Cationic nickel porphyrinoids with unexpected reactivity

Article abstract of DOI:10.1039/c5sc03663a

Cationic nickel(ii) complexes of two ring-contracted porphyrinoid ligands distantly related to the corrins were prepared by metal templated macrocyclisation. The compounds show reversible electron transfer processes and were found to be the first porphyrinoid-based catalysts for C-C cross-coupling.

Full text of DOI:10.1039/c5sc03663a

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DOI: 10.1039/C5SC03663A
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Cationic Nickel porphyrinoids with unexpected reactivity†
Richard Wicht, Stefanie Bahnmüller, Kai Brandhorst, Peter Schweyen and Martin Bröring*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Cationic nickel(II) complexes of two ring-contracted porphyrinoid porphyrin chemistry. As depicted in figure
1 the oxidative
ligands distantly related to the corrins were prepared by metal macrocyclisation of a methyl terminated linear tetrapyrrole 1 leads
templated macrocyclisation. The compounds show reversible to metal chelates of the general form 2 if careful reaction
electron transfer processes and were found to be the first conditions are ensured.6 Using 2,2’-bidipyrrins like 37 for an
porphyrinoid-based catalysts for C-C cross-coupling.
analogous transformation provides an option for the preparation of
metal 9-methylisocorroles like 4. This ligand framework had been
observed in serendipitously isolated rhodium and iridium
complexes before,8 however, no logical synthesis nor any
understanding of the formation of such chelates has so far been
Introduction
The chemistry of ring-contracted porphyrinoids is currently one of
the most intriguing topics in macrocycle research.1 Inspired by the
cobalamins,2 an increasing number of novel porphyrinoid systems
with small cavities and outstanding properties has been introduced
to the field in recent years.3 Corroles have certainly taken the lead
in this ongoing development.4 Artificial partially hydrogenated
corroles, however, are hardly available by selective synthesis and
have been reported on only in a very limited number.5 An inspiring
idea for the synthesis of such a ligand comes from well-known
developed.
A metal template assisted oxidative ring closure
reaction is indeed successful with the precursors 5 and 6, and we
report here about the synthesis and unexpected features of first
cationic nickel(II) complexes 7 and 8 of such 9-methylisocorroles
(scheme 1).
Results and discussion
Treatment of nickel(II)-2,2’-bidipyrrin 57a) or 6 with a slight excess of
silver tetrafluoroborate in dichloromethane solution produces the
desired complexes 7 and 8 in yields of 55% and 76%, respectively.
This procedure contains several critical points which require a
discussion. The choice of the anion of the silver salt is of uttermost
importance for the successful synthesis. Weakly coordinating
-
-
-
anions like BF4 , SbF6 or ClO4 lead to the desired complex cation,
-
-
while more nucleophilic anions like NO3 or CH3CO2 result in the
formation of complex mixtures of different unidentified and often
oxygenated products.
Figure 1 Strategies for hydroporphyrin and isocorrole syntheses.
Institute for Inorganic and Analytical Chemistry, Technical University
Braunschweig, Hagenring 30, 38106 Braunschweig, Germany
† Electronic Supplementary Information (ESI) available: Preparation and analytical
data of 7 and 8; details of DFT calculations and catalytic investigations. See
Scheme 1 Preparation of nickel 9-methylisocorroles 7 and 8, with numbering scheme.
DOI: 10.1039/x0xx00000x
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DOI: 10.1039/C5SC03663A
Figure 3 Cyclic voltammetry and square wave voltammetry results for nickel
9-methylisocorrole 7 (dichloromethane, 0.1 mol L-1 nBu4NPF6; scan rate 200 mV s-1).
Figure 2 Details of the 1H NMR spectrum of the cationic nickel 9-methylisocorrole 7
(300 MHz, CDCl3).
Figure 3 illustrates the results from cyclic voltammetry and square
wave voltammetry measurements on 7. Comparing these data with
the potentials found for the linear tetrapyrrole complexes 5 and 6
under the same conditions (E1/2 = -129/ -161 mV; E1/2 = +475/ +382
mV) reveals unusually large increases of both, oxidation and first
reduction potentials, upon ring closure. With respect to other nickel
porphyrinoids, hydroporphyrinoids, and corrinoids the potentials of
the isocorrole complexes 7 and 8 are shifted by about 0.9 V for the
Secondly, the removal of excess silver ions in an early work-up step
by precipitation with ammonium bromide is essential for the
isolation of a pure product. Other halide salts lead to diminished
yields or even decomposition of the nickel 9-methylisocorrole.
