Mechanism of silver-promoted ligand metathesis in square-planar complexes of d8 ions. Kinetics of formation and molecular structures of a trinuclear intermediate [(Me)(N-N)Pt(μ-Cl)Ag(μ-Cl)Pt(N-N)(Me)]+ and its dinuclear evolution product [(Me)(N-N)Pt(μ-Cl)Pt(N-N)(Me)]+ (N-N = ArN=C(Me)C(Me)=NAr, Ar = 2,6-(i-Pr)2C6H3)

Article abstract of DOI:10.1021/ic011059p

The silver-assisted ligand methatesis reaction involving a platinum(II) complex of formula [PtClMe(N,N-chelate)] with acetonitrile has been investigated. By using a suitably hindered N,N-chelate, an otherwise hardly detectable trinuclear species has been isolated and characterized through X-ray diffractometry. The trinuclear cation consists of two nearly orthogonal [PtCl(Me)(N,N-chelate)] square-planar units entrapping an Ag⁺ cation through the chloride ligands that, acting as bidentate, form a linear AgCl₂ unit with two nonequivalent Ag-Cl bonds. The residual acidity of the silver cation is satisfied by one secondary Ag-Pt interaction [Ag-Pt(1) = 2.82 A] in which the platinum atom acts as a donor. Kinetic studies have demonstrated that the silver assistance operates both through a simple associative step and through a pathway in which the above trinuclear complex is an active intermediate. In a noncoordinating solvent the latter species evolves with AgCl loss and formation of a dinuclear Pt,Pt complex showing a rare single chloride bridge.

Full text of DOI:10.1021/ic011059p

Inorg. Chem. 2002, 41, 2672−2677
Mechanism of Silver-Promoted Ligand Metathesis in Square-Planar
Complexes of d8 Ions. Kinetics of Formation and Molecular Structures
of a Trinuclear Intermediate [(Me)(N−N)Pt(µ-Cl)Ag(µ-Cl)Pt(N−N)(Me)]+
and Its Dinuclear Evolution Product [(Me)(N−N)Pt(µ-Cl)Pt(N−N)(Me)]+
(N−N ) ArNdC(Me)C(Me)dNAr, Ar ) 2,6-(i-Pr)2C6H3)
,†
Vincenzo G. Albano,* Martino Di Serio,‡ Magda Monari,† Ida Orabona,‡ Achille Panunzi,‡ and
,‡
Francesco Ruffo*
Dipartimento di Chimica “G. Ciamician”, UniVersita` di Bologna, Via F. Selmi 2,
I-40126 Bologna, Italy, and Dipartimento di Chimica, UniVersita` di Napoli “Federico II”,
Complesso UniVersitario di Monte S.Angelo, Via Cintia, I-80126 Napoli, Italy
Received October 15, 2001
The silver-assisted ligand methatesis reaction involving a platinum(II) complex of formula [PtClMe(N,N-chelate)]
with acetonitrile has been investigated. By using a suitably hindered N,N-chelate, an otherwise hardly detectable
trinuclear species has been isolated and characterized through X-ray diffractometry. The trinuclear cation consists
of two nearly orthogonal [PtCl(Me)(N,N-chelate)] square-planar units entrapping an Ag+ cation through the chloride
ligands that, acting as bidentate, form a linear AgCl2 unit with two nonequivalent Ag−Cl bonds. The residual acidity
of the silver cation is satisfied by one secondary Ag−Pt interaction [Ag−Pt(1) ) 2.82 Å] in which the platinum atom
acts as a donor. Kinetic studies have demonstrated that the silver assistance operates both through a simple
associative step and through a pathway in which the above trinuclear complex is an active intermediate. In a
noncoordinating solvent the latter species evolves with AgCl loss and formation of a dinuclear Pt,Pt complex showing
a rare single chloride bridge.
Introduction
corresponding neutral species, which are often more stable
and easy to handle. A widely used procedure is the halide
abstraction by a silver salt in the presence of an incoming
neutral L ligand (eq 1).3
This reaction is generally shifted toward the products by
the low solubility of the silver halide in the reaction medium.
It is acknowledged1 that the presence of a positive charge
in a metal complex enhances its ability in promoting several
organic reactions. Within d8 ions this is testified by a
significant number of examples,1,2 spanning from the olefin
polymerization catalyzed by monopositive Ni(II) and Pd(II)
complexes2a to the nucleophilic activation of alkenes pro-
moted by dipositive Pd(II) and Pt(II) centers.1a Therefore,
several synthetic methods have been developed to prepare
positively charged complexes, generally starting from the
L′nM-X + AgY + L ) [L′nM-L]+Y- + AgX (1)
As a part of our research dealing with cationic olefin
complexes of platinum(II),4 we have recently described the
preparation of new square-planar complexes of formula
[Pt(Me)(N,N-chelate)(L)]+ (L ) e.g. olefin or nitrile) by the
* To whom correspondence should be addressed. E-mail: vgalbano@
ciam.unibo.it (V.G.A.); ruffo@unina.it (F.R.).
