A dimeric magnesium(I) compound as a facile two-center/two- electron reductant

Full text of DOI:10.1002/anie.200900331

Angewandte
Chemie
DOI: 10.1002/anie.200900331
Synthetic Methods
A Dimeric Magnesium(I) Compound as a Facile Two-Center/Two-
Electron Reductant**
Simon J. Bonyhady, Shaun P. Green, Cameron Jones,* Sharanappa Nembenna, and
Andreas Stasch*
For more than 100 years, magnesium compounds have proved
indispensable tools for the organic and organometallic
chemist. Of these, Grignard reagents are undoubtedly the
most important and have been utilized in innumerable
synthetic transformations.[1] Some of these are thought to
occur by mechanisms involving single electron transfer (SET)
thermally stable adducts of 2 with several Lewis bases, viz.
[(nacnac)Mg(L’)Mg(L’)(nacnac)] (L’ = thf, dioxane, 4-tert-
butylpyridine or 4-dimethylaminopyridine).[7] Treatment of
= =
these adducts with CyN C NCy led to no reaction, presum-
ably because their magnesium centers were blocked towards
coordination by the carbodiimide. Considering that magne-
sium(I) species have often been implicated as intermediates
in substrate reductions involving organometallic or inorganic
magnesium reagents,[1–4] it seemed that a systematic study of
the reactivity of 2 towards unsaturated compounds would be
of great interest to organic and organometallic chemists.
Herein, we report preliminary results towards this end, which
show that 2 acts as a facile two-center/two-electron reductant,
from the organomagnesium compound to
a substrate,
although the exact nature of these SET processes is not
fully understood. Other magnesium reagents that can act as
one-electron reductants include Mg/MgI2 mixtures, which
have been proposed to be in equilibrium with univalent
magnesium, MgIC, in organic solutions.[2] Although this species
has not been characterized in solution, elegant recent work by
Schnꢀckel, Henke, and Kꢀppe has led to the spectroscopic
characterization of related MgClC and Mg2Cl2 in a solid inert-
gas matrix.[3] In addition, the reactions of the MgCl2CÀ radical
with several ketones have been studied in the gas phase by
mass spectrometry, and SET products were identified.[4]
In late 2007, the first “bottleable” magnesium(I) com-
pounds, [LMgMgL] (L = [(ArN)2CNiPr2]À, PrisoÀ (1) or
[(ArNCMe)2CH]À, nacnacÀ (2; Scheme 1); Ar= 2,6-
À
À
yielding novel doubly reduced, C C- or N N-coupled
products. For comparison, reactions of the closely related
magnesium(II) hydride complex [(nacnac)Mg(m-H)2Mg-
(nacnac)] were carried out with the same substrates, generally
yielding hydromagnesiation products.
= =
NCy gives the doubly reduced product, 3 (Scheme 1).[5]
As already mentioned, the reaction of 2 with CyN C
Similarly, treatment of toluene solutions of 2 with azobenzene
iPr2C6H3), were reported.[5,6] These contained Mg Mg
(PhN NPh) or cyclooctatetraaene (cot) under any stoichi-
À
=
bonds that are kinetically protected from disproportionation
processes by sterically bulky guanidinate or b-diketiminate
ligands. Despite this protection, compound 2 doubly reduces
ometry gave the reduced products, 4 and 5, the latter after
recrystallization from THF. In contrast, reactions of 2 with 1-
adamantyl azide or the isocyanate, tBuNCO, led to reductive
= =
À
À
the carbodiimide, CyN C NCy (Cy = cyclohexyl), at low
N N and C C couplings respectively, to give the unusual
hexazenediide complex, 6, and the magnesium oxamide
compound, 7. All products were obtained in good to high
yields as crystalline solids. The reactions involving N-func-
tionalized substrates were all complete within minutes at
temperatures below À508C, whereas the reduction of cot
required it to be heated with 2 in boiling toluene for 1.5 h.
