Robert D. Profilet

Aryl imido complexes of the group 4 metals : structural aspects and mechanistic study of formation

Abstract The addition of aniline (PhNH2 ⩾ 2 equivalents) to the organometallic compounds [Ti(OC6H3Pri2-2,6)2(η2-ButNCCH2Ph)(CH2Ph)], [Ti(OC6H3Ph2-2,6)2(C4Et4)] and [Zr(OC6H3But2-2,6)2(CH3)2] in hydrocarbon solvents leads to the formation of the mononuclear bis(phenylamido) derivatives [M(OAr)2(NHPh)2] [M = Ti, OAr = OC6H3Pri2-2,6 (1); M = Ti, OAr = OC6H3Ph2-2,6 (2); M = Zr, OAr = OC6H3But2-2,6 (3)]. Treatment of [Hf(CH2Ph)4] first with PhNH2 (4 equivalents), followed by HOC6H3But2-2,6 (2 equivalents), leads to the related bis(phenylamido) compound 4 (M = Hf; OAr = OC6H3But2-2,6). The two homoleptic aryl amido compounds [M(NHC6H3Pri2-2,6)4] [M = Zr (5); Hf (6)] have also been obtained by addition of 2,6-diisopropylaniline to the tetra-benzyl compounds [M(CH2Ph)4] (M = Zr, Hf). The addition of 4-pyrrolidinopyridine (py′) to all of the aryl amido compounds except 4 leads to elimination of 1 equivalent of aryl amine and the formation of a series of five-coordinate aryl amido derivatives of the general formula [M(OAr)2(NPh)(py′)2] [M = Ti, OAr = OC6H3Pri2-2,6 (7); M = Ti, OAr = OC6H3Ph2-2,6 (8); M = Zr, OAr = OC6H3But2-2,6 (9); and [M(NHAr)2(NAr)(py′)2]; M = Zr, Ar = C6H3Pri2-2,6 (10); M = Hf, Ar = C6H3Pri2-2,6 (11)]. In the case of the hafnium bis(phenylamido) complex 4, addition of 4-pyrrolidinopyridine resulted in the formation of a simple adduct. [Hf(OC6H3But2-2,6)2(NHPh)2(py′)], [4·py′]. (A similar adduct, [4·py′], was detected in the conversion of 3–9.) No elimination of aniline from [4·py′] and formation of a phenylimido derivative were observed. Both 2,2′-bipyridine and 1,10-phenanthroline were found to eliminate aniline from compounds 1–3 to produce insoluble products. Addition of 3,4,7,8-tetramethyl-1,10-phenanthroline to 2, however, yielded a soluble phenylimido derivative (12). The four-coordinate aryl amido compounds 2, 3 and 5 were found to be pseudo-tetrahedral in the solid state, while the five-coordinate aryl imido compounds 7, 9, 10 and 11 are best described as distorted trigonal-bipyramidal with trans-axial pyridine ligands. In the phenanthroline derivative 12 a distorted trigonal-bipyramidal geometry exists about the titanium atom with an aryloxide oxygen atom trans to a phenanthroline nitrogen. The bonding of the aryl imido, aryl amido and pyridine groups is described. A detailed study of the reaction of a series of bis(aryl amido) complexes, [Zr(OAr)2(NHC6H4-4X)2] (3)X (X = H, F, CH3, OMe, Br), with a variety of pyridine ligands was undertaken. The mono-pyridine adduct [3X·py] is rapidly formed, followed by the slow formation of the corresponding aryl imido complex [Zr(OAr)2(NHC6H4-4X)(py)2] [9X] and an equivalent of substituted aniline. Equilibrium constants for the reaction [3Xpy] + py = [9X] + ArNH2 were measured. Formation of the aryl imido ligand was found to be favoured by a more basic pyridine ligand and by electron-withdrawing substituents on the aryl ring of the initial aryl amido group. The rate of attainment of the equilibrium situation from [3] and py was investigated and various pathways for the reaction are considered. X-ray crystal structures were determined for 2, 3, 5, 7, 9, 10, 11 and 12.

