A controlled NO-releasing compound:: Synthesis, molecular structure, spectroscopy, electrochemistry, and chemical reactivity of R,R,S,S-trans-[RuCl(NO)(cyclam)]2+(1,4,8,11-tetraazacyclotetradecane)

The synthesis of trans-[RuCl(NO)(cyclam)](2+) (cyclam 1,4,8,11-tetrazacyclotetradecane)can be accomplished by either the addition of cyclam to K-2[RuCl5NO] or by the addition of NO to trans-[RuCl(CF3SO3)(cyclam)](CF3SO3). Crystals of trans- [RuCl(NO)(cyclam)] (ClO4)(2) form in the monoclinic space group P2(1)/c, with unit cell parameters of a = 7.66500(2) Angstrom, b = 24.7244(1) Angstrom, c = 16.2871(2) Angstrom, beta = 95.2550(10)degrees, and Z = 4. One of the two independent molecules in the unit cell lies disordered on a center of symmetry. For the ion in the general position, the Ru-N and N-O bond distances and the [Ru-N-O](3+) bond angle are 1.747(4) Angstrom, 1.128(5) a, 178.0(4)degrees, respectively. In both ions, cyclam adopts the (R,R,S,S) configuration, which is also consistent with 2D COSY H-1 NMR studies in aqueous solution. Reduction (E degrees = -0.1 V) results in the rapid loss of Cl- by first-order kinetics with k = 1.5 s(-1) and the slower loss of NO (k = 6.10 x 10(-4) s(-1), Delta H-double dagger = 15.3 kcal mol(-1), Delta S-double dagger = -21.8 cal mol(-1) K-1). The slow release of NO following reduction causes trans-[RuCl(NO)(cyclam)](2+) to be a promising controlled-release NO prodrug for vasodilation and other purposes. Unlike the related complex trans-[Ru(NO)(NH3)(4)(P(OEt)(3))](PF6)(2), trans-[RuCl(NO)(cyclam)]Cl-2 is inactive in modulating evoked potentials recorded from mice hippocampal slices probably because of the slower dissociation of NO following reduction.