Chemistry of c-trimethylsilyl-substituted heterocarboranes. 23. synthetic, spectroscopic, and structural investigation on half- and full-sandwich Magnesacarboranes of 2,3- and 2,4-C2B4 carborane ligands

Article abstract of DOI:10.1021/om9711133

The reaction of nido-1-Na(L)_n-2-(SiMe₃)-3-(R)-2,3-C₂B ₄H₅ (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-C₂B ₄H₄ (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-C₂B ₄H₄ (I), while when R = Me, the same reaction conditions gave the novel full-exo-sandwich complex commoexo-4,4′,5,5′-Mg(TMEDA)[2-(SiMe₃)-3-(Me)-2,3-C ₂B₄H₅]₂ (II). The full-endo-sandwich magnesacarboranes [Na(THF)₂]₂[commo-1,1′-Mg{2,3-(SiMe ₃)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-(SiMe ₃)₂-2,4-C2B4H4 with MgBr₂ in a 1:1 molar ratio produced closo-1-Mg(TMEDA)-2,4-(SiMe₃)₂-2,4-C₂B ₄H₄ (III), while a 2:1 molar ratio gave [Li(TMEDA)₂]₂[commo-1,1′-Mg{2,4-(SiMe ₃)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-(SiMe ₃)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 methylmag-nesacarborane 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.

Full text of DOI:10.1021/om9711133

1426
Organometallics 1998, 17, 1426-1437
Ch em istr y of C-Tr im eth ylsilyl-Su bstitu ted
Heter oca r bor a n es. 23. Syn th etic, Sp ectr oscop ic, a n d
Str u ctu r a l In vestiga tion on Ha lf- a n d F u ll-Sa n d w ich
Ma gn esa ca r bor a n es of 2,3- a n d 2,4-C2B4 Ca r bor a n e
Liga n d s
Narayan S. Hosmane,*,† Dunming Zhu, J ames E. McDonald, Hongming Zhang,
J ohn A. Maguire, Thomas G. Gray, and Sarah C. Helfert
Department of Chemistry, Southern Methodist University, Dallas, Texas 75275
Received December 19, 1997
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 mag-
nesacarboranes [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,
7
and, in the case of VII, by its Li 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 methylmag-
nesacarborane 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.
In tr od u ction
pentagonal bipyramidal (MC2B4) or icosahedral (MC2B9)
metallacarboranes.1 The most common method of syn-
thesis is the reaction of the particular nido-carborane
anion with a suitable metal halide reagent. The coun-
terions of the precursor carborane ligands are usually
one of the group 1 metals,1,2 although aluminum,3
thallium(I)4,5 and tin(II)6 counterions have also been
There have been a number of reports of the incorpo-
ration of metals, or metal groups, into the polyhedral
cage structures of carboranes.1 Although a variety of
carboranes have been investigated, most cage expansion
studies are those involving the dicarbaboranes having
four or nine boron atoms, which result in the respective
(2) (a) Hawthorne, M. F.; Young, D. C.; Wegner, P. A. J . Am. Chem.
Soc. 1965, 87, 1818. (b) Hawthorne, M. F. Acc. Chem. Res. 1968, 1,
281.
(3) J utzi, P.; Galow, P. J . Organomet. Chem., 1987, 319, 139.
(4) Spencer, J . L.; Green, M.; Stone, F. G. A. J . Chem. Soc., Chem.
Commun. 1972, 1178.
† Camille and Henry Dreyfus Scholar.
(1) (a) Grimes, R. N. In Comprenensive Organometallic Chemistry
II; Abel, E. W., Stone, F. G. A., Wilkinson, G., Eds.; Elsevier Science:
New York, 1995; Vol. 1, Chapter 9. (b) Saxena, A. K.; Maguire, J . A.;
Hosmane, N. S. Chem. Rev. 1997, 97, 2421.
S0276-7333(97)01113-8 CCC: $15.00 © 1998 American Chemical Society
Publication on Web 03/04/1998

C-Trimethylsilyl-Substituted Heterocarboranes
Organometallics, Vol. 17, No. 7, 1998 1427
closo-carborane was subsequently converted to the correspond-
ing N,N,N,N-tetramethylethylenediamine (TMEDA)- or tet-
rahydrofuran (THF)-solvated carbons-apart dilithiacarborane
closo-exo-5,6-[(µ-H)2Li(L)n]-1-Li(L)n-2,4-(SiMe3)2-2,4-C2B4H4 (L
) TMEDA, n ) 1; L ) THF, n ) 2) by reduction with Li/C10H8
as outlined previously.16 The nido-carborane precursor was
converted to the corresponding THF- or TMEDA-solvated
monosodium compound nido-1-Na(L)-2-(SiMe3)-3-(R)-2,3-C2B4H5
(L ) TMEDA or THF; R ) SiMe3, Me), as previously described.7b
Prior to use, TMEDA (Aldrich) was distilled in vacuo and
stored over sodium metal; its purity was checked by IR and
NMR spectra and boiling point measurements. Before use,
naphthalene (Aldrich) was sublimed in vacuo and Li metal
(Aldrich) was freshly cut in a drybox. MgBr2 was freshly
prepared by the reaction of magnesium with 1,2-dibromoet-
hane in anhydrous THF at room temperature. MeMgBr (3.0
M solution in diethyl ether; Aldrich) and Mg(n-C4H9)2 (1.0 M
solution in heptane; Aldrich) were used as received. Benzene,
THF, and n-hexane were dried over LiAlH4 and doubly
distilled; all other solvents, including C6D6 and THF-d8 (Ald-
rich), were dried over 4-8 mesh molecular sieves (Aldrich) and
either saturated with dry argon or degassed before use.
Sp ectr oscop ic a n d An a lytica l P r oced u r es. 1H, 7Li, 11B,
and 13C pulse Fourier transform NMR spectra at 200, 77.7,
64.2, and 50.3 MHz, respectively, were recorded on an IBM-
WP200 SY multinuclear NMR spectrometer. Infrared spectra
were recorded on a Nicolet Magna 550 FT-IR spectrophotom-
eter. Elemental analyses were obtained from E+R Microana-
lytical Laboratory, Inc., Corona, NY.
used. The group 1 metals have generally been assumed
to be innocent spectators whose interactions have been
ignored. However, structural studies on the bis(trim-
ethylsilyl)-substituted C2B4 carborane derivatives have
shown that the group 1 metals can occupy apical
positions above the C2B3 pentagonal faces of the ligands
to form stable half-sandwich complexes7 and in the case
of lithium, a full-sandwich complex.8 It has also been
demonstrated that the structures exert an influence on
the reactivity of these compounds.7a,9 It would be of
great interest to compare the structures and reactivities
of these metallacarboranes with those of the other
s-block elements. Unfortunately, there is little informa-
tion available on the group 2 metallacarboranes. The
only structures of larger cage group 2 metallacarboranes
10
are those of closo-1,1,1,1-(MeCN)4-1,2,4-CaC2B10H12
and [closo-1,1,1-(MeCN)3-1,2,4-SrC2B10H12]n,11 reported
by Hawthorne and co-workers. The beryllacarborane,
Me3NBeC2B9H11 was synthesized and assigned a closo
structure on the basis of its NMR spectrum.12 The only
group 2 metallacarborane structures in the smaller,
C2B4-cage system were those contained in our prelimi-
nary report on half- and full-sandwich magnesacarbo-
ranes that were synthesized by the reaction of nido-1-
Na(L)2-2,3-(SiMe3)2-2,3-C2B4H5 (L ) TMEDA or THF)
with magnesium alkyls.13 We report herein the details
of the synthetic, spectroscopic, and structural studies
of these compounds, as well as the results of an
extension of our investigation of the syntheses and
structures of the group 2 metallacarboranes.
Syn th etic P r oced u r es. All experiments were carried out
in Pyrex round-bottom flasks of 100-250 mL capacities,
containing magnetic stirring bars and fitted with high-vacuum
Teflon valves. Nonvolatile substances were manipulated in
either a drybox or an evacuable glovebag under an atmosphere
of dry nitrogen. All known compounds among the products
were identified by comparing their IR and NMR spectra with
those of authentic samples.
Exp er im en ta l Section
Ma ter ia ls. 2,3-Bis(trimethylsilyl)-2,3-dicarba-nido-hexabo-
rane(8), 2-(trimethylsilyl)-3-(methyl)-2,3-dicarba-nido-hexabo-
rane(8), and 1,2-bis(trimethylsilyl)-1,2-dicarba-closo-hexaborane-
(6) were prepared using the literature methods.7b,14,15 The
Syn th esis of closo-1-Mg(TMEDA)-2,3-(SiMe3)2-2,3-C2B4H4
(I). To a solution of MeMgBr (3.0 mmol) in 1 mL of Et2O, a
benzene solution (20 mL) of nido-1-Na(TMEDA)-2,3-(SiMe3)2-
2,3-C2B4H5 (1.07 g, 3.0 mmol) was added at -78 °C. The
mixture was warmed to room temperature, which resulted in
the slow evolution of gas with the concomitant formation of a
white solid. After the mixture was stirred constantly overnight
at room temperature, it was filtered in vacuo to collect a clear
filtrate. The white solid that remained on the frit was
identified as NaBr (not measured) and was discarded. Re-
moval of the solvent from the filtrate resulted in the formation
of a pale yellow solid, which was washed repeatedly with
n-hexane and later identified as closo-1-Mg(TMEDA)-2,3-
(SiMe3)2-2,3-C2B4H4 (I) (0.94 g, 2.60 mmol, 87% yield; mp 290
°C; soluble in both polar and nonpolar organic solvents).
Compound I was recrystallized from benzene solution to give
colorless platelike crystals. Anal. Calcd for C14H38B4N2Si2-
Mg (I): C, 46.95; H, 10.69; N, 7.81. Found: C, 47.89; H, 10.37;
N, 7.03. The NMR and IR spectral data of I are listed in
Tables 1 and 2, respectively.
Syn th esis of commo-exo-4,4′,5,5′-Mg(TMEDA)[2-(SiMe3)-
3-(Me)-2,3-C2B4H5]2(II). To a solution of MeMgBr (3.0 mmol)
in 1 mL Et2O, a benzene solution of (20 mL) of nido-1-Na-
(TMEDA)-2-(SiMe3)-3-(Me)-2,3-C2B4H5 (0.90 g, 3.0 mmol) was
added at -78 °C. The mixture was warmed to room temper-
ature, which resulted in the formation of a white precipitate,
but no gas evolution was detected. After the mixture was
stirred constantly overnight at room temperature, it was
filtered in vacuo to collect a clear filtrate. Removal of solvents
(5) (a) Schubert, D. M.; Bandman, M. A.; Rees, W. S., J r.; Knobler,
C. B.; Lu, P.; Nam, W.; Hawthorne, M. F. Organometallics 1990, 9,
2046. (b) Bandman, M. A.; Knobler, C. B.; Hawthorne, M. F. Inorg.
Chem. 1989, 28, 8, 1204. (c) Maning, M. J .; Knobler, C. B.; Hawthorne,
M. F. J . Am. Chem. Soc. 1988, 110, 4458.
(6) (a) Islam, M. S.; Siriwardane, U.; Hosmane, N. S.; Maguire, J .
A.; de Meester, P.; Chu, S. S. C. Organometallics 1987, 6, 1936. (b)
Siriwardane, U.; Islam, M. S.; Maguire, J . A.; Hosmane, N. S.
Organometallics 1988, 7, 1893.
(7) (a) Hosmane, N. S.; Siriwardane, U.; Zhang, G.; Zhu, H.;
Maguire, J . A. J . Chem. Soc., Chem. Commun. 1989, 1128. (b)
Hosmane, N. S.; Saxena, A. K.; Barreto, R. D.; Zhang, H.; Maguire, J .
A.; J ia, L.; Wang, Y.; Oki, A. R.; Grover, K. V.; Whitten, S. J .; Dawson,
K.; Tolle, M. A.; Siriwardane, U.; Demissie, T.; Fagner, J . S. Organo-
metallics 1993, 12, 2, 3001. (c) Zhang, H.; Wang, Y.; Saxena, A. K.;
Oki, A. R.; Maguire, J . A.; Hosmane, N. S. Organometallics 1993, 12,
3933.
(8) Hosmane, N. S.; Yang, J .; Zhang, H.; Maguire, J . A. J . Am. Chem.
Soc. 1996, 118, 5150.
(9) (a) Hosmane, N. S.; J ia, L.; Wang, Y.; Saxena, A. K.; Zhang, H.;
Maguire, J . A. Organometallics 1994, 13, 4113. (b) Wang, Y.; Zhang,
H.; Maguire, J . A.; Hosmane, N. S. Organometallics 1993, 12, 3781.
(10) Khattar, R.; Knobler, C. B.; Hawthorne, M. F. J . Am. Chem.
Soc. 1990, 112, 4962.
(11) Khattar, R.; Knobler, C. B.; Hawthorne, M. F. Inorg. Chem.
1990, 29, 2191.
(12) (a) Popp, G.; Hawthorne, M. F. J . Am. Chem. Soc. 1968, 90,
6553. (b) Popp, G.; Hawthorne, M. F. Inorg. Chem. 1971, 10, 391.
(13) Hosmane, N. S.; Zhu, D.; McDonald, J . E.; Zhang, H.; Maguire,
J . A.; Gray, T. G.; Helfert, S. C. J . Am. Chem. Soc. 1995, 117, 12362.
(14) (a) Hosmane, N. S.; Sirmokadam, N. N.; Mollenhauer, M. N.
J . Organomet. Chem. 1985, 279, 359. (b) Hosmane, N. S.; Mollenhauer,
M. N.; Cowley, A. H.; Norman, N. C. Organometallics 1985, 4, 1194.
(c) Barreto, R. D.; Hosmane, N. S. Inorg. Synth. 1992, 29, 89.
(15) Hosmane, N. S.; Barreto, R. D.; Tolle, M. A.; Alexander, J . J .;
Quintana, W.; Siriwardane, U.; Shore, S. G.; Williams, R. E. Inorg.
Chem. 1990, 29, 2698.
(16) (a) Hosmane, N. S.; J ia, L.; Zhang, H.; Bausch, J . W.; Prakash,
G. K. S.; Williams, R. E.; Onak, T. P. Inorg. Chem. 1991, 30, 3793. (b)
J ia, L. M.S. Thesis, Southern Methodist University, Dallas, Texas,
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A.; Hosmane, N. S. Organometallics 1993, 12, 3933.

