RuII and IrIII Complexes Containing ADA and DAD Triple Hydrogen Bonding Motifs: Potential Tectons for the Assembly of Functional Materials

Article abstract of DOI:10.1002/asia.201801748

The synthesis and characterisation of a series of [Ru^II(bpy)₂L] and [Ir(ppy)₂L] complexes containing ligands L with the potential to engage in triple hydrogen bonding interactions is described. L1 and L2 comprise pyridyl triazole chelating units with pendant diaminotriazine units, capable of donor-acceptor-donor (DAD) hydrogen bonding, while L3 and L4 contain ADA hydrogen bonding units proximal to N^N and N^O cleating sites, respectively. X-ray crystallography shows the L1 and L2 containing Ru^II complexes to assemble via R²₂(8) hydrogen bonding dimers, while [Ru^II(bpy)₂L4] assembles via extended hydrogen bonding motifs to form one dimensional chains. By contrast, the expected hydrogen bonding patterns are not observed for the Ru^II and Ir^III complexes of L3. Spectroscopic studies show that the absorption spectra of the complexes result from combinations of MLCT and LLCT transitions. The L1 and L2 complexes of Ir^III and Ru^II complexes are emissive in the solid state and it seems likely that hydrogen bonding to complementary species may facilitate tuning of their ³ILCT emission. Low frequency Raman spectra provide further evidence for ordered interactions in the solid state for the L4 complexes, consistent with the results from X-ray crystallography.

Full text of DOI:10.1002/asia.201801748

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Accepted Article
Title: Ru(II) and Ir(III) complexes containing ADA and DAD triple
hydrogen bonding motifs: Potential tectons for the assembly of
functional materials.
Authors: Aidan P McKay, Joseph I Mapley, Keith C Gordon, and David
Allan McMorran
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10.1002/asia.201801748
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Ru(II) and Ir(III) complexes containing ADA and DAD triple hydrogen
bonding motifs: Potential tectons for the assembly of functional materials.
Aidan P. McKay, Joseph I. Mapley, Keith C. Gordon and David A. McMorran*[a]
Abstract: The synthesis and characterisation of series of
[Ru(II)(bpy)2L] and [Ir(ppy)2L] complexes containing ligands L with the
potential to engage in triple hydrogen bonding interactions is
described. L1 and L2 comprise pyridyl triazole chelating units with
pendant diaminotriazine units, capable of donor-acceptor-donor
(DAD) hydrogen bonding, while L3 and L4 contain ADA hydrogen
bonding units proximal to N^N and N^O cleating sites, respectively.
X-ray crystallography shows the L1 and L2 containing Ru(II)
complexes to assemble via ꢀꢁꢁ(8) hydrogen bonding dimers, while
[Ru(II)(bpy)2L4] assembles via extended hydrogen bonding motifs to
form one dimensional chains. By contrast, the expected hydrogen
bonding patterns are not observed for the Ru(II) and Ir(III) complexes
of L3. Spectroscopic studies show that the absorption spectra of the
complexes result from combinations of MLCT and LLCT transitions.
The L1 and L2 complexes of Ir(III) and Ru(II) complexes are emissive
in the solid state and it seems likely that hydrogen bonding to
complementary species may facilitate tuning of their 3ILCT emission.
Low frequency Raman spectra provide further evidence for ordered
interactions in the solid state for the L4 complexes, consistent with the
results from X-ray crystallography.
crystalline compounds, or co-crystals of pairs of compounds, is
now an area of intense research.[5]
By comparison, the development of such understandings for triple
hydrogen bonding synthons are less explored, and, in particular,
reports of the assembly of transition-metal containing systems in
this way are rare.[6] We have recently reported examples of
Ni(II),[7] Pd(II) and Pt(II)[8] complexes incorporating deprotonated
1,5-diarylbiguanide ligands. These ligands possess a donor-
acceptor-donor (DAD) triple hydrogen bonding motif and co-
crystallization of these with organic molecules containing the
complementary acceptor-donor-acceptor (ADA) motif was shown
to result in the expected assemblies in the solid state.
Unfortunately, the complexes displayed limited solubility in
solvents other than DMF and DMSO, and this solubility was only
due to the hydrogen-bonding ability of the solvents to break up
the ꢀꢁꢁ(8)dimeric homosynthons that assembly the complexes
with each other in the solid state. However, these solvents also
compete with the complementary organic molecules for the
hydrogen bonding sites and so triple hydrogen bond-directed
heterosynthon assembly in solution was not observed. The metal-
biguanide complexes were neutral and all contained square-
planar metal centers, and we supposed that better solubility might
be obtained by moving to charged complexes containing
octahedral metal centers. To that end, and in light of their
desirable photophysical properties, we looked to prepare Ru(II)
and Ir(III) based complexes.
Introduction
Crystal engineering has been described as the understanding of
intermolecular interactions in the context of crystal packing and
the utilization of such understanding in the design of new solids
with desired physical and chemical properties.[1] While the reliable
prediction of the observed three-dimensional assembly of a given
compound, or pair of compounds, in the solid state, which
requires accounting for all the myriad weak interactions present
between the compound(s) (commonly called packing forces),
remains elusive, the use of relatively strong and directional
intermolecular forces as a driver for the assembly process
provides a way to limit the variability of the observed interactions.
