Halide-free diarylcalcium complexes - Syntheses, structures, and stability

Article abstract of DOI:10.1002/chem.201304429

A general procedure was developed for the synthesis of diarylcalcium complexes by addition of KOtBu to arylcalcium iodides in THF. Intermediate arylcalcium tert-butanolate dismutates immediately leading to insoluble tert-butanolate precipitates of calcium. Depending on the steric demand and denticity of additional neutral aliphatic azabases, mononuclear or dinuclear complexes trans-[Ca(α-Naph)₂(thf)₄] (1), [Ca(β-Naph)₂(thf)₄] (2), [Ca(Tol)₂(tmeda)] ₂ (3), [Ca(Ph)₂(tmeda)]₂ (4), [Ca(Ph) ₂(pmdta)(thf)] (5), [Ca(hmteta)(Ph)₂] (6), and [Ca([18]C-6)(Ph)₂] (7) were isolated (Naph=naphthyl; meda=N,N,N′,N′-tetramethylethylenediamine; pmdta= N,N,N′,N′′,N′′-pentamethyldiethylenetriamine; hmteta=N,N,N′,N′′,N′′′,N′′ ′-hexamethyltriethylenetetramine). The Cai￡C bond lengths vary between 250.8 and 263.5apm, the ipso-carbon atoms show low-field-shifted resonances in the ¹³CaNMR spectra. Calcium for your chemistry toolbox! A straightforward procedure allows the preparation of halide-free diarylcalcium complexes (see scheme; hmteta= N,N,N′, N′′,N′′′,N′′′- hexamethyltriethylenetetramine). Despite their enormous reactivity, leading to ether degradation reactions, the stability is sufficient for preparative organic and organometallic chemistry.

Full text of DOI:10.1002/chem.201304429

DOI: 10.1002/chem.201304429
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&
Organometallic Chemistry
Halide-Free Diarylcalcium Complexes—Syntheses, Structures, and
Stability
Jens Langer, Mathias Kçhler, Helmar Gçrls, and Matthias Westerhausen*[a]
Abstract: A general procedure was developed for the syn-
thesis of diarylcalcium complexes by addition of KOtBu to ar-
ylcalcium iodides in THF. Intermediate arylcalcium tert-buta-
nolate dismutates immediately leading to insoluble tert-bu-
tanolate precipitates of calcium. Depending on the steric
demand and denticity of additional neutral aliphatic azabas-
es, mononuclear or dinuclear complexes trans-[Ca(a-Naph)2-
(thf)4] (1), [Ca(b-Naph)2(thf)4] (2), [Ca(Tol)2(tmeda)]2 (3),
[Ca(Ph)2(tmeda)]2
(4),
[Ca(Ph)2(pmdta)(thf)]
(5),
[Ca(hmteta)(Ph)2] (6), and [Ca([18]C-6)(Ph)2] (7) were isolated
(Naph=naphthyl; meda=N,N,N’,N’-tetramethylethylenedia-
mine; pmdta= N,N,N’,N’’,N’’-pentamethyldiethylenetriamine;
hmteta=N,N,N’,N’’,N’’’,N’’’-hexamethyltriethylenetetramine).
The CaꢀC bond lengths vary between 250.8 and 263.5 pm,
the ipso-carbon atoms show low-field-shifted resonances in
the 13C NMR spectra.
Introduction
Organolithium and organomagnesium compounds are among
the most frequently used organometallic reagents and are in-
dispensable tools in every synthetically working chemist’s tool-
box,[1,2] whereas the organometallic derivatives of the heavier
alkaline earth metals were treated as interesting oddities at
best. This is rather surprising given the worldwide accessibility
especially of calcium minerals, guarantying the accessibility of
this metal at low prices. One of the reasons might be the lack
of simple, efficient, and straightforward synthetic strategies for
organocalcium halides and diorganocalcium derivatives, in
marked contrast to magnesium derivatives for which such
strategies are long-known due to the pioneering work of
Grignard[3] and Schlenk.[4] In recent years this gap was partially
closed by the development of refined synthetic strategies for
benzyl,[5] allyl,[6] methanide,[7] and aryl[8] derivatives of calcium
and the higher homologues strontium and barium. Especially,
the development of a reliable protocol for the efficient synthe-
sis of arylcalcium halides[9,10] in analogy to the well-known syn-
thesis of Grignard reagents raises the hope that the applica-
tions of calcium derivatives will one day be as numerous as of
Scheme 1. Simplified Schlenk equilibrium to interchange the Grignard re-
agent PhMgX into MgPh2 and MgX2 (aggregation to oligonuclear complexes
not taken into account).
shift the equilibrium named after him to the right (see
Scheme 1)—is not applicable to calcium. In earlier investiga-
tions, the transmetalation reaction starting from diarylmercury
and calcium shavings was reported to offer a procedure for
the preparation of halide-free diarylcalcium.[11] However, the
isolation of solvent-free products or products that contained
only 0.5THF ligands per calcium center from such solutions in
THF seem unlikely from today’s point of view.
Extending our preliminary study,[12] a straightforward strat-
egy to diarylcalcium complexes is presented in detail and used
for a thorough investigation of these elusive derivatives
with respect to solid-state structures, solution behavior, and
stability.
