Synthesis, structural characterization and cis-trans isomerization of novel (salicylaldiminato)platinum(ii) complexes

Article abstract of DOI:10.1039/c3dt51283e

The reaction of cis-[PtCl₂(dmso)] with the salicylaldimine ligand, N-(2-hydroxybenzylidene)-2,6-di-isopropylaniline, L_A in the presence of sodium acetate in methanol produced both cis- and trans-[PtClL _A(dmso)], 1a and 1b. An analogous reaction for the less bulky ligand, N-(2-hydroxybenzylidene)aniline L_B produced only cis-[PtClL _B(dmso)], 2. The reactions of these dmso complexes with triphenylphosphine also yielded complexes with different geometries depending on the nature of the salicylaldiminato ligand. Thus the cis-trans isomerization of cis-[PtClL_A(PPh₃)] 3a was investigated both experimentally and computationally, and a tetrahedral transition state was detected in this process. A good agreement of the experimental activation parameters with those determined theoretically using DFT was obtained. L _A was also reacted with [PtClMe(cod)] in methanol to yield the corresponding salicylaldiminato complex 6 in which the methyl group is cis to the imine nitrogen. X-ray crystal structures of some compounds obtained are reported.

Full text of DOI:10.1039/c3dt51283e

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Synthesis, structural characterization and cis–trans
isomerization of novel (salicylaldiminato)platinum(II)
Cite this: Dalton Trans., 2013, 42, 11163
complexes†
Feng Zheng,a,b Alan T. Hutton,a Cornelia G. C. E. van Sittert,c John R. Moss‡a and
Selwyn F. Mapolie*b
The reaction of cis-[PtCl2(dmso)] with the salicylaldimine ligand, N-(2-hydroxybenzylidene)-2,6-di-isopro-
pylaniline, LA in the presence of sodium acetate in methanol produced both cis- and trans-[PtClLA(dmso)],
1a and 1b. An analogous reaction for the less bulky ligand, N-(2-hydroxybenzylidene)aniline LB pro-
duced only cis-[PtClLB(dmso)], 2. The reactions of these dmso complexes with triphenylphosphine
also yielded complexes with different geometries depending on the nature of the salicylaldiminato
ligand. Thus the cis–trans isomerization of cis-[PtClLA(PPh3)] 3a was investigated both experimentally and
computationally, and a tetrahedral transition state was detected in this process. A good agreement of
the experimental activation parameters with those determined theoretically using DFT was obtained.
Received 16th May 2013,
Accepted 13th June 2013
L
A was also reacted with [PtClMe(cod)] in methanol to yield the corresponding salicylaldiminato complex
DOI: 10.1039/c3dt51283e
6 in which the methyl group is cis to the imine nitrogen. X-ray crystal structures of some compounds
www.rsc.org/dalton
obtained are reported.
“Grubbs ligands”, have been found to afford highly active cata-
Introduction
lysts for group 4,6 group 6,7 and group 102 metal systems. It
In the development of coordination chemistry, metal com- has been demonstrated via computational studies that the
plexes with Schiff bases as ligands have played an important reaction mechanism of the chain propagation step in olefin
role, and have found extensive applications in a range of oligomerization/polymerization using salicylaldiminato cata-
different fields. Schiff bases derived from salicylaldehydes and lysts is quite complex due to the potential presence of geo-
diamines have, for example, great potential applications in metrical isomers, i.e., the “initiator complexes” for chain
asymmetric catalysis.1 Among other applications, salicylaldimi- propagation can have two different configurations dependent
nato ligands have been used in transition-metal complexes upon whether the alkyl group is trans to N or trans to O
which have been employed as olefin transformation cata- (Fig. 1).8 Higher energies were obtained for isomers with the
lysts.2,3 Moreover, they have also been found to have useful alkyl group trans to the more strongly donating nitrogen
biological applications4 and have been used as functional atom.9 To the best of our knowledge, the cis–trans
materials.5 This has given great impetus to develop a variety of
interesting salicylaldiminato-based metal complexes for appli-
cations in many other fields.
In transition metal-catalyzed olefin polymerization/oligo-
merization reactions, salicylaldiminato ligands, known as
aDepartment of Chemistry, University of Cape Town, Private Bag, Rondebosch, 7700,
South Africa
bDepartment of Chemistry and Polymer Science, Stellenbosch University, Private Bag,
Matieland, 7601, Stellenbosch, South Africa. E-mail: smapolie@sun.ac.za
cCatalysis and Synthesis Research Group, Chemical Resource Beneficiation Focus
Area, North-West University, Potchefstroom, 2520, South Africa
†Electronic supplementary information (ESI) available. CCDC 931185–931189.
For ESI and crystallographic data in CIF or other electronic format see DOI:
Fig. 1 Model for “initiator complexes” and intermediates with cis and trans
geometries for the mechanism of the chain propagation step in ethylene oligo-
merization/polymerization catalyzed by salicylaldiminato (Grubbs) catalysts.8
10.1039/c3dt51283e
‡John Moss is deceased.
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isomerization of salicylaldiminato metal complexes has not coordination chemistry of the salicylaldiminato ligands. The
been reported experimentally. present study is focused on three issues: (1) an evaluation of
Salicylaldiminato platinum(II) complexes have also been the steric effect on coordination behaviour of two salicylaldi-
recently found to exhibit pronounced phosphorescent pro- minato (N^O) ligands in the synthesis of platinum(II) com-
perties.5 For example, ultrasound-induced phosphorescent plexes, (2) a comparative analysis of the reactivity and structure
emission were reported to be observed with chiral, clothes- of the Pt(N^O) complexes, and (3) a combined experimental
pin-shaped
trans-bis(salicylaldiminato)Pt(II)
complexes.5a and theoretical investigation of the cis–trans isomerization of a
However, the use of this class of platinum complexes is very Pt(N^O) complex.
rare in catalysis, probably due to their generally high kinetic
and thermodynamic stability. On the other hand, the
enhanced stability of platinum(II) complexes relative to their
palladium analogues makes them easier to handle and thus
Results and discussion
Synthesis of dmso-ligated (salicylaldiminato)platinum(II)
complexes
easier to study their chemistry. The series of salicylaldiminato
Pt(II) compounds reported here can be potential model com-
plexes for their Pd analogues which have shown catalytic
activity for the transformation of unsaturated hydrocarbons.10 The reaction of the salicylaldiminato ligand LA with cis-
Furthermore, these complexes could potentially be biological [PtCl2(dmso)2] in the presence of sodium acetate in
active as some of their Pd analogues have shown antitumor 1 : 1 : 1 mole ratio was carried out in refluxing methanol for
activities.4a,b
16 h. Two geometrical isomers, in which the dmso group is
a
We are interested in developing the chemistry of (salicyl- either cis to N (cis isomer, 1a) or trans to N (trans isomer, 1b),
aldiminato)platinum(II) complexes before pursuing further were obtained [Scheme 1(A)]. The use of column chromato-
work with possible applications, in particular investigating the graphy allowed the separation of these two isomers.
Scheme 1 Preparation of salicylaldiminato platinum(II) complexes, where cis means L (dmso or PPh3) cis to N, while trans means L trans to N.
11164 | Dalton Trans., 2013, 42, 11163–11179
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Scheme 2 Reactions of dppf with dmso-ligated complexes 1–2.
Different reaction conditions were employed to investigate 5 two individual organoplatinum(II) units are combined in one
potential changes in the isomer ratio. When the reaction was molecular system via a dppf spacer. However, no product is
carried out at room temperature or under reflux for a short formed from the reactions of dppf with the cis complexes 1a
time (30 min), the isolated product contained mainly the cis and 2 due to the steric hindrance (Scheme 2).
isomer, but longer reflux times gave more of the trans isomer
(see Table S1 in ESI†). These observations suggested that the PPh3 derivatives 3–4 were obtained as bright yellow crystalline
cis isomer 1a is the kinetically favoured product. solids after recrystallization from CH2Cl2–MeOH or CH2Cl2–n-
The dmso-ligated platinum(II) complexes 1–2 and their
In order to determine any steric effects of the ligand, plati- hexane, while the diplatinum(II) complex with a dppf spacer, 5,
num complex 2 was synthesized by the reaction of the less was isolated as an orange powder. All the triphenylphosphine
bulky ligand, LB, with cis-[PtCl2(dmso)2] under the same reac- complexes were found to be highly thermally as well as air
tion conditions. In this case, complex 2 precipitated out of the stable in the solid state. The identity of these (salicylaldimi-
solution as a single geometric isomer, in which the dmso group nato)platinum(II) complexes was confirmed by elemental analy-
is cis to the N atom of the chelating ligand [Scheme 1(B)].
sis (C, H, N), mass spectrometry, IR and NMR (1H, 2D-COSY,
The reactions of the monodentate phosphine, PPh3 with 2D-HSQC, 13C and 31P) spectroscopies. The solid state structures
dmso complexes 1a, 1b and 2 were studied and the resulting of two pairs of isomers, 1a/1b and 3a/3b, and 2 were determined
products are depicted in Scheme 1. When the dmso complexes by X-ray crystallography.
