Ruediger Beckhaus

Carbene complexes of titanium group metals — formation and reactivity

Abstract The principles in the formation of carbene complexes of titanium group metals are summarized. Characteristic examples of isolated as well as carbene complexes as intermediates are given. Selected structures and reactivity patterns are discussed. Starting from the Tebbe-type chemistry, new aspects of carbene complexes with only one or without cyclopentadienyl ligands are illustrated. In that direction, the formation of carbene complexes with N and O-donor ligands is described. Applications in co-ordination chemistry, organic syntheses and material sciences are summarized.

Complete Defluorination of 1,2,3,4‐Tetramethyl‐5‐(trifluoromethyl)cyclopentadiene by Titanium Tetrakis(dimethylamide)—Selective Formation of a Cyclic Hexanuclear Titanium Fluoroamide and 6,6‐Dimethylaminotetramethylfulvene

1,2,3,4-Tetramethyl-5-(trifluoromethyl)cyclopentadiene (Cp*CF3-H, 1) reacts with [Ti(NMe2)4] (2) under mild conditions to give [Ti(mu-NMe2)(NMe2)(mu-F)(F)]6 (3) in nearly quantitative yield. The molecular structure of 3 consists of a ring of six [TiF2(NMe2)2] edge-bridged octahedra. Titanium complexes containing the Cp*CF3 ligand, which was the primary intention of these investigations, were not observed. C5Me4=C(NMe2)2 (4) was isolated as a by-product. The complete defluorination of an aliphatic CF3 group occurs during the reaction. The reaction mechanism involves the primary formation of a difluorofulvene intermediate C5Me4=CF2 (5), which was monitored by NMR studies. Density functional theory calculations predict a highly charged C6 atom (+0.87) in 5, which is discussed as the driving force of the reaction.

Reaktionen von Pentafulven-Komplexen des Titans mit Nitrilen und iso-Nitrilen — Synthese und Isomerisierungen von σ, π-Chelatkomplexen mit Cp∼N-Liganden

Die Umsetzungen der Fulvenkomplexe Cp*Ti{η6—C5H4=C(R)(R)}Cl (R = H, R = tBu (1); R = Me, R = iPr (4)) mit Nitrilen und iso-Nitrilen, die zu σ, π-Chelatkomplexen mit Cp∼N-Liganden fuhrten, wurden untersucht und die entstehenden Produkte charakterisiert. Wahrend bei der Umsetzung von 1 bzw. 4 mit Nitrilen eine 1, 2-Monoinsertion der CN-Einheit in die Ti—C(R)(R) (Fv)-Bindung beobachtet wird, fuhrt die Reaktion mit iso-Nitrilen zur Insertion zweier Molekule iso-Nitril. Das Nitril-Insertionsprodukt von 1 zeichnet sich durch eine Imin-Enamin-Tautomerisierung aus. Wahrend die primar gebildete metastabile Imin-Form (3) lediglich durch NMR-Messungen in Losung charakterisiert werden konnte, kristallisiert die Enanim-Form (2) aus n-Hexan, so das die Kristallstuktur ermittelt werden konnte. Der primar gebildete Iminoacylkomplex (7) lagert sich aufgrund der Elektrophilie des Titanatoms unter Bildung einer Ti—N Bindung mit signifikantem N(pπ) Ti(dπ) Bindungscharakter um. n n n nReactions of Pentafulvene Complexes of Titanium with Nitriles and iso-Nitriles — Synthesis and Isomerizations of σ, π-Chelat Complexes with Cp∼N-Ligands n n n nThe reactions of fulvene complexes Cp*Ti{η6—C5H4=C(R)(R)}Cl (R = H, R = tBu (1); R = Me, R = iPr (4)) with nitriles and iso-nitriles, leading to σ, π-chelat complexes with Cp∼N-ligands, have been examined and the formed products characterized. Whereas in the reactions of 1 and 4, respectively, with nitriles a 1, 2-mono-insertion of the CN-group in the Ti—C(R)(R) (Fv) bond is observed, the reaction with iso-nitrils leads to the insertion of two molecules iso-nitrile. The nitrile insertion product of 1 is characterized by an imine-enamine tautomerization. Whereas the primary built meta stable imine species (3) was only identified by NMR measurements in solution, the enamine tautomer (2) crystallized from n-hexane, so that the crystal structure could be determined. The primary formed iminoacyl complex (7) rearranges due to the electrophilicity of the titanium centre and builds a Ti—N bond with significant N(pπ) Ti(dπ) bonding character.

Flexible Structural Features of Pentafulvene Titanium Derivatives: Isolation and Characterization of NHC Complexes

The reaction of η(5),η(1)-pentafulvene titanium complexes with the strong N-heterocyclic carbene (NHC) donor 1,3,4,5-tetramethylimidazole-2-ylidene, leads to the formation of isolable NHC titanium adducts, featuring a haptotropic shift of the pentafulvene ligand, proved by single crystal X-ray diffraction as well as NMR spectroscopy studies.

