Tailoring RuII pyridine/triazole oxygenation catalysts and using photoreactivity to probe their electronic properties

Article abstract of DOI:10.1002/chem.201406441

Tuning of ligand properties is at the heart of influencing chemical reactivity and generating tailor-made catalysts. Herein, three series of complexes [Ru(L)(Cl)(X)]PF₆ (X=DMSO, PPh₃, or CD₃CN) with tripodal ligands (L1-L5) containing pyridine and triazole arms are presented. Triazole-for-pyridine substitution and the substituent at the triazole systematically influence the redox behavior and photoreactivity of the complexes. The mechanism of the light-driven ligand exchange of the DMSO complexes in CD₃CN could be elucidated, and two seven-coordinate intermediates were identified. Finally, tuning of the ligand framework was applied to the catalytic oxygenation of alkanes, for which the DMSO complexes were the best catalysts and the yield improved with increasing number of triazole arms. These results thus show how click-derived ligands can be tuned on demand for catalytic processes.

Full text of DOI:10.1002/chem.201406441

DOI: 10.1002/chem.201406441
Full Paper
&
Tripodal Ligands
Tailoring RuII Pyridine/Triazole Oxygenation Catalysts and Using
Photoreactivity to Probe their Electronic Properties
Fritz Weisser, Hendrik Stevens, Johannes Klein, Margarethe van der Meer, Stephan Hohloch,
and Biprajit Sarkar*[a]
Abstract: Tuning of ligand properties is at the heart of
influencing chemical reactivity and generating tailor-made
catalysts. Herein, three series of complexes [Ru(L)(Cl)(X)]PF6
(X=DMSO, PPh3, or CD3CN) with tripodal ligands (L1–L5)
containing pyridine and triazole arms are presented. Tria-
zole-for-pyridine substitution and the substituent at the
triazole systematically influence the redox behavior and
photoreactivity of the complexes. The mechanism of the
light-driven ligand exchange of the DMSO complexes in
CD3CN could be elucidated, and two seven-coordinate
intermediates were identified. Finally, tuning of the ligand
framework was applied to the catalytic oxygenation of
alkanes, for which the DMSO complexes were the best
catalysts and the yield improved with increasing number of
triazole arms. These results thus show how click-derived
ligands can be tuned on demand for catalytic processes.
Introduction
was rationalized by a combination of steric and electronic
effects caused by the substituents.
Tris(2-pyridylmethyl)amine (TPA) and its derivatives are a family
of versatile ligands. The mobility of its pyridine (py) arms
allows TPA to accommodate metal centers with different coor-
dination geometries and electronic structures.[1] TPA complexes
have been used for enzyme modeling[2] and to study optoelec-
tronics,[3] magnetism,[4] and catalysis.[5] Among the processes
catalyzed by TPA complexes are water oxidation[5b] and the
oxygenation of hydrocarbons.[5c–f]
Catalytic activity of RuII complexes of TPA derivatives were
investigated in the oxygenation of cyclooctane.[5c] Adding elec-
tron-withdrawing methyl ester groups in the 5-position of py
made the RuII center more Lewis acidic and doubled the yield
of oxygenation catalysis compared to the TPA complex.
However, substituting all three py arms with quinolyl moieties
increased the Lewis acidity of the metal center even more but
did not improve the yield of cyclooctanone.
In many cases, the reactivity of the TPA complexes could be
tuned by adding substituents at the py rings[6] or by replacing
py by other N heterocycles.[7] For example, in studies on the
oxygen reactivity of hemocyanin mimic CuI-TPA, it was found
that adding a strongly donating substituent in the 4-position
of py (para to the N atom) resulted in more negative CuI/CuII
oxidation potentials and increased stability of a CuII–superoxi-
do adduct.[6a] Similarly, substituting py by imidazoles increased
the O2 affinity of the corresponding CuI complex.[7]
FeII complexes of TPA derivatives in which the py arms were
stepwise substituted in the 6-position with a halogen atom or
methyl group were studied.[6b–d] With each added halogen or
methyl group the Lewis acidity of the metal center increased.
The reactivity with molecular oxygen increased or decreased
with the number of substituted py arms, depending on the
nature of the additional ligands. This contradictory behavior
A recent development in the derivatization of TPA (L1) is the
substitution of py by the N heterocycle 1,2,3-triazole.[8a–c,h–n]
This is an attractive strategy, because triazoles can be easily
synthesized by copper-catalyzed alkyne–azide cycloaddition
(CuAAC),[8a–g] and the substituent at the triazole can be readily
varied. So far, this approach has mainly been used to substi-
tute one py arm to attach a functional group, for example,
a fluorescent probe, to the tripodal ligand framework.[8i–l]
Complexes of the TPA analogue tris[(1-benzyl-1H-1,2,3-triazol-
4-yl)methyl]amine (TBTA, L4) have been used to catalyze
CuAAC[8o–q] and to study magnetism.[8r–w] However, systematic
investigations of the effect of the triazole-for-pyridine substi-
tution on the properties of the corresponding metal-tripod
complexes are rare.[8h]
Previously, we studied complexes of the formula
[Ru(L)(dmso)(Cl)]+, where L is a tripodal ligand in which the py
rings of TPA (L1) are stepwise substituted by triazoles (L2–L4,
Scheme 1).[8h] This substitution gradually alters the redox-,
photo-, and thermoreactivity of the corresponding complexes
1(dmso)–4(dmso). With increasing number of triazole units, the
electron density at the metal center increases, which leads to
stronger backbonding to the DMSO ligand and decreases the
redox and photoreactivity of the complexes. Herein, we pres-
ent a comparative study of the RuII dmso complexes 1(dmso)–
[a] F. Weisser, H. Stevens, J. Klein, M. van der Meer, Dr. S. Hohloch,
Prof. Dr. B. Sarkar
Institute of Chemistry and Biochemistry
Freie Universität Berlin
Fabeckstrasse 34–36, 14195 Berlin (Germany)
E-mail: biprajit.sarkar@fu-berlin.de
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201406441.
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Scheme 1. Ligands and complexes discussed in this work.
5(dmso) and their RuII–PPh3 and RuII–CH3CN counterparts
(Scheme 1) with a focus on photochemical reactivity and catal-
ysis. The complexes were investigated by X-ray crystallography,
NMR spectroscopy, cyclic voltammetry, and UV/Vis spectroelec-
trochemistry, and they were tested for their catalytic activity in
the oxidation of alkanes.
from the corresponding DMSO complexes by irradiation at
369 nm (sometimes followed by heating) in CD3CN solution.
All complexes have the formula [Ru(L)(X)(Cl)]+ (L=L1–L5; X=
À
DMSO, PPh3, or CD3CN) and contain PF6 as the counterion.
Two possible isomers can be formed with respect to the posi-
tion of the ligand X, which can be trans or cis to the central N
atom of the tripodal ligand.
Whereas 1(dmso)cis can be synthesized exclusively at higher
temperatures and 1(dmso)trans/1(dmso)cis mixtures formed at
room temperature can be separated by recrystallization,
1(PPh3)trans and 1(PPh3)cis cannot be separated in large quanti-
ties. In solution, the PPh3 ligand of 1(PPh3) is prone to dissocia-
tion, which leads to the formation of the already characterized
Results and Discussion
Synthesis and NMR spectroscopy
Complexes
1(dmso)–5(dmso)
were
prepared
from
[Ru(dmso)4Cl2] and the respective ligands L1–L5, whereas
1(PPh3)–5(PPh3) were prepared from [Ru(PPh3)3Cl2]. The
acetonitrile complexes 1(CD3CN)–5(CD3CN) were prepared
dimer [Ru2(L1)2(m-Cl)2]2+ [9]
a small fraction of almost pure 1(PPh3)trans through crystalliza-
.
However, it was possible to obtain
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tion, which was used to obtain NMR and CV data of the
complex. Complexes 1(PPh3)trans and 1(PPh3)cis show distinctive
31P NMR resonances in CD2Cl2 at 47.54 and 42.17 ppm,
respectively.
3(dmso)trans, 4(dmso)trans, and 5(dmso)trans, the corresponding
CD3CN complexes were prepared by a combination of irradia-
tion and subsequent heating at 708C. The complexes formed
by this procedure are most likely 3(CD3CN)trans, 4(CD3CN)trans
,
With L2–L5, only the trans isomers of the DMSO and PPh3
complexes are obtained after column chromatography. How-
ever, mixed ligands L2 and L3 have a second possibility for
isomer formation. For example, if the two py arms of L2 in
2(PPh3)trans are opposite to each other in the two axial positions
(Scheme 1), the resulting isomer 2sym(PPh3)trans has an equatorial
mirror plane and is therefore symmetrical. In 2un(PPh3)trans, one
of the two py arms is in an equatorial position, whereas the
axial positions are occupied by a py and a triazole arm; the re-
sulting isomer is unsymmetric. For 2(dmso)trans, the formation
of the unsymmetrical isomer is favored 2:1, and the two iso-
mers can be separated. For 2(PPh3)trans, a 1:4 ratio of unsym-
metrical and symmetrical isomers was found after chromatog-
raphy, but their separation was less straightforward, and only
a small amount of almost pure 2sym(PPh3)trans could be obtained
and 5(CD3CN)trans (see below). 1H NMR spectra of complexes
1(dmso)–4(dmso), 1(CH3CN), and 2(CH3CN) have been
discussed in a previous report,[8h] and the NMR signal assign-
ment for 1(PPh3)–5(PPh3), 5(dmso), and 3(CD3CN)–5(CD3CN) is
presented in the Supporting Information.
