Dynamics of H-atom exchange in stable cis-dihydrogen/hydride complexes of ruthenium(ii) bearing phosphine and N-N bidentate ligands

Article abstract of DOI:10.1039/c3dt52575a

Synthesis and characterization of cis,trans-[RuH(η²-H ₂)(PPh₃)₂(N-N)][OTf] (N-N = 2,2′-bipyridyl (bpy) 1a, 2,2′-bipyrimidine (bpm) 2a; OTf = trifluoromethane sulfonate (CF₃SO₃)) complexes are reported. The cis-H₂/hydride ligands are involved in H-atom site exchange between the two moieties. This dynamics was investigated by variable temperature NMR spectral studies based on which the mechanism of the exchange process was deduced. The ΔG^≠ for the exchange of H-atoms between the η²-H₂ and hydride ligands was determined to be around 8 and 13 kJ mol^-1, respectively, for 1a and 2a. The H-H distances (d_HH, A) in complexes 1a and 2a have been calculated from the T₁(minimum) and ¹J(H,D) and are found to be 1.07 A (slow) and 0.95 A for 1a and 1.04 A (slow) and 0.94 A for 2a, respectively. The molecular structure of 1a was determined by X-ray crystallography.

Full text of DOI:10.1039/c3dt52575a

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Dynamics of H-atom exchange in stable cis-
dihydrogen/hydride complexes of ruthenium(II)
bearing phosphine and N–N bidentate ligands†
Cite this: Dalton Trans., 2014, 43,
4726
Barun Bera, Yogesh P. Patil, Munirathinam Nethaji and Balaji R. Jagirdar*
Synthesis and characterization of cis,trans-[RuH(η2-H2)(PPh3)2(N–N)][OTf] (N–N = 2,2’-bipyridyl (bpy) 1a,
2,2’-bipyrimidine (bpm) 2a; OTf = trifluoromethane sulfonate (CF3SO3)) complexes are reported. The cis-
H2/hydride ligands are involved in H-atom site exchange between the two moieties. This dynamics was
investigated by variable temperature NMR spectral studies based on which the mechanism of the
exchange process was deduced. The ΔG≠ for the exchange of H-atoms between the η2-H2 and hydride
ligands was determined to be around 8 and 13 kJ mol−1, respectively, for 1a and 2a. The H–H distances
(dHH, Å) in complexes 1a and 2a have been calculated from the T1(minimum) and 1J(H,D) and are found to
be 1.07 Å (slow) and 0.95 Å for 1a and 1.04 Å (slow) and 0.94 Å for 2a, respectively. The molecular struc-
ture of 1a was determined by X-ray crystallography.
Received 18th September 2013,
Accepted 1st January 2014
DOI: 10.1039/c3dt52575a
www.rsc.org/dalton
exchange is facilitated by a cis-interaction which is like a weak
hydrogen bonding interaction between dihydrogen and
Introduction
The binding and activation of molecular hydrogen on a metal hydride ligands observable in the solid state.2 Although inelas-
center continue to attract the attention of researchers from the tic neutron scattering could provide important evidence for
standpoint of the very interesting structural and dynamical fea- this interaction, the requirement of large crystals limits its
tures and catalysis, apart from the interesting reactivity pat- utility to a certain extent. The solution NMR spectral method
terns that they exhibit.1 Complexes possessing dihydrogen and is rather a very useful tool to study such interactions and pro-
hydride ligands in cis conformation exhibit extremely rapid cesses. However, in certain cases, the dynamics of the
dynamics of exchange of hydrogen atoms between the η2-H2 exchange is extremely rapid and cannot be frozen in the NMR
and the hydride ligands in solution (Chart 1). This dynamic time scale even at very low temperatures making it difficult to
detect the η2-H2 ligands in, for example, polyhydride com-
plexes. The dynamic process in cis-dihydrogen/hydride com-
plexes could be considered as a type of σ-bond metathesis
analogous to olefin metathesis (Chart 1).
Previously, cis-dihydrogen/hydride complexes of Mo(II),
Ru(II), Os(II), Rh(III), and Ir(III) have been reported.3 A classical
dihydride complex with the two hydride ligands in cis con-
formation related by a C2 axis could be protonated to give the
corresponding cis-dihydrogen/hydride complex which would
render the two ligands in a dynamic exchange process requir-
ing very limited movement of the ancillary ligands around the
metal center. Examples of such systems include cis,trans-[RuH-
(η2-H2)(PCy3)2(bpy)][OTf] reported by Heinekey and his co-
workers3d
and
cis,trans-[IrH(η2-H2)(PCy3)2(η2-S2CH)][BF4]
reported by us.3b Heinekey’s ruthenium complex was charac-
terized by spin–lattice relaxation time (T1) measurements and
partial deuteration studies. However, structural characteriz-
ation of this complex was not established. Properties of these
complexes are slightly different from the true σ-dihydrogen
complexes. The adjacent hydride ligand spaced close (∼2 Å in
the solid state structure)3e to the η2-H2 ligand has a large
Chart 1 Cis-interaction (top) and σ-bond metathesis (bottom).
