Reductive nitrosylation of tetraoxometallates. Part 14. Generation and proton induced disproportionation of the Re(NO)3+ moiety. Synthesis, characterisation, and electrochemistry of novel hydroxo- and halogenonitrosyl complexes of rhenium

Abstract

In an aqueous alkaline reaction medium, [ReO₄]^– undergoes reductive nitrosylation by NH₂OH·HCl producing [Re(NO)(OH)₄]^–, isolated as APh₄(A = P or As) salts or as a neutral complex [Re(NO)(OH)₃(L–L)][L–L = 1,10-phenanthroline (phen) or 2,2′-bipyridine (bipy)]. The complexes show v(NO) vibrations at ca. 1 680 cm^–1 and their e.s.r. spectra suggest that, besides the hydroxo complex (C_4v), the L–L derivatives also possess axially symmetric structures. These complexes contain the Re(NO)³⁺[or{Re(NO)}⁵] group and show spin-only magnetic moments and almost identical e.s.r: profiles in the polycrystalline condition, both at 298 and 77 K, showing 〈g_av〉≈ 2.0. A well defined sextet due to metal hyperfine structure is observed of which the two outermost peaks are further split to a triplet arising from ¹⁴N (of NO) superhyperfine interaction. However, in frozen acetonitrile or dimethylformamide (dmf) the hyperfine structures collapse. These complexes of Re(NO)³⁺ disproportionate when boiled with HX (X = Cl or Br but not I, which forms only [ReI₆]^3– species), producing [Re(NO)X₅]^– or [Re(NO)X₄(phen)][i.e. Re(NO)⁴⁺], [Re(NO)₂X₄]^–[i.e. Re(NO)₂³⁺], and [Re₂X₈]^4–(Re₂⁴⁺). While a square-pyramidal structure (C_4v) can be suggested for the mononitrosyl halogeno complex anions from i.r. and electronic absorption spectra, these do not distinguish between axial–equatorial (C_s) or equatorial–equatorial (C_2v) dispositions of the two nitrosyl ligands in the dinitrosyl complexes. The seven-co-ordinate complexes [Re(NO)X₄(phen)] undergo thermal and electrochemical (irreversible) reduction corresponding to two one-electron steps. The six-co-ordinate Re(NO)⁴⁺ species, [Re(NO)Cl₅]^–, shows a similar electrochemical reduction process but the five-co-ordinate [Re(NO)(OH)₄]^– is electro-inactive. The cyclic voltammogram of the Re(NO)₂³⁺ species, [Re(NO)₂Cl₄]^–, reveals a reversible, Re(NO)₂³⁺–Re(NO)₂²⁺ couple at a formal potential of –0.04 V vs. a saturated calomel electrode {–0.42 V vs.[Fe(η-C₅H₅)₂]–[Fe(η-C₅H₅)₂]⁺}.