Mechanistic elucidation of the formation of the inverse Ca(I) sandwich complex [(thf)3Ca(μ-C6H3-1,3,5-Ph 3)Ca(thf)3] and stability of aryl-substituted phenylcalcium complexes

Article abstract of DOI:10.1021/ja105534w

The formation of the stable inverse Ca(I) sandwich complex [(thf) ₃Ca(μ-C₆H₃-1,3,5-Ph₃)Ca(thf) ₃] (1) has been investigated mechanistically by the reaction of bromo-2,4,6-triphenylbenzene with calcium

Full text of DOI:10.1021/ja105534w

Published on Web 08/18/2010
Mechanistic Elucidation of the Formation of the Inverse Ca(I)
Sandwich Complex [(thf)3Ca(µ-C6H3-1,3,5-Ph3)Ca(thf)3] and
Stability of Aryl-Substituted Phenylcalcium Complexes
Sven Krieck, Helmar Go¨rls, and Matthias Westerhausen*
Institute of Inorganic and Analytical Chemistry, Friedrich-Schiller-UniVersita¨t Jena,
August-Bebel-Str. 2, D-07743 Jena, Germany
Received July 5, 2010; E-mail: m.we@uni-jena.de
Abstract: The formation of the stable inverse Ca(I) sandwich complex [(thf)3Ca(µ-C6H3-1,3,5-Ph3)Ca(thf)3]
(1) has been investigated mechanistically by the reaction of bromo-2,4,6-triphenylbenzene with calcium in
varying stoichiometric ratios. The key intermediate consists of a solvent-separated ion pair consisting of a
dinuclear calcium cation with a bridging doubly deprotonated triphenylbenzene and a triphenylbenzene
radical counteranion [(thf)3Ca(µ-C6H2-C6H4Ph2)(µ-O-CHdCH2)Ca(thf)3][C6H3Ph3] (4). A precondition of
the formation of 1 is the lability of the heavy Grignard reagent [{2,4,6-Ph3C6H2}Ca(thf)3Br] (2), which has
been studied along with the role of ether degradation reactions. The strong reducing reagent 1 is stable in
THF solution, and ether cleavage does not occur. However, toluene is metalated in good yields, and the
dibenzylcalcium complex [(tmta)2Ca(CH2C6H5)2] (5) is generated after addition of 1,3,5-trimethyl-1,3,5-
triazinane (tmta). The substitution pattern of arylcalcium halides was modified, and it was found that phenyl
substituents at the para position induce lability, leading to an enhanced tendency to cleave ethers. Kinetic
stabilization of the Ca-Cipso bond can be achieved by ortho substitution using m-terphenyl-based ligands.
Direct reaction of iodo-2,6-di(4-tolyl)benzene (6) with activated calcium in THF at low temperatures yielded
the first example of a stable m-terphenylcalcium halide, namely, [{2,6-(4-tol)2C6H3}Ca(thf)3I] (8). The latter
reacts via insertion of carbon dioxide to form the dimeric benzoate [{2,6-(4-tol)2C6H3CO2}Ca(thf)3I]2 (9).
4-dimethylaminopyridine).5 There are also several examples of
Introduction
molecular RMg-MgR derivatives having bulky R groups for
Compounds with subvalent electropositive metals have at-
tracted much attention because, despite the fact that sophisticated
preparative skills often meet exceptional bonding situations
associated with high thermal stability in the solid state and in
solution, these compounds possess reducing properties toward
a wide range of substrates.1 Several approaches have proven to
be effective in stabilizing subvalent compounds of s-block
metals. The suboxides of rubidium and cesium, Rb9O2 and
Cs11O3, contain two or three oxygen-centered metal octahedrons
interconnected via common faces. The bonding situations can
best be formulated as [(Rb9O2)5+ ·5e-] and [(Cs11O3)5+ ·5e-],
respectively; additional metal atoms can offer a metal matrix
for the “free” electrons, leading to more metal-rich compounds
such as Rb6O and Cs4O or Cs7O.2 A similar concept has been
used to discuss the structures and bonding situations in solids
such as M2N (M ) Ca, Sr, Ba)3 and ternary compounds such
as LiBa2N and LiBa3N.4 Another possibility is the formation
kinetic protection of the reactive Mg-Mg bonds, thereby
suppressing dismutation reactions.6 These dimeric magnesium(I)
complexes are surprisingly stable and offer a rich coordination
and redox chemistry, acting as selective double one-electron
reductants.7 In contrast, the subvalent magnesium(I) chloride
or hydride species XMg-MgX (X ) Cl, H) have been
(3) (a) Horvath-Bordon, E.; Riedel, E.; Zerr, A.; McMillan, P. F.;
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2+
of discrete homodinuclear metal-metal bonds leading to M2
2+
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published for the lighter homologous [(L)3Zn-Zn(L)3]2+ (L )
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12492 J. AM. CHEM. SOC. 2010, 132, 12492–12501
10.1021/ja105534w  2010 American Chemical Society

Formation of an Inverse Ca(I) Sandwich Complex
A R T I C L E S
characterized spectroscopically after trapping in an argon or
nitrogen matrix.8 Calculations have shown that Ca-Ca bonds
should also be feasible,9 although the Ca-Ca bond energy is
on the same order of magnitude as the atomization energy of
calcium metal. Therefore, the atomization energy has to be
invested in order to be able to prepare subvalent organometallic
compounds. This can be achieved by employing the coconden-
sation of metal vapor with substrate. Because of the isoelectronic
relationships Sc3+/Ca2+, Sc2+/Ca+, and Sc+/Ca0, subvalent
scandium species are interesting in this context. Cocondensation
of scandium with 1,3,5-tri-tert-butylbenzene yields Sc(0) and
Sc(II) compounds,10 whereas cocondensation of Sc vapor with
2-tert-butyl-1-phosphaethyne leads to the formation of Sc(I) and
Sc(II) complexes.10,11 Scandium(I) bromide can be stabilized
with an organomagnesium matrix.12 Reduction of a cyclopen-
tadienylscandium compound with potassium offers suitable
access to subvalent scandium derivatives as well.11 The stability
of this kind of complexes, the lighter magnesium(I) congener
and the valence isoelectronic d-block metal scandium, suggest
that organic Ca(I) species should also be feasible.
The recently reported synthesis of the inverse Ca(I) sandwich
complex [(thf)3Ca(µ-C6H3-1,3,5-Ph3)Ca(thf)3] (1) shows an
alternative for stabilization of Ae(I) derivatives.13 To date,
however, this compound is the only example of a stable
subvalent organometallic compound of a heavier alkaline earth
(Ae) metal. Surprisingly, the straightforward reduction of an
arene with calcium metal does not lead to this inverse sandwich
complex, but the presence of at least catalytic amounts of
bromoarenes proved to be essential. In order to understand the
formation of 1, the mechanism must be elucidated. Therefore,
we studied the reduction of bromo-2,4,6-triphenylbenzene
(tpbBr) with calcium powder using various stoichiometric ratios
in order to detect intermediates. In addition, the aryl substitution
pattern strongly influences the stability of diverse phenyl-
substituted arylcalcium complexes.
chemical calculations, which showed that metal orbitals with d
symmetry15 stabilize this inverse sandwich complex. However,
the reaction mechanism still remained unknown.
The haloarene tpbBr is very reactive toward insertion
reactions of metals because of the high electron density in the
central arene unit, as a consequence of the symmetric induction
by the electron-donating phenyl substituents. The reaction of
activated calcium with tpbBr and also the direct reaction of
calcium powder with 1,3,5-triphenylbenzene in the presence
of a catalytic amount (10 mol %) of tpbBr both give calcium(I)
complex 1 in good yields of more than 80%. During either
reaction pathway, ether cleavage reactions occur and play a
prominent role. In order to study the mechanism of the formation
of 1, we reduced tpbBr with calcium powder in a strictly
equimolar ratio in THF, which yielded the postulated species
[{2,4,6-Ph3C6H2}Ca(thf)3Br] (2). During the reaction time of
30 min at -60 °C, all of the metal powder dissolved, generating
a slightly yellowish solution of 2. This heavy Grignard solution
proved to be rather reactive and cleaved THF via R-deproto-
nation, yielding 1,3,5-triphenylbenzene nearly quantitatively
over 2 h. For this reason, several attempts to isolate 2 failed.
