Kinetics of formation of dihydrogen complexes by protonation of CpRuHL complexes (L = DPPM, DPPE, 2 PPh3) with HBF4·Et2O in THF and the failure to observe dihydrogen complexes in the reactions with other acids

Article abstract of DOI:10.1021/om9907775

The second-order rate constants for the reactions of the title complexes with HBF₄ in THF do not correlate with the previously reported pK_a's of these complexes. The lack of formation of dihydrogen complexes in their reactions with H

Full text of DOI:10.1021/om9907775

Organometallics 2000, 19, 695-698
695
Notes
Kin etics of F or m a tion of Dih yd r ogen Com p lexes by
P r oton a tion of Cp Ru HL Com p lexes (L ) DP P M, DP P E, 2
P P h 3) w ith HBF 4‚Et2O in THF a n d th e F a ilu r e To
Obser ve Dih yd r ogen Com p lexes in th e Rea ction s w ith
Oth er Acid s
Manuel G. Basallote,* J oaqu´ın Dura´n, M. J esu´s Ferna´ndez-Trujillo, and
M. Angeles Ma´n˜ez
Departamento de Ciencia de los Materiales e Ingenier´ıa Metalu´rgica y Quı´mica Inorga´nica,
Facultad de Ciencias, Universidad de Ca´diz, Apartado 40, Puerto Real 11510, Ca´diz, Spain
Received October 1, 1999
Ta ble 1. Sign a ls in th e NMR Sp ectr a of th e
Summary: The second-order rate constants for the reac-
tions of the title complexes with HBF4 in THF do not
correlate with the previously reported pKa’s of these
complexes. The lack of formation of dihydrogen com-
plexes in their reactions with HCl, HBr, or CF3COOH
in THF and acetone solutions indicates the limited
validity of predictions based on aqueous pKa scales.
Sta r tin g Hyd r id e Com p lexes a n d Th eir P r od u cts
of Rea ction w ith HBF 4‚Et2O in Aceton e-d 6
Solu tion a
CpRuHL
δH δP J HP
CpRu(H2)L+
CpRu(H)2L+
L
δH δP
δH δP J HP
2 PPh3 -11.6(t) 68.0 35 -7.3(s) 49.1 -7.1(t) 59.3 24
DPPM -11.7(t) 20.1 32 -7.4(s) 5.5
DPPE -13.6(t) 91.8 35 -9.2(s) 79.3 -8.6(t) 68.4 29
In tr od u ction
a
The chemical shifts are expressed in ppm and the coupling
constants in Hz. The values reported correspond to the hydridic
or dihydrogen signals referenced to external TMS (δH), and the
singlet in the phosphorus spectra is referenced to external H3PO4
(δP).
The properties of dihydrogen complexes and their
comparison with classical metal hydrides have been the
subject of great interest in recent years.1,2 Although
there is now a reasonable understanding of the synthetic
and structural aspects of the chemistry of dihydrogen
complexes, the kinetics and mechanisms of their reac-
tions are still far from being well understood. We have
recently proposed that protonation of metal hydrides
with acids to form dihydrogen complexes occurs through
a series of dihydrogen-bonded structures with a transi-
tion state that closely resembles the final reaction
Exp er im en ta l Section
Both the preparative and the kinetic work were carried out
under an argon atmosphere using standard Schlenk and
syringe techniques. The solvents were obtained from SDS and
dried by refluxing them from appropriate drying agents; they
were also deoxygenated by bubbling Ar through them im-
mediately before use. The complexes CpRuH(PPh3)2, CpRuH-
(DPPM), and CpRuH(DPPE) were obtained using literature
procedures6-9 and characterized by 1H and 31P{1H} NMR
(DPPM ) Ph2PCH2PPh2, DPPE ) Ph2PCH2CH2PPh2). Hydro-
gen chloride and HBr were generated from methanol and
chloro- or bromotrimethylsilane, respectively. The acids
HBF4‚Et2O and CF3COOH and other reagents were obtained
from Aldrich. The NMR spectra were obtained with a Varian
Unity 400 spectrometer, and the experimental procedure for
the NMR monitoring of the reactions of the complexes with
acids at different temperatures has been previously described.3
The positions of the hydride and phosphorus signals for the
starting compounds and the dihydrogen complexes and clas-
sical dihydrides formed in the protonation reactions (Table 1)
do not differ significantly from those previously reported.6-8
The kinetic experiments were carried out at 25.0 °C using
an Applied Photophysics SX17MV stopped-flow spectropho-
product.3 For the reactions of MH2(phosphine)4 com-
+
plexes (M ) Fe, Ru) there is also a correlation between
the rate constants for protonation with several acids,
and the correlation leads to two parameters (R and S)
that can be used as a measure of the intrinsic reactivity
and selectivity of the metal hydride toward acids.4,5 To
see if the previous findings can be extended to complexes
with very different steric and electronic requirements,
we decided to study the kinetics of protonation of
CpRuHL complexes where L is a bidentate phosphine
or two monodentate phosphines, and the results are
reported in the present paper.
