Hydrogen bonding and proton transfer to ruthenium hydride complex CpRuH(dppe): Metal and hydride dichotomy

Article abstract of DOI:10.1021/ic301585k

The combination of variable temperature (190-297 K) IR and NMR spectroscopy studies with quantum-chemical calculations at the DFT/B3PW91 and AIM level had the aim to determine the mechanism of proton transfer to CpRuH(dppe) (1, dppe = Ph₂P(CH₂)₂PPh₂) and the structures of intermediates. Dihydrogen bond (DHB) formation was established in the case of interaction with weak proton donors like CF₃CH₂OH. Low-temperature protonation (at about 200 K) by stronger proton donors leads via DHB complex to the cationic nonclassical complex [CpRu(η²-H ₂)(dppe)]⁺ (2). Thermodynamic parameters of DHB formation (for CF₃CH₂OH: ΔH _HB = -4.9 ± 0.2 kcal·mol^-1, ΔS _HB = -17.8 ± 0.7 cal·mol^-1·K^-1) and proton transfer (for (CF₃)₂CHOH: ΔH _PT = -5.2 ± 0.3 kcal·mol^-1, ΔS _PT = -23 ± 1 cal·mol^-1·K^-1) were determined. Above 240 K 2 transforms into trans-[CpRu(H)₂(dppe)]⁺ (3) yielding a mixture of 2 and 3 in 1:2 ratio. Kinetic analysis and activation parameters for the [Ru(η²-H₂)]⁺ → trans-[Ru(H)₂]⁺ transformation indicate reversibility of this process in contrast to irreversible intramolecular isomerization of the Cp* analogue. Calculations show that the driving force of this process is greater stability (by 1.5 kcal·mol^-1 in ΔE scale) of the dihydride cation in comparison with the dihydrogen complex. The calculations of the potential energy profile indicate the low barrier for deprotonation of 2 suggesting that the formation of trans-[CpRu(H)₂(dppe)]⁺ proceeds via deprotonation of [Ru(η²-H₂)]⁺ to DHB complex, formation of hydrogen bond with Ru atom and subsequent proton transfer to the metal site.

Full text of DOI:10.1021/ic301585k

Article
pubs.acs.org/IC
Hydrogen Bonding and Proton Transfer to Ruthenium Hydride
Complex CpRuH(dppe): Metal and Hydride Dichotomy
Gleb A. Silantyev,† Oleg A. Filippov,† Peter M. Tolstoy,‡ Natalia V. Belkova,*,† Lina M. Epstein,†
Klaus Weisz,§ and Elena S. Shubina*,†
†A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Vavilov St. 28, 119991 Moscow, Russia
‡Center for Magnetic Resonance, St. Petersburg State University, Universitetskiy pr. 26, 198504 Peterhof, Russia
§Ernst-Moritz-Arndt-Universitat Greifswald, Felix-Hausdorff-Straße 4, 17487 Greifswald, Germany
̈
S
* Supporting Information
ABSTRACT: The combination of variable temperature
(190−297 K) IR and NMR spectroscopy studies with
quantum-chemical calculations at the DFT/B3PW91 and
AIM level had the aim to determine the mechanism of proton
transfer to CpRuH(dppe) (1, dppe = Ph2P(CH2)2PPh2) and
the structures of intermediates. Dihydrogen bond (DHB)
formation was established in the case of interaction with weak
proton donors like CF3CH2OH. Low-temperature protonation
(at about 200 K) by stronger proton donors leads via DHB
complex to the cationic nonclassical complex [CpRu(η2-
H2)(dppe)]+ (2). Thermodynamic parameters of DHB
formation (for CF3CH2OH: ΔH°HB = −4.9 0.2 kcal·mol−1,
ΔS°HB = −17.8 0.7 cal·mol−1·K−1) and proton transfer (for
(CF3)2CHOH: ΔH°PT = −5.2 0.3 kcal·mol−1, ΔS°PT = −23 1 cal·mol−1·K−1) were determined. Above 240 K 2 transforms
into trans-[CpRu(H)2(dppe)]+ (3) yielding a mixture of 2 and 3 in 1:2 ratio. Kinetic analysis and activation parameters for the
“[Ru(η2-H2)]+ → trans-[Ru(H)2]+” transformation indicate reversibility of this process in contrast to irreversible intramolecular
isomerization of the Cp* analogue. Calculations show that the driving force of this process is greater stability (by 1.5 kcal·mol−1
in ΔE scale) of the dihydride cation in comparison with the dihydrogen complex. The calculations of the potential energy profile
indicate the low barrier for deprotonation of 2 suggesting that the formation of trans-[CpRu(H)2(dppe)]+ proceeds via
deprotonation of [Ru(η2-H2)]+ to DHB complex, formation of hydrogen bond with Ru atom and subsequent proton transfer to
the metal site.