† Universita` di Bologna.
‡ Universita` di Napoli “Federico II”.
(3) Liston, D. J.; Lee, Y. J.; Scheidt, W. R.; Reed, C. A. J. Am. Chem.
Soc. 1989, 111, 6643-6648.
(4) (a) Orabona, I.; Macchioni, A.; Ruffo, F.; Zuccaccia, C. Organo-
metallics 1999, 18, 4367-4372. (b) Ganis, P.; Orabona, I.; Ruffo, F.;
Vitagliano, A. Organometallics 1998, 17, 2646-2650. (c) Fusto, M.;
Giordano, F.; Orabona, I.; Panunzi, A.; Ruffo, F. Organometallics
1997, 16, 5981-5987.
(1) (a) Hahn, C.; Morvillo, P.; Vitagliano, A. Eur. J. Inorg. Chem. 2001,
419-429. (b) Maresca, L.; Natile, G. Comments Inorg. Chem. 1994,
16, 95-112.
(2) (a) Ittel, S. D.; Johnson, L. K.; Brookhart, M. Chem. ReV. 2000, 100,
1169-1203. (b) Michelin, R. A.; Mozzon, M.; Bertani, R. Coord.
Chem. ReV. 1996, 147, 299-338.
2672 Inorganic Chemistry, Vol. 41, No. 10, 2002
10.1021/ic011059p CCC: $22.00 © 2002 American Chemical Society
Published on Web 04/19/2002

SilWer-Promoted Ligand Metathesis
CD3CN were used as solvents, and CHCl3 (δ ) 7.26) and
CD2HNO2 (δ ) 4.33) as internal standards. The following ab-
breviations were used for describing NMR multiplicities: s, singlet;
d, doublet; t, triplet; h, heptet; m, multiplet. Infrared spectra were
recorded on a JASCO FT-IR 430 spectrometer.
Synthesis of IIIA. Solid [PtCl(Me)(A)] (0.065 g, 0.10 mmol)
was added to a stirred solution of AgBF4 (0.020 g, 0.10 mmol)
and acetonitrile (0.004 g, 0.10 mmol) in 1 mL of dry THF. After
5 min of stirring, the solvent was removed under vacuum. The red
residue was extracted with dichloromethane and crystallized by slow
addition of n-pentane (0.070 g, 82%). Crystals suitable for X-ray
analysis were obtained from THF/diethyl ether. 1H NMR (CDCl3,
298 K): δ 7.29 (m, 6H, Ar), 2.92 (m, 4H, CHMe2), 1.97 (s, 3H,
NdCMe), 1.77 (s, 3H, NdCMe), 1.31 (d, 6H, CHMe2), 1.26 (d,
6H, CHMe2), 1.54 (d, 6H, CHMe2), 1.49 (d, 6H, CHMe2), 0.73 (s,
Figure 1. Structure of ligands A and B.
just mentioned reaction (eq 2), where N,N-chelate is a
bidentate nitrogen ligand such as A or B (Figure 1).
[PtCl(Me)(N,N-chelate)] (I) + AgY + L )
[Pt(Me)(L)(N,N-chelate)]+Y- (II) + AgCl (2)
3H, JHPt 73 Hz, Pt-Me). IR (in KBr): ν 1100 cm-1 (br) (BF4-).
3
Quite surprisingly, we have found that the type I com-
pound of ligand A (IA) was resistant to undergo reaction 2,
which could be successfully accomplished only by stirring
the reaction mixture for several hours. It should be noted
that A belongs to the class of axially hindered chelates which
afford steric bulk above and below the coordination plane
and, hence, inhibit associative processes.2a On the other hand,
ligand B does not introduce substantial axial hindrance.
The dependence of the substitution rate from the steric
hindrance of the chelate suggested that significant mecha-
nistic insight into the silver-assisted metathesis reaction could
be achieved on examining the above system.
Anal. Calcd for C58H86AgBCl2F4N4Pt2: C, 46.59; H, 5.80; N, 3.75.
Found: C, 46.85; H, 5.63; N, 3.91.
Synthesis of IIB. Solid [PtCl(Me)(B)] (0.048 g, 0.10 mmol) was
added to a stirred solution of AgBF4 (0.020 g, 0.10 mmol) and
acetonitrile (0.004 g, 0.10 mmol) in 1 mL of dry THF. After 5 min
of stirring, the solvent was removed under vacuum and the yellow
residue was extracted with dichloromethane. Removal of the solvent
under vacuum afforded the complex in almost quantitative yield.
1H NMR (CDCl3, 298 K): δ 7.50 (m, 4H, Ar), 7.38 (d, 2H, Ar),
7.26 (t, 2H, Ar), 7.12 (d, 2H, Ar), 2.19 (s, 3H, NdCMe), 2.12 (s,
3H, NdCMe), 2.08 (s, 3H, 4JHPt 14 Hz, MeCN), 0.54 (s, 3H, 3JHPt
75 Hz, Pt-Me). Anal. Calcd for C19H22BF4N3Pt: C, 39.74; H, 3.86;
N, 7.32. Found: C, 39.59; H, 3.81; N, 7.24.
