Influence of the anion of the salt used on the coordination mode of an N-heterocyclic carbene ligand to osmium

Article abstract of DOI:10.1021/om700746h

The reactions of the hexahydride OsH₆(PⁱPr ₃)₂ (1) with 1.0 equiv of the BPh₄^-, BF₄^-, and Br^- salts of 1-(2-pyridylmethyl)-3- methylimidazolium, in tetrahydrofuran under reflux, have been studied. In the three cases, mixtures of the abnormal [OsH(η²-H ₂){_κ(C⁵,N-[1-(2-pyridylmethyl)-3- methylimidazol-5-ylidene]} (PⁱPr₃)₂]A (A = BPh₄ (2a), BF₄ (2b), Br (2c)) and normal [OsH(η₂-H₂) {κC²,N-[1-(2- pyridylmethyl)-3-methylimidazol-2-ylidene]} (P_iPr₃) ₂]A (A = BPh₄ (3a), BF₄ (3b), Br (3c)) isomers are obtained. The formation rate of the abnormal isomer and the abnormal to normal ratio decrease as the coordinating power of the anion of the used salt increases. Treatment of 2b with either HBF₄ · OEt₂ or LiBF₄ gives rise to its isomerization to 3b. The X-ray structure of 2a and T₁(min) values of the OsH₃ resonances of the cations support the hydride-elongated dihydrogen nature of these compounds. The nonclassical interaction between the hydrogen atoms of the OsH₃ unit is more important in the normal isomer than in the abnormal. Treatment of 1 with 2.0 equiv of 1-(2-pyridylmethyl)-3-methylimidazolium bromide yields the bis(normal-NHC) complex [OsH{_κC²,N-[1-(2- pyridylmethyl)-3-methylimidazol-2-ylidene]}₂(P_iPr ₃)]Br (4), which has been also characterized by X-ray diffraction analysis.

Full text of DOI:10.1021/om700746h

6556
Organometallics 2007, 26, 6556–6563
Influence of the Anion of the Salt Used on the Coordination Mode
of an N-Heterocyclic Carbene Ligand to Osmium
Miguel Baya, Beatriz Eguillor, Miguel A. Esteruelas,* Montserrat Oliván,* and
Enrique Oñate
Departamento de Química Inorgánica, Instituto de Ciencia de Materiales de Aragón, UniVersidad de
Zaragoza-CSIC, 50009 Zaragoza, Spain
ReceiVed July 24, 2007
The reactions of the hexahydride OsH6(PiPr3)2 (1) with 1.0 equiv of the BPh4-, BF4-, and Br- salts
of 1-(2-pyridylmethyl)-3-methylimidazolium, in tetrahydrofuran under reflux, have been studied. In the
three cases, mixtures of the abnormal [OsH(η2-H2){κC5,N-[1-(2-pyridylmethyl)-3-methylimidazol-5-
ylidene]}(PiPr3)2]A (A ) BPh4 (2a), BF4 (2b), Br (2c)) and normal [OsH(η2-H2){κC2,N-[1-(2-
pyridylmethyl)-3-methylimidazol-2-ylidene]}(PiPr3)2]A (A ) BPh4 (3a), BF4 (3b), Br (3c)) isomers are
obtained. The formation rate of the abnormal isomer and the abnormal to normal ratio decrease as the
coordinating power of the anion of the used salt increases. Treatment of 2b with either HBF4 · OEt2 or
LiBF4 gives rise to its isomerization to 3b. The X-ray structure of 2a and T1(min) values of the OsH3
resonances of the cations support the hydride-elongated dihydrogen nature of these compounds. The
nonclassical interaction between the hydrogen atoms of the OsH3 unit is more important in the normal
isomer than in the abnormal. Treatment of 1 with 2.0 equiv of 1-(2-pyridylmethyl)-3-methylimidazolium
bromide yields the bis(normal-NHC) complex [OsH{κC2,N-[1-(2-pyridylmethyl)-3-methylimidazol-2-
ylidene]}2(PiPr3)]Br (4), which has been also characterized by X-ray diffraction analysis.
determining the formation of the less favored tautomers and
the influence of their coordination on the behavior of the other
coligands of the complexes.
The abnormal coordination is mainly determined by the steric
requirement of the substituents at the N atoms. By varying their
size, it is possible to control whether abnormal binding will
occur or not.2f For reactions of the pentahydride IrH5(PPh3)2
with 1-(2-pyridylmethyl)-3-methylimidazolium salts,6 it has been
observed that the coordination mode of the NHC ligand in the
Introduction
N-heterocyclic carbenes (NHC) are an emergent class of
versatile ancillary ligands, which are receiving increased atten-
tion due to their ability to stabilize a variety of transition-metal
complexes, some of which are very active in catalytic reactions.1
The common coordination of these types of ligands takes place
through the C-2 atom to the heterocycle. However, there are
also a few reports on NHC-metal complexes that show the
ligand bound through a backbone C-4 or C-5 carbon atom
(Chart 1).2
(2) (a) Hu, X.; Castro-Rodriguez, I.; Meyer, K. Organometallics 2003,
22, 3016. (b) Danopoulos, A. D.; Tsoureas, N.; Wright, J. A.; Light, M. E.
Organometallics 2004, 23, 166. (c) Chianese, A. R.; Kovacevic, A.; Zeglis,
B. M.; Faller, J. W.; Crabtree, R. H. Organometallics 2004, 23, 2461. (d)
Lebel, H.; Janes, M. K.; Charette, A. B.; Nolan, S. P. J. Am. Chem. Soc.
2004, 126, 5046. (e) Stylianides, N.; Danopoulos, A. A.; Tsoureas, N. J.
Organomet. Chem. 2005, 690, 5948. (f) Bacciu, D.; Cavell, K. J.; Fallis,
I. A.; Ooi, L.-L. Angew. Chem., Int. Ed. 2005, 44, 5282. (g) Campeau,
L.-C.; Thansandote, P.; Fagnou, K. Org. Lett. 2005, 7, 1857. (h) Alcarazo,
M.; Roseblade, S. J.; Cowley, A. R.; Fernández, R.; Brown, J. M.; Lassaletta,
J. M. J. Am. Chem. Soc. 2005, 127, 3290. (i) Arnold, P.; Liddle, S. T.
Organometallics 2006, 25, 1485. (j) Kluser, E.; Neels, A.; Albrecht, M.
Chem. Commun. 2006, 4495. (k) Morvan, D.; Capon, J.-F.; Gloaguen, F.;
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The free normal and abnormal NHC species are tautomers.
Theoretical calculations have predicted that the first of them is
20.0 kcal mol-1 more stable than the second one.3 This is
consistent with the scarce number of reported complexes
containing the abnormal tautomers. However, it should be taken
into account that in both biological4 and catalytic5 processes
the energetically less stable tautomer is often an active
intermediate that dictates the mechanism and the formed
products. Thus, it is of great interest to know the factors
* To whom correspondence should be addressed. E-mail: maester@
unizar.es (M.A.E.); molivan@unizar.es. (M.O.).
(1) (a) Herrmann, W. A. Angew. Chem., Int. Ed. 2002, 41, 1290. (b)
Crudden, C. M.; Allen, D. P. Coord. Chem. ReV. 2004, 248, 2247. (c)
Cavallo, L.; Correa, A.; Costabile, C.; Jacobsen, H. J. Organomet. Chem.
2005, 690, 5407. (d) Scott, N. M.; Nolan, S. P. Eur. J. Inorg. Chem. 2005,
1815. (e) Pugh, D.; Danopoulos, A. A. Coord. Chem. ReV. 2007, 251, 610.
