Observations on the Mechanochemical Insertion of Zinc(II), Copper(II), Magnesium(II), and Select Other Metal(II) Ions into Porphyrins

Article abstract of DOI:10.1021/acs.inorgchem.9b00052

Building on a proof of concept study that showed the possibility of the mechanochemical insertion of some M(II) metals into meso-tetraphenylporphyrin using a ball mill as an alternative to traditional solution-based methods, we present here a detailed study of the influence of the many experimental variables on the reaction outcome performed in a planetary mill. Using primarily the mechanochemical zinc, copper, and magnesium insertion reactions, the scope and limits of the type of porphyrins (electron-rich or electron-poor meso-tetraarylporphyrins, synthetic or naturally occurring octaalkylporphyrins, and meso-triphenylcorrole) and metal ion sources suitable for this metal insertion modality were determined. We demonstrate the influence of the experimental metal insertion parameters, such as ball mill speed and reaction time, and investigated the often surprising roles of a variety of grinding agents. Also, the mechanochemical reaction conditions that remove zinc from a zinc porphyrin complex or exchange it for copper were studied. Using some standardized conditions, we also screened the feasibility of a number of other metal(II) insertion reactions (VO, Ni, Fe, Co, Ag, Cd, Pd, Pt, Pb). The underlying factors determining the rates of the insertion reactions were found to be complex and not always readily predictable. Some findings of fundamental significance for the mechanistic understanding of the mechanochemical insertion of metal ions into porphyrins are highlighted. Particularly the mechanochemical insertion of Mg(II) is a mild alternative to established solution methods. The work provides a baseline from which the practitioner may start to evaluate the mechanochemical metal insertion into porphyrins using a planetary ball mill.

Full text of DOI:10.1021/acs.inorgchem.9b00052

Article
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
pubs.acs.org/IC
Observations on the Mechanochemical Insertion of Zinc(II),
Copper(II), Magnesium(II), and Select Other Metal(II) Ions into
Porphyrins
Adewole O. Atoyebi and Christian Bruckner*
̈
Department of Chemistry, University of Connecticut, Unit 3060, Storrs, Connecticut 06269-3060, United States
*
S Supporting Information
ABSTRACT: Building on a proof of concept study that showed the possibility of
the mechanochemical insertion of some M(II) metals into meso-tetraphenylpor-
phyrin using a ball mill as an alternative to traditional solution-based methods, we
present here a detailed study of the influence of the many experimental variables on
the reaction outcome performed in a planetary mill. Using primarily the
mechanochemical zinc, copper, and magnesium insertion reactions, the scope
and limits of the type of porphyrins (electron-rich or electron-poor meso-
tetraarylporphyrins, synthetic or naturally occurring octaalkylporphyrins, and meso-triphenylcorrole) and metal ion sources
suitable for this metal insertion modality were determined. We demonstrate the influence of the experimental metal insertion
parameters, such as ball mill speed and reaction time, and investigated the often surprising roles of a variety of grinding agents.
Also, the mechanochemical reaction conditions that remove zinc from a zinc porphyrin complex or exchange it for copper were
studied. Using some standardized conditions, we also screened the feasibility of a number of other metal(II) insertion reactions
(
VO, Ni, Fe, Co, Ag, Cd, Pd, Pt, Pb). The underlying factors determining the rates of the insertion reactions were found to be
complex and not always readily predictable. Some findings of fundamental significance for the mechanistic understanding of the
mechanochemical insertion of metal ions into porphyrins are highlighted. Particularly the mechanochemical insertion of Mg(II)
is a mild alternative to established solution methods. The work provides a baseline from which the practitioner may start to
evaluate the mechanochemical metal insertion into porphyrins using a planetary ball mill.
9
INTRODUCTION
harvesting. The formation of metalloporphyrins from their
■
10
free bases is a classic metathesis reaction. In its simplest form,
the reaction of a divalent metal salt (ML ) with a dibasic
porphyrin (PorH ) forms the neutral metalloporphyrin
Por]M and corresponding acid of the metal salt anion (LH;
Mechanochemistry is a broad term that defines chemical
syntheses where activation is induced by mechanical force,
such as through grinding or the use of ultrasonic waves. The
use of ball mills in which the dry, solid reagents are intensely
ground together to induce a reaction is particularly attractive.
Possible advantages of solid state reactions over conventional
2
1
,2
2
[
11
Scheme 1):
3
solution-based reactions, in general, and mechanochemical
Scheme 1. Generalized Metal Insertion Reaction into
Porphyrins
methods employing mills, in particular, are the avoidance of a
reaction solvent, frequently decreased reaction times, increased
reaction selectivity, or access to different reaction pathways,
although it is rare to have a single mechanochemical reaction
1
,2
encompass all advantages. There is a growing concern
toward the use of many of the common solvents in chemical
synthesis because of their associated health or environmental
1
,2a
hazards.
Mechanochemical processes avoiding a reaction
solvent altogether offer a greener alternative, particularly also
when factoring in the energetic cost savings and hazards
avoided that are associated with solvent heating, cooling, and
removal. Syntheses under mechanochemical conditions in ball
mills have found broad application in organic synthesis (C−C
and C-heteroatom formations) and the synthesis of metal
coordination complexes, including the formation of inorganic
Many variations of this reaction are known, such as involving
metals of lower oxidation states, the participation of redox
reactions during the metal insertion step, axial ligands to the
metalloporphyrin, the formation of double-decker complexes,
10,12
etc.
Generally, the rate-limiting steps are determined by
the considerable kinetic barrier of the metal insertion step into
1
1
1
,2,4
the planar and relatively rigid porphyrin. The planar
materials such as MOFs.
The formation of metalloporphyrinoids is key for the broad
5
utilization of porphyrinoids in, for example, catalysis,
Received: January 7, 2019
6
,7
8
biomedicine,
molecular electronics, or artificial light-
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.9b00052
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Inorganic Chemistry
Article
porphyrin points its (protonated) donor atoms of low basicity
toward the center of its (small) central cavity. Nature
overcomes these barriers in the metal (iron) insertion enzymes
RESULTS AND DISCUSSION
■
Mechanochemical Insertion of Zinc into TPP. The
grinding together of free base TPP with 1.1 equiv of zinc
acetate (Zn(OAc) ·2H O) in a planetary ball micromill
(
(
ferrochelatases) by inducing a deformation of the porphyrin
protoporphyrin IX) from planarity upon binding to the
2
2
(
Fritsch GmbH, Germany, Pulverisette 7 classic line) equipped
enzyme, thereby exposing the inner NH/N atoms; this
deformation enzyme therefore catalyzes the formation of the
with two 12.5 or 50 mL volume agate grinding vessels
containing with five agate balls (for additional information, see
Experimental Section and Supporting Information) at ambient
conditions (air conditioned to 22 °C, ∼40−50% rel. humidity)
resulted in its quantitative conversion to the corresponding
zinc complex, [TPP]Zn, within 10 to 15 min (entry 1−1). The
time needed for the completion of the reaction was estimated
by retrieval of small aliquots from the vessel at certain time
intervals and their analyses by TLC and UV−vis spectroscopy
see Supporting Information). The precision of the reaction
times quoted is therefore also determined by the sampling
frequency, typically ±5 min in runs shorter than 30 min, and
15 min for longer runs. Because of the retrieval of the
aliquots, the grinding was intermittently stopped, but the run
times listed are the actual run times without inclusion of any
pauses. The interruptions also assured that the vessels did not
warm to any appreciable extent (we note exceptions below),
minimizing temperature effects. We also explicitly monitored
the temperature in some runs, assuring us that the bulk
temperatures in these runs did not exceed a few degrees
Celsius above ambient temperature. Using representative larger
scale reactions, we confirmed the quantitative conversion of
the starting porphyrin by column chromatographic isolation
and gravimetric analysis of the products.
