Synthesis and molecular structures of monooxo aryl complexes of osmium(VI)

Article abstract of DOI:10.1139/v00-192

Reaction of [OsO₄] with C₇H₇MgBr (C₇H₇ = 2-methylphenyl) followed by column chromatography afforded the reported osmium tetraaryl [Os(C₇H₇)₄] along with the oxo-osmium(VI) ([OsO(C₇H₇)₄]) (1) (13%) and the dioxoosmium(VI) ([OsO₂(C₇H₇)₂]) (2) (25%) complexes. Treatment of [OsO₄] with C₈H₉MgBr (C₈H₉ = 2,5-dimethylphenyl) gave a mixture of [Os(C₈H₉)₄] (3) (34%) and [OsO(C₈H₉)₄] (4) (4%) while that with C₈H₉OMgBr (C₈H₉O = 4-methoxy-2-methylphenyl) afforded [OsO(C₈H₉O)₄] (5) in 20% yield. Oxidation of 3 with 3-chloroperoxybenzoic acid afforded 4 in good yield. The solid-state structures of 1 and 4 have been established by X-ray crystallography. Crystals of 1 are tetragonal with a = 13.080(1) and c = 6.6506(5) A, V = 1137.9(1) A³, Z = 2, and space group of P4/n; while those of 4 are tetragonal with a = 13.593(2) and c = 7.377(2) A, V = 1363.0(5) A³, Z = 4, and space group of P4/n. The geometry around osmium in both complexes is square pyramidal with the oxo ligand occupying apical position. The Os - O and Os - C distances in 1 are 1.652(2) and 2.084(1) A, respectively, while those in 4 are 1.688(7) and 2.088(4) A, respectively. The cyclic voltammograms of the monooxo aryl osmium(VI) compounds show reversible Os(VI/V) couple at around -1.4 V vs. ferrocene/ferrocenium couple.

Full text of DOI:10.1139/v00-192

607
Synthesis and molecular structures of monooxo
aryl complexes of osmium(VI)
Man-Kit Lau, Joyce L.C. Chim, Wing-Tak Wong, Ian D. Williams, and
Wa-Hung Leung
Abstract: Reaction of [OsO4] with C7H7MgBr (C7H7 = 2-methylphenyl) followed by column chromatography afforded
the reported osmium tetraaryl [Os(C7H7)4] along with the oxo-osmium(VI) ([OsO(C7H7)4]) (1) (13%) and the dioxo-
osmium(VI) ([OsO2(C7H7)2]) (2) (25%) complexes. Treatment of [OsO4] with C8H9MgBr (C8H9 = 2,5-dimethylphenyl)
gave a mixture of [Os(C8H9)4] (3) (34%) and [OsO(C8H9)4] (4) (4%) while that with C8H9OMgBr (C8H9O = 4-
methoxy-2-methylphenyl) afforded [OsO(C8H9O)4] (5) in 20% yield. Oxidation of 3 with 3-chloroperoxybenzoic acid
afforded 4 in good yield. The solid-state structures of 1 and 4 have been established by X-ray crystallography. Crystals
of 1 are tetragonal with a = 13.080(1) and c = 6.6506(5) Å, V = 1137.9(1) Å3, Z = 2, and space group of P4/n; while
those of 4 are tetragonal with a = 13.593(2) and c = 7.377(2) Å, V = 1363.0(5) Å3, Z = 4, and space group of P4/n.
The geometry around osmium in both complexes is square pyramidal with the oxo ligand occupying apical position.
The Os—O and Os—C distances in 1 are 1.652(2) and 2.084(1) Å, respectively, while those in 4 are 1.688(7) and
2.088(4) Å, respectively. The cyclic voltammograms of the monooxo aryl osmium(VI) compounds show reversible
Os(VI/V) couple at around –1.4 V vs. ferrocene/ferrocenium couple.
Key words: osmium(VI), oxo aryl complexes.
