Terminal Alkynes in the Coordination Sphere of Titanocene Complexes - Elementary Steps in Catalytic Dimerizations and Oligomerizations of Alkynes

Photoinduced Generation of Catalytic Complexes from Substituted-Titanocene−Bis(trimethylsilyl)ethyne Complexes: Contribution to the Mechanism of the Catalytic Head-to-Tail Dimerization of Terminal Alkynes

“…The parameters of the molecular structure of D2 determined by single-crystal X-ray diffraction (Tables and ) confirmed that the HTT dimer is bonded to the titanocene framework as a simple η 2 -alkyne (Figure ). The C−C bond length of the coordinated alkyne corresponds nicely to the values reported for [(η 5 -C 5 Me 5 ) 2 Ti(η 2 -R 1 C⋮CR 2 )] complexes (R 1 /R 2 : (SiMe 3 ) 2 , 1.309(4) Å; Ph/SiMe 3 , 1.308(3)Å;9b Fc/Ph, 1.312(5) Å 17c ) and the (η 5 -C 5 Me 4 H) 2 Ti complexes B1 and 1 mentioned above. The dihedral angle of the cyclopentadienyl planes in D2 (40.2°) is similar to that of [(η 5 -C 5 Me 5 ) 2 Ti(η 2 -PhC⋮CSiMe 3 )] (40.6°), and very slightly lower than that in 2 (41.1°) 9b.…”

“…The titanium atom is pseudotetrahedrally coordinated, though, with angles markedly deviated from the value of the regular tetrahedron; C(1)−Ti−C(3) is more acute (92.1(3)°) because of unlike steric requirements of the cyclopentadienyl and both hydrocarbyl ligands. The Ti−C and Ti−C⋮ bond lengths of 2.127(7) and 2.102(7) Å, respectively, compare well with those observed for other titanium(IV) organometallics: [(η 5 -C 5 Me 5 ) 2 Ti(η 1 -C⋮CPh)(η 1 -CHCHPh)], 2.124(4) and 2.099(5) Å; [(η 5 -C 5 Me 5 ) 2 Ti(η 1 -CHCH 2 )(η 1 -C⋮CPh)], 2.14 and 2.10 Å (in the same order); [(η 5 -C 5 Me 5 ) 2 TiF(η 1 -CHCH 2 )], 2.098(6) Å (Ti−C);10d [(η 5 -C 5 H 5 ) 2 Ti(η 1 -C(Ph)CMe 2 )Br 0.5 Cl 0.5 ], 2.234(4) Å (Ti−C); [{η 5 -C 5 (SiMe 3 )H 4 } 2 Ti(η 1 -C⋮C−C⋮Fc) 2 ], 2.090(7) and 2.099(7) Å (Ti−C⋮); [(η 5 -C 5 Me 5 ) 2 Ti(OH)(η 1 -C⋮Ph)], 2.117(2) Å (Ti−C⋮) . The fact that both Ti−C bond lengths are similar indicates that the η 1 ligands are simply σ-bonded with no significant π-bond contribution to the Ti−C bond such as Ti−CC ↔ TiCC and Ti−C⋮C ↔ TiCC.…”

“…Nevertheless, the chemical picture is unambiguous (Figure , Table ) and verifies the structure assigned on the basis of spectral data. The molecular geometry of A1 corresponds favorably to that of [(η 5 -C 5 Me 5 ) 2 Ti(η-C⋮CPh){η 1 -( E )-CHCHPh}] . The titanocene cyclopentadienyls are eclipsed with the C (Me)− C H edges at the hinge position, thus reflecting mimicked crystallographic C 2 /σ symmetry.…”

“…This was mainly due to a slow conversion of this complex to the catalytically active species, whose concentration remained very low during the alkyne dimerization. The validity of the catalytic cycle has recently been brought in doubt by an observation that [(η 5 -C 5 Me 5 ) 2 Ti(η 2 -Me 3 SiC⋮CSiMe 3 )] reacts with terminal alkynes to afford the decamethyltitanocene (η 1 -( E )-alkenyl)(η 1 -alkynyl) complexes, which cannot obviously be the precursors of the HTT dimers …”

Arylimido‐Bridged Dinuclear Ti(μ‐NAr)2Ti Scaffold for Alkyne Insertion into the ortho‐C−H Bond of Arylimido Ligands

“…Two 1‐(trimethylsilyl)propyne units are carbometallated to form two six‐membered metallacycles, and the carbon atom bearing a trimethylsilyl group is bound selectively to the titanium center. The bond length of Ti−C14 (2.109(2) Å) lies in the typical range of Ti−C(sp 2 ) bonds,,,, and the bond length of C13−C14 (1.356(4) Å) reveals that this bond is a double bond. The six‐membered titanacycles are highly twisted to create a larger bond angle of Ti−N−C1 than in 2 a (i.e., 154.6(2) versus (147.6(2)°, respectively).…”