Structure and dynamics of a dihydrogen/hydride ansa molybdenocene complex

Article abstract of DOI:10.1021/ic0496875

In contrast to [Cp₂MoH₃]⁺, which is a thermally stable trihydride complex, the ansa-bridged analogue [(η-C ₅H₄)₂-CMe₂MoH(H₂)] ⁺ (1) is a thermally labile dihydrogen/hydride complex. Partial deuteration of the hydride ligands allows observation of J_HD = 11.9 Hz in 1-d₁ and 9.9 Hz in 1-d₂ (245 K), indicative of a dihydrogen/hydride structure. There is a slight preference for deuterium to concentrate in the dihydrogen ligand. A rapid dynamic process interchanges the hydride and dihydrogen moieties in complex 1. Low temperature ¹H NMR spectra of 1 give a single hydride resonance, which broadens at very low temperature due to rapid dipole-dipole relaxation (T₁ = 23 ms (750 MHz, 175 K) for the hydride resonance in 1). Low temperature ¹H NMR spectra of 1-d₂ allow the observation of decoalescence at 180 K into two resonances. The bound dihydrogen ligand exhibits hindered rotation with ΔG?₁₅₀ = 7.4 kcal/mol, but H atom exchange is still rapid at all accessible temperatures (down to 130 K). Density functional calculations confirm the dihydrogen/hydride structure as the ground state for the molecule and give estimates for the energy of two hydrogen exchange processes in good agreement with experiment. The presence of the C ansa bridge is shown to decrease the ability of the metallocene fragment to donate to the hydrogens, thus stabilizing the (η²-H₂) unit and modulating the barrier to H₂ rotation.

Full text of DOI:10.1021/ic0496875

Inorg. Chem. 2004, 43, 3475−3483
Structure and Dynamics of a Dihydrogen/Hydride Ansa Molybdenocene
Complex
Vincent Pons,† Stephen L. J. Conway,‡ Malcolm L. H. Green,‡ Jennifer C. Green,‡
,†
Benjamin J. Herbert,‡ and D. Michael Heinekey*
Department of Chemistry, UniVersity of Washington, Box 351700,
Seattle, Washington 98195-1700, and Inorganic Chemistry Laboratory,
UniVersity of Oxford, South Parks Road, Oxford, OX1 3QR, U.K.
Received March 10, 2004
In contrast to [Cp2MoH ]+, which is a thermally stable trihydride complex, the ansa-bridged analogue [(η-C H ) -
3
5 4 2
CMe2MoH(H )]+ (1) is a thermally labile dihydrogen/hydride complex. Partial deuteration of the hydride ligands
2
allows observation of JH-D ) 11.9 Hz in 1-d1 and 9.9 Hz in 1-d2 (245 K), indicative of a dihydrogen/hydride
structure. There is a slight preference for deuterium to concentrate in the dihydrogen ligand. A rapid dynamic
process interchanges the hydride and dihydrogen moieties in complex 1. Low temperature 1H NMR spectra of 1
give a single hydride resonance, which broadens at very low temperature due to rapid dipole−dipole relaxation (T
1
) 23 ms (750 MHz, 175 K) for the hydride resonance in 1). Low temperature 1H NMR spectra of 1-d2 allow the
observation of decoalescence at 180 K into two resonances. The bound dihydrogen ligand exhibits hindered rotation
q
with ∆G 150 ) 7.4 kcal/mol, but H atom exchange is still rapid at all accessible temperatures (down to 130 K).
Density functional calculations confirm the dihydrogen/hydride structure as the ground state for the molecule and
give estimates for the energy of two hydrogen exchange processes in good agreement with experiment. The
presence of the C ansa bridge is shown to decrease the ability of the metallocene fragment to donate to the
hydrogens, thus stabilizing the (η2-H ) unit and modulating the barrier to H rotation.
2
2
Since the report of the first transition metal dihydrogen
complexes,1 the possibility that a fluxional polyhydride
complex might also contain a dihydrogen ligand has been
actively investigated.2 In this context, the dynamic behavior
of prototypical complexes containing one dihydrogen and
one hydride ligand is of particular interest. Excluding the
large group of known complexes with hydride and dihydro-
gen ligands trans to each other, which exhibit relatively high
barriers to site exchange between hydride and dihydrogen
ligands,3 we have focused on complexes with hydride and
dihydrogen ligands in cis positions. Several complexes of
this type have been reported. All show interesting dynamic
properties, exhibiting rapid exchange of hydrogen environ-
ments between the dihydrogen and the hydride ligands, as
revealed by variable temperature NMR studies. Early ex-
amples of this situation include the iridium complex [IrH-
(H2)(bq)(PPh3)2]+ (bq ) benzoquinolate), which was reported
by Crabtree and co-workers to undergo hydrogen atom
exchange between the dihydrogen and hydride ligands with
∆Gq240 ) 10 kcal/mol.4 An iron complex [FeP(CH2CH2CH2-
PMe2)3(H2)H]+, which has ∆Gq
) 9.1 kcal/mol for
200
permutation of the hydrogen environments, was reported by
Field and Bampos.5 In these and most other reported
examples6 of cis dihydrogen/hydride complexes, atom ex-
change between the hydride and dihydrogen environments
* Author to whom correspondence should be addressed. E-mail:
heinekey@chem.washington.edu.
† University of Washington.
‡ University of Oxford.
(1) Kubas, G. J.; Ryan, R. R.; Swanson, B. I.; Vergamini, P. J.;
Wasserman, H. J. J. Am. Chem. Soc. 1984, 106, 451. General review:
Kubas, G. J. Metal Dihydrogen and σ Bond Complexes; Kluwer
Academic/Plenum Publishers: New York, 2001. Review of compu-
tational studies: Maseras, F.; Lledo´s, A.; Clot, E.; Eisenstein, O. Chem.
ReV. 2000, 100, 601.
(2) Cf.: Gusev, D. G.; Kuhlman, R. L.; Renkema, K. B.; Eisenstein, O.;
Caulton, K. G. Inorg. Chem. 1996, 35, 6775 and references therein.
(3) Jessop, P. G.; Morris, R. H. Coord. Chem. ReV. 1992, 121, 155.
