Routes to Heterotrinuclear Metal Siloxide Complexes for Cooperative Activation of O2

Article abstract of DOI:10.1021/acs.inorgchem.0c00279

The assembly of heterometallic complexes capable of activating dioxygen is synthetically challenging. Here, we report two different approaches for the preparation of heterometallic superoxide complexes [^PhL₂Cr^III-η¹-O₂][MX]₂ (^PhL = ^-OPh₂SiOSiPh₂O^-, MX⁺ = [CoCl]⁺, [ZnBr]⁺, [ZnCl]⁺) starting from the Cr^II precursor complex [^PhL₂Cr^II]Li₂(THF)₄. The first strategy proceeds via the exchange of Li⁺ by [MX]⁺ through the addition of MX₂ to [^PhL₂Cr^II]Li₂(THF)₄ before the reaction with dioxygen, whereas in the second approach a salt metathesis reaction is undertaken after O₂ activation by adding MX₂ to [^PhL₂Cr^III-η¹-O₂]Li₂(THF)₄. The first strategy is not applicable in the case of redox-active metal ions, such as Fe²⁺ or Co²⁺, as it leads to the oxidation of the central chromium ion, as exemplified with the isolation of [^PhL₂Cr^IIICl][CoCl]₂(THF)₃. However, it provided access to the hetero-bimetallic complexes [^PhL₂Cr^III-η¹-O₂][MX]₂ ([MX]⁺ = [ZnBr]⁺, [ZnCl]⁺) with redox-inactive flanking metals incorporated. The second strategy can be applied not only for redox-inactive but also for redox-active metal ions and led to the formation of chromium(III) superoxide complexes [^PhL₂Cr^III-η¹-O₂][MX]₂ (MX⁺ = [ZnCl]⁺, [ZnBr]⁺, [CoCl]⁺). The results of stability and reactivity studies (employing TEMPO-H and phenols as substrates) as well as a comparison with the alkali metal series (M⁺ = Li⁺, Na⁺, K⁺) confirmed that although the stability is dependent on the Lewis acidity of the counterions M and the number of solvent molecules coordinated to those, the reactivity is strongly dependent on the accessibility of the superoxide moiety. Consequently, replacement of Li⁺ by XZn⁺ in the superoxides leads to more stable complexes, which at the same time behave more reactive toward O-H groups. Hence, the approaches presented here broaden the scope of accessible heterometallic O₂ activating compounds and provide the basis for further tuning of the reactivity of [^RL₂Cr^III-η¹-O₂]M₂ complexes.

Full text of DOI:10.1021/acs.inorgchem.0c00279

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Routes to Heterotrinuclear Metal Siloxide Complexes for
Cooperative Activation of O2
Marie-Louise Wind, Santina Hoof, Beatrice Braun-Cula, Christian Herwig, and Christian Limberg*
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ABSTRACT: The assembly of heterometallic complexes capable of
activating dioxygen is synthetically challenging. Here, we report two
different approaches for the preparation of heterometallic superoxide
−
complexes [PhL2CrIII-η1-O2][MX]2 (PhL = OPh2SiOSiPh2O−, MX+
= [CoCl]+, [ZnBr]+, [ZnCl]+) starting from the CrII precursor
complex [PhL2CrII]Li2(THF)4. The first strategy proceeds via the
exchange of Li+ by [MX]+ through the addition of MX2 to
[PhL2CrII]Li2(THF)4 before the reaction with dioxygen, whereas in
the second approach a salt metathesis reaction is undertaken after O2
activation by adding MX2 to [PhL2CrIII-η1-O2]Li2(THF)4. The first
strategy is not applicable in the case of redox-active metal ions, such
as Fe2+ or Co2+, as it leads to the oxidation of the central chromium
ion, as exemplified with the isolation of [PhL2CrIIICl]-
[CoCl]2(THF)3. However, it provided access to the hetero-
bimetallic complexes [PhL2CrIII-η1-O2][MX]2 ([MX]+ = [ZnBr]+, [ZnCl]+) with redox-inactive flanking metals incorporated. The
second strategy can be applied not only for redox-inactive but also for redox-active metal ions and led to the formation of
chromium(III) superoxide complexes [PhL2CrIII-η1-O2][MX]2 (MX+ = [ZnCl]+, [ZnBr]+, [CoCl]+). The results of stability and
reactivity studies (employing TEMPO−H and phenols as substrates) as well as a comparison with the alkali metal series (M+ = Li+,
Na+, K+) confirmed that although the stability is dependent on the Lewis acidity of the counterions M and the number of solvent
molecules coordinated to those, the reactivity is strongly dependent on the accessibility of the superoxide moiety. Consequently,
replacement of Li+ by XZn+ in the superoxides leads to more stable complexes, which at the same time behave more reactive toward
O−H groups. Hence, the approaches presented here broaden the scope of accessible heterometallic O2 activating compounds and
provide the basis for further tuning of the reactivity of [RL2CrIII-η1-O2]M2 complexes.
studies on molecular mono- and dinuclear synthetic systems,
capable of reacting with dioxygen or oxygen atom transfer
reagents, have been conducted in the recent past.
