Synthesis and Characterisation of Bridged Titanocene Oxido Complexes and Their Reactions with Water

Abstract

Keywords: Metallocenes / Titanium / Water / IR spectroscopyThe synthesis of two pyridine-stabilised ansa-titanocene oxido complexes is described. Oxidation of the titanocene sources Cp b 2 Ti(η 2 -Me 3 SiC 2 SiMe 3 ) (Cp b 2 = bridged η 5 -cyclopentadienyl, 1a and 1b) with nitrous oxide yields corresponding oxido compounds 2a and 2b, respectively, in high yields. These can be hydrolysed to give dihydroxido complexes 3a

1-Titanacyclobuta-2,3-diene – an elusive four-membered cyclic allene

“…The nature of the Ti species formed by oxygen transfer could not be unequivocally clarified; however, formation of a Ti oxido species is likely. It should however be noted that such species require either kinetic stabilisation by bulky Cp ligands,38 or additional Lewis bases such as pyridines39 to persist as monomeric well-defined complexes. Monitoring of the reaction of 2 and acetone at low temperature, i.e.…”

Recent Synthetic and Catalytic Applications of Group 4 Metallocene Bis(trimethylsilyl)acetylene Complexes

“…Reactions of such organometallic compounds with water as the basis for elementary steps of photoassisted water splitting were summarized in a review and a series of very interesting papers . Detailed investigations were done with respect to the two half reactions of water splitting: the formation of hydrogen and the photoassisted activation of the Ti‐O bond for water oxidation to oxygen . Irradiation of Cp* 2 Ti(OH) 2 resulted in the formation of well‐defined mono‐ and tetranuclear titanium(III) and titanium(IV) complexes, which are formed by Ti‐O bond cleavage and/or Cp*‐elimination .…”

“…This was the reason why an alternative method to obtain such complexes was investigated. The first step for this was the oxidation of the of ansa ‐titanocene bis(trimethylsilyl)acetylene complexes by N 2 O to the pyridine stabilized Ti=O complexes, which was followed by hydrolysis of the Ti=O moiety, a formal 1,2‐addition of water across this bond (Scheme ) …”

Highly selective visible light-induced Ti–O bond splitting in an ansa-titanocene dihydroxido complex

“…Therefore ansa-titanocene dihydroxido complexes were synthesised via a new ''oxido'' route, a method, which prevents the production of metallocene alkenyl-silanolate side products. 9,10 Through extensive explorative testing of various conditions including different light filters and temperatures we discovered that selective Ti-O bond splitting occurred in toluene at 250 K. Irradiation of a solution of complex Me 4 Si 2 (C 5 Me 4 ) 2 Ti(OH) 2 (1) resulted in formation of a single Ti(III) species (2) detected by EPR spectroscopy (Fig. 1, g = 1.978, A Ti = 9.7 G, line width DB = 3.9 G), accompanied by a colour change from yellow to dark green.…”

“…Thus, in order to further corroborate the photo-reduction of complex 1, we prepared complex 2 by an alternative method starting from the aforementioned oxido species Me 4 Si 2 (C 5 Me 4 ) 2 Ti(py)O (py = pyridine). 10 Reduction of the latter with dihydrogen at elevated temperatures in toluene resulted in elimination of pyridine and selective reduction of the terminal oxido group and the Ti(IV) centre to yield the trivalent titanocene(III) monohydroxido complex 2. The EPR signal of 2 exactly corresponds to the Ti(III) complex obtained during irradiation (Fig.…”