Synthesis and NMR studies of [(C5Me5)Os(L)H 2(H2)+] complexes. Evidence of the adoption of different structures by a dihydrogen complex in solution and the solid state

Article abstract of DOI:10.1021/om061017e

Protonation of the osmium(IV) trihydrides (C₅Me ₅)OsH₃(L) with HBF₄ in diethyl ether affords the molecular dihydrogen complexes [(C₅Me₅)Os(H ₂)H₂(L)][BF₄], where L is PPh₃ (1), AsPh₃ (2), or PCy₃ (3). Ruthenium analogues of these species are not stable and instead lose H₂ readily. These compounds adopt four-legged piano-stool geometries in which the phosphine ligand is trans to an elongated dihydrogen ligand. For 1 and 2, the coordinated H₂ ligand is oriented with its H-H vector nearly parallel with the Os-Ct vector, where Ct is the centroid of the C₅Me ₅ ring; in contrast, in 3 the H₂ ligand is oriented with its H-H vector perpendicular to the Os-Ct vector. In the ¹H NMR spectra, exchange between the Os-H and Os-H₂ environments can be slowed at low temperatures for the arsine complex 2 (but not for 1 or 3), and separate resonances could be observed for the hydride and dihydrogen sites; the barrier for exchange is approximately 6.0 kcal/mol. Partially deuterated samples were prepared, and H-H distances within the bound H₂ ligands were deduced from the observed ¹J_HD(av) coupling constants. In addition, H-H distances were deduced from the T₁(min) values for the osmium-bound hydrogen atoms, after correction for exchange and ligand-induced dipolar relaxation effects. In all cases, the two solution measurements were in agreement but differed from that deduced from neutron diffraction data. Specifically, for 1 the solution data gave a distance of ca. 1.07 vs 1.01 A in the solid state; similarly, for 2 the solution value of ca. 1.15 A was longer than the 1.08 A value seen in the solid state. In both cases, the ～0.06 A lengthening in solution, if real, is the result most likely of one or both of two factors: the effect of removing the BF ₄ counterion from the vicinity of the cation and the effect of librational motion that tends to shorten artificially H-H distances deduced from neutron diffraction data. In contrast, for 3 the solution H-H distance of ca. 1.12 A is significantly shorter than the 1.31 A distance determined from the neutron diffraction data. DFT calculations support the hypothesis that different structures are adopted by 3 in solution and in the solid state and that in solution an equilibrium is established between two dihydrogen-dihydride structures, one with a considerably shorter H-H bond than is seen in the solid state.

Full text of DOI:10.1021/om061017e

1658
Organometallics 2007, 26, 1658-1664
Synthesis and NMR Studies of [(C5Me5)Os(L)H2(H2)+] Complexes.
Evidence of the Adoption of Different Structures by a Dihydrogen
Complex in Solution and the Solid State
Christopher L. Gross and Gregory S. Girolami*
The School of Chemical Sciences, The UniVersity of Illinois at Urbana-Champaign,
600 South Mathews AVenue, Urbana, Illinois 61801
ReceiVed NoVember 5, 2006
Protonation of the osmium(IV) trihydrides (C5Me5)OsH3(L) with HBF4 in diethyl ether affords the
molecular dihydrogen complexes [(C5Me5)Os(H2)H2(L)][BF4], where L is PPh3 (1), AsPh3 (2), or PCy3
(3). Ruthenium analogues of these species are not stable and instead lose H2 readily. These compounds
adopt four-legged piano-stool geometries in which the phosphine ligand is “trans” to an elongated
dihydrogen ligand. For 1 and 2, the coordinated H2 ligand is oriented with its H-H vector nearly parallel
with the Os-Ct vector, where Ct is the centroid of the C5Me5 ring; in contrast, in 3 the H2 ligand is
1
oriented with its H-H vector perpendicular to the Os-Ct vector. In the H NMR spectra, exchange
between the Os-H and Os-H2 environments can be slowed at low temperatures for the arsine complex
2 (but not for 1 or 3), and separate resonances could be observed for the hydride and dihydrogen sites;
the barrier for exchange is approximately 6.0 kcal/mol. Partially deuterated samples were prepared, and
H-H distances within the bound H2 ligands were deduced from the observed 1JHD(av) coupling constants.
In addition, H-H distances were deduced from the T1(min) values for the osmium-bound hydrogen atoms,
after correction for exchange and ligand-induced dipolar relaxation effects. In all cases, the two solution
measurements were in agreement but differed from that deduced from neutron diffraction data. Specifically,
for 1 the solution data gave a distance of ca. 1.07 vs 1.01 Å in the solid state; similarly, for 2 the solution
value of ca. 1.15 Å was longer than the 1.08 Å value seen in the solid state. In both cases, the ∼0.06 Å
lengthening in solution, if real, is the result most likely of one or both of two factors: the effect of
removing the BF4 counterion from the vicinity of the cation and the effect of librational motion that
tends to shorten artificially H-H distances deduced from neutron diffraction data. In contrast, for 3 the
solution H-H distance of ca. 1.12 Å is significantly shorter than the 1.31 Å distance determined from
the neutron diffraction data. DFT calculations support the hypothesis that different structures are adopted
by 3 in solution and in the solid state and that in solution an equilibrium is established between two
dihydrogen-dihydride structures, one with a considerably shorter H-H bond than is seen in the solid
state.
Since their discovery, molecular dihydrogen complexes have
generated considerable interest because they may represent a
midway point along the reaction coordinate that leads to the
oxidative addition of dihydrogen to transition-metal centers. The
H-H bond distance is a measure of the extent to which the H2
ligand has been activated. Distances less than 0.85 Å are typical
for “normal” dihydrogen complexes, whereas distances longer
than 1.50 Å are characteristic of classical dihydrides.9 A few
dihydrogen complexes, however, have “elongated” H-H bond
distances of approximately 1.0 to 1.4 Å that constitute an
intermediate situation.4
We have recently described single-crystal neutron diffraction,
inelastic neutron scattering, and density functional studies of
the osmium complexes [(C5Me5)OsH4(L)][BF4], where L )
PPh3, AsPh3, PCy3.10 All three compounds contain a dihydrogen
ligand that falls into the “elongated” category. Interestingly, the
structures of these complexes are not all the same: for the PPh3
Introduction
The chemistry of metal hydrides has taken on renewed interest
in the context of developing transportation systems in which
hydrogen serves as the energy storage medium.1-5 One aspect
of interest is how the structures and properties of transition-
metal hydride complexes depend on the identity of the metal
center. One trend is that the behavior of second- and third-row
metal complexes is often rather different. For example, the
second-row transition-metal complexes (C5H5)2NbH3 and [(C5H5)2-
MoH3][BF4] have classical polyhydride structures but exhibit
pronounced quantum exchange with JHH couplings as large as
1000 Hz.6-8 In contrast, the third-row analogues of these species,
(C5H5)2TaH3 and [(C5H5)2WH3][BF4], exhibit classical behavior
1
in their H NMR spectra.
