1995 Alcan Award Lecture New intermediates in the homolytic and heterolytic splitting of dihydrogen

Abstract

Some of the research of the author and his research group into the structure and reactions of dihydrogen complexes of transition metals is reviewed. The characterization of osmium complexes that can be regarded as having intermediate structures on the way to the homolytic splitting and to the heterolytic splitting of dihydrogen is described. The properties of an iridium complex with novel short proton-hydride contacts is also reviewed.Ke)) words: transition metal, dihydrogen, hydride, complexes, NMR, neutron diffraction, osmium, iridium.Resume : On prCsente une revue des travaux de recherche effectuCs dans les laboratoires de l'auteur sur la structure et les rCactions des complexes dihydrogCnCs des mCtaux de transition. On dCcrit la caracterisation des complexes de l'osmium qui peuvent &tre considCrCs comme possCdant des structures intermediaires entre celles conduisant aux ruptures de la liaison H-H du dihydrogkne soit homolytiques soit hCtCrolytiques. On passe aussi en revue les propriCtCs d'un complexe d'iridium qui comporte de nouveaux contacts courts hydrons-hydrure.

Dihydrogen complexes as prototypes for the coordination chemistry of saturated molecules

“…Thus, H 2 gas can be turned into a strong acid: free H 2 is an extremely weak acid [pK a ϳ 35 in THF (86)], but binding it to an electrophilic cationic metal increases the acidity spectacularly, up to 40 orders of magnitude. The pK a can become as low as Ϫ6, i.e., 2 -H 2 can become more acidic than sulfuric acid as shown by Morris (25,26,82) and later Jia (36). Electron-deficient cationic H 2 complexes with electron withdrawing ligands such as CO and short H-H bonds (Ͻ0.9 Å), i.e., [Re(H 2 )(CO) 4 ϩ becomes acidic enough to protonate weakly basic ethers (89).…”

“…It would take more than a year before others were identified, notably by Morris, Crabtree, Chaudret, and Heinekey. This quartet has since performed elegant synthetic, reactivity, and NMR studies on H 2 and silane complexes (5,(25)(26)(27)(28)(29)(30) (33). However, attempts to prove H 2 binding here was problematic, even long after H 2 binding was established (34).…”

Activation of Molecular Hydrogen

“…Within the large regime of hundreds of L n M-H 2 complexes, the reaction coordinate for the activation of H 2 on a metal (Scheme 5.3) shows d HH varying enormously, from 0.82 to 1.5 A (Scheme 5.7). 3,[10][11][12][13][14][15]17,[21][22][23][24]30,[34][35][36][37][38][39][40][43][44][45][46][47][48][49][50] Although the d HH ranges shown are arbitrary, each category of complexes has distinct properties. The d HH is relatively short (0.8-1.0 A ), and H 2 is reversibly bound, in "true" H 2 complexes best exemplified by W(CO) 3 14,34,48,[49][50][51] were first clearly identified in 1991 in ReH 5 (H 2 )(PR 3 ) 2 where neutron diffraction showed a d HH of 1.357(7) A .…”

“…These researchers have since performed synthetic, reactivity, and NMR studies on H 2 complexes [9][10][11][12][13][14][15][16]114 and were eventually joined by more than 100 investigators worldwide. Remarkably, several complexes initially believed to be hydrides were revealed to be H 2 complexes by Crabtree 9,17 in 1986 using as criteria the short proton NMR relaxation times of H 2 ligands.…”

“…20 Over 600 H 2 complexes are known (most of them stable) for nearly every transition metal and type of coligand and are the focus of 1500 publications, dozens of reviews, and three monographs. [2][3][4][8][9][10][11][12][13][14][15][16]114,[21][22][23][24][25][26][27][28][29] The view on H 2 complexes has shifted from significance in basic science to more practical energy-relevant aspects, for example, H 2 production and storage. An important question after their discovery was are H 2 complexes relevant in catalysis, that is, does direct transfer of hydrogen from a H 2 ligand to a substrate occur?…”