Selective lability of ruthenium(II) arene amino acid complexes

Article abstract of DOI:10.1021/ic502051y

A series of organometallic complexes of the form [(PhH)Ru(amino acid)]⁺ have been synthesized using amino acids able to act as tridentate ligands. The straightforward syntheses gave enantiomerically pure complexes with two stereogenic centers due to the enantiopurity of the chelating ligands. Complexes were characterized in the solid-state and/or solution-state where the stability of the complex allowed. The propensity toward labilization of the coordinatively saturated complexes was investigated. The links between complex stability and structural features are very subtle. Nonetheless, H/D exchange rates of coordinated amino groups reveal more significant differences in reactivity linked to metallocycle ring size resulting in decreasing stability of the metallocycle as the amino acid side-chain length increases. The behavior of these systems in acid is unusual, apparently labilizing the carboxylate residue of the amino acid. This acid-catalyzed hemilability in an organometallic is relevant to the use of Ru(II) arenes in medicinal contexts due to the relatively low pH of cancerous cells.

Full text of DOI:10.1021/ic502051y

Article
pubs.acs.org/IC
Selective Lability of Ruthenium(II) Arene Amino Acid Complexes
Tom G. Scrase,† Michael J. O’Neill,† Andrew J. Peel, Paul W. Senior, Peter D. Matthews, Heyao Shi,†
Sally R. Boss,* and Paul D. Barker*
University of Cambridge, Chemistry Department, Lensfield Road, Cambridge CB2 1EW, United Kingdom
S
* Supporting Information
ABSTRACT: A series of organometallic complexes of the form [(PhH)Ru-
(amino acid)]+ have been synthesized using amino acids able to act as tridentate
ligands. The straightforward syntheses gave enantiomerically pure complexes
with two stereogenic centers due to the enantiopurity of the chelating ligands.
Complexes were characterized in the solid-state and/or solution-state where the
stability of the complex allowed. The propensity toward labilization of the
coordinatively saturated complexes was investigated. The links between complex
stability and structural features are very subtle. Nonetheless, H/D exchange
rates of coordinated amino groups reveal more significant differences in reactivity linked to metallocycle ring size resulting in
decreasing stability of the metallocycle as the amino acid side-chain length increases. The behavior of these systems in acid is
unusual, apparently labilizing the carboxylate residue of the amino acid. This acid-catalyzed hemilability in an organometallic is
relevant to the use of Ru(II) arenes in medicinal contexts due to the relatively low pH of cancerous cells.
pertinent investigations into their reactivity. The complexes
under study exclusively incorporate chelating ligands. The use
of chelating ligands in these complexes will restrict their
promiscuity by slowing any ligand substitutions which will, we
believe, allow for a degree of selectivity in binding.
In order to synthesize chelates with appreciable opportunity
for selectivity, we decided to use amino acids as a cheap and
plentiful source of multidentate, chiral ligands. Consideration of
the different ways that mono- or bidentate ligands could bind
(and the consequences for the configuration of the metal
center) led us to use amino acids able to act as tridentate
ligands in order to “lock-in” the stereochemistry of the metal
center (Scheme 1). This approach has the additional benefit of
ligand sets which would likely act in a hemilabile manner,
allowing fine control over ligand exchange rates. We note that
INTRODUCTION
■
Ruthenium arene complexes have been studied extensively for
their potential as medicinal compounds, particularly for their
anticancer properties.1−6 Evidence is accumulating that
ruthenium complexes with labile ligands are relatively
promiscuous and the speciation observed when such complexes
are challenged with complex biological solutions is exten-
sive.7,8,6 Given the abundance of different Lewis basic groups
on polypeptides, nucleic acids, and even small metabolites, this
is not surprising. Consequently, establishing the link between
ligand exchange properties and cytotoxicity remains a challenge,
although there is already extensive literature which charts the
cytotoxicities of chemically diverse ruthenium species.
An advantage of incorporating metals into complexes that
could be thought of as drug candidates comes from the
inherent strength of the resulting coordination bonds that
could form between the complex and any given target.
However, delivering complexes with labile ligands results in
promiscuous binding to biomolecules and hence lack of
selectivity, which is an undesirable characteristic in a drug.
