Dynamic NMR study of ethene exchange in cationic CNN-type platinum(II) complexes

Article abstract of DOI:10.1021/ic901078u

Catlonic ethylene platinum(II) complexes of the type [Pt(CNN)(C ₂H₄)]⁺, containing a methyl fragment and different diimines (NN), or terdentate (_κC- κ²NN′) anionic ligands, were synthesized and fully characterized both as solids and in solution [NN = 2,2′-dipyridylamine, 1; 2,2′-dipyridylsulfide, 2; 1,10-phenanthroline, 3; 4, 7-diphenyl-1,10- phenanthroline, 4; 3,4,7,8-tetramethyl-1,10-phenanthroline, 5; 2, 2′-bipyridine, 6; HC-NN = 6-tert-butyl-2,2′-bipyridine, 7; 6-neo-pentyl-2,2′-bipyridine, 8; 6-phenyl-2,2′-bipyridine, 9; 6-(α-methyl)benzyl-2,2′-bipyrldine, 10; 6-(α-ethyl)benzyl-2, 2′-bipyridine, 11; 6-(α,α-dimethyl)benzyl-2,2′- bipyridine, 12]. Crystals suitable for X-ray analysis of complexes 5 and 7 were obtained. Ethene exchange at the cyclometalated platlnum(II) complexes 7, 8, and 10-12 was studied by ¹H NMR line-broadening experiments in chloroform-d, as a function of both temperature and olefin concentrations. For the other prepared complexes the process was too fast to be monitored on the NMR time scale even at the lowest temperature. The ethylene exchange rates show a linear dependence on the concentration of the free ligand, with a negligible κ₁ term indicating that either a solvolytic or a dissociative pathway to the products is absent or negligible. The values of the second-order rate constants κ_exc, as obtained by linear regression analysis of the experimental data at 298 K, are in a range of ca. 10⁴-10 ⁵ s^-1 m^-1 . The activation entropies are negative, ranging between - 129 and - 112 J K^-1 mol^-1, as expected for associative processes. The activation process is largely entropy controlled: the TΔS^? contribution to the free energy of activation Is extremely large, amounting to more than 80% for all complexes, with a smaller enthalpy contribution. All the experimental findings evidence that the mechanism takes place via an associative attack by the entering olefin, through a well-ordered, stable pentacoordinated transition state with the two ethene molecules on the trigonal plane. The reactivity of [Pt(CNN)(C ₂H₄)]⁺ complexes Is strongly dependent on the choice of coordinated 6-substltuted-2,2′-blpyrldines, especially when the terdentate anionic fragment is capable of generating steric crowding and congestion on the coordination plane.

Full text of DOI:10.1021/ic901078u

Inorg. Chem. 2010, 49, 407–418 407
DOI: 10.1021/ic901078u
Dynamic NMR Study of Ethene Exchange in Cationic CNN-Type Platinum(II)
Complexes#
M. Rosaria Plutino,*,† Lucia Fenech,‡ Sergio Stoccoro,§ Silvia Rizzato,^ Carlo Castellano,^ and Alberto Albinati^
†
ꢀ
Istituto per lo Studio dei Materiali Nanostrutturati, ISMN-CNR, Unita di Messina-Sezione di Palermo,
‡
ꢀ
Messina, Italy, Dipartimento di Chimica Inorganica, Chimica Analitica e Chimica Fisica, Universita di
Messina, Salita Sperone 31, 98166 Vill. S. Agata, Messina, Italy, Dipartimento di Chimica, Universita di
§
ꢀ
^
ꢀ
Sassari, via Vienna 2, 07100, Sassari, Italy, and Dipartimento di Chimica Strutturale e Facolta di Farmacia, via
ꢀ
Venezian 21, Universita di Milano, 20133, Milano, Italy
Received June 4, 2009
Cationic ethylene platinum(II) complexes of the type [Pt(CNN)(C2H4)]þ, containing a methyl fragment and different
diimines (NN), or terdentate (κC-κ2NN0) anionic ligands, were synthesized and fully characterized both as solids and in
solution [NN = 2,20-dipyridylamine, 1; 2,20-dipyridylsulfide, 2; 1,10-phenanthroline, 3; 4,7-diphenyl-1,10-phenanthroline, 4;
3,4,7,8-tetramethyl-1,10-phenanthroline, 5; 2,20-bipyridine, 6; HC-NN = 6-tert-butyl-2,20-bipyridine, 7; 6-neo-pentyl-2,20-
bipyridine, 8; 6-phenyl-2,20-bipyridine, 9; 6-(R-methyl)benzyl-2,20-bipyridine, 10; 6-(R-ethyl)benzyl-2,20-bipyridine, 11;
6-(R,R-dimethyl)benzyl-2,20-bipyridine, 12]. Crystals suitable for X-ray analysis of complexes 5 and 7 were obtained.
Ethene exchange at the cyclometalated platinum(II) complexes 7, 8, and 10-12 was studied by 1H NMR line-broadening
experiments in chloroform-d, as a function of both temperature and olefin concentrations. For the other prepared
complexes the process was too fast to be monitored on the NMR time scale even at the lowest temperature. The ethylene
exchange rates show a linear dependence on the concentration of the free ligand, with a negligible k1 term indicating
that either a solvolytic or a dissociative pathway to the products is absent or negligible. The values of the second-order
rate constants kexc, as obtained by linear regression analysis of the experimental data at 298 K, are in a range of ca.
104-105 s-1 m-1. The activation entropies are negative, ranging between -129 and -112 J K-1 mol-1, as expected for
associative processes. The activation process is largely entropy controlled: the TΔSq contribution to the free energy of
activation is extremely large, amounting to more than 80% for all complexes, with a smaller enthalpy contribution. All the
experimental findings evidence that the mechanism takes place via an associative attack by the entering olefin, through a
well-ordered, stable pentacoordinated transition state with the two ethene molecules on the trigonal plane. The reactivity of
[Pt(CNN)(C2H4)]þ complexes is strongly dependent on the choice of coordinated 6-substituted-2,20-bipyridines,
especially when the terdentate anionic fragment is capable of generating steric crowding and congestion on the
coordination plane.
Introduction
olefins.1 Apart from thefundamental interest intheir intrinsic
structure and in the nature of the metal-ligand bonding,
these complexes have shown good ability as catalysts in
reactions involving C-C bond formation,2 such as olefin
homo- and copolymerization,3-5 oligomerization, and
dimerization.6-9 Such processes involve platinum and palla-
dium η2-alkene and η2-alkyne divalent complexes, as either
In recent years there has been an increasing interest in the
chemistry of cationic Pd(II) and Pt(II) complexes bearing
substituted diimine ligands and unsaturated π-ligands, which
are key intermediates in many catalytic processes involving
# Dedicated to the memory of Professor Raffaello Romeo.
*Phone: þ39-090-6765734. Fax: þ39-090-3974108. E-mail: rosaria.
plutino@pa.ismn.cnr.it.
(1) Crabtree, R. H. The Organometallic Chemistry of the Transition
Metals, 2nd ed.; Wiley Interscience: New York, 1994; pp 221-271.
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V. C.; Spitzmesser, S. K. Chem. Rev. 2003, 103, 283–315. (c) Scriban, C.;
Glueck, D. S.; Zakharov, L. N.; Kassel, W. S.; DiPasquale, A. G.; Golen, J. A.;
Rheingold, A. L. Organometallics 2006, 25, 5757–5767. (d) Ferreira, A. C.;
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r
2009 American Chemical Society
Published on Web 12/10/2009
pubs.acs.org/IC

408 Inorganic Chemistry, Vol. 49, No. 2, 2010
Plutino et al.
precursors or active species. However, fundamental informa-
tion on metal-olefin bond breaking and formation and on
the factors that control olefin coordination in metal-olefin
complexes is important to exploit the diverse chemistry of
these complexes and can be easily achieved by studying the
dynamic process of olefin exchange. In comparison to
palladium(II), whose catalytic properties are well known,10
the analogous platinum(II) complexes are usually less useful
as catalysts due to their higher kinetic inertness. On the other
hand, they may be exploited as model systems for attaining
mechanistic insights into Pd-catalyzed reactions and other
interesting information from their kinetic behavior. Despite
the great activity in the field, kinetic data relative to olefin
exchange in platinum(II) or palladium(II) square-planar
complexes are scarce in the literature.6,11-14
Scheme 1. Dynamic Behavior for Olefin Complexes in a Type 4
Mechanism
at square-planar platinum(II) complexes, tuning the ex-
change rate by an appropriate selection of ancillary ligands
and temperature, as previously shown in kinetic studies on
exchange15 and substitution16 reactions.
Olefin complexes typically show dynamical structures even
in the solid state,14,17,18 due to rotation, rocking, and/or
librational motions.19 Actually, to explain the dynamic
behavior of the olefin ligand, the following mechanisms have
been taken into account: (1) propeller-like rotation,18,20 (2)
metal-olefin bond cleavage, followed by recombination,21,22
(3) metal-ligand bond cleavage (i.e., the ligand trans to the
olefin moiety), followed by ligand rotation and recombina-
tion,11,21,23,24 and/or (4) intermolecular exchange with the
free olefin ligand.9,25 In most cases the fluxional behavior of
the organometallic compounds has been reported only with-
out a complete discussion of the molecular mechanism
involved. However, definitive arguments for a correct me-
chanistic choice might be provided by NMR line-shape
analysis and by the derived kinetic parameters.
