Atropisomerization, C-H activation, and dissociative substitution at some biphenyl platinum(II) complexes

Article abstract of DOI:10.1021/ja030486u

The reaction of 2,2′-dilithiumbiphenyl with cis-[PtCl ₂(SEt₂)₂] at -10 °C in diethyl ether not only leads to the main product [Pt₂(μ-SEt₂) ₂(bph)₂], containing the planar 2,2′-bi

Full text of DOI:10.1021/ja030486u

Published on Web 04/28/2004
Atropisomerization, C-H Activation, and Dissociative
Substitution at Some Biphenyl Platinum(II) Complexes
Maria Rosaria Plutino,‡ Luigi Monsu’ Scolaro,†,§ Alberto Albinati,| and
Raffaello Romeo*,†
Contribution from the Dipartimento di Chimica Inorganica, Chimica Analitica e Chimica Fisica,
UniVersita` di Messina - Salita Sperone, 31 - Vill. S. Agata - 98166 Messina, Italy,
ISMN - CNR, Sezione di Palermo, Unita` di Messina, INFM, Unita` di Messina, and
Dipartimento di Chimica Strutturale e Stereochimica Inorganica, Facolta` di Farmacia,
UniVersita` di Milano, 20133 Milano, Italy
Received August 7, 2003; E-mail: raf.romeo@chem.unime.it
Abstract: The reaction of 2,2′-dilithiumbiphenyl with cis-[PtCl2(SEt2)2] at -10 °C in diethyl ether not only
leads to the main product [Pt2(µ-SEt2)2(bph)2], containing the planar 2,2′-biphenyl dianion (bph2-), but also
forms a new dinuclear platinum(II) compound of formula [Pt2(µ-SEt2)2(Hbph)4], 1a (Hbph- ) η1-biphenyl
monoanion), in which each metal is in a square-planar environment. NMR spectroscopy and molecular
mechanics (MMFF) calculations were used to characterize 1a. The results suggest that the favored
conformation for the four Hbph biphenyl groups is RââR. In chloroform solution, 1a undergoes atrop-
isomerization to 1b (RâRâ) (kis ) 1.03 × 10-4 s-1, at 298 K) that subsequently cyclometalates (kobs ) 4.48
× 10-6 s-1, at 298 K) to yield [Pt2(µ-SEt2)2(bph)2] and biphenyl. Both processes, atropisomerization and
C-H activation, presumably involve preliminary thioether bridge splitting. The dinuclear complex 1a has
been shown to be a versatile and useful precursor to a variety of mononuclear η1-biphenyl platinum(II)
complexes. By reaction with diethyl sulfide, dimethyl sulfoxide, or with rigid dinitrogen containing ligands,
such as 2,2′-bipyridine or 1,10-phenanthroline, complexes cis-[Pt(Hbph)2(dmso)2] 3, cis-[Pt(Hbph)2(SEt2)2]
4, [Pt(Hbph)2(bpy)] 5, and [Pt(Hbph)2(phen)] 6 were obtained, respectively. The crystal structures of
compounds 5 and 6 were determined. Only the head-to-tail isomer of these compounds was recognized
in the solid state and in solution, where restricted rotation around the Pt-C bond prevents interconversion
to the head-to-head form. A detailed kinetic study of ligand (dmso) exchange and substitution (by 2,2′-
bipyridine and 1,10-phenanthroline) has been performed on complex 3 in CDCl3 and toluene-d8 by 1H
NMR magnetization transfer experiments, and in toluene by UV/vis spectroscopy, respectively. The rates
of both processes show no dependence on ligand concentration, the rate of ligand substitution being in
reasonable agreement with that of ligand exchange at the same temperature. The kinetics are characterized
by largely positive entropies of activation. The results are consistent with a dissociative mode of activation
analogous to the pattern already found for compounds with a similar [Pt(C,C)(S,S)] set of coordinating
ligands. The role of ML3 d8 T-shaped 14-electron species, as elusive reaction intermediates or structurally
characterized compounds, is discussed.
Introduction
picture currently appears to be reasonably clear, and it involves
the direct attack of a nucleophile on the substrate with formation
A rational design of square-planar platinum(II) compounds
with specific therapeutic properties requires a deep knowledge
of their substitution behavior, because it is necessary to predict
what happens along the way to the biological target and what
the reactivity will be in a biological environment.1 Thus,
following Rosenberg’s discovery of the antitumor properties of
cis-platin,2 studies on substitution reactions of Pt(II) complexes
have received an increasing renewal of interest. The mechanistic
of a transient five-coordinate intermediate.3 Old concepts have
been revisited, such as the trans and cis effects in their σ and π
components,4 steric effects, nucleophilic and nucleofuge abilities
(3) (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, 1991. (c) Tobe, M. L. In Comprehensive Coordination Chemistry;
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329. (d) Atwood, J. D. Inorganic Organometallic Reaction Mechanisms;
Brook/Cole: Monterey, CA, 1985. (e) Basolo, F.; Pearson, R. G. Mech-
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H.; Gray, H. B. Ligand Substitution Processes; W. A. Benjamin: New
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† Dipartimento di Chimica Inorganica, Universita` di Messina.
‡ ISMN - CNR, Unita` di Messina.
§ INFM, Unita` di Messina.
| Universita` di Milano.
(1) Cisplatin: Chemistry and Biochemistry of a Leading Anticancer Drug;
Lippert, B., Ed.; Wiley-VCH: Weinheim, 1999.
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222, 385-386.
9
6470
J. AM. CHEM. SOC. 2004, 126, 6470-6484
10.1021/ja030486u CCC: $27.50 © 2004 American Chemical Society

Biphenyl Platinum(II) Complexes
A R T I C L E S
of the ligands, or the discrimination ability of the substrate. New
concepts have been introduced as well, such as a fine electronic
tuning of the lability of Pt(II) complexes on changing systemati-
cally the nature of ancillary noninnocent ligands,5 or making
use of hyperconjugation effects, 6,7 or the possibility of inducing
extra-lability5 or fluxionality8 to ligands through overcrowding
of the coordination plane. Within this mechanistic framework,
a rational control of the molecular architecture and reactivity
of Pt(II) compounds is now at hand, even though the lability of
these compounds will still depend in a manifold and rather
intricate way on the above-mentioned factors.
A most difficult task is to induce a dissociative pathway to
square-planar compounds, in view of the great electronic and
steric propensity of four-coordinated 16-electron species to add
a fifth ligand and to form five-coordinate 18-electron species
either as discrete compounds9 or as reaction intermediates.3
There is a great interest in the possibility of considering
alternative low-energy pathways with three-coordinate 14-
electron d8-ML3 species as key intermediates.10 Dissociation
(usually of a neutral ligand) is often proposed as the initial step
in many reactions involving square-planar d8 organometallic
complexes.11 This dissociation enables subsequent processes
such as â-hydrogen elimination,12,13 insertion of olefins into the
M-H bond,14 reductive elimination,15 electrophilic attack at the
Pt-C bond,16 geometrical16,17 or configurational18 isomerization,
exchange reactions,19 and fluxional motions of coordinated
ligands.8,20 Another promising feature of the unsaturated plati-
num(II) chemistry is the successful C-H activation under mild
conditions by complexes of the form [Pt(N-N)(CH3)(solv)]+,
where N-N is a bidentate nitrogen-centered ligand and “solv”
is a weakly coordinating solvent.21 Knowledge gained from these
studies builds upon the seminal work by Shilov22 and may allow
for the catalytic activation and subsequent functionalization of
one of the less reactive bonds in organic molecules.
After the first unsuccessful attempts, focused on steric block-
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destabilization by use of ligands with a high trans influence,24
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14-e species as key intermediates in nucleophilic substitution
reactions came from kinetic studies of electron-rich complexes
having a set of donor atoms of the type cis-[Pt(C,C)(S,S)] (C is
a strong σ-donor carbon group; S ) thioether or sulfoxide).25-31
The underpinnings of the observed easy dissociation are: (i)
bond weakening at the leaving group due to the trans influence
of strong σ-donors, (ii) high electron density at the metal which
prevents the axial approach of the incoming nucleophile, and
(iii) the stabilization by the remaining set of three in-plane
ligands of a three-coordinated T-shaped 14-e intermediate
without changing the singlet ground state. Also, it has become
evident that other appropriate sets of strong σ-donor groups can
favor sufficient accumulation of electron density at the metal
and stabilize the 14-e intermediate, as for cis-[PtMe2(PR3)-
(dmso)] (PR3 ) isosteric tertiary phosphanes)32 or cis-[Pt-
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A R T I C L E S
Plutino et al.
Scheme 1
ward a tetrahedral arrangement. These favorable factors do not
apply to ligands which exhibit major π-acceptor capabilities,
as for cis-[PtPh2(CO)(SMe2)], where the relief of the excess of
electron density at the metal by carbon monoxide and the
presence of a low-lying LUMO, π-delocalized over the Pt-
C-O atoms, facilitate a changeover of mechanism from
dissociative to associative.34 The 2,2′-biphenyl dianion (bph2-),
a cyclometalating analogue of 2,2′-bipyridine, combines cyclo-
metalation and a favorable in-plane disposition of the aryl rings
for π interaction: contrary to expectation, recent kinetic and
theoretical studies of the complexes [Pt(bph)(SR2)2]25 indicated
that thioether dissociation is much easier than in the diaryl
analogue cis-[PtPh2(SR2)2]. 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.
coordinated to the metal. The complex is slightly soluble in
chlorinated solvents where it undergoes a slow conversion to
the species 1b, as depicted in eq 1 of Scheme 1. Finally,
intramolecular C-H activation on the isomer 1b affords the
dinuclear cyclometalated species [Pt2(µ-SEt2)2(bph)2], the only
product isolated and characterized originally by von Zelewsky
et al.35 using the same procedure (eq 2, Scheme 1).
Compound 1a is a useful synthon for the synthesis of
mononuclear platinum(II) derivatives containing the organo-
metallic skeleton Pt(C,C) in a head-to-tail conformation: (i)
addition of 2 equiv of dimethyl sulfoxide or diethyl sulfide per
platinum(II) yielded complexes 3 and 4, respectively; (ii) bridge
splitting reactions with bidentate nitrogen ligands (bpy or phen)
afforded the diimine-containing species 5 and 6, respectively.
All of the experimental evidence, including the X-ray molecular
structure of 5 and 6 (see below), showed the two Hbph
fragments to be tilted from the coordination plane and in a head-
to-tail conformation. These species maintain an apparently
stereochemically rigid structure in solution (unchanged 1H and
13C NMR spectra up to 333 K) because of restricted rotation of
the Hbph- ligands around the Pt-C bond.36
A view of the structures of compounds 5 and 6, the first
reported so far for platinum(II) compounds with η1-Hbph-
ligands, is given in Figures 1 and 2. Crystallographic data and
selected bond distances and angles are listed in Tables 1 and 2,
respectively. Both complexes possess a slightly distorted square-
planar coordination consisting of the two nitrogen atoms of the
bipy and phen ligands, respectively, and one carbon atom from
each of the two biphenyl moieties. The Pt-C and Pt-N
distances were found to be substantially identical (within 3σ’s)
in both compounds [av. 2.00(3) and 2.10(2) Å, respectively].
The Pt-C distances are as expected37 and can be compared to
We report here a detailed kinetic study of ligand (dmso)
exchange and substitution (by 2,2′-bipyridine and 1,10-phenan-
throline) on the complex cis-[Pt(Hbph)2(dmso)2] (Hbph- ) η1-
biphenyl monoanion). The results are consistent with a disso-
ciative mode of activation by analogy with the behavior already
found for similar [Pt(C,C)(S,S)] compounds. The kinetic data
on these systems are now adequate to establish to what extent
dissociation depends on the nature of the bonded organic moiety
(alkyl or aryl), its size, and its orientation with respect to the
coordination plane. During the synthetic workup, we observed
the formation of a new dinuclear platinum(II) compound with
thioether bridges of formula [Pt2(µ-SEt2)2(Hbph)4], 1a, which
in chloroform undergoes atropisomerization to 1b followed by
C-H activation to yield [Pt2(bph)2(µ-SEt2)2] (bph ) 2,2′-
biphenyl dianion) and biphenyl. The conformations of 1a and
1b were characterized by NMR spectroscopy and molecular
mechanics calculations, and the interconversion and cyclo-
1
metalation were monitored by H NMR.
(35) Cornioley-Deuschel, C.; von Zelewsky, A. Inorg. Chem. 1987, 26, 3354-
3358.
(36) Some other selected examples of restricted aryl rotation around the M-C
bond are as follows: (a) Brown, J. M.; Pe´rez-Torrente, J. J.; Alcock, N.
W. Organometallics 1995, 14, 1195-1203. (b) Rieger, A. L.; Carpenter,
G. B.; Rieger, P. H. Organometallics 1993, 12, 842-847. (c) Anderson,
G. K.; Cross, R. J.; Manojlov´ıc-Muir, L.; Muir, K. W. Organometallics
1988, 7, 1520-1525. (d) Albeniz, A. C.; Casado, A. L.; Espinet, P.
Organometallics 1997, 16, 5416-5423. (e) Casares, J. A.; Coco, S.; Espinet,
P.; Lin, Y. S. Organometallics 1995, 14, 3058-3067. (f) Albeniz, A. C.;
Casado, A. L.; Espinet, P. Inorg. Chem. 1999, 38, 2510-2515. (g)
Anderson, G. K.; Black, D. M.; Cross, R. J.; Robertson, F. J.; Rycroft, D.
S.; Wat, R. K. M. Organometallics 1990, 9, 2568-2574.
Results
Synthesis and Structure of Complexes. In the dinuclear
complex 1a, obtained in a pure form from the reaction of 2,2′-
dilithiumbiphenyl (in a 2:1 stoichiometric ratio) with cis-[Pt-
(SEt2)2Cl2] in Et2O (19% yield), each metal atom is in a square-
planar environment and the biphenyl monoanion is η1-
(34) Romeo, R.; Grassi, A.; Monsu` Scolaro, L. Inorg. Chem. 1992, 31, 4383-
(37) International Tables for X-ray Crystallography, 2nd ed.; Kluwer Aca-
demic: Dordrecht, NL, 1999; Vol. C, Chapters 9.5 and 9.6.
4390.
9
6472 J. AM. CHEM. SOC. VOL. 126, NO. 20, 2004

