see selected recent publications [1] [3] [7] [10] and [11]
In addition to the use of ylidic molecules as ligands we have a complementary interest in the structure and bonding within the molecules themselves. Our strategy has been to synthesise simple (usually air- and moisture-sensitive) compounds and characterise them by X-ray and neutron diffraction. Our interest was stimulated by the observation that coordination of phosphorus ylides to lithium has almost no effect on the P-C(ylide) bond length, which appears to be in conflict with current bonding theories involving significant back bonding from the ylidic carbon atom to phosphorus. The same is so for iminophosphoranes. Indeed, in the latter case we have even observed a decrease in the P-N bond length upon deprotonation. These observations are more in accord with an electrostatic single-bond model to explain the shortness of the P-C(ylide) bond. In order to test this hypothesis we have characterised a number of ylides and iminophosphoranes with some unexpected results. For example, in a recent structure of triphenylphosphonium benzylide we observed short intermolecular C-H…C interactions (Figure 8), which for the first time provides experimental evidence to support previous speculation that such interactions are important in the solution dynamics of ylides. Extension of this work to arsonium ylides (Figure 9) has recently led to some interesting and unexpected results. As a part of this programme we have also obtained a neutron structure of the parent iminotriphenylphosphorane (Figure 10), which enabled accurate location of the NH group. This study suggests that the bonding situation in iminophosphoranes may be more similar to that in ylides than has been previously appreciated. We are currently performing theoretical and experimental charge density studies on a series of these molecules in order to probe the nature of bonding within them more fully.
We have recently discovered a facile and high yield route for the preparation of novel zwitterionic vinylphosphonium salts of the general formula R3P+C(R’)C=C(R’’)C6H4O- by reaction of phosphorus ylides with certain Schiff bases and have characterised products by X-ray (Figure 11) and neutron diffraction (Figure 12). Perhaps of more significance than the new synthetic route, or the unprecedented solid state structures of these compounds is that some of them exhibit both negative solvatochromism and reversible photochromism in solution, both of which are properties relevant to the design of advanced materials. The observed photochromism may have implications for the design of so-called molecular switches for use in optical data storage devices. Theoretical calculations on various aspects of this work are ongoing. The observation of strong negative solvatochromism in these compounds has a direct relevance to the design of nonlinear optical materials. In the light of the interesting properties of these materials we are currently examining other synthetic routes to similar compounds with the aim to study the optical properties of a greater range of materials.
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