New palladium catalysed reactions of bromoporphyrins: Synthesis and crystal structures of nickel(II) complexes of primary 5-aminoporphyrin, 5,5′-bis(porphyrinyl) secondary amine, and 5-hydroxyporphyrin

Article abstract of DOI:10.1039/b608365j

Primary aminoporphyrin, secondary bis(porphyrinyl)amine and hydroxyporphyrin complexes have been isolated and characterised both spectroscopically and crystallographically from the reaction of 5-bromo-10,15,20-triphenylporphyrinatonickel(II) with hydrazine under palladium catalysis. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b608365j

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New palladium catalysed reactions of bromoporphyrins: synthesis and
crystal structures of nickel(II) complexes of primary 5-aminoporphyrin,
5,59-bis(porphyrinyl) secondary amine, and 5-hydroxyporphyrin{
Louisa J. Esdaile,a Mathias O. Sengeb and Dennis P. Arnold*a
Received (in Cambridge, UK) 13th June 2006, Accepted 26th July 2006
First published as an Advance Article on the web 23rd August 2006
DOI: 10.1039/b608365j
reaction mixture deserved closer scrutiny. We therefore investi-
gated the effects of changing the ratios of the various components
and have discovered conditions that favour the formation of 2 or 4
from 1, and also obtained 3 by independent reaction of 2 with 1.
The aminoporphyrin 2 can be isolated in high yield by
increasing the relative amount of hydrazine. Even at 1 : 1 molar
ratio, the primary amine becomes the major product and
formation of 3 and 4 is suppressed. The NMR spectrum of 2 is
typical for a meso-amino porphyrin, the NH2 signal appearing at
5.77 ppm.10 Similar primary aminoporphyrins have previously
been prepared by the NaBH4/Pd/C reduction of the metalloni-
troporphyrin, itself obtained under mild conditions from the free
base unsubstituted porphyrin substrate using I2/AgNO2, followed
Primary aminoporphyrin, secondary bis(porphyrinyl)amine
and hydroxyporphyrin complexes have been isolated and
characterised both spectroscopically and crystallographically
from the reaction of 5-bromo-10,15,20-triphenylporphyrinato-
nickel(II) with hydrazine under palladium catalysis.
Palladium catalysed coupling reactions on haloporphyrins have
revolutionised the functionalisation of the porphyrin periphery.1
There are now many and varied examples of successful C–C and
C–X couplings, expecially on the meso positions(s) of 5,15-diaryl-
or 5,10,15-triarylporphyrins, to produce a range of functionalised
mononuclear derivatives, as well as di- and oligonuclear arrays
with alkynyl, alkenyl and aromatic linkers. Recently, there have
been examples of porphyrin–heteroatom couplings with nucleo-
philes such as amines,2,3 amides,2,4 alcohols and phenols5 thiols,6
and phosphine sources.7
In seeking to extend the amination/amidation reactions, we
reacted bromoporphyrins with tert-butyl and ethyl carbazates, and
uncovered a strong tendency for the bis(carbazate) porphyrins to
oxidize to azocarboxylates or to de-aromatise to form novel di-
iminoporphodimethenes.8 As an extension of this work, we tried
the same conditions (first reported for amidation by Takanami
et al.2) with unsubstituted hydrazine as nucleophile, using palla-
dium(II) acetate and rac-BINAP as catalyst precursors, caesium
carbonate as base, and THF as solvent. The porphyrin precursor,
5-bromo-10,15,20-triphenylporphyrinatonickel(II) (NiTriPPBr, 1),
with just one available meso position, was chosen to avoid
complications of the type seen in our work with the carbazates.8
The results of this experiment (Scheme 1) at a molar ratio of
2(1) : 1(N2H4?H2SO4) were surprising and remarkable. From the
one reaction mixture, we isolated and obtained X-ray crystal
structures for three novel nickel(II) complexes, namely the primary
amine NiTriPPNH2 2, the secondary amine (NiTriPP)2NH 3 and
the hydroxyporphyrin NiTriPPOH 4.{ Logically, the secondary
amine 3, the first bis(porphyrinyl)amine to be isolated, results from
amination of 1 by 2. However, formation of the primary amine by
the apparent Pd-induced cleavage of hydrazine or a porphyrinyl-
hydrazine and the formation of a phenol analogue in an amination
aSynthesis and Molecular Recognition Program, School of Physical and
Chemical Sciences, Queensland University of Technology, G.P.O. Box
2434, Brisbane, 4001, Australia. E-mail: d.arnold@qut.edu.au;
Fax: +61 7 3864 1804; Tel: +61 7 3864 2482
Scheme 1 Palladium catalysed aminations and hydroxylation. Reagents
and conditions: (i) hydrazine sulfate (0.5 eq.), Pd cat., THF, 68 uC, 15 h; (ii)
hydrazine sulfate (1 eq.), Pd cat., THF, 68 uC, 16 h, 51%; (iii) 1, Pd cat.,
dioxane, 100 uC, 3 d, 25%; (iv) Pd cat., THF, 68 uC, 4.5 d, 79% [Pd cat. =
Pd(OAc)2 (7 mol%), rac-BINAP (20 mol%), Cs2CO3 (7 eq.)]; (v) Ac2O, py,
75 uC, 10 min, 77%.
