Synthesis and characterization of the double salts [Pt(bzq)(CNR) 2][Pt(bzq)(CN)2] with significant Pt...Pt and π...π interactions. Mechanistic insights into the ligand exchange process from joint experimental and dft study

Article abstract of DOI:10.1021/om201036z

Double complex salts (DCSs) of stoichiometry [Pt(bzq)(CNR) ₂][Pt(bzq)(CN)₂] (bzq = 7,8-benzoquinolinate; R = tert-butyl (1), 2,6-dimethylphenyl (2), 2-naphtyl (3)) have been prepared by a metathesis reaction between [Pt(bzq)(CNR)₂]ClO₄ and [K(H₂O)][Pt(bzq)(CN)₂] in a 1:1 molar ratio under controlled temperature conditions (range: -10 to 0 °C). Compounds 1-3 have been isolated as air-stable and strongly colored solids [purple (1), orange (2), red-purple (3)]. The X-ray structure of 2 shows that it consists of ionic pairs in which the cationic and anionic square-planar Pt(II) complexes are almost parallel to each other and are connected by Pt-Pt (3.1557(4) A) and π...π (3.41-3.79 A) interactions. Energy decomposition analysis calculations on DCSs 1-3 showed relatively strong ionic-pair interactions (estimated interaction energies of -99.1, -110.0, and -108.6 kcal/mol), which are dominated by electrostatic interactions with small contributions from dispersion (π...π) and covalent (Pt...Pt) bonding interactions involving the 5d and 6p atomic orbitals of the Pt centers. Compounds 1-3 undergo a thermal (165 °C, 24 h) irreversible ligand rearrangement process in the solid state and also in solution at temperatures above 0 °C to give the neutral complexes [Pt(bzq)(CN)(CNR)] as a mixture of two possible isomers (SP-4-2 and SP-4-3). The mechanism of this process has been thoroughly explored by combined NMR and DFT studies. DFT calculations on 1-3 show that the existing Pt...Pt interactions block the associative attack of the Pt(II) centers by the coordinated cyanide and/or isocyanide ligands. Moreover, they support a significant transfer of electron density from the anionic to the cationic component (0.20-0.32 |e|), which renders the isocyanide ligand dissociation more feasible than that in the "free-standing" cationic [Pt(bzq)(CNR) ₂]⁺ components as well as the dissociation of the CN ^- in trans position to the C_bzq in the anionic [Pt(bzq)(CN)₂]^- component. Therefore, the first step in the ligand rearrangement pathway is the dissociation of the isocyanide in trans position to the C_bzq, yielding the [(RNC)(bzq)(μ₂- η¹,η¹-CN)Pt...Pt(bzq)(CN)] intermediates. The rate-limiting step corresponds to the transformation of these intermediates to the neutral [Pt(bzq)(CN)(CNR)] complexes following a synchronous mechanism involving rupture of the Pt-Pt and formation of the Pt-CN bonds through transition states formulated as [(RNC)(bzq)Pt(μ₂- η¹,η¹-CN)Pt(bzq)(CN)].

Full text of DOI:10.1021/om201036z

Article
pubs.acs.org/Organometallics
Synthesis and Characterization of the Double Salts [Pt(bzq)(CNR)2]
[Pt(bzq)(CN)2] with Significant Pt···Pt and π···π Interactions.
Mechanistic Insights into the Ligand Exchange Process from Joint
Experimental and DFT Study
Juan Fornies,*,† Sara Fuertes,† Carmen Larraz,† Antonio Martín,† Violeta Sicilia,*,‡
́
and Athanassios C. Tsipis*,†,§
†Instituto de Síntesis Química y Catal
Zaragoza, Pedro Cerbuna 12, 50009 Zaragoza, Spain
‡Instituto de Síntesis Química y Catal
isis Homogenea (ISQCH), Departamento de Química Inorgan
Zaragoza, Escuela de Ingeniería y Arquitectura de Zaragoza, Campus Río Ebro, Edificio Torres Quevedo, 50018, Zaragoza, Spain
§Laboratory of Inorganic and General Chemistry, Department of Chemistry, University of Ioannina, 451 10 Ioannina,Greece
́
isis Homogen
́
ea (ISQCH), Departamento de Química Inorgan
́
ica, CSIC−Universidad de
́
́
́
ica, CSIC−Universidad de
S
* Supporting Information
ABSTRACT: Double complex salts (DCSs) of stoichiometry [Pt(bzq)-
(CNR)2][Pt(bzq)(CN)2] (bzq = 7,8-benzoquinolinate; R = tert-butyl (1), 2,6-
dimethylphenyl (2), 2-naphtyl (3)) have been prepared by a metathesis
reaction between [Pt(bzq)(CNR)2]ClO4 and [K(H2O)][Pt(bzq)(CN)2] in a
1:1 molar ratio under controlled temperature conditions (range: −10 to 0 °C).
Compounds 1−3 have been isolated as air-stable and strongly colored solids
[purple (1), orange (2), red-purple (3)]. The X-ray structure of 2 shows that it
consists of ionic pairs in which the cationic and anionic square-planar Pt(II)
complexes are almost parallel to each other and are connected by Pt−Pt (3.1557(4) Å) and π···π (3.41−3.79 Å) interactions.
Energy decomposition analysis calculations on DCSs 1−3 showed relatively strong ionic-pair interactions (estimated interaction
energies of −99.1, −110.0, and −108.6 kcal/mol), which are dominated by electrostatic interactions with small contributions
from dispersion (π···π) and covalent (Pt···Pt) bonding interactions involving the 5d and 6p atomic orbitals of the Pt centers.
Compounds 1−3 undergo a thermal (165 °C, 24 h) irreversible ligand rearrangement process in the solid state and also in
solution at temperatures above 0 °C to give the neutral complexes [Pt(bzq)(CN)(CNR)] as a mixture of two possible isomers
(SP-4-2 and SP-4-3). The mechanism of this process has been thoroughly explored by combined NMR and DFT studies. DFT
calculations on 1−3 show that the existing Pt···Pt interactions block the associative attack of the Pt(II) centers by the
coordinated cyanide and/or isocyanide ligands. Moreover, they support a significant transfer of electron density from the anionic
to the cationic component (0.20−0.32 |e|), which renders the isocyanide ligand dissociation more feasible than that in the “free-
standing” cationic [Pt(bzq)(CNR)2]+ components as well as the dissociation of the CN− in trans position to the Cbzq in the
anionic [Pt(bzq)(CN)2]− component. Therefore, the first step in the ligand rearrangement pathway is the dissociation of the
isocyanide in trans position to the Cbzq, yielding the [(RNC)(bzq)(μ2-η1,η1-CN)Pt···Pt(bzq)(CN)] intermediates. The rate-
limiting step corresponds to the transformation of these intermediates to the neutral [Pt(bzq)(CN)(CNR)] complexes following
a synchronous mechanism involving rupture of the Pt−Pt and formation of the Pt−CN bonds through transition states
formulated as [(RNC)(bzq)Pt(μ2-η1,η1-CN)Pt(bzq)(CN)].
