Intramolecular exchange interaction in twofold spin-labelled platinum complexes

Article abstract of DOI:10.1039/b407067d

The synthesis and the crystal structures of two platinum bis-ethynyl-phenyl-nitronyl-nitroxide complexes are reported and solution EPR investigations evidenced the intramolecular exchange interaction through a diamagnetic transition metal based spacer.

Full text of DOI:10.1039/b407067d

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Intramolecular exchange interaction in twofold spin-labelled platinum
complexes
Christophe Stroh,a Marcel Mayor,*a Carsten von Ha¨nischa and Philippe Turekb
a Forschungszentrum Karlsruhe GmbH, Institut fu¨r Nanotechnologie, Postfach 3640, 76021 Karlsruhe,
Germany. E-mail: marcel.mayor@int.fzk.de; Fax: 149 7247 82-5685; Tel: 149 7247 82-6392
b Institut Charles Sadron (CNRS, UPR 22), Universite´ Louis Pasteur, 6 rue Boussingault, BP 40016,
F-67083 Strasbourg Cedex, France
Received (in Cambridge, UK) 10th May 2004, Accepted 13th July 2004
First published as an Advance Article on the web 6th August 2004
The synthesis and the crystal structures of two platinum bis-
(605 nm, e ~ 1250 M21 cm21 for 1, 645 nm, e ~ 1180 M21 cm21
for 2) of the blue nitronyl-nitroxide with an extinction coefficient e
in agreement with a diradical.
ethynyl-phenyl-nitronyl-nitroxide complexes are reported and
solution EPR investigations evidenced the intramolecular
exchange interaction through a diamagnetic transition metal
based spacer.
X-ray crystal structures have been obtained for both radicals.
Figures 1 and 2 show the ORTEP views of the two corresponding
molecules, the first one containing two THF solvate molecules in
the unit cell. The nitronyl-nitroxides have bond lengths charac-
teristic of such radicals having an unpaired electron equally shared
Platinum complexes are very attractive molecules for their
electronic1 and photophysical2 properties. However, few spin-
labelled platinum complexes have been reported in the literature3
and mostly pyridine-nitronyl-nitroxide coordinating ligands have
been used.4 Even if the presence of an intramolecular magnetic
interaction in such diradicals has been shown by solution EPR
spectroscopy,5 precise information about the exchange interaction
through a diamagnetic platinum metal is lacking. To gain better
insight into the mechanisms governing the intramolecular com-
munication in such systems with diamagnetic transition metal ions,
we synthesized and investigated two diradicals based on organo-
metallic trans-platinum(II) complexes of ethynyl-phenyl-nitronyl-
nitroxides (Scheme 1). In this communication, we present the
synthesis of the two new terminal spin-labelled molecular rods 1
and 2 and their characterization by several techniques.{ X-ray
crystal structures of the diradicals are described and the EPR
spectra of the fluid and frozen diluted solutions are discussed.
The diradicals 1{ and 2§ have been prepared by reacting two
equivalents of the corresponding meta- or para-ethynyl-phenyl-
nitronyl-nitroxide radicals with trans-bis-(triphenylphosphine)-
dichloro-platinum in the presence of CuI in a mixture of
THF–iPr2NH 10 : 1. The pure products could be obtained by
chromatography on SiO2 in 72% and 97% yield for 1 and 2
respectively and single crystals were grown from a THF–MeCN
solution for 1 and CH2Cl2–hexane for 2.} The starting radicals
were obtained by an adapted synthesis.6
˚
between two equivalent NO groups (NO ~ 1.28–1.30 A, CN ~
8
1.34–1.35 A). The dihedral angle formed by the mean planes of the
˚
radicals and the adjacent phenyl is also typical of nitronyl-
nitroxides (29.4u for 1, 27.5u for 2). Interestingly, the aromatic rings
are twisted from the Pt–P bond by ca. 35.8u for 1 and 30.4u for 2.
˚
˚
The Pt–C (2.00–2.01 A) and CMC (1.20–1.17 A) bond lengths are in
the expected range for such complexes.
The room temperature EPR spectra of ca. 1024 M CH2Cl2
degassed solution of 1 and 2 exhibit a nine line pattern centred at
g $ 2.0066 (aN $ 3.7 G) for 1 and centred at g $ 2.0094 (aN
$
3.7 G) for 2 (Figs. 3 and 4). Such features are characteristic of a bis-
nitronyl-nitroxide where the unpaired spins of the terminal radical
fragments are in exchange (|J| & aN in energy units) for each
complex. The apparent g-shift observed for 2 may result from spin-
orbit coupling through the molecular orbital overlap of the
p-radical with d-orbitals of the Pt ion. These features are consistent
with an electronic communication within the platinum based
spacer. Even if communication over large distances has been
reported,9 the rigid structure of the molecules supports an exchange
interaction through the diamagnetic transition metal atom. The
hyperfine coupling to the 195Pt nucleus or to 31P is not observed.
The MALDI-TOF spectra of both complexes show the
fragmentations corresponding to the successive loss of four
oxygen atoms, a pattern typical of nitronyl-nitroxide diradicals.7
The characteristic nNO vibration band (1360 cm21 for 1 and
1362 cm21 for 2) together with the nCMC (2107 cm21 for 1,
2114 cm21 for 2) can be seen on the IR spectra. The UV-visible
spectra show the absorption band corresponding to the n A p*
Fig. 1 ORTEP drawing of 1.
Fig. 2 ORTEP drawing of 2.
Scheme 1
2 0 5 0
C h e m . C o m m u n . , 2 0 0 4 , 2 0 5 0 – 2 0 5 1
T h i s j o u r n a l i s ß T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

