Syntheses and X-ray structures of boraamidinate complexes of lithium, phosphorus, and tellurium

Article abstract of DOI:10.1139/v02-111

The dilithiated boraamidinate complexes {Li₂[RB(N-t-Bu)₂]}₂ (1a, R = Ph; 1b, R = t-Bu), prepared by the reaction of B[N(H)-t-Bu]₃ with three equivalents of LiR, are shown by X-ray crystallography to have dimeric structures consisting of a distorted Li₄N₄ cube capped on two opposite faces by RB units. Reactions of 1a with TeCl₄ or PX₃ (X = Cl, Br) yielded complexes PhB(μ-N-t-Bu)₂TeCl₂ (2) and PhB(μ-N-t-Bu)₂PX (4a, X = Cl; 4b, X = Br), respectively. The structures of 2 and 4b were determined by X-ray crystallography. In the solid state, complex 2 forms a dimer with weak Te···Cl contacts (3.2411(6) A). Complex 4b is monomeric with a P-Br bond length of 2.3047(11) A. The reactions of 2 and 4a with the appropriate amounts of Li[N(H)-t-Bu] produce the monomeric tellurium imide PhB(μ-N-t-Bu)₂Te=N-t-Bu and the amido derivative PhB(μ-N-t-Bu)₂PN(H)-t-Bu (4c), respectively. The X-ray structure of 4c was determined. Deprotonation of 4c with n-BuLi produces the dimeric monolithium derivative {Li[PhB(μ-N-t-Bu)₂PN-t-Bu]}₂ (5), which was shown by X-ray crystallography to have a centrosymmetric structure with a central transoid Li₂N₂ ring.

Full text of DOI:10.1139/v02-111

821
Syntheses and X-ray structures of boraamidinate
complexes of lithium, phosphorus, and tellurium
Tristram Chivers, Chantall Fedorchuk, Gabriele Schatte, and Justin K. Brask
Abstract: The dilithiated boraamidinate complexes {Li2[RB(N-t-Bu)2]}2 (1a, R = Ph; 1b, R = t-Bu), prepared by the
reaction of B[N(H)-t-Bu]3 with three equivalents of LiR, are shown by X-ray crystallography to have dimeric structures
consisting of a distorted Li4N4 cube capped on two opposite faces by RB units. Reactions of 1a with TeCl4 or PX3 (X =
Cl, Br) yielded complexes PhB( -N-t-Bu)2TeCl2 (2) and PhB( -N-t-Bu)2PX (4a, X = Cl; 4b, X = Br), respectively.
The structures of 2 and 4b were determined by X-ray crystallography. In the solid state, complex 2 forms a dimer with
weak Te···Cl contacts (3.2411(6) Å). Complex 4b is monomeric with a P—Br bond length of 2.3047(11) Å. The reac-
tions of 2 and 4a with the appropriate amounts of Li[N(H)-t-Bu] produce the monomeric tellurium imide PhB( -N-t-
Bu)2Te=N-t-Bu and the amido derivative PhB( -N-t-Bu)2PN(H)-t-Bu (4c), respectively. The X-ray structure of 4c was
determined. Deprotonation of 4c with n-BuLi produces the dimeric monolithium derivative {Li[PhB( -N-t-Bu)2PN-t-
Bu]}2 (5), which was shown by X-ray crystallography to have a centrosymmetric structure with a central transoid
Li2N2 ring.
Key words: boraamidinates, lithium, tellurium, phosphorus, X-ray structures.
Résumé : Faisant appel à la diffraction des rayons X, on a montré que les structures des complexes boraamidinates
dilithiés {Li2[RB(N-t-Bu)2]}2 (1a, R = Ph; 1b, R = t-Bu) préparés par réaction du B[N(H)-t-Bu]3 avec trois équivalents
de LiR sont des dimères formés d’un cube Li4N4 déformé et portant des unités RB sur deux faces opposées. Les
réactions de métathèse du composé 1a avec le TeCl4 ou le PX3 (X = Cl, Br) fournissent respectivement les complexes
PhB( -N-t-Bu)2TeCl2 (2) et PhB( -N-t-Bu)2PX (4a, X Cl; 4b, X = Br). Les structures des composés 2 et 4b ont été
déterminées par diffraction des rayons X. Le complexe 2 forme un dimère avec de faibles contacts Te···Cl (3,2411(6) Å)
à l’état solide. Le complexe 4b est un monomère dans lequel la longueur de la liaison P—Br est de 2,3047(11) Å.
Les réactions des composés 2 et 4a avec des quantités appropriées de Li[N(H)N-t-Bu)] conduisent à la formation
respectivement de l’imide de tellure monomère PhB( -N-t-Bu)2Te=N-t-Bu et du dérivé amido PhB( -N-t-Bu)2PN(H)-t-
Bu (4c). On a déterminé la structure du composé 4c par diffraction des rayons X. La déprotonation du composé 4c à
l’aide de n-BuLi conduit à la formation du dérivé monolithié dimère {Li[PhB( -N-t-Bu)2PN-t-Bu]}2 (5) dont structure,
telle que déterminé par diffraction des rayons X, est centrosymétrique avec un cycle central Li2N2 transoïde.
Mots clés : boraamidates, lithium, tellure, phosphore, structures par diffraction des rayons X.
[Traduit par la Rédaction] Chivers et al. 831
Introduction
used for the synthesis of complexes of the type PhB( -N-t-
Bu)2MLn (where MLn = Pb(II) (5), ZrCp2 (6), and SnMe2
(6)) as well as the spirocyclic compounds [PhB( -N-t-
Bu)2]2M (where M = Ge, Sn, Ti, or Zr (6), Te (7)). The fluo-
rinated compound C6F5B( -N-t-Bu)2SnMe2 has been ob-
tained in a similar manner (8). Less direct methods have
been employed for the synthesis of the following
boraamidinates: [MeB( -NSiMe3)2Sn]2 (9), t-BuB( -N-t-
Bu)2P(N-t-Bu)-t-Bu (10), RB( -NR>)2S (R = alkyl, aryl,
Amidinate ligands (RC(NR>)2]– (A) have been investi-
gated extensively as ligands for p- and d-block metals (1).
Recent studies have focused on the application of aluminum
(2) and early transition-metal (3) complexes as catalysts for
alkene polymerization. By contrast, the isoelectronic
boraamidinate dianions (RB(NR>)2]2– (B) have received re-
stricted attention. The known complexes of B are limited al-
most exclusively to elements from groups 4, 14, or 16 (4).
The most straightforward route to such complexes in-
volves metathesis between a halide and the reagent
{Li2[PhB(N-t-Bu)2]}2, prepared in situ by dilithiation of
PhB[N(H)-t-Bu]2 with n-BuLi (5). This method has been
C6F5; R>
=
t-Bu or SiMe3) (11), MeB[ -N(2,6-
Me2CH)2C6H3]2SiMe2 (12), MesB( -N-t-Bu)2Si(SiMe3)(Mes)
(13), and PhB( -N-t-Bu)2TeN-t-Bu (14).
-
2-
R
C
R
B
NR'
NR'
R'N
R'N
Received 3 April 2002. Published on the NRC Research Press
Web site at http://canjchem.nrc.ca on 26 June 2002.
A
B
T. Chivers,1 C. Fedorchuk, G. Schatte, and J.K. Brask.
Department of Chemistry, University of Calgary, Calgary,
AB T2N 1N4, Canada.
More recently, we have reported an alternative and poten-
tially more versatile route to boraamidinates that involves
the reaction of B(NH-t-Bu)3 with three equivalents of an
1Corresponding author (e-mail: chivers@ucalgary.ca).
