Cationic dihydrogen/dihydride complexes of osmium: Structure and dynamics

Article abstract of DOI:10.1021/om0700718

Reaction of Cp*Os(CO)₂Cl with [Et₃Si][BAr ₄^F] under hydrogen gas affords the cationic hydrogen complex [Cp*Os(CO)₂(H₂)][BAr₄ ^F] (1) (Cp* = C₅Me₅; Ar^F = C₆F₅). When this reaction is carried out with HD gas, complex 1-d₁ results, with J_HD -24.5 Hz. When solutions of complex 1 are monitored by ¹H NMR spectroscopy over several days, the gradual formation of a trans dihydride species is observed. Similarly, reaction of CpOs(dppm)Br with Na[BAr^F*₄] (Ar ^F* = 3,5-(CF₃)₂C₆H ₃) under hydrogen affords the cationic dihydride complex [CpOs(dppm)H₂][BAr^F*₄] (2). At 295 K, complex 2 exists as a 10:1 mixture of cis and trans isomers. The ¹H NMR spectrum of the cis form in the hydride region exhibits a triplet with JHP = 6.5 Hz, due to rapid exchange of the hydrogen atoms. At low temperature, static spectra of the HH′PP′ spin system can be obtained, revealing quantum mechanical exchange coupling between the two hydride ligands. The observed J_HH′ is temperature dependent, varying from 133 Hz at 141 K to 176 Hz at 198 K. This is the first report of detectable exchange coupling between pairs of chemically equivalent hydrogen atoms.

Full text of DOI:10.1021/om0700718

Organometallics 2007, 26, 2291-2295
2291
Cationic Dihydrogen/Dihydride Complexes of
Osmium: Structure and Dynamics
Jonathan D. Egbert,† R. Morris Bullock,‡ and D. Michael Heinekey*,†
Department of Chemistry, UniVersity of Washington, Box 351700, Seattle, Washington 98195, Chemistry
Department, BrookhaVen National Laboratory, Upton, New York 11973, and Chemical Sciences DiVision,
Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352
ReceiVed January 23, 2007
Reaction of Cp*Os(CO)2Cl with [Et3Si][BArF ] under hydrogen gas affords the cationic hydrogen
4
complex [Cp*Os(CO)2(H2)][BArF ] (1) (Cp* ) C5Me5; ArF ) C6F5). When this reaction is carried out
4
with HD gas, complex 1-d1 results, with JHD ) 24.5 Hz. When solutions of complex 1 are monitored by
1H NMR spectroscopy over several days, the gradual formation of a trans dihydride species is observed.
Similarly, reaction of CpOs(dppm)Br with Na[BArF*4] (ArF* ) 3,5-(CF3)2C6H3) under hydrogen affords
the cationic dihydride complex [CpOs(dppm)H2][BArF*4] (2). At 295 K, complex 2 exists as a 10:1
1
mixture of cis and trans isomers. The H NMR spectrum of the cis form in the hydride region exhibits
a triplet with JHP ) 6.5 Hz, due to rapid exchange of the hydrogen atoms. At low temperature, static
spectra of the HH′PP′ spin system can be obtained, revealing quantum mechanical exchange coupling
between the two hydride ligands. The observed JHH′ is temperature dependent, varying from 133 Hz at
141 K to 176 Hz at 198 K. This is the first report of detectable exchange coupling between pairs of
chemically equivalent hydrogen atoms.
Following the seminal work of Kubas and co-workers in
1984,1 the coordination chemistry of dihydrogen developed very
rapidly into an active field of study.2 Bound dihydrogen exhibits
great variability in structure and reactivity, along with interesting
dynamic behavior. The key structural parameter is the distance
between the two hydrogen atoms, designated as dHH. Complexes
with dHH < 1.0 Å are usually considered to be complexes of
dihydrogen, while complexes with dHH > 1.5 Å are regarded
as conventional dihydride complexes. Molecules with intermedi-
ate dHH values have been variously termed elongated dihydrogen
complexes or compressed dihydride complexes. There are now
several examples of such complexes exhibiting novel structures
and dynamic behavior, including temperature-dependent struc-
tures.3 Particularly interesting is the cationic Ru complex
[Cp*Ru(dppm)(H2)]+, in which the unusual isotope-dependent
structure4 is attributed to a very soft, anharmonic potential
energy surface for the Ru-H and H-H interactions.5 This Ru
complex is one representative of an extensive set of cationic
complexes of the form [Cp/Cp*RuLL′(H2)]+, which exhibit a
wide range of dHH values. The observed dHH seems to depend
upon the steric and electronic contributions of L and L′.6 Many
of the Ru complexes also exhibit equilibria between dihydrogen
and trans dihydride structures (below). In spite of extensive
studies, it remains very difficult to predict which of several
possible structures of similar energy will be adopted by these
molecules.
