Protonation of [tpmRu(PPh3)2H]BF4 [tpm = tris(pyrazolyl)methane]- formation of unusual hydrogen-bonded species

Article abstract of DOI:10.1002/(SICI)1099-0682(200005)2000:5<993::AID-EJIC993>3.0.CO;2-3

Protonation of [tpmRu(PPh₃)₂H](BF₄) with excess HBF₄Et₂O in CD₂Cl₂ yielded, in a straightforward manner, the dicationic η²-dihydrogen complex [tpmRu(PPh₃)₂(H₂)](BF₄)₂, which, as expected, is more acidic than its monocationic Tp [Tp = hydrotris(pyrazolyl)borate] analog [TpRU(PPh₃)₂-(H₂)]BF₄ (pK(a): 2.8 vs. 7.6). The complex [tpmRu(PPh₃)₂(H₂)](BF₄)₂ is unstable towards H₂ loss at ambient temperature. However, acidification of [tpmRu(PPh₃)₂H]BF₄ with excess aqueous HBF₄ or aqueous triflic acid in [D₈]THF gave very interesting results, Variable-temperature ¹H- and ³¹P-NMR studies revealed that the aqueous acid did not fully protonate the metal hydride to form the dihydrogen complex, but a hydrogen-bonded species was obtained. The feature of this species is that the strength of its Ru-H···H-(H₂O)(m) interaction decreases with temperature; this phenomenon is unusual because other complexes containing dihydrogen bonds show enhanced M-H···H-X interaction as the temperature is lowered. Decrease of the dihydrogen-bond strength with temperature in the present case can be attributed to the decline of acidity that results from the formation of larger H+(H₂O)(n) (n > m) clusters at lower temperatures; steric hindrance of these large clusters also contribute to the weakening of the dihydrogen bonding interactions. At higher temperatures, facile H/H exchange occurs in Ru-H···H-(H₂O)(m) via the intermediacy of a 'hydrogen-bonded dihydrogen complex' Ru- (H₂)···(H₂O)(m). To investigate the effect of the H⁺(H₂O)(m) cluster size on the strength of the dihydrogen bonding in [tpmRu(PPh₃)₂H]⁺, molecular orbital calculations at the B3LYP level have been performed on model systems, [tpmRu(PH₃)₂H]⁺ + H⁺(H₂O) and [tpmRu(PH₃)₂H]⁺ + H⁺(H₂O)₂. The results provide further support to the notion that the formation of larger H⁺(H₂O)(n) clusters weakens the Ru-H····H(H₂O)(n) dihydrogen bonding interaction.

Full text of DOI:10.1002/(SICI)1099-0682(200005)2000:5<993::AID-EJIC993>3.0.CO;2-3

FULL PAPER
Protonation of [tpmRu(PPh3)2H]BF4 [tpm ؍ Tris(pyrazolyl)methane] –
Formation of Unusual Hydrogen-Bonded Species
Hei Shing Chu,[a] Zhitao Xu,[b] Siu Man Ng,[a] Chak Po Lau,*[a] and Zhenyang Lin*[b]
Keywords: Ruthenium / Dihydrogen complexes / Dihydrogen bonding / Hydride protonation
Protonation of [tpmRu(PPh3)2H](BF4) with excess HBF4Et2O
in CD2Cl2 yielded, in a straightforward manner, the dicat-
ionic η2-dihydrogen complex [tpmRu(PPh3)2(H2)](BF4)2,
which, as expected, is more acidic than its monocationic Tp
H H–X interaction as the temperature is lowered. Decrease
of the dihydrogen-bond strength with temperature in the
present case can be attributed to the decline of acidity that
results from the formation of larger H+(H2O)n (n Ͼ m) clusters
[Tp
(H2)]BF4
[tpmRu(PPh3)2(H2)](BF4)2 is unstable towards H2 loss at am- bonding interactions. At higher temperatures, facile H/H ex-
=
hydrotris(pyrazolyl)borate] analog [TpRu(PPh3)2- at lower temperatures; steric hindrance of these large clus-
(pKa: 2.8 vs. 7.6). The complex ters also contribute to the weakening of the dihydrogen
bient
temperature.
However,
acidification
of
change occurs in Ru–H H–(H2O)m via the intermediacy of
a ‘‘hydrogen-bonded dihydrogen complex’’ Ru–(H2) (H2O)m.
To investigate the effect of the H+(H2O)m cluster size on the
strength of the dihydrogen bonding in [tpmRu(PPh3)2H]+,
molecular orbital calculations at the B3LYP level have been
performed on model systems, [tpmRu(PH3)2H]+ + H+(H2O)
and [tpmRu(PH3)2H]+ + H+(H2O)2. The results provide further
support to the notion that the formation of larger H+(H2O)n
[tpmRu(PPh3)2H]BF4 with excess aqueous HBF4 or aqueous
triflic acid in [D8]THF gave very interesting results. Variable-
temperature 1H- and 31P-NMR studies revealed that the
aqueous acid did not fully protonate the metal hydride to
form the dihydrogen complex, but a hydrogen-bonded spe-
cies was obtained. The feature of this species is that the
strength of its Ru–H H–(H2O)m interaction decreases with
temperature; this phenomenon is unusual because other clusters weakens the Ru–H H(H2O)n dihydrogen bonding
complexes containing dihydrogen bonds show enhanced M–
interaction.
