Synthesis and characterization of N-silylated, C1-symmetric benzamidinates of lithium, sodium, and tin(II)

Article abstract of DOI:10.1139/V05-250

The synthesis and characterization of complexes obtained from the reactions between Li[N-t-Bu(SiMe₃)] (A) or the sodium analogue Na[N-t-Bu(SiMe₃)] (B) and the cyanoarene RCN (R = Ph or 4-MeOC ₆H₄) are discussed.

Full text of DOI:10.1139/V05-250

269
Synthesis and characterization of N-silylated,
C1-symmetric benzamidinates of lithium, sodium,
and tin(II)1
Floria Antolini, Peter B. Hitchcock, Alexei V. Khvostov, and Michael F. Lappert
Abstract: The synthesis and characterization of complexes obtained from the reactions between Li[N-t-Bu(SiMe3)] (A)
or the sodium analogue Na[N-t-Bu(SiMe3)] (B) and the cyanoarene RCN (R = Ph or 4-MeOC6H4) are discussed. These
are the THF adduct [Li{µ-cis-N(t-Bu)C(Ph)N(SiMe3)}(THF)]2 (1), the TMEDA adduct Li[N(t-Bu)C(Ph)N(SiMe3)]-
(TMEDA) (2), the neutral ligand-free lithium benzamidinate Li[N(t-Bu)C(C6H4OMe-4)N(SiMe3)] (3), and the THF
adduct Li[N(t-Bu)C(C6H4OMe-4)N(SiMe3)](THF) (3a). The preparation and structure of the crystalline compound
[Na{µ-cis-N(t-Bu)C(Ph)N(SiMe3)}(OEt2)]2 (4) is described. From the lithium benzamidinate 1 and Sn(II) chloride the
tin(II) complex [Sn{N(t-Bu)C(Ph)N(SiMe3)}2] (5) was obtained. The molecular structures of the crystalline compounds
1, 4, and 5 were established by X-ray diffraction. In 1 and 4 the benzamidinato ligand is both chelating and bridging,
with the Me3Si-substituted nitrogen atom as the bridging site. The central planar (MN)2 four-membered ring is a rhom-
bus in 1, with almost equal Li—N bond lengths, whereas in 4 the bonds to Na(1) are significantly longer than those to
Na(2). In 5, the ligand is N,N′-chelating.
Key words: alkali metals, tin(II), benzamidinates, NMR spectra, X-ray structures.
Résumé : On discute de la synthèse et de la caractérisation de complexes obtenus par les réactions du Li[N-t-
Bu(SiMe3)] (A) ou de analogue sodique Na[N-t-Bu(SiMe3)] (B) avec des cyanoarènes RCN (R = Ph ou 4-MeOC6H4) et
isolés sous la forme de benzamidinate de lithium neutre, sans ligand, [Li[N(t-Bu)C(C6H4OMe-4)N(SiMe3)] (3) ou sous
la forme d’adduits avec le THF [Li{µ-cis-N(t-Bu)C(Ph)N(SiMe3)}(THF)]2 (1) ou [Li[N(t-Bu)C(C6H4OMe-4)N(SiMe3)]-
(THF) (3a) et avec le TMEDA Li[N(t-Bu)C(Ph)N(SiMe3)](TMEDA) (2). On a aussi préparer le composé [Na{µ-cis-
N(t-Bu)C(Ph)N(SiMe3)}(OEt2)]2 (4) dont on a déterminé la structure cristalline. La réaction du benzamidinate de li-
thium (1) avec le chlorure d’étain(II) permet d’obtenir le complexe d’étain(II) [Sn{N(t-Bu)C(Ph)N(SiMe3)}2] (5). On a
déterminé les structures moléculaires des composés cristallins 1, 4 et 5 par diffraction des rayons X. Dans les compo-
sés 1 et 4, le ligand benzamidinato agit à la fois comme pont et comme agent chélatant alors que l’atome d’azote subs-
titué par un groupe Me3Si agit comme pont. Le cycle central planaire à quatre chaînons (MN)2 est rhombique dans le
composé 1 et les longueurs de ses liaisons Li—N sont pratiquement égales alors que dans le composé 4 les longueurs
des liaisons vers le Na(1) sont beaucoup plus longues que celles vers la Na(2). Dans le composé 5, le ligand est N,N′-
chélatant.
Mots clés : métaux alcalins, étain(II), benzamidinates, spectres RMN, structures déterminées par diffraction des rayons X.
[Traduit par la Rédaction] Antolini et al. 276
Introduction
R2, and R3, (ii) it can display a diversity of bonding modes
in its binding to the metal or metals, and (iii) it often is
firmly bound to the latter. Reviews have dealt with transition
metal amidinates (2a), s-, p-, d-, and f-block metal N,N′-
bis(trimethylsilyl)benzamidinates (R1 = SiMe3 = R3, R2 = an
aryl group) (2b), and “paddle-wheel” dinuclear formamidi-
nates of formula [M2{N(R1)C(H)N(R3)}4(Lax)n] (M = Mo,
Rh, Ni, or Ru; R1 = an aryl group = R3, Lax is the axial
ligand; n = 0–2) (2c).
Amidines and amidinates are valuable reagents in organic
synthesis (1). The amidinato ligand [N(R1)C(R2)N(R3)]–
(Am–) is widely employed in coordination chemistry; there
have already been more than 100 publications in the 21st
century. Its role often is that of a spectator ligand. Attractive
features are that (i) its steric and electronic properties can be
widely varied by judicious selection of the substituents R1,
Received 3 June 2005. Published on the NRC Research Press Web site at http://canjchem.nrc.ca on 17 March 2006.
