Non-classical NHC transfers from the reaction of (IMes)AgCl with osmium carbonyl clusters

Article abstract of DOI:10.1039/b703071c

N-Heterocyclic carbene transfer reactions were attempted using IMesAgCl and two osmium clusters. The products isolated from these reactions suggest that NHC transfers can be unpredictable. The Royal Society of Chemistry.

Full text of DOI:10.1039/b703071c

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Non-classical NHC transfers from the reaction of (IMes)AgCl with osmium
carbonyl clusters†
Craig E. Cooke,a Taramatee Ramnial,a Michael C. Jennings,b Roland K. Pomeroya and Jason A. C. Clyburne*c
Received 28th February 2007, Accepted 2nd March 2007
First published as an Advance Article on the web 26th March 2007
DOI: 10.1039/b703071c
N-Heterocyclic carbene transfer reactions were attempted
using IMesAgCl and two osmium clusters. The products
isolated from these reactions suggest that NHC transfers can
be unpredictable.
transition metal chemistry.4 We also note that although NHCs
are common ligands in transition metal complexes, Os–NHC
complexes are rare, and are unknown for polynuclear osmium
clusters.5
With these features in mind, Os3(l-H)2(CO)10 was reacted with
an equimolar amount of [(IMes)AgCl])], IMes = bis(1,3-(2,4,6-
trimethylphenyl)imidazol-2-ylidene) in a 2 : 3 CH2Cl2 : hexanes
mixture. The reaction vessel was degassed three times using
freeze–pump–thaw techniques, and the purple reaction mixture
was warmed in an oil bath overnight at 60 ◦C. At the end
of this time, the reaction vessel contained an orange–yellow
solution and a small amount of dark solid was present at the
bottom of the vessel. The solvent was removed in vacuo and
the remaining solid dissolved in a minimum of CH2Cl2 and
applied to a silica gel column. Elution under a 30% solution of
CH2Cl2 to hexanes afforded an orange–yellow band of 1.§ This
band was eluted, the solvent removed, and the solid residue was
dissolved in a hot hexanes and toluene solution to afford 1 as
orange–red crystals upon concentration. Other bands were also
eluted and these correspond to Os3(l-H)2(CO)10 and Os3(CO)12
as determined by infrared spectroscopy. Please note that in all
reactions performed in this study complex mixtures of products
were obtained. These mixtures were separated and purified using
column chromatography. Although often a concern, product
rearrangement or decomposition on silica gel columns was not
observed. In all cases crude reaction mixtures were analyzed
by infrared spectroscopy as were the isolated products. The
appearance of v(CO) bands appeared to be consistent in both
position and relative intensity following chromatography, leading
us to believe no rearrangement or decomposition of reaction
products had occurred
N-Heterocyclic carbenes (NHCs) are important ligands in
organometallic chemistry. They provide a versatile alternative
to phosphine ligands, often showing stronger electron-donating
properties while also providing more variable steric environments
than those observed for phosphines.1 The reaction of free NHCs
can be technically demanding owing to their highly reactive
nature. NHC–silver(I) halide complexes are an attractive class of
molecules with the potential for direct transmetallation of the
NHC ligand. This reaction is accompanied by the thermodynami-
cally favourable formation of solid silver(I) halide, which typically
precipitates during the course of the reaction. As NHC–silver(I)
halide complexes are generally air- and moisture-stable, these
reagents allow for an easy starting point into the area of NHC–
metal chemistry. Several research groups have investigated the use
of silver(I) halide complexes as NHC transfer agents and have
successfully prepared NHC complexes of several late transition
metals by this method.2,3 A general scheme depicting the intended
reactivity is outlined in eqn (1).
NHC–Ag–X → NHC–MLn → NHC–MLn + AgX(s)
(1)
We began our investigation on the reaction of NHC–silver(I)
chloride as an NHC transfer agent to various osmium clusters as
an entry point into this emerging field. Due to relativistic effects
and lanthanide contraction, osmium compounds have the ability
to adopt a variety of novel structural architectures possessing
maximized orbital overlap with various ligands. Furthermore,
osmium has a remarkable ability to concatenate. The diversity
of metallic skeletal arrangements provides for a wide variety of
ligand-stabilised osmium cluster compounds. In addition, the
ability to readily remove CO from osmium carbonyl‡ clusters
makes them amenable to both facile ligand substitution and to the
introduction of various metal-containing species. This leads to the
formation of novel mixed metal compounds. Finally, the presence
of CO ligands provides a ready spectroscopic handle for monitor-
ing reaction progress and ligand effects via IR spectroscopy. These
characteristics make osmium carbonyl cluster compounds an ideal
system for the study of NHC transfer reactions and synthetic
1
1H and 13C{ H} NMR solution studies on the crystalline solid
material 1 revealed the presence of an asymmetrically oriented
IMes ligand. 1H NMR studies also revealed a single resonance at
−13.3 ppm that integrates to one hydrogen atom, which indicates
that one of the hydride ligands had been lost from the osmium
cluster. IR solution studies on a hexanes solution of 1 indicated
the presence of eight m(CO) bands. In order to determine the
atom connectivity we performed a crystallographic study.¶* Single
crystals were grown and the results of the study are shown in Fig. 1.