INTRODUCTION
■
In molecular arrangements the placement of two or more
metal centers in close proximity to each other can allow for a
communication and/or cooperation between them, for
instance, with regard to substrate activation in catalytic
cycles.1−8 In nature the activation of small molecules is often
performed by active sites containing several redox-active
centers,9 e.g., the fixation of N2 by nitrogenase10 or O2
reduction by cytochrome c oxidase.11 In addition, for
biological as well as synthetic systems, it has been shown
that properties such as stability and reactivity of multimetallic
sites can also be modulated by redox-inactive Lewis acidic metal
ions. In this context a well-known example for a naturally
occurring multimetallic system is the oxygen evolving complex
(OEC) in Photosystem II: Four manganese ions are active in
the oxidation of water to O2, but the presence of a redox-
inactive Lewis acidic Ca2+ ion is crucial for proper functioning
for reasons not yet entirely understood.12−15
These studies showed that Lewis acids, added deliberately to
reaction solutions, influenced the oxygen reduction rate during
the initial step of dioxygen activation.16 In addition to this, it
could be shown, that the presence of a Lewis acid also effects
the stability and reactivity of the dioxygen adduct complexes
formed.17−23 In 2017 Nam, Fukuzumi, and co-workers
observed a direct correlation between the strength of the
Lewis acid employed and the reactivity of an N-methylated
cyclam iron(III) peroxide complex in electron transfer,
nucleophilic, and electrophilic reactions.24,25 Similar results
Received: January 28, 2020
To gain more insight into how redox-inert metal ions can
cooperate in the activation of small molecules, such as
dioxygen, through their Lewis acidic properties, a variety of
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Table 1. Spectroscopic and Structural Data of Known Chromium(III) Superoxide Complexes with Siloxide Based Ligand
Systems
a
complex
[PhL2Cr-η1-O2]Li2(THF)4 (1)
O−O bond distance/Å
O···M+ distance /Å
UV/vis bands/nm
310, 507, 691, 882
Raman shift/cm−1
ref
28
1.334(4)
2.028
−
3.393
[PhL2Cr-η1-O2]Na2(THF)5 (2)
[PhL2Cr-η1-O2]Li2(EtCN)4 (3)
1.334(7), 1.324(7)
2.500/2.562
2.433/2.373
2.014
3.866
2.014
319, 508, 712
306, 487, 685
309, 489, 693, 927
309, 489, 693
−
27
27
29
29
1.3126(10)
1.337
1130 (16O2)
−
[
iPrL2Cr-η1-O2]Li2(THF)2(EtCN)4 (4)
3.421
−
[
iPrL2Cr-η1-O2]Li2(EtCN)4 (5)
−
1123 (16O2)
a
Measured with Mercury 3.10.
Figure 1. Molecular structures and corresponding UV/vis spectra of previously synthesized chromium(III) superoxide complexes with a siloxide
based ligand system. (A) UV/vis spectrum of [PhL2CrIII-η1-O2]Li2(THF)4 (1) dissolved in THF. (B) UV/vis spectrum of [PhL2CrIII-η1-
O2]Na2(THF)5 (2) dissolved in THF. (C) UV/vis spectrum of [PhL2CrIII-η1-O2]Li2(EtCN)4 (3) dissolved in EtCN. (D) UV/vis spectrum of
[PhL2CrIII-η1-O2]Li2(THF)2(EtCN)2 (4) dissolved in a mixture of THF/EtCN. (Data from refs 27−29.)
were rather recently observed for a chromium(III) superoxide
complex, but as in case of the iron(III) peroxide Lewis acidic
metal ions were added separately to solutions of the already
formed dioxygen adduct complex.26
strongly dependent on these binding modes (M+O2M+ vs
M+O2 interaction), the nature of the metal ions M+ (size and
Lewis acidity), and the solvent employed (THF and nitriles).
Specifically, it could be demonstrated that the stability was
mainly influenced by the ionic radius and Lewis acidity of M+,
whereas the reactivity of these complexes was not solely
dependent on the latter but also on further parameters such as
the accessibility of the active center.27 This shows nicely how
versatile the properties of such heterometallic complexes with
redox-inactive as well as redox-active metal ions can be,
suggesting continuative studies with variation of the flanking
metal ions M+.
We recently investigated a series of chromium(III) super-
oxide complexes, [RL2CrIII-η1-O2]M2 (L = −OR2SiOSiR2O−)
(Table 1), where alkali metal ions (M+ = Li+, Na+, K+) as
Lewis acids were an integral part of both, the precursor
compounds and the O2 adducts. Their influence and also the
effect of the variation of residues R at the siloxide framework
(R = Ph, iPr, Me) on the structures, stabilities, and reactivity
have been investigated.27−29
The superoxide ligands were found to interact with M+ in
varying modes (Figure 1 and Table 1). The UV/vis spectra,
reactivity, and stability of the superoxide complexes were
B
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Scheme 1. Reaction of 6 with Iron(II) Chloride to the Isolated Product [PhL2CrIIICl]Li2(THF)4 and with Cobalt(II) Chloride
to Give [PhL2CrIIICl][CoCl]2(THF)3 (9)
with M = Zn2+, Co2+, Fe2+. For the exchange of Li+ in 6 by
RESULTS AND DISCUSSION
■
[ZnX]+, [FeX]+, and [CoX]+ units (X = Cl−, Br−), ZnCl2,
Synthesis of
[
P h L2 CrI I I Cl][CoCl]2(THF)3 (9),
ZnBr2, FeBr2, FeCl2, and CoBr2 as well as CoCl2 were
employed as metal precursors.