This paper is dedicated to Dr. Arthur J. Carty, in recognition of his many important contributions to organometallic chemistry, and
as a mark of esteem and (by MFL) friendship.
F. Antolini, P.B. Hitchcock, A.V. Khvostov, and M.F. Lappert.2 The Chemical Laboratory, University of Sussex, Brighton
BN1 9QJ, UK.
1This article is part of a Special Issue dedicated to Professor Arthur Carty.
2Corresponding author (e-mail: M.F.Lappert@sussex.ac.uk).
Can. J. Chem. 84: 269–276 (2006)
doi:10.1139/V05-250
© 2006 NRC Canada

270
Can. J. Chem. Vol. 84, 2006
Some group 4 metal amidinates are catalysts for olefin
29Si) instrument, and referenced externally (7Li, using LiCl;
29Si, using SiMe4) or internally to the residual solvent reso-
nances. Unless otherwise stated, all NMR spectra were mea-
polymerization, as summarized in a recent survey on
advances in non-metallocene olefin polymerization (3). For
example, isotactic polypropylene was obtained using
tris(amidinato)zirconium(IV) complexes bearing a chiral N-
substituted ligand (4); and C1-symmetric amidinates,
[Zr(Am)2Me2] (Am: R1 ≠ R3), were moderately active for
formation of isotactic poly(1-hexene) (5). Cationic alumin-
ium amidinates, such as [Al{(N-i-Pr)2C-t-Bu}Me][B(C6F5)3Me],
were shown to be active single site catalysts for the living
polymerization of ethylene (6). Lanthanide metal amidin-
ates, [Ln{(N(c-C6H11))2CR2}3] (Ln = Nd, Gd, Yb; R2 = Me
or Ph), were highly active catalysts for the ring-opening
polymerization of ε-caprolactam (7).
1
sured at 298 K and other than H were proton-decoupled.
Electron impact mass spectra were taken from solid samples
using a Kratos MS 80 RF instrument. Melting points were
taken in sealed capillaries and are uncorrected. Elemental
analyses were determined by Medac Ltd., Brunel University,
Uxbridge, Middlesex, UK.
Preparations
[Li{␮-cis-N(t-Bu)C(Ph)N(SiMe3)}(THF)]2 (1)
A solution of PhCN (0.46 cm3, 4.49 mmol) in diethyl
ether (5 cm3) was added slowly to a cooled (–78 °C) solu-
tion of A (1.01 g, 4.49 mmol) in diethyl ether (15 cm3). The
resulting yellow mixture was allowed to warm to room tem-
perature and was stirred for 12 h. The slightly cloudy mix-
ture was filtered and the filtrate was concentrated and stored
at –25 °C for 24 h, yielding colorless crystals of the 1:1
ether adduct (0.58 g, 44.7%). As the diethyl ether was not
strongly bonded to the molecule, during the drying in vacuo
the complex gradually lost the coordinated solvent, compro-
mising the credibility of the analysis. Thus, the diethyl ether
adduct was recrystallized from a satd. THF solution and a
few colorless crystals of compound 1 were obtained (mp 55–
The more important methods that have been used to pre-
pare metal amidinates are (i) metallation of the amidine
HN(R1)C(R2)N(R3), (ii) ligand transfer reactions from either
an alkali metal (8) or trimethylsilyl (9) amidinate and a
metal chloride, (iii) addition of a metal hydrocarbyl to a
carbodiimide (e.g., LiR2 to R1N=C=NR3, R1 = SiMe3 = R3,
R2 = aryl), and (iv) the reaction between a metal bis(tri-
methylsilyl)amide and a cyanoarene. As for (iv), Sanger (in
our laboratory) found in 1973 that Li[N(SiMe3)2] and PhCN
in Et2O readily yielded Li[{N(SiMe3)}2CPh], which was
characterized by treatment with Me3SiCl, yielding the
benzamidine N(SiMe3)2C(Ph)=N(SiMe3) (10); substantial
extensions to this methodology up to 1990 have been sum-
marized (5b). In the present paper are described reactions of
the related, but C1-symmetric, alkali metal amides M[N-t-
Bu(SiMe3)] (M = Li (A) (11), M = Na (B) (12)) with PhCN
or 4-MeOC6H4CN.
1
80 °C). H NMR (C6D6) δ: 0.06 (s, 9H, SiMe3), 1.26 (s, 9H,
t-Bu), 1.35 (m, 4H, THF), 3.59 (m, 4H, THF), 7.09 (d, 1H,
3
3JHH = 7.20 Hz), 7.12 (m, 2H, Ph), 7.29 (d, 2H, JHH
=
7.12 Hz). 13C NMR (C6D6) δ: 2.91 (SiMe3), 25.43 (THF),
33.49 (CMe3), 51.38 (CMe3), 68.11 (THF), 126.59, 127.49,
Our prior publications in this area dealt with
[K{N(SiMe3)C(C6H3Me2-2,5)=C(H)SiMe3}]2 and [{UCl(µ-
Cl)(L)N(SiMe3)}2][UCl2(L)(Am)]2 (L = {N(SiMe3)C(Ph)}2CH,
Am = N(SiMe3)C(Ph)NC(Ph)=C(H)SiMe3) (13a), M[{N-
(CH=NSiMe3)}2CH]n [n = 3 and M = Li (13b), Na (13c);
n = 4, Tl (13c)); [Li(Am)(CNPh)]2 (13d), [K(Am)]2, [SnCl(Am)],
7
and 127.83 (Ph), 145.37 (ipso-C), 175.65 (NCN). Li NMR
(C6D6) δ: –0.48. 29Si NMR (C6D6) δ at 323 K: –8.08 and
–10.65; at 298 K, δ –10.69. MS (M denotes the monomer)
m/z (% and assignment): 508 (8%, [M2 – 2THF]+), 451
(72%, [M2 – 2THF – t-Bu]+), 395 (8%, [M2 – 2THF – 2t-
Bu + H]+), 375 (48%, [M2 – 2THF – Ph + H]+).