(4) Crabtree, R. H.; Lavin, M.; Bonneviot, L. J. Am. Chem. Soc. 1986,
108, 4032.
(5) Bampos, N.; Field, L. D. Inorg. Chem. 1990, 29, 587.
(6) Other reported cis dihydrogen/hydride complexes: Bianchini, C.;
Peruzzini, M.; Zanobini, F. J. Organomet. Chem. 1988, 354, C19.
Jia, G.; Drouin, S. D.; Jessop, P. G.; Lough, A. J.; Morris, R. H..
Organometallics 1993, 12, 906. Bianchini, C.; Perez, P. J.; Peruzzini,
M.; Zanobini, F.; Vacca, A. Inorg. Chem. 1991, 30, 279. See also ref
3, page 200.
10.1021/ic0496875 CCC: $27.50 © 2004 American Chemical Society
Published on Web 05/05/2004
Inorganic Chemistry, Vol. 43, No. 11, 2004 3475

Pons et al.
also requires significant rearrangement on the part of the
ancillary ligands, which might be expected to contribute
substantially to the activation energy for the process.
Highly symmetric cis dihydrogen/hydride complexes in
which no heavy atom rearrangement is required to effect
hydrogen atom exchange are much less studied. Iridium
dihydrogen/hydride complexes of the form [TpIr(PR3)-
(H2)H]+ 7,8 exhibit a rapid dynamic process which exchanges
the hydride and dihydrogen environments. Unfortunately, this
process is sufficiently rapid that no low temperature limiting
1H NMR spectrum could be obtained. The barrier to the site
exchange process was estimated to be less than 4.5 kcal/
mol.7 Ru complexes such as [(PCy3)2Ru(CO)2(H2)H]+ exhibit
very rapid hydride/dihydrogen exchange, with a barrier
measured by very low temperature NMR spectroscopy of
Figure 1. Partial (hydride region) 1H NMR spectrum of complex 1 at
245 K (750 MHz) with D incorporated in the hydride ligands. JH-D values
are 11.9 Hz for 1-d1 and 9.9 Hz for 1-d2.
∆Gq ) 5.5 kcal/mol.9 The extremely facile reversible
128
cleavage of a relatively strong H-H bond in these complexes
is remarkable and suggests a highly concerted reaction in
which a substantial degree of bond formation accompanies
the cleavage reaction.
Density functional theory (DFT) calculations verify that the
dihydrogen/hydride structure is preferred to the trihydride
structure when an ansa bridge is present. Complex 1 is one
of a small number of reported H2 complexes with a d2
configuration at the metal center and is the first example of
a cis dihydrogen/hydride complex with this electronic
configuration. Complex 1 exhibits novel dynamic behavior
in which hindered dihydrogen rotation can be studied by low
temperature NMR spectroscopy. DFT calculations of the
barrier to rotation are consistent with the experimental
observations.
Metallocene complexes have been used to prepare some
very important examples of early metal dihydrogen com-
plexes. Of particular interest are species such as [Cp2Ta-
(CO)(H2)]+, which have a d2 configuration, leading to
significant barriers to rotation of bound dihydrogen.10 Related
trihydrides such as [Cp2MH3]+ (M ) Mo, W) were reported
by Wilkinson and co-workers very early in the development
of metallocene chemistry.11 While the tungsten trihydride
has a static structure, the Mo analogue is highly fluxional
and exhibits quantum mechanical exchange coupling in low
temperature 1H NMR spectra.12 More recent work by Green
and co-workers has demonstrated that the incorporation of
ansa bridges can significantly modify the dynamic properties
of these molecules. For example, ansa tungstenocene species
such as [(η-C5H4)2CMe2WH3]+ show very rapid hydride
atom exchange at ambient temperature. Low temperature
NMR spectroscopy reveals extremely large exchange cou-
plings.13
We now report our studies of an ansa molybdenocene
complex [(η-C5H4)2CMe2MoH(H2)]+ (1), which is a ther-
mally labile dihydrogen/hydride complex, in contrast to the
unsubstituted Cp complex [Cp2MoH3]+, which is a stable
trihydride species. The presence of the ansa bridge apparently
favors the formation of a bound dihydrogen ligand, which
does not undergo formal oxidative addition to afford a MoVI
trihydride as observed in the absence of the ansa bridge.
Results
Synthesis and Characterization of 1. Protonation of the
neutral ansa dihydride (η-C5H4)2CMe2MoH2 with H(OEt2)2-
(BArF4) (ArF ) 3,5-C6H3(CF3)2) affords a cationic species
1 which is thermally labile above 260 K. Low temperature
1H NMR spectra (250 K) of complex 1 reveal a single new
hydride resonance at δ ) -5.59 ppm (3H) in addition to
appropriate resonances for the Cp ring protons and the methyl
groups of the ansa bridge. At lower observation temperatures,
considerable line broadening of the hydride resonance was
observed. Relaxation time determinations revealed a T1 of
23 ms at 175 K (750 MHz) for the hydride resonance. The
maximum rate of relaxation (min T1) could not be reached,
but this value is indicative of the presence of a dihydrogen
ligand (see Discussion).
Complex 1 decomposes with loss of H2 over the course
of several hours at temperatures g260 K. The rate of
decomposition was not significantly affected by the presence
of an atmosphere of H2. Attempts to incorporate deuterium
in the hydride sites of 1 by exposure of solutions to D2 gas
at low temperature were not successful.
(7) Oldham, W. J., Jr.; Hinkle, A. S.; Heinekey, D. M. J. Am. Chem. Soc.
1997, 119, 11028.
(8) Heinekey, D. M.; Oldham, W. J., Jr. J. Am. Chem. Soc. 1994, 116,
3137.
(9) Heinekey, D. M.; Mellows, H.; Pratum, T. J. Am. Chem. Soc. 2000,
122, 6498.
(10) Sabo-Etienne, S.; Rodriguez, V.; Donnadieu, B.; Chaudret, B.; El
Makarim, H. A.; Barthelat, J. C.; Ulrich, P.; Limbach, H. H.; Mo¨ıse,
C. New J. Chem. 2001, 25, 55.
(11) Green, M. L. H.; McLeverty, J. A.; Pratt, L.; Wilkinson, G. J. Chem.
