Directing protons to the dioxygen ligand of a Ruthenium(II) complex with pendent amines in the second coordination sphere

Article abstract of DOI:10.1002/anie.201105266

Proton relay: A side-on Ru-O₂ complex with pendent amines in the ligand backbone has been synthesized to model proton delivery in O₂ reduction (see scheme and structure; red O, purple Ru, blue N, yellow P). Protonation occurs at the amine near the O₂ ligand, forming a hydrogen bond between the ammonium ion and the O₂ ligand, leading to a small increase in O-O bond length.

Full text of DOI:10.1002/anie.201105266

Communications
DOI: 10.1002/anie.201105266
Dioxygen Activation
Directing Protons to the Dioxygen Ligand of a Ruthenium(II) Complex
with Pendent Amines in the Second Coordination Sphere**
Tristan A. Tronic, Mary Rakowski DuBois, Werner Kaminsky, Michael K. Coggins,
Tianbiao Liu, and James M. Mayer*
The activation and reduction of dioxygen (O2) by transition-
metal centers are key to a variety of biochemical and
industrial processes.[1,2] Efficient reduction of dioxygen to
water is also important in the operation of fuel cells.[3] These
processes are typically proton-coupled electron transfer
(PCET) reactions, requiring the coordinated movement of
multiple protons and electrons.[4] In biological systems, it is
known that initial dioxygen bonding is facilitated by hydrogen
bonding and proton delivery.[5] A few recent synthetic
transition-metal catalysts for oxygen reduction have utilized
directed proton delivery from the metalꢀs second coordina-
tion sphere.[6] Nocera and co-workers have pioneered using a
single pendent carboxylic acid group to improve O2 reduction
catalysts by cobalt porphyrin[6a] or corrole.[6b] Yang et al.
found a stronger acceleration on including non-coordinating
amines as “proton relays” in nickel(II) bis-diphosphine O2
reduction catalysis.[6c] The origins of the catalytic accelera-
Figure 1. Synthesis of [Cp*Ru(P2N2)(O2)][X] and ORTEP diagram of the
tions are not well established, however, because there are few
well characterized catalytic intermediates that show the
interaction of a proton relay with a dioxygen intermediate.
Reported here are ruthenium–O2 complexes with protonated
and deprotonated amine proton relays, showing that the relay
positions protons to form a hydrogen bond with the bound O2.
[Cp*Ru(phosphine)2]+ complexes form stable h2-O2 spe-
cies with a variety of phosphine ligands (Cp* = h5-C5Me5).[7]
This study used a 1,5-diaza-3,7-diphosphacyclooctane ligand
with tert-butyl substituents on the phosphines and benzyl
groups on the amines (P2N2),[8] similar to those used by Yang
et al.[6c] Adding this ligand to [Cp*RuCl]4 yielded [Cp*Ru-
cation in [Cp*Ru(P2N2)(O2)][BPh4]. Thermal ellipsoids are shown at
50% probability. For clarity, hydrogen atoms have been omitted.
and 31P{1H} NMR spectra confirm the structure shown (see
Supporting Information). The chloride complex was con-
verted to the h2-dioxygen compounds [Cp*Ru(P2N2)(O2)][X]
ꢀ
(X = PF6ꢀ, BPh4 ) by chloride abstraction with TlPF6 or
NaBPh4 in air-saturated acetone or ethanol, respectively. The
1H and 31P{1H} NMR spectra of the PF6 and BPh4 salts in
CD2Cl2 are identical, except for those peaks assigned to the
anion, and are representative of this class of compounds.
