New bonding modes for boraamidinate ligands in heavy group 15 complexes: Fluxional behavior of the 1:2 complexes, LiM[PhB(NtBu) 2]2 (M = As, Sb, Bi)

Article abstract of DOI:10.1021/ic0622757

The reactions of MCl₃ with Li₂[PhB(N ^tBu)₂] in 1:1, 1:1.5, and 1:2 molar ratios in diethyl ether produced the monoboraamidinates ClM[PhB(N^tBu)₂] (1a, M = As; 1b, M = Sb; 1c, M = Bi), the novel 2:3 boraamidinate complexes [PhB(N^tBu)₂]M-μ-N(^tBu)B(Ph)N( ^tBu)M[PhB(N^tBu)₂] (2b, M = Sb; 2c, M = Bi), and the bisboraamidinates LiM[PhB(N^tBu)₂]₂ (3a, 3a·OEt₂, M = As; 3b, M = Sb; 3c·OEt₂, M = Bi), respectively. The 2:3 complexes 2b and 2c were also observed in the reactions carried out in a 1:2 molar ratio at room temperature. All complexes have been characterized by multinuclear NMR spectroscopy (¹H, ⁷Li, ¹¹B, and ¹³C) and by single-crystal X-ray structural determinations. The molecular units of the mono-boraamidinates 1a-c are isostructural, but their crystal packing is distinct as a result of stronger intermolecular close contacts going from 1a to 1c. In the novel 2:3 bam complexes 2b and 2c, each metal center is N,N′-chelated by a bam ligand and these two [M(bam)]⁺ units are bridged by the third [bam] ^2- ligand. The structures of the unsolvated bis-boraaminidate complexes 3a and 3b consist of [Li(bam)]^- and [M(bam)]⁺ monomeric units linked by Li-N and M-N bonds to give a tricyclic structure. Solvation of the Li⁺ ion by diethyl ether results in a bicyclic structure composed of four-membered BN₂As and six-membered BN ₃AsLi rings in 3a·OEt₂. In contrast, the analogous bismuth complex 3c·OEt₂ exhibits a tetracyclic structure. Variable-temperature NMR studies reveal that the nature of the fluxional behavior of 3a-c in solution is dependent on the group 15 center.

Full text of DOI:10.1021/ic0622757

Inorg. Chem. 2007, 46, 2627−2636
New Bonding Modes for Boraamidinate Ligands in Heavy Group 15
Complexes: Fluxional Behavior of the 1:2 Complexes, LiM[PhB(NtBu)2]2
(M As, Sb, Bi)
)
Jari Konu, Maravanji S. Balakrishna,‡ Tristram Chivers,*, and Thomas W. Swaddle
Department of Chemistry, UniVersity of Calgary, Calgary, Alberta, Canada T2N 1N4
Received November 30, 2006
The reactions of MCl3 with Li2[PhB(NtBu)2] in 1:1, 1:1.5, and 1:2 molar ratios in diethyl ether produced the
monoboraamidinates ClM[PhB(NtBu)2] (1a, M
As; 1b, M Sb; 1c, M Bi), the novel 2:3 boraamidinate complexes
[PhB(NtBu)2]M- -N(tBu)B(Ph)N(tBu)M[PhB(NtBu)2] (2b, M
Sb; 2c, M Bi), and the bisboraamidinates LiM[PhB-
(NtBu)2]2 (3a, 3a
OEt2, M As; 3b, M Sb; 3c OEt2, M Bi), respectively. The 2:3 complexes 2b and 2c were
)
)
)
µ
)
)
‚
)
)
‚
)
also observed in the reactions carried out in a 1:2 molar ratio at room temperature. All complexes have been
characterized by multinuclear NMR spectroscopy (1H, 7Li, 11B, and 13C) and by single-crystal X-ray structural
determinations. The molecular units of the mono-boraamidinates 1a−c are isostructural, but their crystal packing
is distinct as a result of stronger intermolecular close contacts going from 1a to 1c. In the novel 2:3 bam complexes
2b and 2c, each metal center is N,N
′
-chelated by a bam ligand and these two [M(bam)]+ units are bridged by the
-
third [bam]2 ligand. The structures of the unsolvated bis-boraaminidate complexes 3a and 3b consist of [Li(bam)]-
and [M(bam)]+ monomeric units linked by Li
by diethyl ether results in a bicyclic structure composed of four-membered BN2As and six-membered BN3AsLi
rings in 3a OEt2. In contrast, the analogous bismuth complex 3c OEt2 exhibits a tetracyclic structure. Variable-
−N and M−
N bonds to give a tricyclic structure. Solvation of the Li+ ion
‚
‚
temperature NMR studies reveal that the nature of the fluxional behavior of 3a−c in solution is dependent on the
group 15 center.
Introduction
The dianionic boraamidinate (bam) ligand is formally
isoelectronic with the extensively studied monoanionic
amidinate (am) ligand. The first bam metal complex, a
dimeric tin(II) boraamidinate, was reported in 1979. It was
obtained in low yield from the reaction of Li[N(BMe2)-
(SiMe3)] with SnCl2.1 Dilithio boraamidinates were intro-
duced as reagents for the synthesis of metal complexes in
1990.2,3 Although the use of these reagents in metathesis
represents by far the most general and versatile route to a
wide range of main group and transition metal systems, their
solid-state structures were not established until 2000.4
Recent work on early main group and transition-metal
complexes has established that the 2- charge on the bam
ligand results in some interesting fundamental differences
compared to behavior of the monoanionic am ligand. One
intriguing consequence is the facile tendency for redox
transformations involving the oxidation of the [bam]2-
dianion to the corresponding monoanion radical [bam]-•. This
paramagnetic ligand can be stabilized through complexation
to early main group metals.5,6 In some cases it has been
possible to isolate stable neutral radicals, for example the
* To whom correspondence should be addressed. Telephone: (403) 220-
5741. Fax: (403) 289-9488. E-mail: chivers@ucalgary.ca.
‡ Permanent address: Department of Chemistry, Indian Institute of
Technology, Bombay, Mumbai, India 400 076.
(1) Fusstetter, H.; No¨th, H. Chem. Ber. 1979, 112, 3672.
(2) Heine, A.; Fest, D.; Stalke, D.; Habben, C. D.; Meller, A.; Sheldrick,
G. M J. Chem. Soc., Chem. Commun. 1990, 742.
(3) Fest, D.; Habben, C. D.; Meller, A.; Sheldrick, G. M.; Stalke, D.;
Pauer, F. Chem. Ber. 1990, 123, 703.
(4) Brask, J. K.; Chivers, T.; Schatte, G. Chem. Commun. 2000, 1805.
(5) Chivers, T.; Eisler, D. J.; Fedorchuk, C.; Schatte, G.; Tuononen, H.
M.; Boere´, R. T. Chem. Commun. 2005, 3930.
(6) Chivers, T.; Eisler, D. J.; Fedorchuk, C.; Schatte, G.; Tuononen, H.
M.; Boere´, R. T. Inorg. Chem. 2006, 45, 2119.
10.1021/ic0622757 CCC: $37.00
Published on Web 02/28/2007
© 2007 American Chemical Society
Inorganic Chemistry, Vol. 46, No. 7, 2007 2627

Konu et al.
Scheme 1. Bonding Modes for the Different Classes of bam
all of which fall under the category of mono-boraamidinates
that adopt bonding mode A. The purpose of this investigation
was to explore the coordination behavior of the bam ligand
in complexes of the heavier group 15 elements and,
especially, to assess the stereochemical influence of the lone
pair on the main group metal center in the structures of the
bis-bam complexes Li[M(bam)2] (M ) As, Sb, Bi). In this
paper, we describe the syntheses, solid-state structures, and
spectroscopic characterization of the mono-boraamidinates
ClM[PhB(NtBu)2] (1a, M ) As; 1b, M ) Sb; 1c, M ) Bi),
the 2:3 bam complexes [PhB(NtBu)2]M-µ-N(tBu)B(Ph)N(t-
Bu)-M[PhB(NtBu)2] (2b, M ) Sb; 2c, M ) Bi), and the
bis-boraamidinates LiM[PhB(NtBu)2]2 (3a, 3a‚OEt2, M )
As; 3b, M ) Sb; 3c‚OEt2, M ) Bi). The nature of the
fluxional behavior of 3a-c in solution has been probed by
variable-temperature NMR studies.