Can. J. Chem. 80: 821–831 (2002)
DOI: 10.1139/V02-111
© 2002 NRC Canada

822
Can. J. Chem. Vol. 80, 2002
alkyl-lithium reagent (15). The dilithium salts {Li2[RB(N-t-
Bu)2]}x obtained in this way form either dimeric (R = n-Bu;
x = 2, 1c) or trimeric (R = Me; x = 3) clusters (15). In this
article we describe (i) the synthesis and X-ray structures of
{Li2[RB(N-t-Bu)2]}2 (R = Ph, t-Bu); (ii) the preparation and
X-ray structure of PhB( -N-t-Bu)2TeCl2 and a new route to
the monomeric tellurium imide PhB( -N-t-Bu)2TeN-t-Bu;
and (iii) the preparation, spectroscopic characterization, and
X-ray structure (X = Br) of the P(III) boraamidinates
PhB( -N-t-Bu)2PX (X = Cl, Br, N(H)-t-Bu), and the dimeric
lithium derivative {Li[PhB( -N-t-Bu)2PN-t-Bu]}2.
analyses were provided by the Analytical Services Labora-
tory of the Department of Chemistry, University of Calgary.
Preparation of {Li2[PhB(N-t-Bu)2]}2 (1a)
From B[N(H)-t-Bu]3 and 3 equivalents of LiPh
A solution of LiPh in cyclohexane–ether (1.8 M, 3.70 mL,
6.67 mmol) was added slowly to a solution of B[N(H)-t-
Bu]3 (0.50 g, 2.22 mmol) in Et2O (50 mL) at 23°C and the
mixture was stirred for 18 h. Concentration by removal of
solvent in vacuo, and subsequent cooling (0°C for 18 h) of
the resulting yellow solution (-20 mL) yielded colourless
hexagonal blocks of {Li2[PhB(N-t-Bu)2]}2 (0.82 g,
1
1.69 mmol, 76%); mp. 158–160°C (dec.). H NMR (C6D6)
Experimental
ꢀ: 7.38–7.18 (m, 5H, C6H5), 1.09 (s, 18H, C4H9). 13C NMR
(C6D6) ꢀ: 133.85, 126.41, 125.12 (C6H5), 51.52 (CMe3),
Reagents and general procedures
7
35.33 (CMe3). 11B NMR (C6D6) ꢀ: 35.4 (br). Li NMR
The compounds PhBCl2 (Aldrich, 97%), PCl3 (Aldrich,
98%), PBr3 (Aldrich, 99.99+%), and TeCl4 (Alfa Aesar,
99.9%) were used as received. Li[N(H)-t-Bu] was prepared
by the addition of n-BuLi (2.5 M solution in hexanes,
200 mL, 0.5 mol; Aldrich) to a solution of anhydrous t-
BuNH2 (65 mL, 0.609 mol; Aldrich, 98%) in n-hexane
(C6D6) ꢀ: +2.24 (s). Anal. calcd. for C28H46B2Li4N4: C 68.90,
H 9.50, N 11.48; found: C 67.76, H 9.50, N 10.47.
A 1-mL aliquot of the initial Et2O reaction mixture was
1
pumped to dryness in vacuo and taken up in C6D6. The H
7
and Li NMR spectra indicated the presence of Li[N(H)-t-
1
7
1
Bu]: H NMR ꢀ: 1.36 (s, C4H9), cf. lit.: 1.37 (17). Li NMR
ꢀ: 0.14 (s).
(170 mL) at –10°C and its purity was checked by H NMR
spectroscopy (ꢀ in C7D8, 1.37 (t-Bu) and in THF-d8, 1.07
(t-Bu), –1.55 (NH)). The compound {Li2[n-BuB(N-t-Bu)2]}2
(1c) (15) was obtained in 70% yield from the reaction of
B[N(H)-t-Bu]3 (16) with three equivalents of n-BuLi. Sol-
vents were dried with appropriate drying agents and distilled
onto molecular sieves before use. All reactions and the ma-
nipulation of moisture-sensitive products were carried out
under an atmosphere of argon or under vacuum. All glass-
ware was carefully dried prior to use.
From PhB[N(H)-t-Bu]2 and two equivalents of n-BuLi
PhB[N(H)-t-Bu]2 is obtained by adding a solution of
PhBCl2 (2.000 g, 64 mL, 12.54 mmol) in n-hexane (20 mL)
to a slurry of Li[N(H)-t-Bu] (3.900 g, 49.32 mmol)2 in n-
hexane (30 mL) at –20°C. The reaction mixture was stirred
for 4 h at 23°C and then filtered twice using a glass-sintered
frit (8 m) and then a PTFE filter disk (Acrodisc syringe fil-
ter; diameter: 25 mm; pore size: 0.45 m). The volatile ma-
terials were removed under vacuum to give PhB[N(H)-t-Bu]2
as a golden yellow oil (2.600 g, 11.20 mmol, 89%). IR (cm–1):
Instrumentation
1H NMR spectra were recorded on Bruker AC 200 and
DRX 400 spectrometers, and chemical shifts are reported
1
3429 (NH). H NMR (C6D6) ꢀ: 7.54–7.17 (m, 5H, C6H5),
2.80 (br s, 2H, NH), 1.12 (s, 18H, C4H9). 13C NMR (C6D6)
ꢀ: 133.12, 127.81, 127.61 (C6H5), 49.32 (CMe3), 33.63
(CMe3). 11B NMR (C6D6) ꢀ: 29.6.
7
relative to Me4Si in CDCl3. 13C, 11B, 31P, 125Te, and Li
NMR spectra were measured at 23°C in C6D6, CDCl3, or
C4D8O on a Bruker DRX 400 spectrometer using a 5-mm
broadband probe operating at 100.613, 128.377, 161.975,
126.429, and 155.505 MHz, respectively. The samples were
externally referenced to BF3·Et2O in C6D6, H3PO4 in D2O,
K2TeO3 in D2O (referenced to Me2Te), 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 corre-
lated with shifts to high frequencies (downfield) of the refer-
ence compound. EPR spectra were recorded at 20°C on a
Buker EMX 10/12 EPR spectrometer equipped with a mi-
crowave X-Band bridge (9.1–9.9 GHz), a 10-inch magnet,
and a 12-kW power supply ER083CS. Samples of 1a and 1c
were loaded into a 4-mm glass tube in a glovebox and dis-
solved in oxygen-free toluene. After brief exposure of these
solutions to dry air, the EPR spectra were recorded. Mass
spectra were obtained with a VG Micromass spectrometer
VG7070 (70 eV). Infrared spectra were obtained as Nujol
mulls between KBr plates on a Nicolet Nexus 470 FT-IR
spectrometer in the range of 4000–350 cm–1. Elemental
The product was dissolved in n-hexane (20 mL) and the
solution was cooled to –10°C. Then n-BuLi in hexane (2.5 M,
9 mL, 22.20 mmol) was added, and the reaction mixture was
heated at reflux for 17 h. The white precipitate of 1a was al-
lowed to settle and the yellow solution was removed via a
cannula and discarded. The product was washed twice with
n-hexane (-5 mL) at 0°C giving 1a (2.220 g, 4.55 mmol,
1
81%). The H, 13C, and 11B NMR spectral parameters were
in good agreement with those obtained for 1a prepared by
method A.
An increase in the scale of the reaction of PhBCl2 with
Li[N(H)-t-Bu] by a factor of two results in lower yields (60–
80%) of PhB[N(H)-t-Bu]2.
Preparation of {Li2[t-BuB(N-t-Bu)2]}2 (1b)
tert-Butyllithium (3.30 mL, 5.61 mmol) was added slowly
to a solution of B[N(H)-t-Bu]3 (0.425 g, 1.87 mmol) in n-
pentane (10 mL) at 23°C and stirred for 2.5 h. Concentration
by removal of solvent in vacuo and subsequent cooling (0°C
2 It is necessary to use an excess of Li[N(H)-t-Bu] to optimize the yield of PhB[N(H)-t-Bu]2. Unreacted Li[N(H)-t-Bu] is readily removed by
filtration.
© 2002 NRC Canada

Chivers et al.
