Calcium Hydride Catalysts for Olefin Hydrofunctionalization: Ring-Size Effect of Macrocyclic Ligands on Activity.

The fifteen-membered NNNNN macrocycle Me5 PACP (Me5 PACP=1,4,7,10,13-pentamethyl-1,4,7,10,13-pentaazacyclopentadecane) stabilized the [CaH]+ fragment as a dimer with a distorted pentagonal bipyramidal coordination geometry at calcium. The hydride complex was prepared by protonolysis of calcium dibenzyl with the conjugate acid of Me5 PACP followed by hydrogenolysis or treating with n OctSiH3 of the intermediate calcium benzyl cation. The calcium hydride catalyzed the hydrogenation and hydrosilylation of unactivated olefins faster than the analogous calcium complex stabilized by the twelve-membered NNNN macrocycle Me4 TACD (Me4 TACD=1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane). Kinetic investigations indicate that higher catalytic efficiency for the Me5 PACP stabilized calcium hydride is due to easier dissociation of the dimer in solution when compared to the Me4 TACD analogue.

Reactivity of a Molecular Calcium Hydride Cation ([CaH]+) Supported by an NNNN Macrocycle.

The hydride ligand in the cationic calcium hydride supported by a NNNN-type macrocycle, [(Me4TACD)2Ca2(µ-H)2(THF)][BAr4]2 (1; Me4TACD = 1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane; THF = tetrahydrofuran; BAr4 = B(C6H3-3,5-Me2)4), shows, in addition to its Brönsted basicity toward weak acids, a pronounced nucleophilicity resulting in nucleophilic substitution or insertion (addition) at a silicon or sp2 carbon center. Terminal acetylenes RC≡CH (R = SiMe3, cyclopropyl) as well as 1,4-diphenylbutadiene were deprotonated by 1 to give dinuclear complexes [(Me4TACD)2Ca2(µ-C≡CR)2][BAr4]2 (2a, R = SiMe3; 2b, R = cyclopropyl) and [(Me4TACD)2Ca2(µ2-η4-1,4-Ph2C4H2)][BAr4]2 (3) with H2 evolution. The addition reaction with BH3(THF) gave a tetrahydridoborate complex, [(Me4TACD)Ca(BH4)(THF)2][BAr4] (4), with κ2-H2BH2 coordination in the solid state, suggesting a pronounced Lewis acidic calcium center. The behavior resulting from both Lewis acidity and hydricity becomes apparent in the nucleophilic substitution of fluorobenzene by 1 to give benzene and the dimeric fluoride complex [(Me4TACD)2Ca2(µ-F)2(THF)][BAr4]2·2.5THF (5). Analogous nucleophilic substitution reaction is observed for heterofunctionalized organosilanes XSiR3 [X = I, N(SiHMe2)2, N3; R = Me3 or HMe2], which resulted in the formation of calcium complexes [(Me4TACD)Ca(X)(THF)n][BAr4] (6-8) containing an X ligand along with hydrosilane HSiR3. An insertion reaction by 1 was observed with CO2 and CO to give dinuclear formato complex [(Me4TACD)2Ca2(µ-OCHO)2][BAr4]2 (9) and cis-enediolato complex [(Me4TACD)2Ca2(µ-OCH═CHO)][BAr4]2·3.5THF (10), respectively. The latter is believed to have been formed as a result of the dimerization of an initially generated formyl or oxymethylene complex, [(Me4TACD)Ca(OCH)]+.

Alkali Metal Triphenyl- and Trihydridosilanides Stabilized by a Macrocyclic Polyamine Ligand.

Potassium silanide [KSiH3 ]∞ contains 4.2 wt % of hydrogen and has been intensely studied as hydrogen storage material. The macrocyclic ligand Me4 TACD (1,4,7,10-tetramethyl-1,4,7,10-tetraaminocyclododecane, L) stabilizes the full range of triphenylsilyl complexes [(L)MSiPh3 ]n (M=Li-Cs), which react with H2 or PhSiH3 to form molecular [(L)MSiH3 ]n that can be isolated in soluble form and fully characterized.

Regioselective Hydrosilylation of Olefins Catalyzed by a Molecular Calcium Hydride Cation.