Soc. 1961, 4854.
(12) Heinekey, D. M. J. Am. Chem. Soc. 1991, 113, 6074.
(13) Chernega, A.; Cook, J.; Green, M. L. H.; Labella, L.; Simpson, S. J.;
Souter, J.; Stephens, A. H. H. J. Chem. Soc., Dalton Trans. 1997,
3225.
Complex 1 was prepared in a partially deuterated form
by reaction of (η-C5H4)2CMe2MoH2 with (CF3SO2)2CD2. The
1
hydride region H NMR spectrum at 245 K of a typical
partially deuterated sample is shown in Figure 1. The signal
due to 1-d1 is a 1:1:1 triplet with JH-D ) 11.9 Hz while the
resonance attributed to 1-d2 gives the expected five line
pattern with JH-D ) 9.9 Hz. The signal due to 1-d1 is shifted
3476 Inorganic Chemistry, Vol. 43, No. 11, 2004

A Dihydrogen/Hydride Ansa Molybdenocene Complex
Table 1. Selected Structural Parameters for the Ground State Structures
of 1, 2, and 3
1
2
3
Mo-Ha
Mo-Hb
Mo-Hc
Ha-Hb
1.768
1.782
1.688
0.985
1.832
2.276
1.927
1.423
1.682
1.695
1.669
1.660
1.606
2.288
1.942
1.424
1.667
1.680
1.667
1.648
1.649
2.294
1.951
1.418
Hb-Hc
Mo-Cav
Mo-Cpcent
C-Cav
R
60
128
162
45
138
155
29
151
Cpcent-Mo-Cpcent
â
with no symmetry restrictions, were used to find the ground
state structure in each case. The lowest energy structure for
1 was found to be [(η-C5H4)2CMe2MoH(η2-H2)]+ with a
bound dihydrogen ligand. Compound 3 has a trihydride
ground state of C2V symmetry. The orientation of the
cyclopentadienyl rings is such that four carbons occupy the
back of the metallocene wedge (A); the alternative C2V
structure (B) with two carbons at the back of the wedge was
calculated to be 7.6 kcal/mol higher in energy.
Figure 2. Partial (hydride region) 1H NMR spectrum of complex 1, 1-d1,
and 1-d2 at 145 K (500 MHz): (a) low level of deuteration; (b) high level
of deuteration.
upfield from 1 by ∆δ ) 30 ppb, while the signal due to
1-d2 is upfield by ∆δ ) 80 ppb. The H-D coupling constants
show slight temperature dependence, consistent with the
operation of isotopic perturbation arising from nonstatistical
deuterium distribution. These observations, taken together
with the T1 data, suggest formulation of complex 1 as a
dihydrogen/hydride complex [(η-C5H4)2CMe2MoH(H2)]+
(see Discussion).
The Si ansa bridged species 2 has a trihydride ground state,
which is distorted from C2V symmetry to form a structure
analogous to A with four carbons at the back. The Si sits to
one side of the vector defined by Mo and the central
hydrogen atom, Hb. Attempts to optimize a (H2)H structure
for [(η-C5H4)2SiH2MoH3]+ gave a structure 1.3 kcal/mol
higher in energy but with two imaginary frequencies, one at
-213i corresponding to H-H stretch, and one at -80i
corresponding to ring motion. Calculations by others using
different functionals and basis sets come to an opposite
conclusion for Si bridged species, in that they find a (H2)H
structure to be the ground state; our methodology gives them
as very close in energy.14
Highly deuterated samples of 1 were prepared by reaction
of (η-C5H4)2CMe2MoH2 with excess (CF3SO2)2CD2 at 200
K in methylene chloride or Freon solutions. The resulting
samples contained predominantly 1-d2, with smaller amounts
of 1-d1 and some 1. Low temperature 1H NMR spectra were
recorded in CDF2Cl/CDF3 mixtures. Typical spectra of the
hydride region at low temperature are shown in Figure 2. A
collection of spectra at a variety of temperatures can be found
in the Supporting Information. The assignments of reso-
nances to the various isotopomers is based on the examina-
tion of several samples with varying levels of deuteration.
For 1-d2, decoalescence to give two hydride signals is
observed at ca. 180 K. The signal at -4.25 ppm corresponds
to one hydride and is broad, while the signal at -6.50 ppm
corresponds to two hydrides and is somewhat sharper. Further
cooling allows the resolution of a five line pattern in the
resonance at -4.28 ppm, with JH-D ) 15.6 Hz. No H-D
coupling is resolvable in the resonance at -6.49 ppm. The
signal due to 1 is a single resonance at all accessible
temperatures, while 1-d1 gives two distinct resonances at
-5.35 ppm and -6.55 ppm. Interpretation of the low
temperature spectra of the various isotopomers is detailed
in the Discussion section.
Table 1 gives selected calculated structural parameters for
1, 2, and 3 (R is the angle between the ring planes; â is the
angle between the bridging atom-ipso-carbon vector and
the ipso-carbon ring centroid vector; Ha and Hb form the
rotating H2 unit).
H atom exchange processes were investigated by linear
transits and optimizing transition states. For all three
molecules, interchange of Ha and Hb was preceded by rotation
of Ha-Hb out of the metallocene plane and a transition state
Computational Results. Density functional theory was
used to model the related set of compounds 1, [(η-C5H4)2-
SiMe2MoH3]+ (2), and [Cp2MoH3]+ (3). Energies quoted are
for differences in stationary points on the energy surface,
except where otherwise specified. Geometry optimizations,
(14) Parkin, G. Personal communication.
Inorganic Chemistry, Vol. 43, No. 11, 2004 3477

Pons et al.
structure with parallel rings.15 In contrast, ansa TaV trihydride
complexes have been shown to exhibit enhanced rates of
H2 loss in comparison to their nonbridged analogues.16
Structure of Complex 1. The nonbridged Cp analogue
of complex 1 is a trihydride complex, with interesting
dynamic behavior featuring significant exchange coupling.12
Facile loss of H2 from 1 is suggestive of a dihydrogen/
hydride structure. Two possible structures are depicted below.
