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

Bimetallic 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. Herein, we strengthen 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.