Cis-1,3,5-triaminocyclohexane as a facially capping ligand for ruthenium(II)

Article abstract of DOI:10.1021/ic302819j

Reaction of cis-[RuCl₂(DMSO-S)₃(DMSO-O)] with cis-1,3,5-triaminocyclohexane (tach) results in the formation of [RuCl(tach)(DMSO-S)₂]Cl, a valuable precursor for a wide range of other tach-containing Ru complexes. Reaction of [RuCl(tach)(DMSO-S) ₂]Cl with the chelating nitrogen-based ligands (N-N = bipyridine, phenanthroline, and ethylenediamine) affords [Ru(N-N)(DMSO-S) ₂(tach)][Cl]₂. A similar reaction between [RuCl(tach)(DMSO-S)]Cl with the chelating phosphorus-based ligands (P-P = dppm, dppe, dppp, dppb, dppv, and dppben) leads to the formation of [RuCl(P-P)(tach)]Cl. The structures of 10 examples of the tach-containing complexes have been determined by single crystal X-ray diffraction. An examination of the structural metrics obtained from these studies indicates that the tach ligand is a strong sigma donor. In addition, the presence of the NH₂ groups in the tach ligand allow for participation in hydrogen bonding further modulating the coordinative properties of the ligand.

Full text of DOI:10.1021/ic302819j

Article
pubs.acs.org/IC
cis-1,3,5-Triaminocyclohexane as a Facially Capping Ligand for
Ruthenium(II)
Aimee J. Gamble, Jason M. Lynam,* Robert J. Thatcher, Paul H. Walton,* and Adrian C. Whitwood
Department of Chemistry, University of York, Heslington, York, YO10 5DD, U.K.
S
* Supporting Information
ABSTRACT: Reaction of cis-[RuCl2(DMSO-S)3(DMSO-O)]
with cis-1,3,5-triaminocyclohexane (tach) results in the formation
of [RuCl(tach)(DMSO-S)2]Cl, a valuable precursor for a wide
range of other tach-containing Ru complexes. Reaction of [RuCl-
(tach)(DMSO-S)2]Cl with the chelating nitrogen-based ligands
(N−N = bipyridine, phenanthroline, and ethylenediamine) affords
[Ru(N−N)(DMSO-S)2(tach)][Cl]2. A similar reaction between
[RuCl(tach)(DMSO-S)]Cl with the chelating phosphorus-based
ligands (P−P = dppm, dppe, dppp, dppb, dppv, and dppben) leads
to the formation of [RuCl(P−P)(tach)]Cl. The structures of 10
examples of the tach-containing complexes have been determined
by single crystal X-ray diffraction. An examination of the structural
metrics obtained from these studies indicates that the tach ligand is a strong sigma donor. In addition, the presence of the NH2 groups in
the tach ligand allow for participation in hydrogen bonding further modulating the coordinative properties of the ligand.
Ruthenium complexes with a facially capping arene have
been shown to have potential as new therapeutic agents for the
treatment of cancer. Two leading classes of complex have been
studied, RAPTA-C developed by Dyson contains an η6-bound
cymene ligand and the water-solubilizing PTA phosphine,14
whereas a series of compounds prepared by Sadler and co-workers
contains a range of arene-capping groups and a bidentate
ethylenediamine ligand.15 This theme has been developed further
with different arene capping groups,16 and with [9]aneS317 and
tacn ligands.18
Although the nature of the facially capping ligand in ruthenium
half sandwich compounds may be used primarily to control the
steric and electronic properties of the metal, there are fewer
examples where this ligand may be involved in secondary
interactions, such as hydrogen bonding. This is important in the
light of the discovery by Sadler et al. that NH2 groups in an
ethylenediamine ligand within an arene hand-sandwich complex
assist in the binding of the metal to DNA.19
cis-1,3,5-Triaminocyclohexane (tach) is a ligand which has
previously been used widely in the complexation of first row
transition metal complexes.20 Tach has a number of important
features, first, following a ring “flip” from the all-equatorial
position tach can adopt an all-axial conformation and thus
present the three amino groups to a metal and act as a facially
capping ligand (Scheme 1). In addition the three NH2 groups
also have the potential to act as hydrogen bond donors, a
feature which could potentially be exploited in catalysis or, as is
the case for the ethylenediamine ligand in the complexes
INTRODUCTION
■
Ruthenium half-sandwich compounds1 find applications in a
diverse range of fields including catalysis,2 nonlinear optics,3
and medical chemistry.4 In this context, complexes of the
general type [Ru(fac)L3], where fac represents a general facially
