Anisometric cyclometalated palladium(II) and platinum(II) complexes. Structural and photophysical studies

Article abstract of DOI:10.1021/ic9703540

The potentially tridentate ligand 4′-[4-(dodecyloxy)phenyl]-6′-phenyI-2,2′-bipyridine (HL¹) easily undergoes metalation at the ortho-C atom, yielding the rodlike complexes [Pd(L¹)Cl] (1) and [Pt(L¹)Cl] (2). The solid state features and the photophysical properties of the complexes have been studied. Crystal data for 2: PtC₃₄H₃₉ClN₂O, triclinic, P1?, a = 9.043(2) A?, b = 10.874(2) A?, c = 15.710(4) A?, α = 96.95(2)°, β= 92.51(2)°, γ = 102.32(2)°, V = 1494.3(6) A?³, Z = 2, and final R = 0.0387 (R_w = 0.0335) for 4333 independent reflections. The electronic spectra of complexes 1 and 2 show intense absorption bands in the UV region due to metal-perturbed ligand-centered (LC) transitions. In addition, complex 2 shows a moderately intense metal-to-ligand charge-transfer (MLCT) band in the visible region. Both complexes 1 and 2 show luminescence, although under different conditions. Strong emission has been observed for a 77 K glassy solution (highest energy feature 495 nm; τ = 74 μs) or room-temperature crystal (λ_max = 490 nm) of 1. In both cases, the emission was assigned to a triplet LC excited state. Complex 2 exhibits emission from a ³MLCT level both in solution at room temperature (λ_max, = 590 nm; τ = 700 ns; φ = 0.037) and in glass at 77 K (λ_max = 545 nm; τ = 18 μs). The room-temperature emission of crystalline 2 (highest energy feature 590 nm) is also similar in energy to that observed in fluid solution and exhibits the typical MLCT vibrational progression, thus ruling out a contribution from either metal-metal or strong π-π interactions.

Full text of DOI:10.1021/ic9703540

6150
Inorg. Chem. 1997, 36, 6150-6156
Ar ticles
Anisometric Cyclometalated Palladium(II) and Platinum(II) Complexes. Structural and
Photophysical Studies
Francesco Neve* and Alessandra Crispini
Dipartimento di Chimica, Universita` della Calabria, I-87030 Arcavacata di Rende (CS), Italy
Sebastiano Campagna*
Dipartimento di Chimica Inorganica, Chimica Analitica e Chimica Fisica, Universita` di Messina,
via Sperone 31, I-98166 Messina, Italy
ReceiVed March 27, 1997X
The potentially tridentate ligand 4′-[4-(dodecyloxy)phenyl]-6′-phenyl-2,2′-bipyridine (HL1) easily undergoes
metalation at the ortho-C atom, yielding the rodlike complexes [Pd(L1)Cl] (1) and [Pt(L1)Cl] (2). The solid state
features and the photophysical properties of the complexes have been studied. Crystal data for 2: PtC34H39-
ClN2O, triclinic, P1h, a ) 9.043(2) Å, b ) 10.874(2) Å, c ) 15.710(4) Å, R ) 96.95(2)°, â ) 92.51(2)°, γ )
102.32(2)°, V ) 1494.3(6) Å3, Z ) 2, and final R ) 0.0387 (Rw ) 0.0335) for 4333 independent reflections. The
electronic spectra of complexes 1 and 2 show intense absorption bands in the UV region due to metal-perturbed
ligand-centered (LC) transitions. In addition, complex 2 shows a moderately intense metal-to-ligand charge-
transfer (MLCT) band in the visible region. Both complexes 1 and 2 show luminescence, although under different
conditions. Strong emission has been observed for a 77 K glassy solution (highest energy feature 495 nm; τ )
74 µs) or room-temperature crystal (λmax ) 490 nm) of 1. In both cases, the emission was assigned to a triplet
3
LC excited state. Complex 2 exhibits emission from a MLCT level both in solution at room temperature (λmax
) 590 nm; τ ) 700 ns; Φ ) 0.037) and in glass at 77 K (λmax ) 545 nm; τ ) 18 µs). The room-temperature
emission of crystalline 2 (highest energy feature 590 nm) is also similar in energy to that observed in fluid
solution and exhibits the typical MLCT vibrational progression, thus ruling out a contribution from either metal-
metal or strong π-π interactions.
Introduction
transition metals.5 Rodlike, anisotropic molecular shapes are
nearly ideal for obtaining 1-D nematic and 2-D smectic
mesogenic assemblies. The above prerequisite can be satisfied
when the metal is located either in the middle or at the end of
a promesogenic structure.6 A further condition to be realized
is the absence of strong axial intermolecular interactions in the
mesophase.
Here we describe the synthesis of the anisometric cyclo-
metalated Pd(II) and Pt(II) derivatives of 4′-[4-(dodecyloxy)-
phenyl]-6′-phenyl-2,2′-bipyridine (HL1) (Scheme 1), a ligand
able to supply a C,N,N planar donor set.7,8 Although stacking
(driven mainly by π-π interactions) was foreseen,9,10 the mis-
match between planar cores and hydrocarbon chains was ex-
pected to be high and full of possibly interesting consequences.
Low-dimensional supramolecular structures are highly desir-
able when materials with anisotropic physical properties are
designed. A classical example of such a material is a one-
dimensional metal showing either loose or strong metal-metal
interactions in the solid state.1 A more recent approach, in the
direction of shape anisotropism, is the construction of conjugated
metallopolymers,2 molecular3a or supramolecular3b,c wires, and
channels.3b,c On the other hand, when the focus is on the fruitful
compromise between low-dimensionality and disorder the
archetype would certainly be a columnar liquid crystal.4
For many years some of us have been involved in the
synthesis of new types of thermotropic liquid crystals based on
X Abstract published in AdVance ACS Abstracts, November 1, 1997.
(1) (a) Miller, J. S., Ed. Extended Linear Chain Compounds; Plenum
Press: New York, 1982. (b) Keller, H. J., Ed. Chemistry and Physics
of One Dimensional Metals; Plenum Press: New York, 1977.
(2) For recent examples, see: (a) Irwin, M. J.; Jia, G.; Vittal, J. J.;
Puddephatt, R. J. Organometallics 1996, 15, 5321. (b) James, S. L.;
Verspui, G.; Spek, A. L.; van Koten, G. Chem. Commun. 1996, 1309.