[PhL2CrII][ZnCl]2(THF)3 (10Cl), and [PhL2CrII][ZnBr]2(THF)3
(10Br). We were interested to extend our previous inves-
tigations on the cooperative activation of O2 at M/CrII/M
cores from alkali metal ions to transition metal ions. For this
purpose the corresponding M/CrII/M complexes, with M =
transition metal ion, had to be prepared.
The reaction of complex 6 with 2.1 equiv of iron(II)
chloride led to the isolation of a green solid, which could be
identified as the already known complex, [PhL2CrIIICl]-
Li2(THF)4 (Scheme 1).27 Hence, the reaction with iron(II)
chloride leads to the oxidation of the chromium ion and not to
the envisaged complex with FeII incorporated into the
structure.
The reaction of 6 with 2.1 equiv of cobalt(II) chloride led to
the formation of a blue solid, which was analyzed by means of
ESI-MS, elemental analysis, and IR spectroscopy (see
discussion of the spectrum in the Supporting Information
(SI)). A single crystal X-ray analysis provided the structure of 9
(Figure 2) and revealed that, again, the central CrII center had
In general, for the synthesis of complexes of the formula
[PhL2MT]MM′ (MT = central transition metal ion; M, M′ = p-
block or transition metal ion; PhL = OPh2SiOSiPh2O−) two
−
different approaches have been employed so far:
(1) In the first method the deprotonation of the disilanol
PhLH2 with different bases MR (R = basic residue), e.g., n-
BuLi, LiOtBu, NaOtBu, KOtBu, and KH, is followed by
treatment of the resulting PhLM2 with the transition metal
halides. This led to the isolation of heterometallic complexes of
the type [PhL2MT]M2 (e.g., M = Li+, MT = Co,30 Cr,28,31
Cu;32,33 M = Na+, MT = Cr;34 M = K+, MT = Cr27). However,
the suitability of basic compounds with M other than alkali
metal ions for the deprotonation step is limited and often does
not lead to the formation of the desired product. When, for
instance, transition metal amides were used as precursor
compounds, homometallic complexes were often isolated.35
(2) Sullivan et al.36 as well as Edelmann and co-workers37
found that the exposure of [PhL2Co]Li2(THF)4 to MnCl2 and
[PhL2ScCl]Li2(DME)4 to AlMe3 led to the isolation of crystals
of [PhL2Co]Li(Py2)MnCl(Py) and [PhL2ScCl]AlMe2Li(THF)2,
respectively. In both cases, despite using an excess of reagent,
only one of the two counterions could be exchanged without
losing the overall structure of the complex.
In our previous research we were able to demonstrate for the
−
series [PhL2CrII]M2(THF)n (PhL = OPh2SiOSiPh2O−, M+ =
Li+, n = 4 (6), M+ = Na+, n = 4 (7), M+ = K+, n = 2 (8)) that
varying the size of the alkali metal ions leads to slight changes
in the bond distances and angles within the CrO4 plane, which
extend to the surroundings of the counterions (Supporting
Information, Table S2). Such modifications may explain why in
the complex of Edelmann and co-workers only one Li+ could
be exchanged by “AlMe2”: the difference in ionic radii of four-
coordinated Li+ (0.59 Å) and Al3+ (0.39 Å)38,39 leads, in the
course of the first substitution, to slight structural changes
within the plane inherent to [PhL2ScCl]Li2(DME)4, which
presumably makes the exchange of the second Li+ unfavorable.
The same can be assumed for [PhL2Co]Li(Py2)MnCl(Py)
(Mn2+, 0.66 Å). This suggests that an exchange of both Li+
might be achievable, when ions with comparable radii to Li+
are chosen. The ionic radii for four-coordinated iron, cobalt,
and zinc complexes with the oxidation state +2 are 0.63, 0.58,
and 0.60 Å, respectively; they are thus similar to that of Li+,
and hence we decided to test the previously described method
(2) to gain access to complexes of the type [PhL2CrII][MX]2,
Figure 2. Molecular structure of complex 9 showing 50% probability
ellipsoids. For clarity hydrogen atoms as well as noncoordinating
solvent molecules are omitted. Selected bond lengths (Å): Co1−O4,
1.9621(18); Co1−O1, 1.9806(18); Co1−Cl1, 2.2334(8); Co1···Cr1,
2.9215(6); Cr1−O4, 1.9691(18); Cr1−O3, 1.9846(18); Cr1−O6,
2.0031(18); Cr1−O1, 2.0036(18); Cr1−Cl2, 2.2969(8); Cr1···Co, 2
2.9506(6); Si1−O1, 1.6308(19); Si1−O2, 1.632(2); Co2−O3,
1.9716(18); Co2−O6, 1.9768(18); Co2−Cl3, 2.2233(8); Si2−O3,
1.6312(19); Si2−O2, 1.6338(19).
been oxidized to CrIII and that both Li+ ions had been
exchanged by Co2+ ions, thus leading to the product
[PhL2CrIIICl][CoCl]2(THF)3 (9) (see Scheme 1). The
terminal chlorido ligand is centered above the square-planar
CrO4 unit and does not display any interaction with the cobalt
counterions; a THF molecule completes the octahedral
coordination sphere of the chromium(III) ion. The chlorido
ligands attached to the cobalt counterions are located trans to
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the axial chlorido ligand, and the coordination sphere of the
cobalt ions is saturated by one THF molecule each.
