Platinum complexes from pyridine-imine ligands with pendent functional groups

Article abstract of DOI:10.1016/j.ica.2021.120387

Notable differences are observed in reactions of platinum(II) precursor complexes with pyridine-imine ligands of formula 2-C₅H₄NCH = NCH₂-C₆H₄OR with R = Me or H. The ligand 2-C₅H₄NCH = N-2-C₆H₄OMe, L1, gave simple complexes [PtXY(L1)], XY = Me,Me; Me,Cl; Cl,Cl. However, the ligand 2-C₅H₄NCH = NCH₂-2-C₆H₄OH, L2, gave either [PtCl(κ³-N,N,O-2-C₅H₄NCH₂N = CH-2-C₆H₄O)], in which the ligand L2 had undergone a proton migration and a deprotonation, or a more complex product arising from reaction with solvent. The ligand 2-C₅H₄NCH = NCH₂-4-C₆H₄OH, L3, gave a complex [PtCl₂(L3)], which formed a polymer in the solid state through intermolecular OH.Cl hydrogen bonding.

Full text of DOI:10.1016/j.ica.2021.120387

Inorganica Chimica Acta 522 (2021) 120387
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Research paper
Platinum complexes from pyridine-imine ligands with pendent
functional groups
,
,*
Anwar Abo-Amera b, Paul D. Boylea, Richard J. Puddephatta
a Department of Chemistry, University of Western Ontario, London N6A 5B7, Canada
b Department of Chemistry, Al al-Bayt University, Jordan
A R T I C L E I N F O
A B S T R A C T
Keywords:
Platinum
Notable differences are observed in reactions of platinum(II) precursor complexes with pyridine-imine ligands of
formula 2-C5H4NCH = NCH2-C6H4OR with R = Me or H. The ligand 2-C5H4NCH = N-2-C6H4OMe, L1, gave
simple complexes [PtXY(L1)], XY = Me,Me; Me,Cl; Cl,Cl. However, the ligand 2-C5H4NCH = NCH2-2-C6H4OH,
L2, gave either [PtCl(κ3-N,N,O-2-C5H4NCH2N = CH-2-C6H4O)], in which the ligand L2 had undergone a proton
migration and a deprotonation, or a more complex product arising from reaction with solvent. The ligand 2-
C5H4NCH = NCH2-4-C6H4OH, L3, gave a complex [PtCl2(L3)], which formed a polymer in the solid state through
intermolecular OH..Cl hydrogen bonding.
Diimine
Structure
Pendent group
1. Introduction
During this work, the structure of the widely used complex trans-
[PtClMe(SMe2)2] was determined and is shown in Fig. 1. It has the ex-
The activation of dioxygen by electron-rich platinum(II) complexes
occurs most readily if there is a protic solvent or a ligand at platinum
containing an appended hydroxyl group, in a form of proton-coupled
electron transfer (Scheme 1) [1–8]. In this context, several studies
have been reported using ligands with appended phenol groups [6,8]. As
well as the role in activation of dioxygen (Scheme 1), the phenol group
may also form a hydrogen bond, with electron rich platinum(II) center
as acceptor [9]. This article reports the chemistry of related diimine li-
gands with appended anisole or phenol groups, especially focussing on
the structural differences between these complexes. As well as their role
in bond activation, diimine complexes of platinum are of current interest
for applications in fields ranging from cancer therapy to optoelectronic
and supramolecular materials [10].
pected square planar stereochemistry at platinum(II) with mutually
trans dimethylsulfide ligands. There are two independent but similar
molecules in the unit cell.
