Phosphole complex

The result of the complex forming reaction of the Mathey phosphole (46) is quite different as in this case, the predominant product is the bis(phosphole) complex with cis geometry. (Scheme 18) [62],... 

Fig. 29. Formation of the phosphole complex [(CjPMe2Ph)4Na2(DME)2], 46, and the anion in the potassium compound tranj -[K([18]-crown-6)(THF)2]2[K(C4PMe2Ph)2THF]2. (Reprinted with permission from F. Paul et ai, Angew. Chem. Int. Ed. Engl. 1996, 35, 1125. Copyright 1996 Wiley VCH.)...

Phospholes can behave as simple two electron donors, in the same way as tertiary phosphines, and most of the transition metals have been complexed to phospholes. For example, ruthenium(II) forms a series of complexes [(Phole)2 Ru(CO)2C12] and [(Phole)3 Ru(CO)C12]. The formation of the tris phosphole complex attests to their small size. Because of the ring structure an unusual isomerism has been observed, with the rings either in the basal plane of the square pyramidal complex or normal to the basal plane (Figure 23). 

An important property of metal-bound phosphole ligands is their ability to undergo additional reactions not possible in the noncomplexed form. This is nicely illustrated by the thermally induced reactions of the palladium(ll) complex of 1-phenyl-3,4-dimethylphosphole 341 <1996IC1486>. Heating complex 341 at 145 °C in solution or at 140 °C in the solid state led to the formation of a mixed 7-phosphanorbornene-phosphole complex 343 (Scheme 114). These intramolecular [4-1-2] cycloaddition reactions are believed to proceed via the initial formation of a diallyl 1,4-biradical TS 342. Further examples of this type of reaction may be found in Section 3.15.12.1.1. 

Another intriguing example of reactivity specific to phospholes and their complexes is given by the formation of 3,4-dimethyl-phenylphosphole in the coordination sphere of a metal as illustrated in Scheme 115 (see also Section 3.15.5.2.2) <2004JOMC4647>. Here, treatment of the phospholyl complex 344 with Bu Li affords the corresponding 77 -bound phosphole complex 345, which reacts with Pd(ii) to generate the bimetallic 7] -Mn cr-Pd complex 346 that has been characterized in the solid state by X-ray diffraction. Notably, the P-Mn bite angle of 65° is typical of other dimeric Pd2Cl2 complexes. The 77 -bound C4 moiety shows the expected bond localization. 

In a nice illustration of the impact of metal coordination upon the reactivity of phospholes, a methodology for the functionalization of these heterocycles in the /3-position has been described (see also Scheme 22) <2001JOM105>. Here, coordination of both the P-lone pair and the cyclic diene system was undertaken. The resulting multimetallic complex 79 was treated with lithium diisopropylamide (LDA) to afford the lithium salt 350 (Scheme 118). This readily undergoes nucleophilic substitution with a variety of electrophiles to afford the corresponding substituted phosphole complexes 351-353. The free phospholes can be isolated following decomplexation with cerium(iv) ammonium nitrate (CAN). 

A variety of other bidentate bis(phosphole) complexes have been prepared over the last decade, with the majority incorporating elements of chirality. These are discussed in the section on chiral bidentate ligands (Section 3.15.12.2.3). 

First X-ray diffraction study of an iridium(lll) phosphole complex <2007AC(E)ml818>. 

The P NMR data for the LM(CO)s complexes given in Table 3 a clearly illustrate that the coordination chemical shift of the phosphole complexes is much smaller than those of similar phosphine complexes. This supports the notion of an increased potential for M-P 7t back-donation with phospholes and is also consistent with little phosphorus rehybridization upon coordination. 

Table 3. Infrared data (cm ) for Cr, Mo, and W carbonyl phosphole complexes...

The yields of the (phosphole)nRu(CO) Q2 complexes are rather low because several products are formed in these reactions as the phospholes more readily replace CO than other phosphines do and the phosphole complexes are very soluble. 

Solvent effects are less dominant in the isomerization of the phosphole complexes than in the isomerizations of the phosphine complexes. Likewise changes in internal bond strengths are more important for the phosphole complexes than for the phosphine complexes. This is because both the Pd-P and Pd-X bonds are stronger and shorter in the phosphole than in the phosphine complexes suggesting that DMPP is both a better a-donor and r-acceptor toward Pd(II) than is Me2PPh. 

