Formation of a Cationic Calcium Hydride Cluster with a “Naked” Triphenylsilyl Anion by Hydrogenolysis of Bis(triphenylsilyl)calcium

Protonolysis of bis­(triphenylsilyl)­calcium [Ca­(SiPh₃)₂(THF)₄] (<b>1</b>; THF = tetrahydrofuran) with the NNNN-type macrocyclic amido triamine (Me₃TACD)H (TACD = 1,4,7-triazacyclododecane) gave the heteroleptic calcium complex [Ca­(Me₃TACD)­SiPh₃] (<b>2</b>) in quantitative yield. Hydrogenolysis of <b>2</b> gave the cationic tricalcium dihydride cluster [Ca₃H₂(Me₃TACD)₃]⁺(SiPh₃)⁻·2THF (<b>4a</b>) in high yield with concomitant formation of HSiPh₃. In the crystal, <b>4a</b> consists of a cluster cation and a free triphenylsilyl anion. ¹H NMR spectroscopy and deuterium labeling experiments confirmed the selective cleavage of dihydrogen by the highly polar Ca–Si bond in <b>1</b>

Heterometallic Potassium Rare-Earth-Metal Allyl and Hydrido Complexes Stabilized by a Dianionic (NNNN)-Type Macrocyclic Ancillary Ligand

The macrocyclic diamino diamine (1,7-Me₂TACD)­H₂ (1,7-Me₂TACD = 1,7-dimethyl-1,4,7,10-tetraazacyclododecane, 1,7-Me₂[12]­aneN₄), reacted under propylene elimination with [Ln­(η³-C₃H₅)₃(diox)] (Ln = Y, La) to give the mono­(allyl) complexes [(1,7-Me₂TACD)­Ln­(η³-C₃H₅)]₂ (Ln = Y (<b>1a</b>), La (<b>1b</b>)). A single-crystal X-ray diffraction study shows <b>1b</b> to be a centrosymmetric dimer with lanthanum atoms bridged by one of the two amido nitrogen atoms. Complexes <b>1a</b>,<b>b</b> were treated with 2 equiv of the potassium allyl KC₃H₅ to give the corresponding heterometallic allyl complexes [(1,7-Me₂TACD)­Ln­(η³-C₃H₅)₂K­(THF)]_<i>n</i> (Ln = Y (<b>2a</b>), La (<b>2b</b>)). A single-crystal X-ray diffraction study revealed that <b>2a</b>,<b>b</b> are polymeric in the solid state with allyl ligands bridging the metal centers in addition to the presence of μ₂-amido functions of the 1,7-Me₂TACD ligand. Hydrogenolysis of the yttrium compound <b>2a</b> with 1 bar of H₂ led to the formation of the heterometallic Y₄K₂ hydrido complex [(1,7-Me₂TACD)₂Y₂H₃K­(THF)₂]₂ (<b>3a</b>), which can also be synthesized from a 1:1 mixture of <b>1a</b> and KC₃H₅ with 1 bar of H₂. A single-crystal X-ray diffraction study of <b>3a</b> revealed a dimer of heterotrinuclear Y₂K trihydride aggregate. Treatment of <b>2b</b> with 1 bar of H₂ afforded the heptanuclear La₃K₄ heptahydrido complex [(1,7-Me₂TACD)₃La₃H₇K₄(THF)₇] (<b>3b</b>)

Heterometallic Potassium Rare-Earth-Metal Allyl and Hydrido Complexes Stabilized by a Dianionic (NNNN)-Type Macrocyclic Ancillary Ligand

