Diruthenium(II,III) tetramidates as a new class of oxygenation catalysts

Article abstract of DOI:10.1039/c1dt11530h

Two new diruthenium(II,III) tetramidate compounds, Ru ₂(NHOCC(CH₃)₂)₄Cl (1) and Ru ₂(NHOCCH₂CH₃)₄Cl (2) have been prepared and structurally characterized by X-ray crys

Full text of DOI:10.1039/c1dt11530h

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PAPER
Diruthenium(II,III) tetramidates as a new class of oxygenation catalysts†
Leslie Villalobos, Zhi Cao, Phillip E. Fanwick and Tong Ren*
Received 27th May 2011, Accepted 26th September 2011
DOI: 10.1039/c1dt11530h
Two new diruthenium(II,III) tetramidate compounds, Ru2(NHOCC(CH3)2)4Cl (1) and
Ru2(NHOCCH2CH3)4Cl (2) have been prepared and structurally characterized by X-ray
crystallography. The activity of promoting sulfide oxygenation using simple oxidants such as hydrogen
peroxide (H2O2) and tert-butyl hydroperoxide (TBHP) was studied. A UV-kinetics study indicated that
the initial rates of 1 and 2 are comparable to the previously studied diruthenium tetracarboxylates in
promoting TBHP oxygenation of methyl phenyl sulfide (MPS). Using excess oxidant and CH3CN as
the solvent, organic sulfides MPS and diphenyl sulfide (PPS) were oxidized using 1 mol% of the
catalytic species. Compound 1 is more effective than 2 in converting sulfides to sulfoxide under the same
conditions. Fast conversion was achieved when the reactions were carried out in the solvent-free
conditions, and the major oxidation product was the sulfoxide. The electronic structure of the title
compounds was studied with DFT calculations to gain an understanding of the activation of peroxy
reagents.
propanation and cross-metathesis of alkenes,7 and oxidation of
Introduction
organosulfur compounds.8,9 While the exact mechanism of the
In recent years the synthetic modification of diruthenium tetracar-
latter type reaction remains unknown, it is likely that the Ru2
boxylates with formula [Ru2(O2CR)4L2]0/+/2+ by varying the groups
unit activates the peroxy species using one of its axial positions.
R and L (R = alkyl and aryl, and L = Lewis base) has generated
much interest.1–3 The ligand substitution reaction of Ru2(OAc)4Cl
with amides results in the formation of diruthenium(II,III)
tetramidate compounds of a paddlewheel framework as shown in
Chart 1,4,5 where the triatomic (N–C–O) bridging ligands support
the diruthenium core.
Interesting examples of other diruthenium species as catalysts have
been reported recently, including (i) the generation of transient
axial Ru-nitrene and the isolation of the product of nitrene
insertion into the aromatic CH bond by Berry,10 and (ii) olefin
cyclopropanation with diazo agents catalyzed by diruthenium(I,I)
mixed carbonyl carboxylate compounds.11 Also noteworthy are
the allylic and amine oxidations by tert-butyl hydroperoxide
(TBHP) facilitated by various dirhodium catalysts.12 Past studies
on diruthenium tetramidates have addressed issues including syn-
thetic methods, voltammetric and magnetic characterization, and
electronic structures.4,13 The current study focuses on the influence
of the amidate ligands on the reactivity of the diruthenium
center as peroxide activators for organic sulfide oxygenation. We
are particularly interested in whether diruthenium tetramidates
exhibit faster rates than Ru2(esp)2Cl8 due to the enhanced electron
richness of the Ru2 core.14
The interest in the oxidation of organic sulfides originates from
the utility of sulfoxides and sulfones in medicinal chemistry,15
the removal of refractory sulfur compounds from fossil fuels,16
and the decontamination of V type agents and mustard gas.17,18
Organosulfur compounds in fossil fuels are typically removed
by hydrodesulfurization (HDS), biodesulfurization, and chlorine-
based bleaching processes; these methods present certain draw-
backs. For example, HDS technology requires extremely high
temperatures and pressures, and the lowest sulfur content achieved
is around 500 ppm because of its ineffectiveness in reducing DBT
(dibenzothiophene) derivatives.19 The highest activity obtained
using biodesulfurization is still insufficient to fulfil industrial
Chart 1 Diruthenium amidates of paddlewheel motif; R = isopropyl (1);
R = ethyl (2).
