Migrations of alkyl and aryl groups from silicon to nitrogen in silylated aryloxyiminoquinones

Article abstract of DOI:10.1021/om301028c

Silylation of the oxidized ligand in lead(II) bis(3,5-tert-butyl-1,2- quinone-(3,5-di-tert-butyl-2-hydroxy-1-phenyl)imine), Pb(ONO^Q) ₂, with chlorosilanes RSiX₂Cl (R = Me, Ph; X = Me, Ph, Cl) results in the tetracyclic, pentacoordinate silicon compounds (ON[R]O)SiX ₂ with a reduced, dianionic amine-bisphenolate ligand in which the methyl or phenyl group has migrated to nitrogen. Methyl migration is observed exclusively over phenyl migration, though phenyl migration is observed if no alkyl groups are present on silicon. Chlorotriphenylstannane reacts similarly, with migration of the phenyl group from tin to nitrogen. The five-coordinate products adopt a trigonal-bipyramidal geometry with long bonds to the apical amine donors (d(Si-N) = 2.3-2.9 A). Chlorine occupies the other axial position in strong preference to methyl or phenyl, while methyl has a modest preference for the axial position in comparison to phenyl (8.7:1).

Full text of DOI:10.1021/om301028c

Article
pubs.acs.org/Organometallics
Migrations of Alkyl and Aryl Groups from Silicon to Nitrogen in
Silylated Aryloxyiminoquinones
Sukesh Shekar and Seth N. Brown*
Department of Chemistry and Biochemistry, University of Notre Dame, 251 Nieuwland Science Hall, Notre Dame, Indiana
46556-5670, United States
S
* Supporting Information
ABSTRACT: Silylation of the oxidized ligand in lead(II)
bis(3,5-tert-butyl-1,2-quinone-(3,5-di-tert-butyl-2-hydroxy-1-
phenyl)imine), Pb(ONOQ)2, with chlorosilanes RSiX2Cl (R =
Me, Ph; X = Me, Ph, Cl) results in the tetracyclic, pentacoordinate
silicon compounds (ON[R]O)SiX2 with a reduced, dianionic
amine-bisphenolate ligand in which the methyl or phenyl
group has migrated to nitrogen. Methyl migration is observed
exclusively over phenyl migration, though phenyl migration is
observed if no alkyl groups are present on silicon. Chlorotriphe-
nylstannane reacts similarly, with migration of the phenyl group from tin to nitrogen. The five-coordinate products adopt a trigonal-
bipyramidal geometry with long bonds to the apical amine donors (d(Si−N) = 2.3−2.9 Å). Chlorine occupies the other axial position in
strong preference to methyl or phenyl, while methyl has a modest preference for the axial position in comparison to phenyl (8.7:1).
INTRODUCTION
■
Silicon−carbon bonds are nonpolar and strong, with silicon−
carbon bond dissociation energies over 85 kcal/mol,1 and are
thus typically unreactive. Reactivity of silicon−carbon bonds
can be observed, but generally such reactions involve atypical
hydrocarbyl groups such as allyl,2 or cases in which the silicon
center is bonded to a strong oxidant such as peroxide (as in the
Tamao−Fleming oxidation)3 or to unsaturated transition-metal
centers;4 under most typical chemical conditions, the silicon−
Figure 1. Oxidation states of the “ONO” ligand.
carbon bond is inert. This feature, coupled with the much greater
lability of silicon−heteroatom bonds, has led to widespread use
of trialkylsilyl groups as protecting groups in organic chemistry,
where R3Si groups can be installed and removed under condi-
tions that are strongly influenced by the size of the hydrocarbyl
substituents but where the substituents themselves are generally
inviolate.5
as a synthetic method for such complexes, including ones that
have shown considerable promise in catalytic reactions using
early13 and late14 transition metals. The silylated ligand
R3Si[ONOQ] would supplement the available ONO sources
14,15
such as the fully reduced H3[ONOCat
]
or the oxidized
H[ONOQ]16 or K[ONOQ].17
Trialkylsilyl-substituted species are likewise useful synthons
in inorganic and organometallic chemistry, where metathesis
between a metal halide or alkoxide and a silylated alkoxide,
azide, etc. can often be a mild way of introducing anionic ligands
into a metal’s coordination sphere. We were interested in prepar-
ing a (2-trialkylsiloxy-3,5-di-tert-butylphenyl)imino-4,6-di-tert-
butyl-1,2-benzoquinone in the hopes that this would be a useful
synthetic precursor for preparing metal complexes of the tridentate,
redox-active “ONO” ligand (Figure 1) first reported by Stegmann
and Scheffler as a ligand for group 14 metals.6 Self-assembly of this
ligand from ammonia and 3,5-di-tert-butylcatechol (or 3,5-di-
tert-butyl-1,2-benzoquinone) has been used extensively to pre-
pare neutral homoleptic M(ONO)2 complexes with M = Mg,
Fe, Cu, Ni, Zn, Cd,7 M = Al, Ga, Ca, Ba,8 M = Mn, Co,9 M =
Ti, V, Ge, Sn,10 and M = Pb.11 Although monoligated complexes
have been prepared by self-assembly,12 this is generally not attractive
In a previous study of the interaction of group 14 halides
with the ONO ligand, Flores-Parra, Contreras, and co-workers
observed that reaction of dialkyldichlorostannanes with
Zn(ONOQ)2 resulted in transfer of one alkyl group from tin
to nitrogen and formation of tin complexes of the reduced
18
n
ligand RClSn(ON[R]O) (R = CH3, Bu; eq 1). In contrast,
butyltrichlorostannane was reported to undergo simple trans-
metalation with Zn(ONOQ)2 to give octahedral (ONOQ)SnCl2Bu.
Both the mono- and diarylstannanes PhSnCl3 and Ph2SnCl2 were
reported to give the same simple transmetalation product as well,
(ONOQ)SnCl2Ph, with Ph2SnCl2 apparently transferring phenyl
rather than chloride to zinc.
Received: October 30, 2012
Published: January 14, 2013
© 2013 American Chemical Society
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dx.doi.org/10.1021/om301028c | Organometallics 2013, 32, 556−564