The macrocyclic diamino diamine (1,7-Me₂TACD)­H₂ (1,7-Me₂TACD = 1,7-dimethyl-1,4,7,10-tetraazacyclododecane, 1,7-Me₂[12]­aneN₄), reacted under propylene elimination with [Ln­(η³-C₃H₅)₃(diox)] (Ln = Y, La) to give the mono­(allyl) complexes [(1,7-Me₂TACD)­Ln­(η³-C₃H₅)]₂ (Ln = Y (<b>1a</b>), La (<b>1b</b>)). A single-crystal X-ray diffraction study shows <b>1b</b> to be a centrosymmetric dimer with lanthanum atoms bridged by one of the two amido nitrogen atoms. Complexes <b>1a</b>,<b>b</b> were treated with 2 equiv of the potassium allyl KC₃H₅ to give the corresponding heterometallic allyl complexes [(1,7-Me₂TACD)­Ln­(η³-C₃H₅)₂K­(THF)]_<i>n</i> (Ln = Y (<b>2a</b>), La (<b>2b</b>)). A single-crystal X-ray diffraction study revealed that <b>2a</b>,<b>b</b> are polymeric in the solid state with allyl ligands bridging the metal centers in addition to the presence of μ₂-amido functions of the 1,7-Me₂TACD ligand. Hydrogenolysis of the yttrium compound <b>2a</b> with 1 bar of H₂ led to the formation of the heterometallic Y₄K₂ hydrido complex [(1,7-Me₂TACD)₂Y₂H₃K­(THF)₂]₂ (<b>3a</b>), which can also be synthesized from a 1:1 mixture of <b>1a</b> and KC₃H₅ with 1 bar of H₂. A single-crystal X-ray diffraction study of <b>3a</b> revealed a dimer of heterotrinuclear Y₂K trihydride aggregate. Treatment of <b>2b</b> with 1 bar of H₂ afforded the heptanuclear La₃K₄ heptahydrido complex [(1,7-Me₂TACD)₃La₃H₇K₄(THF)₇] (<b>3b</b>)

Bis(allyl)gallium Cation, Tris(allyl)gallium, and Tetrakis(allyl)gallate: Synthesis, Characterization, and Reactivity

A series of cationic, neutral, and anionic allylgallium complexes has been isolated and fully characterized. It includes neutral [Ga­(η¹-C₃H₅)₃(L)] (<b>1</b>, L = THF; <b>2</b>, L = OPPh₃), cationic [Ga­(η¹-C₃H₅)₂(THF)₂]⁺[A]⁻ (<b>3</b>, [A]⁻ = [B­(C₆F₅)₄]⁻; <b>4</b>, [A]⁻ = [B­(C₆H₃Cl₂)₄]⁻), as well as anionic [Cat]⁺[Ga­(η¹-C₃H₅)₄]⁻ (<b>5</b>, [Cat]⁺ = K⁺; <b>6</b>, [Cat]⁺ = [K­(dibenzo-18-<i>c</i>-6]⁺; <b>7</b>, [Cat]⁺ = [PPh₄]⁺). Binding modes of the allyl ligand in solution and in the solid state have been studied comparatively. Single crystal X-ray analyses revealed a four-coordinate neutral gallium center in <b>2</b>, a five-coordinate cationic gallium center in <b>4</b> and [<b>4</b>·THF], and a four-coordinate anionic gallium center with a bridging μ₂-η¹:η² coordination mode of the allyl ligand in <b>6</b>. The reactivity of this series of allylgallium complexes toward benzophenone and <i>N</i>-heteroaromatics has been investigated. Counterion effects have also been studied. Reactions of <b>1</b> and <b>5</b> with isoquinoline revealed the first examples of organogallium complexes reacting under 1,2-insertion with pyridine derivatives

Alkali Metal Hydridotriphenylborates [(L)M][HBPh₃] (M = Li, Na, K): Chemoselective Catalysts for Carbonyl and CO₂ Hydroboration

Light alkali metal hydridotriphenylborates M­[HBPh₃] (M = Li, Na, K), characterized as tris­{2-(dimethyl­amino)­ethyl}­amine (L) complexes [(L)­M]­[HBPh₃], act as efficient catalysts for the chemoselective hydroboration of a wide range of aldehydes and ketones using pinacolborane HBpin. The lithium derivative showed a remarkably high TOF of ≥17 s^–1. These compounds also catalyze the hydroborative reduction of CO₂ to give formoxyborane HCO₂Bpin without any over-reduction

