Syntheses and structure of the solvent-separated calcium cuprate [(thf)3Ca(μ-Ph)3Ca(thf)3] +[Ph-Cu-Ph]-

Article abstract of DOI:10.1021/om700353w

The transmetalation of phenylcopper yields the solvent-separated cuprate [(thf)₃Ca(μ-Ph)₃Ca(thf)₃] ⁺[Ph-Cu-Ph]^-. This derivative extends the known classes of organic calcium compounds by an organometa

Full text of DOI:10.1021/om700353w

Organometallics 2007, 26, 3269-3271
3269
Syntheses and Structure of the Solvent-Separated Calcium Cuprate
[(thf)3Ca(µ-Ph)3Ca(thf)3]+[Ph-Cu-Ph]-
Reinald Fischer, Helmar Go¨rls, and Matthias Westerhausen*
Institut fu¨r Anorganische und Analytische Chemie der Friedrich-Schiller-UniVersita¨t Jena,
August-Bebel-Strasse 2, D-07743 Jena, Germany
ReceiVed April 12, 2007
Summary: The transmetalation of phenylcopper yields the
solVent-separated cuprate [(thf)3Ca(µ-Ph)3Ca(thf)3]+[Ph-Cu-
Ph]-. This deriVatiVe extends the known classes of organic
calcium compounds by an organometallic complex cation, in
addition to the already well-known neutral diarylcalcium and
trialkylcalciate anions.
obtained. Therefore, aryl compounds of copper, which is nobler
than zinc, were used in this reaction.
The reaction of calcium metal, activated via dissolution in
liquid ammonia and drying under vacuum in the absence of
ammonia, with 2 equiv of phenylcopper in THF led to a copper
colloid and the formation of the sparingly soluble ionic
compound [(thf)3Ca(µ-Ph)3Ca(thf)3]+[Ph-Cu-Ph]- (1) in mod-
erate yield according to the idealized equation (1).11,12 The
Organic homoleptic cuprates of the type M[CuR2] with M )
Li, MgX are a very common class of organocopper reagents,
often called Gilman cuprates.1 These reagents can be prepared
via the metathesis reactions of copper(I) halides with arylmag-
nesium and alkylmagnesium halides or organolithium com-
pounds.2 Sometimes a catalytic amount of copper(I) iodide is
sufficient in order to form the Gilman cuprates as an intermedi-
ate. The addition of small or stoichiometric amounts of copper
salts to the reaction solutions can alter the regiochemistry, as
was observed in the conjugate addition reactions of R,â-
unsaturated esters and ketones.
5“CuPh” + 2Ca + 6THF f
[(thf)3Ca(µ-Ph)3Ca(thf)3]+[Ph-Cu-Ph]- + 4Cu (1)
insolubility of this aryl-rich cuprate in common organic solvents
prevented a further transmetalation and, hence, the formation
of very soluble diarylcalcium, which was accessible via the
Schlenk equilibrium from the direct synthesis of calcium with
iodoarenes.13-15
The lithium cuprates show very rich aggregation chemistry
Despite manifold investigations regarding various applica-
tions,2,3 solution structures,4,5 mechanistic studies,6 and the
generality and variety of the synthetic procedures of the Gilman
cuprates,7 information on the organocuprates of the heavier
alkaline earth metals is very limited. To our knowledge only
one example of such a derivative has been characterized
structurally. The reaction of ethynylsodium with CuI in liquid
ammonia gave the highly soluble Na2[Cu(CtCH)3], which was
transferred into the sparingly soluble calcium derivative via a
metathesis reaction with calcium nitrate.8 This compound
in solution5 and initiated numerous structural investigations.
(11) Synthesis: CuBr (5.0 g, 34.8 mmol) was suspended in 80 mL of
THF at 0 °C. A solution of 38.5 mL of a 0.91 M solution of phenylmag-
nesium bromide (34.58 mmol) in THF was dropped into the solution at 0
°C within 1 h, similar to a literature procedure.12 At this point the reaction
mixture was stirred for 2 h at room temperature. The pale yellow precipitate
was separated with a Schlenk frit. (Caution! Dry phenylcopper is explosive!)
