On the question of mixed-valent states in ligand-bridged dinuclear organoplatinum compounds [RkPt(μ-L)PtRk]n, k = 2 or 4

Article abstract of DOI:10.1021/om980107j

Symmetrically dinuclear complexes between the bis-bidentate bridging ligands μ-L (μ-L = 3,6-bis(2-pyridyl)-1,2,4,5-tetrazine (bptz) or 2,5-bis(1-phenyliminoethyl)pyrazine (bpip)) and the organoplatinum fragments PtMeS₂ (Mes = mesityl), PtMe₂, or PtMe₄ were synthesized as deeply colored compounds. Low-energy charge-transfer transitions from metal d orbitale (Pt¹¹) or metal-carbon σ bond combinations (Pt^IV) to low-lying π* orbitals of the π acceptor ligands are responsible for long-wavelength absorption maxima λ_max(CT) > 700 nm. UV/Vis and EPR spectroelectrochemical results for reversible reduction processes indicate the formation of [Pt^IV]₂(μ-L^.-) and [Pt^II]₂(μ-L^.-) species, however, the latter exhibit a significant metal contribution according to a Pt^II/Pt^I formulation. Cyclic voltammetry reveals that the remarkable system [Mes₂Pt(μ-bptz)PtMes₂]ⁿ forms an enormously stabilized radical anion (n = 1-) with ΔE_1/2 = 1250 V and K_c = 10^21.2 and a Pt^III/Pt^II mixed-valent state (n = 1+) with ΔE_1/2 = 80 mV and K_c = 23. This small K_c value is attributed to the predominantly d_σ orbital character of the redox orbitals on the Pt(II) centers.

Full text of DOI:10.1021/om980107j

3532
Organometallics 1998, 17, 3532-3538
On th e Qu estion of Mixed -Va len t Sta tes in
Liga n d -Br id ged Din u clea r Or ga n op la tin u m Com p ou n d s
[RkP t(µ-L)P tRk]n , k ) 2 or 4†
Axel Klein, Steffen Hasenzahl, and Wolfgang Kaim*
Institut fu¨r Anorganische Chemie, Universita¨t Stuttgart, Pfaffenwaldring 55,
D-70550 Stuttgart, Germany
J an Fiedler
J . Heyrovsky Institute of Physical Chemistry, Academy of Sciences of the Czech Republic,
Dolejsˇkova 3, 18223 Prague, Czech Republic
Received February 19, 1998
Symmetrically dinuclear complexes between the bis-bidentate bridging ligands µ-L (µ-L
) 3,6-bis(2-pyridyl)-1,2,4,5-tetrazine (bptz) or 2,5-bis(1-phenyliminoethyl)pyrazine (bpip))
and the organoplatinum fragments PtMes2 (Mes ) mesityl), PtMe2, or PtMe4 were
synthesized as deeply colored compounds. Low-energy charge-transfer transitions from metal
d orbitals (PtII) or metal-carbon σ bond combinations (PtIV) to low-lying π* orbitals of the
π acceptor ligands are responsible for long-wavelength absorption maxima λmax(CT) > 700
nm. UV/Vis and EPR spectroelectrochemical results for reversible reduction processes
indicate the formation of [PtIV]2(µ-L•-) and [PtII]2(µ-L•-) species, however, the latter exhibit
a significant metal contribution according to a PtII/PtI formulation. Cyclic voltammetry
reveals that the remarkable system [Mes2Pt(µ-bptz)PtMes2]n forms an enormously stabilized
radical anion (n ) 1-) with ∆E1/2 ) 1250 V and Kc ) 1021.2 and a PtIII/PtII mixed-valent
state (n ) 1+) with ∆E1/2 ) 80 mV and Kc ) 23. This small Kc value is attributed to the
predominantly dσ orbital character of the redox orbitals on the Pt(II) centers.
In tr od u ction
2,2′-bipyrimidine7 (bpym) to form compounds [RkPt(µ-
bpym)PtRk]n, k ) 2 or 4. Absorption spectra, calcula-
tions, a structure determination, and spectroelectro-
chemical and EPR studies of generated paramagnetic
species were described.6 In the following, we present
results for dinuclear complexes [RkPt(µ-L)PtRk]n, k ) 2
or 4, with two special bis-bidentate bridging ligands µ-L,
µ-L ) 3,6-bis(2-pyridyl)-1,2,4,5-tetrazine (bptz) and 2,5-
bis(1-phenyliminoethyl)pyrazine (bpip).
Electrochemical, spectroscopic, and reactivity studies1-3
of platinum(II) and -(IV) complexes with unsaturated
nitrogen-donor-containing ligands are of current interest
due to the photoemissive,2 C-H-activating or catalytic4
and pharmacological5 relevance of such species. Instead
of π-accepting monochelate ligands such as aromatic
2,2′-bipyridine1b,c,f and 1,10-phenanthrolines1g or non-
aromatic R-diimines such as 1,4-diaza-1,3-butadienes,1e,n
we have recently6 used the conventional bridging ligand
[Mes2Pt(µ-bptz)PtMes2]n ) 1n
[Me2Pt(µ-bptz)PtMe2]n ) 2n
[Me4Pt(µ-bptz)PtMe4]n ) 3n
[Me4Pt(bpip)]n ) 7n
[Mes2Pt(µ-bpip)PtMes2]n ) 4n
[Me2Pt(µ-bpip)PtMe2]n ) 5n
[Me4Pt(µ-bpip)PtMe4]n ) 6n
† Dedicated to Prof. Heinrich No¨th on the occasion of his 70th
birthday.
