Stabilization and reactivity of the lewis acidic solvated phenylcalcium cation

Article abstract of DOI:10.1002/anie.201209897

Grignard deluxe: The 1,2-dimethoxyethane adduct of phenylcalcium iodide forms the unique solvent-separated species [(dme)₃CaPh] ⁺I^- with a highly Lewis acidic metal center. Degradation of DME occurs, yielding calcium methoxide cages, such as [{(dme)Ca} ₄(CaI)₂(μ₃-OMe)₈(μ ₆-O)], and methyl vinyl ether. The picture shows the crystal structure of [(dme)₃CaPh]⁺ (Ca brown, C gray, O red).

Full text of DOI:10.1002/anie.201209897

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DOI: 10.1002/anie.201209897
Organocalcium Compounds
Stabilization and Reactivity of the Lewis Acidic Solvated
Phenylcalcium Cation**
Jens Langer, Mathias Kçhler, Jçrg Hildebrand, Reinald Fischer, Helmar Gçrls, and
Matthias Westerhausen*
Calcium, the fifth most frequent element on earth, is the ideal
metal for stoichiometric and catalytic applications owing to its
non-toxicity, global abundance, and low cost. Furthermore,
this alkaline-earth metal shows favorable chemical properties
in its compounds, combining the beneficial behavior of the
alkali metals (highly heteropolar bonds, high nucleophilicity
of the counteranions) and Group 3 elements (d orbital
participation, Lewis acidic character of the cation, catalytic
activity[1]) as well as Group 13 metals (highly Lewis acidic
nature).[2]
stabilize the [(14N4)MgMe]+ cation (14N4 = 1,4,8,11-tetra-
methyl-1,4,8,11-tetraazacyclotetradecane).[10] Mononuclear
arylalkaline-earth metal cations are even more elusive
species. [(Cryptand)Mg(Tol)]+[Mg(Tol)3]À (Tol = para-tolyl)
was detected by NMR spectroscopy in solution after addition
of a cryptand to [Mg(Tol)2] but it was not isolated.[11]
Only three mononuclear organocalcium cations have
been characterized, which contain allyl,[12] cyclopentadie-
nide,[13] and pentamethylcyclopentadienide anions,[14] respec-
tively. In all of these compounds, the negative charge of the
anion can be delocalized through the p-system of the ligand.
Derivatives containing purely s-bonded alkyl or aryl rests are
unknown.
Like their magnesium counterparts,[3] arylcalcium halides
are nowadays easily accessible by direct synthesis from
calcium and the appropriate organyl halides. Redistribution
of the anions converts these heteroleptic complexes RCaX
into the homoleptic congeners CaR2 and CaX2, in agreement
with the Schlenk equilibrium observed for magnesium.[4]
However, this simple relationship given above falls short of
the description of the diverse structures and compounds
present in solution. It is well-known in organomagnesium
chemistry that the addition of multidentate strong Lewis
In earlier work,[15] it was established that Lewis base
adducts of phenylcalcium iodide, such as [(thf)4Ca(Ph)I],[16]
[(thp)4Ca(Ph)I],[15] [([18]crown-6)Ca(Ph)I],[15] and [(tme-
da)2Ca(Ph)I][17] (thp = tetrahydropyran, tmeda = N,N,N’,N’-
tetramethylethylendiamine) adopt similar molecular struc-
tures as the corresponding calcium diiodide complexes, while
for the related diarylcalcium compounds,[4] other variants on
these structures were observed. Most of these calcium
diiodide complexes crystallize as discrete neutral molecules;
formation of separated ions is not observed in solid state.
