Role of cyclometalation in controlling the rates of ligand substitution at Platinum(II) complexes

Article abstract of DOI:10.1021/ic0004479

The rates of chloride for triphenylphosphine substitution have been measured in dichloromethane for a series of cyclometalated [Pt(N - N - C)Cl] complexes containing a number of terdentate N - N - C anionic ligands, derived from deprotonated alkyl-, phenyl-, and benzyl-6-substituted 2,2'-bipyridines. These rates have been compared with those of the corresponding [Pt(N - N)(C)Cl] (N - N = 2,2'-bipyridine; C = CH₃ or C₆H₅) complexes having the same set of donor atoms but less constrained arrangements of the ligands. The reactions of the cyclometalated compounds occur as a single-stage conversion from the substrate to the ionic pair [Pt(N - N - C)(PPh₃)]Cl products. There is no evidence by ¹H and ³¹P{¹H} NMR spectroscopy for the formation of other Pt(II) species or of concurrent ring-opening processes. In contrast, in the monoalkyl- or monoaryl-2,2'-bipyridine complexes, chloride substitution is followed by subsequent slower processes which involve the detachment of one arm of the chelated 2,2'-bipyridine, fast cis to trans isomerization of the cis-[Pt(PPh₃)₂(η¹-bipy)(R)]⁺ transient intermediate, and, eventually, the release of free bipy, yielding trans-[Pt(PPh₃)₂(R)Cl] (R = Me or Ph). All reactions are first-order with respect to complex and phosphine concentration, obeying the simple rate law k(obsd) = k₂[PPh₃]. The values of the second-order rate constant k₂ do not seem particularly sensitive to the nature of the bonded organic moiety (alkyl or aryl), to its structure (cyclometalated or not), to the size of the ring, or to the number of alkyl substituents on it. The effects are those foreseen on the basis of an associative mode of activation. The only exception to this pattern of behavior is constituted by the complex [Pt(bipy(φ)-H)Cl] (bipy(φ) = 6-phenyl-2,2'-bipyridine), which features a significant rate enhancement with respect to the analogue [Pt(bipy)(Ph)Cl] complex. The results of this work, together with those of a previous paper, suggest that there is not a specific role of cyclometalation in controlling the reactivity, unless an in-plane aryl ring becomes part of the π-acceptor system of the chelated 2,2'-bipyridine, behaving as a cyclometalated analogue of the nitrogen terdentate 2,2':6',2'-terpyridine.

Full text of DOI:10.1021/ic0004479

Inorg. Chem. 2000, 39, 4749-4755
4749
Role of Cyclometalation in Controlling the Rates of Ligand Substitution at Platinum(II)
Complexes
Raffaello Romeo,*,† Maria Rosaria Plutino,† Luigi Monsu` Scolaro,† Sergio Stoccoro,‡ and
Giovanni Minghetti‡
Dipartimento di Chimica Inorganica, Chimica Analitica e Chimica Fisica e Istituto di Chimica e
Tecnologia dei Prodotti Naturali (ICTPN-CNR), Sezione di Messina, Universita` di Messina, Salita
Sperone 31, Vill. S. Agata, 98166 Messina, Italy, and Dipartimento di Chimica, Universita` di Sassari,
via Vienna 2, 07100 Sassari, Italy
ReceiVed April 25, 2000
The rates of chloride for triphenylphosphine substitution have been measured in dichloromethane for a series of
cyclometalated [Pt(N-N-C)Cl] complexes containing a number of terdentate N-N-C anionic ligands, derived
from deprotonated alkyl-, phenyl-, and benzyl-6-substituted 2,2′-bipyridines. These rates have been compared
with those of the corresponding [Pt(N-N)(C)Cl] (N-N ) 2,2′-bipyridine; C ) CH3 or C6H5) complexes having
the same set of donor atoms but less constrained arrangements of the ligands. The reactions of the cyclometalated
compounds occur as a single-stage conversion from the substrate to the ionic pair [Pt(N-N-C)(PPh3)]Cl products.
There is no evidence by 1H and 31P{1H} NMR spectroscopy for the formation of other Pt(II) species or of concurrent
ring-opening processes. In contrast, in the monoalkyl- or monoaryl-2,2′-bipyridine complexes, chloride substitution
is followed by subsequent slower processes which involve the detachment of one arm of the chelated 2,2′-bipyridine,
fast cis to trans isomerization of the cis-[Pt(PPh3)2(η1-bipy)(R)]+ transient intermediate, and, eventually, the release
of free bipy, yielding trans-[Pt(PPh3)2(R)Cl] (R ) Me or Ph). All reactions are first-order with respect to complex
and phosphine concentration, obeying the simple rate law kobsd ) k2[PPh3]. The values of the second-order rate
constant k2 do not seem particularly sensitive to the nature of the bonded organic moiety (alkyl or aryl), to its
structure (cyclometalated or not), to the size of the ring, or to the number of alkyl substituents on it. The effects
are those foreseen on the basis of an associative mode of activation. The only exception to this pattern of behavior
is constituted by the complex [Pt(bipyφ-H)Cl] (bipyφ ) 6-phenyl-2,2′-bipyridine), which features a significant
rate enhancement with respect to the analogue [Pt(bipy)(Ph)Cl] complex. The results of this work, together with
those of a previous paper, suggest that there is not a specific role of cyclometalation in controlling the reactivity,
unless an in-plane aryl ring becomes part of the π-acceptor system of the chelated 2,2′-bipyridine, behaving as
a cyclometalated analogue of the nitrogen terdentate 2,2′:6′,2′′-terpyridine.
Introduction
effect)6 in a rather different way, through a strong electron
σ-donation along the C-Pt-X bond axis. Generally, changes
Kinetic studies of substitution reactions on square planar
platinum(II) complexes have shown that the lability of these
compounds can be nicely tuned through changes in the electronic
or steric properties of the ancillary ligands. Ligands such as
carbon monoxide,1 R,R′-diimines,2,3 and 2,2′:6′,2′′-terpyridine4
can favor the addition of a fifth ligand and the stabilization of
a five-coordinate transition state through electron back-donation
from filled d orbitals of the metal to empty π*-orbitals of the
ligand. Carbon σ-donor ligands (e.g., alkyl or aryl groups)5 can
enhance the lability of a trans-positioned X group (kinetic trans
of lability do not imply changes in the mode of activation, which
remains associative in nature. The introduction of a carbon-
metal bond is not a sufficient condition to promote dissociative
pathways. On the contrary, if the organic moiety possesses
π-acceptor properties, the lability can be very high with an
associative mechanism, as in the case of solvent exchange in
the cationic complex [Pt(CH3CN)4]2+ 7 or olefin exchange at
chloro ethene complexes.8 In this respect it is worth mentioning
that the presence of olefins as ligands is fundamental for the
isolation of stable five-coordinate platinum(II) complexes.9
Therefore, if the control of the reactivity of Pt(II) complexes
can be achieved rather easily, tuning steric or electronic
* To whom correspondence should be addressed. Phone: +39-090-
391984. Fax: +39-090-393756. E-mail: Raf.Romeo@chem.unime.it.
† Universita` di Messina.
‡ Universita` di Sassari.
(1) Romeo, R.; Grassi, A.; Monsu` Scolaro, L. Inorg. Chem. 1992, 31,
4383.
(2) Annibale, G.; Canovese, L.; Chiessa, G.; Cattalini, L.; Tobe, M. L. J.
Chem. Soc., Dalton Trans. 1990, 405.
