Synthesis and X-ray structures of amidinate, oxoamidate, and thioamidate complexes of boron

Article abstract of DOI:10.1139/v99-221

The reactions of PhBCl₂ with Li[CE(N'Bu)((n)Bu)] or Li[CS(N'Bu)(NH'Bu)] (1:1 molar ratio) in toluene at 23°C produced the heterocycles Ph(Cl)B(μ-N'Bu)(μ-E)C((n)Bu) (1a, E = N'Bu; 1b, E = O; 1c, E = S) or Ph(Cl)B(μ-N'Bu)(μ-S)C(NH'Bu) (2), which were characterized by ¹H, ¹¹B and ¹³C NMR and by mass spectra. X-ray structural determinations revealed that 1a, 1c and 2 contain four-membered rings. In 2 the thioamidate ligand adopts an N,S bonding mode. Crystal data: 1a, monoclinic, space group P2₁, a = 8.816(3), b = 11.311(2), c = 10.168(3) A, β = 98.86(3)°, V = 1001.7(5) A³, Z = 2, R = 0.042, and R(w) = 0.020; 1c, monoclinic, space group P2₁/n, a = 7.617(2) b = 11.200(1), c = 19.568(2), β = 90.74(2)°, V = 1669.1(5) A³, Z = 4, R = 0.046, and R(w) = 0.059; 2, monoclinic, space group P2₁/a, a = 11.357(2), b = 12.289(2), c = 12.620(3) A, β = 95.43(2)°, V = 1753.4(5) A³, Z = 4, R = 0.043, and R(w) = 0.027.

Full text of DOI:10.1139/v99-221

10
Synthesis and X-ray structures of amidinate,
oxoamidate, and thioamidate complexes of boron
Pierre Blais, Tristram Chivers, Andrew Downard, and Masood Parvez
Abstract: The reactions of PhBCl2 with Li[CE(NtBu)(nBu)] or Li[CS(NtBu)(NHtBu)] (1:1 molar ratio) in toluene
at 23°C produced the heterocycles Ph(Cl)B(µ-NtBu)(µ-E)C(nBu) (1a, E = NtBu; 1b, E = O; 1c, E = S) or
1
Ph(Cl)B(µ-NtBu)(µ-S)C(NHtBu) (2), which were characterized by H, 11B and 13C NMR and by mass spectra. X-ray
structural determinations revealed that 1a, 1c and 2 contain four-membered rings. In 2 the thioamidate ligand adopts
an N,S bonding mode. Crystal data: 1a, monoclinic, space group P21, a = 8.816(3), b = 11.311(2), c = 10.168(3) Å,
β = 98.86(3)°, V = 1001.7(5) Å3, Z = 2, R = 0.042, and Rw = 0.020; 1c, monoclinic, space group P21/n, a = 7.617(2),
b = 11.200(1), c = 19.568(2), β = 90.74(2)°, V = 1669.1(5) Å3, Z = 4, R = 0.046, and Rw = 0.059; 2, monoclinic,
space group P21/a, a = 11.357(2), b = 12.289(2), c = 12.620(3) Å, β = 95.43(2)°, V = 1753.4(5) Å3, Z = 4, R = 0.043,
and Rw = 0.027.
Key words: boron, amidinate, oxoamidate, thioamidate, X-ray structures.
