Oxidation of olefins catalyzed by half-sandwich osmium(II) arene complexes

Article abstract of DOI:10.1016/j.jorganchem.2017.12.038

Ten complexes [(η⁶-(arene)OsCl(C₅H₄N-2-CH=N-C₆H₅X)]PF₆ (with arene = p-cymene (1) or benzene (2); X = 4-F (a), 4-Cl (b), 4-Br (c), 4-I(d) and 4–methyl (e)) were synthesized by reacting the corresponding N,N′-bidentate ligands with the osmium arene dimers [(η⁶-arene)Os(μ-Cl)Cl]₂ in a 2:1 ratio. Complexes 1a-e and 2c-d are new. The compounds were fully characterized via ¹H and ¹³C NMR, IR and UV–Vis spectroscopy, and elemental analyses. The x-ray crystal structure of compound 2e is also reported. The Os(II) complex shows the expected “ piano stool” type geometry. These Os(II) compounds were investigated in the catalytic oxidation of olefins to carbonyl compounds with NaIO₄ as terminal oxidant in a H₂O/t-BuOH biphasic system. All of the compounds were very effective catalysts for this reaction and gave the corresponding aldehyde in high yields.

Full text of DOI:10.1016/j.jorganchem.2017.12.038

Accepted Manuscript
Oxidation of olefins catalyzed by half-sandwich osmium(II) arene complexes
Joel M. Gichumbi, Bernard Omondi, Holger B. Friedrich
PII:
S0022-328X(17)30736-2
10.1016/j.jorganchem.2017.12.038
JOM 20233
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 6 September 2017
Revised Date: 20 December 2017
Accepted Date: 30 December 2017
Please cite this article as: J.M. Gichumbi, B. Omondi, H.B. Friedrich, Oxidation of olefins catalyzed by
half-sandwich osmium(II) arene complexes, Journal of Organometallic Chemistry (2018), doi: 10.1016/
j.jorganchem.2017.12.038.
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ACCEPTED MANUSCRIPT
Oxidation of olefins catalyzed by half-sandwich osmium(II) arene complexes
Joel M. Gichumbi, Bernard Omondi, Holger B. Friedrich*
*
School of Chemistry and Physics, University of KwaZulu-Natal, Private Bag X54001, Durban
4001, South Africa.
*Corresponding author. Tel.: +27 312603107; fax: +27 312603091. E-mail address:
friedric@ukzn.ac.za (H.B. Friedrich).
Abstract
Ten complexes [(η6-(arene)OsCl(C5H4N-2-CH=N-C6H5X)]PF6 (with arene = p-cymene (1) or
benzene (2); X = 4-F (a), 4-Cl (b), 4-Br (c), 4-I(d) and 4–methyl (e)) were synthesized by
reacting the corresponding N,Nʹ-bidentate ligands with the osmium arene dimers [(η6-
arene)Os(µ-Cl)Cl]2 in a 2:1 ratio. Complexes 1a-e and 2c-d are new. The compounds were fully
characterized via 1H and 13C NMR, IR and UV-Vis spectroscopy, and elemental analyses. The x-
ray crystal structure of compound 2e is also reported. The Os(II) complex shows the expected “
piano stool” type geometry. These Os(II) compounds were investigated in the catalytic oxidation
of olefins to carbonyl compounds with NaIO4 as terminal oxidant in a H2O/t-BuOH biphasic
system. All of the compounds were very effective catalysts for this reaction and gave the
corresponding aldehyde in high yields.
Key words: Osmium; N,N-bidentate ligands; arene; styrene oxidation; olefin oxidation; x-ray
structure
1. Introduction
The oxidative cleavage of alkenes to give the corresponding aldehydes, ketones or acids is a very
important transformation in organic synthesis [1]. There are two main oxidative pathways
discussed in literature for this oxidative cleavage [2]; the first pathway involves the oxidation of
the alkenes into 1,2-diols which is followed by cleavage with NaIO4 or other oxidants. The
second route involves ozonolysis where the alkene is cleaved into a number of possible
functionalized products, which depend on the workup conditions [2]. Ozonolysis is the standard
method; however, it needs to be very carefully controlled due to the risk of explosion. Therefore,
alternate reactions for the direct cleavage of alkenes, some without 1,2-diol intermediates, are
1

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being developed by researchers [3]. This reaction can be performed by the use of transition metal
complexes. The use of transition metal complexes of iron, ruthenium, manganese and copper
have been widely reported [4]. However, few publications have been devoted to osmium as
catalysts [5,6]. Osmium compounds have been used in the oxidative cleavage of alkyl and aryl
alkenes to their corresponding aldehydes, employing various oxidants [7]. Osmium is also
known to catalyze various organic reactions such as dihydroxylation, oxidation of alkanes with
various oxidants and oxygenation of alkanes [4,8-10]. Since osmium is in the same group as
iron, some osmium catalysts have been viewed as models of iron-containing enzymes and as
biomimetic systems [5].
