Trinuclear iridium and rhodium complexes: the solution to a puzzle involving the multiple coordination possibilities of 1,8-naphthyridine-2-one ligands

Abstract

The multiple coordination possibilities of 1,8-naphthyridine-2-one (HOnapy) and 5,7-dimethyl-1,8-napthyridine-2-one (HOMe 2napy) ligands allow 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(μ-OMe) 2(cod) 2] (cod = 1,5-cyclooctadiene) with HOnapy and HOMe 2napy yield the compounds of the general formula [M( μ-OR 2napy) (cod)] n (M = Ir, R = Me ( 1a, 1b, H ( 2); M = Rh, R = Me ( 3a, 3b). They crystallise as inconvertible 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 spectroscopic data reveal that the yellow forms are mononuclear complexes whilst the dark-coloured crystals contain dinuclear complexes. In solution, the nuclearity of the complexes depends on the solvent. In addition both types of complexes are fluxional. The mixed-ligand complexes [M 2(μ-OMe 2napy) 2(CO) 2(cod)] M = Ir ( 5), Rh ( 6) have been isolated and characterised; they are found to be intermediates in the synthesis of the trinuclear complexes [M 3(μ 3-OMe 2napy) 2(CO) 2(cod) 2] + M = Rh ( 8), Ir ( 9). Reactions of [IrCl(CO) 2(NH 2- p-tolyl] with the complexes [Rh( μ-OR 2napy)(diolefin)] n followed by addition of a poor donor anion is a general one-pot synthesis for the hetertrinuclear complexes [Rh 2Ir( μ 3-OR 2napy) 2(CO) 2(diolefin) 2] + (RMe, diolefin = cod ( 10), tetrafluorobenzo-barrelene (tfbb) ( 11), 2,5-norbornadiene (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 intermediates of the type [(diolefin)Rh(μ-OMe 2napy) 2Ir(CO) 2]. Heteronuclear complexes which possess the core [RhIr 2] 3+, such as [RhIr 2( μ 3-OR 2napy) 2(CO) 2(cod) 2]BF 4 (RMe ( 14), H ( 15)), result from the reaction of 1 or 2 with [Rh(CO) 2S x ] + (S = solvent). The trinuclear complexes undergo two chemically reversible one-electron oxidation processes. The chemical oxidation of 10, 14 and 9 with silver salts gives the mixed-valence trinuclear radicals [Rh 2Ir( μ 3-OMe 2napy) 2(CO) 2(cod) 2] 2+ ( 16), [RhIr 2( μ 3-OMe 2napy) 2(CO) 2(cod) 2] 2+ ( 17) and [Ir 3( μ 3-OMe 2napy) 2(CO) 2(cod) 2] 2+ ( 18), which have been isolated as the perchlorate and tetrafluoroborate salts. The EPR spectrum of 16 indicates that the unpaired electron is essentially 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) A ̊ , α = 88.122(3), β = 83.924(3), γ = 87.038(3)°, Z = 4; 3b crystallises in the Pna2 i orthorhhombic space group, with a = 16.7541(3), b = 11.7500(8), c = 17.7508(7) A ̊ , Z = 4 ; complex 6 is packed in the monoclinic space group P2 i / c, a = 9.6371(1), b = 11.8054(4), c = 27.2010(9) A ̊ , β = 90.556(4)°, Z = 4; crystals of 10b are monoclinic, P2 1/ n, with a = 17.546(7), b = 13.232(6), c = 17.437(8) A ̊ , β = 106.18(1)°, Z = 4; crystals of 16a are triclinic, P-1, with a = 10.318(4), b = 12.562(6), c = 19.308(8) A ̊ , α = 92.12(8), β = 97.65(9) , γ = 90.68(5)°, Z = 2. The five different structures show the coordination versatility of the OMe 2napy 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).