Calcium stilbene complexes: Structures and dual reactivity

Article abstract of DOI:10.1039/c7cc05792j

Addition of a calcium hydride complex to diphenylacetylene gave a complex in which the stilbene dianion symmetrically bridges two Ca²⁺ ions. DFT calculations discuss the effect of the metal stilbene coordination. The stilbene complex reacts as a base (with H₂) or an electron donor (with I₂) and catalyzes the reduction of diphenylacetylene.

Full text of DOI:10.1039/c7cc05792j

ChemComm
View Article Online
View Journal
COMMUNICATION
Calcium stilbene complexes: structures and dual
reactivity†
Cite this:DOI: 10.1039/c7cc05792j
Andrea Causero, Holger Elsen, Gerd Ballmann, Ana Escalona and Sjoerd Harder
*
Received 25th July 2017,
Accepted 17th August 2017
DOI: 10.1039/c7cc05792j
rsc.li/chemcomm
Addition of a calcium hydride complex to diphenylacetylene gave a
complex in which the stilbene dianion symmetrically bridges two
Ca2+ ions. DFT calculations discuss the effect of the metal stilbene
coordination. The stilbene complex reacts as a base (with H2) or
an electron donor (with I2) and catalyzes the reduction of
diphenylacetylene.
The stilbene dianion [Ph(H)C–C(H)Ph]2ꢀ, abbreviated here as SD,
was synthesized already more than 100 years ago by Wilhelm
Schlenk,1 an extraordinary chemist who should not only be
regarded as the father of Schlenk techniques but also as a
pioneer of early main group organometallic chemistry.2
Schlenk and coworkers reacted stilbene with metallic sodium
to obtain Na2(SD) in the form of an extremely air-sensitive
brown-violet powder that in reaction with air emits a cloud of
stilbene vapor (Scheme 1).
The latter reactivity nicely demonstrates the electron storage
properties of this highly-charged dianion. The compound
Na2(SD) can be seen as a hydrocarbon-soluble form of sodium
metal3 that can be used as a tuneable reducing agent for the
degradation of halogenated organic pollutants (Scheme 1).4
Additionally, this multipurpose reagent may function as a double
nucleophile5 or a Brønsted base.6
Scheme 1 Synthesis and reactivity of Na2(SD).
Only a handful of metal stilbene complexes have been
structurally characterized. These have been obtained by four
different routes (Scheme 2) which include: (i) deprotonation
of 1,2-diphenylethane (e.g. by BuLiꢁTMEDA),7 (ii) reduction
of stilbene (e.g. by Cp*2SmII or [ArNC(Me)C(Me)NAr]AlII),8,9
(iii) combined stilbene reduction and salt-metathesis (e.g. by
Scheme 2 Synthetic routes to metal stilbene complexes.
successively adding Na and a lanthanide chloride complex)10,11 describe the synthesis, structure and reactivity of a calcium
or (iv) double addition of a metal hydride complex to diphenyl- stilbene complex by application of route (iv).
acetylene (e.g. by lanthanide hydride complexes).12,13 Herein we
We recently reported the solvent-free amidinate calcium
hydride complex [tBuAmDIPPCaH]214 (1, Fig. 1a) in which (N, aryl)-
coordination of the amidinate ligand saturates the large Ca2+
¨
Inorganic and Organometallic Chemistry, University Erlangen-Nurnberg,
coordination sphere to stabilize the complex; tBuAmDIPP
=
Egerlandstrasse 1, 91058 Erlangen, Germany. E-mail: sjoerd.harder@fau.de
† Electronic supplementary information (ESI) available: Experimental procedures,
selected 1H and 13C NMR spectra, selected IR spectra, crystallographic details, and
DFT calculations. CCDC 1560714–1560716. For ESI and crystallographic data in CIF
or other electronic format see DOI: 10.1039/c7cc05792j
tBuC[N(2,6-iPr-phenyl)]2. Reaction of 1 with diphenylacetylene
in benzene at 80 1C led to a color change from white to brown-red
and gave precipitation of a dark-red microcrystalline product
This journal is ©The Royal Society of Chemistry 2017
Chem. Commun.

View Article Online
Communication
ChemComm
backbone of the amidinate ligand, [AdAmDIPPCa]2(SD), shows
an essentially similar geometry (Fig. 1c and ESI;† AdAmDIPP
(1-adamantyl)C[N(2,6-iPr-phenyl)]2).
