18 research outputs found
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<sup>Mes</sup>PyCar<sup>Mes</sup>, <b>2</b>) into a solution of
CaI<sub>2</sub> in THF leads to microcrystalline [(Car<sup>Mes</sup>PyCar<sup>Mes</sup>)(thf)CaI<sub>2</sub>] (<b>3</b>), in one
case containing a few single crystals of the unique separated ion
pair [(Car<sup>Mes</sup>PyCar<sup>Mes</sup>)<sub>2</sub>(thf)Ca]I<sub>2</sub> (<b>4</b>) with four Ca–C bonds. However, isolation
of single-crystalline [(Car<sup>Mes</sup>PyCar<sup>Mes</sup>)(thf)CaI<sub>2</sub>] (<b>3</b>) succeeds via the addition of <b>2</b> to a solution of [(thf)<sub>5</sub>CaI]<sup>+</sup>[BPh<sub>4</sub>]<sup>−</sup> 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)<sub>5</sub>CaI]<sup>+</sup>[AlPh<sub>4</sub>]<sup>−</sup> with <b>2</b> yields
solvent separated [(Car<sup>Mes</sup>PyCar<sup>Mes</sup>)(thf)<sub>2</sub>CaI]<sup>+</sup>[AlPh<sub>4</sub>]<sup>−</sup> (<b>5</b>). The Ca–C<sub>NHC</sub> bond lengths lie in a typical
range of Ca–C<sub>Aryl</sub> σ bonds and represent the
shortest Ca–C<sub>NHC</sub> bonds known to date. Soluble [(Car<sup>Mes</sup>PyCar<sup>Mes</sup>)(thf)Ca(NPh<sub>2</sub>)<sub>2</sub>] (<b>6</b>) can be prepared via a metathetical approach from <b>4</b> and KNPh<sub>2</sub> as well as via the addition of <b>2</b> to Ca(NPh<sub>2</sub>)<sub>2</sub> in tetrahydrofuran. The
bulkier amido ligands lead to elongated bonds between the calcium
center and the ligand <b>2</b>. Furthermore, the reactions of
MI<sub>2</sub> (M = Sr, Ba) with <b>2</b> yield [(Car<sup>Mes</sup>PyCar<sup>Mes</sup>)(thf)<sub>2</sub>MI<sub>2</sub>] (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)<sub>2</sub>Ca{N(SiMe<sub>3</sub>)<sub>2</sub>}<sub>2</sub>] in tetrahydrofuran and subsequent
crystallization from a mixture of toluene and 1,2-dimethoxyethane
yield [(dme)Ca{C(Pz<sup>th</sup>)<sub>2</sub>Ph}{N(SiMe<sub>3</sub>)<sub>2</sub>}] (<b>3</b>). The α,α-bis(3-thiophen-2-ylpyrazol-1-yl)benzyl
ligand exhibits a κ<sup>2</sup><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 Me<sub>3</sub>SiCH<sub>2</sub>CaX (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<sub>ipso</sub> bond lengths to the ipso-carbon atoms of the aryl groups.
Calciation of 1,3-bis(3-isopropylimidazol-2-ylidene)benzene with Ca(CH<sub>2</sub>SiMe<sub>3</sub>)<sub>2</sub> 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<sub>ipso</sub> bonds to the aryl groups. In all of these complexes, the Ca–C<sub>carbene</sub> 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)<sub>3</sub>K<sub>2</sub>{1,2-(PhN)<sub>2</sub>C<sub>2</sub>H<sub>4</sub>}] (<b>1</b>). Complex <b>1</b> quantitatively reacts with anhydrous CaI<sub>2</sub> in THF yielding insoluble KI and dinuclear [(thf)<sub>5</sub>Ca<sub>2</sub>{1,2-(PhN)<sub>2</sub>C<sub>2</sub>H<sub>4</sub>}<sub>2</sub>] (<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)<sub>2</sub>C<sub>2</sub>H<sub>4</sub>}] (<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 CC triple bonds of diphenylbutadiyne. Addition
of 18-crown-6 ether (18C6) leads to the formation of the sparingly
soluble potassium complex [{(18C6)K}<sub>2</sub>{1,2-(PhN)<sub>2</sub>C<sub>2</sub>H<sub>4</sub>}] (<b>5</b>) and the insoluble calcium
complex [(18C6)Ca{1,2-(PhN)<sub>2</sub>C<sub>2</sub>H<sub>4</sub>}]
(<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)<sub>2</sub>Ca{N(SiMe<sub>3</sub>)<sub>2</sub>}<sub>2</sub>] in tetrahydrofuran and subsequent
