Double cyclometallation of bridging 3,6-bis(2-thienyl)-1,2,4,5-tetrazine in a dinuclear mesityl(dimethylsulfoxide)platinum(II) complex: Structure and properties

Article abstract of DOI:10.1016/j.jorganchem.2007.10.045

3,6-Bis(2-thienyl)-1,2,4,5-tetrazine (bttz) reacts with trans-Pt(dmso)₂(mes)₂, mes = mesityl = 2,4,6-trimethylphenyl, under twofold cyclometallation to yield structurally characterized (μ-bttz-2H⁺)[Pt(dmso)(mes)]₂ with uncoordinated thiophene sulfur atoms and bttz deprotonated in the 3,3′ positions. The structural features include cis-positioned carbanionic ligands, twisted mesityl substituents, S-coordinated dmso ligands with the S{double bond, long}O bonds lying in the molecular plane, shortened inter-ring bonds, and rather short Pt-C bonds at 1.998(9)/2.00(1) A? (Pt-C_mes) and 1.985(9)/1.99(1) A? (Pt-C_bttz-2H+). Reversible reduction to {(μ-bttz-2H⁺)[Pt(dmso)(mes)]₂}^{{radical dot}-} causes a high-energy shift of the charge transfer bands and the appearance of an unresolved EPR signal at g = 1.9905.

Full text of DOI:10.1016/j.jorganchem.2007.10.045

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Journal of Organometallic Chemistry 693 (2008) 1703–1706
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Note
Double cyclometallation of bridging
3,6-bis(2-thienyl)-1,2,4,5-tetrazine in a dinuclear
mesityl(dimethylsulfoxide)platinum(II) complex:
Structure and properties
Biprajit Sarkar a, Thilo Schurr a, Ingo Hartenbach a, Thomas Schleid a,
Jan Fiedler b, Wolfgang Kaim a,*
a Institut fu¨r Anorganische Chemie, Universita¨t Stuttgart, Pfaffenwaldring 55, D-70550 Stuttgart, Germany
b J. Heyrovsky´ Institute of Physical Chemistry, v.v.i., Academy of Sciences of the Czech Republic, Dolejsˇkova 3, CZ-18223 Prague, Czech Republic
Received 28 September 2007; received in revised form 24 October 2007; accepted 24 October 2007
Available online 1 November 2007
Abstract
3,6-Bis(2-thienyl)-1,2,4,5-tetrazine (bttz) reacts with trans-Pt(dmso)2(mes)2, mes = mesityl = 2,4,6-trimethylphenyl, under twofold
cyclometallation to yield structurally characterized (l-bttz-2H+)[Pt(dmso)(mes)]2 with uncoordinated thiophene sulfur atoms and bttz
deprotonated in the 3,30 positions. The structural features include cis-positioned carbanionic ligands, twisted mesityl substituents, S-
coordinated dmso ligands with the S@O bonds lying in the molecular plane, shortened inter-ring bonds, and rather short Pt–C bonds
at 1.998(9)/2.00(1) A (Pt–Cmes) and 1.985(9)/1.99(1) A (Pt–Cbttz-2H+). Reversible reduction to {(l-bttz-2H+)[Pt(dmso)(mes)]2}ꢀꢀ causes a
high-energy shift of the charge transfer bands and the appearance of an unresolved EPR signal at g = 1.9905.
Ó 2007 Elsevier B.V. All rights reserved.
˚
˚
Keywords: Crystal structure; Cyclometallation; EPR spectroscopy; Heterocyclic ligand; Platinum complex
1. Introduction
with electron rich metals coordinated via the N lone pairs
[4] it is also possible to observe intense metal-to-ligand
charge transfer (MLCT, d-p*) transitions at rather long
wavelengths. Most metal complexes of tetrazines have
involved derivatives with coordinating groups in 3,6-posi-
tion [4]. Among the potentially bis-chelating tetrazine accep-
tor ligands the 3,6-bis(2-thienyl)-1,2,4,5-tetrazine (bttz) [5–7]
has occupied a special position (Scheme 1). The dinuclear
redox systems {(l-bttz)[M(bpy)2]2}n+, M = Ru or Os, with
unusually different features of the mixed-valent intermedi-
ates were initially assumed to involve S-bonded thienyl rings
[5] but have now been identified as bis-cyclometallated
species [7] following C–H activation. On the other hand,
[Ru(acac)2(CH3CN)2] reacts with bttz under reductive
ring-opening of the tetrazine to yield a complex {(l-dih-
Th)[Ru(acac)2]2} with dih-Th2ꢀ = 1,2-bis(2-thienylimino)-
hydrazido(2-), i.e. with uncoordinated thiophene [8].
The cumulation of four electronegative nitrogen atoms in
a benzenoid six-membered ring to form 1,2,4,5-tetrazines
has a number of remarkable consequences. In addition to
their high nitrogen content [1] 1,2,4,5-tetrazines are distin-
guished by a very low-lying p* molecular orbital centered
on the N atoms which is responsible for facile reduction
(electron transfer and hydrogenation) [2] and low-energy
electronic transitions [3], observable in absorption and emis-
sion [3]. Typically, the low-energy transitions of 1,2,4,5-tetr-
azines are of internal p–p* or n-p* character [3], however,
*
Corresponding author. Tel.: +49 711 685 4170/71; fax: +49 711 685
4165.
E-mail address: kaim@iac.uni-stuttgart.de (W. Kaim).
0022-328X/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2007.10.045

