Synthesis, structural characterization, and electrochemical studies of nickel porphyrins bearing two peripheral conjugated chelating groups

Article abstract of DOI:10.1021/ic701484r

This Article describes the synthesis of several new nickel porphyrins bearing peripheral chelating groups conjugated with the macrocyclic π system. These monomeric nickel porphyrins display quite unusual chromophoric properties, some of them absorbing in

Full text of DOI:10.1021/ic701484r

Inorg. Chem. 2007, 46, 10241−10251
Synthesis, Structural Characterization, and Electrochemical Studies of
Nickel Porphyrins Bearing Two Peripheral Conjugated Chelating Groups
Se´bastien Richeter,† Christophe Jeandon,† Jean-Paul Gisselbrecht,‡ Romain Ruppert,*,† and
Henry J. Callot†
Laboratoire de Chimie des Porphyrines, Faculte´ de Chimie, UMR 7177 CNRS, UniVersite´ Louis
Pasteur, 1 rue Blaise Pascal, 67000 Strasbourg, France, and Laboratoire d’Electrochimie et de
Chimie Physique du Corps Solide, UMR 7177 CNRS, UniVersite´ Louis Pasteur, 4 rue Blaise
Pascal, 67000 Strasbourg, France
Received July 26, 2007
This Article describes the synthesis of several new nickel porphyrins bearing peripheral chelating groups conjugated
with the macrocyclic system. These monomeric nickel porphyrins display quite unusual chromophoric properties,
π
some of them absorbing in the near-infrared region. Extension of the aromatic core of the porphyrin was realized
by connecting meso-aryl groups with pyrroles by ketone functionalities. Further functionalizations led to
bisenaminoketones or bisenaminothioketones, which are useful building blocks for the elaboration of oligomeric
porphyrins linked by metal ions. All new compounds were studied by electrochemistry, some of them showing up
to six electron transfers and/or splitting of the first oxidation wave.
Introduction
expected between internal and external coordinated metal
ions, thus leading to very promising candidates for magnetic
The expansion and/or functionalization of large aromatic
molecules (or more generally π-conjugated molecules) in
order to build molecular systems with enhanced optoelec-
tronic properties is a topic of broad current interest.1 In
particular, in the field of oligoporphyrins (or porphyrin
analogues, like porphyrazines and phthalocyanines), several
research groups have obtained large molecular architectures
through covalent or noncovalent linkages.2-4 Among these
new chromophores, fused porphyrins have led to the
spectacular enhancement of the electronic properties.5 Using
metal ions as linking units between functionalized porphyrins
allowed the easy preparation of oligoporphyrins in a few
synthetic steps and with excellent yields.6 The use of
metalloporphyrins or metalloporphyrazines functionalized at
the periphery of the macrocycle also allows the anchoring
of metal complexes. In such molecules, interactions can be
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* To whom correspondence should be addressed. E-mail: rruppert@
chimie.u-strasbg.fr.
† Laboratoire de Chimie des Porphyrines.
‡ Laboratoire d’Electrochimie et de Chimie Physique du Corps Solide.
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10.1021/ic701484r CCC: $37.00
Published on Web 10/09/2007
© 2007 American Chemical Society
Inorganic Chemistry, Vol. 46, No. 24, 2007 10241

Richeter et al.
Chart 1. Nickel Ketone 1, Nickel Enaminoketone 2, and Palladium(II)-Linked Diporphyrin 3
molecular materials or to new catalysts where the activity
of the external metal ion might be controlled by the central
metal ions.7
(see Chart 1).8 This compound was obtained by amination
of the known ketone 1.9 The enaminoketone external
chelating group was used to assemble metalloporphyrins such
as 2 into dimers such as 3 around dicationic metal ions.10
Strong interactions between the two linked porphyrins were
found for these dimers. Compared to the previously used
covalent connections (double or triple bonds linking two
porphyrins), the electronic spectra and the electrochemical
study of these dimers demonstrated that the electronic
interactions were in the same range.11 We have extended this
work to porphyrins bearing other external chelating groups
(enaminothioketones, enaminoaldehydes, and enaminothio-
aldehydes), and the interactions between porphyrins found
in the dimers built from these monomers were similar.12
We have described the preparation of nickel porphyrin 2
bearing an enaminoketone conjugated with the aromatic core
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In order to construct higher oligomers using the same
strategy, porphyrins bearing two external chelating groups
were needed. In the same series (enaminoketones as external
chelating groups) as the initially used porphyrins, this implied
that porphyrins bearing two diketones should be aminated.
We have described the synthesis of all possible isomeric
diketones 4-9 (see Chart 2) and the structure of two of them
(compound 4 and an analogue of compound 7, differing only
by the meso-aryl groups) obtained by X-ray diffraction.13
We have already briefly described the use of a nickel
porphyrin bearing two external enaminoketones for the
preparation of a linear heptametallic porphyrinic trimer.14
In this Article, we describe the preparation of 14 nickel
porphyrins bearing two enaminoketones (NO-NO chelating
groups), two enaminothioketones (NS-NS chelating groups),
or mixed external functionalizations (NO-NS chelating
groups). Two of them gave single crystals suitable for an
X-ray structural characterization. The electronic properties
and a detailed electrochemical study of these nickel enamino-
(thio)ketones as well as those of nickel diketones 4-9 will
be presented.
(8) Richeter, S.; Jeandon, C.; Ruppert, R.; Callot, H. J. Tetrahedron Lett.
2001, 42, 2103.
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Org. Chem. 2003, 68, 9200.
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2002, 266.
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Chem. Soc. 2007, 129, 6392.
10242 Inorganic Chemistry, Vol. 46, No. 24, 2007

Electrochemical Studies of Nickel Porphyrins
Chart 2. All Possible Isomeric Porphyrinic Diketones (Compounds 4-9)
11 (from 5, yield 56%): 1H NMR (50 °C): δ ) 8.84, 8.45 (2d,
1+1H, J ) 4.8 Hz, pyrrole), 8.08, 8.05 (d, 1+1H, J ) 4.8 Hz,
pyrrole), 8.52 (dd, 1H, J ) 7.8 and ∼1.5 Hz, cyclized phenyl),
7.46 (ddd, 1H, J ) 7.8, 7.8, and ∼1.5 Hz, cyclized phenyl), 7.69
(ddd, 1H, J ) 7.8, 7.8, and ∼1.5 Hz, cyclized phenyl), 8.12 (dd,
1H, J ) 7.8 and ∼1.5 Hz, cyclized phenyl), 8.48 (dd, 1H, J ) 7.8
and ∼1.5 Hz, cyclized phenyl), 7.51 (ddd, 1H, J ) 7.8, 7.8, and
∼1.5 Hz, cyclized phenyl), 7.63 (ddd, 1H, J ) 7.8, 7.8, and ∼1.5
Hz, cyclized phenyl), 8.12 (dd, 1H, J ) 7.8 and ∼1.5 Hz, cyclized
phenyl), 8.12 (broad, 1H, Ar Hortho), 7.77 (dd, 1H, J ) 1.8 and 1.8
Hz, Ar Hpara), 7.09 (dd, 1H, J ) ∼1.8 and ∼1.8 Hz, Ar Hpara),
8.00-8.05 (broad, 1+1H, Ar Hortho), 7.69 (dd, 1H, J ) 1.8 and 1.8
Hz, Ar Hortho), 1.56, 1.30 (2s, 18+18H, t-butyl), N-H not detected.