The solid state structure of 1 shows the anticipated formation
of an NHC osmium carbonyl cluster complex, but not the one
predicted by eqn (1). The molecule can be viewed as an NHC
complex of the osmium carbonyl cluster Os3(l-H)(l-Cl)(CO)9,
where an NHC occupies a coordination site previously engaged
by a CO ligand in Os3(l-H)2(CO)10, and a hydride ligand has
been replaced by a chloride ligand. By mass balance, the reactants
Os3(l-H)2(CO)10 and [(IMes)AgCl] are incorporated into the final
aDept. of Chemistry, Simon Fraser University, Burnaby, BC, Canada
bDept. of Chemistry, University of Western Ontario, London, ON, Canada
cDept. of Chemistry, Saint Mary’s University, B3H 3C3, Halifax, NS,
Canada. E-mail: jason.clyburne@smu.ca; Tel: +1 902 420 5827
† The HTML version of this article has been enhanced with colour images.
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Fig. 1 ORTEP view of 1. Ellipsoids are drawn at the 50% probability level.
˚
Selected bond distances (A): Os(1)–C(1) 2.121(6), Os(1)–Os(2) 2.8536(4),
Os(1)–Os(3) 2.8629(4), Os(2)–Os(3) 2.8414(3), Os(1)–Cl 2.4715(15),
Os(2)–Cl 2.4566(16), Os(1)–H 1.91(4), Os(2) 1.91(4), C(1)–N(5) 1.352(8),
C(1)–N(2) 1.371(7). Selected bond angles (◦): C(1)–Os(1)–Os(3)
172.70(17), Os(2)–Os(1)–Os(3) 59.609(9), Os(3)–Os(2)–Os(1) 60.359(9),
Os(2)–Os(3)–Os(1) 60.032(9), N(5)–C(1)–N(2) 103.4(5), N(5)–C(1)–Os(1)
126.6(4), N(2)–C(1)–Os(1) 129.8(5).
Fig. 2 ORTEP view of 2. Ellipsoids are drawn at the 50% probability level.
Selected bond distances (A): C(1)–Ag 2.120(12), Os(2)–Ag 2.7983(11),
˚
Os(1)–Ag 2.8071(13), C(1)–N(5) 1.348(15), C(1)–N(2) 1.334(16),
Os(1)–Os(2) 2.8179(8), Cl–Os(1) 2.462(3), Cl–Os(2) 2.455(4). Selected
bond angles (◦): Os(2)–Cl–Os(1) 69.94(10), Os(2)–Ag–C(1) 150.5(4),
Os(1)–Ag–C(1) 148.8(4), Os(1)–Ag–Os(2) 60.36(3), N(2)–C(1)–N(5)
104.9(11).
product with concomitant loss of “AgH”, which presumably
disproportionates to elemental silver and H2.
Another new type of reactivity was observed upon reaction
of equimolar amounts of [(IMes)AgCl] with another activated
osmium species, [Os3(CO)10(CH3CN)2]. The reaction progress was
followed by IR spectroscopy over 2 h at room temperature by
monitoring the disappearance of bands in the CO stretching
region, specifically 2023 cm−1. During the reaction, no observable
precipitate of AgCl was formed, in contrast to standard transfer
reactions involving NHC–silver(I) halide complexes. The solvent
was removed in vacuo, and the resulting solid dissolved in a
minimum of CH2Cl2 and applied to a silica gel column. Elution
and workup afforded two compounds, namely red crystals of 2
and scarlet crystals of 3.
on a dichloromethane solution indicated the presence of six
m(CO) bands. Once again, X-ray crystallographic techniques were
necessary to confidently assign the atom connectivity in 3. Single
crystals of 3 were grown and the results of the study are shown
in Fig. 3. The results clearly show that the solid is composed
of discrete [IMes–H] ions and the new complex anion {[Os3(l-
Cl)(CO)10]2Ag}.