Relatively simple ruthenium(III) coordination complexes, such
as NAMI-A and KP1019, fall into this category. Both initially
showed some promise as anticancer compounds, but their
modes of action have been difficult to define because the
chemical fates of these labile complexes cannot be determined.
At the other end of the spectrum Meggers et al. exploited inert
metal complexes simply as structural platforms able to extend
the valency space available to medicinal chemists beyond
carbon.9,10 In between these extremes, Sadler11,12 and
Dyson1,13 in particular have independently explored ruthenium-
(II) complexes with limited (one or two) labile coordination
sites.
Scheme 1. Generalized Structure of the Tridentate Amino
a
Acid Cations with the Ruthenium(II) Arene Unit
a
n is the number of CH2 groups between the amino acid α carbon and
the coordinating atom provided by the side-chain. Values of n for the
different amino acids and the cation identification numbers are
recorded in Table 1.
In this article, we present the synthesis and characterization
of a series of novel Ru(II) arene complexes, together with some
Received: August 26, 2014
© XXXX American Chemical Society
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others have recently examined reactions of the ruthenium(II)
arene unit with selected amino acids and the cytotoxicity of
some specific compounds, but these compounds have neither
been extensively characterized nor systematically varied.14−16
Additionally, some complexes with arenes functionalized with
pendant amines,17 including phenylalanine,18 which contain a
different class of chelating ligand, have been studied. The
reaction of some of these with DNA has been examined.17
The effect of the chelate ring size in such complexes has not
been studied in detail and certainly not in the context of amino
acid coordination. As shown below, this strongly influences the
stability of the chelate, the properties of coordinating ligand
groups, and hence the selective lability of the whole complex.
Our long-term aim is to use our understanding of this
molecular behavior to rationally design ruthenium drug
candidates with tunable binding lifetimes due to the unique
and orthogonal coordination chemistry of the metal. From this
we can learn how to attenuate the ligand exchange rates of what
are expected to be very stable complexes. This current work
therefore addresses the fundamental inorganic chemistry
underpinning our strategy for medicinal studies.
One rationale behind the particular suite of complexes
chosen was simply to vary the metallocyclic ring size
systematically. 1, 2, 5, and 6 were successfully synthesized,
purified, and crystallized such that high resolution structures
could be obtained. Despite significant effort, complex 3 has not
crystallized, but we have successfully characterized its structure
in solution using NMR spectroscopy (see below and
Supporting Information section 1). To date, we have been
unable to isolate complex 4, but it can be detected by ESI-MS
in reaction mixtures. Complex 7 has been characterized in
solution only. (See Supporting Information section 1.2.)
In principle, complexation of amino acids in a tridentate
fashion as described above should give rise to a minimum of
four diasteromers with the metal becoming a new stereocenter.
Of course, using enantiomerically pure ligands would result in
only two of the diastereomers being possible, but it is clear
(Figure 1) that the single enantiomers of the amino acids
RESULTS AND DISCUSSION
■
A number of related complexes containing the [(PhH)Ru]2+
unit and amino acids have been synthesized and characterized,
exploiting the enantiopurity of the amino acid feedstock to limit
the diastereomeric outcome of the synthesis. Such complexes
are predicted to be classically inert, being chelates of a low-spin
4d6 metal center with no obviously labile ligand.
The structure of these complexes may be described as a half-
sandwich ruthenium complex formed by the coordination of a
benzene ligand and a tridentate amino acidate ligand (Scheme
1). Coordination of the tridentate amino acidate ligand
produces a fused-metallocyclic motif with the ligands under
discussion here. Varying the side-chain length of the ligand
should alter the characteristics of the fused metallocycle. Due to
the use of naturally occurring amino acids as ligands,
coordination of the α-amino and carboxylate groups produces
a 5-membered metallocycle common to all of these complexes.
With the benzene ligand also being conserved across this series
of complexes, the general structure possesses two features that
can be systematically varied: the amino acid side-chain length,
n, and the identity of the side-chain functional group, Z. (see
Scheme 1 and Table 1).
Figure 1. Possible diastereomers of 1 and the absolute configuration at
the Cα and Ru centers.
cannot generate a tridentate binding motif with the metal in
both the R and S configurations. Hence, using pure L-amino
acids results in only one diastereomer with the ruthenium
exclusively in the R configuration.
