Synthesis, characterization, reactivity and molecular structure of arene-osmium complexes: A new synthetic entry into (η6-arene)osmium(II) chemistry

Article abstract of DOI:10.1016/j.poly.2009.03.001

A new convenient one-pot procedure for the preparation of the p-cymene-osmium dimer [(η⁶-p-MeC₆H₄Prⁱ)OsBr₂]₂ (1) from potassium osmate K₂[OsO₂(OH)₄] is desc

Full text of DOI:10.1016/j.poly.2009.03.001

Polyhedron 28 (2009) 1511–1517
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Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis, characterization, reactivity and molecular structure of arene-osmium
complexes: A new synthetic entry into (
g
6-arene)osmium(II) chemistry
1
*
Hadley S. Clayton , Banothile C.E. Makhubela, Hong Su, Gregory S. Smith, John R. Moss
Department of Chemistry, University of Cape Town, Rondebosch 7701, Cape Town, South Africa
a r t i c l e i n f o
a b s t r a c t
A new convenient one-pot procedure for the preparation of the p-cymene-osmium dimer [(g
6-p-MeC6H4-
Article history:
Pri)OsBr2]2 (1) from potassium osmate K2[OsO2(OH)4] is described. The analogous hexamethylbenzene
Received 18 December 2008
Accepted 2 March 2009
Available online 11 March 2009
complex [(
(3–9) (L = CO, PPh3, PEt3, PMePh2, P(OMe)3, P(OBun)3 and P(OPh)3) have been prepared from 1. The com-
plex [(
6-C6Me6)Os(CO)Br2] (10) was obtained from 2. All the complexes were characterized spectro-
scopically by NMR and IR and by elemental analysis. Single crystal X-ray structures of [(
6-p-
MeC6H4Pri)OsBr2]2 (1) and [( 6-p-MeC6H4Pri)Os(PPh3)Br2] (3) are reported. The reactions of some of
these complexes with BrMg(CH2)3CH@CH2 and MeLi are also described.
g g
6-C6Me6)OsBr2]2 (2) and a series of new arene-osmium complexes [( 6-p-MeC6H4Pri)OsLBr2]
g
Keywords:
Arene-osmium
p-Cymene
Potassium osmate
Tertiary phosphine
Phosphite
g
g
Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
of these complexes with lithium and Grignard alkylating reagents
is briefly discussed.
Osmium complexes bearing arene ligands have long been
known and have been well studied [1,2]. The usual method for syn-
thesizing this class of compound is by dehydrogenation of a 1,3- or
2. Experimental
1,4-cyclohexadiene such as
cyclohexadiene using OsCl3 ꢀ 3H2O, OsO4 or Na2[OsCl6] to give di-
meric
6-p-cymene or 6-mesitylene complexes (Scheme 1). The
a-phellandrene or 1,3,5-trimethyl-1,4-
2.1. General experimental procedures
g
g
All manipulations were carried out an under inert nitrogen
atmosphere using a dual vacuum/nitrogen line and standard
Schlenk line techniques unless otherwise stated. All solvents were
obtained commercially and freshly distilled under N2 prior to use.
Diethyl ether, toluene and tetrahydrofuran (THF) were dried over
sodium wire with benzophenone. Dichloromethane, chloroform,
EtOH and n-hexane were dried over calcium hydride.
complexes that have been studied have thus mainly been the
arene-osmium(II) chlorides and prior to this work surprisingly lit-
tle had been reported on the bromide analogues.
The
g
6-arene-osmium(II) dimers are a useful source of mono-
meric osmium(II) complexes as the bridging halide ligands are
readily cleaved by two-electron phosphorus, nitrogen and carbon
donor ligands. These complexes have been used in the preparation
of osmium catalyst precursors for ring-closing, ring-opening and
cross-metathesis [3] and have also been shown to exhibit C–H
bond activation behaviour [4] as well as being useful catalysts
for the cyclopropanation of olefins [5b].
The compounds PMePh2, PPh3, PEt3, P(OMe)3, P(OBun)3,
P(OPh)3, hexamethylbenzene and MeLi were obtained from Al-
drich; while
a-phellandrene (5-isopropyl-2-methylcyclohexa-1,3-
diene) was obtained from Fluka and were all used as received.
K2[OsO2(OH)4] was obtained from Anglo Platinum Corporation.
The Grignard reagent BrMg(CH2)3CH@CH2 was prepared according
to the literature procedures [7].
Bruce and co-workers have recently reported the preparation of
cyclopentadienyl–osmium complexes from potassium osmate [6].
We report here a new, one-pot procedure for the synthesis and
structure of the osmium dimeric synthon [(g
6-p-MeC6H4-
2.2. Instrumentation
Pri)OsBr2]2 from potassium osmate and the preparation and char-
acterization of a number of monomeric derivatives. The reactivity
1H and 31P NMR spectra were recorded on a Varian XR300 MHz
or XR400 MHz spectrometers using tetramethylsilane (TMS) as the
internal standard (for 1H) and H3PO4 as the external standard (for
31P). Mass spectra were recorded at the University of Witwaters-
rand using a VG70SE (Micromass) with 8 kV acceleration and a Ion-
tech Saddlefield FAB gun. The instrument was operated at 8 kV
* Corresponding author.
E-mail address: John.Moss@uct.ac.za (J.R. Moss).
Current address: Department of Chemistry, University of South Africa, Pretoria,
1
South Africa.
0277-5387/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2009.03.001

1512
H.S. Clayton et al. / Polyhedron 28 (2009) 1511–1517
C6H4, 5.47 (d, 2H J = 5.90 Hz) C6H4, 7.30–7.82 (m, 15H) PPh3; 31P
HCl
FeCl2
H2[OsCl6]
OsO4
NMR (121 MHz, CDCl3) d ppm: ꢂ17.57 (s), 1J(187Os–31P) = 283 Hz.
Complexes 4–8 were prepared using the same procedure as de-
scribed for the preparation of complex 3.