(1) (a) Braterman, P. S.; Song, J .-I.; Vogler, C.; Kaim, W. Inorg.
Chem. 1992, 31, 222. (b) Vogler, C.; Schwederski, B.; Klein, A.; Kaim,
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Kaim, W. J . Organomet. Chem. 1992, 440, 207. (d) Braterman, P. S.;
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The ligand bptz was found to have a very low-lying
tetrazine-localized π* orbital8a and to form very stable
anion radical complexes;8b on the other hand, it induced
a very large comproportionation constant Kc ) 1015.0 for
S0276-7333(98)00107-1 CCC: $15.00 © 1998 American Chemical Society
Publication on Web 07/11/1998

Mixed-Valent States in [RkPt(µ-L)PtRk]n, k ) 2 or 4
Organometallics, Vol. 17, No. 16, 1998 3533
the RuIII/RuII mixed-valent state [(H3N)4Ru(µ-bptz)Ru-
nuclear system [Mes2Pt(µ-bpym)PtMes2]n showed no
(NH3)4]5+ 8c
.
The ligand bpip is a rather new bis(R-
separated redox features for the oxidation of the two
diimine) ligand which, like the monochelating 2-pyridi-
necarbaldimines,10 acts through two different chelate
donors, one imine and one azine nitrogen atom per
metal center. The ligand bpip was found to be a
stronger π acceptor than the related 2,5-bis(2-pyridyl)-
pyrazine.9a,d A RhII/RhI mixed-valent intermediate
[(C5Me5)Rh(µ-bpip)Rh(C5Me5)]+ was recently identified
through typical intervalence transfer (IT) transitions in
the near-infrared region.9c
individual metal centers; the nonobservability of a PtIII
/
PtII mixed-valent state was attributed to insufficient
orbital overlap between the π system of the organic
bridge and the dσ redox orbitals on platinum.6 The
methods used to study the electronic structures of
compounds 1-7 were absorption spectroscopy of the
neutral precursors and cyclic voltammetry in combina-
tion with EPR or UV/Vis spectroscopy (spectroelectro-
chemistry).
Concerning the co-ligands, the methyl group (Me) was
used as the most simple organic substituent and the
axially protecting mesityl (Mes ) 2,4,6-trimethylphenyl)
substituent was chosen to avoid the undesired addition
of nucleophiles to the one-electron-oxidized monomeric
platinum(III) states, as shown for several mononuclear
Exp er im en ta l Section
Ma ter ia ls a n d P r oced u r es. The ligands bptz8 and bpip9
and the platinum precursor complexes (DMSO)2PtMes2,10 [Pt2-
Me8(µ-SMe2)2],11 and [Pt2Me4(µ-SMe2)2]12 were obtained fol-
lowing literature procedures. All preparations and physical
measurements were carried out in dried solvents under an
argon atmosphere, using Schlenk techniques. Furthermore,
the tetramethylplatinum(IV) compounds had to be prepared
and studied in the absence of intense light.
species
[PtMes2(R-diimine)]+.1c,f,g
The
corre-
sponding PtIII/PtII half-wave potentials can then be used
to correlate optical data (absorption or emission ener-
gies) with electrochemical results. However, the di-
Bis(d im esitylp la tin u m ) Com p lexes (µ-L)[P tMes2]2: 1
(L ) bp tz) a n d 4 (L ) bp ip ). In a typical reaction, 295 mg
(0.5 mmol) of dimesitylbis(dimethylsulfoxido)platinum(II) was
suspended together with 0.25 mmol of the bridging ligand in
70 mL of toluene and heated under reflux for 5 days. The
sulfoxide vibration ν(SdO) at 1130 cm-1 of the platinum
precursor complex13 had by then disappeared. At the end of
the reaction, the temperature was lowered within another day
to accomplish slow precipitation of the products. The solids
were collected on a microporous frit and washed with diethyl
ether. We thus obtained a poorly soluble blue-black powder
for the bptz complex 1 in a 224 mg (81%) yield. Anal. Calcd
for C48H52N6Pt2: C, 52.26; H, 4.75; N, 7.62. Found: C, 52.50;
H, 4.82, N, 7.74. In the case of the bpip complex 2, we obtained
greenish-black microcrystals in a 243 mg (82%) yield. Anal.
Calcd for C56H66N4Pt2: C, 56.74; H, 5.61; N, 4.73. Found: C,
56.41; H, 5.58; N, 4.74.
(µ-bp tz)[P tMe2]2, 2. A solution of 0.063 g (0.11 mmol) of
[Pt2Me4(µ-SMe2)2] and 0.027 g (0.11 mmol) of bptz in a mixture
of 15 mL of toluene and 5 mL of diethyl ether was stirred
overnight at ambient temperature to yield a black microcrys-
talline precipitate. After filtration, the virtually insoluble solid
was washed with diethyl ether and toluene (66 mg, 87%). Anal.
Calcd for C16H20N6Pt2: C, 27.99; H, 2.94; N, 12.24. Found:
C, 28.95; H, 3.06; N, 11.39. The very poor solubility in all
common solvents precluded further investigation.
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(µ-bp tz)[P tMe4]2, 3. A solution of 0.063 g (0.11 mmol) of
[Pt2Me4(µ-SMe2)2] and 0.027 g (0.11 mmol) of bptz in a mixture
of 15 mL of toluene and 5 mL of diethyl ether was stirred
overnight at ambient temperature in the dark. The poorly
soluble precipitate was filtered and washed with diethyl ether
and toluene to yield 66 mg (87%) of a black microcrystalline
solid. Anal. Calcd for C16H20N6Pt2: C, 27.99; H, 2.94; N, 12.24.
