Steric crowding and redox reactivity in platinum(II) and platinum(IV) complexes containing substituted 1,10-phenanthrolines

Article abstract of DOI:10.1021/ic960125y

The effect of the phenanthroline substituents on the structure and reactivity of platinum(II) and platinum(IV) complexes has been investigated. The X-ray crystal structures of the compounds [PtI₂(4,7-Ph₂phen)]·CHCl₃ (1dz·C

Full text of DOI:10.1021/ic960125y

Inorg. Chem. 1996, 35, 3173-3182
3173
Steric Crowding and Redox Reactivity in Platinum(II) and Platinum(IV) Complexes
Containing Substituted 1,10-Phenanthrolines
Francesco P. Fanizzi,*,† Giovanni Natile,*,† Maurizio Lanfranchi,‡ Antonio Tiripicchio,‡
Franco Laschi,§ and Piero Zanello§
Dipartimento Farmaco-Chimico, Universita` di Bari, Via E. Orabona 4, I-70125 Bari, Italy, Dipartimento
di Chimica Generale ed Inorganica, Chimica Analitica e Chimica Fisica, Universita` di Parma, Centro di
Studio per la Strutturistica Diffrattometrica del CNR, Viale delle Scienze 78, I-43100 Parma, Italy, and
Dipartimento di Chimica, Universita` di Siena, Pian dei Mantellini 44, I-53100 Siena, Italy
ReceiVed December 27, 1995X
The effect of the phenanthroline substituents on the structure and reactivity of platinum(II) and platinum(IV)
complexes has been investigated. The X-ray crystal structures of the compounds [PtI2(4,7-Ph2phen)]‚CHCl3
(1dz‚CHCl3), [PtI4(4,7-Ph2phen)]‚CHCl3 (2dz‚CHCl3), [PtI2(2,9-Me2-4,7-Ph2phen)] (1fz), and [PtI4(2,9-Me2-4,7-
Ph2phen)]‚I2 (2fz‚I2) have shown that complexes 1fz and 2fz, containing ortho-substituted phenanthrolines, exhibit
a remarkable displacement of the equatorial iodine atoms from the N-Pt-N′ plane (average 0.477(2) and 0.199-
(2) Å, respectively), a bending of the phenanthroline [angle between outer rings of 19.9(7) and 14.2(7)°, respectively]
and a rotation of the N-C-C′-N′ plane with respect to the N-Pt-N′ plane [32.3(10) and 26.5(9)°, respectively].
Comparison between the structures of 1fz and 2fz, both having the phenanthroline with methyl substituents in the
ortho position, indicates that, in the latter case, because of the presence of the two axial iodine ligands, the
displacements of the ligands from the equatorial plane are smaller and find a compensation in a narrowing of the
I(1)-Pt-I(1′) angle (5°) and a lengthening of the Pt-N bonds (0.07 Å). The electrochemical behavior of the
four-coordinate platinum(II) complexes shows that compounds possessing regular planar geometry have access
to the one-electron reduced species, whereas those with distorted coordination geometry are irreversibly reduced
by collapsing of the complex geometry. This is in sharp contrast with the behavior of related nickel complexes
for which the pseudo-tetrahedral coordination imposed by bulky 2,9-substituents of phenanthroline stabilizes the
nickel(I) species. Spectroscopic results allow us to assign a significant Pt(I) character to [1d]- monoanions.
The electrogenerated, plus one electron, complexes are not indefinitely stable and, because of conjugation with
the phen ligand, progressively restore the Pt(II) oxidation state by transferring the electron to the peripheral organic
ligand. The latter process can involve multiple electron additions in the macroelectrolysis time scale. The related
platinum(IV) complexes [PtX4(L)] undergo irreversible two-electron reduction accompanied by fast release of
the axial ligands and formation of the corresponding platinum(II) species.
Introduction
For instance the trigonal platinum(0) complex [Pt(C2H4)(2,9-
Me2phen)] easily undergoes oxidative addition of electrophiles
The steric requirements of a chelating ligand can be modified
to increase the selectivity for a specific metal ion. For instance,
the introduction of two methyl substituents in the 2,9-positions
of 1,10-phenanthroline (phen) leads to a very specific reagent
for the analytical determination of copper in the presence of
almost any other type of metal ions (neocuproine).1 The steric
requirement of a chelating ligand can also be designed to tune
the preference of a particular metal ion for a given coordination
geometry and/or oxidation state. For instance, the same 2,9-
Me2phen ligand, because of the steric hindrance created by the
two ortho-methyl substituents, stabilizes three-coordinate platinum-
(0)2 and five-coordinate platinum(II) and palladium(II) species.3
By controlling the coordination geometry of a metal complex,
it is also possibile to induce specific and unusual reactivities.
(XY) to give the trigonal bipyramidal platinum(II) species
[PtXY(C2H4)(2,9-Me2phen)] without loss of olefin.2 Halogena-
tion of these very stable five-coordinate platinum(II) species
offers an easy route to the unprecedented â-haloalkyl platinum-
(IV) complexes [PtX3(CH2CH2X)(2,9-Me2phen)].4 Moreover
the square planar complexes [MX2(2,9-Me2phen)] (M ) Pt, Pd;
X ) halogen) easily react with an extra ligand (L) to give the
addition [MX2(L)(2,9-Me2phen)] rather than the substitution
products [MX(L)(2,9-Me2phen)]X.3
The nature and position of the peripheral phenanthroline
substituents could also influence the redox properties of the
platinum center. In most complexes with aromatic ligands (1,2-
diphenyl-1,2-ethenedithiolate,5 diaminomaleonitrile,6 2-phen-
ylpyridine, 2-phenylpyrazole, and 2,2′-bipyridine (bipy),7-13 4,7-
† Universita` di Bari.
‡ Universita` di Parma.
(4) Fanizzi, F. P.; Natile, G.; Pacifico, C. Fifth International Conference
on The Chemistry of The Platinum Group Metals, St. Andrews,
Scotland, July 11-16, 1993; Abstracts, C135; manuscript in prepara-
tion.
§ Universita` di Siena.
X Abstract published in AdVance ACS Abstracts, April 15, 1996.
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S0020-1669(96)00125-5 CCC: $12.00 © 1996 American Chemical Society

3174 Inorganic Chemistry, Vol. 35, No. 11, 1996
Fanizzi et al.
(DMSO)2] (1 mmol) in the same solvent (40 mL). After 12 h of stirring
the yellow solid was collected, washed with methanol and diethyl ether,
and dried in air; yield, 90%. The elemental analyses are reported in
Table S1 of the Supporting Information.
Chart 1
[PtX2(4,7-Ph2phen)] (X ) Br, 1dy; I, 1dz). An excess of either
tetrabutylammonium bromide or tetrabutylammonium iodide was added
with stirring to a suspension of [PtCl2(4,7-Ph2phen)] (1dz) (0.5 mmol)
in chloroform (200 mL). The reaction mixture was kept in a water
bath (40 °C) under stirring for 3 h. The solvent was then evaporated
under vacuum and the solid residue washed with methanol to remove
excess tetrabutylammonium salt. The deep yellow ([PtBr2(4,7-Ph2-
phen)], 1dy) or orange residue ([PtI2(4,7-Ph2phen)], 1dz) was then
washed with diethyl ether and dried in air; yield, 90%. Elemental
analyses reported in Table S1 of the Supporting Information.
[PtX2(2,9-Me2phen)] (X ) Cl, 1ex; Br, 1ey; I, 1ez) and [PtX2-
(2,9-Me2-4,7-Ph2phen)] (X ) Cl, 1fx; Br, 1fy; I, 1fz). Complexes
1ex-ez were prepared by previously reported procedures,3 and
complexes 1fx-fz were prepared in a similar way; the yields were
g90%. Elemental analyses reported in Table S1 of the Supporting
Information.
