Rates of Dimethyl Sulfoxide Exchange in Monoalkyl Cationic Platinum(II) Complexes Containing Nitrogen Bidentate Ligands. A Proton NMR Study

Article abstract of DOI:10.1021/ic960107g

A series of monoalkyl square-planar complexes of the type [Pt(N-N)(CH₃)(Me₂SO)]PF₆ (1-14), where N-N represents chelating diamines or diimines of widely different steric and electronic characteristics, was synthesized, and the complexes were fully characterized as solids and in solution. The substrates were tailored to offer only one site of exchange to a neutral molecule, i.e. Me₂SO, in a noncoordinating solvent. No evidence for fluxionality of the N-N ligands was found, except for the case of complex 11 formed by 2,9-dimethyl-1,10-phenanthroline. In solution this complex is fluxional with the phenanthroline oscillating between nonequivalent bidentate modes by a mechanism which involves rupture of the metal-nitrogen bond and rapid interconversion of two coordinatively unsaturated T-shaped 14-electron three-coordinate molecular fragments. Rates of this fluxion were measured by NMR spectroscopy from the exchange effects on the ¹H signals of the methyl and aromatic hydrogens. The ΔG? value for the fluxion is 49.6 ± 4 kJ mol^-1. Dimethyl sulfoxide exchange with all the complexes has been studied as a function of ligand concentration by ¹H NMR line-broadening, isotopic labeling, and magnetization transfer experiments with deuterated acetone as the solvent. Second-order rate constants were obtained from linear plots of k_obs vs [Me₂SO] and activation parameters were obtained from exchange experiments carried out at different temperatures. Second-order kinetics and negative entropies of activation indicate an associative mechanism. The lability of dimethyl sulfoxide in the complexes depends in a rather unexpected and spectacular way upon the nature of the coordinate N-N ligands, the difference in reactivity between the first (N-N = N,N,N',N',-tetramethyl-1,2-diaminoethane, k₂²⁹⁸ = (1.15 ± 0.1) × 10^-6 mol^-1 s^-1) and the last (N-N = 2,9-dimethyl-1,10-phenanthroline, k₂²⁹⁸ = (381 ± 0.005) × 10⁴ mol^-1 s^-1) members of the series being greater than 10 orders of magnitude, as a result of a well-known phenomenon of steric retardation (for the first complex) and an unprecedented case of steric acceleration (for the last complex). Other factors of primary importance in controlling the reactivity are (i) the presence of an extensive π system on the ligand N-N, (ii) the ease with which this π system interacts with nonbonding d electrons of the metal, and (iii) the flexibility and ease of elongation of the chelate bite distance. The basicity plays a somewhat minor role, except in the restricted range of the same class of compounds such as substituted phenanthrolines.

Full text of DOI:10.1021/ic960107g

Inorg. Chem. 1996, 35, 5087-5096
5087
Rates of Dimethyl Sulfoxide Exchange in Monoalkyl Cationic Platinum(II) Complexes
Containing Nitrogen Bidentate Ligands. A Proton NMR Study
Raffaello Romeo,* Luigi Monsu` Scolaro, Nicola Nastasi, and Giuseppe Arena
Dipartimento di Chimica Inorganica, Analitica e Chimica Fisica and Istituto Chimica Tecnologia dei
Prodotti Naturali (ITCPN-CNR) Sezione di Messina, Universita` di Messina, Salita Sperone 31, Vill. S.
Agata, 98166 Messina, Italy
ReceiVed January 31, 1996X
A series of monoalkyl square-planar complexes of the type [Pt(N-N)(CH3)(Me2SO)]PF6 (1-14), where N-N
represents chelating diamines or diimines of widely different steric and electronic characteristics, was synthesized,
and the complexes were fully characterized as solids and in solution. The substrates were tailored to offer only
one site of exchange to a neutral molecule, i.e. Me2SO, in a noncoordinating solvent. No evidence for fluxionality
of the N-N ligands was found, except for the case of complex 11 formed by 2,9-dimethyl-1,10-phenanthroline.
In solution this complex is fluxional with the phenanthroline oscillating between nonequivalent bidentate modes
by a mechanism which involves rupture of the metal-nitrogen bond and rapid interconversion of two coordinatively
unsaturated T-shaped 14-electron three-coordinate molecular fragments. Rates of this fluxion were measured by
NMR spectroscopy from the exchange effects on the 1H signals of the methyl and aromatic hydrogens. The ∆Gq
value for the fluxion is 49.6 ( 4 kJ mol-1. Dimethyl sulfoxide exchange with all the complexes has been studied
1
as a function of ligand concentration by H NMR line-broadening, isotopic labeling, and magnetization transfer
experiments with deuterated acetone as the solvent. Second-order rate constants were obtained from linear plots
of kobs Vs [Me2SO] and activation parameters were obtained from exchange experiments carried out at different
temperatures. Second-order kinetics and negative entropies of activation indicate an associative mechanism. The
lability of dimethyl sulfoxide in the complexes depends in a rather unexpected and spectacular way upon the
nature of the coordinate N-N ligands, the difference in reactivity between the first (N-N ) N,N,N′,N′-tetramethyl-
1,2-diaminoethane, k2298 ) (1.15 ( 0.1) × 10-6 mol-1 s-1) and the last (N-N ) 2,9-dimethyl-1,10-phenanthroline,
298
k2 ) (3.81 ( 0.005) ×104 mol-1 s-1) members of the series being greater than 10 orders of magnitude, as a
result of a well-known phenomenon of steric retardation (for the first complex) and an unprecedented case of
steric acceleration (for the last complex). Other factors of primary importance in controlling the reactivity are (i)
the presence of an extensive π system on the ligand N-N, (ii) the ease with which this π system interacts with
nonbonding d electrons of the metal, and (iii) the flexibility and ease of elongation of the chelate bite distance.
The basicity plays a somewhat minor role, except in the restricted range of the same class of compounds such as
substituted phenanthrolines.
Introduction
Another theme of great chemical interest comes from the ease
with which dimethyl sulfoxide can be displaced from square
Neutral and cationic square planar complexes of general
structure [M(N-N)(CH3)X] (M ) Pd(II); N-N ) bidentate
nitrogen ligand; X ) Cl-, CF3SO3-, MeCN) are known to be
good catalysts for the homogeneous copolymerization of olefins
with carbon monoxide.1,2 A key step in the mechanism of the
alternating insertion of CO and alkenes is the removal of the
coordinated X group from the coordination sphere of the metal
promoted by the steric and electronic characteristics of the
“spectator” N-N ligands. Compounds of platinum(II) of the
same type are less useful as catalysts, altough interesting
reactions with alkenes, alkynes, and 1,2- and 1,3-dienes have
been reported.3
planar platinum(II) complexes.4,5 Cationic complexes of the
type [Pt(diam)(Me2SO)Cl]Cl are easily formed by reaction of
diamines with cis-[PtCl2(Me2SO)2] and have proved to be useful
intermediates in the synthesis of neutral antitumor complexes
with the general formula [Pt(diam)Cl2].6,7 The labilization of
the bound sulfoxide is a crucial step in the synthetic procedure.
(3) (a) De Felice, V.; Cucciolito, M. E.; De Renzi, A.; Ruffo, F.; Tesauro,
D. J. Organomet. Chem. 1995, 493, 1. (b) De Felice, V.; De Renzi,
D.; Tesauro, D.; Vitagliano, A. Organometallics 1992, 11, 3669. (c)
De Felice, V.; De Renzi, A.; Giordano, F; Tesauro, D.; J. Chem. Soc.,
Dalton Trans 1993, 1927.
(4) (a) Romeo, R.; Cusumano, M. Inorg. Chim. Acta 1981, 49, 167. (b)
Lanza, S.; Minniti, D.; Romeo, R.; Tobe, M. L. Inorg. Chem. 1983,
22, 2006. (c) Lanza, S.; Minniti, D.; Moore, P.; Sachinidis, J.; Romeo,
R.; Tobe, M. L. Inorg. Chem. 1984, 23, 4428. (d) Romeo, R.; Minniti,
D.; Alibrandi, G.; De Cola L.; Tobe M. L. Inorg. Chem. 1986, 25,
1944. (e) Minniti, D.; Alibrandi, G.; Tobe, M. L.; Romeo, R. Inorg.
Chem. 1987, 26, 3956. (f) Alibrandi, G.; Romeo, R.; Monsu` Scolaro,
L.; Tobe, M. L. Inorg. Chem. 1992, 31, 3061.
(5) (a) Bonivento, M.; Canovese, L.; Cattalini, L.; Marangoni, G.;
Michelon, G.; Tobe, M. L. Inorg. Chem. 1981, 20, 1493. (b) Fanizzi,
F. P.; Natile, G.; Maresca, L.; Manotti-Lanfredi, A. M.; Tiripicchio,
A. J. Chem. Soc., Dalton Trans 1984, 1467. (c) Annibale, G.;
Canovese, L.; Chessa, G.; Cattalini, L.; Tobe, M. L. J. Chem. Soc.,
Dalton Trans 1990, 405.
X Abstract published in AdVance ACS Abstracts, July 1, 1996.
(1) (a) Rix, F. C.; Brookhart, M. J. Am. Chem. Soc. 1995, 117, 1137.
Brookhart, M.; Wagner, M. I.; Balavoine, G. G. A.; Ait Haddou, H.
J. Am. Chem. Soc. 1995, 116, 3641. (b) Brookhart, M.; Rix, F. C.;
DeSimone, J. M.; Barborak, J. C. J. Am. Chem. Soc. 1992, 114, 5894.