capping six-electron donor ligand, are of particular interest as the
nature of the fac group may be used to affect the steric and
electronic properties of the metal. Most conspicuously, complexes
containing the cyclopentadienyl ligand (and its derivatives)
[Ru(η5-C5R5)L3] are widely used, especially since a number of
easily prepared starting materials are available, such as [RuCl-
(η5-C5H5)(PPh3)2].5 In many cases the cyclopentadienyl ligand is
essentially inert, whereas the remaining ligands may be labile, thus
enabling, for example, carbon-carbon and carbon-heteroatom
bond formation.2 Notably the substituents on the cyclopentadienyl
ring can profoundly affect the chemistry of these species. For
example, in the parent cyclopentadienyl case complexes of the type
[Ru(η5-C5R5)L3] almost uniformly obey the 18-electron rule,6
whereas in the corresponding pentamethylcyclopentadienyl case
it is possible to isolate stable 16-electron compounds such as
[Ru(η5-C5Me5)Cl(PCy3)].7
Given the effect that the facially capping group has on a
metal center, ligands other than Cp, such as arenes,8
tris(pyrazolyl)borates,9 tacn,10 cyclotriphosphates11 and [9]-
12
aneS3 have been employed by others to control the relative
steric and electronic properties of ruthenium complexes. The
effect of the capping group may be especially important as
ruthenium half-sandwich compounds have found applications
as catalysts of carbon−carbon bond formation2 and recently in
direct C−H functionalizaton reactions.13
Received: December 21, 2012
Published: March 22, 2013
© 2013 American Chemical Society
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The structure determination demonstrated that many of the
structural features of [1]Cl, predicted from spectroscopic data,
are also present in the solid state structure of [1]PF6. As shown
in Figure 1, the cationic unit [1]+ contains a facially capping
κ3-bound tach ligand, and the remaining coordination sites on the
metal are occupied by two S-bound DMSO ligands and a chloride.
A complete discussion of the structural metrics for all the complexes
reported with a view to ascertaining the properties of the tach ligand
is provided later, but it is important to note the presence of both
intra- [N(1)····O(2) 3.143(3) Å; N(3)····O(1) 2.852(3) Å;] and
intermolecular [N(2)····O(1) 2.843(3) Å] hydrogen bonds between
the NH2 groups and the oxygen atoms of the DMSO groups.
To the best of our knowledge [1]Cl represents the first reported
example of a ruthenium complex containing a facially capping tach
ligand. In addition [1]Cl proved to be a versatile precursor for a
number of other ruthenium complexes containing the tach ligand.
Reaction of an aqueous solution of [1]Cl with a slight excess
of bidentate nitrogen-based ligands bipyridyl (bipy), phenan-
throline (phen), or etheylenediamine (en) at 120 °C for 20 min
gave red solutions from which complexes [Ru(DMSO-S)(κ2-N-N)-
(κ3-tach)][Cl]2, (N−N = bipy [2a][Cl]2, N−N = phen [2b]-
[Cl]2 N−N = en [2c][Cl]2) could be isolated (Scheme 3). In a
similar vein to complex [1]Cl, the resonances for the tach
Scheme 1
prepared by Sadler, may aid with the binding of the ruthenium
complex to DNA.
With this in mind, we considered that tach could add a new
dimension to the chemistry of ruthenium half-sandwich
compounds. We report here the preparations of a range of tach-
ruthenium complexes, which are rare examples of tach acting as a
ligand to a metal outside the first row. We also characterize the
complexes structurally and demonstrate that tach should be
considered as a hard donor ligand under the HSAB classification.
RESULTS AND DISCUSSION
■
Treatment of a dimethylsulfoxide (DMSO) solution of cis-
[RuCl2(DMSO-S)3(DMSO-O)] with tach followed by heating
to 130 °C for 30 min gave a pale yellow solution from which
[RuCl(DMSO-S)2(κ3-tach)]Cl, [1]Cl, could be isolated as a
1
cream solid in excellent yield (Scheme 2). The H NMR
1
ligand in the H NMR spectra of complexes [2]2+ all indicated
a
Scheme 2
that the metal center possessed local Cs symmetry. Furthermore,
in all three cases a singlet resonance of relative intensity six
demonstrated the presence of single DMSO ligand. The structure
of [2a][PF6]2, [2b][PF6]2·MeOH, and [2c][Cl][PF6] were also
confirmed by single crystal X-ray diffraction. The structures of the
cations [2a−c]2+ are presented in Figure 2.
