Ru(ii)-dmso complexes containing azole-based ligands: Synthesis, linkage isomerism and catalytic behaviour

Article abstract of DOI:10.1039/c5dt04376j

The reaction of cis,fac-[RuCl₂(dmso-S)₃(dmso-O)], 1, with different azole (L) ligands leads to new [RuCl₂(L)(dmso-S)₃] compounds (L = CH₃-pz-H, 2; NO₂-pz-H, 3; CF₃-pz-H, 4 and Br-Hind, 5). Complexes 2-5 have been characterized by analytical, spectroscopic and electrochemical techniques as well as by monocrystal X-ray diffraction analysis. Upon oxidation to Ru(iii) the complexes undergo linkage isomerization of a S-bound dmso ligand and the corresponding kinetic rates as well as the thermodynamic properties have been determined for compound 2 and also for the previously described [Ru^IICl₂(pypz-H)(dmso-S)₂] (pypz-H = 2-(3-pyrazolyl)pyridine), 6, from cyclic voltammetries performed at different scan rates. The exposure of compound 2 to visible light in acetonitrile produces the substitution of one dmso ligand by a solvent molecule generating a new compound, 2′. The irradiation of solutions of compounds 2 and 6 in chloroform leads in both cases to the substitution of one dmso by a chlorido ligand in parallel to the oxidation of Ru(ii) to Ru(iii) generating complexes 2′′ and 6′ respectively. The reactivity of compounds 2-6 has been tested with regard to the hydration of nitriles in water as a solvent, displaying in all cases good performance and selectivity for the corresponding amides.

Full text of DOI:10.1039/c5dt04376j

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Ru(II)-dmso Complexes containing Azole-based Ligands:
Synthesis, Linkage Isomerism and Catalytic Behaviour
Íngrid Ferrer, Xavier Fontrodona, Montserrat Rodríguez* and Isabel Romero*
Departament de Química and Serveis Tècnics de Recerca, Universitat de Girona, Campus de Montilivi, Eꢀ17071 Girona, Spain.
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Abstract
Reaction of cis,fac-[RuCl2(dmsoꢀS)3(dmsoꢀO)], 1, with different azole (L) ligands leads to new [RuCl2(L)(dmsoꢀS)3]
compounds (L = CH3ꢀpzꢀH, 2; NO2ꢀpzꢀH, 3; CF3ꢀpzꢀH, 4 and BrꢀHind, 5). Complexes 2-5 have been characterized
by analytical, spectroscopic and electrochemical techniques as well as by monocrystal Xꢀray diffraction analysis.
Upon oxidation to Ru(III) the complexes undergo linkage isomerization of a Sꢀbound dmso ligand and the
corresponding kinetic rates as well as the thermodynamic properties have been determined for compound 2 and also
for the previously described [RuIICl2(pypzꢀH)(dmsoꢀS)2] (pypzꢀH = 2ꢀ(3ꢀpyrazolyl)pyridine), 6, from cyclic
voltammograms performed at different scan rates. Exposure of compound 2 to visible light in acetonitrile produces a
substitution of one dmso ligand by a solvent molecule generating a new compound, 2’. The irradiation of solutions
of compounds 2 and 6 in chloroform leads in both cases to the substitution of one dmso by a chlorido ligand in
parallel to the oxidation of Ru(II) to Ru(III) generating complexes 2" and 6' respectively. The reactivity of
compounds 2-6 has been tested with regard to the hydration of nitriles in water as solvent, displaying in all cases
good performance and selectivity for the corresponding amides.
KEYWORDS: ruthenium /dmso complexes/ azoleꢀbased ligands / linkage isomerism / hydration catalysis.
2

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Introduction
Ruthenium compounds containing dmso ligands combined with a variety of auxiliary ligands have been described
as potent antitumoral compounds,1 as precursors for the synthesis of a large variety of complexes2 and also as
catalysts for a variety of reactions including hydrogenꢀatom transfer and hydrogenation,3 aerobic oxidation of
alcohols,4 oxidation of sulphides to sulfoxides5 and polymerization of olefins.6
The hydration of nitriles to generate the corresponding amides is an important transformation from both academic
and industrial point of view.7 This reaction is also of biotechnological interest since nitrile hydratases, a family of
nonꢀheme iron enzymes,8 are used in the industrial preparation of relevant amides such as acrylamide, nicotinamide
and 5ꢀcyanovaleramide.9 Amides not only constitute versatile building blocks in synthetic organic chemistry10 but
also exhibit a wide range of industrial applications of pharmacological interest.9a,11
The development of new transition metal catalysts able to promote this hydration process under mild conditions
and the study of their mechanism is a challenge of special relevance. Most nitrile hydration catalysts proceed by
coordinating the nitrile to the metal centre thus activating the former toward nucleophilic attack by water or
hydroxyl.12 The vacant coordination site for coordination of the nitrile would be generated by the dissociation of a
labile ligand. Cadierno and others13 have developed excellent hydration protocols in pure water under neutral
conditions using areneꢀruthenium (II) and bis(allyl)ꢀruthenium (IV) complexes containing Pꢀdonor ligands as
catalysts. However, to the best of our knowledge, scarce reports on complexes with Nꢀ or Sꢀdonor ligands applied to
hydration of nitriles in water can be found in the literature. We described in a previous work14 the catalytic hydration
activity of the bisꢀ(dmsoꢀS) Ru complex [RuCl2(pypzꢀH)2(dmsoꢀS)2] (where pypzꢀH is the bidentate 2ꢀ(3ꢀ
pyrazolyl)pyridine) that displayed good degrees of conversion and selectivity in the hydration of several nitrile
substrates, and for which the improved nitrile activation could be related to the remarkable electronꢀwithdrawing
character of the dmso ligands that would accelerate the nucleophilic attack on the Ruꢀcoordinated nitrile substrate.
Also, the lability of the dmso ligands facilitates the generation of vacant coordination sites for the catalytic activity.
On the other hand, dmso ligand is classified as a versatile molecule because of its ambidentate behaviour and
ruthenium compounds containing dmso ligands have shown to undergo rapid photoinduced and electronꢀtransferꢀ
induced RuꢀS/RuꢀO linkage isomerism.15 These properties are crucial to their potential application as optical
memory tools,16 molecular machines17 and information storage devices.18 The linkage isomerization has been
analysed in detail in Ru(II) complexes containing one or two dmso ligands3,5,15a,19 but, to the best of our knowledge,
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no report on such process for Ru(II) complexes with three dmso ligands has been described. An improved catalytic
activity can be expected in these trisꢀdmso complexes thanks to the presence of additional πꢀacceptor dmso ligands.
In this work we present the preparation and exhaustive characterization of new Ruꢀ(dmsoꢀS)3 complexes
containing monodentate azole ligands (Scheme 1). We have studied the electronꢀtransferꢀinduced dmsoꢀS/dmsoꢀO
linkage isomerization of dmso ligands in compounds [RuIICl2(CH3ꢀpzꢀH)(dmsoꢀS)3], 2, and the bisꢀdmso complex
[RuIICl2(pypzꢀH)(dmsoꢀS)2], 6.14 We also report the reactivity of both compounds towards light irradiation in CHCl3
and CH3CN, and the behaviour of the whole family of [RuCl2(L)(dmsoꢀS)3] complexes (where L = pyrazolyl
ligands) as catalysts in the hydration of benzonitrile and acrylonitrile in water medium.
Results and discussion
Synthesis and Structure
ꢀ
Reaction of equimolecular amounts of cis,fac-[RuCl2(dmsoꢀS)3(dmsoꢀO)], 1, and RꢀpzꢀH (R= CH3ꢀ, 2; NO2 , 3;
ꢀ
CF3 , 4) or BrꢀHind (5) ligands in refluxing dichloromethane in the absence of light, results in the substitution of the
Oꢀbound dmso ligand in 1 by the corresponding pyrazolyl ligand leading to the formation of the cisꢀCl, facꢀdmso
complexes 2-5 (see Scheme 1). The octahedral arrangement of six monodentate ligands in compounds 2-5 can
potentially lead to the three isomers depicted in Scheme 2.
