Lewis base phosphane

A systematic NMR spectroscopic study of these adducts suggests that the steric repulsion between the trimethyl aluminum Lewis acid and the phosphane Lewis base rather than the electronic factors account for the detected changes in the P-NMR spectroscopic chemical shifts (Table 1). The change in the chemical shift (A) of the phosphanes on coordination to AlMe3 has been correlated to the... 

Whereas amino groups are used most frequently as intramolecular Lewis bases (Tables 5-7), stable unsupported intermolecular complexes are typically accomplished by carbene-type donors, such as carbenes, isonitriles and ylides (Table 8). To my knowledge, no unsupported intermolecular adduct of ethers, amines or phosphanes have been structurally characterized so far (except for the more ionic cyclopentadienyl derivatives62). 

In Fig. 2.9, we see a moderate downfield shift for the formation of a donor bond upon coordination, almost irrespective of the Lewis acid involved. It is the almost that interests us greatly. We use two phosphanes for comparison, PMe and PPh and note that the downfield shift for PPh upon coordination ranges from A< =21-40ppm, and that for PMOj is in an even narrower band of AJ=46-60ppm. In each case, the actual downfield shift depends on the Lewis base. Defining factors are the geometry around the metal (cis or trans), and the substituents on phosphorus (P or PPh ). Both factors are explainable and will be treated in the respective chapters. 

Phosphanes are characterized, among other things, by their electron lone pair. This electron lone pair can be expected to be utilized in a cr-donor interaction toward a Lewis acid, making the phosphane a Lewis base. In fact, that is the reason for the popularity of phosphanes in transition metal chemistry. Of course, the Lewis basicity not only makes them good ligands, but lets phosphanes develop a rich and diverse main group chemistry as well. 

The same intramolecular Lewis base - Lewis acid interaction can be observed when a chlorophosphane is used instead of a fluorophosphane. However, the chloride is less strongly bonded than fluoride, resulting in the displacement of chloride by the phosphane without the use of an auxiliary Lewis acid. The chemical shift of the tricoordinate phosphorus atom is sensitive to the steric bulk of its carbon substituent. Evidently, sterically demanding substituents like tert-butyl hinder the 2T-bonding interaction from nitrogen, resulting in the observed downfield shift. 

Table 6.3 Intramolecular Lewis base - Lewis acid behavior of phosphanes...

The hypothetical phosphenium cation, PPh, a P(III) species, has recently attracted great interest. It acts as Lewis acid toward a range of Lewis bases, amongst which the phosphanes and carbenes are possibly the best known. The phosphorus resonance of the PPh part of the ensuing Lewis acid - Lewis base adduct is shifted upfield in accord with the nucleophilicity of the Lewis base employed. The Lewis base part, however, does not follow such a clear trend, hi particular, PMCj shows the same chemical shift value as PPhj, an effect that is consistently observed in similar adducts. 

Boron-nitrogen and boron-phosphorous compounds are classical textbook examples of donor-acceptor complexes. The chemical bonds of the Lewis-acid Lewis-base complexes are usually explained in terms of frontier orbital interactions and/or quasiclassical electrostatic attraction in the framework of the Hard and Soft Acid and Base (HSAB) model [73]. We were interested in seeing if the differences between the bond strengths of boron-nitrogen and boron-phosphorous complexes for different boranes, amines and phosphanes can be explained with the EDA method. 

Since phosphanes could be used as Lewis bases to perform electrochemical grafting, Giraudeau and coworkers prepared porphyrin dimers and trimers with... 

Lewis bases 1079 tertiary phosphane 1099 tertiary phosphines 172, 922... 

An interesting example of a Lewis acid/base reaction between neutral reactants is the formation of tris(n-butyl)phosphonium-dithiocarboxylate, ( -Bu)3P" — 82 , from tris(n-butyl)phosphane and carbon disulfide in solution. As expected, this equilibrium is strongly shifted in favour of the dipolar zwitterion with increasing solvent polarity (diethyl ether dimethyl sulfoxide) [272, 273]. 

Whittell, G.R., Balmond, E.L, Robertson, A.P.M., Patra, S.K., Haddow, M.F. and Manners, L, Reactions of amine- and phosphane-borane adducts with frustrated Lewis pair combinations of group 14 triflates and sterically hindered nitrogen bases. Ear. J. Inorg. Chem. 2010 (25), 3967-3975 (2010). 

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