Crystal structures
For 2un(PPh3)trans and 4(PPh3)trans, single-crystal X-ray diffraction
analyses were performed (Figure 2 and Supporting Informa-
1
for H NMR characterization (Figure 1).
Figure 2. ORTEP of 2un(PPh3)trans drawn at the 50% probability level. H atoms,
two CH2Cl2 molecules, and PF6Àion are omitted for clarity.
tion, S29), and their molecular structures were compared
to those of 1(dmso)trans
, 2sym(dmso)trans, 2un(PPh3)trans, and
Figure 1. 1H NMR spectrum of 2sym(PPh3)trans in CD2Cl2.
3un(dmso)trans (Table 1).[5c,8h] The bond lengths and angles of the
PPh3 complexes mainly follow the trends established for the
DMSO analogues. The RuÀNamine distance increases with in-
creasing number of triazole arms from 2.133(5) (for
2un(PPh3)trans) to 2.184(6) Š (for 4(PPh3)trans), and the sum of the
In CD2Cl2, the axial py arms of 2sym(PPh3)trans show four reso-
nances at 8.06, 7.51, 6.74, and 6.64 ppm, and the triazole arm
shows a singlet at 7.22 ppm. The two CH2 groups of the tria-
zole arm in the equatorial position are in a mirror plane and
therefore show two singlet resonances at 4.85 and 4.41 ppm.
The CH2 protons of the axial py arms have different chemical
environments, which result in two pseudodoublets with large
coupling constants (J=14.7 Hz) at 6.28 and 4.74 ppm
(Figure 1, signals d).
RuÀNarm distances decreases from 6.231(15) (for 2un(PPh3)trans
)
to 6.139(19) Š (for 4(PPh3)trans). Because triazoles are weaker ac-
ceptors than py, the electron density at the Ru center increases
from 2un(PPh3)trans to 4(PPh3)trans, and the coordination of the
electron-donating amine N atom becomes weaker. In general,
the RuÀNtriazole distances are shorter than the RuÀNpyridine
distances.
For 3(dmso)trans and 3(PPh3)trans, the formation of the unsym-
metrical isomer is favored [10/1 for 3un(dmso)trans/3sym(dmso)cis;
5/1 for 3un(PPh3)trans/3sym(PPh3)cis]. Complex 3sym(PPh3)trans has
two identical triazole arms in axial positions, which show a
singlet 1H NMR resonance at 7.56 ppm (CD2Cl2), whereas
3un(PPh3)trans shows two triazole singlets at 7.61 and 7.10 ppm
(Supporting Information, Figure S13). All ten CH2 protons in
3un(PPh3)trans have different chemical environments, resulting in
ten pseudodoublets between 6.25 and 4.20 ppm.
Overall, the RuÀNamine bonds are longer in the PPh3 com-
plexes than in the DMSO complexes. This points to a weaker
trans influence between PPh3 and the amine N atom. PPh3 ap-
parently acts as a more donating, less accepting ligand com-
pared to DMSO. Both RuÀSDMSO and RuÀPPPh distances slightly
3
decrease with increasing number of triazole arms. This could
be due to increased backbonding from a more electron-rich
RuII center or to lower steric demand of the triazoles compared
to py. Indicators pointing to the latter explanation are the
Irradiation of 1(dmso)trans, 1(dmso)cis, and 2(dmso)trans in
CD3CN at 369 nm exclusively leads to the formation of cis
CD3CN complexes.[8h] Because of the lower photoreactivity of
N
amine-Ru-P/S angles as well as the angles between the two
planes described by the axially coordinated N heterocycles.
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Table 1. Selected bond lengths [Š] and angles [8] of PPh3 and DMSO complexes. RuÀN bond lengths involving triazole arms are given in italics.
[5c]
[8h]
[8h]
[8h]
2un(PPh3)trans
4(PPh3)trans
1(dmso)trans
2
sym(dmso)trans
2
un(dmso)trans
3un(dmso)trans
RuÀNamine
RuÀNaxial
2.133(5)
2.083(5)
2.063(5)
2.085(5)
6.231(15)
2.3130(17)
2.4336(18)
2.184(6)
2.030(6)
2.058(7)
2.051(6)
6.139(19)
2.285(2)
2.4130(19)
2.095(2)
2.091(2)
2.079(2)
2.065(2)
6.235(6)
2.2653(8)
2.4319(9)
2.119(3)
2.101(2)
2.100(2)
2.031(3)
6.232(7)
2.2432(9)
2.4285(12)
2.112(3)
2.076(3)
2.069(3)
2.066(3)
6.211(9)
2.2470(10)
2.4462(9)
2.134(4)
2.057(4)
2.061(4)
2.052(4)
6.170(12)
2.2359(14)
2.4347(13)
RuÀNequatorial
S RuÀNarm
RuÀP/S
RuÀCl
Namine-Ru-P/S
] ringaxial–ringaxial
171.18(16)
36.5(2)
176.15(16)
13.7(3)
176.13(7)
29.32(8)
179.52(8)
24.02(8)
176.89(8)
25.7(1)
177.07(12)
26.2(2)
Electrochemistry
The cyclic voltammograms of all complexes were measured in
CH3CN. Each shows an RuII/RuIII oxidation process, and most
show one or two irreversible tripod-centered reduction
processes (Table 2 and Supporting Information, Table S3 and
Table 2. Redox potentials E1/2 (peak potentials Ep are given for the irrever-
sible reductions) of all complexes measured in CH3CN with 0.1 m Bu4NPF6
at 100 mVsÀ1. Peak potential ratios Ipc/Ipa for the oxidation processes of all
Figure 3. Cyclic voltammograms of 1(dmso)trans measured in CH3CN with
0.1 m Bu4NPF6 at 100 mVsÀ1 (left) and 10 mVsÀ1 (right).
complexes measured at 10 mVsÀ1
.
E1/2(Ox) [V]
Ipc/Ipa
Ep(Red) [V]
accepting triazole arms, the electron density at the Ru center
increases, which results in stronger backbonding, a more
stable RuÀS bond, and a more reversible oxidation process.
The internal compensation of electron density between tripod,
Ru center, and DMSO results in a small change in the overall
oxidation potential on triazole-for-pyridine substitution. Inter-
estingly, changing the substituent at the triazole from benzyl
to phenyl [in 5(dmso)trans] results in both a higher oxidation
potential (0.64 V) and a weaker RuÀS bond (Ipc/Ipa =0.67).
Compared to the DMSO complexes, there is less backbond-
ing from Ru to the phosphine ligand in the PPh3 complexes.
As a result, the PPh3 complexes are easier to oxidize, and the
variation of the oxidation potentials in the PPh3 series is bigger
[0.35–0.46 V]. Interestingly, 5(PPh3)trans (0.44 V) is harder to oxi-
dize than 4(PPh3)trans (0.35 V): changing the substituent at the
triazole arms from benzyl to phenyl results in lower electron
density at the RuII center, and the electronic properties of com-
plexes with L5 are close to those of complexes with L2.
The acetonitrile complexes are the easiest to oxidize. In pre-
vious studies, we showed that photoactivation favors the for-
mation of the cis CH3CN complexes, whereas thermal activa-
tion favors the trans complexes.[8h] Complexes 1(CD3CN) and
2(CD3CN) could be prepared by irradiation alone and are most
likely cis isomers. To obtain 3(CD3CN)–5(CD3CN), additional
heating for 12 h at 708C after irradiation for 24 h was necessa-
ry, which most likely leads to the formation of the trans CD3CN
species. As observed for 1(dmso), the trans isomers are harder
to oxidize, and this could explain the sudden increase in
oxidation potential from 2(CD3CN)cis to 3(CD3CN)trans. Again,
5(CD3CN)trans is harder to oxidize (0.41 V) than 4(CD3CN)trans
(0.35 V).