It is remarkable that, for compounds 2ꢀ4, we have detected the cis-Cl fac-dmsoꢀS (a) as single geometrical isomer
either when the reflux time is limited to 1h or extended up to 20h. The preferential formation of the cis-Cl fac-dmsoꢀ
S isomer in these complexes can be rationalized taking into account electronic factors as the synergistic πꢀdonor and
πꢀacceptor effects between the Cl and dmso ligands mutually placed in trans: in the cis-Cl,facꢀdmsoꢀS isomers, two
of such ClꢀRuꢀdmsoꢀS axes can be found whereas only one or none is present in the rest of isomers. Moreover, in (b)
and (c) isomers, two πꢀdonor (Cl) or πꢀacceptor (dmso) ligands are necessarily placed in trans, which presumably
disfavours their formation. For compound 5, the cis-Cl,fac-dmsoꢀS isomer (a) is initially formed but a second isomer
is detected when the reflux time is extended for longer than 20 minutes. This minor isomer has not been isolated but
its resonances are consistent with the (c) ligands disposition (Scheme 2) because it settles a trans ClꢀRuꢀdmsoꢀS axis
in contrast to the more disfavoured ClꢀRuꢀCl trans disposition in (b). The (c) isomer could be partially favoured in
complex 5 thanks to the higher πꢀacceptor character of the BrꢀHind ligand that promotes its coordination in trans to
a chlorido πꢀdonor ligand.
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The crystal structures of complexes 2-5 have been solved by Xꢀray diffraction analysis. Figure 1 displays their
molecular structures whereas selected bond parameters as well as the main crystallographic data can be found in
Table 1 and Table S1 (Supporting information) respectively.
In all cases, the Ru metal centres adopt an octahedrally distorted type of coordination with the corresponding
pyrazolyl ligand coordinated in trans with respect to a dmso ligand, thus confirming the formation of the (a) isomer
as discussed above (Scheme 2). All bond distances and angles are within the expected values for this type of
complexes.1e,14,19 The four complexes form intramolecular hydrogen bonds between the pyrazole H atoms and the
oxygen atom from the cis dmso ligands (Hꢀbond distances are Hpz–Odmso = 2.012 Å, 2; 2.120 Å, 3; 1.891 Å, 4; 1.952
Å, 5). These Hꢀbond interactions are in most cases stronger than the analogous Hꢀbonds found in other complexes
containing chelate pyrazolyl ligands, due to the lower geometrical restrictions of the RꢀpzꢀH or BrꢀHind
monodentate ligands with respect to polydentate ligands.3,14,19
Spectroscopic properties
The IR spectra for complexes 2-5 show a band around 1092 cmꢀ1 that can be assigned to υSꢀO stretching of
sulphurꢀcoordinated dmso ligands as Sꢀcoordination increases the SꢀO bond order in the dmso molecule, thus
shifting the corresponding υSꢀO stretch frequency to values above that of free dmso (1050 cmꢀ1). This is also
corroborated by the absence of any SꢀO stretching vibration in the 920ꢀ930 cmꢀ1 range, which would conversely
indicate Oꢀcoordinated dmso ligands.2a
The one dimensional (1D) and twoꢀdimensional (2D) NMR spectra of complexes 2ꢀ5 were registered in CD2Cl2
and are presented in the Supplementary Information (Figures S1ꢀS4). The resonances found for all the complexes
are consistent with the structures obtained in the solid state. The complexes exhibit two sets of signals: one in the
aromatic region corresponding to the nitrogen ligands and the other one in the aliphatic region with three singlets
corresponding to the methyl groups of the bonded dmso ligands (and an additional singlet assigned to the pyrazole
methyl group in the case of complex 2). The complexes display a symmetry plane that contains the pyrazolyl ligand
and the dmso in trans to it and thus the two methyl groups of this trans dmso (for instance C8 and C7 in complex 2,
see Figure 1) are magnetically equivalent. The two remaining dmso show two additional singlets that, from
symmetry considerations, can be tentatively assigned (in the case of complex 2) to C5+C10 (δ = 3.40 ppm) and
C6+C9 (δ = 3.12 ppm), on the basis of the deshielding effect produced by the two Cl ligands, closer to C5 and C10.
The NOE crosspeaks observed between the pyrazolic hydrogen H2 and the singlets at 3.12 and 3.40 ppm (taking
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complex 2 as example) allows identifying unambiguously the two dmso in cis to the pyrazolyl ligand. The
intramolecular hydrogen bonds observed in the structure of these complexes have been also evidenced in solution
because the pyrazolic hydrogen atoms appear downfield in all cases with respect to the corresponding free ligands,
and are also more deshielded than the rest of the hydrogen atoms upon coordination.
The UVꢀVis spectra of complexes 2ꢀ5 are displayed in the Supplementary Information (Figure S5) and the
spectral features of complexes are presented in the experimental section. The complexes exhibit ligand based πꢀπ*
bands below 300 nm and relatively intense bands above 300 nm mainly due to dπꢀπ* MLCT transitions.20 The low
aromatic character of the pyrazole ring involves ligand orbitals of relatively high energy thus leading to the
occurrence of dπꢀπ* absorptions at lower wavelengths than those of analogous complexes containing
pyridylpyrazole ligands, with aromatic pyridine rings directly bound to the metal,3,14 and also increases the πꢀπ*
absorption energies that in some cases appear out of the solvent range.
Redox chemistry and linkage isomerization
The redox properties of complexes 2ꢀ5 have been investigated by cyclic voltammetry (CV) experiments, and the
voltammogram of 2 in acetonitrile, starting at 0 V at a scan rate of 0.1 V, is shown in Figure 2 (solid line). As can be
observed an anodic wave, corresponding to the oxidation of the RuII(dmsoꢀS) species to RuIII(dmsoꢀS), is observed
at Epa = 1.49 V vs Ag/AgCl. This redox process is irreversible and this behaviour can be attributed to a relatively fast
linkage isomerization of a dmso ligand in the oxidized species forming RuIII(dmsoꢀO) (see Scheme 3), as will be
discussed below. Oxygenꢀcoordinated dmso is a favoured form in Ru(III) complexes, consistently with Pearson's
concept of hard and soft acids and bases for the harder Ru(III) Lewis acid. Upon back scanning to negative potential
a cathodic peak at Epc= 0.75 V is observed which is consistent with the reduction of the isomerized species to form
RuII(dmsoꢀO) that, after reduction, would rearrange to restore the initial complex. This process becomes more
evident by starting the scan at the highest potential value (1.8 V) and applying an initial equilibration time of 10 min
(Figure 2, dashed line). Under these conditions the anodic peak corresponding to the oxidation of RuII(dmsoꢀO) to
RuIII(dmsoꢀO) can be observed, and an additional reversible wave a E1/2= 0.98 V appears, corresponding probably to
the substitution of one dmso by acetonitrile solvent.
Complexes 3ꢀ5 display identical behaviour to that described for complex 2 (see Figure S6), whereas the pypzꢀH
complex 6 also undergoes a dmso linkage isomerization process, though it is not complete on the scale time of the
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cyclic voltammetry (see below). The electrochemical data for all the complexes together with those of the pyrazole
compound [RuIICl2(pzꢀH)(dmsoꢀS)3] are gathered in Table 2. In all cases, potential shifts of 0.7ꢀ0.9 V are observed
between the Epa (RuIIꢀS→RuIIIꢀS) and Epc (RuIIIꢀO→RuIIꢀO) values, due to the lower electronꢀwithdrawing ability of
the Oꢀcoordinated dmso that makes the upper oxidation states of Ru more accessible. These shift values are in the
same range as those observed in complexes where the isomerization of only one dmso ligand takes place,3,19 then
suggesting that this is also the case for complexes 2ꢀ6.
