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.

Optimized geometries. In the case of the oxidized and reduced species of 6, the energetically favored mS = 10 high spin state, mS = 12 high spin state, and the BS doublet spin state have been S7 taken into account, see Table S2. No significant differences in bond lengths and bond angles are observed comparing the neutral and oxidized species, see Table S3. Changes in the geometry upon reduction are much more obvious when compared to the neutral geometry. The Ru-N9, Ru-O1 bond lengths become significantly longer and the Fe-Fe, O1-Fe1, O1-Fe2 bond lengths (distances) become shorter, see Table S3. All the reduced species have a bent structure of the [Ru(NO)] moiety, with the Ru-N9-O2 angle of ~ 145° due to the Ru-(NO) character of the LUMO (see Figure S28).
Electronic structure characterization and the frontier orbitals. The iron center is oxidized in the case of the oxidized species 10 [6] + , see Tables S3, S4a, and S5. In the case of reduction, the bent 2 [Ru III -NO] 2+ ({RuNO} 7 ) moiety is the electron acceptor locus, see Tables S3, S4b, and S5.
The Fe1 center oxidation can be seen in the lower α dxz AO population, see Table S5. In addition, the Fe1 spin population is 3.5 while Fe2 population remains close to 4.0, see  Table S4b and Figure S28.
In addition, the Fe1 center has the spin flipped in the case of 2 [6] -, see Table S4b.
B3LYP/def2-SVP spind density and frontier α HOMO-1, HOMO, LUMO, LUMO+1, LUMO+2 orbitals for 10 [6] + and 10 [6]complex are depicted in Figure S28.      Figure S25 reproduces the 82 K Mössbauer spectrum recorded on a powder sample of 6 using a 0.06 T external magnetic field applied parallel to the -beam. A doublet is observed, the high velocity line presenting a larger linewidth and a reduced depth than the low velocity line. This asymmetry may be due to texture effects but may also evidence a slight difference between the two iron sites that are indeed not equivalent according to the X-ray structure. Simulations assuming either a unique iron site or two different iron sites in a 1:1 ratio are reproduced below.