Solid-State Structure of Binuclear Transition Metal Bisphosphine Complexes. Rotation-rate-dependent MAS Spectra of Dipolar and Scalar Coupled Four-Spin Systems of Dimeric Palladium(I) Complexes

Abstract

Straightforward 31P MAS investigations were carried out in the solid state on a series of [Pd2X2(dppm)2] (X=Cl, Br, I) complexes in order to study the effects that ring distortion, conformations, and crystal packing have on the solid-state spectra of these molecules and also to get experimental values for certain scalar couplings (e.g. two-bond trans P–Pd–P) normally not available from solution-state spectra. The phosphorus nuclei in these structures form a tightly dipolar coupled four-spin system which cannot be treated by the averaged Hamiltonian theory. The presence of strong homonuclear dipolar couplings, especially under the so called “rotational resonance” conditions, was expected to add to the complexity of the spectra. It turned out that the molecules are asymmetric in solid state but the rotation-rate dependence of the spectra is simpler than expected. Whereas trans spin-pairs with small isotropic chemical shift differences show a J-recouping phenomenon at modest MAS frequencies, the strongly dipolar coupled cis pairs (outside of the real rotational resonance conditions) do not have a characteristic effect in the rotation range studied or the effect is beyond the spectral resolution (<250 Hz). At higher MAS frequencies under the assumption of “inhomogeneous” behaviour reliable values of the trans J-couplings are available from the spectra.