A β-diketiminato magnesium acetylide and formation of an imido aluminium magnesium hydride compound

Article abstract of DOI:10.1016/j.ica.2011.06.047

The reaction of LiCCMe with [(^Dipnacnac)MgI(OEt₂)] (^Dipnacnac = [(DipNCMe)₂CH], Dip = 2,6-diisopropylphenyl) afforded [{(^Dipnacnac)MgCCMe}₂] 1 in good yield. Treatment of 1 with AlH₃

Full text of DOI:10.1016/j.ica.2011.06.047

Inorganica Chimica Acta 376 (2011) 655–658
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Note
A b-diketiminato magnesium acetylide and formation of an imido aluminium
magnesium hydride compound
Andreas Stasch
School of Chemistry, Monash University, P.O. Box 23, Melbourne, VIC 3800, Australia
a r t i c l e i n f o
a b s t r a c t
The reaction of LiC„CMe with [(Dipnacnac)MgI(OEt2)] (Dipnacnac = [(DipNCMe)2CH], Dip = 2,6-diisopro-
pylphenyl) afforded [{(Dipnacnac)MgC„CMe}2] 1 in good yield. Treatment of 1 with AlH3ꢀNMe3 in toluene
under reflux afforded a low yield of the novel imido aluminium magnesium hydride compound
[{(AlH)(AlH2)3(NDip)3}2Mg] 2. The latter compound features a central Mg2+ ion that is coordinated via
hydride ligands to two monoanionic [(AlH)(AlH2)3(NDip)3]ꢁ fragments. The compounds have been char-
acterised by 1H and 13C NMR spectroscopy, IR spectroscopy and the crystal structures of 1 and 2ꢀ4C7H8
have been determined.
Article history:
Received 9 December 2010
Received in revised form 24 June 2011
Accepted 29 June 2011
Available online 5 July 2011
Keywords:
Acetylides
Aluminium
Hydrides
Imides
Ó 2011 Elsevier B.V. All rights reserved.
Magnesium
1. Introduction
a sterically demanding b-diketiminate magnesium fragment.
Therefore, we prepared LiC„CMe from equimolar amounts of nBuLi
and propyne and treated this with [(Dipnacnac)MgI(OEt2)] (Dipnac-
nac = [(DipNCMe)2CH], Dip = 2,6-diisopropylphenyl) [3] in toluene
and diethyl ether to afford [{(Dipnacnac)MgC„CMe}2] 1 in good
yield, see Fig. 1. Attempts to lithiate compound 1 using nBuLi or
tBuLi in a range of hydrocarbon or donor solvents were unsuccessful
so far and only yielded unaltered 1 back as the only isolated product.
A different concept to obtain compounds with polymetallated
carbon atoms has been realised with hydrometallation reactivity
[1]. For example, Uhl and Breher introduced the compound class of
carbaalanes [4]. These aluminium carbon cluster compounds can
be prepared by hydroalumination reactions of alkynyl aluminium
compounds with small dialkyl aluminium hydrides [5] or similar
reactions involving the alane adduct AlH3ꢀNMe3 [6]; the latter
reactions can also form salt-like products. To investigate the
reactivity of 1 towards aluminium hydrides, we treated [{(Dipnac-
nac)MgC„CMe}2] 1 with 7 equivalents of AlH3ꢀNMe3 in boiling
toluene. This yielded large amounts of insoluble solid and a low yield
of the mixed imido aluminium magnesium hydride compound
[{(AlH)(AlH2)3(NDip)3}2Mg] 2 could be crystallised as a toluene
solvate from the filtered reaction mixture as colourless blocks,
Fig. 1. Furthermore, small amounts of [(Dipnacnac)AlH2] [7] could
be detected in the reaction mixture by 1H NMR spectroscopy. The
formation of the imido complex 2 demonstrates that some of the
b-diketiminate ligand in 1 has been cleaved under these conditions.