(c) Altmann, M.; Bunz, U. H. F. Angew. Chem., Int. Ed. Engl. 1995,
34, 569. (d) Jia, G.; Puddephatt, R. J.; Scott, J. D.; Vittal, J. J.
Organometallics 1993, 12, 3565.
(3) (a) Harriman, A.; Ziessel, R. Chem. Commun. 1996, 1707. (b) van
Nostrum, C. F.; Nolte, R. J. M. Chem. Commun. 1996, 2385. (c) Lehn,
J. M. Supramolecular Chemistry: Concepts and PerspectiVes; VCH:
Weinheim, Germany, 1995; Chapter 8.
(4) (a) Chandrasekkar, S. Liquid Crystals, 2nd ed.; Cambridge University
Press: Cambridge, U.K., 1992; Chapter 6. (b) Vogtle, F. Supramo-
lecular Chemistry; Wiley: Chichester, U.K., 1991; Chapter 8.
(5) (a) Neve, F. AdV. Mater. 1996, 8, 277. (b) Neve, F.; Ghedini, M.; De
Munno, G.; Levelut, A.-M. Chem. Mater. 1995, 7, 688. (c) Neve, F.;
Ghedini, M.; Levelut, A.-M.; Francescangeli, O. Chem. Mater. 1994,
6, 70. (d) Ghedini, M.; Pucci, D.; De Munno, G.; Viterbo, D.; Neve,
F.; Armentano, S. Chem. Mater., 1991, 3, 65. (e) Ghedini, M.;
Armentano, S.; Neve, F.; Licoccia, S. J. Chem. Soc., Dalton Trans.
1988, 1565.
(6) Serrano, J. L., Ed. Metallomesogens; VCH: Weinheim, Germany,
1996.
(7) (a) Bardwell, D. A.; Cargill Thompson, A. M. W.; Jeffery, J. C.;
McCleverty, J. A.; Ward, M. D. J. Chem. Soc., Dalton Trans. 1996,
873. (b) Constable, E. C.; Cargill Thompson, A. M. W.; Cherryman,
J.; Liddiment, T. Inorg. Chim. Acta 1995, 235, 165. (c) Constable, E.
C.; Henney, R. P. G.; Raithby, P. R.; Sousa, L. R. Angew. Chem., Int.
Ed. Engl. 1991, 30, 1363. (d) Constable, E. C.; Henney, R. P. G.;
Leese, T. A.; Tocher, D. A. J. Chem. Soc., Dalton Trans. 1990, 443.
S0020-1669(97)00354-6 CCC: $14.00 © 1997 American Chemical Society

Anisometric Cyclometalated Pd(II) and Pt(II) Complexes
Inorganic Chemistry, Vol. 36, No. 27, 1997 6151
Preparations. 1-(2-Pyridyl)-3-[4-(dodecyloxy)phenyl]propen-1-
one. A solution of 2-acetylpyridine (639 mg, 5.27 mmol) in EtOH
(10 mL) was added dropwise over 10 min to a stirred emulsion of
4-(dodecyloxy)benzaldehyde (1.532 g, 5.27 mmol) and NaOH (287 mg,
7.17 mmol) in EtOH/H2O (40 mL; 3:1, v/v). After 24 h of stirring at
room temperature, the resulting pale yellow solid was filtered off,
washed with diethyl ether, and dried in Vacuo. Yield: 94%. IR (Nujol
Scheme 1
mull): ν(CO), 1673 cm-1 1H NMR (CDCl3): δ (ppm) 8.73 (br d,
.
1H, pyridyl H6), 8.18 (d, 1H, pyridyl H3), 8.17 (d, 1H, olefinic H),
7.91 (d, 1H, olefinic H), 7.86 (br t, 1H, pyridyl H4), 7.68 (d, 2H, phenyl
H2,6), 7.47 (m, 1H, pyridyl H5), 6.91 (d, 2H, phenyl H3,5), 3.99 (t, 2H,
OCH2), 1.79 (m, 2H, OCH2CH2), 1.45-1.26 (m, 18H, (CH2)9), 0.87
(t, 3H, CH3).
4′-[4-(Dodecyloxy)phenyl]-6′-phenyl-2,2′-bipyridine (HL1).
A
mixture of 1-(2-pyridyl)-3-[4-(dodecyloxy)phenyl]propen-1-one (800
mg, 2.03 mmol), 1-(2-oxo-2-phenylethyl)pyridinium bromide (565 mg,
2.03 mmol), and ammonium acetate (1.54 g, 20 mmol) in methanol
(16 mL) was heated to reflux with vigorous stirring. The resulting
orange solution turned dark green after 7 h. Overnight cooling to room
temperature yielded a grayish-violet precipitate. The solid was filtered
off, washed with a small amount of methanol, and dried in Vacuo. The
crude product was purified by column cromatography on SiO2 using
two different eluents consecutively. A yellow band was eluted first
with CH2Cl2, and the yellow eluate was discarded. Further elution of
a brown band with diethyl ether allowed the product to be recovered.
The analytically pure product was obtained as a pinkish-brown solid
upon removal of the solvent under reduced pressure. Yield: 43%.
Mp: 86.5 °C. Anal. Calcd for C34H40N2O: C, 82.88; H, 8.18; N,
5.69. Found: C, 83.01; H, 8.40; N, 5.53. 1H NMR (CDCl3): δ (ppm)
8.71 (br d, 1H, H6), 8.68 (d, 1H, H3), 8.60 (d, 1H, H3′), 8.21 (d, 2H,
H2′′,6′′), 7.95 (d, 1H, H5′), 7.86 (ddd, 1 H, H4), 7.78 (d, 2 H, Ha), 7.50
(m, 3 H, H3′′,4′′,5′′), 7.34 (m, 1 H, H5), 7.02 (d, 2 H, Hb), 4.02 (t, 2 H,
OCH2), 1.82 (m, 2 H, OCH2CH2), 1.48-1.21 (m, 18H, (CH2)9), 0.87
(t, 3H, CH3). 13C{1H} NMR (CDCl3): δ (ppm) 160.2, 157.1, 156.2,
149.8, 149.0, 139.7, 136.7, 130.9, 128.9, 128.7, 128.4, 127.1, 123.6,
121.5, 117.9, 116.9, 115.1, 68.2, 31.9, 29.6, 29.4, 29.3, 26.1, 22.7, 14.1.
