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Title: Isolation, characterisation and reactivity of five-coordinate PtIV complexes
Author: Shaw, Paul Anthony
ISNI:       0000 0004 7224 1436
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 2017
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The oxidative addition to square planar dicyclometallated C^N^C PtII 16 valence electron complexes (C^N^C= 2,6-di(4-fluorophenyl)pyridine and 2-(4-fluorophenyl)-6-tertbutylpyridine) was studied with Cl2 (via PhICl2) and alkyl halides (e.g. MeI, BnBr and allylBr). The two-step nature of the process meant that PtIV 16 valence electron fivecoordinateintermediates could be identified. The product of oxidation was an octahedral 18 valence electron product with the new groups added mutually trans across the metal, with isomerisation seen with time. However, in the case of the chlorine-based oxidant, PhICl2, this was only part of the story, where further reactivity depended on the phosphine coordinated in the fourth coordination site. Oxidation of the PPh3 derivative gave a longlived five-coordinate intermediate (lasting about four hours) before combination with the second chloride. Complexes containing PBu3, PPr3 and P(o-Tol)3 trapped the fivecoordinate complex with agostic interactions, eventually leading to transcyclometallation products. In parallel to these studies, abstraction of X- from the octahedral PtIV oxidation products gave another path to the five-coordinate intermediates, and in most cases, they could be trapped with pyridine. Whilst we did not see any evidence of aryl-Cl coupling, we were able to isolate several three-coordinate monocyclometallated PtII products, which came about through aryl-R reductive elimination via a long-lived five-coordinate complex which were characterised in solution. Addition of a mild base led to the activation of the C(sp2 )-H bond, reforming the five-membered ring. Addition of another equivalent of the alkyl halide, and subsequent addition of AgBF4 showed 100% regioselectivity for reductive coupling of the R group to the ring substituted previously. When C^N^C was 2-(4-fluorophenyl)-6-tertbutylpyridine), the reductive elimination reaction was spontaneous under the reaction conditions for RX addition, showing complete selectivity of C(sp3 )-C(sp2 ) over C(sp3 )-C(sp3 ). Further to these reactions, the PBn3 derivative was also treated with PhICl2, which led to the formation of a PtIV tricyclometallated product. When the halide was removed from the tricyclometallated complex (again with AgBF4), reductive elimination of the coordinated benzyl and aryl group gave a three-coordinate complex with a strained nine-membered ring. Two distinct mechanisms were identified for the inversion of the ring, one of which included the breaking of the C-C bond to reform the five-coordinate intermediate. The fourth coordination site was open enough to allow chloride, CO and H2 to coordinate. Coordination of H2 was not seen directly, instead a product with no cyclometallation and a Pt-H bond was observed.
Supervisor: Not available Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QD Chemistry