Synthesis and spectroscopic properties of elongated ruthenium dihydrogen complexes: Temperature and isotope dependence of H-H distances

Article abstract of DOI:10.1021/ja0118284

Several dihydrogen complexes of ruthenium of the form [Cp/Cp^*Ru(P-P)H₂]⁺ (P-P = chelating diphosphine ligand) have been prepared by reaction of the corresponding neutral chloride complexes with H₂ in the presenc

Full text of DOI:10.1021/ja0118284

Published on Web 01/19/2002
Synthesis and Spectroscopic Properties of Elongated
Ruthenium Dihydrogen Complexes: Temperature and Isotope
Dependence of H-H Distances
James K. Law, Heather Mellows, and D. Michael Heinekey*
Contribution from the Department of Chemistry, Box 351700, UniVersity of Washington,
Seattle, Washington 98195-1700
Received July 27, 2001
Abstract: Several dihydrogen complexes of ruthenium of the form [Cp/Cp*Ru(P-P)H2]+ (P-P ) chelating
diphosphine ligand) have been prepared by reaction of the corresponding neutral chloride complexes with
H2 in the presence of NaB(ArF)4. Treatment with D2 or T2 gas leads to incorporation of deuterium or tritium
in the dihydrogen ligand. Measurement of the resulting H-D and H-T couplings as a function of the
temperature and magnetic field gives results consistent with computational studies which predict that the
H-H bond distance will increase with temperature and will be significantly shortened by isotopic substitution.
The degree of the observed temperature dependence is found to be a critical function of the ancillary
ligand set.
Shortly after the isolation and characterization of the first
transition metal dihydrogen complexes by Kubas and co-
workers,1 a new class of cationic H2 adducts of the form [CpRu-
(L)(L′)H2]+ was reported to result from protonation of the
corresponding neutral hydride complexes. The first synthesis
of this type was reported by Conroy-Lewis and Simpson in
1986. Protonation of CpRu(PPh3)(CN-t-Bu)H affords the stable,
isolable complex [CpRu(PPh3)(CN-t-Bu)H2]+ .2 Subsequently,
a large number of complexes of this type have been prepared
with Cp or Cp* ligands and a wide range of coligands, mostly
chelating phosphines.3 These cationic complexes generally
exhibit tighter binding of H2 than do the original neutral
complexes studied by Kubas, which makes their isolation and
manipulation more straightforward.
phinomethane) has JHD ) 21 Hz, corresponding to rHH ) 1.10
Å. In the case of complex 1, this value for rHH has been
confirmed by a neutron diffraction study.8 Consistent with the
strong metal H2 interaction indicated by the long H-H distance,
complex 1 binds H2 very tightly. Complexes of this type have
been termed “elongated” dihydrogen complexes in that they
seem to have H-H distances intermediate between those of
dihydrogen complexes (e1 Å) and the distances usually
associated with dihydride complexes (g1.5 Å). Such structures
may represent an arrested intermediate state in the very
important oxidative addition reaction.
The utility of determining rHH from measurement of JHD
values was called into question when it was reported that the
H-D isotopomer of complex 1 (1-d1) exhibits a small decrease
in JHD upon increasing the temperature from ca. 200 K to room
temperature, which may signal a slight increase in the H-H
(H-D) bond distance. Thermal population of vibrationally
excited states was proposed to account for the decreased
coupling at higher temperatures.8 This hypothesis is supported
by a computational study using density functional theory (DFT)
calculations. This study also lead to the intriguing suggestion
that the bond distance in complex 1 will be sensitive to isotopic
substitution, with the distance in the H-H complex predicted
to be approximately 10% longer than that in the corresponding
T-T complex.9
On the basis of the well-established inverse correlation
4-6
between H-D coupling and H-H distance rHH
,
complexes
of this family seem to present a very wide range of structures.
At one extreme is the thermally labile species [Cp*Ru(CO)2H2]+
(JHD ) 32 Hz, rHH ) 0.90 Å), which loses H2 readily.7 In
contrast, [Cp*Ru(dppm)H2]+ (1, dppm ) bis-diphenylphos-
(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) Conroy-Lewis, F. M.; Simpson, S. J. J. Chem. Soc., Chem. Commun. 1986,
506-507.
(3) General reviews: (a) Kubas, G. J. Metal Dihydrogen and σ-Bond
Complexes: Structure, Theory, and ReactiVity; Kluwer: New York, 2001.
(b) Heinekey, D. M.; Oldham, W. J., Jr. Chem. ReV. 1993, 93, 913-926.
(c) Morris, R. H.; Jessop, P. G. Coord. Chem. ReV. 1992, 121, 155-289.
(4) Heinekey, D. M.; Luther, T. A. Inorg. Chem. 1996, 35, 4396-4399.
(5) Maltby, P. A.; Schlaf, M.; Steinbeck, M.; Lough, A. J.; Morris, R. H.;
Klooster, W. T.; Koetzle, T. F.; Srivastava, R. C. J. Am. Chem. Soc. 1996,
118, 5396-5407.
(6) The relationship between H-D coupling and rHH has been explored using
quantum chemical calculations: Hush, N. S. J. Am. Chem. Soc. 1997, 119,
1717-1719. An extension to include dihydride complexes has been
reported: Gru¨ndemann, S.; Limbach, H. H.; Buntkowsky, G.; Sabo-Etienne,
S.; Chaudret, B. J. Phys. Chem. A 1999, 103, 4752-4754.
(7) Chinn, M. S.; Heinekey, D. M.; Payne, N. G.; Sofield, C. D. Organome-
tallics 1989, 8, 1824-1826.
We now report a study of the temperature and isotope
dependence of rHH in complex 1 and several related species. In
the course of this work, an improved synthetic procedure has
been developed which allows for a more convenient preparation
(8) Klooster, W. T.; Koetzle, T. F.; Jia, G.; Fong, T. P.; Morris, R. H.; Albinati,
A. J. Am. Chem. Soc. 1994, 116, 7677-7681.
(9) Gelabert, R.; Moreno, M.; Lluch, J. M.; Lledo´s, A. J. Am. Chem. Soc.
1997, 119, 9840-9847.