This behavior is in agreement with earlier reports on arylcalcium
halides.16 Ether degradation reactions are common in the
organometallic chemistry of strongly electropositive s-block
metals, involving in the case of THF R-metalation and subse-
quent cycloreversion combined with liberation of ethenolate and
ethene.17 Addition of iodine to a solution of 2 at low temper-
atures gave 2,4,6-Ph3C6H2I (tpbI, 3), clearly supporting the
conversion of the initial bromoarene to the heavy Grignard-
type species (Scheme 1). This lack of stability contrasts with
that of the well-known 2,4,6-triarylphenyllithium [(Et2O)2Li(C6H2-
2,4,6-Ph3)]18 and -magnesium [(thf)2Mg(Br)C6H2-2,4,6-Ph3]13
derivatives, which have been isolated and structurally character-
ized. Neither excess metal nor storage in THF at ambient
temperature decreased the stability of these lithium and mag-
nesium derivatives.
Results and Discussion
Addition of a slight excess of calcium to an intermediately
generated solution of 2 immediately produced a deep-blue
solution, which indicated the formation of paramagnetic 1,3,5-
triphenylbenzene radical anions. These anions are formed as
one of the products of R-deprotonation of THF and subsequent
reduction with surplus calcium metal. Reactions in [D8]THF
generated monodeuterated [D1]1,3,5-triphenylbenzene, which
also supports the importance of ether solvents acting as Brønsted
acids. A similar reaction (with in situ formation of 2, prepared
via the reaction of tpbBr with 1.5 equiv of activated calcium)
and subsequent storage of the dark-blue reaction solution at low
temperatures led to crystallization of the black, opalescent, and
During the reaction of tpbBr with an excess of activated
calcium in THF at low temperatures, deeply colored solutions
were obtained and the title inverse sandwich complex [(thf)3Ca(µ-
C6H3-1,3,5-Ph3)Ca(thf)3] (1) was isolated.13 The molecular
structure showed that the calcium atoms lie on a C3 axis on
opposite sides of a strictly planar arene dianion. In contrast,
the neutral 1,3,5-triphenylbenzene (tpb) ligand shows a propel-
ler-like arrangement with a twist angle of 38° for the phenyl
substituents.14 The bonding situation was clarified with quantum-
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(16) Ga¨rtner, M.; Go¨rls, H.; Westerhausen, M. Synthesis 2007, 725–730.
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694, 2204–2209. (b) Langer, J.; Krieck, S.; Fischer, R.; Go¨rls, H.;
Walther, D.; Westerhausen, M. Organometallics 2009, 28, 5814–5820.
(c) Kennedy, A. R.; Klett, J.; Mulvey, R. E.; Wright, D. S. Science
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Reiher, M. Chem.sEur. J. 2007, 13, 6292–6306. (b) Velazquez, A.;
Ferna´ndez, I.; Frenking, G.; Merino, G. Organometallics 2007, 26,
4731–4736.
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A R T I C L E S
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Scheme 1. Reaction of Bromo-2,4,6-triphenylbenzene (tpbBr) with Activated Calcium Powder and Its Dependence on the Stoichiometric
Ratio
highly pyrophoric compound [(thf)3Ca(µ-C6H2-C6H4Ph2)-
(µ-O-CHdCH2)Ca(thf)3][C6H3Ph3] (4), as shown in Scheme
1. The solvent-separated ion pair consists of a 1,3,5-triphenyl-
benzene radical monoanion and a dinuclear cation. The latter
contains a bridging vinylate anion that stems from THF
degradation, representing an essential step in the stoichiometric
ratio as the hydrogen source for the formation of calcium(I)
complex 1. The importance of the ether cleavage step was
verified by the necessity of at least 10 mol % tpbBr, as the
direct synthesis of 1 using activated calcium and 1,3,5-
triphenylbenzene failed in each case. The bromoarene does not
simply function as activating reagent (comparable to the addition
of iodine to magnesium chips as a classical Grignard activation
procedure) because the addition of 10 mol % iodine to a mixture
of 1,3,5-triphenylbenzene and excess calcium in THF exclu-
sively led to the formation of the corresponding amount of the
calcium diiodide complex [(thf)4CaI2]. However, the presence
of iodine does not initiate the formation of 1.
triphenylbenzene. The high symmetry of the EPR resonance in
combination with the g value of the free electron (g ) 2.0023)
excludes a metal-centered radical. Additional susceptibility
measurements gave a magnetic moment (µeff) of 1.62µB,
supporting the presence of only “one” unpaired electron and
hence underlining the radical monoanion character of 4. These
The EPR spectrum of 4 in THF solution, pictured in Figure
1,19 is well-resolved and suggests the presence of only one
paramagnetic species, in accordance with the studies of De Boer
and co-workers,20 indicating the free radical monoanion of 1,3,5-
Figure 1. X-band EPR spectrum of
a solution of [(thf)3Ca
(19) For several EPR studies (experimental and via simulation) of 1,3,5-
triphenylbenzene monoanions with varioous metal countercations, see:
Krieck, S.; Go¨rls, H.; Westerhausen, M. J. Am. Chem. Soc., submitted.
(µ-C6H2-C6H4Ph2)(µ-O-CHdCH2)Ca(thf)3]+[C6H3Ph3]- (4) in THF re-
corded at 298 K (g ) 2.0023).
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Formation of an Inverse Ca(I) Sandwich Complex
A R T I C L E S
findings are consistent with studies of alkali-metal monoadducts
of 1,3,5-triphenylbenzene by Lu¨hder.21
After definite quenching of a solution of crystalline 4 in THF
with [D4]methanol, the vinylate moieties were detected via NMR
spectroscopy. Treatment of a THF solution with bromine led
to formation of 2-(2′-bromophenyl)-4,6-diphenylbromoben-
zene,22 whereas tpbBr was not detected.
The directed ortho metalation of a phenyl group in the or-
tho position leads to a moiety of the type 1,1′-biphenyl-2,2′-
diyldicalcium having two additional phenyl groups at the 3-
and 5-positions. The formation of 4 shows that not only THF
molecules (which finally yielded the vinylate anions) but also
the triphenylphenyl system served as proton sources in this
reaction sequence. A similar 1,1′-biphenyl-2,2′-diyldilithium
moiety is well-known and a valuable synthon in numerous ligand
transfer reactions.23
Because of the detected paramagnetism of 4 (particularly of
the 1,3,5-triphenylbenzene monoanion, confirming at the same
time the presence of a delocalized radical anion instead of a
localized carbanion), NMR techniques were not a suitable tool
for fully characterizing this complex. Therefore, we grew single
crystals suitable for determination of the X-ray crystal structure
after numerous recrystallization steps. The molecular structures
and numbering schemes for the cation and anion in 4 are
presented in Figure 2a,b, respectively. The calcium atoms, which
have a Ca · · ·Ca′ distance of 323.24(10) pm, are in distorted
octahedral environments and show an average Ca-C bond
length of 260.1 pm. Because of the stronger electrostatic
attraction, the Ca-O bonds to the bridging vinylate anion
[Ca′-O-Ca, 50.06(8)°] are shorter (av. Ca-O, 231.3 pm) than
those to the THF ligands (av. Ca-O, 237.0 pm) but adopt
characteristic values.17a,24 The C-C bond length [123.2(10) pm]
in the O-CHdCH2 moiety verifies the constitution as an
ethenolate unit.
