Base assisted C-C coupling between carbonyl and polypyridyl ligands in a Ru-NADH-type carbonyl complex

A reaction of a ruthenium(ii) NAD-type complex, [Ru(tpy)(pbn)(Cl)]⁺ (tpy = 2,2′:6′,2′′-terpyridine; pbn = 2-(pyridin-2-yl)benzo[b][1,5]naphthyridine), with pressurized CO (2 MPa) at 150 °C in H₂O selectively produced a two-electron reduced ruthenium(ii)-NADH-type carbonyl complex, [Ru(tpy)(pbnHH)(CO)]²⁺ (pbnHH = 2-(pyridin-2-yl)-5,10-dihydrobenzo[b][1,5]naphthyridine), rather than the oxidized [Ru(tpy)(pbn)(CO)]²⁺ complex. Indeed, [Ru(tpy)(pbnHH)(CO)]²⁺ was quantitatively oxidized to [Ru(tpy)(pbn)(CO)]²⁺ upon treatment with one equiv. of 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ). The reactivity of [Ru(tpy)(pbnHH)(CO)]²⁺ with various bases was studied herein. Treatment of [Ru(tpy)(pbnHH)(CO)]²⁺ with a suitable organic base, 1,5-diazabicyclo[4.3.0]non-5-ene (DBN), resulted in the formation of a new five-membered Ru-CO-bridge metallacycle quantitatively in acetonitrile under air at room temperature. A probable mechanism was proposed for this reaction based on UV-vis, NMR, and EPR spectral studies and other experimental data. Furthermore, a reaction of the five membered Ru-CO-bridge metallacycle with NH₄PF₆ in CH₃CN : H₂O (1 : 1) under air smoothly produced another new six-membered Ru-OCO-bridge complex. A mechanism for the formation of a Ru-OCO-bridge complex was also proposed here on the basis of H₂O¹⁸ experiments, DDQ treatment and other experimental data. These newly synthesized complexes appended with NAD-type ligands may have potential use as renewable hydride sources for organic reductions.