Found: C, 28.95; H, 3.06; N, 11.39. The virtual insolubility
in all common solvents precluded further investigation.
(µ-bp ip )[P tMe2]2, 5. A solution of 0.063 g (0.11 mmol) of
[Pt2Me4(µ-SMe2)2] and 0.031 g (0.031 mmol) of bpip in 15 mL
of benzene and 10 mL of diethyl ether was stirred overnight
at ambient temperature to produce a dark-green precipitate.
After removal of the solvent mixture, the residue was washed
with 10 mL of diethyl ether, filtered, and dried in vacuo. A
greenish-black microcrystalline powder was obtained in a
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3534 Organometallics, Vol. 17, No. 16, 1998
Ta ble 1. 1H NMR Da ta a of bp ip a n d Its Or ga n op la tin u m Com p lexes
Pt-CH3 bpip
H(pyrazine)
Klein et al.
ax
eq
b
compound
CH3
CH3
CH3
2.36
H(phenyl)
bpip
4f
9.43
8.65
6.84,c 7.15,d 7.42e
6.76,c 7.08,d 7.18e
7.08,c 7.37,d 7.57e
7.01,c 7.37,d 7.57e
6.97,c 7.33,d 7.53e
6.95,c 7.18,d 7.44e
7.01,c 7.43,d 7.54e
2.34 (6.8)
1.97
2.82 (7.2)
2.68 (7.5)
2.44
5g
6
1.12 (86.5), 1.89 (87.2)
0.47 (74.1), 0.92 (72.9)
0.49 (73.8), 0.96 (72.4)
0.40 (73.8), 0.86 (72.7)
9.71 (19.0)
9.68 (12.6)
9.58 (13.0)
9.66i
-0.41 (45.3)
-0.37 (45.0)
-0.47 (44.9)
6h
7
2.70 (7.0)
9.78i (7.4)
Measured in acetone-d6 at room temperature unless noted otherwise; chemical shifts δ in ppm, J (195Pt/1H) coupling constants in
a
hertz (in parentheses). Iminoethyl group. c ortho-H, dd, J HH = 8.4 Hz, J HH = 1.1 Hz. para-H, tt, J HH = 7.5 Hz, J HH = 1.1 Hz.
b
3
4
d
3
4
3
g
h
i 5
e meta-H, m, J HH = 8.4 Hz. f Further signals at δ ) 6.27 (m-H), 2.54 (o-CH3), 2.12 (p-CH3). In CD2Cl2. In THF-d8.
J
) 1.2 Hz.
HH
0.034 g yield (66%). Anal. Calcd for C24H30N4Pt2: C, 37.7; H,
5.13; N, 6.79. Found: C, 37.23; H, 4.98; N, 6.77.
(µ-bp ip )[P tMe4]2, 6. A solution of 0.127 g (0.2 mmol) of
[Pt2Me8(µ-SMe2)2] and 0.063 g (0.2 mmol) of bpip was stirred
in 25 mL of diethyl ether at ambient temperature over 10 h
in the dark. Removal of the solvent lead to a black residue,
which was washed with 10 mL of diethyl ether, filtered, and
dried in vacuo to yield a black microcrystalline powder.
Yield: 0.089 g (68%). Anal. Calcd for C28H42N4Pt2: C, 40.77;
H, 5.13; N, 6.69. Found: C, 40.27; H, 4.98; N, 6.77.
(bp ip )P tMe4, 7. A solution of 0.127 g (0.2 mmol) of [Pt2Me8-
(µ-SMe2)2] and 0.129 g (0.41 mmol) of the bpip ligand was
stirred in 25 mL of diethyl ether at ambient temperature over
10 h in the dark. Removal of one-half of the solvent, addition
of 15 mL of n-hexane, and leaving this mixture at -30 °C
overnight lead to precipitation of a green-black solid. This was
collected on a microporous glass frit, washed with 10 mL of
diethyl ether, and dried in vacuo. Yield: 0.120 g (51%). Anal.
Calcd for C24H30N4Pt: C, 50.61; H, 5.31; N, 9.84. Found: C,
50.46; H, 5.09; N, 9.54.
F igu r e 1. Cyclic voltammogram of 1 in THF/0.1 M
Bu4NPF6 at 100 mV/s scan rate (top); simulated cyclic
voltammogram of 1 (bottom). The low solubility of 1 causes
a more soluble impurity to become detectable at about -1.0
V.
In str u m en ta tion . EPR spectra were recorded in the X
band on a Bruker ESP 300 system equipped with a Bruker
ER035M gaussmeter and a HP 5350B microwave counter. 1H
NMR spectra were taken on a Bruker AC 250 spectrometer;
infrared spectra were recorded with a Perkin-Elmer 283
instrument. UV/Vis/NIR-absorption spectra were recorded on
Shimadzu UV160 and Bruins Instruments Omega 10 spec-
trophotometers. Cyclic voltammetry and differential pulse
voltammetry were carried out using a three-electrode config-
uration (glassy carbon working electrode, platinum counter-
electrode, Ag/AgCl reference) and a PAR 273 potentiostat and
function generator with PAR M270/250 software. As an
internal standard, the ferrocene/ferrocenium couple was used.
Cyclic voltammograms were simulated with the help of the
program DigiSim 2.1 (BAS). Spectroelectrochemical measure-
ments were performed using an optically transparent thin-
layer electrode (OTTLE) cell14 for UV/Vis/NIR spectra, a
platinum two-electrode capillary for EPR studies,9a and a Bank
Elektronik Potentioscan Wenking POS 73 potentiostat and
function generator equipped with an XY-plotter PAR RE 0089.