9
1024 VOL. 124, NO. 6, 2002 J. AM. CHEM. SOC.
10.1021/ja0118284 CCC: $22.00 © 2002 American Chemical Society

Elongated Ruthenium Dihydrogen Complexes
A R T I C L E S
Table 1. 1H NMR Data for Hydride and Dihydrogen Resonances in 1-5
a
a
compound
δ RuH , ppm [JHP, Hz]
δ Ru(H ), ppm
Ru(H )/ RuH a
JHD Hza
δRu−(HD) ppba,b
Ru(HD) JH-P, Hza,c
2
2
2
2
1 [Cp*Ru(dppm)H2]+
2 [CpRu(dppe)H2]+
3 [CpRu(dmpe)H2]+
4 [Cp*Ru(dppip)H2]+
5 [Cp*Ru(dmpm)H2]+
-5.975 (28.8)
-8.589 (28.0)
-9.866 (30.7)
-6.276 (27.7)
-7.085 (33.2)
-6.774
-9.042
-10.156
-7.428
-7.881
67/33
40/60
85/15
80/20
94/6
20.6 ( 0.3
24.9 ( 0.1
22.3 ( 0.1
18.6 ( 0.3
15.9 ( 0.1
15e
38
35
16
22
d
2.1 ( 0.1
3.5 ( 0.05
d
d
a CD2Cl2, 300 K, 750 MHz. b Upfield from Ru(H2)+. c Observed in Ru(HD)+ isotopomer. d Not observed. e δRu(HT)+ is 22 ppb upfield from Ru(H2)+
isotopomer.
of complexes of this type. A portion of the results described
here has been previously communicated.10
Results
Synthesis. The previously reported chloride complexes
Cp*Ru(dppm)Cl,11 Cp*Ru(dmpm)Cl,11 CpRu(dppe)Cl,12 and
CpRu(dmpe)Cl13 were prepared using literature procedures
(dppm ) bis-diphenylphosphinomethane; dmpm ) bis-dimeth-
ylphosphinomethane; dppe ) bis-diphenylphosphinoethane;
dmpe ) bis-dimethylphosphinoethane). Reaction of the ap-
propriate chelating phosphine ligand with Cp*Ru(PPh3)2Cl14 or
CpRu(PPh3)2Cl15 affords these complexes in good yield. The
new chloride complex Cp*Ru(dppip)Cl was prepared by reaction
of 2,2-bis(diphenylphosphino)propane (dppip) with Cp*Ru-
(PPh3)2Cl.
Methylene chloride solutions of these chloride complexes in
the presence of NaB(ArF)4 under hydrogen gas (1 atm) react
cleanly to afford the corresponding dihydrogen complexes
[Cp*Ru(dppm)(H2)]+ (1), [CpRu(dppe)(H2)]+ (2), [CpRu(dmpe)-
(H2)]+ (3), [Cp*Ru(dppip)(H2)]+ (4), and [Cp*Ru(dmpm)(H2)]+
(5). Complexes 4 and 5 have not been previously reported.
Complexes 1-3 have been previously prepared by protonation
of the corresponding neutral hydride complexes. 1H NMR data
for complexes 1-5 are summarized in Table 1.
Figure 1. JHD versus temperature for complexes 1-4: [CpRu(dppe)HD]+
(2, open circles); [CpRu(dmpe)HD]+ (3, solid circles); [Cp*Ru(dppm)HD]+
(1, squares); [Cp*Ru(dppip)HD]+ (4, triangles). Error bars indicate estimated
uncertainties.
Isotope Exchange and H-D Coupling. Exposure of meth-
ylene chloride solutions of 1-5 to D2 gas leads to incorporation
of deuterium in the dihydrogen ligand over the course of several
hours, as demonstrated by observation of large H-D couplings
in the monodeuterated isotopomer. Partially tritiated samples
of 1 were also prepared by brief exposure to T2 gas. This
exchange reaction is faster for complexes with a less basic ligand
set.
Dihydrogen/Dihydride Equilibria. Complexes 1-5 exhibit
1
two distinct sets of “hydride” resonances in their H NMR
spectra, consistent with a slowly equilibrating mixture of a
dihydrogen species and a dihydride (dihydrogen/dihydride ratios
are given in Table 1). The dihydride resonance is a triplet, due
to appreciable coupling to the two 31P nuclei of the bidentate
ligand. In some cases, a much smaller H-P coupling (2-3 Hz)
can be resolved in the resonance associated with bound
dihydrogen. In most cases studied here, the dihydrogen form is
predominant, but the position of the equilibrium depends on
the nature of the bidentate ligand, with single-carbon bridges
or the replacement of aryl with alkyl groups very strongly
favoring the dihydrogen form. Chemical shifts for the dihydro-
gen and dihydride resonances in 1-5 and coupling constant
data are tabulated in Table 1. These chemical shifts are observed
to have a small dependence upon the observation temperature.
For example, the resonance due to the dihydride form of
complex 2 shifts to higher field upon lowering the temperature
by ca. 2 ppb/deg. The resonance due to the dihydrogen tautomer
shows a similar but much smaller effect of ca. 0.2 ppb/deg.
At ambient temperature, the observed values for JHD are 24.9
Hz (2), 22.3 Hz (3), 20.6 Hz (1), 18.6 Hz (4), and 15.9 Hz (5).
Field independence of these values was confirmed by observa-
tions at a range of fields (5.8, 11.6, and 17.6 T). The JHD values
for complexes 2, 4, and 5 are independent of temperature within
experimental error, while the couplings observed in complexes
1 and 3 show a small but appreciable increase upon lowering
the temperature from 300 to ca. 200 K. Data for JHD values as
a function of temperature for complexes 1-4 are collected in
Figure 1.
Partially tritiated samples of 1 were prepared by brief
exposure of a partially deuterated sample to carrier free T2 gas,
affording a mixture of all possible isotopomers. 1H NMR spectra
of this mixture allowed for the determination of JHT and JHD
values, while JDT was measured by direct observation in the
3H NMR spectrum (800 MHz). The sample employed had a
preponderance of T2 and relatively small amounts of TD, so an
inversion recovery sequence was employed to null the more
rapidly relaxing resonance (see below) due to bound TT,
allowing for precise determination of JTD (see Figure 2).
