Activation of H2 by a highly distorted RhII complex with a new C3-symmetric tripodal tetraphosphine ligand

Article abstract of DOI:10.1039/b917632b

Facile oxidation of a sterically encumbered Rh^I complex generates a stable Rh^II metalloradical species; the latter is able to activate H₂ under formation of the corresponding Rh^III complex.

Full text of DOI:10.1039/b917632b

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
II
Activation of H2 by a highly distorted Rh complex with a new
C3-symmetric tripodal tetraphosphine ligandw
Jeroen Wassenaar,a Bas de Bruin,a Maxime A. Siegler,b Anthony L. Spek,b
Joost N. H. Reek*a and Jarl Ivar van der Vlugt*a
Received (in Cambridge, UK) 26th August 2009, Accepted 17th December 2009
First published as an Advance Article on the web 12th January 2010
DOI: 10.1039/b917632b
Facile oxidation of a sterically encumbered RhI complex
generates a stable RhII metalloradical species; the latter is able
to activate H2 under formation of the corresponding RhIII
complex.
Rh-complex 2 as well as unusual subsequent reactivity of the
related paramagnetic species [2]PF6 to activate dihydrogen.
The 3-methylindole scaffold seemed a very suitable building
block for a tripodal tetraphosphine ligand, since it is very rigid
and thus minimizes conformational ambiguity.9 Moreover,
we developed the necessary synthetic methods to introduce
phosphorus substituents on the nitrogen as well as on the C2
position of the indole ring previously in the synthesis of chiral
hybrid bidentate ligand IndolPhos and close analogues thereof.10
We set out to construct PP3 ligand 1, which features a
central bridgehead phosphorus atom that is connected to three
3-methylindole fragments at the 2-position (Scheme 1).
Capping of PCl3 with 3 equiv. 2-lithioindole, employing
in situ protection of the nitrogen using CO2,11 followed by
phosphorylation of the nitrogen atom gave the desired
product 1 in good yield.w The pivotal P-atom showed up as
a quartet at d ꢀ75.1 ppm, whilst a doublet was observed at
d 37.3 ppm for the peripheral PPh2 units, with a large coupling
constant JPꢀP of 159 Hz. These large coupling constants
indicate a high degree of rigidity and suggest a conformation
with an overall C3-symmetry in solution.
Activation of dihydrogen provides a basic and often crucial
elementary step in many important transition-metal catalyzed
processes, such as hydrogenation and hydroformylation. This
activation is most commonly achieved by oxidative addition
within the coordination sphere of a transition metal.1,2 Alter-
natively, dihydrogen activation may be achieved by heterolytic
cleavage of the H–H bond, e.g. aided by a strong base as
observed in some Ru hydrogenation catalysts.3 However,
activation of dihydrogen by metal-centered radical complexes
has only been scarcely explored to date. Such a reaction would
be the starting point for hydrogen atom-transfer (HAT)
reactions,4 using molecular hydrogen as the hydrogen atom
donor. The latter type of activation requires a metal–ligand
combination, which is able to stabilize reactive open-shell
metalloradical intermediates. Confined metal geometries are
therefore a prerequisite for selective transformations of this
kind.5 Multipodal and in particular C3-symmetric tripodal
skeletons are attractive frameworks for this purpose.6,7 The
main advantage of these ligands is the effective shielding of the
metal center, preventing side reactions that could lead to
decomposition or loss of selectivity. Secondly, a higher
number of coordinating atoms is advantageous in stabilizing
higher oxidation states, by efficient donation of electron-
density to the transition metal. In particular tetradentate
architectures that feature an all-phosphorus coordination
sphere would be ideally suited.8 Herein we report a new,
sterically demanding, C3-symmetric tripodal tetraphosphine
ligand 1 based on 3-methylindole as a building block, its
In addition to stringent requirements for the ligand, we
believe that also the choice of the transition metal is pivotal in
order to achieve stable metalloradicals for hydrogen activation.