The precipitate was thoroughly washed with three 50 mL portions of THF,
leaving 25 mmol of CuPh, which was suspended in 70 mL of THF. This
suspension was cooled to -78 °C and added to a flask at -78 °C containing
0.5 g of calcium powder (12.47 mmol) and approximately 1000 glass balls
with a diameter of 5 mm. The flask was shaken continuously while it became
gradually warmer. The color of the suspension changed to dark red-brown.
The gray calcium powder disappeared, and fine colorless crystals precipi-
tated. The reaction mixture was shaken for an additional 1 h at room
temperature and finally stored overnight at -20 °C. The colorless
microcrystalline precipitate was collected on a Schlenk frit, washed with
THF, and dried in vacuo. From the filtrate several single crystals of pure
product 1 crystallized at -40 °C. Yield: 4.6 g (76% based on employed
Ca) of fine crystals of crude [Ca2(µ-Ph)3(thf)6][CuPh2]. Purification dif-
ficulties arose from the insolubility of excess Ca, precipitated Cu, and
sparingly soluble CuPh and 1 as well as from the necessity to maintain a
THF-saturated atmosphere in order to prevent aging of the crystals.
Therefore, only a rather poor elemental analysis was obtained. (Anal. Found
for C54H73Ca2CuO6 (Mr ) 961.8): Ca, 8.84; Cu, 7.86. Calcd: Ca, 8.33;
Cu, 6.61.) After removal of all solid materials, the dark mother liquor was
treated overnight with a few grams of mercury in order to remove colloidal
copper. The dark color of the solution disappeared, and the amalgam was
removed. The volume of the solution was reduced to one-fourth of its
original volume and stored at -40 °C. Approximately 0.1 g of colorless
crystals of [{(thf)2Ca(Ph)Br}3‚MgO] (2) precipitated within a few days,
due to residual MgBr2 from the preparation of CuPh.
crystallized as the solvent-separated ion pair [Ca(NH3)6]2+
-
[Cu(CtCH)3]2- with Cu-C bond lengths of 2.031(7) Å and a
nearly trigonal-planar environment of the copper atom (C-
Cu-C ) 119.89(4)°).
Difficulties in obtaining diphenylcalcium prompted us to
consider transmetalation reactions. Reactions of the alkaline-
earth metals with bis[(trimethylsilyl)methyl]zinc led to trans-
metalation reactions; however, only the corresponding M[Zn-
(CH2SiMe3)3]2 species of calcium,9 strontium, and barium10 were
* To whom correspondence should be addressed. Fax: +49 (0) 3641
9-48-102. E-mail: m.we@uni-jena.de.
(1) Gilman, H; Jones, R. G.; Woods, L. A. J. Org. Chem. 1952, 17,
1630-1634.
(2) Heaney, H.; Christie, S. Sci. Synth. 2004, 3, 305-662.
(3) Krause, N.; Ebert, S.; Haubrich, A. Liebigs Ann./Recl. 1997, 2409-
2418.
(4) Nakamura, E.; Yoshikai, N. Organomet. News 2003, 32-35.
(5) Bertz, S. H.; Dabbagh, G. J. Am. Chem. Soc. 1988, 110, 3668-3670.
(6) Ullenius, C.; Christenson, B. Pure Appl. Chem. 1988, 60, 57-64.
(7) Wipf, P. Synthesis 1993, 537-557.
(12) Costa, G.; Camus, A.; Gatti, L.; Marsich, N. J. Organomet. Chem.
1966, 5, 508-572.
(13) Fischer, R.; Ga¨rtner, M.; Go¨rls, H.; Yu, L.; Reiher, M.; Wester-
hausen, M. Angew. Chem. 2007, 119, 1642-1647; Angew. Chem., Int. Ed.