(2) (a) van Asselt, R.; Gielens, E. E. C. G.; Rulke, R. E.; Vrieze, K.;
Elsevier, C. J. J. Am. Chem. Soc. 1994, 116, 977. (b) de Graaf, W.;
Boersma, J; van Koten, G. Organometallics 1990, 9, 1479. (c) Rulke,
R. E.; Han, I. M.; Elsevier, C. J. Vrieze, K.; van Leeuwen, P. W. N.
M.; Roobeek, C. F.; Zoutberg, M. C.; Wang, Y. F.; Stam, C. H. Inorg.
Chim. Acta 1990, 169, 5.
S0020-1669(96)00107-3 CCC: $12.00 © 1996 American Chemical Society

5088 Inorganic Chemistry, Vol. 35, No. 17, 1996
Romeo et al.
Recently, this class of cationic complexes was found to exhibit
by itself chemiotherapeutic activity,8 offering the advantage of
being more soluble in water than the neutral complexes. Here
again the role of the sulfoxide as leaving group is of overriding
importance in order to obtain some insight into the mechanism
of antitumor activity.
Chart 1
In searching for a correlation between the lability of substrates
and the nature of the ancillary ligands, we thought it of interest
to carry out a kinetic study of the exchange reaction
[Pt(N-N)(CH3)(Me2SO)]+ + Me2SO* f
[Pt(N-N)(CH3)(Me2SO)*]+ + Me2SO (1)
where N-N represents a series of chelating diamines or diimines
of widely different steric and electronic properties. Kinetic
studies of solvent exchange on square planar complexes are
scarce.9 The systems studied so far include [Pd(H2O)4]2+ 10
,
[Pt-
(H2O)4]2+ 11
DMF, MeCN, MeNC, Me2S, Et2S),12 [Pd(NH3)4]2+ 13
(MeCN)4]2+ 12 [Pd(CN)4]2- 14 [Pt(CN)4]2- 14
[Pt(1,4-dithiane)2]2+ 15 [Pd(dien)(H2O)]2+ 16
O)]2+ 16 trans-[Pd(Me2S)2Cl2],17 [Pt(Me2SO)2(Me2SO)2]2+ 18
,
non-aqueous tetrasolvates [PdS4]2+ (S ) DMA,
,
[Pt-
,
,
,
,
[Pt(Me2S)4]2+ 15
,
,
[Pd(Et4dien)(H2-
and
,
,
the organometallic complexes cis-[PtR2(Me2S)2]19 and cis-
[PtR2(Me2SO)2]19 (R ) alkyls and aryls). Associative10-18 and
dissociative pathways19 for the exchange have been demon-
strated.
In the complexes of this study the presence of a methyl group
ensures that only the sulfoxide can undergo the substitution
process and provides an array of donor atoms strictly similar
to that of the palladium catalysts.1,2 The organometallic
complexes were synthesized and fully characterized as solids
and in solution. The range of reactivity was so large as to
require the use of various NMR techniques, including isotopic
labeling, magnetization transfer, and line broadening, taking
advantage of the relatively large difference of chemical shift
between the proton signals of coordinated and free sulfoxide.
The overall structure-reactivity relationship for these com-
plexes is presented. The results are discussed within the
framework of an associative mechanism and provide direct
evidence for the great importance of the steric and electronic
properties of the ancillary N-N ligands in governing the lability
of the substrates. The difference in reactivity between the first
and the last members of the series is greater than 10 orders of
magnitude. This enormous variation of lability of the sulfoxide
will be discussed in connection with some factors affecting the
energy of a 5-coordinate transition state or intermediate such
as (i) the presence of an extensive π system on the ligand N-N,
(ii) the ease with which this π system interacts with nonbonding
d electrons of the metal, (iii) the flexibility of the N-N ligand,
and (iv) the relative importance of steric repulsions in the square
planar and in the trigonal bipyramidal configurations.
Experimental Section
Materials. Solvents used in the synthetic procedures were distilled
under nitrogen from appropriate drying agents (diethyl ether from
sodium benzophenone; dichloromethane from barium oxide; dimethyl
sulfoxide, at a low pressure, from CaH2, after preliminary filtration
through an alumina column) and then stored in dried, N2-filled flasks
over activated 4 Å molecular sieves. Acetone-d6 and dimethyl
sulfoxide-d6 were used as received from Aldrich Chemical Co.
Pyridine (py), ethylenediamine (en), and N,N,N′,N′-tetramethyleth-
ylenediamine (Me4en) were purified by distillation from KOH at normal
pressure. N,N′-dicyclohexylethylenediimine (cy2dim), and N,N′-diiso-
propylethylenediimine (pri2dim), were synthesised and purified accord-
ing to literature methods.20
All the other reagents were the highest grade commercially available
and were used as received or were purified by distillation or crystal-
lization when needed. Ligand abbreviations are given in Chart 1.
Synthesis of Complexes. All the complexes were synthesized under
a dry oxygen-free nitrogen atmosphere using standard Schlenk-tube
techniques. The reaction products are air stable and therefore they were
handled without particular precautions.
(6) Romeo, R.; Minniti D.; Lanza, S.; Tobe, M. L. Inorg. Chim. Acta
1977, 22, 87.
(7) (a) Fanizzi, F. P.; Maresca, L.; Natile, G.; Lanfranchi, M.; Manotti-
Lanfredi, A. M.; Tiripicchio, A. Inorg. Chem. 1988, 27, 2422 and
references therein.
(8) (a) Farrell, N.; Kiley, D. M.; Schmidt, W.; Hacker, M. Inorg. Chem.
1990, 29, 397. (b) Lempers, E. L. M.; Bloemink, M. J.; Reedijk, J.
Inorg. Chem. 1991, 30, 201.
(9) Merbach, A. E. Pure Appl. Chem. 1987, 59, 151.
(10) Helm, L.; Elding, L. I.; Merbach, A. E. HelV. Chim. Acta 1984, 67,
1453.
(11) Helm, L.; Elding, L. I.; Merbach, A. E. Inorg. Chem. 1985, 24, 1719.
(12) Hallinan, N.; Besancon, V.; Forster, M.; Elbaze, G.; Ducommun, Y.;
Merbach, A. E. Inorg. Chem. 1991, 30, 1112.
(13) Bronnum, B.; Johansen, H. S.; Skibsted, L. H. Acta Chem. Scand.
1989, 43, 975.
(14) Pesek, J. J.; Mason, R. W. Inorg. Chem. 1983, 22, 2958.
(15) Frey, M.; Elmorth, S.; Moullet, B.; Elding, L. I.; Merbach, A. E. Inorg.
Chem. 1991, 30, 5033.
(16) Berger, J.; Kotowski, M.; van Eldik, R.; Frey, U.; Helm, L.; Merbach,
A. E. Inorg. Chem. 1990, 29, 1719.
trans-[Pt(Me2SO)2(CH3)Cl] was prepared according to a published
method,21 and was crystallized several times from dichloromethane/
diethyl ether mixtures.
(17) Tubino, M.; Merbach, A. E. Inorg. Chim. Acta 1983, 71, 149.
(18) Ducommun, Y.; Helm, L.; Merbach, A. E.; Hellquist, B.; Elding, L.
I. Inorg. Chem. 1989, 28, 377.
(19) Frey, U.; Helm, L.; Merbach, A. E.; Romeo, R. J. Am. Chem. Soc.
1989, 111, 8161.
(20) Kliegman, J. M.; Barnes, R. K. Tetrahedron 1970, 26, 2555.
(21) Eaborn, C. E.; Kundu, K.; Pidcock, P. J. Chem. Soc., Dalton Trans
1981, 933.

Rates of Dimethyl Sulfoxide Exchange
Inorganic Chemistry, Vol. 35, No. 17, 1996 5089
[Pt(en)(Me2SO)(CH3)]PF6 (1). A solution of ethylenediamine (15
mg, 0.25 mmol) in methanol (2 mL) was added drop by drop under
stirring to a solution of trans-[Pt(Me2SO)2(CH3)Cl] (80.4 mg, 0.2 mmol)
in methanol (3 mL). After completion of the reaction, KPF6 (46 mg;
0.25 mmol) was added and the solution set aside for 2 h. A white
solid began to precipitate and the precipitation was completed by adding
Hz, 3H). Anal. Calcd for C13H18F6N3OPSPt: H, 3.00; C, 25.83; N,
6.95. Found: H, 3.10; C, 25.89; N, 7.10.
[Pt(dps)(Me2SO)(CH3)]PF6 (9). IR: ν(SdO) 1129 cm-1 1H NMR
.
3
3
(acetone-d6): δ 8.99 (d, JPtH ) 19.8 Hz, 1H), 8.79 (d, JPtH ) 45.9
Hz, 1H), 8.25 (m, 2H), 8.17 (m, 2H), 7.82 (m, 1H), 7.71 (m, 1H), 3.60
(s, 3JPtH ) 35.8 Hz, 3H), 3.49 (s, 3JPtH ) 35.8 Hz, 3H), 0.84 (s, 2JPtH
)
diethyl ether and cooling to -30 °C. IR: ν(SdO) 1110 cm-1 1H
.
75.6 Hz, 3H). Anal. Calcd for C13H17F6N2OPS2Pt: H, 2.76; C, 25.13;
N, 4.51. Found: H, 2.83; C, 25.02; N, 4.63. FABMS: m/e 475.9
[M+]+.
