Synthesis and Reactivity of Functionalized Electrophilic Phosphinidene Complexes

Abstract

Electrophilic phosphinidene complexes play an important role in organophosphorus synthesis. The chemistry of neutral phosphinidene complexes has been well studied, but cationic phosphinidene complexes are not as well understood. Therefore, new cationic phosphinidene complexes have been synthesized and their reactivity towards bond activation, cycloaddition and nucleophilic addition has been examined. The terminal chloroisopropyl phosphido complex [Cp*Mo(CO)3{P(Cl)i-Pr}] was synthesized, and abstraction of chloride from it generates the transient cationic alkylphosphinidene complex [Cp*Mo(CO)3{P(i-Pr)}]+, which can be trapped with diphenylacetylene to form a phosphirene complex. Trapping with P(C6H5)3 leads to a phosphine coordinated phosphinidene complex. Trapping with diphenylsilane leads to SiH activation and a secondary silyl phosphine complex, while trapping with ferrocene leads to CH activation and a ferrocenyl phosphine complex. Chloride abstraction from dichlorophosphido complex [Cp*Mo(CO)3PCl2] did not lead to a chlorophosphinidene but instead to a bimetallic bridging P2Cl3 complex. Chloride abstraction in the presence of P(C6H5)3 results in [Cp*Mo(CO)3{P(Cl)(PPh3)}][AlCl4]. Reaction of [Cp*Mo(CO)3{P(Cl)(PPh3)}]+ with a second equivalent of P(C6H5)3 and AlCl3 leads to a metal-free triphosphenium salt. Reaction with bis-diphenylphosphinoethane leads to a cyclic triphosphenium salt, while, reaction with bis-diphenylphosphinomethane leads to a dangling phosphine coordinated chlorophosphinidene complex. Photolysis then leads to a chelated phosphine-coordinated chlorophosphinidene complex. Reaction of the dichlorophosphido complex with a stable N-heterocyclic carbene leads to a carbene coordinated chlorophosphinidene complex. Reaction with a second equivalent of carbene and AlCl3 leads to a bis-carbene coordinated phosphenium salt. Reaction of [Cp*Mo(CO)3PCl2] with diisopropylzinc leads to [Cp*Mo(CO)3{P(Cl)(i-Pr)}], the precursor to [Cp*Mo(CO)3{P(i-Pr)}]+. Reaction with phenoxide anions leads to a range of chloroaryloxyphosphido complexes. Chloride abstraction leads to aryloxyphosphinidenes, which can be trapped with diphenylacetylene to form aryloxy phosphirene complexes, or with triphenylphosphine to form phosphine coordinated aryloxyphosphinidene complexes. Reactions of [Cp*Mo(CO)3PCl2] with (-)-menthoxide and (-)-borneoxide result in chloromenthoxy and chloroborneoxyphosphido complexes. Chloride abstraction then generates transient chiral alkoxyphosphinidene complexes. Trapping with phenylacetylene leads to chiral P centers and forms diastereomeric mixtures of alkoxyphosphirene complexes (diastereomeric excess = 20%). With P(C6H5)3, the menthoxyphosphinidene forms a diastereomeric mixture of the phosphine coordinated menthoxyphosphinidene complex (diastereomeric excess = 52%). Reaction of [Cp*Mo(CO)3PCl2] with a thiophenolate anion results in [Cp*Mo(CO)3{P(Cl)(SPh)}], and chloride abstraction generates a transient cationic thiophenoxyphosphinidene complex, which can be trapped with diphenylacetylene to form a thiophenoxy phosphirene complex, or with triphenylphosphine to form a phosphine coordinated thiophenoxyphosphinidene complex. Reactivity of these functionalized phosphinidene complexes have been compared with that of the analogous aminophosphinidene complex experimentally and computationally. Their increasing order of electrophilicity at the phosphinidene phosphorus has been determined as follows amino < < alkyl < chloro ~ phenoxy ~ thiophenoxy.