atoms forming single-atom wide wires.17−26 The first DCS
INTRODUCTION
■
Metallophilic interactions between metals with d8, d10, or s2
prepared and characterized was the Magnus salt [Pt(NH3)4]-
[PtCl4], which exists in two forms that can be distinguished by
electron configurations in extended linear chains have, for many
their color.18 In the green form the platinum atoms show a
years, attracted considerable attention.1−10 Contacts between
linear array with Pt−Pt distances of ca. 3.23(2) Å. However, in
metals in one-dimensional compounds allow them to behave as
the Magnus pink salt, as deduced by powder diffraction, the
efficient conductors with potential application in the field of
Pt−Pt distance is larger than 5 Å. Compounds [Pd(NH3)4]-
molecular electronics.9,11 In addition, intermetallic interactions
[PdCl4], [Pt(NH3)4][PdCl4], and [Pd(NH3)4][PtCl4],19−21
are the cause of the intense colors exhibited by this type of solid
as well as other fascinating features in the optical field.12−16
Some of these linear compounds are double complex salts
Special Issue: F. Gordon A. Stone Commemorative Issue
(DCSs) formed by cationic and anionic complexes stacked in
an alternate way with metallophilic interactions between metal
Received: October 25, 2011
Published: January 18, 2012
© 2012 American Chemical Society
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isostructural with the Magnus green salt but exhibiting pink
color, and DCSs of the type [M(NH2R)4][MCl4] (M = Pd, Pt)
have been also reported.21,22 The green color seems to be
restricted to compounds with the shorter Pt−Pt distances (3.25
Å, R = Me),20,21 while the pink color might be associated with
reduced intermetallic interactions and longer distances (3.62 Å,
R = Et).22,23 In Pt(II) chemistry close attention has been paid
to DCSs because of their increasing use in materials science. A
large number of heterometallic (Pt−M) DCSs have been
synthesized and have sometimes been used as precursors of bi-
and polymetallic powders produced by thermal decomposition
of these salts, attempting to prepare highly effective
heterogeneous catalysts.27−30 However, the really well-known
DCSs are those containing [Pt(CN)4]2−. Compounds such as
[Pt(phen)(CN-Cyclohexyl)2][Pt(CN)4]16 and [Pt(CNR)4]-
[M(CN)4] (M = Pt, Pd; R = alkyl, aryl)24−26 have been
shown to be vapoluminescent and capable of being
incorporated into electronic devices for water vapor and/or
volatile organic compound (VOC) sensing applications.
Thermal rearrangement of the DCS [Pt(p-CN-C6H4-C2H5)4]-
[Pt(CN)4] in refluxing chloroform or by heating in the absence
of solvent gives the neutral complexes trans-[Pt(p-CN-C6H4-
C2H5)2(CN)2]31 and cis-[Pt(p-CN-C6H4-C2H5)2(CN)2],13
with the cis isomer exhibiting vapoluminescent behavior.
More recently Drew and co-workers32 synthesized the neutral
[PtII(CN-i-C3H7)2(CN)2] complex by heating the DCS
[Pt(CN-i-C3H7)4][Pt(CN)4] at 190 °C under N2, and its
solid-state characteristics have been investigated to determine
the viability of the extended linear chain as a useful benzene
sensor.
approach, whereby the rate-limiting step has been assigned and
identified.
RESULTS AND DISCUSSION
■
Synthesis and Characterization of the DCS [Pt(bzq)-
(CNR)2][Pt(bzq)(CN)2]. The double complex salts [Pt(bzq)-
(CNR)2][Pt(bzq)(CN)2] [bzq = 7,8-benzoquinolinate; R =
tert-butyl (tBu, 1), 2,6-dimethylphenyl (Xyl, 2), 2-naphtyl (2-
Np, 3)] were prepared by reaction of [K(H2O)][Pt(bzq)-
(CN)2]38 with an equimolar amount of the corresponding
isonitrile complex, [Pt(bzq)(CNR)2]ClO4,39 under controlled
temperature conditions (see Scheme 1 and Experimental
Scheme 1. Reactions and Numerical Scheme for NMR
Purposes of 1−3
Ligand exchange processes form part of the most
fundamental types of chemical reactions.33 They have been
widely observed in square-planar transition-metal complexes,
where they find extensive synthetic applications, and the
mechanisms by which they take place have been studied in a
few cases.34−37 Such processes have also been observed in
double complex salts, which however have some preparative
drawbacks such as limited stability. In spite of this, as far as we
know, the mechanism of the ligand rearrangement reactions
that DCSs frequently undergo have not been investigated to
date.
Section) and isolated from the reaction mixtures as air-stable
and intensely colored solids [purple (1), orange (2), red-purple
(3)]. Other DCSs have been also synthesized previously by a
metathesis route17,27,29,40,41
In spite of the similarity of the three compounds, the
synthesis conditions are different from each other depending
on the nature of the CNR groups, which determines the
solubility of the starting complexes, [Pt(bzq)(CNR)2]ClO4,
and that of the DCSs and also the temperature range in which
the latter are stable enough to be obtained as pure solids. The
In our previous work on cyclometalated Pt(II) compounds
we prepared luminescent anionic complexes [Pt(bzq)(CN)2]−
(C∧N = bzq, ppy), which could be isolated in two forms: the
yellow [K][Pt(C∧N)(CN)2] and the red (bzq) or purple (ppy)
[K(H2O)][Pt(C∧N)(CN)2], which reversibly transform into
each other.38 In addition, compound [K(H2O)][Pt(bzq)-
(CN)2] exhibits vapochromism when it is exposed to VOCs.
We have also reported the luminescent cationic complexes
t
DCS [Pt(bzq)(CNR)2][Pt(bzq)(13CN)2] [R = Bu, 1′; Xyl, 2′;
2-Np, 3′)], containing 13CN−, were also prepared by a similar
procedure using (NBu4)[Pt(bzq)(13CN)2] (A) as starting
material, with the aim of obtaining a complete characterization
of these compounds.
39
t
[Pt(bzq)(CNR)2]ClO4 (R = Bu, Xyl, 2-Np]. Following on
with our interest in luminescent cyclometalated Pt(II)
compounds we have used the aforementioned complexes to
prepare the DCS [Pt(bzq)(CNR)2][Pt(bzq)(CN)2] [bzq =
7,8-benzoquinolinate; R = tert-butyl (tBu, 1), 2,6-dimethyl-
phenyl (Xyl, 2), 2-naphtyl (2-Np, 3)], consisting of ionic pairs
stabilized by Pt···Pt and π···π stacking interactions in the solid
state. These new DCSs have been found to be metastable
species both in solution and in the solid state, eventually
evolving to the corresponding neutral species [Pt(bzq)(CN)-
(CNR)]. The transmetalation process by which such a
rearrangement takes place has been thoroughly studied by a
combined experimental (NMR) and computational (DFT)
In agreement with the proposed stoichiometry for 1−3 their
mass spectra show peaks due to the anion [Pt(bzq)(CN)2]−
and the corresponding cation [Pt(bzq)(CNR)2]+ operating in a
negative and positive mode, respectively (see Experimental
Section). Conductivity measurements of freshly prepared
methanol solutions (ca. 1 × 10−4 M) of compounds 1−3
indicated that they behave as nonelectrolytes [ΛM (Ω−1 cm2
mol−1): 24, 1; 20, 2; 7, 3].42 Their IR spectra showed the
complete absence of ClO4− absorptions43 and also the ν(C
N) absorptions corresponding to two terminal cyanide and two
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terminal isocyanide ligands, as corresponds to their formula, at
frequencies similar to those observed in their starting
materials.38,39 The X-ray study on a single crystal of compound
2 further confirmed the proposed stoichiometry.
As can be seen in Figure 1, compound 2 consists of ionic
pairs that are stabilized through electrostatic (Coulombic),
cyclometalated,44−48 cyanide,12,13,48−52 and isocya-
nide6,13,31,39,53−55 ligands.