Introduction
The recent discovery of intramolecular[1] and intermol-
ecular[2] M–H H–A dihydrogen bonding between a trans-
ition-metal hydride and a hydrogen-bond donor containing
an O–H or an N–H group has attracted much attention. It
has been shown that such hydride–proton interactions may
significantly contribute to the stabilization of organometal-
lic structures in mononuclear complexes[3] and cluster hy-
dride derivatives.[1h,4] They may also be very important in
the elucidation of reaction pathways occurring on the sur-
face of transition-metal clusters bearing hydride li-
gands.[1h,4] Furthermore, complexes containing dihydrogen
bonding have been proposed as intermediates for the
formation of η2-dihydrogen complexes or heterolytic cleav-
age of η2-H2 ligand.[5] This proposal has been vividly dem-
onstrated by our recent work on intramolecularly hydrogen-
bonded ruthenium complexes containing [2-(dimethylamin-
o)ethyl]cyclopentadienyl and [3-(dimethylamino)propyl]cy-
Scheme 1
clopentadienyl ligands (Scheme 1). The intramolecularly hy-
drogen-bonded complexes were unequivocally characterized
by relaxation time T1 measurements and spin saturation
transfer study; the dihydrogen complexes in Scheme 1 were
unstable toward H2 loss.[6]
Chaudret et al.[7] have recently reported the first direct
observation of an intermolecular dynamic proton transfer
equilibrium:
trans-(dppm)2HRuH HOPh
v
trans-
[a]
Department of Applied Biology Chemical Technology,
[(dppm)2HRu(H2)]ϩ(OPh)–, when excess PhOH was added
to a C6D6 or C7D8 solution of (dppm)2RuH2. The use of
10 equiv. of the more acidic alcohol, hexafluoroisopropyl
alcohol (HFP) in place of PhOH resulted in complete pro-
ton transfer to give the η2-dihydrogen complex trans-
[(dppm)2HRu(H2)]ϩ[OCH(CF3)2]– immediately. Therefore,
it seems that isolation of the hydrogen-bonded complex
The Hong Kong Polytechnic University,
Hung Hom, Kowloon, Hong Kong, China
Fax: (internat.) ϩ 852-2364-9932
E-mail: bccplau@polyu.edu.hk
Department of Chemistry,
[b]
The Hong Kong University of Science and Technology,
Clear Water Bay, Kowloon, Hong Kong, China
Fax: (internat.) ϩ 852-2358-1594
E-mail: chzlin@ust.hk
Eur. J. Inorg. Chem. 2000, 993Ϫ1000
WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2000
1434Ϫ1948/00/0505Ϫ0993 $ 17.50ϩ.50/0
993

H. S. Chu, Z. Xu, S. Man Ng, C. P. Lau, Z. Lin
FULL PAPER
LnM–H HA is very dependent on the acid strength of HA. confirmed by IR spectroscopy, which shows the ν(Ru–H) at
We report here that protonation of the monocationic ruth- 2054 cm–1 (KBr). That the net positive charge in 1 improves
enium hydride complex [tpmRu(PPh3)2H]ϩ [tpm ϭ tris(pyr- the Ru–H binding is evidenced by comparison of its
azolyl)methane] with HBF4 Et2O in CD2Cl2 readily yields ν(Ru–H) with that of TpRu(PPh3)2H [ν(Ru–H) ϭ 2008 cm–1,
the dicationic η2-dihydrogen complex [tpmRu(PPh3)2- KBr].[8] The 31P{1H}-NMR spectrum of 1 shows one res-
(H2)]2ϩ, while on the other hand, acidification of the hy- onance at δ ϭ 68.3, indicative of the chemical equivalence
dride precursor with aqueous HBF4 or aqueous CF3SO3H of the phosphorous atoms. The molecular dihydrogen com-
in [D8]THF generates
a
hydrogen-bonded species plex 2 was prepared in situ by protonation of 1 with
[tpmRu(PPh3)2H H(H2O)m]2ϩ, which displays some un- HBF4 Et2O in [D2]dichloromethane at –50 °C.
1
usual and interesting behaviors in variable temperature H-
The presence of the η2-H2 moiety in 2 was confirmed by
variable-temperature relaxation time T1 measurements and
the observation of a large 1JHD value for the corresponding
isotopomer [tpmRu(PPh3)2(HD)](BF4)2. The 1H-NMR
spectrum of 2 showed a broad signal at δ ϭ –8.04, integrat-
ing for two hydrogen atoms. A minimum T1 value of 24 ms
was recorded at 241 K and 400 MHz. Acidification of 1
with excess DBF4 gave the η2-HD isotopomer, which
showed a 1:1:1 triplet (1JHD ϭ 31.1 Hz) of a 1:2:1 triplet
(2JHP ϭ 5.7 Hz) centered at δ ϭ –8.06 in the 1H-NMR spec-
trum.
and 31P-NMR spectroscopy.
Results and Discussion
Synthesis, Characterization, and Acidity Measurement of
[tpmRu(PPh3)2(H2)](BF4)2 (2)
The dihydrogen complex [tpmRu(PPh3)2(H2)](BF4)2 (2)
was synthesized according to the sequence shown in
Scheme 2. Reaction of RuHCl(PPh3)3 with tpm and NaBF4
in THF gave [tpmRu(PPh3)2H]BF4 (1); adoption of the syn-
thetic route of the analogous Tp complex TpRu(PPh3)2H
[Tp ϭ hydrotris(pyrazolyl)borate][8] to 1 was not successful.
The Tp hydride complex was prepared by sodium borohyd-
ride reduction of the chloro precursor TpRu(PPh3)2Cl,
which in turn was synthesized by reaction of RuCl2(PPh3)3
with KTp.[9] An analogous reaction of RuCl2(PPh3)3 with
tpm in the presence of NaBF4 did not, however, yield
[tpmRu(PPh3)2Cl]BF4 in a pure form, instead it was con-
taminated with some reddish brown solids. We have learned
that the reaction of the neutral facial ligand 1,4,7-trimethyl-
1,4,7-triazacyclononane (MeCn) with RuCl2(PPh3)3 and
NaBF4 is always plagued by impurities too.[10]
The pseudo aqueous pKa of 2 was estimated by studying
the equilibrium shown in Equation 1 with 1H NMR in
CD2Cl2, using the hydride complex [MeCnRu(dppe)H]ϩ
(
MeCn ϭ 1,4,7-trimethyl-1,4,7-triazacyclononane) as the
base.
