Platinum(II) imidazo[4,5-f]-1,10-phenanthroline chloride and thiolate complexes: Synthesis and crystal structures

Article abstract of DOI:10.1016/j.ica.2006.08.007

The planar aromatic imidazo[4,5-f]-1,10-phenanthroline ligands have been used to prepare platinum(II) chloride and thiolate complexes. The X-ray structures of two thiolate compounds are reported, which show column-like packing in the solid state due to intermolecular aromatic π-π interactions. The compounds absorb moderately in the visible region, owing to {charge-transfer-to-diimine} electronic transition.

Full text of DOI:10.1016/j.ica.2006.08.007

Inorganica Chimica Acta 360 (2007) 700–704
www.elsevier.com/locate/ica
Note
Platinum(II) imidazo[4,5-f]-1,10-phenanthroline chloride and
thiolate complexes: Synthesis and crystal structures
*
Nail M. Shavaleev, Harry Adams, Julia A. Weinstein
Department of Chemistry, University of Sheffield, Sheffield S3 7HF, England, United Kingdom
Received 2 July 2006; received in revised form 27 July 2006; accepted 3 August 2006
Available online 18 August 2006
Abstract
The planar aromatic imidazo[4,5-f]-1,10-phenanthroline ligands have been used to prepare platinum(II) chloride and thiolate com-
plexes. The X-ray structures of two thiolate compounds are reported, which show column-like packing in the solid state due to intermo-
lecular aromatic p–p interactions. The compounds absorb moderately in the visible region, owing to {charge-transfer-to-diimine}
electronic transition.
ꢀ 2006 Elsevier B.V. All rights reserved.
Keywords: Platinum; Imidazo[4,5-f]-1,10-phenanthroline; Thiolate; Coordination compounds; X-ray crystal structures; Charge-transfer; Complex
1. Introduction
hyde in the presence of ammonium acetate in acetic acid
[12] (Scheme 1). Four new ligands have been prepared,
which contained either alkyl groups (imP–tBu, L-tBu) to
improve solubility, or extended aromatic chromophore
groups (L-Ph, L-Nap) to potentially modify spectroscopic
properties (Scheme 2).
The ligands were refluxed with Pt(DMSO)2Cl2 [13] in
alcohols to give Pt(imP)Cl2 complexes that were then used
as precursors to other Pt(imP) compounds.
Reaction of Pt(imP)Cl2 with thiophenols in methanol in
the presence of a base gave thiolates Pt(imP)(S-RS)2 as col-
oured precipitates that were purified by column chroma-
tography (Schemes 1 and 2). Sonication was used to
increase the rate of this reaction and drive it to completion
because both the precursor and the product Pt(II) com-
plexes were only sparingly soluble in methanol.
Platinum complexes with the diimine ligands are widely
used as optical materials [1], DNA intercalators [2] and solar
cell dyes [3]. We would like to introduce imidazo[4,5-f]-1,10-
phenanthroline ligands (imP) to Pt(II) coordination chemis-
try, and illustrate it by synthesis of Pt(imP) chloride and
thiolate complexes. The imP ligands are easy to prepare
and modify and therefore have recently gained a lot of inter-
est with respect to synthesis of novel metal compounds [4–
11], amenable to the various applications stated above. In
particular, the potential of flat extended aromatic system
of imP ligands as a DNA intercalating unit has been
exploited [4–6]. In order to increase solubility, and demon-
strate synthetic versatility of the imP ligands, four new rep-
resentatives of this class of diimines have also been prepared.
All new compounds were identified by CHN data and
1H NMR. In the 1H NMR spectra, the NH proton of
imP ligand was observed as either a broad or a sharp sin-
glet, indicating respectively fast or slow exchange of a pro-
ton between the two nitrogens of the imidazole ring. The
fast exchange of a proton resulted in the apparent C2-sym-
2. Results and discussion
The imidazo[4,5-f]-1,10-phenanthrolines were prepared
by a reaction of 1,10-phenanthroline-5,6-dione with alde-
1
*
metric structure of some ligands and complexes in the H
Corresponding author. Tel.: +44 114 222 9408. fax: +44 114 222 9346.
