Low-valent homobimetallic Rh complexes: influence of ligands on the structure and the intramolecular reactivity of Rh-H intermediates

Supporting two metal binding sites by a tailored polydentate trop-based (trop = 5H-dibenzo[a,d]cyclohepten-5-yl) ligand yields highly unsymmetric homobimetallic rhodium(i) complexes. These were studied as models for Rh/C hydrogenation catalysts.

Supporting two metal binding sites by a tailored polydentate trop-based (trop = 5H-dibenzo[a,d]cyclohepten-5-yl) ligand yields highly unsymmetric homobimetallic rhodium(i) complexes. Their reaction with hydrogen rapidly forms Rh hydrides that undergo an intramolecular semihydrogenation of two C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C bonds of the trop ligand. This reaction is chemoselective and converts C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C bonds to a bridging carbene and an olefinic ligand in the first and the second semihydrogenation steps, respectively. Stabilization by a bridging diphosphine ligand allows characterization of a Rh hydride species by advanced NMR techniques and may provide insight into possible elementary steps of H₂ activation by interfacial sites of heterogeneous Rh/C catalysts.

Reduction of Nitrogen Oxides by Hydrogen with Rhodium(I)-Platinum(II) Olefin Complexes as Catalysts

The nitrogen oxides NO₂, NO, and N₂O are among the most potent air pollutants of the 21^st century. A bimetallic Rh^I–Pt^II complex containing an especially designed multidentate phosphine olefin ligand is capable of catalytically detoxifying these nitrogen oxides in the presence of hydrogen to form water and dinitrogen as benign products. The catalytic reactions were performed at room temperature and low pressures (3–4 bar for combined nitrogen oxides and hydrogen gases). A turnover number (TON) of 587 for the reduction of nitrous oxide (N₂O) to water and N₂ was recorded, making these Rh^I–Pt^II complexes the best homogeneous catalysts for this reaction to date. Lower TONs were achieved in the conversion of nitric oxide (NO, TON=38) or nitrogen dioxide (NO₂, TON of 8). These unprecedented homogeneously catalyzed hydrogenation reactions of NO_x were investigated by a combination of multinuclear NMR techniques and DFT calculations, which provide insight into a possible reaction mechanism. The hydrogenation of NO₂ proceeds stepwise, to first give NO and H₂O, followed by the generation of N₂O and H₂O, which is then further converted to N₂ and H₂O. The nitrogen−nitrogen bond‐forming step takes place in the conversion from NO to N₂O and involves reductive dimerization of NO at a rhodium center to give a hyponitrite (N₂O₂²⁻) complex, which was detected as an intermediate.

The coordination chemistry of 2,4,6-oxy functionalised 1,3,5-triphosphinines

A number of stable group 6 metal complexes bearing 2,4,6-oxy functionalised 1,3,5-triphosphinines, phosphorus containing heterocyclic ligands with a central C₃P₃ core, were synthesised such that a complete series of [M{P₃C₃(OX)₃}(CO)₃] compounds is obtained [M = Cr(0), Mo(0), W(0); X = H, SitBuPh₂, B(ipc)₂]. In all complexes, the triphosphinine coordinates in a η⁶-binding mode via the delocalized 6π-system of the ring. The ligand properties can be tuned by changing the substituent on the oxygen centre. The π-electron accepting properties of the ligand increases in the following order: P₃C₃(OH)₃ < P₃C₃(OSi^tBuPh₂)₃ < P₃C₃(OB(ipc)₂)₃. This trend is reflected in the structures determined by X-ray crystallography, and the ν(CO) stretching frequencies determined by IR spectroscopy. The collected data raise questions with respect to the frequently made assumption that phosphinines act as stronger π-acceptors with respect to arenes and thereby deplete electron density at the metal centres. With P₃C₃(OH)₃ as an η⁶-coordinated ligand further molecules can be coordinated in the second coordination sphere via hydrogen bonds, which may be of interest for the construction of coordination polymers.

Selective dehydrogenation of ammonia borane to borazine and derivatives by rhodium olefin complexes

This report presents a selective synthetic approach towards borazine from ammonia borane using a dinuclear rhodium olefin homogeneous catalyst. The synthesis and spectroscopic characterization of a dirhodium ammonia borane complex as an intermediate provides insight into a possible mode of activation.