The redox potential values for these compounds are consistent with the electronic characteristics of the
substituents on the pyrazole ligands. Thus compound 2, containing a methyl σꢀdonor substituent, presents lower Epa
values than compounds [RuIICl2(pzꢀH)(dmsoꢀS)3]14 and 3-5, the latter containing πꢀacceptor substituents. However,
the effect of the πꢀacceptor substituents on the potential values is clearly stronger in complex 4, containing a ꢀCF3
substituent. This fact could probably be due to the more extensive electronic delocalization of the ꢀNO2 and ꢀPh
substituents over the pyrazolic ring in 3 and 5, which would presumably lead to a weaker πꢀacceptor character of
these ligands and therefore to an only moderate increase of the Epa value in comparison to the pzꢀH complex. In the
case of compound RuIICl2(pypzꢀH)(dmsoꢀS)2 6,14 containing the bidentate pyridineꢀpyrazole ligand, a formal
substitution of one dmso ligand by a pyridine ring takes place and this shifts the Epa (RuIIꢀS→RuIIIꢀS) to
significantly lower values (E1/2=1.09 V) when compared to the whole series of trisꢀdmso complexes, thus
manifesting the higher πꢀacceptor capacity of dmso ligands with respect to a pyridine ring.
A throughout kinetic study has been carried out on the linkage isomerization experimented by complexes 2 and 6,
following the method described in the literature by Nicholson and Shain21 that is detailed step by step below.
The scan rates directly influence the intensity of waves of the cyclic voltammograms and this dependence
provides information about the participation of chemical reactions coupled to electrochemical processes (as is the
example of dmso linkage isomerization shown in Scheme 3), allowing the determination of the corresponding rates
and equilibrium constants. We have calculated these constants for the dmso linkage isomerization on compounds 2
and 6 both containing pyrazolyl type of ligands. To do so, cyclic voltammetries at different scan rates, starting the
potential scanning from the lower and upper points of the CV range, have been registered for both complexes in
CH2Cl2 (chosen as solvent with the idea to avoid ligand solvolysis). The set of formulas used are gathered in the
Supplementary Information (Table S3) and the results obtained are gathered in Table 3 together with other Ru
complexes described in the literature.
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The equilibrium constants for the RuIIIꢀO ꢀ RuIIIꢀS reaction (KIIIOꢀS) can be obtained in each case from cyclic
voltammograms recorded through starting the potential scanning from the upper Einit values (1.7 V for 2 and 1.4 V
for 6 with 1 minute of equilibration time at the initial potential in both cases, see Figure 3) and applying equation 1
(Table S3). Plotting the ratio ic1/ic2 vs υꢀ1 and extrapolating υ to infinite in order to obtain the intercept values
(Figures S8 and S9) results in KIIIOꢀS = 0.036 for 2 and KIIIOꢀS = 1.39 for 6 (see Table 3).
The kinetic isomerization constants (kIIIOꢀS and kIIISꢀO) are calculated from plotting υ1/2 vs. the id/ik ratio (eq. 2, Table
S3), where ik represents the measured peak current (ic1) and the id the corresponding diffusional current in the
absence of a chemical reaction (ia1). The ik and id values were obtained respectively from the cyclic voltammetries
registered through reverse (Figure 3) and direct (Figure S7) scan potential. From the value of slope obtained
(Figures S10 and S11), and considering that KIIIOꢀS = kIIIOꢀS/kIIISꢀO, the following kinetic constants can be calculated:
for complex 2, kIIIOꢀS = 0.019 sꢀ1 and kIIISꢀO = 0.53 sꢀ1; for complex 6, kIIIOꢀS = 0.176 sꢀ1 and kIIISꢀO = 0.126 sꢀ1.
With the equilibrium constant KIIIOꢀS and assuming that E° = E1/2 or Epa for each linkage isomer, equation 3 (Table
S3) can be used to calculate the equilibrium constant for the RuIIꢀO ꢀ RuIIꢀS process in the Ru(II) oxidation state,
KIIOꢀS, resulting in KIIOꢀS = 1.53·1012 for 2 and KIIOꢀS = 1.34·1012 for 6. Finally, the kinetic isomerization constants in
the Ru(II) oxidation state can be calculated from eq. 4 in Table S322 and the linear fit of the corresponding plots
(Figures S12 and S13) yield kIIOꢀS= 0.072 sꢀ1 and kIISꢀO= 4.7·10ꢀ14 sꢀ1 for 2 and kIIOꢀS = 0.11 sꢀ1and kIISꢀO = 8.2·10ꢀ14 sꢀ1
for 6.
The high values of KII obtained for compounds 2 and 6 (see Table 3) indicate that, in Ru(II) redox state, the
OꢀS
dmso ligand displays a high preference to be bound to the metal through the S atom in both cases, and the
thermodynamic stability of this form is slightly higher in 2 than in 6. The major stability of complex 6 in the Ru(II)
state with regard to the structurally similar compounds of entries 3ꢀ5 in Table 3 is not easy to explain but could be
related to the distinctive electronic characteristics and the higher volume of the H3p and bpp ligands when compared
to pypzꢀH. Also, in complexes trans,cisꢀ[RuCl2(H3p)(dmsoꢀS)2] and trans,cis-[RuCl2(bpp)(dmsoꢀS)2]ꢀ (entries 4 and
5) the trans disposition of the two chlorido ligands probably diminishes the overall electron density on the Ru metal
centre then facilitating the RuꢀS → RuꢀO isomerization. A similar argument would explain the lower relative
stability of complex outꢀ[Ru(L2)(trpy)(dmsoꢀS)]+ (entry 6) where only one anionic ligand (the deprotonated L2) is
present.
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In the Ru(III) state, linkage isomerization takes place in both complexes (as evidenced by the cyclic
ꢄꢄꢄ
voltammetries discussed above) but, as can be inferred from ꢀ
values, the RuꢀS bound form is still dominant for
ꢁꢂꢃ
complex 6 whereas complex 2 presents a marked preference for the RuIIIꢀO isomer, which is one or two orders of
magnitude higher than the rest of the complexes in Table 3. This is in accordance with the CVs registered for both
complexes, where the cathodic peak corresponding to the RuIIIꢀS→RuIIꢀS reduction process is observed only for
complex 6 whilst complex 2 displays a complete isomerization towards the RuꢀO form immediately after
Ru(II)→Ru(III) oxidation. Steric factors could be responsible for the higher stability of the isomerized form in 2
since three Sꢀbound dmso ligands would probably encounter significant steric hindrance when coordinating in facial
arrangement the smaller Ru(III) ion.
Complex 2 also presents a higher kIII kinetic constant than 6 and this could be due to the presence of an
SꢀO
additional Sꢀbound dmso ligand in 2 instead of the pyridyl ring in 6. A larger number of highly πꢀacceptor auxiliary
ligands will presumably increase the RuꢀS→RuꢀO isomerization rate upon Ru(II)→Ru(III) oxidation, and this is in
accordance with complex outꢀ[Ru(L2)(trpy)(dmsoꢀS)]+ (entry 6) being the one with the largest kIII
value as it
SꢀO
contains four πꢀacceptor pyridyl ligands and only one anionic pyrazolate ring coordinated to Ru. However, it is
noticeable that complex 2, in spite of bearing two anionic πꢀdonor Cl ligands, presents a kIIISꢀO value comparable to
that found for the complex in entry 6, then evidencing the remarkable influence exerted by the third dmso ligand
over the isomerization process in 2. Steric factors arising from the aforementioned facial coordination of three bulky
dmso ligands might also hasten the linkage isomerization in this case. The high kinetic rates for the RuIIIꢀS → RuIIIꢀ
O process in 2 and outꢀ[Ru(L2)(trpy)(dmsoꢀS)]+ are also consistent with the practically quantitative isomerization
observed experimentally for both complexes.5
Photoinduced substitution reactions
In a previous work14 we have investigated the photochemical behaviour of compound 6 in a coordinating solvent
as acetonitrile evidencing the occurrence of lightꢀinduced processes where the substitution of one dmso ligand by
acetonitrile took place. On the other hand, photoinduced decomposition of CHCl3 in presence of Ru(II) compounds
has been also described in the literature.23 In this context, and in order to obtain information about the lability of the
ligands in the synthesized complexes given their potential involvement in some catalytic processes, we have studied
the behaviour of compounds 2 and 6 in CHCl3 and that of complex 2 in CH3CN in presence of light (Scheme 4).