[PtX4(4,7-Ph2phen)] (X ) Cl, 2dx; Br, 2dy; I, 2dz), [PtX4(2,9-
Me2phen)] (X ) Cl, 2ex; Br, 2ey; I, 2ez), and [PtX4(2,9-Me2-4,7-
Ph2phen)] (X ) Cl, 2fx; Br, 2fy; I, 2fz). An excess of halogen in
CCl4 solution was added with stirring to a suspension of the platinum-
(II) complex in chloroform (0.5 mmol in 200 mL of solvent).
Complexes 2ex (pale yellow), 2ey (orange), and 2ez (black), which
have a poor solubility in chloroform, separated from the solution;
complexes 2dx (pale yellow), 2dy (orange), 2dz (red), 2fx (pale yellow),
2fy (orange), and 2fz (deep brown) were precipitated by addition of
diethyl ether to the reaction solution. The products were collected,
washed with diethyl ether, and dried in air. The yields were above
90%. The elemental analyses are reported in Table S1 of the Supporting
Information.
[PtX2(C2H4)(2,9-Me2phen)] (X ) Cl, 3ex; Br, 3ey; I, 3ez) and
[PtX2(C2H4)(2,9-Me2-4,7-Ph2phen)] (X ) Cl, 3fx; Br, 3fy; I, 3fz).
The five-coordinate complexes 3ex-ez were prepared, as already
reported, by direct uptake of ethylene from the corresponding square
planar species 1ex-ez. The analogous complexes 3fx-fz were
prepared in a similar way starting from 1fx-fz and ethylene. The yields
were always greater than 90%. The elemental analyses are reported
in Table S1 of the Supporting Information.
Redox Equilibrium of the Pt(IV)/Pt(II) Iodo Species. Platinum-
(IV) complexes with ortho-substituted phenanthrolines and iodine
ligands (2ez and 2fz) are not stable in chloroform solution but undergo
halogen dissociation and formation of the corresponding platinum(II)
species. The equilibrium constant for dissociation was determined for
the more soluble 2fz species by dissolving a weighed amount of
platinum(IV) complex (1.5 mg) in deuteriochloroform (1 mL) and
evaluating the Pt(IV)/Pt(II) ratio by integration of the corresponding
NMR signals. Addition of iodine shifts the equilibrium toward the
platinum(IV) species.
diphenyl-1,10-phenanthroline,14 and other heterocyclic nitrogen
donor ligands15) the redox process has been localized on the
ligand rather than on the metal. However, in some cases there
were also evidences for either an irreversible8 or a reversible13,16-18
electron addition process involving a metal-based LUMO level.
In this paper we present a systematic study on the effect of
the phenanthroline substituents on the structure and reactivity
of platinum(II) and platinum(IV) complexes (Chart 1).
Materials and Methods
Starting Materials. Commercial reagent grade chemicals, 1,10-
phenanthroline (phen, a), 4,7-dimethyl-1,10-phenanthroline (4,7-Me2-
phen, b), 3,4,7,8-tetramethyl-1,10-phenanthroline (3,4,7,8-Me4phen, c),
4,7-diphenyl-1,10-phenanthroline (4,7-Ph2phen, d), 2,9-dimethyl-1,10-
phenanthroline (2,9-Me2phen, e), and 2,9-dimethyl-4,7-diphenyl-1,10-
phenanthroline (2,9-Me2-4,7-Ph2phen, f) (Aldrich) were used without
further purification. [PtCl2(DMSO)2] (DMSO ) dimethyl sulfoxide)
was prepared as already described.19
Preparation of Complexes. [PtCl2(phen)] (1ax), [PtCl2(4,7-
Me2phen)] (1bx), [PtCl2(3,4,7,8-Me4phen)] (1cx), and [PtCl2(4,7-
Ph2phen)] (1dx). The appropriate phenanthroline ligand (1 mmol) in
methanol (5 mL) was added dropwise to a stirred suspension of [PtCl2-
Reaction of Pt(IV) Complexes with Ethylene. The reaction of
complexes 2ez and 2fz with ethylene was performed in a NMR tube
by flowing ethylene gas through a solution (2fz) or suspension (2ez)
of the platinum(IV) complex (5 mg) in CDCl3 (1 mL). A fading of
the solution of 2fz or a complete dissolution of 2ez took place in a few
minutes in accord with the quantitative formation of the corresponding
five-coordinate species 3ez and 3fz, respectively, and of 1,2-diiodo-
ethane (methylene resonance at δ 3.62 in chloroform solution). In the
case of the chloro and bromo species 2fx,fy, 3 × 10-2 mmol of complex
was dissolved in 2.5 mL of CDCl3 and the solution was placed in a 40
mL Schlenk tube. Air was removed and the tube connected to a 200
mL rubber ballon filled with ethylene. The tube was then cooled in
liquid nitrogen, and after the olefin had passed from the ballon into
the tube, the stop-cock was closed. The reaction flask was then allowed
to reach room temperature and left under stirring. The conversion was
complete for both cases after 1 month. The five-coordinate species
(3fx and 3fy, respectively) were precipitated by addition of pentane;
the solid was collected and dried. The yield was 60% (3fx) and 80%
(3fy), respectively. Five-coordinate complexes obtained by reaction
of the platinum(IV) complexes with ethylene were identical to those
obtained from the platinum(II) species by direct uptake of ethylene.
(10) Braterman P. S.; Song, J. I. Inorg. Chim. Acta 1991, 183, 131-132.
(11) Klein, A.; Hausen, H. D.; Kaim, W. J. Organomet. Chem. 1992, 440,
207-217.
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31, 222-224.
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W.; Klein, A.; Peacock, R. D. Inorg. Chem. 1992, 31, 5084-5088.
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Eisenberg, R. Inorg. Chem. 1992, 31, 2396-2404.
(15) Siedle, A. R.; Mann, K. R.; Bohling, D. A.; Filipovich, G. P.; Toren,
E.; Palensky, F. J.; Newmark, R. A.; Duerst, R. W.; Stebbings, W.
L.; Mishmash, H. E.; Melancon, K. Inorg. Chem. 1985, 24, 2216-
2223.
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104, 2147-2157.
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metallic Compounds; Pombeiro, A. J. L., McCleverty, J. A., Eds.;
NATO ASI Series 385; Kluwer: Dordrecht, The Netherlands, 1993;
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Yu. L. Russ. J. Inorg. Chem. (Engl. Trans.) 1968, 13, 835-838.

Pt(II) and -(IV) Complexes with Substituted phen
Inorganic Chemistry, Vol. 35, No. 11, 1996 3175
Table 1. Experimental Data for the X-ray Diffraction Studies
1dz‚CHCl3
1fz
2dz‚CHCl3
2fz‚I2
mol formula
mol wt
cryst syst
space group
radiationa
a, Å
C24H16I2N2Pt‚CHCl3
900.68
orthorhombic
Pnma
graphite monochromated
11.151(4)
19.647(6)
C26H20I2N2Pt
809.36
orthorhombic
P212121
Nb filtered
10.515(2)
10.016(2)
22.293(4)
C24H16I4N2Pt‚CHCl3
1154.49
monoclinic
C2/c
graphite monochromated
36.823(9)
8.579(2)
20.546(6)
112.38(2)
6002(3)
C26H20I4N2Pt‚I2
1316.97
orthorhombic
Pnma
Nb filtered
10.631(2)
14.834(3)
19.357(4)
b, Å
c, Å
11.448(4)
â, deg
V, Å3
2508(1)
4
2.385
2347.9(8)
4
2.290
3053(1)
4
2.866
Z
D
8
calcd, g cm-3
2.555
F(000)
θ range, deg
1664
3-25
83.94
5304
1619 [I > 2σ(I)]
95
1496
3-27
86.22
2920
2036 [I > 2σ(I)]
361
4176
3-25
90.76
2285
972 [I > 2σ(I)]
187
2344
3-25
106.83
2818
1460 [I > 2σ(I)]
167
µ(Mo KR), cm-1
unique total data
unique obsd data
no. of variables
final R, Rw indicesb
0.0342, 0.0413
0.0570, 0.0701
0.0368, 0.0428
0.0524, 0.0608
a λ ) 0.710 73 Å. b R ) ∑||Fo| - |Fc||/∑|Fo|; Rw ) [∑w(|Fo| - |Fc|)2/∑w(Fo)2]1/2
.