Dimesitylplatinum(II) complexes of aromatic R-di-
imines had been structurally characterized1c,g with
approximately square-planar metal configurations and
sterically protected axial positions; we therefore assume
a similar situation for the compounds Mes2Pt(µ-L)-
PtMes2. The structures of mono- and dinuclear tetra-
methylplatinum(IV) complexes of R-diimines have also
been characterized before,1e,6 illustrating significantly
longer metal-to-carbon bonds to the axial methyl groups
(ca. 2.13 Å) than to the equatorial CH3 substituents (ca.
2.04 Å). In the NMR spectra of the PtMe4 species, the
asymmetric chelate coordination is reflected by dis-
tinctly different chemical shifts of the two nonequivalent
equatorial methyl groups.
Electr och em istr y. All soluble dinuclear compounds
1 and 4-6 exhibit two reversible one-electron reduction
processes in THF/0.1 M Bu4NPF6 (Figure 1, Table 2).
Whereas the soluble methylplatinum complexes are
oxidized in an electrochemically irreversible fashion,
compounds 1 and 4 with the axially shielding mesityl
groups exhibit reversible oxidation waves (Figure 1,
Table 2).
Both the Pt(II) and Pt(IV) species are believed to have
the π*(L) orbital, supported by dxz(Pt), as the lowest
unoccupied MO (LUMO).1n On the other hand, the
HOMO is primarily a metal-centered d orbital for the
diorganoplatinum(II) compounds but an antisymmetric
Pt-Cax σ orbital combination with some px(Pt) contribu-
tions for PtMe4(L) species. These calculation results
have been supported by experimental evidence.1m,n,6
As noted for the mononuclear species PtMen(RNd
CH-CHdNR),1e,n the cyclic voltammograms for com-
Resu lts a n d Discu ssion
Syn th esis a n d Str u ctu r e. The mono- and dinuclear
organoplatinum(II) and -(IV) compounds were obtained
according to established procedures1c,e,f,11,12 as thermally
stable species. However, special care had to be taken
regarding the light-sensitive tetraalkylplatinum(IV)
complexes.1m,n The identity of the compounds was
confirmed by elemental analysis and, except for poorly
soluble (1) or virtually insoluble compounds (2 and 3),
1
by H NMR spectroscopy (Table 1).
(14) Krejcik, M.; Danek, M.; Hartl, F. J . Electroanal. Chem. 1991,
317, 179.

Mixed-Valent States in [RkPt(µ-L)PtRk]n, k ) 2 or 4
Organometallics, Vol. 17, No. 16, 1998 3535
Ta ble 2. Electr och em ica l Da ta a of bp ip a n d bp tz
a n d Th eir Or ga n op la tin u m Com p lexes
Kc values of 1013.9, 1011.0, and 1014.9, respectively. The
significantly lower number for 5•- is in agreement with
the EPR spectroscopically detectable1e,m higher degree
of mixing between the metal centers and the singly
occupied ligand-centered orbital (cf. below). The
established1c,f,g stabilization of the PtIII state through
dimesityl substitution has allowed us to quantitatively
assess, for the first time, the extent of metal-metal
interaction in a compound [RkPtII(µ-L)PtIIIRk]+. Previ-
ous studies of [Mes2Pt(µ-bpym)PtMes2]n 6 and the simi-
lar observation of one two-electron oxidation process for
4 to 42+ (Table 2) have only indicated that such a
“communication” must be very small. The capacity of
the bptz bridging ligand to effect extremely high Kc
values not only for the ligand-reduced form but also for
the mixed-valent metal states has previously been
demonstrated for {(bptz)[Ru(NH3)4]2}5+, for which Kc )
E1/2oxI
(∆Ep)c
E1/2redI
(∆Ep)c
E1/2redII
(∆Ep)c
compound
EpaoxIIb
bptz
1e
bpip
4
-1.33 (59)
-0.69 (60)
-2.05 (80)
-0.93 (63)
-1.11 (84)
-1.06 (90)
-1.46 (85)
-2.34d
0.66 (60)
0.58 (60)f
-1.94 (75)
-2.39d
-1.75 (75)
-1.76 (78)
-1.94 (75)
0.53 (60)f
0.64b
5
6
0.40b
7e
0.40b
a
From cyclic voltammetry in 0.1 M Bu4NPF6/THF solutions at
100 mV/s scan rate. Potentials in volts vs the ferrocene/ferroce-
nium couple. No cyclovoltammograms could be obtained for
Me2Pt(bptz)PtMe2 and Me4Pt(bptz)PtMe4 due to very poor solubil-
b
ity. Anodic peak potentials Epa for irreversible oxidation steps.
c Half-wave potentials E1/2; peak potential differences ∆Ep ) Epa
d
- Epc in millivolts in parentheses. Cathodic peak potentials Epc
for irreversible reduction steps. e Further irreversible reduction
occurs at -2.64 V. f Two-electron process.
1015.0 8c
On oxidation, the corresponding diplatinum
.
pounds n ) 2 (5) and 4 (6) look very much alike. A
reversible ligand-centered reduction and an irreversible
oxidation, either of the metals (PtII compounds) or of
the Pt-Cax σ bonds (PtIV complexes), occur at very
similar potentials (Table 2). Obviously, the binding of
two additional methyl carbanions compensates almost
exactly for the effect of the higher metal oxidation state.
