Alkali metal-stabilized 1,3,5-triphenylbenzene monoanions: Synthesis and characterization of the lithium, sodium, and potassium complexes

Article abstract of DOI:10.1021/om1009632

Alkali metal stabilized monoanions of 1,3,5-triphenylbenzene can be obtained by various reactions starting from the alkali metal. Reduction of the Grignard compound [{(2,4,6-Ph₃-C₆H₂)Mg(dme)Br} ₂(μ-O,O′-dme)] (3

Full text of DOI:10.1021/om1009632

6790 Organometallics 2010, 29, 6790–6800
DOI: 10.1021/om1009632
Alkali Metal-Stabilized 1,3,5-Triphenylbenzene Monoanions: Synthesis
and Characterization of the Lithium, Sodium, and Potassium Complexes†
€
Sven Krieck, Helmar Gorls, and Matthias Westerhausen*
€
Institute of Inorganic and Analytical Chemistry, Friedrich-Schiller-Universitat Jena,
August-Bebel-Strasse 2, D-07743 Jena, Germany
Received October 5, 2010
Alkali metal stabilized monoanions of 1,3,5-triphenylbenzene can be obtained by various reactions
starting from the alkali metal. Reduction of the Grignard compound [{(2,4,6-Ph3-C6H2)Mg-
(dme)Br}2(μ-O,O0-dme)] (3) in DME (1,2-dimethoxyethane) with alkali metals yields solvent-
separated ions [(dme)3A]þ[C6H2-2,4,6-Ph3]- (A=Li, Na (4)) with a localized 2,4,6-triphenylphenyl
carbanion via metal-metal exchange. Contrarily to this observation, radical monoanions of 1,3,5-
triphenylbenzene are generated via transmetalation of the Grignard reagent [{2,4,6-Ph3-C6H2}Mg-
(thf)2Br] (2) with alkali metals in THF (including ether cleavage as the dominant reaction step).
Reduction of 2 with lithium yields the solvent-separated ion pair [(tmta)4Li7Br4O]þ[C6H3-2,4,6-
Ph3]- (6) (tmta=1,3,5-trimethyl-1,3,5-triazinane) with an oxygen-centered lithium cage as complex
cation. However, reactions of 2 with sodium as well as potassium lead to formation of halogen-free
contact ion pairs [(thf)3Na(μ-η6-C6H3-2,4,6-Ph3)] (7) and [(thf)4K(μ-η6-C6H3-Ph3)] (8), respectively.
Treatment of 1,3,5-triphenylbenzene in THF with lithium leads to fragmentation of the inner arene
core, whereas usage of the corresponding heavier alkali metals sodium and potassium, respectively,
also generates 7 or 8. Depending on the metal counterion, reduction of the propeller-shaped 1,3,5-
triphenylbenzene system leads to partial planarization in combination with bond elongations of the
inner arene. Derivatives 7 and 8 consist of a 3,30-di(phenyl)biphenyl moiety with phenyl substituents
twisted by 11ꢀ in the same direction (7) or contrary to each other (8). The same high symmetry was
found in the solid state as well as in solution, which was verified by X-ray structure determinations
and EPR measurements including simulation studies. In the solid state, the alkali metal stabilized
1,3,5-triphenylbenzene radical-monoanions (6, 7, and 8) are pyrophoric and strong black-opalescent,
whereas solutions show solvato- and thermochromic behavior.
Introduction
(ii) arenes (e.g., benzene adducts), and (iii) cyclooctatetrae-
nediides (Cot2-). Metallocenes are generated by deprotona-
tion via metalation under formation of an aromatic ring,
which is the reason for their stability, leading to an enormous
variety of metallocenes,1 varying from ionic alkali metal
derivatives to covalently bound metallocenes of late d- and
p-block metals. In general, the generation of metal π-complexes
of arenes and arene anions involves metalation (e.g., Cp-) or
electron transfer reactions (e.g., Cot2-) with strongly elec-
tropositive metals (such as alkali metals A or alkaline earth
metals Ae). These compounds can function as group transfer
reagents in a salt metathesis reaction in order to prepare π-
complexes of less electropositive metals (such as late transi-
tion metals and p-block metals).
Organometallic compounds can be divided into two major
classes, namely, derivatives with discrete metal-carbon
σ-bonds on one hand and those with multihapto coordina-
tion modes of π-systems to the metals on the other hand. The
latter class contains cyclic aromatic ligands with different
ring sizes, which are able to stabilize low-valent metal ions
and even metal(0) fragments, with the most important ex-
amples being (i) cyclopentadienides (Cp-, e.g. metallocenes),
† Dedicated to Professor Peter Kl€ufers on the occasion of his 60th birthday.
*To whom correspondence should be addressed. Fax: þ49 3641
948102. E-mail: m.we@uni-jena.de.
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The π-system can be extended not only by ring expansion
but also by linkage and annulation of aromatic rings. Alkali
metal π-complexes with several polycyclic unsaturated hy-
drocarbons were intensively investigated by Bock and co-
workers, and quite a few unusual molecular structures
were observed.2 In preparative chemistry these alkali
metal π-complexes are used as strong reducing reagents, as
catalysts in polymerization reactions, in the synthesis of
low-valent transition metal organometallics, or as redox
r
pubs.acs.org/Organometallics
Published on Web 12/02/2010
2010 American Chemical Society

Article
Organometallics, Vol. 29, No. 24, 2010 6791
Scheme 1. Conversion of the Grignard Reagent [{2,4,6-Ph3-C6H2}Mg(thf)2Br] (2) into Its dme Analogue 3 as Well as Solvent Dependency
of Their Reduction with the Lighter Alkali Metals Lithium and Sodium
mediators working in homogeneous phase with an exactly
defined stoichiometry in contrast to the solid metals them-
selves. Despite a wide range of applications, structurally
characterized compounds are rare due to their high reactivity
and sensitivity toward moisture and air. Recently Zhou et al.
have investigated the solid-state structure of potassium
naphthalenide [(thf)K2(C10H8)] (C10H8 = naphthalene).3
Whereas alkali metal π-complexes of anthracene (C14H10)
are soluble in common organic solvents, complexes of the
alkaline earth metals [(thf)nAe(anthracene)] (Ae n: Mg 3,
Ca 4, Sr 2, Ba 1)4 are less soluble and thermolabile. For this
reason dismutation reactions regenerate the original metals
as finely dispersed powders, so-called activated metals (M*)
according to Bogdanovic’s activation procedure.4,5 Complete-
ꢀ
ly different properties were achieved by reduction of anthra-
cene with the β-diketiminate-stabilized homodinuclear
magnesium(I) complex [(mesnacnac)Mg]2, yielding ther-
mally stable and soluble [{(mesnacnac)Mg}2(anthracene)],
very recently shown by Jones and co-workers.6
In the presence of Brønsted acids the metal π-complexes
undergo subsequent reactions that can be used in synthetic
chemistry, e.g., Birch-type reduction of organic substrates.7
With an excess of metal a cascade of several electron transfer
(reduction) and protonation steps occurs, as shown by the
synthesis of the stable organocalcium(I) inverse sandwich
[(thf)3Ca(μ-η6,η6-C6H3-1,3,5-Ph3)Ca(thf)3] (1) consisting of
a strictly planar bridging arene ligand between two (thf)3CaI
moieties that are located on opposite sides of a C3 axis.8,9 The
reaction sequence involves ether degradation of THF via
R-deprotonation reactions by the post-Grignard reagent aryl-
calcium halide (yielding 1,3,5-triphenylbenzene, tpb) and re-
duction finally yielding the 1,3,5-triphenylbenzene dianion.9
Mechanistic elucidations of the formation of 1 showed that
(2) (a) Bock, H.; Havlas, Z; Gharagozloo-Hubmann, K.; Holl, S.;
Sievert, M. Angew. Chem. 2003, 121, 4521–4525. Angew. Chem., Int. Ed.
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€
(d) Bock, H.; Nather, C.; Havlas, Z.; John, A.; Arad, C. Angew. Chem.
1994, 106, 931–934. Angew. Chem., Int. Ed. Engl. 1994, 33, 875-878.