1704
B. Sarkar et al. / Journal of Organometallic Chemistry 693 (2008) 1703–1706
S
N N
N N
S
3,6-bis(2-thienyl)-1,2,4,5-tetrazine
(bttz)
Scheme 1.
In the course of further studies on compounds
between bttz and platinum metal containing complex
fragments we have now been able to obtain a doubly
cyclometallated compound for structural analysis and
electron transfer studies. The precursor compound
[Pt(dmso)2(mes)2] which exists in a trans configuration
[9] and not in a cis arrangement as earlier believed [10]
reacts with bttz to form the dinuclear bis-cyclometallated
species (l-bttz-2H+)- [Pt(dmso)(mes)]2 as evident from a
crystal structure determination and from spectroscopy
(NMR, EPR of the reduced form), cyclic voltammetry
and spectroelectrochemistry.
Fig. 1. Molecular structure of {(l-bttz-2H+)[Pt(dmso)(mes)]2} in the
crystal.
Whereas mononuclear cyclometallated platinum(II)
complexes are well known [11], not in the least due to their
photophysical properties [12], there are but a few reports of
dinuclear such compounds [13] in which the potential for p
conjugatively mediated metal–metal interaction exists.
Dinuclear platinum(II) compounds have been discussed
in connection with cytostatic behaviour [14], and organo-
metallic compounds containing conjugated bridging
ligands were investigated with respect to reduction and oxi-
dation, establishing radical and mixed-valent intermediates
[15]. Considering the stabilization of the Pt(III) state by
two mesityl substituents [16] and the coordination ambiva-
lence of the bttz ligand [5,7] we have reacted these compo-
nents in order to probe the coordination behaviour of that
ligand.
1
and H NMR spectrum clearly point to the composition
indicated, the C–H activation of thienyl rings to effect
cyclometallation is a known kind of reactivity [11–13].
Crystallization of the greenish-brown compound gave
single crystals of suitable quality for X-ray diffraction.
Table 1 contains the bond parameters and Fig. 1 shows a
representation of the molecule in the crystal.
The crystal structure analysis confirms the binding of
both equivalent platinum(II) centers to the 3 or 30 positions
of the thienyl rings, leaving the sulfur atoms uncoordi-
nated. Whereas the doubly deprotonated bttz bridge and
the C,C,N,S donor sets around the platinum centers are
largely coplanar, the mesityl ligand is almost orthogonally
positioned, reflecting the steric repulsion of the ortho-
methyl substituents [16]. The dmso ligand is bound through
the sulfur atom in such a way that the S@O bond comes to
lie within the main molecular plane [17]. Expectedly, the
carbanionic ligands at each platinum center are oriented
in cis position, the Pt–C bond lengths are rather similar
2. Results and discussion
Reaction of bttz with 2 equiv. of Pt(dmso)2(mes)2 for 5
days in refluxing toluene produced (l-bttz-2H+)-
[Pt(dmso)(mes)]2 in 28% isolated yield. In addition to ele-
mental and crystal structure analysis (see below) the IR
˚
and relatively short at about 1.99 A (Table 1).
The bond parameters within the heterocycles of (l-bttz-
2H+)[Pt(dmso)(mes)]2 can be compared with those of free
bttz [5c,7]: The main differences are the shortened inter-ring
distance (1.448(3) A ? 1.406(13) A) and the diminished
intra-chelate angle N1–C1–C2 (117.9(2)° ? 114.3(8)°).
Whereas the latter is caused by the formation of the short
C3–Pt bond, the increased inter-ring bond order reflects the
charge shift from the deprotonated thienyl rings to the
strongly [4a] electron accepting tetrazine.
Electron acceptance is also obvious from electrochemi-
cal experiments: The reversible reduction of the diplati-
num(II) complex as determined by cyclic voltammetry
occurs at ꢀ0.76 V vs. ferrocenium/ferrocene, i.e. at
distinctly less negative values than the reduction of the free
bttz ligand at ꢀ1.26 V [5,7]. This result, observed in spite of
the double deprotonation of the bridging ligand, confirms
˚
˚
Table 1
Selected bond lengths (Aꢀ) and angles (°) for {(l-bttz-2H+)[Pt(dmso)-
(mes)]2}
Bond lengths
Pt1–N1
Pt1–C3
Pt1–C6
Pt1–S2
2.091(7)
1.985(9)
1.998(9)
2.274(3)
C1–C2
1.406(13)
1.316(10)
1.363(11)
1.362(11)
N1–N2
C1–N1
C1–N2A
Bond angles
C3–Pt1–C6
C3–Pt1–N1
C3–Pt1–S2
C6–Pt1–S2
C6–Pt1–N1
91.1(4)
80.3(3)
176.7(3)
91.0(3)
S2–Pt1–N1
C2–C3–Pt1
C3–C2–C1
C2–C1–N1
C1–N1–Pt1
97.6(2)
113.5(7)
118.5(8)
114.3(8)
113.4(6)
171.4(3)

