ortho-Metallated complexes of platinum(ii) and diplatinum(i) containing the carbanions (2-diphenylphosphino)phenyl and (2-diphenylphosphino)-n-tolyl (n = 5, 6)

Abstract

When the ortho-metallated complexes cis-[Pt(κ²-C₆H₃-5-R-2-PPh₂)₂] (R = H 1, Me 2) are either heated in toluene or treated with CO at room temperature, one of the four-membered chelate rings is opened irreversibly to give dinuclear isomers [Pt₂(κ²-C₆H₃-5-R-2-PPh₂)₂(µ-C₆H₃-5-R-2-PPh₂)₂] (R = H 10, Me 11). A single-crystal X-ray diffraction study shows the Pt⋯Pt separation in 10 to be 3.3875(4) Å. By-products of the reactions of 1 and 2 with CO are polymeric isomers (R = H 13, Me 14) in which one of the P–C ligands is believed to bridge adjacent platinum atoms intermolecularly. In contrast to the behaviour of 1 and 2, when cis-[Pt(κ²-C₆H₃-6-Me-2-PPh₂)₂] (cis-3) is heated in toluene, the main product is trans-3, and reaction of cis-3 with CO gives a carbonyl complex [Pt(CO)(κ¹-C-C₆H₃-6-Me-2-PPh₂)(κ²-C₆H₃-6-Me-2-PPh₂)] 15, in which one of the carbanions is coordinated only through the carbon. Formation of a dimer analogous to 10 or 11 is sterically hindered by the 6-methyl substituent. Comproportionation of 1 or 2 with [Pt(PPh₃)₂L] (L = PPh₃, C₂H₄) gives diplatinum(I) complexes [Pt₂(µ-C₆H₃-5-R-2-PPh₂)₂(PPh₃)₂] (R = H 16, Me 17). An X-ray diffraction study shows that 17 contains a pair of planar-coordinated metal atoms separated by 2.61762(16) Å. There is no evidence for the formation of an analogue containing µ-C₆H₃-6-Me-2-PPh₂. The axial PPh₃ ligands of 16 are readily replaced by Bu^tNC giving [Pt₂(µ-2-C₆H₄PPh₂)₂(CNBu^t)₂] 18, which is protonated by HBF₄ to form a µ-hydridodiplatinum(II) salt [Pt₂(µ-H)(µ-2-C₆H₄PPh₂)₂(CNBu^t)₂]BF₄ [21]BF₄. The J_PtPt values in [21]BF₄ and 18, 2700 Hz and 4421 Hz, respectively, reflect the weakening of the Pt–Pt interaction caused by protonation. Similarly, 16 and 17 react with the electrophiles iodine and strong acids to give salts of general formula [Pt₂(µ-Z)(µ-C₆H₃-5-R-2-PPh₂)₂(PPh₃)₂]Y (Y = Z = I, R = H 19⁺, Me 20⁺; Z = H, Y = BF₄, PF₆, OTf, R = H 22⁺; Z = H, Y = PF₆, R = Me 23⁺). A single-crystal X-ray diffraction study of [23]PF₆ shows that the cation has an approximately A-frame geometry, with a Pt⋯Pt separation of 2.7888(3) Å and a Pt–H bond length of 1.62(1) Å, and that the 5-methyl substituents have undergone partial exchange with the 4-hydrogen atoms of the PPh₂ groups of the bridging carbanion. The latter observation indicates that the added proton of [23]⁺ undergoes a reversible reductive elimination–oxidative addition sequence with the Pt–C(aryl) bonds.