UV-vis: (CH2Cl2) λmax ) 368 (ꢀ ) 37 800), 438 (34 600), 536
(64 800), 646 (11 000), 728 (12 300) nm. Elem anal. Calcd for
C62H58N10O2Ni‚H2O: C 74.78, H 6.07, N 8.44. Found: C 74.67,
H 5.90, N 8.35.
Experimental Section
General Procedures. UV-vis spectra were acquired with a
Hewlett-Packard model 8453 instrument (CH2Cl2). NMR (CDCl3
at 25 °C unless otherwise stated, δ (ppm) vs TMS) data acquired
on Bruker AC 300, AM 400, and ARX 500 instruments. COSY
1H-1H NMR have been run on all compounds. Elemental analyses
were performed at the Service de Microanalyse, Universite´ Louis
Pasteur, Strasbourg. Chromatographic separations were obtained
using Merck 9385 silica gel or Merck 1097 alumina. All monomeric
porphyrins gave correct masses (Finnigan TSQ 700, EI, 12 eV or
MALDI-TOF).
Amination of Diketones 4, 5, and 7-9. To a solution of the
diketone (0.18 mmol in 60 mL toluene/ethanol 5/1) was added a
large excess of 4-amino-4H-1,2,4-triazole (350 mg, 4.16 mmoles)
and sodium hydroxide (500 mg, 12.5 mmol). The solution was
heated under reflux, and the progress of the amination was followed
by thin layer chromatography (TLC). After full consumption of
the starting material (ca. 2 h under reflux), trifluoroacetic acid (4
mL) was added and the mixture heated under reflux for an additional
hour. After cooling, dichloromethane (100 mL) was added and the
organic phase washed with water (3 × 1 L). The nickel bisenami-
noketones 10-14 were purified by column chromatography (silica
gel, 300 g, dichloromethane as eluent) and crystallized from
dichloromethane/methanol.
12 (from 7, yield 69%): 1H NMR: δ ) 9.06, 8.50 (2d, 2+2H,
J ) 5.0 Hz, pyrrole), 8.53 (dd, 2H, J ) 7.8 and ∼1.0 Hz, cyclized
phenyl), 8.15 (dd, 2H, J ) 7.8 and ∼1.0 Hz, cyclized phenyl),
7.73 (ddd, 2H, J ) 7.8, 7.8 and ∼1.0 Hz, cyclized phenyl), 7.55
(ddd, 2H, J ) 7.8, 7.8 and ∼1.0 Hz, cyclized phenyl), 7.88, 7.69
(2t, 1+1H, J ) 1.7 Hz, Ar Hpara), 7.20-8.00 (very broad, 4H, Ar
Hortho), 1.47, 1.46 (2s, 18+18H, t-butyl), N-H not detected. UV-
10 (from 4, yield 74%): 1H NMR: δ ) 9.23, 8.10 (2s, 2+2H,
pyrrole), 8.52 (dd, 2H, J ) 7.7 and ∼1.5 Hz, cyclized phenyl),
8.08 (dd, 2H, J ) 7.7 and ∼1.0 Hz, cyclized phenyl), 7.71 (ddd,
2H, J ) 7.7, 7.7, and ∼1.5 Hz, cyclized phenyl), 7.53 (ddd, 2H, J
) 7.7, 7.7, and ∼1.0 Hz, cyclized phenyl), 7.75 (t, 2H, J ) 1.8
Hz, Ar Hpara), 7.58 (broad d, 4H, J ) 1.8 Hz, Ar Hortho), 8.21 (broad
s, 2H, N-H), 5.12 (broad s, 2H, N-H), 1.41 (s, 36H, t-butyl).
UV-vis: (CH2Cl2) λmax ) 318 (ꢀ ) 38 400), 348 (42 400), 480
(66 600), 512 (56 400sh), 544 (69 200), 618 (18 300), 710 (8700)
nm. Elem anal. Calcd for C62H58N6O2Ni‚0.5CH3OH: C 75.53, H
6.08, N 8.46. Found: C 75.74, H 6.22, N 8.50.
vis: (CH2Cl2) λmax ) 416 (ꢀ ) 34 700), 502 (85 500), 672 (16 700),
703 (19 000), 758 (24 800) nm. Elem anal. Calcd for NiC62H58N6O2‚
0.5CH3OH: C 75.53, H 6.08, N 8.46. Found: C 75.56, H 6.00,
N 8.38.
13 (from 8, yield 64%): 1H NMR: δ ) 9.01, 8.45 (2d, 2+2H,
J ) 4.9 Hz, pyrrole), 8.54 (dd, 2H, J ) 7.7 and ∼1.0 Hz, cyclized
phenyl), 8.24 (dd, 2H, J ) 7.7 and ∼1.0 Hz, cyclized phenyl),
7.74 (ddd, 2H, J ) 7.7, 7.7, and ∼1.0 Hz, cyclized phenyl), 7.55
(ddd, 2H, J ) 7.7, 7.7, and ∼1.0 Hz, cyclized phenyl), 7.80 (t, 2H,
J ) 1.7 Hz, Ar Hpara), 7.40-7.80 (broad, 4H, Ar Hortho), 1.46 (s,
36H, t-butyl), N-H not detected. UV-vis: (CH2Cl2) λmax ) 395
Inorganic Chemistry, Vol. 46, No. 24, 2007 10243

Richeter et al.
(ꢀ ) 24 200), 488 (140 500), 667 (20 000), 757 (38 100) nm. Elem
anal. Calcd (%) for C62H58N6O2Ni•CH3OH: C 74.93, H 6.19, N
8.32; found: C 75.13, H 6.11, N 8.24.
7.91, 7.69 (2t, 1+1H, J ) 1.8 Hz, Ar Hpara), 7.40-8.00 (very broad,
4H, Ar Hortho), 11.94, 5.96 (2 broad d, 1+1H, NH2 near S), 1.55,
1.45 (2s, 18+18H, t-butyl), NH2 near O not detected. UV-vis:
(CH2Cl2) λmax ) 362 (ꢀ ) 31 700), 386 (35 300), 442 (47 600),
510 (63 800), 700 (22 500sh), 740 (29 000), 794 (24 700) nm. Elem
anal. Calcd for NiC62H58N6SO‚H2O: C 73.59, H 5.98, N 8.30.
Found: C 73.49, H 5.90, N 8.20.