In the ionic pair, the platinum coordination planes are
basically parallel to each other (interplanar angle 4.7(2)°) and
almost perfectly staggered (torsion angle N(3)−Pt(1)−Pt(2)−
C(26): 32.9(3)°; see Figure S1a), presumably minimizing the
steric repulsion of the ligands and optimizing the Pt···Pt and
π···π interactions.56 The Pt(1)−Pt(2) vector is perpendicular to
both platinum coordination planes (the angles being 7.7(1)°
Pt(1) and 9.0(1)° Pt(2)). The Pt−Pt distance (3.1557(4) Å) is
shorter than the accepted limit of 3.5 Å for Pt···Pt interactions57
and similar to those observed in the crystal structure of other
known DCSs such as [Pt(CN)-iso-C3 H7 )4 ][Pt-
(CN)4]·16H2O.12 The interplanar angle (3.6(1)°) between
the bzq fragments and C−C distances as short as 3.41 Å are
indicative of significant π···π interactions56,58−67 since they are
below the upper limit of 3.8 Å for this kind of interaction in
aromatic compounds.57 The cyanide and the isocyanide ligands
are almost linearly coordinated to the platinum centers, with
bond angles for C and N close to 180°. One of the xylyl groups
(C39−C44) is almost coplanar with the Pt(2) coordination
plane, the interplanar angle being 6.9(2)°, while the other one
(C30−C37) is almost perpendicular (the interplanar angle is
79.9(1)°).
Figure 1. X-ray structure of compound 2.
This double salt crystallizes with one and a half molecules of
methanol, one of them interacting through H-bonds with one
cyanide ligand of the anionic fragment and with the other
molecule of methanol in such a way that there are two kinds of
hydrogen bonds in the crystal: O−H···N(CN) and O−
H···O(OHCH3) (see Figure S1b). All distances and angles
relating to them are in the range of those observed in other
compounds with these kinds of interactions.12,68
covalent Pt···Pt, and dispersive π···π interactions. The cationic
and anionic components of the ionic pairs are square-planar
Pt(II) complexes containing the cyclometalated 7,8-benzoqui-
nolinate (bzq) ligand and two xylylisocyanide or two cyanide
ligands, respectively, to complete the coordination sphere of the
metal atom. Bond angles and distances (Table 1) are in the
range of those observed in platinum complexes with C∧N-
1
The H NMR spectra of freshly prepared solutions of 1−3
show the signals due to the isocyanide ligands in addition to
two sets of signals (1:1) corresponding to the bzq ligand in the
anionic [Pt(bzq)(CN)2]− and the cationic [Pt(bzq)(CNR)2]+
fragments (see Scheme 1 and Experimental Section), which
Table 1. Structural Data for Compound [Pt(bzq)(CN-
Xyl)2][Pt(bzq)(CN)2] (2)
́
Distances (Å)
1
could be unambiguously assigned by 2D NMR H−1H COSY
1
Pt(1)−C(1)
Pt(1)−C(13)
Pt(2)−C(29)
Pt(2)−C(26)
Pt(1)−Pt(2)
C(2)−N(2)
C(38)−N(6)
N(6)−C(39)
2.0037 (73)
2.0398 (74)
1.9455 (78)
2.0459 (58)
3.1557 (4)
Pt(1)−C(2)
Pt(1)−N(3)
Pt(2)−C(38)
Pt(2)−N(4)
C(1)−N(1)
C(29)−N(5)
N(5)−C(30)
1.9720 (89)
2.0535 (63)
1.9529 (75)
2.0293 (66)
1.1432 (96)
1.1756 (102)
1.4075 (104)
experiments. The H NMR signals of the bzq groups appear
significantly upfield shifted with respect to those in their
corresponding precursors, suggesting that the noncovalent π···π
1
interactions are kept in solution. To prove that, the H NMR
spectra of compound 1 and its precursors (NBu4)[Pt(bzq)-
(CN)2]38 and [Pt(bzq)(CNtBu)2]ClO4 were recorded under
39
1.1558 (115)
1.1508 (93)
1.4051 (81)
the same conditions (solutions 10−3 M in CD2Cl2, 298 K) and
compared (see Figure 2).
Angles (deg)
1
As can be seen, the bzq H NMR signals in compound 1
C(1)−Pt(1)−C(2)
N(3)−Pt(1)−C(13)
92.88 (31) C(1)−Pt(1)−C(13)
81.95 (26) N(3)−Pt(1)−C(2)
93.28 (28)
91.91 (30)
89.47 (30)
appear largely upfield shifted with respect to the corresponding
signals in its precursors. This anisotropic shift has been
attributed to the π···π interactions between bzq systems of
anions and cations located in close proximity since it cannot be
imputed to aromatic solvent-induced shifts (ASIS) nor
temperature or concentration.63,69 The anisotropic shift has
sometimes been used as a diagnostic tool in identifying the
formation of supramolecular species in solution through π···π
interactions.70−72 The 195Pt NMR spectra of the more soluble
DCSs 1 and 2 were registered, and both show two signals due
to the anionic and cationic fragments (see Figure S2 for 1′)
downfield shifted with respect to those in the parent complexes,
as can be seen in Table 2. The 195Pt nucleus of the cationic
component in 1 is slightly more shielded than that in 2, as
occurs in the starting cationic complexes [Pt(bzq)(CNR)2]-
PF6.39 This feature could be related to the lesser withdrawing
C(26)−Pt(2)−
95.24 (27) C(29)−Pt(2)−
C(29)
C(38)
N(4)−Pt(2)−C(26)
Pt(1)−C(1)−N(1)
Pt(2)−C(38)−N(6)
82.04 (24) N(4)−Pt(2)−C(38)
176.68 (65) Pt(1)−C(2)−N(2)
176.45 (65) Pt(2)−C(29)−N(5)
93.16 (28)
175.54 (78)
174.81 (68)
170.59 (67)
C(29)−N(5)−
172.50 (77) C(38)−N(6)−
C(30)
C(39)
Pt(2)−Pt(1)−C(1)
Pt(2)−Pt(1)−C(13)
Pt(1)−Pt(2)−C(26)
Pt(1)−Pt(2)−C(38)
D−(H)···A
85.01 (21) Pt(2)−Pt(1)−C(2)
84.12 (17) Pt(2)−Pt(1)−N(3)
84.23 (17) Pt(1)−Pt(2)−C(29)
97.06 (22) Pt(1)−Pt(2)−N(4)
94.39 (22)
95.31 (18)
87.37 (20)
98.40 (17)
∠(DHA)
d(D−H)
d(H···A)
d(D···A)
O(1)−(H1)···N(2)
O(2)−(H2)···O(1)
0.84 (1)
0.84 (1)
2.00 (1)
1.96 (1)
2.777 (10)
2.751 (11)
153.4 (5)
156.2 (7)
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1
Figure 2. H NMR of 1, (NBu4)[Pt(bzq)(CN)2], and [Pt(bzq)(CNtBu)2]ClO4 in 10−3 M CD2Cl2 solution at 298 K, 400 MHz (ppm).
