Structural Influence on the Photochromic Response of a Series of Ruthenium Mononitrosyl Complexes

In mononitrosyl complexes of transition metals two long-lived metastable states corresponding to linkage isomers of the nitrosyl ligand can be induced by irradiation with appropriate wavelengths. Upon irradiation, the N-bound nitrosyl ligand (ground state, GS) turns into two different conformations: isonitrosyl O bound for the metastable state 1 (MS1) and a side-on nitrosyl conformation for the metastable state 2 (MS2). Structural and spectroscopic investigations on [RuCl(NO)py4](PF6)2·1/2H2O (py = pyridine) reveal a nearly 100% conversion from GS to MS1. In order to identify the factors which lead to this outstanding photochromic response we study in this work the influence of counteranions, trans ligands to the NO and equatorial ligands on the conversion efficiency: [RuX(NO)py4]Y2·nH2O (X = Cl and Y = PF6 – (1), BF4 – (2), Br–(3), Cl– (4); X = Br and Y = PF6 – (5), BF4 – (6), Br–(7)) and [RuCl(NO)bpy2](PF6)2 (8), [RuCl2(NO)tpy](PF6) (9), and [Ru(H2O)(NO)bpy2](PF6)3 (10) (bpy = 2,2′-bipyridine; tpy = 2,2′:6′,2″-terpyridine). Structural and infrared spectroscopic investigations show that the shorter the distance between the counterion and the NO ligand the higher the population of the photoinduced metastable linkage isomers. DFT calculations have been performed to confirm the influence of the counterions. Additionally, we found that the lower the donating character of the ligand trans to NO the higher the photoconversion yield.