Résumé : La réaction du [OsO4] avec du C7H7MgBr (C7H7 = 2-méthylphényle), suivie d’une chromatographie sur co-
lonne permet d’isoler le tétraarylosmium attendu [Os(C7H7)4] aux côtés de complexes oxo-osmium(VI) [OsO(C7H7)4]
(1) (13%) et dioxo-osmium(VI) [OsO2(C7H7)2] (2) (25%). Le réaction du [OsO4] avec du C8H9MgBr (C8H9 = 2,5-
diméthylphényle) conduit à un mélange de [Os(C8H9)4] (3) (34%) et de [OsO(C8H9)4] (4) (4%) alors que celle avec
C8H9MgOBr (C8H9 = 4-méthoxy-2-méthylphényle) conduit au [OsO(C8H9O)4] (5) avec un rendement de 20%.
L’oxydation du produit 3 par de l’acide 3-chloroperbenzoïque conduit au produit 4 avec un bon rendement. Les structu-
res des composés 1 et 4 ont été déterminées à l’état solide par diffraction des rayons X. Les cristaux de 1 sont tétrago-
naux, groupe d’espace P4/n, avec a = 13,080(1) et c = 6,6506(5) Å, V = 1137,9(1) Å3 et Z = 2 alors que ceux du
composé 4 sont tétragonaux, groupe d’espace P4/n, avec a = 13,593(2) et c = 7,3776(2) Å et Z = 4 V = 1363,0(5) Å3.
Dans chacun des complexes, la géométrie autour de l’osmium est de type pyramide carrée de laquelle le ligand oxo se
trouve en position apicale. Les distances Os—O et Os—C dans le composé 1 sont respectivement de 1,652(2) et
2,084(1)Å alors que celles dans le composé 4 sont respectivement de 1,688(7) et 2,088(4) Å. Dans les voltampéro-
grammes cycliques des composés monooxo aryl osmium(VI), on observe le couple réversible Os(VI/V) aux environs de
–1,4 V par rapport au couple ferrocène/ferrocénium.
Mots clés : osmium(VI), complexes oxo aryles.
[Traduit par la Rédaction] Lau et al. 612
Introduction
tion of organic compounds (1, 2). Some transition metal oxo
alkyls, notably [Re(CH3)O3], are also used as stoichiometric
and catalytic reagents for organic transformations (2). While
oxo alkyls of oxophilic early transition metals are well-
documented, there are relatively few examples of the later
transition-metal congeners. Wilkinson and co-workers (3, 4)
first reported the synthesis of high-valent osmium aryls and
oxo aryls by arylation of [OsO4] with Grignard reagents
ArMgBr (Ar = aryl). It was found that the ortho-methyl
substituent in Ar has a significant effect on the nature of the
arylation products. Thus, reaction of [OsO4] with diortho-
substituted mesMgBr (mes = 2,4,6-trimethylphenyl) af-
forded [OsO2(mes)2] (3) whereas interactions of [OsO4] with
monoortho-substituted or unsubstituted ArMgBr (e.g., Ar =
2-methylphenyl or phenyl) gave homoleptic Os(IV) aryls
[Os(Ar)4] (4). However, to our knowledge, no
oxo(tetraaryl)osmium(VI) complexes have been isolated al-
Organometallic oxides are of interest due to their rele-
vance to the active intermediates in metal-catalyzed oxida-
Received August 30, 2000. Published on the NRC Research
Press Web site at http://canjchem.nrc.ca on July 2, 2001.
This paper is dedicated to Professor Brian R. James on the
occasion of his 65th birthday.
M.-K. Lau, J.L.C. Chim, I.D. Williams, and W.-H.
Leung.1 Department of Chemistry, The Hong Kong
University of Science and Technology, Clear Water Bay,
Kowloon, Hong Kong, China.
W.-T. Wong. Department of Chemistry, The University of
Hong Kong, Pokfulam Road, Hong Kong, China.
1Corresponding author (e-mail: chleung@ust.hk).
Can. J. Chem. 79: 607–612 (2001)
© 2001 NRC Canada
DOI: 10.1139/cjc-79-5/6-607

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Can. J. Chem. Vol. 79, 2001
1
though oxo(tetraalkyl)osmium(VI) complexes have been
prepared by alkylation of [OsO4] (5) or oxo-osmium
glycolates (6). In view of the low yield for the synthesis of
[Os(C7H7)4], we suspect that there may be other unidentified
product(s) for the arylation of [OsO4]. To this end, we set
out to reinvestigate the reaction between [OsO4] and
ArMgBr, and to separate the products by column chroma-
tography. We here describe the isolation, molecular struc-
tures, and electrochemistry of the first monooxo aryl
complexes of osmium(VI).