Table 2. Selected Structural Parameters for the C2V Transition States
1
2
Mo-Ha
Mo-Hb
Ha-Hb
1.687
1.711
1.523
2.270
1.941
1.422
59
128
161
1.676
1.699
1.557
2.298
1.954
1.422
46
139
157
Mo-Cav
Mo-Cpcent
C-Cav
R
Cpcent-Mo-Cpcent
â
Table 3. Selected Structural Parameters for the Cs Transition States
1
2
3
Mo-Ha
Mo-Hc
Ha-Hc
1.910
1.694
0.828
2.261
1.908
1.427
61
129
163
1.887
1.684
0.836
2.255
1.922
1.427
48
139
159
1.867
1.676
0.842
2.276
1.928
1.422
Mo-Cav
Mo-Cpcent
C-Cav
In principle, these two structures can be distinguished by
measurement of relaxation rates for the hydride ligands. The
measurement of T1 values for the hydride resonance in
complex 1 was performed at temperatures between 240 and
170 K using a 750 MHz instrument. The measured T1
decreases to a value of 23 ms at 182 K. Although a true
minimum could not be attained, this value is a reasonable
estimate for T1(min). This short T1 value observed for the
hydride ligands in complex 1 is qualitatively consistent with
the presence of a bound dihydrogen ligand in a hydride/
dihydrogen structure. The H-H bond length can be estimated
from the mutual dipole-dipole relaxation rate Rd-d of the
1H nuclei in the H2 ligand (Rd-d ) 1/T1(d-d)). The total
R
38
147
Cpcent-Mo-Cpcent
â
with Cs symmetry was found with the unit perpendicular to
the plane. The activation energies for H2 rotation are 10.9,
13.7, and 21.6 kcal/mol for 1, 2, and 3 respectively, showing
the effect of the ansa bridge in reducing the barrier to
rotation. For 2 and 3 the rings rotate in the transition state
to conformation B. For 1 and 2 the exchange of Ha and Hc
was also investigated. For 1 the transition state for this
process is of C2V symmetry and is 2.4 kcal/mol higher in
energy than the dihydrogen/hydride ground state. For 2, the
exchange of Ha and Hc also proceeded via a C2V structure
with the Si atom on the C2 axis passing through Mo and Hb.
The transition state lay 0.9 kcal/mol above the ground state.
Dimensions for the transition states are given in Tables 2
and 3.
relaxation rate RH is the sum of Rd-d and other relaxation
2
rates from interaction with other magnetic dipoles in the
complex, Ro (RH ) Rd-d + Ro). In the case of 1, the Robs
2
value is the weighted average of the relaxation rates of a 1H
atom in all sites. Thus, Robs(1) ) (2RH + RH)/3 where RH
2
2
and RH are relaxation rates of the H2 and terminal hydride
ligands, respectively. We assume that RH is modeled by the
relaxation rate Robs(MoH2) of the hydride ligand in the neutral
dihydride complex (T1(min) of MoH2 ) 1.01 s, 195 K, 750
MHz) and, further, that this rate also approximates Ro.
Therefore, as previously derived,7
Discussion
Preparation of Complex 1. Complex 1 is readily prepared
by protonation of the neutral dihydride precursor with a
variety of acids (HBArF4, trifluoroacetic acid, CH2(SO2-
CF3)2). Isolation of complex 1 is not possible, due to rapid
loss of H2 at temperatures of 270 K and above. A complex
mixture of decomposition products is observed, even in Freon
solvents. No enhancement of thermal stability is observed
under an atmosphere of hydrogen, suggesting that a highly
reactive 16 electron species is formed by hydrogen loss,
which then quickly reacts with solvent or counteranion.
Consistent with these observations, attempts to incorporate
deuterium into the hydride positions in 1 by treatment with
D2 gas were not successful. Ultimately, deuterium was
incorporated by treatment of solutions of 1 with (CF3SO2)2-
CD2. The degree of deuterium incorporation was varied by
varying the ratio of acid to neutral dihydride.
Robs(1) ) (2RH + RH)/3 ) [2(Rd-d + Ro) + RH]/3 )
2
[2Rd-d + 3Robs(MoH2)]/3
Rd-d ) 3[Robs(1) - Robs(MoH2)]/2
Using the experimental values reported above, Rd-d ) 65
s-1.
Applying the method of Halpern, the H-H bond length
can be bracketed between 0.8 Å (fast H2 rotation) and 1.0 Å
(slow rotation).17 In this context, the terms fast and slow are
in comparison to molecular tumbling, which is generally very
rapid. On the basis of the hindered rotation of dihydrogen
(15) Green, J. C.; Jardine, C. N. J. Chem. Soc., Dalton Trans. 1998, 1057.
(16) Shin, J. H.; Parkin, G. Chem. Commun. 1999, 887.
(17) Desrosiers, P. J.; Cai, L.; Lin, Z.; Richards, R.; Halpern, J. J. Am.
Chem. Soc. 1991, 113, 4173. Some confusion about the units employed
in the equations describing dipole-dipole relaxation has been recently
clarified: Bayse, C. A.; Luck, R. L.; Schelter, E. J. Inorg. Chem. 2001,
40, 3463.
