Reactivity of an Unusual Divalent Chromium Aggregate Supported by a Multifunctional Bis(pyrazolate) Pincer Ligand

Abstract

Reaction of Cr[N(SiMe3)2]2(THF)2 with equimolar bis‐pyrazolylpyridine, H2L, yields a product of empirical formula CrL. The kinetic product is [CrL]2, which transforms into the thermodynamic product, the S4 symmetric tetramer [CrL]4, with aggregation by bridging via half of the pyrazolate β‐nitrogen atoms. Dimeric [CrL]2 reacts with DMAP (4‐dimethylamino pyridine) to form square pyramidal LCr(DMAP)2, which weakly binds a third DMAP at low temperatures; that weak binding is associated with the Jahn–Teller effect which already weakens the axial Cr/DMAP bond in LCr(DMAP)2. To confirm its dimeric character, reaction of [CrL]2 with bis‐triphenyl iminium salt of chloride is shown to give the C2 symmetric anion [L2Cr2Cl]–. Reaction of phenol with [CrL]2 in the presence of adventitious water occurs with oxidation of half of the chromium, forming the hydroxyl‐bridged mixed‐valent species [(HL)(L)Cr2(µ‐OH)(OPh)(OH2)], containing five coordinate Cr(II) and six coordinate Cr(III); this multifunctional molecule exhibits several examples of hydrogen bonding. Wholly analogous mixed valent products [(HL)(L)Cr2(µ‐OH)(RCOO)(OH2)] are formed and characterized for benzoic and naphthoic acids. With naphthol, another mixed valent product, containing CrIICrIII, is obtained and characterized. This proton responsive reactivity is generalized to a number of acids HX, to yield mixed valent chromium aggregates where some protons are on pyrazole, and others have been reduced to H2, which is released; in every case the conjugate base of HX binds to the otherwise unsaturated Cr. Mechanistic aspects of reduction of protons to H2 exhibited here are analyzed for their impact on this conversion using various reduced chromium species.