(1) Ogden, J. M. Annu. ReV. Energy EnViron. 1999, 24, 227-279.
(2) Seayad, A. M.; Antonelli, D. M. AdV. Mater. 2004, 16, 765-777.
(3) Clapham, S. E.; Hadzovic, A.; Morris, R. H. Coord. Chem. ReV. 2004,
248, 2201-2237.
(7) Heinekey, D. M. J. Am. Chem. Soc. 1991, 113, 6074-6077.
(8) Camanyes, S.; Maseras, F.; Moreno, M.; Lledo´s, A.; Lluch, J. M.;
Bertra´n, J. J. Am. Chem. Soc. 1996, 118, 4617-4621.
(4) Heinekey, D. M.; Lledos, A.; Lluch, J. M. Chem. Soc. ReV. 2004,
33, 175-182.
(5) Maseras, F.; Lledos, A.; Clot, E.; Eisenstein, O. Chem. ReV. 2000,
100, 601-636.
(9) Maseras, F.; Lledo´s, A.; Costas, M.; Poblet, J. M. Organometallics
1996, 15, 2947-2953.
(6) Sabo-Etienne, S.; Chaudret, B. Coord. Chem. ReV. 1998, 178, 381-
407.
(10) Webster, C. E.; Gross, C. L.; Young, D. M.; Girolami, G. S.; Schultz,
A. J.; Hall, M. B.; Eckert, J. J. Am. Chem. Soc. 2005, 127, 15091-15101.
10.1021/om061017e CCC: $37.00 © 2007 American Chemical Society
Publication on Web 02/22/2007

[(C5Me5)Os(L)H2(H2)+] Complexes
Organometallics, Vol. 26, No. 7, 2007 1659
Figure 1. Inner coordination spheres of 1-3. The phenyl and cyclohexyl groups bound to phosphorus and arsenic, and the methyl hydrogen
atoms on the C5Me5 ligand, are omitted for clarity. Reproduced from ref 10.
and AsPh3 complexes, the coordinated H2 ligand is oriented with
its H-H vector approximately parallel with the Os-Ct vector,
where Ct is the centroid of the C5Me5 ring; in contrast, for PCy3
the H2 ligand is oriented with its H-H vector perpendicular to
the Os-Ct vector.
tained from X-ray crystallographic studies of the PPh3 compound
1 and the PCy3 compound 3 at 198 K can be found in the
Supporting Information of the present article. In order to provide
a context for the following NMR studies, we present only a
summary of the structural details here, along with a brief
description of how the molecular structure changes upon
protonation.
All three compounds adopt four-legged piano-stool geom-
etries in which the four legs are described by the Lewis base L,
two classical hydride ligands, and a nonclassical dihydrogen
ligand (Figure 1). The ligand L is “trans” to the dihydrogen
ligand, and the two classical hydride ligands are “trans” to one
another. For the PPh3 and AsPh3 complexes 1 and 2, the H-H
vector of the H2 ligand lies parallel to the Ct-Os-L plane (Ct
) centroid of the C5Me5 ring). In contrast, in the PCy3 com-
plex 3, the H-H vector is perpendicular to the Ct-Os-L plane.
Not only the orientation of the central two hydrogen atoms
but also the H-H bond length between them depends signifi-
cantly on the nature of L: the H‚‚‚H distances determined from
neutron diffraction are 1.01(1) and 1.08(1) Å for L ) PPh3,
AsPh3, respectively, but 1.31(3) Å for L ) PCy3. All of these
H-H distances are characteristic of “elongated” dihydrogen
ligands.13-17
Density functional calculations showed that the molecular
geometry is a consequence of a delicate balance of electronic
and steric influences created by the L ligand. Highly important
are steric interactions between the C5Me5 and L groups, which
change the Ct-Os-L angle. The change in geometry changes
the relative energies of the frontier orbitals that project toward
the H2 ligand; these frontier orbitals in turn govern the H-H
distance, preferred H-H orientation, and rotational dynamics
of the elongated dihydrogen ligand. The geometry of the
-
dihydrogen ligand is further tuned by interactions with the BF4
counterion.10
In order to complement the above solid-state studies, we now
describe the synthesis and solution characterization of the
[(C5Me5)Os(H2)H2(L)]+ complexes. A significant conclusion is
that, for L ) PCy3, different structures are adopted in solution
and the solid state. Portions of this work have been reported
previously.11
Results
Protonation of the osmium(IV) trihydrides causes a change
in the hybridization of the metal-ligand bonding orbitals, as
shown by a comparison of the structure of the neutral trihydride
(C5Me5)OsH3(PPh3)12 with that of the tetrahydride cation 1. The
angle between the mutually “trans” terminal hydrides in the
square base of the four-legged piano stool opens up considerably
upon protonation from 119(2)° in (C5Me5)OsH3(PPh3)12 to
132.6(5)° in 1. In addition, the Os-P bond distance lengthens
upon protonation from 2.263(1) Å in (C5Me5)OsH3(PPh3) to
2.338(1) Å in 1. These geometrical changes are consistent with
the presumably greater steric requirements of the dihydrogen
ligand in 1 relative to the hydride ligand trans to the phosphine
in (C5Me5)OsH3(PPh3). The geometric changes that take place
Preparation of the Osmium Dihydrogen Complexes
[(C5Me5)Os(L)H2(H2)+]. We have previously described the
synthesis of a series of osmium(IV) trihydrides (C5Me5)-
Os(L)H3, where L ) PPh3, AsPh3, PCy3.12 These compounds
adopt classical hydride structures in both solution and the solid
state. Treatment of these species with HBF4‚Et2O in diethyl ether
generates products with the stoichiometry [(C5Me5)Os(L)H4]-
[BF4], where L ) PPh3 (1), AsPh3 (2), PCy3 (3).
