Chromium Tricarbonyl Catecholate Derivatives. Structural and Reactivity Studies of "16-Electron" Complexes
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The formally “16-electron” chromium tricarbonyl catecholate derivative containing a hydrogen-bonded catechol (1) has been synthesized from the labile ligand displacement reaction of Cr(CO)5THF with 2 equiv of the monoprotonated tetraethylammonium salt of catechol in tetrahydrofuran. Complex 2, the catechol-free derivative, has also been obtained by the removal of the free catechol by the addition of NaOMe. Both complexes have been characterized in solution by infrared and 13C NMR spectroscopies and in the solid-state by X-ray crystallography. Complex 1 crystallizes in the monoclinic space group P21/c with a = 17.532(4) Å, b = 10.541-(4) Å, c = 17.356(7) Å, β = 95.18(3)°, V = 3194(2) Å3, and dcalc = 1.278 g/cm3, for Z = 4. Complex 2 crystallizes in the monoclinic space group P21/n with a = 10.068(8) Å, b = 20.97(2) Å, c = 12.933(9) A, β = 94.58(6)°, V = 2722(4) Å3, and dcalc = 1.233 g/cm3, for Z = 4, from CH3CN/toluene/THF. The coordination geometry of the dianion in each case is intermediate between square pyramidal and trigonal bipyramidal, with average Cr-O bond lengths of 2.021  and 1.998 Å, respectively. Hence, the electronic unsaturation of the chromium center is mitigated in part by π-donation from the catecholate. This phenomenon is enhanced in the absence of intermolecular hydrogen bonding. Evidence for an increase in the π-donating ability of the catecholate ligand when it is not involved in hydrogen-bonding with an added proton donor, such as catechol or methanol, is noted in solution by infrared and,13C NMR spectroscopies. That is, in this instance the v(CO) vibrations occur at lower frequences and the l3C CO resonances are shifted downfield. Both complexes 1 and 2 are unreactive toward good donor ligands such as PMe3; however, they react to varying extents with P(OMe)3 and CO, with complex 1 being more reactive. © 1995, American Chemical Society. All rights reserved.
author list (cited authors)
Darensbourg, D. J., Klausmeyer, K. K., & Reibenspies, J. H.