Synthetic and Structural Investigations of Monomeric Dilithium Boraamidinates and Bidentate NBNCN Ligands with Bulky N-Bonded Groups

Article abstract of DOI:10.1021/ic0352755

The dilithiated boraamidinate complexes [Li₂{PhB(NDipp) ₂}(THF)₃] (7a) (Dipp = 2,6-diisopropylphenyl) and [Li ₂{PhB(NDipp)(N^tBu)}(OEt₂)₂] (7b), prepared by reaction of PhB[N(H)Dipp][N(H)R′] (6a, R′ = Dipp; 6b, R′ = ^tBu) with 2 equiv of ⁿBuLi, are shown by X-ray crystallography to have monomeric structures with two terminal and one bridging THF ligands (7a) or two terminal OEt₂ ligands (7b). The derivative 7a is used to prepare the spirocyclic group 13 derivative [Li(OEt ₂)₄][In{PhB(NDipp)₂}₂] (8a) that is shown by an X-ray structural analysis to be a solvent-separated ion pair. The monoamino derivative PhBCl[N(H)Dipp] (9a), obtained by the reaction of PhBCl₂ with 2 equiv of DippNH₂, serves as a precursor for the synthesis of the four-membered BNCN ring {[R?N(H)](Ph)-B(μ-N ^tBu)₂CⁿBu} (10a, R? = Dipp). The X-ray structures of 6a, 9a, and 10a have been determined. The related derivative 10b (R? = ^tBu) was synthesized by the reaction of {Cl(Ph)B(u-N^tBu)₂CⁿBu} with Li[N(H) ^tBu] and characterized by ¹H, ¹¹B, and ¹³C NMR spectra. In contrast to 10a and 10b, NMR spectroscopic data indicate that the derivatives {[DippN(H)](Ph)B(NR′) ₂CR(NR′)} (11a: R = ^tBu, R′ = Cy; 11b: R = ⁿBu, R′ = Dipp) adopt acyclic structures with three-coordinate boron atoms. Monolithiation of 10a produces the novel hybrid boraamidinate/ amidinate (bamam) ligand {Li[DippN]PhB(N^tBu)CⁿBu(N ^tBu)} (12a).

Full text of DOI:10.1021/ic0352755

Inorg. Chem. 2004, 43, 2643−2653
Synthetic and Structural Investigations of Monomeric Dilithium
Boraamidinates and Bidentate NBNCN Ligands with Bulky N-Bonded
Groups
T. Chivers,* C. Fedorchuk, and M. Parvez
Department of Chemistry, UniVersity of Calgary, Calgary, Alberta, Canada T2N 1N4
Received November 5, 2003
The dilithiated boraamidinate complexes [Li2{PhB(NDipp)2}(THF)3] (7a) (Dipp ) 2,6-diisopropylphenyl) and
t
[Li2{PhB(NDipp)(NBu)}(OEt2)2] (7b), prepared by reaction of PhB[N(H)Dipp][N(H)R′] (6a, R′ ) Dipp; 6b, R′ )
tBu) with 2 equiv of nBuLi, are shown by X-ray crystallography to have monomeric structures with two terminal and
one bridging THF ligands (7a) or two terminal OEt2 ligands (7b). The derivative 7a is used to prepare the spirocyclic
group 13 derivative [Li(OEt2)4][In{PhB(NDipp)2}2] (8a) that is shown by an X-ray structural analysis to be a solvent-
separated ion pair. The monoamino derivative PhBCl[N(H)Dipp] (9a), obtained by the reaction of PhBCl2 with
2 equiv of DippNH , serves as a precursor for the synthesis of the four-membered BNCN ring {[R′′′N(H)](Ph)-
2
t
n
B(µ-NBu)2C Bu} (10a, R′′′ ) Dipp). The X-ray structures of 6a, 9a, and 10a have been determined. The related
t
t
n
t
derivative 10b (R′′′ ) Bu) was synthesized by the reaction of {Cl(Ph)B(µ-NBu)2C Bu} with Li[N(H)Bu] and
characterized by 1H, 11B, and 13C NMR spectra. In contrast to 10a and 10b, NMR spectroscopic data indicate that
the derivatives {[DippN(H)](Ph)B(NR′)2CR(NR′)} (11a: R ) Bu, R′ ) Cy; 11b: R ) Bu, R′ ) Dipp) adopt
acyclic structures with three-coordinate boron atoms. Monolithiation of 10a produces the novel hybrid boraamidinate/
t
n
t
n
t
amidinate (bamam) ligand {Li[DippN]PhB(NBu)C Bu(NBu)} (12a).
Introduction
More recently, derivatives of this ligand with group 13 metals
and with phosphorus(III) centers have been reported5 and
the first group 5 complexes were described.6
Amidinate complexes [RC(NR′)2]- have been thoroughly
investigated for p- and d-block metals.1 Recent studies have
focused on the application of aluminum2 and early transition
metal3 complexes as catalysts for olefin polymerization. By
contrast, the isoelectronic boraamidinate dianions [RB(NR′)2]2-
(1) have received little attention. Although the first examples
of boraamidinate compounds were prepared more than 10
years ago, the emphasis of this earlier work was limited to
coordination complexes of groups 4, 14, or 16 elements,4
and the prevalent boraamidinate ligand was [PhB(NtBu)2]2-.
Although a variety of methods have been employed for
the synthesis of boraamidinate complexes, the most straight-
forward synthesis involves metathesis between an element
halide and the reagents {Li2[PhB(NR′)2]}x, prepared by
dilithiation of bis(organoamino)boranes with n-butyllithium.7
An alternative approach involves the reaction of a tris-
(amino)borane B[N(H)R′]3 with 3 equiv of an organo-lithium
reagent (eq 1).8 The application of this method led to the
first X-ray structural determinations of dilithio boraamidi-
nates. It was shown that the extent of aggregation is
influenced by the substituent (R) on boron.8 Although dimeric
structures are most common for unsolvated clusters, a trimer
was obtained for R ) Me.5b,8
* Author to whom correspondence should be addressed. E-mail: chivers@
ucalgary.ca. Telephone: (403) 220-5741. Fax: (403) 289-9488.
(1) For reviews, see: (a) Edelmann, F. T. Coord. Chem. ReV. 1994, 137,
403. (b) Barker, J.; Kilner, M. Coord. Chem. ReV. 1994, 133, 219.
(2) (a) Dagome, S.; Guzei, I. A.; Coles, M. P.; Jordan, R. F. J. Am. Chem.
Soc. 2000, 122, 274. (b) Duchateau, R.; Meetsma, A.; Teuben, J. H.
Chem. Commun. 1996, 223.
(3) (a) Brussee, E. A. C.; Meetsma, A.; Hessen, B.; Teuben, J. H. Chem.
Commun. 2000, 497. (b) Brussee, E. A. C.; Meetsma, A.; Hessen, B.;
Teuben, J. H. Organometallics 1998, 17, 4090.
(4) For a review, see: Blais, P.; Brask, J. K.; Chivers, T.; Fedorchuk, C.;
Schatte, G. In Group 13 Chemistry: From Fundamentals to Applica-
tions; ACS Symposium Series 822; American Chemical Society:
Washington, DC, 2002; Chapter 14, pp 195-207.
(5) (a) Chivers, T.; Fedorchuk, C.; Schatte, G.; Parvez, M. Inorg. Chem.
2003, 42, 2084; (b) Brask, J. K.; Chivers, T.; Fedorchuk, C.; Schatte,
G. Can. J. Chem. 2002, 80, 821.
(6) Manke, D. R.; Nocera, D. G. Inorg. Chem. 2003, 42, 4431.
(7) Heine, H.; Fest, D.; Stalke, D.; Hebben, C. D.; Meller, A.; Sheldrick,
G. M. J. Chem. Soc., Chem. Commun. 1990, 742.
(8) Brask, J. K.; Chivers, T.; Schatte, G. Chem. Commun. 2000, 1805.
10.1021/ic0352755 CCC: $27.50 © 2004 American Chemical Society
Published on Web 03/10/2004
Inorganic Chemistry, Vol. 43, No. 8, 2004 2643

Chivers et al.
by the literature method.15 Filtrations were performed using either
a PTFE filter disk (Acrodisc syringe filter; diameter: 25 mm; pore
size: 0.45 µm) or a glass-sintered frit (8 µm). Solvents were dried
with appropriate drying agents and distilled onto molecular sieves
before use. All reactions and the manipulation of moisture- and/or
air-sensitive products were carried out under an atmosphere of argon
or under vacuum. All glassware was carefully dried prior to use.
3LiR + B[N(H)R′]3 f
1/x{Li2[RB(NR′)2]}x + LiN(H)R′ + 2RH (1)
The initial objective of this work was to investigate the
influence of a very bulky aryl substituent, 2,6-diisopropyl-
phenyl (Dipp), attached to the nitrogen atoms of a bora-
amidinate ligand on cluster aggregation. During the course
of the investigation, the amino(chloro)phenylborane PhBCl-
[N(H)Dipp] was obtained and structurally characterized. The
availability of this reagent stimulated attempts to generate
hybrid boraamidinate/amidinate (bamam) ligands 2 that are
formally isoelectronic with the extensively studied â-diketim-
inate (nacnac) ligands 3.9 There have been several recent
reports of the synthesis and structures of alkali metal and
alkaline earth complexes of â-ketiminate ligands with Dipp
substituents on the two nitrogen atoms.10-13 It was of interest
to assess the influence of the replacement of an R′CCR′′
unit in 3 by the polar, isoelectronic R′NBR′′ unit in 2 in
these sterically encumbered ligands.
Instrumentation. 1H NMR spectra were recorded on Bruker AC
200 and DRX 400 spectrometers, and chemical shifts are reported
7
relative to Me4Si in CDCl3. 11B,13C, 71Ga, and Li NMR spectra
were measured at 25 °C in C6D6 or D8-thf on a Bruker DRX 400
spectrometer using a 5-mm broadband probe (BBO probe) operating
at 128.336, 100.594, 122.014, and 155.459 MHz, respectively.
