Some studies of the substitution chemistry of [Rh-2(OAc)(2)(CH3CN)(4)]-[BF4](2) with monodentate and bidentate tertiary phosphines

The reactions between [Rh-2(OAc)(2)(CH3CN)(4)][BF4](2) and each of the tert iary phosphines PMe3, PCy3 (Cy = cyclohexyl), Me2PCH2CH2PMe2 (dmpe), Ph2PCH 2CH2PPh2 (dppe), Me2PCH2PMe2 (dmpm) and Ph2PCH2PPh2 (dppm) have been studie d by H-1 and P-31{H-1} NMR spectroscopy in CD3CN. The chelating phosphines dppe and dppm catalyze the exchange of coordinated CH3CN for solvent CD3CN exchange prior to any other observable substitution chemistry. The monodent ate phosphines initially form kinetically labile biaxially ligated complexe s, [Rh-2(OAc)(2)(CH3CN)(4)(PR3)(2)][BF4](2) prior to substitution of the eq uatorial CH3CN by PR3. Over time, the biaxial complex rearranges to form th e monoaxial, monoequatorial complex, involving displacement of a single equ atorial CH3CN ligand. For PCy3 the complex [Rh-2(OAc)(2)(CH3CN)(3)(PCy3)(2) ][BF4](2) has been characterized by H-1 and P-31{H-1} NMR spectroscopy. Wit h time, a further reaction occurs leading to the cleavage of the Rh-Rh bond and the monomeric complex [Rh(CH3CN)(2)(PCy3)(2)][BF4] has been identified . Crystal data at +25 degrees C: space group P2(1)nm, a = 9.879(1) Angstrom , b = 13.275(1) Angstrom, c = 16.705(1) Angstrom and Z = 4. A similar react ion sequence is observed with PMe3 but more isomers of formula [Rh-2(OAc)(2 )(CH3CN)(3)(PMe3)(2)][BF4](2) are observed by P-31{H-1} NMR spectroscopy. R eactions involving dppe lead to axial and equatorial Rh-P bonded complexes. Based on P-31{H-1} NMR data, the bisequatorial complex formulated as [Rh-2 (OAc)(2)(CH3CN)(2)(dppe)][BF4](2) is formed. The formation of the latter, w hich has been followed from 35 to 80 degrees C, is evidently reversible sin ce all attempts to crystallize the complex yielded only the acetonitrile sa lt [Rh-2(OAc)(2)(CH3CN)(4)][BF4](2) and free dppe. With dppm, only axial li gation is observed while for dmpm and dmpe the substitutional behavior is m ore complex and has not been evaluated in detail. The activation parameters for the conversion of the biaxial [Rh-2(OAc)(2)(S)(4)(L)(2)][BF4](2) to th e monoaxial, monoequatorial [Rh-2(OAc)(2)(S)(3)(L)(2)][BF4](2) complex (S = CH3CN and L = phosphine) have been determined. For L = PMe3, Delta H doubl e dagger = 16(1) kcal mol(-1) and Delta S double dagger = -9(3) cal K-1 mol (-1) and for L = PCy3, Delta H double dagger = 21(1) kcal mol(-1) and Delta S double dagger = +2(3) cal K-1 mol(-1). For dppe, the 1∶1 adduct sh ows only one type of P-31 signal for the initial axial complex indicative o f rapid exchange of free and bound PPh2 groups. The rearrangement to the equatorial-axial isomer [Rh-2(OAc)(2)(S)(3)(dppe)] [BF4](2) occurs with Delta H double dagger = 26(1) kcal mol(-1) and Delta S double dagger = +12(1) cal K-1 mol(-1). Collectively these data show that substitution at the Rh-2(4+)-center proceeds via an initial reversible asso ciative process followed by an interchange of labile axial for inert equato rial sites. These results are compared with earlier studies of the substitu tion of M-2(4+)-containing complexes, where M = Mo, Ru and Rh.