For this reason hydrogen bonding has become the most popular
interaction studied.[2] The incorporation of synthons such as
carboxylic acid and amide groups leads to the observation of the
expected ꢀꢁꢁ(8)[3] dimers in a large percentage of the resulting
crystal structures and, for organic molecules at least, hierarchies
of double hydrogen bond synthons have been developed.[4]
Based on these developing sets of rules, modification of
properties such as solubility and mechanical strength of
The feasibility of using hydrogen bonding to mediate electron or
energy transfer between Ru(bpy)3 and Ir(ppy)3 – type systems has
been explored.[9] In particular, studies on Ru(II) and Os(II)-
containing bpy ligands derivatized by nucleobases have been
shown to form triple hydrogen bonded heterosynthons within
which the emission of the ruthenium center is quenched by the
osmium center.[10] Triple hydrogen bonding interactions have also
been shown very recently to be able to non-covalently tune the
emission properties of an Ir(III)-based system.[11] Another recent
report has also highlighted the ability of hydrogen bonding to
facilitate photocatalysis reactions in Ir(III) complexes.[12] There are
fewer reports concerning the incorporation of such centers into
functional materials. For example, while metal-organic
frameworks (MOFs) have become a much studied area of
materials chemistry, examples where Ru(bpy)3 and Ir(ppy)3 – type
centers have been incorporated into the MOF framework are
relatively rare,[13] although the few systems that have been
reported show that such incorporation leads to the MOF gaining
desirable photophysical and/or catalytic properties. It seemed to
us that the triple hydrogen-bond directed assembly of suitable
Ru(II)- and Ir(III)-containing tectons with complementary organic
or metal-containing species might provide an alternate way to
prepare solid state structures with interesting properties.
[a]
A. P. McKay, J. I. Mapley, Prof. K. C. Gordon, Dr. D. A. McMorran
Department of Chemistry Te Tari Hua Ruanuku
University of Otago
Dunedin, New Zealand
E-mail: davidm@chemistry.otago.ac.nz
Supporting information for this article is given via a link at the end of
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Herein we therefore describe a family of [Ru(bpy)2L] and
[Ir(ppy)2L] complexes where L is a bidentate ligand which
incorporates either a DAD or an ADA triple hydrogen bonding
motif. Ligands L1 and L2 incorporate a DAD diaminotriazine
(DAT) synthon[6c, 14] which is remote from the metal ion, whereas
L3 and L4 contain ADA motifs (based on hydantoin[15] and orotic
acid[16] synthons, respectively) which are more proximal to the
metal ion. Furthermore, L1 and L2 bind as neutral ligands to the
respective metal centers, whereas L3 binds as a monoanion and
L4 as a dianion: the resulting metal complexes therefore have
charges ranging from +2 to -1.
L1-d3 as product. The compositions of the new ligands were
confirmed by elemental analysis and HRESI-MS. H NMR were
1
consistent with the expected structures and the deuterated ligand
L1-d3 had absent triazole and methylene signals when compared
to L1, consistent with deuteration at these locations. NH signals
were observed as a broadened singlet at about 6.9 ppm. Both
ligands show limited solubility in common organic solvents so
DMSO-d6 was used for NMR characterisation.
(i)
R
(ii)
Br
CN
N
N
TMS
CN
CN
N
N
N
N
(iii)
NH2
NH2
N
R
N
R
N
N
N
N
N
N
N
N
N
N
N
N
NH2
NH2
L1 R = benzyl
L2 R = hexyl
L1 R = benzyl
L2 R = hexyl
O
O
Scheme 2. Syntheses of ligands L1 and L2. (i) TMS acetylene, [Pd(PPh3)2Cl2],
CuI, THF/DIPA, N2 (ii) (a) Na2CO3, MeOH (b) R-Br, NaN3, CuSO4, Na ascorbate,
DMF/H2O (iii) dicyandiamide, DMSO, 190oC (microwave).
O
NH
O
NH
HO
N
HN
HN
O
HL3
H2L4
Complexes of the type [Ru(bpy)2L] were prepared by the reaction
of a slight excess of the appropriate ligand with one equivalent of
[Ru(bpy)2Cl2] in ethanol which was heated to 125°C under sealed
tube microwave conditions for 2 hours. The cationic complexes of
L1 – L3 were precipitated from solution as their respective
hexafluorophosphate salts by addition of aqueous NH4PF6 and
then further purified through recrystallization from diffusion of
diethyl ether into acetonitrile solution giving orange (L1 – L2) or
red (L3) crystals. The complex of L4 is neutral and therefore could
not be precipitated with aqueous NH4PF6, and instead the ethanol
of the reaction solution was removed via rotary evaporation and
the resulting solid was recrystallized from a warm 1:5 solution of
methanol in water to give the pure product as black/purple crystals.
The compositions of the complexes were confirmed by HRESI-
MS, elemental analysis and 1H and 13C NMR spectroscopies. Due
to the unsymmetrical nature of the tertiary ligand the 1H NMR
spectrum contains individual peaks for all protons. The signals of
the non-bpy ligands were fully assigned by means of COSY,
HSQCAD, and HMBCAD 2D spectroscopies but the symmetry of
the two bipyridine ligands resulted in significant overlap of their
respective proton signals and with a lack of resolved NOSEY
cross peaks mean the individual protons could not be assigned
but conformation of the ligands could be inferred by the number
and types of signals present. The complexes of L1 – L3 had
reasonable solubility so CD3CN was used for NMR
characterisation (spectra in DMSO-d6 solutions contained
significant overlapping of signals, complicate analysis) while the
complex of L4 was much less soluble so DMSO-d6 was used for
characterisation. In acetonitrile the NH signals appear as
broadened singlets at about 5.5 ppm for complexes of L1 and L2
and at about 8.0 ppm for the complex of L3. In DMSO-d6 the NH
signal appears as a much sharper peak and in the case of the
complex of L4 a doublet at 9.76 ppm, which is coupling with the
other proton of the orotic acid ligand. The NH signals of L3 and
Scheme 1. The ligands discussed in this work.