Department of Inorganic & Physical Chemistry, Indian Institute of Science, Bangalore
560012, India. E-mail: jagirdar@ipc.iisc.ernet.in
†Electronic supplementary information (ESI) available: NMR spectral data of the
complexes and details regarding calculations. CCDC 951511. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/c3dt52575a
4726 | Dalton Trans., 2014, 43, 4726–4733
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influence on H2 coordination. In solution, all the three cis,trans-[RuH(η2-H2)(PPh3)2(N–N)][OTf] (N–N = bpy, 1a; bpm,
H-atoms behave as magnetically equivalent nuclei at ambient 2a) (Scheme 1).
temperatures due to fast exchange between the dihydrogen
A high field triplet signal around δ −10 ppm (1a,
and hydride ligands. Only in a few cases, low temperature δ −9.93 ppm; 2a, δ −9.94 ppm) integrating to three H-atoms
NMR spectral studies enabled the observation of the η2-H2 and points to the presence of a RuH3 moiety in these derivatives.
1
hydride ligands separately in the H NMR spectrum since this Three H-atoms of complexes 1a and 2a resonate at a weighted
dynamic process has a rather low free energy of activation.3e
average position between those of the individual chemical
In continuation of our studies on the H-atom site exchange shifts of the dihydrogen and hydride ligands due to a fast
in cis-dihydrogen/hydride complexes, in order to understand exchange process that the 1H NMR spectroscopy at room temp-
the factors that determine their structures and dynamics, we erature cannot arrest. Even at 163 K (in CDCl2F–CDClF2
undertook to examine the case of cis,trans-[RuH(η2-H2)- solvent),6 no decoalescence of the single resonance was noted.
(PPh3)2(N–N)][OTf] (N–N = 2,2′-bipyridyl (bpy), 2,2′-bipyrimi- We, however, noted line broadening of the RuH3 signal due to
dine (bpm); OTf = trifluoromethane sulfonate (CF3SO3)) com- slowing down of the exchange process.7 The ΔG≠ for the
plexes. Since the bulky PPh3 ligands and the rigid chelating exchange process in these complexes were calculated to be
N–N ligand will render the system rather sterically rigid around 8 to 13 kJ mol−1 (see ESI†).8
thereby minimizing the movement of the ancillary ligands
Presence of an η2-H2 ligand in 1a and 2a was established by
around the metal center, we chose to study the H-atom site variable temperature spin–lattice relaxation time (T1, ms)
exchange between the dihydrogen and hydride ligands in measurements, partial deuteration studies, and X-ray crystallo-
these complexes. The analogous cis-H2/hydride derivative cis, graphy, and confirmed by reacting these two dihydrogen com-
trans-[RuH(η2-H2)(PCy3)2(bpy)][OTf] reported earlier has not plexes with CO gas, which resulted in H2 evolution
been structurally characterized as noted before. Herein, we accompanied by the formation of the hydride carbonyl deriva-
report the synthesis and spectroscopic characterization of cis, tives, cis,trans-[RuH(CO)(PPh3)2(N–N)][OTf] (N–N = bpy, 1b;
trans-[RuH(η2-H2)(PPh3)2(N–N)][OTf] complexes together with bpm, 2b) (Scheme 1). Complexes 1b9 and 2b showed spectral
the dynamics studies. We also report the X-ray crystal structure signals at around δ −12 ppm for the RuH moiety and δ 45 ppm
of the cis,trans-[RuH(η2-H2)(PPh3)2(bpy)][OTf] (1a) complex.
for the PPh3 ligands, respectively, in the 1H and 31P NMR
spectra. Additionally, the IR spectrum of 2b showed a distinct
ν(CO) band at 1943 cm−1
.
Variable temperature spin–lattice relaxation time (T1, ms)
measurements of cis,trans-[RuH(η2-H2)(PPh3)2(N–N)][OTf]
(N–N = bpy, 1a; bpm, 2a)
Results and discussion
Synthesis, characterization, and protonation reaction of
cis,trans-[RuH2(PPh3)2(N–N)] (N–N = bpy, 1; bpm, 2)
We carried out variable temperature NMR spectral studies with
a view to obtain limiting spectra that would allow us to observe
two distinct signals for the η2-H2 and hydride ligands of
cis,trans-[RuH(η2-H2)(PPh3)2(N–N)][OTf] (N–N = bpy, 1a; bpm
2a) complexes. In turn, this could enable us to carry out spin–
lattice relaxation time (T1, ms) measurements for the η2-H2
and hydride ligands. However, VT 1H NMR spectral studies
revealed only one signal at all accessible temperatures for both
the ligands. Nevertheless, we carried out T1 measurements for
the RuH3 moiety of complexes 1a and 2a in the temperature
range of 293 K to 183 K. Just like in the previously reported
dihydrogen complexes,10 we noted unusually short T1 values
for complexes 1a and 2a. The T1(minimum) (CD2Cl2, 400 MHz)
observed for complexes 1a and 2a are 21.60 0.06 ms (183 K)
The starting dihydride complexes cis,trans-[RuH2(PPh3)2(N–N)]
(N–N = bpy, 1; bpm, 2) were synthesized from the reaction of
[RuH2(PPh3)4]4 and N–N bidentate ligands (Scheme 1). The
bipyridyl complex 1 was reported earlier by Morris and his co-
1
workers.5 The H NMR spectra of complexes 1 and 2 are com-
prised of high field signal at around δ −14 ppm which could
be attributed to the hydride ligands. A suspension of these
complexes in diethyl ether reacts instantaneously with HOTf to
afford the corresponding cis-dihydrogen/hydride complexes,
1
and 18.80 0.04 ms (191 K), respectively. The VT H T1 data
for complexes 1a and 2a are shown in Fig. 1.