1a or 1b were reacted with PPh3 in a 1 : 1 mole ratio in acetone
at room temperature, two isomers 3a and 3b, with the PPh3
Spectroscopic properties of complexes
ligand in a cis or trans position to the nitrogen atom, were iso-
lated [Scheme 1(A)]. A different behaviour was observed in the
case of complex 2 which contains the less sterically bulky N^O
ligand, as its reaction with PPh3 (1 : 1) in acetone only gave
product 4 with a trans arrangement of the PPh3 and the nitro-
gen atom [Scheme 1(B)]. This is despite the fact that the pre-
cursor is in the cis form.
Reactions of 1a, 1b and 2 with diphosphines (in a 2 : 1
ratio) proved more difficult. The success of the ligand displace-
ment depends on the nature of diphosphines, and the geo-
metry of the dmso–platinum complex. For dppe, no target
products were obtained from reactions with either com-
plexes 1–2. When a diphosphine with a much longer skeletal
backbone, such as dppf, was used, the desired dimeric
complex, 5, was formed from the trans complex 1b. In complex
IR spectra. The most informative peak in the IR spectra, the
ν(CvN) stretch, appears at 1624 and 1613 cm−1 for the salicyl-
aldimine ligands LA and LB, respectively. This is a red-shift
relative to the ligand in all the platinum(II) complexes as
shown in Table 1. The red-shift indicates the coordination of
the imine nitrogen to the platinum centre resulting in a
weakening of the bond between C and N.11 Furthermore, the
disappearance of the band at 2884 cm−1 (due to the OH) con-
firmed the deprotonation of the hydroxyl group and formation
of a σ-bond between O and the metal centre.
A strong peak at ca. 1155 cm−1, assigned to the ν(SvO)
stretch, is observed in the IR spectra of the cis dmso complexes
1a and 2, while this peak for trans 1b appears at lower
wavenumber, around 1140 cm−1. This indicates that the bond
between S and O of the cis complex is stronger than that of the
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trans species. The displacement of the dmso group by phos-
phines in complexes 3–5 was confirmed by the disappearance
of the ν(SvO) band and the appearance of strong absorption
bands at 1434 cm−1 due to the C–P bond.
NMR spectra. In the 1H NMR spectra of dmso complexes
1a, 1b and 2, the disappearance of the phenolic proton around
δ 13.5 ppm and the observation of the upfield shift of the imine
proton (Ha) at ca. 7.8 ppm (see Table 1) relative to that of the
free ligands, suggests successful deprotonation and formation
of a phenoxy σ-bond with the metal centre, consistent with the
IR data for the complexes. For the cis complexes 1a and 2, the
imine protons display large JPt–H values of 93.3 and 86.3 Hz,
respectively, consistent with the presence of a chloride ligand
in a trans position to the nitrogen atom. Conversely, for the
trans complex 1b, the JPt–H for the imine hydrogen is reduced
(63.3 Hz) compared to the cis isomer as shown in Fig. 2, which
is consistent with the presence of a dimethylsulfoxide ligand
in a position trans to the nitrogen atom. A similar phenom-
enon has been observed for other platinum coordination com-
pounds.12 The signal of the dmso methyl protons for the
trans complex 1b appears at lower field compared to that of
the cis complex 1a.
By replacing the dmso group with PPh3, all the imine
protons (Ha) of the complexes with trans geometry (where PPh3
is trans to the imine), 3b and 4, were observed in the region
8.03–8.20 ppm. These observations indicate that π back-
bonding from platinum into empty low lying p-based orbitals
is more pronounced compared to s-based orbitals and
leads to the slight elongation of the Pt–N bonds in the com-
plexes. This consequently results in a downfield shift of
0.08–0.37 ppm with respect to their dmso precursors. In the
case of the trans complexes with phosphine ligands, 3b, 4 and
4
5, the imine proton appears as doublets with J(PH) of
13.4–13.9 Hz, while in the 1H NMR spectrum of the cis
complex 3a the chemical shift of the imine proton shifted to
higher field and appeared as a singlet (see Fig. 3). However, no
coupling of the imine proton with the 195Pt nuclei was observed
in all cases. Similar observations have been previously reported
in the literature for related platinum complexes.13
For the trans phosphine-containing complexes 3b, 4 and 5,
another salient feature observed in the 1H NMR spectra was
the chemical shift of the aromatic proton H5, which moved
up-field to the region 6.14–6.31 ppm compared to their
dmso precursors as shown in Table 1. This has been ascribed
to the anisotropic shielding effect of the aromatic ring current
that a phenyl group in PPh3 has on the H5 proton pointing
towards it.14
For all (salicylaldiminato)platinum(II) complexes containing
LA except 3a (see Fig. 3), signals corresponding to the methyl
protons of the isopropyl moieties (Hc/c′) on the ligands were
split into two doublets, indicating that rotations between the
nitrogen and the ipso carbon of these complexes are restricted.
The 31P NMR spectra show singlets for the phosphine-con-
taining platinum(II) complexes, 3–5, with resolved coupling to
195Pt. With the bulky ligand LA, the cis isomer 3a shows an
even higher-field shift (Δδ = −2.14 ppm) compared to its trans
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Fig. 2 A comparison of the 1H NMR spectra of cis- and trans-[PtClLA(dmso)], 1a and 1b.
Fig. 3 A comparison of the 1H NMR spectra of cis- and trans-[PtClLA(PPh3)], 3a and 3b, as well as the methyl analogue 6.
isomer 3b (Δδ = 8.87 ppm), suggesting that the cis arrange- in 5 appeared at higher field (Δδ = 1.98 ppm) compared to the
ment of the oxygen atom and the imine nitrogen enhances the PPh3-containing mononuclear analogue 3b, due to the electron
shielding effect. In addition, the chemical shift of the P atoms rich ferrocenyl moiety of the spacer ligand.
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1
The JPt–P coupling constants were measured as 4062 Hz in
cis complex 3a and ca. 3850 Hz in trans complexes 3b, 4 and 5
(see Table 1), which are of the order of magnitude expected for
1
a P atom trans to an O atom.13,15,16 The magnitude of JPt–P
coupling constants is a measure of the strength of the Pt–P σ
bonding.17 Thus the lower values for the trans complexes are
indicative of a weakening of the Pt–P σ bonding compared to
those of the cis complex, 3a. Interestingly, there is a ca. 6%
1
increase in the JPt–P for the cis isomer 3a over its trans isomer
3b, probably due to a higher trans influence of the imine nitro-
gen compared to that of the coordinated oxygen, which as a
consequence weakens the Pt–P σ bond in 3b.
In the 13C NMR spectrum, coordination of the imine nitro-
gen was confirmed by the downfield shifts of the imine carbon
signal (Ca) with respect to the free ligands. The signal for the
imine carbon of the cis complex 1a appears more upfield than
that for the trans isomer 1b. The signal for the methyl carbons
of dmso (Cdmso) occurs as a doublet at approximately 47 ppm
for all dmso-ligated complexes except 1b. The peak of Cdmso in
1b appears more upfield (42.31 ppm) than its cis isomer 1a.
No significant shifts in 13C resonances were observed for the
phosphine complexes compared to their dmso precursors. The
differences between the cis/trans isomers 3a and 3b in their
13C spectra are consistent with the trends observed for their
dmso precursors.
Fig. 5 ORTEP view of molecule B in 1b. Ellipsoids are drawn at the 30% prob-
ability level.