19 (from 13): 1H NMR: δ ) 8.99, 8.98, 8.43, 8.40 (4d,
1+1+1+1H, J ) 4.8 Hz, pyrrole), 9.09, 8.53, 8.24, 8.24 (4dd,
1+1+1+1H, J ) 7.7 and ∼1.0 Hz, cyclized phenyl), 7.75 (2ddd,
1+1H, J ) 7.7, 7.7, and ∼1.0 Hz, cyclized phenyl), 7.56 (2ddd,
1+1H, J ) 7.7, 7.7, and ∼1.0 Hz, cyclized phenyl), 7.82, 7.80 (2
broad t, 2H, J ) 1.8 Hz, Ar Hpara), 7.40-7.80 (very broad, 4H, Ar
14 (from 9, yield 75%): 1H NMR (C2D2Cl4, 60 °C): δ ) 8.29
(d, 2H, J ) 5.1 Hz, pyrrole), 8.11 (2d, 2H, J ) 4.9 Hz, pyrrole),
8.67 (d, 2H, J ) 7.5 Hz, cyclized phenyl), 7.62 (t, 1H, J ) 7.5 Hz,
cyclized phenyl), 7.85 (t, 2H, J ) 1.8 Hz, Ar Hpara), 7.70-7.72
(m, 7H, Ar Hortho+para), 1.49 (s, 36H, t-butyl), 1.46 (s, 18H, t-butyl),
N-H not detected (but observed in CDCl3 at 25 °C: δ ) 8.47 and
5.22). UV-vis: (CH2Cl2) λmax ) 400 (ꢀ ) 63 300), 492 (70 600),
644 (21 100), 682 (32 500) nm. High-resolution MS calcd for
C70H74N6O2Ni + H+: 1089.5300; found, 1089.5293.
Thionation of Bisenaminoketones 10-13. A solution of the
nickel bisenaminoketone in benzene was heated under reflux, and
the Lawesson’s reagent was added in small portions (approxima-
tively 1 equiv per addition). The progress of the reaction was
followed by TLC, and the reaction was stopped before full
consumption of the starting material. After evaporation of the
solvent, the residue was chromatographed on a silica gel column
(eluent: dichloromethane) allowing the successive isolation of the
dithionated, the monothionated, and the starting compound (>90%
overall recovery of nickel porphyrins). Crystallization from dichlo-
romethane/methanol afforded the mono- and dithionated products.
15 (from 10): 1H NMR: δ ) 9.19, 9.17, 8.10, 8.08 (4d,
1+1+1+1H, J ) 5.0 Hz, pyrrole), 9.06, 8.52, 8.08, 8.01 (4dd,
1+1+1+1H, J ) 8.0 and ∼1.0 Hz, cyclized phenyl), 7.70, 7.68,
7.53, 7.52 (4ddd, 1+1+1+1H, J ) 8.0, 8.0, and ∼1.0 Hz, cyclized
phenyl), 7.77, 7.75 (2t, 1+1H, J ) 1.8 Hz, Ar Hpara), 7.58 (2 broad
d, 2+2H, Ar Hortho), 11.77, 5.97 (2d, 1+1H, J ) 4.0 Hz, NH2 near
S), 8.21, 5.10 (broad, 1+1H, NH2 near O), 1.41 (2s, 18+18H,
t-butyl). UV-vis: (CH2Cl2) λmax ) 356 (ꢀ ) 37 600), 388
(30 000sh), 468 (39 500), 514 (47 200sh), 566 (61 000), 678
(13 000sh), 742 (9000) nm. Elem anal. Calcd for NiC62H58N6SO‚
H2O: C 73.59, H 5.98, N 8.30. Found: C 73.48, H 5.86, N 8.14.
16 (from 11): 1H NMR: δ ) 8.78, 8.43, 8.02, 7.98 (4d,
1+1+1+1H, J ) 4.8 Hz, pyrrole), 9.05, 8.12 (2dd, 1+1H, J )
7.8 and ∼1.5 Hz, cyclized phenyl S), 7.70, 7.49 (2ddd, 1+1H, J )
7.8, 7.8, and ∼1.5 Hz, cyclized phenyl S), 8.47, 8.21 (2dd, 1+1H,
J ) 7.8 and ∼1.5 Hz, cyclized phenyl O), 7.63, 7.52 (2ddd, 1+1H,
J ) 7.8, 7.8, and ∼1.5 Hz, cyclized phenyl O), 8.00-8.50 (very
broad, 2H, Ar), 7.67 (t, 1H, J ) 1.8 Hz, Ar), 8.09, 7.75, 7.08 (3t,
1+1+1H, J ) 1.8 Hz, Ar), 12.6, 5.15 (2 broad d, 1+1H, NH2 near
S), ∼1.29 (2s, 18+18H, t-butyl), NH2 near O not detected. UV-
vis: (CH2Cl2) λmax ) 378 (ꢀ ) 49 000), 452 (47 000), 570 (68 000),
668 (11 000), 766 (7500) nm. Elem anal. Calcd for NiC62H58N6-
SO‚H2O: C 73.59, H 5.98, N 8.30. Found: C 73.78, H 5.86, N
8.18.
17 (from 11): 1H NMR: δ ) 8.82, 8.45 (2d, 1+1H, J ) 4.8
Hz, pyrrole), 8.04 (s, 2H, pyrrole), 9.00, 8.08 (2dd, 1+1H, J ) 8.0
and ∼1.2 Hz, cyclized phenyl S), 7.66, 7.49 (2ddd, 1+1H, J )
8.0, 8.0, and ∼1.2 Hz, cyclized phenyl S), 8.52, 8.13 (2dd, 1+1H,
J ) 7.5 and ∼1.5 Hz, cyclized phenyl O), 7.70, 7.53 (2ddd, 1+1H,
J ) 7.5, 7.5, and ∼1.5 Hz, cyclized phenyl O), 8.00-8.50 (very
broad, 2H, Ar), 7.67 (t, 1H, J ) 1.8 Hz, Ar), 8.10, 7.77, 7.07 (3t,
1+1+1H, J ) 1.8 Hz, Ar), 11.71, 6.01 (2 broad d, 1+1H, NH2
near S), 1.55, 1.28 (2s, 18+18H, t-butyl), NH2 near O not detected.
UV-vis: (CH2Cl2) λmax ) 392 (ꢀ ) 42 000), 458 (42 500), 570
(60 000), 766 (11 400) nm. Elem anal. Calcd for NiC62H58N6SO‚
H2O: C 73.59, H 5.98, N 8.30. Found: C 73.74, H 5.97, N 8.02.
18 (from 12): 1H NMR: δ ) 9.02, 9.01, 8.49, 8.47 (4d,
1+1+1+1H, J ) 4.8 Hz, pyrrole), 9.07, 8.53, 8.14, 8.11 (4dd,
1+1+1+1H, J ) 8.0 and ∼1.1 Hz, cyclized phenyl), 7.73, 7.72,
7.55 (3ddd, 1+1+2H, J ) 8.0, 8.0 and ∼1.1 Hz, cyclized phenyl),
H
ortho), 11.95, 6.03 (2 broad d, 1+1H, NH2 near S), 1.54, 1.46 (2s,
18+18H, t-butyl), NH2 near O not detected. UV-vis: (CH2Cl2)
λmax ) 346 (ꢀ ) 30 600), 370 (26 600), 436 (38 400sh), 466
(53 800sh), 504 (100 500), 700 (24 200), 738 (18 900sh), 796
(27 100) nm. Elem ana. Calcd for NiC62H58N6SO‚CH3OH: C 73.75,
H 6.09, N 8.19. Found: C 73.75, H 6.11, N 7.83.