It has been reported in the literature that among the T1
values of the entire temperature range measured, the
T1(minimum) is the one that is related to the distance between
the two dipolar H-atoms of the η2-H2 ligands.10a However, in
the case of complexes 1a and 2a, the observed T1(minimum)
values cannot be directly correlated with the H–H distance,
dHH (Å), of the η2-H2 ligand since the adjacent hydride ligand
has some contribution to the measured T1. If we assume that
the T1 of only the hydride ligand of 1a and 2a are the same as
Scheme 1
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Fig. 2 Hydride regions of the 1H{31P, 2H} (left) and 1H{31P} (right) NMR
spectra of cis,trans-[RuH(η2-H2)(PPh3)2(bpy)][OTf] (1a) (CD2Cl2, 296 K)
acquired at 500 MHz.
Fig. 1 Variable temperature T1 measurements of cis,trans-[RuH(η2-H2)-
(PPh3)2(bpy)][OTf] (1a) in CDCl2F–CDClF2 and cis,trans-[RuH(η2-H2)-
(PPh3)2(bpm)][OTf] (2a) in CD2Cl2 at 400 MHz.
that of the RuH2 moiety of 1 and 2, it is possible to calculate
the H–H distance (dHH, Å) for the η2-H2 ligand of the former
complexes.11 For this purpose, we also carried out VT T1
measurements of complexes 1 and 2 (see ESI†). The T1
(CD2Cl2, 400 MHz) values for complexes 1 and 2 were found to
be 477.0 0.2 ms (183 K) and 439.0 0.1 ms (191 K), respect-
ively. The H–H distance (dHH, Å) in the η2-H2 ligand of com-
plexes 1a and 2a could then be calculated to be ∼1.0 Å and
∼0.8 Å, considering the slow and the fast rotation regimes of
the bound H2 molecule, respectively.12 This calculation was
done by following a modified method (see ESI†) established by
Heinekey and co-workers.11 Both the dihydrogen complexes
exhibit the dHH values that are quite close to one another.
Complex 2a (dHH 1.04 Å, slow; 0.82 Å, fast) has a slightly
shorter η2-H2 ligand in comparison to 1a (1.07 Å, slow; 0.84 Å,
fast). This is rather reasonable because complex 2a possesses a
slightly more electron deficient Ru(II) center which results in
less back-bonding from the filled d orbital of the metal center
to the σ* MO of the η2-H2 ligand.
Fig. 3 Hydride regions of the 1H{31P, 2H} (left) and 1H{31P} (right) NMR
spectra of cis,trans-[RuH(η2-H2)(PPh3)2(bpm)][OTf] (2a) (CD2Cl2, 296 K)
acquired at 500 MHz.
Table 1 H–H distances (dHH) in the η2-H2 ligand of cis,trans-[RuH(η2-
H2)(PPh3)2(N–N)][OTf] (N–N = bpy, 1a; bpm, 2a), calculated from T1
measurements and partial deuteration studies
dHH
dHH
(fast, Å) (Hz)
J(H2D)
J(HD2) 1JHD/dHH
Complex (slow, Å)
(Hz)
(Hz/Å)
1a
2a
1.07 (183 K) 0.84
1.04 (191 K) 0.82
7.3 (293 K) 7.5
7.5 (293 K) 7.7
29.6/0.95
30.4/0.94
Hydride dynamics and stability of cis,trans-[RuH(η2-H2)-
(PPh3)2(N–N)][OTf] (N–N = bpy, 1a; bpm, 2a) complexes
Partial deuteration studies of cis,trans-[RuH(η2-H2)-
(PPh3)2(N–N)][OTf] (N–N = bpy, 1a; bpm, 2a) complexes
We carried out VT 1H NMR spectral studies down to 163 K and
183 K for complexes 1a and 2a respectively. Even at those
temperatures, the three H-atoms were quite dynamic. No
decoalescence of the single resonance for the RuH3 moiety
took place except that the signal underwent line broadening at
low temperatures. The mechanism that renders the three
H-atoms highly dynamic is not well understood in similar
systems reported in the literature owing to the difficulty in con-
trolling such a fast dynamic process by using a suitable ligand
environment in the coordination sphere of the metal center.
This fast exchange makes the three H-atoms chemically equi-
valent on the NMR time scale.