This stereochemistry is exactly what is observed for 1, 2, 5,
and 6 both in the solid- and solution-states (Figure 2), and the
NMR spectroscopy data collected for 3 are also consistent with
this.
Additionally, synthesis of 1 using racemic, D,L-DAP and
subsequent X-ray diffraction analysis of the resulting crystals
revealed two cations in the unit cell, related to each other by a
center of inversion; these are one cation of each diastereomer
with configurations SCRRu and RCSRu, respectively.
Table 1. Summary of the Tridentate Amino Acid Complexes
under Investigation and the Generalized Structure of the
a
Cations, [(PhH)Ru(amino acid)]+
side-chain functional
cation
amino acid
n
group (Z)
1
L-2,3-diamino-propionic
acid (DAP)
1
NH2
2
L-2,4-diamino-butanoic
acid (DAB)
2
NH2
3
4
5
6
L-ornithine
L-lysine
3
4
2
2
2
NH2
NH2
L-histidine
L-methionine
L-aspartate
imidazole
SMe
COO−
The existence of enantiopure products is also apparent in the
solution-state. For example, the 1H NMR spectrum of [1][PF6]
shows that the methylene group protons are in unique chemical
environments. Furthermore, the α-CH group proton couples to
b
7
a
We use the complex identifiers (bold numbers in the text) to refer to
3
the monovalent cation. They have all been synthesized as the [PF6]−
salt. 7 is a neutral complex.
both methylene protons with distinct JHH coupling constants
b
(see Supporting Information sections 1.2 and 1.3 for full NMR
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(111.8(4)° and 119.3(2)°, respectively). The increasing Ru−N
bond length and Ru−N−C bond angle reflect poorer orbital
overlap between the amino group lone pair and the vacant
metal orbital, reducing the covalent component to the bonding
and weakening the metal nitrogen interaction.
While it was not possible to characterize [3][PF6] in the
solid-state by X-ray diffraction studies, analysis in the solution-
state using both one- and two-dimensional techniques allowed
for a proposed conformation of the metallocyclic system in
which the pro-chiral protons attached to the α and side-chain
nitrogens and the γ and δ carbon atoms can also be resolved
(Supporting Information section 1.2). The suggested con-
formation of the metallocyclic system of 3 in solution is
depicted in Figure 3.
Figure 2. Ball-and-stick representations of the crystal structures of the
cations 1, 2, 5, and 6. H atoms were not resolved, but the calculated
positions are shown to aid stereochemical appreciation relevant to our
data from NMR spectroscopy.
Figure 3. Proposed structure of [3] (left) compared to the solid-state
structures of [2] (middle) and [1] (right).
assignments). The rigidity of the metallocyclic solid-state
structure of 1 is, therefore, clearly retained in solution as
shown by the nondegeneracy of the protic environments
constrained within the chelating ring. As such the methylene
and amino group protons are pro-chiral. A combination of one-
and two-dimensional methods have enabled us to distinguish
between the pro-chiral methlyene protons as well as those
attached to the metal coordinated α and side-chain nitrogen
atoms (see Supporting Information section 1.3.2).
A similar analysis of [2][PF6] also reveals distinct protic
environments for the protons attached to the pro-chiral
nitrogen and carbon atoms contained within the metallocycles
which indicates that the solid-state structure of this complex is
retained in the solution phase.
As mentioned above, synthesis and isolation of the tridentate
lysine coordinated complex, 4, has not been successful. A
variety of different solvents and anions have been used to try to
capture this cation as a salt but without success. NMR
spectroscopy and ESI-MS studies have shown a variety of
species in the reaction mixtures. The NMR spectra have not
been interpretable, but the MS data show that the tridentate
species is present (m/z = 325.1 Da, expected 325.0). In
addition we also observe species with the lysine coordinated to
the ruthenium in a bidentate manner with either a chloride
(m/z 361 Da) or water (m/z 342) ligand occupying the
remained coordination site of the metal. Two smaller species
that we cannot identify (m/z 307 and 262 Da) were also
apparent.
Varying the side-chain functional group will clearly impact
the structure of the metallocycle which it generates. This can be
seen for the complexes 2, 5, and 6 for which a 6-membered
metallocycle is fixed by the side-chain. Complexes 2 and 6
result in chairlike conformations of this metallocycle while the
two sp2 centers of the imidazole in 5 produce a twist-chairlike
conformation (Figure 4).