[Sn(Am)2], and [Hg(Am)2] (Am
=
N(SiMe3)C(Ph)-
NC(Ph)=C(SiMe3)2) (13e); Li[N(SiMe3)C(Ar)NC(Ar)=C(H)-
SiMe3](D) (D = TMEDA or THF with Ar = Ph; or D =
TMEDA, Ar = 4-MeC6H4) (13f); [Li(Am)]2, [M(Am)Cl2]
(M = Al or Ga) and [{Ln(Am)2(µ-Cl)}2] (Am = N(SiMe3)C-
(Ph)N(CH2)3NMe2, Ln = La or Ce) (13g); Li[N(SiMe3)C-
(H)NC(H)=C(H)C(t-Bu)=NSiMe3] (13h); and Li[N(Si-
Me2OMe)C(C6H3Me2-2,5)NC(C6H3Me2-2,5)=C(H)SiMe3]-
(TMEDA) (13i).
[Li{N(t-Bu)C(Ph)N(SiMe3)}(TMEDA)] (2)
A solution of PhCN (0.32 cm3, 3.11 mmol) in hexane
(5 cm3), was added to a cooled (0 °C) solution of A (0.47 g,
3.11 mmol) in hexane (25 cm3). The reaction mixture was
allowed to warm to room temperature and was stirred for
4 h. TMEDA (0.61 cm3, 4.04 mmol) was added to the mix-
ture, which was stirred for 12 h. The mixture was filtered;
the filtrate was concentrated and stored at 10 °C for 12 h.
Being cloudy, it was again filtered; the filtrate was concen-
trated and stored at 10 °C for a further 12 h to furnish
yellow crystals of compound 2 (0.71 g, 61.3%), mp 132–
135 °C. 1H NMR (C6D6) δ: 0.09 (s, 9H, SiMe3), 1.25 (s, 9H,
t-Bu), 1.78 (s, 4H, TMEDA), 2.02 (s, 12H, TMEDA), 7.06
Experimental
General remarks
All manipulations were performed under argon using stan-
dard Schlenk techniques. Hexane was dried and distilled
over sodium–potassium alloy, THF and diethyl ether were
dried and distilled over sodium–benzophenone and stored
over a sodium mirror under argon. TMEDA (Aldrich) was
distilled from CaH2 prior use. Nitriles (Aldrich) were dried
over molecular sieves. The alkali metal amides M[N-t-
Bu(SiMe3)] (M = Li (A) (14), Na (B) (12)) were synthesized
by published procedures. The NMR spectra were recorded
3
4
(tt, 1H, p-H of Ph, JHH = 7.6 Hz, JHH = 1.2 Hz), 7.15 (m,
3
2H, m-H of Ph), 7.35 (dd, 2H, p-H of Ph, JHH = 7.9 Hz,
4JHH = 1.2 Hz). 13C NMR (C6D6) δ: 4.05 (SiMe3), 34.32
(CMe3), 50.67 (CMe3), 125.80, 127.18 (CH of Ph), 146.47
(ipso-C), 174.56 (NCN). 29Si NMR (C6D6) δ at 50 °C:
7
–15.19. Li NMR (C6D6) δ: –0.34. MS m/z (% and assign-
ment): 375 (34%, [M + Li – H]+), 262 (37%, [M –
TMEDA + Li + H]+), 247 (79%, [M – Li – TMEDA]+), 191
(100%, [247 – t-Bu]+).
1
on a Bruker DPX 300 (300.1 MHz for H, 75.5 MHz for
7
13C, and 116.6 MHz for Li) or an AMX 500 (49.7 MHz for
© 2006 NRC Canada

Antolini et al.
271
sulting solution was allowed to warm to room temperature,
stirred for 12 h, concentrated and stored at –25 °C for 24 h,
yielding colorless crystals of compound 4 (2.24 g, 69%). H
[Li{N(t-Bu)C(C6H4OMe-4)N(SiMe3)}] (3)
4-MeOC6H4CN (0.49 g, 3.62 mmol) was added in small
portions to a cooled (–78 °C) solution of A (0.55 g,
3.63 mmol) in diethyl ether (45 cm3). The reaction mixture
was allowed to warm to room temperature and was stirred
for 12 h. The cloudy mixture was filtered and the filtrate
concentrated and stored at –25 °C for 12 h to afford an
amorphous, ether-solvated product (Et2O–benzamidinate,
1:2; calculated from the NMR integrals), recrystallized from
a saturated hexane solution yielding compound 3 (0.29 g,
28%), as a yellow crystalline material (not suitable for X-ray
1
NMR (C6D6) δ: 0.04 (s, 9H, SiMe3), 1.20 (t, 6H, Et2O), 1.25
(s, 9H, t-Bu), 3.24 (q, 4H, Et2O), 7.00–7.25 (m, 5H, Ph). 13
C
NMR (333 K, C6D6) δ: 3.70 (SiMe3), 15.30 (CMe3), 34.12
(Et2O), 51.38 (CMe3), 65.91 (Et2O), 126.28, 127.49, 129.08
(Ph), 146.23 (ipso-C), 175.10 (NCN). 23Na NMR (C6D6) δ:
8.54 (∆ν1/2 ≈ 2.6 kHz). 29Si NMR (ineptrd, C6D6) δ: –13.37.
Anal. calcd. for C18H33N2NaOSi (%): C 62.8, H 9.65, N
8.13; found: C 62.3, H 9.38, N 8.09.