(5) Basolo, F.; Chatt, J.; Gray, H. B.; Pearson, R. G.; Shaw, B. L. J. Chem.
Soc. 1961, 2207.
(6) Cotton, F. A.; Wilkinson, G. AdVanced Inorganic Chemistry, 5th ed.;
Wiley: New York, 1988.
(3) Romeo, R.; Monsu` Scolaro, L.; Nastasi, L.; Arena, G. Inorg. Chem.
1996, 35, 5087.
(4) (a) Pitteri, B.; Marangoni, G.; Cattalini, L.; Bobbo, T. J. Chem. Soc.,
Dalton Trans. 1995, 3853. (b) Pitteri, B.; Marangoni, G.; Visentin,
L.; Cattalini, L.; Bobbo, T. Polyhedron 1998, 17, 475. (c) Annibale,
G.; Brandolisio, M.; Bugarcic, Z.; Cattalini, L. Transition Met. Chem.
1998, 23, 715.
(7) Hallinan, N.; Besanc¸on, V.; Forster, M.; Elbaze, G.; Ducommun, Y.;
Merbach, A. E. Inorg. Chem. 1991, 30, 1233.
(8) Plutino, M. R.; Otto, S.; Roodt, A.; Elding, L. I. Inorg. Chem. 1999,
38, 1233.
(9) (a) Albano, V. G.; Natile, G.; Panunzi, A. Coord. Chem. ReV. 1994,
133, 67. (b) Maresca, L.; Natile, G. Comments Inorg. Chem. 1993,
14, 349.
10.1021/ic0004479 CCC: $19.00 © 2000 American Chemical Society
Published on Web 09/21/2000

4750 Inorganic Chemistry, Vol. 39, No. 21, 2000
Romeo et al.
properties of the ligands, the promotion of dissociative pathways
is still a very difficult task. We are currently investigating the
reasons which induce a changeover in the mechanism of
substitution. So far, the only well-documented examples of
dissociative ligand exchange and substitution refer to the
reactions of complexes of the type cis-[PtR2S2] (R) Me or Ph;
S ) thioethers or Me2SO).1,10 Recently we extended these
studies to the complexes [Pt(bph)(SR2)2], where the 2,2′-
biphenyl dianion (bph2-), a cyclometalating analogue of 2,2′-
bipyridine, combines cyclometalation and a favorable in-plane
disposition of the aryl rings.11 Contrary to expectation, kinetic
and theoretical results indicated that thioether dissociation is
much easier than in the diaryl analogue cis-[PtPh2(SR2)2].
Electron back-donation from filled d orbitals of the metal to
empty π*-orbitals of the in-plane cyclometalated rings is weak
or absent and is not operative in promoting an associative mode
of activation. These results are quite surprising in view of the
structural similarity between platinum(II) complexes containing
cyclometalating and polypyridine ligands, especially as far as
photochemical and photophysical properties are concerned,12
and strongly suggest investigating in detail the role of cyclo-
metalation in controlling the reactivity. Systematic studies of
the substitution ligand lability of these compounds are quite
few13,14 and have led to different interpretations. In particular,
an important question is whether the considerable acceleration
in rate observed for the substitution of the aqua ligand in the
complexes [Pt(N-C)(N)(H2O)] (N-CH ) N,N-dimethylben-
zylamine, N ) pySO3-3)14a,b and [Pt(N-C-N)(H2O)]+ 14c (N-
CH-N ) 2,6-bis((dimethylamino)methyl)phenyl) stems from
a significant contribution of back-donation into the empty π*-
orbitals of the in-plane aryl ligand or if it is simply the result
of the well-known strong trans-labilizing effect of the Pt-C
bond.15
work is to investigate the sensitivity of the rate of chloride for
phosphine substitution to the nature of the organic bonded
moiety (alkyl or aryl), its size, its orientation with respect to
the coordination plane, and the number and position of alkyl
substituents on the ring. The lability of the chloride in the
cyclometalated complexes does not differ markedly from that
measured for [Pt(N-N)(C)Cl] (N-N ) 2,2′-bipyridine; C )
Me or Ph) complexes having the same set of donor atoms but
less constrained arrangements of the ligands. The results of this
study, together with those of a previous investigation,11 would
suggest that there is not a specific role of cyclometalation in
controlling the substitution lability.
Experimental Section
General Procedures. All solvents were purchased from Aldrich
Chemical Co. The solvents employed in the synthetic procedures were
distilled before use over appropriated drying agents under an oxygen-
free nitrogen atmosphere. Spectrophotometric grade dichloromethane,
used in the kinetic runs, was freshly distilled and degassed by several
freezing-pumping cycles and stored in Schlenk tubes. Methanol was
distilled from magnesium methoxide. Chloroform-d (99.8+%) and
methanol-d4 (99.8+%) were used as received. All the other reagents
were the highest purity grade commercially available and were used
without further purification. All manipulations in the synthesis of the
complexes were carried out under a dry oxygen-free N2 atmosphere
using standard Schlenk-line techniques.
Synthesis of Complexes. [Pt(terpy)Cl]Cl was prepared according
to the method reported by Lippard et al.16 The alkyl- and aryl-2,2′-
bipyridine complexes [Pt(bipy)(R)(Cl)] (R ) methyl (1)17 and phenyl
(4)) were prepared following a previously described general method18
which affords easily and in a quantitative yield compounds of the type
[Pt(N-N)(R)Cl] containing nitrogen ligands.
[Pt(bipy)(Me)(Cl)] (1). The complex trans-[Pt(Me2SO)2(Me)Cl] was
prepared according to Eaborn et al.19 and purified by several crystal-
lizations from dichloromethane/diethyl ether mixtures. A sample of this
complex (0.05 g, 0.125 mmol) was dissolved in the minimum amount
of CH2Cl2 (10 mL) and reacted under stirring with a solution of bipy
(0.021 g, 0.133 mmol) in dichloromethane (5 mL). After a few hours,
most of the solvent was evaporated under vacuum and the solution
added with diethyl ether (1/1, v/v) and cooled at -35 °C, to afford the
pure yellow solid in greater than 90% yield. 1H NMR (CDCl3): δ 9.64
(d, 3JPtH ) 16.2 Hz, 3JHH ) 5.9 Hz, 1H, H6′); 9.22 (d, 3JPtH ) 59.6 Hz,
3JHH ) 5.8 Hz, 1H, H6); 8.17 (ddd, 3Jav ) 8.0 Hz, 4JHH ) 1.5 Hz, 1H,
In this paper we have carried out kinetic studies on cyclo-
metalated compounds of the type [Pt(N-N-C)Cl], containing
a number of terdentate N-N-C anionic ligands derived from
deprotonated alkyl-, phenyl-, or benzyl-6-substituted 2,2′-
bipyridines of different structural properties. The aim of this
(10) (a) Lanza, S.; Minniti, D.; Moore, P.; Sachinidis, J.; Romeo, R.; Tobe,
M. L. Inorg. Chem. 1984, 23, 4428. (b) Lanza, S.; Minniti, D.; Romeo,
R.; Moore, P.; Sachinidis, J.; Tobe, M. L. J. Chem. Soc., Chem.
Commun. 1984, 542. (c) Alibrandi, G.; Bruno, G.; Lanza, S.; Minniti,
D.; Romeo, R.; Tobe, M. L. Inorg. Chem. 1987, 26, 185. (d) Minniti,
D.; Alibrandi, G.; Tobe, M. L.; Romeo, R. Inorg. Chem. 1987, 26,
3956. (e) Frey, U.; Helm, L.; Merbach, A. E.; Romeo, R. J. Am. Chem.
