Reactivity of rhodium and iridium peroxido complexes towards hydrogen in the presence of B(C6F5)3 or [H(OEt2)2][B{3,5-(CF3)2C6H3}4]

Article abstract of DOI:10.1039/c8dt03853h

The peroxido complexes trans-[M(4-C₅F₄N)(O₂)(CNtBu)(PR₃)₂] (1: M = Rh, R = Et; 2a: M = Ir, R = iPr) can be used in the metal-mediated hydrogenation of O₂. The reaction of trans-[Rh(4-C

Full text of DOI:10.1039/c8dt03853h

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Reactivity of rhodium and iridium peroxido
complexes towards hydrogen in the presence of
B(C6F5)3 or [H(OEt2)2][B{3,5-(CF3)2C6H3}4]†
Cite this: Dalton Trans., 2018, 47,
16299
Hanna Baumgarth,‡ Gregor Meier,‡ Cortney N. von Hahmann and
Thomas Braun
*
The peroxido complexes trans-[M(4-C5F4N)(O2)(CNtBu)(PR3)2] (1: M = Rh, R = Et; 2a: M = Ir, R = iPr) can
be used in the metal-mediated hydrogenation of O2. The reaction of trans-[Rh(4-C5F4N)(O2)(CNtBu)
(PEt3)2] (1) with B(C6F5)3 and H2 gave trans-[Rh(4-C5F4N)(CNtBu)(PEt3)2] (3), OPEt3 and (H2O)·B(C6F5)3,
whereas treatment of [H(OEt2)2][B{3,5-(CF3)2C6H3}4] with 1 in the presence of H2 yielded trans-
[Rh(4-C5F4N)(CNtBu)(PEt3)2] (3) and H2O2. The reactivity of 2a towards B(C6F5)3 and BClCy2 was also
studied and an intermediate was detected which is assigned to be trans-[Ir(4-C5F4N)(Cl)(OOBCy2)(CNtBu)
(PiPr3)2] (4a).
Received 24th September 2018,
Accepted 26th October 2018
DOI: 10.1039/c8dt03853h
rsc.li/dalton
bridged binuclear rhodium complex [Rh(Cp*)(µ-NHTs)]2.5
Mechanistic steps might include dioxygen activation, followed
Introduction
The transition metal-mediated hydrogenation of dioxygen is by protonation to form hydroperoxido complexes.3–5,7
an interesting subject in oxygenation chemistry.1 The two However, the water formation step and the fate of the second
feasible products are water and hydrogen peroxide. The oxygen atom of the O2 molecule is unknown. Moreover the
reduction of dioxygen to H2O is an important process, for iridium(III) pincer peroxido complex [Ir{C6H3(CH2PtBu2)2}(O2)]
instance in fuel cells.1 H2O2 is an economically crucial indus- reacts with H2 to yield H2O and [Ir{C6H3(CH2PtBu2)2}(H)4].6
trial product for the manufacture of a wide range of everyday
A variety of catalytic investigations have been carried out to
items.2 The development of a transition metal-mediated access H2O2 from dioxygen and appropriate reducing agents.
selective process, as an alternative to the energy intensive For example, alcohols can be oxidized with O2 using transition
anthraquinone process to access water or hydrogen peroxide metal complexes,8 mainly palladium and copper,9 while H2O2
from dihydrogen and dioxygen, can be considered a funda- is produced as an additional product. Palladium-catalyzed trans-
mental challenge.