Although an oxidation of the chromium center has occurred,
which makes the resulting complex 9 incapable of activating
dioxygen to yield O2 adducts similar to 1−4 (Figure 1), the
formation of complex 9 proves that an exchange of both Li+
counterions via a salt metathesis is possible.
Reactivity of
[
P h L2 CrI I ][ZnCl]2 (THF)3 and
[PhL2CrII][ZnBr]2(THF)3 toward Dioxygen. The behavior of
10Cl/Br toward molecular oxygen was tested by exposing a THF
solution to dioxygen. ESI-MS measurements performed with
the solution showed peaks indicating the formation of a
product resulting from a 1:1 reaction with O2 (SI Figure S4).
To gain further insight into what type of O2 species (e.g.,
peroxo vs superoxo complex) had been formed, a THF
solution of 10Cl/Br was cooled to 10 °C and exposed to an
excess of dioxygen and the reaction was monitored by means
of UV/vis spectroscopy. After completion of the reaction, the
recorded UV/vis spectrum showed two new distinctive
absorption bands at 302 and 675 nm as well as one weak
band at 479 nm (Figure 4). A rather similar absorption
spectrum was observed for 10Br upon addition of dioxygen.
In previous work with β-diketiminato nickel complexes, we
have found that replacing a potassium cation interacting with a
nickel(II) peroxide unit by a zinc ion leads to a further
activation, initiating redox chemistry.40 Hence, attempts to
replace the Li+ ions in 6 by Zn2+ cations suggested itself to gain
further insights concerning the influence of the nature of the
flanking metal ions in subsequent O2 activation studies. On the
basis of the results obtained in the case of CoCl2 combined
with the fact that Zn2+ is redox-inert, a clean metathesis
reaction was anticipated to result from the treatment of 6 with
2.1 equiv of ZnCl2. After workup, a turquoise powder was
isolated, the elemental analysis of which pointed to [PhL2Cr]-
[ZnCl]2(THF)3 (10Cl). ESI-MS measurements of 10Cl
dissolved in THF showed an ion peak at m/z = 1108.8972,
which corresponds to [[PhL2Cr][ZnCl]2+Cl]− (calcd. m/z =
1108.8961) and corroborates this assignment; the m/z value as
well as the isotopic distribution pattern are in good agreement
with the calculated values (SI Figure S3). Crystals grown at
room temperature from a saturated THF solution were
analyzed via X-ray crystallography (Figure 3). The resulting
Figure 4. Absorption spectral changes observed during the reaction
between 10Cl and O2 in THF at 10 °C, leading to the formation of
11Cl.
In our previous work we had analyzed the band patterns in
the UV/vis spectra of chromium(III) superoxide complexes
such as 1−4 in detail and correlated those to the solid-state
structures determined, which were confirmed as relevant also
in solution.27 On the basis of these results we can derive
information also concerning the product of the reaction
between 10Cl/10Br and dioxygen in THF considering its UV/
vis spectrum (Figure 4): Due to the existence of an absorption
band at around 300 nm the product can be assigned
unambiguously as an end-on CrIII superoxo complex (Figure
1 and Table 1).28,41 The absence of a band in the near-infrared
region indicates that both of the zinc ions are interacting
electrostatically with the oxygen atoms of the superoxide
ligand.27 The comparatively high relative intensity of the band
at around 480 nm suggests that the molecule lacks symmetry,
most likely caused by different numbers of solvent molecules
coordinating to the zinc ions. The band in the 670 nm region
implies that in the axial position to the superoxide an
additional ligand is attached to the chromium ion, completing
an octahedral coordination sphere. Hence, we suggest the
formula [PhL2CrIII-η1-O2][ZnX]2(THF)2 (11Br/Cl) for the
product, with a structure as shown in Scheme 2, similar to
the one of 3 (compare UV/vis spectrum in Figure 1). Using
propionitrile as the solvent, where 10Cl/Br is expected27,29 to
convert into [PhL2CrII][ZnX]2(EtCN)3which for formal
distinction we denote separately as *10Cl/Br (Scheme 2)an
almost identical proceeding as in the case of 10Cl/Br was
observed and bands at 300, 467, and 671 nm were detected for
[PhL2CrIII][ZnX]2O2(EtCN)2 (*11Cl/Br), emphasizing that 10
Figure 3. Molecular structure of complex 10Cl showing 50%
probability ellipsoids. For clarity hydrogen atoms as well as
noncoordinating solvent molecules are omitted. Selected bond lengths
(Å): Cr1−O2, 2.0297(11); Cr1−O1, 2.0338(12); O2−Si2,
1.6180(11); Si2−O4, 1.6291(13); Cr1···Zn1, 2.9587(3); Zn1−Cl1,
2.1779(5); Zn1−O2, 1.9489(12); Zn1−O1, 2.0200(12).
structure shows that indeed both lithium ions have been
exchanged and the chromium ion has remained in the
oxidation state +2. The CrII central atom is located in a
square-pyramidal coordination sphere composed of two
disiloxide ligands, which span the plane, and an additional
THF molecule in the apical position. The analogous complex
10Br can be synthesized and isolated by using ZnBr2 instead of
ZnCl2; it was identified as such by X-ray analysis, ESI-MS, and
an IR spectrum that closely resembled the one of 10Cl (SI
Figures S1−S3 and Table S2).