bSchool of Chemistry, SFI Tetrapyrrole Laboratory, Trinity College
Dublin, Dublin 2, Ireland. E-mail: sengem@tcd.ie; Fax: +353 1 608 8536;
Tel: +353 1 608 8537
{ Electronic supplementary information (ESI) available: Details of
synthetic procedures and spectroscopic data. See DOI: 10.1039/b608365j
4192 | Chem. Commun., 2006, 4192–4194
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by metallation.10 This new method from the nickel(II) bromopor-
phyrin and hydrazine is competitive in terms of crude yield and
reaction time, but the nitration/reduction route can be scaled up
more readily. A control experiment in the absence of palladium
salt led only to small amounts of debromination of 1 after 3 days.
The amination of 1 with 2 is a sluggish reaction in THF, but
using 1,4-dioxane at 100 uC, the yield of purified 3 after
recrystallisation has been increased to 25%. The two amines can
be readily separated from each other by column chromatography.
Further optimisation experiments are in progress. While there are
a few examples of diporphyrins linked by just one carbon atom,11
there are no other meso,meso-linked diporphyrins with a single
heteroatom bridge. The unique NH proton appears significantly
downfield at 9.86 ppm, and the effects of close proximity of the
porphyrin rings are apparent in the chemical shifts of the ring
protons on the 10,20-phenyl groups, which are more widely
dispersed than those of 2. As expected, the UV-visible spectrum of
3 differs considerably from that of 2 (Fig. 1), the split Soret band
indicating the expected excitonic coupling between the transition
dipoles of the two rings. The Q bands are also significantly shifted
to lower energy.
Fig. 2 View of the molecular structure of 2 in the crystal. Thermal
ellipsoids are drawn for 50% occupancy; selected hydrogen atoms have
been omitted for clarity.
Fig. 3 View of the molecular structure of 3 in the crystal. Thermal
ellipsoids are drawn for 50% occupancy; selected hydrogen atoms have
been omitted for clarity.
uninterpretable, however upon addition of excess hydrazine
hydrate to the CDCl3 solution, the lines sharpen and appear as
expected for a mono-meso-substituted nickel(II) triarylporphyrin.
The UV-visible spectrum of 4 also implies the presence of small
and variable amounts of the oxyl radical, with a very weak band at
803 nm appearing in some samples.15 Compound 4 was further
characterised by its simple conversion into the corresponding
acetate 5 by reaction with acetic anhydride and pyridine.
Fig. 1 UV-visible spectra of 2 (dashed), 3 (bold), and 4 (solid) in
CH2Cl2.
Lastly, the hydroxyporphyrin 4 can be readily prepared
selectively in 79% yield, by the simple expedient of omitting the
hydrazine in the reaction with 1 and extending the reaction time.
This implies that the caesium carbonate is actually providing the
nucleophile for reaction on the palladium. Although we conducted
experiments with rigorous drying of the base, we cannot yet rule
out the possibility that water is required. To our knowledge, Pd-
catalysed formation of hydroxyarenes by such a method is
unreported. The normal Pd promoted route to phenols from
halobenzenes uses tert-butoxide or tert-butyldimethylsiloxide as
protected hydroxide source.12 Hydroxyporphyrin metal complexes
(the free bases of which exist as the keto ‘‘oxophlorin’’ tautomer)
have been investigated in detail because of their significance (as Fe
complexes) in the natural heme degradation pathways.13 They are
usually prepared by oxidative routes,14 but for the triarylporphyrin
Ni(II) complexes, this new method is very convenient and high-
yielding.
