Long-range electronic connection in picket-fence like ferrocene-porphyrin derivatives

Article abstract of DOI:10.1039/c2dt31667f

The effects of a direct connection between ferrocene and porphyrin units have been thoroughly investigated by electrochemical and spectroscopic methods. These data not only reveal that substitution of the porphyrin macrocycle by one, two, three or four ferrocenyl groups strongly affects the electronic properties of the porphyrin and ferrocenyl moieties, they also clearly demonstrate that the metallocene centres are connected through the porphyrin-based electronic network. The dynamic properties of selected ferrocene-porphyrin conjugates have been investigated by VT NMR and metadynamic calculations. 1,3-Dithiolanyl protecting groups have been introduced on the upper rings of the ferrocene fragments to allow a straightforward and easy access to redox active picket-fence porphyrins. X-ray diffraction analyses of the zinc(ii) 5-[1′-[2-(1,3-dithiolanyl)]ferrocenyl]-10,15,20-tri(p-tolyl)porphyrin and 5,15-bis[1′-[2-(1,3-dithiolanyl)]ferrocenyl]-10,20-bis(p-tolyl)porphyrin complexes reveal the existence of S-Zn bonds involved in supramolecular arrays. The solid state analysis of the trans-5,15-di-(1′-(formyl)ferrocenyl)-10, 20-di-(p-tolyl)-porphyrinatozinc(ii) complex, obtained by deprotection of the dithiolane substituted analog, is conversely found in the crystal lattice as a monomer exhibiting a hexacoordinated zinc metal centre. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c2dt31667f

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Long-range electronic connection in picket-fence like
ferrocene–porphyrin derivatives†
Cite this: Dalton Trans., 2013, 42, 1196
Charles H. Devillers,a Anne Milet,b Jean-Claude Moutet,b Jacques Pécaut,c Guy Royal
,b Eric Saint-Amanb and Christophe Bucher*b,d
The effects of a direct connection between ferrocene and porphyrin units have been thoroughly investi-
gated by electrochemical and spectroscopic methods. These data not only reveal that substitution of the
porphyrin macrocycle by one, two, three or four ferrocenyl groups strongly affects the electronic proper-
ties of the porphyrin and ferrocenyl moieties, they also clearly demonstrate that the metallocene centres
are “connected” through the porphyrin-based electronic network. The dynamic properties of selected
ferrocene–porphyrin conjugates have been investigated by VT NMR and metadynamic calculations. 1,3-
Dithiolanyl protecting groups have been introduced on the upper rings of the ferrocene fragments to
allow a straightforward and easy access to redox active picket-fence porphyrins. X-ray diffraction analyses
of the zinc(II) 5-[1’-[2-(1,3-dithiolanyl)]ferrocenyl]-10,15,20-tri(p-tolyl)porphyrin and 5,15-bis[1’-[2-(1,3-
dithiolanyl)]ferrocenyl]-10,20-bis(p-tolyl)porphyrin complexes reveal the existence of S–Zn bonds involved
in supramolecular arrays. The solid state analysis of the trans-5,15-di-(1’-(formyl)ferrocenyl)-10,20-di-
(p-tolyl)-porphyrinatozinc(II) complex, obtained by deprotection of the dithiolane substituted analog, is
conversely found in the crystal lattice as a monomer exhibiting a hexacoordinated zinc metal centre.
Received 25th July 2012,
Accepted 26th September 2012
DOI: 10.1039/c2dt31667f
www.rsc.org/dalton
considered as an elemental data storage.4,6 Recently, much
efforts have been devoted to conjugated systems featuring
Introduction
Ferrocenes and porphyrins have already been associated in a several metallocenes directly connected to, or fused with, a
wide range of molecular architectures to reach quite different π-conjugated porphyrin, notably to enable an optimized “com-
objectives. Our group has recently published a comprehensive munication” between metallocenes or between the metallo-
review on this topic.1 Their donor–acceptor properties have for cene and the macrocycle.7–30 As a general statement, the
instance been exploited to investigate photoinduced electron intramolecular “communication” between multiple redox
transfer processes and to mimic photosynthesis active sites. centres within molecules might occur through bonds in conju-
Such molecular architectures containing multiple redox active gated structures, or through space as a result of the electrostatic
centres are also of fundamental importance for the deve- repulsion between electrogenerated charges. The magnitude of
lopment of molecular devices for use in analysis or in these phenomena mainly depends on a combination of struc-
electronics.2–5 Their ability to reversibly accept and/or release tural factors (distances, geometry) as well as on the dielectric
electrons at distinct potentials is particularly promising in the constant of the medium used for investigation.31–34 In mixed-
context of molecular electronics as each redox state can be valence chemistry, the interaction is usually characterized by
the Vab parameter34 related to the coupling between metal-
centred orbitals and estimated from the characteristics of the
aInstitut de Chimie Moléculaire de l’Université de Bourgogne, UMR CNRS 6302,
Université de Bourgogne, BP 47870, 21078 Dijon Cedex, France
bDépartement de Chimie Moléculaire, UMR CNRS 5250, Université Joseph Fourier,
BP 53, 38041 Grenoble Cedex 9, France. E-mail: christophe.bucher@ujf-grenoble.fr;
Fax: +(33)476514267; Tel: +(33)476514682
cCEA/DRMFC/SCIB, Laboratoire de coordination et nanochimie, 17 rue des Martyrs,
38054 Grenoble Cedex 9, France
dLaboratoire de Chimie, CNRS-UMR 5182, École Normale Supérieure de Lyon,
69364 Lyon, France. E-mail: christophe.bucher@ens-lyon.fr; Fax: (0033) 472-728-860;
intervalence transition observed in the near IR spectrum. The
interaction between two chemically equivalent redox centres
exhibiting discrete Nernstian electron transfers can also be
revealed by simple electrochemical measurements, for
instance through the observation of two successive CV waves
with distinct half-wave potential values (E1/2). As a matter of
fact, the Vab and ΔE1/2 values depend on the same parameters
and usually exhibit parallel variations,34–36 although electro-
chemical measurements simultaneously involve homovalent
and mixed-valence species produced transitorily at the
electrode interface.
Tel: (0033)472-728-000
†Electronic supplementary information (ESI) available: Experimental details and
characterization data. CCDC 893172–893175. For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/c2dt31667f
1196 | Dalton Trans., 2013, 42, 1196–1209
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We now wish to report the synthesis and characterization of respectively. Electrochemical experiments were conducted in a
such derivatives showing up to four ferrocene subunits intro- conventional three-electrode cell under an argon atmosphere
duced at the meso positions of an aromatic porphyrin skeleton at 20 °C using a CHI 660B electrochemical workstation. The
(Scheme 1). Dithiolanyl protecting groups have been intro- working electrode was a vitreous carbon disc (3 mm in diam-
duced on the upper cyclopentadiene rings to enable further eter) polished with 1 μm diamond paste before each record.
functionalizations of the metallocene-based picket fences sur- The non-aqueous Ag/Ag+ reference electrode was purchased
rounding the porphyrin core. This article also reports on the from CH instrument, Inc. (10 mM AgNO3 in CH3CN contain-
determination of the wave splitting (ΔE1/2) observed in the ing 0.1 M tetra-n-butylammonium perchlorate (TBAP)). Under
electrochemical signature of poly-ferrocenyl-porphyrin conju- these experimental conditions, the potential of the deca-
gates. We also report numerous experimental evidence sup- methylferrocene/decamethylferrocenium (DMFc/DMFc+) redox
porting the existence of efficient electronic “communications” couple, used as an internal reference in dichloromethane and
occurring between multiple chemically-equivalent redox dimethylformamide, was observed at E1/2 = −345 mV and
centres.
−410 mV, respectively.‡ Rotating disc electrode (RDE) voltam-
metry was carried out at a rotation rate of 600 rpm. Cyclic vol-
tammetry (CV) curves were recorded at a scan rate of 0.1 V s−1
.