Presumably, the reductions of the N-functionalized substrates
were facilitated by their initial coordination to the Mg centers
of 2, which is disfavored for cot. In these reactions, each
magnesium(I) center of 2 is formally acting as a one-electron
reductant and, therefore, the complex as a whole behaves as a
two-center/two-electron reductant.
temperature to give the unusual magnesium magnesioamidi-
nate complex, [(nacnac)Mg{(NCy)2C}Mg(nacnac)], 3. It was
postulated that this reaction proceeds via initial coordination
of the carbodiimide at one Mg center of 2, prior to its
reduction, which seems reasonable in light of a subsequent
report that detailed the formation and characterization of
[*] S. J. Bonyhady, S. P. Green, Prof. C. Jones, Dr. S. Nembenna,
Dr. A. Stasch
School of Chemistry, Monash University
PO Box 23, VIC, 3800 (Australia)
Fax: (+61)3-9905-4597
E-mail: cameron.jones@sci.monash.edu.au
andreas.stasch@sci.monash.edu.au
There are a number of interesting features of the
reduction products, 4–7 (Scheme 1), which indicate that 2
and related magnesium(I) compounds may prove useful as
selective reducing agents in organic and organometallic
syntheses. For example, although many reports of the
reduction of azobenzene (mainly by lanthanide(II) reduc-
tants[8]) have been forthcoming, to our knowledge none have
led to the reduced dianionic ligand acting as a bis(k2-azaallyl),
as is the case in 4. Similarly, although magnesium complexes
of the cot2À dianion are known,[9] none have been crystallo-
graphically characterized and little is known of how the
S. P. Green
School of Chemistry, Cardiff University
Main Building, Park Place, Cardiff, CF10 3AT (UK)
[**] We thank the Australian Research Council (fellowships for C.J., A.S.
and S.N.), the Engineering and Physical Sciences Research Council
(partial studentship for S.P.G.), and the US Air Force Asian Office of
Aerospace Research and Development for funding. We also thank
the EPSRC for access to the UK National Mass Spectrometry Facility.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200900331.
Angew. Chem. Int. Ed. 2009, 48, 2973 –2977
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2973

Communications
Scheme 1. Preparation of compounds 3–7; cot=cyclooctatetraene.
relatively small Mg2+ ion coordinates to this ligand. The
structural characterization of 5 sheds light on this coordina-
tion for the first time. Compound 6 is unusual in that it results
from the reductive coupling of two organic azide units and
exhibits a bridging AdN6Ad2À (Ad = 1-adamantyl) ligand.
Despite having six covalently linked N centers, this compound
does not appear to be shock or thermally sensitive to
detonation, as is the case for some related high-energy
hexaazadienes, RNNN(R)N(R)NNR (R = alkyl, aryl etc.).[10]
The only other examples of reductive couplings of an organic
azide were recently reported by Holland and co-workers for
the reactions of b-diketiminate iron(I) fragments with
AdN3.[11] The products of these reactions, [{HC(RCNAr)2}Fe-
(m-N6Ad2)Fe{(ArNCR)2CH}] (R = Me or tBu) were isostruc-
À
tural with 6. Reductive C C couplings of isocyanates have
been reported,[12] but in no case has the resultant oxamide
ligand coordinated two metal centers in the novel N,O,O-
ligating fashion exhibited by 7, presumably because of steric
constraints placed upon the complex by the two bulky nacnac
ligands.
Figure 1. Molecular structure of compound 4. Thermal ellipsoids are
set at 25% probability. Nacnac hydrogen atoms and isopropyl groups
are omitted for clarity. Selected bond lengths [ꢀ] and angles [8]:
The spectroscopic data for 4–7 are largely consistent with
their solid-state structures (see the Supporting Information).
Molecular structures of all compounds were determined
crystallographically. The structure of 4 (Figure 1) confirms
that the PhNNPh2À ligand chelates to both distorted tetrahe-
dral Mg centers in an N,C fashion, and has considerable
bis(azaallylic) character. Evidence for this proposal comes
À
À
À
Mg1 N5 2.0118(16), Mg1 C66 2.404(2), Mg2 N6 2.0145(16),
À
À
À
À
Mg2 C60 2.411(2), N5 N6 1.424(2), N5 C59 1.345(2), N6 C65
À
À
1.348(2), C59 C60 1.427(3), C65 C66 1.426(3); N5-Mg1-C66 75.68(7),
N6-Mg2-C60 76.66(7), N2-Mg1-N1 94.14(7), N3-Mg2-N4 94.64(7).
Those attached to the coordinating C atoms were found to be
significantly out of the phenyl planes (by a mean value of
0.39 ꢁ). In line with this observation is the downfield
chemical shift for these protons in the H NMR spectrum of
the compound at d = 5.68 ppm. The structure of 5 (Figure 2)
À
À
from its long N N and short N C distances, in addition to the
apparent loss of some aromaticity from its phenyl groups. The
latter was verifiable, as all the hydrogen atoms of the phenyl
groups were located from difference maps and freely refined.