20 (from 10): 1H NMR: δ ) 9.19, 8.06 (2s, 2+2H, pyrrole),
9.06 (dd, 2H, J ) 8.0 and 1.1 Hz, cyclized phenyl), 8.05 (dd, 2H,
J ) 8.0 and ∼1 Hz, cyclized phenyl), 7.71 (ddd, 2H, J ) 8.0, 8.0,
and 1.1 Hz, cyclized phenyl), 7.55 (ddd, 2H, J ) 8.0, 8.0, and 1.1
Hz, cyclized phenyl), 7.77 (t, 2H, J ) 1.8 Hz, Ar Hpara), 7.57 (broad
d, 4H, J ) 1.8 Hz, Ar Hortho), 11.72, 5.94 (2 broad d, 2+2H, J )
4.0 Hz, NH2), 1.41 (s, 36H, t-butyl). UV-vis: (CH2Cl2) λmax
)
356 (ꢀ ) 42 400), 410 (44 400), 540 (51 700), 588 (69 300), 720
(16 300), 772 (10 300) nm. Elem anal. Calcd for C62H58N10S2Ni‚
1/2H2O: C 73.08, H 5.84, N 8.25. Found: C 73.06, H 5.88, N 8.16.
21 (from 11): 1H NMR: δ ) 8.76, 8.40 (2d, 1+1H, J ) 4.5
Hz, pyrrole), 7.98 (s, 2H, pyrrole), 9.04 (dd, 1H, J ) 8.0 and ∼1.5
Hz, cyclized phenyl), 7.50 (ddd, 1H, J ) 8.0, 8.0, and ∼1.5 Hz,
cyclized phenyl), 7.70 (ddd, 1H, J ) 8.0, 8.0, and ∼1.2 Hz, cyclized
phenyl), 8.13 (dd, 1H, J ) 8.0 and ∼1.2 Hz, cyclized phenyl),
8.98 (dd, 1H, J ) 8.0 and ∼1.5 Hz, cyclized phenyl), 7.53 (ddd,
1H, J ) 8.0, 8.0, and ∼1.5 Hz, cyclized phenyl), 7.63 (ddd, 1H, J
) 8.0, 8.0, and ∼1.5 Hz, cyclized phenyl), 8.17 (dd, 1H, J ) 8.0
and ∼1.5 Hz, cyclized phenyl), ∼8.00-8.50 (very broad, 2H, Ar),
7.67 (t, 1H, J ) 1.8 Hz, Ar), 8.06, 7.77, 7.09 (3t, 1+1+1H, J )
1.8 Hz, Ar), 12.40 (broad d, 1H, N-H), 11.70, 5.98 (2 broad d,
1+1H, J ∼ 5.0 Hz, N-H) 1.55, 1.29 (2s, 18+18H, t-butyl), one
N-H not detected. UV-vis: (CH2Cl2) λmax ) 398 (ꢀ ) 58 000),
474 (42 000), 600 (67 000), 796 (9700) nm. Elem anal. Calcd for
C62H58N6S2Ni‚2H2O: C 71.19, H 5.97, N 8.03. Found: C 71.12,
H 5.66, N 7.82.
22 (from 12): 1H NMR (50 °C): δ ) 8.97, 8.45 (2d, 2+2H, J
) 4.8 Hz, pyrrole), 9.06 (dd, 2H, J ) 8.0 and 1.1 Hz, cyclized
phenyl), 8.11 (dd, 2H, J ) 8.0 and 1.1 Hz, cyclized phenyl), 7.70
(ddd, 2H, J ) 8.0, 8.0 and 1.1 Hz, cyclized phenyl), 7.54 (ddd,
2H, J ) 8.0, 8.0, and 1.1 Hz, cyclized phenyl), 7.96, 7.71 (2t, 1+1H,
J ) 1.8 Hz, Ar Hpara), 7.60-7.90 (very broad, 4H, Ar Hortho), 11.94,
5.86 (2 broad d, 2+2H, NH2), 1.48, 1.46 (2s, 18+18H, t-butyl).
UV-vis: (CH2Cl2) λmax ) 358 (ꢀ ) 37 800), 404 (46 500), 436
(42 300), 490 (57 800sh), 516 (72 700), 726 (21 800sh), 772
(33 900), 821 (23 800) nm. Elem anal. Calcd for NiC62H58N10S2‚
H2O: C 72.44, H 5.88, N 8.17. Found: C 72.70, H 5.96, N 8.16.
23 (from 13): 1H NMR: δ ) 8.95, 8.38 (2d, 2+2H, J ) 4.8
Hz, pyrrole), 9.08 (dd, 2H, J ) 7.8 and ∼1.0 Hz, cyclized phenyl),
8.23 (dd, 2H, J ) 7.8 and ∼1.0 Hz, cyclized phenyl), 7.75 (ddd,
2H, J ) 7.8, 7.8, and ∼1.0 Hz, cyclized phenyl), 7.57 (ddd, 2H, J
) 7.8, 7.8, and ∼1.0 Hz, cyclized phenyl), 7.82 (broad t, 2H, J )
1.8 Hz, Ar Hpara), 7.40-7.80 (very broad, 4H, Ar Hortho), 11.88,
5.98 (2 broad d, 2+2H, NH2), 1.53 (s, 36H, t-butyl). UV-vis:
10244 Inorganic Chemistry, Vol. 46, No. 24, 2007

Electrochemical Studies of Nickel Porphyrins
Scheme 1. Amination Reaction Leading to Bisenaminoketones (Shown Here on Compound 4)
(CH2Cl2) λmax ) 362 (ꢀ ) 41 900), 396 (29 800), 468 (46 100sh),
508 (108 300), 728 (30 400), 765 (24 400), 827 (27 100) nm. Elem
anal. Calcd for C62H58N6S2Ni‚CH3OH: C 72.62, H 6.00, N 8.06.
Found: C 72.77, H 5.97, N 7.63.
hyde, 3,5-di-tert-butylbenzaldehyde, and pyrrole under Lind-
sey’s conditions,15 followed by metalation of the porphyrins
with nickel(II), hydrolysis of the esters, transformation of
the diacids into acid chlorides, and finally, an intramolecular
Friedel-Crafts reaction. Diketone 9 was obtained following
the same strategy, starting from an o,o′-dicyano-substituted
meso-phenyl group. These diketones in our hands, we first
tried to aminate these compounds with the reagents previ-
ously used for the amination of monofunctionalized porphy-
rins (hydrazine, tosylhydrazine, hydroxylamine, or hydrox-
ylamine O-sulfonic acid).8 Unfortunately, we obtained either
complex mixtures or no reaction products instead of the
expected bisenaminoketones.
Reaction with 4-amino-4H-1,2,4-triazole, an amination
reagent described by Katritzky and Laurenzo,16 allowed us
to prepare all bisenaminoketones in good yields (56-74%).
It was, however, necessary to add a strong Brønsted acid
(such as trifluoroacetic acid) to favor the elimination of
triazole after the 1,4-addition (see Scheme 1).
All nickel bisenaminoketones (see Chart 3) were able to
be characterized by NMR. The amination of the pyrrolic
positions was clearly established by the disappearence of the
diketone pyrrolic singlet signal in 1H NMR. In addition, the
two N-H protons were well-separated (δ ) 8.21 and 5.12
ppm for compound 10) indicating an intramolecular hydrogen
bond between one N-H and the carbonyl oxygen atom. In
the electronic spectra of nickel bisenaminoketones 10-14
compared with those of the corresponding nickel diketones
4-9, a hypsochromic shift was observed (except for com-
pounds 7 and 12). For example, the lowest energy bands for
compound 4 and 10 were located at 736 and 710 nm,
respectively.