NMR (CD3OD/acetone-d6, 1:1, v:v): δ 5.25 (s, broad, 2H), 4.28 (s,
3
broad, 2H), 3.28 (s, JPtH ) 33 Hz, 6H), 3.09 (m, 2H), 3.03 (m, 2H),
2
0.32 (s, JPtH ) 76.5 Hz, 3H). Anal. Calcd for C5H17F6N2OPSPt: H
[Pt(phen)(Me2SO)(CH3)]PF6 (10). IR: ν(SdO) 1123 cm-1 1H
.
3.47; C, 12.17; N, 5.68. Found: H, 3.59; C, 12.01; N, 5.72.
NMR (acetone-d6): δ 10.15 (dd, 3JPtH ) 16.5 Hz, 1H), 9.46 (dd, 3JPtH
) 44.5 Hz, 1H), 9.16 (dd, 1H), 9.09 (dd, 1H), 8.39 (m, 2H), 8.35 (dd,
1H), 8.31 (dd, 1H), 3.75 (s, 3JPtH ) 36.8 Hz, 6H), 1.12 (s, 2JPtH ) 73.7
Hz, 3H). Anal. Calcd for C15H17F6N2OPSPt: H, 2.79; C, 29.37; N,
4.57. Found: H, 2.82; C, 29.41; N, 4.63.
The hexafluorophosphate salts of the complexes with the ligands
Me4en, py, and 2-ampy were prepared by following essentially the same
procedure used for complex 1. The solid compounds separated out in
a good yield from the reaction mixture on adding diethyl ether and
cooling.
[Pt(Me2phen)(Me2SO)(CH3)]PF6 (11). 1H NMR (acetone-d6): δ
[Pt(Me4en)(Me2SO)(CH3)]PF6 (2). IR: ν(SdO) 1128 cm-1
.
1H
4
3
8.82 (d, 2H), 8.20 (s, 2H), 8.04 (d, JPtH ) 6.6 Hz, 2H), 3.57 (s, JPtH
) 36.3 Hz, 6H), 3.10 (s, 4JPtH ) 4.8 Hz, 6H), 1.00 (s, 2JPtH ) 78.6 Hz,
3H). Anal. Calcd for C17H21F6N2OPSPt: H, 3.30; C, 31.83; N, 4.37.
Found: H, 3.32; C, 31.90; N, 4.52.
3
NMR (D2O): δ 3.30 (s, JPtH ) 36.3 Hz, 6H), 2.90 (m, 2H), 2.80 (s,
3
3JPtH ) 16.5 Hz, 6H), 2.74 (m, 2H), 2.67 (s, JPtH ) 44 Hz, 6H), 0.26
(s, 2JPtH ) 72.6 Hz, 3H). Anal. Calcd for C9H25F6N2OPSPt: H, 4.59;
C, 19.68; N, 5.10. Found: H, 4.71; C, 19.62; N, 5.00.
[Pt(Me4phen)(Me2SO)(CH3)]PF6 (12). IR: ν(SdO) 1126 cm-1
.
cis-[Pt(py)2(Me2SO)(CH3)]PF6 (3). 1H NMR (acetone-d6): δ 8.95
1H NMR (acetone-d6): δ 9.91 (s, 3JPtH unresolvable, 1H), 9.09 (s, 3JPtH
) 46.2 Hz, 1H), 8.48 (2H), 3.72 (s, 3JPtH ) 37.4 Hz, 6H), 2.96 (s, 3H),
2.87 (s, 3H), 2.74 (s, 3H), 2.68 (s, 3H), 1.04 (s, 2JPtH ) 72.6 Hz, 3H).
Anal. Calcd for C19H25F6N2OPSPt: H, 3.76; C, 34.08; N, 4.18.
Found: H, 3.70; C, 34.15; N, 4.09.
3
3
(d, JPtH unresolvable, 2H), 8.74 (d, JPtH ) 41.8 Hz, 2H), 8.07 (m,
3
2
2H), 7.63 (m, 4H), 3.41 (s, JPtH ) 34.1 Hz, 6H), 0.72 (s, 3H, JPtH
75.9 Hz, 3H).
)
[Pt(2-ampy)(Me2SO)(CH3)]PF6 (4). Two isomers with a relative
ratio 2:1. Pyridyl trans to Me2SO. IR: ν(SdO) 1128 cm-1 1H NMR
.
[Pt(NO2phen)(Me2SO)(CH3)]PF6 (13). IR: ν(SdO) 1131 cm-1
.
(CD3OD): δ 8.69 (d, 3JPtH ) 41.4 Hz, 1H), 8.16 (t, 1H), 7.77 (d, 1H),
7.56 (t, 1H), 4.43 (s, 3JPtH unresolvable, 2H), 3.44 (s, 3JPtH ) 35.1 Hz,
6H), 0.60 (s, 2JPtH ) 75.1 Hz, 3H). Pyridyl cis to Me2SO. IR: ν(SdO)
1H NMR analysis (acetone-d6) revealed the presence of two geometrical
isomers in an equimolar ratio: δ 10.34 (m, 2H), from 9.68 to 9.36 (m,
8H), 8.52 (m, 4H), 3.77 (s, 3JPtH ) 37.2 Hz, 12H), 1.17(s, 2JPtH ) 73.8
Hz, 3H), 1.15 (s, 2JPtH ) 73.7 Hz, 3H). Anal. Calcd for C15H16F6N3O3-
PSPt: H, 2.45; C, 27.36; N, 6.38. Found: H, 2.35; C, 27.49; N, 6.42.
1110 cm-1
.
1H NMR (CD3OD): δ 9.39 (d, 3JPtH ) 20.0 Hz, 1H), 8.08
3
(t, 1H), 7.71 (d, 1H), 7.50 (t, 1H), 4.41 (s, JPtH unresolvable, 2H),
3
2
3.42 (s, JPtH ) 34.4 Hz, 6H), 0.60 (s, JPtH ) 75.1 Hz, 3H).
[Pt(Ph2phen)(Me2SO)(CH3)]PF6 (14). IR: ν(SdO) 1123 cm-1
.
1H
)
3
3
[Pt(cy2dim)(Me2SO)(CH3)]PF6 (5). A weighed amount of trans-
[Pt(Me2SO)2(CH3)Cl] (0.201 g, 0.5 mmol) was reacted in dimethyl
sulfoxide (2 mL) with AgPF6 (135 mg; 0.53 mmol). After the mixture
was stirred for a few hours, AgCl was filtered off and 0.115 g (0.52
mmol) of dicyclohexylethylenediimine (cy2dim) were added to the
solution. The color turned from pale yellow to orange. The solution
was set aside overnight, some additional solid silver chloride that settled
out was separated by centrifugation, and the excess solvent was
evaporated under reduced pressure (0.1 mmHg, 70 °C). The solid
residue was dissolved in dichloromethane, the solution was filtered on
a cellulose column to remove residual AgCl, and the orange product
NMR (acetone-d6): δ 10.24 (d, JPtH ) 19 Hz, 1H), 9.52 (d, JPtH
45.6 Hz, 3JHH ) 5.8 Hz, 1H), 7.71 (m, 14H), 3.78 (s, 3JPtH ) 36.3 Hz,
2
6H), 1.16 (s, JPtH ) 73.7 Hz, 3H). Anal. Calcd for C27H25F6N2-
OPSPt: H, 3.29; C, 42.36; N, 3.66. Found: H, 3.37; C, 42.45; N,
3.72.
Instrumentation and Measurements. Infrared spectra were re-
corded as Nujol mulls in the range 4000-200 cm-1 using CsI disks on
a Perkin Elmer FT-IR Model 1730 spectrometer. 1H NMR spectra
were obtained on a Bruker AMX R-300 spectrometer equipped with a
broad-band probe operating at 300.13 MHz. 1H chemical shifts are
measured relative to the residual solvent peak and are reported in δ
units downfield from Me4Si. The temperature within the probe was
checked using the methanol or ethylene glycol method.22 Fast atom
bombardment mass spectrometry (FABMS) was performed in a glycerol
matrix with a Kratos instrument equipped with a standard FAB source.
Microanalysis were performed by Analytical Laboratories, Engel-
skirchen. The activation parameters ∆Hq and ∆Sq were calculated by
linear regression analysis of Eyring plots.
separated out on adding diethyl ether and cooling at -30 °C. IR:
3
ν(SdO) 1135 cm-1
.
1H NMR (acetone-d6): δ 9.00 (s, JPtH ) 47.7
3
3
Hz, 1H), 8.91 (s, JPtH ) 85.2 Hz, 1H), 4.52 (s, JPtH ) 18.8 Hz, 1H),
3
3
3.96 (s, JPtH ) 21 Hz, 1H), 3.55 (s, JPtH ) 35.1 Hz, 6H), 2.14 (m, 4
H), 1.89 (m, 4 H), 1.72 (m, 2 H), from 1.62 to 1.09 (m, 10H), 0.92 (s,
2JPtH ) 73.8 Hz, 3H). Anal. Calcd for C17H33F6N2OPSPt: H, 5.09;
C, 31.24; N, 4.29. Found: H, 5.05; C, 31.00; N, 4.36.
All the other hexafluorophosphate salts with the dinitrogen ligands
pri2dim, bpy, dipy, dps, phen, Me2phen, Me4phen, NO2phen, and Ph2-
phen were prepared in satisfactory yields by following essentially the
same procedure used for complex 5.