Preparation of Os analogues of these Ru complexes has been
less explored. In general, the dihydride structures are more
accessible for Os. There is limited evidence for bound dihy-
drogen in this type of osmium complex. Reaction of Cp*Os-
(CO)2H with triflic acid was reported to afford a cationic species
initially thought to be a dihydride.7 Subsequently, studies of
relaxation times were interpreted in terms of an equilibrium
between dihydrogen and dihydride forms.8
Similarly, Jia and co-workers have reported the preparation
and properties of [CpOs(dppm)H2]+ (dppm ) bis-diphenylphos-
phinomethane).9 In this complex, both cis and trans dihydride
isomers were observed, with the cis form predominanting. The
suggestion of a cis dihydride structure is novel, in that such a
structure has never been observed for Ru. This structural
* Corresponding author. E-mail:
edu.
† University of Washington.
heinekey@chem.washington.
‡ Brookhaven National Laboratory and Pacific Northwest National
Laboratory. Current address: Pacific Northwest National Laboratory.
(1) Kubas, G. J.; Ryan, R. R.; Swanson, B. I.; Vergamini, P. J.;
Wasserman, H. J. J. Am. Chem. Soc. 1984, 106, 451-452.
(2) Kubas, G. J. Metal Dihydrogen and σ-Bond Complexes: Structure,
Theory and ReactiVity; Kluwer Academic/Plenum Publishers: New York,
2001.
(6) (a) Jessop, P. G.; Morris, R. H. Coord. Chem. ReV. 1992, 121, 155-
284. (b) Heinekey, D. M.; Oldham, W. J., Jr. Chem. ReV. 1993, 93, 913-
926.
(3) Heinekey, D. M.; Lledo´s, A.; Lluch, J. M. Chem. Soc. ReV. 2004,
(7) Bullock, R. M.; Song, J.-S. J. Am. Chem. Soc. 1994, 116, 8602-
33, 175-182.
8612.
(4) Law, J. K.; Mellows, H.; Heinekey, D. M. J. Am. Chem. Soc. 2002,
124, 1024-1030.
(5) Gelabert, R.; Moreno, M.; Lluch, J. M.; Lledo´s, A. J. Am. Chem.
Soc. 1997, 119, 9840-9847.
(8) Bullock, R. M.; Song, J.-S.; Szalda, D. J. Organometallics 1996, 15,
2504-2516.
(9) Jia, G.; Ng, W. S.; Yao, J.; Lau, C.-P.; Chen, Y. Organometallics
1996, 15, 5039-5045.
10.1021/om0700718 CCC: $37.00 © 2007 American Chemical Society
Publication on Web 03/22/2007

2292 Organometallics, Vol. 26, No. 9, 2007
Egbert et al.
proposal was based on the relatively long value of the minimum
spin lattice relaxation time T1(min) and the low observed value
2
of JHD in the HD derivative (1JHD ) 3 Hz). Averaging of JHP
couplings at the lowest reported temperature suggests that there
is a rapid dynamic process that interchanges the two hydrogen
atoms.
We now report new synthetic approaches to the preparation
of [Cp*Os(CO)2(H2)]+ and [CpOs(dppm)H2]+. Further inves-
tigation of the structure of [Cp*Os(CO)2(H2)]+ gives new insight
into the equilibrium between the dihydrogen and dihydride forms
1
of this complex. Using very low-temperature H NMR, the
dynamic behavior of [CpOs(dppm)H2]+ has been investigated,
revealing quantum mechanical exchange coupling (QMEC)
between the hydrides in the cis isomer. This is the first reported
example of QMEC between two chemically equivalent but
magnetically inequivalent hydrogen atoms.
