β-Diketiminato Calcium Acetylides: Synthesis, Solution Dimerization, and Catalytic Carbon−Carbon Bond Formation

From alkaline earth to coinage metal carboranyls

The β-diketiminato calcium and magnesium complexes, [(BDI)MgnBu] and [(BDI)CaH]2 (BDI = HC{C(Me)NDipp}2; Dipp = 2,6-di-isopropylphenyl), react with ortho-carborane (o-C2B10H12) to provide the respective [(BDI)Ae(o-C2B10H11)] (Ae = Mg or Ca) complexes. While the lighter group 2 species is a monomer with magnesium in a distorted trigonal planar environment, the heavier analogue displays a puckered geometry at calcium in the solid state due to Ca⋯H-B intermolecular interactions. These secondary contacts are, however, readily disrupted upon addition of THF to provide the 4-coordinate monomer, [(BDI)Ca(THF)(o-C2B10H11)]. [(BDI)Mg(o-C2B10H11)] was reacted with [NHCIPrMCl] (NHCIPr = 1,3-bis(isopropyl)imidazol-2-ylidene; M = Cu, Ag, Au) to provide [NHCIPrM(o-C2B10H11)], rare C-bonded examples of coinage metal derivatives of unsubstituted (o-C2B10H11)- and confirming the alkaline earth compounds as viable reagents for the transmetalation of the carboranyl anion.

Enantiopure Calcium Iminophosphonamide Complexes: Synthesis, Photoluminescence, and Catalysis

The synthesis of calcium complexes ligated by three different chiral iminophosphonamide ligands, L-H (L=[Ph₂ P{N(R)CH(CH₃ )Ph}₂ ]), L'-H (L'=[Ph₂ P{NDipp}{N(R)CH(CH₃ )Ph}]), (Dipp=2,6-ⁱ Pr₂ C₆ H₃ ), and L''-H (L''=[Ph₂ P{N(R)CH(CH₃ )naph}₂ ]), (naph=naphthyl) is presented. The resulting structures [L₂ Ca], [L'₂ Ca], and [L''₂ Ca] represent the first examples of enantiopure homoleptic calcium complexes based on this type of ligands. The calcium complexes show blue-green photoluminescence (PL) in the solid state, which is especially bright at low temperatures. Whereas the emission of [L''₂ Ca] is assigned to the fluorescence of naphthyl groups, the PL of [L₂ Ca] and [L'₂ Ca] is contributed by long-lived phosphorescence and thermally activated delayed fluorescence (TADF), with a strong variation of the PL lifetimes over the temperature range of 5-295 K. Furthermore, an excellent catalytic activity was found for these complexes in hydroboration of ketones at room temperature, although no enantioselectivity was achieved.

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)]⁺.

Synthesis of Functionalized Isoindolinones via Calcium Catalyzed Generation and Trapping of N-Acyliminium Ions

Herein we report our full investigation into the calcium catalyzed generation and trapping of N-acyliminium ions from readily available 3-hydroxyisoindolinones. We have successfully employed a range of traditional nucleophiles including carbon, nitrogen, and sulfur containing reactive partners. The reaction is tolerant to a wide range of functionalities and provides high value scaffolds in good to excellent yields.

Synthesis, characterization, and reactivity of dinuclear organo-rare-earth-metal alkyl complexes supported by 2-amidate-functionalized indolyl ligands: substituent effects on coordination and reactivity

Two series of new dinuclear organo-rare-earth-metal alkyl complexes supported by 2-amidate-functionalized indolyl ligands with different haptic modes were synthesized and characterized. The treatment of [RE(CH2SiMe3)3(THF)2] with 1 equiv. of 2-(2,6-iPr2C6H3NHC[double bond, length as m-dash]O)C8H5NH (H2L1) and 2-(2-tBuC6H4NHC[double bond, length as m-dash]O)C8H5NH (H2L2) in toluene yielded the dinuclear organo-rare-earth-metal alkyl complexes {[η1:(μ2-η1:η1)-L1]RE(CH2SiMe3)(THF)2}2 [RE = Gd (1a), Dy (1b), Y (1c), Er (1d), and Yb (1e)] and {[η1:(μ2-η1:η1):η1-L2]RE(CH2SiMe3)(THF)2}2 [RE = Gd (2a), Dy (2b), Y (2c), Er (2d), and Yb (2e)] in good yields. When [RE(CH2SiMe3)3(THF)2] were treated with 2 equiv. of H2L1 or H2L2 in THF, the dinuclear organo-rare-earth-metal complexes {(η1:η1-HL)[η1:(μ2-η1:η1):η1-L]RE(THF)}2 (1ca: RE = Y, L = L1; 2ea: RE = Yb, L = L2) were obtained. The complexes could react with small organic molecules such as N,N'-diisopropylcarbodiimide (DIC), phenyl isocyanate, N-methylallylamine, phenylacetylene, pyridine, N-phenylimidazole, or 4-dimethylaminopyridine (DMAP) to yield a series of new complexes with different reactivity patterns along with the reported rare-earth-metal alkyl complexes. In the presence of cocatalysts, these dinuclear organo-rare-earth-metal alkyl complexes could initiate isoprene polymerization with high activity (100% conversion of 2000 equiv. of isoprene in 12 h), yielding polymers with high regioselectivity (1,4 polymers up to 96.1%).

Synthesis and characterization of 2-t-butylimino-functionalized indolyl rare-earth metal amido complexes for the catalytic addition of terminal alkynes to carbodiimides: the dimeric complexes with the alkynide species in the μ–η1:η2 bonding modes

Dimeric complexes [(μ–η¹:η²-RCC)REL2]₂ (L = 2-(^tBuNCH)C₈H₅N), with alkynyl in the μ–η¹:η² modes, were isolated in the study of the catalytic reaction.