[Pd(L1)Cl] (1). A solution of [Pd(PhCN)2Cl2] (55 mg, 0.142 mmol)
in benzene (5 mL) was added to a suspension of HL1 (70 mg, 0.142
mmol) in MeOH (5 mL). The mixture was then stirred at room
temperature for 3 h, affording a pale green microcrystalline precipitate.
The solid was filtered off, washed with MeOH and diethyl ether, and
dried in Vacuo. Yield: 84%. Mp: 188 °C (DSC). Anal. Calcd for
C34H39ClN2OPd: C, 64.45; H, 6.20; N, 4.40. Found: C, 64.80; H,
6.34; N, 4.05. 1H NMR (CDCl3): δ (ppm) 8.24 (br d, 1 H, H6), 7.89
(d, 1 H, H3), 7.75 (ddd, 1 H, H4), 7.58 (d, 2H, Ha), 7.54 (s, 1 H, H3′),
7.36 (d, 1 H, H3′′ or H6′′), 7.18 (s, 1 H, H5′), 7.08 (m, 2 H, H5 + H6′′ or
H3′′), 6.97 (d, 2 H, Hb), 6.86 (br t, 1 H, H4′′ or H5′′), 6.75 (br t, 1 H, H5′′
or H4′′), 4.04 (t, 2 H, OCH2), 1.85 (m, 2 H, OCH2CH2), 1.52-1.29 (m,
18 H, (CH2)9), 0.90 (t, 3 H, CH3).
Another important aspect to be tested was the potential
luminescence of the cyclometalated complexes11 and its rel-
evance to the preparation of new materials. The present study
confirmed the ability of such systems to act as luminophores,
also allowing a comparison between homologous Pd(II) and
Pt(II) species. Unfortunately, a phenyl-p-dodecyloxy pro-
mesogenic group attached to the 6′-phenyl-2,2′-bipyridine back-
bone did not prove adequate to generate calamitic (based on
rodlike species) mesophases from these dagger-shaped mol-
ecules. Different substituents seem to show a higher propensity
to reach this goal.12 A report on part of this work was recently
communicated.12
Experimental Section
[Pt(L1)Cl] (2). A solution of K2PtCl4 (100 mg, 0.24 mmol) in water
(6 mL) was added to a suspension of HL1 (120 mg, 0.24 mmol) in
MeCN/EtOH (12 mL; 2:1, v/v). The mixture was heated to reflux for
20 h and then cooled. The brownish solid was removed by filtration
and washed with EtOH. Further washing with diethyl ether removed
a brown soluble impurity, leaving the product as a microcrystalline
orange solid on the frit. Yield: 83%. Mp: 184 °C (DSC). Anal.
Calcd for C34H39ClN2OPt: C, 56.54; H, 5.44; N, 3.88. Found: C,
56.58; H, 5.50; N, 4.44. 1H NMR (CDCl3): δ (ppm) 8.59 (br d, 1H,
H6), 7.86-7.77 (m, 2 H, H3,4), 7.58 (d, 2 H, Ha), 7.43 (br s, 1H, H3′),
7.40 (m, 1H, H3′′ or H6′′), 7.26 (m, 1 H, H5), 7.17 (br s, 1H, H5′), 7.09
(m, 1H, H6′′or H3′′), 6.98 (d, 2 H, Hb) 4.04 (t, 2 H, OCH2), 1.85 (m, 2
H, OCH2CH2), 1.51-1.24 (m, 18H, (CH2)9), 0.89 (t, 3H, CH3).
Spectroscopic Characterization. Absorption spectra were recorded
on a Kontron Uvikon 860 spectrophotometer. A Perkin-Elmer LS-5B
spectrofluorimeter equipped with a Hamamatsu R928 phototube was
used to obtain luminescence spectra. The spectra were corrected for
photomultiplier response by using a standard lamp. Luminescence
lifetimes were measured with an Edinburgh FL900 single-photon-
counting spectrometer, using a nitrogen discharge as the pulsed-light
source (pulse width: 3 ns). The emission decay traces were decon-
voluted for the instrumental flash lamp by the Marquadt algorithm.
For each measurement, at least five determinations were carried out.
Materials. 2-Acetylpyridine (Aldrich), 4-(dodecyloxy)benzaldehyde
(Aldrich), ammonium acetate (Lancaster), and 1-(2-oxo-2-phenylethyl)-
pyridinium bromide (Lancaster) were used as received; K2PtCl4 was
purchased from Johnson Matthey. [Pd(PhCN)2Cl2]13 was prepared
according to the literature.
(8) (a) Liu, H.-Q.; Cheung, T.-C.; Che, C.-M. J. Chem. Soc., Chem.
Commun. 1996, 1039. (b) Liu, H.-Q.; Cheung, T.-C.; Peng, S.-M.;
Che, C.-M. J. Chem. Soc., Chem. Commun. 1995, 1787. (c) Chan,
C.-W.; Wong, W.-T.; Che, C.- M. Inorg. Chem. 1994, 33, 1266. (d)
Chan, C.-W.; Lai, T.-F.; Che, C.-M.; Peng, S.-M. J. Am. Chem. Soc.
1993, 115, 11245.
(9) Constable, E. C.; Henney, R. P. G.; Leese, T. A.; Tocher, D. A. J.
Chem. Soc., Chem. Commun. 1990, 513.
(10) Cheung, T.-C.; Cheung, K.-K.; Peng, S.-M.; Che, C.-M. J. Chem. Soc.,
Dalton Trans. 1996, 1645.
(11) (a) Maestri, M.; Deuschel-Cornioley, C.; von Zelewsky, A. Coord.
Chem. ReV. 1991, 111, 117. (b) Maestri, M.; Balzani, V.; Deuschel-
Cornioley, C.; von Zelewsky, A. AdV. Photochem. 1992, 17, 1 and
references therein. (c) Carlson, G. A.; Djurovich, P. I.; Watts, R. J.
Inorg. Chem. 1993, 32, 4483. (d) Spellane, P. J.; Watts, R. J.; Vogler,
A. Inorg. Chem. 1993, 32, 5633. (e) Schmid, B.; Garces, F. O.; Watts,
R. J. Inorg. Chem. 1994, 33, 9.
(12) Neve, F.; Ghedini, M.; Crispini, A. Chem. Commun. 1996, 2463.
(13) Karash, M. S.; Seyler, R. C.; Mayo, F. R. J. Am. Chem. Soc. 1938,
60, 882.

6152 Inorganic Chemistry, Vol. 36, No. 27, 1997
Table 1. Crystal Data and Structure Refinement for 2
Neve et al.
