Novel double-decker phthalocyaninato terbium(iii) single molecule magnets with stabilised redox states

Article abstract of DOI:10.1039/c2dt31171b

Double-decker phthalocyanine lanthanide complexes are single molecule magnets (SMMs) presenting a thermally activated magnetic relaxation with relatively high effective barriers. For this reason they are potential candidates as components for data storage

Full text of DOI:10.1039/c2dt31171b

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PAPER
Novel double-decker phthalocyaninato terbium(III) single molecule magnets
with stabilised redox states†
Mathieu Gonidec,a,b David B. Amabilinoa and Jaume Veciana*a,b
Received 31st May 2012, Accepted 24th July 2012
DOI: 10.1039/c2dt31171b
Double-decker phthalocyanine lanthanide complexes are single molecule magnets (SMMs) presenting a
thermally activated magnetic relaxation with relatively high effective barriers. For this reason they are
potential candidates as components for data storage and spintronic devices. One of the disadvantages of
these compounds is their redox instability: they are oxidized or reduced in the presence of mild oxidizing
and reducing agents. To solve this issue, we designed, prepared and characterized new double-decker
phthalocyanine based SMMs bearing electron withdrawing groups and therefore presenting an increased
redox stability. In the present article, the synthesis and characterization of these novel compounds is
presented and we demonstrate how the magnetic behavior of the complexes is virtually identical to that of
the parent unsubstituted compounds.
problems, and in the framework of a research program aimed at
checking the robustness of the magnetic behavior of double-
Introduction
Single-molecule magnets (SMMs) were initially foreseen as
compounds which present very interesting properties with poten-
tial applications in novel storage devices in which data could be
stored at the single-molecule level.1 For many years, in order to
reach this goal, the science of SMMs has essentially been
focused on deciphering their quantum characteristics2–9 and
trying to produce novel systems with improved properties10–16 so
as to obtain compounds presenting magnetic pseudo-bistability
at reasonably high temperatures. It was not until recently that
they were actually incorporated as active units in hybrid
materials,17–27 in order to assert their use as molecular com-
ponents for spintronic devices. In doing so, the redox instability
of some SMMs was soon detected as a challenge. In particular,
it was shown that Mn12 complexes could undergo reduction on
the Au(111) surface28 and that a similar situation could occur
while depositing terbium double-decker phthalocyanine com-
plexes on Cu(111).17 One of the best candidates for spintronic
applications are lanthanide double-decker phthalocyanine com-
plexes, which present the highest anisotropy barrier of magneti-
zation reversal to date for SMMs.9,16,29–31 However, as a result
of their low redox potentials,32,33 double-decker lanthanide com-
plexes are often inadvertently reduced or oxidized and are also
very sensitive to both acids and bases. In order to solve these
decker complexes as a consequence of different substitution pat-
terns, we designed a series of novel compounds with character-
istics which would give them an improved redox stability.
Electroactive substituents were thus introduced on the outer part
of the skeleton of the double-decker lanthanide complexes, with
the effect of stabilizing the negative charge of the anionic state
of the compounds, which resulted in a strong shift of approxi-
mately 0.7 V of their first oxidation potentials. In the present
article, the synthesis and characterization of these novel double-
decker terbium(III) complexes is presented together with their
SMM behaviour.
Experimental
Materials and methods
All the reagents were used as received from commercial sources.
In particular, dry hexanol and DMF were bought from Sigma-
Aldrich in secure-seal packages. NMR spectra were recorded on
Bruker Avance 250, 360 and 400 MHz spectrometers as
specified. The solvent residual peak was used to calibrate the
spectra with literature reference δ ppm values.34 UV-visible
absorption spectra were collected on a Varian Cary 5000 spec-
trometer at one data point every 0.66 nm and measured at a scan-
ning rate of 1200 nm·min−1. Infrared spectra were recorded on a
Perkin Elmer Spectrum One Fourier Transform spectrometer in
attenuated total reflection (ATR) mode. Mass spectra were
recorded on a Bruker Ultraflex LDI-TOF spectrometer. Cyclic
voltammetry studies were carried out using a VersaSTAT 3
potentiostat from Princeton Applied Research. Standard cyclic
voltammetry was carried out using a three-electrode arrangement
in a single compartment cell. A glassy carbon working electrode,
aInstitut de Ciència de Materials de Barcelona (ICMAB-CSIC), 08193
Bellaterra, Spain. E-mail: vecianaj@icmab.es
bNetworking Center on Bioengineering, Biomaterials and Nanomedicine
(CIBER-BBN), 08193 Bellaterra, Spain
†Electronic supplementary information (ESI) available: Additional
characterization for [TBA][3a–c]: LDI-TOF MS spectra, complementary
cyclic voltammetry data, χ′T(T), χ′′(T), χ′T(ν), Cole–Cole plots and mag-
netization hysteresis plots at 7 K. Magnetization hysteresis plot at several
field sweeping rates for [TBA][3a]. See DOI: 10.1039/c2dt31171b
This journal is © The Royal Society of Chemistry 2012
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(S)-N-methyl(phenyl)methyl-4,5-dichlorophthalimide (1c).
