S–H···Te Hydrogen-Bonded Me2Te2–H2S Complex in an Argon Matrix: What Makes Heavy Chalcogens Prefer a Hydrogen Bond Over a Chalcogen Bond?

The S–H···Te hydrogen-bonded Me2Te2–H2S complex was experimentally identified in a cold and solid argon matrix. Quantum chemical calculations predicted six conformers for the Me2Te2–H2S complex with a S–H···Te hydrogen bond (HB) and only one conformer with a S···Te chalcogen bond (ChB), which was the least stable of all. While five of the HB-bound conformers could be experimentally identified via νS–H spectral shifts, no spectral signature could be found for the ChB-bound conformer. The activation barrier for interconversion of the experimentally observed HB complexes was found to be ≤0.67 kcal mol–1. We show that the absolute strengths of the HB and ChB, as well as the relative strength of the HB compared to the ChB formed between two hydrogen chalcogenides H2X (X = S, Se, and Te), systematically and independently increase due to two structural modifications in the HB acceptor moiety: (i) methyl substitution and (ii) dichalcogen bridge incorporation. In fact, only methyl substitution was found to maximally strengthen the HBs, both absolutely and relative to ChBs, by increasing the proton affinity of the acceptor moiety.