The smaller coordination chemical shifts for the phosphole complexes do not result from weaker Pd-P bonds, as is clear from the X-ray structural results, but rather from the existence of some sr-backbonding between palladium and phosphole as already seen earlier for Cr, Mo, and W(CO)s complexes. The r-backbonding is synergistic and reinforces the Pd-P a bond while at the same time restoring some of the phosphorus electron density removed by this a-donation. The net result is that the drift of electron density away from the phosphole phosphorus upon coordination to palladium is less than that which occurs for phosphines. Thus the coordination chemical shifts also illustrate that the phospholes are softer and more polarizable than similar phosphines such as Mc2PPh. 

The P coordination chemical shifts for both the L2PtX2 and L3PtX2 complexes (Table 10) are related to the free ligand chemical shifts in a fashion analogous to those found for the palladium complexes. Likewise, for platinum phosphole complexes the coordination chemical shifts are less than those found for similar platinum phosphine complexes also indicating some platinum phosphole back donation. 

It is also found (Table 11) that Jpoi is smaller for phosphole complexes than for similar phosphine complexes demonstrating that there is less s-character in the platinum phosphole bond than in the platinum phosphine bond. Since the platinum s-orbital character is probably constant, this signifies less phosphorus s-character in the phosphole... 

The earliest studies of the coordination chemistry of phospholes involved reactions with metal carbonyls. Braye et al. found that 1,2,3,4,5-pentaphenylphosphole (PPP) reacts with Fe3(CO)i2 to form a mixture of complexes among which there is one with a PPP ring > -bonded to Fe(CO)3. These types of complexes must be very unstable and very difficult to prepare due to the high reactivity of the lone pair of die phosphorus atom. In order to obtain them it is necessary to deactivate the phosphorus atom by electron withdrawing substituents and to prevent complexation through the lone pair by steric hindrance. These facts explain why only two i -phosphole complexes have been found, both of them with 2,5-diphenyl substituted phospholes (PPP and TPP ), and why no unambiguous characterization of the bonding mode has been established. 

As a final statement, it must be stressed that much remains to be accomplished with phospholes in coordination chemistry. Practically nothing is known on the C-unsubsti-tuted phosphole complexes coordination with metals such as Ti, Zr, V, W, Ta, Os, Ir, Cu, Ag, Au, Zn, Cd, Hg... has not been seriously investigated. Catalytic activity of phosphole complexes has not been systematically studied. Phosphametallocene complexes are unknown except with Fe(CO)4. Undoubtedly, new exciting developments are at hand. 

FIGURE 22.10 (a) Gold (I) phosphole complex (b) KP46 (c) Gallium tris-maltolate. 

A Lewis salt (192) of 3,4-dimethyl-1-phenylphosphole with BHj is formed by reaction with H3B SMe2. The reaction occurs rapidly at 0°C and is near quantitative <940M925>. The salt can be deprotonated at a methyl group at — 80°C with sec-butyllithium the allylic ion (193) so produced reacts at the 2-position on protonation or methylation, giving the novel phosphine complexes (194) and (196) that are isomeric with phosphole complexes and can generate the free phosphole isomers (195) and (197) on decomplexation (Scheme 44). 

As has been noted in various places in this chapter, the coordination chemistry of phospholes is extensive and of considerable practical importance. The field continues to be a very active one, with new complexes being reported frequently as the list of metals forming complexes with phospholes is extended. Another current activity is the practical one of exploring the use of some of the complexes as homogeneous catalysts for organic reactions, and it is this aspect of phosphole complex chemistry that is summarized here. 

This results in the formation of complexes of phosphiranes and phosphirenes, respectively, which can be decomplexed to give the free phosphines. Phosphirenes were synthesized for the first time by this technique. A one-pot synthesis of phosphirene complexes from a 3,4-dimethyl-1-phenyl-phosphole complex with tungsten has even been developed <85JA4700>. These applications based on phosphole chemistry are included in a review of the three-membered ring systems <87AG(E)275>. 

---