[(C Me )Os(L)H (H )][BF ]
(C5Me5)Os(L)H3 + HBF4 f
5
5
2
2
4
1-3
L ) PPh3 (1), AsPh3 (2), PCy3 (3)
(13) Albinati, A.; Bakhmutov, V. I.; Caulton, K. G.; Clot, E.; Eckert, J.;
Eisenstein, O.; Gusev, D. G.; Grushin, V. V.; Hauger, B. E.; Klooster, W.
T.; Koetzle, T. F.; McMullan, R. K.; O’Loughlin, T. J.; Pe´lissier, M.; Ricci,
J. S.; Sigalas, M. P.; Vymenits, A. B. J. Am. Chem. Soc. 1993, 115, 7300-
7312.
(14) Klooster, W. T.; Koetzle, T. F.; Jia, G.; Fong, T. P.; Morris, R. H.;
Albinati, A. J. Am. Chem. Soc. 1994, 116, 7677-7681.
(15) Maltby, P. A.; Schlaf, M.; Steinbeck, M.; Lough, A. J.; Morris, R.
H.; Klooster, W. T.; Koetzle, T. F.; Srivastara, R. C. J. Am. Chem. Soc.
1996, 118, 5396-5407.
(16) Hasegawa, T.; Li, Z.; Parkin, S.; Hope, H.; McMullan, R. K.;
Koetzle, T. F.; Taube, H. J. Am. Chem. Soc. 1994, 116, 4352-4356.
(17) Brammer, L.; Howard, J. A. K.; Johnson, O.; Koetzle, T. F.; Spencer,
J. L.; Stringer, A. M. J. Chem. Soc., Chem. Commun. 1991, 241-243.
The compounds are isolated as white powders that precipitate
from solution under the reaction conditions employed; re-
crystallization can be accomplished by layering concentrated
CH2Cl2 solutions of the complexes with diethyl ether.
In a companion paper, we reported single-crystal neutron
diffraction studies of 1-3.10 Complementary information ob-
(11) Gross, C. L.; Young, D. M.; Schultz, A. J.; Girolami, G. S. J. Chem.
Soc., Dalton Trans. 1997, 18, 3081-3082.
(12) Gross, C. L.; Girolami, G. S. Organometallics 2006, 25, 4792-
4798.

1660 Organometallics, Vol. 26, No. 7, 2007
Gross and Girolami
may also reflect changes in the energies of the frontier molecular
orbitals upon protonation: these orbitals will decrease in energy
upon protonation, owing to the addition of a positive charge
and the ability of the H2 ligand to act as a π-acceptor. Changes
in the orbital makeup of the Os-H bonds may therefore be
responsible for the increase in the trans H-Os-H angle,
whereas a decrease in π-back-bonding to the PPh3 ligand may
explain the lengthening of the Os-P bond.
It is interesting to note that protonation of the analogous
ruthenium trihydride complex (C5Me5)Ru(PPh3)H3 with HBF4‚
Et2O in toluene does not produce a product containing four
metal-bound hydrogen ligands. Instead, a mixture of species
is obtained that includes the dihydride cation [(C5Me5)Ru-
(PPh3)2H2+] and the toluene π-complex [(C5Me5)Ru(η6-C6H5-
Me)+].18 These products indicate that the initially produced
[(C5Me5)Ru(PPh3)H4+] cation is unstable toward loss of H2. The
stability of [(C5Me5)Os(PPh3)H2(H2)+] vs the ready loss of H2
from its ruthenium analogue reflects the general trend that third-
row transition metals form stronger metal-ligand bonds than
do their second-row congeners.
1
Figure 2. Hydride region of the 500 MHz H NMR spectra of
Spectroscopic Characterization of the Osmium Dihydro-
gen Complexes. The infrared spectrum of the PPh3 compound
1 exhibits two Os-H stretching bands at 2108 and 2063 cm-1
with the higher frequency band at 2108 cm-1 being the more
intense of the two (I2108/I2063 ) 1.9). Most likely, these features
are due to the Os-H stretching vibrations of the classical
hydride ligands. The infrared spectrum of the AsPh3 compound
2 contains two Os-H bands at 2088 and 2029 cm-1 whose
frequencies and intensities are similar to those seen for 1. In
contrast, the infrared spectrum of the PCy3 compound 3 con-
tains two bands in the Os-H stretching region at 2173 and
2102 cm-1, with the lower frequency band being the more
intense by about a factor of 2. These findings are consistent
with the crystallographic results, which show that the structures
of 1 and 2 are similar but fundamentally different from that
of 3.
[(C5Me5)Os(AsPh3)H2(H2)][BF4] (2) in CDFCl2 at -50 °C (top),
-130 °C (middle), and -140 °C (bottom).
1
The room-temperature H NMR spectra of 1-3 in CD2Cl2
all feature a single resonance for the four osmium-bound
hydrogen atoms; the resonances for the PPh3 complex 1 and
the PCy3 complex 3 appear as doublets at δ -9.61 (JHP(av) )
14.2 Hz) and δ -10.61 (JHP(av) ) 15.7 Hz), whereas the
resonance for the AsPh3 complex 2 is a singlet at δ -9.88. Thus,
it is apparent that exchange of the classical and nonclassical
hydrogen atom sites in compounds 1-3 is fast on the NMR
time scale at 25 °C. The hydride resonances of 1 and 3 remain
sharp down to approximately -100 °C, at which point they
begin to broaden. Even at -140 °C (in the solvent CDFCl2),
however, the hydride resonances of 1 and 3 remain broad
singlets. In contrast, the hydride resonance of 2 at -140 °C
decoalesces into two broad equal-intensity features separated
by 1.0 ppm (Figure 2). From this chemical shift separation and
the coalescence temperature, we can calculate that the activation
free energy for the dihydrogen/hydride exchange process in 2
is ∼6.0 kcal/mol.
