Broadening the reactivity spectrum of a phthalocyanine catalyst while suppressing its nucleophilic, electrophilic and radical degradation pathways

Article abstract of DOI:10.1039/c1dt10458f

A robust molecule that resists degradation via nucleophilic, electrophilic and radical attacks is described. Coordinated O₂ is reduced catalytically, producing efficiently thyil radicals in spite of the extreme electronic deficiency of the catalyst.

Full text of DOI:10.1039/c1dt10458f

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Broadening the reactivity spectrum of a phthalocyanine catalyst while
suppressing its nucleophilic, electrophilic and radical degradation pathways†
Andrei Loas, Robert Gerdes, Yongyi Zhang and Sergiu M. Gorun*
Received 17th March 2011, Accepted 23rd March 2011
DOI: 10.1039/c1dt10458f
A robust molecule that resists degradation via nucleophilic,
electrophilic and radical attacks is described. Coordinated O2
is reduced catalytically, producing efficiently thyil radicals in
spite of the extreme electronic deficiency of the catalyst.
iso-perfluoropropyl (i-C3F7) groups gives (i-C3F7)8F8PcM, abbre-
viated to F64PcM (3-M), enhances Pc solubility, produces the first
X-ray quality crystals of a halogenated Pc and severely depresses
the Pc frontier orbitals.3 For 3-M, p–p stacking is disfavored both
in solution4 and in the solid-state.4,5 Diamagnetic 3-Zn catalyzes
the transfer of solar energy to O2 to form O2 that oxygenates
quantitatively an external substrate, (S)-(-)-citronellol.6
3
1
We report an organic-based, thermally and chemically robust
molecule that may suggest ways to design materials refractory
to nucleophilic, electrophilic or radical attack while exhibiting
useful aerobic catalytic properties. Organic-based molecules are
problematic for aerobic oxidations since their C–H bonds are
susceptible to radical attack. In the case of metal phthalocyanines,
Fig. 1, H16PcM (1-M),1 Cythochrome P450 related molecules,
their C–H bonds and p–p stacking limit their utility as oxidation
catalysts. The replacement of H by F, to give F16PcM (2-M)
enhances the already high Pc stability to electrophilic degradation,
for example by sulfuric acid, but favors nucleophilic susceptibility,2
while promoting aggregation. Thus, even the strongest C–X bonds
are insufficient to render this vast class of useful molecules
completely stable. Replacing half of the F atoms of 2-M with
Radical chemistry represents a challenge, which we have
approached by examining a model reaction, viz. the catalyzed
autooxidation of corrosive and foul smelling RSH, a process
practiced industrially (MEROX), catalyzed by partly sulfonated
1-Co.7 The overall reaction stoichiometry is 4 RSH + O2 → 2
RSSR + 2 H2O. Redox reaction pathways, via both Co(II)/Co(III)
and Co(II)/Co(I) pairs are possible. In both cases S- and O-
centered radicals are intermediates. For the relevant Co(II)/Co(I)
pathway (see below), the coordination of RS- to Co(II) is followed
by (i) the reduction of Co(II) to Co(I) and formation of RS∑, (ii)
oxidation of Co(I) by coordinated O2 to regenerate Co(II) and form
∑
O2 - (superoxide). The cycle is repeated to form O22- (peroxide) and
RS∑.8–17 Reaction details are shown in eqn (1):
RS- + PcCo(II) → [RS-–Co(II)Pc] → [RS∑–Co(I)Pc]
(1)
(2)
[RS∑–Co(I)Pc] → RS∑ + PcCo(II) + e-
Soluble (SO3H, SO3Na)4PcCo,8–12 and (COOH)2,4,8PcCo,13 have
been used to reveal mechanistic details in solution. Hetero-
genized systems used 1-Co,14 (COOH)4PcCo,14 (NO2)4PcCo,14
-
(NH2)4PcCo,15 (SO3Na)1,2PcCo,16 (SO3 )4PcCo.17
Polymer composites have also been used.18,19 From a steric point
of view, site-isolation in a matrix hinders the reaction of PcCoO2
with another PcCo to form an inert m-peroxo complex.20 Turnover
numbers increase, for example for C10H21SH from 150 to 770.17b
From an electronic point of view, since the Co(II) to Co(I) reduction
is the rate determining step (r.d.s.), stabilization of Co(I) is desired.