Résumé : Les réactions entre le PhBCl2 et Li[(CE(NtBu)(nBu)] ou Li[CS(NtBu)(NHtBu)] (rapport molaire de 1:1),
dans le toluène, à 23°C, conduisent à la formation d’hétérocycles Ph(Cl)B(µ-NtBu)(µ-E)C(nBu) (1a, E = NtBu; 1b,
1
E = O; 1c, E = S) ou Ph(Cl)B(µ-NtBu)(µ-S)C(NHtBu) (2) que l’on a caractérisés par RMN du H, 11B et 13C ainsi
que par spectrométrie de masse. La diffraction des rayons appliquée aux produits 1a, 1c et 2 révèle qu’ils comportent
des cycles à quatre chaînons. Dans le produit 2, le ligand thioamidate adopte un mode de liaison N,S. Les données
cristallographiques: 1a, monoclinique, groupe d’espace P21, a = 8,816(3), b = 11,311(2) et c = 10,168(3)Å,
β = 98,86(3)°, V = 1001,7(5)Å3, Z = 2, R = 0,042 et Rw = 0,020; 1c, monoclinique, groupe d’espace P21/n,
a = 7,617(2), b = 11,200(1) et c = 19,568(2)Å, β = 90,74(2)°, V = 1669,1(5)Å3, Z = 4, R = 0,046 et Rw = 0,059 et 2,
monoclinique, groupe d’espace P21/a, a = 11,357(2), b = 12,289(2) et c = 12,620(3) Å, β = 95,43(2)°, V = 1753,4(5)
Å3, Z = 4, R = 0,043 et Rw = 0,027.
Mots clés : bore, amidinate, oxoamidate, thiamidate, structures, diffraction des rayons X.
[Traduit par la Rédaction] Blais et al. 15
Introduction
An alternative route to BNCN heterocycles involves the
reaction of BX3 (X = Cl, Br) with Li[CPh(NSiMe3)2] which
produces X2BCPh(NSiMe3)2 (9). Although there have been
extensive studies of amidinate complexes of the heavier
Group 13 elements (Al, Ga, In, Tl) in recent years (10–18),
investigations of boron derivatives are limited to this single
report. The chloride was obtained as a yellow oil and the
bromide gave yellow crystals, but no structural information
was obtained (9).
Here we describe the synthesis and characterization of the
series of BNCE (E = N, O, S) heterocycles 1a–c by the
route shown in eq. [1]. The preparation and X-ray structures
of the hexameric reagents {Li[CE(NtBu)(nBu)]}6 (E = O, S)
were published very recently (19). Although a few transition-
metal complexes of oxoamidate and thioamidate anions
[RC(E)(NR′)]– have been reported (E = O (20), S (21)), none
of them were generated from these anions by metathetical
reactions. Furthermore, heterocycles 1b and 1c are, to our
knowledge, the first examples of p-block element complexes
There is an extensive literature of heterocyclic compounds
involving boron–nitrogen (B-N) linkages (1). Four-
membered rings are uncommon in comparison with larger
ring sizes, but such systems are well established for three-
coordinate boron when another heteroatom, e.g., Si(2), P(2–
4), Te(5, 6), is incorporated into the ring.
Carbon-containing four-membered B-N heterocycles are
relatively rare. Paetzold et al. (7) prepared the three-
coordinate boron system C6F5B(µ-NtBu)2CPh2 by the [2+2]
s
cycloaddition of C6F5B = NtBu and BuN = CPh2. A similar
t
approach employing iso(thio)cyanates was used by Ansorge
et al. (8) to create related four-coordinate boron systems. An
interesting feature of the latter is the thermally induced
isomerization of the N,N′-chelated complex to the unsym-
metrical (N,E-chelation) isomer (Scheme 1). This process
occurs more readily for E = S than for E = O and it is pre-
t
vented by bulky R groups, e.g., Bu (8).
Accepted October 26, 1999.
P. Blais, T. Chivers,1 A. Downard, and M. Parvez. Department of Chemistry, University of Calgary, Calgary,
AB T2N 1N4, Canada.
1Author to whom correspondence may be addressed. Telephone: (403) 220-5741. Fax: (403) 289-9488. e-mail: chivers@ucalgary.ca
Can. J. Chem. 78: 10–15 (2000)
© 2000 NRC Canada

Blais et al.
