TRINUCLEAR IRIDIUM AND RHODIUM COMPLEXES - THE SOLUTION TO A PUZZLE INVOLVING THE MULTIPLE COORDINATION POSSIBILITIES OF 1,8-NAPHTHYRIDINE-2-ONE LIGANDS

The multiple coordination possibilities of 1,8-naphthyridine-2-one (HO napy) and 5,7-dimethyl-1,8-naphthyridine-2-one (HOMe(2)napy) ligands a llow the synthesis of a variety of tri-, di- and mononuclear complexes , showing fluxional behaviour and frequent exchange of the coordinated ML(2) fragments. Thus, reactions of [M(2)(mu-OMe)(2)(cod)(2)] (cod=1, 5-cyclooctadiene) with HOnapy and HOMe(2)napy yield the compounds of t he general formula [M(mu-OR(2)napy) (cod)](n) (M=Ir, R=Me (1a, 1b), H (2); M=Rh, R=Me (3a, 3b). They crystallise as interconvertible yellow (a) and purple/orange (b) forms and also show a puzzling behaviour in solution. X-ray diffraction studies on both forms (3a, 3b) and spectro scopic data reveal that the yellow forms are mononuclear complexes whi lst the dark-coloured crystals contain dinuclear complexes. In solutio n, the nuclearity of the complexes depends on the solvent. In addition both types of complexes are fluxional. The mixed-ligand complexes [M( 2)(mu-OMe(2)napy)(2)(CO)(2)(cod)] (M=Ir (5), Rh (6)) have been isolate d and characterised; they are found to be intermediates in the synthes is of the trinuclear complexes [M(3)(mu(3)-OMe(2)napy)(2)(CO)(2)(cod)( 2)](+) (M=Rh (8), Ir (9)). Reactions of [IrCl(CO)(2)(NH2-p-tolyl)] wit h the complexes [Rh(mu-OR(2)napy)(diolefin)](n) followed by addition o f a poor donor anion is a general one-pot synthesis for the heterotrin uclear complexes [Rh2Ir(mu(3)-OR(2)napy)(2)(CO)(2)(diolefin)(2)](+) (R =Me, diorefrn=cod (10), tetrafluorobenzo-barrelene (tfbb) (11), 2,5-no rbomadiene (nbd) (12); R=H, diolefin=cod (13)). This synthesis follows a stepwise mechanism from the mononuclear to the trinuclear complexes in which mixed-ligand heterodinuclear complexes are involved as inter mediates of the type [(diorefin)Rh(mu-OMe(2)napy)(2)Ir(CO)(2)]. Hetero nuclear complexes which possess the core [RhIr2](3+), such as [RhIr2(m u(3)-OR(2)napy)(2)(CO),(cod)(2)]BF4 (R=Me (14), H (15)), result from t he reaction of 1 or 2 with [Rh(CO)(2)S-x](+) (S=solvent). The trinucle ar complexes undergo two chemically reversible one-electron oxidation processes. The chemical oxidation of 10, 14 and 9 with silver salts gi ves the mixed-valence trinuclear radicals [Rh2Ir(mu(3)-OMe(2)napy)(2)( CO)(2)(cod)(2)](2+) (16), [RhIr2(mu(3)-OMe(2)napy)(2)(CO)(2)(cod)(2)]( 2+) (17) and [Ir-3(mu(3)-OMe(2)napy)(2)(CO)(2)(cod)(2)](2+) (18), whic h have been isolated as the perchlorate and tetrafluoroborate salts. T he EPR spectrum of 16 indicates that the unpaired electron is essentia lly in an orbital delocalised on the metals. The molecular structures of the complexes 3a, 3b, 6, 10b and 16a are described. Crystals of 3a are triclinic, P-1, with a=9.7393(2), b=14.0148(4), c=16.0607(4) Angst rom, alpha=88.122(3), beta=83.924(3), gamma=87.038(3)degrees, Z=4; 3b crystallises in the Pna2(1) orthorhombic space group, with a=16.7541(3 ), b=11.7500(8), c=17.7508(7) Angstrom, Z=4; complex 6 is packed in th e monoclinic space group P2(1)/c, a=9.6371(1), b=11.8054(4), c=27.2010 (9) Angstrom, beta=90.556(4)degrees, Z=4; crystals of 10b are monoclin ic, P2(1)/n, with a=17.546(7), b=13. 232(6), c=17.437(8) Angstrom, bet a=106.18(1)degrees, Z=4; crystals of 16a are triclinic, P-1, with a=10 .318(4), b=12.562(6), c=19.308(8) Angstrom, alpha=92.12(8), beta=97.65 (9), gamma=90.68(5)degrees, Z=2. The five different structures show th e coordination versatility of the OMe(2)napy molecule as ligand, which behaves as a N,N'-chelating (3a), bidentate N,O-donor (3b, 6), or as a tridentate N,N',O-donor bridging ligand (10b, 16a).