Phosphine-Based Platinum(II) Hydroxo and Oxo Complexes¹

Abstract

The platinum(II) hydroxo complexes, [L2Pt(μ-OH)]2(BF4)2 (1), have been prepared from the reaction of L2PtCl2 with AgBF4 for L2 = dppm, dppp, dppb, 2PMe2Ph, and But2bpy (dppm = bis(diphenylphosphino)methane, dppe = bis(diphenylphosphino)ethane, dppp = bis(diphenylphosphino)propane, dppb = bis(diphenylphosphino)butane, But2bpy = 4,4‘-di-tert-butyl-2,2‘-bipyridine). The trifluoroacetate (L2 = dppp) and the nitrate (L2 = But2bpy) salts were also prepared. Two of these new hydroxo complexes, as well as the previously known PPh3 complex, were structurally characterized. Crystals of [(PPh3)2Pt(μ-OH)]2(BF4)2 from CH2Cl2/ether are monoclinic (C2/m) with a = 17.183(5) Å, b = 18.243(4) Å, c = 13.539(4) Å, β = 130.66(2)°, and Z = 2. Crystals of [(dppm)Pt(μ-OH)]2(BF4)2·2DMF from DMF/Et2O are monoclinic (P21/n) with a = 17.193(6) Å, b = 9.341(5) Å, c = 18.666(7) Å, β = 98.73(2)°, and Z = 2. Crystals of [(dppb)Pt(μ-OH)]2(BF4)2·4MeOH from MeOH/Et2O are triclinic (P1̄) with a = 12.547(13) Å, b = 12.858(14) Å, c = 13.039(20) Å, α = 62.89(11)°, β = 63.05(11)°, γ = 60.87(12)°, and Z = 1. The structures of the binuclear dications are all similar, having planar P2Pt(μ-OH)2PtP2 cores with pseudo-D2h symmetry. For all but the dppm complex, addition of 2 equiv of LiN(SiMe3)2 results in deprotonation of the hydroxo groups and formation of the oxo complexes [L2Pt(μ-O)]2. The isolated oxo complexes are associated with one (L = PPh3) or two (L = PMe2Ph; L2 = dppe, dppp, dppb, But2bpy) LiX units (X = BF4 or CF3CO2) by interaction of the oxo ligands with the Li cations. The structure of [(PPh3)2Pt(μ-O)]2·LiBF4 (2) was determined. Crystals of 2·0.5C7H8 from toluene are monoclinic (C2/c), with (173 K) a = 20.469(8) Å, b = 18.085(9) Å, c = 18.152(8) Å, β = 90.34(6)°, and Z = 4. The structure consists of two edge-shared square planar Pt atoms folded at the edge with cis-phosphines and bridging oxygen atoms. The bridging oxo oxygen atoms are coordinated to the Li cation of a LiBF4 contact ion pair. Two fluorine atoms of the BF4 ion also coordinate to the Li cation, resulting in a flattened tetrahedral environment about the Li cation. The oxo complex [(PMe2Ph)2Pt(μ-O)]2·(LiBF4)2 is unstable in solution and converts to the μ3-oxo complex [(L2Pt)3(μ3-O)2](BF4)2 (5) (L = PMe2Ph). Crystals of 5 from THF are orthorhombic (Pbna), with (173 K) a = 16.792(4) Å, b = 17.710(5) Å, c = 19.648(6) Å, and Z = 4. The core structure of the dication consists of a dioxo-bicapped Pt3 triangle. Deprotonation of the dppm hydroxo complex 1 (L2 = dppm) occurs at the dppm methylene group and not at the hydroxo group. The resulting neutral hydroxo complex [(dppm-H)Pt(μ-OH)]2 (6) (dppm-H = bis(diphenylphosphino)methanide) is further deprotonated in THF at the OH group, giving the structurally characterized anionic oxo complex [(dppm-H)Pt(μ-O)]2[Li(THF)2]2 (7). Crystals of 7·4THF from THF are monoclinic (P21/c), with (173 K) a = 10.534(2) Å, b = 21.016(2) Å, c = 18.402(4) Å, β = 103.41(1)°, and Z = 2. The structure has a planar core with the deprotonation of the dppm ligand evident from a shortening of the P−C distances. The Li ions are associated with the oxo groups, not the dppm-H groups, and are each coordinated by two THF molecules, giving a trigonal planar Li coordination geometry