Thus, we have undertaken a thorough investigation of
reaction 2, which has allowed the isolation of an unusual
trinuclear Pt,Ag,Pt complex (IIIA). The X-ray molecular
structure has shown that this cationic compound consists of
two square-planar moieties of type I linked by a chloro-
bridged silver cation. A kinetic study has demonstrated that
this polynuclear compound is one intermediate of reaction
2. Thus, the mechanism involves both a simple associative
step assisted by coordination of the silver cation to the Pt-
Cl bond and a more complicated pathway where the
trinuclear species plays a key role.
Although previous studies already pointed out the active
participation of the silver cation,3,5 this is the first time where
the kinetic data point to a reaction pathway more complicated
than a simple bimolecular step.
Furthermore, when complex IIIA is allowed to slowly
decompose in a noncoordinating solvent, loss of AgCl is
observed with formation of a binuclear Pt,Pt complex (VA)
that an X-ray diffraction study has shown to consist of two
square-planar units linked by a rare Pt-Cl-Pt single chloride
bridge.
Synthesis of VA. Complex IIIA (0.15 g, 0.10 mmol) was
dissolved in 3 mL of chloroform. After 3 days AgCl was filtered
out and the volume of the solution reduced under vacuum. Careful
addition of diethyl ether afforded red crystals of product (0.11 g,
81%). Crystals suitable for X-ray analysis were obtained from
chloroform/diethyl ether. 1H NMR (CDCl3, 298 K): δ 7.35-7.15
(m, 6H, Ar), 2.84 (m, 4H, CHMe2), 1.79 (s, 3H, NdCMe), 1.62
(s, 3H, NdCMe), 1.19 (d, 6H, CHMe2), 1.10 (d, 6H, CHMe2), 1.05
3
(d, 6H, CHMe2), 0.94 (d, 6H, CHMe2), 0.66 (s, 3H, JHPt 72 Hz,
Pt-Me). IR (in KBr): ν 1100 cm-1 (br) (BF4-). Anal. Calcd for
C58H86BClF4N4Pt2: C, 51.53; H, 6.41; N, 4.14. Found: C, 51.77;
H, 6.30; N, 4.23.
Kinetic Measurements. A weighted amount of the appropriate
platinum complex (IA or IIIA) was dissolved in 0.60 mL of a 20:3
mixture of CD3NO2/CD3CN and immediately transferred in an
NMR tube. Eventually, a known amount of AgBF4 was dissolved
in solution. NMR spectra of the reacting mixture were recorded at
regular intervals of time at 303 K. The temperature was calibrated
by using a separate methanol sample, according to a standard
procedure. In all the experiments the reaction was complete within
30-90 min. The concentrations of the platinum species could be
evaluated by integrating suitable separated peaks, i.e. the Pt-Me
1
signals. H NMR for IIA (20:3 CD3NO2/CD3CN, 303 K): δ 7.45
Experimental Section
(m, 6H, Ar), 3.10 (h, 1H, CHMe2), 3.02 (h, 1H, CHMe2), 2.18 (s,
3H, NdCMe), 2.16 (s, 3H, NdCMe), 1.40 (d, 6H, CHMe2), 1.36
(d, 6H, CHMe2), 1.25 (d, 6H, CHMe2), 1.20 (d, 6H, CHMe2), 0.52
All experiments were carried out under nitrogen atmosphere
using Schlenk techniques. Solvents and reagents were of AnalaR
grade and were used without further purification, unless otherwise
stated. THF was dried and distilled from Na/benzophenone.
[PtCl(Me)(A)] and [PtCl(Me)(B)] were prepared according to
literature procedures.4c 1H NMR spectra were recorded on a Varian
XL-200 spectrometer. CDCl3 and a 20:3 mixture of CD3NO2/
3
(s, 3H, JHPt 81 Hz, Pt-Me).
X-ray Diffraction Experiments and Structure Determination
of Cations IIIA and VA as [BF4]- Salts. Crystal data and other
experimental details for both species are reported in Table 1. The
X-ray diffraction data were measured on a Bruker AXS SMART
2000 diffractometer, equipped with a CCD area detector, using Mo
KR radiation (λ ) 0.710 73 Å) at room temperature. Cell dimen-
sions and orientation matrixes were initially determined from least-
(5) Erickson, L. E.; Godfrey, M.; Larsen, R. G. Inorg. Chem. 1987, 26,
992-997.
Inorganic Chemistry, Vol. 41, No. 10, 2002 2673

Albano et al.
