Synthesis of fac-[Os(CO)3(L)Me2] (L = THF or MeCN) and some reactions with PPh3 and trityl salts

Article abstract of DOI:10.1016/j.ica.2004.09.052

The reaction of cis-[Os(CO)₄Me₂] with Me ₃NO in the THF or MeCN yields the complexes fac-[Os(CO) ₃(L)Me₂] (where L = THF or MeCN). Whereas the THF complex is unstable and only characterised spectroscopically, fac-[Os(CO) ₃(MeCN)Me₂] has been isolated as a white solid and fully characterized by both analytical and spectroscopic methods. These complexes fac-[Os(CO)₃(L)Me₂] are shown to be useful intermediates. Thus, reaction with PPh₃ gives fac-[Os(CO)₃(PPh ₃)Me₂] in good yield. Reactions of fac-[Os(CO) ₃(L)Me₂] (L = CO or MeCN) with CPh₃PF ₆ or B(C₆F₅)₃ have been investigated. Whereas cis-[Os(CO)₄Me₂] showed no reaction with either CPh₃PF₆ or B(C₆F₅) ₃, the reaction of fac-[Os(CO)₃(MeCN)Me₂] with CPh₃PF₆ in CH₂Cl₂ occurred over 16 h at room temperature to give an unstable cationic product and CPh₃Me. The reaction was monitored by both IR and NMR spectroscopies. When this reaction of fac-[Os(CO)₃(MeCN)Me₂] was carried out in the presence of a trapping ligand such as MeCN, the stable cationic product [Os(CO)₃(MeCN)₂Me]⁺ could be isolated and identified spectroscopically.

Full text of DOI:10.1016/j.ica.2004.09.052

Inorganica Chimica Acta 358 (2005) 1715–1718
www.elsevier.com/locate/ica
Note
Synthesis of fac-[Os(CO)3(L)Me2] (L = THF or MeCN) and
some reactions with PPh3 and trityl salts
1
Matthew J. Overett , John R. Moss
*
Department of Chemistry, University of Cape Town, Private Bag, Rondebosch 7701, South Africa
Received 5 August 2004; accepted 28 September 2004
Available online 22 October 2004
Dedicated to Professor Gordon Stone in recognition of his many pioneering contributions to organometallic chemistry
Abstract
The reaction of cis-[Os(CO)4Me2] with Me3NO in the THF or MeCN yields the complexes fac-[Os(CO)3(L)Me2] (where L = THF
or MeCN). Whereas the THF complex is unstable and only characterised spectroscopically, fac-[Os(CO)3(MeCN)Me2] has been
isolated as a white solid and fully characterized by both analytical and spectroscopic methods. These complexes fac-
[Os(CO)3(L)Me2] are shown to be useful intermediates. Thus, reaction with PPh3 gives fac-[Os(CO)3(PPh3)Me2] in good yield.
Reactions of fac-[Os(CO)3(L)Me2] (L = CO or MeCN) with CPh3PF6 or B(C6F5)3 have been investigated. Whereas cis-
[Os(CO)4Me2] showed no reaction with either CPh3PF6 or B(C6F5)3, the reaction of fac-[Os(CO)3(MeCN)Me2] with CPh3PF6 in
CH2Cl2 occurred over 16 h at room temperature to give an unstable cationic product and CPh3Me. The reaction was monitored
by both IR and NMR spectroscopies. When this reaction of fac-[Os(CO)3(MeCN)Me2] was carried out in the presence of a trapping
ligand such as MeCN, the stable cationic product [Os(CO)3(MeCN)2Me]+ could be isolated and identified spectroscopically.
ꢀ 2004 Elsevier B.V. All rights reserved.