Figure 2. (a) Molecular structure and numbering scheme for the cation of
4, [(thf)3Ca(µ-C6H2-C6H4Ph2)(µ-O-CHdCH2)Ca(thf)3]+. H atoms have
been omitted for clarity; ellipsoids represent a probability of 40%. Selected
bond lengths (pm) and angles (deg): Ca1-C1, 259.9(3); Ca2-C1, 267.3(3);
Ca1-C8, 259.8(3); Ca2-C8, 253.3(4); Ca1-O1, 231.9(3); Ca2-O1,
230.7(3); O1-C25, 118.2(5); C25-C26, 123.2(10); Ca1-O2, 236.6(3);
Ca1-O3,238.4(3);Ca1-O4,238.1(3);Ca · · ·CaA,323.24(10);Ca1-C1-Ca2,
53.32(8); Ca1-C8-Ca2, 50.06(8); Ca1-O1-Ca2, 45.53(7); O3-Ca1-C8,
173.77(10); O1-Ca1-O4, 174.85(10). (b) Molecular structure and number-
ing scheme of the 1,3,5-triphenylbenzene radical anion [C6H3Ph3]- in 4. H
atoms have been omitted for clarity; ellipsoids represent a probability of
40%. Selected bond lengths (pm) and the torsion angle of the phenyl group,
F (deg), are given.
The 1,1′-biphenyl-2,2′-diyl unit shows only a slight torsion
around the C2-C7 bond. A much stronger deviation from
planarity is realized by the phenyl groups; however, all of the
interaryl bonds represent characteristic single-bond character.
The radical anion shows a remarkable structure. The phenyl
group at C28 is slightly tilted (17°) with respect to the remaining
planar 1,3-diphenylbenzene (m-terphenyl) unit. This feature
strongly influences the bond lengths of the inner carbon ring.
The carbon atom C31 on the opposite side of the twisted phenyl
ring shows remarkably large C31-C bond lengths of 149.8 and
150.2 pm, whereas the other C-C distances have values
characteristic for aromatic systems. In contrast, systems with
localized 1,3,5-triphenylphenyl carbanions possess a propeller-
like arrangement of the phenyl substituents, as realized in, for
example, [(thf)2Mg(Br)C6H2Ph3],13 [(Et2O)2Li(C6H2Ph3)],18 and
[Bi(C6H2Ph3)3].25
Addition of another equivalent of calcium powder to a THF
solution of twice-recrystallized 4 led to the initial formation of
[{(thf)3Ca}2(µ-C6H3-1,3,5-Ph3)] (1), which was verified by
repeated determination of the crystallographic cell parameters
using X-ray diffractometry. The detailed mechanism for the
formation of 1 from 4 remains unclear at present. The sum of
possible reaction sequences (starting from tpbBr, 2, or 4) to
the final formation of the homodinuclear inverse sandwich
complex 1 consists of subsequent steps of deprototation and
protonation reactions (both via ether degradation) as well as
several reduction and probably also synproportionation processes.
In the persistent organic π-diradical 1, the dianionic charge
of the 1,3,5-triphenylbenzene system is stabilized by effective
delocalization, resulting in a reduced nucleophilicity of the
complex. Therefore, ethereal solutions of 1 are stable at ambient
temperature over a long period of time and no tendency toward
(20) van Broekhoven, J. A. M.; Hendriks, B. M. P.; De Boer, E. J. Chem.
Phys. 1971, 54, 1988–1994.
(21) Lu¨hder, K. Z. Chem. 1969, 9, 459–460.
(22) 2-(2′-bromophenyl)-4,6-diphenylbromobenzene: Calcd for C24H16Br2
(464.19 g mol-1): C, 62.10; H, 3.47; Br, 34.43. Found: C, 61.94; H,
3.39; Br, 34.13. ESI-MS (m/z [%]): 463 [100] (M), 383 [43] (M-
Br), 306 [80] (M-2Br; tpbH).
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Organometallics 2000, 19, 5537–5540.
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A R T I C L E S
Krieck et al.
pm, which is slightly larger than that observed for [(tmta)2-
Ca{P(SiMe3)2}2] (av. Ca-N, 264.7 pm)29 and indicative of the
enhanced steric strain. As a result of the bulkiness of the tmta
ligands, a rather small C1-Ca1-C8 angle of 98.1° was
observed. However, the tetrahydrofuran complex [(thf)4Ca-
(CH2C6H5)2] shows a cis arrangement of the benzyl anions with
an average bond angle of 94.1°, whereas the 4-tert-butylbenzyl
derivative crystallizes as the trans isomer of [(thf)4Ca(CH2C6H4-
4-tBu)2].28
The formation of 5 is accompanied by a color change of the
solution from an initial deep-blue to a final dark-violet. EPR
spectroscopic investigations in a sealed tube showed a nearly
constant concentration of 1,3,5-triphenylbenzene anions. These
findings suggest that the reduction equivalents originate pref-
erentially from the oxidation of Ca(I) to Ca(II). In 1, the two
calcium(I) atoms are located on opposite sides of the bridging
arene system, and their d orbitals interact with each other via
an arene-centered orbital, leading to the intrinsic stability of
the subvalent homodinuclear organocalcium complex 1. A
change in the oxidation state of one of the calcium centers
consequently disturbs this bonding situation, and a constitutional
change follows, preserving the radical character on 1,3,5-
triphenylbenzene as a mono- or dianionic species.30 Unfortu-
nately, all attempts to isolate the corresponding species con-
taining {(thf)xCaII(C6H3-1,3,5-Ph3)} failed. Nevertheless, the
metal atoms in ꢀ-diketiminate complexes of MgI with MgI-MgI
bonds can be oxidized with preservation of the magnesium
ꢀ-diketiminate unit.1 Carmona and co-workers31 recently pub-
lished reactions of the decamethyldizincocene species [Cp*Zn-
ZnCp*] (Cp* ) C5Me5) with unusual preservation of the
metal-metal bond. The magnesium and zinc species possess
sterically shielded diamagnetic divalent homodinuclear metal
centers and therefore show binding situations that are funda-
mentally different from that in the intrinsically stable paramag-
netic calcium(I) inverse sandwich complex without a discrete
metal-metal bond.1
Extensive studies of the reaction of 1,3,5-triphenylbenzene
with calcium in liquid ammonia as the solvent at low temper-
atures indicated the formation of polymeric structures. In both
pure ammonia and solvent mixtures of ammonia and donor
solvents [pyridine, THF, 1,2-dimethoxyethane (DME)], the
reaction of 1,3,5-triphenylbenzene and calcium in a 1:1 molar
ratio yielded a violet solid that was completely insoluble in
common organic solvents. Even extraction with strong chelating
bases (DME, tmeda, tmta) failed. Identification via classical
digestion methods and subsequent determination of the metal
content suggested a constitution formula without additional
coordinating ligands as [Ca(C6H3-1,3,5-Ph3)]∞. Similar solubility
properties are well-known for polymeric calciocene [Cp2Ca]∞
(Cp ) C5H5),32 whereas the bisdonor adducts [(thf)2CaCp2]
dissolve easily in donor solvents.33
Figure 3. Molecular structure and numbering scheme for [(tmta)2Ca-
(CH2C6H5)2] (5). H atoms have been omitted for clarity; ellipsoids represent
a probability of 40%. Selected bond lengths (pm) and angles (deg): Ca1-C1,
261.5(3); Ca1-C8, 263.0(3); Ca1-N1, 268.5(3); Ca1-N2, 270.3(2);
Ca1-N3, 268.0(2); Ca1-N4, 270.8(2); Ca1-N5, 270.8(2); Ca1-N6,
264.1(2); C1-Ca1-C8, 98.07(10); C2-C1-Ca1, 111.03(19); C9-C8-Ca1,
113.0(2).