(23 000), 350 (8700). H NMR (300 MHz, CDCl3) δ: 2.57
(s, 6H, CH3), 7.04–7.13 (m, 4H, Hm and Hp), 7.19 (d, J = 7.8
Hz, 2H, Ho), 7.40 (d, J = 7.8 Hz, 2H, Hm′). E1/2 (V): –1.14
[Os(VI/V)]. Anal. calcd. for C14H14O2Os: C 41.6, H 3.5;
found: C 41.3, H 3.9.
[Os(C8H9)4] (3) and [OsO(C8H9)4] (4)
To a solution of [OsO4] (0.5 g, 2.0 mmol) in Et2O
(20 mL) at –78°C were added
7 equiv of 2,5-
dimethylphenylmagnesium bromide (11.5 mL of a 1.2 M so-
lution in Et2O, 14 mmol) dropwise under nitrogen. The re-
sulting reddish-brown mixture was warmed to room
temperature, stirred for 2 h, and evaporated to dryness. The
dark yellow sticky residue was extracted into CH2Cl2, fil-
tered through a Celite pad, and subjected to column chroma-
tography (silica gel). Elution with hexane and hexane–Et2O
(7:3) afforded purple 3 (34%) and orange 4 (2%), respec-
tively.
Experimental
All manipulations, unless otherwise stated, were carried
out in air. Solvents were purified, dried, and distilled prior to
use. [OsO4] was obtained from Strem Ltd. ArMgBr in Et2O
were prepared from commercially available aryl bromides
and Mg.
Spectrometers used were as follows: IR spectra (in KBr
discs) PerkinElmer 16 PC FT IR; mass spectra on a Finnigan
TSQ 7000; UV–vis spectra Milton-Roy Spectronic 3000 di-
ode array; 300 MHz 1H NMR spectra Bruker ALX 300
(chemical shifts in ppm relative to SiMe4). Cyclic
voltammetry was performed with a Princeton Applied Re-
search (PAR) Model 273A potentiostat. The working and
reference electrodes were glassy carbon and Ag/AgNO3
(0.1 M in acetonitrile), respectively, and the scan rate was
100 mV s–1. Formal potentials (E1/2) were measured in
CH2Cl2 solutions with 0.1 M [n-NBu4][PF6] as supporting
electrolyte and reported with reference to the
ferrocenium/ferrocene couple (Cp2Fe+/0). Elemental analyses
were performed by Medac Ltd., Surrey, U.K.
Characterization data for 3
1
FAB-MS m/z: 612 (M+). H NMR (300 MHz, CDCl3) δ:
2.22 (s, 12H, CH3), 2.27 (s, 12H, CH3), 6.57 (d, J = 8.5 Hz,
4H, Hp), 6.68 (d, J = 8.5 Hz, 4H, Hm), 6.70 (s, 4H, Ho). E1/2
(V): –1.48 [Os(IV/III)] and 0.24 [Os(V/IV)]. Anal. calcd. for
C32H36Os: C 62.9, H 5.9; found: C 63.0, H 6.0.
Characterization data for 4
FT IR (KBr, cm–1): 986 (Os=O). FAB-MS m/z: 572 (M+).
UV–vis (CH2Cl2) λmax (nm) (εmax (M–1 cm–1)): 248 (22 000),
1
350 (8250). H NMR (300 MHz, CDCl3) δ: 1.99 (s, 12H,
CH3), 2.25 (s, 12H, CH3), 5.78 (s, 4H, Ho), 6.75 (d, J = 7.5
Hz, 4H, Hp), 7.13 (d, J = 7.5 Hz, 4H, Hm). E1/2 (V): –1.35
[Os(VI/V)]. Anal. calcd. for C32H36OOs: C 59.0, H 4.9;
found: C 58.3, H 4.9.
[OsO(C7H7)4] (1) and [OsO2(C7H7)2] (2) (C7H7 = 2-
methylphenyl)
Preparation of 4 from 3
These complexes were synthesized using a modification
of the Wilkinson’s procedure. To a solution of [OsO4] (1 g,
3.94 mmol) in THF (30 mL) at –78°C were added 7 equiv of
2-methylphenylmagnesium bromide (17.3 mL of a 1.6 M so-
lution in Et2O), and the mixture was stirred for 45 min. The
resulting purple mixture was then warmed to room tempera-
ture, stirred for a further 2 h, and evaporated to dryness. The
residue was extracted into CH2Cl2 in air and subjected to
column chromatography (silica gel). Elution with hexane,
CH2Cl2–hexane (1:4), and acetone afforded purple
[Os(C7H7)4] (10%), orange 1 (13%), and green 2 (25%), re-
spectively. Recrystallization of 1 from CH2Cl2–hexane led to
isolation of orange blocks, which were suitable for X-ray
diffraction.