Catalytic activities of Ru2(II,III) tetracarboxylates have been
reported for the hydrogenation of alkenes and alkynes,6 cyclo-
Department of Chemistry, Purdue University, 560 Oval Drive, West
Lafayette, Indiana 47907, USA. E-mail: tren@purdue.edu
† Electronic supplementary information (ESI) available: Optimised model
structures; electrochemical plots. CCDC reference numbers 827246–
827247. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c1dt11530h
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requirements.20 Processes used for the degradation and decon-
tamination of chemical warfare agents, such as chlorine-based
bleaching processes, are corrosive and can produce undesirable
observed for both compounds and formal potential for 1 is more
cathodic than that of 2, further confirming the electron richness
of 1. It is worth noting that the oxidation of Ru2(esp)2Cl appears
as an irreversible wave at ca. 0.84 V under the same conditions,26
confirming that the tetramidates studied herein are more electron
rich than diruthenium(II,III) tetracarboxylates.
Dinuclear paddlewheel compounds supported by N,O-
bidentate bridging ligands may adopt one of four possible
configuration isomers (Chart 2). The amidate ligand arrangement
in each isomer may influence both the accessibility of axial sites
and the electronic structure, and hence the structural details about
compounds 1 and 2 are relevant to the oxygenation catalysis. Single
crystals for both compounds were obtained by slow diffusion
of ether into a saturated solution, and the molecular structures
determined via single crystal X-ray diffraction are shown in Fig. 1.
reaction byproducts that are harmful to the environment.17
A
convenient and effective method to remove sulfur compounds
from fossil fuels and to detoxify S-containing chemical warfare
agents is sulfide oxygenation, where an organic sulfide is oxidized
to its sulfoxide and then to its sulfone as presented in Scheme 1.21
Scheme 1 Oxygenation of organic sulfides.
Oxidants such as hydrogen peroxide and tert-butyl hydroper-
oxide are ideal, as they are environment-friendly reagents with
water and tert-butyl alcohol as the respective by-products. The
reactions between these oxidants and organic sulfides are slow;
hence extensive studies have been performed for the development
of new catalysts.22 Diruthenium catalysts such as Ru2(OAc)4Cl and
Ru2(esp)2Cl (esp = a,a¢,a¢-tetramethyl-1,3-benzenedipropionic
acid) have been successfully used to promote sulfide oxygenation
by tert-butyl hydroperoxide in either an acetonitrile solution
or neat (solvent-free) conditions.8 Considering the success of
the diruthenium(II,III) tetracarboxylates in facilitating sulfide
oxygenation, we wondered about the catalytic activity of more
electron rich diruthenium species supported by amidate ligands.
Reported herein are the synthesis of two new diruthenium(II,III)
tetramidate compounds (Scheme 1) and their activity in promoting
the oxygenation of methyl phenyl sulfide (MPS) and diphenyl
sulfide (PPS), using hydrogen peroxide or tert-butyl hydroperoxide
as the oxidant.
Results and discussion
A. Synthesis and characterization
Both compounds 1 and 2 were prepared by 24 h refluxing
of Ru2(OAc)4Cl with 6 equiv. of the corresponding amide in
toluene, which was outfitted with a micro-Soxhlet extraction
apparatus containing K2CO3/sand as the scrubber for acetic
acid.23 Recrystallization of compounds from hot methanol yielded
brown crystalline materials. Similar to the previously studied
diruthenium(II,III) tetramidates, compounds 1 and 2 are para-
magnetic with room temperature effective magnetic moments of
3.6 and 3.8 mB, respectively, which correspond to a S = 3/2 ground
state. The vis-NIR absorption spectra of both compounds 1 and 2
feature peaks at ca. 440 nm and 950 nm. The former is assigned to
the n(Cl) → p*(Ru2) LMCT transition,24 and the latter to d(Ru2)
→ d*(Ru2) transition.25 Redox properties of compounds 1 and 2
were examined with both cyclic (CV) and differential voltammetric
(DPV) techniques in acetonitrile solutions (plots provided in the
ESI†). Both compounds 1 and 2 exhibit quasi-reversible 1e-
oxidation at 0.53 V and 0.77 V versus Ag/AgCl, respectively.
