A Highly Active Bidentate Magnesium Catalyst for Amine-Borane Dehydrocoupling: Kinetic and Mechanistic Studies

Article abstract of DOI:10.1002/chem.201901197

A magnesium complex (1) featuring a bidentate aminopyridinato ligand is a remarkably selective catalyst for the dehydrocoupling of amine-boranes. This reaction proceeds to completion with low catalyst loadings (1 mol %) under mild conditions (60 °C), exceeding previously reported s-block systems in terms of selectivity, rate, and turnover number (TON). Mechanistic studies by in situ NMR analysis reveals the reaction to be first order in both catalyst and substrate. A reaction mechanism is proposed to account for these findings, with the high TON of the catalyst attributed to the bidentate nature of the ligand, which allows for reversible deprotonation of the substrate and regeneration of 1 as a stable resting state.

Full text of DOI:10.1002/chem.201901197

A Journal of
Accepted Article
Title: A Highly Active Bidentate Magnesium Catalyst for Amine-Borane
Dehydrocoupling: Kinetic and Mechanistic Studies
Authors: Alexander Ried, Laurence Taylor, Ana Geer, Huw Williams,
William Lewis, Alexander Blake, and Deborah Lorraine Kays
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A Highly Active Bidentate Magnesium Catalyst for Amine-Borane Dehydrocoupling: Kinetic and
Mechanistic Studies
Alexander C. A. Ried,[a],† Laurence J. Taylor,[a], † Ana M. Geer,[a,d], † Huw E. L. Williams,[b] William Lewis,[c]
Alexander J. Blake,[a] Deborah L. Kays[a],*
[a] School of Chemistry, University of Nottingham, University Park, Nottingham, NG7 2RD (UK)
[b] Centre for Biomolecular Sciences, University of Nottingham, University Park, Nottingham, NG7 2RD
(UK)
[c] School of Chemistry, The University of Sydney, F11, Eastern Ave, Sydney NSW 2006 (Australia)
[d] Current address: Department of Chemistry, University of Virginia Charlottesville, Virginia, 22904
(USA)
†
These authors contributed equally to this manuscript.
E-mail: Deborah.Kays@nottingham.ac.uk
Supporting information (full experimental details for the complex synthesis, catalysis, NMR reaction
monitoring,
crystallographic
data
and
CIF
files)
can
be
found
under
http://dx.doi.org/1002/chem.XXXXXXXXXXX.
Abstract
A magnesium complex (1) featuring a bidentate aminopyridinato ligand is a remarkably selective
catalyst for the dehydrocoupling of amine-boranes. This reaction proceeds to completion with low
catalyst loadings (1 mol%) under mild conditions (60 °C), exceeding previously reported s-block
systems in terms of selectivity, rate, and turnover number (TON). Mechanistic studies by in situ NMR
analysis reveals the reaction to be first order in both catalyst and substrate. A reaction mechanism is
proposed to account for these findings, with the high TON of the catalyst attributed to the bidentate
nature of the ligand, which allows for reversible deprotonation of the substrate and regeneration of 1
as a stable resting state.
Introduction
The transition from fossil fuels to a sustainable global energy system is one of the key challenges of
the 21st century. The increasing use of alternative sources has given rise to significant obstacles in
storage and mobility; i.e. how to utilize generated electricity for transportation or later use. Hydrogen
is a promising option for a clean burning fuel, although issues remain over its storage.1,2 Possible
solutions include physical approaches such as high pressures, low temperatures and physisorption, as
well as chemical storage in the form of hydrides.3,4
Of these, amine-boranes have emerged as lead candidates due to their high storage capacity, stability,
and low environmental impact.5 These compounds have been the focus of considerable attention,
with the presence of adjacent protic and hydridic hydrogens providing an excellent system for
hydrogen storage and release.6–8 Hydrogen evolution from amine-boranes may be achieved by
solvolysis, thermolysis, catalysis, or some combination of the above. Of these, solvolysis creates an
1
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10.1002/chem.201901197
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undesirable thermodynamic sink through the formation of very strong B–O bonds;9,10 and thermolysis
temperatures are often high, affording a complex mixture of products.11
In contrast, catalytic hydrogen release can occur at moderate temperatures, often yielding well-
defined products. Given the dominance of transition metals in catalysis, it is unsurprising that the d-
block has received the most attention in this field, with early dehydrocoupling catalysts based on
rhodium and iridium.12,13 Driven by considerations of resource availability and sustainability, recent
investigations have focused on base metals such as titanium,14,15 manganese,16–19 iron,20,21 cobalt,22,23
and nickel.5
Looking beyond the transition metals, there has been significant work in recent years on catalysis with
main group species.24,25 This expansion has carried through to amine-borane dehydrocoupling,26–28
with one of the most noteworthy examples being a bis(borane) reported by Wegner et al. capable of
releasing 2.45 equivalents of H2 from NH3·BH3.29 Despite their ubiquitous nature, low cost, and low
toxicity (barring beryllium and barium), the s-block elements are underrepresented in this area.