Rhodium was selected as the privileged metal as it is known to
form stable odd-electron species, using tetradentate nitrogen
donor ligands.12,13 Complexation with Rh was achieved by
reaction of stoichiometric amounts of the ligand with
[Rh(cod)(m-Cl)]2.w The 31P NMR spectrum for 2 consists of
a doublet-of-quartets at d 43.5 ppm (Dd is 80.8 ppm) for the
central P-atom, and the peripheral phosphorus-units are
present as a doublet-of-doublets at d 74.0 ppm, suggesting a
C3-symmetric trigonal bipyramidal geometry. The latter was
corroborated by single crystal X-ray analysis, see Fig. 1 (left) for
the molecular structure.z The structure displays a slightly dis-
torted trigonal bipyramidal geometry (t-value: 0.93).14 From the
structure, it is apparent that ligand 1 is indeed able to fully
a Supramolecular & Homogeneous Catalysis Group,
van’t Hoff Institute for Molecular Sciences,
University of Amsterdam, Nieuwe Achtergracht 166,
1018 WV Amsterdam, the Netherlands.
E-mail: j.i.vandervlugt@uva.nl, j.n.h.reek@uva.nl;
Fax: +31 20 5255604; Tel: +31 20 5256459
b Department of Crystal and Structural Chemistry,
University of Utrecht, Padualaan 8, 3584 CH Utrecht,
the Netherlands
w Electronic supplementary information (ESI) available: General
experimental methods, synthetic procedures, cyclic voltammetry,
crystallographic details, DFT calculations, and NMR and EPR
spectroscopic details. CCDC 735997 and 735998. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/b917632b
Scheme 1 Straightforward synthesis of tripodal, tetradentate ligand 1.
Conditions: (i) n-BuLi, THF, ꢀ78 1C; CO2; t-BuLi; PCl3. (ii) n-BuLi,
THF, ꢀ78 1C; ClPPh2.
ꢁc
This journal is The Royal Society of Chemistry 2010
1232 | Chem. Commun., 2010, 46, 1232–1234

View Article Online
Fig. 1 Displacement ellipsoid plots (50% probability level) of species
2 (left) and [2]PF6. H-atoms and counter ion are omitted for clarity.
Selected bond lengths (A) and angles (1), for 2: Rh1–P1 2.1592(7),
Rh1–P2 2.2882(8), Rh1–P3 2.2980(7), Rh1–P4 2.3012(8), Rh1–Cl1
2.4402(7), P1–C1 1.803(3); P2–N2 1.738(2); P1–Rh1–Cl1 177.60(3),
P1–Rh1–P3 86.16(3), P2–Rh1–P3 121.79(3), P2–Rh1–P4 120.74(3),
P3–Rh1–P4 115.95(3), P3–Rh1–Cl1 94.30(2). For [2]PF6: Rh1–P1
2.2040(5), Rh1–P2 2.3110(5), Rh1–P3 2.3482(6), Rh1–P4 2.3094(6),
Rh1–Cl1 2.3949(5), P1–C1 1.801(2); P2–N2 1.7128(17); P1–Rh1–Cl1
177.21(2), P1–Rh1–P3 83.83(2), P2–Rh1–P4 107.93(2), P2–Rh1–P3
105.33(2), P3–Rh1–P4 144.21(2), P3–Rh1–Cl1 94.94(2).
Fig. 2 Experimental (black) and simulated (red) X-band EPR spectrum
of [2]PF6. Experimental details: 20 K, frequency 9.3794 GHz, microwave
power 0.2 mW, modulation amplitude 4 Gauss. TBAH (B0.1 M) was
added to obtain a better glass. DFT calculations and data in ESI.w
three P atoms; large coupling with P2 and smaller couplings with
P3 and P4.w The fourth P-atom (pivotal P1) shows no resolved
coupling. A distorted geometry in-between a trigonal bipyramid
(tbp) and a square pyramid (sqpy) is revealed. This is remark-
able, because 5-coordinate RhII-complexes generally adopt a
square pyramidal geometry (Jahn–Teller effect).17
embrace the Rh metal center from the top, whereas the other
side is protected by a strongly coordinating chloride ligand.
