Influence of steric and electronic factors in the stabilization of five-coordinate ethylene complexes of platinum(II): X-ray crystal structure of [PtCl2(2,9-dimethyl-1,10-phenanthroline-5,6-dione)]

Article abstract of DOI:10.1016/S0020-1693(03)00382-7

Reactions of Zeise's salt (K[Pt(??²-C₂H ₄)Cl₃]) with oxidized phenanthroline ligands (1,10-phenanthroline-5,6-dione, phedon, and 2,9-dimethyl-1,10-phenanthroline-5, 6-dione, Me₂phedon) are reported. Comparison with analogous reactions involving unoxidized phen (1,10-phenanthroline) and Me ₂phen (2,9-dimethyl-1,10-phenanthroline) ligands indicates that these latter ligands are less capable to stabilize the five-coordinate species [PtCl₂(η²-C₂H₄)(phenanthroline)] in which the phenanthroline and the olefin share the trigonal plane and two chlorines are in the axial positions. The X-ray structure of the four-coordinate species [PtCl₂(Me₂phedon)] indicates that the major difference between oxidized and unoxidized phenanthrolines is the loss of aromaticity of the central ring of phenanthroline. As a consequence, the oxidized phenanthroline becomes more flexible and can undergo a bow-like distortion so to reduce steric interaction between ortho substituents of phenanthroline and cis chlorine ligands. The increase in stability of the four-coordinate species with Me₂phedon is concomitant with an increase in stability of the five-coordinate precursor complex with ethylene. In the latter case the stabilization is not of sterical origin but stems from reduced electron-donor properties of oxidized phenanthrolines. The balance of the two effects is such that the equilibrium between five- and four-coordinate species is more shifted in favour of the former species in the case of Me ₂phedon than in the case of Me₂phen.

Full text of DOI:10.1016/S0020-1693(03)00382-7

Inorganica Chimica Acta 357 (2004) 149–158
www.elsevier.com/locate/ica
Influence of steric and electronic factors in the
stabilization of five-coordinate ethylene complexes
of platinum(II): X-ray crystal structure of
[PtCl2(2,9-dimethyl-1,10-phenanthroline-5,6-dione)]
a
Nicola Margiotta , Valerio Bertolasi , Francesco Capitelli , Luciana Maresca ,
b
c
a
c
Anna G.G. Moliterni , Francesco Vizza , Giovanni Natile
d
a,
*
a
Dipartimento Farmaco-Chimico, Universitaꢀ di Bari, via E. Orabona 4, 70125 Bari, Italy
b
Dipartimento di Chimica e Centro di Strutturistica Diffrattometrica, Universitaꢀ di Ferrara, Via Borsari 46, 44100 Ferrara, Italy
CNR IC (Istituto di Cristallografia), c/o Dipartimento Geomineralogico, via E. Orabona 4, 70125 Bari, Italy
c
d
Istituto di Chimica dei Composti Organometallici ICCOM-CNR, via J. Nardi 39, 50132 Florence, Italy
Received 30 May 2003; accepted 7 June 2003
Abstract
Reactions of ZeiseÕs salt (K[Pt(g2-C2H4)Cl3]) with oxidized phenanthroline ligands (1,10-phenanthroline-5,6-dione, phedon, and
2,9-dimethyl-1,10-phenanthroline-5,6-dione, Me2phedon) are reported. Comparison with analogous reactions involving unoxidized
phen (1,10-phenanthroline) and Me2phen (2,9-dimethyl-1,10-phenanthroline) ligands indicates that these latter ligands are less
capable to stabilize the five-coordinate species [PtCl2(g2-C2H4)(phenanthroline)] in which the phenanthroline and the olefin share
the trigonal plane and two chlorines are in the axial positions. The X-ray structure of the four-coordinate species
[PtCl2(Me2phedon)] indicates that the major difference between oxidized and unoxidized phenanthrolines is the loss of aromaticity
of the central ring of phenanthroline. As a consequence, the oxidized phenanthroline becomes more flexible and can undergo a bow-
like distortion so to reduce steric interaction between ortho substituents of phenanthroline and cis chlorine ligands. The increase in
stability of the four-coordinate species with Me2phedon is concomitant with an increase in stability of the five-coordinate precursor
complex with ethylene. In the latter case the stabilization is not of sterical origin but stems from reduced electron-donor properties
of oxidized phenanthrolines. The balance of the two effects is such that the equilibrium between five- and four-coordinate species is
more shifted in favour of the former species in the case of Me2phedon than in the case of Me2phen.
Ó 2003 Elsevier B.V. All rights reserved.
Keywords: Platinum; Phenanthroline-dione; Five-coordination; X-ray crystallography
1. Introduction
positions and the bidentate nitrogen ligand and the
ethylene in the equatorial plane [1].
