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)

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Abstract
The synthesis of trans-[RuCl(NO)(cyclam)]2+ (cyclam = 1,4,8,11-tetraazacyclotetradecane) can be accomplished by either the addition of cyclam to K2[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 P21/c, with unit cell parameters of a = 7.66500(2) Å, b = 24.7244(1) Å, c = 16.2871(2) Å, β = 95.2550(10)°, 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) Å, 1.128(5) Å, 178.0(4)°, respectively. In both ions, cyclam adopts the (R,R,S,S) configuration, which is also consistent with 2D COSY 1H NMR studies in aqueous solution. Reduction (E° = −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 × 10-4 s-1, ΔH⧧ = 15.3 kcal mol-1, ΔS⧧ = −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)]Cl2 is inactive in modulating evoked potentials recorded from mice hippocampal slices probably because of the slower dissociation of NO following reduction.

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