Formation of Binuclear Zigzag Hexapentaene Titanium Complexes via a Titanacumulene [Ti=C=C=CH₂] Intermediate

Abstract

The reaction of bis(h 5 :h 1 -pentafulvene)titanium complexes with an allylidenephosphorylide Ph 3 P=C(H)-C(H)=CH 2 leads to binuclear zigzag hexapentaene titanium complexes (Ti2a, Ti2b). The formation of the central C 6 H 4 unit can be described as as pontaneous double C À Hb ond activation process,leading to an R 3 P = C = C = CH 2 intermediate, as as ynthon for atitanabutatriene fragment [(Cp R ) 2 Ti = C = C = CH 2 ]( R: 2-adamantyl, CH(p-tol) 2 ). In as ubsequent dimerization Ti2a and Ti2b are formed, proofed by single-crystal Xray diffraction and NMR measurements.T he reaction sequence is confirmed by DFT calculations.Cumulenes (I), like allenes and its higher homologues are intriguing structure motifs (Scheme 1), due to their structural and physical properties. [1] Generally,t he field of one-dimen-

Advantages of Group 4 Metallocene Bis(trimethylsilyl)acetylene Complexes as Metallocene Sources Towards Other Synthetically used Systems

“…After dimerization the titanabutatriene complex formed a binuclear zig-zag hexapentaene titanium complex (Scheme 9). [53] In summary for the bis(η 5 : η 1 -pentafulvene) complexes (M: Ti, Zr) complexes electrophilic attack, haptotropic shift, insertion of multiple bonds of e. g. nitriles, isonitriles, imines or carbonyl compounds) and EÀ H bond activation reactions were found. NÀ H/CÀ H and double CÀ H bond activation reactions under mild conditions show the special character of these complexes compared to metallocene methyl complexes Cp 2 TiMe 2 .…”

“…Aditionally, by other reactions the conversion of only one pentafulvene [C 5 H 4 =CR 2 ] 2− to Cp′ ligands [C 5 H 4 −CHR 2 ] 1− was described whereas the other remains as [C 5 H 4 =CR 2 ] 2− ligand. This opens the way to a lot of other synthetically very interesting reactions …”

“…N−H/C−H and double C−H bond activation reactions under mild conditions show the special character of these complexes compared to metallocene methyl complexes Cp 2 TiMe 2 . Beckhaus described the weak M−C exo interaction in these complexes as the first examples of heteroatom free organometallic frustrated Lewis pairs in comparison to classical heteroatom containing Lewis pairs …”

“…This opens the way to a lot of other synthetically very interesting reactions. [53][54][55][56][57][58][59][60][61][62][63][64][65][66][67][68][69][70][71] The most spectacular example for unexpected results was the reaction of η 5 : η 1 -pentafulvene titanium complexes with the allylidenephosphorylide Ph 3 P=C(H)À C(H)=CH 2 , giving a titanabutatriene complex by a spontaneous double CÀ H bond activation via an intermediate Ph 3 P=C=C=CH 2 . After dimerization the titanabutatriene complex formed a binuclear zig-zag hexapentaene titanium complex (Scheme 9).…”

“…[39] Beckhaus described the weak MÀ C exo interaction in these complexes as the first examples of heteroatom free organometallic frustrated Lewis pairs in comparison to classical heteroatom containing Lewis pairs. [39,[45][46][47][48][49][50][51][52][53][54][55][56][57][58][59][60][61] Advantages of η 5 : η 1 -pentafulvene complexes in reactions with selected substrates are the high yields of products, formed at room temperature (Scheme 5-10) with a very high selectivity. This means that for selected substrates in a two step process the selective reaction of two or even more substrates is possible.…”