Solid-state interconversions: Unique 100% reversible transformations between the ground and metastable states in single-crystals of a series of nickel( II) nitro complexes

The solid-state, low-temperature linkage isomerism in a series of five square planar group 10 phosphino nitro complexes have been investigated by a combination of photocrystallographic experiments, Raman spectroscopy and computer modelling. The factors influencing the reversible solid-state interconversion between the nitro and nitrito structural isomers have also been investigated, providing insight into the dynamics of this process. The cis-[Ni(dcpe)(NO2)2] (1) and cis-[Ni(dppe)(NO2)2] (2) complexes show reversible 100 % interconversion between the η1-NO2 nitro isomer and the η1-ONO nitrito form when single-crystals are irradiated with 400 nm light at 100 K. Variable temperature photocrystallographic studies for these complexes established that the metastable nitrito isomer reverted to the ground-state nitro isomer at temperatures above 180 K. By comparison, the related trans complex [Ni(PCy3)2(NO2)2] (3) showed 82 % conversion under the same experimental conditions at 100 K. The level of conversion to the metastable nitrito isomers is further reduced when the nickel centre is replaced by palladium or platinum. Prolonged irradiation of the trans-[Pd(PCy3)2(NO2)2] (4) and trans-[Pt(PCy3)2(NO2)2] (5) with 400 nm light gives reversible conversions of 44 and 27 %, respectively, consistent with the slower kinetics associated with the heavier members of group 10. The mechanism of the interconversion has been investigated by theoretical calculations based on the model complex [Ni(dmpe)Cl(NO2)]

High-pressure crystallographic and spectroscopic studies on two molecular dithienylethene switches

Single crystals of the dithienylethene compounds, 1,2-bis(2-methylbenzothiophen-3-yl)perfluorocyclopentene 1 and 1,2-bis(2,5-dimethylthiophen-3-yl)perfluorocyclopentene 2 undergo pressure-induced single-crystal to single-crystal phase transitions between 4.45-5.35 GPa and 4.15-5.70 GPa, respectively. For 1, there is a smooth reduction in unit-cell volume of ~20% from ambient pressure to 4.45 GPa, followed by a dramatic reduction in volume that coincides with a 7.7% increase in the b axis length. Above the pressure of 5.38 GPa a smooth volume reduction continues. In contrast, for 2, there is a continuous change in unit-cell volume with an observed space group change from C2/c to P2₁/c, between the pressures of 4.15 and 5.70 GPa. In the crystals of 1 between 4.45 and 5.38 GPa adjacent molecules slide over each other and the dominant stacking interaction changes from a thiophene...thiophene interaction at 4.45 GPa to a benzothiophene...benzothiophene interaction at 5.38 GPa and, within each molecule, the benzothiophene groups show a significant reorientation at the phase transition. In 2 there is a loss of molecular symmetry, concomitant with the change in space group, at the phase transition with the asymmetric unit changing from containing half a unique molecule to two independent molecules. The molecules show significant reorientations of their ring systems. The nature of the observed transition in 1 was investigated using solid-state computational methods to prove the superior thermodynamic stability of the high-pressure phase to the lower pressure phase at pressures above 5.38 GPa. Solid state UV-Vis spectroscopy of 1, over the pressure range from ambient to 15.4 GPa showed that the compound displayed piezochromism with a significant red shift in the π-π* absorption band and a colour change in the crystal from colourless to red with increasing pressure.We are grateful to the EPSRC for financial support and for studentships to CHW and SS (EP/F021151 and EP/D072859), to the University of Bath for support for SKB, and Diamond Light Source Ltd for the award of beamtime

C–C σ complexes of rhodium

In this article, the complexes [Rh(Binor-S′)(PR(3))][BAr(4)(F)] (R = (i)Pr, Cy, C(5)H(9)) are described. A combination of x-ray crystallography, NMR spectroscopy, density functional theory, and “atoms in molecules” calculations unequivocally demonstrates that the complexes contain rare examples of metal···C [Image: see text] C agostic interactions. Moreover, they are fluxional on the NMR time scale, undergoing rapid and reversible C [Image: see text] C activation. Kinetic data and calculations point to a bismetallacyclobutane, Rh(V), intermediate

Experimental charge density study into C–C σ-interactions in a Binor-S rhodium complex

Transition-metal complexes containing (C–C)→M s-interactions have potential applications in both catalysis and the activation and cleavage of C–C bonds. Fully characterising the bonding and interactions in complexes containing such (C–C)→M s-interactions is vital to understand their chemical behaviour. As a result a high-resolution experimental X-ray charge density study has been undertaken on [Rh(Binor-S)(PCy3)][HCB11Me11] (Binor-S = 1,2,4,5,6,8-dimetheno-s-indacene) which contains a (C–C)→Rh interaction. The data are analysed using Bader’s “Atoms in Molecules” (AIM) approach with particular attention paid to the interactions around the rhodium centre. The results provide clear evidence for the s(C–C)→Rh interaction in the solid-state which is classified as a weak covalent interaction. These results are supported by theoretical calculations

Sequential reduction of high hydride count octahedral rhodium clusters [Rh6(PR3)6H12][BArF 4]2: Redox-switchable hydrogen storage

Cyclic voltammetry on the octahedral rhodium clusters with 12 bridging hydride ligands, [Rh6(PR3)6H12][BArF4]2 (R = Cy Cy-[H12]2+, R = iPr iPr-[H12]2+; [BArF4]- = [B{C6H3(CF3)2}4]-) reveals four potentially accessible redox states: [Rh6(PR3)6H12]0/1+/2+/3+. Chemical oxidation did not produce stable species, but reduction of Cy-[H12]2+ using Cr(eta6-C6H6)2 resulted in the isolation of Cy-[H12]+. X-ray crystallography and electrospray mass spectrometry (ESI-MS) show this to be a monocation, while EPR and NMR measurements confirm that it is a monoradical, S = 1/2, species. Consideration of the electron population of the frontier molecular orbitals is fully consistent with this assignment. A further reduction is mediated by Co(eta5-C5H5)2. In this case the cleanest reduction was observed with the tri-isopropyl phosphine cluster, to afford neutral iPr-[H12]. X-ray crystallography confirms this to be neutral, while NMR and magnetic measurements (SQUID) indicate an S =1 paramagnetic ground state. The clusters Cy-[H12]+ and iPr-[H12] both take up H2 to afford Cy-[H14]+ and iPr-[H14], respectively, which have been characterized by ESI-MS, NMR spectroscopy, and UV-vis spectroscopy. Inspection of the frontier molecular orbitals of S = 1 iPr-[H12] suggest that addition of H2 should form a diamagnetic species, and this is the case. The possibility of "spin blocking" in this H2 uptake is also discussed. Electrochemical investigations on the previously reported Cy-[H16]2+ [J. Am. Chem. Soc. 2006, 128, 6247] show an irreversible loss of H2 on reduction, presumably from an unstable Cy-[H16]+ species. This then forms Cy-[H12]2+ on oxidation which can be recharged with H2 to form Cy-[H16]2+. We show that this loss of H2 is kinetically fast (on the millisecond time scale). Loss of H2 upon reduction has also been followed using chemical reductants and ESI-MS. This facile, reusable gain and loss of 2 equiv of H2 using a simple one-electron redox switch represents a new method of hydrogen storage. Although the overall storage capacity is very low (0.1%) the attractive conditions of room temperature and pressure, actuation by the addition of a single electron, and rapid desorption kinetics make this process of interest for future H2 storage applications