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.