Cl
Os
Cl
Cl
Na2[OsCl6]
Os
Δ
/ EtOH
g
6-p-MeC6H4Pri)Os(PEt3)Br2] (4)
Cl
2.3.4. [(
Complex 4 was prepared using [(
g
6-p-MeC6H4Pri)OsBr2]2 (1)
OsCl3.3H2O
(200 mg, 0.206 mmol) and PEt3 (48 mg, 0.413 mmol). The product
was obtained as an orange solid and dried under vacuum for 2 h.
Yield (191 mg, 73%). Elemental Anal. Calc. for C16H29Br2OsP: C,
31.90; H, 4.85. Found: C, 31.02; H, 4.99%; m.p. 119–122 °C; 1H
NMR (300 MHz, CDCl3) d ppm: 1.04–1.22 (m, 9H) P–(CH2Me)3,
1.27 (d, 6H J = 6.86 Hz) CHMe2, 2.05–2.18 (m, 6H) P–(CH2Me)3,
2.28 (s, 3H) CMe, 2.88 (q, 1H J = 7.04, 7.23, 7.23 Hz) CHMe2, 5.55
(d, 2H J = 4.98 Hz) C6H4, 5.61 (d, 2H J = 5.71 Hz) C6H4; 31P NMR
(121 MHz, CDCl3) d ppm: ꢂ28.35 (s) 1J(187Os–31P) = 269 Hz.
Scheme 1. Reported synthetic methods for dimeric
complexes [1,2].
g
6-p-cymene osmium(II)
using Xenon gas and all matrices were made with 3-nba (3-nitro-
benzyl alchohol). Microanalyses were conducted with a Thermo
Flash 1112 Series CHNS-O Analyzer instrument. FT-IR spectra were
recorded on a Perkin–Elmer Spectrum One FT-IR Spectrometer,
using solution cells with NaCl windows.
Melting points were recorded with a Kofler hot stage micro-
scope (Riechert Thermovar). GC analyses were carried out using a
Varian 3900 gas chromatograph equipped with an FID and a
2.3.5. [(
g
6-p-MeC6H4Pri)Os(PMePh2)Br2] (5)
6-p-MeC6H4Pri)OsBr2]2 (1)
Complex 5 was prepared using [(
g
(460 mg, 0.475 mmol) and PMePh2 (190 mg, 0.950 mmol). The
product was isolated as an orange-yellow solid. Yield (566 mg,
87%). Elemental Anal. Calc. for C23H27Br2OsP: C, 40.36; H, 3.98.
Found: C, 40.11; H, 3.52%; m.p. 196–198 °C; 1H NMR (400 MHz,
CDCl3) d ppm: 1.22 (d, 6H J = 6.97, Hz) CHMe2, 2.02–2.15 (m, 3H)
P–MePh2, 2.18 (d, 3H J = 10.68 Hz) CMe, 2.89 (q, 1H J = 7.04, 7.23,
7.23 Hz) CHMe2, 5.40 (d, 2H J = 5.75 Hz) C6H4, 5.49 (d, 2H
J = 5.71 Hz) C6H4, 7.39–7.76 (m, 10H) P–MePh2; 31P NMR
(121 MHz, CDCl3) d ppm: ꢂ26.68 (s) 1J(187Os–31P) = 274 Hz.
30 m ꢁ 0.32 mm CP-Wax 52 CB column (0.25
lm film thickness).
The carrier gas was helium at 5.0 psi. The oven was programmed
to hold at 32 °C for 4 min and then to ramp to 200 °C at 10 °C/
min and hold 5 min.
2.3. Preparation of arene-osmium complexes
2.3.1. Synthesis of [(g
6-p-MeC6H4Pri)OsBr2]2 (1)
K2[OsO2(OH)4] (5.070 g, 13.760 mmol) was dissolved in HBr
(50 ml, 48% aq) and EtOH (100 ml 96%) and refluxed for 18 h. To
2.3.6. [(
g
6-p-MeC6H4Pri)Os{P(OMe)3}Br2] (6)
6-p-MeC6H4Pri)OsBr2]2 (1)
Complex 6 was prepared using [(
g
the solution was added a-phellandrene (10 ml) and refluxed for a
(200 mg, 0.206 mmol) and P(OMe)3 (51 mg, 0.413 mmol). The
product was obtained as a red-orange crystalline solid. Yield
(185 mg, 74%). Elemental Anal. Calc. for C13H23O3Br2OsP: C,
25.66; H, 3.78. Found: C, 25.29; H, 3.73%; m.p. 154–157 °C; 1H
NMR (400 MHz, CDCl3) d ppm: 1.25 (d, 6H J = 6.95 Hz) CHMe2,
2.33 (d, 3H J = 1.71 Hz) CMe, 2.99 (q, 1H J = 7.04, 7.23, 7.23 Hz)
CHMe2, 3.58 (d, 9H J = 10.90 Hz) P–(OMe)3, 5.49 (d, 2H
J = 5.88 Hz) C6H4, 5.64 (d, 2H J = 5.93 Hz) C6H4; 31P NMR
(121 MHz, CDCl3) d ppm: 68.06 (s) 1J(187Os–31P) = 455 Hz.
further 18 h. The solvent was removed under vacuum to give a
red oil. The product was extracted with CH2Cl2 (5 ꢁ 80 ml) and
the solution concentrated to approximately 50 ml and diethyl
ether added. The solution was cooled at ꢂ15°C overnight to give
orange-red crystals. Yield (4.39 g, 66%); m.p. 246–249 °C. Elemen-
tal Anal. Calc. for C20H28Br4Os2: C, 24.78; H, 2.89. Found: C, 24.93;
H, 2.87%. 1H NMR (300 MHz, CDCl3) d ppm: 1.12–1.30 (m 6H)
CHMe2, 2.17 (s, 3H) CMe, 2.45–2.76 (m, 1H) CHMe2, 6.02 (d, 2H
J = 5.65 Hz) C6H4, 6.15 (d, 2H J = 5.64 Hz) C6H4.