The reaction between ZeiseÕs salt (K[PtCl3(C2H4)])
and bidentate N-donor ligands such as diimines or
aliphatic and aromatic diamines (N–N) leads to the for-
mation of five-coordinate complexes having [PtCl2(g2-
ethylene)(N–N)] composition and trigonal bipyramidal
structure with the two chlorine ligands in the apical
By this procedure a large number of five-coordinate
complexes of platinum have been characterized, in quite
a few cases also via X-ray analysis [2–7]. The bidentate
ligand and the alkene occupy invariably the equatorial
plane of the trigonal bipyramid and this spatial dispo-
sition is a consequence of two concurrent factors: the
good p-acceptor capacity of the unsaturated ligand and
the small bite of the chelate ligand. In the equatorial
plane are located the three pairs of electrons of the do-
nor ligands and two pairs of non-bonding d electrons of
* Corresponding author. Tel.: +390805442774; fax: +390805442230.
E-mail address: natile@farmchim.uniba.it (G. Natile).
0020-1693/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved.
doi:10.1016/S0020-1693(03)00382-7

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N. Margiotta et al. / Inorganica Chimica Acta 357 (2004) 149–158
platinum, altogether five pairs of electrons, which can be
better accommodated if at least one equatorial ligand is
a good electron acceptor and the pairs of electrons of the
other two donors make a small bite [8].
and 2,9-dimethyl-1,10-phenanthroline (Me2phen). The
X-ray structure of the first metal complex with 2,9-
dimethyl-1,10-phenanthroline-5,6-dione is also reported.
It was also found that a great bulk of the chelate li-
gand (particularly when the bulk is located in the ligand
plane) stabilizes the five-coordination since the interli-
gand steric interactions are relieved in the trigonal plane
of a five-coordinate complex (only three donors) as
compared to the square-plane of a four-coordinate
species (four donors).
Since five-coordinate species of d8 transition metals
play a fundamental role not only in reactions of ligand
substitution and cis–trans isomerization, but also in a
large variety of catalytic processes (isomerization, po-
lymerization, etc.) [9–11], we considered worth of fur-
ther investigation the role of the steric and electronic
properties of the bidentate ligand upon the stabilization
of four- versus five-coordination. Therefore, in the
present study, we have extended the investigation to two
new bidentate nitrogen ligands: 1,10-phenanthroline-
5,6-dione (phedon) and 2,9-dimethyl-1,10-phenanthro-
line-5,6-dione (Me2phedon, Scheme 1).
2. Experimental
2.1. General
Elemental analyses were performed using a Carlo
Erba Elemental Analyzer mod. 1106 Instrument. IR
Spectra were recorded on Perkin–Elmer Spectrum One
and Perkin–Elmer 1600 Series FT-IR spectrophotome-
1
ters using KBr as solid support for pellets. H NMR
spectra were recorded on Bruker Avance DPX-WB 300
MHz and Bruker Avance DRX-WB 400 MHz instru-
ments. Chemical shifts were referred to TMS by using
the residual protic peak of the solvent as internal ref-
erence [7.23 ppm (1H) and 77.7 ppm (13C) for CDCl3;
3.30 ppm (1H) for CD3OD and 2.05 ppm (1H) for
acetone-d6].
2.2. Starting materials
1,10-Phenanthroline-5,6-dione has been extensively
used as building block for the synthesis of supramolec-
ular assemblies and metallo-dendrimers because of the
two distinct functionalities present in its skeleton: the
two diiminic nitrogen atoms in positions 1 and 10
(having high propensity to form chelation compounds)
and the ortho-quinonoid system on the back positions
5 and 6 (this latter having a reach redox chemistry)
[12–15]. A much smaller number of papers has dealt
with the analogous ligand bearing methyl substituents in
the ortho positions (Me2phedon) probably because the
steric bulk limits its coordinating ability [16–19].
In the present paper we report the preparation of
four- and five-coordinate platinum(II) complexes with
phedon and Me2phedon and make comparison with the
analogous complexes with 1,10-phenanthroline (phen)
Commercial reagent grade chemicals were used
without further purification.
K[Pt(g2-C2H4)Cl3] (Zeise’s salt) was prepared ac-
cording to the method of Chock from potassium tetra-
chloroplatinate and ethylene gas [20].
1,10-Phenanthroline-5,6-dione (phedon) was prepared
according to the procedure reported by Calderazzo et al.
[16].
2,9-Dimethyl-1,10-phenanthroline-5,6-dione (Me2phe-
don) was prepared according to the procedure reported
by Garas and Vagg [21] with some modifications. A
mixture of 2,9-dimethyl-1,10-phenanthroline (2 g, 8.39
mmol) and potassium bromide (6.8 g, 57.2 mmol) was
cooled to )78 °C. Concentrated H2SO4 (44 ml) was
added dropwise. The temperature was increased to
)50 °C and concentrated HNO3 (24 ml) was slowly
added. The thick orange paste was slowly allowed to
warm to room temperature and then heated at 90 °C for
16 h. The resulting red solution was poured into de-
ionized water (1500 ml), carefully neutralized with
Na2CO3, and then extracted with 5 ꢀ 100 ml of CH2Cl2.
