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.
- This article is part of the themed collection: Reactions Facilitated by Ligand Design