Scheme 1. Two Pathways of Transition-Metal Hydrides
Protonation via syn and anti Hydrogen Bonding
INTRODUCTION
■
Proton transfer involving transition metal hydrides is an
important step of many chemical and biochemical catalytic
processes. Protonation of hydrides is known to yield non-
classical η2-H2 complexes, many of which transform to classical
dihydride isomers upon warming. A classical example is the low
temperature protonation of the Cp*FeH(dppe) complex
described first by Hamon et al.1 Later we have shown that it
proceeds via MH···HA dihydrogen bond formation followed by
η2-H2 → trans-dihydride isomerization which occurs as a direct
intramolecular rearrangement.2−4 The same mechanism has
been established for the ruthenium analogue, Cp*RuH(dppe).5
At that a number of complexes give a dihydrogen-dihydride
mixture as a thermodynamic product of proton transfer.6−9 In
this case the question arises whether dihydride is formed by
simple isomerization of a nonclassical precursor or by
deprotonation of the latter with subsequent proton transfer
to the metal via a M···HA hydrogen bonded intermediate
(Scheme 1).
The protonation of the CpRuH(dppe) complex (1; dppe =
Ph2P(CH2)2PPh2) by strong acids (HPF6, (CF3SO2)2C(Ph)D
or HBF4·Et2O) yields at room temperature the 1:2 mixture of
[CpRu(η2-H2)(dppe)]+ (2) and trans-[CpRu(H)2(dppe)]+
(3)9,10 whereas the dihydrogen complex 2 is the sole
protonation product at 213 K (Scheme 2).11
So, to figure out the reaction mechanism and the possibility
of direct proton transfer to the metal atom in the presence of
hydride ligand we carried out a variable temperature IR and
Received: July 20, 2012
Published: January 25, 2013
© 2013 American Chemical Society
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points on the potential energy surface was confirmed by vibrational
analysis.24 Transition state (TS) structures showed only one negative
eigenvalue in their diagonalized force constant matrices, and their
associated eigenvectors were confirmed to correspond to the motion
along the reaction coordinate under consideration using the intrinsic
reaction coordinate (IRC) method.
Scheme 2. Products of CpRuH(dppe) Protonation by
HBF4·Et2O at Low and Room Temperatures
Natural atomic charges and Wiberg bond indices25 (WBI) were
calculated using the natural bond orbital (NBO) analysis26 option as
incorporated in Gaussian09. Topological analysis of the electron
density distribution function ρ(r) was performed using the AIMALL
program package27 based on the wave function obtained by the
B3PW91 calculations. AIM extended wave function format allows
QTAIM analyses of molecular systems containing heavy atoms
described with ECPs. The energy of the hydrogen bonding interaction
was estimated using the correlation between the energy of the contact
(Econt) and the value of the potential energy density function V(r) in
(3, −1) critical point Econt = 1/2V(r).28,29 Hydrogen bond ellipticity,
εHH, was defined as ε = (λ1/λ2 − 1), where λ1 and λ2 are the negative
eigenvalues of the Hessian of the electron density at the bond critical
point ordered such that λ1 < λ2 < 0.
NMR spectroscopic study and density functional theory (DFT)
calculations of the interaction of 1 with fluorinated alcohols and
other proton donors (CF3CH2OH (TFE), (CF3)2CHOH
(HFIP), CF3COOH (TFA), indole, HBF4·Et2O).
RESULTS AND DISCUSSION
■
Interaction with Weak Proton Donors: Hydrogen
Bonding. Formation of a hydrogen bond between 1 and
weak proton donors such as indole and TFE at low
temperatures in dichloromethane was established by IR
spectroscopy. In the presence of excess 1 the intensity of the
EXPERIMENTAL SECTION
■
Hydride Synthesis. CpRuCl(PPh3)2 was synthesized as de-
scribed12 and then transformed into CpRuCl(dppe).13 The hydride
CpRuH(dppe) was obtained by the reaction of CpRuCl(dppe) with
sodium methylate in MeOH.14
free
νXH bands of the proton donors decreases, and new wide
Dichloromethane was dried by reflux over CaH2 and freshly distilled
under an argon atmosphere prior to use.
bonded
νXH
bands appear at lower frequencies. The band shift,
ΔνXH = νXHfree − νXHbonded, increases from indole to TFE being
223 and 266 cm−1, respectively. These data allow estimating the
enthalpy of hydrogen bond formation ΔH°HB = −4.3 and −4.9
kcal·mol−1 for complexes of 1 with indole and TFE,
respectively. The value obtained for 1·TFE is in good
agreement with that obtained from the temperature depend-
All NMR spectra were measured on Bruker AMX 500 and Bruker
AVANCE 600 NMR spectrometers supplied with a specially designed
low temperature dual probe-head (SEI, 5 mm tube size, 1H/13C).
Eurotherm Variable Temperature Unit was used for temperature
stabilization. The temperature was measured with a build-in copper-
constantan thermocouple positioned about 1 mm below the bottom of
the sample tube. The control unit regulated the heater current to
achieve stable temperature using the Zeigler-Nichols PID controller
procedure. Stability of the temperature during each experiment was
about 0.2 K.
The IR spectra of CH2Cl2 solutions were measured on a Nicolet
6700 FTIR spectrometer in CaF2 cells (l = 2.2 mm). Concentrations
of proton donors were 0.006 M for measurements in the νXH range to
avoid self-association.
ence of 1·TFE hydrogen bond formation constants ΔH°HB
=
−4.9
0.2 kcal·mol−1; ΔS°HB = −17.8 0.7 cal·mol−1·K−1.