The yellow extracts were washed with water, dried over
sodium sulfate, and then the solvent evaporated in va-
cuo. The orange-yellow solid was recrystallized from
methanol (2.1 g, 90% yield). Anal. Calc. for C14H10N2O2
(FW ¼ 238.25): C, 70.58; H, 4.23; N, 11.76. Found: C,
70.22; H, 4.02; N, 11.17%. 1H NMR (300.13 MHz,
3
CDCl3): d (ppm) ¼ 8.41 (2H, d, JH–H ¼ 7:97 Hz), 7.45
3
(2H, d, JH–H ¼ 7:97 Hz), 2.88 (6H, s). 13C NMR
(125.76 MHz, CDCl3) d (ppm) ¼ 26.43 (2C, s, C(13)/
C(14); numbering of carbon atoms as reported in Fig. 3),
125.93 (2C, s, C(2)/C(11)), 126.44 (2C, s, C(4)/C(8)),
Scheme 1.

N. Margiotta et al. / Inorganica Chimica Acta 357 (2004) 149–158
151
137.89 (2C, s, C(3)/C(12)), 153.01 (2C, s, C(1)/C(10)),
167.53 (2C, s, C(5)/C(9)), 179.25 (2C, s, C(6)/C(7)). IR
C(7)). IR (KBr): mmax ¼ 1694 (C@O), 1587 (C@N), and
339 (Pt–Cl) cmꢁ1
.
(KBr): mmax ¼ 1681 (C@O), 1578 (C–N) cmꢁ1
.
[PtCl2(Me2phedon)] (4). 10 mg (0.0177 mmol) of
the five-coordinate complex 3 were dissolved in 3 ml of
CHCl3 and 7 ml of diethyl ether were gently layered on
the previous solution in order to have two phases. Or-
ange crystals of complex 4 (6 mg, 67% yield) formed
after several days at room temperature: an accurately
chosen crystal was analyzed by X-ray spectroscopy.
Anal. Calc. for [PtCl2(Me2phedon)] ꢂ H2O (the sample, if
not kept in dry atmosphere, adsorbs one molecule of
water moisture per molecule of complex), C14H12
Cl2N2O3Pt (FW ¼ 522.26): C, 32.20; H, 2.32; N, 5.36.
2.3. Preparation of complexes
[PtCl2(g2-C2H4)(phedon)] (1). This complex was
not isolated but only prepared in situ and characterized
by NMR spectroscopy. A solution containing 6 mg
(0.0155 mmol) of ZeiseÕs salt in deuterated acetone (0.7
ml) cooled at )20 °C was treated with 1,10-phenan-
throline-5,6-dione (3.4 mg, 0.0155 mmol) and the re-
sulting solution was immediately transferred into an
NMR tube which was inserted in the instrument probe
previously refrigerated at )20 °C.
1
Found: C, 32.14; H, 2.21; N, 5.21%. H NMR (300.13
MHz, CDCl3): d (ppm) ¼ 8.61 (2H, d, 3JH–H ¼ 8:18 Hz),
7.71 (2H, d, 3JH–H ¼ 8:18 Hz), 3.57 (6H, s with satellites,
4JPt–H ¼ 4:10 Hz). IR (KBr): mmax ¼ 1699 (C@O), 1593
(C@N), and 342 and 328 cmꢁ1 (Pt–Cl).
1H NMR (300.13 MHz, acetone-d6): d (ppm) ¼ 9.60
3
4
(2H, dd, JH–H ¼ 5:20 Hz, JH–H ¼ 1:52 Hz), 8.80 (2H,
3
4
dd, JH–H ¼ 8:01 Hz, JH–H ¼ 1:52 Hz), 8.17 (2H, dd,
4JH–H ¼ 8:01 Hz, JH–H ¼ 1:52 Hz), 4.16 (4H, s with
[PtCl2(g2-C2H4)(Me2phen)] (5). This complex was
prepared as reported in [4].
3
2
satellites, JPt–H ¼ 64 Hz).
[PtCl2(phedon)] (2). A solution containing 10 mg
(0.0258 mmol) of ZeiseÕs salt in CH3OH (2 ml) was
treated with a small excess of 1,10-phenanthroline-5,6-
dione (6.2 mg, 0.0295 mmol). The resulting solution was
kept under stirring at room temperature for 1 h mean-
while a yellow precipitate formed. The precipitate was
separated by filtration of the solution, washed with
methanol and dried under vacuum. The yield was 93%
(11.4 mg). Anal. Calc. for [PtCl2(phedon)] ꢂ H2O,
C12H8Cl2N2O3Pt (FW ¼ 494.18): C, 29.16; H, 1.63; N,
5.67. Found: C, 29.26; H, 1.66; N, 5.67%. IR (KBr):
2.4. Decomposition of the five-coordinate complexes
A sample containing [PtCl2(g2-C2H4)(Me2phedon)]
(3) (2.5 mg, 0.0047 mmol) and [PtCl2(g2-C2H4) (Me2-
phen)] (5) (2.4 mg, 0.0047 mmol) in CDCl3 (0.7 ml) was
1
transferred into an NMR tube and H NMR spectra
were recorded from time to time leaving the NMR tube
at constant temperature (45 °C) in the dark. The
amounts of the compounds in solution were evaluated
by integration of selected peaks from the spectra.
mmax ¼ 1703 (C@O), 352 (Pt–Cl) cmꢁ1
.