The crystallographic determinations demonstrated that the
dications [2a]2+ and [2b]2+ are essentially isostructural, with
the tach ligand occupying three coordination sites, and the
remaining positions around the metal being taken by either the bipy
or the phen ligand and an S-bound DMSO. In both cases, the
oxygen atom of the DMSO engages in a weak intramolecular hy-
drogen bonding with two NH2-groups of the tach ligand (Figure 2).
In the structure of [2a][PF6]2 stronger NH2···O hydrogen bonds
[N(1)···O(1) 2.996(4)Å] are observed between the neighboring
molecules (Figure 3a) whereas the cationic units in [2b][PF6]2·
MeOH form a dimer (Figure 3b) with the methanol-of-
crystallization forming a hydrogen bonding bridge between the
two ruthenium centers [N(3)···O(2) 2.951(2) Å, O(2)···O(1)
2.839(2) Å, N(3)···O(1) 3.127(3) Å]. In the case of complex
[2c]2+, despite two equivalents of NaPF6 being added, the structure
determined by single crystal X-ray diffraction indicated that only a
single chloride counteranion had been replaced. Furthermore, in
this structure each chloride anion is involved in weak hydrogen
bonding with N−H groups of two ruthenium dications (Figure 3c)
[N(1)···Cl(1) 3.277(1) Å, N(4)···Cl(1) 3.259(2) Å].
a
(i) DMSO, 30 min, 130 °C.
spectrum of [1]Cl recorded in D2O solution exhibited resonances
consistent with the proposed formulation, and notably the
resonances for the tach ligand proved to be particularly diagnostic
of the symmetry of the Ru complex. Six resonances were observed
for the protons of the cyclohexyl ring and three resonances for the
NH2 groups of this ligand indicating local Cs symmetry at the
metal. Furthermore, the absence of any axial−axial couplings
between neighboring protons indicated that the ring had undergone
the expected flip placing the amine groups in an equatorial position
so that it may act in a facially capping manner. A singlet resonance
of relative intensity 12 was observed at δ 3.36 for the methyl groups
of the coordinated DMSO ligands. The IR spectrum of [1]Cl
contained a band at 1061 cm−1 which was assigned to the S−O
stretch of the DMSO ligands. In general, O-bound DMSO ligands
exhibit stretches in the region 878−1035 cm−1, whereas their S-
bound analogues show bands between 1070 and 1233 cm−1.21
Given the fact that the band for the S-bound DMSO in [RuCl-
(η5-C5Me5)(DMSO)2] was observed at 1060 cm−1,22 the same
coordination mode for the DMSO was assigned in the case of
[1]Cl. This was confirmed by single crystal X-ray diffraction.
Reaction of [1]Cl with KPF6 allowed for exchange of the
counter-anions and the formation of [1]PF6. Slow evaporation
of a methanol solution of this compound resulted in crystals of
[1]PF6 suitable for study by X-ray diffraction. Selected bond
lengths and angles for [1]PF6 are presented in Table 1, and
details of the data collection and structure refinements for all
complexes reported in this work are in Table 3.
