Understanding C–H Bond Activation on a Diruthenium(I) Platform

Abstract

Activation of the C–H bond at the axial site of a [RuI–RuI] platform has been achieved. Room-temperature treatment of 2-(R-phenyl)-1,8-naphthyridine (R = H, F, OMe) with [Ru2(CO)4(CH3CN)6][BF4]2 in CH2Cl2 affords the corresponding diruthenium(I) complexes, which carry two ligands, one of which is orthometalated and the second ligand engages an axial site via a Ru···C–H interaction. Reaction with 2-(2-N-methylpyrrolyl)-1,8-naphthyridine under identical conditions affords another orthometalated/nonmetalated (om/nm) complex. At low temperature (4 °C), however, a nonmetalated complex is isolated that reveals axial Ru···C–H interactions involving both ligands at sites trans to the Ru–Ru bond. A nonmetalated (nm/nm) complex was characterized for 2-pyrrolyl-1,8-naphthyridine at room temperature. Orthometalation of both ligands on a single [Ru–Ru] platform could not be accomplished even at elevated temperature. X-ray metrical parameters clearly distinguish between the orthometalated and nonmetalated ligands. NMR investigation reveals the identity of each proton and sheds light on the nature of [Ru–Ru]···C–H interactions (preagostic/agostic). An electrophilic mechanism is proposed for C–H bond cleavage that involves a C(pπ)–H → σ* [Ru–Ru] interaction, resulting in a Wheland-type intermediate. The heteroatom stabilization is credited to the isolation of nonmetalated complexes for pyrrolyl C–H, whereas lack of such stabilization for phenyl C–H causes rapid proton elimination, giving rise to orthometalation. NPA charge analysis suggests that the first orthometalation makes the [Ru–Ru] core sufficiently electron rich, which does not allow significant interaction with the other axial C–H bond, making the second metalation very difficult.

Bimetallics in a Nutshell: Complexes Supported by Chelating Naphthyridine-Based Ligands

“…[40][41][42] For instance, a BPAN-containing dinuclear Zn complex was 6.4 times faster in performing transesterification of HPNP (2-hydroxypropyl-4-nitrophenyl phosphate, used as an RNA model) than the corresponding mononuclear reference. 40 More recently, Bera and co-workers have successfully utilized a range of substituted naphthyridine ligands to examine the reactivity of binuclear complexes of Cu, [43][44][45] Ru, [46][47][48] and Pd. 49 Naphthyridine-based ligands are also suitable for the synthesis of pentametallic string complexes of Ni 50 and coordination polymers of Ag.…”

“…During the early 1990s, studying rigid-chain Mo-based polymers, Chisholm and co-workers employed 2,7-dioxo-1,8-naphthyridine (DON, Figure ) to isolate a tetranuclear Mo complex featuring two Mo–Mo quadruple bonds as a soluble oligomer. , In the early 2000s the Lippard group synthesized a library of 2,7-functionalized 1,8-naphthyridine ligands containing a variety of nitrogen-based side-arms (Figure ), which afforded a series of homobimetallic complexes mimicking active sites of different metalloenzymes and oxygen-activating proteins. − For instance, a BPAN-containing (see Figure ) dinuclear Zn complex was 6.4 times faster in performing transesterification of HPNP (2-hydroxypropyl-4-nitrophenyl phosphate, used as an RNA model) than the corresponding mononuclear reference . More recently, Bera and co-workers have successfully utilized a range of substituted naphthyridine ligands to examine the reactivity of binuclear complexes of Cu, − Ru, − and Pd . Naphthyridine-based ligands are also suitable for the synthesis of pentametallic string complexes of Ni and coordination polymers of Ag …”