Highly Active Iron Catalyst for Ammonia Borane Dehydrocoupling at

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Highly Active Iron Catalyst for Ammonia Borane Dehydrocoupling at Room Temperature Arne Glüer,† Moritz Förster,¶ Vinicius R. Celinski,‡ Jörn Schmedt auf der Günne,‡ Max C. Holthausen,¶ and Sven Schneider*,† †

Institut für Anorganische Chemie, Georg-August-Universität Göttingen, Tammannstrasse 4, 37077 Göttingen, Germany Insitut für Anorganische und Analytische Chemie, Goethe-Universität, Max-von-Laue-Strasse 7, 60438 Frankfurt am Main, Germany ‡ Inorganic Materials Chemistry, University of Siegen, Adolf-Reichwein-Straße 2, D-57068 Siegen, Germany ¶

S Supporting Information *

ABSTRACT: The iron complex [FeH(CO) (PNP)] (PNP = N(CH2CH2PiPr2)2) is a highly active catalyst for ammonia borane dehydrocoupling at room temperature. Mainly linear polyaminoborane is obtained upon release of 1 equiv of H2. Mechanistic studies suggest that both hydrogen release and B−N coupling are metal-catalyzed and proceed via free aminoborane. Catalyst deactivation results from reaction with free BH3 that is formed by aminoborane rearrangement. Importantly, borane trapping with a simple amine allows for the observation of a TON that is unprecedented for a well-defined base metal catalyst.

KEYWORDS: ammonia borane, dehydrocoupling, iron, pincer ligand, catalyst deactivation, hydrogen storage

T

Scheme 1. Ammonia Borane Dehydrocoupling with Catalyst 3

he dehydrogenation of ammonia borane (NH3BH3, AB) has received considerable attention both for H2 storage and transfer and for the selective formation of B−N polymeric materials.1 New protocols on the regeneration of dehydrogenation products, such as (poly)borazine, (P)BZ, or polyaminoborane, PAB, further fuelled this interest.2 Several catalysts for AB dehydrocoupling were reported, and particularly, some homogeneous second and third row transition metal catalysts showed remarkably high activity and selectivity.3 In contrast, only a few well-defined base metal catalysts were examined.4,5 Importantly, they generally suffer from much lower turnover numbers (TON) and frequencies (TOF), hence requiring high catalyst loading (typically 5 mol %), reaction temperatures (typically 60 °C), and/or photochemical activation. We previously reported the highly active ruthenium catalysts [Ru(H)PMe3(PNP)] (1) and [Ru(H)2PMe3(HPNP)] (2, HPNP = HN(CH2CH2PiPr2)2) for amine borane dehydrogenation.6 The iron complex [Fe(H)CO(PNP)] (3) and related PNP hydrides were recently utilized by several groups as catalysts in challenging de/hydrogenation reactions of organic substrates.7 In this contribution, we report the use of 3 for AB dehydrocoupling to mainly linear PAB at room temperature with low catalyst loading. Importantly, mechanistic examinations enabled 3-fold increase of the TON for 3 and ruthenium catalyst 2 upon simple addition of amine in catalytic amounts. Complex 3 catalyzes the release of 1 equiv of H2 from AB at room temperature without additional activation, such as base or irradiation (Scheme 1). The high catalytic activities (TOF = 30 © XXXX American Chemical Society

h−1) at room temperature are unprecedented for well-defined base metal catalysts.4 Full conversion is obtained with catalyst loadings as low as 0.5 mol % (Figure 1). Furthermore, the TONmax is strongly dependent on catalyst loading, e.g. rising from around 95 (0.1 mol % 3) to 200 (0.5 mol % 3), respectively. This observation suggests that catalysis scales with a higher order in Fe concentration than catalyst deactivation (see below). The 11B MQMAS NMR spectrum of the polymeric, insoluble main product (ca. 90% isolated yield) strongly resembles that Received: October 26, 2015

7214

DOI: 10.1021/acscatal.5b02406 ACS Catal. 2015, 5, 7214−7217

Letter

ACS Catalysis

exceeding 1 equiv. According to experimental and theoretical studies, these products can be attributed to metal-free oligomerization of transient, free aminoborane.10 The release of H 2 NBH 2 as intermediate was confirmed by the observation of H2NB(C6H11)2 upon dehydrogenation in the presence of cyclohexene (Figure S8).10a,11 Note that release of free aminoborane was previously associated with catalysts that produce (P)BZ instead of PAB.10a Hydrogen release at room temperature exhibits first-order rate dependence both in catalyst and in AB (v0 = k [3] [AB], k = 4.6 M−1 s−1; Figure S3), as previously found for catalyst 2 (k = 24 M −1 s −1 ). 6d No induction period is observed. Furthermore, the solution retains a yellow color during catalysis, and addition of mercury does not reduce the reaction rate (Figure S1). These results point toward homogeneous catalysis.12,13 The dihydrides trans- and cis-[Fe(H)2CO(HPNP)] (4a/b),7e are detected by NMR spectroscopy as the main iron species during catalysis (Figure S7), presumably representing the resting state. Further mechanistic details are obtained from DFT computations for the PMe2-truncated model system (Scheme 2).14 Formation of dihydride 4aMe from

Figure 1. Kinetic plots of catalytic ammonia borane dehydrogenation at room temperature in THF (c0(AB) = 0.54 M at 0.2−1 mol % 3 and c0(AB) = 0.27 M at 2 mol % 3).

of PAB obtained with catalyst 2 (Supporting Information, Figure S5).6d The main signal at δiso = −10.6 ppm (second order quadrupolar effect parameter (SOQE) = 1.5 MHz) is assigned to boron atoms in the main chain and low intensity signals at δiso = −21.4 ppm (SOQE = 1.1 MHz) and δiso = −20.8 ppm (SOQE = 1.4 MHz) to BH3 end groups. The small SOQE (0.5 MHz) of a minor signal at δiso = 1.5 ppm indicates a symmetrical environment, and the chemical shift is in agreement with four nitrogen substituents around boron.8 This signal is therefore assigned to B(NH2)4 moieties that link the polymer chains. Notably, the same signals were found for catalysts 1, 2, and [Ir(H)2{C6H3(OPtBu2)2}].6c,d Besides PAB, small amounts of borazine (BZ, B3N3H6, δ11B = +30.7 ppm), polyborazylene (PBZ, B3N3Hx