1(dmso)cis
1(dmso)trans
2sym(dmso)trans
0.57
0.62
0.61
0.63
0.60
0.59
0.64
0.46
0.42
0.39
0.35
0.44
0.27
0.25
0.34
0.35
0.41
0.92
0.45
0.63
0.77
0.99
0.99
0.67
0.95
0.97
0.95
0.97
0.97
0.99
0.98
0.96
0.98
0.95
À2.42
À2.31
À2.36
À2.34
À2.40
À2.57
À2.29; À2.52
À2.23
À2.33; À2.65
À2.48; À2.75
–
À2.36
–
–
À2.49
–
À2.41
2un(dmso)trans
3(dmso)trans
4(dmso)trans
5(dmso)trans
1(PPh3)trans
2(PPh3)trans
3(PPh3)trans
4(PPh3)trans
5(PPh3)trans
1(CH3CN)cis
2(CH3CN)cis
3(CH3CN)trans
4(CH3CN)trans
5(CH3CN)trans
Figures S30–S39). Complex 1(dmso)cis is easier to oxidize than
1(dmso)trans, and there is a slight potential difference between
the isomers 2sym(dmso)trans and 2un(dmso)trans. Nevertheless, the
variation in the oxidation potentials of 1(dmso)trans–4(dmso)trans
is rather small (0.59–0.63 V).
The oxidation of the DMSO complexes is accompanied by
the well-known isomerization from S-bound to O-bound
DMSO.[9,10] This results in a lower peak current in the reverse
sweep and the appearance of a new redox couple for the O-
bound isomer (Figure 3). Hence, the peak-current ratio Ipc/Ipa of
the oxidation process at slow scan rates (10 mVsÀ1) is a mea-
sure of the reversibility of the redox process and of the
strength of the RuÀS bond; a ratio of unity indicates a fully re-
versible process. The reversibility increases from 1(dmso)trans
(Ipc/Ipa =0.45) to 4(dmso)trans (Ipc/Ipa =0.99). With more weakly
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In the DMSO complex series, the potential of the tripod-cen-
tered reduction shifts to more negative values with increasing
number of triazole arms, which is in agreement with the
weaker acceptor character of these N heterocycles. The less
negative reduction potential of 5(dmso)trans (À2.29 V) com-
pared to 4(dmso)trans (À2.57 V) again indicates that the phenyl-
substituted triazole is a better acceptor than the benzyl-substi-
tuted one. The reduction potentials of the PPh3 complexes
follow the trends of the DMSO series; only two of the CD3CN
complexes showed reduction processes in the solvent window.
In summary, both substitution of the N-heterocyclic arms and
the substitution of the monodentate ligand affect the potential
and the reversibility of the metal-centered oxidation process.
The oxidation potentials decrease in the order DMSO>PPh3 >
Figure 4. Changes in the UV/Vis spectrum of 3(PPh3)trans observed on
oxidation in CH2Cl2.
CH3CN. Notably, substitution of the substituent at the triazole character, whereas the lower-energy bands are due to
has an effect on both the oxidation and reduction potentials ligand-to-metal charge transfer (LMCT) transitions.[11]
of the corresponding complexes.
Photoreactivity of the DMSO complexes
UV/Vis spectroelectrochemistry
Previously, we showed that the thermo- and photoreactivity of
UV/Vis spectra of all complexes were recorded in CH2Cl2 (Sup- the DMSO complexes decreases as triazole arms are added.[8h]
porting Information, Figures S41–S43 and Table S5). They show Complexes 1(dmso)cis, 1(dmso)trans, and 2sym(dmso)trans undergo
strong absorption bands in the range 280–500 nm, which we substitution of their DMSO ligand by CD3CN on irradiation at
assign to metal-to-ligand charge-transfer (MLCT) transi- 369 nm or heating. Complex 2un(dmso)trans reacted on irradia-
tions.[5c,9,11] Bands below 280 nm can be assigned to intraligand tion, but only sluggishly on heating. For 3un(dmso)trans and
charge-transfer (ILCT) transitions.[11] For example, the DMSO 4(dmso)trans, neither heating nor irradiation lead to pronounced
complexes show a pyridine p–p* band at 250 nm, the intensity formation of identifiable acetonitrile adducts, although small
of which decreases with increasing number of triazole arms 1H NMR signals of free DMSO were observed.
from 1(dmso) to 4(dmso). Triazole-based p–p* ILCT bands are
Two isomers of 2sym(CD3CN) are formed on heating and irra-
outside of the spectral window for complexes with benzyl- diation. Both products have the MS signature of
1
substituted ligands. However, for the complexes with phenyl- [Ru(L2)(Cl)(CD3CN)]+ and the H NMR signal pattern of a sym-
substituted triazole arms 5(X)trans, the triazole-based ILCT bands metrically coordinated tripodal ligand (Supporting Information,
are shifted to lower energies around 250 nm.
Figure S23). On the basis of ligand-exchange studies in
For all three complex series, the onset of the MLCT bands in [D6]DMSO, we assigned the thermal product as 2sym(CD3CN)trans
the visible region shifts to higher energies with increasing tria- and the irradiation product as 2sym(CD3CN)cis; 2sym(CD3CN)trans
zole content [e.g., ca. 460 nm for 1(dmso)trans and ca. 380 nm can be converted to 2sym(CD3CN)cis by irradiation. In the photo-
for 4(dmso)trans], because the p* orbitals of the weaker-accept- reactions of 2sym(dmso)trans and 2un(dmso)trans, the coordination
ing triazoles are higher in energy than those of py. In general, symmetry of the tripodal ligand is mainly preserved. Only after
the shift of the MLCT band onset correlates nicely with the po- prolonged irradiation is pronounced scrambling of the coordi-
tential gap DE(ox,red) between oxidation (metal-centered) and nation symmetry observed [formation of
reduction (ligand-centered) observed in the CV measurements 2un(CD3CN)cis mixtures; Supporting Information, Figure S53].
(Supporting Information, Figure S44). For example, the PPh3 We reinvestigated the photochemical ligand exchange of
2sym(CD3CN)cis/
complexes have the smallest DE(ox,red) and the lowest-energy 1(dmso)–5(dmso) in CD3CN in more detail and found that
absorption bands. As expected, the MLCT bands of the phenyl- 3un(dmso)trans and 4(dmso)trans do indeed lose their DMSO
substituted complexes 5(X) are at lower energies than those of ligand upon prolonged irradiation, albeit very slowly. All pho-
1
their corresponding benzyl-substituted analogues 4(X), which toreactions in CD3CN were monitored by H NMR spectroscopy,
is further evidence for the electronic influence of the residue and the signals of coordinated and free DMSO were used to
at the triazole.
determine the concentration of the starting species after differ-
On oxidation, the intensity of the MLCT bands of all com- ent times of irradiation (Figure 5). For all complexes, DMSO
plexes decreases (Figure 4, Supporting Information, S45–S47). loss follows first-order kinetics, and the initial reaction rate con-
The spectra of all resulting RuIII complexes show intense bands stant kinit decreases from 1(dmso)trans to 4(dmso)trans (Table 3).
below 300 nm and bands between 300 and 600 nm that are Interestingly, kinit of 5(dmso)trans (7.6”10À3 sÀ1) is significantly
less intense than the MLCT bands of the corresponding RuII larger than that of 4(CD3CN)trans (2.7”10À3 sÀ1), and it is close
species. In a previous work, DFT calculations for similar RuIII to the corresponding values of 2sym(dmso)trans and 2un(dmso)trans
pyridine/triazole complexes showed that the bands at higher (5.9”10À3 and 6.4”10À3 sÀ1, respectively); this is further
energies are mostly due to ILCT transitions or have a mixed evidence for the electronic effect of the triazole substituent.
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Figure 5. Change in concentration of the DMSO complexes over time on
irradiation at 369 nm.
Table 3. Initial reaction rate constants for the light-driven ligand ex-
change of the DMSO complexes in CD3CN.
1
Figure 6. Changes in the H NMR spectrum of 5(dmso)trans in CD3CN on
irradiation (369 nm) and subsequent heating (708C).
Complex
kinit [10À3 sÀ1
]
Complex
un(dmso)trans
4(dmso)trans
5(dmso)trans
kinit [10À3 sÀ1
]
1(dmso)cis
1(dmso)trans
10.53Æ0.04
31.43Æ0.59
5.85Æ0.07
6.42Æ0.09
3
3.01Æ0.02
2.74Æ0.03
7.61Æ0.08
2
2
sym(dmso)trans
un(dmso)trans
The slower ligand exchange of 5(dmso)trans allows intermedi-
ates of the photoreaction to be identified (Figure 6, Scheme 2).
1
In the H NMR spectrum in CD3CN, 5(dmso)trans shows two tria-
zole signals at 8.32 (singlet, 2H, axial arms) and 7.96 ppm (sin-
glet, 1H, equatorial arm; Figure 6A). The CH2 signals show the
pattern of a symmetrically coordinated tripodal ligand (axial
arms: two pseudodoublets at 5.32 and 4.86 ppm; equatorial
arm: singlet at 4.52 ppm, 2H). The signal of coordinated DMSO
is seen at 3.61 ppm (singlet, 6H; Figure 6A). After 10 h of irra-
diation, the starting complex had mainly been transformed (ca.
