Synthesis of Cyclometallated Pt(II) Complexes of a Bulky Bipyridine Ligand

Treatment of 3,4,5,6-tetraphenyl-2,2′-bipyridine (LH) (7) with [PtCl2(dmso)2] (dmso = dimethyl sulfoxide) in chloroform afforded the cyclometallated square-planar platinum(II) complex [PtCl(L)] (8) and the octahedral Pt(IV) complex mer-[PtCl3(L)] (9) containing an anionic tridentate (C^N^N) ligand. The chloride of (8) can be easily replaced by trifluoroacetate to yield [(L)PtO2CCF3] (10). Reaction of (8) with the alkyne HC≡CC6H4Bu-4 resulted in the formation of [(L)PtC≡CC6H4Bu-4] (11). Treatment of (8) with 4-dimetylaminopyridine (DMAP) and PPh3 in dichloromethane, and the subsequent addition of NH4PF6 in methanol produced the salts [Pt(DMAP)(L)]PF6 (12) and [Pt(PPh3)(L)]PF6 (13), respectively. In a similar manner, reaction of (8) with 0.5 equiv. of bis(diphenyl-phosphino)methane (dppm) formed the dppm-bridged binuclear dicationic salt [(L)Pt(dppm)Pt(L)][PF6]2 (14).

Synthesis of the pyridyl-centred polyphenylene (7) (see Scheme 1) and its coordination chemistry particularly with rhodium and palladium have been reported (Ollangnier, et al., 2008;Perera & Draper, 2009;Perera, 2018). It is of interest to explore the chemistry of this pyridyl-centred polyphenylene ligand (7) with platinum centres in order to prepare possible photoactive luminescent complexes of the type [(C^N^N)PtX] (X = halide, trifluoroacetate, acetylides), [(C^N^N)Pt(L′)]PF6 (L′ = pyridine or phosphine) and a binuclear complex containing bridging ligands. In this paper we report studies carried out to devise synthetic routes to such complexes.

Methodology
All the experiments were carried out in an inert atmosphere (dinitrogen or argon). Elemental analyses were carried out on a Carlo Erba 1006 automatic analyser. IR spectra were recorded on a PerkinElmer Spectrum One FT-IR spectrometer fitted with a universal ATR sampling accessory. Mass spectral data were obtained using a micromass LCT electrospray mass spectrometer. NMR spectra were recorded on a Bruker DPX 400 spectrometer (operating frequencies for 1 H and 13 C are 400.13 and 100.62 MHz, respectively) or Bruker Avance Π 600 spectrometer (operating frequencies for 1 H and 13 C are 600.13 and 150.9 MHz). 1 H and 13 C chemical shifts () are in ppm with respect to TMS, and coupling constants (J) are in Hz. Single-crystal analyses were performed on a Bruker SMART APEX CCD diffractometer using graphite monochromised Mo-Kα (λ=0.71073Å) radiation and refinements were obtained using SHELXS software. Figures 3 and 4 feature the images obtained by using Mercury software. 3,4,5,6-Tetraphenyl-2,2′-bipyridine (7) was prepared according to a procedure recorded in relevant literature (Ollangnier, et al., 2008).

Results and Discussion
Treatment of (7) with [PtCl2(dmso)2] (dmso = dimethyl sulfoxide) in boiling chloroform resulted in the formation of a mixture of cyclometallated square-planar platinum(II) complex [PtCl(L)] (8) and an octahedral platinum(IV) complex mer-[(L)PtCl3] (9), both containing an anionic tridentate (C^N^N) ligand. The orange complex (8) was not very soluble in common deuterated solvents and in the proton NMR spectrum, the proton (H6) on the carbon adjacent to nitrogen was the most deshielded and the resonance appeared as a multiplet at 9.37 ppm. The complex (9) was characterised by X-ray crystallography and confirmed the presence of mer-geometry around the platinum centre ( Figure 3). In the proton NMR spectrum, the H6 proton appeared as a multiplet at 9.51 ppm. One can argue that the complex (5)  Scheme 1. Possible mechanism for the formation of complexes (8)-(9); and atom labelling used for the assignment of NMR data Square-planar platinum(II) complexes are being used to study substitution reactions with anionic ligands. The chloride of (8) can be easily replaced by trifluoroacetate ion to produce [(L)PtO2CCF3] (10) a red solid with 93% yield. The IR spectrum of (10) showed an IR band at 1659 cm − for the C=O group. The complex is quite soluble in organic solvents and it was well characterized. The 19 F NMR spectrum showed a singlet at -75.6 ppm for the fluorine nuclei in the CF3 group.
It is interesting to study the substitution of the chloride ion by neutral ligands such as pyridine and phosphine ligands to make the less soluble platinum(II) complex (8) (12) was characterised by X-ray crystallography (Figure 4). In its 1 H-NMR spectrum, the methyl proton resonance of NMe2 group appeared as a singlet at 3.25 ppm.   (12) Replacement of the labile chloride ligand by triphenylphosphine formed the salt (13) as a yellow solid of 94% yield. 31 P-{ 1 H} NMR spectrum of (13) showed a singlet at 27.6 ppm with platinum satellites, 1 J(PtP) = 4144 Hz, for PPh3 and a septet at -143.2 ppm for the PF6 − group. Replacement of two chloride groups from two complexes of (8) by one dppm ligand gave the dppm-bridged binuclear dicationic salt [(L)Pt(-dppm)Pt(L)](PF6)2 (14). The phosphorus resonance of (14) was 13.4 ppm with 1 J(PtP) = 4208 Hz.

Conclusions
The bulky ligand (7) containing four phenyl groups was converted into a quite insoluble cyclometallated terdentate complex [(C^N^N)PtCl]. However, it can be made more soluble by replacing the chloride ligand to produce the corresponding trifluoroacetate and acetylide. Introduction of a neutral ligand such as pyridine or phosphine generated more soluble salts of the type [(C^N^N)Pt(L′)]PF6 where L′ = PPh3, DMAP. A binuclear complex bridging two (C^N^N)Pt units with dppm was also prepared.
Ruether for recording NMR spectra; Dr Martin Feeney for providing mass spectral data; and Dr Thomas McCabe for determining crystal structures. An abstract of this study was presented at the Annual Sessions of the Institute of Chemistry, Ceylon in 2013.