Cationic Heterocycles as Ligands: Synthesis and Reactivity with Anionic Nucleophiles of Cationic Triruthenium Clusters Containing C-Metalated N-Methylquinoxalinium or N-Methylpyrazinium Ligands

The cationic cluster complexes [Ru-3(CO)(10)(mu-H)(mu-kappa N-2,C-(LMe)-Me-1)](+) (3(+); HL1 = quinoxaline) and [Ru-3(CO)(10)(mu-H)(mu-kappa N-2,C-(LMe)-Me-2)](+) (5(+); HL2 = pyrazine) have been prepared as triflate salts by treatment of their neutral precursors [Ru-3(CO)(10)(mu-H)(mu-kappa N-2,C-L-n)] with methyl triflate. The cationic character of their heterocyclic ligands is responsible for their enhanced tendency to react with anionic nucleophiles relative to that of hydrido triruthenium carbonyl clusters that have neutral N-heterocyclic ligands. These clusters react instantaneously with methyl lithium and potassium tris-sec-butylborohydride (K-selectride) to give neutral products that contain novel nonaromatic N-heterocyclic ligands. The following are the products that have been isolated: [Ru-3(CO)(9)(mu-H)(mu(3)-kappa N-2,C-(LMe2)-Me-1)] (6; from 3(+) and methyl lithium), [Ru-3(CO)(9)(mu-H)(mu(3)-kappa N-2,C-(LHMe)-H-1)] (7; from 3(+) and K-selectride), [Ru-3(CO)(9)(mu-H)(mu(3)-kappa N-2,C-(LMe2)-Me-2)] (8: from 5(+) and methyl lithium), and [Ru-3(CO)(9)(mu-H)(mu(3)-kappa N-2,C-(LHMe)-H-2)] (11; from 5(+) and K-selectride). Whereas the reactions of 3(+) lead to products that arise from the attack of the corresponding nucleophile at the C atom of the only CH group adjacent to the N-methyl group, the reactions of 5(+) give mixtures of two products that arise from the attack of the nucleophile at one of the C atoms located on either side of the N-methyl group. The LUMOs and the atomic charges of 3(+) and 5(+) confirm that the reactions of these clusters with anionic nucleophiles are orbital-controlled rather than charge-controlled processes. The N-heterocyclic ligands of all of these neutral products are attached to the metal atoms in nonconventional face-capping modes. Those of compounds 6-8 have the atoms of a ligand C=N fragment sigma-bonded to two Ru atoms and pi-bonded to the other Ru atom. whereas the ligand of compound 11 has a C-N fragment attached to a Ru atom through the N atom and to the remaining two Ru atoms through the C atom. A variable-temperature H-1 NMR spectroscopic study showed that the ligand of compound 7 is involved in a fluxional process at temperatures above -93 degrees C the mechanism of which has been satisfactorily modeled with the help of DFT calculations and involves the interconversion of the two enantiomers of this cluster through conformational change of the ligand CH2 group, which moves from one side of the plane of the heterocyclic ligand to the other, and a 180 degrees rotation of the entire organic ligand over a face of the metal triangle.