Selective formation of a supramolecular coordination complex in the nanometre scale with a ferrocene-based phospholane ligand

Article abstract of DOI:10.1039/d1cc03755b

A straightforward synthesis of the tetradentate phospholane ligand 1 is reported. The 2?:?1 [M?:?L] reaction of 1 with [AuCl(tht)] (tht = tetrahydrothiophene) resulted in the 4?:?2 [M?:?L] supramolecular coordination complex 2 where two ligands 1 are brid

Full text of DOI:10.1039/d1cc03755b

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Selective Formation of a Supramolecular Coordination Complex in
the Nanometre Scale with a Ferrocene-based Phospholane
Ligand†
Received 00th January 20xx,
Accepted 00th January 20xx
Reinhard Hoy,a Toni Grell,b Peter Lönneckea and Evamarie Hey-Hawkins*,a
DOI: 10.1039/x0xx00000x
A straightforward synthesis of the tetradentate phospholane ligand unexplored. Recently, we have shown that highly flexible bis-
1 is reported. The 2:1 [M:L] reaction of 1 with [AuCl(tht)] phospholane ligands based on solely aliphatic alkylene
(tht = tetrahydrothiophen) resulted in a 4:2 [M:L] supramolecular backbones react selectively with [AuCl(tht)] (tht
coordination complex 2 where two ligands 1 are bridging four tetrahydrothiophene) to give nanotubes, polymeric chains and
gold(I) cations. The formation of 2 can be rationalised via a 2:2 [M:L] dimetallamacrocycles, [Au2Cl2{μ-
=
geometrical analysis of the ligand. The coordination mode of the (C4H8P)(CH2)n(PC4H8)-κ2P,P’}2] (n = 5, 7, 9, 11).11 Using a bis-
gold atoms was evaluated based on a CSD search, revealing the phospholane, in which the central part of the alkylidene spacer
geometrical changes for a transition from linear to trigonal planar had been replaced by a ferrocenylene moiety resulted in
coordination environment.
formation of macrocyclic 1:1 [M:L] complexes.12 In both
complexes, the gold(I) cation is in the usual linear coordination
environment but additionally exhibits an interaction with the
chloride counterion ([2+1] coordination). When the number of
phospholane moieties is increased to three (tris-phospholane),
a coordination polymer of gold(I) chloride instead of a
molecular complex was obtained.13 Nevertheless, molecular
gold(I) complexes can be obtained with suitable tridentate
phosphines, as we have recently shown.14–16 To further explore
the coordination chemistry of polyphospholane ligands, we
have now expanded the ligand structure to a tetrakis-
phospholane (1) and have studied its coordination chemistry
with gold(I).
Supramolecular Coordination Complexes (SCCs), constructed
from polydentate organic ligands and transition metals by
reversible metal-ligand bonds, are a field of continuously
growing interest since they provide
a wide range of
applications, such as in sensing,1 inclusion,2 and catalysis3.
There are numerous examples for ferrocene-bearing SCCs, and
a review on this topic has recently been published by Yang and
co-workers, also giving a proposal for the classification of the
different types of SCCs.4 Even though there is a large variety of
ligands available, there are only few examples for SCCs
containing exclusively polyphosphine ligands. Phosphines
require a higher synthetic effort in handling and, due to rotation
around the P–C bond, various orientations of the phosphine
ligand and its lone pair of electrons have to be considered,
resulting in a lower predictability and selectivity of the formed
coordination complexes. One possible attempt to limit this
variety is to use gold(I) as metal cation, since it almost
exclusively favours a linear coordination mode. Literature
provides examples of selectively formed SCCs in the shape of
macrocycles, tetrahedra, helices and polymeric structures
based solely on phosphine ligands and gold(I).5–9
Scheme 1. Structural formula of ligand 1 and schematic representation of the observed
supramolecular 4:2 [M:L] coordination complex 2.
