Boraguanidinates: Synthesis, X-ray structures, and reactions of {Li 2[iPr2NB(NDipp)2]}2 with p-block and group 12 element halides

Article abstract of DOI:10.1021/ic801336t

The new Dipp-substituted boranes (Dipp = 2,6-diisopropylphenyl) ⁱPr₂NB(NHDipp)₂ (5), BrB(NHDipp)₂ (7b), and B(NHDipp)₃ (8) have been prepared in high yields and characterized in solution by multinuclear NMR spectroscopy (¹H, ¹¹B and ¹³C) and in the solid state by X-ray crystallography. Reaction of 5 with 2 equiv of ⁿBuLi in n-hexane produces {Li₂[ⁱPr₂NB(NDipp)₂]} ₂ (6) the first example of a stable dilithiated boraguanidinate. The unsolvated complex 6 has a dimeric structure in the solid state. A survey of the reactions of 6 with p-block and Group 12 element halides revealed various types of reactivity including (a) disproportionation (InCl), (b) reduction to the metal (PbCl₂, CdCl₂, TeBr₄, TeI₄, and TlCl), and (c) simple metathesis (GeCl₂, MgCl₂, and ZnCl₂). The metathetical products were characterized by multinuclear NMR spectra but, in contrast to the dilithiated complex 6, they readily decompose in non-coordinating solvents to form the diprotonated compound 5.

Full text of DOI:10.1021/ic801336t

Inorg. Chem. 2008, 47, 10073-10080
Boraguanidinates: Synthesis, X-ray Structures, and Reactions of
{Li2[iPr2NB(NDipp)2]}2 with p-Block and Group 12 Element Halides
Andrea M. Corrente and Tristram Chivers*
Department of Chemistry, UniVersity of Calgary, Calgary, Alberta T2N 1N4, Canada
Received July 17, 2008
i
The new Dipp-substituted boranes (Dipp ) 2,6-diisopropylphenyl) Pr2NB(NHDipp)2 (5), BrB(NHDipp)2 (7b), and
B(NHDipp)3 (8) have been prepared in high yields and characterized in solution by multinuclear NMR spectroscopy
n
(1H,11B and 13C) and in the solid state by X-ray crystallography. Reaction of 5 with 2 equiv of BuLi in n-hexane
produces {Li2[iPr2NB(NDipp)2]}2 (6) the first example of a stable dilithiated boraguanidinate. The unsolvated complex
6 has a dimeric structure in the solid state. A survey of the reactions of 6 with p-block and Group 12 element
halides revealed various types of reactivity including (a) disproportionation (InCl), (b) reduction to the metal (PbCl2,
CdCl2, TeBr4, TeI4, and TlCl), and (c) simple metathesis (GeCl2, MgCl2, and ZnCl2). The metathetical products
were characterized by multinuclear NMR spectra but, in contrast to the dilithiated complex 6, they readily decompose
in non-coordinating solvents to form the diprotonated compound 5.
Introduction
zirconium(IV) species have also been isolated.8 Jones and
co-workers reported the synthesis and characterization of the
first example of a Mg(I) complex containing the guanidinate
[(Dipp)NC(NiPr2)NDipp]-, where Dipp ) 2,6-diisopropy-
lphenyl.9 X-ray crystallographic analysis of this complex
revealed a unique example of a Mg-Mg single bond, an
exciting development in the area of metal-metal bonding
that illustrates impressively the ability of guanidinate ligands
to stabilize highly reactive species. More recently, the Jones
group has demonstrated that both bulky amidinates and
guanidinates can be employed to isolate thermally stable
lead(II)10 and germanium(II)11 complexes. In addition,
variation of the R groups on the carbon atom of the ligand
backbone has been shown to influence both the oxidation
state of the metal and the coordination mode of group 13
guanidinate complexes.12 Rigid guanidinate-type ligands
Amidinates (1) and guanidinates (2) are monoanionic
ligands that have been extensively investigated as main-group
and transition-metal complexes.1 Both types of ligands have
been incorporated into transition-metal catalysts, and they
have also found use in materials applications such as atomic
layer deposition.1 Another notable feature is their ability to
stabilize reactive metal centers. For example, the first amido
indium hydride incorporated the sterically bulky N,N′-(2,6-
diisopropylphenyl)formamidinate.2 More recently, novel
Au(II) complexes that are stable as solids at room temper-
ature were prepared by using N,N′-(2,6-dimethylphenyl)for-
mamidinate.3 Thermally stable Ge(I) dimers have also been
isolated by employing either bulky amidinate or guanidinate
ligands.4 Guanidinates have also been found to stabilize high
oxidation state molybdenum species,5 low coordinate yt-
trium,6 and low oxidation state gallium.7 Cationic, base-free
(6) Trifonov, A. A.; Lyubov, D. M.; Fedorova, E. A.; Fukin, G. K.;
Schumann, H.; Mu¨hle, S.; Hummert, M.; Bochkarev, M. N. Eur.
J. Inorg. Chem. 2006, 747.
(7) Jones, C.; Junk, P. C.; Platts, J. A.; Stasch, A. J. Am. Chem. Soc.
2006, 128, 2206.
* To whom correspondence should be addressed. E-mail: chivers@
ucalgary.ca. Phone: (403) 220-5741. Fax: (403) 289-9488.
(1) For a recent review, see. Edelmann, F. T., AdV. Organomet. Chem.,
2008, 57, 183.
(2) Baker, R. J.; Jones, C.; Junk, P. C.; Kloth, M. Angew. Chem., Int. Ed.
2004, 43, 3852.
(3) Abdou, H. E.; Mohamed, A. A.; Fackler, J. P., Jr. Inorg. Chem. 2007,
46, 9692.
(4) Green, S. P.; Jones, C.; Junk, P. C.; Lippert, K.-A.; Stasch, A. Chem.
Commun. 2006, 3978.
(5) Cotton, F. A.; Daniels, L. M.; Murillo, C. A.; Timmons, D. J.;
Wilkinson, C. C. J. Am. Chem. Soc. 2002, 124, 9249.
(8) Duncan, A. P.; Mullins, S. M.; Arnold, J.; Bergman, R. G. Organo-
metallics 2001, 20, 1808.
(9) Green, S. P.; Jones, C.; Stasch, A. Science 2007, 318, 1754.
(10) Stasch, A.; Forsyth, C. M.; Jones, C.; Junk, P. C. New J. Chem. 2008,
32, 829.
(11) Jones, C.; Rose, R. P.; Stasch, A. Dalton Trans. 2008, 2871.
(12) Jin, G.; Jones, C.; Junk, P.; Stasch, A.; Woodul, W. D. New J. Chem.
2008, 32, 835.
