A new family of luminescent compounds: Platinum(II) imidoylamidinates exhibiting pH-dependent room temperature luminescence

Article abstract of DOI:10.1039/b602083f

The imidoylamidinate platinum(II) compounds [Pt{NH=C(R)NC(Ph)=NPh} ₂] (R = CH₂Ph 2, p-ClC₆H₄ 3, Ph 4) were prepared by the reaction of the appropriate trans-[PtCl ₂(RCN)₂] with 4 equiv of the amidine PhC(=NH)NHPh giving 2-4 and 2 equivs of the salt PhC(=NH)NHPh·HCl. We also synthesized, by the double alkylation of 4 with MeOSO₂CF₃, complex [Pt{NH=C(Ph)N(Me)C(Ph)=NPh}₂][CF₃SO₃] ₂ (5) which models the bis-protonated form of 4. The complexes were characterized by ¹H, ¹³C NMR, and IR spectroscopies, FAB-MS and by C, H, N elemental analysis. The X-ray crystallography of 4·2CH₂Cl₂ enables the confirmation of the square planar coordination geometry of the metal center with almost planar imidoylamidine ligands, while in 5·2CHC₃ the planarity of the metallacycles is lost and and the central N atom is sp³-hybridized. The imidoylamidinate complexes represent a new family of Pt(II)-based luminescent complexes and they are emissive at room temperature both in solution and in the solid state, with an emission quantum yield ranging from 3. 7 × 10^-4 to 6.2 × 10^-2 in methanol solution; the emission intensity is pH-dependent, being quenched at low pH. UV-visible and luminescence spectroscopies indicate that the lowest excited state of these compounds is ³MLCT or ³IL with significant MLCT character, with emission lifetimes of a few μs. A blue shift of both the absorption and emission with increasing solvent polarity and with decreasing π-electron withdrawing properties of the ligand substituent was observed. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b602083f

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A new family of luminescent compounds: platinum(II) imidoylamidinates
exhibiting pH-dependent room temperature luminescence
Ginka H. Sarova,a Nadezhda A. Bokach,b,c Alexander A. Fedorov,a Ma´rio N. Berberan-Santos,*a
Vadim Yu. Kukushkin,*c Matti Haukka,d Joa˜o J. R. Frau´sto da Silvab and Armando J. L. Pombeiro*b
Received 13th February 2006, Accepted 10th April 2006
First published as an Advance Article on the web 8th May 2006
DOI: 10.1039/b602083f
=
=
The imidoylamidinate platinum(II) compounds [Pt{NH C(R)NC(Ph) NPh}2] (R = CH2Ph 2,
p-ClC6H4 3, Ph 4) were prepared by the reaction of the appropriate trans-[PtCl2(RCN)2] with 4 equiv of
=
=
the amidine PhC( NH)NHPh giving 2–4 and 2 equivs of the salt PhC( NH)NHPh·HCl. We also
synthesized, by the double alkylation of 4 with MeOSO2CF3, complex
=
=
[Pt{NH C(Ph)N(Me)C(Ph) NPh}2][CF3SO3]2 (5) which models the bis-protonated form of 4. The
complexes were characterized by 1H, 13C NMR, and IR spectroscopies, FAB-MS and by C, H, N
elemental analysis. The X-ray crystallography of 4·2CH2Cl2 enables the confirmation of the square
planar coordination geometry of the metal center with almost planar imidoylamidine ligands, while in
5·2CHCl3 the planarity of the metallacycles is lost and and the central N atom is sp3-hybridized. The
imidoylamidinate complexes represent a new family of Pt(II)-based luminescent complexes and they are
emissive at room temperature both in solution and in the solid state, with an emission quantum yield
ranging from 3.7 × 10−4 to 6.2 × 10−2 in methanol solution; the emission intensity is pH-dependent,
being quenched at low pH. UV-visible and luminescence spectroscopies indicate that the lowest excited
state of these compounds is 3MLCT or 3IL with significant MLCT character, with emission lifetimes of
a few ls. A blue shift of both the absorption and emission with increasing solvent polarity and with
decreasing p-electron withdrawing properties of the ligand substituent was observed.
emission properties denoted that we had indeed obtained a novel
type of Pt(II)-based luminescent compound.
Introduction
Luminescent square-planar platinum(II) complexes have attracted
a great deal of interest because of their useful photochemical prop-
erties, i.e. high efficiency and long lifetimes of the emissive states,
and potential applications in many fields, such as chemosensors,1,2
photocatalysts,3 light-emitting diodes (LEDs),4 and photovoltaic
devices.5,6 The most advantageous d8-Pt(II) luminescent systems
are based on [PtX2(diimine)] (X = halide, cyanide, thiolate, alkyl,
aryl or acetylide5,7,8) or terpyridine9,10 complexes, phosphorescent
Pt(II) porphyrins,11 the series of Pt complexes with N,C,N-
In this work we report data on the photophysics of three plat-
=
=
inum(II) complexes, i.e., [Pt{NH C(CH2Ph)NC(Ph) NPh}2] (2),
=
=
=
[Pt{NH C(p-ClC6H4)NC(Ph) NPh}2] (3) and [Pt{NH C(Ph)-
=
NC(Ph) NPh}2] (4), showing pH-dependent room temperature
emission in solution. The single crystal X-ray diffraction analyses
=
=
of [Pt{NH C(Ph)NC(Ph) NPh}2] (4) and its alkylated analogue
=
=
[Pt{NH C(Ph)N(Me)C(Ph) NPh}2][CF3SO3]2 (5) along with
synthetic data for 3 and 5 are also reported. The photophysical
properties have been investigated with respect to the ligand p-
electron withdrawing ability and solvent polarity.