Complexes8 a
Experimental Section
General Procedures. All reactions and the manipulations of
products were performed under an argon atmosphere using standard
Schlenk techniques or in an inert atmosphere glove box. The
compounds PhBCl2 (Aldrich, 97%), AsCl3 (Alfa, 99%), and BiCl3
(Strem, 99.9%) were used as received. SbCl3 (Aldrich, 99%) was
sublimed prior to use. LiN(H)tBu was prepared by the addition of
a Substituents on N are omitted for clarity.
t
nBuLi to a solution of anhydrous BuNH2 in n-hexane at -10 °C,
spirocyclic systems {M[PhB(µ-NtBu)2]2}• (M ) Al, Ga).5,6
The 2- charge also lowers the requirement for ancillary
ligands around high oxidation state metal centers. Conse-
quently, homoleptic species of the type [ML2]x- (x ) 0, 1,
2) and [ML3]2- (L ) bam) are common features of main
group and transition metal complexes.7-9
The versatile coordination ability of the bam ligand is
illustrated by the diversity of complexes that have been
established to date for this bidentate ligand (Scheme 1). The
N,N′-chelating mode, in which both nitrogen centers are
three-coordinate, is observed for homoleptic mono-, bis- and
tris-boraamidinates (A, C, and D). In the case of mono-
boraamidinates, dimerization may occur with the formation
of one four-coordinate nitrogen center (B). The bam ligand
may also function in a bridging mode between two metal
centers with both nitrogen atoms four-coordinate (E). Also,
two bam ligands, in which both nitrogens are three-
coordinate,
may bridge two metal centers (F). This arrangement can
be considered to be an alternative to the dimeric structure
B. Finally, it has recently been shown that three N,N′-
chelating bam ligands may also bridge an MtM unit (G,
M ) Mo, W).10
A survey of the literature of bam complexes of p-block
elements reveals that most attention has been accorded to
groups 13,5,9,11 14,1-3,12-16 and 16.17-19 For the group 15
elements, only phosphorus derivatives have been reported,20,21
and its purity was checked by 1H NMR spectroscopy. The
compounds PhB[N(H)tBu]2 and Li2[PhB(NtBu)2] were prepared
using literature methods.21 The solvents n-hexane, toluene, Et2O,
and THF were dried by distillation over Na/benzophenone under a
nitrogen atmosphere prior to use.
Spectroscopic Methods. The 1H, 7Li, 11B, and 13C NMR spectra
were obtained in THF-d8 or toluene-d8 at 23 °C on a Bruker DRX
400 spectrometer operating at 399.59, 155.30, 128.20, and 100.49
MHz, respectively. 1H and 13C spectra are referenced to the solvent
signal, and the chemical shifts are reported relative to (CH3)4Si.
7Li and 11B NMR spectra are referenced externally, and the chemical
shifts are reported relative to a 1.0 M solution of LiCl in D2O and
to a solution of BF3‚Et2O in C6D6, respectively.
Elemental analyses were performed by Analytical Services,
Department of Chemistry, University of Calgary, Calgary, Alberta,
and by Canadian Microanalytical Service Ltd., Vancouver, British
Columbia.
X-ray Crystallography. Crystals of ClM[PhB(NtBu)2] (1a, M
) As; 1b, M ) Sb; 1c, M ) Bi), [PhB(NtBu)2]M-µ-N(tBu)B(Ph)N-
(tBu)M[PhB(NtBu)2] (2b, M ) Sb; 2c, MdBi) and LiM[PhB(Nt-
(12) Paetzold, P.; Hahnfeld, D.; Englert, U.; Wojnowski, W.; Dreczewski,
B.; Pawelec, Z.; Walz, L. Chem. Ber. 1992, 125, 1073.
(13) Habben, C. D.; Heine, A.; Sheldrick, G. M.; Stalke, D. Z. Naturforsch.
1992, 47b, 1367.
(14) Geschwentner, M.; Noltemeyer, M.; Elter, G.; Meller, A. Z. Anorg.
Allg. Chem. 1994, 620, 1403.
(15) Luthin, W.; Stratman, J.-G.; Elter, G.; Meller, A.; Heine, A.; Gornitzka,
H. Z. Anorg. Allg. Chem. 1995, 621, 1995.
(16) Albrecht, T.; Elter, G.; Noltemeyer, M.; Meller, A. Z. Anorg. Allg.
Chem. 1998, 624, 1514.
(17) Habben, C. D.; Heine, A.; Sheldrick, G. M.; Stalke, D.; Bu¨hl, M.;
von Rague´ Schleyer, P. Chem. Ber. 1991, 124, 47.
(18) Habben, C. D.; Herbst-Irmer, R.; Noltemeyer, M. Z. Naturforsch. 1991,
46b, 625.
(19) Chivers, T.; Gao, X.; Parvez. Angew. Chem., Int. Ed. Engl. 1995, 34,
2549.
(7) Manke, D. R.; Nocera, D. G. Inorg. Chem. 2003, 42, 4431.
(8) For a recent review, see: Fedorchuk, C.; Copsey, M. C.; Chivers, T.
Coord. Chem. ReV. [Online early access] DOI: 10.1016/j.c-
cr.2006.06.010. Published Online: June 27, 2006.
(9) Chivers, T.; Fedorchuk, C.; Schatte, G.; Parvez. Inorg. Chem. 2003,
42, 2084.
(10) Manke, D. R.; Loh, Z-H.; Nocera, D. G. Inorg. Chem. 2004, 43, 3618.
(11) Manke, D. R.; Nocera, D. G. Polyhedron 2006, 25, 493.
(20) Gudat, D.; Niecke, E.; Nieger, M.; Paetzold, P. Chem. Ber. 1988, 121,
565.
(21) Chivers, T.; Fedorchuk, C.; Schatte, G.; Brask, J. K. Can. J. Chem.
2002, 80, 821.
2628 Inorganic Chemistry, Vol. 46, No. 7, 2007

Bonding Modes for Boraamidinate Ligands
Table 1. Crystallographic Data for ClM[PhB(NtBu)2] (1a, M ) As; 1b, M ) Sb; 1c, M ) Bi), [PhB(NtBu)2]M-µ-N(tBu)B(Ph)N(tBu)-M[PhB(NtBu)2]
(2b, M ) Sb; 2c, M ) Bi), and LiM[PhB(NtBu)2]2 (3a, 3a‚OEt2, M ) As; 3b, M ) Sb; 3c‚OEt2, M ) Bi)a
1a
1b
1c
2b
2c
3a
3a‚OEt2
3b
3c‚OEt2
empirical
formula
fw
cryst syst
space group
a (Å)
C14H23As-
ClN2
340.52
triclinic
P1h
6.000(1)
10.653(2)
13.411(3)
94.16(3)
96.76(3)
99.49(3)
835.9(3)
2
C14H23BCl- C14H23BBi- C42H69B3-
C42H69B3-
Bi2N6
1108.42
monoclinic
C2/c
20.239(4)
17.659(4)
15.205(3)
90.00
120.23(3)
90.00
C28H46As-
B2LiN4
542.17
monoclinic
C2/c
25.990(5)
8.672(2)
18.403(4)
90.00
134.22(3)
90.00
2973(2)
4
C32H56As-
B2LiN4O
616.29
triclinic
P1h
10.050(2)
10.148(2)
18.845(4)
82.92(3)
84.57(3)
66.12(3)
1741.9(7)
2
C28H46B2-
LiN4Sb
589.00
monoclinic
C2/c
25.878(5)
8.712(2)
18.521(4)
90.00
133.85(3)
90.00
3011(2)
4
C32H56B2-
BiLiN4O
750.35
monoclinic
P21/c
12.067(2)
20.280(4)
14.304(3)
90.00
94.44(3)
90.00
3490(1)
4
N2Sb
387.35
triclinic
P1h
ClN2
474.58
monoclinic
C2/m
N6Sb2
933.96
monoclinic
C2/c
9.140(2)
11.328(2)
17.090(3)
91.53(3)
102.55(3)
90.16(3)
1726.4(6)
4
33.164(7)
9.341(2)
5.540(1)
90.00
92.13
90.00
20.053(4)
17.3372(4)
15.402(3)
90.00
119.15(3)
90.00
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
1714.8(6)
4
4686(2)
4
4695(2)
4
1.568
F
calcd (g/cm3)
1.353
2.182
1.490
1.838
1.324
1.212
1.166
1.175
1.005
1.299
0.939
1.428
5.081
µ(Mo KR) (mm-1
Θ range (deg)
reflns collected
unique reflns
Rint
)
1.742
10.427
1.187
7.519
3.67-25.03 2.96-25.03 2.86-25.01 3.53-25.03 3.50-25.03 3.39-25.02 3.47-25.03 3.41-25.03 3.36-25.03
5606
2944
0.0257
2598
0.0555
0.1544
1.140
8757
6018
2898
1602
15 577
4123
12 211
4113
0.0499
3132
0.0312
0.0637
1.015
9494
2620
11 591
6097
0.0272
5598
0.0287
0.0724
1.025
8355
2650
11 814
6134
0.0303
5421
0.0274
0.0737
1.048
0.0247
5429
0.0423
1404
0.0320
3485
0.0371
2385
0.0533
0.1342
1.157
0.0979
1758
0.0662
0.1419
1.061
reflns [I>2σ(I)]
R1 [I > 2σ (I)] b
wR2 (all data)c
GOF on F2
0.0333
0.0862
1.139
0.0441
0.1010
1.215
0.0268
0.0650
1.071
a λ(Mo KR) ) 0.71073 Å, T ) -100 °C. b R1 ) ∑||Fo| - |Fc||/∑|Fo|. c wR2 ) [∑w(Fo - Fc )2/∑wFo ]1/2
.