1428 Organometallics, Vol. 17, No. 7, 1998
Ta ble 1. NMR Sp ectr a l Da ta of Ma gn esa ca r bor a n es I-VIIa
Hosmane et al.
11B NMR (64.21 MHz, external BF3‚OEt2)
1
1
I
18.58 (br, 2B, basal BH, J (11B-1H) ) unresolved), -0.28 (br, 1B, basal BH, J (11B-1H) ) unresolved), -49.77 (d, 1B, apical BH,
1J (11B-1H) ) 144.5 Hz)
1
1
II
III
4.50 (br, 2B, basal BH, J (11B-1H) ) unresolved), -5.70 (br, 1B, basal BH, J (11B-1H) ) unresolved), -50.11 (d, 1B, apical BH,
1J (11B-1H) ) 158.2 Hz)
1
1
16.0 (br, 1B, basal BH, J (11B-1H) ) unresolved), 7.96 (br, 2B, basal BH, J (11B-1H) ) unresolved), -49.74 (d, 1B, apical BH,
1J (11B-1H) ) 161.2 Hz)
1
1
IVb 21.28 (br, 1B, basal BH, J (11B-1H) ) unresolved), 17.88 (br, 1B, basal BH, J (11B-1H) ) unresolved), 6.74 (br, 1B, basal BH,
1
1J (11B-1H) ) unresolved), -48.34 (br, 1B, apical BH, J (11B-1H) ) 121.1 Hz)
1
1
Vb
VI
16.66 (br, 2B, basal BH, J (11B-1H) ) unresolved), 1.30 (br, 1B, basal BH, J (11B-1H) ) unresolved), -53.73 (d, 1B, apical BH,
1J (11B-1H) ) 139 Hz)
1
1
16.70 (br, 1B, basal BH, J (11B-1H) ) unresolved), 4.16 (br, 2B, basal BH, J (11B-1H) ) unresolved), -55.46 (d, 1B, apical BH,
1J (11B-1H) ) 160.2 Hz)
1
1
VII 16.00 (br, 1B, basal BH, J (11B-1H) ) unresolved), 4.52 (br, 2B, basal BH, J (11B-1H) ) unresolved), -54.76 (d, 1B, apical BH,
1J (11B-1H) ) 152.4 Hz)
1H NMR (200.13 MHz, external Me4Si)
I
2.19 (s, 12H, Me, TMEDA), 1.67 (s, 4H, CH2, TMEDA), 0.65 (s, 18H, SiMe3)
II
III
2.49 (s, 6H, Me), 1.86 (s, 12H, Me, TMEDA), 1.60 (s, 4H, CH2, TMEDA), 0.51 (s, 18H, SiMe3), -3.9 (vbr, bridge H)
2.16 (s, 12H, Me, TMEDA), 1.85 (s, 4H, CH2, TMEDA), 0.39 (s, 18H, SiMe3)
IVb 3.62 (m, 16H, CH2, THF), 1.77 (m, 16H, CH2, THF), 0.20 (s, 36H, SiMe3)
V
VI
2.20 (s, 24H, Me, TMEDA), 2.03 (s, 8H, CH2, TMEDA), 0.70 (s, 36H, SiMe3)
2.13 (s, 48H, Me, TMEDA), 1.79 (s, 16H, CH2, TMEDA), 0.59 (s, 36H, SiMe3)
VII 3.64 (m, 16 H, CH2, THF), 1.48 (m, 16H, CH2, THF), 0.62 (s, 36H, SiMe3)
13C NMR (50.32 MHz, external Me4Si)
1
I
101.33 (s(br), cage carbons (SiCB)), 56.00 (t, CH2, TMEDA, J (13C-1H) ) 137.0 Hz), 47.89 (q, CH3, TMEDA,
1J (13C-1H) ) 137.5 Hz), 4.13 (q, SiMe3, J (13C-1H) ) 116 Hz)
1
1
II
III
135.26 (s(br), cage carbon, (SiCB)), 112.03 (s(br), cage carbon, (BCMe)), 55.86 (t, CH2, TMEDA, J (13C-1H) ) 137.5 Hz), 47.02
1
1
1
(q, CH3, TMEDA, J (13C-1H) ) 135.4 Hz), 23.50 (q, Me, J (13C-1H) ) 124.6 Hz), 1.56 (q, SiMe3, J (13C-1H) ) 118.2 Hz)
87.97 (s(br), cage carbons (SiCB)), 56.08 (t, CH2, TMEDA, J (13C-1H) ) 135.4 Hz), 46.69 (q, CH3, TMEDA, J (13C-1H) ) 135.4
1
1
Hz), 2.17 (q, SiMe3, J (13C-1H) ) 118.2 Hz)
1
IVb 103.20 (s(br), cage carbons (SiCB)), 68.24 (t, CH2, THF, J (13C-1H) ) 145.0 Hz), 26.37 (t, CH2, THF, J (13C-1H) ) 132.1 Hz),
1
1
1
3.91 (q, SiMe3, J (13C-1H) ) 118.2 Hz)
1
Vb
VI
102.57 (s(br), cage carbons (SiCB)), 57.96 (t, CH2, TMEDA, J (13C-1H) ) 133.2 Hz), 45.62 (q, CH3, TMEDA,
1J (13C-1H) ) 133.2 Hz), 3.28 (q, SiMe3, J (13C-1H) ) 118.2 Hz)
1
93.21 (s(br), cage carbons (SiCB)), 56.37 (t, CH2, TMEDA, J (13C-1H) ) 133.2 Hz), 46.21 (q, CH3, TMEDA,
1
1J (13C-1H) ) 135.4 Hz), 2.37 (q, SiMe3, J (13C-1H) ) 118.2 Hz)
1
VII 93.30 (s(br), cage carbons (SiCB)), 68.88 (t, CH2, THF, J (13C-1H) ) 148.24 Hz), 25.42 (t, CH2, THF, J (13C-1H) ) 133.2 Hz),
1
1
1
2.41 (q, SiMe3, J (13C-1H) ) 116.02 Hz)
7Li NMR (77.7 MHz, external LiNO3)
VII 1.17 (s, br)
a
b
C6D6 was used as the solvent except where indicated. THF-d8 was used as the solvent.
Ta ble 2. IR Sp ectr a l Da ta of Ma gn esa ca r bor a n es I-VIIa
I
3050.3 (w), 2950.7 (s), 2900.8 (s), 2851.0 (s), 2807.4 (m) [(CH)], 2533.2 (s), 2470.9 (s) [(BH)], 1474.0 (s), 1299.6 (m),
1243.5 (s), 1168.7 (m), 1131.3 (m), 1031.6 (m), 950.6 (m), 838.5 (vs), 763.7 (m), 695.2 (m), 640.0 (m)
II
2958.8 (s), 2922.2 (s), 2859.4 (s) [(CH)], 2561.3 (s), 2503.7 (s), 2446.2 (s) [(BH)], 1912.7 (m), 1865.6 (m) [(B-H-B),
bridge], 1447.6 (s), 1332.0 (s), 1253.6 (s), 1175.1 (m), 1107.1 (m), 877.0 (s), 840.3 (s), 819.4 (s), 756 (m)
2967.3 (s), 2891.6 (s), 2795.2 (s) [(CH)], 2547.4 (s), 2492.4 (s), 2451.1 (s) [(BH)], 1460.1 (s), 1363.7 (s), 1301.8 (s),
1239.8 (s), 1177.9 (m), 1129.7 (m), 1074.7 (s), 957.7 (s), 847.5 (vs), 799.4 (s), 634.2 (m), 592.9 (m)
2795.5 (m) [(CH)], 2522.6 (m), 2484.3 (s), 2433.4 (s), 2393.8 (m) [(BH)], 1466.0 (m), 1245.3 (s), 1188.7 (s), 1137.8 (m),
838.0 (vs), 758.8 (s), 685.2 (s), 634.3 (s)
2762.3 (s) [(CH)], 2481.0 (s) 2439.3 (s), 2397.6 (s) [(BH)], 1465.2 (s), 1251.7 (s), 1184.0 (s), 996.5 (m), 840.2 (vs), 762.1 (s),
683.9 (s), 637.0 (s)
2963.6 (s), 2876.9 (s), 2836.8 (s), 2790.2 (s) [(CH)], 2623.4 (s), 2583.4 (s), 2563.3 (s) [(BH)], 1456.0 (s), 1362.6 (m),
1295.9 (s), 1255.8 (s), 1162.4 (m), 1102.4 (m), 1069.0 (m), 955.6 (s), 848.9 (s), 795.5 (s), 588.7 (m), 555.4 (m)
2955.5 (s), 2888.9 (s) [(CH)], 2532.0 (s), 2495.7 (s), 2405.0 (s) [(BH)], 1461.2 (m), 1406.8 (m), 1370.5 (m), 1303.9 (m),
1243.4 (s), 1189.0 (s), 1049.8 (s), 947.0 (m), 886.5 (s), 844.1 (vs), 765.5 (s), 686.8 (s), 632.4 (s)
III
IVb
Vb
VI
VII
a
C6H6 vs C6H6 except where indicated. Legend: v ) very, s ) strong or sharp, m ) medium, w ) weak, sh ) shoulder, and br ) broad.
THF vs THF.
b
from the filtrate resulted in the formation of a pale yellow solid,
which was recrystallized from a benzene solution as colorless
crystals and later identified as commo-exo-4,4′,5,5′-Mg(T-
MEDA)[2-(SiMe3)-3-(Me)-2,3-C2B4H5]2 (II) (0.65 g, 1.40 mmol,
94% yield; mp > 300 °C; soluble in both polar and nonpolar
organic solvents). Anal. Calcd for C18H50B8N2Si2Mg (II): C,
46.84; H, 10.92; N, 6.07. Found: C, 47.85; H, 10.94; N, 6.36.
The NMR and IR spectral data of II are listed in Tables 1 and
2, respectively.
Syn th esis of closo-1-Mg(TMEDA)-2,4-(SiMe3)2-2,4-C2B4H4
(III). A 2.51 g (5.44 mmol) amount of closo-exo-5,6-[(µ-H)2Li-
(TMEDA)]-1-Li(TMEDA)-2,4-(SiMe3)2-2,4-C2B4H4 was reacted
with 1.0 g (5.44 mmol) of MgBr2 in 30 mL of benzene at room
temperature with constant stirring for 24 h. The reaction
mixture was then filtered to collect a pale yellow filtrate. The
white solid on the frit, identified as LiBr (not measured), was
discarded. Removal of the solvent from the filtrate resulted
in the formation of an off-white solid, which was washed with
n-hexane and later identified as closo-1-Mg(TMEDA)-2,4-
(SiMe3)2-2,4-C2B4H4 (III) (1.43 g, 3.99 mmol, 73% yield; mp >
250 °C (dec); soluble in both polar and nonpolar organic
solvents). Compound III was recrystallized from its benzene
solution to yield colorless crystals. Anal. Calcd for C14H38B4N2-
Si2Mg (III): C, 46.95; H, 10.69; N, 7.81. Found: C, 47.12; H,
10.63; N, 8.01. The NMR and IR spectral data of III are listed
in Tables 1 and 2, respectively.

