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, RuCl^sub 3^NO(H^sub 2^O)^sub 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 RuCl^sub 3^NO(H^sub 2^O)^sub 2^ migrates through biological cells. [RuCl^sub 3^NO(imH)(H^sub 2^O)], [RuCl^sub 3^(NO)(imH)^sub 2^] and [RuCl^sub 2^(NO)(imH)^sub 3^]^sup +^ complexes (imH = imidazole) have been detected by ^sup 1^H-n.m.r. and i.r. and electrospray mass spectrometry (e.s.i.-m.s.) methods. Based upon the effect of cis ligand addition on the ν(NO) frequency causing a decrease in frequency, the 1:1 and 1:2 complexes have the imidazole donors in the plane cis to the NO^sup +^ moiety, whereas the 1:3 species has the third imidazole trans to the NO^sup +^. The trans imidazole donor causes 'trans-strengthening' of the N-O bond of the {RuNO}^sup 6^ chromophore. ^sup 1^H-n.m.r. shows that the monodentate imidazole 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 more stable 1:1 complexes with the major isomer having an axially-coordinated histidine imidazole donor, but in-plane donation for histamine. The carboxylate of coordinated histidine remains pendant according to i.r. and ^sup 13^C-n.m.r. data. From syntheses carried out at pH ca. 5, the amino donor is H-bonded to an in-plane H^sub 2^O in the major species (ca. 75%) and coordinated with displacement of the in-plane H^sub 2^O 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 histamine, analogous to the bis imidazole species and the known fac, cis-[RuCl^sub 3^NO(en)] complex. N-Methyliminodiacetate is observed to form one main [RuCl(NO)(mida)(H^sub 2^O)] complex (85%) with two chelated glycinato donor groups with RuCl^sub 3^NO(H^sub 2^O)^sub 2^, one glycinato group chelated 'in-plane' with the central amine donor and one axial coordinated glycinato donor. A second [RuCl(NO)(mida)(H^sub 2^O)] complex (the remaining 15%) has the amine donor trans to NO^sup +^ and chelated glycinato groups which coordinate in the RuClO^sub 2^(OH^sub 2^) plane, either cis or trans to each other, in a 60:40 split (ca. 9% and 6%). The presence of one Cl^sup -^ and one H^sub 2^O in the [RuCl(NO)(mida)(H^sub 2^O)] complexes was es