Unprecedented Reverse Volume Expansion in Spin-Transition Crystals.

The current craze for research around the spin crossover phenomenon can be justified to some extent by the mechanical properties due to the decrease of volume associated with the transition of the metal ion from the HS state to the LS state. As demonstrated here, the molecular complex [Fe(PM-pBrA)2 (NCS)2 ] exhibits, on the contrary, an increase of the unit-cell volume from HS to LS. This counter-intuitive and unprecedented behavior that concerns both the thermal and the photoexcited spin conversions is revealed by a combination of single-crystal and powder X-ray diffraction complemented by magnetic measurements. Interestingly, this abnormal volume change appears concomitant with the wide rotation of a phenyl ring which induces a drastic modification, though reversible, of the structural packing within the crystal. In addition, the light-induced HS state obtained through the Light-Induced Excited Spin-State Trapping shows a remarkably high relaxation temperature, namely T(LIESST), of 109 K, one of the highest so far reported. The above set of quite unusual characteristics opens up new fields of possibilities within the development of spin crossover materials.

Mapping the cooperativity pathways in spin crossover complexes.

Crystal packing energy calculations are applied to the [Fe(PM-L)2(NCS)2] family of spin crossover (SCO) complexes (PM-L = 4-substituted derivatives of the N-(2-pyridylmethylene)-4-aminobiphenyl ligand) with the aim of relating quantitatively the cooperativity of observed SCO transitions to intermolecular interactions in the crystal structures. This approach reveals a linear variation of the transition abruptness with the sum of the magnitudes of the interaction energy changes within the first molecular coordination sphere in the crystal structure. Abrupt transitions are associated with the presence of significant stabilising and destabilising changes in intermolecular interaction energies. While the numerical trend established for the PM-L family does not directly extend to other classes of SCO complex in which the intermolecular interactions may be very different, a plot of transition abruptness against the range of interaction energy changes normalised by the largest change shows a clustering of complexes with similar transition abruptness. The changes in intermolecular interactions are conveniently visualised using energy difference frameworks, which illustrate the cooperativity pathways of an SCO transition.

Pressure-Induced Spin-Crossover Features at Variable Temperature Revealed by In Situ Synchrotron Powder X-ray Diffraction.

An accurate high-pressure X-ray diffraction investigation, at various temperatures, on a powder of a spin-crossover (SCO) complex has allowed the rare deconvolution of the structural features of the high-spin and low-spin phases. As a result, the pressure dependence of the structural parameters of the high-spin and low-spin phases can be discussed independently in the pressure domain where both phases co-exist within the powder. Consequently, crucial unprecedented information is given, such as the variation of bulk moduli with temperature, similar here in amplitude for both spin phases, the temperature-dependence of the pressure-induced SCO abruptness, the temperature dependence of the pressure at which SCO occurs, and arguments for a possible piezo-hysteresis. Performed on the molecular complex [Fe(PM-PeA)2 (NCSe)2 ] (PM-PeA=N-(2'-pyridylmethylene)-4-(phenylethynyl) aniline), this study reveals a pressure-induced SCO at 0.16 GPa and demonstrates that, when increasing temperature, the pressure of transition increases linearly, the abruptness of the pressure-induced SCO strongly increases, and the bulk moduli decrease.

Variation of M···H-C Interactions in Square-Planar Complexes of Nickel(II), Palladium(II), and Platinum(II) Probed by Luminescence Spectroscopy and X-ray Diffraction at Variable Pressure.

Luminescence spectra of isoelectronic square-planar d8 complexes with 3d, 4d, and 5d metal centers show d-d luminescence with an energetic order different from that of the spectrochemical series, indicating that additional structural effects, such as different ligand-metal-ligand angles, are important factors. Variable-pressure luminescence spectra of square-planar nickel(II), palladium(II), and platinum(II) complexes with dimethyldithiocarbamate ({CH3}2DTC) ligands and their deuterated analogues show unexpected variations of the shifts of their maxima. High-resolution crystal structures and crystal structures at variable pressure for [Pt{(CH3)2DTC}2] indicate that intermolecular M···H-C interactions are at the origin of these different shifts.

Characterization of PdH-C interactions in bis-dimethyldithiocarbamate palladium(ii) and its deuterated analog by luminescence spectroscopy at variable pressure.

We present the variable-pressure d-d luminescence spectra of crystalline bis-dimethyldithiocarbamate palladium(ii) and its deuterated analog. The energies and shifts of the band maxima provide evidence for intermolecular PdH-C interactions, with quantitative differences observed for the deuterated complex. Shifts show distinct interactions in three pressure ranges between 1 bar and 85 kbar.