Interestingly, as well as the C and Ru configurations, a third
stereogenic center arises in the case of binding methionine to
the [(PhH)Ru]2+ synthon. While tridentate L-methionine binds
to give the R-configuration at Ru, the sulfur center becomes
either R- or S-, depending on the position of the methyl group
with respect to the stereochemically active lone pair. We
obtained crystals of the complex showing the sulfur center in
the S-configuration, but computational modeling of the two
configurations suggests that they differ in energy in vacuo by
1
only 0.050 kcal mol−1. The H NMR spectrum of the product
In keeping with the results for the other metallocyclic
structures, an analysis of the solution-state structure of
[5][PF6] allows the protons attached to the pro-chiral α
displays two species in a 1:1 ratio, consistent with racemization
at the S center. We believe that although we isolated only one
species in the crystal, both exist in solution. Whether or not this
is due to fluxionality at the sulfur center is unclear from our
studies to date.
It is clear that the length of the side-chain plays a significant
role in the effectiveness of the coordination of the side-chain.
For the complexes we have been able to structurally
characterize in detail, 1 and 2, the differences are subtle. A
small increase in Ru−N bond length on going from 1 (2.131(4)
Å) to 2 (2.153(3) Å) is observed alongside a large change in
the Ru−N−C bond angle for the side-chain amino group
Figure 4. Solid-state structures of complexes 2 (middle), 5 (right), and
6 (left) comparing conformations of 6-membered metallocycles.
C
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nitrogen atom to be distinguished from one another (although
in this case the methylene protons appear coincident in the 1H
NMR spectra).
straightforward correlation between optimized ruthenium
nitrogen bonding (apparent from the bond lengths and angles
observed in the solid-state structures) and faster exchanging
NH protons. We do believe that the differences in the
observable rates of exchange indicate subtle distinctions in
reactivity of the protons and the ruthenium−nitrogen bonds to
which they append, and we interpret these in terms of changing
pKa values (see below).
Reaction of Lys·HCl with [(PhH)RuCl2]2 in the absence of
base resulted in crystals of [8][SbF6] suitable for X-ray
diffraction, showing bidentate lysine chelating [(PhH)RuCl]+
through the carboxylate and α-amino groups (Figure 5).
Such a thorough analysis of the pro-chiral proton environ-
ments within the metallocyclic structures was prompted as a
result of observing distinct and, in some cases, significantly
different H−D proton exchange rates for the coordinated NH
protons, further demonstrating their chemical inequivalence.
We sought to investigate whether these rates of exchange could
be correlated with the strength of the coordination bond
formed between the metal and the amine in the hope that these
exchange rates might find use as “reporters” for the strength
and reactivity of the metal−nitrogen bonds contained in the
complexes. The results are presented in Table 2. The resonance
of the γ-NHproR of 2 is very close to the water signal, and this
precludes accurate measurement of its rate of exchange in
buffered D2O.
Table 2. Observed First-Order Rate Constants (s−1) for H−
D Exchange of Amino Group Protons in Cations 1, 2, 3, and
a
5
H(α‑pro‑R)
H(α‑pro‑S)
H(side‑pro‑R)
H(side‑pro‑S)
d
d
1
2
3
5
3.61 × 10−4
b
2.57 × 10−6
1.35 × 10−3
4.46 × 10−5
1.08 × 10−6
d
d
d
1.75 × 10−3
d
2.07 × 10−3
c
a
measured at 300K in 10 mM sodium phosphate buffer, pH* 6.8.
b
The rate of exchange of the side-chain pro-R NH proton of 2 cannot
c
be measured because of overlap with the solvent resonance. These
protons are not present in 5. The imidazole NH exchanges with a rate
d
constant >5 × 10−2 s−1. Exchange rates too rapid to be measured by
this technique. The fastest exchange rate that can be measured by
NMR spectroscopy has been estimated as 5 × 10−2 s−1.
It is striking that the exchange rates vary by over 4 orders of
magnitude and that, in some instances, the two protons on the
same nitrogen exchange at significantly different rates; for
example the pro-R and pro-S protons of the γ-NH2 in 1
exchange with 30-fold different rates. This difference in rates for
two protons attached to the same coordinating atom suggests
that the exchange does not involve breaking of the Ru−N bond.
