Cobalt-Catalyzed Ammonia Borane Dehydrogenation: Mechanistic Insight and Isolation of a Cobalt Hydride-Amidoborane Complex

Abstract

A Co(I) complex featuring the electron-rich monoanionic bis(carbene) aryl pincer ligand ( Mes CCC) ( Mes CCC = bis(2,4,6-trimethylphenyl-benzimidazol-2-ylidene)phenyl), ( Mes CCC)Co-py (1), was found to catalytically dehydrogenate ammonia borane (NH 3 BH 3 , AB) in THF at 60°C. This process releases 1.7 ± 0.1 equiv of H 2 per AB with simultaneous formation of both soluble (borazine and polyborazylene) and insoluble (poly(aminoborane)) BN-containing species. To help elucidate the Co species present under the catalytic conditions, 1 was reacted with a stoichiometric amount of AB in THF at room temperature, yielding ( Mes CCC)CoH(NH 2 BH 3 ) (2), the first characterized hydride-amidoborane complex of a late transition metal. This complex was characterized by multinuclear ( 1 H, 11 B, and 13 C) NMR and IR spectroscopies as well as X-ray crystallography. Formation of 2 via N−H activation of AB across the Co(I) center of 1 was confirmed by the reaction of 1 with the deuterated isotopologues of AB and was supported computationally by means of density functional theory (DFT) calculations. Isolated 2 was shown to catalyze AB dehydrogenation, forming BN-containing products similar to 1, albeit at a slower rate. In both reactions starting with 1 or 2 as the catalyst, 2 is observed throughout the catalytic dehydrogenation of AB. DFT calculations revealed plausible pathways for the formation of aminoborane (NH 2 BH 2 ) from 2, the generation of soluble BN-containing products, as well as the on-metal oligomerization of AB to produce the insoluble polymeric species. The complexes reported herein represent rare examples of homogeneous cobalt catalysts for the dehydrogenation of AB.

Bimetallic Co–M (M = Cu, Ag, and Au) Carbonyl Complexes Supported by N-Heterocyclic Carbene Ligands: Synthesis, Structures, Computational Investigation, and Catalysis for Ammonia Borane Dehydrogenation

“…Ammmonia–borane (NH 3 BH 3 , AB) dehydrogenation may represent a proof-of-concept target for bifunctional activation by means of heterometallic complexes. Indeed, AB contains both hydridic (B–H) and protic (N–H) hydrogens, which could be activated by interaction with the Lewis acid site and the Lewis base site, respectively, of a heterometallic metal-only Lewis pair . AB dehydrogenation is a very interesting reaction for the potential application of AB as hydrogen storage medium for transportation. − Several monometallic transition metal complexes have been investigated as catalysts for AB dehydrogenation, including NHC complexes and the use of noninnocent ligands and homometallic carbonyls. − Bifunctional AB dehydrogenation may be promoted by [L n M]–H transition metal hydrides, where the [L n M] fragment acts as Lewis acid and the hydride holds a negative charge (Lewis base).…”

Insertion of BH3 into a Cobalt–Aryl Bond: Synthetic Routes to Arylborohydride and Borane-Amino Hydride Complexes

“…20−22 Although direct activation of amines by borohydride complexes has been reported only rarely, Fout et al recently achieved the activation of a N−H bond in H 3 N• BH 3 by the cobalt(I) complex (MesCCC)Co-py, affording a first example of a hydride-amidoborane complex on a late transition metal. 23 Very recently, we demonstrated dehydrogenation-regeneration of iron amido-borane complex Cp*Fe-(η 1 -H 3 B-NHC 6 H 4 PPh 2 ), which was synthesized from an iron tetraborohydride precursor Cp*Fe(η 2 -BH 4 )(NCMe) with a phosphinoamine ligand 1,2-Ph 2 PC 6 H 4 NH 2 . 24 We report here the synthesis of cobalt arylborohydride complexes Cp*Co(κ 3 -…”

“…Buil et al found the tetrahydridoborate osmium hydride, OsH­(η 2 -H 2 BH 2 )­(CO)­(P i Pr 3 ) 2 , is able to activate aryl amines, affording the amino-boryl complex by release of 4 equiv of H 2 (Figure , b) . Such an elementary reaction is promising because it is relevant to catalytic amine-borane dehydrogenation and related transfer hydrogenation reactions. − Although direct activation of amines by borohydride complexes has been reported only rarely, Fout et al recently achieved the activation of a N–H bond in H 3 N·BH 3 by the cobalt­(I) complex (MesCCC)­Co-py, affording a first example of a hydride-amidoborane complex on a late transition metal . Very recently, we demonstrated dehydrogenation-regeneration of iron amido-borane complex Cp*Fe­(η 1 -H 3 B-NHC 6 H 4 PPh 2 ), which was synthesized from an iron tetraborohydride precursor Cp*Fe­(η 2 -BH 4 )­(NCMe) with a phosphinoamine ligand 1,2-Ph 2 PC 6 H 4 NH 2 .…”

Dehydrogenation of iron amido-borane and resaturation of the imino-borane complex

“…Because of relevance to H 2 storage 1-10 and hydrogenation catalysis, [11][12][13][14][15] metal amine-borane complexes [16][17][18] and their dehydrogenated forms, such as amino-boranes [20][21][22] and iminoboranes 4 are arising as a signicant family in organometallic chemistry. In transition metal-catalyzed dehydrocoupling of amine-boranes and related transfer hydrogenations, the interactions between the metal and the borane fragment are essential to dehydrogenation and the consequent transformations.…”

“…In transition metal-catalyzed dehydrocoupling of amine-boranes and related transfer hydrogenations, the interactions between the metal and the borane fragment are essential to dehydrogenation and the consequent transformations. [16][17][18][19][20] Specically, amino-borane complexes containing a M-H 2 B]NR 2 moiety are the primary dehydrogenated species and are oen identied as a resting point in the catalysis (Scheme 1a). [20][21][22] Management of reversible dehydrogenation-regeneration reactions on a M-BH 2 ]NR 2 platform could provide a strategy with which to design efficient catalysts capable of operating sustainable syntheses.…”