We could finally obtain single crystals suitable for an
X-ray diffraction study of compound 10 (shown in Figure
1). The nickel porphyrin is best described as ruffled. The
nickel ion and the four pyrrolic nitrogen atoms are coplanar,
and the four nickel to nitrogen distances are almost identical
(1.90-1.91 Å). The intramolecular hydrogen bond between
one N-H and the carbonyl oxygen expected from the NMR
Electrochemical Studies. Spectroscopic grade dichloromethane
was purchased from Merck, dried over molecular sieves (4 Å), and
stored under argon prior to use. Electrochemical grade NBu4PF6
was purchased from Fluka and used as received. The electrochemi-
cal experiments were carried out at room temperature in dichlo-
romethane containing 0.1 M NBu4PF6 in a three-electrode cell. The
working electrode was either a glassy-carbon disk electrode (3 mm
diameter) or a platinum-disk electrode (2 mm diameter) used either
in motionless mode for cyclic voltammetry (CV: 10 mV‚s-1 to 10
V‚s-1) or as rotating disk electrode for rotating disk voltammetry
(RDV). The electrochemical cell was connected to a computerized
multipurpose electrochemical device AUTOLAB (Eco Chemie BV-
Utrecht, The Netherlands) driven by GPES software running on a
personal computer. All potentials are referenced to the ferrocene/
ferricinium (Fc/Fc+) couple used as an internal standard. The
auxiliary electrode was a Pt wire, and a Pt wire was also used as
a pseudo-reference electrode. The accessible potentials ranged from
+1.2 to -2.4 V vs Fc/Fc+ in dichloromethane.
Crystal Data for 10: C62H58N6NiO2‚CH2Cl2, M ) 1062.84,
triclinic, space group P1h; a ) 13.563(2) Å, b ) 13.818(2) Å, c )
17.639(5) Å, R ) 70.870(5)°, â ) 84.605(5)°, γ ) 60.888(5)°, V
) 2720(1) Å3, Z ) 2, Dc ) 1.30 g‚cm-3. A total of 10 121 ( h (
k + l reflections was collected on a brown crystal of dimensions
0.10 × 0.06 × 0.06 mm3, using a KappaCCD diffractometer,
graphite-monochromated Mo KR radiation, 2.5 < θ < 29.99, T )
173 K. Unique reflections numbering 5622 having I > 3 σ(I) were
used to determine and refine the structure. Final results: R ) 0.102,
Rw ) 0.127, GOF ) 1.101, largest peak in final difference ) 0.916
e‚Å-3
.
Crystal Data for 23: C124H116N12Ni2S4‚CH2Cl2‚H2O‚C4H8O, M
) 2195.12, monoclinic, space group C2/c; a ) 29.5565(2) Å, b )
15.2293(1) Å, c ) 30.1660(3) Å, â ) 116.492(5)°, V ) 12152.7-
(2) Å3, Z ) 4, Dc ) 1.20 g‚cm-3. A total of 18 275 ( h ( k + l
reflections was collected on a violet crystal of dimensions 0.10 ×
0.08 × 0.08 mm3, using a KappaCCD diffractometer, graphite-
monochromated Mo KR radiation, 2.5 < θ < 30.06, T ) 173 K.
Unique reflections numbering 12086 having I > 3 σ(I) were used
to determine and refine the structure. Final results: R ) 0.098, Rw
) 0.129, GOF ) 1.484, largest peak in final difference ) 1.145
e‚Å-3
.
(15) Lindsey, J. S.; Schreiman, I. C.; Hsu, H. C.; Kearney, P. C.;
Marguerettaz, A. M. J. Org. Chem. 1987, 52, 827. Littler, B. J.;
Ciringh, Y.; Lindsey, J. S. J. Org. Chem. 1999, 64, 2864. Lindsey, J.
S. In The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard,
R., Eds; Academic Press: Boston, MA, 2000; Vol. 1, p 45.
(16) Katritzky, A. R.; Laurenzo, K. S. J. Org. Chem. 1986, 51, 5039.
Katritzky, A. R.; Laurenzo, K. S. J. Org. Chem. 1988, 53, 3978.
Results and Discussion
Synthesis of Bisenaminoketones. Diketones 4-8 were
obtained by the condensation of o-carbomethoxybenzalde-
Inorganic Chemistry, Vol. 46, No. 24, 2007 10245

Richeter et al.
Figure 1. X-ray structure of nickel enaminoketone 10 (two meso-aryl groups omitted for clarity, two orthogonal views).
Chart 3. Structures of Nickel Porphyrins 10-23 and of Enamino(thio)ketone 24 (25)
data is indeed observed: the distance between the N-H
hydrogen atom and oxygen is found equal to 2.3 Å. In
addition, the six-membered rings formed by the atoms
involved in the two enaminoketone functions (OCCCNH)
are almost planar.
progress of the reaction it was possible to isolate the
monothionated analogue (see Scheme 2 for compound 10).
The reaction mixture (compounds 10, 15, and 20 in the
example shown) was easily purified by chromatography
because replacement of the oxygen atom by sulfur induced
a drastic decrease in polarity. In the case of the unsym-
metrical compound 5, two monofunctionalized isomers
(compounds 16 and 17) were isolated and their respective
structures determined by 1H NMR nuclear Overhauser effect
experiments. In this way, we isolated and characterized five
monothionated (compounds 15-19) and four dithionated
nickel porphyrins (compounds 20-23) (see Chart 3).
The thionation of the enaminoketones could be easily
Thionation of Porphyrins Bearing Two Enaminoketone
Groups. In the case of the monofunctionalized nickel
porphyrins, we have shown that replacing enaminoketones
by enaminothioketones gave much stronger complexes with
soft metal ions.12 Even dimers built around nickel(II) were
stable enough to allow purification by chromatography.
Therefore, it was obvious that the thionation of doubly
functionalized nickel porphyrins could lead to more stable
oligomers if the assembling was done with soft metal ions.
Treatment of the bisenaminoketones with Lawesson’s reagent
in refluxing benzene led cleanly to bisenaminothioketones.17
In addition, by controlling the amount of reagent and the
1
confirmed from the H NMR spectra. The two NH protons
(17) Pedersen, B. S.; Schreibye, S.; Nilsson, N. H.; Lawesson, S. O. Bull.
Soc. Chim. Belg. 1978, 87, 223. Cava, M. P.; Levinson, M. I.
Tetrahedron 1985, 41, 5061.
10246 Inorganic Chemistry, Vol. 46, No. 24, 2007

Electrochemical Studies of Nickel Porphyrins
Figure 2. X-ray structure of compound 23 (two meso-aryl groups omitted for clarity, two orthogonal views).