6-p-MeC6H4Pri)Os{P(OBun)3}Br2] (7)
6-p-MeC6H4Pri)OsBr2]2 (1)
2.3.2. Synthesis of [(
[(
6-p-MeC6H4Pri)OsBr2]2 (1) (500 mg, 0.516 mmol) and hexa-
g
6-C6Me6)OsBr2]2 (2)
2.3.7. [(
g
Complex 7 was prepared using [(
g
g
(1.000 g, 1.032 mmol) and P(OBun)3 (516 mg, 2.065 mmol). The
product was isolated as a bright orange solid. Yield (1.284 g,
85%). Elemental Anal. Calc. for C22H41O3Br2OsP: C, 35.97; H, 5.58.
Found: C, 35.95; H, 5.39%; m.p. 57–60 °C; 1H NMR (400 MHz,
CDCl3) d ppm: 0.83–0.95 (m, 9H) P–{O(CH2)3CH3}3, 1.21 (d, 6H
J = 6.95 Hz) CHMe2, 1.36 (qd, 6H J = 14.38, 7.24, 7.24, 7.15 Hz) P–
{O(CH2)2CH2CH3}3, 1.52–1.69 (m, 6H) P–{OCH2CH2CH2CH3}3, 2.28
(s, 3H) CMe, 2.87–2.92 (m, 1H) CHMe2, 3.76 (dd, 6H J = 6.90,
13.90 Hz) P–{OCH2(CH2)2CH3}3, 5.43 (d, 2H J = 5.76 Hz) C6H4, 5.57
(d, 2H J = 5.72 Hz) C6H4; 31P NMR (121 MHz, CDCl3) d ppm: 62.43
(s) 1J(187Os–31P) = 449 Hz.
methylbenzene (4.00 g 24.648 mmol) were transferred into a seal-
able tube. The tube was evacuated and heated at 175–180 °C. After
24 h the melt was allowed to reach room temperature and ex-
tracted with CH2Cl2 (25 ml). The solvent was removed using a ro-
tary evaporator and a brown-orange solid was obtained and
washed with (4 ꢁ 50 ml) of n-hexane to remove the excess hexa-
methylbenzene and dried under vacuum for 3 h. Yield (315 mg,
60%); decomposes without melting at 280 °C. Elemental Anal. Calc.
for C24H36Br4Os2: C, 28.13; H, 3.54. Found: C, 28.49; H, 3.57%. 1H
NMR (300 MHz, CDCl3) d ppm: 2.25 (s 36H) 2{C6Me6}.
2.3.3. Synthesis of [(
g
6-p-MeC6H4Pri)Os(PPh3)Br2] (3)
6-p-MeC6H4Pri)OsBr2]2 (1) (1.070 g,
2.3.8. [(
g
6-p-MeC6H4Pri)Os{P(OPh)3}Br2] (8)
6-p-MeC6H4Pri)OsBr2]2 (1)
To suspension of [(
a
g
Complex 8 was prepared using [(
g
1.032 mmol) in n-hexane (80 ml) was added PPh3 (560 mg,
2.064 mmol) and heated to reflux. After 4 h the solvent was re-
moved using a rotary evaporator to give an orange solid residue.
The solid was dissolved in a minimum of CH2Cl2 (15 ml), filtered
and diethyl ether added to give an orange solid. Yield (1.424 g,
92%). Elemental Anal. Calc. for C28H29Br2OsP: C, 45.03; H, 3.88.
Found: C, 45.07; H, 3.86%; m.p. 223–225 °C; 1H NMR (400 MHz,
CDCl3) d ppm: 1.19 (d, 6H J = 6.93 Hz) CHMe2, 1.97 (s, 3H) CMe,
2.99 (q, 1H J = 7.04, 7.23, 7.23 Hz) CHMe2, 5.13 (d, 2H J = 5.82 Hz)
(500 mg, 0.516 mmol) and P(OPh)3 (320 mg, 1.033 mmol). The
product was isolated as an orange solid. Yield (662 mg, 81%). Ele-
mental Anal. Calc. for C28H29Br2O3OsP: C, 42.33; H, 3.68. Found:
C, 42.45; H, 3.18%; m.p. 175–177 °C; 1H NMR (300 MHz, CDCl3) d
ppm: 1.20 (d, 6H J = 6.99 Hz) CHMe2, 2.23 (s, 3H) CMe, 2.55–2.78
(m, 1H) CHMe2, 5.16 (d, 2H J = 5.92 Hz) C6H4, 5.48 (d, 2H
J = 5.92 Hz) C6H4, 7.09–7.36 (m, 15H) P(OPh)3; 31P NMR
(121 MHz, CDCl3) d ppm: 52.28 (s), 1J(187Os–31P) = 476 Hz.

H.S. Clayton et al. / Polyhedron 28 (2009) 1511–1517
1513
2.3.9. Synthesis of [(
g
6-p-MeC6H4Pri)Os(CO)Br2] (9)
J = 6.99 Hz) CHMe2, 1.20–1.88 (m, 8H) CH2, 1.96 (s, 3H) CMe, 2.68
(q, 1H J = 7.04, 7.23, 7.23 Hz) CHMe2, 4.83–5.08 (m, 4H) @CH2,
5.10 (d, 2H J = 5.67 Hz) C6H4, 5.35 (d, 2H J = 5.77 Hz) C6H4, 5.45–
5.90 (m, 2H) CH@.