The continued interest in complexes of osmium has led us to investigate some reactions of [(η6-
arene)Os(µ-Cl)Cl]2 with pyridine-imines and the properties of the products. We report the
preparation and characterization of the new compounds [(η6-(arene)OsCl(C5H4N-2-CH=N-
C6H5X)].PF6 (with arene = p-cymene (1) or benzene (2); X = 4-Br (c), 4-I (d) , as well as arene =
benzene (2); and X = 4-F (a), 4-Cl (b), and 4–methyl (e)). These complexes were then employed
in the oxidation of olefins using NaIO4 as the terminal oxidant in a H2O/tert-butanol biphasic
system.
2. Experimental
The reported reactions were carried out using dry, distilled solvents under nitrogen making use
of standard Schlenk techniques. α-Phellandrene, 1,4-cyclohexadiene, 2-pyridinecarboxaldehyde,
4-flouroaniline, 4-chloroaniline, 4-bromoaniline and 4-methylaniline (all Sigma-Aldrich) were
used as supplied. A Thermal-Scientific Flash 2000 analyzer was used for the elemental analyses.
Solid-state IR spectra were determined with a Perkin Elmer Spectrum 100 ATR spectrometer.
Mass spectra were obtained using a Waters Micromass LCT Premier TOF-MS. Details were as
previously reported [11]. UV-visible spectra were obtained using a Perkin-Elmer LAMBDA 35
1
spectrometer. The H and 13C NMR spectra were obtained from a Bruker Top Spin 400 MHz
spectrometer using DMSO-d6 (Merck). Melting points were determined using an Ernest Leitz
Wetzlar hot stage microscope. The dimers [(η6-arene)Os(µ-Cl)Cl]2, arene = benzene or p-
cymene, were synthesized via reported procedures [12], as were the pyridine-imine ligands
[13,14]. The complexes [(η6-(arene)OsCl(C5H4N-2-CH=N-C6H5X)]PF6 (with arene = benzene
(2); X = 4-F (a), 4-Cl (b), and 4–methyl (e)) were synthesized as reported [15].
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1.0 General method for the preparation of the Os(II) iminopyridyl complexes
The pyridine-imine ligand (0.32 mmol) in methanol (10 ml) was added dropwise to a methanol
(30 ml) solution of [(η6-arene)Os(µ-Cl)Cl]2 (0.15 mmol) at 40 °C. The solution’s colour changed
from yellow to red immediately. The solution was stirred at 40 °C for 3 h, after which the
volume was reduced to ca. 10 ml using a rotary evaporator. This was followed by addition of
NH4PF6 (0.33 mmol) and allowing the solution to stand overnight at 0 °C. The precipitate was
filtered off, followed by washing with cold ethanol, then diethyl ether and finally drying under
vacuum.
