Synthesis and structural characterization of the dinuclear beryllium species [Be2Cl2(μ-Cl)2(PCy3) 2]

Article abstract of DOI:10.1515/znb-2011-0109

The synthesis and full characterization of [Be₂Cl ₂(μ-Cl)₂(PCy₃)₂], which results from the reaction of [Pd(PCy₃)₂] and BeCl₂ with concomitant precipitation of elemental palladium, is reported.

Full text of DOI:10.1515/znb-2011-0109

Synthesis and Structural Characterization of the Dinuclear Beryllium
Species [Be2Cl2(µ-Cl)2(PCy3)2]
Holger Braunschweig and Katrin Gruß
Institut fu¨r Anorganische Chemie, Julius-Maximilians-Universita¨t Wu¨rzburg,
Am Hubland, 97074 Wu¨rzburg, Germany
Reprint requests to Prof. Dr. Holger Braunschweig. Fax: (+49) (0)931 / 888-4623.
E-mail: h.braunschweig@mail.uni-wuerzburg.de
Z. Naturforsch. 2011, 66b, 55 – 57; received October 6, 2010
The synthesis and full characterization of [Be2Cl2(µ-Cl)2(PCy3)2], which results from the reaction
of [Pd(PCy3)2] and BeCl2 with concomitant precipitation of elemental palladium, is reported.
Key words: Beryllium, Dinuclear Compound, Lewis Base, Palladium, X-Ray Diffraction
Introduction
BeCl2, resulting in the formation of a dinuclear beryl-
lium species on a different reaction pathway.
Due to the toxicity of beryllium compounds, the
chemistry of beryllium is far less developed than
that of its neighboring elements [1, 2]. As beryllium-
containing materials feature unique properties, most
of the corresponding research is done in material sci-
ences [3]. With regard to its toxicity, additional work
is focused on the coordination chemistry of Be(II)
in aqueous solutions [4, 5]. Thus, tetrahedral four-
coordinate Be species, which result from the coordi-
nation of ligands containing main group substituents,
are well established [6 – 10]. However, the chemistry
of transition metal-beryllium interactions was limited
to cluster compounds, particularily to examples con-
sisting of Zr [11, 12].
Results and Discussion
The reaction of [Pd(PCy3)2] and BeCl2 was con-
ducted under similar conditions as applied to the
synthesis of 1. Thus, a toluene solution of the pal-
ladium complex was treated with a slight excess
of BeCl2 and heated to 80 ◦C. The reaction was
monitored by 31P NMR spectroscopy, revealing a
new signal at 33.3 ppm, which is slightly highfield
shifted with regard to that of the starting material
(39.2 ppm). Completion of the conversion, though,
required additional BeCl2 – two equivalents in to-
◦
tal – and extended heating to 80 C, after which
Based on previous success with the facile forma-
tion of “metal-only” Lewis pairs between electron-rich
Pt0 complexes and p-block metals, e. g. in the case of
[(Cy3P)2Pt–AlCl3] [13] and [(Cy3P)2Pt–GaCl3] [14],
we sought to extend this rather unusual bonding pattern
to Lewis-base adducts between d- and s-block metals.
BeCl2 as a strong Lewis acid proved to be a promis-
ing starting material, and reaction of [Pt(PCy3)2] with
BeCl2 in benzene resulted in the platinum beryllium
adduct [(Cy3P)2Pt–BeCl2] (1) comprising an unprece-
dented, electron precise bond between beryllium and a
d-block metal [15]. Recent studies showed that related
low-valent palladium complexes also show a propen-
sity to act as metal bases towards metal-coordinate bo-
ryl and borylene ligands, and therefore behave simi-
lar to their platinum congeners [16 – 19]. In the present
paper we report on the reaction of [Pd(PCy3)2] with
a grey solid precipitated from the yellow-green so-
lution, which we assumed to be elemental palla-
dium. Be NMR spectroscopy of the new compound
9
gave a broad signal at 12.7 ppm, which is compara-
ble to three-coordinate beryllium compounds such as
ArBeCl(OEt2) (Ar = C6H3-2,6-Mes2) (12.8 ppm), and
thus more deshielded than corresponding adducts com-
prising beryllium in coordination number four, e. g.