Exceptions were found for the 1,2-dimethoxyethane complex
[(dme)3CaI]+IÀ [18] and the diethylene glycol dimethyl ether
adduct [(diglyme)2CaI]+IÀ.[19] However, earlier efforts to
completely replace the THF ligands in [(thf)4Ca(Ph)I] by
DME failed to yield a cationic phenylcalcium species, and
[(dme)2(thf)Ca(Ph)I][16] was isolated instead. Therefore, our
initial attempts focused on diglyme as solvent in ligand-
exchange reactions starting from [(thf)4Ca(Ph)I]. Unfortu-
nately, no crystalline material was observed after removal of
THF in vacuum und subsequent cooling. Therefore, ether
exchange was repeated using 1,2-dimethoxyethane
(Scheme 1), although this ligand is known to be easily
degradable by strong bases.[20] To ensure complete removal
of THF, the starting material was dissolved in DME, and
thereafter the solvent was removed in vacuo. This procedure
was repeated. The residue was recrystallized from DME,
bases, such as crown ethers or cryptands[5] or even THF, can
2+
lead to solvent-separated ions, such as [{(thf)3Mg}2(m-Cl)3]2
-
[(Ph2Mg)2(m-Cl)2]2À.[6] While homo- and heterobimetallic
magnesiates are well-documented in organomagnesium
chemistry and play key roles when it comes to the adjustment
of reactivity and selectivity (the so-called turbo Grignard
reagent),[7] organomagnesium cations are less common spe-
cies. Only very few examples for mononuclear cationic
alkylmagnesium reagents exist, such as [(dme)2-
(thf)MgMe]+IÀ.[8] Other examples require non-coordinating
anions, as in [(L)Mg(nBu)]+[BPh4]À [9] (L = 4,6-(MesN
=
PPh2)2dibenzofuran) or extremely weak and bulky Lewis
bases, such as cyclopentadienide, fluorenide, and indenide, to
[*] Dr. J. Langer, M. Kçhler, J. Hildebrand, Dr. R. Fischer, Dr. H. Gçrls,
Prof. Dr. M. Westerhausen
Institute of Inorganic and Analytical Chemistry
Friedrich Schiller University Jena
Humboldtstrasse 8, 07743 Jena (Germany)
E-mail: m.we@uni-jena.de
[**] This work was supported by the German Research Foundation
(DFG, Bonn (Germany)). M.K. is grateful to the Fonds der
Chemischen Industrie (Frankfurt/Main (Germany)) for a generous
Ph.D. stipend. We also appreciate the financial support of the
Verband der Chemischen Industrie (VCI/FCI, Frankfurt/Main (Ger-
many)). Infrastructure of our institute was provided by the EU
(European Fonds for Regional Development, EFRE) and the
Friedrich Schiller University Jena.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201209897.
Scheme 1. Formation of [(dme)3CaPh]+IÀ (1).
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yielding a pale orange solution from which colorless crystals
deposited at À208C.
The single crystals consisted of approximately 95% of
[(dme)3CaPh]+IÀ (1) and 5% of [(dme)3CaI]+IÀ, with a super-
position of the phenyl group and the calcium-bound iodide
verifying again the similarity of calcium diiodide and phenyl-
calcium iodide complexes (Figure 1). Despite the large
À
coordination number of seven, a short average Ca C bond
Figure 2. Decay of [(dme)3Ca(Ph)]+IÀ (starting concentration 0.05 m)
in a solvent mixture of [D6]benzene and 1,2-dimethoxyethane (DME/
[D6]benzene 3:1) at ambient temperature.
Figure 1. Molecular structures of the superimposed cations
[(dme)3CaPh]+ (a) and [(dme)3CaI]+ (b). The asymmetric unit consists
of two molecules A and B; only A is displayed. Ellipsoids are set at
40% probability and H atoms are omitted for clarity. Selected bond
lengths of molecule A [molecule B] [pm]: Ca1–C1 251.9(6) [251.5(6)],
Ca1-I1 314.0(15) [312.3(11)], Ca1–O1 245.9(4) [243.3(3)], Ca1–O2
244.6(4) [244.0(3)], Ca1–O3 243.4(4) [247.1(3)], Ca1–O4 246.6(4)
[245.2(3)], Ca1–O5 240.4(4) [246.1(4)], Ca1–O6 243.9(3) [242.3(4)];
angles (deg.): C2-C1-C6 112.5(5) [112.2(5)], Ca1-C1-C2 124.7(4)
[120.7(4)], Ca1-C1-C6 121.5(4) [125.4(4)].[28]
length of 251.7 pm was observed, which is due to a strong
electrostatic attraction. In the related dinuclear [(thf)3Ca(m-
Ph)3Ca(thf)3]+ cation with bridging phenyl groups, an average
Scheme 2. Proposed mechanism for the decomposition of 1.
