Synthesis, Reactivity, and Insights into the Lewis Acidity of Mononuclear Titanocene Imido Complexes Bearing Sterically Demanding Terphenyl Moieties

Abstract

The titanocene imido complex 2 with a m-terphenyl moiety is synthesized by reacting Cp 2 Ti(η 2 -Me 3 SiC 2 SiMe 3 ) (1) with m-terphenyl azide. Subsequent reactivity studies with tetrahydrofuran, pyridine, and nitriles verify that these substrates act as two-electron donors, thus binding end-on to the titanium center to give the donor-stabilized titanocene imides 3a,b and 4a− c. Given the potential of the titanium center to act as an electron acceptor, 2 was reacted with phosphine oxides (OPEt 3 /OPPh 3 ) according to the Gutmann−Beckett method to get information about the Lewis acidity of 2 and to yield 5a,b. Complex 5a is one of the rare examples of structurally characterized titanium complexes with an η 1 -bound cyclopentadienyl ligand. Further insights into the Lewis acidity of 2 were obtained from the nitrile adducts 4a−c through IR spectroscopy and NMR spectroscopy (δ( 19 F) and 1 J C,F employing 4-fluorobenzontrile). In reactions of 2 with terminal alkynes, the respective titanium amido acetylides 6a,b were obtained, whereas reactions with internal alkynes demonstrate that 2 is not very prone to undergoing [2 + 2] cycloaddition reactions.

On the Use of Aqueous Metal-Aqua pKa Values as a Descriptor of Lewis Acidity

“…We were surprised to find that the Gutmann-Beckett method, in which Lewis acidity is interrogated by 31 P nuclear magnetic resonance (NMR) spectroscopy with a suitable phosphorus-containing probe molecule, typically a phosphine oxide, has not been employed for systematic quantification of the Lewis acidity of redox-inactive metal ions. , As this method has been found to be effective for alkaline earth metal complexes and some transition metal compounds, , we anticipated that studies of the wider family of redox-inactive metal ions by this method might enable direct comparisons with the common aqueous p K a scale as well as provide insights for new studies of Lewis acid-modulated chemistry.…”

Titanocene pnictinidene complexes

“…Comparable base-stabilized titanium imido complexes of the general type [Cp2(L)Ti=N Mes Ter] (L = pyridine, THF, RCN) also show Ti-N-CTer angles of over 155°. 30 Scheme 3. Synthesis of the titanocene pnictinidene complexes 3 and 5.…”

An Isolable Azide Adduct of Titanium(II) Follows Bifurcated Deazotation Pathways to an Imide

“…This sterically accessible and highly reducing d 2 complex forms adducts with π-acids such as AdNC and N 2 , [(Tp t Bu,Me )­TiCl­(CNAd)] ( 1 CNAd ) and [{(Tp t Bu,Me )­TiCl} 2 (μ-N 2 )] ( 2 ), respectively, whereas it quantitatively extrudes N 2 upon reaction with Me 3 SiN 3 to afford the prototypical Ti IV silylimide complex, [(Tp t Bu,Me )­TiCl­(NSiMe 3 )] . Although this entry to titanium imide complexes is universal, − the unique geometry of 1 prompted us to further explore its reactivity toward other azides: Under an Ar atmosphere, treatment of a benzene solution of 1 with the organic azide, AdN 3 , led to a dark yellow two-component reaction mixture (Scheme ). 1 H NMR spectroscopy revealed two diamagnetic species; the first, [(Tp t Bu,Me )­TiCl­(γ-N 3 Ad)] ( 3 ), displayed a single set of pyrazole resonances, in line with rapid interconversion on the 1 H NMR time scale.…”