Measuring the Magnetic Anisotropy of Metal-Metal Multiple Bonds: The Importance of Correcting for Ligand Effects

We describe the synthesis and characterization of the quadruply-bonded dimer Mo₂(CH₂NMe₂BH₃)₄ in which each molybdenum(II) center is bound to two chelating boranatodimethylaminomethyl (BDAM) ligands. The BDAM anions bind to the metal at one end by a metal-carbon σ bond and at the other by a three-center M-H-B interaction. Each BDAM ligand chelates to a single Mo atom so that the metal-metal bond is unbridged; the Mo-Mo distance is 2.114(2) Å. Structural and solution NMR data, analyzed via McConnell's equation and supported by DFT calculations, show that the magnetic anisotropies associated with highly polarizable and π-bonding ligands (such as chloride groups and aryl rings) can greatly affect the NMR chemical shifts of reporter groups, so that ignoring their contributions leads to significant overestimates of the anisotropy due just to the metal-metal bond. We propose a method to quantify and correct for the magnetic anisotropy effects arising from the ligands. Application of this method to Mo₂(BDAM)₄ indicates that the magnetic anisotropy of the Mo-Mo quadruple bond in this molecule is about -800 × 10^-36 m³ molecule^-1. Anisotropies significantly higher than this value (as sometimes reported in the prior literature) are most likely incorrect.