10.1021/ic801336t CCC: $40.75  2008 American Chemical Society
Inorganic Chemistry, Vol. 47, No. 21, 2008 10073
Published on Web 09/20/2008

Corrente and Chivers
have been used to prepare tetranuclear Au(I) and dinuclear
Au(II) complexes13 and to provide the first examples of
structurally characterized B(II) monocations.14
for the stabilization of reactive metal and non-metal centers.
To this end we report herein (a) the synthesis and X-ray
structural characterization of the new Dipp-substituted bo-
ranes iPr2NB(NHDipp)2 (5), BrB(NHDipp)2 (7b), and B(N-
HDipp)3 (8), (b) a high yield synthesis and the X-ray structure
of the first stable dilithio boraguanidinate {Li2[iPr2NB-
(NDipp)2]}2 (6), and (c) a preliminary survey of the reactions
of 6 with a variety of p-block and Group 12 element halides.
In comparison to the carbon-based ligands, studies of the
isoelectronic boraamidinate (bam) ligands (3) are limited.15
The introduction of a boron center into the ligand framework
results in a dianionic charge that leads to unique reactivity.
For example, stable spirocyclic radicals containing both the
monoanion radical, [PhB(NtBu)2]-• and the dianion [Ph-
B(NtBu)2]2- N,N′-chelated to a group 13 center have been
isolated.16 Complexes containing a bam ligand can also
display unique bonding modes as evidenced by a recent
investigation of heavy group 15 bam derivatives.17
Experimental Section
Reagents and General Procedures. All reactions and the
manipulation of moisture- and/or air-sensitive products were carried
out under an atmosphere of argon using standard Schlenk line
techniques or in an inert-atmosphere glovebox. Solvents were dried
with appropriate drying agents, distilled before use, and stored over
molecular sieves. Prior to use, all glassware was carefully dried.
The reagents InCl (anhyd. 99.995%) and CdCl2 (anhyd.) were
purchased from Strem, TeI4 (99%) and TlCl (99.99%) were
purchased from Alfa Aesar, and all other chemicals were purchased
from Aldrich Chemical Co. All chemicals were used as received,
with the following exceptions: 2,6-diisopropylaniline (97%, Aldrich)
was purified by distillation (at approximately 100 °C and 10-2 Torr)
and anhydrous ZnCl2 was prepared by heating ZnCl2 ·H2O under
vacuum (150 °C, 10-2 Torr, 18 h). The reagents LiN(H)Dipp and
LiNiPr2 were prepared by the addition of nBuLi (2.5 M in hexane)
As with amidinates and guanidinates, the steric and
electronic properties of bam ligands can be tuned by varying
the substituents at both boron and nitrogen. However, to the
best of our knowledge, the only bam ligand that does not
have an alkyl or aryl group on boron is the all-phenyl
derivative Li2[Ph2NB(NPh)2], which was found to eliminate
LiNPh2 in the presence of pyridine or dimethoxyethane
(DME).18 In contrast to the decomposition of this dilithio
derivative, Bertrand and co-workers have described the
synthesis of 4,19 in which a carbene center is stabilized in a
four-membered ring by a boraguanidinate (bog) ligand;20
electron donation from the -NiPr2 group to the electron-
deficient boron atom restricts the ring π-system to the NCN
fragment, thus preventing the four-membered ring from being
anti-aromatic. Organic synthetic methods were used to
generate 4;19 the synthesis of stable dianionic bog ligands,
for example, [iPr2NB(NDipp)2]2-, has not been reported.
i
in an equimolar amount to DippNH2 or Pr2NH, respectively, in
n-hexanes, and purity was checked by 1H NMR spectroscopy.
Deuterated solvents were purchased from Cambridge Isotope
Laboratories, dried over molecular sieves for at least one week,
and degassed using the freeze-pump-thaw method.
Instrumentation. All NMR spectra were acquired at room
temperature using a Bruker DRX 400 spectrometer. All chemical
shifts are reported in parts per million (ppm) with higher frequency
taken as positive. Chemical shifts for 1H and 13C{1H} NMR spectra
are reported with respect to tetramethylsilane and were calibrated
based on the signal of the residual solvent peak. A solution of 1.0
7
M LiCl in D2O was used as the external standard for Li NMR
spectra and 11B{1H} NMR chemical shifts are reported with respect
to a solution of BF3 ·OEt2 in C6D6. Elemental and mass spectrometry
analyses (using a Waters GCT Premier spectrometer) were per-
formed by the Analytical Services Laboratory of the Department
of Chemistry, University of Calgary.
i
Crystal Structure Determinations. Single crystals of Pr2-
NB(NHDipp)2 (5), {Li2[iPr2NB(NDipp)2]}2 (6), BrB(NHDipp)2
(7b), and B(NHDipp)3 (8) suitable for X-ray analysis were covered
with Paratone oil and mounted on a glass fiber in a stream of N2 at
173 K on a Nonius KappaCCD diffractometer (Mo KR radiation,
λ ) 0.71073 Å) using the COLLECT (Nonius, B.V. 1998) software.
The unit cell parameters were calculated and refined from the full
data set. All crystal cell refinement and data reduction was carried
out using the Nonius DENZO package. After data reduction, the
data were corrected for absorption based on equivalent reflections
using SCALEPACK (Nonius, B.V. 1998). The structures were
solved by direct methods with the SHELXS-9721 program package
and refinement was carried out on F2 against all independent
reflections by the full-matrix least-squares method by using the
SHELXL-97 program.22 All non-hydrogen atoms were refined with
anisotropic thermal parameters. The hydrogen atoms were calculated
The report of carbene 4 stimulated our interest in the
development of bog ligands to assess their broader utility
(13) Mohamed, A. A.; Mayer, A. P.; Abdou, H. E.; Irwin, M. D.; Pe´rez,
L. M.; Fackler, J. P. Inorg. Chem. 2007, 46, 11165.
(14) Ciobanu, O.; Emeljanenko, D.; Kaifer, E.; Mautz, J.; Himmel, H.-J.
Inorg. Chem. 2008, 47, 4774.
(15) For a recent review, see. Fedorchuk, C.; Copsey, M.; Chivers, T.
Coord. Chem. ReV. 2007, 251, 897; the acronym bam is used as a
generic representation of boraamidinate ligands.
(16) Chivers, T.; Eisler, D. J.; Fedorchuk, C.; Schatte, G.; Tuononen, H. M.;
Boere´, R. T. Chem. Commun. 2005, 3930.
(17) Konu, J.; Balakrishna, M. S.; Chivers, T.; Swaddle, T. W. Inorg. Chem.
2007, 46, 2627.
(18) Braun, U.; Habereder, T.; No¨th, H.; Piotrowski, H.; Warchhold, M.