Table 2. 195Pt NMR Data for 1, 2 and Their Starting
Complexes
The double complex salts [Pt(bzq)(CNR)2][Pt(bzq)(CN)2]
[R = Bu, 1; Xyl, 2; 2-Np, 3] are not stable for very long in
a
t
solution at temperatures above 0 °C because each one
compound
anion
cation
transforms into the corresponding neutral complex [Pt(bzq)-
b
t
NBu4[Pt(bzq)(CN)2]
−4103
(CN)(CNR)] (R = Bu, Xyl, 2-Np). These neutral complexes
b
b
[Pt(bzq)(CNtBu)2]PF6
−4246
−4168
−4171
−4078
[Pt(bzq)(CN)(CNR)] result from a ligand rearrangement
process, which is a common feature in DCSs.14 The rate of this
process in solution is lower in methanol (the solvent used in
the synthesis of 1−3) than in dichloromethane and decreases in
the order 2 > 3 > 1. 1H NMR experiments in dichloromethane
(see Figure S3, for 2) reveal that at 286 K the signals due to the
DCSs have completely disappeared in 30 min, 9 h, or 12 h for
2, 3, and 1, respectively, and in their place, the signals due to a
mixture of the SP-4-2 and SP-4-3 isomers (see Scheme 1) of
the corresponding neutral complex [Pt(bzq)(CN)(CNR)] are
observed, with the SP-4-2 being the principal component and
the only one present initially. These two isomers can be
distinguished by their H2, H4, and H9 resonances, which are
very sensitive to the stereochemistry around Pt, by using 2D
[Pt(bzq)(CNXyl)2]PF6
c
[Pt(bzq)(CNtBu)2] [Pt(bzq)(CN)2] (1)
−4034
−4057
d
[Pt(bzq)(CNXyl)2] [Pt(bzq)(CN)2] (2)
a
b
c
d
δ ppm, CD2Cl2, 298 K, 86.02 MHz. 298 K. 283 K. 278 K.
character of the CNtBu in relation to CNXyl, in line with the
estimated electrophilicity index ω+ of 1.08 and 1.56 eV for the
CNtBu and CNXyl, respectively, computed at the B3LYP/6-
31+G(d,p) level. The higher electrophilicity index ω+ = 1.99 eV
calculated for CN-2-Np should be reflected by a greater
electron-withdrawing character of this ligand compared with
CNXyl and CNtBu.
All the above spectroscopic data support the existence of 1−
3 as DCSs formed by the anionic [Pt(bzq)(CN)2]− and the
corresponding cationic [Pt(bzq)(CNR)2]+ components linked
together through Pt···Pt and π···π interactions.
1
NMR H−1H COSY experiments, although the rest of signals
appear partly overlapped. Unfortunately, all our attempts to
obtain pure samples of a single isomer of each neutral complex
[Pt(bzq)(CN)(CNR)] were unsuccessful.
The 13C NMR spectra of the DCSs [Pt(bzq)(CNR)2][Pt-
t
(bzq)(13CN)2] [R = Bu, 1′; Xyl, 2′; 2-Np, 3′)] were also
In the solid state compounds 1−3 also undergo a thermal
(165 °C, 24 h) irreversible ligand rearrangement process to give
the neutral complexes [Pt(bzq)(CN)(CNR)] (R = tBu, Xyl, 2-
Np) as a mixture of two isomers (SP-4-2, SP-4-3), as was
observed by 1H (and/or 13C) NMR. Analogous behavior to 1−
3 in solution or when they are heated in the absence of solvent
has been observed previously for other DCSs such as
[Pt(NH2R)4][PtCl4],22 [Pt(CNR)4][Pt(CN)4],13,31 or [Pt-
(bzq)(CN-2-Np)2][Pt(bzq)Cl2].74
recorded. The chemical shifts and multiplicity of the 13C NMR
signals of 1′ greatly resemble those of the parent cyanide
compound (NBu4)[Pt(bzq)(13CN)2] (A): two doublets (2JC−C
= 4.7 Hz) flanked by 195Pt satellites at 143.8 ppm (1JPt−C = 852
Hz) and 115.8 ppm (1JPt−C = 1428 Hz), which have been
assigned to the cyanide trans to Cbzq and Nbzq, respectively, in
agreement with the smaller trans influence of N with respect to
C.73 Compounds 2′ and 3′ show similar 13C NMR spectra to
that of 1′, although in these cases no C−C coupling was
observed. Therefore, the anion−cation interaction seems not to
affect the 13C signals of the cyanide groups.
Structural, Electronic, and Bonding Properties of the
DCSs [Pt(bzq)(CNR)2][Pt(bzq)(CN)2] and Their [Pt(bzq)-
(CN)2]− and [Pt(bzq)(CNR)2]+ (R = tBu, Xyl, 2-Np)
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Figure 3. Equilibrium geometries (bond lengths in Å, bond angles in deg) of the DCSs [Pt(bzq)(CNR)2][Pt(bzq)(CN)2] (R = tBu, 1; Xyl, 2; 2-Np,
3) and their anionic [Pt(bzq)(CN)2]− and cationic [Pt(bzq)(CNR)2]+ components in the gas phase, optimized at the BP86/TZP level of theory.
Components under Vacuum. Prior to the analysis of the
mechanism of the ligand redistribution reactions in [Pt(bzq)-
(CNR)2][Pt(bzq)(CN)2] DFT calculations were employed to
obtain information about the structures and the electronic and
bonding properties of the DCSs [Pt(bzq)(CNR)2][Pt(bzq)-
(CN)2] and their “free-standing” cationic and anionic
components. The equilibrium geometries of the DCSs
[Pt(bzq)(CNR)2][Pt(bzq)(CN)2] and the anionic [Pt(bzq)-
(CN)2]− and cationic [Pt(bzq)(CNR)2]+ components in the
gas phase, optimized at the BP86/TZP level of theory, are
shown in Figure 3.
It is noteworthy that the BP86/TZP-optimized bond lengths
and bond angles of the DCSs in the gas phase are slightly
overestimated with respect to the solid-state X-ray structural
data. In particular the estimated Pt−Pt distance was found to be
longer by 0.129 Å with respect to the experimental distance of
3.1557(4) Å determined by X-ray crystallography, but in
general, the BP86/TZP-optimized structural parameters of 2
are in good agreement with the X-ray structural analysis data:
platinum coordination interplanar angle 6.3°; Nbzq−Pt−Pt−
CNXyl torsion angle 45.4°; angles between the Pt−Pt vector
and both platinum coordination planes found to be 7.9° Pt(1)
and 9.7° Pt(2). The cyanide and the isocyanide ligands are
almost linearly coordinated to the platinum centers, with Pt−
C−N bond angles around 175°; one of the xylyl groups is
almost coplanar with the Pt coordination plane (interplanar
angle was estimated to be 3.1°), while the other one is almost
perpendicular, with the interplanar angle found to be 78.2°, in
excellent agreement with the X-ray structural data.
The salient structural changes occurring upon formation of
the DCSs 1−3 from the interaction of the cationic and anionic
species are compiled in Table 3. It can be seen that in the ionic
pairs both Pt−CN bonds in the anionic [Pt(bzq)(CN)2]−
component are shortened and the NC−Pt−CN bond angle
Table 3. Structural Changes (bond lengths in Å, bond angles
in deg) Accompanying the Formation of the DCSs 1−3 upon
Interaction of the Cationic and Anionic Components
structural change
1
2
3
a
Δ(Pt−CN)
Δ(Pt−Nbzq
Δ(Pt−Cbzq
−0.071; −0.076 −0.074; −0.075
−0.075; −0.076
−0.037
−0.023
)
−0.040
−0.024
−2.1
0.007;
−0.022
−0.039
−0.026
−3.1
)
Δ(NC−Pt−CN)
Δ(Pt−CNR)
−2.2
b
c
c
c
−0.142;
d
−0.137;
d
d
−0.104
−0.103
Δ(Pt−Nbzq
Δ(Pt−Cbzq
)
−0.041
−0.015
−5.1
−0.044
−0.012
−2.4
)
Δ(RNC−Pt−
−0.08
CNR)
a
b
With respect to the anionic [Pt(bzq)(CN)2]− component. With
c
respect to the cationic [Pt(bzq)(CNR)2]+ component. The Pt−CNR
d
bond in trans position to Nbzq. The Pt−CNR bond in trans position
to Cbzq
.
becomes smaller with respect to the “free-standing” [Pt(bzq)-
(CN)2]− anion, and the same holds true for the Pt−Nbzq and
Pt−Cbzq bonds. Analogous structural changes occur in the
cationic [Pt(bzq)(CNR)2]+ component of DCSs 2 and 3. The
observed structural changes can be well accounted for the
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Figure 4. Eyring plots of the rates of the ligand rearrangement process in 1 and 2.