B. Sarkar et al. / Journal of Organometallic Chemistry 693 (2008) 1703–1706
1705
the efficient compensation of that negative charge by PtII–
C bonding, leaving even sufficient acceptor strength at the
metal to withdraw electron density from the ligand. The
second reduction at ꢀ2.15 V and the oxidation at about
+0.56 V were found irreversible in the cyclic voltammetry
experiment.
EPR spectroscopy of the one-electron reduced form
showed an unresolved broad line at g = 1.9905 which did
not exhibit any g component splitting in frozen solution
(4 K) under X-band conditions (9.5 GHz). In the absence
of detectable 195Pt satellite lines the peak-to-peak linewidth
of about 5 mT suggests an upper limit of 5 mT for the
metal isotope hyperfine coupling which, together with the
small g anisotropy and the isotropic g close to the free elec-
tron value of 2.0023, indicates [18] very little participation
of the heavy metals at the singly occupied MO. We attri-
bute this comparatively [15,16,19] small metal contribution
to the carbanion coordination of PtII while the spin is prob-
ably localized in the tetrazine ring [4a]; formally, the bridge
functions as a radical trianion in that state.
Concluding, we have described a dinuclear complex
(l-bttz-2H+)[Pt(dmso)(mes)]2 in which 2 equiv. diorgano-
platinum(II) centers are connected by a potentially p
conjugated bridge, the 3,30 bis-deprotonated and bis-cyclo-
metallated bttz ligand. The compound with two carbanion-
ic ligands in cis position at each metal center exhibits
reversible reduction to a radical anion complex, formally
containing a radical trianion bridge. EPR and UV/Vis
spectroelectrochemical studies suggest very little metal
participation at the singly occupied MO. As preliminary
studies have indicated, the substitution of the dmso ligand
by nitrogen bases is possible and may be exploited to form
other dinuclear compounds or extended systems (coordina-
tion polymers) (Scheme 1).
3. Experimental
3.1. Instrumentation
EPR spectra in the X-band were recorded with a Bruker
System ESP 300 equipped with a HP frequency counter
5350B, a Bruker ER035M gaussmeter for g-value determi-
nation and a continuous flow cryostat ESR 900 of Oxford
Instruments for measurements at liquid helium tempera-
tures (4 K). 1H NMR spectra were taken on a Bruker
AC 250 spectrometer. IR spectra were obtained using Per-
kin Elmer FTIR 684 and 283 instruments. UV–Vis–NIR
absorption spectra were recorded on J&M TIDAS and Shi-
madzu UV 3101 PC spectrophotometers. Cyclic voltamme-
try was carried out in 0.1 M Bu4NPF6 solutions using a
three-electrode configuration (glassy carbon working elec-
trode, Pt counter electrode, Ag/AgCl reference) and a
PAR 273 potentiostat and function generator. The ferro-
cene/ferrocenium (Fc/Fc+) couple served as internal refer-
ence. Spectroelectrochemistry was performed using an
optically transparent thin-layer electrode (OTTLE) cell. A
two-electrode capillary served to generate intermediates
for X-band EPR studies.
Monitoring the reversible one-electron reduction by
spectroelectrochemistry in the UV–Vis region reveals a
decrease of the long-wavelength charge transfer bands at
560(sh), 434(9550), 395(8970), 340(sh) nm (molar extinc-
tion coefficients in Mꢀ1 cmꢀ1) while new bands emerge at
381(8600), 363(8800), 308(6800) nm (Fig. 2).
The decrease of the absorption bands in the visible fol-
lows from the partial occupation of the low-lying tetr-
azine-centered p* LUMO during reduction and has been
similarly observed for metal-to-ligand charge transfer
(MLCT) bands of reduced dinuclear tetrazine complexes
[4a,15b]. Further transitions including ligand-to-ligand
charge transfer are conceivable, however, a detailed assign-
ment is precluded at this stage by the low symmetry of the
chromophor. At ambient temperatures the neutral precur-
sor compound does not emit as a solid or in solution, most
likely due to energy dissipation through rotation and vibra-
tion of the non-chelate ligands dmso and mes [16].
3.2. Synthesis of (l-bttz-2H+)[Pt(dmso)(mes)]2
A
mixture
containing
307 mg
(0.520 mmol)
Pt(dmso)2(mes)2 and 64 mg (0.260 mmol) bttz in 110 ml
toluene was heated to reflux in toluene for 5 days. The solu-
tion changed its colour from orange-red to dark green
during this time. After removal of the solvent, the product
was recrystallised from dichloromethane/heptane (1/3).
Yield: 150 mg (0.146 mmol, 28%). Anal. Calc. for
C32H38N4O2Pt2S4 (1039.08 g/mol): C, 37.35; H, 3.72; N,
5.44. Found: C, 36.83; H, 3.55; N, 5.02%. 1H NMR
(CD2Cl2): d = 2.28 (s, 6H, p-CH3, mes), 2.41 (s, 12H,
o-CH3, mes, 4J (Pt–H) = 6.41 Hz), 3.08 (s, 12H, CH3,
dmso, 3J (Pt–H) = 13.67 Hz), 6.42 (d, 2H, 40 (bttz),
3J (H–H) = 4.8 Hz, 3J (Pt–H) = 28.6 Hz), 6.79 (s, 4H,
4
C–H, mes, J (Pt–H) = 14.7 Hz), 7.58 (d, 2H, 50 (bttz),
Fig. 2. Spectroelectrochemical reduction of {(l-bttz-2H+)[Pt(dmso)-
(mes)]2} ? {(l-bttz-2H+)[Pt(dmso)(mes)]2}ꢀꢀ in CH2Cl2/0.1 M Bu4NPF6.
3J (H–H) = 4.6 Hz, J (Pt–H) = 19 Hz). UV/Vis (CH2Cl2):
4

