Lewis bases, phosphorus compounds

Towards a simple Lewis base, for example the proton, phosphine is a poorer electron donor than ammonia, the larger phosphorus atom being less able to form a stable covalent bond with the acceptor atom or molecule. Phosphine is, therefore, a much weaker base than ammonia and there is no series of phosphonium salts corresponding to the ammonium salts but phosphonium halides. PH4X (X = Cl, Br, I) can be prepared by the direct combination of phosphine with the appropriate hydrogen halide. These compounds are much more easily dissociated than ammonium halides, the most stable being the iodide, but even this dissociates at 333 K PH4I = PH3 -t- HI... 

Compounds containing carbenium, silyl or phosphonium cations can act as Lewis acids. In addition, phosphorus- and silicon-based hypervalent compounds display a Lewis acid catalytic activity. Furthermore, ionic liquids, organic salts with a melting point below 100 °C, have shown the ability to catalyze a group of reactions either in substoichiometric amount or, if used as the reaction medium, in stoichiometric or even larger quantities. The solvents can be efficiently recovered after the reaction. Each type of these compounds will be discussed in a separate section. 

Adduct formation results in well-defined species. Generally speaking, phosphorus compounds act as Lewis bases [exceptions being penta-valent phosphorus halides as reviewed by Webster <1966 106)] for other examples in which the relevant Lewis acids are metalloid derivatives see references 1966, 107 and 1969, 186. Adducts involving boron have recently been reviewed elsewhere(1969 94 andl02) and are by far the most numerous and use has been made of phosphorus, boron, proton and fluorine resonances, in some cases at varying temperature. 

The manifold intermediates in homogeneous transition-metal catalysis are certainly metal complexes and therefore show a behaviour like ordinary coordination compounds associations of phosphorus donors open up multifarious additional controls. Both, substrates and P ligands are Lewis bases that we have to consider and that compete at the coordination centers of the metal, leading to competitive, non-competitive or uncompetitive activation or inhibition processes in analogy to the terminology of enzyme chemistry... 

The rapid evolution and the multifarious activity in the field of phosphorus chemistry is well illustrated by the proposal (Dupart)332 of a new systematic classification of free and coordinated phosphorus compounds to complete the systems proposed by Wolf333 and by Perkins et a/.334. In this new system, a code number NFE(NPL)D could be used to describe the compound, in which NFE would be the number of free electrons, N the number of valence shell electrons, L the number of ligands and D the number of electron doublets donated or accepted, which would be negative if the P atom acted as a donor (a Lewis base), or positive if P acted as an acceptor (a Lewis acid). 

The ability of the boron atom of 59 to engage in a donor-acceptor interaction was illustrated with DMAP and DABCO (DABCO = diazabi-cyclo-[2.2.2]-octane) that readily formed the corresponding Lewis adducts. Interestingly, a similar behavior was retained after coordination of the phosphorus atom to palladium. The formation of the Lewis base adducts 66a and 66b of complex 65 (Scheme 38) was supported by solid-state 31P and nB CP/MAS-NMR spectroscopy (<5 1 B = 5-6 ppm), although the occurrence of decomposition and/or dissociation processes impeded spectroscopic characterization in solution and recrystallization to obtain X-ray quality crystals. Compounds 66a and 66b substantiate the ability of ambiphilic compounds to engage concomitantly into the coordination of donor and acceptor moieties. Such a dual behavior opens interesting perspectives for the preparation of metallo-polymers and multimetallic complexes. 

It is known that dicoordinated phosphorus compounds can be obtained by the action of Lewis bases, such as triethylamine or DBU, on chlorophosphines ... 

Phosphines are classical Lewis bases or ligands in transition-metal complexes, but the cationic species shown in Fig. 15.4.l(i) are likely to exhibit Lewis acidity by virtue of the positive charge. Despite their electron-rich nature, an extensive coordination chemistry has been developed for Lewis acidic phosphorus. For example, the compound shown below has a coordi-natively unsaturated Ga(I) ligand bonded to a phosphenium cation it can be considered as a counter-example of the traditional coordinate bond since the metal center (Ga) behaves as a Lewis donor (ligand) and the non-metal center (P) behaves as a Lewis acceptor. 

Other possibilities include compounds with MesSi, MesSi2, and FsSi groups, although in the last case the reaction may be complicated by simultaneous disproportionation leading to formation of SiF4 2B, where B is a Lewis base (entry 31). The ClaSi derivative in entry 26 does not form adducts with nitrogen or phosphorus bases, although phosphines cause slow elimination of CO (cf. Section III,E,1). 

A variety of trivalent phosphorus, arsenic, antimony and bismuth compounds, as well as divalent oxygen, sulfur and selenium compounds, can also give complexes with transition metals. These donor molecules are, of course, quite strong Lewis bases and give complexes with acceptors such as BR3 compounds where d orbitals are not involved. However, the donor atoms do have empty dn orbitals and back-acceptance into these orbitals is possible, as shown in Fig. 22-17. 

A similar role has been played by Wittig and co-workers in extending the horizons of aromatic organometallic chemistry. Investigation of the complexes formed between aryl alkali compounds as phonyllithium and Lewis bases of type R M has resulted in the synthesis of ylides of nitrogen and phosphorus, metalloids in rare valence states, and organometallic cr-complexes of unique structure. 

A search on the CSD shows that the nitrogen atoms bound to phosphorus in phosphoramidate compounds aren t involved in normal H-bonding interaction as an acceptor due to the deviation of each N atom environment from pyramidality after binding to P and decreasing its Lewis base character with respect to the initial amine so that, merely one example (refcode HESCEO [111], Scheme 23), belonging to the diazaphosphorinane family, is observed so far with the donor...acceptor (N...N) distance of 3.258(8) A in which it may be considered as a weak N—H... N—P hydrogen bond. 

The ability of boranes to complex and stabilize NHCs for subsequent reaction has been investigated by a number of groups however, this effect can also be thought of as NHC stabilization of the borane. NHCs have been shown to stabilize a multitude of other reactive group 13 compounds. For example, phospha-nylboranes (RHP-BH2) can be stabilized by coordination of a Lewis base at boron and a Lewis acid at phosphorus as illustrated by Adolf et al. in the preparation of the complex shown in Figure 15.22 [112]. 

In general, P(III) compounds adopt the geometry of a trigonal pyramid with the free electron pair on the phosphorus [32]. This property is responsible for the fact that trivalent phosphorus compounds are Lewis bases and can act as soft nucleophiles preferentially in the reaction with soft electrophiles. Upon protonation of the phosphorus, it can also behave as an electrophile. The same situation can be caused by strong electron-withdrawing substituents. 

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