their magnesium analogues. However, while
a growing
number of arylcalcium halides have been isolated and charac-
terized, the diarylcalcium derivatives have stayed singulari-
ties[8b] since the protocol developed by Schlenk[4] for the
straightforward synthesis of the analogous magnesium deriva-
tives—the precipitation of magnesium dihalides from ether Results and Discussion
solutions of Grignard reagents by addition of 1,4-dioxane to
Syntheses and solid-state structures
[a] Dr. J. Langer, M. Kçhler, Dr. H. Gçrls, Prof. Dr. M. Westerhausen
Institute of Inorganic and Analytical Chemistry
Friedrich Schiller University Jena
A more promising route uses arylcalcium halides as starting
materials. Like their magnesium counterparts, these calcium-
based organometallics exhibit a Schlenk-type equilibrium con-
verting arylcalcium halide into diarylcalcium and calcium diha-
lide. Already Bogatskii et al.[13] investigated this equilibrium in
Humboldtstraße 8, 07743 Jena (Germany)
Fax: (+49)43641-948132
E-mail: m.we@uni-jena.de
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THF in dependency of the reaction conditions and found com-
positions of [{CaI(Ph)}·{Ca(Ph)2}n] with 0<n<1.2. Slow fractio-
nated crystallization of a mesitylcalcium iodide solution al-
lowed the isolation of the first structurally characterized com-
plex [Ca(C6H2-2,4,6-Me3)2(thf)3][8b] in low yield. Unfortunately,
other derivatives were not accessible by this approach so far. It
is well known that the Schlenk equilibrium can be shifted to-
wards diarylmagnesium species of the general formula
[(MgR2)n(diox)n+1] (diox=1,4-dioxane) by precipitation of the
magnesium dihalides with 1,4-dioxane.[4,14] However, a simple
adaptation of this protocol to calcium derivatives was not fea-
sible in our hands. Therefore, another strategy was developed.
The metathesis reaction of arylcalcium iodides with potassium
tert-butanolate in THF is supposed to yield intermediate aryl-
calcium tert-butanolate and insoluble potassium iodide that
should precipitate immediately from the reaction solution.
Indeed, the presence of major amounts of iodide in the precip-
itate, obtained from the reaction mixture of [CaI(Ph)(thf)4][10]
and potassium tert-butanolate, was confirmed by treating a hy-
drolyzed and acidified sample with AgNO3. The same test gave
only an extremely weak positive result for an equally treated
sample of the solution. However, additional acidimetric titra-
tion (1m HCl) of the precipitate revealed that the obtained
solid also contained about five ninths of the overall alkalinity
of the system, whereas four ninths stayed in THF solution. The
distribution of benzene and tert-butanol among the two frac-
tions of the reaction mixture was further investigated by gas
chromatography after hydrolysis. While the precipitate con-
tained predominately tert-butanol besides considerable
amounts of benzene, in solution almost exclusively benzene
was detected after hydrolysis. This finding indicates that inter-
mediate phenylcalcium tert-butanolate subsequently formed
homoleptic, highly soluble diphenylcalcium and sparingly solu-
ble tert-butanolate containing calcium species that precipitated
from the reaction mixture. Washing of the residue or use of
more diluted solutions did not alter the distribution of alkaline
species between precipitate and solution significantly. The in-
solubility of the precipitate in solvents that are compatible
with such highly basic s-block organometallics prevents
a more detailed investigation of this solid. The simplified reac-
tion scheme, shown in Scheme 2, summarizes the above-men-
tioned findings.
Figure 1. Molecular structure and numbering scheme of 2. Ellipsoids repre-
sent a probability of 40%, and H atoms are omitted for clarity reasons. Sym-
metry-related atoms (ꢀx+1, ꢀy, ꢀz+1) are marked with the letter “A”.
Selected bond lengths [ꢁ]: Ca1ꢀC1 2.635(3), Ca1ꢀO1 2.4047(19), Ca1ꢀO2
2.3790(18). Angles [8]: C1-Ca1-C1A 180.0, C1-Ca1-O1 92.01(7), C1-Ca1-O2
88.80(7), C1-Ca1-O1A 87.99(7), C1-Ca1-O2A 91.20(7), C2-C1-C10 115.9(3).
1 and 263.5 pm in 2 are similar. The presence of two strongly
basic carbanions in the coordination sphere of calcium leads
to lengthening of the metal carbon bonds in comparison to
naphthylcalcium halide complexes.[9,15–17] For the known com-
pound [Ca(a-Naph){N(SiMe3)2}(thf)3][15] an analogous finding
might be expected due to the presence of a strongly basic
amide, but here the simultaneous reduction of the coordina-
tion number from six to five, which leads to an overall contrac-
tion, overcompensates this effect. Selected bond lengths and
angles of 1 and 2 are listed together with the corresponding
values of naphthylcalcium halides and amides in Table 1.
Additionally, the above-described protocol was used to pre-
pare solutions of di-p-tolylcalcium and diphenylcalcium in THF.
Table 1. Structural parameters of naphthylcalcium complexes (average
values, bond lengths [pm]).
Compound
CaꢀC
CaꢀX
CaꢀO
Ref.
[CaI(a-Naph)(thf)4]
[CaI(b-Naph)(thf)4]
[CaI(b-Naph)(thp)4]
[Ca(m-Br)(a-Naph)(thf)3]2
[Ca(Br)(b-Naph)(thp)4]
[Ca(a-Naph){N(SiMe3)2}(thf)3]
[Ca(a-Naph)2(thf)4] (1)
[Ca(b-Naph)2(thf)4] (2)
255.2
252.8
253.4
252.8
256.9
251.4
262.9
263.5
318.7
319.3
316.6
294.0
299.0
235.0
–
234.7
238.5
241.2
240.5
238.0
239.8
240.2
239.2
[9]
Nevertheless, the reaction proved to be highly valuable
since it offers a straightforward access to halide-free diarylcalci-
um derivatives like trans-[Ca(a-Naph)2(thf)4] (1)[12] and trans-
[Ca(b-Naph)2(thf)4] (2) (Naph=naphthyl). The crystal structure
of 2 is shown in Figure 1. These thf adducts form octahedral
complexes with trans-positioned aryl groups for electrostatic
reasons. The observed average CaꢀC bond lengths of 262.9 in
[17]
[16]
[15]
[17]
[15]
[12]
–
The high solubility of these derivatives hinders their
isolation in crystalline form, although an amorphous
solid of the formal composition [Ca(Ph)2(thf)4][12] was
occasionally observed in the latter case. Nevertheless,
these solutions are highly useful for the synthesis of
a number of donor adducts of diarylcalcium with
Scheme 2. Formation of diarylcalcium.
