Accessing the single-electron manifold: Magnesium-mediated hydrogen release from silanes

Article abstract of DOI:10.1002/anie.201403208

Reactions of TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy) with magnesium hydride species initiate oxidative hydrogen release, which may be elaborated to a catalytic regime within a manifold constructed about sequential TEMPO-mediated redox and Mg?O/Si?H metathesis processes.

Full text of DOI:10.1002/anie.201403208

Angewandte
Chemie
DOI: 10.1002/anie.201403208
s-Bond Metathesis
Accessing the Single-Electron Manifold: Magnesium-mediated
Hydrogen Release from Silanes
David J. Liptrot, Professor Michael S. Hill,* and Mary F. Mahon
Dedicated to the memory of Michael F. Lappert
Abstract: Reactions of TEMPO (2,2,6,6-tetramethyl-1-piper-
idinyloxy) with magnesium hydride species initiate oxidative
hydrogen release, which may be elaborated to a catalytic
There is growing awareness of the ability of heavier
alkaline earth reagents (Mg, Ca, Sr, Ba) to engage in
a plethora of heterofunctionalization and bond-forming
catalyses.[10] While reductive processes, based on Group 2
hydride intermediates generated by metathesis of silane and
borane reagents at amide- and alkoxide-ligated centers, have
been central to recent progress,[11] a severe limitation to
further development is a restriction to catalytic manifolds
derived solely from two electron bond breaking/forming
processes. Herein, we report the activity of TEMPO towards
magnesium hydrides and the utility of subsequent silane
metathesis for the development of oxidative hydrogen release
catalysis. This is, to the best of our knowledge, the first report
of redox-based catalysis by an s-block metal complex and
establishes the viability of SET reactivity in the catalytic
applications of the benign and inexpensive alkaline earth
elements.
regime within
a manifold constructed about sequential
À
À
TEMPO-mediated redox and Mg O/Si H metathesis pro-
cesses.
T
he stable nitroxyl radical, TEMPO (2,2,6,6-tetramethyl-1-
piperidinyloxy), has attracted widespread interest since its
first synthesis in 1959.[1] Much of this attention has centered
on its utility in polymer synthesis,[2] organic oxidation
reactions[3] and, due to its persistent radical character, in
molecular magnetism.[4] Although the use of TEMPO as
a radical donor ligand is dominated by reports of redox-active
d-block complexes,[5] there is also more limited precedent for
the use of TEMPO in d0 f-[6] and s-block[7] complex chemistry.
Mulvey and co-workers reported the first Group 1 and 2
complexes of TEMPO in 2001,[7a] noting not only its
coordination as a neutral two electron donor but also its
ability to adopt an anionic electron configuration when
Our initial investigations centered upon magnesium b-
diketiminate complexes, [CH{C(Me)NAr}2}MgnBu] (1)[12]
and [CH{C(Me)NAr}2}MgH]2 (2)[13] (Ar= 2,6-iPr2C6H3).
Gratifyingly, the action of TEMPO upon both these species
evidenced SET characteristics with almost instantaneous
extinction of the red color associated with TEMPO. Monitor-
ligating Group
2 cations in complexes of the form
[{(Me3Si)2N}Mg{m-TEMPO}]2. A subsequent, elegant report
from this group of single-electron transfer (SET) from
elemental Group 1 metals to TEMPO yielded a structurally
diverse series of [M{TEMPO}] compounds (M = Li, Na, K,
Rb, Cs) once again containing TEMPO anions (hereafter
referred to as TEMPOxide).[7b] A demonstration of TEMPO
reactivity with Group 2 reagents has also been described by
Fedushkin et al. who reported SET with a magnesium com-
plex mediated by a redox-active 1,2-bis(arylimino)acenaph-
thene (BIAN) ligand.[7d] Furthermore, stoichiometric SET to
organomagnesium reagents is well precedented, yielding
[8]
À
À
a variety of C C and C N coupled products. Although
this reactivity has not yet been extended to magnesium
hydride species, it is notable that Jones et al. have reported
that stoichiometric reactions of TEMPO with a range of
Group 13 hydrides yield molecular dihydrogen alongside the
formation of TEMPOxide ligated to the relevant Group 13
center.[9]
1
ing by H NMR spectroscopy revealed that compound 1 had
undergone a well-precedented alkyl coupling to form n-
octane,[7c] while compound 2, in reactivity reminiscent of that
reported for Group 13 hydrides,[9] provided a pronounced
bubbling of molecular dihydrogen [Eq. (1)]. Alongside the
desired small-molecule products of this reaction, a magnesium
complex (3) was isolated and characterized by NMR spec-
troscopy and single-crystal X-ray diffraction analysis
(Figure 1).