(1) J essop, P. G.; Morris, R. H. Coord. Chem. Rev. 1992, 121, 155.
(2) Heinekey, D. M.; Oldham, W. J ., J r. Chem. Rev. 1993, 93, 913.
(3) Basallote, M. G.; Dura´n, J .; Ferna´ndez-Trujillo, M. J .; Ma´n˜ez,
M. A.; Rodr´ıguez de la Torre, J . J . Chem. Soc., Dalton Trans. 1998,
745.
(4) Basallote, M. G.; Dura´n, J .; Ferna´ndez-Trujillo, M. J .; Ma´n˜ez,
M. A. J . Chem. Soc., Dalton Trans. 1998, 2205.
(6) Chinn, M. S.; Heinekey, D. M. J . Am. Chem. Soc. 1990, 112, 5166.
(7) J ia, G.; Morris, R. H. J . Am. Chem. Soc. 1991, 113, 875.
(8) Conroy-Lewis, F. M.; Simpson, S. J . J . Chem. Soc., Chem.
Commun. 1987, 1675.
(5) Basallote, M. G.; Dura´n, J .; Ferna´ndez-Trujillo, M. J .; Ma´n˜ez,
M. A. Inorg. Chem. 1999, 38, 5067.
(9) Davies, S. G.; Moon, S. D.; Simpson, S. J . J . Chem. Soc., Chem.
Commun. 1983, 1278.
10.1021/om9907775 CCC: $19.00 © 2000 American Chemical Society
Publication on Web 01/27/2000

696 Organometallics, Vol. 19, No. 4, 2000
Notes
tometer. All the experiments were carried out at 340-350 nm
under pseudo-first-order conditions (acid excess), and kinetic
traces could be always satisfactorily fitted by a single expo-
nential using the standard software of the instrument. The
HBF4 solutions used in the kinetic work were used within 2 h
from preparation, and the concentration of acid was deter-
mined in every case by dilution of an aliquot of the THF
solution with water followed by titration with KOH using
phenolphthalein indicator.
Resu lts
Low-temperature NMR experiments showed that the
initial reaction products of HBF4‚Et2O with CpRuH-
(PPh3)2, CpRuH(DPPM), and CpRuH(DPPE) in acetone-
d6 are the corresponding dihydrogen complexes. In the
case of the PPh3 complex protonation is followed of
tautomerization to give the corresponding classical
dihydride, whereas a mixture of the dihydrogen com-
pound and the classical dihydride are formed from the
DPPE complex. No classical dihydride was observed for
the DPPM complex. These results are in agreement with
previous observations for these and related com-
plexes,6,8,10-12 and the mechanistic aspects of the tau-
tomerization process have been discussed.6
The kinetics of reaction of the three CpRuHL com-
plexes (L ) DPPM, DPPE, 2 PPh3) with HBF4 in THF13
was studied by monitoring an absorbance decrease at
340-350 nm. The absorbance changes associated with
tautomerization are smaller and much slower than
those corresponding to protonation, and so, they do not
affect the determination of the kobs values for dihydrogen
complex formation (Table S1, Supporting Information).
The observed rate constants show a linear dependence
on the acid concentration (see Figure 1), and the values
derived for the second-order rate constant kHX are given
in Table 2.