Keywords: Osmium; Carbonyl; Methyl; Alkyl
1. Introduction
of cis-[Os(CO)4Me2] gives methane as the primary or-
ganic product; which is believed to form via a radical
pathway [6]. Reactions with Br2, HF and CO have also
been described and these reactions yield cis-[Os(CO)4-
MeBr] [4], cis-[Os(CO)4F2] [7] and [Os(CO)5] [4,8],
respectively. Binuclear compounds may be formed by
reaction of cis-[Os(CO)4Me2] with hydrido-osmium
complexes. For example, reaction with [Os(CO)4H2]
gives [HOs2(CO)4Me] [9]. Thermal [3] and oxidative [9]
decarbonylations of [Os(CO)4Me2] have been used to
prepare fac-[Os(CO)3(PPh3)Me2]. In the latter case, the
oxidative decarbonylation was accomplished by treat-
ment of cis-[Os(CO)4Me2] with Me3NO in the presence
of PPh3. However, no attempt was made to isolate
fac-[Os(CO)3(L)Me2], where L is a labile ligand such
as a nitrile or ether, and to investigate the coordination
chemistry of the resulting activated complex.
Although the reactions of the metal carbonyl alkyls
of the type [M(CO)5R] (M = Mn and Re) have been
extensively studied [1,2], relatively few reactions of metal
carbonyl dialkyls have been reported. An example of
such a dialkyl compound is cis-[Os(CO)4Me2]. This com-
pound was first reported independently by LꢀEplattenier
and his coworker [3,4] and Stone and coworkers [5] in
the early 1970s. Few reactions of cis-[Os(CO)4Me2]
have, however, been described. Thermal decomposition
*
Corresponding author. Tel.: +27 21 6502535; fax: +27 21 6897499.
E-mail addresses: matthew.overett@sasol.com (M.J. Overett),
jrm@science.uct.ac.za (J.R. Moss).
1
Present address: Sastech R&D, SASOL, P.O. Box 1, Sasolburg
1947.
0020-1693/$ - see front matter ꢀ 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2004.09.052

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M.J. Overett, J.R. Moss / Inorganica Chimica Acta 358 (2005) 1715–1718
In this note, we describe a slightly modified and more
NO Æ 2H2O (116 mg, 1.04 mmol) in acetonitrile (6 ml).
The mixture was stirred at room temperature for 24 h,
after which time the reaction was judged to be complete
by IR spectroscopy and TLC (silica, 25% dichlorometh-
ane in hexane, Rf = 0.19). The solvents were removed in
vacuo and the crude residue was purified by column
chromatography (silica gel, 25% dichloromethane in
hexane). Upon removal of solvents in vacuo, a white so-
lid was obtained, which was recrystallised from dichlo-
romethane/hexane to give 2 as white microcrystals (231
mg, 70%), m.p. 104–107 ꢁC; Anal. Calc. for C7H9NO3Os
requires C, 24.3; H, 2.6; N, 4.1. Found: C, 24.6; H, 2.5;
convenient synthesis of cis-[Os(CO)4Me2] and the con-
version of this complex to the useful intermediate fac-
[Os(CO)3(L)Me2] (L = MeCN, THF). Reactions of
these labile intermediates with PPh3 and CPh3PF6 are
described. In a subsequent full paper, we will report
on the reactions of fac-[Os(CO)3(MeCN)Me2] with
mono- and bidentate neutral donor ligands to give a
range of novel mononuclear and binuclear products [11].
2. Experimental
N, 4.0%.
m
max(CO)/cmÀ1 (dichloromethane) 2072s,
2.1. General
1980vs; dH (400 MHz; CDCl3) 2.45 (3H, s, –NCMe),
0.062 (6H, s, Os-Me); dC{H}(100 MHz; CDCl3) 180.4
(CO), 175.9 (CO), 117.9 (MeCN)3.5 (MeCN), À16.3
(Os-Me); m/z (FAB) 347 (M+, 26%), 332 (M+ À Me,
61%), 319 (M+ À CO, 43%), 304 (M+ À CO–Me,
100%), 291 (M+ À 2CO, 35%), 276 (M+ À 2CO–Me,
30%).
All reactions were carried out under nitrogen or ar-
gon using standard Schlenk-tube techniques. Solvents
were purified by distillation under argon from an appro-
priate drying agent (CaH2 for acetonitrile and dichloro-
methane; sodium/benzophenone for THF and toluene).
