Hexaruthenium Carbonyl Cluster Complexes with Basal Edge-Bridged Square Pyramidal Metallic Skeleton: Efficient Synthesis of 2-Imidopyridine Derivatives and Determination of Their Reactive Sites in Carbonyl Substitution Reactions

The reactions of [Ru3(CO)12] with half equivalent of 2-amino-6-methylpyridine (H2ampy) or 2-aminopyridine (H2apy)
in refluxing xylene give the hexanuclear products [Ru6(μ3-H)2(μ5-η2-L)(μ-CO)2(CO)14] (L = ampy, 1; apy, 2). These
reactions represent the first high-yield syntheses of hexanuclear complexes with a basal edge-bridged square
pyramidal metallic skeleton. Five metal atoms of these complexes are bridged by the N-donor ligand in such a way
that the edge-bridging metal atom is attached to the pyridine nitrogen, while the basal atoms of the square pyramid
are capped by an imido fragment that arises from the activation of both N−H bonds of the NH2 group. The reactive
sites of these complexes in CO substitution reactions have been determined by studying the reactivity of 1 with
triphenylphosphine. Two kinetically controlled monosubstitutions take place on the edge-bridging metal atom in
positions cis to the pyridine nitrogen, leading to a mixture of two isomers of formula [Ru6(μ3-H)2(μ5-η2-ampy)(μ-CO)2(CO)13(PPh3)] (3 and 4). On heating at 80 °C, these monosubstituted isomers are transformed, via a dissociative
pathway, into the product of thermodynamic control (5), which has the PPh3 ligand on the apical Ru atom. The di-
and trisubstituted derivatives [Ru6(μ3-H)2(μ5-η2-ampy)(μ-CO)2(CO)12(PPh3)2] (6) and [Ru6(μ3-H)2(μ5-η2-ampy)(μ-CO)2(CO)11(PPh3)3] (7) are stepwise formed from 3−5 and PPh3. Compound 6 has the PPh3 ligands on the edge-bridging and apical Ru atoms, and compound 7 has an additional PPh3 ligand on an unbridged basal Ru atom.
The compound [Ru6(μ3-H)2(μ5-η2-ampy)(μ-CO)2(CO)12(μ-dppm)] (8), in which a basal and the apical Ru atoms are
spanned by the dppm ligand, has been isolated from the reaction of 1 with bis(diphenylphosphino)methane.