As expected,6,8 the dinuclear complex 6 is reduced at
less negative potentials than the mononuclear analogue
7, reflecting the stepwise polarization of the ligand bpip
through successive metal coordination. The potentials
in the bpip ligand series also show that the bis-
(dimesitylplatinum) system 4 is reduced easier than the
analogous dimethylmetal compound 5, which reflects
the stronger electron-donating effect of alkyl vs aryl
groups.
complex 1 does indeed show a small but detectable
splitting between the waves for the 1/1+ and 1+/12+
couples (Figure 1). The potential difference of 80 mV,
as extracted from simulation (Figure 1), translates to
Kc ) 23 for the PtII-PtIII mixed-valent intermediate.
This small value confirms the weak coupling of the d7/
d8 pair through the bridging π system, in agreement
2
with the notion of a singly occupied σ-type orbital (dz )
in the PtIII state. In contrast, the numerous d5/d6 mixed
valent systems16 exhibit a much stronger metal-metal
interaction in such situations due to the involvement
of π-type orbitals in both the bridging ligand and at the
metal oxidation sites.16 In any case, the superior
capacity of the bptz bridging ligand in stabilizing odd-
electron intermediates is confirmed for both the one-
electron oxidized and the one-electron reduced forms of
compound 1.
The splitting of the redox potentials neighboring the
monooxidized and monoreduced paramagnetic states
merits particular attention. The bptz-bridged state 1•-
exhibits an enormous potential range (Figure 1) and
comproportionation constant Kc ) 1021.2, calculated
according to eq 1. This value is significantly higher than
Absor p tion Sp ectr oscop y. Platinum(II) and tet-
raorganoplatinum(IV) complexes of π-acceptor ligands
are distinguished by low-lying solvatochromic charge-
transfer transitions, either from metal d orbitals (PtII)
or from Pt-C σ bond combinations (PtIV) to low-lying
π* orbitals.1n,17 Since bpip and especially bptz have very
low-lying acceptor MOs,8,9 it is not surprising that the
compounds presented here display particularly long-
wavelength absorptions (Figure 2, Table 3), extending
into the near-infrared region.
Kc ) 10∆E/59mV ) [M(n-1)]2/[M][M(n-2)
M + M(n-2) h 2M(n-1)
]
(1)
the Kc ) 1013.3 value found for {(bptz)[Ru(NH3)4]2}3+ 8c
;
it supports previous suggestions8 concerning the gener-
ally very large stability constants of tetrazine anion
radicals which, however, were often deduced from less
reversible second reduction features. Obviously, the
introduction of mesityl substituents does not only
protect the oxidized metal center from nucleophilic
attack1c,f,g but also the doubly reduced ligand from
degradation through electrophiles. We assume that 1•-
exhibits a Kc value more typical of tetrazine redox
systems; the lower value found for {(bptz)[Ru-
(NH3)4]2}3+ 8c probably indicates15 strong metal/ligand
orbital mixing as is also evident from the very rapid
EPR relaxation behavior of that species.8c
Chelating acceptor ligands such as bpip9 or bptz8 are
generally distinguished by several low-lying π* orbitals
of either Ψ or ø local symmetry.1g,18 The three lowest-
lying π* MOs are of au, au (2Ψ), and bg (2Ψ) symmetry
for bptz8 and of au (2Ψ), au (2ø), and bg (2Ψ) symmetry
for bpip (each in the C2h conformation; 2 indicates the
number of corresponding chelate sites), Chart 1.9d
(16) (a) Mixed Valency Systems-Applications in Chemistry, Physics
and Biology; Prassides, K., Ed.; Kluwer: Dordrecht, The Netherlands,
1991. (b) Kaim, W.; Bruns, W.; Poppe, J .; Kasack, V. J . Mol. Struct.
1993, 292, 221. (c) Hornung, F.; Baumann, F.; Kaim, W.; Olabe, J . A.;
Slep, L. D.; Fiedler, J . Inorg. Chem. 1998, 37, 311. (d) Creutz, C. Prog.
Inorg. Chem. 1983, 30, 1. (e) Richardson, D. E.; Taube, H. Coord. Chem.
Rev. 1984, 60, 107.
(17) (a) Kaupp, M.; Stoll, H.; Preuss, H.; Kaim, W.; Stahl, T.; van
Koten, G.; Wissing, E.; Smeets, W. J . J .; Spek, A. L. J . Am. Chem.
Soc. 1991, 113, 5606. (b) Hasenzahl, S.; Kaim, W.; Stahl, T. Inorg.
Chim. Acta 1994, 225, 23.
The dinuclear bpip complexes 4•-, 5•-, and 6•- exhibit
smaller but still rather large redox potential ranges and
(15) (a) Kaim, W.; Kasack, V. Inorg. Chem. 1990, 29, 4696. (b)
Kasack, V.; Kaim, W.; Binder, H.; J ordanov, J .; Roth, E. Inorg. Chem.
1995, 34, 1924.
(18) (a) Orgel, L. E. J . Chem. Soc. 1961, 3683. (b) Ernst, S.; Vogler,
C.; Klein, A.; Kaim, W.; Zalis, S. Inorg. Chem. 1996, 35, 1295.

3536 Organometallics, Vol. 17, No. 16, 1998
Klein et al.
F igu r e 3. Absorption spectra of (1) 1, (2) 1•-, and (3) 12-
from spectroelectrochemistry in THF/0.1 M Bu4NPF6.