1
Figure 3. Hydride region of the 400 MHz H NMR spectrum of
[(C5Me5)Os(PPh3)HD3][BF4] (1-d3) at 25 °C in CD2Cl2. The top
spectrum is resolution enhanced. The doublet splitting is due to
2
31P; the 1:1:1 triplet splitting is due to H.
Additional insight into the solution structures can be obtained
from the NMR spectra of partially deuterated isotopologues of
1 and 2. Addition of a 3:1 mixture of D2O and HBF4‚Et2O to
a diethyl ether solution of (C5Me5)OsD3(PPh3) generates a
distribution of isotopologues of 1, including [(C5Me5)Os(PPh3)-
3); the multiplet splitting gives JHD(av) ) 3.6 Hz. These
coupling constants are averages owing to the dynamic exchange
process; if we assume that there is no site preference for the H
and D atoms, the average coupling constant is equal to [1JHD
+
4(2JHD) + 2J′HD]/6, where1JHD, 2JHD, and 2J′HD are the couplings
within the dihydrogen ligand, between the dihydrogen and
hydride atoms, and between the two classical hydride atoms,
1
D2(D2)][BF4], the fully deuterated material. At 25 °C, the H
NMR resonance for the residual hydrogen atoms in the hydride
sites is a doublet (2JHP(av) ) 14.7 Hz) of multiplets (Figure
2
respectively. If we adopt typical values for the geminal JHD
couplings of between -1 and +1 Hz,19-24 then the intrinsic
1JHD coupling within the bound HD ligand is between 16.6 and
(18) Arliguie, T.; Chaudret, B.; Jalon, F. A.; Otero, A.; Lopez, J. A.;
Lahoz, F. J. Organometallics 1991, 10, 1888-1896.

[(C5Me5)Os(L)H2(H2)+] Complexes
Organometallics, Vol. 26, No. 7, 2007 1661
J
HD coupling constant is consistent with the solid-state results,
whereas for the PCy3 compound 3 the results are inconsistent
with the solid-state structure.
For undeuterated samples of 1-3, we have also carried out
variable-temperature 1H NMR studies of the spin-lattice
relaxation time, a parameter that also can provide information
about the H-H distance.26-29 At 500 MHz in CD2Cl2, the
T1(av, min) values of the Os-H resonances are 99 ms (at -70
°C), 130 ms (at -70 °C), and 41 ms (at -90 °C) for 1-3,
respectively. At these temperatures, the exchange between the
Os-H and Os-H2 sites is in the fast exchange limit, and thus
the observed relaxation time is an average given by the
expression R(av, min) ) 1/2[R(c, min) + R(n, min)], where R(c,
min) and R(n, min) are the relaxation rates (R ) 1/T1) for the
classical and nonclassical hydrogen sites at the T1(av, min)
temperature.
Because all three compounds 1-3 have been characterized
by neutron diffraction, we can use Halpern’s method27,28 to
calculate R(c, min) values for each of the two classical hydride
atoms; this parameter should be equal to the sum of the dipole-
dipole relaxation contributions from all the other nuclear spins
in the molecule (correcting for the natural abundance where
necessary). The dipole-dipole relaxation contributions can be
calculated from the experimentally determined H‚‚‚X distances
between the hydrogen atom of interest and all of the other atoms
in the molecule. From the calculated value of R(c, min) and
the experimental value of R(av, min), we can calculate R(n, min)
from the formula above. After correcting the latter value for
the dipole-dipole relaxations induced by nearby atoms except
that due to the mutual action of the two hydrogen atoms in the
H2 molecule, we can use established formulas to deduce the
value of dHH within the H2 ligand.26-29 If the resulting H-H
distance within the H2 ligand is similar to that in the solid state,
then it is reasonable to conclude that the solution and solid-
state structures are similar. If, however, this procedure gives a
H-H distance significantly different from that in the solid state,
then one can conclude that the solution and solid-state structures
are different.
From the experimental H‚‚‚X distances between each of the
two classical hydrogen atoms in 1 and the other nuclear spins
(1H, 31P, 187Os), we can calculate that R(c, min) should be 4.14
s-1. This rate, combined with the 99 ms value of T1(av, min),
leads to an estimated value of 16.1 s-1 for R(n, min). Subtracting
out the dipole-dipole relaxation rates due to the other nuclear
spins (i.e. excluding the two H atoms within the H2 ligand)
affords a value of 13.0 s-1 (i.e, T1(n, min) ) 77 ms). This value
should be the relaxation rate due just to the dipole-dipole
interaction within the H2 ligand. If we assume that the
dihydrogen ligand rotation rate is fast compared with the
molecular tumbling rate, then from the expression dHH ) 5.81-
(T1(n,min)/4ν)1/6, where the distance is expressed in Å, T1 is in
seconds, and ν is the spectrometer frequency in MHz,26-29 we
obtain dHH ) 1.07 Å, vs 1.01 Å from the neutron diffraction
study.10 The assumption that the H2 rotation rate is fast compared
with the molecular tumbling rate is consistent with the small
rotational barrier of the bound H2 ligand in [(C5Me5)OsH4-
(PPh3)+], the experimental value being 3.1 kcal mol-1, as
determined from inelastic neutron scattering experiments.10
1
Figure 4. Hydride region of the 400 MHz H NMR spectrum of
a mixture of [(C5Me5)Os(AsPh3)H4][BF4] and [(C5Me5)Os(AsPh3)-
H3D][BF4] (2-d0 and 2-d) at 25 °C in CD2Cl2.
26.6 Hz. This span, when substituted into an expression that
relates 1JHD to H-H distances,24,25 affords values of 0.97-1.18
Å, a range that includes the 1.01 Å value determined from the
neutron diffraction study.10
In similar experiments, 2 and 3 were individually mixed with
a small amount of trifluoromethanesulfonic acid-d in CD2Cl2.
1
For 2, the hydride region of the H NMR spectrum of this
mixture features a 1:1:1 triplet due to the monodeuterated
isotopologue of 2 and a singlet due to the unlabeled compound
(Figure 4). The multiplet splitting of the resonance for the
labeled product affords a JHD(av) value of 2.8 Hz. When it is
treated as described above, the JHD(av) value of 2.8 Hz gives
1
an intrinsic JHD coupling of between 11.8 and 21.8 Hz and
distances of 1.06-1.32 Å for the H-H ligand, a range that once
again includes the 1.07 Å value established from the neutron
diffraction study.