Me3NO Æ 2H2O was purified by sublimation at 60 ꢁC,
0.1 mm Hg onto a cold finger and stored in a glovebox.
MeI was distilled under argon from CaH2. Melting
points were recorded on Kofler hotstage microscope
(Reichert Thermovar) and are uncorrected. Microanaly-
sis data were obtained from the University of Cape
Town Microanalytical Laboratory. Infrared spectra
were recorded on a Perkin–Elmer 983 spectrometer in
solution with NaCl windows. NMR spectra were re-
corded on a Varian Mercury-300 spectrometer. Mass
spectrometry was carried out using fast atom bombard-
ment (FAB) ionization at the Cape Technikon.
2.4. Synthesis of fac-[Os(CO)3(THF)Me2] (3)
To a stirred solution of 1 (65 mg, 0.20 mmol) in THF
(5 ml) was added a solution of Me3NO Æ 2H2O (30 mg,
0.27 mmol) in THF (20 ml). The reaction was monitored
by IR spectroscopy. After stirring for 16 h, the reaction
was complete and the reaction mixture was filtered
through silica. Upon removal of the volatiles in vacuo,
a clear oil was recovered, which decomposed after a short
time under vacuum. mmax(CO)/cmÀ1 (THF) 2068s,
1969vs.
2.2. Synthesis of cis-[Os(CO)4Me2] (1)
2.5. Synthesis of fac-[Os(CO)3(PPh3)Me2] (4)
cis-[Os(CO)4Me2] (1) was prepared by a modification
of the method of Stone [5]. Sodium metal (152 mg, 6.6
mmol) was added in small pieces to a suspension of
[Os3(CO)12] (2.95 g, 3.25 mmol) in liquid ammonia (50
ml) at À78 ꢁC. Completion of the reaction was indicated
by the persistence of a blue colour. MeI (1.0 ml, 16
mmol) was added and the mixture was stirred for 1 h.
The ammonia was then allowed to evaporate off, and
residual MeI and ammonia were removed in vacuo for
2 min. Complex 1 was then recovered from the residues
as a white powder by sublimation under passive vacuum
(<1 mm) at 60 ꢁC onto a dry ice cooled cold finger. (2.35
g, 73%); m.p. 63–64 ꢁC, lit. 65–66 ꢁC [5]; mmax(CO)/cmÀ1
(hexane) 2130m, 2044vs, 2011s, 1979w; dH(400 MHz;
CDCl3) 0.05 (6H, s, Os-Me). Spectroscopic data are in
agreement with the literature [5].
2.5.1. From 2
A mixture of 2 (152 mg, 0.440 mmol) and PPh3
(118 mg, 0.45 0mmol) in toluene (10ml) was stirred
at 75 ꢁC. The reaction was monitored by infrared
spectroscopy and judged to be complete after 48 h.
The volatiles were removed in vacuo and after column
chromatography (silica, 20% dichloromethane in hex-
ane, Rf = 0.28), 4 was recovered as a white crystalline
solid. (192 mg, 77%); m.p. 119–122 ꢁC. Anal. Calc. for
C23H21O3POs requires C, 48.8; H, 3.7. Found: C,
48.8; H, 3.7%.
m
max(CO)/cmÀ1 (toluene) 2068vs,
1994s, 1960s; dH (300 MHz; CDCl3) 7.44–7.42 (15H,
m, Haryl), À0.22 (6H, d, 3J(PH) 8 Hz, Os-Me);
d
C{H}(75 MHz; CDCl3) 178.8 (CO), 177.1 (d, 2J(PC)
9Hz, CO), 133.9 (d, 3J(PC) 10 Hz, C3,5 [PPh3]),
131.0 (d, 1J(PC) 50 Hz, C1 [PPh3]), 130.6 (d, 4J(PC)
2 Hz, C4 [PPh3]), 128.3 (d, 3J(PC) 10 Hz, C2,6
[PPh3]), À22.4 (d, 2J(PC) 8 Hz, Os-Me); dP{H}(120
MHz; CDCl3) 0.89; m/z (FAB) 553 (M+ À Me, 7%),
537 (M+ À 2Me–H, 100%), 509 (M+ À 2Me–H–CO,
2.3. Synthesis of fac-[Os(CO)3(MeCN)Me2] (2)
A stirred solution of 1 (316 mg, 0.951 mmol) in aceto-
nitrile (5 ml) was treated with a slight excess of Me3-

M.J. Overett, J.R. Moss / Inorganica Chimica Acta 358 (2005) 1715–1718
1717
60%). Spectroscopic data are in agreement with the lit-
erature [9].