Even though phospholane ligands possess unique properties for
chiral catalysis and are also referred to as privileged ligands,10
their coordination chemistry with gold(I) remains largely
Scheme 1 shows the structural formula of 1 which was prepared
in
a
good overall yield starting from 1,3,5-tris{p-
(diethoxyphosphonylmethyl)phenyl}benzene and 4-bromo-2,6-
diethylbenzaldehyde (details in SI). In order to obtain an SCC
where every gold atom is coordinated by two phosphorus
atoms ([M:L] ratio 2:1), ligand 1 was reacted with two
equivalents of [AuCl(tht)] in dichloromethane (DCM). Crystals of
a. Faculty of Chemistry and Mineralogy, Institute of Inorganic Chemistry, Leipzig
University, Johannisallee 29, 04103 Leipzig, Germany
b. Dipartimento di Chimica, Università degli Studi di Milano, Via Camillo Golgi 19,
20131 Milano, Italy.
c. † Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
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complex 2 could be obtained from the resulting dark red within
Journal Name
a
few minutes, as confirmed by powder XRD
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DOI: 10.1039/D1CC03755B
solution by gas-phase diffusion within a few days (details in SI). measurements, showing only an amorphous phase. This
1
Complex 2 crystallised in the triclinic space group P with one amorphous substance was subsequently dried in vacuum and
molecule in the unit cell (Z = 1) and is located on a further analysed. Elemental analysis showed a C and H content
crystallographic inversion centre. X-ray analysis revealed the which corresponds to complex 2 with 3 DCM molecules. This
formation of a 4:2 [M:L] coordination complex [Au4Cl4(1)2], in was also confirmed by MS-coupled TGA/DSC measurements
which two ligands 1 are bridging four gold(I) cations, thus (Fig. S6) which showed a mass loss of 5% until 198 °C
resulting in a cage structure (Scheme 1, Figure 1). The complex (theoretical 5.3% for 3 DCM), and fragments with m/z 84, 86
has a remarkable size as demonstrated by diameters (distance and 88 (3 isotopic compounds due to presence of 35Cl and 37Cl)
between marginal atoms) of about 3.8 nm (Fe1···Fe1’) and could be detected.
3.3 nm (Au2···Au2’). According to the classification of Yang et
al.,4 complex 2 represents an exo- as well as edge-functionalised
bis-ferrocenyl-based metallacycle. Even though several
complexes are known where two gold(I) atoms are coordinated
by two identical bis-phosphine ligands (see discussion vide
infra), to our knowledge 2 represents the first complex where
two tetrakis-phosphine ligands are coordinating four gold(I)
cations forming an SCC.
Figure 2. Detailed view of the coordination sphere of Au1 and Au2 in complex 2.
Hydrogen atoms and disorders in the phospholane units at P3 and P4 are omitted for
clarity. Selected bond lengths and distances [Å] and angles [°]: Au1–P1 2.296(2), Au1–P4
2.295(2), Au2–P2 2.295(2), Au2–P3 2.306(2), Au1–Cl1 2.882(2), Au2···Cl1 3.309(2),
Au2···Cl2 3.108(3), P1–Au1–P4 168.4(1), P2–Au2–P3 179.0(1), P1–Au1–Cl1 94.2(1), P4–
Au1–Cl1 92.7(1), Cl1···Au2···Cl2 107.0(1).
Figure 1 displays the molecular structure of complex 2. The
cyclopentadienyl rings of the ferrocenylene (fc) moieties
deviate significantly from coplanarity (Θ = 3.4°, Scheme 1) and
furthermore show a slight torsional twist from an ecliptic
conformation (τ = 4.2°) causing the two Y-shaped branches of
the ligand (Scheme 1) to be slightly shifted and instead of being
perfectly superimposed. Even though these conformational
changes may be rather small they become substantial for the
phospholane units due to the size of the branches of the ligand
and thus set the right geometrical conditions for the
coordination of the four gold(I) atoms. Both crystallographically
independent gold(I) atoms are coordinated by two phosphorus
atoms of two different ligands with Au–P bond lengths in the
typical range of about 2.30 Å (Table 1).11,12 Figure 2 shows a
detailed view of the Au coordination sphere. The P–Au–P bond
angles are 179.0(1)° and 168.4(1)° which clearly indicate a linear
coordination environment of the gold(I) atoms but in case of the
latter points to further interactions. In fact, the positions of the
chloride ions show a correlation as the Au–Cl distance for the
gold atom with the smaller P–Au–P bond angle is much shorter
(Au1–Cl1 2.882(2)). This is conspicuously long, but was already
observed for similar coordination compounds.11,12 Conversely,
the Au–Cl distance of the gold atom with almost perfectly linear
P–Au–P bond angle is much longer (Au2···Cl2 3.108(3)) and can
only be considered as an ionic interaction. A similar distance is
observed to Cl1 (Au2···Cl1 3.309(2)) which thus bridges both
gold atoms. The Au1···Au2 distance is about 6.0 Å and therefore
clearly excludes the presence of aurophilic interaction.