4,5-Dichlorophthalic acid (10 mmol, 2.3 g, 1.0 eq.), DMAP
(1.5 mmol, 0.18 g, 0.15 eq.) and (S)-1-phenylethylamine
(11 mmol, 1.3 g, 1.1 eq.) were dissolved in CH2Cl2 (10 mL) in a
round bottom flask under nitrogen. The flask was cooled to 0 °C
and DCC (30 mmol, 6.2 g, 3.0 eq.) was added. The resulting
mixture was stirred at 0 °C for 15 min and then at room tempera-
ture for 12 h. The precipitated urea was then filtered off and
washed thoroughly with CH2Cl2. The combined organic phases
were evaporated on a rotary evaporator and the obtained solids
were purified by silica gel column chromatography using
CH2Cl2 as an eluent, affording (S)-N-methyl(phenyl)methyl-4,5-
dichlorophthalimide as a white solid (2.2 g, 69%). 1H NMR
(400 MHz, CDCl3): 7.87 (Cl–Ar–H, s, 2 H), 7.45–7.55 (–N–CH
(CH3)–Ar–H, m, 2 H), 7.25–7.35 (–N–CH(CH3)–Ar–H, m,
3 H), 5.54 (–N–CH(CH3)–, q, J = 7.3 Hz, 1 H), 1.91 (–N–CH
(CH3)–, d, J = 7.3 Hz, 3 H) ppm. 13C NMR (63 MHz, CDCl3):
a Pt wire secondary electrode and a silver wire as a pseudo refer-
ence electrode were used in the cell. The potentials were then
calibrated with ferrocene as an internal reference. The samples
for magnetometry consisted of gelatine capsules filled with
powder samples (about 15 mg of the sample was used for each
measurement). Ac-magnetic susceptibility measurements were
done on a Quantum Design PPMS magnetometer and for mag-
netization hysteresis, the sample was mounted on a Vibrating
Sample Magnetometer (VSM).
N-Pentadecyl-4,5-dichlorophthalimide (1a). 4,5-Dichlorophtha-
lic acid (10 mmol, 2.35 g, 1.0 eq.), pentadecylamine (16 mmol,
3.6 g, 1.6 eq.) and N,N′-dimethylaminopyridine (DMAP,
1.47 mmol, 0.18 g, 0.15 eq.) were dissolved in CH2Cl2 (10 mL)
in a round bottom flask under nitrogen. The flask was cooled to
0 °C and dicyclohexylcarbodiimide (DCC, 22 mmol, 4.5 g,
2.2 eq.) was added. The resulting mixture was stirred at 0 °C for
15 min and then at room temperature for 12 h. The precipitated
urea was then filtered off and washed thoroughly with CH2Cl2.
The combined organic phases were evaporated on a rotary
evaporator and the obtained solids were purified by silica gel
column chromatography using CH2Cl2 as an eluent, affording
N-pentadecyl-4,5-dichlorophthalimide as a white solid (2.5 g,
59%). MS m/z found: 425.3 (M−); Calcd for C23H33Cl2NO2:
166.3, 139.9, 139.0, 131.2, 128.7, 128.0, 127.5, 125.4, 50.3,
25
546
17.5 ppm. [α]
= 3.75 deg cm2 g−1. FT-IR: 3465 (w),
6094 (w), 3069 (w), 3022 (w), 2985 (w), 2945 (w), 2119 (w),
1771 (m), 1718 (s), 1705 (s), 1614 (m), 1586 (m), 1493 (m),
1456 (m), 1387 (s), 1379 (s), 1354 (s), 1347 (s), 1287 (m),
1214 (m), 1191 (m), 1154 (s), 1138 (m), 1103 (m), 1093 (m),
1050 (m), 1029 (m), 996 (w), 977 (w), 948 (w), 903 (m),
896 (m), 792 (m), 771 (m), 760 (s), 750 (s), 699 (s) cm−1
.
1
426.4. H NMR (360 MHz, CDCl3): 7.91 (Ar–H, s, 2 H), 3.66
(–N–CH2–, t, J = 7.3 Hz, 2 H), 1.59 (–N–CH2–CH2–, m, 2 H),
1.24 (–(CH2)12–CH3, m, 24 H), 0.87 (–CH3, t, J = 7.0 Hz, 3 H)
ppm. 13C NMR (63 MHz, CDCl3): 166.4, 138.7, 131.3, 125.3,
38.5, 31.9, 29.7, 29.6, 29.5, 29.4, 29.3, 29.1, 28.4, 26.8, 22.7,
14.1 ppm. FT-IR: 3100 (w), 2947 (m), 2918 (s), 2850 (s),
1775 (m), 1758 (w), 1688 (s), 1614 (w), 1603 (w), 1470 (m),
1434 (m), 1384 (s), 1350 (s), 1343 (m), 1331 (m), 1310 (m),
1269 (w), 1225 (w), 1191 (m), 1159 (w), 1140 (m), 1103 (m),
1080 (w), 1061 (w), 1041 (m), 1009 (m), 979 (w), 851 (w),
933 (w), 906 (m), 892 (w), 853 (m), 790 (w), 759 (w), 743 (s),
N-Pentadecyl-4,5-dicyanophthalimide (2a). N-Pentadecyl-4,5-
dichlorophthalimide (2.3 mmol, 1.0 g, 1.0 eq.), Pd(PPh3)4
(0.47 mmol, 0.54 g, 0.20 eq.) and zinc(II) cyanide (2.8 mmol,
0.33 g, 1.2 eq.) were introduced into a round-bottomed flask
under argon. Dry DMF (4.7 mL) was added and the mixture was
gently stirred at 120 °C over 2 h. After this time, concentrated
ammonia was added (30 mL) and the solid residues were
filtered, dried and directly purified by silica gel column chrom-
atography using toluene as an eluent, affording the title com-
pound (0.51 g, 54%) as a white crystalline solid. MS m/z found:
407.4 (M−); Calcd for C25H33N3O2: 407.6. 1H NMR (360 MHz,
CDCl3): 8.25 (Ar–H, s, 2 H), 3.74 (–N–CH2–, t, J = 7.2 Hz,
2 H), 1.69 (–N–CH2–CH2–, m, 2 H), 1.25 (–(CH2)12–CH3, m,
24 H), 0.88 (–CH2–CH3, t, J = 6.9 Hz, 3 H) ppm. 13C NMR
(63 MHz, CDCl3): 164.8, 135.5, 127.9, 121.3, 114.2, 39.1, 31.9,
29.7, 29.6, 29.5, 29.4, 29.3, 29.0, 28.3, 26.8, 22.7, 14.1 ppm.
