Reactions of Metal-Metal Quintuple Bonds with Alkynes: [2+2+2] and [2+2] Cycloadditions

Experimental and Computational Studies on Quadruply Bonded Dimolybdenum Complexes with Terminal and Bridging Hydride Ligands

This contribution focuses on complex [Mo₂ (H)₂ (μ-Ad^Dipp2 )₂ ] (1) and tetrahydrofuran and pyridine adducts [Mo₂ (H)₂ (μ-Ad^Dipp2 )₂ (L)₂ ] (1⋅thf and 1⋅py), which contain a trans-(H)Mo≣Mo(H) core (Ad^Dipp2 =HC(NDipp₂ )₂ ; Dipp=2,6-iPr₂ C₆ H₃ ). Computational studies provide insights into the coordination and electronic characteristics of the central trans-Mo₂ H₂ unit of 1, with four-coordinate, fourteen-electron Mo atoms and ϵ-agostic interactions with Dipp methyl groups. Small size C- and N-donors give rise to related complexes 1⋅L but only one molecule of P-donors, for example, PMe₃ , can bind to 1, causing one of the hydrides to form a three-centered, two-electron (3c-2e) Mo-H→Mo bond (2⋅PMe₃ ). A DFT analysis of the terminal and bridging hydride coordination to the Mo≣Mo bond is also reported, along with reactivity studies of the Mo-H bonds of these complexes. Reactions investigated include oxidation of 1⋅thf by silver triflimidate, AgNTf₂ , to afford a monohydride [Mo₂ (μ-H)(μ-NTf₂ )(μ-Ad^Dipp2 )₂ ] (4), with an O,O'-bridging triflimidate ligand.

Formation of a dimeric tungsten(i) complex via C-H activation

An aminopyridinato ligand stabilized and coordinatively unsaturated dimeric tungsten(0) complex having an electronic structure with six metal centred HOMOs (σ²,π²,π²,δ²,δ², and δ*²) has been isolated and structurally characterized. Its reaction with a cryptand leads to a C-H bond activation of one of the isopropyl groups of the N-ligand to form a dimeric tungsten(i) complex.

Reversible Cleavage/Formation of the Chromium-Chromium Quintuple Bond in the Highly Regioselective Alkyne Cyclotrimerization

Herein we report the employment of the quintuply bonded dichromium amidinates [Cr{κ² -HC(N-2,6-ⁱ Pr₂ C₆ H₃ )(N-2,6-R₂ C₆ H₃ )}]₂ (R=iPr (1), Me (7)) as catalysts to mediate the [2+2+2] cyclotrimerization of terminal alkynes giving 1,3,5-trisubstituted benzenes. During the catalysis, the ultrashort Cr-Cr quintuple bond underwent reversible cleavage/formation, corroborated by the characterization of two inverted arene sandwich dichromium complexes (μ-η⁶ :η⁶ -1,3,5-(Me₃ Si)₃ C₆ H₃ )[Cr{κ² -HC(N-2,6-ⁱ Pr₂ C₆ H₃ )(N-2,6-R₂ C₆ H₃ )}]₂ (R=ⁱ Pr (5), Me (8)). In the presence of σ donors, such as THF and 2,4,6-Me₃ C₆ H₂ CN, the bridging arene 1,3,5-(Me₃ Si)₃ C₆ H₃ in 5 and 8 was extruded and 1 and 7 were regenerated. Theoretical calculations were employed to disclose the reaction pathways of these highly regioselective [2+2+2] cylcotrimerization reactions of terminal alkynes.

Synthesis and Characterization of Dirhenadehydro[12]annulenes

The 12-membered-ring metallacycles [mer-Re{≡CCH=C(R)C≡C-}Cl(PMe2 Ph)3 )]2 (R=CMe3 , 1-adamantyl), which are organometallic analogues of antiaromatic octadehydro[12]annulene, are prepared by heating the methyl carbyne complexes mer-Re{≡CCH=C(R)C≡CH}(CH3 )Cl(PMe2 Ph)3 . An intermolecular σ-bond metathesis between the Re-CH3 bond and the acetylenic C-H bond is proposed for their formation.

From chromium-chromium quintuple bonds to molecular squares and porous coordination polymers

Reaction of the quintuply bonded chromium(I) dimer [ApCrCrAp] (Ap = sterically demanding 2-aminopyridinate) with pyrazine yields a chromium(II) complex with a η(4):η(4) face-on coordinated pyrazine dianion. Reaction with 4,4'-bipyridine, on the other hand, completely cleaves the metal-metal bond, leading to a chromium(II)-based molecular square. XRD and magnetic measurements show ligand radical anions and a ferrimagnetic alignment of alternating metal and ligand magnetic moments. Controlled polymerization of the molecular square with pyrazine yields a porous coordination polymer featuring both reduced and nonreduced linkers.

The ligand-based quintuple bond-shortening concept and some of its limitations

Herein, the ligand-based concept of shortening quintuple bonds and some of its limitations are reported. In dichromium-diguanidinato complexes, the length of the quintuple bond can be influenced by the substituent at the central carbon atom of the used ligand. The guanidinato ligand with a 2,6-dimethylpiperidine backbone was found to be the optimal ligand. The reduction of its chromium(II) chloride-ate complex gave a quintuply bonded bimetallic complex with a Cr-Cr distance of 1.7056 (12) Å. Its metal-metal distance, the shortest observed in any stable compound yet, is of essentially the same length as that of the longest alkane C-C bond (1.704 (4) Å). Both molecules, the alkane and the Cr complex, are of remarkable stability. Furthermore, an unsupported Cr(I) dimer with an effective bond order (EBO) of 1.25 between the two metal atoms, indicated by CASSCF/CASPT2 calculations, was isolated as a by-product. The formation of this by-product indicates that with a certain bulk of the guanidinato ligand, other coordination isomers become relevant. Over-reduction takes place, and a chromium-arene sandwich complex structurally related to the classic dibenzene chromium complex was observed, even when bulkier substituents are introduced at the central carbon atom of the used guanidinato ligand.