formations of mixtures of CO, O2, and H2O provide CO2 and
Late transition metal complexes have been reported to H2O2 in the presence of an excess of a Brønsted acid.10 At
reduce molecular oxygen to water, but only a couple of conver- rhodium, we found in model reactions that the peroxido
sions proceed with dihydrogen as reductant.3–6 The reaction of complex trans-[Rh(4-C5F4N)(O2)(CNtBu)(PEt3)2] (1) reacts with
the amido amine hydrido complex [Ir(Cp*)(H){κ2-(N,N)-rac- dihydrogen sources to yield hydrogen peroxide and trans-
NTsCHPhCHPhNH2}] (Cp* = η5-C5Me5; Ts = SO2C6H4CH3) with [Rh(4-C5F4N)(CNtBu)(PEt3)2] (3).11 The metal-bound dioxygen
dioxygen led to the formation of water and the bisamido can be reduced in a catalytic experiment where H2O2 was gen-
complex [Ir(Cp*){κ2-(N,N)-rac-NTsCHPhCHPhNH}].4a,b In the erated at ambient temperature and atmospheric pressure from
presence of dihydrogen and catalytic amounts of the Brønsted dioxygen and ammonium formate.11 A key step in such trans-
acidic borane additive [H(OEt2)2][B{3,5-(CF3)2C6H3}4], the bis- formations is the protonation of a peroxido entity to form
amido complex is converted back to the amido amine hydrido hydroperoxido complexes like trans-[Rh(4-C5F4N){OC(O)H}-
complex to close a putative cycle. A comparable reaction (OOH)(CNtBu)(PEt3)2]11 which subsequently react further to
sequence can be found for the reaction of the bisamido give H2O2. Similarly, the iridium peroxido complex trans-
[Ir(4-C5F4N)(O2)(CNtBu)(PiPr3)2] (2a) releases hydrogen per-
oxide when reacted with various Brønsted acids like HCl,
CF3COOH or HF.12 Note also that trans-[Rh(4-C5F4N)(O2)-
Humboldt-Universität zu Berlin, Department of Chemistry, Brook-Taylor-Straße 2,
D-12489 Berlin, Germany. E-mail: thomas.braun@chemie.hu-berlin.de
(CNtBu)(PEt3)2] (1) reacts with HBpin (HBpin = 4,4,5,5-tetra-
†Electronic supplementary information (ESI) available: Experimental section
methyl-1,3,2-dioxaborolane, pinacolborane) to yield trans-
[Rh(H)(OBpin)(4-C5F4N)(CNtBu)(PEt3)2] and trans-[Rh(4-C5F4N)-
and analytical data. See DOI: 10.1039/c8dt03853h
‡These authors contributed equally to this work.
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(CNtBu)(PEt3)2] (3) with presumably trans-[Rh(H)(OOBpin)-
(4-C5F4N)(CNtBu)(PEt3)2] as an intermediate.13
In this paper, we describe that the dioxygen moiety of the
peroxido complexes trans-[Rh(4-C5F4N)(O2)(CNtBu)(PEt3)2] (1)
and trans-[Ir(4-C5F4N)(O2)(CNtBu)(PiPr3)2] (2a),12,14 which are
generated from O2 and are inert towards dihydrogen, can be
activated by boranes. The peroxido complexes can be hydro-
genated with H2 in the presence of B(C6F5)3. In contrast,
trans-[Rh(4-C5F4N)(O2)(CNtBu)(PEt3)2] (1) forms hydrogen per-
oxide with dihdyrogen in presence of the Brønsted acid
[H(OEt2)2][B{3,5-(CF3)2C6H3}4].15
Scheme 2 Possible mechanism for the reaction of complex 1 with H2
and B(C6F5)3.
Results and discussion
The reaction of trans-[Rh(4-C5F4N)(O2)(CNtBu)(PEt3)2] (1) with
the borane B(C6F5)3 and dihydrogen afforded the rhodium(I)
complex trans-[Rh(4-C5F4N)(CNtBu)(PEt3)2] (3), OPEt3 and the
water adduct (H2O)·B(C6F5)3. There is no indication for the for-
mation of H2O2. In addition to the signal for 3, the 31P{1H}
NMR spectrum of the reaction mixture shows a doublet signal
for a minor compound (ratio 7 : 1) at δ = 20.2 ppm with a coup-
ling constant of 1J(Rh,P) = 134.0 Hz. We were not able to
isolate or identify the unknown rhodium(I) complex. However,
the formation of this compound can be suppressed in the
presence of one equivalent of triethylphosphine (Scheme 1).