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On irradiating a 1 mM solution of 2 in acetonitrile with an 80W lamp, the colour of the solution changes from
pale to deep yellow, indicating the occurrence of lightꢀinduced processes. The changes have been monitored through
UVꢀvis, NMR and cyclic voltammetry experiments.
The UVꢀvis spectra registered during 90 minutes (Figure 4) show two isosbestic points at 327 and 403 nm and a
new MLCT band appears a higher wavelength (432 nm), which would be consistent with the substitution of a dmso
ligand by a less πꢀacceptor acetonitrile ligand (leading to the formation of the new compound [RuCl2(MeCN)(CH3ꢀ
pzꢀH)(dmsoꢀS)2], 2') but could also indicate a geometrical rearrangement from a cis-Cl to a trans-Cl geometry of
complex 2, as has been reported for other dihalide Ruꢀdmso complexes.2b,c Ligand substitution by acetonitrile is
evidenced by NMR and cyclic voltammetry experiments (see below), but simultaneous structural reorganization
cannot be ruled out. Keeping the irradiation for longer led to the disappearance of the isosbestic points, consequently
indicating that multiple lightꢀinduced processes must be taking place.
The changes in the 1HꢀNMR spectrum of the aliphatic region upon 2→2’ photochemical substitution (Figure S14)
clearly show that free dmso (δ 2.6 ppm) is progressively generated along with the disappearance of the three dmso
singlets located at 3.40, 3.36 and 3.1 ppm. Two new resonances with equal integration values appear at 3.04 and
3.26 ppm that are also consistent with the generation of the bisꢀdmso compound 2’ where two types of magnetically
equivalent CH3(dmso) groups are present. These NMR spectra would be consistent with the generation of either cis-Cl
or trans-Cl [RuCl2(MeCN)(CH3ꢀpzꢀH)(dmsoꢀS)2].
The substitution process in 2 has also been followed through cyclic voltammetry (CV) experiments (see Figure
S15). The intensity of the initial redox wave at Epa=1.49 V progressively decreases upon irradiation, in parallel with
the appearance of a new reversible wave at E1/2=1.04 V. The lower halfꢀpotential value for the substituted compound
is consistent with the exchange of one dmso by MeCN since the replacement of an anionic Cl ligand by a neutral
MeCN would generate much higher redox potentials.24 The new wave observed around 1.2 V corresponds to the
oxidation of free dmso which is also in accordance with the formation of 2'.
The substitution process by acetonitrile has been described in the literature with compounds containing one or two
dmso ligands3,5,15a,19,25 but, to the best of our knowledge, this is the first Ru(dmso)3 compound that presents this
behaviour. Unfortunately, all attempts to isolate compound 2' have failed.
On the other hand, similar experiments were carried out for 2 and 6 in chloroform, chosen as nonꢀcoordinating
solvent, with the aim to evaluate the photochemical behaviour of the complexes in particular regarding the dmso
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linkage isomerization. However, after irradiating a solution of complex 6 in chloroform for 12 h the solution colour
changed from yellow to redꢀbrown. On the basis of similar systems described in the literature,19 we could unravel
that the substitution of one dmso ligand by a chlorido ligand and the subsequent oxidation of Ru(II) to Ru(III) had
taken place, leading to the formation of a new complex, mer-[RuIIICl3(pypzꢀH)(dmsoꢀS)] (6'). The lightꢀinduced
reaction of Ru(II) complexes with CHCl3 has already been described and seems to be initiated by highꢀenergy
transitions of the complex.19
The new complex 6' was isolated by precipitation with ether, and suitable crystals for Xꢀray diffraction analysis
were obtained. The corresponding ORTEP plot is shown in Figure 5, where it can be observed that the complex
displays a distorted octahedral geometry with a meridional disposition of the three chlorido ligands. The structural
features of compound 6' are similar to other trisꢀchlorido compounds described in the literature.19 The main
crystallographic data and selected bond distances and angles for the structure can be found in Tables 1 and S1.
It is remarkable that, despite the Ru(II)→Ru(III) oxidation, the remaining dmso ligand in 6' is coordinated by the
sulphur atom so that no linkage isomerization has taken place, probably due to the stabilization of this dmso by an
intramolecular hydrogen bond (Hpz–Odmso = 2.296 Å) and also to the weakened Lewis acid character of the metal by
the presence of an additional strongly σꢀdonor chlorido ligand.
The CV registered after irradiation (Figure S16) shows that this substitution provokes a decrease of the Ru(III/II)
E1/2 value from 1.09 to 0.05 V due to the additional electronꢀdonating ability of the new chlorido ligand. The
evolution of the UVꢀVis spectra throughout the substitution process is depicted in Figure S17 and the final spectrum,
corresponding to 6', shows spectral features such as those displayed by other similar compounds described.19 A shift
to lower energy absorptions is observed due to the higher σ and lower πꢀacceptor capacity of the Cl ligand with
regard to dmso, that provokes a destabilization of the dπ(Ru) donor orbital. Also, a different pattern of absorption
bands was observed with additional ClpπꢀRudπ* LMCT in the visible region.26 Two isosbestic points at 328 and 352
nm are found, confirming the net conversion of 6 into 6'.
On the other hand, the irradiation of a chloroform solution of compound 2 lead to a colour change from yellow to
brown presumably due, as described above for 6, to a dmso substitution by chlorido with concomitant oxidation of
Ru(II) to Ru(III), generating the new complex [RuIIICl3(CH3ꢀpzꢀH)(dmsoꢀS)2], 2". The UVꢀVis spectra registered
during 24 hours (Figure S18) show spectral features such as those displayed by other similar compounds described
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in the literature19 and thus the substitution of one dmso ligand by a chlorido ligand, together with the oxidation of
Ru(II) to Ru(III), has seemingly taken place leading to the generation of complex 2".
The CV registered after irradiation, starting the potential scanning at ꢀ0.2 V vs Ag/AgCl (Figure S19), shows two
new redox processes at E1/2 = 0.1 V and 0.3 V that, unlike the former Ru(III/II) wave in 2, are electrochemically
reversible. We have further investigated the electrochemical behaviour displayed by complex 2" through differential
pulse voltammetry (DPV) at different initial equilibrium times and starting the potential scanning at ꢀ0.2 or at +0.6
V. The DPV obtained are displayed in Figure 6 and, as can be observed, the redox process at E1/2 = 0.3 V is
manifested only when the potential scanning starts at low E values (Figure 6a). Moreover, upon lengthening the
equilibrium time the relative intensity of this redox process (compared to that at E1/2 = 0.1 V) increases from
approximately 1:1 at equilibrium time = 0 s up to 1:2.5 at equilibrium time = 180 s. The reverse scanning potential
(Figure 6b) mainly displays the E1/2 = 0.1 V redox process independently of the equilibrium time applied. Taking
into account these observations a plausible explanation is that the irradiation of 2 leads predominantly to the
formation of a Ru(III) complex (2") where one of the two remaining dmso ligands has undergone linkage
isomerization and is bound through its O atom. This RuꢀOdmso compound, that displays a Ru(III/II) redox process at
E1/2 = 0.1 V, experiences a partial RuꢀO → RuꢀS isomerization upon reduction to Ru(II), generating a new linkage
isomer with E1/2 = 0.3 V where the two dmso ligands are coordinated through their S atoms.