Biphenyl Platinum(II) Complexes
A R T I C L E S
Table 1. Experimental Data for the X-ray Diffraction Study of
Compounds [Pt(Hbph)2(bpy)] (5) and [Pt(Hbph)2(phen)]‚(C7H8)
[6‚(C7H8)]
compound
formula
mol wt
diffractometer
cryst syst
space group
T, K
a, Å
b, Å
c, Å
5
C
6‚(C7H8)
C43H26 N2Pt
765.75
Bruker SMART CCD
monoclinic
Cc
34H24N2Pt
655.64
CAD4
monoclinic
P21/n
298 (2)
14.006 (4)
12.901 (3)
14.637 (6)
107.13 (4)
2527 (2)
4
200 (2)
17.5109 (3)
11.4963 (2)
17.9161 (2)
115.058 (1)
3267.2 (1)
4
â, deg
V, Å3
Z
F
(calcd), g cm-3
1.723
55.78
1.557
43.28
µ, cm-1
radiation
Mo KR (graphite
monochromated λ )
0.71069 Å)
3.0 < θ < 23.0
3643
Figure 1. ORTEP view of a molecule of compound 5 showing 50%
probability ellipsoids for the non-hydrogen atoms.
θ range (deg)
no. data coll
2.2 < θ < 27.5
16 265
no. independent data
no. obs reflns (no)
3487
7312
2076
6072
2
2
2
2
[|Fo| > 4.0σ(|F| )]
0.86-1.00
0.050
[|Fo| > 4.0σ(|F| )]
0.60-0.98
0.040
transmission coeff
a
Rav
Rb (obs reflns)
0.0398
0.0962
0.982
0.0352
0.0757
0.976
2 c
Rw
GOFd
a Rav ) ∑|Fo - Fo ,av|/∑i|Fo |. b R ) ∑(|Fo - (1/k)Fc|)/∑|Fo|. c Rw
)
.
2
2
2
2
[∑w(Fo2 - (1/k)Fc2)2/∑w|Fo | ]. d GOF: [∑w(Fo2 - (1/k)Fc2)2/(no - nv)]1/2
2 2
Table 2. Selected Bond Lengths (Å), Bond Angles (deg), and
Dihedral Angles (deg) for [Pt(Hbph)2(bpy)] (5) and
[Pt(Hbph)2(phen)] (6)
[Pt(Hbph)2(bpy)] (5)
[Pt(Hbph)2(phen)] (6)
Pt-N(1)
Pt-N(2)
Pt-C(2a)
Pt-C(2b)
2.11(1)
2.08(1)
2.01(1)
1.99(1)
2.10(1)
2.11(1)
1.97(2)
2.03(2)
Figure 2. ORTEP view of a molecule of compound 6 showing 50%
probability ellipsoids for the non-hydrogen atoms.
N(1)-Pt-N(2)
N(1)-Pt-C(2b)
N(2)-Pt-C(2a)
C(2a)-Pt-C(2b)
77.6(4)
79.0(2)
96.6(5)
95.3(6)
89.1(2)
97.5(4)
96.5(4)
88.4(5)
those found in similar complexes, for example, in [Pt(mesityl)2-
(dppz)] [av. 2.00(1) Å],38 in [Pt (2,9-dmphen)R2] (R ) Ph,
mesityl), and in [Pt(mesityl)2(phen)] [av. 2.016(5) Å].39
A
N(1)-Pt-C(2b)-C(3b)
N(2)-Pt-C(2a)-C(3a)
C(2a)-C(1a)-C(7a)-C(8a)
C(2b)-C(1b)-C(7b)-C(8b)
-78.8(9)
-69(1)
-71(1)
-53(2)
-51(2)
-75(1)
-47(2)
-48(2)
similar trend can be found for the Pt-N distances: values
around 2.10 Å are found in the above-mentioned complexes
and in the literature.40 As expected, the angles at the Pt atom
are constrained by the bite of the chelating ligand. Thus, for
compound 5, the N1-Pt-N2 angle is 77.6(4)°, while for 6 it
is 79.0(2)°. Moreover, the angles with the trans ligand (i.e.:
N1-Pt-C2a and N2-Pt-C2b), of ca. 175° and 173°, are close
to their ideal values. In both complexes, a small tetrahedral
distortion is observed, with maximum deviations of the atoms
coordinated to the platinum center from their lsq-plane of ca.
(0.03 Å for 6 and (0.06 Å for 5. The bpy and phen ligands
also lie in the coordination plane; the dihedral angle between
the lsq-plane defined by atoms Pt, C2a, C2b and that defined
by the C atoms of the bipyridine is 9.7(5)°, while the
corresponding angle in 6 is 1.3(1)° only. It may also be noted
that the two rings of the bpy ligand are not coplanar, but are
rotated by 5.9(7)°, as a consequence of the coordination to the
metal.
angles in Table 2, and (ii) the dihedral angles between the lsq-
planes C1a-C6a and C1b-C6b and that defined by Pt-N1-
N2. Thus, in 5, these angles are 76.4(3)° and 78.9(3)°,
respectively, while in 6 the corresponding values are 71.2(5)°
and 62.4(3)°. The biphenyl moieties are in both compounds in
a head-to-tail disposition (see Figures 1 and 2). Moreover, the
two rings are not coplanar, but rotated by 43.7(5)° and 46.8(5)°,
respectively, in 5; for 6, the corresponding values are 52.4(5)°
and 62.4(3)°. There is no significant inter- or intramolecular
stacking (π-π interactions). The shortest intermolecular separa-
tions are found in 6 between the phenyl rings and the
phenanthroline ligand of symmetry related molecules (in the
range 4.8-5.1 Å).
NMR Conformational Analysis. All of the proton and
carbon resonances belonging to the species 1-6 were assigned
through NMR spectroscopy from the connectivities in 2D-COSY
and 13C,1H-HMQC experiments. Sketches of complexes 1-6,
together with the adopted numbering scheme, are given in Chart
The rings of the biphenyl moieties are tilted with respect to
the coordination plane, as judged from: (i) the relevant torsion
(38) Klein, A.; Scheiring, T.; Kaim, W. Z. Anorg. Allg. Chem. 1999, 625, 1177-
1180.
(39) Klein, A.; McInnes, E. J. L.; Kaim, W. J. Chem. Soc., Dalton Trans. 2002,
2371-2378.
1
1. The relevant H and 13C NMR data are collected in Tables
1
(40) Allen, F. H. Acta Crystallogr. 2002, B58, 380-388.
3 and 4, respectively. H NMR spectra on freshly dissolved
9
J. AM. CHEM. SOC. VOL. 126, NO. 20, 2004 6473

A R T I C L E S
Plutino et al.
Table 3. Collection of the 1H NMR Resonances of the Platinum(II) Complexes 1-6a
δ1H(JPtH
)
H
H
H
6a,6b
3a,3b
4a,4b
(3JPtH
)
(4JPtH
)
H
(4JPtH
)
H
8a,12a+H
H
9a,11a+H
H
10a,10b
others
2.69 (S-CH2b-CH3), 2.67 (S-CH2a-CH3),
5a,5b
8b,12b
9b,11b
1a
1b
6.28
(71.0)
6.48
6.52
6.64
6.71
6.81
6.84
7.61
7.58
7.41
7.43
7.41
7.33
2.00 (S-CH2-CH3a), 0.29 (S-CH2-CH3b
)
6.94
3.40 (S-CH2a-CH3), 3.19 (S-CH2b-CH3),
2.94 (S-CH2a′-CH3), 2.88 (S-CH2b′-CH3),
(69.0)
1.83 (S-CH2-CH3a), 1.30 (S-CH2-CH3b
5.05 (2JPtH ) 42.8 Hz, CH1,5),
)
2
6.20
(69.3)
6.64
(13.8)
6.76
6.93
(26.4)
7.44
7.33
7.33
4.81 (2JPtH ) 39.2 Hz, CH2,6),
2.46+2.36 ((CH2)4,8), 2.30 ((CH2)3,7
)
3
4
5
6
5.85
(73.4)
6.59
(78.8)
6.79
(70.5)
6.90
(71.8)
6.57
(13.5)
6.59
6.82
6.76
6.87
6.93
7.06
(20.3)
7.03
(23.4)
7.12
7.85
7.96
7.77
7.82
7.39
7.30
7.05
6.98
7.36
7.26
7.05
6.98
3.10 (3JPtH ) 12.1, S-CH3a),
2.29 (3JPtH ) 16.0, S-CH3b
)
2.31 (3JPtH ) 25.8, S-CH2-CH3),
1.13 (S-CH2-CH3)
6.71
6.77
8.41 (3JPtH ) 22.6 Hz, H6,6′), 7.97 (H4,4′),
7.89 (H3,3′), 7.25 (H5,5′
8.68 (3JPtH ) 21.1, H2,9), 8.45 (H4,7),
7.86 (H5,6), 7.57 (H3,8
)
7.17
)
a Recorded in CDCl3 as solvent at 298.2 K. Chemical shifts (δ) in ppm relative to TMS and coupling constants, given in parentheses together with the
assignment, in hertz.
Table 4. Collection of the 13C{1H} NMR Resonances of the Platinum(II) Complexes 1-6a
δ13C(JPtC
)
C
C
C
C
C
C
C
C
C
+
8b,12b
C
C
+
9b,11b
1a,1b
2a,2b
3a,3b
4a,4b
5a,5b
6a,6b
7a,7b
8a,12a
9a,11a
(2JPtC
)
(1JPtC
)
(2JPtC
)
(3JPtC
)
(4JPtC
)
(3JPtC
)
(3JPtC
)
C
10a,10b
others
c
c
c
1ab
1b
-
-
-
-
135.7
136.1
125.2
125.3
122.2
122.5
128.8
129.2
-
-
129.7
127.2
125.8
127.0
33.1 (S-CH2b-CH3), 22.8 (S-CH2a-CH3),
11.9 (S-CH2-CH3a), 9.51 (S-CH2-CH3b
)
c
c
c
127.2
128.8
33.7 (S-CH2b-CH3), 33.3 (S-CH2b′-CH3),
30.5 (S-CH2a-CH3), 30.3 (S-CH2a′-CH3),
12.9 (S-CH2-CH3a), 12.5 (S-CH2-CH3b
)
2
148.0
(39)
151.4
(1117)
136.2
(14)
125.7
(68)
122.4
(9)
129.0
(59)
146.6
(34)
129.7
127.1
125.9
108.2 (1JPtC ) 67 Hz, C1,5),
99.7 (1JPtC ) 41 Hz, C2,6),
30.9 (C4,8), 28.4 (C3,7
)
3
4
5
6
147.1
(41)
148.3
(42)
143.5
(1041)
146.5
(1137)
144.3
136.4
(26)
138.1
(22)
125.8
(73)
125.1
(79)
123.6
(11)
121.7
(10)
128.9
(57)
128.4
(62)
146.5
(28)
146.2
(25)
129.8
130.0
130.1
130.1
127.3
126.8
126.4
126.3
126.2
125.3
124.6
124.6
43.9 (2JPtC ) 20 Hz, S-CH3a),
42.3 (2JPtC ) 37 Hz, S-CH3b
27.6 (S-CH2-CH3),
)
13.1 (3JPtC ) 16 Hz, S-CH2-CH3)
148.6
140.2
125.1
121.9
128.0
147.9
155.7 (C2,2′), 150.1 (C6,6′), 135.7 (C4,4′),
129.9 (C3,3′), 126.9 (C5,5′
155.6 (C10,10′), 151.3 (C4′,6′), 150.1 (C2,9),
135.7 (C4,7), 126.9 (C5,6), 125.4 (C3,8
)
148.6
144.3
140.1
125.1
121.5
128.0
147.9
)
a Recorded in CDCl3 as solvent at 298.2 K. Chemical shifts (δ) in ppm relative to TMS and coupling constants, given in parentheses together with the
assignment, are in hertz. b At T ) 273 K. c Not detectable.
Chart 1. Sketches for Complexes 1-6, Together with the
Adopted Numbering Scheme
samples of 1a in CDCl3 reveal a pattern of signals typical of a
pair of η1-biphenyl ligands spanning mutual cis positions, with
resonances at δ 7.61 (H8a,12a+H8b,12b), 7.41 (H9a,11a+H9b,11b
H
+
10a,10b), 6.84 (H6a,6b), 6.71 (H5a,5b), 6.52 (H4a,4b), and 6.28 (3JPtH
) 71.0 Hz, H3a,3b). The ortho-protons H3a,3b were unambiguously
attributed on the basis of the satellite peaks due to coupling
with 195Pt nuclei (I ) 1/2, 33% natural abundance). The pattern
of the aromatic region in this compound is symmetric and strictly
similar to that observed for the mononuclear complexes 3-6
containing the same molecular fragment cis-[Pt(Hbph)2] in a
head-to-tail conformation. In fact, these mononuclear species
are formed upon bridge-splitting reaction of various ligands on
1a (see Experimental Section) and retain memory of the cis-
[Pt(Hbph)2] conformation of the dinuclear precursor. The
aliphatic region shows a composite multiplet at δ 2.69 (4H,
S-CH2b-CH3) and 2.67 (4H, S-CH2a-CH3), and two well-
separated triplets relative to the methyl protons at δ 2.00 (6H,
S-CH2-CH3a) and 0.29 (6H, S-CH2-CH3b). By lowering the
temperature to 223 K, the multiplet relative to the methylene
protons of the bridging diethyl sulfide separates into a pair of
well-resolved broad quartets at 2.75 and 2.65 ppm, together with
the two expected triplets at 2.09 and 0.27 ppm for the methyl
groups. Interestingly, the structure of strictly similar compounds,
9
6474 J. AM. CHEM. SOC. VOL. 126, NO. 20, 2004