Crystallographic studies
X-ray quality crystals of some of the complexes were obtained
by cooling a concentrated solution of the complex in dichloro-
methane-methanol to 4 °C. The molecular structures of dmso-
ligated complexes 1a, 1b and 2, and the PPh3 derivatives 3a, and
3b were determined by single crystal X-ray structural analysis. The
structure of 1b consists of discrete monomeric molecules with
two different geometries [1b(A) and 1b(B)]. These conformations
are not significantly different in terms of structural parameters
such as bond lengths or angles. This phenomenon is common
place in the literature for related platinum and palladium com-
plexes.12,16,18 The ORTEP plots of 1–3 are shown in Fig. 4–8.
Fig. 6 ORTEP view of 2. Ellipsoids are drawn at the 30% probability level.
Fig. 4 ORTEP view of 1a. Ellipsoids are drawn at the 30% probability level.
11168 | Dalton Trans., 2013, 42, 11163–11179
Fig. 7 ORTEP view of 3a. Ellipsoids are drawn at the 40% probability level.
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shown in Table 3, bond lengths and angles are well within the
range of values obtained for analogous compounds.19
In all cases, the angles between adjacent atoms in the
coordination sphere are close to the expected value of 90°, and
the sum of the angles around platinum is around 360°. The
most noticeable distortion occurs in the O–Pt–Cl angle in the
cis complexes 1a and 3a, where these bond angles are 83.94(7)°
and 82.01(9)°, respectively. This is probably caused by the two
relatively bulky groups, i.e., dmso or PPh3, and the isopropyl
phenyl groups, being cis to each other, forming an angle N(1)–
Pt(1)–S(1) of 97.35(7)° in 1a, and angle N(11)–Pt(2)–P(24) of
103.50(9) in 3a, and consequently resulting in a reduced angle
O–Pt–Cl.
The steric effect of the ligand on the molecular structure is
significant. In the case of ligand LA, the atoms of the metal-
containing ring adopt a practically planar arrangement, as
shown by the sum of their internal bond angles of ca. 720°.20
With reduction of the steric bulk in 2, a certain degree of
strain of the metal-containing ring is observed with a sum of
internal angles of 702.5°. In addition, a nearly facial (perpen-
dicular) orientation21 of the 2,6-diisopropyl phenyl ring to
the metal-containing ring is observed in the complexes with
the bulky ligand LA, as indicated by the torsion angle between
the two planes around 90°, while, this angle is observed to
decrease to 46.9(3)° in 2, i.e., the alternative edge (parallel)
and facial arrangement does not exist in this case.
The trans influence of different ligands around the plati-
num centre can be compared by analysing the selected bond
lengths for the cis/trans isomers, 1a/1b and 3a/3b. For dmso-
ligated complexes, the Pt–O(1)trans-S bond length in cis 1a
[2.015(2) Å] is slightly longer than that trans to a chloride
ligand, i.e., Pt–O(1)trans-Cl in trans 1b [1.991(2) Å], indicating
the higher trans influence of dmso compared to that of Cl.22
The longer Pt–S(1)trans-N [2.2304(8) Å] in trans 1b than the Pt–
S(1)trans-O in cis analogues 1a [2.2181(8) Å] reveals the higher
Fig. 8 ORTEP view of 3b. Ellipsoids are drawn at the 40% probability level.
Relevant crystal data for all structures are given in Table 2
along with selected bond lengths and angles in Table 3.
For all complexes, the coordination sphere of platinum is
square-planar with the κ2-N^O-bound bidentate ligands, the
chlorido ligand and the S-bound dimethylsulfoxide (1a, 1b
and 2), or P-bound triphenylphosphine (3a and 3b) completing
the coordination sphere. The molecular structures provide
decisive evidence of the fact that in 1b and 3b the dmso or
PPh3 ligand occupies the position trans to the imine nitrogen,
while in 1a, 2 and 3a it occupies the cis position. These
complexes have fused bicyclic systems containing a six-
membered metal-containing ring and the phenyl group. As
Table 2 Crystal data and structure refinement for (salicylaldiminato)platinum(II) complexes 1–3
1a
1b
2
3a
3b
Empirical formula
Mr (g mol−1
Crystal system
Space group
C
21H28ClNO2PtS +
C21H28ClNO2PtS
C15H16ClNO2PtS
C37H37ClNOPPt +
2(CH2Cl2)
943.04
Monoclinic
P21/c
C37H37ClNOPPt
CH2Cl2
673.97
Triclinic
)
589.04
Monoclinic
P21/c
504.89
Triclinic
ˉ
P1
773.19
Monoclinic
C2/c
ˉ
P1
a (Å)
b (Å)
c (Å)
α (°)
9.1719(3)
11.1215(2)
14.2096(4)
110.883(2)
108.6750(10)
91.334(2)
1267.38(6)
2
16.4013(18)
19.269(2)
14.4597(16)
90
91.062(2)
90
8.8183(2)
10.3272(9)
18.7983(17)
20.8684(18)
90
100.185(2)
90
35.498(4)
10.2831(11)
23.873(3)
90
130.634(2)
90
10.1831(2)
10.4630(2)
111.9260(10)
93.0980(10)
111.1600(10)
793.23(3)
β (°)
γ (°)
V (Å3)
4569.0(9)
8
3987.4(6)
4
6613.2(13)
8
Z
2
Reflections collected/unique
R (int)
10 198/5197
0.0251
10 6417/11 388
0.0618
6569/3359
0.0095
63 670/7573
0.024
61 047/9659
0.032
Data/restraints/parameters
Final R indices [l > 2σ(l)]
Largest diff. peak and hole
5197/0/272
0.0239, 0.0411
2.272, −0.903
11 388/0/487
0.0247, 0.0460
0.888, −1.057
3359/0/191
0.0144, 0.0325
1.292, −0.764
7573/0/431
0.0264, 0.0681
1.60, −1.56
9659/0/383
0.0191, 0.0444
−0.52, 1.26
(e A−3
)
CCDC number
931185
931186
931187
931188
931189
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Table 3 Selected bond lengths (Å), bond angles (°) and torsion angles (°) for (salicylaldiminato)platinum(II) complexes 1–3
1aa
1b
2a
3aa
3b
(B)b
1.992(2)
2.029(3)
2.2304(8) Pt(1)–S(1)
2.2961(9) Pt(1)–Cl(1)
1.292(4)
2.22(4)
Pt(1)–O(1)
Pt(1)–N(1)
Pt(1)–S(1)
Pt(1)–Cl(1)
N(1)–C(7)
2.015(2)
2.040(3)
2.2181(8)
2.3118(8)
1.300(4)
Pt(1)–O(1)
Pt(1)–N(1)
Pt(1)–S(1)
Pt(1)–Cl(1)
N(1)–C(7)
O(2)⋯H(2)c
Pt(1)–O(1)
Pt(1)–N(1)
2.019(18)
2.029(2)
2.2062(6)
2.2981(16) Pt(2)–Cl(1)
1.297(3)
Pt(2)–O(3)
Pt(2)–N(11)
Pt(2)–P(24)
2.042(3)
2.021(3)
2.2586(9) Pt(1)–P(24)
2.3174(9) Pt(1)–Cl(2)
Pt(1)–O(3)
Pt(1)–N(11)
2.001(2)
2.0707(2)
2.2566(7)
2.2993(8)
1.297(3)
N(1)–C(1)
N(11)–C(10)
1.301(5)
N(11)–C(10)
O(1)–Pt(1)–N(1)
O(1)–Pt(1)–S(1)
N(1)–Pt(1)–Cl(1)
S(1)–Pt(1)–Cl(1)
N(1)–Pt(1)–S(1)
O(1)–Pt(1)–Cl(1)