1
3
hemes. There are also the intrinsic kinetic barriers inherent
to any specific metal ion that need to be overcome. Therefore,
traditional approaches toward the formation of metallopor-
phyrins generally employ the reaction of the porphyrin and a
metal salt (in a low oxidation state) in a high-boiling donor
solvent (pyridine, bp 115 °C; DMF, bp 153 °C; PhCN, bp 191
°C) under reflux conditions over extended periods of time,
(
although some metals insert also under milder conditions (in
10,14
hot or warm CHCl /MeOH mixtures, for example).
The
3
stable oxidation state of the metal in the metalloporphyrin is
then frequently reached by a subsequent (air) oxidation
±
1
0,12
reaction.
As a consequence of the often relatively harsh
conditions, metal insertion reactions into more sensitive
porphyrinoids can be accompanied by considerable macrocycle
degradation or other unwanted side reactions, driving the
15
search for alternative methodologies.
Surprisingly, therefore, the firstand to date loneproof of
concept study on the mechanochemical preparation of
metalloporphyrins was presented by James and co-workers
15b
only most recently.
Their report described the variously
successful mechanochemical insertion of Zn(II), Ni(II),
Cu(II), and Fe(II) salts into a single porphyrin, meso-
tetraphenylporphyrin (TPP), using a shaker mill. The
insertions of Li, Mg, Mn, Co, Au, and Pt were described as
unsuccessful. Some variations of the variables of the
mechanochemical reaction conditions were reported for a
number of failed reactions only.
The mechanical energy applied, i.e., increasing rotational
speed or use of a larger grinding vessel that exposes its contents
2b
to larger impact and shear forces, had a direct effect on the
reaction times: The faster the rotational speed or the larger the
vessel, the faster the zinc insertion reaction (Figure 1). This
clearly demonstrated that the metal insertion is the result of
mechanochemical action.
Herein, we investigate in more depth the use of a planetary
ball mill in the formation of zinc and copper porphyrins with
respect to the type of porphyrins susceptible to this metal
insertion modality (electron-rich or electron-poor meso-
tetraarylporphyrins, octaalkylporphyrins, as well as meso-
triarylcorrole and, peripherally, meso-tetraarylchlorins), the
suitability of various metal ion sources with respect to yield
and reaction times for the formation of the metalloporphyrin,
as well as variation of the insertion conditions (the grinding
energy applied−i.e., ball mill speeds and vessel volumes,
reaction times, addition of grinding agents). We also studied
mechanochemical reaction conditions that remove the zinc ion
or exchange it with a copper ion. We expand on an unexpected
observation, namely, the insertion of Mg(II) mediated by
Florisil, a magnesium silicate. We also report on the (in)ability
to insert of a number of other (transition) metal ions (VO, Ni,
Fe, Co, Ag, Cd, Pd, Pt, Pb) using a broader screening, with
some notable differences or qualifications of the findings by
Figure 1. Rotation speed (mechanical energy) dependence of the
time needed for the quantitative insertion of zinc into TPP.
Conditions: 12.0 mg TPP and 85.7 mg (20 equiv) Zn(OAc) ·2H O.
2
2
15b
James and co-workers.
We will reveal findings that are
important for the mechanistic understanding of the mecha-
nochemical metal insertion pathway as well as demonstrate
that mechanochemical metal insertion conditions are some-
times advantageous over solution state conditions. In any case,
we will report that mechanochemical metal insertion reactions
into porphyrins are fraught with their own sets of difficulties,
surprises, and shortcomings.
The stoichiometric ratio of zinc source to free base
porphyrin had comparably little impact on the speed of
formation of the complex (entries 1−1, 1−3, 1−5, and 1−7).
In a series of runs in which the porphyrin:metal source was
varied from 1:1.1 to 1:20, the time of the reaction to
completion was shortened only by a maximum factor of 2.
B
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The series of experiments leading to Table 1 identified
standard reaction conditions (800 rpm, 50 mL agate vessel,
varying effects on the rates of the metal insertion reaction
(Table 2). In this comparison, we ignore grain size effects;
Table 1. Effect of the Stoichiometric Ratio of the Zinc
Table 2. Effect of Various Additives on the Time to
Completion of the Zinc Insertion into TPP
a
Source and Free Base TPP on the Time to Completion of
a
the Zinc Insertion Reaction
time to
completion or %
entry
x equiv Zn(OAc) ·2H O
additive
time to completion
2
2
entry
0.5 g additives
silica
quartz sand (sea
sand, washed)
product(s)
[TPP]Zn
[TPP]Zn
conversion
1−1
1−2
1−3
1−4
1−5
1−6
1−7
1−8
1.1
1.1
2.5
2.5
5
5
20
20
10−15 min
30 min
10−15 min
15 min
10 min
15 min
2−1
15 min
20 min
0.5 g silica
0.5 g silica
0.5 g silica
0.5 g silica
2−2
2−3
silica-NH2
[TPP]Zn
≤65%
conversion
after 60 min
2
−4
−5
neutral alumina,
Brock activity I
[TPP]Zn
40 min
10 min
15 min
b
d
2
basic alumina,
NR after 60 min
a
Brock activity I
Reaction conditions: Rotational speed of 800 rpm, 50 mL agate
vessel, milled in ∼5 min intervals with ∼2 min breaks.
2−6
acidic alumina,
Brock activity I
[TPP]Zn
>95% conversion
after 35 min
2
2
2
−7
−8
−9
montmorillonite
sodium chloride
cellulose powder
MN 300
[TPP]Zn
[TPP]Zn
[TPP]Zn
30 min
35−40 min
discontinuous runs) that were then used in the bulk of the
subsequent experiments to map the scopes and limits of the
mechanochemical metal insertion reactions. The conditions
used should not suggest that, for instance, lower stoichiometric
amounts of metal source would not lead to reaction
completion. In fact, James and co-workers successfully used
≤70%
conversion
after 50 min
c
2−10
florisil
[TPP]Zn and [TPP]Mg >70% conversion
in a ∼80:20 ratio after 60 min
(MgO:xSiO ·
2
H O)
2
15b
a
1
:1 metal source to TPP ratios.
Mechanochemical Grinding Aids. The addition of
grinding aids has been shown to be advantageous in
Reaction conditions: Rotational speed of 800 rpm, 50 mL agate
vessel, 2.5 equiv Zn(OAc) ·2H O, milled at ∼5 min intervals with ∼2
min breaks. Notable heating of the reaction vessel took place when
using this additive. The reaction was optimized for Mg-insertion, see
Table 8. NR: no reaction observed
2
2
b
c
16
mechanochemical reactions. We tested the effects of the
addition of silica gel (ratio TPP/silica ∼ 1:42 by weight) to
reactions with varying porphyrin/metal ratios (Table 1). The
addition of the silica gel effectively diluted the reactants by a
factor of ∼50, but this slowed the reaction down by only a
factor of ∼2 (cf., e.g., entries 1−1 and 1−2). However, the
presence of the silica gel helped much in forming a
homogeneous mixture of the reagents, minimized the amount
of unreacted porphyrin left in the inevitable “dead space” of the
milling vessel, and assisted in the retrieval of the very fine and
sticky product mixture from the vessel walls and grinding balls,
possibly also by absorbing the (acetic) acid formed during the
reactions. Silica gel was therefore routinely added in
subsequent experiments. Extraction/filtration of the reaction
mixture using, e.g., CH Cl readily separated the product
d
even if initial grain size differences existed, the grains break
down in the mill within the first seconds of any run, i.e., at a
much faster rate than the metal insertion reaction. We found
indications that mechanical as well as chemical factors play a
role in the outcome of the reactions. Some additives seem to
have had no effect beyond being a mechanical aid, like rock salt
(Mohs hardness 2.0−2.5; entry 2−8) or sea sand (Mohs
hardness, depending on purity, ∼6; 7 for pure quartz; entry 2−
2). Softer aids, like montmorillonite (Mohs hardness of 1−2;
entry 2−7) and particularly cellulose (entry 2−9) result in
slower reaction rates. Overall, silica gel (of unspecified
hardness; the hardness of fused silica gels varies and is much
2
2
17
[
[
TPP]Zn from the silica gel. The isolated yields of crystallized
TPP]Zn were identical to comparison solution reactions.
lower than that of crystalline or glassy SiO2) caused the
largest rate acceleration, explaining why the reactions “diluted”
with silica gel were still fast compared to the rate of the
“undiluted” reactions (Table 1).