Surprisingly, the metal hydrides do not react to an
appreciable extent with HCl or HBr after several hours
at room temperature in THF and acetone solutions.
Thus, both the NMR and the stopped-flow experiments
showed that there is no evidence for the formation of
dihydrogen complexes or classical dihydrides in these
reactions. The reactions with CF3COOH were also
monitored, and the NMR spectra (acetone-d6) showed
the disappearance of the starting complex without
evolution of any new signal, whereas the stopped-flow
traces (THF solutions) showed that the reactions occur
with irreproducible kinetics. Again, no evidence was
obtained for the formation of the dihydrogen complexes,
and since there is no formation of any precipitate in the
NMR tubes, we concluded that these reactions lead to
uncharacterized paramagnetic species and did not at-
tempt further characterization or kinetic work.
F igu r e 1. Dependence of the pseudo-first-order rate
constant on the acid concentration for the reaction of HBF4
with the complexes CpRuH(PPh3)2 (a) and CpRuH(DPPE)
(b) at 25.0 °C in THF solution. The points for CpRuH-
(DPPM) are close to those of CpRuH(PPh3)2 and have not
been included for clarity.
Ta ble 2. Secon d -Or d er Ra te Con sta n ts for th e
Rea ction of Cp Ru HL Com p lexes w ith HBF 4‚Et2O
To Give th e Cor r esp on d in g Dih yd r ogen Com p lexes
(THF solu tion , 25.0 °C, Ar a tm osp h er e)a
L
kHX/ M-1 s-1
log kHX
pKa
2 PPh3
DPPE
DPPM
169(4)
70(2)
186(8)
2.23
1.84
2.27
<5b
7.2
7.5
a
The numbers in parentheses indicate the standard deviation
b
in the last significant digit. A pKa of 8.0 has been reported for
CpRu(H)2(PPh3)2 in THF solution; as the equilibrium constant
+
for the tautomerization of the dihydrogen complex to the corre-
sponding dihydride is higher than 103 (equilibrium displaced
toward the dihydride complex), the pKa of the dihydrogen species
can be estimated to be lower than 5.
Discu ssion
The formation of a dihydrogen complex through
protonation of a metal hydride with acids can be
represented in a simplified way by eq 1, which is
formally an acid-base reaction in which a proton is
transferred from HX to LnMH. So, the pKa values of the
HX/X- and LnM(H2)+/LnMH conjugated pairs can be
used to determine the extent of protonation in every
case, and the reaction is expected to be displaced toward
formation of the dihydrogen complex when the pKa of
the dihydrogen complex is higher than that of HX.
LnMH + HX a LnM(H2)+ + X-
(1)
(10) J ia, G.; Lough, A. L.; Morris, R. H. Organometallics 1992, 11,
161.
(11) Ontko, A. C.; Houlis, J . F.; Schnabel, R. C.; Roddick, D. M.;
Fong, T. P.; Lough, A. J .; Morris, R. H. Organometallics 1998, 17, 5467.
(12) Chinn, M. S.; Heinekey, D. M. J . Am. Chem. Soc. 1987, 109,
5865.
(13) The THF solutions were prepared from commercial HBF4‚Et2O.
Although the exact nature of the acid under these conditions is not
well-known, comparison with other acids indicates that the solutions
must contain essentially (H+, BF4-) ion pairs, with both ions being
solvated.14 The solutions of HBF4 and other acids used for the kinetic
work were always freshly prepared dilute solutions in order to avoid
possible polymerization problems; actually, titration of older solutions
leads to different and erratic values of the proton concentration.
