Alkaline Earth Metal–Carbene Complexes with the Versatile Tridentate 2,6-Bis(3-mesitylimidazol-2-ylidene)pyridine Ligand

Diffusion of 2,6-bis­(3-mesitylimidazol-2-ylidene)­pyridine (Car^MesPyCar^Mes, <b>2</b>) into a solution of CaI₂ in THF leads to microcrystalline [(Car^MesPyCar^Mes)­(thf)­CaI₂] (<b>3</b>), in one case containing a few single crystals of the unique separated ion pair [(Car^MesPyCar^Mes)₂(thf)­Ca]­I₂ (<b>4</b>) with four Ca–C bonds. However, isolation of single-crystalline [(Car^MesPyCar^Mes)­(thf)­CaI₂] (<b>3</b>) succeeds via the addition of <b>2</b> to a solution of [(thf)₅CaI]⁺[BPh₄]⁻ due to a subsequent dismutation. Two modifications with the shapes of needles and cubes crystallize simultaneously. In contrast to this finding, the reaction of [(thf)₅CaI]⁺[AlPh₄]⁻ with <b>2</b> yields solvent separated [(Car^MesPyCar^Mes)­(thf)₂CaI]⁺[AlPh₄]⁻ (<b>5</b>). The Ca–C_NHC bond lengths lie in a typical range of Ca–C_Aryl σ bonds and represent the shortest Ca–C_NHC bonds known to date. Soluble [(Car^MesPyCar^Mes)­(thf)­Ca­(NPh₂)₂] (<b>6</b>) can be prepared via a metathetical approach from <b>4</b> and KNPh₂ as well as via the addition of <b>2</b> to Ca­(NPh₂)₂ in tetrahydrofuran. The bulkier amido ligands lead to elongated bonds between the calcium center and the ligand <b>2</b>. Furthermore, the reactions of MI₂ (M = Sr, Ba) with <b>2</b> yield [(Car^MesPyCar^Mes)­(thf)₂MI₂] (M = Sr (<b>7</b>), Ba (<b>8</b>))

Concept for Enhancement of the Stability of Calcium-Bound Pyrazolyl-Substituted Methanides

Metalation of bis­(3-thiophen-2-ylpyrazol-1-yl)­phenylmethane [<b>2</b>, which is accessible from the reaction of bis­(3-thien-2-ylpyrazol-1-yl)­methanone (<b>1</b>) with triphosgene] with [(thf)₂Ca­{N­(SiMe₃)₂}₂] in tetrahydrofuran and subsequent crystallization from a mixture of toluene and 1,2-dimethoxyethane yield [(dme)­Ca­{C­(Pz^th)₂Ph}­{N­(SiMe₃)₂}] (<b>3</b>). The α,α-bis­(3-thiophen-2-ylpyrazol-1-yl)­benzyl ligand exhibits a κ²<i>N</i>,κ<i>C</i>-coordination mode with a Ca–C σ-bond length of 262.8(2) pm. The crystalline compound is stable if air and moisture is strictly excluded; however, in solution; this calcium complex slowly degrades

Directed Ortho Calciation of 1,3-Bis(3-isopropylimidazol-2-ylidene)benzene

The deprotonation of 1,3-bis­(3-isopropylimidazol-2-ylidene)­benzene with Me₃SiCH₂CaX (X = Br, I) in tetrahydrofuran (THF) yields the ether adducts of the corresponding 2,6-bis­(3-isopropylimidazol-2-ylidene)­phenylcalcium halides (X = Br (<b>1</b>·2thf), I (<b>2</b>·2thf)). The crystallization behavior of <b>2</b> can be improved via substitution of ligated thf molecules by tetrahydropyran (thp) ligands, leading to <b>2</b>·2thp. These heteroleptic complexes <b>1</b>·2thf and <b>2</b>·2thp show very small Ca–C_ipso bond lengths to the ipso-carbon atoms of the aryl groups. Calciation of 1,3-bis­(3-isopropylimidazol-2-ylidene)­benzene with Ca­(CH₂SiMe₃)₂ in THP leads to the formation of ether-free homoleptic bis­[2,6-bis­(3-isopropylimidazol-2-ylidene)­phenyl]calcium (<b>3</b>). Intramolecular steric strain causes an elongation of the Ca–C_ipso bonds to the aryl groups. In all of these complexes, the Ca–C_carbene distances are significantly larger than those to the ipso-carbon atoms of the aryl groups