Biphenyl Platinum(II) Complexes
A R T I C L E S
Chart 2. Lowest Energy Calculated Structures of the RââR (1a) and RâRâ (1b) Conformations of [Pt2(Hbph)4(µ-SEt2)2] Together with the
Schematic Representations of the Five Possible Atropisomers and Relative Differences in Strain Energies (kcal/mol)
[Pt2Me4(µ-SEt2)2]41 and [Pt2(p-tol)4(µ-SEt2)2],42 consists of
centrosymmetric dimers with a planar Pt2S2 unit. In the latter
compound, the C6 plane of the p-tolyl groups forms angles of
78.85° [C(1-6)] and 65.56° [C(8-13)] with the Pt2S2 plane.
The diethyl sulfide ligands are symmetrically coordinated to
Pt(1) and Pt(2), and no significant bonding interaction between
the platinum atoms was found.43
isomers or atropisomers can be predicted for 1a. Molecular
mechanics force field (MMFF) minimized molecular models
of such species, together with the calculated differential values
of their potential energy, are shown in Chart 2. As expected,
(43) In view of the recent observation of a cyclic trimeric structure for [PtPh2-
(SMe2)]3 (Song, D.; Wang, S. J. Organomet. Chem. 2002, 648, 302-305),
in addition to the well-known dinuclear structures of [Pt2Ph4(µ-SMe2)2]
and of similar thioether-bridged platinum(II) compounds (see refs 41 and
42), a referee has suggested an additional control of the structure adopted
by the diaryl compound 1a. Fast atom bombardment mass spectrometry
(FABMS) analysis of 1a, with a Xenon pressure of 10-5 Torr, 6 keV energy,
and 1 mA ion current, gave extensive and indeterminate fragmentation. In
the matrix-assisted laser-desorption-ionization mass spectrometry analysis
(MALDI-TOF-MS; matrix: 2,5-dihydroxybenzoic acid in CH2Cl2), the
highest m/z ion observed, at 1001.20, corresponds to the protonated fragment
[M - R1 - R2 + H]+ arising from the loss of a bis-phenyl (R1 ) C12H9)
and an ethyl (R2 ) C2H5) group from 1a. Both the position of the highest
m/z peak in the MALDI-TOF spectrum and the isotope distribution, which
is very close to the calculated one (see Figure S3 in the Supporting
Information), provide unambiguous evidence for a dimeric structure.
Monomeric or trimeric Pt(II) species should exhibit markedly different
isotopic distributions. The spontaneous isomeric conversion of 1a into 1b
in dichloromethane was monitored for 5 h at room temperature by 1H NMR,
after which the sample showed a MALDI-TOF peak pattern coincident
with that obtained for 1a, together with other fragments due to the final
cyclometalated dinuclear product, thus confirming that the conversion of
1a onto 1b is indeed an isomerization process.
On standing at 298 K, 1a progressively converts into the
cyclometalated species [Pt2(bph)2(µ-SEt2)2] and biphenyl, ac-
1
cording to eq 2 in Scheme 1. H NMR spectra showed the
intermediacy of 1b in this process, with a pattern closely similar
to that of the parent 1a, which implies the presence of the
molecular fragments cis-[Pt(Hbph)2], together with bridging
diethylthioethers.
Because of the different relative orientations of the four
surrounding η1-biphenyl anions, a series of at least five rotational
(41) Bancroft, D. P.; Cotton, F. A.; Falvello, L. R.; Schwotzr, W. Inorg. Chem.
1986, 25, 763-770.
(42) Casado Lacabra, M. A.; Canty, A. J.; Lutz, M.; Patel, J.; Spek, A. L.; Sun,
H.; van Koten, G. Inorg. Chim. Acta 2002, 327, 15-19.
9
J. AM. CHEM. SOC. VOL. 126, NO. 20, 2004 6475

A R T I C L E S
Plutino et al.
different atropisomers exhibit different stability and in particular
those having at least a head-to-head disposition of the Hbph
groups on a metal atom (Râââ, ââRR, and RRRR). The most
stable conformations are the alternating RââR and RâRâ,
characterized by a head-to-tail arrangement of the Hbph groups
on both metal centers. Chart 2 reports also side views of the
two atropisomers 1a and 1b, which clearly display largely
different chemical environments for the two ethyl groups on
the sulfur atom of a bridging thioether. The experimental
observation of symmetrical patterns for the aromatic region of
1a and 1b rules out the higher energy atropisomers, and the
NMR evidence, together with 2D-NOE experiments (Table S13
reports a complete map of the observed nOe dipolar contacts),
points to the alternating species RââR and RâRâ.
Now, we must address the problem of establishing the relative
conformations of 1a and 1b. The mutual cis position and the
predominance of a head-to-tail disposition of the two pendant
aryl groups in the cis-[Pt(Hbph)2] fragment of 1a are fully
confirmed by strong interligand nOe’s cross-peaks between the
H8a,12a protons of a Ph-C fragment and the H3b proton that lies
in front of them, on the opposite cis phenyl ring directly
coordinated to platinum(II). Furthermore, the resonances of the
ortho-protons H3a,3b (δ ) 6.28 ppm) are consistently shifted to
lower frequencies with respect to those of the similar H2,6
protons in complexes of the type cis-[Pt(Ph)2(SR2)]27 (R ) a
series of alkyl and aryl groups, δ ) 7.2-7.7 ppm) or in cis-
[Pt(Ph)2(dmso)2]30 (δ ) 7.26 ppm), as a result of ring current
from the phenyl substituent [C(7-12)] on the Hbph group bound
in the cis position to the same metal center. Intense cross-peaks
are also evident between H8a,12a and H9a,11a on Hbph substituents
belonging to different metal centers. Particularly diagnostic in
establishing the conformation of 1a were the intramolecular
cross-peaks between aliphatic (SEt2) and aromatic Hbph protons.
The methylene protons (S-CH2-CH3) of the central diethyl
sulfide show two nOe contacts, at δ 2.69+2.67 ppm, with H8a,12a
and H3a. Most important, while the triplet of the methyl protons
(S-CH2-CH3a) at 2.00 ppm evidences a major nOe cross-peak
with H3a, the signal at 0.29 ppm (S-CH2-CH3b) has only a
selective nOe contact with H8a,12a. The relevant difference of
chemical shift between the two methyl groups is indicative of
a large diversity in the magnetic environments for these groups
on the same sulfur atom, and therefore the RââR conformation
remains the most probable one for 1a.
Figure 3. Aliphatic section of the phase-sensitive 1H-2D NOESY spectrum
relative to a chloroform-d solution of the two atropisomers 1a (empty circles)
and 1b (filled circles), and the cyclometalated species [Pt(bph)(SEt2)]2 (filled
squares) at 273 K. The “filled-in” cross-peaks arise from exchange, whereas
the “open” cross-peaks arise from nOe’s.
1
the H NOESY experiment in Figure 3, it is possible to infer
the occurrence of chemical exchange on the NMR time-scale
between the ethyl groups belonging to the same sulfur atom.
Well-resolved exchange cross-peaks are evident for the eight
protons of the four methylene signals in 1b and for the 12
protons of the two magnetically unequivalent methyl protons
in each isomer. The most probable mechanism for this dynamic
process involves preliminary bridge splitting and subsequent
inversion of configuration at the uncoordinated sulfur atom. This
latter process is well known for chalcogen donor atoms (S, Se,
and Te) in metal complexes,44 usually determining interchange
of diastereotopic protons belonging to prochiral methylene
groups, and for dinuclear species containing the Pt2S2 unit.45
Furthermore, a series of additional intense cross-peaks suggests
the presence of minor fast exchanging species, which could
contribute to the overall process.
1
Phase-sensitive H NOESY experiments on a chloroform-d
Kinetics: (i) Atropisomerization and Cyclometalation.
Freshly prepared chloroform solutions of the dinuclear species
1a (RââR) evolve irreversibly toward the cyclometalated species
[Pt2(bph)2(µ-SEt2)2] and free biphenyl, through the intermediacy
of the unstable conformer 1b (RâRâ). The overall reaction is
described as a sequence of two irreversible consecutive steps
(eqs 1 and 2 in Scheme 1). The NMR features of the final
cyclometalated product were reported previously.35 The integrals
of selected peaks of all species were measured as a function of
time, and the relative calculated concentrations were used in
the fitting procedure (Figure 4). The rate of the first step, the
solution of 1a, cooled at 273 K after 12 h of standing at ambient
temperature, showed the presence of both 1a and 1b, together
with the final cyclometalated species [Pt2(bph)2(µ-SEt2)2] and
free biphenyl (Figure 3). As expected for an RâRâ conformation,
each methylene group of a bridging SEt2 ligand in 1b is
diastereotopic, as evidenced by the two pairs of multiplets (see
Table 3). The separation between the methyl peaks (triplets,
S-CH2-CH3a, 1.83 ppm; S-CH2-CH3b, 1.30 ppm) is smaller
than in 1a. Additional evidence for the RâRâ conformation in
1b comes from other specific nOe contacts between the aromatic
and the aliphatic proton signals listed in Table S13 of the
Supporting Information.
(44) (a) Abel, E. W.; Bhargava, S. K.; Orrell, K. G. Prog. Inorg. Chem. 1984,
32, 1-118. (b) Gummin, D. D.; Ratilla, E. M. A.; Kostic, N. M. Inorg.
Chem. 1986, 25, 2429-2433. (c) Galbraith, J. A.; Menzel, K. A.; Ratilla,
E. M. A.; Kostic, N. M. Inorg. Chem. 1987, 26, 2073-2078.
(45) Abel, E. W.; Evans, D. G.; Koe, J. R.; Hursthouse, M. B.; Maxid, M.;
Mahon, M. F.; Molloy, K. C. J. Chem. Soc., Dalton Trans. 1990, 1697-
1704.
The methylene resonances of all three Pt(II) compounds (1a,
1b, and [Pt2(bph)2(µ-SEt2)2]) show the expected nOe cross-peaks
with the corresponding methyl protons, and in 1b there are nOe
contacts between the couples of diastereotopic protons. From
9
6476 J. AM. CHEM. SOC. VOL. 126, NO. 20, 2004