FT-IR: 3111 (w), 3050 (w), 2962 (w), 2918 (s), 2850 (s), 2240 (w),
1868 (w), 1781 (m), 1717 (s), 1705 (s), 1623 (w), 1470 (m),
1440 (w), 1416 (w), 1397 (s), 1370 (m), 1358 (m), 1331 (m),
1283 (w), 1271 (w), 1250 (w), 1227 (w), 1205 (w), 1186 (w),
1166 (w), 1151 (w), 1088 (s), 1027 (w), 1003 (w), 970 (w),
961 (w), 939 (m), 888 (w), 849 (w), 814 (w), 788 (m), 768 (w),
736 (s), 707 (s) cm−1
.
N-Propyl-4,5-dichlorophthalimide (1b). DMAP (0.23 g,
1.9 mmol, 0.15 eq.) and 4,5-dichlorophthalic acid (3.0 g,
13 mmol, 1.0 eq.) were dissolved in CH2Cl2 (30 mL) in a round
bottom flask under argon. The resulting suspension was cooled
to 0 °C and DCC (7.9 g, 38 mmol, 3.0 eq.) and propylamine
(0.83 g, 14 mmol, 1.1 eq.) were added. The mixture was stirred
at 0 °C for 15 minutes and stirred at room temperature for 2
days. The precipitated urea was then filtered off and washed
thoroughly with CH2Cl2. The organic layer was then evaporated
under vacuum and the residue was purified by silica gel column
chromatography using CH2Cl2 as an eluent, affording the title
compound (2.05 g, 62%) as a white powder. 1H NMR
(360 MHz, CDCl3): 7.90 (Ar–H, s, 2 H), 3.63 (–N–CH2–,
t, J = 7.3 Hz, 2 H), 1.69 (–N–CH2–CH2–, m, 2 H), 0.93 (–CH3,
t, J = 7.4 Hz, 3 H) ppm. 13C NMR (91 MHz, CDCl3): 166.6,
138.9, 131.4, 125.4, 40.2, 21.9, 11.4 ppm. FT-IR: 3095 (w),
3028 (w), 2971 (m), 2939 (w), 2877 (w), 1770 (m), 1741 (w),
1715 (s), 1695 (s), 1619 (m), 1604 (m), 1461 (m), 1448 (m),
1440 (m), 1382 (s), 1361 (s), 1346 (s), 1311 (s), 1192 (m),
1177 (m), 1142 (s), 1103 (m), 1094 (m), 1059 (m), 1044 (s),
956 (w), 919 (w), 896 (w), 885 (w), 854 (s), 790 (s), 750 (s),
745 (s), 719 (s) cm−1
.
N-Propyl-4,5-dicyanophthalimide (2b). N-Propyl-4,5-dichloro-
phthalimide (1.9 mmol, 0.50 g, 1.0 eq.), Pd(PPh3)4
(0.39 mmol, 0.45 g, 0.20 eq.) and zinc(II) cyanide (2.3 mmol,
0.27 g, 1.2 eq.) were introduced into a round-bottomed flask
under argon. Dry DMF (3.2 mL) was added and the mixture was
gently stirred at 120 °C over 2 h. After this time, concentrated
ammonia was added (30 mL) and the solid residues were
filtered, dried and directly purified by silica gel column
chromatography using CH2Cl2 as an eluent, affording the title
737 (s) cm−1
.
Dalton Trans.
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1
compound (0.27 g, 58%) as a white crystalline solid. H NMR
1436 (m), 1404 (m), 1386 (m), 1358 (s), 1215 (m), 115 (s),
(360 MHz, CDCl3): 8.26 (Ar–H, s, 2 H), 3.72 (N–CH2–, t, J =
7.4 Hz, 2 H), 1.73 (–CH2–CH3, m, 2 H), 0.96 (–CH3, t, J =
7.3 Hz, 3 H) ppm. 13C NMR (91 MHz, CDCl3): 164.7, 135.3,
127.8, 121.2, 114.0, 40.7, 21.7, 11.2 ppm. FT-IR: 3117 (w),
3055 (w), 2970 (w), 2940 (w), 2881 (w), 2238 (w), 1779 (m),
1709 (s), 1623 (w), 1544 (w), 1458 (w), 1439 (m), 1418 (m),
1396 (s), 1383 (s), 1363 (s), 1348 (s), 1320 (s), 1242 (w), 1179
(m), 1148 (w), 1098 (w), 1080 (s), 1017 (m), 924 (m), 915 (m),
1066 (s), 911 (m), 813 (w), 786 (w), 735 (s), 660 (m) cm−1
.
[TBA][Bis(N,N,N,N-tetrapropyl-29H,31H-2,3,9,10,16,17,23,24-
phthalocyaninatotetradicarboximide) terbium(III)] ([TBA]-
[3b]). N-Propyl-4,5-dicyanophthalimide (0.42 mmol, 0.10 g,
8.0 eq.) was introduced into a dry Schlenk tube under argon.
The Schlenk was degassed and was purged with argon three
times. Dry hexanol (0.8 ml) was added with a syringe followed
by DBU (0.21 mmol, 31 μl, 4.0 eq.) and after stirring for
5 minutes under argon, Tb(acac)3·[H2O]3 (52 μmol, 27 mg,
1.0 eq.) was added as one aliquot together with a catalytic
amount of potassium acetate. The mixture was then heated in a
sand bath to reflux at 165 °C for 15 h. After that time, a large
excess of tetrabutylammonium acetate was added and the
mixture was precipitated by the addition of methanol and
filtered. The obtained solid was purified by silica gel column
chromatography using CH2Cl2–THF–AcOH (66 : 33 : 1) as an
eluent. Some metal free phthalocyanine was eluted first together
with an unidentified impurity. A deep dark blue band was then
eluted with THF–AcOH (99 : 1) and was characterized as the
anionic form of [TBA][3b]. The obtained fraction was then
purified by repeated size exclusion chromatography on a Bio-
Beads-SX1 medium using CH2Cl2 as an eluent, affording
the title compound (31 mg, 26%) as a dark purple powder.
