Syntheses and structures of zinc and tin(II) compounds with hemilabile N-silyl-tert-butylamido and N-silyl-p-tolylamido ligands that contain pendent tert-butoxy groups

Article abstract of DOI:10.1016/j.poly.2011.08.011

Syntheses and solid-state structures of zinc and tin(II) compounds, containing the N-silyl-amide ligands (O^tBu)(NR)SiMe₂, R = ^tBu (L^tBu), or R = p-tolyl (L^pTol), are reported. The N-silyl amines were

Full text of DOI:10.1016/j.poly.2011.08.011

Polyhedron 30 (2011) 2856–2862
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Polyhedron
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Syntheses and structures of zinc and tin(II) compounds with hemilabile
N-silyl-tert-butylamido and N-silyl-p-tolylamido ligands that contain pendent
tert-butoxy groups
Gregory L. Fondong a, Edmond Y. Njua a, Alexander Steiner b, Charles F. Campana c, Lothar Stahl a,
⇑
a Department of Chemistry, University of North Dakota, Grand Forks, ND 58202, USA
b Department of Chemistry, Crown Street, University of Liverpool, Liverpool L69 7ZD, UK
c Bruker AXS Inc., Madison, WI 53711, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Syntheses and solid-state structures of zinc and tin(II) compounds, containing the N-silyl-amide ligands
(OtBu)(NR)SiMe2, R = tBu (LtBu), or R = p-tolyl (LpTol), are reported. The N-silyl amines were synthesized by
modified published procedures from commercially available Me2SiCl2, tBuOH, and tBuNH2, or
p-Me-C6H4NH2, respectively. Treatment of SnCl2 with LiLpTol furnished Sn(LpTol)2, which was X-ray struc-
turally characterized and shown to contain two covalent Sn–N bonds and two asymmetrical O ? Sn
Received 24 March 2011
Accepted 10 August 2011
Available online 25 August 2011
donor bonds. The single-crystal X-ray structure of Sn(LtBu
)2 revealed a much more symmetrically-coordi-
Keywords:
(N-tert-butoxydimethylsilyl)-tert-
butylamide
(N-tert-butoxydimethylsilyl)-p-tolylamide
N,O-donor ligands
N-silyl-amides
nated, pseudo-trigonal-bipyramidal tin atom. Aminolysis of diethylzinc with HLpTol produced [EtZn(Lp-
Tol)]2, which crystallized as a centrosymmetric dimer, containing four-coordinate zinc atoms connected
by bridging amides. Zinc dichloride, by contrast, reacted with two equivalents of LiLtBu to produce the
homoleptic, pseudo-spirocyclic Zn(LtBu)2.
Ó 2011 Elsevier Ltd. All rights reserved.
Hemilabile ligands
Zinc(II) amides
Tin(II) amides
1. Introduction
made it popular among main group and transition metal chemists
alike.
Secondary amides are versatile ligands for main-group and
transition metals, whose perhaps best known representative is
the bis(trimethylsilyl)-substituted analog N(SiMe3)2, A, shown in
Chart 1 [1,2]. Metal complexes of this ligand and its variants were
first reported almost half a century ago [3], but now representa-
tives for virtually all main-group [4–14] and transition [15–19]
metals are known. It was suggested relatively early in the research
on bulky, secondary amides that these molecules can be
considered steric equivalents of cyclopentadienide, which in turn
led to their widespread acceptance among organometallic chem-
ists. The more recent success of amides of the type NRAr
(Ar = 3,5-C6H3Me2, R = tBu, Me), B, as stabilizing ligands for early
transition metals that are useful in the activation of small mole-
cules renewed interest in the coordination chemistry of these
N-donor ligands [20–22]. Among the most popular main group
derivatives of A is the heterocarbenoid Sn[N(SiMe3)2]2, C, which
was first reported by Lappert and co-workers [23–27]. The diverse
reactivity patterns and reaction products of this stannylene have
We are studying insertions of C and its germanium analog into
the P–Cl bonds of chlorophosphines. Because these reactions are
often too fast to be followed by conventional NMR techniques,
we tried to attenuate the reactivity of the carbenoids by using
amides with intramolecular donor groups. For reasons of synthetic
utility we chose tert-butyl- and p-tolyl-amide-based ligands of the
type (OtBu)(NR)SiMe2, D, because they can be prepared from inex-
pensive dichlorodimethylsilane and the appropriate amine and
tert-butanol. The main-group chemistry of (OtBu)(HNtBu)SiMe2
had been systematically investigated by Veith et al. [28–32]. Later
Teuben and co-workers [33,34] and Edelmann and co-workers [35]
described yttrium and lanthanide complexes, respectively, of this
ligand.
In addition to ease of synthesis and chelating ability, one of the
greatest assets of D is its modular nature, which makes it well sui-
ted for reactivity studies that probe both electronic and steric sub-
stituent effects. Below we report on syntheses and solid-state
structures of N-(tert-butoxydimethylsilyl)-tert-butylamido and
N-(tert-butoxydimethylsilyl)-p-tolylamido derivatives of tin(II).