Crystal Structure Determinations of Complexes 1dz‚CHCl3,
2dz‚CHCl3, 1fz, and 2fz‚I2. Crystals suitable for the X-ray analyses
were obtained by crystallization from chloroform/diethyl ether solutions.
Selected crystallographic data for the four compounds are listed in Table
1. Data were collected at room temperature on Philips PW 1100 (1dz‚
CHCl3 and 2dz‚CHCl3) and Siemens AED (1fz and 2fz‚I2) single-crystal
diffractometers using the graphite-monochromated (1dz‚CHCl3 and
2dz‚CHCl3) and niobium-filtered Mo KR (1fz and 2fz‚I2) radiation.
All reflections, having I > 2σ(I), were considered observed and used
in the analyses. The individual profiles were analyzed following
Lehmann and Larsen.20 Intensities were corrected for Lorentz and
polarization effects. A correction for absorption was applied.21
calomel electrode (SCE). Direct UV-visible spectra on electrolysis
solutions were performed with a Perkin-Elmer Lambda 2 fiber optic
system.
Results and Discussion
Synthesis of the Complexes. The platinum(II) complexes
were synthesized by reaction of [PtCl2(DMSO)2] with the
appropriate phenanthroline ligand. In the case of phenanthro-
lines without substituents in the 2,9-positions (a-d in Chart 1)
the reaction was complete in 2-12 h at room temperature and
gave quantitative yield of product. In contrast phenanthrolines
with substituents in the 2,9-positions (e and f) required higher
temperature (boiling ethanol), longer reaction time (24 h), and
chromatographic purification of the crude product.3 The reac-
tions proceed directly to the formation of the end products
[PtCl2(N-N)] (1ax-fx) without formation of the intermediate
cationic species [PtCl(DMSO)(N-N)]Cl which, instead, are
detected in the case of aliphatic diamines.25,26 Also the reaction
with DMSO of the [PtCl2(N-N)] complexes is different for
phenanthrolines and aliphatic diamines. Complexes with ali-
phatic diamines readily undergo solvolysis with dissociation of
one chloride ion.27 Complexes with phenanthrolines not bearing
substituents in the ortho positions, such as 1ax, in DMSO-d6
solution remain unchanged over a period of weeks (1H NMR
spectra) and do not undergo solvolysis at any appreciable extent.
Finally complexes with ortho-substituted phenanthrolines, such
as 1ex-ez and 1fx-fz, react readily with DMSO to give ad-
dition products ([PtX2(DMSO)(2,9-Me2phen)] and [PtX2-
(DMSO)(2,9-Me2-4,7-Ph2phen)], respectively) in which one
molecule of DMSO has displaced one end of the phenanthro-
line.3
All of the structures were solved by Patterson and Fourier methods
and refined by full-matrix least-squares, first with isotropic and then
with anisotropic thermal parameters in the last cycles, for all of the
non-hydrogen atoms (1fz and 2fz‚I2) or for only the Pt, I, and N atoms
of the complex and the C and Cl atoms of the solvation molecule (1dz‚
CHCl3), or for only the Pt and I atoms of the complex and the C and
Cl atoms of the solvation molecule (2dz‚CHCl3). All hydrogen atoms
were placed at their calculated positions (C-H ) 0.96 Å) and refined
“riding” on the corresponding carbon atoms. Since the space group
P212121 leads to a chiral configuration in the structure, an independent
final cycle of refinement for 1fz was carried out using the coordinates
-x, -y, -z for the non-hydrogen atoms. A remarkable increasing of
the R value was obtained [R(+x,+y,+z) ) 0.0570, R(-x,-y,-z) )
0.0780]. The former model was selected, and the reported data refer
to this model. Atomic scattering factors, corrected for anomalous
dispersion, were taken from ref 22. The SHELX-76 and SHELXS-86
systems of computer programs were used.23 Final atomic coordinates
are given in Tables S2 (1dz‚CHCl3), S3 (1fz), S4 (2dz‚CHCl3), and
S5 (2fz‚I2) of the Supporting Information. All calculations were carried
out on the GOULD POWERNODE 6040 of the Centro di Studio per
la Strutturistica Diffrattometrica del CNR, Parma, Italy.
Physical Measurements. 1H NMR spectra were recorded on a
Bruker AM 300 instrument. IR spectra were recorded as KBr pellets
on Perkin-Elmer 283 and FT 1600 spectrophotometers. Materials and
apparatus for electrochemistry and coupled EPR measurements are
described in ref 24. Potential values are referred to the saturated
The platinum(IV) complexes 2dx-dz, 2ex-ez, and 2fx-fz
(Chart 1) were prepared by halogenation of the corresponding
platinum(II) species in CHCl3 solution. The stability of the
platinum(IV) complexes in organic solvents depends upon the
nature of the halogen and of the phenanthroline ligands. The
(20) Lehmann, M. S.; Larsen, F. K. Acta Crystallogr., Sect. A 1974, 30,
580-584.
(21) Walker, N.; Stuart, D. Acta Crystallogr., Sect. A 1983, 39, 158-166.
Ugozzoli, F. Comput. Chem. 1987, 11, 109-120.
(24) Osella, D.; Gobetto, R.; Milone, L.; Nervi, C.; Ravera, M.; Stein, E.;
Zanello, P.; Laschi, F. Inorg. Chim. Acta 1993, 212, 311-316.
(25) Fanizzi, F. P.; Maresca, L.; Natile, G.; Lanfranchi, M.; Manotti-
Lanfredi, A. M.; Tiripicchio, A. Inorg. Chem. 1988, 27, 2422-2431.
(26) Romeo, R.; Minniti, D.; Lanza, S.; Tobe, M. L. Inorg. Chim. Acta
1977, 22, 87-91.
(22) International Tables for X-Ray Crystallography; Kynoch Press:
Birmingham, England, 1974; Vol. IV.
(23) (a) Sheldrick, G. M. SHELX-76 Program for Crystal Structure
Determination; University of Cambridge: Cambridge, England, 1976.
(b) SHELXS-86 Program for the Solution of Crystal Structures;
University of Go¨ttingen: Go¨ttingen, Germany, 1986.
(27) Fanizzi, F. P.; Intini, F. P.; Maresca, L.; Natile, G. Inorg. Chem. 1990,
29, 29-33.

3176 Inorganic Chemistry, Vol. 35, No. 11, 1996
Fanizzi et al.
