Synthesis of a sterically bulky diphosphine synthon and Ru(ii) complexes of a cooperative tridentate enamide-diphosphine ligand platform

Abstract

In order to generate tridentate enamido diphosphine ligand platforms, we developed procedures for the preparation of tBu₂PCH₂CH₂P(tBu)I, which involve low temperatures, pentane solvent and addition of 4 equiv. of tBuLi to Cl₂PCH₂CH₂PCl₂ or 2 equiv. of tBuLi to known Cl(tBu)PCH₂CH₂P(tBu)Cl also at low temperatures in pentane; an alternate method involves the inverse addition of Cl(tBu)PCH₂CH₂P(tBu)Cl to 2 equiv. of tBuLi in pentane at 0 °C; all of these methods generate good yields of the tetraphosphine dimer (tBu₂PCH₂CH₂P(tBu))₂ contaminated by small amounts of tBu₂PCH₂CH₂PtBu₂ (dtbpe), which can be conveniently separated by sublimation. Subsequent oxidative cleavage of the P–P bond with I₂ or 1,2-diiodoethane results in the formation of the desired tBu₂PCH₂CH₂P(tBu)I, which undergoes C–P bond formation when added to 1 equiv. of the lithium N-2,6-diisopropylphenylenamide of cyclopentylidene imine to generate the HNPP ligand precursor; this species exists as a tautomeric mixture of the corresponding enamine and imine, the ratio of which depends on workup conditions used. This enamine–imine mixture can be used directly to form Ru(II) species either directly with heating to generate the five-coordinate (NPP)RuCl(CO) via loss of H₂ or by inclusion of 1 equiv. of KOtBu to generate (NPP)RuH(CO). X-ray crystallographic studies confirm that the geometry in the solid state matches the solution spectroscopic data. Subsequent studies of (NPP)RuH(CO) indicate that it reacts with benzaldehyde, benzyl alcohol, and H₂ in a cooperative manner to generate a series of hydride carbonyls that have been characterized fully by NMR spectroscopy and X-ray crystallography.