[21]
À
Ca C bond length of 261.4(3) pm was reported.
The
average Ca I distance of 313 pm of superimposed
the degradation of DME with tert-butyllithium.[22] Further-
more, a precipitate formed, formally consisting of ICa·OMe.
Ether adducts of alkoxycalcium iodides were reported earlier
with the calcium atoms in distorted octahedral environments
and both anions embedded in the coordination sphere.[23] This
intermediate methoxycalcium iodide complex can dismutate,
forming the well-known species [(dme)3CaI]+IÀ and calcium
methoxide. In presence of oxido-centered calcium species,
a common impurity of the starting material [(thf)4Ca(Ph)I],[24]
the cage compound [{(dme)Ca}4(CaI)2(m3-OMe)8(m6-O)] (2)
was observed to a minor extent.
A few crystals of hexanuclear species 2 were isolated from
the degradation reaction of phenylcalcium iodide in a solvent
mixture of toluene and DME (Figure 3). This cage compound
consists of an oxygen-centered Ca6 octahedron. All of the
faces are capped by methoxy groups. Such oxide-centered Ca6
octahedral cages seem to be one of the characteristic
degradation products, and they were observed earlier for
the decomposition by a heterobimetallic calcium/zinc system
À
[(dme)3CaI]+IÀ is also smaller than determined for [(dme)2-
(thf)Ca(Ph)I] (Ca C 262.1 pm, Ca I 319.2 pm[16]
)
with
À
À
a seven-coordinate calcium center but in agreement with
the data of pure [(dme)3CaI] I (Ca I 312.0(3) pm).
As expected, narrow endocyclic C2-C1-C6 angles (av.
112.48) were found for the [(dme)3CaPh]+ cation owing to
electrostatic repulsion between the anionic charge in a sp2
+
À
[18a]
À
À
hybrid orbital at C1 and the neighboring C1 C2/6 bonds.
The solvent-separated species 1, a heavier homologue of
a Grignard reagent, is only sparingly soluble in hydrocarbons.
Therefore, the NMR spectra were recorded in a solvent
mixture of [D6]benzene and DME. The ipso-carbon atom C1
exhibits a chemical shift of d = 188.9, which lies in the
characteristic range for calcium-bound terminal phenyl
groups.[16] During the measurements of the NMR spectra in
a sealed NMR tube, DME degradation and the formation of
methyl vinyl ether and benzene was observed (Figure 2 and
Scheme 2). A similar finding has already been observed for
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without the disadvantage of dealing with mixed-metal
reagents.
Received: December 11, 2012
Published online: February 12, 2013
Keywords: calcium · ether decomposition · Lewis acids ·
.
ligand-exchange reactions · phenylcalcium cation
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in THF[25] or as side product of the decomposition of calcium
phosphanides in DME.[26] The iodine atoms are bound to two
opposite calcium atoms of the octahedron, whereas the other
four alkaline-earth metal atoms complete their coordination
spheres with DME ligands. Owing to electrostatic reasons, the
À
Ca O1 bonds to the inner oxide are significantly shorter than
to the methoxy anions. As expected, the DME ligands show
À
À
the largest Ca O distances. The Ca1 I1 distance of
313.9(2) pm lies in the expected range.
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hexane yielded [{(dme)4YbII}{(dme)YbIIIPh4}2],
a quite
unusual mixed-oxidation-state rare-earth metal–organic com-
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center.
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[(thf)4Ca(Ph)I] by bidentate DME enforces dissociation of
À
the Ca I bond, and the formation of a solvent-separated
arylcalcium cation is observed. This unique and highly
reactive complex [(dme)3CaPh]+IÀ easily loses a DME
ligand making the Lewis acidic metal center accessible to
substrates. The high reactivity of the [(dme)3CaPh]+ cation
leads to degradation of DME, yielding calcium methoxides
and methyl vinyl ether. This calcium-based organometallic
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[28] CCDC 912073
([(diglyme)2CaI]+IÀ),
CCDC 912074
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([(dme)3CaPh]+I-, 1), and CCDC 912075 ([{(dme)Ca}4(CaI)2-
(m3-OMe)8(m6-O)], 2) contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
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