Synthesis and Spectroscopic Properties of Dihydrogen Isocyanide Niobocene [Nb(η⁵-C₅H₄SiMe₃)₂(η²-H₂)(CNR)]⁺ Complexes. Experimental and Theoretical Study of the Blocked Rotation of a Coordinated Dihydrogen

Abstract

Synthesis of stable hydride isocyanide derivatives Nb(η5-C5H4SiMe3)2(H)(CNR) has been achieved through the formation of coordinatively unsaturated 16-electron species Nb(η5-C5H4SiMe3)2H by thermolytic loss of H2 followed by the coordination of an isocyanide ligand. Low-temperature protonation with a slight excess of CF3COOH leads to the η2-dihydrogen complexes [Nb(η5-C5H4SiMe3)2(η2-H2)(CNR)]+. NMR spectra of these H−H complexes and their monodeuterated H−D isotopomers present a single high-field resonance at room temperature. By lowering the temperature to 178 K, decoalescence of the signal was observed for the H−D complexes but not for the H−H ones. By combining DFT electronic structure calculations with a monodimensional rotational tunneling model, it has been shown that the absence of decoalescence of the H−H signal is due to the existence of a very large exchange coupling. Conversely, for the H−D isotopomer, the difference in zero point energy corresponding to two nonequivalent (H−D and D-H) positions leads to a slight asymmetry which dramatically reduces the exchange coupling, allowing decoalescence to be observed. Therefore, the H−D classical rotation and the quantum exchange processes will not be practically observed for this complex, whereas only the classical process for the H−H species is quenched out on the NMR time scale.

Molecular Structure of a Hydridoniobocene Complex [Nb(η5‐C5H4SiMe3)2(H)3] and Its Use as Catalyst for the Ring‐Opening Polymerization of Cyclic Esters

“…[15] These two facts seem intuitively to be related, and this led us to envisage a facile H 2 elimination in complex 1 by thermolytic treatment. [2,4] These findings are also consistent with the observed fluxionality of the hydride ligand at high temperature, a process responsible for their special spectroscopic properties. [2,3] …”

“…This intermediate has previously been proposed to be present in different processes. [4,6] Subsequently, a lactone monomer would coordinate to the metal center through the acyl oxygen atom. At this point, two pathways were envisaged for the opening of the lactone ring: cleavage of the bond between the oxygen atom and the alkyl carbon atom (C-O), to form carboxylate active centers, or cleavage of the bond between the oxygen atom and the acyl carbon atom [C(O)-O], resulting in the formation of alkoxide active centers.…”

“…[2] It is now well-accepted that the unusual NMR proton spin couplings between labile hydride protons in transition metal trihydride complexes are due to quantum mechanical hydrogen exchange. [3] Additionally, 1 has shown a broad range of interesting reactivity patterns, namely: (1) easy thermal H 2 elimination as a synthetic method for the preparation of 18-electron Nb III [Nb(η 5 -C 5 H 4 SiMe 3 ) 2 (H)L] (L = π-acid ligand) species; [4] (2) σ-bond activation of a considerable number of chemical species (e.g. H-Ge, H-N, H-Si, H-Sn, etc.…”

First Investigation of Non‐Classical Dihydrogen Bonding between an Early Transition‐Metal Hydride and Alcohols: IR, NMR, and DFT Approach

“…We have also studied the interaction of a strong acid, modelled as H 3 + system: [21] namely, the p-acceptor ligand stabilises the dihydrogen form. The d 2 [MCp 2 LH] complexes behave as Lewis bases and they can therefore be protonated by acids at room temperature to give the corresponding cationic complexes.…”

Theoretical Study of the Effect of Lewis Acids on Dihydrogen Elimination from Niobocene Trihydrides

“…In the lateral adduct corresponding to the catecholborane model case (2 s) the two free hydrogen atoms are still clearly two hydride ligands (the H À H distance is 1.77 as seen in Figure 2). In contrast, when the stronger BF 3 and BH 3 acids are interacting with a side hydride the remaining two hydrogen atoms form a dihydrogen ligand (structures 3 s and 4 s in Figures 3 and 4, respectively) [22] CNR [23] ). Our results show that in order to favor a dihydrogen over a dihydride structure by decrease of the electron density on the metal, there is no need the Lewis acid coordinate directly to the metal.…”