Scheme 2. Thionation of Bisenaminoketone 10
were even more differentiated in the case of the enami-
nothioketones. In compound 15, for example (NO and NS
chelating groups present), the signals were found at 8.21 and
5.10 ppm for the NO chelating group and at 11.77 and 5.97
ppm for the NS chelating group. The aromatic protons next
to the NS chelating group were also shifted downfield (for
compound 15, 9.06 and 8.52 ppm, respectively, for the
protons next to sulfur or oxygen). The replacement of oxygen
by sulfur had also a dramatic influence on the electronic
spectra of the nickel porphyrins. In the sequence shown in
Scheme 2 (enaminoketone 10 giving compounds 15 and 20),
the lowest energy band was bathochromically shifted by ca.
30 nm for each replacement of oxygen by sulfur (from 710
to 742 and 772 nm). To compare these data with that of
simpler compounds, we prepared the known enaminoketone
2418 and enaminothioketone 25.19 The replacement of oxygen
by sulfur induced an even larger bathochromic shift (from
λmax ) 352 nm for 24 to λmax ) 412 nm for 25).
region up to over 1000 nm.20a,b However, in general, even
nickel porphyrins with two additional fused aryl groups do
not show such a large bathochromic shift (lowest energy band
at λmax of 636 nm for recently published examples).20c,d
In the case of bisenaminothioketone 23, we could grow
single crystals suitable for an X-ray diffraction study (see
Figure 2). The porphyrin geometry is again ruffled. The
nickel coordination is square planar with the four nickel to
nitrogen bond lengths almost equal (from 1.89 to 1.91 Å).
The intramolecular hydrogen bond lengths (N-H/S) are
equal to 2.36 Å. As expected, the CdS bond is longer than
the CdO bond in the enaminoketones (1.69 Å instead of
1.34 Å).
Electronic Properties and Electrochemical Studies. The
electrochemical properties of nickel porphyrins are well-
documented.21 In general, the nickelporphyrins are oxidized
in two one-electron steps and reduced in two one-electron
steps (sometimes a third oxidation or reduction step might
These chromophores (lowest energy bands λmax of 796,
821, and 827 nm for compounds 21-23) are quite unusual
for monomeric nickel porphyrins (which absorb generally
below 600 nm) and even for nickel porphyrins with extended
π systems. The electronic data of all new compounds are
summarized in Table 1. Two highly extended porphyrins
described recently show absorbances in the near-infrared
(20) (a) Gill, H. S.; Harmjanz, M.; Santamaria, J.; Finger, I.; Scott, M. J.
Angew. Chem., Int. Ed. 2004, 43, 485. (b) Kurotobi, K.; Kim, K. S.;
Noh, S. B.; Kim, D.; Osuka, A. Angew. Chem., Int. Ed. 2006, 45,
3944. (c) Aihara, H.; Jaquinod, L.; Nurco, D. J.; Smith, K. M. Angew.
Chem., Int. Ed. 2001, 40, 3439. (d) Nath, M.; Huffman, J. C.; Zaleski,
J. M. J. Am. Chem. Soc. 2003, 125, 11484. (e) Fox, S.; Boyle, R. W.
Tetrahedron 2006, 62, 10039. (f) Cammidge, A. N.; Scaife, P. J.;
Berber, G.; Hughes, D. L. Org. Lett. 2005, 7, 3413. (g) Shen, D.-M.;
Liu, C.; Chen, Q.-Y. Chem. Commun. 2005, 4982. (h) Fox, S.; Boyle,
R. W. Chem. Commun. 2004, 1322. (i) Yamane, O.; Sugiura, K.-I.;
Miyasaka, H.; Nakamura, K.; Fujimoto, T.; Nakamura, K.; Kaneda,
T.; Sakata, Y.; Yamashita, M. Chem. Lett. 2004, 33, 40. (j) McCarthy,
J. R.; Hyland, M. A.; Bru¨ckner, C. Chem. Commun. 2003, 1738. (k)
Tanaka, M.; Hayashi, S.; Eu, S.; Umeyama, T.; Matano, Y.; Imahori,
H. Chem. Commun. 2007, 2069. (l) Jasinski, S.; Ermilov, E. A.; Jux,
N.; Ro¨der, B. Eur. J. Org. Chem. 2007, 1075.
(18) Preparation of enaminoketone 24 according to Bredereck, H.; Gomp-
per, R.; Morlock, G. Chem. Ber. 1957, 90, 942.
(19) Thionation of 24 was realized with Lawesson’s reagent, and the
spectroscopic data of 25 obtained in this way were identical to the
ones described previously: Duguay, G. C. R. Acad. Sci., Ser. C 1975,
281, 1077.
Inorganic Chemistry, Vol. 46, No. 24, 2007 10247

Richeter et al.
Table 1. Electronic Spectra of Nickel Porphyrins
leading to a species which was further reduced by one
electron. Protonation gave the neutral nickel dihydroxypor-
phyrin, which then underwent the expected two one-electron
reduction steps (at more cathodic potentials) observed for
nickelporphyrins.
compound
wavelength (λ > 400 nm)
optical gap (eV)
4
10
15
20
5
11
16
17
21
6
412, 480, 524, 736
480, 512, 544, 618, 710
468, 514, 566, 678, 742
410, 540, 588, 720, 772
402, 500, 540, 646, 750
438, 536, 646, 728
452, 570, 668, 766
458, 570, 766
1.68
1.74
1.67
1.60
1.65
1.70
1.61
1.61
1.55
1.50
1.70
1.63
1.56
1.51
1.59
1.63
1.55
1.50
1.77
1.82
This behavior was confirmed by spectroelectrochemical
experiments (see the Supporting Information for spectra and
experimental conditions). Coulometry at potentials R1 or R2
(see arrows in Figure 3) left the porphyrin core intact (no
significant decrease of the Soret band) whereas the reduction
at R3 led to a large intensity decrease of the Soret band
indicative of the generation of the nickel porphyrin radical
anion delocalized over the aromatic core. The dihydroxy
compound obtained after the two-electron reduction gave
back the initial diketone only if the reoxidation was carried
out at a much more positive potential (at O1, see arrow in
Figure 3). Reduction of diketone 6 could also be carried out
chemically with sodium dithionite, leading to a compound
possessing the same electronic spectra as the one resulting
from the electrochemical reduction at potential R2.
A quinone-like behavior also seemed to occur on reduction
of compound 5 (see Figure 4, cyclic voltammetry of
compound 5), but in this case the nickel porphyrin was
reduced first (see the Supporting Information; the spectro-
electrochemical study clearly showed the generation of the
porphyrin radical anion after the first reduction step). The
fourth reduction step was irreversible due to the protonation
steps involved, as seen before for the first two reduction
waves of compound 6.
Compounds 5 and 8 presented an unusual behavior during
oxidation, i.e., the splitting of the first oxidation step into
two waves with an intensity approximately corresponding
to that of 0.5 electron (see Figures 4 and 5). However, the
relative intensities of the two waves varied strongly with the
scan rate used, indicative of an association due to π stacking
of two entities (porphyrin-porphyrin or porphyrin-porphy-
rin radical cation). We had never before observed significant
π stacking between neutral molecules for this type of
compounds (NMR or UV-vis measurements), but the
concentrations used and the high ionic strength required to
run the electrochemical experiments prompted us to check
by NMR the presence or absence of π stacking between
neutral molecules under conditions close to those of the
electrochemical conditions.