C(7)–N(1)–Pt(1)
C(1)–O(1)–Pt(1)
O(1)–C(1)–C(6)
C(1)–C(6)–C(7)
N(1)–C(7)–C(6)
90.26(10)
172.36(7)
174.13(8)
88.46(3)
97.35(7)
83.94(7)
121.8(3)
126.1(2)
124.1(3)
122.5(3)
129.8(3)
O(1)–Pt(1)–N(1)
O(1)–Pt(1)–S(1)
N(1)–Pt(1)–Cl(1)
S(1)–Pt(1)–Cl(1)
N(1)–Pt(1)–S(1)
O(1)–Pt(1)–Cl(1)
C(7)–N(1)–Pt(1)
C(1)–O(1)–Pt(1)
O(1)–C(1)–C(6)
C(1)–C(6)–C(7)
N(1)–C(7)–C(6)
92.17(9)
87.52(7)
92.98(7)
87.85(3)
174.58(7) N(1)–Pt(1)–S(1)
172.70(7) C(1)–Pt(1)–Cl(1)
123.3(2)
126.8(2)
124.6(3)
124.2(3)
128.0(3)
O(1)–Pt(1)–N(1)
O(1)–Pt(1)–S(1)
N(1)–Pt(1)–Cl(1)
S(1)–Pt(1)–Cl(1)
89.01(8)
172.56(6)
173.87(6)
89.39(2)
95.19(6)
86.87(6)
121.41(18) C(10)–N(11)–Pt(2)
119.21(16) C(4)–O(3)–Pt(2)
124.2(2)
122.2(2)
126.4(2)
O(3)–Pt(2)–N(11)
O(3)–Pt(2)–P(24)
N(11)–Pt(2)–Cl(1)
P(24)–Pt(2)–Cl(1)
N(11)–Pt(2)–P(24)
O(3)–Pt(2)–Cl(1)
90.57(12) O(3)–Pt(1)–N(11)
165.90(9) O(3)–Pt(1)–P(24)
172.38(9) N(11)–Pt(1)–Cl(2)
91.75(8)
87.73(6)
91.10(7)
89.45(2)
178.90(5)
176.92(6)
122.5(2)
127.2(2)
125.2(3)
124.5(2)
128.7(2)
85.95(3)
P(24)–Pt(1)–Cl(2)
103.50(9) N(11)–Pt(1)–P(24)
82.01(9)
122.5(3)
127.3(3)
123.6(4)
123.0(3)
130.4(4)
O(3)–Pt(1)–Cl(2)
C(10)–N(11)–Pt(1)
C(4)–O(3)–Pt(1)
O(3)–C(4)–C(9)
C(4)–C(9)–C(10)
N(11)–C(10)–C(9)
C(11)–N(1)–Pt(1)
C(7)–O(1)–Pt(1)
O(1)–C(7)–C(2)
C(7)–C(2)–C(1)
N(1)–C(1)–C(2)
O(3)–C(4)–C(9)
C(4)–C(9)–C(10)
N(11)–C(10)–C(9)
Total 1d
Total 2e
360.0
714.6
Total 1d
Total 2e
360.5
719.1
Total 1d
Total 2e
360.5
702.5
Total 1d
Total 2e
360.0
717.5
Total 1d
Total 2e
360.0
720.0
Pt(1)–N(1)–C(8)–C(13) −104.7(3) Pt(1)–N(1)–C(8)–C(13) −82.8(3)
Pt(1)–N(1)–C(8)–C(13) 46.9(3)
Pt(2)–N(11)–C(12)–C(13) 108.0(3)
Pt(1)–N(11)–C(12)–C(13) −94.70(2)
a L is cis to N atom (L = dmso). b Molecule B. c Intermolecular interactions. d Sum of angles in the coordination environment of the platinum atom. e Sum of internal angles of the metal-
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Scheme 3 Preparation of salicylaldiminato methylplatinum(II) complex 6.
Fig. 9 Projection viewed along [010] for 1b.
precipitate, consisting of a mixture of 6 and a small amount of
the by-product trans-[PtCl(Me)(PPh3)2]. Column chromato-
graphy on SiO2 yielded the pure target complex, 6.
trans influence of the imine nitrogen as compared to the
phenoxy oxygen atom of the N^O ligands.23 This is also
confirmed by the Pt–Cl bond lengths for the cis/trans com-
plexes with either dmso or PPh3 ligands, i.e., Pt–Cl(1)trans-N
in cis 1a/3a > Pt–Cl(1)trans-O in trans 1b/3b. Furthermore,
there is slight lengthening of the Pt–N bond by 0.042 Å for
the trans compound 3b as compared to its dmso precursor
1b, due to the higher trans effect of the P-donor ligand com-
pared to that of the S-donor ligand.24 The lengthening of the
Pt–N distance agrees with the down-field shift of the imine
proton in the 1H NMR spectra for these complexes.
Intermolecular interaction is found in the trans complex
1b. As shown in Fig. 9, the imino hydrogen in 1b is involved in
an interaction with the oxygen atom in the dmso ligand,
NvCH⋯O (d(H(7A)⋯O(1B)) = 2.24(4) Å), increasing the stabi-
lity of 1b as previously suggested for analogous compounds.25
No intermolecular interactions are found for the cis complexes
or the PPh3 derivatives.
For the methylplatinum(II) complex 6, the cis arrangement
of the methyl group and the imine is expected, and this is con-
firmed by the experimental evidence, where coupling of both
the imine proton to the phosphorus (4JP–H = 12.2 Hz, Table 1),
and the coupling of the imine proton to platinum (3JPt–H = 70.9
1
Hz) in a trans position is observed in the H NMR spectrum.
As shown in Fig. 3, the 1H NMR spectra of 6 is almost identical
to that of 3b which has the imine nitrogen trans to PPh3. In
the case of complex 6, however, an additional resonance for the
methyl group on platinum appears as a doublet of doublets at
−0.29 ppm. In fact, all previously reported salicylaldiminato
methylpalladium complexes have a cis geometry in which the
methyl group is cis to the N atom.18,26
cis–trans Isomerization of 3a
We have noticed when triphenylphosphine is added to a solu-
tion of 1a or 1b, the kinetically controlled product 3a with
phosphorus coordinating cis to the imine group is obtained.
Methylplatinum(II) salicylaldiminato complex
It has been mentioned above, that the “initiator complexes” On heating, the kinetic product 3a undergoes a slow conver-
involved in the reaction mechanisms for the chain propagation sion to the thermodynamically more stable trans complex 3b,
step in olefin oligomerization/polymerization using salicylaldi- resulting in an up-field resonance at 6.31 ppm ascribed to
minato catalysts can have cis or trans configurations, in which the H5 proton, as well as a change in both the Pt–P and Pt–H
the cis isomers (with the alkyl group cis to nitrogen) are more coupling constants. In particular, the Pt–P coupling constant
stable.8,9 Herein, we report the synthesis of a model platinum changes from 4063 to 3818 as a result of the isomerization of
“initiator complex”, where R is a methyl group, to evaluate the the PPh3 from the cis to the trans position.
stability of the cis isomer. The reaction of ligand LA with
According to DFT calculations on salicylaldiminato
[PtCl(cod)Me] was studied and the resulting compound, the Ni/Pd catalyzed olefin polymerization, tetrahedral transition
salicylaldiminato methylplatinum(II) complex 6 is depicted in states have been found for cis–trans isomerization of four-
Scheme 3. The first step involves the reaction of the ligand pre- coordinated complexes. The (salicylaldiminato)platinum(II)
cursor with 2 equivalents of sodium acetate, leading to depro- systems under investigation here allow us to determine kinetic
tonation of the hydroxyl group to afford the sodium salt of and computational data for the cis–trans isomerization 3a →
the ligand in situ. Addition of equimolar amounts of 3b (eqn (1)), and to evaluate the possible mechanism for the
[PtCl(cod)Me] and PPh3 to the reaction mixture gave a yellow isomerization process.