The pseudo aqueous pKa value of [MeCnRu(dppe)(H2)]2ϩ
has been determined previously in CD2Cl2 using a similar
method.[12a] The equilibrium mixture could be conveniently
obtained by protonating a mixture of [tpmRu(PPh3)2H]ϩ
and
[ a limited amount of
MeCnRu(dppe)H]ϩ with
HBF4 Et2O. The equilibrium was obtained at –60 °C be-
cause the η2-dihydrogen complex 2 is unstable towards H2
loss at higher temperatures. The relative concentrations of
the dihydrogen complexes and metal hydrides in the equilib-
rium were estimated from integration of the upfield hydride
1
signals in the H-NMR spectrum. The pKa of 2 was estim-
ated to be 3 according to Equation 2. Keq in Equation 2 is
the equilibrium constant of Equation 1.
The dihydrogen complex 2 is a new entry into the rela-
tively small family of dicationic η2-dihydrogen com-
plexes.[12] In contrast to the abundance of monocationic di-
hydrogen complexes, the number of well-characterized di-
Scheme 2
1
The H-NMR spectrum of 1 shows the hydride signal as cationic dihydrogen complexes is still limited. The double
a triplet at δ ϭ –13.98 (JHP ϭ 27.6 Hz). The coupling con- positive charge of the metal center in these complexes
stant is similar to those reported for analogous complexes lowers the effectiveness of dσ(M)Ǟσ*(H2) back bonding,
with three-legged piano-stool structures. For example, J ϭ and increases the σ(H2)Ǟdσ(M) interaction. It is therefore
34.0 Hz for CpRu(PPh3)2H,[11] and 26.4 Hz for expected that depletion of electron density in the η2-H2 li-
TpRu(PPh3)2H.[8] The presence of the hydride ligand is also gand renders the dicationic dihydrogen complexes very
994
Eur. J. Inorg. Chem. 2000, 993Ϫ1000

Protonation of [tpmRu(PPh3)2H]BF4 [tpm ϭ Tris(pyrazolyl)methane]
FULL PAPER
acidic. Dicationic dihydrogen complexes, which demonstrate η2-H2 in [TpRu(PPh3)2(H2)]BF4 may be due the fact that
high reactivity toward heterolysis of η2-H2 (pKa Ͻ –2) the tpm ruthenium fragment [tpmRu(PPh3)2]2ϩ is
a
and high room-temperature stability with respect to loss stronger σ-acceptor than the Tp counterpart. Morris has
of H2, both in solid state and in solution, have recently been pointed out that the lower H–H bond strength of the η2-
reported, including [Os(bpy)(PR3)2(CO)(H2)]2ϩ (PR3
ϭ
H2 in [Os(pys)(PPh3)2(CO)(H2)]BF4 (pys ϭ 2-pyridinethiol-
PPh3, PMePh2; bpy ϭ 2,2Ј-bipyridine),[12b][12c] [Os(phen)- ate) is a result of the strong σ(H2)Ǟdσ(M) interaction.[14]
(PPh3)2(CO)(H2)]2ϩ (phen
[Os(dppp)2(CO)(H2)]2ϩ [12d]
(H2)]2ϩ [12e]
ϭ
1,10-phenanthroline),[12c]
trans-[Os(dppe)2(CH3CN)-
,
Acidification of [tpmRu(PPh3)2H](BF4) (1) with Aqueous
HBF4 or Aqueous CF3SO3H
,
and trans-[Fe(dppe)2(L)(H2)]2ϩ (L ϭ CO,
CNH).[12f] A strong σ(H2)Ǟdσ(M) interaction is believed
to be dominant in these complexes.
While protonation of 1 with excess HBF4 Et2O in
CD2Cl2 generated 2 in a straightforward manner, acid-
ification of 1 in [D8]THF with excess aqueous HBF4 or
aqueous CF3SO3H gave quite different results. No dihydro-
gen gas evolution was detected, even when the acidification
was carried out at room temperature; and no signal seemed
to be observable in the upfield region of the 1H-NMR spec-
trum (Figure 1, left). However, a very broad signal became
detectable at –15°C; and it gradually evolved into a broad
triplet as the temperature was lowered to –45°C; the broad
triplet sharpened when the temperature was further de-
creased to –60°C . The spectral change with temperature
was found to be reversible, i.e. the triplet gradually
broadened into the baseline again with increasing temper-
ature. Concurrent off-resonance decoupled 31P-NMR spec-
tra are also shown in Figure 1 (right). It can be seen that
as the triplet hydride peak starts broadening out at –45 °C,
H–P coupling in the 31P-NMR spectrum disappears. The
phosphorus peak also sharpens as the hydride signal
Less stable (H2 loss at room temperature), but acidic di-
cationic dihydrogen complexes are exemplified by
[Os(PiPr3)2(CH3CN)3(H2)]2ϩ [12g]
,
[Ru(bpy)(PPh3)2(CO)-
(H2)]2ϩ [12c]
,
and [Ru(dppp)2(CO)(H2)]2ϩ [12d]
.