ether cleavage reactions is observed, in accordance with
observations for systems containing homodinuclear Mg(I)-Mg(I)
bonds, which are even stable as various Lewis base adducts.7c,26
Furthermore, solutions of 1 are also stable under reflux
conditions and show no tendency to undergo redox dismutation
of Ca(I) to derivatives of Ca(II) and Ca(0), contrary to the
behavior of the thermolabile anthracene complexes
[(thf)nAeII(anthracene)] (Ae ) Mg, Ca, Sr, Ba), which can
therefore be used for metal activation purposes according to
Bogdanovic´ and co-workers.27 Alkali metal (A) adducts of 1,3,5-
triphenylbenzene also show an equilibrium between A2(tpb) on
the one hand and A(tpb) + A on the other.21
In general, the stability of a compound is associated with its
reactivity, and often these trends are adverse. Despite the
resonance stabilization of the 1,3,5-triphenylbenzene dianion,
complex 1 functions as a strong reducing agent because of the
unusual +1 oxidation state of the calcium atoms. Addition of
iodine to a solution of 1 quantitatively yielded [(thf)4CaI2] and
1,3,5-triphenylbenzene, transferring four reduction equivalents
in all. A solvent change from ether to toluene led within 5 days
to the formation of dibenzylcalcium, which crystallized as the
colorless adduct [(tmta)2Ca(CH2C6H5)2] (5) after addition of
1,3,5-trimethyl-1,3,5-triazinane (tmta). The molecular structure
and numbering scheme for the dibenzylcalcium complex 5 are
displayed in Figure 3. Benzylcalcium compounds are well-
known and have been prepared via salt metathesis of benzylpo-
tassium with CaI2.28
The calcium atom in 5 adopts the large coordination number
of eight. This fact induces steric strain, which leads to a rather
large average Ca-C bond length of 262.3 pm. The Ca-N
distances to the tmta ligands show an average value of 268.1
(29) Westerhausen, M.; Schwarz, W. J. Organomet. Chem. 1993, 463, 51–
63.
(30) (a) Sommerdijk, J. L.; van Broekhoven, J. A. M.; van Willigen, H.;
De Boer, A. J. Chem. Phys. 1969, 51, 2006–2009. (b) van Broekhoven,
J. A. M.; Sommerdijk, J. L.; De Boer, A. J. Mol. Phys. 1971, 20,
993–1003.
(26) Green, S. P.; Jones, C.; Stasch, A. Angew. Chem. 2008, 120, 9219-
9223; Angew. Chem., Int. Ed. 2008, 47, 9079-9083.
(27) (a) Bo¨nnemann, H.; Bogdanovic´, B.; Brinmann, R.; Egeler, N.; Benn,
R.; Topalovic, I.; Seevogel, K. Main Group Met. Chem. 1990, 13,
341–362. (b) Bogdanovic´, B.; Janke, N.; Kinzelmann, H.-G.; West-
eppe, U. Chem. Ber. 1988, 121, 33–37. (c) Bogdanovic´, B.; Liao, S.-
T.; Mynott, R.; Schlichte, K.; Westeppe, U. Chem. Ber. 1984, 117,
1378–1392.
´
(31) Carrasco, M.; Peloso, R.; Rodr´ıguez, A.; Alvarez, E.; Maya, C.;
Carmona, E. Chem.sEur. J. [Online early access]. DOI: 10.1002/
chem.201001011. Published Online: June 11, 2010.
(32) (a) Ziegler, K.; Froitzheim-Ku¨hlhorn, H.; Hafner, H. Chem. Ber. 1956,
89, 434–443. (b) Zerger, R.; Stucky, G. D. J. Organomet. Chem. 1974,
80, 7–17.
(28) Harder, S.; Mu¨ller, S.; Hu¨bner, E. Organometallics 2004, 23, 178–
183.
(33) Fischer, E. O.; Sto¨lzle, G. Chem. Ber. 1961, 94, 2187–2193.
9
12496 J. AM. CHEM. SOC. VOL. 132, NO. 35, 2010

Formation of an Inverse Ca(I) Sandwich Complex
A R T I C L E S
Scheme 2. Representation of the Biphenyl (Round Line) and
m-Terphenyl (Rectangular Line) Subunits of
Halogeno-2,4,6-triphenylbenzene
Scheme 3. Resonance between the Benzenoid and Chinoid Forms
of Biphenyl
Scheme 4. Synthesis and Reactivity of the Classical Grignard
Reagent 7 and Heavy Grignard Reagent 8
A central issue in the reaction cascade that generates 1 is the
enormous instability of the heavy Grignard reagent [{2,4,6-
Ph3C6H2}Ca(thf)3Br] (2) during generation in solution. There-
fore, further studies were performed to address the influence of
the aryl substitution pattern of phenylcalcium halides. Our earlier
studies of a broad portfolio of heavy Grignard reagents showed
that arylcalcium halides tolerate a variety of para substituents,
such as halogen, methoxy, amino, and alkyl groups.34,16 To
date, only arylcalcium halides without substituents that enable
extended increasing delocalization have been isolated, with
1-naphthylcalcium halides being an exception.17d These findings
are contradictory to the additional stabilization effects based
on the reduced nucleophilicity combined with enhanced delo-
calization and charge-stabilization effects. According to Scheme
2, the halogeno-2,4,6-triphenylbenzene system can be regarded
in two ways: (i) a p-phenyl-substituted system representing a
biphenyl subunit and (ii) a 2,6-diphenylphenyl system represent-
ing a m-terphenyl subunit.
To this point, bulky groups were employed in order to
kinetically protect the Ca-C bonds using the m-terphenyl ligand
for stabilization reasons. The reaction of iodo-2,6-di(4-tolyl)-
benzene (6) with activated magnesium (Rieke magnesium) in
THF gave the classical Grignard reagent [{2,6-(4-tol)2C6H3}-
Mg(thf)2I] (7) in high yield (Scheme 4). The structural motif
of 7 is displayed in Figure 4 and shows the expected tetrahedral
coordination sphere of the magnesium center. Treatment of 6
with an excess of magnesium metal under reflux conditions also
exclusively generated the Grignard compound.
The direct reaction of activated calcium with 6 in THF at 20
°C led to complete dissolution of the metal. However, fast
decomposition of intermediate arylcalcium derivatives was
observed. Strict maintenance of the reaction conditions at -60
°C and use of an exact 1:1 stoichiometric ratio of iodoarene to
metal led to the formation of pale-violet [{2,6-(4-
tol)2C6H3}Ca(thf)3I] (8) (Scheme 4), contrary to earlier
In all cases, reactions of 2- and 4-halogenobiphenyl as well
as 2,2′- and 4,4′-dihalogenobiphenyl (halogen ) Br, I) in THF
at low temperatures with stoichiometric amounts of activated
calcium powder led to the formation of the calcium dihalide
complex [(thf)4CaX2] and biphenyl in nearly quantitative yields
(eq 1). However, in neither case was a stable corresponding
heavy Grignard reagent observed.
Phenyl substituents at the 4-position drastically enhance the
tendency for ether degradation reactions, yielding biphenyl after
R-deprotonation of the ether solvent. Perhaps the intrinsic
instability of the Ca-Cipso σ bond in biphenyl derivatives can
be attributed to the formation of chinoid instead of benzenoid
structures (Scheme 3), which would destabilize the arylcalcium
halides. Reactions of an excess of calcium metal (more than 2
equiv) with different substituted (di)halogenobiphenyl deriva-
tives gave deep-green solutions. EPR measurements confirmed
the formation of biphenyl dianions in accordance with Rieke’s
method of metal activation35 as well as with studies of alkali-
metal-biphenyl adducts.36
(34) (a) Westerhausen, M. Z. Anorg. Allg. Chem. 2009, 635, 13–32. (b)
Westerhausen, M. Coord. Chem. ReV. 2008, 252, 1516–1531. (c)
Westerhausen, M.; Ga¨rtner, M.; Fischer, R.; Langer, J. Angew.