Table 2. 1H NMR Data (δ, Downfield from SiMe4; CDCl3 Solvent) for Phenanthroline Ligands and Platinum(II) and Platinum(IV) Complexes
phena
H(3,8)
7.76 dd (8,5)
8.16 dd (8,5)
phen substituentsa
compd
others
H(2,9)
9.09 dd (5,1)
9.69 dd (5,1) [40]
H(4,7)
8.48 dd (8,1)
9.04 dd (8,1)
H(5,6)
7.98
8.28
ab
1axb
Me(4,7)
2.79 d (1)
2.85 d (1)
bc
8.97 d (4)
9.73 d (6) [40]
7.47 dd (1,4)
7.73 dd (6,1)
8.06
8.14
1bxc
Me(3,8)
2.52
2.64
Me(4,7)
2.68
2.72
cc
8.85
9.52 [42]
8.06
8.11
1cxc
Ph(4,7)
d
8.23 d (4)
7.58 d (4)
7.84
8.05
8.04
8.03
8.21
8.19
8.18
7.51 m {10}
7.60 m {10}
7.60 m {10}
7.60 m {10}
7.59 m {4}; 7.66 m {6}
7.60 m {4}; 7.65 m {6}
7.64 m {10}
1dx
1dy
1dz
2dx
2dy
2dz
9.97 d (6) [36]
10.19 d (6) [41]
10.66 d (6) [45]
10.00 d (6) [27]
10.19 d (6) [28]
10.40 d (6) [30]
7.82 d (6) [6]
7.79 d (6) [6]
7.83 d (6) [6]
8.09 d (6) [6]
8.05 d (6) [6]
8.05 d (6) [6]
Me(2,9)
2.93
e
7.47 d (8)
8.11 d (8)
7.70
7.81
7.80
7.80
7.96
7.93
7.89
1ex
1ey
1ez
2exc
2eyc
2ezc
7.56 d (8) [16]
7.56 d (8) [16]
7.57 d (8) [16]
7.74 d (8) [11]
7.72 d (8) [11]
7.69 d (8) [11]
8.35 d (8) [4]
8.34 d (8) [4]
8.34 d (8) [4]
8.48 d (8) [3]
8.45 d (8) [3]
8.42 d (8) [3]
3.25 [7]
3.25 [7]
3.25 [7]
3.61 [6]
3.72 [6]
3.92 [6]
C2H4
3ex
3ey
3ez
7.80 d (8) [3]
7.79 d (8) [3]
7.79 d (8) [3]
8.32 d (8)
8.30 d (8)
8.25 d (8)
7.85
7.85
7.85
3.49 [7]
3.47 [7]
3.44 [7]
Me(2,9)
2.98
3.29 [6]
3.29 [6]
3.29 [6]
3.70 [5]
3.80 [5]
4.00 [5]
3.67 [70]
3.80 [70]
4.20 [71]
Ph(4,7)
7.50 m {10}
f
7.43
7.50
7.50
7.73
7.79
7.78
7.88
7.85
7.81
7.77
1fx
1fy
1fz
2fx
2fy
2fz
7.56 m {6}; 7.49 m {4}
7.56 m {6}; 7.51 m {4}
7.56 m {6}; 7.52 m {4}
7.59 m {6}; 7.52 m {4}
7.59 m {6}; 7.52 m {4}
7.58 m {6}; 7.53 m {4}
7.52
7.63 [11]
7.60 [11]
7.58 [11]
C2H4
3fx
3fy
3fz
7.75
7.75
7.75
7.85
7.85
7.87
3.54 [7]
3.52 [7]
3.48 [7]
7.52 m {10}
7.53 m {10}
7.54 m {10}
3.67 [70]
3.79 [68]
4.20 [71]
a Values of J(H-H) (in parentheses) and J(Pt-H) [in brackets] are given when assignable. Integral values are given in braces. b Solvent )
DMSO-d6. c Solvent ) CD2Cl2.
complexes with phenanthrolines not bearing substituents in the
2,9-positions are stable; in contrast the complexes with ortho-
substituted phenanthroline and iodine ligands (2ez,fz) undergo
reductive elimination to platinum(II) species (1ez,fz) and free
iodine (K ) (4 ( 1) × 10-4 mol‚L-1 for 2fz in CDCl3 at 25
°C). Although the equilibrium is only slightly shifted to the
right, a stream of ethylene through the CDCl3 solution of 2ez,fz
causes immediate and complete disappearance of the platinum-
(IV) complexes and formation of the five-coordinate platinum-
(II) species [PtI2(C2H4)(2,9-Me2phen)] (3ez) and [PtI2(C2H4)(2,9-
Me2-4,7-Ph2phen)] (3fz), respectively, and of 1,2-diiodoethane
([PtI4(L)] + 2C2H4 f [PtI2(C2H4)(L)] + C2H4I2, L ) e, f). The
complexes with ortho-substituted phenanthrolines and either
chloro or bromo ligands (2ex,fx and 2ey,fy) do not undergo
detectable reductive elimination of halogen, but they do react
with ethylene under pressure to give the five-coordinate
platinum(II) species [ca. 15 and 25% conversion for 2fx and
2fy, respectively, in CDCl3 solution (3 × 10-2 mmol of complex
in 2.5 mL of solvent), 5 atm of C2H4, 25 °C, and 108 h reaction
time]. The greater tendency of the iodo complexes (2ez,fz),
with respect to the chloro and bromo species (2ex,fx and 2ey,fy)
to undergo reduction and formation of the five-coordinate
platinum(II) species when treated with ethylene can be related
to both the greater distorsion of the coordination geometry
(which increases by increasing the bulk of the halogen atoms)
and to the lower redox potential of the halogen (which decreases
from Cl2 to I2).
1H NMR data for the free phenanthroline ligands, the four-
and five-coordinate platinum(II) species, and the six-coordinate
platinum(IV) complexes are reported in Table 2. Comparison
of the chemical shifts and 195Pt coupling constants of the ligand
protons, among the four-, five-, and six-coordinate complexes,
gives useful information about the bonding situation in the three
types of complexes. The 4J- and 5J(PtH) of the aromatic protons
have the greatest values in the square planar platinum(II) species
1ex-ez; slightly smaller values are observed in the octahedral
platinum(IV) species 2ex-ez; and much smaller values are
observed in the trigonal bipyramidal five-coordinate platinum-
4
5
(II) complexes 3ex-ez. The much smaller J- and J(PtH)
couplings observed in the five-coordinate species have already
been related to a relevant weakening and lengthening of the
Pt-N bonds.3 In contrast with the aromatic protons, the methyl
substituents in ortho positions have almost identical coupling
with platinum in the four-, five-, and six-coordinate species
(compare values within 1ex-ez-3ex-ez and within 1fx-fz-
3fx-fz), indicating that this coupling originates mainly from a
through-space interaction. Finally, for a given coordination
geometry, the presence of methyl substituents in the 2,9-
positions of the phenanthroline has the effect of slightly reducing
4
the values of J(PtH) (compare values of 1dx-dz with those

Pt(II) and -(IV) Complexes with Substituted phen
Inorganic Chemistry, Vol. 35, No. 11, 1996 3177
Table 3. Selected Bond Distances (Å) and Angles (deg)
a
a
1dz‚CHCl3
1fz
2dz‚CHCl3 2fz‚I2
Pt-I(1)
2.558(2)
2.580(2) 2.606(2) 2.632(2)
2.574(2) 2.602(2)
Pt-I(1′)
Pt-I(2)
2.655(2) 2.665(3)
Pt-I(3)
2.671(2) 2.674(3)
Pt-N
2.02(1)
2.09(2)
2.07(2)
1.31(3)
1.39(3)
1.34(3)
1.38(3)
1.47(3)
1.41(3)
1.36(3)
1.40(3)
1.33(3)
1.48(3)
1.50(4)
1.38(3)
1.38(3)
1.41(3)
1.35(3)
1.47(3)
1.37(3)
1.44(3)
1.48(3)
1.52(4)
2.09(1)
2.08(2)
1.32(3)
1.40(3)
1.34(3)
1.38(3)
1.41(3)
1.41(2)
1.43(3)
1.43(3)
1.35(3)
1.47(2)
2.15(2)
Pt-N′
N-C(1)
1.39(2)
1.35(2)
1.32(2)
1.36(2)
1.44(2)
1.39(2)
1.40(2)
1.44(2)
1.32(2)
1.45(2)
1.32(2)
1.40(2)
1.40(3)
1.40(3)
1.40(3)
1.46(3)
1.36(3)
1.39(3)
1.30(3)
1.50(3)
1.49(3)
N-C(5)
C(1)-C(2)
C(2)-C(3)
C(3)-C(4)
C(4)-C(5)
C(5)-C(5′)
C(4)-C(6)
C(6)-C(6′)
C(3)-C(7)
C(1)-C(Me)
N′-C(1′)
N′-C(5′)
C(1′)-C(2′)
C(2′)-C(3′)
C(3′)-C(4′)
C(4′)-C(5′)
C(4′)-C(6′)
C(3′)-C(7′)
C(1′)-C(Me′)
1.31(3)
1.40(2)
1.36(4)
1.36(3)
1.39(3)
1.40(3)
1.42(2)
1.42(4)
Figure 1. View of the molecular structure of the complex [PtI2(4,7-
Ph2phen)] (1dz) together with the atomic numbering scheme. The
ellipsoids enclose 30% probability. The primed atoms are related to
1
the unprimed ones by the transformation x, /2 - y, z.
