The Electronic Structure of a Beta-Diketiminate Manganese Hydride Dimer

Abstract

The electronic structure of a dimeric manganese hydride catalyst supported by beta-diketiminate ligands, [(2,6-iPr2PhBDI)Mn(mu-H)](2), was investigated with density functional theory. A triple bond between the manganese centres was anticipated from simple electron-counting rules; however, calculations revealed Mn-Mn Mayer bond orders of 0.21 and 0.27 for the ferromagnetically-coupled and antiferromagnetically-coupled extremes, respectively. In accordance with experimentally determined Heisenberg exchange coupling constants of -15 +/- 0.1 cm(-1) (SQUID) and -10.2 +/- 0.7 cm(-1) (EPR), the calculated J(0) value of -10.9 cm(-1) confirmed that the ground state involves antiferromagnetic coupling between high spin Mn(II)-d(5) centres. The effect of steric bulk on the bond order was examined via a model study with the least sterically-demanding version of the beta-diketiminate ligand and was found to be negligible. Mixing between metal- and beta-diketiminate-based orbitals was found to be responsible for the absence of a metal-metal multiple bond. The bridging hydrides give rise to a relatively close positioning of the metal centres, while bridging atoms possessing 2p orbitals result in longer Mn-Mn distances and more stable dimers. The synthesis and characterization of the bridging hydroxide variant, [(2,6-iPr2PhBDI)Mn(mu-OH)](2), provides experimental support for these assessments.