Some advantages of the solvent-less mechanochemical
insertion step were therefore mitigated during workup.
However, since the focus of this study was to broadly screen
the multiple factors that influence the mechanochemical metal
insertion reactions, we chose convenience and comparability of
the various reactions over largest overall practical benefits.
James and co-workers have shown that the use of 1.0 equiv of a
zinc salt of a volatile acid (zinc acetate) avoids this solvent-
based workupa simple drying step was sufficient to remove
the only byproduct (acetic acid) in the quantitative
The three different (neutral, basic, and acidic) aluminas
tested (entries 2−4, 2−5, and 2−6) had the same Brock
activity level I. Assuming all also had the same mechanical
effects because of similar hardness, the starkly different ways in
which they affect the metal insertion reaction rate allow only
the conclusion that their different (surface) chemistries are
responsible for their different effects: Acidic alumina is the
fastest (entry 2−6), neutral alumina much slower (entry 2−4,
rate comparable to that with the hard “neutral” additive
sodium chloride (entry 2−8), or the softer montmorillonite,
entry 2−7), and no reaction was observed in the presence of
15b
conversions they reported.
The additive is not innocent in the mechanochemical zinc
insertion reaction, however. Different additives had starkly
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Article
the basic alumina (entry 2−5). [Hardness of the aluminas used
was not determined but is drastically lower for the noncrystal-
line aluminas than for their very hard crystalline counterparts
(
corundum) of Mohs hardness of up to 9. We observed a
significant warming of the reaction vessel when using basic
alumina, perhaps an indicator of its greater hardness; we also
like to point out the potential hardness incompatibility of the
agate vessel/balls and the aluminas should they contain
crystalline phases. This will lead to mechanical damage of
the vessel. Pitting of the agate vessels/balls over time was
indeed observed, but could not be attributed solely to the runs
using alumina.] Tellingly, the reaction in the presence of basic
silica gel modified with amine groups (entry 2−3) is also very
slow. Inversely, the rates of zinc insertion in the presence of
regular silica (that tends to be slightly acidic) is faster
compared to that of the more neutral sand of approximate
identical bulk composition (entry 2−2).
This suggests that porphyrin protonation and the con-
18
comitant conformational distortion might be an important
aspect for the mechanochemical insertion of zinc into a
porphyrin. In fact, some correlation between the pK of
3
19
porphyrins and copper insertions rates were drawn. Inversely,
assistance in porphyrin deprotonation by the basicity of
particular grinding agents may not be needed. Alas, too acidic
insertion conditions are detrimental (at least for the zinc
insertion reaction), as we show below that visible (bulk)
diprotonation of TPP by strong acids effectively blocks any
zinc insertion. The base conditions could theoretically also
convert the zinc ion to a basic salt, but we will show below that
ZnO and the zinc carbonates/hydroxides/oxides formed on
the surface of zinc powder are still competent sources of zinc
for the formation of zinc porphyrins.
Figure 2. (A) Time course of the degradation of different porphyrins
indicated using the additive silica gel. (B) Time course of the
degradation of TPP on the additives indicated. Conditions: 5.0 mg of
porphyrin milled with 0.5 g of additive at a rotational speed of 800
rpm in a 50 mL agate vessel.
Unexpected was the outcome of the experiment using
Florisil, a magnesium silicate (MgO:xSiO ·H O): The zinc as
2
2
well as the magnesium complexes of TPP were formed (entry
2
−10). We did not pursue the use of Florisil as a grinding aid
Information) or TLC. In fact, we could not extract the gray
product from the silica gel with any solvent tested. T PP
F
any further, but for more details on this surprising pathway of
formation of [TPP]Mg and other magnesium porphyrins, see
below.
degraded ∼30% faster than TPP, and OEP yet another ∼30%
faster. Again, no products could be identified, although a
20
F
21
Overall, we found silica gel to be the most generally suitable
and convenient grinding aid for the insertion of zinc into
porphyrins. We have therefore used silica gel as the grinding
aid in the standardized conditions chosen for the bulk of the
subsequent experiments described. The overall conclusion
from the screening of the grinding aids is that the practitioner
must select a possible grinding aid with great care as it is far
from innocent with respect to the metal insertion reaction. The
following studies will highlight this in a drastic manner.
Mechanochemical Porphyrin Degradation. When
using grinding aidsand particularly silica gelin metal
insertion reactions requiring milling times exceeding 30 min,
we observed severe degradations of the porphyrins. When
milling porphyrins, the metal source, and silica gel over
extended periods of time, we noticed also that the color of the
reaction mixtures invariably started to lose the deep color of
the porphyrin; the mixtures turned over time into a dull gray.
number of direct oxidation products of TPP, T PP, and
22
OEP are known. The rates of degradation vary in the
presence of the metal salts (not shown), likely reflecting the
stability of the metalloporphyrins toward (oxidative) degrada-
20−22
tion.
On the upside, except for the reactions involving the
chlorins (see below, Table 4, entries 4−11 and 4−12), the
degradation products that formed did not interfere with the
isolation or purity of the target products.
The nature of the grinding agent had a profound effect on
the rate of TPP degradation (Figure 2B). Sodium chloride and
sucrose led to a much less severe degradation of the porphyrin
(and some of this “degradation” may be an artifact attributable
to the difficulty of extracting the porphyrin from the ground
sugar that turns over time into a sticky paste). On the other
hand, calcite (CaCO in the form of Iceland spar) degraded
3
TPP even faster than silica gel.
The milling-based mechanochemical degradation of poly-
F
23
We therefore milled TPP, T PP, and OEP with silica gel (in
mers is under active investigation. As well, the mechano-
the absence of metal sources) for up to 120 min. After a given
time, we extracted the mixtures with a standardized amount of
solvent and analyzed the extracts by HPLC (Figure 2A).
All porphyrin peaks gradually decreased over time, with less
than 10% residual TPP after 2 h of milling time, and with no
new product becoming apparent by HPLC (see Supporting
chemical dehalogenation of, for example, environmental
pollutants milled with a number of reactive ingredients (such
as CaO or Fe powder) has been under investigation for some
time. However, we are not aware of reports of the
mechanochemical degradation of the otherwise very robust
porphyrins under milling conditions with supposedly “inert”
2
4
D
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Inorganic Chemistry
Article
grinding agents. We speculate that piezoelectric effects might
play a role. Sodium chloride, for example, is not a piezoelectric
material, and while the others are, including crystalline sucrose
reaction is significantly slower than the reaction with ZnO
(entry 3−7).
Overall, these experiments suggest that zinc sources
containing more basic counteranions are advantageous over
(
albeit its noncrystalline, malleable form it quickly forms in the
−
mill is not piezoelectric), the degradation mechanism operative
here clearly requires further study.
comparable sources with less basic counteranions (AcO over
−
2−
2−
ClO4 and O over S , respectively). This trend is in contrast
to the effects of the additives where the more acidic additives
led to faster rates, possibly pointing at their different roles in
the mechanism of the mechanochemical zinc insertion
reaction.