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The variable-temperature rate constants for the isomeriza-
tion reaction listed in Table 4 were fitted to the Eyring
equation (see eqn (2) in Experimental section) and yielded ΔH‡
ð1Þ
= 26.9 1.1 kcal mol−1, ΔS‡ = 1.2 3.3 eu and ΔG‡298.15
=
26.6 0.1 kcal mol−1 [Fig. 11(B)]. The value of ΔH‡ obtained is
comparable with that of similar systems,27 while the value of
ΔS‡ is known to vary with different isomerization mechanisms.28
Theoretical calculations
The possible mechanisms for cis–trans isomerization depend
on the nature of the solvent, the electronic nature of the
ligands and the temperature.29 Different mechanisms usually
considered for cis–trans isomerization in square-planar com-
plexes include the associative pathway,30 the Berry pseudo-
rotation mechanism,31 the dissociative pathway,32 and direct
geometry change via a tetrahedral four-coordinate transition
state.33
Both the associative and Berry pseudo-rotation mechanisms
need the coordination of a fifth ligand such as an extra PR3
ligand, an ethylene or a solvent molecule.31a,34 In our system,
solvent is the only possible source of the fifth ligand. Since
CDCl3 is a non-coordinating solvent, we can rule out these two
pathways involving five-coordinate intermediates. Therefore,
both the dissociative and the direct geometry change pathways
could be possible mechanisms for the isomerization process
of 3a to 3b. In addition, both the dissociation of PPh3 and the
dissociation of the N-atom of the hemilabile imine ligand
could be possible for the dissociative pathway. Three isomeri-
zation routes were therefore investigated. The energy profiles
are shown in Fig. 12 and the calculated activation parameters
as well as the experimental values are given in Table 5. Fig. 13
shows the optimized structures of the stationary points of
Fig. 12.
Our computational studies were carried out by performing
DFT calculations on the isomerization of 3a through the disso-
ciative pathway, involving the dissociation of the PPh3 ligand,
as well as the direct geometry change via a tetrahedral four-
coordinate transition state. It is worth noting that both the
“Y-shaped” three-coordinate transition state (TS-P) and the tet-
rahedral four-coordinate transition state (TS) were determined
to be extremely close in activation energy (ΔG‡298.15), in which
TS-P is about 2 kcal mol−1 more stable than TS (see Table 5).
On the other hand, TS-P is 10.6 kcal mol−1 higher in enthalpy
(ΔH‡298.15) than TS, due to the very positive value of ΔS‡
for TS-P (45.3 eu), which is expected for the dissociative
mechanism.27a,28b Thus, the small experimental value of ΔS‡
obtained (1.2 3.3 eu) does not correspond to the calculated
ΔS‡ for the dissociation of PPh3 (45.3 eu), and thus we can rule
out this mechanism.
Kinetics of the cis–trans isomerisation
The cis–trans geometrical conversion 3a → 3b in CDCl3 was
investigated quite easily in the temperature range 313–336 K
using 31P NMR spectroscopy. In this temperature range, the
conversion is ca. 95%, yielding the stable trans isomer and a
small amount of the starting cis isomer at the end of the reac-
tion. The reaction rate was determined from the NMR data
and found to be pseudo-first-order. The rate constants at
different temperatures are given in Table 4.
The progress of each reaction was monitored by following
the decrease of the 31P resonance at −2.14 ppm associated
with 3a. Fig. 10 shows the spectral changes during the trans-
formation 3a → 3b at 338 K. The two clearly defined reson-
ances at 8.67 and −2.37 ppm are an indication that the
isomerization is well-behaved and is free of kinetic compli-
cations. The pseudo-first-order plots for the transformation of
3a to 3b at different temperatures are depicted in Fig. 11(A).
Table 4 Experimental pseudo-first-order rate constants for the isomerization
of 3a to 3b
Entry
T/K
kobs/10−5 s−1
1
2
3
4
313
320
328
336
0.19 0.01
0.43 0.02
1.39 0.06
3.85 0.12
The third route proposed for isomerization of 3a is the dis-
sociative pathway involving the dissociation of the imine N.
The dissociation of the labile imine N from 3a affords a cis-like
three-coordinate intermediate, 3a-N, with an agostic inter-
action between the Pt centre and the imine hydrogen (Pt⋯H
distance is 2.467 Å). As shown in Fig. 12, conversion of an
agostic “cis-like” three-coordinate form to its trans form 3b-N
Fig. 10 The relative intensity of the 3b resonance versus time plot for the
experimental data at 338 K (solid black line). Inside: 31P NMR spectral changes
associated with the geometrical isomerization of 3a to 3b in CDCl3 at 338 K.
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Fig. 11 (A) The pseudo-first-order plots for cis–trans isomerization of 3a at variable-temperature. (B) Eyring plot for the isomerization reaction of 3a constructed by
the use of rate data obtained from 31P NMR experiments.
Fig. 12 Isomerization routes: simple rotation via tetrahedral TS (black), dissociative route via the dissociation of PPh3 (red) and dissociative route via the dissociation
of N atom (green). (ΔG298.15/ΔH298.15, kcal mol−1 at 1 atm relative to 3a).
requires additional consumption of energy. Our calculations
Table 5 Summary of theoretical and kinetic data for the cis–trans isomerization
find that TS-N is the highest energy transition structure of the
of 3a
three isomerization routes, and is higher in both enthalpy and
activation energy (ca. 11 kcal mol−1) thus making the imine
Entry
ΔH2‡98.15 (kcal mol−1
)
ΔS‡298.15 (eu)
ΔG2‡98.15 (kcal mol−1
)
dissociation a less likely pathway compared to the one invol-
ving the tetrahedral transition state, TS.
1a
2b
3c
4d
33.9
35.0
23.3
26.9 1.1
45.3
−6.1
4.1
20.4
33.2
22.0
26.6 0.1
Therefore, the calculated activation parameters of ΔH‡
=
23.3 kcal mol−1, ΔS‡ = 4.1 eu for the isomerization of 3a to 3b
via the tetrahedral four-coordinate transition state (TS) are in
1.2 3.3
a Dissociative mechanism via the dissociation of the PPh3 ligand.
b Dissociative mechanism via the dissociation of the N-atom. c One-
step, direct geometry change mechanism. d Experimental values.
good agreement with the experimental values of ΔH‡
=
26.9 1.1 kcal mol−1, ΔS‡ = 1.2 3.3 eu (see Table 5). This
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Fig. 13 Fully optimized geometrical structures of stationary points intercepted along the energy profile for isomerization of complex 3a. Bond lengths are in
angstroms and angles in degrees.
agreement provides support for the computational finding
In the case of the dmso complexes, reaction with PPh3
that the one-step, direct geometry change mechanism is really occurs readily to bring about the displacement of the dmso
preferred to the three-step dissociative mechanism, and agrees ligand. However, in the case of reactions with the dipho-
with the DFT findings on cis–trans isomerization of related sal- sphines (dppf and dppe) the success of the reaction is depen-
icylaldiminato complexes via tetrahedral transition states.9
dent on the backbone of diphosphines and the configuration
of the dmso-ligated precursors.
cis-Methylplatinum(II) complex 6 was obtained as a single
isomeric product from an alternative route using [PtCl(Me)-
(cod)] as starting material, indicating that the (salicylaldimi-
nato)platinum(II) complex with R group (R = Me) cis to the
imine nitrogen is more stable.
The cis–trans isomerization of the N^O complex 3a was
studied both experimentally and theoretically. The results of
DFT calculations suggest that a direct geometry change mech-
anism via a tetrahedral transition state would be the most
likely cis–trans isomerization pathway amongst the studied
mechanisms. The computed activation parameters are in very
Conclusions
The preparation of novel (salicylaldiminato)platinum(II) com-
plexes has been achieved by reacting cis-[PtCl2(dmso)2] with
two salicylaldimine ligands of different steric bulk. This
allowed for a comparative study of their structures and reactiv-
ity. The steric effect of the salicylaldiminato ligands seems
to play an important role in their coordination behaviour
leading to the specific molecular structures obtained for the
complexes.
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good agreement with the experimental activation parameters was eluted using n-hexane–ethyl acetate (70 : 30), to isolate
obtained from kinetic studies.
complex 1a. The third band was eluted by n-hexane–ethyl
acetate (50 : 50) solution to give isomer 1b. These isomers
differ in the position of the Cl− ligand [trans to N atom (1a) or
O atom (1b)].
Experimental section
General
1a, yield 0.550 g, 75%. M.p.: 165–167 °C. IR (KBr):
ν(CHvN) 1606 cm−1, (SvO) 1154 cm−1
.
1H NMR (300 MHz,
3
3
CDCl3): δ = 7.73 [s, JPt–H = 93.27 Hz, 1H, Ha], 7.43 [dd, JH–H
=
The solvents were purified and distilled using standard methods.
Anhydrous HPLC grade methanol (dry, max. 0.005% H2O) was
purchased from Aldrich.
