Isothiocyanate ligand derivatives of platinum(II) terpyridines.
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Date
2008
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Abstract
The novel compounds [Pt(trpy)(NCS)]SbF6 (1) and [Pt{4'-(Ph)trpy}(NCS)]SbF6 (2) where trpy = 2,2':6',2''-terpyridine, have been synthesised and characterised by means of elemental analysis, infrared and 1H NMR spectroscopy, and mass spectroscopy. Compounds 1 and 2 were also prepared with the labelled S13C15N¯ ion as co-ligand; their 13C and 15N NMR spectra recorded at room temperature in CD3CN show that the ambidentate ion is coordinated to the Pt atom mainly (~ 95%) through the N atom, but that a small amount of the S-bound isomer also co-exists in an acetonitrile solution. The synthesis of 1 is preceded by the isolation of yellow 1·CH3CN?Y (Y = yellow) for which the crystal structure has been determined by single crystal X-ray diffraction; this shows that the SCN¯ ion is linearly bound to the Pt atom through the N atom in the solid state, and that the cation is planar. The solvate rapidly loses acetonitrile to form maroon 1-M (M = maroon). The maroon compound exhibits 3MMLCT (metal-metal-to-ligand charge transfer) emission in the solid state as evidenced by a red-shift in the emission maximum from 653 nm at 473 K to 770 nm at 80 K. However, there are anomalous changes in the emission intensity below 200 K; this phenomenon is explained in terms of competitive emission by defect sites in the material. Interestingly, 1-M displays thermochromic behaviour (accompanied by phase changes at Ts > 473 K) that have been documented over the temperature range 80-548 K by means of photography, emission spectral and powder X-ray diffraction measurements. Compound 1-M also exhibits selective and reversible vapochromic behaviour with acetonitrile, DMF and pyridine – the solvates are yellow. We also report solvent specific changes in the emission spectra between 1-M and its acetonitrile, DMF and pyridine solvates. The thermo- and vapochromism of 1-M are linked to the making and breaking of metallophilic Pt...Pt interactions that occur when the planar cations slide in and out of different positions with respect to each other in a π-stack. Single crystals of compound 2 are isolated by desolvation of single crystals of 2·CH3CN. The single-crystal to single-crystal transformation is easily reversed by exposing 2 to vapours of acetonitrile, as confirmed by X-ray structure determinations of 2 and 2·CH3CN performed on the same single crystal. These show that the SCN¯ ion is linearly bound to the Pt atom through the N atom and that the cation is nearly planar. Significantly, there are only very small changes in the cation and anion atom positions between 2 and 2·CH3CN; thus, single crystals of 2 have a porous metal-organic structure with solvent accessible voids/channels. As such, 2 also sorbs methanol and acetone molecules from the vapour phase, the former without loss of single crystallinity, as confirmed by an X-ray crystal structure determination of 2·CH3OH. Single crystals of 2·(CH3)2CO were obtained by direct crystallization from acetone and an X-ray structure determination performed. Interestingly, a single crystal 2·(CH3)2CO desolvates under ambient conditions to give a single crystal of 2 with the original porous metal-organic crystal structure; on the other hand 2·CH3OH does not readily desolvate because of O─H◦◦◦S hydrogen-bonding. Compound 2 and its solvates are yellow, as expected, since the planar cations hardly move on solvent uptake – in marked contrast to the easily moved cations in 1─M – suggestions as to why are given. Finally, we report the solid state photoluminescence (measured at 77 K) of 2, 2·CH3CN, 2·CH3OH, 2·(CH3)2CO and 2·CH3OH_DS where DS denotes desolvation of the methanol solvate. Emission from 2 is characterised by dual emission from 3MLCT (metal-to-ligand charge transfer) and excimeric 3(π-π*) excited states; with the latter being the predominant origin of emission at 77 K. On the other hand, the 2·CH3CN and 2·(CH3)2CO solvates give well defined monomeric 3MLCT emission exclusively. The excimeric emission is representative of the cation packing arrangement in the crystal lattice, and the fact that it is not observed for 2·CH3CN and 2·(CH3)2CO is probably a result of an increase in the potential energy barrier to the formation of excimers when free space is occupied by CH3CN or (CH3)2CO included in the crystal lattice. Emission by 2·CH3OH and 2·CH3OH_DS is further complicated by 3MMLCT emission that arises because of dz2(Pt)-dz2(Pt) orbital interactions present in the solid. As a result multiple emission from 3MLCT, excimeric and 3MMLCT excited states is observed for 2·CH3OH and 2·CH3OH_DS.
Description
Thesis (M.Sc.)-University of KwaZulu-Natal, Pietermaritzburg, 2008.
Keywords
Platinum compounds., Ligands.