1,2-Bis(anilido)ethane Complexes of Calcium and Potassium: Synthesis, Structures, and Catalytic Activity

The metalation of 1,2-bis­(anilino)­ethane with excess KH leads to the formation of the potassium complex [(thf)₃K₂{1,2-(PhN)₂C₂H₄}] (<b>1</b>). Complex <b>1</b> quantitatively reacts with anhydrous CaI₂ in THF yielding insoluble KI and dinuclear [(thf)₅Ca₂{1,2-(PhN)₂C₂H₄}₂] (<b>2</b>) after crystallization from a mixture of THF and hexane. Addition of <i>N</i>,<i>N</i>,<i>N′</i>,<i>­N′′</i>,<i>N′′′</i>,<i>N′′′</i>-hexa­methyl­triethylene­tetraamine (hmteta) yields [(hmteta)­Ca­{1,2-(PhN)₂C₂H₄}] (<b>3</b>). The complexes <b>1</b> and <b>2</b> proved to be inactive as catalysts in hydroamination reactions of diphenylbutadiyne with secondary amines. However, a mixture of <b>1</b> and <b>2</b> (K:Ca ratio of 2:1) mediated the addition of <i>N</i>-methyl-aniline and 1,2-bis­(anilino)­ethane to one of the CC triple bonds of diphenylbutadiyne. Addition of 18-crown-6 ether (18C6) leads to the formation of the sparingly soluble potassium complex [{(18C6)­K}₂{1,2-(PhN)₂C₂H₄}] (<b>5</b>) and the insoluble calcium complex [(18C6)­Ca­{1,2-(PhN)₂C₂H₄}] (<b>6</b>). The calciate-mediated hydroamination reaction is regiocontrolled, but <i>E</i>- and <i>Z</i>-isomeric addition products are observed, regardless of whether the reaction is performed at daylight or in the dark. If toluene is used as solvent for this s-block metal-mediated hydroamination catalysis, (<i>Z,Z</i>)-1,4,5,8,9,12-hexaphenyl-5,8-diazadodeca-3,9-diene-1,11-diyne (<i>Z,Z</i>-<b>8</b>) precipitates, allowing isolation and characterization of this isomer. In solution, this compound isomerizes upon irradiation yielding an equilibrium between (<i>Z,Z</i>)-, (<i>E,Z</i>)- and (<i>E,E</i>)-isomers. The determination of the crystal structures of (<i>Z,Z</i>)- and (<i>E,E</i>)-1,4,5,8,9,12-hexaphenyl-5,8-diazadodeca-3,9-diene-1,11-diyne unequivocally allows the assignment of the NMR parameters to specific isomers

Concept for Enhancement of the Stability of Calcium-Bound Pyrazolyl-Substituted Methanides

Metalation of bis­(3-thiophen-2-ylpyrazol-1-yl)­phenylmethane [<b>2</b>, which is accessible from the reaction of bis­(3-thien-2-ylpyrazol-1-yl)­methanone (<b>1</b>) with triphosgene] with [(thf)₂Ca­{N­(SiMe₃)₂}₂] in tetrahydrofuran and subsequent crystallization from a mixture of toluene and 1,2-dimethoxyethane yield [(dme)­Ca­{C­(Pz^th)₂Ph}­{N­(SiMe₃)₂}] (<b>3</b>). The α,α-bis­(3-thiophen-2-ylpyrazol-1-yl)­benzyl ligand exhibits a κ²<i>N</i>,κ<i>C</i>-coordination mode with a Ca–C σ-bond length of 262.8(2) pm. The crystalline compound is stable if air and moisture is strictly excluded; however, in solution; this calcium complex slowly degrades

Potassium-Mediated Hydrophosphorylation of Heterocumulenes with Diarylphosphane Oxide and Sulfide