Chemical shifts are reported relative to those of BF3‚OEt2 in C6D6,
Me4Si in CDCl3, Ga(NO3)3 in D2O, and 1.0 M LiCl in D2O,
respectively. Line-broadening parameters, used in the exponential
multiplication of the free induction decays, were 50-0.5 Hz.
Chemical shifts with a positive sign are correlated with shifts to
high frequencies (downfield) of the reference compound. Infrared
spectra were obtained as Nujol mulls between KBr plates on a
Nicolet Nexus 470 FT-IR spectrometer in the range 4000-350
cm-1. Elemental analyses were provided by the Analytical Services
Laboratory of the Department of Chemistry, University of Calgary.
Preparation of PhB[N(H)Dipp]2 (6a). A solution of PhBCl2
(0.20 mL, 1.54 mmol) in n-hexane (50 mL) was added slowly to
a stirred slurry of Li[N(H)Dipp] (1.13 g, 6.17 mmol) in n-hexane
(50 mL) at -20 °C. The reaction mixture was allowed to warm
slowly to room temperature and then stirred for 18 h. After filtration
to remove LiCl and excess Li[N(H)Dipp], and removal of solvent
in vacuo, PhB[N(H)Dipp]2 (6a) was isolated as a thick yellow oil
(0.62 g, 1.41 mmol, 91%). Anal. Calcd for C30H41BN2: C, 81.80;
Experimental Section
1
H, 9.38; N, 6.36. Found: C, 81.27; H, 9.31; N, 6.33. H NMR
Reagents and General Procedures. The compounds PhBCl2
(97%), GaCl3 (99.99%), InCl3 (99.999%), 1,3-di-tert-butylcarbo-
(D8-thf, 25 °C): δ 7.29-6.54 (m, 11 H, Ph and aryl of Dipp), 4.13
3
(br s, 2 H, NH), 2.97 [septet, 4 H, -CH(CH3)2, J(1H-1H) )
t
n
diimide (99%), BuNH2 (98%), and BuLi (1.6 or 2.5 M solution
in hexanes) were commercial samples (Aldrich) and used as
received. The reagent 2,6-diisopropylaniline (Dippa) [97%, Aldrich]
was purified by distillation (110 °C, 10-2 Torr). Li[N(H)tBu] was
7 Hz], 1.23 [d, 24 H, -CH(CH3)2, 3J(1H-1H) ) 7 Hz]. 11B NMR
(D8-thf, 25 °C): δ 29 (br s). IR (cm-1): 3393 [ν(N-H)].
Dissolution of PhB[N(H)Dipp]2 in Et2O followed by cooling
(0 °C, 1 week) gave colorless crystals of 6a.
n
prepared by the addition of BuLi (2.5 M, 200 mL, 0.5 mol) to a
Preparation of PhB[N(H)Dipp][N(H)tBu] (6b). A solution of
PhBCl[N(H)Dipp] (9a) (1.49 g, 4.97 mmol) in n-hexane (50 mL)
was added slowly to a stirred slurry of Li[N(H)tBu] (1.57 g, 19.89
mmol) in n-hexane (50 mL) at -20 °C. The reaction mixture was
allowed to warm slowly to room temperature, and the resulting
yellow slurry was stirred for 18 h. The slurry was then filtered,
first by using a glass-sintered frit and then by using an Acrodisc
syringe filter to remove LiCl and excess Li[N(H)tBu]. The filtrate
was then concentrated (ca. 85 mL) by removal of solvent in vacuo
and then cooled to -78 °C to precipitate unreacted Li[N(H)tBu].
Finally, the mixture was filtered a third time. The volatile materials
from the filtrate were removed under vacuum to give 6b as an off-
white powder (0.98 g, 2.91 mmol, 59%). Anal. Calcd for C22H33-
BN2: C, 78.57; H, 9.89; N, 8.33. Found: C, 78.35; H, 9.73; N,
8.10. 1H NMR (D8-thf, 25 °C): δ 7.60-6.48 (m, 8 H, Ph and aryl
of Dipp), 3.91 [septet, 2 H, -CH(CH3)2, 3J(1H-1H) ) 7 Hz], 1.91
solution of anhydrous tBuNH2 (65 mL, 0.61 mol) in n-hexane (170
mL) at -10 °C, and its purity was checked by 1H NMR
spectroscopy [δ in C7D8: 1.37 (-C(CH3)3) and in D8-thf: 1.07
(-C(CH3)3), -1.55 (NH)]. Li[N(H)Dipp] was prepared by the
n
addition of BuLi (1.6 M, 16.6 mL, 0.03 mol) to a solution of
anhydrous DippNH2 (5 mL, 0.03 mol) in Et2O (80 mL) at 0 °C,
and its purity was checked by 1H NMR spectroscopy [δ in D8-thf:
6.69 (m, 2 H, aryl of Dipp), 6.07 (m, 1 H, aryl of Dipp), 3.24
3
(septet, 2 H, -CH(CH3)2, J(1H-1H) ) 7 Hz), 2.67 (br s, 1 H,
NH), 1.22 (d, 12 H, -CH(CH3)2, 3J(1H-1H) ) 7 Hz)] and by 7Li
NMR spectroscopy [δ in D8-thf: 1.16 (s)]. Lithiated carbodiimides
Li{nBuC(NtBu)2} (4a), Li{tBuC(NCy)2} (4b) (Cy ) cyclohexyl),
and Li{nBuC(NDipp)2} (4c) were prepared (97, 98, 70% yields,
respectively) by modification of literature procedures.14a-c The
compound {Cl(Ph)B(µ-NtBu)2CnBu} (5) was obtained in 82% yield
3
(br s, 1 H, NH), 1.26 [d, 6 H, -CH(CH3)2, J(1H-1H) ) 7 Hz],
(9) For a review, see: Bourget-Merle, L.; Lappert, M. F.; Severn, J. R.
Chem. ReV. 2002, 102, 3031.
(10) Dove, A. P.; Gibson, V. C.; Hormnirum, P.; Marshall, E. L.; Segal, J.
A.; White, A. J. P.; Williams, D. J. J. Chem. Soc., Dalton Trans. 2003,
3088.
3
1.14 (br s, 1 H, NH), 1.09 [d, 6 H, -CH(CH3)2, J(1H-1H) )
7 Hz], 0.83 [s, 9 H, -C(CH3)3]. 11B NMR (D8-thf, 25 °C): δ 27
(11) Stender, M.; Wright, R. J.; Eichler, B. E.; Prust, J.; Olmstead, M. M.;
Roesky, H. W.; Power, P. P. J. Chem. Soc., Dalton. Trans. 2001, 3465.
(12) Bailey, P. J.; Dick, C. M. E.; Fabre, E.; Parsons, S. J. Chem. Soc.,
Dalton Trans. 2000, 1655.
(13) Clegg, W.; Cope, E. K.; Edwards, A. J.; Mair, F. S. Inorg. Chem.
1998, 37, 2317.
(14) (a) Chivers, T.; Downard, A.; Yap, G. P. A. J. Chem. Soc., Dalton
Trans. 1998, 2603. (b) Zhou, Y.; Richeson, D. S. J. Am. Chem. Soc.
1996, 118, 10850. (c) Boere´, R. E.; Boere´, R. T.; Masuda, J.;
Wolmersha¨user, G. Can. J. Chem. 2000, 78, 1613.
(15) Blais, P.; Chivers, T.; Downard, A.; Parvez, M. Can. J. Chem. 2000,
78, 10.
2644 Inorganic Chemistry, Vol. 43, No. 8, 2004

Dilithium Boraamidinates and Bidentate NBNCN Ligands
(br s). 13C NMR (C6D6, 25 °C): δ seven resonances in the range
157.15-115.76 (Ph & aryl of Dipp), 47.50 [-C(CH3)3], 33.60
[-CH(CH3)2], 26.67 [-CH(CH3)2], 24.86 [-CH(CH3)2], 23.50
[-C(CH3)3]. IR (cm-1): 3386 [ν(N-H)], 3349 [ν(N-H)].
Preparation of [Li2{PhB(NDipp)2}(THF)3] (7a). A 2.5 M
then filtered to remove LiCl. After removal of solvent in vacuo
from the clear golden yellow filtrate, the residue was redissolved
in n-pentane, forming two phases. The yellow supernatant was
discarded and a light yellow powder was isolated and washed twice
with n-pentane (0.37 g, 0.29 mmol, 51%). 1H NMR (C6D6,
25 °C): δ 7.29-6.74 (m, 22 H, Ph and aryl of Dipp), 4.49 [septet,
4 H, -CH(CH3)2, 3J(1H-1H) ) 7 Hz], 3.81 [septet, 4 H,
n
solution of BuLi in hexanes (1.13 mL, 2.82 mmol) was added
slowly to a stirred solution of PhB[N(H)Dipp]2 (6a) (0.62 g, 1.41
mmol) in n-hexane (200 mL) at -10 °C, producing a clear pale
yellow solution. The resulting reaction mixture was allowed to warm
slowly to room temperature (approximately 30 min), whereupon
the solution became cloudy. The reaction mixture was then set to
reflux for 18 h, and the resulting cloudy solution was bright yellow.
Removal of solvent in vacuo (-20 °C) and several washings with
n-hexane gave pale yellow amorphous [Li2{PhB(NDipp)2}] (0.50
g, 1.11 mmol, 79%); mp 140-142 °C (dec). Dissolution of [Li2-
{PhB(NDipp)2}] in THF followed by cooling (0 °C, 24 h) gave
colorless crystals of 7a. Anal. Calcd for C34H47BLi2N2O (loss of 2
THF): C, 77.86; H, 9.03; N, 5.34. Found: C, 77.34; H, 9.04; N,
3
-CH(CH3)2, J(1H-1H) ) 7 Hz], 2.93 [q, 16 H, (CH3CH2)2O],
3
1.74 [d, 12 H, -CH(CH3)2, J(1H-1H) ) 7 Hz], 1.64 [d, 12 H,
-CH(CH3)2, 3J(1H-1H) ) 7 Hz], 0.82 [d, 12 H, -CH(CH3)2, 3J(1H-
1H) ) 7 Hz], 0.77 [t, 24 H, (CH3CH2)2O], 0.65 [d, 12 H, -CH-
(CH3)2, 3J(1H-1H) ) 7 Hz]. 11B NMR (C6D6, 25 °C): δ 30 (br s).