In order to focus on a pure ethene exchange (type 4
mechanism, Scheme 1), we have synthesized and fully
characterized, either as solids or in solution, a number of
cationic un- and cyclometalated ethylene complexes of the
CNN-type, [Pt(CNN)(C2H4)]þ, containing different methyl
diimine (C = Me and NN) or terdentate orthometalated
diimine (κC-κ2NN0) anionic ligands, respectively.
On these bases, we have thought itinteresting to investigate
the thermodynamics and the mechanism of olefin exchange
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Article
Inorganic Chemistry, Vol. 49, No. 2, 2010 409
coordination sphere of the metal. This approach offers the
opportunity of studying ethene exchange reactions in transi-
tion metal complexes, ruling out a trans olefin exchange
mechanism (type 3 mechanism) in favor of a direct ethene
intermolecular exchange (type 4 mechanism). In addition
these complexes are also characterized by (i) good solubility
in chlorinated solvents due to the presence of the “Broo-
khart” counteranion26 [B(3,5-(CF3)2C6H3)4]- (i.e., the com-
plex [Pt(Me)(phen)(C2H4)]BF4, whose synthesis has been
already reported,27 is rather unsoluble in chloroform-d); (ii)
an extensive planarity of the NN-ligand, as far as the
dipyridyl or phenanthroline moiety is concerned; and (iii) a
different steric crowding above and below the coordin
ation plane due to the different number and nature of the
alkyl substituents bonded to the diimine fragment, to
the alkyl or aryl orthometalated anchoring pendant, and to
the size of the ring, in both the non- and the cyclometalated
complexes. Nevertheless, it is worth noting that cyclometa-
lated platinum(II) species have attracted much attention
due to their potential applications in many fields, such as
in organic synthesis, homogeneous catalysis, and photo-
chemistry.28,29
Instrumentation and Physical Measurements. 1H NMR mea-
surements were performed on a Bruker AMX R-300 spectro-
meter equipped with a broadband probe operating at 300.13
MHz (1H). All chemical shifts were referenced to the protiated
residue in CDCl3 for 1H NMR (7.26 ppm) and are reported in
parts per million (δ/ppm), while coupling constants are given in
hertz (J/Hz). The probe temperature was checked by the metha-
nol (low temperature)31 or ethylene glycol (high temperature)32
method measuring the chemical shift differences between the
-OH and the -CH3/-CH2 resonances of a methanol/ethylene
glycol standard, respectively, as a function of temperature.
Samples were allowed to equilibrate for a minimum of 10 min
at each temperature setting, and the probe was then gradient-
shimmed prior to collection of each data set.
Except where stated, all measurement manipulations were
conducted in the absence of oxygen and water under a nitrogen
or argon atmosphere, either by use of standard Schlenk methods
or in a glovebox apparatus, utilizing glassware that was oven-
dried (130 ꢀC) and evacuated while hot prior to use. NMR
samples were prepared by dissolving, into a 5 mm NMR tube,
about 10 mg of compound in about 0.5 mL of CDCl3. Transfer
of ethene was performed using gastight Hamilton syringes in
an inert atmosphere. Elemental analyses were performed by
Microanalytical Laboratory, Department of Chemistry,
University College Dublin, Ireland.
Here we report the synthesis and spectroscopic character-
ization of platinum(II) complexes of the type [Pt(CNN)-
(C2H4)]þ, together with a detailed 1H NMR dynamic inves-
tigation in chloroform-d on the ethene exchange, as a func-
tion of both temperature and olefin concentration. The aim
of this work is to see to what extent cyclometalation and/or
electronic or steric factors, induced by the substituents on the
ancillary ligands, may affect the ethene lability, and therefore
the olefin exchange rate. To the best of our knowledge this
kinetic study represents one of the few quantitative examples
of a pure concerted intermolecular olefin exchange occurring
with an associative mechanism in square-planar platinum(II)
complexes.9,12
Peak Assignment. Where necessary, the assignment of the
complexes’ resonances was performed by two-dimensional
phase-sensitive H 2D-NOESY experiments using a standard
1
pulse sequence, with a mixing time of 600-800 ms, or alterna-
tively homonuclear proton spin decoupling difference experi-
ments were carried out. In particular as far as this second
method is concerned a “control” spectrum, in which irradiation
is applied in a blank region, is subtracted from one in which the
center of a specific multiplet is irradiated, all the other experi-
mental parameters being identical.
1
Dynamic NMR (DNMR) Studies. Variable-temperature H
NMR spectra of complexes 7, 8, and 10-12 were recorded in
chloroform-d using a BVT200 digital temperature control unit.
Samples were allowed to equilibrate for 7-10 min at the
indicated probe temperature, and spectra were accumulated
using 16-32 scans and 10 s recycle delays.
Experimental Section
1H NMR spectra in chloroform-d were recorded at a mea-
sured temperature, ranging from ca. 200 to 340 K, and full line-
shape analyses were executed on the C2H4 portions of the
resulting spectra. Since the chemical shifts of the hydrogens
for the bound or free ethene are almost unaffected by tempera-
ture, the line shapes were fit at the different temperatures
considering constant both chemical shifts and coupling con-
stants. The chemical shifts observed in the slow limit exchange,
i.e., in the absence of added free ethene, were used to set up the
spin systems. The line widths used in the fitting procedure for
each temperature were determined by comparison with the
width of the solvent peak. First-order rate constants for ethene
exchange (kobs/s-1) were obtained from line shape analysis by
matching the observed 1H NMR (ethene region) spectra in
CDCl3, both at variable ligand concentration and temperature
(see Supporting Information, Table S2 and S3, respectively),
with those simulated using the computer programs gNMR
4.0,33 all spin systems for each complex being processed simul-
taneously. Linear and curvilinear least-squares fit and differen-
tial equations were analyzed using the SCIENTIST software
package.34
General Methods and Materials. Unless otherwise stated, all
synthetic procedures were carried out on the benchtop. The
solvents (AR, LabScan Ltd.; SpS, Romil Ltd.) used in the
synthetic procedures were distilled under nitrogen from appro-
priate drying and deoxygenating agents (diethyl ether from
sodium/benzophenone ketyl; dichloromethane from barium
oxide) according to standard literature procedures30 and then
˚
stored in N2-filled flasks over activated 4 A molecular sieves.
Chloroform-d (D, 99.96% Cambridge Isotope Laboratories)
was dried standing for many days over CaH2, distilled under
nitrogen over activated magnesium sulfate and sodium carbo-
nate, degassed by using three repeated freeze-pump-thaw
˚
cycles, and afterward stored over activated 4 A molecular sieves
and used in a glovebox. Ethene (99.95%) and all other reagents
were of the highest purity grade commercially available and used
without further purification. The salt Na[B(3,5-(CF3)2C6H3)4]
(hereafter, NaBArF) was prepared according to a published
method.26
(26) Brookhart, M.; Grant, B.; Volpe, A. F. Organometallics 1992, 11,
3920–3922.
(27) Fusto, M.; Giordano, F.; Orabona, I.; Ruffo, F.; Panunzi, A.
Organometallics 1997, 16, 5981–5987.
(28) Ryabov, A. D. Chem. Rev. 1990, 90, 403–424.
(29) Zucca, A.; Stoccoro, S.; Cinellu, M. A.; Minghetti, G.; Manassero,
M.; Sansoni, M., Eur. J. Inorg. Chem. 2002, 3336-3346, and references
therein.
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Interscience: New York, 1955; Vol. VII.
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1993-1998.
(34) SCIENTIST 2.0, Data Analysis, Graphing and Curve Fitting Software;
Micromath Scientific Software Inc.: Salt Lake City, UT, 1986-1995.

410 Inorganic Chemistry, Vol. 49, No. 2, 2010
Plutino et al.
1
a. Ligand-Concentration-Dependent Experiments. H NMR
spectra were recorded at constant temperature (298 K). A
weighed sample of the platinum complex was introduced in a
5 mm NMR tube containing CDCl3 (0.5 mL) that was capped
with a rubber septum. The initial NMR spectrum was recorded,
and the subsequent spectra were taken after the addition of the
olefin. Ethene was added to the solution into the sealed NMR
tube by a microsyringe, and the solution was shaken to dissolve
the gas. The ethene concentration was calculated by integration
of the signals of the bound and the free ethene. Alternatively the
integration was made only on a spectrum with the largest
amount of free ethene, in which a good correspondence between
the volume of added gas and the experimental olefin concentra-
tion in solution has been calculated. The ethene concentration
for the diluted solutions was simply derived at each time from
the precise amount of the added olefin volume. This method
showed a good data reproducibility. All concentrations are
expressed in moles per kilogram of solvent (m = mol kg-1).
Chart 1. Structural Formulas for the Complexes 1-6, with the
Numbering Scheme
1
b. Variable-Temperature NMR Experiments. The H NMR
spectra were recorded over the temperature range 198-341 K,
depending on the rate of the process studied at a fixed ethene
concentration.
The calculated second-order exchange rate constants at each
temperature, kexc/s-1 m-1, were used in the subsequent calcula-
tion of thermodynamic parameters using the linear expression
of the Eyring equation
0.149 mmol) dissolved in 50 mL of dichloromethane under
continuous stirring at room temperature. The reaction mixture
was stirred until a powdery off-white solid of complex 2P was
completely formed. The excess solvent was evaporated under
reduced pressure, and the pure product was obtained after
several washing with Et2O and recrystallization from a mix-
ture of dichloromethane/ether (1:1 = v/v). Yield: 80%. Anal.