(10) Law, J. K.; Mellows, H.; Heinekey, D. M. J. Am. Chem. Soc. 2001, 123,
2085-2086.
(11) Lin, W.; Wilson, S. R.; Girolami, G. S. Organometallics 1997, 16, 2987-
2994.
Careful examination of the 1H NMR spectra of partially
deuterated and, in some cases, partially tritiated samples allows
the measurement of the isotope effect on the chemical shifts.
An example of these observations is the case of complex 1,
(12) Ashby, G. S.; Bruce, M. I.; Tomkins, I. B.; Wallis, R. C. Aust. J. Chem.
1979, 32, 1003-1007.
(13) Szczepura, L. F.; Takeuchi, K. J. Inorg. Chem. 1990, 29, 1772-1777
(14) Chinn, M. S.; Heinekey, D. M. J. Am. Chem. Soc. 1990, 112, 5166-5175.
(15) Bruce, M. I.; Hameister, C.; Swincer, A. G.; Wallis, R. C. Inorg. Synth.
1990, 28, 270-272.
9
J. AM. CHEM. SOC. VOL. 124, NO. 6, 2002 1025

A R T I C L E S
Law et al.
(PR3)2H can be protonated to give cationic dihydrides, which
adopt a transoid capped four-legged piano stool structure. The
transoid geometry for the dihydride form has been previously
established by experiments with asymmetric bidentate phosphine
ligands14 and by crystallography in the case of [Cp*Ru(dippe)-
H2]+ 18 and [Cp*Ru(PMe3)2H2]+ .19 When a bidentate phosphine
ligand is employed, an equilibrium mixture of the dihydride
and a dihydrogen complex is obtained. The ratio of these two
tautomers obtained depends on the nature of the bidentate ligand
employed, with one-carbon bridged ligands strongly favoring
the dihydrogen form, while two-carbon bridges favor the
dihydride form. This is understandable in that a single-carbon
bridge between the phosphorus atoms will give a less stable
arrangement when the P atoms are in a transoid geometry.
Within the one-carbon bridged bidentate ligands, intuitive
notions about the relative stability of the formally Ru(II)
dihydrogen complex versus the Ru(IV) dihydride complex based
on the donor properties of the ligand are not borne out by
observation. For example, with dppm (complex 1), 33%
dihydride is observed, while for the better donor dmpm
(complex 5), the proportion of dihydride complex is only 6%.
A possible steric explanation for this is suggested by an
examination of the structural data for the dihydrogen form of
complex 1, which shows some steric congestion between the
Cp* methyl groups and the phenyl rings of the dppm ligand. In
the case of dmpm (complex 5), this steric interaction would be
diminished, leading to a greater proportion of dihydrogen versus
dihydride complex.
H-D Coupling and the Determination of H-H Distances.
Samples containing deuterium in the bound dihydrogen ligand
were generated by exposure of methylene chloride solutions of
complexes 1-5 to HD or D2 gas. Ligand exchange rates were
observed to be faster for complexes with less basic phosphine
coligands. This observation is in contrast to the trend observed
above in the lability of the chloride ligand to replacement by
hydrogen.
In reported dihydrogen complexes which have been structur-
ally characterized by neutron diffraction or solid-state NMR
methods, an inverse correlation between the H-H distance and
JHD of the HD analogue has been observed.4,20 The equation
describing the inverse relationship between rHH and JHD from
corrected neutron diffraction and solid-state NMR data is4
Figure 2. 3H NMR spectrum (800 MHz) of 1. The resonance due to bound
T2 has been nulled by applying a 180-τ-90 pulse sequence with τ ) 15 ms.
where δHD-δΗΗ ) 15 ppb and δHT-δΗΗ ) 22 ppb. Complexes
2-5 exhibit somewhat larger isotope shifts δHD-δΗΗ. These
small shifts of the dihydrogen resonance to higher field upon
substitution with a heavier isotope are independent of the
observation temperature. Values for the isotope effects on the
chemical shifts in 1-5 are tabulated in Table 1.
Relaxation Time Measurements. Using standard inversion
recovery methods, the relaxation time (T1) for the bound
hydrogen in 1 and for the bound tritium in 1-t2 was determined
at a variety of temperatures ranging from 205 to 300 K. For
the bound T2, the maximum rate of relaxation (T1 min ) 21
ms) was observed at 246 K (800 MHz). Analysis using a
modification of the methodology of Halpern and co-workers16
leads to a T-T distance in complex 1 of 1.05 Å.
Discussion
Synthesis. Previous reports of the synthesis of complexes
1-3 have employed the protonation of the corresponding neutral
hydride complexes. The hydride complexes are usually gener-
ated by reduction of the corresponding halide complexes with
alkoxide or borohydride reagents. After the original report of
complex 1, Morris and co-workers reported that reaction of
Cp*Ru(dppm)Cl with AgBF4 in the presence of hydrogen
affords complex 1 in good yield.17 We have found that the direct
reaction of dihydrogen gas with the chloride complexes in the
presence of NaB(ArF)4 affords the cationic dihydrogen com-
plexes 1-5 in quantitative (by NMR) yields. This one-pot
procedure is much more convenient than the two-step method
previously reported. Samples containing deuterium or tritium
in the bound dihydrogen ligand were generated by exposure of
methylene chloride solutions of complexes 1-5 to deuterium
or tritium gas.
rHH ) 1.44 - 0.0168(JHD
)
(1)
Morris and co-workers have used a larger set of distances and
JHD values to develop a similar correlation for the H-H bond
length.5 They have also included uncorrected neutron data and
distances determined from X-ray diffraction data along with the
distances from solid-state NMR and corrected neutron data.