C-Trimethylsilyl-Substituted Heterocarboranes
Organometallics, Vol. 17, No. 7, 1998 1429
Ta ble 3. Cr ysta llogr a p h ic Da ta a for Ma gn esa ca r bor a n es I, II, a n d IV
I
II
IV
formula
fw
C
794.5
34H82N4B8Si4Mg2
C18H50N2B8Si2Mg
461.6
C32H76B8O4Na2Si4Mg
794.1
space group
a, Å
b, Å
P1h
C2/c
C2/c
13.609(2)
14.391(2)
15.021(2)
89.440(10)
63.490(10)
81.280(10)
2596.4(7)
2
12.403(5)
18.807(7)
13.496(5)
90
95.16(3)
90
3136(2)
4
0.978
21.997(6)
13.417(4)
17.386(5)
90
91.28(2)
90
5137(4)
4
1.027
c, Å
R, deg
â, deg
γ, deg
V, Å3
Z
D
calcd, g cm-3
1.016
0.165
abs coeff, mm-1
cryst dimens, mm
scan type
0.142
0.174
0.40 × 0.30 × 0.15
0.25 × 0.35 × 0.20
0.10 × 0.35 × 0.30
ω/2θ
ω/2θ
ω/2θ
scan speed in ω; min, max
2θ range, deg
T, K
5.33, 29.30
3.0-42.0
220
5.33, 29.30
3.0-44.0
228
5.33, 29.30
3.0-42.0
230
decay, %
0
0
0
no. of data collected
no. of obsd reflctns, F>6.0σ(F)
no. of params refined
GOF
5872
4419
494
1.50
0.32, -0.23
0.0387
0.0574
1113
1113
141
1.59
0.38, -0.25
0.0585
0.0751
2862
1139
202
2.50
0.62, -0.36
0.0948
0.1193
∆F(max, min), e/ Å3
Rb
wRc,d
a
b
Graphite-monochromated Mo KR radiation, λ ) 0.710 73 Å. R ) ∑||Fo| - |Fc||/∑|Fo|, wR ) [∑w(Fo - Fc)2/∑w(Fo)2]1/2
.
c w ) 1/[σ2(Fo)
+ 0.001(Fo)2]. w ) 1/[σ2(Fo2) + (0.0410P)2 + 14.4051P], where P ) (Fo + 2 Fc2)/3.
d
2
Syn th esis of [Na (THF )2]2[com m o-1,1′-Mg{2,3-(SiMe3)2-
2,3-C2B4H4}2] (IV). To a solution of (n-C4H9)2Mg (3.2 mmol)
in 3.2 mL of heptane, a benzene solution (25 mL) of nido-1-
Na(THF)2-2,3-(SiMe3)2-2,3-C2B4H5 (2.03 g, 6.45 mmol) was
slowly added in vacuo at 0 °C. The mixture was then warmed
to room temperature and stirred overnight, during which time
slow evolution of a gas and the concomitant formation of a
white precipitate were observed. All of the solvents were then
removed from the mixture to give a pale yellow solid, which
was washed with anhydrous benzene to yield [Na(THF)2]2-
[commo-1,1′-Mg{2,3-(SiMe3)2-2,3-C2B4H4}2] (IV) as a white
solid (2.08 g, 2.64 mmol, 82% yield; mp 320 °C(dec); soluble in
THF and slightly soluble in nonpolar solvents such as ben-
zene). Compound IV was recrystallized from a THF solution
to give colorless crystals. Anal. Calcd for C32H76B8O4Si4Na2-
Mg (IV): C, 48.48; H, 9.59. Found: C, 48.28; H, 9.50. The
NMR and IR spectral data of IV are listed in Tables 1 and 2,
respectively.
Syn th esis of [Na(TMEDA)]2[commo-1,1′-Mg{2,3-(SiMe3)2-
2,3-C2B4H4}2] (V). In a procedure identical to that described
above for IV, the reaction between 3.2 mmol of (n-C4H9)2Mg
in pentane and 6.77 mmol (2.42 g) of the TMEDA-solvated
monosodium compound gave a white solid, identified as [Na-
(TMEDA)]2[commo-1,1′-Mg{2,3-(SiMe3)2-2,3-C2B4H4}2] (V), in
84% yield (2.09 g, 2.84 mmol; mp >250 °C; soluble in polar
organic solvents and slightly soluble in nonpolar organic
solvents). Compound V was later recrystallized from a THF
solution to give colorless crystals. Anal. Calcd for C28H76B8N4-
Si4Na2Mg (V): C, 47.64; H, 10.85; N, 7.93. Found: C, 47.94;
H, 10.79; N, 8.03. The NMR and IR spectral data of V are
listed in Tables 1 and 2, respectively.
n-hexane to collect a white solid. This solid was later identified
as [Li(TMEDA)2]2[commo-1,1′-Mg{2,4-(SiMe3)2-2,4-C2B4H4}2]
(VI) (1.74 g, 1.86 mmol, 85% yield; mp > 250 °C; soluble in
polar solvents and partially soluble in nonpolar solvents).
Compound VI was recrystallized from a benzene solution to
give colorless crystals. Anal. Calcd for C40H108B8N8Si4Li2Mg
(VI): C, 51.20; H, 11.52. Found: C, 52.94; H, 10.69. The NMR
and IR spectral data of VI are listed in Tables 1 and 2,
respectively.
Attem p ted Syn th esis of th e Mixed -Liga n d Ma gn esa -
ca r bor a n e Sa n d w ich [LiNa (THF )4]-[com m o-1,1′-Mg{2,4-
(SiMe3)2-2,4-C2B4H4}{2,3-(SiMe3)2-2,3-C2B4H4}], a n d th e
Syn th esis of [Li(THF )2]2[com m o-1,1′-Mg{2,4-(SiMe3)2-2,4-
C2B4H4}2] (VII). A solution of 4.46 mmol of closo-exo-5,6-[(µ-
H)2Li(THF)2]-1-Li(THF)2-2,4-(SiMe3)2-2,4-C2B4H4 in 25 mL of
benzene was added to a solution of 4.46 mmol of CH3MgBr in
diethyl ether at -78 °C, and the mixture was then warmed to
room temperature and stirred for 4 h. To the resulting
mixture, a solution of 1.59 g (4.46 mmol) of nido-1-Na(THF)-
2,3-(SiMe3)2-2,3-C2B4H5 was added at 0 °C, and the mixture
was then stirred overnight. Filtration of the reaction mixture
gave a brown solution, from which all solvents were removed
to generate a yellow solid. The solid was washed with
n-hexane and was then partially dissolved in 15 mL of
benzene. The resulting mixture was filtered in vacuo to give
a white solid and a brown solution. The white solid was later
spectroscopically identified as [Na(THF)2]2[commo-1,1′-Mg{2,3-
(SiMe3)2-2,3-C2B4H4}2] (IV) (1.39 g, 1.76 mmol, 79% yield).
From the brown solution, colorless crystals were obtained and
later identified as [Li(THF)2]2[commo-1,1′-Mg{2,4-(SiMe3)2-2,4-
C2B4H4}2] (VII) (1.50 g, 1.97 mmol, 88%; mp > 250 °C; soluble
in both polar and nonpolar organic solvents). Anal. Calcd for
Syn th esis of [Li(TMEDA)2]2[commo-1,1′-Mg{2,4-(SiMe3)2-
2,4-C2B4H4}2] (VI).To a suspension of MgBr2 (0.40 g, 2.17
mmol) in 10 mL of benzene, a solution of 2.02 g (4.36 mmol)
of closo-exo-5,6-[(µ-H)2Li(TMEDA)]-1-Li(TMEDA)-2,4-(SiMe3)2-
2,4-C2B4H4 in 20 mL of benzene was added at 0 °C. The
resulting mixture was warmed to room temperature and
stirred overnight, during which time a white precipitate
formed. The mixture was filtered in vacuo to collect the pale
yellow filtrate. The white solid on the frit, identified as LiBr
(not measured), was discarded. Removal of the solvent from
the filtrate gave an off-white solid, which was washed with
C
32H76B8O4Si4Li2Mg (VII): C, 50.42; H, 10.05. Found: C,
49.75; H, 10.26. The NMR and IR spectral data of VII are
listed in Tables 1 and 2, respectively.
Cr ysta l Str u ctu r e An a lysis of closo-1-Mg(TMEDA)-2,3-
(SiMe3)2-2,3-C2B4H4 (I), com m o-exo-4,4′,5,5′-Mg(TMEDA)2-
[2-(SiMe3)-3-(Me)-2,3-C2B4H 5]2 (II), a n d [Na (TH F )2]2-
[com m o-1,1′-Mg{2,3-(SiMe3)2-2,3-C2B4H4}2] (IV). Colorless,
transparent, X-ray quality crystals of the magnesacarboranes
I, II, and IV were grown very slowly from their respective
benzene and THF solutions. The crystals were transferred to