Hydroxyporphyrin complexes readily oxidise and deprotonate
to give oxygen-centred free radicals, which in several cases have
been characterised crystallographically.13 This phenomenon is
evident for 4, as the proton NMR spectrum is broad and
Fig. 4 View of the molecular structure of 4 in the crystal. Thermal
ellipsoids are drawn for 50% occupancy; selected hydrogen atoms have
been omitted for clarity.
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The crystal structures of 2, 3 and 4 (Fig. 2–4) reveal several
interesting features. All compounds show various degrees of ruf
(ruffled) distortion induced by the small Ni(II) ion.16 Compound 4
exhibits only a small degree of distortion [average Ni–N bond
˚
length 1.963(3) A]. Secondary amine 3 has both a relatively planar
˚
macrocycle [Ni1–N = 1.952(4) A] and a severely ruffled one [Ni1–
Notes and references
{ Crystal data (all grown from CH2Cl2/MeOH using standard techniques9):
2: C38H25N5Ni, M = 610.34, orthorhombic, a = 6.4778(2), b = 19.0155(6),
3
˚
˚
c = 22.2824(7) A, U = 2744.72(15) A , T = 90 K, space group P212121, Z =
4, m(Mo-Ka) = 0.746 mm21, 31404 reflections measured, 6313 unique
(Rint = 0.0612) which were used in all calculations. The final wR(F2) was
0.08 (all data). 3: C76H47N9Ni2, M = 1203.65, triclinic, a = 12.4790(9), b =
˚
N = 1.905(4) A], while primary amine 2 shows an intermediate
˚
degree of distortion [Ni–N = 1.939(2) A]. All compounds exhibit
˚
14.1516(9), c = 16.1406(11) A, a = 101.5380(10), b = 99.8540(10), c =
3
˚
¯
95.8160(10)u, U = 2724.3(3) A , T = 90 K, space group P1, Z = 2, m(Mo-
Ka) = 0.75 mm21, 27815 reflections measured, 10802 unique (Rint = 0.079)
which were used in all calculations. The final wR(F2) was 0.1354 (all data).
very close intermolecular packing arrangements. Compound 2
forms p-stacked polymers in which the amino nitrogen atom is
˚
located on top of a neighboring Ni(II) center [Ni–N1 = 3.109 A,
4: C38H24N3NiO, M = 611.32, triclinic, a = 9.6815(19), b = 12.028(2), c =
˚
3
˚
12.986(3) A, a = 69.02(3), b = 77.92(3), c = 85.14(3)u, U = 1380.6(6) A , T =
90 K, space group P1, Z = 2, m(Mo-Ka) = 0.744 mm21, 15377 reflections
˚
Ni–H1B = 2.866 A]. The other side of the macrocycle exhibits a
¯
˚
short phenyl hydrogen–nickel contact [Ni–H15B = 2.819 A].
measured, 5050 unique (Rint = 0.0624) which were used in all calculations.
The final wR(F2) was 0.169 (all data). CCDC 610310 (2); 610308 (3);
610309 (4). For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b608365j
Compound 3 forms polymeric zig-zag chains in which a pyrrole
b-hydrogen atom and a phenyl hydrogen atom are located close to
˚
a neighboring Ni(II) center [Ni1–H37 = 3.037 A, Ni1–H103 =
˚
˚
2.945 A, Ni2–H3A = 2.776 A]. The hydroxyporphyrin 4 forms
very closely packed p-stacked dimers,14,17 where an aryl hydrogen
atom is coordinated to a Ni(II) center and a pyrrole nitrogen atom
1 For reviews on Pd catalysed couplings on tetrapyrroles, see:
W. M. Sharman and J. E. van Lier, J. Porphyrins Phthalocyanines,
2000, 4, 441; J. Setsune, J. Porphyrins Phthalocyanines, 2004, 8, 93; for
general porphyrin functionalisation, including nucleophilic substitutions,
see: M. O. Senge and J. Richter, J. Porphyrins Phthalocyanines, 2004, 8,
934; M. O. Senge, Acc. Chem. Res., 2005, 38, 733.
2 T. Takanami, M. Hayashi, F. Hino and K. Suda, Tetrahedron Lett.,
2003, 44, 7353.
3 Y. Chen and X. P. Zhang, J. Org. Chem., 2003, 68, 4432.
4 G. Y. Gao, Y. Chen and X. P. Zhang, Org. Lett., 2004, 6,
1837.