[PtCl2(g2-C2H4)(Me2phedon)] (3).ZeiseÕssalt(50mg,
0.129 mmol) was dissolved in the minimum amount of
methanol and treated with a slight excess of 2,9-dimethyl-
1,10-phenanthroline-5,6-dione (37 mg, 0.155 mmol). The
resulting yellow solution was kept under stirring at room
temperature for 1 h and then left resting for one additional
hour meanwhile a yellow micro-crystalline solid sepa-
rated out. The yellow product was removed from the so-
lution, washed with methanol and dried under vacuum.
The mother liquor, left standing for one more day, af-
forded another crop of compound. This latter was treated
as the previous one and the two solids were joint together
and analyzed. The overall yield was ca. 66% (48 mg). Anal.
Calc. for [PtCl2(g2-ethylene)(Me2phedon)], C16H14Cl2
N2O2Pt (FW ¼ 532.3): C, 36.10; H, 2.65; N, 5.26. Found:
2.5. X-ray crystallography
2.5.1. Crystal data
C14H10Cl2N2O2Pt, monoclinic, P21=n, a ¼ 8:9570ð2Þ
ꢁ
ꢁ
3
A, b ¼ 10:4390ð2Þ A, c ¼ 15:7160ð3Þ, b ¼ 103:3160
ꢁ
ð12Þ°, V ¼ 1429:97 A , Z ¼ 4; F ð000Þ ¼ 944; Dc ¼ 2:342
g cm3, l ¼ 101:9 cmꢁ1. Orange plate-shaped crystals,
0.05 ꢀ 0.22 ꢀ 0.31 mm3.
2.5.2. Data collection and processing
As many as 8185 reflections (3439 unique) were col-
lected in the range 7:34 6 2h 6 56°, on a Nonius Kappa
CD diffractometer equipped with a fine-focus sealed
tube graphite-monochromated Mo Ka radiation.
Data were corrected for Lorentz and polarization
effects, and for absorption effect according to Blessing
[22]. Omission of intensities with I 6 rðIobsÞ gave 2797
observed reflections, which were employed for the
analysis.
1
C, 35.94; H, 3.12; N, 4.94%. H NMR (300.13 MHz,
3
CDCl3): d (ppm) ¼ 8.54 (2H, d, JH–H ¼ 8:18 Hz), 7.74
3
(2H, d, JH–H ¼ 8:18 Hz), 3.69 (4H, s with satellites,
2JPt–H ¼ 71:32 Hz), 3.39 (6H, s). 13C NMR (75.48 MHz,
CDCl3): d (ppm) ¼ 29.05 (2C, s, C(13)/C(14); numbering
of carbon atoms as reported in Fig. 3), 39.31 (2C, s, eth-
ylene), 128.22 (2C, s, C(2)/C(11)), 129.28 (2C, s, C(4)/
C(8)), 139.70 (2C, s, C(3)/C(12)), 152.52 (2C, s, C(1)/
C(10)), 168.32 (2C, s, C(5)/C(9)), 176.48 (2C, s, C(6)/
2.5.3. Structure solution and refinement
The structure was solved by Direct Methods appli-
cation (SIR97) [23] and refined by a full-matrix least-
squares technique (SHELX-L) [24]. Each asymmetric

152
N. Margiotta et al. / Inorganica Chimica Acta 357 (2004) 149–158
unit contains one independent molecule. All non-
hydrogen atoms were refined anisotropically, while hy-
drogen atoms were placed in idealized positions and had
assigned a common isotropic thermal parameter (C–Har
The remaining two signals in the aromatic region of
3
the spectrum, at 8.80 ppm (2H, dd, JH–H ¼ 8:01 Hz,
3
4JH–H ¼ 1:52 Hz) and 8.17 (2H, dd, JH–H ¼ 8:01 Hz,
4JH–H ¼ 1:52 Hz), were assigned to protons 4/7 and 3/8,
ꢁ
ꢁ
0.93 A, C–Hme 0.96 A, UisoðHÞ ¼ 1:2UisoðCÞ). The final
cycle of least squares refinement included 193 parame-
ters (weighting scheme applied: wꢁ1 ¼ ½r2ðFo2Þ þ
respectively. Finally, the singlet with two satellites at
2
4.16 ppm (4H, intensity 1:4:1, JH–Pt ¼ 64 Hz) was
2
ð0:0908PÞ þ 15:6649Pꢃ, with P ¼ ½ðFo2 þ 2Fc2Þ=3ꢃ). The
Table 1
Crystal data and structure refinement for [PtCl2(Me2phedon)] (4)
final residuals were R ¼ 0:056; wR ¼ 0:135, while GOF
was 1.139. Crystal data and structure refinement are
reported in Table 1.
Empirical formula
Formula weight
Temperature (K)
C14H10Cl2N2O2Pt
504.23
293(2)
ꢁ
Wavelength (A)
0.71070
monoclinic
Crystal system
Space group
Unit cell dimensions
3. Results
P21=n (number 14)
ꢁ
ꢁ
a ¼ 8:9570ð2Þ A; b ¼ 10:4390ð2Þ A;
The reaction between ZeiseÕs salt (K[PtCl3(g2-
C2H4)]) and 1,10-phenanthroline-5,6-dione was carried
on in deuterated acetone at )20 °C. At this temperature
the five-coordinate complex 1 (Scheme 2) is sufficiently
ꢁ
c ¼ 15:7160ð3Þ A a ¼ 90°;
b ¼ 103:3160ð12Þ°; c ¼ 90°
3
ꢁ
Volume (A )
1429.97(5)
4
Z
Calculated density (Mg/m3) 2.342
1
stable to allow its characterization by H NMR.
