NMR and DFT Analysis of [Re₂H₂(CO)₉]:  Evidence of an η²-H₂Intermediate in a New Type of Fast Mutual Exchange between Terminal and Bridging Hydrides

Abstract

Protonation of the anion [Re(2)H(CO)(9)](-) (1) with a strong acid at 193 K affords the neutral complex [Re(2)H(2)(CO)(9)] (2), that in THF above 253 K irreversibly loses H(2) to give [Re(2)(CO)(9)(THF)], previously obtained by room-temperature protonation of 1. Treatment of 2 with NEt(4)OH restores the starting anion 1. Variable temperature (1)H and (13)C NMR spectra as well as T(1) measurements agree with the formulation of 2 as a classical [Re(2)H(mu-H)(CO)(9)] complex, in which two dynamic processes takes place. The "windshield-wiper motion" observed in several related complexes equalizes the two carbonyls trans to the hydrides (E(a) = 44(1) kJ mol(-)(1)), while another much faster process equalizes bridging and terminal hydrides already at 172 K. The variable temperature behavior of the (1)H transverse relaxation times revealed also proton exchange between 2, water, and the parent anion 1 (due to the acidity of 2), but such a process is too slow to account for the fast hydrides exchange in 2. The nature of the latter process has been investigated both experimentally and theoretically. Kinetic data, obtained by the analysis of the variable temperature (1)H spectra (E(a) = 24.5(5) kJ mol(-1)), revealed a small normal kinetic isotope effect (ca. 1.5). The (2)H chemical shift of the fully deuterated isotopomer 2-d(2) was found isochronous with 2, thus ruling out the presence of a significant concentration of a nonclassical [Re(2)(eta(2)-H(2))(CO)(9)] tautomer, in fast exchange with the classical dihydride. Density functional theory (DFT) calculations, carried out at the B3LYP level, confirmed the formulation of [Re(2)H(2)(CO)(9)] as a classical complex. However, when DFT was used to obtain a detailed description of the dynamic behavior of 2 in solution, a new type of hydride fast exchange emerged, involving the nonclassical tautomer as a relatively high energy (12.7 kJ mol(-1)) intermediate. Isotopic perturbation of the equilibrium by partial deuteration of 2 indicated the preference of deuterium for the bridging sites, with Delta H degrees = -475(4) J mol(-1) and Delta S degrees = -0.80(2) J K(-1) mol(-1). The same preference was observed in the anion [Re(2)H(mu-H)Cl(CO)(8)](-).

Reversible Binding of Dihydrogen in Multimetallic Complexes

“…[14] The dihydride complex Re 2 H(µ-H)(CO) 9 irreversibly loses H 2 above 253 K and is a highly fluxional molecule, exchanging hydride sites through a low-energy process (Scheme 2). [15] NMR spectroscopic data (T 1 measurements) suggest a dihydride structure; however, experimental and theoretical studies are in close agreement and show that the barrier for exchange through a dihydrogen intermediate is only 23 [8,16] as do related chelated phosphane complexes [12] and the cluster [Ru 4 (η 6 -C 6 H 6 ) 4 H 6 ] 2+ , [11] both of which have a mixture of dihydrogen and dihydride ligands Scheme 2. as characterised by T 1 relaxation time measurements by NMR spectroscopy at low temperature. The iridium dimer Ir 2 (PiPr 3 ) 2 (pz) 2 (H) 2 (µ-H)(η 2 -H 2 ) is suggested to have a dihydrogen ligand that undergoes rapid exchange with a terminal hydride, with a barrier of 32 kJ mol -1 .…”

“…Kinetic experiments suggest that the rate-determining step is oxidative addition of H 2 at one of the metal centres after CO loss. [35] (15) have been shown to be excellent catalysts for H 2 /D 2 equilibration to form HD. The mechanism proposed invokes reversible addition of both H 2 and D 2 to the metal cluster to afford metal hydride species, [37] which have been observed experimentally to form reversibly on addition of H 2 .…”

NMR Investigation of the Dihydrogen‐Bonding and Proton‐Transfer Equilibria between the Hydrido Carbonyl Anion [HRe2(CO)9]− and Fluorinated Alcohols

“…9 The anionic charge on our hydridic complex makes the situation very different here. The protonated product 3 is neutral, and we can confidently rule out the involvement of its hydrido ligands in H‐bonding with the alcoholate, because the position of the hydride resonance ( δ =−10.7 ppm, averaged signal for bridging and terminal hydrides)12 did not change on varying the nature of the alcohol. We think that in the present case (excess of free alcohol and aprotic, slightly polar solvent) the RO − anion is mainly stabilized by interaction with a second alcohol molecule [Eq.…”

“…Interaction of [NEt 4 ][HRe 2 (CO) 9 ] with HFIP : Addition of HFIP to CD 2 Cl 2 solutions of [NEt 4 ]‐ 1 caused not only the expected upfield shift of the hydride resonance of 1 , due to the formation of 2 b ,27 but also the formation of variable amounts of the neutral complex [H 2 Re 2 (CO) 9 ] ( 3 , δ =−10.7 ppm). Proton transfer, with formation of the RO − anion, most likely affords initially the η 2 ‐H 2 tautomer, which is in fast exchange with the more stable isomer containing a terminal and a bridging hydride ligand (Scheme ) 12…”

“…We considered therefore the dinuclear anionic complex [HRe 2 (CO) 9 ] − ( 1 , Scheme ). Some of us recently showed12 that low‐temperature protonation of this anion by strong acids (e.g., CF 3 SO 3 H, HBF 4 ) gives the neutral species [H 2 Re 2 (CO) 9 ] ( 3 , Scheme ), which contains one terminal and one bridging hydride ligand, in fast exchange via η 2 ‐H 2 tautomer 3′ as a relatively high energy (3 kcal mol −1 ) intermediate. This tautomer is probably involved both in the irreversible H 2 elimination that occurs at T >240 K, and in the formation of the neutral species by protonation of the anion 1 (protonation at the hydridic site is kinetically preferred with respect to the metal) 13…”