However, results with this larger data set are similar to eq 1:
We note that the chloride complexes with the more basic
phosphine ligands (such as dmpm) react faster with hydrogen
in the above reaction than do the complexes with less basic
coligands such as dppm. This is consistent with labilization of
chloride due to destabilizing interactions between chloride lone
pairs and nonbonding d electrons at the metal center.
rHH ) 1.42 - 0.0167(JHD
)
(2)
Using eq 1 and the ambient temperature value for JHD observed
in complex 1 gives rHH ) 1.092 Å. The use of eq 2 leads to
rHH ) 1.071 Å. Since the distances derived from neutron
diffraction and solid-state NMR data are viewed as more
Dihydrogen/Dihydride Equilibria. It is well established that
complexes of the general form CpRu(PR3)2H and Cp*Ru-
(16) Desrosiers, P. J.; Cai, L.; Lin, Z.; Richards, R.; Halpern, J. J. Am. Chem.
Soc. 1991, 113, 4173-4184. 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, 3463-3467.
(17) Facey, G. A.; Fong, T. P.; Gusev, D.; Macdonald, P. M.; Morris, R. H.;
Schlaf, M.; Xu, W. Can. J. Chem. 1999, 77, 1899-1910.
(18) de los Rios, I.; Tenorio, M. J.; Padilla, J.; Puerta, M. C.; Valerga, P.
Organometallics 1996, 15, 4565-4574.
(19) Lemke, F. R.; Brammer, L. Organometallics 1995, 14, 3980-3985.
(20) Luther, T. A.; Heinekey, D. M. Inorg. Chem. 1998, 37, 127-132.
9
1026 J. AM. CHEM. SOC. VOL. 124, NO. 6, 2002

Elongated Ruthenium Dihydrogen Complexes
A R T I C L E S
Table 2. Couplings (Hz) and Derived Distances (Å) in Isotopomers of 1
T (K)
JHD (Hz)a
rHD (Å)
JHT (Hz)b
rHT (Å)
JDT (Hz)c
rDT (Å)
286
275
267
251
236
220
204
20.8 ( 0.3
21.3 ( 0.2
21.5 ( 0.2
21.8 ( 0.2
22.1 ( 0.2
22.4 ( 0.2
22.6 ( 0.3
1.091 ( 0.005
1.082 ( 0.004
1.079 ( 0.004
1.075 ( 0.004
1.070 ( 0.004
1.064 ( 0.004
1.060 ( 0.005
149.2 ( 0.5
150.4 ( 0.4
151.8 ( 0.4
153.6 ( 0.4
155.8 ( 0.5
158.7 ( 0.5
160.8 ( 0.6
1.079 ( 0.001
1.076 ( 0.001
1.073 ( 0.001
1.069 ( 0.001
1.063 ( 0.001
1.056 ( 0.001
1.051 ( 0.0015
23.4 ( 0.3
24.0 ( 0.3
24.1 ( 0.2
24.4 ( 0.2
25.1 ( 0.3
25.6 ( 0.3
25.8 ( 0.4
1.072 ( 0.005
1.062 ( 0.005
1.061 ( 0.004
1.056 ( 0.004
1.045 ( 0.005
1.037 ( 0.005
1.034 ( 0.0055
a Recorded at 750 MHz. b Average of couplings obtained at 750 and 800 MHz. c Recorded at 800 MHz.
reliable, we favor the use of eq 1 in determining distances from
coupling data. On the basis of a measurement uncertainty in
upfield shift of the HD resonance from the H2 signal (δHD
-
δ
ΗΗ), which ranges from 15 to 35 ppb (see Table 1). These
J
HD of (0.3 Hz, the uncertainty in these distances arising from
isotope shifts are typical of reported dihydrogen complexes23
and are smaller than what is observed in H2/HD gas (∆δ )
+36 ppb).24 These small chemical shift differences are inde-
pendent of temperature.
measurement error is estimated to be (0.005 Å. Since the line
widths of the resonances being examined vary with temperature
and the nature of the ligand set, it is actually an oversimplifica-
tion to attribute a single uncertainty to these measurements. In
general, line widths for the resonance due to bound H-D
increase slightly at lower temperatures, due to relaxation and
solvent viscosity effects. For complex 1, an additional source
of line broadening is the onset of dihydrogen/dihydride ex-
change, which becomes significant as the sample temperature
approaches 300 K, as established by spin saturation transfer
experiments by Morris and co-workers.5 Thus the most precise
data for H-D coupling in complex 1 is obtained for tempera-
tures in the middle of the studied range. In contrast to complex
1, the resonances due to bound HD in CpRu(dmpe)HD+ (3)
are quite sharp at higher temperatures, affording more precise
values for the coupling (see below).
Complex 1 has been thoroughly studied by Morris and co-
workers, who report rHH ) 1.10(3) Å on the basis of neutron
diffraction data obtained at low temperature. This distance was
confirmed by a study of the relaxation time (T1) for the
dihydrogen ligand in 1, which gave rHH ) 1.10 Å, with the
assumption that the dihydrogen ligand is in the slow rotation
regime.8 Uncertainties in H-H distances derived from T1
measurements are difficult to evaluate, with a reasonable
uncertainty from measurement error alone being ca. (3-5%.21
Additional uncertainty arises from assumptions made about the
rotational regime of the bound dihydrogen ligand and sources
of relaxation other than H-H dipole-dipole relaxation.
Determination of rHH from the JHD value has the advantage
that correction factors concerning the rotation regime of the
dihydrogen ligand and other sources of relaxation do not have
to be considered.16,22 In the case of complex 1, the H-H
distances determined by neutron diffraction, relaxation, and
H-D coupling are in excellent agreement.
Temperature and Isotope Dependence of Bond Distances
in Bound Dihydrogen. In complex 1, the H-H distance derived
from coupling data was found to vary significantly with
observation temperature, increasing about 3% when the tem-
perature was increased from 204 to 286 K (see Table 2). The
observed couplings are independent of the magnetic field
employed, which rules out the possibility that residual dipolar
coupling could be responsible for the temperature dependence
of the coupling.25
These observations are consistent with a detailed computa-
tional study of complex 1 using DFT methods by Lledo´s, Lluch,
and co-workers.9 These workers report a very unusual potential
energy surface for the bound hydrogen in this molecule. The
H-H distance can be significantly elongated with very little
increase in energy. The calculations indicate that a vibrationally
excited state (with a longer H-H distance of about 1.3 Å) can
be significantly populated at moderate temperatures. Further
analysis of this potential surface in terms of the reported Raman
spectrum of 1 and the corresponding vibrational modes has also
been reported.26 Our observations of the temperature dependence
of the H-D coupling in 1 are in excellent agreement with the
computational predictions.