We have re-
cently reported the synthesis and acidity of some dicationic
dihydrogen complexes containing the facial triazacyclonon-
ane ligands [RCnRu(L)(LЈ)(H2)]2ϩ [HCn ϭ 1,4,7-triazacy-
clononane, MeCn ϭ 1,4,7-trimethyl-1,4,7-triazacyclonon-
ane; L,LЈ ϭ (PPh3)2, dppe, and CO, PPh3]; the acidity of
the complexes [HCnRu(PPh3)2(H2)]2ϩ (pKa ϭ 4.5) and
[
MeCnRu(dppe)(H2)]2ϩ (pKa ϭ 3.8), although low in com-
parison to those of the aforementioned dicationic dihydro-
gen complexes, are nevertheless significantly higher than
those of their monocationic hydrotris(pyrazolyl)borato (Tp)
and Cp analogoues (by 3–4 pKa units). The carbonyl-con-
taining complexes [HCnRu(PPh3)(CO)(H2)]2ϩ (pKa ϭ –1.3)
and [MeCnRu(PPh3)(CO)(H2)]2ϩ (pKa ϭ –2.6) are also
more acidic than their monocationic Tp counterpart
[TpRu(PPh3)(CO)(H2)]ϩ (pKa ϭ –0.6), but to a smaller ex-
tent. The non-carbonyl-containing RCn dihydrogen com-
plexes are stable in the solid state at room temperature, but
in solution the η2-H2 ligand is displaced by strongly coordi-
nating solvent such as acetonitrile.[12a] It should be men-
tioned that dicationic dihydrogen complexes are not neces-
sarily acidic, for example, [Os(NH3)4(L)(H2)]2ϩ [12h,12i] and
1
broadens into the baseline. Like the VT H-NMR study,
the spectral changes with temperature in the 31P-NMR
measurements are also reversible. In fact, if the protonation
reaction was carried out at –60°C, and the temperature
gradually raised to 20°C, the sets of 1H- and 31P-NMR
spectra collected at various temperatures were identical to
those depicted in Figure 1. It is interesting to note that
throughout the whole variable-temperature NMR study,
signals pertaining to η2-H2 ligand and free H2 were not ob-
served.
[Os(en)2(L)(H2)]2ϩ [12j]
reported by Taube and co-workers,
,
which tightly bind H2, are not acidic, probably due to the
strong electron-donating effect of the NH3 and en ligands.
The fact that complex 2 is more acidic than the analog-
ous dicationic RCn dihydrogen complexes [HCnRu-
(PPh3)2(H2)]2ϩ [12a] and [MeCnRu(PPh3)2(H2)]2ϩ [12a] seems
to indicate that the tpm ligand is less electron donating than
RCn. As expected, 2 is significantly more acidic than the
analogous monocationic Tp and Cp complexes. On the
R
other hand, the stability of 2 is lower than that of the Cn,
Tp, and Cp counterparts. The η2-H2 of 2 is labile at –15 °C.
In fact, addition of HBF4 Et2O to 1 in CD2Cl2 at room
temperature resulted in immediate evolution of H2, which
1
gave a signal at δ ϭ 4.62 in the H-NMR spectrum. The
major product of the reaction was the aquo complex
[tpmRu(PPh3)2(H2O)](BF4)2 (3),[13] which was formed by
displacement of η2-H2 by residual H2O in the solvent, only
a trace amount of 2 was detected (vide infra). It is note-
1
Figure 1. Variable-temperature (left) H-NMR spectra (upfield re-
gion) and (right) 31P-NMR spectra of [tpmRu(PPh3)2H]ϩ ϩ aq
HBF4 in [D8]THF
worthy that the T1(min) of 2 is larger than that of the mono-
The fact that acidification of 1 with aqueous HBF4 or
1
cationic analog [TpRu(PPh3)2(H2)]BF4,[8] and its JHD aqueous CF3SO3H in [D8]THF gave identical result, as re-
value is smaller than that of the latter. The weaker H–H vealed by VT 1H- and 31P-NMR studies, seemed to indicate
interaction of the η2-H2 ligand in 2 relative to that of the that the two acids were leveled to the same species in the
Eur. J. Inorg. Chem. 2000, 993Ϫ1000
995

H. S. Chu, Z. Xu, S. Man Ng, C. P. Lau, Z. Lin
FULL PAPER
two systems. We assume that the acids were both leveled to and Hϩ(H2O)m (5 v 5Ј) via the intermediacies of a pair
Hϩ(H2O)m, and have to admit that the precise state of sol- of transient η2-dihydrogen complexes (6 and 6Ј) leads to
vation of Hϩ was unknown.[15] It seems, however, that the broadening-out of the hydride signal. The feature of this
pKa value (–1.74)[16] commonly assigned to H3Oϩ does not pair (6 and 6Ј) is that the η2-H2 ligand, which is hydrogen-
reflect the actual acidity of the aqueous HBF4 or aqueous bonded to the (H2O)m, does not dissociate readily from the
CF3SO3H in our case. Failure to generate the dihydrogen metal center, and that might be the reason for not ob-
1
complex 2 with aqueous HBF4 or aqueous CF3SO3H im- serving free H2 in the course of the VT H-NMR study.
plies that acidity of either acid might only be close to or Analogously, Crabtree et al. have very recently reported
even lower than that of [tpmRu(PPh3)2(H2)]2ϩ, and there- that hydrogen-bonding provided by a pendant amine group
fore is not able to effect complete proton transfer to 1. In is the key factor that allows stabilization of an Ir–(HF)
our recent work, we have attributed deprotonation of the complex.[17] Species similar to 5 (or 5Ј) and 6 (or 6Ј) have
η2-H2 ligands in [TpRu(L2)(H2)]ϩ [L2
ϭ
(PPh3)2, been proposed as intermediates in the protonation of cis-
dppe][8][12a] and [RCnRu(L2)(H2)]2ϩ [RCn ϭ 1,4,7-triazacy- [FeH2{PCH2CH2PPh2)3}] to form the η2-dihydrogen com-
clononane, 1,4,7-trimethyl-1,4,7-triazacyclononane; L2
plex.[18]
(PPh3)2, dppe][12a] by H2O in THF/H2O or organic/aqueous
At low temperature (–60° C), acidity decreases due to
ϭ
mixed solvents to strong solvation of Hϩ by H2O; all these higher degree of solvation of Hϩ, i.e. formation of larger
complexes have pKa values higher than 3.
cluster Hϩ(H2O)n; Ru–H H(H2O)n, dihydrogen bonding in
A weakens and therefore results in sharpening of the hy-
dride signal.