823
for 3 h) of the resulting solution (-2 mL) yielded colourless
tion at 100°C (10–3 torr) to give 4a (2.028 g, 6.848 mmol,
1
1
rods of 1b (0.342 g, 0.76 mmol, 81%). H NMR (C6D6) ꢀ:
84%) as a white solid; mp 104°C. H NMR (C6D6) ꢀ: 7.4–
1.48 (s, 9H, BC4H9), 1.38 (s, 18H, NC4H9). 7Li NMR (C6D6)
ꢀ: –0.55 (s). 11B NMR (C6D6) ꢀ: 35.1 (s). Anal. calcd. for
C12H27BLi2N2: C 64.33, H 12.15, N, 12.50; found: C 64.70,
H 11.32, N 12.58.
7.0 (m, 5H, C6H5), 1.15 (d, JH,P = 1.20 Hz, 18H, C4H9);
4
(THF-d8) ꢀ: 7.44–7.30 and 7.13–7.08 (m, 5H, C6H5), 1.16
4
(d, JH,P = 0.96 Hz, 18H C4H9). 13C NMR (THF-d8) ꢀ:
131.93, 129.54, 128.65 (3s, C6H5), 52.77 (d, 2JC,P = 6.10 Hz,
3
CMe3), 32.28 (d, JC,P = 6.90 Hz, CMe3). 11B NMR (THF-
d8) ꢀ: 35.4 (s). 31P NMR (THF-d8) ꢀ: 181.2 (s). MS m/z: 281
([M – Me]+), 225 ([M – N-t-Bu]+). Anal. calcd. for
C14H23BClN2P: C 56.69, H 7.82, N 9.44; found: C 56.13,
H 7.34, N 9.28.
Preparation of PhB( -N-t-Bu)2TeCl2 (2)
The addition of a colourless solution of 1a (1.000 g,
2.049 mmol) in toluene (10 mL) to a yellow solution of
TeCl4 (1.104 g, 4.098 mmol) in toluene (15 mL) at –78°C
produced a dark red solution, which became black upon
warming to 23°C. After 1 h, the volatiles were removed un-
der vacuum and the solid black residue was extracted with
acetonitrile (2 × 10 mL). Elemental tellurium (50 mg) was
separated by filtration through an Acrodisc syringe filter
(PTFE membrane, pore size 0.45 m) to give an orange so-
lution. Removal of solvent under vacuum produced PhB( -
N-t-Bu)2TeCl2 (1.604 g, 3.742 mmol, 91%) as an orange-
Preparation of PhB( -N-t-Bu)2PBr (4b)
A colourless solution of 1a (0.500 g, 1.024 mmol) in THF
(15 mL) was added to a solution of PBr3 (0.195 mL,
0.555 g, 2.049 mmol) in toluene (10 mL) at ꢁ40°C. After
5 min, the reaction mixture was allowed to reach room tem-
perature, whereupon a pale yellow solution containing a yel-
low-orange solid (LiBr) was obtained. The reaction mixture
was filtered through a PTFE filter disk after 3.5 h. The vola-
tile materials were removed under vacuum to give a pale
sticky yellow solid. n-Hexane (-10 mL) was added and the
cloudy solution was transferred into another Schlenk vessel
leaving a microcrystalline yellow solid (0.293 g) (mainly
LiBr). After removal of the solvent from the decanted solu-
tion an off-white, slightly sticky solid was recovered. The
solid was identified as [PhB( -N-t-Bu)2]PBr (4b, 0.600 g,
1.764 mmol, 86%) by multinuclear NMR spectroscopy and
single crystal X-ray structure analysis. Two 20-mg samples
of 4b were dissolved in Et2O and in a mixture of Et2O–n-
pentane (1:1) (-3 mL) in test tubes and colourless prism-like
crystals were obtained from both samples after four days at
1
yellow solid; mp 110–114°C dec. H NMR (C6D6) ꢀ: 7.51–
7.16 (m, 5H, C6H5), 1.22 (s, 18H, C4H9); (CDCl3) ꢀ: 7.60–
7.38 (m), 1.29 (s). 13C NMR (C6D6) ꢀ: 131.08, 129.75,
128.49 (C6H5), 55.76 (CMe3), 33.70 (CMe3). 11B NMR
(C6D6) ꢀ: 43.5; (CDCl3) ꢀ: 43.3. 125Te NMR (CDCl3) ꢀ:
1677.1; (C6D6) ꢀ: 1673.9. Anal. calcd. for C14H23BCl2N2Te:
C 39.23, H 5.41, N 6.53; found: C 37.42, H 5.35, N 6.05.
Preparation of PhB( -N-t-Bu)2TeN-t-Bu (3)
A slurry of Li[N(H)-t-Bu] (0.222 g, 2.800 mmol) in n-
hexane (15 mL) was added to a yellow-orange solution of 2
(0.600 g, 1.400 mmol) in n-hexane (20 mL) cooled to
–78°C. The reaction mixture was allowed to reach room
temperature after 15 min, whereupon an orange solution
containing a white solid (LiCl) was obtained. The mixture
was stirred for 18 h, after which time the colour of the solu-
tion had changed to dark orange. The solution was filtered
and the volatile materials were removed under vacuum to
give a sticky red solid. Et2O (5 mL) was added and at –60°C
PhB( -N-t-Bu)2TeN-t-Bu (3) crystallized from this solution.
The red supernatant was removed via cannula, and the re-
maining yellow-brown solid was washed with Et2O (2 ×
5 mL) at –60°C to give 3 (0.233 g, 0.543 mmol, 39%). The
1
–19°C. H NMR (C6D6) ꢀ: 7.6–7.2 (m, 5H, C6H5), 1.22 (d,
4JH,P = 1.00 Hz, 18H, C4H9); (THF-d8) ꢀ: 7.44 (m, 2H,
4
C6H5), 7.34 (m, 3H, C6H5), 1.19 (d, JH,P = 1.12 Hz, 18H,
C4H9). 13C NMR (THF-d8) ꢀ: 131.88, 129.76, 128.62 (3s,
2
3
C6H5), 53.21 (d, JC,P = 5.84 Hz, CMe3), 32.07 (d, JC,P
=
6.65 Hz, CMe3). 11B NMR (THF-d8) ꢀ: 34.5 (s). 31P NMR
(THF-d8) ꢀ: 199.5 (s). MS m/z: 340 (M+), 325 ([M – Me]+),
261 ([M – N-t-Bu]+). Anal. calcd. for C14H23BBrN2P: C 49.30,
H 6.80, N 8.21; found: C 48.57, H 8.63, N 7.81.
1
final product was LiCl-free (7Li NMR). H NMR (C6D6) ꢀ:
Preparation of PhB( -N-t-Bu)2P[N(H)-t-Bu] (4c)
A
colourless solution of Li[N(H)-t-Bu] (0.133 g,
7.50–7.10 (m, 5H, C6H5), 1.71 (s, 9H, C4H9), 1.11 (s, 18H,
C4H9); cf. lit. (14) ꢀ: 7.6–7.2, 1.70, 1.11 (in C6D6). 13C
NMR (THF-d8) ꢀ: 132.03, 128.15, 128.00 (C6H5), 64.71
(CMe3), 53.91 (CMe3), 34.85 (CMe3), 34.48 (CMe3). 11B
NMR (THF-d8) ꢀ: 33.4 (br). 125Te NMR (THF-d8) ꢀ: 1491.5
(s).
1.688 mmol) in THF (10 mL) was added to a solution of 4a
(0.500 g, 1.688 mmol) in THF (15 mL) at –20°C. The reac-
tion mixture was allowed to reach room temperature after
10 min. The volatile materials were removed under vacuum
after 4 h to give a white solid. n-Hexane (10 mL) was added
and the resulting solution was filtered through a PTFE filter
disk to remove LiCl. The solvent was removed under vac-
uum to give 4c as a white solid (0.454 g, 1.362 mmol, 81%);
Preparation of PhB( -N-t-Bu)2PCl (4a)
A colourless solution of 1a (2.000 g, 4.098 mmol) in THF
(15 mL) was added to a solution of PCl3 (0.72 mL, 1.125 g,
8.195 mmol) in toluene (15 mL) at –78°C. The reaction mix-
ture was allowed to reach room temperature after 0.5 h,
whereupon a pale yellow solution containing a white solid
(LiCl) was obtained. After 4 h, the reaction mixture was fil-
tered through a PTFE filter disk (0.45 m). The volatile ma-
terials were removed under vacuum to give a white solid.