To a solution of 3 (0.5 g, 0.08 mmol) in CH2Cl2 (10 mL)
was added 1.5 equiv of 3-chloroperoxybenzoic acid or t-
BuOOH at room temperature. The reaction mixture was
stirred for 2 h, evaporated to dryness, and subjected to col-
umn chromatography (silica gel). The product was eluted
with hexane and recrystallized from CH2Cl2–hexane to give
orange crystals (80% yield), which was identified as 4 by 1H
NMR spectroscopy and mass spectrometry. Recrystallization
of 4 from CH2Cl2–hexane afforded orange blocks, which
were suitable for X-ray diffraction study.
[OsO(C8H9O)4] (5)
To a solution of [OsO4] (0.5 g, 2.0 mmol) in Et2O
(20 mL) at –40°C was added 7 equiv of 4-methoxy-2-
methylphenylmagnesium bromide (20.6 mL of a 0.67 M so-
lution in Et2O), and the mixture was slowly warmed to room
temperature and stirred for 2 h. The solvent was pumped off
and the residue was extracted into CH2Cl2 in air. Column
chromatography (silica gel, eluant: CH2Cl2–Et2O (1:5)) fol-
lowed by recrystallization from CH2Cl2–hexane afforded a
brown solid. Yield: 0.25 g, 20%. FAB-MS m/z: 693 [M + 1]+.
FT IR (KBr, cm–1): 982 (OsO). UV–vis (CH2Cl2) λmax (nm)
Characterization data for 1
FT IR (KBr, cm–1): 986 (Os=O). FAB-MS m/z: 572 (M+).
1H NMR (300 MHz, CDCl3) δ: 2.32 (s, 12H, CH3), 5.96 (d,
J = 7.6 Hz, 4H, Ho), 6.87 (t, J = 7.6 Hz, 4H, Hm), 6.96 (t,
J = 7.6 Hz, 4H, Hp), 7.27 (d, J = 7.6 Hz, 4H, Hm′). E1/2 (V):
–1.41 [Os(VI/V)]. Anal. calcd. for C28H28OOs: C 59.0, H
4.9; found: C 58.3, H 4.9.
1
(εmax (M–1 cm–1)): 254 (26 700), 350 (11 700). H NMR
Characterization data for 2
FT IR (KBr, cm–1): 912, 956 (OsO2). FAB-MS m/z: 407
[M + 1]+. UV–vis (CH2Cl2) λmax (nm) (εmax (M–1 cm–1)): 248
(300 MHz, CDCl3) δ: 2.28 (s, 12H, CH3), 3.76 (s, 12H,
OCH3), 5.77 (d, J = 8.8 Hz, 4H, Ho), 6.41 (d, J = 8.8 Hz,
© 2001 NRC Canada

Lau et al.
609
Table 1. Crystallographic data for [OsO(C7H7)4] (1) and [OsO(C8H9)4] (4).
Compound
1
4
Formula
C28H28OOs
C36H36OOs
Formula weight
Crystal system
Space group
a (Å)
570.73
Tetragonal
P4/n
13.080(1)
626.81
Tetragonal
P4/n
13.593(2)
c (Å)
V (Å3)
6.6505(5)
1137.9(1)
7.377(2)
1363.0(5)
Z
2
2
ρcalcd (g cm–3)
µ (Mo Kα) (cm–1)
F(000)
1.666
56.16
560
1.527
46.99
624
Crystal size (mm)
Transmission factor
Total reflections
Unique reflections
No. of observed reflections
No. of variables
R (F)
0.22 × 0.12 × 0.12
—
6851
—
6217 (I > 1.5σ (I))
70
0.022
0.037
—
—
1.38
0.04
–0.51–1.06
0.7 × 0.6 × 0.2
—
1636
1522 (Rint = 0.0221)
—
79
—
Rw (F)
—
R (F2) (all data)
wR2 (F2) (all data)
GoF
0.0304
0.0842
1.088
—
Max ∆ / σ (final cycle)
Residual density (e Å–3)
–0.549–1.153
4H, Hm), 6.88 (s, J = 8.8 Hz, 4H, Hm′). E1/2 (V): –1.37
[Os(VI/V)] and 0.93 irreversible [Os(VII/VI)]. Anal. calcd.
