Synthesis of a mononuclear, non-square-planar chromium(ii) bis(alkoxide) complex and its reactivity toward organic carbonyls and CO₂

Abstract

In this paper, we report the synthesis and reactivity of a rare mononuclear chromium(ii) bis(alkoxide) complex, Cr(OR')2(THF)2, that is supported by a new bulky alkoxide ligand (OR' = di-t-butyl-(3,5-diphenylphenyl)methoxide). The complex is prepared by protonolysis of square-planar Cr(N(SiMe3)2)2(THF)2 with HOR'. X-ray structure determination disclosed that Cr(OR')2(THF)2 features a distorted seesaw geometry, in contrast to nearly all other tetra-coordinate Cr(ii) complexes, which are square-planar. The reactivity of Cr(OR')2(THF)2 with aldehydes, ketones, and carbon dioxide was investigated. Treatment of Cr(OR')2(THF)2 with two equivalents of aromatic aldehydes ArCHO (ArCHO = benzaldehyde, 4-anisaldehyde, 4-trifluorbenzaldehyde, and 2,4,6-trimethylbenzaldehyde) leads cleanly to the formation of Cr(iv) diolate complexes Cr(OR')2(O2C2H2Ar2) that were characterized by UV-vis and IR spectroscopies and elemental analysis; the representative complex Cr(OR')2(O2C2H2Ph2) was characterized by X-ray crystallography. In contrast, no reductive coupling was observed for ketones: treatment of Cr(OR')2(THF)2 with one or two equivalents of benzophenone forms invariably a single ketone adduct Cr(OR')2(OCPh2) which does not react further. QM/MM calculations suggest the steric demands prevent ketone coupling, and demonstrate that a mononuclear Cr(iii) bis-aldehyde complex with partially reduced aldehydes is sufficient for C-C bond formation. The reaction of Cr(OR')2(THF)2 with CO2 leads to the insertion of CO2 into a Cr-OR' bond, followed by complex rearrangement to form a diamagnetic dinuclear paddlewheel complex Cr2(O2COR')4(THF)2, that was characterized by NMR, UV-vis, and IR spectroscopy, and X-ray crystallography.

Synthesis of Chromium(II) Complexes with Chelating Bis(alkoxide) Ligand and Their Reactions with Organoazides and Diazoalkanes

“…Homodinuclear complex 2 is structurally related to the previously reported alkoxide-bridged Cr 2 (OC t Bu 2 Ph) 4 [31], Cr 2 (OC t Bu 3 ) 2 (µ 2 -OC t Bu 2 H) 2 [39], and siloxide-bridged Cr 2 (OSi t Bu 3 ) 4 [40]. We note that monodentate alkoxides [OC t Bu 2 Ph] and [OC t Bu 2 (3,5-Ph 2 C 6 H 3 )] formed dinuclear or mononuclear complexes selectively [31,38], whereas the chelating ligand appears to support both types of geometries in the solid state, depending on the nature of the crystallization medium.…”

“…Treatment of the chromium(II) amide precursor with one equivalent of H 2 [OO] Ph in THF produces a green solid. Recrystallization of the product from the toluene/THF mixture yields mononuclear pale green Cr[OO] Ph (THF) 2 (1) related to the mononuclear Cr(OC t Bu 2 (3,5-Ph 2 C 6 H 3 )) 2 (THF) 2 [38]. In contrast, recrystallization of the product from a mixture of two polar solvents (CH 2 Cl 2 and THF) forms pale green-blue homodinuclear complex Cr 2 [OO] Ph 2 (2, Scheme 1) isolated in 79% yield.…”

A bis‐Pyrazolate Pincer on Reduced Cr Deoxygenates CO2: Selective Capture of the Derived Oxide by CrII

“…[12] Ac hromium example includes the reaction of Cr(OR') 2 (THF) 2 carrying ab ulky alkoxide with CO 2 promptly gives the diamagneticp addlewheel compound Cr 2 (O 2 COR') 4 (THF) 2 . [13] Another example is the reaction of aC o I synthon with CO 2 ,w hich proceeds in a3:2 Co:CO 2 stoichiometry,t og ive aC o I carbonyl and aC o 2 II (CO 3 2À )p roduct. [4] Furthercharacterization…”

“…Carbonate is not a frequently characterized co‐product of CO 2 deoxygenation, and carbonate bridging two Cr II is also unusual . A chromium example includes the reaction of Cr(OR′) 2 (THF) 2 carrying a bulky alkoxide with CO 2 promptly gives the diamagnetic paddlewheel compound Cr 2 (O 2 COR′) 4 (THF) 2 . Another example is the reaction of a Co I synthon with CO 2 , which proceeds in a 3:2 Co:CO 2 stoichiometry, to give a Co I carbonyl and a Co 2 II (CO 3 2− ) product …”

Hydrodechlorination of Dichloromethane by a Metal-Free Triazole-Porphyrin Electrocatalyst: Demonstration of Main-Group Element Electrocatalysis

“…[6][7][8] Transition metals are particularly attractive because of their unfilled d-orbitals that could facilitate electron exchange. 2,[9][10][11] While these conditions can afford high yields and efficiencies, they often require the use of expensive and rare metals, and also raise stability concerns such as catalyst poisoning, fouling, or decomposition. 12 Metal-free catalysts are versatile because they can provide a less expensive and more stable reaction pathway due to the absence of metal centers.…”