The instability of 1 with respect to loss of H2 is in contrast
to the enhancement of thermal stability reported for neutral
ansa MoIV and WIV complexes. It has been established that
this stabilization arises from the inability of the metallocene
fragment resulting from reductive elimination to achieve a
3478 Inorganic Chemistry, Vol. 43, No. 11, 2004

A Dihydrogen/Hydride Ansa Molybdenocene Complex
[Cp2MoH3]+ exhibits rapid exchange of the hydride ligands,
leading to a single resonance for the hydride ligands at room
temperature.11 Subsequent investigation of the low temper-
ature NMR spectrum of [Cp2MoH3]+ revealed that the
hydride resonances correspond to an AB2 spin system at low
temperature, with JAB ) 450 Hz at 153 K.12 Recent studies
on ansa derivatives of the tungsten trihydride reveal that the
installation of a bridge between the two cyclopentadienyl
rings has a dramatic effect on the hydride dynamics,
significantly lowering the activation energy for hydrogen
Table 4. Mulliken Atom-Atom Overlap Populations for Ground State
Structures
complex
Mo-Ha
Mo-Hb
Mo-Hc
Ha-Hb
1
2
3
0.0182
0.1813
0.1902
0.0027
0.0158
0.0758
0.2464
0.1589
0.1903
0.3389
0.0966
0.0583
Table 5. Population of the σ and σ* Orbitals of Ha-Hb in the Ground
State and Cs Transition State for 1
σ
σ*
1 ground state
1 Cs transition state
1.87
1.94
0.25
0.06
1
atom exchange. Study of the low temperature H NMR
spectra of complexes such as [(η-C5H4)2CMe2WH3]+ reveals
Very large exchange couplings, with values up to 16000 Hz
reported at 200 K.13 Smaller, but still significant, exchange
couplings were reported for analogues with SiMe2 and C2-
Me4 bridges.18
In the case of complex 1, we anticipated that the d2
configuration would lead to a substantial barrier to H2 rotation
and that the degenerate atom exchange would be slowed
sufficiently at low temperature that three distinct hydride
resonances could be observed, corresponding to a hydride
environment and inner/outer dihydrogen resonances. The
diagram below depicts the dynamic process envisaged for
this molecule, with the central structure representing a
transition state for atom transfer from one side of the
molecule to the other.
described below, the latter distance is more likely to be
correct. A second method based on measurement of JH-D in
the HD ligand is described below which yields a more
accurate value for the H2 bond length.
H-D Coupling in Complex 1. Partial deuteration of the
hydride ligands in complex 1 leads to the observation of
1
distinct resonances for 1, 1-d1, and 1-d2, in the H NMR
spectrum, with the deuterated species exhibiting substantial
H-D coupling. The resonances due to 1-d1 and 1-d2 are
shifted modestly upfield from that due to 1, by ∆δ ) 30
and 80 ppb, respectively. At 245 K, the H-D coupling in
1-d1 is 11.9 Hz, while the H-D coupling in 1-d2 is 9.9 Hz
(see Figure 1). These values for JH-D are indicative of the
presence of a dihydrogen ligand. If the deuterium atoms were
distributed statistically in a hydride/dihydrogen structure, the
1
two values of JH-D would be equal and JH-D would be
exactly three times this value. The observation of different
values of JH-D in the different isotopomers is characteristic
of isotopic perturbation of an equilibrium and a nonstatistical
distribution of deuterium. The actual value for JH-D in
complex 1 must be between 36 and 30 Hz, which leads to
1
1
Surprisingly, H NMR spectra of complex 1 in the hydride
region exhibit a single resonance, even when the temperature
of observation was lowered to 130 K. This observation may
be consistent with a very rapid dynamic process of the type
depicted above, in which both atom exchange and dihydrogen
ligand rotation are extremely facile. Partially deuterated
samples reveal a much more nuanced situation.
Examination of the 1H NMR spectra of mixtures of 1, 1-d1,
and 1-d2 reveals complex dynamic behavior in the partially
deuterated isotopomers. At 140 K, two resonances are
observed for complex 1-d2. The lower field resonance at
-4.28 ppm is a five line pattern due to coupling to two D
nuclei, with JH-D ) 15.6 Hz. This coupling pattern results
from a rapid atom exchange between the two structures
depicted below, with the result that the observed JH-D is equal
to 1JH-D/2, consistent with the high temperature observations
noted previously, where a value of 1JH-D ) 30-36 Hz was
deduced.
an H-H distance of 0.84-0.94 Å.
In contrast, the ∆δ values are essentially independent of
temperature, suggesting that the chemical shift differences
between the different chemical environments are modest and
that much of the observed isotope effect on the chemical
shifts can be attributed to intrinsic isotope effects. This has
been partially verified by low temperature NMR spectra of
1-d2 (vide infra).
The calculated minimum energy structure for 1 is in good
agreement with the experimental findings in that the H3
system is found to be of the dihydrogen/hydride type. The
Mo-H distances correlate with the Mulliken overlap popula-
tions given in Table 4.
The calculated H-H distance in the dihydrogen ligand is
0.985 Å (Table 1), slightly outside the range indicated from
the H-D coupling constant, though within that given by the
relaxation time. Populations of the H2 σ and σ* orbitals are
given in Table 5. The Ha-Hb distance in the rotated Cs
symmetry transition state is 0.828 Å for 1 (see Table 3),
showing that back-donation to the H2 unit has decreased
substantially. The occupation of the H2 σ* orbitals drops from
0.25 in the ground state to 0.06 in the Cs transition state
(Table 5).
It is important to note that these observations establish that
Dynamic Behavior of Complex 1. The tungsten complex
[Cp2WH3]+ has static hydride ligands, but the Mo analogue
(18) Conway, S. L. J.; Dijkstra, T.; Doerer, L. H.; Green, M. L. H.; Labella,
L.; Stephens, A. H. H. J. Chem. Soc., Dalton Trans. 1998, 2689.
Inorganic Chemistry, Vol. 43, No. 11, 2004 3479

Pons et al.
while rotation of the dihydrogen ligand is hindered, degener-
ate H atom exchange as depicted above is still occurring
very rapidly.
The higher field resonance of intensity 2 attributed to 1-d2
exhibits no resolvable H-D coupling. The expected H-D
coupling in this resonance is JH-D/2, which would be ca.
15.6 Hz. This outcome is readily explicable in terms of the
atom exchange process depicted below. The single hydrogen
atom in 1-d2 interacts with a single deuteron, but only one
of the two structures depicted below leads to a significant
coupling.
1
environment and the two H atoms are adjacent, as depicted
1
below. Since the anticipated H-D coupling is again JH-D
/
2, this modest coupling is not resolved. Given that H2 rotation
was found to be hindered in 1-d1, it is reasonable to anticipate
that two distinct chemical environments for H atoms would
result from the situation below, corresponding to “inside”
and “outside” locations for the H atom.
The line width of this resonance at 140 K is ca. 15 Hz. Our
failure to observe this coupling is attributed to the occurrence
of a large DD exchange coupling, which has precedent in
the observations of Otero and co-workers19 for [(η-C5H4-
SiMe3)2Nb(CNR)(H2)]+ and Chaudret and co-workers20 for
[(η-C5H4SiMe3)2Nb(PR3)(H2)]+.
