Coordination of RuCl3(NO)(H2O)(2) by imidazole, histidine and iminodiacetate ligands: a study of complexation of 'Caged NO' by simple bio-cellular donors

The 'caged NO' reagent, RuCl3NO(H2O)(2), has been studied by n.m.r. and i.r . methods with imidazole, histidine, histamine, and N-methyliminodiacetate as complexing ligands. These ligands are representative of cellular donors that would be encountered as RuCl3NO(H2O)(2) migrates through biological ce lls. [RuCl3NO(imH)(H2O)], [RuCl3(NO)(imH)(2)] and [RuCl2(NO)(imH)(3)](+) co mplexes (imH = imidazole) have been detected by H-1-n.m.r. and i.r. and ele ctrospray mass spectrometry (e.s.i.-m.s.) methods. Based upon the effect of cis ligand addition on the nu (NO) frequency causing a decrease in frequen cy, the 1:1 and 1:2 complexes have the imidazole donors in the plane cis to the NO+ moiety, whereas the 1:3 species has the third imidazole trans to t he NO+. The trans imidazole donor causes 'trans-strengthening' of the N-O b ond of the {RuNO}(6) chromophore. H-1-n.m.r. shows that the monodentate imi dazole donor(s) is (are) in rapid exchange with free imidazole in solution for each of the n = 1-3 species. Histidine and histamine make kinetically m ore stable 1:1 complexes with the major isomer having an axially-coordinate d histidine imidazole donor, but in-plane donation for histamine. The carbo xylate of coordinated histidine remains pendant according to i.r. and C-13- n.m.r. data. From syntheses carried out at pH ca. 5, the amino donor is H-b onded to an in-plane H2O in the major species (ca. 75%) and coordinated wit h displacement of the in-plane H2O in the lesser isomer (25%). By contrast, the histamine ligand binds with an in-plane bound imidazole and a pendant protonated amino group (94%). The remaining 6% has an in-plane chelated his tamine, analogous to the bis imidazole species and the known fac, cis-[RuCl 3NO(en)] complex. N-Methyliminodiacetate is observed to form one main [RuCl (NO)(mida)(H2O)] complex (85%) with two chelated glycinato donor groups wit h RuCl3NO(H2O)(2), one glycinato group chelated `in-plane' with the central amine donor and one axial coordinated glycinato donor. A second [RuCl(NO)( mida)(H2O)] complex (the remaining 15%) has the amine donor trans to NO+ an d chelated glycinato groups which coordinate in the RuClO2(OH2) plane, eith er cis or trans to each other, in a 60:40 split (ca. 9% and 6%). The presen ce of one Cl- and one H2O in the [RuCl(NO)(mida)(H2O)] complexes was establ ished by e.s.i.-m.s. These results show that RuCl3NO(H2O)(2) is likely to b e freely mobile within a cellular environment, forming stable complexes via bidentate chelation with `two-point' nitrogen donors (en, his, etc).