NMR experiments were carried out on compound 5 by
varying the nickel porphyrin concentration (from 6.10-5 to
3.10-3 mol/L, which was close to the solubility limit) or by
varying the amount of supporting electrolyte present for a
given concentration of nickelporphyrin (NBu4PF6 from 0 to
8.10-2 mol/L with 10-3 mol/L of nickel porphyrin). In all
cases, the chemical shifts measured did not change signifi-
cantly (e0.02 ppm chemical shift differences were observed),
demonstrating clearly the absence of association due to π
stacking between two neutral molecules. Therefore, the
association probably takes place between the radical cation
generated at the anode and a neutral molecule present in close
proximity. Under our conditions, this kind of association
seems to be almost quantitative in the medium used since
474, 600, 796
480, 516, 738, 826
420, 490, 550, 726
416, 502, 672, 703, 758
442, 510, 700, 740, 794
404, 436, 490, 516, 726, 772, 821
472, 698, 776
7
12
18
22
8
13
19
23
9
488, 667, 757
436, 466, 504, 700, 738, 796
468, 508, 728, 765, 827
482, 522, 698
14
400, 492, 644, 682
be observed, but close to the electrolyte oxidation or
reduction potential). In most cases, the initial oxidations and
reductions are localized on the aromatic π system and do
not involve the nickel(II) ion. We have studied by cyclic
voltammetry the electrochemical behavior of the porphyrins
described in this Article and of the six isomeric diketones
4-9 described previously.13 All data are summarized in
Table 2.
In the case of the first nickelporphyrins studied, i.e.,
diketones 4-9, instead of the expected usual reversible two
oxidation and reduction steps, we observed several additional
steps indicative of either the participation of the reducible
keto functionalities to the reduction steps (especially for
diketone 6) or an aggregation due to the flattening of the
conjugated system of some diketones (for compounds 5 and
8). A good agreement (except for compound 9 whose low
solubility led to electrode inhibition) was found between the
optical gap values and the electrochemical gap between the
first oxidation and the first reduction step.
Diketone 6 showed quite unusual optical properties for a
monomeric nickelporphyrin (λmax ) 826 nm), and the close
proximity of the two oxygen atoms (dO-O ) 2.86 Å) led to
an easy protonation of the diketone (quantitative protonation
was observed with p-toluenesulfonic acid), reminiscent of
the one found for 1,8-bis(dialkylamino)naphthalene (proton
sponge).13 This conjugated 1,4-diketone might also be seen
as a quinone, and the electrochemical results validated this
comparison. The reduction of compound 6 occurred in four
one-electron steps (see the cyclic voltammetry curves in
Figure 3), with the second one being irreversible even at
high scan rates (up to 5 V/s).
The first two reduction steps were similar to the reduction
of a quinone, as detailed in Scheme 3. After the uptake of
the first electron, protonation of the radical anion occurred,
(21) Kadish, K. M.; Van Caemelbecke, E.; Royal, G. In The Porphyrin
Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds; Academic
Press: Boston, MA, 2000; Vol. 8, p 3. Kadish, K. M.; Lin, M.; Van
Caemelbecke, E.; De Stefano, G.; Medforth, C. J.; Nurco, D. J.;
Nelson, N. Y.; Krattinger, B.; Muzzi, C. M.; Jaquinod, L.; Xu, Y.;
Shyr, D. C.; Smith, K. M.; Shelnutt, J. A. Inorg. Chem. 2002, 41,
6673.
10248 Inorganic Chemistry, Vol. 46, No. 24, 2007

Electrochemical Studies of Nickel Porphyrins
Table 2. Cyclic Voltammetry Dataa
reduction E′° in V vs Fc/Fc+
(number of electrons exchanged)
oxidation E′° in V vs Fc/Fc+
(number of electrons exchanged)
compound
4
5
6
7
-2.12(1i)
-2.15(li)
-2.18(1)
-1.84(1)
-1.77(1)
-1.73(1)
-1.82(1)
-1.33(1)
-1.35(1)
-1.17(1i)
-1.32(1)
-1.16(1)
-1.29
-1.67(1)
-1.52(1)
-1.49(1)
-1.51(1)
-1.66(1)
-1.52(1)
-1.40(1)
-1.38(1)
-1.35(1)
-1.32(1)
-1.40(1)
-1.33(1)
-1.23(1)
-1.24(1)
-0.99(1)
-1.00(1)
-0.78(1)
-1.01(1)
-0.88(1)
-0.95
-1.37(1)
-1.21(1)
-1.15(1)
-1.16(1)
-1.31(1)
-1.25(1)
-1.07(1)
-1.15(1)
-1.05(1)
-1.00(1)
-1.17(1)
-1.05(1)
-0.96(1)
-0.95(1)
0.71(1)
1.12(1i)
1.05(1)
1.12(1i)
1.08(1i)
1.04(1)
0.64(0.5)
0.70(1)
0.72(1)
0.64(0.5)
0.94
0.73(0.5)
0.75(0.5)
8
9b
10
11
12
13
14
15
16
17
18
19
20
21
22
23
-2.11
-2.23(1)
-2.19(1)
-2.01(1)
-2.00(1)
-2.37(i)
-2.38(i)
-2.41(1)
-1.92(1i)
-1.87(1i)
-1.91(1)
-2.19(1)
-2.23(1)
-1.74(1)
-1.75(1)
-2.00(1)
-2.04(1)
0.40(1)
0.39(1)
0.40(1)
0.38(1)
0.41(0.5)
0.38(i)
0.37(1i)
0.39(1i)
0.41(i)
0.39(i)
0.37(i)
0.39(i)
0.42(i)
0.36(i)
0.83(1)
0.69(1)
0.81(1)
0.69(1)
0.52(0.5)
0.68(1)
0.72(1i)
0.68(1)
0.65(1)
0.69(1)
0.62(i)
0.66(i)
0.65(i)
0.65(i)
1.26(i)
0.83(1)
-2.20(1i)
-2.25(i)
-2.19(1)
0.87(1i)
0.80(1)
0.76(1)
0.84(1)
0.97(i)
0.81(1)
0.89(2)
1.00(i)
-2.13(2i)
-2.11(i)
-2.15(i)
a Experimental conditions: see Experimental Section. The number of exchanged electrons is indicated in parenthesis following the peak potential when
it could be determined accurately and confirmed by RDV experiments. All reduction or oxidation steps are electrochemically reversible (∆Ep ≈ 60-80 mV)
except when indicated by the letter “i” next to the number of exchanged electrons. b Approximative values due to the low solubility of compound 9.
Scheme 3. Proposed Detailed Electron and Proton Uptake for Compound 6
the two electrochemical waves were of equal intensity (0.5
+ 0.5 electron at the scan rate of 100 mV/s). This
phenomenon is not very common but is known for com-
pounds where large flat aromatic systems are present. For
example, for the ruthenium complexes of eilatin or isoeilatin
(extended bipyridines), an analogous splitting (two waves
accounting for 0.5 electron) was observed for the first
reduction wave (RuII/RuI).22 Splitting of the first oxidation
wave was also described for several metal complexes (M )
Mn, Co, Ni, Cu) of octaethylcorrole.23 The authors studied
extensively the mechanism of the oxidations and came to
the conclusion that, in these examples, the splitting was due
to the association of two neutral molecules prior to oxidation.