The latter forms an adduct with the Lewis acidic borane (see
ESI†).16 After 2d the conversion of 1 was about 80% and the
ratio of 3 to phosphine oxide to (H2O)·B(C6F5)3 was 3.4 : 6.7 : 1.
The water adduct (H2O)·B(C6F5)3 was identified by a compari-
son of the 1H and 19F NMR data with an authentic sample and
shows a broad signal for (H2O)·B(C6F5)3 at δ = 4.66 ppm in the
1H NMR spectrum.17 This signal cannot be detected when D2
is used as the reducing agent.
transfer of an oxygen atom of a putative RhOOB-moiety to PEt3
may lead to the formation of OPEt3 and the borato complex
trans-[Rh(4-C5F4N){OB(C6F5)3}(H2)(CNtBu)(PEt3)2]. It is known
that complexes containing a RhOOR group can oxygenate
phosphines.11,21 As mentioned above, the intramolecular oxy-
genation of the phosphine ligand in the perborate complex
trans-[Rh(4-C5F4N)(H){OOBpin}(CNtBu)(PEt3)2] at low tempera-
ture has been described.13 A stepwise transfer of hydrogen
atoms22 to the {Rh-OB(C6F5)3} entity may then lead to trans-
[Rh(4-C5F4N)(H){HOB(C6F5)3}(CNtBu)(PEt3)2] and subsequently
to 3 and the water adduct (H2O)·B(C6F5)3. Alternatively, the
reaction could proceed via the formation of an ion pair trans-
[Rh(4-C5F4N)(OOH)(CNtBu)(PEt3)2][HB(C6F5)3] in a first step.
The heterolytic cleavage of H2 using frustrated Lewis pairs is
well established.23 However, the NMR and IR data of the reac-
tion of 1 with the borane B(C6F5)3 and dihydrogen did not
show any evidence for the presence of the borate
[HB(C6F5)3]−.24 In addition, treatment of the peroxido complex
1 with [2,6-(CH3)2C5H3NH][HB(C6F5)3]23a led to the formation
of an unstable compound which decomposed to OPEt3 and
2,3,5,6-tetrafluoropyridine. A rhodium species could not be
identified. Therefore, we consider a mechanism involving
[HB(C6F5)3]− to be unlikely.
The reaction of the iridium peroxido complex trans-
[Ir(4-C5F4N)(O2)(CNtBu)(PiPr3)2] (2a) with the borane B(C6F5)3
in a dihydrogen atmosphere also afforded the water adduct
(H2O)·B(C6F5)3, similar to the analogous reaction at rhodium.
However, at iridium, no distinct metal species could be iso-
lated, even in the presence of additional phosphine, but the
NMR studies support the assumption of a pre-coordination of
the Lewis acid at the peroxido ligand. Addition of B(C6F5)3 to a
solution of 2a in argon atmosphere resulted initially in a
broad signal for a new species in the 31P{1H} NMR spectrum at
δ = 83 ppm. This might indicate the formation of a Lewis acid–
base adduct between the nucleophilic oxygen moiety of the
iridium complex12 and the acidic borane. When the reaction
solution was cooled down to −50 °C and a base (pyridine or
lutidine) was added, the starting compound 2a was isolated
again after work-up.
It seems to be plausible, that a pre-coordination of the
borane at the peroxido entity of 1 may lead to a weakening or
even a cleavage of a rhodium oxygen bond to generate a vacant
coordination site (Scheme 2).18,19 In the presence of dihydro-
gen this might result in the formation of a non-classical dihy-
drogen complex.20 Then several reactions pathways are conceiv-
able to generate (H2O)·B(C6F5)3 and phosphine oxide. The
Scheme 1 Reduction of 1 with dihydrogen in the presence of B(C6F5)3
or [H(OEt2)2][B{3,5-(CF3)2C6H3}4].