Biphenyl Platinum(II) Complexes
A R T I C L E S
Figure 4. Time dependence of the concentration of starting dinuclear
complex 1a, the intermediate 1b, and the final biphenyl cyclometalated
species for the cyclometalation reaction as described in Scheme 1 [k1
)
(1.03 ( 0.01) × 10-4 s-1; k2 ) (4.48 ( 0.05) × 10-6 s-1, in chloroform
at 298 K].
Figure 5. 3D-plot showing the dependence of the observed pseudo-first-
order rate constants (kobsd/s-1) for reaction 6 (N-N ) 2,2′ bipyridine) as a
function of different amounts of entering ligand (bpy) and free dimethyl
sulfoxide. (Experimental conditions: [dmso]/mM ) 10.5; 14.0; 17.0; 22.4
in toluene at 298 K.)
atropisomerization process, was measured by following the
disappearance with time of the methylene signals relative to
the coordinated diethyl sulfide in 1a (at δ 2.69-2.67). The value
of the rate constant k1 ) (1.03 ( 0.01) × 10-4 s-1 was
calculated by a nonlinear least-squares fit to the equation
reaction proceeds, and dmso is generated from the complex.
Deliberate addition of the nucleofuge (dmso) has a retarding
effect on the rate, and the first-order plots become perfectly
linear. At a constant sulfoxide concentration, a curvilinear
dependence on the concentration of the entering nucleophile
(bpy or phen) is observed which levels off to a limiting value
at high concentration. A global 3-D representation of the [dmso]
and [N-N] dependencies is given in Figure 5 for the substitution
of cis-[Pt(Hbph)2(dmso)2] by bpy in toluene at 298 K. The rate
data appear to fit the nonlinear rate law
[1a]t ) [1a]0 exp(-k1t)
(3)
where [1a]0 is the concentration of the starting complex at the
beginning of the reaction. The rate of the second step (cyclo-
metalation) was obtained by monitoring the diethyl sulfide
methylene resonances of both the intermediate species 1b (four
peaks in the 3.40-2.88 ppm range) and the final cyclometalated
complex [Pt2(bph)2(µ-SEt2)2] (bph) (at δ 3.82). The values of
the concentrations derived from the integrals were fitted to the
equations:
kobsd ) a[N-N]/(b[dmso] + [N-N])
(7)
The values of kobsd, [N-N], and [dmso] were fitted to this
expression using a nonlinear least-squares curve-fitting pro-
gram,46 and the best values of the constants a and b were
obtained together with their standard deviations.
(iii) Dimethyl Sulfoxide Exchange. The kinetics of ligand
exchange between coordinated and free dimethyl sulfoxide, see
eq 8
[1b]t ) [1a]0[k1/(k2 - k1)][exp(-k1t) - exp(-k2t)] (4)
[bph]t ) [1a]0 - [1a]t - [1b]t
(5)
which describe the time dependence of [1b] and [bph],
respectively. The values of k1 and k2 derived from a global best
fitting analysis using eqs 3-5 and the SCIENTIST program46
were in good agreement with the values obtained from the
separate application of eqs 3 and 4.
(ii) Dimethyl Sulfoxide Substitution. The displacement
reactions of dimethyl sulfoxide from complex 3 by 2,2′-
bipyridine and 1,10-phenanthroline are represented in eq 6
cis-[Pt(Hbph)2(dmso)2] + 2dmso* f
cis-[Pt(Hbph)2(dmso*)2] + 2dmso (8)
were investigated in chloroform-d or in toluene-d8 as solvent
by the 1H NMR magnetization transfer technique. The calculated
rate constants (kexch/s-1) are collected in Table S17. The ligand
exchange was found to be independent of the concentration of
added sulfoxide. A variable temperature study was undertaken
for the calculations of the activation parameters.
cis-[Pt(Hbph)2(dmso)2] + N-N f
[Pt(Hbph)2(N-N)] + 2dmso (6)
The experiments were carried out in toluene at 298.2 ( 0.05
K. All reactions went to completion and obeyed a first-order
rate law up to 90% conversion. A collection of the pseudo-
first-order rate constants kobsd for reaction 6, at various
concentrations of entering diimine ligand (N-N ) bpy, phen)
and dimethyl sulfoxide, is reported in Tables S14 and S15. The
kinetic data for the temperature dependence of the same
reactions are given in Table S16.
Discussion
Atropisomerization and Intramolecular C-H Activation.
The cis-bis(η1-biphenyl) platinum derivatives 2-6 display static
1H NMR spectra up to 333 K. No fluxional behavior was
observed, and only one atropisomer was found in solution,
suggesting restricted rotation about the Pt-C bond that preserves
the head-to-tail conformation observed in the solid state for 5
and 6.
In the absence of added free dimethyl sulfoxide, the pseudo-
first-order plots show some deviation from linearity as the
Particularly useful for the identification of the atropisomer
in solution were the results of phase-sensitive 1H NOESY
(46) SCIENTIST, Micro Math Scientific Software, Salt Lake City, UT.
9
J. AM. CHEM. SOC. VOL. 126, NO. 20, 2004 6477

A R T I C L E S
Plutino et al.
Chart 3
monomers:
[Pt2(Hbph)4(µ-SEt2)2] / 2[Pt(Hbph)2(SEt2)]
(9)
The monomeric species [Pt(Hbph)2(SEt2)] might be stabilized
by an agostic interaction involving the empty σ orbital on Pt
and the electron pair of the C-H bond at the C8a or the C12a
sites. This hypothesis is strongly supported by the observation
that bulky ligands favor the formation of agostic interactions
between the metal and C-H bonds48 and that cyclometalation
is initiated by an agostic interaction with the σ(C-H) orbital
followed by back-donation to the σ*(CH) orbital.49 It might be
claimed that the transient agostic [Pt(Hbph)2(SEt2)] should be
considered a 16-electron species. We will turn our attention to
this problem later, when discussing the characteristics of three-
coordinate d8 species as reaction intermediates or structurally
characterized compounds. Right now, we observe that the 16-
electron full four-coordinate monomers cis-[Pt(Hbph)2(SEt2)2]
or cis-[Pt(Hbph)2(dmso)2] complexes do not undergo cyclo-
metalation under the same conditions illustrated above for the
dinuclear [Pt2(Hbph)4(µ-SEt2)2] compound. Oxidative addition
of the C-H bond at the C8a,12a sites follows this preliminary
Pt-CH interaction, yielding a cyclometalated-arylhydrido 16-
electron Pt(IV) five-coordinate intermediate. Several theoretical
studies have indicated that these reaction intermediates should
have square-pyramidal geometry,50 but only recently have two
papers appeared describing the isolation and structural charac-
terization of such compounds.51 Finally, reductive elimination
should yield the dinuclear cyclometalated species [Pt2(bph)2-
(µ-SEt2)2] and free biphenyl. Cyclometalation on 1b develops
along the same key steps recognized by the Whitesides’ group
for [Pt(PEt3)2(CH2CMe3)2]:52 (i) reversible dissociation of the
phosphane ligand, (ii) oxidative addition of the C-H bond to
the coordinatively unsaturated intermediate, (iii) rate-limiting
reductive elimination of neopentane, and (iv) reassociation of
triethylphosphane. The reason why intramolecular C-H activa-
tion exhibits a higher energy barrier in 1a (RââR conformation)
than in 1b (RâRâ) is still not clear. We are inclined to think
that in the latter conformation the equilibrium of eq 9 is more
shifted toward the couple of monomers where the C-H8a,12a
bonds of the noncoordinated phenyl fragment have easy access
to the vacant coordination site on the metal. The good computer-
fit of the kinetic data in Figure 4 does not exclude that the
proposed unsaturated intermediate for metalation should be to
some extent reached also from 1a.
experiments and the strong through-space cross-peaks observed
between hydrogen atoms of mutually cis Hbph groups. In
particular, the predominance of a head-to-tail disposition of the
two aryl groups is clearly indicated by strong nOe’s cross-peaks
between the H8a,12a protons on the pendant arm in one Ph-C
fragment and the H3b proton that lies on the phenyl ring opposite
to it and directly coordinated in the cis position to the platinum-
(II) metal center. An additional consequence of the mutual
interaction between pendant and coordinated aryl rings in a head-
to-tail conformation is the marked anisotropic shift to low
frequencies of the resonance of the ortho-protons H3a,3b
.
The high activation energy for atropisomerization of the cis-
bis(η1-biphenyl) platinum derivatives 2-6 is undoubtedly steric
in origin. Similar steric effects are operative also in the dinuclear
1a complex and are expected to be responsible for restricted
Pt-C rotation as well. In this case, however, thioether bridge
splitting, as depicted in Chart 3, can produce a relevant relief
of steric congestion, allowing for a slow rotation of at least a
pendant η1-biphenyl fragment in the monobridged species.
Thus, atropisomerization of 1a into 1b could involve prior
bridge splitting of the anterior thioether ligand (as seen in Chart
3) with rotation of the pendant R′ Hbph arm followed by bridge
splitting of the posterior thioether and subsequent rotation of
the â′ organic fragment to afford the final RâRâ conformation
of 1b. Other sequences of bridge splitting and Pt-C rotation
processes are conceivable which could involve, as a transient
intermediate, the high energy Râââ species. The presence of
this latter elusive component has been inferred by intense cross-
peaks in the 1H NOESY experiments (Figure 3) attributed to a
fast exchanging species other than 1a, 1b, or the final cyclo-
metalated complex. The same experiments showed the occur-
rence of chemical exchange on the NMR time-scale between
ethyl groups belonging to the same sulfur atom which imply
preliminary bridge splitting and subsequent inversion of con-
figuration at the uncoordinated sulfur atom.
Thioether dissociation seems to be also at the origin of the
intramolecular C-H activation that leads to the formation of
the cyclometalated [Pt2(bph)2(µ-SEt2)2] compound. The most
conceivable mechanism involves preliminary bond dissociation
at a thioether bridge, as depicted in Chart 3. The dinuclear [Pt2-
(Hbph)4(µ-SEt2)2] compound exhibits the same propensity to
dissociation as found by Puddephatt for [Pt2Me4(µ-SMe2)2]19
as a precursor to platinum compounds containing bidentate
(C,N) or terdentate (C,N,N′) ligands.47 Dissociation of the Pt-S
bond on 1b originates coordinative and electronic unsaturation
at a single metal atom (three-coordinate, 14-electron species).
The behavior observed is also consistent with a preequilibrium
in which a molecule of dimer gives two 14-electron T-shaped
Dissociative Exchange and Substitution. The exchange of
the coordinated dmso in compound 3 with free ligand in toluene-
(48) (a) Brookhart, M.; Green, M. L. H. J. Organomet. Chem. 1983, 250, 395-
408. (b) Cotton, F. A.; LaCour, T.; Stanislowski, A. G. J. Am. Chem. Soc.
1974, 96, 754-760.
(49) (a) Crabtree, R. H.; Holt, E. M.; Lavin, M.; Morehouse, S. M. Inorg. Chem.
1985, 24, 1986-1992. (b) Lavin, M.; Crabtree, R. H. Organometallics 1989,
8, 99-104.
(50) (a) Hill, G. S.; Puddephatt, R. J. Organometallics 1998, 17, 1478-1486.
(b) Siegbahn, P. E. M.; Crabtree, R. H. J. Am. Chem. Soc. 1996, 118, 4442-
4450. (c) Bartlett, K. L.; Goldberg, K. I.; Borden, W. T. J. Am. Chem.
Soc. 2000, 122, 1456-1465. (d) Bartlett, K. L.; Goldberg, K. I.; Borden,
W. T. Organometallics 2001, 20, 2669-2678. (e) Heiberg, H.; Johansson,
L.; Gropen, O.; Ryan, O. B.; Swang, O.; Tilset, M. J. Am. Chem. Soc.
2000, 122, 10831-10845. (f) Gilbert, T. M.; Hristov, I.; Ziegler, T.
Organometallics 2001, 20, 1183-1189.
(51) (a) Fekl, U.; Kaminsky, W.; Goldberg, K. I. J. Am. Chem. Soc. 2001, 123,
6423-6424. (b) Reinartz, S.; White, P. S.; Brookhart, M.; Templeton, J.
L. J. Am. Chem. Soc. 2001, 123, 6425-6426.
(52) Foley, P.; DiCosimo, R.; Whitesides, G. M. J. Am. Chem. Soc. 1980, 102,
6713-6725.
(47) (a) Anderson, C. M.; Crespo, M.; Jennings, M. C.; Lough, A. J.; Ferguson,
G.; Puddephatt, R. J. Organometallics 1991, 10, 2672-2679. (b) Crespo,
M.; Martinez, M.; Sales, J.; Solans, X.; Font-Bard´ıa, M. Organometallics
1992, 11, 1288-1295.
9
6478 J. AM. CHEM. SOC. VOL. 126, NO. 20, 2004