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Douthwaite, R. E. Coord. Chem. ReV. 2007, 251, 702. (h) Gade, L. H.;
Bellemin-Laponnaz, S. Coord. Chem. ReV. 2007, 251, 718. (i) Colacino,
E.; Martinez, J.; Lamaty, F. Coord. Chem. ReV. 2007, 251, 726. (j) Dragutan,
V.; Dragutan, I.; Delaude, L.; Demonceau, A. Coord. Chem. ReV. 2007,
251, 765. (k) Mata, J. A.; Poyatos, M.; Peris, E. Coord. Chem. ReV. 2007,
251, 841. (l) Sommer, W. J.; Weck, M. Coord. Chem. ReV. 2007, 251,
860. (m) Diez-Gonzalez, S.; Nolan, S. P. Coord. Chem. ReV. 2007, 251,
874. (n) Kascatan-Nebioglu, A.; Panzner, M. J.; Tessier, C. A.; Cannon,
C. L.; Youngs, W. J. Coord. Chem. ReV. 2007, 251, 884.
(3) Sini, G.; Eisenstein, O.; Crabtree, R. H. Inorg. Chem. 2002, 41, 602.
(4) Raczyn´ska, E. D.; Kosin´ska, W.; Oówski, B.; Gawinecki, R. Chem.
ReV. 2005, 105, 3561.
(5) See for example: Lewis, J. C.; Bergman, R. G.; Ellman, J. A. J. Am.
Chem. Soc. 2007, 129, 5332.
(6) (a) Gründemann, S.; Kovacevic, A.; Albrecht, M.; Faller, J. W.;
Crabtree, R. H. Chem. Commun. 2001, 2274. (b) Kovacevic, A.; Gründe-
mann, S.; Miecznikowski, J. R.; Clot, E. Eisenstein, O.; Crabtree, R. H.
Chem. Commun. 2002, 2580. (c) Gründemann, S.; Kovacevic, A.; Albrecht,
M.; Faller, J. W.; Crabtree, R. H. J. Am. Chem. Soc. 2002, 124, 10473. (d)
Crabtree, R. H. Pure Appl. Chem. 2003, 75, 435. (e) Appelhans, L. N.;
Zuccaccia, D.; Kovacevic, A.; Chianese, A. R.; Miecznikowski, J. R.;
Macchioni, A.; Clot, E.; Eisenstein, O.; Crabtree, R. H. J. Am. Chem. Soc.
2005, 127, 16299.
10.1021/om700746h CCC: $37.00
2007 American Chemical Society
Publication on Web 11/27/2007

Coordination of an N-Heterocyclic Carbene to Os
Organometallics, Vol. 26, No. 26, 2007 6557
Chart 1
products also depends upon the counterion of the used salt. This
surprising and interesting fact certainly needs additional studies
with other systems where abnormal binding also occurs to
establish how often it takes place.
Figure 1. Molecular structure of the cation of 2a. Selected bond
lengths (Å) and angles (deg): Os-P(1) ) 2.3605(14), Os-P(2) )
2.3683(14), Os-N(3) ) 2.213(4), Os-C(1) ) 2.123(6), H(1A)-
H(1B) ) 1.44(5); P(1)-Os-P(2) ) 164.39(5), P(1)-Os-N(3) )
99.70(11), P(2)-Os-N(3) ) 93.44(11), P(1)-Os-C(1) ) 98.58(14),
P(2)-Os-C(1) ) 90.78(14), N(3)-Os-C(1) ) 85.40(18).
The complex OsH6(PiPr3)2 is a d2 polyhydride which is
obtained in high yield from the known compound OsH2Cl2-
(PiPr3)2,7 via the trihydride tetrahydrideborate intermediate
OsH3(η2-H2BH2)(PiPr3)2.8 This hexahydride reacts with Lewis
bases to afford d4 polyhydride derivatives, where the hydride
ligands undergo thermally activated site exchange processes and
show quantum exchange coupling,9 and activates C-H bonds
of amines,10 ketones,11 and aldehydes12 to give species remi-
niscent of the intermediates proposed for Murai’s reactions.13
In spite of that, Os-NHC compounds are extremely rare,14
in particular those containing abnormal tautomers, and we
have now studied the reactions of the hexahydride
OsH6(PiPr3)2 with 1-(2-pyridylmethyl)-3-methylimidazolium
salts.
NHC ligand in the reaction products, and (iii) the preparation
and X-ray structure of a novel bis(normal NHC)Os compound.
Results and Discussion
1. Preparation and Characterization of an Abnormal
Os-NHC Complex. Treatment under reflux of tetrahydrofuran
solutions of OsH6(PiPr3)2 (1) with 1.1 equiv of 1-(2-pyridyl-
methyl)-3-methylimidazolium tetraphenylborate, over 6 h, pro-
duces the release of 2.0 equiv of molecular hydrogen and the
formation of an 84:16 mixture of the abnormal- and normal-
NHC complexes 2a and 3a, according to eq 1.
In this paper, we report the following: (i) the formation and
X-ray structure of an osmium complex containing an NHC
ligand with abnormal coordination, (ii) the influence of the
counterion of the used salt on the coordination mode of the
(7) Aracama, M.; Esteruelas, M. A.; Lahoz, F. J.; López, J. A.; Meyer,
U.; Oro, L. A.; Werner, H. Inorg. Chem. 1991, 30, 288.
(8) (a) Esteruelas, M. A.; Jean, Y.; Lledós, A.; Oro, L. A.; Ruiz, N.;
Volatron, F. Inorg. Chem. 1994, 33, 3609. (b) Demachy, I.; Esteruelas,
M. A.; Jean, Y.; Lledós, A.; Maseras, F.; Oro, L. A.; Valero, C.; Volatron,
F. J. Am. Chem. Soc. 1996, 118, 8388.
(9) (a) Esteruelas, M. A.; Lahoz, F. J.; López, A. M.; Oñate, E.; Oro,
L. A.; Ruiz, N.; Sola, E.; Tolosa, J. I. Inorg. Chem. 1996, 35, 7811. (b)
Castillo, A.; Esteruelas, M. A.; Oñate, E.; Ruiz, N. J. Am. Chem. Soc. 1997,
119, 9691. (c) Castillo, A.; Barea, G.; Esteruelas, M. A.; Lahoz, F. J.; Lledós,
A.; Maseras, F.; Modrego, J.; Oñate, E.; Oro, L. A.; Ruiz, N.; Sola, E.
Inorg. Chem. 1999, 38, 1814. (d) Esteruelas, M. A.; Lledós, A.; Martín,
M.; Maseras, F.; Osés, R.; Ruiz, N.; Tomàs, J. Organometallics 2001, 20,
5297.
(1)
The major product, the abnormal complex 2a, was separated
from its normal isomer 3a as yellow crystals suitable for an
X-raydiffractionanalysis,bycrystallizationindichloromethane–di-
ethyl ether. Figure 1 shows a view of the cation of the salt. The
structure proves the abnormal coordination of the imidazolium
ring and suggests that this complex is a hydride-elongated
dihydrogen species.15
The geometry around the osmium atom can be rationalized
as a distorted pentagonal bipyramid with the phosphine ligands
occupying axial positions (P(1)-Os-P(2) ) 164.39(5)°). The
metal coordination sphere is completed by the pyridinic nitrogen
atom N(3) and the abnormal carbon atom C(1) of the chelate
group and the hydrogen atoms H(1A), H(1B), and H(1C) of
(10) (a) Barea, G.; Esteruelas, M. A.; Lledós, A.; López, A. M.; Oñate,
E.; Tolosa, J. I. Organometallics 1998, 17, 4065. (b) Barrio, P.; Esteruelas,
M. A.; Oñate, E. Organometallics 2004, 23, 3627.
(11) (a) Barrio, P.; Castarlenas, R.; Esteruelas, M. A.; Lledós, A.;
Maseras, F.; Oñate, E.; Tomàs, J. Organometallics 2001, 20, 442. (b) Barrio,
P.; Castarlenas, R.; Esteruelas, M. A.; Oñate, E. Organometallics 2001,
20, 2635.