=
Apart from the Yb stilbene complex, not many structures are
known for comparison. Crystal structures of [Li(TMEDA)]2(SD)
and [Li(PMDTA)]2(SD) show similar connectivities.7 The metals
bridge the stilbene dianion with equal Li–C bond distances but
standard deviations are high on account of disorder. Also
lanthanide(III) metals show symmetrical bridging but an exact
bond analysis is plagued either by poor crystal quality (Sm)8 or
stilbene disorder (La, Y).10 A singular example of a bis-amide Al
complex in which stilbene bridges between Al centers shows
unequal Al–C distances (Fig. 1d).9 The stilbene fragment should
be described as the dianion PhCꢀ(H)–(H)CꢀPh with sp3-hybridized
C atoms that form 2-center–2-electron C–Al bonds of considerable
covalent character. Charge-localization on the central C atoms
by Lewis acidic Al3+ is also evident from long C–C and
C–Ph bonds.
The few available crystal structures of stilbene complexes
suggest that SD in predominantly ionic compounds interacts
symmetrically with metal cations and are highly delocalized
whereas more covalent complexes contain SD fragments with
localized negative charges. Given the lack of reliable structural
data, a series of metal stilbene complexes was studied by
computational methods. Centrosymmetric stilbene complexes
with group 1–3 metals of increasing electronegativity have been
optimized at the B3PW91//6-311++G** level (see the ESI† for
calculations on the full complex). Selected geometric parameters
and NPA charges are compared with the two extremes: SD and
PhCH2CH2Ph (Table 1).
Fig. 1 (a) Synthesis of [tBuAmDIPPCa]2(SD) (2). (b) Crystal structure of
[tBuAmDIPPCa]2(SD); hydrogen atoms have been omitted for clarity (except
those at the stilbene dianion). Selected bond distances (Å): Ca–N2 2.384(1),
Ca–C30 2.617(2), Ca–C300 2.591(2), Ca–C31 2.616(1), Ca–C36 2.860(2), and
Ca–Ar 2.750(1)–2.922(2) (average: 2.839(1)). (c) Comparison of bond distances
(Å) for [tBuAmDIPPCa]2(SD) (red), [tBuAmDIPPYb]2(SD) (blue) and [AdAmDIPP-
Ca]2(SD) (green). (d) Selected bond distances for an Al stilbene complex.
The C–C bond in SD is extremely short (1.388 Å) and close to
that of a standard CQC double bond (1.337(6) Å).20 Short Ph–C
bonds indicate a considerable transfer of electron density from
the benzylic Cs to the Ph groups which is supported by NPA
charge analysis. In fact, the Ph groups carry more than 90% of
the negative charge: q(Ph) = ꢀ0.904. Coordination of alkali
[tBuAmDIPPCa]2(SD). This synthetic route is comparable to that for metal cations leads to elongation of the C–C bond. This is due
the analogue YbII complex, [tBuAmDIPPYb]2(SD),12 underscoring the to the localizing effect of these cations that increases along the
great similarity between CaII and YbII chemistry.15
row K+ o Na+ o Li+ (i.e. with decreasing size and increasing
Due to the nearly equal ionic radii for Ca2+ and Yb2+,16 Lewis acidity). NPA analysis indeed shows a strong increase of
the Ca and Yb stilbene complexes crystallize isomorphically the negative charge on C with a concomitant decrease of q(Ph)
with closely related geometries. The centrosymmetric dimer along the same row K+ o Na+ o Li+. The bonding between SD
[tBuAmDIPPCa]2(SD) is composed of two tBuAmDIPPCa+ units and the alkali metals is quite ionic (80–90%) and metals bridge
that are symmetrically bridged by a stilbene dianion (Fig. 1b): the C–C bond symmetrically (Q defined as M–C/M–C0 E 1).
the Ca–C distances range from 2.591(2) to 2.617(2) Å. There are
additional attractive interactions between Ca2+ and the ring gives a larger variation in geometries. It is peculiar that
ipso and Cortho atoms. The H atoms at the stilbene dianion have although the ionicity of the metal-SD bonds and the negative
Bonding with group 2 metal cations Be2+, Mg2+ and Ca2+
C
been refined and the sum of the valence angles of 352(1)1 at charge on SD increase along the row Be2+ o Mg2+ o Ca2+, the
C30 indicates an essentially flat structure with sp2-hybridized highest q(C) is found in the Be complex. This rather unusual
carbon atoms. The double negative charge is significantly situation is due to the very strong localizing effect of the small
delocalized over the Ph substituents resulting in nearly equal and highly Lewis-acidic Be2+ cation that, like Al3+, polarizes
C30–C300 (1.453(2) Å) and C30–C31 (1.437(2) Å) bond distances. electron density from the Ph rings towards the central Cs. This
Also the very acute C–C–C angle at Cipso (115.4(1)1) and elongated leads to extensive repulsion and a very long C–C bond of 1.570 Å.