1a
Red solid, yield 85%, m.p. 161 °C (dec.). 1H NMR: δ 9.52 (d, JHH = 5.5 Hz, 1H, Py); 9.30 (s, 1H,
CH=N); 8.39 (m, 1H, Py); 8.26 (m, 1H, Py); 7.86 (m, 3H, Ar); 7.79 (m, 2H, Ar); 6.38 (d, JHH
=
5.8 Hz, 1H, p-cymene ); 6.00 (d, JHH = 5.8 Hz, 1H, p-cymene); 5.90 (d, JHH = 5.8 Hz, 1H, Ar, p-
cymene); 5.8 (d, JHH = 5.75 Hz, 1H, Ar, p-cymene); 2.40 (m, 1H, CH(CH3)3); 2.24 (s, 3H, CH3);
0. 93 (m, 6H, Me2, cymene). 13C NMR: 168.99 (CH=N); 155.84 (Py); 155.67 (Py); 140.10 (Py);
129.97 (Py); 129.80 (Py); 125.20 (Py); 125.11 (Ar); 116.51 (Ar); 116.29 (Ar); 97.64 (Ar); 96.82
(Ar); 78.83 (Ar, p-cymene); 78.80 (Ar, p-cymene); 75.47 (Ar, p-cymene); 75.35 (Ar, p-
cymene); 30.67 (CH3, p-cymene); 21.91 (CH3, p-cymene); 18.25 (CH3, p-cymene). IR (solid
state): ν(C=N) 1610.0 cm-1, ν(P-F) 830.5 cm-1. Anal. Calculated for C22H23ClF7N2OsP: C, 37.48;
H, 3.29; N, 3.97. Found: C, 37.68; H, 3.35 H; N 4.37. MS (ESI, m/z): 561.0 [C22H23ClFN2Os]+
1b
Red solid, yield 80%, m.p. 171 °C (dec.). 1H NMR: δ 9.53 (d, JHH = 5.4 Hz, 1H, Py); 9.36 (s, 1H,
CH=N); 8.40 (d, JHH = 7.6 Hz, 1H, Py); 8.30 (d, JHH = 7.6 Hz, 1H, Ar); 7.86 (m, 1H, Ar); 7.76
(m, 4H, Ar); 6.42 (m, 1H, Ar, p-cymene ); 6.05 (m, 1H, Ar, p-cymene); 5.99 (m, 1H, Ar, p-
cymene); 5.77 (m, 1H, Ar, p-cymene); 2.42 (d, JHH = 6.9 Hz, 1H, CH(CH3)3); 2.24 (s, 3H, CH3);
0. 92 (m, 6H, Me2, cymene). 13C NMR: δ 169.37 (CH=N); 155.77 (Py); 150.48 (Py); 140.11
(Py); 134.24 (Py); 130.18 (Py); 129.93 (Ar); 129.55 (Ar); 124.76 (Ar); 97.82 (Ar); 96.89 (Ar);
78.94 (Ar, p-cymene); 78.16 (Ar, p-cymene); 75.39 (Ar, p-cymene); 75.34 (Ar, p-cymene));
30.68 (CH3, p-cymene); 21.92 (CH3, p-cymene); 18.27 (CH3, p-cymene). IR (solid state):
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ν(C=N) 1615.4 cm-1, ν(P-F) 825.2 cm-1. Anal. Calculated for C22H23ClF7N2OsP: C, 36.62; H, 3.21;
N, 3.88. Found: C, 36.89; H, 3.12; N 4.05. MS (ESI, m/z): 577.0 [C22H23ClFN2Os]+
1c
1
Red solid, yield 80%, m.p. 191 °C (dec.). H NMR: δ 9.52 (d, JHH = 5.3 Hz, 1H, Py); 9.32 (s,
1H, CH=N); 8.40 (d, JHH = 7.6 Hz, 1H, Py); 8.30 (d, JHH = 7.6 Hz, 1H, Py); 7.84 (d, JHH = 8.4 Hz,
3H, Ar); 7.69 (d, JHH = 8.4 Hz, 2H, Ar); 6.39 (d, JHH = 5.6 Hz, 1H, p-cymene); 6.00 (d, JHH
=
5.6 Hz, 1H, Ar, p-cymene); 5.92 (d, JHH = 5.6 Hz, 1H, Ar, p-cymene); 5.74 (d, JHH = 5.6 Hz,
1H, Ar, p-cymene); 2.24 (m, 1H, CH(CH3)3); 2.30 (s, 3H, CH3); 0. 92 (m, 6H, Me2, cymene).
13C NMR: δ 169.35 (CH=N); 155.77 (Py); 155.71 (Py); 150.88 (Py); 140.10 (Py); 132.47 (Py);
130.17 (Py); 129.92 (Ar); 124.98 (Ar); 122.87 (Ar); 97.86 (Ar); 96.91 (Ar); 78.96 (Ar, p-
cymene); 78.14 (Ar, p-cymene); 75.36 (Ar, p-cymene); 30.69 (CH3, p-cymene); 21.92 (CH3, p-
cymene); 18.25 (CH3, p-cymene). IR (solid state): ν(C=N) 1613.9 cm-1, ν(P-F) 825.8 cm-1. Anal.
Calculated for C22H23ClBrF6N2OsP : C, 34.50; H, 3.03; N, 3.66 Found: C, 34.39; H, 2.98; N 3.33.