1
2974 www.angewandte.org
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2009, 48, 2973 –2977

Angewandte
Chemie
tural iron compounds and are indicative of significant
À
À
delocalization over the N3 N4 N5 fragments. In compound
7 (Figure 4), the N2C2O2 core of the molecule is close to
planar and chelates the Mg centers in both O,Oꢂ- and N,O-
coordination modes. The distorted tetrahedral Mg atoms
share coordination by O1, and the bond lengths within the
core do not indicate substantial delocalization over the
coupled isocyanate unit.
Figure 2. Molecular structure of compound 5. Thermal ellipsoids are
set at 25% probability. Hydrogen atoms and isopropyl groups are
À
omitted for clarity. Selected bond lengths [ꢀ] and angles [8]: Mg1 O1
À
À
À
2.016(2), Mg1 C60 2.312(3), Mg1 C59 2.328(3), Mg2 O2 2.012(2),
À
À
Mg2 C63 2.295(3), Mg2 C62 2.353(3); N1-Mg1-N2 92.32(9),
C60-Mg1-C59 35.92(11), N3-Mg2-N4 92.25(9), C63-Mg2-C62 35.70(11).
reveals its cot2À ligand to be essentially planar with C C
À
Figure 4. Molecular structure of compound 7. Thermal ellipsoids are
distances ranging from 1.387(4)–1.431(4) ꢁ. The ligand
bridges two {(nacnac)Mg(thf)} fragments which are each
coordinated by the ring in a h2 fashion. Reminiscent of the
structure of 4, the N6 chain in compound 6 (Figure 3) is
essentially planar and chelates to both Mg centers in an N,N’
set at 25% probability. Hydrogen atoms and isopropyl groups are
À
omitted for clarity. Selected bond lengths [ꢀ] and angles [8]. Mg1 O1
À
À
À
2.071(2), Mg1 N5 2.097(3), Mg2 O2 1.925(2), Mg2 O1 2.036(2),
À
À
À
À
O1 C59 1.354(4), O2 C60 1.298(4), N5 C59 1.306(4), N6 C60
À
1.289(4), C59 C60 1.520(4); N1-Mg1-N2 93.55(11), O1-Mg1-N5
À
fashion, yielding two planar MgN4 rings. The N N bond
64.41(9), N4-Mg2-N3 92.77(11), O2-Mg2-O1 82.85(10).
lengths within the chain are similar to those in the isostruc-
As we have recently prepared and structurally character-
ized the magnesium(II) hydride analogue of 2, [(nacnac)Mg-
(m-H)2Mg(nacnac)] 8,[7] the opportunity presented itself to
investigate the ability of the complex to reduce the substrates
chosen for this study, and compare the results with the
reductions involving 2. These results were also of interest as,
although reductions involving poorly characterized com-
pounds of the type RMgH have been previously reported,
they can lead to varying outcomes, for example, the formation
[13]
À
of hydromagnesiation products by Mg H addition, or the
formation of paramagnetic species by SET processes.[14]
Moreover, comparisons of reduction reactions of 8 with
those of Harder and co-workersꢂ closely related calcium
hydride complex, [(nacnac)(thf)Ca(m-H)2Ca(thf)(nacnac)],[15]
would be instructive.
All substrates were treated with 8 in toluene at À808C and
the reaction mixtures subsequently warmed to room temper-
ature. No reaction occurred with cot, though when the
reaction mixture was subsequently heated at reflux for 1 hr,
an intractable mixture of products resulted. Similarly, the
reaction with tBuNCO gave a number of unidentified
products which could not be isolated. In contrast, the
reductions of the other substrates led to the hydromagnesi-
Figure 3. Molecular structure of compound 6. Thermal ellipsoids are
set at 25% probability. Hydrogen atoms and isopropyl groups are
omitted for clarity. Selected bond lengths [ꢀ] and angles [8]:
À
À
À
À
Mg1 N5’ 2.024(2), Mg1 N1 2.037(2), Mg1 N2 2.051(2), Mg1 N3
À
À
À
2.0888(19), N3 N4 1.301(3), N4 N5 1.315(3), N5 N5’ 1.410(3);
N1-Mg1-N2 95.82(8), N5’-Mg1-N3 75.61(8). Symmetry operation:
’ (Àx+1,Ày,Àz).