2007, 46, 1618-1623.
(8) Cremer, U.; Disch, S.; Ruschewitz, U. Z. Anorg. Allg. Chem. 2004,
630, 2304-2310.
(9) Westerhausen, M.; Gu¨ckel, C.; Piotrowski, H.; Vogt, M. Z. Anorg.
Allg. Chem. 2002, 628, 735-740.
(14) Ga¨rtner, M.; Go¨rls, H.; Westerhausen, M. Synthesis 2007, 725-
730.
(10) Westerhausen, M.; Gu¨ckel, C.; Habereder, T.; Vogt, M.; Warchhold,
M.; No¨th, H. Organometallics 2001, 20, 893-899.
(15) Westerhausen, M.; Ga¨rtner, M.; Fischer, R.; Langer, J. Angew. Chem.
2007, 119, 1994-2001; Angew. Chem., Int. Ed. 2007, 46, 1950-1956.
10.1021/om700353w CCC: $37.00 © 2007 American Chemical Society
Publication on Web 06/05/2007

3270 Organometallics, Vol. 26, No. 14, 2007
Communications
16
Tetranuclear contact ion pairs in Li2Cu2R4 and solvent-
separated ions with oligonuclear cuprate anions are well-known.
In pentanuclear cuprates such as [Cu5Ph6]- the copper atoms
formed a trigonal bipyramid with Cu-C bond lengths of 1.99-
(2) and 2.19(2) Å to the equatorial and axial copper atoms,
respectively.17 One18 or both19 of the axial copper atoms can
be substituted by electropositive lithium or magnesium atoms.
A higher content of electropositive metals led to more open
structures of the higher order cuprates.20 Often not all halogen
atoms are substituted by aryl groups, and heteroleptic cuprates
such as [Cu5(C6H4CHdCH2)2Br4]- crystallized.21 The cyanide,
which can be regarded as a pseudohalide, bridges the cations
and anions, thus leading to chain structures.22 The diphenyl-
cuprate anion can be part of the solvent-separated [Li(12-crown-
4)2][Ph-Cu-Ph] (average Cu-C ) 1.925(10) Å)23 or contact
ion pairs such as [Li2Cu(aryl)2Br] (average Cu-C ) 1.941(4)
Å; aryl ) C6H4-2-CH2N(R)CH2CH2NR2 with R ) Me, Et).24
In the chemistry of solvent-separated organocuprates cited
above, the aryl groups were bound to the copper atoms and the
electropositive cations (lithium and magnesium) were coordi-
nated by ether molecules or halide anions, thus forming, for
Figure 1. Molecular structure of [(thf)3Ca(µ-Ph)3Ca(thf)3]+[Ph-
Cu-Ph]- (1). The ellipsoids represent a probability of 40%; H
atoms are omitted for clarity reasons. Selected bond lengths (Å)
and angles (deg): Ca-O1 ) 2.404(2), Ca-O2 ) 2.414(2), Ca-
O3 ) 2.418(2), Ca-C1 ) 2.625(2), Ca-C1A ) 2.613(2), Ca-
C7 ) 2.605(3), Cu-C23 ) 1.910(3), Ca‚‚‚CaA ) 3.1799(9); Ca-
C1-CaA ) 74.76(7), Ca-C7-CaA ) 75.2(1), C23-Cu-C23A
) 180.0(2), C2-C1-C6 ) 113.4(2), C8-C7-C8A ) 112.5(3),
C24-C23-C28 ) 114.1(3).