1430 Organometallics, Vol. 17, No. 7, 1998
Hosmane et al.
Ta ble 4. Selected Bon d Len gth s (Å) a n d Bon d An gles (d eg) for Ma gn esa ca r bor a n es I, II, a n d IV
Bond Lengths (Å)
Compound I
Mg(1)-Mg(2)
Mg(1)-C(11)
Mg(1)-C(12)
Mg(1)-B(13)
Mg(1)-B(14)
Mg(1)-B(15)
Mg(1)-B(24)
Mg(1)-N(51)
Mg(1)-N(52)
Mg(2)-B(14)
Mg(2)-C(21)
Mg(2)-C(22)
Mg(2)-B(23)
3.426 (2)
2.649 (3)
2.635 (3)
2.493 (3)
2.393 (3)
2.472 (4)
2.540 (5)
2.327 (2)
2.252 (2)
2.534 (5)
2.621 (4)
2.596 (3)
2.485 (3)
Mg(2)-B(24)
Mg(2)-B(25)
Mg(2)-N(61)
Mg(2)-N(62)
C(11)-C(12)
C(11)-B(15)
C(11)-B(16)
C(12)-B(13)
C(12)-B(16)
B(13)-B(14)
B(13)-B(16)
B(14)-B(15)
B(14)-B(16)
2.404 (3)
2.471 (3)
2.325 (3)
2.256 (2)
1.521 (4)
1.565 (5)
1.699 (4)
1.566 (6)
1.714 (4)
1.645 (5)
1.786 (6)
1.648 (6)
1.778 (6)
B(15)-B(16)
C(21)-C(22)
C(21)-B(25)
C(21)-B(26)
C(22)-B(23)
C(22)-B(26)
B(23)-B(24)
B(23)-B(26)
B(24)-B(25)
B(24)-B(26)
B(25)-B(26)
1.781 (4)
1.507 (4)
1.573 (5)
1.692 (4)
1.564 (5)
1.708 (3)
1.661 (4)
1.782 (5)
1.635 (6)
1.761 (6)
1.771 (5)
Compound II
Mg-B(13)
2.383 (8)
2.480 (7)
2.183 (5)
2.383 (8)
2.480 (7)
2.183 (5)
1.458 (8)
C(11)-B(15)
C(11)-B(16)
C(12)-B(13)
C(12)-B(16)
C(12)-C(24)
B(13)-B(14)
B(13)-B(16)
1.525 (9)
1.723 (9)
1.502 (9)
1.753 (9)
1.515 (8)
1.663 (10)
1.787 (10)
B(14)-B(15)
B(14)-B(16)
B(15)-B(16)
N(25)-C(26)
N(25)-C(27)
N(25)-C(28)
C(26)-C(26A)
1.767 (10)
1.740 (10)
1.762 (10)
1.476 (7)
1.491 (8)
1.475 (8)
1.600 (11)
Mg-B(14)
Mg-N(25)
Mg-B(13A)
Mg-B(14A)
Mg-N(25A)
C(11)-C(12)
Compound IV
Mg-C(11)
Mg-C(12)
Mg-B(13)
Mg-B(14)
Mg-B(15)
Mg-C(11A)
Mg-C(12A)
Mg-B(13A)
Mg-B(14B)
Mg-B(15B)
Na-B(13)
2.446 (13)
2.461 (14)
2.420 (16)
2.398 (16)
2.405 (18)
2.446 (13)
2.461 (14)
2.420 (16)
2.398 (16)
2.405 (18)
2.976 (17)
Na-B(14)
Na-B(16)
Na-O(31)
Na-O(32)
Na-O(41)
Na-C(42)
Na-Na(A)
Na-B(14A)
Na-B(15A)
C(11)-C(12)
C(11)-B(15)
2.867 (17)
3.055 (18)
2.234 (23)
2.409 (24)
2.308 (14)
3.117 (30)
3.801 (11)
2.883 (19)
2.831 (19)
1.509 (21)
1.572 (23)
C(12)-B(13)
C(11)-B(16)
C(12)-B(16)
B(13)-B(14)
B(13)-B(16)
B(14)-B(15)
B(14)-B(16)
B(14)-Na(A)
B(15)-B(16)
B(15)-Na(A)
1.552 (22)
1.723 (22)
1.676 (23)
1.694 (25)
1.749 (25)
1.593 (24)
1.779 (24)
2.883 (19)
1.776 (26)
2.831 (19)
Bond Angles (deg)
Compound I
Mg(2)-Mg(1)-C(11)
107.5(1)
93.8(1)
33.5(1)
58.6(1)
59.3(1)
35.4(1)
47.7(1)
61.0(1)
61.1(1)
39.3(1)
85.1(1)
99.5(1)
131.6(1)
151.3(1)
115.3(1)
87.4(1)
97.9(1)
137.6(1)
105.6(1)
99.5(1)
122.2(1)
160.2(1)
136.0(1)
98.9(1)
79.9(1)
44.3(1)
107.1(1)
151.1(1)
92.4(1)
126.7(1)
33.6(1)
56.8(1)
93.7(1)
59.7(1)
35.8(1)
47.8(1)
91.5(1)
C(11)-Mg(1)-B(15)
35.4(1)
Mg(2)-Mg(1)-C(12)
C(11)-Mg(1)-C(12)
Mg(2)-Mg(1)-B(13)
C(11)-Mg(1)-B(13)
C(12)-Mg(1)-B(13)
Mg(2)-Mg(1)-B(14)
C(11)-Mg(1)-B(14)
C(12)-Mg(1)-B(14)
B(13)-Mg(1)-B(14)
Mg(2)-Mg(1)-B(15)
C(11)-Mg(1)-N(51)
C(12)-Mg(1)-N(51)
B(13)-Mg(1)-N(51)
B(14)-Mg(1)-N(51)
B(15)-Mg(1)-N(51)
B(24)-Mg(1)-N(51)
Mg(2)-Mg(1)-N(52)
C(11)-Mg(1)-N(52)
C(12)-Mg(1)-N(52)
B(13)-Mg(1)-N(52)
B(14)-Mg(1)-N(52)
B(15)-Mg(1)-N(52)
B(24)-Mg(1)-N(52)
N(51)-Mg(1)-N(52)
Mg(1)-Mg(2)-B(14)
Mg(1)-Mg(2)-C(21)
B(14)-Mg(2)-C(21)
Mg(1)-Mg(2)-C(22)
B(14)-Mg(2)-C(22)
C(21)-Mg(2)-C(22)
Mg(1)-Mg(2)-B(23)
B(14)-Mg(2)-B(23)
C(21)-Mg(2)-B(23)
C(22)-Mg(2)-B(23)
Mg(1)-Mg(2)-B(24)
B(14)-Mg(2)-B(24)
C(12)-Mg(1)-B(15)
B(13)-Mg(1)-B(15)
B(14)-Mg(1)-B(15)
Mg(2)-Mg(1)-B(24)
C(11)-Mg(1)-B(24)
C(12)-Mg(1)-B(24)
B(13)-Mg(1)-B(24)
B(14)-Mg(1)-B(24)
B(15)-Mg(1)-B(24)
Mg(2)-Mg(1)-N(51)
C(22)-Mg(2)-B(24)
B(23)-Mg(2)-B(24)
Mg(1)-Mg(2)-B(25)
B(14)-Mg(2)-B(25)
C(21)-Mg(2)-B(25)
C(22)-Mg(2)-B(25)
B(23)-Mg(2)-B(25)
B(24)-Mg(2)-B(25)
Mg(1)-Mg(2)-N(61)
B(14)-Mg(2)-N(61)
C(21)-Mg(2)-N(61)
C(22)-Mg(2)-N(61)
B(23)-Mg(2)-N(61)
B(24)-Mg(2)-N(61)
B(25)-Mg(2)-N(61)
Mg(1)-Mg(2)-N(62)
B(14)-Mg(2)-N(62)
C(21)-Mg(2)-N(62)
C(22)-Mg(2)-N(62)
B(23)-Mg(2)-N(62)
B(24)-Mg(2)-N(62)
B(25)-Mg(2)-N(62)
N(61)-Mg(2)-N(62)
Mg(1)-C(11)-C(12)
Mg(1)-C(11)-B(15)
C(12)-C(11)-B(15)
59.5(1)
64.1(1)
39.6(1)
44.5(1)
152.0(1)
129.3(1)
96.2(1)
91.6(1)
124.6(1)
119.2(1)
61.4(1)
39.7(1)
85.3(1)
125.3(1)
35.8(1)
59.9(1)
64.4(1)
39.2(1)
119.1(1)
98.9(1)
100.7(1)
133.3(1)
151.4(1)
114.0(1)
87.4(1)
138.0(1)
99.4(1)
104.5(1)
99.1(1)
122.7(1)
160.4(1)
135.1(1)
80.4(1)
72.8(2)
66.1(2)
110.5(3)