The complex basicity in hydrogen bonding,30−32 Ej = ΔH°HB
/
(Pi·ΔH°11) or Ej = ΔH°HB/Pi/ΔH°11 = 1.21, is lower than that
of Cp*RuH(dppe) (Ej = 1.395 and higher than that of
CpRuH(CO)(PCy3) (Ej = 1.02).33
The IR spectrum of 1 in the range of M-H stretching
vibrations features a wide (Δν1/2 = 67 cm−1) band at 1937
cm−1, which is asymmetric because of the overlap with dppe
ligand vibrations (overtones of the phenyl rings). In the
presence of increasing amount of TFE the νRuH band intensity
increases by 11−23% whereas its maximum shifts to lower
For the reference measurements the complexes 2 and 3 were
prepared in situ by protonation of 1 by 1.2 equiv of HBF4·Et2O in
CH2Cl2 or CD2Cl2. Their spectroscopic parameters are in full
agreement with those reported in ref 9.
1
2: H NMR (600 MHz, CD2Cl2, 200 K): δ = −9.21 ppm (2H,
Ru(η2-H2)); 31P{1H} NMR (212 MHz, CD2Cl2, 200 K): δ = 79.3
ppm.
frequencies (Figure 1). Band deconvolution shows the presence
1
band at 1928 cm−1 (Figure 1). These
bonded
3: H NMR (600 MHz, CD2Cl2, 200 K): δ = −8.72 ppm (2H,
of a new νMH
RuH2); 31P{1H} NMR (212 MHz, CD2Cl2, 200 K): δ = 68.4 ppm; IR
(CH2Cl2): νas(RuH2) = 1984 cm−1, νs(RuH2) = 2014 cm−1.
Computational Details. Calculations were performed with the
Gaussian0915 package at the DFT/B3PW9116−18 level without any
ligand simplification. The basis set for the Ru and P atoms was that
associated with the pseudopotential,19,20 with a standard double-ζ
LANL2DZ contraction,15 supplemented in the case of P with a set of
d-polarization functions.21 The carbon and hydrogen atoms of the Cp
and dppe ligands together with the atoms of the proton donor
molecules (C, F, H) that are not involved in hydrogen bonds were
described with a 6-31G basis set.22 The hydridic hydrogen atom and
the hydrogen and oxygen atoms of the proton donor molecules
involved in hydrogen bonding were described with a 6-31G(d,p) set of
basis functions.23
changes indicate the interaction of the proton donor with the
hydride ligand, that is, formation of a dihydrogen bond (DHB)
(1′).34,35
The DHB formation was confirmed by NMR spectra under
1
similar conditions. The hydride triplet in H NMR shifts from
δRuH −14.12 ppm for 1 to −14.30 in the presence of 7 equiv of
TFE at 200 K in CD2Cl2 (Figure 2) whereas the 31P{1H}
resonance shifts from 90.3 ppm to 89.4. ppm. Such changes are
typical of dihydrogen bond formation32,36,37 resulting from the
fast equilibrium between the complex 1′ and free hydride 1.
The minimum of the longitudinal relaxation time of the hydride
resonance T1 min decreases from 1050 ms (210 K, 500 MHz) to
647 ms (210 K, 500 MHz) in the presence of 4 equiv of TFE.
That allowed estimating the H···H distance dHH = 1.96 Å in the
dihydrogen bonded complex taking into account the
The structures of the reactants, hydrogen bonded complexes, ion
pairs, and transition states were fully optimized with this basis set
without any symmetry restrictions. The nature of all of the stationary
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can safely assume that 2 (and 3) exists as contact ion pairs
stabilized by a hydrogen bond between the cation and anion
(Scheme 3).
With the temperature increase the proton transfer equili-
brium shifts toward free (1) and dihydrogen-bonded (1′)
hydride (Scheme 3) as is evident from the redistribution of the
signals intensities (Figure 3, Supporting Information, Figure
S1). Integration of signals in the temperature range 195−225 K
allowed estimating the proton transfer equilibrium constants
KPT. The analysis of their temperature dependence (Supporting
Information, Figure S1) gave the thermodynamic parameters of
the proton transfer step: ΔH°PT = −5.2 0.3 kcal·mol−1 and
ΔS°PT = −23 1 cal·mol−1·K−1. These values are in accord
with those reported for proton transfer to various hydride
complexes37,39 when the negative entropy change reflects the
more organized solvent structure around the hydrogen bonded
ion pair.
To get more information on dynamic exchange processes, a
two-dimensional 1H-1H EXSY experiment was carried out for a
solution of 1 in CD2Cl2 in the presence of 10 equiv of HFIP at
235 K. As shown by the cross-peak in Figure 4, the hydrogen
exchange occurs between 2 and the neutral forms 1/1′ in line
with the proton transfer mechanism proposed above (Scheme
3).
Figure 1. IR spectra (νMH region) of CpRuH(dppe) (1, 0.025 M; red
line) and 1 in the presence of 2, 4, and 6 equiv of TFE. CH2Cl2, 200 K,
free
l = 2.2 mm. Deconvolution of the blue line: νMH (dashed) and
bonded
νMH
(dotted).