Results
Synthesis and Characterization of Complexes. The com-
plex trans-[PtMeCl(Me2SO)2] is a useful synthon to synthesize
square planar platinum(II) organometallic compounds containing
the molecular fragments [Pt(Me)(Me2SO)] or [PtMeX] (X )
halide ions).23 As shown in the reaction scheme, two different
procedures were applied to the synthesis of the [Pt(N-N)Me-
(Me2SO)]PF6 complexes, depending on the nature of the
dinitrogen ligand N-N and on the lability of the coordinated
Me2SO in the ensuing product. When N-N is a diamine,
pyridine or a mixed aminopyridine, as for compounds 1-4, the
reaction of the precursor in methanol with a slight excess of
the chelating ligand leads to the easy formation of [Pt(N-N)-
[Pt(pri2dim)(Me2SO)(CH3)]PF6 (6). IR: ν(SdO) 1128 cm-1
.
1H
)
3
3
NMR (acetone-d6): δ 9.10 (s, JPtH ) 46.7 Hz, 1H), 9.00 (s, JPtH
3
84.7 Hz, 1H), 4.94 (m, 1H), 4.45 (m, 1H), 3.56 (s, JPtH ) 35.2 Hz,
3
3
6H), 1.46 (d, 6H), 1.38 (d, JHH ) 6.0 Hz, 6H), 0.95 (s, JPtH ) 73.1
Hz, 3H). Anal. Calcd for C11H25F6N2OPSPt: H, 4.39; C, 23.04; N,
4.89. Found: H, 4.33; C, 22.93; N, 4.78.
[Pt(bpy)(Me2SO)(CH3)]PF6 (7). IR: ν(SdO) 1128 cm-1 1H NMR
.
(acetone-d6): δ 9.85 (dd, 3JPtH ) 19.8 Hz, 1H), 9.08 (dd, 3JPtH ) 45.1
3
Hz, 1H), 8.76 (m, 2H), 8.56 (m, 2H), 8.02 (m, 2H), 3.69 (s, JPtH
)
36.3 Hz, 6H), 0.92 (s, 2JPtH ) 72.6 Hz, 3H). Anal. Calcd for C13H17-
F6N2OPSPt: H, 2.91; C, 26.49; N, 4.75. Found: H, 3.10; C, 26.53;
N, 4.86.
[Pt(dipy)(Me2SO)(CH3)]PF6 (8). IR: ν(SdO) 1130 cm-1
.
1H
NMR (acetone-d6): δ 9.99 (s, 1H), 8.76 (dd, 3JPtH ) 17 Hz, 1H), 8.43
(22) Van Geet, A. L. Anal. Chem. 1968, 40, 2227. Van Geet, A. L. Anal.
Chem. 1970, 42, 679.
(23) Romeo, R.; Monsu` Scolaro, L., submitted for publication.
3
(dd, JPtH ) 46.5 Hz, 1H), 8.08 (m, 2H), 7.55 (t, 2H), 7.38 (t, 1H),
3
2
7.29 (t, 1H), 3.51 (s, JPtH ) 36.3 Hz, 6H), 0.68 (s, JPtH ) 73.7

5090 Inorganic Chemistry, Vol. 35, No. 17, 1996
Romeo et al.
1
Table 1. Selected H NMR Data for [Pt(N-N)Me(Me2SO)] PF6 Complexesa
N-N
δ(CH3Pt)
δ(Me2SO)
δ(H aliphatics)
δ(H aromatics)
1
2
3
4a
4b
5
6
7
enb
0.32 (76.5)
0.26 (72.6)
0.72 (75.9)
0.60 (75.1)
0.60 (75.1)
0.92 (73.8)
0.95 (73.1)
0.92 (72.6)
0.68 (73.7)
0.84 (75.6)
3.28 (33.0)
3.30 (36.3)
3.41 (34.1)
3.44 (35.1)
3.42 (34.4)
3.55 (35.1)
3.56 (35.2)
3.69 (36.3)
3.51 (36.3)
3.60 (35.8)
3.49 (35.8)
3.75 (36.8)
3.57 (36.3)
3.72 (37.4)
3.77 (37.2)
Me4enc
py
2.80 (16.5); 2.67 (44.0)
8.95; 8.74 (41.8)
8.69 (41.4)
9.39 (20.0)
9.00 (47.7); 8.91 (85.2)
9.10 (46.7); 9.00 (84.7)
9.85 (19.8); 9.08 (45.1)
8.76 (17.0); 8.43 (46.5)
8.99 (19.8); 8.79 (45.9)
trans-2-ampyd,f
cis-2-ampye,f
cy2dim
pri2dim
bpy
4.43
4.41
4.52 (18.8); 3.96 (21.0)
1.46; 1.38
8
9
dipy
dps
3.49 (35.8)
10
phen
1.12 (73.7)
1.00 (78.6)
1.04 (72.6)
1.17 (73.8)
1.15 (73.7)
1.16 (73.7)
10.15 (16.5); 9.46 (44.5)
11
12
13
Me2phen
Me4phen
NO2phen
3.10 (4.8)g
2.96; 2.87; 2.74; 2.68
9.91; 9.09 (46.2)
10.34
14
Ph2phen
3.78 (36.3)
10.24 (19.0); 9.52 (45.6)
2
3
a Resonances in ppm from TMS at 298 K; solvent ) acetone-d6, unless noted otherwise; JPtH in Hz for PtCH3 and JPtH in Hz for Pt(CH3)2SO,
for the aliphatic and the aromatic protons are given in parentheses. b Solvent ) 1:1 CD3OD/(CD3)2CO v:v. c Solvent ) D2O. d py trans to Me2SO.
e py cis to Me2SO. f Solvent ) CD3OD.
J
in Hz.
PtH
g 4
Me(Me2SO)]Cl, which can be isolated directly as chloride salts
or as hexafluorophosphate salts on adding KPF6.
coupling (47.6 Hz) and it is attributable to the iminic proton
trans with respect to the strong activating methyl group.25 The
corresponding proton trans to Me2SO (at δ 9.00 ppm) exhibits
a much higher coupling constant (84.7 Hz). The coordinated
3
Me2SO gives a singlet at δ 3.56 with a JPtH ) 35.2 Hz
consistent with the retention of the S-bonded mode. The methyl
2
group gives a singlet at δ 0.95 ppm with JPtH ) 73.1 Hz, a
1
value in the range of similar cationic species. The H NMR
spectra of the complexes with the ligands 2-ampy (4) and NO2-
phen (13) clearly show the presence of the two possible isomers.
In the case of complex 4, the presence of markedly different
coupling constants with 195Pt for the ortho proton on the pyridine
ring is particularly diagnostic for assigning the two species
indicated in Table 1 as 4a (pyridine trans to S-bonded Me2SO)
and 4b (pyridine in the cis position), respectively. On the
contrary, a close inspection of the spectrum for the mixture
relative to complex 13 does not allow a definitive assignment
to be made.
With all the phenanthrolines and diimines (compounds 5-14),
it is necessary first to add AgPF6 to the solution of trans-
[PtMeCl(Me2SO)2] in dimethyl sulfoxide, followed by the
addition of the proper chelating ligand. A slight excess of the
latter serves to shift the equilibrium toward complete precipita-
tion of AgCl. In the presence of chloride ion the only product
is [Pt(N-N)MeCl].
1
The H NMR spectrum of the complex [Pt(dps)(CH3)(Me2-
SO)]+ is of interest in that two methyl resonances are observed
at δ 3.60 and at δ 3.49, each with 195Pt satellites. A molecular
model shows that the six-membered ring formed by coordination
of the two nitrogens of the ligand assumes conformations in
which the sulfur atom lies well outside of the square planar
coordination plane. The dimethyl sulfoxide ligand becomes
therefore prochiral, and the two methyl groups are diastereotopic.
The same pattern of behavior is expected for the complex
containing dipy, the only difference with the dps complex being
the presence of an out-of-plane nitrogen atom instead of the
sulfur atom. Contrary to these expectations, only one methyl
signal is observed for the compound 8 at δ 3.51 (3JPtH ) 36.3
Hz). If we exclude that such a single signal is the result of a
perfect coincidence of resonances of two diastereotopic methyls,
it must be concluded that the secondary nitrogen of the
coordinated dipy ring moves rapidly from one side of the
coordination plane to the other with a rate that is fast in the
NMR time scale and faster than that of the dps ring. The
reasons for this are unclear. The suspicion of a possible
oxidation “in situ” of the sulfide to sulfoxide with an increased
rigidity of the ring, was ruled out by the results of the elemental
analysis and the FAB MS spectrum carried out on the complex
recovered from the NMR measurements. One reviewer sug-
All the complexes were fully characterized by elemental
analyses, IR and 1H NMR spectroscopy. A collection of
selected spectroscopic data for the various complexes is reported
in Table 1. The presence of a strong peak at 1125 ( 7 cm-1
,
which is assigned to the SdO stretch, is taken to indicate
bonding through sulfur, since this is shifted to higher frequency
than the corresponding vibration in the free ligand.24 All the
1H NMR spectra, with the exception of that for the complex
(11) containing the Me2phen ligand (see below), are consistent
with the proposed structures showing the pattern of an S-bound
sulfoxide (δ in the range 3.30-3.80 ppm), of a methyl group
directly coordinated to the metal (δ in the range 0.15-1.20
ppm), and of a chelate ligand coordinated to an asymmetric
platinum metal center. As an example, the complex (6)
containing pri2dim as chelating ligand displays two distinct
resonances in the low field region at δ 9.10 and 9.00 due to the
imino protons of the diimine ligand, as well as two easily
distinguishable isopropyl groups in the aliphatic region. The
assignment of the iminic protons was facilitated by the presence
of largely different coupling constants with the isotopically
abundant 195Pt (33%, I ) 1/2). The most downfield shifted signal
at δ 9.10 ppm (1H) shows two satellite peaks due to 195Pt
(25) Appleton, T. G.; Clark, H. C.; Manzer, L. E. Coord. Chem. ReV. 1973,
10, 335.