(12) Barrio, P.; Esteruelas, M. A.; Oñate, E. Organometallics 2004, 23,
1340.
(13) Kakiuchi, F.; Murai, S. Acc. Chem. Res. 2002, 35, 826.
(14) (a) Hitchcock, P. B.; Lappert, M. F.; Pye, P. L. J. Chem. Soc.,
Dalton Trans. 1978, 826. (b) Lappert, M. F.; Pye, P. L. J. Chem. Soc.,
Dalton Trans. 1978, 837. (c) Herrmann, W. A.; Elison, M.; Fischer, J.;
Köcher, C.; Artus, G. R. J. Chem. Eur. J. 1996, 2, 772. (d) Castarlenas, R.;
Esteruelas, M. A.; Oñate, E. Organometallics 2005, 24, 4343. (e) Cabeza,
J. A.; da Silva, I.; del Río, I.; Sánchez-Vega, M. G. Dalton Trans. 2006,
3966. (f) Cooke, C. E.; Ramnial, T.; Jennings, M. C.; Pomeroy, R. K.;
Clyburne, J. A. C. Dalton Trans. 2007, 1755. (g) Castarlenas, R.; Esteruelas,
M. A.; Oñate, E. Organometallics 2007, 26, 2129. (h) Castarlenas, R.;
Esteruelas, M. A.; Oñate, E. Organometallics 2007, 26, 3082.
(15) Barrio, P.; Esteruelas, M. A.; Lledós, A.; Oñate, E.; Tomàs, J.
Organometallics 2004, 23, 3008 and references therein.

6558 Organometallics, Vol. 26, No. 26, 2007
Baya et al.
the hydride-elongated dihydrogen unit. The chelate ligand acts
with a C(1)-Os-N(3) bite angle of 85.40(18)° and forms a
six-membered ring with the metal center. As a result of the boat
conformation of the ring, the phosphines are inequivalent. Thus,
at 143 K, the 31P{1H} NMR spectrum in CDCl2F shows an
RH(1C) ) R* + RH(1B)-H(1C) + RH(1A)-H(1C)
(4)
For exchange rates higher than 100 s-1, as in this case at
263 K (vide infra), the relaxation rate is the weighted average
of the relaxation rate of each hydride (eq 5).18
AB spin system centered at 22.1 ppm and defined by JAB
)
R ) (RH(1B) + RH(1A) + RH(1C))/3
(5)
240 Hz and ∆ν ) 860 Hz. At temperatures higher than 143 K,
the interconversion of the conformers produces the equilibration
of the phosphine ligands. Thus, at 293 K in CD2Cl2, the
spectrum contains a singlet at 23.6 ppm.
Using the H-H separations obtained from the X-ray diffrac-
tion study, an R value of 16.6 s-1 is calculated by means of
eqs 2-5. This value leads to a T1(min) value of 60 ms, which
1
is in very good agreement with that determined by H NMR
The Os-C(1) bond length of 2.123(6) Å is between 0.01 and
0.05 Å longer than the Os-NHC distances found in the normal
1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) com-
plexes [(η6-p-cymene)OsCl(IPr)]+ (2.078(2) Å), [(η6-p-cymene)-
OsCl(dCHPh)(IPr))]+ (2.090(3) Å),14d [OsCl(dCHPh)(CH3CN)4-
(IPr)]+ (2.069(6) Å), [OsHCl(t CPh)(IPr)(PiPr3)]+ (2.108(2)
Å),14g and [OsCl{κ3C,N,O-[dCHC(O)pyC(CH3)O]}(NCCH3)-
(IPr)]+ (2.074(5) Å),14h and about 0.12 Å longer than the
separation between the metal and the NH tautomer of 8-meth-
ylquinoline in the complex OsCl2(η2-H2){κ-C-(HNC8H6Me)}-
(PiPr3)2 (2.005(6) Å),16 where the joint with the metal also
implies a carbon atom in an R-position with regard to only one
nitrogen atom. In agreement with the abnormal nature of C(1),
its resonance in the 13C{1H} NMR spectrum in CD2Cl2 at room
temperature appears as a triplet at 153.8 ppm, with a C-P
coupling constant of 1.2 Hz. Singlets at 133.5 and 127.9 ppm
were assigned to the carbon atoms C(4) and C(2), respectively,
spectroscopy.
The presence of only one resonance for the OsH3 unit in the
1H NMR spectrum in CD2Cl2 at room temperature is consistent
with the operation of a thermally activated site exchange process
between the hydride and the elongated dihydrogen, which
proceeds at a rate sufficient to lead to the single signal. Lowering
the sample temperature produces broadening of the resonance.
At about 248 K decoalescence occurs, and between 243 and
213 K two signals are observed at -9.25 and -14.61 ppm in
a 2:1 intensity ratio. Line-shape analyses of the 1H{31P} NMR
spectra in the high-field region between 213 and 283 K allow
the calculation of the rate constants for the hydride-elongated
dihydrogen exchange at these temperatures. The activation
parameters obtained from the corresponding Eyring analysis are
∆Hq ) 11.9 ( 0.4 kcal mol-1 and ∆Sq ) 3.8 ( 1 eu. At 203
K in both CD2Cl2 and CDCl2F, the decoalescence of the
elongated dihydrogen resonance takes place. Thus, at 163 K,
the spectrum in CDCl2F shows resonances at -8.2 (H(1A)),
-10.2 (H(1B)), and -14.6 (H(1C)) ppm (Figure 2). Line-shape
analyses of the spectra in CDCl2F between 168 and 253 K allow
the calculation of the rate constants for the exchange between
the hydrogen atoms of the elongated dihydrogen ligand at these
temperatures. In this case, the activation parameters obtained
from the corresponding Eyring analysis are ∆Hq ) 7.6 ( 0.3
kcal mol-1 and ∆Sq ) -5 ( 1 eu. The value of the activation
enthalpy lies in the range previously reported for other blocked
rotation processes in elongated dihydrogen ligands.10a,15,19
2. Influence of the Anion of the Starting Salt:
Tetraphenylborate versus Tetrafluoroborate. Figure 3 shows
the composition in metal compounds as a function of the time
of the reaction mixture resulting from the treatment of 1 with
1.0 equiv of 1-(2-pyridylmethyl)-3-methylimidazolium tetraphe-
nylborate, in tetrahydrofuran under reflux. The results clearly
show that under the reaction conditions the formation of the
salt 2a is favored from a kinetic point of view and that, once
2a is formed, its conversion into 3a does not take place in a
spontaneous manner.
1
on the basis of the HSQC spectrum. In the H NMR spectrum
the resonance of the normal C(4)-H proton appears at 5.95
ppm, in accordance with the chemical shift found in the free
salt (δ 5.37), whereas the abnormal C(2)-H resonance is
observed at 6.22 ppm.
The separation between the hydrogen atoms H(1A) and
H(1B), which are disposed transoid to C(1), of 1.44(5) Å lies
within the range of distances found in the so-called elongated
dihydrogen complexes,15 and it is about 0.14 Å shorter than
that between H(1B) and H(1C) (1.58(5) Å). The separation
between H(1A) and H(1C) is 2.65(4) Å. Although the three
hydrogen atoms are inequivalent, at room temperature, they give
rise to only one signal centered at -10.90 ppm in the high-
1
field region of the H NMR spectrum. For this resonance, a
T1(min) value of 68 ms was obtained at 263 K in the 300 MHz
scale.