CO gas was bubbled through a suspension of [(
g
6-p-MeC6H4-
Pri)OsBr2]2 (1) (500 mg, 0.195 mmol) in n-hexane (25 ml). After
45 min the solvent was removed using a rotary evaporator to give
a red-orange solid which was dried under vacuum for 1 h. Yield
(482 mg, 91%). Elemental Anal. Calc. for C11H14Br2OOs: C, 25.79;
H, 2.75. Found: C, 25.55; H, 2.66%; decomposes without melting
2.3.14. [(
Complex 14 was prepared using [Os[(
g
6-C6Me6)(CO)Os{(CH2)3CH@CH2}2] (14)
g
6-C6Me6)(CO)Br2]
above 200 °C; IR (CH2Cl2, cmꢂ1):
m
(C„O) 2022; 1H NMR
(60 mg, 0.111 mmol) in dry benzene and BrMg(CH2)3CH@CH2
(300 MHz, CDCl3) d ppm: 1.20 (d, 6H J = 6.99 Hz) CHMe2, 2.13 (s,
3H) CMe, 2.77 (q, 1H J = 7.04, 7.23, 7.23 Hz) CHMe2, 5.14 (d, 2H
J = 5.82 Hz) C6H4, 5.47 (d, 2H J = 5.90 Hz) C6H4.
(1.66 M, 0.26 ml, 0.444 mmol). The product was obtained as a
brown oil. Yield (28 mg, 49%); IR (CHCl3, cmꢂ1):
m(C„O) 1897;
LRMS (FAB) C23H26OOs: m/z = 519.3 [M]+, 451.2 [Mꢂ(CH2)3
CH@CH2]+, 383.1 [Mꢂ2((CH2)3CH@CH2)]+; 1H NMR (300 MHz,
C6D6) d ppm: 0.82–0.89 (m, 4H) Os–CH2, 1.02–1.80 (m, 8H) CH2,
2.03 (s, 18H) {C6Me6}, 4.93–5.20 (m, 4H) @CH2, 5.55 (d, 2H
J = 5.18 Hz) CH@.
2.3.10. Synthesis of [(
CO gas was bubbled through a solution of [Os(
g
6-C6Me6)Os(CO)Br2] (10)
g
6-C6Me6)Br2]2
(2) (200 mg, 0.195 mmol) dissolved in dry chloroform (40 ml).
After 45 min the solvent was removed using a rotary evaporator
to give a dark orange solid which was dried under vacuum for
1 h. Yield (89 mg, 42%). Elemental Anal. Calc. for C13H18Br2OOs: C,
28.90; H, 3.33. Found: C, 28.52; H, 3.85%; decomposes without
2.3.15. [(
g
6-p-MeC6H4Pri)(CO)Os(CH3)2] (15)
6-p-MeC6H4Pri)Os(CO)Br2] (9) (200 mg,
A
solution of [(
g
0.390 mmol) in dry THF (30 ml) was cooled to ꢂ78 °C. MeLi
(0.66 M, 2.5 ml, 1.560 mmol) was then added drop wise to this
solution. The solution was allowed to reach room temperature
and stirred for 10 h, during which time it turned from orange to
yellow. After 10 h the solvent was removed using a rotary evapora-
tor to give a brown oil residue. The product was extracted with n-
hexane (3 ꢁ 15 ml) and the solvent was removed to give a brown
melting above 280 °C; IR (CHCl3, cmꢂ1):
m
(C„O) 2005; 1H NMR
(300 MHz, CDCl3) d ppm: 1.98 (s, 18H) {C6Me6}.
2.3.11. Synthesis of [(g
6-p-MeC6H4Pri)(PPh3)Os{(CH2)3CH@CH2}2]
(11)
[(g
6-p-MeC6H4Pri)Os(PPh3)Br2] (3) (1.000 g, 1.420 mmol) was
transferred into a Schlenk tube and dissolved in freshly dried
THF (30 ml). To this was added BrMg(CH2)3CH@CH2 (0.54 M,
10.56 ml, 5.690 mmol) under N2 at ꢂ78 °C. The solution was then
allowed to reach room temperature and stirred for 18 h, during
which time it had turned from orange to yellow. A saturated NH4Cl
solution (15 ml) was added drop wise to the solution at ꢂ78 °C in
order to hydrolyze the excess Grignard reagent. After reaching
room temperature dichloromethane (20 ml) was added and the or-
ganic layer was separated, dried over anhydrous MgSO4 and fil-
tered. The volatiles were removed using a rotary evaporator to
give a yellow-brown oil which was dried under vacuum for
30 min. Yield (745 mg, 72%); 1H NMR (300 MHz, C6D6) d ppm:
0.82–0.91 (m, 4H) Os–CH2, 1.21 (d, 6H J = 6.99 Hz) CHMe2, 1.40–
1.88 (m, 8H) CH2, 1.99 (s, 3H) CMe, 2.98 (q, 1H J = 7.04, 7.23,
7.23 Hz) CHMe2, 4.52–5.04 (m, 4H) @CH2, 5.32 (d, 2H J = 5.90 Hz)
C6H4, 5.53 (d, 2H J = 5.88 Hz) C6H4, 5.61–5.79 (m, 2H) CH@, 7.28–
7.82 (m, 15H) PPh3; 31P NMR (121 MHz, CDCl3) d ppm: 12.61 (s).
Complexes 12–14 were prepared using the same procedure as
described for the preparation of complex 11.
oil of [(
IR (CH2Cl2, cmꢂ1):
g
6-p-MeC6H4Pri)(CO)Os(CH3)2] (15). Yield (106 mg, 71%);
m
(C„O) 1923; 1H NMR (400 MHz, C6D6) d
ppm: 0.95 (s, 6H) Os–CH3, 0.99 (d, 6H J = 6.90 Hz) CHMe2, 1.71 (s,
3H) CMe, 2.88 (q, 1H J = 7.04, 7.23, 7.23 Hz) CHMe2, 5.47(d, 2H
J = 5.80 Hz) C6H4, 5.81 (d, 2H J = 5.80 Hz) C6H4.
2.4. X-ray crystal structures
The data for 1 were collected at 293 K on a Siemens P4 diffrac-
tometer fitted with a Bruker 1 K CCD detector and SMART control
software using graphite-monochromated, Mo K
a radiation by
means of a combination of phi and omega scans. Data reduction
was performed using SAINT+ and the intensities were corrected for
absorption using SADABS [8]. The structure was solved by direct
methods using SHELXTS [8] and refined by full-matrix least-squares
using SHELXTL [8] and SHELXL-97 [9].