1
70%) into one new species. In the H NMR spectrum, this new
Figure 7. 1H NMR spectra of 5(CD3CN)2 (top) and 5(CD3CN)trans (bottom) in
CD3CN.
species showed three triazole signals at 8.61, 8.19, and
8.14 ppm (singlets, 1H each; Figure 6C) and no signal of coor-
dinated DMSO (instead, a large signal of free DMSO at
2.47 ppm). Hence, each of the tripod arms in the new species
has a different chemical environment. Similarly, each CH2
proton has a different environment, which leads to six pseudo-
doublets with large coupling constants. For the CH2 group of
a noncoordinated triazole arm, one would expect a singlet or
two pseudodoublets that are close to each other or almost
merged. Here, the coupling constants and roof effects of the
pseudodoublets point to a signal assignment (Figure 7, top)
Scheme 2. Starting complex (A), intermediates (B, C), and product (D) of the CD3CN-for-DMSO ligand exchange of 5(dmso)trans
.
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that precludes a noncoordinated tripod arm. Hence, the prod-
uct obtained after 10 h of irradiation features three coordinat-
ed tripod arms, no coordinated DMSO ligand, and it is highly
unsymmetrical (three different triazole positions). These obser-
vations can be best explained by a seven-coordinate species
containing L5, ClÀ, and two CD3CN molecules, namely,
5(CD3CN)2 (Scheme 2C).
The 1H NMR spectrum recorded after 2 h of irradiation of
5(dmso)trans shows the triazole signals of the starting complex
as well as of 5(CD3CN)2 and three additional singlets at 8.65,
8.24, and 8.23 ppm that match in intensity (1:1:1; Figure 6B).
1
Around the H NMR signal of the coordinated DMSO ligand of
5(dmso)trans, there are two new singlets at 3.67 and 3.45 ppm
that match the new triazole signals in intensity (3H each; Fig-
ure 6B). This signal pattern points to the formation of an inter-
mediate with a coordinated DMSO ligand and an unsymmetri-
cal coordination environment (three different triazole positions,
DMSO methyl groups have different environments) and can
be explained by a seven-coordinate species containing L5,
ClÀ, DMSO, and a CD3CN ligand, namely, 5(dmso)(CD3CN)
(Scheme 2B).
Scheme 3. Proposed mechanism of the light-driven ligand exchange in
CD3CN, shown for the example of 2sym(dmso)trans
.
Apparently, the photochemical ligand exchange of DMSO
complexes in CD3CN proceeds in three steps: Firstly, the Ru–
dmso bond is loosened, and CD3CN is added to form seven-co-
ordinate [Ru(L)(dmso)(CD3CN)(Cl)]+. Secondly, DMSO is substi-
tuted by a second CD3CN molecule to form seven-coordinate
[Ru(L)(CD3CN)2(Cl)]+. Finally, one of the CD3CN ligands dissoci-
ates to give rise to the six-coordinate, octahedral acetonitrile
complex [Ru(L)(CD3CN)(Cl)]+. For 5(dmso)trans, the photoreac-
tion stops at the intermediate 5(CD3CN)2. Therefore, at the end
of the irradiation, we heated 5(CD3CN)2 at 708C for 12 h. The
mentioned observations: formation of two seven-coordinate
and unsymmetrical intermediates, favored formation of species
with CH3CN cis to the central amine N atom, and preservation
of the coordination symmetry of the tripodal ligand.
For 3un(dmso)trans, 4(dmso)trans, and 5(dmso)trans, samples of
the photoreaction mixtures were subjected to CV. The resulting
voltammograms showed waves of four oxidation processes of
different intensity (Figure 8 and Supporting Information,
1
resulting product showed a H NMR signal pattern that is con-
sistent with six-coordinate, octahedral species 5(CD3CN) having
a mirror plane (Figure 7, bottom; Scheme 2D). The last step of
the photoreaction seems to be driven by steric repulsion, and
for 5(CD3CN)2, which has less sterically demanding triazole
arms, the repulsion is only strong enough at elevated tempera-
tures. When 5(CD3CN) is irradiated again for 2 h, the signals of
5(CD3CN)2 reappear.
Figure 8. Cyclic voltammogram (CH3CN, 0.1 m Bu4NPF6, 100 mVsÀ1) of a
reaction mixture obtained during the photochemical ligand exchange of
5(dmso)trans in CD3CN (see Scheme 2). The differential-pulse voltammogram
(20 mVsÀ1) is shown as dashed curve at the bottom.
MS analyses of samples taken during the photoreaction of
1(dmso)–5(dmso) corroborate this proposed mechanism. The
samples were taken from the NMR tubes (CD3CN) and diluted
with a 20-fold volume of CH3CN before they were subjected to
MS: any acetonitrile species formed in the spectrometer
should contain CH3CN ligands, whereas species formed in the
NMR tube during the photoreaction should contain CD3CN li-
Figure S40). By comparison to the corresponding 1H NMR
spectrum (signal intensity compared to redox-wave intensity)
and to CVs of the starting complex and of the final product
(obtained after irradiation and subsequent heating), the four
oxidation waves could be assigned to the corresponding
species involved in the photoreaction (Figure 6, Scheme 2).
Their potentials follow the order Eox[X(dmso)(CH3CN)]>
Eox[X(dmso)]>Eox[X(CH3CN)2]>Eox[X(CH3CN)] (Supporting Infor-
mation, Table S4).
gands. For the photoreactions of 2sym(dmso)trans, 3un(dmso)trans
,
4(dmso)trans and 5(dmso)trans the MS signals of [Ru(L)
,
,
(dmso)(Cl)]+, [Ru(L)(dmso)(CD3CN)(Cl)]+, [Ru(L)(CD3CN)2(Cl)]+,
and [Ru(L)(CD3CN)(Cl)]+ were found (Supporting Information,
Figure S59–S62 and Table S7). With more sterically demanding
py arms, the seven-coordinate intermediates are less stable.
Hence, the corresponding MS signals are very small for
2
sym(dmso)trans and are not found for 1(dmso).
On the basis of these results, we propose a new mechanism
for the light-driven ligand exchange, shown for the example of
sym(dmso)trans in Scheme 3. It accounts for all of the above-
2
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Catalysis
slightly better than the precursor [Ru(dmso)4Cl2] (18%). The dif-
ference in coordination geometry between 2sym(dmso)cis and
2un(dmso)trans also had no effect on the catalytic abilities of the
two complexes. The yield of cyclooctanone increased with in-
creasing number of triazole arms, from 23% with 1(dmso)trans
to 65% with 4(dmso)trans. This is an almost threefold increase
and also a significantly better yield than the maximum value
obtained with RuII complexes of substituted pyridines (50%).[5c]
In contrast to the redox- and photoreactivity, the substituent
at the triazole does not affect the yield [62% with 5(dmso)trans].
The PPh3 complexes show similar or slightly lower yields than
their DMSO counterparts [24% with 1(PPh3) and 54% with
4(PPh3)trans]. The worst results in the test reaction in CH2Cl2
were obtained with 1(CD3CN)cis.
The DMSO and PPh3 complexes were tested for their ability to
catalyze the oxidation of alkanes with mCPBA (Scheme 4,
Table 4).[5c] Testing of the CD3CN complexes was hampered by
difficulties in preparing sufficient amounts of the complexes
with more than one triazole arm because of their decreased
Scheme 4. Oxygenation of cyclooctane with meta-chloroperoxybenzoic acid
For 4(dmso)trans, the yield of cyclooctanone could be im-
proved to 76% by using five equivalents of mCPBA, and the
catalyst loading could be reduced to 0.5 mol% (Table 5). We
further tried to oxidize cyclohexane and ethylbenzene. Cyclo-
hexane was converted to e-caprolactone in 39% yield, and eth-
ylbenzene was converted to acetophenone in only 6% yield. In
a recent examples a homogeneous Ru catalysts achieved
a higher conversion of cyclohexane, but this catalyst system
featured a very large excess of oxidant and stopped at the cy-
clohexanol stage.[12a] Interestingly, the oxidation of cyclohexa-
nol and 1-phenylethanol catalyzed by 4(dmso)trans led to excel-
lent yields of caprolactone and acetophenone, respectively.
However, because the oxidation of secondary alcohols with
mCPBA in CH2Cl2 can already be achieved with metal-free
2,2,6,6-tetramethylpiperidine-1-oxy (TEMPO) as catalyst,[12b] the
improvement of the catalytic performance in the first step, the
activation of aliphatic CÀH bonds, will be the focus of future
studies on these systems.
(mCPBA) catalyzed by RuII complexes.
Table 4. Catalytic performance of the RuII complexes in the oxygenation
of cyclooctane.
Catalyst[a]
Yield[b] of
Catalyst[a]
Yield[b] of
cyclooctanone [%]
cyclooctanone [%]
1(dmso)cis
1(dmso)trans
23
23
5(dmso)trans
1(PPh3)
4(PPh3)trans
1(CH3CN)cis
[Ru(dmso)4Cl2] 18
no catalyst
62
24
55
10
2sym(dmso)trans 34
2un(dmso)trans
35
52
65
3(dmso)trans
4(dmso)trans
2
[a] Reaction
conditions:
[cyclooctane]=0.25 mmol,
[mCPBA]=
0.625 mmol, [Ru]=0.0025 mmol, [1,2-dichlorobenzene]=0.1 mmol, 5 mL
CH2Cl2, 408C, 16 h. For workup conditions, see Experimental Section.