Soc. 1989, 111, 8161. (f) Alibrandi, D.; Minniti, D.; Monsu` Scolaro,
L.; Romeo, R. Inorg. Chem. 1989, 28, 1939.
3
4
H4); 8.11 (ddd, Jav ) 8.0 Hz, JHH ) 1.5 Hz, 1H, H4′); 8.02 (m, 2H,
H3+ H3′); 7.64 (ddd, 3JHH ) 5.9, 7.4 Hz, 4JHH ) 1.5 Hz, 1H, H5′); 7.47
(ddd, 3JHH ) 5.9, 7.4 Hz, 4JHH ) 1.5 Hz, 1H, H5); 1.22 (s, 2JPtH ) 77.9
Hz, 3H, CH3Pt).
[Pt(bipy)(Ph)(Cl)] (4). The complex was obtained from the reaction
of 2,2′-bipyridine with trans-[Pt(Ph)(Cl)(Me2SO)2]19 following the same
procedure described above. 1H NMR (CDCl3): δ 9.68 (d, 3JPtH ) 12.9
Hz, 3JHH ) 5.2 Hz, 1H, H6′); 8.72 (d, 3JPtH ) 59.1 Hz, 3JHH ) 5.8 Hz,
1H, H6); 8.13 (m, 2H, H4′ + H4); 8.07 (dd, 3JHH ) 8.0 Hz, 4JHH ) 1.4
(11) Plutino, M. R.; Monsu` Scolaro, L.; Romeo, R.; Grassi, A. Inorg. Chem.
2000, 39, 2713.
(12) (a) Sandrini, D.; Maestri, M.; Balzani, V.; Chassot, A.; von Zelewsky,
A. J. Am. Chem. Soc. 1987, 109, 7720. (b) Chan, C.-W.; Lai, T.-F.;
Che C.-M.; Peng, S.-M. J. Am. Chem. Soc. 1993, 115, 11245. (c)
Cheung, T.-C.; Cheung, K.-K.; Peng, S.-M.; Che, C.-M. J. Chem. Soc.,
Dalton Trans. 1996, 1645. (d) Maestri, M.; Deuchel-Cornioley, C.;
von Zelewsky, A. Coord. Chem. ReV. 1991, 111, 117. (e) Mdeleni,
M. M.; Bridgewater, J. S.; Watts, R. J.; Ford, P. C. Inorg. Chem. 1995,
34, 2331. (f) Constable, E. C.; Cargill Thompson, A. M. W.; Tocher,
D. A. In Supramolecular Chemistry; Balzani, V., De Cola, L., Eds.;
Kluwer: Dordrecht, The Netherlands, 1992. (g) Constable, E. C.;
Cargill Thompson, A. M. W.; Cherryman, J.; Liddiment, T. Inorg.
Chim. Acta 1995, 235, 165 and references therein.
3
4
Hz, 1H, H3′); 8.02 (dd, JHH ) 8.2 Hz, JHH ) 1.1 Hz, 1H, H3); 7.69
3
4
3
(ddd, JHH ) 5.5, 7.4 Hz, JHH ) 1.7 Hz, 1H, H5′); 7.47 (dd, JPtH
36.8 Hz, 3JHH ) 8.0 Hz, 4JHH ) 1.4 Hz, 2H, H2′′,6′′); 7.31 (ddd, 3JHH
5.8, 7.4 Hz, 4JHH ) 1.7 Hz, 1H, H5); 7.12 (ddd, 3Jav ) 7.4 Hz, 4JHH
)
)
)
1.4 Hz, 2H, H3′′,5′′); 6.99 (ddd, Jav ) 7.4 Hz, 4JHH ) 1.4 Hz, 1H, H4′′).
Cyclometalated complexes [Pt(N-N-C)Cl] [N-N-CH ) 6-tert-
butyl-2,2′-bipyridine, bipyt (2);20 6-neopentyl-2,2′-bipyridine, bipyn (3);21
6-phenyl-2,2′-bipyridine, bipyφ (5);22 6-benzyl-2,2′-bipyridine, bipyâ
(6);23 6-(R-methyl)benzyl-2,2′-bipyridine, bipyRMe (7);24 and 6-(R,R-
(13) Romeo, R.; Plutino, M. R.; Monsu` Scolaro, L.; Stoccoro, S. Inorg.
Chim. Acta 1997, 265, 225.
(14) (a) Schmu¨lling, M.; Ryabov, A. D.; van Eldik, R. J. Chem. Soc., Chem.
Commun. 1992, 1609. (b) Schmu¨lling, M.; Ryabov, A. D.; van Eldik,
R. J. Chem. Soc., Dalton Trans. 1994, 1257. (c) Schmu¨lling, M.;
Grove, D. M.; van Koten, G.; van Eldik, R.; Veldman, N.; Spek, A.
L. Organometallics 1996, 15, 1384.
(15) (a) Elding, L. I.; Romeo, R. J. Chem. Soc., Dalton Trans. 1996, 1471.
(b) Wendt, O. F.; Oskarsson, A.; Leipold, J. G.; Elding, L. I. Inorg.
Chem. 1997, 36, 4514.
(16) Howe-Grant, M.; Lippard, S. J. Inorg. Synth. 1980, 20, 101.
(17) Clark, H. C.; Manzer, L. E. J. Organomet. Chem. 1973, 59, 411.
(18) Romeo, R.; Monsu` Scolaro, L. Inorg. Synth. 1998, 32, 153.
(19) Eaborn, C.; Kundu, K.; Pidcock, A. J. J. Chem. Soc., Dalton Trans.
1981, 933.
(20) Minghetti, G.; Cinellu, M. A.; Stoccoro, S.; Zucca, A.; Manassero,
M. J. Chem. Soc., Dalton Trans. 1995, 777.
(21) Minghetti, G.; Stoccoro, S.; et al. Manuscript in preparation.

Role of Cyclometalation in Ligand Substitution
Inorganic Chemistry, Vol. 39, No. 21, 2000 4751
dimethyl)benzyl-2,2′-bipyridine, bipyc (9)]23 were prepared following
a broad-band probe operating at 300.13 and 121.49 MHz, respectively.
1H chemical shifts were measured relative to the residual solvent peak
and are reported in δ units downfield from SiMe4. 31P{1H} chemical
shifts, in parts per million, are given relative to external phosphoric
acid. The temperature within the probe was checked using the methanol
or ethylene glycol method.25 The resonances were assigned by
performing selective decoupling experiments of the signals. Stopped-
flow kinetic runs were recorded on an HP8452A spectrophotometer
published methods. The purity and identity of the complexes were
1
checked by elemental analyses and H NMR spectroscopy.
[Pt(bipyrEt-H)Cl] (bipyrEt ) 6-(1-ethylbenzyl)-2,2′-bipyridine) (8).
A 0.274 g (1 mmol) sample of racemic bipyREt and 3.5 mL of HCl (2
M) were added to a solution of K2[PtCl4] (0.415 g, 1 mmol) in water
(30 mL). The mixture was heated on a water bath until the solution
was colorless and then cooled to afford a yellow-orange precipitate
which was filtered off, washed successively with water, ethanol, and
diethyl ether, and recrystallized from CH2Cl2/Et2O, yield 64%, mp 273-
274 °C. Anal. Calcd for C19H17N2PtCl: C, 45.28; H, 3.4; N, 5.56.