7Li NMR (C6D6, 25 °C): δ -7.06 (s). 13C NMR (C6D6, 25 °C): δ
eight resonances in the range 149.38-120.81 (Ph and aryl of Dipp),
65.51 [(CH3CH2)2O], 29.06 [-CH(CH3)2], 28.30 [-CH(CH3)2],
24.33 [-CH(CH3)2], 24.03 [-CH(CH3)2], 23.37 [-CH(CH3)2],
22.93 [-CH(CH3)2], 14.68 [(CH3CH2)2O]. Dissolution of 8a in Et2O
followed by cooling (0 °C, 48 h) gave colorless X-ray quality
crystals. Several attempts to obtain CHN analyses gave inconsistent
results, owing to the loss of Et2O from the complex.
1
5.86. H NMR (D8-thf, 25 °C): δ 7.29-6.33 (m, 11 H, Ph and
aryl of Dipp), 3.66 [septet, 4 H, -CH(CH3)2, 3J(1H-1H) ) 7 Hz],
3.62 [m, 12 H, O(CH2)2(CH2)2], 1.77 [m, 12 H, O(CH2)2(CH2)2],
Preparation of [Li(OEt2)x][Ga{PhB(NDipp)2}2] (8b). Complex
8b (0.47 g, 0.38 mmol, 51%) was obtained as a golden yellow
powder using the procedure described for 8a [7a: 0.67 g, 1.47
3
1.11 [d, 24 H, -CH(CH3)2, J(1H-1H) ) 7 Hz]. 11B NMR (D8-
thf, 25 °C): δ 29 (br s). 7Li NMR (D8-thf, 25 °C): δ 0.24 (s). 13
C
1
NMR (D8-thf, 25 °C): δ 158.06 (C1 aryl of Dipp), 140.00 (C2 and
C6 aryl of Dipp), 134.90 (C3 and C5 aryl of Dipp), 122.75 (C4 aryl
of Dipp), four resonances in the range 126.94-115.30 (Ph), 68.28
[O(CH2)2(CH2)2], 28.33 [-CH(CH3)2], 26.43 [O(CH2)2(CH2)2],
14.11 [-CH(CH3)2]. IR (cm-1): No N-H stretch.
mmol; GaCl3: 0.13 g, 0.74 mmol]. H NMR (C6D6, 25 °C): δ
7.73-6.96 (m, 22 H, Ph and aryl of Dipp), 4.02 [septet, 4 H,
3
-CH(CH3)2, J(1H-1H) ) 7 Hz], 3.62 [septet, 4 H, -CH(CH3)2,
3J(1H-1H) ) 7 Hz], 2.97 [q, 16 H, (CH3CH2)2O], 1.59 [d, 12 H,
-CH(CH3)2, 3J(1H-1H) ) 7 Hz], 1.30 [d, 12 H, -CH(CH3)2,
3
3J(1H-1H) ) 7 Hz], 1.17 [d, 12 H, -CH(CH3)2, J(1H-1H) ) 7
Preparation of [Li2{PhB(NDipp)(NtBu)}(OEt2)2] (7b). A 2.5
M solution of nBuLi in hexanes (2.33 mL, 5.83 mmol) was added
slowly to a stirred solution of 6b (0.98 g, 2.91 mmol) in n-hexane
(150 mL) at -10 °C, producing a clear yellow solution. The
resulting reaction mixture was allowed to warm slowly to room
temperature (approximately 30 min) and then set to reflux for 18
h. The clear yellow solution was cooled to -20 °C and then
concentrated (ca. 140 mL) by removal of solvent in vacuo. The
resulting solution was further cooled to -78 °C, at which point a
white solid was deposited. Removal of the clear yellow supernatant
via decantation and several washings with n-hexane gave off-white
amorphous [Li2{PhB(NDipp)(NtBu)}] (0.76 g, 2.18 mmol, 75%).
Dissolution of [Li2{PhB(NDipp)(NtBu)}] in Et2O followed by
cooling (0 °C, 0.5 h) gave colorless crystals of 7b. Several attempts
to obtain CHN analyses gave inconsistent results, owing to the loss
of Et2O from the complex. 1H NMR (C6D6, 25 °C): δ 7.29-6.63
(m, 8 H, Ph and aryl of Dipp), 3.54 [septet, 1 H, -CH(CH3)2,
3J(1H-1H) ) 7 Hz], 3.26 [q, 8 H, (CH3CH2)2O], 2.99 [septet, 1 H,
-CH(CH3)2, 3J(1H-1H) ) 7 Hz], 1.40 [d, 3 H, -CH(CH3)2, 3J(1H-
1H) ) 7 Hz], 1.37 [d, 3 H, -CH(CH3)2, 3J(1H-1H) ) 7 Hz], 1.31
[s, 9 H, -C(CH3)3], 1.10 [t, 12 H, (CH3CH2)2O], 0.99 [d, 3 H,
-CH(CH3)2, 3J(1H-1H) ) 7 Hz], 0.34 [d, 3 H, -CH(CH3)2, 3J(1H-
1H) ) 7 Hz]. 11B NMR (C6D6, 25 °C): δ 35 (br s). 7Li NMR (C6D6,
25 °C): δ 2.33 (s). 13C NMR (C6D6, 25 °C): δ eight resonances
in the range 150.43-119.65 (Ph and aryl of Dipp), 67.20
[(CH3CH2)2O], 51.46 [-C(CH3)3], 37.57 [-CH(CH3)2], 35.78
[-CH(CH3)2], 28.97 [-CH(CH3)2], 27.40 [-CH(CH3)2], 24.03
[-CH(CH3)2], 23.24 [-CH(CH3)2], 22.47 [-C(CH3)3], 14.20 [(CH3-
CH2)2O]. IR (cm-1): No N-H stretch.
3
Hz], 1.05 [d, 12 H, -CH(CH3)2, J(1H-1H) ) 7 Hz], 0.79 [t, 24
H, (CH3CH2)2O]. 11B NMR (C6D6, 25 °C): δ 28 (br s). 71Ga NMR
(C6D6, 25 °C): δ 430.9 (s). 7Li NMR (C6D6, 25 °C): δ -0.72 (s).
13C NMR (C6D6, 25 °C): δ eight resonances in the range 147.52-
118.26 (Ph and aryl of Dipp), 65.71 [(CH3CH2)2O], 28.78
[-CH(CH3)2], 28.52 [-CH(CH3)2], 25.45 [-CH(CH3)2], 25.03
[-CH(CH3)2], 24.89 [-CH(CH3)2], 24.59 [-CH(CH3)2], 15.02
[(CH3CH2)2O]. Several attempts to obtain CHN analyses gave
inconsistent results, owing to the loss of Et2O from the complex.
Preparation of PhBCl[N(H)Dipp] (9a). A solution of DippNH2
(0.58 mL, 3.08 mmol) in Et2O (20 mL) was added to a stirred
solution of PhBCl2 (0.20 mL, 1.54 mmol) in Et2O (50 mL) at -40
°C, instantly producing a white slurry. The reaction mixture was
allowed to warm slowly to room temperature and then stirred for
2 h. A clear colorless filtrate was isolated after filtration. The volatile
materials were removed under vacuum to give 9a as a white
crystalline solid (0.43 g, 1.44 mmol, 93%). Anal. Calcd for C18-
H23BClN: C, 72.15; H, 7.74; N, 4.67. Found: C, 71.92; H, 7.93;
1
N, 4.60. H NMR (C6D6, 25 °C): δ 7.74-6.93 (m, 8 H, Ph and
aryl of Dipp), 5.71 (br s, 1 H, NH), 3.23 [septet, 2 H, -CH(CH3)2,
3
3J(1H-1H) ) 7 Hz], 1.16 [d, 12 H, -CH(CH3)2, J(1H-1H) )
7 Hz]. 11B NMR (C6D6, 25 °C): δ 40 (br s). 13C NMR (C6D6,
25 °C): δ 145.31 (C1 aryl of Dipp), 135.72 (C2 and C6 aryl of
Dipp), 135.51 (C3 and C5 aryl of Dipp), 135.19 (C4 aryl of Dipp),
four resonances in the range 133.38-123.47 (Ph), 28.67 [-CH-
(CH3)2], 24.15 [-CH(CH3)2].
t
Preparation of PhBCl[N(H)tBu] (9b). A solution of BuNH2
(0.82 mL, 7.71 mmol) in Et2O (20 mL) was added to a stirred
solution of PhBCl2 (0.50 mL, 3.85 mmol) in Et2O (50 mL) at -40
°C, instantly producing a white slurry. The reaction mixture was
allowed to warm slowly to room temperature and then stirred for
2 h. The slurry was then filtered and the volatile materials were
removed from the filtrate under vacuum to give a viscous oil. Pure
PhBCl[N(H)tBu] (0.70 g, 3.58 mmol, 93%) was obtained as a clear
Preparation of [Li(OEt2)4][In{PhB(NDipp)2}2] (8a). A slurry
of 7a (0.51 g, 1.13 mmol) in Et2O (150 mL) was added to solid
InCl3 (0.12 g, 0.56 mmol) at -78 °C, producing a yellow slurry
with a pink tinge. The stirred reaction mixture was allowed to warm
slowly to room temperature, at which point it became a paler yellow
slurry. After 18 h, the reaction mixture was allowed to settle and
Inorganic Chemistry, Vol. 43, No. 8, 2004 2645

Chivers et al.
colorless liquid by distillation (80 °C, 10-2 Torr). Anal. Calcd for
C10H15BClN: C, 61.44; H, 7.73; N, 7.16. Found: C, 61.29; H, 7.79;
N, 6.72. 1H NMR (C6D6, 25 °C): δ 7.67-7.15 (m, 5 H, Ph), 4.66
(br s, 1 H, NH), 1.23 [s, 9 H, (-C(CH3)3]. 11B NMR (C6D6,
25 °C): δ 36 (s). 13C NMR (C6D6, 25 °C): δ four resonances in
the range 132.7-127.9 (Ph), 50.96 [-C(CH3)3], 31.3 [-C(CH3)3].