BeCl2(OEt2)2 and Be{[N(SiMe3)]2CPh}2, which ex-
hibit resonances at 2.6 and 5.5 ppm, respectively [20].
It should be noted though, that in case of the corre-
9
sponding Pt-Be adduct 1 no Be NMR signal could
be detected due to unresolved coupling to platinum
and phosphorus nuclei. The aforementioned spectro-
scopic data as well as the required twofold excess of
BeCl2 already indicate that [Pd(PCy3)2] does not form
a Lewis base adduct with BeCl2 in analogy to its plat-
c
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56
H. Braunschweig – K. Gruß · The Dinuclear Beryllium Species [Be2Cl2(µ-Cl)2(PCy3)2]
that in [Be2Cl6]2− (95.6(1)◦). Somewhat surprising,
a CCDC search provided no information as to other
structurally characterized beryllium chloride species
of the type trans-(L)ClBe-(µ-Cl)2-BeCl(L) (L = neu-
tral donor), wherein one donating ligand stabilizes a
tetrahedral beryllium center. However, similar struc-
tural motifs are known from d-block metal species
such as the corresponding dinuclear mercury com-
pound [Hg2(Cl)2(µ-Cl)2(PCy3)2] [23] and the palla-
dium complex [Pd2(Cl)2(µ-Cl)2(PCy3)2] [24]. How-
ever, the latter compound displays the expected square-
planar geometry at the palladium centers.
In conclusion, we have shown that the reaction of
[Pd(PCy3)2] with BeCl2 takes a completely different
course than that of the corresponding platinum phos-
phine species. In case of the former, BeCl2 abstracts
the phosphine ligands with formation of a dinuclear,
structurally rare Be-P adduct, without any indication
for the formation of a palladium-beryllium complex.
Presumably, this finding can be ascribed to a decreased
Lewis basicity of Pd in comparison to Pt.
inum congener. Indeed, after work up, 62 % of a col-
orless, crystalline material was isolated. X-Ray analy-
sis revealed the formation of the dinuclear phosphine
adduct [Be2Cl2(µ-Cl)2(PCy3)2] (2).
In the crystal, 2 displays C2h symmetry, and each
beryllium center is surrounded by a phosphine group,
two bridging and one terminal chloride substitu-
tent, thus exhibiting a distorted tetrahedral geometry
(Fig. 1).
˚
The Be–P bond length in 2 (1.932(2) A) is signifi-
cantly shorther than in a related bisphosphine complex
˚
of beryllium [BeCl2(Ph2PCH2PPh2)2] (2.206(3) A),
recently reported by Dehnicke et al. [21]. The P–Be–
Cl1 plane in 2 is orientated almost perpendicular
with respect to the central Be–Cl2–Be a–Cl2 a plane
(88.2◦). As to be expected, the exocyclic Be–Cl1 dis-
˚
tance of 1.932(2) A is shorter than the endocyclic
Experimental Section
˚
Be–Cl separations (Be–Cl2 2.088(2) A). Compari-
Safety note: in view of the toxicity of beryllium and its
compounds, all necessary safety measures were undertaken.
All reactions were carried out on a small scale, and for NMR
spectroscopy we used exclusively J. Young NMR tubes. The
glassware was cleaned separately, and all waste was collected
in suitable containers.
son to the structurally related (Ph4P)2[Be2Cl6] reveals
˚
many similarities [22]. Thus, the terminal (1.952(3) A)
˚
and the bridging Be–Cl bonds (2.102(3) A) in the latter
species are only slighthly longer than those in 2. Like-
wise, the central four-membered ring in 2 displays an
angle of 97.13(9)◦ (Cl2–Be–Cl2 a), which resembles
General considerations: All manipulations were per-
formed under an inert atmosphere of dry argon using ei-
ther standard Schlenk-line or glovebox techniques. Toluene
was distilled over sodium and stored over molecular sieves
prior to use. C6D6 was dried over molecular sieves and de-
gassed by three freeze-pump-thaw cycles before use. Anhy-
drous BeCl2 was purchased from Aldrich, [Pd(PCy3)2] was
prepared according to known methods [25]. The NMR spec-
tra were recorded on a Bruker Avance 500 (1H: 500.13 MHz;
13C: 125.76 MHz; 31P: 202.45 MHz; 9Be: 70.28 MHz) FT-
1
NMR spectrometer. 1H and 13C{ H} NMR spectra were
referenced to external TMS via the signal of the residual
protons of the solvent (1H) or via the solvent itself (13C).
