Synthesis and characterization of meso-[Ru(NO)Cl(dioxocyclam)] and the 1H NMR comparison with [MII(dioxocyclam)] complexes (MII=NiII and PdII) (dioxocyclam=1,4,8,11-tetraazacyclotetradecane-5,7-dione)

trans-[Ru(NO)X(dioxocyclam)], X=Cl− and OH−, dioxocyclam=(1,4,8,11-tetraazacyclotetradecane-5,7-dione) have been prepared and characterized by NMR, IR and ESI-MS techniques. The trans-[Ru(NO)Cl(dioxocyclam)] shows the nitrosyl stretch at 1846 versus 1875 cm−1 in the cyclam analogue, indicative of strong π-donation from the deprotonated dioxocyclam ligand to the RuII center and, in turn, to the NO+ group. Upon coordination, the dioxocyclam ligand no longer undergoes facile H/D exchange of the NH and C-6 protons. The methylene protons exist in symmetric patterns indicative of the meso isomer with both C-12 and C-14 methylene carbons projecting below the RuN4 plane toward the side of the axial Cl− for the major product. A lesser amount of the rac isomer, wherein the C-12 and C-14 methylene carbons reside on opposite sides of the RuN4 coordination plane, is detected. mmff94 calculations determined that the meso form is more stable than rac by 3.33 kcal mol−1. The RuII–N(amide) bonds were calculated as having normal lengths (2.07 Å), but the calculated RuII–N(amine) bonds are elongated to 2.35 and 2.36 Å. (It is known that molecular mechanics can overestimate bond lengths for second and third row metal centers by 0.10–0.15 Å). Even after correcting for the over-calculated bond distance factor, it is seen that the amine–ruthenium distances are longer, and hence the bonds are weaker, than for ‘normal’ RuII–amines. The presence of the axial Cl− is established by the ESI-MS ion fragments at m/z=394 for {d2-[Ru(NO)Cl(dioxocyclam)]H}+. Ions for {[Ru(NO)(H2O)(dioxocyclam)]}+ (m/z=377) and {[Ru(NO)(dioxocyclam)]}+ (m/z=359), the latter from loss of HCl or H2O from the 394 and 377 ions, are detected. 1H and 13C NMR data for meso-[Ru(NO)Cl(dioxocyclam)] are compared to the meso-[PdII(dioxocyclam)] complex, and to the NiII derivative that was synthesized at lower temperature than in previous literature reports. The meso-[NiII(dioxocylam)] complex was identified previously. In the present work, it is shown that below 50°C the product for the NiII system is rac-[NiII(dioxocyclam)]. The assignments for the {Ru(NO)}3+, NiII and PdII dioxocyclam complexes as having the dominant isomeric forms as meso for {Ru(NO)}3+ and PdII, and rac for NiII are supported by the H–H COSY spectrum for {RuNO)}3+, H–H and C–H COSY spectra for NiII and the H–H COSY spectrum for PdII derivatives.