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 (Et 4 N)[(Tp)MoFe 3 S 4 Cl 3 ] cubane have both been proposed to contain unusual spin‐coupled Mo III sites with an S (Mo)=1/2 non‐Hund configuration at the Mo atom. Herein, we present Fe and Mo L 3 ‐edge X‐ray magnetic circular dichroism (XMCD) spectroscopy of the (Et 4 N)[(Tp)MoFe 3 S 4 Cl 3 ] cubane and Fe L 2,3 ‐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 L 2,3 ‐edge XMCD within the intact MoFe protein. Further, Mo L 3 ‐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.