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multidentate nitrogen and oxygen donor ligands. Utilization of
such chelating ligands facilitates the isolation of diarylcalcium
derivatives and provides an insight into the coordination
chemistry of these derivatives in comparison to their arylcalci-
um halide analogues. The use of N,N,N’,N’-tetramethyl-
ethylenediamine (tmeda) disclosed that the structural analo-
gies found for phenylcalcium iodides and calcium diiodides
cannot be simply extended to the arylcalcium derivatives.
While [CaI(Ph)(tmeda)2][8d] and [CaI2(tmeda)2][19] are mononu-
clear octahedral complexes with a trans alignment of the
anionic ligands, the use of an excess of bidentate TMEDA leads
in the case of the diarylcalcium derivatives to the crystallization
of dimers of the type [(Aryl)Ca(m-Aryl)(tmeda)]2 with two termi-
nal and two bridging aryl ligands. The coordination sphere of
calcium in [(Tol)Ca(m-Tol)(tmeda)]2 (3) (see Figure 2; Tol=4-
methylphenyl), and [(Ph)Ca(m-Ph)(tmeda)]2 (4)[12] is remarkable
and best described as distorted square pyramidal with the two
nitrogen atoms of tmeda and the carbon atoms of the bridg-
Since both aryl groups induce comparable intramolecular
strain, intermolecular forces and packing effects might be re-
sponsible for these unexpected findings. Enhancement of the
denticity of the utilized Lewis azabase leads to mononuclear
complexes with significantly bent C-Ca-C fragments. Hexa-co-
ordination of the calcium atoms is achieved by an additional
THF ligand for the N,N,N’,N’’,N’’-pentamethyldiethylenetriamine
(pmdta)-stabilized
complex
cis-[Ca(Ph)2(pmdta)(thf)]
(5)
(Figure 3), whereas the tetradentate N,N,N’,N’’,N’’’,N’’’-hexame-
thyltriethylenetetramine ligand (hmteta) itself shields the calci-
um atom in cis-[Ca(hmteta)(Ph)2] (6) effectively (Figure 4). The
observed cis-alignment of the phenyl groups is unique for
mononuclear octahedral arylcalcium complexes. Here, electro-
static repulsion between the anions only leads to a widening
of the angle C-Ca-C to 111.09(10)8 in 5 and 109.67(11)8 in 6,
Figure 3. Molecular structure and numbering scheme of 5. Ellipsoids repre-
sent a probability of 40%. H atoms and a disorder of C15 and C16 are omit-
ted for clarity reasons. Selected bond lengths [ꢁ]: Ca1ꢀC1 2.594(3), Ca1ꢀC7
2.581(3), Ca1ꢀO1 2.465(2), Ca1ꢀN1 2.626(3), Ca1ꢀN2 2.616(3), Ca1ꢀN3
2.655(3). Angles [8]: C1-Ca1-C7 111.09(10), C1-Ca1-O1 83.99(9), C7-Ca1-O1
95.40(9), C1-Ca1-N1 151.05(9), C1-Ca1-N2 92.75(9), C1-Ca1-N3 94.66(10),
C7-Ca1-N1 93.79(10), C7-Ca1-N2 151.56(10), C7-Ca1-N3 91.57(10), O1-Ca1-N1
78.98(9), O1-Ca1-N2 102.56(9), O1-Ca1-N3 172.93(9), C2-C1-C6 112.6(3),
C8-C7-C12 113.0(3).
Figure 2. Molecular structure and numbering scheme of 3. Ellipsoids repre-
sent a probability of 40%, and H atoms are omitted for clarity reasons. Sym-
metry-related atoms (ꢀxꢀ1, ꢀy, ꢀz+2) are marked with the letter “A”.
Selected bond lengths [ꢁ]: Ca1ꢀC1 2.600(4), Ca1ꢀC8 2.590(4), Ca1ꢀC8A
2.589(4), Ca1ꢀN1 2.592(3), Ca1ꢀN2 2.570(3). Angles [8]: C1-Ca1-C8 112.02(12),
C1-Ca1-C8A 114.69(12), C8-Ca1-C8A 92.49(11), C1-Ca1-N1 101.65(12),
C1-Ca1-N2 98.30(11), C8-Ca1-N1 87.92(10), C8-Ca1-N2 146.14(11), C8A-Ca1-
N1 140.36(12), C8A-Ca1-N2 88.01(11), C2-C1-C6 116.0(4), C9-C8-C13 112.4(3).
ing aryl ligands occupying the basal and the terminal aryl
ligand the apical position. The two pyramids share a common
edge of the basal face with the tips of the pyramids pointing
in opposite directions. The calcium atom itself is displaced to-
wards the apical ligand by d=77 pm in 3 (61 pm in 4) from
the least-squares plane defined by N1, N2, C8, and C8A (C7
and C7A in 4).
Furthermore it was noticed that the terminal and bridging
CaꢀC bond lengths of [(Tol)Ca(m-Tol)(tmeda)]2 exhibit very simi-
lar values of 259.0(4) and 260.0(4) pm, respectively, whereas for
sterically comparable [(Ph)Ca(m-Ph)(tmeda)]2 (4) the terminal
Ca–Ct distances of 250.8(3) pm are significantly shortened.
Figure 4. Molecular structure and numbering scheme of 6. Ellipsoids repre-
sent a probability of 40%, and H atoms are omitted for clarity reasons. Se-
lected bond lengths [ꢁ]: Ca1ꢀC1 2.564(3), Ca1ꢀC7 2.607(3), Ca1ꢀN1 2.631(3),
Ca1ꢀN2 2.636(3), Ca1ꢀN3 2.644(3), Ca1ꢀN4 2.579(3). Angles [8]: C1-Ca1-C7
109.67(11), C1-Ca1-N1 94.69(9), C1-Ca1-N2 94.34(10), C1-Ca1-N3 148.53(10),
C1-Ca1-N4 89.67(10), C7-Ca1-N1 92.58(9), C7-Ca1-N2 151.68(10), C7-Ca1-N3
95.77(10), C7-Ca1-N4 93.12(10), C2-C1-C6 112.1(3), C8-C7-C12 111.2(3).
