Line Shape Analysis of Dynamic NMR Spectra for Characterizing Coordination Sphere Rearrangements at a Chiral Rhenium Polyhydride Complex

Summary

Line shape analysis of NMR spectra collected over a range of temperatures serves as a guide for the rearrangement of inner coordination-sphere atoms at a chiral, eight-coordinate, rhenium(V) polyhydride complex, ReH₅(PPh₃)₂(sec-butyl amine). Line shape analysis is also used to determine the activation parameters ΔH^‡, ΔS^‡, and ΔG^‡ for those atom rearrangements.

Abstract

Dynamic solution nuclear magnetic resonance (NMR) spectroscopy is the typical method of characterizing the dynamic rearrangements of atoms within the coordination sphere for transition metal polyhydride complexes. Line shape fitting of the dynamic NMR spectra can lead to estimates for the activation parameters of the dynamic rearrangement processes. A combination of dynamic ³¹P-{¹H} NMR spectroscopy of metal-bound phosphorus atoms with dynamic ¹H-{³¹P} NMR spectroscopy of hydride ligands may identify hydride ligand rearrangements that occur in conjunction with a phosphorus atom rearrangement. For molecules that exhibit such a coupled pair of rearrangements, dynamic NMR spectroscopy can be used to test theoretical models for the ligand rearrangements. Dynamic ¹H-{³¹P} NMR spectroscopy and line shape fitting can also identify the presence of an exchange process that moves a specific hydride ligand beyond the metal's inner coordination sphere through a proton exchange with a solvent molecule such as adventitious water. The preparation of a new compound, ReH₅(PPh₃)₂(sec-butyl amine), that exemplifies multiple dynamic rearrangement processes is presented along with line shape fitting of dynamic NMR spectra of the complex. Line shape fitting results can be analyzed by the Eyring equation to estimate the activation parameters for the identified dynamic processes.

Introduction

NMR spectroscopy is commonly used to characterize dynamic processes that occur within or between molecules. For many simple intramolecular rearrangements, estimation of ΔG^‡ is as straight-forward as measuring the frequency difference, Δν, between two resonances at the slow exchange limit and determining the coalescence temperature for those same resonances (Figure 1)¹. The relationship,

ΔG^‡ = 4.575 x 10^-3 kcal/mol x T_c [9.972 + log (T_c/Δν)]

where T_c is the coalescence temperature for a pair of r....

Protocol

1. Sample preparation

1. Preparation of ReH₇(PPh₃)₂³⁵
   1. Combine 0.15 g of sodium borohydride and 0.41 g of ReOCl₃(PPh₃)₂ in a two- or three-necked 100 mL round-bottomed flask fitted with a rubber septum and gas port, or a 100 mL Kjeldahl flask (with a side arm gas port) fitted with a rubber septum (Supplementary Figure 1).
   2. Add a spin bar to the reaction vessel.
   3. In a fume hood, use a piece of rubber pressure tubing to connect the gas port of the reaction vessel with one of the stopcocks of a dual glass mani....

Results

The characterizations of both rhenium polyhydride products described in this manuscript are best accomplished by ¹H-{³¹P} and ³¹P-{¹H} NMR spectroscopy. In a room temperature d₆-benzene solution, the hydride ligand resonance of ReH₇(PPh₃)₂ appears as a binomial triplet at δ = -4.2 ppm with ²J_PH = 18 Hz by ¹H NMR spectroscopy (Supplementary Figure 2). The same d₆-benzene solu.......

Discussion

There are four items in the preparation of ReH₇(PPh₃)₂ that can impact the quantity and purity of the material that is produced. First, the use of an ice bath during the first 15 min of the reaction is important to remove heat from the reaction that occurs between sodium borohydride and water. Higher initial temperatures lead to a decreased yield of the ReH₇(PPh₃)₂ product due to formation of the thermal decomposition product Re₂H₈(PP.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Bruker Avance II 400 MHz NMR spectrometer	Bruker Biospin		The instrument includes a two channel probe (1H and X) with the X channel tunable from 162 MHz to 10 Mhz. The instrument is also VT capable with a dewar and heat exchanger for VT work.
d8-toluene	MilliporeSigma	434388
Powerstat variable transformer	Powerstat
sec-butyl amine	MilliporeSigma	B89000
Sodium borohydride	MilliporeSigma	452882
Tetrahydrofuran	MilliporeSigma	186562
Thermowell C3AM 100 mL	Thermowell
Topspin 3.0 or 4.1.4 with dNMR	Bruker Biospin		Data was acquired with Topspin version 3.0 and data handling was performed on a second computer that was running Topspin version 4.1.4..
Trichlorooxobis(triphenylphosphine) rhenium(V)	MilliporeSigma	370193
Vacuubrand PC3000 vacuum pump with a CVC 3000 controller	Vacuubrand