Catalytic conversion of nitrogen to ammonia by a molecular Fe model complex

The reduction of N(2) to NH(3) is a requisite transformation for life.(1) While it is widely appreciated that the iron-rich cofactors of nitrogenase enzymes facilitate this transformation,(2-5) how they do so remains poorly understood. A central element of debate has been the exact site(s) of nitrogen coordination and reduction.(6,7) The synthetic inorganic community placed an early emphasis on Mo(8), because Mo was thought to be an essential element of nitrogenases(3) and because pioneering work by Chatt and coworkers established that well-defined Mo model complexes could mediate the stoichiometric conversion of N(2) to NH(3).(9) This chemical transformation can be performed in a catalytic fashion by two well-defined molecular systems that feature Mo centres.(10,11) However, it is now thought that Fe is the only transition metal essential to all nitrogenases,(3) and recent biochemical and spectroscopic data has implicated Fe instead of Mo as the site of N(2) binding in the FeMo-cofactor.(12) In this work, we describe a tris(phosphine)borane-supported Fe complex that catalyzes the reduction of N(2) to NH(3) under mild conditions, wherein >40% of the H(+)/e(-) equivalents are delivered to N(2). Our results indicate that a single Fe site may be capable of stabilizing the various N(x)H(y) intermediates generated en route to catalytic NH(3) formation. Geometric tunability at Fe imparted by a flexible Fe-B interaction in our model system appears to be important for efficient catalysis.(13-15) We propose that the interstitial light C-atom recently assigned in the nitrogenase cofactor may play a similar role,(16,17) perhaps by enabling a single Fe site to mediate the enzymatic catalysis via a flexible Fe-C interaction.(18)