Di- and trivalent chromium bis(pyrazol-3-yl)pyridine pincer complexes with good leaving groups

Recent Developments in Reactions and Catalysis of Protic Pyrazole Complexes

Protic pyrazoles (N-unsubstituted pyrazoles) have been versatile ligands in various fields, such as materials chemistry and homogeneous catalysis, owing to their proton-responsive nature. This review provides an overview of the reactivities of protic pyrazole complexes. The coordination chemistry of pincer-type 2,6-bis(1H-pyrazol-3-yl)pyridines is first surveyed as a class of compounds for which significant advances have made in the last decade. The stoichiometric reactivities of protic pyrazole complexes with inorganic nitrogenous compounds are then described, which possibly relates to the inorganic nitrogen cycle in nature. The last part of this article is devoted to outlining the catalytic application of protic pyrazole complexes, emphasizing the mechanistic aspect. The role of the NH group in the protic pyrazole ligand and resulting metal-ligand cooperation in these transformations are discussed.

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

Reduction of the bis-pyrazolyl pyridine complex [CrL]₂ with 4 KC₈ , followed by addition of one azobenzene (overall mole ratio 1:4:1), PhNNPh, transfers reducing equivalents to three azobenzenes, to form [K₃ Cr(PhNNPh)₃ ]. This has three κ² 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 [K₂ CrL(PhNNPh)₂ ], which has C₂ 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 [Cr₂ L₂ (BCC)], in which the reducing equivalents originate purely from Cr^II . An analogous study of the reaction of [CrL]₂ with azobenzene yields [Cr₂ L₂ (PhNNPh)(THF)], an adduct in which one THF has displaced one of four hydrazide nitrogen/Cr bonds. Together these illustrate different modes for the Cr₂ L₂ unit to bind and reduce the N=N bond. Collectively, these results show that two divalent Cr, without added K⁰ , have the ability to reduce the N=N bond. Further KC₈ reduction of preformed Cr₂ L₂ (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 KC₈ 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.

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

Dimeric [CrL]₂ , where L is the conjugate base of bis-pyrazolyl pyridine, is evaluated for its ability to undergo inner sphere and outer sphere redox chemistry. It reacts with Cp₂ Fe⁺ to give [Cr₄ (HL)₄ (μ₄ -O)]²⁺ , still containing divalent Cr. Reduction (KC₈ ) of [CrL]₂ by two electrons gives [K₂ (THF)₃ Cr₃ L₃ (μ₃ -O)], and by four electrons gives [K₄ (THF)₁₀ Cr₂ L₂ (μ-O)], each of which has scavenged (hydr)oxide from glass surface because of the electrophilicity of the underligated Cr. [K₄ (THF)₁₀ Cr₂ L₂ (μ-O)], is shown by comprehensive DFT calculations and analysis of intra-ligand bond lengths to contain a pyridyl radical L^3- and Cr^II , illustrating that this pincer is proton-responsive, redox active, and a versatile donor to associated K⁺ ions here. The K⁺ electrophiles interact with electron-rich oxo, but do not significantly (>5 kcal mol^-1 ) alter spin state energies. Inner sphere oxidation of [CrL]₂ with a quinone gives [Cr₂ L₂ (semiquinone)₂ ], while pre-reduced [CrL]₂ ^2- reacts with quinone to give [K₃ (THF)₃ Cr₂ L₂ (catecholate)₂ (μ-OH)], a product of capture of two undercoordinated LCr(catecholate)^1- by hydroxide.

[Cr(pincer2-)]2 as an electron shuttle for reductively promoted hydrazine disproportionation

We describe here delivery of hydrazine to a reducing, low oxidation state chromium bound to a proton responsive ligand which has already been deprotonated. Reaction of PhHNNH₂ at a 4 : 1 mole ratio with the bis-(pyrazolate)pyridyl pincer ligated reducing agent [Cr^IIL]₂ gives prompt conversion to [CrL₂(PhNH₂)₂(μ-PhHNN)], with release of NH₃ and C₆H₆, a new disproportionation of the hydrazine, with trapping of PhHNN as its dianion, bridging the two chromium centers. The redox balance of the reaction is discussed, and participation by Brønsted basic sites on the bis-(pyrazolate)pyridyl pincer ligand L^2- is suggested, but no hydrazine protons remain on the pincer in the product.