Formation of enol and acyl phosphine ligands via alkylation of a tungsten phosphaallyl complex: crystal structures of [WI{C6H4(PH2)[PMeC(O)CH2(C6H4Me-4)]-1,2}(CO)(η-C5H5)]·0.5C4H8O and [WI{C6H4(PH2)[P{(CH2)4I}C(O)CH2(C6H4Me-4)]-1,2}(CO)(η-C5H5)]

Abstract

The π-phosphaallyl complex [W{σ,η³-C₆H₄(PH₂)[PC(OH)CH(C₆H₄Me-4)]-1,2}(CO)(η-C₅H₅)]1 has an unused lone pair of electrons on the central phosphorus atom of the chelate ring and like a conventional PR₃ ligand will undergo facile P-alkylation reactions. Methylation (Me₃OBF₄, CH₂Cl₂, 20°C) at the allylic phosphorus affords the co-ordinated enol-phosphine salt [W{σ²,η²-C₆H₄(PH₂)[PMeC(OH)CH(C₆H₄Me-4)]-1,2}(CO)(η-C₅H₅)][BF₄]2. Complex 2 is stable in CH₂Cl₂ but MeCN displaces the enol group (1 d, 20°C), affording [W{σ²-C₆H₄(PH₂)[PMeC(OH)CH(C₆H₄Me-4)]-1,2}(MeCN)(CO)(η-C₅H₅)]⁺3, which subsequently undergoes very slow (10 d, 20°C) enol–keto tautomerisation to the ketone isomer [W{σ²-C₆H₄(PH₂)[PMeC(O)CH₂(C₆H₄Me-4)]-1,2}(MeCN)(CO)(η-C₅H₅)]⁺4. Deprotonation of the hydroxyl group in 2(Et₃N, MeCN) affords the neutral metallaphosphacyclobutanone complex [W{σ³-C₆H₄(PH₂)[PMeC(O)CH(C₆H₄Me-4)]-1,2}(CO)(η-C₅H₅)]5 which undergoes facile O-methylation (Me₃OBF₄, CH₂Cl₂) affording [W{σ²,η²-C₆H₄(PH₂)[PMeC(OMe)CH(C₆H₄Me-4)]-1,2}(CO)(η-C₅H₅)][BF₄]6, a methoxy derivative of the hydroxy precursor 2. The methoxyvinyl group in 6 is slowly displaced by MeCN (3 d, 20°C), affording the cation [W{σ²-C₆H₄(PH₂)[PMeC(OMe)CH(C₆H₄ Me-4)]-1,2}(MeCN)(CO)(η-C₅H₅)]⁺7. The presence of the methoxy group in 7 prevents enol–keto tautomerisation. The reaction of complex 1 with Mel (CH₂Cl₂, 20°C) initially affords a yellow precipitate of [W{σ², η²-C₆H₄(PH₂)[PMeC(OH)CH(C₆H₄Me-4)]-1,2}(CO)(η-C₅H₅)]I 8 which then slowly dissolves and simultaneously isomerises to [Wl{σ²-C₆H₄(PH₂)[PMeC(O)CH₂(C₆H₄Me-4)]-1,2}(CO)(η-C₅H₅)]9. The related reaction of 1 with 1,4-diiodobutane gives [Wl{σ²-C₆H₄(PH₂)[P{(CH₂)₄I}C(O)CH₂(C₆H₄Me-4)]-1,2}(CO)(η-C₅H₅)]10, an n-butyl iodide analogue of 9. The molecular structures of 9 and 10 have been established by single-crystal X-ray diffraction studies and are extremely similar. The tungsten atoms carry a cyclopentadienyl ring and are ligated by the o-phenylene bidentate phosphine, iodide, and CO ligands. In both cases the iodide ligand lies trans to the PH₂ end of the bidentate phosphine ligand. The alkylated phosphorus atom of the bidentate phosphine ligand carries a methyl (9) or an n-butyl iodide (10) group, and an acyl group C(O)CH₂C₆H₄Me-4 which has arisen from isomerisation of the enol moiety C(OH)C(H)C₆H₄Me-4 in the precursor 1. The alkyl groups lie syn to the cyclopentadienyl rings which is consistent with stereospecific alkylation of the phosphorus lone pair in complex 1. Spectroscopic data (IR and ¹H, ¹³C-{¹H}, and ³¹P NMR) for the new tungsten complexes are discussed and mechanisms proposed to account for their formation.