Chem. 2007, 119, 1994-2001; Angew. Chem., Int. Ed. 2008, 46,
1950-1956.
Figure 4. Representation of the structural motif of [{2,6-(4-tol)2C6H3}-
Mg(thf)2I] (7).
9
J. AM. CHEM. SOC. VOL. 132, NO. 35, 2010 12497

A R T I C L E S
Krieck et al.
Figure 5. Molecular structure and numbering scheme for [{2,6-(4-
tol)2C6H3}Ca(thf)3I] (8). Ellipsoids represent a probability of 40%; H atoms
have been omitted for clarity. Selected bond lengths (pm) and angles (deg):
Ca1-I1, 307.54(7); Ca1-C1, 251.7(4); Ca1-O1, 238.4(4); Ca1-O2,
239.0(3); Ca1-O3, 234.4(3); C1-Ca1-I1, 108.20(9); C1-Ca1-O1,
84.8(1); C1-Ca1-O2, 144.7(1); C1-Ca1-O3, 115.1(1); I1-Ca1-
O1, 160.6(1); I1-Ca1-O2, 85.54(7); I1-Ca1-O3, 97.50(8); O1-Ca1-O2,
75.9(1); O1-Ca1-O3, 89.5(1); O2-Ca1-O3, 94.1(1); Ca1-C1-C2,
116.9(3); Ca1-C1-C6, 124.6(3); C2-C1-C6, 114.7(3).
Figure 6. Molecular structure and numbering scheme for [{2,6-(4-
tol)2C6H3CO2}Ca(thf)3I]2 (9). Symmetry-related atoms (-x + 1, -y, -z)
are marked with an “A”. Ellipsoids represent a probability of 40%; H atoms
have been omitted for clarity. Selected bond lengths (pm): Ca1-I1,
312.84(9); Ca1-O1, 228.2(3); Ca1-O2A, 226.1(4); Ca1-O3, 236.0(3);
Ca1-O4, 241.1(3); Ca1-O5, 242.5(4); C1-O1, 125.4(5); C1-O2, 124.4(5);
C1-C2, 151.6(6).
mulene carbon dioxide was supplied, the corresponding p-di-
tolylbenzoate dimer [{2,6-(4-tol)2C6H3CO2}Ca(thf)3I]2 (9) was
isolated (Scheme 4), representing the well-known reactivity
observed for classical Grignard reagents.
findings.16,34 An ethereal solution of 8 showed ∼20% decom-
position during storage for 24 h at -40 °C, and in the solid
state, storage at -20 °C for several days without noticeable
decomposition was possible.
The reactions of excess activated calcium with 6 and 8 in
ethereal solutions led to formation of deep-red-violet solutions.
However, because of the thermolability of these solutions,
several attempts to isolate the species failed, and spontaneous
decolorization of the solutions combined with metal precipitation
was observed, comparable to the findings of Bogdanovic´ and
co-workers.27
The molecular structure and numbering scheme for the first
structurally characterized m-terphenyl-based Grignard-type com-
plex, [{2,6-(4-tol)2C6H3}Ca(thf)3I] (8), are shown in Figure 5.
In comparison to [{2,6(4-tol)2C6H3}Mg(thf)2I] (7), a rather
similar molecular structure is found for 8, with a pentacoordinate
calcium atom. In the distorted trigonal bipyramidal coordination
sphere, the bulky aryl fragment is positioned in the equatorial
plane and the atoms O1 and I1 occupy the axial positions
[Ca1-O1, 238.4(4) pm; Ca1-I1, 307.54(7) pm; O1-Ca1-I1,
160.6(1)°], in accordance with the VSEPR model. The bond
lengths of Ca1-C1 [251.7(4) pm] and Ca1-I1 are rather short
because of the low coordination number of calcium but lie in
the characteristic region for arylcalcium derivatives.34,16 In the
equatorial plane, the C1-Ca1-O2 angle is very wide as a result
of an additional agostic interaction between Ca1 and C15 of
The molecular structure and numbering scheme for 9 are
shown in Figure 6. In this complex, which has a hexacoordinate
calcium center, the THF molecules show a facial arrangement.
The benzoate anions in 9 show a µ,η1,η1-O,O′ coordination
mode with an average Ca-O distance of 227.2 pm, leading to
an eight-membered (Ca-O-C-O)2 dimetallacycle with an
orthogonal orientation of the m-terphenyl units. The inner eight-
membered (Ca-O-C-O)2 ring shows charge delocalization
within the carboxylate units. The Ca-Othf distances are sig-
nificantly larger than those to the carboxylate oxygens because
in the latter an additional electrostatic attraction strengthens the
interactions. The carboxylate moiety and the aryl fragment show
no π interaction [C1-C2, 151.6(6) pm]. Because of the larger
coordination number, the Ca1-I1 bond length of 312.84(9) pm
is larger than that in 8.
Conclusion
These investigations have clearly demonstrated the complexity
of redox reactions of halogenoarenes with activated calcium.
Whereas lithium and magnesium quantitatively insert into the
carbon-halogen bond, a similar arylcalcium halide is strongly
destabilized when p-aryl substituents are bound. In this case,
the strongly enhanced reactivity leads to fast ether degradation
reactions. In the presence of additional calcium powder, the ether
degradation is accompanied by a directed ortho metalation
(DOM) reaction of the triphenylphenyl group, leading to the
formation of a 3,5-diphenyl-1,1′-biphenyl-2,2′-diyl group (at-
tached as a bridging ligand between two calcium atoms) and a
1,3,5-triphenylbenzene radical anion, which crystallize as [(thf)3-
Ca(µ-C6H2-C6H4Ph2)(µ-O-CHdCH2)Ca(thf)3][C6H3Ph3] (4).
This complex can be isolated, redissolved in THF, and reduced
with another equivalent of activated calcium, yielding the inverse
calcium(I) sandwich complex [{(thf)3Ca}2(µ-C6H3-1,3,5-Ph3)]
(1). Solvent THF molecules as well as the 2,4,6-triphenylphenyl
anion (yielding the 1,1′-biphenyl-2,2′-diyl anion) act as proton
the tolyl group [Ca1-C15, 313.0(4) pm]. The ipso C (Ca-Cipso
)
shows a 13C NMR shift of 196.4 ppm, which is a typical low-
field-shifted resonance for arylcalcium halides.16,17d
The reaction of 8 with iodine regenerated the starting material
iodo-2,6-di(4-tolyl)benzene (6), showing that ether degradation
reactions were minor side reactions in this case. Substitution
of the iodide in 8 via a metathesis reaction with potassium
triethylborohydrideledtotheformationof[{2,6-(4-tol)2C6H3}Ca(µ-
H)BEt3] as a contact ion pair.37 However, when the heterocu-
(35) Wu, T.-C.; Xiong, H.; Rieke, R. D. J. Org. Chem. 1990, 55, 5045–
5051.
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12498 J. AM. CHEM. SOC. VOL. 132, NO. 35, 2010

Formation of an Inverse Ca(I) Sandwich Complex
A R T I C L E S
(1.00 g, 3.94 mmol) in THF (20 mL) at -20 °C. The mixture was
stirred for 12 h at room temperature and then extracted with
chloroform; the organic layer was separated, washed with Na2S2O3
and several times with brine, and dried over Na2SO4. Next, the
solvent was evaporated, and the resulting yellowish oil was treated
with ethanol (150 mL) under reflux conditions. After filtration,
storage at -20 °C afforded colorless needles of 3 (1.03 g, 2.38
mmol, 92%).
sources, which are mandatory for the conversion of the
triarylphenyl groups into 1,3,5-triphenylbenzene molecules. A
direct synthesis of 1 from calcium and 1,3,5-triphenylbenzene
was not successful, and at least catalytic amounts of the
corresponding bromoarene are needed to initiate the reaction.