Predominantly investigated as storage materials rather than catalysts,30–33 their lack of popularity may
be due to poor selectivity and extended reaction times of e.g. 124 h (72% conversion) with Me2NH·BH3
as a substrate, as reported for Group 1 bis(trimethylsilyl)amides.34 However, recent work is yielding
more active systems.35,36 Whilst Group 2 systems have shown greater activity towards more complex
amine-borane substrates,37–39 it should be noted that some systems studied have subsequently been
shown to exhibit “spontaneous” dehydrocoupling.40 In contrast, the Me2NH·BH3 and iPr2NH·BH3
substrates investigated here showed no hydrogen release in the absence of catalyst.
Herein, we report a highly active magnesium catalyst for the dehydrocoupling of Me2NH·BH3 and
dehydrogenation of iPr2NH·BH3. In addition to being readily synthesized on a multi-gram scale, the
catalyst exceeds previously published examples in terms of selectivity, substrate scope, turnover
number (TON), and reaction rate. We present a detailed mechanistic investigation into this catalyst,
including stoichiometric reactivity studies and kinetic measurements, and propose a plausible catalytic
cycle to account for these data.
Results and Discussion
Catalyst Synthesis and Investigation
2
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Scheme 1. Synthesis of compounds 1 (a) and 2 (b), and catalytic dehydrocoupling/dehydrogenation of Me2NH·BH3 (c) and iPr2NH·BH3 (d) by 1.
Two magnesium complexes, 1 and 2, were synthesized from the reaction of the proligands (L1H41 or
L2H42) with MeMgI·(OEt2)1.5 and tetramethylethylenediamine (TMEDA) in diethyl ether (Scheme 1a and
b), and obtained in high yields and purity (1: 63% yield; 2: 86% yield; see ESI, Figures S2–S5, sections
S3.2 and S4.1). The complexes were investigated as catalysts for the dehydrocoupling of Me2NH·BH3.
While 2 was found to be unreactive, complex 1 gave excellent conversion of Me2NH·BH3 to the dimeric
dehydrocoupling product [Me2NBH2]2 (3) with concomitant formation of H2 in C6D6 at 60 °C (Scheme
1c, Figures S17–18).
11
1
11
The reaction was monitored by in situ B NMR spectroscopy (Figure 2a), discontinuous H and B
NMR measurements (Figure S15), and volumetrically by the measurement of hydrogen evolution
(Figures S25–26). The reaction is remarkably selective, affording [Me2NBH2]2 (3) almost exclusively (<
3% Me2N=BH2, 4, formed as side product, Figure S18). High conversions are achieved in a relatively
short time with low catalyst loadings (at 60 °C; ca. 99% conversion after 80 mins at 10 mol% [1], 150
mins at 5 mol% [1]; Figure 1a). High conversions were obtained even at catalyst loadings as low as 1
mol%, with >99% in 60 h at 60 °C (implying a TON ≥100; Figure S19). Increasing the temperature to
80 °C gives >95% conversion in ca. 23 minutes with a catalyst loading of just 1.5 mol% (Figure S29d).
The catalyst also works in THF, albeit at a significantly reduced rate (Figure S16), which is attributed
to co-ordination of the solvent to the catalyst active site.43 A similar solvent effect was seen with a
lithium-based dehydrocoupling catalyst studied by Mulvey and co-workers.44
3
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Figure 1. a) Conversion (mol%) vs. time (min) for the dehydrocoupling of Me2NH·BH3 with 5 mol% () and 10 mol% () of 1 in C6D6 at 60 °C. Data
obtained by in situ monitoring of the reaction by 11B NMR. b) Graph tracking concentration of substrate (Me2NH·BH3), product ([Me2NBH2]2; 3), side-
product (Me2N=BH2; 4), and intermediate (Me2NH–BH2–NMe2–BH3; 5) over reaction course. Concentrations determined by in situ 11B NMR
measurements at 60 °C in C6D6. Initial concentrations [3] = 0.29 M, [1] = 15 mM.