Synthesis and catalytic activity of tridentate N-(2-pyridylethyl)-substituted bulky amidinates of calcium and strontium

Sterically protected tridentate amidinates of calcium have been prepared to study the intramolecular hydroamination of an aminoalkene.

Calcium formamidinate derivatives by hydride insertion of carbodiimides

Insertion reactions of N,N′-dialkyl and N,N′-diaryl carbodiimides with a dimeric calcium hydride provide formamidinate derivatives whose structures on the steric demands of the carbodiimide reagent.

Calcium-catalyzed reactions of element-H bonds

Investigation on organocalcium catalysis is just unfolding during the past decade. Beside conventional Ca salts with strong electron-withdrawing counter anions that may serve as Lewis acid catalysts, many Ca complexes have also been designed recently and found to be good catalysts in activation of element-H (EH) bonds like transition metal catalysts. These findings are interesting and may attract the interest of the chemists. Due to the great abundance, non-toxicity, and biocompatible features of Ca element, Ca-catalyzed reactions can be of great significance from the viewpoint of industry. This short review summarizes the recent advances on Ca-catalyzed reactions of EH bonds. We hope that it may provide a useful guide for interested readers from both the academy and industry.

Simple ZnEt2as a catalyst in carbodiimide hydroalkynylation: structural and mechanistic studies

Simple ZnEt₂is an efficient catalyst for the addition of terminal alkynes to carbodiimides, through amidinate complexes, and consecutive isocyanate addition and intramolecular cyclohydroamination.

Alkaline earths as main group reagents in molecular catalysis

The past decade has witnessed some remarkable advances in our appreciation of the structural and reaction chemistry of the heavier alkaline earth (Ae = Mg, Ca, Sr, Ba) elements. Derived from complexes of these metals in their immutable +2 oxidation state, a broad and widely applicable catalytic chemistry has also emerged, driven by considerations of cost and inherent low toxicity. The considerable adjustments incurred to ionic radius and resultant cation charge density also provide reactivity with significant mechanistic and kinetic variability as group 2 is descended. In an attempt to place these advances in the broader context of contemporary main group element chemistry, this review focusses on the developing state of the art in both multiple bond heterofunctionalisation and cross coupling catalysis. We review specific advances in alkene and alkyne hydroamination and hydrophosphination catalysis and related extensions of this reactivity that allow the synthesis of a wide variety of acyclic and heterocyclic small molecules. The use of heavier alkaline earth hydride derivatives as pre-catalysts and intermediates in multiple bond hydrogenation, hydrosilylation and hydroboration is also described along with the emergence of these and related reagents in a variety of dehydrocoupling processes that allow that facile catalytic construction of Si-C, Si-N and B-N bonds.

Catalytic bond forming reactions promoted by amidinate, guanidinate and phosphaguanidinate compounds of magnesium

The synthesis and catalytic properties of a series of magnesium compounds consisting of monoanionic, N,N'-chelating ligands (N∩N = amidinates, guanidinates, phosphaguanidinates) is reported. The compounds were synthesized by (i) insertion of a carbodiimide into an existing Mg-C or Mg-N bond, or (ii) protonolysis of an organomagnesium compound by a neutral pre-ligand. Structural analyses of mono- or bis-(chelate) compounds with general formula Mg(N∩N)X(L)n and Mg(N∩N)2(L)n (X = halide, aryloxide, amide; L = Et2O, THF; n = 0, 1 or 2) have been performed and the influence that the ligand substituent patterns have on the solid-state structures has been probed. Selected examples of the compounds were tested as (pre)catalysts for the polymerization of lactide, the dimerization of aldehydes and the hydroacetylenation of carbodiimides.

Catalytic hydroacetylenation of carbodiimides with homoleptic alkaline earth hexamethyldisilazides

The homoleptic alkaline earth hexamethyldisilazides, [M{N(SiMe3)2}2(THF)2] (M = Mg 1a; Ca 1b; Sr 1c), have been shown to act as efficient precatalysts for the hydroacetylenation of organic carbodiimides with alkyl- and arylacetylenes. Catalytic activity was observed to increase with the size of the group 2 metal centre employed and to be strongly influenced by the steric properties of the carbodiimide substrate. The intermediate dimeric calcium and strontium bis(amidinate) complexes, [{PhC≡CC(N(i)Pr)2}2M]2 (M = Ca 2b, Sr 2c), have been isolated and crystallographically characterised. Kinetic studies using the strontium precursor, 1c, provided a reaction rate law independent of [acetylene] but proportional to [carbodiimide](2) and inversely proportional to the concentration of the amidine product in solution.

Heterofunctionalization catalysis with organometallic complexes of calcium, strontium and barium

Despite the routine employment of Grignard reagents and Hauser bases as stoichiometric carbanion reagents in organic and inorganic synthesis, a defined reaction chemistry encompassing the heavier elements of Group II (M = Ca, Sr and Ba) has, until recently, remained unreported. This article provides details of the recent progress in heavier Group II catalysed small molecule transformations mediated by well-defined heteroleptic and homoleptic complexes of the form LMX or MX 2 ; where L is a mono-anionic ligand and X is a reactive σ-bonded substituent. The intra- and intermolecular heterofunctionalization (hydroamination, hydrophosphination, hydrosilylation and hydrogenation) of alkenes, alkynes, dienes, carbodiimides, isocyanates and ketones is discussed.