We must conclude that the mechanochemical reaction
conditions in the planetary mill in the presence of some
grinding aids cannot be considered mild, and the reaction
times should be minimized. Furthermore, the grinding aids are
in more than one way noninnocent, again highlighting that the
practitioner must select a grinding aid with great care and be
aware of possible degradation reactions.
However, as we will show below (Table 7), this
interpretation is not fully holding true for the insertion of
Cu(II) that is much less sensitive to the acidity of the
environment at which metal insertion is still observed. Our
results vary significantly from those by James and co-workers
who reported the inability to insert zinc into TPP using ZnO
or [Zn (CO ) (OH) ], even upon the addition of small
Zinc Sources for the Mechanochemical Preparation
of [TPP]Zn. Next to zinc acetate, various other zinc sources
proved to be competent in the delivery of zinc ions to free base
TPP (Table 3). Zinc perchlorate (Zn(ClO ) ·6H O) was
4
2
2
5
3 2
6
15b
amounts of solvent (DMF, MeOH, or H O). Our utilization
2
Table 3. Effect of Various Zinc Sources on the Time to
Completion of the Zinc Insertion into TPP
of a more aggressive planetary mill instead of the shaker mill
used by James and co-workers might be source of the observed
differences.
a
2d
Mechanochemical Zinc Insertion into Other Porphyr-
ins. Other porphyrins were also susceptible toward the
mechanochemical insertion of Zn(II) using a number of zinc
sources, each with their own insertion rates (Table 4). The β-
octaalkylporphyrin OEP reacted (entries 4−1 to 4−3) at rates
comparable to that of TPP with zinc acetate (cf. entries 1−2
and 4−1) but comparably faster with zinc powder (cf. entries
x equiv Zn(II)
entry
source
additives time to completion or % conversion
3
−1 2.5 equiv
0.5 g
silica
0.5 g
silica
>90% conversion after 50 min
Zn(ClO
−2 5 equiv Zn(ClO
O + 5 equiv
4 2 2
) ·6H O
3
4
)
2
·
40 min
3
−6 and 4−2) and zinc oxide (cf. entries 3−8 and 4−3).
Protoporphyrin IX, as its dimethyl ester (PPIX-DME, entry
−4), and etioporphyrin I (ETIO-I, entry 4−5) were chosen
as derivatives closely related to the naturally occurring β-
6H
2
DABCO
c
4
3
3
3
3
−3 5 equiv ZnCl
2
0.5 g
silica
0.5 g
silica
NR after 115 min (protonation of
TPP visible)
25
−4 5 equiv ZnCl + 5
>96% conversion after 30 min
2
alkylporphyrins. Both are faster than TPP and more in line
with the structurally related OEP. The electron-poor meso-
equiv DABCO
−5 5 equiv zinc powder,
44 μm grain size
−6 5 equiv zinc powder, 0.5 g
44 μm grain size silica
30−40 min
F
tetrakis(pentafluorophenyl)porphyrin T PP is slower overall
<
(
entry 4−6 to 4−8), with 5 equiv of zinc oxide accomplishing
>60% conversion after 80 min
<
within 60 min only a conversion of about 50%. The metal
3
3
−7 5 equiv ZnO
−8 5 equiv ZnO
55 min
complexes of electron-rich meso-tetrakis(thien-2- and 3-yl)-
2
3
0.5 g
silica
>60% conversion after 80 min
porphyrins T( Thiop)P and T( Thiop)P have found utility in
26
a number of electrocatalytic and sensing applications.
b
3
−9 5 + 10 equiv ZnS
∼50% after 40 min and 5 equiv ZnS, >
2
3
T( Thiop)P and T( Thiop)P react somewhat slower than
TPP (entries 4−9 and 4−10, respectively) but nonetheless
form smoothly the expected zinc chelates in under 60 min.
9
0% after 70 min and additional 10
equiv ZnS
a
Reaction conditions: rotational speed of 800 rpm, 50 mL agate
vessel, milled at ∼5 min intervals with ∼2 min breaks; silica gel (40−
5 μm particle size). Unspecified polymorph. NR: no reaction
The meso-tetraaryl-2,3-dihydroxychlorins (OH) TPC and
2
b
c
F
7
(OH) T PC are representatives of a class of relatively robust
2
27
observed.
chlorins but are nonetheless prone to acid- or thermally
induced dehydration and HF-elimination reactions; metal
insertion reactions into these chlorins are therefore not always
suitable, but much slower (>50 min, entry 3−1) than zinc
acetate (15 min, entry 1−4) under otherwise identical
conditions. The addition of base to the zinc perchlorate
reaction accelerated the reaction (entry 3−2). Surprisingly,
15c,27b
unproblematic.
Indeed, the mechanochemical insertion
of zinc, even under the most optimal conditions (entries 4−11
and 4−12), was possible but were accompanied by extensive
(20−40% decomposition), complex, and ill-defined degrada-
tion reactions. We found that the corresponding solution state
zinc chloride (ZnCl ) failed to insert zinc without the addition
2
of a solid base (DABCO; entries 3−3 and 3−4, respectively).
In the absence of this base, the mixture turned green over time,
the color of the diprotonated porphyrin.
insertion of zinc using zinc acetate in a CHCl /MeOH mixture
3
27
takes place readily upon slight warming. We thus abandoned
experiments using these chlorin diols and focused on the
principal factors controlling the metal insertions into
porphyrins.
Zinc oxide and zinc powder were also suitable for the
insertion of zinc into TPP; moreover, the addition of silica gel
to these reactions proved detrimental (entries 3−5 to 3−8). In
the case of zinc powder, we assume that the surface zinc
oxides/hydroxides/carbonates are the source of the zinc ions.
Because of their insolubility in organic solvents, neither source
is suitable for solution state zinc insertion reactions. Powdered
ZnS can also be used as a zinc source (entry 3−9), but the
Mechanochemical Demetalation of [TPP]Zn. The acid-
induced demetalation of zinc porphyrins is a well-known and a
frequently practiced reaction, traditionally requiring the
washing of a solution of [TPP]Zn dissolved in organic
solvents with a ∼3 M aqueous solution of a strong mineral acid
E
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Inorganic Chemistry
Article
Table 4. Effect of Various Free Base Porphyrins and Varying Zn(II) Sources on the Time of Completion of the Zinc Insertion
a
Reaction
entry
substrate
OEP
OEP
x equiv Zn(II) source
product(s)
[OEP]Zn
[OEP]Zn
time to completion or % conversion
4
4
4
4
4
4
4
4
4
4
4
4
−1
−2
−3
−4
−5
−6
−7
−8
1.1 equiv Zn(OAc) ·2H O
20 min
20 min
20 min
<15 min
10 min
25 min
30 min
2
2
5 equiv zinc powder
5 equiv ZnO
OEP
[OEP]Zn
PPIX-DME
ETIO-I
2.5 equiv Zn(OAc) ·2H O
[PPIX-DME]Zn
[ETIO I]Zn
2
2
2.5 equiv Zn(OAc) ·2H O
2
2
F
F
T PP
1.1 equiv Zn(OAc) ·2H O
[T PP]Zn
2
2
F
F
T PP
5 equiv zinc powder
5 equiv ZnO
[T PP]Zn
F
F
T PP
[T PP]Zn
>50% conversion after 60 min
2
2
b
−9
T( Thiop)P
3.0 equiv Zn(OAc) ·2H O
[T( Thiop)P]Zn
60 min
2
2
3
3
−10
−11
−12
T( Thiop)P
2.5 equiv Zn(OAc) ·2H O
[T( Thiop)P]Zn
40 min
2
2
(OH) TPC
2.5 equiv Zn(OAc) ·2H O
[(OH) TPC]Zn
60 min (40% decomposition)
60 min (20% decomposition)
2
2
2
2
F
F
(OH) T PC
2.5 equiv Zn(OAc) ·2H O
[(OH) T PC]Zn
2
2
2
2
a
Reaction conditions: rotational speed of 800 rpm, 50 mL agate vessel, milled at ∼5 min intervals with ∼2 min breaks; silica gel (40−75 μm
b
particle size). Reaction was run initially with 1.5 equiv Zn(OAc) ·2H O and an additional 1.5 equiv of Zn(OAc) ·2H O were introduced 40 min
into the reaction.