1
For deuterated samples of 3, the hydride region of the H
NMR spectrum contains a number of overlapping doublets of
multiplets due to the presence of several isotopologues. The
multiplet splitting affords a JHD(av) value of 3.4 Hz, which, in
turn, gives an intrinsic 1JHD coupling of between 15.4 and 25.4
Hz and H-H distances of 0.99-1.21 Å (vs 1.31 Å from the
neutron diffraction study). For 3, the experimental H-H distance
falls outside the range estimated from the solution data.
It is clear from the above results that uncertainties in the
values of the two-bond JHD coupling constants (i.e., those
involving the classical hydrides) make it difficult to derive
accurate H-H bond distances for polyhydrides in which
exchange between the classical and nonclassical hydride sites
is fast on the NMR time scale. Nevertheless, we can conclude
that for 1 and 2 the H-H distance calculated from the average
2
(19) An upper limit on the magnitude of the two-bond JHH coupling
constant between mutually cis classical hydrides is 6 Hz (trans couplings
can be as large as 15 Hz).20-24. Correcting for the H/D gyromagnetic ratio
2
of 6.51 gives a maximum JHD coupling constant between cis classical
hydrides of about 1 Hz. Interestingly, it is thought that the sign of this
coupling constant is negative.24
(20) Herrmann, W. A.; Okuda, J. Angew. Chem., Int. Ed Engl. 1986,
25, 1092-1093.
(21) Lantero, D. R.; Motry, D. H.; Ward, D. L.; Smith, M. L., III. J.
Am. Chem. Soc. 1994, 116, 10811-10812.
(22) Ditzel, E. J.; Robertson, G. B. Aust. J. Chem. 1995, 48, 1183-
1191.
(26) Morris, R. H. Can. J. Chem. 1996, 74, 1907-1915.
(27) Desrosiers, P. J.; Cai, L.; Lin, Z.; Richards, R.; Halpern, J. J. Am.
Chem. Soc. 1991, 113, 4173-4184.
(23) Delpech, F.; Sabo-Etienne, S.; Donnadieu, B.; Chaudret, B. Orga-
nometallics 1998, 17, 4926-4928.
(24) Gru¨ndemann, S.; Limbach, H.-H.; Buntkowsky, G.; Sabo-Etienne,
S.; Chaudret, B. J. Phys. Chem. A 1999, 103, 4752-4754.
(25) Gelabert, R.; Moreno, M.; Lluch, J. M.; Lledos, A.; Pons, V.;
Heinekey, D. M. J. Am. Chem. Soc. 2004, 126, 8813-8822.
(28) Bayse, C. A.; Luck, R. L.; Schelter, E. J. Inorg. Chem. 2001, 40,
3463-3467.
(29) See: Gusev, D. G.; Kuhlman, R. L.; Renkema, K. B.; Eisenstein,
O.; Caulton, K. G. Inorg. Chem. 1996, 35, 6775-6783 and references
therein.

1662 Organometallics, Vol. 26, No. 7, 2007
Gross and Girolami
be derived from DFT calculations on 1 and 2.10 When the BF4
counterion is included in the calculations (as a gas-phase ion
pair; i.e., in the absence of solvent), there is a single, minimum
energy structure (containing one dihydrogen ligand) that is very
similar to that determined by neutron diffraction. Interestingly,
when the BF4 counterion is removed and the structure of the
free cation optimized, there are two principal low-energy
structures of approximately equal energy: the observed dihy-
dride-dihydrogen structure is slightly lower in energy than a
tetrahydride isomer by 0.17 kcal mol-1. Removal of the BF4
counterion increases the optimized H-H distance in the
dihydride-dihydrogen structure by approximately 0.1 Å. Thus,
the DFT calculations present two possibilities for why the H-H
distance is longer in solution: the H-H distance in the
dihydrogen ligand increases when the BF4 counterion is removed
from the vicinity of the cation, and another structure with four
classical hydride ligands becomes similar in energy to the
dihydride-dihydrogen structure. Thus, the theoretical results
are consistent with the present solution experiments, which
suggest that the H-H distance is longer in solution than in the
solid state.
For the PCy3 compound 3, the T1(min) value affords a H-H
distance of 0.89 Å if we assume that the H2 ligand is rotating
rapidly relative to the molecular tumbling rate in solution but
1.12 Å if we assume that rotation is slow. The former value is
unlikely, in view of the DFT calculations (see below); thus, 3
differs from 1 and 2 in that the rate of H2 rotation is slow rather
than fast relative to the rate of molecular tumbling. The reason
for this difference in rotation rates may be related to the
electronic factors responsible for the different structure adopted
by 3 in the solid state; perhaps also relevant is the proposal
that cis dihydride-dihydrogen interactions can affect the barrier
to H2 rotation.32,33
Unlike the behavior seen for 1 and 2, in which the solution
H-H distances are longer than that deduced by neutron
diffraction by about 0.06 Å, for 3 the solution H-H distances
are shorter by about 0.15 Å. DFT calculations again afford some
insight into this finding.10 When the BF4 counterion is included
in the calculations, there is a single minimum-energy structure
in which the two central hydrogens form a very elongated H2
ligand (H-H ) 1.51 Å) oriented with the H-H vector, as seen
in the solid state (perpendicular to the Ct-Os-L plane). For
the free cation, the DFT calculations reveal that there are
multiple minima. Significantly, in one of them, which lies only
1.5 kcal mol-1 above the global minimum, the H-H vector is
still perpendicular to the Ct-Os-L plane but the H‚‚‚H distance
of 1.10 Å is much shorter than that in the global minimum
structure. We suggest that this structure is present in solution,
perhaps in equilibrium with the structure with the longer H-H
bond, and is responsible for the shorter H-H distance deduced
from the solution NMR data. It is unlikely that the different
H-H distance deduced from the relaxation data is due to
changes in the conformation of the cyclohexyl rings of the PCy3
ligand, because these atoms contribute relatively little to the
dipole-dipole relaxation rates.