The ability of [1]Cl to act as a precursor for ruthenium
phosphine complexes was also investigated. Heating a solution
of [1]Cl and a range of bidentate phosphine (P−P) ligands in
methanol solution at reflux afforded a series of complexes
[RuCl(κ2-P-P)(κ3-tach)]Cl, (P−P = dppm [3a]Cl, P−P = dppe
[3b]Cl, P−P = dppp [3c]Cl, P−P = dppb [3d]Cl, P−P = dppv
[3e]Cl, P−P = dppben [3f]Cl) (Scheme 4). The cations with
general structure [3]+ all shared a common series of spectroscopic
features. Using [3a]Cl as an example, the 31P{1H} NMR spectrum
displayed a single resonance for the coordinated dppm ligand at δP
1
10.1, and the H NMR spectrum indicated that the tach ligand
possessed local Cs symmetry. In contrast to the corresponding
reactions between the nitrogen-based ligands, no evidence for
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Table 2. [9]aneS = 1,4,7-trithiacyclononane, Cp = η5-1-methoxy-2,4-tert-butyl-3-neopentylcyclopentadienyl, (L
=
OEt
̂
3
(η5-C5H5)Co{P(O)(OEt)2}3), Ind = η5-C5H9, TpiPr = hydrotris(3,5-diisopropylpyrazolyl)borate, Tpm = tris(pyrazolyl)methane
fac
Ru−S(1)
Ru−S(2)
S(1)−O(1)
S(2)−O(2)
Ru−Cl
ref
[RuCl(fac)(S-DMSO)2]
tach
2.2581(5)
2.288(1)
2.186(2)
2.325(1)
2.303(3)
2.2524(5)
2.250(1)
2.190(2)
2.338(4)
2.299(3)
1.5011(18)
1.480(2)
1.472(5)
1.485(3)
1.485(3)
1.4883(17)
1.482(2)
1.476(6)
1.481(3)
1.471(3)
2.4170(6)
2.414(1)
2.362(2)
2.420(1)
2.447(3)
Tp
26
27
28
29
ref
LOEt
[9]aneS3
Cp*
Ru−N(4)
Ru−N(5)
S(1)−O(1)
Ru−S(1)
[Ru(fac)(bipy)(S-DMSO)]
1.500(2)
tach
2.077(3)
2.071(7)
Ru−P(1)
2.076(2)
2.100(7)
Ru−P(2)
2.2223(8)
2.285(3)
[9]aneS3
1.487(8)
17a
Ru−Cl
P(1)−Ru−P(2)
[RuCl(fac)(dppm)]
2.4139(12)
2.4302(6)
tach
Cp
2.2625(12)
2.2724(5)
2.282(2)
2.2610(12)
2.2833(6)
2.294(2)
72.30(4)
72.07(2)
71.53(6)
71.32(6)
71.29(6)
22
30
31
32
Cp*
2.434(2)
Cp
̂
2.3289(16)
2.316(2)
2.3459(16)
2.309(2)
2.4567(15)
2.397(2);
p-cymene
[RuCl(fac)(dppe)]
2.4431(14)
2.4466(7)
tach
Cp
2.230(11)
2.2688(7)
2.275(2)
2.2882(5)
2.353(3)
2.3657(11)
2.313(3)
2.285(3)
2.2314(4)
2.329(1)
2.320(13)
2.2863(7)
2.282(2)
2.2812(5)
2.360(3)
2.3496(12)
2.287(3)
2.279(3)
2.2970(4)
2.336(1)
82.7(4)
83.04(3)
83.5(1)
22
21
30
31
33
34
35
36
32
2.452(2)
Cp*
2.4532(5)
82.15(2)
82.22(9)
82.30(4)
84.46(11)
85.3(1)
Cp
̂
2.452(2)
C60Me5
2.4397(11)
2.397(3)
Tpm
TpiPr
2.359(4)
Ind
2.4331(4)
82.171(13)
83.03(3)
p-cymene
2.430(1)
[RuCl(fac)(dppp)]
2.4404(4)
tach
2.2721(5)
2.3235(9)
2.2836(5)
2.3035(9)
89.481(17)
93.58(3)
Tpm
2.4056(8)
37
[RuCl(fac)(dppb)]
2.4379(12)
2.399(2)
tach
toluene
Cp
2.2872(13)
2.322(2)
2.2860(14)
2.349(2)
92.98(5)
92.30(7)
93.64(2)
94.88 (3)
38
39
40
2.2810(5)
2.2502 (9)
2.2809(6)
2.2968 (8)
2.4404(4)
Ind
2.4467 (8)
a
Scheme 3
Figure 1. ORTEP diagram of the cation [1]+, ellipsoids are shown at
the 50% probability level and hydrogen atoms (except for the NH2-
groups) omitted for clarity.
a
(i) H2O, 120 °C, 20 min.
chloride-containing complexes (e.g., for [3a]+ a peak with the
expected isotope pattern at m/z 650.1213 corresponding to
[RuCl(dppm)(tach)]+ was observed).