Table 1. Crystal Data and Diffraction Experimental Details for Cations
IIIA and VA in Their [BF4]- Salts
formula
fw
C58H86AgBCl2F4N4Pt2·0.5thf C59H86BCl4F4N4Pt2
1547.1
1470.1
temp, K
λ, Å
cryst symmetry
space group
a, Å
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
293(2)
0.710 73
triclinic
293(2)
0.710 73
monoclinic
P21/n (No. 14)
16.170(1)
24.522(1)
18.464(1)
90
112.189(1)
90
6779.2(6)
4
P1h (No. 2)
10.5016(3)
17.9486(5)
18.7605(5)
109.921(1)
90.684(1)
91.212(1)
3323.1(2)
2
Z
σ
calcd, Mg m-3
1.510
1.440
4.327
0.0374, 0.0728
µ(Mo KR), mm-1 4.620
R1(F),a wR2(F2)b
0.0419, 0.0947
a R1 ) Σ||Fo| - |Fc|/Σ|Fo|. b wR2 ) [Σw(Fo2 - Fc2)2/Σw(Fo2)2]1/2, where
Figure 2. ORTEP drawing of cation IIIA as determined by X-ray
diffraction in its [BF4]- salt. Thermal ellipsoids are drawn at 30% of the
probability level. For the sake of clarity, the atom labeling has been omitted
where not necessary.
2
2
2
w ) 1/[σ2(Fo ) + (aP)2 + bP], where P ) (Fo + 2Fc )/3.
squares refinements on reflections measured in three sets of 20
exposures collected in three different ω regions and eventually
refined against all reflections. Full spheres of the diffraction space
were measured by 0.3° ω steps, 40 s exposures, and the sample-
detector distance kept at 5.0 cm. Intensity decay was monitored by
recollecting the initial 50 frames at the end of each data collection
and analyzing the duplicate reflections. The collected frames were
processed for integration by using the program SAINT, and an
empirical absorption correction was applied using SADABS6 on
the basis of the Laue symmetry of the reciprocal space. The
structures were solved by direct methods (SIR 97)7 and subsequent
Fourier syntheses and refined by full-matrix least-squares calcula-
tions on F2 (SHELXTL)8 using anisotropic thermal parameters for
all non-hydrogen atoms. The hydrogen atoms were located experi-
mentally, but their positions were idealized and refined riding the
pertinent carbon atoms [C(sp3)-H ) 0.98, C(sp2)-H ) 0.93 Å].
The refinement of the structure model of VA was straightforward,
and the final difference electron density map was featureless. On
the contrary, the Fourier map for IIIA showed some significant
residual peaks of electron density. Two peaks were located in the
vicinity of the silver cation, and three were grouped around the
inversion center at 0.5, 0.5, 0.5, forming a sort of cyclohexane.
The latter were reasonably explained as a tetrahydrofuran molecule
exhibiting a double image for the oxygen atom and single images
for the CH2 groups. The rationalization of the stronger peaks (4.5
and 1.8 e Å-3) proved more tricky because they were placed at
1.87 Å from each other and at 1.03 and 1.18 Å from Ag,
respectively. The distances from Ag were clearly inconsistent with
the position of this atom and were tentatively rationalized as
alternative positions of the silver cation. A refinement of the site
occupation factors afforded the following values: Ag(a) 0.87; Ag(b)
0.09; Ag(c) 0.04. An explanation of the reason the silver cation
resulted as split over three sites came from an analysis of the
bonding distances of its fainter images (Å): Ag(b)-Cl(1), 2.23;
Ag(b)-C(2)(methyl), 2.56; Ag(c)-Cl(2), 2.48; Ag(c)-C(1)(methyl),
2.68. The supposed silver-methyl contacts would become chemi-
cally acceptable if they actually were silver-chloride interactions.
Therefore, disorder was postulated in the crystal at the level of the
molecular units; i.e., parts of the platinum complexes were rotated
or reflected in accord with the symmetry of the diimine ligands
leaving these ligands substantially unmoved but determining Cl/
Me scrambling [9% around Pt(1) and 4% around Pt(2)]. As a
consequence, the silver cation had to move in accord with the
displacements of its ligands producing the observed multiple images
and allowing one to detect and measure the phenomenon that
otherwise would have remained unnoticed.
Results and Discussion
Isolation and X-ray Solid-State Structure of the
Trinuclear Complex Cation [(Me)(N-N)Pt(µ-Cl)Ag(µ-
Cl)Pt(N-N)(Me)]+ (IIIA). The study of the mechanism of
reaction 2 has been accomplished by using MeCN as
incoming L ligand and AgBF4 as the source of Ag+:
[PtCl(Me)(A)] (IA) + AgBF4 + MeCN )
[Pt(Me)(A)(MeCN)] [BF4] (IIA) + AgCl (3)
When solid [PtCl(Me)(A)] (IA) is added to a solution in dry
THF containing 1 equiv of both AgBF4 and MeCN, im-
mediate formation of a clear red solution ensues. Slow
formation of AgCl is subsequently observed. After 24 h the
solvent is removed under vacuum and product IIA can be
extracted in quantitative yield with dichloromethane. Alter-
natively, removal of the solvent from the early-formed red
solution affords an orange solid, which can be crystallized
1
as orange needles (IIIA). The H NMR spectrum of these
crystals discloses the presence of a single species. Elemental
analyses reveal the presence of silver in the molecule, while
the IR spectrum shows the presence of [BF4]-. The data
suggest a cationic structure where both Pt and Ag are present,
possibly linked via a Cl bridge. This hypothesis has been
confirmed by an X-ray diffraction experiment.