To better understand the ligand effects in the absence of lone-pair
electrons, we also synthesized and X-ray structurally characterized
their mono- and di-substituted zinc complexes.
⇑
Corresponding author. Tel.: +1 701 777 2242; fax: +1 701 777 2331.
E-mail address: lstahl@chem.und.edu (L. Stahl).
0277-5387/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2011.08.011

G.L. Fondong et al. / Polyhedron 30 (2011) 2856–2862
2857
Scheme 2. Synthesis of 3.
Chart 1. Selected secondary amides and a tin(II) derivative of N(SiMe3)2.
2. Results and discussion
The syntheses of all ligands reported herein are based on the
intermediate chloro/tert-butoxydimethylsilane, 1, which was pre-
pared from dichlorodimethylsilane. We encountered problems in
the selective mono-alkoxylation of the dichlorodimethylsilane
with NaOtBu, as described in the original report [28], often obtain-
ing di-tert-butoxydimethylsilane that was difficult to separate
from the chloro-tert-butoxydimethylsilane. This may be related
to the quality of the commercial NaOtBu, whose purity is difficult
to assess. In our hands the synthesis of 1 via the addition of one
equivalent of tert-butanol to dichlorodimethylsilane in the pres-
ence of pyridine (Scheme 1) proved superior, as it gave consistently
only the desired mono-tert-butoxydimethylsilane. Removal of the
pyridinium chloride by filtration and vacuum distillation (38 °C,
5 torr) of the filtrate afforded 1 in ca. 50% yield.
Scheme 3. Syntheses of 4 and 5.
is clearly stereochemically active the bond angels about the tin
atom correspond to neither tetrahedral nor to trigonal-bipyramidal
(tbp) geometry. Thus, while the O–Sn–O angle is essentially linear
(177.16(4)°), as would be expected for a tbp structure, the N–Sn–N
angle spans only 101.68(6)° and is thus closer to the value ex-
pected for tetrahedral geometry. For comparison, the N–Sn–N an-
gle in C is only slightly larger at 104.7(2)°. As is typical for
spirocycles, the N–Sn–O angles are extremely acute. In 4a, how-
ever, the angle compression is even more severe, with values of
63.11(5)° and 63.27(4)°, respectively, because the O ? Sn donor-
bonds are not subject to the same angle strain as covalent bonds.
At 2.1548(16) and 2.1486(16) Å the almost symmetrical tin–nitro-
gen bonds are elongated compared to those in C, where they are ca.
2.09 Å long [27]. The equidistant Oꢀ ꢀ ꢀSn contacts (Sn1–
O1 = 2.6184(14) and Sn1–O2 = 2.6240(14) Å) are much longer than
covalent Sn–O bonds (1.995–2.071 Å) [36–39], but significantly
shorter than the sum of the van der Waals radii of these elements
(3.69 Å) [40,41].
The N-silyl-tert-butylamine (OtBu)(NHtBu)SiMe2, 2, was synthe-
sized according to a published procedure from 1 and two equiva-
lents of tert-butylamine [28]. For the p-toluidine-based ligand,
we chose the modified procedure shown in Scheme 2. Removal
of lithium chloride by filtration, followed by vacuum distillation
of the residue furnished (OtBu)(NH-4-C6H4Me)SiMe2, 3, in 82%
yield.
Using a slightly modified literature method [29], the bis[(tert-
butoxydimethylsilyl)amide]tin compounds 4 and 5 were synthe-
sized as shown in Scheme 3 and isolated as crystalline, light-yellow
solids in good yields. Compound 4 was previously reported to have
a melting point of 40 °C [29], which may have precluded a single-
crystal X-ray analysis. The crystals we isolated melted at ca. 60 °C;
they were thus easier to manipulate and subjected to a single-crys-
tal X-ray study. Crystal and refinement parameters for 4 are listed
in Table 1, while selected bond parameters are given in the caption
of Fig. 1.
The second independent molecule, 4b, is shown in Fig. 2 in a
perspective that emphasizes its close structural relationship to
stannylene C. In essence, 4b is an analog of Lappert’s stannylene
with two additional O ? Sn donor interactions, which may com-
promise the carbenoid nature of the molecule. A comparison of
the metric parameters, listed in the captions of Figs. 1 and 2, re-
veals the isostructural natures of 4a and 4b.