The pKa scale proposed by Morris1 is now widely
accepted to measure the acidity of dihydrogen com-
plexes, and the values reported for the CpRuHL com-
plexes are included in Table 2. As these values are
referenced to an aqueous scale, one would then expect
protonation of the hydrides with all the four acids
tested, but our experimental observations indicate that
there is no protonation of CpRuHL with acids as strong

Notes
Organometallics, Vol. 19, No. 4, 2000 697
as HCl or HBr, which suggests a much higher acidity
of the CpRuL(H2)+ species. Actually, the closely related
CpRuHL complex with L ) PR2CH2CH2PR2 (R ) C2F5)
is so acidic that it is not protonated by HBF4, and
conversion to a mixture of the corresponding dihydrogen
complex and the classical dihydride is only achieved
using triflic acid.11 However, there are also literature
reports showing formation of the dihydrogen complexes
upon protonation of CpRuHL (L ) DPPM, DPPE, 2
PPh3) complexes with acids both stronger (HBF4, CF3-
SO3H) and weaker (CpRuHL′2, HPR3+) than HCl or
HBr.7,15
The lack of formation of dihydrogen complexes in the
reactions with a large excess of HCl, HBr, or CF3COOH
is unlikely to be due to kinetic reasons because we have
observed that protonation of other metal hydrides with
these acids is usually faster than with HBF4.3-5 Thus,
the lack of formation of dihydrogen complexes in our
experiments with acids other than HBF4 must be
related to the thermodynamics of the process and clearly
indicate the limited validity of simple pKa scales to
measure the acidity in a solvent such as THF.
expressed by the Bronsted correlation.18 The operation
of a Bronsted-type correlation for proton transfer in-
volving acidic metal hydrides was demonstrated years
ago by Norton and co-workers,19-21 and we have shown
more recently a similar correlation for the formation of
dihydrogen complexes from basic metal hydrides.4,5
According to this correlation, the values of log kHX for
the reaction of a series of metal hydrides with a common
acid to form the corresponding dihydrogen complexes
are expected to show a linear dependence with the pKa
of the dihydrogen complexes.
The values in Table 2 for the DPPE and DPPM
complexes show that a modest increase in the pKa leads
to a small increase of log kHX, but the values for the
PPh3 complex do not appear to follow this trend.
Although the pKa of the dihydrogen complex cannot be
measured in this case, the quantitative conversion to
the dihydride species (pKa ) 8.0) indicates a signifi-
cantly smaller value, probably close to 5, which places
this complex clearly out of the tendency observed for
the DPPE and DPPM complexes.
In THF solutions, eq 1 is an oversimplification of the
chemical processes that actually take place in solution
because the low dielectric constant favors the formation
of ion pairs (eq 2) and homoconjugated species (eq 3)
that affect equilibrium 1.16 The values of the equilibrium
constant for ion pair formation (Kip) can be as high as
105-107 M-1 in THF solution,14 and a significant
difference between the values for A+ ) H+ and LnM-
(H2)+ can make reaction 1 go in the opposite direction.
We are unable at this time to explain satisfactorily
the lack of correlation between the pKa and log kHX
values, but there is the possibility that it is caused by
a different kinetic selectivity of the complexes toward
acids. Actually, we have shown that two parameters (R
and S) are required to express the correlation between
the kinetic and the acidity data for the reaction of a
series of complexes with several acids.4,5 The impos-
sibility of obtaining kinetic data for the reaction of the
Cp complexes with the other acids tried hinders a
detailed analysis of the problem, and comparison of
kinetic data for reactions with a single acid would be
meaningless if the complexes have a very different
selectivity.
A+ + X- a (A+,X-); Kip
(2)
(3)
HX + X- a HX2
; Khc
-
As a consequence of the large values possible for both
Kip and Khc, the effective acidity of a species in THF and
other organic solvents is very different from its acidity
in water, which becomes especially important when
comparing the acidity of two species. For example, CF3-
SO3H and picric acid have extremely different acidity
in water, but their pKa values in THF differ by less than
4 units.14,17 Thus, the pKa values of dihydrogen com-
plexes in the aqueous scale maintain their utility to
measure the relative tendency of these compounds to
deprotonate, but they do not appear to be valid for the
comparison with acids of a very different nature whose
acidity has not been directly measured in the same
solvent. Probably, it would be preferable to use a
nonaqueous scale including exclusively the dihydrogen
complexes and those species such as HPR3 that have
been used in the equilibrium measurements required
for the determination of their pKa.
The dependence of the rate constants for proton
transfer on the difference of acidity between the re-
agents was recognized long ago, and it is usually
An alternative interpretation to the lack of kHX-pKa
correlation would be that protonation of the CpRuHL
hydrides does not lead directly to the dihydrogen
complexes. If the latter compounds are formed through
a mechanism of the type shown in eqs 4 and 5, with
the rate-determining step being formation of an un-
stable and undetectable intermediate A that converts
rapidly into the dihydrogen complex, the values of log
k
HX would correlate with the acidity of the intermediate.
