Aligned Insulating Cavities for Metal Atom Binding on Conductive Surfaces

Precise control over multiple coordination sites is critical for directing molecular assembly on surfaces, a challenge well suited to N-heterocyclic carbenes (NHCs) owing to their strong σ-donation and tunable steric properties. In previously reported bisNHC systems, on-surface carbene generation from salt precursors requires elevated temperatures, driving adsorption into thermodynamically favored flat-lying configurations that obscure intrinsic binding behavior. Here, we employ isolable bisNHC derivatives bearing isopropyl and tert-butyl substituents, enabling their direct deposition as unprotected molecular precursors. Scanning tunneling microscopy and near-edge X-ray absorption fine structure measurements reveal that both derivatives adopt flat-lying geometries; however, their lateral assembly behavior differs markedly. The isopropyl substituted derivative forms mutually repulsive chains, whereas the more sterically hindered tert-butyl derivative (bisNHCtBu) unexpectedly assembles into a close-packed, dipole-driven molecular lattice. The bisNHCtBu lattice is stable at room temperature but thermally fragile, underscoring the difficulty of accessing this architecture via protected-precursor approaches. The bisNHCtBu lattice exhibits a wide spectroscopic bandgap and contains aligned metal-binding cavities that enable observation of spin-flip excitations of deposited dysprosium atoms. This molecular lattice provides a versatile platform for probing emergent electronic and magnetic properties and the single molecule reactivity of surface-confined, low-coordinate metal complexes.