The DFT study also leads to the novel prediction that the
observed bond distance will be quite sensitive to isotope
substitution. For example, the bond distance for the T-T
complex was predicted to be ca. 10% shorter than that in the
corresponding H-H species. This remarkable difference in bond
distances upon isotopic substitution is attributed to the highly
anharmonic nature of the vibrational potential.9
To probe for the predicted isotope effects on bond distance,
the use of tritium NMR spectroscopy is advantageous. The
Using eq 1 and the ambient temperature H-D coupling data
for complexes 1-5 tabulated in Table 1, values for the H-H
distance in the bound dihydrogen ligand ranging from 1.02 (2)
to 1.17 Å (5) can be derived. By using JHD values to derive
H-H distances, we are explicitly assuming that the distance is
independent of isotope substitution. In at least some cases, this
is not correct (see below).
(23) Reported isotope shifts in dihydrogen complexes are generally <50 ppb.
Some reported exceptions are: (a) ∆δ ) +90 ppb for Ru(H2)(OEP)(THF)
(OEP ) octaethylporphyrin), +130 ppb for Os(H2)(OEP)(*Im) (*Im )
3-tert-butyl-4-phenylimidazole), and -200 ppb for Ru2(H2)(DPB)(*Im)2
(DPB ) 1,8-bis[5-(2,8,13,17-tetraethyl-3,7,12,18-tetramethyl)porphyrin]-
biphenylene). [Collman, J. P.; Wagenknecht, P. S.; Hutchison, J. E.; Lewis,
N. S.; Lopez, M. A.; Guilard, R.; L’Her, M.; Bothner-By, A. A.; Mishra,
P. K. J. Am. Chem. Soc. 1992, 114, 5654-5664.] (b) ∆δ ) +200 ppb for
[Cp2Ta(CO)(H2)]+. [Moreno, B.; Sabo-Etienne, S.; Chaudret, B.; Rodriguez,
A.; Jalo´n, F.; Trofimenko, S. J. Am. Chem. Soc. 1994, 116, 2635-2636.]
(c) ∆δ ) +80 ppb for [Os(H2)(en)2OAc]2+ (en ) ethylenediamine;
OAc ) acetate). [Hasegawa, T.; Li, Z.-W.; Parkin, S.; Hope, H.; McMullan,
R. K.; Koetzle, T. F.; Taube, H. J. Am. Chem. Soc. 1994, 116, 4352-
5356.]
The observation of mixtures of bound HH and bound HD in
complexes 1-5 also allows for the measurement of the small
(21) Bakhmutov, V. I.; Voronstov, E. V. ReV. Inorg. Chem. 1998, 18, 183-
221.
(22) Gusev, D. G.; Kuhlman, R. L.; Renkema, K. B.; Eisenstein, O.; Caulton,
K. G. Inorg. Chem. 1996, 35, 6775-6783.
(24) Evans, D. F. Chem. Ind. 1961, 1960
(25) Luther, T. A.; Heinekey, D. M. J. Am. Chem. Soc. 1997, 119, 6688-6689.
(26) Gelabert, R.; Moreno, M.; Lluch, J. M.; Lledos, A. Chem. Phys. 1999,
241, 155-166.
9
J. AM. CHEM. SOC. VOL. 124, NO. 6, 2002 1027

A R T I C L E S
Law et al.
magnetogyric ratio of 3H is 6.949 times larger than that of 2H,
so couplings between H and T should be correspondingly larger
than couplings between H and D, making the detection of small
changes in the coupling much more reliable. The couplings JHT
and JDT can be converted to the equivalent H-D couplings using
γT/γD ) 6.949 and γT/γΗ ) 1.067. The most precise data arise
from the H-T coupling measurements, where the experimental
uncertainty is estimated as (0.4 Hz, which corresponds to an
uncertainty in the bond distance of (0.001 Å. The bond
distances for bound H-D, H-T, and T-D as a function of
temperature are tabulated in Table 2.
ligand, it is of interest to explore this phenomenon as a function
of the H-H distance, which is readily varied by changing the
nature of the phosphine ligand. To this end, we have reexamined
some related complexes from the literature. For example,
[CpRu(dppe)H2]+ (2) exhibits an H-D coupling of 24.9 Hz in
2-d1, consistent with an H-H distance of ca. 1.02 Å. Careful
examination shows that the H-D coupling in this complex is
invariant with temperature (see Figure 2). This result is
intuitively reasonable, since a shorter H-H bond is reasonably
expected to be stiffer and more resistant to elongation.
We reasoned that greater elongation of the H-H distance
would result in a softer potential and perhaps an even greater
effect of temperature upon the bond distances. To make a more
elongated dihydrogen ligand, we envisaged that use of a more
basic ligand set should lead to a more electron-rich Ru center
and enhance back-donation to the H-H σ* orbital. To test this
idea, the new complexes [Cp*Ru(dppip)H2] + (4) and [Cp*Ru-
(dmpm)H2]+ (5) were prepared. The H-D couplings at ambient
temperature are 18.6 and 15.9 Hz, respectively. This shows that
our strategy has been successful and that a highly elongated
H-H distance of ca. 1.17 Å has been achieved in complex 5.
Surprisingly, the H-D couplings in 4 and 5 are found to be
invariant with temperature. This unexpected result suggests that
the longer and weaker H-H bond in these complexes is not in
fact accompanied by a suitably soft potential energy surface to
allow for facile thermal excitation of low energy vibrational
modes corresponding to further elongation of the H-H distance.
Complexes such as 5 are difficult to describe in simple valence
bond terms since the H-H distance seems to be intermediate
between those expected for a dihydride and a dihydrogen
complex. There may be justification for thinking of them as
having significant dihydride character, with the hydrogen atoms
quite strongly localized in their respective positions.