C-Trimethylsilyl-Substituted Heterocarboranes
Organometallics, Vol. 17, No. 7, 1998 1431
Ta ble 4 (Con tin u ed )
Compound I
C(21)-Mg(2)-B(24)
C(12)-C(11)-B(16)
B(15)-C(11)-B(16)
Mg(1)-C(12)-C(11)
Mg(1)-C(12)-B(13)
C(11)-C(12)-B(13)
Mg(1)-C(12)-B(16)
C(11)-C(12)-B(16)
B(13)-C(12)-B(16)
Mg(1)-B(13)-C(12)
Mg(1)-B(13)-B(14)
C(12)-B(13)-B(14)
Mg(1)-B(13)-B(16)
C(12)-B(13)-B(16)
B(14)-B(13)-B(16)
Mg(1)-B(14)-Mg(2)
Mg(1)-B(14)-B(13)
Mg(2)-B(14)-B(13)
Mg(1)-B(14)-B(15)
Mg(2)-B(14)-B(15)
B(13)-B(14)-B(15)
Mg(1)-B(14)-B(16)
Mg(2)-B(14)-B(16)
B(13)-B(14)-B(16)
B(15)-B(14)-B(16)
Mg(1)-B(15)-C(11)
Mg(1)-B(15)-B(14)
C(11)-B(15)-B(14)
C(22)-B(23)-B(24)
Mg(2)-B(23)-B(26)
C(22)-B(23)-B(26)
B(24)-B(23)-B(26)
Mg(1)-B(24)-Mg(2)
Mg(1)-B(24)-B(23)
Mg(2)-B(24)-B(23)
Mg(1)-B(24)-B(25)
Mg(2)-B(24)-B(25)
B(23)-B(24)-B(25)
Mg(1)-B(24)-B(26)
Mg(2)-B(24)-B(26)
B(23)-B(24)-B(26)
B(25)-B(24)-B(26)
Mg(2)-B(25)-C(21)
Mg(2)-B(25)-B(24)
C(21)-B(25)-B(24)
Mg(2)-B(25)-B(26)
C(21)-B(25)-B(26)
B(24)-B(25)-B(26)
C(21)-B(26)-C(22)
61.1(1)
Mg(1)-C(11)-B(16)
94.1(2)
98.3(2)
60.6(2)
62.3(2)
52.9(2)
93.8(2)
53.1(2)
95.3(2)
94.5(2)
55.0(2)
53.4(2)
93.1(2)
95.2(2)
55.2(2)
72.3(2)
66.9(2)
110.5(3)
94.2(2)
64.2(2)
65.6(2)
74.1(2)
68.2(2)
111.8(2)
94.7(2)
63.2(2)
65.8(2)
76.0(2)
67.5(2)
94.1(2)
53.2(2)
95.8(2)
95.1(2)
55.9(2)
53.9(2)
93.3(2)
96.0(2)
55.2(2)
103.6(2)
114.1(2)
112.7(2)
105.9(2)
112.1(2)
114.2(2)
102.9(2)
115.8(2)
110.7(2)
105.4(2)
113.3(2)
113.0(2)
64.0(2)
66.0(2)
73.8(1)
67.3(1)
111.1(2)
94.3(2)
63.0(2)
65.8(2)
77.3(1)
67.1(1)
105.9(3)
97.4(2)
61.1(2)
62.3(2)
88.1(1)
73.6(2)
89.4(2)
72.8(2)
150.2(2)
106.2(3)
101.3(2)
145.9(2)
62.7(2)
62.5(2)
78.5(2)
67.6(2)
106.2(3)
105.0(3)
96.7(2)
61.0(2)
61.4(2)
87.7(1)
85.7(2)
72.8(1)
151.9(2)
72.6(2)
106.5(3)
142.7(2)
100.2(2)
62.7(2)
62.8(2)
77.3(2)
68.2(2)
106.0(2)
97.5(2)
60.5(2)
62.1(2)
52.6(2)
Mg(1)-B(15)-B(16)
C(11)-B(15)-B(16)
B(14)-B(15)-B(16)
C(11)-B(16)-C(12)
C(11)-B(16)-B(13)
C(12)-B(16)-B(13)
C(11)-B(16)-B(14)
C(12)-B(16)-B(14)
B(13)-B(16)-B(14)
C(11)-B(16)-B(15)
C(12)-B(16)-B(15)
B(13)-B(16)-B(15)
B(14)-B(16)-B(15)
Mg(2)-C(21)-C(22)
Mg(2)-C(21)-B(25)
C(22)-C(21)-B(25)
Mg(2)-C(21)-B(26)
C(22)-C(21)-B(26)
B(25)-C(21)-B(26)
Mg(2)-C(22)-C(21)
Mg(2)-C(22)-B(23)
C(21)-C(22)-B(23)
Mg(2)-C(22)-B(26)
C(21)-C(22)-B(26)
B(23)-C(22)-B(26)
Mg(2)-B(23)-C(22)
Mg(2)-B(23)-B(24)
C(21)-B(26)-B(23)
C(22)-B(26)-B(23)
C(21)-B(26)-B(24)
C(22)-B(26)-B(24)
B(23)-B(26)-B(24)
C(21)-B(26)-B(25)
C(22)-B(26)-B(25)
B(23)-B(26)-B(25)
B(24)-B(26)-B(25)
Mg(1)-N(51)-C(53)
Mg(1)-N(51)-C(55)
Mg(1)-N(51)-C(56)
Mg(1)-N(52)-C(54)
Mg(1)-N(52)-C(57)
Mg(1)-N(52)-C(58)
Mg(2)-N(61)-C(63)
Mg(2)-N(61)-C(65)
Mg(2)-N(61)-C(66)
Mg(2)-N(62)-C(64)
Mg(2)-N(62)-C(67)
Mg(2)-N(62)-C(68)
Compound II
B(14)-B(13)-B(16)
B(13)-Mg-B(14)
59.3(13)
39.9(2)
B(13)-Mg-B(13A)
B(14)-Mg-B(13A)
N(25)-Mg-B(13A)
B(13)-Mg-B(14A)
Mg-B(14)-B(13)
Mg-B(14)-B(15)
B(13)-B(14)-B(15)
Mg-B(14)-B(16)
B(13)-B(14)-B(16)
B(15)-B(14)-B(16)
C(11)-B(15)-B(14)
C(11)-B(15)-B(16)
B(14)-B(15)-B(16)
C(11)-B(16)-C(12)
C(11)-B(16)-B(13)
C(12)-B(16)-B(13)
C(11)-B(16)-B(14)
C(12)-B(16)-B(14)
B(13)-B(16)-B(14)
C(11)-B(16)-B(15)
C(12)-B(16)-B(15)
B(13)-B(16)-B(15)
B(14)-B(16)-B(15)
Mg-N(25)-C(26)
Mg-N(25)-C(27)
Mg-N(25)-C(28)
90.9(4)
108.0(2)
141.3(2)
108.0(2)
66.9(3)
135.6(4)
102.3(5)
130.2(4)
63.3(4)
60.3(4)
103.9(5)
62.8(4)
59.1(4)
49.6(3)
90.5(4)
50.2(4)
97.2(5)
93.1(5)
56.2(4)
51.9(3)
89.7(4)
97.6(5)
60.6(4)
105.1(3)
114.9(4)
111.4(3)
B(13)-Mg-N(25)
B(14)-Mg-N(25)
B(14)-Mg-B(14A)
N(25)-Mg-B(14A)
B(13A)-Mg-B(14A)
B(13)-Mg-N(25A)
B(14)-Mg-N(25A)
N(25)-Mg-N(25A)
B(13A)-Mg-N(25A)
B(14A)-Mg-N(25A)
C(12)-C(11)-B(15)
C(12)-C(11)-B(16)
B(15)-C(11)-B(16)
C(11)-C(12)-B(13)
C(11)-C(12)-B(16)
B(13)-C(12)-B(16)
C(11)-C(12)-C(24)
B(13)-C(12)-C(24)
B(16)-C(12)-C(24)
Mg-B(13)-C(12)
105.1(2)
106.7(2)
141.9(3)
101.4(2)
39.9(2)
141.3(2)
101.4(2)
84.3(3)
105.1(2)
106.7(2)
112.4(5)
66.3(4)
65.4(4)
114.8(5)
64.1(4)
66.1(4)
121.6(5)
122.3(5)
130.7(5)
138.2(5)
73.2(4)
106.4(5)
133.6(4)
63.7(4)
Mg-B(13)-B(14)
C(12)-B(13)-B(14)
Mg-B(13)-B(16)
C(12)-B(13)-B(16)
B(14)-B(13)-B(16)
60.4(4)