The data for 3 were collected at 293 K on a Nonius Kappa-CCD
diffractometer using graphite-monochromated Mo K
a radiation.
The strategy for the data collection was evaluated using the Bruker
Nonius ‘‘COLLECT” program. Data were reduced and scaled using DEN-
ZO-SMN software [10]. Face indexed absorption corrections were
made using the program XPREP. The structure was solved by direct
methods and refined employing full-matrix least-squares with
the program SHELXL-97 [9] refining on F2, using the graphic interface
X-seed [11]. All non-H atoms were first refined isotropically fol-
lowed by anisotropic refinements. Hydrogen atoms were first lo-
cated in the difference electron density maps then positioned
geometrically and allowed to ride on their respective parent atoms.
2.3.12. [(
g
6-p-MeC6H4Pri){P(OBun)3}Os{(CH2)3CH@CH2}2] (12)
6-p-MeC6H4Pri)Os{P(O-
Complex 12 was prepared using [(
g
Bun)3}Br2] (7) (190 mg, 0.259 mmol) in freshly dried THF (35 ml)
and BrMg(CH2)3CH@CH2 (2.0 M, 0.5 ml, 1.035 mmol). The product
was obtained as a yellow oil. Yield (178 mg, 97%); 1H NMR
(300 MHz, C6D6) d ppm: 0.78–0.89 (m, 13H) Os–CH2 and P–
{O(CH2)3CH3}3, 1.24 (d, 6H J = 6.93 Hz) CHMe2, 1.40–1.70 (m,
16H) CH2 and P–{OCH2(CH2)2CH3}3, 1.87–2.03 (m, 4H) CH2, 2.28
(s 3H) CMe, 2.92 (m, 1H) CHMe2, 3.77 (dd, 6H J = 6.90, 13.90 Hz)
P–{OCH2(CH2)2CH3}3, 4.72–5.05 (m, 4H) @CH2, 5.56 (d, 2H
J = 5.76 Hz) C6H4, 5.62 (d, 2H J = 5.72 Hz) C6H4, 5.68–5.84 (m, 2H)
3. Results and discussion
CH@; 31P NMR (121 MHz, CDCl3)
1J(187Os–31P) = 476 Hz.
d
ppm: 79.41 (s),
3.1. Syntheses and characterization of arene-osmium dimeric
complexes
2.3.13. [(
g
6-p-MeC6H4Pri)(CO)Os{(CH2)3CH@CH2}2] (13)
6-p-MeC6H4Pri)Os(CO)Br2]
The dimer [(g
6-p-MeC6H4Pri)OsBr2]2 (1) was prepared by
heating a solution of potassium osmate, K2[OsO2(OH)4], in concen-
trated HBr and aqueous ethanol at reflux for 18 h followed by addi-
Complex 13 was prepared using [(
g
(9) (150 mg, 0.291 mmol) in freshly dried THF (25 ml) and
BrMg(CH2)3CH@CH2 (0.32 M, 3.64 ml, 1.167 mmol). This com-
pound was obtained as a dark brown oil after an aqueous work-
tion of
a-phellandrene and further heating for an additional 18 h
up. Yield (96 mg, 68%); IR (CH2Cl2, cmꢂ1): (C„O) 1965; 1H NMR
m
(Scheme 2). The complex was isolated in a moderate yield of 66%
as a red-brown stable solid which was also found to be stable in
(300 MHz, C6D6) d ppm: 0.77–0.91 (m, 4H) Os–CH2, 1.18 (d, 6H

1514
H.S. Clayton et al. / Polyhedron 28 (2009) 1511–1517
K2[OsO2(OH)4]
HBr
Br
Br
Br
excess
C6Me6
Br
α-phellandrene
Os
Os
Os
Os
H2OsBr6
reflux
Br
170-180oC
EtOH
reflux
Br
Br
Br
1
2
CHCl3
room temp.
CO
hexane
reflux
2L
Br
Br
Br
Br
2
2
OC
Os
Os
L
3 L = PPh3
10
4 L = PEt3
5 L = PMePh2
6 L = P(OMe)3
7 L = P(OBun)3
8 L = P(OPh)3
9 L = CO
Scheme 2. The syntheses of arene-osmium dimers 1 and 2 and osmium dibromide complexes 3–10.
[12]. The iodo complex [(
halide metathesis of the analogous chloro complex with NaI
[12d]. Synthesis of [(
6-C6Me6)OsBr2]2 (2) was achieved by follow-
g
6-p-MeC6H4Pri)OsI2]2 was prepared by
Table 1
Crystal data and structure refinement details.
g
1
3
ing a reported protocol for the chloride analogue [13]. Using this
method, complex 2 and excess hexamethylbenzene were heated
at 170–180 °C in a sealed and evacuated tube for 24 h (Scheme
2). After extraction with dichloromethane, the complex 2 was ob-
tained as a stable brown solid which decomposes without melting
at 280 °C.
1
Empirical formula
Formula weight
Crystal system
Space group
Crystal dimensions (mm)
a (Å)
C20H28Br4Os2
ꢀ
3H2O C28H29Br2OsP
746.50
974.52
monoclinic
C2/c
monoclinic
P21/n
0.46 ꢁ 0.40 ꢁ 0.36
42.739(5)
8.0787(9)
24.322(3)
90
0.09 ꢁ 0.07 ꢁ 0.06
9.20140(10)
21.59420(10)
13.54940(10)
90
106.9670(10)
90
2575.04(4)
1.926
b (Å)
c (Å)
The 1H NMR spectrum of complex 1 displayed characteristic p-
cymene ligand chemical shifts, while a singlet at 2.25 ppm corre-
sponding to the methyl groups of the hexamethylbenzene ligands
in complex 2 was observed in its 1H NMR spectrum (Table 2). The
signal for these methyl groups appeared at a slightly higher fre-
quency when compared to the chloride analogue (1.92 ppm)
[13]. This is evidence of less electronic interaction between the
bromide ion and the metal centre as opposed to when the more
electronegative chloride ion is present. Satisfactory elemental anal-
yses were obtained for both complexes 1 and 2 (Table 2).