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but not to the formation of the trans-isomer. Obviously, the
employed chelating ligands pmdta and hmteta enforce the for-
mation of the cis-form in these derivatives. This tendency of
tetradentate hmteta to stabilize mononuclear octahedral calci-
um complexes with a cis-alignment was observed earlier for
[CaI2(hmteta)][19] and [Ca(hmteta)(PPh2)2].[20] However, in the
case of calcium phosphanides it was shown that the difference
in energy between the isomers is rather small since [Ca(PPh2)2-
(thf)4] can be isolated in both forms depending on the crystalli-
zation conditions.[20,21] A small difference in energy of 1.97 kcal
molꢀ1 was also calculated (DFT) between the cis and trans
form of [Ca(Bz)2(thf)4].[5b] For the arylcalcium derivatives, a simi-
lar assumption is reasonable with respect to the results dis-
cussed above. Additionally, the dinuclear arylcalcium derivative
[Ca(m-Br)(Phenanthryl)(thf)3]2 was reported recently, in which
the terminal phenanthryl ligand occupies a cis-position relative
to both bridging bromide ions resulting in a facial alignment
of the three anionic ligands.[22]
Figure 5. Molecular structure and numbering scheme of 7. Ellipsoids repre-
sent a probability of 40%, and H atoms are omitted for clarity reasons. Only
molecule A of two crystallographically independent molecules is displayed.
Selected bond lengths [ꢁ]: Ca1AꢀC1A 2.536(7), Ca1AꢀC7A 2.514(7),
Ca1AꢀO1A 2.694(5), Ca1AꢀO2A 2.600(5), Ca1AꢀO3A 2.672(5), Ca1AꢀO4A
2.711(5), Ca1AꢀO5A 2.662(5), Ca1AꢀO6A 2.758(5). Angles [8]: C1A-Ca1A-C7A
167.5(2), C1A-Ca1A-O1A 81.9(2), C1A-Ca1A-O2A 84.94(19), C1A-Ca1A-O3A
98.2(2), C1A-Ca1A-O4A 89.86(19), C1A-Ca1A-O5A 96.46(19), C1A-Ca1A-O6A
88.0(2), C7A-Ca1A-O1A 85.9(2), C7A-Ca1A-O2A 91.59(19), C7A-Ca1A-O3A
90.59(19), C7A-Ca1A-O4A 102.3(2), C7A-Ca1A-O5A 87.18(19), C7A-Ca1A-O6A
83.50(19), C2A-C1A-C6A 112.1(6), C8A-C7A-C12A 112.7(6).
A mononuclear derivative of diphenylcalcium with an unusu-
ally large coordination number of eight is realized for the
[18]crown-6 adduct (Figure 5). Despite this large coordination
number of the alkaline earth metal center a rather short aver-
age CaꢀC bond of 253.4 pm is observed in [Ca([18]C-6)(Ph)2]
(7) as the result of the greatly reduced steric pressure of the
crown ether relative to four thf ligands. A similar observation
was made in the case of [Ca([18]C-6)I2].[19] The Ca–O distances
in 7 are large, which indicates that the coordination gap of
this crown ether is slightly too large for a calcium cation. Like
the related phenylcalcium iodide and calcium diiodide com-
plexes with this ligand,[19] 7 is insoluble in common organic
solvents, such as THF, toluene, benzene, and diethyl ether. At-
tempts to apply other chelating oxygen donor ligands, such as
dimethoxyethane, failed probably due to the rapid degrada-
tion of arylcalcium compounds in the presence of this li-
gand.[8e]
of 1 suggests that solution structure and observed solid-state
structure are in good agreement with each other. Simple spec-
tra also predestined 1 for further investigation of its stability in
THF by NMR spectroscopic measurement. It was determined
that half of the compound decomposes by THF degradation
within two days at ambient temperature in a two-to-one mix-
ture of THF and [D6]benzene when using cyclohexane as the
internal standard (see Figure 6); much faster than related aryl-
calcium halide derivatives.[16] Among the decomposition prod-
ucts, naphthalene and ethene were identified, which indicated
that a-deprotonation of THF and its subsequent cycloreversion
occurred as one of the decomposition pathways.
Table 2 summarizes selected bond lengths and angles of the
investigated complexes 1–7.
Investigations in solution
The solubility of all synthesized diarylcalcium derivatives,
except for 7, in [D8]THF is rather high allowing an investigation
of these compounds in solution. The derivative 1 shows
a single set of resonances for
Already in the case of the b-naphthyl derivative 2 it became
obvious that the structures of such THF-ligated diarylcalcium
derivatives are less uniform in solution than observed for the
the
metallated
a-naphthyl
groups in the 1H and 13C NMR
spectra. The signal of the calci-
um-bound carbon atom was ob-
served at d=198.9 ppm in the
13C NMR spectrum, significantly
shifted downfield relative to the
related carbon atom in naphtha-
lene (d=128 ppm) under identi-
cal conditions. This shift is slight-
ly more pronounced than in a-
naphthylcalcium iodide (d=
195.4 ppm).[9] The observed data
Table 2. Structural parameters of diarylcalcium complexes.
[a]
Compound
CN
CaꢀCt[a]
CaꢀCbr
CaꢀN[a]
259.0
258.1
–
CaꢀO[a]
–
Ct-Ca-Ct[a]
Ref.
