A Dimeric Chromium(II) Pincer as an Electron Shuttle for N=N Bond Scission.

Reduction of the bis-pyrazolyl pyridine complex [CrL] with 4 KC , followed by addition of one azobenzene (overall mole ratio 1:4:1), PhNNPh, transfers reducing equivalents to three azobenzenes, to form [K Cr(PhNNPh) ]. This has three κ PhNNPh ligands and K bound to nitrogen atoms of azobenzene. When the stoichiometry is modified to 1:4:3, the product is changed to [K CrL(PhNNPh) ], which has C symmetry except for the intimate ion pairing of two K ions to reduced azobenzene nitrogen atoms, and to pyrazolate and phenyl rings. The origin of the observed delivery of reducing equivalents to several, not to a single N=N bond, is traced to the resistance of the one-electron-reduced substrate to receiving a second electron, and is thus a general phenomenon. [CrL] alone is shown to be a two-electron reductant towards benzo[c]cinnoline (BCC) resulting in a product of formula [Cr L (BCC)], in which the reducing equivalents originate purely from Cr . An analogous study of the reaction of [CrL] with azobenzene yields [Cr L (PhNNPh)(THF)], an adduct in which one THF has displaced one of four hydrazide nitrogen/Cr bonds. Together these illustrate different modes for the Cr L unit to bind and reduce the N=N bond. Collectively, these results show that two divalent Cr, without added K , have the ability to reduce the N=N bond. Further KC reduction of preformed Cr L (RNNR) inevitably gives products in which K stabilizes the charge in the increasingly electron-rich nitrogen atoms, in a phenomenon which mimics proton coupled electron transfer: K performs the role of H . A least-squares fit of the two singly reduced DFT structures shows that the only major change is a re-orientation of one of the two phenyl rings in order to avoid repulsion with potassium but to still allow interaction of that phenyl π system with K . This shows both the impact of K , being modest to nitrogen/chromium interactions, but nevertheless accommodating some π donation of phenyl to potassium. Finally, delivering increasing equivalents of KC leads to complete cleavage of the N=N bond, and both N bind to three Cr . The varied impacts of the K electrophile on NN multiple bond reduction is discussed.