a
(°)
b (°)
(°)
V (Å3)
119.162(2)
90
7333.5(14)
2.648
12
5320
18297
6894 [0.0555]
1.024
R1 = 0.0629,
wR2 = 0.1706
R1 = 0.0711,
wR2 = 0.1795
0.00041(3)
c
q
(Mg/m3)
Z
4
F(000)
1432
90817
4872 [0.0465]
1.076
R1 = 0.0199,
wR2 = 0.0394
R1 = 0.0261,
wR2 = 0.0411
0.00089(6)
0.544 and ꢂ0.624
Total reflections measured
Independent reflections [Rint
Goodness-of-fit on F2
]
Final R indices [I > 2
r(I)]
R indices (all data)
3.2. Syntheses and characterization of arene-osmium dibromide
complexes
Extinction coefficient
Largest difference in peak and hole 4.715 and ꢂ2.911
(e Åꢂ3
)
A series of arene-osmium dibromide complexes 3–8 of the type
[(g
6-p-MeC6H4Pri)OsLBr2] (L = tertiary phosphine and phosphite li-
gands) were prepared by heating suspensions of the dimeric com-
pound 1 in refluxing n-hexane with two equivalents of the
appropriate ligand over 4 h (Scheme 2). These complexes were ob-
tained as stable red-orange to orange solids in yields ranging from
solution, including in chlorinated solvents. Syntheses of the chlo-
ride and iodide analogues of this complex have been reported be-
fore using OsO4 and OsCl3 ꢀ 3H2O as starting materials, respectively
Table 2
1H NMR data, elemental analyses and melting points of complexes 1 and 2.
Complex
1
d (ppm)a
J (Hz) assignment
Cb (%)
Hb (%)
m.p. (°C)
1.12–1.30 (m)
2.17 (s)
2.78 (m)
6.02 (d) J = 5.65
6.15 (d) J = 5.64
CH(CH3)2
C(CH3)
CH(CH3)2
C6H4
24.93(24.78)
2.87(2.89)
246–249
C6H4
2
2.25 (s)
C6(CH3)6
28.49(28.13)
3.57(3.54)
Decomposes without melting at 280
a
Spectra recorded in CDCl3, internal standard was tetramethylsilane (TMS).
Calculated values in parentheses.
b

H.S. Clayton et al. / Polyhedron 28 (2009) 1511–1517
1515
73% to 92%. Furthermore, complexes 3–8 were found to be stable in
solution including in chlorinated solvents, with compound 3 show-
ing remarkable stability in chloroform on the bench top for
2 weeks and thereafter decomposing to a brown unidentified
species.
logues reveals that the complexes exhibit halide dependence, with
deshielding of the 31P NMR signal in the order of Cl > Br > I, thus
replacement of the bromide ligands of complexes 3–8 with chlo-
rides results in an upfield shift of the 31P NMR signal. The opposite
effect can be seen when the bromide ligands are replaced by iodide
ligands (Table 3) [14]. A slight increase in 1J(187Os–31P) coupling
constants can also be seen from Iꢂ to Brꢂ and Clꢂ containing
complexes.
Only one example was found in the literature of a similar dibro-
mide complex, i.e. [(
was prepared by a method involving metathesis of the chlorides
in [(
6-p-MeC6H4Pri)Os(PMe3)Cl2] with NaBr [14]. All other reports
g
6-p-MeC6H4Pri)Os(PMe3)Br2]. This complex
g
Complexes 9 and 10 showed expected terminal m(CO) bands at
found described dichloride and di-iodide complexes [5,12,14].
Thus, to the best of our knowledge complexes 3–8 and its precur-
sor 1 are all new. Complexes 9 and 10 were formed by bubbling CO
gas through a suspension of compound 1 in n-hexane or a solution
of compound 2 in chloroform at room temperature over 45 min
(Scheme 2). Red-orange, solids of complexes 9 and 10 were affor-
ded in good yields of 91% and 62%, respectively.
2022 and 2005 cmꢂ1, respectively.
3.3. Crystal structures of [(
MeC6H4Pri)Os(PPh3)Br2
g g
6-p-MeC6H4Pri)OsBr2]2 and ( 6-p-
Red crystals of 1 and 3 were obtained by slow evaporation of
dichloromethane solutions at room temperature. The crystallo-
graphic details and selected bond distances and angles for both
structures are given in Tables 1 and 4, respectively. The molecular
structures and atomic numbering schemes for 1 and 3 are pre-
sented in Figs. 1 and 2, respectively.
The carbonyl complexes [(
g
g
6-p-MeC6H4Pri)Os(CO)Br2] (9) and
[(
6-C6Me6)Os(CO)Br2] (10) were remarkably thermally stable,
only decomposing to black unidentified species at and above
200 °C, respectively. The complexes with tertiary phosphine li-
gands 3–5 all had high melting points of between 196 and 225 °C
while, phosphite containing complexes 6–8 showed melting points
of between 85 and 177 °C.
