Role of Substrate Deformation in the Reactivity of N-Heterocyclic Silylenes and Germylenes

Abstract Controlling reactivity and pathway selectivity in N-heterocyclic silylenes (NHSi) and germylenes (NHGe) has lacked a unified mechanistic framework. Using density functional theory together with the activation strain model (ASM), energy decomposition analysis (EDA), and aromaticity metrics, we establish substrate deformation as the principal determinant of activation barriers across diazoalkane decomposition, alkyne functionalization, and small-molecule activation. Ring aromaticity, though measurable, predicts neither barrier heights nor pathway selectivity; instead, activation barriers scale with the geometric cost of distorting the incoming substrate to accommodate the larger Si and Ge centers. Excessive diazoalkane deformation, not ring distortion, drives the prohibitive C–H activation barriers, whereas the preferred N2-release pathway benefits from strong orbital interactions requiring little substrate reorganization. The paradigm holds across 5-ItBu and 5-Dipp frameworks and six-membered scaffolds (with pathway selectivity governed by deformation for the heavier congeners and by orbital interactions where deformation demands are comparable), and extends to Si–H, B–H, and C–F activation. N2-release yields a silene showing no Si═C bond cleavage over 2.6 ps of Born–Oppenheimer molecular dynamics (BOMD) up to 900 K, with low predicted barriers for PhOH, PhCCH, and NH3, providing a design framework for heavier-tetrylene reagents.