12
or S,C,S-13pincer ligands and some other cyclometalated Pt
compounds.14
Very recently we reported the coupling between the ami-
=
dine PhC( NH)NHPh and ligated nitriles in the platinum(II)
Results and discussion
complex [PtCl2(RCN)2] (R = Et, CH2Ph, Ph, NEt2) giv-
=
ing Pt-bound chelated imidoylamidines(ates) [Pt{NH C(R)-
Pt(II)-mediated nitrile–amidine coupling
15
=
NC(Ph) NPh}2]. It has been observed that the complexes with
Amidines can act as nucleophiles towards the triple CN bond
in coordinated nitriles to give imidoylamidines, but the sub-
ject is still underdeveloped. Besides two collateral evidences
favoring the metal-mediated nitrile–amidine coupling,16,17 Kilner
et al.18 obtained the platinum(II) imidoylamidinate complexes
R = Ph and CH2Ph exhibit luminescent properties in solution,
in the solid state and on being spread onto TLC SiO2 plates; the
aCentro de Qu´ımica-F´ısica Molecular, Instituto Superior Te´cnico, 1049-001
Lisboa, Portugal. E-mail: berberan@ist.utl.pt
=
=
[Pt{NH C(Ph)NC(Ph) NH}2] by reaction of [PtCl2(PhCN)2]
18
bCentro de Qu´ımica Estrutural, Instituto Superior Te´cnico, 1049-001,
Lisboa, Portugal. E-mail: pombeiro@ist.utl.pt
=
with 2 equiv of PhC(NH2) NLi. In accordance with the authors,
the lithiation of benzamidine was necessary to enhance the
nucleophilicity of this species and to promote the reaction. The
work we have recently been reported15 and the current results
on preparation of 3 (see later) demonstrate that the amidine
cDepartment of Chemistry, St.Petersburg State University, Universitetsky
pr. 26, 198504 Stary Petergof, Russian Federation. E-mail: kukushkin@
VK2100.spb.edu
dDepartment of Chemistry, University of Joensuu, P.O. Box 111, FIN-80101
Joensuu, Finland. E-mail: matti.haukka@joensuu.fi
=
PhC( NH)NHPh is a sufficiently good nucleophile to react
3798 | Dalton Trans., 2006, 3798–3805
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Table 1 Crystal data for 4·2(CH2Cl2) and 5·2(CHCl3)
without additional activation (by, e.g., lithiation), even under
relatively mild conditions (40 ◦C), with Pt(II)-bound nitriles thus
providing easy access to (imidoylamidinate)Pt(II) complexes.
We observed that for the success of the reaction another
stoichiometry, as compared to that employed by Kilner and
4·2(CH2Cl2)
5·2(CHCl3)
Empirical formula
FW
C42H36Cl4N6Pt
961.66
100(2)
0.71073
Monoclinic
P21/c
15.6529(9)
6.0112(2)
22.0709(12)
90
110.086(2)
90
1950.40(17)
2
1.637
3.910
C46H40Cl6F6N6O6PtS2
1358.75
120(2)
0.71073
Triclinic
¯
P1
10.1713(7)
14.0242(10)
18.5465(10)
95.531(4)
90.497(4)
99.482(4)
2596.4(3)
2
1.738
3.166
15746
9361
colleagues,18 should be used. Thus, the platinum(II) compounds
Temp./K
˚
k/A
15
[Pt{NH C(R)NC(Ph) NPh}2] (2, 3 (this work), and 415;
Scheme 1) were prepared by reaction of the appropriate trans-
[PtCl2(RCN)2] (R = CH2Ph,19 Ph,20 and p-ClC6H4 1; for synthesis
of the latter starting material see Experimental) with 4 equiv of
=
=
Cryst syst.
Space group
˚
a/A
˚
b/A
˚
=
c/A
the amidine PhC( NH)NHPh. The addition of the amidine to the
a/◦
b/◦
c /◦
coordinated nitriles followed by ring-closure of the newly formed
imidoylamidines is accompanied by dehydrochlorination and HCl
is trapped with another amidine molecule to form (Scheme 1, route
3
˚
V/A
=
Z
A) the salt PhC( NH)NHPh·HCl which was isolated as the other
qcalc/Mg m−3
product of the reaction. We also synthesized (Scheme 1, route
B), by the double alkylation of 4 with MeOSO2CF3, complex 5
(characterized by X-ray crystallography) which models the bis-
protonated form of 4 and the latter model was needed to interpret
some photophysics experiments (see later).
l(Mo Ka)/mm−1
No. of collected reflns
No. of unique rflns
RInt.
12271
3673
0.0699
0.0352
0.0452
0.0482
0.1161
R1a (I ≥ 2 r)
wR2b (I ≥ 2 r)
0.0794
a R1 = RꢀFo| − |Fcꢀ/R|Fo|. b wR2 = [R[w(Fo − Fc2)2]/R[w(Fo2)2]]1/2
.
2
◦
˚
Table 2 Selected bond lengths (A) and angles ( ) for 4·2(CH2Cl2) and
5·2(CHCl3)
5·2(CHCl3)
4·2(CH2Cl2)
A
B
Pt(1)–N(1)
Pt(1)–N(3)
N(1)–C(1)
C(1)–N(2)
N(2)–C(8)
N(2)–C(9)
C(9)–N(3)
1.986(4)
2.041(5)
1.304(8)
1.349(8)
1.994(6)
2.028(7)
1.283(10)
1.389(10)
1.452(10)
1.379(11)
1.318(10)
1.977(6)
2.033(6)
1.305(10)
1.374(10)
1.500(10)
1.385(10)
1.288(10)
Scheme 1
1.336(8)
1.310(8)
N(1)–Pt(1)–N(3)
C(1)–N(2)–C(9)
87.7(2)
122.9(5)
87.2(3)
124.7(6)
87.1(3)
124.9(6)
=
=
Molecular structures of [Pt{NH C(Ph)NC(Ph)
=
=
NPh} ]·2CH2Cl2 (4·2CH2Cl2) and [Pt{NH C(Ph)N(Me)C(Ph)
NPh}22][CF3SO3]2·2CHCl3 (5·2CHCl3)
Collected X-ray data for both complexes are presented in Table 1,
while selected bond lengths and angles are shown in Table 2. Com-
˚
plex 4 has a square planar geometry. The Pt–N bonds [1.986(4) A
˚
and 2.041(5) A for Pt(1)–N(1) and Pt(1)–N(3), correspondingly]
18
=
=
agree well with those found in [Pt{NH C(Ph)NC(Ph) NH}2].
Atoms of the metallacycles lie in one plane with a mean devi-
˚
=
ation of 0.027 A. The N C bond lengths [N(1)–C(1) 1.304(8)
˚
and N(3)–C(9) 1.310(8) A] and the N–C bond lengths [N(2)–
˚
C(9) 1.336(8) and N(2)–C(1) 1.349(8) A] in the metallacycle
have values typical for the corresponding bonds in both Pt(II)
and Ni(II) imidoylamidine(ato) complexes and exhibit a similar
small degree of delocalization in the ring.16–18 The chelate angle
N(1)–Pt(1)–N(3) [87.7(2)◦] agrees well with that observed in
18
=
=
[Pt{NH C(Ph)NC(Ph) NH}2].