(24) Davies, J. A. AdV. Inorg. Chem. Radiochem. 1981, 24, 115.

Rates of Dimethyl Sulfoxide Exchange
Inorganic Chemistry, Vol. 35, No. 17, 1996 5091
gested the possibility of (partial) deprotonation and aromatization
of the 2.2′-dipyridylamine ligand and another the formation of
five-coordinate species, but both hypotheses are not in keeping
with the NMR spectral evidence.
Flipping of the 2,9-Dimethyl-1,10-phenanthroline Ligand.
As for all the other compounds the coordinated phenanthroline
in compound 11 should result in the halves of the molecule
being inequivalent. In fact, at 298 K, the two methyls and the
aromatic proton pairs H3 and H8, H4 and H7, and H5 and H6 are
chemically equivalent, indicating a fast site exchange of the two
nitrogen atoms of the Me2phen ligand. As the temperature is
lowered below 230 K, the signals of the phenanthroline ligand
start to decoalesce and at 200 K the NMR spectrum shows two
singlets, one for each methyl, four doublets, one for H3, H8,
H4, and H7 protons, and an AB system for H5 and H6 protons,
as expected as a result of the asymmetry of a firmly bonded
bidentate phenanthroline. Both the methyl protons and the
aromatic protons of the substituted phenanthroline were used
for the line-shape analysis of the spectra. The exchange rates
were calculated using the computer program DNMR 5,26 and
the computer-calculated spectra were visually compared to the
experimental spectra. The results of the two sets of data agreed
well. Values of ∆Hq ) 44.8 ( 2 kJ mol-1, ∆Sq ) -20 ( 8 J
K-1 mol-1 and ∆Gq ) 49.6 ( 4 (kJ mol-1), at the coalescence
temperature, were derived from a linear regression analysis of
1
the Eyring plot. The aromatic region of the H NMR spectra
of compound 11, taken at different temperatures, is shown in
Figure 1.
Kinetics of Sulfoxide Exchange. The exchange of sulfoxide
for the various complexes takes place according to the reaction
scheme in (1) with no evidence throughout of ring opening or
of other concurrent processes, except for the case of Me2phen
(compound 11) discussed above. The exchange rates have been
Figure 1. Upper plot (in the frame): 1H NMR spectrum of the complex
[Pt(Me2phen)(CH3)(Me2SO)]PF6 in acetone-d6 at 298 K. Intensities are
given in arbitrary units and in different scales for the aromatic and the
aliphatic regions of the spectrum. Lower plot: experimental (on the
left) and simulated NMR spectra (on the right) of the signals of the
phenanthroline aromatic protons. The temperatures and the “best-fit”
rate constants for the fluxional motion of the nitrogen ligand are as
follows: (a) T ) 238 K, kobs, s-1 ) 58; (b) T ) 228 K, kobs, s-1 ) 23;
(c) T ) 223 K, kobs, s-1 ) 12; (d) T ) 218 K, kobs, s-1 ) 7; (e) T )
208 K, kobs, s-1 ) 2.
1
measured by H NMR using different experimental methods
according to the rate of the process.
(i) Isotopic Exchange. In the case of slowly exchanging
systems, the kinetic runs were performed by adding with a
microsyringe a known volume of dimethyl sulfoxide-d6 on a
prethermostated solution of weighted [Pt(N-N)Me(Me2SO)]PF6
complex and acetone-d6. At least a 5-fold excess of Me2SO-d6
over complex was ensured in any run. All concentrations are
expressed in moles per kilogram of solvent (m ) mol kg-1).
The isotopic exchange was followed by monitoring the increase
in intensity of the proton NMR signal of free Me2SO at δ 2.60
and the matching decrease of the signal of the sulfoxide
coordinated to the metal (values of the chemical shifts and of
the 195Pt coupling constants for the various complexes are
reported in the third column of Table 1). The spectra were
recorded at appropriate intervals of time (Figure 2). The mole
fraction F ) [Me2SO]f/([Me2SO]f + [Me2SO]b) of the free
nondeuterated dimethyl sulfoxide was obtained by integration
of the signals, and the first-order rate constants kexch (s-1) for
the exchange of the label were obtained from a nonlinear least-
squares fit of the experimental data to F ) c1 + c2 exp(-kexcht),
with c1, c2, and kexch as the parameters to be optimized. A
similar analysis can be performed by using the mole fraction
of the nondeuterated bound dimethyl sulfoxide. An example
of a fit is shown in Figure S2. From the McKay equation, Rexch
) kexch ab(a + b)-1 (where Rexch is the rate of the exchange
process, a the concentration of complex, and b the concentration
Figure 2. 300-MHz spectra of a 0.008 m [Pt(Me4phen)(CH3)(Me2-
SO)]+and 0.292 m (CD3)2SO solution in deuterated acetone at 298 K
as a function of time. Upper plot: coordinated Me2SO (δ ) 3.72 ppm,
3JPtH ) 37.4 Hz). Lower plot: free Me2SO (δ ) 2.56 ppm).
of free sulfoxide) the pseudo-first-order rate constants, kobsd
)
Rexch/a, were calculated and are collected in Table 2. The
exchange is first-order with respect to free ligand and the linear
plots of kobs Vs [Me2SO] pass through the origin. The exchange
therefore follows the rate law kobs ) k2[Me2SO]. The values
of k2, obtained from a linear regression analysis of the rate law,
are collected in Table 2.
(26) Stephenson, D. S.; Binsch, G. QCPE 1978. 11, 365.

5092 Inorganic Chemistry, Vol. 35, No. 17, 1996
Romeo et al.
Table 2. Pseudo-First-Order Rate Constants kobs, as a Function of Concentration of Free Me2SO, and Second-Order Rate Constants, k2, for
a
Me2SO Exchange on [Pt(N-N)(CH3)(Me2SO)]PF6 Complexes in Acetone-d6
N-N
en
N-N
dipy
c
d
c
d
[Me2SO]b
103kobs
k2
[Me2SO]b
103kobs
k2
1
0.571
0.974
1.50
0.500
1.00
0.00548
0.00936
0.0143
0.00055
0.00112
0.00174
0.354
0.706
1.32
8
14
7
0.124
0.244
0.410
6.55
12.9
21.6
4.44
7.10
14.8
2.21
4.91
7.00
6.62
13.0
26.3
13.5
25.7
39.9
(9.49 ( 0.05) × 10-6
(1.15 ( 0.01) × 10-6
(3.5 ( 0.3) × 10-3
(1.0 ( 0.1) × 10-2
(1.6 ( 0.1) × 10-2
(2.66 ( 0.05) × 10-2
0.053 ( 0.002
0.10 ( 0.01
0.16 ( 0.01
0.180 ( 0.04
2
Me4en
Ph2phen
bpy
0.0400
0.0713
0.142
0.0150
0.0311
0.0453
0.0352
0.0708
0.142
1.53
4a
9
2-ampy
dps
0.100
0.188
0.373
0.100
0.251
0.496
0.090
0.196
0.400
0.145
0.292
0.410
0.840
2.22
4.93
1.44
3.20
6.34
3.65
7.49
10.7
10
13
5
phen
3
py
NO2phen
cy2dime
0.040
0.080
0.119
0.33 ( 0.04
0.920
12
Me4phen
6
11
pri2dimf
0.79 ( 0.06
Me2pheng
38100
a At 298 K. b m. c s-1
. .
d m-1 s-1 e From Eyring plots (data in Table 3). f From magnetization transfer experiments (data in Table 3). g From line
broadening experiments.
Table 3. Temperature Dependence of Observed kobs, and Second-Order Rate Constants, k2, for Me2SO Exchange with
[Pt(cy2dim)(CH3)(Me2SO)]PF6 (5) and [Pt(i-pr2dim)(CH3) (Me2SO)]PF6 (6) Complexes in Acetone-d6
compound 5
103kobs
compound 6
103kobs
b
c
b
c
T/K
[Me2SO]a
102k2
T/K
[Me2SO]a
102k2
238d
0.0452
0.081
0.151
0.0382
0.0767
0.151
0.0383
0.0.697
0.158
0.045
0.082
0.150
0.110
0.226
0.333
0.386
0.697
1.29
0.710
1.38
2.76
1.76
3.49
7.33
4.31
7.86
248d
0.054
0.100
0.150
0.033
0.0697
0.158
0.033
0.0752
0.155
0.169
0.316
0.497
0.762
1.39
2.10
1.55
3.26
7.41
2.87
6.50
13.4
136
242
395
0.85 ( 0.01
1.82 ( 0.05
4.6 ( 0.4
1.39 ( 0.03
4.69 ( 0.02
8.65 ( 0.01
79 ( 6
248d
258d
268d
313e
258d
268d
298e
14.3
295
583
878
9.4 ( 0.2
261 ( 12
a m. b s-1 c m-1 s-1 d From isotopic exchange experiments. e From magnetization transfer experiments.
. .