Ta ble 4. P a r a m eter sa of Lin ea r Regr ession s ν˜ ) A
b
+ B × E*MLCT for th e Solva toch r om ism of
Lon g-Wa velen gth Absor p tion Ma xim a
compound
band
A
B
r
nd
4
4
5
5
6
6
λ1
λ2
12 280
14 450
13 060
22 530
12 540
23 860
1790
1830
1880
2450
1980
2170
0.820
0.940
0.999
0.996
0.828
0.913
6
6
4
4
6
6
c
λ1
c
λ2
λ1
λ2
a
b
A and B in cm-1; r is the correlation coefficient. Solvent
F igu r e 2. Absorption spectra of compounds 5 (top) and 6
(bottom) in dichloromethane.
parameters E*MLCT from ref 21a. c Main features of corresponding
band system (see Table 3). Number of solvents used for correla-
tion.
d
Ta ble 3. Lon g-Wa velen gth Absor p tion Da ta a of
Mon o- a n d Din u clea r Or ga n op la tin u m Com p lexes
the typical1f,19 magnitude 1300 ( 150 cm-1, could only
be observed for the complexes with bpip (Figure 2, Table
3). The lack of such features for bptz compound 1 is
attributed to strong band broadening on MLCT excita-
tion involving tetrazine π* orbitals; 1,2,4,5-tetrazines
exhibit extensive structural reorganization on π* orbital
occupation.20 As Figure 2 illustrates, the spectral
resolution is better for the PtII compound 5 than for the
PtIV analogue 6 with more degrees of geometrical
freedom.
complex
λ3
440b
λ2
λ1
1
4
5
6
7
690b
852b
418
557, 603, 654 765
414 (6.7) 465sh
400 (3.8) 510sh
388
588sh, 641sh, 702 (6.15)
670sh, 734 (2.85), 805sh
612sh, 671sh, 731, 796sh
503
a
Wavelengths, λ, of maxima in nanometers (extinction coef-
ficients, ꢀ, in 103 M-1 cm-1); measurements in CH2Cl2. For
b
assignments of transitions, see text. Values in THF: 421 (2.99),
639 (3.45), and 782 (3.02) nm.
Ch a r t 1
The charge-transfer bands of organoplatinum com-
pounds with π-acceptor ligands are usually solvent
dependent (solvatochromism).1b,c,f,n,2 The long-wave-
length absorption maxima of the centrosymmetric com-
pounds 4-6 were thus measured (Table S1, Supporting
Information) in various solvents. For the less soluble
complex 1, the database was too small to extract
meaningful correlations with solvent parameters. The
complexes of bpip exhibit the typical negative solvato-
chromism,21 i.e., hypsochromic shifts with increasing
solvent polarity. Correlations with the established8a,21a,b
Several intense (allowed) charge-transfer transitions are
thus expected and observed in the visible region (Figure
2).
We have previously pointed out that the d8 configu-
ration of square-planar PtII typically results in a situ-
ation where the allowed transitions from the stabilized
dxz to π*(Ψ) orbital and from the less stabilized dxy
orbital to the π*(ø) MO come to lie in comparable energy
ranges, the frequent result being an overlap of several
often vibrationally structured band systems.1c,f Such a
situation is also observed here for compounds 1, 4, and
5, however, an exact assignment cannot be made yet
with reasonable confidence. The platinum(IV) com-
pounds 6 and 7 also exhibit long-wavelength charge-
transfer bands which we assign to transitions from
Pt-C σ bond combinations to low-lying π* orbitals of
appropriate symmetry.1e,f,n,17 Vibrational structuring of
21a
solvent parameter E*MLCT were only partly satisfac-
tory in terms of a linear regression (Table 4); the
sensitivity B from the equation ν ) A + B × E*MLCT
(19) Nieuwenhuis, H. A.; Stufkens, D. J .; McNicholl, R.-A.; Al-
Obaidi, A. H. R.; Coates, C. G.; Bell, S. E. J .; McGarvey, J . J .; Westwell,
J .; George, M. W.; Turner, J . M. J . Am. Chem. Soc. 1995, 117, 5579.
(20) (a) Kaim, W.; Moscherosch, M.; Kohlmann, S.; Field, J . S.;
Fenske, D. In The Chemistry of the Copper and Zinc Triads; Welch,
A. J ., Chapman, S. K., Eds.; Royal Society of Chemistry: Cambridge,
1993; p 248. (b) Schwach, M. Ph.D. Thesis, Universita¨t Stuttgart, 1997.
(21) (a) Manuta, C. M.; Lees, A. J . Inorg. Chem. 1983, 22, 3825. (b)
Kaim, W.; Kohlmann, S.; Ernst, S.; Olbrich-Deussner, B.; Bessen-
bacher, C.; Schulz, A. J . Organomet. Chem. 1987, 321, 215. (c)
Dodsworth, E. S.; Lever, A. B. P. Coord. Chem. Rev. 1990, 97, 271.
Dodsworth, E. S.; Lever, A. B. P. Inorg. Chem. 1990, 29, 499.

Mixed-Valent States in [RkPt(µ-L)PtRk]n, k ) 2 or 4
Organometallics, Vol. 17, No. 16, 1998 3537
Ta ble 5. Sp ectr oelectr och em ica l Da ta :a UV/Vis/
NIR-Absor p tion Ma xim a , λ, of Ion ic Din u clea r
Or ga n op la tin u m Com p lexes
complex
λ
1
421 (2.99), 639 (3.45), 782 (3.02)
553 (9.6), 613sh (5.1)
555 (18.0), 878 (11.4)
406 (10.73), 639 (7.07), 728 (6.13)
313 (13.0), 540 (13.65), 611sh (6.92), 1056 (0.4)
280 (18.8), 542 (25.0), 577 (27.1)
325, 375, 468, 596, 735, 840
394, 711
398, 475, 575, 630
585 br
1390 (0.13)
922 (0.78), 1335sh (0.45), 1427 (0.54)
1•-
12-
4
4•-
42-
5•-
6
6•-
62-
12+ b
42+ b
F igu r e 4. EPR spectrum (110 K) of electrogenerated 5•-
in THF/0.1 M Bu4NPF6.
a
Wavelengths, λ, in nanometers (extinction coefficients in 103
M-1 cm-1) from measurements following in situ electrolysis in
b
Special features of the mixed-valent monocation 1+
could not be discerned as a consequence of the small Kc
value of 23.