The crystal structure of 5 contains two symmetry independent
halves of 5 in the asymmetric unit. The complex 5 also has a
square planar geometry (Fig. 2); the bonds Pt(1)–N(1) and Pt(1)–
Fig. 1 X-Ray molecular structure of 4. Thermal ellipsoids are drawn at
50% probability.
˚
N(3) [1.994(6), 2.028(7) and 1.977(6), 2.033(6) A for unit A and B,
metallacycles is lost (mean deviation from the plane 0.101 and
respectively] as well as the chelate angle [87.2(3) and 87.1(3)◦]
0.102 A for units A and B, correspondingly) due to, at least one
˚
resemble closely those found in 4. In 5, the planarity of the
of these reasons, the steric strain caused by the methyl group and
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Fig. 2 X-Ray structure of 52+; the counterions were omitted for clarity.
Thermal ellipsoids are drawn at 50% probability.
N(2) and Pt(1) are bent away from the plane defined by N(1)–C(1)–
C(9)–N(3) [Pt(1), N(2) distances from the plane 0.317, 0.179(7) and
˚
0.305, 0.192(7) A. Angles between the lines defined by bonds Pt(1)–
N(1#1) and N(2)–C(8) and the plane N(1)–C(1)–C(9)–N(3) are
80.9(3), 72.7(5) and 81.2(3), 73.6(5) for units A and B, respectively
(#1 for A is −x, −y, −1 −z and for B −x, −y, −z)]. Moreover,
C(1)–N(2) and N(2)–C(9) distances in 5 have more of a single
bond character as compared to 4 and this, along with the angles
around the N(2) atom, implies that the N atom is sp3-hybridized.
CCDC reference numbers 297957 and 297958.
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b602083f
Absorption spectra of the (imidoylamidine)Pt(II) complexes
The UV-visible absorption spectra of compounds 2–4 in CH2Cl2
solutions are shown in Fig. 3.
Fig. 3 Absorption spectra of 2, 3 and 4 solutions in CH2Cl2.
The absorption data (wavelengths of maxima and respective
molar absorption coefficients) is summarised in Table 3. All
complexes exhibit high-energy absorption bands in the region 220–
280 nm, assignable as 1(p–p*) IL (intraligand) transitions based on
energy, intensity and similarity with the absorption spectrum of
=
the free amidine PhC( NH)NHPh. The lowest-energy absorption
bands at 380–430 nm are intrinsic of the d8 platinum complexes, as
the absorption of the free amidine is significant only below 370 nm.
3800 | Dalton Trans., 2006, 3798–3805
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We assign these bands as of MLCT character based on their low
energy, ligand dependence and intensity (e > 1000 M−1 cm−1), and
by analogy with the published data.21–25
(UL < 1 × 10−5). This quenching by the chlorinated solvent is
attributable in both cases to the formation of a weakly emissive
(3 and 4) or essentially non-emissive (2) exciplex by means of an
excited-state charge-transfer interaction.28
At 77 K, the emission intensity increases and a blue shift of
7–8 nm is observed. For complex 4 the two vibronic components
at 504 nm and 544 nm are well defined, the shoulder at 592 nm
is also better resolved. A small change in the relative intensities
of the vibronic components is also seen, i.e. the peak at 504 nm
is resolved better and is slightly more intense. Fig. 5 shows the
emission spectra of compound 3 in EtOH : MeOH (4 : 1) solution
at 293 K and 77 K.
Two further features of the absorption spectra deserve mention:
(i) All complexes display a significant solvatochromic shift. In
accordance with their CT character,26 absorption bands red shift
when going from MeOH to CH2Cl2 solution (Dk = 6 nm for 2,
20 nm for 3 and 9 nm for 4); (ii) The lowest energy absorption band
is red-shifted for 3 and 4 in comparison with 2, see Fig. 3. This
difference is understandable taking into account that the aromatic
part of the R group can participate in p-conjugation in 3 and 4
but not in 2. The kmax variation with the change of the substituent
is consistent with a ligand based LUMO, in agreement with the
assignment of this band as of CT origin.
Emission spectra of the (imidoylamidine)Pt(II) complexes
All compounds are emissive in methanol solution at 298 K (Fig. 4)
with a luminescence quantum yield that increases on going from
complex 2 to complexes 3 and 4.
Fig. 5 Emission spectra of complex 3 in EtOH : MeOH (4 : 1) at 293 K
and 77 K.
In addition to the higher vibronic resolution, a blue shift is also
apparent, changing the maximum emission wavelengths from 500,
537, 581 nm at 293 K to 493, 532, 575, 634 (sh) nm in this solvent
mixture at 77 K.
In the solid state all complexes showed strong emission with
emission maxima occurring at approximately the same emission
wavelengths as in the methanol solutions, and with lifetimes in the
range of 1.6 to 1.8 ls.
Lifetime measurements were also carried out at room temper-
ature and in low temperature methylcyclohexane : toluene (7 :
2) glass (110 K) and are presented in Table 3. The luminescence
decays for all complexes at 110 K were well fitted with mono-
exponential functions with lifetimes in the range of 2 to 3 ls
(Fig. 6).
For complexes 3 and 4, a lifetime of 2 ls and a luminescence
quantum yield of 6 × 10−2 (room temperature data) imply a
radiative lifetime of 30 ls, compatible with an IL spin-forbidden
transition (phosphorescence). For complex 2, the radiative lifetime
is even higher, 5 ms, characteristic again of a spin-forbidden
transition, in this case mainly of the MLCT type.
Room temperature luminescence decays of in MeOH solutions
are shorter and bi-exponential, with lifetimes of 1.8 ls (75%) and
600 ns (25%) for complex 4. In CH2Cl2 solutions, the luminescence
decays are even more complex and considerably faster than in
MeOH, in agreement with a solvent quenching effect.
Fig. 4 Emission spectra of compounds 2–4 in degassed MeOH solution
at room temperature. The excitation wavelengths were 370 nm, 395 nm
and 400 nm, respectively.
Excitation of complex 2 resulted in a broad and structureless
emission band with a maximum at 463 nm and a quantum
efficiency of 1.3 × 10−4 and 3.7 × 10−4 in air-saturated and
degassed solutions, respectively. Excitation of compounds 3 and
4, on the other hand, resulted in a strong, structured emission
with maxima at 502 nm, 537 nm and 578 nm for complex 3 and
512 nm and 549 nm, with a shoulder at 596 nm for complex 4.
