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

In order to generate tridentate enamido diphosphine ligand platforms, we developed procedures for the preparation of tBu PCH CH P(tBu)I, which involve low temperatures, pentane solvent and addition of 4 equiv. of tBuLi to Cl PCH CH PCl or 2 equiv. of tBuLi to known Cl(tBu)PCH CH P(tBu)Cl also at low temperatures in pentane; an alternate method involves the inverse addition of Cl(tBu)PCH CH P(tBu)Cl to 2 equiv. of tBuLi in pentane at 0 °C; all of these methods generate good yields of the tetraphosphine dimer (tBu PCH CH P(tBu)) contaminated by small amounts of tBu PCH CH PtBu (dtbpe), which can be conveniently separated by sublimation. Subsequent oxidative cleavage of the P-P bond with I or 1,2-diiodoethane results in the formation of the desired tBu PCH CH P(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 H 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 H in a cooperative manner to generate a series of hydride carbonyls that have been characterized fully by NMR spectroscopy and X-ray crystallography.