2
2
2
2
(
HCl is typically used) or the addition of p-TSA monohydrate
smoothly with the electron-rich or -poor tetraarylporphyrins
TPP, T PP, T( Thiop)P, and T( Thiop)P and the
octaalkylporphyrin OEP (Table 6). This confirms the results
10
F
2
3
or TFA to an organic solution of the zinc porphyrin. We find
now that the demetalation of [TPP]Zn can also be achieved
mechanochemically using a variety of solid acids (Table 5).
Table 6. Various Free Base Porphyrins and Their Time of
Completion of the Mechanochemical Copper Insertion
Reaction
Table 5. Reaction Conditions and Times for the Removal of
Zinc(II) from [TPP]Zn
a
a
time to
Cu(OAc) ·
time to
2
entry
reagent
product
completion
entry
substrate
H O
product(s)
[TPP]Cu
completion
2
5
5
5
−1 10 equiv pTsOH·H O
−2 10 equiv ClCH CO H
−3 0.5 g Dowex HCR-W2, H -form, 16−40
TPP
TPP
TPP
30 min
20 min
<25 min
6−1 TPP
6−2 OEP
6−3 T PP
1.5 equiv
1.5 equiv
1.5 equiv
2.5 equiv
15 min
15 min
15 min
30 min
2
[OEP]Cu
2
2
+
F
F
[T PP]Cu
mesh size
2
2
6−4 T( Thiop)P
[T( Thiop)P]
b
5−4 5 equiv EDTA
NR after 60 min
NR after 60 min
Cu
b
3
3
5−5 5 equiv Na EDTA·2H O
6−5 T( Thiop)P
2.5 equiv
2.5 equiv
[T( Thiop)P]
30 min
30 min
2
2
a
Cu
Reaction conditions: Rotational speed of 800 rpm, 50 mL agate
vessel, milled at ∼5 min intervals with ∼2 min breaks; 0.5 g silica gel
6−6 TPCorrole
[TPCorrole]Cu
b
a
(
40−75 μm particle size). NR: no reaction observed.
Reaction conditions: Rotational speed of 800 rpm, 50 mL agate
vessel, milled at ∼5 min intervals with ∼2 min breaks; 0.5 g silica gel
40−75 μm particle size).
(
Thus, milling [TPP]Zn with p-TSA monohydrate (entry 5−1),
monochloroacetic acid (entry 5−2), or Dowex, a sulfonated
polystyrol-based ion-exchange resin in the acidic form (entry
−3), resulted in the conversion of the purple color of the
metalloporphyrin to bright green, the color of the diprotonated
porphyrin, indicating that an irreversible demetalation had
taken place. In contrast, strong chelating agents like EDTA or
its disodium salt Na EDTA·2H O did not elicit a mechano-
chemical demetalation of [TPP]Zn (entries 5−4 and 5−5).
Mechanochemical Copper Insertion into Porphyrins.
The insertion of copper into porphyrins generally requires
higher boiling solvents or longer reaction times than the
insertion of zinc; in turn, the resulting copper complexes are
much more resistant to acid-induced demetalation (requiring
15b
by James and co-workers for TPP. Since all these copper
insertions proceeded satisfactorily under the standard con-
ditions tested, we did not test a variation of the metal source or
the effects of other additives than silica gel. However, it is
known from solution experiments that the anion of the metal
source plays a role in the metal insertion rate (cf. also the zinc
5
1
8
2
2
28
insertion experiments). A mechanochemical demetalation of
[TPP]Cu could not be achieved under any of the reaction
conditions tested.
meso-Τriarylcorroles form stable copper(III) complexes that
are produced by spontaneous air oxidation of the initially
2
9
formed copper(II) complex. We found now that mecha-
nochemical conditions are also suited for the formation of the
[meso-triphenylcorrolato]copper(III) complex [TPCorrole]-
10
concentrated H SO for their demetalation). In general, the
mechanochemical copper insertion reactions proceeded
2
4
F
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Inorganic Chemistry
Article
a
Table 7. Outcomes of Zinc/Copper Insertion Competition and Metal Replacement Reactions
substrate
x equiv Zn(OAc) ·2H O
x equiv Cu(OAc) ·H O
product(s)
time to completion or % conversion
2
2
2
2
7−1
7−2
7−3
7−4
7−5
7−6
7−7
TPP
T PP
OEP
2.5 equiv
2.5 equiv
2.5 equiv
2.5 equiv
2.5 equiv
2.5 equiv
2.5 equiv
2.5 equiv
2.5 equiv
[TPP]Cu: [TPP]Zn
90:10 after 10 min 100:0 after 40 min
98:2 after 10 min
98:2 after 10 min
40 min
F
F
F
[T PP]Cu: [T PP]Zn
[OEP]Cu: [OEP]Zn
[TPP]Cu
[TPP]Zn
[OEP]Zn
[OEP]Cu
40 min
F
F
[T PP]Zn
[T PP]Cu
<50% conversion after 60 min
60 min
F
F
[T PP]Zn
2.5 equiv +7.0 equiv of p-TSA
[T PP]Cu
a
Reaction conditions: rotational speed of 800 rpm, 50 mL agate vessel, milled at ∼5 min intervals with ∼2 min breaks; 0.5 g silica gel (40−75 μm
particle size).
F
Table 8. Outcomes of the Mechanochemical Metal Insertion Reactions of TPP, T PP, and OEP Using Florisil and Mg(II)
a
Sources
time to completion or %
entry
substrate
x equiv Mg(II) source
additives
product
conversion
8−1
8−2
8−3
8−4
8−5
8−6
8−7
TPP or OEP
TPP
TPP
TPP
TPP
0.5 g Florisil
0.5 g Florisil
0.5 g Florisil, 10 equiv DABCO
[TPP]Mg or [OEP]Mg <10% conversion after 120 min
10 equiv MgCl2
10 equiv MgCl2
[TPP]Mg
[TPP]Mg
[TPP]Mg
[TPP]Mg
90 min
90 min
<60 min
<20% conversion after 60 min
after 120 min
<60 min
2.5 equiv MgBr or MgI 0.5 g Florisil, 2.5 equiv DABCO
2
2
2.5 equiv MgBr2
0.5 g talc
with or without 5 equiv DABCO
0.5 g Florisil, with or without 5 equiv
DABCO
b
TPP
OEP
5 equiv MgBr or MgI
NR
2
2
2
5 equiv MgBr or MgI
[OEP]Mg
2
8
8
8
8
−8
−9
OEP
5 equiv MgBr2
0.5 g talc
[OEP]Mg
<20% conversion after 60 min
after 120 min
<20% conversion after 120 min
120 min
b
OEP
F
5 equiv MgBr or MgI
5 equiv DABCO
0.5 g Florisil, 5 equiv DABCO
5 equiv DABCO
NR
2
2
2
2
F
−10 T PP
−11 T PP
5 equiv MgBr or MgI
[T PP]Mg
2
F
b
5 equiv MgBr or MgI
NR
2
a
Reaction conditions: Rotational speed of 800 rpm, 50 mL agate vessel, milled at ∼ 5−20 min intervals with ∼2 min breaks; with or without
b
additives (silica gel 40−75 μm particle size, Florisil 100−200 mesh). NR: no reaction observed.