The quantum yields of the emission in air-saturated and degassed
MeOH solutions are respectively 3.0 × 10−2 and 5.8 × 10−2 for
complex 3, and 3.3 × 10−2 and 6.2 × 10−2 for complex 4. Using
an average lifetime of 2 ls at room temperature in methanol
solution, and a concentration of O2 in air-equilibrated MeOH
at 298 K of 1.0 × 10−2 M,27 bimolecular quenching rate constants
for compounds 2–4 of 9 × 107, 4 × 107 and 6 × 107 M−1 s−1,
respectively, are obtained. These rate constants, whilst large, are
well below diffusion control (ca. 109 M−1 s−1 for triplet state
quenching by O2).27
Solvent and ligand dependence of the emission
In contrast to methanol solutions, the emission is considerably
suppressed in CH2Cl2 solution. Although a weak emission was
observed for complexes 3 and 4 with a quantum yield in air-
saturated CH2Cl2 of 3.5 × 10−5 and 4.0 × 10−5, respectively,
complex 2 is essentially non-emissive in fluid CH2Cl2 at 298 K
The emission of the complexes studied shifts with ligand modifi-
cation and solvent polarity. The emission maxima shift to lower
energy as the ligand becomes a stronger p-electron-acceptor. This
shift is parallel to that observed in the absorption spectra but
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Scheme 2
which models the bis-protonated form and it was observed that
the dicationic complex 5 does not exhibit emission neither in the
solid state nor in solution. Based on the crystallographic data (see
above), we believe that alkylation and, apparently, protonation
might change hybridization of the N atom from sp2 to sp3 and
these structural changes could affect the electronic distribution in
the complex and, consequently, the luminescence. The correlations
between acid–base properties of these (imidoylamidine)Pt(II)
complexes and the pH-dependence of the emission will be subject
to further studies.
Fig. 6 Luminescence decay of complex 4 in methylcyclohexane : toluene
(7 : 2) glass at 100 K. Excitation and emission wavelengths were 415 nm
and 501 nm, respectively. The timescale was 7.46 ns per channel. The decay
is well fitted (reduced v2 = 1.09) by an exponential function with a lifetime
of 2.8 ls.
Nature of the excited state in the (imidoylamidine)Pt(II) complexes
The low energy (∼550 nm), long radiative lifetime and the
broad and structureless emission of complex 2 indicate a 3MLCT
excited state. For complexes 3 and 4, the structured and red-
shifted emission and the much higher emission quantum yields
greater than that for the ∼400 nm absorption band. The phenyl
substituent, attached directly to the chelate cycle in complex 4
increases the ligand p-electron-withdrawing ability and emission
occurs at 512 nm, while the maxima occur at 463 nm for complex
2 and at 502 nm for complex 3 (all wavelengths refer to MeOH
solutions). The emission band position was found to depend also
on solvent polarity. A red shift of about 20 nm was observed in
CH2Cl2 solutions compared to the methanol solutions. Although
the greatest effect of solvent polarity on the absorption spectra was
observed for complex 3 (the absorption maxima in MeOH, EtOH :
MeOH (4 : 1) and CH2Cl2 solutions were observed at 400 nm,
412 nm and 420 nm), the greatest dependence of emission on
solvent was observed for compound 4. The emission maxima for
the latter in CH2Cl2 occurs at 532 nm whereas in MeOH solution it
is observed at 512 nm. For complex 3 the first emission maximum
also red shifts parallel to the absorption shift from 502 nm in
MeOH to 516 nm in CH2Cl2 solution.
3
indicate a mainly IL CT (p–p*) excited state, with some MLCT
contribution.25 As mentioned above, the difference between 2, 3
and 4 is understandable taking into account that only for the
latter can the aromatic part of the R group participate in p-
conjugation, resulting in a lowered LUMO energy of the ligand.
The dependence of kem with the substituent and solvent polarity
(a 50 nm shift when changing the substituent from CH2Ph to Ph
and a 20 nm shift when going from MeOH to CH2Cl2) also agrees
with the assignments made.
Final remarks
We have shown that neutral (imidoylamidinate)Pt(II) complexes
constitute a new family of room temperature luminescent
square-planar compounds. They are planar chelates simply
derived from nucleophilic addition, under mild conditions, of
pH-Dependence of the luminescence
=
Another interesting aspect of the photophysics of the complexes
under study is the emission dependence on the pH of the solution.
In acidic media, we observed the complete quenching of the
emission. The emission is reversible upon neutralization and, in
basic solutions (NaOH in methanol), the complex characteristic
emission bands at about 500 nm appear in the spectra. The same
observation holds for the absorption spectra, i.e. the CT bands
are not visible in acid solutions; instead, very weak absorption
bands, which are blue shifted in respect to the CT bands can be
distinguished. Quenching of the emission upon protonation could
be due to an enhanced non-radiative deactivation via short lived
d–d states as a result of increased energy of the emissive CT excited
states.
We believe that the pH-dependence of the emission is due
to the acid–base properties of the central nitrogen atom which,
similarly to analogous nickel(II) complexes,16,29 can be protonated
(Scheme 2).
In order to obtain additional data favoring the relevance of the
quenching with the protonation of the N atoms, we synthesized
(Scheme 1, route B), by the double alkylation of 4, complex 5
PhC( NH)NHPh to each of the nitrile ligands activated by
coordination at [PtCl2(RCN)2], with ring closure, in the presence
of an excess of the amidine to trap the HCl formed upon
dehydrochlorination. The generality of this simple route to the
synthesis of other metal chelates with imidoylamidinate ligands
is anticipated. It does not require the previous synthesis of
imidoylamidinate salts to be used as reagents, in contrast to
some fluoroalkyl imidoylamidinate complexes of various met-
als with bis(perfluoroalkyl)triazapentadienyl ligands, typically
N(Ph) C(C3F7)NC(C3F7) N(Ph) ,30–32 for which no luminescent
−
=
=
behaviour has been quoted.
p-Conjugation effects appear to play a relevant role in the
luminescent behaviour which is strong only when the R group
of the organonitrile is capable of p-conjugation. It is barely or
not detected for R groups such as an alkyl or an amide in
the (imidoylamidinate)Pt(II) chelates related to 2–4 but derived
from amidine coupling to, e.g., acetonitrile or organocyanamide
(Et2NCN) ligands. Moreover, an increase of the p-electron with-
drawing character of R significantly increases the luminescence
quantum efficiency and leads to a shift of the emission to lower
3802 | Dalton Trans., 2006, 3798–3805
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energy. The emission is also dependent on pH, being quenched in
acidic media probably due to ligand protonation.