1
1H and 13C{ H} NMR solution studies on the red crystalline
solid material 2 revealed the presence of a symmetrical IMes
ligand. IR solution studies on a hexanes solution of 2 indicated the
presence of seven m(CO) bands. Curiously, elemental analysis and
LSIMS (P+ 1299.9 m/z) were consistent with the incorporation of
[(IMes)AgCl] into the cluster, with concomitant loss of acetoni-
trile. In order to determine the atom connectivity we again used
crystallographic techniques. Single crystals of 2 were grown and
the results of the study are shown in Fig. 2.
As shown in the figure, the Ag–C bond is maintained in 2, but
the Ag–Cl bond has been cleaved. Two new Ag–Os interactions
are formed, leading to a rare NHC hetero-bimetallic complex.
Fig. 3 ORTEP view of the anionic component of 3. Ellipsoids are
drawn at the 50% probability level. Selected bond distances (A): Os(2)–Cl
2.462(7), Os(1)–Cl 2.455(6), Ag–Os(2) 2.902(1), Ag–Os(1) 2.860(1),
Os(2)–Os(1) 2.781(1), Os(2)–Os(3) 2.858(1), Os(1)–Os(3) 2.858(1). Selected
bond angles (◦): Os(1)–Cl–Os(2) 68.86(16), Os(1)–Ag–Os(2) 57.71(3).
˚
˚
The silver–carbon bond distance 2.120(12) A, is longer than in the
6
˚
[(IMes)AgCl], 2.056(7) A. The chloride atom was incorporated
into a bridging position within the cluster such that the complex
can be formulated as [Os3(l-Cl)(CO)10(l-AgIMes)]. Complex 2 is
structurally related to [Os3(CO)10(l-PPh2)(l-AgP(C6H5)3], which
was prepared in a multistep procedure,7 but the preparation of
this class of bimetallic butterfly complex is best achieved using the
synthetic methodology used in our current report.
This anion was confirmed by the observation of a major ion
detected by LSIMS studies (P+ 1883.4 m/z) and elemental analysis
on the bulk sample was in accord with this formulation. Formation
of the imidazolium species suggests the presence of water in the
reaction vessel, but repeated experiments to generate compounds 2
and 3 under rigorous conditions resulted in similar isolated yields,
Finally, 1H solution NMR on the scarlet crystalline solid
material 3 revealed the presence of [IMes–H]+. IR solution studies
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thus suggesting abstraction of H+ from another source, such as
CH2Cl2.
applied to a silica gel column. Elution under a 30% solution of CH2Cl2
to hexanes afforded an orange–yellow band of 1. The resulting solid was
recrystallised from a concentrated hot hexanes and toluene solution to
afford 1 as orange–red crystals, (41 mg, 19%). Other bands were also eluted
and these correspond to Os3(l-H)2(CO)10 and Os3(CO)12 as determined by
infrared spectroscopy. IR (in hexanes) m(CO): 2093m, 2051s, 2012vs, 2002s,
1990w, 1976m, 1971m, 1939m cm−1. 1H NMR (500 MHz, 295 K, CD2Cl2)
d −13.28 [s, 1H, l-H], 2.10 [s, 6H, ortho-CH3], 2.13 [s, 6H, ortho-CH3],
2.36 [s, 6H, para-CH3], 7.05 [s, 2H, meta-H], 7.07 [s, 2H, meta-H], 7.14 [s,
2H, imidazo-H]. 13C{1H} NMR (100.61 MHz, 295 K, CD2Cl2) d 18.6 [s,
ortho-CH3], 19.0 [s, ortho-CH3] 21.4 [s, para-CH3], 124.2 [s, NCC], 129.7
[s, Ar-C-3,5], 130.4 [s, Ar-C-3,5], 136.3 [s, Ar-C-2,6], 136.6 [s, Ar-C-2,6]
137.7 [s, Ar-C1,], 140.2 [s, Ar-C-4]. MS (LSIMS) 1164.1 (P+). Anal. Calc.
for C30H25ClN2O9Os3: C, 30.96; H, 2.17; N, 2.41. Found: C, 30.93; H,
2.14; N, 2.47. 2: [Os3(l-Cl)(CO)10(l-AgIMes)] was prepared by literature
methods. To a Schlenk tube was added [Os3(CO)10(CH3CN)2] (101 mg,
0.11 mmol), [(IMes)AgCl] (49 mg, 0.11 mmol) and dry CH2Cl2 (30 mL).