After addition of Et2O (5 mL), the solution was filtered to
remove LiCl. Further purification was achieved by sublima-
1
mp 60°C. IR (cm–1): 3347 (NH). H NMR (C7D8) ꢀ: 7.5–7.0
4
(m, 5H, C6H5), 2.37 (s, 1H, NH), 1.90 (d, JH,P = 0.90 Hz,
9H, C4H9), 1.25 (s, 18H, C4H9); (THF-d8) ꢀ: 7.36–7.30 and
2
7.27–7.20 (m, 5H, C6H5), 3.26 (d, JH,P = 6.0 Hz, 1H, NH),
4
1.31 (d, JH,P = 1.07 Hz, 9H, C4H9), 1.12 (s, 18H, C4H9).
13C NMR (THF-d8) ꢀ: 132.09, 128.01, 127.96 (3s, C6H5),
3
51.74 (s, CMe3), 51.67 (s, CMe3), 33.27 (d, JC,P = 6.14 Hz,
3
CMe3), 33.03 (d, JC,P = 9.34 Hz, CMe3). 11B NMR (THF-
d8) ꢀ: 32.66 (s). 31P NMR (THF-d8) ꢀ: 94.02 (s). Anal.
© 2002 NRC Canada

824
Can. J. Chem. Vol. 80, 2002
calcd. for C18H33BN3P: C 64.87, H 9.98, N 12.61; found:
C 63.06, H 9.69, N 11.90.3
HKL, DENZO, and SCALEPACK (18c) software, which
corrects for beam non-homogeneity, possible crystal decay,
and Lp effects, respectively. A multiscan absorption correc-
tion was applied (SCALEPACK) (18c).
Preparation of {Li[PhB( -N-t-Bu)2N-t-Bu]}2 (5)
Relevant parameters for the data collections and crystallo-
graphic data for 1a, 1b, 2, 4b, 4c, and 5 are summarized in
Tables 1 and 2.5
A solution of n-BuLi (2.5 M, 0.60 mL, 1.50 mmol) in
hexanes was added to a solution of 4c (0.500 g, 1.50 mmol)
in n-hexane (10 mL) at –78°C. After 2.5 h, the solvent was
removed from the golden yellow solution under vacuum to
give 5 as a white solid (0.443 g, 1.306 mmol, 87%); mp
All structures were solved using direct methods (1a, 1b,
4b: SHELXS-97 (19a); 4c: SHELXS-97–2 (19b); 2: SIR-92
(20a); 6: SIR-97 (20b)) and refined by full-matrix least-
squares methods on F2 with SHELXL97–2 (19b). The non-
hydrogen atoms were refined anisotropically. Hydrogen
atoms were included at geometrically idealized positions
(C—H and N—H bond distances 0.95 Å) and were not re-
fined. The isotropic thermal parameters of the hydrogen at-
oms were fixed at 1.2 times that of the corresponding carbon
or nitrogen atom.
Two of the carbon atoms (C(50) and C(53)) of the t-Bu
group attached to B(1) in 1b were disordered around the
mirror plane with partial occupancy factors of 0.5 each,
leading to two possible orientations of the disordered t-Bu
group: C(52), C(50), C(51), C(53) and C(53)*, C(50)*,
C(52), C(51) (*: x, –y + 1/2, z).
1
7
151°C (dec.). The H, 13C, Li, and 11B NMR spectra of
THF-d8 solutions of this product at 23°C indicate the pres-
ence of two components (5A and 5B) in the approximate
molar ratio 3:1.
1
4
5A: H NMR ꢀ: 7.38–7.14 (m, 5H, C6H5), 1.24 (d, JH,P
=
1.13 Hz, 9H, exo-C4H9), 1.13 (s, 18H, endo-C4H9). 13C
NMR ꢀ: 127.59, 127.41, 127.21 (3s, C6H5), 53.09 (d,
2JC,P = 25.6 Hz, exo-CMe3), 52.13 (d, JC,P = 7.7 Hz, endo-
2
3
3
CMe3), 36.47 (d, JC,P = 6.5 Hz, exo-CMe3), 33.49 (d, JC,P
= 6.5 Hz, endo-CMe3). Li NMR ꢀ: 0.39 (s).4 11B NMR ꢀ:
7
31.68 (s). 31P NMR ꢀ: 129.89 (s).
1
5B: H NMR ꢀ: 7.38–7.14 (m, 5H, C6H5), 1.32 (s, 9H,
exo-C4H9), 1.18 (s, 18H, endo-C4H9). 13C NMR ꢀ: 133.03,
2
132.49, 132.18 (3s, C6H5), 56.84 (d, JC,P = 18.7 Hz,
2
In 4b, one of the carbon atoms (C(13), C(13)>) of one of
the t-Bu groups was disordered around the mirror plane with
partial occupancy factors of 0.62(2) and 0.38(2), respec-
tively. In 4c, the hydrogen atom connected to N(3) was lo-
cated from the Fourier difference map.
exo-CMe3), 51.58 (d, JC,P = 8.2 Hz, endo-CMe3), 35.92 (d,
3JC,P = 10.9 Hz, exo-CMe3), 33.38 (d, JC,P = 4.8 Hz, endo-
3
CMe3). 11B NMR ꢀ: 37.21 (s). 31P NMR ꢀ: 68.10 (s). X-ray
quality crystals were obtained from a solution of this solid
(0.040 g) in C7D8 after 5 days at –19°C in an NMR tube.
Satisfactory CHN analyses could not be obtained because of
the extremely moisture-sensitive crystalline samples of 5.
Results and discussion
X-ray analyses
Synthesis and X-ray structures of {Li2[RB(N-t-Bu)2]}2
(1a, R = Ph; 1b, R = t-Bu)
Crystals suitable for X-ray diffraction were obtained from
solutions of the desired compounds in n-pentane at 23°C
(1a, 1b), in CDCl3 at 23°C (2), in Et2O or Et2O–n-pentane at
–19°C (4b), in n-hexane at –19°C (4c), and in C7D8 at
–19°C (5). Single crystals of 1a, 1b, 2, 4b, 4c, and 5 were
coated with Paratone oil, mounted on thin glass fibres, and
frozen in the cold nitrogen stream of the goniometer or on a
CryoLoop (nylon fibre).
Measurements for 1a and 1b were made on a Bruker AXS
SMART/Platform 1000 CCD (ꢂ-scans) and on a Bruker
AXS SMART/PY/RA/SMART 1000 CCD (ꢃ- and ꢂ-scans)
diffractometer, respectively. The data were reduced with
SAINT (18a), which corrects for Lorentz and polarization
effects, and corrected for absorption with SADABS (18b).
X-ray data for 2, 4b, 4c, and 5 were collected on a Nonius
Kappa CCD 4-circle Kappa FR540C diffractometer using
ꢃ- and ꢂ-scans. Data reduction was performed with the
In a preliminary communication (15), we reported that the
dilithio boraamidinate complexes {Li2[RB(N-t-Bu)2]}x (R =
n-Bu, x = 2 (1c); R = Me, x = 2,3) were obtained by treat-
ment of B[N(H)-t-Bu]3 with three equivalents of the appro-
priate organolithium reagent. In this transformation the
alkyl-lithium reagent functions as a base to dilithiate
B[N(H)-t-Bu]3 and as a nucleophile to displace the third
t-BuNH group with an alkyl substituent.6 Here we have
demonstrated that aryl-lithium reagents can also be used in
this new synthesis of boraamidinates. Thus {Li2[PhB(N-t-
Bu)2]}2 (1a) is obtained in 76% yield via eq. [1]. The pre-
ferred synthesis of 1a, however, is the dilithiation of
PhB[N(H)-t-Bu]2, for which an improved procedure is de-
scribed in the Experimental section (eq. [2]). The overall
yield of 1a from PhBCl2 is -73%; it is not necessary to sep-
arate the product from Li[N(H)-t-Bu] (cf. eq. [1]).