involatile powder after flash chromatography. Com-
pound 3 decomposes upon removal of the tetrahydrofu-
ran in vacuo; however, it may be identified by its infrared
spectrum and its similar reactivity to 2. Treatment of 2 or
3 with PPh3 gives the previously reported [9] fac-[Os(-
CO)3(PPh3)Me2] (4) as expected, by displacement of
the labile ligand with PPh3 (Scheme 1). Full characterisa-
tion of 4 is reported for the first time.
2.5.2. From 3
A solution of 3 was prepared by treatment of 2 (50
mg, 0.15 mmol) with an excess of Me3NO Æ 2H2O in
THF. The solution was filtered through silica, PPh3
(40 mg, 0.15 mmol) was added, and the mixture was ref-
luxed for 48 h. The reaction was monitored by IR
spectroscopy. After removal of the volatiles in vacuo
and column chromatography (silica, 20% dichlorometh-
ane in hexane), a white crystalline solid was recovered
and identified as 4 by IR and NMR spectroscopies (59
mg, 69%).
An interesting feature of 2 is the effect of solvent on
1
its H NMR spectrum (Fig. 1). Substantial differences
in chemical shifts are apparent when 2 is dissolved in
CDCl3 and C6D6. The fine structure in the signals when
dissolved in C6D6 is due to non-equivalence of the
methyl protons on the NMR timescale. The restricted
rotation about the Os–CH3 and NC–CH3 bonds may
be attributed to the p-stacking present in the benzene
solvent.
2.6. Synthesis of fac-[Os(CO)3(MeCN)2Me]PF6 (5)
A solution of CPh3PF6 (46 mg, 0.12 mmol) in aceto-
nitrile (5.5 ml) was added dropwise over 4 h to a stirred
solution of 2 (41 mg, 0.12 mmol) in acetonitrile (5 ml).
The progress of the reaction was monitored by IR
spectroscopy. After stirring for 24 h, the volatiles were
removed in vacuo and the white residue was washed
with pentane (3 · 10 ml). Recrystallisation from dichlo-
romethane/hexane gave 5 as a white crystalline solid. (48
mg, 78%); m.p. 131–133 ꢁC. Anal. Calc. C8H9F6N2
O3POs requires C, 18.6; H, 1.8; N, 5.4. Found: C,
18.9; H, 1.6; N, 5.3%. mmax(CO)/cmÀ1 (dichloromethane)
2123s, 2051vs, 2039vs; dH (400 MHz; CD2Cl2) 2.59 (6H,
s, Os-NCMe), 0.52 (3H, s, Os-Me); dC{H}(100 MHz;
CD2Cl2) 170.9 (CO), 168.4 (CO), 123.4 (NCMe), 3.6
(NCMe), À15.9 (Os-Me); m/z (FAB) 373 (Os(CO)3(NC-
Me)2Me+, 100%), 345 (Os(CO)3(NCMe)2Me+ À CO,
66%), 304 (Os(CO)3(NCMe)2Me+ À CO–MeCN, 12%).