The resulting reaction mixture was allowed to stir at room temperature
for 2 h. The reaction progress was followed by IR spectroscopy and
the disappearance of bands in the CO stretching region associated with
[Os3(CO)10(CH3CN)2], specifically 2023 cm−1. An orange–red solution
resulted and appeared to be completely reacted by IR spectroscopy. The
solvent was removed under vacuum and the resulting solid dissolved in
a minimum of CH2Cl2 and applied to a silica gel column. Elution under
a hexanes and CH2Cl2 solvent system afforded two bands. An orange
band eluted under 20% dichloromethane had the solvent removed and
the resulting solid was recrystallised from a concentrated hexanes solution
to afford 2 as red crystals (49 mg, 35%). A second deep red band eluted
under 100% dichloromethane, the solvent was removed and the resulting
solid was recrystallised from a concentrated dichloromethane solution to
afford 3 as scarlet crystals (15 mg, 13%). 2: IR (in hexanes) m(CO): 2092w,
2036vs, 2010s, 2003m, 1981w, 1974m, 1951m cm−1. 1H NMR (400 MHz,
295 K, CDCl3) d 1.83[s, 12, ortho-CH3], 2.11 [s, 6H, para-CH3], 6.76 [s, 4H,
Compounds 2 and 3 represent unusual cases in NHC transfer
reactions.3 In compound 2 it appears that facile insertion into the
Ag–Cl bond can occur, and in the case of 3, anion exchange can
occur with [(IMes)2Ag]+[AgCl2]−, a species that is available due
to a dynamic equilibrium with [(IMes)AgCl].8 We suggest that
[AgCl2]− ion reacts with the trinuclear cluster in such a way as to
lead to its complete incorporation into 3.
The reaction of NHC–silver(I) chloride compounds with Group
4 metals containing Lewis acids have been shown to transfer
the chloride ion to yield complex anions and the well known
[(IMes)2Ag]+ ion.8 In the case discussed in this paper, incorpo-
ration of both silver and chloride groups into the metal cluster is
most likely the result of the large soft late transition metal centres
(osmium) providing a large source of electron density to complex
the AgCl fragment. It is also possible that the steric constraints
imposed by the open edge of the triangular cluster may have
prevented transmetallation.
The use of NHC–silver(I) halides in transmetallation reactions
involving a variety of metallic reagents has been well documented,3
however the results of the current study clearly suggest a word
of caution: while NHC–silver(I) halide complexes are widely
used, their chemistry is often complex. The molecules themselves
engage in complex equilibria, involving ionic species as well as
free carbenes,9 and in this report we show that non-classical
reactions can occur. These results suggest that clean or efficient
transmetallation reactions may not occur in all cases and in situ
reactions involving silver(I) halides should be approached with
caution as Ag containing products or intermediates may persist.
The ability of silver(I) halides to form hetero-bimetallic species like
2 and 3 provides evidence for a new synthetic use for NHC–silver(I)
halide complexes and is a synthetically attractive alternative to
pyrolysis approaches to bimetallic species, which are often plagued
with decomposition problems.
1
meta-H], 6.89 [d, 2H, imidazo-H]. 13C{ H} NMR (100.61 MHz, 295 K,
CD2Cl2) d 17.6 [s, ortho-CH3], 21.4 [s, para-CH3], 122.6 [d, NCC], 129.8
[s, Ar-C-3,5], 134.9 [s, Ar-C-2,6], 135.4 [s, Ar-C1,], 139.7 [s, Ar-C-4]; MS
(LSIMS) 1297.8 (P+). Anal. Calc. for C31H26ClAgN2O10Os3: C, 28.63; H,
2.02; N, 2.15. Found: C, 28.40; H, 1.94; N, 2.01. 3: IR (in CH2Cl2) m(CO):
2095.5w, 2085m, 2037br,s, 2000br,m, 1976m,sh, 1959br,m cm−1; 1H NMR
(500 MHz, 295 K, CD2Cl2) d 2.14 [s, 12H, ortho-CH3], 2.41 [s, 6H, para-
1
CH3], 7.16 [s, 4H, meta-H], 7.55 [d, 2H, imidazo-H], 8.25 [s, C+–H] 13C{ H}
NMR (100.61 MHz, 295 K, CDCl2) d 17.7 [s, ortho-CH3], 21.5 [s, para-
CH3], 125.7 [s, NCC], 130.8 [s, Ar-C-3,5], 134.4 [s, C1], 136.1 [s, Ar-C-2,6],
136.9 [s, Ar-C1,], 143.3 [s, Ar-C-4]; MS (LSIMS) 1883.4 (P+). Anal. Calc.
for C41H25AgCl2N2O20Os6: C, 22.53; H, 1.15; N, 1.28. Found: C, 22.56; H,
1.20; N, 1.40.