3 Slightly low CHN values were obtained for three different samples. The product was, however, spectroscopically pure (1H, 13C, and
7
31P NMR spectra) and showed no resonance for LiCl in the Li NMR spectrum.
7
4 Two very weak resonances were also observed in the Li NMR spectrum at ꢀ 1.07 and 1.60, but neither of these resonances were suffi-
ciently intense to be attributable to 5B. It is possible that the 7Li NMR resonance for 5B is hidden under the resonance at ꢀ 0.39 for 5A.
5 Supplementary material may be purchased from the Depository of Unpublished Data, Document Delivery, CISTI, National Research
Council Canada, Ottawa, ON K1A 0S2, Canada. For information on obtaining material electronically go to
http://www.nrc.ca/cisti/irm/unpub_e.shtml. Crystallographic information has also been deposited with the Cambridge Crystallographic Data
Centre (CCDC Nos. 181868 (1a), 181864 (1b), 181865 (2), 181866 (4b), 181867 (4c), and 181863 (5)). Copies of the data can be obtained
free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Centre, 12, Union Road, Cam-
bridge CB2 1EZ, UK; fax: +44 1223 336033; or deposit@ccdc.cam.ac.uk).
6 A similar transformation has been reported by Nöth et al. (21) for the lithiation of MesP(NH-t-Bu)2 with n-BuLi, which produces
{n-BuP[N(Li)-t-Bu]2}2·(OEt2)2 via the nucleophilic substitution of the mesityl group at the phosphorus(III) centre by an n-butyl group.
© 2002 NRC Canada

Chivers et al.
825
Table 1. Crystallographic data for 1a, 1b, and 2.
1a
1b
2
Formula
FW
C24H54B2Li4N4
448.09
C28H46B2Li4N4
488.07
C14H23BCl2N2Te
428.65
Crystal size (mm)
Colour and habit
T (K)
0.39 × 0.30 × 0.13
Colourless prism
193(2)
0.48 × 0.21 × 0.15
Colourless prism
193(2)
0.30 × 0.30 × 0.25
Orange plate
173(2)
Wavelength (Å)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
ꢄ (°)
V (Å3)
Z
Dcalcd. (g cm–3)
(cm–1)
0.71073
Monoclinic
P21/m
10.1301(11)
11.0986(12)
13.4048(13)
100.8559(19)
1480.1(3)
2
0.71073
Monoclinic
C2/c
19.844(4)
8.9232(18)
17.876(4)
100.970(3)
3107.6(11)
4
0.71073
Orthorhombic
Pbca
14.8178(2)
11.5449(2)
21.3800(3)
3657.48(10)
8
1.557
1.005
0.55
1.043
0.58
19.11
No. of reflns. collected
No. of indep. reflns./Rint
Reflns. with I > 2ꢅ(I)
S (GoF) on F2 a
R [I O 2ꢅ(I)]b
wR (all data)c
Largest diff. peak/hole (e Å–3)
5783
2624/0.0446
1650
1.025
0.0571
7320
3177/0.0529
1784
0.923
0.0543
10310
5226/0.0196
4140
1.021
0.0265
0.0655
0.625/–0.583
0.1652
0.188/–0.136
0.1452
0.293/–0.220
aS = {ꢆ[w(F 2o – F 2c )2]/(n – p)}1/2
.
bR = ꢆ||Fo| – |Fc||/ꢆ|Fo|.
cwR = {[ꢆw(F o2 – F c2 )2]/[ꢆw(F 2o)2]}1/2
.
Table 2. Crystallographic data for 4b, 4c, and 5.
4b
4c
5
Formula
FW
C14H23BBrN2P
341.02
C18H33BN3P
333.25
C36H64B2Li2N4P2
678.37
Crystal size (mm)
Colour and habit
T (K)
0.30 × 0.30 × 0.20
Colourless prism
223(2)
0.15 × 0.15 × 0.10
Colourless plate
173(2)
0.12 × 0.12 × 0.05
Colourless plate
173(2)
Wavelength (Å)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
ꢄ (°)
V (Å3)
Z
Dcalcd. (g cm–3)
(cm–1)
0.71073
Monoclinic
P21/m
6.2890(1)
13.8520(3)
10.0331(2)
91.306(2)
873.80(3)
2
0.71073
Monoclinic
C2/c
36.1860(6)
6.0030(2)
25.2640(6)
131.0551(11)
4138.35(18)
8
0.71073
Monoclinic
P21/n
11.2310(2)
10.4010(2)
18.4790(4)
107.2790(8)
2061.18(7)
2
1.296
24.33
1.070
1.36
1.093
1.37
No. of reflns. collected
No. of indep. reflns./Rint
Reflns. with I > 2ꢅ(I)
S (GoF) on F2 a
R [I O 2ꢅ(I)]b
wR (all data)c
Largest diff. peak/hole (e Å–3)
4064
2423/0.0139
1785
1.042
0.0517
6619
3507/0.0371
2520
1.030
0.0381
9039
4695/0.0264
3703
1.030
0.0419
0.1442
0.800/–0.949
0.0998
0.210/–0.262
0.1146
0.563/–0.382
aS = {ꢆ[w(F 2o – F 2c )2]/(n – p)}1/2
bR = ꢆ||Fo| – |Fc||/ꢆ|Fo|.
.
cwR = {[ꢆw(F o2 – F c2 )2]/[ꢆw(F 2o)2]}1/2
.
© 2002 NRC Canada

826
Can. J. Chem. Vol. 80, 2002
Table 3. Selected bond lengths and bond angles (deg) for 1a and 1b.
Bond lengths (Å)
Bond angles (deg)
1a
B(1)—N(1)
B(1)—N(2)
B(1)—C(31)
N(1)—Li(1)>
N(1)—Li(2)
N(1)—Li(1)
N(2)—Li(2)
N(2)—Li(1)
N(2)—Li(2)>
1b
1.448(3)
1.449(3)
1.598(3)
2.017(4)
2.027(4)
2.077(4)
2.022(4)
2.052(4)
2.074(4)
B(1)-N(1)-Li(1)>
B(1)-N(1)-Li(1)
B(1)-N(2)-Li(2)
B(1)-N(2)-Li(1)
B(1)-N(1)-Li(2)
B(1)-N(2)-Li(2)>
Li(1)>-N(1)-Li(2)
Li(1)>-N(1)-Li(1)
Li(2)-N(1)-Li(1)
132.46(18)
75.12(15)
78.15(16)
75.93(16)
78.02(16)
131.29(17)
69.94(15)
69.62(17)
87.84(16)
Li(2)-N(2)-Li(1)
88.65(16)
68.34(15)
109.50(18)
110.64(18)
109.59(17)
69.93(14)
71.52(13)
109.94(18)
109.34(18)
Li(1)-N(2)-Li(2)>
N(1)-B(1)-N(2)
N(1)>-Li(1)-N(2)
N(1)>-Li(1)-N(1)
N(2)-Li(1)-N(1)
N(2)-Li(2)-N(1)
N(2)-Li(2)-N(2)>
N(1)-Li(2)-N(2)>
B(1)—N(1)
B(1)—N(2)
B(1)—C(50)
B(2)—N(3)
B(2)—C(60)
N(1)—Li(1)
N(1)—Li(2)
N(2)—Li(1)
N(2)—Li(3)
N(3)—Li(1)
N(3)—Li(2)
N(3)—Li(3)
1.476(4)
1.479(4)
1.660(5)
1.476(2)
1.662(4)
1.980(4)
2.154(5)
1.996(4)
2.080(5)
2.125(4)
1.976(4)
1.999(4)
N(1)-B(1)-N(2)
N(1)-B(1)-C(50)
N(2)-B(1)-C(50)
N(3)-B(2)-N(3)>
N(3)-B(2)-C(60)
B(1)-N(1)-Li(1)
B(1)-N(1)-Li(2)
Li(1)-N(1)-Li(2)
B(2)-N(3)-Li(2)
B(2)-N(3)-Li(3)
B(2)-N(3)-Li(1)
Li(1)-N(3)-Li(2)
Li(1)-N(3)-Li(3)
105.2(2)
126.6(3)
126.4(3)
105.5(2)
127.25(11)
78.33(15)
132.1(2)
67.43(14)
79.65(15)
76.36(16)
131.94(15)
68.07(16)
66.79(15)
Li(2)-N(3)-Li(3)
N(1)-Li(1)-N(2)
N(1)-Li(1)-N(3)
N(2)-Li(1)-N(3)
N(1)-Li(2)-N(3)
N(3)-Li(2)-N(3)>
N(2)-Li(3)-N(3)
N(3)-Li(3)-N(3)>
Li(1)-N(1)-Li(1)>
B(1)-N(2)-Li(1)
B(1)-N(2)-Li(3)
Li(1)-N(2)-Li(3)
Li(1)-N(2)-Li(1)>
85.51(17)
72.41(13)
112.25(17)
111.41(17)
111.22(19)
72.94(18)
113.23(19)
71.99(18)
86.4(2)
77.73(15)
132.1(2)
67.74(14)
85.5(2)
Note: Symmetry transformations used to generate equivalent atoms >: –x + 1/2, y, –z.