H−D exchange may occur via an acid- or base-catalyzed
mechanism. The acid-catalyzed mechanism would involve
ligand dissociation of the amino group as the rate-determining
step prior to a rapid H−D exchange, whereas for the base-
catalyzed mechanism, deprotonation of the coordinated amino
group would be rate-determining. The H−D exchange
experiments were carried out in D2O at pH* 6.8. As such,
the base is the solvent, and the observed rate of exchange is
expected to be first order for either mechanism. The base-
catalyzed mechanism is favored by stronger Ru−N bonds
stabilizing the conjugate base, implying a lower energy
transition state on achieving the intermediate. Further to this,
the acid-catalyzed mechanism would necessitate a highly
organized intermediate for the pro-R and pro-S amino group
protons to exhibit the distinct exchange rates that are observed
for them.
Figure 5. Computed gas-phase structures of 8 (top left), its
diastereomer (top right), and the crystallographically obtained
structure of 8 for comparison (bottom).
Charge balancing suggests that the side-chain amino group is
protonated in this structure. Although there was insufficient
material for NMR spectroscopic analysis, ESI-MS gave signals
consistent with the composition of the crystal structure. Only
one enantiomer was observed in the solid-state. Computation
of the respective energies of the different binding modes
(Figure 5) suggests that the in vacuo energies of the isomers
differ by 1.141 kcal/mol, with the stereochemistry found in the
solid-state being the more stable. It should be noted, however,
that the calculated geometry differed significantly from that
found in the crystal structure, apparently favoring internal
−
hydrogen bonding in the absence of the SbF6 counterion.
More interesting still, the crystal structure of [8][SbF6]
suggests that the bidentate binding mode could be favored over
the tridentate arrangement by simple protonation of a would-be
ligand. The possibility of pH-triggered hemilability was one we
found interesting. We reacted one of the tridentate complexes,
1, with acetic acid in water in an attempt to labilize a donor
atom by trapping it with a proton as a pendant conjugate acid.
This would allow another Lewis base such as acetate or water to
fill the now-vacant site.
The decrease in exchange rate as the metallocycle ring size
increases is significant, and we believe reflects, paradoxically, a
weakening of the side-chain coordination. However, this
interpretation is not obvious from the structural data we have
gathered thus far, as there does not appear to be a
In the event, analysis of the reaction mixture by MS
suggested that [1·HOAc] had formed (as well as [1·HCl]); we
attribute the presence of chloride to impurities in the solvent.
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Consistent with the MS signals, the NMR spectra of the
reaction mixture display NOESY cross-peaks between the
methyl group of acetic acid and both the β-CH2 and α-CH
signals from the DAP ligand, as well as two new η6-benzene
environments and two new 13C signals in the carbonyl region.
These results show that an “inert” complex could become
primed to react by a lowering of pH. This is particularly
relevant to medicinal ruthenium research, as cancerous cells are
typically more acidic than healthy cells. An acidic pH “trigger”
like this could be incorporated into a Ru(II) drug, increasing its
specificity. While searching the literature for such reactivity is
challenging, we do not believe that such acid-catalyzed
hemilability has been reported for small-molecule complexes
before, though the behavior is well-known in metalloproteins.
Becoming interested in the potential of [1][PF6] to react
with biomolecular targets, we attempted to react it with N-
acetyl methionine methyl ester. This N,O-protected amino acid
better mimics methionine in a peptide than the free amino acid,
leaving only the soft thioether group as a possible coordination
site.
6) corroborate this: The HOMO of 1 has large, orthogonal
orbital coefficients on the oxygen atoms.
Figure 6. HOMO of 1. Note the large coefficients on the oxygen
atoms, which seem to become labilized in acid in preference to the
amino groups.
Noting that this reaction still yielded only a small proportion
of [1 N-Ac-met-OMe]+, and that it was extremely slow, we
considered how a pH buffer might affect the reactivity. Having
already established that acid labilizes the DAP, we incubated
[1][PF6] with N-Ac-met-OMe in acidic buffer.