Methylation of 4. Complex 4 (40 mg, 0.051 mmol) was
suspended in dry THF (1 mL) and MeOSO2CF3 (0.106 mmol)
was added dropwise to the suspension and left to stand on stirring
for 2 h at the room temperature. The almost colorless suspension
was then concentrated under vacuum to give an oily residue which
was washed twice with Et2O (two 1-mL portions), then dissolved
in CHCl3/Et2O (1 : 1, 2 mL) and the solution was left to stand
in a vial at room temperature for evaporation. After evaporation,
almost colorless crystals were formed on the top of the vial (yield
is 13.6 mg, 23%) along with some amount of oily residue on the
bottom. The crystals were collected for X-ray analysis and the
oily residue was dried under vacuum to give a pale-yellow powder
(yield is 30.8 mg, 52%).
Hence, this study presents a type of complexes whose lumi-
nescence properties can be easily tuned by changing the electronic
properties of the R group of the parent organonitrile and the pH of
the solution. It is expected that a further tuning can be achieved by
playing with the nature of the organic group of the amidine applied
to their synthesis, e.g., by using a substituted phenylamidine such
=
as ArC( NH)NHAr (Ar = substituted phenyl). Hence, they
provide an opportunity to investigate and establish “structure–
basicity–luminescence” relationships. The luminescence proper-
ties of complexes 3 and 4, namely an intense emission in the visible,
a relatively long lifetime, and pH and O2 sensitivity, render these
platinum complexes potential candidates for sensing applications.
=
=
[Pt{NH C(Ph)N(Me)C(Ph) NPh}2][CF3SO3]2
(5). Anal.
Calcd for C44H38N6F6S2O6Pt·2/3CHCl3: C, 44.72; H, 3.25; N,
7.01. Found: C, 44.60; H, 3.28; N, 6.85%. FAB+–MS, m/z: 821
Experimental
1
[M − H]+. H NMR in CDCl3, d: 7.52, 7.33 and 7.15 (m’s, 15H,
1
Materials and syntheses
Ph), 6.65 (s, br, 1H, NH), 3.81 (s, 3H, Me). 13C{ H} NMR in
=
CDCl3, d: 165.97 and 156.12 (two C N), 143.91, 132.52, 129.80,
For synthetic needs commercially available solvents were used after
distillation, while for the spectroscopic measurements solvents of
spectroscopic grade were used. MeOSO2CF3 was purchased from
128.97, 128.81, 128.09, 127.54, and 126.55 (carbons in Ph’s),
42.47 (Me).
33
=
Aldrich, whereas PhC( NH)NHPh, trans-[PtCl2(RCN)2] (R =
Physical measurements
CH2Ph,19 Ph ), [Pt{NH C(CH2Ph)NC(Ph) NPh}2] (2) and
20
15
=
=
[Pt{NH C(Ph)NC(Ph) NPh}2] (4)15 were prepared according to
=
=
C, H and N elemental analyses were carried out by the Micro-
analytical Service of the Instituto Superior Te´cnico. Positive-ion
FAB mass spectra were obtained on a Trio 2000 instrument by
bombarding 3-nitrobenzyl alcohol (NBA) matrices of the samples
with 8 keV (ca. 1.28 × 1015 J) Xe atoms. Mass calibration for data
system acquisition was achieved using CsI. Infrared spectra (4000–
400 cm−1) were recorded on a JASCO FT/IR-430 instrument in
KBr pellets (separate experiments show that the spectra of all the
compounds are similar in KBr and Nujol. Hence, only the spectra
in KBr which are better resolved and not overlapped with Nujol
the published methods.
trans-[PtCl2(p-ClC6H4CN)2] (1). The compound was pre-
pared by a modified procedure described in the literature.34
[PtCl2(MeCN)2] (300 mg, 0.86 mmol) and p-ClC6H4CN (400 mg,
2.91 mmol) were suspended in 2 mL of toluene and the reaction
mixture was refluxed with stirring for 6 h, whereupon Et2O (10 mL)
was added to form a greenish–yellow powder which was filtered
off, washed with Et2O (four 5 mL portions) and dried in air. Yield
380 mg, 82%. Anal. Calcd for C14H8N2PtCl4: C, 31.17; H, 1.50;
N, 5.20. Found: C, 31.30; H, 1.52; N, 5.20%. FAB+–MS, m/z:
1
1
bands are given). H and 13C{ H} NMR spectra were measured
on a Varian UNITY 300 spectrometer at ambient temperature.
Absorption spectra were recorded on a Shimadzu UV-3101PC
UV-VIS-NIR spectrophotometer using 1.0 cm path length cells.
All spectra were run against solvent as a blank. The emission
spectra were obtained on a SPEX Fluorolog F112A fluorimeter
and corrected for instrumental response. Fluid solution samples
were degassed by freeze–pump–thaw cycles at least four times.
Emission spectra at low temperature were obtained in EtOH :
MeOH (4 : 1) (at 77 K) or methylcyclohexane : toluene (7 : 2)
(at 110 K) solvent mixtures using an Oxford DN 1704 cryostat.
Excitation wavelengths for quantum yield determinations were
330 nm in the case of complex 4, 360 nm for complex 2 and
407 nm for complex 3. A Corion WG-360-S 650F cut-off filter was
used in the emission when performing luminescence measurements
with kex < 360 nm. 9-Bromoanthracene in MeOH (φF = 0.02027)
was used as a standard for the quantum yield determinations
in the case of complexes 2 and 4. For determination of the
emission quantum yields of complex 3, a tetracene solution in
MeOH was used (UF = 0.16 in polar solvents27). Time resolved
picosecond luminescence measurements were performed using the
single-photon timing method with laser excitation. The set-up
consisted of a mode-locked Coherent Innova 400-10 argon-ion
laser that synchronously pumped a cavity dumped Coherent 701-
2 dye (Rhodamine 6G or DCM) laser, delivering fundamental or
539 [M]+. IR spectrum in KBr, selected bands, cm−1: 2291 m–s
1
=
m(C≡N), 1582 m–s m(C C from Ar), 1095 s m(C–Cl). H NMR in
CDCl3, d: 7.75 (d, 2H) and 7.56 (d, 2H)(p-ClC6H4). 13C{ H} NMR
1
in CDCl3, d: 142.62 (C–Cl), 134.87 and 130.14 (m- and p-C6H4),
116.04 (C≡N).