1
[b] Yield determined by H NMR spectroscopy.
Overall, there is a correlation between the number of tria-
zole arms and the yield. However, there is no correlation be-
tween the Lewis acidity of the RuII center, that is, the oxidation
potential in the CV, and the catalytic performance. This has al-
ready been noticed for Ru and Fe complexes of TPA derivatives
with substituted py rings.[5c,f] Because of the long reaction
times, the yields could reflect the stability of the complexes
rather than their activity. It was shown before that oxidation
photoreactivity. The reaction conditions for alkane oxidation
used here differ slightly from those used by Yamaguchi et al. in
previous studies with 1(dmso)cis and 1(dmso)trans (Experimental
Section).[5c] Most importantly, we used 2.5 instead of 1.5
equivalents of oxidant.
There was no difference in yield between the two isomers
1(dmso)cis and 1(dmso)trans (both 23%), which are both only
Table 5. Oxygenation of different substrates catalyzed by 4(dmso)trans
.
Conditions[a]
Substrate
Products
Yield[b] [%]
1.0 mol% RuII, 2.5 equiv mCPBA
1.0 mol% RuII, 5.0 equiv mCPBA
0.5 mol% RuII, 5.0 equiv mCPBA
0.25 mol% RuII, 5.0 equiv mCPBA
1.0 mol% RuII, 5.0 equiv mCPBA
1.0 mol% RuII, 1.0 equiv mCPBA
1.0 mol% RuII, 2.5 equiv mCPBA
1.0 mol% RuII, 2.5 equiv mCPBA
1.0 mol% RuII, 5.0 equiv mCPBA
1.0 mol% RuII, 10.0 equiv mCPBA
1.0 mol% RuII, 1.5 equiv mCPBA
1.0 mol% RuII, 3.0 equiv mCPBA
cyclooctane
cyclooctane
cyclooctane
cyclooctane
ethylbenzene
1-phenylethanol
1-phenylethanol
cyclohexane
cyclohexane
cyclohexane
cyclohexanol
cyclohexanol
cyclooctanone
cyclooctanone
cyclooctanone
cyclooctanone
acetophenone
acetophenone
acetophenone
cyclohexanone, e-caprolactone
e-caprolactone
65
76
73
31
6
49
88
3, 25
39
e-caprolactone
cyclohexanone, e-caprolactone
e-caprolactone
39
65, 18
85
[a] [substrate]=0.25 mmol, [1,2-dichlorobenzene]=0.1 mmol, 5 mL CH2Cl2, 408C, 16 h. For workup conditions, see Experimental Section. [b] Yield
1
determined by H NMR spectroscopy.
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Scheme 5. Proposed catalytic cycle for the oxygenation reactions catalyzed by the Ru complexes presented in this paper.
reactions with Ru–TPA complexes proceed via a reactive
RuIV=O intermediate.[12c,d] It is known that such reactive
metal–oxygen species can undergo autooxidation, which can
lead, for example, to the hydroxylation of CÀH bonds close to
the metal center.[12e,f]
than improving the catalytic performance through tuning the
electronic properties.
Conclusion
Scheme 5 shows a proposed catalytic cycle that is based on
the hydrogen-abstraction, oxygen-rebound mechanism.[13a–d]
For the formation of the RuIV=O intermediate, a vacant coordi-
nation site is needed, and the chlorido ligand most likely disso-
ciates. If PPh3 or DMSO were to decoordinate to form an active
species, there should be a more pronounced difference in cata-
lytic performance between DMSO and PPh3 complexes, and
the complex 1(dmso)trans should be more active than 1(dmso)cis
or 4(dmso)trans. For TPA and TPA derivatives, most of the high-
valent ruthenium oxido and iron oxido species for which struc-
tures have been determined by X-ray diffraction or calculated
by DFT methods have the metal–oxido bond trans to the cen-
tral amine N atom of the ligand framework.[13b,e–j] Scheme 5
shows that, during the catalytic cycle, aromatic CÀH bonds of
the py rings in 1(X) would necessarily be in close proximity to
oxyl, hydroxyl, or alkyl radicals, which makes the pyridine-rich
complexes prone to degradation reactions. The triazoles in
4(X) or 5(X) only have one CÀH bond, which points away from
the Ru center, preventing the attack of radical species. There-
fore, the incorporation of triazole arms into the ligand frame-
work may increase catalyst stability towards degradation rather
We have synthesized, characterized, and compared three series
of complexes [Ru(L)(Cl)(X)]+ in which L is a tripodal ligand con-
taining different numbers of py and triazole arms and X is
DMSO, PPh3, or CH3CN. With increasing number of triazole
arms, the RuII center becomes more electron-rich, which is re-
flected by the decreasing RuII/RuIII oxidation potentials and the
decreasing photoreactivity of the DMSO complexes. The sub-
stituents at the triazole rings also have a significant influence.
We elucidated the mechanism of the light-driven ligand
exchange of the DMSO complexes in CD3CN and identified
two intermediates.
The complexes were tested as catalysts in the oxidation of
cyclooctane to cyclooctanone with mCPBA. Here, the yields in-
creased with increasing number of triazole arms in both the
DMSO and PPh3 series of complexes, but we observed no gen-
eral correlation between the Lewis acidity of the complexes
and their catalytic performance. We assume that the better
catalytic performance of the triazole-rich complexes is due to
better stability towards degradation through radical stress.
Thus, RuII complexes with tripodal tris-triazole ligands are an
ideal platform for tailoring catalyst systems and reactivity: The
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centrations of 3 mmolLÀ1. The tubes were fixed 1 cm away from
a UV lamp (369 nm, 4 W; UVP, Cambridge, UK), the emission profile
of which is shown in the Supporting Information (Figure S48), and
ligands are readily synthesized by click chemistry, the substitu-
ents at the triazole arms can be varied with ease, and the tria-
zole rings are highly stable towards chemical stress (acids,
bases, oxidizing agents). These ligands allow the electronic
properties of the complex to be changed by means of the sub-
stituents at the triazole arms without affecting the steric prop-
erties around the metal center. The redox and photoreactivity
of the DMSO complexes can be used to probe the effect of
the ligand framework on the electronic properties of the RuII
center. Finally, the complexes can be tested as catalysts in dif-
ferent organic transformations, and the variation of the triazole
substituent can be used to establish correlations between elec-
tronic properties and catalytic performance of the complexes.
Further studies to explore the effect of the residues at the
triazole rings are currently on-going.
1
irradiated. The H NMR signals of coordinated DMSO in the original
species (3.65–3.35 ppm) and the signal of free DMSO (2.47 ppm)
were used to determine the concentration of the starting complex
after different times of irradiation.
General procedure for oxygenation catalysis
The protocol used in this work comprised an aqueous workup to
remove m-chlorobenzoic acid and residual mCPBA, which inter-
1
fered with both H NMR and GC-MS analyses of the reaction mix-
1
ture. Initially, yields were determined by using H NMR spectra and
GC-MS analyses. Since the two methods afforded identical results,
only 1H NMR spectra were used.
The catalyst (0.0025 mmol) and mCPBA (70%, 0.625 mmol, 143 mg)
were placed in a Schlenk flask (50 mL), which was evacuated and
purged with nitrogen three times. Then, a stock solution (5 mL)
containing the substrate (0.05 molLÀ1) and 1,2-dichlorobenzene
(0.02 molLÀ1) in CH2Cl2 was added, and the reaction mixture was
heated at 408C. After 16 h, the reaction was quenched by adding
aqueous ammonia (5%, 2.5 mL). The aqueous phase was extracted
with CH2Cl2 (2”3 mL), and the combined organic phases were
washed with demineralized water and dried with Na2SO4. After fil-
tration, the solvent was removed on a rotary evaporator (408C,
500 mbar), and the residue was dissolved in CDCl3 and subjected
to 1H NMR analysis. The resonances of 1,2-dichlorobenzene and
the product were used to determine the yield of the reaction
(shown for the example of cyclooctanone in Figure S63 of the Sup-
porting Information). All experiments were performed in triplicate,
and average yields are given.
Experimental Section
General procedures
All chemicals were commercially available and used as purchased
unless otherwise noted. Solvents used for synthesis were dried
with appropriate drying agents. All manipulations were carried out
under a nitrogen atmosphere unless otherwise noted. The precur-
sors [Ru(dmso)4Cl2][14] and [Ru(PPh3)3Cl2],[15] ligands L1–L5,[8g,16] and
complexes 1(dmso)–4(dmso)[8g] were prepared according to litera-
ture procedures. 1H and 31P NMR data were recorded on a Jeol
Lambda 400 (400 MHz and 162 MHz, respectively), JEOL ECP500
(1H, 500 MHz), or JEOL ECX400 (31P, 202 MHz) by using the chemical
shift of the solvent as an internal standard; elemental analyses
(C,H,N) were measured with an Elementar Vario EL III and a Perki-
nElmer Analyzer 240. Mass spectra were recorded with an Agilent
6210 ESI-TOF spectrometer (Agilent Technologies, Santa Clara, CA,
USA).
