The Synthesis, Characterization and Reactivity of a Series of Ruthenium N-triphosPh Complexes

Summary

Ruthenium phosphine complexes are widely used for homogeneous catalytic reactions such as hydrogenations. The synthesis of a series of novel tridentate ruthenium complexes bearing the N-triphos ligand N(CH₂PPh₂)₃ is reported. Additionally, the stoichiometric reaction of a dihydride Ru–N-triphos complex with levulinic acid is described.

Abstract

Herein we report the synthesis of a tridentate phosphine ligand N(CH₂PPh₂)₃ (N-triphos^Ph) (1) via a phosphorus based Mannich reaction of the hydroxylmethylene phosphine precursor with ammonia in methanol under a nitrogen atmosphere. The N-triphos^Ph ligand precipitates from the solution after approximately 1 hr of reflux and can be isolated analytically pure via simple cannula filtration procedure under nitrogen. Reaction of the N-triphos^Ph ligand with [Ru₃(CO)₁₂] under reflux affords a deep red solution that show evolution of CO gas on ligand complexation. Orange crystals of the complex [Ru(CO)₂{N(CH₂PPh₂)₃}-κ³P] (2) were isolated on cooling to RT. The ³¹P{¹H} NMR spectrum showed a characteristic single peak at lower frequency compared to the free ligand. Reaction of a toluene solution of complex 2 with oxygen resulted in the instantaneous precipitation of the carbonate complex [Ru(CO₃)(CO){N(CH₂PPh₂)₃}-κ³P] (3) as an air stable orange solid. Subsequent hydrogenation of 3 under 15 bar of hydrogen in a high-pressure reactor gave the dihydride complex [RuH₂(CO){N(CH₂PPh₂)₃}-κ³P] (4), which was fully characterized by X-ray crystallography and NMR spectroscopy. Complexes 3 and 4 are potentially useful catalyst precursors for a range of hydrogenation reactions, including biomass-derived products such as levulinic acid (LA). Complex 4 was found to cleanly react with LA in the presence of the proton source additive NH₄PF₆ to give [Ru(CO){N(CH₂PPh₂)₃}-κ³P{CH₃CO(CH₂)₂CO₂H}-κ²O](PF₆) (6).

Introduction

Ruthenium phosphine based complexes are some of the most widely studied and chemically versatile molecular catalysts.^1-9 Typically, such ruthenium catalysts contain either mono- or bi-dentate ligands that dictate the electronics, sterics, geometry and solubility of the complex, and which profoundly impact on catalytic activity. Multidentate phosphine systems have been less widely studied for catalysis, as they are known to impart greater stability on the metal center owing to the greater chelate effect of multiple phosphorus donors on the metal center. Such stabilization can be undesirable for catalysis, however, under harsher reaction conditions (higher te....

Protocol

Note: Carry out all syntheses in a fume hood, and only after appropriate safety issues have been identified and measures taken to protect against them. Personal protective equipment include a lab coat, gloves and safety goggles and should be worn at all times.

1. Synthesis of N,N,N-tris(diphenylphosphinomethylene)amine, N(CH₂PPh₂)₃ (N-triphos^Ph) (1)

1. To a 200 ml oven dried Schlenk flask add diphenyl(hydroxymethylene)phosphonium chloride¹¹ (6.99 g, 24.7 mmol) and place under nitrogen via three sequential vacuum-nitrogen cycles on a dual-manifold Schlenk line.
2. Add degass....

Results

The N-triphos^Ph ligand (1) and the ruthenium complex series: Ru(CO)₂{N(CH₂PPh₂)₃}-κ³P] (2), [Ru(CO₃)(CO){N(CH₂PPh₂)₃}-κ³P] (3) and [Ru(H)₂(CO){N(CH₂PPh₂)₃}-κ³P] (4) were characterized via ¹H, ¹³C{¹H}, ³⁰

Discussion

Herein we have described efficient synthetic procedures for the synthesis of a tridentate phosphine ligand and a series of ruthenium complexes. The N-triphos^Ph ligand (1) can be easily prepared in high yield with a minimalistic work-up procedure. This phosphorus based Mannich reaction used to synthesize these types of ligands is quite general and can be used for other ligand derivatives with differing R-groups on the P-atoms.^10-12,15-18 Additionally, this synthetic methodol.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Methanol			Obtained from in-house solvent purification system: Innovative Technology, inc "pure solv" drying tower. Stored in ampules over activated molecular sieves under nitrogen.
Toluene
Diethyl Ether
Tetrahydrofuran (THF)
Acetonitrile
d6-Acetone	VWR	VWRC87152.0011	Store in fridge
Triethylamine	Sigma-Aldrich	TO886-1L	Distilled and stored over activated molecular sieves under N2
2 M Ammonia solution in methanol	Sigma-Aldrich	341428-100ML	Solution comes in a "Sure-Seal" bottle
NH4PF6	Sigma-Aldrich	216593-5G	Store in desiccator
Levulinic Acid	Acros Organics	125142500	Solid but melts close to room temperature
3 Å Molecular sieves	Alfa Aesar	LO5359	Activate by heating over night under vacuum
Schlenk flasks	GPE	Custom design
Dual-manifold Schlenk line	GPE	Custom design	Dual-manifold of i) N2 that has been passed through a silica drying column and ii) vacuum.
Rotary vacuum pump	Edwards	RV3 A652-01-903
100 ml Autoclave Engineer's high pressure reactor	Autoclave Engineer	Custon design
Vortex Stirrer	VWR	444-1378