2.2. Complexes with the ligand L1
New platinum(II) complexes 4–6 were prepared by reaction of L1
with each of the precursor complexes 1 – 3 (Scheme 2). The complexes
were characterized by their 1H NMR spectra and by structure de-
terminations (Fig. 2, Table 1). Complex 5 was formed as a mixture of two
isomers 5a and 5b, with the methylplatinum group trans to the pyridyl
and imine group respectively. The ratio [5a]:[5b] = 5, and the major
isomer 5a was structurally characterized (Fig. 2b). In the 1H NMR
spectrum the methylplatinum resonances for 5a and 5b were at d 1.21,
2J(PtH) = 80 Hz, and 1.18, 2J(PtH) = 77 Hz, respectively. The coupling
constants from 195Pt to the imine proton and to the ortho proton of the
pyridyl group were particularly sensitive to the trans-influence of the
2. Results and discussion
2.1. Synthetic methods
3
trans group [16]. Thus, the coupling constant for the imine proton J
The synthetic procedure involved displacement of the dime-
(PtN = CH) = 104 Hz in 5a (trans to Cl) but 36 Hz in 5b (trans to Me),
while for the ortho pyridyl proton 3J(PtNCH) = 12 Hz in 5a (trans to Me)
but 52 Hz in 5b (trans to Cl) (Table 1). Similarly, in the structures (Fig. 2,
Table 1), the Pt-N bonds are longer when trans to methyl than when trans
to chloride.
thylsulfide ligands from a precursor molecule [Pt2Me4(μ-SMe2)2],
[PtClMe(SMe2)2] or [PtCl2(SMe2)2], 1 – 3 (Chart 1, [11,12]) with the
ligand 2-C5H4NCH = N-2-C6H4OMe, 2-C5H4NCH = N-2-C6H4OH, or 2-
C5H4NCH = N-4-C6H4OH, L1 – L3 (Chart 1, [13–15]).
* Corresponding author.
E-mail address: pudd@uwo.ca (R.J. Puddephatt).
https://doi.org/10.1016/j.ica.2021.120387
Received 8 March 2021; Accepted 6 April 2021
Available online 9 April 2021
0020-1693/© 2021 Elsevier B.V. All rights reserved.

A. Abo-Amer et al.
Inorganica Chimica Acta 522 (2021) 120387
Scheme 1. Reactions of platinum(II) complexes with dioxygen [4,8].
Fig. 1. A view of the structure of trans-[PtClMe(SMe2)2], showing 30% prob-
ability ellipsoids. Selected bond distances: Pt(1A)C(1A) 2.052(3), Pt(1A)Cl(1A)
2.4341(9), Pt(1A)S(1A) 2.2886(7), Pt(1A)S(2A) 2.2930(8) Å.
Chart 1. The platinum and ligand reagents.
2.3. Complexes with the ligands L2 and L3
The reactions of the precursor complex [PtCl2(SMe2)2], 3, with the
ligands L2 [14] and L3 [15] gave complexes 7 and 8 as major products
(Scheme 3). The complex [PtCl2(L3)], 8, was expected but the formation
of 7 involves both isomerization of the ligand L2 [14] to L2a [17,18]
(Chart 1) and deprotonation of L2a to give the anionic tridentate ligand
in [PtCl(L2a-H)], 7. Isomerization of free imines by transamination is
well known, and typically catalyzed by base, but it is less studied in
coordination complexes of imines [19–21]. From a reaction of the pre-
cursor complex [PtCl2(SMe2)2], 3, with the ligand L2 in a mixture of
acetone and ethanol, the unusual complex 9 was obtained. Complex 9
was isolated in very low yield as a yellow crystalline solid, and it was
characterized only by structure determination. From stoichiometry, its
formation involves incorporation of both an acetone and an ethanol
molecule to a likely intermediate complex [PtCl2(L2a)], with loss of a
water molecule (Scheme 3).