Treatment of cis,trans-[RuH(η2-H2)(PPh3)2(N–N)][OTf] (N–N =
bpy, 1a; bpm, 2a) complexes with HD gas resulted in a slow
incorporation of deuterium. Two isotopomers, RuH2D and
RuHD2, together with the original RuH3 species were observed
in the 1H NMR spectrum of the partially deuterated samples
(Fig. 2 and 3). The distinct quintet signal expected for RuHD2
1
species in the H{31P} NMR spectrum was not observed due to
its partial superimposition on the intense 1 : 1 : 1 triplet
pattern of the RuH2D isotopomer. Three chemical shift values
are closely spaced (ΔδH = 20–40 ppb), which is evident from
the 1H{31P, 2H} NMR spectrum (Fig. 2 and 3). The isotopic per-
turbation of resonance (IPR)13 resulted in an upfield isotopic
shift in 1a and 2a and the distribution of the deuterium atom
is statistical since we noted nearly equal J(H2D) and J(HD2)
Three plausible pathways that facilitate the exchange
process could be envisioned: (a) oxidative addition of the η2-H2
ligand, followed by reductive elimination of a H2 molecule
involving the hydride ligand, (b) fast proton transfer (heteroly-
sis) from the dihydrogen to hydride ligand, and (c) through a
1
values (Table 1). The coupling constant J(H,D) and H–H dis-
tances (dHH, Å) were calculated to be ∼30 Hz and ∼0.95 Å,
respectively.11
4728 | Dalton Trans., 2014, 43, 4726–4733
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concerted trihydrogen anionic species. Oxidative addition of PPh3 ligands create a specific coordination pocket for the η2-H2
the bound H2 ligand resulting in the formation of a trihydride ligand. Thus, the η2-H2 ligand is protected from substitution by
species followed by the re-formation of a dihydrogen ligand an external nucleophile. The bound H2 ligand, as mentioned
involving the hydride moiety is known to require a large ΔG≠ above, could not be substituted by sterically bulky ligands such
and thus is less likely.3h,j Free energy barriers required for the as PPh3 and Ph2PCH2CH2PPh2. However, sterically less
H-atom exchange between the dihydrogen and hydride ligands demanding ligands were found to substitute the η2-H2 ligand.
via heterolysis of H2 vary in the range of 20–50 kJ mol−1 3d,i,7
.
On the other hand, in an effort to prepare a σ-methane
Therefore, these two pathways of H-atom site exchange are less complex of the type cis,trans-[RuH(η2-H–CH3)(PPh3)2(N–N)]-
favored in our case. Next, we carried out reactions of 1a and 2a [OTf], we treated the starting dihydride complex cis,trans-
with certain Lewis bases such as CO, NMe3, C5H5N, CH3CN, [RuH2(PPh3)2(N–N)] (N–N = bpy, 1; bpm, 2) with CH3OTf in the
C6H5CN, PPh3, and Ph2PCH2CH2PPh2 to evaluate the heteroly- temperature range of 188–203 K. This reaction resulted in
tic behaviour of the bound H2 ligand. Whereas no deprotona- methane evolution (1H NMR spectrum: δ 0.2 ppm, singlet).
tion took place in any of these reactions, the bound H2 ligand The spectral details have been deposited in the ESI.† The reac-
was substituted by CO, C5H5N, CH3CN, and C6H5CN ligands. tion presumably proceeds through the intermediacy of an
Here as well, we rule out the H-atom site exchange via heteroly- η2-H–CH3 species which could not be observed; the reason
sis of H2 followed by the formation of a new dihydrogen ligand could be that the M–H–C interaction energy falls below the
involving the hydride ligand and the proton (abstracted from typical range, i.e., 37–63 kJ mol−1 16
.
the H2 ligand), which is captured by the Lewis base. The third
plausible pathway rendering the three H-atoms equivalent via
the involvement of a trihydrogen species would mean the
requirement of a ΔG≠ of ∼17 kJ mol−1 (theoretical).14 Since we
did not observe decoalescence of the RuH3 signal in the 1H
NMR spectrum up to a temperature of 163 K, it is reasonable
to expect that the ΔG≠ for the exchange process in our case is
≤17 kJ mol−1 (approximate ΔG≠ values were calculated to be 8
and 13 kJ mol−1 for 1a and 2a, respectively) which points to
the involvement of a trihydrogen species.
X-ray crystal structure of cis,trans-[RuH(η2-H2)(PPh3)2(bpy)]-
[OTf] (1a)
We obtained single crystals of complex 1a from a dilute
CH2Cl2 solution by layer diffusion of Et2O at 273 K under a H2
atmosphere over a period of several days. X-ray diffraction data
were collected at 130 K. Selected bond lengths and angles have
been summarized in Table 2 and the ORTEP diagram is shown
in Fig. 4. The RuH3 fragment and the bpy ligand occupy a
A closely related cis-dihydrogen/hydride complex cis,trans-
[RuH(η2-H2)(PCy3)2(bpy)][OTf] reported by Heinekey and his
co-workers3d exhibits a dHH of ∼1.1 Å, which is longer than
that in 1a and 2a. This is due to the presence of an electron
rich PCy3 ligand. In this species as well, no decoalescence of
the single resonance for the RuH3 could be detected even at
low temperature. However, another similar system cis,trans-
[RuH(η2-H2)(PCy3)2(CO)2][OTf] showed a low temperature limit-
ing spectrum in the 13C NMR spectrum at 120 K.3d The ΔG≠
(23.1 kJ mol−1) measured for this system is the lowest among
the related complexes known. The presence of the π-acceptor,
CO ligand in the trans position to the η2-H2 moiety is respon-
sible for the short H–H bond in this complex, ∼0.9 Å. The idea
in this case was to have an η2-H2 ligand with a large H–H bond
energy, which in turn could result in a relatively large and thus
measurable ΔG≠ required for the exchange process.3d
Table 2 Selected bond lengths (Å) and angles (°) of cis,trans-[RuH(η2-
H2)(PPh3)2(bpy)][OTf] (1a)
H(1a)–H(1b)
Ru(1)–H(1b)
Ru(1)–H(1a)
Ru(1)–H(1c)
Ru(1)–N(1)
Ru(1)–P(1)
0.99(4)
1.54(3)
1.61(3)
1.55(3)
2.10(1)
2.33(1)
H(1a)–Ru(1)–H(1b)
H(1b)–Ru(1)–H(1c)
H(1c)–Ru(1)–H(1a)
P(1)–Ru(1)–P(2)
36.6(13)
62.3(15)
99.0(14)
169.1(1)
76.31(6)
N(1)–Ru(1)–N(2)
It is rather intriguing to note that cis,trans-[RuH(η2-H2)-
(PPh3)2(bpy)][OTf] (1a) is isolable and is quite stable in the
solid state. This observation prompted us to crystallize it and
explore its structure (see below). Stability of a dihydrogen
complex depends mainly on two bonding components: (i) σ-
donation from the filled MO of H2 to an empty d orbital of
metal and (ii) back donation of electron density from a filled d
orbital of metal to the empty σ* MO of H2. In our case, the
unusual stability of 1a and 2a could be explained as follows.