We report here the characterisation of these complexes and an
assessment of their suitability as tectons for the construction of
functional assemblies.
Results and Discussion
The 5-(2-pyridylmethylene)hydantoin ligand, HL3, was prepared
as described in literature by reacting 2-pyridinecarboxaldehyde
with a slight excess of hydantoin in a piperidine solution,[15b] and
the orotic acid ligand, H2L4, was used as purchased. The two
pyridyl triazole based ligands L1 and L2 are new in this work and
were prepared through a three-step synthesis (Scheme 2). In
each case. 2-bromo-5-cyanopyridine was initially converted to the
trimethylsilane
protected
acetylene
through
standard
Sonogashira techniques, give the known product 2-(2-
trimethylsilyl)acetyl-5-cyanopyridine product which gave spectra
consistent with reported literature.[17] The benzyl and hexyl
substituted 1,2,3-triazoles were then synthesized from the
deprotected acetylene and the appropriately substituted azide,
which was formed in situ from the substituted bromide and sodium
azide, through a CuAAC type “click” reaction.[18] Finally the cyano-
group underwent cyclisation with dicyandiamide in DMSO at
190°C under sealed tube microwave conditions to give the 2,4-
diamino-1,3,5-triazine moiety as has been observed for other
cyano- substituted heterocycles.[19]
When the benzyl substituted pyridyltriazole underwent the
cyclisation reaction in deuterated solvent (DMSO-d6) exchange of
the triazole and methylene protons for deuterons occurred giving
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L4 are more deshielded than L1 and L2 due to both the proximity
of the carboxyl groups and coordination of the ruthenium ion to
the same ring as the amine moiety. While we might expect the
complexes to self-assemble via hydrogen bonding in solution, no
concentration dependence is observed in any of the complexes in
CD3CN or DMSO-d6. The complexes were not soluble in common
chlorinated solvents so the concentration dependence could not
be assessed in less hydrogen bonding solvents.
The absorption spectra for all complexes are shown in Figure 1
and it can be seen that the Ru complexes have a lower band gap
than their Ir counterparts. The spectra for [Ru(bpy)2(L1)]2+ and
[Ru(bpy)2(L2)]2+ are comparable to [Ru(bpy)2(pytri)]2+ which
displays an 1MLCT absorption at 440 nm, but has reduced
absorption between 340 and 390 nm.[21] This higher energy
absorption process, which is also present in the free ligand, is
attributed to a charge transfer transition from the DAT unit to the
Complexes of the type fac-[Ir(ppy)2L] were prepared from the
reaction of the [(Ir(ppy)2-μ2-Cl2] dimer with two and a half
equivalents of the appropriate ligand and an excess of sodium
carbonate in dry DMSO solution under Ar at 125°C for 6 hours.
The cationic complexes of L1 and L2 were precipitated from the
reaction solution by slow addition of aqueous NH4PF6 and the
isolated solid was purified by filtration through celite as a
dichloromethane solution and precipitated by addition of diethyl
ether to give the products as yellow solids. The neutral complex
of L3 was extracted from a solution of the DMSO reaction solution
and water:brine solution with chloroform and the combined
organic extracts were repeatedly washed with water to remove
residual DMSO. The resulting oil was taken up in minimum
chloroform and the product precipitated as an orange solid by
addition of excess petroleum ether. The anionic complex of L4
precipitated overnight as the tetrabutylammonium salt after the
addition of an aqueous solution of tetrabutylammonium chloride
and then acetone to the reaction solution. The compositions of the
complexes were confirmed by HRESI-MS, elemental analysis and
1H and 13C NMR spectroscopies. Due to the unsymmetrical nature
of the ligands the 1H NMR spectrum contains individual peaks for
all protons but as expected from the [(Ir(ppy)2-μ2-Cl2] dimer only
the fac- isomers of the complexes are observed where the
coordinated C atoms are trans to the tertiary ligand.[20] The proton
signals of all the ligands are assigned by means of COSY,
NOSEY, HSQCAD, and HMBCAD 2D spectroscopies with the
cross peaks between the coordinated carbon of the ppy ligands
and the protons of the same ring as well as to the proton para- to
the coordinated nitrogen allowing the full assignment of the
ligands (although the two ppy ligands cannot be differentiated).
pyridyl-triazole unit (Figure S1). At lower energy a weak
absorption is observed which is attributed to the spin-forbidden
3MLCT transition.[21a]
L3 and L4, unlike L1 and L2, differ from bpy due to the σ
coordinating nitrogen and, in the case of L4, coordinating oxygen.
This induces a splitting of the Ru d orbitals and a clear separation
of the MLCT transitions is observed. For [Ru(bpy)2(L4)] a
shoulder on the low energy MLCT is observed at 480 nm which is
attributed to the degeneracy of the Ru d orbitals being lifted. This
perturbation is not observed for [Ru(bpy)2(L3)]+ which is
consistent with calculations (vide infra)(Tables S1-S12). The
overall splitting is relatively similar for both ligands with a
difference in energy between the highest and lowest MLCT bands
of 0.99 eV for [Ru(bpy)2(L3)]+ and 1.04 eV for [Ru(bpy)2(L4)]. At
lower energies than the MLCT bands ligand to ligand charge
transfer (LLCT) transitions are observed for both [Ru(bpy)2(L3)]+
and [Ru(bpy)2(L4)].