Biphenyl Platinum(II) Complexes
A R T I C L E S
( 3) kJ mol-1, ∆Sq ) (-5 ( 9) J K-1 mol-1] are substantially
identical and comparable to those obtained from the kinetics of
ligand exchange, within the experimental error.
Scheme 2
The experimental kinetic evidence points unequivocally to a
dissociative mode of activation (D, according to the Langford-
Gray nomenclature)3f for the system here examined. The
assessment of a dissociative mechanism is accredited by: (i)
the dissociation rate k1 being independent of the nature and
concentration of the entering group N-N, (ii) the rate of dmso
dissociation k1 being independent of the nature of N-N and
substantially identical to the rate constant kexch for dmso
exchange, (iii) the mass-law retardation plots suggesting a
competition between the nucleofuge dmso and the nucleophile
N-N for an intermediate of a lower coordination number; the
intermediate has a lifetime long enough to discriminate between
different entering groups (k2/k-1 ) 0.18 for bpy and 0.621 for
phen, respectively), and (iv) the magnitude of the positive
entropy of activation. The solvents used have an insufficiently
coordinating power to give an associatively activated contribu-
tion. Accordingly, a study on the pressure dependence of ligand
exchange rate constants of similar complexes, including cis-
[PtPh2(dmso)2], in nonpolar solvents, led to positive values of
the activation volumes.26
d8 (eq 8) takes place with a rate law of the form rate ) kexch
-
[complex], with kexch ) (114 ( 3) × 10-3 s-1 at 298 K.53 There
is no sulfoxide-dependent contribution, even at the highest
sulfoxide concentrations. The temperature dependence of kexch
gives ∆Hq ) (95 ( 3) kJ mol-1 and ∆Sq ) (56 ( 10) J K-1
mol-1. As expected for dissociatively activated processes, the
entropy of activation is positive. The exchange rate constant at
298 K and the activation parameters in CDCl3 are as follows:
kexch ) (288 ( 8) × 10-3 s-1, ∆Hq ) (93 ( 5) kJ mol-1, and
∆Sq ) (56 ( 17) J K-1 mol-1. Such differences may be due to
the different solvating power of toluene and chloroform.
It is of interest to focus shortly on some attractive features
emerging from the comparison of the kinetic data in Table 5 of
compounds exhibiting a dissociative behavior. The change from
CH3 (entry 1, kexch ) 0.0112 s-1) to C6H5 (entry 2, kexch
)
0.0140 s-1) does not greatly affect the dissociative activation.
Inductive σ effects are dominant, the extent of the aryl-platinum
π interaction being small. The stabilization of the three-
coordinate intermediate by interaction between Pt and the phenyl
hydrogens in [PtPh2(dmso)] plays little, if any, part in promoting
the dissociative nature of the process. The dissociative mech-
anism is expected to be enhanced by extra electron release from
the ligands, but changing from C6H5 to 4-CH3C6H4 (entry 3)
leads to no significant change in the magnitude of kl. A
pentafluorophenyl group (entry 4) in place of the phenyl group
induces a significant decrease of lability. Because molecular
structures of similar compounds show that phenyl and pen-
tafluorophenyl groups exert a comparable trans influence,54 there
are no arguments to suggest that this result may be ascribed to
a ground-state effect, that is, to the strength of the Pt-S bond.
Rather, it seems that a ligand-to-metal reduction of electron
density brings about a noticeable destabilization of the three-
coordinated intermediate. Substitution in the complex containing
the planar 2,2′-biphenyl dianion (entry 7) is still dissociative
and largely easier than in its analogue containing single aryl
ligands (entry 6), despite its presumed π-acceptor properties. It
must be concluded that back-donation from filled d orbitals to
empty π* of the in-plane cyclometalated rings is weak or absent
and is not operative in promoting a changeover from dissociative
to associative mode of activation. The complex cis-[Pt(Hbph)2-
(dmso)2] (entry 5), despite the remarkable steric congestion of
the two noncoordinated aryl groups lying above and below the
coordination plane in a “pseudo-octahedral” fashion, reacts at
a similar rate as the planar [Pt(bph)(Me2S)2] (entry 7). Finally,
the reactivity appears to be sensitive to the electron-donating
ability of the substituents on the sulfur atom of the leaving group
Proton NMR and electronic spectra clearly indicate that the
reaction of cis-[Pt(Hbph)2(dmso)2] with 2,2′-bipyridine and 1,10-
phenanthroline according to eq 6 occurs in a single observable
stage, ring closing being much faster than displacement of the
first dmso, and is retarded by the addition of free dmso. The
pattern is strictly similar to that found in our previous studies
on cis-[Pt(C,C)(S,S)] (S ) thioethers or sulfoxides) systems
where the easy dissociation of the ligand S was shown to be
the rate-determining step of the substitution. The same dis-
sociative mechanism (Scheme 2) applies which involves: (i)
dissociative loss of dmso from the substrate (k1 path) to yield
a three-coordinated 14-electron intermediate, and (ii) competition
for it between the re-entry of dmso (via k-1) and the attack of
a ligand (either dmso* or N-N, via k2) to yield the observed
products. The rate law is given by the equation:
kobsd ) k1[N-N]/{(k-1/k2)[dmso] + [N-N]} (10)
that reduces to kobsd ) k1 for k2 . k-1. The rate constants are
related to the empirical parameters a and b in eq 7 by the
expressions a ) k1 and b ) k-1/k2. The dissociation constant
k1 measures the rate of sulfoxide leaving the coordination sphere
of the metal, while the k2/k-1 ratio measures the competition
between N-N and the leaving sulfoxide for the three-coordinate
intermediate. Rate constants and activation parameters derived
from the kinetic analysis of nucleophilic substitution of cis-[Pt-
(Hbph)2(dmso)2] with bpy [k1 ) (76 ( 1) × 10-3 s-1, ∆Hq )
(84 ( 1) kJ mol-1, ∆Sq ) (15 ( 3) J K-1 mol-1, in toluene at
298 K] and with phen [k1 ) (78 ( 2) × 10-3 s-1, ∆Hq ) (78
(54) Bresciani Pahor, N.; Plazzotta, M.; Randaccio, L.; Bruno, G.; Ricevuto,
(53) Derived from the Eyring plot.
V.; Romeo, R. Inorg. Chim. Acta 1978, 31, 171-175.
9
J. AM. CHEM. SOC. VOL. 126, NO. 20, 2004 6479

A R T I C L E S
Plutino et al.
Table 5. Selected Kinetic Data for Dissociation of Ligands from Pt(II) Complexes at 303 K
103kexch
103k1
(k2/ k-1
)
∆V
ref
a
b
c
q d
complex
solvent
ligand
1
2
3
4
5
6
7
cis-[Pt(Me)2(dmso)2]
cis-[Pt(Ph)2(dmso)2]
cis-[Pt(p-tolyl)2(dmso)2]
cis-[Pt(C6F5)2(dmso)2]
cis-[Pt(Hbph)2(dmso)2]
cis-[Pt(Ph)2(Me2S)2]
[Pt(bph)(Me2S)2]
benzene
benzene
benzene
toluene
toluene
benzene
toluene
chloroform
toluene
chloroform
chloroform
toluene
dmso
21e
79f
11.2
14.0
15.9
0.89
140
5.3
107g
0.17
+4.9
+5.5
26, 29
26, 30
30
dmso
dmso
dmso
dmso
Me2S
Me2S
0.049
0.044
0.055
0.18
0.68
0.096
31
220e
this work
26, 28
25
3.8e
+4.7
1060g
8
[Pt(bph)(Et2S)2]
Et2S
30g
0.46
25
84g
114e
56
9
10
cis-[Pt(Me)2(dmso)(PPh3)]
cis-[Pt(MePh2Si)2(Me2PhP)2]
dmso
Me2PhP
114e
32
33
a In s-1, from magnetization transfer experiments. b In s-1, from substitution reaction studies with bpy. c Competition ratio. d Activation volume in cm3.
1
e Calculated from the Eyring equation. f By flow H NMR. g At 298 K.
(cf. entries 7 and 8), in agreement with previous findings on a
series of compounds of the type cis-[PtPh2L2],27 where L
encompasses a wide range of thioethers of different electron-
donating characteristics.
of this intermediate in solution is always very low and the
possibility of detecting it with spectroscopic techniques is a hard,
or impossible, task. The coordinative and electronic unsaturation
increases enormously the electrophilicity of the intermediate and
its tendency to coordinate solvent molecules or any other
electron-donating moiety, including the electron pair of a C-H
bond (agostic interaction) or electrons coming from a donor
atom of noninnocent ligands. Interactions of this type are of
greatest importance in the approach to the isolation and structural
characterization of such three-coordinate species, having in
mind, however, that the stabilization of transient species
generated in situ by ligand dissociation is reflected in a sharp
increase of reactivity.
Three-Coordinate Intermediates. This work has confirmed
that a ligand set of two cis σ-bonded carbon atoms and two
sulfur atoms induces dissociation of one of the sulfur ligands,
no matter whether the carbon atom comes from an alkyl, an
aryl, or a metalated aryl ligand. Cyclometalation and in-plane
disposition of aryl rings do not favor an associative pathway,
through the increase of electrophilicity of the platinum center,
because the expected back-donation toward the aromatic π*
orbitals does not come into play. The low-energy dissociative
pathway is therefore a combined result of: (i) the presence of
a high electronic barrier at the metal which prevents the
approach of the nucleophile lone-pair, (ii) bond weakening at
the leaving group due to the trans influence of strong σ-donors
(in the present cases, the organic carbanions which form strong
Pt-C σ-bonds), and (iii) the stabilization of the remaining set
of three in-plane ligands in a T-shaped structure which allows
the 14-electron intermediate to maintain the original singlet
ground state. These favorable electronic factors still apply to
cis-[PtMe2(PPh3)(dmso)] (entry 9 in Table 5), where the
introduction of the σ-donor PR3 produces no changeover of
mechanism or does not greatly affect the easily achieved
dissociative activation. The same reasoning applies to the
tetrahedral distorted Werner-type compound cis-[Pt(MePh2Si)2-
(Me2PhP)2] (entry 10), where an appropriate set of strong
σ-donor silyl groups can favor sufficient accumulation of
electron density at the metal and stabilization of the 14-e
intermediate. If one of the thioethers is displaced by CO, as in
cis-[PtPh2(CO)(SMe2)],34 the mechanism changes again from
a dissociative to an associative one, as discussed previously.
The â-Hydrogen Kinetic Effect. Orpen and Spencer55 have
described the structure of â-agostic cationic platinum(II)
complexes of the type [Pt(P-P)(R)]+ (P-P ) chelating
diphosphane; R ) alkyl), where the chelating ligand prevents
isomerization and the platinum interaction with a â-hydrogen
of the alkyl group is so extended as to favor the formation of
a well-defined 3-center-2-electron Pt-H-C bond. The same
structure can be envisaged for the three-coordinate transition
state formed upon solvent dissociation in the uncatalyzed cis
to trans isomerization16a of cis-[Pt(PEt3)2(R)(S)]+ (R ) linear
or branched alkyls; S ) MeOH). In these solvento complexes,
when R ) Et, n-Pr, n-Bu, the rate of isomerization is much
higher than that of complexes containing alkyl groups with no
â-hydrogen atoms, such as methyl, neopentyl, and trimethylsilyl
groups. Inductive or steric effects cannot account for the large
â-hydrogen kinetic effect. A specific interaction of these
hydrogen atoms with the metal is likely to be responsible for
the large rate enhancement. The extent of Pt-H interaction in
the cis-[Pt(PEt3)2(Et)]+ transition state is presumably less than
in the structurally characterized [Pt(P-P)(Et)]+, but it is
sufficient to partly satisfy the coordinative unsaturation, to
decrease its energy, and to accelerate the fluxionality of the
intermediate. A tentative calculation of the free energy involved
in the â-hydrogen kinetic effect can be performed, based on
the reasonable assumption that the relative ground-state energies
of the square-planar cis-[Pt(PEt3)2(Me)(S)]+ and cis-[Pt(PEt3)2-
(Et)(S)]+ complexes are not greatly different. The free energy
Once the main features to induce dissociation on a four-
coordinate d8 metal complex have been recognized, one starts
to wonder to what extent the knowledge gained in mechanistic
studies can be exploited for synthetic purposes. We know we
must deal with electron-rich four-coordinate complexes. The
value of the dissociation rate constant is an indication of the
ease with which the bond breaking process occurs and of
stabilization of the three-coordinate intermediate. The competi-
tion ratio between nucleofuge and nucleophile measures the
discrimination ability of the intermediate and its lifetime.
Molecular orbital calculations suggest that three-coordinate 14-e
d8-ML3 species have a T-shaped geometry, but the concentration
of activation for the methyl solvento complex is 87.2 kJ mol-1
,
whereas that for the ethyl complex is 63.4 kJ mol-1. The
difference between these two values should reflect entirely the
(55) Carr, N.; Mole, L.; Orpen, A. G.; Spencer, J. L. J. Chem. Soc., Dalton
Trans. 1992, 2653-2662. (b) Mole, L.; Spencer, J. L.; Carr, N.; Orpen, A.
G. Organometallics 1991, 10, 49-52.
9
6480 J. AM. CHEM. SOC. VOL. 126, NO. 20, 2004

