Electronic structure of Schiff-base peroxo{2,2'-[1,2-phenyl-enebis(nitrilo-methanylyl-idene)]bis-(6-meth-oxy-phenolato)}titanium(IV) monohydrate: a possible model structure of the reaction center for the theoretical study of hemoglobin

The Ruthenium Nitrosyl Moiety in Clusters: Trinuclear Linear μ-Hydroxido Magnesium(II)-Diruthenium(II), μ3-Oxido Trinuclear Diiron(III)-Ruthenium(II), and Tetranuclear μ4-Oxido Trigallium(III)-Ruthenium(II) Complexes

The ruthenium nitrosyl moiety, {RuNO}⁶, is important as a potential releasing agent of nitric oxide and is of inherent interest in coordination chemistry. Typically, {RuNO}⁶ is found in mononuclear complexes. Herein we describe the synthesis and characterization of several multimetal cluster complexes that contain this unit. Specifically, the heterotrinuclear μ₃-oxido clusters [Fe₂RuCl₄(μ₃-O)(μ-OMe)(μ-pz)₂(NO)(Hpz)₂] (6) and [Fe₂RuCl₃(μ₃-O)(μ-OMe)(μ-pz)₃(MeOH)(NO)(Hpz)][Fe₂RuCl₃(μ₃-O)(μ-OMe)(μ-pz)₃(DMF)(NO)(Hpz)] (7·MeOH·2H₂O) and the heterotetranuclear μ₄-oxido complex [Ga₃RuCl₃(μ₄-O)(μ-OMe)₃(μ-pz)₄(NO)] (8) were prepared from trans-[Ru(OH)(NO)(Hpz)₄]Cl₂ (5), which itself was prepared via acidic hydrolysis of the linear heterotrinuclear complex {[Ru(μ-OH)(μ-pz)₂(pz)(NO)(Hpz)]₂Mg} (4). Complex 4 was synthesized from the mononuclear Ru complexes (H₂pz)[trans-RuCl₄(Hpz)₂] (1), trans-[RuCl₂(Hpz)₄]Cl (2), and trans-[RuCl₂(Hpz)₄] (3). The new compounds 4–8 were all characterized by elemental analysis, ESI mass spectrometry, IR, UV–vis, and ¹H NMR spectroscopy, and single-crystal X-ray diffraction, with complexes 6 and 7 being characterized also by temperature-dependent magnetic susceptibility measurements and Mössbauer spectroscopy. Magnetometry indicated a strong antiferromagnetic interaction between paramagnetic centers in 6 and 7. The ability of 4 and 6–8 to form linkage isomers and release NO upon irradiation in the solid state was investigated by IR spectroscopy. A theoretical investigation of the electronic structure of 6 by DFT and ab initio CASSCF/NEVPT2 calculations indicated a redox-noninnocent behavior of the NO ancillary ligand in 6, which was also manifested in TD-DFT calculations of its electronic absorption spectrum. The electronic structure of 6 was also studied by an X-ray charge density analysis.

Mononuclear trans-[Ru(OH)NO(Hpz)₄]²⁺ proved to be a source of μ-hydroxido and μ₃- and/or μ₄-oxido bridging groups, which could be incorporated into the heterotrinuclear complexes {[Ru(μ-OH)(μ-pz)₂(pz)(NO)(Hpz)]₂Mg}, [Fe₂RuCl₄(μ₃-O)(μ-OMe)(μ-pz)₂(NO)(Hpz)₂], and [Fe₂RuCl₃(μ₃-O)(μ-OMe)(μ-pz)₃(MeOH)(NO)(Hpz)][Fe₂RuCl₃(μ₃-O)(μ-OMe)(μ-pz)₃(DMF)(NO)(Hpz)] (7·MeOH·2H₂O) and the heterotetranuclear μ₄-oxido complex [Ga₃RuCl₃(μ₄-O)(μ-OMe)₃(μ-pz)₄(NO)]. The structures obtained were all confirmed by SC-XRD, including an X-ray charge density analysis that revealed the electronic structure of the RuFe2 cluster. Two of these nitrosyl complexes underwent photoinduced isomerization with generation of the nitrosyl linkage isomers MS1 and MS2, as revealed by IR spectroscopy at 10 K.