Found: C, 44.73; H, 3.55; N, 5.36. MS-FAB (m/z): 504 [M + H]+. 1H
equipped with a Hi-Tech SFA-20 Rapid Kinetic Accessory; for t1/2
<
5 s an Applied Photophysics Bio Sequential SX-17 MX stopped-flow
ASVD spectrophotometer was used.
Kinetics. The chloride for phosphine substitution reactions were
followed by stopped-flow spectrophotometry under pseudo-first-order
conditions. Initial concentrations of starting complex were in the range
0.05-0.1 mM, and pseudo-first-order conditions were achieved by
having the ligand in at least 10-fold excess. Rate constants were
evaluated with the SCIENTIST software package26 by fitting the
absorbance/time data to the exponential function At ) A00 + (A0 -
A00) exp(-kobsd t). Rate constants are reported as average values from
five to seven kinetic runs.
3
3
NMR (CDCl3): δ 9.69 (d, JPtH ) 15.4 Hz, JHH ) 5.5 Hz, 1H, H6′);
8.09 (d, 3JPtH ) 48.4 Hz, 3JHH ) 7.9 Hz, 4JHH ) 1.6 Hz, 1H, H6′′); 8.05
(m, 1H, H4′); 7.95 (m, 2H, H4 + H3′); 7.83 (d, 1H, H3); 7.63 (dd, 1H,
3
H5′); 7.45 (d, 1H, H5); 7.04 (m, 3H, H4′′+ H3′′+ H5′′); 3.99 (t, JHH
)
7.4 Hz, 1H, CH-CH2-); 2.44, 2.34 (m, 2H, CH-CH2-CH3); 0.803
3
(t, JHH ) 7.4 Hz, 3H, -CH2-CH3).
The phosphine complexes [Pt(bipy)(PPh3)(R)]Cl (10, 13) and [Pt-
(N-N-C)(PPh3)]Cl (11, 12, 14-18) were prepared in situ by reacting
in an NMR tube weighted amounts of complexes 1-9 dissolved in
CDCl3 with the stoichiometric amount of PPh3. A detailed collection
of 1H and 31P{1H} NMR resonances of the neutral chloride complexes
1-9 and of the corresponding phosphine derivatives 10-18 is reported
as Supporting Information Table S1 together with the assignment (10
is the corresponding PPh3 complex of 1, 11 of 2, and so on).
[Pt(bipy)(Me2SO)Me]CF3SO3 (19). AgCF3SO3 (0.137 g, 0.53
mmol) and the complex trans-[Pt(Me2SO)2(Me)(Cl)] (0.200 g, 0.5
mmol) were reacted in dimethyl sulfoxide (4 mL). After a few hours
of stirring, AgCl was filtered off and 0.080 g of 2,2′-bipyridine was
added to the solution. The yellow solution was filtered on cellulose
powder to remove traces of residue AgCl, and the excess solvent was
evaporated under reduced pressure (0.1 mmHg, 70 °C). The solid yellow
residue was washed several times with ether, and recrystallized from a
dichloromethane/ether (1/1, v/v) mixture with a yield of 80%. 1H NMR
(CDCl3): δ 9.74 (dd, 3JPtH ) 14.3 Hz, 3JHH ) 6.0 Hz, 4JHH ) 1.7 Hz,
Results
We have already reported the use of the complex trans-[Pt-
(Me2SO)2(Me)Cl] as a useful synthon to introduce the moieties
{PtMe} and {PtMeCl}.18 The method was applied successfully
to the formation of the cationic complexes [Pt(N-N-N)(Me)]-
Cl (N-N-N ) 2,2′:6′,2′′-terpyridine and 1,5-diamino-3-aza-
pentane (dien))27 and [Pt(N-N)(Me2SO) (Me)]PF6 (N-N ) a
wide series of diamines and diimines),3 to the synthesis of cis-
(Me,am) and trans-(Me,am) [Pt(Me2SO)(am)(Me)Cl] (am )
monodentate nitrogen ligands),28 and as a new route to [Pt(1,-
10-phenanthroline)(Me)Cl].18 The complex [Pt(Me2SO)2(Ph)-
Cl] behaves likewise.29 Thus, the addition of the equimolar
amount of 2,2′-bipyridine to a dichloromethane solution of [Pt-
(Me2SO)2(Me)Cl] led to the rapid substitution of both sulfoxide
ligands, yielding compound 1 in a pure crystalline form and in
an almost quantitative yield. In a similar fashion compound 4
was formed starting from [Pt(Me2SO)2(Ph)Cl], prepared ac-
cording to ref 19. When the reaction is carried out in dimethyl
sulfoxide as the solvent and in the presence of a silver salt
(AgCF3SO3), the sulfoxide-containing compounds [Pt(bipy)-
(Me2SO)(R)]CF3SO3 (R ) Me or Ph) are obtained in high yield
(ca. 80%).
3
4
1H, H6′); 8.86 (dd, JHH ) 8.2 Hz, JHH ) 1.1 Hz, 1H, H3); 8.83 (d,
3JPtH ) 46.2 Hz, JHH ) 5.5 Hz, 1H, H6); 8.77 (d, JHH ) 8.2 Hz, 1H,
H3′); 8.45 (ddd, 3Jav ) 8.0 Hz, 4JHH ) 1.1 Hz, 1H, H4); 8.34 (ddd, 3Jav
) 8.0 Hz, 4JHH ) 1.1 Hz, 1H, H4′); 7.80 (ddd, 3JHH ) 5.5, 7.7 Hz, 4JHH
) 1.7 Hz, 1H, H5); 7.71 (ddd, 3JHH ) 6.0, 8.2 Hz, 4JHH ) 1.7 Hz, 1H,
H5′); 3.57 (s, JPtH ) 35.7 Hz, 6H, CH3S); 0.87 (s, JPtH ) 71.5 Hz,
3H, CH3Pt).
3
3
3
2
[Pt(bipy)(PPh3)Me]CF3SO3 (20). Triphenylphosphine (0.049 g, 0.19
mmol) in 20 mL of dichloromethane was added dropwise to a solution
of the complex 19 (0.100 g, 0.17 mmol) dissolved in CH2Cl2 (20 mL).
Rotary-vacuum concentration, addition of ether (1/1, v/v), and cooling
at -35 °C yielded a pure white solid in 90% yield. 1H NMR (CDCl3):
The synthesis and the spectroscopic properties of cyclom-
etalated platinum(II) [Pt(N-N-C)Cl] complexes, containing
6-alkyl-2,2′-bipyridine (2, 3),20,21 6-benzyl-2,2′-bipyridine (6,
7, 9),23,24 and 6-phenyl-2,2′-bipyridine (5) ligands,22 have been
described in detail elsewhere. Compound 8, where N-N-C is
the deprotonated form of 6-(R-ethyl)benzyl-2,2′-bipyridine, is
new and was prepared as reported in the Experimental Section.
3
2
δ 8.93 (d, JPtH ) 37.7 Hz, 2H, H3 + H6); 0.78 (s, JPtH ) 70.4 Hz,
3JPH ) 3.3 Hz, 3H, CH3Pt). 31P{1H} NMR: δ 19.7 (1JPtP ) 4355 Hz).