Reactions with the related amine-borane iPr2NH·BH3 afforded the dehydrogenated compound
iPr2N=BH2 as the dominant product (>95%, Scheme 1d). This reaction proceeded (albeit slowly) at
room temperature (Figure S21 and S22) and increasing the temperature to 60 °C gave near total
conversion in less than 1 hour (5 mol% catalyst loading, Figure S23). The formation of iPr2N=BH2 has
been observed in previous work on the catalytic dehydrogenation of iPr2NH·BH3.45–47
To put this reactivity into context, the most effective previous alkaline earth catalyst for the reaction
in Scheme 1c was published by Hill et al. in 2010.38 This catalyst required 72 h at 60 °C to effect high
(> 90%) conversions, and was less selective than 1, affording several side products including
(Me2N)2BH. The same catalyst proved more effective at amine-borane dehydrocoupling with other
substrates, catalysing the reaction between secondary and primary amines with either pinacolborane
(HBPin) or 9-borabicyclo[3.3.1]nonane (9-BBN).48 In most cases these reactions proceeded to
completion in 1 h at room temperature (10 mol% catalyst loading).48 A related magnesium complex
was effective at catalysing the dehydrogenation of iPr2NH·BH3 to iPr2N=BH2, but could only effect
stoichiometric conversions of Me2NH·BH3 to 3.47 There have also been reports of magnesium and
calcium bis(trimethylsilyl)amides catalysing the dehydrocoupling of amine-boranes with amines to
afford asymmetric diaminoboranes.49 In most cases, however, these transformations required high
4
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temperatures and long reaction times (i.e. 70 °C, 48 h for coupling Me2NH·BH3 and tBuNH2, 2.5 mol%
catalyst loading).49
Looking at Group 1 species, we can compare 1 to a recent lithium-based catalyst published by Mulvey
et al.36 Although relatively selective for the formation of [Me2NBH2]2 from Me2NH·BH3, the catalyst
showed significantly lower activity than species 1, with 89% conversion after 60 h at 80 °C in toluene
(2.5 mol% catalyst loading). Broadening our scope to the rare-earth metals, a lanthanum hydride
complex published by Okuda et al. catalysed the dehydrocoupling of Me2NH·BH3 at a comparable rate
to species 1 (100% conversion in 2 h at 60 °C, 3 mol% catalyst loading).50 However, the catalyst was
less selective than 1, affording [Me2NBH2]2 and (Me2N)2BH in a ca. 80:20 ratio.50 To the best of our
knowledge, there are no examples of an alkaline earth catalyst (for any amine-borane
dehydrocoupling reaction) that are reported to be effective at <2.5 mol% loading,39 with 1 the first
catalyst for which this has been demonstrated.51
Catalyst reusability was tested by repeated injections of Me2NH·BH3 to a solution of 1 in toluene, with
reaction progress monitored by hydrogen evolution. At 10 mol% catalyst loading, 1 was found to
tolerate two consecutive injections before a significant deterioration of activity was observed (Figure
S26). This is likely due to catalyst decomposition from the gradual ingress of air and moisture with
repeated injections. There are very few studies in the literature where the recyclability of main group
catalysts for the dehydrocoupling of amine-boranes has been ascertained,29 and to the best of our
knowledge this is the first such determination for an s-block metal catalyst.
Using the data gathered from in situ reaction monitoring by 11B NMR spectroscopy (Figures 1 and 2;
Figures S27–S31), it is possible to gain significant mechanistic insight into this reaction. In addition to
starting material, product (3), and trace amounts of side product (Me2N=BH2, 4), the linear species
Me2NH–BH2–NMe2–BH3 (5) is also observed during the reaction.52 Compound 5 is a commonly
observed intermediate in the dehydrocoupling of Me2NH·BH3,14,16 and its concentration remains low
throughout the reaction, gradually dropping to zero as the reaction nears completion (Figure 1b). Side
product 4 appears to form in the early stages of the reaction, and its concentration remains relatively
constant throughout (Figure 1b). Unlike 5, its concentration does not drop to zero. Monitoring by 1H
NMR reveals that 1 is regenerated at the end of the reaction, with no appreciable change in the signals
arising from the catalyst (Figure S20).
