An aryl-bridged dixanthene scaffold for building multinucleating ligands and supramolecular assemblies: Syntheses and structures

Ruthenium(II) Complexes of a Xanthene-Spanned Dicarbene Ligand

Octahedral ruthenium(II) complexes of a xanthene-di(N-heterocyclic carbene) ancillary ligand (XdC) have been prepared and structurally characterized. Examples catalyze the transfer hydrogenation of ketones {[Ru(CO)I2(C,O,C'-XdC)] (1) and [Ru(CO)(MeCN)2(C,O,C'-XdC)]2+ (22+)} and the selective electrochemical reduction of CO2 to CO {[Ru(N,N'-bpy)(CO)(C,O,C'-XdC)]2+ (32+) at 0.40 V overpotential in MeCN-H2O (1 M)}. The reaction of 1 with KBEt3H afforded isomers of [(C,C'-XdC)Ru(μ-H)(H)]2 dimers, which are stable to reductive elimination of the XdC ligand, thereby suggesting similar (XdC)Rh(coligand)(H)x species may be viable intermediates in catalyses. The electrochemical reduction of CO2 involves a double reduction of 32+ to 3••, which has been characterized by IR-SEC and DFT calculations. The DFT calculations suggest the Ru-Oxanth bond breaks in 3••, opening a metal site for CO2 binding with selectivity over protons enabled by the diffuse nature of the HOMO delocalized over the metal and the bipyridine and carbonyl coligands. The results point to the promise of metal complexes of flexible and hemilabile xanthene-(NHC)2 ancillary ligands in catalysis.

Synthesis of Mono- and Bis-Pyrazoles Bearing Flexible p-Tolyl Ether and Rigid Xanthene Backbones, and Their Potential as Ligands in the Pd-Catalysed Suzuki–Miyaura Cross-Coupling Reaction

The present work describes the synthesis of mono- and bis-pyrazole compounds bearing flexible p-tolyl ether and rigid 4,5-dibromo-2,7-di-tert-butyl-9,9-dimethyl-9H-xanthene backbones as pyrazolyl analogues of DPEphos and Xantphos ligands, respectively. The synthesis of new pyrazolyl analogues was accomplished following an Ullmann coupling protocol, and the resulting products were isolated in overall good yields. In addition, a hybrid imidazolyl–pyrazolyl analogue bearing a xanthene backbone was synthesized using the same protocol, whereas a hybrid selanyl–pyrazolyl analogue with a xanthene backbone was synthesized in a good yield employing a second C–H activation step. The symmetrical bis-pyrazolyl and the hybrid imidazolyl–pyrazolyl analogues were found to be the most active among the new ligands evaluated in the Pd-catalysed Suzuki-Miyaura cross-coupling of aryl halides with aryl boronic acids. A simple catalytic system based on Pd(OAc)2/2a was developed, which efficiently catalyses the Suzuki–Miyaura reaction of aryl halides and aryl boronic acids and provides moderate to excellent yields of the corresponding cross-coupling products.