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Scheme 2. Schematic Representation of the Reaction between 6 and ZnX2 (X = Br, Cl) to give 10Cl/Br and *10Cl/Br and Further
Reaction with O2 to Yield 11Cl/Br and *11Cl/Br, Respectively
and *10 and correspondingly also 11 and *11 are basically the
same core species. Attempts to obtain resonance Raman data
remained unsuccessful, despite measuring at different concen-
trations as well as temperatures and in different solvents.
Stability of [PhL2CrIII-η1-O2][ZnCl]2(Sol)2 (Sol = THF
(11Cl), EtCN (*11Cl)) in THF and EtCN. To gain more insight
into how far [ZnX]+ affects the reactivity and stability, self-
decay rates of 11Cl and *11Cl were determined. Therefore, 11Cl
and *11Cl were generated in THF or EtCN, respectively, and
the decay of the bands at 302 nm (THF) or 300 nm (EtCN)
as well as the formation of a band typical for the decay product
(350 nm) were monitored by UV/vis spectroscopy at 10 °C,
respectively (SI Figure S5). First-order rate constants
determined for the self-decay of 11Cl revealed that although
to the number of M+/O2 interactions (Li+/O2 in THF vs Li+/
O2/Li+ in EtCN) and the CrO4 backbone (planar vs roof-
like),27 the decay rates of 11Cl and *11Cl (kobs
= (1.02
EtCN
0.05) × 10−4 s−1) are almost identical. From this (and on the
basis of the comparable bond dissociation energies of THF and
EtCN) it can be concluded that the structures are the same in
THF and EtCN. Finally, we note thatunlike in the case of
the above-mentioned Ni-peroxide-K/Zn system40the zinc
ions in the systems investigated here do not lead to a
significant increase of superoxide activation compared to alkali
metal cations.
Reactivity of [PhL2CrIII-η1-O2][ZnCl]2(Sol)2 (Sol = THF
(11Cl), EtCN (*11Cl)) in THF and EtCN. To assess the
reactivity of 11Cl and *11Cl, the decay of their characteristic
absorption bands (λTHF = 302 and 675 nm, λEtCN = 300 and
673 nm) upon addition of 1-hydroxy-2,2,6,6-tetramethylpiper-
idine (TEMPO−H) was monitored UV/vis spectroscopically.
Indeed, the bands disappeared according to a pseudo-first-
order rate law and the corresponding rate constants increased
proportionally with an increase of the concentration of
TEMPO−H, giving second-order rate constants (k2) of 11.8
1.0 M−1 s−1 (THF) and 10.4 0.7 M−1 s−1 (EtCN). The
formation of TEMPO was confirmed by means of EPR
spectroscopy (SI Figure S9). The second-order rate constants
determined for the reactions in THF and EtCN are almost
identical within the error margin. This matches the above
inferences on the solvent-independent Zn/O2/Zn interactions,
and therefore, further experiments were only performed in
THF. A comparison of the second-order rate constant of 11Cl
11Cl features a M/O2/M interaction similar to that of the Na+
analogue 2 (kobs
= (5.39 0.02) × 10−3 s−1), it exhibits a
THF
THF
self-decay rate of kobs
= (1.11 0.01) × 10−4 s−1, which is
THF
similar to the one determined for the Li+ analogue 1 (kobs
=
(1.89 0.12) × 10−4 s−1) in THF. This may be rationalized
first by the fact that zinc ions are more Lewis acidic than
sodium ions (and thus lead to more stable Cr−O2···M
entities). Moreover, its has been hypothesized and supported
previously27,42 that the self-decay proceeds via the oxidation of
the solvent and this could explain the stability of 11Cl
additionally: the precoordination of the THF molecules at
the flanking metals likely facilitates this process, and while
there are four such molecules in the case of 2 and three in 1,
there is only one coordinated THF molecule in 11Cl, where
chlorido ligands occupy two of the potential binding sites.
Hence, 11Cl is even slightly more stable than 1. That the
solvent indeed plays a role in the decay reaction could be
confirmed: The oxidation products of THF could be detected
by means of NMR spectroscopy, and UV/vis spectroscopic
monitoring of the decay reaction in deuterated THF revealed a
kinetic isotopic effect of KIE = 4.75 (SI Figure S7). Despite
many attempts to crystallize 11Cl/Br at −80 °C from THF or
EtCN solutions, no crystals suitable for X-ray analysis could be
grown. Studying the solvent dependency of the self-decay an
observation could be made that further supports the inferences
with the ones of 1 and 2 shows that 11Cl exhibits a much
THF
higher reactivity than 1 (k2
= 1.6 0.30 M−1 s−1) and 2
THF
(k2
= 0.27
0.01 M−1 s−1) in THF. In our previous
investigations on the alkali metal series we found that the
reactivity is determined by two factors, namely, the Lewis
aciditiy of the heterometal and the steric conditions around the
superoxide moiety.27 Decreasing Lewis acidity will lead to
more reactive complexes (as the superoxide moiety is less
stabilized) but this effect can be overruled by the accessibility
of the superoxide function, when structural changes occur with
the variation of the metal. The importance of the sterics also in
the case of 11Cl became obvious in further investigations
performed with phenols as substrates. Compound 11Cl does
not react with 2,6-di-tert-butylphenol at −80 °C, but it reacts
concerning the structures made above: in contrast to
[PhL2CrIII-η1-O2]Li2(EtCN)4 (3) (kobs
= (4.78 0.30) ×
EtCN
10−5 s−1), which is more stable than [PhL2CrIII-η1-O2]-
Li2(THF)4 (1), likely due to structural changes with regard
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with 2,6-dimethylphenol (forming 3,3′,5,5’-tetramethyl-4,4′-
diphenoquinone), although the bond dissociation energy of
the latter is 1.7 kcal/mol higher. A likely reason is the bulkiness
of the tert-butyl groups, which impede the approach of the
substrate. Since the replacement of Li+ by XZn+ was found to
lead to more stable complexes, the higher reactivity of 11Cl
compared to 1 and 2, not only toward substituted phenols but
also TEMPO−H, will also have its origin mainly in the higher
accessibility of the superoxide unit, which can be rationalized:
in the case of 11Cl two chlorido ligands coordinate to the
flanking metal ions instead of solvent molecules, and because
the former are less bulky, a more accessible superoxide moiety
results, which leads to a higher reactivity.