The total relaxation rate for a H(n) hydride ligand (R(n) )
1/T1(min)H(n)) is the addition of the relaxation rate due to the
hydride dipole–dipole interactions (at 300 MHz RH-H ) 129.18/
rHH6) and that due to all other of the relaxation contributors
(R*).17 Thus, for these types of osmium compounds, one can
obtain values of T1(min) converted to the 300 MHz scale for
each hydride by using eqs 2-4, when one knows the separations
between the hydrides (rHH), since R* can be estimated to be
2.5 s-1, which is the average value of those found in the osmium
trihydride compounds [OsH3(diolefin)(PiPr3)2]BF4.9b
Treatment of dichloromethane solutions of the isomeric
mixture of 2a and 3a with 2.0 equiv of HBF4 · OEt2 produces
the quantitative decomposition of the tetraphenylborate anion,
(18) Desrosiers, P. J.; Cai, L.; Lin, Z.; Richards, R.; Halpern, J. J. Am.
Chem. Soc. 1991, 113, 4173.
(19) (a) Jalón, F. A.; Otero, A.; Manzano, B. R.; Villaseñor, E.; Chaudret,
B. J. Am. Chem. Soc. 1995, 117, 10123. (b) Sabo-Etienne, S.; Chaudret,
B.; el Makarim, H. A.; Barthelat, J.-C.; Daudey, J.-P.; Ulrich, S.; Limbach,
H.-H.; Moïse, C. J. Am. Chem. Soc. 1995, 117, 11602. (c) Antiñolo, A.;
Carrillo-Hermosilla, F.; Fajardo, M. J.; García-Yuste, S.; Otero, A.;
Camanyes, S.; Maseras, F.; Moreno, M.; Lledós, A.; Lluch, J. M. J. Am.
Chem. Soc. 1997, 119, 6107. (d) Sabo-Etienne, S.; Rodríguez, V.;
Donnadieu, B.; Chaudret, B.; el Makarim, H. A.; Barthelat, J.-C.; Ulrich,
S.; Limbach, H.-H.; Moïse, C. New J. Chem. 2001, 25, 55. (e) Esteruelas,
M. A.; Lledós, A.; Oliván, M.; Oñate, E.; Tajada, M. A.; Ujaque, G.
Organometallics 2003, 22, 3753. (f) Eguillor, B.; Esteruelas, M. A.; Oliván,
M.; Oñate, E. Organometallics 2004, 23, 6015. (g) Eguillor, B.; Esteruelas,
M. A.; Oliván, M.; Oñate, E. Organometallics 2005, 24, 1428. (h) Eguillor,
B.; Esteruelas, M. A.; Oliván, M. Organometallics 2006, 25, 4691.
RH(1B) ) R* + RH(1B)-H(1A) + RH(1B)-H(1C)
RH(1B) ) R* + RH(1B)-H(1A)+RH(1B)-H(1C)
(2)
(3)
(16) Esteruelas, M. A.; Fernández-Alvarez, F. J.; Oñate, E. J. Am. Chem.
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(17) Jessop, P. G.; Morris, R. H. Coord. Chem. ReV. 1992, 121, 155.

Coordination of an N-Heterocyclic Carbene to Os
Organometallics, Vol. 26, No. 26, 2007 6559
Figure 2. Variable-temperature 1H{31P} NMR spectra (400 MHz, CDCl2F) in the high-field region of [OsH(η2-H2){κC5,N-[1-(2-
pyridylmethyl)-3-methylimidazol-5-ylidene]}(PiPr3)2]BPh4 (2a): experimental (left); simulated (right).
ppm. The 31P{1H} NMR spectrum shows a behavior similar to
that of 2a. At 293 K a singlet at 19.9 ppm is observed. Lowering
the sample temperature produces a broadening of the resonance.
At 163 K in CDCl2F the decoalescence takes place, and at 143
K the spectrum contains two broad signals corresponding to an
1
AB spin system. In the H NMR spectrum in CD2Cl2 at room
temperature, the most noticeable resonance is a triplet at -10.78
ppm, with an H-P coupling constant of 13.3 Hz, corresponding
to the OsH3 unit. Lowering the sample temperature produces
the broadening of the signal. However, decoalescence does not
occur down to 143 K in CDCl2F.
A variable-temperature 300 MHz study in the temperature
range where the signal shape allows us to obtain accurate values,
273–223 K, gives a T1 value of 89 ms at 273 K, which decreases
to 55 ms at 223 K. Although this value does not correspond to
the T1(min) value of the resonance, it is lower than that of 2a,
which indicates that the nonclassical interaction is more
important in the OsH3 unit of the normal cation than in the
abnormal one. In this context, it should be noted that a normal
carbon atom is stabilized by two mesomeric nitrogens, while
an abnormal atom is stabilized by only one. Thus, an NHC
ligand should be a better σ-donor in the abnormal coordination
than in the normal coordination.21 Since the nonclassical
interaction is disfavored by back-donation from filled metal
orbitals of predominant d character to the σ* orbital of the
coordinated hydrogen molecule, it appears to be clear why the
abnormal coordination disfavors the nonclassical interaction with
regard to the normal coordination.
Figure 3. Time vs composition of the reaction mixture of 1 with
1-(2-pyridylmethyl)-3-methylimidazolium tetraphenylborate.
into benzene and triphenylborane,20 and the isomerization of
the abnormal cation to the normal one (eq 6), which is isolated
as the tetrafluoroborate salt 3b. Treatment of the latter with
sodium tetraphenylborate affords 3a as a pure yellow solid. In
tetrahydrofuran under reflux, complex 3a is stable and does not
isomerize into 2a after 6 h.
Figure 4 shows the composition in metal compounds as a
function of the time of the reaction mixture resulting from the
treatment of 1 with 1.0 equiv of 1-(2-pyridylmethyl)-3-meth-
ylimidazolium tetrafluoroborate, in tetrahydrofuran under reflux.
As is the case for the starting tetraphenylborate salt, the
formation of the abnormal BF4 salt 2b is kinetically favored
with regard to the normal BF4 salt 3b. However, in this case
(6)
The coordination of the normal carbon atom of the five-
membered ring is supported by the 13C{1H} NMR spectrum in
CD2Cl2 at room temperature, which contains the OsC resonance
at 182.0 ppm, as a triplet with a C-P coupling constant of 6.8
(21) Owen, J. S.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 2004,
126, 8247.
(20) Cooper, J. N.; Powell, R. E. J. Am. Chem. Soc. 1963, 85, 1590.

6560 Organometallics, Vol. 26, No. 26, 2007
Baya et al.
Figure 4. Time vs composition of the reaction mixture of 1 with
1-(2-pyridylmethyl)-3-methylimidazolium tetrafluoroborate.
Figure 6. Molecular structure of the cation of 4. Selected bond
lengths (Å) and angles (deg): Os-P ) 2.395(2), Os-N(5) )
2.177(6), Os-N(6) ) 2.196(7), Os-C(1) ) 2.035(8), Os-C(11)
) 1.993(9); P-Os-N(5) ) 93.09(19), P-Os-N(6) ) 107.71(19),
P-Os-C(1))163.6(2), P-Os-C(11))95.7(2), N(5)-Os-N(6)
) 89.4(2), N(5)-Os-C(1) ) 82.8(3), N(5)-Os-C(11) )
171.0(3), N(6)-Os-C(1) ) 88.2(3), N(6)-Os-C(11) ) 86.1(3),
C(1)-Os-C(11) ) 89.3(3).
Figure 5. Time vs composition of the reaction mixture of 1 with
1-(2-pyridylmethyl)-3-methylimidazolium bromide.
reflux, the formation of the normal Br salt 3c is kinetically
favored with regard to the formation of the abnormal Br salt
2c. During the reaction, traces of the previously reported
the difference between the formation rates is significantly smaller
than in the tetraphenylborate case. These results are consistent
with previous observation of Crabtree’s group for iridium,6
which related the isomers ratio to the ion pairing between the
normal CH proton of the starting salt and the counterions, as
shown by the sensitivity of the proton NMR shift of the normal
CH resonance to the nature of the anion (δBPh4, 5.37; δBF4, 8.98).
