X-ray Magnetic Circular Dichroism Spectroscopy Applied to Nitrogenase and Related Models: Experimental Evidence for a Spin-Coupled Molybdenum(III) Center.

Nitrogenase enzymes catalyze the reduction of atmospheric dinitrogen to ammonia utilizing a Mo-7Fe-9S-C active site, the so-called FeMoco cluster. FeMoco and an analogous small-molecule (Et4 N)[(Tp)MoFe3 S4 Cl3 ] cubane have both been proposed to contain unusual spin-coupled MoIII sites with an S(Mo)=1/2 non-Hund configuration at the Mo atom. Herein, we present Fe and Mo L3 -edge X-ray magnetic circular dichroism (XMCD) spectroscopy of the (Et4 N)[(Tp)MoFe3 S4 Cl3 ] cubane and Fe L2,3 -edge XMCD spectroscopy of the MoFe protein (containing both FeMoco and the 8Fe-7S P-cluster active sites). As the P-clusters of MoFe protein have an S=0 total spin, these are effectively XMCD-silent at low temperature and high magnetic field, allowing for FeMoco to be selectively probed by Fe L2,3 -edge XMCD within the intact MoFe protein. Further, Mo L3 -edge XMCD spectroscopy of the cubane model has provided experimental support for a local S(Mo)=1/2 configuration, demonstrating the power and selectivity of XMCD.