First, the nitrogen donor present trans to the η2-H2 ligand opti-
mizes both the components of the M–H2 interaction resulting
in a substantially large M–H2 bond energy.15 Second, the steric
Fig. 4 ORTEP view of cis,trans-[RuH(η2-H2)(PPh3)2(bpy)][OTf] (1a)
(thermal ellipsoids are shown at the 50% probability level); counter-ions,
solvent molecules, and H-atoms of the bpy and PPh3 ligands are not
bulkiness and suitable arrangement of the phenyl rings of the shown in the diagram for clarity.
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single plane around the metal center. The two PPh3 ligands dihydrogen and the hydride ligands with a free energy of acti-
are perpendicular to this plane with a slightly bent confor- vation of ≤17 kJ mol−1 and are thought to proceed through the
mation (P(1)–Ru(1)–P(2) = 169.1(1)°) towards the η2-H2 and intermediacy of a trihydrogen species. Reaction of cis,trans-
hydride ligands. This arrangement creates a sterically encum- [RuH2(PPh3)2(N–N)] complexes with CH3OTf resulted in
bered environment around the dihydrogen ligand. The H–H methane evolution, and presumably proceeds through the
bond length, d(H(1a)–H(1b)), is 0.99(4) Å and the H(1a)–Ru(1)– intermediacy of an η2-CH4 species, cis,trans-[RuH(η2-H–CH3)-
H(1b) angle is about 36.6(13)°, which implies that 1a is a true (PPh3)2(N–N)][OTf],
η2-H2 complex; this is also supported by the partial deuteration spectroscopically.
studies.17 The H–H distance in free H2 is 0.74 Å from which it
which
could
not
be
observed
could be calculated that H2 underwent about 34% elongation
upon coordination in 1a. The H(1b) atom of the H2 ligand is
Experimental section
about 1.60 Å away from the H(1c) atom, which, in the solid
state structure, stays as an isolated hydride ligand with a
Ru(1)–H(1c) distance of 1.55(3) Å. Instead of being in linear
arrangement, the H(1c)–Ru(1)–N(2) angle is sufficiently bent
(168°) away from the coordination site trans to the N(1) atom
thus creating enough space for accommodating the η2-H2
ligand. A straight line is drawn along N(1) and Ru(1) and
extrapolated through the mid-point of H(1a)–H(1b) bond, con-
firming an η2 coordination mode of the dihydrogen ligand.
X-ray crystal structure and computational studies of related
complexes cis,trans-[RuH(η2-H2)(PiPr3)2(X)] (X = 2-phenylpyri-
dine (ph-py), benzoquinoline (bq), phenylpyrazole (ph-pz))
have been reported recently.3e The η2-H2 ligand in those com-
plexes occupies the coordination site that is trans to the nega-
tively charged carbon ligand (a strong σ-donor). This typical
situation intensifies the back donation effect. Consequently,
those complexes were found to be stable under ambient con-
ditions. The H–H distances, dHH, in those cases were ∼0.9 Å
(X-ray).
General procedures
All reactions and manipulations were performed under a dry
argon or nitrogen atmosphere using standard Schlenk line
techniques. Solvents were distilled over respective drying
agents prior to use.18 2,2′-Bipyridyl (bpy) (Spectrochem), 2,2′-
bipyrimidine (bpm), HOTf and CH3OTf (Aldrich) were used as
received. The 1H, 13C (100 MHz), 19F (376 MHz) and 31P
(160 MHz) NMR spectral data were acquired using an Avance
Bruker 400 MHz instrument. The NMR spectral studies of the
H/D isotopomers were carried out at the NMR Research
Centre, Indian Institute of Science, Bangalore by using an
Avance Bruker 500 MHz instrument. All 1H and 13C NMR spec-
tral values noted herein are relative to the signals of the
respective deuterated solvents used.19 The 31P and 19F NMR
spectral signals are calibrated, respectively, to 85% H3PO4
(aqueous solution) and CFCl3 as external references at
δ 0.0 ppm. IR spectra of the neat powder samples were
recorded using a Bruker ALPHA-P spectrometer working in the
400–4500 cm−1 range with a 4 cm−1 resolution.