Weakening of the dihydrogen bonding in A at low tem-
perature can also be rationalized in terms of steric effect of
the large cluster Hϩ(H2O)n.
Another possible scenario for the protonation of 1 with
aqueous acid is the occurrence of a rapid proton transfer
between 1 and H2O (Equation 3); lowering the temperature
would slow down this proton exchange and the low temper-
ature spectrum closely resembles that of complex 1. But the
equilibrium depicted in Equation 3 does not seem to be in
congruence with the VT NMR spectra shown Figure 1.
If such a rapid proton transfer occurs, it is most likely
that a weighted average of hydride signals of 1 and 2 is
observed, and the phosphorus signal is also an average of
those of 1 and 2. However, careful examination of the VT
1H- and 31P-NMR spectra does not reveal the existence of
these average signals. It is also noteworthy that no H2 loss
was observed when the system was allowed to stand at
room temperature. Had the equilibrium depicted in Equa-
Scheme 3
Spectral changes of the hydride signal in the VT 1H- tion 3 existed, the equilibrating 2 should have evolved H2
NMR study of the acidification reaction of 1 with aqueous while standing at room temperature. We have learned that
acid are contrary to the behaviors of the hydride signals 2 is unstable with respect to H2 loss at –15 °C.
of complexes containing dihydrogen bonding. The hydride
Dihydrogen bonding between the protonated water clus-
signals of these complexes usually broaden as the temper- ter H(H2O)ϩm and the hydride in 5 (or 5Ј) was characterized
ature goes down, due to strengthening of the M–H H–X by IR spectroscopy in THF at room temperature. A THF
interaction, and they sharpen as the temperature is in- solution of 2 showed the ν(RuH) band at 2040 cm–1, upon
creased because of weakening of the dihydrogen bonding. addition of 10 equiv. of aqueous HBF4, this band was
The unusual spectral behavior displayed by the hydride li- shifted to a lower wavenumber at 1967 cm–1. Berke, Epstein,
gand of 1 in the acidification process can be reconciled by and co-workers have characterized intermolecular hydrogen
the sequence depicted in Scheme 3. Acidification of 1 with bonding of acidic alcohols to the hydride ligand of
aqueous acid generates the hydrogen-bonded species 5; at WH(CO)2(NO)L2 by IR spectroscopy in hexane at 200 K.
higher temperature, rapid H/H exchange between Ru–H The IR spectrum of the tungsten hydride complex showed
996
Eur. J. Inorg. Chem. 2000, 993Ϫ1000

Protonation of [tpmRu(PPh3)2H]BF4 [tpm ϭ Tris(pyrazolyl)methane]
FULL PAPER
1
a low-wavenumber shoulder in the ν(WH) band, in the pres- itored this transformation by using H-NMR spectroscopy.
ence of 3 equiv. of hexafluoro-2-propanol (HFIP).[2e] Un- Complex 2 was first formed in a 5-mm NMR tube by acid-
fortunately, it is not possible to characterize the dihydrogen ification of a [D8]THF solution of 1 (0.4 mL) with 1 equiv.
bonding between 2 and protonated water cluster by meas- of HBF4 Et2O at –30 °C, subsequent addition of 50 µL of
urement of T1(min) because the strength of the Ru– H2O to the solution led to the disappearance of the η2-H2
H H(H2O)m or n dihydrogen bond in the system varies with signal of 2 and concomitant appearance of the very broad
temperature. Furthermore, it is not possible to measure T1 peak of the hydrogen-bonded hydride of 5 (Scheme 4).
at –30 °C or above since the hydride signal broadens into
the baseline at these temperatures. Acidification of 1 with
HBF4 Et2O in CD3OD/[D8]THF gave practically identical
results as the protonation of 1 with aqueous HBF4 in
[D8]THF. This is not surprising since it is expected that
methanol behaves very similarly to H2O in this process.
Quantum chemical calculations described in the following
section also seem to support our proposal of Scheme 3.
At this point, we would like to comment on the forma-
tion of aquo complex [tpmRu(PPh3)2(H2O)](BF4)2 (3) in
the room-temperature protonation reaction of 1 with
HBF4 Et2O in CD2Cl2. The predominant form of acid in
this system is Et2OHϩ, which is strong enough to fully pro-
tonate 1 to form 2. The trace amount of water present in
Scheme 4
Quantum Chemical Study
The broadening/sharpening changes of the hydride signal
the system does not form the hydrogen-bonded species 5 (or for the dihydrogen-bonded species 5 observed in the vari-
5Ј) with 2 because it is protonated by Et2OHϩ (Equation 4), able-temperature NMR experiments show unusual behavior
which is a stronger acid than 2. At –15°C or above, the when compared to other dihydrogen bonded systems.[2e,7]
η2-H2 ligand in 2 becomes labile, and the coordinatively In order to provide an insight into the understanding of the
unsaturated species generated upon dissociation of H2 from unusual behavior, quantum mechanical calculations at the
2 abstracts H2O from the equilibrium (Equation 4) to form density-functional B3LYP level of theory have been under-
the thermodynamically stable aquo complex 3.
taken to investigate the structural details for the following
model systems: [tpmRu(PH3)2H]ϩ, [tpmRu(PH3)2(H2)]2ϩ
[tpmRu(PH3)2H]ϩ ϩ Hϩ(H2O), and [tpmRu(PH3)2H]ϩ
,
ϩ
Hϩ(H2O)2. The [tpmRu(PH3)2(H2)]2ϩ system represents the
complete proton transfer product in organic solvent. The
latter two model systems represent the corresponding situ-
ation in aqueous/THF solution, and the relevant calcula-
tions allow us to study the effect of water aggregation on
the dihydrogen bonding. They can also provide information
on the structural details of protonated products under dif-
ferent acidity because one can consider that HBF4 Et2O is
more acidic than Hϩ(H2O), which in turn, is more acidic
than Hϩ(H2O)2. It should be noted that the model systems
cannot be directly related to the experimental systems be-
cause the actual degree of water aggregation in each experi-
mental system cannot be well-defined. Therefore, the calcu-
lated H H distances for different numbers of water molec-
ules in the model systems are only useful in considering the
trend, which qualitatively accounts for the effect of water
aggregation on the dihydrogen bonding.