example, [Li(thf)4]+,17 [Li4Cl2(OEt2)10]2+ 19
,
[Mg(thf)6]2+, and
[Mg(thf)5Cl]+.21
The molecular structure of 1 consists of solvent-separated
ions and is shown in Figure 1.25 The dinuclear cation contains
the hexacoordinate calcium atoms in distorted-octahedral en-
vironments with bridging phenyl groups and terminally bound
THF molecules which leads to a facial arrangement of the
substituents. Due to the bridging position of the phenyl groups,
the Ca-C bond lengths vary between 2.605(3) and 2.625(2) Å
and are slightly larger than those observed for arylcalcium
iodides. The C-Ca-C angles have values in the rather narrow
range 84.80(9)-87.98(7)°, and the Ca-C-Ca bonds can be
described as three-center-two-electron bonds. Neglect of the
THF ligands leads to a pyramidal arrangement of the hydro-
carbyl ligands at calcium, which was also observed for the tris-
[bis(trimethylsilyl)methyl]calciate anion with a trigonal-pyra-
midal environment of the three-coordinate calcium atom, similar
to the case for the isotypic Yb derivative.26 In this anion, the
Ca-C distances display values between 2.474(4) and 2.556(4)
Å and C-Ca-C angles between 107.9(1) and 116.1(1)°. With
this anion in mind, the cation of 1 can also be regarded as a
pyramidal triphenylcalciate coordinating to another calcium
atom, leading to a Ca‚‚‚Ca distance of 3.1799(9) Å; the vacant
coordination sites are occupied by THF molecules. Comparable
structures are unknown for the magnesium derivatives. In [Ph2-
Mg(µ-Ph)2MgPh2]2- the magnesium atoms display distorted-
tetrahedral coordination spheres.27 In oligomeric arylmagnesium
compounds, the molecular structures can also be derived
according to VSEPR rules.28,29
(16) (a) van Koten, G.; Jastrzebski, J. T. B. H.; Noltes, J. G. J.
Organomet. Chem. 1977, 140, C23-C27. (b) van Koten, G.; Noltes, J. G.
J. Am. Chem. Soc. 1979, 101, 6593-6599. (c) van Koten, G.; Jastrzebski,
J. T. B. H. J. Am. Chem. Soc. 1985, 107, 697-698.
(17) Edwards, P. G.; Gellert, R. W.; Marks, M. W.; Bau, R. J. Am. Chem.
Soc. 1982, 104, 2072-2073.
(18) Khan, S. I.; Edwards, P. G.; Yuan, H. S. H.; Bau, R. J. Am. Chem.
Soc. 1985, 107, 1682-1684.
(19) Hope, H.; Oram, D.; Power, P. P. J. Am. Chem. Soc. 1984, 106,
1149-1150.
(20) (a) Olmstead, M. M.; Power, P. P. J. Am. Chem. Soc. 1989, 111,
4135-4136. (b) Olmstead, M. M.; Power, P. P. J. Am. Chem. Soc. 1990,
112, 8008-8014.
The diphenylcuprate anion is strictly linear, due to crystal-
lographic inversion symmetry. The Cu-C distances with values
of 1.910(3) Å are slightly shorter than those observed for
solvent-separated [Li(12-crown-4)2][Ph-Cu-Ph].23
(21) Eriksson, H.; O¨ rtendahl, M.; Håkansson, M. Organometallics 1996,
15, 4823-4831.
(22) (a) Kronenburg, C. M. P.; Jastrzebski, J. T. B. H.; Spek, A. L.; van
Koten, G. J. Am. Chem. Soc. 1998, 120, 9688-9689. (b) Kronenburg, C.
M. P.; Jastrzebski, J. T. B. H.; van Koten, G. Polyhedron 2000, 19, 553-
555.
The magnesium halide from the metathesis reaction of CuBr
with PhMgBr has to be washed out thoroughly in order to avoid
side reactions. When MgBr2 was present during the transmeta-
lation reaction, not only did the sparingly soluble [(thf)3Ca(µ-
Ph)3Ca(thf)3][Ph-Cu-Ph] precipitate but also the black solution
still contained phenyl groups. After workup procedures,11
[{(thf)2Ca(Ph)Br}3‚MgO] (2) with an oxygen-centered MgCa3
tetrahedron precipitated at low temperatures in the shape of
colorless crystals. The oxygen stems from ether cleavage, as
described earlier.13,15 Heterobimetallic Mg-Ca organometallic
compounds are rare, and examples include R2N-Ca(µ-NR2)2-
Mg-NR2 with R ) SiMe3.30 The molecular structure of 2 is
(23) Hope, H.; Olmstead, M. M.; Power, P. P.; Sandell, J.; Xu, X. J.