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Ar during 1 to 2 h. The solvent is evaporated or the reaction mixture is
filtered through a short column packed with SiO2 (elution: THF) before
evaporation. The pure metallic-blue products are obtained by chromato-
graphy on SiO2 (CH2Cl2–MeCN).
{ SiO2 (CH2Cl2–MeCN
9 : 1). Yield: 72%. MALDI-TOF (1,8,9-
anthracenetriol): 1232.7 [M]1, 1216.7 [M 2 O]1, 1201.7 [M 2 2O 1
H]1, 1185.7 [M 2 3O 1 H]1, 1169.7 [M 2 4O 1 H]1. IR (KBr pellets):
3438, 3056, 2989, 2107 (nCMC), 1590, 1572, 1481, 1435, 1417, 1387, 1360
(nNO), 1305, 1218, 1166, 1140, 1098, 794, 746, 708, 691, 540, 521, 456. UV-
vis (CH2Cl2) l, nm (e, M21 cm21): 605 (1250), 350 (53840), 268 (69510),
226 (95560). Rf (SiO2, CH2Cl2–MeCN 9 : 1): 0.42. EPR (CH2Cl2, rt): 9
lines, aN ~ 3.7 G, g ~ 2.0066. Anal. calcd for C66H62N4O4P2Pt (Mr ~
1232.25): C, 64.33; H, 5.07; N, 4.55. Found: C, 64.28; H, 4.82; N, 4.23%.
Mp: 207–209 uC (dec).
§ SiO2 (CH2Cl2–MeCN 97 : 3). Yield: 97%. MALDI-TOF (1,8,9-
anthracenetriol): 1217.2 [M 2 O 1 H]1, 1201.2 [M 2 2O 1 H]1,
1186.2 [M 2 3O 1 2H]1, 1170.2 [M 2 4O 1 2H]1. IR (KBr pellets): 3443,
3055, 2982, 2937, 2114 (nCMC), 1598, 1480, 1435, 1418, 1385, 1362 (nNO),
1300, 1219, 1184, 1166, 1132, 1098, 837, 743, 707, 689, 512. UV-vis
(CH2Cl2) l, nm (e, M21 cm21): 645 (1180), 372 (71350), 308 (31760), 225
(77310). Rf (SiO2, CH2Cl2–MeCN 94 : 6): 0.25. EPR (CH2Cl2, rt): 9 lines,
Fig. 3 EPR spectrum of 1 in fluid solution. Inset: DMS ~ ¡2 of the powder
at 4 K.
aN ~ 3.7 G, g ~ 2.0094. Anal. calcd for C66H62N4O4P2Pt (Mr ~ 1232.25):
C, 64.33; H, 5.07; N, 4.55. Found: C, 64.61; H, 4.81; N, 4.24%. Mp: 212–
215 uC (dec).
} Crystal data and structure refinement for 1 and 2: Data were collected at
200 K on a STOE diffractometer with graphite-monochromated Mo-Ka
˚
radiation (l ~ 0.71073 A). The structures were solved by direct methods
and refined by full-matrix least-squares analysis. 1, M ~ 1376.43, triclinic,
Fig. 4 EPR spectrum of 2 in fluid solution.
¯
˚
space group P1, with a ~ 9.752(2), b ~ 12.068(2), c ~ 14.920(3) A, a ~
3
˚
76.54(3), b ~ 89.08(3), c ~ 69.46(3)u, V ~ 1595.0(6) A , Z ~ 1, Dc ~
Frozen CH2Cl2 diluted solution at 4 K showed a ‘‘half-field’’
DMS ~ ¡2 resonance, associated with the existence of a triplet
state, for compound 1. Interestingly, such a pattern could not be
observed for compound 2, presumably due to a slightly larger inter-
spin distance in this compound. The temperature dependence of the
integrated intensity of the DMS ~ ¡1 line shows a pure Curie law
within experimental error.
1.433 Mg m23, m ~ 2.307 mm21. 369 parameters were refined using 5796
unique observed reflections [I w 1s(I)] to give R (all data) ~ 0.0476 and
wR2 (all data) ~ 0.1217. 2, M ~ 1232.23, monoclinic, space group C2/c,
˚
with a ~ 34.963(7), b ~ 9.4928(19), c ~ 19.323(4) A, b ~ 119.33(3)u, V ~