MS m/z found: 2074.3 (M3b−); Calcd for C104H72N24O16Tb:
2072.8. UV-vis (log(ε)): 359 (5.1), 434 (4.5), 539 (4.1),
582 (4.4), 644 (5.2), 724 (5.0) nm. FT-IR: 2964 (w), 2935 (w),
2875 (w), 1760 (s), 1699 (s), 1482 (w), 1456 (w), 1436 (m),
1404 (s), 1379 (s), 1359 (s), 1345 (s), 1215 (s), 1147 (w),
1111 (s), 1073 (m), 1054 (s), 1002 (m), 976 (m), 955 (m),
907 (m), 889 (m), 842 (w), 814 (w), 785 (w), 733 (s), 699 (m),
893 (m), 861 (m), 792 (m), 767 (m), 740 (s), 699 (w) cm−1
.
(S)-N-methyl(phenyl)methyl-4,5-dicyanophthalimide (2c).
(S)-N-Methyl(phenyl)methyl-4,5-dichlorophthalimide (3.1 mmol,
1.0 g, 1.0 eq.), Pd(PPh3)4 (0.62 mmol, 0.72 g, 0.20 eq.) and
zinc(II) cyanide (3.7 mmol, 0.44 g, 1.2 eq.) were introduced into
a round-bottomed flask under argon. Dry DMF (6.2 mL) was
added and the mixture was gently stirred at 120 °C over 2 h.
After this time, concentrated ammonia was added (30 mL) and
the solid residues were filtered, dried and directly purified by
silica gel column chromatography using toluene as an eluent,
affording the title compound (0.68 g, 72%) as a white crystalline
1
solid. H NMR (250 MHz, CDCl3): 8.21 (NC–Ar–H, s, 2 H),
7.4–7.55 (–N–CH(CH3)–Ar–H, m, 2 H), 7.27–7.40 (–N–CH
(CH3)–Ar–H, m, 3 H), 5.58 (–N–CH(CH3)–, q, J = 7.2 Hz,
1 H), 1.94 (–N–CH(CH3)–, d, J = 7.2 Hz, 3 H) ppm. 13C NMR
(63 MHz, CDCl3): 164.6, 139.0, 135.3, 128.9, 128.5, 128.0,
127.6, 121.2, 114.2, 51.2, 17.4 ppm. FT-IR: 3121 (w), 3062 (w),
2922 (w), 2243 (w), 1779 (m), 1713 (s), 1494 (w), 1457 (w),
1416 (w), 1389 (m), 1362 (s), 1318 (s), 1239 (w), 1214 (w),
1161 (m), 1113 (w), 1096 (m), 1072 (m), 1023 (w), 1003 (w),
993 (w), 920 (m), 904 (s), 849 (w), 805 (m), 771 (m), 741 (s),
723 (m), 701 (s) cm−1. [α] = −0.26 deg cm2 g−1
25
.
659 (m) cm−1
.
546
[TBA][Bis(N,N,N,N-tetrapentadecyl-29H,31H-2,3,9,10,16,17,-
23,24-phthalocyaninatotetradicarboximide) terbium(III)] ([TBA]-
[3a]). N-Pentadecyl-4,5-dicyanophthalimide (0.17 mmol, 70 mg,
8.0 eq.) was introduced into a dry Schlenk tube under argon.
The Schlenk was degassed and was purged with argon 3 times.
Dry hexanol (0.5 ml) was added with a syringe followed by
1,8-diazabicycloundec-7-ene (DBU, 86 μmol, 13 mg, 4.0 eq.)
and after stirring for 5 minutes under argon Tb(OAc)3·[H2O]6
(0.027 mmol, 12 mg, 1.0 eq.) was added as one aliquot. The
mixture was then warmed in a sand bath to reflux at 160 °C for
14 h. After that time, a large excess of tetrabutylammonium
acetate was added and the mixture was dried under vacuum to
remove the hexanol. The obtained waxy solid was purified by
silica gel column chromatography using chloroform–AcOH
(99 : 1) as an eluent. Two thin bands were eluted containing the
metal free phthalocyanine and an unidentified impurity, respect-
ively. The deep dark blue band remaining at the top of the
column was then eluted with THF and was characterized as
[TBA][3a]. The obtained fraction was purified by repeated size
exclusion chromatography on a BioBeads-SX1 medium using
toluene as an eluent, affording the title compound (27 mg, 37%)
as a waxy green-blue material. MS m/z found: 3419.7 (M3a−);
Calcd for C200H264N24O16Tb: 3419.3. UV-vis (log(ε)): 359
(5.0), 434 (4.5), 541 (3.9), 582 (4.3), 644 (5.2), 724 (5.0) nm.
FT-IR: 2921 (s), 2851 (s), 1767 (s), 1705 (s), 1465 (m),
[TBA][Bis(N,N,N,N-tetra((S)-methyl(phenyl)methyl)-29H,31H-
2,3,9,10,16,17,23,24-Phthalocyaninatotetradicarboximide)
terbium(III)]
([TBA][3c]). (S)-N-Methyl(phenyl)methyl-4,5-
dicyanophthalimide (0.60 mmol, 0.18 g, 8.0 eq.) was introduced
into a dry Schlenk tube under argon. The Schlenk was degassed
and was purged with argon 3 times. Dry hexanol (1.1 ml) was
added with a syringe followed by DBU (0.30 mmol, 45 μl,
4.0 eq.) and after stirring for 5 min under argon, Tb(acac)3·
[H2O]3 (75 μmol, 38 mg, 1.0 eq.) was added as one aliquot
together with a catalytic amount of potassium acetate. The
mixture was then heated in a sand bath to reflux at 165 °C for
15 h. After that time, a large excess of tetrabutylammonium
acetate was added and the mixture was precipitated by the
addition of methanol and filtered. The obtained solid was
purified by silica gel column chromatography using CH2Cl2–
THF–AcOH (66 : 33 : 1) as an eluent. Some metal free phthalo-
cyanine was eluted first together with an unidentified impurity. A
deep dark blue band was then eluted with THF–AcOH (99 : 1)
and was characterized as the anionic form of [TBA][3c]. The
obtained fraction was then purified by repeated size exclusion
chromatography on a BioBeads-SX1 medium using CH2Cl2 as
an eluent, affording the title compound (56 mg, 30%) as a shiny
dark purple powder. MS m/z Found: 2570.1 (M3c−); Calcd for
C
144H88N24O16Tb: 2569.3. UV-vis (log(ε)): 362 (5.1), 434 (4.5),
553 (4.3), 587 (4.4), 647 (5.1), 721 (5.1) nm. FT-IR: 2964 (w),
This journal is © The Royal Society of Chemistry 2012
Dalton Trans.