Further information about the structure of the hydrogen
bonded complex and the proton transfer product was obtained
from the study of the interaction of 1 with CF3COOH by IR
and NMR spectroscopies. Interestingly, the interaction of 1
with this acid at 200 K leads to partial proton transfer and
formation of both 2 and 3. Addition of 0.5 to 1.5 equiv of
CF3COOH to a 0.025 M solution of 1 in CH2Cl2 causes a
decrease of the νRuH(1) band intensity and the appearance of
νRuH(3) bands at 2014 and 1989 cm−1 assigned to νRuH2 and
s
νRuH2as, respectively (Figure 5).35 Full proton transfer is
observed in the presence of over 2 equiv of acid as confirmed
1
by H NMR spectra according to which both 2 and 3 are
present in solution in the ratio 2:1 as determined by integration
of the corresponding high-field resonances. This ratio is
independent of the acid excess and does not change up to
240 K. Above this temperature the 2:3 ratio increases reaching
the equilibrium value of 1:2 at 298 K similar to that reported
for the BF4 counteranion.10 These changes correspond to the
free energy (ΔG) values of −0.4 (at 298 K) and −0.3 (at 200
K) kcal·mol−1 for 2 ⇄ 3 equilibrium, and a van’t Hoff plot gives
ΔH = −1.8 0.1 kcal·mol−1, ΔS = −5.7 0.4 cal·mol−1·K−1.
IR spectra of 1/CF3COOH mixture (0.5 equiv of acid) in the
Figure 2. 1H NMR spectra (hydride region, 500 MHz) of
CpRuH(dppe) in the presence of TFE: 0 (a), 4 (b), and 7 (c)
equiv. CD2Cl2, 200 K.
range of νCO(CF3COOH) and νas
(CF3COO−) show the
OCO
appearance of the νas
band of the anion at 1685 cm−1 as
OCO
expected for partial proton transfer (Scheme 4). The less
intense bands at 1782 and 1735 cm−1 belong to the νCO
vibrations of the acid in the hydrogen bonded complex of 1
with two CF3COOH molecules (Scheme 4a).40 These three
bands increase upon further addition of the acid (Supporting
Information, Figure S2) in agreement with the right shift of
hydrogen bonding and proton transfer equilibrium (Scheme
4b). At 2.5-fold acid excess the additional band appears in the
equilibrium constants obtained from the IR data.36,38 Similarly
in the presence of 6 equiv of HFIP the T1 min of hydride
resonance decreases from 1383 ms (190 K, 600 MHz) to 487
ms (195 K, 600 MHz) yielding the H···H distance in the DHB
complex dHH = 1.91 Å.
Interaction with Stronger Proton Donors: Proton
Transfer Equilibrium. Interaction of 1 with 7 equiv of
HFIP at 195 K in CD2Cl2 leads to partial proton transfer with
the formation of dihydrogen complex 2 which is characterized
by the hydride resonance at −9.21 ppm (Figure 3, the
insignificant amount of dihydride 3 is observed at −8.72 ppm
probably because of the local warming of the sample). On the
basis of the data reported for other hydride complexes,39 one
spectrum at 1645 cm−1 belonging to νas
vibrations of
OCO
homoconjugated anion [CF3COO(CF3COOH)n]−. This in-
dicates dissociation of hydrogen bonded ion pair in the
presence of excess acid.40
Proton Transfer and Formation of trans-Dihydride. As
mentioned above, when fluorinated alcohols are used as proton
donors the dihydrogen complex 2 is the only proton transfer
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1
Figure 3. H NMR spectra of 1 (hydride region, 600 MHz) in the presence of 7 equiv of HFIP. 195 and 235 K, CD2Cl2.
dihydride 3 begins above 240 K. When 10 equiv of HFIP were
used at low temperature (190 K) in CD2Cl2 the formation of
the dihydrogen complex 2 was quantitative (only traces of 3
were observed, Figure 6). With the temperature increase to 210
K (with 5 K step) a slight change in the 2 to 3 intensity ratio
was observed in favor of the latter. At 215 K a triplet at −14.66
ppm appears in the spectrum that belongs to the averaged
signal of the hydride ligand in free (1) and hydrogen-bonded
(1′) complexes. The position of this signal is determined by the
position of the dihydrogen bond formation equilibrium, shifting
upfield at higher content of 1′. Analysis of the data acquired at
different temperatures showed that the temperature increase
Scheme 3. Mechanism of Proton Transfer to CpRuH(dppe)
(1) via Dihydrogen Bonded Complex (1′) Formation
product below 240 K. The temperature increase shifts proton
transfer equilibrium to the left, toward the DHB complex 1′
and starting hydride 1 (Figure 3). Formation of the trans-
Figure 4. 2D EXSY spectrum of 1 in the presence of 10 equiv of HFIP in CD2Cl2 at 235 K (100 ms mixing time). Corresponding 1D spectral
regions are shown at the top and to the left.
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Figure 6. 1H NMR spectra (hydride region, 600 MHz) of
CpRuH(dppe) (1) in the presence of 10 equiv of HFIP at different
temperatures, CD2Cl2.
Figure 5. IR spectra (νMH region) of CpRuH(dppe) (1, 0.025 M) and
1 in the presence of increasing amount of TFA: 0 (a), 0.5 (b), 1 (c),
1.5 (d), 2 (e), 2.5 (f) equiv. The intensity increase on going from
spectrum e to f is due to the impact of νCO absorption of the acid (νCO
= 1800 cm−1). CH2Cl2, 200 K, l = 1.2 mm.
shifts the equilibrium between 1 and 1′ to the left; that is
manifested by the decrease of Δδ = δ1 − δ(1+ROH) values
(Supporting Information, Table S1).
