Reversible H2 splitting between Ru(ii) and a remote carbanion in a zwitterionic compound

Article abstract of DOI:10.1039/b919606d

We report a reversible long-range splitting of H₂ between a Ru(ii) center and a remote carbanion in a zwitterionic compound.

Full text of DOI:10.1039/b919606d

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Reversible H2 splitting between Ru(II) and a remote carbanion
in a zwitterionic compoundw
Elzbieta Stepowska, Huiling Jiang and Datong Song*
Received (in Berkeley, CA, USA) 21st September 2009, Accepted 25th November 2009
First published as an Advance Article on the web 11th December 2009
DOI: 10.1039/b919606d
We report a reversible long-range splitting of H2 between
a Ru(II) center and a remote carbanion in a zwitterionic
compound.
electron-rich to show the required Lewis acidity. As a result,
when [RhL(PPh3)2] was treated with H2, only oxidative
addition occurred to form the corresponding dihydride.
Recently, we have extended our study to a more Lewis acidic
Ru(II) system using the Lꢀ ligand. This increased Lewis acidity
in combination with the unquenched basicity of the Lꢀ ligand
indeed created a FLP situation. Herein, we report a reversible
long-range heterolytic cleavage of H2 between Ru(II) and a
remote uncoordinated carbanion in a zwitterionic compound
cis,trans-[RuH(L)(N2)(PPh3)2] (2).
The formation and cleavage of dihydrogen have attracted
much research effort in recent years because of their applications
in hydrogenation and relevance to biological H2 processing
and the hydrogen economy.1,2 Particularly, heterolytic splitting
of dihydrogen is of vital importance in the hydrogenation of
polar bonds2,3 and the catalytic activity of hydrogenase.4
The general scheme of heterolytic splitting of H2 has been
established: upon coordination to an electrophilic metal
center, the H2 molecule becomes more acidic and thus can
release a mobile proton to an internal or external base, leaving
a hydride on the metal center.2,5 For example, in a key step of
Noyori’s catalytic system, H2 is split into a hydride and a
proton between the Ru(II) center and a highly basic amido
ligand.5 Variations of heterolytic H2 splitting have been
reported between various metal centers and adjacent nucleo-
philic heteroatoms (such as N, O, and S), facilitated by the
close proximity between the metal center and the heteroatom.6
The recent advances by Stephan and others7 demonstrated
that heterolysis of H2 can be achieved simply by unquenched
Lewis acidity and basicity. Such frustrated Lewis pairs (FLPs)
play a key role in polarizing H2 and in turn, splitting it into
H+ and Hꢀ.7,8 What intrigued us most is Stephan’s integrated
phosphane–borane system in which the sterically encumbered
phosphorus and boron centers in the same molecule are well
spaced by a rigid C6F4 linker group.9 This discovery prompted
Compound
2 can be prepared by reacting cis,cis-
[RuHCl(LH)(PPh3)2] (1) (where LH = 4,5-diazafluorene) with
KOtBu in a THF solution under a dinitrogen atmosphere
(Scheme 1). Compound 2 is air- and moisture-sensitive both in
the solid state and in solution. The 1H NMR spectrum of
compound 2 in C6D6 shows an unsymmetrical structure with
two sets of signals for the two pyridine rings of the Lꢀ ligand.
The proton on the central ring of Lꢀ resonates at 6.44 ppm
indicating an increased aromatic character.11 The Ru–H
2
resonates at ꢀ12.23 ppm with a JP–H of 20 Hz. The proton
decoupled 31P NMR spectrum of 2 in C6D6 has only one
singlet at 49.61 ppm. The N2 stretching frequency appears
at 2092 cmꢀ1 in the IR spectrum, hinting that not much
back-donation is involved.12
The solid-state structure of 2 (Fig. 1) confirmed by X-ray
crystallographyz is consistent with the solution NMR data.
The Ru(II) center adopts a distorted octahedral geometry with
two nitrogen donor atoms from Lꢀ, two phosphorus donor
atoms from two triphenylphosphine ligands that are trans to
each other, a hydride and an end-on dinitrogen occupying the
six coordination sites. The N–N bond length is 1.111(2) A,
slightly longer than that of a free N2 (1.0975 A), consistent
with the IR data.12 The Ru1–N2 bond (2.282(1) A) is longer
than the Ru1–N1 bond (2.114(1) A) as a result of the strong
trans-influence of the hydride. Similar to the previously
reported zwitterionic Rh complexes of Lꢀ, the Cpꢀ moiety
of the chelating ligand is dangling and thus retains its
unquenched basicity.10 Although the Ru(II) center in 2 is
us to investigate
a related scenario—zwitterionic metal
complexes with unquenched basicity on negatively charged
ligands and Lewis acidity on positively charged metal centers.
Previously, we demonstrated the ability of the negatively
charged 4,5-diazafluorenide (Lꢀ) ligand to form zwitterionic
Rh complexes, in which the Cpꢀ moiety of the Lꢀ ligand
remains uncoordinated and thus has unquenched basicity.10
We envisioned that if the Lewis acidity of the metal center is
sufficiently unquenched at the same time, the complex may
have the capability of polarizing and heterolyzing H2 molecules,
resembling the integrated phosphane–borane system. The
Rh(I) center in the complex [RhL(PPh3)2], however, is too
Davenport Chemical Research Laboratories, Department of
Chemistry, University of Toronto, 80 St. George Street, Toronto,
Ontario, Canada. E-mail: dsong@chem.utoronto.ca;
Fax: 1-416-978-7013; Tel: 1-416-978-7014
w Electronic supplementary information (ESI) available: Experimental
and spectroscopic details of 2 and 3. CCDC 743623 and 743624. For
ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/b919606d
Scheme 1 Synthesis of compound 2.
ꢁc
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556 | Chem. Commun., 2010, 46, 556–558