E-mail address: julia.weinstein@sheffield.ac.uk (J.A. Weinstein).
NMR spectra in solutions.
0020-1693/$ - see front matter ꢀ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2006.08.007

N.M. Shavaleev et al. / Inorganica Chimica Acta 360 (2007) 700–704
701
The thiolate complexes Pt(imP)(S-RS)2 were obtained as
red or brown solids. Their colour was determined by the
broad and structureless absorption band in the visible
region, centred at 503–526 nm, with the values of extinc-
tion coefficients of (2.7–4.3) · 103 Mꢀ1 cmꢀ1 (Table 1), typ-
RN
N
NH
O
N
O
N
a
ical for
a charge-transfer transition. The negative
solvatochromic behaviour (e.g., a shift of the absorption
maximum from 498 nm in CH3CN to 519 nm in CH2Cl2
for Pt(imP–tBu)(S-Ph–tBu)2), typical for Pt(diimine)(thio-
late)2 complexes [1], also indicates the charge-transfer nat-
ure of the corresponding electronic transition. The energy
of the lowest absorption band was red-shifted with the
increase of the electron donating ability of thiophenols in
the order S-MetBu > S-tBu > S-CO2Me, suggesting that
the HOMO is largely localised on the thiolate part of the
molecule. This assignment is in agreement with the absence
of absorption bands at wavelengths >450 nm in the
Pt(imP)Cl2 precursors. At the same time, the energy of
the lowest absorption band of Pt(imP)(S-RS)2 was not very
sensitive to the RN group of imP ligand, and remains sim-
ilar to that of Pt(1,10-phenanthroline)(S-RS)2 [1c]. These
observations tentatively suggest that the corresponding
electronic transition is mainly localised on the [phenanthro-
line–Pt–S-RS] core with a rather limited RN group partici-
pation. The lowest absorption band has therefore been
assigned to a mainly thiolate-to-diimine charge-transfer
transition.
N
N
imP
b
RN
RN
N
NH
N
NH
c
N
N
N
S
N
S
Pt
Pt
Cl
Cl
Pt(imP)Cl2
RS
RS
Pt(imP)(S-RS)2
Scheme 1. Synthesis of imP ligands and Pt2+ complexes: (a) aldehyde
RNCHO (small excess), ammonium acetate (excess), acetic acid, N2, reflux,
3 h; (b) PtCl2(DMSO)2, ethanol, N2, reflux, 24 h; (c) thiophenol HS-RS
and base (both in small excess), methanol, N2, sonication, rt, 24 h.
The compounds were non-luminescent in degassed
CH2Cl2 solutions at rt.
Crystal structures of Pt(L-NMe2)(S-tBu)2 and Pt(L-
NO2)(S-MetBu)2 complexes are shown in Figs. 1 and 2.
The structural features are similar for both complexes.
The Pt(II) is in a distorted square planar coordination envi-
ronment. A particular distortion towards a tetrahedral
coordination is observed for Pt(L-NO2)(S-MetBu)2 with a
dihedral angle of 7.32ꢁ between the planes defined by
N(1) Pt(1) N(2) and S(1) Pt(1) S(2) atoms (Fig. 2).
The imP ligands are planar in both structures. The two
thiol ligands are situated above the imP plane resulting in
an overall L-shaped molecule. The o-hydrogen atoms of
thiol ligands are located above the coordination plane
of Pt(II). In the Pt(L-NO2)(S-MetBu)2 structure such
R
N
NH
N
N
NH
N
N
NH
N
N
N
N
Table 1
imP-tBu
Absorption spectra of the Pt(imP)(S-RS)2 complexes
L-R
L-Nap
k )
max/nm (10ꢀ3e/Mꢀ1 cmꢀ1
R = H, tBu, OMe,
Complex
Pt(imP-tBu)(S-CO2Me)2
Pt(imP-tBu)(S-tBu)2
503 (2.9), 364 (11), 295 (38), 257 (68)
519 (2.7), 402 (5.7), 386 (6.2),
288 (43), 258 (81)
523 (3.4), 394 (6.4), 279 (72)
510 (2.9), 394 (6.4), 283 (75)
510 (3.7), 285 (62)
NMe2, NO2, Ph
O
SH
SH
SH
Pt(L-H)(S-MetBu)2
Pt(L-tBu)(S-tBu)2
OCH3
Pt(L-tBu)(S-CO2Me)2
Pt(L-OMe)(S-MetBu)2
Pt(L-NMe2)(S-tBu)2
Pt(L-NMe2)(S-CO2Me)2
Pt(L-NO2)(S-MetBu)2
Pt(L-Ph)(S-tBu)2
526 (2.8), 285 (62)
507 (3.4), 349 (42), 287 (53), 257 (43)
505 (4.3), 353 (46), 290 (48)
520 (3.5), 358 (30), 288 (42), 260 (55)
515 (2.9), 287 (67)
S-tBu
S-MetBu
S-CO2Me
Pt(L-Nap)(S-tBu)2
512 (3.1), 390 (7.8, sh), 274 (85)
Scheme 2. imP Ligands and thiophenols used in the study.