Despite the structural similarity between the two compounds, the
formal potential of 1e- oxidation of 1 is 0.24 V more cathodic
than that of 2, indicating that the former is significantly more
electron rich than the latter. Quasi-reversible reductions were also
Fig. 1 Molecular structures of Ru2(ONHCC(CH3)2)4Cl (1, a) and
Ru2(ONHC(CH2CH2)4Cl (2, b). Hydrogen atoms were omitted for clarity
and ellipsoids were plotted at 30% probability level.
It is clear from Fig. 1 that the amidate ligands adopt the cis-(2,2)
arrangement similar to what has been previously reported for other
diruthenium amidate compounds,27 where the like atoms bound to
the same Ru center are cis to each other. There is a crystallographic
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of 1 is predominantly a s*(Ru–Ru) orbital, as noted in the previous
studies of similar compounds.3,30 Of special notice is the electron
density (dz2) stacked along the axial direction at the unsaturated
5-coordinated Ru center in the LUMO. Upon the introduction
of the H2O molecule (namely 1¢·H2O), the stacked dz2 electron
density partially shifts to the H2O, leading to a more relaxed Ru–
˚
˚
Ru bond distance, 2.375 A (2.355 A in 1¢). The addition of the
axial H2O seems to destabilize all Ru2 based MOs when compared
with those of 1¢ and the LUMO being the most destabilized. The
coordination to Ru2 also brought subtle changes in H2O: the O–H
Chart 2 Configuration isomers in M2(N–O)4 type compounds.
◦
˚
Table 1 Selected bond lengths (A) and angles ( ) for compounds 1 and 2
1
2
˚
˚
bond elongated from 0.975 A in free H2O to 0.977 A, and the H–
O–H angle relaxed from 104.5 to 112.1 degree. The latter reflects
the conversion of one of the H2O’s lone pairs into a dative bonding
pair. One may infer from the DFT result of 1¢·H2O that the electron
density along the Ru–Ru axial position would be imparted onto
H2O2 upon its coordination, which leads to weakened Ru–Ru and
O–O bonds. The latter would lead to the activation of the peroxy
species.
The preceding conjecture was verified with the DFT study of
1¢·H2O2. Similar to the cases of 1¢ and 1¢·H2O, the calculation
of 1¢·H2O2 converged to a configuration with SOMO, SOMO-1,
and SOMO-2 being singly occupied, and the compositions of
these SOMOs are comparable across the series as shown in Fig. 2.
Clearly, the presence and nature of the dative ligand (H2O or H2O2)
has a minimal impact on the electronic structure of the Ru2 core.
Ru1–Ru1¢
Ru1–O1
Ru1–O2
Ru1–N1
Ru1–N2
Ru1–Cl
2.2997(7)
2.067(3)
2.050(3)
2.033(4)
2.026(4)
2.5671(9)
2.288(1)
2.033(6)
2.037(6)
2.048(6)
2.035(7)
2.581(2)
Ru1¢–Ru1–O1
Ru1¢–Ru1–O2
Ru1¢–Ru1–N1
Ru1¢–Ru1–N2
Ru1–Ru1¢–Cl
91.9(1)
91.51(9)
86.9(1)
87.2(1)
178.16(2)
90.5(2)
90.3(2)
88.6(2)
88.5(2)
177.7(5)
inversion center that bisects the Ru1–Ru1¢ bond in both cases.
Furthermore, both compounds form one-dimensional zig-zag
chain structures with axial Cl bridging the adjacent Ru2 units,
similar to many diruthenium(II,III) species supported by either
carboxylate or amidate ligands.1,28 The Ru–Ru bond lengths in
˚
There is a slight lengthening of Ru–Ru bond in 1¢·H2O2 (2.388 A)
˚
in comparison with that of 1¢·H2O (2.375 A). On the other hand,
˚
1 and 2 are 2.2997(7) and 2.288(1) A, respectively, which are
the O–O bond in 1¢·H2O2 has been significantly elongated to 1.618
31
comparable to the previously reported tetramidate compounds27
and consistent with a formal bond order of 2.5. These distances are
also similar to those found in analogous tetracarboxylates such as
A from that of free H2O2 (1.475 A). This result points to the
pivotal role of the dative coordination of peroxy species to Ru2 in
the eventual O–O bond cleavage.