Eur. J. Inorg. Chem. 2002, 1132.
(19) Ishida, Y.; Donnadieu, B.; Bertrand, G. Proc. Natl. Acad. Sci. U.S.A.
2006, 103, 13585.
(20) The acronym bog is suggested as a generic representation for
boraguanidinate ligands.
(21) Sheldrick, G.M. SHELXS-97, Program for crystal structure determi-
nation; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
(22) Sheldrick, G. M. SHELXL-97, Program for refinement of crystal
structures; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
10074 Inorganic Chemistry, Vol. 47, No. 21, 2008

Boraguanidinates
geometrically and were riding on their respective atoms. Crystal-
lographic data are summarized in Table 1. These data can be
obtained free of charge from the Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/dat_arequest/cif.
g, 4.15 mmol) in n-hexane (15 mL) at -80 °C by using a procedure
similar to that described for 7a. Anal. Calcd for C24H36BN2Br: C,
65.03; H, 8.19; N, 6.32. Found: C, 64.64; H, 8.34; N, 6.34. H
NMR (C6D6, 25 °C): δ 7.19-7.10 (m, 6 H, Dipp), 4.37 (br, 1 H),
3.94 (br, 1 H), 3.60 (br s, 2 H), 3.41 (br, 2 H), 1.19 (overlapping
doublets, 24 H, CH(CH3)2). 11B NMR (C6D6, 25 °C): δ 25.7. 13C
NMR(C6D6, 25 °C): δ 147 (Dipp), 136.8 (Dipp), 135.9 (Dipp),
123.9 (Dipp), 29.0 (CH(CH3)2 of Dipp groups), 24.3 (-CH(CH3)2
of Dipp groups). LRMS: Calculated m/z for [C24H36N2BBr]: 442.2.
Found: 442.2. MP: 109-112°.
1
Preparation of iPr2NB(NHDipp)2 (5). A solution of iPr2NBCl2
(1.50 g, 8.20 mmol) in n-hexane (10 mL) was added to a stirred
slurry of LiN(H)Dipp (3.05 g, 16.6 mmol) in n-hexane (25 mL) at
0 °C. The ice bath was removed after addition, and the reaction
was allowed to warm to room temperature and stirred for 18 h.
The reaction mixture was filtered and volatiles were removed in
vacuo yielding 5 as a pale yellow solid (3.16 g, 6.80 mmol, 82%).
Colorless, X-ray quality crystals were grown from two different
n-hexane solutions, one exposed to air and the other air-protected.
Anal. Calcd for C30H50BN3: C, 77.73; H, 10.87; N, 9.06. Found:
C, 77.35; H, 10.22; N, 8.96. 1H NMR (C6D6, 25 °C): δ 7.13 (m, 6
H, Dipp), 3.68 (sept, 4 H, -CH(CH3)2 of Dipp groups, 3JH-H ) 6.8
Hz), 3.30 (2 H, br s, -NH), 3.24 (sept, 2 H, -CH(CH3)2 of -NiPr2
groups, 3JH-H ) 6.8 Hz), 1.28 (d, 12 H, -CH(CH3)2 of Dipp groups,
3JH-H ) 6.8 Hz), 1.18 (d, 12 H, -CH(CH3)2 of Dipp groups, 3JH-H
) 6.8 Hz), 1.11 (d, 12 H, CH(CH3)2 of -NiPr2 groups, 3JH-H ) 6.8
Hz). 11B NMR (C6D6, 25 °C): δ 23.4. 13C NMR (C6D6, 25 °C): δ
146.5 (Dipp), 139.8 (Dipp), 126.0 (Dipp), 123.6 (Dipp), 45.3
(-CH(CH3)2 of -NiPr2 groups), 29.1 (-CH(CH3)2 of Dipp groups),
24.5 (-CH(CH3)2 of Dipp groups), 24.3 (-CH(CH3)2 of -NiPr2
groups), 24.0 (-CH(CH3)2 of Dipp groups). Mp: 123-126 °C.
Preparation of Li2[iPr2NB(NDipp)2](6). A 2.5 M solution of
nBuLi in hexanes (2.0 mL, 5.0 mmol) was added to a solution of
iPr2NB(NHDipp)2 (1.16 g, 2.49 mmol) in n-hexane (20 mL) at -30
°C. The reaction mixture was allowed to warm to room temperature
and stirred for 18 h. Volatiles were removed in vacuo to give 6 as
a pale yellow solid (1.004 g, 2.10 mmol, 85%). Colorless, X-ray
quality crystals were grown from an n-hexane/toluene mixture.
Anal. Calcd for C66H110N6B2Li4 (dimer + co-crystallized hexane
molecule): C, 76.44; H, 10.69; N, 8.10. Found: C, 76.12; H, 10.93;
Preparation of B(NHDipp)3 (8). A solution of DippNH2 (6.38
g, 36 mmol) in n-hexane (40 mL) was added to a stirred solution
of BCl3 in n-hexane (6.0 mL, 6.0 mmol) at -80 °C. A white
precipitate began to form upon addition. The reaction mixture was
allowed to warm to room temperature and stirred for 18 h. After
filtration, volatiles were removed in vacuo yielding 8 as a white
sticky solid (3.0 g, 5.6 mmol, 93%). Colorless, X-ray quality crystals
were grown from an air-protected n-hexane solution. Anal. Calcd
for C36H54BN3: C, 80.12; H, 10.09; N, 7.79. Found: C, 79.62; H,
1
10.12; N, 7.72. H NMR (C6D6, 25 °C): δ 7.13 (m, Dipp), 7.02
(m, Dipp), 3.73 (sept, 6 H, CH(CH3)2 of Dipp groups, 3JH-H ) 6.9
Hz), 3.26 (s, 3 H, -NH), 1.24, 1.19 (overlapping d, 36 H, CH(CH3)2
of Dipp groups). 11B NMR (C6D6, 25 °C): δ 22.6. 13C NMR (C6D6,
25 °C): δ 147.1 (Dipp), 137.4 (Dipp), 126.6 (Dipp), 123.9 (Dipp),
28.9 (-CH(CH3)2 of Dipp groups), 24.1 (-CH(CH3)2 of Dipp groups).
HRMS: Calculated m/z for [C36H54BN3]: 539.4411. Found: 539.4416.
MP: 167-170 °C.
Alternative Synthesis of Li2[iPr2NB(NDipp)2] (6). A solution
of B(NHDipp)3 (0.334 g, 0.62 mmol) in n-hexane (12 mL) was
added to a slurry of LDA (0.20 g, 1.87 mmol) in n-hexane (12
mL) at -40 °C. The reaction was maintained at this temperature
for 45 min then allowed to warm to room temperature and stirred
for 18 h. The mixture was filtered and volatiles were removed in
1
vacuo to give a yellow solid (0.285 g). The H NMR spectrum
showed the presence of Li2[iPr2NB(NDipp)2](6) as the major
product, together with a minor unidentified impurity.