The molecular structure of IIIA, as determined in its
[BF4]- salt, is shown in Figure 2, and selected bond distances
and angles are reported in Table 2. The cation consists of
two [PtCl(Me)(A)] planar molecules and an Ag+ cation
(6) Sheldrick, G. M. SADABS, Program for empirical absorption cor-
rection; University of Go¨ttingen: Go¨ttingen, Germany, 1996.
(7) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.; Giaco-
vazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna,
R. J. Appl. Crystallogr. 1999, 32, 115-119.
(8) Sheldrick, G. M. SHELXTLplus Version 5.1 (Windows NT Version)-
Structure Determination Package; Bruker Analytical X-ray Instruments
Inc.: Madison, WI, 1998.
2674 Inorganic Chemistry, Vol. 41, No. 10, 2002

SilWer-Promoted Ligand Metathesis
Scheme 1
Scheme 2
Table 2. Selected Bond Lengths (Å) and Angles (deg) for IIIA
Pt(1)-N(1)
Pt(1)-N(2)
Pt(1)-C(1)
Pt(2)-N(3)
Pt(2)-N(4)
Pt(2)-C(2)
Pt(2)-Cl(2)
Pt(2)-Ag(a)
Pt(1)-Cl(1)
Ag(a)-Cl(1)
Ag(a)-Cl(2)
N(1)-C(15)
N(1)-C(3)
2.006(2)
2.091(2)
2.064(3)
2.016(2)
2.094(2)
2.100(3)
2.298(1)
2.895(1)
2.291(1)
2.398(1)
2.471(1)
1.290(4)
1.419(3)
N(2)C(16)
N(2)C(19)
1.282(4)
1.420(3)
1.288(4)
1.422(3)
1.284(4)
1.417(3)
1.491(4)
1.498(4)
1.470(4)
1.480(5)
1.489(5)
1.496(5)
N(3)-C(43)
N(3)-C(31)
N(4)-C(44)
N(4)-C(47)
C(15)-C(16)
C(16)-C(17)
C(15)-C(18)
C(43)-C(44)
C(43)-C(46)
C(44)-C(45)
Pt(1)-Cl(1)-Ag(a)
N(1)-Pt(1)-N(2)
N(1)-Pt(1)-C(1)
C(1)-Pt(1)-Cl(1)
113.34(5)
77.2(1)
98.2(1)
88.1(1)
Pt(2)-Cl(2)-Ag(a)
N(3)-Pt(2)-N(4)
N(3)-Pt(2)-C(2)
Cl(1)-Ag(a)-Cl(2)
74.64(3)
77.3(1)
97.2(1)
164.87(5)
entrapped between the chloride ligands in an almost linear
AgCl2 unit [Ag(a)-Cl(1) ) 2.398(1), Ag(a)-Cl(2) )
2.471(1) Å, Cl(1)-Ag(a)-Cl(2) ) 164.87(5)°].9 The result-
ing trinuclear supercation adopts an asymmetric overall
conformation. As a consequence, Ag+ is differently posi-
tioned with respect to the two [PtCl(Me)(A)] moieties. It
sits in the coordination plane of Pt(1) (distance from the
average plane 0.15 Å) and is well out of the coordination
plane of Pt(2) [dihedral angle between coordination and
Pt(2)Cl(2)Ag(a) planes 71.26(6)°]. In this geometry Ag+ falls
within bonding distance of Pt(2) [2.895(1) Å] and remains
far apart from Pt(1) [3.84 Å]. An analysis of the Pt-Cl-
Ag(a) angles [Pt(1)-Cl(1)-Ag(a) ) 113.34(4)°, Pt(2)-
Cl(2)-Ag(a) ) 74.64(3)°] strongly suggests an attractive
Pt(2)‚‚‚Ag(a) interaction; otherwise more balanced values
would have been observed. The consistence of the dative
Pt(2)fAg(a) interaction could have been estimated by
comparing the corresponding bond distances around Pt(2)
and Pt(1). Unfortunately an accurate analysis of the electron
density map has given evidence of some methyl-chloride
disorder [ca. 9% and 4% scrambling around Pt(2) and Pt(1),
respectively]. Therefore we are discussing the cation con-
formation of highest weight (ca. 87%) and the platinum-
ligand bond distances are weighted average values over the
different molecules packed in the crystal and cannot be used
for detecting differences in electron saturation of the two
platinum atoms. The degree of the Pt(2)fAg(a) interaction
could also be evaluated from the displacements of the Pt
atoms out of their coordination planes. Actually both atoms
are strictly in plane (deviation 0.001 Å), and therefore, the
Pt(2)‚‚‚Ag(a) interaction is a weak one and the asymmetric
conformation of the entire molecule is motivated more by
the optimization of the molecular bulk than by the formation
of some metal-metal bond. Platinum-silver interactions are
not a novelty,10 but the present cation displays the peculiarity
of the neutral nature of the platinum units. A comparable
stereochemical situation has been found in the polymeric
complex [PtAgCl2(C6Cl5)(PPh3)],11 even though this species
is substantially different from an electronic point of view,
the platinum complex being an anion.