N-Pt-N′
78.1(4)
97.3(3)
78.8(7)
95.2(5)
95.8(5)
80.5(6)
96.1(5)
95.2(4)
88.6(5)
89.3(5)
92.3(1)
90.7(1)
88.2(1)
89.3(5)
87.9(5)
90.3(1)
91.3(1)
129(1)
112(1)
130(2)
113(1)
119(2)
116(2)
78.4(6)
99.0(4)
N-Pt-I(1)
I(1′)-Pt-N′
I(2)-Pt-N
91.7(4)
92.7(1)
I(2)-Pt-N′
I(1)-Pt-I(2)
I(2)-Pt-I(1′)
I(1′)-Pt-I(1)
87.3(1)
88.1(1)
83.2(1)
88.6(4)
I(3)-Pt-N
I(3)-Pt-N′
I(3)-Pt-I(1)
87.0(1)
I(3)-Pt-I(1′)
Pt-N-C(1)
128(1)
116(1)
134(1)
105(1)
133(1)
108(1)
120(2)
118(2)
119(2)
121(2)
120(2)
124(2)
125(2)
114(2)
121(2)
124(2)
120(2)
116(2)
118(2)
121(2)
120(2)
124(2)
118(2)
124(2)
119(2)
117(2)
121(2)
124(2)
115(2)
120(2)
121(2)
119(2)
120(2)
121(2)
118(2)
123(2)
131(1)
108(1)
Pt-N-C(5)
Pt-N′-C(1′)
Pt-N′-C(5′)
C(1)-N-C(5)
C(5′)-N′-C(1′)
N-C(1)-C(Me)
N-C(1)-C(2)
C(2)-C(1)-C(Me)
C(1)-C(2)-C(3)
C(2)-C(3)-C(7)
C(2)-C(3)-C(4)
C(4)-C(3)-C(7)
C(3)-C(4)-C(6)
C(3)-C(4)-C(5)
C(5)-C(4)-C(6)
N-C(5)-C(4)
C(4)-C(5)-C(5′)
N-C(5)-C(5′)
C(4)-C(6)-C(6′)
C(6)-C(6′)-C(4′)
C(6′)-C(4′)-C(3′)
C(6′)-C(4′)-C(5′)
C(5′)-C(4′)-C(3′)
C(5)-C(5′)-C(4′)
N′-C(5′)-C(4′)
N′-C(5′)-C(5)
N′-C(1′)-C(2′)
C(1′)-C(2′)-C(3′)
C(4′)-C(3′)-C(2′)
C(2′)-C(3′)-C(7′)
C(4′)-C(3′)-C(7′)
N′-C(1′)-C(Me′)
C(2′)-C(1′)-C(Me′)
116(1)
122(2)
120(2)
123(2)
121(2)
116(2)
122(2)
121(2)
118(2)
124(2)
124(1)
120(1)
116(1)
124(1)
125(1)
118(1)
116(1)
124(1)
122(1)
115(1)
122(1)
121(2)
119(2)
118(2)
123(2)
126(2)
119(2)
114(2)
119(2)
123(2)
118(2)
122(2)
125(2)
127(2)
114(2)
119(2)
121(2)
123(2)
116(2)
122(2)
124(2)
116(2)
122(2)
122(2)
Figure 2. View of the molecular structure of the complex [PtI2(2,9-
Me2-4,7-Ph2phen)] (1fz) together with the atomic numbering scheme.
The ellipsoids enclose 30% probability.
125(2)
117(2)
117(2)
121(2)
120(2)
119(2)
123(2)
of 1fx-fz and values of 2dx-dz with those of 2fx-fz), which
is indicative of a slight weakening of the Pt-N bonds.
Crystal Structures of Compounds 1dz‚CHCl3 and 1fz.
The crystal structures of two four-coordinate platinum(II)
complexes, containing phenanthroline ligands with and without
methyl substituents in ortho positions, have been determined.
Views of the molecular structures with the atomic numbering
schemes are given in Figures 1 and 2. Relevant bond distances
and angles are reported in Table 3.
The complex 1dz‚CHCl3 has a crystallographically imposed
Cs symmetry; two cis positions of the square planar coordination
geometry are occupied by the nitrogen atoms of the 4,7-Ph2-
phen ligand and the other two by iodide ions (Figure 1). The
four donor atoms and the central metal are perfectly coplanar.
The Pt-I and Pt-N bond distances are 2.558(2) and 2.02(1)
Å, respectively, and the I-Pt-I′ and N-Pt-N′ angles 87.3(1)
and 78.1(4)°, respectively. The phen moiety exhibits a small
bowlike distorsion with a dihedral angle between the outer rings
of 5.0(4)°; moreover the plane of the chelating moiety N-C(5)-
a The primed atoms are related to the unprimed ones by the
transformation x, /2 - y, z.
1

3178 Inorganic Chemistry, Vol. 35, No. 11, 1996
Fanizzi et al.
C(5′)-N′ forms a dihedral angle of 3.7(6)° with the N-Pt-N′
plane. An analogous bowlike distorsion of the phenanthroline
was observed in the palladium compound [PdCl2(4,7-Ph2-
phen)],28 while a bending, on opposite directions, of the outer
rings with respect to the central ring was found in the
bis(phenanthrolinium) cation29 and in a series of [M(phen)2]2+
complexes (M ) Pt and Pd).7,30
The structure of 1fz, with a phenanthroline ligand carrying
methyl substituents in the two ortho positions (Figure 2), shows
remarkable differences from that of the previously discussed
1dz. (a) The platinum atom is displaced by 0.222(1) Å from
the mean plane through I(1), I(1′), N, and N′ (while it was
coplanar with the four donor atoms in 1dz). (b) The Pt-I
[2.574(2) and 2.580(2) Å] and Pt-N bond distances [2.07(2)
and 2.09(2) Å] are longer than in 1dz, the difference is rather
small for Pt-I (ca. 0.02 Å) but notably larger for Pt-N (ca.
0.06 Å). (c) The I(1)-Pt-I(1′) and N-Pt-N′ angles [88.1(1)
and 78.8(7)°, respectively] remain practically unchanged in the
two complexes, but the N‚‚‚N′ separation is greater in 1fz [2.64-
(2) Å] than in 1dz [2.55(2) Å]. (d) The phen ligand exhibits
much greater bowlike distortion [dihedral angles of 19.9(7)°
between the mean planes of the outer rings, N-C(1)-C(2)-
C(3)-C(4)-C(5) and N′-C(1′)-C(2′)-C(3′)-C(4′)-C(5′)]
and rotation with respect to the coordination plane [dihedral
angle of 32.3(9)° between the N-C(5)-C(5′)-N′ chelating
plane and the N-Pt-N′ coordination plane]. (e) The two iodo
ligands are displaced from the N-Pt-N′ plane by 0.508(2) and
0.447(2) Å, respectively (while they were perfectly coplanar
with the N-Pt-N′ moiety in 1dz). The steric interactions
between the ortho methyl groups and the cis iodo ligands appear
to be responsible for the observed differences.3ac,30,31
Figure 3. View of the molecular structure of the complex [PtI4(4,7-
Ph2phen)] (2dz) together with the atomic numbering scheme. The
ellipsoids enclose 30% probability.
Crystal Structures of Compounds 2dz‚CHCl3 and 2fz‚I2.