crystallization from a mixture of toluene and 1,2-dimethoxyethane
yield [(dme)Ca{C(Pz<sup>th</sup>)<sub>2</sub>Ph}{N(SiMe<sub>3</sub>)<sub>2</sub>}] (<b>3</b>). The α,α-bis(3-thiophen-2-ylpyrazol-1-yl)benzyl
ligand exhibits a κ<sup>2</sup><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(OPMes<sub>2</sub>)]<sub>4</sub> (<b>1a</b>) and [(thf)<sub>3</sub>{K(OPMes<sub>2</sub>)}<sub>4</sub>] (<b>1b</b>) as well as [(thp)KSPPh<sub>2</sub>]<sub>∞</sub> (<b>2</b>) with a strand-like structure in the
crystalline state. In ethereal solution these complexes very slowly
degrade into KPAr<sub>2</sub> and KE<sub>2</sub>PAr<sub>2</sub> (E
= O, S). The catalytic conversion of iPr-NCE′
(E′ = O, S) and of R-NCN-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 Ar<sub>2</sub>PEK (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–NCN-<i>i</i>Pr yielding
Ph<sub>2</sub>P(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)<sub>2</sub>Ca{N(SiMe<sub>3</sub>)<sub>2</sub>}<sub>2</sub>] in tetrahydrofuran and subsequent
crystallization from a mixture of toluene and 1,2-dimethoxyethane
yield [(dme)Ca{C(Pz<sup>th</sup>)<sub>2</sub>Ph}{N(SiMe<sub>3</sub>)<sub>2</sub>}] (<b>3</b>). The α,α-bis(3-thiophen-2-ylpyrazol-1-yl)benzyl
ligand exhibits a κ<sup>2</sup><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
K<sub>2</sub>[Ca{N(H)Dipp}<sub>4</sub>] (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(OPAryl<sub>2</sub>)]<sub>4</sub> and [(thf)<sub>4</sub>Ca(OPAryl<sub>2</sub>)<sub>2</sub>] yields the corresponding mononuclear complexes [(18C6)K(OPAryl<sub>2</sub>)] [Aryl = Ph (<b>1a</b>), Mes (<b>1b</b>)] and
[(18C6)Ca(OPAryl<sub>2</sub>)<sub>2</sub>] [Aryl = Ph (<b>2a</b>), Mes (<b>2b</b>)]. The metathesis reaction of [(thf)K(OPAryl<sub>2</sub>)]<sub>4</sub> with CaI<sub>2</sub> yields the calciate [K<sub>2</sub>Ca(thf)<sub><i>x</i></sub>{OPMes<sub>2</sub>}<sub>4</sub>]. The addition of dimesitylphosphane oxide and crystallization
from a hexane solution gives [K<sub>2</sub>Ca{OPMes<sub>2</sub>}<sub>4</sub>{Mes<sub>2</sub>P(O)H}] (<b>3</b>). The complexes [(thf)K(OPMes<sub>2</sub>)]<sub>4</sub>, [(thf)<sub>4</sub>Ca(OPMes<sub>2</sub>)<sub>2</sub>], <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(SiMe<sub>3</sub>)<sub>2</sub>}<sub>2</sub>] (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(SiMe<sub>3</sub>)<sub>2</sub>}<sub>2</sub>] (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<sup>Tp</sup>)<sub>4</sub>Mg(Pz<sup>Tp</sup>)<sub>2</sub>] (<b>3b</b>),
dinuclear [(tmeda)Ca(Pz<sup>Tp</sup>)<sub>2</sub>]<sub>2</sub> (<b>3c</b>), mononuclear [(pmdeta)Sr(Pz<sup>Tp</sup>)<sub>2</sub>]
(<b>3d</b>), and [(hmteta)Ba(Pz<sup>Tp</sup>)<sub>2</sub>] (<b>3e</b>) have been structurally characterized. Regardless of the
applied stoichiometry, magnesiation of thienylpyrazole <b>3a</b> with dibutylmagnesium yields [(HPz<sup>Tp</sup>)<sub>4</sub>Mg(Pz<sup>Tp</sup>)<sub>2</sub>] (<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<sup>R</sup>)<sub>3</sub>}<sub>2</sub>] (R = Ph and Tp) has been studied by quantum chemical
methods, the results of which propose an intermediate complex of the
nature [{(Pz<sup>R</sup>)<sub>2</sub>C}<sub>2</sub>Ca{Pz<sup>R</sup>}<sub>2</sub>]; thereafter, the singlet carbenes ([:C(Pz<sup>R</sup>)<sub>2</sub>]) 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