1
analysis); mp 127–130 °C. H NMR (C6D6) δ: 0.01 (s, 9H,
[Sn{N-t-BuC(Ph)N(SiMe3)}2] (5)
SiMe3), 1.25 (s, 9H, t-Bu), 3.30 (s, 3H, MeO), 6.75 (d, 2H,
3
3JHH = 8.37 Hz), 7.17 (d, 2H, JHH = 9.42 Hz). Anal. calcd.
SnCl2 (0.481 g, 2.54 mmol) was added to a cooled (0 °C)
solution of 1 (1.48 g, 5.08 mmol) in diethyl ether (40 cm3).
The mixture was warmed to room temperature and stirred
for 12 h. The volatiles were removed at 30 °C and 10–2 Torr
(1 Torr = 133.3224 Pa). The residue was extracted into di-
ethyl ether. The extract was filtered and the filtrate concen-
trated and stored at –25 °C for 4 months to furnish pale
yellow crystals of compound 5 (1.0 g, 64%), mp 73–80 °C.
1H NMR (C6D6) δ: 0.11 (s, 9H, SiMe3), 1.24 (s, 9H, t-Bu),
7.01 (m, 5H, Ph). 13C NMR (C6D6) δ: 2.86 (SiMe3), 33.22
(CMe3), 53.94 (CMe3), 126.88, 128.32, 128.99 (Ph), 142.15
(ipso-C), 170.01 (NCN). 29Si NMR (C6D6) δ: –1.75. 119Sn
NMR (C6D6) δ: –258.5. MS m/z (% and assignment): 614
(25%, [M]+), 557 (6%, [M – t-Bu]+), 367 (27%, [M – (N-t-
Bu)C(Ph)NSiMe3)]+), 247 (48%, [(N-t-Bu)C(Ph)(NSiMe3)]+),
233 (50%, [(N-t-Bu)C(Ph)(NSiMe3) – t-Bu]+), 191 (100%,
[SiMe3N=CPh]+). Anal. calcd. for C28H46N4Si2Sn (%): C
54.8, H 6.59, N 9.11; found: C 54.0, H 7.57, N 9.13.
for C15H25LiN2OSi (%): C 63.5, H 9.41, N 9.83; found: C
62.5, H 8.49, N 9.81.
Variable temperature (V.T.) experiments
1
IR (cm–1): 2246 ν(CN). H NMR (C6D5CD3 at ca. 231 K)
δ: 0.09, 0.18, 0.23 (s, 9H, SiMe3), 1.28, 1.32, 1.35 (s, 9H, t-
Bu), 3.17, 3.22, 3.34 (s, 3H, MeO), 6.66 (m, 2H), 7.18 (d,
3
3
1H, JHH = 8.21 Hz), 7.30 (d, 1H, JHH = 7.95 Hz); at ca.
243 K δ: 0.08, 0.16, 0.21 (s, 9H, SiMe3), 1.26, 1.31, 1.34 (s,
9H, t-Bu), 3.17, 3.22, 3.24, 3.30 (s, 3H, MeO), 6.65 (d, 1H,
3JHH = 7.93 Hz), 6.67 (d, 1H, JHH = 6.86 Hz), 7.16 (d, 1H,
3
3JHH = 8.24 Hz), 7.25 (d, 1H, JHH = 7.97 Hz); at ca. 253 K
3
δ: 0.07, 0.14, 0.19 (s, 9H, SiMe3), 1.24, 1.29, 1.34 (s, 9H, t-
Bu), 3.17, 3.22, 3.24, 3.27 (s, 3H, MeO), 6.72 (m, 2H), 7.17
(m, 2H); at ca. 273 K δ: 0.90 (m, 9H, SiMe3), 1.20 (s, 9H, t-
3
Bu), 3.23, 3.25 (m, 3H, MeO), 6.67 (m, 3H, (d, 2H, JHH
=
6.92 Hz)), 7.13 (d, 1H, 3JHH = 7.27 Hz); at ca. 298 K δ: 0.01
(s, 9H, SiMe3), 1.18 (s, 9H, t-Bu), 3.26 (s, 3H, MeO), 6.66
3
(d, 2H, o-H, JHH = 7.44 Hz), 7.09 (m, 2H, m-H); at ca. 338
X-ray crystallographic analysis of [Li{µ-cis-N(t-
Bu)C(Ph)N(SiMe3)}(THF)]2 (1), [Na{µ-cis-N(t-
K δ: 0.09 (s, 9H, SiMe3), 1.25 (s, 9H, t-Bu), 3.43 (s, 3H,
3
3
3
MeO), 6.66 (d, 2H, JHH = 6.82 Hz), 7.25 (d, 2H, JHH
=
Bu)C(Ph)N(SiMe3)}(OEt2)]2 (4), and [Sn{η -N(t-
8.52 Hz). 13C NMR (C6D6 at ca. 298 K) δ: 2.78 (SiMe3),
33.41 (CMe3), 54.68 (CMe37), 113.05 (MeO), 126.32, 128.40,
129.23 (Ph), 158.95 (CN). Li NMR (C6D6 at ca. 298 K) δ:
–0.24. 29Si NMR (C6D6 at 338 K) δ: –9.92. MS m/z (% and
assignment): 277 (85%, [M – Li]+), 263 (38%, [M – Li –
Me]+), 221 (100%, [M – t-Bu – H]+), 206 (65%, [M – t-Bu –
Me – H]+), 73 (100%, [SiMe3]+), 57 (12%, [t-Bu]+).