Figure 1. Hydride region of the 1H NMR spectrum of complex 1
as a function of time.
reaction of Cp*Os(CO)2H with CCl4. This is more convenient
than the previously reported preparation from Cp*Os(η4-COD)-
Cl.15 Reaction of Cp*Os(CO)2Cl with [Et3Si][BArF ] in meth-
Experimental Section
4
ylene chloride under 1 atm of hydrogen gas affords the hydrogen
All complexes were prepared using standard Schlenk techniques
under argon. NMR spectra were obtained at 500 MHz on a Bruker
Avance DRX series spectrometer. Spin lattice relaxation times (T1)
as a function of temperature were measured using a standard
inversion recovery pulse sequence. CDFCl2/CDF2Cl mixtures were
prepared by the method of Siegel and Anet.10 NMR spectra were
simulated using gNMR 4.1 (Ivorysoft). Cp*Os(CO)2H was prepared
as previously reported.11
complex [Cp*Os(CO)2(H2)][BArF ] (1) (ArF ) C6F5). In the
4
hydride region of the 1H NMR spectrum, complex 1 exhibits a
single resonance at -7.61 ppm. The T1 of this resonance was
measured as a function of temperature using an inversion
recovery sequence. The maximum rate of relaxation (T1(min)) is
24 ms at 180 K (500 MHz). When the preparative reaction was
carried out under HD gas, a mixture of 1 and 1-d1 was obtained.
The resonance due to 1-d1 exhibits JHD ) 24.5 Hz.
Cp*Os(CO)2Cl. Cp*Os(CO)2H was dissolved in CCl4 at room
temperature. Cp*Os(CO)2Cl formed cleanly in >95% yield. Cp*Os-
(CO)2Cl is a light yellow air-stable solid. IR (CH2Cl2): 2016 (s),
Reaction of CpOs(dppm)Br in methylene chloride with Na-
[BArF*4] under H2 gas (1 atm) over the course of 3 days affords
[CpOs(dppm)H2][BArF*4] (2) (ArF* ) 3,5-(CF3)2C6H3). Com-
plex 2 was characterized by 1H and 31P NMR spectroscopy and
by measurement of the T1(min), which is consistent with the
previous observations of Jia and co-workers.9
1
1956 (s) cm-1. H NMR (CD2Cl2): δ 2.01 (s, C5Me5).
[Cp*Os(CO)2(H2)]/[Cp*Os(CO)2H2][BArF ] (1). A J. Young
4
NMR tube was charged with [Et3Si][BArF ] prepared from [Ph3C]-
4
[BArF ] (20 mg, 25 µmol) and excess Et3SiH.12 To this was added
4
Cp*Os(CO)2Cl (5 mg, 12 µmol). CD2Cl2 was added by vacuum
transfer. The head space was filled with H2 and kept frozen with
liquid N2 until just before the first NMR spectrum was acquired.
1H NMR (CD2Cl2): δ 2.42 (s, Cp*, Os(H2)), 2.35 (s, Cp*, trans-
OsH2), -7.61 (s, Os(H2)), -10.05 (s, trans-OsH2). T1 (ms, trans-
OsH2): 2900 (156 K), 2700 (161 K), 2900 (166 K), 3100 (171 K),
3500 (177 K), 3900 (182 K); (ms, Os(H2)): 29 (156 K), 26
(161 K), 25 (166 K), 25 (171 K), 27 (177 K), 29 (182 K), 35
(192 K), 56 (218 K).
