CHROMIUM TRICARBONYL CATECHOLATE DERIVATIVES - STRUCTURAL AND REACTIVITY STUDIES OF 16-ELECTRON COMPLEXES

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)(5)THF 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 C-13 NMR spectroscopies and in the solid-state by X-ray crystallography. Complex 1 crystallizes in the monoclinic space group P2(1)/c with a 17.532(4) Angstrom, b = 10.541-(4) Angstrom, c = 17.356(7) Angstrom, beta = 95.18(3)degrees, V = 3194(2) Angstrom(3), and d(calc) = 1.278 g/cm(3), for Z = 4. Complex 2 crystallizes in the monoclinic space group P2(1)/n with a = 10.068(8) Angstrom, b = 20.97(2) Angstrom, c = 12.933(9) Angstrom, beta = 94.58(6)degrees, V = 2722(4) Angstrom(3), and d(calc) = 1.233 g/cm(3), 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[3] and 1.998[9] Angstrom, respectively. Hence, the electronic unsaturation of the chromium center is mitigated in part by pi-donation from the catecholate. This phenomenon is enhanced in the absence of intermolecular hydrogen bonding. Evidence for an increase in the pi-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 C-13 NMR spectroscopies. That is, in this instance the v(CO) vibrations occur at lower frequences and the C-13 CO resonances are shifted downfield. Both complexes 1 and 2 are unreactive toward good donor ligands such as PMe(3); however, they react to varying extents with P(OMe)(3) and CO, with complex 1 being more reactive.