Neutral and Zwitterionic Low-Coordinate Titanium Complexes Bearing the Terminal Phosphinidene Functionality. Structural, Spectroscopic, Theoretical, and Catalytic Studies Addressing the Ti−P Multiple Bond

Abstract

Alpha-hydrogen abstraction and alpha-hydrogen migration reactions yield novel titanium(IV) complexes bearing terminal phosphinidene ligands. Via an alpha-H migration reaction, the phosphinidene ((tBu)nacnac)Ti=P[Trip](CH(2)(tBu) ((tBu)nacnac(-) = [Ar]NC((t)Bu)CHC((t)Bu)N[Ar], Ar = 2,6-(CHMe2)(2C6H3, Trip = 2,4,6-(i)Pr3C6H2) was prepared by the addition of the primary phosphide LiPH[Trip] to the nucleophilic alkylidene triflato complex ((tBu)nacnac)Ti=CH(t)Bu(OTf), while alpha-H abstraction was promoted by the addition of LiPH[Trip] to the dimethyl triflato precursor ((tBu)nacnac)Ti(CH)(2)(OTf) to afford ((tBu)nacnac)Ti=P[Trip](CH3). Treatment of ((tBu)nacnac)Ti=P[Trip](CH3) with B(C6F5)(3) induces methide abstraction concurrent with formation of the first titanium(IV) phosphinidene zwitterion complex ((tBu)nacnac)Ti=P[Trip]{CH3B(C6F5)(3)}. Complex ((tBu)nacnac)Ti=P[Trip]{CH3B(C6F5)(3)} [2 + 2] cycloadds readily PhCCPh to afford the phosphametallacyclobutene [((tBu)nacnac)Ti(P[Trip]PhCCPh)][CH3B(C6F5)(3)]. These titanium(IV) phosphinidene complexes possess the shortest Ti=P bonds reported, have linear phosphinidene groups, and reveal significantly upfielded solution 31P NMR spectroscopic resonances for the phosphinidene phosphorus. Solid state 31P NMR spectroscopic data also corroborate with all three complexes possessing considerably shielded chemical shifts for the linear and terminal phosphinidene functionality. In addition, high-level DFT studies on the phosphinidenes suggest the terminal phosphinidene linkage to be stabilized via a pseudo Ti[triple bond]P bond. Linearity about the Ti-P-C(ipso) linkage is highly dependent on the sterically encumbering substituents protecting the phosphinidene. Complex ((tBu)nacnac)Ti=P[Trip]{CH3B(C6F5))(3)} can catalyze the hydrophosphination of PhCCPh with H(2)PPh to produce the secondary vinylphosphine HP[Ph]PhC=CHPh. In addition, we demonstrate that this zwitterion is a powerful phospha-Staudinger reagent and can therefore act as a carboamination precatalyst of diphenylacetylene with aldimines.

Nucleophilic Phosphinidene Complexes: Access and Applicability

“…The generality of the a-hydrogen migration [28] was further demonstrated by the synthesis of titanium(IV) phosphinidene 34 (Scheme 10 c), which bears the PNP-pincer ligand N[2-P-(CHMe 2 ) 2 -4-methylphenyl] 2 À ; imide and alkylidene functionalities can be obtained by the same approach.…”

“…Analogously, Menye-Biyogo et al have reported the formation of the putative phosphinidenes 51 a from the interaction of phosphinidene complex [h 6 -pCym(Cy 3 P)Ru = P À Mes*] and alkynes by loss of the phosphine ligand. [62] Zwitterionic titanium phosphinidene 18 with its labile borate group was shown to undergo [2+2]-cycloaddition with diphenylacetylene to generate phosphatitanocyclobutene 96 (Scheme 28 a), [28] which was identified on the basis of its characteristic chemical shifts in the 31 P (d = 160.7 ppm) and 13 C NMR spectra (d = 253.5 ppm). Complex 18 was demonstrated to be able to function as a precatalyst in the catalytic hydrophosphination of PhCCPh with PhPH 2 .…”

“…The proposed mechanism (Scheme 28 b) involves PAr transfer of the primary phosphine and subsequent [2+2]-cycloaddition of diphenylacetylene to form 97, which generates vinylphosphine PhHP(Ph)C = CHPh 98 upon reaction with phenylphosphine. [28] A diphosphorus analogue of the versatile Dötz intermediate that is common in the chemistry of complexed carbenes has been reported by the group of Lammertsma. h 3 -Diphosphavinylcarbene complex 100 resulted on the DBU-induced reaction of Ru-and Ir-complexed primary phosphines 46 a and 48 a with phosphaalkyne Mes*CP (Scheme 29).…”

Recent Advances in Metal-Catalyzed C–P Bond Formation

“…[2+2]-cycloaddition, as observed for related imido complexes, then gives metallacycle 14, whose protonation with phenylphosphine yields Ti-bis(phosphido) complex 15. a-elimination would then yield the alkenylphosphine product and regenerate the catalyst [42].…”

Linear versus bent bonding in metal-phosphinidene complexes: Theoretical studies of the electrophilic phosphinidene complexes [(Cp)(CO)2MPMe)]+, [(Cp)(CO)3MPMe)]+ (M = Cr, Mo, W)

“…The philicity is influenced by the type of spectator ligand L rather than for the nature of the transition metal M [62]. DFT [58]. These titanium(IV) phosphinidene complexes possess the shortest Ti=P bonds reported, have linear phosphinidene groups, and reveal significantly upfielded 31 P NMR chemical shifts.…”