Pentafulvenkomplexe des Titans — Synthese, Struktur und Fluktuierendes Verhalten von Cp′Ti{η6-C5H4=C(p-Tol)2}Cl (Cp′ = Cp*, Cp)

Die Umsetzung von Cp′TiCl3 (Cp′ = Cp* bzw. Cp) mit Magnesium und 6, 6-Di-para-tolylpentafulven liefert in guten Ausbeuten die Pentafulvenkomplexe Cp*Ti{η6-C5H4=C(p-Tol)2}Cl (4) bzw. CpTi{η6-C5H4=C(p-Tol)2}Cl (5). Die Kristallstruktur von 4 konnte durch Rontgenstrukturanalyse geeigneter Einkristalle ermittelt werden und zeigt verglichen mit bereits bekannten η6-Fulvenkomplexen einen ungewohnlich grosen Ti—C(p-Tol)2 (Fv)-Abstand (2.535(5)A), der durch die sterisch anspruchsvollen Substituenten am exocyclischen Kohlenstoffatom des Fulvenliganden hervorgerufen wird. Mit Hilfe dynamischer 1H-NMR- und Sattigungstransfer-Experimente last sich eine Rotation des Fulvenliganden um die Ti—Ct2-Achse (Ct2 = Schwerpunkt der Fulvenring-Kohlenstoffatome) mit einer Aktivierungsbarriere ΔG≠C = 60.6 ± 0.5 kJ mol−1 (TC = 314 ± 2 K) nachweisen. Fur 5 liegt diese deutlich hoher. Ein analoges dynamisches Verhalten ist bereits fur Dienkomplexe bekannt und war Gegenstand zahlreicher Untersuchungen, wird aber unseres Wissens hier erstmalig fur einen Pentafulvenkomplex beschrieben. n n n nComplexes of Titanium — Synthesis, Structure, and Fluxional Behaviour of CpTi{η6-C5H4=C(p-Tol)2}Cl (Cp′ = Cp*, Cp) n n n nThe reaction of Cp′TiCl3 (C′ = Cp* or Cp) with magnesium and 6, 6-di-para-tolylpentafulvene generates good yields of pentafulvene complexes Cp*Ti{η6-C5H4=C(p-Tol)2}Cl (4) and CpTi{η6-C5H4=C(p-Tol)2}Cl (5), respectively. The crystal and molecular structure of 4 have been determined from X-ray data and exhibits compared to known η6-pentafulvene complexes an unusual large Ti—C(p-Tol)2 (Fv)-distance (2.535(5)A) evoked by the bulky substituents at the exocyclic carbon. Dynamic 1H-NMR and spin saturation transfer experiments point out a rotation of the fulvene ligand around the Ti—Ct2 axis (Ct2 = centroid of the fulvene ring carbon atoms) with an activation barrier ΔG≠C = 60.6 ± 0.5 kJ mol−1 (TC = 314 ± 2 K). For 5 this barrier is significantly larger. Analogous dynamic behaviour is well known for diene complexes, but to our knowledge, it is here first-time described for a pentafulvene complex.

From Five to Seven: Ring Expansion of Monoazadiene Titanium Complexes by Insertion of Aldehydes, Ketones and Nitriles

The ring enlargement reactions at ambient temperatures of non C-terminus substituted monoazabutadiene (η4 -RN=CHCH=CH2 ) titanium complexes 2 are investigated. The insertion of aldehydes/ketones (five examples) and nitriles (four examples) into the Ti-C bonds result in expansion of the five-membered rings to uncommon seven-membered titanacycles 3 and 4 in good yields. These new compounds are fully characterized by NMR spectroscopy and single-crystal X-ray diffraction. In subsequent reactions, the seven-membered ring systems are protolyzed and the released organic fragments are isolated. Whereas the aldehyde/ketone insertion products 3 form substituted δ-amino alcohols 5 after reduction with NaBH3 CN, the nitrile insertion products 4 form substituted pyrroles 6 via cyclization.

Crystal structure of (η5-pentamethylcyclopentadienyl)(η5-1-(2,2-dimethylpropyl) cyclopentadienyl)(hydroxy)titaniumchloride, C20H31ClOTi

C20H31CIOTÌ, monocl in ic , P2k ( N o . 14), a = 1 2 . 8 0 4 1 ( 9 ) Â , b = 9 .2935(4 ) A , c = 1 7 . 3 4 8 ( 1 ) A , 1 1 0 . 0 2 7 ( 7 ) ° , V= 1939.5 Â 3 , Ζ = 4 , R g í F ) = 0.033, wRkÚF 2 ) = 0.078, T = 193 Κ .