[12]
[Ca(Ph)2(tmeda)]2 (4)
[Ca(Tol)2(tmeda)]2 (3)
[Ca(Mes)2(thf)3]
[Ca(a-Naph)2(thf)4] (1)
[Ca(b-Naph)2(thf)4] (2)
[Ca(Ph)2(pmdta)(thf)] (5)
[Ca(hmteta)(Ph)2] (6)
[Ca([18]C-6)(Ph)2] (7)
5
5
5
6
6
6
6
8
250.8
260.0
252.0
262.9
263.5
258.8
258.6
253.4
259.5
259.0
–
–
–
–
–
–
–
–
–
238.9
240.2
239.2
246.5
–
119.6
176.8
180.0
111.1
109.7
171.0
[8b]
[12]
–
–
263.2
262.3
–
269.2
[a] Average values, bond lengths [pm] and angles [8]; CN coordination number of Ca, Ct terminally bound to
the ipso-carbon atom, Cbr ipso-carbon atom in a bridging position.
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In case of the hmteta-ligated complex 6, the opposite obser-
vation was made. Here the signals of the calcium bound
phenyl groups are well defined in [D8]THF, whereas the signals
of the chelating ligand are extremely broad. Although these
broad signals indicate fluxionality of the complex and changes
in conformation and denticity of the hmteta ligand might be
expected, a complete replacement of the ligand by solvent
molecules can be excluded. This is somewhat surprising since
the NMR spectroscopic data in the case of the related calcium
complex [Ca(hmteta)(PPh2)2] in [D8]THF clearly underlines that
such an substitution is possible and was achieved for this
system.[20] Obviously, the nature of the anion (steric demand,
hardness/softness etc) influences the ease of substitution of
the neutral ligand in these calcium compounds. Additionally it
was noticed during the NMR spectroscopic investigation of 6
that crops of the product that have stayed in contact with the
toluene containing mother liquor for extended time spans
were contaminated with increasing amounts of benzylcalcium
species[5b] as judged by the appearance of typical signals at
d=1.74, 5.84, 6.51, and 6.56 ppm in the 1H NMR spectrum.
This observation underlines the highly basic character of com-
plexes like 6.
Figure 6. Decomposition of a 0.08m solution of 1 in THF/[D6]benzene (2:1).
solid state. Despite the use of a pure sample of 2, a second
broad incomplete signal set of a b-naphthyl group became no-
ticeable beside the expected ten signals of this moiety in the
13C NMR spectrum (see the Experimental Section). Additionally
the presence of this second species was also observed in the
1H NMR spectrum. Here, two signals for each naphthyl proton
in ortho-position to the calcium were observed in the region
between d=7.9 and 8.3 ppm. Nevertheless, the ratio between
the sum of the b-naphthyl groups and THF of two to four is in
agreement with the solid-state structure.
Since the presence of an excess of [D8]THF leads to reversion
of the formation of the azabase ligated systems or at least
strongly influences their structures in solution, a reinvestigation
of these systems with hydrocarbons as solvents was envi-
sioned. While the insolubility of the mononuclear complexes 5
and 6 in benzene and toluene hampered this endeavor, the di-
nuclear derivatives showed high solubility in these solvents. It
was observed that 3 and 4 are fluxional molecules even in
[D6]benzene and only one averaged signal set for the intercon-
In the case of diphenylcalcium, solutions in [D8]THF show
1
only very broad signals in both H and 13C NMR spectra point-
1
ing to a highly fluxional structure of this compound in solu-
tion. Similar broad signals were observed in the case of 4 and
5 in [D8]THF besides rather sharp signals of the non-coordinat-
ed nitrogen donor ligands tmeda and pmdta, respectively (see
Figure 7). This observation clearly shows that the chelating
effect of these ligands is not sufficient to suppress their re-
placement in the coordination sphere of calcium by the excess
of deuterated THF used as the solvent.
verting bridging and terminal aryl groups was found in the H
and 13C NMR spectra of these complexes. Therefore, the chemi-
cal shift of the metallated carbon atom of the phenyl group in
4 (d=186.0 ppm) expectedly falls between the values found
for terminal phenyl groups like in parent [CaI(Ph)(thf)4] (d=
190.3 ppm)[10] and bridging phenyl groups as observed in
[Ca(Cp)(dme)(m-Ph)]2 (d=184.5 ppm; Cp=cyclopentadienide;
dme=1,2-dimethoxyethane).[23]
In general, the 13C NMR spectroscopic data of the investigat-
ed complexes show characteristics similar to arylcalcium hal-
ides and related aryllithium and arylmagnesium systems. All
those derivatives have a strong downfield shift of the metallat-
ed carbon atom relative to the parent organic derivative in
common, which becomes stronger in the order Mg<CaꢁLi.
This downfield shift is slightly more pronounced in terminal
than in bridging groups. On this connection, a disagreeing ex-
ception should be mentioned. The synthesis of the structurally
not characterized diarylcalcium compound [Ca(2-C5H3N-4-
CMe3)2(thf)2],[24] containing a metallated pyridine derivative,
was described recently. Here, an upfield shift of the calcium
bound carbon atom to d=120.3 ppm was mentioned. Al-
though this observation might be connected to the introduc-
tion of an heteroatom into the aromatic system, it is not only
in marked contrast to the results reported above but also to
the structurally characterized 2-pyridylmagnesium derivative
[Mg2Cl2(m-2-C5H4N)2(thf)3],[25] which in our hands showed
Figure 7. 1H NMR spectrum of 5.(*=benzene) in [D8]THF.
Chem. Eur. J. 2014, 20, 3154 – 3161
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residual signals of [D8]THF or [D6]benzene were used as an internal
standard.
a strong downfield shift of the metallated ortho carbon atom
of pyridine to d=210.4 ppm (ortho CH of pyridine: d=
150.6 ppm) in the 13C NMR spectrum in [D8]THF.
