Facile Ru-H2 heterolytic activation and intramolecular proton transfer assisted by basic N-centers in the ligands

Article abstract of DOI:10.1021/ja055116f

The use of the phosphine PPh₂py instead of PPh₃ in complexes of the type [Cp*RuH(P)₂] enormously alters the kinetic control of the proton-transfer reactions over this compound and its chemical behavior. The reaction at low temperature of [Cp*RuH(PPh₂py)₂], 2, with HBF₄ gives as products the classical dihydride trans-[Cp*RuH₂(PPh₂py)₂](BF₄), 3 (1 equiv of HBF₄) or the dihydrogen-bonded complex [Cp*RuHH(PPh₂pyH)(PPh₂py)](BF₄)₂, 4 (2 equiv of HBF₄). These complexes exhibit very accessible intramolecular processes of proton transfer, and finally, a slow release of H₂ takes place at room temperature. Derivatives 2 and 3 are active catalysts for the deuterium labeling of H₂ using methanol-d₄ as an isotopic source. This demonstrates that the release of hydrogen is reversible, that the heterolytic activation of H₂ is an easy process, and that acid species participate in the intramolecular proton-transfer processes. These observations are supported by reaction-coordinate calculations at the DFT/B3LYP level that show the existence of a low-energy reaction path that easily transforms the classical trans dihydride complex into the nonclassical cis dihydrogen compound in a reversible way, through the involvement of hydrogen- and dihydrogen-bonded intermediates and the essential participation of the pyridine centers. The different energy minima of this reaction profile are very accessible through low-energy transition states, all of which have been located. Copyright

Full text of DOI:10.1021/ja055116f

Published on Web 10/12/2005
Facile Ru-H2 Heterolytic Activation and Intramolecular Proton Transfer
Assisted by Basic N-Centers in the Ligands
Fe´lix A. Jalo´n,*,† Blanca R. Manzano,† Agust´ın Caballero,† M. Carmen Carrio´n,† Luc´ıa Santos,‡
Gustavo Espino,§ and Miquel Moreno
Departamento de Qu´ımica Inorga´nica, Orga´nica y Bioqu´ımica, IRICA, Departamento de Qu´ımica F´ısica,
UniVersidad de Castilla-La Mancha, AVda. C. J. Cela 10, 13071 Ciudad Real, Spain, Facultad de Ciencias,
UniVersidad de Burgos, Pza. Misael Ban˜uelos s/n, 09001 Burgos, Spain, and Departamento de Qu´ımica, Edifici Cn,
UniVersidad Auto`noma de Barcelona, 08193 Bellaterra, Spain
Received July 28, 2005; E-mail: Felix.Jalon@uclm.es
Cationic dihydride derivatives of formulas [Cp′RuH2L2]+ and
Scheme 1
[Cp′Ru(H2)L2]+ have proven to be particularly useful complexes
concerning proton-transfer reactions and the dihydride/dihydrogen
tautomerism.1 From the classical work of Chin and Heinekey,2 some
conclusions were obtained that nowadays are considered as solidly
established, namely, (i) at low temperature, the proton transfer to
transfer over [Cp*RuH(PPh3)2] that at low temperature is trans-
the derivatives [Cp′RuHL2] (L ) phosphine) gives first the
formed into the nonclassical cis-[Cp*Ru(H2)(PPh3)2]BF4.1,2 The
nonclassical dihydrogen isomer under kinetic control; (ii) upon
observed decrease of the T1(min) in 4 with respect to 3 and two
increasing the temperature, the trans-[Cp′RuH2L2] (classical hy-
other experimental observations concerning 4, namely, (i) the
dride) is formed either in equilibrium with the cis form or as the
identical T1’s measured for the hydrides and the NH resonances in
sole species; (iii) although the proposed mechanism for this
all the range of temperature studied, and (ii) the significantly
isomerization process is only speculative, its intramolecular char-
broadening of these two resonances when the temperature increases,
acter is well established. In this context, our goal was the design
are indicative of the existence, in 4, of a dihydrogen bond RuH‚‚
of monohydride and dihydride complexes of this type bearing basic
‚HN with a rapid proton-hydride exchange.5 The two hydrides and
and uncoordinated centers that could favor proton-transfer pro-
the two N(py) centers of 4 must alternate in this bond since their
cesses3 and offer new perspectives and information in this field.
respective resonances are equivalent in the NMR.
