HS (t(2g)(4) e(g)(2)) electron configurations and indicate significant sigma donation to the Fe d(x)2(-)(y)2 and d(z)2 atomic orbitals. The Fe-N(NCS) and Fe-N(btr) coordination interactions are characterized using the laplacian distribution of the ED, the molecular electrostatic potential, and the fragment charges obtained by integration over the topological atomic basins. A combination of electrostatic and covalent contributions to these interactions is pointed out. Interlayer interactions are evidenced by the presence of bond critical points in N...H hydrogen bonds involving the non-coordinated water molecule. Systematic differences in the atomic displacement parameters between the LS and HS states have been described and rationalized in terms of modifications of bond force constants.

Intermolecular control of thermoswitching and photoswitching phenomena in two spin-crossover polymorphs

“…This contraction is known to be associated with the electronic population of more bonding states in the LS electronic distribution, as observed by electron density analysis. 50 Therefore the two polymorphs have the same average Fe-N bonds in the two HS (high-temperature or photoinduced) and LS states. Differences are observed by looking at the deformation of the Fe-N 6 octahedron, through the angular distortion parameter 39,51 and on the length distortion parameter ζ [see geometry are similar and small in the LS state, the molecular HS state of BIA I appears significantly more distorted than the one of BIA II, both for the HS stable state above 170 K and the photoinduced one at low temperature.…”

A critical review of the T(LIESST) temperature in spin crossover materials − What it is and what it is not

“…the mechanism of photoswitching from the molecule to the material scale, the different nature and timescales of the intervening transitions and intersystem crossings, and the lifetime of the final photo-induced state [10]. In particular structural investigations went from the first crystal structure of a sample in the light-induced HS state in 2001 [11] to charge density studies of the metastable state [12] and recent pioneering ultrafast crystallography studies of the photo-induced SCO mechanism [13]. Indeed spectroscopic ultrafast techniques, with a strong feedback of advanced ab initio calculations [14], have been intensively used in the last decade on some simple Fe(II) compounds such as [Fe(bipy) [16] (tpy= 2,2′:6′,2″-terpyridine), taken as models for the photophysical processes in other metal transition complexes.…”

Crystallography Under Extreme Conditions: State of the Art and Perspectives

“…The success of kind experiments consists in in situ laser irradiation of a cryogenically cooled crystal, with a N2 or He gas-flow system for less restricted access than closed cryostat (White et al, 1994), mounted on a diffractometer equipped with a CCD detector for fast data collection (Graafsma et al, 1997;Muchmore, 1999). More recently, Legrand (2005) has developed a crystallographic experimental methodology approach 2007a;2007b) and shown that it is possible to measure with accuracy photo-induced metastable states and to refine its experimental electron density below 35 K using conventional X-ray sources (Legrand et al, 2006;Pillet et al, 2008).…”