Preparation of {[DippN(H)]PhB(NDipp)CnBu(NDipp)} (11b).
A solution of 9a (0.58 g, 1.94 mmol) in n-hexane (50 mL) was
added to a stirred solution of 4c (0.83 g, 1.94 mmol) in n-hexane
(50 mL) at -78 °C. The reaction mixture was allowed to slowly
warm to room temperature (pale yellow cloudy solution) and then
stirred for 18 h. The reaction mixture was then filtered, and the
solvent was removed in vacuo from the clear yellow filtrate to give
a very viscous yellow oil (1.01 g, 1.48 mmol, 76%). Anal. Calcd
for C47H66BN3: C, 82.55; H, 9.73; N, 6.14. Found: C, 82.07; H,
9.98; N, 6.11. 11B NMR (C6D6, 25 °C): δ 32 (br s).
Preparation of {[DippN(H)](Ph)B(µ-NtBu)2CnBu} (10a). A
solution of 9a (1.02 g, 3.40 mmol) in n-hexane (50 mL) was added
to a stirred mixture of Li{nBuC(NtBu)2} (4a) (0.74 g, 3.40 mmol)
in n-hexane (50 mL) at -78 °C. The reaction mixture was allowed
to slowly warm to room temperature (white cloudy solution), stirred
for 18 h, and then filtered to remove LiCl. The solvent was removed
in vacuo from the clear colorless filtrate to give a white powder
{[DippN(H)](Ph)B(µ-NtBu)2CnBu} (10a) (1.42 g, 2.99 mmol, 88%).
X-ray quality crystals were isolated from a concentrated solution
of 10a in toluene (-10 °C, 2 d). Anal. Calcd for C31H50BN3: C,
78.29; H, 10.60; N, 8.84. Found: C, 78.18; H, 10.85; N, 8.88. 1H
NMR (C6D6, 25 °C): δ 7.80-7.04 (m, 8 H, Ph and aryl of Dipp),
4.13 (br s, 1 H, NH), 3.70 [septet, 2 H, -CH(CH3)2, 3J(1H-1H) )
7 Hz], 2.36 (m, 2 H, -CH2CH2CH2CH3), 1.71 (m, 2H, -CH2CH2-
CH2CH3), 1.40 (m, 2 H, -CH2CH2CH2CH3), 1.27 [d, 12 H, -CH-
Preparation of {Li[DippN]PhB(NtBu)CnBu(NtBu)} (12a). A
2.5 M solution of nBuLi in hexanes (1 mL, 2.50 mmol) was added
slowly to a stirred solution of 10a (1.00 g, 2.10 mmol) in n-hexane
(100 mL) at room temperature, producing a clear very pale yellow
solution. A white slurry is formed approximately 30 s after complete
addition. The resulting reaction mixture was stirred for an additional
2 h, and the precipitate was isolated by filtration using a glass-
sintered frit. The product was then washed with cold n-hexane three
times to give {Li[DippN](Ph)B(NtBu)CnBu(NtBu)} (12a) (0.63 g,
1.31 mmol, 62%). Anal. Calcd for C31H49BN3Li: C, 77.33; H, 10.26;
1
N, 8.73. Found: C, 76.75; H, 10.50; N, 8.45. H NMR (C6D6,
25 °C): δ 7.89-7.10 (m, 8 H, Ph and aryl of Dipp), 3.79 [septet,
3
(CH3)2, J(1H-1H) ) 7 Hz], 1.03 [s, 18 H, -C(CH3)3], 0.78 (t, 3
1 H, -CH(CH3)2, 3J(1H-1H) ) 7 Hz], 3.20 [septet, 1 H,
H, -CH2CH2CH2CH3). 11B NMR (C6D6, 25 °C): δ 5.32 (s). 13C
NMR (C6D6, 25 °C): δ 169.8 (-CnBu), 153.3-117.7 (Ph and aryl
of Dipp), 51.8 [-C(CH3)3], 31.5 [-C(CH3)3], 30.4 (-CH2CH2-
CH2CH3), 28.9 (-CH2CH2CH2CH3), 27.6 [-CH(CH3)2], 27.1
[-CH(CH3)2], 22.9 (-CH2CH2CH2CH3), 12.6 (-CH2CH2CH2CH3).
IR (cm-1): 3420 [ν(N-H)].
3
-CH(CH3)2, J(1H-1H) ) 7 Hz], 2.21 (m, 2 H, -CH2CH2CH2-
CH3), 2.07 (m, 2H, -CH2CH2CH2CH3), 1.53 (m, 2 H, -CH2-
CH2CH2CH3), 1.44 [d, 3 H, -CH(CH3)2, 3J(1H-1H) ) 7 Hz], 1.40
[s, 9 H, -C(CH3)3], 1.38 [d, 3 H, -CH(CH3)2, 3J(1H-1H) )
3
7 Hz], 1.28 [d, 3 H, -CH(CH3)2, J(1H-1H) ) 7 Hz], 1.08 [d, 3
H, -CH(CH3)2, 3J(1H-1H) ) 7 Hz], 0.99 [s, 9 H, -C(CH3)3], 0.81
(t, 3 H, -CH2CH2CH2CH3). 11B NMR (C6D6, 25 °C): δ 29 (br s).
7Li NMR (C6D6, 25 °C): δ 0.13 (s). 13C NMR (C6D6, 25 °C): δ
eight resonances in the range 152.90-118.72 (Ph and aryl of Dipp),
54.95 [-C(CH3)3], 52.85 [-C(CH3)3], 36.94 (-CCH2CH2CH2CH3),
31.24 [-C(CH3)3], 30.23 [-C(CH3)3], 28.41 [-CH(CH3)2], 28.14
[-CH(CH3)2], 27.62 [-CH(CH3)2], 24.72 [-CH(CH3)2], 24.51
[-CH(CH3)2], 24.14 [-CH(CH3)2], 23.75 (-CH2CH2CH2CH3),
23.47 (-CH2CH2CH2CH3), 23.10 (-CH2CH2CH2CH3), 13.59
(-CH2CH2CH2CH3). IR (cm-1): No N-H stretch.
Preparation of {[tBuN(H)](Ph)B(µ-NtBu)2CnBu} (10b). A
solution of 5 (0.07 g, 0.21 mmol) in THF (15 mL) was added to a
stirred mixture of Li[N(H)tBu] (0.03 g, 0.42 mmol) in THF (10
mL) at ambient temperature. The reaction mixture was allowed to
stir for 48 h. Solvent was removed in vacuo and the residue was
taken up in n-hexane. After filtration and removal of solvent in
vacuo, {[tBuN(H)](Ph)B(µ-NtBu)2CnBu} (10b) was isolated as an
off-white crystalline solid (0.04 g, 0.11 mmol, 51%). Anal. Calcd
for C23H42BN3: C, 74.38; H, 11.40; N, 11.31. Found: C, 73.95;
1
H, 10.99; N, 11.24. H NMR (C6D6, 25 °C): δ 7.70-7.24 (m,
X-ray Analyses. Single crystals of 6a, 7a, 8a, and 7b (colorless
block), 10a (colorless prismatic crystal), and 9a (colorless plate)
were coated with Paratone 8277 oil (Exxon), mounted onto thin
glass fibers or inside a mounted CryoLoop (Hampton Research,
diameter of the nylon fiber 20 and 10 µm), and quickly frozen in
the cold nitrogen stream of the goniometer. Measurements for 6a,
7a, 8a, 9a, 7b, and 10a were made on a Nonius CCD four-circle
Kappa FR540C diffractometer using graphite-monochromated Mo
KR radiation (6a, 7a, 8a, 7b, 10a: λ ) 0.71073 Å; 9a: λ ) 0.71069
Å). Data were measured using φ and ω scans. Data reduction was
performed by using the HKL DENZO and SCALEPACK soft-
ware.16 A multiscan absorption correction was applied to the data
(SCALEPACK).16
5 H, Ph), 2.31 (m, 2 H, -CH2CH2CH2CH3), 1.68 (m, 2 H,
-CH2CH2CH2CH3), 1.43 [s, 9 H, exo-C(CH3)3], 1.21 [s, 18 H,
endo-C(CH3)3], 1.17 (m, 2 H, -CH2CH2CH2CH3), 1.11 (br s, 1 H,
NH), 0.80 (t, 3 H, -CH2CH2CH2CH3). 11B NMR (C6D6, 25 °C):
δ 5.1 (s). 13C NMR (C6D6, 25 °C): δ 171.03 (-CCH2CH2CH2-
CH3), 132.60 (Ph), 127.61 (Ph), 127.06 (Ph), 125.98 (Ph), 52.09
[endo-C(CH3)3], 49.52 [exo-C(CH3)3], 33.25 [exo-C(CH3)3], 30.70
[endo-C(CH3)3], 29.12 (-CH2CH2CH2CH3), 28.99 (-CH2CH2CH2-
CH3), 23.31 (-CH2CH2CH2CH3), 13.31 (-CH2CH2CH2CH3).
Preparation of {[DippN(H)]PhB(NCy)CtBu(NCy)} (11a). A
solution of 9a (1.05 g, 3.50 mmol) in n-hexane (50 mL) was added
to a stirred solution of 4b (0.95 g, 3.50 mmol) in n-hexane (50
mL) at -78 °C. The reaction mixture was allowed to slowly warm
to room temperature (white cloudy solution), stirred for an additional
18 h, and then filtered to remove LiCl. The solvent was removed
in vacuo from the filtrate to give a very viscous yellow oil. The oil
was redissolved in a minimal amount of Et2O and then the solvent
was removed in vacuo. This process was repeated twice to give
11a as a yellow sticky residue (1.64 g, 3.11 mmol, 89%). Anal.
Calcd for C35H54BN3: C, 79.67; H, 10.32; N, 7.96. Found: C,
79.11; H, 10.70; N, 7.88. 11B NMR (C6D6, 25 °C): δ 30 (br s). A
Relevant parameters for the data collections and crystallographic
data for 6a, 7a, 8a, 9a, 7b, and 10a are summarized in Table 1.