The X-ray structure analyses of the platinum(IV) complexes
2dz and 2fz were also carried on in order to make comparison
with those of the corresponding platinum(II) species. The
structure of 2dz is shown in Figure 3; that of 2fz (which has an
imposed Cs symmetry), in Figure 4. Relevant bond distances
and angles are reported in Table 3. In both cases the platinum-
(IV) atom exhibits the expected octahedral coordination involv-
ing two nitrogen atoms of the phenanthroline and four iodo
anions.
The differences between 2dz and 2fz resemble those already
described for the corresponding platinum(II) species 1dz and
1fz. The presence of methyl substituents in the ortho positions
of the phenanthroline causes the following: (a) lengthening of
the Pt-I [2.602(2) and 2.606(2) Å in 2dz and 2.632(2) Å in
2fz] and Pt-N bond distances [2.08(2) and 2.09(1) Å in 2dz
and 2.15(2) Å in 2fz], (b) narrowing of the I(1)-Pt-I(1′) [88.20-
(6) in 2dz and 83.19(6)° in 2fz] and N-Pt-N′ bite angles [80.5-
(6) in 2dz and 78.4(6)° in 2fz], (c) bowlike distorsion of the
phen ligand [dihedral angle between the outer rings of 1.4(7)°
in 2dz and 14.2(7)° in 2fz], (d) tilting of the plane of the
chelating moiety N-C(5)-C(5′)-N′ with respect to that of
coordination N-Pt-N′ [dihedral angle of 3.3(8)° in 2dz and
26.5(9)° in 2fz], and (e) displacement of the iodine atoms from
the N-Pt-N′ plane [0 in 2dz and 0.199(2) Å in 2fz]. The
bond lengths of the apical iodines [2.655(2) and 2.671(2) Å in
Figure 4. View of the molecular structure of the complex [PtI4(2,9-
Me2-4,7-Ph2phen)] (2fz) together with the atomic numbering scheme.
The ellipsoids enclose 30% probability. The primed atoms are related
1
to the unprimed ones by the transformation x, /2 - y, z.
Figure 5. Projection of the structures of complexes 1fz (solid line)
and 2fz (dashed line) along the N-N′ vector, showing the bending of
the phen ligands and their rotation with respect to the platinum
equatorial coordination planes.
2dz and 2.665(3) and 2.674(3) Å in 2fz] are greater than those
of the equatorial ones by ∼0.05 Å due to the greater trans
influence of iodine with respect to nitrogen atoms.
Comparison between the structures of 1fz and 2fz (Figure
5), both having the phenanthroline with methyl substituents in
the ortho position, indicates that, in the latter case, because of
the presence of the two axial iodine ligands, the displacements
of the ligands from the equatorial plane are smaller and find a
(28) Anbei, J.; Kruger, C.; Pfeil, B. Acta Crystallogr., Sect. C 1987, 43,
2334-2338.
(29) Maresca, L.; Natile, G.; Fanizzi, F. P. J. Am. Chem. Soc. 1989, 111,
1492-1493.
(30) Milani, B.; Alessio, E.; Mestroni, G.; Sommazzi, A.; Garbassi, F.;
Zangrando, E.; Bresciani Pahor, N.; Randaccio, L. J. Chem. Soc.,
Dalton Trans. 1994, 1903-1911.
(31) De Felice, V.; Albano, V. G.; Castellari, C.; Cucciolito, M. E.; De
Renzi, A. J. Organomet. Chem. 1991, 403, 269-277.

Pt(II) and -(IV) Complexes with Substituted phen
Inorganic Chemistry, Vol. 35, No. 11, 1996 3179
compensation in a narrowing of the I(1)-Pt-I(1′) angle (5°)
and a lengthening of the Pt-N bonds (0.07 Å).
Finally comparison between the structures of 1dz-fz and
those of 2dz-fz indicates that, on passing from a square planar
to an octahedral environment, the equatorial Pt-N and Pt-I
bonds become longer (by 0.06 and 0.05 Å, respectively).
In the crystals of 2fz‚I2 there is one molecule of iodine per
molecule of complex. The iodine molecule of solvation [I(4)-
I(5)] has an internuclear distance of 2.710(5) Å which is
comparable to that found in crystalline I2 [2.715(6) Å].32 This
molecule interacts with an axial iodine ligand, I(3), and with
another I2 molecule [I(4)‚‚‚I(3) ) 3.634(4) and I(4)‚‚‚I(5′′) )
3.622(5) Å (double primed species ) 1/2 + x, y, 3/2 - z)]. Both
interactions are weaker than those found in crystalline I2 [I‚‚‚I
) 3.496(6) Å].
Electrochemistry of Platinum(II) Complexes. In dichlo-
romethane solution the cyclic voltammetric response exhibited
by 1ax (unsubstituted phenanthroline) consists of a first reduc-
tion process showing features of chemical reversibility (ipa/ipc
) 1 at scan rates varying from 0.02 to 10.24 V s-1, (E°)′ )
-1.24 V)33 followed by a second cathodic step very close to
the solvent discharge. Controlled potential coulometry in
correspondence to the first reduction (Ew ) -1.4 V) quickly
consumes 1-electron/molecule, but a residual current, slightly
greater than the background current, remains up to the slow
consumption of about 4 electrons/molecule. Accordingly, the
original lemon-yellow solution turns initially deep blue and then,
more slowly, green. This indicates that the reduction process,
which involves 1-electron/molecule in the short time of cyclic
voltammetry, in the longer time of electrolysis is coupled to a
subsequent reaction. Any attempt to identify the final green
product by mass spectrometric techniques and NMR spectros-
copy failed. The same behavior was displayed in tetrahydro-
furan, in which, however, the complex is less soluble.
Figure 6. Cyclic voltammetric responses recorded at a platinum
electrode on (a) a CH2Cl2 solution containing 1dx (1.24 × 10-3 mol
L-1) and [Fe(C5H5)2] (1.29 × 10-3 mol L-1) and (b) THF solution of
1dx (1.00 × 10-3 mol L-1): [NBu4][ClO4] supporting electrolyte, 0.2
mol L-1; scan rate, 0.1 V s-1
.
Complex 1dx (Ph substituents in the 4,7-positions of phenan-
throline) exhibits, in dichloromethane solution, a redox pro-
pensity quite similar to that of the previous complex. The one-
electron character of the first reduction step in the cyclic
voltammetry time scale (Figure 6a) was proved by comparison
with the one-electron oxidation of an equimolar amount of
ferrocene ((E°)′ ) +0.43 V). Moreover, as in the case of 1ax,
controlled potential coulometry (Ew ) -1.3 V) consumes
quickly 1-electron/molecule (concomitantly, the pale yellow
solution progressively turns blue to emerald green), and then a
residual current slowly flows up to the consumption of more
than 3 electrons/molecule (the solution turns finally pale green).
In tetrahydrofuran solution a second reduction step becomes
well-visible that also displays features of chemical reversibility,
Figure 6b. In view of the fact that the free ligand d, in
tetrahydrofuran solution, undergoes a quite irreversible reduction
at Ep ) -1.79 V, we preliminarily assign the two primary
cathodic steps to the sequential reductions Pt(II)/Pt(I)/Pt(0).
Figure 7. Cyclic voltammograms recorded at a platinum electrode on
CH2Cl2 solutions containing [NBu4][ClO4] (0.2 mol L-1) and (a) 1cx
(1.1 × 10-3 mol L-1; scan rate, 5.12 V s-1) and (b) 1fx (1.0 × 10-3
mol L-1) (scan rate, 20.48 V s-1).
reduction process is also not fully reversible and only scan rates
greater than 5.0 V s-1 allow the anodic-to-cathodic peak-current
ratio to reach unity, Figure 7a. In the latter case, the chemical
stability of the one-electron reduced species is significantly
greater in tetrahydrofuran solution (where complex 1cx is less
soluble) than in dichloromethane and the ipa/ipc ratio is equal to
The other platinum(II) complexes with phenanthrolines not
bearing substituents in the 2,9-positions (b and c) exhibit
behavior roughly similar to those of the previous two complexes.
