The reactivity of binuclear rhodium hydrides : fundamental processes involving two metal centres
Creators: name: Piers, Warren Edward
Publisher: name: University of British Columbia
Year: 2010

description: Current knowledge of mechanistic organometallic chemistry has resulted largely from the study of mononuclear transition metal complexes. The possibility that different primary organometallic processes involving two or more metal centres may exist has been addressed only recendy. Reactivity studies on a simple, well defined binuclear system ought to provide fundamental insights into the nature of such polynuclear primary processes. The binuclear rhodium hydrides {[R₂P(CH₂)nPR₂]Rh(μ-H)}₂ (R = Pri, n = 2-4, 1a-1c; R = OPri, n = 2, 1d) were thus reacted with a variety of organic compounds in an attempt to define primary processes involving two metal centres. The reactions of 1a-c with dihydrogen proceed rapidly to produce fluxional binuclear tetrahydrides whose structure is dependent on the chelate ring size of the diphosphine ligand. The dihydrides also catalyze the hydrogenation of olefins. Two mechanistic pathways for this cycle are proposed to exist as supported by chemical and kinetic evidence. One utilizes binuclear intermediates, the other mononuclear; the latter predominates in the 1b catalyzed system (chelate ring size of six) while the former is favoured in the cycle mediated by la (chelate ring size of five). The reactions of 1a and 1b with 1,3-dienes led to the solid (X-ray diffraction) and solution state characterization of binuclear complexes incorporating bridging dienyl ligands in the previously unobserved μ-ƞ⁴-ϭ and μ-ƞ³-ƞ³ "partial sandwich" bonding modes. A fluxional process interconverting the two bonding modes was observed spectroscopically in the products of the 1a/butadiene reaction and a model accounting for this is proposed. Labelling and alternate synthetic studies, as well as the observation of an intermediate at low temperature, support a mechanism for these reactions which involves the dehydrogenation of the dihydrides followed by further reaction of [(P₂)Rh]₂ with a second equivalent of diene. Bridging amido hydrides of general formula [(P₂)Rh]₂(μ-NR'CH₂R")(μ-H) are produced in the reactions of 1a and 1d with imines (R'N=CHR"). Mechanistic studies reveal that initial ϭ-donation of the imine lone pair; of electrons to one Rh followed by π-coordination of the C=N bond to the other precedes formal insertion of the C=N bond into Rh-H. This proposal is consistent with the results of labelling and kinetic studies, but the crux of its support lies in the spectroscopic observation of two intermediates en route to the product amido hydrides. The specific synthesis of cationic μ-ƞ²-ϭ imine complexes closely related to one of the proposed intermediates in the reaction was carried out to confirm the plausibility of such an intermediate. Reaction of the amido hydrides with dihydrogen was slow in producing free amine and the hydrogen adducts of 1a or 1d, precluding the use of these dihydrides as catalyst precursors in the homogeneous hydrogenation of imines. Reaction of 1a and 1d with nitriles (R"'C=N) produces μ-alkylideneimido hydride complexes of general formula [(P₂)Rh]₂(μ-N=CHR'")(μ-H). One derivative (P₂, R = Pri, n = 2; R"' = CH₃) has been characterized by X-ray crystallography. Further reaction of these complexes with dihydrogen yield the amido hydrides [(P₂)Rh]₂(μ-NHCH₂R"')(μ-H). No intermediates in these reactions were observed, precluding meaningful mechanistic proposals for this stepwise reduction of nitriles as mediated by two metal centres.