4
3
8.53 Hz, JH–H = 1.60 Hz, 1H, H2], 7.24 [t, JH–H = 7.74 Hz, 1H,
3
3
H4′], 7.13 [d, JH–H = 8.34 Hz, 2H, H3,4], 7.07 [d, JH–H
=
3
4
7.74 Hz, 2H, H3′,5′], 6.63 [dd, JH–H = 7.33 Hz, JH–H = 1.86 Hz,
NMR spectra were recorded on a Varian Mercury-300 MHz
or Varian Unity-400 MHz spectrometer. Residual solvent
signals were used as reference for 1H and 13C NMR. H3PO4
(85% in D2O) were used as reference for 31P NMR. Abbrevi-
ations used: s = singlet, d = doublet, t = triplet, m = multiplet,
b = broad, NMR labelling is as shown in Schemes 1–3. Infrared
spectra were recorded as KBr pellets and measured on a Perkin
Elmer Spectrum One FT-IR spectrophotometer. Mass spectral
analyses were carried out at Stellenbosch University on a
Waters Q-TOF Ultima API or Waters Quattro Micro API mass
spectrometer and using the electrospray ionization technique.
Elemental analyses were carried out on a Fisons EA 1108
CHNS Elemental Analyzer at the microanalytical laboratory of
the University of Cape Town. Melting points were recorded on
a Kofler hotstage microscope (Reichert Thermovar).
X-ray single crystal intensity data for structures were col-
lected on Nonius Kappa-CCD (1a, 1b and 2) or Bruker
KAPPA APEX II DUO (3a and 3b) diffractometers using graph-
ite monochromated MoKα radiation (λ = 0.71073 Å). The temp-
erature was controlled by an Oxford Cryostream cooling system
(Oxford Cryostat). The strategy for the data collections was
evaluated using the Bruker Nonius “Collect” program. Data
were scaled and reduced using DENZO-SMN software.35 An
empirical absorption correction using the program SADABS36
was applied. The structure was solved by direct methods and
refined employing full-matrix least-squares with the program
SHELXL-97,37 refining on F2. Packing diagrams were produced
using the program PovRay and the graphic interface X-Seed.38
All the non-hydrogen atoms were refined anisotropically. The
hydrogen atoms were placed in idealised positions in a riding
model with Uiso set at 1.2 or 1.5 times those of their parent
atoms and fixed C–H bond lengths.
1H, H5], 3.32 [s, 6H, Hd], 3.51 [hept, 2H, Hb], 1.28 [d, JH–H
=
3
3
6.88 Hz, 6H, Hc], 1.03 [d, JH–H = 6.79 Hz, 6H, Hc′]. 13C NMR
(CDCl3): δ = 163.05 [s, Ca], 162.47 [s, C1′], 148.19 [s, C6], 141.78
[s, C2′,6′], 136.68 [s, C2], 133.66 [s, C3], 128.00 [s, C4′], 123.37
[s, C3′,5′], 123.06 [s, C6], 120.79 [s, C4], 117.25 [s, C5], 47.37
[s, Cd], 27.93 [s, Cb], 24.71 [s, Cc′], 22.54 [s, Cc]. EI-MS: m/z
590.12 [M]+, 556.18 [M − OMe]+, 553.15 [M − Cl]+, 515.15
i
[M − Pr − Me]+, 474.12 [M − Cl − dmso]+. Anal. found (calc.
for C21H28ClNO2PtS): C: 43.08 (42.82), H: 4.92 (4.79), N: 2.26
(2.59), S: 5.19 (5.43).
1b, yield 0.164 g, 23%. M.p.: 193–195 °C. IR: ν (CHvN)
1
1606 cm−1, (SvO) 1140 cm−1. H NMR (300 MHz, CDCl3): δ =
3
3
7.73 (s, JPt–H = 59.47 Hz), 7.73 [s, JPt–H = 93.27 Hz, 1H, Ha],
7.41 [dd, JH–H = 8.68 Hz, JH–H = 1.81 Hz, 1H, H2], 7.20
3
4
[t, JH–H = 7.74 Hz, 1H, H4′], 7.15–7.09 [m, 2H, H3,4], 7.04 [dd,
3
3JH–H = 8.67 Hz, JH–H = 0.54 Hz, 2H, H3′,5′], 6.65 [dd, JH–H
=
4
3
7.96 Hz, JH–H = 1.08 Hz, 1H, H5], 3.32 [s, 6H, Hd], 3.26 [hept,
4
2H, Hb], 1.27 [d, JH–H = 6.80 Hz, 6H, Hc], 1.03 [d, JH–H
=
3
3
6.88 Hz, 6H, Hc′]. 13C NMR (CDCl3): δ = 162.83 [s, C1′], 161.45
[s, Ca], 146.36 [s, C6], 141.78 [s, C2′,6′], 136.09 [s, C2], 134.38
[C3], 127.66 [C4′], 123.38 [s, C1], 123.07 [C3′,5′], 120.62 [s, C4],
117.11 [s, C5], 42.31 [s, Cd], 28.09 [s, Cb], 24.40 [s, Cc′], 22.79
[s, Cc]. EI-MS: m/z 590.12 [M]+, 556.18 [M − OMe]+, 553.15
i
[M − Cl]+, 515.15 [M − Pr − Me]+, 474.12 [M − Cl − dmso]+.
Anal. found (calc. for C21H28ClNO2PtS): C: 43.07 (42.82), H:
4.86 (4.79), N: 2.38 (2.59), S: 5.45 (5.43).
[PtCl{(OC6H4)CHvN(C6H5)}(SOMe2)] (2). cis-[PtCl2(SOMe2)2]
(0.540 g, 1.28 mmol) and the imine LB (0.252 g, 1.28 mmol) in
the presence of sodium acetate (0.105 mg, 1.28 mmol) were
allowed to react in dry methanol (20 ml) for 16 h at room tem-
perature. A yellow precipitate of complex 2 formed. The product
was collected by filtration in vacuo. Yield: 0.435 g, 67%. M.p.:
Preparation of the compounds
170–172 °C. IR (KBr): ν (CHvN) 1607 cm−1, (SvO) 1155 cm−1
.
The starting material cis-[PtCl2(SOMe2)2]39 and salicylaldimi- 1H NMR (300 MHz, CDCl3): δ = 7.83 [s, JPt–H = 86.34 Hz, 1H,
3
3
nato ligands LA–LB2a were prepared as reported elsewhere.
Ha], 7.47 [d, JH–H = 7.19 Hz, 2H, H2′,6′], 7.36 [m, 3H, H3′,4′,5′],
[PtCl{(OC6H4)CHvN{2,6-(Me2CH)2(C6H3)}}(SOMe2)] (1). cis- 7.27 [d, JH–H = 7.26 Hz, 1H, H2], 7.21 [dt, JH–H = 8.08 Hz,
3
3
[PtCl2(SOMe2)2] (0.522 g, 1.24 mmol) and the imine LA 4JH–H = 1.30 Hz, 1H, H4], 7.11 [t, JH–H = 8.60 Hz, 1H, H3], 6.64
3
(0.348 g, 1.24 mmol) in the presence of sodium acetate [d, JH–H = 7.42 Hz, 1H, H5], 3.24 [s, 6H, Hd]. 13C NMR
3
(0.101 g, 1.24 mmol) were allowed to react in dry methanol (101 MHz, CDCl3): δ = 163.47 [s, C1′], 162.69 [s, Ca], 153.52
(20 ml) under reflux for 2 h. The solvent was removed on a [s, C6], 136.79 [s, C2], 133.66 [s, C4], 128.82 [s, C2′,6′], 128.00
rotary evaporator, and the residue obtained was dissolved in a [s, C4′], 124.18 [s, C3′,5′], 121.06 [s, C3], 120.08 [s, C1], 117.58
minimum amount of CH2Cl2 and then passed through a SiO2 [s, C5], 46.36 [s, Cd]. EI-MS: m/z 506.03 [M]+, 469.05 [M − Cl]+,
column. Two isomeric forms of this product (1a and 1b) 391.04 [M − Cl − Ph]+, 390.04 [M − Cl − dmso]+. Anal. found
were isolated. n-Hexane–ethyl acetate (95 : 5) was used to (calc. for C15H16ClNO2PtS): C, 35.90 (35.68); H, 3.02 (3.19); N,
elute the unreacted ligand (first band) while the second band 2.38 (2.77), S 5.98 (6.35).