A
M∞A ) M0 (τ1A/T1A)
(3)
(4)
(ii) Magnetization Transfer. The rate constants for the
moderately fast exchanging complexes 5 and 6 at the highest
temperatures were measured by applying a magnetization
transfer technique as proposed by Forsen and Hoffman.27 The
two sites are those of free and bound Me2SO. For both
complexes the two signals are well-separated without scalar
coupling, and no complications arise from the triplet-like
structure of the bound signal. In a typical procedure the
resonance of the free Me2SO was completely saturated by
selective decoupling pulses of variable length. The experimental
heights of the bound Me2SO peak, corrected for the contribution
of the 195Pt satellites (33% abundance), were fitted by linear
1/τ1A ) 1/τA + 1/T1A
experiments in Table 3 were fitted to the Eyring equation,
298
leading to k2 ) 0.920 m-1 s-1, ∆Hq ) 45.5 ( 3 kJ mol-1
and ∆Sq ) -93 ( 9 J K-1 mol-1 for the [Pt(cy2dim)(CH3)(Me2-
SO)]+ complex (5) and to k2298 ) 0.79 ( 0.06 m-1 s-1, ∆Hq )
46.2 ( 4 kJ mol-1 and ∆Sq ) -91 ( 10 J K-1 mol-1 for the
[Pt(pri2dim)(CH3)(Me2SO)]+ complex (6).
(iii) Line-Broadening Analysis. The rate of sulfoxide
exchange with the [Pt(Me2phen)(CH3)(Me2SO)]+ complex was
measured in the slow-exchange region from the width of the
NMR signals at half-height using eq 5 where τa is the residence
A
regression analysis to eq 2, where M0A, MtA, and M∞ are the
A
(MtA - M∞A) ) M0 τ1A/τA exp (-t/τ1A)
(2)
1/τa ) π(ωA - ωo )
(5)
experimental intensities before saturation, at the time t, and after
a prolonged saturation, respectively. τ1A is correlated to τA (the
lifetime of the observed site) and T1A (spin-lattice relaxation
time) by eqs 3 and 4.
The variable-temperature rate constants including data ob-
tained from isotopic exchange and from magnetization transfer
A
time in site A and ωA and ωoA are the linewidths in the presence
and in the absence of exchange, respectively.28 The latter was
measured at low temperature, where the reaction is frozen on
(28) Becker, E. D. High Resolution NMR. Theory and Chemical Applica-
tions; Academic Press: New York, 1980; pp 240-245.
(27) Forse`n, S.; Hoffman, R. A. J. Chem. Phys. 1963, 39, 2892.

Rates of Dimethyl Sulfoxide Exchange
Inorganic Chemistry, Vol. 35, No. 17, 1996 5093
the NMR time scale. The experimental linewidths were
corrected by subtracting the line width of TMS to minimize
the effect of instrumental line-broadening. Values of observed,
The complex [Pt(Me2phen)(CH3)(Me2SO)]+, in contrast with
those mentioned above, has only two coordination sites occupied
in the asymmetric molecular fragment [Pt(CH3)(Me2SO)] op-
posite to the half of the molecule which contains the coordina-
tion positions accessible to the two nitrogens of the chelate
ligand. Thus, any conceivable mechanism for the flipping of
the phenanthroline does not need to take into account the
presence or the role of additional ligands L and therefore the
system appears to be by far simpler than those described in
previous studies.29,30 We cannot rigorously exclude pathways
which involve five-coordinate complexes promoted by nucleo-
philic attack by the solvent, by traces of free ligand, or even by
weak coordination of hexafluorophosphate. However, a simple
reaction scheme which accounts for all experimental findings
is the following one
k
obs, and second-order rate constants, k2, for the exchange, at
various temperatures and dimethysulfoxide concentrations, are
given as Supporting Information (Table S1).
Discussion
The reaction of trans-[Pt(CH3)Cl(Me2SO)2] with an extended
series of nitrogen N-N bidentate ligands, carried out according
to the procedures described above, afforded a series of new
cationic complexes of general formula [Pt(N-N)(CH3)(Me2-
SO)]+, that have been fully characterized analytically, chemi-
1
cally, and spectroscopically by infrared and H NMR. The
assignment of the 1H NMR spectra was facilitated by the
presence of coupling constants associated with the isotopically
abundant 195Pt and the data in Table 1 are consistent with the
pattern of a S-bound sulfoxide, a methyl group directly
coordinated to the metal and a chelate ligand coordinated to an
asymmetric platinum metal center. The complexes formed by
the asymmetric ligands 2-ampy and NO2phen showed the
presence of the two expected isomers that could be definitely
identified as compounds 4a and 4b only in the case of the mixed
aminopyridine ligand. The prochirality showed by the dimethyl
sulfoxide of the complex containing dps has been interpreted
as an indication that the sulfur of the pyridyl sulfide does not
move rapidly from one side to the other of the coordination
plane.
A significant feature of the 1H NMR spectra of the Me2phen
complex is that, at ambient temperature, the halves of the ligand
show coincident signals that separate out into different sets of
signals only when the temperature is lowered (see Figure 1).
Fast exchange of the two nitrogen atoms at a single coordination
site (flipping) has been previously investigated for platinum
complexes of the type cis-[Pt(PR3)2Cl(N-N)] and the fluxional
process found to be strongly dependent on the orientation of
the lone pairs of the chelate.29 Complexes containing mono-
coordinate 2,9-dimethyl-1,10-phenanthroline of the type cis- and
trans-[Pt(Me2phen)X2(PPh3)] (X ) Cl, Br, and I),30 trans-
which involves rupture of the metal-nitrogen bond and formation
of the coordinatively unsaturated T-shaped 14-electron three
coordinate species I1 that rapidly interconverts into its I2
counterpart. Thereby, ring closure follows. The mechanism
is reminiscent of that proposed to account for the apparent
rotation of a π-allyl group relative to nitrogen ligands on
palladium, which involves a monocoordinate chelating ligand
on tricoordinated palladium.33,34 This is obviously a simplified
reaction scheme that does not account for the role of the solvent
as a potential nucleophile in promoting ring rupture or in
trapping the coordination sites of the highly unstable I1 and I2
three-coordinate intermediates after the opening of the ring.
There is no reasonable doubt that the driving force for ring
opening is a specific steric interaction of the ortho substituents
of the chelated ligand with the other cis atoms in the square
planar coordination plane, nicely evidenced in some molecular
structures of platinum(II) containing chelated 2,9-dimethyl-1,-
10-phenanthroline such as those of [Pt(Me2phen)X2] (X ) Br
and I)31 in which a major distortion from the square-planar
geometry is indicated by the dihedral angle between the plane
of the chelating moiety (N-C-C-N) and the plane of
coordination (X-Pt-X). The fast interconversion of the two
T-shaped platinum cations intermediates I1 and I2 containing
monodentate phen is accounted for by the low-energy barrier
indicated by theoretical calculations for the fluxionality of
coordinatively unsaturated three-coordinate species of this type.35
It is now widely accepted that the formation of such 3-coordinate
[Pt(Me2phen)XL2]30 (L ) PPh3 and PBun ) and [Pt(Me2phen)-
3
X2L] (L ) CO, PPh3, Me2SO, Me2S, nitrosobenzene, pyridine,
amine)31 were recently shown to exhibit the same fluxional
motion. The ∆Gq values for the fluxion were measured and
found to be strongly dependent upon the nature of the trans
activating group (for a phosphine in position trans to the
phenanthroline only an upper limit of 25 kJ mol-1 could be
estimated for the exchange process) and upon the covalent radius
of the halide ion. The mechanism for the exchange of the donor
atoms of a monocoordinated Me2-phen at a single coordination
site requires the formation of a five-coordinated transition state
in which both ends of the phenanthroline are coordinated to
the metal. Ring closure on [Pt(Me2phen)X2L] yields fairly
stable trigonal-bypiramidal five-coordinate compounds only in
the case of L ) alkenes32 or alkynes.3c
(29) (a) Bushnell, G. W.; Dixon, K. R.; Khan, M. A. Can. J. Chem. 1978,
56, 450. (b) Bushnell, G. W.; Dixon, K. R.; Khan, M. A. Can. J.
Chem. 1978, 56, 879. (c) Dixon, K. R. Inorg. Chem. 1977, 16, 2618.
(d) Alvarez, S.; Bermejo, M. J.; Vinaixa J. J. Am. Chem. Soc. 1987,
109, 5316. (e) Brandon, J. B.; Collins, M.; Dixon, K. R. Can. J. Chem.
1987, 66, 950.
(32) (a) Fanizzi, F. P.; Maresca, L.; Natile, G.; Lanfranchi, M.; Tiripicchio,
A.; Pacchioni, G. J. Chem. Soc., Chem. Commun. 1992, 333. (b)
Fanizzi, F. P.; Intini, F. P.; Maresca, L.; Natile, G.; Lanfranchi, M.;
Tiripicchio, A. J. Chem. Soc., Dalton Trans. 1991, 1007.
(33) Albinati, A.; Kunz, R. W.; Ammann, C. J.; Pregosin, P. S. Organo-
metallics 1991, 10, 1800.
(30) Fanizzi, F. P.; Natile, G.; Lanfranchi, M.; Tiripicchio, A. Inorg. Chem.
1994, 33, 3331.
(31) Clark, R. J. H.; Fanizzi, F. P.; Natile, G.; Pacifico, C.; van Rooyen,
C. G.; Tocher, D. A. Inorg. Chim. Acta 1995, 235, 205.
(34) Gogoll, A.; Ornebro, J.; Grennberg, H.; Bakwall, J. E. J. Am. Chem.
Soc. 1994, 116, 3631.

5094 Inorganic Chemistry, Vol. 35, No. 17, 1996
Romeo et al.