The preparation of the hydrophosphorylation catalysts succeeds via the metalation of dimesitylphosphane oxide and diphenylphosphane sulfide with potassium hydride in ethereal solvents such as tetrahydropyran (THP) and tetrahydrofuran (THF) yielding the tetramers [(thp)­K­(OPMes₂)]₄ (<b>1a</b>) and [(thf)₃{K­(OPMes₂)}₄] (<b>1b</b>) as well as [(thp)­KSPPh₂]_∞ (<b>2</b>) with a strand-like structure in the crystalline state. In ethereal solution these complexes very slowly degrade into KPAr₂ and KE₂PAr₂ (E = O, S). The catalytic conversion of iPr-NCE′ (E′ = O, S) and of R-NCN-R (R = <i>i</i>Pr, <i>c</i>Hex) to the addition products Ar2P­(E)-C­(=E′)-NHR (Ar = Ph, Mes; E = O, S; E′ = O, S, NR) was studied in the presence of catalytic amounts of Ar₂PEK (Ar = Ph, Mes; E = O, S). Steric hindrance prevents the addition of dimesitylphosphane oxide to <i>N</i>,<i>N</i>′-diisopropylcarbodiimide, whereas diphenylphosphane oxide and sulfide smoothly add to <i>i</i>Pr–NCN-<i>i</i>Pr yielding Ph₂P­(E)-C­(N-<i>i</i>Pr)-NH<i>i</i>Pr (E = O, S)

Concept for Enhancement of the Stability of Calcium-Bound Pyrazolyl-Substituted Methanides

Metalation of bis­(3-thiophen-2-ylpyrazol-1-yl)­phenylmethane [<b>2</b>, which is accessible from the reaction of bis­(3-thien-2-ylpyrazol-1-yl)­methanone (<b>1</b>) with triphosgene] with [(thf)₂Ca­{N­(SiMe₃)₂}₂] in tetrahydrofuran and subsequent crystallization from a mixture of toluene and 1,2-dimethoxyethane yield [(dme)­Ca­{C­(Pz^th)₂Ph}­{N­(SiMe₃)₂}] (<b>3</b>). The α,α-bis­(3-thiophen-2-ylpyrazol-1-yl)­benzyl ligand exhibits a κ²<i>N</i>,κ<i>C</i>-coordination mode with a Ca–C σ-bond length of 262.8(2) pm. The crystalline compound is stable if air and moisture is strictly excluded; however, in solution; this calcium complex slowly degrades

s‑Block-Metal-Mediated Hydroamination of Diphenylbutadiyne with Primary Arylamines Using a Dipotassium Tetrakis(amino)calciate Precatalyst

The hydroamination of diphenylbutadiyne with primary arylamines requires a reactive catalyst. In the presence of heterobimetallic K₂[Ca­{N­(H)­Dipp}₄] (Dipp = 2,6-diisopropylphenyl) the performance of this reaction in THF yields 2-<i>tert</i>-butyl-6,7,10,11-tetraphenyl-9<i>H</i>-cyclohepta­[<i>c</i>]­quinoline (<b>1a</b>) and 2-fluoro-6,7,10,11-tetraphenyl-9<i>H</i>-cyclohepta­[<i>c</i>]­quinoline (<b>1b</b>) within 3 days at room temperature when 4-<i>tert</i>-butyl- and 4-fluoroaniline, respectively, have been used. During this catalysis <i>o</i>-CH activation occurs and quinoline derivatives are formed. Blocking the <i>o</i>-CH positions by methyl groups and use of 2,4,6-trimethylaniline under similar reaction conditions leads to the formation of <i>N</i>-mesityl-7-(<i>E</i>)-((mesitylimino)­(phenyl)­methyl)-2,3,6-triphenylcyclohepta-1,3,6-trienylamine (<b>2</b>) containing a β-diketimine unit with a N–H···N hydrogen bridge. NMR experiments with labeled 4-<i>tert</i>-butylaniline verify the transfer of N-bound hydrogen atoms to the newly formed cycloheptatriene ring. If the s-block-metal-mediated hydroamination of diphenylbutadiyne is performed in refluxing THF for 6 days, <i>N-</i>aryl-2,5-diphenylpyrroles <b>3a</b>–<b>d</b> (<b>3a</b>, R = tBu, R′ = H; <b>3b</b>, R = F, R′ = H; <b>3c</b>, R = R′ = Me; <b>3d</b>, R = R′ = H) are obtained regardless of the substitution pattern of the arylamines

Influence of 18-Crown‑6 Ether Coordination on the Catalytic Activity of Potassium and Calcium Diarylphosphinites in Hydrophosphorylation Reactions