[1]
[2]
B[N(H)-t-Bu]3 + 3LiPh J 1/2{Li2[PhB(N-t-Bu)2]}2
+ Li[N(H)-t-Bu] + 2C6H6
Fig. 1. ORTEP drawing of {Li2[PhB(N-t-Bu)2]}2 (1a) (30%
probability ellipsoids). Primed atoms are related by the symmetry
transformation: –x + 1/2, y, –z.
PhBCl2 + 2LiN(H)-t-Bu J PhB[N(H)-t-Bu]2
2n-BuLi
+ 2LiCl ZZZZJ 1/2{Li2[PhB(N-t-Bu)2]}2
+ 2H-n-Bu
We have also shown that the tert-butyl derivative {Li2[t-
BuB(N-t-Bu)2]}2 (1b) can be prepared in 81% yield by the
reaction of B[N(H)-t-Bu]3 with three equivalents of t-BuLi.
It is interesting to compare this result with the related chem-
istry of transition-metal amides. The amido–niobium com-
plex [(THF)2Li]2[Nb(NMes)3(NHMes)] (Mes
=
2,4,6-
Me3C6H2) is lithiated by t-BuLi to give the tetra-
kisimidoniobate (THF)4Li3[Nb(NMes)4] (22a), whereas the
reaction of the same complex with n-BuLi results in
nucleophilic displacement of the amido ligand to give the
alkyl–niobium complex [[(THF)2Li]2n-BuNb(NMes)3] (22b).
An intriguing observation, which was not reported in the
earlier work (5, 6), is the appearance of persistent bright
pink solutions when 1a–c are exposed to air.7 This transfor-
mation is reminiscent of the behaviour of solutions of the re-
lated dimeric clusters {Li2[E(N-t-Bu)3]}2 (E = S, Se), which
form deep blue (E = S) or green (E = Se) solutions upon air
oxidation (23–25). These colours were attributed to the radi-
(7Li, 92.6%, I = 3/2) nuclei (24, 25). These neutral radicals
C
C–
cals {Li3[E(N-t-Bu)3]2} on the basis of their EPR spectra,
are comprised of the radical anion [E(N-t-Bu)3] and the
which consisted of a septet of decets with the appropriate
relative intensities for the coupling of the unpaired electron
with three nitrogen (14N, 99.6%, I = 1) and three lithium
dianion [E(N-t-Bu)3]2– bridged by three Li+ ions. The pink
solution of air-oxidized 1a in toluene at 23°C exhibits a
five-line EPR spectrum, but the hyperfine coupling with
7 T. Chivers, C. Fedorchuk, G. Schatte, J.K. Brask, and R.T. Boeré. Unpublished results.
© 2002 NRC Canada

Chivers et al.
827
Table 4. Selected bond lengths (Å) and bond angles (deg) for 2.
Bond lengths (Å)
Bond angles (deg)
78.29(2)
94.53(2)
168.06(2)
108.97(2)
172.73(2)
69.33(7)
94.48(5)
97.38(5)
88.98(5)
89.82(5)
B(1)-N(1)-C(10)
B(1)-N(1)-Te(1)
C(10)-N(1)-Te(1)
B(1)-N(2)-C(20)
B(1)-N(2)-Te(1)
C(20)-N(2)-Te(1)
N(2)-B(1)-N(1)
N(2)-B(1)-C(1)
N(1)-B(1)-C(1)
137.35(18)
92.21(13)
127.02(14)
136.69(18)
92.41(13)
129.46(14)
103.49(19)
127.08(18)
129.23(18)
Te(1)—N(2)
Te(1)—N(1)
Te(1)—Cl(1)
Te(1)—Cl(2)
N(1)—B(1)
N(2)—B(1)
C(1)—B(1)
1.9954(16)
1.9998(16)
2.4356(6)
2.6452(6)
1.447(3)
Cl(2)-Te(1)···Cl(2)*
Cl(1)-Te(1)···Cl(2)*
Cl(2)*-Te(1)-N(1)
Cl(2)*-Te(1)-N(2)
Cl(1)-Te(1)-Cl(2)
N(2)-Te(1)-N(1)
N(2)-Te(1)-Cl(1)
N(1)-Te(1)-Cl(1)
N(2)-Te(1)-Cl(2)
N(1)-Te(1)-Cl(2)
1.447(3)
1.563(3)
Te(1)···Cl(2)*
3.2411(6)
Note: Symmetry transformations used to generate equivalent atoms *: –x, –y, –z.
Fig. 2. ORTEP drawing of [PhB( -N-t-Bu)2TeCl2]2 (2) (30%
probability ellipsoids). Starred atoms are related by the symmetry
transformation: –x, –y, –z.
Scheme 1.
tBu
N
tBu
N
tBu
NLi
Cl
tBu
N
2 LiNHtBu
PhB
Te
+ TeCl4
PhB
Te :
Cl
PhB
N
NLi
tBu
N
tBu
tBu
3
2
-2 LiCl
1
2 {Li2[Te(NtBu)3]}2
+
PhBCl2
/
lithium centres is not resolved.7 Consequently, we will not
speculate on the identity of the radical species.
Because the structures of 1a and 1b were shown by X-ray
crystallography to be similar, only the drawing of 1a is
shown (see Fig. 1). Pertinent structural parameters are com-
pared in Table 3. The dimeric arrangement of 1a and 1b is
similar to that previously reported for {Li2[n-BuB(N-t-
Bu)2]}2 (1c) (15); a detailed discussion here of the structures
of 1a and 1b is therefore not warranted. All three structures
consist of a distorted Li4N4 cube capped on two opposite
faces by RB units, which are in a trigonal planar environ-
ment. Molecule 1b, however, crystallizes in the space group
P21/m (cf. P21/n for 1c), imposing a mirror plane that bisects
the cluster core at the atoms B(1), B(2), Li(2), Li(3), N(1),
and N(2). Carbon atoms C(50) and C(53) of the t-Bu group
attached to B(1) are disordered over two positions, each with
site occupancy factors of 0.5. The replacement of the n-Bu
groups attached to the boron atoms in 1c with bulkier t-Bu
groups results in lengthening of the B—N and B—C bonds
(by -0.02 and 0.05 Å, respectively) and contraction of the
N-B-N angles (by -3°). The weighted mean Li—N distance
of 2.032 Å in 1b is similar to that found for 1c (2.037 Å),
but the range of Li—N distances (1.976(4)–2.154(5) Å) is
broader. The weighted mean distance of 2.226 Å for the
agostic C–H···Li interactions in 1b is significantly shorter
(by -0.2 Å) than that in 1c, presumably as a result of the in-
fluence of the bulky t-Bu groups. Consistently, the mean C–
H···Li contacts in 1a (2.421 Å) are similar to the correspond-
ing interaction exhibited by 1c (2.440 Å).