Calcium was activated by dissolution in liquid NH3 and subsequent
reduction of the deep-blue solution to dryness, resulting in finely
divided calcium powders. The complexes [CaI(Ph)(thf)4],[10] [CaI(Tol)-
(thf)4],[10] and [CaI(b-Naph)(thf)4][17] were prepared according to
known procedures and recrystallized from THF prior to use. Com-
pounds 1 and 4 were synthesized as preliminarily reported.[12] The
calcium content of the products was determined by complexomet-
ric titration of a hydrolyzed aliquot with 0.05m EDTA by using Erio-
chrome BlackT as an indicator.[26] In the case of complexes contain-
ing chelating ligands, prior wet-chemical acid digestion with boil-
ing HNO3 was necessary.
Conclusion
A straightforward strategy towards diarylcalcium derivatives
was developed. To observe these products arylcalcium iodides
were reacted with potassium tert-butanolate in THF under
elimination of insoluble KI. Intermediate arylcalcium tert-buta-
nolate dismutates immediately and an insoluble precipitate,
mainly consisting of calcium bis(tert-butanolate) formed and
were removed by filtration. Isolation of crystalline THF adducts
of diarylcalcium succeeded by cooling of a concentrated THF
solution in the case of the naphthyl derivatives [Ca(a-Naph)2-
(thf)4] (1) and [Ca(b-Naph)2(thf)4] (2). Although this strategy is
also suitable for the phenyl derivative, the addition of multi-
dentate azabases was found to be superior for the isolation of
well-defined crystalline products, such as [Ca(Ph)2(tmeda)]2 (4),
the closely related tolyl derivative [Ca(Tol)2(tmeda)]2 (3),
[Ca(Ph)2(pmdta)(thf)] (5), and [Ca(hmteta)(Ph)2] (6). The last two
complexes show bent C-Ca-C moieties with bond angles of
111.1 and 109.78, respectively, and have to be considered the
first mononuclear octahedral arylcalcium derivatives with a cis-
alignment of the anionic ligands. Furthermore, addition of
[18]crown-6 ether to solutions of diphenylcalcium in THF gave
sparingly soluble [Ca([18]C-6)(Ph)2] (7) with an octa-coordinat-
ed calcium center.
Synthesis of [Ca(b-Naph)2(thf)4] (2)
[CaI(b-Naph)(thf)4] (1.45 g, 2.50 mmol) was dissolved in THF (25 mL)
and potassium tert-butanolate (281 mg, 2.50 mmol) was added.
The resulting suspension was stirred for half an hour at ambient
temperature. Thereafter, the formed precipitate was removed by
filtration by using a Schlenk frit covered with diatomaceous earth.
The volume of the filtrate was reduced to a half by distillation
under reduced pressure and the remaining solution was stored at
ꢀ408C for 5 days. The formed yellow crystals were isolated by fil-
tration and were dried in vacuo. The crystals lost non-coordinated
1
THF upon drying. Yield: 213 mg of 1 (0.37 mmol, 29.2%); H NMR
(400 MHz, [D8]THF): d=1.78 (m, 16H; CH2 thf), 3.63 (m, 16H; OCH2
thf), 7.07 (m, 2H; CH naphthyl), 7.14 (m, 2H; CH naphthyl), 7.32 (d,
2H; CH naphthyl), 7.52 (m, 4H; CH naphthyl), 7.95+8.08 (brm, 2H;
CH naphthyl), 8.14+8.28 ppm (brm, 2H; CH naphthyl);
13C{1H} NMR (100.6 MHz, [D8]THF): d=25.2 (8C; CH2 thf), 67.3 (8C;
OCH2 thf), 122.2 (2C; CH naphthyl), 122.4 (2C; CH naphthyl), 123.4
(2C; CH naphthyl), 127.3 (2C; CH naphthyl), 127.8 (2C; CH naph-
thyl), 132.8 (2C; quart. C naphthyl), 133.9 (2C; quart. C naphthyl),
139.4 (2C; CH naphthyl), 140.7 (2C; CH naphthyl), 189.0 ppm (2C;
C-Ca); an additional incomplete set of very broad signals was ob-
served: d=121.9, 123.2, 139.9, 141.6 ppm; elemental analysis calcd
(%) for C36H46CaO4 (582.84 gmolꢀ1): Ca 6.88; found: Ca 7.00. Suita-
ble crystals for X-ray diffraction experiments of the composition
[Ca(b-Naph)2(thf)4]·2THF were obtained by recrystallization in THF
and cooling to ꢀ408C.
The application of tmeda as a ligand revealed that diphenyl-
calcium complexes are not necessarily isostructural to phenyl-
calcium iodides and calcium diiodides that adopt identical
structures in the presence of a variety of neutral donor ligands.
As in the case of the related tolyl compound 3, the use of
tmeda yielded a dinuclear phenyl derivative of the formula
[Ca(Ph)2(tmeda)]2 (4) instead of a mononuclear complex. The
coordination sphere of the two calcium atoms in these com-
pounds is best described as square pyramidal with two bridg-
ing phenyl groups that connect two neighboring pyramids by
a shared edge of the basal face. As a consequence of this
unique structure the solubility of these dinuclear complexes in
hydrocarbons, such as benzene and toluene, is greatly en-
hanced. This feature might help to overcome the dependency
of the arylcalcium chemistry on solvents like THF and THP
which limit the long-term stability of these reagents due to
subsequent decomposition reactions.
Synthesis of [Ca(Tol)2(tmeda)]2 (3)
[CaI(Tol)(thf)4] (2.72 g, 4.98 mmol) was dissolved in THF (40 mL) and
solid potassium tert-butanolate (559 mg, 4.98 mmol) was added to
the stirred colorless solution. The resulting slurry was stirred for
half an hour at 08C. Thereafter, the formed colorless precipitate
was removed by filtration using a Schlenk frit covered with dia-
tomaceous earth and discarded. The orange-colored filtrate was re-
duced to dryness in vacuo. The resulting orange-colored foam was
taken up in tmeda (20 mL) resulting in immediate precipitation (or
in some attempts crystallization) of a colorless solid. After 1 h at
ambient temperature, the reaction mixture was stored at ꢀ208C
overnight. Afterwards, the formed colorless solid was isolated by
filtration and dried in vacuum. The mother liquor was discarded.