5 G. Y. Gao, A. J. Colvin, Y. Chen and X. P. Zhang, Org. Lett., 2003, 5,
3261.
6 G. Y. Gao, A. J. Colvin, Y. Chen and X. P. Zhang, J. Org. Chem.,
2004, 69, 8886.
7 F. Atefi, J. C. McMurtrie, P. Turner, M. Duriska and D. P. Arnold,
Inorg. Chem., 2006, 45, 6479.
8 L. J. Esdaile, J. C. McMurtrie, P. Turner and D. P. Arnold, Tetrahedron
Lett., 2005, 46, 6931.
9 H. Hope, Prog. Inorg. Chem., 1994, 41, 1; M. O. Senge and K. M. Smith,
Acta Crystallogr., Sect. C, 2005, 61, o537.
˚
˚
[Ni–H156 = 3.037 A, N21–H156 = 2.488 A]. In the absences of
any real acceptor groups, the donor –OH or –NH2 hydrogen
atoms are not involved in any other binding in either structure.
In summary, the novel reactions we have uncovered suggest
several new avenues of study, in both porphyrin chemistry and
more generally in palladium catalysis of unsuspected processes.
While there are many cases of reductive cleavage of hydrazine(s)
by transition metal species,18 the use of hydrazine as an ammonia
surrogate in Pd catalysed aminations of aryl bromides is
unprecedented and invites further investigation. Primary amine
synthesis by Pd catalysed amination is normally achieved in two
steps using (di)allylamines, tert-butyl carbamate, imines, or
sulfoximines as intermediates.12 Likewise, the formation of the
hydroxyporphyrin from a carbonate base suggests that the direct
substitution of haloarenes to form phenol analogues may be
viable. Our own interests lie in the new porphyrin chemistry and
especially in the further study of the secondary amines such as 3.
We are currently pursuing the coupling reactions and redox
chemistry of 3, as bis(diarylamines) linked by conjugated bridges
are currently of significant interest due to the delocalised electronic
structures of their mixed-valence radical cations. Porphyrin
analogues of such species may offer favourable electronic
delocalisation and stability of relevance to molecular electronics
applications.19
10 D. P. Arnold, R. C. Bott, H. Eldridge, F. Elms, G. Smith and M. Zojaji,
Aust. J. Chem., 1997, 50, 495.
11 M. O. Senge, M. G. H. Vicente, K. R. Gerzevske, T. P. Forsyth and
K. M. Smith, Inorg. Chem., 1994, 33, 5625.
12 D. Prim, J.-M. Campagne, D. Joseph and B. Andrioletti, Tetrahedron,
2002, 58, 2041.
13 A. L. Balch, Coord. Chem. Rev., 2000, 200–202, 349 and refs.
therein.
´
´ ´
14 J. Wojaczynski, M. Ste˛pien and L. Latos-Graz˙ynski, Eur. J. Inorg.
Chem., 2002, 1806.
15 A. L. Balch, M. Mazzanti and M. M. Olmstead, Inorg. Chem., 1993, 32,
4737.
16 M. O. Senge, Chem. Commun., 2006, 243.
DPA thanks the Australian Research Council for financial
support through Discovery Grant DP0663774, MOS gratefully
acknowledges support by Science Foundation Ireland (SFI
Research Professorship 04/RP1/B482), and LJE thanks the
Faculty of Science and the Synthesis and Molecular Recognition
Program, Queensland University of Technology for a postgrad-
uate scholarship.
17 M. O. Senge and K. M. Smith, J. Chem. Soc., Chem. Commun., 1992,
1108; M. O. Senge, C. W. Eigenbrot, T. D. Brennan, W. R. Scheidt and
K. M. Smith, Inorg. Chem., 1993, 32, 313.
18 For example: I. Takei, K. Dohki, K. Kobayashi, T. Suzuki and
M. Hidai, Inorg. Chem., 2005, 44, 3768; M. P. Shaver and M. D. Fryzuk,
J. Am. Chem. Soc., 2005, 127, 500.
19 For a porphyrin example, see: J.-C. Chang, C.-J. Ma, G.-H. Lee,
S.-M. Peng and C.-Y. Yeh, Dalton Trans., 2005, 1504.
4194 | Chem. Commun., 2006, 4192–4194
This journal is ß The Royal Society of Chemistry 2006