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Fig. 2 POV-Ray drawing of 3. Color key: C, black; P, pink; N, blue;
Fig. 1 POV-Ray drawing of 2. Color key: C, black; P, pink; N,
H, white; Ru, green.
blue; H, white; Ru, green.
saturated with six ligands, the N2 ligand is sufficiently labile,
rendering the Ru(II) center Lewis acidic upon dissociation.
In solution (in C6H6 or THF), compound 2 reacts with
1 atm of H2 at 60 1C to form cis,cis,trans-[Ru(H)2(LH)(PPh3)2]
(3) quantitatively within 1.5 h (Scheme 2, Fig. 2),z whereas the
reaction is much slower at ambient temperature. The use of
D2 instead of H2 results in deuterium incorporation to the
9-position of the LH ligand and Ru–D formation.
Interestingly, the ortho positions of the phenyl groups of
PPh3 are deuterated in this process, indicating a concomitant
reversible cyclometallation process. When the partially
deuterated 3 is treated with 1 atm of H2 at 60 1C in solution,
protium can be incorporated to the 9-position of the LH
ligand, the ortho position of the phenyl groups of PPh3, and
the Ru center, indicating the reversible nature of the hydrogen
splitting and releasing process. When a solution of compound
3 (in benzene or THF) is heated at 60 1C in a sealed Schlenk
bomb under a dinitrogen atmosphere for 12 h, compound 2
can be detected by NMR spectroscopy in 12% conversion.
Prolonged heating does not change the conversion
significantly, suggesting that the H2 split and release
equilibrium is lying on the split side.
(PNP)IrPh (where PNP is 2,6-bis(di-tert-butylphosphinomethyl)-
pyridine) system,13c where the carbon atom and the Ir
center are B3.2 A apart, resulting in to formation of a
trans-dihydride rather than the expected cis-dihydride. Dyson
and co-workers have reported H2 splitting between the Ru(II)
center and the carbon backbone of the b-diketiminate ligand
in a Ru(II) arene b-diketiminate complex, in which the carbon
atom and the Ru center are 3.4 A apart.14 More recently,
Stradiotto and co-workers have reported a heterolysis of
H2 between a Ru(II) center and a carbon atom in a
Cp*Ru(k3-P,C,C0) species,15 in which the distance between
the Ru center and the carbon atom is B4.2 A. In contrast, the
Ru center and C-9 of the Lꢀ ligand are B5.0 A apart in
compound 2.
In the (PNP)Ir system, a unique mechanism involving
deprotonation/protonation–aromatization/dearomatization
has been supported by both experimental and DFT data;16 in
the Cp*Ru(k3-P,C,C0) case, a C–Ru bond is present, hinting
the possibility of hydrogenolysis of the C–Ru bond followed
by isomerization. A concerted mechanism has been suggested
by DFT for the Ru(II) arene b-diketiminate case, whereas in
compound 2 case, a concerted mechanism appears unlikely
because of the large separation between the Ru center and the
carbon atom, unless the proton transfer is assisted by a bridge
of multiple solvent (e.g. H2O) molecules.16a Although at first
glance, H2 splitting by compound 2 may also be the Noyori
type, via a resonance structure with amido-Ru moiety,
followed by isomerization, the D2 experiment disfavors this
mechanistic proposal.17 Interestingly, even for Milstein’s
(PNP)Ir system, despite the presence of an amido nitrogen,
DFT calculations suggested that the Noyori type of H2
splitting followed by isomerization is unlikely.16b Mechanistic
studies on the long-range H2 splitting by 2 are currently
underway and will be published in due course.
The Milstein group has reported reversible H2 splitting
between the metal center and a non-adjacent carbon atom in
(PNN)Ru and (PNP)Ru pincer compounds.13a,b The same
group has also reported an intriguing H2 splitting by the
In summary, we have demonstrated a unique reversible
heterolysis of dihydrogen between a Ru(II) center and a distant
uncoordinated carbanion in the zwitterionic compound 2.
Although the mechanism of such heterolysis of H2 is yet to
Scheme 2 Reversible H2 activation by compound 2.
ꢁc
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Chem. Commun., 2010, 46, 556–558 | 557

