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 |
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| Authors: | Mithun Sarkar Prosenjit Daw Tapas Ghatak Prof Jitendra K Bera |
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| Institution: | Department of Chemistry, Center for Environmental Science and Engineering, Indian Institute of Technology Kanpur, Kanpur 208016 (India), Fax: (+91)?512‐2597436 |
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| Abstract: | Ferrocene‐amide‐functionalized 1,8‐naphthyridine (NP) based ligands {(5,7‐dimethyl‐1,8‐naphthyridin‐2‐yl)amino]carbonyl}ferrocene (L1H) and {(3‐phenyl‐1,8‐naphthyridin‐2‐yl)amino]carbonyl}ferrocene (L2H) have been synthesized. Room‐temperature treatment of both the ligands with Rh2(CH3COO)4 produced Rh2(CH3COO)3(L1)] ( 1 ) and Rh2(CH3COO)3(L2)] ( 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 L1H and the inertness of the bridging carboxylate groups prevented the incorporation of the second ligand on the {RhII–RhII} unit. The use of the more labile Rh2(CF3COO)4 salt with L1H produced a cis bis‐adduct Rh2(CF3COO)4(L1H)2] ( 3 ), whereas L2H resulted in a trans bis‐adduct Rh2(CF3COO)3(L2)(L2H)] ( 4 ). Ligand L1H exhibits chelate binding in 3 and L2H 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 Rh2(CH3COO)2(CH3CN)4(L2H)2](BF4)2 ( 6 ). However, the axial ligands reorganize at reflux into a bridge‐chelate coordination mode and produce Rh2(CH3COO)2(CH3CN)2(L1H)](BF4)2 ( 5 ) and Rh2(CH3COO)2(L2H)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 efficacyof dirhodium(II,II) compounds. |
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| Keywords: | catalysis C H activation hemilability hydrogen bonds naphthyridine rhodium |
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