Earlier,
a
16-electron system cis,trans-[Ru(H)(η2-H2)-
(PCy3)2(I)] showed unusual geometry around the metal
center.3k The structural features of the RuH3 moiety of that
complex are quite similar to that of 1a. In general, the hydride
and η2-H2 ligands of a cis-dihydrogen/hydride complex are
Synthesis of cis,trans-[RuH2(PPh3)2(N–N)] (N–N = bpy, 1;
bpm, 2)
A
toluene solution (15 mL) of [RuH2(PPh3)4] (800 mg,
found to be present in the same plane, mainly due to weak 0.70 mmol) and N–N bidentate ligand (160 mg, 1.03 mmol
interaction between the hydride ligand and one hydrogen bpy; 160 mg, 1.01 mmol bpm) was heated at 368 K for 1 h in
atom of the η2-H2 ligand as mentioned earlier. From the struc- an oil bath. The resulting dark green (bpy) or dark black (bpm)
tural data of 1a, it is difficult to say that there is an interaction colored solution was cooled to room temperature and then
between the hydride and η2-H2 ligands. However, the reason diethyl ether (50 mL) was added to cause precipitation. After
behind all three hydrogen atoms (of 1a) being in a plane could the mixture was stirred for 10 min (to allow complete dissolu-
be the presence of such a kind of weak interaction.
tion of PPh3), the resulting precipitate was filtered and washed
with diethyl ether (3 10 mL) to afford cis,trans-
×
[RuH2(PPh3)2(bpy)] (1) as dark violet powder and cis,trans-
[RuH2(PPh3)2(bpm)] (2) as a dark black solid. Yield: complex 1,
460 mg (84%); complex 2, 390 mg (70%). Complex 1 was
Conclusions
Protonation of cis,trans-[RuH2(PPh3)2(N–N)] (N–N = bpy, 1; reported previously by Morris and his co-workers.5 Characteriz-
1
bpm, 2) complexes using HOTf resulted in the corresponding ation details of complex 2: H NMR (400 MHz, CD2Cl2, 293 K,
dihydrogen complexes cis,trans-[RuH(η2-H2)(PPh3)2(N–N)][OTf] δ ppm): −13.98 (t, J(H, P) = 27.0 Hz, 2H, RuH2), 6.51 (t, J(H,
(N–N = bpy, 1a; bpm, 2a). These derivatives are stable under H) = 5.0 Hz, 2H, bpm: 5,5′-H), 7.16–7.41 (m, 30H, PPh3), 8.38
ambient conditions and, in fact, 1a was isolable and could be (d, J(H, H) = 5.0 Hz, 2H, bpm: 6,6′-H), 8.48 (s, 2H, bpm: 4,4′-
crystallized. The X-ray crystal structure of cis,trans-[RuH(η2-H2)- H); 13C{1H} NMR (100 MHz, CD2Cl2, 293 K, δ ppm): 121.1 (s,
(PPh3)2(bpy)][OTf] (1a) was established and all the hydrogen bpm: 5,5′-C), 127.8 (t, J(C, P) = 4.0 Hz, PPh3: meta-C), 128.2 (s,
atoms were located unambiguously which revealed the pres- PPh3: para-C), 133.4 (t, J(C, P) = 6.0 Hz, PPh3: ortho-C), 139.1 (t,
ence of a planar RuH3 moiety. In solution, the H-atoms in J(C, P) = 22.0 Hz, PPh3: ipso-C), 150.3 (s, bpm: 4,4′-C), 161.5 (s,
these two complexes undergo site exchange between the bpm: 2,2′-C), 162.2 (s, bpm: 6,6′-C) (for atomic numbering of the
4730 | Dalton Trans., 2014, 43, 4726–4733
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4.0 Hz, 2H, bpm: 4,4′-H); 13C{1H} NMR (100 MHz, CD2Cl2,
273 K, δ ppm): 123.6 (s, bpm: 5,5′-C), 129.3 (t, J(C, P) = 4.0 Hz,
PPh3: meta-C), 130.9 (s, PPh3: para-C), 131.7 (t, J(C, P) = 22.0
Hz, PPh3: ipso-C), 133.1 (t, J(C, P) = 6.0 Hz, PPh3: ortho-C),
157.4 (s, bpm: 4,4′-C), 159.2 (s, bpm: 2,2′-C), 162.8 (s, bpm:
6,6′-C) (atomic numbering of the bpm ligand of 2a is the same
as that of bpy of 2); 31P{1H} NMR (160 MHz, CD2Cl2, 293 K,
δ ppm): 48.3 (s, PPh3); 19F NMR (376 MHz, CD2Cl2, 293 K,
δ ppm): −78.7 (s, CF3).
Spin–lattice relaxation time (T1, ms; 400 MHz) measurements
Variable temperature T1 data were obtained for complexes 1a
and 2a using the standard inversion recovery method at
400 MHz by applying a 180–τ–90° pulse sequence with a 5 K or
10 K temperature interval.20 Errors in the T1 data were calcu-
lated from the fitting of delay time (τ) vs. intensity plot. The
error in the T1(minimum) was calculated from fitting of the
T vs. T1 curve.