The chloride co-ligand was stable in the presence of Ag-salts,
as no abstraction to form AgCl was observed. Instead, an
instantaneous colour change from burgundy-red to dark-purple
with formation of a Ag0-mirror indicated redox-based reactivity
resulting in the formation of the corresponding (PP3)RhIICl
complex [2]PF6. Disproportionation of (PP3)RhIICl is likely
hindered by the strained (ligand) geometries of the hypothetical
(trigonal pyramidal) (PP3)RhI and (constrained octahedral)
(PP3)RhIIICl products. In line with this, cyclic voltammetry
for complex 2 showed a fully reversible RhI/RhII oxidation
wave at ꢀ0.4 V, referenced vs. Fc/Fc+.w The relatively low
potential indicates facile oxidation, likely due to the coordina-
tion of four strongly electron-donating phosphines. RhII
complex [2]PF6 was independently prepared by one-electron
DFT calculations show that about 51% of the spin density
of 2+ resides on Rh and about 24% on the ‘‘apical’’
P2-donor.w The species is thus best described as a strongly
covalent radical. It has substantial metalloradical character,
but with a strong contribution of the square pyramidal ‘axial’
P2 donor. Despite the long-term stability of solid samples of
metalloradical complex 2+ and the small accessible molecular
surface, we wondered if this species would exhibit reactivity
towards molecular hydrogen. When [2]PF6 was exposed to
molecular hydrogen for 12 hours (5 bar, CD2Cl2), RhIII–H
species 3 was formed quantitatively. In the 1H NMR spectrum,
oxidation of
2
with ferrocenium hexafluorophosphate.
a
complex doublet-of-doublet-of-doublet-of-triplets was
2
Solution-state RT magnetization measurements by Evans’
method are consistent with the formulation of complex [2]PF6
as a radical species, with a meff value of 2.29(1) mB.15 This value
is somewhat larger than the spin-only value (1.73 mB), indicating
spin–orbit influences.16 Species [2]PF6 is a bench-stable solid
and no appreciable decomposition is observed in common
solvents, indicating highly effective shielding of the RhII center
by the ligand sphere. The proposed formulation of [2]PF6 was
confirmed by X-ray crystallography (Fig. 1, right).z
observed at d ꢀ6.52 ppm, with a large JPꢀH coupling of
174.8 Hz (trans-disposition with phosphorus).w This strongly
suggests the presence of a hydride ligand coupled to three
inequivalent P-atoms. In the corresponding 31P NMR
spectrum an AM2X spin pattern could be discerned.w
This led us to assign this species as the hydrido complex 3,
formulated as [Rh(Cl)(H)(1)]PF6 (Scheme 2), with the hydrido
ligand trans to one of the side-arm P-atoms and the chloro
ligand trans to the pivotal P donor. Independent synthesis of
this complex (as the analogous triflate salt) was carried out by
protonation of 2 with HOTf. Furthermore, when D2 instead
of H2 was reacted with [2]PF6, deuterido complex 3-D,
[Rh(Cl)(D)(1)]PF6, was obtained, unambiguously proving that
the hydride stems from molecular hydrogen. The NMR
spectrum of 3-D shows only traces of the hydride caused by
the presence of H2 and HD in the D2 gas.w
The molecular structure features a distorted trigonal bi-
pyramidal geometry around the Rh-center, with a t-value of
0.55. Clearly, the geometric constraints in ligand framework 1
dictate and govern the unusual structural characteristics of the
corresponding metalloradical complex. The RhII-ion is largely
shielded and protected by the rigid ligand scaffold.w
The X-band EPR spectrum of [2]PF6 is shown in Fig. 2.
Identical data were obtained for stored (one-month old) and
in situ generated samples, indicative of a highly stable radical
species. Simulation of the experimental EPR spectrum of
[2]PF6 at 20 K reveals a rhombic g-tensor with moderate
g-anisotropy and resolved superhyperfine couplings with mainly
The formation of 3 is proposed to proceed via an outer-
sphere redox process, in which the RhII complex is reduced to
RhI complex 2 by dihydrogen (H2 - 2H+ + 2e), with
subsequent or concerted protonation of RhI to RhIII–H. The
mechanism of net hydrogen transfer from H2 to the sterically
ꢁc
This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 1232–1234 | 1233

View Article Online
(Rint = 0.062), R = 0.0386, wR2 = 0.0767 (all data); CCDC 735997.
[2]PF6: C63H51ClN3P4RhꢃF6Pꢃ3(CH2Cl2), FW = 1512.06, 0.26 ꢄ 0.31 ꢄ
ꢀ
0.32 mm, T = 110 K, triclinic, P1, a = 13.2110(6)A, b = 15.3608(8)A,
c = 17.6791(10)A, a = 93.963(2)1, b = 110.874(2)1, g = 101.654(3)1,
=
V = 3243.8(3)A3, Z = 2, 14 892 reflections, 83 759 unique (Rint
0.024), R = 0.0309, wR2 = 0.0759 (all data); CCDC 735998.
Scheme 2 Activation of H2 by complex [2]PF6, resulting in the
formation of RhIII complex 3 via intermediacy of species 2.