The iridium complexes show better solubility than the ruthenium
complexes but DMSO-d6 was still used for NMR characterisation,
both for giving clearer, less overlapped, signals than other
solvents and to give sharper NH signals for assignment. For the
complexes of L1 and L2 these signals are still broadened and
both appear at about 6.4 ppm. For the complexes of L3 and L4
the NH signals appear as very sharp singlets at 10.17 and 9.52
ppm, respectively. As for the ruthenium complexes the complexes
of L3 and L4 have downfield shifted NH signals compared to L1
and L2 because of both the close proximity to carbonyl groups
and coordination of the metal ion to the same ring as the amine is
contained in. Also like the ruthenium complexes, no concentration
dependent peak shifting was observed in DMSO-d6 nor was it
observed in the less hydrogen bonding solvent CDCl3 although in
this solvent the NH peaks, which are the most likely to shift due
to self-assembly through hydrogen bonding, cannot be observed
likely due to proton exchange with the solvent.
Figure 1. UV-Vis spectra for all complexes. Ru complexes measured in DMSO
and Ir complexes measured in DCM.
The absorption spectra of [Ir(ppy)2(L1)]+ and [Ir(ppy)2(L2)]+
displays a weaker absorption spectrum as expected for Ir-ppy
systems.[22] The lowest energy transitions are assigned as spin
3
1
forbidden MLCT transitions with MLCT transitions occurring at
higher energies. The Ir to ppy MLCT transitions are not perturbed
compared to the previously reported methyl-pytri analogue
[Ir(ppy)2(Me-pytri)]+.[22c] For the Ir complexes with L3 and L4
ligands some spectral differences are observed. For [Ir(ppy)2(L4)]-
similar MLCT transitions are present with an additional transition
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occurring at 460 nm that is attributed to a LLCT from L4 to ppy.
There is a redshift of the MLCT transitions for [Ir(ppy)2(L3)]
indicating that the accepting MOs are located on the L3 instead
of ppy ligands which consistent with calculations. The absorption
profile for all compounds were independent of concentration
indicating that negligible dimerisation occurs in solution.
transition results in [Ir(ppy)2(L3)] having greater absorption above
350 nm than the other Ir complexes. The predicted transitions for
[Ir(ppy)2(L4)]- show mixed MLCT and LLCT from Ir and L4 to ppy
which are similar to those of [Ru(bpy)2(L4)], despite occurring at
higher energy although there is increased LLCT contribution from
L4 to all transitions.
Optimised geometries were calculated using the B3LYP method
with a split basis set which has previously been shown to provide
a good model for other Ru and Ir complexes.[23] A LanL2DZ basis
set was employed for Ru and Ir with 6-31G(d) for all other
atoms.[24] As L1 and L2 complexes are structurally similar only
the L1 complexes were calculated. Raman spectra were
measured to validate the optimised geometries and it was found
that the measured Raman bands differed from predicted values
with a mean absolute deviation of less than 10 cm-1 (Figures S2-
S4). This value does not take into account the carbonyl bands, for
which frequencies are often overestimated in these types of
calculations.[25] The optimised geometries show good correlation
to structures obtained from X-ray crystallography (vide infra) with
differences in bond lengths of less than 2.2 pm.
In summary the lowest energy chromophores for these
complexes are: (a) MLCT from Ru to bpy for Ru(bpy)2(L1)]2+ and
Ru(bpy)2(L2)]2+,
(b)
LLCT from L3 or L4 to bpy for
[Ru(bpy)2(L3)]+ and [Ru(bpy)2(L4)], (c) MLCT/LLCT from Ir and
ppy to L1 for [Ir(ppy)2(L1)]+ and [Ir(ppy)2(L2)]+, (d) MLCT/LLCT
from Ir and ppy to L3 for [Ir(ppy)2(L3)] and (e) MLCT/LLCT from
Ru and L4 to ppy.
The emission spectra for the Ir complexes are shown in Figure 2.
No emission was observed for the Ru complexes which is partially
due to the presence of a lower energy 3metal centred (MC) states
being thermally accessible from the 3MLCT state.[26] It has
previously been reported that the inclusion of the pytri unit
3
improves the overlap of the 3MLCT and MC states of Ru which
results in a quenching of emission even when the emissive state
is not located on the pytri moiety. [21a, 28] The emission processes
exhibited by [Ir(ppy)2(L3)] and [Ir(ppy)2(L4)]- display vibronic
progressions. These features are consistent with a ppy based
mixed MLCT/LC states which is seen in other Ir-ppy
complexes.[22c, 26] Emission is observed from both triplet (~500
nm) and singlet (~400 nm) states with the singlet state displaying
considerably lower intensity which is consistent with the rapid
intersystem crossing observed in other Ir(III) complexes.[29] The
measured lifetimes (Table 1) are consistent with the assignments
of the states as singlet and triplet for the high and low energy
emission, respectively. [Ir(ppy)2(L3)] displays considerably
shorter triplet state lifetime than the other Ir complexes. This may
be attributable to the methylene linker between the pyridine and
hydantoin components of L3. Similar lifetimes were reported for
an analogous Ir complex with a ligand containing a pyridine and
Once the optimised geometries were validated time dependant-
density functional theory (TD-DFT) calculations were performed
to model the complexes electronic properties. The calculated
molecular orbitals (MOs) and electronic transitions are shown in
Tables S1-S12. The predicted transitions are in good agreement
with the absorption spectra and are consistent with the
assignments made above. For the Ru complexes the LUMO is
consistently located on the bpy ligands. This is unsurprising as
both the anionic ligands (L3 and L4) as well as the electron rich
triazole containing ligands (L1 and L2) are expected to be higher
energy than that of bpy.[21, 26] In Ru(bpy)2(L1)]2+ the HOMO was
predicted to be located on the DAT unit of L1 however, due to the
isolation of the HOMO from the bpy based LUMOs, the lowest
energy transition with appreciable oscillator strength occurs from
HOMO-1 which is located on the Ru. For both [Ru(bpy)2(L3)]+ and
[Ru(bpy)2(L4)] the HOMO is located on the anionic ligand with
lower energy MOs based on the Ru which show splitting based
on the asymmetric ligand field. Thus the lowest energy transitions
are LLCT from L3 or L4 with MLCT transitions to bpy occurring at
higher energy. The MLCT transitions for [Ru(bpy)2(L4)] include
notable LLCT contribution from L4.
