A New Class of Rhodium(I) k1-P and k2-P,N Complexes with Rigid PTN(R) Ligands (PTN = 7-Phospha-3-methyl-1,3,5-triazabicyclo[3.3.1]nonane)

The open cage ligands PTN(R) (PTN = 7-R-phospha-3-methyl-1,3,5-triazabicyclo[3.3.1]nonane; R = Me, Ph), derived from cage cleavage of the water-soluble phosphine PTA (PTA = 1,3,5-triaza-7-phosphaadamantane), were used to synthesize neutral [RhCl(cod)(PTN(R)] (10) and [RhI(CO){PTN-(Me)}] (12) and cationic [Rh(cod){PTN(R)}][BArF4] (11) rhodium(I) complexes, which were comprehensively characterized both in solution and in the solid state (cod = 1,5-cyclooctadiene; BArF4 = B[3,5-(CF3)2C6H3]4). Two modes of coordination to the metal were identified, k1-P (for 10) and k2-P,N (11,12). For k1-P coordination, the free N donor site rests in close proximity to the Rh center imparted by the rigid PTN framework. The complexes were tested as catalysts for (biphasic) olefin hydroformylation and regioselective C=C bond reduction under transfer hydrogenation conditions, together with acetophenone hydrogenation. DFT calculations on simplified models helped to assess the bonding properties within the complexes and to determine the amount of cage strain that accompanies the structural modifications of the ligand when transformed from a k1-P to k2-P,N coordination mode.