Synthesis and reactions of complexes of 1,3-bis(diphenyl-phosphino)-2-methylallyl with molybdenum or tungsten tetracarbonyls

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Abstract
Treatment of molybdenum or tungsten hexacarbonyls with the diphosphine CH2 C(CH2PPh2)2 gives complexes [M{(Ph2PCH2)2CCH2}(CO)4](M = Mo, 3a; or W, 3b). Either of these complexes with sodium ethoxide in boiling ethanol exhibits prototropic rearrangement to give [M{Ph2PCHCMeCH2PPh2}(CO)4](M = Mo, 4a; or W, 4b). Treatment of either complexes 3 or 4 with strong bases such as methyllithium, lithium 2,2,6,6-tetramethylpiperidide or lithium bis(trimethylsilyl)amide gave bright orange solutions of the symmetrical 1,3-bis(diphenylphosphino)-2-methylallylmetal tetracarbonyl anions [M{Ph2PCHC(Me)CHPPh2}(CO)4]–5. The molybdenum complex 5a, when treated with D2O, gave [Mo(Ph2PCHCMeCHDPPh2)(CO)4]6, which was shown to be a monodeuterio-complex by 2H, 1H and 13C NMR spectroscopy. Both the 31P-{1H} and the 13C-{1H} NMR spectra of this compound showed secondary isotope effects and the 13C-{1H} spectrum showed that there was no plane of symmetry, whereas with the corresponding perhydro complex 4a, there is such a plane (going through the Mo and the two P). The carbanions 5 react with electrophiles such as Mel, SiMe3Cl, Etl, allyl bromide, PhCH2Br or PPh2Cl to give substitution products [M(Ph2PCHCMeCHR′PPh2)(CO)4](R′= Me, 7; SiMe3, 8; Et, 9; CH2 CHCH2, 10; CH2Ph, 11, or PPh2, 13); these complexes were fully characterised. Treatment of 7a(M = Mo) with LiMe and then an equivalent amount of methanol, followed by more LiMe and methanol, gave the isomerised product [Mo(Ph2PCMeCMeCH2PPh2)(CO)4]12. When a solution of complex 13a M = M) was heated in toluene the tricarbonyl [Mo{(Ph2P)2CHCMeCHPPh2}(CO)3]14a, in which all three phosphoruses are co-ordinated, was formed; similarly the corresponding tungsten complex 13b, when heated in decane, gave [W{(Ph2P)2CHCMeCHPPh2}(CO)3]14b. Proton, 2H, 13C and 31P NMR data, infrared [ν(CO)] and some mass spectral data are given and discussed.

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