[Pt(bipy)(Me2SO)(Ph)]CF3SO3 (21). In a fashion identical to the
preparation of 19 the compound was obtained in 85% yield by reaction
1
of bipy and trans-[Pt(Me2SO)2(Ph)(Cl)]. H NMR (CDCl3): δ 9.67
3
1
(d, JHH ) 5.5 Hz, 1H, H6′); 8.82 (m, 2H, H3 + H3′); 8.35 (m, 2H, H4
NMR Measurements. The most evident feature in the H
3
3
+ H4′); 7.75 (dd, JHH ) 5.5, 8.2 Hz, 1H, H5′); 7.59 (dd, JPtH ) 51.6
NMR spectra of [Pt(bipy)(R)Cl] compounds (Table 1) is the
signal of the H6′ proton of the pyridine ring in a position trans
to the carbon atom (at about δ 9.6 ppm, 3JPtH ) 16.2 and 12.9
Hz for 1 and 4, respectively) which can be distinguished from
that of the H6 proton trans to chloride (3JPtH ) 59.6 and 59.1
Hz for 1 and 4, respectively) on the basis of the magnitude
3
3
3
Hz, JHH ) 5.5 Hz, 1H, H6); 7.44 (d, br, JPtH ) 36.2 Hz, JHH ) 6.0
Hz, 2H, H2′′, 6′′); 7.39 (d, 3JHH ) 6.0, 8.2 Hz, 1H, H5); 7.23 (m, br, 3H,
3
H3′′, + H4′′); 3.28 (s, JPtH ) 35.7 Hz, 6H, CH3S).
5′′
[Pt(bipy)(PPh3)(Ph)]CF3SO3 (22). In a fashion identical to the
preparation of 20 the compound was obtained by reacting stoichiometric
1
3
amounts of 21 and PPh3. H NMR (CDCl3): δ 7.78 (d, JPtH ) 40.6
Hz, 3JHH ) 5.5 Hz, 1H, H6). 31P{1H} NMR: δ 17.1 (1JPtP ) 4299 Hz).
Instrumentation and Measurements. 1H and 31P{1H} NMR spectra
were recorded on a BRUKER AMX-R 300 spectrometer equipped with
3
observed for the JPtH coupling constant. On substituting the
(25) (a) Van Geet, A. L. Anal. Chem. 1968, 40, 2227. (b) Van Geet, A. L.
Anal. Chem. 1970, 42, 679.
(26) SCIENTIST, Micro Math Scientific Software, Salt Lake City, UT.
(27) Arena, G.; Monsu` Scolaro, L.; Pasternack, R. F. Romeo, R. Inorg.
Chem. 1995, 34, 2994.
(28) Romeo, R.; Monsu` Scolaro, L.; Nastasi, N.; Mann, B. E.; Bruno, G.;
Nicolo’, F. Inorg. Chem. 1996, 35, 7691.
(22) Constable, E. C.; Henney, R. P. G.; Leese, T. A.; Tocher, D. A. J.
Chem. Soc., Dalton Trans. 1990, 443.
(23) Sanna, G.; Minghetti, G.; Zucca, A.; Pilo, M. I.; Seeber, R.; Laschi,
F. Inorg. Chim. Acta 2000, 305/2, 189.
(24) Cinellu, M. A.; Gladiali, S.; Minghetti, G. J. Organomet. Chem. 1989,
363, 401.
(29) Arena, G.; Calogero, G.; Campagna, S.; Monsu` Scolaro, L.; Ricevuto,
V.; Romeo, R. Inorg. Chem. 1998, 37, 2763.

4752 Inorganic Chemistry, Vol. 39, No. 21, 2000
Table 1. Relevant Proton and 31P{1H} NMR Dataa
Romeo et al.
31P
compd no.
CH3
CH2
CH
H6
H6′
H6′′
1
10
2
11
3
1.22 (77.9)
0.76 (70.3)
1.46 (10.4)
1.34
9.22 (59.6)
8.90 (35.5)
9.64 (16.2)
7.58
9.25 (14.3)
7.51
19.8 [4346]
20.6 [4078]
2.86 (84.4)
2.13 (68.2)
2.45 (87.4)
2.84
1.11
9.60 (13.2)
12
0.86
1.59 (65.4)
2.90
7.59 (13.2)
22.6 [4444]
4
13
5
14
6
15
7
16
8
8.72 (59.1)
7.76 (39.9)
9.68 (12.9)
7.00
9.07 (12.6)
6.45 (13.1)
9.63 (14.8)
7.64
9.71 (15.4)
7.66
9.69 (15.4)
7.70
7.47 (36.8)
7.03 (45.1)
7.75 (44.0)
6.40 (53.3)
8.07 (41.8)
6.77 (55.0)
8.11 (46.2)
6.87 (58.2)
8.09 (48.4)
6.92 (57.2)
17.2 [4288]
25.8 [4098]b
20.4 [4374]
20.6 [4408]
20.0 [4417]
21.1 [4453]
4.35 (9.3)
4.38 (8.8)
1.87
2.07
0.803
0.847
2.16
2.24
4.39
4.44
3.99
4.05
2.44, 2.34
2.72, 2.37
17
9
18
9.70 (15.4)
7.65
6.93 (54.9)
a Chemical shifts in parts per million from internal SiMe4 and external H3PO4, room temperature, coupling constants in hertz; JPtH in parentheses,
JPtP in square brackets, CDCl3 as solvent. The assignment follows the numeration pattern shown in Chart 1. b At 273 K.
Chart 1. Structural Formulas of the Complexes
chloride ion with PPh3, as for compounds 10 and 13, the signal
of the H6′ proton experiences a marked upfield shift (e.g., δ )
7.58 ppm, 10; 7.00 ppm, 13) due to the anisotropic shielding
by the adjacent aromatic rings.
1
The H NMR spectra of the cyclometalated compounds are
in agreement with the formulation proposed in Chart 1. The
complexes are characterized by the presence of [5,6]- or [5,5]-
ring systems, arising from the coordination of the metal to the
two nitrogen atoms and to a linking methyl or o-phenyl carbon.
1
As for compounds 1 and 4, the average value of the H NMR
signal of the H6′ proton (Table 1) is found at δ ) 9.52 ( 0.2
3
ppm with a coupling to the 195Pt nucleus JPtH ) 14.4 ( 1 Hz
and moves to higher frequencies (δ ) 7.46 ( 0.5 ppm) as a
result of the substitution of the chloride ion with triphenylphos-
phine. The six-membered cyclometalated rings derived from the
6-benzyl-2,2′-bipyridine ligands adopt a boat conformation in
the solid state,30-32 but in solution, at room temperature, they
exhibit a fluxional motion between two boatlike conformations
as shown by the equivalence of the two protons of the methylene
group in compounds 6 and 15 and of the two methyl groups in
1
compounds 9 and 18. The H NMR spectra of the complexes
[Pt(bipyREt)Cl] (8) and [Pt(bipyREt)(PPh3)]Cl (17) are of interest
in that two resonances are observed for the diastereotopic
methylene protons of the ethyl substituent, due to the presence
of the chiral R carbon.
The 31P{1H} NMR spectra of the cationic phosphine [Pt(N-
N-C)(PPh3)]Cl derivatives show a singlet signal with an
average value centered at δ ) 20.9 ( 2 ppm. The values of the
1JPtP coupling constants are in a range typical for a phosphorus
atom trans to a pyridine nitrogen donor atom,33 and setting apart
the rogue values for complexes 11 and 14, they encompass a
1
range of 165 Hz with JPtP ) 4390 ( 60 Hz. The large
deviations from this value for compound 11 (1JPtP ) 4078 Hz)
and for compound 14 (1JPtP ) 4098 Hz) could be associated
with stereoelectronic effects arising from the smaller five-
(30) Cinellu, M. A.; Zucca, A.; Stoccoro, S.; Minghetti, G.; Manassero,
M.; Sansoni, M. J. Chem. Soc., Dalton Trans. 1996, 4217-4225.