MS (ESI, m/z): 621[C22H23 BrClN2Os]+
1d
Red solid, yield 82%, m.p. 201 °C (dec.). 1H NMR: δ 9.52 (d, JHH = 5.4 Hz, 1H, Py); 9.31 (s, 1H,
CH=N); 8.40 (d, JHH = 7.6 Hz, 1H, Py); 8.29 (d, JHH = 7.6 Hz, 1H, Py); 7.85 (m, 1H, Ar); 7.53
(d, JHH = 8.4 Hz, 2H, Ar); 6.39 (d, JHH = 5.8 Hz, 1H, p-cymene); 5.99 (d, JHH = 5.7 Hz, 1H, Ar,
p-cymene); 5.90 (d, JHH = 5.6 Hz, 1H, Ar, p-cymene), 5.73 (d, JHH = 5.7 Hz, 1H, Ar, p-cymene);
2.41 (m, 1H, CH(CH3)3); 2.23 (s, 3H, CH3); 0.92 (m, 6H, Me2, cymene). 13C NMR: δ 169.11
(CH=N); 155.79 (Py); 155.73 (Py); 151.32 (Py); 140.10 (Py); 138.29 (Py); 130.16 (Py); 129.91
(Ar); 124.96 (Ar); 97.93 (Ar); 96.83 (Ar); 96.43 (Ar); 78.98 (Ar, p-cymene); 78.18 (Ar, p-
cymene); 75.34 (Ar, p-cymene); 75.29 (Ar, p-cymene); 30.70 (CH3, p-cymene); 21.90 (CH3, p-
cymene); 18.28 (CH3, p-cymene). IR (solid state): ν(C=N) 1615.7 cm-1, ν(P-F) 815.8 cm-1. Anal.
Calculated for C22H23ClIF6N2OsP: C, 32.50; H, 2.85; N, 3.45 Found: C, 31.86; H, 2.75; N 3.29.
MS (ESI, m/z): 669 [C22H23 IClN2Os]+
1e
Red solid, yield 80%, m.p. 189 °C (dec.). 1H NMR: δ 9.51 (d, JHH = 5.5 Hz, 1H, Py); 9.28 (s, 1H,
CH=N); 8.38 (d, JHH = 7.6 Hz, 1H, Py); 8.28 (t, JHHH = 7.6 Hz, 1H, Py); 7.84 (t, JHHH = 6.2 Hz,
4

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1H, Ar); 7.62 (d, JHH = 8.2 Hz, 2H, Ar); 7.43 (d, JHH = 8.1 Hz, 2H, Ar); 6.37 (d, JHH = 5.7 Hz,
1H, p-cymene); 5.97 (d, JHH = 5.8 Hz, 1H, Ar, p-cymene); 5.82 (d, JHH = 5.8 Hz, 1H, Ar, p-
cymene); 5.73 (d, JHH = 5.7 Hz, 1H, Ar, p-cymene); 2.45 (s, CH3); 2.20 (s, 3H, CH(CH3)3); 0. 92
(m, 6H, Me2, cymene). 13C NMR: δ 167.96 (CH=N); 155.95 (Py); 155.64 (Py); 149.47 (Py);
140.05 (Py); 139.83 (Py); 129.88 (Ar); 129.76 (Ar); 129.64 (Ar); 122.80 (Ar); 97.52 (Ar); 96.59
(Ar); 78.66 (Ar, p-cymene); 78.37 (Ar, p-cymene); 75.60 (Ar, p-cymene), 75.26 (Ar, p-cymene);
30.68 (CH3, p-cymene); 21.89 (CH3, p-cymene); 21.89 (CH3); 18.25 (CH3, p-cymene). IR (solid
state): ν(C=N) 1615.9 cm-1, ν(P-F) 824.6 cm-1. Anal. Calculated for C23H26ClF6N2OsP: C, 39.40;
H, 3.74; N, 4.00 Found: C, 38.80; H, 3.81 H; N 3.66. MS (ESI, m/z): 557 [C23H26ClN2Os]+
2c
Red solid. Yield 82%. m.p. 210 °C (dec.). 1H NMR: δ 9.62 (d, JHH = 5.4 Hz, 1H, Py); 9.31 (s, 1H,
CH=N); 8.40 (m, 1H, Py); 8.28 (m, 1H, Py); 7.84 (m, 3H, Ar); 7.69 (d, JHH = 8.8 Hz, 2H, Ar);
6.11 (s, 6H, C6H6). 13C NMR: δ 169.34 (CH=N); 155.82 (Py); 155.76 (Py); 150.73 (Py); 140.23
(Py); 132.37 (Py); 129.99 (Py); 129.18 (Ar); 124.89 (Ar); 122.76 (Ar); 78.78 (C6H6). IR (KBr,
cm-1): υ(C=N) 1613.2, ν(P-F) 822.2. Anal. Calculated for C18H15ClBrF6N2OsP: C, 30.45; H, 2.13;
N, 3.95. Found C, 30.68; H, 2.00; N, 3.47. MS (ESI, m/z): 564.97 [C18H15ClBrN2Os]+
2d
Red solid. Yield 82%. m.p. 205 °C (dec.). 1H NMR: δ 9.61 (d, JHH = 8.8 Hz, 1H, Py); 9.29 (s, 1H,