2
9a
Cu. This suggests that metal insertion reactions requiring a
more complex insertion mechanism are also accessible using
mechanochemical approaches (cf. also to the silver(II) and
silver(III) insertions using a Ag(I) source into porphyrins and
corrole, respectively, Table 10).
the sole product at higher metal/porphyrin ratios (Table S1),
the zinc-to-copper replacement reactions are not observed
under neutral solution conditions (Table S2) but require the
addition of acid to enable rapid metal exchange. The acid
added needs to be strong enough to push out the zinc without
being too strong to prevent copper insertion. In transference,
the transmetalation experiments indicated that using silica gel
as an additive in the mechanochemical reactions rendered a
significant acidic character to the reaction conditions.
Mechanochemical Competition Metal Insertions. The
rapid insertion of copper prompted the question of what the
outcome of zinc and copper competition insertion experiments
would reveal (Table 7). Starting with 2.5 equiv of each of the
zinc and copper acetates, the copper insertion product
Mechanochemical Insertion of Magnesium into
Porphyrins. Building on the unexpected observation that
the use of magnesium silicate, Florisil, as grinding agent proved
to be a source of Mg(II) for the formation of [TPP]Mg (entry
2−10), we investigated this reaction in more depth (Table 8).
The classic Mg(II) insertion methods involve Grignard
reagents or hindered phenoxy magnesium iodide complexes
[
TPP]Cu rapidly becomes the dominating species and, in
fact, replaces over time all of zinc in the [TPP]Zn initially
formed (entry 7−1). These dynamics are also observed for
F
T PP (entry 7−2) and OEP (entry 7−3). In fact, grinding the
pure preformed zinc complexes [TPP]Zn (entry 7−4),
F
[
OEP]Zn (entry 7−5), or [T PP]Zn (entry 7−6) with
1
0
copper acetate converts them over time to the corresponding
copper complexes, although the latter less basic ligand reacted
slower and required the addition of acid for completion (entry
at elevated temperatures or MgCl in refluxing DMF or
pyridine,
2
14,15
whereby more recent methods using organic
amines and inorganic magnesium salts (particularly MgBr or
2
15a
7
−7).
MgI ) have allowed milder conditions.
2
While [TPP]Cu is also the predominant product in
To bring the mechanochemical magnesium insertion
corresponding solution state competition experiments and is
reaction to completion in TPP and OEP, we found that
G
DOI: 10.1021/acs.inorgchem.9b00052
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Inorganic Chemistry
Article
Florisil alone was not sufficient (entry 8−1), but the addition
Table 9. Outcomes of the Mechanochemical Metal Insertion
F
of 10 equiv of MgCl with (entry 8−2) or without (entry 8−3)
Reactions of TPP, T PP, and OEP Using select other Period
2
a
the addition of a solid base (DABCO) was needed.
3 and 4 M(II) Sources
Mg(OAc) ·4H O, in the presence or absence of a base and/
2
2
or Florisil, proved unsuitable for the metal insertion reaction
(
see Supporting Information). However, the use of MgBr or
2
MgI in the presence of base is rapid (entry 8−4), even when
2
using a smaller stoichiometric excess of metal salt. Most
crucially, however, the use of Florisil is essential. Thus, the
reaction of TPP with MgBr or MgI in the presence or
time to
completion/
degree of
2
2
entry
substrate x equiv reagent(s), additive product
conversion
absence of base fails when Florisil is absent (entry 8−6). Silica
gel as an additive also had no effect on the magnesium
insertion reactions (see Supporting Information). Correspond-
ingly, the insertion of Mg into TPP was reported to be
9
−1
TPP
2.5 equiv VO(acac) , 0.5 g [TPP]
>50% conversion
after 100 min
2
silica gel
VO
9
−2
TPP,
5−10 equiv FeCl ·4H O,
[TPP]
FeCl
<5% conversion
after 80 min
2
2
F
T PP,
0.5 g silica gel
15b
unsuccessful by James and co-workers. This key finding is
OEP
F
also replicated with OEP (entries 8−7 versus 8−8) and T PP
9−3
−4
TPP
1.5 equiv Fe(OAc)
2
[TPP]
FeOAc
b
<15% conversion
after 180 min
(
entries 8−10 versus 8−11) that otherwise insert magnesium
9
TPP
2.5 equiv CoCl
2
·6H
2
O or
NR after 80 min
more rapidly or more slowly than TPP, respectively, mirroring
the trends seen also for other metals. In comparison to
reactions that use the crystalline magnesium silicate talc
Co(OAc)
silica gel
2
·4H O, 0.5 g
2
9−5
OEP
TPP
2.5 equiv Co(OAc) ·
2
[OEP]
Co
>50% conversion
after 60 min
2
4H O, 0.5 g silica gel
(
Mg Si O (OH) ) as a grinding aid (entries 8−4 versus 8−5
3
4
10
2
9
−6
5 equiv NiCl
silica gel
2 2
·6H O, 0.5 g [TPP]Ni 40 min
and entries 8−8 versus 8−9), we find talc to be much inferior
to Florisil, though still better than the organic base DABCO
9
9
−7
−8
TPP
TPP
1.5 equiv Ni(OAc)
2
·4H
·4H
2
O
[TPP]Ni 40 min
(
entry 8−11). We cannot offer an explanation for these
2.5 equiv Ni(OAc)
0.5 g silica gel
2
2
O, [TPP]Ni 40 min
observations, except to say that we do not believe it is the
basicity of the silicate that enables this reaction.
F
F
9−9
T PP
5 equiv NiCl ·6H O, 0.5 g [T PP]
2
2
∼ 65% conversion
after 100 min
silica gel
Ni
b
In summary, the amorphous, synthetic magnesium silicate
Florisil, in combination with the magnesium salts of the
heavier halogens, mediates the insertion of magnesium into
porphyrins cleanly under astoundingly mild methods,
introducing mechanochemical methods as an alternative to
traditional solution-based methods to accomplish magnesium
insertions. Once again, the “grinding aid” Florisil plays a much
more complex role in the mechanochemical metal insertion
reaction than we anticipated.
F
9
−10 T PP
1.5 equiv Ni(OAc) ·4H O, NR after 90 min
2 2
0.5 g silica gel
9
−11 OEP
1.5 equiv Ni(OAc) ·4H O [OEP]Ni 40 min
2
2
a
Reaction conditions: rotational speed of 800 rpm, 50 mL agate
vessel, milled at ∼5−20 min intervals with ∼2 min breaks; with or
without additives (silica gel 40−75 μm particle size, Florisil 100−200
b
mesh). NR: no reaction observed.
Mechanochemical Insertion of Select Other Period 3
and 4 Metals into Porphyrins. In an extension to the
copper, zinc, and magnesium insertion experiments, we
screened a range of other two-valent metals for the suitability
of their mechanochemical insertion into a number of synthetic
porphyrins. The mechanochemical reaction of VO acetyl
acetonate with TPP in the presence of silica gel is slow (Table
silica gel) after 3 h in the shaker mill (i.e., a 6-fold longer time
than for a comparable zinc insertion at a 50% slower oscillation
frequency).