Angew. Chem. Int. Ed. 2009, 48, 2973 –2977
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
2975

Communications
0.1197 (all data). 5·(thf)2: C82H122Mg2N4O4; Mr = 1276.46; monoclinic;
C2/c; a = 49.722(10), b = 24.652(5), c = 30.501(6) ꢁ; b = 125.27(3)8;
V= 30525(11) ꢁ3; Z = 16; 1calcd = 1.111 MgmÀ3; T= 123(2) K; l =
0.71073 ꢁ. 46954 reflections collected, 26254 independent reflections
(Rint = 0.0322); R1 = 0.0710 (F2 > 2s(F2)); wR2 = 0.2148 (all data).
ation products, 9–11, in good yields (Scheme 2). Compound 9
was also obtained in high yield from the reaction of 8 with the
=
formamidine, CyN C(H)N(H)Cy. These results reveal that 8
displays similar reactivity towards unsaturated compounds,
compared to the recently studied complex, [(nacnac)(thf)Ca-
(m-H)2Ca(thf)(nacnac)].[15] In addition, Holland and co-work-
ersꢂ isostructural iron(II) hydride, [{HC(tBuCNAr)2}Fe(m-
¯
6·(toluene)2: C92H128Mg2N10; Mr = 1422.66; triclinic; P1; a = 12.836(3),
b = 13.850(3), c = 13.912(3) ꢁ; a = 109.13(3), b = 111.80(3), g =
101.91(3)8; V= 2089.8(7) ꢁ3; Z = 1; 1calcd = 1.130 MgmÀ3
;
T=
123(2) K; l = 0.71073 ꢁ. 15233 reflections collected, 8178 independ-
ent reflections (Rint = 0.0309); R1 = 0.0638 (F2 > 2s(F2)); wR2 = 0.1864
(all data). 7·(toluene)1.5: C78.50H112Mg2N6O2; Mr = 1220.36; mono-
clinic; P21/n; a = 14.886(3), b = 23.938(5), c = 20.879(4) ꢁ; b =
H)2Fe{(ArNCtBu)2CH}], displayed almost identical reactivity
[16]
=
= =
towards AdN3, PhN NPh and iPrN C NiPr.
97.99(3)8; V= 3382.7(12) ꢁ3; Z = 4; 1calcd = 1.100 MgmÀ3
;
T=
123(2) K; l = 0.71073 ꢁ. 24137 reflections collected, 12932 inde-
pendent reflections (Rint = 0.0451); R1 = 0.0753 (F2 > 2s(F2)); wR2 =
0.1711 (all data). 10: C41H52MgN4; Mr = 625.18; monoclinic; P21/n;
a = 14.889(3), b = 15.238(3), c = 16.978(3) ꢁ; b = 102.41(3)8; V=
3762.0(13) ꢁ3; Z = 4; 1calcd = 1.104 MgmÀ3
;
T= 123(2) K; l =
0.71073 ꢁ. 11912 reflections collected, 6424 independent reflections
(Rint = 0.0350); R1 = 0.0465 (F2 > 2s(F2)); wR2 = 0.1184 (all data).
[11(AdNH2)]: C49H74MgN6; Mr = 771.45; monoclinic; P21/c; a =
21.296(4), b = 10.099(2), c = 20.949(4) ꢁ; b = 100.63(3)8; V=
4428.1(15) ꢁ3; Z = 4; 1calcd = 1.157 MgmÀ3
;
T= 123(2) K; l =
0.71073 ꢁ. 15053 reflections collected, 7794 independent reflections
(Rint = 0.0522); R1 = 0.0567 (F2 > 2s(F2)); wR2 = 0.1647 (all data).
CCDC 716772 (4), 716773 (5), 716774 (6), 716775 (7), 716776 (10) and
716777 (11) contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
Scheme 2. Preparation of compounds 9–11.
Full synthetic and spectroscopic details for 4–7 and 9–11, and
ORTEP representations of 10 and [(nacnac)Mg(k2-N,N’-AdN3H)-
(AdNH2)] can be found in the Supporting Information.
The spectroscopic data for 9–11 are consistent with the
proposed formulations of these compounds. The X-ray crystal
structure of 10 shows the compound to be monomeric with
the hydrazide ligand coordinated to the Mg center in a h2
mode, as was detected in several zirconium[17] and sama-
rium[18] complexes. From the reaction that gave 11, a very low
yield (< 2%) of the crystalline primary-amine-coordinated
product, [(nacnac)Mg(k2-N,N’-AdN3H)(AdNH2)], was also
isolated. The amine ligand presumably originates from the
full reduction of AdN3 by 8. The structure of this monomeric
complex, that of 10, and the spectroscopic data for 9–11 are
included in the Supporting Information.
In summary, a dimeric magnesium(I) complex acts as a
facile and selective two-center/two-electron reductant
towards a series of unsaturated substrates. Different out-
comes resulted from the reductions of the substrates with an
analogous magnesium(II) hydride complex. The novel prod-
ucts obtained from the reductions with the magnesium(I)
complex prompt us to believe that it may find use as an
alternative to other reductants, for example, SmII compounds
and s-block metals, that are widely utilized in organic and
organometallic syntheses. We are currently exploring this
possibility.