˚
˚
Ru2(O2CCH2CH3)4Cl with a Ru1–Ru1¢ distance equal to 2.292(7)
29
˚
A. Additional bond lengths and angles for compounds 1 and 2
are presented in Table 1.
B. Oxygenation catalysis
To gain insight into the electronic structure of diruthenium
tetramidates and the impact of H2O2 binding, spin-unrestricted
DFT calculations were performed for both compound 1 and
two hypothetical compounds 1¢·H2O and 1¢·H2O2, where the
vacant axial position of 1 is occupied by a water and a hydrogen
peroxide molecule, respectively. The geometry of model 1¢ was
fully optimized from the crystal structure of 1 at BP86/LanL2DZ
level using the DFT method. The structures of 1¢·H2O and 1¢·H2O2
were constructed by the further full optimization of 1¢ with a water
molecule and a hydrogen peroxide molecule added to the five-
coordinated Ru center, respectively. The bond lengths and angles
around the Ru2 core in the optimized geometry of 1 match well
with those from the crystal structure, and the optimized geometries
for all three model compounds are given in the ESI.† Fig. 2 shows
the computed frontier molecular orbital diagrams for compounds
1¢, 1¢·H2O, and 1¢·H2O2.
Successful preparation and characterization of 1 and 2 was
followed by the examination of their ability in facilitating the
oxygenation of methyl phenyl sulfide (MPS) and phenyl sulfide
(PPS) using H2O2 and TBHP as the oxidants in CH3CN. The data
obtained through GC analysis are presented in Tables 2 and 3 for
oxidants as H2O2 and TBHP, respectively. As shown in Table 2,
compounds 1 and 2 are active for the oxygenation of MPS and
PPS. The highest rate of conversion was obtained for the MPS
oxidation using 1: MPS was completely consumed in 24 h to yield
71% MPSO and 27% MPSO2. Oxygenation of PPS is slower than
that of MPS and a significant amount of sulfide was still present
at 24 h with 1 (entry 6) under the same conditions as that for MPS
(entry 3). It is clear from comparing the turn-over-frequencies
(TOFs) of entries 3 and 9 that catalyst 2 is slightly less efficient than
1. Previously, we reported TOFs as high as 5000 for organic sulfide
oxygenation by H2O2 with Mn–Me3TACN catalysts.32 Clearly, the
diruthenium tetramidate catalysts 1 and 2 are inferior in terms of
reaction rate. However, the amide ligands are much less expensive
than Me3TACN, which make 1 and 2 suitable candidates when the
rate is not the crucial factor in the selection of catalysts.
Calculations for both model compounds 1¢ and 1¢·H2O con-
verged to similar configurations with SOMO, SOMO-1, and
SOMO-2 being singly occupied. It is clear from Fig. 2 that the
SOMOs in both cases are predominantly the d*(Ru–Ru) orbital
with an additional contribution from the lone pairs of N and O
centers of the amidate ligands. The SOMO-1 and SOMO-2 are
nearly degenerate with a separation of 0.029 and 0.025 eV in 1¢ and
1¢·H2O, respectively. They are primarily the p*(Ru–Ru) orbitals
with significant contribution from the p(Cl) lone pair. The LUMO
The proficiency of compounds 1 and 2 in facilitating TBHP
oxygenation was similarly examined and the results are given in
Table 3. It is clear from Table 3 (entries 13–24) that both 1 and 2
are capable of activating TBHP, with 1 being slightly faster than
2. Furthermore, the reactions using TBHP are universally slower
646 | Dalton Trans., 2012, 41, 644–650
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Fig. 2 Molecular orbital diagram model compounds 1¢ (left), 1¢·H2O (middle) and 1¢·H2O2 (right) obtained from DFT calculations.
Table 2 Results for the analysis of MPS and PPS oxidation using 1 mol %
during the early phase of the reactions, they did not result in the
complete consumption of MPS at 24 h (entries 15 and 21) as
did Ru2(esp)2Cl.8 Initial rate studies based on UV spectroscopic
techniques were performed to address this issue as described below.