2
2
4
Bu)2]2 (3a, 3a‚OEt2, MdAs; 3b, MdSb; 3c‚OEt2, M ) Bi) were
coated with Paratone 8277 oil and mounted on a glass fiber.
Diffraction data were collected on a Nonius KappaCCD diffrac-
tometer using monochromated Mo KR radiation (λ ) 0.71073 Å)
at -100 °C. The data sets were corrected for Lorentz and
polarization effects, and empirical absorption correction was applied
to the net intensities. The structures were solved by direct methods
using SHELXS-9722 and refined using SHELXL-97.23 After the
full-matrix least-squares refinement of the non-hydrogen atoms with
anisotropic thermal parameters, the hydrogen atoms were placed
in calculated positions [C-H ) 0.98 Å for C(CH3)3 and 0.95 Å
for phenyl hydrogens]. The isotropic thermal parameters of the
hydrogen atoms were fixed at 1.2 times that of the corresponding
carbon for phenyl hydrogens and 1.5 times for C(CH3)3. In the final
refinement, the hydrogen atoms were riding with the carbon atom
to which they were bonded.
Calcd for C14H23AsBClN2: C, 49.38; H, 6.81; N, 8.23. Found: C,
1
48.77; H, 6.92; N, 7.92. H NMR (toluene-d8, 23 °C): δ 7.08-
7.36 [m, 5H, C6H5], 1.08 [s, 18H, C(CH3)3]. 13C{1H} NMR: δ
127.9-131.2 [C6H5], 52.6 [s, C(CH3)3], 32.8 [s, C(CH3)3]. 11B
NMR: δ 37.3. Colorless crystals of 1a were obtained from n-hexane
after 4 days at 5 °C.
Synthesis of ClSb[PhB(NtBu)2] (1b). Compound 1b was
obtained as a pale yellow powder (0.180 g, 78%) from the reaction
of Li2[PhB(NtBu)2] (0.147 g, 0.60 mmol) with SbCl3 (0.137 g, 0.60
mmol) in diethyl ether via the method described for 1a. Anal. Calcd
for C14H23SbBClN2: C, 43.41; H, 5.98; N, 7.23. Found: C, 42.82;
1
H, 6.50; N, 6.86. H NMR (toluene-d8, 23 °C): δ 7.10-7.43 [m,
5H, C6H5], 1.00 [s, 18H, C(CH3)3]. 13C{1H} NMR: δ 127.7-131.2
[C6H5], 51.7 [C(CH3)3], 34.2 [C(CH3)3]. 11B NMR: δ 39.4.
Pale yellow crystals of 1b were obtained from n-hexane in 24 h at
-20 °C.
In the structures of 3a and 3b, the lithium and group 15 atom
positions were disordered with the atoms statistically distributed
over the two atomic sites. In both structures, the two atoms were
not constrained to locate in the same position, but the anisotropic
thermal parameters were restricted to be equal. The site occupation
factors were fixed to be 50% in both cases. The structure of 1c
was also disordered, the second molecule being generated by
symmetry over the bismuth and a nitrogen atom (50% site
occupation factors). The scattering factors for the neutral atoms
were those incorporated with the programs. Crystallographic data
are summarized in Table 1.
Synthesis of ClAs[PhB(NtBu)2] (1a). A solution of AsCl3 (0.181
g, 1.00 mmol) in diethyl ether (15 mL) was cooled to -80 °C, and
a solution of Li2[PhB(NtBu)2] (0.244 g, 1.00 mmol) in diethyl ether
(15 mL) was added via cannula. The reaction mixture was stirred
for 1/2 h at -80 °C and ∼4 h at room temperature. LiCl was
removed by filtration, and the solvent was evaporated under vacuum
to give 1a as an analytically pure, white solid (0.296 g, 87%). Anal.
Synthesis of ClBi[PhB(NtBu)2] (1c). Compound 1c was obtained
as a red powder (0.188 g, 79%) from the reaction of Li2[PhB(Nt-
Bu)2] (0.122 g, 0.50 mmol) with BiCl3 (0.158 g, 0.50 mmol) in
diethyl ether via the method described for 1a. Anal. Calcd for
C14H23BiBClN2: C, 35.43; H, 4.88; N, 5.90. Found: C, 36.10; H,
5.22; N, 5.71. 1H NMR (toluene-d8, 23 °C): δ 7.15-7.59 [m, 5H,
C6H5], 0.95 [s, 18H, C(CH3)3]. 13C{1H} NMR: δ 127.4-131.2
[C6H5], 50.7 [C(CH3)3], 36.2 [C(CH3)3]. 11B NMR: δ 38.5. Orange
crystals of 1c were grown from n-hexane in 3 h at -20 °C.
Synthesis of [PhB(NtBu)2]Sb-µ-N(tBu)B(Ph)N(tBu)-Sb[PhB-
(NtBu)2] (2b). A solution of Li2[PhB(NtBu)2] (0.220 g, 0.90 mmol)
in 15 mL of diethyl ether was added to a solution of SbCl3 (0.137
g, 0.60 mmol) in 15 mL of Et2O at -80 °C. The reaction mixture
was stirred for 1/2 h at -80 °C and 4 h at 23 °C. LiCl was removed
by filtration, and the solvent was evaporated under vacuum to give
2b as an amorphous, pale yellow powder (0.203 g, 73%). Anal.
Calcd for C42H69B3N6Sb2: C, 54.01; H, 7.45; N, 9.00. Found: C,
1
53.61; H, 7.53; N, 8.62. H NMR (toluene-d8, 23 °C): δ 7.17-
7.92 [m, 15H, C6H5], 1.65 [s, 18H, bridging C(CH3)3], 1.33 [s, 18H,
chelating C(CH3)3], 1.32 [s, 18H, chelating C(CH3)3]. 13C{1H}
NMR: δ 127.3-134.9 [C6H5], 61.4 [bridging C(CH3)3], 52.4
[chelating C(CH3)3], 52.3 [chelating C(CH3)3], 36.3 [C(CH3)3], 35.5
(22) Sheldrick, G. M. SHELXS-97, Program for Crystal Structure Deter-
mination; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
(23) Sheldrick, G. M. SHELXL-97, Program for Crystal Structure Refine-
ment; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
Inorganic Chemistry, Vol. 46, No. 7, 2007 2629

Konu et al.
[C(CH3)3], 35.4 [C(CH3)3]. 11B NMR: δ 39.4 (br). X-ray quality
crystals of 2b were obtained from a concentrated solution of toluene
or diethyl ether in 24 h at 23 °C.
Results and Discussion
Synthesis, NMR Spectra, and Crystal Structures of
ClM[PhB(NtBu)2] (1a, M ) As; 1b, M ) Sb; 1c, M )
Bi). Analytically pure mono-boraamidinates ClM[PhB(Nt-
Bu)2] (1a-c) are readily obtained in excellent yields by the
reaction of MCl3 with Li2[PhB(NtBu)2] in a 1:1 molar ratio
in diethyl ether (eq 1). The compounds 1a-c were character-
ized by multinuclear NMR spectroscopy in solution and by
X-ray crystallography in the solid state.