5
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Figure 2. Mechanistic data from the dehydrocoupling of Me2NH·BH3 by 1. a) Stacked 11B NMR spectra from in situ NMR measurements. b) Plot
of [Me2NH·BH3] (mM, as determined by integration of 11B NMR spectra) vs time for one set of kinetic data (toluene, 60 °C, 4.5 mM 1). c) Plot of
turnover frequency (TOF, υ·[1]−1) vs concentration of Me2NH·BH3 (mM) for three experiments with different catalyst concentrations. d) Eyring plot
for data collected over the temperature range 50–80 °C.
Plotting reaction rate (υ) against substrate concentration [Me2NH·BH3] (Figure S28) indicates that the
reaction is pseudo-first order with respect to substrate at high concentrations, with a more complex
rate dependence at low substrate concentrations.53 A plot of turnover frequency (TOF = υ·[1]−1)
against substrate concentration for three different catalyst loadings shows overlay of the data sets
(Figure 2c), indicating that the reaction is first order with respect to catalyst.53 From this, a rate
equation (valid for [Me2NH·BH3] >> [1]) of υ = kobs[Me2NH·BH3][1] has been derived.
Further measurements were carried out over a range of temperatures (50–80 °C) in toluene (ESI,
section S4.3.2). Extraction of the pseudo-first order rate constants for these reactions (Figure S30)
allowed for the construction of an Eyring plot (Figure 2d) and determination of the activation
parameters (Δ‡H° = +155(8) kJ mol−1 and Δ‡S° = +140(20) J K−1 mol−1).
Kinetic isotope effect (KIE) experiments were performed using deuterated substrate analogues.
Reactions with Me2ND·BH3 proceeded at approximately the same rate (within error) as those with
Me2NH·BH3, suggesting that N–D bond cleavage does not feature in the rate-limiting step of the
catalytic cycle. This stands in contrast to, for example, the iron catalysed dehydrocoupling of
Me2NH·BH3 reported by Webster et al., which displayed a KIE of 2.5 ± 0.2 with Me2ND·BH3.45 The
1
distinctive 1:1:1 triplet of HD was also observed by H NMR as the sole by-product in reactions with
Me2ND·BH3 (Figure S32). It was not possible to obtain a pure sample of Me2NH·BD3 from theliterature
6
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procedure, which afforded mixtures of Me2NH·BD3 and Me2NH·BH3 (ESI, section S4.1.4). The kinetic
isotope effect for B–D bonds was thus calculated from an intermolecular competition experiment with
this mixed sample,54 which gave a value of kH/kD = 1.6 ± 0.1 (ESI, section 4.3.3). A mixture of H2 and HD
was observed by 1H NMR spectroscopy in this reaction.
Stoichiometric reactions between Me2NH·BH3 and 1 (ESI, section S4.4.1) resulted in the slow build-up
of 3, 4, 5, and two additional signals at δB 3.4 (t) and −14.6 (q) (Figure S35). This compound cannot be
fully identified, but the signals are similar to previously reported magnesium complexes of amine-
boranes38,47 and thus may correspond to an NMe2BH2NMe2BH3 chain bound to a magnesium atom.
This compound is likely to be an intermediate of the catalytic cycle. Heating the sample to 60 °C
resulted in the disappearance of these signals in the 11B NMR spectrum. Despite repeated attempts,
no reaction intermediates containing coordinated magnesium could be crystallized from
stoichiometric or substoichiometric experiments with Me2NH·BH3.
Mechanistic Discussion
Scheme 2. Proposed catalytic cycle for dehydrocoupling of Me2NH·BH3 by 1.
We propose the catalytic cycle shown in Scheme 2 to account for the observed mechanistic data. This
mechanism is broadly similar to those previously proposed for previous alkaline earth metal
dehydrocoupling catalysts,24 but with some key differences due to the nature of the ligand L1. It should
be noted that there has been little work on the kinetics of such systems, with the mechanism being
inferred from the isolation of plausible intermediates and qualitative reaction monitoring by NMR
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spectroscopy.38,47 As such, the kinetic data presented in this work provides useful supporting evidence
for both this mechanism and the generalized mechanism of alkaline earth dehydrocoupling catalysts.