Overstabilization, however, could hinder catalyst reoxidation to
Co(II), eqn 2, and thus the catalytic process. Indeed, a Sabatier
(volcano) plot of the rate of electrocatalytic oxidation of RSH
vs. the PcCo(II)/Co(I) reduction potentials exhibits a negative
slope, indicating that the reoxidation to Co(II) controls the r.d.s.21
The potentials, in turn, correlate with substituents’ Hammett
constants, Fig. 2. Previously, 2-Co was the extreme low-rate
point due to the strongest F-induced stabilization of Co(I). The
paramagnetic 3-Co,3b is electronically related to other PcCo’s, all
exhibiting a singly occupied dz2 and equivalent dxz and dyz orbitals
Fig. 1 Cobalt phthalocyanines. Color code: F, green; N, blue; O, red;
C, gray; Co, orange. (a) 1-Co: R1 = R2 = H; 2-Co: R1 = R2 = F; 3-Co:
R1 = i-C3F7, R2 = F. (b) F64PcCoO2 (3-CoO2) reaction intermediates
∑
-
(O2 stands for both O2 and O22-) drawn based on the X-ray structure
of 3-Co(acetone)2 with the F groups shown as van der Waals spheres
and the Co coordination sphere depicted as balls-and-sticks. The atomic
coordinates of all atoms except O2 have been determined experimentally.3b
Department of Chemistry and Environmental Science, New Jersey Institute
of Technology, Newark, NJ, 07102, USA. E-mail: gorun@njit.edu
† Electronic supplementary information (ESI) available: Materials and
methods, ESR spectra of F64PcCo, ESR parameters of selected phthalo-
cyanines, UV-Vis titration of F64PcCo, stability of PcCo complexes and O2
consumption in the oxidation of PBT. See DOI: 10.1039/c1dt10458f
5162 | Dalton Trans., 2011, 40, 5162–5165
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Fig. 2 (a) Plot of Pc(Co(II)/Co(I)) reduction potentials vs. the sum of substituents Hammett s constants. (SO3-)4Pc: R1 = SO3-, H; (NH2)4Pc: R1 =
NH2, H; (NO2)4Pc: R1 = NO2, H; (OCH3)8Pc: R1 = OCH3; (OC8H17)4Pc: R1 = OC8H17, H.21 Equation: y = -0.579 + 0.0518x; correlation coefficient:
0.9955. Inset: calculated reduction potentials for hypothetical (Rf)8F8Pc, using Rf substituents with known Hammett constants.25 R2 = F, R1 = Rf (in
ascending order of the Eo¢Co(II)/Co(I) potentials): propyl, isopropyl (F64Pc, experimental point), ethyl, methyl, t-butyl. (b) O2 consumption in the catalyzed
autooxidation of 2-mercaptoethanol in aqueous THF.
(ESR in solution and solid-state, Table S1†). Axial binding by the
weakly coordinating acetone is noted in solid-state.3 Coordination
of N-methylimidazole (ESR, Fig. S1, S2†) and ligand-independent
site-isolation, for example for M = Zn,6 and Cu,4 in solution and
in films5 are characteristics imparted by the F64Pc scaffold. The
thermal stability of 3-Co is high; the complex sublimes in air at
~380 ◦C without decomposition. Interestingly, 3-Co cannot be
electrochemically oxidized to Co(III) in DMF within the solvent
limits, but its reduction occurs at Eo¢ = -0.22 V (vs. SCE), thus
justifying the choice of the Co(II)/Co(I) catalytic pathway. The
3-Zn reduction value is -0.30 V.22
soft–soft type interaction and (iii) a high affinity for axial ligands.
Thus, DFT frontier orbital energy calculations for 1-Co, 2-Co
and (C2F5)8F8PcCo (F48PcCo, 3¢-Co) a surrogate for 3-Co (too
large for the calculations) reveal that the ionization potentials
increase by ~1.3 eV and ~1.1 eV from 1-Co to 2-Co and 2-
Co to 3¢-Co, respectively. Since C2F5 and i-C3F7 have similar
Hammett constants25 (Fig. 2a, inset) 3-Co and 3¢-Co should have
similar potentials.3d Electron affinity varies similarly, establishing
progressively more difficult oxidation/easier reduction and more
favorable axial binding as the F content increases.
The results of thiol coupling studies using 1-, 2- and 3-Co and
2-mercaptoethanol (2-ME) are shown in Fig. 2b. The reactions
produce only the expected 2-hydroxyethyl disulfide (1H and 13C
NMR). No other S-oxidized products are observed, thus allowing
a 4 : 1 direct correlation between the number of moles of RSH and
O2 consumed, respectively. In the presence of a 1000-fold molar
excess of thiol, but in the absence of base, 3-Co(II) is not reduced. In
contrast, the formation of the thiolate ion upon addition of NaOH,
[thiol]/[NaOH] = 110/1, results in instantaneous appearance of
3-Co(I) (UV-Vis, Fig. S6†). Immediate O2 uptake occurs only
when both RS- and the catalyst are present. Light makes no
difference indicating absence of solar energy transfer. The catalysis
parameters are listed in Table 1.
