Reactivity of titanium imidazolin-2-iminato complexes with 2,6-diisopropylaniline and 2-{(2,6-diisopropylphenyl)-iminomethyl}pyrrole

Molecular Capacitors: Accessible 6- and 8-Electron Redox Chemistry from Dimeric "Ti(I)" and "Ti(0)" Synthons Supported by Imidazolin-2-Iminato Ligands

Reduction of the diamagnetic Ti(III)/Ti(III) dimer [Cl₂Ti(μ-NIm^Dipp)]₂ (1) (NIm^Dipp = [1,3-bis(Dipp)imidazolin-2-iminato]^-, Dipp = C₆H₃-2,6-ⁱPr₂) with 4 and 6 equiv of KC₈ generates the intramolecularly arene-masked, dinuclear titanium compounds [(μ-N-η⁶-Im^Dipp)Ti]₂ (2) and {[(Et₂O)₂K](μ-N-μ-η⁶:η⁶-Im^Dipp)Ti}₂ (3), respectively, in modest yields. The compounds have been structurally characterized by X-ray crystallographic analysis, and inspection of the bond metrics within the η⁶-coordinated aryl substituent of the bridging imidazolin-2-iminato ligand shows perturbation of the aromatic system most consistent with two-electron reduction of the ring. As such, 2 and 3 can be assigned respectively as possessing metal centers in formal Ti(III)/Ti(III) and Ti(II)/Ti(II) oxidation states. Exploration of their redox chemistry reveal the ability to reduce several substrate equivalents. For instance, treatment of 2 with excess C₈H₈ (COT) forms the novel COT-bridged complex [(Im^DippN)(η⁸-COT)Ti](μ-η²:η³-COT)[Ti(η⁴-COT)(NIm^Dipp)] (4) that dissociates in THF solutions to give mononuclear (Im^DippN)Ti(η⁸-COT)(THF) (5). Addition of COT to 3 yields heterometallic [(Im^DippN)(η⁴-COT)Ti(μ-η⁴:η⁵-COT)K(THF)(μ-η⁶:η⁴-COT)Ti(NIm^Dipp)(μ-η⁴:η⁴-COT)K(THF)₂]_n (6). Compounds 4 and 5 are the products of the 4-electron oxidation of 2, while 6 stands as the 8-electron oxidation product of 3. Reduction of organozides was also explored. Low temperature reaction of 2 with 4 equiv of AdN₃ gives the terminal and bridged imido complex [(Im^DippN)Ti(═NAd)](μ-NAd)₂[Ti(NIm^Dipp)(N₃Ad)] (7) that undergoes intermolecular C-H activation of toluene at room temperature to afford the amido compound [(Im^DippN)Ti(NHAd)](μ-NAd)₂[Ti(C₆H₄Me)(NIm^Dipp)] (8-tol). These complexes are the 6-electron oxidation products of the reaction of 2 with AdN₃. Furthermore, treatment of 3 with 4 equiv of AdN₃ produces the thermally stable Ti(III)/Ti(III) terminal and bridged imido [K(18-crown-6)(THF)₂]{[(Im^DippN)Ti(NAd)](μ-NAd)₂K[Ti(NIm^Dipp)]} (10). Altogether, these reactions firmly establish 2 and 3 as unprecedented Ti(I)/Ti(I) and Ti(0)/Ti(0) synthons with the clear capacity to effect multielectron reductions ranging from 4 to 8 electrons.

Titanium complexes with unsymmetrically substituted imidazolin-2-iminato ligands

The reaction of the unsymmetrical N-heterocyclic carbenes 1-(2,4,6-trimethylphenyl)-3-(adamantyl)imidazolin-2-ylidene (IAdMes, 1a) and 1-(2,6-diisopropylphenyl)-3-(adamantyl)imidazolin-2-ylidene (IAdDipp, 1b) with trimethylsilyl azide furnished the 2-(trimethylsilylimino)imidazolines 2a (Im^AdMesNSiMe₃) and 2b (Im^AdDippNSiMe₃). Desilylation by stirring in methanol gave the corresponding imidazolin-2-imines 3a (Im^AdMesNH) and 3b (Im^AdDippNH). 2a and 2b were treated with [TiCl₄(THF)₂] (THF = tetrahydrofuran) and [CpTiCl₃] (Cp = η⁵-C₅H₅) to form the mono- and bis(imidazolin-2-iminato) titanium(IV) complexes [(Im^AdRN)TiCl₃] (4, R = Mes, Dipp), [Cp(Im^AdRN)TiCl₂] (5, R = Mes, Dipp), and [(Im^AdRN)₂TiCl₂] (6, R = Mes, Dipp). The crystal structures of all compounds except 2b were determined by X-ray diffraction analysis.