Janet R. Clark

Formation and reactivity of 1,3-cyclohexadiene complexes of niobium and tantalum containing aryloxide ligation: Selectivity differences in the hydrogenation of 1,3-cyclohexadiene☆

Abstract The sodium amalgam reduction (2 Na per M) of hydrocarbon solutions of [M(OAr)3Cl2] (M = Nb, Ta) or [Nb(OAr)2Cl3]2 (OAr = 2,6-di-isopropylphenoxide) in the presence of either 1,3- or 1,4-cyclohexadiene yielded the complexes [M(OAr)3(η4-C6H8)] ( 1 a : M = Nb ; 1 b : M = Ta ) and [Nb(OAr)2Cl(η4-C6H8)], 2, respectively. The solid state structures of 1a, isomorphous 1b and 2 showed the 1,3-cyclohexadiene strongly bound to the metal. The ligand is not symmetrically bound to the metal in 1 but in 2 there is a crystallographic mirror plane. The solution NMR spectra of 1a and 1b show only one set of aryloxide ligand signals and only four proton and three carbon resonances for the C6H8 group. The hydrolysis of 1 or 2 yielded 2,6-di-isopropylphenol and one equivalent of cyclohexane (1H NMR). The niobium compounds 1a and 2 will catalyse the dis-proportionation and hydrogenation of 1,3-cyclohexadiene with differing selectivity.

Intramolecular activation of aromatic C–H bonds by tantalum alkylidene groups: evaluating ‘cyclometallation resistant’ aryloxide ligation

The ligands 2,3,5,6-tetraphenylphenoxide and 3,5-dimethyl-2,6-diphenylphenoxide undergo intramolecular aromatic C–H bond activation by tantalum alkylidene groups at rates 20 and 100 times slower than simple 2,6-diphenylphenoxide.

SYNTHESIS AND STRUCTURE OF NIOBIUM AND TANTALUM DERIVATIVES OF BIS(DICYCLOHEXYLPHOSPHINO)METHANE (DCPM)

The sodium amalgam reduction (2 Na per M) of the Group 5 metal chlorides [M 2 Cl 10 ] (M=Nb, Ta) in the presence of bis(dicyclohexylphosphino)methane (dcpm) leads to the binuclear compounds [(dcpm)Cl 2 M(μ 2 -Cl) 2 MCl 2 (dcpm)] (M=Nb, 1 ; Ta, 2 ). Solution NMR spectroscopic properties of 1 and 2 indicate that the dcpm ligand does not bridge (binucleate) the di-metal unit but is chelated to a single metal center. A single crystal X-ray diffraction study of 1 confirms this and shows two independent molecules both with an edge shared bis-octahedral geometry with NbNb distances of 2.738(1) and 2.749(1) A. The addition of dcpm to the tantalum trichloride [Ta(OC 6 H 3 Pr i  2 -2,6) 2 Cl 3 ] 2 initially leads to the adduct cis - mer -[Ta(OC 6 H 3 Pr i  2 -2,6) 2 Cl 3 (dcpm)] ( 3 ). The molecular structure of 3 is found to contain an η 1 -bound dcpm ligand. In the 31 P NMR spectrum of 3 well resolved doublets are present for the coordinated and ‘dangling’ phosphorus atoms. The sodium amalgam reduction of 3 in hydrocarbon solvents does not lead to any isolable binuclear compounds. Instead the reaction leads to the compound [Ta(OC 6 H 3 Pr i -η 2 -CMeCH 2 )Cl(dcpm)] ( 4 ). Compound 4 contains an aryloxide ligand chelated to the metal via an η 2 -interaction with a vinyl group formed by the dehydrogenation of an ortho -isopropyl group. The structural parameters are consistent with a metallacyclopropane bonding description for this interaction.

Studies of six-coordinate adducts all-trans-[M(OAr)2(X)2(L)2] (M=Nb, Ta; X=Cl, Br; L=tertiary phosphine, pyridine)

An extensive series of d 1 -derivatives of niobium and tantalum [M(OAr)2(X)2(L)2 ]( M Nb, Ta; X Cl, Br; Ltertiary phosphine, pyridine) have been isolated by reduction of the corresponding adducts [M(OAr)2(X)3(L)] in the presence of excess L. Crystallographic studies show an all-trans, octahedral arrangement of ligands about the metal center. The effect of the halide, donor ligand and aryloxide substituents upon the structural parameters is discussed.

INTRAMOLECULAR ACTIVATION OF AROMATIC C-H BONDS AT TANTALUM(V) METAL CENTERS : EVALUATING CYCLOMETALLATION 'RESISTANT' AND 'IMMUNE' ARYLOXIDE LIGATION

The trichloride compounds [Ta(OC 6 HPh 2 -2,6-R 2 -3,5)Cl 3 ] (1: R = H a, Ph b, Me c, Pr i d or Bu t e) have been obtained by treating [Ta 2 Cl 10 ] with the corresponding 3,5-disubstituted-2,6-diphenylphenols Ia–Ie. The solid-state structures of 1c and 1d show a square-pyramidal structure with an axial aryloxide ligand. The reaction of 1 with LiCH 2 SiMe 3 (3 equivalents) led to the isolation of the tris(alkyls) [Ta(OC 6 HPh 2 -2,6-R 2 -3,5) 2 (CH 2 SiMe 3 ) 3 ] (4a–4d) except in the case of the 3,5-di-tert-butyl derivative 1e which generated the alkylidene compound [Ta(OC 6 H 3 Ph 2 -2,6-Bu t -3,5) 2 (CHSiMe 3 )(CH 2 SiMe 3 )] 6e. The alkylidenes 6a–6d can be produced by photolysis of the corresponding tris(alkyls) 4a–4d. The alkylidenes 6a–6d undergo intramolecular cyclometallation of the aryloxide ligand (addition of an aromatic C–H bond to the tantalum alkylidene) at a rate which is extremely dependent on the meta substituents on the phenoxide nucleus. Kinetic studies show that conversion of 6a–6d into monometallated 7a–7d is first order with the phenyl, methyl and isopropyl substituents slowing the ring closure down by factors of 20, 90 and 360 respectively. The tert-butyl substituent completely shuts down cyclometallation of the adjacent phenyl ring. It is argued that bulky substituents inhibit rotation of the ortho-phenyl ring into a conformation necessary for C–H bond activation. Structural analysis of the torsion angles between ortho-phenyl and phenoxy rings has been carried out. The use of 1 H NMR chemical shifts has been demonstrated to be a valuable tool to probe the average conformations adopted in solution.

Aryloxide ligand dependent reactivity of tantalum dihydride compounds with alkenes

The tantalum dihydride compounds [Ta(OAr)2(L)n(Cl)(H)2](n= 2, OAr = 2,6-diphenyl- and 2,6-diisopropylphenoxide; n= 1, OAr = 2,6-di-tert-butylphenoxide) react with styrene to produce different organometallic products depending upon the nature of the aryloxide ligand.

Reaction pathways for the oligomerization of organic isocyanides by tantalum hydride reagents

The tantalum monohydride compound [Ta(OAr)2Cl2(H)(PMe2Ph)2](OAr = 2,6-diisopropylphenoxide) will react with one, two or three equivalents or organic isocyanides to produce a sequence of organometallic products resulting from initial insertion into the Ta–H bond and subsequent coupling reactions.