Palladium Complexes of Constrained Polyphosphines - Discovery and Investigation of “Through-Space”NMR Spin-Spin Couplings in Organometallic Compounds

Among the most common high-resolution nuclear magnetic resonance parameters (NMR parameters), nuclear spin-spin coupling provides decisive data for organic, organometallic, biological and coordination compounds characterization. This electron-mediated coupling characterized by J constant is very classically thought as transmitted by unambiguously covalently bonded atoms. Yet, experimental and theoretical NMR studies have highlighted since the sixties the existence of scalar J spin couplings operating via clearly nonbonded interactions, these couplings are often called “through-space” internuclear spin-spin couplings (TS couplings). We discovered that the through-space spin coupling was not only a phenomenon detectable in fluorinated constrained organic molecules, but also in organometallic and coordinating compounds, and especially in palladium and group 10 coordination complexes of constrained polyphosphine ligands. The present article aims at revealing the TS contribution of indirect nuclear spin-spin in ferrocenyl polyphosphine and their palladium coordination complexes. Theoretical and fundamental aspects are introduced in continuous relationship with experimental spectroscopic and structural data. The role of lone-pairs in nonbonded spin-spin coupling is disclosed and correlated with through-space distance dependence of J constant intensity within palladium and nickel halide complexes. Some important consequences in structural characterization of the compounds in solution are discussed. The spatial proximity of phosphorus atoms — recognized from TS spin couplings — is at the origin of new reactivity and mechanistic understanding in C-C palladium-catalyzed cross-coupling, which are quoted herein. Future trends in the modelling of this fascinating phenomenon, towards a better understanding, are briefly evoked.