Synthesis, Reactivity, and Computational Studies of the Cationic Tungsten Methyl Complex [W(NPh)(N₂N_py)Me]⁺ and Related Compounds (N₂N_py = MeC(2-C₅H₄N)(CH₂NSiMe₃)₂)

Abstract

Me(L)] + (L ) MeCN (4 + ) or THF (5 + )). For comparison the neutral tantalum derivatives Ta(N t Bu)(N 2 N py )R (R ) Me (6) or η 1 -allyl (7)) were synthesized by reaction of Ta(N t Bu)(N 2 N py )Cl(py) with MeLi or (allyl)MgCl. Compound 6, valence isoelectronic with 2 + , was crystallographically characterized. Although both 2 + and 6 possess trigonal bipyramidal geometries at the metal, the methyl ligand in 2 + lies in the equatorial plane (with NPh trans to pyridyl), whereas in 6 the opposite arrangement of methyl and imido ligands is found. Reaction of 1 with 0.5 equiv of BAr F 3 gave the fluxional Me-bridged cation [{W(NPh)(N 2 N py )Me} 2 (µ-Me)] + (8 + ); 8 + was also formed by direct reaction of 1 with 2 + . The methyl cation 2 + underwent facile methyl group exchange with Cp 2 ZrMe 2 and ZnMe 2 as established by spin saturation transfer and deuterium labeling studies. Although a stable intermediate was not spectroscopically observed for either reaction, for the latter case a likely adduct was identified by DFT calculations on a model system and features coordination of Zn to the imido nitrogen and a Zn-Me‚‚‚W interaction. Reaction of 2 + with AlMe 3 formed [W{MeC(2-C 5 H 4 NAlMe)(CHNSiMe 3 )(CH 2 NSiMe 3 )}(µ-NPh)Me 2 ] + (9 + ) and CH 4 by deprotonation of a CH 2 linkage of N 2 N py . DFT (B3PW91) calculations on model systems of the type M(NR){HC(2-C 5 H 4 N)(CH 2 SiH 3 ) 2 }(X) (X ) Cl, Me) showed that there is an unambiguous electronic preference for the imido ligand to lie trans to the pyridyl nitrogen. This geometry allows optimal π-donation from the imido and the amido nitrogen atoms. Inclusion of the steric bulk of the SiMe 3 groups and the R group (Ph or t Bu) on the imido ligand through ONIOM(B3PW91:UFF) calculations showed that the underlying electronic preference for the imido ligand to be trans to pyridyl can be reversed because of increased steric repulsions between the imido and amido N-substituents in this isomer. These cause a misdirection of the amido lone pair π-donation, which in turn destabilizes the metal-imido ligand π-bonding.

Reaction Site Diversity in the Reactions of Titanium Hydrazides with Organic Nitriles, Isonitriles and Isocyanates: TiNα Cycloaddition, TiNα Insertion and NαNβ Bond Cleavage

“…The molecular structures of 14 and 15, as determined by X-ray crystallography, are shown in Figure 2 and selected distances and angles are given in Table 2. [5h,i, k, 17,19] However, more generally it has been found that five-coordinate compounds of this general type can have the NR ligand in either an equatorial position (e.g., [20] or an axial one (e.g.,…”

Synthesis of Titanium Borylimido Compounds Supported by Diamide-Amine Ligands and Their Reactions with Alkynes

“…The electronic preference is for the multiple bond to be oriented axially (termed the trans arrangement), as when it is oriented in the equatorial site (termed the cis arrangement), the π-donor components of the Ti−N imide triple bond compete more with the N amide lone pairs for π-acceptor orbitals on the metal center. 19 In compounds 9−11, steric demands clearly dominate, placing the borylimido ligand cis to the NMe donor. In all three structures, the boryl moiety is oriented such that the boryl ring lies nearly coplanar with the N(4)−Ti(1)−N(7) axis, this presumably being to avoid steric clash between the bulky Ar′ groups and the N 2 R N Me ligand substituents.…”

“…The positioning of the titanium–nitrogen multiple bond in such complexes depends on both steric and electronic considerations. The electronic preference is for the multiple bond to be oriented axially (termed the trans arrangement), as when it is oriented in the equatorial site (termed the cis arrangement), the π-donor components of the Ti–N imide triple bond compete more with the N amide lone pairs for π-acceptor orbitals on the metal center . In compounds 9 – 11 , steric demands clearly dominate, placing the borylimido ligand cis to the NMe donor.…”

A Remarkable Switch from a Diamination to a Hydrohydrazination Catalyst and Observation of an Unprecedented Catalyst Resting State

“…This geometry is also predominantly maintained in solution according to NOE experiments. The hydrazide ligand in the solid‐state structure of 1 occupies the electronically preferred11 axial position trans to the NMe donor, whereas in I it lies exclusively in the equatorial position, which is the sterically preferred site. Minor resonances in the 1 H NMR spectrum of 1 are attributed to the isomer with NNPh 2 in the equatorial position.…”