Electrolyses were performed at controlled potential using a Pt
plate (2 cm2). Electrochemical simulations and best fitting of
experimental data were performed by the Digisim® software
(vs. 3). High resolution mass spectra (HRMS) were recorded on
a MicrOTOF Q Bruker instrument in ESI (positive mode) or on
a Bruker Daltonics Ultraflex II spectrometer in the MALDI/TOF
Experimental
Reagents and instrumentation
Dichloromethane and dimethylformamide (Rathburn, HPLC
grade) have been distilled over calcium hydride under argon
and under reduced pressure over 3 Å molecular sieves,
Scheme 1 Syntheses of the meso-(dithiolanyl)ferrocenyltolylporphyrins 2H2, 3H2, 4H2, 5H2 and 6H2; (i) TFA (1.5 equiv.), CH2Cl2, Ar, 298 K, 30 min; (ii) Et3N (1.5
equiv.), p-chloranil/THF, 3 h.41
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reflectron mode with dithranol as a matrix and polyethylene added to the eluent to give 2H2 (340 mg, 10%), 4H2 (541 mg,
glycol ion series as an internal calibrant, at the Plateforme 13%), 3H2 (240 mg, 6%) and 5H2 (45 mg, ≈1%) isolated as
d’Analyse Chimique et de Synthèse Moléculaire de l’Université dark green solids.
de Bourgogne (PACSMUB). NMR spectra were recorded on a
2H2: 1H NMR (250 MHz, CDCl3, 298 K) δ (ppm): −2.31
Bruker AC-2000 250 MHz. 1H chemical shifts (ppm) were refer- (s, 2H, –NH); 2.69 (s, 3H, –Me); 2.71 (s, 6H, –Me); 3.05–3.40
enced to residual solvent peaks. UV-vis spectra were recorded (m, 4H, –S(CH2)2S–); 4.09 (m, 2H, –Fc); 4.36 (m, 2H, –Fc); 4.88
on a Varian Cary 100 spectrophotometer using quartz cells.
(m, 2H, –Fc); 5.51 (s, 1H, –HC(S–)2); 5.57 (m, 2H, –Fc); 7.55
(m, 6H, –Tol); 8.09 (m, 6H, –Tol); 8.80 (m, 6H, β-pyrr); 9.94
(d, 3J = 5.00 Hz, 2H, β-pyrr). UV-vis. (CH2Cl2) λmax, nm
(ε, L mol−1 cm−1): 422 (331 000); 510 (10 800); 588 (9200); 671
(8300). HRMS (ESI/TOF) m/z calcd for C54H45N4FeS2: 869.2431;
found: 869.2450 [M + H]+.
Synthesis
1-[2-(1,3-Dithiolanyl)]-1′-formylferrocene (1). 1,2-Dithioethane
(5.40 mL, 64.2 mmol) was added to a cold (0 °C) CH2Cl2 sol-
ution (450 mL) of 1,1′-diformylferrocene37 (15.9 g, 64.2 mmol).
Trifluoroboride etherate (15.92 mL, 129.4 mmol) dissolved in
160 mL of CH2Cl2 was then added dropwise (30 min) at 0 °C.
After stirring the resulting solution at 0 °C for 5 h, an aqueous
NaHCO3 solution (50 mL, 10%) was added. The organic layer
was then washed with 100 mL of an aqueous solution satu-
rated with sodium bicarbonate, with 2 × 100 mL of water and
finally with 100 mL of brine. The organic layer was then dried
over anhydrous sodium sulphate, filtered and the solvent was
evaporated under reduced pressure. The crude compound
was purified by column chromatography on silica gel using
n-hexane, with increasing amounts of ethyl acetate (0 to 2%),
as the eluent to afford 14.3 g (yield: 70%) of pure 1-[2-
(1,3-dithiolanyl)]-1′-formylferrocene isolated as a red solid.
3H2: 1H NMR (250 MHz, CDCl3, 298 K) δ (ppm): −1.83
(s, 2H, –NH); 2.70 (s, 6H, –Me); 3.08–3.36 (m, 8H, –S(CH2)2S–);
3.99 (m, 4H, –Fc); 4.31 (m, 4H, –Fc); 4.87 (m, 4H, –Fc); 5.48
3
(s, 2H, –HC(S–)2); 5.52 (m, 4H, –Fc); 7.54 (d, J = 7.25 Hz, 4H,
3
–Tol); 8.05 (d, J = 8.25 Hz, 4H, –Tol); 8.68 (s, 2H, β-pyrr); 8.74
3
(d, J = 4.75 Hz, 2H, β-pyrr); 9.80 (s, 2H, β-pyrr); 9.85 (d, 3J =
5.00 Hz, 2H, β-pyrr). UV-vis. (CH2Cl2) λmax, nm (ε, L mol−1
cm−1): 427 (237 000); 616 (12 500); 692 (10 400). HRMS (ESI/
TOF) m/z calcd for C60H51N4Fe2S4: 1067.1693; found:
1067.1696 [M + H]+.
4H2: 1H NMR (250 MHz, CDCl3, 298 K) δ (ppm): −1.70
(s, 2H, –NH); 2.71 (s, 6H, –Me); 3.06–3.36 (m, 8H, –S(CH2)2S–);
4.01 (m, 4H, –Fc); 4.31 (m, 4H, –Fc); 4.86 (m, 4H, –Fc); 5.48
3
(s, 2H, –HC(S–)2); 5.52 (m, 4H, –Fc); 7.55 (d, J = 8.25 Hz, 4H,
1H NMR (250 MHz, CDCl3, 298 K)
δ (ppm): 3.30
–Tol); 8.06 (d, 3J = 7.50 Hz, 4H, –Tol); 8.69 (d, 3J = 4.75 Hz, 4H,
3
(m, 4H, thioethane); 4.27 (s, 2H, –Fc); 4.40 (s, 2H, –Fc); 4.61
(s, 2H, –Fc); 4.78 (s, 2H, –Fc); 5.42 (s, 1H, –HC(S–)2); 9.66
(s, 1H, –CHO).
β-pyrr); 9.78 (d, J = 5.00 Hz, 4H, β-pyrr). UV-vis. (CH2Cl2) λmax
,
nm (ε, L mol−1 cm−1): 425 (247 000); 615 (13 700); 695 (14 200).
HRMS (ESI/TOF) m/z calcd for C60H51N4Fe2S4: 1067.1693;
found: 1067.1736 [M + H]+.
These data are consistent with those reported in ref. 38.
5-[1′-[2-(1,3-Dithiolanyl)]ferrocenyl]-10,15,20-tri(p-tolyl)-por-
phyrin (2H2); 5,10-bis[1′-[2-(1,3-dithiolanyl)]ferrocenyl]-15,20-
di(p-tolyl)porphyrin (3H2); 5,15-bis[1′-[2-(1,3-dithiolanyl)]ferro-
cenyl]-10,20-bi(p-tolyl)porphyrin (4H2); 5,10,15-tris[1′-[2-(1,3-
dithiolanyl)]ferrocenyl]-20-(p-tolyl)porphyrin (5H2). 5-Tolyldi-
pyrromethane39 (1.84 g, 7.8 mmol) and 1 (2.50 g, 7.8 mmol)
were dissolved in 400 mL of anhydrous CH2Cl2 and argon was
bubbled through the solution for about 15 min. After protect-
ing the mixture from light, trifluoroacetic acid (0.60 mL,
7.8 mmol) was added dropwise. The solution was stirred at
room temperature for an additional period of 30 min and neu-
tralized with 2,4,6-trimethylpyridine (1.04 mL, 7.8 mmol).
p-Chloranil (1.92 g, 7.8 mmol) was then added and the
mixture was kept under stirring at room temperature for 3 h.
The solvent was evaporated under reduced pressure. The
resulting crude oil was suspended in 500 mL of a NaOH (2 M)
aqueous solution and the mixture was stirred for 1 h at room
temperature. The dark precipitate was filtered off, washed with
water and dried under vacuum. The crude compound was puri-
fied by chromatography on silica gel using CH2Cl2–n-hexane
(75/25 v/v) as the eluent. Four successive fractions were col-
lected when increasing amounts of ethyl acetate (0 to 2%) were
5H2: 1H NMR (250 MHz, CDCl3, 298 K) δ (ppm): −1.14
(s, 2H, –NH); 2.69 (s, 4H, –Me); 3.06–3.36 (m, 12H, –S
(CH2)2S–); 3.89 (m, 6H, –Fc); 4.25 (m, 6H, –Fc); 4.84 (m, 6H,
–Fc); 5.42 (s, 3H, –HC(S–)2); 5.44 (m, 6H, –Fc); 7.54 (d, 3J =
8.50 Hz, 2H, –Tol); 8.01 (d, 3J = 8.75 Hz, 2H, –Tol); 8.61 (d, 3J =
5.00 Hz, 2H, β-pyrr); 9.61 (d, 3J = 3.75 Hz, 4H, β-pyrr); 9.74
(s, 4H, β-pyrr). UV-vis. (CH2Cl2) λmax, nm (ε, L mol−1 cm−1):
430 (187 000); 635 (14 700); 711 (12 900). HRMS (ESI/TOF)
m/z calcd for C66H57N4Fe3S6: 1265.0957; found: 1265.0990
[M + H]+.