1
31P{ H} NMR spectra were referenced to 85 % H3PO4,
9Be NMR spectra to an aqueous solution of BeCl2.
Di-µ-chloro-trans-dichloro-bis[tricyclohexylphosphine]-
diberyllium (2)
Fig. 1. Molecular structure of [Be2Cl2(µ-Cl)2(PCy3)2] (2).
Displacement ellipsoids are at the 50 % probability level.
Symmetry related positions (−x+1, −y, −z+1) are marked
A small excess of BeCl2 (2.8 mg, 0.035 mmol) was
added to a pale-yellow solution of [Pd(PCy3)2] (20 mg,
0.030 mmol) in toluene (0.4 mL). The reaction was heated
for 18 h at 80 ◦C. A second portion of BeCl2 (2.0 mg,
˚
with a. Selected bond lenghts (A) and angles (deg): Be–
Cl1 1.932(2), Be–P 2.216(2), Be–Cl2 2.088(2); P–Be–Cl1
114.47(10), P–Be–Cl2 110.54(10), Cl2–Be–Cl2 a 97.13(9).
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H. Braunschweig – K. Gruß · The Dinuclear Beryllium Species [Be2Cl2(µ-Cl)2(PCy3)2]
57
0.025 mmol) was added◦, and the reaction mixture was again monochromatized MoKα radiation. The structure was solved
heated for 18 h at 80 C to complete the conversion. No using Direct Methods, expanded using Fourier techniques
ligand exchange was observed in solution. Palladium pre- and refined with the SHELX software package [28]. All
cipitated as a dark-grey solid from the yellow-green solu- non-hydrogen atoms were refined anisotropically. Hydro-
tion, and after filtration the latter was layered with hex- gen atoms were assigned idealized positions and were in-
ane. After slow evaporation in a glovebox at r. t., 2 was cluded in structure factor calculations. Crystal data for 2:
obtained as colorless crystals (13 mg, 62 %). The crystals C42H72Be2Cl4P2, Mr = 798.76, colorless block, 0.28×0.1×
were redissolved in C6D6 for spectroscopic characteriza- 0.1 mm3, monoclinic space group P21/c, a = 15.583(1),
◦
tion. – 1H NMR (500.13 MHz, C6D6): δ = 1.26 (br s, 18H, b = 8.2539(6), c = 18.4827(13) A, β = 112.999(1) ,
˚
3
1.21 g cm−3
,
˚
Cy), 1.75 – 1.61 (m, 30H, Cy), 2.11 – 2.05 (m, 18H, Cy). –
V
=
2188.3(3) A ,
Z
=
2, ρcalcd
=
13C NMR (125.76 MHz, C6D6): δ = 26.55 (s, C4, Cy), 27.81 µ = 0.4 mm−1, F(000) = 860 e, T = 168(2) K, R1 = 0.0441,
(virtual triplet, N [26] = 11 Hz, C2, C6, Cy), 31.45 (s, C3, wR2 = 0.0911 for 4281 independent reflections [2θ ≤
C5, Cy), 34.14 (virtual triplet, N [27] = 18 Hz, C1, Cy). – 52.02◦] and 226 refined parameters, ∆ρ (max / min) =
1
−3
.
31P{ H} NMR (202.46 MHz, C6D6): δ = 33.25. – 9Be NMR 0.495 / −0.363 e A
˚
(70.28 MHz, C6D6): δ = 12.72 (br s). – C36H66Be2Cl4P2
(720.70): calcd. C 60.00, H 9.23; found C 59.19, H 8.58.
CCDC 795511 contains the supplementary crystallo-
graphic 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 request/cif.
X-Ray structure determination
Acknowledgement
The crystal data of 2 were collected on a Bruker
APEX diffractometer with CCD area detector and graphite-
Financial support by the DFG is gratefully acknowledged.
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