1
Pt···Pt interactions existing in 1−3, which support transfer of
electron density between the ionic components of the DCSs.
The transfer of electron density from the anionic to cationic
component amounted to 0.20, 0.32, and 0.29 |e| for 1, 2, and 3,
respectively. The charge transferred is indicative of the
contribution of covalent Pt···Pt interactions to the bonding
mode following the trend 2 > 3 > 1. According to the natural
bond orbital (NBO) population analysis, the Pt centers of the
cationic (anionic) components acquire a positive natural atomic
charge of 0.22(0.16), 0.19(0.13), and 0.20(0.15) |e| in 1, 2, and
3 DCSs, respectively. Furthermore, the natural electron
configuration of the Pt centers of the cationic and anionic
components is 5d8.736s0.636p0.43 and 5d8.776s0.666p0.43, respec-
tively, in 1, 5d8.736s0.616p0.47 and 5d8.796s0.656p0.44, respectively,
in 2, and 5d8.726s0.626p0.45 and 5d8.776s0.656p0.43, respectively, in
3, illustrating the participation of the 6p atomic orbitals of the
Pt centers in the bonding mode as well.
Energy decomposition analysis calculations (EDA), at the
SSB-D/TZ2P level, carried out on DCSs 1−3 showed relatively
strong anion−cation interactions. The interaction energies are
estimated to be −99.1, −110.0, and −108.6 kcal/mol for DCSs
1, 2, and 3, respectively. According to EDA, these anion−cation
interactions are mainly electrostatic (Coulombic) with small
contributions from dispersion forces (π···π-type interactions)
and covalent Pt···Pt bonding interactions. The estimated Eel
component of the interaction energy for 1−3 was found to be
72.5, 76.0, and 74.4 kcal/mol, respectively. Accordingly the
estimated Edisp component of the interaction energy for 1−3
was found to be 22.7, 24.2, and 22.4 kcal/mol, respectively,
while the Ecov component was found to be 3.9, 9.8, and 11.7
kcal/mol, respectively.
As previously mentioned, H NMR experiments in dichloro-
methane revealed that the rate of the ligand exchange process
decreases in the order 2 > 3 > 1 and that the DCSs evolved to
the corresponding neutral complexes [Pt(bzq)(CN)(CNR)] as
a mixture of two isomers, SP-4-2 and SP-4-3, with SP-4-2 being
the principal component and the only one present initially.
With the aim of gaining insight into the mechanism of this
process and the role of the Pt···Pt and π···π interactions in the
reaction pathway, we studied the ligand rearrangement process
undergone by DCSs 1−3 by combined experimental and
computational calculations. For this purpose 1H NMR
spectroscopy was used to follow the processes undergone by
compounds 1 and 2 in CD2Cl2 at three different temperatures77
in the range −1 to +30 °C (see Experimental Section) because
the reaction does not work at low temperatures (below −5 °C)
and at RT it is very fast, especially for compound 2. Compound
3 could not be investigated because the resulting neutral
complexes precipitate in the NMR tube, which precluded the
possibility of obtaining accurate data from the integral values of
the NMR signals. The linear Eyring plot, as shown in Figure 4,
provides the estimation of the activation parameters (ΔH⧧,
ΔS⧧), which are compiled in Table 4. These studies showed
that the rearrangement process on 1 and 2 obeys first-order
Table 4. Rate Constants and Activation Parameters for the
Ligand Rearrangement Process in 1 and 2
⧧
⧧
compound
K (s−1) (T)
ΔH (kJ mol−1) ΔS (J K−1 mol−1)
1
5.3354 × 10−6 (285 K)
1.0624 × 10−5 (292 K)
4.5707 × 10−5 (303 K)
1.3885 × 10−5 (272 K)
2.8327 × 10−5 (277 K)
7.0134 × 10−5 (286 K)
84.1 ( 8)
−51 ( 27)
2
72.2 ( 3)
−72 ( 11)
Mechanistic Insights into Ligand Exchange Processes
in DCSs [Pt(bzq)(CNR)2][Pt(bzq)(CN)2] (R = tBu, Xyl, 2-Np)
by Combined NMR and DFT Study. Ligand substitution
reactions on square-planar Pt(II) complexes usually proceed by
an associative mechanism,75 although clear-cut evidence of a
dissociative pathway has been found in ligand exchange or
ligand substitution reactions,34−36 as in complexes of the type
cis-[PtR2S2] (R = Me, Ph; S = R2S, DMSO),76 which use the
strong labilizing effects of Me and Ph groups to proceed
through unsaturated three-coordinated intermediates. However,
as far as we know, mechanistic studies of ligand exchange
processes in DCSs have been not carried out so far.
kinetics (see Figure S4) and that the reaction rate for 1 is lower
than for 2 (see rate constants in Table 4).
Additional experiments proved that (a) DCSs transform into
the neutral complexes faster in the presence of free isocyanide
(i.e., a 2.26 × 10−3 M solution of DCS 1 in CD2Cl2 at 297 K
takes 12 h to transform completely in the neutral complexes
and with CNtBu in 1:1 and 1.2 molar ratio takes 6.5 and 3 h,
respectively); (b) at RT complex [Pt(bzq)(CN)2]− does not
react with CNR (R = tBu, Xyl; 1:1 or 2:1 molar ratios) to give
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Figure 5. Equilibrium geometries (bond lengths in Å, bond angles in deg) of the DCS intermediates [(RNC)(bzq)(μ2-η1,η1-CN)Pt···Pt(bzq)(CN)]
4−6 and the respective transition states in the gas phase optimized at the BP86/TZP level of theory.
the neutral complexes [Pt(bzq)(CN)(CNR)], so the sub-
stitution of CN− by CNR does not take place; and (c) the
intermediates [(RNC)(bzq)(μ2-η1,η1-CN)Pt···Pt(bzq)(CN)]
(R = Bu, 4; Xyl, 5; 2-Np, 6).
t
t
reactions between [Pt(bzq)(CNR)2]+ (R = Bu, Xyl) and CN−
The optimized equilibrium geometries of the intermediates
4−6 with the corresponding transition states, TS1−4, TS2−5, and
TS3−6, are shown in Figure 5. In the intermediates 4−6 the
Pt···Pt interactions are significantly strengthened, as is reflected
by the shortening of the Pt−Pt bond lengths by 0.587, 0.368,
and 0.467 Å with respect to the Pt−Pt bond lengths of the
DCSs 1−3, respectively. Moreover, the C donor atom of the
cyanide ligand in trans position to Nbzq interacts weakly with
the platinum atom of the cationic component, forming a weak
asymmetric Pt1(μ2-CN)Pt2 bridge. The C···Pt2 distances are
predicted to be 3.180, 3.092, and 2.987 Å in intermediates 4, 5,
and 6, respectively.
