Thermal Stability, Decomposition Paths, and Ph/Ph Exchange Reactions of [(Ph₃P)₂Pd(Ph)X] (X = I, Br, Cl, F, and HF₂)

Abstract

Complexes of the type [(Ph3P)2Pd(Ph)X], where X = I (1), Br (2), Cl (3), F (4), and HF2 (5), possess different thermal stability and reactivity toward the Pd−Ph/P−Ph exchange reactions. While 1 decomposed (16 h) in toluene at 110 °C to [Ph4P]I, Pd metal, and Ph3P, complexes 2 and 3 exhibited no sign of decomposition under these conditions. Kinetic studies of the aryl−aryl exchange reactions of [(Ph3P)2Pd(C6D5)X] in benzene-d6 demonstrated that the rate of exchange decreases in the order 1 > 2 > 3, the observed rate constant ratio, kI:kBr:kCl, in benzene at 75 °C being ca. 100:4:1 for 1-d5, 2-d5, and 3-d5. The exchange was facilitated by a decrease in the concentration of the complex, polar media, and a Lewis acid, e.g., Et2O·BF3. Unlike [Bu4N]PF6, which speeded up the exchange reaction of 2-d5, [Bu4N]Br inhibited it due to the formation of anionic four-coordinate [(Ph3P)Pd(C6D5)Br2]-. The latter and its iodo analogue were generated in dichloromethane and benzene upon addition of [Bu4N]X or PPN Cl to [(Ph3P)2Pd2(Ph)2(μ-X)2] (X = I, Br, or Cl) and characterized in solution by 1H and 31P NMR spectral data. The mechanism of the aryl−aryl exchange reactions of [(Ph3P)2Pd(C6D5)X] in noncoordinating solvents of low polarity may not require Pd−X ionization but rather involves phosphine dissociation, the ease of which decreases in the order X = I > Br > Cl, as suggested by crystallographic data. Two mechanisms govern the thermal reactions of [(Ph3P)2Pd(Ph)F], 4. One of them is similar to the aryl−aryl exchange and decomposition path for 1−3, involving a tight ion pair intermediate, [Ph4P][(Ph3P)PdF], within which two processes were shown to occur. At 75 °C, the C−P oxidative addition restores the original neutral complex (4). At 90 °C, reversible fluoride transfer from Pd to the phosphonium cation resulted in the formation of covalent [Ph4PF] and [(Ph3P)Pd], which was trapped by PhI to produce [(Ph3P)2Pd2(Ph)2(μ-I)2]. The other decomposition path of 4 leads to the formation of [(Ph3P)3Pd], Pd, Ph2, Ph3PF2, and Ph2P−PPh2 as main products. Unlike the aryl−aryl exchange, this decomposition reaction is not inhibited by free phosphine. The formation of biphenyl was shown to occur due to PdPh/PPh coupling on the metal center. Mechanisms accounting for the formation of these products are proposed and discussed. The facile (4 h at 75 °C) thermal decomposition of [(Ph3P)2Pd(Ph)(FHF)] (5) in benzene resulted in the clean formation of PhH, Ph3PF2, Pd metal, and [(Ph3P)3Pd].