Fluxionality, substitution, and hydrogen exchange at eight-coordinate rhenium(V) polyhydride centers

Describing Polytopal Rearrangement Processes of Octacoordinate Structures. I. Renewed Insights into Fluxionality of the Rhenium Polyhydride Complex ReH5(PPh3)2(Pyridine)

“…On this basis, we will further examine the rotational flexibility of the pyridine ligand 27 and the hydrogen exchange with a water molecule in other rhenium polyhydride structures similar to ReH 5 (PPh 3 ) 2 (pyridine). 28 In addition, the observation of three separate coalescences in the hydride NMR peaks 62 and the occurrence of coalescence in the phosphorus resonances 29 are part of the unsolved fluxionality puzzles in octacoordinate rhenium polyhydrides. Computational studies of these problems will help us to depict a more complete picture of fluxionality in rhenium polyhydrides.…”

“…As a part of our enduring interest in describing the polytopal rearrangement mechanisms with theoretical chemistry, we chose the octacoordinate rhenium polyhydride complexes ReH 5 (PPh 3 ) 2 L (L = pyridine and 2-(acetylamino)­pyridine) as the subject to be investigated in this work. We chose rhenium polyhydrides as the first target in this series of investigations over other octacoordinate compounds − because fluxionality in rhenium polyhydrides is a current topic of interest in the experimental organometallic community. − …”

“…lateral and basal three-arm turnstile rotations) that can jointly lead to the complete scrambling of all five hydrides based on the theoretical calculations with DFT and coupled-cluster theory. Although some recent NMR studies by Moehring and co-workers emphasized on (1) the rotational flexibility of the Re−N bond 27 and (2) the intermolecular exchange between one hydride ligand and a proton of the surrounding water molecule, 28 we believe that to fully understand the fluxionality concerning the five hydrides in ReH 5 (PPh 3 ) 2 (pyridine) (i.e. polytopal rearrangement) is an ideal starting point to solve the puzzles of fluxionality in rhenium polyhydrides.…”

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 ].…”

“…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 ,…”