triazole linked via
a methylene bridge; [Ir(ppy)2(py-Me-tri)],
t = 200 ns.[22b] As the emissive state is ppy based this suggests
that emission is not occurring from the lowest energy state for
[Ir(ppy)2(L3)], which is L3 centred, and this is consistent with
excitation emission map (Fig S7) which shows that the lowest
energy transitions leads to a dark state. The shorter triplet lifetime
alongside the presence of a dark state can be used to rationalise
the decreased quantum yield observed for [Ir(ppy)2(L3)]. The
In the Ir complexes the energy of the ppy ancillary ligands is
higher than that of the bpy ligands in the Ru complexes. This
results in a blueshifting of the absorption spectra and increased
influence of the accompanying ligand.[27] For [Ir(ppy)2(L1)]+ the
LUMO is located on L1 while the HOMO is delocalised over Ir and
ppy, thus the lowest energy transition is a mixed MLCT/LLCT
from Ir and ppy to L1. The HOMO and HOMO-1 of [Ir(ppy)2(L3)]
are spread across all three ligands and the Ir while the unoccupied
MOs are mostly located on either ppy or L3 which results in the
transitions having mixed MLCT, LLCT and ligand centred (LC)
character. The lowest energy transition results in population of the
LUMO, which is located primarily on L3 (87%), with higher energy
transitions populating ppy based MOs. The Ir to L3 MLCT/LLCT
1
excitation emission map also revealed that the MLCT emission
only occurred upon direct excitation to that state (320 to 350 nm)
which supports the assignment of the lower energy transitions as
having L3 or L4 involvement.
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Solid state emission is also observed for the all Ru complexes
except for [Ru(bpy)2(L3)]+. The emission for [Ru(bpy)2(L1)]2+ and
[Ru(bpy)2(L2)]2+ is similar but slightly redshifted to their Ir
counterparts indicating the same 3ILCT emission is occurring. The
emission for [Ru{bpy)2(L4)] is considerably redshifted compared
with its Ir counterpart. This is unsurprising as [Ir{ppy)2(L4)]- is
emitting from a ppy based MLCT/LC state while the lowest energy
states for [Ru{bpy)2(L4)] are MLCT and LLCT. The occurrence of
solid state emission for the Ru complexes, which is not observed
in solution, is attributed to a loss of non-radiative decay pathways,
due to restriction of their geometries in the solid state. The
flexibility of the methylene bridge in [Ru(bpy)2(L3)]+ allows for
access to these pathways which explains the lack of emission for
this complex.
Figure 2. Normalised emission spectra for Ir complexes in DCM with an
excitation at 325 nm.
Crystals
of
[Ru(bpy)2(L1-d3)](PF6)2·CH3CN
and
[Ru(bpy)2(L2)](PF6)2·CH3CN suitable for X-ray analysis were
grown from diffusion of diethyl ether into acetonitrile solutions of
[Ru(bpy)2(L1-d3)](PF6)2 and [Ru(bpy)2(L2)](PF6)2, respectively.
While crystals of the non-deuterated L1 complex were obtained,
none were found to be suitable for X-ray analysis. However, the
deuterated complex was serendipitously crystallised and these
crystals, which were suitable for X-ray analysis, possessed the
same unit cell and was found for the crystals of the non deuterated
version. Both complexes crystallize in the triclinic space group P-
1 with the asymmetric unit containing one complex unit with the
accompanying hexafluorophosphate anions and one acetonitrile
The emission spectra for [Ir(ppy)2(L1)]+ and [Ir(ppy)2(L2)]+ display
a redshifted emission with a loss of vibronic features. This is in
stark contrast with the previously reported, DAT lacking derivative,
[Ir(ppy)2(pytri-Me)]+ which displayed a similar triplet emission as
[Ir(ppy)2(L3)] and [Ir(ppy)2(L4)]-. [22c] This suggests the emission is
occurring from a 3ILCT state which is consistent with the
measured lifetimes. While the 3ILCT state is lower in energy than
the 3MLCT state the 1ILCT state is higher in energy than the
1MLCT indicating that there is either improved spin-orbit coupling
for the ILCT state or that there is a shift in excited state geometry
which lowers the energy of the ILCT states below the MLCT states.
solvent molecule.