1
N, 7.92. H NMR (THF-d8, 25 °C): δ 6.77 (m, 4 H, Dipp), 6.23
(m, 2 H, Dipp), 3.84 (br sept, 4 H, -CH(CH3)2 of Dipp groups),
3.65 (br sept, 2 H, CH(CH3)2 of -NiPr2 groups), 1.20 (d, 12 H,
Reaction of Li2[PhB(NDipp)2] with InCl. A solution of
Li2[PhB(NDipp)2]23 (0.20 g, 0.45 mmol) in Et2O was added to a
slurry of InCl (0.069 g, 0.46 mmol) in Et2O at -70 °C. The reaction
mixture immediately turned dark brown with formation of a
precipitate. The mixture was allowed to warm to room temperature
and stirred for 4 h. After filtration to remove indium metal and
LiCl and evaporation of volatiles in vacuo, an orange-yellow solid
was isolated and the major product was determined to be the
spirocyclic indium(III) complex LiIn[PhB(NDipp)2]2.18 1H NMR
(C6D6, 25 °C): δ 7.29-6.73 (m, 22 H, aryl of Dipp and Ph), 4.47
(sept, 4 H, -CH(CH3)2, 3JH-H ) 7 Hz), 3.81 (sept, 4 H, -CH(CH3)2,
3
-CH(CH3)2 of Dipp groups, JH-H ) 6.8 Hz), 1.07 (d, 12 H,
3
-CH(CH3)2 of Dipp groups, JH-H ) 6.8 Hz), 0.92 (d, 12 H,
3
CH(CH3)2 of -NiPr2 groups, JH-H ) 6.9 Hz). 11B NMR (THF-d8,
25 °C): δ 22.8. 7Li NMR (THF-d8): δ 0.87. 13C NMR (THF-d8, 25
°C): δ 157.9 (Dipp), 139.2 (Dipp), 122.7 (Dipp), 46.9 (-CH(CH3)2
of -NiPr2 groups), 27.9 (-CH(CH3)2 of Dipp groups), 26.5
(-CH(CH3)2 of Dipp groups), 25.8 (-CH(CH3)2 of -NiPr2 groups),
24.9 (-CH(CH3)2 of Dipp groups).
Preparation of ClB(NHDipp)2 (7a). A solution of DippNH2
(4.26 g, 24.0 mmol) in n-hexane (40 mL) was added to a solution
of BCl3 in n-hexane (6 mL, 6 mmol) at -80 °C. A white precipitate
formed shortly after the addition. The reaction mixture was warmed
to room temperature and stirred for 18 h. It was then filtered and
volatiles were removed in vacuo to give 7a as a colorless solid
(1.71 g, 72%). Anal. Calcd for C24H36BN2Cl: C, 72.28; H, 9.10;
3JH-H ) 7 Hz), 2.93 (q, Et2O), 1.73 (d, 12 H, -CH(CH3)2, 3JH-H
)
3
7 Hz), 1.64 (d, 12 H, -CH(CH3)2, JH-H ) 7 Hz), 0.82 (d, 12 H,
3
-CH(CH3)2, JH-H ) 7 Hz), 0.77 (t, Et2O), 0.65 (d, 12 H,
3
-CH(CH3)2, JH-H ) 7 Hz). 11B NMR (C6D6, 25 °C): δ 30 ppm
(br, s).
Synthesis of {Ge[iPr2NB(NDipp)2]}2 ·dioxane (10). A solution
of 6 (0.012 g, 0.025 mmol) in THF-d8 was added to solid
GeCl2 ·(C4H8O2) (0.006 g, 0.026 mmol) in an NMR tube. As the
GeCl2 ·(C4H8O2) dissolved and reacted, the color of the solution
changed from pale yellow to golden orange. NMR data were
1
N, 7.02. Found: C, 73.08; H, 9.45; N, 7.05. H NMR (C6D6, 25
°C): δ 7.17-7.06 (m, 6 H, Dipp), 4.14 (br, 1 H), 3.72 (br, 1 H),
3.57 (br s, 2 H), 3.38 (br, 2 H), 1.20 (overlapping doublets, 24 H,
CH(CH3)2 of Dipp groups). 11B NMR (C6D6, 25 °C): δ 26.4. 13C
NMR (C6D6, 25 °C): δ 147 (Dipp), 137 (Dipp), 124 (Dipp), 28.9
(-CH(CH3)2 of Dipp groups), 24.3 (-CH(CH3)2 of Dipp groups).
HRMS: Calculated m/z for [C24H36N2BCl]: 398.2660. Found:
398.2680. MP: 67-69 °C.
Preparation of BrB(NHDipp)2 (7b). The bromo derivative 7b
was obtained as a colorless solid (0.87 g, 4.2 mmol, 47%) from
DippNH2 (2.85 g, 16.1 mmol) in n-hexane (15 mL) and BBr3 (1.04
1
collected on this solution. H NMR (THF-d8, 25 °C): δ 7.04 (d, 4
H, Dipp), 6.87 (t, 2 H, Dipp), 3.80 (sept, 4 H, CH(CH3)2 of Dipp
3
groups, JH-H ) 6.9 Hz), 3.57 (s, 4H, dioxane), 3.26 (sept, 2 H,
3
CH(CH3)2 of NiPr2 groups, JH-H ) 6.9 Hz), 1.30 (d, 12 H,
(23) Chivers, T.; Fedorchuk, C.; Parvez, M. Inorg. Chem. 2004, 43, 2643.
Inorganic Chemistry, Vol. 47, No. 21, 2008 10075

Corrente and Chivers
Scheme 1
3
CH(CH3)2 of Dipp groups, JH-H ) 6.9 Hz), 1.16, (d, 12 H,
solvent. The molecular structure of 6 reveals the formation
of a dimeric cluster in the solid state with a co-crystallized
solvent molecule (Figure 2, hexane molecule omitted for
clarity).
3
CH(CH3)2 of Dipp groups, JH-H ) 6.9 Hz), 0.86 (d, 12 H,
CH(CH3)2 of NiPr2 groups, JH-H ) 6.9 Hz). 11B NMR (THF-d8,
3
25 °C): δ 28.04. 13C NMR (THF-d8, 25 °C): δ 145.2, 123.4, 123.2,
68.0, 45.7, 28.8, 28.0, 25.0, 23.4.