The X-ray crystal structure of [Cp*Ru(P2N2)(O2)][BPh4]
ꢀ
ꢀ
1
(P2N2)Cl] (Figure 1). An X-ray crystal structure and the H
(Figure 1) confirms the assignment. The O2 ligand is bound
2
ꢀ
essentially symmetrically h to the Ru center, with Ru O
[*] T. A. Tronic, Dr. W. Kaminsky, M. K. Coggins, Prof. J. M. Mayer
Department of Chemistry, University of Washington
Box 351700, Seattle, WA 98195 (USA)
ꢀ
bond lengths of 2.019(1) and 2.023(1) ꢁ. The O O bond
length of 1.401(1) ꢁ is within the range of ca. 1.36–1.40 ꢁ
observed for other [Cp*Ru(phosphine)2(O2)]+ complexes,[7]
E-mail: mayer@chem.washington.edu
ꢀ
and is in between the O O distances in superoxide (KO2,
Dr. M. Rakowski DuBois, Dr. T. Liu
Chemical and Materials Sciences Division
Pacific Northwest National Lab, Richland, WA 99352 (USA)
1.28 ꢁ)[9] and hydrogen peroxide (1.46 ꢁ).[10] IR spectra show
nO-O = 935 cmꢀ1 (n
¼880 cmꢀ1), consistent with an h2-O2
18Oꢀ18O
[11]
ꢀ
complex with this O O bond length. This complex thus
could be formally described as a RuIV–peroxo complex.
[Cp*Ru(P2N2)(O2)]+ and [Cp*Ru(P2N2)Cl] have similar
1H and 31P{1H} NMR spectra, suggesting that the changes in
electronic structure on replacing Clꢀ by O2 are not very
extensive. The Cp* and tert-butyl resonances are slightly more
downfield in the O2 complex, by ca. 0.1 ppm, and the 31P
resonance is 10.6 ppm upfield. The [Cp*Ru(P2N2)(O2)]+ salts
are stable under vacuum and CH2Cl2 solutions are stable to
sparging with N2 or freeze–pump–thawing, indicating that the
binding of O2 is not reversible. The O2 and chloride structures
[**] This work is supported as part of the Center for Molecular
Electrocatalysis, an Energy Frontier Research Center funded by the
U.S. Department of Energy, Office of Science, Office of Basic Energy
Sciences, under FWP 56073. We thank Dr. Rajan Paranji for NMR
assistance and Dr. Jenny Yang for 15N-labeled P2N2 ligand. M.R.D.
and T.L. were supported by the Division of Chemical Sciences,
Biosciences and Geosciences, Office of Basic Energy Sciences,
Office of Science of the U.S. Department of Energy. Pacific
Northwest National Lab is operated by Battelle for the U.S.
Department of Energy.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201105266.
10936
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 10936 –10939

both have the P2N2 ligand bound to the metal center only
through the phosphorus atoms. As found in P2N2 complexes
with other metals, the ligand structure deters the amines from
chelating the metal center, as this would form strained four-
membered rings.[12] The uncoordinated amines are therefore
potentially able to act as second-coordination sphere proton
relays, although the chair conformation of the proximal side
of the P2N2 ligand orients N1 away from the O2 ligand. The
other amine nitrogen, N2, is on the opposite side of the
ruthenium center and cannot interact with the O2 ligand.
Treatment of [Cp*Ru(P2N2)Cl] with excess LiPF6 and
tosylic acid in aerobic methanol yielded yellow-brown
crystals. NMR spectra and an X-ray crystal structure
showed these to be the protonated derivative, [Cp*Ru-
(P2N2H)(O2)][PF6]2 (Figure 2). In the absence of tosylic acid
Structures A and B, and the NMR spectra, show that this
compound has a protonated amine group. There are two PF6
ꢀ
ions per Ru center, indicating that the metal complex has a
2 + charge consistent with a protonated species. The two
structures have somewhat different metrical parameters,
apparently because the methanol or water molecules of
crystallization are involved in different hydrogen bonding
networks with the PF6ꢀ ions. In neither structure is the solvent
interacting with the protonated or unprotonated amines, or
with the O2 ligand. In both structures, the conformation of the
proximal amine is inverted relative to the structures of
[Cp*Ru(P2N2)Cl] and [Cp*Ru(P2N2)(O2)][BPh4], bringing
N1 into close proximity with the O2 ligand. The shorter of
the N···O distances in each structure, 2.846(2) ꢁ in A and
2.746(2) ꢁ in B (Table 1), are indicative of substantial hydro-
gen-bonding interactions.[13] The protonated nitrogen is some-
what farther from the other oxygen of the O2 ligand, 2.934(2),
2.905(2) ꢁ in A and B, respectively (Figure 2 and Supporting
Information, Figure S10). In each of these structures, the h2
ꢀ
binding mode of the O2 ligand is retained. The O O bond is
essentially unchanged in form A (dO1-O2 = 1.405(2) ꢁ, within
error of [Cp*Ru(P2N2)(O2)][BPh4]), but is lengthened slightly
Figure 2. ORTEPs of two different structures of [Cp*Ru(P2N2H)(O2)]-
[PF6]2: A (·1= H2O), left, and B (·1= MeOH), right, with thermal
ellipsoids shown at 50% probability. For clarity, the solvent molecules,
counterions, and hydrogen atoms except for the hydrogen-bonding H1,
have been omitted.