During this process, no binding of the halide ion to the metal center
is observed. This is remarkable as in all related cases, e.g. with the
monoanionic N-methylporphyrins and analogues, neutral high spin
(S = 1) halogenido nickel(II) complexes form immediately under
such conditions, and are isolated as robust compounds.9 We
assume that the small cavity sizes of the 9-methylisocorrole ligands
of 7 and 8 inhibit a spin change from a small low spin (S = 0) to a
large high spin (S = 1) nickel(II) ion, thus enforcing a square planar
coordination mode of the 3d8 ion. The attempt to purify crude 7 or
8 by chromatography was unsuccessful due to the sensitivity of the
products. Once purified by recrystallisation, however, the
complexes are sufficiently stable for investigation under typical
laboratory conditions, and could be stored as solids under nitrogen.
The cationic nickel 9-methylisocorroles 7 and 8 were identified by
ESI-MS (m/z for [M-X]+: 593.3, and 717.3, respectively) and by NMR
spectroscopy. At first glance, the complete loss of molecular
first
reduction
step,
and
by
about
0.4 V for the first oxidation.10 DFT calculations on the one-electron
reduced 7’ suggest a ligand centered reduction process. The
calculations are in agreement with the results from an EPR study
(figure 4). Here, chemical reduction of 7 was performed by Na/Hg
treatment in dry tetrahydrofurane, and the one-electron reduced
product was shown to be a purely organic radical without apparent
Ni(I) contributions.
Other than the reduction, chemical oxidation of 7 by dioxygen from
air occurs over a day in solution, and results in the formation of a
mixture of unassigned species. In one case, a single crystal grew
1
symmetry is apparent in the H- and 13C NMR spectra through the
increased number of resonance lines. Particularly striking are the
AB systems of the protons of the bridging CH2 groups, which appear
well resolved in the spectrum of 7 (figure 2). Resonances for these
protons occur at 3.49 ppm and 1.74 ppm, and are as characteristic
as the singlet for the protons of the C9-bound methyl group at 1.13
ppm for the tetrapyrrole framework. In addition, the unusual
quaternary carbon atom C9 and the methylene bridge carbon atom
C10 appear as prominent signals at 77.9 ppm and 36.6 ppm,
respectively, in the 13C NMR spectrum of 7. Similar data was
obtained for 8, and the constitution of both compounds is further
supported by 2D NMR spectra (see ESI).
The redox behaviour of the new 9-methylisocorrole nickel
complexes 7 and 8 was investigated in dichloromethane solution by
cyclic voltammetry. Quasi-reversible reductions at E1/2 = –907/ -920
mV and at –1515/ -1548 mV, and quasi-reversible oxidations at E1/2
= +655/ +603 mV vs. Fc/Fc+ are observed for 7 and 8, respectively.
Figure 4 Graphical representation of the singly occupied molecular orbital (SOMO) from
DFT calculations (B3LYP/TZVP,WTBS)[11] and X-band EPR spectrum (tetrahydrofurane,
28 K) of the one-electron reduced nickel 9-methylisocorrole 7’.
2 | J. Name., 2012, 00, 1-3
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Table 1 Catalytic aryl-aryl cross-coupling reactions catalysed by 7 or 8.
View Article Online
DOI: 10.1039/C5SC03663A
Cat.