11
n
solution of Li[C(NtBu)2 Bu] (1.267 g, 5.29 mmol) in tolu-
ene (10 mL) at 23°C. The mixture was stirred for 2 h to give
a white precipitate and a pale yellow solution. Lithium
chloride was removed by filtration and removal of solvent
yielded a viscous yellow oil, which was washed with diethyl
ether (2 × 5 mL) to give pale yellow crystals of 1a (1.46 g,
Scheme 1.
1
4.36 mmol, 82%); mp 76–79°C. H NMR (in C6D6, 25°C,
δ): 8.1–7.2 (m, 5H, C6H5), 2.14 (m, 2H, CH2), 1.60 (m,
2H, CH2), 1.17 (s, 18H, C(CH3)3), 1.11 (m, 2H, CH2),
0.73 (t, 3H, CH3). 13C NMR (in C6D6, 25°C, δ): 176.3
n
(s, C(NtBu)2 Bu), 134.0–128.2 (m, C6H5B-), 54.1 (s, -C(CH3)3),
31.6 (s, -C(CH3)3), 49.3 (s, CH2CH2CH2CH3), 30.4 (s,
CH2CH2CH2CH3), 24.4 (s, CH2CH2CH2CH3), 6.4 (s,
-CH2CH2CH2CH3). 11B NMR (in C6D6, 25°C, δ): 6.8 (s).
EIMS: m/z 334 (M+, C19H32N211B35Cl; good agreement be-
tween calculated and observed isotopic distribution). Anal.
calcd. for C19H32N2BCl: C 68.16, H 9.65, N 8.37; found: C
67.76, H 9.97, N 8.45.
of these bidentate ligands. The structure of the related
heterocycle 2 is included for comparison.
Preparation of Ph(Cl)B(µ-NtBu)(µ-O)CnBu (1b)
A solution of PhBCl2 (0.889 g, 5.60 mmol) in toluene
(5 mL) was added dropwise to
a
solution of
Li[CO(NtBu)nBu] (0.885 g, 5.42 mmol) in toluene (10 mL)
at 23°C. Observations and work-up procedure were similar
to those described for 1a. The crude product was washed
with cold (–80°C) hexane (5 mL) (6 × 5 mL) to give 1b as a
1
yellow solid (1.03 g, 3.69 mmol, 68%); mp 111–112°C. H
NMR (in C6D6, 25°C, δ): 8.3–7.2 (m, 5H, C6H5), 1.93 (m,
2H, CH2), 1.42 (m, 2H, CH2), 1.14 (m, 2H, CH2), 0.98 (s,
9H, C(CH3)3), 0.70 (t, 3H, CH3). 13C NMR (in C6D6, 25°C,
δ): 184.2 (s, CO(NtBu)nBu), 136–127 (m, C6H5B-), 54.1 (s,
-C(CH3)3), 29.9 (s, -C(CH3)3), 31.0 (s, -CH2CH2CH2CH3),
27.1 (s, -CH2CH2CH2CH3), 23.1 (s, -CH2CH2CH2CH3), and
14.5 (s, -CH2CH2CH2CH3). 11B NMR (in C6D6, 25°C, δ):
11.6 (s). FAB–MS: m/z 280 [(M+H)+]. Anal. calcd. for
C15H23NOBCl: C 64.56, H 8.33, N 5.02; found: C 63.91, H
8.41, N 4.67.
Experimental
t
The reagents BuNCE (E = NtBu, O, S), LinBu (2.5 M in
t
hexane), BuNH2, and PhBCl2 were all obtained from Aldrich
and used as received. The reagents Li[CE(NtBu)nBu] were
prepared by the addition of LinBu to an equimolar amount of
tBuNCE (E = NtBu, O, S) in toluene at 23°C (19, 22).
