Synthesis, Structure and Density Functional Theory (DFT) Study of a Rhenium(I) Pyridylpyrazol Complex as a Potential Photocatalyst for CO2 Reduction

The Re(I) complex, [Re(PyPzH)(CO)(3)Cl] where PyPzH = 2-(1H-pyrazol-3-yl)pyridine, was successfully synthesised and characterised with an infrared (IR), ultraviolet-visible (UV-Vis), H-1 and C-13 nuclear magnetic resonance (NMR) spectroscopies and X-ray crystallography. The IR spectrum featured three v(C O), v(N-H), v(C=N) and v(C=C) signals at (1860-2020), 3137, 1614 and 1513 cm(-1), respectively. The UV-Vis spectrum of the complex exhibited ligand-centred (pi (R)>*) electronic excitations [lambda(max) = 227 nm, epsilon = 1.942 x 10(4) M(-1)cm(-1); lambda(max) = 292 nm, epsilon = 0.853 x 10(4) M(-1)cm(-1)] and a metal-to-ligand charge transfer (MLCT) band [lambda(max) = 331 nm, epsilon = 0.467 x 10(4) M(-1)cm(-1)]. The C-13 and H-1-NMR spectra exhibited the characteristic signals of the three C O (189.0 - 199.0 ppm) and NH (14.84 ppm), respectively. The X-ray structure of [Re(PyPzH)(CO)(3)Cl] showed the crystal adopted a monoclinic system with a C2/c space group [unit cell dimensions: a = 27.7422(14) angstrom, b = 11.1456(5) angstrom, c = 9.2461(4) angstrom with alpha = gamma = 90 degrees and beta = 92.552(2)degrees]. Density functional theory (DFT) and time-dependent (TD) DFT calculations were performed to investigate the optimised structural geometry and electronic properties of the title complex. The results showed that the highest-occupied molecular orbital (HOMO) was predominantly found on the d pi-orbitals of Re(I), Cl and CO. While the lowest-unoccupied molecular orbital (LUMO) was located on the PyPzH moiety. The structural and photophysical properties of the [Re(PyPzH)(CO)(3)Cl] were established and the reaction enthalpies for the dissociation of Cl atom in the formation of [Re(PyPzH)(CO)(3)](center dot) were discussed in view of its potential application for photocatalytic CO2 reduction.