Although the diimine ligands are not appreciably affected
by disorder, their dominating contribution in determining the
molecular shape, together with their ability to conform to
C2, Cs, or C2V idealized symmetries, is at the origin of the
partial disorder of the chloride-methyl groups. The packing
interactions do not completely discriminate molecules rotated
according to the symmetry of the dominating ligand, and
some Cl/Me mixing takes place in the crystal. Actually the
ligand at Pt(1) adopts an idealized C2V symmetry, while that
at Pt(2) is better described by a Cs symmetry with the mirror
plane orthogonal to the coordination plane. The multisite
locations of the chloride ligands generate corresponding
displacements of the silver cation, but the Coulombic
component of the packing energy seems little affected. In
fact Ag+ is incapsulated in the core of a large molecular
assemblage and is far away from the [BF4]- counterions
(shortest Ag(a)‚‚‚F distance 9.82 Å). Anyway the increase
of configurational entropy generated by the disorder is
beneficial to the crystal stability.
Kinetic Studies. The successful isolation of the poly-
nuclear species IIIA has stimulated us to gain insight into
the mechanism of reaction 3. Previous related works3,5
suggest that silver may have either a simple scavenger role
(Scheme 1) or a more active function (Scheme 2). In the
former circumstance, the incoming MeCN ligand would
substitute the chloride with formation of the cationic
[Pt(Me)(A)(MeCN)]+Cl- intermediate (reaction i in Scheme
1), and then AgCl would precipitate (reaction ii). In the latter
case, the Ag+ ion would first link the metal-bonded chloride
of IA (reaction i in Scheme 2) by forming the undetected
IVA binuclear cation, which may evolve into the final
product through reaction ii of Scheme 2. However, our results
suggest that this Pt,Ag complex is able to interact with a
second molecule of IA with formation of the isolated
trinuclear complex IIIA (reaction iii). Thus, the mechanistic
investigation must take into account the possibility that also
IIIA may afford the final product IIA (reaction iv). The
(9) For the meaning of the labels on silver ion, see the Experimental
Section.
(10) Uso`n, R.; Fornie´s, J. Inorg. Chim. Acta 1992, 198-200, 165-177.
(11) Uso`n, R.; Fornie´s, J.; Toma`s, M.; Ara, I. Inorg. Chem. 1994, 33, 4023-
4028.
Inorganic Chemistry, Vol. 41, No. 10, 2002 2675

Albano et al.
relative rates of reactions ii and iv can indicate if IIIA is
actually an intermediate of reaction 3.
The equations 8-11 correspond to a nonlinear equations
system that must be solved before every integration step.
The parameters of the model [K(ei), K(eiii), kii, and kiv] can
be obtained by nonlinear regression of the model on the
experimental data of the runs reported in Table S1.
Several kinetic experiments have been performed aiming
to discriminate among the plausible mechanisms (see Table
S1 of the Supporting Information). All runs have been
performed at 303 K, using a mixture of CD3NO2 and CD3CN
(20:3) as solvent (0.60 mL) and different initial amounts of
IA, IIIA, and AgBF4.
The experimental data of runs performed at different silver
concentrations clearly indicate that the reaction rate strongly
depends on the concentration of this ion. Therefore, the
mechanism shown in Scheme 1 is certainly not operating,
since silver would take part only in reaction ii, i.e. in the
precipitation of the silver chloride that surely cannot be the
slow reaction of the mechanism. This agrees with previous
results on a related system.5
Once clarified that the role played by Ag+ is not a passive
one, the mechanism proposed in Scheme 2 has been
investigated. Reactions i and iii can be considered as fast
equilibria. Actually, even at low temperatures (223 K)
average NMR signals are observed for the involved com-
plexes.12 On the other hand, the slow reactions of the
mechanism can be considered reactions ii and iv. In fact,
success in isolating a stable complex such as IIIA undoubt-
edly stems from the ability of the chelate to inhibit associative
processes;2a i.e., the hindrance above and below the coor-
dination plane inhibits the approach of acetonitrile. In keeping
with this hypothesis, when reaction 3 is performed on the
complex of the poorly crowded ligand B, the cationic product
[Pt(Me)(MeCN)(B)]BF4 (IIB) is rapidly formed.