¶ Crystal data: for 1: C33.5H29ClN2O9Os3, triclinic, space group P-◦1, a =
Acknowledgements
˚
˚
˚
8.3651(2)◦A, b = 10.8743(3) A, c = 20.4975(7) A, a = 80.968(2) , b =
◦
3
˚
81.369(2) , c = 71.935(2) , V = 1740.42(9) A , Z = 2, Dc = 2.308 Mg
m−3, absorption coefficient = 11.056 mm−1, R1 (2r data) = 0.0344, wR2
Funding was provided by the Natural Sciences and Engineering
Council of Canada (NSERC) through the Discovery Grants
Program to J. A. C. C. and R. K. P. J. A. C. C. acknowledges
support from the Canada Research Chairs Program, the Canadian
Foundation for Innovation and the Nova Scotia Research and
Innovation Trust.
(all data) = 0.0856 GOF = 1.034.
˚
For 2: C31H24AgClN2O11Os3, triclinic, space group ◦P-1, a = 8.1388◦(6) A,
˚
˚
b = 12.53◦07(9) A, c = 18.0480(10) A, a = 93.746(4) , b = 97.217(4) , c =
3
97.567(3) , V = 1803.9(2) A , Z = 2, Dc = 2.391 Mg m−3, absorption
˚
coefficient = 11.195 mm−1, R1 (2r data) = 0.0540, wR2 (all data) = 0.1196,
GOF = 0.944.
For 3: C41H25AgCl2N2O20Os6, monoclinic, space group C2/c◦, a =
˚
˚
˚
14.5755(8)◦A, b = 21.6908(16) A, c = 17.6188(11) A, a = 90 , b =
◦
106.714(3) , c = 90 , V = 5334.9(6) A , Z = 4, Dc = 2.721 Mg m−3
,
3
˚
Notes and references
absorption coefficient = 14.763 mm−1, R1 (2r data) = 0.0669, wR2 (all
data) = 0.1855, GOF = 1.007. The three data sets were collected at 150 K,
150 K, and 296 K, respectively, on a Nonius Kappa-CCD diffractometer
with COLLECT (Nonius B.V., 1998).
‡ General considerations: Care should be exercised when working with
osmium carbonyls due to health and exposure risks. Unless otherwise
stated, manipulations of starting materials and products were carried out
under a nitrogen atmosphere with the use of standard Schlenk techniques.
Solvents were thoroughly dried before use.10
* CCDC reference numbers 629182–629184. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/b703071c
§ Synthetic procedures and selected characterisation data: 1: [Os3(l-H)(l-
Cl)(CO)9(IMes)]: To a Carius tube was added (160 mg, 0.19 mmol) Os3(l-
H)2(CO)10, (84 mg, 0.19 mmol) [(IMes)AgCl])], CH2Cl2 (20 mL) and
hexanes (30 mL). The reaction vessel was degassed three times by freeze–
pump–thaw methods and the purple reaction mixture placed in an oil bath
overnight at 60 ◦C. The following morning the reaction vessel was removed
from heat and the solution cooled to room temperature. The reaction
vessel contained an orange–yellow solution and a small amount of dark
solid was present at the bottom of the vessel. The solvent was removed
in vacuo and the remaining solid dissolved in a minimum of CH2Cl2 and
1 W. A. Herrmann, Angew. Chem., Int. Ed., 2002, 41, 1290 or; R. H.
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6 T. Ramnial, C. D. Abernethy, M. D. Spicer, I. D. McKenzie, I. D. Gay
and J. A. C. Clyburne, Inorg. Chem., 2003, 42, 1391.
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P. R. Raithby, S. J. Teat and A. D. Woods, Dalton Trans., 2003,
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and references therein.
9 To illustrate this point, a recent paper: (A. C. Sentman, S. Csihony,
R. M. Waymouth and J. L. Hedrick, J. Org. Chem., 2005, 70, 2391.)
reported on the catalytic behaviour of [(IMes)2Ag][AgCl2]. The isolated
materials clearly behave as a source of catalytically active NHC, the
mass spectral data suggested the presence of an ionic compound,
[(IMes)2Ag], but the solution 1H NMR spectrum was dominated by
signals for IMesAgCl (see ref. 6).
10 S. R. Drake and P. A. Loveday, Inorg. Synth., 1989, 25, 230.
1758 | Dalton Trans., 2007, 1755–1758
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