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Donor-acceptor properties of phosphorus-sulphur compounds Le Geyt, Michael Robert

Abstract

The cage molecule -P₄S₃ has been examined by a variety of spectroscopic and chemical methods in order to determine some information about its stability, its distribution of electronic density, and also about its donor-acceptor properties. P₄S₃ has been shown to be a poor σ-donor as indicated by its lack of reactivity toward either BC1₃ or BF₃, its failure to be protonated by HBF₄, and its failure to be quaternized by either MeSO₃F or Et₃OBF₄. It does however show evidence of acceptor properties by its reaction with certain thiophiles. Numerous attempts were made to prepare transition metal complexes of P₄S₃. The formation of complexes is most favoured with the later and heavier transition metals in low oxidation states. In these complexes, optimum donor-acceptor interaction is expected and results reveal that P₄S₃ is a good π-acceptor. The infrared and Raman spectra of P₄S₃ were obtained and complete vibrational assignments have been made. The spectra are in substantial agreement with recently published information but there is some difference in interpretation. Tetraphosphorus trisulphide diiodide, α-P₄S₃I₂ has been shown to behave chemically as little more than a molecular association of and I2« Attempts to prepare the cation P₄S₃²⁺ were unsuccessful and experiments designed to replace iodide by other groups such as -CN, -SON and -OEt invariably resulted in loss of molecular iodine and regeneration of P₄S₃. Continuing experimental difficulties with P₄S₃ and its complexes brought about a change in the direction of this research. Instead, various cyclic organophosphorus- sulphur compounds were examined chemically and spectro- scopically, a major portion of the work involving the interrelationship between the compounds Ph₄P₄S, Ph₃P₃S₃ and Ph₂P₂S₄ At the time that this work was begun, the structures of Ph₄P₄S and Ph₃P₃S₃ were uncertain and the establishment of these structures was naturally essential for an understanding of their chemistry. The crystal structure of Ph₄P₄S has been determined, and. the molecule, contrary to literature reports, consists of a non-planar five-membered P₄S ring. The structure of Ph₃P₃S₃ has been elucidated by interpretation of the phosphorus and fluorine n.m.r. spectra of its para-fluorophenyl analogue. Ph₃P₃S₃ consists of an un symmetrical five-membered P₃S₂ ring, again contrary to literature reports, the remaining sulphur atom being bonded exocyclically to one of the phosphorus atoms. The meta- and para-fluorophenyl analogues of the compounds Ph₄P₄S, Ph₃P₃S₃ and Ph₂P₂S₄ and also those of (PhP)[sub n] and (PhPI)₂, have been prepared; using the methods of Taft et al, inductive and resonance parameters have been calculated from the ¹⁹F n.m.r. shifts. Acceptor properties (both σ- and π-) are greatest in those compounds richest in sulphur and Ph₂P₂S₄, in particular, exhibits very large inductive and conjugative withdrawal. The infrared and Raman spectra of the phenyl and p-fluorophenyl derivatives have been obtained and a number of vibrational assignments have been made. The mass spectra of the phenyl and p-fluorophenyl derivatives have been obtained, and considerable rearrangement of molecular fragments is a common feature.

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