The observation of a single hydride resonance at low
temperature for the molecule depicted above suggests that
some other process is operating to render the two chemical
environments equivalent. We suggest that this is the result
of a large exchange coupling between the two H atoms of
the bound dihydrogen moiety. In an AB spin system, large
values of JAB can lead to a single resonance if JAB is ca.
50∆δ. Our data do not permit an evaluation of the chemical
shift difference between the two ends of the bound dihy-
drogen ligand. Essentially the same situation was reported
by Chaudret and co-workers10 for [Cp2Ta(CO)(H2)]+, where
hindered hydrogen rotation was observed for bound HD, but
a single resonance was seen for bound H2. This was attributed
to the operation of a very large exchange coupling in the
bound dihydrogen ligand. Chaudret and co-workers were able
to directly observe the chemical shift difference between the
two ends of the bound H-D, which was reported to be 0.64
ppm. Assuming a similar chemical shift difference in
complex 1, we estimate an H-H exchange coupling in the
bound dihydrogen ligand in 1 of at least 24000 Hz, based
on the observation of a single resonance for this AB spin
system even at 750 MHz. We note that this value is critically
dependent on the assumed chemical shift difference, and
would be correspondingly reduced if ∆δ in complex 1 is
smaller than observed in the complex studied by Chaudret
and co-workers.
Upon raising the temperature of observation, the reso-
nances attributed to 1-d2 broaden and coalesce at ca. 180 K.
Computed line shapes for spectra collected in the temperature
range 140-180 K were matched to experimental data to
extract the rate of hydrogen ligand rotation. An Eyring plot
of this data gives ∆Hq ) 7.3 kcal/mol, ∆Sq) -0.3 cal/mol
deg, and ∆Gq ) 7.4 kcal/mol. This barrier for hydrogen
150
rotation is comparable to previously reported observations
in d2 Ta and Nb dihydrogen complexes.10 Frequency calcula-
tions on 1 were used to determine ∆Gq for 1 for HaHb
150
exchange at 150 K giving a value of 10 kcal/mol (see
Supporting Information). In this case, with the H-H bond
formed in the ground state, zero point energy differences
between the ground and the transition state made little
difference to the estimated energy barrier. The calculated
solvent effects at 298 K using the COSMO solvent model21
indicate that the activation energy may be reduced by a
further 1.5 kcal/mol. Thus the experimental and calculated
values are in reasonable agreement. A thermodynamic
analysis for all three systems is included in the Supporting
Information.
Frequency calculations on 1 were used to estimate ∆Gq
Very different NMR signals are observed at 140 K for
the resonances attributed to 1-d1. Two signals are again
observed, with a broad singlet at -5.35 ppm (2H) and a 1:1:1
triplet (JH-D ) 15.6 Hz) at -6.55 ppm (1H) (see Figure 2).
The upfield resonance is attributed to a situation where H
atoms are interacting with a single deuteron as shown below.
As noted above, this leads to an observed H-D coupling
150
for 1 for HaHc exchange at 150 K giving a value of 0.69
kcal/mol (see Supporting Information). This very low barrier
would indicate extremely large exchange coupling, as is
found experimentally.
These considerations also allow us to understand the
observation of a single resonance for complex 1 at all
accessible temperatures when no deuterium is incorporated
in the hydride ligands. Rapid atom exchange combined with
large exchange coupling when two H atoms are adjacent
leads to the observed single resonance.
1
equal to JH-D/2.
The broad singlet at lower field is assigned to the form of
1-d1 where the single deuteron is in the “outside” chemical
(19) Antinolo, A.; Carillo-Hermosilla, F.; Fajardo, M.; Garcia-Yuste, S.;
Otero, A.; Camanyes, S.; Maseras, F.; Moreno, M.; Lledos, A.; Lluch,
J. M. J. Am. Chem. Soc. 1997, 119, 6107.
(20) Jalon, F. A.; Otero, A.; Manzano, B. R.; Villasenor, E.; Chaudret, B.
J. Am. Chem. Soc. 1997, 119, 10123.
Isotopic Perturbation Effects. As previously reported for
an iridium hydride/dihydrogen complex, the observed chemi-
cal shifts and JH-D coupling data can in principle be analyzed
quantitatively to calculate the limiting chemical shifts of the
(21) Pye, C. C.; Ziegler, T. Theor. Chem. Acc. 1999, 101, 396.
3480 Inorganic Chemistry, Vol. 43, No. 11, 2004

A Dihydrogen/Hydride Ansa Molybdenocene Complex
dihydrogen ligand (δH ) and the terminal hydride ligand (δH),
In a d2 bent metallocene the two electrons occupy a d(x2)
orbital pointing across the metallocene plane.26 This orbital
has the capacity to back-bond to the cyclopentadienyl rings.
The degree of back-bonding is very sensitive to the ring
orientation and is maximized with conformation B, and
minimized with conformation A.21 The d(x2) orbital also
provides a nodal match with the 1sa - 1sb + 1sc linear
combination of the trihydride 1s orbitals. This antibonding
combination of the three H 1s orbitals lies higher in energy
than the metal d(x2) orbital and competes for back-donation
with the cyclopentadienyl rings. The more the rings stabilize
the d(x2) orbital, the less it is available to bond to the three
hydrogens.
2
1JH-D for the HD ligand, and the energy difference which
arises from deuterium substitution at the different sites.7 In
the case of complex 1, this analysis is complicated by the
relatively small isotope effects observed upon deuteration,
which are attributed in part to the intrinsic isotope effects
on the hydride chemical shifts expected to result from partial
deuteration. A further complication arises from the rapid
atom exchange process described above, which leads to
averaging of chemical shifts even at low temperature.
1
A value of JH-D can be extracted from the low temper-
1
ature H NMR spectra of the isotopomers of complex 1. In
1-d2, the low field resonance is a five line pattern where the
1
observed H-D coupling is equal to JH-D/2. Similarly,
complex 1-d2 exhibits in the upfield resonance a three line
1
pattern with coupling equal to JH-D/2. Both of these
1
couplings are observed to be 15.6 Hz, so JH-D ) 31 Hz.