With this proposal, we have used the phosphine PPh2py (py )
Considering that in a piano stool structure, very probably, only
pyridine) that, according with our experience, favors intramolecular
one pyridine fragment can be involved in the possible RuH‚‚‚Hpy
proton-transfer reactions.4
interaction (see below), and subtracting the contribution of the N(py)
The reaction of [Cp*RuCl2]n with PPh2py and zinc gave [Cp*Ru-
atom to the excess of relaxation in 4 with respect to 3, we have
(κ2-P,N-PPh2py)(PPh2py)]Cl (1). This complex reacted with KBH4
calculated6 an H‚‚‚H distance of 1.64 Å, which is very close to
to give [Cp*RuH(PPh2py)2] (2), whose hydride appears at -12.35
that expected for a dihydrogen bond.7
1
ppm (t, JHP ) 32.8 Hz) in the H NMR spectrum.
Complexes 3 and 4 give rise, after the release of hydrogen, to
Proton-transfer reactions over 2, using HBF4 as acid, in complex:
products with a 31P NMR pattern very similar to that of 1 (1′ and
acid molar ratios of 1:1 and 1:2 were carried out in acetone-d6 at
5, respectively). The more remarkable difference between 1′ and 5
-80 °C (see Scheme 1). D/H isotopic exchange with the solvent
is the presence of a NH resonance for 5 at 12.27 ppm. Therefore,
was not observed.
we ascribe the formula [Cp*Ru(κ2-P,N-PPh2py)(PPh2py)]BF4 for
Products 3 and 4 showed singlet 31P NMR resonances. In the
1′ and [Cp*Ru(κ2-P,N-PPh2py)(PPh2pyH)](BF4)2 for 5.
1H NMR spectrum, a broad resonance for each compound, assigned
Both 2 and 3 were used as catalysts in a deuterium labeling
to the hydride groups (2H), appeared around -6 ppm. Significantly,
1
experiment of H2 monitored by H NMR using methanol-d4 as
the broad hydride resonance of 3 (but not that of 4) resolves as a
deuterium source at room temperature. Complex 3 is particularly
triplet between -80 and -30 °C with JHP ) 20.7 Hz and is
active since 50% of the initial H2 is labeled after 7 min. After 4 h,
unaffected by the temperature. The most remarkable difference
H2 has practically disappeared from the spectrum, and HD and D2
between 3 and 4 was the existence of a broad resonance at 12.90
are in an equimolecular ratio. However, the instability of 3 at room
ppm in 4 (1H) that could be assigned to a (py)NH group. For both
temperature prevents this process at longer times. Complex 2 is
complexes, a broadening of the hydride resonance and also that of
not so active (see Figure 1). However, 2 is more stable than 3, and
NH for 4 was observed as the temperature increased. At room
after 15 h of activity, when practically only D2 was present in the
temperature, the slow release of H2 was detected. The T1(min) of
solution, 2 was unaffected. Only the deuteration of the hydride
3 and 4 and that of 2 were determined using a 500 MHz spectro-
ligands of 2 and 3 was detected.
meter. The values found were 414 ms at 0 °C (2), 400 ms at -45
These results show that labile dihydrogen compounds, able to
°C (3), and 203 ms at -20 °C (4). The stated JHP coupling constant
heterolytically activate H2, are accessible from 2 and 3 in methanol-
for the hydride of 3 and this high T1(min) allows one to conclude
d4. This conclusion is especially striking since for 3 no evidence
that 3 is the classical dihydride trans-[Cp*RuH2(PPh2py)2]BF4. This
of such species had been observed in the NMR spectra and, more
result contrasts with the previously reported behavior of the proton
notably, because the trans-to-cis isomerization was considered to
† Dpto. Q.I.O.B., Universidad C-LM, IRICA.
‡ Dpto. O.F., Universidad C-LM.
§ Universidad de Burgos.
be a very unfavorable process.
To shed light on the mechanism of this process, we have carried
out a theoretical study at a DFT (B3LYP) level. First, the geometry
Universidad Auto`noma de Barcelona.