The degradation of sterically demanding b-diketiminate complexes
with hydride sources or low oxidation state species has previously
The chemistry of molecular compounds with polyanionic carbon
or nitrogen ligands with multiple charges on one central atom is
less developed compared with related monoanionic examples. For
example, geminal organodimetallics with electropositive main
group elements are less common in the synthetic laboratory, despite
offering a unique reactivity and often feature interesting structural
motifs [1]. An intriguing example is the tetralithiation of propyne
using n-butylithium (nBuLi) in hexane that can afford a metastable
hexane solution of C3Li4 or an orange–red pyrophoric solid after pre-
cipitation [2]. These have previously been used to synthesize, for
example, the substituted compounds (Me3E)2C@C@C(EMe3)2 (E@Si,
Ge, Sn) [2], however, the substitution chemistry of these and related
complexes [1] remains underdeveloped.
2. Results and discussion
We carried out the reaction of propyne with four equivalents of
nBuLi by adding a hexane solution of propyne to four equivalents
of nBuLi at low temperature followed by heating under reflux to
obtain an insoluble, highly pyrophoric orange–red solid. Unfortu-
nately, this material could not be dissolved in coordinating solvents
such as THF. This could indicate a highly polymeric nature of this
solid. In order to prepare a soluble model complex of this system
for further studies, we attempted to substitute one Li+ cation with
been observed and involves the formation of
a substituted
E-mail address: Andreas.Stasch@monash.edu
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.06.047

656
A. Stasch / Inorganica Chimica Acta 376 (2011) 655–658
Fig. 1. Synthesis of compounds 1 and 2; (i) LiC„CMe, toluene, Et2O; (ii) 7 AlH3ꢀNMe3, toluene, reflux.
azabutadiene and element imido fragments via a C–N cleavage reac-
tion [8] and a similar process is believed to occur during the forma-
tion of the imido compound 2.
(2069 cmꢁ1) is at slightly higher wave numbers compared to those
of [{(Dipnacnac)CaC„CR}2] (2029–2048 cmꢁ1).
Compound 2 crystallises with half a molecule in the asymmetric
unit, with the Mg atom sitting on a centre of inversion, and can be
described as a contact ion pair of two monoanionic [(AlH)(AlH2)3
(NDip)3]ꢁ moieties coordinating solely via bridging hydrides to a
The molecular structure of compound 1 (Fig. 2) shows that
dimerisation of two (Dipnacnac)MgC„CMe fragments occur via
contacts of the Mg atom to the acetylide
p-system. The end on
bound acetylide magnesium fragment Mg–C„C is almost linear
central Mg2+ cation, see Fig. 3. Each DipN imido ligand acts as a l3
-
(Mg1–C30–31 angle of 178.2(2)°) with a short Mg–C bond of
cap between two AlH2 and one AlH fragment with Al–N distances
0
0
2.132(2) ÅA and longer MgꢀꢀꢀC contacts (2.301(2) to C30 and
between ca. 1.92–1.94 ÅA. For comparison, some related anionic ami-
no and/or imino alanate complexes incorporating Li or Na cations
have beencharacterised by thegroups of Roesky [11] and Henderson
[12]. The latter work describes the structure of [Li(OEt2)3][(AlH)4
(NPh)6{Li(OEt2)}3] and contains an anionic open cage fragment with
solely four coordinate Al atoms with Al–N distances slightly shorter
than those in 2. Also, a range of products from the reaction of DipNH2
with AlH3ꢀNMe3 under various stoichiometries and conditions
yielded amino and imino alanes that have been studied by Schulz
and co-workers [13]. In compound 2, the two [(AlH)(AlH2)3
(NDip)3]ꢁ half spheres coordinate via hydride bridges of the six
0
2.515(2) ÅA to C31) in the bridging
p interaction. A number of re-
lated b-diketiminate calcium and strontium acetylides have re-
cently been crystallographically characterised showing similar
bridging coordination modes of the acetylide ligands [9]. Further-
more, interactions of acetylide ions with heavier group 2 metals
have recently been investigated theoretically and experimentally
[10]. The NMR and IR spectroscopic data of 1 is in accordance with
its solid state structure. The comparison with data for related
[{(Dipnacnac)CaC„CR}2] complexes [9a] suggest a less flexible
solution behaviour for 1 at room temperature. In 1, 1H and 13C
NMR resonances for two different iPr groups of the (Dipnacnac) li-
gand can be detected, whereas only resonances for one iPr environ-
ment are found in [{(Dipnacnac)CaC„CR}2] at room temperature.