CH=N); 8.37 (m,1H, Py); 8.26 (m, 1H, Py); 7.99 (m, 2H, Ar); 7.85 (m, 1H, Ar); 7.53 (d, JHH
=
8.5 Hz, 2H, Ar); 6.11 (s, 6H, C6H6). 13C NMR: δ 169.13 (CH=N); 155.79 (Py); 151.17 (Py);
140.22 (Py); 138.19 (Py); 129.96 (Py); 129.76 (Ar); 124.86 (Ar); 98.30 (Ar); 78.77 (C6H6). IR
(KBr, cm-1): υ(C=N) 1613.9, ν(P-F) 822.9 . Anal. Calculated for C18H15ClIF6N2OsP: C, 28.56; H,
2.00; N, 3.70. Found C, 29.06; H, 1.92; N, 3.28. MS (ESI, m/z): 610.0 [C18H15ClIN2Os]+
Suitable crystals for X-ray diffraction were obtained by layering a fourfold volume of hexane on
a dry acetone solution of compound 2e, then leaving the mixture at RT in the dark for 2 days.
Data collection was carried out using a Bruker Smart APEXII diffractometer, while the reduction
was done with the program SAINT+ [14]. The structure was solved by direct methods via
SHELXS [16] and refined with SHELXL [14]. The details are as previously reported [16].
Structure refinement and crystal data information for compound 2e are given in Table 1.
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Table 1. Summary of the crystal data of 2e
Empirical formula
Formula weight
Temperature
C22H24ClF6N2OOsP
703.05
173(2) K
Wavelength
0.71073 Å
Crystal system
Space group
Monoclinic
C2/c
Unit cell dimensions
a = 27.7514(10) Å
b = 8.4921(3) Å
c = 21.7276(8) Å
β
= 112.055(2)°
4745.8(3) Å3
V
Z
8
Densitycalc
1.968 mg/m3
5.620 mm-1
Absorption coefficient
F(000)
2720
Crystal size
0.58 x 0.53 x 0.37 mm3
Theta range for data collection
Index ranges
2.023 to 27.999°
-36
≤h
≤36, -11
≤k
≤11, -8≤l
≤29
Collected reflections
Independent reflections
Completeness to theta = 25.242°
Max. and min. transmission
Absorption correction
Max. and min. transmission
Refinement method
55233
5728 [R(int) = 0.0493]
100%
0.2283 and 0.1396
Semi-empirical from equivalents
0.175 and 0.089
Full-matrix least-squares on F2
5959 / 0 / 305
Data / restraints / parameters
Final R indices [I>2sigma(I)]
R indices (all data)
R1 = 0.0232, wR2 = 0.0533
R1 = 0.0306, wR2 = 0.0569
1.402 and -1.217 e.Å3
Largest diff. peak and hole
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2.5 Alkene oxidation reactions
All catalytic reactions were carried out in Schlenk tubes heated at 60 °C. The products were
identified and quantified by GC (Perkin Elmer Auto system) equipped with a capillary column
(Varian wax, 25 m x 0.15 mm x 2 µm) and a flame ionization detector. The injector temperature
was 250 °C. The ratio of catalyst to substrate was 1:100. The products were confirmed by
comparison of their retention times with those of standards obtained commercially (Sigma
Aldrich), as well as GC-MS analyses. The alkene (0.5 mmol), internal standard (biphenyl, 0.5
mmol) and catalyst (0.5 mol %) were dissolved in a water (3 ml) and tert-butanol (3 ml) mixture.