Scheme 2. Synthesis of the complexes [PtMe2(L1)], 4, [PtClMe(L1)], 5a and
5b, and [PtCl2(L1)], 6.
deprotonated ligand L2a acting as a tridentate N2O donor. The isomeric
form of the ligand is clearly shown by the difference between the amine
and imine distances N(2)-C(6) = 1.468(2) Å and N(2) = C(7) = 1.293(2)
The structure of complex 7 is shown in Fig. 3. Each platinum(II)
center is square planar with PtN2OCl coordination and with the
Å. The molecules form π-stacked dimers in the crystalline state, with the
shortest intermolecular contact to platinum as Pt(1)..N(2A) = 3.46 Å and
2

A. Abo-Amer et al.
Inorganica Chimica Acta 522 (2021) 120387
Scheme 3. Synthesis of complexes 7 – 9.
Fig. 2. The structures of the complexes (a) [PtCl2(L1)], (b) [PtClMe(L1)], (c)
[PtMe2(L1)], showing 30% probability ellipsoids.
Table 1
Selected bond distances (Å), angles (o) and coupling constants (Hz) in complexes
1 – 3.
Complex
4
5a
5b
6
Pt-N(py)
2.1018(11)
2.0861(12)
2.0373(14)b
2.0373(14)b
77.63(5)
85.31(6)b
36
2.0896(11)
2.0006(11)
2.1068(13)b
2.2776(6)a
79.29(5)
88.33(4)c
104
2.0110(18)
1.9896(17)
2.2973(6)a
2.2976(6)a
80.32(7)
90.29(2)a
96
Pt-N(imine)
Pt-X(t-py)
Pt-X(t-im)
N-Pt-N
X-Pt-X
3J(PtH)imine
36
52
3J(PtH)py
18
12
39
a
X = Cl; b X = Me; c Angle Me-Pt-Cl.
with the molecules related by inversion symmetry (Fig. 3). No isomer-
ization or deprotonation is observed in the formation of complex 8,
whose structure is shown in Fig. 4. The molecules of [PtCl2(L3)] form
supramolecular polymer chains by hydrogen bonding, with O(1)..Cl(2A)
= Cl(2)..O(1B) 3.1858(15) Å.
Fig. 3. The structure of the complex [PtCl(2-C5H4NCH2N = CH-2-C6H4O)], 7,
The unusual structure of complex 9 is shown in Fig. 5. The substit-
uent on N(2) is a benzodihydropyran derivative and there are chiral
centers at N(2) and at two of the carbon atoms of the 6-membered ring.
showing a π-stacked dimer. Selected distances: Pt(1)N(1) 2.0063(16), Pt(1)N(2)
1.9577(15), Pt(1)O(1) 1.9919(14), Pt(1)Cl(1) 2.3187(5), N(2)C(6) 1.468(2), N
(2)C(7) 1.293(2) Å. Equivalent atoms: x, y, z; 1-x, 1-y, 1-z.
3

A. Abo-Amer et al.
Inorganica Chimica Acta 522 (2021) 120387
Fig. 4. The structure of complex 8, with 25% probability ellipsoids, showing the supramolecular polymeric structure formed by intermolecular hydrogen bonding.
Selected bond distances: Pt(1)N(1) 2.0148(12), Pt(1)N(2) 2.0023(11), Pt(1)Cl(1) 2.2902(5), Pt(1)Cl(2) 2.3102(6), N(2)C(6) 1.2886(16), N(2)C(7) 1.4860(16) Å; H-
bond distance: O(1)..Cl(2A) = Cl(2)..O(1B) 3.1858(15) Å. Equivalent atoms: x, y, z; 2-x, y-1, z-1; x-1, y + 1, z + 1.
Chart 2. NMR labeling scheme.
ethanamine [L1], N-[(2-phenol)methylene]-2-pyridinemethanamine
[L2], and N-[(4-phenol)methylene]-2-pyridinemethanamine [L3]
1
were prepared using the literature methods [11–15]. H NMR spectra
were recorded using a Varian Inova 400 NMR or a Varian Inova 600
NMR spectrometer at room temperature. The NMR labeling scheme is
shown in Chart 2.