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Fig. 1 Molecular structure of the phthalimide substituted double-
decker tebium(III) complexes [TBA][3a–c].
2939 (w), 2875 (w), 1763 (s), 1700 (s), 1482 (m), 1450 (m),
1401 (m), 1377 (s), 1334 (s), 1206 (m), 1144 (m), 1111 (m),
1092 (m), 1049 (s), 1029 (m), 1013 (m), 906 (s), 854 (w),
814 (w), 802 (w), 774 (w), 749 (m), 735 (s), 693 (s),
Scheme 1 Synthesis of the imide substituted double-decker complexes
[TBA][3a–c].
663 (s) cm−1
.
Results and discussion
Design and synthesis
In order to stabilize the negative charge of the anionic double-
decker complexes, cyclic imide derivatives were introduced on
the periphery of the phthalocyanine ligands (Fig. 1). The choice
of the substituents was based on the electron withdrawing capa-
bility of the carbonyl groups and the symmetric functionalization
thus obtained, yielding a symmetric molecular structure. More-
over, phthalimides are notoriously stable moieties used as amine
protecting groups.35 While carbonyl substituted phthalocyanines
are known,36 to the best of our knowledge the [TBA][3a–c] salts
are the first reported examples of phthalocyanine double-decker
complexes bearing these substituents, in this case in the form of
an N-substituted phthalimide.
The target double-decker complexes [TBA][3a–c] were syn-
thesized in good yields using an original three step pathway
from commercially available dichlorophthalic acid (see
Scheme 1). The dichlorophthalimides 1a–c were first prepared in
yields of approximately 60% by reacting the desired primary
amines with dichlorophthalic acid in conditions similar to those
of the Steglich esterification, that is in the presence of dicyclo-
hexylcarbodiimide (DCC) and N,N′-dimethylaminopyridine
(DMAP).37 Subsequently, the chlorides were displaced by cya-
nides in a tandem catalyzed reaction with zinc cyanide and tetra-
kis-triphenylphosphine palladium(0),25,38 affording the imide
phthalonitriles 2a–c with yields varying from 54 to 72%.
Finally, the salts of the anionic double-decker complexes [TBA]-
[3a–c] were prepared by boiling the corresponding phthaloni-
triles in n-hexanol with the acetylacetonate salts of the terbium(III)
ion in the presence of 1,8-diazabicycloundec-7-ene (DBU)
and a catalytic amount of potassium acetate, in a variation of the
classical conditions performed by De Cian et al.,39 followed by
treatment with an excess amount of tetrabutyl ammonium acetate
([TBA][OAc]).
Fig. 2 Normalized UV-visible absorption spectra of [TBA][3a–c] in
dichloromethane solutions at concentrations of 6.9, 8.6 and 7.5 ×
10−5 M, respectively.
The moderate yields obtained (26–37%) could be accounted
for by partial degradation of the imide groups in the harsh reac-
tion conditions necessary to prepare the double-decker com-
plexes, as partially hydrolized fragments were detected in the
crude reaction products by mass spectroscopy and were elimi-
nated during the purification process.
UV-visible spectroscopy
The absorption spectra of the salts [TBA][3a–c] (Fig. 2) were
globally quite similar and consistent with their structures, pres-
enting features akin to those of classical anionic double-decker
phthalocyanine lanthanide complexes40 but also exhibiting some
major differences.
While anionic terbium(III) double-decker complexes usually
present an intense Q band at 623 nm with a shoulder at around
Dalton Trans.
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Fig. 3 Absorption spectral changes upon the heterogeneous oxidation
of the anionic form [TBA][3a] in a dichloromethane solution containing
a small crystal of iron(III) perchlorate.
675 nm,41 the Q band of [TBA][3a] ([TBA][3b] and [TBA][3c],
respectively) is shifted to a higher wavelength and appears as
two resolved bands at 644 (644 and 647, respectively) and 724
(724 and 721, respectively) nm. The characterization of the
anionic redox state was confirmed by adding a small crystal of
iron(III) perchlorate to a dichloromethane solution of compound
[TBA][3a]. The iron perchlorate is insoluble in dichloromethane
and so the oxidation took place slowly and could be monitored
by recording the spectral changes of the solution over time
(Fig. 3). The conversion of the native anionic state [TBA]+[3a]−
to its neutral form, 3a0, proceeded with well defined isosbestic
points at 700 and 666 nm and the final neutral state exhibited an
absorption spectrum consistent with that of neutral double-
decker complexes.40
Notably, two bands appear in the UV-visible and NIR regions
at 467 and 918 nm, respectively, which is a signature of the
π-radical ligands present in neutral double-decker complexes.
The main difference between the spectral features of 3a0 and the
unsubstituted complex [Pc2Tb]0 is a small shift of the Q band to
a lower energy (681 nm for 3a0 instead of the usual 667 nm for
previously reported complexes).