Synthesis of [PhB(NtBu)2]Bi-µ-N(tBu)B(Ph)N(tBu)-Bi[PhB-
(NtBu)2] (2c). Compound 2c was obtained as an orange powder
(0.151 g, 68%) from the reaction of Li2[PhB(NtBu)2] (0.146 g, 0.60
mmol) with BiCl3 (0.126 g, 0.40 mmol) in Et2O via the method
described for 2b. Anal. Calcd for C42H69B3N6Bi2: C, 45.51; H,
6.27; N, 7.58. Found: C, 45.30; H, 6.08; N, 7.39. 1H NMR (toluene-
d8, 23 °C): δ 7.12-7.89 [m, 15H, C6H5], 1.64 [s, 18H, bridging
C(CH3)3], 1.26 [s, 18H, chelating C(CH3)3], 1.21 [s, 18H, chelating
C(CH3)3]. 13C{1H} NMR: δ 127.4-134.9 [C6H5], 61.5 [bridging
C(CH3)3], 52.1 [chelating C(CH3)3], 51.9 [chelating C(CH3)3], 37.6
[C(CH3)3], 37.4 [C(CH3)3], 37.1 [C(CH3)3]. 11B NMR: δ 34.3 (br).
X-ray quality crystals of 2c were obtained from n-hexane in 5 days
at -20 °C.
MCl3 + Li2[PhB(NtBu)2] f ClM[PhB(NtBu)2] + 2 LiCl
M ) As (1a), Sb (1b), Bi (1c)
(1)
The NMR spectra of 1a-c in toluene-d8 are consistent
with the replacement of two Cl atoms in MCl3 by the bam
Synthesis of LiAs[PhB(NtBu)2]2 (3a). A solution of Li2[PhB(Nt-
Bu)2] (0.244 g, 1.00 mmol) in 15 mL of Et2O was added
to a solution of AsCl3 (0.091 g, 0.50 mmol) in 15 mL of Et2O at
-80 °C. The reaction mixture was stirred for 1/2 h at -80 °C and
16 h at 23 °C. LiCl was removed by filtration, and the solvent was
concentrated to ∼2 mL. Crystallization from diethyl ether afforded
3a after 5 days at 5 °C (0.151 g, 56%). Although a few crystals of
3a‚OEt2 were also obtained, the NMR data indicates that unsolvated
3a is the main product. Anal. Calcd for C28H46AsB2LiN4: C, 62.03;
1
ligand. The H NMR spectrum is composed of a singlet
attributed to the C(CH3)3 hydrogens and a multiplet for the
phenyl hydrogens. The 13C{1H} NMR spectrum shows
singlets for the C(CH3)3 and C(CH3)3 carbon atoms, as well
as the characteristic phenyl resonances. The 1H NMR
resonance of the C(CH3)3 hydrogens shifts to lower fre-
quency, and the multiplet observed for the phenyl group
exhibits a slight displacement to higher frequency on going
from 1a to 1c. Concomitantly, the 13C NMR resonance for
the C(CH3)3 carbons shifts to higher frequency and the
R-carbon of the C(CH3)3 groups exhibits a singlet at lower
frequency along the series 1a-c. The changes in the 13C
chemical shifts of the phenyl carbon signals are rather small.
The 11B NMR spectra of 1a-c display a broad singlet in
the range of 37.3-39.4 ppm consistent with the presence of
three-coordinate boron centers.
The crystal structures of 1a-c are depicted in Figure 1,
and the pertinent bond parameters are presented in Table 2.
The observed disorder in structure 1c results in high standard
deviations in the bond parameters. The molecular units of
1a-c (Figure 1a) are isostructural with the previously
reported group 15 mono-bam, BrP[PhB(NtBu)2] (1d),21 and
exhibit bonding mode A (Scheme 1). The crystal packing,
however, is distinct because of the increasing strength of
the intermolecular M‚‚‚Cl (M ) As, Sb, Bi) close contacts
for the heavier metal centers. Whereas the phosphorus- and
arsenic-containing compounds, 1d21 and 1a, adopt a packing
arrangement without significant intermolecular close contacts
in the crystal lattice, the antimony complex 1b shows weak
Sb‚‚‚Cl contacts between the neighboring molecules giving
rise to infinite chains of the mono-boraamidinates (Figure
1b). The bismuth analogue 1c exhibits similar infinite chains.
The Bi‚‚‚Cl close contacts along the chain, however, are
significantly stronger than in 1b [3.495(1) Å in 1b vs 3.134-
(5) Å in 1c]. In addition, 1c displays a second, significantly
weaker, Bi‚‚‚Cl contact [3.784(6) Å], thus inducing infinite
chains of dimeric units in the solid state (Figure 1c).
The M-Cl bond length increases by ∼0.2 Å from 1a to
1b and from 1b to 1c corresponding to an increase in the
size of the group 15 center. The calculated M-Cl bond
order24 is somewhat lower in 1c than that in 1a and 1b (0.54
for 1c and ∼0.84 for 1a and 1b). This difference is attributed
1
H, 8.55; N, 10.33. Found: C, 62.01; H, 8.41; N, 10.02. H NMR
(toluene-d8, 23 °C): δ 7.13-7.68 [m, 10H, C6H5], 1.39 [s, 9H,
C(CH3)3], 1.30 [s, 18H, C(CH3)3], 1.22 [s, 9H, C(CH3)3]. 13C{1H}
NMR: δ 126.9-134.0 [s, C6H5], 56.8 [s, C(CH3)3], 53.1 [s,
C(CH3)3], 50.1 [s, C(CH3)3], 35.3 [s, C(CH3)3], 34.3 [s, C(CH3)3],
7
33.6 [s, C(CH3)3]. 11B NMR: δ 34.1. Li NMR: δ 1.27.
Synthesis of LiSb[PhB(NtBu)2]2 (3b). A solution of Li2[PhB(Nt-
Bu)2] (0.195 g, 0.80 mmol) in 15 mL of THF was added to a
solution of SbCl3 (0.091 g, 0.40 mmol) in 15 mL of THF at
-80 °C. The reaction mixture was stirred for 1/2 h at -80 °C and
then heated to 65 °C for 6 h. Solvent was removed in vacuo, and
the pale yellow residue was dissolved in∼20 mL of Et2O. LiCl
was removed by filtration, and the solvent was concentrated to ∼2
mL. Crystallization from Et2O afforded 3b as pale yellow crystalline
material after 2 days at 5 °C (0.043 g, 18%). 1H NMR (toluene-d8,
23 °C): δ 7.10-7.61 [m, 10H, C6H5], 1.21 [s, 36H, C(CH3)3]. 13C-
{1H} NMR: δ 127.1-134.1 [C6H5], 50.2 [C(CH3)3], 34.8 [C(CH3)3].
11B NMR: δ 34.9. 7Li NMR: δ 1.05. The purity of 3b was
indicated by the NMR data; however, several attempts to obtain
accurate CHN data were unsuccessful.
Synthesis of LiBi[PhB(NtBu)2]2‚OEt2 (3c‚OEt2). A solution of
Li2[PhB(NtBu)2] (0.195 g, 0.80 mmol) in 15 mL of Et2O was added
to a suspension of BiCl3 (0.126 g, 0.40 mmol) in 15 mL of Et2O
at -80 °C. The reaction mixture was allowed to reach room
temperature in 1/2 h and was then heated to 35 °C for 16 h. LiCl
was removed by filtration, and the solvent was evaporated in vacuo.
The precipitate was washed with cold n-hexane (0 °C) giving 3c‚
OEt2 as a yellow powder (0.195 g, 65%). 1H NMR (toluene-d8, 23
°C): δ 7.15-7.50 [m, 10H, C6H5], 3.46 [q, 4H, (CH3CH2)2O], 1.31
[s, 36H, C(CH3)3], 1.108 [t, 6H, (CH3CH2)2O]. 13C{1H} NMR: δ
125.9-134.9 [C6H5], 65.1 [(CH3CH2)2O], 53.8 [C(CH3)3] 36.8
[C(CH3)3], 14.7 [(CH3CH2)2O]. 11B NMR: δ 35.1. 7Li NMR: 0.19.
Recrystallization from Et2O gave yellow X-ray quality crystals of
3c‚OEt2 after 6 h at 23 °C. Accurate CHN data could not be
obtained for 3c‚OEt2 because of partial loss of OEt2 during
manipulation of the sample.