for C32H36O5Os: C 57.0, H 5.3; found: C 56.0, H 5.3.
reported the synthesis of [Os(C7H7)4] by the reaction of
[OsO4] with C7H7MgBr. The osmium tetraaryl was obtained
in 29% yield after extraction into light petroleum and subse-
quent recrystallization under nitrogen. However, we found
that when the reaction mixture was exposed to air and then
chromatographed (silica gel) in air, in addition to
[Os(C7H7)4], the oxo-osmium species [OsO(C7H7)4] (1) and
[OsO2(C7H7)2] (2) species were isolated in 13 and 25%
yield, respectively. The relative yields of [Os(C7H7)4], 1,
and 2 for the reaction were, however, found to be dependent
on reaction conditions, workup procedure, and some un-
known factors that have not been optimized yet. It appears
that the oxo complexes 1 and 2 were derived directly from
reaction of [OsO4] with C7H7MgBr, instead of via aerobic
oxidation of [Os(C7H7)4], because [Os(C7H7)4], 1, and 2 are
all air stable in solution and under chromatographic condi-
tions. Like [Os(C7H7)4] (3a), complex 1 exhibits a symmet-
ric 1H NMR spectrum featuring a single ortho-methyl
resonance at δ 2.32 at room temperature, indicating that the
2-methylphenyl ligands are free to rotate around the Os—C
bonds at the NMR timescale. The UV–vis spectrum of 1 in
CH2Cl2 displays absorption bands at ca. 350 (εmax = 8700
M–1 cm–1) and 248 nm (sh, εmax = 23 000 M–1 cm–1). The
former band is tentatively attributed to a charge transfer
transition due to the Os=O moiety while the latter probably
arises from an intraligand transition because a similar ab-
sorption was also found for [Os(C7H7)4]. The IR Os—O
X-ray crystallography
Pertinent crystallographic data and other experimental de-
tails for complexes 1 and 4 are summarized in Table 1. Data
for 1 and 4 were collected on a Bruker CCD diffractometer
and a Siemens P4 diffractometer, respectively, at room tem-
perature using graphite-monochromated Mo Kα radiation (λ
= 0.71073 Å). Data were corrected for Lorentz and polariza-
tion effects. Absorption corrections are semiempirical based
on ψ-scan data. Structures 1 and 4 were solved by direct
methods and refined by full-matrix least-squares on F and
F2, respectively. Hydrogen atoms were included and fixed in
their idealized positions (C-H 0.95 Å). Calculations were
carried out using the teXsan (7) (for 1) and SHELXL (8)
(for 4) crystallographic software packages. Complete listings
of bond lengths and angles, final atomic coordinates of non-
hydrogen atoms with thermal parameters, and anisotropic
thermal parameters are included as supplementary material.2
Results and Discussion
Synthesis
The preparations of osmium oxo aryl complexes are sum-
marized in Scheme 1. Wilkinson and co-workers (4a) first
2 Crystal data for 1 and 4 may be purchased from the Depository of Unpublished Data, Document Delivery, CISTI, National Research Coun-
cil Canada, Ottawa, ON K1A 0S2, Canada DUD No. 3276. For information on obtaining material electronically go to
http://www.nrc.ca/cisti/irm/unpub_e.shtml. This supplementary material has also been deposited with the Cambridge Crystallographic Cen-
tre (deposition nos. 148 810 and 148 809, respectively), and can be obtained on request from: The Director, Cambridge Crystallographic
Centre, University Chemical Laboratory, 12 Union Road, Cambridge, CB2 1EZ, U.K.
© 2001 NRC Canada

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Can. J. Chem. Vol. 79, 2001
Scheme 1.