The calculated geometric and energetic reaction profiles of
the intramolecular ligand rearrangement processes in DCSs 1−
3 are depicted schematically in Figure 6. As can be seen, the
first reaction step involving the dissociation of the isocyanide
ligand in trans position to the Cbzq is endothermic, and the
estimated endothermicity was found to be around 92−112 kJ
mol−1. The next step of the reaction path involves the migration
of the bridging cyanide ligand in intermediates 4−6 to the
platinum center of the cationic component, affording the SP-4-
2 isomers of the neutral [Pt(bzq)(CN)(CNR)] complexes and
the neutral three-coordinated [Pt(bzq)(CN)] species with the
(1:1 molar ratio) are very quick and mainly render the (SP-4-
3)-[Pt(bzq)(CN)(CNR)] isomer with only small traces of the
SP-4-2 one. In view of these results it seems plausible to accept
that the cation−anion interaction in the DCSs 1 and 2 plays an
important role in determining the ligand exchange mechanism.
The strong anion−cation interactions that were described in
the previous subsection should block the associative attack of
the Pt(II) centers by the coordinated cyanide and/or
isocyanide ligands. Moreover, the Pt−Pt interactions are
predicted to support a significant transfer of electron density
from the anionic to cationic component, which shields the
vacant 6pz orbital of the Pt center of the cationic component
and hinders nucleophilic attack by the coordinated cyanide
ligands of the anionic species. Furthermore, the charge transfer
renders the platinum center of the cationic component less
electrophilic, and thus the isocyanide ligand dissociation
process becomes more feasible. The estimated Pt−CNR
energies for the dissociation of the CNR in trans position to
the Cbzq are found to be 107.9, 91.6, and 112.1 kJ/mol for
DCSs 1, 2, and 3, respectively. It should be noted that for the
“free-standing” cationic [Pt(bzq)(CNtBu)2]+, [Pt(bzq)-
(CNXyl)2]+, and [Pt(bzq)(CN-2-NP)2]+ components the
estimated Pt−CNR bond dissociation energies for the
dissociation of the CNR in trans position to the Cbzq are
predicted to be 160.2, 148.1, and 141.8 kJ/mol, respectively.
The dissociation of the cyanide ligand in trans position to the
Cbzq in the anionic [Pt(bzq)(CN)2]− component is much
higher, amounting to 268.6 kJ/mol. Thus, the first step in the
ligand rearrangement pathway should correspond to the
isocyanide ligand dissociation process yielding the DCS
cyanide ligand in trans position to Cbzq
.
The neutral [Pt(bzq)(CN)] species, being a transient
species, easily captures “free” isocyanide ligands to yield the
SP-4-2 isomers or rapidly isomerizes to the more stable
[Pt(bzq)(CN)] species with the cyanide ligand in trans position
to Nbzq. The geometric and energetic profiles of the
isomerization process are depicted schematically in Figure 7.
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Figure 6. Energetic (ΔRH, in kJ mol−1) and geometric reaction profiles for the ligand rearrangement processes in DCSs 1−3, computed at the BP86/
TZP level of theory.
It can be seen that the isomerization process of the neutral
three-coordinated [Pt(bzq)(CN)] species is almost barrierless
(activation barrier of 51.5 kJ mol−1) and exothermic by −56.9
kJ mol−1. The more stable three-coordinated isomer easily
captures “free” isocyanide ligands to yield the SP-4-3 isomers,
thus accounting well for the formation of the mixture of the two
isomers detected experimentally.
The rate-limiting step corresponds to the transformation of
the intermediates 4−6 to the neutral [Pt(bzq)(CN)(CNR)]
complexes. The transformation of intermediates 4, 5, and 6
proceeds via transition states TS1−4 (ν = 208i cm−1), TS2−5 (ν =
214i cm−1), and TS3−6 (ν = 217i cm−1), surmounting relatively
low activation barriers of 159.4, 144.3, and 157.3 kJ mol−1,
respectively. In the vibrational modes corresponding to the
imaginary frequencies of TS1−4, TS2−5, and TS3−6, the
dominant motions involve the movement of the bridging
cyanide ligand toward the platinum center of the cationic
component. It should be noted that in the transition states the
formation of the μ2-CN bridge enforces the rupture of the Pt−
Pt bond. The calculated gas-phase ΔS⧧ values for the
intramolecular ligand rearrangement processes in DCSs 1−3
are predicted to be −153, −208, and −149 J K−1 mol−1,
respectively, at the BP86/TZP level following the same trend as
the experimental values (see Table 4). The negative values of
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cationic [Pt(bzq)(CNR)2]+ components and for the dissocia-
tion of the CN− in trans position to the Cbzq in the anionic
[Pt(bzq)(CN)2]− component, so that the first step in the
ligand rearrangement pathway is the dissociation of the
isocyanide in trans position to the Cbzq, yielding the
[(RNC)(bzq)(μ2-η1,η1-CN)Pt···Pt(bzq)(CN)] intermediates.
Thus, the rate-limiting step corresponds to the transformation
of these intermediates to the neutral [Pt(bzq)(CN)(CNR)]
complexes following a synchronous mechanism involving
rupture of the Pt−Pt and formation of the Pt−CN bonds
through transition states formulated as [(RNC)(bzq)Pt(μ2-
η1,η1-CN)Pt(bzq)(CN)]. The total energy demand for the
formation of the transition states of the DCSs 1, 2, and 3
accounts well for the experimentally observed reaction rate that
follows the trend 1 < 3 < 2.
Figure 7. Energetic (ΔRH, in kJ mol−1) and geometric (bond lengths
in Å, bond angles in deg) reaction profiles for the isomerization of the
neutral three-coordinated [Pt(bzq)(CN)] species, computed at the
BP86/TZP level of theory.
EXPERIMENTAL SECTION
■
General Procedures and Materials. The starting materials
[K(H2O)][Pt(bzq)(CN)2]38 and [Pt(bzq)(CNR)2]ClO4 (R = tert-
butyl (1), 2,6-dimethylphenyl (2), 2-naphtyl (3))39 were prepared as
described elsewhere. K13CN and KCN were used as purchased from
Isotec. Elemental analyses were carried out with a EuroEa elemental
analyzer. IR spectra were recorded on a Perkin-Elmer Spectrum 100
FT-IR spectrometer (ATR in the range 250−4000 cm−1). Mass
spectral analyses were performed with a Microflex MALDI-TOF
Bruker or an Autoflex III MALDI-TOF Bruker instrument.
ΔS⧧ indicate that the formation of the μ2-CN bridge precedes
the Pt−Pt bond breaking. Overall the transformation process is
exothermic; the estimated exothermicities are predicted to be
−126.1, −200.4, and −129.3 kJ mol−1 for intermediates 4, 5,
and 6, respectively (see Figure 7). Furthermore, according to
the total energy demand for the formation of the transition
states of the DCSs 1, 2, and 3 amounting to 159.4, 144.3, and
157.3 kJ mol−1, respectively (Figure 7), the rate of trans-
formation process follows the trend 1 < 3 < 2, as was
experimentally observed. It should be noticed that the relatively
large discrepancy between the calculated and experimentally
determined “effective” activation enthalpies could be attributed
to the fact that gas-phase results, despite the restrictions
introduced to the theoretical model, would give a simple
relationship to the experimental results obtained in solution.
Moreover it is well established that kinetic and thermodynamic
results are highly dependent on the functional employed. The
proposed mechanism is further supported by the fact that the
ligand exchange process is faster in the presence of free
isocyanide and the reaction rate increases with the isocyanide
concentration.
NMR spectra were recorded on a Varian Unity-300 and Bruker 400
spectrometer using the standard references: SiMe4 for H and 13C;
1
Na2PtCl6 in D2O for 195Pt. All of the 13C and 195Pt were proton-
decoupled, J is given in Hz, and assignments are based on H−1H
1
1
COSY and H−195Pt-HMQC experiments.