5,10,15,20-Tetra[1′-[2-(1,3-dithiolanyl)]ferrocenyl]porphyrin
(6H2). 1-[2-(1,3-Dithiolanyl)-1′-formylferrocene (1) (700 mg,
2.2 mmol) and pyrrole (150 μL, 2.2 mmol) were dissolved in
200 mL of anhydrous CH2Cl2 and argon was bubbled through
the solution for about 15 min. After protecting the mixture
from light, trifluoroacetic acid (250 μL, 3.25 mmol) was added
dropwise. The solution was kept under stirring at room tempera-
ture for 2 h. p-Chloranil (810 mg, 3.3 mmol) and triethylamine
(460 μL, 3.3 mmol) were then added. After stirring the resulting
solution at room temperature for 4 h, the solvent was evaporated
under reduced pressure. The crude compound was purified by
column chromatography on silica gel using CH2Cl2 as the
eluent to give 240 mg (29%) of 6H2 isolated as a violet solid.
6H2: 1H NMR (250 MHz, CDCl3, 298 K) δ (ppm): −0.53
(s, 2H, –NH); 3.02–3.38 (m, 16H, –S(CH2)2S–); 3.80 (m, 8H,
‡Under these conditions (DCM/TBAP), we found that E1/2[Fc/Fc+] = E1/2[DMFc/
DMFc+] + 0.545 V.
1198 | Dalton Trans., 2013, 42, 1196–1209
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–Fc); 4.19 (m, 8H, –Fc); 4.81 (m, 8H, –Fc); 5.34 (m, 8H, –Fc); was removed under reduced pressure. The resulting crude oil
5.40 (s, 4H, –HC(S–)2); 9.57 (s, 8H, β-pyrr). UV-vis. (CH2Cl2) was then suspended in 500 mL of aqueous NaOH (2 M) and
λmax, nm (ε, L mol−1 cm−1): 435 (144 000); 663 (15 000); the mixture was stirred for 1 h at room temperature. The black
726 (12 800). HRMS (ESI/TOF) m/z calcd for C72H63N4Fe4S8: precipitate was filtered off, washed with water and dried. After
1463.0221; found: 1463.0224 [M + H]+.
evaporation of the solvent, the resulting solid was purified by
5-(1′-(Formyl)ferrocenyl)-10,15,20-tri(p-tolyl)porphyrin (7H2). A column chromatography on silica gel using CH2Cl2–n-hexane
solution of 2H2 (50 mg, 0.057 mmol) in THF (100 mL) was (75/25 v/v) as the eluent. The first fraction collected was the tar-
added to
a mixture of N-chlorosuccinimide (45.7 mg, geted 5-ferrocenyl-10,15,20-tri(p-tolyl)porphyrin 9H2 (90 mg,
0.34 mmol) and AgNO3 (58.1 mg, 0.34 mmol) dissolved in 5.5%).
CH3CN–H2O (35 mL, 85/15, v/v). After stirring the resulting sol-
9H2: 1H NMR (400 MHz, CDCl3, 298 K) δ (ppm): −2.29
ution at room temperature for 10 min, 1 mL of an aqueous sol- (s, 2H, –NH); 2.69 (s, 3H, –Me); 2.71 (s, 6H, –Me); 4.18 (m, 5H,
ution saturated with Na2SO3, 1 mL of an aqueous solution –Fc); 4.82 (m, 2H, –Fc); 5.55 (m, 2H, –Fc); 7.51–7.60 (m, 6H,
saturated with Na2CO3 and 1 mL of an aqueous solution satu- –Tol); 8.05–8.13 (m, 6H, –Tol); 8.82–8.74 (m, 6H, β-pyrr); 9.98
3
rated with NaCl were successively added to the mixture. 50 mL (d, J = 4.80 Hz, 2H, β-pyrr). HRMS (MALDI-TOF) m/z calcd for
of CH2Cl2 were then added and the mixture was filtered. The C51H41FeN4: 765.2682; found: 765.2714 [M + H]+.
filtrate was washed with CH2Cl2. The organic phases were col-
5,10,15,20-Tetra(ferrocenyl)porphyrin (10H2). Argon was
lected, washed with water and dried over anhydrous Na2SO4. bubbled for 15 min through a solution of ferrocenecarboxalde-
The black crude product obtained upon evaporation of the hyde (470 mg, 2.2 mmol) and pyrrole (150 μL, 2.2 mmol) in
solvent under reduced pressure was purified by column 200 mL of anhydrous CH2Cl2. After protecting the mixture
chromatography on silica gel using CH2Cl2 as the eluent to from light, trifluoroacetic acid (250 μL, 3.3 mmol) was slowly
1
afford 32 mg (70%) of 7H2 isolated as a violet solid. H NMR added. The solution was kept under stirring at room tempera-
and MS analyses of 7H2 are consistent with those reported in ture for 2 h and then neutralized with triethylamine (456 μL,
ref. 8.
3.3 mmol). p-Chloranil (440 mg, 3.3 mmol) was then added
5,15-Di(1′-(formyl)ferrocenyl)-10,20-di(p-tolyl)porphyrin and after stirring the resulting solution at room temperature
(8H2). A solution of 4H2 (100 mg, 0.094 mmol) in THF for 3 h, the solvent was removed under reduced pressure. The
(100 mL) was added to a mixture of N-chlorosuccinimide crude compound was purified by column chromatography on
(150 mg, 1.1 mmol) and AgNO3 (191 mg, 1.1 mmol) dissolved silica gel using CH2Cl2 as the eluent to give 136 mg (24%) of
in CH3CN–H2O (70 mL, 85/15, v/v). After stirring the resulting 10H2. Spectroscopic data found for 10H2 are consistent with
solution at room temperature for 10 min, 2 mL of an aqueous those reported in the literature.40
solution saturated with NaSO3, 2 mL of an aqueous solution
saturated with Na2CO3 and 2 mL of an aqueous solution satu-
Metallation of the free bases
rated with NaCl were successively added to the mixture. In a typical experiment, 1 mL of a saturated solution of Zn-
100 mL of CH2Cl2 were then added and the mixture was fil- (OAc)2 in CH3OH was added to a stirred CH2Cl2 solution of the
tered. The filtrate was washed with CH2Cl2. The organic free base (50 μmol/20 mL). The mixture was kept under stirring
phases were collected, washed with water and dried over anhy- for 10 h at room temperature and then washed with water. The
drous Na2SO4. The black crude product obtained upon evapor- organic layer was dried over anhydrous Na2SO4, filtered and
ation of the solvent under reduced pressure was purified by then removed under reduced pressure. The crude product was
column chromatography on silica gel using CH2Cl2 as the purified by column chromatography on silica gel using CH2Cl2
eluent to yield 43 mg (50%) of 8H2 isolated as a violet solid.
as the eluent to afford the targeted Zn(II) complexes in high
8H2: 1H NMR (250 MHz, CDCl3, 298 K) δ (ppm): −1.76 yields (>90%).
(s, 2H, –NH); 2.72 (s, 6H, –Me); 4.38 (m, 4H, –Fc); 4.80 (m, 4H,
2Zn: 1H NMR (500 MHz, CDCl3, 298 K) δ (ppm): 2.21
3
–Fc); 4.92 (m, 4H, –Fc); 5.57 (m, 4H, –Fc); 7.57 (d, J = 7.50 Hz, (m, 2H, –S(CH2)2S–); 2.65 (s, 3H, –Me); 2.67 (m, 8H, –Me and
4H, –Tol); 8.04 (d, 3J = 7.00 Hz, 4H, –Tol); 8.73 (d, 3J = 4.75 Hz, –S(CH2)2S–); 3.97 (m, 2H, –Fc); 3.99 (m, 2H, –Fc); 4.45 (s, 1H,
4H, β-pyrr); 9.69 (d, 3J = 4.50 Hz, 4H, β-pyrr); 9.89 (s, 2H, –HC(S–)2); 4.58 (m, 2H, –Fc); 5.37 (m, 2H, –Fc); 7.52 (m, 6H,
–CHO). Mass spectroscopy (FAB+–MS), m/z: [M + 1]+ = 915. –Tol); 8.03 (m, 6H, –Tol); 8.83 (m, 6H, β-pyrr); 10.03 (d, 3J =
HRMS (ESI/TOF) m/z calcd for C56H42O2N4Fe2Na: 937.1902; 3.50 Hz, 2H, β-pyrr). UV-vis. (CH2Cl2) λmax, nm (ε, L mol−1
found: 937.1921 [M + Na]+.
cm−1): 425 (369 000); 568 (12 600); 619 (13 800). HRMS
5-Ferrocenyl-10,15,20-tri(p-tolyl)porphyrin (9H2). Argon was (ESI/TOF) m/z calcd for C54H42N4FeS2Zn: 930.1488; found:
bubbled for 15 min through a solution of 5-tolyldipyrro- 930.1508 [M]+.