‘1,3-dimetallabenzene’ derivatives of niobium and tantalum ☆

Abstract The alkylidyne bridged compounds [(cb)2M(μ-CSiMe3)2M(cb)2] (1a M  Nb; 1b, M  Ta; cb = carbazole) react thermally with one equivalent of alkynes EtCCEt and Me3SiCCH to produce new organometallic derivatives 2 and 3, respectively. The molecular structures of 2 and 3 are shown to consist of non-planar six-membered di-metallacycles originating from insertion of an alkyne unit into one of the alkylidyne bridges of 1. Treatment of 1b with 3,5-di-tert-butyl-2,6-diphenylphenol (ArOH) leads to the monophenoxide [(ArO)(cb)Ta(μ-CSiMe3)2Ta(cb)2] 4. Treatment of 4 with Me3SiCCH leads to the corresponding metallacycle [(ArO)(cb)Ta(μ-CSiMe3){μ-C(SiMe3)CHC(SiMe3)}Ta(cb)2] 5. Structural parameters for 2b, 3a, 3b and 5 obtained by X-ray diffraction show delocalization is present with four equivalent MC and two equivalent CC distances. The non-planar structure is best described as a twist of the planar [M(μ-C)M] and [CCC] units within the ring. The non-planar structure of 2a and 2b is maintained in solution as evidenced by diastereotopic methylene protons for the CH2CH3 substituents. At elevated temperatures coalescence to a simple A2B3 pattern is observed. From the coalescence temperatures of 328 K for 2b an activation barrier of 16.1(5) kcal mol−1 can be estimated for the adoption of a planar structure. The insertion of Me3SiCCH produces a 2,4,6-substitution pattern for 3a, 3b and 5 with the 5-H (beta to two metal centers) being downfield shifted to σ 8.43 (3a), 8.64 (3b) and 9.03 ppm (5).

Synthesis and structure of a di-tantalum(IV) hexahydride compound containing ancillary amido ligation

Abstract Hydrogenation of the alkylidyne bridged dimer [(cb) 2 Ta(μ-CSiMe 3 ) 2 Ta(cb) 2 ] (cbH = 9H-carbazole) in the presence of PMe 2 Ph leads to the green hexahydride compound [(cb)(PMe 2 Ph) 2 (H)Ta(μ-H) 4 Ta(H)(PMe 2 Ph) 2 (cb)] which contains a TaTa distance of 2.5359(4) A and TaN (carbazole) distance of 2.184(4) A.

Crystal and molecular structure of penta(4-methylbenzyl)tantalum, [Ta(CH2C6H4-4Me)5]

Reaction of the dichloride [TaCl 2 (CH 2 C 6 H 4 -4Me) 3 ] ( 1 ) (CH 2 C 6 H 4 -4Me) 2 ] leads to the homoleptic 4-methylbenzyl compound [Ta(CH 2 C 6 H 4 -4Me) 5 ] ( 2 ). The solid state structure of 2 shows a distorted square-pyramidal geometry for the central TaC 5 core. Four of the 4-methylbenzyl ligands are σ-bound to the tantalum with TaCH 2 C angles of 108.4(5)−125.7(5)°, while the remaining ligand has an acute TaCH 2 C angle of 90.4(5)°. In solution all alkyl groups are equivalent by 1 H and 13 C NMR even at −60°C, indicating facile exchange of non-equivalent alkyl groups on the NMR time-scale.

Surface-supported group 5 metal organometallic compounds for catalytic arene hydrogenation

The Group 5 metal alkylidyne bridged compounds [(Me3SiCH2)2M(µ-CSiMe3)2M(CH2SiMe3)2](M = Nb, Ta) supported on silica will catalyse the exhaustive hydrogenation of a variety of aromatic substrates.