Preparation of Ph(Cl)B(µ-NtBu)(µ-S)CnBu (1c)
LiNHtBu was made by the reaction of dry BuNH2 with
t
A solution of PhBCl2 (0.997 g, 6.28 mmol) in hexane
(5 mL) was added dropwise to a solution of Li[CS(NtBu)nBu]
(0.880 g, 4.91 mmol) in hexane (10 mL) at 23°C. Observa-
tions and work-up procedure were similar to those described
for 1a. The crude product was washed with diethyl ether (2
× 3 mL) to give 1c as a yellow powder (0.455 g, 1.54 mmol,
LinBu. All solvents were dried with appropriate drying agents
and distilled. Deuterated solvents were degassed immedi-
ately before use. All reactions and manipulations of moisture
and air-sensitive products were carried out using a glove box
and Schlenk techniques under an atmosphere of high purity,
dried argon.
1
31%); mp 67–69°C. H NMR (in C6D6, 25°C, δ): 8.4–7.0
1H NMR spectra were recorded on a Bruker ACE200
spectrometer and the chemical shifts are reported relative to
Me4Si in C6D6. 11B and 13C NMR spectra were recorded on
a Bruker AMX300 spectrometer and the chemical shifts are
reported relative to BF3·OEt2 in C6D6 and TMS in C6D6, re-
spectively. Elemental analyses were provided by the Analyti-
cal Services Laboratory, Department of Chemistry,
University of Calgary. IR spectra were obtained as Nujol
mulls on a Mattson 4030 FTIR spectrophotometer. Mass
spectra (EI, 70eV) were measured on a Kratos MS80RFA in-
strument.
(m, 5H, C6H5), 2.23 (m, 2H, CH2), 1.40 (m, 2H, CH2), 1.11
(m, 2H, CH2), 1.07 (s, 9H, C(CH3)3), 0.71 (t, 3H, CH3). 13C
NMR (in C6D6, 25°C, δ): 203.7 (s, CS(NtBu)nBu), 136–128
(m, C6H5B-), 60.5 (s, -C(CH3)3), 29.5 (s, -C(CH3)3) and
(CH2CH2CH2CH3), 37.0 (s, -CH2CH2CH2CH3), 22.8 (s,
-CH2CH2CH2CH3), and 14.2 (s, -CH2CH2CH2CH3). 11B
NMR (in C6D6, 25°C, δ): 6.5 (s). EIMS: m/z 295 (M+). Anal.
calcd. for C15H23NSBCl: C 60.92, H 7.86, N 4.74; found: C
61.82, H 8.49, N 4.55.
Preparation of Ph(Cl)B( -NtBu)( -S)CN(H)tBu (2)
µ
µ
A
solution of tert-butyl isothiocyanate (1.94 g,
Preparation of Ph(Cl)B(µ-NtBu)2CnBu (1a)
A solution of PhBCl2 (0.925 g, 5.82 mmol) in toluene
(5 mL) was added dropwise via a stainless steel cannula to a
16.8 mmol) in toluene (10 mL) was added dropwise to a
slurry of LiNHtBu (2.68 g, 33.9 mmol) in toluene (15 mL) at
20°C. The reaction mixture was stirred for 18 h. Solvent and
© 2000 NRC Canada

12
Can. J. Chem. Vol. 78, 2000
Table 1. Crystallographic data for 1a, 1c, and 2.
1a
1c
2
Formula
Fw
C19H32N2BCl
334.74
C15H23NBClS
295.68
C15H24N2BClS
310.69
Space group
a, Å
b, Å
P21 (no. 4)
8.816 (3)
11.311 (2)
10.168 (3)
98.86 (3)
1001.7 (5)
2
P21/n (no. 14)
7.617 (2)
11.200 (1)
19.568 (2)
90.74 (2)
1669.1 (5)
4
P21/a (no. 14)
11.357 (2)
12.289 (2)
12.620 (3)
95.43 (2)
1753.4 (5)
4
c, Å
β, deg
V, Å3
Z
T, °C
λ, Å
–103.0
0.71069
1.110
1.92
0.042
–103.0
0.71069
1.177
3.35
0.046
–103.0
0.71069
1.177
3.29
0.043
ρ
calcd, g cm–3
µ, cm–1
Ra
Rw
b
0.020
0.059
0.027
aR = Σ||Fo| – |Fc||/Σ|Fo|.
bRw = [Σwδ2/ΣwFo ]1/2
.
