No abstract

A comparison between models for the substrate−catalyst adduct in hydrodenitrogenation (HDN) catalysis is made with respect to oxygen vs sulfur ancillary ligands. Reacting [η2(N,C)-NC5 tBu3H2]Ta(OAr)2Cl (1, Ar = 2,6-C6H3 iPr2) with KOtBu affords orange crystals of the alkoxide [η2(N,C)-NC5 tBu3H2]Ta(OAr)2(OtBu) (2), while 1 and LiStBu react to form the red thiolate analogue [η2(N,C)-NC5 tBu3H2]Ta(OAr)2(StBu) (3). Structural studies of both complexes 2 and 3 are reported and compared with other η2(N,C)-NC5 tBu3H2 derivatives. A trace of the bromide complex [η2(N,C)-NC5 tBu3H2]Ta(OAr)2Br (4) is isolated from reacting [η2(N,C)-NC5 tBu3H2]Ta(OAr)2Cl (1) with EtMgBr in THF/Et2O solution and is also structurally characterized for comparison. Complexes 2−4 reveal a severe interruption of aromaticity within the heterocycle, different rotational preferences of the pyridine NC5 plane with respect to the Ta(OAr)2X moiety, and various aryloxide ligand structural differences. From this comparison, arguments will be presented that support the ancillary ligand π-donor ability decreasing as OtBu > OAr > StBu > Cl ≈ Br > Et, although evidence suggests that the StBu ligand is a better σ + π donor overall than OAr or OtBu.

The reactions of TaCl5 with Me3SiNC9H10 or LiNC9H10, where [NC9H10]- = tetrahydroquinolinyl (the amido anion of tetrahydroquinoline), afford selective preparative routes to the complete series of amido halide complexes of tantalum(V) Ta(NC9H10) n Cl5 - n for n = 1−5 (compounds 1−5, respectively). The monokis(tetrahydroquinolinyl) complex is isolated as an ether adduct Ta(NC9H10)Cl4(OEt2) while the complexes Ta(NC9H10) n Cl5 - n (n = 2−5) are found to be base-free, monomeric species. The related complexes of indolinyl [NC8H8]- (the amido anion of indoline), Ta(NC8H8) n Cl5 - n (THF) for n = 1 (6) or 2 (7), have been prepared from TaCl5, Me3SiNC8H8, and THF. An X-ray structural determination of Ta(NC9H10)2Cl3 (2) reveals that it adopts a trigonal bipyramidal geometry with equatorial amido ligands that are closer to lying parallel (within) than perpendicular to the TBP equatorial plane. Routes to mixed-ligand aryloxide−amide complexes have been developed from either aryloxide or amido precursors but not from both. Thus, Ta(NC9H10)(OAr)Cl3(OEt2) (8), where Ar = 2,6-C6H3 iPr2, and Ta(NC8H8)(OAr)Cl3(OEt2) (9) are available from reacting Ta(OAr)Cl4(OEt2) with Me3SiNC9H10 and Me3SiNC8H8, respectively, while Ta(NC9H10)2(OAr)2Cl (10) is available from Ta(NC9H10)2Cl3 (2) and excess LiOAr·OEt2. The alkyl derivatives Ta(NC9H10)(OAr)Me2Cl (11), Ta(NC9H10)(OAr)Et2Cl (12), Ta(NC9H10)2(OAr)2Me (13), and Ta(NC9H10)Me2Cl2 (14) are prepared from AlR3 or ZnR2 reagents and the appropriate precursor. Thermolyzing compounds 4, 5, and 11−14 in solution afforded no evidence for the formation of any η2(N,C)-heterocyclic complexes arising from metalation of a NC9H10 ligand.

A series of η6-hexamethylbenzene alkyl and aryl complexes of tantalum(III) supported by aryloxide and arenethiolate ligands have been prepared, characterized, and compared to their halide analogues. Thus, (η6-C6Me6)Ta(OAr)2Cl (1, Ar = 2,6-C6H3 iPr2) reacts with MeMgBr at low temperature to afford (η6-C6Me6)Ta(OAr)2Me (3). Low-temperature alkylation of (η6-C6Me6)Ta(OAr)Cl2 (2) with 2 equiv of RMgBr forms (η6-C6Me6)Ta(OAr)R2 (4, R = Me; 5, R = Et) and with 2 equiv of RLi affords (η6-C6Me6)Ta(OAr)R2 (6, R = CH2SiMe3; 7, R = Ph). Complexes 3−7 are more stable than their halide precursors; no products arising from α- or β-H elimination processes were identified upon thermolysis. In addition to NMR studies of these compounds, cyclic voltammetry experiments show two oxidation processes; the Ta(III) ⇄ Ta(IV) couple is quasi-reversible, and the Ta(IV) → Ta(V) process is irreversible. Molecules of 5 exhibit a folded arene ligand with π-electron localization (diene−diyl structure) and normal ethyl ligands (no evidence for agostic interactions). Under the appropriate conditions, (η6-C6Me6)Ta(OAr)Cl2 (2) can be monoalkylated using 1 equiv of LiCH2SiMe3 or LiPh to afford (η6-C6Me6)Ta(OAr)(CH2SiMe3)Cl (8) and (η6-C6Me6)Ta(OAr)(Ph)Cl (9). However, attempts to monoalkylate (η6-C6Me6)Ta(OAr)Cl2 with 1 equiv of either MeMgBr or EtMgBr provide the “double-exchange” products (η6-C6Me6)Ta(OAr)(Me)Br (10) and (η6-C6Me6)Ta(OAr)(Et)Br (11), respectively. The metathesis product (η6-C6Me6)Ta(OAr)(Et)Cl (12) is isolated in good yield upon attempts to alkylate (η6-C6Me6)Ta(OAr)(CH2SiMe3)Cl (8) with ZnEt2. However, (η6-C6Me6)Ta(OAr)(CH2SiMe3)Cl (8) reacts with PhLi to afford (η6-C6Me6)Ta(OAr)(CH2SiMe3)Ph (13). The halide alkyl complexes (η6-C6Me6)Ta(OAr)(Et)Br (11) and (η6-C6Me6)Ta(OAr)(CH2SiMe3)Cl (8) react with LiBEt3H to provide the hydrido complexes (η6-C6Me6)Ta(OAr)(Et)H (14) and (η6-C6Me6)Ta(OAr)(CH2SiMe3)H (15), respectively. The arenethiolate complexes (η6-C6Me6)Ta(OAr)(SAr‘)Cl (16) (Ar‘ = 2,4,6-C6H2 iPr3) and (η6-C6Me6)Ta(OAr)(S(mes))Cl (17) (mes = 2,4,6-C6H2Me3) are formed upon reacting (η6-C6Me6)Ta(OAr)Cl2 (2) with the appropriate lithium arenethiolate reagent, and the characterization of these species is discussed.

No abstract