Ar
Os
O
O
7 ArMgBr
Ar
Ar
Ar
Ar
[OsO ]
4
+
+
Os
Os
Ar
Ar
Ar
Ar
chromatography
O
Ar
CH
3
(10%)
1 (13%)
2 (25%)
Ar =
CH
3
3 (34%)
4 (4%)
H C
3
CH
3
5 (20%)
H CO
3
stretching frequency for 1 was observed at 990 cm–1, which
is comparable to that for [OsO(CH3)4] (994 cm–1) (6a) but is
lower than that for [OsO(CH2SiMe3)4] (1054 cm–1) (5b).
Complex 2, which presumably has a similar tetrahedral
geometry as [OsO2(mes)2] (4a), exhibits ν(OsO)as at 1000
and 980 cm–1. Neither 1 nor 2 reacted with oxygen abstrac-
tors such as tertiary phosphines and Si2Cl6. Attempts to re-
duce 2 by cobaltocene led to recovery of the starting material.
Fig. 1. Optical spectral trace for the reaction between
[Os(C8H9)4] (3) and t-BuOOH in CH2Cl2.
To further investigate the substituent effect of Ar on the
arylation of [OsO4], disubstituted phenyl Grignard were used
as the alkylation agents. Treatment of [OsO4] with 7 equiv
of 2,5-dimethylphenylmagnesium bromide (C8H9MgBr) in
Et2O afforded purple [Os(C8H9)4] (3) (33%) and orange
[OsO(C8H9)4] (4) (4%), which were separated by column
chromatography. On the other hand, reaction of [OsO4] with
4-methoxy-2-methylphenylmagnesium bromide (C8H9OMgBr)
in Et2O gave the oxo-Os(VI) compound [OsO(C8H9O)4] (5)
as the sole isolable product in 20% yield. Complexes 3–5 are
soluble in common organic solvents including hexanes,
ether, and methanol. They are air-stable in solution and
could be chromatographed in air without decomposition. It
appears that the presence of the 5-methyl substituent in the
aryl group enhances the production of Os(IV) tetraaryl while
the 4-methoxy substituent favors oxo-Os(VI) formation al-
though the origin of such a substituent effect is not well un-
from t-BuOOH to 3 is a clean, single-step process. The
charge transfer band for methoxy-substituted 5 (385 nm)
was found at a longer wavelength relative to those for 4
(352 nm) and 1 (350 nm). The 1H NMR spectrum of 4
shows two singlets at δ 1.99 and 2.25 due to the meta- and
ortho-methyl protons while the methyl and methoxy proton
signals for 5 were observed at δ 2.28 and 3.76, respectively.
The IR Os—O stretching frequencies for 4 and 5 (982 and
986 cm–1, respectively) are comparable to that for 1.
1
derstood. The H NMR spectrum of 3 shows two singlets at
δ 2.22 and 2.27 due to the ortho- and meta-methyl protons.
Oxidation of 3 with [Ag(CF3SO3)] afforded the osmium(V)
species [Os(C8H9)4]+, which exhibits an isotropic EPR
signal at g = 2.04 at –120°C. A similar g value was found
for the structurally characterized osmium(V) complex
[Os(C7H7)4][CF3SO3] (3b, 9). Treatment of 3 with single-
oxygen donors such as 3-chloroperoxybenzoic acid and t-
BuOOH gave 4 in good yield. Figure 1 shows a spectral
trace for the reaction of 3 with t-BuOOH in CH2Cl2. Upon
addition of ca. 1 equiv of t-BuOOH, the d-d bands for 3
centered at 447, 579, and 662 nm disappeared and concomi-
tantly an absorption at 352 nm attributable to the charge
transfer band for 4 appeared. The observation of isosbestic
points at ca. 325 and 420 nm suggests that the oxo transfer
Crystal structures
Complexes 1 and 4 have been unambiguously character-
ized by X-ray crystallography. The corresponding molecular
structures with selected bond lengths and angles are shown
in Figs. 2 and 3. Complex 1 is isostructural with the previ-
ously reported Re(VI) analogue [ReO(C7H7)4], which also
exhibits a fourfold molecular symmetry (10). The geometry
around osmium in 1 and 4 is pseudo-square pyramidal with
the oxo ligand occupying the apical position. The Os atoms
are situated above the C4 plane with the O-Os-C angles of
110.4(4) and 110.82(12)° for 1 and 4, respectively, (cf.
© 2001 NRC Canada

Lau et al.
