Tertiary phosphine binding to pyridylazole chelated rhenium via substitution in phosphine oxide precursors: geometrical preference, twin isomerization and effects of diphosphine spacer length and metal oxidation state

Abstract

The bimolecular reaction of [Re^VL(O)Cl₃] [L = 2-(2-pyridyl)benzoxazole (L¹), 2-(2-pyridyl)benzthiazole (L²)], 1 with excess diphosphine [Ph₂P(CH₂)_xPPh₂ (x = 1–4)] has furnished [Re^IIIL(OP(Ph)₂(CH₂)_xP(Ph)₂)Cl₃], 2 which is spontaneously converted in solution to [Re^IIIL(P(Ph)₂(CH₂)_xP(O)(Ph)₂)Cl₃], 4. The reaction of [Re^IIIL(OPMe_yPh_{3 − y})Cl₃], 3 with PMe_yPh_{3 − y} (y = 0–2) has afforded [Re^IIIL(PMe_yPh_{3 − y})Cl₃], 5. Oxidation of 2 and 3 by dilute nitric acid has furnished nitrates of the rhenium(IV) species, 2⁺ and 3⁺. Structure determination vis-à-vis spectral and electrochemical comparisons have revealed a meridional geometry for 2, 3, 2⁺, and 3⁺ and a facial geometry for 4 and 5. The transformation 2 → 4 is a twin isomerization (linkage-cum-geometrical), the geometrical part of which recurs in the conversion 3 → 5. Rate studies have revealed that the reaction 2 → 4 is intramolecular in nature. It is initiated by nucleophilic attack of the metal by the dangling phosphine function. The process slows down nearly exponentially as the diphosphine spacer length (x) increases. The oxidised complex 2⁺ does not isomerize.