1
methane (1 g, 4.23 mmol) and 1-carboxaldehyde–ferrocene
3Zn: H NMR (250 MHz, CDCl3, 298 K) δ (ppm): 2.40–2.60
(905 mg, 4.23 mmol) in anhydrous CH2Cl2 (200 mL). After pro- (m, 4H, –S(CH2)2S–); 2.70 (s, 6H, –Me); 2.80–3.00 (m, 4H, –S
tecting the mixture from light, trifluoroacetic acid (0.47 mL, (CH2)2S–); 4.00 (m, 4H, –Fc); 4.11 (m, 4H, –Fc); 4.68 (m, 4H,
6.1 mmol) was slowly added. The solution was kept under stir- –Fc); 4.79 (s, 2H, –HC(S–)2); 5.44 (m, 4H, –Fc); 7.55 (d, 3J =
3
ring at room temperature for 30 min and neutralized with tri- 8.00 Hz, 4H, –Tol); 8.07 (d, J = 8.00 Hz, 4H, –Tol); 8.80 (s, 2H,
ethylamine (0.88 mL, 6.3 mmol). p-Chloranil (1.549 g, β-pyrr); 8.85 (d, 3J = 4.50 Hz, 2H, β-pyrr); 9.96 (s, 2H, β-pyrr);
6.3 mmol) was then added. After stirring for 3 h, the solvent 10.03 (d, 3J = 4.75 Hz, 2H, β-pyrr). UV-vis. (CH2Cl2) λmax, nm
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(ε, L mol−1 cm−1): 429 (248 000); 586 (10 200); 640 (19 500). were conducted with SMART (v. 5.054) and SAINT (v. 6.36A),
HRMS (ESI/TOF) m/z calcd for C60H48N4Fe2S4Zn: 1128.0751; respectively, from Bruker Analytical X-ray Instruments.
found: 1128.0775 [M]+.
A summary of the crystallographic data and structure refine-
4Zn: 1H NMR (250 MHz, CDCl3, 298 K) δ (ppm): 2.71 (s, 6H, ment is given in the ESI.† All non-hydrogen atoms were refined
–Me); 2.90–3.08 (m, 4H, –S(CH2)2S–); 3.09–3.24 (m, 4H, –S with anisotropic thermal parameters except for disordered
(CH2)2S–); 4.12 (m, 4H, –Fc); 4.31 (m, 4H, –Fc); 4.82 (m, 4H, solvent molecules of THF in 4. Hydrogen atoms were generated
–Fc); 5.31 (s, 2H, –HC(S–)2); 5.50 (m, 4H, –Fc); 7.56 (d, 3J = in idealized positions for compounds 3 and 4; riding on the
7.50 Hz, 4H, –Tol); 8.07 (d, 3J = 6.75 Hz, 4H, –Tol); 8.81 (d, 3J = carrier atoms and were found and refined in complex 2, with
3
5.00 Hz, 4H, β-pyrr); 10.01 (d, J = 4.00 Hz, 4H, β-pyrr). UV-vis. isotropic thermal parameters for all. CCDC 893172–893175.
(CH2Cl2) λmax, nm (ε, L mol−1 cm−1): 428 (261 000); 583 (9600);
648 (21 000). HRMS (ESI/TOF) m/z calcd for C60H48N4Fe2S4Zn:
1128.0751; found: 1128.0783 [M]+.
1
5Zn: H NMR (250 MHz, CDCl3, 298 K) δ (ppm): 2.24–2.50
(m, 6H, –S(CH2)2S–); 2.62–2.90 (m, 9H, –Me and –S(CH2)2S–);
Results and discussion
3.76–4.08 (m, 12H, –Fc); 4.44–4.72 (m, 9H, –HC(S–)2 and Fc);
3
3
5.30 (m, 6H, –Fc); 7.54 (d, J = 7.25 Hz, 2H, –Tol); 8.06 (d, J =
7.50 Hz, 2H, –Tol); 8.75 (d, 3J = 5.25 Hz, 2H, β-pyrr); 9.79 (d, 3J =
Synthesis of 2H2–6H2 and their zinc complexes 2Zn–6Zn. In
5.25 Hz, 2H, β-pyrr); 9.89 (m, 4H, β-pyrr). UV-vis. (CH2Cl2) λmax
,
previous articles,7–28 we, and the other research groups, have
nm (ε, L mol−1 cm−1): 432 (207 000); 605 (10 000); 661(29 000). reported the synthesis of a range of meso-ferrocenyl-porphyrins
HRMS (ESI/TOF) m/z calcd for C66H54N4Fe3S6Zn: 1326.0015; from commercially available ferrocenecarboxaldehyde and
found: 1326.0044 [M]+.
pyrrole using a standard Lindsey’s synthetic strategy.41 Purifi-
1
6Zn: H NMR (250 MHz, CDCl3, 298 K) δ (ppm): 2.66–2.82 cation of the crude products unfortunately proved quite
(m, 8H, –S(CH2)2S–); 2.92–3.10 (m, 8H, –S(CH2)2S–); 3.89 difficult and could only be achieved efficiently on small scales.
(m, 8H, –Fc); 4.11 (m, 8H, –Fc); 4.70 (m, 8H, –Fc); 5.00 (s, 4H, These significant drawbacks have hitherto considerably
–HC(S–)2); 5.33 (m, 8H, –Fc); 9.79 (s, 8H, β-pyrr). UV-vis. restricted the application scope of such molecules, like for
(CH2Cl2) λmax, nm (ε, L mol−1 cm−1): 437 (138 000); 627 (8700); instance as intermediates in the synthesis of ferrocene-based
680 (29 400). HRMS (ESI/TOF) m/z calcd for C72H61N4Fe4S8Zn: “picket-fence” or “basket handled” porphyrins. Our strategy to
1524.9357; found: 1524.9380 [M + H]+.
overcome these limitations and promote an easy and straight-
7Zn: 1H NMR (250 MHz, CDCl3, 298 K) δ (ppm): 2.70 (s, 3H, forward post-functionalization involves the use of dithiolanyl-
–Me); 2.72 (s, 6H, –Me); 4.32 (m, 2H, –Fc); 4.44 (m, 2H, –Fc); protected ferrocene-based starting materials. Synthesis of the
4.56 (m, 2H, –Fc); 5.35 (m, 2H, –Fc); 7.44–7.64 (m, 6H, –Tol); targeted ferrocene–porphyrin conjugates is summarized in
8.00–8.21 (m, 6H, –Tol); 8.63 (s, 1H, –CHO); 8.80–9.03 (m, 6H, Scheme 1. It starts with the mono-protection of diformyl ferro-
3
β-pyrr); 9.86 (d, J = 4.75 Hz, 2H, β-pyrr). HRMS (ESI/TOF) m/z cene to afford 1 in 70% yield38 (Scheme 1). The dithiolanyl
calcd for C52H38ON4FeZn: 854.1683; found: 854.1709 [M]+.
group has been selected (i) for its ability to resist to acidic con-
8Zn: 1H NMR (250 MHz, CDCl3, 298 K) δ (ppm): 2.72 (s, 6H, ditions, (ii) to enhance the solubility and the polarity of the
–Me); 4.54 (m, 4H, –Fc); 4.74 (m, 4H, –Fc); 4.79 (m, 4H, –Fc); porphyrin products and (iii) to allow an easy post-functionali-
3
3
5.57 (m, 4H, –Fc); 7.55 (d, J = 7.25 Hz, 4H, –Tol); 8.06 (d, J = zation of the ferrocenyl fragments. The acid-catalyzed Mac
7.75 Hz, 4H, –Tol); 8.79 (d, 3J = 5.25 Hz, 4H, β-pyrr); 9.01 Donald-type condensation of 1 with one equivalent of 5-tolyldi-
3
(s, 2H, –CHO); 9.86 (d, J = 4.75 Hz, 4H, β-pyrr). HRMS (MAL- pyrromethane in dichloromethane, followed by oxidation with
DI-TOF) m/z calcd for C56H40Fe2N4O2Zn: 976.1142; found: p-chloranil, led to a crude mixture from which 2H2–5H2 (path
976.1105 [M]+.
A, Scheme 1) could be isolated in 10, 6, 13 and ≈1% yield,
9Zn: 1H NMR (500 MHz, CDCl3, 298 K) δ (ppm): 2.70 (s, 3H, respectively. The tetra-substituted derivative 6H2 was obtained
–Me); 2.71 (s, 6H, –Me); 4.24 (s, 5H, –Fc); 4.82 (m, 2H, –Fc); in 29% yield using the same procedure starting from pyrrole
5.55 (m, 2H, –Fc); 7.48–7.63 (m, 6H, –Tol); 8.00–8.16 (m, 6H, (path B, Scheme 1).