In that case, the absence of correlation with the pKa of
the dihydrogen complexes would simply reflect a mecha-
nistic change with respect to that proposed for the MH2-
(phosphine)4 complexes. Several possibilities of proton
attack at a metal hydride are possible and have been
discussed in the literature,8,10,22 including attacks at the
hydride ligand, at the metal center, and at an ancillary
ligand.
+
(18) Bell, R. P. The Proton in Chemistry; Cornell University Press:
Ithaca, NY, 1973; p 200.
(19) Moore, E. J .; Sullivan, J . M.; Norton, J . R. J . Am. Chem. Soc.
1986, 108, 2257.
(20) Edidin, R. T.; Sullivan, J . M.; Norton, J . R. J . Am. Chem. Soc.
1987, 109, 3945.
(21) Kristja´nsdo´ttir, S. S.; Norton, J . R. In Transition Metal Hy-
drides; Dedieu, A., Ed.; VCH: New York, 1992; p 309.
(22) Bautista, M. T.; Cappellani, E. P.; Drouin, S. D.; Morris, R. H.;
Schweitzer, C. T.; Sella, A.; Zubkowski, J . J . Am. Chem. Soc. 1991,
113, 4876.
(14) Barbosa, J .; Barro´n, D.; Bosch, E.; Rose´s, M. Anal. Chim. Acta
1992, 264, 229.
(15) J ia, G.; Morris, R. H. Inorg. Chem. 1990, 29, 581.
(16) We have also discussed previously3 the role of the homoconju-
gation and ion pair formation equilibria on the kinetics of formation
of dihydrogen complexes.
(17) Coetzee, J . F.; Deshmukh, B. K.; Liao, C. C. Chem. Rev. 1990,
90, 827.

698 Organometallics, Vol. 19, No. 4, 2000
CpRuHL + HX f A; kHX
Notes
for other compounds,23-25 there is no experimental
evidence favoring their operation in the reactions of the
CpRuHL complexes. In any case, with the information
available at this time, we think that a mechanistic
change in the protonation reaction or a different selec-
tivity of the CpRuHL complexes toward acids appears
as the most reasonable explanation for the lack of
correlation between log kHX and pKa values.
(4)
(5)
A f CpRu(H2)L+; fast
Attack at the hydride is not compatible with the
mechanism in eqs 4 and 5 because it would lead directly
to the dihydrogen complex without the need of any
intermediate. Attack at the metal would lead to a metal
dihydride as intermediate, and the NMR observations
in this work and previous literature reports clearly show
that the classical dihydrides are slowly formed from the
dihydrogen complexes, so ruling out this interpretation.
Thus, the only possibility is that the intermediate
results from proton attack at an ancillary ligand, either
the phosphine or the Cp ring. For the CpRuHL com-
plexes, phosphine protonation requires the opening of
a chelate ring, whereas protonation of the cyclopenta-
dienyl ring requires a ring slippage process. Although
both of these processes have been observed or proposed
Ack n ow led gm en t. Financial support from J unta de
Andaluc´ıa (Grupo FQM-137) and EU (FEDER 96-0044)
is acknowledged.
Su p p or tin g In for m a tion Ava ila ble: A table including
pseudo-first-order rate constants for the reactions of CpRuHL
complexes with HBF4 in THF at 25.0 °C. This material is
available free of charge via the Internet at http://pubs.acs.org.
OM9907775
(23) Henderson, R. A. J . Chem. Soc., Dalton Trans. 1988, 509.
(24) Basallote, M. G.; Dura´n, J .; Ferna´ndez-Trujillo, M. J .; Gonza´lez,
G. Ma´n˜ez, M. A.; Mart´ınez, M. Inorg. Chem. 1998, 37, 1623.
(25) Basallote, M. G.; Dura´n, J .; Ferna´ndez-Trujillo, M. J .; Gonza´lez,
G. Ma´n˜ez, M. A.; Mart´ınez, M. J . Chem. Soc., Dalton Trans. 1999, 3379.