The reaction of
nido-1-Na(L)
n
-2-(SiMe3)-3-(R)-2,3-C2B4H5
(n = 2, L = THF, R = SiMe3;
n
= 1, L = TMEDA, R = SiMe3 or Me) or
closo-exo-5,6-[(μ-H)2Li(L)
n
]-1-Li(L)
n
-2,4-(SiMe3)2-2,4-C2B4H4 (n = 2, L =
THF; n = 1, L = TMEDA) with various magnesium reagents
produced
a number of different magnesacarboranes. The product of a 1:1
molar ratio reaction of the
2,3-C2B4 monosodium compound (R =
SiMe3 and L = TMEDA) with MeMgBr was the
half-sandwich magnesacarborane
closo-1-Mg(TMEDA)-2,3-(SiMe3)2-2,3-C2B4H4
(I), while when
R = Me, the same reaction conditions gave the novel full-exo-sandwich
complex commo-exo-4,4‘,5,5‘-Mg(TMEDA)[2-(SiMe3)-3-(Me)-2,3-C2B4H5]2
(II). The full-endo-sandwich magnesacarboranes
[Na(THF)2]2[commo-1,1‘-Mg{2,3-(SiMe3)2-2,3-C2B4H4}2]
(IV) and [Na(TMEDA)]2[commo-1,1‘-Mg{2,3-(SiMe3)2-2,3-C2B4H4}2]
(V) were the exclusive products when
the appropriate monosodium compound reacted with
Mg(Bu)2 in 2:1 molar ratios.
Reaction
of
closo-exo-5,6-[(μ-H)2Li(TMEDA)]-1-Li(TMEDA)-2,4-(SiMe3)2-2,4-C2B4H4
with MgBr2 in a
1:1 molar ratio produced
closo-1-Mg(TMEDA)-2,4-(SiMe3)2-2,4-C2B4H4
(III), while a 2:1 molar
ratio gave
[Li(TMEDA)2]2[commo-1,1‘-Mg{2,4-(SiMe3)2-2,4-C2B4H4}2]
(VI). The yields ranged
from 73% for III to 94% for II. On the
other hand, the 1:1 molar ratio reaction of the THF-solvated 2,4-C2B4 disodium compound with MeMgBr,
followed by the addition of 1 equiv of
the THF-solvated monosodium compound of the
2,3-C2B4 carborane did not give the
expected
mixed-ligand complex but produced a 50:50 mixture of IV and
[Li(THF)2]2[commo-1,1‘-Mg{2,4-(SiMe3)2-2,4-C2B4H4}2]
(VII) in nearly quantitative yields. The
magnesacarboranes were
characterized by their infrared spectra, chemical analysis,
H1, 11B, and 13C NMR
spectra,
and, in the case of VII, by its 7Li NMR
spectrum. Compounds I, II, and IV
were further
characterized by single-crystal X-ray analysis. Compound
I crystallizes as a dimer in which
a Mg occupies the apical position above the pentagonal face of one
carborane and is bonded
to the unique boron of the other carborane in the dimer through a
Mg−H−B bridge. The
structure of II is one in which a TMEDA-solvated Mg is
exo-polyhedrally bonded to two
2,3-C2B4 monoanionic ligands through a pair of
Mg−H−B bridges, while in IV, the two
carborane dianions are η5-bonded to a Mg in a more
conventional endo-sandwich complex.
The reactions with MeMgBr are thought to proceed through the
formation of a methylmagnesacarborane intermediate which undergoes further reaction to give the
final products.
The 11B NMR spectra of I−VII
were analyzed with the aid of ab initio GIAO molecular
orbital calculations.