We are interested in alkyl abstraction reactions from
neutral dialkyl complexes to form cationic, coordina-
tively unsaturated alkyl species. Such reactions take
place during the activation of many alkene oligomerisa-
tion and polymerisation catalysts. To this end, we inves-
tigated the reaction of 1 and 2 with alkyl abstraction
reagents such as CPh3PF6 and B(C6F5)3. Both 1 and 2
were unreactive towards methyl abstraction with
B(C6F5)3, and no reaction was again observed between
1 and CPh3PF6. However, treatment of 2 with one
equivalent of CPh3PF6 in dichloromethane resulted in
the clean abstraction of a methyl group to yield a cati-
onic species. The formation of the by-product CPh3Me
could be observed by NMR (dH = 2.19 ppm, CPh3Me)
[10] and the shift in the infrared absorption bands to
higher wave number (mCO = 2114, 2040, 2013 cmÀ1) is
consistent with this result. However, the postulated 16-
electron product [Os(CO)3(MeCN)Me]PF6 was found
to be unstable and could not be isolated or observed
1
3. Results and discussion
in situ by H NMR. However, by performing the reac-
tion in the presence of a donor ligand such as acetonit-
rile, the cationic species could be ꢁtrappedꢀ as a stable 18-
electron salt, and thus fac-[Os(CO)3(MeCN)2Me]PF6 (5)
was successfully isolated and characterised (Scheme 2).
The rich coordination chemistry of 2 with monoden-
tate and bidentate ligands will be described in a full
paper [11].
fac-[Os(CO)3(L)Me2] (L = MeCN, 2; L = THF, 3)
may be prepared by treatment of 1 with Me3NO in aceto-
nitrile and tetrahydrofuran, respectively (Scheme 1). An
excess of Me3NO may be used without evidence of a sec-
ond decarbonylation and the reactions give exclusively
the fac isomer. Compound 2 may be isolated as a white
CO
CO
Os
CO
Me3NO
L
OC
OC
Me
Me
OC
OC
Me
Me
PPh3
OC
OC
Me
Me
Os
Os
toluene
CO
L
PPh3
∆
1
2 (L= MeCN)
3 (L= THF)
4
Scheme 1.

1718
M.J. Overett, J.R. Moss / Inorganica Chimica Acta 358 (2005) 1715–1718
CO
OC
OC
Me
Me
2
Os
NCMe
1
1
2
3.0
2.8
2.6
2.4
2.2
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
-0.2
-0.4
2
1
3.0
2.8
2.6
2.4
2.2
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
-0.2
-0.4
Fig. 1. 1H NMR spectra of 2 in CDCl3 (top) and C6D6 (bottom).
References
PF6
CPh3PF6
- CPh3Me
CO
Os
CO
Os
OC
OC
Me
Me
OC
OC
Me
[1] E.W. Abel, F.G.A. Stone, G. Wilkinson (Eds.), Comprehen-
sive Organometallic Chemistry, Pergammon, New York,
1982.
NCMe
MeCN
r.t.
NCMe
NCMe
[2] E.W. Abel, F.G.A. Stone, G. Wilkinson (Eds.), Comprehensive
Organometallic Chemistry II, Pergammon, New York, 1995.
[3] F. LꢀEplattenier, Inorg. Chem. 8 (1969) 965.
[4] F. LꢀEplattenier, C. Pelichet, Helv. Chim. Acta 53 (1970)
1091.
2
5
´
Scheme 2.
[5] R.D. George, S.A.R. Knox, F.G.A. Stone, J. Chem. Soc. Dalton
(1973) 972.
[6] W.J. Carter, S.J. Okrasinski, J.R. Norton, Organometallics 4
(1985) 1376.
Acknowledgements
[7] S.A. Brewer, L.A. Buggey, J.H. Holloway, E.G. Hope, J. Chem.
Soc. Dalton (1995) 2941.
The authors thank Sasol Technology (Pty) Ltd, the
NRF, THRIP and the University of Cape Town for
financial support, as well as Johnson Matthey and An-
glo Platinum Corporation for donation of osmium salts.
The authors acknowledge Prof. Graham Jackson and
Gaelle Ramon for helpful discussions.
[8] N.J. Bristow, B.D. Moore, M. Poliakoff, G. Ryott, J.J. Turner,
J. Organomet. Chem. 260 (1984) 181.
[9] W.J. Carter, J.W. Kelland, S.J. Okrasinski, K.E. Warner, J.R.
Norton, Inorg. Chem. 21 (1982) 3955.
[10] R.E. Dixon, A. Streitwieser, J. Org. Chem. 57 (1992) 6125.
[11] M.J. Overett, J.R. Moss, H. Su, manuscript in preparation.