2630 Inorganic Chemistry, Vol. 46, No. 7, 2007

Bonding Modes for Boraamidinate Ligands
Figure 2. Molecular structure of [PhB(NtBu)2]M-µ-N(tBu)B(Ph)N(tBu)-
M[PhB(NtBu)2] (2b, M ) Sb) with the atomic numbering scheme. The
bismuth complex 2c (M ) Bi) is isostructural with 2b. Hydrogen atoms
have been omitted for clarity. Symmetry operation: (A) 1 - x, y, 1.5 - z.
lengths show the expected elongation with increasing size
of the group 15 atom. The endocyclic bond angles at the
group 15 center decrease significantly along the series 1d-
1a-1b-1c, while there is a corresponding increase in the
endocyclic bond angle at boron. The stereochemical influence
of the lone pair is readily apparent from the trend in the
sum of the bond angles at the group 15 center [287.5° (1d),
280.2° (1a), 269.5° (1b), and 263.2° (1c)], reflecting the
increasing s character as one descends the group.
Synthesis, Crystal Structures, and NMR Spectra of
[PhB(NtBu)2]M-µ-N(tBu)B(Ph)N(tBu)-M[PhB(NtBu)2] (2b,
M ) Sb; 2c, M ) Bi). The novel 2:3 boraamidinate
complexes 2b and 2c are obtained in good yields by the
reaction of MCl3 with Li2[PhB(NtBu)2] (MdSb, Bi) in a 1:1.5
molar ratio in diethyl ether (eq 2). Attempts to produce the
arsenic analog 2a with this method or the reaction between
1a and Li2[PhB(NtBu)2] in Et2O resulted in a mixture of the
mono- and bis-boraamidinates, 1a and 3a, as revealed by a
combination of NMR spectroscopy and X-ray crystal-
lography. A small amount of unidentified byproducts was
also evident in the NMR spectra.
Figure 1. Molecular structures of ClM[PhB(NtBu)2] with the atomic
numbering schemes: (a) the monomeric unit of 1a (M ) As), (b) Sb‚‚‚Cl
close contacts in the infinite chains of 1b (M ) Sb), and (c) Bi‚‚‚Cl close
contacts in the dimeric infinite chains of 1c (M ) Bi). Hydrogen atoms
have been omitted for clarity. Symmetry operations: (A) x, y + 1, z. (B)
x, y - 1, z. (C) x, y, z - 1. (D) 1 - x, y, 2 - z. (E) x, y, z + 1.
Table 2. Selected Bond Lengths (Å) and Angles (deg) in
XM[PhB(NtBu)2] [X ) Br, M ) P (1d);21 X ) Cl, M ) As (1a), Sb
(1b), Bi (1c)]a
1db
1ac
1bd
1ce
M1-N1
M1-N2
M1-Cl1
1.688(3)
1.688(3)
2.305(1)f
[0.81]
1.846(5)
1.839(5)
2.229(2)
[0.85]
2.024(3)
2.031(3)
2.427(1)
[0.83]
2.11(2)
2.15(1)
2.638(5)
[0.54]
2MCl3 + 3Li2[PhB(NtBu)2] f M2[PhB(NtBu)2]3 + 6LiCl
N1-B1
N2-B1
M...Cl1′
M...Cl1′′
1.440(4)
1.440(4)
1.448(8)
1.441(8)
1.443(5)
1.443(5)
3.495(1)g
1.46(4)
1.46(3)
3.134(5)h
3.784(6)i
M ) Sb (2b), Bi (2c)
(2)
The molecular structures of 2b and 2c with the atomic
numbering scheme are depicted in Figure 2, and selected
bond parameters are summarized in Table 3. The isostructural
2:3 boraamidinate complexes 2b and 2c are composed of
two metal centers N,N′-chelated by a bam ligand; the two
[M(bam)]+ units are bridged by the third [bam]2- ligand,
thus combining the bonding modes A and F (Scheme 1).
The compounds show C2 symmetry through the boron atom
B2 and the phenyl carbon atoms C19 and C22 of the bridging
N1-M1-N2
N1-M1-Cl1
N2-M1-Cl1
M1-N1-B1
M1-N2-B1
N1-B1-N2
80.1(1)
103.7(1)f
103.7(1)f
90.4(2)
90.4(2)
97.9(3)
74.5(2)
102.5(2)
103.2(2)
90.9(4)
91.4(3)
101.0(5)
69.0(1)
66.0(2)
96.9(4)
100.3(3)
93(1)
92(1)
105(2)
100.64(9)
99.83(9)
91.3(2)
91.6(2)
105.5(3)
a Calculated bond orders in square brackets.24 b M ) P. c M ) As. d M
) Sb (bond parameters of the second independent molecule are within
experimental error). e M ) Bi. f Cl ) Br. g Symmetry operation: x, y - 1,
z. h Symmetry operation: x, y, z - 1. i Symmetry operation: 1 - x,
y, 2 - z.
(24) The bond orders were calculated by the Pauling equation N )
10(D-R)/0.71 25
where R is the observed bond length (Å). The single
,
to the strong intermolecular Bi‚‚‚Cl close contacts observed
for 1c. The B-N bond lengths of the bam unit in 1a-d are
all equal within the experimental error, while the M-N bond
bond length D is estimated from the sums of appropriate covalent
radii (Å):25 P-Br ) 2.24, As-Cl ) 2.18, Sb-Cl ) 2.37, Bi-Cl )
2.45.
Inorganic Chemistry, Vol. 46, No. 7, 2007 2631

Konu et al.
Table 3. Selected Bond Lengths (Å) and Angles (deg) in
[PhB(NtBu)2]M-µ-N(tBu)B(Ph)N(tBu)-M[PhB(NtBu)2] (M ) Sb, Bi)
2ba
2cb
2ba
2cb
M1-N1
M1-N2
M1-N3
2.067(2)
2.077(2)
2.082(2)
2.162(5) B1-N1
2.195(4) B1-N2
2.213(3) B2-N3
1.430(4) 1.439(8)
1.439(4) 1.450(8)
1.463(4) 1.457(6)
N1-M1-N2
N1-M1-N3
N2-M1-N3
N1-B1-N2
67.3(1)
64.7(2)
N3-B2-C19
121.2(2) 122.6(3)
109.1(2) 106.5(3)
125.3(2) 124.5(3)
124.6(2) 126.5(4)
106.12(9) 106.1(2) M1-N3-B2
109.22(9) 108.0(2) M1-N3-C9
106.3(3)
107.7(5) B2-N3-C9
114.8(7) M1-N3-
N3-B2-N3(A)c 117.6(3)
50.5(2)
50.6(3)
B2-N3(A)c
a M ) Sb. b M ) Bi. c Symmetry operation: -x, y, 1.5 - z.
Figure 3. Molecular structure of LiM[PhB(NtBu)2]2 (3a, M ) As) with
the atomic numbering scheme. Hydrogen atoms have been omitted for
clarity. The antimony complex 3b (M ) Sb) is isostructural with 3a.
Symmetry operation: (A) -x, y, 0.5 - z.
[bam]2- ligand. The nitrogen and boron atoms of the bridging
[bam]2- ligand adopt trigonal planar geometry (sum of the
bond angles on N(3) and B(2) are 359.0 and 360.0° in 2b
and 357.5 and 360.0° in 2c, respectively). The two [M(bam)]+
units are arranged in a cis fashion with respect to the bridging
[bam]2- ligand. The nearly planar MNBN rings are tilted in
a manner that reflects the influence of the lone pair on the
metal. Despite the size difference between antimony and
bismuth, and the resulting differences in the M-N bond
lengths, the torsion angles M1-N3-B2-N3(A) are equal
at ∼50.5°. In addition to controlling the structural features
of 2b and 2c, we also tentatively attribute the lack of
formation of the arsenic analogue 2a to the steric require-
ments of the lone pair, that is, the shorter As-N bonds would
lead to untenable interference between the tBu-groups of the
chelating and bridging bam ligands under the influence of
the lone pair on arsenic.
the third [bam]2- ligand. The 11B NMR spectra for 2b and
2c show only one very broad resonance, presumably as a
result of overlap of broad signals with similar chemical
shifts for the different three-coordinate boron environments,
B1/B1(A) and B2.