Synthesis of (NBu4)[Pt(bzq)(13CN)2] (A). (NBu4)[Pt(bzq)-
(13CN)2] (A) was prepared in the same way as (NBu4)[Pt(bzq)-
(CN)2], but using K13CN. IR (cm−1): ν(CN−) 2073s, 2061s. 13C
NMR (CD2Cl2, 100.6 MHz, 293 K, δ): 144.2 (d, 13CNtrans‑C, JC−C
=
2
1
1
4.7 Hz JPt−C = 832 Hz), 115.8 (d, 13CNtrans‑N, JPt−C = Hz 1424 Hz).
Synthesis of [Pt(bzq)(CN-tBu)2][Pt(bzq)(CN)2] (1). To an orange
suspension of [Pt(bzq)(CN-tBu)2](ClO4) (0.200 g, 0.31 mmol) in
methanol (25 mL) was added [K(H2O)][Pt(bzq)(CN)2] (0.150 g,
0.31 mmol). The mixture was stirred at 0 °C for 30 min, and then it
was allowed to reach room temperature. After 1 h, the orange-red
solution was evaporated to dryness and the residue treated with cold
CH2Cl2 (10 mL, 0 °C) and filtered through Celite. The resulting
solution was evaporated to 2 mL, and diethyl ether (15 mL) was added
to it to give pure 1 as a red solid. Yield: 0.22 g, 75%. Anal. Calcd for
C38H34N6Pt2 (964.87): C, 47.30; H, 3.55; N, 8.71. Found: C, 47.26; H,
3.46; N, 8.70. MS(ES+): m/z 539 ([Pt(bzq)(CNtBu)2]+, 100%).
MS(ES−): m/z 425 ([Pt(bzq)(CN)2]−, 60%). IR (cm−1): ν(CNR)
2232s, 2204s, ν(CN−) 2122s, 2107s. ΛM (1.1 × 10−4 M, methanol
solution): 24 Ω−1 cm2 mol−1. 1H NMR (CD2Cl2, 400.13 MHz, 298 K,
δ): 8.8 (dd, 3J(H2−H3) = 5 Hz, 4J(H2−H4) = 1 Hz, 3J(Pt−H2) = 30
CONCLUSIONS
■
The double complex salts [Pt(bzq)(CNR)2][Pt(bzq)(CN)2]
(bzq = 7,8-benzoquinolinate; R = tert-butyl (1), 2,6-
dimethylphenyl (2), 2-naphtyl (3)) can be prepared by a
metathesis reaction between [Pt(bzq)(CNR)2]ClO4 and [K-
(H2O)][Pt(bzq)(CN)2] in a 1:1 molar ratio under controlled
temperature conditions. The X-ray structure of 2 shows that it
consists of ionic pairs stabilized through Pt···Pt and π···π
interactions. Theoretical calculations indicate that, in the DCSs,
the bonding mode is dominated by ionic (Coulombic)
interactions, but it is also supported by dispersive π···π and
covalent Pt···Pt interactions.
3
3
Hz, H2, bzq), 8.5 (d, J(H2′-H3′) = 5 Hz, J(Pt−H2′) = 35 Hz, H2′,
3
3
bzq), 8.1 (d, J(H9−H8) = 7 Hz, J(Pt−H9) = 44 Hz, H9, bzq), 8.0
(d, 3J(H4′-H3′ = 8 Hz, H4′, bzq), 7.6 (dd, 3J(H4−H3) = 8 Hz, 4J(H4−
H2) = 1 Hz, H4, bzq), 7.4 (AB, H5′, J(H5′-H6′) = 9 Hz, bzq, νA),
3
7.36−7.2 (m, H3′, H5, H6′, H7, H7′, H8, H8′, H9′, bzq), 7.0 (AB, H6,
3J(H6−H5) = 9 Hz, bzq, νB), 6.8 (dd, J(H3−H4) = 8 Hz, J(H3−
H2) = 5 Hz, H3, bzq), 1.91 (s, 9H, CH3, CNtBu), 1.89 (s, 9H, CH3,
CNtBu). 195Pt NMR (CD2Cl2, 86.02 MHz, 283 K, δ): −4034
([Pt(bzq)(CN)2]−, −4171 ([Pt(bzq)(CNtBu)2]+). 13C NMR (1′,
3
3
Compounds 1−3 are metastable species both in solution and
in the solid state, evolving to the corresponding neutral
complexes [Pt(bzq)(CN)(CNR)], as a result of a ligand
rearrangement process. DFT calculations on 1−3 show that the
existing Pt···Pt interactions block the associative attack of the
Pt(II) centers by the coordinated cyanide and/or isocyanide
ligands. They also render the isocyanide ligand dissociation
process in DCSs more feasible than for the “free-standing”
CD2Cl2, 100.6 MHz, 263 K, δ): 143.9 (d, 13CNtrans‑C, JC−C = 5 Hz,
2
1JPt−C = 852 Hz), 115.8 (d, 13CNtrans‑N, JPt−C = 1428 Hz).
1
Synthesis of [Pt(bzq)(CN-Xyl)2][Pt(bzq)(CN)2] (2). To a yellow
suspension of [Pt(bzq)(CN-Xyl)2](ClO4) (0.110 g, 0.15 mmol) in
methanol (20 mL) was added [K(H2O)][Pt(bzq)(CN)2] (0.072 g,
0.15 mmol) at −10 °C, and the mixture was stirred for 1 h at this
temperature to give a bright orange suspension. Then the solvent was
evaporated to dryness, and cold H2O (5 mL, 5 °C) added to the
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residue to give an orange solid, which was filtered and washed with
MeOH (1 mL) and OEt2 (15 mL), 2. Yield: 0.11 g, 69%. Anal. Calcd
for C46H34N6Pt2 (1060.96): C, 52.07; H, 3.23; N, 7.92. Found: C,
51.86; H, 3.37; N, 7.94. MS(ES+): m/z 504 ([Pt(bzq)(CNXyl)2]+,
100%). MS(ES−): m/z 425 ([Pt(bzq)(CN)2]−, 35%). IR (cm−1):
ν(CNR) 2205s, 2176s, ν(CN−) 2125s, 2114s. ΛM (1.1 × 10−4 M,
theory, as implemented in the ADF2010.01 software, while scalar
relativistic effects have been considered using ZORA. The SSB-D is a
meta-GGA functional that includes dispersion corrections.88
X-ray Structure Determination. Suitable crystals for X-ray
diffraction studies were obtained by slow diffusion of diethyl ether
into a solution of the complexes in methanol. Crystals were mounted
at the end of a quartz fiber. The radiation used was graphite-
monochromated Mo Kα (λ = 0.71073 Å). X-ray intensity data were
collected on an Oxford Diffraction Xcalibur diffractometer. The
diffraction frames were integrated and corrected for absorption by
using the CrysAlis RED program.89 The structure was solved by
Patterson and Fourier methods and refined by full-matrix least-squares
on F2 with SHELXL-97.90 All non-hydrogen atoms were assigned
anisotropic displacement parameters and refined without positional
constraints, except as noted below. All hydrogen atoms were
constrained to idealized geometries and assigned isotropic displace-
ment parameters equal to 1.2 times the Uiso values of their attached
parent atoms (1.5 times for the methyl hydrogen atoms). A well-
defined methanol solvent molecule was found in the density maps and
refined with occupancy 1. This molecule establishes a H bond with
N(2). A second methanol molecule was also found and was refined
with a 0.5 partial occupancy. This second methanol established an H
bond too, this time with the oxygen atom of the first MeOH molecule,
O(1). Full-matrix least-squares refinement of these models against F2
converged to final residual indices given in Table S4.