THF/0.1 M Bu4NPF6 at ambient temperatures. Ligand-field
transitions of low-spin d7 system.
exhibits typical values for “normal” MLCT absorption
bands.21,22a
EP R Sp ectr oscop y. The stability of several kinds
of odd-electron dinuclear organoplatinum species has
stimulated our efforts in characterization through EPR
spectroscopy. Recent results of paramagnetic platinum
complexes with R-diimine and related ligands1a-m,24,25
have led to different formulations of the proper oxidation
states involved. Dinuclear platinum compounds26 are
of particular interest in that respect because of possible
mixed-valency contributions.
As observed previously,1c,f,g the persistent oxidized
species containing Mes2PtIII centers remained EPR
silent even at 3.5 K. (There is no indication for PtIII
dimer formation, which would be hindered by the
mesityl groups.) The very rapid relaxation is attributed
to the presence of close-lying d orbitals for the single-
electron and to correspondingly close-lying paramag-
netic states. The high spin-orbit coupling constant27
of the 5d element adds to the EPR line broadening. On
the other hand, the monoanionic states, obtained through
in situ one-electron reduction in a two-electrode cell,
exhibit partially resolved EPR spectra (Figures 4 and
5); data analysis yielded the results summarized in
Table 6.
The availability of half-wave potentials E1/2 from
reversible processes for both oxidation and reduction of
bis(dimesitylplatinum) compounds 1 and 4 allowed us
to estimate the Franck-Condon contributions øFC from
intra- and intermolecular reorganization after long-
wavelength CT excitation (Eop).1f,n,22 Using the ap-
proximation given in eq 2 and the data from Tables 2,
3, and S1, we arrive at values of 0.27 eV for 1 in THF;
compound 4 yields 0.25 eV in THF and 0.18 eV in
dichloromethane. Despite the dinuclear composition,
øFC ) Eop [eV] - (E1/2 oxI - E1/2 redI) [V]
(1 eV ) 8065.5 cm-1
(2)
)
the numbers for øFC are still rather small and only
slightly higher than those of mononuclear species with
R-diimine ligands,1f,n which supports the notion of little
structural change1n and largely independent metal/
ligand interfaces.21c
Spectroelectrochemical data of the dinuclear com-
pounds, as obtained with an OTTLE cell,14 are sum-
marized in Table 5. A corresponding spectrum is shown
for the system 10/1-/2- in Figure 3.
The reversibility of the electron-transfer processes
was confirmed in the spectroelectrochemistry experi-
ments. The data for the reduction processes show no
particular long-wavelength shifts, presumably due to
the stabilization of the π* orbital occupied by one or two
electrons. More detailed assignments of the transitions
will require high-level open-shell calculations and spec-
troelectrochemical data for the ligands and more simple
complexes.
The weak long-wavelength bands detected above 900
nm for the dicationic species 12+ and 4+ (Table 5) are
readily assigned to dfd (ligand-field) transitions in a
low-spin d7 configuration (PtIII).1f Up to three such
weak, low-energy bands may be observed1f,23 from
doubly occupied d orbitals to the singly occupied level.
All dinuclear anions [RkPt(µ-L)PtRk]- exhibit a rhom-
bic g factor pattern. The total g anisotropy ∆g ) g1 -
g3 is smallest for the reduced diplatinum(IV) compound
6•- and highest for the related 5•-. In the series of
(24) See also paramagnetic dithiolene platinum complexes formu-
lated with “PtIII” (g1 ≈ 2.2, g2 ≈ 2.07, g3 ≈ 1.84): Mo¨ller, E.; Kirmse,
R. Inorg. Chim. Acta 1997, 257, 273.
(25) PtI species are postulated in the following: Fanizzi, F. P.; Natile,
G.; Lanfranchi, M.; Tiripicchio, A.; Laschi, F.; Zanello, P. Inorg. Chem.
1996, 35, 3173.
(26) (a) Barton, J . K.; Szalda, D. J .; Rabinowitz, M. N.; Waszczak,
J . V.; Lippard, S. J . J . Am. Chem. Soc. 1979, 101, 1434. (b) Micklitz,
W.; Mu¨ller, G.; Huber, B.; Riede, J .; Rashwan, F.; Heinze, J .; Lippert,
B. J . Am. Chem. Soc. 1985, 110, 70684. (c) Matsumoto, K.; Sakai, K.;
Nishio, K.; Tokisue, Y.; Ito, R.; Nishide, T.; Shichi, Y. J . Am. Chem.
Soc. 1992, 114, 8110. (d) Uso´n, R.; Fornie´s, J .; Falvello, L. R.; Toma´s,
M.; Casas, J . M.; Mart´ın, A.; Cotton, F. A. J . Am. Chem. Soc. 1994,
116, 7160. (e) Fornie´s, J .; Menjo´n, B.; Sanz-Carrillo, R. M.; Toma´s,
M.; Connelly, N. G.; Crossley, J . G.; Orpen, A. G. J . Am. Chem. Soc.
1995, 117, 4295. (f) Prenzler, P. D.; Heath, G. A.; Lee, S. B.; Raptis, R.
G. J . Chem. Soc., Chem. Commun. 1996, 2271. (g) Uso´n, R.; Fornie´s,
J .; Toma´s, M.; Casas, J . M.; Cotton, F. A.; Falvello, L. R.; Feng, X. J .