1 R. H. Crabtree, The Organometallic Chemistry of the
Transition Metals, Wiley-Interscience, New York, 3rd edn, 2001,
pp. 152–155.
2 For an overview of H2 activation by frustrated Lewis pairs, see:
D. W. Stephan and G. Erker, Angew. Chem., Int. Ed., 2010, 49, 46.
3 R. Noyori, M. Kitamura and T. Ohkuma, Proc. Natl. Acad. Sci.
U. S. A., 2004, 101, 5356; R. J. Hamilton and S. H. Bergens, J. Am.
Chem. Soc., 2008, 130, 11979.
protected metalloradical species should differ substantially
from that proposed by Rauchfuss for H2 oxidation by an
Ir-based ligand-centered radical species.18 The redox-potential
of [2]PF6 vs. NHE is 0.28 V, which implies that the overall
oxidation of H2 to 2H+ is thermodynamically allowed even
without subsequent metal protonation. The process might
thus involve the concerted trimolecular two-electron oxidation
of H2 simultaneously by two complexes 2+ (which may
proceed over a rather long distance between the species so
that steric hindrance plays a minor role). However, we cannot
completely exclude other mechanisms, such as two consecutive
4 A. Wu, J. Masland, R. D. Swartz, W. Kaminsky and J. M. Mayer,
Inorg. Chem., 2007, 46, 11190, and references therein.
5 J. I. van der Vlugt, T. Koblenz, J. Wassenaar and J. N. H. Reek,
in Molecular Encapsulation: Reactions in Constrained Systems,
ed. J.-L. Mieusset and U. H. Brinker, Wiley-VCH, Weinheim,
2009, in press; T. Koblenz, J. Wassenaar and J. N. H. Reek, Chem.
Soc. Rev., 2008, 37, 247; M. Kuil, T. Soltner, P. W. N. M. van
Leeuwen and J. N. H. Reek, J. Am. Chem. Soc., 2006, 128, 11344;
K. Drauz and H. Waldmann, Enzyme Catalysis in Organic Synthesis:
A Comprehensive Handbook, Wiley-VCH, Weinheim, 2nd edn, 2002.
6 S. Trofimenko, Chem. Rev., 1993, 93, 943; C. Moberg, Angew.
Chem., Int. Ed., 1998, 37, 248; I. Kuzu, I. Krummenacher,
J. Meyer, F. Armbruster and F. Breher, Dalton Trans., 2008, 5836.
7 L. Turculet, J. D. Feldman and T. D. Tilley, Organometallics,
2004, 23, 2488; C. Foltz, M. Enders, S. Bellemin-Laponnaz,
H. Wadepohl and L. H. Gade, Chem.–Eur. J., 2007, 13, 5994;
C. C. Lu, C. T. Saouma, M. W. Day and J. C. Peters, J. Am. Chem.
Soc., 2007, 129, 4; M. Sircoglou, S. Bontemps, G. Bouhadir,
N. Saffon, K. Miqueu, W. X. Gu, M. Mercy, C. H. Chen,
B. M. Foxman, L. Maron, O. V. Ozerov and D. Bourissou,
J. Am. Chem. Soc., 2008, 130, 16729.
bimolecular single-electron transfer steps. The formation of
+
ꢂ
high-energy species (H2 , or Hꢂ after immediate proton
transfer to RhI) in the latter case speaks against such a
stepwise process, but this could be possible if tunneling is
involved. Direct homolytic splitting of H2, as observed for
RhII–porphyrin systems,19 also seems less likely, because this
reaction should suffer from extensive steric shielding provided
by the ligand protecting the RhII center (again, unless tunnel-
ing is involved). We are currently investigating possibilities to
apply the observed activation of dihydrogen by RhII metallo-
radicals based on these novel, modular tripodal phosphorus
ligands for hydrogen atom-transfer (HAT) reactions.