[CpOs(dppm)H2][BArF*4] (2). A J. Young NMR tube was
charged with CpOs(dppm)Br (4.4 mg, 6.1 µmol)9,13 and Na[BArF*4]
(6.3 mg, 7.0 µmol).14 CD2Cl2 was added via vacuum transfer, and
the head space was backfilled with H2 gas. Complete formation of
[CpOs(dppm)H2][BArF*4] took several days. CD2Cl2 was replaced
with a mixture of CDCl2F and CDClF2 for low-temperature
experiments. 1H NMR spectrum (CD2Cl2, hydride region, 295 K):
δ -10.50 (t, JHP ) 32 Hz, (H)2), -11.33 (t, JHP ) 6.5 Hz, H2). T1
(ms, trans-OsH2): 2540 (166 K), 1400 (171 K), 1280 (177 K),
1120 (187 K), 1080 (198 K), 1150 (208 K), 1110 (218 K), 1220
(223 K), (ms, cis-OsH2), 480 (171 K), 415 (177 K), 319 (187 K),
271 (198 K), 248 (208 K), 240 (218 K), 243 (223 K).
Isomerization of Complex 1. At 295 K, samples of 1 in
methylene chloride slowly isomerize to give a new hydride
resonance at -10.05 ppm. The minimum relaxation time T1 of
this signal is 2700 ms, and it is assigned to the trans dihydride
complex. Over the course of several days, equilibrium is reached
with a dihydride:dihydrogen ratio of 3.5:1 (Figure 1). The rate
of approach to equilibrium exhibited considerable variability
across samples (see Discussion).
Low-Temperature 1H NMR Spectroscopy of Complex 2.
In CD2Cl2, the 1H NMR spectrum of complex 2 in the hydride
region consists of two triplet resonances in the ratio of 10:1 at
-11.3 ppm (JPH ) 6.5 Hz) and -10.6 ppm (JPH ) 32 Hz). The
major resonance is assigned to the cis dihydride complex based
on the prior observations of Jia and co-workers.8
1
The H NMR spectrum of complex 2 in the hydride region
has now been examined at lower temperature using CDFCl2/
CDF2Cl mixtures. The resonance due to 2-cis obtained at 187
K is shown in Figure 2. The fitting of the observed spectra to
the calculated spectra is described in the Discussion section.
The observed value of JHH is dependent on the observation
temperature, varying from 133 Hz at 141 K to 176 Hz at
198 K. Simulations of spectra were carried out using JPP ) 70
Hz, JPH ) (12, κ ) 25 Hz, and fitting for JHH. The HH coupling
cannot be determined above 198 K due to the onset of rapid
hydride ligand permutation, which leads to line broadening.
Results
Synthesis and Characterization of [Cp*Os(CO)2(H2)]+ (1)
and [CpOs(dppm)H2]+ (2). Cp*Os(CO)2Cl was prepared by
(10) Siegel, J. S.; Anet, F. A. L. J. Org. Chem. 1988, 53, 2629-2630.
(11) Zhang, J.; Huang, K.-W.; Szalda, D. J.; Bullock, R. M. Organo-
metallics 2006, 25, 2209-2215.
Discussion
(12) Lambert, J. B.; Zhang, S.; Ciro, S. M. Organometallics 1994, 13,
2430-2443.
Preparation and Isomerization of Complex 1. Complex 1
was previously prepared by protonation of Cp*Os(CO)2H with
(13) Ashby, S. A.; Bruce, M. I.; Tomkins, I. B.; Wallis, R. C. Aust. J.
Chem. 1997, 32, 1003-1016.
