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.