Multiple Modes for Coordination of Phenazine to Molybdenum: Ring Fusion Promotes Access to η⁴-Coordination, Oxidative Addition of Dihydrogen and Hydrogenation of Aromatic Nitrogen Compounds

Mo(PMe3)6 reacts with phenazine (PhzH) to give (η6-C6-PhzH)Mo(PMe3)3, (μ-η6,η6-PhzH)[Mo(PMe3)3]2 and (η4-C4-PhzH)2Mo(PMe3)2, each of which displays previously unknown coordination modes for phenazine. Both mononuclear (η6-C6-PhzH)Mo(PMe3)3 and dinuclear (μ-η6,η6-PhzH)[Mo(PMe3)3]2 react with H2 at room temperature to give the respective dihydride complexes, (η4-C4-PhzH)Mo(PMe3)3H2 and (μ-η6,η4-PhzH)[Mo(PMe3)3][Mo(PMe3)3H2]. A comparison of (η6-C6-PhzH)Mo(PMe3)3 with the anthracene (AnH) and acridine (AcrH) counterparts, (η6-AnH)Mo(PMe3)3 and (η6-C6-AcrH)Mo(PMe3)3, indicates that oxidative addition of H2 is promoted by incorporation of nitrogen substituents into the central ring. Furthermore, comparison of (η6-C6-PhzH)Mo(PMe3)3 with the quinoxaline (QoxH) analogue, (η6-C6-QoxH)Mo(PMe3)3, indicates that ring fusion also promotes oxidative addition of H2. The mononitrogen quinoline (QH) and acridine compounds, (η6-C6-QH)Mo(PMe3)3 and (η6-C6-AcrH)Mo(PMe3)3, which respectively possess two and three fused six-membered rings, exhibit a similar trend, with the former being inert towards H2, while the latter reacts rapidly to yield (η4-C4-AcrH)Mo(PMe3)3H2. Ring fusion also promotes hydrogenation of the heterocyclic ligand, with (η6-C6-AcrH)Mo(PMe3)3 releasing 9,10-dihydroacridine upon treatment with H2 in benzene at 95 °C. Furthermore, catalytic hydrogenation of acridine to a mixture of 9,10-dihydroacridine and 1,2,3,4-tetrahydroacridine may be achieved by treatment of (η6-C6-AcrH)Mo(PMe3)3 with acridine and H2 at 95 °C.