A Brønsted Acidic Gallium Hydride: Facile Interconversion of NNNN‐Macrocycle Supported [Ga^I]⁺ and [Ga^IIIH]²⁺

Abstract

Protonolysis of [Cp*M] (M=Ga, In, Tl) with [(Me4TACD)H][BAr4Me] (Me4TACD=N,N′,N′′,N′′′‐tetramethyl‐1,4,7,10‐tetraazacyclododecane; [BAr4Me]−=[B{C6H3‐3,5‐(CH3)2}4]−) provided monovalent salts [(Me4TACD)M][BAr4Me], whereas [Cp*Al]4 yielded trivalent [(Me4TACD)AlH][BAr4Me]2. Protonation of [(Me4TACD)Ga][BAr4Me] with [Et3NH][BAr4Me] gave an unusually acidic (pKa(CH3CN)=24.5) gallium(III) hydride dication [(Me4TACD)GaH][BAr4Me]2. Deprotonation with IMe4 (1,3,4,5‐tetramethyl‐imidazol‐ylidene) returned [(Me4TACD)Ga][BAr4Me]. These reversible processes occur with formal two‐electron oxidation and reduction of gallium. DFT calculations suggest that gallium(I) protonation is facilitated by strong coordination of the tetradentate ligand, which raises the HOMO energy. High nuclear charge of [(Me4TACD)GaH]2+ facilitates hydride‐to‐metal charge transfer during deprotonation. Attempts to prepare a gallium(III) dihydride cation resulted in spontaneous dehydrogenation to [(Me4TACD)Ga]+.

Hydroboration and Deoxygenation of CO2 Mediated by a Gallium(I) Cation

“…The carbonate ligand binds in a κ 2 -manner and is coplanar with N1 and N3, such that the equatorial plane of the distorted octahedron can be defined by N1, O2, O3, N3. The Ga1−N1 and −N3 distances (2.123(3), 2.20(2) Å) are longer than the near-axial Ga1−N2 and Ga−N4 bonds (2.105(5), 2.07(4) Å; N2−Ga1−N4 = 155.0(3)°), but all Ga−N bonds are substantially contracted compared to compound 1 (2.376− 2.485 Å), 19 consistent with oxidation of the metal center. Compound 3 is, to the best of our knowledge, the first structurally characterized example of a molecular gallium carbonate complex.…”

“…In the knowledge that coordination of a Lewis acid to the terminal oxygen atom can be an effective way to stabilize reactive oxide complexes, − a 1:1 mixture of 1 and BPh 3 was exposed to N 2 O. Rapid elimination of N 2 was accompanied by the clean formation of a new species 4·BPh 3 , which was characterized in the 1 H NMR spectrum by well-defined NC H 2 multiplets whose downfield chemical shift is indicative of Me 4 TACD bound to a Ga 3+ ion (Scheme b) . In addition to the sharp peak of the borate counterion, the 11 B NMR spectrum exhibited a broader resonance at δ B −10.3 ppm, upfield shifted relative to “free” BPh 3 in this solvent (δ B −0.2 ppm) and consistent with four-coordinate boron and a Ga­(μ-O)B unit.…”

“…Gallium as the most electronegative element of group 13 is less reducing than its lighter congeners, in part due to stabilization of the 4 s electrons by the “scandide-contraction” induced by the poor shielding of the intervening filled 3 d -subshell. , In the absence of strong donors, [Ga] + is a stronger oxidant than a ferrocenium ion . The 4 s “lone pair” can be made more energetic by coordination of neutral σ-donating ligands to the cation, but such species undergo disproportionation to elemental gallium and Ga III species. − ,− Chelation by the NNNN macrocycle Me 4 TACD imparts reducing character to the metal center, such that [(Me 4 TACD)­Ga] + may reduce CO 2 to CO and form an unstable oxido gallium­(III) cation [(Me 4 TACD)­GaO] + , which rapidly adds CO 2 to provide a carbonate complex [(Me 4 TACD)­Ga­(κ 2 -O 2 CO)] + . In the presence of HBpin, hydroboration of CO 2 to formate equivalent (HCO 2 )­Bpin occurs, with concomitant consumption of the gallium carbonate cation to an unidentified species.…”

“…29 The 4s "lone pair" can be made more energetic by coordination of neutral σ-donating ligands to the cation, but such species undergo disproportionation to elemental gallium and Ga III species. 30−32,59−62 Chelation by the NNNN macrocycle Me 4 TACD imparts reducing character to the metal center, 19 such that [(Me 4 TACD)Ga] + may reduce CO 2 to CO and form an unstable oxido gallium(III) cation [(Me 4 TACD)GaO] + , which rapidly adds CO 2 to provide a carbonate complex [(Me 4 TACD)Ga(κ 2 -O 2 CO)] + . In the presence of HBpin, hydroboration of CO 2 to formate equivalent (HCO 2 )Bpin occurs, with concomitant consumption of the gallium carbonate cation to an unidentified species.…”

“…17,18 We recently reported the gallium(I) complex cation [(Me 4 TACD)Ga][BAr 4 ] (1; Me 4 TACD = N,N′,N″,N′′′tetramethyl-1,4,7,10-tetraazacyclododecane; Ar = C 6 H 3 -3,5-Me 2 ), which can be reversibly protonated to give its conjugate acid, the dicationic gallium(III) hydride [(Me 4 TACD)GaH]-[BAr 4 ] 2 (2). 19 Although the polarized Ga δ+ −H δ− bond and high Lewis acidity of the dicationic gallium center might be expected to endow compound 2 with activity in the hydroboration of CO 2 with HBpin, 20−22 this compound exhibits an unusual electronic situation with a low-lying LUMO (Ga−H σ*) and a ligand-based HOMO. On the other hand, while neutral and anionic gallium(I) complexes have been well-studied in small-molecule activation, 23−26 less strongly reducing monovalent cations such as 1 are little explored in this regard.…”