14-electron compounds offers a favourable reaction route to a
number of fundamental processes as an alternative to the
participation of 4- and 5-coordinate species.36 Among them,
the uncatalyzed cis to trans isomerization of complexes of the
type cis-[Pt(PEt3)2(R)X] (R ) alkyl or aryl groups; X ) halide
ion),37 â-hydride elimination from dialkyl-38 and monoalkyl-
diphosphinoplatinum(II)39 complexes, olefin insertion into a
Pt-H bond,39a reductive elimination of hydrogen from a three-
coordinate PtLH2 (L ) phosphine) fragment,40 some sym-
metrization reactions,41 and nucleophilic substitutions.19,42 The
fluxional motion of the phenanthroline seen in this study for
the complex [Pt(Me2phen)(CH3)(Me2SO)]+ can be safely added
to the previous list. Moreover, it suggests the possibility for
the flipping to occur also in other [Pt(Me2phen)XY] molecules
and that for the [Pt(Me2phen)X2] species it is not seen only
because of the symmetry of the molecule. A detailed investiga-
tion of this aspect of the problem is underway.
At temperatures below 210 K the Me2phen NMR signals
show two singlets, one for each methyl, and separated signals
for the aromatic protons, as a consequence of the freezing of
the flipping of the ligand. When a stoichiometric amount of
Me2SO is added to a solution of the complex [Pt(Me2phen)
(CH3)(Me2SO)]+ at this temperature, the NMR signals of bound
and free dimethyl sulfoxide are broad and the broadening
increases with increasing the temperature or sulfoxide concen-
tration. The dependence of the rate constant upon [Me2SO]
and the temperature (in the range 250-300 K) was determined
from the width of the NMR signals by using eq 5. The second-
order rate law of eq 6 was assumed for this exchange reaction.
by a family of straight lines with a zero intecept (Figure S3)
and obeys eq 6. Linear regression analysis of these plots gave
the values of k2, the second-order rate constants for the
bimolecular attack of Me2SO on the substrate. Thus, the
exchange is perfectly in keeping with an associative mode of
activation, with no evidence of a significant contribution to the
reactivity of a concurrent solvolytic pathway. This is supported
by the low enthalpies of activation and the negative entropies
of activation found for the complexes 5 and 6 (see Results
section).
Steric Effects. The experimental findings indicate that the
exchange is dominated by the direct attack of the nucleophile
on the metal. The lability of the sulfur-bonded Me2SO in the
[Pt(N-N)(CH3)(Me2SO)]+ complexes appears to be profoundly
affected by the nature and the characteristics of the coordinated
N-N ligand, the difference of reactivity between the first and
the last members of the series being greater than 10 orders of
magnitude (for complexes 2 and 11 the values of k2 are 1.15 ×
10-6 m-1 s-1 and 38.1 × 103 m-1 s-1, respectively). The
circumstance that 2 and 11 are formed by the most sterically
demanding ligands Me4en and Me2phen, calls for a careful
analysis of the specificity of steric effects. A significant feature
is that steric bulk due to alkyl substituents at the nitrogens of
the chelated diamine or in positions 2 and 9 of the phenanthro-
line, affects the lability of the corresponding complexes in
opposite directions. For 2 a decrease of rate of 10 times is
observed with respect to 1, the complex formed by unsubstituted
ethylenediamine, in agreement with similar results obtained in
a study of the solvolysis of alkyl-substituted ethylenediamine
platinum(II) complexes43 and in a number of other studies of
steric retardation on square planar complexes.44 The interpreta-
tion is straightforward. The moderate decrease of rate can be
ascribed to the additional difficulty of the nucleophile in
approaching the metal center and in forming the new bond or,
in other words, to a fairly modest destabilization of the
5-coordinate transition state. In contrast, the substrate containing
Me2phen exchanges the coordinated Me2SO at a rate that is 5
orders of magnitude higher than that of 10, the complex
containing unsubstituted phen (k2 ) 0.18 m-1 s-1) or even of
12 and 14, complexes with alkyl substituents (k2 ) 0.026 m-1
s-1) or aryl substituents (k2 ) 0.10 m-1 s-1) on the phenan-
throline in positions different from 2,9.
kobs ) k2[Me2SO]
(6)
The second-order rate constants k2 were fitted to the Eyring
equation leading to k2298 ) 38.1 × 103 m-1 s-1, ∆Hq ) 13.5 (
0.8 kJ mol-1 and ∆Sq ) -112 ( 3 JK-1 mol-1
.
The systematic kinetics of the slow reactions, studied at
different Me2SO concentrations, were followed by isotopic
exchange. During the exchange there was no indication of a
loss of asymmetry in the NMR signals of the two halves of the
coordinated bidentate N-N ligand. The dependence of the
pseudo-first-order rate constants, listed in Table 2, is described
(35) (a) Tatsumi, K.; Hoffmann, R.; Yamamoto, A.; Stille, J. K. Bull. Chem.
Soc. Jpn. 1981, 54, 1857. (b) Thorn, D. L.; Hoffman, R. J. Am. Chem.
Soc. 1978, 100, 2079. (c) Mc Carthy, T. J.; Nuzzo, R. J. Whitesides,
G. M. J. Am. Chem. Soc. 1981,103, 1676. (d) Komiya, S.; Albright,
T. A.; Hoffmann, R.; Kochi, J. K. J. Am. Chem. Soc. 1976, 98, 7255.
(36) For a review on dissociative pathways in platinum(II) chemistry, see:
Romeo, R. Comments Inorg. Chem. 1990, 11, 21-57.
This pattern of behavior finds its origin in the specific relevant
nonbonding repulsions by the methyl groups in positions 2,9
with the two cis groups in the square-planar configuration, as
seen in the congestion and distortion of the molecular structures
of the [Pt(Me2phen)X2] complexes.31 There is significant steric
relief when the metal adds a fifth ligand in the formation of a
5-coordinate trigonal-bypiramidal structure, where the phenan-
throline occupies two positions of the equatorial plane. The
molecular structures of a number of 5-coordinated complexes
containing Me2phen show32 that, in this new position, the ligand
is completely planar with the carbon atoms of the methyl groups
far away from short range steric repulsions. In conclusion,
observed consequences of the great steric destabilization of the
square planar configuration are (i) a fluxional motion of the
ligand (flipping), (ii) the easy uptake of an additional ligand
(expecially olefins or alkynes)45 yielding fairly stable 5-coor-
dinate species, and (iii) a marked acceleration of the rate of
ligand exchange.
(37) Alibrandi, G.; Minniti, D.; Monsu` Scolaro, L.; Romeo, R. Inorg. Chem.
1988, 27, 318 and references therein.
(38) (a) Whitesides, G. M. Pure Appl. Chem. 1981, 53, 287. (b) Yamamoto,
A.; Yamamoto, T.; Komiya, S.; Ozawa, F. Pure Appl. Chem. 1984,
56, 1621. (c) Mc Carthy, T. J.; Nuzzo, R. J.; Whitesides, G. M. J. Am.
Chem. Soc. 1981, 103, 3396. (d) Whitesides, G. M.; Gaash, J. F.;
Sdedronsky, E. R. J. Am. Chem. Soc. 1972, 94, 5258. (e) Nuzzo, R.
G.; Mc Carthy, T. J.; Whitesides, G. M. J. Am. Chem. Soc. 1981,
103, 3404. (f) Foley, P.; Di Cosimo, R.; Whitesides, G. M. J. Am.
Chem. Soc. 1980, 102, 6713. (g) Komiya, S.; Morimoto, Y.; Yama-
moto, A.; Yamamoto, T. Organometallics 1982, 1, 1528. (h) Miller,
T. M.; Whitesides, G. M. Organometallics 1986, 5, 1473. (i) Moore,
S. S.; DiCosimo, R.; Sowinki, A. F.; Whitesides, G. M. J. Am. Chem.
Soc. 1981, 103, 948.
(39) (a) Romeo, R.; Alibrandi, G.; Monsu` Scolaro, L. Inorg. Chem. 1993,
32, 4688. (b) Alibrandi, G.; Cusumano, M.; Minniti, D.; Monsu`
Scolaro, L.; Romeo, R. Inorg. Chem. 1989, 28, 342. (c) Alibrandi,
G.; Monsu` Scolaro, L.; Minniti, D.; Romeo, R. Inorg. Chem. 1990,
29, 3467. (d) Brainard, R. L.; Miller, M. T.; Whitesides, G. M.
Organometallics 1986, 5, 1481.
(43) Fanizzi, F. P.; Intini, F. P.; Maresca, L.; Natile, G.; Uccello-Barretta,
G. Inorg. Chem. 1990, 29, 29.
(44) Kotowsky, M.; van Eldick, R. in Inorganic High Pressure Chemis-
try: Kinetics and Mechanisms; van Eldik, R., Ed.; Elsevier: Amster-
dam, 1986.
(40) Packett, D. L.; Trogler, W. C. Inorg. Chem. 1988, 27, 1768.
(41) Scott, J. D.; Puddephatt, R. J. Organometallics 1983, 2, 1643.
(42) Romeo, R.; Grassi, A.; Monsu` Scolaro, L. Inorg. Chem. 1992, 31,
4383 and references therein.

Rates of Dimethyl Sulfoxide Exchange
Inorganic Chemistry, Vol. 35, No. 17, 1996 5095
Figure 3. Relative changes of the free energy ∆Gq, calculated from the rate constants in Table 2, on going from the square planar ground state to
the trigonal bypiramidal transition state. The first two illustrate a case of steric retardation, as result of nonbonding interactions at the transition
state; the last two refer to a rare example of steric acceleration in a bimolecular substitution.
A qualitative view of the different ways in which steric effects
come into play can be seen in Figure 3, based on the reasonable
assumption that the relative energies of the square-planar
complexes 1, 2, and 10 as well as those of the trigonal
bypiramidal transition states 10 and 11 are not greatly different.