The averaged JH-D observed for 1-d2 at high temperature is
1
9.9 Hz, which would give a value for JH-D ) 29.7 Hz if
deuterium distribution is statistical. We can thus conclude
that, in this isotopomer, the distribution of deuterium among
the different chemical environments is only slightly perturbed
from a statistical distribution, with a very slight preference
for D to concentrate in the dihydrogen ligand.
Thus the ground-state structure of 3 (MoVI) has ring
conformation A whereas the transition state for HaHb
exchange which has the rotated H2 unit (MoIV) has ring
conformation B. In the rotated transition state there is no
back-bonding to the H ligands from the d(x2) orbital; they
lie in its nodal plane; thus it is stabilized by the rings adopting
conformation B. Compound 1 with the ansa-C bridge is
sufficiently strained that the rings are fixed in conformation
B; this cuts down on back-donation to the hydrogen atoms
and favors the IV oxidation state for Mo with the conse-
quence that the ground state is of the H2/H type and the
barrier to H2 rotation is modest. For 2, the Si bridge is less
strained and both ring conformations are possible, as is found
for 3.
Contour plots for the HOMOs of 1 and 3, both in their
ground states and their Cs transition states, are shown in
Figure 3. In the ground states the greater localization of the
HOMO of 1 on the metal is evident. For both Cs transition
states both the rotated H2 unit and the hydride lie in the nodal
plane of the HOMO, which is consequently localized on the
metal.
The average value of JH-D for 1-d1 observed at high
temperature is 11.9 Hz, which suggests a greater perturbation
from a statistical deuterium distribution in this isotopomer,
with D concentrated significantly in the dihydrogen ligand.
These observations are qualitatively similar to those previ-
ously reported for a rhodium dihydrogen/hydride complex,
where deuterium was also concentrated in the dihydrogen
ligand.22 In contrast, an iridium dihydrogen/hydride complex
exhibits concentration of deuterium in the hydride, rather
than the dihydrogen ligand.7 Since second row metals
generally form weaker M-H bonds than third row metals
and D tends to concentrate in the stronger bond, it is
reasonable that complex 1 exhibits concentration of D in the
dihydrogen ligand. Frequency calculations at 298 K on 1-d1
give relative free energies for the three isotopomers of 0,
0.071, and 0.071 kcal/mol for the D in Ha, Hb, and Hc sites,
respectively, indicating a small preference for deuterium to
accumulate in the H2 ligand.
Electronic Structure Considerations. The change from
a H2/H structure to a trihydride structure can be formally
represented as an oxidation of the metal, in the present case
from MoIV to MoVI. The difference in the structure of 1 and
its W analogue, which has a trihydride structure, is readily
understood in these terms as W is easier to oxidize than Mo.
The role of ansa bridges in affecting the ease of oxidation
at the metal is also well documented.23-25
Conclusion
We find that [(η-C5H4)2CMe2MoH(H2)]+ is the first
example of a dihydrogen/hydride complex with a d2 con-
figuration. DFT calculations are consistent with the experi-
mentally determined structure. The dihydrogen ligand ex-
hibits hindered rotation as revealed by low temperature NMR
spectroscopy of partially deuterated samples. The measured
barrier to rotation is in good agreement with the results of
DFT calculations. While hydrogen rotation has a significant
barrier, H atom transfer between the dihydrogen moiety and
the adjacent hydride remains extremely rapid at all accessible
temperatures. Significant differences in the low temperature
NMR spectra of the isotopomers of 1 are attributed to the
(22) Taw, F. L.; Mellows, H.; White, P. S.; Hollander, F. J.; Bergman, R.
G.; Brookhart, M.; Heinekey, D. M. J. Am. Chem. Soc. 2002, 124,
5100.
(23) Green, J. C.; Scottow, A. New J. Chem. 1999, 23, 651.
(24) Zachmanoglou, C. E.; Docrat, A.; Bridgewater, B. M.; Parkin, G.;
Brandow, G.; Bercaw, J. E.; Jardine, C. N.; Lyall, M.; Green, J. C.;
Keister, J. B. J. Am. Chem. Soc. 2002, 124, 9525.
(25) Ackerman, L. J.; Green, M. L. H.; Green J. C.; Bercaw, J. E.
Organometallics 2003, 22, 188.
(26) Green, J. C. Chem. Soc. ReV. 1998, 27, 263.
Inorganic Chemistry, Vol. 43, No. 11, 2004 3481

Pons et al.
CD2Cl2 (1-2 mL) was condensed into the NMR tube, which was
1
then sealed at 200 K. H NMR, δ (CD2Cl2, 750 MHz, 245 K):
5.55, 5.17 (s, 8H, C5H4), 1.24 (s, 6 H, CMe2), -5.59 (s, 3 H Mo-
(H2)H).
1-d1 and -d2. Deuterium incorporation was achieved by proto-
nation of a methylene chloride (or Freon) solution of (η-C5H4)2-
CMe2MoH2 with 1 to 20 equivalents of deuterated acid CF3COOD
or D2C(SO2CF3)2. 1H NMR, δ (CD2Cl2, 750 MHz, 245 K): -5.59
(s), -5.61 (t), -5.68 (quint), (MoH3-d0, -d1, -d2).
Computational Methods. Calculations were performed using
ADF v2002.0328-30 using the Vosko, Vilke, and Nusair local
functional31 with the Becke 8832,33 and Perdew 8634 nonlocal
exchange and correlation gradient corrections. The basis sets used
were uncontracted triple-ú Slater-type orbitals (STOs). Hydrogen,
deuterium, and carbon were given extra polarization functions (2p
on H and 3d on C). The cores of atoms were frozen, C up to the
1s level, Si up to the 2p level, and Mo up to the 3d level. Scalar
ZORA relativistic corrections35-38 were used. Stationary points were
verified as either a ground state or transition state by full frequency
calculations39,40 with either zero or one imaginary eigenvector in
the Hessian. Transition states were verified with IRC calcula-
tions.41,42 Cartesian coordinates for all stationary points are given
in the Supporting Information. During a linear transit, one or more
internal variables (internuclear distance or angle) were fixed to
successive values as a reaction coordinate and all other variables
were optimized. The calculation of solvent effects used the COSMO
model implemented into ADF by Pye and Ziegler.20 The calculation
was done on the solvent excluding surface using the default radii
for the program, using a dielectric constant of 9.08 corresponding
to dichloromethane. Thermodynamic activation energies were
calculated using the formula
Figure 3. Contour plots for the HOMOs of 1 and 3. Left: ground state.