and a subsequent fast oxidation of Fe2+ to Fe3+ did not allow its
further characterization.
However, employing CoCl2 led to the formation of a more
stable Co/Cr(O2)/Co superoxide: Upon addition of CoCl2 to
a THF solution of 1 the band of 1 at 882 nm disappeared,
while simultaneously a shift of the “CoCl” absorption band
from 686 to 696 nm occurred (Figure 5).
The band at 696 nm had also been observed for 9 and
therefore indicates an incorporation of [CoCl]+ into the
structure. The presence of a band at 303 nm, which has shifted
from 310 nm (1), corroborates the formation of a superoxide
species with an end-on bound O2 ligand. Density functional
calculations support our assignment as a superoxide (Support-
ing Information). The band at 696 nm unfortunately masks the
usually occurring bands in that region so that more detailed
information on the Co/O2 interaction cannot be derived from
the UV/vis spectrum. Nevertheless, the obtained results
suggest the successful exchange of the counterions and thus
the formation of the envisaged complex [PhL2CrIII-η1-O2]-
The findings described above align therefore quite nicely
with the observations made so far.
Exchange of Li+ in [PhL2CrIII-η1-O2]Li2 (THF)4 (1) by
Redox-Active and -Inactive Metal Fragments. Since the
exchange of both counterions in the CrII precursor complexes
prior to the reaction with dioxygen (route 1, Scheme 3)
[CoCl]2 (12). Compound 12 exhibits a self-decay rate of
= (7.59 0.02) × 10−5 s−1 at 10 °C, which is slightly
THF
Scheme 3. Formation of 11Cl/Br via 10Cl/Br Prepared from 6
by Treatment with ZnX2 (X = Br, Cl) (Route 1) or through
the Reaction of 1 with ZnX2 (Route 2)
kobs
lower than the rates determined for 1 (kobsTHF = (1.89 0.12)
× 10−4 s−1) and 11Cl (kobs
= (1.11 0.01) × 10−4 s−1) in
THF
THF. To compare the reactivities of 12 with the reactivity of
11Cl and the starting compound 1, the disappearance of the
absorption band characteristic of 12 (302 nm) upon addition
of TEMPO−H was monitored UV/vis spectroscopically.
Pseudo-first-order rate constants determined at −80 °C
increased proportionally with an increase of the concentration
of TEMPO−H, giving a second-order rate constant (k2) of
(5.0 0.2) × 10−3 M−1 s−1 (THF). This very low reactivity is
surprising, and at this point we can only assume that either
changes in the M+/O2 interaction or the amount of
coordinating solvent molecules result in a less accessible
superoxide moiety and are therefore responsible for the
decreased reactivity.
CONCLUSION
■
We herein present two methods to extend the chemistry of
disiloxandiolate based hetero-bimetallic superoxide complexes
from chromium/alkali metal to chromium/transition metal
combinations. The reaction of 6 with ZnCl2 or ZnBr2 allowed
for the isolation of [PhL2CrII][ZnX]2(THF)2 (X = Cl, Br),
which react with dioxygen forming the Zn/Cr(O2)/Zn
superoxide complexes [PhL2CrIII-η1-O2][ZnX]2(THF)3 (X =
Cl, Br). Similar attempts to prepare corresponding compounds
with redox-active ions in place of Zn2+ failed due to the
oxidation of the CrII central atom, but we were able to
demonstrate for the examples of Zn and Co that the exchange
works also at the superoxide level. Since, in the superoxide
[PhL2CrIII-η1-O2]Li2(THF)4, the Cr center is already in the
oxidation state +3, no redox chemistry takes place then in
contact with redox-active transition metal ions, so that the salt
metathesis to the desired Zn/Cr(O2)/Zn and Co/Cr(O2)/Co
compounds can be carried out. Results of the investigation on
the reactivity and stability of [PhL2CrIII-η1-O2][ZnCl]2(THF)2
confirm that the intrinsic stability is mainly governed by the
Lewis acidity of the metal ions and the number of coordinated
solvent molecules, whereas the reactivity of such hetero-
bimetallic complexes is majorly influenced by the accessibility
of the superoxide moiety and, thus, by the structural
arrangements around the superoxide moiety; this nicely
complements our previous findings. With this information
apparently only works for redox-inert transition metal ions
(redox-active metal ions employed so far led to the oxidation
of the central chromium(II) atom), we contemplated
alternative avenues to heterometallic M/Cr(O2)/M complexes,
as this would increase the variability of this system. A
conceivable synthetic route to such compounds would be a
metal exchange with the already formed superoxide complex 1
(route 2, Scheme 3). To test this, we generated 1 in situ at 10
°C in THF and added an excess of ZnCl2 to the solution. The
immediate disappearance of the band at 882 nm, which is
typical for 1, and the shift of the other three bands to 302, 479,
and 675 nm corroborate the clean conversion of 1 into 11Cl
(SI Figure S10). The exchange reaction can also be performed
in the other direction: 1 can be generated by adding an excess
of LiCl to a solution of 11Cl at 10 °C.
