Synthesis and reactivity of the osmium methylidene complex [(C5Me5)Os(=CH2)(dppm)][OTf]

Article abstract of DOI:10.1039/a901644i

Treatment of the hydride complex (C₅Me₅)Os(dppm)H, [dppm = bis(diphenylphosphino)methane] with 2 equivalents of methyl trifluoromethanesulfonate (MeOTf) affords the methylidene complex [(C₅Me₅)Os(=CH₂

Full text of DOI:10.1039/a901644i

Synthesis and reactivity of the osmium methylidene complex
[(C5Me5)Os(NCH2)(dppm)][OTf]
Julia L. Brumaghim and Gregory S. Girolami*
School of Chemical Sciences, University of Illinois at Urbana-Champaign, 600 South Mathews Avenue, Urbana,
Illinois 61801. E-mail: girolami@scs.uiuc.edu
Received (in Bloomington, IN, USA) 26th February 1999, Accepted 12th April 1999
Treatment of the hydride complex (C5Me5)Os(dppm)H,
[dppm = bis(diphenylphosphino)methane] with 2 equiva-
lents of methyl trifluoromethanesulfonate (MeOTf) affords
the methylidene complex [(C5Me5)Os(NCH2)(dppm)][OTf];
the molecular structure and dynamic NMR behavior of this
methylidene complex are described.
the free energy of activation is 14.7 ± 0.5 kcal mol21 at
25 °C.
The rotation barrier is a measure of how much the p bonding
is weakened upon rotating the methylidene ligand by 90° about
the MNC axis. Somewhat surprisingly, the activation entropies
and enthalpies of rotation have never been measured for any
terminal methylidene complex.15,17,18 For four molecules,
however the free energies of activation have been reported: 8.3
± 0.1 kcal mol21 at 285 °C for [(C5Me5)W(NCH2)(PPh3)-
(CO)2][AsF6],15 9.0 ± 0.1 kcal mol21 at 270 °C for
[(C5Me5)W(NCH2)(PEt3)(CO)2][AsF6],15 10.7 ± 0.2 kcal mol21
at 228 °C for [(C5Me5)Fe(NCH2)(dppe)][BF4],18 and ! 19 kcal
mol21 at 114 °C for [(C5Me5)Re(NCH2)(NO)(PPh3)][PF6].17
An X-ray crystallographic study‡ of 1 revealed that the Os–C
distance for the methylidene ligand is 1.926(9) Å (Fig. 2). This
value is comparable to the MNC bond distances seen for other
late transition metal methylidene complexes: 1.87(1) Å in
Ir(NCH2)[N(SiMe2CH2PPh2)2],11 1.90(2) Å in [(C5Me5)Re-
(NCH2)(NO){P(OPh)3}]+,17 and 1.92(1) Å in Os(NCH2)(PPh3)-
(NO)Cl.14
Transition metal alkylidenes have been of interest for the last 20
years owing to their role as intermediates in olefin metathesis1–7
and Fischer–Tropsch8 reactions. In most cases, the alkylidene
ligand is substituted; in contrast, there are relatively few
examples of the simplest alkylidene, a terminal methylidene
( = CH2) ligand. In 1975, Schrock described the first such
complex, Cp2Ta(NCH2)(CH3),9 and the number of complexes
containing a terminal methylidene ligand has slowly grown
since.1,10–19
Most terminal methylidene complexes have been synthesized
by one of two routes: by abstraction of a hydride from a methyl
group, or by abstraction of a proton from a cationic methyl
compound.13 We now describe the synthesis of an osmium
methylidene complex from the reaction of an osmium hydride
with methyl trifluoromethanesulfonate (MeOTf).
Like other cationic methylidene complexes,14 the methyli-
dene complex 1 is susceptible to attack by nucleophiles. Thus,
treatment of 1 with LiBH4 yields the corresponding methyl
complex, (C5Me5)Os(dppm)CH3.§
At least two mechanisms could account for the formation of
1 upon treatment of (C5Me5)Os(dppm)H with MeOTf. In one
mechanism [eqn. (1)], an osmium methyl/hydride intermediate
is generated initially, and dihydrogen is lost to form 1.
Treatment of (C5Me5)Os(dppm)H20 with 2 equivalents of
MeOTf in pentane at room temperature for 18 h affords a yellow
powder, which has been identified as the new methylidene
compound [(C5Me5)Os(NCH2)(dppm)][OTf] 1.† The 1H NMR
spectrum of 1 at 230 °C shows that the two hydrogen atoms of
the methylidene ligand are inequivalent. In the 13C NMR
spectrum of 1, the methylidene carbon gives rise to a triplet at
d 261.9 (JCH 144 Hz). The inequivalence of the hydrogen atoms
is a result of the preferred orientation of the methylidene ligand,
which places one hydrogen atom proximal to, and the other
distal from, the C5Me5 ligand. The same orientation is seen for
the methylidene ligands in other [(C5R5)M(NCH2)L2]n+ com-
plexes.15,17,18
+
H
CH3
–H2
+
+
LnOs
H
LnOs
LnOs CH2
(1)
CH3
In another mechanism [eqn. (2)], the osmium methyl/hydride
intermediate undergoes reductive elimination of methane;
As the temperature is raised, the methylidene signals in the
1H NMR spectrum broaden and finally coalesce at 60 °C as
rotation of the methylidene ligand around the OsNC bond
becomes fast on the NMR time scale. The variable-temperature
1H NMR line shapes and the simulations of the spectra are
shown in Fig. 1. An Eyring plot showed that the activation
parameters for rotation of the methylidene ligand in 1 are DH‡
= 16.4 ± 0.5 kcal mol21 and DS‡ = 5.7 ± 1.5 cal mol21 K21
;
Fig. 1 Variable temperature 500 MHz 1H NMR line shape for the
methylidene protons in 1 (thf-d8, left) and simulated spectra (right). Rate
Fig. 2 Crystal structure of [(C5Me5)Os(NCH2)(dppm)][OTf], molecule 1;
35% probability density surfaces are shown. The hydrogen atoms and
triflate anion are omitted for clarity.
constants are given in s21
.
Chem. Commun., 1999, 953–954
953

