Stepwise Reactions of TiCl₄ and Ti(OⁱPr)Cl₃ with 2-Propanol. Variable-Temperature NMR Studies and Crystal Structures of [TiCl₂(OⁱPr)(HOⁱPr)(μ-Cl)]₂ and [TiCl₂(OⁱPr)(HOⁱPr)(μ-OⁱPr)]₂

Abstract

The reactions of TiCl 4 and Ti(O i Pr)Cl 3 with 1-3 molar equiv of 2-propanol give the dimeric complexes [TiCl 3 (HO i -Pr)(µ-Cl)] 2 (1), [TiCl 2 (O i Pr)(HO i Pr)(µ-Cl)] 2 (2), and [TiCl 2 (O i Pr)(HO i Pr)(µ-O i Pr)] 2 (3). The reactions are shown to proceed stepwise with the consecutive addition of 2-propanol. Complexes 2 and 3 are dynamic in solution. However, the proton VT NMR studies show that they are static and remain the dimeric features in solution at low temperatures. Two inequivalent methyl environments are observed for the 2-propanol ligand of complex 3, probably due to the intramolecular hydrogen bonding from the isopropanolic hydrogen to the chloride on the second titanium metal center. 2 crystallizes in the orthorhombic space group Pbca with cell parameters of a ) 12.605(3) Å, b ) 12.841(4) Å, c ) 15.414(4) Å, Z ) 4, D calcd ) 1.456 g/cm 3 , R ) 0.049, R w ) 0.062, and G OF ) 1.38. 3 crystallizes in the monoclinic space group P2 1 /c with cell parameters of a ) 9.457(2) Å, b ) 15.277(2) Å, c ) 11.354(4), β ) 112.76(2)°, Z ) 2, D calcd ) 1.305 g/cm 3 , R ) 0.032, R w ) 0.048, and G OF ) 1.29. Through the analysis of the bonding features of the solid-state structures 2 and 3, the relative bonding sequence of -O i Pr > Cl -> HO i Pr > µ--O i Pr > µ-Clis established. This bonding order is useful for the prediction of the geometries of sixcoordinate titanium complexes containing those ligands with the rule "the strong ligand prefers a position trans to the weak ligand in the complexes". Besides, the reaction features demonstrated in this study may apply to the reaction of TiCl 4 or Ti(O i Pr)Cl 3 with diols, especially for chiral diols used as ligands in titanium complexes for various asymmetric syntheses of organic compounds.

A Combined Theoretical and Experimental Investigation on the Enantioselective Oxidation of Aryl Benzyl Sulfides in the Presence of a Chiral Titanium Catalyst

“…A mechanistic scheme was suggested for vanadium catalysts, [21] but nothing similar has been reported for active titanium species, even though titanium was the first metal that was proposed for these reactions [9] and a huge amount of literature relating to its use has been published in more than 25 years. [6,7] The proposal of a plausible reaction mechanism encountered serious problems, [35][36][37] the most significant being the experimental evidence for a fast exchange of titanium ligands in solution [35] and the formation of supramolecular assemblies. It has been demonstrated that the complexes formed between titanium and tartrates [36] and between titanium and BINOL [14,37] yield intricate patterns when observed by spectroscopic techniques.…”

Bis(μ-2-hydroxymethyl-2-methylpropane-1,3-diolato)bis[dichloridotitanium(IV)] diethyl ether disolvate

“…The short Ti-O2 bond length [1.760 (1) Å] is associated with an alkoxo ligand. In comparison, terminal alkoxo ligand Ti-O bond lengths range from 1.702 (4) to 1.742 (6) Å in a variety of iso-propoxo Ti complexes (Gau et al 1996) than observed for isopropyl alcohol ligated to titanium [bond lengths 2.087 (4) and 2.093 (4) Å] (Wu et al 1996). The Ti-Cl1 and Ti-Cl2 bond lengths [2.3184 (4) and 2.3292 (4) Å] do not differ significantly from each other and are typical of Ti-Cl bonds observed elsewhere.…”

“…For general background to Ti-O and Ti-Cl bonds, see: Gau et al (1996); Wu et al (1996). For closely related structures, see: Talbot-Eeckelaers et al (2006); Chang et al (1993); Salta & Zubieta (1997); Chen et al (1997).…”

“…The Ti-Cl1 and Ti-Cl2 bond lengths [2.3184 (4) and 2.3292 (4) Å] do not differ significantly from each other and are typical of Ti-Cl bonds observed elsewhere. (Gau et al 1996;Wu et al 1996).…”

Synthesis and Characterisation of Various Diester and Triester Adducts of TiCl4

“…Wu et al reacted 2-propanol with TiCl 4 to give a Ti 2 Cl 8 dimer. [20] What is of note in the case of this species is that despite the complexity of the molecule formed, there is no elimination of any chloride species as a result of the reaction. Only addition followed by dimerisation of the resulting bimetallic species.…”