Figure 1. Molecular structure of 2. Hydrogen atoms and disorders of ethyl groups as well
as phospholane rings are omitted for clarity.
The close arrangement of the rigid Y-shaped branches of the
ligand in 2 creates a large cavity inside the cage structure. These
cavities form channels along the crystallographic [211] direction
(Fig. S1) with an approximately rectangular size of about 2 Å x 5
Å. This arrangement in the crystal structure is stabilised by
multiple intermolecular π-π-stacking interactions between the
aromatic moieties of the supramolecular complexes. In these
solvent-accessible channels, merely eight very poorly defined
DCM molecules could be located during the X-ray structure
refinement. Using the SQUEEZE routine implemented in
PLATON for the final refinement (details in SI) indicated the
presence of an additional 20 DCM molecules (thus 28 in total)
which are completely disordered within these channels.
When removed from solution, crystals of 2·28DCM rapidly lose
the co-crystallised solvent, thereby also losing crystallinity
This relationship between the P–Au–P bond angle and the Au–
Cl distance is often observed in three-coordinate gold(I) bis-
2 | J. Name., 2012, 00, 1-3
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phosphine complexes. While the so-called T-shaped or [2+1] significantly from linearity as was also speculatedVpierweAvritiocluesOlnyl.in1e8
DOI: 10.1039/D1CC03755B
coordination mode of Au2 observed in 2 represents one end of When examining individual structures, it seems in fact that
the spectrum (with P–Au–P close to 180° and weak interaction intermolecular interactions, namely hydrogen bonds, are
with the halide), the trigonal planar coordination environment responsible for stabilising a halide position further away from
in compounds such as [AuCl(PPh3)2]17 (with a P–Au–P bond the Au atom.18–20 This is also true for the molecular structure of
angle far from 180° and a short gold-chloride bond) represents 2·28DCM where the remote halide Cl2 forms two weak
the other end. In contrast, the coordination environment of the hydrogen bonds of type CH–Cl with H atoms of the phospholane
second gold atom in 2 (Au1) and in fact of numerous other groups. Furthermore, due to its vicinity to the void filled with
complexes reported in the literature lie between both these solvent molecules, interactions with the DCM molecules can
extremes. For example, Bowmaker et al.18 described different also be assumed.
polymorphs for complexes [AuX(PCy3)2] (X = Cl, Br, and I) where In Figure 3 (bottom), the Au–X distance is plotted versus the
the gold-halide bond length varies strongly. Based on their data, respective Au–P distances. Obviously, there is neither any
they proposed a linear dependency between the P–Au–P bond dependency at all between the two geometrical parameters nor
angle and the Au–X distance and furthermore explained the any influence of the type of halide. The Au–P distances vary
variation of the halide position with a stabilisation through within a close range about 2.3 Å. The reason could be relativistic
weak interactions in the crystal structures, such as weak effects of gold22–24 as suggested by 197Au Mößbauer
hydrogen bonds. As no such dependency is observed in spectroscopy of selected complexes;21 the Au–P bonding
compound 2, we have conducted a comprehensive search in the orbitals have largely s-character, Au–X mainly p-character,
Cambridge Structural Database (CSD) to ascertain to what involving only negligible π-acceptor interactions.
extent these findings can be applied to related gold(I) bis- Interestingly, when the Au–X distance falls below a critical value
phosphine complexes with an {AuP2} motif involving a halide (X lying between 2.334 and 2.437 Å for Cl (no cases for Br and I),
= Cl, Br, and I) as a third donor atom. The search and subsequent the coordination environment rearranges to a distorted T-
individual inspection of the results (comprehensive details in SI) shaped or distorted trigonal-planar mode involving one very
gave 232 hits in 165 structures of which the P–Au–P bond angle, long Au–P distance. This is observed for merely three
Au–X, Au–P as well as potential Au···Au distances were complexes25,26
,
[AuCl(L)]
with
L
=
1,2-bis[bis(2-
analysed.
methylphenyl)phosphino]benzene (VIJHAA) and [(AuCl)n({cyclo-
(P4tBu3)}2)] with n = 2, 3 (SUCYOH, SUCYUN) even though more
results can be found when other atoms than halides are
included as a third donor27,28. For all the findings mentioned
here, there seems to be no systematic influence of aurophilic
interactions, although commonly present (Section S5, Fig. S2
and S3, SI).