X-ray crystallography
Single-crystal X-ray diffraction data were collected with a Bruker
Smart AXS diffractometer. Data were collected at 140(2) K by using
graphite-monochromated MoKa radiation (l=0.71073 Š). The strat-
egy for the data collection was evaluated by using the Smart soft-
ware. The data were collected by the standard –w scan techniques
and were scaled and reduced by using Saint++ software. The
structures were solved by direct methods by using SHELXS-97
and refined by full-matrix least-squares methods on F2 with
SHELXL-97.[18a]
Electrochemistry
Cyclic voltammogramms were recorded with a PAR VersaStat 4 po-
tentiostat (Ametek) by working in anhydrous dichloromethane
(H2Oꢀ0.005%, puriss., SigmaAldrich) or acetonitrile (H2Oꢀ0.01%,
puriss., SigmaAldrich) distilled from calcium hydride or calcium
chloride, respectively. A three-electrode setup was used with
a glassy-carbon working electrode, a coiled platinum wire as the
counter electrode, and a coiled silver wire as the pseudoreference
electrode. Ferrocene was used as internal standard, and 0.1 m
NBu4PF6 (Fluka,ꢁ99.0%, electrochemical grade) was used as an
electrolyte.
All non-hydrogen atoms were refined anisotropically. The remain-
ing hydrogen atoms were placed in geometrically constrained po-
sitions and refined with isotropic temperature factors, generally 1.2
times the Ueq values of their parent atoms. The asymmetric unit of
5(PPh3)trans contains a strongly disordered and partly occupied di-
chloromethane molecule, which could not be refined and was
therefore removed by using SQUEEZE techniques.[18b] Table S2
(Supporting Information) contains the parameters for the data
collection and refinement.
UV/Vis spectroscopy and spectroelectrochemistry
UV/Vis spectra were recorded with an Avantes spectrometer con-
sisting of a light source (AvaLight-DH-S-Bal), a UV/Vis detector
(AvaSpec-ULS2048), and an NIR detector (AvaSpec-NIR256-TEC).
UV/Vis spectroelectrochemical measurements were carried out in
an optically transparent thin-layer electrochemical (OTTLE) cell[17]
CCDC 1035972
(2un(PPh3)trans·2CH2Cl2)
and
1035975
(4(PPh3)trans·CH2Cl2) 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/data_request/cif.
with
a platinum-mesh working electrode, an platinum-mesh
counter electrode, and a silver-foil pseudoreference electrode.
Synthesis
Photoreactivity experiments
[Ru(L5)(dmso)(Cl)]PF6 {5(dmso)trans}: L5 (76 mg, 0.15 mmol),
[Ru(dmso)4Cl2] (74 mg, 0.15 mmol), and KPF6 (34 mg, 0.2 mmol)
were allowed to react in methanol (15 mL). After cooling to room
The photoreactivity experiments on 1(dmso)–5(dmso) were carried
out in CD3CN in NMR tubes sealed with Parafilm with complex con-
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temperature, the reaction mixture was concentrated (to ca. 5 mL)
by evaporation of the solvent, and Et2O (ca. 30 mL) was added to
precipitate the crude product. The crude product was dried in
vacuum and purified by column chromatography on aluminum
oxide (5 wt% water) with CH2Cl2/methanol (99/1 v/v) as eluent,
and the product was collected as the major yellow fraction. Evapo-
ration of the solvent afforded 5(dmso)trans as crystalline yellow
14.2, 3.9 Hz, 2H, NCH2(ax)), 4.41 ppm (s, 2H, NCH2(eq)); 31P NMR for
2
sym(PPh3)trans (202 MHz, CD2Cl2): d=51.95 (s, PPh3), À144.38 ppm
1
(sept, PF6À); H NMR for 2un(PPh3)trans (500 MHz, CD2Cl2): d=7.76 (s,
1H, triazole), 7.71 (d, J=5.3 Hz, 1H, py), 7.15 (d, J=7.2 Hz, 2H, py),
6.98 (d, J=7.7 Hz, 1H, py), 6.56 (t, J=6.6 Hz, 1H, py), 6.46 (t, J=
6.5 Hz, 1H, py), 6.13 (d, J=14.5 Hz, 1H, CH2), 5.59 (d, J=13.7 Hz,
1H, CH2), 5.26 (d, J=14.8 Hz, 1H, CH2), 4.64 ppm (dd, J=13.8,
3.9 Hz, 1H, CH2); 31P NMR for 2un(PPh3)trans (202 MHz, CD2Cl2): d=
51.29 (s, PPh3), À144.38 ppm (sept, PF6À); MS (ESI) calcd for
C40H37N6PRuCl [Ru(L2)(Cl)(PPh3)]+: 769.1549; found: 769.1536; ele-
mental analysis calcd for C40H37N6RuClP2F6·C3H6O: C 53.12, H 4.46,
N 8.64; found: C 53.32, H 4.37, N 8.86.
1
powder (90 mg, 71% yield). H NMR (400 MHz, CDCl3): d=8.26 (s,
2H, triazoleax), 7.97 (s, 1H, triazoleeq), 7.68–7.63 (m, 4H, Ph), 7.62–
7.58 (m, 2H, Ph), 7.52–7.34 (m, 9H, Ph), 5.43 (d, J=13.7 Hz, 2H,
CH2(ax)), 5.12 (d, J=14.0 Hz, 2H, CH2(ax)), 4.86 (s, 2H, CH2(eq)),
3.75 ppm (s, 6H, dmso); MS (ESI) calcd for C29H30N10RuClSO
[Ru(L5)(Cl)(dmso)]+: 703.1058; found: 703.1101; elemental analysis
calcd for C29H30N10RuClSOPF6·0.8CH3OH: C 40.96, H 3.83, N 16.03;
found: C 41.00, H 3.87, N 16.07.
[Ru(L3)(PPh3)(Cl)]PF6 {3(PPh3)trans}: The preparation was analogous
to that of 1(PPh3), and [Ru(PPh3)3Cl2] (0.1 mmol, 100 mg), L3
(0.1 mmol, 45 mg), and KPF6 (0.5 mmol, 92 mg) were used. The re-
sulting residue was purified by column chromatography on alumi-
num oxide with CH2Cl2/MeOH (99/1) as the eluent. After evapora-
tion of the solvent, a 1/5 mixture of 3sym(PPh3)trans/3un(PPh3)trans was
obtained as a yellow solid in 65% yield (0.065 mmol, 65 mg). In
a second column-chromatographic separation, with pure CH2Cl2 as
the eluent, almost pure 3un(PPh3)trans was obtained. Single crystals
of 3un(PPh3)trans suitable for X-ray diffraction analysis were obtained
by slow diffusion of n-hexane into a concentrated solution of the
complex in CH2Cl2 at À358C. 1H NMR for 3un(PPh3)trans (500 MHz,
CD2Cl2): d=8.01–7.94 (m, Ph), 7.87 (d, J=5.7 Hz, 1H, py), 7.83 (dd,
J=9.6, 7.7 Hz, 1H, py), 7.61 (s, 1H, triazole), 7.41–7.32 (m, Ph),
7.32–7.24 (m, Ph), 7.24–7.19 (m, Ph), 7.10 (s, 1H, triazole), 7.07–7.04
(m, Ph), 6.98 (d, J=7.0 Hz, 1H, py), 6.77–6.73 (m, Ph), 6.53 (t, J=
6.5 Hz, 1H, py), 6.19 (d, J=14.2 Hz, NCH2), 5.64 (d, J=13.6 Hz, 1H,
NCH2), 5.24 (d, J=14.6 Hz, 1H, PhCH2 triazoleax), 5.07 (d, J=14.7 Hz,
1H, PhCH2 triazoleax), 4.79 (d, J=15.1 Hz, 1H, PhCH2 triazoleeq), 4.75
(d, J=15.1 Hz, 1H, PhCH2 triazoleeq), 4.64 (dd, J=14.2, 3.5 Hz, 1H,
NCH2), 4.56 (dd, J=13.6, 3.6 Hz, 1H, NCH2), 4.33 (d, J=17.3 Hz, 1H,
NCH2(eq)), 4.25 ppm (d, J=17.2 Hz, 1H, NCH2(eq)); 31P NMR for
3un(PPh3)trans (202 MHz, CD2Cl2): d=54.25 (s, PPh3), À144.38 ppm
[Ru(L1)(PPh3)(Cl)]PF6 {1(PPh3)}: [Ru(PPh3)3Cl2] (0.1 mmol, 100 mg)
and L1 (0.1 mmol, 29 mg) were heated at reflux in toluene (8 mL)
for 18 h. After cooling to room temperature a precipitate was fil-
tered off and washed with diethyl ether. The precipitate was dis-
solved in methanol (5 mL) and stirred with KPF6 (0.5 mmol, 92 mg)
for 3 h. The solvent was then removed under reduced pressure,
and the resulting residue was purified by column chromatography
on aluminum oxide with CH2Cl2/MeOH (99.5/0.5) as the eluent.