(14) Reger, D. L.; Wright, T. D.; Little, C. A.; Lamba, J. J. S.; Smith,
M. D. Inorg. Chem. 2001, 40, 3810-3814.
(15) Albers, M. O.; Liles, D. C.; Robinson, D. J.; Shaver, A.; Singleton,
E. Organometallics 1987, 6, 2347-2354.

Cationic Dihydrogen/Dihydride Complexes of Osmium
Organometallics, Vol. 26, No. 9, 2007 2293
analogous to the previously reported study of [CpRu(dmpe)-
(H2)]+, where protonation of the corresponding neutral hydride
at low temperature gives exclusively the dihydrogen complex
as a kinetic product, which then undergoes isomerization to an
equilibrium mixture in which the dihydrogen form remains
predominant.20
While several cationic ruthenium dihydrogen complexes of
the general form [Cp/Cp*RuL2(H2)]+ are known, the observation
of a significant concentration of dihydrogen complex at equi-
librium rather than a dihydride is unusual for osmium.2 We
attribute this to the relatively electron-deficient metal center
resulting from the presence of two carbonyl ligands. Attempts
to quantify the rate of approach to equilibrium were frustrated
by a lack of reproducibility from sample to sample. The previous
studies of complex 1 by Bullock and co-workers noted facile
interconversion of the dihydrogen and dihydride forms, suf-
ficiently rapid to cause T1 averaging.8 This isomerization likely
proceeds via rapid proton exchange possibly mediated by the
presence of triflate. When complex 1 is prepared directly from
hydrogen using an unreactive anion, isomerization is slowed
by approximately 6 orders of magnitude. Our observations of
somewhat irreproducible rates of isomerization are consistent
with the presence of small, variable amounts of basic impurities
such as water, which can provide a pathway to facilitate
isomerization via reversible proton transfer from 1. The observed
isomerization reaction may be consistent with a minor proto-
nation pathway that leads directly from Cp*Os(CO)2H to the
trans dihydride complex. Alternatively, the isomerization could
be a strictly intramolecular process, as reported20 for [CpRu-
(dmpe)(H2)]+. The latter possibility seems less likely, since it
is not clear why such an intramolecular process would be
accelerated by the presence of triflate.
Figure 2. Partial 1H NMR spectrum of complex 2-cis at 187 K in
CDFCl2/CDF2Cl. Bottom: Experimental spectrum, inset with 3 Hz
line broadening. Top: Calculated spectrum with JPP ) 70 Hz, JPH
) ( 12, κ ) 25 Hz, JHH ) 148 Hz, and hydride permutation at
5 s-1
.
triflic acid.7 On the basis of T1 data, a hydridic resonance at
-7.6 ppm was attributed to the dihydrogen form of 1. This
assignment was complicated by a proton exchange process
between the highly acidic complex 1 and triflate. We have now
prepared complex 1 directly from hydrogen gas, which allows
for an unambiguous determination of the structure. The reso-
nance at -7.6 ppm exhibits a T1(min) value of 24 ms at 180 K
(500 MHz). Using the method of Halpern and co-workers,16
this value corresponds to dHH ) 1.1 Å, suggestive of a
moderately elongated dihydrogen complex. This is confirmed
by the preparation of 1-d1 (from HD gas), which exhibits
JHD ) 24.5 Hz. On the basis of the correlation between HD
coupling and dHH, this coupling is consistent with dHH ) 1.02
Å.17 The two methods of determination of dHH are reasonably
consistent, and the presence of a dihydrogen ligand in complex
1 is confirmed. This value of dHH can be compared to the
previously reported ruthenium analogue, [Cp*Ru(CO)2(H2)]+,
which exhibits JHD ) 32 Hz, consistent with a somewhat shorter
dHH of 0.89 Å.18 While this ruthenium complex is quite labile
to loss of H2, complex 1 is stable at room temperature in the
absence of oxygen and water.
While the dihydrogen form of complex 1 is formed initially
when hydrogen gas binds to the metal center, monitoring by
1H NMR spectroscopy at room temperature reveals a slow
isomerization to an equilibrium mixture of dihydrogen complex
and a new species, which exhibits a resonance at -10.0 ppm
(Figure 1). The T1(min) for this resonance is observed to be
2700 ms at 161 K (500 MHz), consistent with assignment as a
trans dihydride complex.19 At equilibrium, the trans dihydride
form of complex 1 is slightly favored, with a dihydride:
dihydrogen ratio of 3.5:1 at 295 K. This situation is directly
Structure and Dynamic Behavior of Complex 2. In contrast
to 1, phosphine donors in complexes such as [CpOs(dppm)-
H2]+ (2) lead to a dihydride structure. Complex 2 was previously
studied by Jia and co-workers, who observed the occurrence of
cis and trans forms of this complex, with the former predomi-
nant. Apparently due to geometric constraints, the dppm ligand
disfavors the formation of the trans dihydride structure, so
complex 2 adopts a very unusual cis dihydride structure. In
contrast, the direct ruthenium analogue [CpRu(dppm)(H2)]+ is
a dihydrogen complex, with no observed dihydride form.21
(16) Desrosiers, P. J.; Cai, L.; Lin, Z.; Richards, R.; Halpern, J. J. Am.