Organometallics
Article
2,4,8,10-Tetra-tert-butyl-6,6,12-trimethyl-12H-dibenzo[d,g]-
[1.3.6.2]dioxaazasilocine, Me2Si(ON[Me]O). To a solution of
0.2313 g of Pb(ONOQ)2 (0.2197 mmol) in 3 mL of methylene
chloride was added 54.5 μL of chlorotrimethylsilane (1.98 equiv) via
syringe. Within 5 min of stirring at room temperature, the solution
changed color to a deep magenta. The reaction mixture was stirred
18 h, and solid lead chloride was removed by filtration. The mother
liquor was layered with methanol and allowed to stand overnight to
provide crystals, which were filtered and washed with 2 × 2 mL of
methanol to yield 0.1100 g of Me2Si(ON[Me]O) (50%). 1H NMR: δ
t
0.34, 0.46 (s, 3H each, SiCH3), 1.27, 1.28 (s, 18H each, Bu), 2.98
(s, 3H, NCH3), 7.33, 7.44 (d, 2 Hz, 2H each, ArH). 13C{1H} NMR: δ
0.71, 2.81 (SiCH3), 29.82, 31.77 (C(CH3)3), 34.71, 35.24 (C(CH3)3),
45.62 (NCH3), 118.65, 121.31, 138.13, 139.90, 142.90, 148.32 (CArO).
29Si{1H} NMR: δ −16.11. IR (cm−1): 3084 (w), 2925 (s), 2859 (s),
1576 (s), 1262 (w), 1205 (w), 1140 (m), 1119 (m), 1070 (m), 1021
(w), 972 (m), 914 (m), 661 (w). ESI-MS: 496.3591 (M + H+, calcd
496.3605). Anal. Calcd for C31H49NO2Si: C, 75.10; H, 9.96; N, 2.83.
Found: C, 75.41; H, 10.15; N, 2.89 .
Alkyl- and aryltin compounds are well-known to readily
undergo C−Sn bond cleavage, leading to their widespread use
in reactions such as Stille couplings as sources of nucleophilic
hydrocarbyl groups.19 Given the much greater tendency of tin
in comparison to silicon to donate its alkyl group and the fact
that migration appeared to be only sporadic in the (ONO)-
stannanes, we did not expect silylation to be accompanied by
migration. In fact, we report here that attempted silylation of
the (ONOQ)− group with methyl- and phenylsilyl chlorides invari-
ably results in migration of alkyl or aryl groups from silicon to
nitrogen. We also describe the analogous migration of phenyl
groups from tin to nitrogen.
2,4,8,10-Tetra-tert-butyl-6-chloro-6,12-dimethyl-12H-dibenzo-
[d,g][1.3.6.2]dioxaazasilocine, ClMeSi(ON[Me]O). The compound
was prepared analogously using 0.1729 g of Pb(ONOQ)2 (0.1643
mmol) and 45 μL of dichlorodimethylsilane (0.3710 mmol) in 4 mL
of CHCl3. After the mixture was stirred overnight, lead chloride was
removed by filtration and the filtrate was evaporated to dryness in
vacuo. The compound was crystallized by dissolving the crude product
in chloroform, layering with acetonitrile, and cooling to −18 °C. The
crystals were filtered and washed with 2 × 2 mL of acetonitrile to yield
EXPERIMENTAL SECTION
1
■
0.0449 g of ClMeSi(ON[Me]O) (27%). H NMR: δ 0.90 (s, 3H,
t
All procedures were carried out under an inert atmosphere in a