3
Absorption coefficient
(mmꢁ1
10.190
The signal at 9.60 ppm (2H, dd, JH–H ¼ 5:20 Hz,
)
4JH–H ¼ 1:52 Hz) was assigned to protons 2 and 9
(numbering of protons as in Scheme 2). The ca. 0.6 ppm
shift at lower field of this signal (the corresponding
signal for free phedon falls at 8.98 ppm in CD3OD) is a
clear evidence of N–N coordination of phedon to the
metal. The preferential coordination of phedon through
the two nitrogen atoms rather than through the ortho
quinonoid system was also expected on the basis of the
greater basicity of the diiminic system with respect to the
quinonoid functionality [16,25].
F ð000Þ
944
Crystal shape and colour
Crystal size (mm)
plate, orange
0.05 ꢀ 0.22 ꢀ 0.31
3.67–28.00°
h range for data collection
Reflections collected/unique 8185/3439
Refinement method
full-matrix least-squares on F 2
2797/0/193
Reflections [I > 2rðIÞ]/
restraints/parameters
Final R indices [I > 2rðIÞ]
R indices (all data)
R ¼ 0:056; wR ¼ 0:135
R ¼ 0:078; wR ¼ 0:171
1.139
Goodness-of-fit on F 2
Scheme 2.

N. Margiotta et al. / Inorganica Chimica Acta 357 (2004) 149–158
153
characteristic of a platinum(II) coordinated ethylene in a
five-coordinate environment (in four-coordinate plati-
num(II) complexes the chemical shift is, in the average,
1 ppm at lower field).
assigned to protons 4/7 and 3/8 of Me2phedon, respec-
tively. The singlet with two satellites at 3.69 ppm (4H,
intensity 1:4:1, 2JH–Pt ¼ 71:32 Hz) was assigned to ethyl-
ene and showed that the values of chemical shift and
coupling constant are very close to those found in the
analogous complex with Me2phen (3.67 ppm and 70 Hz).
Finally, the singlet at 3.39 ppm (6H) was assigned to the
two methyls of Me2phedon. The downfield shift of this
signal (corresponding signal in the free ligand at 2.88
ppm) is a clear evidence in favour of the N–N coordina-
tion of Me2phedon to the metal [2,4,27]. The coordination
of Me2phedon through the two iminic nitrogens was also
confirmed by the presence of a C@O stretching at 1694
cmꢁ1 (very close to those of the free ligand at 1681 cmꢁ1).
The four-coordinate complex 4 was obtained unex-
pectedly in an attempt of crystallization of 3 from a
chloroform/diethylether solution. We discovered after-
wards that compound 3 is sensitive to light which greatly
accelerates its decomposition to 4.
The reaction between phedon and ZeiseÕs salt was re-
peated at room temperature and led to the immediate
precipitation of a yellow product (2 in Scheme 2) which
was characterized by elemental analysis and IR spec-
troscopy. The elemental analysis indicated a single phe-
don ligand per platinum atom as well as two chlorine
ligands. The yellow product was totally insoluble in water
and methanol as well as in most common organic solvents
(with the exception of dimethylsulfoxide and dimethyl-
formamide where decomposition was observed). The
coordination of the phedon ligand to platinum through
the two iminic nitrogens was supported by the presence,
in the IR spectrum, of a sharp band at 1703 cmꢁ1 assigned
to the C@O stretching. Such a band is not observed in the
reaction products of low-valence organometallic com-
plexes with the quinonoid functionality in which the metal
gets oxidized while the quinonoid system is reduced
[12,26]. Furthermore, the small shift of this band to high
Complex 4 was found to be almost insoluble in most
common solvents, it is only slightly soluble in chloro-
1
form where the H NMR spectrum was taken (Fig. 2).
Two doublets at 8.61 and 7.71 ppm (each doublet inte-
wavenumbers with respect to the free ligand (1686 cmꢁ1
)
3
indicates a small increase in the carbonyl bond order
consequent to coordination of the two nitrogen atoms to
platinum [14,16,26].
grates for 2H, JH–H ¼ 8:18 Hz) were assigned to the
pairs of Me2phedon protons in position 4/7 and 3/8,
respectively.
Performing the reaction of ZeiseÕs salt with 2,9-di-
methyl-1,10-phenanthroline-5,6-dione at room temper-
ature a yellow solid started separating out after ca. 1 h.
This product, however, resulted to be the five-coordinate
species [PtCl2(g2-C2H4)(Me2phedon)] (3) and not the
four-coordinate species obtained under similar experi-
mental conditions in the case of phedon.
