Acetylenic Carbon-13 Chemical Shift Tensors for Diphenylacetylene and (η²-Diphenylacetylene)Pt(PPh₃)₂:  A Solid-State NMR and Theoretical Study

The structure of (η²-diphenylacetylene)Pt(PPh₃) ₂, as well as those of its dichloromethane and benzene solvates, is determined via X-ray crystallography. An investigation of the chemical shift (CS) tensors of the ¹³C-labeled carbons in Ph¹³C⋮¹³CPh and (η²-Ph¹³C⋮¹³CPh)Pt(PPh₃)₂·(C₆H₆) is carried out via analysis of ¹³C NMR spectra from stationary solid samples. The principal components of the CS tensors as well as their orientations with respect to the ¹³C,¹³C internuclear vector are determined. DFT calculations of these CS tensors are in close agreement with the experimental values. For diphenylacetylene (tolane), the orientations and principal-component magnitudes of the alkynyl carbon CS tensors are comparable to those for other alkynyl carbons, although the CS tensor is not axially symmetric in this case. Coordination to platinum causes a change in the CS tensor orientation and a net increase in the isotropic chemical shift, resulting from a significant increase in two principal components (δ₁₁ and δ₃₃) while the third (δ₂₂) decreases only slightly. The measured carbon CS tensors in the platinum complex bear a striking similarity to those of the alkenyl carbons in trans-Ph(H)CC(H)Ph, and a short theoretical discussion of these observations is presented.