The mole fractions of the species in the equilibrium
(Supporting Information, Table S1) were calculated in the
temperature range 215−265 K from the data obtained by
integration of the corresponding signals (Figure 6). Changes of
mole fractions with temperature are plotted in Figure 7,
showing that the decrease of the concentration of 2 is
accompanied by the formation of the neutral forms (due to
the deprotonation of 2) and of trans-dihydride 3. However, the
concentration of 3 practically does not increase above 255 K
and decreases above 260 K pointing to the reversible formation
of trans-dihydride due to the left-hand shift of the protonation
equilibrium leading to the steady growth of the neutral forms.
At 265 K the predominant species in the presence of 10
equiv of HFIP in CD2Cl2 is 1/1′ (74.1%). The small amount of
dihydride 3 formed under these conditions does not change
when the temperature is decreased to 200 K (Supporting
Information, Figure S3) whereas the resonance of 2 appears at
250 K and grows with cooling. This confirms the reversibility of
the formation of the cationic complexes 2 and 3 and the kinetic
preference of the hydride protonation.
Figure 7. Temperature dependence of the mol fraction of the
molecular (1 + 1′) and ionic (2 and 3) species formed at 1/HFIP ratio
1:10.
neutral forms (1 + 1′) signal. However the data described so far
do not allow to discriminate whether 3 is formed by direct
isomerization of 2 (as for its Cp* analogue4) or by
deprotonation-reprotonation pathway via anti hydrogen bond
to the metal. To address this point we studied the reaction
kinetics and performed DFT calculations (see below).
Transformation of 2 to 3 takes time at low temperatures. The
kinetics of this transformation was studied by 1H NMR
monitoring of corresponding hydride resonances at 250−265 K
(Figure 8a). Complex 2 was generated by in situ protonation of
1 with HFIP or HBF4 at a temperature of about 200 K, when
proton transfer quantitatively gives a η2-H2 complex. Kinetic
curves of isomerization were obtained from integration of 2 and
Thus, the dihydrogen bonded complex 1′ is the intermediate
of the proton transfer yielding the nonclassical complex 2.
Formation of 2 is reversible as evidenced by the increase of its
resonance upon cooling accompanied by disappearance of the
Scheme 4. (a) Hydrogen Bonded Complex 1·(TFA)2. (b) Equilibria between Molecular and Ionic Forms of CF3COOH and
Corresponding νCO or νas
Vibration Frequencies
OCO
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1
Figure 8. H NMR monitoring of the 2 to 3 transformation (left) and corresponding kinetic plot linearized in the coordinates of reversible first-
order reaction (right). 250 K, CD2Cl2.
Table 1. Isomerization Equilibrium Constant Kisom, Rate Constants of Direct (k1) and Reverse (k2) Reactions, the Reaction Free
Energy (ΔGisom), and the Activation Free Energy (ΔG⧧) for the 2 to 3 Transformation
HA
T, K
Kisom
104 k1, s−1
104 k2, s−1
ΔGisom, kcal·mol−1
ΔG⧧, kcal·mol−1
HFIP
246
250
253
258
253
260
2.12
1.98
1.91
1.92
1.85
2.07
0.49 0.01
0.86 0.02
1.5 0.02
2.4 0.02
1.2 0.02
3.1 0.03
0.23 0.01
0.43 0.02
0.78 0.02
1.3 0.02
0.65 0.02
1.5 0.03
−0.37
−0.34
−0.33
−0.33
−0.31
−0.37
19.14
19.21
19.18
19.32
19.3
HBF4
19.3
Figure 9. Computed structural parameters of the hydrogen bonded complexes between CpRuH(dppe) and CH3OH, CF3CH2OH (bold),
(CF3)2CHOH (italic), and CF3OH (bold, blue).
3 signals at different reaction times τ. Interestingly, kinetic plots
become linear in ln(x∞/(x∞ − x)) coordinates (Figure 8b),
where x is the amount of nonreacted complex 2 (or the amount
of complex 3 formed) at time τ, x∞ is the amount of 2 at
equilibrium. The slope of this line is k1 + k2 and together with
Kisom = k1/k2 = [3]/[2] gives direct (k1) and reverse (k2) rate
constants of complex 3 formation. Such rate law corresponds to
a reversible first-order reaction of the isomerization of
[CpRu(η2-H2)(dppe)]+ (2) into [CpRu(H)2(dppe)]+ (3) in
contrast to irreversible isomerization of [Cp*Ru(η2-H2)-
(dppe)]+ to [Cp*Ru(H)2(dppe)]+.5
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Table 2. Interaction Energies (in kcal·mol−1) Relative to the
The temperature range from 240 to 260 K turned to be the
most convenient for NMR monitoring of the isomerization
process. The values of the equilibrium constant Kisom, the rate
constants of direct (k1) and reverse (k2) reactions obtained in
the presence of 20 equiv of HFIP are gathered in Table 1
together with the free energies ΔGisom and ΔG⧧ calculated from
these data.
