Stepwise formation of a molecular square with bridging NH,O-substituted dicarbene building blocks

Article abstract of DOI:10.1021/ja205021p

The β,β′-bis(triisopropylsiloxy)phenyl-1,4-diisocyanide 3 and [Ir(Cp*)Cl₂]₂ were used for the stepwise assembly of the [Ir(Cp*)Cl] cornered molecular square [6](Cl)₄. Synthesis of the tetrakis(diisocyanide) bridged molecular square [Ir(Cp*)Cl(3)]₄(BF₄)₄ [5](BF ₄)₄ followed by cleavage of the O-Si(i-Pr)₃ bonds of the diisocyanide bridges with HCl/i-PrOH led to an intramolecular attack of the liberated hydroxyl groups at the isocyanide carbon atoms with formation of molecular square [6](Cl)₄ featuring four dicarbene linkers.

Full text of DOI:10.1021/ja205021p

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Stepwise Formation of a Molecular Square with Bridging NH,O-
Substituted Dicarbene Building Blocks
Fabian M. Conrady,† Roland Fr€ohlich,‡ Christian Schulte to Brinke,† Tania Pape,† and F. Ekkehardt Hahn*,†
†Institut f€ur Anorganische und Analytische Chemie, Universit€at M€unster, Corrensstrasse 30, D-48149 M€unster, Germany
‡Organisch-Chemisches Institut, Universit€at M€unster, Corrensstrasse 40, D-48149 M€unster, Germany
S Supporting Information
b
Scheme 1. Synthesis of Complex [2]
ABSTRACT: The β,β0-bis(triisopropylsiloxy)phenyl-1,
4-diisocyanide 3 and [Ir(Cp*)Cl2]2 were used for the stepwise
assembly of the [Ir(Cp*)Cl] cornered molecular square
[6](Cl)4. Synthesis of the tetrakis(diisocyanide) bridged
molecular square [Ir(Cp*)Cl(3)]4(BF4)4 [5](BF4)4 followed
by cleavage of the OꢀSi(i-Pr)3 bonds of the diisocyanide
bridges with HCl/i-PrOH led to an intramolecular attack of
the liberated hydroxyl groups at the isocyanide carbon atoms
with formation of molecular square [6](Cl)4 featuring four
dicarbene linkers.
he self-assembly of metallosupramolecular structures has
become a field of intensive research after Lehn et al.
T
demonstrated the spontaneous formation of dinuclear helicates
from bipyridine and CuI.1 Subsequently, a large number of
metallosupramolecular assemblies have been described.2 Most
of these compounds are built up from metal centers coordinated
by nitrogen and/or oxygen donor atoms of polydentate ligands,
and some have been used as molecular hosts for the encapsula-
tion and selective chemical transformation of small molecules.3
Molecular squares built from end-capped PdII and 4,40-bipyr-
idine building blocks have been among the first metallosupra-
molecular assemblies to be studied in detail by the groups of
Fujita,4 Stang,5 and others.6 Related architectures featuring poly-
dentate ligands with carbon donor atoms are rare although some
examples with bridging diisocyanide,7 acyclic diaminocarbene,8
or NHC ligands9 have been described.
Figure 1. Molecular structure of [2]; hydrogen atoms (except NH) are
omitted for clarity. The complex resides on a mirror plane passing
through atoms Ir and Cl. Selected bond lengths (Å) and angles (deg):
Ir1ꢀC1 2.004(3), Ir1ꢀCl 2.4244(13); C1ꢀIr1ꢀC1* 88.8(2).
(Scheme 1; see Supporting Information (SI)). The initially
formed diisocyanide complex was not isolated but instead treated
with KF/H2O which caused cleavage of the OꢀSiMe3 bonds
followed by intramolecular nucleophilic attack of the liberated
hydroxyl groups at the isocyanide carbon atoms with generation
of the di-NHC complex [2]. Complex [2] was identified by
1H NMR spectroscopy revealing the characteristic resonance for
the NꢀH proton at 10.04 ppm and by 13C NMR spectroscopy
showing the resonance for the heterocarbene C2 carbon atom
at 178.7 ppm.
An X-ray diffraction analysis (Figure 1) confirmed the forma-
tion of [2]. The metric parameters found in [2] fall in the range
previously described for IrꢀNHC complexes.20 The angle C1ꢀ
IrꢀC1* measures 88.8(2)° which is a perfect value for the
construction of a molecular square with {[Ir(Cp*)Cl(NHC)2}
N-Heterocyclic carbenes (NHCs) are CII donor ligands which
form stable MꢀC bonds.10 A number of poly-NHC ligands have
been described,11 but only recently have the first metallosupra-
molecular structures emerged which are held together by poly-
NHC ligands. Among these are molecular squares12 featuring
rigid ditopic benzobis-NHCs13 and 4,40-bipyridine linkers as well
as cylindrical assemblies obtained from macrocyclic14 and other
poly-NHC ligands.15
We have studied the metal template controlled transformation
of β-functionalized isocyanides into N,O-16 and N,N-substituted
NHC ligands.17 Here we present a novel β,β0-disubstituted
phenyl-1,4-diisocyanide,18 its use for the generation of a molec-
ular square featuring four bridging diisocyanide ligands, and the
template controlled intramolecular conversion of the bridging
diisocyanides into four bridging dicarbene ligands.
First we studied the conversion of two isocyanide ligands
into NH,O-NHCs at the iridium template by reacting [Ir(Cp*)-
Cl2]2 with 4 equiv of 2-trimethylsiloxyphenyl isocyanide 119
Received: May 31, 2011
Published: July 05, 2011
r
2011 American Chemical Society
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|