This reaction was initially followed by MS. Within 2 days, a
significant peak due to [1·N-Ac-Met-OMe]+ was observed at
m/z = 488.1 Da (expected 488.08 Da), though 1 was still
clearly present. As mentioned above, one-dimensional NMR
spectroscopy was difficult to interpret due to signal overlap, but
a DOSY spectrum (see Supporting Information Figure 3.2) was
consistent with the arene and methionine fragments being
present in the same species, a strong indication of covalent
bonding between the ruthenium starting material and the
protected methionine molecule.
Further, this result indicates that the carboxylate residue
(rather than an amine ligand) has been displaced from the
metal. Displacement of an amine ligand would yield an
ammonium group whose presence would be detected by ESI-
MS, but the gain of an additional proton is not observed.
Reducing the hapticity of the arene ring seems unlikely to yield
a product so stable in aerobic water.
In principle there are two possible stereochemical outcomes
at the Ru center from the proposed carboxylate displacement
by N-Ac-Met-OMe: either with retention or inversion of
ruthenium’s stereoconfiguration. These possibilities were
investigated computationally. The optimized gas-phase struc-
tures (Figure 7) differed in energy by only 0.027 kcal/mol,
suggesting that product stereocontrol (if any) would be kinetic
rather than thermodynamic. Spectroscopy did not show
evidence that would distinguish the species (for example, no
NOESY cross-peak was observed between the DAP or
methionine ligands and the arene ring). Although we remain
unsure about the dominant product stereochemistry at Ru, the
increased speed and extent of the substitution reaction in the
presence of acid is noteworthy.
Incubating excess N-Ac-Met-OMe with [1][PF6] at 40 °C
over 6 weeks resulted in a solution whose MS showed a peak
1
due to [1·N-Ac-Met-OMe]+. The one-dimensional H NMR
spectrum showed no new, resolved signals (see Supporting
Information section 3), but we believe that this is simply
because the product peaks overlap with those of the starting
material; the NOESY spectrum indicates through-space
coupling between the aliphatic α-CH and β-CH2 protons on
the DAP ligand and the methionine γ-CH2 group, adjacent to
the sulfur atom (see Supporting Information section 3). We do
not believe that these cross-peaks can be explained by simple
intermolecular interaction: they suggest the formation of a Ru−
S bond.
Interestingly, the MS data suggest that the carboxylate ligand
has dissociated in preference to the amino group, because the
resulting free amine would become rapidly protonated in
aqueous acid, and should be seen as a peak due to [1-H]2+,
which is absent from the ESI-MS spectrum. We note that this is
consistent with expected Ru−X bond strengths: the soft N
center is a better σ-donor than the hard O center. Further, the
carboxylate ligand is vulnerable to activation by protonation
(Scheme 2) in a way that the coordinatively saturated amine
ligand is not: the carbonyl group has multiple lone pairs of
electrons which could conceivably act as a Bronsted base,
resulting in the decomplexation of the neutral −COOH
fragment. Under most pH regimes, this would be then expected
to revert to the carboxylate. DFT calculations (see Supporting
Information section 5) displaying the forms of the MO (Figure
Scheme 2. Proposed Mechanism for Acid-Catalyzed Ligand
Substitution
CONCLUSIONS
■
Controlling the diastereomer formed in reactions between
[(PhH)Ru]2+ and a tridentate amino acid is quite straightfor-
ward, and depends upon the enantiomer of amino acid chosen.
In this set of examples, we can see that the ease of synthesis and
practical stability of the tridentate complex formed is correlated
with the ring size of the resultant metallocycle formed when the
side-chain coordinates. From our observations, 5- or 6-
membered rings appear relatively easy to synthesize and
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acid, one could make either the side-chain ligand or the “main
chain” carboxylate the labile ligand.
The acid-triggered lability of the resulting chelates is an
extremely interesting feature, and appears particularly relevant
to anticancer applications, as it could provide a high degree of
selectivity between rates of reaction in healthy and diseased
cells. Building upon the synthetic and structural understanding
discussed here, we can continue to study the reaction of this
suite of complexes with simple and more complex biochemical
targets. These pro-complexes may also be of interest to other
medicinal applications or as biocompatible catalysts19 with the
potential to provide specificity and enantioselectivity, an
attractive behavior in a drug candidate; we hope that the use
of these chelates will provide a new strategy in the move away
from cisplatin-type lability that is thought to be the origin of
highly nonselective binding and general cytotoxicity.