C
ipso–Cortho bonds (1.423(2)–1.426(2) Å) are tell-tales for the extensive
Hybridization at these C atoms is quantified by the sum of
delocalization of electron density into the Ph rings.17,18 The slightly the non-metal valence angles (Sangles) and changes from sp2
different complex with an adamantyl substituent in the (Ca) to sp3 (Be). The latter is also strongly reflected in the Ph–C
Chem. Commun.
This journal is ©The Royal Society of Chemistry 2017

View Article Online
ChemComm
Communication
Table 1 Comparison of stilbene geometries (distances in Å and angles in 1) and NPA charges (q)
Xa
C–C0
Ph–C
Sangles
M–C/M–C0 = Q
q(M)
q(SD)
q(C)
q(Ph)
b
Complex
SD
K2(SD)
Na2(SD)
Li2(SD)
(HCa)2(SD)
(HMg)2(SD)
(HBe)2(SD)
(H2Al)2(SD)
(H2B)2(SD)
PhCH2CH2Ph
—
1.388
1.459
1.467
1.488
1.487
1.543
1.570
1.529
1.531
1.543
1.428
1.414
1.426
1.433
1.437
1.466
1.506
1.503
1.526
1.507
360.0
358.2
354.6
354.4
354.7
341.3
332.2
335.2
332.2
331.4
—
—
ꢀ2
ꢀ0.272
ꢀ0.558
ꢀ0.560
ꢀ0.645
ꢀ0.695
ꢀ0.759
ꢀ0.853
ꢀ0.764
ꢀ0.283
ꢀ0.396
ꢀ0.904
ꢀ0.543
ꢀ0.432
ꢀ0.388
ꢀ0.337
ꢀ0.186
ꢀ0.062
ꢀ0.069
+0.022
ꢀ0.026
0.91
0.93
0.97
1.04
1.23
1.47
1.47
2.01
2.20
2.763/2.843 = 0.972
2.472/2.560 = 0.965
2.053/2.088 = 0.983
2.548/2.565 = 0.990
2.168/2.439 = 0.888
1.698/2.591 = 0.655
1.994/2.973 = 0.671
1.564/2.659 = 0.588
1.095/2.162 = 0.506
0.905
0.791
0.809
1.513
1.266
1.152
1.344
0.233
0.211
ꢀ1.810
ꢀ1.582
ꢀ1.618
ꢀ1.614
ꢀ1.436
ꢀ1.368
ꢀ1.192
ꢀ0.294
ꢀ0.422
a
Allred–Rochow electronegativity of the metal (or B or H).19 b Sum of the Ph–C–C, Ph–C–H and H–C–C valence angles defining the planarity/
hybridization of the C atom (3601 is planar/sp2 and 3271 is tetrahedral/sp3).
and C–C bond lengths as well as in the metal–SD bonding
which becomes less symmetric along the row Ca2+ (Q = 0.990) 4
Mg2+ (Q = 0.888) 4 Be2+ (Q = 0.655). Bonding of the SD unit with
group 3 elements B and Al is considerably more covalent with
ionicities of 15% (B) and 60% (Al). Both B and Al bridge asym-
metrically with Q values of 0.588 (B) and 0.671 (Al). In agreement
with the long (essentially single) C–C bonds, the central Cs are
hybridized close to sp3.
The geometry of the calcium model complex (HCa)2(SD)
Scheme 3 Reactivity of [tBuAmDIPPCa]2(SD) with I2 and H2 and the
catalytic cycle for alkyne hydrogenation.
closely resembles that of [tBuAmDIPPCa]2(SD) (Fig. 1b). Although
the diagonal relationship21 between Li and Mg would predict close
similarities between Li2(SD) and (HMg)2(SD), Table 1 clearly shows
that the Li and Ca stilbene complexes match better.