Reaction of trans-[PtCl2(p-ClC6H4CN)2] with the Ami-
dine. trans-[PtCl2(p-ClC6H4CN)2] (28.7 mg, 0.053 mmol) and
=
PhC( NH)NHPh (42 mg, 0.212 mmol) were dissolved in CH2Cl2
(0.5 mL) and left at 40 ◦C. Yellow needles appeared on the
top of the vial after 6 h, whereupon they were filtered off
and then mechanically separated from the colourless crystals of
=
PhC( NH)NHPh·HCl. Yield: is 32 mg, 70%.
=
=
[Pt{NH C(p-ClC6H4)NC(Ph) NPh}2] (3). Anal. Calcd for
C40H30N6PtCl2 ·0.2CH2Cl2: C, 55.07; H, 3.50; N, 9.59. Found: C,
55.18; H, 3.32; N, 9.49%. FAB+–MS, m/z: 859 [M]+. IR spectrum
in KBr, selected bands, cm : 33310 m–s m(N–H), 1537 s m(C N
−1
=
1
=
=
and C C from Ar), 1420 s m(C C from Ar), 695 m d(C–H). H
NMR in CDCl3, d: 7.33 (m, 4H) and 7.13 (m, 10H) (Ph), 6.29
(s, br, 1H, NH). 13C{ H} NMR in CDCl3, d: 160.50 and 154.93
1
=
(two C N), 146.18, 136.03, 129.19, 128.47, 128.83, 127.82, 127.54,
127.33 and 125.96 (carbons in Ph and p-ClC6H4).
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View Article Online
frequency-doubled 5 ps pulses at a repetition rate of 3.4 MHz,
or alternatively, of a Spectra-Physics Millenia Xs Nd:YVO4
diode pumped laser, pumping a pulse picked Spectra-Physics
Tsunami titanium-sapphire laser, delivering 100 fs frequency-
doubled pulses at a repetition rate of 4 MHz. Intensity decay
measurements were made by alternate collection of impulse and
decay, with the emission polarizer set at the magic angle position.
Impulse was recorded slightly away from excitation wavelength
with a scattering suspension (1 cm cell). For the decays, a
cut-off filter was used, effectively removing all excitation light.
The emission signal passed through a depolarizer, a Jobin-Yvon
HR320 monochromator with a grating of 100 lines nm−1, and
was recorded on a Hamamatsu 2809U-01 microchannel plate
photomultiplier as a detector. The instrument response function
had an effective FWHM of 35 ps.35
2 Q.-Z. Yang, L.-Z. Wu, H. Zhang, B. Chen, Z.-X. Wu, L.-P. Zhang and
C.-H. Tung, Inorg. Chem., 2004, 43, 5195; P. K. M. Siu, S.-W. Lai, W.
Lu, N. Zhu and C.-M. Che, Eur. J. Inorg. Chem., 2003, 2749; C. E. Buss
and K. R. Mann, J. Am. Chem. Soc., 2002, 124, 1031; Y. Kunugi, K. R.
Mann, L. L. Miller and C. L. Exstrom, J. Am. Chem. Soc., 1998, 120,
589; K. A. Van Houten, D. C. Heath and R. S. Pilato, J. Am. Chem.
Soc., 1998, 120, 12359; C. S. Peyratout, T. K. Aldridge, D. K. Crites
and D. R. McMillin, Inorg. Chem., 1995, 34, 4484; V. H. Houlding and
A. J. Frank, Inorg. Chem., 1985, 24, 3664; K.-Y. Wong, C.-M. Che, W.
Lu and Z. He, US Pat., 2003, 2003228701; K.-Y. Wong, C.-M. Che, W.
Lu and Z. He, Chem. Abstr., 2003, 140, 35072; M. Kato, Jpn. Kokai
Tokkyo Koho, 2003, JP 2003064091; M. Kato, Chem. Abstr., 2003, 138,
214472.
3 D. Zhang, L.-Z. Wu, L. Zhou, X. Han, Q.-Z. Yang, L.-P. Zhang and
G.-H. Tung, J. Am. Chem. Soc., 2004, 126, 3440; W. B. Connick and
H. B. Gray, J. Am. Chem. Soc., 1997, 119, 11620; W. A. Kalsbeck, D. M.
Gingell, J. E. Malinsky and H. H. Thorp, Inorg. Chem., 1994, 33, 3313.
4 for recent works see:M. Cocchi, D. Virgili, C. Sabatini, V. Fattori, P.
Di Marco, M. Maestri and J. Kalinowski, Synth. Met., 2004, 147, 253;
H.-F. Xiang, S.-C. Yu, C.-M. Che and P. T. Lai, Appl. Phys. Lett.,
2003, 83, 1518; C.-M. Che, Y.-J. Hou, M. C. W. Chan, J. Guo, Y. Liu
and Y. J. Wang, J. Mater. Chem., 2003, 13, 1362; J. M. Lupton and J.
Klein, Chem. Phys. Lett., 2002, 363, 204; E. M. Gross, N. R. Armstrong
and R. M. J. Wightman, J. Electrochem. Soc., 2002, 149, E137; R. W. T.
Higgins, A. P. Monkman, H.-G. Nothofer and U. J. Scherf, Appl. Phys.,
2002, 91, 99; S. Lamansky, R. C. Kwong, M. Nugent, P. I. Djurovich
and M. E. Thompson, Org. Electron., 2001, 2, 53.
5 for review see:M. Hissler, J. E. McGarrah, W. B. Connick, D. K. Geiger,
S. D. Cummings and R. Eisenberg, Coord. Chem. Rev., 2000, 208, 115.
6 E. A. M. Geary, L. J. Yellowlees, L. A. Jack, I. D. H. Oswald, S. Parsons,
N. Hirata, J. R. Durrant and N. Robertson, Inorg. Chem., 2005, 44, 242
and references therein.