Results and Discussion
empirical formula
fw
temp
wavelength
crystal system
space group
unit-cell dimens
C34H39ClN2OPt
722.2
298 K
0.71073 Å
triclinic
P1h
The cyclometalating ligand HL1 can be conveniently prepared
through a typical Kro¨hnke20 synthesis (Scheme 1). The reaction
of the appropriate enone with N-phenacylpyridinium bromide
in the presence of an excess of ammonium acetate resulted in
the formation of a crude product. Purification by chromatog-
raphy afforded the pure ligand in 43% yield.
a ) 9.043(2) Å
b ) 10.874(2) Å
c ) 15.710(4) Å
R ) 96.95(2)°
â ) 92.51(2)°
γ ) 102.32(2)°
1494.3(6) Å3
2
The cyclometalated species [Pd(L1)Cl] and [Pt(L1)Cl] were
then obtained by different standard methods, according to the
different ease of metalation at Pd(II) and Pt(II) centers. A room-
temperature reaction between HL1 and [Pd(PhCN)2Cl2] in
benzene/methanol was sufficient to give [Pd(L1)Cl] (1) as a pale
green solid in high yield. Proton NMR spectroscopy confirmed
the presence of a deprotonated HL1 ligand. As cycloplatina-
tion often requires more drastic conditions, the cyclometalated
[Pt(L1)Cl] (2) was obtained only under reflux conditions by
reaction of K2[PtCl4] and HL1 in acetonitrile/water/ethanol. This
mixture of solvents was found to be preferable to aqueous
acetonitrile, a medium commonly used for similar reactions of
6′-phenyl-2,2′-bipyridine (HL2)7d,10 and 6′-(2-thienyl)-2,2′-bi-
pyridine (HL3).7c This prevented the formation of a black solid
in the early stage of the reaction. [Pt(2-Me-allyl)Cl]2 could not
be used as a starting material for a cycloplatination to be carried
out under milder conditions.21 In fact, attempts to use [Pt(2-
Me-allyl)Cl]2 to cyclometalate HL1 only gave an unknown
species that contained a “Pt(HL1)” fragment with an N,N-
coordination and which is not simply [Pt(HL1)Cl2]. Complex
2 formed directly from the the reaction mixture as a pure, orange
solid. However, recrystallization gave two different forms,
depending on the method used. Whereas rapid precipitation
from a chloroform solution with diethyl ether gave 2 as a dark
green solid, slow diffusion of ethanol through the chloroform
solution gave well-formed, orange crystals of 2. In both cases,
solutions of the two forms have identical proton NMR spectra.
A similar behavior has been previously observed by Constable
for [Pt(L2)(MeCN)]PF6.9
volume
Z
density (calcd)
abs coeff
F(000)
1.605 g/cm3
48.14 cm-1
720
scan type
2θ-θ
no. of independent reflns
no. of obsd reflns [I > 2σ(I)]
no. of parameters
final R indicesa,b
goodness-of-fit
largest and mean ∆/σ
5225 [R(int) ) 0.017]
4333
352
R ) 0.0387, R′ ) 0.0335
1.50
0.001, 0.000
a R ) ∑||Fo| - |Fc||/∑|Fo|. b R′ ) [∑w(|Fo| - |Fc|)2/∑w|Fo| )1/2
.
2
Luminescence quantum yields were obtained by using the optically
dilute method14 (quantum yield standards were [Ru(bpy)3]2+ in aerated
aqueous solution (Φ ) 0.02815) for the visible emission and anthracene
in degassed ethanol (Φ ) 0.2716) for the UV emission). Experimental
errors were as follows: absorption and emission maxima, (2nm; molar
absorption coefficients, 10%; emission lifetimes, 10%; luminescence
quantum yields, 20%.
1H and 13C NMR spectra were recorded on a Bruker AC300
spectrometer.
Crystal Structure Determination. Orange crystals of [Pt(L1)Cl]
(2) suitable for diffraction analysis were obtained from a chloroform/
ethanol mixture. Diffraction measurements were carried out on a
Siemens R3m/V automated four-circle diffractometer equipped with
graphite-monochromated Mo KR radiation (λ ) 0.710 73 Å). The unit
cell dimensions were obtained by application of the automatic diffrac-
tometer indexing routine to the positions of 33 reflections, with 2θ
angles in the range 16-28°. The data were corrected for Lorentz,
polarization, and X-ray absorption effects, and an empirical absorption
correction was applied using a method based upon azimuthal (Ψ) scan
data.17 Details of crystal data collection are listed in Table 1. The
structure was solved by Patterson and Fourier methods and refined by
full-matrix least-squares procedures. The weighting scheme used in
The thermal behavior of complexes 1 and 2 was studied by
differential scanning calorimetry (DSC) (rate of 10 °C min-1
)
and optical microscopy. Both species have similar melting
points, although 2 undergoes several endothermic crystal-to-
crystal transitions before melting into a red-orange isotropic
liquid. In addition, 2 shows an exothermic transition at about
158 °C, also visible from polarizing optical microscopy. The
latter technique confirmed the absence of liquid crystalline
phases.
X-ray Crystal Structure of [Pt(L1)Cl] (2). The atomic
coordinates and isotropic displacement coefficients, U(eq), are
listed in Table 2. Table 3 lists relevant bond lengths and angles.
A view of the structure of 2 is shown in Figure 1. Although
already briefly reported,12 the X-ray crystal structure of 1 will
be recalled for comparison, where appropriate.
The Pt atom in 2 reveals a distorted square-planar geometry
with the N(2)-Pt-C(2) angle of 161.4(3)° deviating from
linearity. This type of distortion is common for Pt(II) square-
planar centers bound to terdentate ligands that form two fused
five-membered chelate rings.8d,9,10,22,23 The corresponding angle
for [Pd(L1)Cl] (1) has been found to be 160.9(4)°.12 The bond
the last refinement cycle was w-1 ) σ2|Fo| + q|Fc| , with q ) 0.0001.
2
All the non-hydrogen atoms were refined anisotropically, and the
hydrogen atoms were included as idealized atoms riding on the
respective carbon atoms with C-H bond lengths appropriate to the
carbon atom hybridization. The isotropic displacement parameter of
each hydrogen atom was fixed at U ) 0.08 Å2. Convergence for 4333
observed reflections [I > 2σ(I)] and 352 parameters was reached at R
) 0.039 and R′ ) 0.034 with a goodness of fit of 1.50. The final
Fourier-difference map was featureless, with maximum positive and
negative peaks of 1.43 (1.1 Å from the Pt atom) and -0.69 e Å-3
,
respectively. All calculations were performed with SHELXTL PLUS18
and PARST19 programs. Atomic scattering factors were as implemented
in the SHELXTL PLUS program.
distances within the two chelate rings [PtNCCN and [PtCCCN]
(14) Demas, J. N.; Crosby, G. A. J. Phys. Chem. 1971, 75, 991.