The model described above is general and foresees the
possibility to arrive to IIA either through the dinuclear IVA
or the trinuclear IIIA. Possible particular cases can be
devised as, for instance, a mechanism in which only IIIA
(mechanism 2a, kii ) 0) or only IVA (mechanism 2b, kiv )
0) evolves to IIA or that IVA and IIIA have comparable
reactivity (mechanism 2c, kii ∼ kiv). The last assumption is
the most reasonable because species IIIA and IVA exhibit
similar steric hindrance. Nevertheless, to discriminate be-
tween the three models, these have all been submitted to
nonlinear regression13 on the experimental data. In Figure
S5 of the Supporting Information the deviations between
calculated and experimental values can be appreciated. Model
2c provides lower deviations and, on the other hand, also a
better correlation index (99.40 against 97.97 of mechanism
2a and 97.35 of mechanism 2b). In Figures S1-S4 the
accords obtained with the parameters of the model 2c
determined by regression [K(ei) ) 6.6 ( 1.5 mL/mmol, K(eiii)
) 71 ( 25 mL/mmol, kii ) kiv ) 0.070 ( 0.010 1/min]
between experimental data and calculated values can be
appreciated. The same result was obtained when the calcula-
tions were performed by allowing different values for kii and
kiv.
The analysis on the models shows that both IVA and IIIA
evolve to IIA, and therefore, they are both intermediates of
the silver salt metathesis under investigation.
Therefore, the concentration of IIA and of all the species
containing Ag (CAg) is given by the numerical integration
of the following equations:
X-ray Structure of the Dinuclear Cation [(Me)(N-N)-
Pt(µ-Cl)Pt(N-N)(Me)]+ (VA) Obtained from IIIA by
AgCl Loss. Complex IIIA has revealed to be unstable in
deuteriochloroform solution. AgCl quantitatively precipitates
within few days with formation of a new complex (VA).
Also in this case the molecular structure has been determined
by X-ray diffraction and has clearly revealed that cation VA
is the product of AgCl elimination from IIIA. The molecule
is illustrated in Figure 3, and selected bond distances and
angles are reported in Table 3. The stereogeometry of the
cation shows the presence of two equivalent square-planar
Pt(II) units sharing the chloride ligand. The molecular
conformation is determined by the need to avoid close
intraligand contacts, especially between the methyl ligands
and the bulky 2,5-(i-Pr)2C6H3 groups that are placed almost
orthogonally to the PtN2C2 chelate rings. The Pt(1)-Cl-
Pt(2) angle is obtuse [119.39(5)°] and the Pt(1)‚‚‚Pt(2)
contact quite long [3.965(1) Å]. The two square moieties
are flat (deviations from the average planes in the range
(0.02 Å) and symmetrically rotated out of the Pt(1)ClPt(2)
plane ((37.5°); therefore, the cation as a whole conforms
to a C2 idealized symmetry. The bond distances in the two
chemically equivalent units are equal within experimental
d[IIA]/dt ) rii + riv
dCAg/dt ) -rii - riv
(4)
(5)
Since CD3CN is in wide excess, the rates of reactions ii and
iv are surely of order 0 for this reagent, while they will be
dependent on the concentration of IVA and IIIA:
rii ) kii[IVA]
riv ) kiv[IIIA]
(6)
(7)
The concentrations of IVA and IIIA are given by the solution
of the mass balances (8) and (9), where C°Pt is the initial
total platinum concentration. By using the equilibrium
equations 10 and 11, it is obtained
[IA] + [IIA] +2[IIIA] + [IVA] ) C°Pt
[Ag+] + [IIA] + [IIIA] + [IVA] ) CAg
(8)
(9)
K(ei) ) [IVA]/([IA][Ag+])
K(eiii) ) [IIIA]/([IA][IVA])
(10)
(11)
(12) The strong MLCT band displayed by the involved species and
precipitation of AgCl prevented an equilibrium study through UV-
vis spectroscopy.
(13) Buzzi Ferraris, G. Ing. Chim. Ital. 1968, 12, 171-176.
2676 Inorganic Chemistry, Vol. 41, No. 10, 2002

SilWer-Promoted Ligand Metathesis
ment accompanied by a short Pt‚‚‚Pt contact [2.9696(6) Å],
an acute Pt-Cl-Pt angle [77.48(9)°], and significant dis-
placements of the bridging chloride ligand out of the
coordination planes. We must conclude that this kind of
conformation is energetically favored but not accessible to
the title cation because of the cumbersome substituted aryl
appendages. The platinum-ligand bonds in the two species
exhibit similar trends except for the Pt-Cl interactions
(average values 2.296(1) Å in VA and 2.372(5) Å in the
other cation) as a consequence of the bond strain originated
by the conformation of the cation.
A database search has revealed no molecule of similar
structure except the palladium cation [Pd2(µ-Cl){C6H3-
(CH2NMe2)2-o,o′}2]+,16 whose overall conformation is similar
to that of the present cation, despite significant differences
in bond values, due to a nonequivalent bond disposition in
the coordination plane.
