N.M.R. Investigation of Dynamic Molecules Containing Elements of Groups V or VI

Green, Thomas Houston (1975) N.M.R. Investigation of Dynamic Molecules Containing Elements of Groups V or VI. PhD thesis, University of Glasgow.

Abstract

a) Chalcogen inversion in (Et2X)2MHal2 complexes. The objective of this work was to establish pyramidal chalcogen (S,Se and Te) inversion and to ascertain which factors might influence the inversion rate in a series of transition metal complexes. Monodentate chalcogen complexes of the type (Et2X)2MHal2 (X = S,Se or Te; M = Pt or Pd) were examined by variable temperature NMR spectroscopy the issues arising from this choice forming the basis of further research. Both sulphur and selenium invert when coordinated to platinum, this being the first ever report of pyramidal selenium inversion. The rate of sulphur inversion depends on the nature of the ligand trans to the inverting centre. The geometries of the complexes in solution were established by dipole-moment measurements, which revealed in almost every case a trans-configuration. Contrary to popular belief, the (Et2Te)2MHal2 complexes are trans in solution. A cis arrangement of inverting centres was effected by the synthesis of (EtSeC2H4SeEt)MHal2 complexes and the VTNMR results showed that the rate of selenium inversion also depends on the nature of the trans-ligand. The retention of 195pt-x-C-H coupling (195Pt, I = 1/2, 33.7% abundant) after coalescence served to differentiate between a pyramidal inversion and a ligand exchange mechanism in the platinum monodentate complexes, but the absence of a suitable magnetic isotope of palladium of sufficient abundance meant that pyramidal chalcogen inversion in palladium complexes alone could not be unequivocally established. Addition of excess ligand to these complexes showed that ligand exchange was not involved at these temperatures and unambiguous sulphur inversion in the (Et2S)2PdHal2 complexes was demonstrated. The order of ease of ligand exchange was shown to be Et2Te " Et2Se > Et2S. The pronounced effect of traces of diethyltelluride on the spectra of the (Et2Te)2PtHal2 complexes offered a rationalisation of the high temperature spectra of these complexes when no excess ligand had been added. 195Pt-Te-C-H coupling was never observed at temperatures above those of coalescence, almost certainly due to a ligand exchange-recombination mechanism as a result of the decomposition of these complexes at high temperatures. The question of whether or not tellurium inverts led to many attempts to produce a suitable tellurium-containing complex, culminating in the synthesis of cis-(Et2Te)2Pt(p-tolyl)2 which showed VTNMR characteristics consistent with pyramidal tellurium inversion. This represents the first unambiguous proof of such a mechanism. The order of ease of inversion within the chalcogens is S > Se > Te, and palladium complexes have faster inversion rates than analogous platinum complexes. Lastly, a series of benzyl complexes, trans-(Bz2S)2PdHal2 was examined by VTNMR with the intention of calculating Free Energies of Activation for the inversion process by computer methods. These were not obtained, since at low temperatures other signals and coalescences were observed. Rationalisations in terms of cis-isomers are presented. b) Hindered rotation about P-N bonds. A series of compounds of the type Ph2P-N(R)-P(Ph)Cl (I) was prepared, (R = Me, Et, Prn, Pri or Bu t). 31P and 1H{31P} NMR spectra were obtained and from the coalescence temperatures it was established that the torsional barriers are dependent on the steric requirement of the R group on nitrogen -the order of ease of rotation being R = Me>Et"Prn > Pr i> Bu t. The variation in P-N-P coupling constant with temperature is related to the two dihedral angles between the lone pairs on both phosphorus atoms and the P-N bonds of the other phosphorus atoms. This enabled low temperature conformational preferences to be assigned. For (I), (R = Me, Et, Prn), this is the (0,0) conformer (ie. two 0

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Keywords: Inorganic chemistry, Molecular chemistry
• Date of Award: 1975
• Depositing User: Enlighten Team
• Unique ID: glathesis:1975-78694
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 30 Jan 2020 15:01
• Last Modified: 30 Jan 2020 15:01
• URI: https://theses.gla.ac.uk/id/eprint/78694