7 J. E. McGarrah and R. Eisenberg, Inorg. Chem., 2003, 42, 4355; T. J.
Wadas, R. J. Lachicotte and R. Eisenberg, Inorg. Chem., 2003, 42, 3772;
J. E. McGarrah, Y.-J. Kim, M. Hissler and R. Eisenberg, Inorg. Chem.,
2001, 40, 4510; S. Huertas, M. Hissler, J. E. McGarrah, R. J. Lachicotte
and R. Eisenberg, Inorg. Chem., 2001, 40, 1183; W. B. Connick, D.
Geiger and R. Eisenberg, Inorg. Chem., 1999, 38, 3264; W. Paw, S. D.
Cummings, M. A. Mansour, W. B. Connick, D. K. Geiger and R.
Eisenberg, Coord. Chem. Rev., 1998, 171, 125.
8 F. Hua, S. Kinayyigit, J. R. Cable and F. N. Castellano, Inorg. Chem.,
2005, 44, 471; I. E. Pomestchenko and F. N. Castellano, J. Phys. Chem.
A, 2004, 108, 3485; I. E. Pomestchenko, C. R. Luman, M. Hissler,
R. Ziessel and F. N. Castellano, Inorg. Chem., 2003, 42, 1394; K.
Kobayashi, H. Sato, S. Kishi, M. Kato, S. Ishizaka, N. Kitamura and
A. Yamagishi, J. Phys. Chem. B, 2004, 108, 18665; Y.-D. Chen, L.-Y.
Zhang, L.-X. Shi and Z.-N. Chen, Inorg. Chem., 2004, 43, 7493; Y.-
D. Chen, Y.-H. Qin, L.-Y. Zhang, L.-X. Shi and Z.-N. Chen, Inorg.
Chem., 2004, 43, 1197; Y. Kang, J. Lee, D. Song and S. Wang, Dalton
Trans., 2003, 3493; A. Klein, J. van Slageren and S. Zalis, Eur. J. Inorg.
Chem., 2003, 1917; M. K. Kuimova, M. Ya. Mel’nikov, J. A. Weinstein
and M. W. George, J. Chem. Soc., Dalton Trans., 2002, 2857; W. L.
Fleeman and W. B. Connick, Comments Inorg. Chem., 2002, 23, 205;
B.-C. Tzeng, S.-C. Chan, M. C. W. Chan, C.-M. Che, K.-K. Cheung
and S.-M. Peng, Inorg. Chem., 2001, 40, 6699; S.-C. Chan, M. C. W.
Chan, Y. Wang, C.-M. Che, K.-K. Cheung and N. Zhu, Chem.–Eur. J.,
2001, 7, 4180.
9 D. R. McMillin and J. J. Moore, Coord. Chem. Rev., 2002, 229, 113.
10 T. J. Wadas, Q.-M. Wang, Y.-J. Kim, C. Flaschenreim, T. N. Blanton
and R. Eisenberg, J. Am. Chem. Soc., 2004, 126, 16841; K. M.-C. Wong,
W.-S. Tang, B. W.-K. Chu, N. Zhu and V. W.-W. Yam, Organometallics,
2004, 23, 3459; H. Tannai, K. Tsuge, Y. Sasaki, O. Hatozaki and N.
Oyama, Dalton Trans., 2003, 2353; V. W.-W. Yam, K. M.-C. Wong and
N. Zhu, Angew. Chem., Int. Ed., 2003, 42, 1400; C.-K. Hui, B. W.-K.
Chu, N. Zhu and V. W.-W. Yam, Inorg. Chem., 2002, 41, 6178.
11 for recent works see: S. Pan and L. J. Rothberg, J. Am. Chem. Soc.,
2005, 127, 6087; X. Wang, M. R. Andersson, M. E. Thompson and
O. Inganas, Thin Solid Films, 2004, 468, 226; S. A. Bagnich, C. Im,
H. Bassler, D. Neher and U. Scherf, Chem. Phys., 2004, 299, 11; K.
Eaton, B. Douglas and P. Douglas, Sens. Actuators, B, 2004, 97, 2;
S. A. Bagnich and H. Bassler, Chem. Phys. Lett., 2003, 381, 464.
12 J. A. G. Williams, A. Beeby, E. S. Davies, J. A. Weinstein and C. Wilson,
Inorg. Chem., 2003, 42, 8609; M. V. Kulikova, K. P. Balashev and Kh.
Erzin, Russ. J. Gen. Chem., 2003, 73, 1839; M. V. Kulikova, K. P.
Balashev and Kh. Erzin, Chem. Abstr., 2004, 141, 174294.
Crystal structure determinations of 4·2CH2Cl2 and 5·2CHCl3
Crystals of both complexes were obtained directly from the
reaction mixtures. The X-ray diffraction data were collected with
a Nonius KappaCCD diffractometer using Mo Ka radiation (k =
˚
0.71073 A). The single crystals were mounted in inert oil within
the cold gas stream of the diffractometer. The Denzo-Scalepack36
program package was used for cell refinements and data reduction.
Structures were solved by direct methods using the SHELXS
program.37 A multiscan absorption correction based on equivalent
reflections (XPREP in SHELXTL v. 6.14 (4)38 or Denzo-Scalepack
(5) was applied to data (Tmin/Tmax values were 0.2427/0.8262 and
0.5504/0.8506 for 4 and 5, respectively). The structure was refined
with SHELXL-9739 and WinGX graphical user interface.40 In 4,
the NH hydrogens were located from the difference Fourier map
and refined isotropically. Other hydrogens were placed in idealized
position and constrained to ride on their parent atom. In 4, the
Pt atom lies on an inversion centre and has planar coordination
geometry while in 5 the asymmetric unit has two independent half
molecules with the Pt atoms on inversion centres. Again, molecules
have planar coordination geometry. The crystallographic data are
summarised in Table 1 and selected bond lengths and angles in
Table 2.
Acknowledgements
This work was supported by the Fundac¸a˜o para a Cieˆncia e a
Tecnologia (FCT), Portugal, through the POCI 2010 programme
(FEDER funded) and by a HRM Marie Curie Research Training
Network (AQUACHEM project MRTN-CT-2003-503864). G. S.
and N. A. B. are grateful to FCT (grants SFRH/BPD/5728/2001
and SFRH/BPD/11448/2002, respectively) and V. Yu. K. for a
grant for Cientista Convidado/Professor at the Instituto Superior
Te´cnico. N. A. B. and V. Yu. K. thank the Russian Fund for
Basic Research for grants 05-03-32140 and 06-03-32065 and the
Scientific Council of the President of the Russian Federation (grant
MK-1040.2005.3). N. A. B., M. H., and V. Yu. K. express their
gratitude to the Academy of Finland for financial support.