In the starting salt the normal CH bond of the imidazolium group
is weakened by a hydrogen bond with the anion. This facilitates
its addition to the metal center and, therefore, the formation of
the normal isomer. Thus, stronger ion pairings correspond to
smaller abnormal to normal rate ratios.
complexes OsH2Br2(PiPr3)2 and OsH3Br(PiPr3)2 are also
detected. Under the reaction conditions, the normal salt 3c is
unstable and evolves to the abnormal salt 2c and the bis(normal
NHC) derivative 4. The formation of 2c from 3c appears to
point out the idea of that both the relative stability of the normal
and abnormal salts and activation energy for the isomerization
depend upon the nature of the anion.
22
23
The bis(normal NHC) complex 4 can be also selectively
obtained as a pure red solid in 66% yield by treatment of 1
with 2.0 equiv of 1-(2-pyridylmethyl)-3-methylimidazolium
bromide, in tetrahydrofuran under reflux, according to eq 7.
The behavior of the mixture of 2b and 3b is similar to that
of the mixture of 2a and 3a when it is treated with HBF4 · OEt2.
In agreement with the isomerization shown in eq 6, the addition
of 1.0 equiv of this acid to a dichloromethane solution of the
mixture of 2b and 3b gives rise to the quantitative transformation
of 2b to 3b. Although this appears to support an isomerization
by acidic catalysis, it must be pointed out that the same result
is reached when the tetrahydrofuran solutions of the mixture
are treated with lithium tetrafluoroborate.
The isomerization of 2b to 3b promoted by lithium tetrafluo-
roborate appears to rule out an acidic catalysis and opens the
door to an interesting question: in solution, do the thermody-
namic stabilities of the complex salts and/or the activation
energy for the isomerization depend upon the nature of the
anions and their concentrations? The transformation of 2b into
3b promoted by both HBF4 · OEt2 in dichloromethane and
lithium tetrafluoroborate in tetrahydrofuran certainly suggests
(7)
Figure 6 shows a view of the cation of 4. The coordination
geometry around the osmium atom can be rationalized as a
distorted octahedron with the phosphorus atom of the phosphine
and the normal C(1) carbon atom of one of the chelating ligands
occupying mutually trans positions (C(1)-Os-P ) 163.6(2)°).
-
a favorable effect of the BF4 anion on the normal cation.
5. Reaction with the Bromide Salt: Formation of a
Bis(normal NHC) Complex. The reaction of 1 with 1.0 equiv
of 1-(2-pyridylmethyl)-3-methylimidazolium bromide is much
more complex than those with the starting BPh4 and BF4 salts
(Figure 5). In agreement with an ion pairing between the normal
CH proton and the anion stronger in the starting Br salt than in
those previously studied (δBr, 10.49), in tetrahydrofuran under
(22) (a) δOs-H, -14.86 (t, JP-H ) 33.3 Hz); δPiPr3, 45.9. (b) Gusev, D. G.;
Kuhlman, R.; Rambo, J. R.; Berke, H.; Eisenstein, O.; Caulton, K. G. J. Am.
Chem. Soc. 1995, 117, 281.
(23) (a) δOs-H, -19.80 (br); δPiPr3, 23.2 (223 K). (b) Gusev, D. G.;
Kuhlman, R.; Sini, G.; Eisenstein, O.; Caulton, K. G. J. Am. Chem. Soc.
1994, 116, 2685.

Coordination of an N-Heterocyclic Carbene to Os
Organometallics, Vol. 26, No. 26, 2007 6561
lium
bromide,25
1-(2-pyridylmethyl)-3-methylimidazolium
The nitrogen atom N(5) of this chelated group is disposed trans
to the normal carbon atom C(11) of the other chelated ligand
(N(5)-Os-C(11) ) 171.0(3)°). The bite angles C(1)-Os-N(5)
and C(11)-Os-N(6) are 82.8(3) and 86.1(3)°, respectively. In
spite of the asymmetric coordination of the NHC ligands, the
Os-C bond lengths of 1.993(9) (Os-C(11)) and 2.035(8) Å
(Os-C(1)) are statistically identical and about 0.1 Å shorter
than that of 2a.
tetrafluoroborate,6c and CDCl2F26 were prepared as previously
described.
Preparation of 1-(2-Pyridylmethyl)-3-methylimidazolium Tet-
raphenylborate. Anion Exchange. A solution of 1-(2-pyridylm-
ethyl)-3-methylimidazolium bromide (200 mg, 0.79 mmol) in
acetone/MeOH (15 mL/5 mL) was treated with the stoichiometric
amount of NaBPh4 (269.3 mg, 0.79 mmol), and the resulting
solution was stirred for 2 h at room temperature. After this time
the solvent was removed and CH2Cl2 added, resulting in a
suspension. The suspension was filtered through Celite and the
solution concentrated to ca. 0.5 mL and diethyl ether added to afford
a white solid that was washed with further portions of diethyl ether
The asymmetry of the coordinated carbon atoms is revealed
by the 13C{1H} NMR spectrum in CD2Cl2 at room temperature.
The resonance due to C(1) appears at 177.4 ppm as a doublet
with a C-P coupling constant of 78 Hz, while that correspond-
1
1
ing to C(11) is observed at 172.5 ppm as a singlet. In the H
(3 × 5 mL) and dried in vacuo. Yield: 330 mg (80%). H NMR
NMR spectrum, the most noticeable feature is the hydride
resonance, which appears at –16.17 ppm as a doublet with a
H-P coupling constant of 20.8 Hz. A singlet at 17.8 ppm in
the 31P{1H} NMR spectrum is also characteristic of 4.
(400 MHz, CD2Cl2, 293 K): δ 8.50 (d, JH-H ) 5.0, 1H, Hpy), 7.71
(t, JH-H ) 7.6, 1H, Hpy), 7.46 (m, 8H, o-BPh4), 7.29 (dd, JH-H
)
7.6, JH-H ) 5.0, 1H, Hpy), 7.15 (d, JH-H ) 7.6, 1H, Hpy), 6.97 (t,
JH-H ) 7.2, 8H, m-BPh4), 6.84 (d, JH-H ) 1.6, 1H, imidazole),
6.80 (t, JH-H ) 7.2, 4H, p-BPh4), 6.54 (d, JH-H ) 1.6, 1H,
imidazole), 5.37 (s, 1H, NCHN), 4.69 (s, 2H, -CH2-), 3.15 (s,
3H, -CH3). 13C{1H} NMR (100.56 MHz, CD2Cl2, 293 K, plus
apt): δ 164.5 (q, JC-B ) 65.6, Cipso BPh4), 152.2 (s, Cpy), 150.3 (s,
Cpy), 137.8 (s, Cpy), 136.2 (s, NCHN), 136.1 (o-BPh4), 126.4 (q,
JC-B ) 3, m-BPh4), 124.3 (s, Cpy), 122.93 (s, CH imidazole), 122.7
(s, CH imidazole), 122.6 (s, Cpy), 122.5 (s, p-BPh4), 54.1 (s,
-CH2-), 36.4 (s, -CH3).
Concluding Remarks. As a new chapter of our work on the
Os-C bond chemistry,24 in this paper we describe the prepara-
tion and X-ray structure of an osmium complex containing an
NHC ligand with abnormal coordination. This novel OsH3
compound and its normal isomer are obtained from the reactions
of the hexahydride OsH6(PiPr3)2 with 1-(2-pyridylmethyl)-3-
methylimidazolium salts. In agreement with Crabtree’s group
we note that, in fact, the anion of the starting salt has a marked
influence on the obtained isomer ratio.