Table 1. H-H Distances (Å) for [(C5Me5)Os(L)H2(H2)][BF4]
Complexes Determined by Different Methods
L
method
PPh3 (1)
AsPh3 (2)
PCy3 (3)
neutron diffa
X-ray diff
JHD
1.01(1)
0.78(8)
0.97-1.18
1.07c
1.08(1)
not detd
1.06-1.32
1.15c
1.31(3)
1.14(7)
0.99-1.21
1.12d
b
T1(min)
2
a From ref 10. b The range corresponds to varying the JHD coupling
constants involving the classical hydride ligands between -1 and +1 Hz.
c Fast rotation assumption. d Slow rotation assumption.
A similar analysis has been applied to the T1 data for the
AsPh3 compound 2. If we assume that the H2 ligand is in the
fast rotation limit, this procedure affords a value of 1.15 Å for
the H-H distance in 2, vs 1.08 Å for from the neutron
diffraction study. Thus, for both 1 and 2, the H-H distance
derived from the relaxation data is slightly longer than the
distance seen in the solid state.
For the PCy3 compound 3, by following the same procedure
as that above and applying the fast-rotation assumption, we
obtain a calculated H-H distance of 0.89 Å for the dihydrogen
ligand in 3. If we assume instead that the H2 ligand in 3 is in
the slow rotation limit, then we calculate a value of 1.12 Å for
the H-H distance. Both values differ considerably from the
neutron diffraction value of 1.35 Å, and this finding strongly
supports the conclusion from the JHD data that the solution
structure differs from the solid-state structure.
Esteruelas has recently described the preparation of a similar
dihydride-elongated dihydrogen derivative, [(C5H5)Os(PiPr3)-
H2(H2)+], as both the BF4 and triflate salts.30 The average JHP
coupling constant of 14.1 Hz is very similar to the value we
observe for 1. The T1 value decreases from 950 ( 20 ms at 0
°C to 106 ( 1 ms at -93 °C but did not reach a minimum in
this temperature range. Partial deuteration afforded an average
JHD coupling constant of 3.6 Hz, corresponding to a H-H
distance of about 1.1 Å, which agrees well with that found in
1 by neutron diffraction.
Comparison of H-H Distances Determined by Different
Methods. Table 1 summarizes the H-H distances derived from
neutron diffraction, X-ray diffraction, H-D coupling constants,
and variable-temperature proton relaxation rates. In making these
comparisons, it should be kept in mind that neutron diffraction
data are collected at 20 K, vs ∼200 K for the JHD(av) and
T1(min) measurements. Because the H-H distances derived
from the average H-D coupling constants are especially
uncertain, owing to the unknown values of the intrinsic two-
bond coupling constants involving the classical hydride ligands,
we will focus our discussion on the distances determined from
the relaxation experiments. Similarly, the H-H distances
determined from the X-ray studies are highly uncertain and we
will instead focus on the neutron diffraction results.
One principal conclusion of this work is that the H-H bond
distances for 1 and 2 determined from the T1(min) measurements
are both ca. 0.06 Å longer than the values obtained by neutron
diffraction. The difference may be ascribable in part to the
effects of librational motion on the H-H distance derived from
neutron diffraction: such motion artificially shortens the H-H
bond distance by 0.02-0.1 Å.14,31 Another explanation of the
longer H-H distances deduced in the solution experiments can
Few other examples of polyhydrides are known that adopt
more than one structure in solution or that adopt different
structures in solution vs the solid state.34 The molecule
(32) Eisenstein, O.; Jackson, S. A. Inorg. Chem. 1990, 29, 3910-3914.
(33) Bianchini, C.; Masi, D.; Perruzzini, M.; Casarin, M.; Maccato, C.;
Rizzi, G. A. Inorg. Chem. 1997, 36, 1061-1069.
(34) We exclude from consideration polyhydrides in which a single
ground-state structure is dynamic via a high-energy intermediate and instead
restrict our discussion to structures that have non-negligible populations at
equilibrium.
(30) Esteruelas, M. A.; Hernandez, Y. A.; Lopez, A. M.; Olivan, M.;
Onate, E. Organometallics 2005, 24, 5989-6000.
(31) Kubas, G. J.; Burns, C. J.; Eckert, J.; Johnson, S. W.; Larson, A.
C.; Vergamini, P. J.; Unkefer, C. J.; Khalsa, G. R. K.; Jackson, S. A.;
Eisenstein, O. J. Am. Chem. Soc. 1993, 115, 569-581.

[(C5Me5)Os(L)H2(H2)+] Complexes
Organometallics, Vol. 26, No. 7, 2007 1663
[ReH4(CO)(PMe2Ph)3+] exists in solution as a mixture of the
solid was collected by filtration. Yield: 0.14 g (81%). Anal. Calcd
for C28H34BF4OsAs: C, 46.6; H, 4.74; As, 10.4. Found: C, 46.7;
H, 4.82; As, 10.2. 1H NMR (CD2Cl2): δ 7.53 (m, m + p-CH, 9H),
7.39 (m, o-CH, 6H), 2.13 (s, C5Me5, 15H), -9.88 (s, Os-H, 4H).
13C{1H} NMR (CD2Cl2): 132.6 (s, i-C), 132.2 (s, o-C), 132.0 (s,
p-C), 129.9 (s, m-C), 99.6 (s, C5Me5), 11.1 (s, C5Me5). IR (Nujol,
cm-1): 2088 (w), 2029 (w), 1568 (w), 1563 (w), 1476 (m,sh), 1439
(s), 1420 (w), 1366 (m, sh), 1340 (w), 1313 (w), 1307 (w), 1281
(w), 1184 (w), 1166 (w), 1151 (w), 1096 (m), 1081 (s), 1054 (s),
1035 (s), 999 (m), 988 (w), 977 (w), 971 (w), 961 (w), 933 (w),
911 (w), 865 (w), 848 (w), 833 (w), 807 (w), 795 (w), 754 (s), 748
(s), 723 (w), 702 (w), 694 (m), 668 (w), 634 (w), 613 (w), 576
(w), 522 (w), 486 (w), 475 (m), 460 (w), 416 (w), 407 (w).
