Synthesis and characterization of the hexagonal prismatic cage {THF?[PhB(CN)3]6[Cp*Rh]6} 6+

Article abstract of DOI:10.1039/b401505c

Condensation of [Cp*Rh(CH₃NO₂) _n]²⁺ and the tricyanoborate [PhB(CN)₃] ^- affords the hexagonal bipyramidal cage {[PhB(CN)₃] ₆[Cp*Rh]₆}⁶⁺, demonstrating that tetrahedral tricyanide building blocks can lead to novel cage structures.

Full text of DOI:10.1039/b401505c

Synthesis and characterization of the hexagonal prismatic cage
{THF7[PhB(CN)3]6[Cp*Rh]6}6+†
Matthew L. Kuhlman, Haijun Yao and Thomas B. Rauchfuss*
Department of Chemistry, University of Illinois at Urbana-Champaign, Urbana, IL, 6180, USA.
E-mail: rauchfuz@uiuc.edu
Received (in Cambridge, UK) 2nd February 2004, Accepted 29th April 2004
First published as an Advance Article on the web 18th May 2004
Condensation of [Cp*Rh(CH3NO2)n]2+ and the tricyanoborate
[PhB(CN)3]2 affords the hexagonal bipyramidal cage
{[PhB(CN)3]6[Cp*Rh]6}6+, demonstrating that tetrahedral tri-
cyanide building blocks can lead to novel cage structures.
11B NMR spectroscopy. Preliminary studies suggest that numerous
cyanoborate ligands can be prepared analogously.
The reaction of [PhB(CN)3]2 and MeNO2 solutions of
[Cp*Rh(CH3NO2)n]2+ (generated in situ from four equiv. AgOTf
and Cp*2Rh2Cl4) afforded a single major product, 1 (eqn. (1)). The
1H NMR spectrum consisted of one Cp* signal as well as one set of
multiplets for the phenyl region, with Ph/Cp* ratio ~ 1 : 1. ESI-MS
analysis of fresh CH3NO2 solutions indicated the that 1 (404 m/z)
Cages based on coordination frameworks with substantial interior
volumes are of topical interest for their potential in host–guest
behavior.1–5 The continuing, critical requirement in this area is for
predictable synthetic routes, and key here are modular precursors –
sometimes called tectons or building blocks that have geometrically
well-defined bridging tendencies.6–8 Through systematic modifica-
tion of these subunits, one can optimize conditions that favor cage
formation. Within this context, cyanide has long been recognized as
a particularly useful linking group, and cyanometallates are useful
building blocks for the synthesis of polymeric coordination solids.
These binary cyanometallates have not proven suitable for the
synthesis of molecular cages, but related mixed-ligand tricyanome-
tallates are reliable precursors to molecular cages,9–11 some of
which exhibit highly selective host–guest behavior.12,13 Character-
istically, the tricyanometallate approach gives rise to cuboidal or
box-like cages (Scheme 1). The box-like structures are a natural
consequence of the ~ 90° NC–M–CN angles in the tricyanome-
tallates. In an effort to develop new families of cyanometallate
cages, we sought tricyano building blocks where the NC–M–CN
angles are > 90°. Preliminary results described below demonstrate
the promise of this approach.
z+
undergoes fragmentation to numerous [PhB(CN)3]x-[Cp*Rh]y
4+
species such as [PhB(CN)3]6 [Cp*Rh]5
(547 m/z),
2+
2+
[PhB(CN)3]4[Cp*Rh]3 (689 m/z), and [PhB(CN)3]6[Cp*Rh]4
(974 m/z) although solvento adducts (e.g., {PhB(CN)3:
RhCp*(NCMe)x}+) were not observed. The IR spectrum of 1
revealed nCN at 2253 and 2207 cm21, vs. 2134 cm21 for
[PhB(CN)3]2, indicating all three CN units are coordinated.
(1)
Our attention was drawn to the recently reported coordination
solid {Ag[FB(CN)3]}, formed serendipitously via the reaction of
The aforementioned data support the formation of a cage but it
remained unclear if in fact the cage structure was cuboidal. X-ray
crystallographic analysis on crystals obtained from THF-MeNO2
AgNO3, NaF, and K[B(CN)4].14 In our initial approach we
2 15
examined the reaction of [Cp*Rh(NCMe)3]2+ with HB(CN)3
,
established that
1
consists of the hexacationic cage
which gave the adduct [H(CN)2BCN–RhCp*(NCMe)2]+ charac-
terized by ESI-MS (molecular ions for [H(CN)2B–CN–
RhCp*(NCMe)x]+, where x = 0, 1, 2). The inability of the
cyanoborate to compete with the MeCN solvent encouraged us to
modify the tricyanoborate and to avoid coordinating solvents. To
this end we prepared the new cyanoborate [K(18-c-6)]-
[PhB(CN)3], via the reaction of PhBCl2 and three equiv. of [K-
18-crown-6]CN in THF solution. This air stable, colorless salt was
obtained in analytically pure form after chromatographic purifica-
tion on alumina and was further characterized by ESI-MS, 1H, and
{[PhB(CN)3]6[Cp*Rh]6}6+ (Fig. 1)‡. The Rh6B6(CN)18 framework
in 1 is hexagonal prismatic. Two 18-membered Rh3B3(CN)6 rings
are inter-connected by six cyanides. The resulting cage has
idealized D3d symmetry. The structural results explain the observa-
tion of two bands for nm-CN, 12 of the cyanide ligands form the
hexagonal faces and the other six cyanides connect the two
hexagonal faces. The edge dimension (B–C–N–Rh) of the hexago-
nal prism is 4.8 Å yielding a volume of 287 Å3, whereas the free
volume or van der Waals’ volume of the hexagonal prism is
calculated (Cerius2 version 4.9) at 173 Å3, considerably large than
our previously described molecular boxes11 (free volume of ca. 57
Å3).
Both NMR spectroscopy and crystallography indicate the
presence of a molecule of THF at the interior of the cage, which
demonstrates its considerable volume. We have verified by 1H
NMR spectroscopy that THF can be removed from crystalline 1 in
vacuum; space-filling models suggests the THF can easily escape
through the hexagonal face (Fig. 2). THF is not however required
for the formation of the cage, 1 can be synthesized in neat MeNO2
solution.
Hexagonal prisms are ideally composed of 90° and 120° angles,
whereas the molecular building blocks feature 90° (N–Rh–N) and
109° (C–B–C) angles. The resulting strain is accommodated in a
number of ways. A slight chair-like ruffling of the two Rh3B3(CN)6
hexagons results in diamondoid distortions of the four-sided
Rh2B2(CN)4 rings (Fig. 1B.). The angles of C–B–C and N–Rh–N
that compose the hexagonal face have values typical for tetrahedral
and octahedral geometries, 109–106° and 93–85°, respectively, vs.
Scheme 1
† Electronic supplementary information (ESI) available: experimental
details for syntheses and crystallographic analyses. See http://www.rsc.org/
suppdata/cc/b4/b401505c/
1370
C h e m . C o m m u n . , 2 0 0 4 , 1 3 7 0 – 1 3 7 1
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