Three equivalents of oxidant, NaIO4, were subsequently added in one batch and the immiscible
mixture stirred vigorously, at 60 °C. The reactions were monitored by removing aliquots hourly
and injecting 1 µL into the GC each time. Furthermore, control experiments were done under the
same conditions, where the reactions were carried out without either the catalyst or NaIO4.
3.0 Results and Discussion
3.1 The preparation and characterization of the osmium compounds
Reacting the N,N-bidentate ligands with the osmium dimers [(η6-arene)Os(µ-Cl)Cl]2 in methanol
at 40 °C gave the mononuclear compounds [(η6-arene)OsCl(C5H4N-2-CH=N-C6H5X)]PF6 (with
arene = p-cymene (1) or benzene (2); X = 4-F (a), 4-Cl (b), 4-Br (c), 4-I(d) and 4–methyl (e)).
They were crystalised as hexafluorophosphate salts as shown in Scheme 1. These air-stable
complexes are not hygroscopic and dissolve in polar solvents including acetonitrile, acetone,
DMF and DMSO, but do not dissolve in non-polar solvents like diethyl ether, dichloromethane
and hexane.
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Scheme 1: Preparation of [(η6-arene)OsCl(C5H4N-2-CH=N-C6H4-X)]PF6 (arene = p-cymene (1a-
1e) or C6H6 (2a-2e))
Confirmation of formation of complexes 1a-1e and 2c and 2d is found by comparing the 1H and
13C NMR spectra by monitoring the proton and carbon signal of the imine (CH=N) group of the
ligands and the complexes. On complexation the imine proton peaks shift downfield to δ = 9.25 -
9.36 ppm from δ = 8.49 - 8.58 ppm of the uncoordinated ligands (Table 2), because the imine
proton is deshielded since the nitrogen donates a lone pair of electrons to the osmium [15]. In
addition, the 13C NMR spectra showed that the imine carbon peak shifted from δ = 160.1 - 163.2
for the free ligands to δ = 167.9 - 170.3 for the complexes. This observation agrees well with
those on related compounds [17]. Also, the bonding of the bidentate ligands to the Os para-
cymene moiety is further confirmed because the proton resonances of the phenyl ring of the p-
cymene group resolve into separate peaks because the arene ring loses symmetry once of the
bidentate ligand coordinated to the osmium centre, which also has been observed for related
compounds [18].
Furthermore, comparison of the IR spectra supported complex formation. Thus, a strong
absorption band is seen for all the complexes between1610 - 1616 cm-1, due to the symmetrical
υ(C=N) vibration. This peak was at lower wavenumbers than those of the corresponding band of
the free pyridine-imine ligands at 1623 - 1626 cm-1. This shift indicates that the bidentate ligands
8

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have bonded to the osmium (Table 2). A distinctive peak is seen for the complexes between 816
-
and 830 cm-1 and is assigned to the PF6 counter ion [19,20].
High-resolution mass spectra of 1a-2e further confirm the proposed formulation of the
mononuclear complexes. All complexes give the base peak [(η6-arene)OsCl(C5H4N-2-CH=N-
C6H5X)]+. Furthermore, each fragment shows the expected characteristic multiple peaks that are
due to the isotopes of osmium.
13
Table 2. The 1H NMR, C NMR and IR shifts observed for the imine functionality of the free
ligands and the complexes 1a-e and 2c-d
Complex 1H NMR (ppm)
13C NMR (ppm)
IR (CH=N)imine (cm-1)
Ligand
Complex Ligand Complex Ligand
Complex
1610.0
1615.4
1613.9
1615.7
1615.9
1613.2
1613.9
1a
1b
1c
1d
1e
2c
2d
8.57
8.58
8.59
8.56
8.49
8.59
8.56
9.30
9.36
9.32
9.31
9.28
9.31
9.29
161.4
160.6
160.1
162.1
163.2
160.1
162.2
168.9
169.4
169.3
155.8
169.1
169.3
169.1
1624.5
1623.3
1622.7
1627.1
1625.6
1622.7
1627.1
The UV-vis spectra of the ligands and their complexes (Figure 1) were obtained in CH3CN. For
the ligands, absorption bands occur between 230 nm - 249 nm and 282 nm - 327 nm. These are
assigned to n-π* and π-π* transitions, respectively. A bathochromic shift of these bands to 255
nm - 273 nm and 311 nm - 316 nm, respectively, is seen on complexation. The complexes further
showed bands in the region 410 nm - 423 nm, due to metal to ligand (dπ-π*) charge transfer
transitions from the filled 5d orbitals to the empty π* orbitals [11,17]. The UV-vis spectra of the
ligands do not show these bands, which confirms also that the proposed complexes formed.
