Chemistry of azopyrimidine chelates of Re^VO, Re^IIIOPPh₃ and Re^VNAr

Abstract

The concerned azo ligands are L¹, L² and L³ (general abbreviation L) in which the aryl groups are phenyl, p-chlorophenyl and o,p-dichlorophenyl, respectively. The reaction of L¹ with KReO₄ in hot concentrated HCl affording [Re^VOCl₃(L²)] (2a), is attended with aryl chlorination. A similar reaction with L² furnished [Re^VOCl₃(L³)] (2b). The oxo complexes spontaneously reacted with PPh₃ and ArNH₂ furnishing [Re^III(OPPh₃)Cl₃(L)] (3) and [Re^V(NAr)Cl₃(L)] (4), respectively. Unlike 2 and 4, 3 displays large paramagnetic ¹H NMR shifts. The type 2 species exhibit an irreversible electrochemical Re^V→Re^VI oxidation near 2.2 V versus SCE. In type 4 species the Re^VI/Re^V couple occurs near 1.5 V and in type 3 species the Re^IV/Re^III couple lies near 1.2 V. Structure determination of 2b, [Re^III(OPPh₃)Cl₃(L¹)] (3a) and [Re^V(NC₆H₄Cl-p)Cl₃(L²)] (4b) have revealed uniform meridional geometry. In 2b the Re---O and in 4b the Re---N (imide) distances are 1.666(10) and 1.719(6) Å, respectively, consistent with triple bonding. The Re---N bonds lying trans to the oxo oxygen 2b and to the imide nitrogen in 4b are substantially lengthened. The Re---N(azo) lengths are 2.072(13), 1.957(9) and 2.003(6) Å in 2b, 3a and 4b, the corresponding N---N distances being 1.25(2), 1.318(11) and 1.305(9) Å, respectively. In 3a dπ(Re)-π (azo) back-bonding is clearly present and in 4b electron flow to the azo (p) orbital occurs via the route ArN→Re^V→azo(π^∗). The corresponding route in 2b is less effective because the oxo group is a poorer donor compared with the imido group. In the lattice of 4b noncovalent Cl-Cl and aromatic π-stacking are present.