Calcd for PtC11H11N2ClS: C, 30.45; H, 2.56; N, 6.46. Found: C,
30.51; H, 2.61; N, 6.41.
!
ꢀ ꢁ
k
kB ΔSq
ΔHq
RT
ln
¼
þ
-
ð1Þ
T
h
R
where ΔHq is the activation enthalpy, ΔSq is activation entropy,
kB, h, and R are the Boltzmann, Planck, and gas constants,
respectively, and T is the absolute temperature.35
3
1H NMR (CDCl3, T=298 K): δ 9.71 (dd, JPtH=16.0 Hz,
The activation free energy (ΔGq/kJ mol-1) was then calcu-
lated at T = 298 K by use of the following equation:
3
3
JHH=5.5 Hz, 4JHH=1.1 Hz, 1H, H6 ), 9.20 (d, JPtH=41.0 Hz,
0
3JHH=5.5 Hz, 1H, H6), 7.90 (dd, 3JHH = 8.8, 7.4 Hz, 1H, H4),
7.83 (dd, 3JHH=8.8, 7.4 Hz, 1H, H4 ), 7.82 (d, JHH3 = 8.8 Hz,
3
0
1H, H3), 7.71 (d, 3JHH = 8.8 Hz, 1H, H3 ), 7.51 (dd, JHH = 7.4,
ΔGq ¼ ΔHq -TΔSq
ð2Þ
0
5.5 Hz, 1H, H5 ), 7.43 (ddd, JHH = 7.4, 5.5 Hz, 4JHH = 1.5 Hz,
3
0
1H, H5), 1.16 (s, 2JPtH=77.6 Hz, 3H, Pt-CH3).
Synthetic Procedures. Alkyl-dinitrogen uncyclometalated
and alkyl/aryl-dinitrogen cyclometalated complexes of the type
[Pt(CNN)(C2H4)]X (X = BArF, CF3SO3, PF6) were prepared,
unless otherwise described, starting from the corresponding
chloride complexes [Pt(CNN)Cl] by ethene for chloride substi-
tution. The purity of all starting materials and reaction products
The complexes [Pt(Cl)(Me)(Ph2phen)], 4P, and [Pt(Cl)(Me)-
(Me4phen)], 5P, were obtained following the same procedure
described above for the synthesis of complex 2P (see Supporting
Information for the complexes’ 1H NMR characterization and
elemental analysis).
1
was determined by H NMR spectroscopy, and clean spectra
with correct integration were obtained in all cases.
[Pt(Me)(dipy)(C2H4)]BArF, 1. A weighed amount of complex
1P (0.040 g, 0.096 mmol) was dissolved in CH2Cl2; then a
stoichiometric amount of NaBArF (0.085 g, 0.096 mmol) was
added, followed by ethene bubbling into the solution. After
stirring about 1 h, NaCl was filtered off, the solvent excess was
evaporated under reduced pressure, and the pure product was
obtained after washing with several aliquots of petroleum ether.
Yield: 70%. Anal. Calcd for PtBC45H28F24N3: C, 42.47; H, 2.22;
N, 3.30. Found: C, 42.19; H, 2.28; N, 3.33.
Uncyclometalated Platinum(II) Complexes. Complexes of the
type [Pt(Me)(NN)(C2H4)]BArF [NN = 2,20-dipyridylamine
(dipy), 1; 2,20-dipyridylsulfide (dps), 2; 1,10-phenanthroline
(phen), 3; 4,7-diphenyl-1,10-phenanthroline (Ph2phen), 4; 3,4,7,8-
tetramethyl-1,10-phenanthroline (Me4phen), 5; 2,20-bipyridine
(bipy), 6], as shown in Chart1, and [Pt(Me)(Ph2phen)(C2H4)]-
CF3SO3, 4b, were prepared, from the corresponding chloride
complexes [Pt(Cl)(Me)(NN)], 1P-6P, respectively, by chloride
abstraction with NaBArF or AgCF3SO3 in the presence of a
continuous ethene flow. The precursor platinum(II) complexes
trans-[Pt(Cl)(Me)(Me2SO)2]36 and [Pt(Cl)(Me)(NN)] (NN=dipy,
1P;37 phen, 3P;38 bipy, 6P16) were prepared according to already
published synthetic procedures.
1H NMR (CDCl3, T = 298 K): δ 8.25 (d, 3JPtH = 40.3 Hz,
3JH3H = 5.9 Hz, 1H, H6), 7.81 (dd, 3Jav = 7.4 Hz, 1H, H4), 7.76
(d, JPtH = 17.1 Hz, JHH=5.9 Hz, 1H, H6’), 7.69 (s, br, 8H,
3
0
0
0
H
H
o -BArF), 7.68 (obscured by Ho -BArF, 1H, H4 ), 7.49 (s, br, 4H,
0
p -BArF), 7.36 (s, br, 1H, NH), 7.18 (dd, 3JHH = 7.4, 5.9 Hz, 1H,
H5), 7.09 (dd, 3JHH = 7.4, 5.9 Hz, 1H, H5 ), 7.01 (d, JHH = 8.1
3
0
Hz, 1H, H3), 6.96 (d, 3JHH = 8.1 Hz, 1H, H3 ), 4.01 (s, br, 4H,
0
[Pt(Cl)(Me)(dps)], 2P. trans-[Pt(Cl)(Me)(Me2SO)2] (0.060 g,
0.149 mmol) was reacted with 2,20-dipyridylsulfide (0.028 g,
C2H4), 0.57 (s, 2JPtH = 69.1 Hz, 3H, Pt-CH3).
The other complexes 2-6 were obtained by following the
same procedure previously reported for compound 1 (see Sup-
porting Information for the complexes’ 1H NMR characteriza-
tion and elemental analysis).
[Pt(Me)(Ph2phen)(C2H4)]CF3SO3, 4b, was obtained with a
procedure similar to that of the parent complex 4 by reaction of
4P (0.030 g, 0.052 mmol) and AgCF3SO3 (0.013 g, 0.052 mmol)
in dichloromethane under continuous stirring and a constant
(35) Activation parameters, ΔHq/kJ mol-1 and ΔSq/J K-1 mol-1, were
calculated from the slope and the intercept of the best line obtained by a
least-squares analysis performed with the SCIENTIST software.
(36) Eaborn, C.; Kundu, K.; Pidock, A. J. J. Chem. Soc., Dalton Trans.
1981, 933–938.
(37) Romeo, R.; Nastasi, N.; Scolaro, L. M.; Plutino, M. R.; Albinati, S.;
Macchioni, A. Inorg. Chem. 1998, 37, 5460–5466.
ꢁ
(38) Romeo, R.; Monsu Scolaro, L. Inorg. Synth. 1998, 32, 153–158.

Article
Inorganic Chemistry, Vol. 49, No. 2, 2010 411
ethene atmosphere. After the complete formation of 4b and the
removal of AgCl, the filtrated solution was concentrated under
reduced pressure on a rotary evaporator; then petroleum ether
(10 mL, 1:1 = v/v) was added to obtain 4b as a pure product with
a 60% yield. Anal. Calcd for PtC28H23F3N2O3S: C, 46.73; H,
3.22; N, 3.89. Found: C, 46.81; H, 3.26; N, 3.85.
Scheme 2. Synthetic Pathway to Methyl Chloride Platinum(II) Com-
plexes 1P-6P
1H NMR (CDCl3, T = 298 K): δ 9.24 (d, 3JPtH = 51.6 Hz, 3JHH
=5.5 Hz, 1H, H2), 9.05(d, 3JPtH=14.2 Hz, 3JHH=5.5 Hz, 1H, H9),
8.23 (d, 3JHH = 5.5 Hz, 1H, H8), 8.14 (AB system, 3JHH =9.4 Hz,
4
3
JHH = 1.3 Hz, 1H, H4 ), 7.86 (d, JHH = 8.0 Hz, 1H, H3 ), 7.70
1H, H5), 8.08 (AB system, 3JHH=9.4 Hz, 1H, H6), 8.06 (d, 3JHH
=
0
0
5.5 Hz, 1H, H3), 7.66-7.51 (m, 10H, Ph), 4.58 (s, br, 2JPtH = 68.2
0
0
(d, 1H, obscured by Ho -BArF, H3), 7.68 (s, br, 8H, Ho -BArF), 7.56
0
(ddd, 3JHH = 8.0, 5.3 Hz, 4JHH = 1.3 Hz, 1H, H5 ), 7.51 (d, JHH
3
Hz, 4H, C2H4), 0.93 (s, 2JPtH=69.8 Hz, 3H, Pt-CH3).
2
= 8.0 Hz, 1H, H5), 7.47 (s, br, 4H, Hp -BArF), 4.32 (s, JPtH
0
=
Cyclometalated Platinum(II) Complexes. Similarly to the
noncyclometalated species, complexes of the type [Pt(CNN)-
(C2H4)]BArF [HC-NN = 6-tert-butyl-2,20-bipyridine (bipyt), 7;
6-neo-pentyl-2,20-bipyridine (bipyn), 8; 6-phenyl-2,20-bipyridine
(bipyφ), 9; 6-(R-methyl)benzyl-2,20-bipyridine (bipyRMe), 10;
6-(R-ethyl)benzyl-2,20-bipyridine (bipyREt), 11; 6-(R,R-dimeth-
yl)benzyl-2,20-bipyridine (bipyc), 12], as shown in Chart 2 and
63.4 Hz, 4H, C2H4), 2.20 (s, 2JPtH = 69.4 Hz, 2H, CCH2Pt), 1.41
(s, 6H, CCH3).