The first two steps involve reversible deprotonation of Me2NH·BH3 by the silylamide group of 1, to give
species I1, followed by an irreversible β-hydride elimination; affording hydrogen, the side
product/intermediate 4, and species 1. This step likely proceeds via a short-lived magnesium hydride,
which rapidly deprotonates the pendant silyl amine to regenerate 1. This mechanism is somewhat
different to previous alkaline earth catalysts, which were added as pre-catalysts and underwent an
initial irreversible deprotonation reaction, typically with elimination of an alkyl or amino species.24
Here, because of the bidentate nature of L1, the amine generated is held proximal to the magnesium
and continues to take part in the reaction. This also helps explain the inactivity of 2 as a catalyst, as
the less basic pyrrolide is unlikely to deprotonate Me2NH·BH3.
Species I1 can also undergo an insertion reaction with 4 to generate species I2. This intermediate is
considered to correspond to the unknown species observed by 11B NMR in the stoichiometric
experiments (Figure S35), and similar insertion reactions are commonly invoked in mechanisms of this
type.24 While it has not been possible to isolate the intermediates I1 and I2, we consider them plausible
based on previously proposed mechanisms for similar catalysts.24,38 Furthermore, the silylamide is the
only group on the complex sufficiently basic to deprotonate the substrate. The partial dissociation of
the ligand, with the pyridine nitrogen remaining bound to the magnesium atom, seems a reasonable
explanation for the regeneration of 1 at the end of the reaction with no loss of ligand. Species I2, once
formed, can undergo either a reversible proton exchange (affording linear intermediate 5) or an
irreversible δ-hydride elimination to afford the major product (3) and hydrogen.
Scheme 3. Rate controlling steps of the proposed catalytic cycle
Based on the first order dependence on both 1 and Me2NH·BH3, the first two steps of the proposed
mechanism (formation of I1 and subsequent β-hydride elimination) must control the overall rate of
the catalytic cycle. This is best modelled using the pre-equilibrium approximation,55 in which an
equilibrium is established between (Me2NH·BH3 + 1) and I1, followed by a slow β-hydride elimination
(Scheme 3). A schematic potential energy surface is shown in Figure 3 for illustrative purposes.
Denoting the rate constants as k1, k−1, and k2 (Scheme 3); it can be shown that the overall rate equation
for this process is given by:55
8
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푘1푘2
푘−1
[
]
휐 =
푀푒2푁퐻 · 퐵퐻3 [ퟏ]
Use of Me2ND·BH3 as a substrate in this reaction will reduce the rate of both k1 and k−1. As a result,
the overall rate of reaction does not change significantly. This can also be understood as the overall
rate of reaction being determined by the position of equilibrium for this deprotonation, rather than
the rate of deprotonation. By contrast, use of Me2NH·BD3 will reduce the rate of k2, resulting in a
modest KIE for this substrate. The fact that β-hydride elimination is the rate-limiting step (as indicated
by the KIE) suggests that the initial deprotonation must be a reversible process. If it were irreversible,
one would expect intermediate I1 to accumulate in the reaction, which is not observed.
Figure 3. Schematic Potential Energy Surface (PES) for the first two steps of the proposed catalytic cycle.