(Pentamethylcyclopentadienyl)dihydrido(η2-dihydrogen)(tri-
cyclohexylphosphine)osmium(IV) Tetrafluoroborate, [(C5Me5)-
Os(PCy3)H2(H2)][BF4] (3). To a solution of (C5Me5)OsH3(PCy3)
(0.15 g, 0.25 mmol) in diethyl ether (10 mL) was added HBF4‚
Et2O (0.15 mL, 2.0 mmol). A white precipitate formed immediately.
The solution was stirred at room temperature for 30 min, and the
white solid was collected by filtration. Yield: 0.09 g (52%). Anal.
Calcd for C28H52BF4OsP: C, 48.3; H, 7.52; P, 4.45. Found: C,
48.5; H, 7.98; P, 4.27. 1H NMR (CD2Cl2): δ 2.27 (s, C5Me5, 15H),
1.90 (m, PCy3, 6H), 1.76 (m, PCy3, 9H), 1.68 (m, PCy3, 3H), 1.27
(m, PCy3, 15H), -10.61 (d, JPH ) 15.7 Hz, Os-H, 4H). 13C{1H}
NMR (CD2Cl2): δ 99.8 (s, C5Me5), 38.7 (d, JPC ) 30.4 Hz, 1-C),
29.8 (d, JPC ) 2.8 Hz, 3-C), 27.5 (d, JPC ) 12.0 Hz, 2-C), 26.5 (s,
4-C), 11.5 (s, C5Me5). 31P{1H} NMR (CD2Cl2): δ 31.3 (s). IR
(Nujol, cm-1): 2173 (w), 2102 (w), 1423 (w), 1366 (w), 1348 (w),
1343 (w), 1329 (w), 1321 (w), 1306 (w), 1298 (w), 1279 (w), 1265
(w), 1226 (w), 1199 (w), 1185 (w), 1174 (m), 1127 (w), 1102 (m),
1086 (s), 1060 (s), 1047 (s), 1032 (s), 960 (w), 900 (w), 889 (w),
852 (w), 846 (w), 827 (w), 813 (m), 798 (w), 740 (w), 722 (w),
531 (w), 517 (w), 490 (w).
dihydrogen-dihyride and classical tetrahydride forms,35-37
a
finding that has been supported by DFT calculations.38,39
+
Gusev40 has shown that [FeH3(PMe3)4+] and [RuH3(PEt3)4
]
isomerize between six-coordinate MII dihydrogen-hydride and
seven-coordinate MIV trihydride geometries, and similar systems
have been studied theoretically.41
Experimental Section
All operations were carried out under argon or vacuum by using
standard Schlenk techniques. Solvents were distilled under nitrogen
from sodium benzophenone (diethyl ether, pentane) or magnesium
(ethanol). The osmium compounds (C5Me5)Os(L)H3 were synthe-
sized as described elsewhere.12 Tetrafluoroboric acid (Aldrich) and
deuterium oxide (Cambridge) were used without further purification.
Elemental analyses were performed by the University of Illinois
Microanalytical Laboratory. Field desorption mass spectra were
recorded on a Finnigan-MAT 731 mass spectrometer from samples
loaded as solutions in dichloromethane. The IR spectra were
recorded on a Perkin-Elmer 1700 FT-IR instrument as Nujol mulls
1
between KBr plates. The H and 31P NMR data were recorded on
a Varian Unity 400 spectrometer at 400 and 161 MHz, respectively;
13C NMR spectra were recorded on a Varian Unity 500 spectrometer
at 101 MHz. Some NMR spectra were recorded on General Electric
QE-300, GN-300, and GN-500 instruments. Chemical shifts are
reported in δ units (positive shifts to high frequency) relative to
SiMe4 (1H and 13C) or H3PO4 (31P). T1 measurements were made
by the inversion recovery method, with the time-dependent intensi-
ties fitted by least squares to an exponential formula.
(Pentamethylcyclopentadienyl)dihydrido(η2-dihydrogen)(tri-
phenylphosphine)osmium(IV) Tetrafluoroborate, [(C5Me5)Os-
(PPh3)H2(H2)][BF4] (1). To a solution of (C5Me5)OsH3(PPh3) (0.45
g, 0.76 mmol) in diethyl ether (25 mL) was added HBF4‚Et2O (0.20
mL, 2.7 mmol). A white precipitate formed immediately. The
solution was stirred at room temperature for 1.5 h, and the white
solid was collected by filtration. Yield: 0.48 g (93%). Anal. Calcd
for C28H34BF4OsP: C, 49.6; H, 5.05; P, 4.56. Found: C, 49.6; H,
5.05; P, 4.27. 1H NMR (CD2Cl2): δ 7.52 (m, m-CH + p-CH, 9H),
7.34 (m, o-CH, 6H), 2.06 (d, JPH ) 1.4 Hz, C5Me5, 15H), -9.62
(d, JPH ) 14.2 Hz, Os-H, 4H). 13C{1H} NMR (CD2Cl2): δ 133.2
(d, JPC ) 62.6 Hz, i-C), 133.1 (d, JPC ) 10.7 Hz, o-C), 132.1
(d, JPC ) 3.1 Hz, p-C), 129.3 (d, JPC ) 11.4 Hz, m-C), 100.4 (s,
C5Me5), 10.8 (s, C5Me5). 31P{1H} NMR (CD2Cl2): δ 10.9 (s). IR
(Nujol, cm-1): 2108 (m), 2063 (m), 2008 (w), 1979 (w), 1822 (w),
1586 (w), 1578 (w), 1483 (s), 1436 (s), 1342 (w), 1334 (w), 1313
(m), 1284 (m), 1181 (w), 1163 (m), 1094 (s), 1062 (s), 1051 (s),
1036 (s), 1027 (s), 1006 (m), 997 (m), 977 (w), 965 (w), 934 (w),
920 (w), 894 (w), 856 (w), 834 (w), 786 (m), 757 (m), 752 (m),
747 (s), 697 (s), 593 (w), 533 (s), 519 (m), 511 (s), 503 (s), 467
(m), 434 (w), 426 (w).
Crystallographic Studies.42 Single crystals of [(C5Me5)Os-
(PPh3)H2(H2)][BF4] (1), grown from 1:1 CH2Cl2/Et2O, were
mounted on glass fibers with Paratone-N oil (Exxon) and im-
mediately cooled to -75 °C in a cold nitrogen gas stream on the
diffractometer. [Single crystals of [(C5Me5)Os(PCy3)H2(H2)][BF4]
(3), grown from CH2Cl2/Et2O, were treated similarly. Subsequent
comments in brackets will refer to this compound.] The cell
dimensions in Table 1 were calculated from 8192 [7099] reflections.