In complex 1bx, the one-electron reduction process is ac-
companied by a slow chemical reaction even in the short times
of cyclic voltammetry and the ipa/ipc ratio reaches unity only at
a scan rate greater than 0.2 V s-1. In the case of 1cx, the
1 already at a rate of 0.5 V s-1
. The free ligand c, in
tetrahydrofuran solution, undergoes irreversible reduction at Ep
) -2.11 V.
The formal electrode potentials of these redox changes are
compiled in Table 4. Comparison of the data indicates that the
change from chloride to iodide ligands does not affect ap-
preciably the redox potentials, but it slightly lowers the stability
of the reduced species. Also the introduction of electron-
releasing substituents (methyl groups) in the phenanthroline
ligand (compounds 1b,c) lowers the stability of the reduced
species. In contrast, electron-withdrawing substituents (phenyl
groups in 1d) stabilize the reduced species. It is also interesting
(32) Van Bolhuis, F.; Koster, P. B.; Migchelsen, T. Acta Crystallogr. 1967,
23, 90-91.
(33) Brown, E. R.; Sandifer, J. R. In Physical Methods of Chemistry.
Electrochemical Methods; Rossiter, B. W., Hamilton, J. F., Eds.;
Wiley: New York, 1986; Vol. II, Chapter IV.

3180 Inorganic Chemistry, Vol. 35, No. 11, 1996
Fanizzi et al.
Table 4. Redox Potentials for the Electron Transfers Exhibited by
the Platinum(II) Complexes
a
bc
a
bc
c
complex
1ax
(E°)′0/- ∆Ep (E°)′-/2- ∆Ep Ep0/2- solvent ref
-1.24
-1.13
-1.20
-1.05
-1.20
-1.20
-1.04
-1.20
-1.06
-1.43
76
82
66
85
84
68
88
86h
82
80
-1.9de
-1.92e
-1.80e
-1.70
-1.75
-1.80e
-1.68
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
CH2Cl2
THF
f
f
f
f
f
f
f
f
f
f
f
f
f
f
f
f
f
f
f
f
1dx
-
CH2Cl2
THF
75
-
MeCNg
CH2Cl2
THF
1dy
1dz
-
144h
-
CH2Cl2
THF
CH2Cl2
CH2Cl2
THF
-1.68e
-2.05de
-
-
1bx
1cx
-
-1.50 114i
-
-1.33
96
-
-
1ex
-1.27 CH2Cl2
-1.18 THF
-1.14 THF
-1.25 CH2Cl2
-1.15 THF
-1.19 CH2Cl2
-1.08 THF
-1.05 THF
1ey
1fx
1fy
1fz
Figure 8. Cyclic voltammograms obtained at a platinum electrode on
(a) a THF solution of 2dx (1.1 × 10-3 mol L-1) and (b) a CH2Cl2
solution of 2fx (9.0 × 10-4 mol L-1): [NBu4][ClO4] supporting
[Pt(bipy)Cl2] -1.24
80
70
-
-1.82
-
110
-
-
MeCN 15
electrolyte, 0.2 mol L-1; scan rate, 0.2 V s-1
.
-1.17
DMF
16
a In volts vs SCE. b In millivolts. c Measured at 0.1 V s-1
.
d Difficult
Table 5. Peak Potential Values (in volts vs SCE) for the
Platinum(IV)/Platinum(II) Reduction Process
f
to evaluate. e Irreversible process. Present work. g Very low solubility.
h Measured at 0.5 V s-1 i Measured at 2.0 V s-1
. .
a
complex
Ep
solvent
ref
2dx
2dy
2dz
2fx
-0.35
-0.29
-0.23
-0.18
-0.19
+0.11
-0.16
-0.22
-0.20
-0.23
-1.35
THF
CH2Cl2
THF
CH2Cl2
THF
CH2Cl2
THF
CH2Cl2
THF
THF
b
b
b
b
b
b
b
b
b
b
15
to note that the first one electron reduction occurs at about the
same potential value for [Pt(phen)Cl2] (1ax) and [Pt(bipy)-
Cl2].16,17
Complexes 1ex,fx, having substituents in the 2,9-positions
of the phenanthroline ligand, behave differently from those
previously investigated and containing 2,9-unsubstituted phenan-
throlines (1ax-dx). They undergo an irreversible reduction
which does not exhibit any directly associated response in the
2fy
2fz
reverse scan, even at a rate of 20.48 V s-1
. The cyclic
MeCN
[Pt(CH2CH2CH2)Cl2(bipy)]
voltammetric profile exhibited by 1fx is shown in Figure 7b. It
is most likely that the same compounds (1ex,fx) have a
remarkably distorted coordination geometry similar to that of
compound 1fz discussed in the crystallographic section.
Therefore the bulk of the data suggests that for platinum(II)
complexes the planarity of the platinum center is a prerequisite
for the thermodynamic access to the corresponding one-electron
reduced species. This is in sharp contrast with the behavior of
related [NiCl2(2,9-substituted-1,10-phen)] complexes, for which
the pseudo-tetrahedral coordination imposed by the bulky 2,9-
substituents stabilizes the corresponding nickel(I) species.34
Electrochemistry of the Platinum(IV) Complexes. The
cyclic voltammetric responses exhibited by the platinum(IV)
complexes 2dx and 2fx are shown in Figure 8. Both complexes
display a first irreversible two-electron cathodic step (measured
by controlled potential coulometry) followed by a reduction
pattern identical to those exhibited by 1dx and 1fx, respectively
(Figures 6b and 7b). Therefore the platinum(IV) complexes,
upon two-electron addition, lose the two axial ligands, affording
the corresponding platinum(II) species. Such a behavior was
already described by Kochi in the study of the platinum(IV)-
-1.26
MeCN
15
[Pt(CH2CH2CH2)Cl2(4,7-Ph2phen)]
a Measured at 0.2 V s-1 b Present work.
.
the nature of the halide ligands (this was not the case for the
platinum(II)/platinum(I) reduction, which most likely takes place
without releasing of halide ions); (ii) the redox-induced removal
of the axial halides is notably easier in the actual complexes
than in the platinum-cyclobutane complexes (this can be
attributed to the greater ability of the equatorial halide ligands,
with respect to the hydrocarbon chain, to remove electron
density from the platinum center).
Spectroscopy of the First Reduction Products of 1ax and
1dx: Visible Spectra. Figure 9 shows the visible spectra
recorded at different stages of the controlled potential coulom-
etry of 1dx (Ew ) -1.3 V) performed at ambient temperature.
In the initial stage of the electrolysis, when the solution is blue
colored, a well-shaped absorption at λmax ) 633 nm is
predominant (spectrum c); by an increase in the electrolysis time,
this band is progressively replaced by two peaks, at 460 and
600 nm, which characterize the multielectron-reduced green
solution (spectrum f).
EPR Spectra. The liquid nitrogen X-band EPR spectrum
(100 K) recorded at the first stage of the electrolysis of 1dx
performed at 253 K is shown in Figure 10a. The line shape
features of the λmax ) 633 nm species are indicative of a S )
1/2 paramagnetic system exhibiting a broad axial structure (g|
> g > ge) with significant orbital contribution: g| ) 2.055 (
0.006, g ) 2.005 ( 0.006, g ) 1/3(g| + 2g ) ) 2.022 (
0.006. By considering that the metal d9 configuration should
display typical spectra with g| > g > 2.0023,13,17,35 these data
cyclobutane complexes [Pt(CH2CH2CH2)Cl2(bipy)] and [Pt(CH2-
CH2CH2)Cl2(4,7-Ph2phen)] (the latter complex contains the same
phenanthroline ligand as 2dx).15 The values of the electro-
chemical potentials for the platinum(IV)/platinum(II) reductions
are reported in Table 5. Two points are worth noting: (i) the
platinum(IV)/platinum(II) reduction is significantly affected by
(34) Masood, M. A.; Hodgson, D. J.; Zacharias, P. S. Inorg. Chim. Acta
1994, 221, 99-108.