[Pt(bipyt-H)(C2H4)]CF3SO3, 7b, was obtained by a similar
route to 7, reacting complex 7P (0.025 g, 0.056 mmol) with the
stoichiometric amount of AgCF3SO3 (0.014 g, 0.056 mmol) in
dichloromethane as solvent and flushing ethene through the
solution. Yield: 55%. Anal. Calcd for PtC17H19F3N2O3S: C,
34.99; H, 3.28; N, 4.80. Found: C, 34.95; H, 3.30; N, 4.78.
Chart 2. Structural Formulas for the Complexes 7-12, Together with
the Adopted Numbering Scheme
1H NMR (CDCl3, T = 298 K) δ: 8.70 (d, 3JHH = 8.2 Hz, 1H,
3
H3 ), 8.59 (dd, JHH = 8.2 Hz, 4JHH = 10.2 Hz, 1H, H3), 8.37
0
(dd, 3Jav = 7.7 Hz, 1H, H4), 8.35 (d, 3JHH = 5.5 Hz, 1H, H6 ),
0
8.31 (ddd, JHH = 8.2, 7.7 Hz, 4JHH = 1.3 Hz, 1H, H4 ), 7.77
3
0
3
4
0
(ddd, JHH = 7.7, 5.5 Hz, JHH = 1.3 Hz, 1H, H5 ), 7.57 (dd,
3JHH = 8.2 Hz, 4JHH = 1.1 Hz, 4JPtH = 10.1 Hz, 1H, H5), 4.46
(s, JPtH = 62.1 Hz, 4H, C2H4), 2.20 (s, 2JPtH = 69.8 Hz, 2H,
2
CCH2Pt), 1.46 (s, 6H, CCH3).
[Pt(bipyt-H)(C2H4)]PF6, 7c, was prepared and characterized
in situ by anion exchange reaction of 7 with an excess of
(nBu4N)PF6 in 0.5 mL of chloroform-d directly into the NMR
tube (CPt = 5.5 mm, [(nBu4N)PF6] = 54.3 mm).
1H NMR (CDCl3, T = 298 K) δ: 8.37 (dd, 3JHH = 8.0 Hz,
4
3
JHH = 1.2 Hz, 1H, H3 ), 8.34 (d, JHH = 5.3 Hz, 1H, H6 ), 8.31
=
0
0
(dd, 3Jav = 8.0 Hz, 1H, H4), 8.25 (ddd, 3Jav = 8.0 Hz, 4JHH
3
1.1 Hz, 1H, H4 ), 8.24 (d, JHH = 8.0 Hz, 4JHH = 1.2 Hz, 1H,
0
H3), 7.80 (ddd, 3JHH = 7.8, 5.3 Hz, 4JHH = 1.2 Hz, 1H, H5 ),
0
7.60 (dd, 3JHH = 8.0 Hz, 4JHH = 1.1 Hz, 4JPtH = 10.1 Hz, 1H,
H5), 4.44 (s, 2JPtH = 63.6 Hz, 4H, C2H4), 2.21 (s, 2JPtH = 69.2
Hz, 2H, CCH2Pt), 1.45 (s, 6H, CCH3).
Complexes 8-12 were obtained following the same procedure
described for 7 by using the complexes 8P-12P as starting
materials. The triflate derivative 11b was prepared as well as
compound 7b (see Supporting Information for the detailed 1H
NMR and elemental analysis complexes’ characterization).
X-ray Data Collection and Structure Determination for
[5]BArF and [7]BArF. It proved impossible to grow crystals of
good quality of 5 and 7. Moreover, as shown by the structural
determinations, both compounds contain disordered counter-
ions; thus the accuracy of the structural determinations is
limited. However, the mean features of the coordination are
unambiguously determined. Data collections, structural details
and Table S1 with all the experimental crystallographic data are
given as Supporting Information.
[Pt(CNN)(C2H4)]CF3SO3 [HC-NN = 6-tert-butyl-2,20-bipyridine
(bipyt), 7b; 6-(R-ethyl)benzyl-2,20-bipyridine (bipyREt), 11b] were
prepared from the analogous chloride complexes [Pt(CNN)Cl]
[HC-NN = bipyt, 7P;39 bipyn, 8P;29 bipyφ, 9P;40 bipyRMe, 10P;41
bipyREt, 11P;16 bipyc, 12P42] by reaction with ethene and a stoichio-
metric amount of NaBArF or AgCF3SO3 to remove the bound
chloride ligand.
[Pt(bipyt-H)(C2H4)]BArF, 7. A 0.047 g amount of complex 7P
(0.106 mmol) was dissolved in CH2Cl2, and a stoichiometric
amount of NaBArF (0.094 g, 0.106 mmol) was added under
stirring, while ethene was bubbled through the solution. After
NaCl precipitated completely from the reaction mixture, the
solution was filtered and reduced in volume under low pressure.
Then an equal volume of petroleum ether was added until
complete precipitation of a pale yellow solid product. Yield:
60%. Anal. Calcd for PtBC48H31F24N2: C, 44.43; H, 2.41; N,
2.16. Found: C, 44.18; H, 2.41; N, 2.17.
Results and Discussion
Synthetic Procedures. The neutral chloride platinum-
(II) complexes [Pt(Cl)(Me)(NN)] (NN = dipy, 1P; dps,
2P; phen, 3P; Ph2phen, 4P; Me4phen, 5P, bipy, 6P) have
been prepared following a general method38 reported in
the literature, using the complex trans-[Pt(Cl)(Me)-
(Me2SO)2] as a useful synthon to introduce the organo-
metallic fragments {PtMe} and {PtClMe}. Thus, the
addition of an equimolecular amount of the diimine
ligand NN to a dichloromethane solution of trans-[Pt-
(Cl)(Me)(Me2SO)2] led to the rapid substitution of both
sulfoxide ligands, yielding compounds 1P-6P in a pure
1H NMR (CDCl3, T = 298 K): δ 8.08 (d, 3JHH = 5.3 Hz, 1H,
3
H6 ), 8.02 (dd, Jav = 8.0 Hz, 1H, H4), 7.98 (ddd, 3Jav = 8.0 Hz,
0
(39) Minghetti, G.; Cinellu, M. A.; Stoccoro, S.; Zucca, A.; Manassero,
M. J. Chem. Soc., Dalton Trans. 1995, 777–781.
(40) Constable, E. C.; Henney, R. P. G.; Leese, T. A.; Tocher, D. A. J.
Chem. Soc., Dalton Trans. 1990, 443–449.
(41) Cinellu, M. A.; Gladiali, S.; Minghetti, G. J. Organomet. Chem. 1989,
363, 401–408.
(42) Sanna, G.; Minghetti, G.; Zucca, A.; Pilo, M. I.; Seeber, R.; Laschi,
F. Inorg. Chim. Acta 2000, 305, 189–205.

412 Inorganic Chemistry, Vol. 49, No. 2, 2010
Plutino et al.
Scheme 3. Synthetic Pathway to Uncyclometalated (1-6) and Cyclo-
metalated (7-12) Platinum(II) Ethene Complexesa
NMR tube by moving the equilibrium shown in eq 3
toward the products with an excess of (nBu4N)PF6.
n
½Ptðbipyt-HÞðC2H4ÞꢀBArF þ ð Bu4NÞPF6
Keq
s
F
s
R
7
n
½Ptðbipyt-HÞðC2H4ÞꢀPF6 þ ð Bu4NÞBArF
ð3Þ
7c
An increase of the counteranion concentration shifts
the resonances of the aromatic protons to higher frequen-
cies (see Supporting Information, Figure S1, upper plot),
while the aliphatic ones (not shown in the figure for the
sake of clarity) are just slightly affected. The most shifted
a Chloride abstraction was performed by using the salt MX =
NaBArF or AgCF3SO3.
0
protons appear to be H3 and H3 , suggesting that com-
pound 7c exists in chloroform-d solution in the form of a
tight ion pair. As already observed for similar platinum-
(II) or palladium(II) complexes containing aromatic dini-
trogen ligands,3,8,37,44 the counteranion lies above or
crystalline form and in almost quantitative yield, accord-
ing to Scheme 2.
The synthesis and the spectroscopic properties of the
platinum(II) species [Pt(Me)(N-N)(Cl)] (NN = dipy,
1P;37 phen, 3P;38 bipy, 6P16) and of the cyclometalated
platinum(II) [Pt(CNN)Cl] complexes [containing 6-alkyl-
2,20-bipyridine (7, 8),29,39 6-phenyl-2,20-bipyridine (9),40
and substituted 6-benzyl-2,20-bipyridine (10-12)]16,41,42
have been described in detail elsewhere except for com-
plexes 2P, 4P, and 5P (containing dinitrogen ligands such
as dps, Ph2phen, and Me4phen, respectively), whose synth-
esis and spectroscopic characterization are reported in the
Experimental Section or given as Supporting Information.
The cationic ethene platinum(II) complexes of general
formula [Pt(CNN)(C2H4)]þ, 1-12, have been prepared
as previously shown for other similar cationic diimine
methyl platinum(II) complexes [Pt(CNN)X] (X = sulf-
oxides or phosphanes),37,43 in dichloromethane solution,
starting from the corresponding chloride complexes
[Pt(CNN)(Cl)], by use of an appropriate chloride-remov-
ing reagent, under a constant olefin atmosphere.