Data were collected with an area detector by using the measure-
ment parameters listed in Table 1. The measured intensities were
reduced to structure factor amplitudes and their esd’s by correction
for background, scan speed, Lorentz, and polarization effects.
Systematic absences for h0l (l * 2n) and 0k0 (k * 2n) were only
consistent with space group P21/c. [For 3, the cell constants were
only consistent with space groups P1 and Phl and refinement was
successful in the latter space group.] An empirical absorption
correction was applied, the maximum and minimum transmission
factors being 0.989 and 0.607. Systematically absent reflections
were deleted, and symmetry-equivalent reflections were averaged
to yield the set of unique data. Five reflections (11,16,2, -8,11,6,
(Pentamethylcyclopentadienyl)dihydrido(η2-dihydrogen)(tri-
phenylarsine)osmium(IV) Tetrafluoroborate, [(C5Me5)Os(AsPh3)-
H2(H2)][BF4] (2). To a solution of (C5Me5)OsH3(AsPh3) (0.15 g,
0.24 mmol) in diethyl ether (15 mL) was added HBF4‚Et2O (0.10
mL, 1.3 mmol). A white precipitate formed immediately. The
solution was stirred at room temperature for 20 min, and the white
2
2
-12,13,1, 0,2,10, and -10,19,6) with Fo < -3σ(Fo ) were
suppressed. [For 3, a face-indexed absorption correction was
applied, the maximum and minimum transmission factors being
0.844 and 0.529. The 0,0,1 reflection was obscured by the beam
stop and was deleted.] The remaining 6463 [6629] data were used
in the least-squares refinement.
(35) Luo, X. L.; Crabtree, R. H. J. Am. Chem. Soc. 1990, 112, 6912-
6918.
(36) Gusev, D. G.; Nietlispach, D.; Eremenko, I. L.; Berke, H. Inorg.
Chem. 1993, 32, 3628-3636.
(37) Gusev, D. G.; Berke, H. Chem. Ber. 1996, 129, 1143-1155.
(38) Lin, Z. Y.; Hall, M. B. J. Am. Chem. Soc. 1994, 116, 4446-4448.
(39) Gusev, D. G. Organometallics 2003, 22, 5148-5151.
(40) Gusev, D. G.; Hubener, R.; Burger, P.; Orama, O.; Berke, H. J.
Am. Chem. Soc. 1997, 119, 3716-3731.
(41) Li, J.; Dickson, R. M.; Ziegler, T. J. Am. Chem. Soc. 1995, 117,
11482-11487.
The correct positions for the osmium and phosphorus atoms were
deduced from a sharpened Patterson map. The quantity minimized
2
2
by the least-squares program was ∑w(Fo - Fc ),2 where w )
{[σ(Fo )]2 + (0.0166P)2 + 13.017P}-1 and P ) (Fo + 2Fc2)/3
2
2
2
[for 3, w ) {[σ(Fo )]2 + 7.0947P}-1]. The analytical approxima-
(42) For a description of the crystallographic programs and procedures
used, see: Brumaghim, J. L.; Priepot, J. G.; Girolami, G. S. Organometallics
1999, 18, 3139-2144.

1664 Organometallics, Vol. 26, No. 7, 2007
Gross and Girolami
tions to the scattering factors were used, and all structure factors
were corrected for both real and imaginary components of
anomalous dispersion. Independent anisotropic displacement factors
were refined for the non-hydrogen atoms. Hydrogen atoms on the
phosphine and C5Me5 ligands were placed in idealized tetrahedral
locations with C-H ) 0.98 Å (methyl) and 1.00 Å (methine); the
methyl groups were treated as rigid rotators, and their positions
were optimized by rotation about the C-C bond. The displacement
parameters for the methyl hydrogen atoms were set equal to 1.5
times Ueq for the attached carbon atom, and the displacement
parameters for the methine hydrogen atoms were set equal to 1.2
times Ueq. The osmium-bound hydrogen atoms were located in the
difference Fourier maps, and their locations were independently
refined with a common isotropic displacement factor. Convergence
of the hydride locations was slow, especially for H4, and thus some
doubt must be associated with their positions. [For 3, methyl and
methine hydrogens were treated as above; methylene hydrogens
were placed in idealized positions with C-H ) 0.99 Å and
displacement parameters set equal to 1.2 times Ueq for the attached
carbon atom. Likely positions for the osmium-bound hydrogen
atoms surfaced in the latter stages of the refinement, and their
positions were refined independently, except that a common
displacement parameter was assigned for them.] An isotropic
extinction parameter was refined to a final value of x ) 1.0(1) ×
10-6 [2.5(2) × 10-6], where Fc is multiplied by the factor k[1 +
Fc2xλ3/sin 2θ]-1/4 with k being the overall scale factor. Successful
convergence was indicated by the maximum shift/error of 0.002
[0.001] for the last cycle. Final refinement parameters are given in
Table 1. The largest peak in the final Fourier difference map (2.29
e Å-3) was located 3.17 Å from the boron atom and 2.18 Å from
F(4). [For 3, the largest peak in the final Fourier difference map
(0.94 e Å-3) was located 1.18 Å from osmium.] A final analysis
of variance between observed and calculated structure factors
showed no apparent errors.
Acknowledgment. We thank the National Science Founda-
tion (Grant Nos. CHE 00-76061 and CHE 04-20768) for support
of this work, and Dr. Scott Wilson and Ms. Teresa Prussak-
Wieckowska of the University of Illinois Materials Chemistry
Laboratory for collecting the crystallographic data for 1 and 3.
C.L.G. thanks the University of Illinois at Urbana-Champaign
for a departmental fellowship. We thank a reviewer for valuable
comments about the interpretations of the solution NMR data.
Supporting Information Available: CIF files giving X-ray
crystallographic data for [(C5Me5)Os(PPh3)H2(H2)][BF4] (1) and
[(C5Me5)Os(PCy3)H2(H2)][BF4] (3). This material is available free
of charge via the Internet at http://pubs.acs.org.
OM061017E