Uncovering Intramolecular π-Type Hydrogen Bonds in Solution by NMR Spectroscopy and DFT Calculations

Reaction between the phosphinito bridged diplatinum species [(PHCy2)Pt(μ-PCy2){κ2P,O-μ-P(O)Cy2}Pt(PHCy2)](Pt-Pt) (1), and (trimethylsilyl)acetylene at 273 K affords the σ-acetylide complex [(PHCy2)(η1-Me3SiC≡C)Pt(μ-PCy2)Pt(PHCy2){κP-P(OH)Cy2}](Pt-Pt) (2) featuring an intramolecular π-type hydrogen bond. Scalar and dipolar couplings involving the POH proton were detected by 2D NMR experiments. Relativistic DFT calculations of the geometry, relative energy, and NMR properties of model systems of 2 confirmed the structural assignment and allowed the energy of the π-type hydrogen bond to be estimated (ca. 22 kJ mol-1). Hydrogen bonds in solution: The existence of a π-type hydrogen bond between the PO-H and the C≡C triple bond in the σ-acetylide complex [(PHCy2)(η1-Me3SiC≡C)Pt(μ-PCy2)Pt(PHCy2){κP-P(OH)Cy2}](Pt-Pt) (2) in solution was proven by multinuclear NMR spectroscopy and confirmed by relativistic DFT calculations on model systems of 2. The estimated hydrogen-bonding energy and the calculated coupling values and chemical shift indicate a remarkably strong π-type interaction (see scheme).