The addition of an equivalent amount of NaBArF salt
to a dichloromethane solution of the neutral chloride-
containing platinum(II) compounds (CNN) 1P-12P led
to the easy replacement of chloride from the coordination
sphere of the complexes, yielding NaCl and a very labile
and reactive cationic solvento species that, in the presence
of bubbling ethene, afforded the desired corresponding
compounds 1-12.
0
below the coordination plane close to the H3 and H3
protons (see Supporting Information, lower plot of
Figure S1). Indeed the shift of their resonances to higher
frequencies is pronounced due to the progressive weak-
ening of the ring current effect due to the replacement of
the tetra-arylborate anion BArF- by an excess of the
PF6- one (according to eq 3).
n
½7cꢀ½ð Bu4NÞBArFꢀ
x2
Keq
¼
¼
ð4Þ
ð5Þ
n
½7ꢀ½ð Bu4NÞPF6ꢀ
ðA - xÞðB - xÞ
χ7
χ7
c
δav ¼ δ7 þ δ7c
Aδ7 þ xðδ7c - δ7Þ
δav
¼
ð50Þ
A
Chemical shifts (δav(1H)/ppm) measured for all the
0
0
aromatic signals (H3-5, H3 -6 ) versus the concentration
of the added hexafluorophosphate salt ([(nBu4N)PF6]/m)
were globally fitted to eqs 4 and 50 with a nonlinear least-
squares routine of the SCIENTIST program,34 yielding
the fits shown in Figure S1, which interpolate the experi-
mental data with a calculated value of the equilibrium
constant of Keq = 0.93 ( 0.09. In eqs 4-50, A denotes the
total starting concentration of 7, B is the concentration of
added (nBu4N)PF6, x is the equilibrium concentration of
the products 7c and (nBu4N)BArF, and δav is the average
observed chemical shift at the equilibrium between 7 and
7c, with molar fractions χ7 and χ7c, respectively. The
equilibrium concentrations of all the species involved in
eq 3 were calculated for every single addition of
(nBu4N)PF6.
The trifluoromethanesulfonate complex salts 4b, 7b,
and 11b were obtained following the same procedure
described above using silver triflate, AgCF3SO3, as the
reagent for chloride abstraction (see Scheme 3).
All the obtained complexes (cyclometalated and not),
1-6 and 7-12 (in Charts 1 and 2, respectively), are air
stable and can be stored at 277 K without appreciable
decomposition for several months; they are very soluble
in chlorinated solvents, especially the derivatives with the
Brookhart counteranion (BArF-), on which all the NMR
investigations have been carried out (see Experimental
Section).
Solid-State Structures of 5 and 7. Figure 1 shows
ORTEP views of cations 5 and 7, respectively, while a
discussion of the structures is given in the Supporting
˚
As far as compound [Pt(bipyt-H)(C2H4)]PF6, 7c, is
concerned, the synthesis has been performed in situ in a
Information, together with selected bond lengths (A) and
angles (deg) in Figures S2 and S3.
The immediate coordination sphere of both platinum-
(II) complexes consists of the two N donor atoms of
(43) (a) Romeo, R.; Fenech, L.; Carnabuci, S.; Plutino, M. R.; Romeo, A.
Inorg. Chem. 2002, 41, 2839–2847. (b) Romeo, R.; Carnabuci, S.; Fenech, L.;
Plutino, M. R.; Albinati, A. Angew. Chem., Int. Ed. 2006, 45, 4494–4498.
(44) Macchioni, A.; Magistrato, A.; Orabona, I.; Ruffo, F.; Rothlisberger,
U.; Zuccaccia, C. New J. Chem. 2003, 27, 455–458.

Article
Inorganic Chemistry, Vol. 49, No. 2, 2010 413
Figure 1. ORTEP view for the platinum(II) cationic complexes 5 and 7.
Table 1. Relevant 1H NMR Data for the Uncyclometalated Platinum(II) Complexes of the Type [Pt(Me)(NN)(C2H4)]X (X = BArF, CF3SO3)a
δ
n
complex
NN ligand
C2H4-Pt
Me-Pt
1
2
3
4
4b
5
6
[Pt(Me)(dipy)(C2H4)]BArF
[Pt(Me)(dps)(C2H4)]BArF
8.25 (40.3, H6), 7.76 (17.1, H6’)
8.40 (50.4, H6), 8.18 (22.0, H6’)
9.13 (50.4, H2), 8.36 (16.7, H9)
9.16 (50.6, H2), 7.90 (15.4, H9)
9.24 (51.6, H2), 9.05 (14.2, H9)
8.83 (51.5, H2), 8.06 (14.8, H9)
4.01b
0.57 (69.1)
0.68 (70.4)
0.90 (69.0)
0.92 (70.4)
0.93 (69.8)
0.86 (69.9)
0.78 (70.2)
4.07c
[Pt(Me)(phen)(C2H4)]BArF
[Pt(Me)(Ph2phen)(C2H4)]BArF
[Pt(Me)(Ph2phen)(C2H4)]CF3SO3
[Pt(Me)(Me4phen)(C2H4)]BArF
[Pt(Me)(bipy)(C2H4)]BArF
4.33 (63.0)
4.32 (68.7)
4.58 (68.2)
4.27 (66.9)
4.20 (64.7)
0
8.81 (50.0, H6), 7.95 (14.0, H6 )
a Chemical shifts in parts per million, coupling constants in hertz; JPtH in parentheses, CDCl3 as solvent, T = 298 K. The assignment follows the
numeration pattern shown in Chart 1. b Broad singlet. c Broad AA0BB0 system.
Table 2. Relevant 1H NMR Data for the Cyclometalated Platinum(II) Complexes of the Type [Pt(CNN)(C2H4)]X (X = BArF, CF3SO3, PF6)a
δ
CNN ligand
0
00
n
complex
H6
H6
CH
CH2
CH3
C2H4-Pt
4.32 (63.4)
4.46 (62.1)
4.44 (63.6)
4.08 (69.9)
7
[Pt(bipyt-H)(C2H4)]BArF
[Pt(bipyt-H)(C2H4)]CF3SO3
[Pt(bipyt-H)(C2H4)]PF6
8.08
2.20 (69.4)
2.20 (69.8)
2.21 (69.2)
1.68 (70.3)
1.41
1.46
1.45
1.06
7b
7c
8
8.37
8.37
7.93
[Pt(bipyn-H)(C2H4)]BArF
[Pt(bipyφ -H)(C2H4)]BArF
[Pt(bipyRMe-H)(C2H4)]BArF
[Pt(bipyREt-H)(C2H4)]BArF
[Pt(bipyREt-H)(C2H4)]CF3SO3
[Pt(bipyc-H)(C2H4)]BArF
9
7.94 (14.0)
8.10 (17.2)
8.08 (18.8)
8.41 (16.4)
8.09 (18.0)
6.57 (43.0)
6.84 (47.0)
6.83 (44.0)
6.86 (43.8)
6.79 (47.0)
4.66 (60.2)
10
11
11b
12
4.48
4.10
4.08
1.75
0.76
0.76
2.05
4.72, 4.64 (65.0)b
4.71, 4.61 (67.2)b
4.80, 4.75 (65.0)b
4.64 (65.6)
2.12
2.12
a Chemical shifts in parts per million, coupling constants in hertz; JPtH in parentheses, CDCl3 as solvent, T = 298 K. The assignment follows the
numeration pattern shown in Chart 2. b AA0BB0 system.
the substituted di-imine ligand, the methyl group, and the
η2-coordinated ethylene molecule. The coordination in 5
(see Figure 1) is square planar and comparable to that
found in similar square-planar complexes,45 while that in
7 is distorted square planar, with the two rings of the
dipyridyl ligand slightly twisted (dihedral angle between
rings of 3.1(4)ꢀ).
the corresponding nuclei in the two halves of the dinitro-
gen NN ligands are in agreement with the higher trans
influence exerted by the alkyl group in comparison with
the chloride or the ethylene moiety.46 As an example, the
3JPtH coupling constant of the H2 proton for the 4,7-
diphenyl-1,10-phenanthroline ligand in complex 4P is
62.5 Hz, while this value drops to 17.3 Hz for the H9
proton that is trans to the methyl ligand. Similarly, the
ethene derivatives exhibit coupling constants, 3JPtH, of ca.
40-50 Hz for the protons in ortho position to the nitrogen
donor atom trans to the ethylene and of ca. 15-20 Hz for
those trans to the methyl group.
Solution Structure and NMR Characterization. The 1H
NMR spectra of the cationic complexes 1-12 were
assigned on the basis of 1D selective irradiation or 2D
1
NOESY experiments. Selected H NMR resonances at
298 K for all the synthesized un- and cyclometalated
compounds are reported in Tables 1 and 2, respectively.
1H NMR spectra of compounds [Pt(Cl)(Me)(NN)]
(1P-6P) as well as those of the cationic complexes
[Pt(Me)(NN)(C2H4)]þ (1-6) show, as expected, the none-
quivalence of the two halves of the complexes, especially
for the signals belonging to the di-imine fragment. The
differences in the 195Pt coupling constants, 3JPt-H/Hz, of
Proton resonances of the Pt-Me groups, as reported for
related methyl/alkene and methyl/alkyne derivatives,27,47
appear just below δ 1 ppm, with typical 195Pt satellites of
ca. 70 Hz.