The derived bond distances at the higher temperatures are in
reasonable agreement with the value of rHH ) 1.10(3) Å for
complex 1 reported by neutron diffraction. It is clear from our
data that substitution with heavier isotopes leads to a significant
shortening of the bond distance in H-T and D-T complexes,
as predicted by the DFT study of Lledos, Lluch, and co-
workers.9 For example, at 267 K, rHD ) 1.079 ( 0.004 Å, while
rTD ) 1.061 ( 0.004 Å. It is particulary noteworthy that the
value for rHD is significantly shorter than the neutron diffraction
value for rHH and that replacing H with T leads to further
shortening of the bond distance.
Interestingly, the temperature dependence of the bond distance
is slightly different for the various isotopomers. For example,
an increase in temperature from 204 to 286 K leads to a decrease
in the H-D coupling in 1 and a corresponding increase in the
derived bond distance of ca. 2.9%. Examination of the data for
the D-T complex shows a similar trend, but with a change of
ca. 3.7% over the same temperature range. This observation is
also consistent with the DFT study, in that the heavier
isotopomer is predicted to have more closely spaced vibrational
energy levels, leading to greater population of excited vibrational
states as the temperature is raised.9
Since the temperature-dependent behavior of the coupling
seemed to occur only in species such as 1, with H-D couplings
of ca. 21 Hz, we chose to reexamine the well-known complex
[CpRu(dmpe)H2]+ (3), which we first reported in 1987 and
identified as a dihydrogen complex on the basis of a measured
H-D coupling of 22 Hz.27 Examination of the H-D coupling
in 3-d1 as a function of temperature reveals a small but
significant variation with temperature. We find that the H-D
coupling in 3 is 23.05 Hz at 200 K, decreasing to 22.30 Hz at
300 K (see Figure 1). While these changes are very small, the
line widths of the proton resonances in 3-d1 are extremely
narrow, leading to very precise data, with an estimated uncer-
tainty in the measured couplings of (0.05 Hz. (see Figure 3).
The narrow lines observed for 3-d1 allowed us in our original
observations of this complex to report the presence of a small
H-P coupling (2JH-P ) 3.6 Hz) in the bound dihydrogen
ligand.27 Several reports of such couplings are now in the
literature. It is generally observed in Ru complexes of the type
under study here that small (e5 Hz) H-P couplings are
indicative of the presence of a bound dihydrogen ligand, while
larger H-P couplings are found for dihydride structures. In the
course of our detailed examination of the NMR spectra of 3-d1
as a function of temperature, a pronounced temperature depen-
dence of the H-P coupling was observed. At 200 K, a value
An estimate of the bond distance in the T-T complex (1-t2)
can be obtained by measurement of the maximum rate of
relaxation (T1 min) in this complex. For the bound T2, the
maximum rate of relaxation (T1 min ) 21 ms) was observed at
246 K (800 MHz). Analysis using the methodology of Halpern
and co-workers16 leads to a T-T distance of 1.06 Å in complex
1. A more reliable bond distance of 1.05 Å was obtained by
fitting the complete data set for all temperatures to the equation
for dipole-dipole relaxation, using data for τc versus temper-
ature derived by determining the maximum rate of relaxation
1
3
of H and H at various fields.14
As expected from the trend of shorter bond distances for the
heavier isotopes, this bond distance for bound T-T is slightly
shorter than that found from coupling data for the D-T analogue
at a similar temperature (1.056 Å for the D-T complex at 251
K). In light of the experimental uncertainty associated with the
measurement of bond distances by the relaxation method, which
is inherently less precise than the coupling methodology, the
agreement between the two methods is gratifying. A slightly
shorter distance of ca. 1.01 Å was anticipated for the T-T
complex, on the basis of the DFT study, suggesting that the
computational study may underestimate the bond distance in
this case.
Temperature Dependence of H-D Coupling as a Function
of Ligand Set. Since the novel temperature dependence of the
H-D coupling in complex 1 is attributed to an unusually soft
vibrational potential for stretching of this elongated dihydrogen
2
of JH-P ) 2.7 Hz was observed, increasing with temperature
to 3.5 Hz at 300 K (see Figure 4). It is remarkable that this two
(27) Chinn, M. S.; Heinekey, D. J. Am. Chem. Soc. 1987, 109, 5865-5867.
9
1028 J. AM. CHEM. SOC. VOL. 124, NO. 6, 2002

Elongated Ruthenium Dihydrogen Complexes
A R T I C L E S
the dihydrogen form (as seen in the low-temperature neutron
structure of 1) and a cis-dihydride complex, each representing
distinct minima in the PES. If this hypothesis is correct, the
observed couplings and chemical shifts would represent a
population weighted average of these two isomers, which would
be expected to shift with temperature. If such an equilibrium
prevails, a temperature-dependent chemical shift for the reso-
nance due to bound dihydrogen would be expected. Careful
measurement shows that the chemical shift due to the bound
dihydrogen in 1 is essentially independent of temperature,
moving upfield by only 10 ppb upon changing the temperature
from 220 to 300 K. This temperature dependence is actually
smaller than that observed for the dihydride tautomer of complex
1, which shifts by ca. 100 ppb over the same temperature range.
A dihydrogen/dihydride equilibrium should also lead to
isotopic perturbation effects arising from nonstatistical distribu-
tion of deuterium (and tritium). These effects will be manifested
by large and temperature-dependent values of the isotope shift.
However, the isotope effects on the chemical shifts are small
1
Figure 3. Partial H NMR spectrum (750 MHz) of [CpRu(dmpe)(HD)]+
(3-d1) at (a) 298 K and (b) 215 K. * indicates partially obscured resonance
due to H2 isotopomer.
and temperature independent. For example, in complex 1, δHD
-
δΗΗ ) 15 ppb and δHT-δΗΗ ) 22 ppb. The former of these
values is actually in the low end of the range of isotope effects
reported for bound dihydrogen in complexes of this type.28
Similar observations were made for complex 3. The lack of
temperature dependence of the isotope shift is incompatible with
the equilibrium hypothesis unless the chemical shifts of the
dihydrogen and cis-dihydride tautomers are very similar or the
occupation of these two different chemical environments by
deuterium is essentially statistical. The former requirement is
unlikely to be met, since the chemical shift difference between
the dihydrogen and trans-dihydride forms of complexes 1-5 is
substantial (see Table 1). Statistical occupancy of the two
different chemical environments by a deuteron (or Triton) seems
unlikely, since in a related Ru dihydrogen/hydride complex,
there is a clear preference for deuterium to accumulate in the
hydride site in preference to the dihydrogen ligand.29
The complexes studied in this work present a wide range of
H-H distances (from ca. 1 Å in 2 to nearly 1.2 Å in 5). If
these outcomes are the result of a dihydride/dihydogen equi-
librium, then a considerable range of equilibrium constants must
be represented (with complex 1 being in the middle of the
range), all of which should exhibit some temperature depen-
dence. Since only complex 1 (and to a lesser extent complex
3) exhibits temperature-dependent couplings, these observations
are not compatible with the equilibrium hypothesis. While the
equilibrium hypothesis involving two different minima in the
potential surface cannot be conclusively ruled out, we believe
it to be unlikely.