Isolated Reactants
a
ROH
CH3OH
adduct
ΔE
ΔEZPVE
Econt
syn-1′
−6.3
−4.9
−1.7
−7.4
−6.7
−5.0
−8.0
−10.1
−6.2
−12.2
−12.0
−7.6
−5.3
−3.8
−0.5
−6.9
−6.3
−4.1
−7.6
−9.4
−5.4
−11.8
−11.5
−7.2
−4.2
−3.4
−2.0
−4.9
−3.6
−2.2
−6.1
−6.1
−2.4
−6.2
−6.8
−3.8
syn-1′a
anti-1′
syn-1′
syn-1′a
anti-1′
syn-1′
syn-1′a
anti-1′
syn-1′
syn-1′a
anti-1′
CF3CH2OH
(CF3)2CHOH
CF3OH
Analysis of the temperature dependence of the rate constant
for direct reaction k1 gives activation enthalpy (ΔH⧧) and
entropy (ΔS⧧) of 16
1 kcal·mol−1 and −13
5
cal·mol−1·K−1, respectively. These numbers are very similar to
ΔH⧧ = 17.6 0.9 kcal·mol−1 and ΔS⧧ = −7.8
0.6
cal·mol− 1 ·K− 1 reported for the closely related
[CpRuH2(dmdppe)]+.6 The negative activation entropy
indicates an ordered (associative) transition state typical for
proton transfer reactions41 and is in contrast with positive
values found for isomerization of [Cp*M(η2-H2)(dppe)]+.4,5,42
Kinetics of the transformation of 2 into 3 was also studied in
the presence of higher HFIP excess (30 equiv) at 258 K. The
values of rate constants for direct (k1) and reverse (k2)
reactions are practically the same as in the presence of 20 equiv
of HFIP giving the same values of free energies (compare to
Table 1): k1 = 2.4 × 10−4 s−1, k2 = 1.2 × 10−4 s−1, ΔG⧧ = 19.33
kcal·mol−1, and ΔGisom = −0.34 kcal·mol−1. When tetrafluor-
oboric acid (1.1−1.5 equiv) was used to generate 2 the rate
constants of direct (k1) and reverse (k2) reaction showed rather
small difference from those obtained in the presence of HFIP at
the same temperature (Table 1), not allowing the solid
conclusion about the counterion effect on the 2 ⇄ 3
transformation rate. But the kinetic data obtained do allow
suggesting that transformation of the dihydrogen 2 to the
dihydride 3 is reversible for any strong acid.
a
The values determined from the AIM analysis data.
−1) in either complex that binds two hydrogens (Supporting
Information, Figure S4, Table S2). The (O)H···H interaction is
predominant in both complexes independent of ROH:
delocalization indices (DI)43,47 are always higher for the
(O)H···H contacts than for the (O)H···M ones (Supporting
Information, Table S2). However, the deviation of the bond
path from linearity is evident from higher ellipticity values for
syn-1′a complexes. Thus, both geometrical and electronic
parameters indicate that syn-1′ and syn-1′a are DHB complexes
with higher metal involvement in syn-1′ complexes.
Inspection of the hydrogen bonding energies reveals
interesting trends. The syn-1′ structure is most favored for
CH3OH and TFE; the increase of proton donor strength makes
the two syn-structures (syn-1′ and syn-1′a) of either close or the
same energy. The same trend can be seen in the AIM derived
Econt values, though the absolute numbers are somewhat lower
than ΔE (ΔEZPVE) because only HH interaction is taken into
account under the AIM approach. Dihydrogen bonding (syn-
complexes) is clearly preferred over bonding to the metal (anti-
1′); a similar trend was observed for Cp*MH(dppe)
analogues.3,5,42,43 These data are in agreement with the
experimental evidence of DHB formation and explain why it
is impossibile to observe the hydrogen bonded anti-1′ complex
in the spectra.
Computational Study: Proton Transfer and Formation
of trans-Dihydride. The most relevant to our study is the
DFT/B3LYP study of proton transfer to the Cp*FeH(dhpe)
and CpFeH(dhpe) complexes (dhpe = H2PCH2CH2PH2) as
models of Cp*FeH(dppe) using either 1 or 2 molecules of
HFIP or CF3COOH are reported in ref 3. It has shown the
clear preference of hydrogen bonding to the hydride ligand and
its intermediacy in the formation of the [Fe(η2-H2)]+ complex.3
Both theory and experiment have revealed no evidence for
proton transfer to the metal in this case.2,3 Formation of trans-
[Cp*Fe(H)2(dppe)]+ has been shown to be intramolecular and
occur via the “direct” pathway involving simultaneous H−H
bond breaking and cis−trans isomerization. This pathway is
slightly preferred over the “via Cp” pathway involving agostic
C5Me5H intermediates.4 Its occurrence was strongly supported
experimentally by KIE studies.4 Our calculations give the
transition state for the “direct” transformation of [CpRu(η2-
H2)(dppe)]+ (2) in trans-[CpRu(H)2(dppe)]+ (3) at 24.1
(ΔE⧧) or 23.7 (ΔG⧧) kcal·mol−1, and the highest transition
state for the “via Cp” pathway at 27.1 (ΔE⧧), 28.5 (ΔG⧧)
kcal·mol−1 (Supporting Information, Figure S6). These values
Computational Study: Hydrogen Bonding. In the case
of half-sandwich cyclopentadienyl Group 8 metal hydride
complexes of type (C5R5)MHL2 (L2 = 2 PR3 or bidentate
diphosphine) a proton donor attack on the hydride ligand side
(syn) would lead to a DHB complex and η2-H2 proton transfer
products, whereas the attack at the opposite side (anti) would
result in M···HA hydrogen bonding and trans dihydride
formation as the result of proton transfer.