EXPERIMENTAL SECTION
■
Figure 7. Optimized structures of [1·N-Ac-met-OMe]+. The bottom
Syntheses. The syntheses of [(PhH)RuCl2]2, [(PhH)Ru(Aa)]-
[PF6] (Aa = diaminopropionic acid (DAP) [1][PF6], diaminobutanoic
acid (DAB) [2][PF6], ornithine [3][PF6], histidine [5][PF6],
methionine [6][PF6], aspartic acid [7]) as well as that of
[(PhH)Ru(Lys-H)(Cl)][SbF6] ([8][SbF6]) and the attempted syn-
thesis of lysine [4][PF6] are detailed in the Supporting Information.
All reagents were used as received. Syntheses involving air- and
moisture-sensitive reagents and products were undertaken using the
appropriate precautions.
In the representative synthesis of [1][PF6], [(PhH)RuCl2]2 (149
mg, 0.299 mmol) and 2,3-diaminopropionic acid monohydrochloride
(84.9 mg, 0.604 mmol) were dissolved in MeOH (15 mL).
Triethylamine (0.08 mL, 0.6 mmol) was added, and the solution
stirred overnight at 40 DEG under nitrogen. The solution was filtered,
and the filtrate exposed to vacuum at 40 °C until precipitate was
observed. NH4PF6 (299 mg, 1.83 mmol) was then added, precipitating
the product as a yellow solid, which was filtered, washed with ice-cold
MeOH, and dried in vacuo to give a daffodil-yellow powder (103.8 mg,
0.243 mmol, 40%). Recrystallization from hot MeOH gave needle-like
crystals suitable for X-ray diffraction analysis.
structure is more stable by 0.027 kcal/mol.
crystallize readily, while the single example we have of a 7-
membered ring has only been characterized in solution by
NMR. We can see that with side-chain amino groups as the
ligands, varying this ring size from 5 to 8 has dramatic
consequences on the stabilities of the complex cations
generated, to the point that the 8-membered ring will not
stably form.
By definition, coordination of an amino group to the metal
lowers the pKa values of both the protonation of the neutral
amino group and for the protonation of a coordinated imido
group (−NH−). At pH 7, it is clear that the amino acid side-
chains are coordinated as NH2 but then surprisingly, in the
examples shown above, the H/D exchange rates for the two
protons on this coordinated amine exchange more slowly than
those of the coordinated α amino group protons. In addition,
the exchange rate for these side-chain amino group protons
decreases as the side-chain gets longer, perhaps indicating that
the pKa values for the side-chain amino groups are being raised
back toward their values in the free amino acid. This may
indicate weaker bonding of the nitrogen to the metal to the
point that, in the tridentate lysine complex, protons compete
with the metal for the lysine ε amino group and the tridentate
complex will not stably form. Lysine is, however, perfectly
stable in a bidentate coordinated mode. DAP has been used in a
diamino coordinated bidentate mode before.14
Instrumentation. Mass spectrometry was performed on a
Micromass Quattro LC ESI-mass spectrometer. Samples were typically
prepared in ultrapure water or 50:50 ultrapure water/acetonitrile.
Ionization was achieved with a capillary voltage of 2.8 kV, cone voltage
of 30 V and a collection voltage of 3 V. Desolvation and capillary
temperatures were 40 °C.
1H, 13C, and two-dimensional NMR spectra were collected using a
1
DRX-500 FT-NMR or ATM DPX-400 FT-NMR spectrometer. H
NMR spectra were collected at 500 and 400 MHz on the respective
spectrometers and 13C NMR spectra at 125 MHz.
Data from proton−deuterium exchange experiments were collated
and analyzed with Bruker’s Dynamic Centre Package. Integrals were
normalized relative to the 6H singlet of the benzene ligand (assumed
to remain constant). A detailed description of experiments is provided
in the Supporting Information.
In the case of DAP the amino acid carboxylate can be
protonated and shown to be labile. This suggests that, within
the coordinated complex, the apparent pKa for protonation of
this conjugate base must be higher than that for protonation of
either the α or β amino groups in this ligand. This lowering of
the amino group pKa values has not been explored before in
this context.
Single-crystal X-ray diffraction studies were performed at 180 K
using a Nonius Kappa CCD. Structures were solved by direct methods,
and refinement on F2 was by full-matrix least-squares techniques.