The addition of 18-crown-6 ether (1,4,7,10,13,16-hexaoxacyclooctadecane) to tetranuclear [(thf)­K­(OPAryl₂)]₄ and [(thf)₄Ca­(OPAryl₂)₂] yields the corresponding mononuclear complexes [(18C6)­K­(OPAryl₂)] [Aryl = Ph (<b>1a</b>), Mes (<b>1b</b>)] and [(18C6)­Ca­(OPAryl₂)₂] [Aryl = Ph (<b>2a</b>), Mes (<b>2b</b>)]. The metathesis reaction of [(thf)­K­(OPAryl₂)]₄ with CaI₂ yields the calciate [K₂Ca­(thf)_<i>x</i>{OPMes₂}₄]. The addition of dimesitylphosphane oxide and crystallization from a hexane solution gives [K₂Ca­{OPMes₂}₄{Mes₂P­(O)­H}] (<b>3</b>). The complexes [(thf)­K­(OPMes₂)]₄, [(thf)₄Ca­(OPMes₂)₂], <b>1b</b>, <b>2b</b>, and the calciate <b>3</b> are tested as catalysts in the hydrophosphorylation of isopropylisocyanate with dimesitylphosphane oxide, quantitatively yielding <i>N</i>-isopropyl­(dimesitylphosphoryl)­formamide. The potassium complexes are more efficient catalysts than the calcium congeners, and coordination of 18-crown-6 decelerates the catalytic conversion

Tris(pyrazolyl)methanides of the Alkaline Earth Metals: Influence of the Substitution Pattern on Stability and Degradation

Tris­pyra­zolyl­methanides commonly act as strong tridentate bases toward metal ions. This expected coordination behavior has been observed for tris­(3,4,5-tri­methyl­pyra­zolyl)­meth­ane (<b>1a</b>), which yields the alkaline-earth-metal bis­[tris­(3,4,5-tri­methyl­pyra­zolyl)­meth­anides] of magnesium (<b>1b</b>), calcium (<b>1c</b>), strontium (<b>1d</b>), and barium (<b>1e</b>) via deprotonation of <b>1a</b> with di­butyl­magnesium and [Ae­{N­(SiMe₃)₂}₂] (Ae = Mg, Ca, Sr, and Ba, respectively). Barium complex <b>1e</b> degrades during recrystallization that was attempted from aromatic hydrocarbons and ethers. In these scorpionate complexes, the metal ions are embedded in distorted octahedral coordination spheres. Contrarily, tris­(3-thienyl­pyra­zolyl)­methane (<b>2a</b>) exhibits a strikingly different reactivity. Di­butyl­magnesium is unable to deprotonate <b>2a</b>, whereas [Ae­{N­(SiMe₃)₂}₂] (Ae = Ca, Sr, and Ba) smoothly metalates <b>2a</b>. However, the primary alkaline-earth-metal bis­[tris­(3-thienyl­pyra­zolyl)­meth­anides] of Ca (<b>2c</b>), Sr (<b>2d</b>), and Ba (<b>2e</b>) represent intermediates and degrade under the formation of the alkaline-earth-metal bis­(3-thienyl­pyra­zolates) of calcium (<b>3c</b>), strontium (<b>3d</b>), and barium (<b>3e</b>) and the elimination of tetra­kis­(3-thienyl­pyra­zolyl)­ethene (<b>4</b>). To isolate crystalline compounds, 3-thienyl­pyra­zole has been metalated, and the corresponding derivatives [(HPz^Tp)₄Mg­(Pz^Tp)₂] (<b>3b</b>), dinuclear [(tmeda)­Ca­(Pz^Tp)₂]₂ (<b>3c</b>), mononuclear [(pmdeta)­Sr­(Pz^Tp)₂] (<b>3d</b>), and [(hmteta)­Ba­(Pz^Tp)₂] (<b>3e</b>) have been structurally characterized. Regardless of the applied stoichiometry, magnesiation of thienyl­pyra­zole <b>3a</b> with di­butyl­magnesium yields [(HPz^Tp)₄Mg­(Pz^Tp)₂] (<b>3b</b>), which is stabilized in the solid state by intramolecular N–H···N···H–N hydrogen bridges. The degradation of [Ae­{C­(Pz^R)₃}₂] (R = Ph and Tp) has been studied by quantum chemical methods, the results of which propose an intermediate complex of the nature [{(Pz^R)₂C}₂Ca­{Pz^R}₂]; thereafter, the singlet carbenes ([:C­(Pz^R)₂]) dimerize in the vicinity of the alkaline earth metal to tetrapyrazolylethene, which is liberated from the coordination sphere as a result of it being a very poor ligand for an s-block metal ion