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1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
1a
1b
1c
1d
1e
2c
2d
210
260
310
360
410
460
510
560
Wavenumber (nm)
Figure 1: The UV-Vis spectra of complexes 1a - 1e and 2c-2d
3.2
Molecular structure of 2e
Compound 2e crystallizes in red block-like crystals with single molecules of the cation, [(η6-
benzene)Os(C5H4N-2-CH=N(4-Me-Ph))]+, acetone and the counter ion in the asymmetric unit.
The molecular structure is given in Figure 2, while Table 3 gives selected bond angles and
distances. In the structure, osmium is bonded to the bidentate ligand via the N atoms of the imine
and pyridine groups, which together with the chloride make up three legs of a “piano stool”,
while the disordered arene group forms the apex of the three-legged piano stool”. This leads to a
“pseudo-octahedral geometry around the Os(II) center.. The Os—Cl distance in the complex is
2.3920(8) Å and this is comparable to reported values for similar compounds [21-23]. The
lengths of the Os—N bonds are 2.057(3) Å and 2.091(3) Å, which are also similar to those of
comparable compounds with values of between 2.074(4) Å and 2.08(5) Å [23]. The N—Os—N
bond angle was found to be 75.70(10)° and the N—Os—Cl angles were found to be 84.07(7)° to
86.57(7)°, which again are comparable to those of similar compounds [21,23].
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Figure 2: A diagram showing the molecular structure of 2e, which also shows the atom
numbering and the displacement ellipsoids at a 50% probability level. Hydrogen atoms and the
disordered component of the complex have been omitted from the diagram for clarity purposes,
as have the counter anion and the acetone molecule.
Table 3: Selected bond distances (Å) and angles (deg)
2e
Bond Lengths
Os(1)—N(1)
Os (1) —N(2)
Os (1) —Cl(1)
(Å)
Bond angles (°)
2.057(3)
2.091(3)
2.3920(8)
N(1)—Os(1) —N(2)
N(1)—Os(1) —Cl(1)
N(2)—Os(1) —Cl(1)
75.70(10)
86.570(7)
84.070(7)
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3.3 The oxidation of styrene
3.3.1 Optimization
From a previous study on the oxidation of olefins using ruthenium arene complexes, the
following reaction conditions were chosen [17]: a 1:1 tert-butanol: water ratio, 60 °C reaction
temperature and a co-oxidant to oxidant ratio of 3:1. However, for the osmium compounds, the
optimum catalytic loading was found to be 0.5 mol %. At lower loadings, the reaction was very
slow and at higher loadings, there was no improvement in reaction time or yield. Blank
experiments, which were done under the same conditions, revealed that the osmium(II)
complexes gave only very low conversions without NaIO4 present. Furthermore, the oxidant,
NaIO4, without catalyst present, gave low conversion also.
3.3.2 Catalytic results discussion
Complexes 1a-e and 2a-e were investigated for catalytic activity in the oxidation of styrene
(Table 4). The main oxidation product obtained from this reaction was benzaldehyde, which is
either due to direct oxidative cleavage of the C=C bond of styrene or the rapid further reaction
of the intermediate styrene oxide to give benzaldehyde [24]. In this work, the second route seems
most likely because the reaction of styrene oxide under identical catalytic conditions gave
benzaldehyde quantitatively. The highest conversion was reached after one hour with all the
catalysts, with TOF values ranging from 175 to 195.
Once the optimum conditions for the reaction were established, to establish the scope and
limitations of the reaction, the oxidation of a number of other olefins was examined. For this
extended study, catalyst 1d was chosen, since it showed the highest TOF of the complexes
investigated. Complex 1d gave high conversions for all of the styrene derivatives studied (Table
5). These contained electron donating (entries 1 and 2) and withdrawing (entry 6) groups, and
gave the expected aldehydes in high yields. The catalyst further cleaved the double bond in trans-
stilbene to give benzaldehyde and reacted with a linear alkene, where 1-octene gave 1-heptanal
(Table 5).
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Table 4: Oxidation of styrene catalyzed by catalysts 1a-d and 2a–d.