The sum of the bond angles about boron is 360°; however,
thegeometryisdistortedfromtrigonal,withtheN(1)-B(1)-N(2)
angle being about 7° smaller than the corresponding bond
angle in the acyclic neutral ligand 5 (109° vs 116°),
presumably as a result of the constraints imposed by the
cluster structure. The dimeric nature of 6 parallels our
previous observations for the unsolvated dilithio bam’s:
Synthesis of Mg[iPr2NB(NDipp)2] (11). A mixture of 6 (0.189
g, 0.40 mmol) and MgCl2 (0.038 g, 0.40 mmol) was dissolved in
THF (15 mL) and heated at 60 °C for 4 h. Volatiles were removed
1
in vacuo to give 11 as an orange solid (0.085 g, 44%). H NMR
(THF-d8 25 °C): δ 6.67 (d, 4 H, Dipp), 6.25 (t, 2 H, Dipp), 4.15
(sept, 4 H, CH(CH3)2 of Dipp groups, 3JH-H ) 6.74 Hz), 3.43 (sept,
2 H, -CH(CH3)2 of -NiPr2 groups 3JH-H ) 6.72 Hz), 1.18 (d, 12 H,
3JH-H ) 6.74 Hz), 1.09 (d, 12 H, 3JH-H ) 6.72 Hz), 0.76 (d, 12 H,
3JH-H ) 6.88 Hz). 11B NMR (THF-d8, 25 °C): δ 23.2. 13C NMR
(THF-d8, 25 °C): δ 157.5, 141.7, 122.5, 115.0, 47.1, 27.9, 26.5,
24.8.
Synthesis of Zn[iPr2NB(NDipp)2] (12). A solution of 6 (0.372
g, 0.78 mmol) in THF (10 mL) was added to solid ZnCl2 (0.107 g,
0.79 mmol) and stirred for 30 min. Volatiles were removed in vacuo
and the product was extracted with Et2O (20 mL) and filtered.
Removal of volatiles in vacuo gave 12 as pale yellow solid (0.335
24
24
25
{Li2[RB(NtBu)2]}2 (R ) Me, nBu, Ph,25 and Bu ). In
addition to the dimer {Li2[MeB(NtBu)2]}2, a trimeric struc-
ture is also observed for the B-methyl derivative {Li2[MeB-
(NtBu)2]}3.24 A comparison of relevant bond lengths and
bond angles in 6 with those of selected dilithio bam’s is found
in Table 2.
t
To fully appreciate any structural effects imposed by the
diisopropylamino group in 6, a comparison with an analogous
N-Dipp-substituted bam complex {Li2[RB(NDipp)2]}2 is
desirable. However, the only known example is the tris-
solvated complex (µ-THF)[Li(THF)]2[PhB(NDipp)2], which
exists as a monomer in the solid state owing to solvation of
the Li+ cations.23 In contrast to {Li[PhB(NDipp)2]}2, the
boraguanidinate 6 is hexane-soluble so crystals can be grown
without exposure to a coordinating solvent; dimer formation
increases the coordination number of the lithium cation
through an additional nitrogen contact, as previously ob-
served in unsolvated dilithio bam’s.24,25 The N(1)-B(1)-N(2)
bond angle in 6 is slightly narrower than that in the monomer
(µ-THF)[Li(THF)]2[PhB(NDipp)2] (109.1° versus 111.4°),
likely as a result of the constrained geometry imposed by
dimer formation. The B-N bond lengths in 6 are also slightly
elongated with respect to those in (µ-THF)[Li(THF)]2-
[PhB(NDipp)2], as a consequence of the additional amino
group contributing electron density to the boron center.
1
g, 0.50 mmol, 63%). H NMR (THF-d8, 25 °C): δ 6.74 (d, 4 H,
Dipp), 6.42 (t, 2 H, Dipp), 4.15 (sept, 4 H, CH(CH3)2 of Dipp
groups, 3JH-H ) 6.8 Hz), 3.30 (sept, 2 H, CH(CH3)2 of NiPr2 groups,
3JH-H ) 6.87 Hz), 1.21 (d, 12 H, CH(CH3)2 of Dipp groups, 3JH-H
) 6.8 Hz), 1.13, (d, 12 H, CH(CH3)2 of Dipp groups, 3JH-H ) 6.8
Hz), 0.76 (d, 12 H, CH(CH3)2 of NiPr2 groups, JH-H ) 6.8 Hz).
3
11B NMR (THF-d8, 25 °C): δ 25.2. 13C NMR (THF-d8, 25 °C): δ
154.2, 143.1, 121.9, 117.0, 46.7, 28.2, 25.0, 23.3.
Results and Discussion
i
Synthesis and Characterization of Pr2NB(NHDipp)2
(5) and {Li2[iPr2NB(NDipp)2]}2 (6). The reaction of
iPr2NBCl2 with 2 equiv of lithium 2,6-diisopropylphenyl
amide, LiN(H)(Dipp), in n-hexane produces the neutral
i
ligand Pr2NB(NHDipp)2 (5) in >80% yield (Scheme 1).
Colorless crystals of 5 suitable for X-ray diffraction were
grown from a n-hexane solution in air, and the molecular
structure is shown in Figure 1. Selected bond lengths and
angles are presented in Table 3. The geometry at boron is
slightly distorted from trigonal with bond angles in the range
116.0-123.3°; however, the sum of these angles is 360°.
The nitrogen atoms also display distorted trigonal geometries,
having bond angles ranging from about 114 to 131°, though
the sum of the angles about each nitrogen is 360°. There is
no significant difference in the B-NiPr2 and B-N(H)Dipp
bond lengths, which are intermediate between single and
double bond values, as expected for N(2p)fB(2p) π-bond-
ing.
Deprotonation of 5 with 2 equiv of nbutyllithium in
hexanes (Scheme 1) gives the dianionic ligand as the dilithio
derivative {Li2[iPr2NB(NDipp)2]}2 (6) in isolated yields of
>90%. This new boraguanidinate reagent is soluble in
hexane, and X-ray quality crystals were grown from this
Figure 1. Molecular structure of 5. Hydrogen atoms are omitted and only
the R-carbon atoms of Dipp groups are shown for clarity.
10076 Inorganic Chemistry, Vol. 47, No. 21, 2008

Boraguanidinates
Table 1. Crystallographic Data for 5, 6, 7b, and 8a
5
6
7b
8
empirical formula
fw
C30 H50 B N3
463.54
C33H55BLi2N3
518.49
C24H36BBrN2
443.27
C36H54BN3
539.63
cryst. system
space group
a, Å
b, Å
c, Å
R, deg.
ꢀ, deg.
γ, deg.