–Tol); 8.81–8.99 (m, 6H, β-pyrr); 10.20 (d, 3J = 4.75 Hz, 2H,
It is worth mentioning that the yields of 2H2–5H2 fall in the
β-pyrr). HRMS (ESI/TOF) m/z calcd for C51H38N4FeZn: same range as those reported by Chandrashekar and co-
826.1734; found: 826.1756 [M]+.
workers14 (1–10%), who used a ferrocenyl-dipyrromethane as a
starting material, but that the statistical distribution of pro-
ducts is quite different; the main difference being that our pro-
Crystallography
Crystals of 2Zn, 4Zn, 8Zn and 10Zn were used for data collec- cedure allows us to isolate simultaneously the cis and trans
tion on a SMART CCD diffractometer using Mo-Kα graphite- bisferrocenyl derivatives 3H2 and 4H2 in satisfactory yields
monochromatic radiation (λ = 0.71073 Å). Intensity data were (6 and 13%, respectively). Another interesting point is that the
corrected for Lorentz, polarization effects and absorption. yield of tetraferrocenylporphyrin 6H2 (∼30%) is significantly
Structure solution and refinement were performed with lower than those reported by Loim10–13 or Nemykin22,25,40
the SHELXTL (v. 5.10; Bruker Analytical X-ray Instruments: (∼40%) using BF3-catalyzed Lyndsey-like procedures involving
Madison, WI, 1997 package). Data collection and reduction pyrrole and ferrocenecarboxaldehyde as starting materials.
1200 | Dalton Trans., 2013, 42, 1196–1209
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Metallation of these free-base porphyrins with Zn2+, to yield
2Zn–6Zn, was then achieved quantitatively using zinc acetate
in a MeOH–CH2Cl2 solvent mixture.
Crystallography
Single crystals of 2Zn were grown by slow evaporation of a
deuterated dichloromethane solution (Fig. 1). 2Zn crystallizes
in the C2/c space group of the monoclinic system, with 8 crys-
tallographically independent molecular entities self-assembled
in four distinct columnar structures. The Zn(II) atom is found
to lie ∼0.2 Å above the mean plane formed by the four nitrogen
atoms. The ferrocene unit is slightly twisted with an interpla-
nar angle between both cyclopentadienyl (Cp) rings of ∼5.6°.
The zinc ion is pentacoordinated in a square pyramidal geo-
metry with four nitrogen atoms in equatorial positions and
one sulfur atom, from the dithiolanyl of a neighbouring por-
phyrin, in an apical position ((Zn–S(1)) = 2.658(2) Å (Fig. 2).
Iteration of this intermolecular coordination mode leads to an
infinite columnar self-assembled network wherein each
monomer interacts with two neighbours through Zn–S bonds.
The interplanar angle between the covalently linked Cp and
the porphyrin plane is ∼51°. In the coordination polymer, the
interplanar angle between the porphyrin plane and the closest
Cp ring, bearing the coordinated dithiolane, is 88.6°. This
intermolecular arrangement allows the observation of rather
short HFc–πporph distances ranging from 2.2 to 2.8 Å.
Fig. 2 Ortep42 side view of the crystal packing in 2Zn. Solvent molecules,
hydrogen atoms and tolyl rings have been omitted for clarity reasons. Thermal
ellipsoids are scaled to a 50% probability level.
Single crystals of 4Zn have been obtained in THF. The
resulting solid state structure is depicted in Fig. 3. The inter-
planar angle between the mean porphyrin plane and the cova-
lently linked Cp ring is 50.65° and both ferrocenes are found
in a slightly more open conformation (Cp^Cp ∼7.79°) than in
2Zn (∼5.63°). This compound is isolated as a single isomer
with both ferrocenyl groups in a syn configuration (α,β-atropoi-
somer), both ferrocenes pointing towards opposite directions.
Fig. 3 Ortep42 view of 4Zn. Solvent molecules and hydrogen atoms have been
omitted for reasons of clarity. Thermal ellipsoids are scaled to a 50% probability
level.
It should be emphasized that prior to this work, every solid-
state structure of 5,15-diferrocenylporphyrins reported in the
literature was found in the presumably more stable α,α-confor-
mation.40 The unexpected α,β-conformation adopted by 4Zn is
thus most probably imposed by the formation of self-
assembled coordination oligomers/polymers involving the
dithiolane substituents and the zinc(II) cations. The Zn(II)
atom lies in the plane defined by the four nitrogen atoms. The
Fig. 1 Ortep42 view of 2Zn, solvent molecules and hydrogen atoms have been
omitted for clarity reasons. Thermal ellipsoids are scaled to a 50% probability
level.
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NMR spectroscopy
NMR spectroscopy allowed us to investigate the steric and elec-
tronic effects of the ferrocene fragments on the porphyrin
ring. For the free base compounds (2H2–6H2), the inner NH’s
resonate at high fields, below 0 ppm, as a result of the ring
current associated with the aromatic porphyrin (Fig. 6).12
The corresponding broad singlets are observed at −2.31,
−1.83, −1.70, −1.14 and −0.53 ppm on the NMR spectra of
2H2, 3H2, 4H2, 5H2 and 6H2, respectively. As reported by
Nemykin et al.,40 the NH resonance undergoes a downfield
shift as the number of ferrocenyl substituents increases. These
changes are clearly related to a progressive decrease in the por-
phyrin ring current which might result from a significant
sharing of electron density between the ferrocene and por-
phyrin units and/or from the distortion of the porphyrin skel-
eton following its substitution by an increasing number of
bulky ferrocenyl substituents.15,43 These steric effects are for
instance revealed at the solid state for 10Zn through the saddle
shape of the porphyrin ring (Fig. 5). As expected, metallation
of 2H2–6H2 with zinc(II) was found to enhance the rigidity of
the porphyrin skeleton leading to a larger ring current revealed
on the 1H-NMR spectra by the shifts of the β-pyrrolic signals
towards lower fields (0.03 < Δδ < 0.23 ppm). The 1H NMR
signals attributed to the hydrogen atoms in the dithiolane ring
Fig. 4 Partial Ortep42 side view of the crystal packing in 4Zn. Solvent mol-
ecules, hydrogen atoms and tolyl rings have been omitted for clarity reasons.
Thermal ellipsoids are scaled to a 50% probability level.
interatomic Fe–Fe distance reaches 13.326(4) Å. The Zn(II)
atom is hexacoordinated by four nitrogen atoms in equatorial
positions and by two sulphur atoms from the dithiolane
groups of two adjacent molecules ((Zn–S(2)) = 3.160(2) Å and S
(2)–Zn–S(2) = 180°, Fig. 4). In the resulting coordination
polymer, each monomer is linked to four neighbours through
Zn–S coordination bonds.
Single crystals of 10Zn have been obtained in THF (Fig. 5).
It crystallizes in the C2/c space group of the monoclinic
system, with eight crystallographically independent molecular
entities including the title compound, THF and water. The Zn(II)
atom is found to lie ∼0.27 Å above the mean plane of the por-
phyrin skeleton. It is pentacoordinated by four nitrogen atoms
in equatorial positions and by one oxygen atom from a THF
molecule in an axial position ((Zn–O(1)) = 2.20(2) Å). As pre-
viously reported for the free base,18,40 the Zn(II) complex is iso-
lated as a single isomer adopting an α,β,α,β conformation.§
Both ferrocene moieties located on the side of the coordi-
nated THF molecule are pointing towards opposite directions
(backward and frontward on the drawing depicted in Fig. 5), as
opposed to the other two ones pointing in the same direction.
The ferrocenes are tilted to about 41–52° relative to the por-
phyrin plane and the Fe–Fe distance ranges from 8.572(3) to
11.732(3) Å measured between two adjacent (α,β; Fe(2)–Fe(3))
or non-adjacent (α,α; Fe(1)–Fe(3)) ferrocenes, respectively.
Fig. 6 1H NMR spectra of 2H2–6H2 (250 MHz, CDCl3, 295 K). Attribution of
each signal is detailed in Scheme 1. The numbers represent the relative inte-
gration values calculated for each signal.
Fig. 5 Side Ortep42 view of 10Zn. Thermal ellipsoids are scaled to a 50% prob-
ability level. The carbon atoms of the THF molecule (O(1)) have been omitted
for clarity reasons.
§It should be mentioned that previous work carried out on a trans-dichlorotin
(IV)–tetraferrocenylporphyrin suggests that the configuration observed in the
solid state might be imposed by solvent effects (see ref. 60).
