Carbon Nitride as a Ligand: Synthesis, Characterisation and Application

Carbon nitride’s properties can be tuned through the coordination of metal atoms, which can lead to enhanced catalytic activity. However, to date, there are few reported examples of inner-sphere coordination of metal complex fragments to carbon nitride. Therefore, the effects of coordination of metal complex fragments to carbon nitride haves been investigated. Reaction of the rhenium carbonyl complex, [ReCl(CO)5] with the surface of urea-derived carbon nitride (UCN) results in [ReCl(CO)3(UCN)], with a rhenium concentration of 0.39 mmol g-1. The synthesis of the manganese analogue resulted in manganese oxidation leading to [Mn(UCN)] ([Mn] = 0.24 mmol g-1). Infrared spectroscopy, along with crystal structures of molecular analogues, [MCl(CO)3(DMNA-κ2N, N’)] (M = Re, Mn) was used to gain insight into the coordination of metals complex fragments to carbon nitride. Two morphologies of carbon nitride, unstructured urea-derived carbon nitride (UCN) and porous cyanamide derived carbon nitride (CCN), were then decorated with [Ru(bpy)2]2+ fragments. The carbon nitride structure affected metal loading, as [Ru(bpy)2(UCN)](PF6)2 ([Ru] = 0.016 mmol g-1) showed lower metal loading compared to [Ru(bpy)2(CCN)](PF6)2 ([Ru] = 0.076 mmol g-1). [Ru(bpy)2(DMNA-κ2N, N’)](PF6)2 was synthesised as a molecular analogue to gain insight into the coordination mode. The photocatalytic activities of [Ru(bpy)2(UCN)](PF6)2 and [Ru(bpy)2(UCN)](PF6)2 were completely inhibited compared to the undecorated materials. EPR and photoluminescence suggested the presence of rapid, efficient quenching of excited states in ruthenium decorated carbon nitride. [IrCl2Cp*(UCN)] ([Ir] = 0.069 mmol g-1) was synthesised to design a novel, recyclable hydrogenation catalyst. Direct hydrogenation reactions were carried out using hexane as a solvent, and despite low activity, [IrCl2Cp*(UCN)] showed good selectivity toward terminal alkenes and over 80% of catalytic activity was retained after 5 catalytic runs. Direct coordination of metal complex fragments to carbon nitride is shown to be a viable route to tuning the properties of carbon nitride and developing recyclable novel catalysts.