The Dean−Evans Relation in ³¹P NMR Spectroscopy and Its Application to the Chemistry of Octahedral Tungsten Sulfide Clusters

One of the challenges in studying the chemistry of hexanuclear octahedral metal clusters is analyzing the many possible complexes, including stereoisomers, when these complexes consist of mixed axial ligands (two or more). In the case of W6S8L6-n(PR3)n (n = 0−6; L = nonphosphine Lewis base ligands, PR3 = phosphines) clusters, in situ identification of the 10 possible complexes is possible by 31P NMR due to P−W−W−P coupling. A linear relation for 31P NMR shifts (δ(31P)) of these W6S8L6-n(PR3)n complexes, analogous to the Dean−Evans relation for 19F NMR shifts of octahedral tin complexes, is found and expressed as δ(31P) = δref + pC + qT with two variables (p and q, the number of ligands L in the cis or trans position to PR3, respectively) with two constants (C and T, characteristic of a given ligand L). 31P NMR investigation of over 200 complexes in 26 W6S8L6-n(PR3)n systems show that this relation is generally valid for W6S8 clusters. Such a relation helps spectroscopic assignments and demonstrates the trans and cis influence on hexanuclear clusters. Large bulky ligands cause deviations from the linear behavior due to steric effects. With the help of 2-D 31P NMR spectroscopy, mixtures of W6S8(PR3)6-n(PR‘3)n (n = 0−6) complexes can also be unequivocally interpreted. The Dean−Evans relation is expanded to account for different phosphine ligands. Partial substitution reactions of these W6S8 complexes by phosphines were investigated using 31P NMR, and four single crystals of mixed-ligand clusters are characterized with X-ray diffraction. In summary, 31P NMR and other NMR techniques, combined with Dean−Evans relations, are invaluable analytical tools for studying molecular W6S8 cluster chemistry and are likely to be useful for studying other mixed-ligand metal clusters.