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4
[PtCl{(OC6H4)CHvN{2,6-(Me2CH)2(C6H3)}}(PPh3)] (3). 1a or (300 MHz, CDCl3): δ = 8.20 [d, JP–H = 13.39 Hz, 1H, Ha], 7.77
3
1b or the mixture of 1a and 1b (0.566 g, 1.0 mmol) and triphe- (m, 6H, Ph–H), 7.47 [d, JH–H = 6.84 Hz, 1H, H2], 7.45–7.37
3
nylphosphine (0.296 g, 1.0 mmol) were allowed to react in (m, 9H, Ph–H), 7.35 (m, 1H, H4) 7.28 (d, JH–H = 7.48 Hz, 2H,
3
3
dichloromethane (20 ml) at room temperature for 2 h. The H2′,6′), 7.22 (t, JH–H = 7.64 Hz, 2H, H3′,5′) 7.17 (t, JH–H = 8.60
solvent was removed using a rotary evaporator, and the residue Hz, 1H, H4′), 6.55 (t, JH–H = 7.53 Hz, 1H, H3), 6.16 (d, JH–H
=
3
3
obtained was dissolved in a minimum amount of CH2Cl2 and 8.62 Hz, 1H, H5). 13C NMR (101 MHz, CDCl3): δ = 163.01 [s,
then passed through a SiO2 column. Two isomeric forms of C1′], 160.89 [s, Ca], 151.12 [s, C6], 135.06 [d, JP–C = 11.52 Hz, 6C,
this product (3a and 3b) were isolated. Elution with n-hexane– Ph–C], 134.96 [s, C4′], 130. 67 [s, C2,2′,6′], 128.20 [d, JP–C = 2.42
ethyl acetate (98 : 2) solution produced a yellow band, which Hz, 3C, Ph–C], 127.96 [d, JP–C = 10.32 Hz, 6C, Ph–C], 124.83
yielded 3b; elution with n-hexane–ethyl acetate (80 : 20) [s, C3′,5′], 121.86 [s, C5], 119.64 [s, C1], 116.52 [s, C3]. 31P NMR
solution yielded a second band that was collected and (121 MHz, CDCl3): δ = 7.40 [s, JP–Pt = 3869 Hz]. EI-MS: m/z
concentrated to give 3a. These isomers differ in the coordi- 688.2 [M]+, 653.8 [M
nation site of the PPh3 ligand [cis to N atom (3a) or trans to N C31H25ClNOPPt): C, 54.64 (54.04); H, 3.85 (3.66); N, 1.87 (2.03).
atom (3b)]. [{PtCl(OC6H4)CHvN{2,6-(Me2CH)2(C6H3)}}2(μ-dppf)] (5).
was obtained from compound 1b (0.119 g,
−
Cl]+. Anal. found (calc. for
3a, yield 0.294 mg, 34%. M.p.: 212–214 °C. IR: ν (CHvN) Complex
5
1606 cm−1
.
1H NMR (300 MHz, CDCl3): δ = 7.61 [s, 1H, Ha], 0.202 mmol) and dppf (0.056 g, 0.1014 mmol) which were
3
7.49 [d, JH–H = 8.32 Hz, 1H, H2], 7.48–7.43 [m, 6H, Ph–H], allowed to react in dichloromethane (20 ml) at room temp-
3
7.29–7.25 [m, 3H, Ph–H], 7.19 [d, JH–H = 8.76 Hz, 1H, H5], erature for 16 h. The solvent was removed on a rotary evapor-
7.16–7.12 [m, 6H, Ph–H], 7.09 [dt, 3JH–H = 8.07 Hz, 4JH–H = 1.48 ator, and the residue obtained was dissolved in a minimum
3
3
Hz, 1H, H4], 6.83 [t, JH–H = 7.77 Hz, 1H, H4′], 6.64 [d, JH–H
=
amount of CH2Cl2 and then passed through a SiO2 column.
7.75 Hz, 2H, H3′,5′], 6.59 [t, 3JH–H = 7.29 Hz, 1H, H3], 3.66 [hept, n-Hexane was used to elute the unreacted dppf, and the
2H, Hb], 0.94–0.92 [m, 12H, Hc,c′]. 13C NMR (101 MHz, CDCl3): second band was removed by an n-hexane–ethyl acetate (95 : 5)
δ = 163.94 [s, Ca], 163.22 [s, C1′], 151.50 [s, C6], 140.99 [s, C2′,6′], mixture to produce complex 5. Yield: 68 mg, (75%). M.p.:
136.17 [s, C2], 134.92 [d, JP–C = 10.29, 6C, Ph–C], 133.48 [s, C4], decompose without melting at 281–283 °C. IR (KBr):
129.83 [d, JP–C = 2.33 Hz, 3C, Ph–C], 128.50 [s, C4′], 127.81 ν (CHvN) 1610 cm−1
.
1H NMR (400 MHz, CDCl3): δ = 8.03
3
[d, JP–C = 11.05, 6C, Ph–C], 123.59 [s, C3′,5′], 122.05 [s, C5], [d, JH–H = 14.16 Hz, 2H, Ha], 7.67–7.58 [m, 8H, Ph–H], 7.41
3
3
118.85 [s, C1], 116.25 [s, C3], 27.78 [s, Cb], 25.48 [s, Cc′], 21.91 [d, JH–H = 6.63 Hz, 2H, H2], 7.39 [d, JH–H = 6.59 Hz, 4H,
[s, Cc]. 31P NMR (121 MHz, CDCl3): δ = −2.14 [s, JP–Pt = 4063.88]. Ph–H], 7.35–7.25 [m, 8H, Ph–H], 7.22–7.20 [m, 2H, H4], 7.18
3
EI-MS: m/z 774.2 [M]+, 737.2 [M − Cl]+. Anal. found (calc. for [d, JH–H = 7.43 Hz, 4H, H3′,5′], 7.17–7.14 [m, 2H, H4′], 6.53
3
3
C37H37ClNOPPt): C, 57.70 (57.47); H, 5.02 (4.82); N, 1.73 (1.81).
[t, JH–H = 7.12 Hz, 2H, H3], 6.14 [d, JH–H = 8.60 Hz, 2H, H5],
3b, yield 0.423 g, 48%. M.p.: 236–238 °C. IR: ν (CHvN) 4.74 [d, JH–H = 0.98 Hz, 4H, He], 4.63 [d, JH–H = 1.55 Hz, 4H,
3
3
1609.8 cm−1
.
1H NMR (300 MHz, CDCl3): δ = 8.06 [d, JP–H
=
=
=
Hd], 3.67–3.33 [hept, 4H, Hb], 1.33 [d, JH–H = 6.81 Hz, 12H,
4
3
13.92 Hz, 1H, Ha], 7.83–7.76 [m, 6H, Ph–H], 7.50 [d, JH–H
Hc′], 1.08 [d, JH–H = 6.84 Hz, 1H, Hc]. 13C NMR (101 MHz,
3
3
13.92 Hz, 1H, H2], 7.48–7.36 [m, 9H, Ph–H], 7.26 [t, JH–H
CDCl3): δ = 163.26 [s, C1′], 160.68 [s, Ca], 146.39 [s, C6], 141.98
3
1.82 Hz, 1H, H4], 7.24 [t, JH–H = 1.80 Hz, 1H, H4′], 7.20 [s, C2′,6′], 134.93 [s, C2], 134.53 [s, C6], 134.27 [d, JP–C = 10.41
3
[d, 3JH–H = 1.61 Hz, 2H, H3′,5′], 6.57 [dt, 3JH–H = 7.50 Hz, 4JH–H
=
Hz, 8C, Ph–C], 130.33 [s, C4′], 127.54 [d, JP–C = 11.15 Hz, 12C,
1.07 Hz, 1H, H3], 6.31 [dd, JH–H = 8.57 Hz, JH–H = 0.49 Hz, Ph–C], 127.01 [s, C4], 122.85 [s, C3′,5′], 120.98 [s, C5], 119.68
3
4
1H, H5], 3.53 [hept, 2H, Hb], 1.41 [d, JH–H = 6.83 Hz, 6H, Hc], [s, C1], 116.07 [s, C3], 76.51 [d, JP–C = 10.52 Hz, Ce], 75.37 [d,
3
1.14 [d, JH–H = 6.83 Hz, 6H, Hc′]. 13C NMR (101 MHz, CDCl3): JP–C = 7.98 Hz, Cd], 28.16 [s, Cb], 24.79 [s, Cc], 23.05 [s, Cc′].