Although the coordination environment of compound 3 is
otherwise comparable to Fedushkinꢀs BIAN derivative,
wherein the nitroxide nitrogen center displays significant
pyramidalization indicative of single-electron reduction of the
radical starting material,[7d] the two compounds differ signifi-
cantly in the mode of binding of the TEMPOxide ligand.
[*] D. J. Liptrot, P. M. S. Hill, Dr. M. F. Mahon
Department of Chemistry, University of Bath
Claverton Down, Bath, BA2 7AY (UK)
E-mail: msh27@bath.ac.uk
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201403208.
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Analysis of the resultant solution by NMR spectroscopy also
confirmed the formation of a new silane-containing species,
proposed to be TEMPOSi(H)2Ph, formed via a catalytic
manifold reliant upon both SET and s-bond metathesis
(Scheme 1). The reaction was optimized with respect to
a small range of solvents, with the donor solvent [D8]THF
providing significantly enhanced reaction rates in comparison
to the aromatic solvents investigated, C6D6 and [D8]toluene.
A series of stoichiometric investigations were undertaken
to elucidate the intermediate species participating in this
reaction. Reaction of 5 with two equivalents of TEMPO in
THF yielded stepwise activation of the magnesium-bound
butyl groups giving rise to a crystallographically characterized
heteroleptic TEMPOxide-alkyl [(THF)Mg(TEMPO)(Bu)]2
Figure 1. ORTEP representation of 3 (30% probability ellipsoids).
Isopropyl methyl groups and hydrogen atoms omitted for clarity.
Selected bond lengths [ꢀ] and angles [8]: Mg1–O2 1.8887(13), Mg1–
O1 2.0534(14), Mg1–N2 2.0844(14), Mg1–N1 2.0915(15), O(2)–N(3)
1.4423(1); O2-Mg1-O1 111.07(6), O2-Mg1-N2 117.99(6), O1-Mg1-N2
108.31(6), O2-Mg1-N1 119.43(6), O1-Mg1-N1 105.45(6), N2-Mg1-N1
92.65(6), O2-N3-C40 108.71(13), O2-N3-C34 108.82(13).
Whereas the TEMPOxide binds in an unambiguous h1
coordination mode through the terminal oxygen center in 3,
coordination to the BIAN-coordinated magnesium occurs
through both the nitroxide oxygen and nitrogen centers.
Although the causes of the structural change are unclear, this
Figure 2. ORTEP representation of 7 (30% probability ellipsoids).
Hydrogen atoms omitted for clarity. Selected bond lengths [ꢀ] and
angles [8]: Mg1–O1 1.8848(11), Mg1–O2 1.9869(11), Mg1–O2’
1.9891(1), Mg1–O3 2.0763(12), O1–N1 1.4420(15), O2–N2
1.4551(15); O1-Mg1-O2 126.70(5), O1-Mg1-O2’ 127.26(5), O2-Mg1-
O2’ 79.54(5), O1-Mg1-O3 104.77(5), O2-Mg1-O3 110.22(5), O2’-Mg1-
O3 105.78(5), N1-O1-Mg1 130.91(9), N2-O2-Mg1 106.85(8), N2-O2-
Mg1’ 150.75(8), Mg1-O2-Mg1’ 100.20(5). Symmetry transformations
used to generate equivalent atoms: (’) Àx,y,Àz+1/2.
À
adjustment results in a significant shortening of the Mg O
bond in compound 3 [1.8887(13) versus 1.9061(10) ꢁ].
Attempts to extend this reactivity to a catalytic manifold,
reliant upon the conversion 3 into 2 through s-bond meta-
thesis, were undertaken by utilizing phenylsilane as a hydride
source. Unfortunately no reactivity between 3 and PhSiH3
was observed which we attribute to the steric demands of the
various substituents surrounding the magnesium center
precluding approach of the silane reagent. Consequently,
our attention turned to the less sterically congested homo-
leptic magnesium amide and alkyl pre-catalysts, [Mg{N-
(SiMe3)2}2(THF)2] (4) and Bu2Mg (5). While TEMPO and
PhSiH3 displayed no background reactivity in the absence of
the magnesium reagents, reactions in the presence of
10 mol% 4 or 5 in C6D6 induced an observable bubbling
and the decolorization of the solution over the course of days.