References
1 For reviews see: J. N. Demas and B. A. DeGraff, Coord. Chem. Rev.,
2001, 211, 317; A. Mills, Platinum Met. Rev., 1997, 41, 115.
3804 | Dalton Trans., 2006, 3798–3805
This journal is
The Royal Society of Chemistry 2006
©

View Article Online
13 T. Kanbara, K. Okada, T. Yamamoto, H. Ogawa and T. Inoue,
J. Organomet. Chem., 2004, 689, 1860.
24 J. A. Bailey, M. G. Hill, R. E. Marsh, V. M. Miskowski, W. P. Schaefer
and H. B. Gray, Inorg. Chem., 1995, 34, 4591.
25 S. E. Hobert, J. T. Carney and S. P. Cummings, Inorg. Chim. Acta, 2001,
318, 89.
26 P. M. Gidney, R. D. Gillard and B. T. Heaton, J. Chem. Soc., Dalton
Trans., 1973, 132.
27 S. L. Murov, I. Carmichael and G. L. Hug, Handbook of Photochem-
istry, 1993, Marcel Dekker, p. 291.
28 S. Foley, M. N. Berberan-Santos, A. Fedorov, R. Bensasson, S. Leach
and B. Gigante, Chem. Phys., 2001, 263, 437.
29 J. Guo, W.-K. Wong and W.-Y. Wong, Eur. J. Inorg. Chem., 2004, 267.
30 A. R. Siedle, R. J. Webb, F. E. Behr, R. A. Newmark, D. A. Weil, K.
Erickson, R. Naujok, M. Brostrom, M. Mueller, S.-H. Chou and V. G.
Young, Jr., Inorg. Chem., 2003, 42, 932.
31 A. R. Siedle, R. J. Webb, M. Brostrom, R. A. Newmark, F. E. Behr and
V. G. Young, Jr., Organometallics, 2004, 23, 2281.
32 H. V. R. Dias and S. Singh, Inorg. Chem., 2004, 43, 5786.
33 F. Cooper and M. Partridg, Org. Synth., 1958, 36, 62.
34 Yu. N. Kukushkin and T. B. Pakhomova, Zh. Obshch. Khim., 1995, 65,
330.
14 for recent works see: S.-W. Lai and C.-M. Che, Top. Curr. Chem.,
2004, 241, 27; P. Kit-Man Siu, D.-L. Ma and C.-M. Che, Chem.
Commun., 2005, 1025; A. Diez, J. Fornies, A. Garcia, E. Lalinde and
M. T. Moreno, Inorg. Chem., 2005, 44, 2443; S. J. Lee, S. Kang, Jae S.
Lee, S. H. Lee, K. J. Hwang, Y. K. Kim and Y. S. Kim, J. Photosci.,
2003, 10, 185; H. Jude, J. A. Krause Bauer and W. B. Connick, Inorg.
Chem., 2005, 44, 1211; H. Jude, J. A. Krause Bauer and W. B. Connick,
Inorg. Chem., 2004, 43, 725; A. S. Ionkin, W. J. Marshall and Y. Wang,
Organometallics, 2005, 24, 619; B. K.-W. Chiu, M. H.-W. Lam, D. Y.-K.
Lee and W.-Y. Wong, J. Organomet. Chem., 2004, 689, 2888; A. Caubet,
C. Lopez, X. Solans and M. Font-Bardia, J. Organomet. Chem., 2003,
669, 164; C.-M. Che, W.-F. Fu, S.-W. Lai, Y.-J. Hou and Y.-L. Liu,
Chem. Commun., 2003, 118.
15 N. A. Bokach, T. V. Kuznetsova, S. A. Simanova, M. Haukka,
A. J. L. Pombeiro and V. Yu. Kukushkin, Inorg. Chem., 2005, 44,
5152.
16 M. N. Kopylovich, A. J. L. Pombeiro, A. Fischer, L. Kloo and V. Yu.
Kukushkin, Inorg. Chem., 2003, 42, 7239.
17 R. Norrestam, Acta Crystallogr., Sect. C, 1984, 40, 955.
18 J. Baker, M. Kilner, M. M. Mahmoud and S. C. Wallwork, J. Chem.
Soc., Dalton Trans., 1989, 837.
19 V. Yu. Kukushkin, V. M. Tkachuk and N. V. Vorobiov-Desiatovsky,
Inorg. Synth., 1998, 32, 144.
35 A. A. Fedorov, S. P. Barbosa and M. N. Berberan-Santos, Chem. Phys.
Lett., 2006, 421, 157.
36 Z. Otwinowski and W. Minor, Processing of X-ray Diffraction Data
Collected in Oscillation Mode In Methods in Enzymology, Volume
276, Macromolecular Crystallography, Part A, ed. C. W. Carter, Jr. and
R. M. Sweet, Academic Press, New York, 1997, pp. 307–326.
37 G. M. Sheldrick, SHELXS97, Program for Crystal Structure Determi-
nation, University of Go¨ttingen, 1997.
20 T. Uchiyama, Y. Nakamura, T. Miwa, S. Kawaguchi and S. Okeya,
Chem. Lett., 1980, 337; T. Uchiyama, Y. Toshiyasu, Y. Nakamura, T.
Miwa and S. Kawaguchi, Bull. Chem. Soc. Jpn., 1981, 54, 181.
21 M. Hissler, W. B. Connick, D. K. Geiger, J. E. McGarrah, D. Lipa, R. J.
Lachicotte and R. Eisenberg, Inorg. Chem., 2000, 39, 447.
22 H. Kunkely and A. Vogler, Inorg. Chim. Acta, 2001, 319, 183.
23 V. M. Miskowski, V. H. Houlding, C.-M. Che and Y. Wang, Inorg.
Chem., 1993, 32, 2518.
38 G. M. Sheldrick, SHELXTL v. 6.14, Bruker Analytical X-ray Systems,
Bruker AXS, Inc., Madison, Wisconsin, USA, 2003.
39 G. M. Sheldrick, SHELXL-97, Program for Crystal Structure Refine-
ment, University of Go¨ttingen: Go¨ttingen, Germany, 1997.
40 L. J. Farrugia, J. Appl. Crystallogr., 1999, 32, 837.
This journal is
The Royal Society of Chemistry 2006
Dalton Trans., 2006, 3798–3805 | 3805
©