Phosphenium complex

The direct synthesis of neutral phosphenium complexes was reported from ionic... 

Phosphenium complexes, with tungsten carbonyls, 5, 624 Phosphides... 

IV. Competitive Reactions with Cationic Phosphenium Complex Formation. . 120... 

The chemistry of transition metal complexes bearing a cationic phosphenium ligand dates back to the synthesis and characterization of [(CO)4Fe P(NR2)2 ][PF6] by Parry in 1978.2 Since then, many cationic phosphenium complexes have been investigated because a phosphenium may serve as a a-donor (a Lewis base) and also as a 7i-acceptor (a Lewis acid) due to the lone pair electrons and a vacant p orbital, and a few review articles have appeared.3 7 [LraM(PR2)], being an electrically neutral transition metal complex, is sometimes treated as a phosphenium complex because it can be considered to consist of LraM and PR. 8 11 In this chapter, the focus is on electrically cationic transition metal complexes described as [LraM(PRR )]+ -... 

Phosphines and their derivatives are known to be very useful ligands toward transition metals and a variety of complexes with phosphine as a ligand have been prepared for all kinds of transition metals. If one of the substituents on a coordinating tertiary phosphorus compound is abstracted as an anion, it would form a cationic phosphenium complex. Actually this strategy has been widely used, and halide, hydride, and alkoxide have been abstracted as an anion by an appropriate Lewis acid. An alternative method to prepare cationic phosphenium complexes is a direct reaction of a phosphenium cation with a transition metal complex having appropriate... 

Hydride abstraction is not a common way to generate a cationic phosphenium complex. One example has been reported [Eq. (3)].13 The hydride ion transfer is in equilibrium at — 80 °C the P-H compound (5) exists in solution dominantly, whereas at — 23 °C, 5 and phosphenium species (6) are present in approximately equal concentrations. 

A phosphenium cation reacts with a transition metal complex having an appropriate leaving ligand such as carbonyl, phosphine, and acetonitrile to generate a cationic phosphenium complex [Eqs. (10)-( 14)].2,26-29 This reaction seems to be the simplest method for the preparation of a cationic phosphenium complex. The drawback of this method is that known stable phosphenium cations are limited in number. 

As shown in Eq. (14), W(bpy)(CO)3(MeCN) reacts with the phosphenium cation (17) to give a cationic phosphenium complex (18). The same phosphenium complex (though the counter anion is different) (19) is also prepared in the reaction of the same starting W complex with the corresponding chlorophosphine [Eq. (15)].29 In this reaction, P-Cl heterolytic cleavage spontaneously occurs to form the phosphenium complex. Complex 19 is unstable and is gradually converted into 20 by CP/CO exchange. 

In the reaction of HBF4 with the neutral complex Cp(CO)2W-[PPh N(SiMe3)2 ] (24) which possesses a planar geometry around the phosphorus, protonation takes place at the metal center to give a cationic phosphenium complex (25) [Eq. (17)].31... 

Although electrophilic character is expected for cationic phosphenium complexes, only a few examples have been reported. Mer-10 readily reacts with Nu- (Me-, OEt-) to generate 29 [Eq. (20)].20 This shows that mer-10 is susceptible to nucleophilic attack at the phosphenium phosphorus. Complex 29 is not obtained in the reaction of the corresponding phosphite complex (30) with Me- or OEt-. 

When E is carbon, the produced phosphenium complex is too reactive to be detected. The alkyl ligand immediately migrates to the phosphenium phosphorus to give 37. The 16 electron species can be converted into an isolable complex 38 by the addition of PPh3.22,23... 

When E is Si or Ge, the cationic phosphenium complex (36) is stable and Fe-Si and Fe-Ge bonds remain unreacted.22,23 The formation of the silyl phosphenium complex, [Cp(CO)(SiMe3)Fe P(NMeCH2)2 ]BPh4 was confirmed by X-ray analysis.23... 

Eq. (22)].38 Although there is no evidence supporting the formation of a cationic phosphenium complex, it may be generated and serve as a catalyst for hydroformylation of styrene. 

COMPETITIVE REACTIONS WITH CATIONIC PHOSPHENIUM COMPLEX FORMATION... 

Alkoxide anion abstraction by a Lewis acid from phosphorus ligand on a transition metal is one of the most useful methods to prepare cationic phosphenium complexes. It has been reported that OR anion abstraction from an alkoxyalkyl ligand, C(OR)R2, on a transition metal complex leads to the formation of a cationic carbene complex.39-42 It is similarly expected that OR anion abstraction from an alkoxysilyl ligand, Si(OR)R2, would cause the formation of a cationic silylene complex. Therefore, increased attention has been focused on the reaction with a Lewis acid of transition metal complexes bearing both alkoxyalkyl and phosphite ligands and of complexes bearing both alkoxysilyl and phosphite ligands. 

PHYSICAL PROPERTIES AND M-P BOND NATURE OF CATIONIC PHOSPHENIUM COMPLEXES... 

P NMR Data fora Phosphenium Group in Cationic Phosphenium Complexes... 

In contrast to 76 cationic phosphenium complexes for which 31P NMR data have been reported, only 7 complexes have been structurally characterized (Table IV). Five of them are diamino-substituted phosphenium complexes, one is a monoaminomonoalkoxy phosphenium complex,32 and one is a dialkoxy phosphenium complex.16,18 The geometry... 

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