In CH2Cl2 or CH2Cl2 + 0.5% CH3OH.

702
N.M. Shavaleev et al. / Inorganica Chimica Acta 360 (2007) 700–704
Fig. 1. Molecular structure of Pt(L-NMe2)(S-tBu)2 (top-view) and pack-
ing of molecules into column of dimers (side-view) (50% probability
˚
ellipsoids, H atoms omitted). Selected bond lengths (A) and angles (ꢁ):
Fig. 2. Molecular structure of Pt(L-NO2)(S-MetBu)2 (top-view) and
packing of molecules into column of dimers (side-view) (50% probability
ellipsoids, H atoms and co-crystallised molecule of DMSO omitted).
Pt(1)–N(1) 2.062(5), Pt(1)–N(2) 2.066(5), Pt(1)–S(1) 2.2945(15), Pt(1)–S(2)
2.2822(16), N(1)–Pt(1)–N(2) 79.55(19), N(1)–Pt(1)–S(2) 95.44(15), N(2)–
Pt(1)–S(1) 94.82(14), S(2)–Pt(1)–S(1) 90.03(6), N(1)–Pt(1)–S(1) 173.94(14),
N(2)–Pt(1)–S(2) 173.86(14).
˚
Selected bond lengths (A) and angles (ꢁ): Pt(1)–N(1) 2.080(4), Pt(1)–N(2)
2.068(4), Pt(1)–S(1) 2.2996(14), Pt(1)–S(2) 2.2915(16), N(1)–Pt(1)–N(2)
79.71(16), N(1)–Pt(1)–S(2) 96.75(12), N(2)–Pt(1)–S(1) 96.00(12), S(2)–
Pt(1)–S(1) 87.81(6), N(1)–Pt(1)–S(1) 172.05(12), N(2)–Pt(1)–S(2)
175.61(13).
arrangement results in a short intramolecular Ptꢁ ꢁ ꢁH–C
˚
contact (<2.9 A) [14] with the Pt(1)ꢁ ꢁ ꢁH(21) and
˚
˚
˚
Pt(1)ꢁ ꢁ ꢁH(36) distances of 2.772 A and 2.838 A respec-
tively and with the Ptꢁ ꢁ ꢁH–C angles of ca. 122ꢁ.
The shortest Ptꢁ ꢁ ꢁPt distances are 8.035 A for Pt(L-
t
t
˚
NMe2)(S- Bu)2 and 8.923 A for Pt(L-NO2)(S-Me Bu)2 indi-
cating absence of metal–metal interaction.
The intermolecular aromatic p–p interactions of imP
ligands result in the packing of their Pt(II) complexes into
dimers (Figs. 1 and 2). The thiol ligands are pointing out-
wards from the centre of the dimer, thus allowing a closer
approach and a better overlap between aromatic rings of
imP ligands. The dimers are further packed into columns
running along the a-axis. Each molecule within the column
is related to the next one by a centre of inversion. The
intra- and inter-dimer distances between imP ligands
The co-crystallised molecule of DMSO in the structure
of Pt(L-NO2)(S-MetBu)2 forms a hydrogen bond through
sulfoxide oxygen with NH proton of imidazole ring with
˚
Oꢁ ꢁ ꢁH distance 1.895 A and Oꢁ ꢁ ꢁHN angle 165.28ꢁ.