All of the complexes [3]Cl could be isolated in an analytically
pure form without the need for anion metathesis. Crystals
DMSO coordination to the metal was obtained. No resonances
were present in the 1H NMR spectrum for a coordinated DMSO,
and in the mass spectrum the only ions observed were due to the
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Figure 3. Hydrogen bonding motifs observed in the solid state
structures of compounds (a) [2a](PF6)2, (b) [2b](PF6)2·MeOH, (c)
[2c]Cl(PF6).
a
Scheme 4
Figure 2. ORTEP diagrams of the dications (a) [2a]2+, (b) [2b]2+, and
(c) [2c]2+. Ellipsoids are shown at the 50% probability level and
hydrogen atoms (except for the NH2-groups) omitted for clarity.
suitable for single crystal X-ray diffraction of [3c−f]Cl could be
obtained as the chloride salts, for the dppm and dppe analogue
crystals of the corresponding PF6 salts ([3a]PF6 and [3b]PF6)
were employed for the structural determination. The
subsequent structural determinations demonstrated that the
cationic components of the complexes were essentially
isostructural (Figure 4), and in each case the ruthenium was
shown to be coordinated to a tach, chelating phosphine and a
chloride ligand. With the use of [3a]PF6 as an example, the
structure of the complex adopts that of a distorted octahedron
a
(i) MeOH, reflux, 18 h.
at the metal center and the ruthenium tach adamantane moiety
is also distorted and tilted in comparison to the octahedron.
This is evident from the Ru−N bond lengths of 2.119(4) Å for
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Figure 4. ORTEP diagrams of the cations (a) [3a]+, (b) [3b]+, (c) [3c]+, (d) [3d]+, (e) [3e]+, and (f) [3f]+. Ellipsoids are shown at the 50%
probability level and hydrogen atoms (except for the NH2-groups) omitted for clarity.
the amine trans- to the chlorido ligand versus 2.171(4) and
2.162(4) Å for those trans- to phosphine as a result of the
different trans-influence of the two ligands. The dppm ligand
forms a four-membered ring with the ruthenium center, with a
bite angle of 72.30(4)°, significantly smaller than the idealized
90°. The chelating phosphine is also angled away from the
N(1) of tach by approximately 20°, which may be the result of
N−H···π hydrogen-bonds between the phenyl rings and the amine of
the tach group: this is a common feature of all of the cations.
Therefore, the phenyl rings adjacent to the chloride ligand are twisted
apart, removing the bulky groups from the locality of the metal center.
The structures of [3a]PF6 and [3b]PF6 also show the
presence of long-range order. The cations in [3a]PF6 form
linear hydrogen-bonded chains; (Figure 5a) which involve a
bifurcated hydrogen bond between the chloride ligand on one
ruthenium with an NH2 group on neighboring tach ligand
[Cl(1)···N(4) 3.179(5) Å]. The crystals of [3b]PF6 grew in the
trigonal space group P3c1, resulting in a hexagonal morphology.
The cations of [3b]+ form trigonal and hexagonal motifs around
the hexafluorophosphate anions, located on special positions
(Figure 5b). A single anion and the dppe ligand are disordered
across two sites in the structural solution. A hydrophobic solvent
channel is present around one of the anions, but it did not prove to
be possible to model this channel; therefore the SQUEEZE
algorithm within PLATON was employed.23 The remaining
complexes of general type [3]+ do not exhibit strong hydrogen-
bonding interactions between the cations within the solid state
structure; hydrogen-bonding between the solvent and the cation
are the dominant intermolecular interactions.
EVALUATION OF THE TACH LIGAND AS A
FACIALLY CAPPING GROUP
■
Selected structural metrics for the complexes prepared in this
study are presented in Table 1, in addition, the comparison of
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although the data must be treated with some caution as the
DMSO ligand in [1]PF6 is involved in hydrogen bonding to the
tach ligand. The ruthenium−sulfur bond in DMSO complexes
is dependent on both σ- and π-components and the length
of the S−O bond may be used to probe the nature of the
π-interaction.24 In general, the longer the S−O bond, the greater
amount of π-back-donation is occurring into the S−O π* orbital,25
thus indicating that the ruthenium center in the tach-containing
complex is more electron rich that at least the Tp, [9]aneS3, or
Cp* analogues. The metal chloride bond lengths in this series do
not show any significant variation.
