Alkyne[hydrotris(pyrazolyl)borato]tantalum Complexes – An Ethyl Group is a Better α-Agostic Donor Than a Methyl Group

Abstract

The synthesis of unprecedented alkyne hydrotris(3,5‐dimethylpyrazolyl)borato (TpMe2) tantalum(III) complexes is reported. The parent dichloro complex TpMe2TaCl2(PhC≡ CCH3) (2) is obtained from TaCl3(DME)(PhC≡CCH3) and KTpMe2. 2 reacts with methyllithium to give the dimethyl complex TpMe2Ta(CH3)2(PhC≡CCH3) (3) and with ethylmagnesium chloride to give the X‐ray characterized α‐agostic ethyl complex TpMe2Ta(Cl)(μ‐H–CHCH3)(PhC≡CCH3) (4). Reaction of 4 with methyllithium generates the mixed methyl ethyl complex TpMe2Ta(CH3)(μ‐H–CHCH3)(PhC≡CCH3) (5) in solution. Spectroscopic data indicate that 5 is α‐agostic at the ethyl group only.

Tantalum Complexes Incorporating Tris(pyrazolyl)Borate Ligands: Syntheses, Structures, and Ethylene Polymerization Behavior

“…The average N−Ta−N angles formed by the tridentate Tp′ ligand ( 3 : 80.08°; 4 : 81.76°) are similar for the two cases and are within the range of those for the reported Tp′Ta complexes (78−83°). − Although the C(37)−N(10)−Ta(1) angle in 3 (178.0°) is almost linear, the C(37)−N(7)−Ta(1) angle in complex 4 (159.7°) considerably deviates from 180°. A similar deviation was observed for Ta(NR)Cl(diamidopyridine)(pyridine) complexes (R = t Bu, 2,6- i Pr 2 C 6 H 3 ) .…”

“…The elongation of the Ta−N distance may be attributed to a more electron-donating nature of the imido group than that of the Cl atom. The extent of the trans influence of the imido auxiliary ligand is greater than that of the alkyne auxiliary ligands, Tp*TaCl 2 (η 2 -phenylpropyne) and Tp*TaEtCl(η 2 -phenylpropyne) . The average Ta−N distances for the pyrazolyl groups ( 3 : 2.29 Å; 4 : 2.31 Å) are somewhat longer than those of the reported Tp*TaCl 2 (η 2 -phenylpropyne) (average 2.25 Å), Tp*TaEtCl(η 2 -phenylpropyne) (average 2.28 Å), TpTaMe 3 Cl (average 2.27 Å), Tp*TaCl(η 2 -CH 2 NPh)(CH 2 t Bu) (average 2.27 Å), and [Tp*TaCl 3 ][TaCl 6 ] (average 2.16 Å) …”

Equilibria between α- and β-Agostic Stabilized Rotamers of Secondary Alkyl Niobium Complexes

“…Similarly, in 5CR-AS, the β-ethyl group is found in this same wedge, leaving a methyl group in the agostic position. It is worth noting that this same structural feature is also observed in the R-agostic primary alkyl complexes Tp Me2 MCl(CH 2 Me)-(PhCtCR) (M ) Nb, Ta) 17,20 and Tp Me2 NbCl(CH 2 SiMe 3 )-(PhCtCMe) 21 in which the alkyl group sits between two pyrazolyl rings. For 5AR-CS, however, the situation is reversed because of the configuration change at either Nb or CR.…”

Synthesis and characterization of poly(pyrazolyl)borate tantalum amide complexes and their reactivities toward oxygen

“…Although numerous studies have been conducted on polypyrazolyborate complexes of main group and transition metals, only a few studies of Ta complexes containing the ligands are known [8][9][10][11][12][13][14][15][16][17][18] . Notably, no Ta complex containing only polypyrazolyborate and amides has been reported.…”