The behaviour displayed by the Ru(III) trisꢀchlorido complex 2" differs from that discussed above for complex
6', where the remaining dmso ligand was coordinated through sulphur despite being a Ru(III) compound. Some
factors could explain this distinctive behaviour: 1) as discussed earlier, a dmso ligand presents higher πꢀacceptor
character than a pyridyl ring and, consequently, the overall electronꢀwithdrawing character of the ligands is
enhanced in 2" when compared to 6', thus facilitating the isomerization towards the less πꢀacceptor Oꢀbound dmso;
2), the remaining dmso ligand in 6' is stabilized by Hꢀbonding interactions with the pyrazolyl H atom.
On the other hand, the difference between the potential values for the Oꢀ and Sꢀbound dmso forms in 2"
(approximately 0.2 V) is lower than that displayed by complexes 2-5 and 6, which is around 0.6ꢀ0.8 V. This could
be explained by the increased electron density at the Ru metal centre in 2" thanks to the third anionic chlorido
ligand that probably balances to a higher extent the electronꢀwithdrawing character of a Sꢀbound dmso.
Unfortunately, the isolation of suitable crystals for the Xꢀray diffraction structure of this species has not been
achieved so the presence of an Oꢀbound dmso cannot be unambiguously confirmed. Yet, the low potential values
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observed are in agreement with the substitution of one dmso by a chlorido ligand owing to the higher electronꢀ
donating character of the latter.
Catalytic hydration of nitriles
We have checked the activity of complexes 2-6 as precatalysts in the hydration process of benzonitrile and
acrylonitrile as substrates, in neutral conditions using water as solvent at 80ºC. The remaining nitrile has been
quantified through GC chromatography with biphenyl as internal standard and the hydrolysis products have been
analysed by NMR spectroscopy and compared to pure samples of the corresponding amide and acid derivatives.
Conversion and selectivity values are summarized in Table 4 together with the results previously obtained for the
analogous complex [RuCl2(pzꢀH)(dmsoꢀS)3], containing the pyrazole ligand.14 Blank experiments without any
catalyst were carried out by keeping the substrates in water at 80ºC for 20h. In all cases, the nitrile was
quantitatively recovered.
As we can observe in Table 4, the complexes were found to be active towards nitrile hydration with moderate
conversion values (though for benzonitrile hydration with complex 3 a good conversion value of 85% is attained).
However, the most remarkable feature is the excellent selectivity observed for the corresponding amides in all cases.
It is interesting to emphasize the performance of these compounds in the hydration of acrylonitrile, where the
industrially relevant acrylamide product is quantitatively obtained.
A possible mechanism currently accepted in the hydration of nitriles is a ligand substitution process, where a
labile ligand is initially replaced by the corresponding nitrile substrate.12 Then, the electronic properties of the
catalysts will likely influence the extent of the hydration reaction, providing that a nucleophilic attack of water (or
hydroxo anions) on the nitrile carbon atom takes place.27 The relatively good performance of compounds 2-5
could be explained by electronic factors (a high number of electronꢀwithdrawing ligands that would render the
ruthenium metal centre more electrophilic and consequently would lead to a better nitrile activation upon
coordination) and also by the occurrence of a large number of potentially labile sites, presumably those
occupied by dmso ligands. Decoordination of dmso during the catalytic process is supported by the fact that
free dmso is found in all cases when analysing the hydrolysis products by NMR spectroscopy. To further
confirm dmso decoordination, a sample of catalyst 2 was kept in water at 80ºC and the evolution was
monitored through UVꢀvis spectroscopy (Figure S20). A new band at 516 nm progressively appears which
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would be in accordance with the substitution of a dmso ligand by a less πꢀacceptor aqua or hydroxo ligands.
1
This is also consistent with the displacement of the ligands resonances observed in the final HꢀNMR spectrum
together with an increase of the free dmso signal (Figure S21). However, the final NMR shows the presence of
multiple resonances which can be indicative of the formation of distinctive hydrolysed Ru species.
Clear differences in the conversion values are observed in Table 4 among catalysts 2ꢀ5 (containing monodentate
pyrazolyl ligands) in the hydration of a specific substrate that are likewise originated by the distinctive electronic
characteristics of the monodentate azole ligands, as the rest of ligands and the catalyst geometry are identical for the
whole set of complexes 2ꢀ5. This influence can be observed for instance when using benzonitrile as substrate.
Indeed, an enhanced performance is found, as expected, for complex 3 having the NO2 electronꢀwithdrawing
substituent when compared to [RuCl2(pzꢀH)(dmsoꢀS)3], and a decrease in conversion is shown for the methylꢀ
pyrazole complex 2, with a more σꢀdonor ligand. The indazole complex 5 would also be expected to display better
activity than [RuCl2(pzꢀH)(dmsoꢀS)3] but this is not the case, probably due to a certain delocalization effect of the π
electron density of the Brꢀphenyl ring on the pyrazole. Complex 4 is also expected to display improved activity, but
it undergoes a colour change during the catalytic process that most certainly indicates a catalyst deactivation, thus
yielding the lowest conversion value for this substrate. For complex 6, the formal replacement of a dmso ligand by a
pyridyl ring slightly decreases the conversion value with regard to [RuCl2(pzꢀH)(dmsoꢀS)3] but still displaying a
relatively good performance. Structural factors might also be influencing the performance of complex 6 due to the
presence of a more rigid bidentate ligand.
The comparison of catalysts 2ꢀ5 in the hydration of acrylonitrile follows a tendency similar to that described
above for benzonitrile: complex 2 displays lower activity than [RuCl2(pzꢀH)(dmsoꢀS)3] and complex 5 performance
is lower than expected. Yet, in this case complex 4, with the CF3 substituent, does not undergo any remarkable
change of colour that could be related to catalyst decomposition as was the case for benzonitrile, and it displays a
relatively high activity. Finally, complex 6 displays low performance for the hydration of acrylonitrile, with only
22% conversion. These results would manifest a certain influence of the electronic characteristics of the substrates
as the more electrophilic catalysts 3 and 6 attain relatively good conversion values for the more activated substrate
benzonitrile, whereas for acrylonitrile (where electronic factors of the substrate are not expected to have a relevant
influence) the performances are moderate.
Conclusions
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We have synthesized and fully characterized a new family of Ru(dmso)3 complexes containing azoleꢀbased
ligands. The substituents on the ligands do not have any remarkable influence on the structural parameters but some
differences are exhibited in the redox potential values and in the performance of the complexes towards the
hydration of nitriles in water. All compounds have displayed good selectivity for the amide products being the first
Ru(dmso)3 compounds capable to carry out this reaction in water.
Upon oxidation to Ru(III), linkage isomerization of one of the Sꢀcoordinated dmso takes place in the case of
compounds 2 and 6, and scan rate dependent voltammograms permitted to estimate the kinetic linkage isomerization
rates and the thermodynamic equilibrium constants evidencing a markedly higher preference of complex 2 for the
Oꢀdmso form in the oxidation state Ru(III) when compared to that of 6, which is in accordance with the high degree
of linkage isomerization found for the former in cyclic voltammetry experiments. The differences observed between
both compounds arise from electronic factors promoted by the presence of an additional dmso ligand in compound 2
with regard to 6. Exposure of compounds 2 and 6 to visible light in acetonitrile and chloroform produces the
substitution of one dmso ligand by CH3CN or a chlorido ligand respectively, with a concomitant Ru(II)→Ru(III)
oxidation in the latter case.
Experimental Section
Materials
All reagents used in the present work were obtained from Aldrich Chemical Co and were used without further
purification. Reagent grade organic solvents were obtained from SDS and high purity deꢀionized water was obtained
by passing distilled water through a nanoꢀpure MiliꢀQ water purification system. RuCl3·2H2O, was supplied by
Johnson and Matthey Ltd. and was used as received.