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(e) Bock, H.; Ruppert, K.; Nather, C.; Havlas, Z.; Hermann, H.-F.;
Arad, C.; Gobel, I.; John, A.; Meuret, T.; Nick, S.; Rauschenbach, A.;
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ꢀ
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ꢀ
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(5) (a) Bogdanovic, B.; Janke, N.; Kinzelmann, H.-G.; Westeppe, U.
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Chem. Ber. 1988, 121, 33–37. (b) Bogdanovic, B. Angew. Chem. 1985, 97,
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2009, 43, 384–390.

6792 Organometallics, Vol. 29, No. 24, 2010
Krieck et al.
the key intermediate of the formation reaction consists of a
solvent-separated ion pair of the constitution [(thf)3Ca-
(μ-C6H2-C6H4-Ph2)(μ-O-CHdCH2)Ca(thf)3][C6H3-Ph3].10
Alkali metals (A) and alkaline earth metal monocations
(Aeþ, ns1, open-shell systems) are isoelectronic, whereas
alkali metal ions (Aþ) are isoelectronic to the divalent alka-
line earth metal ions (Ae2þ, ns0, closed-shell systems). Hence,
the replacement of Caþ in derivative 1 by K yields an
isoelectronic complex anion that should exhibit functional
relationships to 1 with a similar reducing power. Substitution
of Caþ by Kþ leads to the formation of a neutral complex
with a reduced reduction power because solely the 1,3,5-
triphenylbenzene anion acts as electron donor. Early inves-
tigations on the reduction of 1,3,5-triphenylbenzene with, for
example, potassium showed that in contrast to calcium this
alkali metal is able to reduce this arene once or twice, yielding
radical anions of tpb.
Figure 1. Molecular structure and numbering scheme of
[{(2,4,6-Ph3-C6H2)Mg(dme)Br}2(μ-O,O0-dme)] (3). The H atoms
are omitted for clarity reasons. The ellipsoids represent a prob-
˚
ability of 40%. Selected bond lengths [A] and angles [deg]:
Mg1-C1 2.178(4), Mg1-Br1 2.5973(14), Mg1-O1 2.209(3),
Mg1-O2 2.132(3), Mg1-O3 2.087(3); O2-Mg1-C1 139.77(13),
O3-Mg1-C1 121.50(14), O3-Mg1-O2 92.05(12), O1-Mg1-
Br1 165.68(11), C1-Mg1-Br1 105.68(11).
Results and Discussion
˚
˚
Mg-Br distances (2: 2.4683(9) A, 3: 2.597(1) A) andthe Mg-O
The reaction of bromo-2,4,6-triphenylbenzene (tpb Br)
with Rieke magnesium in THF yielded the expected
Grignard reagent [{2,4,6-Ph3-C6H2}Mg(thf)2Br] (2).8 A simi-
lar reaction at low temperatures with activated calcium
powder led to formation of the highly reactive heavy post-
Grignard reagent [{2,4,6-Ph3-C6H2}Ca(thf)3Br], which de-
grades promptly under formation of 1,3,5-triphenylbenzene
via ether cleavage of THF.10 Ether cleavage reactions are
common in the organometallic chemistry of strongly electro-
positive metals, particularly s-block metals, and in some
cases this type of degradation dominates the synthesis.11
Only in THF did an excess of calcium powder lead to for-
mation of the inverse sandwich complex [(thf)3Ca(μ-η6,
η6-C6H3-1,3,5-Ph3)Ca(thf)3] (1) via subsequent reduction
steps, yielding the planar 1,3,5-triphenylbenzene dianion.
In order to investigate the influence of the solvent on the
degradation reaction, the Grignard compounds [{2,4,6-Ph3-
C6H2}Mg(thf)2Br] (2)8 and [{(2,4,6-Ph3-C6H2)Mg(dme)Br}2-
(μ-O,O0-dme)] (3) were reacted with alkali metals, resulting
in a different reactivity pattern (Scheme 1). The latter com-
pound 3 was prepared from 2 via repeated crystallization
from 1,2-dimethoxyethane (DME).
˚
˚
bond lengths (average values, 2: 2.037 A, 3: 2.143 A) and is in
agreement with comparable magnesium m-terphenyl-based
compounds.12 As is common for aryl groups bound to very
electropositive metals, a rather narrow endocyclic C2-C1-
C6 angle of 114.0(4)ꢀ is observed. A rather similar structure
was observed for the m-terphenylcalcium iodide [{2,4,6-Ph3-
C6H2}Ca(thf)3I].10
Reaction of 3 with lithium and sodium sand, respectively,
in DME (Scheme 1) led to metal-metal exchange by pre-
cipitation of magnesium metal, yielding solvent-separated
ion pairs [(dme)3A]þ[C6H2-2,4,6-Ph3]- (A = Li, Na (4)).
Alkali metals reduce Mg(II) to Mg(0), in agreement with
ꢀ 4,5
the activation methods according to Rieke13 and Bogdanovic,
and the strong chelating effect of the bidentate Lewis base
DME leads to a separation of the generated organyl alkali
metal into solvent-separated ions. The presence of a localized
phenyl anion instead of a radical anion of 1,3,5-triphenyl-
benzene is clearly demonstrated by NMR experiments
(δ(13C) Ci 183.1 ppm) as well as by the absence of an EPR
resonance. Furthermore, 4 reacted with iodine in THF
solution nearly quantitatively to form iodo-2,4,6-triphenyl-
benzene (5).14 On one hand, these findings support the
assumption that 1,2-dimethoxyethane exhibits a larger kinetic
stability toward ether degradation reactions. On the other
hand binding interaction versus solvation effects are in direct
competition with each other. Metal-halogen exchange of
iodo-2,6-di(mesityl)benzene and n-BuLi in hexane yields
the dimeric, aryl-bridged lithium derivative [{Li(μ-C6H3-
Mes2)}2],15 and metathesis reactions give the corresponding
organyl sodium complex [{Na(μ-C6H3-Mes2)}2].16
The molecular structure and numbering scheme of
[{(2,4,6-Ph3-C6H2)Mg(dme)Br}2(μ-O,O0-dme)] (3) (Figure 1)
contains a pentacoordinate magnesium atom with a bridging
dme molecule. The environment of Mg consists of a distorted
trigonal bipyramid with the axial atoms being O1 and Br1
(O1-Mg1-Br1 165.7(1)ꢀ). Distortions are caused by the
small bite (O1 O2 distance) of the bidentate dme ligand
3 3 3
and the bulkiness of the 2,4,6-triphenylphenyl group. Due to
the larger coordination number, the Mg-C bond length of
Reactions in THF follow different pathways: Treatment
of one equivalent of lithium sand with a solution of [{2,4,6-
Ph3-C6H2}Mg(thf)2Br] (2) in THF led to precipitation of
magnesium metal and to a slightly green coloration of the solu-
tion during a short time. Addition of further lithium equiva-
lents led to a deeply colored (þ2 equiv: green; þ3 equiv: violet)
˚
2.178(4) A is slightly elongated compared to 2 (Mg-C
2.148(3) A). This effect is even more pronounced for the
8
˚
€
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Article
Organometallics, Vol. 29, No. 24, 2010 6793
and extremely air- and moisture-sensitive solutions. Surpris-
ingly, however, by addition of further excess of lithium the
metal dissolves completely during several hours, forming
“black” solutions. Biphenyl displays a similar behavior by
treatment with excess lithium metal.17 Hydrolysis of a reac-
tion mixture containing 2 and 10 equivalents of lithium in
THF after 24 h reaction time and analyses via HPLC and
GC/MS techniques underly the various reactions by several
fragments of the triphenylbenzene system, e.g., terphenyls,
biphenyls, phenyls, styrene, and oligomers (via polymeriza-
tion of fragments). Due to its Pearson hardness, lithium
preferably interacts with centers of large charge den-
sities (σ-bond skeleton), with an excess of reduction equiva-
lents leading to σ-bond cleavage reactions. In contrast,
treatment of bromo-2,4,6-triphenylbenzene with excess of
n-BuLi in ether yields only the expected product of metal-
halogen exchange reactions, namely, [{2,4,6-Ph3-C6H2}Li-
(Et2O)2].18
Reaction of [{2,4,6-Ph3-C6H2}Mg(thf)2Br] (2) with four
equivalents of lithium in THF at -60 ꢀC, interruption of the
reaction by filtration (separation of excess of reducing
agents) after 2 h, followed by several recrystallization steps
after addition of small amounts of 1,3,5-trimethyl-1,3,5-
triazinane (tmta) yield black, opalescent, pyrophoric, and
toluene-soluble [(tmta)4Li7Br4O]þ[C6H3-2,4,6-Ph3]- (6) as
an ion pair complex (Scheme 1) with a yield of 83% (based
on bromine content). The mother liquor contains a similar
pattern of arene fragments to that described above.