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Paper
The chlorinated and less Lewis acidic borane BClCy2 was
chosen to get more information on the reactivity of Lewis
acidic boranes towards the oxygen moiety of the peroxido
complex 2a. The reaction of the iridium complex trans-
[Ir(4-C5F4N)(O2)(CNtBu)(PiPr3)2] (2a) with BClCy2 afforded the
literature known dichlorido complex trans-[Ir(4-C5F4N)-
(Cl)2(CNtBu)(PiPr3)2] (5) as the main product as well as Cy2BOBCy2
and (CyBO)3 as oxygenated boron species.12a The identification
of the latter was done by 11B NMR spectroscopy and GC-MS
measurements. If the reaction is carried out in presence of
PiPr3, an intermediate 4a was detected in the reaction mixture.
We assign it to be trans-[Ir(4-C5F4N)(Cl)(OOBCy2)(CNtBu)-
(PiPr3)2]. However, we cannot exclude trans-[Ir(4-C5F4N){OO⋯B
(Cl)(Cy2)}(CNtBu)(PiPr3)2] as a possible alternative entirely. The
31P{1H} NMR spectrum of the reaction mixture displays a
singlet at δ = −5.9 ppm for the iridium species 4a as well as
the signal for 5. The 19F NMR spectrum for 4a shows four mul-
tiplets of equal intensity at δ = −99.3, −100.1, −112.8 and
−122.0 ppm for the inequivalent fluorine atoms of the tetra-
fluoropyridyl ligand. A low temperature NMR study of the reac-
tion mixture shows two broad resonances in the 11B NMR spec-
trum at δ = 4 and 0 ppm. Possibly, one resonance results from
the boron atom of 4a and the other from Cy2BOOBCy2. The
peroxoborane Cy2BOOBCy2 could not be detected at room
temperature anymore. It is presumably not stable and under-
goes decomposition. Stable neutral peroxoboranes are rather
unusual and a subporphyrinato derivative was described.25 For
the molybdenum complex [Mo(O)(O2){B(C6F5)3}{η2-PhN(O)C(O)-
Ph}2], the authors propose the coordination of the Lewis acid
at the peroxido moiety and observe a resonance in the 11B
NMR spectrum at δ = 4 ppm.26 The IR spectrum of the reaction
mixture provided evidence for iridium in the oxidation state
+III for 4a and exhibits a strong absorption band at ˜ν =
2160 cm−1 for the stretching vibration of the CNtBu ligand.27
In addition, an absorption band at ˜ν = 821 cm−1 was detected.
This area is characteristic for O–O stretching vibrations of per-
oxido compounds.14,21,25a,28 For the reaction of the isotopolo-
gue trans-[Ir(4-C5F4N)(18O2)(CNtBu)(PiPr3)2] (2b) with BClCy2,
this absorption band shifts to ˜ν = 787 cm−1 for the proposed
Scheme 3 Proposed mechanism for the reaction of 2a with BClCy2.