However, cooling of the latter complex (for R = tBu) leads to split-
ting of the iPr resonances [9a]. It is worth mentioning, that both 1
and [{(Dipnacnac)CaC„CR}2] display six resonances in the aromatic
region of the 13C NMR spectrum for the Dip substituents at room
AlH2 units to the central Mg atom. The six MgꢀꢀꢀH contacts range
0
from ca. 1.87 to 2.00 ÅA giving a slightly distorted octahedral coordi-
nation sphere. Mixed element hydride bridges to Mg are not an
uncommon structural motif. For example, a monoetherate complex
of magnesium alanate has been crystallographically characterised
and features a central Mg2+ ion octahedrally coordinated to five hy-
drides and one diethyl ether ligand [14]. The mean MgꢀꢀꢀH contact of
0
0
temperature. The C„C stretch in the IR spectrum of
1
2.0 ÅA and the mean Al–H bond length of 1.55 ÅA in the alanate com-
plex are similar to those in 2. The Mg environmentin 2 is reminiscent
of the Mg packing in solid MgH2, as determined from a neutron dif-
fraction structure of rutil type MgD2 with six MgꢀꢀꢀD contacts of ca.
0
1.95 ÅA in a regular octahedral environment [15]. Accordingly, com-
pound 2 can be viewed as a soluble model complex for the coordina-
tion of magnesium in solid MgH2. Very recently, hydride rich
2þ
6þ
magnesium complexes with Mg4H6
and Mg8H10
cores sup-
ported by sterically demanding ligands have been prepared and
structurally characterised that feature distorted tetrahedral coordi-
nation spheres around the Mg atoms [16].
The spectroscopic data for compound 2 is consistent with its so-
lid state structure. The 1H NMR spectrum shows two doublets and
two septets for the protons of the iPr groups and, accordingly in its
13C NMR spectrum, two resonances each for the methyl and
methine carbon atoms of the iPr groups, as well as six resonances
for the aromatic carbons. The IR spectrum displays two sharp
stretches at 1999 and 1900 cmꢁ1, the latter being the stronger
one, and a broad one at around 1693 cmꢁ1 with a shoulder peak
at around 1753 cmꢁ1. The first two stretches are tentatively as-
signed to the terminal Al–H bonds and the latter ones to the bridg-
ing hydrides.
In summary, we have synthesised the first structurally charac-
terised b-diketiminate magnesium acetylide and its reaction with
AlH3ꢀNMe3 yielded a novel imido aluminium magnesium hydride
compound with a central Mg2+ ion solely coordinated to hydride
ligands. The latter reaction involves some degradation of the
b-diketiminate ligand.
Fig. 2. ORTEP diagram of [{(Dipnacnac)MgC„CMe}2] 1 (30% ellipsoids). Hydrogen
atoms have been omitted for clarity. Selected bond lengths (ÅA) and angles (°): Mg1–
C30 2.132(2), Mg10ꢀꢀꢀC30 2.301(2), Mg10ꢀꢀꢀC31 2.515(2), C30–C31 1.234(3) Mg1–N1
2.0717(19), Mg1–N2 2.0613(18); N1–Mg1–N2 92.95(7), Mg1–C30–C31 178.2(2).
0

A. Stasch / Inorganica Chimica Acta 376 (2011) 655–658
657
Fig. 3. ORTEP diagram of [{(AlH)(AlH2)3(NDip)3}2Mg] 2 (30% ellipsoids). Four toluene molecules per formula unit and hydrogen atoms except hydride ligands have been
0
omitted for clarity. Selected bond lengths (ÅA) and angles (°): Al1–N1 1.9410(19), Al1–N2 1.9374(18), Al1–N3 1.9368(18), Al2–N1 1.9254(18), Al2–N2 1.9301(19), Al3–N2
1.9250(18), Al3–N3 1.9265(18), Al4–N1 1.9318(18), Al4–N3 1.9220(19), Al1–H1 1.48(3), Al2–H2 1.48(2), Al2–H5 1.66(4), Al3–H3 1.49(3), Al3–H6 1.62(4), Al4–H4 1.56(3),
Al4–H7 1.60(3), Mg1–H5 1.87(4), Mg1–H6 1.88(3), Mg1–H7 2.00(3); N1–Al1–N2 90.08(8), Al1–N1–Al2 88.59(8), Al2–N1–Al4 111.64(9).
