Chiral amine ligands at rhenium(V) pentahydride complexes allow for characterization of an energetically accessible and reversible steric inversion of diastereotopic phosphorus atoms

Dynamic Processes of Rhenium Polyhydride Complexes

“…Line shape fitting of dynamic NMR spectra [ 10 , 23 , 31 , 33 , 34 , 37 , 58 , 59 ], molecular modeling [ 22 , 27 , 35 , 36 ], and isotopic substitution [ 1 , 4 , 31 , 33 , 60 , 61 ] are the most commonly used techniques to examine the dynamic properties of rhenium polyhydride complexes. Longitudinal 1 H NMR relaxation measurements (T 1 ) of hydride ligand relaxations have been frequently used to explore the possibility of dihydrogen ligand coordination, but the technique has been shown to have issues that make it less than ideal [ 11 , 14 , 16 , 58 , 62 ].…”

“…Line shape fitting of such variation is one of the techniques used, when the slow-exchange domain of the NMR spectra can be reached, to identify the dynamic processes of rhenium polyhydride complexes and to quantify the thermodynamic parameters for such processes. An alternative NMR method of quantifying the free energy of activation for a dynamic process in which an exchange occurs between two nuclides with an equal population uses the frequency difference between the two resonances and the coalescence temperature to estimate the energy barrier at the coalescence temperature (ΔG ‡ ) for the process ( Figure 8 ) [ 37 ]. When such an exchange occurs between two resonances of unequal populations, as occurs when isomers are present [ 32 ], an estimate of the two populations, the frequency difference between the resonances, and the coalescence temperature can produce an estimate of the energy barrier for conversion of each isomer into the other isomer [ 64 ].…”

“…The inclusion of a chiral substituent in the 4 position of a pyridine ligand or inclusion of a chiral aliphatic amine into complexes of the form ReH 5 (PPh 3 ) 2 (amine) produces two doublet resonances for diastereotopic phosphorus atoms in the slow-exchange domain of the 31 P-{ 1 H} NMR spectra for some such complexes [ 37 , 65 ]. In the fast-exchange domain of the 31 P-{ 1 H} NMR spectra of such complexes, the pair of doublets coalesce into a sharp singlet through, presumably, the same dynamic process that occurs when the amine ligand is pyridine.…”

“…Figure 16 shows that fast rotation about the Re-N bond in the complex ReH 5 (PPh 3 ) 2 [4-CH 3 CH(OH)py] will make the 3 and 5 1 H NMR resonances equivalent in the fast-exchange spectrum but will not affect the coalescence of the 31 P-{ 1 H} diastereotopic phosphorus atom resonances. For the complex of interest, the energy barriers for the exchange of 3 and 5 protons or for the coalescence of the diastereotopic phosphorus atom resonances are essentially the same [ 37 ]. Of the two choices depicted in Figure 16 , the results of the investigation of the complex ReH 5 (PPh 3 ) 2 [4-CH 3 CH(OH)py] support a mechanism in which the 3 and 5 proton of aromatic amine ligands and the interconversion of E and Z isomers occurs through a process that inverts the steric perspective of the two phosphorus atoms with respect to the nitrogen atom found in a B site on the second trapezoid.…”

“…Rhenium polyhydride complexes (rhenium complexes supported by four or more hydride ligands) have been the focus of numerous studies. The large number of reports on rhenium polyhydride complexes reflect several factors: (1) convenient synthetic routes into such complexes [ 1 , 2 , 3 , 4 ]; (2) the relative stability of such complexes, which makes for convenient materials to work with; (3) intrinsically interesting aspects of rhenium polyhydride complexes such as the homoleptic dianion, (ReH 9 ) 2− or the presence of dihydrogen as a ligand on a metal center with multiple hydride ligands [ 5 , 6 , 7 , 8 , 9 , 10 ]; (4) the technical challenge of characterizing a dihydrogen ligand in the presence of multiple hydride ligands bound to a metal center [ 11 , 12 , 13 , 14 , 15 , 16 , 17 ]; (5) the convenient ability to prepare mixed-metal polyhydride complexes and examine the properties of such complexes [ 18 , 19 , 20 , 21 ]; (6) an interest in understanding the dynamic properties and solution behavior of high coordination metal centers stabilized by several hydride ligands [ 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 , 33 , 34 , 35 , 36 , 37 , 38 ]; and (7) the demonstrated ability for such complexes to serve as precatalysts for the transformation of organic molecules [ 39 , 40 , 41 ,…”