Studies on the chemical reduction of polynuclear titanium(IV) nitrido complexes.

The redox chemistry of cube-type titanium(IV) nitrido complexes [{Ti4(η5-C5Me5)3(R)}(µ3-N)4] (R = η5-C5Me5 (1), N(SiMe3)2 (2), η5-C5H4SiMe3 (3), and η5-C5H5 (4)) was investigated by electrochemical methods and chemical reactions. Cyclic voltammetry studies indicate that 1-4 undergo a reversible one-electron reduction at ca. -1.8 V vs. ferrocenium/ferrocene. Thus, complex 1 reacts with sodium sand in tetrahydrofuran to produce the highly reactive ionic compound [Na(thf)6][{Ti(η5-C5Me5)}4(µ3-N)4] (5). The treatment of complexes 1-4 in toluene with one equivalent of [K(C5Me5)] in the presence of macrocycles (L) leads to C10Me10 and the formation of more stable derivatives [K(L)][{Ti4(η5-C5Me5)3(R)}(µ3-N)4] (R = η5-C5Me5, L = 18-crown-6 (6), crypt-222 (7); R = N(SiMe3)2, L = 18-crown-6 (8), crypt-222 (9); R = η5-C5H4SiMe3, L = 18-crown-6 (10), crypt-222 (11); R = η5-C5H5, L = crypt-222 (12)). However, the analogous reaction of 4 with [K(C5Me5)] and 18-crown-6 affords [{(18-crown-6)K}2(µ-η5:η5-C5H5)][{Ti4(η5-C5Me5)3(η5-C5H5)}(µ3-N)4] (13) via abstraction of one cyclopentadienide group from a putative intermediate [(18-crown-6)K(µ-η5:η5-C5H5)Ti4(η5-C5Me5)3(µ3-N)4]. In contrast to the cube-type nitrido systems 1-4, the cyclic voltammogram of the trinuclear imido-nitrido titanium(IV) complex [{Ti(η5-C5Me5)(µ-NH)}3(µ3-N)] (14) does not reveal any reversible redox event and 14 readily reacts with [K(C5Me5)] to afford C5Me5H and the diamagnetic derivative [{K(µ4-N)(µ3-NH)2Ti3(η5-C5Me5)3(µ3-N)}2] (15). The treatment of 15 with two equiv. of 18-crown-6 polyethers produces the molecular species [(L)K{(µ3-N)(µ3-NH)2Ti3(η5-C5Me5)3(µ3-N)}] (L = 18-crown-6 (16), dibenzo-18-crown-6 (17)). Complex 17 further reacts with one equiv. of dibenzo-18-crown-6 to yield the ion-separated compound [K(dibenzo-18-crown-6)2][Ti3(η5-C5Me5)3(µ3-N)(µ-N)(µ-NH)2] (18) similar to the ion pair [K(crypt-222)][Ti3(η5-C5Me5)3(µ3-N)(µ-N)(µ-NH)2] (19) obtained in the treatment of 15 with cryptand-222.

Successive Protonation and Methylation of Bridging Imido and Nitrido Ligands at Titanium Complexes.