611
Fig. 2. Molecular structure of [OsO(C7H7)4] (1). Selected bond
lengths and angles are: Os(1)—O(1) 1.652(2) and Os(1)—C(1)
2.084(1) Å; O(1)-Os(1)-C(1) 110.04(4), C(1)-Os(1)-C(1*)
82.59(3), and C(1)-Os(1)-C(1*) 137.91(8)°.
Fig. 4. Cyclic voltammogram of [OsO(C8H9)4] (4) in CH2Cl2 at
a glassy carbon electrode. Scan rate = 100 mV s–1.
Electrochemistry
The formal potentials for the osmium aryl complexes have
been determined by cyclic voltammetry in CH2Cl2. The
tetraaryl complex 3 exhibits reversible couples at 0.24 and
–1.48 V vs. Cp2Fe+/0, which are assigned to the metal-
centered Os(V/IV) and Os(IV/III) couples, respectively. The
E1/2 ([Os(V/IV)]) for 3 is less anodic than that for
[Os(C7H7)4] (0.33 V) due to inductive effect of meta-methyl
substitutents in the aryl groups. The cyclic voltammogram of
4 (Fig. 4) shows a reversible reduction couple at –1.35 V,
which is assigned to the Os(VI/V) couple. Similarly, the
Os(VI/V) couples for 1 and 5 were observed at –1.41 and
–1.37 V, respectively. An irreversible oxidation wave at 0.93
V, which is tentatively assigned to the Os(VII/VI) oxidation,
was also found for 5. For comparison, the Os(VII/VI) couple
for [Os(O)(CH2SiMe3)4] was observed at 1.19 V vs.
Ag/AgCl (5b). The dioxo-osmium(VI) complex 2 exhibits
the Os(VI/V) couple at –1.14 V, which is similar to that for
[OsO2(mes)2] (–1.28 V) (4b). In general, the E1/2
([Os(VI/V)]) for [OsO(aryl)4] are more negative than those
for [OsO2(aryl)2], indicating that the σ-aryls are very strong
donor ligands.
Fig. 3. Molecular structure of [OsO(C8H9)4] (4). Selected bond
lengths and angles are: Os(1)—O(1) 1.688(7) and Os(1)—C(1)
2.088(4) Å; O(1)-Os(1)-C(1) 110.82(12), C(1)-Os(1)-C(1B)
82.74(8), and C(1)-Os(1)-C(1A) 138.4(2)°.
Conclusions
In summary, we found that depending on the nature of
aryl ligand, reaction of [OsO4] with arylmagnesium bromide
produced a mixture of osmium(IV) tetraaryl, oxo(tetra-
aryl)osmium(VI), and dioxo(diaryl)osmium(VI) complexes,
which could be separated by column chromatography. It ap-
pears that the presence of the 5-methyl substituent in the
aryl ligand enhances the formation of Os(IV) tetraaryl while
the 4-methoxy substituent favors the formation of
[OsO(aryl)4]. Oxidation of [Os(C8H9)4] with 3-choroperoxy-
benzoic acid gave [OsO(C8H9)4] in good yield. The
oxo(tetraaryl)osmium(VI) complexes exhibit reversible
Os(V/IV) couples at ca. –1.4 V while the Os(VI/V) couples
106.0(2)° for [ReO(C7H7)4] (10)). Like [ReO(C7H7)4], the
ortho-methyl groups in 1 and 4 are found on the same side
as the oxo function possibly due to steric reasons. The
Os—O and average Os—C distance for 1 are 1.652(2) and
2.084(1) Å, respectively, while those for 4 are 1.688(7) and
2.088(4) Å, respectively. The Os—C distances are compara-
ble to those in [OsO(CH2SiMe3)4] (average 2.078 Å) (5b)
and [ReO(C7H7)4] (2.063 Å) (10) but is slightly shorter than
that in [Os(C7H7)4] (average 1.997 Å) (4a). The Os—O dis-
tances are short and similar to that in [OsO(CH2SiMe3)4]
(1.692(6) Å) (5b).
for dioxodiarylosmium(VI) occur at ca. –1.1 V vs. Cp2Fe+/0
.
Acknowledgement
The work described in this paper was supported by a grant
from the Research Grants Council of the Hong Kong SAR,
China (project no. HKUST 6189/00P).
© 2001 NRC Canada

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© 2001 NRC Canada