its further reaction with H2 back to the initial calcium hydride
The highly air-sensitive complexes [tBuAmDIPPCa]2(SD) and complex suggests that Ca-catalyzed alkyne hydrogenation
[AdAmDIPPCa]2(SD) are poorly soluble in C6D6 and partially may be possible (Scheme 3). With few exceptions,23 the latter
decompose into homoleptic species (see the ESI†). Addition process is generally catalyzed by noble transition metals
of hexane, however, regenerated crystals of the [RAmDIPPCa]2(SD) (Ru, Os, Ir, Rh).24 Indeed, nearly quantitative conversion of
complexes, demonstrating that decomposition is indeed due to diphenylacetylene into 1,2-diphenylethane could be achieved:
ligand scrambling. In contrast, attempted NMR measurements C6D6, 80 1C, 6 bar H2, 10 mol% cat, 48 h, 97% conversion (see
of [tBuAmDIPPYb]2(SD) resulted in exclusive hydrolysis of the ESI†). Considering the stoichiometric reactions discussed
PhCH2CH2PH and the amidine tBuAmDIPPH and therefore no above, a likely catalytic cycle involves double hydride addition
further reactivity has been described.12 The C6D6 solution of followed by double H2 deprotonation (Scheme 3, right). Although
[tBuAmDIPPCa]2(SD) shows a characteristic triplet at 5.88 ppm there is precedence for a frustrated Lewis pair catalyzed alkyne-to-
that may be assigned to the para-H of the SD ion. Its unusually alkene reduction,25 the current Ca-catalyzed conversion repre-
high-field shift is comparable to that in benzyl alkali metal sents the first transition metal-free alkyne-to-alkane reduction.
complexes22 and signifies considerable charge delocalization The further substrate scope and selectivity of this catalytic
into the Ph ring. The ambivalent nature of SD, which can be procedure are currently under investigation.
seen as either the dianion PhCꢀ(H)–(H)CꢀPh or the delocalized
In summary, double addition of a calcium hydride
dianion [Ph(H)CQC(H)Ph]2ꢀ, is also clear from its reactivity. complex to diphenylacetylene gave the calcium stilbene complex
In reaction with I2, SD acts as an electron reservoir leading to [tBuAmDIPPCa]2(SD) that closely resembles its Yb analogue
the formation of Ph(H)CQC(H)Ph (cis/trans: 10/90) and a [tBuAmDIPPYb]2(SD). The crystal structure of this compound con-
calcium iodide complex (Scheme 3). The latter crystallized as tains a highly delocalized stilbene dianion that is symmetrically
[tBuAmDIPPCaIꢁ(THF)2]2 (3) in 40% yield (see the ESI† for details bridged by Ca2+ ions. This bonding situation compares well to
and crystal structures). In reaction with H2, however, SD acts as that in ionic alkali metal stilbene complexes. DFT-calculations
a strong base yielding PhCH2CH2Ph and [tBuAmDIPPCaH]2 demonstrate close similarities of electronics and structures in
which could be isolated in a crystalline yield of 36%. Although di-lithio stilbene complexes Li2(SD) and (HCa)2(SD). The ambiva-
the reaction with I2 at low temperature (ꢀ30 1C) is also very fast, lent nature of the Ca stilbene complex is demonstrated by its
the latter reaction with H2 is much slower (6 bar, 80 1C, 2 days). reactivities with I2 and H2. In reaction with I2, the stilbene dianion
The high temperature needed results in a partial ligand functions as an electron reservoir for the reduction of I2 to 2Iꢀ,
exchange (Schlenk equilibrium) to give Ca(tBuAmDIPP
) as a giving stilbene as a side product. In this case, the Ca stilbene
2
side product. The double addition of a calcium hydride complex can be seen as a Ca(I) synthon, i.e. it reacts similar to
complex to diphenylacetylene to give [tBuAmDIPPCa]2(SD) and the hitherto hypothetical tBuAmDIPPCaI. Given the very negative
This journal is ©The Royal Society of Chemistry 2017
Chem. Commun.

View Article Online
Communication
ChemComm
reduction potential of SD (ꢀ2.72 V),26 its reducing power is similar
to that of Na. We are currently exploring the use of 2 as a synthon
in organocalcium chemistry. In reaction with H2, however,
it reacts as a double Brønsted base that deprotonates H2 giving
the initial calcium hydride reagent [tBuAmDIPPCaH]2 and the side
product 1,2-diphenylethane. The latter reactivity is the key to
catalytic alkyne hydrogenation. The first Ca-catalyzed alkyne-to-
alkane reduction demonstrates the growing scope and possibili-
ties of alkaline-earth metal catalysis.27
10 W. Huang, P. M. Abukhalil, S. I. Khan and P. L. Diaconescu, Chem.
Commun., 2014, 50, 5221–5223.
11 P. L. Diaconescu and C. C. Cummins, Inorg. Chem., 2012, 51,
2902–2916.
