Di-manganese(0) carbonyl mediated azo bond cleavage and recombination reaction in azo-aromatic ligand: An effect of extended phenolate donor

Abstract

Three manganese complexes of the anionic azo-phenolate ligand [L1]−, obtained from the reaction between HL1 and Mn(OAc)2.4H2O (OAc = acetate) or Mn2(CO)10, are reported. The chosen manganese precursors are of different characters: the hydrated Mn(OAc)2 is a common source of Mn(II), while the carbonyl complex is known for its strong reducing character. The reaction between the Mn(II)-salt and HL1 (HL1 = 2,4-di-tert-butyl-6-(pyridin-2-ylazo)-phenol) produced a low-spin Mn(III) complex, ls-[MnIII(L1)−(L1)2−]; 1 via substitution and internal electron transfer (Mn(II) → [L1]−). The chemical oxidation of this molecular complex with I2 provided access to the cationic complex, ls-[MnIII(L1)−2]I3; [1]+[I3]− in quantitative yield. On the other hand, the reaction between Mn2(CO)10 and 4 equivalent of HL1 is complex. The reaction produced a heteroleptic high-spin Mn(III)-complex, hs-[MnIII(L1)−(L2)2−]; 2 in moderate yield. The ligand H2L2 (H2L2 = 2,2′-dihydroxo-3,5,3′,5′-tetra-tert-butylazobenzene) produced in situ via simultaneous azo-cleavage of [L1]− and recombination of two amido-phenolate fragments. The electronic structures of all three complexes coordinated by the two redox noninnocent ligands [L1/2]−/2− have been elucidated by using a host of physical methods: X-ray crystallography, magnetic susceptibility measurements, cyclic voltammetry, absorption spectroscopy and density functional theory (DFT). Their experimental structures are well reproduced by DFT calculations and support the overall electronic structures of the above compounds. The complexes 1, [1]+[I3]− and 2 have one, two and four unpaired electrons respectively, as evidenced by room temperature magnetic susceptibility measurements. The redox process between [1]+/1 is ligand centred which does not affect the trivalent oxidation state of the metal ion. The chemical conversion [L1]− → [L2]2− occurs only in aerobic condition and a rationale for the reaction is discussed.