4
2
ꢀ
in form B to 1.4161(17) ꢁ (D = 0.152(20) ꢁ). The O O bond
in structure B is, to our knowledge, longer than in any
previously reported [Cp*Ru(phosphine)2O2]+ complex.[7] It
ꢀ
seems likely that the longer O O bond in structure B is
related to the shorter N···O hydrogen bond in that structure.
Though the structural changes are slight, the perturbation of
the O2 ligand on protonation is also indicated by the 30 cmꢀ1
the same compound is formed (determined by NMR spec-
troscopy), but in much lower yields. Crystals were obtained
from the tosylic acid reaction which contained one water of
crystallization for every four cations (structure A); similar
crystals from a reaction without added acid were the 0.5
methanol solvate (structure B; Figure 2, Table 1). [Cp*Ru-
shift in nO-O to 905 cmꢀ1 (n
¼840) cmꢀ1.
18Oꢀ18O
1H NMR spectra of [Cp*Ru(P2N2H)(O2)][PF6]2 in CD2Cl2
show sharp resonances that are consistent with the crystal
structures. Half of the benzylic CH2 and PCH2N resonances
are shifted downfield by ca. 1 ppm relative to [Cp*Ru-
(P2N2)(O2)][PF6], consistent with protonation of one of the
two amine nitrogens. Additionally, the downfield benzylic
CH2 is split into a doublet (JHH = 3 Hz) due to coupling with
the NH, while the upfield benzyl resonance is a singlet (as in
the spectra of the unprotonated complexes). The 31P{1H}
spectrum shows a broad resonance, rather than the sharp
singlet observed for [Cp*Ru(P2N2)(O2)][PF6]. Lowering the
temperature to ꢀ608C causes decoalescence into two signals.
The barrier to this process at ꢀ208C is ca. 10 kcalmolꢀ1 (see
Supporting Information). This process is not exchange
between protonated and unprotonated species, as NMR
spectra of [Cp*Ru(P2N2)(O2)][PF6] with less than 1 equiv of
dimethoxypyridine·HPF6 show both this broad resonance and
the sharp resonance of [Cp*Ru(P2N2)(O2)][PF6]. These
spectra may reflect a fluxional process in which the proton-
Table 1: Relevant distances [ꢀ] from X-ray crystal structures of [Cp*Ru-
(P2N2)(O2)][BPh4] and [Cp*Ru(P2N2H)(O2)][PF6]2 forms A and B.
Bond
[Cp*Ru(P2N2)(O2)][BPh4]
[Cp*Ru(P2N2H)(O2)][PF6]2
Form A
Form B
ꢀ
Ru O1
ꢀ
Ru O2
2.0229(7)
2.0190(7)
1.4009(11)
3.959(1)
2.0393(14)
2.0362(14)
1.405(2)
2.846(2)
2.934(2)
2.0260(11)
2.0350(11)
1.4161(17)
2.905(2)
ꢀ
O1 O2
N1···O1
N1···O2
3.978(1)
2.746(2)
(P2N2H)(O2)]2+ can also be generated in situ in CD2Cl2 and
observed by NMR spectroscopy, using ca. 1 equiv of 2,6-
dimethoxypyridine·HPF6 [Eq. (1)].