Figure 2. Representation of the proposed structural motif of
[(tmta)4Li7Br4O]þ[C6H3-2,4,6-Ph3]- (6), based on a rather poor
diffractometer data set.
The structural motif (represented in Figure 2) of 6 shows
once again the unwillingness of lithium π-coordination of the
soft arenes in the presence of harder Lewis bases, but prefers
aggregation and cage formation with various geometries.19
In general, cages are formed containing triangular lithium
faces, which build platonic bodies.20 In compound 6 the
cationic lithium cage [(tmta)4Li7Br4O]þ contains an inner
oxygen-centered lithium octahedron with nearly ideal octa-
hedral geometry due to anionic (O2-) compensation of the
electrostatic Li Li repulsion forces. Three lithium atoms
3 3 3
Figure 3. Simulated and measured EPR spectrum (X band) of a
solution of crystalline [(tmta)4Li7Br4O]þ[C6H3-2,4,6-Ph3]- (6)
in toluene at room temperature (g = 2.0023).
of the octahedron form a face-sharing tetrahedron with a
further lithium atom with bridging bromide μ3-Br on each
face, similar to the octahedral face in trans position to this Li4
tetrahedron. The coordination sphere is completed by three
tridentate ligands tmta (1,3,5-trimethyl-1,3,5-triazinane).
Contrary to derivative 4, in complex 6 the anion [C6H3-
2,4,6-Ph3]- is a π-radical monoanion of 1,3,5-triphenylben-
zene, characterized by EPR and susceptibility measurements
(μeff=1.31 μB). The recorded EPR spectrum of a solution of
crystalline 6 in toluene and a simulated spectrum are compared
in Figure 3. In accordance with the molecular structure of the
1,3,5-triphenylbenzene monoanion in 6 (Figure 2) the simu-
lation suggests two chemically equivalent phenyl substitu-
ents as in the case of a planar m-terphenyl system with a
twisting phenyl group in the 5 position. The presence of the
1,3,5-triphenylbenzene anion and the oxygen-centered lith-
ium cage indicates ether cleavage reactions (Wittig rearran-
gement, particularly in the case of lithium organyls)11e in a
stoichiometric ratio during the reaction of lithium metal with
organyl magnesium [{2,4,6-Ph3-C6H2}Mg(thf)2Br] (2).
Completely different reactivity was observed for sodium
and potassium, because this system remains stable also after
addition of an excess of these softer metals in comparison to
harder lithium. After reaction times of several days, the ex-
cess reducing agent was removed and no kind of ligand
fragmentation was found. In THF, ether cleavage was the
dominant reaction by treatment of 2 with the corresponding
alkali metal, and the time-delayed formation of an initially
deep green and finally deep violet solution in combination
with magnesium metal precipitation was observed. Extrac-
tion with mixtures of toluene/THF and cooling of these solu-
tions led to the crystallization of halide-free [(thf)3Na-
(μ-η6-C6H3-2,4,6-Ph3)] (7) and [(thf)4K(μ-η6-C6H3-Ph3)] (8),
€
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Jain, D. C.; Raghavachari, K. In Lithium Chemistry: A Theoretical and
Experimental Overview; Sapse, A.-M., von Schleyer, P., Eds.; Wiley: New
York, 1995. (e) Pauer, F.; Power, P. P. In Lithium Chemistry: A Theoretical
and Experimental Overview; Sapse, A.-M., von Schleyer, P., Eds.; Wiley:
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6794 Organometallics, Vol. 29, No. 24, 2010
Scheme 2. Reactivity Pattern of Sodium- and Potassium-Stabilized 1,3,5-Triphenylbenzene Monoanions
Krieck et al.
respectively, which are extremely sensitive toward moisture
and air as well as pyrophoric in the solid state. These com-
pounds were also accessible via the direct reduction of 1,3,5-
triphenylbenzene with the corresponding metal in THF
(Scheme 2). Treatment of bromo-2,4,6-triphenylbenzene
with potassium sand in THF yields 8, also. These findings
underlie the instability of the in situ generated alkali metal
organyls (via transmetalation reaction) in the presence of
further reducing equivalents. Ether degradation yields the
1,3,5-triphenylbenzene system. An excess of sodium or po-
tassium gave deep violet solutions of the respective dinuclear
complexes {A2(C6H3-1,3,5-Ph3)} (A=Na, K), which showed
a disproportionation equilibrium with 7 and 8, respectively,
and the elemental alkali metals. Heating of a deep violet
THF solution of 1,3,5-triphenylbenzene containing two
equivalents of sodium metal led to a color change to green
under metal separation, whereas cooling completely dis-
solved the metal in combination with re-formation of the
initial violet color. Addition of 1,3,5-triphenylbenzene to a
solution of the dianion led to a green solution of the
corresponding singly reduced arene system. This behavior
[(thf)3Ca(μ-η6,η6-C6H3-1,3,5-Ph3)Ca(thf)3] (1). Susceptibil-
ity measurements gave 1.62 μB for 7 and 1.73 μB for 8,
respectively, and represents a singly reduced triphenylben-
zene anion with one unpaired electron, whereas in 1 (2.33 μB)
the dianion was found to correlate to a triplet ground state
(S = 1).8 Aromatic hydrocarbons containing a 3- or 6-fold
axis of rotation (e.g., benzene, triphenylene, 1,3,5-triphenyl-
benzene) possess 2-fold degenerate highest bonding and
lowest antibonding π-orbitals, resulting in a triplet ground
state after reduction.23 1,3,5-Triphenylbenzene can be re-
duced reversibly only once or twice according to previous
investigations.24 Addition of a larger amount of cold
n-pentane to a solution of 7 in THF led to discoloration and
separation of a black, pyrophoric solid with variable com-
positions containing traces of elemental sodium (1.83-2.06
μB, mixture of several compounds).
The molecular structure of 1 showed a strictly planar 1,3,5-
triphenylbenzene anion (D3h symmetry), whereas the neutral
tpb molecule exhibits a propeller-like arrangement of the
phenyl substituents (C3 symmetry) with a twisting angle of
38ꢀ.25 The C-C distances of the neutral arene show an
was already noticed earlier by Luhder,21 and this fact is also
average value of 1.387 A, and the inter-ring distance lies at
˚
€
˚
described for multiply reduced perphenyl-substituted
benzenes.22 The colors are strongly influenced by the con-
centration, the temperature (thermochromism), and the
solvents (solvatochromism). Reduction of 1,2,4,5-tetraphe-
nylbenzene with lithium and sodium, respectively, displays
similar phenomena.22 Due to the extremely large extinction
coefficients and the above-mentioned equilibrium, the color
of the solution is not an unequivocal indicator for the
presence of singly or doubly reduced systems. Interpretation
and additional studies of color effects were impeded by the
extraordinarily large extinction coefficients in combination
with the high sensitivity of 7 and 8, respectively, in solution.
Bock specified the knowledge of these systems as “tip of the
iceberg of equilibria networks in solution”.2e
1.487 A, which is a characteristic value for a single bond
between two sp2-hybridized C atoms. In 1 the average C-C
bond (1.464 A) within the inner arene ring is elongated, and
˚
˚
between the rings a smaller value of 1.435 A is observed in
accordance with an effective charge delocalization. From the
equilibrium of sodium with 1,3,5-triphenylbenzene crystals
of black [(thf)3Na(μ-η6-C6H3-2,4,6-Ph3)] (7) were obtained.
The molecule of 7 contains a mirror plane, and its structure is
shown in Figure 4. The sodium atom is located in a distorted
tetrahedral environment of four ligands with the arene
6
˚
binding in a η -fashion (average Na-O 2.292 A, Na-C
2.887 A). In this compound the triphenylbenzene fragment
is nearly planar (the biphenyl unit is strictly planar and the
˚
All attempts to isolate dinuclear alkali metal complexes of
1,3,5-triphenylbenzene failed, whereas calcium stabilizes the
dianion in the case of the inverse sandwich Ca(I) complex
(23) (a) Li, S.; Ma, J.; Jiang, Y. J. Phys Chem. A 1997, 101, 5587–5592.