The structure of the corrole used (octaethyl-substituted
corrole) favored this type of association, since the molecules
might be roughly described as flat. In our case, the nickel
porphyrins are substituted in at least two meso positions with
3,5-di-tert-butyl phenyl groups, thus making such associa-
tions between two neutral molecules much less favorable.
However, association between one neutral and one charged
nickel porphyrin should be much stronger and is known even
for encumbered meso-substituted tetraarylporphyrins. X-ray
structures have been reported for mixed-valence species (one
neutral and one radical cation molecule) or between two
radical cations, leading to dimeric entities or even to
polymeric materials.24 Therefore, a strong interaction between
a radical cation generated at the anode and the starting
material is the most plausible explanation for the peculiar
behavior observed for some compounds during the initial
oxidation step.
(22) Gut, D.; Goldberg, I.; Kol, M. Inorg. Chem. 2003, 42, 3483. Bergman,
S. D.; Reshef, D.; Frish, L.; Cohen, Y.; Goldberg, I.; Kol, M. Inorg.
Chem. 2004, 43, 3792.
(23) Kadish, K. M.; Adamian, V. A.; Van Caemelbecke, E.; Gueletii, E.;
Will, S.; Erben, C.; Vogel, E. J. Am. Chem. Soc. 1998, 120, 11986.
In the case of the bisenaminoketones 10-14, all potentials
are considerably shifted toward negative potentials when
Inorganic Chemistry, Vol. 46, No. 24, 2007 10249

Richeter et al.
Figure 3. Cyclic voltammetry of compound 6 (see Experimental Section
for conditions).
Figure 4. Cyclic voltammetry of compound 5 (see Experimental Section
for conditions).
Figure 6. Comparison of compounds 10, 15, and 20 (cyclic voltammetry
curves A, B, and C respectively. See Experimental Section for conditions).
For the thionated compounds 15 and 20, different scanning limits shows
the irreversibility of the first oxidation waves.
compared with the first reduction potentials of diketone 4),
except for compounds 7 and 12. These electrochemical
data correlate with the shifts of their respective electronic
spectra.
Figure 5. Cyclic voltammetry of compound 8 (see Experimental Section
for conditions).
compared with those of the analogous diketones 4-9 (for
example, a cathodic shift of 380 mV was observed for
the first reduction potential of enaminoketone 10 when
The opposite is true after replacement of the oxygen atom
by sulfur. In all series of compounds, the general trend was
the same as the one observed for the series shown in Figure
6 (compounds 10, 15, and 20). The values of the first
oxidation potentials did not vary much within the same series.
By contrast, the first reduction potential is anodically shifted
by approximatively 100 mV for each replacement of an
oxygen atom by sulfur (see in the Supporting Information a
schematic representation of the potential evolution for three
series of compounds).
The splitting of the first oxidation waves was no longer
observed for the bisenaminoketones 10-13 or their thionated
analogues (except for the sparingly soluble compound 14,
but the low solubility did not allow us to study this compound
correctly). As soon as one thioenaminoketone moiety was
present in the structure, the first oxidation potential was
always irreversible, as observed before for the monofunc-
tionalized nickel porphyrins.12
(24) Scheidt, W. R. In The Porphyrin Handbook; Kadish, K. M., Smith,
K. M., Guilard, R., Eds; Academic Press: Boston, MA, 2000; Vol. 3,
p 49. Examples of mixed-valence X-ray structures in: Song, H.; Reed,
C. A.; Scheidt, W. R. J. Am. Chem. Soc. 1989, 111, 6867. Scheidt,
W. R.; Cheng, B.; Haller, K. J.; Mislankar, A.; Rae, A. D.; Reddy, K.
V.; Song, H.; Orosz, R. D.; Reed, C.; Cukiernik, F.; Marchon, J. C.
J. Am. Chem. Soc. 1993, 115, 1181. Scheidt, W. R.; Brancato-
Buentello, K. E.; Song, H.; Reddy, K. V.; Cheng, B. Inorg. Chem.
1996, 35, 7500. Brancato-Buentello, K. E.; Kang, S.-J.; Scheidt, W.
R. J. Am. Chem. Soc. 1997, 119, 2839. Polymeric columnar stacks of
partly oxidized metalloporphyrins; see, for example: Liou, K.;
Newcomb, T. P.; Heagy, M. D.; Thompson, J. A.; Heuer, W. B.;
Musseman, R. L.; Jacobsen, C. S.; Hoffman, B. M.; Ibers, J. A. Inorg.
Chem. 1992, 31, 4517. For radical cations, dimers are known even
for the sterically encumbered meso-substituted metalloporphyrins. See,
for example: Spaulding, L. D.; Eller, P. G.; Bertrand, J. A.; Felton,
R. H. J. Am. Chem. Soc. 1974, 96, 982. Gans, P.; Buisson, G.; Due´e,
E.; Marchon, J. C.; Erler, B. S.; Scholz, W. F.; Reed, C. A. J. Am.
Chem. Soc. 1986, 108, 1223. Song, H.; Rath, N. P.; Reed, C. A.;
Scheidt, W. R. Inorg. Chem. 1989, 28, 1839.
10250 Inorganic Chemistry, Vol. 46, No. 24, 2007

Electrochemical Studies of Nickel Porphyrins
Conclusion
These nickelporphyrins bearing two external enamino-
(thio)ketone functionalities are well-suited for the construc-
tion of oligoporphyrins linked by metal ions. Furthermore,
the presence of a hard external ligand (NO chelate) and a
soft ligand (NS chelate) in the same molecules should allow
selective complexation of soft or hard metal ions on each
site of the porphyrins. This should permit much more
efficient and elegant synthetic pathways toward oligomers
than the one used previously with the bis NO chelates.
For all compounds studied (4-23), the electrochemical
data were in good agreement with the electronic spectra
recorded in dichloromethane. Some compounds showed, in
addition to the classical two oxidation and two reduction
waves generally measured for nickel porphyrins, quinone-
like (two electrons + two protons) reduction steps, which
were observed before or after the porphyrin core reduction
depending on the structure of the extended nickel porphyrins.
In addition, some compounds showed a splitting of the first
oxidation waves that might be due to the stacking of the
generated radical cation with the starting neutral molecule.
For all different structures, the electronic influence might
be summarized by the following: (a) Bisamination of the
diketones led to a hypsochromic shift in the electronic
spectra, an increase of the HOMO-LUMO gap, and a
cathodic shift of all electrochemical potentials (with the
exception of compounds 7 and 12). (b) Thionation of the
enaminoketones led to a bathochromic shift, a decrease of
the HOMO-LUMO gap, and an anodic shift of all reduction
potentials.
Acknowledgment. We thank A. Decian and N. Kyrit-
sakas (Service Commun de Rayons X, Universite´ Louis
Pasteur) for solving the structures, R. Graff and L. Allouche
for the NMR spectra, and E. Mastio, P. Wehrung, and C.
Dietrich for the mass spectra.
Supporting Information Available: CV voltammetry and
spectroelectrochemical studies of compounds 5 and 6; evolution
of the reduction and oxidation potential for three series of
compounds in PDF format. This material is available free of charge
via the Internet at http://pubs.acs.org.
IC701484R
Inorganic Chemistry, Vol. 46, No. 24, 2007 10251