MECHANISTIC STUDY OF THE CYCLOMETALATION OF ORTHO-ARYLPHENOXIDE LIGANDS AT GROUP-5 METAL CENTERS

A series of 4-methylbenzyl derivatives of niobium and tantalum of general formula M(OAr)2-(CH2C6H4-4-Me)3 (OAr = various 2,6-diaryl- or 2,4,6-triarylphenoxides) have been synthesized. Thermolysis of these compounds at temperatures ranging from 100 to 150-degrees-C is found to lead to the elimination of 1 equiv of p-xylene and formation of a series of cyclometalated compounds in which intramolecular activation of the aromatic CH bond of one of the substituent aryl rings of an aryl oxide ligand had taken place. A mechanistic investigation of the reactivity was carried by utilizing 2,4,6-triarylphenoxide groups in which the 2-aryl group contained the para substituent CH3, CH3O, and Cl. The product distribution of these competitive ring-closure reactions was estimated by using H-1 NMR spectroscopy. The effect of substituents within the aryl ring upon the rate of ring closure was found to be slight. It was found, however, that irrespective of whether an electron-donating (CH3) or electron-withdrawing (Cl) substituent was used the substituted ring underwent metalation preferentially over the nonsubstituted ring. The results of this study are inconsistent with these reactions following an electrophilic aromatic substitution pathway proceeding via an arenium ion intermediate being operative for these reactions. A structural study of the tris(4-methylbenzyl) substrates M(OC6H3Ph2-2,6)2(CH2C6H4Me)3 (M = Nb, Ta) showed the compounds to be isomorphous in the solid state with trans, axial aryl oxide ligands and three equatorial 4-methylbenzyl groups about the metal center. Crystal data at 22-degrees-C for Nb(OC6H3Ph2-2,6)2(CH2C6H4Me)3 are as follows: a = 13.008 (3) angstrom, b = 14.356 (3) angstrom, c = 15.235 (2) angstrom, alpha = 105.49 (1)-degrees, beta = 100.98 (1)-degrees, gamma = 91.46 (2)-degrees, Z = 2, d(calcd) = 1.227 g cm-3 in space group P1BAR. Crystal data for Ta(OC6H3Ph2-2,6)2(CH2C6H4Me)3 at 20-degrees-C are as follows: a = 12.982 (2) angstrom, b = 14.348 (2) angstrom, c = 15.210 (5) angstrom, alpha = 105.49 (2)-degrees, beta = 100.81 (2)-degrees, gamma = 01.36 (2)-degrees, Z = 2, d(calcd) = 1.341 g cm-3 in space group P1BAR. Both compounds contain one toluene solvate molecule per metal.