(31) Minghetti, G.; Cinellu, M. A.; Chelucci, G.; Gladiali, F.; Demartin,
F.; Manassero, M. J. Organomet. Chem. 1986, 307, 107.
(32) Cinellu, M. A.; Minghetti, G.; Pinna, M. V.; Stoccoro, S.; Zucca, A.;
Manassero, M. J. Chem. Soc., Dalton Trans. 1999, 2823.
(33) Appleton, T. G.; Clark, M. C.; Manzer, L. E. Coord. Chem. ReV. 1973,
10, 335.

Role of Cyclometalation in Ligand Substitution
Inorganic Chemistry, Vol. 39, No. 21, 2000 4753
Table 2. Second-Order Rate Constants of Chloride for Triphenyl-
phosphine Substitution on Cyclometalated [Pt(N-N-C)Cl] (2, 3,
5-9) and Uncyclometalated [Pt(N-N)(C)Cl] (1, 4) Complexesa
complex k2, M-1 s-1 complex k2, M-1 s-1 complex k2, M-1 s-1
1
2
3
1854 ( 37
903 ( 18
311 ( 5
4
5
6
343 ( 3
65610 ( 267
1046 ( 6
7
8
9
166 ( 1
85 ( 2
45.5 ( 0.9
a By spectrophotometric kinetic runs carried out in dichloromethane
at T ) 298.16 K.
from linear regression analysis of the dependence of kobs on
[PPh3], are listed in Table 2, together with their standard
deviations.
Ring-Opening Reactions. Under the same experimental
conditions adopted for the cyclometalated complexes, the
compounds 1, 4, and [Pt(terpy)Cl], after the first fast chloride
for phosphine substitution, undergo dechelation in a series of
subsequent slower steps. In fact, the addition of a slight excess
of phosphine to 1 leads to the formation of trans-[Pt(PPh3)2-
(Me)Cl]34 and free 2,2′-bipyridine. Similar results were obtained
for the phenyl derivative 4, with formation of trans-[Pt(PPh3)2-
(Ph)Cl].35 To shed some light on the sequence of dechelation
steps on the cationic complex, avoiding any possible interference
by chloride ion, we synthesized compounds 20 and 22 contain-
Figure 1. Dependence of the pseudo-first-order rate constants (kobsd
)
on phosphine concentration for the chloride substitution from cyclo-
metalated [Pt(N-N-C)Cl] and uncyclometalated [Pt(N-N)(C)Cl]
complexes (in dichloromethane, at 298.2 K). Numbers refer to the
compounds as listed in Chart 1.
-
ing the weakly coordinating CF3SO3 counteranion. Addition
of the stoichiometric amount of phosphine to 20 in CDCl3 at
220 K led to the formation of trans-[Pt(PPh3)2(η1-bipy)(Me)]CF3-
SO3.36 On adding further small amounts of phosphine, the
monodentate nitrogen ligand was easily displaced, yielding [Pt-
membered cyclometalated rings, but this hypothesis deserves
confirmation.
2
(PPh3)3(Me)]CF3SO3 (1H NMR: δ 0.30 (m, JPtH ) 56.0 Hz,
3H, CH3Pt). 31P{1H} NMR: δ 29.1 (1JPtP ) 2929 Hz); 20.8
Substitution Reactions. The substitution of chloride for
triphenylphosphine from the cyclometalated complexes [Pt(N-
N-C)Cl] (N-N-CH ) bipyt (2), bipyn (3), bipyφ (5), bipyâ
(6); bipyRMe (7), bipyREt (8), bipyc (9)) takes place in a single
step according to the reaction
2
(1JPtP ) 1915 Hz; JPP ) 20 Hz)). Any attempt to reveal the
presence of the transient cis-[Pt(PPh3)2(η1-bipy)(Me)]CF3SO3
complex, containing 2,2′-bipyridine bound in an η1-mode, was
unsuccessful. This is probably due to the fact that its conversion
into the most thermodynamically stable trans isomer is very
fast. The mechanism of geometrical isomerization of organo-
metallic diphosphine complexes of the type [Pt(PEt3)2(Me)X]
(X ) halide ion) has been studied in great detail and proved to
be dissociative in nature in protic solvents.37 However, these
species are relatively stable in nonpolar solvents as well as their
cationic [Pt(PEt3)2(Me)(am)]X derivatives. Most likely, the fast
geometrical conversion of the transient cis-[Pt(PPh3)2(η1-bipy)-
(Me)]+ could be associated with a fast exchange between the
free and the bound ends of the nitrogen chelating ligand. This
hypothesis is currently checked in our laboratory. On reacting
[Pt(bipy)(Me2SO)(Me)]CF3SO3 (19) with a chelating phosphine
(1,2-diphosphinoethane, dppe), it is possible to isolate easily
the [Pt(dppe)(η1-bipy)(Me)]CF3SO3 complex, a stable chelated
analogue of the transient bisphosphine intermediate. Its NMR
spectrum features the proton pattern of bound η1-2,2′-bipyridine
and of the methyl group and two 31P resonances at δ 36.3 (1JPtPa
) 4078 Hz) and 49.4 (1JPtPb ) 1838 Hz; Pa, phosphorus atom
[Pt(N-N-C)Cl] + PPh3 f [Pt(N-N-C)(PPh3)]Cl (1)
The fused [5,5]- and [5,6]-ring systems are robust, and there
is no evidence of ring opening or of other concurrent processes.
1
The H and the 31P{1H} NMR spectra taken after completion
of the reactions showed that the process under study was indeed
the simple replacement of chloride with a phosphine.
The spectral changes of reaction 1 occur in the near-visible
region between 500 and 320 nm and are associated mainly with
the disappearance of the starting complex. The systematic
kinetics of these reactions were studied at different ligand
concentrations and required the use of stopped-flow techniques.
At the concentrations of phosphine used, the reactions went to
completion and excellent fits were obtained from the regression
analysis of the absorbance vs time data. The dependence of the
pseudo-first-order rate constants (Table S2, in the Supporting
Information) is described by a family of straight lines (Figure
1) and obeys eq 2.
(34) 1H NMR (CDCl3): δ 7.73 (m, 12H, Ho,o′(P)); 7.41 (m, br, 12H, Hm,m′
-
2
(P)); 7.39 (m, br, 6H, Hp(P)); -0.08 (m, JPtH ) 78.6 Hz, 3H, CH3-
kobs ) k1 + k2[PPh3]
(2)
Pt). 31P{1H} NMR: δ 29.8 (1JPtP ) 3150 Hz).
(35) 1H NMR (CDCl3): δ 7.53 (d, 3JHH ) 6.2 Hz, 12H, Ho,o′(P)); 7.34 (m,
3
3Jav ) 6.9 Hz, 6H, Hp(P)); 7.25 (m, Jav ) 6.9 Hz, 12H, Hm,m′(P));
The contribution of the reagent-independent term k1 is
negligible (k1 refers to a solvolytic path, and k2 is the second-
order rate constant for the bimolecular attack of PPh3 on the
substrate). The absence of a k1 term indicates that the poor
nucleophile dichloromethane, as expected, cannot provide a
solvolytic pathway to the products, but also proves the absence
of a possible parallel dissociative pathway. The values of k2,
6.67 (d, 3JPtH ) 55.0 Hz, 3JHH ) 7.4 Hz, 2H, H2′′,6′′); 6.29 (dd, 3Jav
)
8.0 Hz, 1H, H4′′); 6.12 (dd, 3Jav ) 7.6 Hz, 2H, H3′′,5′′). 31P{1H} NMR:
δ 25.0 (1JPtP ) 3151 Hz).