Bu)C(Ph)N(SiMe3)}2] (5)
Diffraction data for compound 1 were collected on a
Nonius CAD4 and for 4 and 5 on a Nonius Kappa-CCD
diffractometer using monochromated Mo Kα radiation
(λ = 0.710 73 Å). Crystals were directly mounted on the
diffractometer under a stream of cold nitrogen gas. In com-
plex 4 there was unresolved end-for-end disorder in one
ligand giving rise to partial Si/C occupancy for the sites la-
beled Si(2) and C(25) and some unrealistic ADPs for this
ligand. DELU restraints were applied. Dimensions in the
disordered ligand are unreliable. In both ligands of complex
5 there was unresolved end-for-end disorder of the SiMe3
and t-Bu groups; 0.86:0.14 for Si1 and C8, and 0.65:0.35 for
Si2 and C22. Bonds and angles involving these atoms are
therefore unreliable. The structures were refined on all F2
using SHELXL 97 (15). Further details are in Table 1.
[Li{␮-cis-N(t-Bu)C(C6H4OMe-4)N(SiMe3)}(THF)] (3a)
Evaporation of a THF solution of 3 produced the amor-
1
phous complex 3a. H NMR (C6D6) δ: 0.11 (s, 9H, SiMe3),
1.29 (s, 9H, t-Bu), 1.39 (m, 4H, CH2 of THF), 3.33 (s, 3H,
MeO), 3.64 (m, 4H, OCH2 of THF), 6.78 (d, 2H, m-H of
3
3
C6H4, JHH = 8.65 Hz), 7.22 (d, 2H, o-H of C6H4, JHH
=
8.48 Hz). 13C NMR (C6D6) δ: 3.22 (SiMe3), 25.51 (THF),
33.65 (CMe3), 51.35 (CMe3), 54.63 (MeO), 68.16 (THF),
112.90 (m-C of C6H4), 129.05 (o-C of C6H4), 138.07 (ipso-C
7
of C6H4), 158.75 (p-C of C6H4), 175.58 (NCN). Li NMR
(C6D6) δ: 1.86. 29Si NMR (inverse gated, d1 = 25 s, C6D6) δ:
Results and discussion
1
–11.30. H NOE (C6D6, η, %) irradiation of MeO: 5.7 (m-H
1
of C6H4). H{7Li} NOE (C6D6, – η, %): 0.08 (SiMe3), 0.12
Reactions between Li[N-t-Bu(SiMe3)] (A) and the nitrile
RCN (R = Ph or 4-MeOC6H4)
(OCH2 of THF), 0.15 (o-H of C6H4).
The initial targets in the nitrile reactions were lithium
amidinates derived from Li[N-t-Bu(SiMe3)] (A) (11). This
was based on an analogy with reactions of Li[N(SiMe3)2]
(14) with a cyanoarene (10, 2b). The results of treating A
with PhCN and 4-MeOC6H4CN are summarized in Scheme 1.
[Na{␮-cis-N(t-Bu)C(Ph)N(SiMe3)}(OEt2)]2 (4)
A solution of PhCN (0.98 cm3, 9.56 mmol) in diethyl
ether (5 cm3) was added slowly to a cooled (0 °C) solution
of B (1.60 g, 9.56 mmol) in diethyl ether (50 cm3). The re-
© 2006 NRC Canada

272
Can. J. Chem. Vol. 84, 2006
Table 1. Crystal data and structure refinements for 1, 4, and 5.
Compound
1
4
5
Empirical formula
C36H62Li2N4O2Si2
C36H66N4Na2O2Si2
C28H46N4Si2Sn
Formula weight
Crystal system
Space group
a (Å)
b (Å)
c (Å)
652.96
689.09
Tetragonal
P43 (No. 78)
12.0845(4)
12.0845(4)
29.6425(6)
90
613.56
Monoclinic
C2/c (No. 15)
11.413(2)
17.370(3)
20.526(4)
90
99.27(2)
90
4016.3(13)
4
Triclinic
P1 (No. 2)
10.7754(2)
11.9247(3)
14.0436(4)
67.740(1)
73.941(1)
86.438(2)
1 603.01(7)
2
α (°)
β (°)
90
90
γ (°)
V (Å3)
4 328.8(2)
4
1.06
Z
Dc (Mg m–3)
1.08
1.27
µ(Mo Kα) (mm–1)
Reflections collected
0.12
3712
0.13
12 138
0.89
25 339
Independent reflections
Reflections with I > 2σ(I)
Data/restraints/parameters
Final R indices [I > 2σ(I)]
3533 [R(int) = 0.029]
2376
7 216 [R(int) = 0.043]
5 783
9 307 [R(int) = 0.035]
7 955
3533/0/208
R1 = 0.057,
wR2 = 0.120
R1 = 0.098,
wR2 = 0.139
7 216/209/416
R1 = 0.062,
wR2 = 0.148
R1 = 0.084,
wR2 = 0.164
9 307/0/318
R1 = 0.033,
wR2 = 0.072
R1 = 0.044,
wR2 = 0.077
R indices (all data)
Scheme 1. Synthesis of complexes 1–3 and 5.