On going from 1 to 2 we observe a classic case of steric
retardation in bimolecular processes, due to steric congestion
at the 5-coordinate transition state;46 the passage from 10 to 11
offers an unprecedented example of steric acceleration, as a
result of steric destabilization of the square-planar ground state.
Electronic Effects. We will refer to the lability of complexes
7 and 14 as a term of comparison to examine changes of
reactivity due primarily to electronic effects. Diimines, such
as 2,2′-bipyridyl and phenanthroline, exert a considerable
labilizing effect when compared to saturated nitrogen donors
such as ethylendiamine, so that the displacement of the neutral
ligand from [Pt(N-N)(Me2SO)(CH3)]+ (N-N ) bpy or phen) is
at least 4 orders of magnitude faster than from [Pt(en)(Me2-
SO)(CH3)]+. This is not a new phenomenon5c,47,48 and it can
be ascribed to the planarity of the imine ligands and to their
ability to act as π acceptors and stabilize intermediates of higher
coordination number. An increased electrophilicity of the metal,
besides the increase of reactivity, leads also to a significant
increase of the nucleophilic discrimination ability of the
substrate.47,48 It is of interest to note that in the complex cis-
[Pt(py)2(Me2SO)(CH3)]+, which has the same array of donor
atoms around the metal as for bpy and phen, but a different
orientation of the pyridine aromatic rings, the reactivity is
significantly less (2 orders of magnitude) than that of 7 and 14.
Thus, back-donation from filled d orbitals on the metal to empty
antibonding orbitals of the ligands appears to be somewhat
reduced, although there is a wide range of kinetic,49 spectro-
scopic,50 and structural51 evidence in favor of a tendency of
pyridine for π-bonding interactions.
Much of the reduction of reactivity of complexes containing
2,2′-dipyridylamine and 2,2′-dipyridyl sulfide is due to the fact
that the bidentate ligands are no longer R-diimines. The
introduction of a spacer group between the pyridine rings, such
as the sulfur atom for dps, has little effect on the reactivity which
remains comparable to that of the pyridine complex, whereas
comparison of the rates of 8 and 9 suggests that to a certain
extent NH is more effective than the sulfur in assisting sulfoxide
exchange.
The mixed ligand 2-(aminomethyl)pyridine is coordinated to
the metal through a sp3 amine nitrogen and a sp2 imine nitrogen.
The rate of exchange in Table 2 (k2 ) 3.5 × 10-3 m-1 s-1
)
refers to the isomer 4a that contains the pyridyl moiety trans to
the leaving sulfoxide. The reactivity appears to be somewhat
lower than that of the pyridine complex but still more than 100
(49) (a) Romeo, R.; Tobe, M. L. Inorg. Chem. 1974, 13, 548. (b) Kennedy,
B. P.; Gosling, R.; Tobe, M. L. Inorg. Chem. 1977, 16, 1744. (c)
Gosling, R.; Tobe, M. L. Inorg. Chim. Acta 1980, 42, 223. (d) Gosling,
R.; Tobe, M. L. Inorg. Chem. 1983, 22, 1235.
(45) (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.
(46) (a) Wilkins, R. G. Kinetics and Mechanisms of Reactions of Transition
Metals Compexes; VCH: Weinheim, Germany, 1991. (b) Atwood, J.
D. Inorganic and Organometallic Reaction Mechanisms; Brooks/Cole
Publishing Co.: Monterey, CA, 1985. (c) Tobe, M. L. In Compre-
hensiVe Coordination Chemistry; Wilkinson, G., Ed.; Pergamon:
Oxford, U.K., 1987; Vol. 1, pp 311-329. (d) Langford C. H.; Gray,
H. B. Ligand Substitution Processes; W. A. Benjamin: New York,
1965; pp 18-51. (e) Basolo, F.; Pearson, R. G. Mechanisms of
Inorganic Reactions; John Wiley: New York, 1968; pp 351-453.
(47) Cusumano, M.; Guglielmo, G.; Ricevuto, V. J. Chem. Soc. Dalton
Trans. 1980, 2044.
(50) (a) Dennenberg, R. J.; Darensbourg, D. J. Inorg. Chem. 1972, 11, 72.
(b) Seligson, A. L.; Trogler, W. C. J. Am. Chem. Soc. 1991, 113,
2520. (c) Shepherd, R. E.; Taube, H. Inorg. Chem. 1973, 12, 1392.
(51) (a) Albinati, A.; Anklin, C. G.; Ganazzali, F.; Ruegg, H.; Pregosin, P.
S. Inorg. Chem. 1987, 26, 503. (b) Colamarino, P.; Arioli, P. L. J.
Chem. Soc., Dalton Trans. 1975, 1656. (c) Caruso, F.; Spagna, R.;
Zambonelli, L. Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst.
Chem. 1980, 36, 713. (d) Nyburg, S. C.; Simpson, K.; Wong-Nu, W.
J Chem. Soc., Dalton Trans. 1976,1685 (e) Rochon, F. D.; Kong, P.
C.; Melanson, R. Can. J. Chem. 1980, 58, 97. (f) Rochon, F. D.;
Melanson, R. Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst.
Chem. 1980, B36, 691. (g) Melanson, R.; Rochon, F. Acta Crystallogr.,
Sect. B: Struct. Crystallogr. Cryst. Chem. 1978, B34, 1125.
(48) Romeo, R.; Arena, G.; Monsu` Scolaro, L.; Plutino, M. R. Inorg. Chim.
Acta 1995, 240, 81.

5096 Inorganic Chemistry, Vol. 35, No. 17, 1996
Romeo et al.
times higher than that of the ethylenediamine complex having
two saturated nitrogen atoms. It is therefore confirmed that the
nature of the trans-activating group and its π bonding ability
play a major role in controlling the rates of bimolecular
exchange.
pri2dim that possess a higher flexibility around the N-C-C-N
bond chain. No previous kinetic studies can be found on these
systems or information on the extent to which such ligands favor
electron delocalization from platinum. However, the results of
this study seem to suggest that the ease with which both the
backbone and the bite angle53 of these molecules can be distorted
on going from the square planar to the trigonal bypiramidal
configuration does play a significant role in controlling the
substitution rate.
In conclusion, the overall structure-reactivity relationship for
these complexes having the same array of donor atoms around
the metal, has been rationalized on the basis of an interplay of
electronic and steric effects. It is worth of interest to note that
a fine tuning of the properties of the “spectator” ligands can
lead to platinum(II) substrates of very high reactivity, compa-
rable or even by far greater (as for the Me2phen complex) to
that of similar palladium(II) species.47 A study to ascertain
whether these highly reactive Pt(II) compounds possess catalytic
properties is underway.
Further evidence for a relationship between the lability of
Me2SO and the electrophilicity of the reaction center, as dictated
by the extent of displacement of charge by the spectator ligands,
is given by the dependence of the reactivity of complexes of
substituted phenanthrolines upon their basicity. Setting apart
the highly hindered Me2phen discussed previously, and taking
into account only data points for phenanthrolines in which the
substituents are in positions far away from the reaction center
with little if any possibility of steric interactions, one finds a
linear relationship between the pKa of the phenanthrolines, taken
as a measure of their electron donicity, and the logarithms of
rates constants for ligand exchange, according to the equation
log k2 ) 0.663 - 0.341 pKa (four data points, R ) 0.931, from
linear regression analysis). The correlation is very much
improved (three data points, R ) 0.997) if the point for the
phen complex, which lies above the line determined by the
remainder of the data, is excluded from the analysis. As can
be seen from the slope of the plot, the sensitivity of the reaction
rates to the electron density brought about by substituents on
the phenanthroline is significant (on going from the least basic
NO2phen, pKa ) 3.23, to the most basic Me4phen, pKa ) 6.35,
the reactivity is decreased by a factor of 10) but still modest in
comparison to the overall change of reactivity observed for the
entire set of ligands examined.
Acknowledgment. We wish to thank Dr. B. E. Mann for
helpful discussions and the CNR and MURST for financial
support. Thanks are due also to the reviewers for helpful
suggestions.
Supporting Information Available: Table S1, reporting the
dependence of dimethyl sulfoxide exchange rates, kobs (s-1), of 11 upon
the temperature and [Me2SO], Figure S2, a typical kinetic plot for
isotopic labeling experiment, and Figure S3, an overall plot of the
dependence of kobs (s-1) on [Me2SO] for isotopic labeling experiments
on various complexes (3 pages). Ordering information is given in any
current masthead page.
The reactivity shown by the complexes containing flexible
diimine ligands such as cy2dim and pri2dim, despite their strong
tendency to transfer electron density to the metal,52 appears to
be somewhat greater than that of complexes with the more rigid
and less basic bpy and phen. All these ligands, on coordinating
to the metal in a square planar configuration, form five-
membered rings, whose rigidity varies significantly on going
from the completely rigid phen to bpy, which can undergo some
rotation around the C(2)-C(2′) bond axis, to cy2dim and
IC960107G
(52) A value of pKa ) 10.3 for pri2dimH+ was measured potentiometrically
in water at 298 K and µ ) 0.1 (NaCl) and it can be compared with
the value of pKa ) 4.86 for phenH+, taken from: Smith, R. M.;
Martell, A. E. Critical Stability Constants; Plenum Press: New York,
1975; Vol. 2.
(53) Casey, C. P.; Whiteker, G. T.; Melville, M. G.; Petrovich, L. M.;
Gavney, J. A.; Powell, D. R. J. Am. Chem. Soc. 1992, 114, 5535.