IR spectra were collected from solids on a PerkinElmer Spectrum
TWO FT-IR machine and analyzed using the Spectrum software
package.
We have shown that [(PhH)Ru(amino acid)]+ complexes are
relatively stable and long-lived in buffered aqueous solution but
could be triggered into ligand exchange in the right
environment, which could either be a change in the bulk
solution properties (in a specific biological compartment for
instance) or by local provision of alternative, strong Lewis basic
donor atoms, such as when bound to a specific protein or
nucleic acid structure where ligand exchange can be triggered
by the structural environment. By appropriate choice of amino
DFT calculations were performed using the Gaussian09 package.20
After trialing several different methods (see Supporting Information),
we settled on using the B3LYP hybrid functional and the LANL2DZ
basis set for Ru and the 6-31G(d,p) basis set for all other elements. We
have included a more detailed workflow in the Supporting Information
to enable reproducibility, and hope that the wider chemical
community will consider sharing workflows in a similar manner.
F
DOI: 10.1021/ic502051y
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
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M.; Laurenczy, G.; Geldbach, T. J.; Sava, G.; Dyson, P. J. J. Med. Chem.
2005, 48, 4161−4171.
ASSOCIATED CONTENT
■
S
* Supporting Information
Details of the synthesis of all compounds, NMR analyses, X-ray
diffraction and ORTEP diagrams, details of computational
methods. Crystallographic data in CIF format. This material is
available free of charge via the Internet at http://pubs.acs.org.
(14) Barragan
Canto, A.; Sadler, P. J.; de Llorens, R.; Moreno, V.; Gonzal
Massaguer, A.; Marchan, V. Bioconjugate Chem. 2012, 23, 1838−1855.
(15) Bíro, L.; Balogh, E.; Buglyo, P. J. Organomet. Chem. 2013, 734,
61−68.
́
, F.; Carrion-Salip, D.; Gom
́
ez-Pinto, I.; Gonzal
́
ez-
́
́
ez, C.;
́
AUTHOR INFORMATION
■
́
́
Corresponding Authors
*E-mail: srb26@cam.ac.uk.
*E-mail: pdb30@cam.ac.uk.
(16) Egbewande, F. A.; Paul, L. E. H.; Therrien, B.; Furrer, J. Eur. J.
Inorg. Chem. 2014, 2014, 1174−1184.
(17) Melchart, M.; Habtemariam, A.; Novakova, O.; Moggach, S. A.;
Fabbiani, F. P. A.; Parsons, S.; Brabec, V.; Sadler, P. J. Inorg. Chem.
2007, 46, 8950−8962.
Present Address
†H.S. current address is the Chemistry Department, University
of Oxford, South Parks Road, Oxford, United Kingdom.
(18) Reiner, T.; Jantke, D.; Miao, X.-H.; Marziale, A. N.; Kiefer, F. J.;
Eppinger, J. Dalton Trans. 2013, 42, 8692−8703.
Author Contributions
†The first two authors contributed equally to the work.
(19) Dougan, S. J.; Habtemariam, A.; McHale, S. E.; Parsons, S.;
Sadler, P. J. Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 11628−11633.
(20) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci,
B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H.
P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.;
Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima,
T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.;
Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin,
K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.;
Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega,
N.; Millam, M. J.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.;
Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.;
Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.;
Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.;
Author Contributions
T.G.S., M.J.O., A.J.P., P.W.S., P.D.M., and H.S. designed,
performed, and analyzed experiments. M.J.O. carried out DFT
calculations. T.G.S. and M.J.O. wrote the initial draft. S.R.B.
and P.D.B. designed experiments, analyzed data, and wrote the
manuscript. All authors have given approval to the final version
of the manuscript.
Funding
T.G.S. and M.J.O. thank the EPSRC for Studentships EP/
P505445/1 and EP/K503/009/1, respectively.
Notes
The authors declare no competing financial interest.
̈
Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.;
Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09,
Revision D.01; 2009.
ACKNOWLEDGMENTS
■
We thank Dr. Alex Simperler, and the NSCCS training facilities,
for assistance with DFT calculations, Duncan Howe, of the
Chemistry Department NMR service, Dr. John Davies for
crystallographic data collection and refinement, and BP for a
summer studentship to H.S.
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DOI: 10.1021/ic502051y
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