Received: January 19, 2009
Published online: March 17, 2009
Keywords: hydrides · magnesium · metal–metal interactions ·
.
reduction · synthetic methods
[1] a) Grignard Reagents: New Developments (Ed.: H. G. Ritch-
ey, Jr.), Wiley, Chichester, 2000; b) The Chemistry of Organo-
magnesium Compounds, Part 1 (Eds.: Z. Rappoport, I. Marek),
Wiley, Chichester, 2008.
[2] M. D. Rausch, W. E. McEwen, J. Kleinberg, Chem. Rev. 1957, 57,
417 – 437.
[3] R. Kꢀppe, P. Henke, H. Schnꢀckel, Angew. Chem. 2008, 120,
8868 – 8872; Angew. Chem. Int. Ed. 2008, 47, 8740 – 8744.
[4] C. C. L. Thum, G. N. Khairallah, R. A. J. OꢂHair, Angew. Chem.
2008, 120, 9258 – 9261; Angew. Chem. Int. Ed. 2008, 47, 9118 –
9121.
[5] S. P. Green, C. Jones, A. Stasch, Science 2007, 318, 1754 – 1757.
[6] M. Westerhausen, Angew. Chem. 2008, 120, 2215 – 2217; Angew.
Chem. Int. Ed. 2008, 47, 2185 – 2187.
[7] S. P. Green, C. Jones, A. Stasch, Angew. Chem. 2008, 120, 9219 –
9223; Angew. Chem. Int. Ed. 2008, 47, 9079 – 9083.
[8] See for example: W. J. Evans, D. K. Drummond, L. R. Cham-
berlain, R. J. Doedens, S. G. Bott, H. Zhang, J. L. Atwood, J.
Am. Chem. Soc. 1988, 110, 4983 – 4994.
[9] See for example: T. Alonso, S. Harvey, P. C. Junk, C. L. Raston,
B. Skelton, A. H. White, Organometallics 1987, 6, 2110 – 2116.
[10] See for example: a) D. Mackay, D. D. McIntyre, N. J. Taylor, J.
Org. Chem. 1982, 47, 532 – 535; b) C. M. Fitchett, C. Richardson,
P. J. Steel, Org. Biomol. Chem. 2005, 3, 498 – 502.
Experimental Section
Crystallographic data: 4·(toluene): C77H100Mg2N6; Mr = 1158.25;
monoclinic; P21/c; a = 22.385(5), b = 16.364(3), c = 19.056(4) ꢁ; b =
100.86(3)8; V= 6855(2) ꢁ3; Z = 4; 1calcd = 1.122 MgmÀ3
;
T=
123(2) K; l = 0.71073 ꢁ; 18641 reflections collected; 11855 inde-
[11] R. E. Cowley, J. Elhaꢃk, N. A. Eckert, W. W. Brennessel, E. Bill,
P. L. Holland, J. Am. Chem. Soc. 2008, 130, 6074 – 6075.
pendent reflections (Rint = 0.0226); R1 = 0.0471 (F2 > 2s(F2)); wR2 =
2976 www.angewandte.org
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2009, 48, 2973 –2977

Angewandte
Chemie
[12] See for example: F. Yuan, Q. Shen, J. Sun, Polyhedron 1998, 17,
2009 – 2012.
[13] E. C. Ashby, J. J. Lin, A. B. Goel, J. Org. Chem. 1978, 43, 1557 –
1560.
[14] A. B. Goel, E. C. Ashby, J. Organomet. Chem. 1981, 214, C1 –
C6.
[15] a) J. Spielmann, S. Harder, Chem. Eur. J. 2007, 13, 8928 – 8938;
b) S. Harder, J. Brettar, Angew. Chem. 2006, 118, 3554 – 3558;
Angew. Chem. Int. Ed. 2006, 45, 3474 – 3478.
[16] Y. Yu, A. R. Sadique, J. M. Smith, T. R. Dugan, R. E. Cowley,
W. W. Brennessel, C. J. Flaschenriem, E. Bill, T. R. Cundari, P. L.
Holland, J. Am. Chem. Soc. 2008, 130, 6624 – 6638.
[17] P. J. Walsh, F. J. Hollander, R. G. Bergman, J. Organomet. Chem.
1992, 428, 13 – 47.
[18] W. J. Evans, G. Kociok-Kohn, V. S. Leong, J. W. Ziller, Inorg.
Chem. 1992, 31, 3592 – 3600.
Angew. Chem. Int. Ed. 2009, 48, 2973 –2977
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
2977