Phenylboraamidinate complexes of tellurium(IV)
We have shown earlier that a boraamidinate ligand stabi-
lizes a monomeric tellurium imide in the complex PhB( -N-
t-Bu)2Te=N-t-Bu (3) (14); however, the yield of this com-
plex in the synthesis from PhBCl2 and {Li2[Te(N-t-Bu)3]}2
was only 29%. Consequently, an alternative route to 3, as
depicted in Scheme 1, was investigated. The reaction of the
monomer of 1a with TeCl4 in a 1:1 molar ratio produces
PhB( -N-t-Bu)2TeCl2 (2) in 91% yield. The spirocyclic com-
plex [PhB( -N-t-Bu)2]2Te is obtained in 43% yield when a
2:1 stoichiometry is used for this reaction (7). Complex 2
was characterized by CHN analyses, multinuclear (1H, 13C,
11B, and 125Te) NMR spectroscopy,8 and by an X-ray struc-
tural determination (vide infra). The subsequent reaction of
2 with two equivalents of Li[N(H)-t-Bu] produced 3 in
-40% yield. Although the improvement in the yield of 3 is
modest, this new synthesis is preferable to the existing
method using {Li2[Te(N-t-Bu)3]}2, (14) in view of the easier
preparation of the reagents.
The structure of 2 was determined by X-ray crystallogra-
phy (see Fig. 2). Selected bond lengths and bond angles are
given in Table 4. Complex 2 forms a centrosymmetric dimer
linked by weak Te···Cl interactions of 3.2411(6) Å (cf. sum
of van der Waals radii for Te and Cl of 3.81 Å (27a) or 4.0
Å (27b)). Several bisamido or bisimido–tellurium dichloride
complexes have been structurally characterized, including
monomeric [(Ph3PN)2TeCl2 (28), (Ph2CN)2TeCl2 (29)],
8 The 125Te NMR resonance for 2 appears at ꢀ 1677, which may be compared with the 125Te chemical shifts of ꢀ 1575 and 1693 observed for
the inequivalent tellurium atoms in t-BuNTe( -N-t-Bu)2TeCl2 (26).
© 2002 NRC Canada

828
Can. J. Chem. Vol. 80, 2002
Fig. 3. ORTEP drawing of [PhB( -N-t-Bu)2PBr] (4b) (30%
Table 5. Selected bond lengths (Å) and bond angles (deg) for 4b.
probability ellipsoids).
Bond lengths (Å)
Bond angles (deg)
80.09(17)
103.67(10)
112.85(11)
90.41(19)
136.3(2)
130.6(2)
97.9(3)
131.03(15)
Br(1)—P(1)
P(1)—N(1)
N(1)—B(1)
N(1)—C(10)
C(1)—B(1)
2.3047(11)
1.688(3)
1.440(4)
1.470(5)
1.567(5)
N(1)-P(1)-N(1)*
N(1)-P(1)-Br(1)
B(1)-P(1)-Br(1)
B(1)-N(1)-P(1)
B(1)-N(1)-C(10)
C(10)-N(1)-P(1)
N(1)*-B(1)-N(1)
N(1)-B(1)-C(1)
Note: Symmetry transformations used to generate equivalent atoms *: x,
–y + 1/2, z.
dimeric [(Me3PN)2TeCl2 (30), CF3SNTeCl2·THF (31)], or
extended structures [{Cl2Te( -N-t-Bu)2TeCl2}3 (26),
[(Me3Si)2N]2TeCl2 (32), MeN[PhBN(Me)]2TeCl2 (7),
(Ph2SN)2TeCl2 (29), and t-BuN(H)Sb( -N-t-Bu)2TeCl2
(33)9]. The different degrees of oligomerization observed for
closely related molecules suggest that packing consider-
ations, influenced by subtle steric effects, are responsible for
the presence (or absence) of association.
Fig. 4. ORTEP drawing of [PhB( -N-t-Bu)2PN(H)-t-Bu] (4c)
(30% probability ellipsoids).
The boraamidinate ligand in 2 is symmetrically bonded to
tellurium with equal B–N and Te–N bond lengths. The rele-
vant bond angles (N(2)-Te(1)-N(1) = 69.33(7)°, Cl(2)*-
Te(1)-N(1) = 168.06(2)°, Cl(2)*-Te(1)-N(2) = 108.97(2)°,
Cl(1)-Te(1)-Cl(2) = 172.73(2)°) indicate that the tellurium
atom in 2 is in a distorted pseudo-octahedral environment,
with the lone pair occupying an octahedral site. The Te—N
bond lengths (av 1.997 Å) can be compared with the values
of 2.036 and 2.147 Å reported for the equatorial and axial
bonds, respectively, in the spirocyclic compound [PhB( -N-
t-Bu)2]2Te, which is a distorted trigonal bipyramid (7). The
Te—Cl bond distances in 2 are significantly different
(2.4356(6) and 2.6452(6) Å) as a result of the weak Te···Cl
contacts, which are reflected by the non-linearity of the Cl-
Te-Cl unit (172.73(2)°) and the Cl(2)-Te(1)···Cl(2)* bond an-
gle (78.29(2)°) in the Te2Cl2 rectangle.
[3]
1/2{Li2[PhB(N-t-Bu)2]}2 + PX3
J PhB( -N-t-Bu)2PX
(X = Cl, Br)
The X-ray structure of 4b is illustrated in Fig. 3 and se-
lected bond lengths and bond angles are given in Table 5.
The four-membered PNBN ring is slightly puckered (P(1)-
N(1)-B(1)-N(1)* = 8.8(3)°) with symmetrically bridging N-
t-Bu groups. The B–N bond lengths are in the typical range
for phenylboraamidinate complexes (14) and the geometry at
nitrogen is essentially planar. A search of the Cambridge
Crystal Data Base revealed no structural information for cy-
clic P(III)–N compounds with exocyclic P–Br bonds. For ex-
ample, the structure of the cyclodiphosph(III) azane BrP( -
N-t-Bu)2PBr is unknown (34). The P—N bond distance of
1.688(3) Å can be compared with values in the range
1.674(3)–1.689(9) Å for cyclodiphosph(III) azanes with Cl
substituents attached to phosphorus (35, 36). The P—Br dis-
tance of 2.3047(11) Å is only slightly longer than the sum of
the covalent radii for P and Br (2.24 Å) (27b), consistent
with a strong covalent contribution to the P–Br bond.
Phenylboraamidinate complexes of phosphorus(III)
We have also obtained boraamidinate complexes contain-
ing a phosphorus(III)–halogen bond in 85–95% yields by the
reaction of 1a with phosphorus trihalides (eq. [3]). The only
previously reported P(III) complex is MeB( -N-t-Bu)2PMe
prepared by treatment of MeBBr2 with (LiN-t-Bu)2PMe
(34). The complexes PhB( -N-t-Bu)2PX (4a, X = Cl; 4b,
X = Br) were characterized by CHN analyses, multinuclear
(1H, 13C, 11B, and 31P) NMR spectroscopy, mass spectrome-
1
try, and an X-ray structure determination of 4b. The H
NMR spectra of 4a or 4b in C6D6 or THF-d8 show reso-
nances for the Ph and N-t-Bu groups with the appropriate in-
tensities. The latter appears as a doublet owing to the four-
The nucleophilic substitution reaction of 4a with one
equivalent of Li[N(H)-t-Bu] produced PhB( -N-t-
Bu)2PN(H)-t-Bu (4c) in -80% yield. Amido derivative 4c
(see Table 6) was characterized by multinuclear (1H, 13C,
11B, and 31P) NMR and by X-ray crystallography (Fig. 4).