The singlet with two satellites at 3.57 ppm (6H,
4JH–Pt ¼ 4:10 Hz) was assigned to the protons of the
methyl substituents in positions 2 and 9. The deshielding
of the signal belonging to the methyl substituents as well
as its small coupling with 195Pt clearly support N–N
coordination of Me2phedon. Coordination through the
nitrogen atoms was also supported by IR evidences such
as the C@O stretching band at 1699 cmꢁ1. The square-
planar coordination was also supported by the two
bands at 342 and 329 cmꢁ1 assigned to the symmetric
The 1H NMR spectrum of 3 in CDCl3 (Fig. 1) showed
two doublets in the aromatic region at 8.54 (2H,
3
3JH–H ¼ 8:18 Hz) and 7.74 ppm (2H, JH–H ¼ 8:18 Hz)
Fig. 1. 1H NMR spectrum (300 MHz) of complex [PtCl2(g2-
C2H4)(Me2phedon)] (3) in CDCl3 (the asterisk indicates the solvent
peak).
Fig. 2. 1H NMR spectrum (300 MHz) of [PtCl2(Me2phedon)] (4) in
CDCl3 (the asterisk indicates the solvent peak flanked by weak side
bands caused by coupling with 13C).

154
N. Margiotta et al. / Inorganica Chimica Acta 357 (2004) 149–158
and antisymmetric Pt–Cl stretchings stemming from the
two chloro ligands coordinated in cis position.
It is worth noting the greater downfield shift (0.18
ppm) of the methyl substituents in the positions 2 and 9
observed in the four-coordinate complex 4 as compared
to the five-coordinate species 3. This feature is indicative
of a stronger ligand–metal ion interaction in 4 than in 3.
We have already pointed out that in the trigonal plane
of a trigonal bipyramidal platinum(II) species insist five
pairs of electrons, three from the three donors and two
from non-bonding d-orbitals of the metal. Therefore
metal–ligand bonds in the trigonal plane will be basi-
cally weaker than those formed in square-planar four-
coordinate compounds.
4. Crystal structure
A view of the structure of complex 4, together with
the atomic numbering scheme, is given in Fig. 3, frac-
tional atomic coordinates are given in Table 2. There are
no atoms in special position. The Pt atom shows square
planar coordination, with two nitrogen and two chlorine
atoms in cis positions; it is evident a pyramidal distor-
ꢁ
tion with the platinum atom displaced by 0.17 A from
the mean plane defined by the four donors, the latter
ꢁ
atoms are coplanar within ꢄ 0.03 A. The N(1)–Pt–N(2)
angle of 79.6(3)° is largely determined by geometrical
constrains inside the chelate ring, while the deviation
from ideality (90°) of other angles between cis-ligands is
mainly due to interligand steric interactions (see Table 2).
In particular the close contacts between chlorine atoms
and contiguous Me groups [Cl(1)ꢂ ꢂ ꢂC(14) ¼ 3.24(1) and
ꢁ
Cl(2)ꢂ ꢂ ꢂC(13) ¼ 3.27(1) A and, correspondingly, Cl(1)ꢂ ꢂ ꢂ
ꢁ
H(143) ¼ 2.73 and Cl(2)ꢂ ꢂ ꢂH(133) ¼ 2.46 A; sum of the
Fig. 3. View of the asymmetric unit of [PtCl2(Me2phedon)] (4) with the
atomic numbering scheme. Ellipsoids are drawn at 50% probability.
Above is shown the top view and below the front view.
ꢁ
Cl and H van der Waals radii ¼ 2.95 A] [28] determine
considerable distortions within the phenanthroline li-
gand and within the coordination sphere (Fig. 3, bottom
view).
86.8(1)° [85.8(1)° for Me2phen]; (ii) a widening of the
Cl–Pt–N angles to 97.7(3)° and 94.3(3)° [97.8(3)° and
95.4(3)° in the case of Me2phen].
Within the phenanthroline ligand relevant distortions
are: (i) a bending of the phenanthroline plane, which
assumes a bowlike conformation with a dihedral angle
between the mean planes through the external rings
[N(1)–C(1)–C(2)–C(3)–C(4)–C(5) and N(2)–C(10)–
C(11)–C(12)–C(8)–C(9)] of 22.7(4)° [16.6(4)° in the
analogous complex with Me2phen]; (ii) a remarkable
rotation of the ligand plane around the N(1)–N(2) vec-
tor so that the chelating moiety [N(1)–C(5)–C(9)–N(2)]
forms with the coordination plane [N(1)–Pt–N(2)] a di-
hedral angle of 29.5(5)° [27.5(5)° in the complex with
Me2phen]; (iii) a displacement of the methyls from the
phenanthroline plane as indicated by the torsion angles
Me–C–N–C of 13.6° and 16.5° (14.0° and 10.2° in the
case of Me2phen).
Oxygen atoms O(1) and O(2) and carbon atoms C(6)
and C(7) show sp2 hybridization, with C@O double
ꢁ
bond lengths of 1.218(14) and 1.199(15) A, respectively,
in good agreement with literature values [16,29,30]. The
presence of C@O bonds determines the loss of aroma-
ticity of the central ring, with respect to the external
ones, as indicated by the lengthening of the C–C bonds
which, with the only exception of the C(4)–C(5) and
C(8)–C(9) bonds shared with the external rings, are, on
ꢁ
the average, 0.11 A longer. The same bonds are, on the
ꢁ
average, 0.09 A longer in 4 than in the corresponding
compound with Me2phen [31,32]. Particularly indicative
ꢁ
is the C(6)–C(7) distance which results to be 1.52 A in
ꢁ
the present case, while it was 1.34 A in the case of
Me2phen.