The compound crystallizes with two independent molecules in
the monoclinic unit cell, space group P21/n, of which one, 4a, is
shown in Fig. 1. As can be seen in the thermal ellipsoid plot, the
molecule is a pseudo-spirocycle in which the central tin(II) atom
is chelated by both N,O ligands. Although the lone pair of electrons
The p-tolyl-substituted analog of 4, namely Sn[(OtBu)(N-4-
C6H4Me)SiMe2]2 (5), crystallizes in the monoclinic system, space
group P21/n, but with the conventional four molecules in the unit
cell. Crystal and refinement data for 5 are collected in Table 1, and
the caption of Fig. 3 contains selected bond parameters. As the ther-
mal-ellipsoid plot shows, the solid-state structure of this compound
is noticeably less symmetrical than that of 4. Thus, the N1, O1 ligand
chelates the tin atom in a manner similar to that in 4, with a compa-
rable Sn–N bond of 2.1438(19) Å, but a shorter O ? Sn donor bond
2.3753(17) Å. The tin–nitrogen bond (Sn–N2) of the second ligand is
similarly short (2.095(2) Å) as those in C, but it is accompanied by a
much longer Snꢀ ꢀ ꢀO contact (2.97(3) Å). Conventional O ? Sn donor
bonds in complexes like SnClBr(THF)2 [42], for example, are ca.
2.40–2.50 Å long; separations approaching 3 Å thus fall well outside
of the range of donor bonds between these elements. It is therefore
better to view the tin atom in this molecule as three coordinate. It
Scheme 1. Synthesis of 1.

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G.L. Fondong et al. / Polyhedron 30 (2011) 2856–2862
Table 1
Crystal, data collection, and refinement parameters for 4–7.
Compound
4
5
6
7
Molecular formula
Formula weight
Space group (no.)
T (K)
C
20H48N2O2Si2Sn
C26H44N2O2Si2Sn
591.50
P21/n(14)
C30H54N2O2Si2Zn2
661.68
C20H48N2O2Si2Zn
470.15
P21/c(14)
523.47
P21/n(14)
173
ꢀ
P1ð2Þ
173
173
173
a (Å)
b (Å)
c (Å)
16.7929(16)
11.9038(11)
28.263(3)
9.0822(17)
15.450(3)
22.607(4)
10.9560(15)
12.8020(17)
12.9093(18)
89.575(2)
87.730(2)
74.677(2)
1744.9(4)
2
18.3798(18)
18.1299(18)
18.4131(18)
a
(°)
b (°)
98.777(2)
99.040(3)
112.321(3)
c
(°)
V (Å3)
Z
5583.7(9)
8
3132.9(10)
4
5675.9(10)
8
k (Å)
0.71073
1.245
1.016
0.71073
1.254
0.914
0.71073
1.259
1.470
0.71073
1.100
0.965
qcalc g cmꢁ3
l
(mmꢁ1
)
F(000)
2208
1232
704
2048
h Range (°)
1.86–28.26
12909/0/519
1.028
0.0277
0.0727
1.60–28.32
7271/68/360
1.067
0.0343
0.0895
1.58–27.50
7638/0/357
1.048
0.0375
0.1039
1.12–22.26
7323/416/539
1.053
0.0758
0.2094
Data/restraints/parameters
Goodness-of-fit (GOF) on F2
R(F)a [I > 2
r
(I)]
wR2 (F2)b [all data]
Largest difference in peak and hole (e Åꢁ3
)
1.197, ꢁ0.294
1.103, ꢁ0.235
0.772, ꢁ0.258
1.599, ꢁ0.370
a
R =
wR2 = {[
R
|Fo ꢁ Fc|/
R|Fo|.
b
2
R
w(Fo ꢁ Fc2)]/[
R
w(Fo2)2]}1/2; w = 1/[ 2(Fo)2 + (xP)2 + yP] where P = (Maximum (Fo2 + 0) + 2 Fc2)/3.
r
Fig. 2. Solid-state structure and partial labeling scheme of 4b, in a perspective that
emphasizes its relationship to C. All atoms are drawn as 50% thermal ellipsoids and
hydrogen atoms have been omitted for clarity. Selected bond lengths (Å) and angles
(°): Sn2–N3 2.1406(16); Sn2–N4 2.1479(16); Sn2–O3 2.5970(13); Sn2–O4
2.6829(14); N3–Sn2–N4 101.89(7); O3–Sn2–O4 174.81(4); O3–Sn2–N3 63.31(5);
O3–Sn2–N4 112.55(5); O4–Sn2–N4 62.57(5); O4–Sn2–N3 115.09(5).
Fig. 1. Solid-state structure of 4a, emphasizing the pseudo trigonal-bipyramidal
structure of the compound. All atoms are drawn as 50% thermal ellipsoids and
hydrogen atoms have been omitted for clarity. Selected bond lengths (Å) and angles
(°): Sn1–N1 2.1545(16); Sn1–N2 2.1488(16); Sn1–O1 2.6184(14); Sn1–O2
2.6240(14); N1–Sn1–N2 101.68(6); O1–Sn1–O2 177.16(4); O1–Sn1–N1 63.27(4);
O1–Sn1–N2 115.24(5); O2–Sn1–N2 63.11(5); O2–Sn1–N1 114.44(5).
acute than in the tert-butylamido based ligand; this may be due to
the lesser steric demands of the p-tolyl substituents.