(b) Jesse, R. E.; Biloen, P.; Prins, R.; van Voorst, J. D. W.; Hoijtink, G. J. Mol.
Phys. 1963, 6, 633–635.
(24) (a) Sommerdijk, J. L.; van Broekhoven, J. A. M.; van Willigen,
H.; de Boer, E. J. Chem. Phys. 1969, 51, 2006–2009. (b) Bates, R. B.; Hess,
B. A., Jr.; Ogle, C. A.; Schaad, L. J. J. Am. Chem. Soc. 1981, 103, 5052–
5058.
€
(21) Luhder, K. Z. Chem. 1969, 9, 459–460.
(22) Eshdat, L.; Ayalon, A.; Beust, R.; Shenhar, R.; Rabinovitz, A.
J. Am. Chem. Soc. 2000, 122, 12637–12645.
(25) Lin, Y. C.; Williams, D. E. Acta Crystallogr. 1975, B31, 318–320.

Article
Organometallics, Vol. 29, No. 24, 2010 6795
Figure 5. Molecular structure and numbering scheme of
[(thf)4K(μ-η6-C6H3-2,4,6-Ph3)] (8). The H atoms are neglected
and the η6-K-benzene interactions are drawn as a stick to the
centroid of the inner arene for clarity reasons. The ellipsoids
Figure 4. Molecular structure and numbering scheme of [(thf)3-
Na(μ-η6-C6H3-2,4,6-Ph3)] (7). The H atoms are neglected and
the η6-Na-benzene interactions are drawn as a stick to the cen-
troid of the inner arene for clarity reasons. The ellipsoids re-
˚
represent a probability of 40%. Selected bond lengths [A] and
angles [deg]: K-O1 2.7347(19), K-O2 2.728(2), K-O3
2.7313(19), K-O4 2.712(2), K-C1 3.243(3), K-C2 3.305(2),
K-C3 3.295(3), K-C4 3.251(2), K-C5 3.209(3), K-C6
3.196(3), C1-C2 1.430(3), C2-C3 1.429(3), C3-C4 1.420(3),
C4-C5 1.449(3), C5-C6 1.448(3); O1-K-O2 91.59(6), O1-
K-O3 143.14(7).
˚
present a probability of 40%. Selected bond lengths [A] and
angles [deg]: Na-O1 2.2908(14), Na-O2 2.293(2), Na-C1
2.864(3), Na-C2 2.893(2), Na-C3 2.908(2), Na-C4 2.857(3),
C1-C2 1.416(2), C2-C3 1.447(2), C3-C4 1.435(2); O1-
Na-O1A 108.24(8), O1-Na-O2 103.02(6).
phenyl groups in the 3/30 positions (at C2/C2A, see Figure 4)
are twisted by the same angle of 11ꢀ of the plane with a mirror
plane orthogonal to the 4,40-plane of biphenyl) with average
˚
˚
C-C bonds of 1.433 A (arene) and 1.465 A (ring-ring
bond), adopting rather similar values.
The molecular structure of [(thf)4K(μ-η6-C6H3-Ph3)] (8) is
represented in Figure 5. The potassium atom occupies the
center of a distorted square pyramid binding to four THF
molecules (average K-O 2.727 A) and in a η -fashion
(average K-C 3.250 A) to the nearly planar arene. The
6
˚
˚
1,3,5-triphenylbenzene monoanion consists of a planar bi-
phenyl moiety with two twisted phenyl groups in 3/30 posi-
tion (at C2 and C6, Figure 5) against each other (twisting
angle 11ꢀ). The C-C bonds in the central ring are elongated
˚
(average C-C 1.433 A), as also observed for 7.
An important issue in the context of the alkali metal-
arene adducts is given by the comparatively high symmetry
of the reduced 1,3,5-triphenylbenzene ligand resulting from
crystal-packing effects in the solid state or mainly by ex-
tensive metal π-arene interactions. The EPR spectrum of a
solution of crystalline [(thf)3Na(μ-η6-C6H3-2,4,6-Ph3)] (7) in
THF (Figure 6) is well resolved and shows a highly sym-
metric resonance, typical for organic π-radicals with the
g value of 2.0023 (g value of the “free” electron), in accor-
dance to 1 and earlier studies,24 of organic compounds with
delocalized electrons. Furthermore, the EPR spectra are
nearly independent of the kind of solvent (THF, toluene),
as well as of temperature effects; raising the temperature
causes a slight decrease in spectral intensity without mark-
edly altering the hyperfine structure. For comparison rea-
sons, simulated spectra of a strictly planar 1,3,5-triphenyl-
benzene system with three principal mirror planes (D3h: four
different types of protons in the ratio 3:3:6:6) and 3,30-
di(phenyl)biphenyl with a lower symmetry of a vertical
mirror plane (Cs: 8 or 10 different types of protons in the
Figure 6. Simulated and measured EPR spectra (X band) of a
solution of crystalline [(thf)3Na(μ-η6-C6H3-2,4,6-Ph3)] (7) in
THF at room temperature (g = 2.0023).
ratio 1:1:2:2:2:2:4:4 or 1:1:2:2:2:2:2:2:2:2, depending on the
rotational position of the outer aryl moieties) or a bisecting C2
axis are included in Scheme 3. The resulting Cs/C2 symmetry of
the solid-state structure of 7 was verified by EPR experiments in
solution. Solutions of crystalline [(thf)4K(μ-η6-C6H3-Ph3)] (8)

6796 Organometallics, Vol. 29, No. 24, 2010
Krieck et al.
Scheme 3. Different Kinds of Protons Depending on the Symmetry of the 1,3,5-Triphenylbenzene Monoanion
Scheme 4. Observed Equilibrium of Mono- and Dianionic 1,3,5-
Triphenylbenzene Sodium Adducts in THF Solution
in THF also display a complicated EPR spectrum caused by
decreased symmetry.
As a consequence of several disproportionation equilibria,
attempts to isolate solid (crystalline) di(sodium) adducts of
1,3,5-triphenylbenzene failed (Scheme 4). Figure 7 represents
an EPR spectrum at low temperatures received by measuring
a frozen solution of crystalline [(thf)3Na(μ-η6-C6H3-2,4,6-
Ph3)] (7) in THF in a tube containing a sodium mirror. The
EPR spectrum is well resolved and almost identical to the
spectrum of the dianionic arene in the calcium(I) complex
[(thf)3Ca(μ-η6,η6-C6H3-1,3,5-Ph3)Ca(thf)3] (1).8 Simulation
experiments verified this assumption, finding four different
types of protons in the ratio 3:3:6:6, verifying the strictly
planar structure of the 1,3,5-triphenylbenzene unit (D3h
symmetry), whereby the coupling constants are multiples
of each other. Two coupling constants are found [n=2 ꢀ 3H
(A=0.85 G), 2 ꢀ 6H (A=1.70 G)], identical to 1. Hence, a
2-fold reduction of the arene leads to increased symmetry as
a consequence of complete planarization. These findings
suggest for the di(sodium) adduct of 1,3,5-triphenylbenzene
a similar structural motif to that in 1 with bifacial coordina-
tion of the metal ions on opposite sides of the arene plane
(Scheme 4), in contrast to the di(lithio) hexakis(trimethyl-
silyl)benzenediide [(thf)2Li]2[C6(SiMe3)6], showing syn-facial
location of the lithium atoms of the benzene dianion with a
boat conformation.26
Figure 7. Simulated and experimental EPR spectra (X band) at
77 K of a frozen solution of crystalline [(thf)3Na(μ-η6-C6H3-
2,4,6-Ph3)] (7) in THF measured in a tube containing a sodium
mirror (g = 2.0023).