(36) 1H NMR (CDCl3): δ 8.39 (s, br, 2H); 7.72 (s, br, 2H); 7.50-7.15
(m, br, 32H); 7.08 (s, br, 2H); 0.17 (s, br, 2JPtH ) 68.4 Hz, 3H, CH3-
Pt).31P{1H} NMR: δ 27.0 (1JPtP ) 3209 Hz).
(37) Romeo, R.; Alibrandi, G. Inorg. Chem. 1997, 36, 4822 and references
therein.

4754 Inorganic Chemistry, Vol. 39, No. 21, 2000
Romeo et al.
trans to the pyridyl nitrogen; Pb, phosphorus atom trans to the
methyl group).
respect to the complex containing 1,5-diamino-3-azapentane
(dien) as chelating ligand.4a The origin of this rate enhancement
lies in the noticeable π-acceptor capability of the terpy ligand.
By relieving the excess of electron density at the metal, the
polypyridine facilitates the addition of the incoming nucleophile
and the stabilization of a 18-electron five-coordinate transition
state. A marked increase of reactivity was observed also on
going from an a diamine to an R,R′-diimine,3 and it again reflects
the increase of electrophilicity of the reaction center as the ability
to transfer electron density to the ancillary ligands increases.
Following this reasoning, the decreased reactivity of the [Pt-
(N-N-C)Cl] complexes with respect to [Pt(N-N-N)Cl]+ (N-
N-N ) terpy), except for [Pt(bipyφ-H)Cl] as we will see below,
is a combined effect of a partial loss of aromaticity of the
terdentate ligand, due to the formation of a cyclometalated ring,
and of a specific cis effect of the carbon atom. Electron donation
by the strong σ-donor carbon group can favor a sufficient
accumulation of electron density at the metal to compensate
the π-electron withdrawal by the bipyridyl moiety.
Factors of primary importance in controlling the reactivity
and the mode of activation of complexes containing a planar
R,R′-diimine, such as 2,2′-bipyridine, are the presence of an
extensive π-system on the chelating ligand and the ease with
which this π-system interacts with the nonbonding d electrons
of the metal. The sequence of rate data in Table 2 strongly
suggests that the introduction of a cyclometalated ring has little
effect on these factors and on the reactivity. Setting apart the
case of [Pt(bipyφ-H)Cl], the lability of the chloride appears to
be affected very little by changes in the nature and in the
structural properties of the coordinated N-N-C or (N-N)(C)
ligands, the difference of reactivity between the first and the
last members of the series in Table 2 being less than 2 orders
of magnitude (for complexes 1 and 9 the values of k2 are 1850
and 45 M-1 s-1, respectively).
On going from [Pt(bipy)(CH3)Cl] (1) to [Pt(bipyt-H)Cl] (2)
the second-order rate constant (k2 ) 1850 M-1 s-1) is reduced
by half (k2 ) 903 M-1 s-1) and becomes 6 times slower for
[Pt(bipyn-H)Cl] (3) (k2 ) 311 M-1 s-1). Thus, the closure of
an alkyl chain to form a five- or six-membered ring does not
cause a great difference of reactivity with respect to compound
1. Both the size of the ring and the position of the two methyl
substituents on the aliphatic chain play little if any role in
controlling the reactivity. The increase of steric hindrance by
the methyl substituents is brought about at the periphery of the
coordination plane, and it does not significantly affect the rate
of attack of the reagent on the metal.
The reactivity of [Pt(bipy)(Ph)Cl] (4), in which the phenyl
ring lies perpendicular to the square coordination plane, is 6
times smaller than that of [Pt(bipy)(CH3)Cl] (1) (k2 ) 343 M-1
s-1). The magnitude of the trans and cis effect of the methyl
and phenyl groups being comparable, the moderate decrease of
rate is steric in nature, and it can be ascribed to the additional
difficulty of the nucleophile in approaching the metal center
and in forming the new bond. Similar results were obtained for
substitution reactions on trans-[Pt(PEt3)2(R)Cl]41 and in the
protonolysis of alkylarylplatinum(II) complexes.42
The same procedure of a stepwise addition of phosphine
applied to [Pt(terpy)Cl]Cl in CDCl3/CD3OD (1/4, v/v) revealed
the presence of only one relatively stable open-ring species in
solution, [Pt(η2-terpy)(PPh3)2]Cl2 (31P{1H} NMR: δ 18.8 (1JPtPa
2
) 3701 Hz), 7.30 (1JPtPb ) 3286 Hz; JPP ) 19 Hz)), in
agreement with other cases in which terpy coordinates in a
bidentate fashion.38,39 Complete dechelation yields [Pt(PPh3)3-
1
Cl]Cl (31P{1H} NMR: δ 27.3 (d, JPtP ) 2450 Hz), 16.8 (t,
2
1JPtP ) 3643 Hz; JPP ) 19 Hz)) and free terpyridine ligand.
Discussion
In keeping with the usual pattern of behavior for square planar
platinum(II) complexes,40 the chloride for phosphine substitution
from cyclometalated [Pt(N-N-C)Cl] complexes takes place
with an associative mode of activation. The process is entirely
dominated by the direct attack of the nucleophile on the
substrate, and the values of the second-order rate constants k2,
collected in Table 2, offer an interesting structure-reactivity
relationship. Before this set of rate data is examined in detail,
it is of interest to focus on some attractive features emerging
from the comparison between the behavior exhibited by the
cyclometalated compounds and that of the other compounds
investigated. Interestingly, upon nucleophilic attack, the ter-
dentate N-N-C donor skeleton remains firmly bound to the
metal center and the chloride ion is the only labile group
undergoing substitution. Thus, the fused [5,5]- and [5,6]-ring
systems formed by the cyclometalated N-N-C ligands are
better stabilizing than the [5,5] N-N-N ring system formed
by the terdentate terpy ligand. The latter, as described above,
is easily removed from the metal in a series of consecutive
substitution steps. We can exclude, however, that the anionic
character of the N-N-C must be held responsible for its
stability because the strictly similar set of (N-N)(C) ligands,
in which the donor carbon atom is not contained in an
intramolecular coordination system, is by far less stable. In fact,
in compounds 1 and 4, chloride substitution by phosphine is
followed by subsequent substitution steps which involve the
detachment of one arm of 2,2′-bipyridine, fast geometrical cis
to trans isomerization of the transient species in which the
bidentate ligand is coordinated in a monodentate fashion, and,
eventually, the complete removal of the chelating nitrogen
ligand. Since the carbon atom of [Pt(N-N-C)Cl] (as well as
that of [Pt(N-N)(C)Cl]) is hardly removed from the coordina-
tion sphere upon nucleophilic attack, it puts an anchoring effect
into action which avoids the detachment of the bipyridine
molecular fragment.