The computational analysis of hydrogen bonding and proton
transfer was carried out for the real complex CpRuH(dppe) at
the DFT/B3PW91 level to allow comparison with the data
reported for the Cp*RuH(dppe) complex.5 The two expected
minima (syn-1′ and anti-1′) were indeed found (Figure 9).
Surprisingly, another minimum was found for hydrogen
bonded adducts with alcohol being at the hydride side (syn-
1′a, Figure 9), which differs by the orientation of the alkyl
substituent of the alcohol and the parameters of the MH···HO
moiety. The formation energies of all adducts are gathered in
Table 2. The details of the electron density analysis and AIM
analysis are given in Supporting Information, Tables S2 and S3.
Inspection of the data in Figure 9 shows that both syn-1′ and
syn-1′a complexes can be described as bifurcate adducts with
simultaneous interaction of the ROH proton with the hydride
hydrogen and the metal. Detailed analysis of such bifurcate
hydrogen bonds was recently described by some of us for
Cp*MH(dppe) complexes.43 The major difference between the
two structures syn-1′ and syn-1′a is in the O−H···M and O−
H···H angles, the latter being more acute in syn-1′a. At that the
(O)H···H distance is longer in syn-1′a than in syn-1′, whereas
(O)H···M distance is shorter. Notably, “Atoms in molecule”
(AIM) analysis44−46 showed just one bond critical point (3,
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Figure 10. Optimized geometries of hydrogen bonded complexes of 1 with two CF3OH molecules (syn-1″a and anti-1″) showing selected bond
lengths (in Å) and angles (in degrees). Hydrogen atoms of Cp and dppe ligands are omitted for clarity.
are comparable to those calculated for [Cp*MH2(dppe)]+
cations (M = Fe, Os).4,42 However, these values are too high
to support equilibrated [Ru(η2-H2)]+ to [Ru(H)2]+ trans-
formation observed experimentally.
kcal·mol−1 more favorable on the free energy (ΔGgas) scale.
This energy difference is in agreement with the experimentally
observed formation of a 2/3 mixture. The positions of both
transition states, TS-syn and TS-anti (Figure 11), are
substantially lower than the TSs for the intramolecular
isomerization of [CpRu(η2-H2)(dppe)]+ (2) to trans-[CpRu-
(H)2(dppe)]+ (3) along the “direct” or “via Cp” pathways (vide
supra). Thus, the calculations show for the first time the
preference of the intermolecular “deprotonation-reprotonation”
pathway of [CpRu(η2-H2)(dppe)]+ (2) transformation to trans-
[CpRu(H)2(dppe)]+ (3) where the deprotonation of 2 is the
rate determining step. At that the transformation of DHB
intermediate syn-1″a into hydrogen bonded anti-1″ does not
require complete dissociation of syn-1″a; the hydrogen bonded
complex containing the alcohol molecules bonded simulta-
neously at syn and anti positions (syn-,anti-1″; Figure 12) is a
feasible reaction intermediate (5.2 kcal·mol−1 above syn-1″a;
Figure 11).
The energy profiles of proton transfer to CpRuH(dppe)
were calculated using two CF3OH molecules. The correspond-
ing hydrogen bonded complexes (syn-1″a and anti-1″) were
proven to be the reaction intermediates (Figure 10), which are
connected with corresponding proton transfer products 2 and 3
via transition states TS-syn and TS-anti according to the IRC
calculations. Interestingly the geometry of the dihydrogen
bonded intermediate syn-1″a is closely related to that of syn-1′a
complexes featuring same angles but shorter H···H(O) and
M···H (O) distances in agreement with the higher proton
donating strength of the CF3OH dimer.
Proton transfer easily occurs in both hydrogen bonded
complexes: the reaction transition states (TS-syn and TS-anti)
are found less than 2.5 kcal·mol−1 above the corresponding
hydrogen bonded minima (Figure 11). The proton transfer
productshydrogen bonded ion pairs [CpRu-
(H)2(dppe)]+···[CF3OHOCF3]− and [CpRu(η2-H2)-
(dppe)]+···[CF3OHOCF3]− (Supporting Information, Figure
S5)have comparable energies, the trans-dihydride 3 being 1.5
Our experimental data show that the mechanism of [M(η2-
H2)]+ to [M(H)2]+ transformation is different for semi-
sandwich pentamethylcyclopentadienyl and cyclopentadienyl
complexes of ruthenium. The irreversible first order reaction
occurs in the case of [Cp*MH2(dppe)]+ complexes (M = Fe,
Ru, Os),4,5,42 but it is reversible first order in the case of
[CpRuH2(dppe)]+. The DFT calculations performed suggest
that it is the [Ru(η2-H2)]+ deprotonation-reprotonation process
what is responsible for trans-[Ru(H)2]+ formation in case of
[CpRuH2(dppe)]+. Following this mechanism the energy
profile can be drawn on the basis of experimental data (Figure
13). In the presence of alcohol excess the DHB complex (syn-
1′) should dominate at low temperatures, yielding the
nonclassical cation 2 as the result of proton transfer. Following
the computational results we believe that the activation
parameters determined experimentally for [M(η2-H2)]+ to
[M(H)2]+ transformation (ΔH⧧ = 16 1 kcal·mol−1, ΔS⧧ =
−13
5 cal·mol−1·K−1) are those of the [M(η2-H2)]+
deprotonation step. Under this assumption, the barrier for
proton transfer to yield 2 from 1′ (ΔH⧧PT) could be easily
calculated as 10.8 kcal·mol−1. This value is too small to study
the protonation kinetics by spectroscopic techniques at regular
time scales. The temperature increase should allow formation
of a hydrogen bond to the metal (anti-1′), which undergoes
Figure 11. Electronic energy (ΔE, dashed line, italic numbers in
parentheses) and free energy (ΔG, solid line, regular numbers) profiles
for 1·2CF3OH system in gas phase.