2
tBuNH2 were removed under vacuum and the product was
washed with n-hexane (2 × 5 mL) to give Li2[CS(NtBu)2] as
a very moisture-sensitive yellow powder (2.26 g, 67%).
1H NMR (in C6D6, 25°C, δ) 1.18 [s, C(CH3)3].
4.0° min–1, scan width (1.31 + 0.34 tan θ)°, and monochro-
matic MoKα radiation in the range 4.0° < 2θ < 55.1°.
The intensities of 2443 unique reflections were measured
of which 1346 had I > 3σ (I). Data were corrected for Lo-
rentz-polarization and absorption effects. The structure was
solved by direct methods (23) and expanded using Fourier
techniques (24). The non-hydrogen atoms were refined
anisotropically. Hydrogen atoms were included but not re-
fined. Scattering factors were those of Cromer and Waber
(25) and allowance was made for anomalous dispersion (26).
All calculations were performed using teXsan.3
A solution of PhBCl2 (0.318 g, 2.00 mmol) in hexane
(5 mL) was cooled to –35°C and added dropwise to a slurry
of Li2[CS(NtBu)2] (0.400 g, 2.00 mmol) in hexane (10 mL)
at –35°C. The reaction mixture was stirred for 5 h at –35°C.
The mother liquor was decanted by cannula from the white
precipitate and stored at –25°C for 3 days to give colourless
crystals of 2 (0.055 g, 15%); mp 124–126°C. The precipitate
was extracted with diethyl ether (2 × 20 mL) to give (PhBS)3
(0.075 g, 31% based on boron); EIMS: m/z 360 (M+).
1H NMR (in C6D6, 25°C): δ 8.2–7.3 (m, 5H, C6H5), 5.2 (s,
1H, NH), 1.09 (s, 9H, C(CH3)3), 1.00 [s, 9H, C(CH3)3)].
13C NMR (in C6D6, 25 °C, δ): 172.3 [CS(NtBu)(NHtBu)],
136–128 (m, C6H5B), 56.3 [HNC(CH3)3], 54.6 [NC(CH3)3],
29.6 [HNC(CH3)3], 28.8 [NC(CH3)3]. 11B NMR (in C6D6,
25°C, δ): 6.5 (s). EIMS: m/z 312 [(M+1)+]. Anal. calcd. for
C15H24BClN2S: C 57.99, H 7.79, N 9.02; found: C 57.83,
H 7.31, N 8.53.
1c
A colourless, prismatic crystal (0.50 × 0.48 × 0.45 mm)
was obtained from pentane–Et2O at –14°C. Data collection,
structure solution and refinement procedures were similar to
those described for 1a except for the use of a different direct
methods program (27). Range of data collected 4.0° < 2θ <
55.1°, scan speed 8.0° min–1, scan width (1.57 + 0.34 tan θ)°,
4063 unique reflections of which 2431 had I > 3σ (I).
2
A colourless, prismatic crystal (0.50 × 0.35 × 0.30 mm)
was obtained from diethyl ether at –18°C. Data collection,
structure solution and refinement procedures were similar to
those described for 1c. Range of data collected 4.0° < 2θ <
51.0°, scan speed 8.0° min –1, scan width (1.63 + 0.34 tan θ)°,
4241 unique reflections of which 2330 had I > 3σ (I).