Because of the pre-equilibrium in this reaction, the overall kinetic energy barrier for this process (Δ‡G°)
will depend on the energy barriers for k1, k−1, and k2 according to the equation:55
Δ‡G° = Δ‡G1° + Δ‡G2° − Δ‡G−1°
This is shown visually with the schematic potential energy surface in Figure 3. This means that the
experimentally determined activation parameters for this reaction will depend on all three of these
processes. As Δ‡G1° is the largest single term in this expression (Figure 3), the activation parameters
will be dominated by the transition state for the deprotonation step, even though the β-hydride
elimination is the slowest step. This results in the large positive entropy of activation (Δ‡S° = +140(20)
J K−1 mol−1, consistent with partial dissociation of the bidentate ligand) and large enthalpy of activation
(Δ‡H° = +155(8) kJ mol−1, consistent with significant bond cleavage as we approach the transition
state). It should be noted that similar activation parameters were obtained for the dehydrocoupling
of 9-BBN and N-methylaniline by Hill’s magnesium catalyst (Δ‡H° = +125.1(3) kJ mol−1 and Δ‡S° = +85(2)
J K−1 mol−1), and that this was also attributed to deprotonation of the amine.48
The observed deviation from first-order kinetics in Me2NH·BH3 can be explained by the insertion
reaction between I1 and 4 being non-rate limiting provided [Me2NH·BH3] >> [1], and becoming rate
limiting as the reaction nears completion. The fact that compound 5 is fully consumed at the end of
the reaction, while some 4 always remains unreacted, can be explained by both species being steady-
state intermediates of the catalytic cycle (i.e. their rate of formation and consumption is
approximately equal). This means the concentration of both 4 and 5 remains low and relatively
constant throughout. As the reaction nears completion, 5 is fully converted to 3 through reaction with
the catalyst. However, compound 4 can only be consumed in the presence of both Me2NH·BH3 and 1
(see Scheme 2). As the reaction nears completion, all the Me2NH·BH3 will be converted to 4 by reacting
with the catalyst. This means that there is no Me2NH·BH3 left for 4 to react with, and thus a small
9
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amount of 4 is left unreacted once all substrate is consumed. Finally, for the reaction of iPr2NH·BH3
with 1, it is likely that the insertion reaction is prevented by the increased steric bulk, leading to
iPr2N=BH2 as the major product. Thus, the mechanism shown in Scheme 2 accounts for all the
observed mechanistic data; including the rate dependencies, kinetic isotope effects, activation
parameters, and observed intermediates.
Conclusions
In conclusion, magnesium complex 1 is an effective catalyst for the dehydrocoupling/dehydrogenation
of Me2NH·BH3 and iPr2NH·BH3. The dehydrocoupling of Me2NH·BH3 afforded [Me2NBH2]2 more cleanly,
rapidly, and at lower catalyst loadings than any previously reported alkaline earth metal catalyst. We
propose that the bidentate ligand L1 allows the catalyst to reversibly regenerate 1 as the catalytic
resting state, rather than irreversibly forming a magnesium hydride species. This distinguishes the
system from previous catalysts and may account for the remarkably high efficacy and stability of 1 as
a catalyst for amine-borane dehydrocoupling.
Experimental Methods
All experimental details including synthetic methods, catalytic studies, kinetic measurements, and
crystallographic data are provided in the Electronic Supplementary Information (ESI).
Acknowledgements
This work was supported by the Engineering and Physical Sciences Research Council [grant number
EP/R004064/1]; The Leverhulme Trust [grant number RPG-2014-317]; and the University of
Nottingham. We also thank Mr. Stephen Boyer (London Metropolitan University) for elemental
analyses.
Keywords: dehydrocoupling • magnesium • hydrogen storage • amido ligands • homogeneous
catalysis
References
1
2
3
G. Marbán and T. Valdés-Solís, Int. J. Hydrog. Energy, 2007, 32, 1625–1637.
L. Barreto, A. Makihira and K. Riahi, Int. J. Hydrog. Energy, 2003, 28, 267–284.
A. F. Dalebrook, W. Gan, M. Grasemann, S. Moret and G. Laurenczy, Chem. Commun., 2013,
49, 8735–8751.
4
5
6
7
8
9
A. Züttel, Naturwissenschaften, 2004, 91, 157–172.
R. J. Keaton, J. M. Blacquiere and T. R. Baker, J. Am. Chem. Soc., 2007, 129, 1844–1845.
S. Bhunya, T. Malakar, G. Ganguly and A. Paul, ACS Catal., 2016, 6, 7907–7934.
M. Bowden, T. Autrey, I. Brown and M. Ryan, Curr. Appl. Phys., 2008, 8, 498–500.
F. H. Stephens, V. Pons and R. T. Baker, Dalton Trans., 2007, 2613–2626.
T. W. Graham, C.-W. Tsang, X. Chen, R. Guo, W. Jia, S.-M. Lu, C. Sui-Seng, C. B. Ewart, A. Lough,
D. Amoroso and K. Abdur-Rashid, Angew. Chem. Int. Ed., 2010, 49, 8708–8711.
T. L. Cottrell, The Strengths of Chemical Bonds, Butterworths, London, UK, 2nd edn., 1958.
10
10
This article is protected by copyright. All rights reserved.

