Novel rhodium(I) complexes with (2-hydroxyphenyl)diphenylphosphine ligand: catalytic properties and X-ray structures of Rh(OC6H4PPh2)(CO)(PPh3) and Rh(OC6H4PPh2){P(OPh)3}2 · 0.5C6H6

Unexpected Vulnerability of DPEphos to C-O Activation in the Presence of Nucleophilic Metal Hydrides

C-O bond activation of DPEphos occurs upon mild heating in the presence of [Ru(NHC)2 (PPh3 )2 H2 ] (NHC=N-heterocyclic carbene) to form phosphinophenolate products. When NHC=IEt2 Me2 , C-O activation is accompanied by C-N activation of an NHC ligand to yield a coordinated N-phosphino-functionalised carbene. DFT calculations define a nucleophilic mechanism in which a hydride ligand attacks the aryl carbon of the DPEphos C-O bond. This is promoted by the strongly donating NHC ligands which render a trans dihydride intermediate featuring highly nucleophilic hydride ligands accessible. C-O bond activation also occurs upon heating cis-[Ru(DPEphos)2 H2 ]. DFT calculations suggest this reaction is promoted by the steric encumbrance associated with two bulky DPEphos ligands. Our observations that facile degradation of the DPEphos ligand via C-O bond activation is possible under relatively mild reaction conditions has potential ramifications for the use of this ligand in high-temperature catalysis.

The reaction of mixtures of [Rh4(CO)12] and triphenylphosphite with carbon monoxide or syngas as studied by high-resolution, high-pressure NMR spectroscopy

The fragmentation and redistribution reactions of [Rh4(CO)12-x{P(OPh)3}x] (x = 1-4) with carbon monoxide have been studied using high-resolution, high-pressure NMR spectroscopy. Under the conditions of efficient gas mixing in a high-pressure NMR bubble column, [Rh4(CO)9{P(OPh)3}3] fragments to give mainly [Rh2(CO)6{P(OPh)3}2]; [Rh4(CO)11{P(OPh)3}] is also observed,implying redistribution of the phosphite ligand and/or recombination of the dimers to tetrameric clusters. Fragmentation of[Rh4(CO)10{P(OPh)3}2] is found to be pressure-dependent giving predominantly [Rh2(CO)6{P(OPh)3}2] at low CO pressure (1-40 bar), and increasing amounts of [Rh2(CO)7{P(OPh)3}] at higher (40-80 bar) pressure. Using Syngas (CO : H2 (1 : 1)) instead of CO in the above fragmentations, homolytic addition of H2 to the dimer [Rh2(CO)6{P(OPh)3}2] to give [RhH(CO)3{P(OPh3}] and [RhH(CO)2{P(OPh)3}2] is observed. The distribution of tetrameric species obtained is similar to that obtained under the same partial pressure of CO. On depressurisation/out-gassing of the sample, the original mixture of tetrameric clusters is obtained.

Supramolecular bidentate phosphorus ligands based on bis-zinc(ii) and bis-tin(iv) porphyrin building blocks

Selective metal-ligand interactions have been used to prepare supramolecular bidentate ligands by mixing monodentate ligands with a suitable template. For these assemblies pyridine phosphorus ligands and a zinc(II) porphyrin dimer were used. In the rhodium-catalysed hydroformylation of 1-octene and styrene improved selectivities have been obtained for some of the assembled bidentate ligand systems. In the palladium catalysed asymmetric allylic alkylation similar effects were observed; the enantioselectivity increased by using a bisporphyrin template. The preparation of supramolecular catalyst systems was also explored using tin-oxygen interactions. Dihydroxotin(IV) porphyrin and carboxylic phosphorus ligands assemble into supramolecular ligands and the phosphorus donor atom coordinates to transition metals. The stronger oxygen-tin bond, compared to pyridine-zinc does not result in a better performance of the catalyst.