C‒H activation of caffeine at triruthenium and triosmium centers

Abstract

Reactions of caffeine with triruthenium and triosmium clusters are described. Caffeine reacts with [Ru3(CO)12] at 110°C to afford [Ru3(CO)10{µ,η1,κ1-C5N4(CH3)3O2}(µ-H)] (1) and [Ru3(CO)8{µ,η1,κ1-C5N4(CH3)3O2}2(µ-H)2] (2) which contain metalated caffeine ligand(s) formed via C‒H bond activation at imidazole ring carbon of caffeine. Similar reactions between caffeine and [Os3(CO)12] or [Os3(CO)10(µ-H)2] furnish only the osmium analogue of 1 namely [Os3(CO)10{µ,η1,κ1-C5N4(CH3)3O2}(µ-H)] (3). In all these products, the metalated caffeine ligand acts in bridging capacity using the free imidazole ring nitrogen and carbon atoms. Cluster 1 readily reacts with PPh3 at room temperature to yield mono- and bis-phosphine substituted derivatives [Ru3(CO)9(PPh3){µ,η1,κ1-C5N4(CH3)3O2}(µ-H)] (4) and [Ru3(CO)8(PPh3)2{µ,η1,κ1-C5N4(CH3)3O2}(µ-H)] (5), respectively. All the new clusters have been spectroscopically characterized together with single crystal X-ray diffraction studies of 1 and 3.