The structures were solved by direct methods (SIR-9217) and refined
by a full-matrix least-squares method based on F2 using SHELXL-
97.18 The non-hydrogen atoms were refined anisotropically (unless
(16) HKL DENZO and SCALEPACK vl.96: Otwinowski, Z.; Minor, W.
Processing of X-ray Diffraction Data Collected in Oscillation Mode;
Methods in Enzymology, Vol. 276: Macromolecular Crystallography,
Part A.; Carter, C. W., Jr., Sweet, R. M., Eds.; Academic Press: San
Diego, CA, 1997; pp 307-326.
(17) Altomare, A.; Cascarano, M.; Giacovazzo, C.; Guagliardid, A. SIR-
92, A package for crystal structure solution by direct methods and
refinement. J. Appl. Crystallogr. 1993, 26, 343.
1
sharp very minor resonance was also observed at δ 1.55. The H
NMR spectrum was uninformative, owing to the complex reso-
nances of the inequivalent cyclohexyl groups.
2646 Inorganic Chemistry, Vol. 43, No. 8, 2004

Dilithium Boraamidinates and Bidentate NBNCN Ligands
Table 1. Selected Crystal Data and Data Collection Parameters for PhB[N(H)Dipp]2‚(C4H10O)0.25 (6a), [Li2{PhB(NDipp)2}(THF)3] (7a),
[Li(OEt2)4][In{PhB(NDipp)2}2]‚(C7H8)2 (8a), PhBCl[N(H)Dipp] (9a), [Li2{PhB(NDipp)(NtBu)}(OEt2)2] (7b), and {[DippN(H)](Ph)B(µ-NtBu)2CnBu}
(10a)
6a
7a
8a
9a
7b
10a
formula
C30H41BN2‚CH2.5O0.25
C42H63BLi2N2O3
C60H78B2InN4‚C16H40LiO4‚
C14H16
C18H23BClN
C30H51BLi2N2O2
C31H50BN3
fw
458.99
P1h
668.63
P21/n
10.5766(4)
20.9002(8)
18.2412(10)
90
93.037(2)
90
4026.6(3)
4
1479.39
P1h
299.63
P21/a
11.7907(5)
13.1664(5)
11.9232(6)
90
113.866(2)
90
1692.70(13)
4
496.42
P21/n
19.019(3)
18.461(3)
20.330(4)
90
117.523(9)
90
6330.2(19)
8
475.55
P21
10.634(7)
9.724(7)
14.733(13)
90
99.06(3)
90
1504(2)
2
space group
a, Å
b, Å
11.092(2)
15.078(2)
19.185(3)
67.634(6)
88.134(6)
82.276(9)
2939.7(8)
4
12.580(2)
18.196(2)
19.164(3)
99.140(8)
104.177(6)
95.163(6)
4160.9(10)
2
c, Å
R, deg
â, deg
γ, deg
V, Å3
Z
T, K
λ, Å
173(2)
0.71073
1.037
0.060
1002
123(2)
0.71073
1.103
0.067
1456
173(2)
0.71073
1.181
0.34
1592
173(2)
0.71069
1.176
0.22
640
123(2)
0.71073
1.042
0.062
2176
173(2)
0.71073
1.050
0.060
522
d
calcd, g cm-3
µ, mm-1
F(000)
Ra
0.063
0.163
0.077
0.171
0.066
0.179
0.042
0.109
0.062
0.148
0.061
0.159
b
Rw
2
2
2
a R ) Σ||Fo| - |Fc||/Σ|Fo| (I > 2.00σ(I)). b Rw ) {[Σw(Fo - Fc )2]/[ Σw(Fo )2]}1/2 (all data).
otherwise stated). Hydrogen atoms were included at geometrically
idealized positions (C-H bond distances 0.95 Å) and were not
refined. The NH protons in 6a, 9a, and 10a were initially located
in the difference Fourier map and then included at geometrically
idealized positions (N-H bond distances 0.95 Å) and were not
refined. The isotropic thermal parameters of the hydrogen atoms
were fixed at 1.2 times that of the preceding carbon or nitrogen
atom.
Thermal ellipsoids are shown at the 30% probability level and only
selected carbon atoms are labeled.
Results and Discussion
Syntheses and X-ray Structures of PhB[N(H)Dipp]2
(6a) and [Li2{PhB(NDipp)2}(THF)3] (7a). The usual route
to the synthesis of dilithio boraamidinate complexes {Li2-
[RB(NR′)2]}x is the dilithiation of the phenylbis(organoami-
no)boranes PhB[N(H)R′]2 with n-butyllithium.5b The bis-
amino derivative PhB[N(H)Dipp]2 (6a) can be prepared by
reaction of Li[N(H)Dipp] with PhBCl2 over a period of
18 h at room temperature (eq 2). An excess (4 equiv) of the
former reagent was necessary in order to drive this reaction
to completion. The structure of 6a was determined by a
single-crystal X-ray analysis, and the molecular geometry
and atomic numbering scheme are shown in Figure 1, while
pertinent structural parameters are summarized in Table 2.
Typically, organobis(organoamino)borane species exist as
oils;22a the only structurally characterized example is MeB-
[N(H)Me]2.22b As expected, the boron atom in 6a adopts a
trigonal planar geometry (Σ B ) 360.0°). The mean B-N
bond length is 1.417 Å (cf. 1.414 Å in MeB[N(H)Me]2).22b
The orientation of the bulky Dipp substituents is similar to
that of the N-Me groups in MeB[N(H)Me]2 and the phenyl
rings are rotated with respect to each other to minimize steric
interactions. The intermolecular N...H bridges observed in
The asymmetric unit of 6a is composed of two independent
molecules of C30H41BN2 [since data are very similar for both
molecules, only selected bond lengths and angles are included for
one of the molecules (Table 2) and only one molecule is depicted
in the thermal ellipsoid plot (Figure 1)] and a half molecule of
diethyl ether lying close to an inversion center that is disordered.
The disordered solvent molecule was refined with isotropic thermal
parameters. The H-atoms of the disordered solvate molecule were
ignored. Two carbon atoms [labeled as C(36), C(37), C(36′), and
C(37′)] in one of the three coordinating THF molecules in 7a were
disordered over two sites with refined site occupancy factors of
0.496(2) and 0.504(2), respectively. For 8a, the non-hydrogen atoms
of the In-complex were refined anisotropically, while the other
atoms were allowed isotropic thermal parameters. The structure
contained one ordered and two disordered half molecules of toluene
solvate and four disordered molecules of diethyl ether coordinated
to Li. Two of the ether molecules had full occupancy factors while
the other two molecules were located over three sites with partial
site occupancy factors. Two H-atoms of the disordered toluene
molecules were ignored. For 7b, there are two molecules of C30H51-
BLi2N2O2 in an asymmetric unit; since data are very similar for
both molecules, only selected bond lengths and angles are included
for one of the molecules (Table 6) and only one molecule is depicted
in the thermal ellipsoid plot (Figure 5). A diethyl ether molecule
[labeled as O(3), C(53), C(54), C(55), and C(56)] was disordered;
the smaller fraction of C-atoms of this molecule were refined with
isotropic thermal parameters.
22b
1
MeB[N(H)Me]2 are not observed in 6a. In the H NMR
spectrum of 6a, only one resonance is observed for both the
CH and CH3 groups of the Dipp substituents. These signals
(19) Farrugia, L. J. WinGX v1.64.05 2003: An Integrated System of
Windows Programs for the Solution, Refinement and Analysis of
Single-Crystal X-ray Diffraction Data. J. Appl. Crystallogr. 1999, 32,
837.
(20) SHELXTL-NT 5.1, XPREP, Program Library for Structure Solution
and Molecular Graphics; Bruker AXS, Inc.: Madison, WI, 1998.
(21) CorelDRAWTM 9; Corel Corporation: Ottawa, Ontario, Canada, 2000.
(22) (a) For an example, see: Burch, J. E.; Gerrard, W.; Mooney, E. F. J.
Chem. Soc. 1962, 2200. (b) Niederpruem, N.; Boese, R.; Schmid, G.
Z. Naturforsch., Teil B 1991, 46, 84.
Thermal ellipsoid plots for the crystal structures were generated
using the programs CAMERON19 and XP (part of the SHELXTL-
NT 5.120 program library) and then imported into CorelDRAW 9.21
(18) Sheldrick, G. M. SHELXL 97-2: Program for the Solution of Crystal
Structures; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
Inorganic Chemistry, Vol. 43, No. 8, 2004 2647

Chivers et al.
Figure 2. Thermal ellipsoid plot of [Li2{PhB(NDipp)2}(THF)3] (7a). For
clarity, H atoms are omitted and only the oxygen atoms of the THF
molecules are shown.
Table 3. Selected Bond Lengths and Bond Angles for
[Li2{PhB(NDipp)2}(THF)3] (7a)
Figure 1. Thermal ellipsoid plot of PhB[N(H)Dipp]2 (6a). For clarity, H
atoms are omitted except H1 on N1 and H2 on N2.