1
methanol solution): 20 Ω−1 cm2 mol−1. H NMR (CD3OD, 400.13
3
MHz, 298 K, δ): 8.8 (d, J(H2′−H3′) = 5 Hz, H2′, bzq), 8.5 (d,
3J(H2−H3) = 5 Hz, H2, bzq), 8.2 (d, 3J(H4′−H3′) = 8 Hz,, H4′, bzq),
3
3
8.1 (d, J(H9−H8 = 7 Hz, J(Pt−H9) = 43 Hz, H9, bzq), 7.9 (d,
3J(H4−H3) = 8 Hz, H4, bzq), 7.6 (d, 3J(H9′−H8′ = 7 Hz, 3J(Pt−H9)
3
= 42 Hz, H9′, bzq), 7.5 (AB, H5′, J(H5′−H6′) = 9 Hz, bzq, νA), 7.5
3
(AB, H5, J(H5−H6) = 9 Hz, bzq, νA), 7.44−7.29 (m, H7, H7′, H8,
3
H8′, H3′), 7.4 (AB, H6′, J(H6′−H5′) = 9 Hz, bzq, νB), 7.2 (AB, H6,
3J(H6−H5) = 9 Hz, bzq, νB), 6.9 (dd, J(H3−H4) = 8 Hz, J(H3−
H2) = 5 Hz, H3, bzq). 195Pt NMR (CD2Cl2, 86.02 MHz, 278 K, δ):
−4057 ([Pt(bzq)(CN)2]−, −4078 ([Pt(bzq)(CN-Xyl)2]+). 13C{1H}
3
3
NMR (2′,CD2Cl2, 100.6 MHz, 253 K, δ): 144.0 (s, 13CNtrans‑C, 1JPt−C
=
845 Hz), 116.1 (s, 13CNtrans‑N, JPt−C= 1421 Hz).
1
Synthesis of [Pt(bzq)(CN-2-Np)2][Pt(bzq)(CN)2] (3). To an
orange suspension of [Pt(bzq)(CN-2-Np)2](ClO4) (0.116 g, 0.15
mmol) in a mixture of MeCN/H2O (10:10 mL) was added
[K(H2O)][Pt(bzq)(CN)2] (0.072 g, 0.15 mmol) at −10 °C, and
the mixture stirred at −10 °C for 1 h to give a purple suspended solid,
which was filtered and washed with H2O (5 mL), MeOH (5 mL), and
OEt2 (15 mL), 3. Yield: 0.15 g, 90%. Anal. Calcd for C50H30N6Pt2
(1104): C, 54.35; H, 2.74; N, 7.61. Found: C, 54.08; H, 2.51; N, 7.63.
MS(ES+): m/z 679 ([Pt(bzq)(CN-2-Np)2]+, 100%). MS(ES−): m/z
425 ([Pt(bzq)(CN)2], 50%). IR (cm−1): ν(CNR) 2221s, 2189s,
ν(CN−) 2119s, 2108s. ΛM (1 × 10−4 M, methanol solution): 7 Ω−1
ASSOCIATED CONTENT
■
S
* Supporting Information
View of 2. 1H−195Pt NMR spectrum of compound 1′ in CD2Cl2
1
at 293 K. H NMR spectrum showing the transformation of 2
1
cm2 mol−1. H NMR (CD2Cl2, 400.13 MHz, 298 K, δ): 8.74 (s, H1,
into [Pt(bzq)(CN)(CNXyl)] as a mixture of two isomers at
286 K in CD2Cl2. First-order kinetic plots for the ligand
redistribution process of 1 in CD2Cl2 at 285, 292 and 303 K.
First-order kinetic plots for the ligand redistribution process of
2 in CD2Cl2 at 272, 277, and 286 K. Cartesian coordinates of all
the stationary points (Table S1) and energetic results (Tables
S2 and S3). Crystal data and structure refinement for complex
2·1.5MeOH (Table S4). This material is available free of charge
via the Internet at http://pubs.acs.org.
Np), 8.65 (m, 2H, H2, H2′, bzq), 8.58 (s, H3,Np), 8.20−6.48 (m,
26H, Np/bzq), 13C{1H} NMR (3′, CD2Cl2, 100.6 MHz, 253 K, δ):
1
1
143.6 (s, 13CNtrans‑C, JPt−C = 852 Hz), 116.7 (s, 13CNtrans‑N, JPt−C
=
1427 Hz).
Kinetic Studies of the Ligand Rearrangement Process in
Compounds 1−3. Solutions of compounds [Pt(bzq)(CNR)2][Pt-
t
(bzq)(CN)2] [R = Bu, 1; Xyl, 2] in CD2Cl2 were prepared at low
temperatures and used as samples to follow their conversion into the
1
neutral complexes [Pt(bzq)(CN)(CNR)] by H NMR spectroscopy.
The spectra were recorded every few minutes at each temperature for
both compounds. The rate of reaction was followed in each case by
integration of the H2 signal corresponding to the three identified
compounds: [Pt(bzq)(CNR)2][Pt(bzq)(CN)2], (SP-4-2)-[Pt(bzq)-
(CN)(CNR)], and SP-4-3-[Pt(bzq)(CN)(CNR)]. The sum of all
three signals was considered as [DCS]0 since the neutral compounds
originate from the corresponding DCSs. Plots of ln([DCS]0/([DCS]t)
versus time (t) gave a straight line of slope k; plots of ln(k/T) versus
1/T gave slope ΔH⧧/R and intercept ΔS⧧/R + ln(kB/h). Errors were
determined from error propagation formulas, which were derived from
the Eyring equation.78 The temperature of the NMR probe was
calibrated using a methanol temperature standard, and the estimated
error in the temperature measurements was 1 K.
Computational Details. The geometries of all stationary points
were fully optimized, without symmetry constraints, employing the
standard BP86 pure functional as implemented in the ADF 2010.01
program suite.79 The latter consists of Becke’s 1988 functional, which
includes the Slater exchange along with corrections involving the
gradient of the density80 combined with a local correlation functional
of Perdew that includes gradient corrections.81 Relativistic effects were
treated by applying the zero-order regular approximation
(ZORA).82−86 For optimization of geometries we have used a
Slater-type orbitals basis set for all atoms, denoted as TZP, which is
a core double-ζ, valence triple-ζ, polarized basis set. This basis set is
specially designed for ZORA calculations, primarily to include much
steeper core-like functions. Hereafter the method used in DFT
calculations is abbreviated as BP86/TZP. All stationary points have
been identified as minima (number of imaginary frequencies
NImag=0) or transition states (NImag=1). Energy decomposition
analysis87 calculations were performed at the SSB-D/TZ2P level of
AUTHOR INFORMATION
Corresponding Author
*Fax: (+34)976-762189. E-mail: juan.fornies@unizar.es;
sicilia@unizar.es; attsipis@uoi.gr.
■
ACKNOWLEDGMENTS
This work was supported by the Spanish MICINN (Projects
CTQ2008-06669-C02-01) and the Gobierno de Arago
■
́
n
́
(Grupo Consolidado: Quimica Inorgan
Organometalicos). A.C.T. thanks the University of Ioannina for
the sabbatical leave granted and all his colleagues at the
́
ica y de los Compuestos
́
́
Departamento de Quimica Inorgan
́
ica, Instituto de Ciencia de
Materiales de Aragon, Facultad de Ciencias, Universidad de
́
Zaragoza−CSIC, Zaragoza (Spain), for their generous hospital-
ity.
DEDICATION
Dedicated to the memory of Prof. F. Gordon A. Stone.
■
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