Instrumentation and Measurements
IR spectra were recorded on an ATR MKꢀII Golden Gate Single Reflection. UVꢀVis spectroscopy was performed
on a Cary 50 Scan (Varian) UVꢀVis spectrophotometer with 1 cm quartz cells. Cyclic voltammetric (CV) and
Differential Pulse Voltammetry (DPV) experiments were performed in an IJꢀCambria IHꢀ660 potentiostat using a
three electrode cell. Glassy carbon electrode (3 mm diameter) from BAS was used as working electrode, platinum
wire as auxiliary and Ag/AgCl as the reference electrode. All cyclic voltammograms presented in this work were
recorded under nitrogen atmosphere. The complexes were dissolved in solvents containing the necessary amount of
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ꢀ
nꢀBu4NH+PF6 (TBAH) as supporting electrolyte to yield a 0.1 M ionic strength solution. All E1/2 values reported in
this work were estimated from cyclic voltammetric experiments as the average of the oxidative and reductive peak
potentials (Epa+Epc)/2, or directly from DPV. Unless explicitly mentioned the concentration of the complexes was
approximately 1mM. The NMR spectroscopy was performed on a Bruker DPX 300 and 400 MHz. Samples were
run in CD2Cl2 or d3ꢀacetonitrile with internal references (residual protons and/or tetramethylsilane). Elemental
analyses were performed using a CHNSꢀO Elemental Analyser EAꢀ1108 from Fisons. Monochromatic irradiations
were carried out by using a 80 W lamp source from Phillips on complex solutions, typically 1mM. Gas
chromatography experiments were performed by capillary GC, using a GCꢀ2010 Gas Chromatograph from
Shimadzu, equipped with an Astec CHIRALDEX GꢀTA Column (30 m x 0.25 mm diameter) incorporating a FID
detector. All the product analyses in the catalytic experiments were performed by means of GC using biphenyl as
internal standard.
Crystallographic Data Collection and Structure Determination
Measurement of the crystals were performed on a Bruker Smart Apex CCD diffractometer using graphiteꢀ
monochromated Mo Kα radiation (λ = 0.71073Å) from an XꢀRay tube. Data collection, Smart V. 5.631 (BrukerAXS
1997ꢀ02); data reduction, Saint+ Version 6.36A (Bruker AXS 2001); absorption correction, SADABS version 2.10
(Bruker AXS 2001) and structure solution and refinement, structure solution and refinement, SHELXLꢀ2013
(Sheldrick, 2013). The crystallographic data as well as details of the structure solution and refinement procedures
are reported in supplementary information. CCDC 1423204 (2), 1423200 (3), 1423202 (4), 1423201 (5) and
1423203 (6’) contain the supplementary crystallographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/products/csd/request/
Catalytic studies
The ruthenium catalyst (0.01 mmol), water (3 ml) and the corresponding nitrile (1 mmol) were introduced into a
sealed tube and the reaction mixture stirred at 80ºC. The nitrile was extracted with chloroform and quantified by GC,
whereas the identity and purity of the resulting amides was assessed by 1HꢀNMR on the reaction crude.
Preparations
cis,fac-[RuCl2(dmsoꢀS)3(dmsoꢀO)], 1,2a and [RuIICl2(pypzꢀH)(dmsoꢀS)2], 6,14 complexes were prepared according
to literature procedures. All synthetic manipulations were routinely performed under nitrogen atmosphere using
vacuum line techniques. Electrochemical experiments were performed under N2 atmosphere with degassed solvents.
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All spectroscopic, electrochemical and synthetic experiments were performed in the absence of light unless
explicitly mentioned.
[RuIICl2(CH3-pz-H)(dmso-S)3], 2. A 0.034g (0.414 mmol) sample of fomepizole ligand and cis,fac-
[RuCl2(dmsoꢀS)3(dmsoꢀO)] (0.2 g, 0.413 mmol) were dissolved in 10ml of dichloromethane and the resulting
solution refluxed for 1 h at 60ºC. After this time, the mixture was allowed to cool to room temperature and the
volume was reduced; a yellowꢀorange precipitate was formed and was collected on a frit, washed with ether and
vacuumꢀdried. Yield: 144.9 mg (71.84 %). Anal. Found (Calc.) for C10H24Cl2N203RuS3: C, 24.61 (24.60); H, 4.94
(4.90); N, 5.69 (5.70). 1HꢀNMR (CD2Cl2, 400MHz): δ 2.09 (s, 3H, H4), 3.12 (s, 6H, H6, H9), 3.40 (s, 6H, H5, H10),
3.45 (s, 6H, H7, H8), 7.48 (s, 1H, H1), 8.25 (s, 1H, H3), 13.71 (s, 1H, H2). 13CꢀNMR (CD2Cl2, 400 MHz): 9.3 (C4),
46.84 (C7, C8), 47.16 (C6, C9), 47.67 (C5, C10), 117.98 (C2), 129.37 (C1), 141.4 (C3). For the NMR assignments
we use the same labelling scheme as for the Xꢀray structures (Figure 1). IR (ν, cmꢀ1): 3016, 2925, 1410, 1300, 1091,
1052, 1019, 676, 611. Epa (CH3CN +0.1M TBAH): 1.49V vs Ag/AgCl. UVꢀVis (CH2Cl2) [λmax, nm (ɛ, Mꢀ1ꢁcmꢀ1)]:
359 (573.8).
[RuIICl2(NO2-pz-H)(dmso-S)3], 3.This compound was prepared following a method analogous to that described
for 2 starting from compound cis,fac-[RuCl2(dmsoꢀS)3(dmsoꢀO)] (0.2g, 0.413 mmol) and 4ꢀnitroꢀ1Hꢀpyrazole
(0.046 g, 0.407 mmol). Yield: 122mg (56.89%). Anal. Found (Calc.) for C9H21Cl2N305RuS3: C, 20.7(20.8); H,
1
3.9(4.04); N, 7.9(8.1). HꢀNMR (CD2Cl2, 400MHz): δ 3.20 (s, 6H, H5, H9), 3.45 (s, 6H, H4, H8), 3.47 (s, 6H, H6,
H7), 8.38 (s, 1H, H1), 9.14 (s, 1H, H3), 15.1 (s, 1H, H2). 13C (CD2Cl2, 400MHz): δ 46.20 (C6, C7), 47.17 (C4, C8),
47.64 (C6, C7), 19.76 (C1), 139.33 (C3). IR (ν, cmꢀ1): 3150, 3024, 2915, 1515, 1405, 1335, 1108, 1039, 998, 819,
752, 672. Epa (CH3CN +0.1M TBAH): 1.60V vs Ag/AgCl. UVꢀVis (CH2Cl2) [λmax, nm (ɛ, Mꢀ1ꢁcmꢀ1)]: 310 (328.7).
[RuIICl2(CF3-pz-H)(dmso-S)3], 4. This compound was prepared following a method analogous to that described
for 2 starting from compound cis,fac-[RuCl2(dmsoꢀS)3(dmsoꢀO)] (0.1g, 0.2064 mmol) and 3ꢀtrifluoromethylꢀ
pyrazole (0.028g, 0.206 mmol). Yield: 34.66mg (30.95%). Anal. Found (Calc.) C10H21Cl2F3N203RuS3: C,
1
21.1(22.1); H, 3.3(3.8); N, 4.5(5.1). HꢀNMR (CD2Cl2, 400MHz): δ 3.15 (s, 6H, H6, H10), 3.42 (s, 6H, H5, H9),
3.46 (s, 6H, H7, H8), 6.74 (s, 1H, H2A), 8.59 (s, 1H, H3), 15.33 (s, 1H, H2). 13C (CD2Cl2, 400MHz): δ 46.02 (C9,
C10), 47.06 (C7, C8), 47.58 (C5, C6), 107.37 (C2), 134.67 (C1), 135.08 (C4), 143.7 (C3). IR (ν, cmꢀ1): 3115, 3010,
2925, 1458, 1350, 1318, 1100, 1014, 919, 794, 676. Epa (CH3CN +0.1M TBAH): 1,73V vs Ag/AgCl. UVꢀVis
(CH2Cl2) [λmax, nm (ɛ, Mꢀ1ꢁcmꢀ1)]: 356 (560.34).
