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

Abstract

Treatment of molybdenum or tungsten hexacarbonyls with the diphosphine CH₂C(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; CH₂CHCH₂, 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.