Coordinatively Unsaturated Derivatives of Group 6 Metal Carbonyls Containing the pi-Donating Ligand 3,5-Di-tert-butylcatecholate

The series of group 6 metal tricarbonyl derivatives of di-tert-butylcatecholate have been synthesized from the reactions of M(CO)(5)THF (M = Cr, Mo, W) with 2 equiv of Et(4)N]3,5-(t)Bu(2)OC(6)H(2)OH]. Subsequent removal of the free catechol was achieved by the addition of NaOMe. The complexes were shown by X-ray crystallography to exhibit coordinatively unsaturated M degrees centers. These metal dianions which have formally 16e(-) configurations are stabilized by pi-donation from the oxygen atoms of the catecholate ligand. This is evident from the short M-O bond distances, e.g., for M = W, 2.059(6) ? vs 2.151(4) ? for a single bond. The structures of these five-coordinate dianions can be loosely defined as trigonal bipyramidal with the more electron-rich oxygen donor of the catecholate (ortho to the electron-releasing tert-butyl substituent) occupying an equatorial site as indicated by a shorter M-O bond length. The tungsten derivative was shown to reversibly react with CO or phosphines to afford the 18e(-), saturated complexes. Although the molybdenum tricarbonyl derivative reacts with CO to partially provide the tetracarbonyl complex, the analogous process involving chromium did not occur. That is, the formation of an O-->M pi bond vs an additional M-CO bond is favored for M = chromium. Complex 2, Et(4)N](2)W(CO)(4)DTBCat], crystallizes in the monoclinic space group P2(1)/c with a = 10.013(5) ?, b = 43.921(14) ?, c = 9.113(4) ?, beta = 115.76(3) degrees, V = 3609(3) ?(3), and d(calc) = 1.429 g/cm(3), for Z = 4. Complex 4, Et(4)N](2)Mo(CO)(3)DTBCat], crystallized in the monoclinic space group C2 with a = 18.255(7) ?, b = 8.596(3) ?, c = 22.369(7) ?, beta = 91.05(6) degrees, V = 3510(2) ?(3), and d(calc) = 1.251 g/cm(3), for Z = 4. Similarly, complex 5, Et(4)N](2)Cr(CO)(3)DTBCat], crystallized in the monoclinic space group C2 with a = 18.09(2) ?, b = 8.553(3) ?, c = 21.927(11) ?, beta = 91.09(8) degrees, V = 3393(4) ?(3), and d(calc) = 1.208 g/cm(3), for Z = 4.