3
δ = 163.46 [s, C1′], 160.48 [s, Ca], 146.33 [s, C6], 141.86 [s, C2′,6′], 31P NMR (162 MHz, CDCl3): δ = 1.98 [s, JP–Pt = 3851 Hz]. EI-MS:
135.13 [d, JP–C = 10.52 Hz, 6C, Ph–C], 134.89 [s, C2], 134.48 [s, C4], m/z 1576.3 [M]+, 1540.4 [M − Cl]+. Anal. found (calc. for
130.63 [d, JP–C = 2.38 Hz, 3C, Ph–C], 128.71 [s, C1], 127.85 C72H72Cl2FeN2O2P2Pt2): C, 55.12 (54.86); H, 4.73 (4.60); N, 1.63
[d, JP–C = 11.11 Hz, 6C, Ph–C], 126.98 [s, C4′], 122.84 [s, C3′,5′], (1.78).
121.10 [s, C5], 116.10 [s, C3], 28.18 [s, Cb], 24.73 [s, Cc′], 22.86
[Pt(CH3){(OC6H4)CHvN{2,6-(Me2CH)2(C6H3)}}(PPh3)]
(6).
[s, Cc]. 31P NMR (121 MHz, CDCl3): δ = 8.87 [s, JP–Pt = 3818.82]. To a methanol solution (10 ml) of imine LA (0.206 g,
EI-MS: m/z 774.2 [M]+, 737.2 [M − Cl]+, Anal. found (calc. for 0.73 mmol), sodium acetate (0.06 g, 0.73 mmol) was added.
C37H37ClNOPPt): C, 57.78 (57.47); H, 4.93 (4.82); N, 1.72 (1.81).
[PtCl{(OC6H4)CHvN(C6H5)}(PPh3)] (4). Complex
The reaction mixture was allowed to stir at room temperature
was for 30 min. This mixture was slowly added to a methanol solu-
4
obtained from 2 (0.108 g, 0.21 mmol) and triphenylphosphine tion (10 ml) of [PtCl(cod)Me] (0.258 g, 0.73 mmol) at room
(56 mg, 0.21 mmol), which were allowed to react in dichloro- temperature, giving a clear yellow mixture. PPh3 (0.192 g,
methane (20 ml) at room temperature for 2 h. The solvent was 0.73 mmol) was then added, resulting in the formation of a
removed on a rotary evaporator, and the residue obtained was yellow precipitate. The reaction mixture was left to stir at room
dissolved in a minimum amount of CH2Cl2 and then passed temperature for 16 h, after which it was filtered through Celite,
through a SiO2 column. Elution with an n-hexane–ethyl acetate and the solvent was removed in vacuo to give a light yellow
(90 : 10) solution gave a yellow band. Yield 0.102 g (69.3%). solid. The crude product was dissolved in a minimum amount
M.p.: 210–212 °C. IR: ν (CHvN) 1608 cm−1
.
1H NMR of CH2Cl2 and then passed through a SiO2 column. Elution
11176 | Dalton Trans., 2013, 42, 11163–11179
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with an n-hexane–ethyl acetate (95 : 5) solution produced a field (SCF) density was set to 1 × 10−5 hartrees, while the toler-
yellow band that was collected and concentrated to give 6. ance for energy convergence was set to 1 × 10−6 hartrees.
Yield 0.472 g (86%). M.p.: 254–256 °C. IR (KBr): ν (CHvN) Additional convergence criteria include the tolerance for con-
4
1608 cm−1
.
1H NMR (300 MHz, CDCl3): δ = 8.11 [d, JP–H
=
verged gradient (0.002 hartrees Å−1) and the tolerance for con-
12.25 Hz, JPt–H = 70.88 Hz, 1H, Ha], 7.73–7.67 (m, 6H, Ph–H), verged atom displacement (0.005 Å). The thermal smearing
3
7.44–7.33 (m, 9H, Ph–H), 7.27 [dt, 3JH–H = 8.62 Hz, 3JH–H = 1.81 option in Materials Studio makes use of a fractional electron
Hz, 1H, H4], 7.21–7.16 [m, H, H3′,4′,5′], 7.10 [dd, JH–H = 7.92 occupancy scheme at the Fermi level according to a finite-
3
3
Hz, JH–H = 1.86 Hz, 1H, H2], 6.45–6.42 [m, 2H, H3,5], 3.59 temperature Fermi function.41,43
4
[hept, 2H, Hb], 1.37 [d, JH–H = 6.87 Hz, 6H, Hc′], 1.14 [d, JH–H
In all cases optimized geometries were subjected to full fre-
3
3
= 6.85 Hz, 6H, Hc], −0.29 [d, JP–H = 3.19 Hz, JPt–H = 73.34 Hz, quency analyses at the same GGA/PW91/DNP level of theory to
3H, Hd]. 13C NMR (101 MHz, CDCl3): δ = 166.74 [s, C1′], 162.12 verify the nature of the stationary points. Equilibrium geome-
[s, Ca], 146.76 [s, C6], 141.54 [s, C2′,6′], 135.00 [s, C2], 134.89 tries were characterized by the absence of imaginary frequen-
[Ph–C], 134.64 [s, C4], 130.25 [d, JP–C = 2.08 Hz, Ph–C], 129.73 cies. Preliminary transition state geometries were obtained by
[s, C1], 127.75 [d, JP–C = 10.80 Hz, Ph–C], 126.65 [s, C4′], 123.15 either the DMol3 PES scan functionality in Cerius44 (Version
[s, C5,3′,5′], 113.95 [s, C3], 27.56 [s, Cb], 25.13 [s, Cc], 22.54 5.5, Accelrys, Inc.) or the integrated linear synchronous transit/
[s, Cc′], −18.93 [d, JP–C = 8.97 Hz, Cd]. 31P NMR (121 MHz, quadratic synchronous transit (LST/QST) algorithm45 available
CDCl3): δ = 20.43 [s, JP–Pt = 4399.74 Hz]. EI-MS: m/z 753.3 [M]+, in Materials Studio. These preliminary structures were then
513.1 [M − (O-Ph) − (2,6-iPr-Ph)]+. Anal. found (calc. for subjected to full TS optimizations using an eigenvector-follow-
C31H25ClNOPPt): C, 60.04 (60.63); H, 5.66 (5.36); N, 1.63 (1.86). ing algorithm. All transition structure geometries exhibited
only one imaginary frequency in the reaction coordinate.
3
2
All calculations were performed without the incorporation
of solvent effects. All results were mass balanced for the iso-
lated system in the gas phase. The reported energies refer to
Gibbs free energy at 298.15 K and 1 atm.
Isomerization kinetics (3a → 3b)
An NMR tube (5 mm) was charged with 15 mg of 3a, which
was then dissolved at room temperature (293 K) in chloroform-d
to a fixed volume of 600 5 μl. The tube was placed in the ther-
mostated probe. The conversion 3a → 3b was then followed by
conventional 31P NMR spectroscopy. All reactions obeyed a
first-order rate law until over 90% completion of the reaction.
Relative concentration of 3a vs. time data were acquired
Acknowledgements
from the 31P NMR signal areas of 3a and 3b and plotted as Acknowledgements go to the National Research Foundation
ln(a0/at) vs. t (a0 = relative concentration of 3a before heating = (South Africa) and the DST/NRF COE in Catalysis (c*change)
100%; at = relative concentration of 3a at time, t) to obtain for funding. This work was also supported by Research Com-
first-order rate constants kobs from least-squares slopes (stan- mittees of the University of Cape Town and Stellenbosch Uni-
dard errors are also given). Activation parameters were derived versity. The authors wish to acknowledge Centre for High
from a linear least-squares analysis of ln(kobs/T) vs. T−1 data Performance Computing (CHPC) in Cape Town, South Africa
according to the linear expression of the Eyring equation:5
for computational resources as well.
k
T
ꢀΔHz
kB ꢀΔSz
ln
¼
þ ln
þ
ð2Þ
R
h
R
(where R = 1.986 cal mol−1 K−1, kB = 8.62 × 10−5 eV K−1, h =
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