Measuring the Magnetic Anisotropy of Metal–Metal\nMultiple Bonds: The Importance of Correcting for Ligand Effects

We\ndescribe the synthesis and characterization of the quadruply-bonded\ndimer Mo<sub>2</sub>(CH<sub>2</sub>NMe<sub>2</sub>BH<sub>3</sub>)<sub>4</sub> in which each molybdenum(II) center is bound to two chelating\nboranatodimethylaminomethyl (BDAM) ligands. The BDAM anions bind to\nthe metal at one end by a metal–carbon σ bond and at\nthe other by a three-center M–H–B interaction. Each\nBDAM ligand chelates to a single Mo atom so that the metal–metal\nbond is unbridged; the Mo–Mo distance is 2.114(2) Å. Structural\nand solution NMR data, analyzed via McConnell’s equation and\nsupported by DFT calculations, show that the magnetic anisotropies\nassociated with highly polarizable and π-bonding ligands (such\nas chloride groups and aryl rings) can greatly affect the NMR chemical\nshifts of reporter groups, so that ignoring their contributions leads\nto significant overestimates of the anisotropy due just to the metal–metal\nbond. We propose a method to quantify and correct for the magnetic\nanisotropy effects arising from the ligands. Application of this method\nto Mo<sub>2</sub>(BDAM)<sub>4</sub> indicates that the magnetic anisotropy\nof the Mo–Mo quadruple bond in this molecule is about −800\n× 10<sup>–36</sup> m<sup>3</sup> molecule<sup>–1</sup>. Anisotropies significantly higher than this value (as sometimes\nreported in the prior literature) are most likely incorrect.