Astonishingly these reactions are temperature-controlled and
do not take place at low temperatures. Upon applying the same
procedure to the reaction of 1 with FeBr2, the immediate
disappearance of the band at 882 nm with a simultaneous shift
of the band at 310 to 308 nm and a formation of a shoulder at
710 nm indicated an incorporation of Fe2+. Unfortunately, the
strong absorption of the newly formed Cr/O2/Fe intermediate
F
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Figure 5. (A) UV/vis spectrum of a THF solution of CoCl2 at 25 °C. (B) UV/vis spectrum of compound 9 at 25 °C for comparison. (C) UV/vis
spectral changes observed for a THF solution of 1 upon addition of CoCl2. Red: UV/vis spectrum of 1 before addition of CoCl2. Blue: UV/vis
spectrum after addition of a substoichiometric amount of CoCl2. Yellow: UV/vis spectrum after addition of 2.1 equiv of CoCl2. Green: UV/vis
spectrum after addition of an excess of CoCl2. Inset shows the band at 696 nm slowly increasing and then shifting to the region of unbound CoCl2
upon addition of an excess of CoCl2. The decrease of the band at 882 nm can also be observed upon addition of CoCl2.
this work. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
and the methods to create rather different M/Cr(O2)/M cores
at hand, we hope to be able to design and control further novel
reactive complexes, where two selected metal centers
cooperate in O2 activation, in future.
Chemicals. 1,1,3,3-Tetraphenylsilan-1,3-diol was synthesized
according to literature.46 1,3-Dichloro-1,1,3,3-tetraphenylsiloxane,
TEMPO, CoCl2, FeCl2, and CrCl2 were purchased at ABCR and
used as received. Compounds 1, 3, and 6 were synthesized according
to literature,28 and TEMPO−H was produced according to a
previously described modified literature procedure.27,47 O2 was
added via a balloon, which had been thoroughly flushed with argon
before being filled with dioxygen.
Solvents. THF was purchased from Acros Organics and was
distilled over sodium, degassed, and stored over activated molecular
sieves (3 Å) prior to use. Chloroform was dried over CaCl2, refluxed
over P2O5, degassed, and stored over activated molecular sieves (4 Å)
prior to use. Hexane, diethyl ether, and acetonitrile were dried with an
MBraun solvent purification system and degassed prior to use.
Propionitrile was purchased from Acros Organics as well as from
Sigma-Aldrich, degassed, and stored over activated molecular sieves
(3 Å) prior to use.
EXPERIMENTAL SECTION
■
General Considerations. All manipulations were carried out in a
glovebox, or by means of Schlenk-type techniques involving the use of
a dry argon atmosphere. Microanalyses were performed with a Leco
CHNS-932 elemental analyzer. Infrared (IR) spectra were recorded
with samples prepared as KBr pellets with a Shimadzu FTIR-8400S
spectrometer. HR-ESI-MS data were collected using an Agilent
Technologies 6210 time-of-flight LC-MS instrument. UV/vis spectra
were obtained at variable temperatures using an Agilent 8453 UV−
visible spectrophotometer equipped with a Unisoku USP-203-A
cryostat. EPR spectra were recorded at an ESR MiniScope MS5000
(Magnettech), equipped with a quartz dewar. Crystal data were
collected at a Bruker D8 Venture diffractometer at 100 K, using Mo
Kα radiation (λ = 0.71073 Å). The structures were solved by direct
methods using SHELXS-97 and refined by full matrix least-squares
procedures based on F2 with all reflections measured with SHELXL-
2013.43,44 Multiscan corrections were applied to the data.45 All non-
hydrogen atoms were refined anisotropically. All hydrogen atom
positions were introduced at their idealized positions and were refined
using a riding model. CCDC 1949950 (9), 1949952 (10Cl), and
1949951 (10Br) contain the supplementary crystallographic data for
[PhL2CrIIICl][CoCl]2(THF)3 (9). To a solution of 6 (50 mg, 0.04
mmol, 1 equiv) in THF, CoCl2 (11.6 mg, 0.09 mmol, 2.1 equiv) was
added, and the solution was stirred overnight at room temperature.
The resulting blue solution was filtered and the solvent subsequently
removed. The solid residue was washed with hexane, small amounts of
Et2O, MeCN, and CHCl3 and redissolved in THF. The solution thus
obtained was layered with hexane, which within a few days led to blue
G
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Article
crystals of 9 (17.2 mg, 0.01 mmol, 32%) suitable for single crystal X-
ray diffraction analysis. By employing 4 equiv of CoCl2, the yield can
be increased to 65%.
IR spectra, ESI-MS spectra, crystal structure analysis
data, UV/vis spectra, EPR spectra, and density func-
tional calculations (PDF)
Elem. Anal. Found for C60H64Cl3Co2CrO9Si4: H, 4.53; C, 50.95%.