Partial deuteration experiments
Chart 2
Partial deuteration of complexes 1a and 2a was achieved by
bubbling HD gas (generated by a reaction between NaH and
D2O) through their CH2Cl2 solutions for 1 h at 213 K, 273 K,
and 288 K. The extent of incorporation of deuterium into the
cis-H2/hydride complexes was found to be maximum at 288 K.
bpm ligand, see Chart 2); 31P{1H} NMR (160 MHz, CD2Cl2, 293 K,
δ ppm): 61.3 (s, PPh3); IR (neat, cm−1): 1920 (νRuH); ESI-MS
(CH3CN): m/z 785 (M+ − H); Anal. Calc. for C44H38N4P2Ru:
C 67.25, H 4.87, N 7.13%; Found: C 67.43, H 4.50, N 6.80%.
Synthesis of cis,trans-[RuH(CO)(PPh3)2(N–N)][OTf] (N–N = bpy,
1b; bpm, 2b)
Synthesis of cis,trans-[RuH(η2-H2)(PPh3)2(N–N)][OTf] (N–N =
bpy, 1a; bpm, 2a)
CO gas was bubbled through CH2Cl2 (10 mL) solutions of cis,
To a suspension of cis,trans-[RuH2(PPh3)2(N–N)] (100 mg, trans-[RuH(η2-H2)(PPh3)2(N–N)][OTf] (N–N = bpy, 1a; bpm, 2a)
0.12 mmol 1; 100 mg, 0.11 mmol 2) in Et2O (10 mL) was added complexes (120 mg, 0.13 mmol 1a; 120 mg, 0.12 mmol 2a) at
HOTf (12 μL, 0.12 mmol) in an atmosphere of H2 or N2 or Ar. 273 K for 10 min. The solutions were then stirred at room
The dark violet (1) or dark black (2) colored suspension turned temperature for 30 min to allow for completion of the reaction
yellow (1a) or brown (2a) within 20 min of stirring. The reac- and then filtered through Celite. The resulting filtrates were
tion mixture was left undisturbed for the precipitate to settle concentrated to 1 mL and cis,trans-[RuH(CO)(PPh3)2(N–N)]-
down at the bottom of the Schlenk tube. Unreacted HOTf (if [OTf] (N–N = bpy, 1b; bpm, 2b) complexes were precipitated by
any) was removed by washing the resulting solid of complexes addition of Et2O (10 mL). Yield, 1b: 80 mg (64%); 2b: 62 mg
1a and 2a with Et2O (2 × 10 mL). Yield: complex 1a, 100 mg (50%). Complex 1b was reported by Luther and Heinekey
(90%); complex 2a, 60 mg (51%).
Characterization details of complex 1a: H NMR (400 MHz,
previously.9c
1
Characterization details of cis,trans-[RuH(CO)(PPh3)2(bpm)]-
1
CD2Cl2, 293 K, δ ppm): −9.93 (t, J(H, P) = 13.0 Hz, 3H, RuH3), [OTf] complex 2b: H NMR (400 MHz, CDCl3, 293 K, δ ppm):
6.91 (t, J(H, H) = 5.0 Hz, 2H, bpy: 5,5′-H), 7.22–7.36 (m, PPh3), −11.65 (t, J(H, P) = 19.0 Hz, 1H, RuH), 6.67 (t, J(H, H) = 5.0 Hz,
7.80 (t, J(H, H) = 5.0 Hz, 2H, bpy: 4,4′-H), 7.81 (d, J(H, H) = 1H, bpm: 3-H), 7.27–7.39 (m, PPh3), 7.57 (t, J(H, H) = 5.0 Hz,
5.0 Hz, 2H, bpy: 3,3′-H), 8.65 (d, J(H, H) = 5.0 Hz, 2H, bpy: 6,6′- 1H, bpm: 10-H), 7.75 (d, J(H, H) = 5.0 Hz, 1H, bpm: 2-H), 8.74
H); 13C{1H} NMR (100 MHz, CD2Cl2, 293 K, δ ppm): 123.4 (s, (d, J(H, H) = 5.0 Hz, 1H, bpm: 4-H), 8.83 (d, J(H, H) = 5.0 Hz,
bpy: 3,3′-C), 126.3 (s, bpy: 5,5′-C), 128.8 (t, J(C, P) = 4.0 Hz, 1H, bpm: 11-H), 9.24 (d, J(H, H) = 5.0 Hz, 1H, bpm: 9-H);
PPh3: meta-C), 130.4 (s, PPh3: para-C), 132.8 (t, J(C, P) = 13C{1H} NMR (100 MHz, CDCl3, 293 K, δ ppm): 124.2 (s, bpm:
21.0 Hz, PPh3: ipso-C), 133.3 (t, J(C, P) = 6.0 Hz, PPh3: ortho-C), 3-C), 124.5 (s, bpm: 10-C), 129.5 (t, J(C, P) = 4.0 Hz, PPh3: meta-
137.1 (s, bpy: 4,4′-C), 154.3 (s, bpy: 2,2′-C), 155.7 (s, bpy: 6,6′-C) C), 131.0 (t, J(C, P) = 22.0 Hz, PPh3: ipso-C), 131.2 (s, PPh3:
(for atomic numbering of the bpy ligand, see Chart 2); 31P{1H} para-C), 133.4 (t, J(C, P) = 6.0 Hz, PPh3: ortho-C), 158.0 (s, bpm:
NMR (160 MHz, CD2Cl2, 293 K, δ ppm): 48.9 (s, PPh3); 19F 4-C), 158.3 (s, bpm: 11-C), 159.8 (s, bpm: 6-C), 160.0 (s, bpm:
NMR (376 MHz, CD2Cl2, 293 K, δ ppm): −80 (s, CF3).