Fig. 4 Cyclic voltammograms of 1.0 mM solutions of [TBA][3a]
(top), [TBA][3b] (middle) and [TBA][3c] (bottom) in dichloromethane
−
containing 0.1 M TBA+PF6
.
other redox processes, suggests that the compounds form aggre-
gates in the conditions of the experiment. This hypothesis is also
supported by the fact that the branched phenylethyl substituted
compound [TBA][3c] presents reversible voltammograms for all
the studied redox processes, which can be attributed to the
absence of aggregation due to the bulkiness of its side groups.
Electrochemistry
In order to characterize the redox properties of [TBA][3a–c],
cyclic voltammograms were recorded in dichloromethane solu-
tions. For the sake of simplicity, only the data concerning the
two oxidation processes is shown in Fig. 4. For more detailed
information see Fig. S4 to S9 in the ESI.† The data confirms that
the compounds present an increased redox stability of the
anionic state under ambient conditions. Indeed, the first oxi-
dation potentials are found to be around 0.3 V vs. Fc/Fc+ for all
three compounds, which corresponds to a shift of approximately
0.7 V with respect to the unsubstituted Pc2Tb33,42 (the full list of
half-wave potentials is given in Table 1). It is noticeable that for
[TBA][3a] and [TBA][3b], the first oxidation process is not well
defined and appears as two waves of lower intensity for
[TBA][3a] and as an asymmetrical shape for [TBA][3b]. This
fact, combined with the reversible waves associated with the
Magnetic properties
The slow magnetic relaxation of single molecule magnets is
reflected in the temperature and frequency dependent maxima in
the ac-magnetic characteristics.1 In the case of [TBA][3a–c], the
frequency dependent in-phase and out-of-phase ac-magnetic sus-
ceptibilities, χ′M(ν) and χ′′M(ν), were measured at temperatures
ranging from 5 to 55 K. Their temperature dependent counter-
parts (χ′M(T) and χ′′M(T)) were measured at several frequencies
between 111 and 9999 Hz. For the sake of simplicity, only
χ′′M(ν) is represented here (Fig. 5) and the rest of the data is pro-
vided in the ESI.†
The in-phase and out-of-phase components were fitted to a
Cole–Cole model,43 optimizing the parameters so as to fit both
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Dalton Trans.

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Table 1 Measured oxidation and reduction potentials for [TBA][3a–c] in dichloromethane–0.1 M TBAPF6, and the reported values for the
unsubstituted compound in the same conditions.32 The potentials are quoted versus Fc/Fc+ which is taken as 0.49 V vs. SCE. The calculated ΔE
values correspond to the potential shifts with respect to Pc2Tb
Compound
Ox2
Ox1
Red1
Red2
ΔEOx2
ΔEOx1
ΔERed1
ΔERed2
[TBA][3a]
[TBA][3b]
[TBA][3c]
+0.84
+0.76
+0.78
+0.02
+0.28a
+0.28a
+0.31
−0.93
−0.96
−0.93
−1.57
−1.13
−1.19
−1.12
−1.81
+0.80
+0.74
+0.76
—
+0.68a
+0.68a
+0.71
—
+0.64
+0.61
+0.64
—
+0.68
+0.62
+0.69
—
[TBA][Pc2Tb]
−0.40
a These values are approximated since this redox process was poorly defined in the experimental conditions.
Fig. 6 Arrhenius plots for [TBA][3a] (top), [TBA][3b] (middle) and
[TBA][3c] (bottom) extracted from the susceptibility plots of χac(ν) and
χac(T). The dotted lines correspond to the linear fits of the thermally acti-
vated regimes of all three compounds.
Fig. 5 Frequency dependent plot of the out-of-phase ac-magnetic sus-
ceptibility χ′′M(ν) plots for [TBA][3a] (top), [TBA][3b] (middle) and
[TBA][3c] (bottom), between 5 and 50 K. The continuous lines are best
fits to a Cole–Cole model.
ðωτÞ1ꢀα cosðπα=2Þ
χ′′ðωÞ ¼ ðχT ꢀ χsÞ
ð2Þ
zꢀza
1 þ 2ðωτÞ1ꢀα sinðπα=2Þ þ ðωτÞ
The fitted relaxation times, τ, together with those extracted
from the temperature dependent plots were used to build the
Arrhenius plots for compounds [TBA][3a–c] (Fig. 6). As usually
occurs with double-decker SMMS,29,44 the Arrhenius plots pres-
ents two regimes: a linear part at higher temperature, corres-
ponding to a thermally activated Orbach relaxation process, and
sets of data, with ω = 2πν:
χ′ðωÞ ¼ χs þ ðχT ꢀ χsÞ
1 þ ðωτÞ1ꢀα sinðπα=2Þ
ꢁ
ð1Þ
1 þ 2ðωτÞ1ꢀα sinðπα=2Þ þ ðωτÞ
zꢀza
Dalton Trans.
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a deviation from the same at lower temperature where tunnelling
is predominant. The observed effective barrier heights for
[TBA][3a–c] were found to be Δobs = 445, 428 and 463 cm−1
respectively, with pre-exponential factors of τ0−1 = 6.35 × 10−11
,
,
1.34 × 10−10 and 2.22 × 10−11, respectively. The values obtained
here are in good agreement with those estimated from the
f-electronic structure of the unsubstituted anionic double-decker
phthalocyanine terbium(III) complex, with the energy difference
between J6 and J5 estimated at about 420 cm−1 for
[TBA][Pc2Tb]. The Cole–Cole plots at 35 K (ESI†) show that
[TBA][3a–c] presents a single magnetic relaxation process with
a small dispersion of the relaxation rates (1 − α = 0.75–0.80).