V, Å3
orthorhombic
Pbca
18.7744(4)
15.687(3)
20.218(4)
90.00
90.00
90.00
5954(2)
8
orthorhombic
Pbcn
14.103(3)
21.061(4)
21.701(4)
90.00
90.00
90.00
6446(2)
8
monoclinic
P2(1)/n
19.626(4)
6.426(1)
20.311(4)
90.00
103.98(3)
90.00
2486(1)
2
monoclinic
P2(1)/n
10.744(2)
28.035(6)
11.875(2)
90.00
112.08(3)
90.00
3314(1)
4
Z
T, °C
-100
-100
-100
-100
F
calcd, g/cm3
1.034
1.069
1.184
1.081
µ(Mo KR), mm-1
crystal size, mm3
F(000)
0.059
0.060
1.665
0.062
0.40 × 0.36 × 0.28
0.40 × 0.16 × 0.08
0.08 × 0.08 × 0.04
0.28 × 0.28 × 0.16
2048
2280
936
1184
Θ range, deg
reflns collected
unique reflns
Rint
3.38-25.03
87203
5242
0.0234
0.0468
2.56-25.03
34800
5663
0.0502
0.0627
3.35-25.03
46407
4379
0.1371
0.0643
2.99-25.03
42938
5830
0.0393
0.0583
b
R1 [I > 2σ(I)]
wR2 (all data)
c
0.1238
1.033
0.1749
1.030
0.1566
1.039
0.1507
1.032
GOF on F2
completeness
0.996
0.994
0.995
0.996
a λ(Mo KR) ) 0.71073 Å. b R1 ) ∑||Fo| - |Fc||/∑|Fo|. c wR2 ) [∑w(Fo - Fc )2/∑wFo ]1/2
.
2
2
4
Table 2. Bond Lengths (Å) and Bond Angles (deg) for 6 and Dimeric
Dilithioboraamidinates
{Li2[nBuB
{Li2[PhB
{Li2[tBuB
b
a
b
6
(NtBu)2]}2
(NtBu)2]}2
(NtBu)2]}2
B1-N1
1.471(3)
1.470(3)
2.055(5)
2.121(5)
2.144(5)
2.063(5)
2.063(5)
2.145(5)
109.1(2)
126.1(2)
124.8(2)
1.455(4)
1.464(4)
2.032(5)
2.039(6)
2.027(5)
2.032(5)
2.022(6)
2.091(5)
108.2(2)
N/A
1.448(3)
1.449(3)
2.077(4)
2.052(4)
2.017(4)
2.027(4)
2.022(4)
2.074(4)
109.5(2)
N/A
1.476(4)
1.479(4)
1.980(4)
2.154(5)
2.125(4)
2.154(5)
1.976(4)
1.976(4)
105.2(2)
N/A
B1-N2
Li1-N1
Li1-N2
Li1-N2A
Li2-N1
Li2-N2A
Li2-N1A
N1-B1-N2
N3-B1-N2
N3-B1-N1
N/A
N/A
N/A
a Ref 24. b Ref 25.
Table 3. Selected Bond Lengths (Å) and Bond Angles (deg) for 5, 7b,
and 8
5
7b
8
B1-N1
1.438(2)
1.437(2)
1.383(7)
1.402(7)
1.951(6)
1.423(3)
1.427(3)
B1-N2
B1-Br1
B1-N3
1.429(2)
1.426(3)
N3-B1-N1
N3-B1-N2
N1-B1-N2
N1-B1-Br1
N2-B1-Br1
120.66(13)
123.32(14)
116.01(13)
120.61(19)
119.04(18)
120.35(18)
Figure 2. Molecular structure of 6. Hydrogen atoms are omitted and only
the R-carbon atoms of Dipp groups are shown for clarity. Symmetry
123.5(5)
119.2(4)
117.2(4)
1
elements used to generate equivalent atoms: #1, -x + 1, y, -z + /2; #2,
-x, -y, -z + 1.
The B-N distances in 6 are not significantly different from
those in the dimeric B-alkyl bam’s {Li2[RB(NtBu)2]} (R )
nBu, tBu), but they are slightly elongated when compared to
those in the B-phenyl derivative{Li2[PhB(NtBu)2]}2; this is
likely a result of the electron-donating nature of the dialky-
lamino or alkyl substituents on boron compared to the
electron-withdrawing phenyl group. The nitrogen-lithium
distances in 6 fall within the expected range for three-
coordinate nitrogen centers.
an excess of n/sBu2Mg.26 By contrast, the neutral bog ligand
5 was recovered unchanged from attempted reactions with
dibutyl magnesium, dimethyl zinc, or trimethyl aluminum
at various temperatures in either coordinating or non-
coordinating solvents.
Oxidation with I2 - Radical Formation. Exposure of
dilithio bam’s to air produces persistent bright pink solutions,
(24) Brask, J. K.; Chivers, T.; Schatte, G. Chem. Commun. 2000, 1805.
(25) Chivers, T.; Fedorchuk, C.; Schatte, G.; Brask, J. K. Can. J. Chem.
2002, 80, 821.
(26) Chivers, T.; Eisler, D. J.; Fedorchuk, C.; Schatte, G.; Tuononen, H. M.;
Boeré, R. T. Inorg. Chem. 2006, 45, 2119.
The direct metalation reaction (alkane elimination) is
successful for the preparation of the magnesium bam
complex Mg(OEt2)2[PhB(NDipp)2] from PhB(NHDipp)2 and
Inorganic Chemistry, Vol. 47, No. 21, 2008 10077

Corrente and Chivers
Scheme 2
indicative of radical formation.25 The one-electron oxidation
of {[PhB(NtBu)2][Li2]}2 with 0.5 equiv of I2 generates the
thermally unstable, monomeric neutral radical, {[PhB(µ-
NtBu)2]Li(OEt)x}•, in which the radical anion [PhB(NtBu)2]-•
is N,N′-chelated to a solvated Li+ cation.26 This radical was
characterized by a combination of low-temperature electron
paramagnetic resonance (EPR) spectroscopy and density-
functional theory (DFT) calculations. To compare the
behavior of boraguanidinates upon oxidation with that of
boraamidinates, the reaction of 6 with 0.5 equiv of I2 was
carried out at -100 °C. Upon addition of the orange-red I2
solution a transient violet color was observed, which disap-
peared rapidly to give a golden yellow solution and an EPR
spectrum could not be obtained. Subsequently, a pale cream
solid was isolated and was found to contain the diprotonated
derivative iPr2NB(NHDipp)2 (5) as the major product by 1H
NMR spectroscopy. These qualitative observations indicate
that the neutral bog radical {[iPr2B(µ-NDipp)2]Li(OEt)x}• has
lower thermal stability than the bam radical {[PhB(µ-
NtBu)2]Li(OEt)x}•.26
rable three-coordinate boron environment containing a
bromine and two nitrogen substituents. To minimize steric
interactions, the Dipp groups are oriented perpendicular to
the N-B-N plane, with dihedral angles of 90.0° and 91.5°
for B(1)-N(1)-C(1)-C(2) and B(1)-N(2)-C(13)-C(14),
respectively.
