THE CYCLOMETALLATION OF 2,6-DIPHENYLPHENOXIDE (OAR-2,6PH2) LIGANDS AT NIOBIUM(V) AND TANTALUM(V) METAL CENTERS - THE SOLID-STATE STRUCTURES OF THE MONOCYCLOMETALLATED COMPOUNDS TA(OC6H3PH-C6H4)(OAR-2,6PH2)2(R) (R = CH3, NORMAL-C4H9) AND NB(OC6H3PH-C6H4)(OAR-2,6PH2)2(CL)

Title full: The cyclometallation of 2,6-diphenylphenoxide (OAr-2,6Ph2) ligands at niobium(V) and tantalum(V) metal centres: The solid state structures of the monocyclometallated compounds Ta(OC6H3Ph-C6H4)(OAr-2,6Ph 2)2(R) (R = CH3, n-C4H9) and Nb(OC6H3Ph-C6H4)(OAr-2,6Ph 2)2(Cl). The dimethyl compounds M(OAr-2,6Ph2)3(CH3)2 (M = Nb, Ta; OAr-2,6Ph2 = 2,6-diphenylphenoxide) undergo the thermal loss of methane in hydrocarbon solvents to generate the monocylometallated compounds M(OC6H3Ph-C6H4)(OAr-2,6Ph 2)2(CH3) (M = Nb, 1 ; Ta, 2). Kinetic studies show the reactions to be first-order with activation parameters ΔH‡ = 25.3±0.8 kcal mol-1 and ΔS‡ = -15±5 cal mol-1 K-1 for the tantalum compound. The use of the labelled compound Ta(OAr-2,6Ph2)3(CD3)2 shows no change in the rate of metallation and also the absence of a methylidene intermediate for the reaction. Attempts to generate a bis(n-butyl) compound by treatment of Ta(OAr-2,6Ph2)3Cl2 with LiBun (2 equivs) led instead to the immediate formation of the metallated compound Ta(OC6H3Ph-C6H4)(OAr-2,6Ph 2)2(Bun) (3) along with butane. Thermolysis of the bis(p-methylbenzyl) compound Ta(OAr-2,6Ph2)3(CH2C6H4 -4Me)2 also leads to a monocyclometallated compound (4) and p-xylene. However, attempts to obtain a corresponding niobium bis(p-methylbenzyl) compound by treatment of Nb(OAr-2,6Ph2)3Cl2 with Mg(CH2C6H4-4Me)2 led instead directly to the monocyclometallated derivatives Nb(OC6H3Ph-C6H4)(OAr-2,6Ph 2)2(Cl) (5) and Nb(OC6H3Ph-C6H4)(OAr-2,6Ph 2)2(CH2C6H4-4Me) (6). Single-crystal X-ray diffraction studies of 2, 3 and 5 show the molecules to be similar in the solid state. A trigonal bipyramidal environment about the metal atom is observed with trans,axial oxygen atoms. The new MC(aryl) bond is chelated to an oxygen atom in one of the axial positions. Crystal data for TaC64H50O3; 2·3/2C6H6 at -155°C: a = 13.497(3), b = 17.033(6), c = 10.744(2) Å, α = 94.76(1), β = 96.34(1), γ = 79.80(1)°, Z = 2, dcalc = 1.444 g cm-3 in space group P1; for TaC64H53O3; 3·C6H6 at -130°C: a = 18.202(5), b = 17.930(3), c = 15.282(3) Å, β = 96.05(2)°, Z = 4, dcalc = 1.408 g cm-3 in space group P21/c; for NbC54H38ClO3; 5 at 20°C: a = 18.830(4), b = 11.2856(9), c = 20.725(5) Å, β = 102.8(1)°, Z = 4, dcalc = 1.335 g cm-1 in space group P21/c. © 1990.