Coordination-induced weakening of ammonia, water, and hydrazine X–H bonds in a molybdenum complex

Abstract

Although scores of transition metal complexes incorporating ammonia or water ligands have been characterized over the past century, little is known about how coordination influences the strength of the nitrogen-hydrogen and oxygen-hydrogen bonds. Here we report the synthesis of a molybdenum ammonia complex supported by terpyridine and phosphine ligands that lowers the nitrogen-hydrogen bond dissociation free energy from 99.5 (gas phase) to an experimentally measured value of 45.8 kilocalories per mole (agreeing closely with a value of 45.1 kilocalories per mole calculated by density functional theory). This bond weakening enables spontaneous dihydrogen evolution upon gentle heating, as well as the hydrogenation of styrene. Analogous molybdenum complexes promote dihydrogen evolution from coordinated water and hydrazine. Electrochemical and theoretical studies elucidate the contributions of metal redox potential and ammonia acidity to this effect.A mmonia and water are ubiquitous small molecules with strong bonds between hydrogen and the central atom (1). For over a century, transition metal-ammine (NH 3 ) and -aquo (H 2 O) compounds have defined bonding paradigms in chemistry (2), found application in cancer therapy (3), and promoted important fundamental chemical reactions such as electron transfer that rely on the inertness of the N-H or O-H bonds in the supporting ligands ( Fig. 1) (4).Common strategies for activation of ammonia and water include oxidative addition to a transition metal center (5-7), deprotonation by transition metal hydrides (8), reaction with bimetallic compounds (9), cooperative chemistry between a transition metal and a supporting ligand (10-12), and element-hydrogen (X-H) bond cleavage through reaction with main group compounds (13-16). Using most of these strategies, activation of the strong X-H bond is not typically coupled to H-H bond formation. One exception is the observation of H 2 elimination following oxidative addition of ammonia to a tantalum(III) complex (17).An alternative and less commonly explored strategy is homolytic cleavage of the X-H bond. Because of the high gas-phase bond dissociation free energies (BDFEs; 99.5 and 111.0 kcal/mol for NH 3 and H 2 O, respectively) (1), interaction with a transition metal or other appropriate reagent is necessary to induce bond weakening. As shown in Fig. 1, most classical transition metal compounds with ammine (aquo) ligands have N-H (O-H) bond strengths that are only slightly perturbed from the gas-phase value. Because experimental data are lacking, we used density functional theory (DFT) to compute N-H BDFEs.Coordination-induced bond weakening, whereby interaction of a ligand results in a considerable lowering of the X-H BDFE, has recently been identified or implicated in rare instances (18-23) and has been applied by Knowles's group (24) and others (25-27) in reactions of organic molecules involving N-H and O-H bonds, respectively. Cuerva's group (26, 27) and ours (28) However, this strategy has not yet been shown to be capable of ...

Ruthenium Complexes for Catalytic Dehydrogenation of Hydrazine and Transfer Hydrogenation Reactions

“…[5] In this regard,t reating 4-methoxys- [25] Similara ctivation of hydrazine, by weakening of NÀHb ond of hydrazine upon coordination of hydrazinew ith molybdenum-terpyridine-phosphine has also been demonstrated. [26] It has been observedt hat such metal hydrazinec omplexation lower the nitrogen-hydrogen bond dissociation free energy (34.6 kcal mol À1 )a nd hence enable H 2 evolution and hydrogenation of styrene. [26] Analogous Ru-hydride and Ru-diazene intermediates werea lso observed during dehydrogenation of hydrazine over Ru-tetradentate phosphine complexes via a" coordination induced activation pathway".…”

“…[26] It has been observedt hat such metal hydrazinec omplexation lower the nitrogen-hydrogen bond dissociation free energy (34.6 kcal mol À1 )a nd hence enable H 2 evolution and hydrogenation of styrene. [26] Analogous Ru-hydride and Ru-diazene intermediates werea lso observed during dehydrogenation of hydrazine over Ru-tetradentate phosphine complexes via a" coordination induced activation pathway". [27] Based on the above findings and spectrald ata (mass and 1 HNMR) we proposed ap lausible general reaction pathway for the hydrazine activation with ruthenium complexes shown in the Scheme 1.…”

“…num-terpyridine-phosphine in [( Ph Tpy)(PPh 2 Me) 2 Mo(k 2 -N 2 H 4 )] complex. [26] Field et al also investigated the base-induced dehydrogenation of hydrazinev ia a "coordination induced activation pathway" over Ru-tetradentate phosphine complexes, where ligand exerted significant impact on the coordination of hydrazinet oR u( side-on [RuCl(k 2 -N 2 H 4 )(k 3 -PP 3 ipr )] + or end-on bound hydrazine [RuCl(k 1 -N 2 H 4 )(k 4 -PP 3 Ph )] + )a nd the reactivity pathway of the resultingc omplexes. [27] Apart from homogeneous catalysts there are several reports availablei nl iterature for the dehydrogenation of hydrazine using heterogeneousc atalysts but exploration of homogeneous complexes for hydrazine activation is of particulari mportanceb ecause of several applications.…”

Activation of O−H and C−O Bonds in Water and Methanol by a Vanadium‐Bound Thiyl Radical

“…As mentionedi nt he introduction, there are several known mechanisms for water activation,i ncluding (1) oxidative addition to as inglet ransitionm etal center, [10a, 22] ab inuclear transition metal center, [10] or ag eometrically constrained main group element, [11] (2) OÀHc ooperative cleavage by at ransition metal and its supporting ligands, [12] and (3) coordination-induced weakening of OÀHb onds to form H 2(g) . [13] In the current system,w ef ound that OÀHb onds of water were cleaved through au nique ligand-ligand cooperation fashion.T his represents an ovel pathwayf or water activation.…”

“…[12] Recently,i tw as also found that OÀHb ond can be weakened via water coordinationt oaMo complex, then activated with the evolution of H 2 . [13] H 2 Oi sw eakeneda nd activated by coordinationt oatitanium center or through the second coordination sphere interactioni n[ Cp 2 TiCl], [14] thus serving as ah ydrogen-atomt ransfer reagent. [15] In this work, we demonstrate that an isolated vanadium-bound thiyl radicals peciesh as the reactivity to activate H 2 Oa nd CH 3 OH through the cleavage of strong OÀHa nd CÀO bonds.…”