(Nujol)
m
(cmꢁ1): 2069w (C„C), 1623w, 1519m, 1365s, 1314m,
3. Experimental
1261m, 1252m, 1174m, 1098m, 1057m, 1018m, 934m, 792s,
760s, 727m; Anal. Calc. for C64H88Mg2N4: C, 79.90; H, 9.22; N,
5.82. Found: C, 79.12; N, 9.04; H, 5.63%.
3.1. General
All manipulations were carried out using standard Schlenk and
glove box techniques under an atmosphere of high purity dinitro-
gen. Toluene and hexane were distilled over molten potassium,
whilst diethylether was distilled over Na/K alloy (1:3). 1H and
13C{1H} NMR spectra were recorded on a Bruker DPX 300 spec-
trometer in deuterated benzene and were referenced to the resid-
ual 1H or 13C resonances of the solvent used. IR spectra were
recorded using a Perkin–Elmer RXI FT-IR spectrometer as Nujol
mulls between NaCl plates. Melting points were determined in
sealed glass capillaries under dinitrogen, and are uncorrected.
LiC„CMe was prepared from nBuLi and propyne in hexane [2]
and [(Dipnacnac)MgI(OEt2)] [3] was prepared according to the liter-
ature procedure.
3.3. Preparation of [{(AlH)(AlH2)3(NDip)3}2Mg] 2
A solution of AlH3ꢀNMe3 (2.4 mL of a 0.67 m solution in toluene,
1.61 mmol) was added to a mixture of [{(Dipnacnac)MgC„CMe}2] 1
(0.22 g, 0.23 mmol) and toluene (10 mL). The mixture was heated
under reflux for 1 h (forming a large amount of a solid), filtered
hot, concentrated to ca. 3 mL and cooled to 4 °C to afford colourless
blocks of 2ꢀ4C7H8. Yield: 30 mg (12%, based on DipN); Mp: slow de-
comp. above ca. 190 °C; 1H NMR (300 MHz, C6D6, 303 K): d 1.28 (d,
3
3JH–H = 6.9 Hz, 36H, CH(CH3)2), 1.39 (d, JH–H = 6.9 Hz, 36H,
3
CH(CH3)2), 2.93 (sept, JH–H = 6.9 Hz, 6H, CH(CH3)2), 3.19 (sept,
3JH–H = 6.9 Hz, 4H, CH(CH3)2), 4.52 (br, with shoulder at ca. 4.3,
ca. 12H, AlH), 5.1 (vbr, ca. 2H, AlH), 6.84–7.20 (m, 18H, ArH);
13C{1H} NMR (75.5 MHz, C6D6, 303 K): d = 26.6 (CH(CH3)2), 26.8
(CH(CH3)2), 33.5 (CH(CH3)2), 33.7 (CH(CH3)2), 124.0 (Ar-C), 124.9
(Ar-C), 126.4 (Ar-C), 141.6 (Ar-C), 141.7 (Ar-C), 142.3 (Ar-C); IR
3.2. Preparation of [{(Dipnacnac)MgC„CMe}2] 1
Toluene (40 mL) and Et2O (20 mL) were added to a mixture of
[(Dipnacnac)MgI(OEt2)] (1.73 g, 2.69 mmol) and LiC„CMe
(0.133 g, 2.90 mmol) at ꢁ80 °C. The mixture was slowly warmed
to room temperature and stirred for 1 h. The mixture was filtered,
concentrated to ca. 20 mL and cooled to 4 °C to afford a toluene sol-
vate of 1. Further concentration of the supernatant solution and
cooling afforded a second crop. The solid was dried under vacuum.
Single crystals of 1 were obtained by crystallisation from hexane.
Yield: 0.92 g (71%); Mp >300 °C; 1H NMR (300 MHz, C6D6, 303 K):
(Nujol)
m
(cmꢁ1): 1999w (AlH), 1901m (AlH), 1693s (br, with
shoulder at 1753, AlH), 1426m, 1366s, 1314m, 1225m, 1163s,
1101s, 1032m, 946s, 888s, 853s, 758s, 729m, 696s.