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J. AM. CHEM. SOC. 2005, 127, 15364-15365
10.1021/ja055116f CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
tions, from both P3 and P2, allowed the optimization of the other
two minima, denoted as A1 and A2 in Figure 2, that are the most
stable structures along the whole process. These minima possess a
hydrogen bond between the two pyridines. The proton transfer from
one pyridine group to the other connects A1 and A2 and takes place
through TSA, which is just 1.1 kcal/mol above A1. The existence
of A1 and A2 is one of the most important results of this DFT
study because it connects the two proton-transfer processes. In this
way, the protonation of one N(py) center could be the initial step
in the proton transfer. The low-energy cost allows the proton transfer
both to the Ru (TRANS) and to the Ru-H bond (CIS) that could
interconvert through A1-A2. The existence of 3 (TRANS) as a
sole species at -80 °C in acetone solution could be the consequence
of an unexplored additional stabilization at low temperature of this
tautomer. To the best of our knowledge, this mechanism, including
all the necessary TS’s, has never been theoretically seen.
Figure 1. H2/HD/D2 ratio versus time in the D+-H2 exchange with
2/methanol-d4 at room temperature.
The reaction scheme is completed with the minimum energy
structure P1, obtained by H2 release from CIS and formation of a
Ru-py bond. In terms of free energy, P1 will be probably very
favored due to the increase of entropy that supposes the loss of
H2. This fact would support the experimental evolution of 3 to give
1′. The full sequence represented in Figure 2 also supports our
labeling experiments. Accepting that the release of dihydrogen is
a reversible process and that the involvement of acid species of
pyridinium as A1, A2, P2, and P3 allows the incorporation of
deuterons from methanol-d4, the labeling of external H2 must be a
feasible process.
In conclusion, the substitution of PPh3 by PPh2py dramatically
changes the behavior of [Cp*RuHP2] against proton-transfer
processes. In contrast with the PPh3 derivative, 2 gives a trans
dihydride even at low temperature. Probably the kinetic control is
suppressed at this temperature due to the easy intramolecular proton
transfer favored by the presence of the N(py) basic centers. At room
temperature, the cis-(H2) tautomer must be easily formed (H2
release) and a heterolytic activation of H2 takes place. A complete
theoretical study, including all of the transition states for the proton-
transfer processes, supports all of the experimental observations.
Figure 2. Calculated reaction profile (kcal/mol) for the proton transfer
over [CpRuH2(PMe2py)2]+. Energy values are given in brackets in kcal/
mol (vacuum/acetone).
for the model compound [CpRuH(PMe2py)2] was optimized. The
starting parameters for this structure were based on the X-ray
structure of [CpRuH(PPh3)2].8 Theoretical results are depicted in
Figure 2 (see also Supporting Information), where the geometries
of the located stationary points are shown along with a scheme of
their relative energy (values in kcal/mol in brackets).
Acknowledgment. Financial support from the Spanish DGES/
MCyT (CTQ2005-01430/BQU) and from the J. Castilla-La Man-
cha-FEDER Funds (PBI-05-003) and facilities in the use of the
Computational Service of the UCLM are acknowledged. Dedicated
to Prof. Elguero on the occasion of his 70th birthday.
Supporting Information Available: Experimental section, and
computational details (22 pages, print/PDF). This material is available
free of charge via the Internet at http://pubs.acs.org.
The energy was calculated in vacuum and also by introducing
acetone as solvent correction through the PCM cavity method.9 The
energy levels in the figure and the data in the discussion refer to
the vacuum calculations, but the conclusions in acetone are similar.
These results reveal the existence of both the dihydrogen (CIS)
and the dihydride (TRANS) structures, which are very close in
energy (CIS 1.3 kcal/mol more stable than TRANS). As seen in
the left part of Figure 2, a proton transfer transforms the TRANS
structure into another minimum labeled P3 that exhibits an intra-
molecular hydrogen bond, Ru‚‚‚H-N(py). Fortunately, we were
able to find the transition state (TST) between these two minima.
The energetic accessibility of TST ensures that deprotonation of
the TRANS form to give P3 is theoretically possible. Another
proton-transfer process is presented on the right of Figure 2 that
goes from the CIS minimum to a dihydrogen-bridged structure
termed P2. The participation of unconventional hydrogen bonds,
as those found in P2 and P3, in proton-transfer processes has been
previously proposed.10 Again, a low-energy transition state TSC
was found linking CIS and P2 minima. This fact implies that not
only the proton transfer from a fragment pyH to a Ru-H bond is
feasible but also the reverse process of heterolytic breaking of the
coordinated H2 molecule in CIS, thanks to the participation of the
pyridine moiety. A combination of Ru-P and P-C(py) bond rota-
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