The H/H exchange proposed in Scheme 3 was further
supported by H/D exchange, which occurred in the acid-
ification of a [D8]THF solution of 1 with CF3SO3D/D2O.
It was found that addition of CF3SO3D/D2O (v/v, 1:1) to a
[D8]THF solution of 1 at room temperature led to immedi-
ate broadening of the hydride signal into the baseline. The
upfield triplet signal, which re-appeared as the temperature
was lowered to –60 °C, diminished considerably due to H/
D exchange.
Structure 5 (or 5Ј) can be viewed as an intermediate on
the reaction path that leads to transient 6 (or 6Ј), and then
to the product of complete proton transfer from an acid to
the metal hydride precursor. Gusev and Reinhart have very
recently proposed that reactions of anionic ruthenium and
rhenium hydrides with phenylacetylene first generate the
hydrogen-bonded species [LnM–H H–CϵCPh]–, which
equilibrates with the transient [LnM(µ-H2)(CCPh)]– (Equa-
tion 5). The latter, which is analogous to 6 (or 6Ј), then
undergoes hydride transfer to the Cα atom of phenylacetyl-
ene to form the [LnM–C(Ph)ϭCH2]– vinyl intermediate.[19]
The optimized structures for the four model systems are
shown in Figure 2. Consistent with the experimental finding
that the direct protonation of [tmpRu(PPh3)2H]ϩ with
HBF4 Et2O in an organic solvent gives a non-classical dihy-
drogen complex, [tpmRu(PH3)2(H2)]2ϩ. The H–H distance
˚
in this dihydrogen complex is calculated to be 0.81 A with
a symmetrical coordination of the η2-H2 unit (Ru–H: 1.82
[20]
˚
A).
The calculations for the [tpmRu(PH3)2H]ϩ ϩ Hϩ(H2O)
Generation of the Hydrogen-Bonded Complex from 2
We have been able to generate the hydrogen-bonded com- system give a similar result. The system can be described as
plex 5 from the η2-dihydrogen complex 2, and have mon- [tpmRu(PH3)2(H2) OH2]2ϩ, again a non-classical dihydro-
Eur. J. Inorg. Chem. 2000, 993Ϫ1000
997

H. S. Chu, Z. Xu, S. Man Ng, C. P. Lau, Z. Lin
FULL PAPER
These calculations indicate that the mono-water system
prefers a dihydrogen non-classical structure, while the di-
water system adopts a structure close to a classical hydride.
Though weakly bound, the presence of an additional water
molecule surprisingly switches the non-classical structure to
a classical one with M–H H–X dihydrogen bonding. This
result suggests that a fast exchange can occur between clas-
sical and non-classical structures, as proposed in Scheme 3,
when temperature increases. The association and dissoci-
ation of the weakly bound water molecule can be strongly
influenced by temperature.
The current work deals with the effect of water aggrega-
tion on the H H dihydrogen bonding. The stability of these
dicationic hydrogen-bonded systems is another important
topic that requires further attention. Theoretically, the
evaluation of the H H bonding strengths in the studied
systems involves the charge-separation processes. Therefore,
much higher levels of theory are necessary in order to pro-
duce some meaningful results. Clearly, the stability of these
systems should be further studied both theoretically and ex-
perimentally.
Conclusion
We have shown in this study, the species that are pro-
posed as intermediates along the reaction pathway of pro-
tonation of metal hydrides to form η2-dihydrogen com-
plexes. One of these species is a hydrogen-bonded metal hy-
dride, [tpmRu(PPh3)2H H(H2O)n]2ϩ (A), which is formed
by acidification of [tpmRu(PPh3)2H]ϩ with aqueous HBF4
Figure 2.
Optimized
structures
for
model
systems:
[tpmRu(PH3)2H]ϩ, [tpmRu(PH3)2(η2-H2)]2ϩ, [tpmRu(PH3)2H]ϩ
Hϩ(H2O), and [tpmRu(PH3)2H]ϩ ϩ Hϩ(H2O)2
ϩ
gen complex, a weak hydrogen bond exists between one of in [D8]THF at –60 °C. The comparatively weak Ru–
the two hydrogen atoms and the oxygen atom of H2O H H(H2O)n dihydrogen interaction is attributable to lower
2
(H O: 1.67 A). The H–H distance (0.84 A) in the η -H2 acidity and larger steric hindrance of the bulky Hϩ(H2O)n
unit lengthens only slightly, although the coordination of clusters, which are the major forms of acid at low temper-
the η2-H2 ligand becomes unsymmetrical. The Ru–H dis- ature. At higher temperatures, the degree of aquation of Hϩ
tance on the side of the O H hydrogen bonding is decreases and the smaller clusters Hϩ(H2O)m (m Ͻ n) ex-
˚
˚
˚
˚
lengthened (1.97 A) while the other is shortened (1.78 A). hibit relatively higher acidity, therefore the species 5 (or 5Ј)
The results of these calculations for the mono-water system with stronger dihydrogen bonding become the major species
suggest that Hϩ(H2O) is still very acidic, resulting in the in solution. Finally, a full-fledged η2-dihydrogen complex
formation of non-classical structure upon protonation.