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Scheme 2. Synthesis of Complex [4]
Scheme 3. Synthesis of the Molecular Squares [5](BF4)4 and
[6](Cl)4
Figure 2. Molecular structure of one molecule of [4]; hydrogen atoms
are omitted for clarity. The complex resides on a crystallographic
inversion center. Selected bond lengths (Å) and angles (deg): IrꢀC1
1.918(5), IrꢀCl1 2.4097(13), IrꢀCl2 2.4039(13), C1ꢀN1 1.163(6),
Ir Ir* 11.5787(8); Cl1ꢀIrꢀCl2 89.88(5), Cl1ꢀIrꢀC1 87.13(14),
3 3 3
Cl2ꢀIrꢀC1 87.30(15), Ir1ꢀC1ꢀN1 177.1(5), C1ꢀN1ꢀC2 164.8(5).
building blocks. The two NHC ligands share one NH proton via
a NꢀH N hydrogen bond. Similar behavior, allowing the
3 3 3
formation of a neutral complex, has been observed before.16c,21
Next the β,β0-functionalized phenyl-1,4-diisocyanide 3 was
prepared starting from 2,5-diaminohydroquinone dihydrochlor-
ide (see SI). This diisocyanide could only be prepared with the
sterically demanding tris(isopropyl)silyl (TIPS) protection
groups instead of the trimethylsilyl groups used for 1. Reaction
of 3 with 1 equiv of [Ir(Cp*)Cl2]2 in CH2Cl2 yielded the
dinuclear complex [4] in almost quantitative yield (Scheme 2;
see SI). The progress of the reaction can be monitored by IR
spectroscopy where the wavenumber for the isocyanide stretch-
ing vibration shifts from υ~ = 2130 cmꢀ1 for free 3 to υ~ =
2152 cmꢀ1 for [4]. 13C NMR spectroscopy also confirmed the
formation of [4] showing the resonance for the isocyanide
carbon atom at 136.0 ppm compared to the corresponding signal
for the free ligand at 169.8 ppm.18
The dinuclear complex with just one bridging di-NHC ligand
was not studied further. Instead, the tetranuclear iridium com-
plex featuring four bridging diisocyanide ligands [5](BF4)4 was
synthesized from 2 equiv of [4] and 2 equiv of the diisocyanide 3
using the method described by Tatsumi for related diisocyanide-
bridged molecular squares (Scheme 3; see SI).7
Due to the cationic nature of complex cation [5]4+, the IR
spectrum shows the isocyanide stretching vibration at a slightly
higher wavenumber (υ~ = 2166 cmꢀ1) compared to the neutral
complex [4] (υ~ = 2152 cmꢀ1). The 13C NMR spectrum of
[5](BF4)4 reveals the resonance for the isocyanide carbon atom
at 124.6 ppm, slightly upfield from the equivalent resonance in
[4] (136.0 ppm). The HR-ESI mass spectrum shows a peak at
m/z = 1142.7933 for the molecular ion [[5](BF4)]3+ (calcd
1142.7920).
An X-ray diffraction analysis confirmed the composition of [4]
(the asymmetric unit contains two essentially identical indepen-
dent halves of [4]; only one molecule of [4] is shown in
Figure 2). Complex [4] features an IrꢀC1 separation of
1.918(5) Å and a slightly bent C1ꢀN1ꢀC2 (164.8(5)°) iso-
cyanide group. The Cp* ligands are oriented pseudo-trans to each
Dissolving complex [5](BF4)4 in a 5 M isopropanolic hydro-
gen chloride solution and stirring the mixture for 12 h at 100 °C
yielded the molecular square [6](Cl)4 as a yellow, air-stable solid
in excellent yield (Scheme 3). In this one-step reaction all eight
OꢀSi(i-Pr)3 bonds of [5](BF4)4 are cleaved and the generated
hydroxyl groups attack the isocyanide carbon atoms in their R-
position in an intramolecular nucleophilic fashion under forma-
tion of eight new NH,O-NHC donor groups (see SI). The result
is compound [6](Cl)4, the first molecular square exclusively
bridged by ditopic dicarbene ligands.
other, and the Ir Ir* separation measures 11.5787(8) Å.
3 3 3
Cleavage of the OꢀSi(i-Pr)3 bonds in [4] with tetrabutylam-
monium fluoride in fluorobenzene converts the bridging diiso-
cyanide ligand into a bridging di-NHC ligand featuring two NH,
O-substituted heterocarbene donors. The bridging dicarbene
ligand was detected by the characteristic resonances for the
NꢀH protons at 12.12 ppm and the resonance for the carbene
carbon atoms at 185.2 ppm which is similar to the chemical shift
for the equivalent resonance for the carbene carbon atoms in
complex [2] (178.7 ppm).
The formation of [6](Cl)4 was confirmed by NMR spectros-
copy. The 1H NMR spectrum shows the characteristic signal for
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NHC complexes in different oxidation states like the couples
FeII/FeIII and ReI/ReIII.
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental details for the
b
synthesis of all compounds and X-ray crystallographic files for
compounds [2], [4], and [6](Cl)4 in CIF format. This material is
available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
fehahn@uni-muenster.de
Figure 3. Molecular structure of [6]4+ in [6](Cl)4 with hydrogen
atoms, except for NꢀH, omitted for clarity. Selected bond lengths
(Å) and angles (deg): IrꢀC5 2.013(11), Ir1***ꢀC1 2.001(10), IrꢀCl1
2.399(3), C1ꢀO1 1.366(13), C1ꢀN1 1.321(4), C5ꢀO2 1.351(13),
’ ACKNOWLEDGMENT
The authors thank the Deutsche Forschungsgemeinschaft
(IRTG 1444 and SFB 858) for financial support.
C5ꢀN2 1.292(15); C1ꢀIr1***ꢀC5*** 85.7(4), Ir Ir 10.5000(9).
3 3 3
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