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Reactivity of an Unusual Divalent Chromium Aggregate Supported by a Multifunctional Bis(pyrazolate) Pincer Ligand
Author(s) -
Labrum Nicholas S.,
Pink Maren,
Chen ChunHsing,
Caulton Kenneth G.
Publication year - 2019
Publication title -
european journal of inorganic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 1434-1948
DOI - 10.1002/ejic.201900033
Subject(s) - chemistry , chromium , reactivity (psychology) , iminium , medicinal chemistry , pyridine , nucleophile , ligand (biochemistry) , pyridinium , inorganic chemistry , stereochemistry , crystallography , organic chemistry , ion , catalysis , medicine , biochemistry , alternative medicine , receptor , pathology
Reaction of Cr[N(SiMe 3 ) 2 ] 2 (THF) 2 with equimolar bis‐pyrazolylpyridine, H 2 L, yields a product of empirical formula CrL. The kinetic product is [CrL] 2 , which transforms into the thermodynamic product, the S 4 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 C 2 symmetric anion [L 2 Cr 2 Cl] – . 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)Cr 2 (µ‐OH)(OPh)(OH 2 )], 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)Cr 2 (µ‐OH)(RCOO)(OH 2 )] are formed and characterized for benzoic and naphthoic acids. With naphthol, another mixed valent product, containing Cr II Cr III , 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 H 2 , which is released; in every case the conjugate base of HX binds to the otherwise unsaturated Cr. Mechanistic aspects of reduction of protons to H 2 exhibited here are analyzed for their impact on this conversion using various reduced chromium species.