Synthesis of LiM[PhB(NtBu)2]2 (3a, M ) As; 3b, M )
Sb; 3c, M ) Bi). The reaction of MCl3 (M ) As, Bi) with
Li2[PhB(NtBu)2] in diethyl ether and treatment of SbCl3 with
Li2[PhB(NtBu)2] in THF in a 1:2 molar ratio produce the
bis-boraamidinates LiM[PhB(NtBu)2]2 (3a-c) (eq 3).
MCl3 + 2Li2[PhB(NtBu)2] f LiM[PhB(NtBu)2] + 3LiCl
[M ) As (3a), Sb (3b), Bi (3c)]
(3)
The three M-N and B-N bond lengths span the narrow
ranges of 2.067(2)-2.082(2) and 1.430(4)-1.463(4) Å in
2b and 2.162(5)-2.213(3) and 1.439(8)-1.457(6) Å in 2c.
For both compounds, the bond lengths increase in the order
M1-N1 < M1-N2 < M1-N3 and B1-N1 < B1-N2 <
B2-N3, that is, the longest bonds involve the bridging bam
ligand. The average M-N bond lengths involving the
chelating bam ligand in 2b and 2c are ∼0.05 Å longer than
those in the mono-boraamidinates 1b and 1c, whereas the
corresponding differences in the B-N bond lengths of the
chelating boraamidinates are not significant. Expectedly, the
N-B-N bond angle in the bridging bam ligand is signifi-
cantly wider than that in the chelating ligand, by ∼10° in
2b and 7° in 2c. For both compounds, the N2-M1-N3 angle
is somewhat wider than the N1-M1-N3 angle. The sum
of the bond angles at the pyramidal group 15 center are
282.6° (2b) and 278.8° (2c).
After recrystallization, both unsolvated (3a) and solvated
(3a‚OEt2) complexes were obtained for arsenic, while only
the unsolvated complex was isolated for antimony (3b) and
the solvated complex (3c‚OEt2) crystallized for bismuth.
Although the arsenic-containing bis-bam 3a is readily
obtained by conducting the reaction at room temperature,
the formation of the bismuth analogue, 3c‚OEt2, requires
elevated temperature for completion. The room-temperature
reaction resulted in a mixture of the bis-bam 3c‚OEt2, the
2:3 complex 2c, and unreacted Li2[PhB(NtBu)2] as revealed
by NMR spectroscopy. The antimony complex 3b was
obtained only in moderate yields after a reaction in boiling
THF, while the room-temperature reactions in various
solvents and the reaction in boiling Et2O produced mainly
the 2:3 complex 2b.
Crystal Structures of LiM[PhB(NtBu)2]2 (3a, M ) As;
3b, M ) Sb) and Li(OEt2)M[PhB(NtBu)2]2 (3a‚OEt2, M
) As; 3c‚OEt2, M ) Bi). The disordered structures of the
unsolvated bis-boraamidinates 3a and 3b exhibit a ladder
arrangement (bonding mode B in Scheme 1), which is
analogous to that previously observed for the tin(II) and lead-
(II) complexes, {Sn[MeB(NSiMe3)(NSiMe3)]}2 and {Pb-
[PhB(NtBu)(NtBu)]}2.1,2 The molecular structures of 3a and
3b are composed of two four-membered rings, BN2Li and
BN2M (M ) As, Sb), connected by Li-N and M-N bonds
thus forming tricyclic compounds with three- and four-
coordinate nitrogens (Figure 3). Selected bond parameters
for 3a and 3b are compared to those of 3a‚OEt2 and 3c‚
OEt2 in Table 4. The B-N bond lengths in 3a and 3b show
1
The H NMR spectra of 2b and 2c in toluene-d8 are
composed of three singlets in a 1:1:1 intensity ratio attributed
to the C(CH3)3 hydrogens and a multiplet for the phenyl
hydrogens. Consistently, the 13C{1H} NMR spectra display
three singlets for both the C(CH3)3 and C(CH3)3 carbons, as
well as the expected resonances for phenyl carbons. The three
t
equally intense singlets observed for the Bu groups are
assigned to the units bonded to N(1), N(2), and N(3),
respectively. The influence of the lone pair on the group 15
atom is evident from the NMR data of 2b and 2c, which
are consistent with a stereochemically rigid structure in
solution involving nonfluxional [M(bam)]+ units bridged by
2632 Inorganic Chemistry, Vol. 46, No. 7, 2007

Bonding Modes for Boraamidinate Ligands
Table 4. Selected Bond Lengths (Å) and Angles (deg) in
LiM[PhB(NtBu)2]2 (M ) As, Sb, Bi)
3aa
3bb
3aa
3bb
M1-N1
1.789(3) 1.943(5) Li1(A)c-N1(A)c 2.07(2)
1.64(4)
2.28(4)
M1-N2
1.967(3) 2.145(5) Li1c-N2(A)c
1.972(3) 2.198(5) B1-N1
2.04(3)
M1-N2(A)c
Li1(A)c-N2
1.408(4) 1.407(8)
1.461(5) 1.455(8)
1.95(3)
2.30(5)
B1-N2(A)c
N1-M1-N2
108.0(1) 104.2(2) M1-N2(A)c-B1 79.2(1)
80.5(3)
N1-M1-N2(A)c 76.7(1)
69.1(2)
N1-B1-N2(A)c 109.2(3) 111.0(5)
N2-M1-N2(A)c 101.3(1) 98.5(2)
N1(A)c-
109.0(3) 110.5(5)
B1(A)c-N2
M1-N1-B1
87.3(2)
91.4(4)
B1(A)c-N2-M1 126.4(2) 126.8(4)
a
d
a
d
3a‚OEt2 3c‚OEt2
3a‚OEt2
3c‚OEt2
M1-N1
M1-N2
M1-N3
M1-N4
B1-N1
B1-N2
1.962(2) 2.274(3) B2-N3
1.876(2) 2.283(3) B2-N4
1.845(2) 2.239(3) Li1-N1
3.327(2) 2.444(3) Li1-N3
1.481(2) 1.485(5) Li1-N4
1.412(3) 1.398(5) Li1...O1
1.527(2)
1.371(3)
2.092(3)
2.853(3)
1.902(3)
2.012(3)
1.456(5)
1.418(5)
2.126(8)
2.128(7)
2.360(8)
1.922(7)
N1-M1-N2
N1-M1-N3
N1-M1-N4
N2-M1-N3
72.45(6) 63.2(1)
109.38(7) 91.2(1)
72.37(5) 91.6(1)
N2-M1-N4
N3-M1-N4
N1-B1-N2
73.18(6) 149.6(1)
46.91(5) 59.6(1)
103.3(2) 111.7(3)
116.6(2) 108.3(5)
111.05(7) 101.9(1) N3-B2-N4
a M ) As. b M ) Sb. c Symmetry operation: -x, y, 0.5 - z. d M ) Bi.
a slight inequality of ∼0.04 Å for both compounds. As
expected, the M-N bonds to the three-coordinate nitrogen
in 3a and 3b are ∼0.18 and 0.24 Å, respectively, shorter
than those involving the four-coordinate nitrogens. The
observed disorder in the structures results in inaccuracy in
the lithium position preventing a detailed discussion of the
bond parameters involving the lithium center.
The central four-membered MN2Li (M ) As, Sb) rings
in 3a and 3b are nonplanar with mean torsion angles
involving the four atoms of ∼12 and 5°, respectively. The
three-coordinate boron and nitrogen atoms in 3a and 3b show
a cis arrangement with respect to the central four-membered
ring similar to that in {Sn[MeB(NSiMe3)(NSiMe3)]}2,1 but
in contrast to the trans geometry observed in {Pb[PhB(Nt-
Bu)(NtBu)]}2.2 This divergence indicates that crystal packing,
rather than the nature of the groups on nitrogen and boron,
is the determining factor in the geometrical arrangement of
these dimers.