[tpmRu(PPh3)2(H2)]2ϩ is generated when tpmRu(PPh3)2H
Surprisingly, further association with one more water is protonated with HBF4 Et2O in CD2Cl2. The results of
molecule, which gives the [tpmRu(PH3)2H]ϩ ϩ Hϩ(H2O)2 our quantum chemical calculations at B3LYP level on
system, leads to a species that is close to a classical hydride. model systems, [tpmRu(PH3)2H]ϩ
ϩ
Hϩ(H2O) and
The metal-hydride distance, calculated to be 1.67 A, is [tpmRu(PH3)2H]ϩ ϩ Hϩ(H2O)2, indeed suggest that the
˚
within the range of classical M–H distance[20g,21] {1.61 A in Ru–H H(H2O)m dihydrogen bonding interaction greatly
˚
[tpmRu(PH3)2H]ϩ}. In this system, the hydride ligand was depends on the size of the water cluster. For the mono-
also found to have a significant interaction with the proton water model system, a non-classical dihydrogen complex is
from the Hϩ(H2O)2 unit. The H H distance was calculated preferred. The di-water system, however, gives a Ru–
˚
to be 1.18 A. The typical H H distance in other M–H H– H H(H2O)2 dihydrogen bonded species.
[5]
˚
X systems is around 1.8 A. Apparently, in the current
The acidification of metal hydrides normally leads to
system, the hydride acceptor [Hϩ in Hϩ(H2O)2] is very pro- either non-classical η2-H2 dihydrogen complexes or forma-
tonic when compared to other systems studied previ- tion of species with the M–H H–X interaction (dihydrogen
ously.[2e,7] In the structure of [tpmRu(PH3)2H]ϩ
bonding). Strongly acidic conditions give non-classical
Hϩ(H2O)2, one can clearly see the aggregate of Hϩ(H2O)2, complexes, while weakly acidic conditions result in dihydro-
which can be formulated as Hϩ(H2O) OH2. The additional gen bonding. Here, we demonstrate that the strength of hy-
H2O is only weakly bounded through normal hydrogen dride-proton interaction can be tuned through the associ-
bonding.
ation of water molecules to Hϩ.
998
Eur. J. Inorg. Chem. 2000, 993Ϫ1000

Protonation of [tpmRu(PPh3)2H]BF4 [tpm ϭ Tris(pyrazolyl)methane]
FULL PAPER
pz3CH). – 31P{1H} NMR (CD2CL2, 161.70 MHz, –50 °C): δ ϭ
48.2 (s). Variable- temperature T1 measurements on the H2 signal
were carried out by the inversion-recovery method using standard
180°–τ–90° pulse sequence. – T1 (400 MHz, ms): 41.3 (203 K), 32.9
(213 K), 28.2 (223 K), 25.2 (233 K), 23.9 (243 K), 28.7 (253 K).
T1(min) (400 MHz, 241 K): 24.0 ms.
Experimental Section
All reactions were performed under dry nitrogen using standard
Schlenk techniques. Solvents were distilled under nitrogen from ap-
propriate drying agents[22] (solvent/drying agent): methanol/Mg/I2,
ethanol/Mg/I2, acetonitrile/CaH2, dichloromethane/CaH2, tetrahy-
drofuran/Na/benzophenone, diethyl ether/Na, n-hexane/Na. Ruth-
enium trichloride, RuCl3 3H2O, pyrazole, sodium tetrafluorobor-
ate, and [D1]trifluoromethanesulfonic acid were purchased from
Aldrich. Tetrafluoroboric acid in ethereal solution and aqueous so-
lution were obtained from Fluka. Triphenylphosphane was ob-
In Situ Preparation of [tpmRu(PPh3)2(HD)](BF4)2 ([D1]-2): A
sample of 1 (18 mg, 20 µmol) was dissolved in 0.4 mL of [D2]dich-
loromethane. After cooling the solution to –50 °C, 2.1 µL of DBF4
solution was added. – 1H NMR (CD2Cl2, 400 MHz, –50 °C): δ ϭ –
8.06 (tt, JHD ϭ 31.1 Hz, JHP ϭ 5.8 Hz).
tained from Merck and was recrystallized from ethanol before use.
and [
MeCnRu(dppe)H]BF4
[23]
The complexes RuHCl(PPh3)3
[tpmRu(PPh3)2(H2O)](BF4)2 (3): A sample of 1 (300 mg, 0.3 mmol)
was dissolved in 20 mL of CH2Cl2, excess HBF4 Et2O (100 µL, 54%
solution in diethyl ether) was added, followed by addition of 5 µL
of H2O. The solution was stirred at room temperature for 30 min,
and was then concentrated to dryness under vacuum to yield a pale
yellow solid, which was washed with 20 mL of diethyl ether. Yield:
270 mg (88%). – C46H42B2F8N6OP2Ru: calcd. C 53.55, H 4.10, N
8.15; found C 53.36, H 3.93, N 8.07. – 1H NMR (CD2Cl2,
400 MHz, 20 °C): δ ϭ 9.91 (s, 1 H, pz3CH), 8.79 (d, 1 H, pz,
(
MeCn ϭ 1,4,7-trimethyl-1,4,7-triazacyclononane)[12a] were synthe-
sized according to literature methods. – Infrared spectra were ob-
tained from a Nicolet Magna 750 Ft-IR spectrophotometer. 1H-
NMR spectra were recorded with a Bruker DPX-400 spectrometer;
chemical shifts were reported relative to residual protons of the
deuterated solvents. 31P-NMR spectrum were measured at
161.7 MHz with a Bruker DPX-400 spectrometer; chemical shifts
of these spectra were externally referenced to 85% H3PO4 in D2O
(δ ϭ 0). Relaxation-time T1 measurements were carried out at
400 MHz by the inversion-recovery method using the standard
180°–τ–90° pulse sequences. Elemental analyses were performed by
M–H–W laboratories, Phoenix, AZ.