The addition of a solution of chloroborane 7a in toluene
to 3 equiv of LDA at 0 °C produced a pale yellow solid,
which was shown by the 1H NMR spectrum to be primarily
the dilithio bog 6, together with a minor unidentified
impurity. Although this reaction represents an alternative
synthesis of 6, the method shown in Scheme 1 is preferable.
However, the novel haloboranes 7a and 7b are potentially
useful reagents for the preparation of other bam or bog
ligands via reactions with alkyl-, aryl-, or amido-lithium
reagents.
We have previously shown that treatment of the tris(alky-
lamino)boranes (tBuNH)3B with 3 equiv of an organolithium
n
reagent RLi (R ) Me, Bu) produces the dilithio bam’s
{Li2[RB(NtBu)2]}n (R ) Me, n ) 2, 3; R ) nBu, n ) 2) via
Alternative Syntheses of 6: Synthesis and X-ray
Structures of BrB(NHDipp)2 and B(NHDipp)3. An alter-
native synthetic approach to 6 involves the reaction of BCl3
with 4 equiv of DippNH2 to form ClB(NHDipp)2 (7a),
followed by the addition of 3 equiv of lithium diisopropyl
amide LiNiPr2 (LDA) (Scheme 2). The reagent LDA serves
the dual purpose of installing the diisopropylamino group
as a substituent on boron and deprotonating both -NHDipp
groups in a one-pot process.
The chloroborane, Cl(BNHDipp)2 (7a), was produced as
a colorless solid in 72% yield and characterized by elemental
analysis, high resolution mass spectrometry and multinuclear
(1H, 11B, and 13C) NMR spectroscopy; however, crystals
isolated from a concentrated n-hexane solution were fibrous
needles not suitable for X-ray diffraction. Consequently, the
bromine derivative, BrB(NHDipp)2 (7b), was prepared as a
colorless solid in 47% yield by the analogous reaction of
BBr3 and 4 equiv of DippNH2. Spectroscopic data of 7b
revealed the expected small upfield shift of the 11B resonance
(25.7 vs 26.4 ppm), and the 1H NMR data were comparable
with that of 7a. Compound 7b was also characterized by
elemental analysis and mass spectrometry and was found to
have a higher melting range than its chlorine counterpart
(109-112° vs 67-69°). Crystals suitable for X-ray diffrac-
tion formed from an n-hexane solution, and the molecular
structure is shown in Figure 3.
a process that involves the concomitant nucleophilic dis-
placement of one BuNH by an R group on boron and the
t
deprotonation of the other two RNH groups.24 Thus, the
reaction of (DippNH)3B (8) with 3 equiv of LDA represents
another possible route to 6. With this objective in mind the
tris(arylamino)borane 8 was obtained in 93% yield as
colorless crystals from the reaction of BCl3 and 6 equiv of
DippNH2 in n-hexane. The borane 8 was characterized by
elemental analysis, high resolution mass spectrometry, multi-
nuclear (1H, 11B, and 13C) NMR spectroscopy, and a single
crystal X-ray structural determination. The 11B NMR chemi-
cal shift of 8 is about 4 ppm upfield from that of the
chloroborane 7a.
The X-ray analysis of 8 confirmed the expected trigonal
geometry (Figure 4). A comparison of selected bond lengths
and bond angles for 5, 7b, and 8 is found in Table 3. The
B-N(H)Dipp bond distances in 8 are similar to those found
The B-N(H)Dipp bond distances in 7b are slightly shorter
than those found in 5 (Table 3), a result of the replacement
of one of the amino groups by the more weakly π-bonding
bromine atom. The geometry about boron is distorted from
trigonal planar although the sum of the angles is 360°. The
B-Br distance is, within experimental error, the same as
found for a bromo-diazaborole,27 which provides a compa-
Figure 3. Molecular structure of 7b. Hydrogen atoms are omitted, and
only the R-carbon atoms of Dipp groups are shown for clarity.
10078 Inorganic Chemistry, Vol. 47, No. 21, 2008

Boraguanidinates
reducing power of the bog ligand, which is attributed to its
increased electron-richness when compared to the dilithio
bam’s.
The reduction of Tl(I) to Tl in the reaction of 6 and 2
equiv of TlCl is not unprecedented, since Manke and Nocera
observed the formation of thallium metal in the reaction of
30
i
t
Li2[PhB(NR)2] (R ) Pr, Bu) with TlCl. However, these
authors were able to isolate the polymeric thallium(I) bam
complexes {Tl2[PhB(NR)2]}∞ in moderate yields from these
reactions. By contrast, the major product of the reaction of
i
1
6 and TlCl was identified as Pr2NB(NHDipp)2(5) by H
NMR spectroscopy.
(b) Disproportionation. The reaction of 6 with an
equimolar amount of indium(I) chloride results in the
immediate deposition of indium metal upon mixing of the
reagents, even at -80 °C. For comparison, the equivalent
reaction was carried out using the dilithio bam Li2[PhB-
(NDipp)2] and similar observations were made. After removal
of indium metal by filtration, the isolated solid was deter-
mined by 1H NMR spectroscopy to be the previously reported
spirocyclic indium(III) complex 9,31 confirming that dispro-
portionation of the initially formed bamIn(I) complex had
occurred (Scheme 3). It seems reasonable to suggest that a
similar disproportionation takes place in the reaction of 6
with InCl. However, attempts to grow X-ray crystals of the
product of this reaction were unsuccessful.
(c) Metathesis. In a few cases simple metathesis occurred
in the reactions of 6 with p-block or group 12 element
halides. For example, the reactions of 6 with GeCl2 ·dioxane,
MgCl2, and ZnCl2 proceeded cleanly to give products that
are tentatively identified as {Ge[iPr2NB(NDipp)2]}2 ·dioxane
(10) and M(THF)x[iPr2NB(NDipp)2] (11, M ) Mg; 12, M
) Zn) on the basis of their NMR spectra. The 1H NMR data
for the two types of isopropyl groups in the new bog
complexes (10, M ) Ge; 11, M ) Mg; 12, M ) Zn) are
compared with those of 5 and 6 in Table 4. From these data
it is clear that the CH3 and CH resonances of the isopropyl
groups and the CH3 resonances of the Dipp groups in 10-12
are significantly shifted with respect to those in 5; conse-
quently, the presence of small amounts of 5 in the reaction
products is readily detected. Unfortunately, the attempted
separation of LiCl from these products using non-coordinat-
ing solvents, for example, toluene and hexane, invariably
resulted in the formation of some of the diprotonated
derivative 5. This observation is surprising in view of the
lack of decomposition of the dilithio derivative 6 in toluene
or hexane.