The colorless, crystalline, binuclear THF-bound lithium
benzamidinate [Li{µ-cis-N(t-Bu)C(Ph)N(SiMe3)}(THF)]2
Compounds 1, 2, and 3 gave satisfactory multinuclear (1H,
7
13C, Li and 29Si) NMR and mass (EI) spectra, as well as
(1) was obtained after crystallizing the initially formed
weakly bound diethyl ether adduct from THF. Alternatively,
by carrying out the reaction between A and PhCN in hexane,
followed by addition of TMEDA, the product was the yel-
low, crystalline, and probably (MS) mononuclear TMEDA
adduct, Li[N(t-Bu)C(Ph)N(SiMe3)](TMEDA) (2), as in the
X-ray-characterized compounds, [Li{N(SiMe3)C(Ph)N(SiMe3)}-
(TMEDA)] (16), [Li{N(myrtanyl)C(Ph)N(SiMe3)}(TMEDA)]
(4), and [Li{N(i-Pr)C(terpenyl)N(i-Pr)}(TMEDA)] (17).
microanalysis for 3. Complex 1 was further authenticated by
its molecular structure determination by single crystal X-ray
diffraction; X-ray quality crystals were not obtained for 2, 3,
and 3·THF. Whereas the mass spectrum of 3 was consistent
with it being a mononuclear complex, the 1H NMR spectrum
in C6D5CD3 at ambient temperature gave broad signals, which
sharpened only upon heating. In the 231–338 K range, there
appeared to be three species present. The heteronuclear
1H{7Li} NOE experiment of the THF adduct 3a suggests
that in benzene solution the compound has the cis-dimeric
structure, as found in the crystalline THF adduct 1 (Fig. 1).
The THF-solvated lithium benzamidinate [Li{µ-cis-N(t-
Bu)C(Ph)N(SiMe3)}(THF)]2 (1) was crystallized after stor-
age for 1 day at –25 °C. An ORTEP representation of com-
plex 1 is depicted in Fig. 1. Selected bond distances (Å) and
angles (°) are reported, together with data for the related
The reaction between A and 4-MeOC6H4CN in diethyl
ether yielded the expected lithium benzamidinate as an
amorphous powder containing weakly bound diethyl ether;
crystallization from hexane afforded the neutral donor-free,
pale-yellow, crystalline compound [Li{N(t-Bu)C(C6H4OMe-
4)N(SiMe3)}] (3), which in THF solution yielded the adduct
[Li{N(t-Bu)C(C6H4OMe-4)N(SiMe3)}(THF)] (3a).
© 2006 NRC Canada

Antolini et al.
273
Fig. 1. Molecular structure of [Li{µ-cis-N(t-
Bu)C(Ph)N(SiMe3)}(THF)]2 (1) (20% ellipsoids, H atoms are
omitted for clarity).
Table 2. Selected bond distances (Å) and angles (°) for com-
pounds 1 and [Li{µ-N(SiMe3)C(C6H4OMe-4)N(SiMe3)2}(THF)]2
(6) (19).
Compounds
1
6 (19)
Bond distances (Å)
Li—N(1)
Li—N(2)
2.035(5)
2.199(5)
2.063(5)
2.099(9)
2.145(8)
2.073(8)
Li—N(2)′
Bond angles (°)
N(1)-C(1)-N(2)
N(1)-Li-N(2)
117.5(2)
65.2(2)
120.1(4)
65.5(3)
MeC6H4)N(SiMe3)}(NCC6H4Me-4)]2 (20e), and [Li{N-
(SiMe3)C(Ph)N(SiMe3)}(NCPh)]2 (20f).
Reactions between Na[N-t-Bu(SiMe3)] (B) and PhCN
It had previously been shown that the sodium
benzamidinate, [Na{µ-N(SiMe3)C(Ph)N(SiMe3)}(OEt2)]2-
(OEt2), had been obtained from the reaction of Na[N-
(SiMe3)2] with PhCN (21). Hence, Na[N-t-Bu(SiMe3)] (B)
was treated with PhCN in Et2O–hexane at low temperature;
an inseparable mixture of starting materials and insertion
products was formed. Various attempts to isolate the sodium
benzamidinate (the presence of which was inferred by means
of 13C NMR spectroscopy of the reaction mixture (δ 174.8
NCN)) failed. It may be that some of the unidentified prod-
ucts consisted of oligomerized PhCN, as in the case of treating
PhCN with Li-n-Bu (10, 22). In contrast, when the reaction
was carried out in Et2O, the expected product, [Na{µ-cis-
N(t-Bu)C(Ph)N(SiMe3)}(OEt2)]2 (4), was obtained (eq. [1]).
compound, [Li{µ-N(SiMe3)C(C6H4OMe-4)N(SiMe3)2}(THF)]2
(6) (18, 19), in Table 2.
A fused array of three, four-membered rings forms the
core of molecule 1. The dihedral angles between each of the
outer rings and the central are 129.85° and 112.19°. The
central [LiN(2)Li′N(2)′] rhomboid core is puckered, being
folded by 13.7° around the Li(1)-Li(2) vector. The presence
of a tert-butyl group imposes a distinct asymmetry on the N-
C-N system (N(1)—C(1) = 1.302(3) Å and C(1)—N(2) =
1.370(3) Å). This corresponds to differences in the Li—N(1)
and Li—N(2) bond lengths. The two nitrogen atoms of each
benzamidinate ligand differ in that the tert-butyl-bound N is
three-coordinate, whereas the four-coordinated trimethylsilyl-
bound N atom bridges the two lithium atoms; thus, each
ligand is N,N′-chelating, but also N(SiMe3)-bridging. Due to
steric impediments, the tert-butyl groups in complex 1 are
arranged in a cis fashion. This creates narrower folding an-
gles of the ladder structure core (112.89° and 129.85°) than
in 6 (130.4° and 145.3°) (19). Thus, compound 1 exhibits a
more compact architecture than that of 6, evident also in the
narrower N(1)-C(1)-N(2) angle in 1 than in 6 (19). A further
structural detail concerns the orientation of the phenyl rings.
The steric influence of the SiMe3 and t-Bu groups imposes,
at the aromatic substituents, a nearly orthogonal orientation
(80.5° for one phenyl group and 78.2° for the other) with re-
spect to the N-C-N heteroallylic units, which precludes any
significant conjugation between the two π systems.