by repulsion between the ortho hydrogen atoms and the
H atoms of the inner arene ring) has to be overcompensated
by an effective delocalization of the anionic charge.27 The
limitation of rotation of the phenyl substituents around the
C-C σ-bonds rises with the degree of reduction, suggesting
an increase of the π-bond order by extended charge deloca-
lization, whereas the C-C bonds of the inner arene ring show
values nearly characteristic for single bonds between sp2-
hybridized carbon atoms.22 The stabilization of the carba-
nion is additionally influenced by the association between
the metal counterion and the 1,3,5-triphenylbenzene radical
anion.28 In general, increasing charge delocalization within
the carbanion weakens the electrostatic anion-cation
interactions.29 Investigations of alkali metal trityl adducts
(trityl=triphenylmethyl anion, Ph3C-) gave similar results;
due to multihapto coordination modes, heavier alkali metals
are able to increasingly planarize the ligand.29
The reduction of the propeller-shaped 1,3,5-triphenylbenzene
ligand leads to enhancement of the symmetry by complete
planarization (dianion, 1) or partial planarization (monoanion,
6, 7, and 8). During planarization considerable strain (caused
As mentioned above, the symmetry of the arene system
influences its bonding situation (Table 1). In neutral,
€
(26) (a) Ebata, K.; Setaka, W.; Inoue, T.; Kabuto, C.; Kira, M.;
Sakurai, H. J. Am. Chem. Soc. 1998, 120, 1335–1336. (b) Sygula, A.;
Rabideau, P. W. J. Am. Chem. Soc. 1991, 113, 7797–7799. (c) Sekiguchi, A.;
Ebata, K.; Kabuto, C.; Sakurai, H. J. Am. Chem. Soc. 1991, 113, 1464–1465.
(d) Sakurai, H.; Ebata, K.; Kabuto, C.; Sekiguchi, A. J. Am. Chem. Soc.
1990, 112, 1799–1803.
(27) Huber, W.; May, A.; Mullen, K. Chem. Ber. 1981, 114, 1318–
1336.
(28) Gerson, F.; Kowert, B.; Peake, B. M. J. Am. Chem. Soc. 1974, 96,
118–120.
ꢀ
(29) Lambert, C.; von Rague Schleyer, P. Angew. Chem. 1994, 106,
1187–1199. Angew. Chem., Int. Ed. Engl. 1994, 33, 1129-1140.

Article
Table 1. Comparison of Selected Bond Lengths [A] of 1, 7, and 8
Organometallics, Vol. 29, No. 24, 2010 6797
˚
Table 2. Comparison of Calculated versus Experimental Struc-
tural Data of Sodium and Potassium Arene Adducts (average
parameter
1
7
8
˚
values in A)
C1-C2
C2-C3
C3-C4
C4-C5
C5-C6
C6-C1
av C-C
av Ci-Co
M-C1
M-C2
M-C3
M-C4
M-C5
M-C6
av M-C
1.478(7)
1.449(7)
1.416(2)
1.447(2)
1.435(2)
1.430(3)
1.429(3)
1.420(3)
1.449(3)
1.448(3)
1.424(3)
1.433
system
method
parameter
Na
K
b
M(η3-C3H3)37 MP2/6-31G** M-C
M(η6-C6H6)38 MP2/6-31G** M-C
2.425-2.491 2.791-2.811
2.804
b
3.217
2.894
3.137
2.805
2.870
3.250
2.917
M-Ccentr 2.426
M(η6-C6H6)39 MP2/6-311G** M-C
2.803
M-Ccentr 2.425
M-Ccentr
1.464
1.435
1.433
1.464
2.864(3)
2.893(2)
2.908(2)
2.857(3)
M(ηn-CPh3)35
7, 8
1.463
2.598(3)
2.586(3)
3.243(3)
3.305(2)
3.295(3)
3.251(2)
3.209(3)
3.196(3)
3.250
M-C
M-Ccentr 2.501
2.881
b
carbon contacts in [(thf)3Na(μ-η6-C6H3-2,4,6-Ph3)] (7) are
comparable to those in [(Et2O)2Na(μ2-η6-Ph2CdCPh2)]¥ (av
30
˚
˚
Na-C 2.70-2.82 A, Na-O 2.32 A). Neutral benzene
ligation leads to slight elongation, e.g., in the weak coordi-
nating, icosahedral carboranate [CB11Me12]- stabilized di-
2.592
2.139
2.405(3)
2.836
2.477
2.2908(14)
2.293(2)
a
M-Ccentr
2.917
M-L
2.728(2)
2.7347(19)
2.7313(19)
2.712(2)
34
˚
(benzene)sodium(I) sandwich (Na-Ccentr 2.69 A). The
(thf)4Kþ fragments in 8 bind via a η6-coordination mode
˚
to the arene radical-monoanion (av K-C 3.250 A, K-Ccentr
a Arithmetical averaged M-C distance. b Presence of mirror planes.
˚
˚
2.917 A, K-O 2.727 A). Comparable data are found for
trityl potassium compounds [(thf)(pmdta)K(η6-CPh3)] (av
propeller-shaped 1,3,5-triphenylbenzene the C-C equilibrium
distance in the central benzene is determined to be 1.387 A.
˚
Monoreduction led to elongation of about 0.046 A to a value
˚
of 1.433 A (6, 7, and 8), and 2-fold reduction led to further
˚
elongation by approximately 0.031 A (1, in sum 0.077 A).
Due to charge delocalization, the exocyclic phenyl substitu-
ents are shortened (6-8 0.027 A, 1 0.052 A). These trends are
independent of the interactions via contact-ion- (1, 7, 8) or
solvent-separated ion couples (6).
˚
˚
˚
25
˚
K-C 3.204 A, K-Ccentr 2.781 A, K-O 2.721 A; pmdta=N,
N,N0,N00,N00-pentamethyldiethylenetriamine),35 for the re-
duction product of 1,1,4,4-tetraphenylbutadiene (C28H22)
withpotassiumforming[{(dme)K}2(μ-η6-C28H22)] (K-C3.02-
˚
3.12 A),2e and for benzyl potassium derivatives [(pmdta)K(μ-
˚
η3-CH2Ph) (CH3Ph)]¥ (K-C 3.171(2)-3.297(2) A) and
˚
˚
˚
3
6
˚
[(pmdta)K(μ-η -CH2Ph)]¥ (K-C 3.150(2)-3.288(2) A).
In Table 2 a comparison of calculated and experimentally
obtained data is given for selected complexes. The differ-
ences of the Na-C and K-C distances reflect the sodium
and potassium radii. In general, the calculated distances are
slightly smaller than those found in single-crystal structure
determinations. Furthermore, the nature and diversity of
interactions (such as electrostatic, dipole-dipole, quadru-
pole, polarization, solvation effects) proved to be challeng-
ing for theoretical methods.8,40
Similar properties are observed for the reduction of tetra-
phenylethylene (Ph2CdCPh2) with sodium in diethyl ether,
yielding polymeric [(Et2O)2Na(μ2-η6-Ph2CdCPh2)]¥ with an
elongated olefinic double bond (1.49 A).2a,30 Reaction of 1,2-
˚
diphenylbenzene (C6H4-1,2-Ph2) with potassium in diglyme
[diglyme=di(2-methoxyethyl) ether] generates the arene dia-
nion [{(diglyme)K}2(C6H4-1,2-Ph2)] with elongation of the
˚
C-C bonds in the central arene (0.04-0.11 A) and short-
ening of the exocyclic C-C bonds to the phenyl substituents
(0.07 A). However, in sterically more encumbered arenes
30
˚
Conclusions
these effects are much more pronounced: Reduction of 1,2,3-
triphenylbenzene (C6H3-1,2,3-Ph3) with sodium in diglyme
yields solvent-separated [(diglyme)Na)]þ[(diglyme)Na-μ-
(η6-C6H3-1,2,3-Ph3)]- with both elongated C-C bonds in the
central arene and a shortening of the exocyclic C-C bonds to
The reactivity of the Grignard complexes [{2,4,6-Ph3-
C6H2}Mg(thf)2Br] (2) and [{(2,4,6-Ph3-C6H2)Mg(dme)Br}2-
(μ-O,O0-dme)] (3) toward alkali metals strongly depends on
the solvent. On one hand, in DME a metal-metal exchange is
observed leading to aryl metal derivatives [(dme)3A]þ[C6H2-
2,4,6-Ph3]- (A=Li, Na(4)), which are solvent-separated due to
the effective chelating behavior toward the alkali metal cations.
On the other hand, a transmetalation reaction in THF is ac-
companied by prominent ether attack and arene radical mono-
anions are formed. Hard lithium ions prefer harder coordina-
tion environments, yielding an oxygen-centered lithium cage
in solvent-separated [(tmta)4Li7Br4O]þ[C6H3-2,4,6-Ph3]- (6).