1432 Organometallics, Vol. 17, No. 7, 1998
Hosmane et al.
Ta ble 4 (Con tin u ed )
Compound IV
C(11)-Mg-C(12)
C(11)-Mg-B(13)
C(12)-Mg-B(14)
B(13)-Mg-B(14)
C(11)-Mg-B(15)
C(12)-Mg-B(15)
B(13)-Mg-B(15)
B(14)-Mg-B(15)
B(13)-Na-B(14)
B(13)-Na-B(16)
B(14)-Na-B(16)
B(13)-Na-O(31)
B(14)-Na-O(31)
B(16)-Na-O(31)
B(13)-Na-O(312)
B(14)-Na-O(312)
B(16)-Na-O(312)
O(31)-Na-O(312)
B(13)-Na-O(41)
B(14)-Na-O(41)
B(16)-Na-O(41)
O(31)-Na-O(41)
O(312)-Na-O(41)
B(13)-Na-C(42)
B(14)-Na-C(42)
B(16)-Na-C(42)
O(31)-Na-C(42)
O(312)-Na-C(42)
O(41)-Na-C(42)
B(13)-B(14)-B(15)
Mg-B(14)-B(16)
Na-B(14)-B(16)
B(13)-B(14)-B(16)
B(15)-B(14)-B(16)
Mg-B(15)-C(11)
Mg-B(15)-B(14)
C(11)-B(15)-B(14)
Mg-B(15)-B(16)
C(11)-B(15)-B(16)
B(14)-B(15)-B(16)
Na-B(16)-C(11)
Na-B(16)-C(12)
35.8(5)
C(12)-Mg-B(13)
C(11)-Mg-B(14)
Mg-C(11)-C(12)
Mg-C(11)-B(15)
C(12)-C(11)-B(15)
Mg-C(11)-B(16)
C(12)-C(11)-B(16)
B(15)-C(11)-B(16)
Mg-C(12)-C(11)
Mg-C(12)-B(13)
C(11)-C(12)-B(13)
Mg-C(12)-B(16)
C(11)-C(12)-B(16)
B(13)-C(12)-B(16)
Mg-B(13)-Na
Mg-B(13)-C(12)
Na-B(13)-C(12)
Mg-B(13)-B(14)
Na-B(13)-B(14)
C(12)-B(13)-B(14)
Mg-B(13)-B(16)
Na-B(13)-B(16)
C(12)-B(13)-B(16)
B(14)-B(13)-B(16)
Mg-B(14)-Na
Mg-B(14)-B(13)
Na-B(14)-B(13)
Mg-B(14)-B(15)
Na-B(14)-B(15)
C(11)-B(16)-C(12)
Na-B(16)-B(13)
C(11)-B(16)-B(13)
C(12)-B(16)-B(13)
Na-B(16)-B(14)
C(11)-B(16)-B(14)
C(12)-B(16)-B(14)
B(13)-B(16)-B(14)
Na-B(16)-B(15)
C(11)-B(16)-B(15)
C(12)-B(16)-B(15)
B(13)-B(16)-B(15)
B(14)-B(16)-B(15)
37.1(5)
63.9(5)
72.6(7)
69.7(8)
109.6(12)
95.3(8)
62.1(10)
65.0(10)
71.6(7)
70.0(8)
111.8(12)
96.1(9)
65.3(10)
65.5(10)
136.5(7)
72.9(8)
130.8(10)
68.7(8)
69.7(8)
105.0(12)
95.6(9)
62.8(5)
64.1(5)
41.2(6)
37.8(6)
62.3(5)
65.6(6)
38.7(6)
33.6(5)
33.7(5)
34.8(5)
91.1(7)
110.7(7)
123.8(7)
97.3(6)
108.0(6)
130.9(6)
17.1(8)
102.5(5)
129.2(5)
95.6(5)
85.6(7)
99.8(6)
108.7(7)
122.8(7)
88.6(7)
109.2(8)
121.5(8)
24.9(6)
105.3(13)
95.5(9)
78.4(8)
60.4(10)
63.3(10)
72.5(8)
70.4(9)
108.2(13)
95.3(10)
61.6(10)
63.5(10)
159.7(10)
120.2(10)
75.7(8)
60.7(10)
62.2(10)
144.5(8)
70.1(8)
76.7(8)
70.9(9)
131.9(10)
52.7(9)
70.7(8)
93.8(11)
53.8(9)
66.8(7)
94.1(11)
96.4(11)
57.4(10)
113.7(10)
53.4(9)
93.6(11)
95.7(11)
53.2(10)
a drybox, coated with mineral oil, and sequentially mounted
on a Siemens R3m/V diffractometer under a low-temperature
nitrogen stream. The pertinent crystallographic data and
conditions for data collection are summarized in Table 3. Final
unit cell parameters were obtained by a least-squares fit of
the angles of 24-30 accurately centered reflections in the 2θ
range from 16° to 32.0°. Intensity data were corrected for
Lorentz and polarization effects. The structures were solved
by direct methods using the SHELXTL-Plus package and
subsequent Fourier syntheses.17 Full-matrix least-squares
refinements were performed for all three structures. Scatter-
ing factors and anomalous-dispersion corrections for the heavy
atoms were taken from ref 18. All non-H atoms, except for
the C and O atoms of the disordered THF groups in IV, were
refined isotropically. Bonds in the disordered groups were
restrained during the final cycles of refinements. Methyl and
methylene H’s in the nondisordered groups were placed in
calculated positions. The cage H’s of I were isotropically
refined, while those of II and IV were located in DF maps but
were not refined. The final values of R and Rw are listed in
Table 3, while selected bond lengths and bond angles are given
in Table 4. The atomic coordinates, a full list of bond lengths
and angles, isotropic thermal parameters, and positions of the
H atoms are given in the Supporting Information.
atomic orbital)19 ab initio molecular orbital calculations at the
HF/6-311G** level of theory on the model compounds closo-
1-(en)-1,2,3-MgC2B4H6 (VIII), commo-exo-4,4′,5,5′-Mg(en)-[2,3-
C2B4H7]2 (IX), closo-1-(en)-1,2,4-MgC2B4H6 (X), {commo-1,1′-
[1,2,3-MgC2B4H6]2}- (XI), and {commo-1,1′-[1,2,4-MgC2B4H6]2}-
(XII) (en ) ethylenediamine, (NH2CH2)2). These compounds
differ from their I-VII analogues in that H’s replace the SiMe3
groups on the cage carbons and en is used in place of TMEDA
as the metal-solvating group. Such substitutions have been
successfully used in generating model systems for interpreting
the 11B NMR spectra of a number of trimethylsilyl-substituted
carboranes and metallacarboranes.20,21 The structures of the
model compounds were optimized by approximate density
function theory (DFT) using Becke’s three-parameter method22
and the correlation functional of Lee, Yang, and Parr23 with
the 6-31G* basis set (B3LYP/6-31G*).24 The GIAO-HF/6-
311G**//B3LYP/6-31G* calculations were carried out on a Dec-
RA or a Silicon Graphics Indigo2 Impact10000 workstation
using the Gausian 94 series of programs.25 The 11B chemical
shifts calculated for the model compounds, listed in Table 5,
(19) Wolinski, K.; Hilton, J . F.; Pulay, P. J . Am. Chem. Soc. 1990,
112, 8251.
(20) Ezhova, M. B.; Zhang, H.; Maguire, J . A.; Hosmane, N. S. J .
Organomet. Chem., in press.
(21) Hosmane, N. S.; Lu, K.-J .; Zhang, H.; Maguire, J . A. Organo-
metallics 1997, 16, 5163.
Ca lcu la tion s. The 11B NMR spectra of compounds I-VII
were analyzed with the aid of GIAO (gauge-independent
(17) Sheldrick, G. M. Structure Determination Software Programs;
Siemens X-ray Analytical Instrument Corp.: Madison, WI, 1991.
(18) International Tables For X-ray Crystallography; Kynoch
press: Birmingham, U.K., 1974; Vol. IV.
(22) Becke, A. D. J . Chem. Phys. 1993, 98, 5648.
(23) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785.
(24) For a discussion of DFT methods, see: Ziegler, T. Chem. Rev.
1991, 91, 651 and references therein.

C-Trimethylsilyl-Substituted Heterocarboranes
Organometallics, Vol. 17, No. 7, 1998 1433
Ta ble 5. Ca lcu la ted 11B NMR Ch em ica l Sh ifts of Som e Ma gn esa ca r bor a n esa
δ, ppmb,c
uniquee
compound
basald
13.67 (2)
apicalf
closo-1-(en)-1,2,3-MgC2B4H6 (VIII)
commo-exo-4,4′,5,5′-Mg(en)-[2,3-C2B4H6]2 (IX)g
closo-1-(en)-1,2,4-MgC2B4H6 (X)
8.88 (1)
-4.24 (1)
10.84 (2)
0.69 (1)
5.90 (2)
9.81 (2)
6.59 (1)
3.83 (1)
-49.62 (1)
-52.48 (1)
-48.52 (1)
-54.80 (1)
-55.09 (1)
-49.84 (1)
-49.02 (1)
-48.76 (1)
5.44 (1) 4.35 (1)
14.82 (1)
{commo-1,1′-[1,2,3-MgC2B4H6]2}2- (XI)
{commo-1,1′-[1,2,4-MgC2B4H6]2}2- (XII)
closo-[1,2,4-LiC2B4H6]- h
11.61 (2)
12.35 (1)
12.87 (1)
15.48 (2)
closo-[1,2,3-LiC2B4H6]- h
closo-exo-5,6(µ-H2)2Li-1,2,3-LiC2B4H6]h,i
16.25 (1) 8.52 (1,BLi)
a
b
Geometries optimized at the B3LYP/6-31G* level. GIAO-calculated chemical shifts at the HF/6-311G** level, realtive to BF3‚OEt2
calculated at the same level. c Relative areas in parentheses. Equivalent to B(13,15) in Figures 1-3. e Equivalent to B(14) in Figures
d
1-3. f Equivalent to B(16) in Figures 1-3. The order is B(13), B(15), B(14), and B(16) in Figure 2. From ref 20. The sequence in order
g
h
i
of increasing shielding is B(basal, no bridge), B(basal, Li bridge), B(unique), B(apical).
Sch em e 1. Syn th eses of Ha lf- a n d F u ll-Sa n d w ich Ma gn esa ca r bor a n es of th e C2B4-Ca ge System s
are relative to BF3‚OEt2 as a standard. The standard was
subjected to the same optimization/GIAO cycle as were the
compounds.
compounds
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 metalating reagents, as outlined
in Scheme 1. As can be seen from this scheme, the
products of these reactions are complex functions of the
stoichiometries of the reactions, the nature of the
carborane precursors, and the nature of the metalating
reagents. The most straightforward results are those
obtained from the reactions of the TMEDA-solvated
dilithium carborane [Li(TMEDA)]2[2,4-(SiMe3)2-2,4-
C2B4H4] and MgBr2, where the products were deter-
mined by the stiochiomtery of the reactions. A 1:1
MgBr2-to-carborane molar ratio produced, in 73% yield,
the half-sandwich magnesacarborane closo-1-Mg(TME-
DA)-2,4-(SiMe3)2-2,4-C2B4H4 (III), while a 1:2 MgBr2-
to-carborane molar ratio gave the full-sandwich com-
pound [commo-1,1′-Mg-{2,4-(SiMe3)2-2,4-C2B4H4}2]2- (VI)
Resu lts a n d Discu ssion
Syn th esis. The magnesacarboranes were prepared
by the reactions of either the carbons-adjacent mono-
sodium carboranes 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 the carbons-apart dilithium
(25) Frisch, M. J .; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.;
J ohnson, B. G.; Robb, M. A.; Cheeseman, J . R.; Keith, T.; Peterson, G.
A.; Montgomery, J . A.; Raghavachari, K.; Al-Laham, M. A.; Zakrzewski,
V. G.; Ortiz, J . V.; Foresman, J . B.; Peng, C. Y.; Ayala, P. Y.; Chen,
W.; Wong, M. W.; Andres, J . L.; Replogle, E. S.; Gomperts, R.; Martin,
R. L.; Fox, D. J .; Binkley, J . S.; Defrees, D. J .; Baker, J .; Stewart, J .
P.; Head-Gordon, M.; Gonzalez, C.; Pople, J . A. Gaussian 94, Revision
D.4; Gaussian, Inc.: Pittsburgh, PA, 1995.