Et2O
[1]
[Na{N-t-Bu(SiMe3)}]3 + 3PhCN ⎯⎯⎯→
3/2[Na{µ-cis-N(t-Bu)C(Ph)N(SiMe3)}(OEt2)]2
(4, 69%, X-ray)
The sodium benzamidinate 4 was crystallized from the re-
action mixture at –25 °C. An ORTEP representation of its
molecular structure is shown in Fig. 2. Selected bond dis-
tances (Å) and angles (°), together with data for the related
compound, [Na{µ-N(SiMe3)C(Ph)N(SiMe3)}(OEt2)]2(OEt2)
(7) (21), are listed in Table 3.
There is disorder between the silicon and carbon atoms
(Si2 and C25) attached to N3 and N4, respectively, of one of
the benzamidinato ligands. The N-silylated, four-coordinate
nitrogen atom N1 bridges the sodium atoms Na1 and Na2,
whereas the tert-butyl-bound nitrogen atom N2 is in a three-
coordinate environment. Thus, the ligand is both N1,N2-
chelating and N1-bridging. The difference between the
Na1—N bond lengths of the bridging (N1) and terminal
(N2) atoms is 0.38 Å, whereas the corresponding value for
compound 7 is 0.19 Å (21). While the angle N1-Na1-N2 is
similar in 4 and 7, the angle N1-C1-N2 is slightly smaller in
4 than in 7 (21). The dihedral angle between the C2-C7
phenyl ring plane and the N1-C1-N2 plane is 78.1°.
Other examples of dimeric crystalline N-silylated lithium
benzamidinates include [Li{µ-N(n-Pr)C(Ph)N(SiMe3)}(OEt2)]2
(20a), [Li(Am)]2 (Am = N(SiMe3)C(Ph)N(CH2)nNMe2, n =
2 (20b) or 3 (13g)), [Li{N(SiMe3)C(Ph)NC(Ph)=C(SiMe3)2}-
(NCPh)]2 (13d), [Li{N(SiMe3)C(Ph)NC(Ph)=C(H)SiMe3}-
(THF)]2 (13f), [Li{N((1R,2R)-(–)-2-(trimethylsilylamino)-
cyclohexyl)propyl)C(Ph)N(SiMe3)}]2 (20c), several phenyl-
fluorinated benzamidinates (20d), [Li{N(SiMe3)C(4-
Synthesis and characterization of [Sn{N(t-
Bu)C(Ph)N(SiMe3)}2] (5)
As indicated in Scheme 1, the reaction between 1 and
SnCl2 in diethyl ether yielded crystalline [Sn{N(t-
Bu)C(Ph)N(SiMe3)}2] (5), which was characterized by ele-
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274
Can. J. Chem. Vol. 84, 2006
Fig. 2. Molecular structure of [Na{µ-cis-N(t-Bu)C(Ph)N(SiMe3)}(OEt2)]2 (4) (20% ellipsoids, H atoms are omitted for clarity).
Table 3. Selected bond distances (Å) and angles (°)
for compounds 4 and [Na{µ-N(SiMe3)C(Ph)N-
(SiMe3)}(OEt2)]2(OEt2) (7) (21).
Fig. 3. Molecular structure of [Sn{N-t-BuC(Ph)N(SiMe3)}2] (5)
(20% ellipsoids, H atoms are omitted for clarity).
Compounds
4
7 (21)
Bond distances (Å)
Na(1)—N(1)
Na(1)—N(2)
Na(1)—N(3)
Na(1)—N(4)
Na(2)—N(1)
Na(2)—N(4)
2.752(4)
2.376(4)
2.412(4)
2.758(5)
2.380(4)
2.336(4)
2.671(5)
2.482(5)
2.426(5)
2.725(5)
2.459(5)
2.397(5)
Bond angles (°)
N(1)-Na(1)-N(2)
N(3)-Na(1)-N(4)
N(1)-Na(1)-N(4)
N(1)-Na(2)-N(4)
N(1)-C(1)-N(2)
N(3)-C(15)-N(4)
52.07(12)
51.02(19)
100.02(12)
127.07(16)
118.3(3)
52.8(2)
52.8(2)
105.9(2)
124.9(2)
121.0(5)
121.0(5)
Crystalline
5
is
a
monomer with N,N′-chelating
benzamidinato ligands. In each of the latter, there is unre-
solved end-for-end disorder of the SiMe3 and CMe3 groups:
0.86:0.14 for Si1 and C8, and 0.65:0.35 for Si2 and C22;
bond lengths and angles involving these atoms are therefore
unreliable. The structure of 5 is closely similar to that of 8
(24). The geometric parameters of each of the two Sn-N-C-
N′ rings in both 5 and 8 are almost identical. The dihedral
angle between the N1-Sn-N2 and N3-Sn-N4 planes in 5 is
86°, while that between each N-C-N plane and the attached
aromatic C6 plane is ca. 78°. There clearly is a stereo-
chemically active lone pair of electrons at the Sn atom.
Other Sn(II) benzamidinates related to 5 and 8 (24) that
have been reported include [Sn{N(SiMe3)C(Ph)NC-
(Ph)=C(SiMe3)2}2] (13d), [Sn{N(SiMe2Ph)C(Ph)N(SiMe2-
Ph)}2] (24), [Sn{N(SiMe3)C(Me)N(SiMe3)}2] (25), and
[Sn{N(SiMe3)C(C6H4Ph-4)N(Ph)}2] (26).3
118.8(5)
mental analysis, multinuclear NMR spectra in C6D6, and its
EI mass spectrum. The 119Sn NMR chemical shift value of δ
–258.5 for 5 may be compared with the δ –387 for the tin(II)
azaallyl Sn[N(SiMe3)C(t-Bu)C(H)SiMe3]2, which like 5 has
a four-coordinate tin(II) atom at the spiro junction of two
strained four-membered rings (23).