(Figure 3) In the structure of
-
[Ru(bpy)2(L2)](PF6)2 the second PF6 anion (P2 and F7-F12) is
disordered with the major component being 70% present. The
ruthenium ions are octahedral with the pyridyltriazole ligands
coordinating N^N as expected through the pyridine and N-1 of the
triazole. The octahedral coordination geometry of both structures
is partially distorted with the three chelate rings having bond
angles at Ru1 ranging from 78.0° to 79.1°. Each structure
contains a racemic mixture of the Λ - and Δ - enantiomers which,
in pairs, form hydrogen bonded dimers where the two DAT rings
hydrogen bond to each other in an offset manner with supporting
interactions from the PF6 anions. (Figure 4)
Table 1. Emission properties for all complexes in degassed dichloromethane.
An excitation of 325.4 nm was used for all measurements.
λEm / nm
t / ns
Φ
[Ir(ppy)2(L1)] a
[Ir(ppy)2(L2)] a
[Ir(ppy)2(L3)] a
586
582
496
398
496
398
389
389
655
730
254
8.1
790
8.4
2.3
2.1
0.03
0.03
0.01
[Ir(ppy)2(L4)]- a
0.05
In the structure of [Ru(bpy)2(L1-d3)](PF6)2 the hydrogen bonding
L1
L2
0.02
0.02
ꢂ
forms a ꢀꢂꢂ 10 ꢀꢁ 8 ꢀꢂ 10 motif with a strong hydrogen bonds
between N9 and N6 of the adjacent molecule (H···N 2.24 Å,
NH···N 165°) as well as between N8 and F10, while the bond
observed between N9 and F9 is weaker (H···F 2.17 and 2.36 Å,
NH···F 158° and 125° respectively). In the structure of
(
)
ꢁ( )
(
)
[a] Highest energy emission reported for processes with vibronic structure.
-
[Ru(bpy)2(L2)](PF6)2 however the PF6 anions are arranged
Solid state emission spectra are shown in Figure S9.
[Ir(ppy)2(L1)]+ and [Ir(ppy)2(L2)]+ display similar solution and solid
state emission indicating that the solid is amorphous. The slightly
broadened emission of [Ir(ppy)2(L2)]+ implies that it has a greater
disorder in the solid than [Ir(ppy)2(L1)]+ which is consistent with
low frequency Raman measurements (vide infra). The solid state
emission of [Ir(ppy)2(L3)] and [Ir(ppy)2(L4)]- display a small
bathochromic shift compared with the solution phase emission
which is consistent with other Ir(III) complexes.[30]
differently and have more interactions with the amines of the DAT
ꢁ
ꢁ( ) ꢁ( ) ꢁ( )
moiety giving a ꢀꢃꢁ(4)ꢀꢂ 8 ꢀꢁ 8 ꢀꢂ 8 ꢀꢃ (4) motif with a strong
hydrogen bond between N9 and N6 of the adjacent molecule
(H···N 2.17 Å, NH···N 157°) and a series of weaker bonds from
N8 to F7A and F8A as well as N9 to F8A (H···F 2.37, 2.39, 2.42
Å, NH···F 134°, 161°, 117° respectively).
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Figure 4. (top) X-ray crystal structure of [Ru(bpy)2(L1-d3)](PF6)2 showing the
ꢂ
ꢀꢂꢂ 10 ꢀꢁ 8 ꢀꢂ 10 motif hydrogen bonding pairs. (bottom) X-ray crystal
(
)
ꢁ( )
(
)
structure of [Ru(bpy)2(L2)](PF6)2 showing the ꢀꢃꢁ(4)ꢀꢂ 8 ꢀꢁ 8 ꢀꢂ 8 ꢀꢃ (4)
ꢁ
ꢁ( ) ꢁ( ) ꢁ( )
motif hydrogen bonding pairs.
Crystals of [Ru(bpy)2(L3)](PF6)·CH3CN suitable for X-ray analysis
were grown from diffusion of diethyl ether into an acetonitrile
solution of [Ru(bpy)2(L3)](PF6). The complex crystallizes in the
monoclinic space group P21 with the asymmetric unit containing a
single enantiomer of the complex, with a Λ configuration, its
accompanying hexafluorophosphate anion and one acetonitrile
solvent molecule. (Figure 5) The ruthenium ion is octahedral with
the singly deprotonated L3 ligand coordinating N^N to form a six
membered chelate ring and an overall 1+ complex. The
octahedral coordination geometry is partially distorted with the
three chelate rings having bonding angles at Ru1 ranging from
78.9° to 89.3°. Unexpectedly, the hydrogen bonding observed in
the structure is merely a series of discrete single hydrogen bonds.
N3 of the hydantoin singly hydrogen bonds to N8 of the
acetonitrile solvate (H···N 2.25 Å, NH···N 162°). Both O1 and O2
engage in non-classical CH···O hydrogen bonding with aromatic
protons from adjacent bi-pyridine moieties (H···O 2.55, and 2.34
Å, CH···O 142°, and 175° for O1 and O2 respectively) but
otherwise do not show any strong interactions, consistent with the
results of the low frequency Raman studies. This is also observed
Figure.
3 X-ray crystal structure of [Ru(bpy)2(L1-d3)](PF6)2 (top) and
[Ru(bpy)2(L2)](PF6)2 (bottom). Ellipsoids are drawn at 50% and only major
components of disorder shown.