12 I. V. Basalov, D. M. Lyubkov, G. K. Fukin, A. S. Shaviryn and A. A.
Trifonov, Angew. Chem., Int. Ed., 2012, 51, 3444–3447.
13 J. Cheng, H. Wang, M. Nishiura and Z. Hou, Chem. Sci., 2012, 3,
2230–2233.
¨
14 A. Causero, G. Ballmann, J. Pahl, H. Zijlstra, C. Farber and S. Harder,
Organometallics, 2016, 35, 3350–3360.
15 S. Harder, Angew. Chem., Int. Ed., 2004, 43, 2714–2718.
16 Ionic radii for 6-coordinate cations: Ca2+ 1.00 Å; Yb2+ 1.02 Å.
R. D. Shannon, Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen.
Crystallogr., 1976, 32, 751.
´
17 S. Harder, C. Ruspic, N. Nı Bhriain, F. Berkermann and M. Schu¨rmann,
Conflicts of interest
Z. Naturforsch., 2008, 63b, 267–274.
18 D. Hoffmann, W. Bauer, F. Hampel, N. J. R. van Eikema Hommes,
P. von R. Schleyer, P. Otto, U. Pieper, D. Stalke, D. S. Wright and
R. Snaith, J. Am. Chem. Soc., 1994, 116, 528–536.
There are no conflicts to declare.
19 A. L. Allred, J. Inorg. Nucl. Chem., 1961, 17, 215–221.
20 Handbook of Chemistry & Physics, ed. R. C. Weast and M. J. Astle,
CRC Press, 63rd edn, 1982–1983, p. F-180.
21 D. F. Shriver and P. W. Atkins, Inorg. Chem., Oxford University Press,
UK, 2006, 4th edn, p. 259.
Notes and references
1 W. Schlenk, Chem. Ber., 1914, 47, 473.
2 T. T. Tidwell, Angew. Chem., Int. Ed., 2001, 40, 331–337.
3 W. Schlenk, Liebigs Ann. Chem., 1928, 463, 98–227.
4 (a) U. Azzena, L. Pisano, S. Antonello and F. Maran, J. Org. Chem., 22 S. Harder, Z. Anorg. Allg. Chem., 2010, 636, 2205–2211.
2009, 74, 8064–8070; (b) U. Azzena and M. Pittalis, Tetrahedron, 23 See for Fe catalysis: S. C. Bart, E. Lobkovsky and P. J. Chirik, J. Am.
2011, 67, 3360–3362; (c) U. Azzena, L. Pisano, F. Kondrot and
M. Pittalis, Appl. Organomet. Chem., 2012, 26, 180–184.
5 (a) J. G. Smith, Organometallics, 1983, 2, 1577–1582; (b) U. Azzena,
Chem. Soc., 2004, 126, 13794–13807.
´
24 R. A. Sanchez-Delgado and M. Rosales, Coord. Chem. Rev., 2000, 196,
249–280.
´
¨
G. Dettori, C. Lubinu, A. Mannu and L. Pisano, Tetrahedron, 2005, 25 K. Chernichenko, A. Madarasz, I. Papai, M. Nieger, M. Leskela and
61, 8663–8668. T. Repo, Nat. Chem., 2013, 5, 718–723.
6 U. Azzena, G. Dettori, L. Pisano and M. Pittalis, Tetrahedron, 2011, 26 trans-Stilbene measured in dimethylamine/[Bu4N+][Brꢀ]: K. Meerholz
67, 3470–3475.
and J. Heinze, J. Am. Chem. Soc., 1989, 111, 2325–2326.
7 M. Walczak and G. Stucky, J. Am. Chem. Soc., 1976, 98, 5531–5539.
8 W. J. Evans, T. A. Ulibarri and J. W. Ziller, J. Am. Chem. Soc., 1990,
112, 219–223.
9 Y. Zhao, Y. Lei, Q. Dong, B. Wu and X. J. Yang, Chem. – Eur. J., 2013,
19, 12059–12066.
27 See for reviews: (a) S. Harder, Chem. Rev., 2010, 110, 3852–3876;
(b) M. R. Crimmin and M. S. Hill, Alkaline-Earth Metal Compounds:
Oddities and Applications, in Topics in Organometallic Chemistry, ed.
S. Harder, Springer, 2013, ch. 5, vol. 45, pp. 191–241; (c) M. S. Hill,
D. J. Liptrot and C. Weetman, Chem. Soc. Rev., 2016, 45, 972–988.
Chem. Commun.
This journal is ©The Royal Society of Chemistry 2017