The most interesting feature in the 1H NMR spectra of
all the cationic complexes [Pt(CNN)(C2H4)]þ (1-12) is
(46) Purcell, K. F.; Kotz, J. C. Inorganic Chemistry; Saunders: London,
1985; pp 704-705.
(47) Cucciolito, M. E.; De Felice, V.; Orabona, I.; Ruffo, F. J. Chem.
Soc., Dalton Trans. 1997, 351–355.
(45) Plutino, M. R.; Scolaro, L. M.; Albinati, A.; Romeo, R. J. Am. Chem.
Soc. 2004, 126, 6470–6484.

414 Inorganic Chemistry, Vol. 49, No. 2, 2010
Plutino et al.
Figure 2. 1H NMR spectrum of complex 7b recorded at 300.13 MHz in chloroform-d at 298 K.
relative to the ethylene protons. In all these compounds
the corresponding signal is shifted upfield with respect to
free ethene (δ 4.01-4.80 ppm), indicating that olefin is
coordinated to platinum through the CdC double bond
[that is, for Pt(II) ethylene compounds typical signals
occur at δ 5.4 (free); 4.8-4.3 (four-coordinate complex-
es); 3.7-1.7 (trigonal-bipyramidal complexes)].48
At room temperature, the 1H NMR spectra of all these
complexes display an averaged peak for the four protons
(a singlet for complexes 7, 8, and 12; an AA0BB0 system
for complexes 10 and 11,11b), which is not consistent with
an olefin in a static asymmetric square-planar arrange-
ment.
Thus, the chemical equivalence of the four ethene
protons indicates a fast dynamic process of the coordi-
nated C2H4 molecule around the Pt-ethene bond. This
motion averages one or both pairs of signals for the
protons HA,C and HB,D, as shown in Scheme 4, where
the 180ꢀ rotation of the ligand is observed along the
Pt-ethene bond. On the other hand, the ethylene AA0BB0
pattern observed in complexes 10 and 11 could be
ascribed to a sterically retarded rotation, combined with
a lower symmetry given by the orthometalated ancillary
ligands bipyRMe-H and bipyREt-H, respectively.
Scheme 4. Ethene Rotation around the Pt-C2H4 Bonda
a The sketches are shown with a view in front of the platinum-
(II)-alkene bond, where the metal center lies behind the olefin moiety.
expected singlets, one at δ 2.20 ppm with 2JPtH = 69.8 Hz,
for the methylene group bound to the platinum(II), and
the other at δ 1.46 ppm, assigned to the two methylic
protons. The aromatic region exhibits seven different
resonances for the asymmetric bipy ligand. The related
compound 7, containing the BArF- counteranion, evi-
dences clearly detectable upfield shifts of the aromatic
protons (see Figure S4 in the Supporting Information),
while the aliphatic signals are almost unaffected. As
already described in the previous section, this phenom-
enon is attributable to the ring current effect exerted by
the four 3,5-(CF3)2C6H3 aromatic pendants of the anion
0
on the central protons, H3 and H3 , of the bipyridine
moiety.
1
The aromatic region of the H NMR spectra of the
complex [Pt(bipyt)(C2H4)]BArF (7) is also affected by
temperature variation or changing the solvent and/or the
counteranion. Addition of different amounts of CD3OD
or benzene-d6 to a sample of compound 7 in chloroform-d
solution led to significant shifts of all the aromatic pro-
tons. In particular, downfield shifts of the bipyridine
proton signals are observed upon CD3OD addition, while
upfield shifts are detected with benzene-d6. These phe-
nomena usually are related to ion-pair formation and to
the relative strength of opposite charges’ interactions.
Accordingly, addition of an increasing amount of
(nBu4N)PF6 salt in solution produced similar chemical
shift changes (see Supporting Information, Figure S1). In
all these experiments, the most shifted protons were
For all these complexes, with the exception of com-
plexes 1 and 2, the signals for the ethylene protons
showed 195Pt satellites of ca. 60-70 Hz. On the other
hand, as far as complexes 1 and 2 are concerned, 195Pt
satellites are undetectable and the olefin peak is clearly
broad. In any case, the olefin protons resonate in a
range considered typical for square-planar platinum(II)
complexes. At low temperature, where the rotation is
almost frozen, the 1H NMR spectra of all the compounds
(except 9) displayed a static structure for the ethene,
showing an AA0BB0 or AA0XX0 system for complexes 7,
8, and 12 and an ABCD system for complexes 10 and 11,
respectively.
0
always H3 and H3 . These findings suggest that compound
7 exists as a tight ion pair in CDCl3 and that the counter-
ion lies on the side and in proximity to the bipyridine
Figure
2
shows the spectrum of [Pt(bipyt-H)-
(C2H4)]CF3SO3, 7b. In the aliphatic region the sharp
2
signal at δ 4.46 ppm with JPtH = 62.1 Hz is clearly
0
fragment close to H3 and H3 .
attributable to the four protons of the bound ethylene.48
The compound [Pt(Me)(bipy)(C2H4)]BArF, 6, posses-
sing a similar CNN skeleton in the series of uncyclome-
talated species, shows the expected pattern for the
aromatic proton signals, together with two singlets in
the aliphatic region relative to the ethene protons (δ 4.20,
The tert-butyl fragment in the bipyt-H ligand gives two
(48) Albano, V. G.; Natile, G.; Panunzi, A. Coord. Chem. Rev. 1994, 133,
67–114.

Article
Inorganic Chemistry, Vol. 49, No. 2, 2010 415
2JPtH = 64.7 Hz) and to the methyl fragment (δ 0.78,
2JPtH = 70.2 Hz; see 1H NMR spectrum of 6 at T = 298 K
in chloroform-d, Figure S5).
Dynamic Processes in 1-12. In order to determine the
ethene exchange rate constants, line-shape analyses were
performed for all the investigated complexes at different
ethene concentrations and/or temperature. For almost all
the complexes the ethylene proton resonances, at room
temperature, appeared with 195Pt satellites (3JPtH
=
60.2-70.4 Hz). On adding free ethene, the olefinic proton
signals lose their satellites, showing a typical concentra-
tion-dependent broadening, which is accounted for by a
relatively fast exchange between free and coordinated
ethene. Only in the case of the uncyclometalated species
[Pt(Me)(dipy)(C2H4)]BArF (1) and [Pt(Me)(dps)(C2H4)]-
BArF (2) can a dominant chemical shift anisotropy
relaxation mechanism explain the unresolved spin-spin
coupling to 195Pt; ethylene protons appear as a slightly
broadened singlet, as already reported in the literature for
the related [PtMe(daph)(ethylene)]þ or [PtMe(4,40-But-
2,20-bipy)(ethylene)]þ complexes.27,49 The broad olefinic
signal exhibited at room temperature indicates that these
complexes, even in the absence of added external ethene,
might be involved in a fast dynamic process, most prob-
ably an exchange between coordinated and free ethene
that may be produced after dissolution of the complexes,
either by ligand dissociation or by an associative process
involving the solvent.50 Any attempt to study the ethene
exchange on these two species, as well as for the other
uncyclometalated compounds 3-6 and for the highly
planar cyclometalated complex [Pt(bipyφ)(C2H4)]BArF
(9), failed even when we used a very low ethene concen-
tration at the lowest investigated temperature (T = 203
K). This fact led us to conclude that, in these cases, ethene
exchange reaction was too fast to be monitored on the
NMR time scale, maybe due to a less congested platinum-
(II) environment brought about by the CNN-type spec-
tator ligands.
1
Figure 3. Aliphatic portion of the H NMR spectra of 8 showing the
line-broadening effect on the signal of the coordinated olefin as a function
of ethene concentration (chloroform-d; 300.13 MHz; experimental con-
ditions: T = 298 K; CPt = 3.55 mm; ligand concentrations, [C2H4]/mm,
and best-fitfirst-order rateconstants, kobs/s-1, for the ethene exchange are
reported on the left and the right side of the spectra, respectively).
Scheme 5. Ethene Exchange on Cationic Platinum(II) Complexes of
the Type [Pt(CNN)(C2H4)]þ
Therefore, the intermolecular exchange between coor-
dinated and free ethylene was studied by the DNMR
technique for the cyclometalated complexes 7, 8 and
10-12, which react at room temperature with free ethy-
lene with rates suitable for the proton NMR technique.
The process takes place according to Scheme 5, with no
evidence throughout of olefin insertion into the Pt-C
bond or of five-coordinated species formation.
ordinated ligand. Comparison between experimental and
calculated peak shapes afforded the values of the rate
constants for each olefin concentration.
For all the investigated systems, these pseudo-first-
order rate constants (kobs/s-1) when plotted against the
ethylene concentrations give straight lines with no inter-
cept value (Figure 4). The second-order rate law (eq 6)
assumed for this exchange reaction is as follows:
The exchange rates were determined by line-broad-
ening H NMR experiments at room temperature, as a
1
function of the concentration of ethylene added in solu-
tion, while the temperature dependence of the ethene
exchange rates was obtained by full line-shape analysis
of spectra taken at different temperatures.
