The reaction between Pt- and Pd-cis di-chloro complexes and 4,4′-diethynylbiphenyl: synthesis and characterisation of a ‘zigzag’ metal/poly-yne polymer

doi:10.1016/S0032-3861(01)00646-2

Polymer

Volume 43, Issue 3, February 2002, Pages 655-663

https://doi.org/10.1016/S0032-3861(01)00646-2 Get rights and content

Abstract

The synthesis of poly-yne polymers containing transition metals inserted in the main chain has been attempted by reacting a dialkyne molecule, 4,4′-diethynylbiphenyl (or DEBP), with [PtCl₂(dppe)] and [PdCl₂(dppe)], the platinum- and palladium-cis square-planar di-chlorine complexes containing diphenylphosphine ethane (dppe) as bidentate ligand. The aim of this work was to obtain organometallic polymers ([Pt(dppe)DEBP]_n and [Pd(dppe)DEBP]_n, respectively) having an all-cis ‘zigzag’ structure, by formation of a σ-acetylide bond between the transition metal complexes and the dialkyne molecule. When [PtCl₂(dppe)] was reacted with DEBP, the formation of a chlorine-terminated [Pt(dppe)DEBP]_n oligomer was evidenced; in the reaction involving the Pd(II) complex, on the other hand, [PdCl₂(dppe)] seems to catalyse the polymerisation of DEBP via opening of the triple bond, producing a poly-DEBP polymer containing Pd(II) atoms inserted in the main chain.

Introduction

Organometallic π-conjugated polymers containing σ-bonded transition metals in the main chain form a new class of molecular materials that has been the object of large interest in connection with the possible applications in the field of molecular electronics [1], [2], [3]. The delocalisation of the π-system involves the metal d-orbitals [4], producing peculiar physical properties, such as photoluminescence [5] and third-order non-linear optical (NLO) properties [6], which make these materials promising candidates for the preparation of electronic devices.

Various types of organometallic polymers containing transition metals σ-bonded to carbon atoms in the main chain have been prepared, as reviewed by Hagihara et al. [7] and more recently by Nguyen et al. [8]. Σ-bonds between acetylide groups and transition metals are stabilised, with respect to other M–C σ-bonds, by the favourable interaction between the metal d-orbitals and the π-orbitals of the ethynyl carbons, which strengthens the bond [7]; ligands such as tertiary phosphines produce a further stabilisation of the bond. Metal–poly-yne polymers can be easily produced by a dehydroalogenation reaction between the corresponding dichloro complexes and terminal dialkynes, according to the reaction scheme: $MeCl_{2} (PR_{3})_{2} + HC  C – S – C  CH →[– Me (PR_{3})_{2} – C  C – S – C  C –]_{n}$ where PR₃=tertiary phosphine, S=organic spacer, Me=Pd, Pt. The reaction, catalysed by cuprous halogenydes, takes place in a basic environment (diethylamine is used as solvent) and has been successfully employed in the preparation of Pt– and Pd–poly-yne polymers containing various phosphine ligands and organic spacers. For instance, some of us reported the preparation of Pt(II) and Pd(II) polymeric complexes containing DEBP as the organic spacer [9]; when triphenylphosphine complexes were used as precursors, insoluble [(PPh₃)₂MeDEBP–]_n polymers were obtained; the use of tributylphosphine complexes, on the other hand, leads to soluble polymers [(PBu₃)₂MeDEBP]_n (Me=Pd, Pt) that can be cast in the form of thin films. In all the reported systems the central metal is present in square-planar trans configuration, generating a rigid rod-like structure of the polymer chain. Near-edge X-ray absorption fine structure (NEXAFS) investigations on thin films of [(PBu₃)₂MeDEBP]_n evidenced the ordered structure of the polymer backbone and the unexpected alignment of the polymer chains even in thick films [10]. More recently, the preparation of soluble Pd– and Pt–poly-yne polymers containing thiophene spacers has been reported [11].

It is worth noticing that in the reported experimental conditions, for all the investigated systems, when a cis dichloro complex is used as precursor a cis–trans isomeristaion occurs and the central metal atom is always found in trans configuration in the product polymer, which has therefore a linear rod-like structure. In the present paper, we report an investigation on the reactions between DEBP and two square planar chelate complexes [PtCl₂(dppe)] and [PdCl₂(dppe)] containing bis(diphenylphosphino)ethane Ph₂PCH₂CH₂PPh₂ (dppe) as the ligand. It was our aim to obtain metal-intercalated poly-yne polymers containing the metal blocked in the cis configuration by the bidentate ligand and having therefore the ‘zigzag’ structure shown in Fig. 1. These polymers might be considered for the tailoring of third order NLO and photoluminescence properties. However, this goal was only partially achieved, because unexpected results were obtained; apparently, a ‘locked’ cis configuration of the precursor complex [MeCl₂(dppe)] leads to more complex reaction pathways, particularly for Me=Pd. The chemical structures of the metal–poly-yne polymers were elucidated with the help of X-ray photoelectron spectroscopy (XPS) measurements.

Section snippets

Instrumentation

Infrared spectra were recorded on nujol mulls using KBr (4000–400 cm⁻¹) optical cells on a Bruker Vector 22 FTIR interferometer.

UV spectra were recorded from CHCl₃ solutions on a Perkin–Elmer Lambda 5 spectrophotometer.

Cleanliness of the samples was checked by thin layer chromatography analysis on silica gel using CHCl₃ or tetrahydrofurane as solvent.

Elemental analyses were carried out at the Chemistry Department of the University ‘La Sapienza’, Rome.

X-ray photoelectron spectroscopy (XPS)

Results and discussion

The reactions of DEBP with the Pt- and Pd-complexes yielded products having a completely different structure; therefore they will be examined separately.

Conclusions

The synthesis of new polymeric all-cis square planar organometallic Pt- and Pd-complexes, having a zigzag structure and containing diphenylphospino ethane as bidentate ligand and diethynylbiphenyl as organic spacer, [Pt(dppe)DEBP]_n and [Pd(dppe)DEBP]_n, was attempted by reacting the corresponding dichloro complexes, [Pt(dppe)Cl₂] and [Pd(dppe)Cl₂], with the dialkyne DEBP.

The Pt-material is actually a chlorine terminated oligomer, as evidenced by XPS spectroscopy, containing on average 4–5

Acknowledgements

The authors wish to thank Prof. M.V. Russo for helpful discussion. Financial support from MURST and CNR MSTA II is gratefully acknowledged.

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The reaction between Pt- and Pd-cis di-chloro complexes and 4,4′-diethynylbiphenyl: synthesis and characterisation of a ‘zigzag’ metal/poly-yne polymer

Abstract

Introduction

Section snippets

Instrumentation

Results and discussion

Conclusions

Acknowledgements

J Organomet Chem

Synth Met

Polymer

J Electron Spectrosc Relat Phenom

J Electron Spectrosc Relat Phenom

Chem Phys Lett

Polymer

Polymer

Polymer

J Chem Soc Chem

J Chem Phys