Figure 4. Schematic representation of the angles α and β in the two different
coordination modes – left: intramolecular, side view of the ligand; right: intermolecular,
top view onto the plain constructed by two bridging ligands. Only one half of the ligand
backbone is shown.
The comprehension of the Au coordination environment also
allows to better understand why of all theoretical possibilities
the SCC with the M:L [4:2] stoichiometry is formed. Accordingly,
the ligand geometry and the possible orientations of the lone
pairs of electrons at the phosphorus atom of the phospholane
moieties must be taken into account. Coordination of the gold(I)
atoms occurs along the direction of the lone pairs of electrons
of the donor atoms (Figure 4, top). Therefore, the Caryl–P–Au
bond angle α is typically found in the range of 113 to 114° (in 2
115.31° and 118.81°) if no steric hindrance is present. The
Figure 3. Results of the CSD search. Plot of the P–Au–P bond angles versus the Au–X
distances (top) and plot of the Au–P bond lengths versus the Au–X distances (bottom).
In Figure 3 (top), the plot of Au–X distances versus P–Au–P bond
angles demonstrates that the P–Au–P angle approaches 180°
for longer Au-X bonds or distances. This dependency deviates
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thermodynamically most favoured P–Au–P angle β is expected
to be 180°. Even though several complexes with trigonal-planar
coordination environment are known, a linear coordination
mode is still by far the most favoured for gold(I).24,29,30 As can be
understood from Figure 4 (left), in a hypothetical M:L [2:1]
complex both arms of ligand 1 at 1,1´-position of the ferrocene
unit would be oriented in parallel fashion to each other to bind
the single Au atom (intramolecularly) thus resulting in an angle
β of around 132° (180° – 2·|α–90°|, internal angle sum
4
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6
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triangle‡). The ligand backbone of
1 allows no free
conformational changes (e.g., a rotation of the fc unit, dihedral
angle Θ) that would change this coordination angle on gold(I)
which is far from the favoured linear arrangement and
therefore highly unfavourable. In contrast, this is avoided in the
M:L [4:2] complex 2 by coordinating the gold atoms by two
8
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deviates much less from ideal linearity. Furthermore, the two 12 A. Schmied, A. Straube, T. Grell, S. Jähnigen and E. Hey-
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Conclusions
In conclusion, a new tetradentate tetrakis-phospholane ligand
1 was synthesised and used to create a new M:L [4:2] SCC (2)
with gold(I) in the nanometre scale with Au in a linear
coordination environment. The realisation of the M:L [4:2]
stoichiometry instead of [2:1] can be rationalised by careful
consideration of the geometrical parameters of the ligand.
Furthermore, a comprehensive CSD search on related gold(I)
complexes has shown that with increasing gold-halide
distances, the P–Au–P bond angles approach 180° while the Au–
P bond lengths are usually unaffected. These results are
generally applicable for three-coordinated gold(I) halide
complexes.
Notes and references
‡ For calculation of the P–Au–P bond angle β, an isosceles triangle
between these three atoms is considered. The sum of angles
(always 180°) is composed of β and two identical angles γ. In case
of the hypothetical M:L [2:1] complex the P–C bond is
perpendicular to the P···P connection line and therefore, γ can be
obtained by subtracting 90° from α. Instead, in the idealised M:L
[4:2] complex the trigonal arrangement of the ligand causes a
relative angle of 120° between these two elements and thus
necessitates subtracting 120° from α to obtain γ.
1
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30 M. A. Carvajal, J. J. Novoa and S. Alvarez, J. Am. Chem.
Soc., 2004, 126, 1465–1477.
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