It was found in the early study of Ru2(esp)2Cl that the solubility
of the catalyst in non-polar solvents allows solvent free reaction
conditions.8 The same type of solvent free catalytic reactions were
performed by dissolving either 1 or 2 directly into organic sulfide
and initiating the reactions by the addition of TBHP. As seen on
entries 25 and 27, the TOFs of MPS oxygenation were drastically
enhanced compared to those obtained in CH3CN (entries 13 and
19). Nevertheless, these results are not nearly as good as those
from using Ru2(esp)2Cl, where nearly quantitative conversion of
MPS to MPSO was achieved with only one equiv. of TBHP.8
To better understand the less than desired performance of
compounds 1 and 2 as oxygenation catalysts, the initial rates of
MPS oxygenation reactions were investigated using UV absorp-
tion spectroscopy. The consumption of MPS can be monitored
by the disappearance of the band at 290 nm (n(S) → p*(ph)) as
described in prior studies.8,33 The dependence of reaction rates
on both [1] and [2] was monitored for the first 30 min under
pseudo first order conditions (100 equiv. TBHP) and the initial
rates (kobs) were extracted from the natural logarithm of the
a
of catalyst and 8 equiv. of H2O2
Catalyst RSR¢ Entry t/h RSR¢% RSOR¢% RSO2R¢% TOFb
1
2
MPS
1
2
3
4
5
6
7
8
2
6
24
2
6
24
2
6
24
2
6
19
5
0
73
81
71
33
45
51
71
82
89
34
43
56
6
43
18
5.2
21
10
2.9
38
15
4.5
19
9
14
27
4
6
9
2
5
10
2
4
PPS
62
48
38
28
12
1
64
51
37
MPS
PPS
9
10
11
12
24
7
2.9
a Reaction conditions: 10 mL of solvent, 0.5 mmol of substrate, 4 mol
H2O2, 0.005 mmol of catalyst; room temperature. b Turn-over-frequency
(hour-1) = {[RR¢SO] + 2[RR¢SO2]}/{[Cat]*t (h)}.
than the corresponding reactions using H2O2. The comparison of
catalytic efficiency of 1 and 2 with that of Ru2(esp)2Cl is interesting
and perplexing. While both 1 and 2 (entries 13 and 19) are
comparable to Ru2(esp)2Cl in terms of TOF of MPS oxygenation
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Table 3 TBHP oxygenation of organic sulfides
course of catalytic reactions. Indeed, when MPS oxygenation
was performed using 100 equiv. of TBHP (entries 29 and 30),
significant conversion of MPS was achieved during the first 1.5 h
but little reactivity was detected thereafter (1.5–24 h), suggesting
that the majority of the catalyst was degraded during the first
1.5 h. Furthermore, the reaction mixture with 1 or 2 generally
changes from an initial yellow color to a persistent dark purple
during the course of the catalytic reaction with TBHP. Upon the
reaction with 8 equiv. of TBHP in the absence of organic sulfide,
both catalysts 1 and 2 were converted to dark purple species that
are spectroscopically distinct from the pristine catalysts (see Fig.
S1 and S2 in the ESI for spectral comparison†). Similar changes
were observed with H2O2 as the oxidant.
Catalyst RSR¢ Entry t/h RSR¢ % RSOR¢ % RSO2R¢ % TOF
1
MPS 13
2
6
40
39
52
54
69
16
19
24
35
38
54
16
22
34
5
5
5
4
5
6
5
6
6
2
3
12
31
11
3
12
5
1.5
23
13
3
10
5
14
15
16
17
18
24 26
2
6
24 70
2
4
24 40
2
6
PPS
78
75
2
MPS 19
59
54
20
21
22
23
24
PPS
77
73
24 51
2
Solvent free conditionsb
1
MPS 25
PPS 26
MPS 27
PPS 28
Catalyst stability testc
1
1
1
1
0
58
2
90
40
84
36
9
0
14
0
108
40
112
36
Conclusion
2
64
Two new diruthenium tetramidate compounds (1 and 2) were
synthesized and characterized with spectroscopic, voltammetric
techniques and theoretical calculations. The ability of both
1 and 2 in promoting oxygenation of organic sulfides using
H2O2 and TBHP at ambient conditions has been demonstrated.