Do steric factors favor Co lower oxidation states? A statistical
X-ray analysis of all Co porphyrins (Por) and Pcs in the Cambridge
Crystallographic Database23 indicates that Co deviates by less than
˚
0.1 A from the ligand N4 coordination plane regardless of its
oxidation state (I, II or III) or coordination number. For Pcs the
mean Co–N distances differ by approx. 1 e.s.d. when Co(II) and
˚
Co(III) are considered, viz. 1.927 0.003 A on average. For the only
˚
PcCo(I) complex, Co–N distances range is 1.879–1.914 A, mean
24
˚
1.896 A. The shortening of the Co–N distances upon reduction
˚
from Co(II) to Co(I) viz. 0.035 A is identical for both Por’s and Pc’s.
˚
Note that the mean Co(II)–N distance in 3-Co, 1.926 A, is typical
for both Co(II) and Co(III) and thus Co(I) is not favored. Taken
together, the X-ray data suggests neither a structural hindrance for
oxidation of Co(II) to Co(III), nor a preference for the reduction
of Co(II) to Co(I). Thus, the 3-Co’s record electronic deficiency,
Fig. 2a, beyond 2-Co, is determined by electronic factors: aromatic
F replacement by Rf groups exacerbates electronic deficiency due
to loss of aromatic F p-back bonding. Relevant for catalysis, eqn
(1a), the reversible chemical reduction 3-Co(II) ↔ 3-Co(I) occurs
in the presence of HO- ions, as indicated by isosbestic points
and the increase of the 710 nm Q-band of the Co(I) complex
at the expense of the 670 nm Q-band of the Co(II) one (see
Fig. S3†). HCl completely reverses the reduction. In contrast,
the isostructural F64Pc(2-)Zn(II) ↔ F64Pc(3-)Zn(II) reduction is
ligand centered. The actual catalytic activity of 3-Co was far
from certain given (i) the inverse correlation between electron
deficiency and thiol oxidation rates,21 (ii) strong S–Co bonds, a
3-Co is highly stable at 25 ◦C under the reaction conditions
with nucleophiles and radicals present. Moreover, 3-Co showed
Table 1 Parameters of the catalyzed autooxidation of 2-mercaptoethanol
Catalyst
Stabilitya
Rateb
TOFc
TONd
H16PcCo
F16PcCo
F64PcCo
75%
35%
>99%
23.8
4.9
12.8
3.0
0.84
1.74
12 600
7700
13 000
a Stability, defined as the ratio of (Q-bands intensities after 24 h/initial
intensities) ¥ 100. See also Fig. S4–S7†. Pc degradation products have not
been identified. b Initial reaction rates, mmol O2 min-1, calculated from the
linear fit portion of Fig. 2a. c Turnover frequency, mol RSH s-1 mol Pc-1,
calculated under pseudo-first order conditions. d Total oxidation number
after 5 h, limited by the RSH batch reaction to ~13 000.
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no degradation for at least 2 days in refluxing, basic aqueous
THF, or in concentrated H2SO4. Since the aromatic F substituents
in 3-Co should be more susceptible to nucleophilic attack relative
to 2-Co, the protective steric effect imparted by the i-C3F7 groups
becomes apparent.
thiolate binding is believed to weaken the M–O bond thus favoring
the release of H2O2,28 an effect relevant here since H2O2 released
from the Co center contributes to thiol coupling.
In summary, we report the first member of a family of
three-dimensional, metal–organic aerobic catalysts whose organic
ligand framework is designed to stabilize it against all possible
degradation pathways. Coordination and reduction of O2 within
a fluorinated active site pocket leads to both O- and S-centered
radicals, the latter coupling to disulfides.
The stabilization of ligand composition, while offering labile
sites for catalysis is a challenge29 that responds to identified
future technology needs.30 The fluoro-perfluoroalkyl substituents
might offer an answer within phthalocyanines and, maybe, other
frameworks.
Dedicated to the memory of the late Prof. Philip H. Rieger
of Brown University. Financial support from the National Sci-
ence Foundation and the US Army is gratefully acknowledged.
B. Bench is thanked for parts of the electrochemistry and ESR
data.
The initial oxidation rates are partly incongruent with the
reduction potentials. The calculated ratio of initial reaction rates
for 2-Co/1-Co based on reduction potentials is 0.16, vs. the
observed value of 0.84/3.0 = 0.28. In contrast, 3-Co, presumably
less efficient than 2-Co, has a rate twice as high, ~20 times faster
than predicted based on reduction potentials. Since the reoxidation
of Co(I) to Co(II) (the r.d.s.) is proceeding as expected based
on free energy correlations, the discrepancy is unexplainable on
electronic grounds alone. Possible reasons for the enhanced rate
of 3-Co include: (i) Rf steric crowding leading to an accelerated
departure of the thyil radical (product), a classical feature of
enzymatic reactions and consistent with the limited miscibility of
hydrocarbons and fluorinated solvents, (ii) an Rf-induced extra
loss of Co2+ polarizability, making it unlikely to bind soft S-
radicals, (iii) hydrophobic preference for neutral (thyil radical)
over charged (thiolate) species in the immediate Rf catalytic
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∑
-
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©