1706
B. Sarkar et al. / Journal of Organometallic Chemistry 693 (2008) 1703–1706
(c) I. Janowska, F. Miomandre, G. Clavier, P. Audebert, J.
k
max/nm (e/Mꢀ1 cmꢀ1) = 565(1.99), 434(9.55), 395(8.97),
Zakrzewski, K.H. Thi, I. Ledoux-Rak, J. Phys. Chem. A 110 (2006)
12971.
341(6.9).
[4] (a) W. Kaim, Coord. Chem. Rev. 230 (2002) 127;
(b) S. Kohlmann, S. Ernst, W. Kaim, Angew. Chem., Int. Ed. 97
(1985) 698;
3.3. Crystallography
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(1985) 684.
Crystallographic data and refinement parameters of (l-
bttz-2H+)[Pt(dmso)(mes)]2: green needles, 0.2 ꢁ 0.1 ꢁ
[5] (a) B. Sarkar, W. Kaim, A. Klein, B. Schwederski, J. Fiedler, C.
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0.05 mm, C32H38N4O2Pt2S4, M = 1029.08 g molꢀ1, space
˚
a = 15.9138(5) A, b =
group
P21/c, monoclinic,
˚
˚
11.4844(3) A, c = 11.9364(3) A, b = 124.68(10)°, V =
3
˚
1793.81(9) A , Z = 2,
q
calc = 1.905 g cmꢀ3
,
l
(Mo
Ka) = 0.71073 A, absorption coefficient 8.057 mmꢀ1
,
`
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restraints, Rint/Rr = 0.0774/0.0303, GOF/F2 = 1.144,
R1 = 0.0676 and wR2 = 0.1254 for all data, R1 = 0.0577
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and wR2 = 0.1204 foꢀr3I > 2r(I), largest residual densities
˚
ꢀ1.083 and 1.514 e A
.
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CCDC 660655 contains the supplementary crystallo-
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e-mailing data_request@ccdc.cam.ac.uk, or by contacting
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