Am. Chem. Soc. 1993, 115, 4145. (h) Cotton, F. A.; Matonic, J . H.;
Murillo, C. A. Inorg. Chem. 1996, 35, 498. (i) Bandoli, G.; Caputo, P.
A.; Intini, F. P.; Sivo, M. F.; Natile, G. J . Am. Chem. Soc. 1997, 119,
10370.
(22) (a) Kober, E. M.; Goldsby, K. A.; Narayana, D. N. S.; Meyer, T.
J . J . Am. Chem. Soc. 1983, 105, 4303. (b) Dodsworth, E. S.; Lever, A.
B. P. Chem. Phys. Lett. 1985, 119, 61; 1986, 124, 152.
(23) (a) Ceulemans, A.; Dendooven, M.; Vanquickenborne, L. G.
Inorg. Chem. 1985, 24, 1159. (b) Falvello, L.; Gerloch, M. Inorg. Chem.
1980, 19, 472.
(27) Weil, J . A.; Bolton, J . R.; Wertz, J . E. Electron Paramagnetic
Resonance; Wiley: New York, 1994.

3538 Organometallics, Vol. 17, No. 16, 1998
Klein et al.
reduced diorganoplatinum(II) species exhibit a higher
degree of metal contribution to the spin distribution
than tetraorganoplatinum(IV) analogues.1e,m Inciden-
tally, the isotropic values giso do not reflect these details;
however, they indicate that the reduced (PtIV)2 systems
differ from the reduced (PtII)2 species by the sign of the
g factor deviation from the free electron value of ge )
2.0023. The deviation g > ge for the latter radicals
reflects the closeness of occupied d orbitals to the
SOMO.7a,28
195Pt metal hyperfine coupling was only observed for
1•- (a2) and 6•- (aiso, a1, a2).1p The parameters lie in
the typical range for other known platinum complexes
of R-diimine anion radicals.1a-m
Su m m a r y. The results presented in this study not
only confirm that double metal coordination to the good
π-accepting bridging ligands bpip and bptz causes
facilitated reduction and low-energy charge transfer
absorptions, they also illustrate the superior capacity
of bptz, particularly, to stabilize odd-electron intermedi-
ates, both for the reduced and oxidized forms of the
compounds [RkPt(µ-L)PtRk].
(i) Starting from R2PtII(µ-L)PtIIR2, the persistent and
stable (Kc > 1011) one-electron reduced form is predomi-
nantly formulated as [R2PtII(µ-L-I)PtIIR2]- with only
slight but detectable contributions from a metal mixed-
valent form [R2PtII(µ-L0)PtIR2]- (alternatively [R2PtIII
-
(µ-L-II)PtIIR2]-).
(ii) If sterically protected as in system 1, it may even
be possible to observe the mixed-valent intermediate in
the comproportionation equilibrium
F igu r e 5. EPR spectra at 300 (top) and 110 K (bottom) of
electrogenerated 6•- in THF/0.1 M Bu4NPF6.
Ta ble 6. ESR Da ta a of An ion ic Din u clea r
Or ga n op la tin u m Com p lexes
[R2PtII(µ-L)PtIIR2] + [R2PtIII(µ-L)PtIIIR2]2+
h
2[R2PtIII(µ-L)PtIIR2]+
c
anionb
giso
aPt
g1
g2
g3
∆gd solvent
1•-
4•-
4•-
5•-
6•-
2.0264 n.o. 2.0824 2.0244e 1.9727 0.109 DCE
2.0090 n.o. 2.1083 2.0062 1.9033 0.205 CH2Cl2
2.0053 n.o. 2.1110 2.0059 1.9150 0.196 THF
However, the metal-metal interaction across the π
ligands is much smaller for these d7/d8 systems than
for d5/d6 species.
2.0080 n.o. 2.153
2.0054 1.867
0.286 THF
0.080 THF
1.9856 4.8 2.018f 2.001g 1.938
(iii) Finally, reduction of R4PtIV(µ-L)PtIVR4 yields
[R4PtIV(µ-L-I)PtIVR4]- with negligible contributions from
[R4PtIV(µ-L0)PtIIIR4]-.
a
b
Coupling constants a in mT (1T ) 104 Gauss). Generated
by in situ electrolysis at ambient temperatures in 0.1 M Bu4NPF6
solvent systems. giso measured at room temperature, anisotropic
g factors at 110 K. c 195Pt, I ) 1/2, 33.8% natural abundance. ∆g
d
Ack n ow led gm en t. We thank the Volkswagenstif-
tung and Deutsche Forschungsgemeinschaft for finan-
cial support.
) g1 - g3. e a2
(
195Pt) ) 3.0 mT. f a1(195Pt) ) 4.7 mT. a2(195Pt) )
g
3.8 mT.
dimesitylplatinum systems, the bptz complex 1•- ex-
hibits a distinctly smaller g anisotropy than 4•-. Since
the large spin-orbit coupling contributions from plati-
num27 are a major factor in influencing g values,1a-m,24
we conclude that the metal participation at the SOMO
is lower in 1•- than in 4•- (due to a lower lying π* orbital
of bptz) and in 6•- relative to 5•-. Previous studies on
mononuclear systems have similarly revealed that
Su p p or tin g In for m a tion Ava ila ble: Table S1, giving
absorption spectral data of complexes 1 and 4-6 in various
solvents (1 page). Ordering information is given on any
current masthead page.
OM980107J
(28) Kaim, W. Coord. Chem. Rev. 1987, 76, 187.