Dr Paul Newman - MSc PhD MRSC
Unlike acyclic phosphines and azacycles, the coordination chemistry of cyclic phosphines (phosphacycles) is relatively poorly developed. We are interested in the synthesis of innately asymmetric, structurally elaborate phosphines derived from readily available ‘chiral pool’ precursors. The structure of one such ligand (phenop) is shown below. Phenop has eight stereogenic carbon centres (four of which derive from the camphor starting material and four of which are generated stereoselectively during the synthesis) and a pseudo-chiral phosphorus. Phenop forms numerous complexes with a range of transition metals in various oxidation states and undergoes facile cyclometallation (internal C-H activation) with Pd(II) and Pt(II) to give P,C chelates as shown in 2. The observed cyclometallation is unusual for two reasons: firstly the activated C-H bond is of an alkyl type rather than the more common aryl; and secondly, the actual site of C-H activation depends on the nature of the metal precursor. For example the use of Na2[PdCl4] gives the complex 1 whereas palladium(II) acetate produces 2.
We are also interested in other fused cyclic systems, a bicyclic example of which is shown in 3 together with a bidentate derivative (4) and the molecular structure of its complex with Mo(CO)4.
Fused expanded-ring NHCs
N-heterocyclic carbenes (NHCs) have achieved great prominence in recent years as ligands of outstanding promise and application in coordination chemistry and homogeneous catalysis. NHCs where the constituent heterocyclic ring(s) is >5 membered are referred to as expanded-ring derivatives and they possess unusual properties such as increased basicity / nucleophilicity and enhanced sterics that may lead to novel reactivity in their metal complexes. We (together with Prof Cavell) are interested in fused 6/7-membered NHCs with exo-cyclic N-substituents containing other potential donors such as those derived from azolium salts of the type shown below (5). The N,N’-dipyridyl derivative of 5 undergoes room temperature oxidative addition to Ni(1,5-COD)2 to give the pincer complex shown in the figure below. This is unusual chemistry for expanded-ring azolium salts which do not tend to undergo oxidative addition as readily as 5-membered NHCs.