New ligands for gold : bonding mode and structural complex characterisation

Strasser, Christoph Erik (2008-12)

Thesis (PhD (Chemistry and Polymer Science))--Stellenbosch University, 2008.

Thesis

Novel gold(I) trithiophosphite complexes were synthesised by utilising the ligands P(SR)3 (R = Me, Ph) and 1,2-bis(1,3,2-dithiaphospholan-2-ylthio)ethane (2L). Reaction with (tht)AuCl or (tht)AuC6F5 readily yielded the corresponding complexes (RS)3PAuX and 2L(AuX)2 (X = Cl, C6F5) as well as {Au[P(SMe)3]2}CF3SO3. Structural characterisation by X-ray diffraction revealed linear complexes in part associating by Au…Au and/or Au…S contacts, two polymorphs of one compound associating by either Au…S interactions or p-stacking was also obtained. (MeS)3PAuCl and (MeO)3PAuCl were found to be isostructural in the solid state. The complex chloro[tris(4-methylthiazol-2-yl)phosphane]gold, A, was used to probe the electronic influence tris(azol-2-yl)phosphanes exert upon gold(I) by substituting the chloride with various thiolates. In contrast to Ph3PAuCl, only NCS– and PhC(O)S– afforded stable compounds which could be attributed to a weaker donating capability of the tris- (azolyl)phosphane ligand class. The compounds A and chloro[tris(thiazol-2-yl)phosphane]- gold, B, were shown to crystallise in 4 new polymorphs and solvates bringing the total to an exceptional seven. Among the solid-state structures of A the rare instance of a polymorph and a thf solvate not exhibiting aurophilic interactions as opposed to the original structure were observed. Complex B was shown to crystallise in polymorphs where dimers are associated either by Au…Au or Au…Cl interactions but otherwise exhibit similar arrangements of the ligand, this set of polymorphs is unprecedented amongst gold complexes. An NMR experiment proved that tris(thiazolyl)phosphane complexes are subject to hydrolysis under alkaline conditions. A trimeric gold(I) heterometallacycle, obtained by reacting (tht)AuCl with 4,4-dimethyl-2-(2- thienyl)oxazoline deprotonated at C-5 of the thiophene ring, was structurally characterised. Intramolecular Au…S interactions were found to be present which precluded interaction of the gold atoms with other metal centres such as Me3CNCAuCl or AgNO3. A second solvate obtained additionally exhibits Au…Au interactions. The scope of uncommon bis-imine coordination to AuI was expanded by utilising 1,2-bis(1-imidazolylmethyl)-2,4,6-trimethylbenzene (2L) to synthesise the [Au2(μ-2L)2]2+ cation. The triflate salt forms the first porous crystal structure of gold and the co-crystallised solvent could be partially removed by evacuation at elevated temperatures. Utilising a ditopic phosphite ligand instead of the commonly used ditopic phosphane ligands, a new cationic species of the type [Au2(μ-2L)3]2+ was characterised in the solid state for the first time. Finally, employing 2-phenylthiazole and 1-(thiazol-2-yl)piperidine which can be deprotonated at C-5 of the thiazole ring, Fischer-type pentacarbonyltungsten carbeniate complexes were prepared and structurally characterised. Starting from these complexes, the analogous Fischertype methoxycarbene as well as carbyne complexes could be obtained by alkylation and formal oxide abstraction, respectively. The latter products readily formed dinuclear adducts with AuCl. A Fischer-type methoxycarbene could be transferred to AuI affording the first such gold(I) complex exhibiting Au…Au interactions in the solid state as well as a rare agostic Au…H interaction which was examined by low-temperature 1H NMR measurements. Transfer of the carbeniate ligand derived from 1-(thiazol-2-yl)piperidine to Ph3PAu+ afforded an aurated thiazole product (by an unprecedented loss of CO) which may be represented as a pseudoabnormal azolylidene complex owing to W(CO)5-coordination at a distant nitrogen. The carbeniate originating from 2-phenylthiazole, on the other hand, afforded, by rare W(CO)5- trapping and without CO-loss, a pseudo Fischer-type carbene complex. Carbene transfer to gold was complemented by the first transfers of rNHC ligands from chromium and tungsten to gold(I) affording a novel class of complexes, all of which were structurally characterised. This work bridges the unnatural divide created between Fischer and N-heterocyclic carbene complexes.

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