Dihydrogen Cleavage by a Zinc–Zinc Bond of a Heteroleptic Dizinc(I) Cation

Oxidative addition of dihydrogen across a metal–metal bond to form reactive metal hydrides in homogeneous catalysis is known for transition metals but not for zinc(I)–zinc(I) bond as found in Carmona’s eponymous dizinconene [Zn2Cp*2] (Cp* = η5-C5Me5). Dihydrogen reacted with the heteroleptic zinc(I)–zinc(I) bonded cation [(L2)Zn–ZnCp*][BAr4F] (L2 = TMEDA, N,N,N′,N′-tetramethylethylenediamine, TEEDA, N,N,N′,N′-tetraethylethylenediamine; ArF = 3,5-(CF3)2C6H3) under 2 bar at 80 °C to give the zinc(II) hydride cation [(L2)ZnH(thf)][BAr4F] along with zinc metal and Cp*H derived from the intermediate [Cp*ZnH]. DFT calculations show that the cleavage of dihydrogen occurs through a highly unsymmetrical transition state. Mechanistic studies agree with a heterolytic cleavage of dihydrogen as a result of the cationic charge and unsymmetrical ligand coordination. To explore the existence of zinc(I) hydride, thermally unstable hydridotriphenylborate complexes of zinc(I) [(L2)Zn(HBPh3)–ZnCp*] (L2 = TMEDA, TEEDA; TMPDA, N,N,N′,N′-tetramethyl-1,3-propylenediamine) have been prepared by salt metathesis and were shown to undergo fast exchange with both BPh3 and [HBPh3]−

Me₆TREN-Supported Alkali Metal Hydridotriphenylborates [(L)M][HBPh₃] (M = Li, Na, K): Synthesis, Structure, and Reactivity

Triphenylborane (BPh₃) abstracted the β-Si<i>H</i> in [(L)­M­{N­(SiHMe₂)₂}] (L = Me₆TREN; M = Li, Na, K) in THF to give the hydridotriphenylborates [(L)­M]­[HBPh₃]. Reactions in benzene favored silazide instead of hydride abstraction to give [(L)­M]­[Ph₃B–N­(SiHMe₂)₂]. The hydridotriphenylborates [(L)­M]­[HBPh₃] catalyzed the chemoselective hydroboration of benzophenone by pinacolborane (HBpin), with the lithium derivative being the most active. The solution structure of [(L)­Li]­[HBPh₃] was qualitatively investigated in the context of its superior catalytic activity. Fluxional coordination of L in [(L)­Li­(THF)]⁺ in tandem with a THF solvent molecule was revealed by NMR spectroscopy. <i>para</i>-Substituents in [HB­(C₆H₄-<i>p</i>-X)₃]⁻ influenced the rate which is apparently determined by the addition of the B–H function to the isolable alkoxy borate intermediate [(L)­Li]­[Ph₂CHOBPh₃]

Silyl–Hydrosilane Exchange at a Magnesium Triphenylsilyl Complex Supported by a Cyclen-Derived <i>NNNN</i>-Type Macrocyclic Ligand

The magnesium triphenylsilyl complex [(Me₃TACD)­Mg­(SiPh₃)] (<b>2</b>) was synthesized from magnesium bis­(triphenylsilyl) [Mg­(SiPh₃)₂(THF)₂]·THF (<b>1</b>; THF = tetrahydrofuran) and the <i>NNNN</i>-type macrocyclic amidotriamine proligand (Me₃TACD)H ((Me₃TACD)­H = Me₃[12]­aneN₄ = 1,4,7-trimethyl-1,4,7,10-tetraazacyclododecane). Treating <b>2</b> with AlR₃ (R = Me, Et) gave the magnesium triphenylsilyl complexes with “blocked” amido function [(Me₃TACD·AlR₃)­Mg­(SiPh₃)] (<b>3a</b>: R = Me; <b>3b</b>: R = Et). Instead of forming a Mg–H bond upon reaction with dihydrogen or hydrosilanes, <b>2</b> and <b>3a</b>,<b>b</b> underwent rapid silyl–silane exchange with hydrosilanes. Treating the ethyl complex [(Me₃TACD·AlEt₃)­MgEt] with H₃SiPh gave [(Me₃TACD·AlEt₃)­MgH] (<b>4</b>), albeit not in a reproducible manner. The silyl–hydrosilane exchange allows access to other magnesium silyls of the type [(Me₃TACD)­Mg­(SiR′₃)] (<b>5a</b>: SiR′₃ = SiH₂Ph; <b>5b</b>: SiR′₃ = SiHPh₂) and [(Me₃TACD·AlR₃)­Mg­(SiR′₃)] (<b>6a</b>: SiR′₃ = SiH₂Ph, R = Me; <b>6b</b>: SiR′₃ = SiH₂Ph, R = Et; <b>7a</b>: SiR′₃ = SiHPh₂, R = Me; <b>7b</b>: SiR′₃ = SiHPh₂, R = Et). The reaction of <b>2</b> with H₂SiMePh in THF at room temperature resulted in an equilibrium (<i>K</i>_eq ≈ 1). Protonolysis of <b>2</b> with Brønsted acids (HX) 2,5-di-<i>tert</i>-butylphenol, phenylacetylene, acetophenone, aniline, and triethylammonium chloride each gave a compound [(Me₃TACD)­Mg­(X)] with the conjugated base coordinated at the magnesium along with HSiPh₃. The magnesium silyls <b>1</b>, <b>2</b>, and <b>7b</b> as well as the magnesium hydride <b>4</b> contain a distorted square-pyramidal magnesium center according to single-crystal X-ray diffraction