The molecular structures of 3a‚OEt2 and 3c‚OEt2 are
depicted in Figure 4, and the relevant bond parameters are
listed in Table 4. The solvation of the lithium center by Et2O
in 3a disrupts the ladder structure so that the transannular
Li1‚‚‚N3 and As1‚‚‚N4 interactions are extremely weak
(2.853(3) and 3.327(2) Å, respectively) resulting in es-
sentially three-coordinate lithium and arsenic atoms in a
bicyclic compound composed of a four-membered BN2As
and a six-membered BN3AsLi ring (Figure 4a). For com-
parison, the spirocyclic group 13 complex Li(OEt2)Ga[PhB(µ-
NtBu)2]2 exhibits an almost perpendicular arrangement of the
two four-membered GaNBN rings with four Ga-N and two
Li-N bonds.10 Similar M-N-Li interactions have been
observed in homoleptic group 4 metal complexes of the type
M(bam)3Li2 (M ) Zr, Hf).7 The lone pair on arsenic in 3a‚
OEt2, prevents the perpendicular arrangement of the bam
ligands and therefore also contributes to the observed bonding
arrangement. The lithium atom in 3a‚OEt2 is strongly bonded
Figure 4. Crystal structures of (a) Li(OEt2)As[PhB(NtBu)2]2 (3a‚OEt2)
and (b) Li(OEt2)Bi[PhB(NtBu)2]2 (3c‚OEt2) with the atomic numbering
scheme. Hydrogen atoms have been omitted for clarity.
to one bam ligand [Li1-N4 ) 1.902(3) Å], and it is also
coordinated to the chelating bam ligand [Li1-N1 ) 2.092-
(3) Å]. The lack of a significant As1-N4 interaction results
in a disparity of ∼0.16 Å in the B-N bond lengths of the
bam ligand that bridges As1 and Li1. The B2-N3 distance
of 1.371(3) Å is unusually short compared to the range of
values reported for bam complexes.8 A similar B-N bond
length has been observed only in the gallium and indium
complexes {[PhB(NtBu)2]GaCl}2, Li(THF)4[PhB(NtBu)2]-
GaCl2‚GaCl3, and [PhB(NtBu)2]InCl2‚LiCl(THF)2 [B-N )
1.394(3), 1.377(4), and 1.393(5) Å, respectively].9 The
asymmetric bonding arrangement in 3a‚OEt2 is also reflected
in a difference of 0.07 Å in the B-N bond lengths of the
chelating bam ligand. As expected, the As-N bond involving
the four-coordinate nitrogen atom is significantly longer than
those involving the three-coordinate nitrogen atoms [As1-
N1 ) 1.962(2) Å vs As1-N2 ) 1.876(2) and As1-N3 )
1.845(2) Å].
The structure of 3c‚OEt2 exhibits four-coordinate bismuth
and lithium centers in a tetracyclic arrangement (Figure 4b).
The structural disparity between 3c‚OEt2 and 3a‚OEt2 is
tentatively attributed to the smaller size of arsenic compared
to bismuth, which in conjunction with lithium coordination
to three nitrogens, impedes As1-N4 bond formation because
t
of steric repulsions between the Bu-groups on N4 and N1.
Three of the Bi-N bond lengths in 3c‚OEt2 span a narrow
range [2.239(3)-2.283(3) Å], whereas the fourth contact
(Bi1-N4) is ∼0.18 Å longer. Similarly, two of the Li-N
Inorganic Chemistry, Vol. 46, No. 7, 2007 2633

Konu et al.
bonds are equal within the experimental error, but the third
contact (Li1-N4) is longer by ∼0.23 Å. All Li-N and Bi-N
bonds are slightly longer than the single bond length (sum
of the covalent radii for Li-N and Bi-N is 1.93 and 2.16
Å, respectively).25 The average Bi-N bond length in the
chelating bam ligand increases significantly along the series
1c, 2c, and 3c‚OEt2 with values of ∼2.13, 2.18, and 2.28 Å,
respectively. Lithium coordination results in a significant
perturbation (0.07 Å) of the B-N bond lengths in the
chelating bam ligand, since N1 is four-coordinate and N2 is
three-coordinate, whereas the difference is only ∼0.04 Å in
the bam ligand that is coordinated to both Li1 and Bi1.
Variable-Temperature NMR Studies of LiM[PhB-
(NtBu)2]2 (3a, M ) As; 3b, M ) Sb; 3c, M ) Bi). The 1H
NMR spectrum of 3a at room temperature shows three
singlets in a 1:2:1 intensity ratio for the C(CH3)3 hydrogens
and a multiplet for phenyl hydrogens. Consistently, three
singlets in a 1:2:1 ratio are observed in the 13C{1H} NMR
spectrum for the C(CH3)3 and C(CH3)3 carbons, in addition
to the characteristic resonances of the phenyl carbons. The
7Li and 11B NMR spectra exhibit singlets at 1.27 and 34.1
ppm, respectively. Since identical resonances are observed
after crystallization from either n-hexane or diethyl ether,
the NMR spectra (measured in toluene-d8 and in THF-d8)
suggest that the formation of the unsolvated complex 3a is
preferred over the solvated 3a‚OEt2.
Figure 5. C(CH3)3 resonances in the VT 1H NMR spectra of (a) LiSb-
[PhB(NtBu)2]2 (3b) and (b) Li(OEt2)Bi[PhB(NtBu)2]2 (3c‚OEt2). Only
selected temperatures (K) are shown.
a 1:1 intensity ratio at 264 K (Figure 5). By contrast, as the
temperature is lowered, the broad resonance of 3b divides
first into three singlets with a 1:1:2 intensity ratio at 277 K,
and then the two low-field resonances coalesce at 200 K
resulting in two singlets with equal intensities. These
In contrast to the 1:2:1 pattern observed for the C(CH3)3
1
1
hydrogens in the H NMR spectrum of 3a, the H NMR
spectrum of the isostructural complex 3b in toluene-d8 at
room temperature displays one broad singlet for the C(CH3)3
hydrogens. Consistently, singlets are also observed in the
13C{1H} NMR spectrum for the C(CH3)3 and C(CH3)3
1
differences in the VT H NMR spectra suggest that two
kinetic processes occur for the antimony complex 3b,
whereas only one takes place for the solvated bismuth
complex 3c‚OEt2. In contrast to 3b and 3c‚OEt2 and despite
the structural similarity between 3a and 3b in the solid state,
the arsenic complex 3a exhibits nonfluxionality in solution;
the three singlets with a 1:2:1 intensity ratio observed for
the C(CH3)3 groups in the 1H NMR spectrum remain
unchanged in the temperature range of 182-334 K.
7
carbons. The Li and 11B NMR spectra exhibit singlets at
1.05 and 34.9 ppm, respectively. The observation of a single
environment for the C(CH3)3 groups in 3b suggests that a
fluxional process occurs in solution at 296 K that results in
t
four equivalent Bu environments on the NMR time scale.
1
The room-temperature H NMR spectrum of 3c‚OEt2 in
toluene-d8 shows a singlet for the C(CH3)3 hydrogens, in
addition to the characteristic resonances for Et2O coordinated
to lithium and the phenyl hydrogens. Singlets are also
observed for the C(CH3)3 and C(CH3)3 carbons in the 13C-
{1H} NMR spectrum. The 7Li and 11B NMR spectra exhibit
singlets at 0.19 and 35.1 ppm, respectively. The half-width
The fluxional process observed for 3c‚OEt2 can be
explained by a mechanism related to Berry pseudorotation26
t
in which the equatorial and axial BuN groups of a trigonal
bipyramid exchange positions via, ideally, a square pyramidal
intermediate while the pivotal position, occupied by the lone
pair, remains unchanged. We propose that, at low temper-
atures (i.e., below the coalescence temperature of 264 K),
the two equally intense 1H NMR resonances can be attributed
1
t
of the H NMR resonance of Bu groups for 3c‚OEt2 is
significantly narrower than that observed for 3b at room
temperature (2.14 vs 5.16 Hz) suggesting a faster fluxional
process at room temperature.
t
to the axial and equatorial BuN groups of the [Bi(bam)2]-
anion in a trigonal bipyramidal geometry in which the
counterion lithium is coordinated to the two equatorial
nitrogens (Scheme 2). When the temperature is increased, a
fluxional process occurs via intermediate IIc in which the
solvated lithium ion is coordinated to three nitrogens. The
proposed pseudo-square pyramidal geometry of the inter-
mediate IIc resembles that of the solid-state structure of 3c‚
OEt2 (Figure 4b). At room temperature, a rapid exchange
To gain some insights into the nature of the fluxional
processes observed for the bis-bam complexes LiM[PhB-
(NtBu)2]2, variable-temperature (VT) 1H NMR studies of 3a,
3b, and 3c‚OEt2 in toluene-d8 were carried out. This dynamic
NMR investigation revealed different behavior for each of
the bis-boraamidinates 3a-c in solution. The singlet observed
for 3c‚OEt2 at room-temperature splits into two singlets with
(25) Pauling, L. The Nature of the Chemical Bond, 3rd ed.; Cornell
University Press; Ithaca, NY, 1960.