The geometrical parameters of the phenylboraamidinate
ligand in 4c are similar to those discussed above for 4b, al-
though the four-membered ring is more puckered (torsion
angle P(1)-N(1)-B(1)-N(2) = –12.50(13)°). The exocyclic
P—N bond length of 1.6544(16) Å is -0.1 Å shorter than
1
bond H–31P coupling. 31P NMR resonances are observed at
ꢀ 181 (4a) and 199 (4b) in THF-d8 solution (cf. XP( -N-t-
Bu)2PX (ꢀ 211, X = Cl; ꢀ 224, X = Br)) (34). Surprisingly,
the mass spectra of 4a and 4b show fragment ions corre-
sponding to [M – Me]+ and [M – N-t-Bu]+ rather than [M –
X]+ implying a relatively strong covalent P–X (X = Cl, Br)
bond. A molecular ion is observed for 4b.
9 The complex t-BuN(H)Sb( -N-t-Bu)2TeCl2 forms an extended structure involving weak Sb···Cl contacts rather than Te···Cl contacts (33).
© 2002 NRC Canada

Chivers et al.
829
Table 6. Selected bond lengths (Å) and bond angles (deg) for 4c.
Bond lengths (Å)
Bond angles (deg)
77.37(7)
B(1)-N(2)-P(1)
C(20)-N(2)-P(1)
C(30)-N(3)-P(1)
C(30)-N(3)-H
P(1)-N(3)-H
N(2)-B(1)-N(1)
N(2)-B(1)-C(1)
N(1)-B(1)-C(1)
90.81(11)
P(1)—N(1)
P(1)—N(2)
P(1)—N(3)
N(1)—B(1)
N(2)—B(1)
B(1)—C(1)
1.7395(14)
1.7452(14)
1.6544(16)
1.438(2)
1.436(2)
1.570(3)
N(1)-P(1)-N(2)
N(3)-P(1)-N(1)
N(3)-P(1)-N(2)
B(1)-N(1)-P(1)
B(1)-N(1)-C(10)
C(10)-N(1)-P(1)
B(1)-N(2)-C(20)
105.28(7)
106.66(7)
90.98(11)
135.11(14)
130.26(11)
132.45(14)
128.08(11)
129.85(13)
112.0(14)
118.1(14)
98.55(14)
131.32(16)
130.02(16)
Table 7. Selected bond lengths (Å) and bond angles (deg) for 5.
Bond lengths (Å)
Bond angles (deg)
109.02(6)
105.69(6)
75.99(5)
136.94(12)
121.40(9)
92.11(9)
C(30)-N(3)-P(1)
C(30)-N(3)-Li(1)*
P(1)-N(3)-Li(1)*
C(30)-N(3)-Li(1)
P(1)-N(3)-Li(1)
Li(1)*-N(3)-Li(1)
N(1)-B(1)-N(2)
N(1)-B(1)-C(1)
N(2)-B(1)-C(1)
N(3)*-Li(1)-N(3)
N(3)*-Li(1)-N(2)
N(3)-Li(1)-N(2)
116.16(9)
109.49(11)
128.94(10)
136.83(11)
90.13(9)
70.38(12)
99.13(12)
130.33(13)
129.94(13)
109.62(12)
136.97(14)
80.60(10)
P(1)—N(3)
P(1)—N(1)
P(1)—N(2)
N(1)—B(1)
N(2)—B(1)
N(2)—Li(1)
N(3)—Li(1)
N(3)—Li(1)*
C(1)—B(1)
1.6403(12)
1.7661(12)
1.8114(12)
1.4171(19)
1.4762(19)
2.171(3)
2.084(3)
2.047(3)
1.573(2)
N(3)-P(1)-N(1)
N(3)-P(1)-N(2)
N(1)-P(1)-N(2)
B(1)-N(1)-C(10)
C(20)-N(2)-P(1)
B(1)-N(1)-P(1)
B(1)-N(2)-P(1)
C(10)-N(1)-P(1)
B(1)-N(2)-C(20)
B(1)-N(2)-Li(1)
C(20)-N(2)-Li(1)
P(1)-N(2)-Li(1)
88.41(9)
127.41(10)
126.64(11)
113.87(11)
112.92(11)
83.10(8)
The reaction of 4c with one equivalent of n-BuLi pro-
duced the monolithium derivative {Li[Ph( -N-t-Bu)2PN-t-
Bu]}2 (5), which was shown by X-ray crystallography to be
a centrosymmetric dimer (Fig. 5). The monomeric units in 5
are formed by N,N>-chelation of the [PhB( -N-t-Bu)2N-t-
Bu]– anion to Li+ through both the exocyclic and one of the
endocyclic N atoms. As a result, one of the nitrogen atoms
(N1) in the puckered PNBN ring (torsion angle P(1)-N(1)-
B(1)-N(2) = 17.58(10)°) is three-coordinate, while the other
(N2) is four-coordinate. As indicated in Table 7, this differ-
ence is reflected in an asymmetry in the B—N and P—N
bond distances (1.4171(19) vs. 1.4762(19) Å and 1.7661(12)
vs. 1.8114(12) Å, respectively). The relatively short P(1)—
N(3) bond length (1.6403(12) Å) is comparable to the mean
value of -1.65 Å reported for the corresponding P—N bonds
in {Li[P(N-t-Bu)2]}4 (37). The two monomeric units in 5 are
linked by Li–N interactions to give a transoid ladder struc-
ture (38) with typical Li–N bond distances. The chelation of
the [PhB( -N-t-Bu)2PN-t-Bu]– anion to Li+ in 6 does not
change the bond angles at the P(III) centre from those ob-
served for 4c.
Fig. 5. ORTEP drawing of {Li[PhB( -N-t-Bu)2PN-t-Bu]}2 (5)
(30% probability ellipsoids). Only the ꢈ-carbon atoms of t-Bu
groups are shown. Starred atoms are related by the symmetry
transformation: –x + 2, –y, –z.
7
The multinuclear (1H, 13C, Li, 11B, and 31P) NMR data
for 5 in THF-d8 indicated the presence of two components,
5A and 5B, in solution in the approximate molar ratio 3:1.
1
The relative intensities of the H and 13C NMR resonances
for 5A and 5B both indicate C6H5, endo-t-Bu, and exo-t-Bu
tBu
N
tBu
N
tBu
N
P
PhB
P
Li(thf)3
PhB
Nt
Bu
N
the mean endocyclic P–N bond distance and the geometry at
N(3) is planar, suggesting a significant pꢇ–pꢇ interaction
between N(3) and P(1).
N
tBu
tBu
Li
(thf)2
5B
5A
© 2002 NRC Canada

830
Can. J. Chem. Vol. 80, 2002
groups in the ratio 1:2:1, consistent with the empirical for-
mula Li[t-BuN( -N-t-Bu)2BPh], in addition to a resonance
for a three-coordinate boron atom. Both 5A and 5B exhibit
singlets in the 31P NMR spectra with chemical shifts of ꢀ
129.9 and 68.1, respectively. It is well established that sol-
vation of Li+ centres in lithium derivatives of polyimido an-
ions of p-block elements may lead to the disruption of
polycyclic (cluster) structures or the stabilization of smaller
aggregates (39). A pertinent example is the tetramer
[Li{P(N-t-Bu)2}]4 (40), which forms the solvated dimer
[(THF)Li{P(N-t-Bu)2}]2 in THF solution (41). Another,
more dramatic, demonstration of this effect is the unraveling
of the cyclic ladder structure of [Li2{C(N-t-Bu)3}]2 (42)
the formation of long-lived radicals upon oxidation. Detailed
EPR studies will be necessary to characterize these radicals.
This propensity for radical formation is likely to lead to in-
teresting chemistry in future investigations of metal–halogen
exchange reactions of lithium boraamidinates, especially for
those metals which have two (or more) readily accessible
oxidation states.
Acknowledgments
We thank the Natural Science and Engineering Research
Council of Canada (NSERC) for financial support, Dr. R.
McDonald (University of Alberta) for X-ray data collection
of 1a and 1b, and Dr. M. Parvez for advice in the refinement
of the X-ray structures.
upon
solvation
to
give
the
open
ladder
[(THF)3Li2{C(NPh)3}]2 (43). Although the imido (NR)
groups are different in these two derivatives, the primary
cause of the different structures is clearly the Li+ centres.
For 5, in the presence of an excess of THF, the most likely
solvated complexes are 5A and 5B. The former has equiva-
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