Within the coordination sphere most relevant dis-
tortions are: (i) a narrowing of the Cl–Pt–Cl angle to

N. Margiotta et al. / Inorganica Chimica Acta 357 (2004) 149–158
155
Table 2
ꢁ
Bond distances (A) and angles (°) for [PtCl2(Me2phedon)] (4)
Bond distances
Bond angles
Pt(1)–N(1)
Pt(1)–N(2)
Pt(1)–Cl(1)
Pt(1)–Cl(2)
O(1)–C(7)
O(2)–C(6)
N(1)–C(1)
C(1)–C(2)
C(2)–C(3)
C(3)–C(4)
C(4)–C(5)
C(5)–N(1)
C(4)–C(6)
C(5)–C(9)
C(1)–C(13)
N(2)–C(10)
C(10)–C(11)
C(11)–C(12)
C(8)–C(12)
C(8)–C(9)
C(9)–N(2)
C(7)–C(8)
C(6)–C(7)
C(10)–C(14)
2.028(10)
2.032(8)
2.302(3)
N(1)–Pt–N(2)
79.6(3)
Cl(1)–Pt–Cl(2)
Cl(1)–Pt–N(2)
Cl(2)–Pt–N(1)
86.81(15)
94.3(3)
97.7(3)
2.290(3)
1.218(14)
1.199(15)
1.366(14)
1.39(2)
Pt–N(1)–C(1)
Pt–N(1)–C(5)
132.2(9)
110.4(7)
116.7(10)
119.3(12)
119.6(12)
120.9(12)
122.2(12)
117.9(13)
117.2(13)
121.8(12)
120.9(11)
125.7(10)
113.2(9)
121.0(10)
117.9(10)
130.7(7)
109.7(7)
119.5(9)
120.0(10)
118.8(11)
121.1(11)
120.8(11)
119.0(11)
117.1(11)
122.7(10)
120.2(11)
123.2(10)
115.7(9)
120.6(10)
117.5(10)
121.7(12)
120.9(12)
122.4(13)
119.8(13)
C(1)–N(1)–C(5)
N(1)–C(1)–C(13)
N(1)–C(1)–C(2)
C(2)–C(1)–C(13)
C(1)–C(2)–C(3)
C(2)–C(3)–C(4)
C(3)–C(4)–C(5)
C(3)–C(4)–C(6)
C(5)–C(4)–C(6)
C(4)–C(5)–N(1)
C(9)–C(5)–N(1)
C(4)–C(5)–C(9)
C(4)–C(6)–C(7)
Pt–N(2)–C(10)
Pt–N(2)–C(9)
1.37(2)
1.399(19)
1.358(16)
1.349(14)
1.495(19)
1.495(14)
1.488(19)
1.360(14)
1.391(17)
1.380(19)
1.387(18)
1.394(15)
1.321(14)
1.487(17)
1.519(19)
1.493(17)
C(9)–N(2)–C(10)
N(2)–C(10)–C(14)
N(2)–C(10)–C(11)
C(11)–C(10)–C(14)
C(10)–C(11)–C(12)
C(11)–C(12)–C(8)
C(12)–C(8)–C(9)
C(12)–C(8)–C(7)
C(9)–C(8)–C(7)
C(8)–C(9)–N(2)
C(5)–C(9)–N(2)
C(8)–C(9)–C(5)
C(8)–C(7)–C(6)
O(1)–C(7)–C(8)
O(1)–C(7)–C(6)
O(2)–C(6)–C(4)
O(2)–C(6)–C(7)
Two interesting observations stem from comparison
between the X-ray data of compound 4 and the analo-
gous compound with Me2phen, these are: (i) distortions
within the phenanthroline ligand generally greater for
Me2phedon than for Me2phen and (ii) distortions within
the coordination sphere generally smaller for Me2phe-
don than for Me2phen. Both features originate from the
loss of aromaticity of the central ring of Me2phedon
which renders the ligand slightly more flexible and, as a
consequence, more adaptable to the requirements of the
coordination sphere which, therefore, becomes slightly
less distorted.
to the other, so that the chlorine atoms of one subunit
interact with the phenanthroline of the other subunit
ꢁ
[shortest distances Cl(1) ꢂ ꢂ ꢂ C(6) ¼ 3.14(1) A, Cl(1) ꢂ ꢂ ꢂ
ꢁ
ꢁ
ꢁ
C(7) ¼ 3.12(1) A, Cl(2) ꢂ ꢂ ꢂ C(7) ¼ 3.21(1) A, and Cl(2) ꢂ ꢂ ꢂ
C(8) ¼ 3.33(1) A] (Fig. 4). Adjacent pairs are quasi or-
thogonal to each other so that each phenanthroline has,
on one side, the chlorine atoms of the second subunit
and, on the other side, the chlorine atoms of a neigh-
bouring pair.