Conversion (%)
Catalyst
1a
Yield (%)
TONa
181
183
183
195
186
183
191
190
195
181
TOF (h-1)b
181
98
91
91
92
97
93
91
95
95
97
90
100
100
100
100
100
99
183
1b
183
1c
195
1d
186
1e
183
2a
191
2b
100
100
99
190
2c
195
2d
181
2e
a : Turnover number = mol product/mol catalyst,
b: Turnover frequency = mol product/mol catalyst/hour (determined once the reaction was
complete)
When the osmium complex precursor [(η6-p-cymene)Os(µ-Cl)Cl]2 (0.5 mol%) was employed as
a catalyst for styrene oxidation, the reaction took three hours with a benzaldehyde yield of 86 %.
Thus, the precursor required a longer reaction time than the complex although it has double the
concentration of the osmium. Furthermore, when the osmium precursor was mixed with the
ligand d in a 1:2 ratio and used for the styrene oxidation reaction, the reaction took two hours
with an 82% yield to benzaldehyde, suggesting that the formation of the complex is important in
the oxidation reaction.
Investigations were made towards determining the mechanism of the osmium(II) arene catalysed
oxidation of olefins. The UV-Vis spectrum obtained for the yellow compound recovered at the
end of the reaction with catalyst 1d showed a peak at ca. 250 nm. A similar peak was observed
by Sugimoto and coworkers [25] who assigned the band at around 250-300 nm to Os=O species.
In addition, the solid-state IR spectrum obtained for the recovered catalyst, from complex 1d,
showed a peak at 881cm-1 which could be assigned to the symmetric stretches υ(Os=O) for an
osmium-dioxo species [25-27]. The proton NMR of the recovered catalyst shows that the arene
ring remains attached. This has been observed for a similar compound of ruthenium in oxidation
reactions [17]. Also, the ESI-MS spectrum of the catalyst residue after the oxidation reaction
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showed an Os fragment peak at 539, which was the maximum of the isotopic cluster. This peak
might be assigned to [(benzene)OsClO2]+.(MeCN)(H2O)9. Thus, this peak may suggest the
presence in solution of an osmium species which contains a coordinated benzene group, and it
implies also that the N, N-bidentate ligand cleaves.
Table 5: Data for the oxidative cleavage of selected olefins by 1d with the co-oxidant NaIO4
Conversion Yield Time
TOF
h-1
Substrate
Product
TON
181
%
%
h
1
2
4-methyl benzaldehyde
100
90
0.5
362
4-methoxy benzaldehyde 99
89
91
1
1
163
183
163
O
3
benzaldehyde
100
183
4
5
4-chloro benzaldehyde
1-heptanal
100
99
99
85
1
1
196
170
196
170
Cl
From the observations mentioned and literature reports, a mechanism involving an Os(VI) oxo
species is suggested. Thus, the complex reacts with the oxidant NaIO4 to give an Os(VI)-cis
dioxo intermediate. Electrophilic attack of this high-valent dioxo-intermediate on the C=C bond
of the olefin via a concerted [3+2] cycloaddition reaction gives an Os(IV) cyclo adduct.
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Scheme 2: The mechanistic scheme proposed for the oxidation of styrene using complexes 1a-
2e.
Conclusion
The mononuclear complexes 1a-1e and 2a-2e were synthesized, isolated and characterized. The
molecular structure of compound 2e reveals the expected pseudo octahedral three legged piano
stool geometry. These compounds oxidized styrene very effectively and showed very good
selectivity to benzaldehyde, with the yield to benzaldehyde being 91-97%. Furthermore, the
reactions of 1-octene, stilbene, 4-methoxy styrene, 4-methyl styrene and 4-chloro styrene over
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compound 1d gave the corresponding aldehydes via oxidative cleavage in equally good yield.
The mechanism is believed to involve an Os(VI) oxo species, which is indicated by the UV, IR,
1H NMR and ESI spectral data.
Acknowledgements
We thank the NRF and UKZN for financial support. JMG thanks Prof. E. N. Njoka for his
support.
Supplementary material
CCDC 1571550 contain the supplementary crystallographic data for this paper. These data can
be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html (or from the
Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44
1223 336033).
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Research Highlights
•
•
•
•
New half-sandwich Osmium(II) pyridine-imine complexes prepared and characterized
They show good catalytic activity in the oxidation of olefins to aldehydes
They work best in a biphasic medium (H2O/t-butanol)
The crystal structure of [(η6-benzene)Os(C5H4N-2-CH=N(4-Me-Ph))]+ is reported