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[RuIICl2(Br-Hind)(dmso-S)3], 5. This compound was prepared following a method analogous to that described
for 2 starting from compound cis,fac-[RuCl2(dmsoꢀS)3(dmsoꢀO)] (0.2g, 0.4129 mmol) and 6ꢀbromoꢀ1Hꢀindazole
(0.081g, 0.412 mmol) except that the reflux was performed 20 min. Yield: 154.1mg (61.89 %) Anal.Found (Calc.)
1
for C13H23BrCl2N203RuS3: C, 23.9 (25.1); H, 3.4 (3.8); N, 4.3 (4.04). HꢀNMR (CD2Cl2, 400MHz): δ 3.17 (s, 6H,
H9, H12), 3.45 (s, 6H, H8, H13), 3.49 (s, 6H, H10, H11), 7.30 (d, 1H, H4), 7.62 (s, 1H, H5), 7.76 (s, 1H, H2A),
9.11 (s, 1H, H7), 14.16 (s, 1H, H2B). 13C (CD2Cl2, 400MHz): δ 46.1 (C10, C11), 47.26 (C12, C13), 47.75 (C8, C9),
113.81 (C2), 122.10 (C6), 122.78 (C3), 123.3 (C5), 126.19 (C4), 139.03 (C7), 141.38 (C1). IR (ν, cmꢀ1): 3128,
3020, 2916, 2802, 1624, 1097, 1056, 1006, 950, 676, 590. Epa (CH3CN +0.1M TBAH): 1,66V vs Ag/AgCl. UVꢀVis
(CH2Cl2) [λmax, nm (ɛ, Mꢀ1ꢁcmꢀ1)]: 343 (532.4).
mer-[RuIIICl3(pypz-H)(dmso-S)], 6’. A 5 mg sample of 6 was dissolved in 10 ml of CHCl3 and was irradiated
overnight with light (80W tungsten lamp). The volume of the solution was reduced and a precipitate was formed
after addition of Et2O yielding complex 6' in quantitative yield. Anal. Found (Calc.) for C10H13Cl3N3ORuS: C, 28.0
(27.8); H, 3.4 (3.0); N, 9.5 (9.7). E1/2 (CH2Cl2+0.1M TBAH): 0.05V vs Ag/AgCl. UVꢀVis (CH2Cl2) [λmax, nm (ɛ, Mꢀ
1ꢁcmꢀ1)]: 276 (15112), 318 (5336), 360 (4464), 406 (9782).
mer-[RuIIICl3(CH3-pz-H)(dmso-S)2], 2". A 5 mg sample of 2 was dissolved in 10 ml of CHCl3 and was
irradiated overnight with light (80W tungsten lamp). The volume of the solution was reduced and a precipitate was
formed after addition of Et2O yielding complex 2" in quantitative yield. E1/2 (CH2Cl2+0.1M TBAH): 0.1 V vs
Ag/AgCl.
Acknowledgements
This research has been financed by MINECO of Spain (CTQ2010ꢀ21532ꢀC02ꢀ01). IF thanks UdG for a
predoctoral grant. Johnson & Matthey LTD are acknowledged for a RuCl3·nH2O loan.
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Scheme 1. Synthetic strategy and ligands used in this work.
CH2Cl2
cis, f acꢀ[RuCl2(L)(dmsoꢀS)3]
cis,f ac-[RuCl2(dmsoꢀS)3(dmsoꢀO)] +
L
[1]
2
3
L = CH3ꢀpzꢀH,
L = NO2ꢀpzꢀH,
L = CF3ꢀpzꢀH,
4
5
L = BrꢀHind,
CH3-pz-H
NO2-pz-H
CF3-pz-H
Br-Hind
pypz-H
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Scheme 2. Possible stereoisomers for complexes 2-5.
cisꢀCl facꢀdmsoꢀS (a)
transꢀCl merꢀdmsoꢀS (b)
cisꢀCl merꢀdmsoꢀS (c)
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Scheme 3. Electron transfer and linkage isomerization processes observed for Ruꢀdmso complexes.
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Scheme 4. Photochemical transformation of complexes 2 and 6 in different solvents.
hυ
MeCN
[RuCl (MeCN)(CH -pz-H)(dmso) ] 2’
2
3
2
cis, fac-[RuCl (CH -pz-H)(dmso) ]
2
3
3
hυ
CHCl3
2
[RuCl (CH -pz-H)(dmso) ]
2"
3
3
2
hυ
cis, cis-[RuCl (pypz-H)(dmso) ]
[RuCl (pypz-H)(dmso)]
3
6’
2
2
CHCl3
6
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Table 1. Selected bond lengths (Å) and angles (°) for complexes 2ꢀ5 and 6'
2
3
4
5
6'
Ru(1)ꢀN(1)
2.1461(18)
2.2682(7)
2.3003(6)
2.2866(6)
2.4267(7)
2.1461(18)
2.120(2)
2.145(3)
2.141(3)
Ru(1)ꢀN(3)
Ru(1)ꢀN(1)
Ru(1)ꢀS(1)
Ru(1)ꢀCl(2)
Ru(1)ꢀCl(1)
Ru(1)ꢀCl(3)
2.019(3)
Ru(1)ꢀS(1)
Ru(1)ꢀS(2)
Ru(1)ꢀS(3)
Ru(1)ꢀCl(1)
Ru(1)ꢀCl(2)
2.2455(15)
2.2690(17)
2.2760(15)
2.4028(17)
2.3928(16)
2.2817(10)
2.2944(11)
2.2610(11)
2.3977(11)
2.4187(11)
2.2780(15)
2.2816(13)
2.2901(11)
2.4212(12)
2.4211(16)
2.118(3)
2.2639(14)
2.3170(13)
2.3438(13)
2.3464(13)
N(1)ꢀRu(1)ꢀS(1)
N(1)ꢀRu(1)ꢀS(2)
N(1)ꢀRu(1)ꢀS(3)
S(1)ꢀRu(1)ꢀS(2)
S(1)ꢀRu(1)ꢀS(3)
S(2)ꢀRu(1)ꢀS(3)
N(1)ꢀRu(1)ꢀCl(1)
N(1)ꢀRu(1)ꢀCl(2)
S(1)ꢀRu(1)ꢀCl(1)
S(2)ꢀRu(1)ꢀCl(1)
S(3)ꢀRu(1)ꢀCl(1)
S(1)ꢀRu(1)ꢀCl(2)
S(2)ꢀRu(1)ꢀCl(2)
S(3)ꢀRu(1)ꢀCl(2)
Cl(1)ꢀRu(1)ꢀCl(2)
90.52(5)
171.03(5)
94.43(5)
96.14(2)
91.16(2)
91.44(2)
84.09(5)
88.02(5)
174.59(2)
89.17(2)
89.77(2)
90.79(2)
85.89(2)
176.86(2)
88.53(2)
88.82(5)
173.07(4)
94.80(5)
92.82(4)
93.48(6)
91.83(3)
85.69(5)
86.46(5)
174.487(19)
92.69(3)
86.52(6)
92.09(5)
86.76(3)
174.31(2)
88.05(6)
94.64(9)
172.82(9)
89.13(9)
91.91(4)
92.74(4)
93.48(4)
87.09(9)
85.49(9)
174.64(4)
87.35(4)
92.35(5)
87.35(4)
91.90(4)
174.61(4)
87.73(5)
92.63(9)
174.29(8)
88.35(9)
92.46(4)
90.25(5)
94.24(5)
85.53(9)
85.08(9)
87.12(5)
92.12(5)
173.22(4)
175.40(4)
89.67(4)
93.66(5)
88.73(5)
N(3)Ru(1)N(1)
N(3)Ru(1)S(1)
N(1)Ru(1)S(1)
N(3)Ru(1)Cl(2)
N(1)Ru(1)Cl(2)
S(1)Ru(1)Cl(2)
N(3)Ru(1)Cl(1)
N(1)Ru(1)Cl(1)
S(1)Ru(1)Cl(1)
Cl(2)Ru(1)Cl(1)
N(3)Ru(1)Cl(3)
N(1)Ru(1)Cl(3)
S(1)Ru(1)Cl(3)
Cl(2)Ru(1)Cl(3)
Cl(1)Ru(1)Cl(3)
77.91(12)
97.44(9)
174.81(9)
88.07(10)
86.27(9)
91.30(4)
86.28(10)
92.33(9)
89.66(4)
174.34(4)
172.46(9)
95.14(9)
89.62(4)
94.46(5)
91.12(5)
23

Dalton Transactions
Page 24 of 39
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DOI: 10.1039/C5DT04376J
Table 2. Electrochemical data (CH3CN +0.1M TBAP; E in V vs Ag/AgCl) for the complexes described in this work
and others for purposes of comparison.