Am. Chem. Soc. 1985, 107, 4337-4338.
(24) (a) Kronenburg, C. M. P.; Amijs, C. H. M.; Jastrzebski, J. T. B. H.;
Lutz, M.; Spek, A. L.; van Koten, G. Organometallics 2002, 21, 4662-
4671. (b) Kronenburg, C. M. P.; Jastrzebski, J. T. B. H.; Boersma, J.; Lutz,
M.; Spek, A. L.; van Koten, G. J. Am. Chem. Soc. 2002, 124, 11675-
11683.
(25) X-ray structure determination of 1: intensity data were collected
on a Nonius Kappa CCD diffractometer using graphite-monochromated Mo
KR radiation. Data were corrected for Lorentz-polarization and for
absorption effects. Crystallographic data as well as structure solution and
refinement details for 1: C54H73Ca2CuO6, Mr ) 961.82, colorless prism,
size 0.04 × 0.04 × 0.03 mm3, monoclinic, space group C2/c, a ) 22.5330-
(7) Å, b ) 11.2561(4) Å, c ) 20.6374(6) Å, â ) 91.917(2)°, V ) 5231.4-
(3) Å3, T ) -90 °C, Z ) 4, Fcalcd ) 1.221 g cm-3, µ(Mo KR) ) 6.59
cm-1, multiscan, minimum/maximum transmission 0.8898/0.9198, F(000)
) 2056, 18 399 reflections in h (-29 to +29), k (-14 to +12), l (-26 to
(26) Hitchcock, P. B.; Khvostov, A. V.; Lappert, M. F. J. Organomet.
Chem. 2002, 663, 263-268.
(27) Thoennes, D.; Weiss, E. Chem. Ber. 1978, 111, 3726-3731.
(28) Markies, P. R.; Schat, G.; Akkerman, O. S.; Bickelhaupt, F.; Smeets,
W. J. J.; van der Sluis, P.; Spek, A. L. J. Organomet. Chem. 1990, 393,
315-331.
+26), measured in the range 2.72° e θ e 27.51°, completeness θmax
)
99.5%, 6000 independent reflections, Rint ) 0.0545, 3943 reflections with
Fo > 4σ(Fo), 287 parameters, 0 restraints, R1(obsd) ) 0.0488, wR2(obsd)
) 0.1136, R1(all) ) 0.090, wR2(all) ) 0.1316, GOF ) 1.010, largest
(29) Wehmschulte, R. J.; Twamley, B.; Khan, M. A. Inorg. Chem. 2001,
40, 6004-6008.
difference peak/hole 0.547/-0.429 e Å-3
.

Communications
Organometallics, Vol. 26, No. 14, 2007 3271
With [(thf)3Ca(µ-Ph)3Ca(thf)3][Ph-Cu-Ph] the range of
organocalcium compounds has been extended: examples of
anionic [Ca{CH(SiMe3)2}3]-,26 neutral [(thf)3Ca(Mes)2],13 and
cationic [(thf)3Ca(µ-Ph)3Ca(thf)3]+ species have been structurally
characterized. The field of heavy Grignard reagents is gaining
generality, a requirement for the implementation of these highly
reactive compounds in synthetic organic and organometallic
chemistry.
Acknowledgment. We thank the Deutsche Forschungsge-
meinschaft (DFG, Bonn-Bad Godesberg, Germany) for generous
financial support. We also acknowledge the funding of the Fonds
der Chemischen Industrie (Frankfurt/Main, Germany).