Bond Distances (Å)
B(1)-N(1)
B(1)-N(2)
B(1)-C(1)
N(1)-C(7)
N(1)-Li(2)
N(1)-Li(1)
N(2)-C(19)
1.415(5)
1.446(4)
1.599(5)
1.400(4)
2.115(6)
2.022(6)
1.390(4)
N(2)-Li(1)
N(2)-Li(2)
O(1)-Li(1)
O(2)-Li(2)
O(2)-Li(1)
O(3)-Li(2)
2.010(6)
2.049(7)
1.916(6)
2.013(6)
2.270(7)
1.922(7)
Table 2. Selected Bond Lengths and Bond Angles for
PhB[N(H)Dipp]2‚(C4H10O)0.25 (6a)
Bond Distances (Å)
B(1)-N(1)
N(1)-C(1)
B(1)-N(2)
1.420(3)
1.441(3)
1.414(3)
N(2)-C(13)
B(1)-C(25)
1.433(3)
1.576(3)
Bond Angles (deg)
Bond Angles (deg)
N(1)-B(1)-N(2)
N(1)-B(1)-C(1)
N(1)-Li(1)-O(2)
B(1)-N(1)-Li(2)
B(1)-N(2)-Li(1)
B(1)-N(2)-Li(2)
B(1)-N(1)-Li(1)
C(7)-N(1)-Li(1)
C(7)-N(1)-Li(2)
C(7)-N(1)-B(1)
C(19)-N(2)-Li(2)
C(19)-N(2)-B(1)
C(19)-N(2)-Li(1)
O(3)-Li(2)-N(2)
O(3)-Li(2)-O(2)
111.4(3)
125.2(3)
87.6(2)
77.5(2)
83.4(2)
N(2)-B(1)-C(1)
N(2)-Li(1)-N(1)
N(2)-Li(2)-N(1)
N(2)-Li(1)-O(2)
Li(1)-N(1)-Li(2)
Li(1)-N(2)-Li(2)
Li(2)-O(2)-Li(1)
O(1)-Li(1)-N(2)
O(1)-Li(1)-N(1)
O(1)-Li(1)-O(2)
O(2)-Li(2)-N(1)
O(2)-Li(2)-N(2)
O(3)-Li(2)-N(1)
122.9(3)
71.8(2)
69.1(2)
101.9(3)
70.1(2)
71.7(3)
67.1(2)
121.7(3)
163.0(4)
98.6(3)
92.2(3)
110.0(3)
137.3(3)
B(1)-N(1)-C(1)
B(1)-N(2)-C(13)
N(2)-B(1)-N(1)
124.6(2)
128.0(2)
118.5(2)
N(2)-B(1)-C(25)
N(1)-B(1)-C(25)
122.8(2)
118.7(2)
are very broad at ambient temperature, indicating that there
is rapid rotation of the N-aryl groups in solution, resulting
in the observed equivalence in the spectrum.
79.2(2)
83.7(3)
135.5(3)
135.9(3)
130.4(3)
148.1(3)
126.4(3)
124.4(3)
127.9(3)
111.5(3)
n
Addition of 2 equiv of BuLi to a solution of 6a in
n-hexane followed by an 18 h reflux provided yellow
amorphous Li2{PhB(NDipp)2} (7a) in a 79% yield (eq 3).
Dissolution of 7a in THF provided colorless crystals, which
were characterized by CHN analyses, multinuclear (1H, 11B,
7Li, and 13C) NMR spectroscopy, IR spectroscopy, and a
parameters are summarized in Table 3. In all previous
structures of dilithio derivatives of boraamidinates, the
fundamental building block is the Li2N2B unit A. In the case
1
single-crystal X-ray diffraction experiment. The H NMR
spectrum showed resonances for the Ph and Dipp substituents
with intensities indicative of symmetrically equivalent Dipp
substituents. Deprotonation was also indicated by the lack
of an N-H stretch in the IR spectrum.
n
t
of {Li2[RB(NtBu)2]}2 (R ) Ph, Bu, Bu),5b,8 two of these
units participate in a face-to-face interaction through lithium-
nitrogen contacts to give the bicapped cube B (R′ ) Bu).
t
In the unique example of a trimer {Li2[RB(NtBu)2]}3 (R )
Me),8 three Li2N2B units associate edge-on through lithium-
nitrogen contacts to give the tricapped hexagonal prism C.
The boraamidinate 7a is the first example of a monomeric
dilithio boraamidinate. Each Li+ ion is coordinated by one
terminal and one asymmetrically bridging THF molecule.
Since all previous boraamidinate structures involve unsol-
vated Li+ centers,5b,8 the monomeric structure of 7a may be
the result of solvation of the Li+ ions or the steric bulk of
the Dipp substituents on nitrogen (or a combination of both
effects). A monomeric structure similar to that of 7a has been
PhBCl2 + 2Li[N(H)Dipp] f PhB[N(H)Dipp]2 + 2LiCl
6a
(2)
PhB[N(H)Dipp]2 + 2nBuLi f
6a
Li2{PhB(NDipp)2} + 2nBuH (3)
7a
The molecular geometry and atomic numbering scheme
for 7a are shown in Figure 2, and pertinent structural
2648 Inorganic Chemistry, Vol. 43, No. 8, 2004

Dilithium Boraamidinates and Bidentate NBNCN Ligands
reported for {(THF)3Li2[PhAs(NtBu)2]} in which the smaller
tBu substituents are attached to the nitrogen atoms.23 Thus,
it seems likely that solvation accounts for the monomeric
structure of 7a.
Figure 3. Thermal ellipsoid plot of the anion in [Li(OEt2)4][In{PhB-
(NDipp)2}2] (8a). For clarity, H atoms are omitted.
Table 4. Selected Bond Lengths and Bond Angles for
[Li(OEt2)4][In{PhB(NDipp)2}2]‚(C7H8)2 (8a)
Bond Distances (Å)
In(1)-N(2)
In(1)-N(1)
In(1)-N(4)
In(1)-N(3)
2.123(4)
2.129(4)
2.132(3)
2.133(3)
N(1)-B(1)
N(2)-B(2)
N(3)-B(2)
N(4)-B(1)
1.437(7)
1.442(6)
1.427(6)
1.431(6)
The [PhB(NR′)2]2- framework in 7a chelates each of the
two lithium centers in an N,N′ manner. However, the B-N
bond lengths in the boraamidinate ligand in 7a differ by ca.
0.03 Å [B(1)-N(1) ) 1.415(5) Å; B(1)-N(2) ) 1.446(4)
Å]. This small difference may be a response to the asym-
metric bonding of the bridging THF molecules that, in turn,
may result from packing effects. In addition, the bite angle
[ N(1)-B(1)-N(2)] increases from 109.50(18)° in [Li2-
Bond Angles (deg)
91.3(3)
91.4(3)
90.9(3)
90.9(3)
125.3(4)
143.3(1)
67.1(1)
124.3(4)
127.0(1)
67.4(1)
141.8(2)
110.8(4)
124.9(4)
B(1)-N(1)-In(1)
B(1)-N(4)-In(1)
B(2)-N(2)-In(1)
B(2)-N(3)-In(1)
N(1)-B(1)-C(49)
N(1)-In(1)-N(3)
N(1)-In(1)-N(4)
N(2)-B(2)-C(55)
N(2)-In(1)-N(1)
N(2)-In(1)-N(3)
N(2)-In(1)-N(4)
N(3)-B(2)-N(2)
N(3)-B(2)-C(55)
N(4)-B(1)-N(1)
N(4)-B(1)-C(49)
N(4)-In(1)-N(3)
C(1)-N(1)-B(1)
C(1)-N(1)-In(1)
C(13)-N(2)-B(2)
C(13)-N(2)-In(1)
C(25)-N(3)-B(2)
C(25)-N(3)-In(1)
C(37)-N(4)-B(1)
C(37)-N(4)-In(1)
110.3(4)
124.4(4)
125.7(1)
131.0(4)
135.1(3)
129.5(4)
134.5(3)
131.3(4)
135.3(3)
132.7(4)
134.2(3)
{PhB(NtBu)2}]2 to 111.4(3)° in 7a, presumably in order
5b
to accommodate the bulkier -NDipp groups. Least-squares
planes24 were calculated for the two four-membered rings
containing N1-B1-Li1-N2 (plane 1) and N1-B1-Li2-
N2 (plane 2), which were found to be puckered rings with
the two nitrogen atoms below the plane while the boron and
lithium atoms reside above the plane [deviations (Å) from
the least-squares planes: plane 1 ) -0.154(2) N1, 0.212(3)
B1, 0.094(1) Li1, -0.152(2) N2; plane 2 ) -0.223(2) N1,
0.331(3) B1, 0.115(1) Li2, -0.224(2) N2]. Furthermore, the
two phenyl rings of the Dipp substituents are offset from
one another with an angle between the two planes [least-
squares planes25 calculated for plane 3: C7-C12 and for
plane 4: C19-24] of 33.8(1)°. In addition, in contrast to
the previously reported structures of {Li2[RB(NtBu)2]}2
amidinate anions [RC(NR′)2]- has revealed novel structural
chemistry for In26 and catalytic activity for cationic Al or
Ga complexes.27 Recently, we reported the first boraamidi-
nate complexes of group 13 elements including [In{PhB-
(NtBu)2}2Li(OEt2)] in which the spirocyclic anion [In-
{PhB(NtBu)2}2]- is N,N′-chelated to a monosolvated lithium
cation.5a The reaction of Li2[PhB(NDipp)2] (7a) with InCl3
in a 2:1 stoichiometry also gives a spirocyclic compound
[Li(OEt2)4][In{PhB(NDipp)2}2] (8a). However, the X-ray
structural analysis reveals that this is a solvent-separated ion
pair.
n
t
8
(R ) Ph, Bu, Bu)5b,8 and {Li2[MeB(NtBu)2]}3 there are
The molecular geometry and atomic numbering scheme
for 8a are shown in Figure 3, and pertinent structural
parameters are summarized in Table 4. The spirocyclic anion
[In{PhB(NDipp)2}2]- is presumably stabilized by the steric
bulk provided by the four Dipp substituents attached to the
nitrogen atoms, while the lithium cation is solvated by four
no C(-H)‚‚‚Li agostic interactions in 7a.
Synthesis and X-ray Structure of [Li(OEt2)4][In{PhB-
(NDipp)2}2] (8a). Recent work on group 13 complexes of
(23) Briand, G. G.; Chivers, T.; Parvez, M. J. Chem. Soc., Dalton Trans.
2002, 3785.
(24) x, y, z in crystal coordinates: plane 1 (Å) ) -1.92(3)x + 4.74(5)y +
17.61(2)z ) 6.17(2); plane 2 (Å) ) -6.68(2)x + 10.50(4)y - 10.14-
(5)z ) 0.62(3).
(25) x, y, z in crystal coordinates: plane 3 (Å) ) -6.28(1)x + 15.63(2)y
+ 6.00(2)z ) 6.593(6); plane 4 (Å) ) -8.176(9)x + 12.34(2)y -
3.48(3)z ) 2.63(2).
(26) (a) Zhou, Y.; Richeson, D. S. Inorg. Chem. 1996, 35, 1423. (b) Zhou,
Y.; Richeson, D. S. Inorg. Chem. 1996, 35, 2448.
(27) Dagome, S.; Guzei, I. A.; Coles, M. P.; Jordan, R. F. J. Am. Chem.