Biphenyl Platinum(II) Complexes
A R T I C L E S
Chart 4
characterized compounds so far, relies on the use of ligands
that guarantee some form of protection of the fourth coordination
site and of electron transfer to the vacant σ orbital of the metal.
To what extent such electron transfer occurs can be a matter of
debate, but essential is the knowledge acquired from kinetic
studies to detect new synthetic procedures and to explore the
role of these fascinating elusive species in fundamental processes
such as C-H bond activation, hydroformylation, or polymer-
ization reactions.
free energy involved in the â-hydrogen kinetic effect. This effect
has consequences on the rates of dissociative processes that
parallel the spectroscopic and structural properties of structurally
characterized compounds with full â-agostic interactions such
as [Pt(P-P)(Et)]+. If steric effects play little, if any, role in
promoting dissociation (compare in Table 5 the reactivity of
the highly congested cis-[Pt(Hbph)2(dmso)2] and the flat planar
[Pt(bph)(Me2S)2]), then the rate acceleration exhibited by cis-
[Pt(Hbph)2(dmso)2] (entry 5) with respect to cis-[Pt(Ph)2(dmso)2]
(entry 2) can be attributed to hydrogen interactions in the
transition state of the type described in Chart 4. These hydrogen
interactions, while lowering the energy of the TS below that of
a formally three-coordinate 14-electron species (by only 8.4 kJ
mol-1, as derived from the rate data in Table 5), are insuf-
ficiently stabilizing to allow such species to be isolated and
characterized. An alternative explanation, kinetically indistin-
guishable from the stabilization of the TS, could involve steric
congestion in the ground state and a direct participation of the
dangling ortho-phenyl substituents in promoting steric relief and
easier removal of dmso, even though little bond distortion is to
be expected on going from a four-coordinate square-planar
arrangement to a T-shaped three-coordinate geometry. It is
possible that both effects, steric and electronic, come contem-
porarily into play, within the framework of a dissociative
substitution.
Conclusion
The chemistry of the novel dinuclear compound [Pt2(µ-SEt2)2-
(Hbph)4], 1a, appears to be complex and intriguing. The
presence of pendant η1-biphenyl groups determines an interest-
ing atropisomerization between two preferential low-energy
conformations and the intramolecular C-H activation leading
to cycloplatination. Opening of the thioether bridge and forma-
tion of a coordinatively unsaturated species is a prerequisite
for the occurrence of both processes. By analogy with well-
known dinuclear species, such as [Pt2(µ-SEt2)2(C6H5)4]62 and
[Pt2(µ-SMe2)2(Me)4],19,63 1a can be exploited as a synthetic
precursor to a large variety of organometallic species containing
the [Pt(Hbph)2] moiety. Two such compounds, with 2,2′-
bipyridine and 1,10-phenanthroline, have been structurally
characterized by X-ray diffraction studies. A detailed kinetic
study of ligand (dmso) exchange and substitution (by 2,2′-
bipyridine and 1,10-phenanthroline), performed on the novel
complex cis-[Pt(Hbph)2(dmso)2], has confirmed that a ligand
set of two cis σ-bonded carbon atoms and two sulfur atoms
induces dissociation of one of the sulfur ligands, no matter
whether the carbon atom comes from an alkyl, an aryl, or a
metalated aryl ligand. The new data allow for an updated
understanding of the factors controlling the changeover from
associative to dissociative pathways in square-planar d8 metal
complexes. The question is addressed at what extent the
knowledge gained in mechanistic studies on ML3 d8 T-shaped
14-electron species can be exploited for synthetic purposes. It
is pointed out that, when such species have been isolated and
structurally characterized, they show C-H bonds being involved
in agostic interactions with the metal center. These interactions
guarantee some form of protection of the fourth coordination
site and electron transfer to the vacant σ orbital of the metal.
When occurring within a transition state or a three-coordinate
intermediate, it is classified as the â-hydrogen kinetic effect,
presumably leading to a sharp increase of the ligand dissociation
rate.
A survey of the X-ray data on a few isolated three-coordinate
14-e d8-ML3 species shows C-H bonds involved in agostic
interactions to the empty coordination site of otherwise T-shaped
d8 complexes. These examples include the anion56 [Ni(mesityl)3]-,
t
t
the cation57 [Rh(PPh3)3]+, the very recent [Rh(Bu2 PCH2PBu2 )-
(np)]58 (np ) neopentyl), and [PdArX(PBut2R)] (Ar ) Ph, X
) Br, R ) adamantyl; Ar ) 2,4-xylyl, X ) I, R ) But).59
Agostic interactions stabilize also the novel “T-shaped 14-
electron” platinum(II) cations trans-[Pt(CH3)L2]+ [L ) PR2-
(2,6-Me2C6H3), R ) Ph, Cy].60 A formally three-coordinate
cationic complex [Pd(CH2CMe2Ph)(dmpe)]+ appears to be
stabilized by an auxiliary π, η1 interaction with the ipso-carbon
of the phenyl group.61
Espinet11c argues strongly against such species being con-
sidered three-coordinate 14-electron species. However, the
challenge to isolate “14-electron T-shaped” compounds and to
confirm their structures experimentally, according to what is
discussed above and to the characteristics of the few structurally
Experimental Section
General Procedures and Chemicals. All syntheses were performed
on a double-manifold Schlenk vacuum line under a dry and oxygen-
free dinitrogen atmosphere using freshly distilled, dried, and degassed
solvents. Solvents employed in the synthetic procedures (Analytical
Reagent Grade, Lab-Scan, Ltd.) and in the kinetic runs (A.C.S.
spectrophotometric grade, Aldrich Chem. Co.) were distilled under
dinitrogen from sodium-benzophenone ketyl (tetrahydrofuran, diethyl
ether, toluene) or barium oxide (dichloromethane).64 Chloroform-d (99.8
(56) Hay-Motherwell, R.; Wilkinson, G.; Sweet, T. K. N.; Hursthouse, M. B.
Polyhedron 1996, 15, 3163-3166.
(57) Yared, Y. W.; Miles, S. L.; Bau, R.; Reed, C. A. J. Am. Chem. Soc. 1977,
99, 7076-7078.
(58) Urtel, H.; Meier, C.; Eisentra¨nger, F.; Rominger, F.; Joschek, J. P.;
Hofmann, P. Angew. Chem., Int. Ed. 2001, 40, 781-784.
(59) Stambuli, J. P.; Bu¨hl, M.; Hartwig, J. F. J. Am. Chem. Soc. 2002, 124,
9346-9347.
(62) Steele, B. R.; Vrieze, K. Transition Met. Chem. 1977, 2, 140-144.
(63) (a) Rashidi, M.; Hashemi, M.; Khorasani-Motlagh, M.; Puddephatt, R. J.
Organometallics 2000, 19, 2751-2755. (b) Rashidi, M.; Fakhroeian, Z.;
Puddephatt, R. J. J. Organomet. Chem. 1991, 406, 261-267.
(64) Riddick, J. A.; Bunger, W. B.; Sakano, T. K. In Organic SolVents, 4th ed.;
Weissberger, A., Ed.; Wiley & Sons: New York, 1986.
(60) Baratta, W.; Stoccoro, S.; Doppiu, A.; Herdtweck, E.; Zucca, A.; Rigo, P.
Angew. Chem., Int. Ed. 2003, 42, 105-108.
(61) Campora, J.; Gutie´rrez-Puebla, E.; Lo´pez, J. A.; Monge, A.; Palma, P.; del
R´ıo, D.; Carmona, E. Angew. Chem., Int. Ed. 2001, 40, 3641-3644.
9
J. AM. CHEM. SOC. VOL. 126, NO. 20, 2004 6481