Reaction of 1 with 1-(2-Pyridylmethyl)-3-methylimidazo-
lium Tetraphenylborate. 1-(2-Pyridylmethyl)-3-methylimidazo-
lium tetraphenylborate (120.3 mg, 0.231mmol) was added to a
solution of 1 (100 mg, 0.193 mmol) in THF (15 mL) and refluxed
for 6 h. The progress of the reaction was monitored by 31P{1H}
NMR spectroscopy, which showed quantitative conversion to a 84:
16 mixture of isomers 2a and 3a (vide infra). After this time, the
solution was filtered through Celite and was taken to dryness.
Subsequent addition of diethyl ether caused the precipitation of a
yellow solid, which was washed with diethyl ether and dried in
As has been previously proposed, the anion of the starting
salt interacts with the normal CH proton.6 As a consequence of
the resulting hydrogen bond, the normal CH bond is weakened.
This facilitates its addition to the metal center, and therefore
the formation rate of the normal isomer increases as the
coordination power of the anion of the starting salt also increases
-
(Br- > BF4 > BPh4-).
vacuo. Yield: 146 mg (63%). The H and 31P{1H} NMR spectra
of this solid in CD2Cl2 showed that the ratio 84:16 is maintained.
Anal. Calcd for C52H76BN3OsP2: C, 62.07; H, 7.61; N, 4.18. Found:
1
The relative stability of the abnormal and normal salts is
-
difficult to rationalize. It appears that BF4 favors the normal
-
coordination, while with BPh4 both salts can be prepared as
pure solids and none of them isomerizes in solution into the
other. The anion Br- favors the decomposition of the mono-
(normal NHC) salt to afford a bis(normal NHC) derivative of
osmium.
C, 62.19; H, 7.70; N, 4.14. Complex 2a could be isolated in pure
form by slow diffusion of diethyl ether into a concentrated solution
of the isomeric mixture in dichloromethane at –30 °C.
Spectroscopic data of 2a: IR (cm-1): ν(OsH) 2022 (w), ν(NdC)
1571 (w). 1H NMR (400 MHz, CD2Cl2, 293 K): δ 9.61 (d, JH-H
)
In conclusion, these results show that both the rates of the
organometallic reactions and the stability of the formed products
can have a marked dependence upon the nature of the anions
of the used salts.
7.0, 1H, Hpy), 7.67 (t, JH-H ) 7.0, 1H, Hpy), 7.48 (br, 8H, o-BPh4),
7.10 (d, JH-H ) 7.0, 1H, Hpy), 7.00 (t, JH-H ) 7.2, 8H, m-BPh4),
6.93 (t, JH-H ) 7.0, 1H, Hpy) 6.79 (t, JH-H ) 7.2, 4H, p-BPh4),
6.22 (s, 1H, Him), 5.95 (s, 1H, Him), 4.23 (s, 2H, -CH2-), 3.32 (s,
3H, -CH3), 1.87 (m, 6H, PCH(CH3)2), 0.93 (dvt, N ) 12.4, JH-H
) 6.8, 18H, PCH(CH3)2), 0.87 (dvt, N ) 12.8, JH-H ) 6.8, 18H,
PCH(CH3)2), -10.91 (br, 3H, OsH). 1H NMR (400 MHz, CDFCl2,
168 K, hydride region): δ -8.19 (br, 1H, OsH), –10.24 (br, 1H,
OsH), -14.55 (br, 1H, OsH). 31P{1H} NMR (161.9 MHz, CD2Cl2,
293 K): δ 23.6 (s). 31P{1H} NMR (161.9 MHz, CDFCl2, 143 K):
δ 22.1 (AB spin system, ∆ν ) 860 Hz, JA-B ) 240 Hz). 13C{1H}
Experimental Section
All reactions were carried out under an argon atmosphere using
Schlenk tube techniques. Solvents were obtained oxygen- and water-
free from an MBraun solvent purification apparatus. Infrared spectra
were recorded on a Perkin-Elmer Spectrum One spectrometer as
solids (Nujol mull). 1H, 13C{1H}, and 31P{1H} NMR spectra were
recorded on a Varian Gemini 2000, a Bruker AXR 300, or a Bruker
Avance 400 instrument. Chemical shifts are referenced to residual
solvent peaks (1H and 13C{1H}), external H3PO4 (31P{1H}),
NMR (100.56 MHz, CD2Cl2, 293 K, plus apt): δ 164.5 (q, JC-B
)
49.2, Cipso BPh4), 164.5 (s, Cpy), 153.8 (t, JC-P ) 1.2, Ccarbene), 137.5
(s, Cpy), 136.2 (q, JC-B ) 1.3, o-BPh4) 133.5 (s, Cim), 127.9 (s,
Cim), 126.7 (s, Cpy), 126.3 (q, JC-B ) 2.8, m-BPh4), 124.7 (s, Cpy),
122.3 (s, p-BPh4), 55.9 (s, -CH2-), 34.8 (s, -CH3), 26.9 (vt, N )
24.3, PCH(CH3)2), 20.1, 19.6 (both s, PCH(CH3)2). T1(min) (ms,
OsH, 300 MHz, CD2Cl2, 263 K): 68 ( 1 (-10.90 ppm).
1
Coupling constants J and N (N ) JP-H + JP-H for H; N ) JP′-C
+ JP′-C for 13C{1H}), are given in hertz. C, H, and N analyses
were measured on a Perkin-Elmer 2400 CHNS/O analyzer. The
complex OsH6(PiPr3)2 (1),7 1-(2-pyridylmethyl)-3-methylimidazo-
Spectroscopic data of 3a: IR (cm-1): ν(OsH) 2045 (w), ν(NdC)
1580 (w). 1H NMR (400 MHz, CD2Cl2, 293 K): δ 9.57 (d, JH-H
)
(24) (a) Esteruelas, M. A.; López, A. M. Organometallics 2005, 24,
3584. (b) Esteruelas, M. A.; López, A. M.; Oliván, M. Coord. Chem. ReV.
2007, 251, 795.
(25) McGuiness, D. S.; Cavell, K. J. Organometallics 2000, 19, 741.
(26) Siegel, J. S.; Anet, F. A. L. J. Org. Chem. 1988, 53, 2629.

6562 Organometallics, Vol. 26, No. 26, 2007
Baya et al.
Table 1. Crystal Data and Data Collection and Refinement Details
for 2a and 4
7.3, 1H, Hpy), 7.70 (t, JH-H ) 7.3, 1H, Hpy), 7.37 (m, 8H, o-BPh4),
7.14 (d, JH-H ) 7.3, 1H, Hpy), 7.06 (t, JH-H ) 7.2, 5H, m-BPh4,
and Him), 7.02 (t, JH-H ) 7.2, 8H, m-BPh4), 6.94 (d, JH-H ) 2.0,
1H, Him), 4.82 (s, 2H, -CH2-), 3.76 (s, 3H, -CH3), 1.99 (m, 6H,
PCH(CH3)2), 0.94 (dvt, N ) 12.8, JH-H ) 7.2, 18H, PCH(CH3)2),
0.86 (dvt, N ) 12.8, JH-H ) 6.8, 18H, PCH(CH3)2), -10.78 (t,
JH-P ) 13.2, 3H, OsH). 31P{1H} NMR (121.42 MHz, CD2Cl2, 293
K): δ 19.9 (s). 13C{1H} NMR (75.42 MHz, CD2Cl2, 293 K, plus
apt): δ 182.0 (t, JC-P ) 6.9, Ccarbene), 164.3 (q, JC-B ) 49.2, Cipso
BPh4), 164.3 (s, Cpy), 154.1 (t, Cpy), 138.2 (s, Cpy),136.2 (br s,
o-BPh4) 126.7 (s, Cpy), 126.1 (q, JC-B ) 2.7, m-BPh4), 123.1 (s,
Cim), 123.0 (s, Cim), 56.7 (s, -CH2--), 39.4(s, --CH3), 28.6 (vt,
N ) 24.9, PCH(CH3)2), 19.5, 19.4, (both s, PCH(CH3)2). T1
(ms, OsH, 300 MHz, CD2Cl2, 223 K): 55 ( 1 (-10.75 ppm).