As expected, there is no evidence for this asymmetry in the
room temperature, solution-phase 1H NMR spectrum of 5, which
consists of two doublets in the aromatic region at 6.51 and
6.84 ppm, and three singlets in the aliphatic region at 2.17, 1.36,
and 0.050 ppm, the latter signals being assigned to the p-methyl,
tert-butyl, and trimethylsilyl groups, respectively. The 1H NMR
spectrum of 4 exhibits singlets at d = 1.37, 1.34, and 0.31 ppm for
the O-tert-butyl, N-tert-butyl, and equivalent silyl methyl groups,
respectively. This, too, suggests fluxional ligands, because in the
solid the MeSi groups are diastereotopic and therefore should yield
two signals if this conformation were maintained in solution.
Because of the obvious stereochemical activity of the lone pair
of electrons in 4 and 5 we chose to obtain structural information
on zinc complexes which are free of this complicating feature. Zinc
may be noted that the p-tolyl groups in both ligands of this com-
pound have decidedly different conformations, the aromatic ring
in ligand 1 (N1 and O1) being coplanar with the heterocycle, while
that in ligand 2 (N2 and O2) is perpendicular to it. Whether these
conformational differences reflect real effects – be they inductive
or resonance – exerted by the aryl groups or whether they are
merely packing artifacts is not clear. The asymmetry of 5 is also
reflected in the bond angles, which at least in some cases differ
significantly from those of 4a and 4b. For example, the N(1)–
Sn(1)–N(2) angle is only 95.19(7)° and thus substantially more

G.L. Fondong et al. / Polyhedron 30 (2011) 2856–2862
2859
Fig. 4. Thermal-ellipsoid plot and partial labeling scheme of one (6a) of the two
dimers of 6. All but the carbon atoms (35%) are drawn as 50% ellipsoids. Selected
bond lengths (Å) and angles (°): Zn1–N1 2.1019(19); Zn1–N10 2.057(2); Zn1–O1
2.4446(18); Zn1–C5 1.976(3); N1–Zn1–N10 91.57(7); Zn1–N1–Zn10 88.43(7); N1–
Zn1–O1 67.93(7); N1–Zn1–C5 130.40(11); O1–Zn1–C5 106.19(10); O1–Si1–N1
97.69(9).
Fig. 3. Thermal ellipsoid plot and partial labeling scheme of 5. All but the carbon
atoms (35%) are drawn as 50% ellipsoids. Selected bond lengths (Å) and angles (°):
Sn–N1 2.1438(19); Sn–N2 2.095(2); Sn–O1 2.3753(17); Sn–O2 2.97(3); N1–Sn–N2
95.19(7); O1–Sn–O2 144.1(5); O1–Sn–N1 92.56(9); O1–Sn–N2 94.02(7); O2–Sn–N2
56.1(5); O2–Sn–N1 130.1(6).
2.1026(18) Å in length. This confirms that 6a is really a dinuclear
complex in which the secondary amides bridge two zinc centers in
a slightly asymmetrical fashion. Because of this bridging interaction
both zinc–nitrogen bonds are substantially longer than those in
two-coordinate complexes, like Zn[N(SiMe3)2]2, where they are
merely 1.833(11) Å long [47]. For comparison, in the methyl-zinc
guanidinate monomer Zn[Me2NC(NiPr)2]OBMes2 the Zn–N bonds
are 2.0287(17) and 2.0645(16) Å long, while they measure
2.063(3) and 2.101(3) Å, respectively, in the related dimer {[MeZn(-
NiPr)2CN(SiMe3)2]}2 [48,49]. The coordination environments of the
zinc centers in 6a are completed by normal-length zinc–ethyl bonds
(1.975(3) Å) and O ? Zn donor bonds (2.4447(17) Å), which are
comparatively shorter than those in the tin compounds 4 and 5. To
a first approximation the zinc atoms are tetrahedrally coordinated,
but because of their inclusion in a polycyclic structure the bond an-
gles naturally differ from tetrahedral values.
has an extensive chemistry involving primary and secondary
amides, with both hydrocarbyl and silyl substituents [43–46].
Although some of these show zinc in a linear, two-coordinate envi-
ronment, three- and four-coordinate zinc complexes are far more
common, particularly in those cases where the ligands have addi-
tional donor sites. Divalent zinc with its closed-shell d10 configura-
tion is somewhat similar to tin(II), but zinc is more Lewis acidic
and with a covalent radius of 1.25 Å also significantly smaller than
the Group 14 metal. The attempted synthesis of Zn[(OtBu)(N-4-
C6H4Me)SiMe2]2 via the aminolysis of ZnEt2 with one or two equiv-
alents of HLpTol, even on prolonged refluxing, invariably gave only
the mono-ligand derivative 6, shown in Scheme 4.
