Redox-Disproportionation of a Decamethyltitanocene(III) Isonitrile Alkynyl Complex

A General Concept for the Electronic and Steric Modification of 1-Metallacyclobuta-2,3-dienes: A Case Study of Group 4 Metallocene Complexes

The synthesis of group 4 metal 1-metallacyclobuta-2,3-dienes as organometallic analogues of elusive 1,2-cyclobutadiene has so far been limited to SiMe3 substituted examples. We present the synthesis of two Ph substituted dilithiated ligand precursors for the preparation of four new 1-metallacyclobuta-2,3-dienes [rac-(ebthi)M] (M=Ti, Zr; ebthi=1,2-ethylene-1,10-bis(η5-tetrahydroindenyl)). The organolithium compounds [Li2(RC3Ph)] (1 b: R=Ph, 1 c: R=SiMe3) as well as the metallacycles of the general formula [rac-(ebthi)M(R1C3R2)] (2 b: M=Ti, R1=R2=Ph, 2 c: M=Ti, R1=Ph, R2=SiMe3; 3 b: M=Zr, R1=R2=Ph; 3 c: M=Zr, R1=Ph, R2=SiMe3) were fully characterised. Single crystal X-ray diffraction and quantum chemical bond analysis of the Ti and Zr complexes reveal ligand influence on the biradicaloid character of the titanocene complexes. X-band EPR spectroscopy of structurally similar Ti complexes [rac-(ebthi)Ti(Me3SiC3SiMe3)] (2 a), 2 b, and 2 c was carried out to evaluate the accessibility of an EPR active triplet state. Cyclic voltammetry shows that introduction of Ph groups renders the complexes easier to reduce. 13C CPMAS NMR analysis provides insights into the cause of the low field shift of the resonances of metal-bonded carbon atoms and provides evidence of the absence of the β-C-Ti interaction.

Frustrated Lewis pair adducts of alkynyl-capped tetrelenes

The frustrated Lewis pair (FLP) adducts PB{ECl2} (PB = iPr2P(C6H4)BCy2; Cy = cyclohexyl; E = Si, Ge) were used to access a bis(alkynyl)-functionalized silylene and a germylene; the goal behind preparing these species was to obtain new unsaturated main group polymers [E(CCSiMe3)2]n upon heating. While the silylene adduct PB{Si(CCSiMe3)2} was stable up to 150 °C, the heavier element congener PB{Ge(CCSiMe3)2} underwent a complicated rearrangement process accompanied by Cy-group migration and Ge(II)-alkyne coordination. Density functional theory computations were performed to understand the mechanistic pathway for the unusual rearrangement of PB{Ge(CCSiMe3)2}.

1-Zirconacyclobuta-2,3-dienes: synthesis of organometallic analogs of elusive 1,2-cyclobutadiene, unprecedented intramolecular C-H activation, and reactivity studies

The structure, bonding, and reactivity of small, highly unsaturated ring systems is of fundamental interest for inorganic and organic chemistry. Four-membered metallacyclobuta-2,3-dienes, also referred to as metallacycloallenes, are among the most exotic examples for ring systems as these represent organometallic analogs of 1,2-cyclobutadiene, the smallest cyclic allene. Herein, the synthesis of the first examples of 1-zirconacyclobuta-2,3-dienes of the type [Cp'₂Zr(Me₃SiC₃SiMe₃)] (Cp'₂ = rac-(ebthi), (ebthi = 1,2-ethylene-1,1'-bis(η⁵-tetrahydroindenyl)) (2a); rac-Me₂Si(thi)₂, thi = (η⁵-tetrahydroindenyl), (2b)) is presented. Both complexes undergo selective thermal C-H activation at the 7-position of the ansa-cyclopentadienyl ligand to produce a new type of "tucked-in" zirconocene system, 3a and 3b, that possesses a η³-propargyl/allenyl ligand. Both types of complexes react with carbonyl compounds, producing enynes in the case of 2a and 2b, as well as η¹-allenyl complexes for 3a and 3b. Computational analysis of the structure and bonding of 2a and 3a reveals significant differences to a previously described related Ti complex. All complexes were fully characterised, including X-ray crystallography and experimental results were supported by DFT analysis.

Electronic Structure Analysis and Reactivity of the Bimetallic Bis-Titanocene Vinylcarboxylate Complex, [(Cp2Ti)2(O2C3TMS2)]†

Multimetallic redox cooperativity features heavily in both bioinorganic and synthetic reactions. Here, the electronic structure of the bimetallic Ti/Ti complex 11, [(Cp₂Ti)₂(O₂C₃TMS₂)] has been revisited with EPR, confirming a predominantly Ti^III/Ti^III electronic structure. Reactions of 11 with 2,6-dimethylphenyl isocyanide (CNXyl), TMSCl, MeI, and BnCl were explored, revealing differential redox chemistry of the bimetallic core. In reactions with CNXyl and TMSCl, the metallacyclic Ti^III center remained unperturbed, with reactions taking place at the pendent κ²(O,O)-titanocene fragment, while reaction with MeI resulted in remote oxidation of the metallacyclic Ti center, indicative of a cooperative redox process. All structures were studied via X-ray diffraction and EPR spectroscopic analysis, and their electronic structures are discussed in the context of the covalent bond classification (CBC) electron counting method.

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

The synthesis and characterisation of a 1-titanacyclobuta-2,3-diene complex, an organometallic analog of elusive 1,2-cyclobutadiene, is presented.

The synthesis of an unusual 1-metalla-2,3-cyclobutadiene complex [rac-(ebthi)Ti(Me₃SiC₃SiMe₃)] (rac-ebthi = rac-1,2-ethylene-1,1′-bis(η⁵-tetrahydroindenyl)), a formal metallacyclic analogue of a non-existent four-membered 1,2-cyclobutadiene, is described. By variation of the cyclopentadienyl ligand of the titanocene precursor it was possible to stabilise this highly exotic compound which selectively reacts with ketones and aldehydes to yield enynes by oxygen transfer to titanium. Analysis of the bonding and electronic structure of the metallacycle shows that the complex is best described as an unusual antiferromagnetically coupled biradicaloid system, possessing a formal Ti(iii) centre coordinated with a monoanionic radical ligand.

Titanocene Silylpropyne Complexes: Promising Intermediates en route to a Four-Membered 1-Metallacyclobuta-2,3-diene?

Coordination of the alkyl-substituted alkynes Me₃ SiC₂ CH₂ R (1: R=SiMe₃ ; 2: R=N(SiMe₃ )₂ ) to titanocene centres [Cp'₂ Ti] (Cp'=Cp, Cp*) yields stable alkyne complexes of the type Cp'₂ Ti(η² -Me₃ SiC₂ CH₂ R) (3: Cp'=Cp, R=SiMe₃ ; 5: Cp'=Cp, R=N(SiMe₃ )₂ ; 6: Cp'=Cp*, R=SiMe₃ ) that are not prone to alkyne/allene isomerisation. When reacting alkyne 2 with Cp^*₂ TiCl₂ and Mg formation of the complex Cp*₂ Ti(III)(η³ -Me₃ SiC₂ CH₂ ) (7) which displays a propargylic unit coordinated to the Ti^III centre takes place. All complexes were fully characterised, the molecular structures for 5, 6, and 7 are discussed.