Subtle reactivity patterns of non-heteroatom-substituted manganese alkynyl carbene complexes in the presence of phosphorus probes

Abstract

The non-heteroatom-substituted manganese alkynyl carbene complexes (η⁵-MeC₅H₄)(CO)₂MnC(R)CCR′ (3; 3a: R = R′ = Ph, 3b: R = Ph, R′ = Tol, 3c: R = Tol, R′ = Ph) have been synthesised in high yields upon treatment of the corresponding carbyne complexes [(η⁵-MeC₅H₄)(CO)₂MnCR][BPh₄] ([2][BPh₄]) with the appropriate alkynyllithium reagents LiCCR′ (R′ = Ph, Tol). The use of tetraphenylborate as counter anion associated with the cationic carbyne complexes has been decisive. The X-ray structures of (η⁵-MeC₅H₄)(CO)₂MnC(Tol)CCPh (3c), and its precursor [(η⁵-MeC₅H₄)(CO)₂MnCTol][BPh₄] ([2b](BPh₄]) are reported. The reactivity of complexes 3 toward phosphines has been investigated. In the presence of PPh₃, complexes 3 act as a Michael acceptor to afford the zwitterionic σ-allenylphosphonium complexes (η⁵-MeC₅H₄)(CO)₂MnC(R)CC(PPh₃)R′ (5) resulting from nucleophilic attack by the phosphine on the remote alkynyl carbon atom. Complexes 5 exhibit a dynamic process in solution, which has been rationalized in terms of a fast [NMR time-scale] rotation of the allene substituents around the allene axis; metrical features within the X-ray structure of (η⁵-MeC₅H₄)(CO)₂MnC(Ph)CC(PPh₃)Tol (5b) support the proposal. In the presence of PMe₃, complexes 3 undergo a nucleophilic attack on the carbene carbon atom to give zwitterionic σ-propargylphosphonium complexes (η⁵-MeC₅H₄)(CO)₂MnC(R)(PMe₃)CCR′ (6). Complexes 6 readily isomerise in solution to give the σ-allenylphosphonium complexes (η⁵-MeC₅H₄)(CO)₂MnC(R′)CC(PMe₃)R (7) through a 1,3 shift of the [(η⁵-MeC₅H₄)(CO)₂Mn] fragment. The nucleophilic attack of PPh₂Me on 3 is not selective and leads to a mixture of the σ-propargylphosphonium complexes (η⁵-MeC₅H₄)(CO)₂MnC(R)(PPh₂Me)CCR′ (9) and the σ-allenylphosphonium complexes (η⁵-MeC₅H₄)(CO)₂MnC(R)CC(PPh₂Me)R′ (10). Like complexes 6, complexes 9 readily isomerize to give the σ-allenylphosphonium complexes (η⁵-MeC₅H₄)(CO)₂MnC(R′)CC(PPh₂Me)R (10′). Upon gentle heating, complexes 7, and mixtures of 10 and 10′ cyclise to give the σ-dihydrophospholium complexes (η⁵-MeC₅H₄)(CO)₂MnCC(R′)PMe₂CH₂CH(R) (8), and mixtures of complexes (η⁵-MeC₅H₄)(CO)₂MnCC(Ph)PPh₂CH₂CH(Tol) (11) and (η⁵-MeC₅H₄)(CO)₂MnCC(Tol)PMe₂CH₂CH(Ph) (11′), respectively. The reactions of complexes 3 with secondary phosphines HPR¹₂ (R¹ = Ph, Cy) give a mixture of the η²-allene complexes (η⁵-MeC₅H₄)(CO)₂Mn[η²-{R¹₂PC(R)CC(R′)H}] (12), and the regioisomeric η⁴-vinylketene complexes (η⁵-MeC₅H₄)(CO)Mn[η⁴-{R¹₂PC(R)CHC(R′)CO}] (13) and (η⁵-MeC₅H₄)(CO)Mn[η⁴-{R¹₂PC(R′)CHC(R)CO}] (13′). The solid-state structure of (η⁵-MeC₅H₄)(CO)₂Mn[η²-{Ph₂PC(Ph)CC(Tol)H}] (12b) and (η⁵-MeC₅H₄)(CO)Mn[η⁴-{Cy₂PC(Ph)CHC(Ph)CO}] (13d) are reported. Finally, a mechanism that may account for the formation of the species 12, 13 and 13′ is proposed.