Diruthenium tetramidates are less active than the previously
studied diruthenium tetracarboxylates. It appears that the en-
hanced electron richness actually led to faster degradation of
the catalysts. Currently, we are investigating the feasibility of the
other extreme—maintaining catalytic activity while increasing the
longevity by using more electron deficient Ru2 species.
1
MPS 29
30
1.5
24
5
5
62
60
31
35
83
5
Conditions:a (top half) 0.5 mmol of substrate, 4 mmol TBHP, 0.005 mmol
of catalyst, 10 mL of solvent; room temperature b Solvent free: 0.5 mmol
of substrate, 4 mmol TBHP, 0.005 mmol of catalyst; room temperature.
c Catalyst stability test: 0.5 mmol of substrate, 50 mmol TBHP, 0.005 mmol
of catalyst; room temperature.
absorbance values plotted versus time. The initial rates plotted
versus catalyst concentration are shown in Fig. 3 for compounds 1,
2 and Ru2(esp)2Cl, from which the pseudo first order rate constant
k¢ was obtained.
Experimental
kobs = k¢[cat]
(1)
General considerations
MPS, PPS and the internal standard 1,2-dichlorobenzene were
purchased from ACROS Organics. The ligands, isobutyramide
and propionamide, were purchased from Sigma–Aldrich, the
30% H2O2 from Mallinkroft Chemicals and the 70% tert-butyl
hydroperoxide also from ACROS Organics. All reagents were
used as received. Ru2(AcO)4Cl was prepared as described in the
literature.34 Spectroscopic absorption spectra were collected with
a JASCO V-670 spectrophotometer. The GC data were recorded
on an Agilent 7890A Gas Chromatographic equipped with a
HP-5 capillary column. Infrared spectra were recorded using a
JASCOFT/IR-ATR 6300 spectrometer. All HR-nESI-MS data
were performed on a modified QqTOF tandem mass spectrometer
in CH2Cl2 (QSTAR XL; mass resolving power ~8000 amu; mass
accuracy ~20 ppm; Applied Biosystems/MDS Sciex, Concord,
ON, Canada). Masses were calculated by isotopic distribution
utilizing Analyst 1.4 software (Applied Biosystems/MDS Sciex,
Concord, ON, Canada). Both cyclic voltammetry (CV) and differ-
ential pulse voltammetry (DPV) were performed on a CHI620A
voltammetric analyzer with a glassy-carbon working electrode
(diameter 2 mm), a Pt-wire auxiliary electrode, and a Ag/AgCl
reference electrode. All the reactions reported were carried out at
room temperature and environmental pressure unless otherwise
specified.
Fig. 3 Dependence of kobs on [1] (diamond), [2] (circle) and [Ru2(esp)2Cl]
(square) in MeCN in the oxygenation of MPS using TBHP.
The k¢ values obtained for 1 and 2 are 0.22 and 0.095 min-1,
respectively, and the former is comparable to that found for
Ru2(esp)2Cl (0.17 min-1) under the same conditions.8 While the
k¢ values of 1 and 2 would predict similar catalytic efficiency to
Ru2(esp)2Cl, their sluggish performance implies that some other
limiting factors, such as the stability of the catalysts, affect the
Ru2(HNOC(CH3)2)4Cl (1)
A mixture of Ru2(OAc)4Cl (475 mg, 1 mmol) and isobutyramide
(522 mg, 6 mmol) was suspended in 60 mL of toluene and refluxed
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with a condenser outfitted by a Soxhlet extraction apparatus
containing K2CO3 for approximately 24 h. After the solvent
removal, the residue was dissolved in hot methanol, filtered and
recrystallized by the addition of ether. Compound 1 was isolated
as an orange-brown microcrystalline material (0.401 g, 62%). Data
for 1: HR-nESI-MS based on 101Ru, m/z 547.053, corresponding
to [M-Cl]+ (calc. 547.057). UV-Vis-NIR, lmax (nm, e (M-1 cm-1)):
335(4490), 465(960), 999(200); IR (cm-1) N–H 3299(m), 3351(m),
were performed with the generalized gradient approximation,
using Becke’s non-local correction to exchange and Perdew’s non-
local correction to correlation (BP86).38 The basis set used was
the LanL2DZ effective core potential39 for the metal centers
and 6-31G(d,p) for the ligand atoms. No negative frequency was
observed in the vibrational frequency analysis.