Calcd: H, 4.90; C, 54.69%. Elemental analysis with metal siloxide
compounds can lead to low carbon values, even if pure, crystalline
material is investigated, likely due to incomplete combustion (SiC
formation). In the case of 9 repeated measurements with crystalline
and amorphous product led to similar results, where the H content
acceptably agrees with the calculated value, while the determined C
content is somewhat too low. UV/vis [λmax(THF)/nm (ε/(cm−1
Accession Codes
CCDC 1949950−1949952 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
mol−1 L))]: 590 (93), 660 (95), 696 (105). IR (νmax/cm−1 (KBr)):
̅
3069 (w), 2976 (w), 2903 (w), 1590 (w), 1429 (m), 1122 (s), 1114
(s), 1041 (s), 1024 (s), 996 (m), 888 (s), 748 (m), 715 (s), 699 (s),
544 (s), 523 (s). HR-ESI-MS. [M − 3THF + H2O + CH3CN]− (m/
z): Calcd for [C50H45Cl3Co2CrNO7Si4], 1157.9413; found,
1157.9197. [M − 3THF + CH2CH2CN]− (m/z): Calcd for
[C51H47Cl3Co2CrNO6Si4], 1153.9401; found, 1153.9589.
AUTHOR INFORMATION
Corresponding Author
■
Christian Limberg − Chemistry Department, Humboldt-
̈
Universitat zu Berlin, 12489 Berlin, Germany; orcid.org/
[PhL2CrII][ZnCl]2(THF)3 (10Cl). To a solution of 6 (22.0 mg, 0.02
mmol, 1 equiv) in THF, ZnCl2 (5.4 mg, 0.04 mmol, 2.1 equiv) was
added, and the solution was stirred overnight at room temperature.
The resulting turquoise solution was filtered, and subsequently the
solvent was removed under reduced pressure. The solid residue was
washed with hexane and small amounts of Et2O and redissolved in
THF. The solution thus obtained was layered with hexane, which led
within a few days to turquoise crystals of 10Cl (14.4 mg, 0.01 mmol,
56%), which were suitable for single crystal X-ray diffraction analysis.
Elem. Anal. (of the solid before crystallization). Found for
C60H64Cl2CrO9Si4Zn2: H, 4.83; C, 53.74%. Calcd: H, 4.98; C,
55.64%. (See comments made in the case of 9.) UV/vis [λmax(THF)/
0000-0002-0751-1386; Phone: +4930-20937382;
Email: Christian.limberg@chemie.hu-berlin.de
Authors
Marie-Louise Wind − Chemistry Department, Humboldt-
̈
Universitat zu Berlin, 12489 Berlin, Germany
Santina Hoof − Chemistry Department, Humboldt-Universitat
̈
zu Berlin, 12489 Berlin, Germany
Beatrice Braun-Cula − Chemistry Department, Humboldt-
̈
Universitat zu Berlin, 12489 Berlin, Germany; orcid.org/
0000-0001-6262-4925
Christian Herwig − Chemistry Department, Humboldt-
nm (ε/(cm−1 mol−1 L))]: 672 (28). IR (νmax/cm−1 (KBr)): 3069
̅
̈
Universitat zu Berlin, 12489 Berlin, Germany
(m), 3045 (m), 2979 (m), 2957 (m), 2882 (m), 1963 (vw), 1893
(vw), 1826 (vw), 1590 (w), 1429 (m), 1123 (s), 1114 (s), 1040 (s),
1023 (s), 996 (s), 893 (s), 744 (m), 717 (s), 700 (s), 553 (s), 526
(w). HR-ESI-MS. [M − 3THF + Cl]− (m/z): Calcd for
[C48H40Cl3CrO6Si4Zn2]−, 1108.8961; found, 1108.8972.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c00279
Notes
[PhL2CrII][ZnBr]2(THF)3 (10Br). To a solution of 6 (22.0 mg, 0.02
mmol, 1 equiv) in THF, ZnBr2 (8.8 mg, 0.04 mmol, 2.1 equiv) was
added, and the solution was stirred overnight at room temperature.
The turquoise solution was filtered, the solvent removed under
reduced pressure, and the solid residue washed with hexane as well as
small amounts of Et2O. After redissolving the solid in THF, the
solution was layered with hexane, which led to turquoise crystals of
10Br (15.2 mg, 0.01 mmol, 59%) suitable for single crystal X-ray
diffraction analysis.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We are grateful to the Deutsche Forschungsgemeinschaft (LI
714/10-1 and the cluster of excellence “Unifying Systems in
■
̈
Catalysis” EXC 2008) as well as the Humboldt-Universitat zu
Berlin for financial support.
UV/vis [λmax(THF)/nm (ε/(cm−1 mol−1 L))]: 672 (28). IR (νmax
/
̅
cm−1 (KBr)): 3069 (m), 3045 (m), 2980 (m), 2958 (m), 2885 (m),
1963 (vw), 1893 (vw), 1826 (vw), 1590 (w), 1429 (m), 1124 (s),
1114 (s), 1041 (s), 1024 (s), 995 (s), 887 (s), 743 (m), 718 (s), 699
(s), 554 (s), 524 (w). HR-ESI-MS. [M − 3THF + H2O + MeCN]−
(m/z): Calcd for [C50H45Br2CrO7NSi4Zn2]−, 1220.8633;
found,1220.8484.
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ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c00279.
H
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