1434 Organometallics, Vol. 17, No. 7, 1998
Hosmane et al.
tions at the HF-3-21G* level of theory show the meth-
ylmagnesacarborane to be a stable compound whose
decomposition, according to eq 1, would be exothermic,
with a calculated ∆E ) -55.5 kJ /mol. While this energy
change must be viewed with some caution, it does
suggest that the assumption of a kinetically formed
intermediate is not unreasonable. Similar intermedi-
1-(en)Mg(CH3)-2,3-C2B4H5 f
1-(en)Mg-2,3-C2B4H4 + CH4 (1)
ates are thought to be important in the other magne-
sacarborane formation reactions (vide infra). One of the
most surprising results in this study was the formation
of the exo-sandwich magnesacarborane commo-exo-
4,4′,5,5′-Mg(TMEDA)[2-(SiMe3)-3-(Me)-2,3-C2B4H5]2 (II),
from the reaction of MeMgBr with [Na(TMEDA)][2-
(SiMe3)-3-(Me)-2,3-C2B4H5] under conditions identical
to those used in the synthesis of the closo-magnesacar-
borane I. This compound was produced in 94% yield,
the highest for any magnesacarborane, from a 1:1
Grignard-to-carborane molar ratio. Scheme 1 outlines
our tentative rationalization for this reaction. As in the
formation of I, we postulate that the initial product in
the reaction is the methylmagnesacarborane closo-1-
(TMEDA)Mg(CH3)-2-(SiMe3)-3-(Me)-2,3-C2B4H5. This
intermediate, instead of dimerizing with the loss of
methane, undergoes a disproportionation reaction to
give II and a dialkylmagnesium compound (see Scheme
1) through a Schlenk-type of equilibrium,26 as shown
in eq 2. The postulated (Me)2Mg(TMEDA) product was
F igu r e 1. A perspective view of the “carbons adjacent”
dimeric magnesacarborane half-sandwich, closo-1-Mg(T-
MEDA)-2,3-(SiMe3)2-2,3-C2B4H4 (I), showing the atom-
numbering scheme. The thermal ellipsoids are drawn at
the 40% probability level. For clarity, the exo-polyhedral
SiMe3 groups were drawn with circles of arbitrary radii
and the methyl and methylene H’s are omitted.
in 85% yield. It is not obvious what specific interactions
determine the product distributions in these reactions.
The driving force for the formation of the half-sandwich
magnesacarborane, III, could well be the removal from
solution of the insoluble LiBr coproduct which forms
along with the metallacarborane. However, the forma-
tion of the full-sandwich complex, VI, which involves,
at least formally, the displacement of a TMEDA mol-
ecule on III by a carborane dianion is not accompanied
by the formation of an insoluble coproduct. Thus, the
commo-magnesacarborane, VI, seems to be an inher-
ently very stable compound whose formation drives the
reaction essentially to completion (85% yield). In a
similar manner, the full-sandwich carbons-adjacent
commo-magnesacarboranes, IV and V, were produced
in high yields (82% and 85%, respectively) from a 1:2
stoichiometry of the dialkylmagnesium with either the
THF-(IV) or the TMEDA-solvated (V) monosodium
compounds of the [2,3-(SiMe3)2-2,3-C2B4H5]- monoanion.
While the results of the reactions of the mono- and
dianions of the carboranes with either the magnesium
dialkyl or the magnesium dihalide reagents can be
readily rationalized, the reactions with the mixed-alkyl/
halide Grignard reagent, MeMgBr, are more difficult
to explain. The half-sandwich magnesacarborane closo-
1-Mg(TMEDA)-2,3-(SiMe3)2-2,3-C2B4H4 (I) was formed
from the 1:1 molar ratio reaction of the Grignard
reagent with [Na(TMEDA)][2,3-(SiMe3)2-2,3-C2B4H5].
The solid-state structure of I shows it to be a dimer of
two opposing half-sandwich magnesacarboranes in which
a TMEDA-solvated Mg occupies the apical position
above the C2B3 face of one carborane and is bonded to
the unique boron of the other carborane through a Mg-
H-B bridge (see Figure 1). Two coproducts, insoluble
NaBr and gaseous CH4, are formed along with the
dimer. Although it is experimentally impossible to
determine which coproduct forms first, a reasonable
sequence would be the initial formation of NaBr and
an alkyl-substituted magnesacarborane of the form
closo-1-(TMEDA)Mg(CH3)-2,3-(SiMe3)2-2,3-C2B4H5, which
would then dimerize with the elimination CH4. This
sequence is supported by ab initio molecular orbital
calculations on the model system 1-(en)Mg(CH3)-2,3-
C2B4H5 [en ) (NH2CH2)2]. Molecular orbital calcula-
2[1-(TMEDA)Mg(CH3)-2-(SiMe3)-3-
(Me)-2,3-C2B4H5] h 4,4′,5,5′-Mg(TMEDA)-
[2-(SiMe3)-3-(Me)-2,3-C2B4H5]2 (II) +
(Me)2Mg(TMEDA) (2)
reported to be soluble in benzene and would remain in
solution when II was recrystallized from that solvent.27
Irrespective of the mechanism, the almost quantitative
formation of II shows that it is an extremely stable
compound. The formation of the exo-sandwich magne-
sacarborane instead of a more conventional closo-
product, similar to I, could be the result of less steric
crowding in the initially formed closo-methylmagnesa-
carborane intermediate, which in turn would allow
dimerization directly between the two capping magne-
sium atoms via a double Me and carborane bridge. Such
doubly bridged structures have been proposed as transi-
tion states in the ligand exchange processes that proceed
through Schlenk equilibria.27 In the formation of I, the
presence of the second bulky SiMe3 group could prevent
such bridging and restrict interaction of the two meth-
ylmagnesacarborane intermediates to the sides opposite
the cage carbons as found in I (see Figure 1).
A half-sandwich methylmagnesacarborane intermedi-
ate may also play a role in the sequential reaction of
the Grignard reagent with equimolar amounts of first
(26) For references and discussion of Schlenk equilibria, see: (a)
Linsell, W. E. In Comprehensive Organometallic Chemistry I; Abel, E.
W., Stone, F. G. A., Wilkinson, G., Eds.; Pergamon: Oxford, 1982; Vol.
1. (b) Linsell, W. E. In Comprehensive Organometallic Chemistry II;
Abel, E. W., Stone, F. G. A., Wilkinson, G., Eds.; Elsevier: New York,
1995; Vol. 1.
(27) Coates, G. E.; Heslop, J . A. J . Chem. Soc. A 1966, 26.

C-Trimethylsilyl-Substituted Heterocarboranes
Organometallics, Vol. 17, No. 7, 1998 1435
[Li(THF)2]2[2,4-(SiMe3)2-2,4-C2B4H4] and then [Na-
(THF)2][2,3-(SiMe3)2-2,3-C2B4H5]. The reaction did not
produce the expected mixed-ligand carborane sandwich
compound; instead, a 50:50 mixture of the carbons-apart
and carbons-adjacent full-sandwich magnesacar-
boranes, [Li(THF)2]2[commo-1,1′-Mg{2,4-(SiMe3)2-2,4-
C2B4H4}2] (VII) and [Na(THF)2]2[commo-1,1′-Mg{2,3-
(SiMe3)2-2,3-C2B4H4}2] (IV) was obtained, in yields of
88% and 79%, respectively. There has been one re-
ported synthesis of a mixed-ligand metallacarborane,
that of the dimeric erbacarborane {[Li(TMEDA)2]2-
[commo-1-(2,3-(SiMe3)2-2,3-C2B4H4)-1-Er-(2,4-(SiMe3)2-
2,4-C2B4H4)]}2, which was formed in 95% yield from the
reaction of the carbons-adjacent, full-sandwich erbac-
arborane with the dilithium compound [Li(TMEDA)]2-
[2,4-(SiMe3)2-2,4-C2B4H4].28 In view of this result, there
seems little reason to assume that the mixed-ligand
metallacarborane would be inherently less stable than
its homoligated analogues. A plausible explanation for
the exclusive formation of IV and VII involves the initial
formation of a carbons-apart closo-methylmagnesacar-
borane that exists in equilibrium with IV and a THF-
solvated dimethylmagnesium product. In a reaction
similar to that observed for Mg(Bu)2, the dimethylmag-
nesium compound could react irreversibly with the
bridged hydrogens of [Na(THF)2][2,3-(SiMe3)2-2,3-C2B4H5]
to give VII and gaseous methane. Thus, the various
products found in the study of the reaction of the
Grignard reagent, MeMgBr, with different carboranes
can be rationalized, although not predicted, by assuming
the initial formation of a closo-methylmagnesacarborane
intermediate, which, depending on the nature of the
carborane and the metal-solvating molecules, could
further react to produce compounds I, II, IV, or VII.
F igu r e 2. A perspective view of the exo-sandwich mag-
nesacarborane, commo-exo-4,4′,5,5′-Mg(TMEDA)2[2-(SiMe3)-
3-(Me)-2,3-C2B4H5]2 (II), showing the atom-numbering
scheme. The thermal ellipsoids are drawn at the 40%
probability level. For clarity, the methyl and methylene H’s
are omitted.
Cr ysta l Str u ctu r es of closo-1-Mg(TMEDA)-2,3-
(SiMe3)2-2,3-C2B4H 4 (I), com m o-exo-4,4′,5,5′-Mg-
(TMEDA)2[2-(SiMe3)-3-(Me)-2,3-C2B4H5]2 (II), a n d
[N a (T H F )2 ]2 -[c om m o-1,1′-Mg {2,3-(S iMe 3 )2 -2,3-
C2B4H4}2] (IV). The crystal structures of the magne-
sacarboranes I, II, and IV are shown in Figures1-3,
respectively; some important bond distances and angles
are given in Table 4. Figure 1 shows that compound I
crystallizes as a dimer of two oppositely oriented closo-
1-Mg(TMEDA)-2,3-(SiMe3)2-2,3-C2B4H4 half-sandwich
complexes. As described earlier, each magnesium oc-
cupies an apical position above an open C2B3 pentagonal
face of one carborane unit and is bonded to the unique
boron of the other carborane via a Mg-H-B bridge. The
relevant bond distances in Figure 1, in Å, are Mg(1,2)-
C(11,21) ) 2.649(3), 2.621(4); Mg(1,2)-C(12,22) ) 2.635-
(3), 2.596(3); Mg(1,2)-B(13,23) ) 2.493(3), 2.485(3);
Mg(1,2)-B(15,25) ) 2.472(4), 2.471(3); Mg(1,2)-B(14,-
24) ) 2.393(3), 2.404(3). Although some variations
exist, the structures of both complexes are essentially
equivalent in that each capping magnesium atom
resides in the pseudomirror plane of its respective
carborane ligand,29 but is displaced slightly away from
the cage carbons and toward the unique borons of each
cage, borons B(14) and B(24) in Figure 1. These
F igu r e 3. A perspective view of the polymeric full-
sandwich magnesacarborane, [Na(THF)2]2-[commo-1,1′-Mg-
{2,3-(SiMe3)2-2,3-C2B4H4}2] (IV), showing the atom-num-
bering scheme. The thermal ellipsoids are drawn at the
40% probability level. For clarity, the solvated THF mol-
ecules and the exo-polyhedral SiMe3 units are drawn with
circles of arbitrary radii.
distortions result in metal-cage carbon distances that
are about 0.1-0.3 Å longer than the metal-boron
distances. Similar slip distortions were found in the
full- and half-sandwich group 13 and group 14 metal-
lacarboranes and have been attributed to preferential
bonding with the borons by the metal p orbitals oriented
tangentially to the cage.1 Since the p orbitals on the
magnesium would be very diffuse, it is doubtful that
such interactions play a major role in the distortions
(28) Hosmane, N. S.; Wang, Y.; Zhang, H.; Oki, A. R.; Maguire, J .
A.; Waldho¨r, E.; Kaim, W.; Binder, H.; Kremer, R. K. Organometallics
1995, 14, 1101.
(29) In Figure 1, the plane containing the apical borons (B(16) and
B(26)), the unique borons (B(14) and B(24)), and the midpoint of the
bond between the cage carbons.