Kinetics of Ethene Exchange. i. Ligand Concentra-
tion Dependence. Figure 3 shows typical 1H NMR spectra
as a function of ethene concentration, recorded at 298 K
on a chloroform-d solution of complex 8. At comparable
ethene and complex concentrations, the spectra show
broad averaged signals relative to the free and the co-
kobs ¼ kexc½C2H4ꢀ
ð6Þ
Pseudo-first-order rate constants for complexes 7, 8,
and 10-12 as a function of ethene concentrations, to-
gether with the corresponding second-order rate con-
stants (kexc/s-1 m-1), obtained from linear regression
analysis of the rate law, are given as Supporting Informa-
tion in Table S2.
ii. Variable-Temperature Experiments. The intermole-
cular ethylene exchange process on complexes 7, 8, and
10-12 was investigated in the temperature range
198-341 K in CDCl3.
(49) Hill, G. S.; Rendina, L. M.; Puddephatt, R. J. J. Chem. Soc., Dalton
Trans. 1996, 1809–1813.
(50) Kaplan, P. D.; Schmidt, P.; Brause, A.; Orchin, M. J. Am. Chem. Soc.
1969, 91, 85-88, and references therein.

416 Inorganic Chemistry, Vol. 49, No. 2, 2010
Plutino et al.
Figure 6. Eyring plots obtained for ethene exchange on cyclometalated
platinum(II) complexes 7, 8, and 10-12 from 1H DNMR data measured
in chloroform-d.
Figure 4. Dependence of the observed pseudo-first-order rate constants
(kobs/s-1) for ethene exchange on complexes 7, 8, and 10-12 as a function
ofdifferent amounts of free ethylene(experimental conditions:CPt = 3-5
mm, [C2H4] = 0-30 mm in chloroform-d at 298 K).
only one of the coordinated ligands, namely, the olefin
group in this case, must undergo substitution. Accord-
ingly, the ethene exchange in these cyclometalated com-
plexes (where HC-NN = bipyt, 7; bipyn, 8; bipyRMe, 10;
bipyREt, 11; bipyc, 12) takes place in a single step accord-
ing to Scheme 5.The fused [5,5]- and [5,6]-ring systems are
robust, and there is no evidence of ring-opening or of
other concurrent processes. The systematic kinetics of
these reactions studied at different olefin concentrations
by DNMR techniques prove the dependence of the
pseudo-first-order rate constants described by straight
lines (Figure 4). The contribution of the reagent-indepen-
dent term k1 is negligible (k1 usually refers to a solvolytic
path, and k2, i.e., kexc, is the second-order rate constant
for the bimolecular attack of the olefin on the substrate,
as shown in Supporting Information Scheme S1). The
absence of a k1 term indicates that, as expected, a poor
nucleophilic solvent such as chloroform cannot provide a
solvolytic pathway to the products, but also proves the
absence of a possible parallel dissociative pathway.
The values of kexc, with their standard deviations, from
the linear regression analysis of the dependence of kobs on
[C2H4], at 298 K, are listed in Table 3, together with the
obtained corresponding activation parameters. The va-
lues of the activation entropies, obtained by Eyring
analysis of variable-temperature kinetic data, are nega-
tive, as expected for associative processes. Significantly,
the TΔSq contribution to the free energy of activation is
extremely large, amounting to more than 80% for all
complexes, while the enthalpy terms are correspondingly
small. Analogous situations with formation of a well-
ordered, stable transition state have been previously
reported in other substitution or exchange reactions
involving leaving groups trans to an olefin.51-53,12,54
All the results illustrated above strongly suggest that a
nondissociative alkene exchange is operative,12 a process
already discussed for square-planar platinum(II) com-
plexes27 and also found in the case of the isostructural
Figure 5. Variable-temperature 1H NMR spectra of complex 8 showing
the free and coordinated ethylene protons peaks (chloroform-d; 300.13
MHz; experimental conditions: CPt = 3.55 mm; [C2H4] = 6.26 mm;
temperature, T/K, and rate constants, kobs/s-1, for the ethene exchange
are reported on the left and the right side of the spectra, respectively).
At a sufficiently low temperature a significant amount
of ethene (about 2:1 with respect to the complex con-
centration) was added in solution and the 1H NMR
spectrum was recorded. Separated broad signals for free
and coordinated ethene were observed (Figure 5), which
averaged on raising the temperature, according to the
corresponding increase of the exchange rate constants.
The activation parameters for the exchange of ethene
were obtained from Eyring plots (Figure 6) over the
temperature range 198-341 K.
Rate constants of the exchange reaction as determined
by full line shape analysis and the activation parameters
for the process derived from Eyring plots are collected
and given as Supporting Information in Table S3.
iii. The Mechanism. Square-planar un- and cyclome-
talated platinum(II) complexes of the type [Pt(CNN)-
(C2H4)]þ (1-6 and 7-12, respectively) were particularly
designed to have the following characteristics: (i) they
must be soluble and thermally stable in organic solvents;
(ii) in the cationic complex ion all but one coordination
sites must be blocked by robust ligands in order to prevent
side reactions or possible ring-opening or -closures; (iii)
(51) Steyn, G. J. J.; Roodt, A.; Poletaeva, I.; Varshavsky, Y. S. J.
Organomet. Chem. 1997, 536/537, 197–205.
(52) Holloway, C. E.; Fogelman, J. Can. J. Chem. 1970, 48, 3802–3806.
(53) Guillot-Edelheit, G.; Chottard, J.-C. J. Chem. Soc., Dalton Trans.
1984, 169–173.
(54) (a) Fanizzi, F. P.; Intini, f. P.; Maresca, L.; Natile, G.; Gasparini, F.
J. Chem. Soc., Dalton Trans. 1990, 1019–1022. (b) Hahn, C.; Morvillo, P.;
Herdtweck, E.; Vitagliano, A., Organometallics 2002, 2002, 1807-1818.

Article
Inorganic Chemistry, Vol. 49, No. 2, 2010 417
Table 3. Second-Order Rate Constants and Activation Parameters for Ethene Exchange in Some Square-Planar Platinum(II) Complexes
298a
complex
10-3kexc
ΔHqb
ΔSqc
ΔGqd
ref
[Pt(bipyt-H)(C2H4)]BArF (7)
[Pt(bipyt-H)(C2H4)]CF3SO3 (7b)
[Pt(bipyt-H)(C2H4)]PF6 (7c)
[Pt(bipyn-H)(C2H4)]BArF (8)
[Pt(bipyφ-H)(C2H4)]BArF (9)
[Pt(bipyRMe-H)(C2H4)] BArF (10)
[Pt(bipyREt-H)(C2H4)] BArF (11)
[Pt(bipyREt-H)(C2H4)]CF3SO3 (11b)
[Pt(bipyc-H)(C2H4)]BArF (12)
[PtCl3(C2H4)]-f
109 ( 2
88.7 ( 0.7
113 ( 1
99 ( 1
7.3 ( 0.2
-121.5 ( 0.9
43.5
44.7e
44.1e
44.5
,40
46.8
49.5
50.8e
41.4
53.8
40.6
60.6
53.5
46.2
52.8
this work
this work
this work
this work
this work
this work
this work
this work
this work
12
5.9 ( 0.1
-129.4 ( 0.4
39.3 ( 0.1
8.4 ( 0.1
7.8 ( 0.1
94 ( 1
2.1 ( 0.1
500 ( 20
0.148 ( 0.01
2.6 ( 0.2
4.3 ( 0.4
3.7
13.4 ( 0.5
14.5 ( 0.8
-112 ( 2
-118 ( 3
8 ( 1
19.1 ( 0.3
10.2 ( 0.4
29.3 ( 0.6
8.8 ( 0.7
45 ( 8
-112 ( 5
-117 ( 1
-102 ( 2
-105 ( 2
-150 ( 20
-140 ( 20
trans-[PtCl2(C2H4)MeOH] f
trans-[PtCl2(C2H4)(imidazole)] g
[PtCl(C2H4)(acac)]
12
51
52
53
trans-[PtCl2(ol)(py)]
[Pt(Et)(N-N)(C2H4)]BArFh
9
a At 298 K, s-1 m-1, in chloroform-d unless otherwise stated. b kJ mol-1
.
c J K-1 mol-1
.
d At 298 K, kJ mol-1 e Calculated from the single value of
.
kexc at 298 K. f In methanol-d4. g In acetone-d6. h In CD2Cl2 at 300 K. N-N = [(2,6-Me2C6H3)NdC(An)-C(An)dN(2,6-Me2C6H3)] (An = 1,8-
naphthalenediyl).
Scheme 6. Associative Mechanism for the Ethene Exchange on
Square-Planar Cyclometalated Platinum(II) Species 7-12 in Chloro-
form-d Solutions
rhodium(I) ethylene complex, by freezing the five-coor-
dinated species as the intermediate at low temperature.55
Scheme 6 illustrates the suggested associative mechan-
ism, similarly to that proposed for dynamic processes in
alkene complexes.23,27 It involves a five-coordinated bis-
(ethylene) intermediate that accounts for the dynamic
NMR behavior through a twisting of the trigonal faces of
the square-planar pyramid, thus resulting in the inter-
conversion of two square-planar pyramid intermediates
(II and IV) via a five-coordinated bis(ethylene) trigonal-
bipyramid intermediate (III).
