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

Abstract

Reduction of the bis-pyrazolyl pyridine complex [CrL] 2 with 4KC 8 ,f ollowed 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 k 2 PhNNPh 2À 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 pairingo ft wo K + ions to reduceda zobenzene nitrogen atoms, andt op yrazolate and phenylr ings. The origin of the observed delivery of reducinge quivalents to several, not to as ingle N=N bond, is traced to the resistance of the one-electron-reduced substrate to receiving as econd electron, and is thus ag eneral phenomenon. [CrL] 2 alone is shown to be atwo-electron reductant towards benzo[c]cinnoline (BCC) resulting in a product of formula [Cr 2 L 2 (BCC)],i nw hich the reducing equivalents originate purely from Cr II .A na nalogous study of the reaction of [CrL] 2 with azobenzene yields [Cr 2 L 2 (PhNNPh)(THF)],a na dduct in whicho ne THF has displaced one of four hydrazide nitrogen/Cr bonds.T ogether these illustrate different modes for the Cr 2 L 2 unit to bind and reduce the N=Nb ond. Collectively,t heser esultss how that two divalentC r, without added K 0 ,h avet he ability to reduce the N=Nb ond. 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 nitrogena toms, in ap henomenonw hich mimics proton coupled electron transfer:K + performs the role of H + .Aleast-squares fit of the two singlyr educed DFT structures shows that the only major change is ar e-orientationo fo ne of the two phenyl rings in ordert oa void repulsion with potassium but to still allow interaction of that phenyl p system with K + .T his shows both the impact of K + ,b eing modestt on itrogen/ chromium interactions, but nevertheless accommodating some p donation of phenyl to potassium. Finally,d elivering increasing equivalents of KC 8 leads to complete cleavage of the N=Nb ond, and both Nb ind to three Cr II .T he varied impacts of the K + electrophile on NN multiple bond reduction is discussed.

Recent Developments in Reactions and Catalysis of Protic Pyrazole Complexes

“…Only the benzo[c]cinnoline derivative undergoes N-N bond cleavage upon treatment with an excess of the reductant. Caulton's group demonstrated that the reaction of dichromium(II) complex 35 (vide supra) [57] with azobenzene yields the paramagnetic chromium(III) complex 77 having an unsymmetrically bridged PhNNPh unit (Scheme 23) [76]. The N-N distance of 1.471( 9 chloride counteranion.…”

“…An analogous complex of a tetradentate ligand having non-protic pyridylmethyl arms displays no catalytic activity, even in the presence of external protic pyrazole as a proton source, indicating that the proton-responsive unit in the second coordination sphere is responsible for the N-N bond cleavage. Caulton s group demonstrated that the reaction of dichromium(II) complex 35 (vide supra) [57] with azobenzene yields the paramagnetic chromium(III) complex 77 having an unsymmetrically bridged PhNNPh unit (Scheme 23) [76]. The N-N distance of 1.471( 9 Electron transfer from the dichloromium(II) complex 35 to phenylhydrazine gives rise to the formation of the phenylhydrazido(2−)-bridged di(aniline)dichromium(III) complex 78 (Scheme 23) [77].…”

The Influence of Nucleophilic and Redox Pincer Character as well as Alkali Metals on the Capture of Oxygen Substrates: The Case of Chromium(II)

“…This is a distinct difference from reaction of [CrL]2 with azobenzene, which reduces this by two electrons, in a sawhorse structure [Cr2L2(azobenzene 2-)] and then subsequent reduction by KC8 delivers electrons first to each of the two Cr, and only then are the electrons delivered to hydrazide, to give complete scission of the NN bond. [3] The established that [CrL]2 is not reducing enough to effect the twoelectron reduction of one benzoquinone to catecholate. We were interested in whether pre-reduction of [CrL]2 could further reduce the quinone.…”

“…In addition to the electrophile-responsiveness at the βN of the pincer, the 1,4-diazabutadiene moiety is envisioned to temporarily store electrons and thereby aid the metal in the reduction of substrate. [2] In a previous report, we leveraged the Cr II /Cr III redox couple for single-or two-electron transfer from [CrL]2 for N=N bond reduction [3] and reductively promoted phenylhydrazine disproportionation. [4] Prompted by the rich cyclic voltammogram of [CrL]2 (see SI) we sought to explore here the outer sphere oxidation and reduction of [CrL]2, and progressive electron transfer to 3,5-di-tertbutyl benzoquinone.…”