Figure 4. Molecular structure of B(NHDipp)3, 8. Hydrogen atoms are
omitted, and only the R-carbon atoms of Dipp groups are shown for clarity.
in 5 (vide supra), and the geometry about boron is almost
perfectly trigonal planar with the sum of the angles being
360°. To minimize steric interactions, two of the three Dipp
groups are oriented perpendicular to the N-B-N plane, with
dihedral angles of 93.0 and 88.6° for B(1)-N(1)-C(1)-C(2)
and B(1)-N(2)-C(13)-C(14), respectively, while the third
Dipp group is twisted slightly further, with a B(1)-N-
(3)-C(25)-C(30) torsion angle of 102.1°.
The addition of 3 equiv of LDA to 8 produced 6 (1H
NMR), together with the same impurity previously observed
in the synthesis depicted in Scheme 2. Although 6 can be
produced by these two alternative methods, the synthesis
i
using the commercially available Pr2NBCl2 (Scheme 1) is
the most efficient route to the new bog ligand.
Reactions of 6 with p-Block and Group 12 Element
Halides. To compare the applications of the new Li2bog 6
as a metathetical reagent with the well-established uses of
dilithio bam’s,15 the reactions of 6 with selected p-block and
group 12 element halides were investigated. Although 6 was
effective in metathesis in some cases, it was also found to
be a strong reducing agent toward many p-block element
halides or to give products that disproportionate with
deposition of the metal.
(a) Reduction. The reactions of 6 with a variety of p-block
or group 12 metal halides, for example, SnCl2, PbCl2, TeBr4,
TeI4, CdCl2, and TlCl, resulted in the instantaneous deposi-
tion of the respective metal upon mixing of the reagents
either at room temperature or below. In most cases, after
removal of the metal via filtration, the isolated solid from
these reactions contained two or more major products by
1H NMR spectroscopy.
The behavior of 6 in the reactions with tin(II) and lead(II)
dichlorides differs from that of dilithio bam’s, which produce
stable dimeric complexes M(bam)2 (M ) Sn, Pb) via
metathesis.28,29 In fact, the lead(II) dimer {Pb[PhB(NtBu)2]}2
was initially prepared by the reduction of PbCl4 with
Li2[PhB(NtBu)2], although it is more conveniently prepared
directly from PbCl2.29 The immediate precipitation of Pb
metal in the reaction of 6 with PbCl2 demonstrates the strong
The complexes 10, 11, and 12 all show broad singlets in
the 11B NMR spectra at δ 28.0, 23.2, and 25.2, respectively,
cf. δ 22.8 for the dilithio derivative 6. A distinct downfield
shift in these resonances is evident upon replacement of an
(27) Weber, L.; Rausch, A.; Wartig, H. B.; Stammler, H.-G.; Neumann,
B. Eur. J. Inorg. Chem. 2002, 2438.
(28) Fuꢀstetter, H.; No¨th, H. Chem. Ber. 1979, 112, 3672.
(29) Heine, A.; Fest, D.; Stalke, D.; Habben, C. D.; Meller, A.; Sheldrick,
G. M. J. Chem. Soc., Chem. Commun. 1990, 742.
(30) Manke, D. R.; Nocera, D. G. Polyhedron 2006, 25, 493.
(31) Chivers, T.; Fedorchuk, C.; Schatte, G.; Parvez, M. Inorg. Chem. 2003,
42, 2084.
Inorganic Chemistry, Vol. 47, No. 21, 2008 10079

Corrente and Chivers
Scheme 3
1
Table 4. H NMR Data (in ppm) for the Isopropyl Groups in 5, 6, and 10-12 (in THF-d8)
(5)
(6)
(10)
3.80
(11)
4.15
(12)
-CH(CH3)2 of Dipp groups
-CH(CH3)2 of -NiPr2 groups
-CH(CH3)2 of Dipp groups
-CH(CH3)2 of NiPr2 groups
3.61
3.25
3.84
3.65
4.15
3.30
3.26
3.43
1.22, 1.16
1.12
1.20, 1.07
0.92
1.30, 1.16
0.86
1.18, 1.09
0.76
1.21, 1.13
0.76
electropositive metal (Li or Mg) by zinc or germanium.
12 metal halides reveals that the bog ligand is a much
stronger reducing agent, presumably as a result of the
electron-donating influence of the diisopropylamino sub-
stituent on boron. Consequently, the formation of a metal is
frequently observed in attempted metathetical reactions. In
some cases, for example, InCl, metal formation may result
from disproportionation of the initial product rather than
direct reduction. Metathesis does occur cleanly in some cases,
for example, with MgCl2, ZnCl2, and GeCl2, but the
characterization of the resulting bog complexes was limited
to multinuclear NMR spectra because, in contrast to the
dilithio derivative {Li2[iPr2NB(NDipp)2]}2, the complexes
of these divalent metals show a propensity to form
iPr2NB(NHDipp)2 in non-coordinating solvents. Modification
of the reducing ability of the bog ligand by replacement of
the diisopropyl amino substituent with weaker π-donating
amino groups is under investigation.
1
The integration of the H NMR resonances was used to
estimate the extent of solvation in the complex
{Ge[iPr2NB(NDipp)2]}2 · dioxane (11) and, on this basis, a
structure in which the dioxane ligand bridges the two
germanium centers is proposed. Coordinated THF resonances
were also evident in the 1H NMR spectra of the magnesium
and zinc complexes M(THF)x[iPr2NB(NDipp)2] (12, M )
Mg; 13, M ) Zn); the value of x is most likely 2 to complete
the tetrahedral coordination sphere at these metals. Attempted
crystallizations of 12 and 13 in the presence of coordinating
ligands such as tetramethylethylene diamine (TMEDA) or
PPh3 gave either glassy non-crystalline materials or decom-
position to 5.
Acknowledgment. The authors gratefully acknowledge
financial support from the Natural Sciences and Engineering
Council (Canada) and the Alberta Ingenuity Fund (A.M.C).
Conclusion
We have developed a high yield synthesis of the first stable
dilithio bog, {Li2[iPr2NB(NDipp)2]}2, which adopts a dimeric
cluster structure similar to those of dilithio bam’s in the solid
state. A comparison of the reactions of this new dilithio bog
and those of dilithio bam’s with selected p-block and Group
Supporting Information Available: X-ray crystallographic files
in CIF format. This material is available free of charge via the
Internet at http://pubs.acs.org.
IC801336T
10080 Inorganic Chemistry, Vol. 47, No. 21, 2008