5591.3(19) A , Z ~ 4, Dc ~ 1.464 Mg m23, m ~ 2.620 mm21. 349
3
˚
parameters were refined using 3530 unique observed reflections [I w 1s(I)]
to give R (all data) ~ 0.0452 and wR2 (all data) ~ 0.1018. CCDC 238541
and 238542. See http://www.rsc.org/suppdata/cc/b4/b407067d/ for crystal-
lographic data in .cif or other electronic format.
As a consequence of the shape of the spectra in fluid solution
featuring the limit of strong exchange interaction, i.e. |J| & aN, a
lower limit of the exchange interaction would correspond to ca.
0.1 cm21 for a few hundreds of hyperfine splitting. The absence of
significant departure of the I.T ~ f(T) curve from a constant
behaviour at low temperature may be qualitatively used for the
estimation of the higher limit of |J|. Assuming a singlet–triplet
equilibrium for this two-spin system, a simulation shows no
deviation of the I.T ~ f(T) curve within 10% variation around the
1 (a) M. Mayor, C. von Ha¨nisch, H. B. Weber, J. Reichert and
D. Beckmann, Angew. Chem., Int. Ed., 2002, 41, 1183; (b) T. L. Schull,
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F. Diederich, Chem. Eur. J., 2001, 7, 1333.
high temperature limit down to 4 K when |J| ¡ 1 cm21
.
In conclusion, despite the weak spin density located on the triple
bond carbon atoms of ethynyl-phenyl-nitronyl-nitroxide radicals,
an electronic communication takes place in these two new
complexes. In view of the molecular structure of both compounds,
a reasonable pathway for this exchange interaction involves the d
orbitals of the square planar d8 platinum metal.
3 F. Hintermaier, S. Helding, L. B. Volodarsky, K. Su¨nkel, K. Polborn
and W. Beck, Z. Naturforsch., Teil B, 1998, 53, 101.
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H. Mori, Inorg. Chim. Acta, 2003, 348, 123.
The synthesis of cis-platinum complexes is currently under
investigation and further studies to gain better insight into the
intramolecular exchange interaction will be performed.
We thank the Networkproject MOLMEM, German Ministry of
Education and Research (BMBF-FZK 13 N 8360) for financial
support and the Alexander von Humboldt Foundation for a post-
doctoral fellowship to C. S.
5 (a) K. E. Schwarzhans and A. Stuefer, Z. Naturforsch., Teil B, 1981, 36,
195; (b) K. E. Schwarzhans and A. Stuefer, Monatsh. Chem., 1983, 114,
137.
6 D .G. B. Boocock and E. F. Ullman, J. Am. Chem. Soc., 1968, 90, 5945.
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Notes and references
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{ Synthesis of 1 and 2: trans-PtCl2(PPh3)2 (80 mg scale), ethynyl-(4,4,5,5-
tetramethyl-4,5-dihydro-1H-imidazol-1-yloxyl)-phenyl (ca. 2 equiv.) and
CuI (ca. 10 mol%) in THF (10 ml) and iPr2NH (1 ml) are stirred at rt under
C h e m . C o m m u n . , 2 0 0 4 , 2 0 5 0 – 2 0 5 1
2 0 5 1