The heavier cations of sodium and potassium are markedly
softer Lewis acids, resulting in contact ions pairs with the soft
arene anion in [(thf)3Na(μ-η6-C6H3-2,4,6-Ph3)] (7) and [(thf)4-
K(μ-η6-C6H3-Ph3)] (8), respectively.
30
˚
the phenyl substituents of approximately 0.10 A.
The elongation of the C-C equilibrium distance in the
central arene core could not be verified by theoretical
investigations regarding metal π-arene interactions with
alkali metal ions,31 but with alkaline earth metal ions.32
˚
The cation radius of potassium (1.49 A) is at least 27%
˚
larger than that of sodium (1.17 A), whereas the metal
centroid distance increases by only about 17% (Na-Ccentr
2.501 A, K-Ccent 2.917 A).33 The average multihapto metal
˚
˚
(30) Bock, H.; Havlas, Z; Gharagozloo-Hubmann, K.; Holl, S.;
Sievert, M. Angew. Chem. 2003, 121, 4521–4525. Angew. Chem., Int.
Ed. 2003, 42, 4385-4389.
(31) Amicangelo, J. C.; Armentrout, P. B. J. Am. Chem. Soc. 2000,
104, 11420–11432.
(32) Tan, X. J.; Zhu, W. L.; Cui, M.; Luo, X. M.; Gu, J. D.; Silman, I.;
Sussman, J. L.; Jiang, H. L.; Ji, R. Y.; Chen, K. X. Chem. Phys. Lett.
2001, 349, 113–122.
(33) Massey, A. G. Main Group Chemistry; John Wiley & Sons, Ltd:
New York, 2000.
(34) King, B. T.; Noll, B. C.; Michl, J. Collect. Czech. Chem. Com-
mun. 1999, 64, 1001–1012.
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(35) Hoffmann, D.; Bauer, W.; von Rague Schleyer, P.; Pieper, U.;
Stalke, D. Organometallics 1993, 12, 1193–1200.
(36) Hoffmann, D.; Bauer, W.; Hampel, F.; van Eikema Hommes,
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N. J. R.; von Rague Schleyer, P.; Otto, P.; Pieper, U.; Stalke, D.; Wright,
D. S.; Snaith, R. J. Am. Chem. Soc. 1994, 116, 528–536.

6798 Organometallics, Vol. 29, No. 24, 2010
Krieck et al.
The colors of the solutions of these anions clearly show the
degree of reduction, solutions of monoanions being green
and of dianions being deep violet, whereas dependencies on
the metal, on the temperature (thermochromism), and on the
solvent (solvatochromism) were also observed. The negative
charges lead to an alignment of the inner arene ring with
bond elongations and increasing planarity of the arene
moiety. It was shown that reduction of the phenyl-substituted
arene restricted the free rotation of the phenyl substituents
around the C-C σ-bond both in the solid state and in solution.
Phenyl as well as silyl substituents26 are able to stabilize the
anionic state of the benzene core.
The peculiar redox properties of 1,3,5-triphenylbenzene
lead to an equilibrium of {A2(C6H3-1,3,5-Ph3)} with {A-
(C6H3-1,3,5-Ph3)} þ A, with the monoanion being favored in
the crystallization process. The corresponding dianion in the
case of the di(sodium) 1,3,5-triphenylbenzene adduct was
verified via EPR measurements, being an isostructural frag-
ment of the inverse organocalcium(I) sandwich [(thf)3Ca(μ-
η6,η6-C6H3-1,3,5-Ph3)Ca(thf)3] (1). The first ionization en-
ergies of Na (IENa=5.14 eV)41 and K (IEK=4.34 eV)41 are
comparable to the first IE of calcium (IECa1=6.11 eV),41 but
the second IECa2 of 11.87 eV41 is significantly larger. This
fact and the possibility of calcium to employ d-type orbitals42
with the arene π-system8 seem to stabilize the unusual inverse
sandwich complex [(thf)3Ca(μ-η6,η6-C6H3-1,3,5-Ph3)Ca-
(thf)3] (1).
For the NMR assignments, atoms in the central ring are
unprimed, atoms in the ortho-phenyl groups are primed, and
atoms in the para-phenyl group are doubly primed. EPR spectra
were measured on a Bruker ESP 300 E X-band spectrometer
with a modulation amplitude of 0.104 G. EPR spectrum simula-
tions were carried out with the Easyspin software package.46
Synthesis of [{(2,4,6-Ph3-C6H2)Mg(dme)Br}2(μ-O,O0-dme)]
(3). Solid [{2,4,6-Ph3-C6H2}Mg(thf)2Br] (2) (1.11 g, 2.00 mmol)
was dissolved in DME (20 mL) at 80 ꢀC and stirred for
additional 2 h. The resulting solution was filtered off, and storage
at -20 ꢀC led to crystallization of 3 as large colorless cubes.
Separation and gently drying under vacuum gave 0.96 g (0.9 mmol,
90%) of 2.
Physical Data of 3. Melting point: 168 ꢀC. Anal. Calcd for
C60H64Br2Mg2O6 (1089.56 g mol-1): Mg, 4.39 Found: Mg, 4.21.
1H NMR (400.25 MHz, 25 ꢀC, [D8]THF): δ 3.27 (18H, s, CH3,
3
dme), 3.43 (12H, s, CH2, dme), 7.27 (6H, m, JH-H =7.6 Hz,
3
p0/p00-CH), 7.37 (4H, t, JH-H = 7.6 Hz, m00-CH), 7.43 (8H, t,
3JH-H = 8.0 Hz, m0-CH), 7.60 (4H, s, m-CH), 7.76 (8H, d,
3JH-H=8.0 Hz, o0-CH), 7.91 (4H, d, 3JH-H=7.6 Hz, o00-CH).
13C{1H} NMR (100.65 MHz, 25 ꢀC, [D8]THF): δ 58.8 (6C,
CH3, dme), 72.7 (8C, CH2, dme), 124.4 (4C, m-CH), 125.5 (4C,
o00-CH), 126.6 (2C, p00-CH), 127.6 (4C, p0-CH), 127.9 (8C, m0-
CH), 128.1 (4C, m00-CH), 129.5 (8C, o0-CH), 139.5 (2C, i00-C),
143.3 (2C, p-C), 150.6 (4C, i0-C), 154.2 (4C, o-CH), 164.1 (2C, i-
C). IR (Nujol, KBr, cm-1) ν: 2922, vs(br); 2727, m; 2347, m;
2312, m; 1949, w; 1889, w; 1823, w; 1774, w; 1595, m; 1576, m;
1518, m; 1491, m; 1456, s; 1411, m; 1378, s; 1291, m; 1246, m;
1221, w; 1190, m; 1156, m; 1102, s; 1054, s; 1028, s; 988, m; 912,
m; 884, m; 871, m; 827, w; 752, s; 732, m; 697, s; 634, m; 623, m;
611, m.
Experimental Section
Synthesis of [(dme)3Na]þ[C6H2-2,4,6-Ph3]- (4). Sodium sand
(0.06 g, 2.61 mmol) was melted, resulting in a metal mirror, and a
solution of [({2,4,6-Ph3-C6H2}Mg(dme)Br)2(μ-O,O0-dme)] (3)
(0.40 g, 0.37 mmol) in DME (15 mL) was placed in the flask.
Stirring for 3 days at room temperature led to formation of a
light green solution combined with magnesium metal precipita-
tion. After filtration, the solution was stored at -20 ꢀC, which
led to precipitation of light green blocks of 6. Separation and
gently drying under vacuum gave 0.29 g (0.48 mmol, 65%) of 4.
Physical Data of 4. Dec: 105 ꢀC. Anal. Calcd for C36H47NaO6
(598.74 g mol-1): C, 72.22; H, 7.91. Found: C, 71.93; H, 7.72. 1H
NMR (400.25 MHz, 25 ꢀC, [D6]benzene): δ 3.24 (18H, s, CH3,
Manipulation and handling of all compounds were per-
formed in an argon atmosphere using standard Schlenk techni-
ques. Solvents were dried thoroughly and distilled under argon.