Angew. Chem. Int. Ed. 2011, 50, 10936 –10939
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www.angewandte.org 10937

Communications
ated amine exchanges between hydrogen bonding to O1 and
O2, as suggested by the asymmetric structures in the solid
state.
The location of the proton on the amine has been further
confirmed by 1H-15N NMR spectra in CH2Cl2 of the 15N-
labeled compound. The 1-D heteronuclear single quantum
coherence (HSQC) spectrum of [Cp*Ru(P215N2H)(O2)][PF6]2
has a single peak at 7.6 ppm and the J1Hꢀ15N of 76 Hz shows the
1
ꢀ
presence of an N H bond. The downfield H NMR chemical
shift is suggestive of a hydrogen-bonding environment.[13] The
chemical shift and coupling constant are also consistent with
the proton being located on one benzylamine rather than
bridged between two benzylamines. Such benzylamine-
bridged species have previously been determined to have
Figure 3. Cyclic voltammograms of [Cp*Ru(P2N2)(O2)][PF6] (1.2 mm)
(gray, dashed line) and [Cp*Ru(P2N2)(O2)][PF6] with HOTf added
(black, solid line) in CH2Cl2, 0.1m [nBu4N][PF6], scan rate 0.1 Vsꢀ1
.
chemical shifts of ca. 15 ppm and J1Hꢀ ꢁ30 Hz.[14]
15N
Density functional theory (DFT) calculations were per-
formed to further support the proposed protonated structure.
Gas-phase calculations (BP86/6-31G**(SDD)) were per-
formed with Gaussian09.[15] Optimizations of [Cp*Ru-
(P2N2)(O2)]+ and [Cp*Ru(P2N2H)(O2)]2+ gave structures
very similar to those found in the crystal structures. In the
latter, the protonated amine nitrogen N1 is calculated to form
a short hydrogen bond to O1 (dN1-O1 = 2.65 ꢁ), with a
identical peak is seen in the CV of isolated [Cp*Ru-
(P2N2H)(O2)][PF6]2 (Figure S15). Ongoing work is examining
the products of these reductions.
In summary, a ruthenium h2-O2 complex has been
synthesized with a pendent amine in the second coordination
sphere of the metal. The crystal structures, NMR and IR data
indicate that the pendent amine can bind a proton, and that
the resulting ammonium ion forms a hydrogen bond with the
considerably longer distance to the other oxygen (dN1-O2
=
ꢀ
3.00 ꢁ), as in the crystal structures. The calculated dO1-O2
lengthens upon protonation, by 0.015 ꢁ, consistent with the
change observed crystallographically. The use of other func-
tionals or larger basis sets gave similar geometries, and
consistently showed a change in dO1-O2 of ca. 0.015 ꢁ (see
O2 ligand, slightly lengthening the O O bond. Experiments
are underway to test how protonation affects the reactivity of
this complex and how the relays affect its ability to act as a
catalyst for O2 reduction.
Received: July 27, 2011
Revised: August 30, 2011
Published online: September 26, 2011
Supporting Information), suggesting that the change in dO1-O2
,
though relatively small, is due to the presence of the proton.
The calculated n
is shifted from 976 cmꢀ1 to 951 cmꢀ1
16Oꢀ16O
with protonation, consistent with the ca. 30 cmꢀ1 shift
observed experimentally.
Keywords: dioxygen ligands · hydrogen bonding ·
.
O-O activation · protonation · proton-coupled electron transfer
Another minimum was located with the proton bound to
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However, the energy of this species is calculated to be
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N2-protonated structure, and in the unprotonated O2 struc-
ture, the N1···O distances are both long, ca. 3.1 ꢁ. Further-
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