(CNtBu)(PR3)2] (X = Cl, {OC(O)H}; M = Rh, R = Et; M = Ir, R =
iPr) is discussed.11,12a,14a In all cases, the corresponding anion
of the Brønsted acid is bound to the metal center. Therefore,
the utilization of a weakly coordinating anion may open up a
pathway for further reactions. Indeed, treatment of the
Brønsted acid [H(OEt2)2][B{3,5-(CF3)2C6H3}4]15 with 1 in the
presence of dihydrogen yielded the rhodium(I) complex trans-
[Rh(4-C5F4N)(CNtBu)(PEt3)2] (3) and H2O2. The latter was
identified by a positive peroxide test (see ESI†). A broad signal
at δ = 2.89 ppm in the 1H NMR spectrum of the reaction
mixture is also attributed to H2O2. This signal cannot be
detected when D2 is used as the reducing agent. When
equimolar amounts of [H(OEt2)2][B{3,5-(CF3)2C6H3}4] were
used, after two days the reaction mixture showed signals in the
NMR spectrum for
3 and trans-[Rh(CNtBu)2(PEt3)2][B{3,5-
(CF3)2C6H3}4] in a ratio of 3 : 2, along with some OPEt3 and
2,3,5,6-tetrafluoropyridine. The generation of the latter is due
to a competing protonation of the Rh–C bond in 3. This was
confirmed in an independent experiment by protonation of 3
with [H(OEt2)2][B{3,5-(CF3)2C6H3}4]. However, the formation of
the ionic rhodium compound was diminished when catalytic
amounts of [H(OEt2)2][B{3,5-(CF3)2C6H3}4] were applied
(Scheme 1). NMR spectroscopic analyses of the reaction
mixture revealed small amounts of OPEt3 and 2,3,5,6-tetra-
fluoropyridine (approx. 10%), the rhodium(I) complex 3 and
trans-[Rh(CNtBu)2(PEt3)2][B{3,5-(CF3)2C6H3}4] with a ratio of
the latter complexes of 8 : 1. In case of the iridium complex
trans-[Ir(4-C5F4N)(O2)(CNtBu)(PiPr3)2] (2a), no defined reaction
with [H(OEt2)2][B{3,5-(CF3)2C6H3}4] was observed. Instead,
decomposition occurred and no iridium species could be
identified.
intermediate
trans-[Ir(4-C5F4N)(Cl)(18O18OBCy2)(CNtBu)-
(PiPr3)2] (4b). The HR-FT-ESI mass spectra gave m/z = 991.405
and m/z = 995.400 for the ions [M + H]+ of the isotopologues.
Note that only very few metallaperoxoborane species were
structurally characterized.13,25,29 A conceivable intermediate,
trans-[Ir(4-C5F4N)(Cl)(OOBCy2)(CNtBu)(PiPr3)2] (4a) supports
the suggested mechanism, which involves an activation of the
peroxido complexes 1 and 2a at the oxygen moiety by
B(C6F5)3.18,26 With BClCy2 the metal–oxygen bond is cleaved
and the chloride anion binds at the resulting free coordination
site to give 4a (Scheme 3). The reaction with another equi-
Conclusions
valent of BClCy2 would then afford the dichlorido complex In conclusion we have demonstrated that the peroxido ligand
trans-[Ir(4-C5F4N)(Cl)2(CNtBu)(PiPr3)2] (5) and the peroxo- in the rhodium complex trans-[Rh(4-C5F4N)(O2)(CNtBu)-
borane Cy2BOOBCy2 as observed (Scheme 3).
(PEt3)2] (1) can be reduced using dihydrogen. In the presence
For the protonation reaction of the peroxido units of 1 or 2a of the Lewis acid B(C6F5)3 the water adduct (H2O)·B(C6F5)3
with HCl or HCOOH at low temperature, the formation of was generated. With catalytic amounts of the Brønsted acid
hydroperoxido complexes like trans-[M(4-C5F4N)(X)(OOH)- [H(OEt2)2][B{3,5-(CF3)2C6H3}4] hydrogen peroxide is produced
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in case of the rhodium complex 1. The Rh(I) product trans-[Rh
(4-C5F4N)(CNtBu)(PEt3)2] (3) can again be converted into the
peroxido derivative 1 with O2.14a The latter reaction is very slow
which hampers the development of a catalytic process. To the
best of our knowledge, 3 is the only transition metal complex
that produces water or hydrogen peroxide from O2 and H2
using the same complex by varying the acid which is required
as additive. Other systems use Brønsted acids instead of H2
and often require an additional reducing agent like
decamethylferrocene.3d,30
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
We would like to acknowledge the Deutsche Forschungs-
gemeinschaft (DFG) as well as the cluster of excellence
“Unifying Concepts in Catalysis” for financial support.
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