Austin, Elliot John Winston; (1996) Halogen-bridged mixed-valence complexes of platinum: Solid-state NMR and resonance Raman spectroscopic studies. Doctoral thesis (Ph.D), UCL (University College London).

Text Halogen-bridged mixed-valence complexes of platinum - Solid-state NMR and resonance Raman spectroscopic studies.pdf Download (15MB)

Abstract

Solid-state NMR spectroscopy has been combined with resonance Raman spectroscopy in the study of platinum compounds belonging to the class known as Halogen-bridged Mixed-valence Metal Linear-Chain complexes (HMMCs). This class is characterised by the repeat unit of -[-X-MIVL4-X-MIIL4-]-, where M is the metal (Ni, Pd or Pt), X is the halogen (Cl, Br or I) and L4 represents the four equatorial metal-ligand bonds. The metals can bond to a combination of halogens and amines of varying denticity; the net charge of the chain is balanced by interstitial counterions. Much of the interesting chemistry of this class is associated with the intervalence charge transfer (IVCT: MIV -MII), which gives rise to an intense, polarised absorption, usually in the visible region. Excitation within this band can couple with motion along the chain to enhance the intensity of the Raman active symmetric (X-MIV-X) stretch, which is termed the v1 mode. The influence of metal or halogen on the properties of HMMCs is usually attributed to the degree of charge delocalisation that occurs on chain formation. The application of solid-state NMR spectroscopy to the analysis of HMMCs has been investigated. Studies on [Pt(2,3,2-tet)][Pt(2,3,2-tet)X2](CIO4)4 (2,3,2-tet = 3,7-diazanonane-1,9-diamine) show that it is possible to probe nuclei at natural abundance, although it is not very practical; subsequent solid-state 15N NMR studies used 15N-enriched ligands. Solid-state 15N and 195Pt NMR analyses of [Pt(en)X2][Pt(en)X4] (en = ethylenediamine) show that, contrary to expectation, the effect of chain formation is small and is similar for PtII and PtIV nuclei alike. The influence of counterions is examined in the study of the cationic chain complexes [Pt(en)2][Pt(en)2X2]Y4 (Y = CIO4, BF4 or PF6) and their monomers. The variation in 15N chemical shift with Y is accounted for by the hydrogen-bonding strength of the counterion. For Y = CIO4 or BF4, the relationship between monomers and chain is similar to that observed in the neutral-chain systems, but for Y = PF6 it is more complicated. The results of the analysis of the mixed-halide HMMCs [Pt(en)2][Pt(en)2Cl2-2[alpha]Br2[alpha]](CIO4)4 show that the number of [Cl-PtIV-Br] units in these species is close to that predicted for a purely random distribution. and hence much larger than that assumed previously. Simulated vibrational spectra have been computed and are compared with Raman and infrared spectra to help determine the most likely distribution of halogens. The unusual traits of some platinum ammine complexes have been examined. Solid-state 15N NMR spectra demonstrate that there are two distinct forms of cis-Pt(NH3)2Cl2, cis-Pt(NH3)2Br2 and trans-Pt(NH3)2CI2, and that the properties of some tetraammine complexes are dependent on the preparative conditions (solvent, ratios of reagents, etc.). The HMMCs cis-[Pt(NH3)2Br2][Pt(NH3)2Br4] and cis-[Pt(NH3)2I2][Pt(NH3)2I4] are reported for the first time, but the complex thought to be cis-[Pt(NH3)2Cl2][Pt(NH3)2CI4] is shown to contain significant trans impurities. A new kind of HMMC, made by treating [Pt(opd)2][superscript 0] (opd = ortho-phenylenediamine) with [Pt(en)2Cl2](CIO4)2, is reported. It contains many defects and its unusual vibrational spectra are discussed briefly.

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