Synthesis of New Magnesium Imide and
Magnesium Guanidinate Complexes
Alexander Fuchs,, Elisabeth Kaifer,, Hans-Jörgg Himmel
Anorganisch-Chemisches Institut, Universität Heidelberg, Im Neuenheimer Feld 270, D-69120 Heidelberg, Germany,
Fax: ++49 - 6221 - 545707;
not isolated
Several magnesium imides were synthesised as the
above general scheme shows. All have an aromatic
substituent RAryl directly bonded to the imide nitrogen
atom, e.g. Phenyl,[2] Naphthyl,[2] and a coordinating
solvent molecule stabilising the magnesium atom.
Depending on the space of the substituent the imide
crystallises in a hexameric or tetrameric structure.[1]
We discovered that it is also possible to stabilise the
imide with an α-silicon. Therefore we used Ph3SiNH2 as
amine source.[3] The corresponding magnesium imide
crystallises as the tetrameric [(THF)MgNSiPh3]4. As the
space-filling model shows the heterocubane is almost
completely covered by the ligand shell.
Quantum chemical calculations - B3LYP/TZVPP - were
accomplished to understand the reaction pathway.
Instead of Ph3SiNMg we used PhNMg.[3] Although no
solvent effects were considered, these calculations show
that ethane elimination from monomeric EtMgNHPh is
very unlikely. Probably dimeric imide is directly formed
from dimeric alkylmagnesium amide and subsequently
oligomerises further.
ΔG
G0 / kJ mol-1
Graduierttenkolleg 8
850: Modellierung von Moleküle
eigenschafften
Synthesis of a Magnesium Imide[1]
MgNPh + EtH
(+194)
100
0
EtMgNHPh 1/2 [MgNPh]2 + EtH
(+3)
1/2 [EtMgNHPh]2
(-55)
1/6 [MgNPh]6 + EtH
(-127)
X-ray structure (Hydrogen atoms are omitted) and space-filling model of [(THF)MgNSiPh3]4. Colour code: Mg green, N blue,
Si yellow, O red, C gray, H white.
Calculated standard Gibbs free energies of several relevant
species relative to that of monomeric Ethyl magnesium
(phenylamide).
Magnesium Imides as Imide Transfer Reagents[2]
Magnesium imides are versatile synthetic reagents.
Especially [(THF)MgNPh]6 featuring a hexagonal
prismatic Mg6N6 cage structure was investigated
intensely.[2] One of its properties is to transfer its imide
substituent to the main group 4 dihalides GeCl2 . dioxane,
SnCl2 or PbCl2.
It is possible to assign the metal transfer reaction
successfully
to
our
magnesium
imide
[(THF)MgNSiPh3]4. We accomplished this reaction with
GeCl2.dioxane to obtain the corresponding germanium
imide. The X-ray analysis shows that the crystal structure
consists of the heterocubic species [GeNSiPh3]4.
X-ray structure of [GeNSiPh3]4 (Hydrogen atoms are omitted)
Colour code: Ge dark gray, N blue, Si yellow, O red, C gray.
Synthesis of the First Magnesium hppH Complex (hppH = 1,3,4,6,7,8-Hexahydropyrimido[1,2-a]pyrimidinide)[4]
Reaction between MeMgBr and the bicyclic guanidine
hppH affords a new magnesium guanidinate compound.
It crystallises as the tetrameric cluster [MgBr(hpp)]4.
The core consists of four magnesium atoms which are
square pyramidal distorted. Reduction of the Mg (+II) to
Mg (+I) was unsuccessful using Na or K mirror.
N
Key bond lengths [Å] for [MgBr(hpp)]4
N
[Mg]
Mg – Mg
N
[Mg]
[Mg]
2.943(3)
3.4086(16)
3.4181(16)
Binding mode of hpp in [MgBr(hpp)]4: κ1,2N – κ2,3 N
Mg – N
2.097(3)
Mg – N
2.113(3)
2.163(3)
2.132(3)
2.188(3
2.188(3)
2.293(4)
2.322(3)
X-ray structure of [MgBr(hpp)]4 (Hydrogen atoms are omitted)
Colour code: Mg green, N blue, Br brown, C gray.
[1] e.g.Jeffrey A. Rood, Bruce C. Noll, and Kenneth W. Henderson, Inorg. Chem. 2007, 46, 7259 – 7261.
[2] Warren J. Grigsby, Tony Hascall, Jeffrey J. Ellison, Marilyn M. Olmstead, and Philip P. Power, Inorg. Chem. 1996, 35, 3254-3261.
[3] A. Fuchs, Elisabeth Kaifer and Hans-Jörg Himmel , Eur. J. Inorg. Chem. 2008, 41-43.
[4] Oxana Ciobanu, Alexander Fuchs, Matthias Reinmuth, Anna Lebkücher, Elisabeth Kaifer, Hubert Wadepohl and Hans-Jörg Himmel, ZAAC. 2009, in press.