The molecular structures are similar to those reported for the
analogous chloride complexes [(
6-p-MeC6H4Pri)Os(PPh3)Cl2 [14]. The C6 ring in both 1 and 3 is
planar in contrast to the puckering of the aromatic ring observed
in (
6-p-MeC6H4Pri)Os(PPh3)Cl2. For 1 the asymmetric unit con-
g
6-p-MeC6H4Pri)OsCl2]2 [15] and
(g
The 1H NMR spectra of complexes 3–9 all exhibited signals
characteristic of p-cymene ligands. These signals are similar to
those reported for the analogous dichloride and di-iodide com-
pounds [14], with slight shifts as a result of the different halides
present. In general, the aromatic proton signals for the bromide
complexes appeared downfield relative to the chloride analogues
g
sists of one complete dimeric complex plus half of a second one
as this molecule lies across a centre of inversion. Each asymmetric
unit was found to contain half a water molecule and hence there is
one water molecule per three molecules of the complex. As in the
chloride complex, each Os of the dimer is bonded to two bridging
Br ligands and one terminal Br ligand, with a slightly long bond
measured for the bridging Br ligand. No direct interaction was ob-
served between the osmium centres.
and upfield relative to the iodo complexes. [(g
6-C6Me6)Os(CO)Br2]
(10) displayed a singlet corresponding to the methyl groups of
hexamethylbenzene at 1.98 ppm, slightly more downfield than
that reported for the chloride derivative [(
(1.96 ppm) [13].
g
6-C6Me6)Os(CO)Cl2]
Complex 3 exhibits a three-legged piano-stool geometry at os-
mium and many of the corresponding distances and angles are
The phosphine containing complexes 3–5 showed low field 31P
NMR peaks together with satellites due to coupling with 187Os (Ta-
ble 3). Larger coupling constants were seen in the phosphite bear-
ing complexes 6–8, whose 31P NMR signals appeared as singlets in
the more deshielded region, when compared to their phosphine
counterparts (Table 3). This clearly shows that the type of phos-
Table 4
Selected bond lengths (Å), bond angles (°) and torsion angles (°) for 1 and 3.
[(g
6-p-
(g
6-p-
phorus donor ligand, affects the 31P NMR chemical shifts in (g6
-
MeC6H4Pri)OsBr2]2 (1)
MeC6H4Pri)Os(PPh3)Br2 (3)
p-MeC6H4Pri)OsLBr2] type complexes. Similar coupling of 187Os
Bond lengths (Å)
Os(1)–C(1)
Os(1)–C(2)
Os(1)–C(3)
Os(1)–C(4)
Os(1)–C(5)
Os(1)–C(6)
Os(1)–Br(1)
Os(1)–Br(2)
Os(2)–C(11)
Os(2)–C(12)
Os(2)–C(13)
Os(2)–C(14)
Os(2)–C(15)
Os(2)–C(16)
Os(3)–C(21)
Os(3)–C(22)
Os(3)–C(23)
Os(3)–C(24)
Os(3)–C(25)
Os(3)–C(26)
and 31P has been reported for related (
plexes [14].
g
6-arene)osmium(II) com-
2.220(15)
2.132(14)
2.169(14)
2.175(14)
2.167(13)
2.164(14)
Os(1)–C(2)
Os(1)–C(3)
Os(1)–C(4)
Os(1)–C(5)
Os(1)–C(6)
Os(1)–C(7)
2.248(3)
2.199(3)
2.173(3)
2.263(3)
2.235(3)
2.185(3)
2.5548(3)
2.5505(4)
2.3546(8)
Comparison of the 31P NMR signals and 1J(187Os–31P) coupling
constants of complexes 3–8 with those of chloride and iodide ana-
2.5353(14) Os(1)–Br(1)
2.5811(13) Os(1)–Br(2)
Table 3
31P NMR data for complexes 3–8 and previously reported related complexes.
2.205(15)
2.168(14)
2.126(16)
2.162(18)
2.141(17)
2.186(15)
2.198(13)
2.192(13)
2.168(17)
2.138(18)
2.109(17)
2.181(16)
Os(1)–P(1)
Complexa
db
(
31P)
1Jc
(
187Os,31P)
Reference
[(g
[(g
[(g
[(g
[(g
[(g
[(g
[(g
[(g
[(g
[(g
[(g
[(g
6-p-MeC6H4Pri)Os(PPh3)Br2] (3)
6-p-MeC6H4Pri)Os(PEt3)Br2] (4)
6-p-MeC6H4Pri)Os(PMePh2)Br2] (5)
6-p-MeC6H4Pri)Os{P(OMe)3}Br2] (6)
6-p-MeC6H4Pri)Os{P(OBun)3}Br2] (7)
6-p-MeC6H4Pri)Os{P(OPh)3}Br2] (8)
6-p-MeC6H4Pri)Os(PPh3)Cl2]
ꢂ17.5
ꢂ28.3
ꢂ26.6
68.0
62.4
52.2
ꢂ13.1
ꢂ20.3
73.6
68.0
58.4
283
269
274
455
449
476
282
275
453
447
477
275
285
this work
this work
this work
this work
this work
this work
[14]
[14]
[14]
[14]
[14]
6-p-MeC6H4Pri)Os(PMePh2)Cl2]
6-p-MeC6H4Pri)Os{P(OMe)3}Cl2]
6-p-MeC6H4Pri)Os{P(OBun)3}Cl2]
6-p-MeC6H4Pri)Os{P(OPh)3}Cl2]
6-p-MeC6H4Pri)Os(PMePh2)I2]
6-p-MeC6H4Pri)Os(PPh3)I2]
Bond angles (°)
Br(1)–Os(1)–Br(2)
Br(4)–Os(2)–Br(5)
87.16(5)
81.34(4)
Br(1)–Os(1)–Br(2)
P(1)–Os(1)–Br(1)
P(1)–Os(1)–Br(2)
86.672(13)
86.46(2)
88.99(2)
ꢂ37.6
[14]
[14]
ꢂ23.7
a
Spectra recorded in CDCl3 at room temperature with chemical shifts relative to
Torsion angles (°)
H3PO4.
C(3)–C(4)–C(8)–C(9)
C(3)–C(4)–C(8)–C(10)
27(2)
ꢂ96.5(18)
C(6)–C(5)–C(8)–C(9)
C(6)–C(5)–C(8)–C(10)
26.9(4)
ꢂ97.9(4)
b
d in ppm.
J in Hz.
c

1516
H.S. Clayton et al. / Polyhedron 28 (2009) 1511–1517
similar to those found for the analogous chloride complex. As ex-
pected, slightly longer Os–X bond lengths were measured in the
(g
6-p-MeC6H4Pri)Os(PPh3)Br2 complex (2.55 Å) when compared
to the analogous Cl complex (2.42 Å) since the difference in cova-
lent radii of Cl and Br is 0.15 Å. However, exchange of the chloride
ligands for bromide ligands left the Os–P bond length unchanged at
2.35 Å.