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Figure 12. Optimized geometry of hydrogen bonded complex syn-,anti-1″ featuring simultaneous Ru···HO and RuH···HO bonds. Selected bond
lengths (in Å) and angles (in degrees) are shown. Hydrogen atoms of Cp and dppe ligands are omitted for clarity.
Figure 13. Enthalpy profile (ΔH, italic numbers, in kcal·mol−1) for the reaction between 1 and HFIP based on the experimental data. Gray bars are
drawn approximately since parameters of this step can not be observed experimentally.
proton transfer even more easy to yield the classical cation 3.
The driving force for the overall process is the thermodynamic
favorability of the trans-dihydride 3.
NMR spectroscopic data are consistent with dihydrogen bond
formation, and two structures found computationally, when the
proton donor is on the same side as the hydride ligand (syn),
can be described as a dihydrogen bond with additional
interaction to the metal. The ligand change lowers the hydride
basicity, but it also diminishes the steric hindrance allowing
better access of the proton donor to the metal in anti position.
As the result the two proton accepting sites, hydride ligand and
metal atom, become competitive, and direct proton transfer to
CONCLUSIONS
■
Our results show that substitution of Cp* by less bulky and less
electron rich Cp in Cp′RuH(dppe) (Cp′ = Cp or Cp*) still
does not make it possible for hydrogen bonding to the metal in
the presence of the hydride ligand. Variable temperature IR and
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the metal to yield a trans-dihydride becomes operative in case
of the Cp complex.
(6) Chinn, M. S.; Heinekey, D. M. J. Am. Chem. Soc. 1990, 112,
5166−5175.
The low temperature (190−210 K) proton transfer leads via
dihydrogen bonding to the nonclassical hydride [CpRu(η2-
H2)(dppe)]+ (2) only. Upon the temperature increase (above
250−260 K) the latter transforms into classical dihydride trans-
[CpRu(H)2(dppe)]+ (3) yielding a 1:2 mixture of 2 and 3.
According to both experiment and calculations the dihydrogen
to dihydride isomerization mechanism involves deprotonation
of dihydrogen complex to initial hydride with subsequent
formation of Ru···HA hydrogen bond and proton transfer to
the metal site. However being energetically less favorable, the
hydrogen-bond to the metal atom (Ru···HA) (anti-1′) is still
low populated and could not be observed experimentally.
Evidences of such reaction mechanism are found for the first
time, though the possibility of anti-proton transfer to the metal
has been suggested for the formation of trans-[Cp*Os-
(H)2(CO)2]+ from [Cp*Os(η2-H2)(CO)2]+.48 Thus, the
metal-hydride dichotomy is a subject of modification of the
ligand environment: change of steric and electronic properties
upon substitution of Cp* by Cp ring induces not only
quantitative but very significant qualitative change in the
hydride complex reactivity. The reversibility of trans-[CpRu-
(H)2(dppe)]+ formation is important for its operation as ionic
hydrogenation catalyst,49 so both [Ru(η2-H2)]+ and [Ru(H)2]+
can transfer proton to the product.
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ASSOCIATED CONTENT
* Supporting Information
■
S
Additional IR and NMR spectra, details of the AIM analysis,
optimized geometries (Cartesian coordinates) of all the species.
This material is available free of charge via the Internet at
http://pubs.acs.org.
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H., Eds.; Akademie Verlag: Berlin, Germany, 1991; p 11.
AUTHOR INFORMATION
Corresponding Author
*E-mail: nataliabelk@ineos.ac.ru (N.V.B.), shu@ineos.ac.ru
(E.S.S.). Fax: +7 499 1355085 (N.V.B., E.S.S.).
■
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(21) Hollwarth, A.; Bohme, M.; Dapprich, S.; Ehlers, A.; Gobbi, A.;
̈
̈
The authors declare no competing financial interest.
Jonas, V.; Kohler, K. F.; Stegmann, R.; Veldkamp, A.; Frenking, G.
̈
Chem. Phys. Lett. 1993, 208, 237−240.
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ACKNOWLEDGMENTS
■
This work was supported by the Russian Foundation for Basic
Research (projects 11-03-01210, 12-03-31326, 12-03-33018),
the German-Russian Interdisciplinary Science Center (G-
RISC) funded by the German Federal Foreign Office via the
German Academic Exchange Service (DAAD) (projects C-
2010b-6 and C-2011a-4), and by the Division of Chemistry and
Material Sciences of RAS.
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