Santina Hoof

Pyridyl-Functionalised 3H-1,2,3,4-Triazaphospholes: Synthesis, Coordination Chemistry and Photophysical Properties of Low-Coordinate Phosphorus Compounds

Novel conjugated, pyridyl-functionalised triazaphospholes with either tBu or SiMe3 substituents at the 5-position of the N3 PC heterocycle have been prepared by a [3+2] cycloaddition reaction and compared with structurally related, triazole-based systems. Photoexcitation of the 2-pyridyl-substituted triazaphosphole gives rise to a significant fluorescence emission with a quantum yield of up to 12 %. In contrast, the all-nitrogen triazole analogue shows no emission at all. DFT calculations indicate that the 2-pyridyl substituted systems have a more rigid and planar structure than their 3- and 4-pyridyl isomers. Time-dependent (TD) DFT calculations show that only the 2-pyridyl-substituted triazaphosphole exhibits similar planar geometry, with matching conformational arrangements in the lowest energy excited state and the ground state; this helps to explain the enhanced emission intensity. The chelating P,N-hybrid ligand forms a Re(I) complex of the type [(N^N)Re(CO)3 Br] through the coordination of nitrogen atom N(2) to the metal centre rather than through the phosphorus donor. Both structural and spectroscopic data indicate substantial π-accepting character of the triazaphosphole, which is again in contrast to that of the all-nitrogen-containing triazoles. The synthesis and photophysical properties of a new class of phosphorus-containing extended π systems are described.

The Effect of Substituents at Lewis Acidic Bismuth(III) Centers on Its Propensity to Bind a Noble Metal Donor

To investigate the effect of X in ambiphilic compounds BiX(o-PPh2-C6H4)2, PBiP-X, on metallophilic Pt-Bi interactions in its PtCl2 complexes two new derivatives PBiP-Me and PBiP-C6F5 were synthesized. Reaction with dichloro(1,5-cyclooctadiene)platinum(II) led to the platinum(II) complexes [PtCl2(PBiP-Me)], 3, and [PtCl2(PBiP-C6F5)], 4, which together with the halide [PtCl2(PBiP-Cl)], 2, reported previously, establish a series of related PBiP-X complexes differing only in X. This could be complemented by accessing [PtCl2(PBiP-OTf)], 5, through the reaction of 2 with AgOTf. Analysis of the geometrical and electronic structures of these complexes revealed that in all cases the platinum(II) centers act as donors (through their filled d(z(2)) orbitals) to the bismuth(III) centers (possessing σ*(Bi-X)/6p acceptor orbitals). The strength of these interactions increases with increasing electron-withdrawing character of X, which supports the conceptual approach in constructing this new class of compounds.

A Biomimetic Nickel Complex with a Reduced CO2 Ligand Generated by Formate Deprotonation and Its Behaviour towards CO2

Reduced CO2 species are key intermediates in a variety of natural and synthetic processes. In the majority of systems, however, they elude isolation or characterisation owing to high reactivity or limited accessibility (heterogeneous systems), and their formulations thus often remain uncertain or are based on calculations only. We herein report on a Ni-CO22- complex that is unique in many ways. While its structural and electronic features help understand the CO2 -bound state in Ni,Fe carbon monoxide dehydrogenases, its reactivity sheds light on how CO2 can be converted into CO/CO32- by nickel complexes. In addition, the complex was generated by a rare example of formate β-deprotonation, a mechanistic step relevant to the nickel-catalysed conversion of Hx COyz- at electrodes and formate oxidation in formate dehydrogenases.