Interestingly, as far as compounds 8, 10, and 11 are
concerned, a good correlation is found between the rates
of ethene exchange (kexc) and the rates of chloride for
triphenylphosphane associative substitution on the ana-
logous chloride compounds.16 A comparison of the bi-
molecular rate constants indicates that the second process
is at least 3 orders of magnitude slower than olefin
exchange (in that case kinetics were followed by UV-Vis
stopped-flow technique) and the changes in rate constants
are less sensitive than kexc to structural variations in the
substrates. It is worthwhile to remark that the former
compound [Pt(bipyφ-H)(Cl)], 9P, in the case of the phos-
of this table reveals that the most closely related kinetic
parameters are those of the cyclometalated complexes
(7, 8, 10-12) together with that of complex [Pt(Et)(N-N)-
(C2H4)]BArF,9 recently reported by Shiotsuki et al.
Undoubtedly for all of them ethene exchange takes place
with the associative mechanism reported above.
Further comments on the data included in Table 3
relative to platinum(II) complexes with very different
arrangement of ancillary ligands could be tricky, as we
do not have any information whether they refer to a
concerted olefin exchange (see Scheme 6) or to an asso-
ciative mechanism involving consecutive displacement of
ligands. This latter mechanism was shown to be operative
in the ethene exchange on [PtCl3(C2H4)]- and in the
corresponding methanol solvolysis product, trans-[PtCl2-
(C2H4)(MeOH)].56 Here the much higher reactivity of the
phane substitution process showed the fastest kinetic rate,
i.e., almost 2 orders of magnitude greater than 2P. This
result must be compared to that obtained in the present
study on olefin exchange processes, thus further confirm-
ing our deduction of a quite low activation free energy for
9, ΔGq , 40 kJ mol-1 vs the related value of 43.5 kJ mol-1
calculated for [Pt(bipyt-H)(C2H4)]BArF, 7. Indeed, complex
9 evidences the fastest observable exchange rate constant
within the series of the studied cyclometalated platinum(II)
complexes. Interestingly, its extreme lability could also be
explained in terms of ground-state destabilization, as the
quite low value of the 195Pt coupling constant for the bound
ethene in this species (2JPtH = 60 Hz) strongly suggests a
longer and weaker platinum(II)-(η2-olefin) bond.
Table 3 collects rate constants and free activation
energies for ethene exchange reactions in square-planar
platinum(II) complexes already reported in the literature,
together with those from the present study. An inspection
(56) (a) Benedetti, M.; Fanizzi, F. P.; Maresca, L.; Natile, G. Chem.
Commun. 2006, 1118–1120. (b) Barone, C. R.; Benedetti, M.; Vecchio, V. M.;
Fanizzi, F. P.; Maresca, L.; Natile, G. Dalton Trans. 2008, 5313–5322.
(55) Hahn, C.; Sieler, J.; Taube, R. Chem. Ber. 1997, 130, 939–945.

418 Inorganic Chemistry, Vol. 49, No. 2, 2010
Plutino et al.
solvento species (kexc = 500 ꢁ 103 M-1 s-1) with respect
to the chloride species (kexc = 2.1 ꢁ 103 M-1 s-1) is
justified only by the much greater lability of MeOH with
respect to Cl-, while it is clearly in disagreement with the
poor trans-activating ability of MeOH with respect to the
chloride anion. The mechanism for olefin exchange in
trans-[PtCl2(C2H4)(MeOH)] and [PtCl3(C2H4)]- 12 thus,
as already reported in other nucleophilic processes in
olefinic complexes,50,57 most probably involves rate-de-
termining associative attack by the entering olefin at the
labile site trans to the coordinated olefin. This site is
occupied by either a chloride or a solvent molecule
through the formation of the bis-ethylene platinum(II)
complex trans-[PtCl2(C2H4)2], whose presence in solution
was confirmed later.58
the platinum(II) center (i.e., the carbon dinitrogen CNN
skeleton). In the case of the uncyclometalated complexes 1-6
and of the cyclometalated species 9, all showing lower steric
hindrance above and below the coordination plane and a
lower congestion around the platinum(II) center, a fast
exchange between coordinated and free olefin takes place
in solution (even at the lower accessible temperature). The
ethylene exchange in 7, 8, and 10-12 complexes is first order
in ethylene concentration and is characterized by large
negative activation entropy. As in the reaction of chlo-
ride for phosphanes substitution in uncharged parent chlor-
ide complexes 7P-12P,16 the kinetic data for olefin exchange
are compatible with an associative mechanism involving the
interconversion of two short-lived labile square-pyramid
intermediates via a five-coordinated bis(η2-ethylene) trigo-
nal-bipyramid intermediate, in which both ethene moieties lie
in the trigonal coordination plane. All the experimental
findings indicate that, in such cationic platinum(II)
[Pt(CNN)(C2H4)]þ complexes, ethene exchange is particularly
sensitive to the choice of the coordinated 6-substituted-2,20-
bipyridines, depending on the nature of the bonded organic
moiety or anchoring pendant (alkyl or aryl), on the size of the
ring, or on the number of alkyl substituents on it. Considering
the data as a whole, a crucial role seems to be played by the
capability of the anionic CNN-type fragments to introduce
steric hindrance above and below the square-planar coordina-
tion plane and the involved trigonal- and square-pyramidal
intermediates increasing the reaction energetics, as in 10 and 11,
where the ethene lability is strongly reduced.
In the case of the cyclometalated complexes bipyt (7),
bipyn (8), and bipyc (12), the values of the rate constants
and of the free energy of activation are compared for the
reactions listed in Table 3. The differences of reactivity
among the three members of the series are negligible (kexc
ca. 105 s-1 m-1), suggesting that the energy barrier
involved in the formation of the five-coordinate transi-
tion state appears to be marginally affected by changes in
the nature and in the structural properties of the coordi-
nated CNN ligands.
As previously suggested by literature data, the ex-
change process can be strongly retarded or frozen by
steric hindrance induced by bulky substituents on the
adjacent ligands.27,59 In the series of studied cyclometa-
lated complexes, we may conclude that when the ligands’
steric hindrance comes into action, as for bipyRMe and
bipyREt, the lability of ethene is decreased at least 1 order
of magnitude (kexc = 39.3 vs 8.4 ꢁ 105 s-1 m-1 for 10 and
11, respectively). In these complexes the steric congestion
of the CNN-terdentate ancillary orthometalated ligands,
combined with the reduced symmetry brought about on
the square-planar coordination plane, seems to inhibit
even ethene rotation, and at room temperature both
compounds show an AA0BB0 system for the four ethylene
protons.
Acknowledgment. This work was financially supported
0
ꢀ
by Ministero dell Universita e della Ricerca (MIUR,
PRIN 2004, prot: 2004030719_008) and CNR. The paper
is dedicated to the memory of Professor Raffaello Ro-
meo, who died on February 23, 2009, and who inspired
this study. All the authors wish to honor and gratefully
acknowledge Prof. R. Romeo for the continuous support
of his experience, helpful discussions, and advice. We also
thank the Microanalytical Laboratory, Department of
Chemistry, University College Dublin, Ireland, for mi-
croanalyses.
Conclusions
Supporting Information Available: Experimental Section in-
cludes the 1H NMR characterization and elemental analysis for
the precursor complexes 4P and 5P, the non-cyclometalated
species 2-6, and the cyclometalated compounds 8-12 and 11b;
X-ray data collection and structure determination for 5 and 7,
together with their structural studies; a paragraph on the error
analysis. Tables include the experimental data for the X-ray
diffraction study of compounds 5 and 7; the ethene and tem-
perature dependence, respectively, of the observed exchange
rate constants (kobs/s-1) together with derived second-order rate
constants (kexc/s-1 m-1) and the activation parameters (ΔHq,
ΔSq, and ΔGq) for the ethene exchange reaction on
[Pt(CNN)(C2H4)]BArF complexes in CDCl3. Figures include
the chemical shift changes for the aromatic protons of complex 7
as a function of the added counteranion, (nBu4N)PF6 (upper
side) and ball-and-stick representation of the tight ion-pair
complex 7c, together with numbering (lower side); ORTEP view
of the platinum(II) cationic complexes 5 and 7, with selected
In this study, we have described the synthesis and the
reactivity of a large class of cationic platinum(II) complexes
[Pt(CNN)(C2H4)]þ, containing a η2-ethene moiety, different
diimines (N-N), and a methyl fragment or terdentate (κC-
κ2NN0) anionic cyclometalating ligands.
The two kinds of platinum(II) complexes, [Pt(Me)(NN)-
(C2H4)]þ (1-6) and [Pt(CNN)(C2H4)]þ (7-12), exhibit a fair
difference in the coordinated alkene lability. A detailed
kinetic study of the dynamic behavior of ethene was per-
formed by line-broadening NMR techniques as a function of
ligand and temperature dependence in complexes 7, 8, and
10-12, where the process was not too fast on the NMR time
scale to be investigated. The ethene lability may be correlated
to differences in the bulkiness of the spectator ligands around
(57) (a) Otto, S.; Elding, L. I. J. Chem. Soc., Dalton Trans. 2002, 2354–
2360. (b) Otto, S.; Roodt, A.; Elding, L. I. Dalton Trans. 2003, 2519–2525.
(58) Otto, S.; Roodt, A.; Elding, L. I. Inorg. Chem. Commun. 2006, 9, 764–
766.
(59) Natile, G.; Maresca, L.; Cattalini, L. J. Chem. Soc., Dalton Trans.
1977, 651–655.
˚
bond lengths (A) and angles (deg) given in the figure captions;
1H NMR spectra of complexes 6 and 7. Crystallographic data
for structures 5 and 7 are available as CIF files. This material is
available free of charge via the Internet at http://pubs.acs.org.