X-ray analysis
Crystal data for 1a, 1c, and 2 are summarized in Table 1.2
1a
A colourless, prismatic crystal (0.45 × 0.40 × 0.55 mm)
was obtained from a mixture of pentane and diethyl ether
(1:1) at 23°C. It was coated with epoxy and mounted on a
glass fibre. Accurate cell dimensions and a crystal orienta-
tion matrix were obtained on a Rigaku AFC6S diffracto-
meter by a least-squares fit of the setting angles of 25
reflections in the range 18.39° < 2θ < 23.96°. Intensity data
were collected by the ω/2θ method using a scan speed of
Results and discussion
Compounds 1a–c were obtained as moisture-sensitive pale
yellow solids by the metathetical reaction illustrated in
eq. [1]. They were characterized by elemental analyses and
2 Copies of material on deposit may be purchased from: The Depository of Unpublished Data, Document Delivery, CISTI, National Research
Council Canada, Ottawa, Canada K1A 0S2. Tables of hydrogen atom coordinates and bond lengths and angles involving hydrogen atoms
have also been deposited with the Cambridge Crystallographic Data Centre and can be obtained on request from: The Director, Cambridge
Crystallographic Data Centre, University Chemical Laboratory, 12 Union Road, Cambridge, CB2 1EZ, U.K.
3 teXsan. Single crystal structure analysis software. Version 1.2. Molecular Structure Corp., The Woodlands, Tex. 1992.
© 2000 NRC Canada

Blais et al.
13
Fig. 3. ORTEP drawing of PhB(Cl)(µ-NtBu)(µ-S)CN(H)tBu (2).
Fig. 1. ORTEP drawing of PhB(Cl)(µ-NtBu)2CnBu (1a).
Table 2. Selected bond lengths (Å), bond angles (deg) for 1a,
1c, and 2.
1a
1c
2
a
B—Car
B—Cl
1.589(6)
1.882(6)
1.567(6)
1.590(6)
1.321(6)
1.341(6)
1.522(7)
1.491(5)
1.470(5)
—
113.8(4)
81.6(3)
89.2(4)
132.5(4)
137.8(4)
87.6(4)
136.0(4)
135.2(4)
101.6(4)
128.4(5)
130.0(5)
—
1.581(5)
1.847(4)
1.571(5)
2.032(4)
1.302(4)
1.740(4)
1.493(5)
1.503(4)
—
1.581(4)
1.872(3)
1.546(4)
2.008(3)
1.323(3)
1.742(3)
—
Fig. 2. ORTEP drawing of PhB(Cl)(µ-NtBu)(µ-S)CnBu (1c).
B—N(1)
B—Eb
C(1)—N(1)
C(1)—Eb
C(1)—C(2)
N(1)—C(6)
N(2)—C(10)
C(1)—Nexo
CAr-B-Cl
N(1)-B-Eb
B-N(1)-C(1)
C(1)-N(1)-C(6)
B-N(1)-C(6)
B-Eb-C(1)
B-N(2)-C(10)
C(1)-N(2)-C(10)
N(1)-C(1)-Eb
N(1)-C(1)-C(2)
Eb-C(1)-C(2)
N(1)-C(1)-Nexo
Eb-C(1)-Nexo
1.495(3)
—
c
—
1.323(3)
113.2(2)
84.9(2)
99.9(2)
127.8(2)
131.9(2)
71.4(1)
—
—
103.7 (2)
—
—
128.8 (3)
127.4 (2)
115.6(3)
83.7(2)
100.1(3)
128.9(3)
131.3(3)
71.2(2)
—
—
105.0 (3)
131.3 (3)
123.7 (3)
—
1
by H, 13C, and 11B NMR spectra. The 11B NMR chemical
shifts are in the range δ 6–12 ppm consistent with
tetracoordinate boron (28). The highest molecular ions ob-
served in the EI and FAB mass spectra indicate monomeric
structures in the solid state. We note, however, that four-
coordinate boron compounds involving the [Me2P(S)NR]–
ligand, cf. [RC(S)NtBu]–, exist in solution as an equilibrium
mixture of monomers and dimers (29).
—
—
aCAr = ipso carbon of C6H5 group.
b1a, E = N(2); 1c, E = S; 2, E = S.
cNexo = exocyclic N atom.