4.2. X-ray structure determinations
Single-crystal X-ray diffraction measurements were made using a
Bruker APEX-II CCD diffractometer with graphite-monochromated Mo
Kα (λ = 0.71073 Å) radiation. Single crystals of the complexes were
Fig. 5. The structure of complex 9, with 30% probability ellipsoids, showing
the supramolecular dimeric structure formed by intermolecular hydrogen
bonding. Selected bond distances: Pt(1)N(1) 2.000(3), Pt(1)N(2) 2.056(3), Pt
(1)Cl(1) 2.3105(11), Pt(1)Cl(2) 2.3006(10) Å; H-bond distance: N(2)..Cl(1A) =
Cl(1)..N(2A) 3.257(4) Å. Equivalent atoms: x, y, z; 1/2-x, y, 1-z.
immersed in paraffin oil and mounted on MiteGen micromounts. The
structures were solved using dual space methods and refined by the full-
matrix least-squares procedure of SHELXL [22]. Crystallographic data
are given in the crystallographic information files (CCDC
1946961–1946966, 2062853). The following specific comments are for
individual structures. In the structure of complex 2, there were two in-
dependent molecules in the unit cell and one was disordered between
[PtClMe(SMe2)2] and [PtCl2(SMe2)2]. Data in the text are for the
molecule that did not contain Me/Cl disorder, though it did have dis-
order of one SMe2 group. The checkcif for complexes 2 and 5 contained
A level alerts, which are likely due to inadequate absorption corrections,
with unassigned electron density close to the Pt atoms in each case.
The complex forms head to tail dimers by intermolecular hydrogen
bonding. The molecules in each dimer have the same chirality, but the
lattice contains equal numbers of dimers with the opposite chirality.
3. Conclusions
The results described above show that the reactions of ligands L1 or
L3 with platinum(II) complexes 1 – 3 can act as a model for the first step
in the more complex reactions of the ligand L2. In particular, the re-
actions of L1 or L3 with [PtCl2(SMe2)2], 3, give the simple complexes
[PtCl2(L1)] and [PtCl2(L3)] respectively, whereas the similar reaction of
L2 gives mostly [PtCl(κ3-N,N’,O-L2a-H)], 7, involving both isomeriza-
tion and deprotonation of the ligand L2.
4.3. Synthetic procedures
4.3.1. [PtMe2(L1)], 4. A mixture of [Pt2Me4(
μ
-SMe2)2] (151 mg,
0.263 mmol)) and
N-[(2-methoxyphenyl)methylene]-2-pyr-
idinemethanamine [L1] (119 mg, 0.525 mmol) in ether (30 mL) was
stirred for 4 h. to give the product as a red suspension. The product was
separated by filtration, washed with ether, dried in vacuo and then
recrystallized from CH2Cl2. Yield: 65% (154 mg). NMR in CD2Cl2: δ(1H)
4. Experimental section
4.1. General methods
3
3
3
= 9.15 [d, 1H, J(HH) = 6 Hz, J(PtH) = 18 Hz, H6], 8.85 [s, 1H, J
(PtH) = 36 Hz, H7], 8.04 [t, 1H, 3J(HH) = 8 Hz, H4], 7.56 [d, 1H, 3J
(HH) = 8 Hz, H3], 7.52 [dd, 1H, 3J(HH) = 8 Hz, 5 Hz, H5], 7.34 [t, 1H,
3J(HH) = 7 Hz, H4a], 7.29 [d, 1H, 3J(HH) = 7 Hz, H6a], 6.90–7.00 [m,
The compounds cis/trans-[PtCl2(SMe2)2], trans-[PtClMe(SMe2)2],
[Pt2Me4(µ-SMe2)2], N-[(2-methoxyphenyl)methylene] ꢀ 2-pyridinem
4

A. Abo-Amer et al.
Inorganica Chimica Acta 522 (2021) 120387
2H, H3a, H5a], 5.34 [s, 2H, 3J(PtH) = 36 Hz, H7a], 3.82 [s, 3H, Ome],
1.10 [s, 3H, 2J(PtH) = 84 Hz, PtMe], 1.06 [s, 3H, 2J(PtH) = 88 Hz,
PtMe].