It should also be noted that attempts to substitute the DMSO
ligand in complexes [2]2+ were unsuccessful, which is consistent
with the short metal−sulfur bond lengths as shown by X-ray
diffraction and also the unusual position of the S−O stretching
frequency in the IR spectrum of the complexes ([2a]2+ 1015 cm−1,
[2b]2+ 1016 cm−1, and [2c]2+ 1035 cm−1). The difference in
reactivity exhibited by complex [1]PF6 toward diamine and
diphosphine ligands is also worthy of note. In the case of the
nitrogen-based ligands the dications [2]2+ are formed in which a
DMSO ligand remains in the coordination sphere of the metal. In
contrast, for the reaction with diphosphine ligands the
monocations [3]+ are generated in which the coordination sphere
is completed by tach and a chloride ligand. This difference in
behavior may be rationalized on the basis of the different π-
accepting character of the ligands in question. The diamine ligands
are relatively poor π-acceptors; therefore in the electron-rich metal
environment created by the tach ligand extensive π-back-donation
to the DMSO ligand will occur, thus strengthening the metal−
sulfur interaction and disfavoring substitution. Such an argument is
supported by the structural arguments presented above. In
contrast, diphosphine ligands are considerably better π-acceptors
than the diamines, hence favoring an additional π-donor (chloride),
as opposed to a π-acceptor (DMSO) in the coordination sphere of
the metal although such a preference is clearly apparent, it is
exacerbated by the presence of hard nature of the tach ligand.
In conclusion, versatile synthetic routes to a range of
complexes containing the facially capping tach ligand have been
developed. An analysis of the structural parameters and
chemical reactivity indicates that this ligand is a hard donor
and that the ruthenium center is extremely electron rich. This
factor may also permit the isolation of the ruthenium cations as
their chloride salts which exhibit good water solubility.
Figure 5. (a) Diagram of the asymmetric unit and the intermolecular
hydrogen-bonds within the linear chains of cations in [3a]PF6. (b)
Diagram of the trigonal structures in [3b]PF6.
complexes containing the tach ligand and other facially capping
groups are in Table 2. In all cases the complexes exhibit a
distorted octahedral coordination environment with the tach
ligand occupying three coordination sites; however, because of
the geometrical constraints of the cyclohexane ring, a
nonidealized geometry is obtained. With the use of complex
[1]PF6 as an example, the angles subtended by the three
nitrogen atoms of the tach ligand are all less than 90° [N(1)−
Ru(1)−N(2) 88.30(8) °, N(2)−Ru(1)−N(3) 85.89(8) °,
N(1)−Ru(1)−N(3) 88.49(8) °] whereas the angles between
the remaining three donor atoms are all considerable greater
[S(1)−Ru(1)−Cl(1) 96.04(2) °, S(1)−Ru(1)−S(2) 97.49(2)
°, S(2)−Ru(1)−Cl(1) 92.37(2) °]. In addition, all of the angles
between the nitrogen atoms of the tach ligand and the chloride
and DMSO ligands are notably less than the idealized 180°
[N(1)−Ru(1)−Cl(1) 173.36(5) °, N(2)−Ru(1)−S(1)
171.02(6) °, N(3)−Ru(1)−S(2) 176.42(6) °]. This general
pattern is repeated in the structures of all of the complexes,
although in the cases where bidentate ligands are present, the
nature of the bridge linking the donor atoms ensure that angles
between these groups are often less than 90°.
ASSOCIATED CONTENT
* Supporting Information
■
S
CIF with details of X-ray data collection and refinement for
complexes reported. Complete experimental details for the
synthesis and characterization of all novel compounds. This
material is available free of charge via the Internet at http://
pubs.acs.org.
A comparison of the bond lengths found within these tach-
containing complexes and others containing common facially
capping groups appears to demonstrate that tach may be
considered to be a hard-type donor ligand, resulting in an
electron-rich ruthenium center. For example, in the case of
complexes of structure [RuCl(fac)(DMSO-S)], the Ru−S bond
lengths found in complex [1]PF6 are shorter than those found
in the cases where the facially coordinating ligand (fac) is either
Tp, [9]aneS3, or Cp*, but longer than those found when the
hard π-donor L(OEt) is present. In addition, an examination of
the sulfur−oxygen bonds reveals that the S(1)−O(1) bond in
the structure of [1]PF6 is the longest in this type of complex,
AUTHOR INFORMATION
Corresponding Author
*E-mail: jason.lynam@york.ac.uk (J.M.L.), paul.walton@york.
ac.uk (P.H.W.).
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to the EPSRC (DTA Studentship for A.J.G.)
for funding.
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