Figure 3. ORTEP drawing of cation VA as determined by X-ray
diffraction in its [BF4]- salt. Thermal ellipsoids are at the 30% probability
level. For the sake of clarity, the atom labeling has been omitted where not
necessary.
Conclusions
This study provides a detailed investigation on the mech-
anism of one of the most widely used inorganic reactions,
i.e. the silver-assisted methatesis reaction:
Table 3. Selected Bond Lengths (Å) and Angles (deg) for VA
L′nM-X + AgY + L ) [L′nM-L]+Y- + AgX
Pt(1)-Cl(1)
Pt(2)-Cl(1)
Pt(1)-N(1)
Pt(1)-N(2)
Pt(2)-N(3)
Pt(2)-N(4)
Pt(1)-C(1)
Pt(2)-C(30)
N(1)-C(2)
N(2)-C(4)
Pt(1)‚‚‚Pt(2)
2.296(1)
2.296(1)
1.990(3)
2.091(3)
2.103(3)
2.000(3)
2.069(4)
2.068(4)
1.288(5)
1.289(5)
3.965(3)
C(2)-C(4)
C(2)-C(3)
C(4)-C(5)
N(1)-C(18)
N(2)-C(6)
N(3)-C(31)
N(4)-C(33)
C(31)-C(33)
N(3)-C(47)
N(4)-C(35)
1.479(6)
1.515(6)
1.503(6)
1.413(4)
1.410(4)
1.285(5)
1.281(5)
1.484(5)
1.426(4)
1.426(4)
The presence of an axially hindered N,N-chelate in the
coordination sphere of a Pt(II) complex has allowed the
isolation and crystallization of the trinuclear Pt,Ag,Pt cation
IIIA, whose structural features have been disclosed by an
X-ray diffraction study. A kinetic study has clearly confirmed
the involvement of IIIA in the reaction mechanism, which
therefore consists of two parallel pathways. The first one
involves formation of a dinuclear Pt,Ag intermediate, as
already suggested in previous related studies.3,5 Alternatively,
this latter species can afford the trinuclear intermediate IIIA,
which also can give rise to the final cationic product.
In a noncoordinating solvent, the trinuclear complex
decomposes with AgCl loss and formation of a rare example
of single chloro-bridged Pt,Pt dimer (VA).
Pt(1)-Cl(1)-Pt(2)
N(1)-Pt(1)-N(2)
N(3)-Pt(2)-N(4)
N(1)-Pt(1)-C(1)
119.39(5)
78.0(1)
77.8(1)
96.5(1)
N(1)-Pt(1)-Cl(1)
N(4)-Pt(2)-Cl(1)
Cl(1)-Pt(1)-C(1)
173.0(1)
172.5(1)
90.3(1)
errors (see Table 3), and the average values of the platinum-
ligand interactions are as follows (Å): Pt-Cl ) 2.296(1);
Pt-C(methyl) ) 2.068(4); Pt-N ) 1.995(5) and 2.097(5)
for the nitrogens trans to the chloride and methyl groups,
respectively. These values agree well with those recently
reported for the mononuclear species [PtClMe(ArNdCHCHd
NAr)] (Ar ) 2-(MeOCH2)-4,6-t-BuC6H2) (Å): Pt-Cl )
2.283(3); Pt-C(methyl) ) 2.053(10); Pt-N ) 1.994(9) and
2.122(9).14 Also interesting is a comparison with the stereo-
chemically more similar cation [Pt2(µ-Cl)(S,S-trans-1-(Nd
CH(C6H4)-2-(NdCHPh)C6H10)2]+.15 The most significant
difference concerns the orientations of the square-planar
moieties in the latter that, in contrast to the skewed
conformation in VA, feature an almost antiparallel arrange-
Acknowledgment. The authors thank the Consiglio
Nazionale delle Ricerche, the MURST (Programmi di
Ricerca Scientifica di Rilevante Interesse Nazionale, Co-
finanziamento 2000-2001), and the Centro Interdipartimen-
tale di Metodologie Chimico-Fisiche, Universita` di Napoli
“Federico II”, for NMR facilities.
Supporting Information Available: A table of kinetic runs,
figures showing the accord between the kinetic model and the
experimental data, and X-ray crystallographic files in CIF format
for the structure of complexes IIIA and VA. This material is
available free of charge via the Internet at: http://pubs.acs.org.
IC011059P
(14) Yang, K.; Lachicotte, R. J.; Eisenberg, R. Organometallics 1998, 17,
5102-5113.
(15) Baar, C. R.; Jenkins, H. A.; Jennings, M. C.; Yap, G. P. A.; Puddephatt,
R. J. Organometallics 2000, 19, 4870-4877.
(16) Terheijden, J.; van Koten, G.; Grove, D. M.; Vrieze, K.; Spek, A. L.
J. Chem. Soc., Dalton Trans. 1987, 1359-1366.
Inorganic Chemistry, Vol. 41, No. 10, 2002 2677