Pt(II) and -(IV) Complexes with Substituted phen
Inorganic Chemistry, Vol. 35, No. 11, 1996 3181
complexes which are predominantly ligand centered,14,17 this
could constitute EPR evidence for the generation of a platinum
having marked Pt(I) character, complexes with inherently Pt(I)
character have recently been reported by other authors.35
The EPR spectra recorded when the number of electrons/
molecule spent in the reduction process is greater than 1 are
shown in Figure 10b,c. The liquid nitrogen spectrum (Figure
10b) exhibits a complex spectral pattern, which, at the glassy-
fluid transition temperature, converts into a single, relatively
broad and isotropic absorption (Figure 10c). These spectral
features are reversible on refreezing the solution. A similar
absorption was observed in the electrochemical reduction of [Pt-
(bipy)Cl2] and was assigned to a ligand centered paramagnetic
monoanion [PtII(bipy)*Cl2]- (asterisk denotes reduced species).17
The liquid nitrogen line shape analysis can be carried out in
terms of a S ) 1/2 spin Hamiltonian of a paramagnetic species
Figure 9. Visible spectra recorded on a CH2Cl2 solution of 1dx (0.8
× 10-3 mol L-1) at different stages of controlled potential electrolysis
(Ew ) -1.3 V): (a) initial and after (b) 0.5, (c) 1.0, (d) 2.0, (e) 2.7,
and (f) 3.0 electrons/molecule. [NBu4][ClO4] supporting electrolyte,
exhibiting a largely structured rhombic symmetry (gl > gm
>
gh * ge ) 2.0023) and experiencing a significant metal
contribution.36 The nuclear hyperfine (hpf) splittings (al * am
* ah) of the Pt-195 satellites are now detectable; they appear
well-resolved at low and medium fields but unresolved at higher
fields. Second derivative analysis allowed a better characteriza-
tion of this spectral pattern, which was also confirmed by
computer simulation (SIM 14a program).37 Therefore, in
agreement with previous reports,17,18 the spectra of Figure 10b,c
are assigned to a [PtII(4,7-Ph2phen)*Cl2]n- species. The fluid
parameters fit well the glassy computed ones. Also, in the
present case, the lack of the hyperfine isotropic peaks of Pt-
195 satellites as well as of the corresponding ligand superhy-
perfine splittings is attributable to the large isotropic linewidth
∆Hiso.
0.2 mol L-1
.
In summary the EPR data indicate that the reduction process
of the platinum(II) complex 1dx generates primarily the
corresponding Pt(I) species [1dx]- which, however, because of
conjugation with the nonredox innocent phen ligand, progres-
sively restores the Pt(II) oxidation state through internal redox
changes based on the peripheral organic ligand. The change
of solvent from dichloromethane to tetrahydrofuran causes some
variations in the EPR parameters as a consequence of either a
line-narrowing or a slightly better hpf resolution.18 In the case
of 1ax, the EPR spectrum recorded at the first stages of the
electrolysis reveals a pattern which can be interpreted as being
due to the ovelapping signals of equimolar amounts of rhombic
and axial platinum forms, i.e., [PtII(phen*)Cl2]- and [PtI(phen)-
Cl2]-. Upon exhaustive electrolysis only the PtII radical
survives. Table 6 summarizes the relevant EPR data. Finally,
in agreement with the already discussed lower stability of the
one-electron reduced complexes [1bx]- and [1cx]-, we were
unable to detect EPR absorptions for both of them.
Conclusions
Comparison of the X-ray structures of strictly related platinum
compounds has allowed an estimate of the distortions caused
by the steric interaction between the Me substituents in the ortho
position of the phenanthroline and the cis iodo ligands in four-
and six-coordinate environments. These involve displacement
of the equatorial iodine atoms from the N-Pt-N′ plane [average
0.477(2) and 0.199(2) Å for four- (1fz) and six-coordinate
complexes (2fz), respectively], bending of the phenanthroline
(angle between the planes of the outer rings of 19.9(7) and 14.2-
(7)°, respectively), and rotation of the overall phenanthroline
plane with respect to the N-Pt-N′ plane [32.3(10) and 26.5-
Figure 10. X-band EPR spectra recorded on a CH2Cl2 solution of 1dx
(1.9 × 10-3 mol L-1) at different stages of controlled potential
electrolysis (performed at 253 K, Ew ) -1.3 V): (a) after 0.3 electrons/
molecule, 100 K; (b) after 2.0 electrons/molecule, 100 K (top, first
derivative; bottom, second derivative); (c) after 2.0 electrons/molecule,
300 K. [NBu4][ClO4] supporting electrolyte, 0.2 mol L-1
.
strongly support a significant metal character for the anionic
complex [1dx]- in which the noticeable anisotropic line width
overlaps the relevant satellite aaniso(Pt-195) hyperfine features
as well as those of the ligands. To the best of our knowledge,
within the reduction processes of polypyridine-platinum(II)
(36) Goodman, B. A.; Raynor J. B. AdV. Inorg. Chem. Radiochem. 1970,
13, 135-362.
(37) Lozos, J. P.; Hoffman, B. M.; Franz, C. G. QCPE 1973, 11, 243.
(35) Klein, A.; Kaim, W. Organometallics 1995, 14, 1176.

3182 Inorganic Chemistry, Vol. 35, No. 11, 1996
Fanizzi et al.
Table 6. X-Band EPR Parameters for the Platinum(I) and Platinum(II) Species Electrogenerated upon Reduction of 1dx and 1ax (gi ( 0.006;
ai ( 6 G)
complex
gl
gm
gh
g
giso
al
am
ah
a
solvent
[1dx]-
2.055a,b
2.005a,c
2.022a
1.980
CH2Cl2
CH2Cl2
THF
CH2Cl2
CH2Cl2
2.034
2.034
2.002
2.001
1.902
1.900
56.0
57.0
83.0
84.0
e22.0
e23.0
e53.0
e54.0
1.979
1.967
[1ax]-
2.051a,b,d
2.007a,c,d
2.022a,d
1.983
2.032
2.010
1.907
58.0
77.0
e20.0
e52.0
a Platinum(I) species. b g|. c g . d Calculated from a complex spectral pattern (see text).
(9)°, respectively]. The smaller deviations imposed by the axial
iodo ligands in the six-coordinate complex find a compensation
in a narrowing of the I-Pt-I′ angle (5°) and a lengthening of
the Pt-N bonds (0.07 Å).
Distortions in the coordination geometries affect the redox
properties of these complexes. Species possessing regular
square planar geometry afford initially monoanions having Pt-
(I) character, whereas species with distorted geometry are
irreversibly reduced. This is in sharp contrast with the behavior
of related nickel complexes for which the tetrahedral distortion
imposed by the 2,9-dimethyl substituents of the phenanthroline
ligand stabilizes the nickel(I) species. In the long times of
macroelectrolysis the instantaneously electrogenerated Pt(I)
species convert to the corresponding Pt(II) anion radicals.
Acknowledgment. This work was supported by the Minis-
tero della Universita` e della Ricerca Scientifica e Tecnologica
(MURST, quota 40%), the Italian National Research Council
(CNR), and European Community (EC, Contract Cl1-CT92-
0016). The authors gratefully acknowledge Prof L. G. Marzilli
(Emory University, Atlanta, GA) for helpful discussion.
Supporting Information Available: Elemental analyses of the
compounds (Table S1); atomic coordinates and isotropic thermal
parameters (Tables S2-5), anisotropic thermal parameters (Tables S6-
9), and full crystallographic data (Table S10-13) for compounds 1dz‚
CHCl3, 1fz, 2dz‚CHCl3, and 2fz‚I2, respectively, and experimental data
for the X-ray diffraction studies (Table S14) (12 pages). Ordering
information is given on any current masthead page.
IC960125Y