An ORTEP representation of complex 5 is depicted in
Fig. 3. A list of selected bond distances (Å) and angles (°),
together with complementary data on [Sn{N(SiMe3)C(Ph)N-
(SiMe3)}2] (8) (24), is presented in Table 4.
3 Supplementary data for this article are available on the journal Web site (http://canjchem.nrc.ca) or may be purchased from the Depository
of Unpublished Data, Document Delivery, CISTI, National Research Council Canada, Ottawa, ON K1A 0R6, Canada. DUD 5000. For more
information on obtaining material refer to http://cisti-icist.nrc-cnrc.gc.ca/irm/unpub_e.shtml. CCDC 273350–273352 contain the crystallo-
graphic data for this manuscript. These data can be obtained, free of charge, via http://www.ccdc.cam.ac.uk/conts/retrieving.html (Or from
the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax +44 1223 336033; or de-
posit@ccdc.cam.ac.uk).
© 2006 NRC Canada

Antolini et al.
275
Table 4. Selected bond distances (Å) and angles (°) for 5 and [Sn{N(SiMe3)C(Ph)N(SiMe3)}2]
(8) (24).
Compounds
5
8 (24)
Bond distances (Å)
Sn—N(1,3)
Sn—N(2,4)
2.224(2), 2.229(2)
2.384(2), 2.382(2)
2.239(2), 2.245(2)
2.375(2), 2.374(4)
Bond angles (°)
N(1)-Sn-N(3)
N(2)-Sn-N(3)
N(2)-Sn-N(4)
N(1)-Sn-N(4)
94.9(1)
97.5(1)
146.3(1)
98.1(1)
58.2(1)
115.1(2)
93.3(1)
95.3(1)
143.1(1)
94.2(1)
58.7 (1) and 58.9(1)
117.2(2) [117.4(2)]
N(1,3)-Sn-N(2,4)
N(1)-C(1)-N(2) [N(4)-C(15)-N(3)]
11. (a) R.P Bush, N.C. Lloyd, and C.A. Pearce. J. Chem. Soc.
Chem. Commun. 1270 (1967); (b) R. Brüning, W. Kalk, and I.
Schumann-Ruidisch. Z. Naturforsch. B, 23, 307 (1968).
12. F. Antolini, P.B. Hitchcock, A.V. Khvostov, and M.F. Lappert.
Eur. J. Inorg. Chem. 3391 (2003).
Conclusions
Five new crystalline benzamidinates bearing the C1 sym-
metric [N(t-Bu)C(R)N(SiMe3)]– ligand (Am–) have been
prepared
and
characterized:
[Li(Am)(THF)]2
(1),
13. (a) P.B. Hitchcock, M.F. Lappert, and D.-S. Liu. J. Organomet.
Chem. 488, 241 (1995); (b) W.M. Boesveld, P.B. Hitchcock,
and M.F. Lappert. J. Chem. Soc. Dalton Trans. 4041 (1999);
(c) W.M. Boesveld, P.B. Hitchcock, and M.F. Lappert. Angew.
Chem. Int. Ed. 39, 222 (2000); (d) P.B. Hitchcock, M.F.
Lappert, and M. Layh. J. Chem. Soc. Dalton Trans. 3113
(1998); (e) C.F. Caro, P.B. Hitchcock, M.F. Lappert, and M.
Layh. Chem. Commun. (Cambridge), 1297 (1998); (f) P.B.
Hitchcock, M.F. Lappert, M. Layh, D.-S. Liu, R. Sablong, and
T. Shun. J. Chem. Soc. Dalton Trans. 2301 (2000); (g) D.
Doyle, Yu.K. Gun’ko, P.B. Hitchcock, and M.F. Lappert. J.
Chem. Soc. Dalton Trans. 4093 (2000); (h) W.M. Boesveld,
P.B. Hitchcock, and M.F. Lappert. J. Chem. Soc. Perkin Trans.
1, 1103 (2001); (i) P.B. Hitchcock, M.F. Lappert, and X.-H.
Wei. J. Organomet. Chem. 683, 83 (2003).
Li(Am)(TMEDA) (2), Li(Am) (3), [Na(Am)(OEt2)]2 (4), and
[Sn(Am)2] (5) (R = Ph in 1, 2, 4, and 5; R = 4-MeOC6H4 in
3). The X-ray structures of 1, 4, and 5 are reported. The
Am– ligand in 1 and 4 is both chelating and via N(SiMe3) is
also bridging. The two structures differ in that the core of
the planar M-N-CN′-M′ ring is rhomboidal in 1 (M = Li),
but C2 symmetric with unequal contiguous Na—N bond
lengths in 4. The Am– ligand in 5 functions as a chelate.
Acknowledgments
1
1
We thank Dr. A.G. Avent for recording H NOE, H{7Li}
NOE, 29Si-, 119Sn-, and 23Na-NMR spectra, the EU, and the
University of Bologna, Bologna, Italy for provision of a
studentship for FA, and the UK Engineering and Physical
Sciences Research Council (EPSRC) for a fellowship for
AVK.
14. (a) D. Mootz, A. Zinnius, and B. Böttcher. Angew. Chem. Int.
Ed. Engl. 8, 378 (1969); (b) R.D. Rogers, J.L. Atwood, and R.
Grüning. J. Organomet. Chem. 157, 229 (1978).
15. G.M. Sheldrick. SHELXL-97 [computer program]. University
of Göttingen, Göttingen, Germany. 1997.
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