Chemo- and regioselectivity are often difficult to control during olefin hydrosilylation catalyzed by d- and f-block metal complexes. The cationic hydride of calcium [CaH]+ stabilized by an NNNN macrocycle was found to catalyze the regioselective hydrosilylation of aliphatic olefins to give anti-Markovnikov products, while aryl-substituted olefins were hydrosilyated with Markovnikov regioselectivity. Ethylene was efficiently hydrosilylated by primary and secondary hydrosilanes to give di- and monoethylated silanes. Aliphatic hydrosilanes were preferred over other commonly employed hydrosilanes: Arylsilanes such as PhSiH3 underwent scrambling reactions promoted by the nucleophilic hydride, while alkoxy- and siloxy-substituted hydrosilanes gave isolable alkoxy and siloxy calcium derivatives.

A tetranuclear calcium hydride cluster with a highly symmetric [Ca4H6]2+ core.

A highly symmetric, dicationic tetranuclear calcium hydride cluster with an adamantoid [Ca4H6]2+ core stabilized by the NNN macrocycle Me3TACN (1,4,7-trimethyl-1,4,7-triazacyclononane) was prepared. DFT calculations indicate delocalized bonding within the [Ca4H6]2+ fragment with d-orbital contribution.

Molecular hydrides of divalent ytterbium supported by a macrocyclic ligand: synthesis, structure and olefin hydrofunctionalization catalysis.

Bis(triphenylsilyl) and dibenzyl ytterbium(ii) complexes supported by the macrocyclic polyamine ligand Me4TACD (1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane) reacted with H2 to give the cationic ytterbium hydrides [(Me4TACD)2Yb2(µ2-H)(2+n)](2-n)+ (n = 0, 1). The hydrides catalyzed the H2/D2 isotope exchange as well as hydrogenation and hydrosilylation of 1-hexene.

Hypervalent Hydrosilicates Connected to Light Alkali Metal Amides: Synthesis, Structure, and Hydrosilylation Catalysis.

When light alkali metal amides [M(Me3 TACD)]n (M=Li, Na, K; (Me3 TACD)H=1,4,7-trimethyl-1,4,7,10-tetraazacyclododecane) were treated with H2 SiPh2 in THF, [M{(H2 SiPh2 )Me3 TACD}(thf)x ] containing a pendant hypervalent dihydridosilicate were formed and characterized by elemental analysis, NMR/IR spectroscopy, and single-crystal X-ray crystallography. The lithium complex catalyzed the hydrosilylation of styrene derivatives under mild conditions with anti-Markovnikov regioselectivity.

Hydrido Complexes of Calcium: A New Family of Molecular Alkaline-Earth-Metal Compounds.

The application of solid calcium hydride CaH2 has been mostly confined to its use as a desiccant, although its catalytic activity has long been known. Since the first isolation of a well-defined molecular calcium hydride in 2006, the past decade has witnessed a gradual emergence of this new family of compounds. Although the detrimental Schlenk equilibrium has kept the number of examples low, the novelty of their reactivity, especially in small-molecule activation, holds great promise. This Minireview gives an overview of the molecular calcium hydrides reported to date, highlighting their synthesis, structure, and reactivity.

Calcium Hydride Cation [CaH]+ Stabilized by an NNNN-type Macrocyclic Ligand: A Selective Catalyst for Olefin Hydrogenation.

Reaction of dibenzyl calcium complex [Ca(Me4 TACD)(CH2 Ph)2 ], containing the neutral NNNN-type macrocyclic ligand Me4 TACD (Me4 TACD=1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane), with triphenylsilane gave the cationic dinuclear calcium hydride [Ca2 H2 (Me4 TACD)2 ](PhCHSiPh3 )2 which was characterized by NMR spectroscopy and single-crystal X-ray diffraction. The cation can be regarded as the ligand-stabilized dimeric form of hypothetical [CaH]+ . Hydrogenolysis of benzyl calcium cation [Ca(Me4 TACD)(CH2 Ph)(thf)]+ gave dicationic calcium hydrides [Ca2 H2 (Me4 TACD)2 ][BAr4 ]2 (Ar=C6 H4 -4-t Bu; C6 H3 -3,5-Me2 ) containing weakly coordinating anions. In THF, they catalyzed the isotope exchange of H2 and D2 to give HD and the hydrogenation of unactivated 1-alkenes.

Isolation and properties of a palladium PBP pincer complex featuring an ambiphilic boryl site.

The palladium diphosphinoboryl pincer [{(o-PPh2C6H4)2B}PdIII] 8 was prepared from the diphosphinoborane complex 6a. The boryl functionality of the pincer displayed ambiphilic properties allowing: (1) coordination of Lewis basic pyridines to the boryl site and (2) access to palladium diphosphinoborane complex derivatives.