Ruthenium Nitrosyls Derived from Polypyridine Ligands with Carboxamide or Imine Nitrogen Donor(s): Isoelectronic Complexes with Different NO Photolability

As part of our search for photoactive ruthenium nitrosyls, a set of {RuNO}6 nitrosyls has been synthesized and
structurally characterized. In this set, the first nitrosyl [(SBPy3)Ru(NO)](BF4)3 (1) is derived from a polypyridine
Schiff base ligand SBPy3, while the remaining three nitrosyls are derived from analogous polypyridine ligands
containing either one ([(PaPy3)Ru(NO)](BF4)2 (2)) or two ([(Py3P)Ru(NO)]BF4 (3) and [(Py3P)Ru(NO)(Cl)] (4))
carboxamide group(s). The coordination structures of 1 and 2 are very similar except that in 2, a carboxamido
nitrogen is coordinated to the ruthenium center in place of an imine nitrogen in case of 1. In 3 and 4, the ruthenium
center is coordinated to two carboxamido nitrogens in the equatorial plane and the bound NO is trans to a pyridine
nitrogen (in 3) and chloride (in 4), respectively. Complexes 1−3 contain N6 donor set, and the NO stretching
frequencies (νNO) correlate well with the N−O bond distances. All four diamagnetic {RuNO}6 nitrosyls are photoactive
and release NO rapidly upon illumination with low-intensity (5−10 mW) UV light. Interestingly, photolysis of 1
generates the diamagnetic Ru(II) photoproduct [(SBPy3)Ru(MeCN)]2+ while 2−4 afford paramagnetic Ru(III) species
in MeCN solution. The quantum yield values of NO release under UV illumination (λmax = 302 nm) lie in the range
0.06−0.17. Complexes 3 and 4 also exhibit considerable photoactivity under visible light. The efficiency of NO
release increases in the order 2 < 3 < 4, indicating that photorelease of NO is facilitated by (a) the increase in the
number of coordinated carboxamido nitrogen(s) and (b) the presence of negatively charged ligands (like chloride)
trans to the bound NO.