(mol%)
Ar1X
Ar2MgY
(equiv)
Time
(h)
Conv.a
(%)
yielda,b
(%)
7 (5)
7 (5)
7 (5)
7 (5)
7 (1)
7 (5)
7 (5)
7 (5)
8 (5)
8 (5)
8 (5)
8 (5)
8 (1)
8 (5)
8 (5)
8 (5)
8 (5)
1-BrNaph PhMgCl (1.5)
1-BrNaph PhMgCl (5)
1-BrNaph PhMgCl (6×0.5)
1-BrNaph PhMgBr (6×0.5)
1-BrNaph PhMgCl (5×1)
0,5
0,5
46
100
100
100
93
44
81
45
100
38
70
51
10
100
91
41
71
86
88
77
42
76
34
95
15
61
44
9
3
8
8
9
9
2
5
6
5
4
9
6
2
20
20 70
5
2
1,25
1,25
23,33
1,33
23,33
0,33
40
40
23,33
0,5
0,5
0,5
1
23,33
1
43
35
7 182
1-ClNaph
1-ClNaph
1-INaph
PhBr
PhMgCl (1.5)
PhMgCl (6×0.5)
PhMgCl (1.5)
AniMgBr (3×2)
PhMgBr (4×1)
AniMgBr (4×1)
0
0
5
21
52
19
0 163
19 111
0 108
4-BrAni
1-BrPhF
1-BrNaph PhMgBr (1.5)
1-BrNaph PhMgCl (1.5)
1-BrNaph PhMgBr (15)
1-BrNaph AniMgBr (4×1)
1-BrNaph PhF2MgBr (4×1)
1
11
4
35
<1 <1
Figure 5 Molecular structure of the dicationic dimer of 7. Selected bond lengths [Å]: Ni-
N1 1.846(3), Ni-N2 1.832(3), Ni-N3 1.868(3), Ni-N4 1.873(3), C9-C10 1.565(4), C10-C10’
1.586(4). Thermal ellipsoids are set at 50% probability. The second subunit is shown as
stick representation only, and most hydrogen atoms are removed for clarity.
81 12
64
69
7
7
0
0
20 67
0 107
0 27
69
100
5-BrAnth
AniMgBr (4×1)
100
a determined by GC, rel. to the arylhalide component Ar1X. b Ar1Ar2 Ar1Ar1 HAr1
from this complex mixture and allowed the crystallographic
identification of an oxidation product. Quite surprisingly, this
analysis showed a species very similar to 7 but dimerised through
the bridging methylene unit C10.[3b),12] Figure 5 shows the molecular
structure and provides details of the coordination environment of
the nickel ion as well as of the conformational state of the
macrocycle.
Ar2Ar2. Anth = anthracene; Ani = anisole; PhF = 4-CF3C6H4; PhF2 = 3,5-(CF3)2C6H3.
arylhalides (scheme 2). Best results are obtained if the Grignard
reagent is added in small aliquots over the course of 2 hrs to a
solution of the nickel complex 7 or 8 (5 mol%) and the arylhalide in
dry THF at ambient temperature, with the two complexes behaving
basically identical. These conditions generally give high to complete
conversion of the halide component, with high selectivity for the
cross-coupling product. The reaction can be run in dichloromethane
or toluene solution, too, but with reduced yield of the desired
heterodiaryl. A reduced catalyst loading elongates the reaction time
as expected. However, excess Grignard reagent (2.5 – 4 equiv.) is
required in all cases, and significant amounts of the homocoupling
product from the organometallic component are present
throughout. As judged by the stoichiometry, another and as yet
unassigned by-reaction of the Grignard reagent must occur,
presumably by solvolysis. These by-products form in particular upon
long reaction times, e.g. upon attempts to reduce the catalyst
loading. In the absence of a nickel 9-methylisocorrole 7 or 8, or in
the presence of nickel tetraphenylporphyrin, no cross-coupling
reaction and no homocoupling of the Grignard component occurs,
and upon using simple nickel(II) acetate as catalyst only the
unwanted symmetric biaryl and the very slow formation of the
cross-coupling product could be detected (< 9% after 300 hrs).
Table 1 summarises the results using different catalysts, reaction
conditions, and starting materials. Both components, the aryl halide
as well as the Grignard reagent, are variable within successful
protocols. Beside the solvent, the influence of the size of the
arylhalide is remarkable as the larger anthracenyl bromide shows
the fastest reaction, followed by the naphthyl and phenyl systems.