Acknowledgments
We thank the NSERC (Canada) for financial support.
4.3.2. [PtClMe(L1)], 5. A mixture of trans-[PtClMe(SMe2)2] (264
mg, 0.714 mmol) and N-[(2-methoxy phenyl)methylene] ꢀ 2-pyr-
idinemethanamine [L1], (160 mg, 0.714 mmol) in diethyl ether (25
mL) was stirred for 2 h., to give the product as an orange suspension. The
product was separated by filtration, washed with n-pentane and dried in
vacuo as a yellow/orange solid. The product was recrystallized from
CH2Cl2. Yield: 58% (195 mg). NMR analysis indicated the presence of
two isomers 5a and 5b, with K = [5a]/[5b] = 5. NMR in CD2Cl2: 5a,
δ(1H) = 9.39 [d, 1H, 3J(HH) = 5 Hz, 3J(PtH) = 12 Hz, H6], 8.71 [s, 1H,
3J(PtH) = 104 Hz, H7], 8.02 [t, 1H, 3J(HH) = 8 Hz, H4], 7.74 [dd, 1H, 3J
(HH) = 8 Hz, 5 Hz, H5], 7.67 [d, 1H, 3J(HH) = 8 Hz, H3], 7.40 [t, 1H, 3J
(HH) = 7 Hz, H4a], 7.30 [d, 1H, 3J(HH) = 7 Hz, H6a], 6.95–7.05 [m, 2H,
H3a, H5a], 5.35 [s, 2H, 3J(PtH) = 32 Hz, H7a], 3.84 [s, 3H, OMe], 1.21
[s, 3H, 2J(PtH) = 80 Hz, PtMe]; 5b, δ(1H) = 9.12 [d, 1H, 3J(HH) = 5 Hz,
3J(PtH) = 52 Hz, H6], 8.52 [m, 1H, 3J(PtH) = 36 Hz, H7], 8.08 [t, 1H, 3J
(HH) = 8 Hz, H4], 7.74 [dd, 1H, 3J(HH) = 8 Hz, 5 Hz, H5], 7.57 [d, 1H,
3J(HH) = 8 Hz, H3], 7.52 [d, 1H, 3J(HH) = 7 Hz, H6a], 7.47 [t, 1H, 3J
(HH) = 7 Hz, H4a], 7.35 [t, 1H, 3J(HH) = 7 Hz, H5a], 6.95 [d, 1H, H3a],
5.39 [s, 2H, 3J(PtH) = 14 Hz, H7a], 3.84 [s, 3H, OMe], 1.18 [s, 3H, 2J
(PtH) = 77 Hz, PtMe].
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.ica.2021.120387.
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mg, 2.15 mmol) and N-[(2-phenol) methylene]-2-pyridinemethanamine
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(PtH) = 70 Hz, H7a], 7.89 [t, 1H, 3J(HH) = 7 Hz, H4], 7.45 [dd, 1H, 3J
(HH) = 7 Hz, 6 Hz, H5], 7.44 [d, 1H, 3J(HH) = 7 Hz, H3], 7.40 [d, 1H, 3J
(HH) = 8 Hz, H6a], 7.22 [t, 1H, 3J(HH) = 7 Hz, H4a], 7.01 [d, 1H, 3J
(HH) = 7 Hz, H3a], 6.66 [t, 1H, 3J(HH) = 7 Hz, H5a], 5.40 [s, 2H, H7].
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mg, 1.07 mmol) and ligand L3 (228 mg, 1.07 mmol) in ether (35 mL)
was stirred for 24 h. to give the product as a yellow precipitate, which
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Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
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