INTRAMOLECULAR VS INTERMOLECULAR ALPHA-HYDROGEN ABSTRACTION IN THE GENERATION OF MULTIPLE IMIDO COMPLEXES - SYNTHESIS, REACTIVITY, AND STRUCTURAL STUDIES OF THE D(0) TRIS(IMIDO) FUNCTIONAL-GROUP OF TUNGSTEN

The synthesis, reactivity, and structural characterization of the d(0) tris(imido)complex [Li(THF)(4)][W(NAr)(3)Cl] (2, Ar = 2,6-C6H3-i-Pr-2 ) are reported. When W(NAr)Cl-4(THF) reacts with 2 equiv of Me(3)SiNHA r in THF, the bis(imido) compound W(NAr)(2)Cl-2(THF)(2) (1) is isolate d. Reacting W(NAr)(2)Cl-2(THF)(2) with 2 equiv of LiNHAr in THF afford s the yellow, crystalline tris(imido) complex [Li(THF)(4)][W(NAr3)Cl] (2). Complex 2 is shown to be the kinetic product of this reaction sin ce it reacts with byproduct H2NAr to afford W(NAr)(2)(NHAr)(2)(3). Exp eriments are described that support the d(0) W(=NR)(2) functional grou p in W(NAr)(2)Cl-2(THF)(2) (1) arising by an intramolecular alpha-H ab straction in [W(NAr)(NHAr)(2)Cl-2(THF)(n)] (with loss of H2NAr), while the formation of [W(NAr)(3)Cl](-)(2) from W(NAr)(2)Cl-2(THF)(2) (1) m ost likely proceeds by an intermolecular deprotonation of nascent W(NA r)(2)(NHAr)Cl by the second equivalent of [NHAr](-). These experiments include synthesis and reactivity studies of W(NAr)(2)Cl-2(NH2Ar) (4), W(NAr)(NEt(2))Cl-3(THF) (5), and W(NAr)(2)(NEt(2))Cl (6). The d(0) me tal center in [W(NAr)(3)Cl](-)(2) is susceptible to nucleophilic attac k as seen in its reactions with PMePh(2), PMe(3), [n-Bu(4)N]Br, MeLi, and LiCH(2)SiMe(3) to afford the substitution products W(NAr)(3)(PMePh (2)) (7), W(NAr)(3)(PMe(3)) (8), [n-Bu(4)N][W(NAr)(3)Br] (9), [Li(THF) (4)] [W(NAr)(3)Me] (10), and [Li(THF)(4)][W(NAr)(3)(CH(2)SiMe(3))] (11 ), respectively. Kinetic and mechanistic evidence is presented that su ggests these reactions proceed by a bimolecular, S(N)2 attack at the d (0) tungsten center. [Li(THF)(4)][W(NAr)(3)Cl] (2) crystallizes in the monoclinic space group P2(1)/n (No. 14) with a = 13.787(4), Angstrom, b = 17.348(5) Angstrom, c = 22.781(8) Angstrom, beta = 90.426(28)degr ees, and V = 5448.5(30) Angstrom(3), with Z = 4 and D(calc) = 1.268 g cm(-3). W(NAr)(3)(PMe(3)) (8) crystallizes in the orthorhombic space g roup Pbca (No. 61) with a = 18.572(3) Angstrom, b = 25.966(4) Angstrom , c = 16.819(3) Angstrom, and V = 8111(4) Angstrom(3), with Z = 8 and D(calc) = 1.29 g cm(-3). The tungsten atom of [Li(THF)(4)][W(NAr)(3)Cl ] (2) is tetrahedrally coordinated with three virtually identical imid o ligands with an average W-N bond length of 1.78 Angstrom and an aver age W-N-C-ipso bond angle of 171 degrees. W(NAr)(3)(PMe(3)) (8) is als o tetrahedrally coordinated and displays an average imido W-N bond len gth of 1.79 Angstrom and slightly bent W-N-C-ipso bond angles (av 167 degrees), though one imido ligand is more strongly bent than the other two. The electronic structure of the C-3v W(NAr)(3)L compounds sugges ts a ligand-based, nonbonding a(2) HOMO comprised of a N(p pi) orbital combination oriented perpendicular to the molecule's C-3 axis. Accord ingly, an imido nitrogen is subject to electrophilic attack, as seen i n the reactions of W(NAr)(3)(PMe(3)) (8) with HOAr'(Ar' = 2,6-C(6)H(3) Me(2)), Me(3)SiI, MeI, and PhNCO that afford W(NAr)(2)(NHAr)(OAr') (12 ), W(NAr)(2)[N(SiMe(3))Ar]I (13), W(NAr)(2)(NMeAr)I (14), and W[NArC(O )NPh](NAr)(2)(PMe(3)) (15), respectively. Similarly, the reaction of [ Li(THF)(4)][W(NAr)(3)Me] (10) with [HNMe(3)]BPh(4) does not protonate the W-Me bond but rather attacks the imido nitrogen to afford W(NAr)(2 )(NHAr)Me (16). W(NAr)(2)(NHAr)Me (16) does not eliminate CH4 upon the rmolysis to afford base-free [W(NAr)(3)]; other attempts to. generate this species are described.