7-C), 161.8 (s, bpm: 2-C), 162.2 (s, bpm: 9-C), 203.5 (t, J(C, P) =
1
Characterization details of complex 2a: H NMR (400 MHz, 14.0 Hz, CO) (for atomic numbering of the bpy ligand, see
CD2Cl2, 293 K, δ ppm): −9.94 (t, J(H, P) = 12.0 Hz, 3H, RuH3), Chart 2); 31P{1H} NMR (160 MHz, CDCl3, 293 K, δ ppm): 45.2
7.03 (t, J(H, H) = 5.0 Hz, 2H, bpm: 5,5′-H), 7.23–7.42 (m, PPh3), (s, PPh3); 19F NMR (376 MHz, CDCl3, 293 K, δ ppm): −77.9 (s,
8.78 (d, J(H, H) = 5.0 Hz, 2H, bpm: 6,6′-H), 8.88 (d, J(H, H) = CF3); IR (neat, cm−1): 1943 (νCO); ESI-MS (CH3CN): m/z 813
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(0.3, M − OTf), 590 (0.7, M − OTf − PPh3 + K+), 521 (1.0, M − using the SADABS program. The structure was solved by direct
OTf − PPh3 − H − CO), 364 (0.5, M − OTf − PPh3 − CO − methods and refined using the SHELX programs.21 All the
bpm); Anal. Calc. for C46H47F3N4O9P2RuS: C 52.98, H 3.67, non-hydrogen atoms were refined anisotropically. Hydrogen
N 5.37%; Found: C 52.34, H 3.87, N 6.28%.
atoms were located by difference Fourier synthesis and refined
isotropically. Complex 1a crystallized in P21/n space group with
four molecules in the unit cell. The complete crystallographic
data of complex 1a are summarized in Table 3.
Reaction of cis,trans-[RuH2(PPh3)2(N–N)] (N–N = bpy, 1; bpm, 2)
with CH3OTf
A NMR tube provided with a high vacuum stopcock was
charged with cis,trans-[RuH2(PPh3)2(N–N)] (20 mg, 0.03 mmol
1; 20 mg, 0.025 mmol 2) and CD2Cl2 (0.5 mL) inside a glove
box. CH3OTf (2.4 μL, 0.04 mmol) was added to the solution
under frozen conditions. Then, the NMR tube was evacuated
for 30 min before flame sealing. The frozen solid was melted
slowly in a low temperature slush bath (183 K, mixture of Et2O
and liq. N2). The sample was then inserted into a NMR probe
that was precooled and maintained at 183 K before acquiring
the 1H and 31P NMR spectral data at that temperature.
Acknowledgements
We are grateful for the financial support from the Department
of Science & Technology (Science & Engineering Research
Board), India. We are thankful to Prof. Siddhartha P. Sarma
(Molecular Biophysics Unit) and Dr S. Ragothama (NMR
Research Centre) for helpful discussions on NMR
spectroscopy. B. B. thanks the Indian Institute of Science,
Bangalore and Y. P. P. thanks CSIR, India for fellowships.
X-ray crystal structure determination of cis,trans-[RuH(η2-H2)-
(PPh3)2(bpy)][OTf] (1a)
A single crystal of cis,trans-[RuH(η2-H2)(PPh3)2(bpy)][OTf] (1a)
suitable for X-ray diffraction study was chosen carefully under
a microscope and coated with paraffin oil to avoid direct
exposure to air. The unit cell parameters and the intensity data
were collected at 130 K on a Bruker KAPPA APEX II CCD diffr-
actometer equipped with a fine focus Mo Kα X-ray source. The
SMART software was used for data acquisition and the SAINT
software for data reduction. Absorption corrections were made
Notes and references
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Table 3 Crystallographic data of cis,trans-[RuH(η2-H2)(PPh3)2(bpy)]-
[OTf] (1a)
CCDC number
Formula
951511
C47H41N2P2SO3F3Ru·CH2Cl2
1018.81
Yellow
130(2)
Monoclinic
P21/n
9.5167(5)
22.4523(12)
21.4491(12)
96.349(2)
4555.0(4)
4
Formula weight
Colour of crystal
Temperature (K)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
β (°)
V (Å3)
Z
µ (Mo Kα) (cm−1
F(000)
)
6.34
2080
Crystal size (mm)
Radiation
Limiting indices
0.31 × 0.15 × 0.01
Mo Kα (λ = 0.71073 cm−1
)
−11 ≤ h ≤ 11, −27 ≤ k ≤ 27,
−26 ≤ l ≤ 26
θ (°) range
1.81–25.99
Absorption correction
Refinement method
Empirical
Full-matrix least squares
on F2
Number of data/unique data
Maximum and minimum
transmission
79482/8954
0.9937 and 0.8277
Rint
0.0362
Number of parameters refined
R (final) [I > 2σ(I)]
R (all)
731
R1 = 0.0265, wR2 = 0.0640
R1 = 0.0331, wR2 = 0.0698
1.073
4 J. J. Levison and S. D. Robinson, J. Chem. Soc. A, 1970,
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