Supporting Information Available: CIF files giving data
collection and refinement procedures as well as positional coordi-
nates of all atoms. This material is available free of charge via the
Internet at http:/pubs.acs.org. In addition, the data deposited at the
Cambridge Crystallographic Data Centre under CCDC-637913 (1)
and CCDC-642882 (2) contain the supplementary crystallographic
data, excluding structure factors; these data can be obtained free
of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or from
the Cambridge Crystallographic Data Centre, 12, Union Road,
Cambridge CB2 1EZ, U.K.; fax (+44) 1223-336-033; deposit@
ccdc.cam.ac.uk).
Figure 2. Molecular structure of [{(thf)2Ca(Ph)Br}3‚MgO] (2). The
ellipsoids represent a probability of 40%; H atoms are neglected
for clarity reasons. Symmetry-related atoms are marked with one
(-x + y - 2, -x - 1, z) or two primes (-y - 1, x - y + 1, z).
Selected bond lengths (Å) and angles (deg): Mg-C1 ) 2.238(5),
Mg-O1 ) 1.995(5), Ca-C1′ ) 2.779(5), Ca-O1 ) 2.169(1), Ca-
O2 ) 2.397(3), Ca-O3 ) 2.420(4), Ca-Br ) 2.968(1); C1-Mg-
C1′ ) 115.9(1), Mg-C1-Ca′′ ) 77.2(2), Mg-O1-Ca ) 98.5(1),
Ca-O1-Ca′ ) 117.85(6), Ca-Br-Ca′′ ) 77.07(3), Br-Ca-Br′
) 160.40(4), C2-C1-C6 ) 113.3(5).
shown in Figure 2.31 The phenyl groups bridge the hetero-
metallic Mg-Ca edges (Mg-C1 ) 2.238(5) Å, Ca-C1 )
2.779(5) Å), whereas the bromine atoms bridge the homo-
metallic Ca-Ca edges (Ca-Br ) 2.968(1) Å and Ca-Br′ )
2.995(1) Å). Due to strong electrostatic attractions, short Mg-
O1 (1.995(5) Å) and Ca-O1 (2.169(1) Å) bonds were observed,
whereas the Ca-O2 bond lengths to the THF molecules lie in
a characteristic region (Ca-O2 ) 2.397(3) Å and Ca-O3 )
2.420(4) Å). This strong electrostatic attraction leads to rather
small Mg‚‚‚Ca and Ca‚‚‚Ca contacts of 3.156(2) and 3.715(1)
Å, respectively.
OM700353W
(31) X-ray structure determination of 2: intensity data were collected
on a Nonius Kappa CCD diffractometer using graphite-monochromated Mo
KR radiation. Data were corrected for Lorentz-polarization and for
absorption effects. Crystallographic data as well as structure solution and
refinement details for 2: C42H63Br3Ca3MgO7, Mr ) 1064.20, colorless prism,
size 0.05 × 0.05 × 0.04 mm3, rhombohedral, space group R3c, a ) 19.5010-
(4) Å, b ) 19.5010(4) Å, c ) 22.1204(5) Å, V ) 7285.1(3) Å3, T ) -90
°C, Z ) 6, Fcalcd ) 1.455 g cm-3, µ(Mo KR) ) 28.65 cm-1, multiscan,
minimum/maximum transmission 0.6221/0.6846, F(000) ) 3288, 16 070
reflections in h (-23 to +25), k (-23 to +25), l (-24 to +28), measured
in the range 4.18° e θ e 27.46°, completeness θmax ) 99.5%, 3578
independent reflections, Rint ) 0.0518, 3036 reflections with Fo > 4σ(Fo),
156 parameters, 1 restraint, R1(obsd) ) 0.0433, wR2(obsd) ) 0.1049, R1-
(all) ) 0.0579, wR2(all) ) 0.1128, GOF ) 1.032, Flack parameter 0.28-
(30) Wendell, L. T.; Bender, J.; He, X.; Noll, B. C.; Henderson, K. W.
Organometallics 2006, 25, 4953-4959.
(1), largest difference peak/hole 0.727/-0.450 e Å-3
.