exchange between the diethyl ether solvent and the MeOTf reagent. There
are three molecules in the asymmetric unit, but one of these sites is occupied
exclusively by the OsNCH2 complex. The metric parameters discussed in
the text are for this molecule. The presence of the ethylene complex in the
sample was confirmed by NMR spectroscopy and by mass spectrometry.
CCDC 182/1226.
attack by a second equivalent of MeOTf followed by loss of a
proton affords 1.
+
+
–H+
H
CH3
–CH4
CH3
LnOs
H
LnO+s
LnOs+
LnOs CH3
LnOs CH2 (2)
2+
+
CH3
§ Selected spectroscopic data for (C5Me5)Os(dppm)Me: MS(FD): m/z
726[M+]. 1H NMR (C6D6, 25 °C): d 0.30 (t, JHP 7.9 Hz, Os–CH3). 13C{1H}
NMR (C6D6, 25 °C): d 226.8 (t, JCP 8.6 Hz, Os–CH3). 31P{1H} NMR
(C6D6, 25 °C): d 232.6 (s).
In both of these mechanisms, the first step is formation of an
osmium methyl/hydride cation. This step has precedence in our
study of the analogous (C5Me5)Os(dmpm)H system [dmpm =
bis(dimethylphosphino)methane].21 The mechanism responsi-
ble for the formation of 1 was determined by following the
reaction of (C5Me5)Os(dppm)H with MeOTf in a sealed NMR
¶ Selected spectroscopic data for (C5Me5)Os(dppm)(OTf): MS(FD): m/z
860[M+]. 19F NMR (C6D6, 25 °C): d 280.0 (s, CF3). 31P{1H} NMR (C6D6,
25 °C): d 231.0 (s).
1
tube. As judged by H NMR spectroscopy, no dihydrogen is
1 P. Schwab, R. H. Grubbs and J. W. Ziller, J. Am. Chem. Soc., 1996, 118,
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generated, but a peak attributable to methane (d 0.11) grows in
during the course of the reaction. On a preparatory scale, if
(C5Me5)Os(dppm)H and MeOTf are allowed to react for only 1
h in pentane, the triflate complex (C5Me5)Os(dppm)OTf can be
isolated.¶ Subsequent treatment of isolated samples of
(C5Me5)Os(dppm)OTf with additional MeOTf affords 1 in high
yield. These results suggest that 1 is generated by the second of
the two mechanisms shown above.
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Further studies of these new osmium complexes are in
progress.
We thank the Department of Energy (Grant DEFG02-
96-ER45439) for support of this work, and the University of
Illinois School of Chemical Sciences for a departmental
fellowship to J. L. B. NMR spectra were collected on
instruments in the Varian Oxford Instrument Center for
Excellence at the University of Illinois at Urbana-Champaign;
external funding for this instrumentation was obtained from the
Keck Foundation, NIH, and NSF.
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Chem. Soc., 1997, 119, 3171.
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Notes and references
† Selected spectroscopic data for 1: MS(FD); m/z 725[M+]. 1H NMR (thf-
d8, 230 °C): d 15.49 (td, 3JHP 6.5, 1JHH 1.2 Hz, OsNCH2), 17.36 (d, 1JHH 1.2
Hz, OsNCH2). 13C{1H} NMR (CD2Cl2, 230 °C): d 261.9 (s, OsNCH2). 19
F
NMR (thf-d8, 25 °C): d 280.0 (CF3). 31P{1H} NMR (thf-d8, 25 °C): d
237.7 (s).
‡ Crystal data for 1 at 198 K: monoclinic, space group P21/n, with a =
11.5525(10), b = 50.217(4), c = 18.410(2) Å, b = 96.866(2)°, V =
10603(2) Å3, Z = 12, R1 (obs. data) = 0.0754, wR2 (all data) = 0.2182 for
1078 parameters and 101 restraints refined against 18671 unique data. The
crystal chosen was grown from diethyl ether by treating
(C5Me5)Os(dppm)H with MeOTf; we have not been able to grow crystals
from other solvents. Under these conditions, the crystals obtained were a
mixture of
1 with a second compound, the ethylene complex
[(C5Me5)Os(dppm)(C2H4)][OTf], which was evidently generated by alkyl
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Chem. Commun., 1999, 953–954