In the structure of [Ru(bpy)2(L1-d3)](PF6)2 a L1-d3 ligand overlays
the hydrogen bonding DAT moieties between two other
complexes with the benzyl ring offset π-π stacking with one DAT
ring (centroid–N6 3.50 Å) and the triazole deuteron hydrogen
bonding to an amine of the other DAT ring (D···N 2.62 Å, CD···N
152o) supported by an additional offset π-π interaction between
two pyridines (centroid–C3 3.62 Å), overall forming a chain of
complexes down the crystallographic b axis (Figure 4).[5] The
structure of [Ru(bpy)2(L2)](PF6)2 has a similar arrangement of
complexes although in a more extended manner, relying more on
bridging interactions through the PF6- anions (H···F 2.27 – 2.63 Å,
115° - 180°) and not displaying any significant π-stacking
interactions. (Figures S10-S11)
-
from the PF6 anion which also shows a series of CH···F
interactions (H···F 2.43 – 2.61 Å, 127° – 164°).
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It is noted that, where ꢀꢁꢁ(8) dimers are found, the two component
molecules are generally spanning a center of inversion – in this
case, the symmetry of the adopted space group may be what
precludes the formation of the expected hydrogen bonding motifs.
additional hydrogen bonds from the carbonyl oxygens to the O5
and O6 water molecules (H···O 2.10, 2.05, and 2.07 Å, OH···O
171°, 171°, and 167° for H6A, H6B, and H5A respectively) to form
an overall ꢀꢄꢂ(10)ꢀꢁꢁ(8)ꢀꢄꢂ(10) motif. The packing of the
molecules is further facilitated by weak non-classical hydrogen
bonds between H3 and O4 (H···O 2.73 Å, CH···O 162°) of an
adjacent molecule forming a second ꢀꢁꢁ(10) like motif, and is
supported by O5 which hydrogen bonds to the carboxylic acid
(H···O
1.89
Å,
OH···O
163°).
Thus,
a
{ ꢀꢄꢂ(10)ꢀꢁꢁ(8)ꢀꢄꢂ(10)ꢀꢂꢁ(8)ꢀꢁꢁ(10)ꢀꢂꢁ(8) }¥ motif assembles the
complexes into infinite one-dimensional chains along the
crystallographic a axis. (Figure 7) The extensive hydrogen
bonding that the carbonyl groups enjoy is consistent with the
results of the low frequency Raman studies (vide infra). The
partial occupancy water solvate has a weak non-classical
hydrogen bond to C6 (H···O 2.52, CH···O 135).
Figure 5. X-ray crystal structure of [Ru(bpy)2(L3)]PF6. Ellipsoids are drawn at
the 50% probability level.
Crystals of [Ru(bpy)2(L4)] suitable for X-ray analysis were
obtained from an aqueous solution of the complex. The complex
crystallises in the monoclinic space group P21/c with the
asymmetric unit containing one complex unit and two full water
molecules with a third in partial (40%) occupancy (Figure 6). The
ruthenium ion is octahedral with the doubly deprotonated orotic
acid ligand coordinating N^O bidentate, as expected, to give an
overall neutral complex. The octahedral coordination geometry is
partially distorted with the three chelate rings having bonding
angles at Ru1 ranging from 78.3° to 79.6°.
Figure 7. X-ray crystal structure of [Ru(bpy)2(L4)] showing hydrogen bonding
chains
down
the
ꢁ( )
crystallographic
a
axis
with
repeating
ꢂ
ꢁ
{ꢀꢄꢂ 10 ꢀꢁ 8 ꢀꢄ 10 ꢀꢂ 8 ꢀꢁ 10 ꢀꢂ 8 }ꢅ motif.
(
)
ꢁ( )
(
)
(
)
ꢁ( )
Crystals of fac-[Ir(ppy)2(L3)] suitable for X-ray were grown from
diffusion of diethyl ether into a chloroform solution of the complex.
The complex crystallizes in the monoclinic space group C2/c with
the asymmetric unit containing one complex and half a disordered
diethyl ether solvate. (Figure 8) The iridium ion is octahedral with
the singly deprotonated L3 ligand coordinating N^N and the
coordinated C atoms of the ppy ligands both trans- to the L3
ligand giving the fac- geometry[20] and an overall neutral complex
as expected. The octahedral coordination geometry is partially
distorted with the three chelate rings having bonding angles at Ir1
ranging from 80.2° to 88.3°.
Figure 6. X-ray crystal structure of [Ru(bpy)2(L4)]. Ellipsoids are drawn at the
50% probability level and water solvates O6 and O7 are omitted for clarity.
The structure contains a racemic mixture of the Λ - and Δ -
enantiomers and the orotate ligand forms an ꢀꢁꢁ(8) hydrogen
bond to the orotate ligand of an adjacent molecule between N2
and O2 (H···O 2.15 Å, NH···O 157°). This is supported by
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The crystal contains a racemic mix of the Λ - and Δ - enantiomers
which for hydrogen bonded pairs via O3 of the ether solvate
(H···O 1.99 Å, NH···O 175°) with the angle of the hydrogen
bonding through the oxygen (N···O···N) and the overall angle
between the two complexes being 113°. These pairs of
complexes then stack into chains down the b axis supported by a
series of weak non-classical hydrogen bonds from C18 and C19
to O1 (H···O 2.997 and 2.965 Å, CH···O 127° and 126°,
The Raman spectra for all compounds are shown in Figure 10 and
it can be observed from the low frequency region that there is a
range of crystallinity among the compounds. [Ir(ppy)2(L1)]+ and
[Ir(ppy)2(L2)]+ are the most amorphous and display a single broad
band in the LFR region. Some crystallinity is observed for
[Ir(ppy)2(L3)], which is made apparent by the resolution of low
frequency bands, but even greater crystallinity is seen for the
remaining complexes. Further information is revealed in the mid
respectively). (Figure 9) As was the case with [Ru(bpy)2(L3)](PF6), frequency region. (Figure S4) Carbonyl bands can be clearly
no ꢀꢁꢁ(8) hydrogen bonding motif is observed.
resolved for [Ru(bpy)2(L3)]+ but only a broad band is observed for
[Ir(ppy)2(L3)] consistent with the low frequency region which
indicates that [Ir(ppy)2(L3)] is more amorphous. The carbonyl
bands for the L4 complexes could not be resolved. The lack of
carbonyl bands is also observed with an excitation of 1064 nm.