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are conversely observed at higher fields in the metallated
species than in the free bases. For instance, the resonance of
the thioacetal proton Hi (see Scheme 1 for attribution) res-
onates at 5.51 and 4.45 in the spectrum of 2H2 and 2Zn,
respectively. This metal-induced shift is attributed to the for-
mation of self-assembled coordination oligomers/polymers in
solution, wherein the dithiolane protons dive into the shield-
ing cone of the macrocycle, as observed in the X-ray structures
of 2Zn and 4Zn.
The long-range “communication” observed between metal-
locene fragments covalently linked through a porphyrin ring
has often been attributed to steric effects prohibiting atropoi-
somerization.16,18 These dynamic issues have been addressed
in the present study through detailed VT-NMR investigations
conducted with the monoferrocenyl-porphyrin derivatives 2Zn
and 9Zn. For both species, decreasing the temperature from
300 K to 185 K led to a splitting of the initial doublet attribu-
ted to the β-pyrrolic protons Ha and Ha′ (Fig. 7). At 185 K,
rotation does not occur and the ferrocene fragment adopts the
tilted conformation observed at the solid state. As a result, Ha
and Ha′ become chemically and magnetically unequivalent
with a chemical shift between both signals reaching almost
2 ppm. The activation energies corresponding to the rotational
motion of ferrocene in 2Zn† and 9Zn were estimated from
coalescence temperatures (Tc)44,45 at 10.4 kcal mol−1 (Tc = 230
5 K) and 9.2 kcal mol−1 (Tc = 215 5 K), respectively. Here
again, the higher value found for 2Zn most probably results
from the existence of intermolecular S–Zn coordination bonds
occurring in deuterated chloroform. This assumption is
further confirmed by the important upfield shift of the
Fig. 8 Free energy profile corresponding to the rotation motion of one of the
two ferrocenes of 11Zn around the Cmeso–CFc bond.
–S–CH2–CH2–S– proton signals observed upon cooling (from
2.28 ppm at 298 K to −0.24 ppm at 233 K†).
Ab initio dynamics calculations
These results have been further confirmed by Born–Oppenhei-
mer dynamics calculations at the DFT level conducted on the
most symmetric species 11Zn (Fig. 8). The ab initio dynamics
calculations were performed using the CP2K-QuickStep46,47
program at the DFT level with the BLYP functional.48,49 The
basis set was a double zeta polarized set of Gaussian orbitals
for the second and third row atoms and double zeta for the
iron and zinc atoms with the GTH pseudo-potentials.50,51 To
work around the issue of rare events, the metadynamics52 or
“Hill’s methods” has been used. A series of small repulsive
Gaussian potentials (Hills) centred on the current values of the
collective variables were added during the dynamics to prevent
the system from revisiting points in the configurational space
of the collective variables (further details on the calculation
procedure can be found in the ESI† section).
In the present study, we considered the dihedral angle
CFc–CFc–Cporph–Cporph (see Fig. 8) as a collective variable.
A total free energy activation of 10–10.5 kcal mol−1 was
obtained from the resulting energy profile (Fig. 8). This value
is in very good agreement with the VT NMR experimental data
detailed above and it is also found close to values previously
reported for the meso-tetraferrocenyl free base22 and zinc44
porphyrins (10.4 kcal mol−1 and 11.7 kcal mol−1, respectively).
The low activation energy found for the ferrocenyl-substituted
porphyrins 11Zn is thus in agreement with a free rotation of
the ferrocenyl subunit at room temperature.
UV-visible absorption spectroscopy
UV-vis. absorption spectrophotometry experiments carried out
with 2H2–6H2 also suggest a progressive decrease of the por-
phyrin-based aromaticity with the increasing number of cova-
lently linked ferrocenyl substituents.† As shown in Fig. 9, this
effect is clearly revealed by the increasing bathochromic and
Fig. 7 Partial 1H NMR spectra of 9Zn in the 185.3–302.9 K temperature range
(9 × 10−3 M, CD2Cl2, 500 MHz, 9.2 ≤ δ ≤ 11.2 ppm).
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the CV curve displays two distinct waves whereas a single
signal flanked by two shoulders is seen on the CV curve of the
cis isomer 3H2. According to RDE voltammetry experiments,
the overall number of exchanged electrons is in agreement
with the number of ferrocene units, i.e. two electrons/
molecule.
The electrochemical signatures of the tri- and tetra-substi-
tuted ferrocenyl compounds feature two consecutive waves
observed with 1/2 and 1/3 intensity ratios, respectively. The
shape of these signals indicates that the chemically equivalent
Fc subunits in the di-, tri- and tetra-substituted porphyrins are
electrochemically non-equivalent, the oxidation of one Fc
centre shifting the oxidation of the remaining ones towards
higher potential values.
Fig. 9 UV-vis. absorption spectra of 2Zn–6Zn recorded in DCM at 295 K.
Simulation and best fitting of the experimental data were
used to determine the formal potentials of each ferrocenyl
subunit in 3H2, 4H2 and 5H2.† These data are collected in
Table 2. Unfortunately, adsorption of the oxidized complex
hypochromic shifts of the Soret band occurring from 2H2 to
6H2 or from 2Zn up to 6Zn (Table 1).
It should also be noted that the larger ε values found
for the trans isomers (4H2 and 4Zn) than for the cis isomers
(3H2 and 3Zn) suggest a more planar structure for the trans-
disubstituted derivatives (Table 1). For all the investigated
species, metallation with Zn2+ resulted in the bathochromic
shifts of the main absorption bands along with a significant
increase of the associated molar extinction coefficients.
n+
6H2 onto the electrode surface, as revealed by the obser-
vation of a desorption peak on the reverse scan of the CV
curve, precluded the determination of the corresponding
formal potentials. Theoretical developments carried out with
molecules submitted to n successive one-electron transfers
centred on fully independent and chemically equivalent redox
sites have established that the shift between each successive
formal potential should equal 35.6, 28.5 and 23.7 mV when
n = 2, 3 and 4, respectively, the resulting CV curve however still
displaying the same peak to peak separation of 60 mV.29,30 The
latter feature is not observed on the ferrocene centred oxi-
dation signals depicted in Fig. 10 showing multiple shoulders
and splitting attributed to the interactions occurring between
each redox site in the oxidized and/or reduced states.16,35,43
The extent of the “communication” between ferrocene centres
furthermore evolves with the relative position of each metallo-
cene on the porphyrin skeleton. The differences in the shape
of the CV waves recorded with the bisferrocenyl isomers 3H2
and 4H2 for instance suggest that the coupling is stronger in
the trans isomer than in the cis isomer. These findings con-
trast with previous work, carried out by Swarts23 or Nemykin40
on similar molecules.