3.4. X-ray crystallography
Suitable crystals were mounted in silicone oil and were mea-
0
sured with Mo Ka radiation (k = 0.71073 ÅA) at T = 123(2) K using a
3
3
d 0.57 (d, JH–H = 6.9 Hz, 12H, CH(CH3)2), 1.20 (d, JH–H = 6.9 Hz,
Nonius Kappa CCD (1) or a Bruker X8 Apex II (2ꢀ4C7H8) diffractom-
eter and were refined using SHELX [17]. All non-hydrogen atoms
were refined anisotropically. The hydride ligands in 2ꢀ4C7H8 were
located from difference maps and their positions were freely re-
fined, all other hydrogen atoms were included in calculated posi-
3
24H, CH(CH3)2), 1.48 (d, JH–H = 6.9 Hz, 12H, CH(CH3)2), 1.48 (s,
12H, NCCH3), 1.59 (s, 6H, C„CCH3), 3.00 (sept, JH–H = 6.9 Hz, 4 H,
3
3
CH(CH3)2), 3.42 (sept, JH–H = 6.9 Hz, 4H, CH(CH3)2), 4.65 (s, 2H,
CH), 6.97–7.22 (m, 12H, ArH); 13C{1H} NMR (75.5 MHz, C6D6,
303 K): d = 23.9, 24.3, 24.4, 24.5, 25.1, 25.9 (4 ꢂ CH(CH3)2, NCCH3,
C„CCH3), 27.6 (CH(CH3)2), 29.0 (CH(CH3)2), 94.9 (CH), 112.8
(MgC„C), 115.8 (C„CCH3), 124.1 (Ar-C), 125.7 (Ar-C), 129.4
(Ar-C), 142.2 (Ar-C), 143.5 (Ar-C), 145.9 (Ar-C), 169.3 (NCCH3); IR
tions. Crystal data for 1:
C64H88Mg2N4; M = 962.00 g/mol;
0
0
monoclinic space group P21/n; a = 11.028(2) ÅA, b = 14.289(3) ÅA,
0
0
c = 18.326(4) ÅA, b = 95.07(3)°, V = 2876.5(10) ÅA3; Z = 2,
q = 1.111 g/
cm3,
l
= 0.083 mmꢁ1, ꢁ13 6 h 6 13, ꢁ16 6 k 6 16, ꢁ21 6 l 6 21,

658
A. Stasch / Inorganica Chimica Acta 376 (2011) 655–658
[3] (a) J. Prust, K. Most, I. Müller, E. Alexopoulos, A. Stasch, I. Uson, H.W. Roesky, Z.
Anorg. Allg. Chem. 627 (2001) 2032;
(b) S.J. Bonyhady, C. Jones, S. Nembenna, A. Stasch, A.J. Edwards, G.J. McIntyre,
Chem. Eur. J. 16 (2010) 938.
[4] (a) W. Uhl, F. Breher, Eur. J. Inorg. Chem. (2000) 1;
(b) W. Uhl, Coord. Chem. Rev. 252 (2008) 1540.
[5] (a) W. Uhl, F. Breher, Angew. Chem., Int. Ed. 38 (1999) 1578;
(b) W. Uhl, F. Breher, A. Lützen, W. Saak, Angew. Chem., Int. Ed. 39 (2000) 406.
[6] (a) A. Stasch, M. Ferbinteanu, J. Prust, W. Zheng, F. Cimpoesu, H.W. Roesky, J.
Magull, H.-G. Schmidt, M. Noltemeyer, J. Am. Chem. Soc. 124 (2002) 5441;
(b) A. Stasch, H.W. Roesky, P.v.R. Schleyer, J. Magull, Angew. Chem., Int. Ed. 42
(2003) 5507;
refl. collected: 8877, refl. unique: 5050, Goodness-of-fit = 1.022, R1
(I > 2r(I)) = 0.0520, wR2 (all data) = 0.1352, completeness to theta
0
(25.00°): 99.6%, largest difference peak/hole: 0.449/ꢁ0.220 e ÅAꢁ3
.