The reactions of nitrido complexes [{Ti(η5-C5Me5)(µ-NH)}3(µ3-N)] (1) and [{Ti(η5-C5Me5)}4(µ3-N)4] (2) with electrophilic reagents ROTf (R = H, Me; OTf = OSO2CF3) in different molar ratios have allowed the structural characterization of a series of titanium intermediates en route to the formation of the ammonium salts [NR4]OTf and [NR4][Ti(η5-C5Me5)(OTf)4]. The treatment of the trinuclear imido-nitrido complex 1 with 5.5 equiv of triflic acid in toluene at room temperature led to the dinuclear complex [Ti2(η5-C5Me5)2(µ-N)(NH3)(µ-O2SOCF3)2(OTf)] (3) and [NH4]OTf. Compound 3, along with the ammonium salts [NMe4]OTf and [NMe4][Ti(η5-C5Me5)(OTf)4] (5), was also obtained in the reaction of 1 with 8 equiv of methyl triflate in toluene at 100 °C. The trinuclear complex [Ti3(η5-C5Me5)3(µ-N)(µ-NH)2(µ-O2SOCF3)(OTf)] (4), an intermediate in the formation of 3, was isolated in the treatment of 1 with 4 equiv of MeOTf, although compound 4 was prepared in better yield by treatment of 1 with Me3SiOTf (2 equiv). Addition of a large excess of MeOTf or HOTf reagents to solutions of 3 resulted in the clean formation of ammonium salts [NR4][Ti(η5-C5Me5)(OTf)4] (R = Me (5), H (6)). Treatment of the tetranuclear nitrido complex [{Ti(η5-C5Me5)}4(µ3-N)4] (2) with 1 equiv of ROTf in toluene afforded the precipitation of the ionic compounds [{Ti(η5-C5Me5)}4(µ3-N)3(µ3-NR)][OTf] (R = H (8), Me (9)), while a large excess of HOTf led to the formation of [{Ti(η5-C5Me5)}4(µ3-N)3(µ3-NH)][Ti(η5-C5Me5)(OTf)4(NH3)] (10) by rupture of a fraction of tetranuclear molecules. Complex 2 reacted with 1 equiv of [M(η5-C5H5)(CO)3H] (M = Mo, Cr) via hydrogenation of one nitrido ligand to give the molecular derivative [{Ti(η5-C5Me5)}4(µ3-N)3(µ3-NH)] (11) and [{M(η5-C5H5)(CO)3}2], while a second 1 equiv of [M(η5-C5H5)(CO)3H] produced the ionic compounds [{Ti(η5-C5Me5)}4(µ3-N)2(µ3-NH)2][M(η5-C5H5)(CO)3] (M = Mo (12), Cr (13)) by protonation of another nitrido group. The X-ray crystal structures of 3-5, 9, 10, and 13 were determined.

Homo and heteropolymetallic Group 4 molecular nitrides.

Treatment of [{Ti(η(5)-C5Me5)(µ-NH)}3(µ3-N)] (1) with zirconium or hafnium tetrachloride adducts [MCl4(thf)2] affords complexes [Cl3M{(µ3-N)(µ3-NH)2Ti3(η(5)-C5Me5)3(µ3-N)}] (M = Zr (2), Hf (3)). Titanium chloride complexes [Cl2Ti{(µ3-N)2(µ3-NH)Ti3(η(5)-C5Me5)3(µ3-N)}] (4) and [(Me2NH)ClTi{(µ3-N)3Ti3(η(5)-C5Me5)3(µ3-N)}] (5) are obtained by reaction of 1 with [TiCl4-x(NMe2)x] (x = 2, 3). The dimethylamine ligand in 5 is displaced by pyridine to give the analogue complex [(py)2ClTi{(µ3-N)3Ti3(η(5)-C5Me5)3(µ3-N)}] (6). Treatment of the titanium chloride complexes 4 and 5 with sodium cyclopentadienide or lithium bis(trimethylsilyl)amide reagents leads to the cube-type nitrido derivatives [RTi{(µ3-N)3Ti3(η(5)-C5Me5)3(µ3-N)}] (R = η(5)-C5H5 (7), N(SiMe3)2 (8)). The reaction of the zirconium trichloride complex 2 with [Tl(C5H5)] yields exclusively the dichloride-monocyclopentadienyl derivative [(η(5)-C5H5)Cl2Zr{(µ3-N)(µ3-NH)2Ti3(η(5)-C5Me5)3(µ3-N)}] (9). However, the treatment of 2 with excess [Na(C5H5)] causes further chloride replacement in 9 and subsequent cyclopentadiene elimination to give [(η(5)-C5H5)Zr{(µ3-N)3Ti3(η(5)-C5Me5)3(µ3-N)}] (10) via intermediates [(η(5)-C5H5)2ClZr{(µ3-N)Ti3(η(5)-C5Me5)3(µ-NH)2(µ3-N)}] (11) and [(η(5)-C5H5)ClZr{(µ3-N)2(µ3-NH)Ti3(η(5)-C5Me5)3(µ3-N)}] (12). Treatment of 2 or 9 with [Mg(C5H5)2] leads to the magnesium derivative [(η(5)-C5H5)Mg(µ-Cl)2(η(5)-C5H5)Zr{(µ4-N)(µ3-N)(µ3-NH)Ti3(η(5)-C5Me5)3(µ3-N)}] (13), which can be visualized as a result of the incorporation of one [Mg(η(5)-C5H5)Cl] moiety to complex 12. In contrast to the metathesis process with [M(C5H5)x] derivatives and subsequent C5H6 eliminations, the reaction of 2 with potassium pentamethylcyclopentadienide in toluene produces the paramagnetic derivative [K(µ-Cl)3Zr{(µ3-N)(µ3-NH)2Ti3(η(5)-C5Me5)3(µ3-N)}] (14) and C10Me10. Complex 14 reacts with one equivalent of 18-crown-6 or cryptand-222 to give the molecular complex [(18-crown-6)K(µ-Cl)3Zr{(µ3-N)(µ3-NH)2Ti3(η(5)-C5Me5)3(µ3-N)}] (15) or the ion pair [K(crypt-222)][Cl3Zr{(µ3-N)(µ3-NH)2Ti3(η(5)-C5Me5)3(µ3-N)}] (16). The X-ray crystal structures of 2, 8, 13 and 15 have been determined.