Electrochemistry
The electrochemical behavior of 2H2–6H2 and of their Zn2+
complexes has been investigated by cyclic voltammetry (CV)
and by voltammetry at rotating disk electrodes (RDE) in
dichloromethane (DCM) or in N,N-dimethylformamide (DMF)
solutions. All potentials have been referenced towards the half-
wave potential of decamethyl-ferrocene (DMFc/DMFc+) used as
an internal reference. Electrochemical data determined from
CV experiments are collected in Table 2. The electrochemical
signature of all the investigated compounds arises from elec-
tron transfers centred on the ferrocene (Fc) and on the por-
phyrin (P) moieties. In DCM, the oxidation of the dithiolane
substituents is also observed as two irreversible waves at Epa
=
1.20 and 1.46 V.† As a general statement, the signatures of the
ferrocene and porphyrin units are strongly affected by the
extended π conjugation within the molecules.15,16,18,43 The fer-
rocene groups are reversibly oxidized at potential values
ranging from +500 to +750 mV (Fig. 10). The electron with-
drawing effect of the porphyrin ring on the ferrocene centre is
revealed on the CV of 2H2 through the anodic shift of the ferro-
cene-centred one-electron oxidation wave (E1/2 = +0.64 V) as
Swarts and coworkers found no significant differences
between the first and second ferrocene-centered oxidations of
cis and trans porphyrin isomers (111 and 115 mV for
“2HPtrans” and “2HPcis”, respectively and 102 and 103 mV for
“ZnPtrans” and “ZnPcis”, respectively), whereas Nemykin
reported a much higher ΔE value for a cis isomer (208 mV)
than for a trans isomer (150 mV). Under our experimental con-
ditions, the potential shifts between both ferrocene oxidation
processes in 3H2 and 4H2 were found to reach 85 mV and
compared to that of ferrocene used as a reference (E1/2
=
+0.54 V). For the trans disubstituted ferrocenyl compound 4H2,
Table 1 Absorption wavelengths λmax (nm) and molar extinction coefficients ε (M−1 cm−1) measured for 2H2–6H2 and 2Zn–6Zn in DCM
2H2
3H2
4H2
5H2
6H2
2Zn
3Zn
4Zn
5Zn
6Zn
λmax
422
331
427
237
425
247
430
187
435
144
425
369
429
248
428
261
432
207
437
138
10−3 × ε
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Table 2 Electrochemical data for 2H2, 3H2, 4H2, 5H2, 6H2 and their Zn complexes (5 × 10−4 M, 0.25 M TBAP, 0.1 V s−1, E in V vs. DMFc/DMFc+)
Solvent
E1/2
2H2
3H2
4H2
5H2
6H2
2Zn
3Zn
4Zn
5Zn
6Zn
DMF
DMF
DMF
DMF
(P−˙/P2−
(P/P−˙)
(Fc/Fc+)
(P/P+˙)
)
)
−1.530
−1.090
0.590
−1.505
−1.510a
−1.495
−1.530
−1.720
−1.350
0.535
−1.675
−1.700a
−1.690a
−1.680a
−1.100
−1.080
−1.110
−1.120
−1.345
−1.325
−1.340
−1.335
b
b
b
b
b
b
b
b
b
b
b
1.080
1.210
1.020
0.855
0.910
0.920
1.000
DCM
DCM
DCM
(P−˙/P2−
(P/P−˙)
(Fc/Fc+)
−1.520
−1.125
0.640
−1.590a
−1.160
0.605c
−1.745a
−1.305
0.600
−1.740a
−1.310
0.545c
0.615c
0.700c
0.585c
0.695c
0.725c
1.135d
0.555c
0.645c
0.510c
0.665c
0.685c
∼1.11
b
b
0.720c
0.655c
DCM
(P/P+˙)
1.135d
1.210d
1.180d
1.070d
0.895
∼1.070
0.975
∼1.085
a Epc, irreversible. b Extensive overlaps occurring between successive redox systems (ferrocene, porphyrin or electrolyte-centered) prohibit an
accurate experimental determination of this E1/2 value. c From best fitting. d Epa, irreversible.
ferrocenyl group(s) on the porphyrin ring is enlightened by
comparison between the reduction potentials of 2H2–6H2 or
2Zn–6Zn and those of the reference compounds TPPH2 and
TPPZn. The first reductions of 2Zn and TPPZn are at −1.305 V
and −1.205 V, respectively. In addition, the nature of the
solvent used for analyses has been shown to affect quite sig-
nificantly the number and the reversibility of the observed
redox processes.
Two successive one-electron reduction waves are systemati-
cally observed in a DMF medium, whereas formation of the di-
cation P2+ species is only observed in CH2Cl2. The potential
values as well as the reversibility of the porphyrin-centred elec-
Fig. 10 Cyclic voltammograms of 2H2–6H2 limited to the potential range cor-
tron transfers also turn out to undergo significant variations
responding to the oxidation of the ferrocene units (5 × 10−4 M, 100 mV s−1
,
with the substitution pattern and with the insertion of Zn2+.
As observed with most porphyrins,16,53 the redox processes of
the free bases are shifted towards more negative potentials
(ca. −200 mV) upon metallation with Zn2+. In dichloro-
methane, the irreversibility of the first oxidation of the free
base porphyrins suggests the existence of a follow-up chemical
reaction. In contrast, the first oxidation of the zinc porphyrins
appears reversible at the cyclic voltammetry time scale.
DCM 0.1 M TBAP, glassy carbon WE Ø = 3 mm, E vs.: DMFc/DMFc+).
115 mV, respectively. These data confirm the fact that the elec-
tronic “coupling” between both ferrocenyl substituents is
stronger in 4H2, although the Fe⋯Fe distance happens to be
much higher in the trans isomer (11.5 < d < 13.5 Å) than for in
the cis one 3 (6.2 < d < 11.1 Å). The extent of the bent or ruffled
deformations of the porphyrin skeleton induced by the pres-
ence of two bulky ferrocenyl substituents in the 5,10 or 5,15-
positions of the macrocycle is another key factor which will
undoubtedly affect the mixing of molecular orbitals involved
in the through-bond “communication” between ferrocene
centres. Unfortunately, failure to obtain X-ray data for the cis
isomer did not allow us to draw clear cut conclusions on these
structural aspects. In that regard, it should be mentioned that
1H NMR data do not support the assumption of a weaker ring
current in 3H2 than in 4H2.
Spectroelectrochemistry
Absorption spectra were recorded periodically throughout bulk
electrolyses experiments carried out in CH2Cl2. In all cases,
oxidation of the ferrocene units leads to a decrease in the
intensity of the Soret band along with a slight hypsochromic
shift of its maximum wavelength. These changes are in agree-
ment with the electron withdrawing character of the electro-
generated ferrocenium and with the delocalization of the
positive charge over the whole aromatic macrocycle.18,35
CV curves of porphyrins (P) usually display two successive
one-electron oxidation waves involving formation of P˙+ and
P2+, as well as two successive one-electron reduction waves
leading to the formation of the radical anion (P˙−) and di-
anion (P2−).53 In all studied compounds, the potential separ-
ations between the first porphyrin oxidation and first por-
phyrin reduction potentials are in accordance with the
standard HOMO–LUMO gap of ca. 2.2 V commonly measured
with porphyrins.54,55 The electrodonating effect of the
The exhaustive oxidation of the ferrocene unit in 2H2 (Eapp
=
0.81 V) was for instance accompanied with a progressive color
change from brown/green to pale green. After the uptake of
one electron, the initial Soret band at 422 nm (ε = 331 × 103
M−1 cm−1) had lost 2/3 of its initial intensity (ε = 118 300 M−1
cm−1) at the expense of a new band developing at 448 nm (ε =
76 200 M−1 cm−1). The reversibility of these transformations
was checked through a bulk back reduction (Eapp = 0.51 V) of
the resulting ferricinium appended porphyrin allowing us to
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Fig. 11 Electrolysis of a 10−4 M solution of 2H2 followed by UV-vis. spec-
troscopy (l = 1 mm, 0.25 M TBAP in CH2Cl2, Eapp = 1.26 V, −2.1 e).
Fig. 12 Electrolysis of a 10−4 M solution of 2Zn followed by UV-vis. spec-
troscopy (l = 1 mm, 0.25 M TBAP in CH2Cl2, Eapp = 1.07 V, −2.1 e).
restore the initial spectrum. Further oxidation carried out at completely different from that observed with 2H2. As seen in
1.26 V and uptake of another one electron/molecule (1 < the spectra shown in Fig. 12, the intensity of the Soret band
ΔQtotal < 2.1 electron/molecule, Fig. 11) led to the development decreases down to ε = 151 000 M−1 cm−1 while a new broad
of the signal at 448 nm (ε = 2 × 105 M−1 cm−1) and to the dis- band appears at 822 nm. The magnitude of the modifications
appearance of the initial Soret band at 422 nm. The irreversi- in the porphyrin-based signature, proceeding through well
bility of the oxidation was revealed by the fact that the defined isosbestic points at 415, 558, 577, 601 and 622 nm,
signature of the starting material could not be restored by has been attributed to delocalization of the electron hole
back reduction of the doubly oxidized species. This irreversibil- (radical cation character) over the whole molecule from the fer-
ity was further confirmed by CV analysis of the electrolyzed sol- rocene centre to the porphyrin ring.
ution (obtained after uptake of two electrons from 2H2 at Eapp
Oxidation of 2Zn+˙ carried out at 1.07 V, corresponding to
= 1.26 V) showing a reversible oxidation wave at 0.81 V and a the first porphyrin-centred electron transfer, is accompanied
reversible reduction at −0.36 V. This signature was not only by a further decrease in the intensity of the Soret and Q bands.
found to be different from that of the starting material, as These changes turned out to be however fully irreversible at
expected for a species produced irreversibly from the electro- the electrolysis time scale since the signature of the starting
generated dicationic species 2H22+, it was most importantly solution could not be restored by back reduction. In summary,
found to hold great similarities with that of the doubly-proto- these results not only demonstrate that the doubly oxidized
nated H4TPP2+ species.56 To confirm our hypothesis that proto- species 2Zn2+ is poorly stable at the electrolysis time scale, it
nation of the porphyrin ring might be involved, we carried out also supports our assumption that the bulk oxidation of 2H2
2+
2+
a detailed characterization of 2H4 produced in CH2Cl2 from yields quantitatively 2H4
.