The Nature of the Heavy Alkaline Earth Metal–Hydrogen Bond: Synthesis, Structure, and Reactivity of a Cationic Strontium Hydride Cluster

The molecular strontium hydride [(Me₃TACD)₃Sr₃(μ₃-H)₂]­[SiPh₃] (<b>2</b>) was isolated as the dark red benzene solvate <b>2·C</b>_<b>6</b><b>H</b>_<b>6</b> in 69% yield from the reaction of [Sr­(SiPh₃)₂(thf)₃] (<b>1′</b>) with (Me₃TACD)H (1,4,7-trimethyl-1,4,7,10-tetraazacyclododecane). This reaction can be considered as a redox process, with the Brønsted acidic amine proton in (Me₃TACD)H transformed into the hydride by the anion [SiPh₃]⁻. Trace amounts of water resulted in the formation of [(Me₃TACD)₃Sr₃(μ₃-H)­(μ₃-OH)]­[SiPh₃] (<b>2*</b>), which cocrystallized with <b>2</b>. Single-crystal X-ray diffraction of <b>2</b> revealed a substitutional disorder of a bridging hydride with a hydroxide ligand. Hydride complex <b>2</b> was also obtained by hydrogenolysis of [(Me₃TACD)­Sr­(SiPh₃)] (<b>3</b>), although pure <b>3</b> proved difficult to isolate. In the presence of a 2-fold excess of (Me₃TACD)­H, the reaction with disilyl <b>1′</b> gave [(Me₃TACD)­SiPh₃] (<b>4</b>). Complex <b>2</b> underwent facile H/D exchange with D₂ (1 bar), with the anion [SiPh₃]⁻ decomposing concurrently. In the reaction of <b>2</b> with 1,1-diphenylethylene (DPE), the anion [SiPh₃]⁻ was added to the CC bond in DPE to give [(Me₃TACD)₃Sr₃H₂]­[Ph₂CCH₂SiPh₃] (<b>5</b>), whereas the cationic cluster [(Me₃TACD)₃Sr₃H₂]⁺ remained unchanged. 9-Fluorenone underwent one-electron reduction with <b>2</b> to give the paramagnetic ketyl complex [{(Me₃TACD)­H}­Sr­(OC₁₃H₈^•)₂(thf)₂] (<b>6</b>). These strontium compounds are structurally similar to the lighter calcium congeners, but more reactive, in particular with regard to fast H/D exchange and [SiPh₃]⁻ anion decomposition. DFT studies on the cationic hydride clusters suggest a more pronounced covalent character for strontium compared to calcium. Disilyl <b>1</b>, strontium diketyl <b>6</b>, and the calcium congener of <b>6</b>, [{(Me₃TACD)­H}­Ca­(OC₁₃H₈<b>·</b>)₂] (<b>10</b>), were also characterized by X-ray diffraction

Bis(allyl)zinc Revisited: Sigma versus Pi Bonding of Allyl Coordination

The reinvestigation of two allyl zinc compounds, parent bis­(allyl)­zinc [Zn­(C₃H₅)₂] (<b>1</b>) and 2-methallyl chloro zinc [Zn­(C₄H₇)­Cl] (<b>2</b>), revealed two new coordination modes in the solid state for the allyl ligand, <i>viz cis</i>- and <i>trans</i>-μ₂-η¹:η¹. These results call for modification of the conventional interpretation of zinc–allyl interactions. Computational results indicate that the classical η³-bonding mode of the allyl ligand is not favored in zinc compounds. A rare case of a zinc–olefin interaction in the dimer of [Zn­(η¹-C₃H₅)­(OC­(C₃H₅)­Ph₂)] was found in the monoinsertion product of <b>1</b> with benzophenone