15b
Neither we nor James and co-workers succeeded in the
insertion of cobalt (using Co(OAc) or CoCl ) into TPP
2
2
15b
(entry 9−4). However, we found OEP to convert to ∼50%
forming [OEP]Co after milling with Co(OAc) and silica gel
2
9
, entry 9−1), with a [TPP]VO/TPP ratio exceeding 1:1 only
for 60 min (entry 9−5).
after 100 min of reaction time. The high insertion barrier of
The insertion of nickel into porphyrins is traditionally
14
VO into TPP can also be gleaned from the high-temperature
performed in refluxing DMF but also takes place in hot
10
10
conditions needed for solution state VO insertions. The
porphyrin degradation reaction described above becomes
prominent during such longer reaction times in the ball mill,
severely reducing the overall isolated yield of product. This
makes long mill runs in the presence of silica gel unattractive,
even though the (unknown) porphyrin degradation products
are not hindering the clean elution of the product [TPP]VO
via column chromatography.
pyridine (bp = 115 °C) or even in refluxing CHCl /MeOH
3
30
(∼60 °C) over the course of 5 days. The mechanochemical
insertion of nickel into porphyrins is possible, with notable
differences between varying nickel sources. The general trends
observed for the different porphyrins are preserved: Inserting
nickel into TPP and OEP using NiCl (with silica gel) or
2
Ni(OAc) ·4H O (with or without silica gel) is facile (entries
2
2
9−6 through 9−8, and 9−11, respectively) and much slower
F
The solution state insertion of iron(II) and cobalt(II) into
porphyrins requires reflux conditions of high temperature
for T PP (entry 9−9), whereby the use of Ni(OAc) ·4H O
2
2
with silica gel is inadequate to effect any reaction (entry 9−
10). In comparison to the corresponding zinc insertion
reactions (Tables 1 and 3), therefore, the suitability of the
respective acetate and chloride salts is inverted. Our results for
TPP are commensurate with those reported by James and co-
1
0,14
solventsF.
The mechanochemical reaction of FeCl with
2
TPP, T PP, or OEP yielded even after 80 min reaction time
less than 5% conversion to a product of the expected UV
spectra for the corresponding [porphyrin]FeCl complexes
15b
(
entry 9−2). The use of Fe(OAc) only marginally improved
workers.
2
the outcome for TPP (entry 9−3). These results are in
concord with the 7% yield James and co-workers reported for
Mechanochemical Insertion of Select Other Period 5
and 6 Metals into Porphyrins. Some period 5 and 6 metals
proved also susceptible to mechanochemical insertion into a
the formation of [TPP]Fe using Fe(OAc) (in the absence of
2
H
DOI: 10.1021/acs.inorgchem.9b00052
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Inorganic Chemistry
Article
F
Table 10. Outcomes of the Mechanochemical Metal Insertion Reactions of TPP, T PP, and OEP Using a Period 5 and 6 Metal
a
Source
entry
substrate
x equiv reagent(s)
20 equiv AgOAc
20 equiv AgOAc
20 equiv AgOAc
5 eq AgOAc
5 equiv Pd(OAc)2
2.5 equiv PdCl2
5 equiv Pd(OAc)2
product
time to completion/degree of conversion
1
0−1
TPP
T PP
OEP
[TPP]Ag
[T PP]Ag
[OEP]Ag
[TPCorrole]Ag(III)
[TPP]Pd
[TPP]Pd
[T PP]Pd
[T PP]Pd
[OEP]Pd
[OEP]Pd
<70% conversion after 120 min
20 min
<30% conversion after 60 min
30 min
F
F
1
1
1
1
1
1
1
1
1
1
1
1
1
0−2
0−3
0−4
0−5
0−6
0−7
0−8
0−9
0−10
0−11
0−12
0−13
0−14
TPCorrole
TPP
85% conversion after 60 min
40 min
<45% conversion after 30 min
TPP
F
F
b
T PP
F
F
T PP
5 equiv PdCl with/without 5 equiv DABCO
<30% conversion after 50 min
∼ 75% conversion after 60 min
<30 min
2
OEP
OEP
5 equiv PdCl2
5 equiv Pd(OAc)2
F
c
TPP or T PP or OEP
10 equiv CdCl ·2.5H O, with/without silica gel
NR after 90 min
NR after 90 min
[TPP]Pb
[TPP]Pt
2
2
F
c
TPP or T PP or OEP
5 equiv Pb(OAc) ·3H O
2
2
TPP
TPP
5 equiv Pb(OAc) ·3H O, no silica gel
<15% conversion after 180 min
<40% conversion after 60 min
2
2
2.5 equiv PtCl with/without 2.5 equiv DABCO
2
a
Reaction conditions: rotational speed of 800 rpm, 50 mL agate vessel, milled at ∼5 min intervals with ∼2 min breaks; 0.5 g silica gel (40−75 μm
b
c
particle size). Reaction did not proceed significantly with time (we tested up to 85 min), even after the addition of additional Pd(OAc) . NR: no
2
reaction observed
porphyrin (Table 10). The stable oxidation state of silver in
conclusion of whether PdCl or Pd(OAc) is a generally better
(faster) metal source cannot be derived.
2
2
31
porphyrins is Ag(II), in corroles Ag(III). The standard
conditions to insert silver into porphyrins is the use of
Both Cd(II) and Pb(II) can be inserted into porphyrins
heating of the porphyrin and the corresponding salts in
10
(
Ag(OAc) in warm pyridine. Thus, the insertion reaction is
characterized by a disproportionation reaction: 2 equiv of
Ag(OAc) reacts with one porphyrin to form the silver(II)
pyridine or DMF), albeit the resulting metalloporphyrins are,
by virtue of the large metal ions, not sitting in the plane of the
3
6
14
porphyrin, rather sensitive to acid-induced demetala-
porphyrin and elemental silver and acetic acid; for the
10,14
tions.
None of the porphyrins tested showed any sign
tribasic corroles, three Ag(OAc)’s react with one macrocycle to
32
that Cd(II) or Pb(II) could be inserted mechanochemically
form the silver(III) corrole and 2 equiv of elemental silver.
(
entries 10−11 to 10−14).
Particularly the solution state insertion of the kinetically
We found that the mechanochemical insertion of silver into
porphyrins is possible, but it requires a larger stoichiometric
excesses of Ag(OAc) than in solution. Like for other metals,
the insertion rates are much porphyrin-dependent: The
electron-rich porphyrins TPP and OEP insert silver only
partially after 1 or 2 h reaction times (entries 10−1 and 10−3),
while the corresponding reaction involving the electron-poor
rather inert Pt(II) into porphyrins requires extended periods of
10,12b,15c
high heat or forcing microwave conditions.
In light of
this, the 40% insertion of Pt(II) into TPP after 60 min
grinding using 2.5 equiv of PtCl with or without the addition
2
of a solid base is remarkable. However, we did not attempt to
optimize this mechanochemical insertion reaction, but none-
theless like to point out its promise.
F
porphyrin T PP is completed after 20 min (entry 10−2).
Triphenylcorrole TPCorrole forms quantitatively the Ag(III)
complex after 30 min (entry 10−4), another indication for the
unusually facile oxidation of the silver to its unusually high
SUMMARY AND CONCLUSIONS
■
32
This work began to lay out for the practitioner the scope and
limits of using a planetary ball mill for the mechanochemical
insertion of a range of metal ions into porphyrins as an
alternative to traditional solution-based methods. We tested a
wide range of porphyrins, metals, and metal sources and
detailed the influences of porphyrins, metal salts, time, grinding
oxidation state imposed upon it by the ligand. We attribute
the negative result James and co-workers reported for their
attempt to insert silver into TPP primarily to their using an
15b
insufficient amount (1 equiv) of Ag(OAc).
Palladium(II) and platinum(II) (hydro)porphyrins have
33
found multiple uses as photosensitizers, optical oxygen
additives, and, in some cases, bases, on the outcome of the
3
4
35
sensors, or photocatalysts. Their preparation involves the
use of Pd(II) and Pt(II) salts under high temperature
15b
reactions. We thereby much extended precedent work.
A
solid acid-induced mechanochemical removal of zinc from zinc
porphyrins could also be accomplished.