Chem. Soc. 1991, 113, 4173-4184. See also: Bayse, C. A.; Luck, R. L.;
Schelter, E. J. Inorg. Chem. 2001, 40, 3463-3467.
Consistent with the observations reported by Jia and co-
workers, we find that the T1(min) value for the hydride resonance
due to the cis form of complex 2 is 240 ms at 218 K (500 MHz).
Application of the method of Halpern and co-workers16 gives
(17) Gelabert, R.; Moreno, M.; Lluch, J. M.; Lledos, A.; Pons, V.;
Heinekey, D. M. J. Am. Chem. Soc. 2004, 126, 8813-8822.
(18) Chinn, M. S.; Heinekey, D. M.; Payne, N. G.; Sofield, C. D.
Organometallics 1989, 8, 1824-1826.
(19) In the four-legged piano stool geometry, intraligand bond angles
are not 90/180°. We use cis/trans as geometric designations rather than the
more correct but cumbersome cisoid/transoid designations.
(20) Chinn, M. S.; Heinekey, D. M. J. Am. Chem. Soc. 1990, 112, 5166-
5175.
(21) Jia, G.; Morris, R. H. J. Am. Chem. Soc. 1991, 113, 875-883.

2294 Organometallics, Vol. 26, No. 9, 2007
Egbert et al.
1
couplings must be 13 Hz. The observed low-temperature H
NMR spectrum for complex 2-cis in the hydride region can be
simulated using HP couplings of 25 and 12 Hz (with opposite
sign), along with JPP ) 70 Hz. The anticipated JHH′ value of
19.5 Hz does not give a satisfactory simulation. A JHH value of
148 Hz is required to fit the spectrum (Figure 2). The outer
wings in the spectrum shown in Figure 2 are broader than
expected. This line broadening can be fit by adding hydride
permutation at 5 s-1 to the simulation.
Figure 3. Exchange coupling (Jex) in complex 2 as a function of
temperature. Errors in J values increase with temperature due to
line broadening caused by the onset of thermal exchange.
an HH distance of 1.6 Å. This is similar to the distance derived17
from T1 data for the related dicationic Ir complex [Cp*Ir-
(dmpm)H2]2+. This Ir dihydride has JHD ) 7.5-9 Hz, consistent
with the relaxation time data. In the case of complex 2, the
observed JHD of 3.0 Hz corresponds to an HH distance of ca.
1.8 Å, inconsistent with the HH distance derived from the
relaxation time data. While the correlation between HD coupling
and HH distance seems to be quite reliable for shorter HH
distances, this inconsistency demonstrates that this correlation
may become unreliable at very long HH distances. For these
structures, the coupling is dominated by two bond terms that
depend upon the identity of the metal and the bond angles. For
molecules of this type, the HH distance is best defined by
relaxation time measurements.
The fact that the observed value of JHH′ greatly exceeds the
expected value suggests that QMEC is operative here. Further
evidence for this is provided by the observation that JHH′ is
highly temperature dependent, ranging from 133 Hz at 140 K
to 176 Hz at 198 K. For two hydride ligands bonded to a metal
center, if each H atom is strongly localized in its respective
potential energy well, then the vibrational motions of each can
be treated independently. In some cases, with relatively short
HH distances dHH and soft potential energy surfaces, the orbital
wave functions may overlap. The vibrational motions of the
two atoms can no longer be treated independently. This overlap
gives rise to symmetric and antisymmetric combinations of the
two single particle wave functions, which differ in energy by
2Jex, where Jex is the exchange coupling. This interaction is
manifest in the 1H NMR spectra in combination with the
magnetic component of the coupling according to Jobs ) Jmag
- 2Jex, where Jmag is the contribution to the overall observed
coupling Jobs due to the Fermi contact term. This term can be
evaluated by noting as mentioned above that JHD ) 3.0 Hz, so
the magnetic contribution to the observed HH coupling must
be 19.5 Hz. In previous studies of related four-legged piano
stool complexes [CpIr(PR3)H3]+, it was found that the sign of
Jmag is positive.24 Assuming this to be true in complex 2, the
actual values of Jex as a function of temperature range from
-57 Hz at 140 K to -67 Hz at 198 K (Figure 3).