8 M. J. Baker and P. G. Pringle, J. Chem. Soc., Chem. Commun., 1993,
314; R. B. King and R. N. Kapoor, J. Am. Chem. Soc., 1969, 91, 5191;
J. W. Dawson and L. M. Venanzi, J. Am. Chem. Soc., 1968, 90, 7229.
9 M. Ciclosi, J. Lloret, F. Estevan, P. Lahuerta, M. Sanau
J. Perez-Prieto, Angew. Chem., Int. Ed., 2006, 45, 6741; M. Ciclosi,
F. Estevan, P. Lahuerta, V. Passarelli, J. Perez-Prieto and
M. Sanau, Dalton Trans., 2009, 2290.
´ and
Summarizing, we have reported the straightforward syn-
thesis of a mononuclear RhI complex 2 and the corresponding
stable RhII species 2+, based on a novel tripodal, tetradentate
phosphorus ligand 1. The isolated metalloradical complex 2+
is a rare RhIICl compound with an unusual intermediate
coordination geometry. Notwithstanding its bench-stable
character, 2+ reacts with molecular hydrogen to quantitatively
form the corresponding RhIII hydrochloride complex 3. This is
the first example of dihydrogen activation by a metal-centered
RhII metalloradical, which may proceed through complete
oxidation of molecular hydrogen followed by protonation of
the resulting RhI species. This reaction can be considered as a
starting point of HAT reactions, using molecular hydrogen as
hydrogen atom donor. Further studies aimed at the reactivity
of these unusual RhII metalloradicals are ongoing, including
studies derived from the related cationic RhI species. More-
over, we are investigating the use of enantiomerically pure
C3-symmetric derivatives of 1 in asymmetric transformations.
This research was supported by the NRSC-C and NWO-CW.
We thank Dr D. G. H. Hetterscheid, J. W. H. Peeters and
T. Mahabiersing for technical assistance.
´
´
´
10 J. Wassenaar and J. N. H. Reek, Dalton Trans., 2007, 3750;
J. Wassenaar, M. Kuil and J. N. H. Reek, Adv. Synth. Catal.,
2008, 350, 1610; J. Wassenaar, S. van Zutphen, G. Mora, P. Le
Floch, M. Siegler, A. L. Spek and J. N. H. Reek, Organometallics,
2009, 28, 2724; J. Wassenaar and J. N. H. Reek, J. Org. Chem.,
2009, 74, 8403.
11 A. R. Katritzky, K. Akutagawa and R. A. Jones, Synth. Commun.,
1988, 18, 1151; J. O. Yu, C. S. Browning and D. H. Farrar, Chem.
Commun., 2008, 1020.
12 B. de Bruin, P. H. M. Budzelaar and A. W. Gal, Angew. Chem.,
Int. Ed., 2004, 43, 4142; H. R. L. Davies and J. R. Manning,
Nature, 2008, 451, 417; H. R. L. Davies and S. J. Hedley, Chem.
Soc. Rev., 2007, 36, 1109; C. A. Merlic and A. L. Zechman,
Synthesis, 2003, 1137.
13 W. I. Dzik, J. M. M. Smits, J. N. H. Reek and B. de Bruin,
Organometallics, 2009, 28, 1631; W. I. Dzik, J. N. H. Reek and
B. de Bruin, Chem.–Eur. J., 2008, 14, 7594.
14 A. W. Addison, T. N. Rao, J. Reedijk, J. van Rijn and
G. C. Verschoor, J. Chem. Soc., Dalton Trans., 1984, 1349.
15 D. F. Evans, J. Chem. Soc., 1959, 2003.
16 The meff value is similar to that of a PNP-based mononuclear RhII
species, see: M. Feller, E. Ben-Ari, T. Gupta, L. J. W. Shimon,
G. Leitus, Y. Diskin-Posner, L. Weiner and D. Milstein, Inorg.
Chem., 2007, 46, 10479.
17 D. G. H. Hetterscheid, A. J. J. Koekoek, H. Grutzmacher and
¨
Notes and references
B. de Bruin, Prog. Inorg. Chem., 2007, 55, 247.
18 M. R. Ringenberg, S. Latha Kokatam, Z. M. Heiden and
T. B. Rauchfuss, J. Am. Chem. Soc., 2008, 130, 788.
19 W. Cui and B. B. Wayland, J. Am. Chem. Soc., 2004, 126, 8266,
and references therein.
z Crystal data. 2: C63H51ClN3P4RhꢃCH2Cl2, FW
= 1197.23,
, a = 11.8086(5),
0.10 ꢄ 0.14 ꢄ 0.27 mm, T = 110 K, triclinic, P1
ꢀ
b = 14.6492(4), c = 17.0348(5) A, a = 83.286(2)1, b = 87.505(2)1, g =
71.163(2)1, V = 2769.79(17)A3, Z = 2, 12 704 reflections, 62 878 unique
ꢁc
This journal is The Royal Society of Chemistry 2010
1234 | Chem. Commun., 2010, 46, 1232–1234