In conclusion, this contribution has exploited syntheti-
cally versatile imidazo[4,5-f]-1,10-phenanthroline, imP,
ligands to form a new series of stable Pt(II) chloride and
thiolate complexes. The compounds absorb moderately in
the visible region, owing to {charge-transfer-to-diimine}
electronic transition. The X-ray crystallography has shown
column-like packing in the solid state due to the intermo-
lecular aromatic p–p interactions between the neighbouring
imP ligands.
˚
˚
within the column are 3.364 A and 3.453 A for Pt(L-
NMe2)(S-tBu)2 and 3.360 A and 3.374 A for Pt(L-
NO2)(S-MetBu)2 (the distances were measured between
the planes defined by all non-hydrogen atoms of imP ligand
except the NO2 or N(CH3)2group).
˚
˚

N.M. Shavaleev et al. / Inorganica Chimica Acta 360 (2007) 700–704
703
The extended planar aromatic system of the imP ligands
renders their Pt(II) complexes potential intercalators into
DNA. The notable lack of influence of the RN substituent
on the imP ligand on the electronic absorption spectra of
these chromophores opens up future possibilities for alter-
ing their molecular properties, such as, for example, solubil-
ity, charge, or functionalisation required for incorporation
into polynuclear assemblies and in solar cells, without alter-
ing absorption characteristics. These compounds thus offer
a valuable alternative to the 2,20-bipyridyl or 1,10-phenan-
throline ligands that are widely used in Pt(II) chemistry.
red, green or brown suspension of the product. The solid
was filtered and washed with CH3OH (10 ml). Purification
by column chromatography (SiO2, CH3OH:CH2Cl2, 1:99–
3:97) gave main coloured fraction that was reduced to 5 ml.
Addition of hexane precipitated the product that was fil-
tered, washed with hexane or a mixture of hexane and ether
(50:50) and dried in vacuum. The yields were in the range
30–70%. The complexes were obtained as dark red or
brown solids that were insoluble in alkanes and alcohols,
soluble to various extents in CH2Cl2, and highly soluble
in DMF, DMSO and in the mixtures of CH2Cl2 with
alcohols.
3. Experimental
3.4. X-ray crystallography
All reactions were carried out in the dark under N2 using
general grade solvents that were degassed by bubbling N2
for 10 min. Purification, crystal growth and handling of
all compounds were carried out under air with a minimum
exposure to light. All products were air- and moisture-
stable solids which were stored in the dark when not used.
Single crystals were obtained by vapour diffusion of
diethyl ether to DMF [Pt(L-NMe2)(S-tBu)2] or DMSO
[Pt(L-NO2)(S-MetBu)2] solutions of the complexes.
Data were collected on a Bruker Smart CCD area detec-
tor with Oxford Cryosystems low temperature system.
Reflections were measured from a hemisphere of data col-
lected of frames each covering 0.3ꢁ in omega. All reflections
were corrected for Lorentz and polarization effects and for
absorption by semi empirical methods based on symmetry-
equivalent and repeated reflections. The structure was
solved by direct methods and refined by full matrix least
squares methods on F2. Complex scattering factors were
taken from the program package SHELXTL (An integrated
system for solving and refining crystal structures from dif-
fraction data, Revision 5.1, Bruker AXS LTD) as imple-
mented on the IBM PC. Hydrogen atoms were placed
geometrically and refined with a riding model (including
torsional freedom for methyl groups) and with Uiso con-
strained to be 1.2 (1.5 for methyl groups) times Ueq of
the carrier atom. Data in common: T = 150(2) K,
3.1. Synthesis of imP ligands
1,10-Phenathroline-5,6-dione, the corresponding alde-
hyde (small excess) and ammonium acetate (excess) were
refluxed in degassed acetic acid (10 ml) under N2 for 3 h.
The reaction mixture was diluted with water (20 ml) and
neutralized with NH4OH to give a precipitate of the prod-
uct which was filtered, washed with dilute NH4OH (pH 8),
water and ether and dried in vacuum. The compounds were
often obtained as solvates with water and acetic acid. The
1
amount of acetic acid was estimated by H NMR. The
ligands were soluble in alcohols, DMSO and mixtures of
CH2Cl2 with alcohols but were insoluble in ether, hexane
and CH2Cl2. The ligands L-H, L-NO2, L-OMe and L-
NMe2 were known from previous studies [4]. The ligands
imP-tBu, L-tBu, L-Ph and L-Nap are new.