A R T I C L E S
Plutino et al.
+ %, C.I.L., Inc.) and toluene-d8 (99 + %, Aldrich Chem. Co.) were
used as received. Dimethyl sulfoxide was purified by filtration through
a 2 × 25 cm chromatographic column filled with alumina (activated
grade I, neutral, 150 mesh, Aldrich), under a dinitrogen atmosphere.
All of the other reagents were purchased from commercial suppliers
and used without further purification. Elemental analyses were per-
formed by Redox SNC (Milan, Italy).
Synthesis of Complexes. Complexes cis-[PtCl2(SEt2)2]65 and [PtI2-
(COD)]66 were synthesized following the procedures reported in the
literature.
[Pt(Hbph)2(COD)], 2. A suspension of [PtI2(COD)] (200 mg, 0.36
mmol) in diethyl ether (20 mL) was cooled at -10 °C and treated with
2,2′-Li2bph (5.4 mL, 0.135 M) in diethyl ether. The mixture was stirred
for 2 h while being warmed to room temperature and then worked up
with 10 mL of water. After extraction of the aqueous solution with
dichloromethane, the combined organic extracts were dried over sodium
sulfate and filtered off. Evaporation of the solvent under reduced
pressure and recrystallization from toluene/petroleum ether (1/1 v/v)
gave 2 as an off-white solid (170 mg, 78%). Anal. Calcd for C32H30Pt:
1
C, 63.0; H, 4.9. Found: C, 63.5; H, 4.8. H NMR (CDCl3): δ 7.44
(m, 4H, H8a,12a+H8b,12b), 7.33 (m, 6H, H9a,11a+H9b,11b+H10a,10b), 6.93 (dd,
[Pt2(µ-SEt2)2(Hbph)4], 1a (Hbph- ) η1-2,2′-biphenyl anion), was
obtained adventitiously as a side product in the course of the synthesis
of the dinuclear cyclometalated species [Pt2(µ-SEt2)2(bph)2] according
to the procedure reported by von Zelewsky et al.35 A solution of 2,2′-
Li2bph in ethyl ether67 (40 mL, 0.135 M) was added dropwise to a
stirred suspension of cis-[PtCl2(SEt2)2] (1.2 g, 2.69 mmol) in 40 mL
of diethyl ether at -10 °C. The reaction mixture was kept for 1 h at
-10 °C, subsequently warmed to 0 °C, kept for an additional hour at
this temperature, and then hydrolyzed with 20 mL of water. The organic
phase was separated out, and the residual product was extracted from
the aqueous solution with dichloromethane (4 × 10 mL); the CH2Cl2
extract was combined with the original organic fraction and left
overnight over Na2SO4. After evaporation to dryness, the residue was
dissolved in the minimum amount of dichloromethane. Chromatography
over a 2 × 20 cm silica gel column with a mixture of dichloromethane/
hexane (3/2 ) v/v) afforded 200 mg of the dinuclear complex [Pt2(µ-
SEt2)2(bph)2] (17% yield) from the yellow band, isolated by recrystal-
lization from dichloromethane/pentane (1/1 ) v/v). Collection of the
colorless initial fractions before the yellow band and subsequent workup
according to the procedure described above afforded the dinuclear
uncyclometalated species [Pt2(µ-SEt2)2(Hbph)4] (1a) as a crystalline off-
white solid (300 mg, 19% yield). Anal. Calcd for C56H56Pt2S2: C, 56.8;
4JPtH ) 26.4 Hz, 3JHH ) 7.2 Hz, 4JHH ) 1.1 Hz, 2H, H6a,6b), 6.76 (ddd,
4
4
3Jav ) 7.2 Hz, JHH ) 1.1 Hz, 2H, H5a,5b), 6.64 (ddd, JPtH ) 13.8 Hz,
4
3
3Jav ) 7.2 Hz, JHH ) 1.1 Hz, 2H, H4a,4b), 6.20 (d, JPtH ) 69.3 Hz,
3JHH ) 7.2 Hz, 2H, H3a,3b), 5.05 (m, br, JPtH ) 42.8 Hz, 2H, CH1,5),
2
2
4.81 (m, br, JPtH ) 39.2 Hz, 2H, CH2,6), 2.46+2.36 (m, br, 4H,
(CH2)4,8), 2.30 (m, br, 4H, (CH2)3,7). 13C{1H} NMR (CDCl3): δ 151.4
(1JPtC ) 1117 Hz, C2a,2b), 148.0 (2JPtC ) 39 Hz, C1a,1b), 146.6 (3JPtC
)
34 Hz, C7a,7b), 136.2 (2JPtC ) 14 Hz, C3a,3b), 129.7 (C8a,12a+C8b,12b), 129.0
(3JPtC ) 59 Hz, C6a,6b), 127.1 (C9a,11a+C9b,11b), 125.9 (C10a,10b), 125.7
(3JPtC ) 68 Hz, C4a,4b), 122.4 (4JPtC ) 9 Hz, C5a,5b), 108.2 (1JPtC ) 67
Hz, C1,5), 99.7 (1JPtC ) 41 Hz, C2,6), 30.9 (C4,8), 28.4 (C3,7).
cis-[Pt(Hbph)2(dmso)2], 3. Method A: 1a (100 mg, 0.0845 mmol)
was dissolved in dimethyl sulfoxide (4 mL) under continuous stirring.
The solution was allowed to evaporate to dryness at 80 °C under
vacuum. The solid residue was washed with diethyl ether (4 × 3 mL),
dried, and then dissolved in the minimum amount of dichloromethane.
Addition of 10 mL of diethyl ether and cooling to -35 °C led to the
separation of 94 mg of 3 (85% yield). Method B: A solution of 2 (100
mg, 0.164 mmol) in dimethyl sulfoxide (4 mL) was stirred for 1 h,
and then the solvent was removed with a rotavapor in vacuo at 80 °C.
The resulting off-white solid was washed several times with diethyl
ether to remove residual traces of the sulfoxide and COD, and then
recrystallized from a dichloromethane/diethyl ether mixture (1/1 v/v)
at -35 °C (92 mg, 85% yield). Anal. Calcd for C28H30S2O2Pt: C, 51.1;
H, 4.6; O, 4.9; S, 9.8. Found: C, 51.3; H, 4.7; O, 5.0; S, 9.9. 1H NMR
1
H, 4.8; S, 5.4. Found: C, 56.1; H, 4.9; S, 5.3. H NMR (CDCl3): δ
7.61 (d, br, 8H, H8a,12a+H8b,12b), 7.41 (m, br, 12H, H9a,11a+H9b,11b+H10a,10b),
3
4
3
6.84 (dd, JHH ) 7.5 Hz, JHH ) 1.4 Hz, 4H, H6a,6b), 6.71 (dd, Jav
)
3
3
7.1 Hz, 4H, H5a,5b), 6.52 (dd, Jav ) 7.3 Hz, 4H, H4a,4b), 6.28 (d, JHH
) 7.2 Hz, JPtH ) 71.0 Hz, 4H, H3a,3b), 2.69 (m, br, 4H, S-CH2b-
CH3), 2.67 (m, br, 4H, S-CH2a-CH3), 2.00 (t, JHH ) 7.1 Hz, 6H,
(CDCl3): δ 7.85 (dd, 3JHH ) 6.6 Hz, 4JHH ) 1.7 Hz, 4H, H8a,12a+H8b,12b),
3
4
7.39 (m, 4H, H9a,11a+H9b,11b), 7.36 (m, 2H, H10a,10b), 7.06 (dd, JPtH
20.3 Hz, 3JHH ) 7.7 Hz, 4JHH ) 1.7 Hz, 2H, H6a,6b), 6.82 (ddd, 3JHH
7.7 Hz, 4JHH ) 1.1 Hz, 2H, H5a,5b), 6.57 (ddd, 4JPtH ) 13.5 Hz, 3JHH
)
)
)
)
3
S-CH2-CH3a), 0.288 (t, 3JHH ) 7.2 Hz, 6H, S-CH2-CH3b). 13C{1H}
NMR (CDCl3, T ) 273 K): δ 135.7 (C3a,3b), 129.7 (C8a,12a+C8b,12b),
128.8 (C6a,6b), 127.2 (C9a,11a+C9b,11b), 125.8 (C10a,10b), 125.2 (C4a,4b),
122.2 (C5a,5b), 33.1 (S-CH2b-CH3), 22.8 (S-CH2a-CH3), 11.9 (S-
CH2-CH3a), 9.51 (S-CH2-CH3b).
4
3
3
7.7 Hz, JHH ) 1.7 Hz, 2H, H4a,4b), 5.85 (dd, JPtH ) 73.4 Hz, JHH
4
3
7.7 Hz, JHH ) 1.1 Hz, 2H, H3a,3b), 3.10 (s, JPtH ) 12.1 Hz, 6H,
S-(CH3)a), 2.29 (s, JPtH ) 16.0 Hz, 6H, S-(CH3)b). 13C{1H} NMR
3
(CDCl3): δ 147.1 (2JPtC ) 41 Hz, C1a,1b), 146.5 (3JPtC ) 28 Hz, C7a,7b),
143.5 (1JPtC ) 1041 Hz, C2a,2b), 136.4 (2JPtC ) 26 Hz, C3a,3b), 129.8
(C8a,12a+C8b,12b), 128.9 (3JPtC ) 57 Hz, C6a,6b), 127.3 (C9a,11a+C9b,11b),
126.2 (C10a,10b), 125.8 (3JPtC ) 73 Hz, C4a,4b), 123.6 (4JPtC ) 11 Hz,
1H NMR spectrometry in CDCl3 showed compound 1a to evolve
with time toward the conformer 1b, which was separated almost
quantitatively from 1a by several subsequent crystallizations from a
dichloromethane/petroleum ether mixture (1/1 ) v/v).
C
5a,5b), 43.9 (2JPtC ) 20 Hz, S-CH3a), 42.3 (2JPtC ) 37 Hz, S-CH3b).
cis-[Pt(Hbph)2(SEt2)2], 4, was prepared and characterized in situ
1
3
[Pt2(µ-SEt2)2(Hbph)4], 1b. H NMR (CDCl3): δ 7.58 (d, br, JHH
3
) 7.7 Hz, 8H, H8a,12a+H8b,12b), 7.43 (m, br, JHH ) 7.7 Hz, 8H,
by bridge-splitting reaction of 1a with an excess of SEt2 in chloroform-
3
d. 1H NMR (CDCl3): δ 7.96 (d, br, 3JHH ) 7.2 Hz, 4H, H8a,12a+H8b,12b),
H9a,11a+H9b,11b), 7.33 (m, br, JHH ) 7.7 Hz, 4H, H10a,10b), 6.94 (dd,
3JHH ) 7.7 Hz, 4H, H6a,6b), 6.81 (dd, Jav ) 7.7 Hz, 4H, H5a,5b), 6.64
3
3
7.30 (m, br, JHH ) 7.1 Hz, 4H, H9a,11a+H9b,11b), 7.26 (m, br, 2H,
3
3
3
H
10a,10b), 7.03 (d, 4JPtH ) 23.4 Hz, 3JHH ) 7.5 Hz, 2H, H6a,6b), 6.76 (m,
(dd, Jav ) 7.3 Hz, 4H, H4a,4b), 6.48 (d, JHH ) 7.3 Hz, JPtH ) 69.0
Hz, 4H, H3a,3b), 3.40 (m, br, JHH ) 7.9 Hz, 2H, S-CH2a-CH3), 3.19
(m, br, JHH ) 7.7 Hz, 2H, S-CH2b-CH3), 2.94 (m, br, JHH ) 7.9
Hz, 2H, S-CH2a′-CH3), 2.88 (m, br, JHH ) 7.7 Hz, 2H, S-CH2b′
CH3), 1.83 (t, JHH ) 7.9 Hz, 6H, S-CH2-CH3a), 1.30 (t, JHH ) 7.7
Hz, 6H, S-CH2-CH3b). 13C{1H} NMR (CDCl3): δ 136.1 (C3a,3b), 129.2
(C6a,6b), 128.8 (C9a,11a+C9b,11b), 127.2 (C8a,12a+C8b,12b), 127.0 (C10a,10b),
3
3
2H, H5a,5b), 6.59 (m, JPtH ) 78.8 Hz (H3a,3b), 4H, H3a,3b+H4a,4b), 2.31
3
3
3
3
3
(m, JPtH ) 25.8 Hz, JHH ) 7.5 Hz, 8H, S-CH2-CH3), 1.13 (t, JHH
) 7.5 Hz, 12H, S-CH2-CH3). 13C{1H} NMR (CDCl3): δ 148.3 (2JPtC
) 42 Hz, C1a,1b), 146.5 (1JPtC ) 1137 Hz, C2a,2b), 146.2 (3JPtC ) 25 Hz,
3
-
3
3
C
7a,7b), 138.1 (2JPtC ) 22 Hz, C3a,3b), 130.0 (C8a,12a+C8b,12b), 128.4 (3JPtC
) 62 Hz, C6a,6b), 126.8 (C9a,11a+C9b,11b), 125.3 (C10a,10b), 125.1 (3JPtC
)
79 Hz, C4a,4b), 121.7 (4JPtC ) 10 Hz, C5a,5b), 27.6 (S-CH2-CH3), 13.1
(3JPtC ) 16 Hz, S-CH2-CH3).
125.3 (C4a,4b), 122.5 (C5a,5b), 33.7 (S-CH2b-CH3), 33.3 (S-CH2b′
-
CH3), 30.5 (S-CH2a-CH3), 30.3 (S-CH2a′-CH3), 12.9 (S-CH2-
CH3a), 12.5 (S-CH2-CH3b).
[Pt(Hbph)2(bpy)], 5. A dichloromethane solution (25 mL) of 2,2′-
bipyridine (26.1 mg, 0.167 mmol) was added dropwise to a solution
of 3 (100 mg, 0.152 mmol) in dichloromethane (25 mL). The color
changed instantaneously to dark yellow. The solution was refluxed
under continuous stirring for 2 h. Evaporation of most of the solvent,
(65) Kauffman, G. B.; Cowan, D. O. Inorg. Synth. 1960, 6, 211-215.
(66) Clark, H. C.; Manzer, L. E. J. Organomet. Chem. 1973, 59, 411-428.
(67) Gardner, S. A.; Gordon, H. B.; Rausch, M. D. J. Organomet. Chem. 1973,
60, 179-188.
9
6482 J. AM. CHEM. SOC. VOL. 126, NO. 20, 2004