2
Figure 4. JHPvalues between the bound HD and the phosphine ligand as
a function of temperature: [CpRu(dmpe)(HD)]+ (3, triangles); [CpRu(dppe)-
(HD)]+ (2, squares).
bond coupling shows a much larger (ca. 30%) change upon
increasing the observation temperature than does the one bond
H-D coupling, which decreased by only 3.4% over a similar
temperature range. In contrast, the values of JH-P in complex
2 are independent of the observation temperature.
2
We surmise that this temperature-dependent H-P coupling
is also due to thermal population of a vibrational excited state,
which presumably has stronger Ru-H interactions, leading to
more efficient transmission of coupling between H and 31P.
1
An additional factor which is difficult to assess is the presumed
angle dependence of the coupling. That this angle dependence
can be significant is confirmed by the observation that com-
plexes 2 and 3 (with a two-carbon bridge in the bidentate
phosphine ligand) exhibit observable H-P couplings, while
complexes with a one-carbon bridge (1, 4, and 5) show no such
coupling. In contrast to the dihydrogen complexes, the transoid
dihydride tautomers exhibit large and temperature independent
values of JH-P. We tentatively conclude that under circum-
stances where the structure of an elongated dihydrogen complex
allows measurement of JH-P, temperature dependence of this
parameter can be a sensitive indicator of H-H bond stretching
due to thermal excitation of low energy vibrational modes.
Possible Dihydrogen/cis-Dihydride Equilibrium. Although
the DFT study found a single minimum in the PES, the observed
temperature dependence of the coupling constants described
above might also be explained by a rapid equilibrium between
Summary and Conclusions
The results described here provide direct experimental
verification of the conclusions of the DFT study of Lledo´s,
Lluch, and co-workers.9 The remarkable isotope dependence
of the bond distance in the bound dihydrogen ligand of
complexes 1 and 3 is attributed to the extremely flat and highly
anharmonic potential energy surface which defines the H-H
and M-H interactions in this complex. This allows the zero
2
2
(28) cf. Chinn, M. S.; Heinekey, D. M. J. Am. Chem. Soc. 1990, 112, 2, 5166-
5175.
(29) Heinekey, D. M.; Mellows, H.; Pratum, T. J. Am. Chem. Soc. 2000, 122,
6498-6499.
9
J. AM. CHEM. SOC. VOL. 124, NO. 6, 2002 1029

A R T I C L E S
Law et al.
crystals which were filtered in air, rinsed with ice cold pentane, and
dried in vacuo (459 mg, 94%). The complex is indefinitely air stable
in crystalline form and may be manipulated in solution in air for short
periods (hours) at ambient temperature with no signs of decomposition.
1H NMR (C6D6): δ 7.4-7.3 (br, 20H, C6H5), 1.82 (t, 3H, 3JPH 15 Hz,
point energy differences among the various isotopomers to be
directly reflected in the bond distances. The striking change of
the bond distance upon small changes in temperature is due to
thermal population of vibrational excited states which are only
slightly higher in energy than the ground state. That these effects
are exhibited by molecules which are readily isolable at ambient
temperatures is very unusual.
Our studies of phosphine ligand variation show that this effect
is acutely dependent upon the choice of coligands. In this
particular family of cationic Ru complexes, measurable tem-
perature dependence of H-H distances is only observed when
the H-H distance is ca. 1.1 Å. Complexes with H-H distances
slightly shorter or longer than this exhibit temperature inde-
pendent H-H distances. It is important to note that the DFT
study made use of the computationally expedient “ligand” H2-
PCH2PH2. Thus subtle effects on the Ru-H2 interaction caused
by variations in the electronic properties of the bidentate
phosphine ligand were not addressed. Our results suggest that
such ligand variation has a significant effect on the vibrational
potential experienced by the bound dihydrogen.
3
CH3), 1.62 (s, 15H, Cp*), 1.61 (t, 3H, JPH 16 Hz, CH3). 31P{1H}: δ
44 (s).
Cp*Ru(dppm)Cl and Cp*Ru(dmpm)Cl. These were prepared as
above using dppm and dmpm, respectively. The yields were 76% for
Cp*Ru(dppm)Cl and 46% for Cp*Ru(dmpm)Cl. 1H and 31P{1H} NMR
spectra matched those reported in the literature.
[Cp*Ru(dmpm)H2]B(ArF)4 (5). A 40 mL Schlenk tube was charged
with 38 mg (0.093 mmol) of Cp*Ru(dmpm)Cl, 88 mg (0.098 mmol)
of NaB(ArF)4, and a Teflon stir bar. The tube was evacuated, and 15
mL of fluorobenzene was added by vacuum transfer. After back filling
with 1.1 atm of H2, the tube was warmed slowly to ambient temperature
with vigorous agitation to afford a colorless solution and a white
precipitate of NaCl. After stirring for 1 h at room temperature, the
solution was filtered under argon. The dihydrogen complex was
crystallized by slow diffusion of cyclopentane into the solution. The
solvents were removed via syringe under argon, and the crystals were
rinsed with 2 × 5 mL of cyclopentane and dried in vacuo. Yield: 51
1
mg, 55%. H NMR (CD2Cl2) Ru(H2): δ 3.47 (m, 1H, CH2), 2.98 (m,
Experimental Section
1H, CH2), 1.97 (t, 15H, 4JPH 1.5 Hz, Cp*), 1.66 (m, 12H, CH3), -7.77
(s, 2H, br, H2). RuH2: δ 3.27 (t, 2H, 2JPH 8.5 Hz, CH2), 2.03 (s, 15H,
General Procedures. Unless stated otherwise, all manipulations were
carried out under argon using Schlenk techniques. Complexes Cp*Ru-
(PPh3)2Cl, CpRu(dmpe)Cl, CpRu(dppe)H2+, and [Cp*Ru(dppm)H2]+
were prepared according to literature procedures. Dihydrogen complexes
1 and 3 were prepared according to literature procedures by the
protonation of neutral ruthenium hydrides with HBF4/diethyl ether.