Overall, the susceptibility data of [TBA][3a–c] proves to be very
consistent from one substituent to the other, demonstrating the
robustness of the properties of this new family of complexes
towards subtitution on the imido nitrogen atom, which makes
this system a promising and versatile platform for potential
future applications. Magnetization hysteresis plots were recorded
for all three compounds at 2 and 7 K, at a field sweeping rate of
1 T·min−1, and for |μ0H| ≤ 5 T. The curves were normalized to
their 5 T values and are plotted in Fig. 7 for |μ0H| ≤ 1 T (the
hystereses curves measured at 2 K are provided in the ESI†). As
occurs with previously reported anionic double-decker
SMMs,9,41,45 [TBA][3a], [TBA][3b] and [TBA][3c] present
butterfly-shaped hysteresis curves with a narrowing of the loops
for |μ0H| ≤ 0.09 T, which is known to be a result of the quantum
tunnelling of magnetization (QTM) occurring between the eight
low-lying |Jz>|Iz> states of the anionic compounds. As a result
of this phenomenon, the remnant magnetization and coercive
fields are minimal.
It is worth mentioning that the magnetization behavior
does not deviate much from that of the unsubstituted [TBA]-
[Pc2Tb] and that all three compounds present similar features.
Nevertheless, the width of the hystereses close to the zero
field varies slightly from one compound to another. In parti-
cular, [TBA][3b] presents an almost closed hysteresis loop,
which could be the result of slightly more efficient tunnel-
ling in the solid state of this compound with respect to the other
two.
Fig. 7 Hysteresis of magnetization for [TBA][3a] (top), [TBA][3b]
(middle) and [TBA][3c] (bottom) measured at 2 K and a sweeping rate
of 1 T·min−1
.
for future applications in hybrid molecular devices, limiting the
occurrence of charge transfer between the inorganic substrates
and the SMMs.
Conclusions
In the present work, the synthesis of a new family of double-
decker phthalocyanine complexes was demonstrated. Their
absorption, electrochemical and magnetic characterization
showed that the oxidation and reduction potentials of the new
compounds all present strong shifts of 0.6–0.8 V with respect to
complexes with more electron rich phthalocyanine rings, which
provides an unprecedented redox stability to this new family of
molecules. Indeed, while the unsubstituted neutral Pc2Tb can be
easily oxidized and reduced, often not on purpose, by the action
of mild chemical agents, the window of stability for the new
reported compounds spans about 1.2 V and is almost centered on
0 V vs. SCE. Despite this strong shift in redox potentials, we
have shown how the magnetic behaviour of the complexes is
intact, as can be seen in both the ac-magnetic susceptibility data
and the hystereses of magnetization. It is therefore expected that
the new system presented in this article will be a good candidate
Acknowledgements
This work has been carried out within the 6th and 7th FPs of the
EU, under contracts MAGMANet (no. 515767), FuMaSSEC
(MEST-CT-2005-020992) and supported by the Spanish projects
EMOCIONA (CTQ2006-06333/BQU), POMAs (CTQ22010-
19501), the Generalitat de Catalunya AGAUR (2009SGR-
00516) and the CIBER de Bioingeniera, Biomateriales y Nano-
medicina (CIBER-BBN), promoted by ISCIII, Spain. We
warmly thank Amable Bernabé (ICMAB-CSIC) for recording
the IR and LDI-TOF spectra and Bernat Bozzo for the PPMS
susceptibility and hysteresis measurements.
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22 D. Gatteschi, A. Cornia, M. Mannini and R. Sessoli, Inorg. Chem., 2009,
48, 3408–3419.
References
23 L. Margheriti, M. Mannini, L. Sorace, L. Gorini, D. Gatteschi,
A. Caneschi, D. Chiappe, R. Moroni, F. B. de Mongeot, A. Cornia,
F. M. Piras, A. Magnani and R. Sessoli, Small, 2009, 5, 1460–1466.
24 M. Mannini, F. Pineider, P. Sainctavit, C. Danieli, E. Otero,
C. Sciancalepore, A. M. Talarico, M. A. Arrio, A. Cornia, D. Gatteschi
and R. Sessoli, Nat. Mater., 2009, 8, 194–197.
25 M. Gonidec, R. Biagi, V. Corradini, F. Moro, V. De Renzi, U. del
Pennino, D. Summa, L. Muccioli, C. Zannoni, D. B. Amabilino and
J. Veciana, J. Am. Chem. Soc., 2011, 133, 6603–6612.
1 D. Gatteschi, R. Sessoli and J. Villain, Molecular Nanomagnets, Oxford
University Press, Oxford, U.K., 2006.
2 R. Sessoli, D. Gatteschi, A. Caneschi and M. A. Novak, Nature, 1993,
365, 141–143.
3 R. Sessoli, H. L. Tsai, A. R. Schake, S. Wang, J. B. Vincent, K. Folting,
D. Gatteschi, G. Christou and D. N. Hendrickson, J. Am. Chem. Soc.,
1993, 115, 1804–1816.
4 J. R. Friedman, M. P. Sarachik, J. Tejada, J. Maciejewski and R. Ziolo,
J. Appl. Phys., 1996, 79, 6031–6033.
26 R. Biagi, J. Fernandez-Rodriguez, M. Gonidec, A. Mirone, V. Corradini,
F. Moro, V. De Renzi, U. del Pennino, J. C. Cezar, D. B. Amabilino and
J. Veciana, Phys. Rev. B: Condens. Matter Mater. Phys., 2010, 82, 224406.
27 M. Mannini, F. Pineider, C. Danieli, F. Totti, L. Sorace, P. Sainctavit,
M. A. Arrio, E. Otero, L. Joly, J. C. Cezar, A. Cornia and R. Sessoli,
Nature, 2010, 468, 417–421.
28 M. Mannini, P. Sainctavit, R. Sessoli, C. Cartier dit Moulin, F. Pineider,
M. A. Arrio, A. Cornia and D. Gatteschi, Chem.–Eur. J., 2008, 14, 7530–
7535.
5 J. R. Friedman, M. P. Sarachik, J. Tejada and R. Ziolo, Phys. Rev. Lett.,
1996, 76, 3830–3833.