The three-dimensional array is also stabilized by
strong electrostatic interactions between protons of
methyl C(14) and oxygen atom O(1) [O(1) ꢂ ꢂ ꢂ H(143)
ꢁ
(x þ 1=2; ꢁy þ 1=2; z þ 1=2) ¼ 2.65(1) A and O(1) ꢂ ꢂ ꢂ
ꢁ
H(142)(ꢁx þ 1=2; y ꢁ 1=2; ꢁz þ 1=2) ¼ 2.87(1) A], be-
tween aromatic proton H(11) and oxygen atom O(1)
ꢁ
5. Crystal packing
[O(1) ꢂ ꢂ ꢂ H(11)(ꢁx þ 1=2; y ꢁ 1=2; ꢁz þ 1=2) ¼ 2.49(1) A],
and between aromatic proton H(12) and chlorine ligand
Cl(2) [Cl(2) ꢂ ꢂ ꢂ H(12)(x þ 1=2; ꢁy þ 1=2; z ꢁ 1=2) ¼ 2.85
The three-dimensional array is made of pairs of quasi
parallel [PtCl2(Me2phedon)] moieties. Within each pair
the two molecules are rotated by 180°, one with respect
ꢁ
(1) A].

156
N. Margiotta et al. / Inorganica Chimica Acta 357 (2004) 149–158
Fig. 4. Crystal packing of [PtCl2(Me2phedon)] (4) down the a axis.
6. Discussion and conclusion
(2), exactly as in the case of phen. Compound 2, as the
analogous complex with phen, resulted to be insoluble in
most common organic solvents preventing the investi-
gation of the dissociation equilibrium:
Two different types of coordination compounds are
expected from a d8 metal ion like platinum(II): four-
coordinate square-planar and five-coordinate trigonal
bipyramidal (or square pyramidal) species. Although com-
plexes of the second type are coordinatively saturated,
they are much less common than four-coordinate com-
plexes which, instead, have an unsaturated coordination
shell [2]. The common procedure for the preparation of
a five-coordinate complex from a four-coordinate pre-
cursor is the substitution of a bidentate ligand (usually a
N-donor ligand with a small bite) for a monodentate
ligand in the starting substrate. This synthetic pathway
was used by Maresca et al. [1] for the preparation of the
five-coordinate complex with phen. Starting with ZeiseÕs
salt and free ligand in methanol and performing the
reaction at 0 °C, the five-coordinate complex [PtCl2(g2-
C2H4)(1,10-phenanthroline)] was obtained as a crystal-
line solid. In solution in common organic solvents the
compound tends to decompose releasing ethylene and
affording the corresponding square-planar complex
[PtCl2(1,10-phenanthroline)]. The decomposition can be
repressed by keeping the solution at low temperature
(below )10 °C).
½PtCl2ðg2 ꢁ C2H4ÞðN–NÞꢃ ꢀ ½PtCl2ðN–NÞꢃ þ C2H4 ð1Þ
If 2,9-dimethyl-1,10-phenanthroline (Me2phen) is
used instead of phenanthroline, the five-coordinate
complex, [PtCl2(g2-C2H4)(Me2phen)], is much more
stable even at ambient temperature [4,33]. The steric
hindrance of the methyl substituents stabilizes the five-
coordinate species which places the Me2phen ligand in the
equatorial plane in which insist only three donor groups.
The reaction between ZeiseÕs salt and the Me2phedon
also afforded the five-coordinate complex 3 as a stable
species. In order to study the effect of the electron-
attracting ortho-quinonoid system upon the stabilization
of the five-coordinate species, we investigated the de-
composition equilibrium (1) in a NMR sample containing
both [PtCl2(g2-C2H4)(Me2phedon)] (3) and [PtCl2(g2-
C2H4)(Me2phen)] (5). The percentages of different spe-
cies, as a function of time, are reported in Fig. 5. At
equilibrium only ca. 20% of complex 3 had decomposed to
4, while decomposition was ca. 60% for complex 5.
The greater stability of the five-coordinate complex
with Me2phedon, as compared to the analogous com-
plex with Me2phen, stems from reduced electron-donor
capacity of the oxidized phenanthroline which helps in
diminishing the electron charge accumulation in the
trigonal plane of the five-coordinate species.
The same trend was observed by us using the phedon
ligand instead of phen. At )20 °C we were able to obtain
and characterize the five-coordinate complex [PtCl2(g2-
C2H4)(phedon)] (1) which resulted to be stable at that
temperature. On the other hand, when we repeated the
reaction at room temperature we obtained the precipi-
tation of the four-coordinate complex [PtCl2(phedon)]
Moreover such a stabilization could be greater than
what appears from the shift in the equilibrium between

N. Margiotta et al. / Inorganica Chimica Acta 357 (2004) 149–158
157
MMO3027791), the EC (COST Chemistry Projects D20/
0001/2000 and D20/0003/01), and Consiglio Nazionale
delle Ricerche (CNR-Agenzia 2000 CNR00Aaf-002) for
financial support.
Complex 3
Complex 4
Complex 5
Complex 6
60
50
40
30
20
10
0
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