Amide-Functionalized Naphthyridines on a RhII-RhII Platform: Effect of Steric Crowding, Hemilability, and Hydrogen-Bonding Interactions on the Structural Diversity and Catalytic Activity of Dirhodium(II) Complexes

Ferrocene-amide-functionalized 1,8-naphthyridine (NP) based ligands {[(5,7-dimethyl-1,8-naphthyridin-2-yl) amino] carbonyl} ferrocene ((LH)-H-1) and {[(3-phenyl-1,8-naphthyridin-2-yl) amino] carbonyl} ferrocene ((LH)-H-2) have been synthesized. Room-temperature treatment of both the ligands with Rh-2(CH3COO)(4) produced [Rh-2(CH3COO)(3)(L-1)] (1) and [Rh-2(CH3COO)(3)(L-2)] (2) as neutral complexes in which the ligands were deprotonated and bound in a tridentate fashion. The steric effect of the ortho-methyl group in (LH)-H-1 and the inertness of the bridging carboxylate groups prevented the incorporation of the second ligand on the {Rh-II-Rh-II} unit. The use of the more labile Rh-2(CF3COO)(4) salt with (LH)-H-1 produced a cis bis-adduct [Rh-2(CF3COO)(4)((LH)-H-1)(2)] (3), whereas (LH)-H-2 resulted in a trans bis-adduct [Rh-2(CF3COO)(3)(L-2)((LH)-H-2)] (4). Ligand (LH)-H-1 exhibits chelate binding in 3 and (LH)-H-2 forms a bridge-chelate mode in 4. Hydrogen-bonding interactions between the amide hydrogen and carboxylate oxygen atoms play an important role in the formation of these complexes. In the absence of this hydrogen-bonding interaction, both ligands bind axially as evident from the X-ray structure of [Rh-2(CH3COO)(2)(CH3CN)(4)((LH)-H-2)(2)](BF4)(2) (6). However, the axial ligands reorganize at reflux into a bridge-chelate coordination mode and produce [Rh-2(CH3COO)(2)(CH3CN)(2)((LH)-H-1)](BF4)(2) (5) and [Rh-2(CH3COO)(2)((LH)-H-2)(2)](BF4)(2) (7). Judicious selection of the dirhodium(II) precursors, choice of ligand, and adaptation of the correct reaction conditions affords 7, which features hemilabile amide side arms that occupy sites trans to the Rh-Rh bond. Consequently, this compound exhibits higher catalytic activity for carbene insertion to the C-H bond of substituted indoles by using appropriate diazo compounds, whereas other compounds are far less reactive. Thus, this work demonstrates the utility of steric crowding, hemilability, and hydrogen-bonding functionalities to govern the structure and catalytic efficacy of dirhodium(II,II) compounds.