View Article Online
be studied, the net result of this reaction represents an
unusually long range heterolytic cleavage of H2 between a
metal center and a carbanion in the same molecule. The
equilibrium favors hydrogen splitting. The mechanism of this
reversible heterolytic H2 splitting, the possibility to activate
other small molecules with compound 2, and the potential
catalytic activity of 2 and 3 are being investigated in our
laboratory.
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This research was supported by grants to D. S. from
Canadian Foundation for Innovation, Natural Sciences and
Engineering Research Council of Canada, Ontario Ministry of
Research and Innovation, the University of Toronto,
Connaught Foundation. E. S. is grateful for a postgraduate
scholarship from the OGS program of Ontario. We thank
Professors Robert H. Morris and Douglas W. Stephan for
helpful discussions.
¨
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z Selected crystallographic data, for 2ꢂC6H6: a = 18.4215(5), b =
9.9438(2), c = 25.1806(7) A, b = 100.457(1)1, V = 4321.68(19) A3,
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4
R1 = 0.0262, wR2 = 0.0631; for 3: a = 13.8338(4), b = 17.7363(6),
c = 15.9357(3) A, b = 105.997(2)1, V = 3758.57(18) A3, Z = 4,
P21/n, T = 150 K, unique data: 8519 (Rint = 0.0438), observed data
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17 If the H2 splitting is Noyori type followed by isomerization
through resonance structures, one would expect deuterium
incorporation into the meta-position of the pyridine moiety during
the D2 splitting experiment to certain extent. However, this
deuterium incorporation was not observed. For details, see ESIw.
ꢁc
This journal is The Royal Society of Chemistry 2010
558 | Chem. Commun., 2010, 46, 556–558