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A Dimeric Chromium(II) Pincer as an Electron Shuttle for N=N Bond Scission
Author(s) -
Labrum Nicholas S.,
Cabelof Alyssa C.,
Caulton Kenneth G.
Publication year - 2020
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.202001749
Subject(s) - chemistry , crystallography , azobenzene , bond cleavage , medicinal chemistry , adduct , pyridine , electron deficiency , stereochemistry , molecule , organic chemistry , catalysis
Reduction of the bis‐pyrazolyl pyridine complex [CrL] 2 with 4 KC 8 , followed by addition of one azobenzene (overall mole ratio 1:4:1), PhNNPh, transfers reducing equivalents to three azobenzenes, to form [K 3 Cr(PhNNPh) 3 ]. This has three κ 2 PhNNPh 2− ligands and K + bound to nitrogen atoms of azobenzene. When the stoichiometry is modified to 1:4:3, the product is changed to [K 2 CrL(PhNNPh) 2 ], which has C 2 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] 2 alone is shown to be a two‐electron reductant towards benzo[ c ]cinnoline (BCC) resulting in a product of formula [Cr 2 L 2 (BCC)], in which the reducing equivalents originate purely from Cr II . An analogous study of the reaction of [CrL] 2 with azobenzene yields [Cr 2 L 2 (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 2 L 2 unit to bind and reduce the N=N bond. Collectively, these results show that two divalent Cr, without added K 0 , have the ability to reduce the N=N bond. Further KC 8 reduction of preformed Cr 2 L 2 (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 8 leads to complete cleavage of the N=N bond, and both N bind to three Cr II . The varied impacts of the K + electrophile on NN multiple bond reduction is discussed.