3.4. Reactivity of arene-osmium dibromide complexes
In an attempt to prepare osmium bis(alkenyl) complexes, the
dibromide complexes 3, 7, 9 and 10 were reacted with excess 5-
pentenyl Grignard reagents in THF and yellow to brown oily prod-
ucts were isolated from these reactions. These oils proved to be very
unstable, therefore difficult to characterize completely. However,
1H and 31P NMR data as well as IR data obtained for the products,
were consistent with the formation of bis(1-pentenyl)osmium(II)
complexes 11–14 (Scheme 3). In addition, the bis(1-pentenyl) com-
plexes 11–14 all decomposed at room temperature to give black
species and when the residues were extracted and analysed by
GC, the results showed that the complexes decompose to yield
1,4-pentadiene and 1-pentene exclusively as the organic products.
These are similar decomposition products to those reported in the
thermal decomposition studies of platinum bis(1-alkenyl) com-
plexes containing mono- and diphosphine ligands [16].
Fig. 2. Molecular structure of [(g
6-p-MeC6H4Pri)Os(PPh3)Br2] (3) with ellipsoids
shown at the 30% probability density level.
signal from 62.43 ppm in complex 7 to 79.41 ppm, together with
an increase in 1J(187Os–31P) coupling constant from 449 to 476 Hz.
IR spectra of complexes 13 and 14 displayed strong m(C„O)
bands at 1965 and 1897 cmꢂ1, respectively. These bands occurred
at lower frequencies when compared to their starting materials 9
and 10, and these shifts may be attributed to the change in basicity
of the ligands on the osmium.
In addition to shifts corresponding to the protons of the respec-
tive tertiary phosphine or phosphite ligands, the 1H NMR spectra of
complexes 11–13 all displayed expected signals for the p-cymene
ligand. Complex 14 showed a signal for the methyl groups of the
hexamethylbenzene at 2.03 ppm.
A satisfactory mass spectrum was obtained for complex 10. The
spectrum showed a peak corresponding to the parent ion (m/
z = 519.3[M]+) with a fragmentation pattern that corresponds to
the sequential loss of the pentenyl moieties (m/z = 451.2[MꢂC5H9]+)
and (m/z = 383.1[Mꢂ(C5H9)2]+).
The alkenyl chains of all the complexes showed similar 1H NMR
patterns. The Os–CH2 protons appeared furthest downfield as
broad multiplets integrating for four protons at ca. 0.78–
0.91 ppm. Osmium has a shielding effect on the methylene protons
adjacent to it. This shielding effect has also been observed in sim-
ilar Pt and Pd bis(1-alkeny) [17]. Signals for the remaining methyl-
enes of the pentenyl chains gave rise to broad multiplets in the
region of ca. 1.02–2.03 ppm, while the pendant alkenyl functional-
ity gave rise to distinct multiplets in the region of ca. 4.52 and 5.20
for the @CH2 protons and ca. 5.45 and 5.90 for the CH@ protons.
The PPh3 derivative 11 exhibited deshielding of its 31P NMR sig-
nal from ꢂ17.57 ppm in complex 3 to 12.61 ppm. The P(OBun)3
containing complex 12 also exhibited deshielding of its 31P NMR
Reaction of [(g
6-p-MeC6H4Pri)(CO)OsBr2] (9) with excess MeLi
resulted in the formation of the di-methyl complex 15 (Eq. (1))
as evidenced by 1H NMR and IR spectral data. We expected this
complex 15 to be more stable than the bis(1-pentenyl) analogue
13 since it has no b-hydrogens, however the compound was iso-
lated as a brown oil and was also found to be very unstable and dif-
ficult to handle. Unlike complex 15, the di-alkyl complex cis-
[Os(CO)4Me2] exists as a stable white crystalline solid [18]. The
related complex [(
a brownish yellow oil by Maitlis and co-workers [12c].
g
6-p-MeC6H4Pri)(PPh3)OsMe2] was isolated as
Fig. 1. Molecular structure of two crystallographically independent [(g
6-p-MeC6H4Pri)OsBr2]2 (1) molecules with ellipsoids shown at the 30% probability density level. A co-
crystallised water molecule is included.

H.S. Clayton et al. / Polyhedron 28 (2009) 1511–1517
1517
excess
BrMg
Ar
Br
Ar
THF, RT
16-18 hrs
Os
Os
L
Br
L
11 L = PPh3 ,
Ar = p-cymene
12 L = P(OBun)3, Ar = p-cymene
13 L = CO,
14 L = CO,
Ar = p-cymene
Ar = C6Me6
Scheme 3. The synthesis of osmium bis(1-pentenyl) complexes 11–14.
References
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4
(1979) 1. and
Me
Me
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OC
OC
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The dimers [(
g
6-p-MeC6H4Pri)OsBr2]2 and [( 6-C6Me6)OsBr2]2
g
can be prepared from potassium osmate following the procedures
reported herein. From these dimers a series of new arene-osmium
dibromide complexes containing CO, tertiary phosphines and
phosphites were obtained. The osmium dibromide complexes re-
acted with 5-pentenyl Grignard and MeLi to give the putative
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g
6-p-MeC6H4Pri)(-
CO)OsMe2], respectively, which were isolated as unstable yellow
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(b) R.D. George, S.A.R. Knox, F.G.A. Stone, J. Chem. Soc., Dalton Trans. (1973)
972.
We would like to thank the Anglo Platinum Corporation, Uni-
versity of Cape Town and University of South Africa. We also thank
the National Research Foundation (NRF) of South Africa and NRF-
DST Centre of Excellence in Catalysis (C Change) for financial sup-
port and David Liles at the University of Pretoria for solving the
*
crystal structure of complex 1.