Yield: 447 mg of 2 (0.66 mmol, 53.1%); 1H NMR ([D6]benzene,
200 MHz): d=1.47 (s, 8H; CH2 tmeda), 1.54 (s, 24H; CH3 tmeda),
2.39 (s, 12H; CH3 tolyl), 7.37 (AA’ part of an AA’BB’ spin system,
8H; m-CH tolyl), 8.53 ppm (BB’ part of an AA’BB’ spin system, 8H;
o-CH tolyl); 13C{1H} NMR ([D6]benzene, 50.3 MHz): d=21.9 (4C; CH3
tolyl), 45.1 (br, 8C; CH3 tmeda), 56.3 (br, 4C; CH2 tmeda), 128.3
(8C; m-CH tolyl), 134.0 (s, 4C; C tolyl), 142.1 (8C; o-CH tolyl),
181.4 ppm (4C; Ca-C tolyl); elemental analysis calcd (%) for
Experimental Section
General
All manipulations were carried out under an inert argon atmos-
phere by using standard Schlenk techniques. THF was dried over
KOH and distilled over sodium/benzophenone under an argon at-
mosphere; deuterated THF and benzene was dried over sodium,
degassed, and saturated with argon. The yields given are not opti-
mized. 1H and 13C{1H} NMR spectra were recorded on Bruker AC
200, AC 400, or AC 600 spectrometers. Chemical shifts are reported
in parts per million relative to Me4Si as an external standard. The
Chem. Eur. J. 2014, 20, 3154 – 3161
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C40H60Ca2N4 (677.104 gmolꢀ1): Ca 11.84; found: Ca 12.07. The
almost colorless crystals isolated directly from the reaction mixture
in one case were suitable for X-ray diffraction experiments.
CH3 hmteta), 54.2–59.2 (br, 6C; CH2 hmteta), 122.4 (s, 2C; p-CH
phenyl), 125.3 (s, 4C; m-CH phenyl), 142.0 (s, 4C; o-CH phenyl),
191.3 ppm (s, 2C; Ca-C phenyl); elemental analysis calcd (%) for
C24H40CaN4 (432.658 gmolꢀ1): Ca 9.14; found: Ca 9.26.
Synthesis of [Ca(Ph)2(pmdta)(thf)] (5)
Synthesis of [Ca([18]C-6)(Ph)2] (7)
[CaI(Ph)(thf)4] (1.66 g, 3.11 mmol) was dissolved in THF (12 mL) and
potassium tert-butanolate (349 mg, 3.11 mmol) was added. The re-
sulting suspension was stirred for half an hour at ambient temper-
ature. Thereafter, the formed precipitate was removed by filtration
using a Schlenk frit covered with diatomaceous earth. Toluene
(5 mL) and pmdta (3.5 mL, 16.76 mmol) were added to the filtrate.
Thereafter, THF was removed by distillation under reduced pres-
sure and the remaining solution was stored at ꢀ408C for one
week. The formed pale-orange crystals of the composition [Ca(Ph)2-
(pmdta)(thf)] were suitable for X-ray diffraction experiments. The
crystals were isolated by filtration and were dried in vacuo. Yield:
Potassium tert-butanolate (276 mg, 2.46 mmol) was added to a so-
lution of [CaI(Ph)(thf)4] (1.31 g, 2.46 mmol) in THF (7 mL). The re-
sulting suspension was stirred for half an hour at ambient temper-
ature. The formed precipitate was removed by filtration using
a Schlenk frit covered with diatomaceous earth. Colorless crystals
precipitated during diffusion of a 0.2m solution of [18]crown-6 in
THF in the filtrate at 08C. These crystals of the composition
[Ca([18]C-6)(phenyl)2] were suitable for X-ray diffraction experi-
ments. The colorless crystals were isolated by filtration and were
dried in vacuo. Yield: 195 mg of 5 (0.43 mmol, 35.0%); elemental
analysis calcd (%) for C24H34CaO6 (458.61 gmolꢀ1): Ca 8.74; found:
Ca 8.41. Because of the insolubility of the compound in common
organic solvents no NMR spectroscopic data could be obtained.
1
160 mg of 3 (0.36 mmol, 23.4%); H NMR ([D8]THF, 400 MHz): d=
1.81 (m, 4H; CH2 thf), 2.20 (s, 12H; CH3 pmdta), 2.26 (s, 3H; CH3
pmdta), 2.35 (m, 4H; CH2 pmdta), 2.47 (m, 4H; CH2 pmdta), 3.65
(m, 4H; OCH2 thf), 6.52–7.26 (brm, 6H; m- and p-CH phenyl), 7.57–
8.50 ppm (brm, 4H; o-CH phenyl); 13C{1H} NMR ([D8]THF,
100.6 MHz): d=26.4 (s, 2C; CH2, thf), 43.4 (s, 1C; NCH3, pmdta),
46.3 (s, 4C; NCH3, pmdta), 57.3 (s, 2C; NCH2, pmdta), 58.8 (s, 2C;
NCH2, pmdta), 121–131 (very broad overlapping signals, 6C; CH
phenyl), 141–145 (very broad overlapping signals, 4C; CH phenyl),
189–194 ppm (very broad signals, 2C; Ca-C phenyl); elemental
analysis calcd (%) for C25H41CaN3O (439.70 gmolꢀ1): Ca 9.12; found:
Ca 9.03.