˚
k = 0.71073 A.
Pt(L-NMe2)(S-tBu)2. CCDC Ref. 607251; C41H43N5-
PtS2; MW = 865.01; red plates; size (mm): 0.32 ·
0.12 · 0.10; monoclinic; space group P21/c (C52 h, No.
3.2. Synthesis of complexes Pt(imP)Cl2
˚
14); a/b/c (A) = 8.0354(6)/23.8089(16)/21.3169(14); a/b/c
3
˚
PtCl2(DMSO)2 [13] and imP in 1:1 molar ratio (0.3–
0.6 mmol scale) were refluxed in degassed ethanol (30 ml)
under N2 for 24 h to give a precipitate of the product that
was filtered, washed with methanol and ether and dried in a
vacuum. The yields were in the range 80–90%. The prod-
ucts were of yellow colour except for the deep red coloured
Pt(L-NMe2)Cl2. The complexes Pt(L-tBu)Cl2 and Pt(L-
NMe2)Cl2 were soluble in DMSO. Other complexes were
insoluble in all common solvents.
(ꢁ) = 90/95.276(3)/90; V = 4060.9(5) A ; Z = 4; qcalc
=
1.415 g cmꢀ3; l = 3.591 mmꢀ1; F(000) = 1736; h range
(ꢁ) = 1.29–29.05; reflections collected/Rint/data/restraints/
parameters = 52304/0.0726/10698/323/443; goodness-of-
fit on F2 = 1.050; R(I > 2r(I)): R1 = 0.0487, wR2 = 0.1199;
R
(all data): R1 = 0.0746, wR2 = 0.1288; Dqmin/max
(e Aꢀ3) = ꢀ1.629/1.807.
˚
Pt(L-NO2)(S-MetBu)2. CCDC Ref. 607252; C41H41N5-
O2PtS2 Æ (C2H6OS); MW = 973.14; red blocks; size (mm):
0.32 · 0.16 · 0.12; monoclinic; space group P21/n (a non-
standard setting of P21/c C52 h, No. 14); a/b/c
3.3. Synthesis of complexes Pt(imP)(thiolate)2
˚
(A) = 12.4893(18)/20.476(3)/17.300(2); a/b/c (ꢁ) = 90/
tBuOK or triethylamine (0.31 mmol) and thiol
(0.31 mmol) were stirred at rt for 5 min in degassed
CH3OH (15 ml) under N2. Pt(imP)Cl2 (0.15 mmol) was
added to the resulting solution and the reaction mixture
was sonicated for 8 h at rt and stirred overnight to give a
105.269(2)/90; V = 4268.0(10) A ; Z = 4;
q
calc = 1.514
3
˚
g cmꢀ3; l = 3.478 mmꢀ1; F(000) = 1960; h range (ꢁ) =
1.57–27.55; reflections collected/Rint/data/restraints/
parameters = 48169/0.1059/9689/6/514; goodness-of-fit on
F2 = 0.920; R(I > 2r(I)): R1 = 0.0425, wR2 = 0.0861; R

704
N.M. Shavaleev et al. / Inorganica Chimica Acta 360 (2007) 700–704
(all data): R1 = 0.0874, wR2 = 0.0999; Dqmin/max (e Aꢀ3) =
ꢀ0.997/2.209.
˚
(d) E. Rotondo, A. Rotondo, F. Nicolo, M.L. Di Pietro, M.A.
Messina, M. Cusumano, Eur. J. Inorg. Chem. (2004) 4710;
(e) W. Lu, D.A. Vicic, J.K. Barton, Inorg. Chem. 44 (2005) 7970.
[3] (a) A. Islam, H. Sugihara, K. Hara, L.P. Singh, R. Katoh, M.
Yanagida, Y. Takahashi, S. Murata, H. Arakawa, New J. Chem. 24
(2000) 343;
Acknowledgement
(b) E.A.M. Geary, N. Hirata, J. Clifford, J.R. Durrant, S. Parsons, A.
