Cationic phosphenium complexes

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 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... 

With a cationic phosphenium complex of a transition metal, 7c-donation from the transition metal to phosphenium phosphorus is also conceivable. A cationic phosphenium complex can be described in the resonance forms shown in Scheme 6. 58 corresponds to a complex where a plus charge is located on the phosphorus and a phosphenium cation coordinates to a transition metal through its lone pair. Thus, the bond between M and P in 58 can be seen as a dative bond. If a sufficient electron density flows from the filled d orbital of a transition metal into the vacant p orbital on the phosphorus, the plus charge would be located on a transition metal and the M-P bond would become a double bond (57). The Ti-electron donation... 

Systematic study on the influence of these substituents on the stability of cationic phosphenium complexes for [(bpy)(CO)3M PXY ]+ revealed that the stability increases upon going to a heavier congener in terms of the transition metal, with increasing the number of amino substituents on the phosphorus, and with adding an ethylene bridge between X and Y.25... 

An amino substituent on the phosphenium phosphorus makes cationic phosphenium complexes stable, but the ji-donation from an amino substituent is only a minor contribution. So what is the role of an amino substituent The role may be, as is proposed for Arduengo carbene, to protect the approach of a nucleophile to phosphenium phosphorus by high p7i lone pair density flanking the phosphenium center.32... 

Theoretical description of cationic phosphenium complexes has been reported in only a few papers. A simple representation is shown in Fig. 2 with diaminophosphenium and a 16e-ML5 fragment.6 There are two main interactions an interaction of the phosphenium HOMO (lone pair) with the metal LUMO and that of the phosphenium LUMO with one of the occupied d orbitals. The former constitutes a dative bond from P to M, and the latter a Ji-back donation from M to P. 

COMPARISON BETWEEN CATIONIC PHOSPHENIUM COMPLEXES AND SILYLENE COMPLEXES... 

As mentioned above, cationic phosphenium is isoelectronic with carbene, silylene, germylene, stannylene, and plumbylene, because the central element has lone pair electrons and a vacant p orbital as well as two substituents. Therefore, their transition metal complexes have attracted considerable attention. It might be of special interest to compare cationic phosphenium complexes with silylene complexes because phosphorus and silicon are both situated in the third row of the periodic table. 

A cationic phosphenium complex has positive charge. It has been estimated that the positive charge is located on the phosphorus atom to considerable extent and that the central metal is electronically neutral or slightly negative.51,52 With regard to silylene complexes, strong M5+ =Si5 polarization has been reported.53,54 Therefore, these species are expected to form an adduct with a 2e-donor base such as an amine or ether [Eq. (24)]. 

All cationic phosphenium complexes reported to date are base-free species. In contrast, silylene complexes tend to be stabilized by forming base adducts. Most silylene complexes are base-stabilized species54 59 and base-free silylene complexes are still rare.59-63 Typical examples are shown in Scheme 7. A phosphite-phosphenium complex exists in a solid state and even in solution, and does not take an OMe bridging form between the two P atoms,64 whereas a silyl-silylene complex is not detected, but exists as an OMe bridging form between the two Si atoms.65... 

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