3
3JHH ϭ 2.90 Hz), 8.55 (d, 2 H, pz, JHH ϭ 2.78 Hz), 7.60–7.34 (m,
3
30H of PPh3), 6.90 (d, 1 H, pz, JHH ϭ 1.98 Hz), 6.43 (t, 2 H, pz,
3
3JHH ϭ 2.48 Hz), 5.86 (t, 1 H, pz, JHH ϭ 2.68 Hz), 5.52 (d, 1 H,
pz, 3JHH ϭ 2.31 Hz), 3.71 (s, 2 H, H2O). – 31P{1H} NMR (CD2Cl2,
162 MHz, 20 °C): δ ϭ 44.1 (s).
Trispyrazolylmethane (tpm): The ligand tpm was prepared by fol-
lowing a literature method,[24] with some modifications. A solution
of pyrazole (10.00 g, 0.15 mmol) in 30 mL of THF was added to a
vigorously stirred suspension of potassium (1.9 g, 50 mmol) in
200 mL of THF. After the potassium had completely reacted, chlo-
roform (6.0 mL, 6.5 mmol) was slowly added over a period of 2 h,
and the colloidal mixture was stirred under reflux for 36 h. The
mixture was filtered at room temperature to remove the KCl, and
the solvent of the filtrate was removed by using a rotary evaporator
to yield a dark brown sludge. Pure trispyrazolylmethane was ob-
tained by sublimation of the dark brown sludge. Yield: 3.1 g (9.5%
based on pyrazole used).
Acidity Measurement: The pKa value of [tpmRu(PPh3)2(H2)](BF4)2
was estimated by studying the equilibrium shown in Equation 1. In
the experiment, appropriate amounts of [tpmRu(PPh3)2H]BF4 and
[
MeCnRu(dppe)H]BF4 were dissolved in CD2Cl2 in an NMR tube,
and a limited amount of HBF4 Et2O was then added. The 1H-
NMR spectrum was immediately taken at –60 °C. Relative molar
concentrations of the equilibrated species were derived from the
1H-NMR integrations of the hydride signals.
Protonation of [tpmRu(PPh3)2H]BF4 (1) with Aqueous HBF4: A
sample of [tpmRu(PPh3)2H]BF4 (1) (10 mg, 0.011 mmol) was dis-
solved in 0.4 mL of [D8]THF in a 5-mm NMR tube, and 10 µL of
aqueous HBF4 (8 ) was added. The VT 1H-NMR spectra were
immediately taken.
[tpmRu(PPh3)2H]BF4 (1): Samples of RuHCl(PPh3)3 (0.60 g,
0.60 mmol), tpm (0.15 g, 0.70 mmol), and NaBF4 (0.072 g
0.66 mmol) were dissolved is 20 mL of THF and the solution was
stirred at room temperature for 1 h. The solution was then concen-
trated to dryness under vacuum, and the residue was washed with
methanol. Pure product was obtained by recrystallization of the
Computational Details: The quantum mechanical calculations at
the density-functional B3LYP level of theory used the effective core
potentials of Wadt and Hay with double-ζ valence functions[25] for
the Ru and P atoms. The standard 3–21G basis sets were applied
to the first-row elements H, C, and N, with those hydrogens directly
involved in hydrogen bonding (and dihydrogen bonding) aug-
mented by single-ζ polarization functions (i.e. 3–21 g**).[26] The use
of the 3–21G basis set in the calculations allows us to keep the
computational expense reasonable due to the large size of the tmp
ligand. All structures were fully optimized with the Gaussian98
package.[27]
residue with dichloromethane/hexane. Yield: 0.48 g (87%).
–
C46H41BF4N6P2Ru: calcd. C 59.55, H 4.45, N 9.06; found C 59.48,
H 4.48, N 8.94. – IR (KBr, cm–1): ν(Ru–H) ϭ 2054 (m). – 1H NMR
3
([D6]acetone, 400 MHz, 25 °C): δ ϭ –13.82 (t, 1 H, JHP
ϭ
27.8 Hz), 5.99 (t, 2 H, pz, 3JHH ϭ 2.36 Hz), 6.31 (t, 1 H, pz, 3JHH ϭ
3
2.35 Hz), 6.83 (d, 1 H, pz, JHH ϭ 1.70 Hz), 6.88 (d, 2 H, pz,
3JHH ϭ 1.82 Hz), 7.13–7.37 (m, 30 H of PPh3), 8.21 (d, 2 H, pz,
3JHH ϭ 2.59 Hz), 8.48 (d, 1 H, pz, JHH ϭ 2.57 Hz), 9.33 (s, 1
3
H, pz3CH). – 31P{1H} NMR (CD2Cl2, 161.70 MHz, 25 °C): δ ϭ
70.6 (s).
Acknowledgments
In Situ Preparation of [tpmRu(PPh3)2(H2)](BF4)2 (2): A sample of
1 (10 mg, 11 µmol) was dissolved in 0.4 mL of [D2]dichlorome-
thane, the solution was cooled to –50 °C inside the NMR probe,
1.3 µL of HBF4 (13 µmol, 54% solution in diethyl ether) was then
added. A period of 15 min was allowed for completion of the reac-
tion before NMR spectra were recorded. – 1H NMR (CD2Cl2,
400 MHz, –50 °C): δ ϭ –8.04 [br. s, 2 H, Ru–(H2)], 5.34 (s, 1 H,
pz), 5.76 (s, 1 H, pz), 5.93 (s, 2 H, pz), 6.32 (s, 2 H, pz), 6.98–7.50
(m, 30 H of PPh3), 7.98 (s, 2 H, pz), 8.41 (s, 1 H, pz), 9.33 (s, 1 H,
This work was supported by the Hong Kong Research Grants
Council (Project No. PolyU5178/99P), the Hong Kong Polytechnic
University, and the Hong Kong University of Science and Techno-
logy.
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