(Figures S2, S3) Alongside the crystallinity of the L4 complexes,
this suggests that the carbonyls are occupied with intramolecular
bonds in a crystalline lattice which is consistent with the X-ray
crystallography data.
Figure 8. X-ray crystal structure of fac-[Ir(ppy)2(L3)]. Ellipsoids are drawn at the
50% probability level and with only the major components of disorder shown.
Figure 10. Low and mid frequency Raman spectra for all compounds with a
785 nm excitation in solid state.
Hirshfeld Analysis[31] provides further insight into the relative
importance of the various intermolecular interactions found in the
crystal structures. In particular, the fingerprint region plots di vs.
de, graphically depict the distances from the calculated Hirshfeld
surface to the nearest nuclei inside and outside the surface,
respectively, with the colours changing from blue to green as the
frequency with which a particular (di/de) increases. Figures S12-
S16 show the fingerprint plots for the five complexes.
Figure 9. X-ray crystal structure of fac-[Ir(ppy)2(L3)] showing how two
complexes hydrogen bond (blue) via the diethyl ether solvate, then the hydrogen
bonded complexes stack down the crystallographic b axis with support from
weak non-classical hydrogen bonds (green).
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Figure 11 shows the relative amounts of the major interaction
types observed in the structures, obtained from the fingerprint
plots. Comparison of the plots for [Ru(bpy)2(L1)](PF6)2 and
[Ru(bpy)2(L2)](PF6)2 shows that the percentage of H-H
interactions for the L2 complex is larger than for the L1 complex,
due to the hexyl chains, while the percentage of C-C interactions
is greater for the L1 complex due to the benzyl substituent being
able to engage in p-p stacking interactions.
promise for the formation of the desired heterosynthon
assemblies, use of the L3 complexes might be less successful.
The measured emission spectra of the complexes show that there
is limited scope for using hydrogen bonding through the L ligand
to tune the emission of the Ru(II) complexes in solution (as they
are non-emissive) nor for the [Ir(ppy)2(L3)] and [Ir(ppy)2(L4)]-
complexes, as these emit from ppy-based states. However, the
Ru(II) and Ir(III) complexes of L1 and L2 do show promise,
particularly in the solid state, as in these the emissive state lies
on the L ligand.
The low frequency Raman studies support what is observed in the
X-ray crystallography, specifically that the assembly of the
compounds in the solid-state structures of the L3-containing
complexes does not significantly involve the carbonyl groups
while the opposite is true for the L4-containing complexes. This,
for these complexes at least, the LFR spectra provide a non-
crystallographic guide to the nature of the hydrogen bonding
interactions present.
With these results in hand, work on preparing co-crystals of these
complexes with each other and with other suitable organic
species is underway and these results will be reported in due
course.
Figure 11. Quantification of intermolecular interactions in the crystal structures
from HIrshfeld fingerprint analysis.
Experimental Section
The percentage of N-H interactions are similar between them, and
the large percentage of H-F interactions reflects the hydrogen
bonding interactions to the PF6- anions. The plots for the two L3
complexes show very similar percentages of H-O interactions, but
differ in that the [Ru(bpy)2(L3)]PF6 complex has 25.6% of the
interactions as H-F hydrogen bonds to the anion. By comparison,
the plot for [Ru(bpy)2(L4)] shows that over one third of all the
interactions are H-O hydrogen bonds, as expected given the
extensive hydrogen bonding interactions observed.
Experimental details, crystal structures, Hirshfeld Analysis and
spectroscopic data are provided in the Supporting Information.
Acknowledgements
APM and JIM thank the University of Otago for doctoral
scholarships and funding from the McDiarmid Institute is gratefully
acknowledged.
Conclusions
Keywords: Hydrogen bonding • crystal engineering • ruthenium
• iridium • X-ray crystallography
The aim of the work was to prepare transition metal complexes
that could act as potential tectons for triple hydrogen bond-
directed assembly. The new ligands and the Ru(II) and Ir(III)
complexes described here were able to be readily synthesized
and while they display good solubility in low-donicity solvents, 1H
NMR studies disappointingly showed no evidence of appreciable
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10.1002/asia.201801748
Chemistry - An Asian Journal
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Ru(bpy)2(L)]
and
[Ir(ppy)2(L)]
ligand
Aidan P McKay, Joseph I Mapley, Keith
C. Gordon and David A. McMorran*
complexes, where
L
is
a
containing either a DAD or ADA triple
hydrogen bonding motif, have been
prepared and characterised and their
potential to act as components in
supramolecular assembles assessed,
through analysis of their solid-state
structures and their spectroscopic
properties.
Page No. – Page No.
Ru(II) and Ir(III) complexes containing
ADA and DAD triple hydrogen
bonding motifs: Potential tectons for
the assembly of functional materials.
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