10.1002/chem.201901197
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11
12
13
C. W. Hamilton, R. T. Baker, A. Staubitz and I. Manners, Chem. Soc. Rev., 2009, 38, 279–293.
C. A. Jaska, K. Temple, A. J. Lough and I. Manners, Chem. Commun., 2001, 962–963.
M. C. Denney, V. Pons, T. J. Hebden, D. M. Heinekey and K. I. Goldberg, J. Am. Chem. Soc., 2006,
128, 12048–12049.
14
M. E. Sloan, A. Staubitz, T. J. Clark, C. A. Russell, G. C. Lloyd-Jones and I. Manners, J. Am. Chem.
Soc., 2010, 132, 3831–3841.
15
16
T. J. Clark, C. A. Russell and I. Manners, J. Am. Chem. Soc., 2006, 128, 9582–9583.
H. R. Sharpe, A. M. Geer, T. J. Blundell, F. R. Hastings, M. W. Fay, G. A. Rance, W. Lewis, A. J.
Blake and D. L. Kays, Catal. Sci. Technol., 2018, 8, 229–235.
17
18
M. Gediga, C. M. Feil, S. H. Schlindwein, J. Bender, M. Nieger and D. Gudat, Chem. Eur. J., 2017,
23, 11560–11569.
S. Muhammad, S. Moncho, E. N. Brothers and A. A. Bengali, Chem. Commun., 2014, 50, 5874–
5877.
19
20
T. Kakizawa, Y. Kawano, K. Naganeyama and M. Shimoi, Chem. Lett., 2011, 40, 171–173.
A. Glüer, M. Förster, V. R. Celinski, J. Schmedt auf der Günne, M. C. Holthausen and S.
Schneider, ACS Catal., 2015, 5, 7214–7217.
21
22
R. T. Baker, J. C. Gordon, C. W. Hamilton, N. J. Henson, P.-H. Lin, S. Maguire, M. Murugesu, B.
L. Scott and N. C. Smythe, J. Am. Chem. Soc., 2012, 134, 5598–5609.
S. Todisco, L. Luconi, G. Giambastiani, A. Rossin, M. Peruzzini, I. E. Golub, O. A. Filippov, N. V
Belkova and E. S. Shubina, Inorg. Chem, 2017, 56, 4296–4307.
T.-P. Lin and J. C. Peters, J. Am. Chem. Soc., 2013, 135, 15310–15313.
R. L. Melen, Chem. Soc. Rev., 2016, 45, 775–788.
23
24
25
26
27
28
P. P. Power, Nature, 2010, 463, 171–177.
Z. Tang, X. Chen, H. Chen, L. Wu and X. Yu, Angew. Chem. Int. Ed., 2013, 52, 5832–5835.
A. J. M. Miller and J. E. Bercaw, Chem. Commun., 2010, 46, 1709–11.
D. W. Himmelberger, C. W. Yoon, M. E. Bluhm, P. J. Carroll and L. G. Sneddon, J. Am. Chem.
Soc., 2009, 131, 14101–14110.
29
For an example of a recyclability study with a boron-based Lewis acid catalyst, Z. Lu, L.
Schweighauser, H. Hausmann and H. A. Wegner, Angew. Chem. Int. Ed., 2015, 54, 15556–
15559.
30
31
T. E. Stennett and S. Harder, Chem. Soc. Rev., 2016, 45, 1112–1128.
Y. S. Chua, H. Wu, W. Zhou, T. J. Udovic, G. Wu, Z. Xiong, M. W. Wong and P. Chen, Inorg. Chem.,
2012, 51, 1599–1603.
32
33
H. V. K. Diyabalanage, R. P. Shrestha, T. A. Semelsberger, B. L. Scott, M. E. Bowden, B. L. Davis
and A. K. Burrell, Angew. Chem. Int. Ed., 2007, 46, 8995–8997.
S. Orimo, Y. Nakamori, J. R. Eliseo, A. Züttel and C. M. Jensen, Chem. Rev., 2007, 107, 4111–
4132.
34
35
P. Bellham, M. S. Hill and G. Kociok-Köhn, Dalton Trans., 2015, 44, 12078–12081.
R. Nolla-Saltiel, A. M. Geer, W. Lewis, A. J. Blake and D. L. Kays, Chem. Commun., 2018, 54,
1825–1828.