The selectivity of the formation of the cross-coupling product is
higher for electron poor substrates, accompanied with a significant
decrease of the reaction rate. The expected influence was observed
for different halides at the aryl halide component where the
reaction rate increases while the selectivity decreases in the order
The dimer crystallises as a chloroform solvate in the space group
P21/c. Two nickel atoms in distorted square-planar coordination
geometry and with Ni-N distances ranging from 1.832(3) Å to
1.873(3) Å are found in the central N4 cavities of the macrocyclic
subunits and support the expected low spin-d8 configuration of the
metal ions. As a single positive charge is calculated per subunit, two
-
BF4 anions are found for each dimer. These anions are isolated
from the dication and do not bind to the Nickel(II) atoms. The
conformation of the macrocyclic 9-methylisocorrole is mainly
governed by the steric requirements of the C9 bound methyl group
and the methylene bridge C10 involved in the dimer formation. At
this position of the macrocycle an offset is observed, resulting in a
weakly helical conformation. Additional characteristic non-planar
distortions, which may be expected from the peripheral
substitution, are not detected.[13]
The persistent low spin configuration of the cationic complexes 7
and 8, and the unusual potential of the first reduction step tempted
us to investigate whether organometallic reactions could be
conducted with this compound. Indeed, catalytic C-C cross-coupling
reactions were observed upon treatment of the nickel 9-
methylisocorroles 7 and 8 with Grignard reagents in the presence of
Scheme 2 Catalytic Caryl-Caryl cross-coupling with 7 and 8 (X = Cl, Br, I).
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porphyrinoids. Our first investigations havVeiew ArrteicvleeOanlleinde
DOI: 10.1039/C5SC03663A
interesting redox and ligand exchange behaviour for the nickel
complexes which differ largely from what is known for nickel
porphyrins. Indeed, no change in the oxidation state of the
nickel atom is observed upon oxidation or reduction, and the
low spin character of the Ni(II) ion in the 9-methylisocorrole
complexes remains persistent even in the presence of
additional donors like halogenido or phenyl ligands. These
features, which are unprecedented in the chemistry of nickel
porphyrins, apparently lead to catalytic activity in C-C cross-
coupling reactions. Such organometallic reactivity is typically
unexpected for metal porphyrins due to the lack of two
adjacent coordination sites at the metal atom.[16] In this case,
however, the catalytic transformation seems to proceed via
kinetically advantageous multi-step radical reactions in very
good selectivity, with the non-innocent 9-methylisocorrole
ligand playing the role of an electron relay. This example
shows that nickel porphyrinoids can be very attractive
compounds and may well become useful in fields like catalysis
and sensors,[17] and that nickel can do more than stabilising
macrocycles in porphyrin research.
Scheme 3 Proposed radical mechanism for the nickel 9-methylisocorrole catalysed Caryl
Caryl cross-coupling of PhMgBr with 1-bromonaphthalene.
-
chlorine, bromine, iodine. Sterically hindered mesityl derivatives
could not be coupled, and the reaction stops at -78°C. Beside the
homocoupling product from the Grignard component, which seems
to be formed during the activation of the nickel 9-methylisocorrole
cations 7 and 8, the major by-products stem from dehalogenation
and from homocoupling of the aryl halide. These by-products point
to a radical mechanism of the catalytic reaction. Addition of TEMPO
indeed quenches the cross-coupling reaction. The reaction is
initiated upon the addition of the Grignard component to 7 and 8,
and UV/Vis and EPR spectroscopy indicate the formation of the
one-electron reduced neutral radicals 7’ and 8’, respectively, upon
this treatment (see ESI for reference). All further steps are
speculative at this point. However, as the nickel 9-methylisocorrole
scaffold most probably does not allow the binding of two organic
residues to the nickel at the same side of the macrocycle, a step-
wise mechanism as depicted in scheme 3 is proposed. As the rate
determining step the reduced catalyst 7’ or 8’ activates the aryl
halide by one-electron transfer to the radical [Ar1X]·- and is itself
reactivated by the rapid reaction with the Grignard component,
producing the radical [Ar2]· to close the catalytic cycle. Product and
by-product formation may be rationalised as the possible radical
combination processes. Other than for the known nickel catalysed
C-C coupling reactions like the Kumada coupling[14] or the example
recently described by Tilley et al.,[15] this new process takes place
without any redox change at the metal atom and can be related to
the non-innocent behaviour of the 9-methylisocorrole ligand. The
selectivity for the cross-coupling may be understood kinetically if
the activation of the nickel complex by the Grignard reagent is
faster than the diffusion of the aryl halide radical. However, the
origin of the H atom necessary for the dehalogenation products
remains unclear, and further studies will be necessary to provide
solid grounds for the proposed mechanism.
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The successful synthesis of cationic nickel(II) 9-
methylisocorroles from 2,2'-bidipyrrin-type tetrapyrroles by a
templated oxidative macrocyclisation reaction using silver salts
of weakly coordinating anions as oxidants paves the road to
the investigation of a group of new contracted metal
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