1,3,5-Triphenylbenzene was purchased from Aldrich, and bromo-
2,4,6-triphenylbenzene was prepared from 1,3,5-triphenyl-
benzene and bromine in tetrachloromethane.43 The Grignard
reagent [{2,4,6-Ph3-C6H2}Mg(thf)2Br] (2) was prepared accord-
ing to a literature procedure.8 Calcium (granules) was used for
the activation process44 as purchased from Aldrich without
further purification. Rieke magnesium was prepared according
to a literature procedure,45 washed several times with hot
acetone, and dried under 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 difficult. The bromide content
was determined via argentometric titration with potentiometric
indication of the end-point.
3
dme), 3.39 (12H, s, CH2, dme), 7.25 (3H, m, JH-H =7.6 Hz,
p0/p00-CH), 7.36 (2H, t, 3JH-H=7.6 Hz, m00-CH), 7.41 (4H, t, 3JH-H
=8.0 Hz, m0-CH), 7.61 (2H, s, m-CH), 7.74 (4H, d, 3JH-H=8.0
3
Hz, o0-CH), 7.92 (2H, d, JH-H = 7.6 Hz, o00-CH). 13C{1H}
NMR (100.65 MHz, 25 ꢀC, [D6]benzene): δ 58.6 (6C, CH3,
dme), 72.7 (8C, CH2, dme), 124.2 (2C, m-CH), 125.3 (2C, o00-
CH), 126.5 (1C, p00-CH), 127.4 (2C, p0-CH), 127.9 (4C, m0-CH),
128.2 (2C, m00-CH), 129.4 (4C, o0-CH), 139.4 (1C, i00-C), 143.5
(1C, p-C), 150.4 (2C, i0-C), 154.3 (2C, o-CH), 183.1 (1C, i-C).
Synthesis of [(tmta)4Li7Br4O]þ[C6H3-2,4,6-Ph3]- (6). Small
portions of lithium sand (0.07 g, 10.09 mmol) were added during
one hour to a solution of [{2,4,6-Ph3-C6H2}Mg(thf)2Br] (2) (1.49 g,
2.69 mmol) in toluene (20 mL)/THF (10 mL) at -60 ꢀC. An
exothermic reaction took place under magnesium metal separa-
tion, and after half an hour the mixture turned green. After
additional stirring for 1.5 h at -20 ꢀC the reaction was aborted
by filtration (separation of the metal excess) of the deep red-
violet mixture, and 1,3,5-trimethyl-1,3,5-triazinane (1 mL, tmta)
was added. The volume of the solution was reduced to half, and
filtration and storage at -40 ꢀC led to formation of black scales.
Fivefold recrystallization steps (toluene/THF/tmta: 20:20:1, 10 ꢀC)
€
ꢀ
(37) Hommes, N. J. R. v. E.; Buhl, M.; von Rague Schleyer, P.; Wu,
Y.-D. J. Organomet. Chem. 1991, 409, 307–320.
(38) Nicholas, J. B.; Hay, B. P.; Dixon, D. A. J. Phys. Chem. A 1991,
103, 1394–1400.
(39) Tsuzuki, S.; Uchimaru, T.; Mikami, M. J. Phys. Chem. A 2003,
107, 10414–10418.
(40) Ma, J. C.; Dougherty, D. A. Chem. Rev. 1997, 97, 1303–1324.
(41) Holleman, A. F.; Wiberg, E.; Wiberg, N.: Lehrbuch der Anorga-
nischen Chemie, 102th ed.; W. de Gruyter: Berlin, 2007.
(42) For importance of d-orbitals at Ca see e.g.: (a) Deng, S.; Simon,
€
A.; Kohler, J. Angew. Chem. 2008, 120, 6805–6808. Angew. Chem., Int.
Ed. 2008, 47, 6703-6706. (b) Garcia-Fernandez, P.; Bersuker, I. B.;
Boggs, J. E. J. Phys. Chem. A 2007, 111, 10409–10415. (c) Kaupp, M.
Angew. Chem. 2001, 113, 3642–3677. Angew. Chem., Int. Ed. 2001, 40,
3534-3565.
(43) (a) Haaland, A.; Rypdal, K; Verne, H. P.; Scherer, W.; Thiel,
W. R. Angew. Chem. 1994, 106, 2515–2517. (c) Angew. Chem., Int. Ed.
Engl. 1994, 33, 2443-2445. (b) Kohler, E. P.; Blanchard, L. W., Jr. J. Am.
Chem. Soc. 1935, 57, 367–371.
led to crystallization of black opalescent cubes of 6 2(PhMe)
3
(crystalline yield: 0.78 g, 0.56 mmol, 83%, based on Br).
Physical data of 6 2(PhMe). Dec: >46 ꢀC. Anal. Calcd for
3
C62H94Br4Li7N12O (1391.69 g mol-1): C, 53.51; H, 6.8; N,
€
€
(44) Fischer, R.; Gartner, M.; Gorls, H.; Westerhausen, M. Angew.
Chem. 2006, 118, 624–627. Angew. Chem., Int. Ed. 2006, 45, 609-612.
(45) Rieke, R. D.; Xiong, H. J. Org. Chem. 1991, 56, 3109–3118.
(46) Stoll, S.; Schweiger, A. J. Magn. Reson. 2006, 178 (1), 42–55.

Article
Organometallics, Vol. 29, No. 24, 2010 6799
Table 3. Crystal Data and Refinement Details for the X-ray Structure Determinations of the Compounds 3, 6, 7, and 8
3
6
7
8
formula
fw (g mol-1
C
60H64Br2Mg2O6, 0.4 C4H10O2
C24H58Br4Li7N12O, C24H18, 1.5(C7H8)
C36H42NaO3
545.69
-90(2)
monoclinic
P21/m
7.7567(4)
18.6392(7)
10.7397(5)
90
100.621(2)
90
1526.13(12)
2
1.187
0.86
10 719
2012
3569/0.0600
0.1651
0.0555
C16H32KO4, C24H18
633.90
-140(2)
monoclinic
P21/n
10.7269(3)
16.5121(7)
19.7339(6)
90
93.107(2)
90
3490.2(2)
4
1.206
1.92
24 570
4770
7984/0.0747
0.1733
0.0644
3
)
1143.60
-90(2)
triclinic
P1
7.5104(3)
10.7441(5)
19.2715(9)
87.816(3)
79.614(3)
76.216(3)
1485.52(11)
1
1.278
14.36
10 104
4446
6602/0.0362
0.1753
0.0624
1.034
1.288/-0.600
none
1343.63
-140(2)
monoclinic
P21/n
15.3259(5)
25.4392(8)
18.1212(6)
90
94.099(2)
90
7047.0(4)
4
1.266
3
T (ꢀC)
cryst syst
space group
˚
a (A)
˚
b (A)
˚
c (A)
R (deg)
β (deg)
γ (deg)
3
˚
V/A
Z
F (g cm-3
)
3
μ (mm-1
measd data
data with I > 2σ(I)
)
23.28
41 504
6981
15 578/0.1373
unique data (Rint
)
wR2 (all data, on F2)a
R1 (I > 2σ(I)) a
Sb
1.021
1.023
-
˚
res dens (e A 3)
3
0.259/-0.260
none
745338
0.381/-0.391
none
782317
absorpt method
CCDC no.
none
motif
745337
P
P
P
P
a Definition of the R indices: R1 = ( ||Fo| - |Fc||)/ |Fo|; wR2 = { [w(Fo2 - Fc2)2]/ [w(Fo2)2]}1/2 with w-1 = σ2(Fo2) þ (aP)2 þ bP; P = [2Fc
þ
2
P
Max(Fo2]/3. b s = { [w(Fo2 - Fc2)2]/(No - Np)}1/2
.
12.08; Br, 22.97. Found: C, 51.24; H, 6.32; N, 11.81; Br, 30.21.
EPR (THF solution, rt): g=2.0023; various hyperfine coupling
pattern. Susceptibility measurements (Gouy magnetic balance,
294 K): χv=4.182 ꢀ 10-7, χg=1.020 ꢀ 10-6 cm3 g-1, χm=1.420 ꢀ
temperature. After a second color change from initial green to
deep violet the solution was filtered.
Method 2. Small portions of potassium sand (0.36 g, 9.21
mmol) were added during one hour in a solution of [{2,4,6-Ph3-
C6H2}Mg(thf)2Br] (2) (1.23 g, 2.23 mmol) in THF (25 mL) and
stirred for 28 h at room temperature. After separation of
magnesium metal the mixture was filtered, resulting in a deep
violet solution.