36
R. McLellan, A. R. Kennedy, R. E. Mulvey, S. A. Orr and S. D. Robertson, Chem. Eur. J., 2017, 23,
16853–16861.
37
38
39
40
A. Harinath, S. Anga and T. K. Panda, RSC Adv., 2016, 6, 35648–35653.
D. J. Liptrot, M. S. Hill, M. F. Mahon and D. J. MacDougall, Chem. Eur. J., 2010, 16, 8508–8515.
J. Spielmann, M. Bolte and S. Harder, Chem. Commun., 2009, 6934–6936.
H. Helten, A. P. M. Robertson, A. Staubitz, J. R. Vance, M. F. Haddow and I. Manners, Chem.
Eur. J., 2012, 18, 4665–4680.
41
42
43
F. Ortu, G. J. Moxey, A. J. Blake, W. Lewis and D. L. Kays, Inorg. Chem., 2013, 52, 12429–12439.
J. J. Klappa, A. E. Rich and K. McNeill, Org. Lett., 2002, 4, 435–437.
Rates in C6D6 and THF were compared by discontinuous NMR measurements with stop-start
heating (ESI, section S4.2.1). It should be noted that the apparent rate when monitoring by this
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10.1002/chem.201901197
Chemistry - A European Journal
method (Figures S15 and S16) is substantially lower than that observed by in situ NMR
measurements (Figure S27) This indicates a significant systematic error associated with
stopping and starting the reaction. As such, discontinuous NMR measurements cannot be used
to draw any quantitative conclusions about rates or mechanisms. They are, however, sufficient
for a qualitative comparison of approximate reaction rates in different solvents.
R. McLellan, A. R. Kennedy, S. A. Orr, S. D. Robertson and R. E. Mulvey, Angew. Chem. Int. Ed.,
2017, 56, 1036–1041.
N. T. Coles, M. F. Mahon and R. L. Webster, Organometallics, 2017, 36, 2262–2268.
C. A. Jaska, K. Temple, A. J. Lough and I. Manners, J. Am. Chem. Soc., 2003, 125, 9424–9434.
M. S. Hill, M. Hodgson, D. J. Liptrot and M. F. Mahon, Dalton Trans., 2011, 40, 7783–7790.
D. J. Liptrot, M. S. Hill, M. F. Mahon and A. S. S. Wilson, Angew. Chem. Int. Ed., 2015, 54, 13362–
13365.
44
45
46
47
48
49
50
51
P. Bellham, M. S. Hill, G. Kociok-Köhn and D. J. Liptrot, Chem. Commun., 2013, 49, 1960–1962.
P. Cui, T. P. Spaniol, L. Maron and J. Okuda, Chem. Eur. J., 2013, 19, 13437–13444.
To the best of our knowledge, there is only one example of an alkaline earth metal catalyst (for
any transformation) that was shown to be effective at lower catalyst loadings. This was a
barium species for the cross dehydrocoupling of amines and silanes. Barium is, of course, a less
attractive prospect for catalysis due to its high toxicity. See C. Bellini, J.-F. Carpentier, S. Tobisch
and Y. Sarazin, Angew. Chem. Int. Ed., 2015, 54, 7679–7683.
52
53
Species identified by comparison with literature data, J. R. Vance, A. P. M. Robertson, K. Lee
and I. Manners, Chem. Eur. J., 2011, 17, 4099–4103.
We have employed the graphical techniques of Reaction Progress Kinetic Analysis (RPKA) in
determining the reaction order with respect to both catalyst and substrate. For an overview of
RPKA, see D. G. Blackmond, Angew. Chem. Int. Ed., 2005, 44, 4302–4320.
L. Melander and W. H. Saunders, Reaction Rates of Isotopic Molecules, John Wiley & Sons, Ltd.,
2nd edn., 1960.
54
55
E. V Anslyn and D. A. Dougherty, Modern Physical Organic Chemistry, University Science Books,
Sausalito, CA, 1st edn., 2006.
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Table of Contents
Reversible coordination: A Mg(II) complex featuring an aminopyridinato ligand is a remarkably
selective catalyst for amine-borane dehydrocoupling. The bidentate ligand allows the catalyst to
reversibly regenerate the Mg(II) complex as the resting state, rather than irreversibly forming a
magnesium hydride species, leading to high efficiency and stability.
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