After evaporation of the solvent, a 2/1 mixture of 1(PPh3)trans
/
1(PPh3)cis was obtained as an orange solid in 18% yield
(0.018 mmol, 15 mg). Single crystals of 1(PPh3)trans suitable for X-ray
diffraction analysis were obtained by slow diffusion of n-hexane
into a concentrated solution of the complex in CH2Cl2 at À358C.:
1H NMR of 1(PPh3)trans (400 MHz, CD2Cl2): d=8.09 (d, J=5.7 Hz, 2H,
pyax), 7.94–7.87 (m, 6H, PhP), 7.57–7.48 (m, 2H, pyax), 7.47–7.40 (m,
2H), 7.40–7.26 (m, 8H), 7.26–7.20 (m, 1H, pyeq), 7.18–7.14 (m, 3H),
6.93–6.90 (m, 2H), 6.73–6.67 (m, 2H), 6.29 (td, J=6.6, 1.6 Hz, 1H,
pyeq), 6.20 (d, J=14.6 Hz, 1H, CH2(ax)), 4.72 (dd, J=14.4, 4.1 Hz,
2H, CH2(ax)), 4.68 ppm (s, 2H, CH2(eq)); 31P NMR of 1(PPh3)trans
1
(162 MHz, CD2Cl2): d=47.40 (s, PPh3), À143.89 (sept, PF6À); H NMR
1
of 1(PPh3)cis (400 MHz, CD2Cl2): d=9.94 (d, J=6.0 Hz, 1H, pyeq), 8.67
(d, J=5.4 Hz, 2H, pyax), 7.58–7.10 (m, PhP+py), 7.04–7.01 (m, 1H,
pyeq), 7.00–6.95 (m, 2H, pyax), 4.49 (dd, J=15.0, 1.6 Hz, 2H,
CH2(ax)), 4.28 (s, 2H, CH2(eq)), 4.18 ppm (d, J=15.0 Hz, 2H,
CH2(ax)); 31P NMR of 1(PPh3)cis (162 MHz, CD2Cl2): d=41.82 (s, PPh3),
À143.89 ppm (sept, PF6À); MS (ESI) calcd for C36H33N4PRuCl
[Ru(L1)(Cl)(PPh3)]+: 689.1175; found: 689.1169; elemental analysis
calcd for C36H33N4RuClP2F6·C3H6O: C 52.50, H 4.41, N 6.28; found: C
52.71, H 4.27, N 6.33.
(sept, PF6À); H NMR for 3sym(PPh3)trans (500 MHz, CD2Cl2): d=8.09 (d,
J=5.9 Hz, 1H, pyeq), 7.56 (s, 2H, triazoleax), 7.14–7.11 (m, 1H, pyeq),
6.89 (d, J=7.9 Hz, 1H, pyeq), 6.36 (t, J=6.8 Hz, 1H, pyeq), 5.45 (d,
J=13.8 Hz, 2H, CH2), 5.18 (d, J=14.8 Hz, 2H, CH2), 5.11 (d, J=
14.9 Hz, 2H, CH2), 4.58 ppm (s, 2H, CH2); 31P NMR for 3sym(PPh3)trans
(202 MHz, CD2Cl2): d=56.78 (s, PPh3), À144.38 ppm (sept, PF6À); MS
(ESI) calcd for C44H41N8PRuCl [Ru(L3)(Cl)(PPh3)]+: 849.1924; found:
849.1958; elemental analysis calcd for C44H41N8RuClP2F6: C 53.15, H
4.16, N 11.27; found: C 53.26, H 4.19, N 11.27.
[Ru(L2)(PPh3)(Cl)]PF6 {2(PPh3)trans}: The preparation was analogous
to that of 1(PPh3), and [Ru(PPh3)3Cl2] (0.1 mmol, 100 mg), L2
(0.1 mmol, 37 mg), and KPF6 (0.5 mmol, 92 mg) were used. The re-
sulting residue was purified by column chromatography on alumi-
num oxide with CH2Cl2/MeOH (99/1) as eluent. After evaporation
of the solvent, a 4/1 mixture of 2sym(PPh3)trans/2un(PPh3)trans was ob-
tained as a yellow solid in 35% yield (0.035 mmol, 32 mg). In
a second column-chromatographic separation, by carefully varying
the eluent from pure CH2Cl2 to CH2Cl2/MeOH (99.5/0.5), fractions
with 2sym(PPh3)trans/2un(PPh3)trans ratios of 10/1, 1/1, and 1/4 were ob-
tained. Single crystals of 2un(PPh3)trans suitable for X-ray diffraction
analysis were obtained by slow diffusion of n-hexane into a concen-
trated solution of 2sym(PPh3)trans/2un(PPh3)trans (1/4) in CH2Cl2 at
À358C.1H NMR of 2sym(PPh3)trans (500 MHz, CD2Cl2): d=8.06 (dd, J=
5.7, 1.2 Hz, 2H, pyax), 7.97–7.92 (m, 5H, PhP), 7.51 (td, J=7.7,
1.5 Hz, 2H, pyax), 7.50–7.43 (m, 3H, Ph), 7.39–7.33 (m, 5H, PhP),
7.33–7.28 (m, 1H, PhP), 7.26–7.23 (m, 1H, PhP), 7.22 (s, 1H, tria-
zole), 6.76–6.71 (m, 2H, pyax), 6.64 (t, J=6.6 Hz, 2H, pyax), 6.28 (d,
J=14.1 Hz, 2H, NCH2(ax)), 4.85 (s, 2H, PhCH2 triazole), 4.74 (dd, J=
[Ru(L4)(PPh3)(Cl)]PF6 {4(PPh3)trans}: The preparation was analogous
to that of 1(PPh3), and [Ru(PPh3)3Cl2] (0.1 mmol, 100 mg), L4
(0.1 mmol, 53 mg), and KPF6 (0.5 mmol, 92 mg) were used. The re-
sulting residue was purified by column chromatography on alumi-
num oxide with CH2Cl2/MeOH (99/1) as eluent. After evaporation
of the solvent, 4(PPh3)trans was obtained as a light yellow solid in
67% yield (0.067 mmol, 76 mg). Single crystals of 4(PPh3)trans suita-
ble for X-ray diffraction analysis were obtained by slow diffusion of
n-hexane into a concentrated solution of the complex in CH2Cl2 at
1
À358C. H NMR (500 MHz, CD2Cl2): d=7.92–7.86 (m, 6H, PhP), 7.46
(s, 2H, triazoleax), 7.39–7.33 (m, 9H, PhCH2 triazole), 7.33–7.28 (m,
3H, PhP), 7.28–7.23 (m, 6H, PhP), 7.20 (s, 1H, triazoleeq), 7.09–7.05
(m, 4H, PhCH2 triazoleax), 6.97–6.93 (m, 2H, PhCH2 triazoleax), 5.53
(d, J=13.8 Hz, 2H, NCH2(ax)), 5.17 (d, J=14.7 Hz, 2H, PhCH2 triazo-
leax), 5.04 (d, J=14.7 Hz, 2H, PhCH2 triazoleax), 4.83 (s, 2H, PhCH2
triazoleeq), 4.52 (dd, J=13.7, 3.5 Hz, 2H, NCH2(ax)), 4.31 ppm (s, 2H,
NCH2(eq)).31P NMR (162 MHz, [D3]acetonitrile): d=60.05 (s, PPh3),
À148.39 ppm (sept, PF6À); MS (ESI) calcd for C48H45N10PRuCl
[Ru(L4)(Cl)(PPh3)]+: 929.2298; found: 929.2289; elemental analysis
Chem. Eur. J. 2015, 21, 8926 – 8938
www.chemeurj.org
8936
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Full Paper
calcd for C48H45N10RuClP2F6: C 53.66, H 4.22, N 13.04; found: C
53.61, H 4.59, N 13.01.
7.41 (s, 1H, b), 7.38–7.32 (m, 3H, Ph), 7.32–7.27 (m, Ph), 7.25–7.21
(m, Ph), 7.19–7.14 (m, 1H, py), 7.08–7.04 (m, 1H, py), 7.04–6.99 (m,
6H, py+Ph), 6.92–6.88 (m, 1H, py), 5.50 (d, J=15.2 Hz, 2H, PhCH2
triazole, a) 5.48 (s, 2H, PhCH2 triazole, b), 5.44 (d, J=15.2 Hz, 2H,
PhCH2 triazole, a), 5.43 (d, J=15.1 Hz, 1H, PhCH2 triazole, b), 5.33
(d, J=15.5 Hz, 2H, PhCH2 triazole+NCH2, b), 4.94 (d, J=14.0 Hz,
1H, NCH2, b), 4.89 (d, J=14.3 Hz, 2H, NCH2(ax), a), 4.65 (d, J=
14.6 Hz, 1H, NCH2, b), 4.50 (d, J=13.9 Hz, 1H, NCH2, b), 4.46 (d, J=
13.9 Hz, 2H, NCH2(ax), a), 4.32 (s, 2H, NCH2(eq), a), 4.19 (d, J=
17.5 Hz, 1H, NCH2, b), 4.0 ppm3 (d, J=17.5 Hz, 1H, b); MS (ESI)
calcd for C28H26D3N9RuCl [Ru(L3)(Cl)(CD3CN)]+: 631.1466; found:
631.1448.
