Preparation, Structural Characterization, and Reactions of Tantalum-Alkyne Complexes TaCl₃(R¹C⋮CR²)L₂ (L₂ = DME, Bipy, and TMEDA; L = Py)

Abstract

Tantalum-alkyne complexes with the general formula TaCl3(R1C⋮CR2)(dme) (1a, R1 = R2 = Et; 1b, R1 = R2 = n-C5H11; 1c, R1 = Ph, R2 = Me; 1d, R1 = R2 = Ph; DME = 1,2-dimethoxyethane) were synthesized by treatment of the corresponding alkynes with a low-valent tantalum derived by reduction of tantalum pentachloride with zinc powder in a mixed solvent of toluene and DME. The DME ligand can be replaced by 2 equiv of pyridine to afford the corresponding dipyridine complexes TaCl3(R1C⋮CR2)(py)2 (2a, R1 = R2 = Et; 2b, R1 = R2 = n-C5H11; 2c, R1 = Ph, R2 = Me; 2d, R1 = R2 = Ph). Additionally, the reaction of 1a with the bidentate nitrogen ligands bipyridine and N,N,N‘,N‘-tetramethyethylenediamine (= TMEDA) gave TaCl3(EtC⋮CEt)(bipy) (3) and TaCl3(EtC⋮CEt)(tmeda) (4), respectively. The η2- and 4e-coordination mode of an alkyne with a large contribution of a metalacyclopropene canonical structure was revealed by spectroscopic methods (NMR and IR) and crystallographic analyses of 1a, 1c, 2a, and 4. The reactivies of the tantalum-3-hexyne complexes 1a, 2a, 3, and 4 toward 3-phenylpropanal were investigated. Only the pyridine complex 2a reacted with a stoichiometric amount of the aldehyde to afford the corresponding allylic alcohol in 77% yield upon hydrolysis. The reaction proceeded via an oxatantalacyclopentene species.

Direct Evidence for a [4+2] Cycloaddition Mechanism of Alkynes to Tantallacyclopentadiene on Dinuclear Tantalum Complexes as a Model of Alkyne Cyclotrimerization

“…In addition, the bending structure of the C 6 Et 6 ligand is ascertained by the dihedral angle (37.5°) between the C1‐C2‐C3‐C4 and C1‐C6‐C5‐C4 planes, the value of which is less acute than that of [(C 6 Et 6 )Ta(DIPP) 2 Cl] (34.4°) 11a. The bond length of C21C22 (1.391(18) Å) in 4 is typical for a dianionic η 2 ‐alkyne ligand found in mononuclear complexes, [(η 2 ‐3‐hexyne)TaCl 3 (L) 2 ] (L=py; L 2 =DME, TMEDA) 7. 12b…”

“…In contrast, fast rotation of the arene ligands on tantalum and dinuclear vanadium centers were observedf or previously reported[ ( h 6 -arene)Ta(OAr) 3 CH 3 ]. [7,12] Although the mechanism of the reaction shown in Eq. (5) is unclear,w e presumed isproportionation of chloridel igandst ake place during the formation of complex 4.…”

“…Reaction of [(η 2 ‐EtCCEt)TaCl 3 (DME)] ( 1 )7 with anhydrous AlCl 3 (1 equiv) in toluene afforded dinuclear complex 2 as a dark‐purple solid in 75 % yield, along with the formation of AlCl 3 (DME), in which anhydrous AlCl 3 functioned to capture DME [Eq. (3)].…”

“…The upfield shift of the methylene protons bound to Cα compared with that of Cβ indicated the sp 3 carbon character of Cα. The μ‐η 2 :η 2 ‐3‐hexyne moiety was characterized as a dianionic alkyne ligation bridging two tantalum centers because 1 H NMR signals due to ethyl groups resonated at lower magnetic field at δ H =3.98 ppm [q, 3 J (H,H)=7.4 Hz, C H 2 CH 3 ] and δ H =1.53 ppm [t, 3 J (H,H)=7.4 Hz, CH 2 C H 3 ] 7. 12 Although the mechanism of the reaction shown in Eq.…”

“…Reactiono f[ ( h 2 -EtCCEt)TaCl 3 (DME)] (1) [7] with anhydrous AlCl 3 (1 equiv) in toluene afforded dinuclear complex 2 as ad arkpurple solid in 75 %y ield, along with the formation of AlCl 3 (DME),i nw hich anhydrous AlCl 3 functioned to capture DME [Eq. ( 3)].T he molecular structure of 2 was characterized spectroscopically,a nd further established by an X-ray diffraction study (Figure 1a nd Ta ble 1), which clearly indicated the formation of ad imetallacyclopentadiene structure bridging two tantalum atoms.…”