(6) (see Figure S1 in the Supporting Information) and the
anticipated homoleptic magnesium TEMPOxide complex,
[(THF)Mg(TEMPO)2]2 (7), which was isolated and charac-
terized by NMR spectroscopy and a further single-crystal X-
ray diffraction analysis (Figure 2). The dimeric structure of
compound 7 comprises terminal and bridging TEMPOxide
À
ligands. Most notably, the bridging Mg O interactions within
the structure of 7 [1.9869(11), 1.9891(1) ꢁ] are similar to
those of Mulveyꢀs structurally related complex,
[{(Me3Si)2N}Mg{m-TEMPO}]2,[7a] but are significantly elon-
gated in comparison to both the terminal distances within the
dimeric unit itself [1.8848(11) ꢁ] and the unique TEMPOxide
ligand of compound 3. Although monitoring of reactions of
isolated samples of 7 with an excess of phenylsilane by
1H NMR spectroscopy evidenced the formation of the
proposed silane containing product, TEMPOSi(H)2Ph, the
magnesium hydride intermediate implicated in the proposed
mechanism was not observed. Rather, under these stoichio-
Scheme 1.
2
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Table 1: The results of the scope study into silane (0.1 mmol) coupling
partners with TEMPO mediated by [Mg{N(SiMe3)2}2(THF)2] (4)
(0.01 mmol) in 4:1 THF/C6D6 (0.5 mL).
In summary, we have described the first example of SET
reactivity which is catalytic in an s-block metal. This oxidative
coupling is mediated by a magnesium(II) center and yields
dihydrogen and a series of TEMPO silylethers. Catalytic
turnover most likely proceeds via a catalytically active
Entry Silane
Silane: Product
TEMPO
t
T
Conv.
[d] [8C] [%]
À
À
magnesium TEMPOxide which engages in Si H/Mg O s-
bond metathesis to yield an intermediate magnesium hydride
species and the silylether. The action of TEMPO upon this
magnesium hydride proceeds by SET and, to the best of our
knowledge, constitutes the first example of the action of
TEMPO on a Group 2 hydride. We are continuing to explore
this reactivity and to elaborate further Group 2-centered
protocols which couple s-bond metathesis to SET steps for
productive bond-forming catalysis.
1
2
3
4
5
6
7
PhSiH3
PhSiH3
Ph2SiH2
Ph2SiH2
Ph(Me)SiH2 1:1
Et3SiH
Ph3SiH
1:1
1:2
1:1
1:2
PhSi(H)2OTEMP
1
6
3
4
1
3
3
60 99
80 96
60 97
80 99
80 77
PhSi(H)OTEMP2
Ph2Si(H)OTEMP
Ph2Si(H)OTEMP
Ph(Me)Si(H)OTEMP
N/A
1:1
1:1
80
80
0
0
N/A
metric conditions an intractable colorless solid, proposed to
be bulk magnesium hydride, was observed to precipitate from
THF solution.
Experimental Section
Details of the synthesis, characterization data and the crystallographic
protocols employed in this study are given in the Supporting
Information. CCDC 990851 (3), 990852 (6), 990853 (7) contain the
supplementary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
The influence of substitution on the silane coupling
partner for reactions catalyzed by compound 4 was inves-
tigated through an evaluation of the reaction scope (Table 1).
Although limited, this study allowed the delineation of
several trends, most notably a significant dependence on
substrate bulk. The least hindered primary silane (entry 1)
was found to react far more rapidly than secondary silanes
(entries 2, 3 and 5) while tertiary silanes were too sterically
encumbered to engage in onward reaction with the magne-
sium TEMPOxide, even at elevated temperatures and
extended reaction times (entries 4, 6 and 7). These data are
consistent with limitations previously observed for the
magnesium-mediated coupling of amines with silanes.[11g]
Further TEMPO substitution of the silane substrate was
also only possible for the least sterically congested silane
coupling partner, PhSiH3 (entry 2).
Received: March 11, 2014
Published online: && &&, &&&&
Keywords: homogeneous catalysis · magnesium · metathesis ·
.
single-electron transfer · TEMPO
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4
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Angew. Chem. Int. Ed. 2014, 53, 1 – 5
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Angewandte
Chemie
Communications
s-Bond Metathesis
Upping the tempo: Reactions of TEMPO
(2,2,6,6-tetramethyl-1-piperidinyloxy)
with magnesium hydride species initiate
catalytic hydrogen release through
sequential TEMPO-mediated redox and
D. J. Liptrot, P. M. S. Hill,*
M. F. Mahon
&&&& — &&&&
À
À
Accessing the Single-Electron Manifold:
Magnesium-mediated Hydrogen Release
from Silanes
Mg O/Si H metathesis processes (see
scheme). This is the first example of
catalytic single-electron transfer reactivity
involving an s-block metal.
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These are not the final page numbers!