(15) Nakamaru, K. Bull. Chem. Soc. Jpn. 1982, 55, 2697.
(16) Dawson, W. R.; Windsor, M. W. J. Phys. Chem. 1968, 72, 3251.
(17) North, A. C. T.; Phillips, D. C. Acta Crystallogr., Sect. A 1968, 24,
351.
(18) SHELXTL PLUS, Version 4.21; Siemens Analytical X-ray Instruments
Inc.: Madison, WI, 1990.
(20) Kro¨hnke, F. Synthesis 1976, 1.
(21) Pregosin, P. S.; Wombacher, F.; Albinati, A.; Lianza, F. J. Organomet.
Chem. 1991, 418, 249.
(22) Yip, H.-K; Cheng, L.-K.; Cheung, K.-K.; Che, C.-M. J. Chem. Soc.,
Dalton Trans. 1993, 2933.
(23) Minghetti, G.; Cinellu, M. A.; Stoccoro, S.; Zucca, A.; Manassero,
M. J. Chem. Soc., Dalton Trans. 1995, 777.
(19) Nardelli, M. Comput. Chem. 1983, 7, 95.

Anisometric Cyclometalated Pd(II) and Pt(II) Complexes
Inorganic Chemistry, Vol. 36, No. 27, 1997 6153
Table 2. Atomic Coordinates (×104) and Equivalent Isotropic
Displacement Coefficients (Å2 × 103) for 2
atom
x
y
z
U(eq)a
Pt
Cl
N(1)
N(2)
O(1)
C(1)
C(2)
C(3)
C(4)
C(5)
C(6)
C(7)
C(8)
C(9)
C(10)
C(11)
C(12)
C(13)
C(14)
C(15)
C(16)
C(17)
C(18)
C(19)
C(20)
C(21)
C(22)
C(23)
C(24)
C(25)
C(26)
C(27)
C(28)
C(29)
C(30)
C(31)
C(32)
C(33)
C(34)
823(1)
2916(2)
-891(6)
1793(5)
3623(1)
2915(2)
4249(5)
5588(5)
6888(5)
2097(6)
1978(6)
767(6)
4088(1)
4533(1)
3679(3)
4175(3)
1585(3)
3594(4)
3905(4)
4079(5)
3970(5)
3655(5)
3458(5)
3452(4)
3131(4)
3030(4)
3285(4)
3600(4)
3876(4)
3875(5)
4157(5)
4465(5)
4455(4)
2648(4)
2468(5)
2102(4)
1926(4)
2098(5)
2447(5)
1311(5)
1028(5)
602(5)
47(1)
71(1)
44(2)
47(2)
77(2)
52(3)
53(3)
59(3)
67(3)
75(3)
64(3)
49(2)
50(2)
46(2)
49(2)
46(2)
47(2)
56(3)
65(3)
62(3)
53(3)
45(2)
60(3)
59(3)
58(3)
74(3)
63(3)
68(3)
80(4)
76(4)
82(4)
85(4)
90(4)
86(4)
81(4)
74(3)
81(4)
78(4)
120(5)
-8179(5)
-2153(7)
-681(7)
-573(8)
-1809(8)
-3210(9)
-3391(8)
-2244(7)
-3465(7)
-3345(7)
-1950(7)
-755(7)
-234(7)
-102(7)
1072(7)
3395(6)
3811(6)
5094(6)
5916(6)
5486(6)
6252(6)
7544(6)
8177(7)
7502(7)
6216(7)
5527(6)
4738(7)
5145(7)
6375(7)
7174(7)
6760(7)
6123(7)
7048(8)
6425(8)
7401(8)
6883(9)
7901(9)
7443(9)
8492(8)
8050(8)
9105(8)
8669(8)
9763(8)
804(7)
1223(8)
2667(8)
3682(8)
Figure 1. Molecular structure of [Pt(L1)Cl] (2) showing the numbering
scheme adopted. Thermal ellipsoids are depicted at the 50% probability
level.
3206(7)
-4644(7)
-6077(8)
-7275(7)
-7077(8)
-5674(9)
-4487(8)
-9638(7)
-10559(8)
-12120(8)
-12975(8)
-14542(8)
-15335(8)
-16938(9)
-17673(8)
-19284(8)
-20025(8)
-21630(8)
-22326(10)
2.02(2) Å, respectively. In our case, the longer Pt-N(2) dis-
tance also results from the strong trans influence of the phenyl
carbon C(2).
Both complexes 1 and 2 show essentially planar chelate rings.
The maximum deviations from planarity are 0.007(8) Å [N(2)
in [PdNCCN]] and 0.027(6) Å [C(12) in [PtNCCN]], respec-
tively. The dihedral angles between the two five-membered
rings are very similar (1.2(2)° in 1 and 1.3(2)° in 2 ). For both
complexes 1 and 2, the whole coordinated portion of the ligand
is planar, as proved by the dihedral angles between the central
and the lateral aromatic rings [3.1(3) and 4.1(2)° for complex
1; 4.2(2) and 1.1(2)° for complex 2]. The rotationally free
phenyl ring becomes coplanar with the rest of the aromatic
system on going from complex 1 to complex 2, with tilt angles
of 21.5(1)° in the former case and 8.0(1)° in the latter. A further
difference between the structures of 1 and 2 is related to the
conformation of the peripheral hydrocarbon chain. While in 1
the dodecyloxy chain deviates strongly from an all-anti con-
formation [maximum deviation of 58(4)° about C(30)-C(31)12],
in 2 it shows a regular anti conformation with mean torsion
angle of 178(7)°.
The packing of complex 2 in the crystal is shown in Figure
2 (top view). The shortest Pt---Pt intermolecular separation of
4.426(1) Å occurs between centrosymmetrically related mol-
ecules, which are approximately superimposed along the c axis.
The two molecules are slipped-stacked along the a axis, with a
transverse slip around 4 Å. This is reminiscent of the molecular
packing observed in complex 1, where the shortest Pd---Pd
contact is 5.667(2) Å.12 In both cases, the two molecules
superimpose in a head-to-tail orientation and the two square
planes are twisted with respect to each other, with average
torsion angles of 37.9° (complex 1) and 37.0° (complex 2).