Method 3. Small portions of potassium sand (0.43 g, 11.00
mmol) were added during one hour in a solution of 2,4,6-
triphenylbromobenzene (1.72 g, 4.46 mmol) in THF (10 mL).
After stirring for 14 h at room temperature the mixture was
filtered.
General Procedure. All volatiles were distilled off, and the
black residue was extracted with a mixture of toluene (10 mL)
and THF (10 mL) at room temperature. The extract was filtered
once more, and storage at -20 ꢀC led to precipitation of a black
solid. Twofold recrystallization led to formation of black opa-
lescent and pyrophoric prisms of 8 at -20 ꢀC (method 1,
crystalline yield: 1.32 g, 2.08 mmol, 45%; method 2, crystalline
yield: 0.85 g, 1.34 mmol, 60%; method 3, crystalline yield: 1.53 g,
2.41 mmol, 54%).
10-3 cm3 mol-1, μeff = 1.31 μB. Li{1H} NMR (155.55 MHz,
7
25 ꢀC, [D6]benzene): δ 1.48 m(br). IR (Nujol, KBr, cm-1) ν:
2922, vs(br); 2726, m; 2662, m; 1594, w; 1566, w; 1456, vs; 1377,
vs; 1308, m; 1267, s; 1233, m; 1158, m; 1116, m; 1004, w; 978, w;
942, m; 914, m; 752, m; 725, s; 697, m.
Synthesis of [(thf)3Na(μ-η6-C6H3-2,4,6-Ph3)] (7). Method A.
Small portions of sodium sand (0.86 g, 37.40 mmol) were added
during one hour to a solution of 1,3,5-triphenylbenzene (2.86 g,
9.33 mmol) in THF (20 mL) and stirred for 18 h at room
temperature. The resulting dark green solution was filtered, all
volatiles were distilled off, and the black residue was extracted
with a mixture of toluene (10 mL) and THF (10 mL) at room
temperature. The extract was filtered once more, and storage
at -20 ꢀC led to separation of a black solid. Several steps of
recrystallization initiated precipitation of black opalescent and
pyrophoric cubes of 7 at -20 ꢀC (crystalline yield: 2.13 g,
3.9 mmol, 42%).
Method B. Small portions of sodium sand (0.25 g, 10.87 mmol)
were added during 30 min in a solution of [{2,4,6-Ph3-C6H2}Mg-
(thf)2Br] (2) (1.00 g, 1.81 mmol) in THF (15 mL) and stirred for
12 h at room temperature. Magnesium metal precipitated, and a
dark green-brown solution was formed. After filtration all
volatiles were distilled off and the black residue was extracted
with a mixture of toluene (15 mL) and THF (5 mL) at room
temperature. The extract was filtered once more, and storage at
-20 ꢀC led to separation of an opalescent solid. Several steps of
recrystallization generated by storage at -20 ꢀC black opales-
cent and pyrophoric cubes of 7 (crystalline yield: 0.81 g,
1.48 mmol, 82%).
Physical Data of 8. Dec: >34 ꢀC. Anal. Calcd for C40H50KO4
(633.92 g mol-1): C, 75.79; H, 7.95. Found: C, 76.19; H, 8.23. 8:
EPR (THF solution, rt): g=2.0024; various hyperfine coupling
pattern in accordance to a tpb-monoanion. Susceptibility mea-
surements (Gouy magnetic balance, 294 K): χv=1.276 ꢀ 10-6
,
=
χg=2.493 ꢀ 10-6 cm3 g-1, χm=1.580 ꢀ 10-3 cm3 mol-1, μeff
1.73 μB.
EPR Measurements. EPR spectra were measured on a Bruker
ESP 300 E X-band spectrometer with a modulation amplitude
of 0.104 G. EPR spectrum simulations were carried out with the
Easyspin software package using a genetic algorithm and results
after 16 h of iteration (40 000 generations).46
6: EPR (toluene solution, rt): g = 2.0023; various hyperfine
coupling patterns. Simulation (two equivalent phenyl groups,
ΔHpp=0.0413 G, g=2.0023, Lorentz form, eight different kinds
of protons): n=1H (A=0.3436 G), 1H (A=6.2838 G), 2H (A=
6.0279 G), 2H (A = 2.6683 G), 2H (A = 6.1949 G), 2H (A =
0.5170 G), 4H (A=7.3986 G), 4H (A=2.8422 G).
Physical Data of 7. Dec: >40 ꢀC. Anal. Calcd for C36H42-
NaO3 (545.71 g mol-1): C, 79.23; H, 7.76. Found: C, 79.84; H,
7.91. Susceptibility measurements (Gouy magnetic balance,
294 K): χv=1.381 ꢀ 10-6, χg=2.527 ꢀ 10-6 cm3 g-1, χm=1.379
ꢀ 10-3 cm3 mol-1, μeff=1.62 μB.
Synthesis of [(thf)4K(μ-η6-C6H3-Ph3)] (8). Method 1. Small
portions of potassium sand (0.32 g, 8.18 mmol) were added
during one hour in a solution of 1,3,5-triphenylbenzene (1.43 g,
4.67 mmol) in THF (20 mL) and stirred for 18 h at room
7: EPR (THF solution, rt): g = 2.0023; various hyperfine
coupling patterns. Simulation (Cs symmetry, ΔHpp=0.19535 G,

6800 Organometallics, Vol. 29, No. 24, 2010
Krieck et al.
g=2.0023, Lorentz form, eight different kinds of protons): n=
1H (A=1.5795 G), 1H (A=10.3807 G), 2H (A=-6.7406 G),
2H (A=-4.0131 G), 2H (A=-2.0089 G), 2H (A=-1.2012 G),
4H (A=2.4895 G), 4H (A=4.8224 G).
7þNa (THF solution, 77 K): g = 2.0023. Simulation (D3h
symmetry, ΔHpp =0.85 G, g=2.0023, Lorentz form, four dif-
ferent kinds of protons): n=2 ꢀ 3H (A=0.8574 G), 2 ꢀ 6H (A=
1.7068 G).
crystals of 6 were extremely thin and of low quality, resulting
in a substandard data set; however, the structure is sufficient to
show connectivity and geometry despite the high final R value.
We will be publishing only the conformation of the molecule and
the crystallographic data. We will not deposit the data in the
Cambridge Crystallographic Data Centre. Crystallographic
data as well as structure solution and refinement details are
summarized in Table 3. XP (SIEMENS Analytical X-ray In-
struments, Inc.) was used for structure representations.
Crystal Structure Determination. The intensity data for the
compounds were collected on a Nonius KappaCCD diffract-
ometer using graphite-monochromated Mo KR radiation. Data
were corrected for Lorentz and polarization effects but not for
absorption effects.47,48
Acknowledgment. We thank the Deutsche For-
schungsgemeinschaft (DFG, Bonn/Germany) for gener-
ous financial support of this research initiative. We are
grateful to Dr. Manfred Friedrich, University of Jena, for
EPR measurements and simulation experiments.
The structures were solved by direct methods (SHELXS49)
2
and refined by full-matrix least-squares techniques against Fo
(SHELXL-9749). All hydrogen atoms were included at calculated
positions with fixed thermal parameters. All nondisordered,
non-hydrogen atoms were refined anisotropically.49 The
Supporting Information Available: Crystallographic data
(excluding structure factors) have been deposited with the
Cambridge Crystallographic Data Centre as supplementary
publications CCDC-745120 for 3, CCDC-745122 for 7, and
CCDC-782317 for 8. Copies of the data can be obtained free of
charge on application to CCDC, 12 Union Road, Cambridge
CB2 1EZ, UK [e-mail: deposit@ccdc.cam.ac.uk]. This material
is available free of charge via the Internet at http://pubs.acs.org.
(47) COLLECT, Data Collection Software; Nonius, B. V.: Netherlands,
1998.
(48) Processing of X-Ray Diffraction Data Collected in Oscillation
Mode: Otwinowski, Z.; Minor, W. In Methods in Enzymology, Vol. 276,
Macromolecular Crystallography, Part A, Carter, C. W.; Sweet, R. M.,
Eds.; Academic Press: 1997; pp 307-326.
(49) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112–122.