The cation [Pt(terpy)Cl]+ is characterized by extensive
planarity and π-acceptor properties of the terdentate nitrogen
ligand. We find that the rate of chloride substitution by PPh3 in
dichloromethane is too fast to be followed, even with stopped-
flow techniques (k2 g 105 M-1 s-1). The extreme lability of
the chloride in this complex is not surprising, since previous
kinetic studies with nucleophiles weaker than PPh3, in methanol,
showed a rate enhancement of 5-6 orders of magnitude with
As pointed out by Constable et al.43 the ligand 6-phenyl-
2,2′-bipyridine behaves as a true cyclometalating analogue of
2,2′:6′,2′′-terpyridine. The molecular structure of the cation [Pt-
(38) Constable, E. C. Tetrahedron 1992, 48, 1013.
(39) Abel, E. W.; Dimitrov, V. S.; Long, N. J.; Orrel, K. G.; Osborne, A.
G.; Sik, V.; Hursthouse, M. B.; Mazid M. A. J. Chem. Soc., Dalton
Trans. 1993, 291.
(40) (a) Tobe, M. L.; Burgess, J. Inorganic Reaction Mechanisms; Addison
Wesley Longman: Essex, England, 1999. (b) Wilkins, R. G. Kinetics
and Mechanisms of Reactions of Transition Metal Complexes; VCH:
Weinheim, Germany. (c) Basolo, F. Coord. Chem. ReV. 1996, 154,
151.
(41) Cusumano, M.; Marricchi, P.; Romeo, R.; Ricevuto, V.; Belluco, U.
Inorg. Chim. Acta 1979, 34,169.
(42) Romeo, R.; Plutino, M. R.; Elding, L. I. Inorg. Chem. 1997, 36, 5909.
(43) Constable, E. C.; Henney, R. P. G.; Leese, T. A.; Tocher, D. A. J.
Chem. Soc., Chem. Commun. 1990, 513.

Role of Cyclometalation in Ligand Substitution
Inorganic Chemistry, Vol. 39, No. 21, 2000 4755
(bipyφ-H)(MeCN)]+ shows strict similarities with the structure
of terpy complex cations such as [Pt(terpy)Cl]+,44,45 [Pt(terpy)-
(CH2NO2)]+,46 and [Pt(terpy)(CH3)]+,47 as far as planarity and
bond lengths and angles are concerned. Likewise, this cation
forms discrete dimeric units held together by stacking interac-
tions between the aromatic ligands. Most importantly, the
equivalence between the pyridyl and the phenyl rings is notable.
In the compound [Pt(bipyφ-H)Cl] (5) the lability of the chloride
ion is very high (k2 ) 65 × 103 M-1 s-1), almost 2 orders of
magnitude higher than that of compound 4, and this sharp
increase of reactivity finds its origin in the aromaticity of the
cyclometalated 6-phenyl-2,2′-bipyridine ligand, comparable to
or not much less than that of 2,2′:6′,2′′-terpyridine. Only in this
particular case, the in-plane phenyl ring concurs to the electron
withdrawal from filled d orbitals of the metal to empty π* of
the terdentate ligand.
Further evidence for a relationship between the lability of
chloride and the aromaticity of the terdentate ligand is given
by the fact that the introduction of a spacer group in the
cyclometalated ring, as for the benzyl derivatives, produces a
sharp decrease in rate with respect to compound 5. The
compound [Pt(bipyâ-H)Cl] (k2 ) 1046 M-1 s-1) reacts at a rate
comparable to that of [Pt(bipy)(CH3)Cl], and the rate is still
reduced as a result of methyl ([Pt(bipyRMe-H)Cl], k2 ) 166 M-1
s-1), ethyl ([Pt(bipyREt-H)Cl], k2 ) 85 M-1 s-1), and dimethyl
([Pt(bipyc-H)Cl], k2 ) 45 M-1 s-1) substitution on the methylene
carbon. Since the six-membered cycle adopts a boat conforma-
tion, the substituent alkyl groups are directed toward the metal
atom and their proximity to the reaction center is at the origin
of the steric retardation.
by the planar diimine is crucial in controlling the reactivity and
activation mode of compound 1. The introduction of a cyclo-
metalated in-plane aryl ring, separated through a spacer meth-
ylene group from the bipy moiety, hardly affects the rate of
chloride substitution. This fact rules out any effective back-
donation toward the aromatic π*-orbitals in the 18-e five-
coordinated transition state. In principle, some flow of electron
density from the metal to the in-plane aryl ring of 2 cannot be
excluded. However, all the kinetic evidence11,15 and the
comparison with similar noncyclometalated organometallic
systems suggest that the presence of the strong trans-labilizing
platinum-carbon σ bond is the main, if not the only, factor
responsible for the high lability of H2O in compound 2.
Conclusions
The lability of chloride in platinum(II) [Pt(N-N-C)Cl]
complexes, containing terdentate N-N-C anionic ligands
derived from deprotonated 6-substituted 2,2′-bipyridines, de-
pends very little on the various factors which characterize a
cyclometalated ring, such as its size, the nature of the Pt-C
bond (whether metalation comes from phenyl or alkyl C-H
activation), and the number of substituents on the ring. The main
factor controlling the reactivity is the electrophilicity of the
metal, as dictated by the strong π-electron withdrawal of the
2,2′-bipyridine fragment. There is not a specific role of
cyclometalation in increasing the reactivity, as shown by the
comparison with the lability of chloride in the [Pt(N-N)(C)-
Cl] (C ) CH3 or C6H5) complexes having the same set of donor
atoms but less constrained arrangements of the ligands. The only
exception to this pattern of behavior is constituted by the
complex [Pt(bipyφ-H)Cl], which experiences a significant rate
enhancement with respect to [Pt(bipy)(Ph)Cl]. In the latter the
in-plane phenyl ring of the 6-phenyl-2,2′-bipyridine ligand
becomes part of an extensive π-acceptor system, like that of
the nitrogen terdentate 2,2′:6′,2′′-terpyridine. The 2,2′-biphenyl
dianion (bph2-) ligand, which is a cyclometalating analogue of
2,2′-bipyridine, behaves in a completely different way. In the
complexes [Pt(bph)(SR2)2], back-donation into the empty π*-
orbitals of the in-plane aryl rings does not come into play, and
the mechanism of ligand exchange is dissociative.
Finally, it is of interest to compare briefly the behavior of
platinum(II) complexes with N-N-C and N-C-N anionic
ligands, at least as far as the lability of the fourth coordinated
group is concerned. Both the cyclometalated compounds 1 and
2 exhibit high reactivity. The results of this study show that
Acknowledgment. We thank the Ministero dell’Universita`
e della Ricerca Scientifica e Tecnologica (MURST), Programmi
di Ricerca Scientifica di Rilevante Interesse Nazionale, Cofi-
nanziamento 1998-9, the Universita` degli Studi di Messina and
Sassari, and CNR for funding this work.
the relief of electron density from the metal through π-bonding
Supporting Information Available: Tables listing resonances of
(44) Bailey, J. A.; Hill, M. G.; Marsh, R. E.; Miskowski, V. M.; Schaefer,
W. P.; Gray, H. B. Inorg. Chem. 1994, 34, 4591.
(45) Yip, H.-K.; Cheng, L.-K.; Cheung, K.-K.; Che, C.-M. J. Chem. Soc.,
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(46) Akiba, M.; Sasaki, Y. Inorg. Chem. Commun. 1998, 1, 61.
(47) Romeo, R.; Monsu` Scolaro, L.; Plutino, M. R.; Albinati, A. J.
Organomet. Chem. 2000, 593-594, 403.
compounds 1-18 and observed pseudo-first-order rate constants (kobsd
/
s-1) for nucleophilic substitution on compounds 1-9 as a function of
[PPh3]. This material is available free of charge via the Internet at
http://pubs.acs.org.
IC0004479