The colorless compound crystallizes in the triclinic space group
ꢀ
P1, with two crystallographically-independent dimers in the unit
The failure of ZnEt2 to furnish diligated zinc derivatives served
as a reminder that such species are better accessed via zinc diam-
ides, like Zn[N(SiMe3)2]2 [49], or by treating zinc chloride with al-
kali metal amides [50,51]. To obtain a monomeric zinc compound
with two of the title ligands we treated anhydrous zinc chloride
with two equivalents of the lithium salt of 2 in hexanes, as shown
in Scheme 5. The reaction proceeded cleanly and furnished color-
less 7 in high yield. The simple 1H NMR spectra, revealing only
two sharp singlets for the chemically-inequivalent tert-butyl
groups and one singlet for the SiMe2 groups, were consistent with
a symmetrical molecule. The compound proved to be exceedingly
soluble even in hexanes, suggesting a monomeric compound free
of intermolecular Oꢀ ꢀ ꢀZn interactions. A single-crystal X-ray analy-
sis confirmed this assumption, and Fig. 5 shows the solid-state
structure of 7, which crystallizes in the monoclinic space group
P21/c with two crystallographically-independent molecules. In
both molecules the zinc atom and one of the ligands are
cell. Crystal data and refinement parameters for 6a are listed in Ta-
ble 1, while selected bond parameters are provided in the caption
of Fig. 4. The second dimer has virtually identical bond parameters,
which may be accessed in the supplementary crystallographic
data.
As Fig. 4 shows, the compound has a ladder-type structure, so
commonly found for main-group metal compounds. Such dimeric,
or oligomeric, ladders are often the result of intermolecular Lewis
acid–base interactions. At first glance 6a appears to be of this type,
consisting of dimers of EtZnLpTol moieties in which the nitrogen
atom of one unit is connected to the zinc atom of the opposing unit
via a donor bond. Closer inspection, however, reveals that the puta-
tive N ? Zn donor bond is actuallyshorter than the ‘‘intramolecular’’
bond, the former being 2.058(2) Å long, while the latter is
Scheme 4. Synthesis of 6.
Scheme 5. Synthesis of 7.

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G.L. Fondong et al. / Polyhedron 30 (2011) 2856–2862
Scheme 6. Interconversion from ladder structure (E) to butterfly structure (F).
nor ligands, like THF, dioxane, and bipyridine, shows that bridging
amides are present even in these ionic compounds.
Fig. 5. Thermal-ellipsoid plot and partial labeling scheme of one (7a) of the
independent molecules of 7. For clarity hydrogen atoms have been omitted; all but
the carbon atoms (30%) are drawn as 50% ellipsoids. Selected bond lengths (Å) and
angles (°): Zn1–N1 1.879(5); Zn1–N2 1.840(5); Zn1–O1 2.381(7); Zn1–O2 2.352(5);
N1–Zn1–N2 164.0(4); O1–Zn1–O2 100.3(3); N1–Zn1–O1 70.8(2); N1–Zn1–O2
117.9(2); N2–Zn1–O1 122.3(4); N2–Zn1–O2 71.1(2); N1–Si1–O1 95.9(2); N2–Si2–
O2 95.6(2).
Conceptually, the ladder isomer E may be interconverted to the
butterfly form F by the release of one O ? M donor bond and the
rotation of the ‘‘free’’ SiOtBu group about an Si–N bond, as shown
in Scheme 6. It is thus conceivable that 6 exhibits structure F in
solution and structure E in the solid state, but no experimental evi-
dence for such a rearrangement exists.
Based on the X-ray structural data, the title ligands are best con-
sidered bulky secondary amides with pendant alkoxy groups, in
which the latter interact to a varying degree with the central metal
atom. In the tin complexes this interaction is likely very weak in
solution, because it barely exists in the solid state. The more Le-
wis-acidic zinc centers interact more strongly with the oxygen
atoms, as reflected in their shorter O ? M bonds.
disordered; the latter in such a manner that the nitrogen and oxy-
gen atoms and their respective tert-butyl groups share sites. Fig. 5
shows one of these molecules, 7a, but with the disorder removed
and only one of two positions of the zinc atom and of the disor-
dered ligand shown.
Although 7 appears to be a spirocycle with an almost tetrahe-
drally-coordinated zinc atom, the bond lengths and angles suggest
that the metal is better viewed as 2 + 2 coordinated, having two
short (covalent) and two long (donor) bonds. Thus, the Zn–N bonds
in 7 are barely longer than those in truly two-coordinate zinc
amides, c.f., Zn(NtBu2)2, with Zn–N = 1.824(3) and 1.831(3) Å [50],
respectively, and Zn[N(SiPh2Me)2]2 with an average Zn–N length
3. Conclusion
The
tert-butoxydimethylsilylamides
[(OtBu)(NR)SiMe2]ꢁ,
R = tBu, p-tolyl, are versatile hemilabile ligands that are character-
ized by ease of synthesis and coordinative flexibility, which is re-
flected in the solid-state structures of their tin(II) complexes. Thus,
while LtBu creates four-coordinate tin centers with pseudo trigo-
nal-bipyramidal structures, the LpTol ligands produces a three-coor-
dinate metal center in which one ligand chelates the tin(II) ion while
the second ligand is bound through its nitrogen atom only. This dif-
ference suggests weak O ? Sn donor bonds, a feature which should
aid in the utility of these ligands for Group 14 heterocarbenoids
where such hemilability is desirable. The failure of diethylzinc to un-
dergo complete aminolysis by (OtBu)(NHR)SiMe2 can be ascribed to
the steric hindrance of the second Zn–C bond. Instead, two of these
ligands bridge two metal centers to form a ladder-type structure in
the solid state. A homoleptic Zn(LtBu)2 complex, featuring a pseudo-
tetrahedrally coordinated zinc ion, was obtained by treating ZnCl2
of 1.850(3) Å [51].
