Chemistry of azopyrimidine chelates of Re<SUP>V</SUP>O, Re<SUP>III</SUP>OPPh<SUB>3</SUB> and Re<SUP>V</SUP>NAr

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 [ReVOCl3(L2)] (2a), is attended with aryl chlorination. A similar reaction with L2 furnished [ReVOCl3(L3)] (2b). The oxo complexes spontaneously reacted with PPh3 and ArNH2 furnishing [ReIII(OPPh3)Cl3(L)] (3) and [ReV(NAr)Cl3(L)] (4), respectively. Unlike 2 and 4, 3 displays large paramagnetic 1H NMR shifts. The type 2 species exhibit an irreversible electrochemical ReV&#8594;ReVI oxidation near 2.2 V versus SCE. In type 4 species the ReVI/ReV couple occurs near 1.5 V and in type 3 species the ReIV/ReIII couple lies near 1.2 V. Structure determination of 2b, [ReIII(OPPh3)Cl3(L1)] (3a) and [ReV(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) &#197;, 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) &#197; in 2b, 3a and 4b, the corresponding N---N distances being 1.25(2), 1.318(11) and 1.305(9) &#197;, respectively. In 3a d&#960;(Re)-&#960; (azo) back-bonding is clearly present and in 4b electron flow to the azo (p) orbital occurs via the route ArN&#8594;ReV&#8594;azo(&#960;&#8727;). 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 &#960;-stacking are present