This behavior contrasts with the reaction pattern for lithium and
magnesium compounds (which yield stable aryllithium and
arylmagnesium halides, respectively) and therefore allows the
synthesis of an organocalcium(I) complex. To the best of our
knowledge, no isolated and structurally characterized derivatives
of mono- or dimetalated 1,1′biphenyl-4,4′-diyl systems with
lighter s-block metals are known. A similar inverse sandwich
complex of magnesium(I) is unknown at present. The inverse
sandwich complex 1 is a strong reducing agent that is able to
metalate toluene with generation of the dibenzylcalcium complex
[(tmta)2Ca(CH2C6H5)2] (5). In contrast to p-phenyl-substituted
arylcalcium halides, 2,6-diaryl-substituted congeners are stable,
both in solution and in the solid state. The synthesis of the first
m-terphenylcalcium compound, [{2,6-(4-tol)2C6H3}Ca(thf)3I]
(8), and its derivatization as the corresponding benzoate dimer,
[{2,6-(4-tol)2C6H3CO2}Ca(thf)3I]2 (9), succeeded.
Physical Data for 3. Mp: 128 °C (EtOH). Anal. Calcd for
C24H17I (432.3 g mol-1): C, 66.68; H, 3.96; I, 29.36. Found: C,
66.45; H, 3.72; I, 29.21. 1H NMR (400.25 MHz, 25 °C, CDCl3): δ
7.32 (1H, t, 3JH-H ) 8.0 Hz, p′′-CH), 7.38 (2H, t, 3JH-H ) 7.4 Hz,
p′-CH), 7.50 (2H, t, 3JH-H ) 7.6 Hz, m′′-CH), 7.54 (4H, t, 3JH-H
)
7.6 Hz, m′-CH), 7.58 (2H, s, m-CH), 7.60 (4H, d, 3JH-H ) 7.2 Hz,
3
o′-CH), 7.70 (2H, d, JH-H ) 7.2 Hz, o′′-CH). 13C{1H} NMR
(100.65 MHz, 25 °C, CDCl3): δ 96.3 (1C, Cipso), 126.1 (2C, o′′-
CH), 126.4 (2C, m-CH), 127.1 (2C, p′-CH), 127.5 (1C, p′′-CH),
128.0 (4C, m′-CH), 128.9 (2C, m′′-CH), 129.9 (4C, o′-CH), 139.0
(1C, C′ip′so), 140.1 (1C, p-C), 141.5 (2C, Ci′pso), 146.9 (2C, o-CH).
ESI-MS (m/z [%]): 432 [78] (M), 306 [100] (M - I).
Synthesis of [(thf)3Ca(µ-C6H2-C6H4Ph2}(µ-O-CHdCH2)-
Ca(thf)3][C6H3Ph3] (4). Calcium (0.16 g, 3.99 mmol) was activated
and dried according to standard procedures and suspended in 25
mL of THF. A solution of tpbBr (1.00 g, 2.59 mmol) in 10 mL of
THF was added at -78 °C. Thereafter, the mixture was shaken for
4 h at -60 °C. Filtration and storage of the deep-blue-violet mother
liquor led to crystallization of a black, opalescent, and pyrophoric
solid. Several steps of recrystallization allowed the isolation of
black, opalescent, highly pyrophoric crystals of 4 (1.07 g, 0.91
mmol, 70%).
This study has demonstrated once again the limitations and
importance of the solvent used in organocalcium chemistry:
ether solvents often are easily cleaved under decomposition of
the organometallic derivative, and aromatic hydrocarbons can
be deprotonated, whereas aliphatic hydrocarbons are poor
solvents for saltlike organoalkaline-earth metal compounds with
strongly heteropolar metal-carbon bonds.
Physical Data for 4. Decomp.: 37 °C. Anal. Calcd for C75H88-
Ca2O7 (1181.65 g mol-1): Ca, 6.78. Found: Ca, 6.51. Susceptibility
Experimental Section
measurements (Gouy magnetic balance, 294 K): ꢁv ) 1.381 × 10-6
,
)
ꢁg ) 2.527 × 10-5 cm3 g-1, ꢁm ) 1.379 × 10-3 cm3 mol-1, µeff
Manipulation and handling of all compounds were performed
under strictly anaerobic conditions using standard Schlenk tech-
niques in an argon atmosphere. Solvents were dried thoroughly and
distilled under argon. 1,3,5-Triphenylbenzene was purchased from
Aldrich, and tpbBr was prepared from 1,3,5-triphenylbenzene and
bromine in tetrachloromethane.38,39 Calcium granules as purchased
from Aldrich were used for the activation process40 without further
purification. Rieke magnesium was prepared according to a literature
procedure,41 washed several times with hot acetone, and dried in
vacuum at 80 °C. The necessity to maintain a THF-saturated
atmosphere in order to prevent aging of the crystals and the extreme
sensitivity toward moisture and air, especially the highly pyrophoric
nature of some compounds, made the analytical characterization
challenging.
For the NMR assignments, atoms in the central ring are
unprimed, atoms in the o-phenyl groups are primed, and atoms in
the p-phenyl group are doubly primed.
Synthesis of 2,4,6-Ph3C6H2I (3) via [{2,4,6-Ph3C6H2}Ca(thf)3Br]
(2). A solution of tpbBr (1.00 g, 2.60 mmol) in THF (10 mL) was
cooled to -40 °C and added to a suspension of activated calcium
powder (0.104 g, 2.60 mmol) in THF (20 mL) at -78 °C. The
mixture was shaken at -60 °C for 30 min, yielding a clear yellowish
solution. Thereafter, the solution of 2 was quenched with iodine
1.62µB. EPR measurements (thf, r.t.): g ) 2.0024, hyperfine
structure pattern corresponding to a 1,3,5-triphenylbenzene radical.
Characterization of the Vinylate Species (after Addition of
3 Equiv of [D4]Methanol). 1H NMR (400.25 MHz, 25 °C,
[D8]THF): δ 4.09 (2H, m, O-CHdCH2), 6.32 (1H, m, O-
CHdCH2). 13C{1H} NMR (100.65 MHz, 25 °C, [D8]THF): δ 69.7
(1C, O-CHdCH2), 80.1 (1C, O-CHdCH2).
Synthesis of [(tmta)2Ca(CH2C6H5)2] (5). All volatiles of a
solution of [(thf)3Ca(µ-Ph3C6H3)Ca(thf)3] (10 mL, 2.41 mmol, 0.241
M) in THF were removed in vacuo at 4 °C. The black residue was
then dried (1 h) and extracted with toluene (8 mL). The extract
was filtered off and stirred at room temperature (r.t.) for 24 h, during
which time the color of the solution changed from dark-blue to
dark-violet. Addition of tmta (1 mL) and subsequent filtration led
to the formation of 5 (0.95 g, 1.98 mmol, 82%) as colorless prisms
at r.t. within 5 days.
Physical Data for 5. Decomp.: 58 °C. Anal. Calcd for
C26H44CaN6 (480.75 g mol-1): C, 64.96; H, 9.23; N, 17.47; Ca,
8.34. Found: C, 65.09; H, 9.41; N, 17.13; Ca, 8.50. 1H NMR (400.25
MHz, 25 °C, [D6]benzene): δ 1.35 (4H, s, CH2-Ph), 2.06 (18H, s,
CH3, tmta), 3.00 (12H, s(br), CH2, tmta), 6.92-7,23 (10H, m, Ph).
13C{1H} NMR (100.65 MHz, 25 °C, [D6]benzene): δ 30.2 (2C,
CH2-Ph), 40.4 (6C, CH3, tmta), 77.7 (6C, CH2, tmta), 106.2 (2C,
p-C), 120.2 (4C, o,o′-CH), 128.3 (4C, m,m′-CH), 157.8 (2C, Cipso).