1436 Organometallics, Vol. 17, No. 7, 1998
Hosmane et al.
seen in Figure 1. They are most probably related to the
rather strong interaction of each magnesium with the
unique boron on the opposing cage; these distances are
Mg(1,2)-B(24,14) ) 2.540(5) Å, 2.534(5), which are
significantly shorter than the Mg-C(cage) distances in
each complex. Support for this explanation is found in
the structure of the full-sandwich magnesacarborane
IV. Inspection of the equivalent Mg-C2B3 bond dis-
tances in IV, as shown in Figure 3 and listed in Table
4, provides no evidence for a slip distortion; the two
carborane ligands are equivalent and are η5-bonded to
the commo-magnesium metal, with the average Mg-
C2B3 bond distances being 2.426 ( 0.022 Å.30 In
contrast to the very symmetric bonding in IV, the exo-
sandwich magnesacarborane II shows such extreme
slippage that the compound is better described as one
in which a TMEDA-solvated magnesium is exo-polyhe-
drally bound to the two cages through a pair of Mg-
H-B bridges of unequal strength. The pertinent bond
distances are Mg-B(13) ) 2.383(8) Å; Mg-B(14) )
2.480(7) Å (see Figure 2). These distances are compa-
rable to the shorter metal-carborane atom bond dis-
tances in compounds I and IV (see Table 4). The
attenuated metal-cage bonding in II is somewhat
compensated by stronger bonding by the TMEDA. The
Mg-N bond distances in II are 2.183 Å, compared to
average distances of 2.290 ( 0.036 Å in I.
15.68, 1.92 and -49.73 ppm,7b while a 1:2:1 pattern at
δ 18.02, 6.56, and -48.78 ppm is found for closo-exo-
5,6-[(µ-H)2Li(TMEDA)]-1-Li(TMEDA)-2,4-(SiMe3)2-2,4-
C2B4H4.7c Substitution of a group 1 metal with a more
covalently bonding metal group incorporating a group
13 or 14 metal generally leads to a significant downfield
shift of the apical boron resonance, with the facial
borons being less affected.33 This has been rationalized
on the basis of withdrawal of total electron density from
the cage to the capping metal.34 However, the general
2:1:1 and 1:2:1 peak area ratios are maintained for the
respective carbons-adjacent and carbons-apart metal-
lacarcoranes, although the three facial boron resonances
sometimes cannot be resolved, which results in 3:1
patterns.33b,35 With the exception of IV, the peak area
ratios found in the 11B NMR spectra of magnesacarbo-
ranes, shown in Table 1, all show the expected patterns,
with the 2,3-C2B4-based magnesacarboranes (I, II, and
V) having 2:1:1 patterns and those based on the carbons-
apart cages (III, VI, and VII) having 1:2:1 patterns.
However, several questions arise regarding the 11B
NMR spectra of the magnesacarboranes. The 2:1:1
pattern found in the 11B NMR spectra of III is surpris-
ing in view of its solid-state structure (see Figure 2),
which clearly shows nonequivalent basal borons, one
[B(15)] involved in a B-H-B bridge bond while the
other [B(13)] is participating in a BH-Mg-BH bridge.
On the other hand, the 11B NMR spectra of IV shows a
1:1:1:1 pattern indicating nonequivalent borons, even
though Figure 3 shows equivalency or at least near
equivalency of the basal borons [B(13,15)]. The spec-
trum of IV also differs significantly from that of V,
which is formulated to have the same commo-structure
as that of IV; the only difference in these two compounds
is in the nature of the molecules that solvate the sodium
counterions. Since the 11B NMR chemical shifts are
determined by internal cage bonding, it is surprising
that such a minor change in the counterions would
cause that much of a difference in their spectra,
especially in view of the fact that similar differences
found in VI and VII produced almost no effect on their
spectra (see Table 1).
Due to the severe disorder of the solvating TMEDA
molecules the structure of the carbons-apart full-
sandwich compound, VI, could not be completely re-
fined.31 However, a full-sandwich magnesacarborane,
more similar to IV than to II, was clearly evident from
the difference Fourier maps. This structure is consis-
tent with its analytical and spectroscopic data.
Sp ectr a . Compounds I-VII were characterized by
their 1H, 11B, and 13C NMR spectra, with VII being
further characterized by its 7Li NMR spectrum (see
Table 1). The spectra show resonances consistent with
the formulations given in the Experimental Section and,
in the cases of compounds I, II, and IV, with their solid-
state crystal structures. The 11B NMR spectra of other
main-group metallacarboranes in the C2B4 cage system
have proven especially useful as characterization tools.
In general, the group 1 mono- and dianionic carborane
precursors of I-VII all show a resonance in the δ -50
ppm range due to the apical boron (B(16) in Figures
1-3), with the resonances of the less-shielded borons
of the C2B3 open faces being shifted downfield by some
50-80 ppm.7 The main difference in the two systems
lies in the relative positions of the unique and basal
boron resonances.32 In the carbons-adjacent carboranes,
the basal borons are the least shielded and a 2:1:1 peak
area ratio pattern is found,7b while in the carbons-apart
system, the relative shieldings are reversed, which
results in a 1:2:1 pattern.7c For example, the 11B NMR
spectrum of [Na(TMEDA)][2,3-(SiMe3)2-2,3-C2B4H5]
shows three resonances of 2:1:1 peak area ratios at δ
In an effort to assess the changes expected in the 11B
NMR spectra of the metallacarboranes on substituting
a magnesium for a lithium or sodium, the 11B NMR
shielding constants for the model compounds closo-1-
(en)-1,2,3-MgC2B4H6 (VIII), commo-exo-4,4′,5,5′-Mg(en)-
[2,3-C2B4H6]2 (IX), closo-1-(en)-1,2,4-MgC2B4H6 (X),
{commo-1,1′-[1,2,3-MgC2B4H6]2}2- (XI), and {commo-
1,1′-[1,2,4-MgC2B4H6]2}2- (XII) (en ) ethylenediamine,
(NH2CH2)2] were determined using GIAO-HF/6-311G**/
/B3LYP/6-31G* molecular orbital calculations; the re-
sults are listed in Table 5. Also shown in this table are
some calculated chemical shifts for several lithiacarbo-
ranes taken from ref 20. The calculated 11B NMR
(33) (a) Hosmane, N. S.; Saxena, A. K.; Lu, K.-J .; Maguire, J . A.;
Zhang, H.; Wang, Y.; Thomas, C. J .; Zhu, D.; Grover, B. R.; Gray, T.
G.; Eintracht, J . F.; Isom, H.; Cowley, A. H. Organometallics 1995,
14, 5104. (b) Cowley, A. H.; Galow, P.; Hosmane, N. S.; J utzi, P.;
Norman, N. C. J . Chem. Soc., Chem. Commun. 1984, 1564. (c)
Hosmane, N. S.; Sirmokadam, N. N.; Herber, R. H. Organometallics
1984, 3, 1665. (d)Hosmane, N. S.; J ia, L.; Zhang, H.; Maguire, J . A.
Organometallics 1994, 13, 1411.
(34) Maguire, J . A.; Ford, G. P.; Hosmane, N. S. Inorg. Chem. 1988,
27, 3354.
(35) Hosmane, N. S.; de Meester, P.; Maldar, N. N.; Potts, S. B.;
Chu, S. S. C. Organometallics 1986, 5, 772.
(30) Whenever an average value of a parameter is given, the
indetermination listed is the average deviation.
(31) Compound VI crystallized in the monoclinic space group C2
with a ) 17.685 Å, b ) 14.358 Å, c ) 19.952 Å, and â ) 93.59°. Because
of disorder, the cage could only be partially refined and the data are
not included.
(32) Following normal usage, the facial boron that resides in the
pseudomirror plane of the cage is the unique boron (B(14) in Figures
1-3), with the basal borons being the two off-plane atoms.

C-Trimethylsilyl-Substituted Heterocarboranes
Organometallics, Vol. 17, No. 7, 1998 1437
chemical shifts of the model compounds show the
expected general pattern, that is the carbons-adjacent
magnesacarboranes, VIII and XI, show 2:1:1 peak area
ratio patterns while their carbons-apart analogues, X
and XII, exhibit 1:2:1 patterns. A comparison of the
experimental chemical shifts of the apical boron reso-
nances of I-VII, and those of the model compounds with
their respective group 1 precursors shows very little
change when replacing the group 1 metal with magne-
sium. This is in marked contrast to the changes
observed for the group 13 and group 14 metallacarbo-
ranes and most likely reflects the predominately ionic
interactions between the carboranes and the s-block
elements. The calculated 11B chemical shifts for IX,
which is a model for III, show that the chemical shift
of the boron involved in B-H-B bridging is very similar
to that of the boron participating in Mg bridge bonding;
the calculated chemical shifts are δ 4.35 and 5.44 ppm,
respectively (see Table 5). Therefore, the broadened
2:1:1 pattern, as exhibited by II, is quite consistent with
its solid-state structure (see Table 1 and Figure 2). A
comparison of the chemical shifts of I with those of VIII
shows a large difference in the positions of the unique
boron resonances, with that of I (δ -0.28 ppm) being
significantly upfield from that of its model compound
(δ 8.88 ppm). This discrepancy is larger than can be
accounted for by the substitutions made in the model
compound and reflects the effects of dimerization; bridge
bonding by either a H or a metal leads to deshielding.
The 11B NMR chemical shifts of X, XI, and XII agree
quite well with the spectra of their respective trimeth-
ylsilyl-substituted analogues III, V, and VI or VII,
indicating that the structures denoted by their formula-
tions are valid. However, it is difficult to reconcile the
11B NMR spectrum of IV with either its solid-state
structure or that of its model compound (XI). Since the
spectrum is broad, it may represent a conglomerate of
several different clusters involving [Na(THF)]+ and
[commo-1,1′-Mg(2,3-(SiMe3)2-1,2,3-MgC2B4H4)2]2-. Such
extended clusters are known to exist in the THF-
solvated natria- and lithiacarboranes.7
Ack n ow led gm en t. Dedicated with all best wishes
to Prof. Heinrich No¨th (Universita¨t Munchen) on the
occasion of his 70th birthday. This work was supported
by grants from the Robert A. Welch Foundation (Grant
Nos. N-1016 and N-1322), the donors of the Petroleum
Research Fund, administered by the American Chemi-
cal Society, and the National Science Foundation.
Su p p or tin g In for m a tion Ava ila ble: Tables of atomic
coordinates (Table S-1), a full listing of bond lengths and bond
angles (Table S-2), anisotropic displacement parameters (Table
S-3), and H-atom coordinates and isotropic displacement
coefficients (Table S-4) for I, II, and IV (23 pages). Ordering
information is given on any current masthead page.
OM9711133