Crystal data for 2ꢀ4C7H8: C100H148Al8MgN6; M = 1674.39 g/mol;
0
0
monoclinic space group P21/n; a = 15.2559(16) ÅA, b = 19.292(2) ÅA,
0
0
c = 16.8874(17) ÅA, b = 94.294(4)°, V = 4956.3(9) ÅA3; Z = 2,
q =
1.122 g/cm3,
l
= 0.135 mmꢁ1, ꢁ19 6 h 6 18, ꢁ24 6 k 6 24, ꢁ21 6
l 6 21, refl. collected: 50507, refl. unique: 10811, Goodness-
of-fit = 1.088, R1 (I > 2r(I)) = 0.0535, wR2 (all data) = 0.1439, com-
pleteness to theta (27.00°): 99.9%, largest difference peak/hole:
(c) A. Stasch, H.W. Roesky, M. Noltemeyer, H.-G. Schmidt, Inorg. Chem. 44
(2005) 5854.
[7] C. Cui, H.W. Roesky, H. Hao, H.-G. Schmidt, M. Noltemeyer, Angew. Chem., Int.
Ed. 39 (2000) 1815.
[8] (a) K.D. Conroy, W.E. Piers, M. Parvez, Organometallics 28 (2009) 6228;
(b) W.D. Woodul, E. Carter, R. Müller, A.F. Richards, A. Stasch, M. Kaupp, D.M.
Murphy, M. Driess, C. Jones, J. Am. Chem. Soc. 133 (2011) 10074.
[9] (a) A.G. Avent, M.R. Crimmin, M.S. Hill, P.B. Hitchcock, Organometallics 24
(2005) 1184;
0
0.474/ꢁ0.279 e ÅA–3
.
Acknowledgements
AS is grateful to the Australian Research Council for
fellowship.
a
(b) A.G.M. Barrett, M.R. Crimmin, M.S. Hill, P.B. Hitchcock, S.L. Lomas, P.A.
Procopiou, K. Suntharalingam, Chem. Commun. (2009) 2299;
(c) A. Stasch, S.P. Sarish, H.W. Roesky, K. Meindl, F. Dall’Antonia, T. Schulz, D.
Stalke, Chem. Asian J. 4 (2009) 1451.
Appendix A. Supplementary data
[10] M.A. Guino-o, J.S. Alexander, M.L. McKee, H. Hope, U.B. Englich, K. Ruhlandt-
Senge, Chem. Eur. J. 15 (2009) 11842.
[11] M.L. Montero, H. Wessel, H.W. Roesky, M. Teichert, I. Uson, Angew. Chem., Int.
Ed. Engl. 36 (1997) 629.
[12] K.W. Henderson, A.R. Kennedy, A.E. McKeown, R.E. Mulvey, Angew. Chem., Int.
Ed. 39 (2000) 3879.
[13] T. Bauer, S. Schulz, H. Hupfer, M. Nieger, Organometallics 21 (2002) 2931.
[14] M. Fichtner, O. Fuhr, J. Alloys Comp. 345 (2002) 286.
[15] W.H. Zachariasen, C.E. Holler Jr., J.F. Stamper Jr., Acta Crystallogr., Sect. 16
(1963) 352.
CCDC-793605 (1) and CCDC-793606 (2ꢀ4C7H8) contains the
supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via http://www.ccdc.cam.ac.uk/data_request/
cif.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.ica.2011.06.047.
[16] (a) M. Arrowsmith, M.S. Hill, D.J. MacDougall, M.F. Mahon, Angew. Chem., Int.
Ed. 48 (2009) 4013;
References
(b) S. Harder, J. Spielmann, J. Intemann, H. Bandmann, Angew. Chem., Int. Ed.
50 (2011) 4156.
[17] G.M. Sheldrick, Acta Cryst. A64 (2008) 112.
[1] I. Marek, J.-F. Normant, Chem. Rev. 96 (1996) 3241.
[2] (a) R. West, P.A. Carney, I.C. Mineo, J. Am. Chem. Soc. 87 (1965) 3788;
(b) R. West, P.C. Jones, J. Am. Chem. Soc. 91 (1969) 6156;
(c) W. Priester, R. West, T.L. Chwang, J. Am. Chem. Soc. 98 (1976) 8413.