IR (nujol, KBr) ν (cm-1): 2853 (vs), 2280 (s), 1888 (m), 1455 (s),
1377 (s), 1259 (m), 1234 (s), 1157 (s), 1115 (vs), 1048 (m), 1026
(m), 1003 (s), 914 (s), 860 (s), 811 (m), 727 (s), 693 (m).
Synthesis of 2,6-(4-tol)2C6H3I (6). The synthesis of 6 was
performed according to slightly modified literature procedures.42-44
Physical Data for 6. Mp: 140 °C (EtOH). Anal. Calcd for
C20H17I (384.26 g mol-1): C, 62.51; H, 4.46; I, 33.03. Found: C,
62.40; H, 4.28; I, 32.80. 1H NMR (400.25 MHz, 25 °C, CDCl3): δ
2.45 (6H, s, CH3), 7.22-7.43 (11H, m, aryl). 13C{1H} NMR (100.65
MHz, 25 °C, CDCl3): δ 21.3 (2C, CH3), 104.2 (1C, Cipso), 127.5
(2C, m-CH), 128.5 (5C, o′,p-CH), 129.3 (4C, m′-CH), 137.2 (2C,
(36) (a) Eshdat, L.; Ayalon, A.; Beust, R.; Shenhar, R.; Rabinovitz, M.
J. Am. Chem. Soc. 2000, 122, 12637–12645. (b) De Boer, E.; Klaassen,
A. A. K.; Mooij, J. J.; Nordik, J. H. Pure Appl. Chem. 1979, 51, 73–
83. (c) Lu¨hder, K. Z. Chem. 1969, 10, 387–388.
(37) Krieck, S.; Go¨rls, H.; Westerhausen, M. Inorg. Chem. Commun. 2010,
in press; DOI: 10.1016/j.inoche.2010.08.018.
(38) Haaland, A.; Rypdal, K.; Verne, H. P.; Scherer, W.; Thiel, W. R.
Angew. Chem. 1994, 106, 2515-2517; Angew. Chem., Int. Ed. 1994,
33, 2443-2445.
(39) Kohler, E. P.; Blanchard, L. W., Jr. J. Am. Chem. Soc. 1935, 57, 367–
371.
(40) Fischer, R.; Ga¨rtner, M.; Go¨rls, H.; Westerhausen, M. Angew. Chem.
2006, 118, 624-627; Angew. Chem., Int. Ed. 2006, 45, 609-612.
(41) Rieke, R. D.; Xiong, H. J. Org. Chem. 1991, 56, 3109–3118.
9
J. AM. CHEM. SOC. VOL. 132, NO. 35, 2010 12499

A R T I C L E S
Krieck et al.
Table 1. Crystal Data and Refinement Details for the X-ray Structure Determinations of Compounds 4, 5, 7, 8, and 9
4
5
7
8
9
formula
[C50H67Ca2O7][C24H18]·3.5C4H8O
C26H38CaN6
474.70
-140(2)
orthorhombic
Pca21
16.2058(3)
8.3923(2)
20.4013(7)
90.00
90.00
90.00
2774.66(13)
4
1.136
2.49
18124
4804
5985/0.0482
0.1346
0.0491
1.043
0.434/-0.290
none
C28H33IMgO2
552.75
C32H41CaIO3
640.63
-90(2)
monoclinic
Cc
10.3305(2)
39.9525(9)
8.7003(3)
90
121.825(2)
90
3051.03(14)
4
1.395
12.47
10805
5257
6359/0.0326
0.0771
0.0354
0.994
0.484/-0.590
none
745124
C66H82Ca2I2O10 ·2C4H8O
1513.48
-90(2)
monoclinic
P21/c
13.6756(6)
18.0856(4)
16.8440(6)
90
93.290(2)
90
4159.2(3)
2
1.209
9.3
29291
5600
9507/0.0687
0.1957
0.0611
fw (g mol-1
T (°C)
)
1418.94
-140(2)
monoclinic
P21/c
24.6771(5)
14.9476(4)
22.7395(5)
90.00
92.546(1)
90.00
8379.5(3)
4
1.125
1.91
57700
10570
19010/0.0895
0.2772
0.0847
-90(2)
crystal system
space group
a (Å)
b (Å)
c (Å)
R (deg)
ꢀ (deg)
γ (deg)
V (Å3)
triclinic
j
P1
7.9095(3)
8.8456(3)
37.1188(15)
91.238(2)
91.204(2)
90.096(2)
2595.80(17)
4
1.414
12.79
12092
7031
10098/0.0349
0.3744
0.1218
1.059
1.624/-1.850
none
Z
F (g cm-3
)
µ (mm-1
)
measured data
data with I > 2σ(I)
unique data/Rint
wR2 (all data, on F2)a
R1 [I > 2σ(I)]a
Sb
1.037
1.428/-0.395
none
1.025
0.882/-0.582
none
resid. density (e Å-3
absorption method
CCDC no.
)
771312
771313
motif
745125
a
2
2
2
Definitions of the R indices: R1 ) ∑||Fo| - |Fc||/∑|Fo|; wR2 ) {∑[w(Fo - Fc2)2]/∑[w(Fo )2]}1/2, in which w-1 ) σ2(Fo ) + (aP)2 + bP, where P )
b
[2Fc2 + max(Fo )]/3. S ) {∑[w(Fo - Fc2)2]/(No - Np)}1/2
.
2
2
p′-CH), 142.9 (2C, C′ipso), 148.0 (2C, o-C). ESI-MS (m/z [%]): 384
[65] (M), 258 [32] (Tol2Ph), 169 [100] (C13H13), 258 [32] (Tol2Ph).
Synthesis of [{2,6-(4-tol)2C6H3}Mg(thf)2I] (7). A solution of 6
(1.25 g, 3.25 mmol) in THF (15 mL) was added dropwise to a
suspension of Rieke magnesium (0.97 g, 4.00 mmol) in THF (20
mL). The mixture was heated for 2 h under reflux and then filtered,
after which storage of the mother liquor at +4 °C led to crys-
tallization of colorless needles. Separation, washing with n-pentane
(10 mL, cooled to -40 °C), and gentle drying in vacuum gave
1.51 g of 7 (2.73 mmol, 84%).
(100.65 MHz, 25 °C, [D8]THF): δ 21.0 (2C, CH3), 26.3 (6C, CH2,
thf), 68.1 (6C, CH2O, thf), 126.1 (2C, m-CH), 127.6 (4C, o′-CH),
129.7 (1C, p-CH), 130.1 (4C, m′-CH), 137.6 (2C, o-C), 139.2 (2C,
p′-C), 142.4 (2C, C′ipso), 196.4 (1C, Cipso). IR (Nujol, KBr) ν (cm-1):
2894 (vs), 2246 (m), 1913 (w), 1803 (w), 1735 (w), 1719 (w), 1648
(w), 1603 (m), 1566 (w), 1517 (m), 1457 (vs), 1377 (vs), 1307
(m), 1248 (m), 1172 (m), 1111 (m), 1074 (s), 1029 (s), 915 (m),
888 (m), 822 (s), 786 (vs), 754 (m), 721 (m), 696 (m), 669 (m).
Synthesis of [{2,6-(4-tol)2C6H3CO2}Ca(thf)3I]2 (9). A solution
of 8 (50 mL, 4.54 mmol) in THF was prepared at -60 °C in an
argon atmosphere. The argon atmosphere was exchanged for an
atmosphere of carbon dioxide. The pale-yellow solution was stored
at -78 °C, which led to the crystallization of colorless cubes within
4 days. Separation, washing with Et2O (5 mL, cooled to -78 °C),
and gentle drying in vacuum yielded 2.12 g of 9 (1.55 mmol, 68%).