(26) Berry, R. S. J. Chem. Phys. 1960, 32, 933.
2634 Inorganic Chemistry, Vol. 46, No. 7, 2007

Bonding Modes for Boraamidinate Ligands
Scheme 2. Fluxional Behavior (Berry Pseudorotation, I-III) Observed for 3b and 3c‚OEt2 in Solutiona
a Substituents on B and N and the OEt2 solvate in 3c‚OEt2 are omitted for clarity. The dashed arrows indicate the similarity between intermediates IIb
and IIc and the solid-state structures of 3b and 3c‚OEt2, respectively.
Table 5. Kinetic Data for LiSb[PhB(NtBu)2]2 (3b) and Li(OEt2)Bi[PhB(NtBu)2]2 (3c‚OEt2)
3b
3b
3c‚OEt2
k1 (s-1, 226 K)
∆Hq1 (kJ mol-1
707
12.0 ( 0.5
-135 ( 3
k2 (s-1, 296 K)
∆Hq2 (kJ mol-1
170
80.7 ( 2.7
72 ( 10
k (s-1, 296 K)
∆Hq (kJ mol-1
)
3.9 × 103
76.3 ( 2.9
80 ( 11
)
)
∆Sq1 (J K-1 mol-1
)
∆Sq2 (J K-1 mol-1
)
∆Sq (J K-1 mol-1
)
takes place in which the uncoordinated nitrogen center
alternates, since all four BuN groups are indistinguishable
for lithium coordination.
bonds compared to the Sb-N bonds, that is, the tbp geometry
with four As-N and two Li-N bonds (I and III in Scheme
2) is hindered. Similarly, the cleavage of the fourth M-N
bond (M ) As, Sb) in both 3a and 3b is attributed to the
steric strain caused by the smaller size of arsenic and
antimony compared to bismuth, as was inferred from the
comparison of the solid-state structures of 3a‚OEt2 and 3c‚
OEt2.
t
On the basis of the VT NMR data, the fluxional behavior
of 3b appears to be separated into two processes. The
observation of two equally intense resonances below 200 K
implies a trigonal bipyramidal structure I (Scheme 2) similar
to that described above for 3c‚OEt2. To explain the 1:1:2
t
The kinetic data for 3b and 3c‚OEt2 were analyzed using
the line-shape method for rate constant determination, and
the enthalpies, ∆Hq, and entropies, ∆Sq, of activation were
calculated from the Eyring equation.27 The results of the
analyses are summarized in Table 5, and full details can be
found in the Supporting Information. As suggested by the
broadness of the 1H NMR singlets at room temperature, the
rate of the fluxional process in 3b is significantly slower
than that in 3c‚OEt2, thus supporting the proposed Sb-N
bond cleavage in 3b (k2 ) 170 s-1 and k ) 3.9 × 103 s-1 at
296 K for 3b and 3c‚OEt2, respectively). The enthalpy of
activation for this kinetic process exhibits only a slightly
higher barrier for the transition state in 3b than that in 3c‚
pattern for the BuN group resonances that evolves upon
raising the temperature, we propose the formation of the
pseudo-square pyramidal intermediate IIb in which one of
the Sb-N bonds is cleaved. This results in symmetry-related
t
tBu groups on nitrogen atoms N3 and N4, while the Bu
groups on N1 and N2 are in unique environments. This
structure gives rise to the ladderlike arrangement observed
for 3b in the solid state (Figure 3). Similar to that for 3c‚
OEt2, the collapse of these three resonances into a singlet
upon a further increase in temperature (above 277 K) is
attributed to rapid exchange in which the nitrogen coordina-
tion alternate between antimony and lithium centers (Scheme
2).
OEt2 (∆Hq ) 80.7 ( 2.7 kJ mol-1 and ∆Hq ) 76.3 ( 2.9
The arsenic-containing bis-bam 3a does not exhibit flux-
ionality in solution, and as in 3b, the fourth As-N bond is
absent in the solid-state structures of 3a and 3a‚OEt2. The
2
kJ mol-1 for 3b and 3c‚OEt2, respectively). Values of the
entropies and enthalpies of activation for these processes are
comparable within the standard deviation, but the opposite
enthalpy-entropy balance results in a significant difference
between k2 and k. As expected, the low-temperature process
for 3b (I f IIb) has a relatively small activation enthalpy
1
1:2:1 pattern observed in the H NMR spectrum of 3a can
be attributed to a pseudo-square pyramidal geometry analo-
gous to that of the antimony intermediate IIb. The lack of
fluxionality in 3a is attributed primarily to the higher
activation enthalpy (for the Berry pseudorotation) caused by
the greater steric repulsion that results from the shorter As-N
(27) Sandstro¨m, J. Dynamic NMR Spectroscopy; Academic Press; New
York, 1982.
Inorganic Chemistry, Vol. 46, No. 7, 2007 2635

Konu et al.
of 12.0 ( 0.5 kJ mol-1 indicating a very low energy barrier
for the Sb-N bond cleavage and the formation of a third
Li-N coordination upon going from I to IIb. The negative
value for the entropy of activation indicates a more ordered
arrangement for the transition state between I and IIb.
The exchange process observed for 3b and 3c‚OEt2 is
reminiscent of the fluxionality observed in group 13 spiro-
cyclic complexes Li(OEt2)M[PhB(NtBu)2]2 (M ) Ga, In),
for which only a single tBuN resonance was observed at room
temperature.9 In those cases, however, VT NMR studies were
not conducted. More generally, such fluxional processes in
lithium derivatives of polyimido anions of p-block elements
have low activation energies.28
2b and 2c is more favorable than the production of the bis-
boraamidinates 3b and 3c for antimony and bismuth. The
stereochemical activity of the lone pair on the group 15
element center apparently prevents the formation of an
arsenic-containing 2:3 bam complex and also contributes to
the stereochemical rigidity observed for the antimony and
bismuth 2:3 complexes in solution. The influence of the lone
pair is also evident in the structures of the bis-boraamidinates,
LiM[PhB(NtBu)2]2 (M ) As, Sb, Bi). Whereas the bismuth
complex, Li(OEt2)Bi[PhB(NtBu)2]2, is composed of two
chelating bam ligands, the second bam ligand in the arsenic
and antimony 1:2 compounds is bonded to both the lithium
center and the group 15 element as a result of the shorter
t
M-N bonds and the resulting steric interaction of the Bu
groups.
Conclusions
The fluxional process observed in solution for 3b and 3c‚
OEt2 can be explained by a mechanism related to Berry
pseudorotation in which axial and equatorial pairs of BuN
groups exchange via a distorted square pyramidal intermedi-
ate. In the case of the antimony-containing bis-bam complex
3b, two kinetic processes are involved as a result of cleavage
of the fourth Sb-N bond during the tbp f square pyramid
conversion.
The versatile coordinating ability of the bam ligand is
highlighted in this initial study of heavy group 15 derivatives.
The mono-boraamidinates ClM[PhB(NtBu)2] (M ) As, Sb,
Bi) with an M-Cl reactive site are potentially useful reagents
for further functionalization of the group 15 element center.
The 2:3 bam complexes, M2[PhB(NtBu)2]3 (M ) Sb, Bi),
display a unique bonding arrangement in which two metal
centers are N,N′-chelated by a bam ligand and the two
[M(bam)]+ units are bridged by the third [bam]2- ligand.
Both solvated and unsolvated structures were observed for
the bis-boraamidinates LiM[PhB(NtBu)2] (M ) As, Sb, Bi).
For the arsenic and antimony compounds, the unsolvated
complexes exhibit a ladder structure; only the monosolvated
complex was isolated for bismuth.
t
Acknowledgment. The authors gratefully acknowledge
financial support from the Academy of Finland (J.K.) and
the Natural Sciences and Engineering Research Council
(Canada) and thank Dr. Dana Eisler for helpful comments.
Supporting Information Available: X-ray crystallographic files
1
in CIF format, variable-temperature H NMR spectra of 3b and
Although the arsenic-containing 1:2 complex 3a is readily
obtained at room temperature, formation of the 2:3 complexes
3c‚OEt2, and details of the kinetic data analysis. This material is
available free of charge via the Internet at http://pubs.acs.org.
(28) Brask, J. K.; Chivers, T. Angew. Chem., Int. Ed. Engl. 2001, 40, 3960.
IC0622757
2636 Inorganic Chemistry, Vol. 46, No. 7, 2007