Fig. 1 Molecular structure of the cation in {THF7[PhB(CN)3]6[Cp*Rh]6}(OTf)6} and two perspectives of the inorganic framework. Thermal ellipsoids are
drawn at the 50% level.
b = 18.325(6), c = 13.871(9) Å, V = 4034(3) Å3, T = 193(2) K, space
group P3m1, Z = 1, m(MoKa, l = 0.71073 Å), m = 0.760 mm21, 31922
¯
data collected, 2741 unique (Rint = 0.069). Final R1 [I > 2s(I)] = 0.0406,
wR2 (all data) = 0.1362. CCDC 230644. See http://www.rsc.org/suppdata/
cc/b4/b401505c/ for crystallographic data in .cif or other electronic
format.
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120° for an idealized hexagon. The large difference is absorbed by
distortions in the B–C–N and especially Rh–N–C angles, which are
175° and 167°, respectively. The B–CN and Rh–NC distances of
1.61 Å and 2.09 Å, respectively, are typical.
The present work demonstrates that tricyanometallate building
blocks with C–M–C angles more open than 90° enable the
formation of a new generation of molecular cages. The potential to
vary the organic substituent in the organotricyano-borate building
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Notes and references
‡ Crystal data: for 1 (yellow hexagon prism 0.36 3 0.20 3 0.14 mm):
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C
118H128B6N18ORh66+6OTf·THF, M = 3463.23, trigonal, a = 18.325(6),
C h e m . C o m m u n . , 2 0 0 4 , 1 3 7 0 – 1 3 7 1
1371