Variable-Temperature Kinetic Analysis of a Three-Site Terminal Ligand Exchange in Nickel A-Frames by Quantitative ³¹P{¹H} EXSY NMR

Abstract

Variable-temperature two-dimensional phase-sensitive 31P{1H} EXSY NMR spectroscopy was used to determine the kinetics and thermodynamics of terminal ligand redistribution in the nickel A-frames Ni2(dppm)2(CCH2)X2 (X = SCN (1), Cl (3); dppm = bis(diphenylphosphino)methane). Solutions containing 1:1 mixtures of 1 and 3 undergo terminal ligand exchange via a two-step process involving a mixed-ligand intermediate, Ni2(dppm)2(CCH2)(Cl)(SCN) (2). This type of terminal ligand exchange is characteristic of this class of compounds. One-dimensional 31P{1H} NMR spectra for the mixed-halogen or halogen/pseudo-halogen species show a strong solvent dependence. The observed NMR spectra for 2 range from a broad singlet (DMSO) to a well-resolved AA‘BB‘ multiplet (benzene) and are dependent on the rate of ligand substitution. Rate constants for the terminal ligand exchange processes in DMSO-d 6 were calculated by three methods from the 31P{1H} EXSY NMR data. From these measurements, a complete thermodynamic characterization of this exchange process was achieved. Entropies of activation calculated from Eyring plots range from −11.4 to −19.0 cal/(K mol). This implies ordering of the system during the transition state and is consistent with an associative interchange (Ia) mechanistic model. Free energy calculations show an intrinsic thermodynamic stability associated with the mixed-ligand species 2.

The role of dynamic ligand exchange in the oxidation chemistry of cerium(iii)

“… 15 The technique has been successfully applied to a number of diverse chemical systems and nuclei, including 1 H and 7 Li nuclei and paramagnetic metal cations. 15b , 16 Recently, we have employed this technique to determine BINOLate and Li + exchange for 1-RE(homo) under catalytically relevant conditions. 17 …”

Kinetics and Mechanism of the Stereochemical Isomerization of an Arene−Ruthenium Complex of the Atropisomeric Ligand 1,1‘-Biphenyl-2,2‘-diamine

“…Errors in these measurements are propagated into the rate calculations . Another disadvantage is that several mixing times must be used, leading to the need for large amounts of instrument time . The advantage to this method is in calculating rate constants in multisite systems.…”

“…52 Another disadvantage is that several mixing times must be used, leading to the need for large amounts of instrument time. 53 The advantage to this method is in calculating rate constants in multisite systems. For a two-site system such as this one, there is no advantage in using 2D EXSY to determine rate constants.…”

Mechanisms and Beyond: Elucidation of Fluxional Dynamics by Exchange NMR Spectroscopy

“…Analysis of six exchanging sites for 13 C and nine for 205 Tl at 25 °C along with 1 H correlations yielded otherwise non‐accessible information. A similar approach furnished the dynamic parameters for the ligand exchange in tripodal phosphine‐nitrile complex 146 and Cl/SCN replacement in Ni A‐frame complexes 147 …”

“…Analysis of six exchanging sites for 13 Ca nd nine for 205 Tl at 25 8C along with 1 Hc orrelationsy ieldedo therwise non-accessible information.Asimilara pproach furnished the dynamic parameters for the ligand exchange in tripodalp hosphine-nitrile complex 146 [223] and Cl/SCN replacement in Ni A-frame complexes 147. [224] The cis-trans isomerisation and step-wise replacement of ligands by solventm olecules (wet acetone-d, À56 8C) in aN i complex, 148,w as seen by as pectator 19 F-19 FE XSY, [225] which afforded hard-to-obtain rate constants for hydrolytic cleavage and back-formation (k ex = 0.074 m À1 s À1 )o ft he coordinated acetone ligand.Aproposed fac-mer isomerisation of ag allium Scheme44. Interlocked catenane 140 is in dynamic equilibrium with its constituentr ings.…”