Some studies of the substitution chemistry of [Rh2(OAc)2(CH3CN)4][BF4]2 with monodentate and bidentate tertiary phosphines †

Abstract

The reactions between [Rh2(OAc)2(CH3CN)4][BF4]2 and each of the tertiary phosphines PMe3, PCy3 (Cy = cyclohexyl), Me2PCH2CH2PMe2 (dmpe), Ph2PCH2CH2PPh2 (dppe), Me2PCH2PMe2 (dmpm) and Ph2PCH2PPh2 (dppm) have been studied by 1H and 31P{1H} 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 monodentate phosphines initially form kinetically labile biaxially ligated complexes, [Rh2(OAc)2(CH3CN)4(PR3)2][BF4]2 prior to substitution of the equatorial CH3CN by PR3. Over time, the biaxial complex rearranges to form the monoaxial, monoequatorial complex, involving displacement of a single equatorial CH3CN ligand. For PCy3 the complex [Rh2(OAc)2(CH3CN)3(PCy3)2][BF4]2 has been characterized by 1H and 31P{1H} NMR spectroscopy. With 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 °C: space group P21nm, a = 9.879(1) A, b = 13.275(1) A, c = 16.705(1) A and Z = 4. A similar reaction sequence is observed with PMe3 but more isomers of formula [Rh2(OAc)2(CH3CN)3(PMe3)2][BF4]2 are observed by 31P{1H} NMR spectroscopy. Reactions involving dppe lead to axial and equatorial Rh–P bonded complexes. Based on 31P{1H} NMR data, the bisequatorial complex formulated as [Rh2(OAc)2(CH3CN)2(dppe)][BF4]2 is formed. The formation of the latter, which has been followed from 35 to 80 °C, is evidently reversible since all attempts to crystallize the complex yielded only the acetonitrile salt [Rh2(OAc)2(CH3CN)4][BF4]2 and free dppe. With dppm, only axial ligation is observed while for dmpm and dmpe the substitutional behavior is more complex and has not been evaluated in detail. The activation parameters for the conversion of the biaxial [Rh2(OAc)2(S)4(L)2][BF4]2 to the monoaxial, monoequatorial [Rh2(OAc)2(S)3(L)2][BF4]2 complex (S = CH3CN and L = phosphine) have been determined. For L = PMe3, ΔH‡ = 16(1) kcal mol−1 and ΔS‡ = −9(3) cal K−1 mol−1 and for L = PCy3, ΔH‡ = 21(1) kcal mol−1 and ΔS‡ = +2(3) cal K−1 mol−1. For dppe, the 1∶1 adduct shows only one type of 31P signal for the initial axial complex indicative of rapid exchange of free and bound PPh2 groups. The rearrangement to the equatorial–axial isomer [Rh2(OAc)2(S)3(dppe)][BF4]2 occurs with ΔH‡ = 26(1) kcal mol−1 and ΔS‡ = +12(1) cal K−1 mol−1. Collectively these data show that substitution at the Rh24+-center proceeds via an initial reversible associative process followed by an interchange of labile axial for inert equatorial sites. These results are compared with earlier studies of the substitution of M24+-containing complexes, where M = Mo, Ru and Rh.