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.