6 L. Thomas, F. Lionti, R. Ballou, D. Gatteschi, R. Sessoli and B. Barbara,
Nature, 1996, 383, 145–147.
7 D. Gatteschi and R. Sessoli, Angew. Chem., Int. Ed., 2003, 42, 268–297.
8 J. Tejada, R. F. Ziolo and X. X. Zhang, Chem. Mater., 1996, 8,
1784–1792.
9 N. Ishikawa, M. Sugita and W. Wernsdorfer, Angew. Chem., Int. Ed.,
2005, 44, 2931–2935.
29 N. Ishikawa, M. Sugita, T. Ishikawa, S.-Y. Koshihara and Y. Kaizu,
J. Phys. Chem. B, 2004, 108, 11265–11271.
30 N. Ishikawa, M. Sugita and W. Wernsdorfer, J. Am. Chem. Soc., 2005,
127, 3650–3651.
10 E. K. Brechin, Chem. Commun., 2005, 5141–5153.
11 A. L. Barra, A. Caneschi, A. Cornia, F. Fabrizi de Biani, D. Gatteschi,
C. Sangregorio, R. Sessoli and L. Sorace, J. Am. Chem. Soc., 1999, 121,
5302–5310.
31 N. Ishikawa, Polyhedron, 2007, 26, 2147–2153.
32 P. Zhu, F. Lu, N. Pan, Dennis P. Arnold, S. Zhang and J. Jiang,
Eur. J. Inorg. Chem., 2004, 2004, 510–517.
33 K. M. Kadish, T. Nakanishi, A. Gurek, V. Ahsen and I. Yilmaz, J. Phys.
Chem. B, 2001, 105, 9817–9821.
12 A. J. Tasiopoulos, A. Vinslava, W. Wernsdorfer, K. A. Abboud and
G. Christou, Angew. Chem., Int. Ed., 2004, 43, 2117–2121.
13 C. J. Milios, R. Inglis, R. Bagai, W. Wernsdorfer, A. Collins,
S. Moggach, S. Parsons, S. P. Perlepes, G. Christou and E. K. Brechin,
Chem. Commun., 2007, 3476–3478.
34 H. E. Gottlieb, V. Kotlyar and A. Nudelman, J. Org. Chem., 1997, 62,
7512–7515.
35 T. W. Green and P. G. M. Wuts, Protective Groups in Organic Synthesis,
Wiley-Interscience, New York, 1999.
36 B. Tylleman, R. Gomez-Aspe, G. Gbabode, Y. H. Geerts and
S. Sergeyev, Tetrahedron, 2008, 64, 4155–4161.
14 C. J. Milios, A. Vinslava, W. Wernsdorfer, S. Moggach, S. Parsons,
S. P. Perlepes, G. Christou and E. K. Brechin, J. Am. Chem. Soc., 2007,
129, 2754–2755.
15 A. Cornia, A. C. Fabretti, P. Garrisi, C. Mortalo, D. Bonacchi,
D. Gatteschi, R. Sessoli, L. Sorace, W. Wernsdorfer and A. L. Barra,
Angew. Chem., Int. Ed., 2004, 43, 1136–1139.
37 B. Neises and W. Steglich, Angew. Chem., Int. Ed. Engl., 1978, 17,
522–524.
16 N. Ishikawa, M. Sugita, T. Ishikawa, S.-Y. Koshihara and Y. Kaizu,
J. Am. Chem. Soc., 2003, 125, 8694–8695.
38 A. G. M. Barrett, G. R. Hanson, A. J. P. White, D. J. Williams and
A. S. Micallef, Tetrahedron, 2007, 63, 5244–5250.
17 L. Vitali, S. Fabris, A. M. Conte, S. Brink, M. Ruben, S. Baroni and
K. Kern, Nano Lett., 2008, 8, 3364–3368.
39 A. De Cian, M. Moussavi, J. Fischer and R. Weiss, Inorg. Chem., 1985,
24, 3162–3167.
40 N. Ishikawa, J. Porphyrins Phthalocyanines, 2001, 5, 87–101.
41 M. Gonidec, E. S. Davies, J. McMaster, D. B. Amabilino and J. Veciana,
J. Am. Chem. Soc., 2010, 132, 1756–1757.
18 S. Stepanow, J. Honolka, P. Gambardella, L. Vitali, N. Abdurakhmanova,
T.-C. Tseng, S. Rauschenbach, S. L. Tait, V. Sessi, S. Klyatskaya,
M. Ruben and K. Kern, J. Am. Chem. Soc., 2010, 132, 11900–11901.
19 A. Candini, S. Klyatskaya, M. Ruben, W. Wernsdorfer and M. Affronte,
Nano Lett., 2011, 11, 2634–2639.
42 D. P. Arnold and J. Jiang, J. Phys. Chem. A, 2001, 105, 7525–7533.
43 K. S. Cole and R. H. Cole, J. Chem. Phys., 1941, 9, 341–351.
44 M. Gonidec, F. Luis, À. Vílchez, J. Esquena, David B. Amabilino and
J. Veciana, Angew. Chem., Int. Ed., 2010, 49, 1623–1626.
45 M. Waters, F. Moro, I. Krivokapic, J. McMaster and J. v. Slageren,
Dalton Trans., 2012, 41, 1128–1130.
20 M. Mannini, E. Tancini, L. Sorace, P. Sainctavit, M.-A. Arrio, Y. Qian,
E. Otero, D. Chiappe, L. Margheriti, J. C. Cezar, R. Sessoli and
A. Cornia, Inorg. Chem., 2011, 50, 2911–2917.
21 L. Margheriti, D. Chiappe, M. Mannini, P. Car, P. Sainctavit,
M. A. Arrio, F. B. de Mongeot, J. C. Cezar, F. M. Piras, A. Magnani,
E. Otero, A. Caneschi and R. Sessoli, Adv. Mater., 2010, 22, 5488–5493.
Dalton Trans.
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