μ2‐η1:η1‐N2 Bridged Bimetallic Dinitrogen Complexes: Geometry of the First Excited State in Connection to N2 π‐Photoactivation

Abstract

AbstractBimetallic end‐on μ2‐η1:η1‐N2 bridging dinitrogen complexes have served as the platform for photochemical N2 activation, mainly for the N−N cleavage. However, the alternate N−N π‐photoactivation route has remained largely unexplored. This study strengthens the notion of weakening the N−N bond through the population of π* orbital upon electronic excitation from the ground to the first excited state using four prototypical complexes based on Fe (1), Mo (2), and Ru (3,4). The complexes 1–4 possess characteristic N−N π* based LUMO (π*‐π*‐π*) centered on their M−N−N−M core, which was earlier postulated to play a central role in the N2 photoactivation. Vertical electronic excitation of the highest oscillator strength involves transitions to the N−N π*‐based acceptor orbital (π*‐π*‐π*) in complexes 1–4. This induces geometry relaxation of the first excited metal‐to‐nitrogen (π*) charge transfer (1MNCT) state leading to a “zigzag” M−N−N−M core in the equilibrium structure. Obtaining the equilibrium geometry in the first excited state with the full‐sized complexes widens the scope of N−N π‐photoactivation with μ2‐η1:η1‐N2 bridging dinitrogen complexes. Promisingly, the elongated N−N bond and bent ∠MNN angle in the photoexcited S1 state of 1–4 resemble their radical‐ and di‐anion forms, which lead toward thermodynamically feasible N−N protonation in the S1 excited state.