Different rhodium­(III) complexes [Rh­(C,C)­(P,P)­X2]+ bearing both a cis-chelating dicarbene and a diphosphine ligand were synthesized (C,C = methylene­(4,4′-diimidazolylidene); P,P = 1,2-bis­(diphenylphosphino)­ethane (dppe), (R)-(+)-2,2′-bis­(diphenylphosphino)-1,1′-binaphthalene (R-BINAP); X = halide, carbanion, NCMe). Solution analysis by NMR spectroscopy indicate a dynamic behavior of the complexes and cis/trans isomerization processes, likely through dissociation of the nonchelating ligands X (X = halide, NCMe), and eventually also involving the diphosphine ligand, identified by the formation of phosphine oxides. The presence of a diphosphine ligand in addition to the dicarbene substantially enhances the catalytic activity of the rhodium center in the transfer hydrogenation of ketones in iPrOH/KOH, reaching over 4000 turnover numbers and turnover frequencies around 1000 h–1 vs 330 h–1 for the phosphine-free analogue. Optimization of the catalytic conditions allowed transfer hydrogenation to be run with only 1 mol % base instead of the often used 10 mol %. The chiral R-BINAP ligand enhances catalytic activity, though no enantioselectivity was induced in the transfer hydrogenation of fluoroacetophenone as prochiral substrate.

Metalation of novel ditriazolium salts containing a trimethylene (-CHCHCH-) or dimethylether linker (-CHOCH-) was probed with different rhodium(iii) and iridium(iii) precursors. When using [MCp*Cl], a transmetalation protocol via a triazolylidene silver intermediate was effective, while base-assisted metalation with MClvia sequential deprotonation of the triazolium salt with KOtBu and addition of the metal precursor afforded homoleptic complexes. The N-substituent on the triazole heterocycle directed the metalation process and led to C,C,C-tridentate chelating ditriazolylidene complexes for N-phenyl substituents. With ethyl substituents, only C,C-bidentate complexes were formed, while metalation with mesityl substituents was unsuccessful, presumably due to steric constraints. Through modification of the reaction conditions for the metalation step, an intermediate species was isolated that contains a C,C-bidentate chelate en route to the formation of the tridentate ligand system. Accordingly, C-H bond activation occurs prior to formation of the second metal-triazolylidene bond. Stability studies with a C,C,C-tridentate chelating ditriazolylidene iridium complex towards DCl showed deuterium incorporation at both N-phenyl groups and indicate that C-H bond activation is reversible while the C-Ir bond is robust. The flexible linker between the two triazolylidene donor sites provides access to both facial and meridional coordination modes.

Treatment of an abnormal dicarbene ligated rhodium(iii) dimer with 2,2'-bipyridine (bipy), 1,10-phenanthroline (phen) or 2,2':6',2''-terpyridine (terpy) results in coordination of the N-donor ligands and concomitant cleavage of the dimeric structure. Depending on the denticity of the pyridyl ligand, this situation retains one (L = terpy) or two (L = bipy, phen) flexible sites for substrate coordination. In the case of the bipy complexes, modification of the electron density at Rh, without directly affecting the steric environment about the metal centre, was achieved by the incorporation of electron-donating or electron-withdrawing substituents on the bipy backbone. The dicarbene pyridyl complexes were active in transfer hydrogenation catalysis of benzophenone at 0.15 mol% catalyst loading in a iPrOH/KOH mixture. The catalysts displayed a strong characteristic colour change (yellow to purple) after activation which allowed for visual monitoring of the status of the reaction. The colour probe and the robustness of the active catalysts proved useful for catalyst recycling. The catalytic activity sustained over five consecutive substrate batch additions and gave a maximum overall turnover number of 3100.

The Monte Carlo technique of Ferrenberg and Swendsen [Phys. Rev. Lett. 61, 2635(1988] is improved by efficiently determining the tails of the Boltzmann distribution at the appropriate temperature. This is achieved by combining several distributions generated at different temperatures to form a composite distribution. The composite distribution leads to values of the specific heat and energy which are accurate over the entire temperature range of interest. Results illustrating these improvements are reported for the square two-dimensional Ising model.

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