Right: Cs transition states.
occurrence of exchange coupling, which is important when
two H or D atoms are adjacent, leading to deceptively simple
spectra. We note that related trihydride complexes such as
[(η-C5H5)2MoH3]+ have higher barriers to hydrogen atom
permutation. In light of the current observations on complex
1, the intermediacy of a dihydrogen/hydride species in the
hydrogen atom permutation process seems plausible for
metallocene trihydride complexes in general.
We are continuing to investigate the structure and dynam-
ics of dihydrogen/hydride complexes with d2 configurations.
Such complexes are structurally very close to the long sought
trihydrogen complexes.
∆GqT ) GT(products) - GT(reactants)
GT ) ESCF + EZPE + Eint(T) - TS(T)
They therefore represent gas phase free energies. Solvent corrections
Experimental Section
(28) Baerends, E. J.; Autschbach, J. A.; Berces, A.; Bo, C.; Boerringter,
P. M.; Cavallo, L.; Chong, D. P.; Deng, L.; Dickson, R. M.; Ellis, D.
E.; Fan, L.; Fischer, T. H.; Fonseca-Guerra, C.; van Gisbergen, S. J.;
Groeneveld, J. A.; Gritsenko, O. V.; Gru¨ning, M.; Harris, F. E.; van
den Hoek, P.; Jacobsen, H.; van Kessel, G.; Kootstra, F.; van Lenthe,
E.; Osinga, V. P.; Patchkovskii, S.; Philipsen, P. H. T.; Post, D.; Pye,
C. C.; Ravenek, W.; Ros, P.; Schipper, P. R. T.; Schreckenbach, G.;
Snijiders, J. G.; Sola, M.; Swart, M.; Swerhone, D.; te Velde, G.;
Vernooijis, P.; Versluis, L.; Visser, O.; van Wezenbeek, E.; Wie-
senekker, G.; Wolff, S. K.; Woo, T. K.; Ziegler, T. Amsterdam Density
Functional, 2nd ed.; Scientific Computing & Modelling NV: Vrije
Universiteit, Amsterdam, 2002.
(29) te Velde, G.; Bickelhaupt, F. M.; Baerends, E. J.; Fonseca Guerra,
C.; Van Gisbergen, S. J. A.; Snijders, J. G.; Ziegler, T. J. Comput.
Chem. 2001, 22, 931.
(30) Guerra, C. F.; Snijders, J. G.; te Velde, G.; Baerends, E. J. Theor.
Chem. Acc. 1998, 99, 391.
(31) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys. 1980, 58, 1200.
(32) Becke, A. D. Phys. ReV. A 1988, 38, 3098.
(33) Becke, A. D. J. Chem. Phys. 1988, 88, 1053.
(34) Perdew, J. P. Phys. ReV. B 1986, 33, 8800.
(35) van Lenthe, E.; Baerends, E. J.; Snijders, J. G. J. Chem. Phys. 1993,
99, 4597.
(36) van Lenthe, E.; Baerends, E. J.; Snijders, J. G. J. Chem. Phys. 1994,
101, 9783.
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General Methods. Unless stated otherwise, all manipulations
were carried out under argon using Schlenk or drybox techniques.
[H(Et2O)2][BArF4] and (η-C5H4)2CMe2MoH2 were synthesized as
described in the literature. Deuterated solvents (Cambridge Isotope
Laboratories) were dried over CaH2. Deuterated Freon 21 (CDFCl2),
22 (CDF2Cl), and 23 (CDF3) were prepared by the method of Anet27
and stored in a glass bomb over calcium hydride. (CF3SO2)2CH2
was the generous gift of Dr. Allen Siedle (3M). The deuterated
acid (CF3SO2)2CD2 was made by exchanging (CF3SO2)2CH2 with
CH3OD followed by sublimation. CF3COOD was purchased from
Aldrich and used as received. Hydrogen gas was purchased from
Airgas and passed through a column of activated molecular sieves
prior to use. D2 gas was purchased from Cambridge Isotope
Laboratories. NMR spectra were recorded on Bruker AV 500, DRX
499, and DMX 750 spectrometers. Proton NMR spectra were
referenced to the solvent resonance with chemical shifts reported
relative to TMS. The NMR studies were carried out in high quality
5 mm NMR tubes. The workup of spectra used for precise
measuring of coupling constants used zero filling to 128K data
points prior to Fourier transform.
[(η-C5H4)2CMe2Mo(H2)H]B(ArF)4. A screwcap NMR tube (J-
Young) was charged with 5 mg of (η-C5H4)2CMe2MoH2 (0.018
mmol) and 20 mg of [3,5-(CF3)2C6H3)4B]H(OEt2)2 (0.019 mmol).
(27) Siegel, J. S.; Anet, F. A. J. Org. Chem. 1988, 53, 2629.
3482 Inorganic Chemistry, Vol. 43, No. 11, 2004

A Dihydrogen/Hydride Ansa Molybdenocene Complex
were not included as their calculation requires temperature depend-
ent variables which are only reliably available for 298.15 K. Ha
and Hb form the rotating H2 units.
using the facilities at the Oxford Supercomputer Center. We
thank EPSRC for studentship support (B.J.H. and S.L.J.C.).
Supporting Information Available: Computational details for
1-3, including Cartesian coordinates for all stationary points.
Variable temperature 1H NMR spectra of 1. This material is
available free of charge via the Internet at http://pubs.acs.org.
Acknowledgment. The research at the University of
Washington was supported by the National Science Founda-
tion. Acquisition of the 750 MHz NMR spectrometer was
supported by the National Science Foundation and the
Murdoch Charitable Trust. Calculations were performed
IC0496875
Inorganic Chemistry, Vol. 43, No. 11, 2004 3483