Abstract
Whereas synthetically catalyzed nitrogen reduction (N(2) R) to produce ammonia is widely studied, catalysis to instead produce hydrazine (N(2) H(4) ) has received less attention despite its considerable mechanistic interest. Herein, we disclose that irradiation of a tris(phosphine)borane (P(3) (B) ) Fe catalyst, P(3) (B) Fe(+) , significantly alters its product profile to increase N(2) H(4) versus NH(3) ; P(3) (B) Fe(+) is otherwise known to be highly selective for NH(3) . We posit a key terminal hydrazido intermediate, P(3) (B) Fe=NNH(2) , as selectivity-determining. Whereas its singlet ground state undergoes protonation to liberate NH(3) , a low-lying triplet excited state leads to reactivity at N(α) and formation of N(2) H(4) . Associated electrochemical and spectroscopic studies establish that N(2) H(4) lies along a unique product pathway; NH(3) is not produced from N(2) H(4) . Our findings are distinct from the canonical mechanism for hydrazine formation, which proceeds via a diazene (HN=NH) intermediate and showcase light as a tool to tailor selectivity.