Biphenyl Platinum(II) Complexes
A R T I C L E S
addition of hexane, and cooling led to the separation of a dark-orange
solid that was washed with hexane or Et2O, dried, and separated out in
an almost quantitative yield (95 mg, 95%). Anal. Calcd for C34H26N2-
measured every hour. The collected intensities were corrected for
Lorentz and polarization factors.69 An empirical absorption correction70
was also applied by using azimuthal (Ψ) scans of three “high-ø” (ø g
80°) reflections. Of the 3487 independent data collected, 2076 were
considered as observed and used for the solution and refinement of
the structure. Selected crystallographic and other relevant data are listed
in Table 1 and in Table S1 of the Supporting Information. The standard
deviations on intensities were calculated in term of statistics alone. The
1
Pt: C, 62.1; H, 4.0; N, 4.3. Found: C, 61.7; H, 3.8; N, 4.0. H NMR
(CDCl3): δ 8.41 (dd, 3JPtH ) 22.6 Hz, 3JHH ) 5.3 Hz, 4JHH ) 1.8 Hz,
3
4
2H, H6,6′), 7.97 (ddd, JHH ) 8.0, 8.4 Hz, JHH ) 1.8 Hz, 2H, H4,4′),
7.89 (d, 3JHH ) 8.0 Hz, 2H, H3,3′), 7.77 (dd, 3JHH ) 7.4 Hz, 4JHH ) 1.7
Hz, 4H, H8a,12a+H8b,12b), 7.25 (ddd, 3JHH ) 8.4, 5.3 Hz, 4JHH ) 1.3 Hz,
3
4
2H, H5,5′), 7.12 (dd, JHH ) 7.5 Hz, JHH ) 1.5 Hz, 2H, H6a,6b), 7.05
structure was solved by direct and Fourier methods and refined by full
(m, 6H, H9a,11a+H9b,11b+H10a,10b), 6.87 (ddd, 3Jav ) 7.4 Hz, 4JHH ) 1.5
Hz, 2H, H5a,5b), 6.79 (dd, JPtH ) 70.5 Hz, JHH ) 7.3 Hz, JHH ) 1.5
Hz, 2H, H3a,3b), 6.71 (ddd, Jav ) 7.4 Hz, JHH ) 1.5 Hz, 2H, H4a,4b).
13C{1H} NMR (CDCl3): δ 155.7 (C2,2′), 150.1 (C6,6′), 148.6 (C1a,1b),
147.9 (C7a,7b), 144.3 (C2a,2b), 140.2 (C3a,3b), 135.7 (C4,4′), 130.1
(C8a,12a+C8b,12b), 129.9 (C3,3′), 128.0 (C6a,6b), 126.9 (C5,5′), 126.4
(C9a,11a+C9b,11b), 125.1 (C4a,4b), 124.6 (C10a,10b), 121.9 (C5a,5b).
matrix least-squares71 (the function minimized being [∑w(Fo
-
2
(1/k)Fc2)2]) using anisotropic displacement parameters for all atoms.
No extinction correction was deemed to be necessary. The contribution
of the hydrogen atoms in their calculated positions [C-H ) 0.95 (Å),
B(H) ) 1.3-1.5 × B(Cbonded) (Å2)] was included in the refinement
using a riding model. Upon convergence (see Table S1), no significant
features were found in the Fourier difference map. All calculations were
carried out by using the PC version of the SHELX-97 and ORTEP
programs.71 The scattering factors used, corrected for the real and
imaginary parts of the anomalous dispersion, were taken from the
literature.72
3
3
4
3
4
[Pt(Hbph)2(phen)], 6, was obtained from 3 and 1,10-phenanthroline
following essentially the same procedure described for 5. Anal. Calcd
for C36H26N2Pt: C, 63.4; H, 3.8; N, 4.1. Found: C, 63.4; H, 4.0; N,
1
3
3
4.0. H NMR (CDCl3): δ 8.68 (dd, JPtH ) 21.1 Hz, JHH ) 5.3 Hz,
4JHH ) 1.2 Hz, 2H, H2,9), 8.45 (ddd, JHH ) 8.2 Hz, JHH ) 1.2 Hz,
3
4
Structural Study of [Pt(Hbph)2(phen)]‚(C7H8), 6‚(C7H8). The
crystal was cooled to 200(2) K, and the space group was determined
from the systematic absences; the cell constants were refined, at the
end of the data collection, using the data reduction software SAINT.73
Data were collected by using an ω scan in steps of 0.3°; a list of
experimental conditions for the data collection is given in Table S1.
The collected intensities were corrected for Lorentz and polarization
factors and empirically for absorption using the SADABS program.74
Selected crystallographic and other relevant data are listed in Table 1
and in the Supporting Information Table S1. The standard deviations
on intensities were calculated in term of statistics alone as shown in
Tables 1 and S1. The structure was solved by Patterson and Fourier
methods and refined by full matrix least-squares,71 minimizing the
3
4
2H, H4,7), 7.86 (s, 2H, H5,6), 7.82 (dd, JHH ) 8.2 Hz, JHH ) 1.7 Hz,
4H, H8a,12a+H8b,12b), 7.57 (dd, 3JHH ) 8.2, 5.3 Hz, 2H, H3,8), 7.17 (dd,
3JHH ) 7.5 Hz, 4JHH ) 1.2 Hz, 2H, H6a,6b), 6.98 (m, 6H, H9a,11a+H9b,11b
+
H
10a,10b), 6.93 (ddd, 3Jav ) 7.5 Hz, 4JHH ) 1.2 Hz, 2H, H5a,5b), 6.90 (dd,
3JPtH ) 71.8 Hz, 3JHH ) 7.5 Hz, 4JHH ) 1.2 Hz, 2H, H3a,3b), 6.77 (ddd,
3Jav ) 7.5 Hz, 2H, H4a,4b). 13C{1H} NMR (CDCl3): δ 155.6 (C10′,10′′),
151.3 (C4′,6′), 150.1 (C2,9), 148.6 (C1a,1b), 147.9 (C7a,7b), 144.3 (C2a,2b),
140.1 (C3a,3b), 135.7 (C4,7), 130.1 (C8a,12a+C8b,12b), 128.0 (C6a,6b), 126.9
(C5,6), 126.3 (C9a,11a+C9b,11b), 125.4 (C3,8), 125.1 (C4a,4b), 124.6 (C10a,10b),
121.5 (C5a,5b).
Instrumentation and Measurements. NMR analyses were per-
formed on a Bruker AMX-R 300 spectrometer equipped with a broad-
band probe operating at 300.13 and 75.46 MHz for 1H and 13C nuclei,
respectively. 1H and 13C NMR chemical shifts are reported in ppm (δ)
with respect to TMS and referenced to the residual protiated impurities
of the deuterated solvent. Coupling constants are given in hertz. Usually
samples were degassed prior to use by freeze-pump methodology. The
temperature within the probe was checked using the methanol method.68
2D-COSY experiments were performed in a phase-sensitive mode.
Normal-range 13C,1H-correlation experiments (HMQC, heteronuclear
multiple quantum coherence) were recorded by a standard inverse
function [∑w(Fo - (1/k)Fc2)2] and using anisotropic displacement
2
parameters for all atoms. No extinction correction was deemed
necessary. Toward the end of the refinement, a clathrated toluene
molecule was found on a difference Fourier map. This molecule is
highly disordered, and therefore the refined geometry is only ap-
proximate; however, this disorder is not affecting the refined geometry
of the complex. The contribution of the hydrogen atoms in their
calculated positions [C-H ) 0.95(Å), B(H) ) 1.3-1.5 × B(Cbonded
)
(Å2)] was included in the refinement using a riding model. The
handedness of the structure was tested by refining the Flack’s
parameter.75 Upon convergence, the final Fourier difference map showed
no significant peaks. All calculations were carried out, as above, using
the PC version of the SHELX-97 programs.71
1
detection sequence considering JCH ) 145 Hz. Relaxation times T1
for protons were set at 0.55 s (1), 0.9 s (4-6), and 0.85 s (3). Phase-
sensitive 1H 2D-NOESY experiments were performed with a standard
pulse sequence by using a mixing time of 0.8 s (3, 6) or 0.6 s (1).
UV-vis spectra were run with a rapid-scanning Hewlett-Packard model
8452A spectrophotometer or with a Perkin-Elmer Lambda3 spectro-
photometer, interfaced with a microcomputer for data collection and
equipped with a cell compartment thermostated by a Perkin-Elmer PTP
(Peltier temperature programmer). The temperature accuracy was (0.05
°C.
Computational Details. Molecular mechanics calculations were
performed on a Pentium IV personal computer using the Spartan
package version ’02.76 A modified version of the MMFF94 force field
implemented for transition metal parameters was used in the minimiza-
tion calculations. The X-ray structure of [Pt2(p-tol)4(µ-SEt2)2],42 im-
ported from the Cambridge Crystallographic Database (CSD), was
properly modified and used as input for the calculations to gauge the
performance of the MMFF calculations. Different lowest-energy
conformations of the dinuclear complex 1 were generated by the
Crystallography. Crystals of 5 and 6, suitable for X-ray diffraction,
were obtained by slow diffusion of hexane into a concentrated toluene
solution and are air stable. Crystals of 5 were mounted on an CAD4
diffractometer for the unit cell and space group determinations and for
the data collection. The unit cell constants, space group determination,
and the data collection for 6 were carried out, at 200(2) K, on a Bruker
SMART diffractometer equipped with a CCD detector.
(69) MolEN: Molecular Structure Solution Procedure; Enraf-Nonius: Delft,
The Netherlands, 1990.
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Structural Study of [Pt(Hbph)2(bpy)], 5. Data were measured with
variable scan speed to ensure constant statistical precision on the
collected intensities. Three standard reflections were used to check the
stability of the crystals and of the experimental conditions and were
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A R T I C L E S
Plutino et al.
systematic variation of the dihedral angles S-Pt-C2-C1 and C2-
C1-C7-C8 for all four Hbph fragments. Full geometry optimization
for each structure to a gradient convergence limit of less than 10-5
was carried out before a final single point energy calculation. During
the minimization, no symmetry restriction was imposed, but the
positions of all atoms in both of the square coordination planes, that
is, Pt-S1-S2-C2a-C2b, were kept frozen.
without perturbing the magnetization of the other signal. The integrals
of Sbound (Mb) were fitted by linear regression analysis to eq 11
ln[(Mb)t - (Mb)∞] ) -ln(Mb)0(τ1b/τb) + t/τ1b
(11)
where (Mb)0, (Mb)t, and (Mb)∞ are the integrals of the peak before
saturation, at the time t, and after a prolonged saturation, respectively.
τ1b is correlated to T1b, the spin-lattice relaxation time, and to τb, the
lifetime of the observed site, by eqs 12-13.
Kinetic Studies. (a) Spectrophotometric Kinetics. Nucleophilic
substitution reactions of 3 with bpy and phen to yield 5 and 6 were
carried out at 298.2 ( 0.05 K in toluene as the solvent and were
followed by repetitive scanning of the spectrum at suitable times in
the wavelength range 380-540 nm or, at fixed wavelength, where the
absorbance of the starting sulfoxide complex is negligible in comparison
to that of the final diimine product [Pt f (N-N) MLCT] band at λmax
) 484 nm, 5; 478 nm, 6. The reactions, carried out in the presence of
at least a 10-fold excess of chelating ligand and a 20-fold excess of
dmso over the complex, went to completion with the final spectra being
identical to those of authentic samples of [Pt(Hbph)2(N-N)]. Abstract
factor analysis77 of the spectral changes confirmed that the diimine-
containing product is the only absorbing species in solution, and there
was no indication of the presence of significant amounts of any
intermediate species. Rate constants, kobsd/s-1, were evaluated by using
the SCIENTIST software package,46 from nonlinear least-squares fits
of the experimental data to the equation At ) A∞ + (A0 - A∞)
exp(-kobsdt), with A0, A∞, and kobsd as the parameters to be optimized
(A0 ) absorbance after mixing of reagents, A∞ ) absorbance at
completion of reaction), and are collected in Tables S3 and S4 of the
Supporting Information, as a function of the nucleophile (N-N) and
nucleofuge (dmso) concentrations.
(Mb)∞ ) (Mb)0(τ1b/T1b)
1/τ1b ) 1/τb + 1/T1b
(12)
(13)
-1
The pseudo-first-order constants were calculated as kexch ) 2τb
,
taking into account the fact that the exchange involves two equivalent
sites. All of the rates of ligand exchange were independent of the
concentration of free sulfoxide. The rate constants at different temper-
atures from magnetization transfer experiments (in Supporting Informa-
tion Table S6) were fitted to the Eyring equation
k ) (kT/h) exp(-∆Hq/RT) exp(∆Sq/R)
(14)
leading to the activation parameters reported in Table S6.
Acknowledgment. We thank the Ministero dell’Istruzione,
dell’Universita` e della Ricerca (MIUR), Programmi di Ricerca
Scientifica di Rilevante Interesse Nazionale, Cofinanziamento
2002-2003, the Universita` degli Studi di Messina, and CNR
for funding this work.
(b) Magnetization Transfer Experiments. The rate constants for
the moderately fast exchange of dmso between two nonequivalent sites,
Sbound and Sfree, in complex 3, were determined as a function of both
temperature and added sulfoxide concentration by magnetization transfer
experiments, as described by Forse`n-Hoffman.78 In a typical experi-
ment, the resonance of the free site, Sfree (at δ 2.6), in CDCl3 was
completely saturated through a selective rf pulse, while the resonance
of Sbound (at δ 3.1) was monitored as a function of the variable irradiation
time. Thus, the condition of a good separation of the signals of free
and bound ligands was fulfilled, allowing saturation of the free signal
Supporting Information Available: Text giving experimental
details and a full listing of crystallographic data for 5 and 6-
(C7H8) (CIF), including tables of positional and isotropic
equivalent displacement parameters, calculated positions of the
hydrogen atoms, anisotropic displacement parameters, bond
distances, and bond and torsion angles. ORTEP figures showing
the full numbering schemes are also included. Table of NOESY
connectivities for 1a and 1b. Tables giving the concentration
and temperature dependencies of primary kinetic data. A figure
showing experimental and calculated isotopic distribution of (M
- C12H9 - C2H5 + H)+ for 1a. This material is available free
of charge via the Internet at http://pubs.acs.org.
(77) (a) Uguagliati, P.; Benedetti, A.; Enzo, S.; Schiffini, L. Comput. Chem.
1984, 8, 161-168. (b) Sandrini, P. L.; Mantovani, A.; Crociani, B.;
Uguagliati, P. Inorg. Chim. Acta 1981, 51, 71-80. (c) Malinowski, E. R.;
Hovery, D. G. Factor Analysis in Chemistry; Wiley-Interscience: New
York, 1980; pp 159-161.
(78) Forse`n, S.; Hoffmann, R. A. J. Chem. Phys. 1963, 39, 2892-2901.
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