Catalytic runs
C
O 1637(m), C–N 1442(m). Cyclic voltammogram [E1/2/V,
For all the reactions presented, 2.5 mmol of the organic sulfide
was placed in a flask with 2.0 mmol of the internal standard (1,2-
dichlorobenzene) and 0.025 mmol of the catalyst and dissolved in
10 mL of MeCN. Solvent free reactions were done by dissolving the
catalyst directly into the organic sulfide. Reactions were started by
the addition of the corresponding amount of 9.7 M H2O2 solution
or 7.6 M TBHP solution. Oxidant solution concentrations were
determined by iodometric analysis. Results for entries 1–30 are
an average of two runs. The turn-over-frequencies (TOF h-1) were
calculated using the equation ([RSOR¢] + 2[RSO2R¢])/(1*t (h-1)).
DEp/V, ibackward/iforward]: 0.562, 0.038, 0.70. meff: 3.56 mB.
Ru2(HNOC(CH2CH3))4Cl (2)
A mixture of Ru2(OAc)4Cl (475 mg, 1 mmol) and propionamide
(430 mg, 6 mmol) was dissolved in 60 mL of toluene and
refluxed as described above for 24 h. The color of the mixture
changed from red-purple to dark brown. Compound 2 was
recrystallized as described for compound 1 and isolated as dark
brown microcrystalline material (0.573, 73%). Data for 2: HR-
nESI-MS based on 101Ru, m/z 486.959, corresponding to [M-
Cl] (486.960). UV-Vis-NIR, lmax (nm, e (M-1 cm-1)): 341(3260),
Kinetic studies
467(1090), 1014(150); IR (cm-1) N–H 3318(m), 3342(m), C
1635(m), C–N 1436(m). Cyclic voltammogram [E1/2/V, DEp/V,
backward/iforward]: 0.862, 0.094, 0.60. meff: 3.83 mB.
O
UV experiments to measure the effects of catalyst concentration on
the MPS oxidation was studied in a pseudo first order environment
by using an excess of TBHP (100 equiv. in relation to MPS).
Reaction rates were calculated when using 1, 2, 4 and 5 mol % of
catalyst. Results were collected by monitoring the disappearance
of the UV absorption band at 290 nm as previously reported.40
Further details are given in the Results and discussion section.
i
Crystal structure determination
Single crystals suitable for X-ray diffraction were isolated via
slow diffusion of ether into a saturated methanol solution. For
the determination of the crystal structure, a brown needle of
compound 1 having approximate dimensions of 0.16 ¥ 0.08 ¥
0.05 mm and an orange needle of 2 having approximate dimensions
of 0.16 ¥ 0.04 ¥ 0.01 mm were mounted on a fiber in a random
orientation. Diffraction data was collected on a Rigaku Rapid
II diffractometer using Cu-Ka at 150(1) K and Frames were
integrated with DENZO-SMN.35 The structures were solved by
direct methods using SIR2004,36 and refinement was performed
using SHELX-97.37 Crystallographic data for 1 and 2 is presented
in Table 4.
Acknowledgements
This work was supported by both the US Army Research Office
(Grant W911NF-06-1-0305) and Purdue University. We thank Mr.
Yang Gao for his assistance in acquiring HR-MS data.
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Formula
C18H40ClN4O6Ru2
646.14
C12H20ClN4O4Ru2
521.91
Formula weight
Space group
P2/c (No. 13)
11.6827(8)
9.4837(5)
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1364.2(2)
2
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2357 (0.057)
0.047, 0.135
C2/c (No. 15)
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4
2.043
150
5703
1403 (0.043)
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˚
a/A
˚
b/A
˚
c/A
b/◦
3
˚
V/A
Z
rc/g cm-3
T/K
Data collected
Unique data (Rint
R1(obs), wR2(obs)
)
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The Royal Society of Chemistry 2012
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