Zn[N(SiMe3)2]2 revealed a linear molecule with symmetrical Zn–
bonds that are 1.824(14) Å long [47]. At 2.3198(6) and
A
gas electron diffraction study on
N
2.3444(6) Å the O ? Zn donor bonds in 7 are substantially shorter
than those in dimeric 6 (2.4446(18) Å), but longer than those in
ether adducts or related four-coordinate zinc compounds, where
these bonds can be as short as 2.193(4) Å [52]. It is illustrative that
the O ? Zn donor bonds in 7 are almost equidistant with those in
Zn[(CH2)nOMe]2, n = 3, 4, where they are 2.37(3) and 2.38(5) Å
long, respectively, suggestive of similar interactions [24]. In keep-
ing with the bond-length trends, the N–Zn–N angle (164.0°) is
much closer to linear than to tetrahedral. The zinc derivative 7 is
a structural analog of Mg[(OtBu)(NtBu)SiMe2]2, which was reported
as early as 1982 but whose solid-state structure was never deter-
mined [28]. Based on 1H NMR evidence the authors proposed a spi-
rocyclic molecule, and the structure of 7 supports this assignment.
The zinc complexes 6 and 7 were synthesized mainly to observe
the ligand coordination in a divalent metal devoid of lone pair elec-
trons. We mention parenthetically that zinc amide chemistry has
seen enormous growth in the past decades [2], and that zinc
amides with internal donor groups are being investigated, among
others, as zinc source materials for vapor deposition techniques
[53]. Contrary to our expectations monosubstitution of ZnEt2 by
LpTol did not give monomeric EtZnLpTol, but furnished instead the
with two equivalents of the LiLtBu
.
4. Experimental
4.1. General considerations
All experiments were performed under an atmosphere of puri-
fied nitrogen or argon, using standard Schlenk techniques. Dichlo-
romethane was distilled from CaH2 and stored over molecular
sieves. THF was pre-dried with CaH2 and like the hydrocarbon sol-
vents dried and freed of molecular oxygen by distillation under an
atmosphere of nitrogen from sodium or potassium benzophenone
ketyl immediately before use. NMR spectra were recorded on a
Bruker Avance 500 NMR spectrometer. The NMR spectra were ref-
erenced relative to benzene-d6 (1H = 7.15, 13C = 128.0 ppm), THF-
d8 (1H = 3.58 and 1.73 ppm, 13C = 67.57 and 25.37 ppm) dichloro-
methane-d2 (1H = 5.32, 13C = 54.00 ppm), respectively. Melting
dinuclear EtZn(l
-LpTol)2ZnEt, which contains two bridging amido
groups. In such dimers the zinc centers suffer greater steric conges-
tion, thereby preventing the coordination of a second amine and
the subsequent aminolysis of the remaining Zn–C bond. The so-
lid-state structure of 6 Fig. 3, is remarkably similar to the ladder
structure (E) of alkali metal derivatives of LtBu and related ligands
[30,31]. The failure of these alkali metal dimers of LtBu to dissociate
to monomers in solution upon addition of mono- and bidentate do-
points were obtained on sealed samples with
a Mel-Temp

G.L. Fondong et al. / Polyhedron 30 (2011) 2856–2862
2861
apparatus; they are uncorrected. Midwest Micro Labs, LLC, India-
napolis, IN, and Desert Analytics, Tucson, AZ performed elemental
analyses. The reagents Me2SiCl2 and BuNH2 were purchased from
Aldrich and distilled prior to use. Tert-butanol was obtained from
Aldrich and stored over molecular sieves. Anhydrous SnCl2 and
ZnCl2 were obtained from Strem, and ZnEt2 was purchased from
Aldrich; all were used as received. The Me2Si(tBuO)(NHtBu) was
synthesized by a published method [28].
C6H4Me)SiMe2, 3 (2.78 g, 11.7 mmol) and nBuLi (2.90 M, 4.00 mL,
11.7 mmol). This solution was added dropwise to a solution of
SnCl2 (1.11 g, 5.85 mmol) in THF (10 mL). After 12 h of stirring
the solvent was removed in vacuo, and the residue was extracted
into hexanes. The extract was filtered on a medium-porosity frit,
concentrated in vacuo and stored at ꢁ21 °C for 1 week. This affor-
ded yellow, X-ray quality crystals in 80% yield. M.p.: 100–101 °C.