Treatment of the Mixture of 2a and 3a with HBF4. Two
equivalents of HBF4 · OEt2 (55 µL, 0.397 mmol) was added to a
solution of a mixture of 2a and 3a (200 mg, 0.198 mmol) in CH2Cl2.
After the solution was stirred for 30 min at room temperature, the
color had changed from yellow to brown. The resulting solution
was filtered through Celite and was taken to dryness. Addition of
diethyl ether caused the precipitation of a yellow solid, which was
washed with diethyl ether and dried in vacuo. Yield: 125 mg (81%).
The 1H and 31P{1H} NMR spectra of this solid in CD2Cl2 showed
only peaks assigned to 3b.
2a
4
Crystal Data
C52H76BN3OsP2
1006.11
formula
mol wt
color, habit
size, mm
sym, space group
a, Å
C30H46BrCl2N6OsP
862.71
pale yellow, plate
0.10,0.10,0.03
red, irregular block
0.36, 0.06, 0.02
j
j
triclinic, P1
triclinic, P1
11.4772(18)
13.186(2)
16.507(3)
84.376(3)
82.862(3)
88.430(3)
2466.5(7)
2
8.664(3)
11.597(4)
16.999(5)
89.303(5)
86.032(6)
86.220(6)
1700.2(9)
2
b, Å
c, Å
R, deg
ꢀ, deg
γ, deg
V, Å3
Z
Dcalcd, g cm-3
1.355
1.685
Data Collection and Refinement
Bruker Smart APEX
0.710 73
diffractometer
λ(Mo KR), Å
monochromator
scan type
graphite oriented
ω scans
2.686
µ, mm-1
5.159
2θ range, deg
temp, K
3-58
100.0(2)
30 683
3-57
105.0(1)
20 937
no. of data collected
no. of unique data
no. of params/restraints 568/3
11 759 (Rint ) 0.0846) 8037 (Rint ) 0.0882)
Reaction of 1 with 1-(2-Pyridylmethyl)-3-methylimidazo-
lium Tetrafluoroborate. 1-(2-Pyridylmethyl)-3-methylimidazolium
tetrafluoroborate (50.2 mg, 0.193 mmol) was added to a solution
of 1 (100 mg, 0.193 mmol) in THF (15 mL) and refluxed for 5 h.
The solution changed from pale to deep yellow. After the mixture
was cooled to room temperature, the solvent was evaporated.
Subsequent addition of diethyl ether caused the precipitation of a
beige solid, which was washed with diethyl ether and dried in vacuo.
382/0
R1a (F2 > 2σ(F2))
wR2b (all data)
Sc (all data)
0.0481
0.0806
0.752
0.0620
0.1369
0.995
2
a R1(F)
)
∑||Fo|
.
-
|Fc||/∑|Fo|. b wR2(F2)
2 2
)
{∑[w(Fo
-
Fc ) ]/∑[wFo2)2]}1/2
2 2
c GOF ) S ) {∑[Fo - Fc ) ]/(n - p)}1/2, where
n is the number of reflections and p is the number of refined parameters.
2
1
Yield: 95 mg (42%). The H and 31P{1H} NMR spectra of this
Reaction of 1 with 1 Equiv of 1-(2-Pyridylmethyl)-3-meth-
ylimidazolium Bromide. 1-(2-Pyridylmethyl)-3-methylimidazolium
bromide (49 mg, 0.193 mmol) was added to a solution of 1 (100
mg, 0.193 mmol) in THF (15 mL) and heated at the reflux
temperature. At regular intervals (30 min) a portion (1 mL) of the
resulting mixture was transferred via cannula to a Schlenk flask
and it was evaporated to dryness. A 0.5 mL portion of dichlo-
romethane-d2 was added and the solution transferred to an NMR
tube. The progress of the reaction was periodically (30 min)
solid in CD2Cl2 showed a 56:44 mixture of complexes 2b and 3b.
Anal. Calcd for C28H56BF4N3OsP2: C, 43.46; H, 7.29; N, 5.43.
Found: C, 43.31; H, 7.43; N, 5.03.
Spectroscopic data of 2b: 1H NMR (500 MHz, CD2Cl2, 293 K):
δ 9.67 (d, JH-H ) 7.5, 1H, Hpy), 8.74 (s, 1H, Him), 7.74 (t, JH-H
)
7.5, 1H, Hpy), 7.62 (d, JH-H ) 7.5, 1H, Hpy), 6.97 (t, JH-H ) 7.5,
1H, Hpy), 6.47 (s, 1H, Him), 5.22 (s, 2H, -CH2-), 3.75 (s, 3H,
-CH3), 1.94 (m, 6H, PCH(CH3)2), 0.98 (dvt, N ) 12.5, JH-H
)
1
monitored by H and 31P{1H} NMR spectroscopy.
6.5, 18H, PCH(CH3)2), 0.92 (dvt, N ) 13, JH-H ) 6.0, 18H,
PCH(CH3)2), -10.60 (very br, 3H, OsH). 31P{1H} NMR (202.34
MHz, CD2Cl2, 293 K): δ 23.6 (s).
Spectroscopic data of 2c: 1H NMR (400 MHz, CD2Cl2, 293 K):
δ 9.50 (d, JH-H ) 6, 1H, Hpy), 8.46 (t, JH-H ) 6, 1H, Hpy), 8.20 (d,
JH-H ) 1.6, 1H, Him), 7.75 (t, JH-H ) 6, 1H, Hpy), 7.62 (t, JH-H
)
Spectroscopic data of 3b: 1H NMR (300 MHz, CD2Cl2, 293 K):
δ 9.61 (d, JH-H ) 6.5, 1H, Hpy), 7.82 (t, JH-H ) 6.5, 1H, Hpy),
7.63 (d, JH-H ) 6.5, 1H, Hpy), 7.40 (d, JH-H ) 1.8, 1H, Him), 7.13
(d, JH-H ) 1.8, 1H, Him), 7.06 (t, JH-H ) 6.5, 1H, Hpy), 5.26 (s,
2H, -CH2-), 3.81 (s, 3H, -CH3) 2.02 (m, 6H, PCH(CH3)2), 0.95
(dvt, N ) 12.6, JH-H ) 6.9, 18H, PCH(CH3)2), 0.87 (dvt, N )
12.9, JH-H ) 6.9, 18H, PCH(CH3)2), -10.78 (t, JH-P ) 13.3, 3H,
OsH). 31P{1H} NMR (121.42 MHz, CD2Cl2, 293 K): δ 19.9 (s).
13C{1H} NMR (75.42 MHz, CD2Cl2, 293 K, plus apt): δ 182.0 (t,
JC-P ) 6.8, NCHC), 164.4 (s, Cpy), 154.5 (t, JC-P ) 1.2 Cpy), 138.5
(s, Cpy), 126.9 (s, Cpy), 123.4 (s, Cim), 123.2 (s, Cim), 56.7 (s,
-CH2-), 39.5 (s, -CH3), 28.7 (vt, N ) 24.9, PCH(CH3)2), 19.7,
19.5 (both s, PCH(CH3)2).