Physical Data for 9. Decomp.: 48 °C. Anal. Calcd for
C66H82Ca2I2O10 (1369.32 g mol-1): C, 57.89; H, 6.04; Ca, 5.85; I,
18.54. Found: C, 57.13; H, 5.84; Ca, 5.92; I, 18.16. 1H NMR
(400.25 MHz, 25 °C, [D8]THF): δ 1.68 (24H, m, CH2, thf), 2.34
(12H, s, CH3), 3.58 (24H, m, CH2O, thf), 7.12 (12H, m, m′,m-
CH), 7.23 (2H, t, 3JH-H ) 7.6 Hz, p-CH), 7.47 (8H, d, 3JH-H ) 8.0
Hz, o′-CH). 13C{1H} NMR (100.65 MHz, 25 °C, [D8]THF): δ 21.1
(4C, CH3), 26.2 (12C, CH2, thf), 68.1 (12C, CH2O, thf), 126.6 (2C,
p-CH), 129.1 (8C, m′-CH), 129.3 (4C, m-CH), 130.0 (8C, o′-CH),
135.7 (2C, Cipso), 136.3 (4C, Ci′pso), 139.0 (4C, o-C), 140.9 (4C,
p′-C), 160.1 (2C, COO). ESI-MS (m/z [%]): 169 [100] (C13H13),
258 [25] (tol2Ph), 341 [72] (tol2PhCOOCa). IR (Nujol, KBr) ν
(cm-1): 2928 (vs), 2727 (m), 2371 (m), 1944 (w), 1846 (w), 1831
(w), 1794 (w), 1735 (w), 1647 (w), 1610 (s), 1560 (s), 1508 (m),
1457 (vs), 1377 (vs), 1305 (m), 1212 (m), 1170 (m), 1120 (m),
1072 (m), 1035 (s), 973 (m), 888 (m), 842 (m), 805 (m), 768 (m),
721 (s), 680 (s).
Physical Data for 7. Mp: 108 °C. Anal. Calcd for C28H33MgIO2
(552.78 g mol-1): C, 60.84; H, 6.02; Mg, 4.40; I, 22.96. Found: C,
61.20; H, 6.43; Mg, 4.21; I, 22.51. 1H NMR (400.25 MHz, 25 °C,
[D8]THF): δ 1.77 (8H, m, CH2, thf), 2.36 (6H, s, CH3), 3.62 (8H,
3
m, CH2O, thf), 7.22 (4H, d, JH-H ) 8.00 Hz, m′-CH), 7.44 (1H,
3
3
t, JH-H ) 8.1 Hz, p-CH), 7.53 (2H, dd, JH-H ) 7.6 Hz, m-CH),
7.72 (4H, d, 3JH-H ) 8.00 Hz, o′-CH). 13C{1H} NMR (100.65 MHz,
25 °C, [D8]THF): δ 20.9 (2C, CH3), 26.2 (4C, CH2, thf), 67.7 (4C,
CH2O, thf), 126.0 (2C, m-CH), 127.5 (4C, o′-CH), 129.7 (1C,
p-CH), 130.0 (4C, m′-CH), 137.6 (2C, o-C), 139.1 (2C, p′-C), 142.4
(2C, C′ipso), 164.7 (1C, Cipso). ESI-MS (m/z [%]): 169 [100] (C13H13),
258 [32] (Tol2Ph). IR (Nujol, KBr) ν (cm-1): 2862 (vs(br)), 2372
(m), 1915 (m), 1856 (w), 1795 (w), 1648 (w), 1604 (w), 1561 (w),
1537 (m), 1509 (s), 1548 (vs), 1631 (s), 1378 (s), 1347 (s), 1298
(m), 1251 (m), 1230 (m), 1211 (w), 1181 (s), 1111 (m), 1095 (m),
1074 (m), 1026 (vs), 950 (m), 917 (m), 882 (s), 832 (s), 786 (vs),
752 (w), 720 (m), 693 (m).
Synthesis of [{2,6-(4-tol)2C6H3}Ca(thf)3I] (8). A solution of 6
(1.50 g, 3.90 mmol) in THF (10 mL) was added to a suspension of
activated calcium (0.156 g, 3.90 mmol) in THF (25 mL) at -60
°C. The mixture was shaken for 6 h at -60 °C and then filtered at
-40 °C. The resulting orange solution was stored at -40 °C, which
led to the crystallization of pale-violet platelets. Separation, washing
with Et2O (8 mL, cooled to -78 °C), and gentle drying in vacuum
gave 1.89 g of 8 (2.95 mmol, 76% based on employed aryliodide).
Physical Data for 8. Mp: 213 °C. Anal. Calcd for C32H41CaIO3
(640.65 g mol-1): Ca, 6.21; I, 19.81. Found: Ca, 6.12; I, 19.69. 1H
NMR (400.25 MHz, 25 °C, [D8]THF): δ 1.64 (12H, m, CH2, thf),
Crystal Structure Determination. The intensity data for the
compounds were collected on a Nonius KappaCCD diffractometer
using graphite-monochromatized Mo KR radiation. The data were
corrected for Lorentz and polarization effects but not for absorption
effects.45,46
2.32 (6H, s, CH3), 3.55 (12H, m, CH2O, thf), 7.20 (4H, d, 3JH-H
)
(42) Saednya, A.; Hart, H. Synthesis 1996, 1455–1458.
(43) Csoergeh, I.; Gallardo, O.; Weber, E.; Pollex, R.; Doerpinghaus, N.
J. Inclusion Phenom. Mol. Recognit. Chem. 1995, 20, 253–266.
3
8.00 Hz, m′-CH), 7.40 (1H, t, JH-H ) 6.8 Hz, p-CH), 7.49 (2H,
3
dd, JH-H ) 6.8 Hz, m-CH), 7.53 (4H, d, o′-CH). 13C{1H} NMR
9
12500 J. AM. CHEM. SOC. VOL. 132, NO. 35, 2010

Formation of an Inverse Ca(I) Sandwich Complex
A R T I C L E S
The structures were solved by direct methods (SHELXS47) and
refined by full-matrix least-squares techniques against Fo2 (SHELXL-
9747). All of the hydrogen atoms were included at calculated
positions with fixed thermal parameters. All non-disordered non-
hydrogen atoms were refined anisotropically.47 After repeated
recrystallization, the crystals of 7 were extremely thin and of low
quality, resulting in a substandard data set; however, the structure
was sufficient to show connectivity and geometry despite the high
final R value. Here we are publishing only the conformation of the
molecule and the crystallographic data. Deposition of the data in
the Cambridge Crystallographic Data Centre is not intended because
of this. Crystallographic data as well as structure solution and
refinement details are summarized in Table 1. XP (Siemens
Analytical X-ray Instruments, Inc.) was used for structure
representations.
Acknowledgment. We thank the Deutsche Forschungsgemein-
schaft (DFG, Bonn, Germany) for generous financial support of
this research initiative.
Supporting Information Available: Crystallographic data in
CIF format. This material is available free of charge via the
Internet at http://pubs.acs.org. The crystallographic data (exclud-
ing structure factors) have been deposited with the Cambridge
Crystallographic Data Centre as supplementary publications
CCDC-771312 (4), CCDC-771313 (5), CCDC-745124 (8), and
CCDC-745125 (9). Copies of the data can be obtained free of
charge upon application to the CCDC, 12 Union Road, Cam-
bridge CB2 1EZ, U.K. (E-mail: deposit@ccdc.cam.ac.uk).
(44) Du, C.-J. F.; Hart, H.; Ng, K.-K. D. J. Org. Chem. 1986, 51, 3162–3165.
(45) COLLECT Data Collection Software; Nonius BV: Delft, The Neth-
erlands, 1998.
(46) Otwinowski, Z.; Minor, W. Methods Enzymol. 1997, 276, 307–326.
(47) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112–122.
JA105534W
9
J. AM. CHEM. SOC. VOL. 132, NO. 35, 2010 12501