1H NMR (500.1 MHz, THF-d8, 27 °C): d 6.80 (d, JHH = 8.05 Hz, 4H,
Ar), 6.51 (d, JHH = 8.22 Hz, 4H, Ar), 2.17 (s, 6H, p-Me), 1.36 (s,
18H, tBu), 0.05 (s, 12H, Me). 13C{1H}NMR (125.8 MHz, THF-d8,
27 °C): d 148.92 (s, ipso-Ar), 129.59 (s, o-Ar), 128.31 (s, p-Ar),
125.17 (s, m-Ar), 75.16, (s, OC(CH3)3), 32.49 (s, OC(CH3)3), 20.87
(s, p-Me), 4.19 (s, SiMe2). Anal. Calc. for C26H44N2O2Si2Sn: C,
52.79; H, 7.50; N, 4.74. Found: C, 52.60; H, 7.27; N, 4.63%.
t
4.2. Syntheses
4.2.1. Me2(OtBu)SiCl (1)
Dichlorodimethylsilane (67.0 mL, 0.550 mol), pyridine (45.3 mL,
0.560 mol), and THF (550 mL) were combined in a 2000 mL, three-
necked flask, equipped with a magnetic stirbar. The mixture was
chilled to 0 °C and treated dropwise with a solution of tert-butanol
(52.6 mL, 550 mmol) in THF (50 mL), which produced a cloudy-
white suspension that was allowed to stir at room temperature
overnight. The reaction mixture was filtered on a medium-porosity
frit and distilled (5 torr, 38 °C) using a Vigreux column to obtain
analytically-pure 1 (by NMR) as a colorless oil. Yield: 46.2 g, 49%.
The spectroscopic and physical properties of this compound were
identical to those described in the literature [28].
4.2.5. {EtZn[(OtBu)(N-4-C6H4Me)SiMe2]}2 (6)
A cold (ꢁ78 °C) solution of (OtBu)(NH-4-C6H4Me)SiMe2, 3,
(1.85 g, 7.80 mmol) in THF (14 mL) was treated dropwise with a
solution of diethylzinc (1.0 M, 7.80 mL, 7.8 mmol) in THF (10 mL).
The reaction mixture was stirred at ꢁ78 °C for 1 h, allowed to
warm to room temperature and then stirred overnight, during
which time it became bright yellow. All solvents were removed
in vacuo, and the residue was extracted into hexanes, filtered,
and stored at ꢁ21 °C. After several days colorless, X-ray quality
crystals formed. Yield: 1.9 g, 73%. M.p.: 156–160 °C. 1H NMR
(500.1 MHz, dichloromethane-d2, 27 °C): d 6.97 (d, JHH = 8.20 Hz,
4H, Ar), 6.84 (d, JHH = 8.20 Hz, 4H, Ar), 2.24 (s, 6H, p-Me), 1.35 (t,
JHH = 8.10 Hz, 6H, –CH3), 0.86 (s, 18H, OtBu), 0.41 (q, JHH = 8.10 Hz,
4H, ZnCH2), 0.15 (s, SiMe2); 13C{1H} NMR (125.8 MHz, dichloro-
methane-d2, 27 °C): d 148.30 (s, i-Ar), 130.77 (s, p-Ar), 130.3 (s,
o-Ar), 126.1 (s, m-Ar), 74.6 (s, OC(CH3)3), 31.3 (s, OC(CH3)3), 20.8
(s, ZnCH2–), 13.2 (s, –CH3), 3.22 (s, SiMe2). Anal. Calc. for
4.2.2. (OtBu)(NH-4-C6H4Me)SiMe2 (3)
A 100 mL two-necked, round-bottom flask was charged with
p-toluidine (2.67 g, 24.9 mmol) and hexanes (50 mL). The con-
tents was cooled to 0 °C and treated dropwise with nBuLi
(2.90 M in hexanes, 8.60 mL, 24.9 mmol), resulting in the forma-
tion of a white precipitate. The mixture was stirred at 0 °C for
3 h, allowed to warm to room temperature and then treated with
a solution of 1 (4.20 g, 24.9 mmol) in hexanes (10 mL). It was fur-
ther diluted with hexanes (20 mL), allowed to stir overnight and
then filtered through a medium-porosity frit. Vacuum distillation
on a Vigreux column furnished a viscous, yellow oil. Yield: 5.71 g,
C15H27NOSiZn: C, 54.46; H, 8.23; N, 4.23. Found: C, 54.26; H,
7.91; N, 4.34%.
82%. 1H NMR (500.1 MHz, benzene-d6, 27 °C): d 6.93 (d, JHH
=
4.2.6. Zn[(OtBu)(NtBu)SiMe2]2 (7)
8.10 Hz, 2H, Ar), 6.68 (d, JHH = 8.37 Hz, 2H, Ar), 3.26 (s, 1H, NH),
t
A slurry of ZnCl2 (1.05 g, 7.66 mmol) in cold hexanes (ꢁ78 °C)
was treated dropwise with a solution of (OtBu)(LiNtBu)SiMe2, pre-
pared from (OtBu)(NHtBu)SiMe2 (3.11 g, 15.3 mmol) and nBuLi
(2.5 M, 6.5 mL, 16.3 mmol), as described in the synthesis of 4.