Soc. 2000, 122, 274 and references therein.
Inorganic Chemistry, Vol. 43, No. 8, 2004 2649

Chivers et al.
Table 5. Selected Bond Lengths and Bond Angles for
PhBCl[N(H)Dipp] (9a)
Bond Distances (Å)
B(1)-N(1)
B(1)-C(1)
1.386(3)
1.559(3)
N(1)-C(7)
B(1)-Cl(1)
1.447(2)
1.798(2)
Bond Angles (deg)
B(1)-N(1)-C(7)
N(1)-B(1)-C(1)
128.0(2)
126.0(2)
N(1)-B(1)-Cl(1)
C(1)-B(1)-Cl(1)
115.9(2)
118.1(2)
to the electronegative chlorine atom attached to the central
boron atom, which enhances B-N π-bonding. The boron-
chlorine bond length in 9a [B(1)-Cl(1) ) 1.798(2) Å] is
comparable to those in Me2N-N[B(Mes)Cl]2 [B(1)-Cl(1)
) 1.843(4) Å; B(1)-Cl(2) ) 1.796(5) Å].30
In view of the possible application of 9a and related
derivatives in the preparation of new ligand systems, it was
desirable to have a high-yield synthesis. This is achieved
through the reaction of 2 equiv of DippNH2 with phenyldi-
chloroborane, which produces 9a in 93% yield (eq 4).
Figure 4. Thermal ellipsoid plot of PhBCl[N(H)Dipp] (9a). For clarity,
H atoms are omitted except H1 on N1.
PhBCl2 + 2DippNH2 f
PhBCl[N(H)Dipp] + [DippNH3]Cl (4)
molecules of diethyl ether. The boron atoms in 8a adopt a
distorted trigonal planar geometry (Σ B ) 360.0°) with bond
angles N-B-N of 110.3(4) and 110.8(4)°. In the contact
ion-pair [In{PhB(NtBu)2}2Li(OEt2)], there is a significant
difference (ca. 0.13 Å) in the two B-N bond distances in
the planar, four-membered BN2In rings, which is related to
the different coordination numbers of the nitrogen atoms in
this complex. Consistently, the B-N bond distances in 8a,
in which all nitrogen atoms are three-coordinate, are ap-
proximately equal. The bond angles at the central In atom
in 8a deviate markedly from tetrahedral (range ca. 67-141°).
The reaction of 7a with GaCl3 (2:1 stoichiometry) gave a
similar product [Li(OEt2)x][Ga{PhB(NDipp)2}2] (8b), which
9a
Synthesis and X-ray Structure of [Li2{PhB(NDipp)-
(NtBu)}(OEt2)2] (7b). With a direct method to the reagent
PhBCl[N(H)Dipp] available, it was possible to prepare
[Li2{PhB(NDipp)(NtBu)}(OEt2)2] (7b), the first boraamidi-
nate with different substituents attached to the two nitrogen
atoms. The borane PhB[N(H)Dipp][N(H)tBu] (6b) was
prepared by the reaction of an excess of Li[N(H)tBu] with
1
PhBCl[N(H)Dipp]. The H NMR spectrum of 6b exhibits
one septet for the two equivalent methine hydrogens and a
pair of doublets of equal intensity for the diastereotopically
i
inequivalent Pr methyl pairs consistent with a Cs structure
7
has been characterized by multinuclear (1H, 11B, Li, and
n
in solution. The reaction of 2 equiv of BuLi with 6b in
13C) NMR spectroscopy. Integration of the resonances due
to the diethyl ether molecules (with respect to those of the
alkyl and aryl of the anion moiety) in the 1H NMR spectrum
indicates x ) 4.
boiling n-hexane solution provided off-white amorphous
Li2{PhB(NDipp)(NtBu)}. Dissolution of Li2{PhB(NDipp)-
(NtBu)} in diethyl ether yielded colorless crystals of 7b,
which were characterized by CHN analyses, multinuclear
(1H, 11B, Li, and 13C) NMR spectroscopy, and a single-
7
Synthesis and X-ray Structure of PhBCl[N(H)Dipp]
(9a). During the synthesis of PhB[N(H)Dipp]2 (6a) (eq 2),
it was found that incomplete reaction yields the monoamido
derivative PhBCl[N(H)Dipp] (9a). The bis(amino)borane 6a
is isolated as a viscous oil that slowly crystallizes over
24 h. When incomplete reaction occurs, the monosubstituted
product 9a crystallizes first and can be separated from 6a.
Crystals of 9a were isolated and characterized by CHN
analysis and multinuclear (1H, 11B, and 13C) NMR spectros-
copy. As expected, successive replacement of the chlorine
atoms on the central boron atom by organoamino groups
causes an upfield shift of the 11B NMR resonance {PhBCl2:
δ 63; PhBCl[N(H)Dipp]: δ 40; PhB[N(H)Dipp]2: δ 29}.
The solid-state structure of 9a was determined by X-ray
crystallography. The molecular geometry and atomic num-
bering scheme are shown in Figure 4, and pertinent structural
parameters are summarized in Table 5. The boron-nitrogen
bond length of 1.386(3) Å is significantly shorter than those
of typical triaminoboranes28 [cf. B-N ) 1.432 Å in tris(2-
pyridylamino)borane29]. This shortening is most likely due
crystal X-ray diffraction experiment. The 1H NMR spectrum
t
showed resonances for the Ph, Bu, and Dipp substituents
with the appropriate relative intensities. Two separate meth-
ine signals are observed in the spectrum and four doublets
for the iPr methyl groups, reflecting both the lower symmetry
of the molecule and hindered rotation of the N-aryl groups
in solution. Deprotonation was indicated by the lack of an
N-H stretch in the IR spectrum.
The molecular geometry and atomic numbering scheme
for 7b are shown in Figure 5, and pertinent structural
parameters are summarized in Table 6. The [PhB(NDipp)-
(NtBu)]2- framework in 7b chelates each of the two lithium
atoms in an N,N′ manner with each lithium center solvated
by one molecule of diethyl ether. As in the case of 7a, this
solvation prevents further association. The boron atom in
(28) No¨th, H. Z. Naturforsch., Teil B 1983, 38, 692.
(29) Braun, U.; Habereder, T.; No¨th, H.; Piotrowski, H.; Warchhold, M.
Eur. J. Inorg. Chem. 2002, 1132.
(30) Diemer, S.; No¨th, H.; Storch, W. Eur. J. Inorg. Chem. 1999, 1765.
2650 Inorganic Chemistry, Vol. 43, No. 8, 2004

Dilithium Boraamidinates and Bidentate NBNCN Ligands
Figure 5. Thermal ellipsoid plot of [Li2{PhB(NDipp)(NtBu)}(OEt2)2] (7b).
For clarity, H atoms are omitted and only the oxygen atoms of the OEt2
molecules are shown.
Figure 6. Thermal ellipsoid plot of {[DippN(H)](Ph)B(µ-NtBu)2 CnBu}
(10a). For clarity, H atoms are omitted except H1 on N1.
Table 6. Selected Bond Lengths and Bond Angles for
[Li2{PhB(NDipp)(NtBu)}(OEt2)2] (7b)
Table 7. Selected Bond Lengths and Bond Angles for
{[DippN(H)](Ph)B(µ-NtBu)2CnBu}(10a)
Bond Distances (Å)
B(1)-N(1)
B(1)-N(2)
B(1)-C(17)
N(1)-Li(1)
N(1)-Li(2)
N(1)-C(1)
1.428(3)
1.451(3)
1.601(3)
1.988(4)
1.985(5)
1.468(3)
N(2)-Li(1)
N(2)-Li(2)
N(2)-C(5)
O(1)-Li(1)
O(2)-Li(2)
2.010(4)
2.003(4)
1.397(3)
1.921(4)
1.937(4)
Bond Distances (Å)
B(1)-N(3)
B(1)-N(2)
B(1)-N(1)
B(1)-C(26)
N(3)-C(1)
1.633(5)
1.606(5)
1.499(5)
1.618(5)
1.330(4)
N(3)-C(22)
N(2)-C(18)
N(2)-C(1)
N(1)-C(6)
1.481(5)
1.487(4)
1.336(5)
1.424(5)
Bond Angles (deg)
Bond Angles (deg)
B(1)-N(1)-C(1)
B(1)-N(1)-Li(2)
B(1)-N(1)-Li(1)
B(1)-N(2)-C(5)
B(1)-N(2)-Li(1)
B(1)-N(2)-Li(2)
N(1)-B(1)-N(2)
N(1)-B(1)-C(17)
128.1(2)
79.3(2)
79.5(2)
125.3(2)
78.2(2)
78.2(2)
109.9(2)
130.3(2)
N(2)-B(1)-C(17)
C(1)-N(1)-Li(2)
C(1)-N(1)-Li(1)
C(5)-N(2)-Li(2)
C(5)-N(2)-Li(1)
Li(2)-N(1)-Li(1)
Li(2)-N(2)-Li(1)
119.7(2)
130.2(2)
139.1(2)
140.2(2)
133.6(2)
78.7(2)
C(1)-N(3)-B(1)
C(1)-N(2)-B(1)
C(6)-N(1)-B(1)
N(3)-C(1)-N(2)
89.4(3)
90.3(2)
128.3(3)
101.2(3)
N(2)-B(1)-N(3)
N(1)-B(1)-N(2)
N(1)-B(1)-N(3)
N(1)-B(1)-C(26)
79.0(2)
117.2(3)
118.7(3)
112.5(3)
[ν(N-H)]), along with the equivalence of the NtBu groups
(δ 1.03). The 11B NMR chemical shift (δ 5.3 ppm) is
consistent with a tetracoordinate boron environment, sug-
gesting a cyclic structure.31
77.8(2)
An X-ray structural analysis confirmed that 10a contains
a four-membered BNCN ring. The molecular geometry and
atomic numbering scheme are shown in Figure 6, and
selected bond lengths and bond angles are summarized in
Table 7. The heterocyclic ring is essentially planar with a
torsion angle [B(1)-N(3)-C(1)-N(2)] of 3.0(3)°. The C-N
bond distances [1.330(4) and 1.336(5) Å] are almost equal,
approximately intermediate between C-N double-bond and
C-N(sp2) single-bond values. The sums of the bond angles
at the three-coordinate N and C centers are ca. 360°. These
structural features indicate delocalized bonding in the NCN
unit with approximately equal contributions from the two
canonical forms D and E.
