Phenylcyanamide ligands and their metal complexes

Dedicated to Professor A.B.P. Lever
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Abstract

This review examines the various synthetic routes to phenylcyanamide ligands, and their physical characterization by electronic, NMR and IR spectroscopies, crystallography and theoretical methods. While the coordination chemistry of phenylcyanamide ligands is largely unexplored, a significant number of mononuclear coordination complexes of ruthenium, copper and nickel group ions have been synthesized. Topics to be covered are UV–vis, NMR and IR spectroscopy, linkage isomerism and crystallography, and finally cyclic voltammetry. Studies of the remarkable ability of the 1,4-dicyanamidobenzene dianion bridging ligand to mediate antiferromagnetic or resonance exchange in dinuclear ruthenium complexes will be reviewed.

Introduction

The coordination chemistry of phenylcyanamide ligands is expected to be as potentially rich as that of the pseudohalides (for example, azide [1] or thiocyanate [2]). However, very little has been done and it is only through recent efforts that this chemistry is being elucidated. The attachment of a phenyl ring to the cyanamide group adds an extra dimension not present in azide or thiocyanate ligands. An extensive π conjugation between the cyanamide group and the phenyl ring provides an energetically favorable means by which a metal ion can couple into a conjugated organic π system. This is demonstrated in this review by the extraordinary ability of 1,4-dicyanamidobenzene to mediate metal–metal coupling, the magnitude of which is dramatically dependent upon the nature of both inner and outer coordination spheres. The wealth of data provided by the study of these dinuclear systems has permitted a quantitative evaluation of metal–metal coupling within the context of Marcus–Hush theory [3] and the determination of metal–metal and metal–ligand coupling elements using charge-transfer band oscillator strengths [4]. The recent interest in the field of inorganic chemistry to develop novel hybrid materials that combine coordination and organic chemistry provides further impetus to this research.

Section snippets

Neutral phenylcyanamides (pcydH)

Phenylcyanamide derivatives can be readily prepared in high yields from the corresponding anilines [5], [6]. An aniline is reacted with benzoylthiocyanate in acetone to generate the benzoylthiourea. This is isolated and hydrolyzed in aqueous solution to form the thiourea, which is then desulfurized using Pb(II), as shown in Scheme 1.

Lead sulfide is filtered off and to the filtrate glacial acetic acid is added, precipitating the neutral phenylcyanamide. The above reaction does not appear to be

Physical properties of phenylcyanamides

The cyanamide group is a three-atom π-system in which the amine non-bonding electrons can delocalize into the nitrile π-bonds. Accordingly, the cyanamide group is expected to be a poorer π-acceptor but better donor than analogous nitrile ligands [18]. For this reason, cyanamides are expected to be less sensitive to base hydrolysis. The extent of delocalization of the amine lone pair into the phenyl group is influenced by the nature and the number of phenyl ring substitutents. Table 1 gives 13

Coordination geometry

The phenylcyanamides are ambidentate ligands whether neutral or in anionic form and so the possibility of linkage isomerism must be recognized. As shown below, neutral phenylcyanamides can coordinate to a metal ion through either the nitrile or the amine nitrogen. However, at this point in time, there are no crystal structures of neutral phenylcyanamides coordinated to metal ions. The amine nitrogen is sterically crowded by the phenyl ring and so terminal coordination to the nitrile is

Future studies

The coordination chemistry of phenylcyanamide ligands still requires much effort to complete. There are no examples of complexes of the early transition metals and of the middle and late transition metals, complexes of Fe, Os, Rh, Ir, Au and the Zn group are unknown. Neutral phenylcyanamide ligands should stabilize low-valent metals and may provide some unusual organometallic complexes. In this regard, the acidity of the amine proton and the possibility of side-on cyanamide group coordination

Acknowledgements

The financial support of the Natural Sciences and Engineering Research Council of Canada is gratefully recognized.

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