Although it is reasonable to exclude a metal-metal interaction
in complex 2 (Pt---Pt > 4 Å), the existence of ligand π-π
interactions within the dimeric units is apparent from the
intermolecular plane separation (3.42 Å) between the coordi-
nated portions of the ligand. The stacked parts of the ligand
356(5)
-84(5)
-300(6)
-712(5)
-952(5)
-1343(5)
-1546(6)
-1955(5)
-2150(7)
a Equivalent isotropic U defined as one-third of the trace of the
orthogonalized Uij tensor.
Table 3. Selected Bond Lengths and Angles for [Pt(L1)Cl] (2)
Bond Distances (Å)
Pt-Cl
2.302(2)
2.115(5)
1.372(7)
1.365(9)
1.475(10)
Pt-N(1)
1.935(5)
1.984(6)
1.345(8)
1.434(10)
1.493(8)
Pt-N(2)
Pt-C(2)
N(1)-C(7)
N(2)-C(12)
C(1)-C(7)
N(1)-C(11)
C(1)-C(2)
C(11)-C(12)
Bond Angles (deg)
177.8(2) Cl-Pt-N(2)
79.1(2) Cl-Pt-C(2)
Cl-Pt-N(1)
99.6(1)
98.9(2)
161.4(3)
121.0(4)
114.5(5)
111.9(6)
N(1)-Pt-N(2)
N(1)-Pt-C(2)
Pt-N(1)-C(7)
Pt-N(2)-C(12)
Pt-C(2)-C(1)
82.4(2) N(2)-Pt-C(2)
118.4(4) Pt-N(1)-C(11)
112.4(3) C(2)-C(1)-C(7)
112.8(5) N(1)-C(7)-C(1)
N(1)-C(11)-C(12) 112.3(6) N(2)-C(12)-C(11) 115.1(5)
are in good agreement with those reported for other platinum-
(II) 6′-phenyl-2,2′-bipyridine complexes9,10 and are similar to
those seen for 1. Moreover, the presence of a short Pt-N bond
length to the central pyridine ring [1.935(5) Å] and a longer
one to the lateral pyridine ring [2.115(5) Å] is common to the
platinum(II)-2,2′:6′,2′′-terpyridine derivatives. The analysis of
nine structures containing the 2,2′:6′,2′′-terpyridine coordinated
ligand recorded in the Cambridge Structural Database (CSD)24
has given mean values of the Pt-N bonds as 1.93(2) and
are the cycloplatinated ring [PtCCCN] of one molecule and the
lateral pyridine ring of the centrosymmetrically related molecule.
The repetition of the dimeric unit along the a axis gives rise to
the shortest interdimeric Pt---Pt separation of 7.787(2) Å. In
addition, molecules of dimeric units show partial overlap
between the rotationally free aromatic ring of one molecule and
(24) Allen, F. H.; Davies, J. E.; Galloy, J. J.; Johnson, O.; Kennard, O.;
Macrae, C. F.; Mitchell, E. M.; Smith, J. M.; Watson, D. G. J. Chem.
Inf. Comput. Sci. 1991, 31, 187.

6154 Inorganic Chemistry, Vol. 36, No. 27, 1997
Neve et al.
Figure 3. Absorption spectrum (on the left) of [Pt(L1)Cl] in dichlo-
romethane solution at room temperature and luminescence spectra of
the same complex in butyronitrile rigid matrix at 77 K (a), in
dichloromethane fluid solution at 298 K (b, dotted line), and in the
solid state at 298 K (c). The luminescence spectra shown here are not
corrected for photomultiplier response. Corrected values are given in
the text.
metal-perturbed ligand-centered (LC) transitions. Complex 1
does not exhibit (Figure 4) a MLCT band in the visible region,
as expected because the palladium(II) center is much more
difficult to oxidize than the platinum(II) center. Therefore,
MLCT transitions should occur at higher energy in 1 than in
the platinum complex and probably contribute to the strong
absorption at about 320 nm (ꢀ ) 15 690 M-1 cm-1), overlapping
with the usual LC bands. In the absorption spectra of both
complexes, a weak shoulder is present at about 365 nm. The
fact that the energy of such a shoulder is unaffected on going
from palladium to platinum complexes rules out metal-centered
(MC) and MLCT attributions. This absorption feature is
reminescent of the shoulder at 360 nm which is present in the
spectrum of the protonated ligand, so that we have tentatively
assigned it to n f π* transitions involving the oxygen of the
alkoxy group, which is far away from the chelation sites and is
practically unperturbed by metal coordination.
The protonated ligand HL1 exhibits strong UV luminescence
when excited in the π-π* absorption band (see Figure S1;
highest energy feature of the emission spectrum 350 nm;
luminescence lifetime 1.6 ns; quantum yield 0.18). Because
of the energy, lifetime, and quantum yield, the luminescence is
assigned to the singlet π f π* excited state. On excitation of
the absorption tail at wavelengths longer than 380 nm, another
very weak, broad, and unstructured luminescence band peaking
at 500 nm can be seen (Φ < 10-4; excitation wavelength 390
nm). This band is tentatively assigned to the alkoxyphenyl
moiety, suggesting that in the protonated free ligand this
chromophore is essentially uncoupled with the phenyl-bipy-
ridine framework, in which the π f π* fluorescence originates.
Photophysical properties of platinum(II)- and palladium(II)-
polypyridine complexes have been extensively studied.27 The
lack of room-temperature luminescence in fluid solution for
many Pd(II) and Pt(II) complexes is usually due to the presence
of low-lying MC excited states, which can be easily populated
by thermal activation and provide fast deactivation pathways
Via molecular distortion.11 The situation is quite different in a
rigid matrix at 77 K. Under these conditions, the thermally-
activated deactivation processes are blocked and luminescence
from MLCT and/or LC levels can be obtained.11 Besides the
MLCT and LC excited states, dσ* f π*(polypyridine) charge-
Figure 2. Crystal packing of [Pt(L1)Cl] (2) (top view) and [Pd(L1)Cl]
(1) (bottom view).
the lateral pyridine ring of the other, with the shortest interatomic
distance at 3.40(1) Å.
Finally, crystal packing shows that a compromise is at work
between two different inherent tendencies of such complexes.