1
Jia and co-workers studied the H NMR spectrum of 2 in
methylene chloride at various temperatures down to 190 K and
reported that the triplet resonance due to 2-cis became a poorly
resolved broad resonance at low temperature.22 As noted by Jia
and co-workers, the single triplet hydride resonance of the cis
structure is suggestive of a dynamic process that rapidly
interchanges the two hydride environments. In the absence of
such a process, the hydride resonance for 2-cis should be quite
complex, since the two hydrides are magnetically inequivalent,
with the spin system corresponding to HH′PP′. Alternatively,
the same triplet spectrum can be obtained if there is a very large
coupling JHH between the two hydride nuclei. Such large
couplings are attributed to the operation of quantum mechanical
exchange coupling (QMEC). QMEC has been observed in a
number of transition metal polyhydride complexes. The previous
studies have been summarized in an excellent review article.23
1
We have found that by recording the H NMR spectrum at
very low temperature in Freon solvents, it is possible to obtain
well-resolved static spectra. A typical low-temperature spectrum
is shown in Figure 2. Input of the HP, HH, and PP couplings
in this species allows for the calculation of the appearance of
the low-temperature spectrum. The HH coupling can be
estimated from the reported value of JHD ) 3.0 Hz for 2-d1.
Since γH/γD ) 6.514, the expected value of JHH is approximately
19.5 Hz. Based on previously reported dppm complexes, JPP
will be approximately 70 Hz. The two values for the cis and
trans HP coupling are more difficult to estimate. It is known
that cis and trans HP couplings in four-legged piano stool
complexes of this type are opposite in sign.24 In the case of
complex 2, the observed averaged HP coupling is 6.5 Hz,
suggesting that the difference between the cis and trans
The increase in the magnitude of Jex with increasing tem-
perature is consistent with previous examples of exchange
coupling. In the case of the related Ir complexes of the form
[CpIr(PR3)H3]+, coupling data were available over a wide range
of temperatures, making it possible to fit the data quantitatively
to a two-dimensional harmonic oscillator model.24 In the case
of complex 2, limited data for Jex as a function of temperature
are available, preventing the quantitative application of this
model.
Conclusions
Convenient preparations of complexes 1 and 2 directly from
hydrogen gas have been developed. In the case of complex 1,
the initially formed product is confirmed as a dihydrogen
complex by measurement of HD coupling. Very slow isomer-
ization to an equilibrium mixture of dihydride and dihydrogen
(22) Ng, W. S. Hydride and Acetylene Chemistry of Cyclopentadienyl
Group 8 Metal Complexes. Ph.D. Thesis, Hong Kong University of Science
and Technology, September 1998.
(23) Sabo-Etienne, S.; Chaudret, B. Chem. ReV. 1998, 98, 2077-2092.
(24) Heinekey, D. M.; Hinkle, A. S.; Close, J. D. J. Am. Chem. Soc.
1996, 118, 5353-5361.

Cationic Dihydrogen/Dihydride Complexes of Osmium
Organometallics, Vol. 26, No. 9, 2007 2295
forms is observed. Samples of 1 prepared from hydrogen gas
exhibit rates of dihydrogen to dihydride isomerization that are
several orders of magnitude slower than previous observations
using samples with triflate counterion.
With the more basic ligand environment provided by the
dppm ligand, a greater degree of HH bond activation is
observed, and the structure of 2 is a cis dihydride. The dynamics
of the hydride ligands in complex 2 have been studied by very
low-temperature NMR spectroscopy, which reveals that QMEC
is operative. This is the first report of QMEC in a chemically
equivalent pair of hydrogen atoms.
Acknowledgment. Research at the University of Washington
was supported by the NSF. Research at Brookhaven National
Laboratory was carried out under contract DE-AC02-98CH10886
with the U.S. Department of Energy and was supported by its
Division of Chemical Sciences, Office of Basic Energy Sciences.
Research at Pacific Northwest National Laboratory (PNNL) was
funded by LDRD funds of PNNL. PNNL is operated by Battelle
for the U.S. Department of Energy. We thank Dr. Jie Zhang
for the preparation of Cp*Os(CO)2Cl.
OM0700718