Structure determinations
The intensity data for the compounds were collected on a Nonius
KappaCCD diffractometer using graphite-monochromated MoKa ra-
diation. Data were corrected for Lorentz and polarization effects
but not for absorption effects.[27,28] The structures were solved by
direct methods (SHELXS[29]) and refined by full-matrix least-squares
techniques against Fo2 (SHELXL-97[29]). The hydrogen atoms of
compound 2 were located by difference Fourier synthesis and re-
fined isotropically. All other hydrogen atoms were included at cal-
culated positions with fixed thermal parameters. The crystal of 7
was a non-merohedral twin. The twin law was determined by
PLATON[30] to (ꢀ0.499, ꢀ0.013, ꢀ0.5/0.0, ꢀ1.0, 0.0/ꢀ1.502, 0.038,
Synthesis of [Ca(hmteta)(Ph)2] (6)
[CaI(Ph)(thf)4] (1.76 g, 3.31 mmol) was dissolved in THF (15 mL) and
solid potassium tert-butanolate (371 mg, 3.31 mmol) was added to
the stirred colorless solution. The
resulting suspension was stirred
for half an hour at 08C. Thereafter,
Table 3. Crystal data and refinement details for the X-ray structure determinations of the compounds 2, 3, and
5–7.
the formed colorless precipitate
was removed by filtration using
a Schlenk frit covered with dia-
tomaceous earth and discarded.
Toluene (10 mL) and hmteta
(762 mg, 3.31 mmol) were added
to the orange-colored filtrate by
syringe. To remove most of the
THF, the volume of the solution
was afterwards reduced to 10 mL
by distillation under reduced pres-
sure. Thereafter, the concentrated
solution was stored at ꢀ408C for
two days. The formed, almost col-
orless crystals of the composition
[Ca(hmteta)(phenyl)2] were suitable
for X-ray diffraction experiments.
The crystals were isolated by filtra-
tion and were dried in vacuo.
Compound
Formula
2
3
5
6
7
C36H46CaO4·2(C4H8O) C40H60Ca2N4
727.02
C25H41CaN3O
439.69
ꢀ140(2)
monoclinic
P21/c
8.4679(4)
11.6465(6)
26.0532(11)
90
98.79(3)
90
2539.2(2)
4
C24H40CaN4
424.68
ꢀ140(2)
monoclinic
P21/c
8.7260(3)
11.0921(3)
25.8776(8)
90
98.223(1)
90
2478.93(13)
4
C24H34CaO6
458.59
Fw [gmolꢀ1
T [8C]
]
677.08
ꢀ90(2)
monoclinic
P21/c
8.1649(9)
19.301(3)
12.8576(15)
90
93.173(8)
90
2023.1(5)
2
ꢀ140(2)
ꢀ140(2)
crystal system
space group
a [ꢁ]
b [ꢁ]
c [ꢁ]
Triclinic
triclinic
¯
¯
P1
P1
8.6069(2)
9.3903(2)
12.8338(4)
75.886(1)
79.536(1)
86.601(2)
989.08(4)
1
10.5018(13)
13.4564(14)
19.074(3)
102.566(5)
93.812(6)
111.010(5)
2424.9(5)
4
a [8]
b [8]
g [8]
V [ꢁ3]
Z
1 [gcmꢀ3
]
1.221
2.05
1.111
3.12
1.150
2.67
1.138
2.69
1.256
2.94
m [cmꢀ1
]
measured data
data with I>2s(I)
6692
4245
12821
1943
12611
4566
14436
4864
10971
7506
unique data (Rint
)
4404/0.0174
0.1718
0.0654
1.154
1.011/ꢀ0.474
969132
4560/0.1204
0.1721
0.0703
0.978
5411/0.0337
0.2174
0.0763
1.123
5562/0.0314
0.2225
0.0801
1.037
11024/0.0000
0.1675
0.0560
Yield: 263 mg of
36.8%);
4
(0.61 mmol,
([D8]THF,
wR2 (all data, on F2)[a]
R1 (I>2s(I))[a]
S[b]
1H NMR
400 MHz): d=1.9–3.2 (brm, 30H;
CH2 +CH3 hmteta), 6.72 (m, 2H; p-
CH phenyl), 6.86 (m, 4H; m-CH
phenyl), 7.97 ppm (m, 4H; o-CH
phenyl); 13C{1H} NMR ([D8]THF,
100.6 MHz): d=42.5–49.0 (br, 6C;
1.079
Res. dens./e ꢁꢀ3
CCDC no.
0.299/ꢀ0.245 0.776/ꢀ0.626 1.187/ꢀ0.648 0.672/ꢀ0.384
969133 969134 969135 969136
2
[a] Definition of the
R
indices: R1 =(Sj jFo jꢀjFc j j)/SjFo j ; wR2 ={S[w(Fo ꢀFc2)2]/S[w(Fo2)2]}1/2 with wꢀ1
=
2
s2(Fo2)+(aP)2+bP; P=[2Fc2+Max(Fo2)]/3; [b] s={S[w(Fo ꢀFc2)2]/(NoꢀNp)}1/2
.
Chem. Eur. J. 2014, 20, 3154 – 3161
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ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
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0.499). The contribution of the main component was refined to
0.482(1). All non-disordered, non-hydrogen atoms were refined ani-
sotropically.[29] Crystallographic data as well as structure solution
and refinement details are summarized in Table 3. XP (SIEMENS An-
alytical X-ray Instruments) was used for structure representations.
CCDC-969132 (2), 969133 (3), 969134 (5) 969135 (6), and 969136
(7) contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_re-
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Acknowledgements
This work was supported by the German Research Foundation
(DFG, Bonn, Germany). M.K. is grateful to the Fonds der Chemi-
schen Industrie (Frankfurt/Main, Germany) for a generous PhD.
stipend. We also appreciate the financial support of the Ver-
band der Chemischen Industrie (VCI/FCI, Frankfurt/Main, Ger-
many). Infrastructure of our institute was provided by the EU
(European Fonds for Regional Development, EFRE) and the
Friedrich Schiller University Jena.
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Keywords: calcium · diarylcalcium · direct synthesis · N
ligands · Schlenk-type equilibrium
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Received: November 12, 2013
Published online on February 12, 2014
Chem. Eur. J. 2014, 20, 3154 – 3161
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