Journal of the Chemical Society, Dalton Transactions p. 1045 - 1050 (1996)
Update date:2022-08-04
Topics:
Estevan, Francisco
Gonzalez, Gabriel
Lahuerta, Pascual
Martinez, Manuel
Peris, Eduardo
Van Eldik, Rudi
The reaction of compounds [Rh2(O2CMe)(μ-O2CMe) 2{μ-(XC6H3)P(C6H 4X-p)2}{P(C6H4X′-p) 3}] (X = H, X′ = H or Me; X = Me, X′ = H or Me), having a metallated and an equatorial phosphine in a head-to-tail cisoid configuration, to produce the corresponding doubly metallated compounds [Rh2(μ-O2CMe)2· {μ-(XC6H3)P(C6H4X-p) 2}2], has been studied in detail. The thermal, acid-, and base-assisted conditions include toluene solution, neat acetic acid, and different bases [PPh3, P(C6H4Me-p)3 or pyridine] in toluene solution and as a function of temperature, pressure and base concentration (when applicable). The doubly metallated compounds always show retention of the head-to-tail cisoid configuration according to 31P NMR spectroscopy. The reported activation parameters allow a clear differentiation between the spontaneous, acid-assisted and base-assisted processes, some of which had been previously studied (ΔH? = 80 ± 5, 68 ± 1, 38 ± 3 kJ mol-1; ΔS? = -105 ± 15, -85 ± 3, -150 ± 30 J K-1 mol-1; ΔV? = -22.7 ± 0.2, -15.3 ± 0.2, -35.0 ± 0.5 cm3 mol-1 respectively, for the compound with X, X′ = H). For the reactions under base-assisted conditions, when long reaction times and/or high added Lewis-base concentrations were applied, the non-metallated phosphine was displaced from equatorial to axial co-ordination. Under these conditions a mixture of the bis adducts of the monometallated [Rh2(μ-O2CMe)3{μ-(XC6H 3)P(C6H4X-p)2}], and the doubly-metallated compounds was detected by 31P NMR spectroscopy. The activation parameters obtained for this slower process agree with those expected for substitution reactions and are interpreted in terms of the steric demands of the mono and bis adducts formed immediately on mixing the reaction solutions.
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