6, 1H, Hpy), 7.04 (d, JH-H ) 1.6, 1H, Him), 5.07 (s, 2H, -CH2-),
3.73 (s, 3H, -CH3), 1.94 (m, 6H, PCH(CH3)2), 0.96 (dvt, N )
12.8, JH-H ) 6.8, 18H, PCH(CH3)2), 0.89 (dvt, N ) 12.8, JH--H
)
6.4, 18H, PCH(CH3)2), -12.50 (br, 3H, OsH). 31P{1H} NMR
(161.9 MHz, CD2Cl2, 293 K): δ 23.6 (s).
Spectroscopic data of 3c: 1H NMR (300 MHz, CD2Cl2, 293 K):
δ 9.59 (d, JH-H ) 6.5, 1H, Hpy), 7.97 (d, JH--H ) 6.5, 1H, Hpy),
7.86 (t, JH-H ) 6.5, 1H, Hpy), 7.85 (s, 1H, Him), 7.14 (s, 1H, Him),
7.06 (t, JH-H ) 6.5, 1H, Hpy), 5.63 (s, 2H, -CH2-), 3.82 (s, 3H,
-CH3) 2.03 (m, 6H, PCH(CH3)2), 0.96 (dvt, N ) 12.9, JH-H
)
6.9, 18H, PCH(CH3)2), 0.86 (dvt, N ) 12.9, JH-H ) 6.6, 18H,
PCH(CH3)2), -10.79 (t, JH-P ) 13.2, 3H, OsH). 31P{1H} NMR
(121.42 MHz, CD2Cl2, 293 K): δ 19.9 (s).
Treatment of the Mixture of 2b and 3b with LiBF4. One
equivalent of LiBF4 (12.1 mg, 0.129 mmol) was added to a solution
of a mixture of 2b and 3b (100 mg, 0.129 mmol) in THF (10 mL).
After the solution was stirred for 30 min at room temperature, the
resulting solution was filtered through Celite and was taken to
dryness. Addition of diethyl ether caused the precipitation of a
yellow solid, which was washed with diethyl ether and dried in
vacuo. Yield: 85 mg (85%). The 1H and 31P{1H} NMR spectra of
this solid in CD2Cl2 showed only peaks assigned to 3b.
Reaction of 1 with 2 Equiv of 1-(2-Pyridylmethyl)-3-meth-
ylimidazolium Bromide: Formation of the Complex [OsH{KC2,N-
[1-(2-pyridylmethyl)-3-methylimidazol-2-ylidene]}2(PiPr3)]Br
(4). 1-(2-Pyridylmethyl)-3-methylimidazolium bromide (147.6 mg,
0.581 mmol) was added to a solution of 1 (150 mg, 0.289 mmol)
in THF (20 mL) and refluxed for 5 h. The solution changed from
colorless to deep red. When the solution was cooled to room
temperature, a red solid appeared. The solid was separated by
decantation, washed repeatedly with diethyl ether, and dried in

Coordination of an N-Heterocyclic Carbene to Os
Organometallics, Vol. 26, No. 26, 2007 6563
vacuo. Yield: 148 mg (66%). Anal. Calcd for C29H44BrN6OsP: C,
44.78; H, 5.70; N, 10.80. Found: C, 44.98; H, 5.87; N, 10.88. IR
(Nujol, cm-1): ν(OsH) 2018 (w), ν(NdC) 1594 (w), 1568 (w). 1H
NMR (400 MHz, CD2Cl2, 293 K): δ 8.93 (d, JH-H ) 6.8, 1H, Hpy),
7.77 (t, JH-H ) 6.8, 1H, Hpy), 7.66–7.63 (m, 2H, Hpy), 7.58 (d,
JH-H ) 6.5, 1H, Hpy), 7.29 (d, JH-H ) 6.5, 1H, Hpy), 7.26 (d, JH-H
) 2, 1H, Him), 7.22 (d, JH-H ) 2, 1H, Him), 7.04 (t, JH-H ) 6.8,
1H, Hpy), 6.88 (d, JH-H ) 2, 1H, Him), 6.69 (t, JH-H ) 6.5, 1H,
Hpy), 6.57 (d, JH-H ) 2, 1H, Him), 5.31 (AB spin system, ∆ν )
79.8 Hz, JAB ) 13.6), 5.44 (d, JH-H ) 2.4, 2H, -CH2-), 3.21 (s,
3H, -CH3), 3.21 (s, 3H, -CH3), 2.10 (m, 3H, PCH(CH3)2), 1.68
least squares on F2 with SHELXL97,28 was similar for both
complexes, including isotropic and subsequently anisotropic dis-
placement parameters. The high quality and extended range of
diffraction data allowed location of the hydride ligands of 2a and
4 in the difference Fourier maps. However, for complex 2a we
observed short Os-H distances due to the well-known behavior
of the X-ray experiments that usually show M-H distances shorter
than those based on neutron diffraction, a radiation much more
appropriate for the precise localization of lighter elements. Then a
restrained geometry was used in the last cycles of refinement for
these ligands. Hydrogen atoms were included in calculated positions
and refined riding on their respective carbon atoms with the thermal
parameter related to the bonded atoms. All the highest electronic
residuals were observed in the close proximity of the Os centers
and make no chemical sense. Crystal data and details of the data
collection and refinement are given in Table 1.
(s, 3H, -CH3), 0.89 (m, 18H, PCH(CH3)2), -16.17 (d, JH-P
)
20.8, 1H, OsH). 31P{1H} NMR (161.9 MHz, CD2Cl2, 293 K): δ
17.8 (s). 13C{1H} NMR (100.56 MHz, CD2Cl2, 293K, plus apt): δ
177.4 (d, JC-P ) 78.0, NCN), 172.5 (s, NCN), 161.7, 159.6, 157.8,
155.8, 135.7, 133.9, 125.2, 125.1, 125.0 (all s, Cpy), 121.4, 120.6,
120.2, 120.1 (all s, Cim), 59.3 (s, -CH2-), 58.3 (s, -CH2-), 37.7
(s, -CH3), 33.5 (s, -CH3), 30.6 (d, JC-P ) 19.0, PCH(CH3)2), 20.8,
20.6 (both s, PCH(CH3)2).
Structural Analysis of Complexes 2a and 4. Crystals of 2a
suitable for X-ray diffraction were obtained by slow diffusion of
diethyl ether into a concentrated solution of the isomeric mixture
of complexes 2a and 3a in dichloromethane and of pentane into a
solution of 4 in dichloromethane. X-ray data for both complexes
were collected on a Bruker Smart APEX CCD diffractometer
equipped with a normal-focus, 2.4 kW sealed tube source (Mo
radiation, λ ) 0.710 73 Å) operating at 50 kV and 40 mA. Data
were collected over the complete sphere by a combination of four
sets. Each frame exposure time was 10 s covering 0.3° in ω. Data
were corrected for absorption by using a multiscan method applied
with the SADABS program.27 The structures of both compounds
were solved by the Patterson method. Refinement, by full-matrix
Acknowledgment. Financial support from the Spanish
MEC (Projects CTQ2005-00656 and Consolider Ingenio
2010 CSD2007-00006) and the Diputación General de
Aragón (E35) is gratefully acknowledged. B.E. thanks the
Spanish MEC for her grant. M.B. thanks the Spanish MEC/
Universidad de Zaragoza for funding through the “Ramón y
Cajal” program.
Supporting Information Available: CIF files giving positional
and displacement parameters, crystallographic data, and bond
lengths and angles of compounds 2a and 4. This material is available
free of charge via the Internet at http://pubs.acs.org.
OM700746H
(28) SHELXTL Package V. 6.10; Bruker-AXS, Madison, WI, 2000.
Sheldrick, G. M. SHELXS-86 and SHELXL-97; University of Göttingen,
Göttingen, Germany, 1997.
(27) Blessing, R. H. Acta Crystallogr. 1995, A51, 33. SADABS: Area-
Detector Absorption Correction; Bruker-AXS, Madison, WI, 1996.