The related complex 2 was obtained unexpectedly, and in
low yield, from the reaction of PhBCl2 with Li2[CS(NtBu)2].
A major by-product of this reaction is (PhBS)3 (30),4 which
accounts for the low yield of 2.5 Surprisingly, we were un-
able to obtain 2 from the reaction of PhBCl2 with
Li[CS(NHtBu)(NtBu)] (prepared from the reaction of
NMR chemical shift for 2 is 6.5 ppm consistent with
tetracoordinate boron.
The monomeric, four-membered ring structures of 1a, 1c,
and 2 were confirmed by X-ray crystallography (see Figs. 1–3).
Numerous attempts to obtain crystals of 1b suitable for
X-ray crystallography were unsuccessful. Pertinent metrical
parameters for these heterocyclic systems are compared in
Table 2. All three ring systems are essentially planar with
torsion angles (BNCE) of –1.9(4)°, 1.2(3)°, and –3.9(2)° for
tBuNCS with LiNHtBu in a 1:1 molar ratio). The 1H and 13
C
NMR spectra of 2 both show the presence of inequivalent
1
NtBu environments and the H NMR spectrum also exhibits
a broad singlet at δ 5.2 attributable to the NH group. The 11
B
4 In one experiment a small amount of the five-membered ring (PhB)2S3 was isolated and identified by X-ray crystallography. This structure
has been reported previously in a review (31).
5 The metathetical reactions of Li2[CSNtBu]2 with other organometallic halides also produce the corresponding sulfides. For example,
(Cp2ZrS)2 (32) and (Me2SnS)3 (33) were obtained from the reactions with Cp2ZrCl2 and Me2SnCl2, respectively, and identified by compari-
son of spectroscopic data with those reported in the literature (P. Blais and T. Chivers, unpublished results).
© 2000 NRC Canada

14
Can. J. Chem. Vol. 78, 2000
1a, 1c, and 2, respectively. The anionic ligand in 2 is
chelated to boron in an N,S rather than N,N′ bonding mode
consistent with the previous observations of Ansorge et al.
(8) for related ligands in four-coordinate boron complexes.
The C—N bond distances (1.321(6) and 1.341(6) Å) in
the amidinate 1a are almost equal, approximately intermedi-
ate 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 centres are ca. 360° (as is the case for the other two
heterocycles 1c and 2). These structural features indicate
delocalized bonding in the NCN unit with approximately
equal contributions from the two canonical forms A and B
(E = NtBu). Consistently, the B—N bond lengths are also
approximately equal (1.590(6) and 1.567(6) Å) and fall
within the range of 1.55–1.61 Å found for B—N bonds in-
volving four-coordinate boron linked to three-coordinate ni-
trogen (34). Interestingly, the endocyclic and exocyclic
C—N bond lengths in 2 are identical at 1.323(3) Å. The
shortness of the exocyclic bond suggests a significant contri-
bution from the resonance form C to the overall structure
(cf. Scheme 1 (8)).
respectively. However, there are substantial differences in
the other two endocyclic bond angles for the S-containing
rings 1c and 2 compared to the corresponding values for 1a
as a result of the ability of two-coordinate sulfur to accom-
modate relatively small bond angles. Thus the endocyclic
bond angles are ca. 71° at sulfur and ca. 100° at nitrogen in
1c and 2, cf. 88–89° at the N atoms in 1a.
Conclusions
The first p-block element complexes of oxo- and thio-
amidate anions have been prepared by metathetical reactions
using the novel reagents Li[CE(NtBu)(nBu)] (E = O, S). By
analogy with the extensive chemistry of amidinates it is
likely that this approach can be applied to the synthesis of a
wide range of main group element and transition metal com-
plexes of these hetero-bidentate ligands.
Acknowledgements
We thank the Natural Sciences and Engineering Research
Council of Canada (NSERC) for financial support.
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