1
0,12b
conditions.
Surprisingly, therefore, the mechanochemical
F
insertion of Pd(II) into TPP, T PP, and OEP is relatively
effortless (entries 10−5 through 10−10), but completion of
the reaction cannot always be achieved within 60 min under
the standard conditions tested. The influences of the reactivity
of the porphyrin observed for other metals (not involving a
redox reaction as part of the metal insertion) are preserved:
Given the ostensibly simple mechanochemical reactions
under investigation, their details proved to be surprisingly
complex. Some findings have significance for the fundamental
mechanisms underlying mechanochemical metal insertion
reactions, but some findings must remain unexplained. Perhaps
the easiest to understand are the influences of the porphyrin:
the more basic, the easier (i.e., faster) is the metal insertion.
F
OEP inserts faster than TPP that is much faster than T PP. A
I
DOI: 10.1021/acs.inorgchem.9b00052
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
+
The influence of the metal ion source (anion influence) is
often straightforward for a given metal but not readily
explained, or therefore predictable, across different metals. In
some cases, the more basic anion acetate is more advantageous
to use than the less basic chloride, but in other cases the
reverse is true.
of multicm dimensions), and Dowex HCR-W2, H -form (J.T. Baker,
USA; spherical beads, 16−40 mesh).
All metal salts were sourced from commercial suppliers and were
reagent grade, or better.
Instruments. A Fritsch GmbH, Germany, Pulverise planetary
micro mill (Pulverisette 7 classic line) equipped with two agate
grinding vessels was used in the milling experiment, with main disc
speeds ranging from 100 to 800 rpm. Small vessel A: volume 12.5 mL,
equipped with five agate balls (10 mm), total weight ∼7.0 g. Large
vessel B: Inner dimensions were 45 mm diameter, 37 mm height,
volume 50 mL, equipped with eight agate balls (10 mm) with a total
weight of ∼13.6 g (for additional information, see Supporting
Information).
HPLC (Agilent 1100 Series) equipped with a Grace analytical
normal-phase Apollo silica column (4.6 × 250 mm, 5 μm). Samples
were dissolved in ethyl acetate prior to injection. Detection
wavelength was set to 400 nm.
A number of metal ions are suitable to be inserted
mechanochemically, whereby their ease of insertion roughly
correlates with the severity of the classic solution state
insertion reaction conditions. Nonetheless, distinct differences
in the corresponding solution state reactions are notable. For
instance, in the solution state, copper ions at neutral conditions
do not compete the zinc out of zinc porphyrins, but in the solid
state reactions this is readily accomplished. Also, Mg(II)
porphyrins can be formed under unusually mild conditions by
grinding a porphyrin together with common magnesium
sources but needing a specific insertion mediatorthe
magnesium silicate Florisil. The group 10 metals Pd(II) and
Pt(II) are also notoriously hard to insert into porphyrins, but
they inserted under comparably mild mechanochemical
reaction conditions, albeit an optimization of these reactions
was not attempted.
UV−vis spectra were recorded on a Varian Cary 50 spectropho-
tometer in the solvents indicted.
Typical Procedure. In a typical experiment, a 50 mL agate
grinding jar equipped with eight 10 mm agate balls was charged with
2.0 mg of free base porphyrin, the appropriate equivalents of divalent
metal ion source, and a solid additive (500 mg; 1:42 (w/w) ratio of
with respect to the free base porphyrin). This mixture was milled in a
planetary ball mill at the disc speeds indicated (typically 800 rpm) for
the time indicated. During this time, aliquots were retrieved at
different time intervals to monitor the progress of the reaction. These
aliquots were placed into a small pipet with a cotton plug and
extracted with CH Cl , and the eluent was analyzed by TLC (silica
1
Perhaps the most enigmatic factors involve the influence of
the grinding reagents. While they provide some practical
advantages with respect to product retrieval and mixing
efficiencies, particularly when running small scale reactions,
many are not at all inert. While some, depending on their
nature, stunt or accelerate the metal insertion reactions, some
lead over time also to an extensive mechanochemical
degradation of the porphyrin. Others, like Florisil, are
obligatory for the insertion of magnesium. This suggests that
a judicious choice of grinding reagent needs to be made to
balance all their inherent advantages and disadvantages. At this
point, their positive or negative influences cannot be predicted
and would need to be determined experimentally. This clearly
highlights that much still remains to be understood regarding
2
2
gel), UV−vis spectrophotometry, or HPLC, as appropriate.
ASSOCIATED CONTENT
■
*
S
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.9b00052.
Details to the ball mill used, experimental details, and a
reproduction of a number of representative TLCs,
UV−vis spectra, and HPLC traces used in the
compilation of the data tables (PDF)
1
,2
mechanochemical syntheses.
However, the study also
demonstrates that mechanochemical methods clearly possess
potential for the insertion of metals into porphyrins.
EXPERIMENTAL SECTION
AUTHOR INFORMATION
■
■
Materials. All solvents and reagents (Aldrich, Acros) were used as
Corresponding Author
*Tel.: +01 (860) 486-2743. E-mail: c.bruckner@uconn.edu.
3
7
F
38
39
40
41
received. TPP, T PP, OEP, ETIO-I, PPIX-DME, meso-
42
2
43
3
43b
27a
TPCorrole, T( Thiop)P, T( Thiop)P,
(OH) TPC,
and
2
F
27b
(
OH) T PC
were prepared as described in the literature, were
ORCID
2
sourced from commercial suppliers, or were gifted to us. The known
metalloporphyrins that were used as comparison materials were
prepared by metal insertions into the corresponding free base
Adewole O. Atoyebi: 0000-0002-9495-5072
̈
Christian Bruckner: 0000-0002-1560-7345
Author Contributions
10
chromophores using traditional methods.
Analytical (aluminum backed, silica gel 60, 250 μm thickness) and
preparative (20 × 20 cm, glass backed, silica gel 60, 500 μm
thickness) TLC plates and standard grade, 60 Å, 32−63 μm flash
column silica gel were used. Additives: silica gel (Sorbent
Technologies, USA; particle size, 40−75 μm; surface area, 450−550
The manuscript was written through contributions of both
authors; both have also given approval to the final version of
the manuscript.
Notes
2
m /g; pH, 6.0−7.0), quartz sand (sea sand, washed, Aldrich, particle
The authors declare no competing financial interest.
size 0.1−0.315 mm, pH of 400 g/L slurry 5−8; slurry), silica-NH (Si-
2
WAX from Silicycle, Canada; propylamine-functionalized silica gel,
pK = 9.8, particle size: 40−63 μm), neutral alumina, Brock activity I
ACKNOWLEDGMENTS
a
■
(
Sorbent Technologies, USA; particle size: 50−200 μm), basic
Funding for this work was provided by the U.S. National
Science Foundation (NSF) through grants CHE-1465133 and
CHE-1800631. We thank David Dolphin, University of British
Columbia, and Chi-Kwong (Chris) Chang, Michigan State
University, for a donation of the octaalkylporphyrins used in
this study.
alumina, Brock activity I (Sorbent Technologies, USA; particle size:
0−200 μm), acidic alumina, Brock activity I (M. Woelm, Germany),
5
2
montmorillonite (Aldrich, USA; 250 m /g, pH 3−4), sodium chloride
(
Aldrich, USA), cellulose powder (Macherey, Nagel & Co., Germany;
MN 300), Florisil (Aldrich, USA; 100−200 mesh), talc (Aldrich, 10
μm powder), calcite (Educational Innovations; Iceland spar, crystals
J
DOI: 10.1021/acs.inorgchem.9b00052
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Chlorins and Observation of Hydroporphyrin Intermediates. Inorg.
Chem. 2018, 57, 3177−3182.
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