Dawson, L.J. Yellowlees, N. Robertson, Dalton Trans. (2003) 3757;
(c) B.W. Smucker, J.M. Hudson, M.A. Omary, K.R. Dunbar, Inorg.
Chem. 42 (2003) 4714.
The research was funded by EPSRC (Grants GR/
T03352 and GR/T03345).
[4] (a) J.Z. Wu, L. Li, T.X. Zeng, L.N. Ji, J.Y. Zhou, T. Luo, R.H. Li,
Polyhedron 16 (1996) 103;
Appendix A. Supporting information
(b) J.Z. Wu, G. Yang, S. Chen, L.N. Ji, J.Y. Zhou, Y. Xu, Inorg.
Chim. Acta 283 (1998) 17.
Additional experimental details are available as Sup-
porting Information. Supplementary data associated with
this article can be found, in the online version, at
doi:10.1016/j.ica.2006.08.007.
[5] P.T. Selvi, M. Palaniandavar, Inorg. Chim. Acta 337 (2002) 420.
[6] (a) M. Mariappan, B.G. Maiya, Eur. J. Inorg. Chem. (2005) 2164;
(b) M.J. Han, L.H. Gao, K.Z. Wang, New J. Chem. 30 (2006) 208.
[7] B. Gholamkhass, H. Mametsuka, K. Koike, T. Tanabe, M. Furue,
O. Ishitani, Inorg. Chem. 44 (2005) 2326.
[8] Y.G. Sun, R. Xu, J. Zhao, X.M. Song, E.J. Gao, Shenyang Huagong
Xueyuan Xuebao 15 (2001) 252.
References
[9] (a) M. Guan, Z.Q. Bian, F.Y. Li, H. Xin, C.H. Huang, New J.
Chem. 27 (2003) 1731;
[1] (a) W. Paw, S.D. Cummings, M.A. Mansour, W.B. Connick, D.K.
Geiger, R. Eisenberg, Coord. Chem. Rev. 171 (1998) 125;
(b) J.A. Weinstein, M.T. Tierney, E.S. Davies, K. Base, A.A.
Robeiro, M.W. Grinstaff, Inorg. Chem. 45 (2006) 4544;
(c) J.A. Weinstein, N.N. Zheligovskaya, I.V. Lileev, A.M. Galin,
M.Ya. Mel’nikov, Russ. J. Inorg. Chem., Engl. Transl. 43 (1998)
1361;
(d) A. Vogler, H. Kunkely, J. Am. Chem. Soc. 103 (1981) 1559;
(e) S.D. Cummings, R. Eisenberg, J. Am. Chem. Soc. 118 (1996) 1949.
[2] (a) A. Klein, T. Scheiring, W. Kaim, Z. Anorg. Allg. Chem. 625
(1999) 1177;
(b) Y.J. Bing, L.M. Leung, G. Menglian, Tetrahedron Lett. 45 (2004)
6361.
[10] T. Cardinaels, K. Driesen, T.N. Parac-Vogt, B. Heinrich, C.
Bourgogne, D. Guillon, B. Donnio, K. Binnemans, Chem. Mater.
17 (2005) 6589.
[11] (a) C.R. Mayer, M. Herve, H. Lavanant, F. Secheresse, Tetrahedron
Lett. 45 (2004) 7805;
(b) Y.C. Hsu, H. Zheng, J.T. Lin, K.C. Ho, Sol. Energy Mater. Sol.
Cells 87 (2005) 357.
[12] E.A. Steck, A.R. Day, J. Am. Chem. Soc. 65 (1943) 452.
[13] J.H. Price, A.N. Williamson, R.F. Schramm, B.B. Wayland, Inorg.
Chem. 11 (1972) 1280.
(b) C.M. Che, M. Yang, K.H. Wong, H.L. Chan, W. Lam, Chem.
Eur. J. 5 (1999) 3350;
[14] W. Yao, O. Eisenstein, R.H. Crabtree, Inorg. Chim. Acta 254 (1997)
105.
(c) C.R. Brodie, J.G. Collins, J.R. Aldrich-Wright, Dalton Trans.
(2004) 1145;