7b adopts a distorted trigonal planar geometry (Σ B )
359.9°) with a bond angle N-B-N of 109.9(2)°. The B-N
bond lengths in 7b differ by 0.023 Å with the longer bond
[B(1)-N(2) ) 1.451(3) Å] attributed to the steric influence
of the bulkier Dipp substituent on N(2). There are no
C(-H)‚‚‚Li agostic interactions in the structure of 7b.
Synthetic Approaches to Hybrid Boraamidinate/
Amidinate (bamam) Ligands. Two approaches to the
synthesis of the novel bamam ligands 2 were investigated.
The first makes use of the reaction of lithium amidinates
with organoamino(chloro)phenyl boranes (9) (Scheme 1).
The reaction of PhBCl[N(H)Dipp] (9a) and Li{nBuC(NtBu)2}
(4a) in a 1:1 stoichiometry in n-hexane produced {[DippN-
(H)](Ph)B(µ-NtBu)2CnBu} (10a) in 88% yield. The product
was characterized by a single-crystal X-ray diffraction
experiment in addition to CHN analysis, multinuclear (1H,
Consistently, the endocyclic B-N bond lengths are also
approximately equal [1.633(5) and 1.606(5) Å], but are
1
11B, and 13C) NMR spectra, and IR spectroscopy. The H
(31) No¨th, H.; Wrackmeyer, B. In Nuclear Magnetic Resonance Spectros-
copy of Boron Compounds; Springer-Verlag: Berlin, 1978; pp 74-
101.
NMR spectrum indicates the presence of the amino proton
(δ 4.13), which is also supported by IR analysis (3420 cm-1
Inorganic Chemistry, Vol. 43, No. 8, 2004 2651

Chivers et al.
Scheme 1. Syntheses of Hybrid Boraamidinate/Amidinate (BAMAM) Ligands 2.
significantly longer than those in {Cl(Ph)B(µ-NtBu)2CnBu}
(5) [1.567(6) and 1.590(6) Å]15 and fall just outside the range
of 1.55-1.61 Å found for B-N bonds involving four-
coordinate boron linked to three-coordinate nitrogen.32 These
structural features indicate that the [DippN(H)]PhB unit in
10a is coordinated more weakly by the amidinate ligand than
the (Cl)PhB unit in 5,15 likely as a result of stronger
π-donation from the exocyclic [DippN(H)] group to boron
in 10a. The exocyclic B-N bond length is shorter [ca. 0.12
Å] than the mean endocyclic B-N length and is slightly
shorter [by ca. 0.051 Å] than the mean B-N bond distances
involving four-coordinate boron linked to three-coordinate
nitrogen.
indicates that the three-coordinate form represents ca. 90%
of the mixture. Although a comparison of 10b and 11b
suggests that the steric influence of the two Dipp substituents
induces an acyclic structure, steric effects cannot explain the
formation of an acyclic arrangement in 11a, which has
cyclohexyl groups attached to both nitrogens. Attempts to
monolithiate 11a and 11b have been unsuccessful.
The second synthetic approach to bamam ligands involves
the nucleophilic replacement of the exocyclic chloro sub-
stituent in {Cl(Ph)B(µ-NtBu)2CnBu} (5) by an organoamino
group followed by deprotonation with n-butyllithium (Scheme
1). The reaction of 5 with Li[N(H)Dipp] in a 1:1 molar ratio
was unsuccessful even in boiling THF. When a 1:2 molar
ratio was employed, however, a mixture of {[DippN(H)]-
(Ph)B(µ-NtBu)2CnBu} (10a) and Li[N(H)Dipp] was obtained
according to multinuclear (1H, 11B, and 13C) NMR analyses,
but this mixture could not be separated. When the analogous
reaction of 5 with Li[N(H)tBu] was carried out in a 1:2 molar
ratio at room temperature, {[tBuN(H)](Ph)B(µ-NtBu)2CnBu}-
(10b) was obtained in 51% yield. The derivative 10b was
characterized by CHN and multinuclear (1H, 13C, and 11B)
NMR analyses, which indicated a four-coordinate boron
The monoamino borane PhBCl[N(H)tBu] (9b) was ob-
tained in 93% yield from the reaction of PhBCl2 and 2 equiv
of tert-butylamine and characterized by CHN elemental
analyses and 1H, 11B, and 13C NMR spectroscopy. Attempts
to make the related ring system 10b by the reaction of 9b
with Li{nBuC(NtBu)2} (4a) resulted in incomplete reaction,
1
even after prolonged reflux according to H and 11B NMR
spectra of the reaction mixture. Subsequently, 10b was
prepared using a different synthetic approach (vide infra).
The reactions of PhBCl[N(H)Dipp] (9a) and Li{tBuC-
(NCy)2} (4b) or Li{nBuC(NDipp)2} (4c) in a 1:1 stoichi-
ometry in n-hexane proceed to completion to produce 11a
and 11b in 89% and 76% yields, respectively (Scheme 1).
In contrast to the observations for 10a, the 11B NMR spectra
for 11a and 11b exhibit a major, broad resonance at ca. 30-
32 ppm, indicative of a three-coordinate boron31 (i.e., an
acyclic structure as depicted in Scheme 1), and a sharp, minor
resonance at ca. 1.5 ppm attributable to the cyclic isomer,
cf. 10a. Variable temperature 11B NMR spectra of 11a in
C6D6 in the range 280-342 K show that the broad resonance
at 30 ppm grows at the expense of the sharp singlet at 1.5
ppm as the temperature is increased. We infer that, in C6D6,
there is an equilibrium between the acyclic and cyclic forms
of 11a and that, as expected, the former is increasingly
favored at high temperatures. At room temperature, the
integration of the resonances in the 11B NMR spectra
n
t
center and exhibited the expected resonances for Bu, Bu,
and Ph groups with the appropriate relative intensities.
Interestingly, when the reaction of 5 with Li[N(H)tBu] was
carried out in a 1:2 molar ratio under reflux in THF, the
known amidinate complex {(Li[nBuC(NtBu)2])2‚LiCl‚THF}
1
was formed and identified by comparison of the H NMR
spectrum and unit cell parameters with the literature data.33
1
The other product was identified by H NMR spectroscopy
data as the bis(amino)borane PhB[N(H)tBu]2.5b Thus, it
appears that, under these reaction conditions, the second
equivalent of Li[N(H)tBu] acts as a nucleophile rather than
a base toward {[tBuN(H)](Ph)B(µ-NtBu)2CnBu} (10b) to
generate the observed products (Scheme 2). This conclusion
was confirmed by a separate reaction in which treatment of
1
10b with Li[N(H)tBu] in THF was shown by H NMR to
produce the known products (Li[nBuC(NtBu)2])2 (4a)14a and
PhB[N(H)tBu]2.5b
(32) Allen, F. H.; Kennard, O.; Watson, D. G.; Brammer, L.; Orpen, A.
G.; Taylor, R. J. Chem. Soc., Perkin Trans. 2 1987, S1.
(33) Chivers, T.; Downard, A.; Parvez, M. Inorg. Chem. 1999, 38, 4347.
2652 Inorganic Chemistry, Vol. 43, No. 8, 2004

Dilithium Boraamidinates and Bidentate NBNCN Ligands
t
Scheme 2. Displacement of the Amidinate Ligand in 5 by a BuNH
Group.
or yield of the product. Crystals of 12a were obtained from
THF, but these crystals are unstable in the absence of solvent
and attempts to mount them on the diffractometer have been
unsuccessful. Nevertheless, the NMR data indicate that the
bamam ligand is N(R′),N(R′′′)-chelated to Li as expected.
Conclusions
The first monomeric dilithium boraamidinates have been
prepared and structurally characterized. Unlike previously
reported dilithio boraamidinates, the lithium ions in these
new derivatives are solvated and this structural feature, rather
than the bulkiness of the Dipp substituent, may account for
the monomeric structures. The preparation of hybrid bora-
amidinate/amidinate ligands by two different routes has
revealed that either four-membered BNCN rings, with an
exocyclic amido (NHR) substituent, or an acyclic NBNCN
chelating ligand are formed depending on the substituents
attached to the nitrogen or carbon atoms. These structural
Treatment of 10a with n-butyllithium produces the lithiated
bamam ligand {Li[DippN]PhB(NtBu)CnBu(NtBu)} (12a) in
a 62% yield. The product has been characterized by CHN
elemental analyses, and multinuclear (1H, 13C, 11B, and 7Li)
NMR and IR spectroscopy. Monolithiation was indicated by
isomers are readily distinguished by their characteristic 11
B
NMR chemical shifts, which fall in the range δ 0-5 for the
cyclic systems (four-coordinate boron) and δ 30-35 for the
acyclic ligands (three-coordinate boron). Monolithiation of
the exocyclic amido substituent on the ring system in
{[DippN(H)](Ph)B(µ-NtBu)2CnBu} provides the alkali de-
rivative {Li[DippN]PhB(NtBu)CnBu(NtBu)}. Future studies
will focus on magnesium complexes of this ligand.
7
the lack of an N-H stretch in the IR spectrum and the Li
NMR resonance at δ 0.13. Deprotonation promotes opening
of the four-membered BN2C ring in 10a as indicated by 11
B
1
NMR chemical shifts (δ 29 for 12a vs δ 5 for 10a) and H
NMR data (inequivalence of the NtBu groups in 12a: δ 1.40
and 0.99 vs equivalent NtBu groups in 10a: δ 1.03). The
NMR spectra reveal that 12a can be isolated in a reasonably
pure form with 10a as a minor (<8%) impurity. Variation
of the reaction temperature, reaction time, or purification
method of the isolated product has little effect on the purity
Supporting Information Available: X-ray crystallographic
information in CIF format. This material is available free of charge
via the Internet at http://pubs.acs.org.
IC0352755
Inorganic Chemistry, Vol. 43, No. 8, 2004 2653