Partial hydrogenation of a tetranuclear titanium nitrido complex with ammonia borane.

The treatment of [{Ti(η(5)-C5Me5)}4(µ3-N)4] with NH3BH3 leads to the paramagnetic imidonitrido complex [{Ti(η(5)-C5Me5)}4(µ3-N)3(µ3-NH)], which can also be obtained by stepwise proton and electron transfer with HOTf and [K(C5Me5)].

Reactivity with electrophiles of imido groups supported on trinuclear titanium systems.

Several trinuclear titanium complexes bearing amido µ-NHR, imido µ-NR, and nitrido µn-N ligands have been prepared by reaction of [{Ti(η(5)-C5Me5)(µ-NH)}3(µ3-N)] (1) with 1 equiv of electrophilic reagents ROTf (R = H, Me, SiMe3; OTf = OSO2CF3). Treatment of 1 with triflic acid or methyl triflate in toluene at room temperature affords the precipitation of compounds [Ti3(η(5)-C5Me5)3(µ3-N)(µ-NH)2(µ-NH2)(OTf)] (2) or [Ti3(η(5)-C5Me5)3(µ3-N)(µ-NH)(µ-NH2)(µ-NMe)(OTf)] (3). Complexes 2 and 3 exhibit a fluxional behavior in solution consisting of proton exchange between µ-NH2 and µ-NH groups, assisted by the triflato ligand, as could be inferred from a dynamic NMR spectroscopy study. Monitoring by NMR spectroscopy the reaction course of 1 with MeOTf allows the characterization of the methylamido intermediate [Ti3(η(5)-C5Me5)3(µ3-N)(µ-NH)2(µ-NHMe)(OTf)] (4), which readily rearranges to give 3 by a proton migration from the NHMe amido group to the NH imido ligands. The treatment of 1 with 1 equiv of Me3SiOTf produces the stable ionic complex [Ti3(η(5)-C5Me5)3(µ3-N)(µ-NH)2(µ-NHSiMe3)][OTf] (5) with a disposition of the nitrogen ligands similar to that of 4. Complex 5 reacts with 1 equiv of [K{N(SiMe3)2}] at room temperature to give [Ti3(η(5)-C5Me5)3(µ3-N)(µ-N)(µ-NH)(µ-NHSiMe3)] (6), which at 85 °C rearranges to the trimethylsilylimido derivative [Ti3(η(5)-C5Me5)3(µ3-N)(µ-NH)2(µ-NSiMe3)] (7). Treatment of 7 with [K{N(SiMe3)2}] affords the potassium derivative [K{(µ3-N)(µ3-NH)(µ3-NSiMe3)Ti3(η(5)-C5Me5)3(µ3-N)}] (8), which upon addition of 18-crown-6 leads to the ion pair [K(18-crown-6)][Ti3(η(5)-C5Me5)3(µ3-N)(µ-N)(µ-NH)(µ-NSiMe3)] (9). The X-ray crystal structures of 2, 5, 6, and 8 have been determined.

Redox-active behavior of the [{Ti(η(5)-C5Me5)(µ-NH)}3(µ3-N)] metalloligand.

Treatment of [Cl3Y{(µ3-NH)3Ti3(η(5)-C5Me5)3(µ3-N)}] with [K(C5Me5)] in toluene gives C10Me10 and the paramagnetic [K(µ-Cl)3Y{(µ3-NH)3Ti3(η(5)-C5Me5)3(µ3-N)}] (3) derivative. Crystallization of 3 in pyridine affords the potassium-free [Cl2(py)2Y{(µ3-NH)3Ti3(η(5)-C5Me5)3(µ3-N)}] (4) complex. Whereas the reaction of 3 with 1 equiv of 18-crown-6 leads to the molecular complex [(18-crown-6)K(µ-Cl)3Y{(µ3-NH)3Ti3(η(5)-C5Me5)3(µ3-N)}] (5), the analogous treatment of 3 with cryptand-222 affords the ion pair [K(crypt-222)][Cl3Y{(µ3-NH)3Ti3(η(5)-C5Me5)3(µ3-N)}] (6). The X-ray crystal structures of 4, 5, and 6 have been determined. Density functional theory (DFT) calculations have elucidated the electronic structure of these species, which should be regarded as containing trivalent Y bonded to the {(µ3-NH)3Ti3(η(5)-C5Me5)3(µ3-N)} metalloligand radical anion.