The reaction mixture was allowed to stir overnight, filtered, con-
centrated, and stored at ꢁ21 °C for several days. This afforded a
large crop of clear, colorless crystals. Yield: 2.98 g, 83%. M.p.:
130–133 °C. 1H NMR (500.1 MHz, benzene-d6, 27 °C): d 1.405 (s,
9H, OtBu), 1.322 (s, 9H, NtBu), 0.353 (s, 6H, SiMe2); 13C{1H} NMR
(125.8 MHz, benzene-d6, 27 °C): d 74.03 (s, OC(CH3)3), 51.24 (s,
NC(CH3)3), 37.85 (s, OC(CH3)3), 32.83 (s, NC(CH3)3), 7.21 (s, SiMe2).
Anal. Calc. for C20H48N2O2Si2Zn: C, 51.09; H, 10.29; N, 5.96. Found:
C, 50.81; H, 9.95; N, 6.07%.
2.14 (s, 3H, p-Me), 1.22 (s, 9H, Bu), 0.19 (s, 6H, Me). 13C{1H}NMR
(125.8 MHz, benzene-d6, 27 °C): d 144.84 (s, i-Ar), 130.32 (s, o-Ar),
127.40 (s, p-Ar), 117.55 (s, m-Ar), 73.33 (s, OC(CH3)3), 32.25
(s, OC(CH3)3), 20.99 (s, SiMe2) 1.01 (s, p-Me). Anal. Calc. for
C13H23NOSi: C, 65.77; H, 9.76; N, 5.90. Found: C, 65.61; H, 9.44;
N, 5.72%.
4.2.3. Sn[(OtBu)(NtBu)SiMe2]2 (4)
A solution of (tBuO)(NHtBu)SiMe2, 2 (4.28 g, 21.0 mmol) in cold
(0 °C) hexanes (20 mL) was treated dropwise with a solution of
nBuLi (2.90 M, 8.27 mL, 24.0 mmol) in hexanes (15 mL). The light-
yellow mixture was refluxed for 3 h, allowed to cool to RT, and
then added dropwise to a cold (ꢁ78 °C) SnCl2 (2.00 g, 10.5 mmol)
solution in THF (20 mL). The reaction mixture was stirred over-
night while it warmed to room temperature. All liquids were re-
moved in vacuo, the residue was extracted into hexanes, and the
extract was filtered on a medium-porosity frit. The filtrate was
concentrated in vacuo and stored at ꢁ21 °C for 10 days, to afford
5.33 g of light-yellow crystals. Yield: 97%. M.p.: 60–62 °C. 1H
NMR (500.1 MHz, THF-d8, 27 °C): d 1.37 ppm (s, 18H, OtBu), 1.34
(s, 18H, NtBu), 0.310 (s, 12H, CH3); 13C{1H} NMR (125.8 MHz,
THF-d8, 27 °C) d 73.6 ppm (s, OC(CH3)3), 52.6 (s, NC(CH3)3), 36.1
(s, OC(CH3)3), 30.8 (s, NC(CH3)3), 3.52 (s, SiMe2). Anal. Calc. for
4.3. X-ray crystallography
4.3.1. Compounds 4–7
Suitable, single crystals were coated with Paratone oil, attached
to Litholoop or Mitegen sample holders, and manually centered on
the diffractometer in a stream of cold nitrogen. Reflection intensi-
ties were collected with a Bruker Apex CCD diffractometer,
equipped with an Oxford Cryosystems 700 Series Cryostream cool-
er, operating at 173 K. Data were measured using
x scans of 0.3°
C
20H48N2O2Si2Sn: C, 45.89; H, 9.24; N, 5.35. Found: C, 45.81; H,
per frame for 20 s until a complete hemisphere had been collected.
Cell parameters were retrieved using SMART [53] software and re-
fined with SAINT [54] on all observed reflections. Data were reduced
with SAINT, which corrects for Lp and decay. Empirical absorption
corrections were applied with SADABS [55]. The structures were
solved by direct methods with the SHELXS-97 [56] program and
9.44; N, 5.22%.
4.2.4. Sn[(OtBu)(N-4-C6H4Me)SiMe2]2 (5)
The procedure follows the synthesis of 4 described above. A
lithium amide solution was prepared by combining (OtBu)(NH-4-

2862
G.L. Fondong et al. / Polyhedron 30 (2011) 2856–2862
refined by full-matrix least squares methods on F2 with SHELXL-97
[57], incorporated in SHELXTL-PC, Version 5.03 [58].
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We thank the University of North Dakota for funds to purchase
the Bruker Apex Diffractometer.
Appendix A. Supplementary data
CCDC 818111, 818112, 818113 and 818114 contain the supple-
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free of charge via http://www.ccdc.cam.ac.uk/conts/retriev-
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