complex peptides and are unable to discriminate between even chemically distinct amides. Loss of stereochemical integrity and generation of dangerous waste plague the reactions even with simple substrates. In this regard, bis-oxo molybdenum catalysts display a unique ability to selectively activate amides. While the earlier catalysts could efficiently promote reactions with simple substrates, they were inefficient due to decomposition and undesired side-reactions. To overcome these limitations, we investigated N-capped tripodal tetradentate ligands 1 hypothesized to extend the lifetime of the catalyst and eliminate non-selective pathways such as dehydroalanine formation (Figure ). Ligands 1 with the general formula N2O2 contain two phenolic oxygen atoms and a spectator pyridine, which acts as semilabile group opening a coordination site for the incoming substrates. Figure depicts a series of complexes prepared by direct exchange of 1 with [MoO2(acac)2] (acac = acetylacetonate). When the reactivity of catalysts 2 was benchmarked against other Mo(VI) complexes with bidentate ligands (e.g., 6methylpicolinic acid or 8-hydroxyquinoline), they consistently resulted in cleaner conversions. Furthermore, the activity of 2 could be modulated by changing the sterics of the pyridine group directly shielding one of the oxo ligand (Figure ). While replacing the proton in the ortho position with a methyl group had little effects on the rate of cyclodehydration with 3 (entries 1 and 2), installation of the phenyl group resulted in a two-fold increase (2h, entry 8). This change of reactivity is a highly desirable feature as it reduces detrimental elimination to dehydroalanine and shortens the reaction time to several hours with a complete consumption of 3 (98% yield). As expected, catalysts with large groups in the close neighborhood to the metal (e.g., 2a and 2b) are significantly slower in catalyzing cyclodehydration of 3, and a beneficial nonlinear effect of substituting the meta-position with a large alkyl substituent (R 3 ) on the phenoxide group was observed in reactions catalyzed by 2d and 2f-2h. We attribute the increase of reactivity to the changes in the overall structure of the complex as demonstrated by analysis of single crystal X-ray structures of 2e and 2h (Figure ). Both complexes show a distorted octahedral geometry and cis-configuration of the dioxo ligands, but the bond between pyridine nitrogen and molybdenum underwent a small but noticeable elongation (~0.05Å), a structural change sufficient to increase the catalytic activity. Furthermore, by adding tert-butyl substituents, solid-state structure of 2h rearranged into a more closed isomer, which is likely the consequence of crystal packing. The bulky tert-butyl groups at the ortho-(2a-2b, 2e) and meta-positions (2d, 2f-2h) serve to increase solubility in organic solvents, extend the catalyst's lifetime at elevated temperatures (60-140 o C), and introduce overall flexibility. All complexes 2a-2h are crystalline solids stable at ambient conditions and can be recovered (>85%) by chromatographic purification after completed cyclodehydration.