Electrophilic attack on trinuclear titanium imido-nitrido systems.

Alkylation of [{Ti(η(5)-C(5)Me(5))(µ-NH)}(3)(µ(3)-N)] with MeOTf occurs at the imido ligands to produce the methylamido derivative [Ti(3)(η(5)-C(5)Me(5))(3)(µ(3)-N)(µ-NH)(2)(µ-NHMe)(OTf)] which readily rearranges to form the methylimido complex [Ti(3)(η(5)-C(5)Me(5))(3)(µ(3)-N)(µ-NH)(µ-NH(2))(µ-NMe)(OTf)].

Molecular nitrides with titanium and rare-earth metals.

A series of titanium-group 3/lanthanide metal complexes have been prepared by reaction of [{Ti(η(5)-C(5)Me(5))(µ-NH)}(3)(µ(3)-N)] (1) with halide, triflate, or amido derivatives of the rare-earth metals. Treatment of 1 with metal halide complexes [MCl(3)(thf)(n)] or metal trifluoromethanesulfonate derivatives [M(O(3)SCF(3))(3)] at room temperature affords the cube-type adducts [X(3)M{(µ(3)-NH)(3)Ti(3)(η(5)-C(5)Me(5))(3)(µ(3)-N)}] (X = Cl, M = Sc (2), Y (3), La (4), Sm (5), Er (6), Lu (7); X = OTf, M = Y (8), Sm (9), Er (10)). Treatment of yttrium (3) and lanthanum (4) halide complexes with 3 equiv of lithium 2,6-dimethylphenoxido [LiOAr] produces the aryloxido complexes [(ArO)(3)M{(µ(3)-NH)(3)Ti(3)(η(5)-C(5)Me(5))(3)(µ(3)-N)}] (M = Y (11), La (12)). Complex 1 reacts with 0.5 equiv of rare-earth bis(trimethylsilyl)amido derivatives [M{N(SiMe(3))(2)}(3)] in toluene at 85-180 °C to afford the corner-shared double-cube nitrido compounds [M(µ(3)-N)(3)(µ(3)-NH)(3){Ti(3)(η(5)-C(5)Me(5))(3)(µ(3)-N)}(2)] (M = Sc (13), Y (14), La (15), Sm (16), Eu (17), Er (18), Lu (19)) via NH(SiMe(3))(2) elimination. A single-cube intermediate [{(Me(3)Si)(2)N}Sc{(µ(3)-N)(2)(µ(3)-NH)Ti(3)(η(5)-C(5)Me(5))(3)(µ(3)-N)}] (20) was obtained by the treatment of 1 with 1 equiv of the scandium bis(trimethylsilyl)amido derivative [Sc{N(SiMe(3))(2)}(3)]. The X-ray crystal structures of 2, 7, 11, 14, 15, and 19 have been determined. The thermal decomposition in the solid state of double-cube nitrido complexes 14, 15, and 18 has been investigated by thermogravimetric analysis (TGA) and differential thermal analysis (DTA) measurements, as well as by pyrolysis experiments at 1100 °C under different atmospheres (Ar, H(2)/N(2), NH(3)) for the yttrium complex 14.

Yttrium and erbium halide complexes with [{Ti(eta5-C5Me5)(mu-NH)}3(mu3-N)] as a neutral tridentate ligand.

Treatment of [{TiCp*(mu-NH)} 3(mu 3-N)] ( 1; Cp* = eta (5)-C 5Me 5) with yttrium and erbium halide complexes [MCl 3(THF) 3.5] and [MCpCl 2(THF) 3] (Cp = eta (5)-C 5H 5) gives cube-type adducts [Cl 3M{(mu 3-NH) 3Ti 3Cp* 3(mu 3-N)}] and [CpCl 2M{(mu 3-NH) 3Ti 3Cp* 3(mu 3-N)}]. An analogous reaction of 1 with [{MCp 2Cl} 2] in toluene affords [Cp 3M(mu-Cl)ClCpM{(mu 3-NH) 3Ti 3Cp* 3(mu 3-N)}] (M = Y, Er).