2H2 upon addition of increasing amounts of trifluoroacetic The UV-Vis absorption spectra recorded periodically as 4H2
acid (TFA). Addition of up to 2 molar equivalents of TFA led to was subjected to electrochemical oxidations were found to be
the progressive disappearance of the Soret band at 422 nm similar to those monitored with 2H2. The successive oxi-
along with the development of a new band at 448 nm attribu- dations carried out at 0.71 V (1 electron/molecule) and then at
ted to 2H42+. Additionally, the CV curve of the diprotonated 0.86 V (1 additional electron/molecule) were accompanied by
2+
species 2H4 and that of the species produced by bulk oxi- the progressive decrease in the intensity of the initial Soret
dation of 2H2 turned out to display similar key features includ- band at 426 nm at the expense of a new band developing at
ing the reversible waves at 0.80 V and −0.36 V. These results 453 nm (ε = 132 500 M−1 cm−1†). The overall process was
2+
led us to conclude that the electrogenerated dication 2H2 is found to be fully irreversible at the time scale of electrolysis
not stable in anhydrous CH2Cl2 and readily evolves towards and a CV analysis of the resulting oxidized solution revealed
2H42+. It should be mentioned that similar proton exchanges two reversible oxidation waves at 0.78 V and 0.90 V attributed
involving oxidized tetrapyrrolic macrocycles or nitrogen con- to the oxidation of the Fc centres and one reduction process at
taining aromatic compounds35,57,58 have been explained as −0.37 V attributed to the protonated porphyrin ring electroche-
resulting from the presence of an oxidizable and protic sub- mical response. As seen with the mono-ferrocenyl derivative,
strate in the electrolyte (solvent, traces of water …).
the electrochemical and spectrophotometric signatures of the
The proposed course of events was further confirmed with electrolyzed solution match those of the diprotonated species
2+
spectroelectrochemical investigations carried out with the 4H4 which could be produced in situ by addition of TFA to a
metallated species 2Zn. The first, ferrocene-centred, electron solution of the free base 4H2.†
transfer remains reversible at the time scale of electrolysis but
The oxidized forms of this bisferrocenylporphyrin derivative
the UV-vis. signature of the electrogenerated species 2Zn+˙ is were found to be greatly stabilized by insertion of a zinc atom
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within the porphyrin ring and the reversibility of both ferro-
cene-centred oxidation processes was checked from back-
reduction experiments allowing us to recover the UV-vis
absorption spectrum of the starting material 4Zn. The electro-
chemical oxidation of 4Zn into 4Zn2+ was revealed by the hypo-
chromic shift of the Soret band from ε = 261 000 M−1 cm−1
down to ε = 126 000 M−1 cm−1 and by the growth of a new
band at 850 nm (ε = 9700 M−1 cm−1). As discussed for the
monoferrocenyl-porphyrin analog 2Zn, the magnitude of these
changes, involving porphyrin-based π–π* transitions, supports
the assumption of an extended delocalization of the electro-
generated positive charges over the entire molecule, i.e. from
the ferrocene centres to the porphyrin macrocycle.
Fig. 13 Ortep42 views of 8Zn. Thermal ellipsoids are scaled to a 50% prob-
ability level.
Removal of the 1,3-dithiolanyl protecting group
further confirmed by the disappearance of the multiplets near
3.05–3.40 ppm for 2H2 and 4H2 along with the appearance of
the characteristic singlet of the aldehyde function(s) at 9.89
and 9.90 ppm for 8H2 and 7H2, respectively. The correspond-
ing Zn2+ complexes 7Zn and 8Zn could be obtained using Zn
(OAc)2 as a metal source following classical metallation
procedures.
The 1,3-dithiolanyl protecting groups have been introduced on
the upper ring of the ferrocene fragments to allow a straight-
forward post-functionalization and an easy access to a range of
redox active picket-fence porphyrins. Numerous chemical or
electrochemical strategies have been considered to remove the
dithiolane protecting groups in 2H2 and 4H2 and we found
that the most efficient and mildest procedure for synthesizing
the targeted formylated compounds 7H2 and 8H2 in good
yields (Scheme 2) involves Ag+ in the presence of N-chlorosuc-
Single crystals of 8Zn were obtained by diffusion of MeOH
into
a
solution of 8Zn in CHCl3. It is formulated as
1
cinimide.7 The H-NMR spectra of 2H2 and 7H2 indicate that
ˉ
C58H48N4Fe2O4Zn and crystallizes in the P1 space group of the
triclinic system, revealing one crystallographically independent
molecular entity consisting of one 8Zn entity and two
methanol molecules. 8Zn is isolated as a single isomer with
both ferrocenyl groups adopting a syn conformation (α,β-
atropoisomer), the latter being favored due to hydrogen bonds
between the two coordinated MeOH molecules and the alde-
hyde functions (Fig. 13).
the inner NH resonances are shifted upfield upon hydrolysis
of the dithiolane substituents (−2.31 and −2.36 ppm, respect-
ively). This shift is also observed for 4H2 and 8H2 (−1.70 and
−1.76 ppm, respectively). Removal of the dithiolane group is
The interplanar angle between the covalently linked Cp and
porphyrin planes is ∼34.7° whereas the ferrocene is slightly
twisted with an interplanar angle between both Cp rings of
∼6.3°.
The Zn(II) atom lies inside the porphyrin plane and is coor-
dinated by four nitrogen atoms of the macrocycle, with bond
distances ranging from 2.043(2) to 2.059(2) Å and by two
oxygen atoms from two methanol molecules bound in axial
positions (Zn–O(2)) = 2.494(2) Å). The Fe–Fe distance is 12.775
(2) Å and both hydroxyl groups are H-bonded to the oxygen
atoms of the aldehyde units (O(1)⋯H–O(2) = 2.882(4) Å).
Further evidence of the high electronic coupling between
the ferrocene and porphyrin units came from electrochemical
investigations carried out with the protected and deprotected
derivatives 2H2, 4H2 and 7H2, 8H2, respectively. While the con-
version of the dithiolane rings into formyl groups proved to
have no significant effect on the shape of the observed CV
waves (Fig. 14), it led to large shifts of the associated half-wave
oxidation and reduction potentials towards more positive
values. These results thus reveal that (i) the magnitude of the
electronic coupling between both iron centres is similar in
both species, and that (ii) the electron-withdrawing character
of the formyl groups is efficiently transmitted to the ferrocene
Scheme 2 (i) NCS (6 equiv.), AgNO3 (6 equiv.) in THF/CH3CN/H2O, 70%; (ii)
NCS (12 equiv.), AgNO3 (12 equiv.) in THF/CH3CN/H2O, 50%.59
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ferrocenyl macrocycle than for the cis isomer. The effective
electronic “communication” through the whole molecule was
further confirmed by bulk electrolyses experiments. Oxidation
of the ferrocene centers in 2H2 or in 4H2 was found to produce
2+
the protonated species 2H4 or 4H42+, respectively. The exact
succession of electrochemical and chemical steps leading to
these species is still unknown but their formation supports the
notion that the electrogenerated positive charge is significantly
delocalized over the macrocycle. It is noteworthy to mention
that deprotection of the dithiolanyl-substituted ferrocenes
could be readily achieved without loss of the ferrocene and
porphyrin-based electrochemical activity.
Fig. 14 Cyclic voltammograms of 4H2 (dotted line) and 8H2 (solid line) (5 ×
10−4 M, 100 mV s−1, CH2Cl2 0.1 M TBAP, WE: glassy carbon Ø = 3 mm, CE: Pt
wire, Ref: DMFc/DMFc+).
Future work will be devoted to the synthesis of functiona-
lized ferrocene-based picket fence porphyrins which could
find applications in supramolecular chemistry, in electroche-
mical recognition or in molecular electronics.
Table 3 Electrochemical data for 2H2, 2Zn, 7H2, 7Zn (5 × 10−4 M in CH2Cl2,
0.25 M TBAP, ν = 0.1 V s−1, E (in V) vs. DMFc/DMFc+)
P/P˙−
Fc/Fc+˙
P/P+˙
2H2
2Zn
7H2
7Zn
−1.125a
−1.305a
−1.090a
−1.240a
0.640a
0.600a
0.835a
0.725a
1.135b
0.895a
1.165b
0.970a
Acknowledgements
The authors would like to thank the “centre national de la
recherche scientifique”, the “université Joseph Fourier”, the
“conseil régional de Bourgogne” and the “université de Bour-
gogne” for their financial support. C. H. D. would like to thank
Dr Fanny Chaux for carrying out the ESI-MS analyses. A. M.
wishes to thank the “région Rhône-Alpes” as well as the CECIC
calculation facility.
a E1/2, reversible. b Epa, irreversible.
subunit and to a lesser extent to the porphyrin, their electro-
chemical signatures being shifted positively to +120 mV and
+75 mV between 2Zn and 7Zn, respectively (Table 3).
Notes and references
Conclusions
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