Thermal Coupling Reactions of 1-Phenyl-3,4-dimethylphosphole within the Coordination Sphere of Palladium(II)

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Abstract
The thermolyses of dihalobis(1-phenyl-3,4-dimethylphosphole)palladium(II) complexes [(DMPP)2PdX2, X = Cl, Br, I] were investigated in 1,1,2,2-tetrachloroethane solutions at 145 °C and in the crystalline state at 140 °C. For cis-(DMPP)2 PdCl2 and cis- or trans-(DMPP)2 PdBr2 four types of products were formed: (1) [4 + 2] cycloaddition products, (2) [2 + 2] cycloaddition products, (3) compounds that result from 1,5-hydrogen migration from a methyl group on one phosphole to the β-carbon of an adjacent phosphole (exo-methylene), and (4) products that result from an intermolecular [4 + 2] coupling of two phospholes followed sequentially by phosphinidene elimination and intramolecular [4 + 2] cycloaddition to another phosphole to give diphosphatetracyclotetradecatrienes (DPTCT). trans-(DMPP)2PdBr2 undergoes thermal isomerization to cis-(DMPP)2PdBr2 in the solid state, and cis- and trans-(DMPP)2PdBr2 give the same products in both their solid- and solution-state thermolyses. In contrast, trans-(DMPP)2 PdI2 neither isomerizes to the cis-isomer nor undergoes any of the phosphole coupling reactions in either the solution or solid state. The crystal structures of trans-(DMPP)2PdX2 (X = Br, I), {(DMPP)2[2 + 2]}PdBr2, {(DMPP)2(exo-methylene)}PdBr2, and (DPTCT)PdCl2 were determined. They crystallize in the monoclinic P21/c, triclinic P1̄, monoclinic P21/c, monoclinic P21/n, and orthorhombic P212121 space groups in units cells of the following dimensions: a = 10.158 (3) Å, b = 14.876 (4) Å, c = 16.829 (5) Å, β = 104.25(2)°, ρcalc = 1.732 g/cm3, Z = 4; a = 9.025(1) Å, b = 11.023(1) Å, c = 13.833 (1) Å, α = 101.15(1)°, β = 98.82(1)°, γ = 105.30(1)°, ρcalc = 1.886 g/cm3, Z = 2; a = 13.090 (2) Å, b = 17.637 (2) Å, c = 21.834 (2) Å, β = 100.51 (1)°, ρcalc = 1.738 g/cm3, Z = 4, a = 10.721 (1) Å, b = 16.929 (1) Å, c = 14.675(1) Å, β = 97.86 (1)°, ρcalc = 1.663 g/cm3, Z = 4; and a = 15.532 (3) Å, b = 19.401 (4) Å, c = 9.910 (2) Å, ρcalc = 1.490 g/cm3, Z = 2, respectively. Least-squares refinements converged at final values of R(F) of 0.041, 0.0354, 0.0624, 0.0533, and 0.035 for 2770, 2672, 2729, 2159, and 2525 independent observed reflections, respectively. Kinetic studies suggest that the reaction mechanisms are the same in both the solid and solution states and that the reaction mechanisms are substantially different from those previously reported for the thermolyses of the analogous cis-(DMPP)2PtX2 complexes.