Corresponding monodeuterated isotopomers were prepared in situ by
exchange with D2 (g) or HD (g) in NMR tubes. The phosphine ligand
dppip was prepared from 2,2-dichloropropane and Ph2PNa as described
in the literature.30 All other phosphines were obtained from Strem
Chemicals and used as received. Hydrogen gas was purchased from
Airgas and passed through a column of activated molecular sieves prior
to use. HD (g) and D2 (g) were used as received from Cambridge
Isotopes. NaB(ArF)4 was prepared by the published method.31 Elemental
analyses were performed by Galbraith. NMR spectra were recorded
on Bruker AC-200, DPX-200, DRX-499, AM-500, and DMX-750
spectrometers. Proton NMR spectra were referenced to the solvent
resonance with chemical shifts reported relative to TMS. 31P chemical
shifts were referenced to external 85% H3PO4. The NMR studies were
carried out in high quality 5 mm NMR tubes, utilizing deuterated solvent
distilled from standard drying agents. The conventional inversion-
recovery method (180-τ-90) was used to determine the relaxation times
T1 of various isotopomers of 3 at 750 MHz. In each experiment, the
waiting period was longer than 10 times the expected relaxation rate.
Ten variable delays were employed, utilizing appropriate pulse widths
determined for both 1H and 3H. The workup of spectra used for precise
measuring of coupling constants used zero filling to 128 K data points
prior to Fourier transform.
2
Cp*), 1.72 (br m, 12H, CH3), -7.09 (t, 2H, JPH 32.5 Hz, RuH2).
31P{1H} NMR: Ru(H2), δ -41.1 (s); RuH2, δ -31.1 (s). Anal. Calcd:
C, 45.61; H, 3.51. Found: C, 45.65; H, 3.76.
[Cp*Ru(dppip)H2]B(ArF)4 (4). This was prepared as above in 58%
1
yield. H NMR (CD2Cl2): Ru(H2), δ 1.77 (s, 15H, Cp*), 1.55 (t, 3H,
3JPH 14.6 Hz, CH3), 0.83 (t, 3H, 3JPH 17.6 Hz, CH3), -6.28 (s, br, 2H,
H2); RuH2, δ 1.88 (s, 15H, Cp*), 1.06 (t, 6H, 3JPH 15.5 Hz, CH3), -7.43
(t, 2H, 2JPH 27.7 Hz). Additionally, complex overlapping aryl resonances
for both the dihydrogen and the dihydride isomers were observed at δ
7.10-7.60. 31P{1H} NMR: Ru(H2), δ 53.7 (s); RuH2, δ 44.0 (s).
In Situ Preparation of Isotopomers of Dihydrogen Complexes.
Dihydrogen complexes 1 and 3 were deuterated by dissolving the
1
complexes in CD2Cl2 and treating with D2 gas (1 atm) until H NMR
indicated that 90% deuteration had been achieved. This allows for the
observation of the monodeuterated isotopomer without interference from
the H2 isotopomer. A simpler method was developed for compounds
2, 4, and 5. The dihydrogen complexes were prepared in situ as
described above from the corresponding ruthenium chloride and
NaB(ArF)4 in CD2Cl2. The reaction was carried out under 1.1 atm of
HD gas, affording samples that were nearly 100% HD isotopomer by
1H NMR. Atom scrambling occurs slowly (hours) for these complexes
at ambient temperatures. Storage of samples at temperatures lower than
273 K prevents isotopomer scrambling on a time scale of several days.
3
Preparation of H Labeled 1. In a typical procedure, a medium-
walled NMR tube attached to a Kontes valve was charged with 1 mg
of 1. After dissolution in CD2Cl2, the solution was placed under 1.1
atm HD (g) overnight to afford a mixture of isotopomers with a
Cp*Ru(dppip)Cl. A 500 mL Schlenk flask was charged with 550
mg (0.72 mmol) of Cp*Ru(PPh3)2Cl, 302 mg (0.73 mmol) of dppip,
and 200 mL of freshly distilled toluene. This slurry was purged for 10
min with argon and brought to reflux under argon producing an orange
solution. After being refluxed for 3 h, the solution was allowed to cool
and was stirred at ambient temperature overnight. The solution was
chromatographed on a 12 cm column of silica packed in hexanes.
Following elution of PPh3 by benzene, Cp*Ru(dppip)Cl was rapidly
eluted with diethyl ether as an orange band. The ether solution was
concentrated in vacuo and cooled to -35 °C overnight, affording red
1
deuterium content of ca. 50% as indicated by H NMR spectroscopy.
Following three freeze-pump-thaw cycles, the tube was attached to
the stainless steel tritium manifold and placed under 200 Torr of T2
gas for 2 h. This procedure affords a sample containing about 20%
tritium in the bound dihydrogen. The T2 (g) was removed via three
freeze-pump-thaw cycles, and the tube was flame-sealed under
vacuum.
Acknowledgment. This research was supported by the
National Science Foundation. Acquisition of the 750 MHz NMR
spectrometer was supported by the National Science Foundation
and the Murdoch Charitable Trust.
(30) Pons, A.; Rossell, O.; Sexo, M.; Solans, X.; Font-Bardia, M. J. Organomet.
Chem. 1996, 514, 177-182.
(31) Brookhart, M.; Grant, B.; Volpe, A. F., Jr. Organometallics 1992, 11, 3920-
3922.
JA0118284
9
1030 J. AM. CHEM. SOC. VOL. 124, NO. 6, 2002