Preparation, Structure, and Reactivity of Dipalladium(I) Complexes Containing the Carbanion 2‑C6F4PPh2: Coexistence of Distinct, Noninterconverting Head-to-Head [Dipalladium(0/II)] and Head-to-Tail [Dipalladium(I)] Species

Comproportionation of trans-[Pd­(κ2-2-C6F4PPh2)2] with [PdL4] (L = PPh3, AsPh3) gives metal–metal-bonded dipalladium­(I) complexes [Pd2I(μ-2-C6F4PPh2)2(L)2] [L = PPh3 (5), AsPh3 (6)] in which the bridging ligands adopt a head-to-tail arrangement. The corresponding diplatinum­(I) complex [Pt2I(μ-2-C6F4PPh2)2(PPh3)2] (9) is obtained similarly from [Pt­(κ2-2-C6F4PPh2)2] and [Pt­(PPh3)3]. The separations between the metal atoms in the dipalladium­(I) complexes [2.5740(3) Å (5), 2.5511(3) Å (6)] are slightly less than that in the diplatinum­(I) complex 9 [2.61179(15) Å]. The axial triphenylarsine ligands of 6 are replaced by tert-butyl isocyanide to give the dipalladium­(I) complex [Pd2I(μ-2-C6F4PPh2)2(CNtBu)2] (7). However, treatment of trans-[Pd­(κ2-2-C6F4PPh2)2] with [Pd­(CNtBu)2], generated in situ from a mixture of tert-butyl isocyanide and [Pd­(η5-Cp)­(η3-allyl)], gives a formally mixed-valent palladium(0)–palladium­(II) complex [Pd20/II(μ-2-C6F4PPh2)2(CNtBu)2] (8), in which the bridging ligands are arranged head-to-head. In contrast, comproportionation of [Pt­(κ2-2-C6F4PPh2)2] with [Pt3(CNtBu)6] gives the diplatinum­(I) complex [Pt2I(μ-2-C6F4PPh2)2(CNtBu)2] (10) analogous to 7; there was no evidence for the formation of a dinuclear mixed-valent species. Although complexes 7 and 8 are isomers, with similar Pd–Pd separations [2.5803(4) Å (7), 2.5580(2) Å (8)], they do not interconvert in solution. The diplatinum­(I) complex 9 undergoes oxidative addition with iodine to give an A-frame cation, [Pt2(μ-I)­(μ-2-C6F4PPh2)2(PPh3)2]+, isolated as its PF6– salt (11). In contrast, the dipalladium­(I) complex 5 eliminates one of the PPh3 ligands when it undergoes oxidative addition with halogens and with methyl iodide, the products being A-frame dipalladium­(II) complexes [Pd2X­(μ-Y)­(μ-2-C6F4PPh2)2(PPh3)] [X = Y = I (12), Cl (13); X = Me, Y = I (14)]. The metal–metal distances in 11–14 [2.9478(5) Å (11), 2.8078(7) Å (12), 2.8241(3) Å (13), and 2.8013(5) Å (14)] are ca. 0.3 Å greater than in their dimetal­(I) precursors, consistent with a weaker metal–metal interaction in the dimetal­(II) complexes. Unexpectedly, reaction of 5 with iodobenzene gives a mononuclear palladium­(II) complex, cis-[Pd­(κ2C,P-2-C6F4PPh2)­(κC-2-C6F4PPh2)­(PPh3)] (15). This is suggested to be formed by a sequence of (a) oxidative addition of iodobenzene to 5 to give a σ-phenyl complex analogous to the σ-methyl complex 14, (b) a change in the binding mode of one of the 2-C6F4PPh2 ligands from μ- to κ2-, and (c) reductive elimination of tetraphenylphosphonium iodide and loss of Pd(0).