Compound
[RuCl2(pzꢀH)(dmsoꢀS)3]a
Epa RuIIꢀS→RuIIIꢀS
Epc RuIIIꢀO→RuIIꢀO
1.60
1.49
1.62
1.73
1.66
1.09b
0.82
0.75
0.86
0.85
0.85
0.33
[RuCl2(CH3ꢀpzꢀH)(dmsoꢀS)3], 2
[RuCl2(NO2ꢀpzꢀH)(dmsoꢀS)3], 3
[RuCl2(CF3ꢀpzꢀH)(dmsoꢀS)3], 4
[RuCl2(BrꢀHind)(dmsoꢀS)3], 5
[RuCl2(pypzꢀH)(dmsoꢀS)2], 6a
a Reference 14 bE1/2 value, in CH2Cl2 +0.1M TBAP
24

Page 25 of 39
Dalton Transactions
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DOI: 10.1039/C5DT04376J
Table 3. Thermodynamic and kinetic parameters for the linkage isomerization in complexes 2 and 6, together with
related Ruꢀdmso complexes.a
ꢈꢈꢈ
ꢈꢈꢈ
ꢈꢈ
ꢈꢈ
ꢉ
ꢉ
ꢉ
ꢉ
ꢆꢂꢇ
ꢇꢂꢆ
ꢆꢂꢇ
ꢇꢂꢆ
ꢈꢈꢈ
ꢈꢈ
Entry Compound
ꢅ
ꢅ
ꢆꢂꢇ
ꢆꢂꢇ
(s-1)
(s-1)
(s-1)
(s-1)
4.7·10ꢀ14
8.2·10ꢀ14
9.3·10ꢀ14
1.6·10ꢀ10
2.1·10ꢀ11
4.6·10ꢀ10
cis,facꢀ[RuCl2(CH3ꢀpzꢀH)(dmsoꢀS)3], 2
cis,cisꢀ[RuCl2(pypzꢀH)(dmsoꢀS)2], 6
cis,cisꢀ[RuCl2(H3p)(dmsoꢀS)2] (ref 19)
trans,cisꢀ[RuCl2(H3p)(dmsoꢀS)2] (ref 19)
trans,cis-[RuCl2(bpp)(dmsoꢀS)2]ꢀ (ref 3)
outꢀ[Ru(L2)(trpy)(dmsoꢀS)]+ (ref 5)
0.036 1.9·10ꢀ2
5.3·10ꢀ1
1.53·1012 7.2·10ꢀ2
1
2
3
4
5
6
1.39
1.7
1.76·10ꢀ1
2.8·10ꢀ1
5.7·10ꢀ2
1.7·10ꢀ2
7.7·10ꢀ2
1.26·10ꢀ1 1.34·1012 1.1·10ꢀ1
1.7·10ꢀ1
2.2·10ꢀ1
6.5·10ꢀ2
6.0·10ꢀ1
5.2·1011
5.3·108
6.5·109
5.5·108
4.9·10ꢀ1
8.7·10ꢀ2
1.3·10ꢀ1
2.5·10ꢀ1
0.27
0.26
0.13
aH3p is 5ꢀphenylꢀ3ꢀ(2ꢀpyridyl)ꢀ1Hꢀpyrazole, L2 is 5ꢀphenylꢀ3ꢀ(pyridinꢀ2ꢀyl)pyrazolate and bpp is 3,5ꢀ(2ꢀpyridyl)pyrazolate
(see Scheme S1 in the supplementary information for a schematic drawing of the complexes)
25

Dalton Transactions
Page 26 of 39
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DOI: 10.1039/C5DT04376J
Table 4. Ruꢀcatalyzed hydration of nitriles to amides in water using complexes 2-6 as catalysts.a
Substrate
Cat.
Yield [%]
80c
29
Select.(%)b
>99c
>99
[RuCl2(pzꢀH)(dmsoꢀS)3]c
2
3
85
>99
4
24
>99
5
30
>99
6
70
>99
[RuCl2(pzꢀH)(dmsoꢀS)3]c
61c
35
>99c
>99
2
3
4
5
6
40
>99
53(30)d
43
>99
>99
22
>99
a Reactions performed at 80ºC using 1 mmol of nitrile in 3ml of water.
[Substrate]:[Ru] ratio = 100:1. Time: 20 h reaction
b Selectivity = (amide yield/substrate conversion) x 100
c reference 14
d In parentheses, reaction performed at 50ºC
26

Page 27 of 39
Dalton Transactions
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DOI: 10.1039/C5DT04376J
Figure Captions
Figure 1. Ortep plots and labelling schemes for compounds 2-5.
Figure 2. Cyclic voltammetries of complex 2 recorded in CH3CN: solid line, potential scan starting at Einit = 0 V
with 2 s equilibrium time; dashed line, potential scan starting at Einit = 1.8 V with 10 min equilibrium time.
Figure 3. CV registered in CH2Cl2 (TBAH, 0.1M) vs Ag/AgCl starting the scanning potential at Einit = 1.7 V for 2
and 1.4 V for 6, at scan rates between 0.20 and 8 V/s and applying an equilibration time of 1 minute. a) complex 2,
b) complex 6.
Figure 4. UVꢀvisible spectra corresponding to the photochemical transformation of 2 into 2' upon light irradiation in
acetonitrile.
Figure 5 . Ortep plot and labelling schemes for the Xꢀray structure of compound 6'.
Figure 6. DPV of a solution of complex 2" in CH2Cl2 + 0.1M TBAP, starting the scanning of potential at a) ꢀ0.2 V
and b) 0.6 V. The equilibrium time applied in each case (0ꢀ180 s) is indicated.
27

Dalton Transactions
Page 28 of 39
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DOI: 10.1039/C5DT04376J
Figure 1.
C4
O2
N3
C1
O1
C2
C1
C2
N2
C3
H2
N2
N1
C3
H2
N1
Cl1
C6
O3
Cl1
Cl2
O5
C5
S3
Cl2
C10
C5
S1
S3
C4
S1
S2
C8
C8
O1
C9
C6
C9
O3
C7
O2
S2
O4
C7
2
3
Br1
F3
C4
F1
C3
C4
C1
C5
C2
C2
F2
C6
N2
C1
N2
C3
N1
H2
C7
N1
H2B
C13
C10
O1
Cl2
Cl2
C9
Cl1
O3
S1
C9
Cl1
S3
S3
C5
C7
C6
S1
C8
O3
S2
C8
O1
S2
C11
C12
O2
O2
C10
4
5
28

Page 29 of 39
Dalton Transactions
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DOI: 10.1039/C5DT04376J
Figure 2.
8.E-05
6.E-05
4.E-05
2.E-05
0.E+00
-2.E-05
-4.E-05
I (A)
0
0.5
1
1.5
2
E (V)
29