Whereas the planar cores (that represent the polar, or at least
polarizable, part of the molecules) roughly pile up along one
direction, the nonpolar chains tend to confine themselves to a
nonpolar region. This segregation effect is maximized for the
platinum homologue 2. Reasons for a somewhat different
packing for 1 (Figure 2, bottom view) are to be found either in
the different conformation of the hydrocarbon chain or in the
reduced tendency of planar Pd(II) centers to stack.
Absorption Spectra and Photophysical Properties. The
free protonated HL1 ligand and its Pd(II) and Pt(II) complexes
are fairly stable in CH2Cl2, as demonstrated by the constancy
of their absorption spectra over 1 week.
The absorption spectrum of HL1 (Figure S1 (Supporting
Information)) is dominated by a strong band peaking at 295
nm (ꢀ ) 28 480 M-1 cm-1) followed at longer wavelengths by
a weakly absorbing tail exhibiting a shoulder at about 360 nm
(ꢀ ) 1030 M-1 cm-1). The strong UV band is attributed to a
π f π* transition involving the aromatic framework, on the
basis of the extinction coefficient and the similarity with the π
f π* band reported for other polypyridine complexes.11,25 The
tail at lower energy is assigned to n f π* transitions, probably
involving pyridine nitrogens and the alkoxy fragment.
The absorption spectrum of complex 2 (Figure 3, Table S5
(Supporting Information)) shows a moderately intense visible
absorption feature (λmax ) 420 nm ; ꢀ ) 5400 M-1 cm-1) which
can be safely assigned to a metal-to-ligand charge-transfer
(MLCT) transition from the dπ metal orbitals to a π* orbital of
the polypyridine ligand, by similitude with Pt(II)-terpypyri-
dine26 and cyclometalated11 compounds, and intense absorption
bands in the UV region. The latter bands are attributable to
(27) For some representative examples, see refs 8, 10, 11, 26, and 28.
(28) (a) Sandrini, D.; Maestri, M.; Balzani, V.; Chassot, A.; von Zelewsky,
A. J. Am. Chem. Soc. 1987, 109, 7720. (b) Kunkely, H.; Vogler, A.
J. Am. Chem. Soc. 1990, 112, 5625. (c) Blanton, C. B.; Rillema, D.
P. Inorg. Chim. Acta 1990, 168, 145. (d) Cummings, S. D.; Eisenberg,
R. Inorg. Chem. 1995, 34, 2007. (e) Rosace, G.; Giuffrida, G.; Saitta,
M.; Guglielmo, G.; Campagna, S.; Lanza, S. Inorg. Chem. 1996, 35,
6816.
(25) Lytle, F. E.; Hercules, D. M. J. Am. Chem. Soc. 1969, 91, 253.
(26) (a) Aldridge, T. K.; Stacy, E. M.; McMillin, D. R. Inorg. Chem. 1994,
33, 722. (b) Bailey, J. A.; Hill, M. G.; Marsch, R. E.; Miskowski, V.
M.; Schaefer, W. P.; Gray, H. B. Inorg. Chem. 1995, 34, 4591.

Anisometric Cyclometalated Pd(II) and Pt(II) Complexes
Inorganic Chemistry, Vol. 36, No. 27, 1997 6155
3
attributed to MLCT level(s),30 both in glassy matrix at 77 K
and in fluid solution at room temperature. The excitation
spectrum of 2 closely matches the corresponding absorption
spectrum. In particular, even the shoulder at about 365 nm,
assigned to the transition within the alkoxyphenyl chromophore
(see above), contributes to the excitation spectrum, so indicating
that excitation in the latter chromophore also results in popula-
3
tion of the luminescent MLCT level. This contrasts with the
double emission exhibited by the HL1 free ligand, suggesting
that, in the platinum(II) complex, coupling between the alkoxy-
phenyl-centered excited state and the triplet MLCT state occurs.
Figure 4. Absorption spectrum (on the left) of [Pd(L1)Cl] in dichlo-
romethane solution at room temperature and luminescence spectra of
the same complex in butyronitrile rigid matrix at 77 K (a) and in the
solid state at 298 K (b). The luminescence spectra shown here are not
corrected for photomultiplier response. Corrected values are given in
the text.
It is now interesting to compare the photophysical properties
of [Pt(L1)Cl] (2) with those reported for the related cyclometa-
lated complex [Pt(L2)Cl] (as previously stated, HL2 ) 6′-phenyl-
3
2,2′-bipyridine),10 which is also luminescent from a MLCT
excited state both in fluid solution at room temperature and in
glass at 77 K. The luminescent MLCT state of [Pt(L1)Cl] is
displaced toward the red with respect to the luminescent MLCT
state of [Pt(L2)Cl] (compare the 590 nm emission maximum of
[Pt(L1)Cl] with the 565 nm emission maximum of [Pt(L2)Cl]
in dichloromethane fluid solution and the 545 nm highest
energy emission feature of [Pt(L1)Cl] in butyronitrile at 77 K
and the 515 nm highest energy emission feature of [Pt(L2)Cl]
in acetonitrile at 77 K). Such an energy difference can be
attributed to the presence of an additional phenyl ring in L1.
The longer luminescence lifetime and higher quantum yield of
the room-temperature emission of [Pt(L1)Cl] compared with
those for [Pt(L2)Cl] (0.70 µs and 0.037 Vs 0.51 µs and 0.025
for luminescence lifetimes and quantum yields of [Pt(L1)Cl]
and [Pt(L2)Cl],10 respectively) can be attributed to the in-
creased energy difference in [Pt(L1)Cl] between the luminescent
transfer (hereafter MMLCT) excited states have to be taken into
account in d8 square planar compounds. These excited states
originate from metal-metal interactions usually occurring in
stacked dimers (or polymers).29 They are typical of solid-state
systems and have been demonstrated to be responsible for the
emission of a number of metal-metal-bonded systems.29
Furthermore, when face-to-face oligomerization between Pt(II)
or Pd(II) complexes carrying large aromatic frameworks
leads to suitable arrangements, even excimeric emission can
occur.10,26b,28b Oligomer formation, and the corresponding
MMLCT and/or excimeric emission, is not limited to solid or
glassy systems but is also found in fluid solution,28b and can
give useful information about the extent of intermolecular
interactions between the individual molecules, topics which are
of basic importance for a number of interesting properties of
the compounds, including liquid crystal behavior.
