A Simple Homoleptic Gallium(I) Olefin Complex: Mimicking Transition‐Metal Chemistry at a Main‐Group Metal?

Abstract

The earth-metal olefin complex [Ga I (COD) 2 ] + [Al-(OR F) 4 ] À (COD = 1,5-cyclooctadiene; R F = C(CF 3) 3) constitutes the first homoleptic olefin complex of any main-group metal accessible as a bulk compound. It is straight forward to prepare in good yield and constitutes an olefin complex of a main-group metal that-similar to many transition-metalsmay adopt the + 1 and + 3 oxidation states opening potential applications. Crystallographic-, vibrational-and computational investigations give an insight to the atypical bonding between an olefin and a main-group metal. They are compared to classical transition-metal relatives. Transition metal olefin complexes are widely known and used as (pre-)catalysts in a variety of reactions. Since their first discovery in the 1820ies by Zeise, [1] they developed into a mature class of compounds with a manifold of applications. Especially the properties of chelating 1,5-cyclooctadiene (COD) to stabilize low valent transition-metal complexes like [M(COD) 2 ] 0,+ (M = Ni, Pd, Pt) [2, 3] or [ClM(COD)] 2 /[M-(COD) 2 ] + (M = Rh, Ir) [4, 5] sufficiently to be isolated, furnished a large interest into follow-up chemistry. Since the COD-ligands are easily displaced by other ligands such as phosphines, or alternatively may be hydrogenated in the course of the reaction and thus are removed from the metal atom leaving free and reactive coordination sites, they rapidly developed into very useful (pre-)catalysts for a variety of reactions. [6] For example, Crabtrees famous catalyst [Ir-(COD)(py)(PCy 3)] + [PF 6 ] À [4] or the Rhodium analogues [7] activate H 2 and hydrogenate, but also isomerize and hydroborate alkenes. By contrast to transition-metal olefin complexes, homoleptic olefin complexes of a main-group metal remain unknown as isolable bulk compounds. Only a few examples are known under the special conditions of matrix isolation spectroscopy, that is, Li(C 2 H 4) n [8] or M(C 2 H 4) n (M = Al, [9] Ga, [10, 11] In; [12] n = 1-3) as well as inside a mass spectrometer, that is, [M(C 2 H 4) n ] + (M = Na [13] or Al [14]). Turning to group 13, only a few bulk compounds are related; for example, the Al atom in AlCp 3 , but not GaCp 3 , is h 2-coordinated by at least one charged [C 5 H 5 ] À = Cp À ligand showing a related behaviour to olefins. [15] Somewhat related are the classical Gacyclophane and-arene complexes. [16] The first true group 13 olefin complex, the cyclohexene complex [Al(C 6 F 5) 3 •(C 6 H 10)], was published by Stephan et al. [16c] Very recently, Crimmin reported systems that could be described as Al I olefin complexes or Al III metallacyclopropanes, [17] Aldridge added versions, which include such situations as intermediates of Al I reactions with arenes [18] and Inoue reactions of a dialumene with olefins and acetylenes giving the respective dialuminacyclobutanes. [19] Still, the simple access to an isolable bulk homoleptic olefin complex of any main-group metal-most favourably to an entry with switchable redox states that supports ligand substitution by steering ...

The Coordination Chemistry and Clustering of Subvalent Ga+ and In+ upon Addition of σ‐Donor Ligands

“…Hence, [pf] (H atoms omitted for clarity) and QTAIM analysis, i. e. the Laplacian of the electron density, of the bonding interaction between Ga and the C=C double bonds. [54] considering the significant stabilization of the ns 2 -electron pair (n = 4,5) in cationic complexes outlined previously, the introduction of a positive charge represents a promising pathway to overcome this hurdle. In fact, [Ga(PhF) 2 ][CHB 11 Cl 11 ] was reported to be accessible in high yield via spontaneous H 2 -elimination from the [H 2 Ga III (PhF) 2 ][CHB 11 Cl 11 ] complex.…”

“…). [18,[52][53][54] Figure 4. a) Frontier orbitals of the [Ga{P( t Bu) 3 } 2 ] + -complex as calculated on PBE0-D3BJ/def2-TZVPP//BP86-D3BJ/def2-SVP level of DFT (isovalue of 0.09 a.u.).…”

“…Figure 7. Molecular cation structure of [Ga(COD) 2 ][pf] (H atoms omitted for clarity) and QTAIM analysis, i. e. the Laplacian of the electron density, of the bonding interaction between Ga and the C=C double bonds [54]. …”

Low Valent Main Group Compounds in Small Molecule Activation

“…Krossing and co-workers successfully isolated [Ga(COD)2] + complex in high yields. [187] The cationic Ga + is coordinated to four double bonds of the two COD ligands in a pseudo-tetrahedral arrangement. Theoretical studies showed there is no back-bonding to the COD ligands, implying a dominant electrostatic interaction of Ga + and COD.…”

Quaternary and Tertiary Carbon Centers Synthesized via Gallium-Catalyzed Direct Substitution of Unfunctionalized Propargylic Alcohols with Boronic Acids

“…Mechanistic routes for the cationic Ga- or Ag-catalyzed propargylic substitution reaction were postulated (Scheme ). It was thought that the metal could coordinate with the hydroxyl of 13 (see 14 ) or both the alkyne and the hydroxyl to form pi-coordinated 15 , which corresponds to the need for a soft Lewis acid. , Since trifluoroborate salts did not serve as good nucleophiles in this reaction, the hydroxyl group of the boronic acid might be playing a role in the reaction mechanism. Recruitment of the boronic acid by the metal could then produce adduct 17 .…”