[Ru(L5)(PPh3)(Cl)]PF6 {5(PPh3)trans}: The preparation was analogous
to that of 1(PPh3), and [Ru(PPh3)3Cl2] (0.1 mmol, 100 mg), L5
(0.1 mmol, 49 mg), and KPF6 (0.5 mmol, 92 mg) were used. The re-
sulting residue was purified by column chromatography on alumi-
num oxide with CH2Cl2/MeOH (99/1) as eluent. After evaporation
of the solvent, 5(PPh3)trans was obtained as a yellow solid in 51%
yield (0.051 mmol, 53 mg). Single crystals of 5(PPh3)trans suitable for
X-ray diffraction analysis were obtained by slow diffusion of n-
hexane into a concentrated solution of the complex in CH2Cl2 at
1
À358C. H NMR (500 MHz, CD2Cl2): d=8.07 (s, 2H, triazoleax), 7.96–
[Ru(L4)(CD3CN)(Cl)]PF6 {4(CD3CN)trans}: 4(dmso)trans (0.005 mmol,
4.45 mg) was dissolved in CD3CN (0.7 mL) in a standard NMR tube
and irradiated with a 4 W UV lamp at 369 nm for 24 h. The sample
7.89 (m, 6H, PhP), 7.75 (s, 1H, triazoleeq), 7.47–7.40 (m, 6H, Ph),
7.40–7.34 (m, 3H, Ph), 7.34–7.27 (m, 13H, Ph), 7.05–7.00 (m, 2H,
PhP), 5.73 (d, J=14.2 Hz, 2H, NCH2(ax)), 4.83 (dd, J=13.8, 3.5 Hz,
2H, NCH2(ax)), 4.65 (s, 2H, NCH2(eq)); 31P NMR (202 MHz, CD2Cl2):
d=56.70 (s, PPh3), À144.30 ppm (sept, PF6À); MS (ESI) calcd for
C45H39N10PRuCl [Ru(L5)(Cl)(PPh3)]+: 887.1829; found: 887.1848; ele-
mental analysis calcd for C45H39N10RuClP2F6·2H2O: C 50.59, H 4.06, N
13.11; found: C 50.59, H 3.74, N 12.77.
1
was then heated in an oil bath at 708C for 12 h. H NMR (400 MHz,
[D3]acetonitrile): d=7.75 (s, 2H, triazoleax), 7.44 (s, 1H, triazoleeq),
7.38–7.31 (m, 9H, PhCH2 triazole), 7.20–7.14 (m, 6H, PhCH2 tria-
zole), 5.46 (s, 4H, PhCH2 triazoleax), 5.36 (s, 2H, PhCH2 triazoleeq),
4.92 (d, J=14.1 Hz, 2H, NCH2(ax)), 4.49 (d, J=14.0 Hz, 2H,
NCH2(ax)), 4.07 ppm (s, 2H, NCH2(eq)); MS (ESI) calcd for
C32H30D3N11RuCl [Ru(L4)(Cl)(CD3CN)]+: 711.1841; found: 711.1794.
[Ru(L1)(CD3CN)(Cl)]PF6 {1(CD3CN)cis}: 1(dmso)cis (0.005 mmol,
3.25 mg) was dissolved in CD3CN (0.7 mL) in a standard NMR tube
and irradiated with a 4 W UV lamp at 369 nm. The reaction was
[Ru(L5)(CD3CN)(Cl)]PF6 {5(CD3CN)trans}: 5(dmso)trans (0.005 mmol,
4.05 mg) was dissolved in CD3CN (0.7 mL) in a standard NMR tube
and irradiated with a 4 W UV lamp at 369 nm for 24 h. The sample
1
monitored by H NMR spectroscopy and stopped when the trans-
formation was complete (ca. 4 h). 1H NMR (400 MHz,
[D3]acetonitrile): d=9.67 (dd, J=5.8, 1.6 Hz, 1H, pyeq), 8.93 (dd, J=
5.5, 1.6 Hz, 2H, pyax), 7.66 (td, J=7.8, 1.6 Hz, 2H, pyax), 7.46 (td, J=
7.7, 1.6 Hz, 1H, pyeq), 7.34 (d, J=7.8 Hz, 2H, pyax), 7.22 (d, J=
7.7 Hz, 2H, pyax), 7.14 (dd, J=6.7, 5.9 Hz, 1H, pyeq), 6.93 (dd, J=7.9,
1.5 Hz, 1H, pyeq), 4.82 (d, J=15.1 Hz, 2H, NCH2(ax)), 4.73 (d, J=
15.1 Hz, 2H, NCH2(ax)), 4.31 ppm (s, 2H, NCH2(eq)); MS (ESI) calcd
for C20H18D3N5RuCl [Ru(L1)(Cl)(CD3CN)]+: 471.0717; found:
471.0745.
1
was then heated in an oil bath at 708C for 12 h. H NMR (400 MHz,
[D3]acetonitrile): d=8.29 (s, 2H, triazoleax), 7.94 (s, 1H, triazoleeq),
7.75–7.70 (m, 4H, Ph), 7.67–7.63 (m, 2H, Ph), 7.57–7.42 (m, 9H, Ph),
5.12 (d, J=14.1 Hz, 2H, NCH2(ax)), 4.72 (d, J=14.1 Hz, 2H,
NCH2(ax)), 4.31 ppm (s, 2H, NCH2(eq)); MS (ESI) calcd for
C29H24D3N11RuCl [Ru(L5)(Cl)(CD3CN)]+: 669.1371; found: 669.1398.
Acknowledgements
[Ru(L2)(CD3CN)(Cl)]PF6 {2(CD3CN)cis}: 2sym(dmso)trans or 2un(dmso)trans
(0.005 mmol, 3.65 mg) was dissolved in CD3CN (0.7 mL) in a stan-
dard NMR tube and irradiated with a 4 W UV lamp at 369 nm. The
reaction was monitored by 1H NMR spectroscopy and stopped
when the transformation was complete (ca. 10 h). 1H NMR for
Fonds der Chemischen Industrie (FCI), the Freie Universität
Berlin, and the Deutsche Forschungsgemeinschaft (DFG) are
kindly acknowledged for financial support of this work.
2sym(CD3CN)cis (400 MHz, [D3]acetonitrile): d=8.96 (dd, J=5.4,
1.6 Hz, 2H, pyax), 7.66 (td, J=7.8, 1.7 Hz, 2H, pyax), 7.39 (s, 1H, tria-
zoleeq), 7.31 (d, J=8.3 Hz, 2H, pyax), 7.30–7.26 (m, 3H, PhCH2 triazo-
leeq), 7.26–7.21 (m, 2H, pyax), 6.93–6.88 (m, 2H, PhCH2 triazoleeq),
4.84 (d, J=15.1 Hz, 2H, NCH2(ax)), 4.74 (d, J=15.1 Hz, 2H,
NCH2(ax)), 4.00 ppm (s, 2H, NCH2(eq)). 1H NMR for 2un(CD3CN)cis
(400 MHz, [D3]acetonitrile): d=9.64 (dd, J=5.7, 1.5 Hz, 1H, py), 9.02
(dd, J=5.5, 1.6 Hz, 1H, py), 7.78 (s, 1H, triazoleax), 7.64 (td, J=7.7,
1.6 Hz, 1H, py), 7.48 (td, J=7.7, 1.6 Hz, 1H, py), 7.37–7.25 (m,
PhCH2 triazoleax), 7.21 (t, J=6.6 Hz, 1H, py), 7.13 (t, J=7.0 Hz, 1H,
py), 7.07–7.04 (m, PhCH2 triazoleax), 6.94–6.90 (m,PhCH2 triazoleax),
5.51 (d, J=15.0 Hz, 1H, PhCH2 triazoleax), 5.45 (d, J=15.0 Hz, 1H,
PhCH2 triazoleax), 4.84 (d, J=15.2 Hz, 1H, NCH2), 4.73 (d, J=15.2 Hz,
1H, NCH2), 4.54 (d, J=14.3 Hz, 1H, NCH2), 4.47 (d, J=14.2 Hz, 1H,
NCH2), 4.27 (d, J=18.4 Hz, 1H, NCH2), 4.17 ppm (d, J=18.4 Hz, 1H,
NCH2); MS (ESI) calcd for C24H22D3N7RuCl [Ru(L2)(Cl)(CD3CN)]+:
551.1092; found: 551.1088.
Keywords: N ligands
ruthenium · tripodal ligands
· oxygenation · photochemistry ·
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Received: December 11, 2014
Published online on May 8, 2015
Chem. Eur. J. 2015, 21, 8926 – 8938
www.chemeurj.org
8938
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