3
3MLCT level and an upper-lying MC excited state, which is
Complex 1 shows strong emission (highest energy feature
495 nm; τ ) 74 µs) in a butyronitrile rigid matrix at 77 K (Table
S5, Figure 4). The 77 K glass emission spectrum is highly
structured, exhibiting a vibronic progression of about 1300 cm-1
typical of C-N and C-C aromatic stretchings, and it can be
essentially attributed to a 3LC excited state partially mixed with
a closely-lying 3MLCT level, on the basis of the spectral shape
and lifetime.11,30 The luminescence is totally quenched in fluid
solution at room temperature, as usual for Pd(II)-polypyridine
complexes, because of the presence of low-lying, thermally-
held responsible for the dominant radiationless deactivation
pathway of the luminescent level. Indeed, the lowest 3MC levels
of [Pt(L1)Cl] and [Pt(L2)Cl] should lie at practically the same
energy.
A further interesting difference between the photophysical
properties of [Pt(L1)Cl] and [Pt(L2)Cl] concerns their lumines-
cence behavior in the solid state. In fact, the solid state emission
of [Pt(L2)Cl] measured at room temperature (λmax ) 665 nm)
is substantially lower in energy than the emission of the complex
at 77 K in a glassy matrix and at room temperature in fluid
3
activated MC states.11,28 Room-temperature luminescence is
3
solution. This emission was assigned to a MMLCT excited
also exhibited by microcrystalline samples of 1 (λmax ) 490
state,10 in line with the crystal structure of the complex, which
shows an intermolecular Pt-Pt separation of 3.28 Å, sufficient
for inducing strong metal-metal interactions. On the contrary,
the emission of [Pt(L1)Cl] in the solid state at room tempera-
ture (highest energy feature at 590 nm, Figure 4) is quite similar
in energy to the emission recorded in fluid solution, so that a
3MLCT origin is proposed. The presence of a vibrational
progression in the solid state emission strongly supports such
an assignment.
nm). Although the emission spectrum is unstructured (Figure
3
4), we assign this emission to the same LC level responsible
for the 77 K glassy emission, essentially because it occurs at a
similar energy.
Complex 2 exhibits emission both at room temperature in
dichloromethane fluid solution (λmax ) 590 nm; τ ) 700 ns; Φ
) 0.037) and at 77 K in butyronitrile glasses (λmax ) 545 nm;
τ ) 18 µs; see Table S5 and Figure 3). The emission spectrum
at 77 K is structured, and the vibrational progression is typical
of luminescence originating from either 3MLCT or 3LC
levels.11,30 The emission spectrum at room temperature in fluid
solution is slightly displaced to the red with respect to the 77
K emission, and the structure is almost totally lost. Because
of the luminescence lifetimes and of the rate constant of the
radiative transition (kr ) 5.3 × 104 s-1), obtained from room-
temperature τ and Φ values, the emission of [Pt(L1)Cl] is
The absence of any trace of MMLCT as well as of excimeric
emission in [Pt(L1)Cl] (2) indicates that axial metal-metal
interactions and π-π aromatic interactions between the poly-
pyridine frameworks are not strong enough to affect the
photophysical properties of the “isolated” molecules in both fluid
solution and glasses. Although the situation can obviously be
different in the fluid and glassy states, metal-metal distances
are not expected to decrease and π-π interactions are not
expected to be enhanced. Furthermore, the absence of the above
type of emission even in the solid state is in fair agreement
with the crystal structure of the complex, which shows a Pt-Pt
separation exceeding 4 Å and a head-to-tail arrangements of
the complex molecules in the crystal packing. The Pt-Pt
(29) (a) Rounhill, D. M.; Gray, H. B.; Che, C.-M. Acc. Chem. Res. 1989,
22, 55. (b) Houlding, V. H.; Miskowski, V. M. Inorg. Chem. 1991,
30, 4446. (c) Nocera, D. G. Acc. Chem. Res. 1995, 28, 209. (d) Hill,
M. G.; Bailey, J. A.; Miskowski, V. M.; Gray, H. B. Inorg. Chem.
1996, 35, 4585.
(30) Crosby, G. A. Acc. Chem. Res. 1975, 8, 231.

6156 Inorganic Chemistry, Vol. 36, No. 27, 1997
Neve et al.
separation is too large to allow significant axial metal-metal
interaction from spectroscopic and photophysical viewpoints,
and π-π interactions between polypyridine moieties are not
effective as a consequence of the unfavorable arrangement of
the molecules within the crystal.
under different conditions. This study correlates the solid state
properties of the cyclometalated complexes with the photo-
physical behavior. It is apparent that the introduction of long-
chain alkoxy substituents in the periphery of the ligand leads
to a reduced tendency toward metal-metal and interplanar
stacking. This is reflected in luminescence properties signifi-
cantly different from those of analogous species.
Conclusions
Acknowledgment. Financial support from the italian Min-
istero dell’Universita` e della Ricerca Scientifica e Tecnologica
(MURST) and Consiglio Nazionale delle Ricerche (CNR) is
gratefully acknowledged.
We have shown that metalation of the HL1 ligand effectively
promotes lamellar or pseudolamellar crystal packing, depending
on the nature of the metal ion. Yet the strong anisometry of
the complexes is not sufficient to generate mesomorphism, as
the factors that control the stability of the crystal phase are of
increased importance.
Supporting Information Available: Full tables listing atomic
coordinates for hydrogens, complete bond lenghts and angles, and
anisotropic thermal parameters for compound 2 (Tables S1-S4),
absorption and emission spectra for the HL1 ligand (Figure S1), and a
listing of photophysical data for the complexes (Table S5) (6 pages).
Ordering information is given on any current masthead page.
Whereas the desired liquid-crystalline behavior failed to
appear, photophysical studies have revealed interesting aspects,
confirming the attractiveness of cyclometalated10,11,31 and poly-
pyridine32 Pd(II) and Pt(II) species from a photophysical
viewpoint. In fact, both 1 and 2 exhibit luminescence, although
IC9703540
(32) For recent examples, see: (a) Kato, M.; Sasano, K.; Kosuge, C.;
Yamazaki, M.; Yano, S.; Kimura, M. Inorg. Chem. 1996, 35, 116.
(b) Connick, W. B.; Henling, L. M.; Marsh, R. E.; Gray, H. B. Inorg.
Chem. 1996, 35, 6261.
(31) (a) Mdleleni, M. M.; Bridgewater, J. S.; Watts, R. J.; Ford, P. C. Inorg.
Chem. 1995, 34, 2334. (b) Craig, C. A.; Watts, R. J. Inorg. Chem.
1989, 28, 309.