nitrogen-filled glovebox or on a vacuum line. Chlorinated solvents and
acetonitrile were dried over 4 Å molecular sieves, followed by CaH2.
Benzene was dried over sodium and ether over sodium benzophenone
ketyl. Alcohols were dried over 4 Å molecular sieves. Deuterated
solvents were obtained from Cambridge Isotope Laboratories, dried
using the same procedures as their protio analogues, and stored in the
drybox prior to use. Zn(ONOQ)2 was prepared as described in the
literature.7 Other reagents were commercially available and used with-
out further purification. 1H and 13C{1H} NMR spectra were measured
as CDCl3 solutions on a Varian VXR-300 and Bruker Avance DPX 400
spectrometers. 29Si{1H} and 119Sn{1H} NMR spectra were measured
Si-Me), 1.26, 1.36 (s, 18H each, Bu), 3.02 (s, 3H, NCH3), 7.15, 7.33
(d, 2 Hz, 2H each, ArH). 13C{1H} NMR: δ 3.63 (SiCH3), 29.52, 31.42
(C(CH3)3), 34.56, 35.20 (C(CH3)3), 45.92 (NCH3), 116.72, 122.53,
138.41, 138.75, 144.10, 147.63 (CArO). 29Si{1H} NMR: δ −44.89. IR
(cm−1): 2958 (s), 2929 (s), 2851 (s), 1605 (m), 1589 (m), 1303 (m),
1258 (w), 1237 (w), 1172 (m), 972 (m), 927 (m), 865 (m). ESI-MS:
498.3382 (M + H+, calcd for Me[OH]Si(ON[Me]O) 498.3398). Anal.
Calcd for C30H46ClNO2Si: C, 69.80; H, 8.98; N, 2.71. Found: C,
68.18; H, 8.78; N, 2.61.
2,4,8,10-Tetra-tert-butyl-6,6-dichloro-12-methyl-12H-dibenzo-
[d,g][1.3.6.2]dioxaazasilocine, Cl2Si(ON[Me]O). This compound was
generated in a scintillation vial using 33.8 mg of Pb(ONOQ)2 (0.0321
mmol) and 8.0 μL of methyltrichlorosilane (0.068 mmol) in 0.8 mL of
CDCl3. After it was stirred for 5 min, the violet solution was filtered
through a plug of sand into a screw-cap NMR tube for character-
ization. 1H NMR: δ 1.31, 1.41 (s, 18H each, tBu), 3.16 (s, 3H, NCH3),
7.23, 7.41 (d, 2 Hz, 2H each, ArH). 13C{1H} NMR: δ 29.54, 31.62
(C(CH3)3), 34.91, 35.13 (C(CH3)3), 50.09 (NCH3), 116.10, 123.17,
136.90, 138.03, 144.96, 145.61. 29Si{1H} NMR: δ −80.41. IR (evap-
orated film, cm−1): 2958 (s), 2917 (s), 2868 (m), 2851(m), 1486 (s),
1458 (s), 1413 (m), 1360 (s), 1294 (m), 1266 (m), 1245 (w), 1209
(w), 1188 (w), 1102 (m), 1061 (m), 963 (m), 927 (m), 894 (w), 878
(w), 853 (m), 780 (w).
2,4,8,10-Tetra-tert-butyl-6-chloro-12-methyl-6-phenyl-12H-
dibenzo[d,g][1.3.6.2]dioxaazasilocine, ClPhSi(ON[Me]O). After reac-
tion as described above (0.2511 g of Pb(ONOQ)2, 78 μL of PhMeSiCl2,
4 mL of CH2Cl2, 18 h) and removal of lead chloride by filtration, the
brown-red filtrate was allowed to stand for 2 days. Light pink crystals
deposited from CH2Cl2 and were isolated by filtration to produce
0.1219 g of ClPhSi(ON[Me]O) (44%). 1H NMR: δ 1.29, 1.51 (s, 18H
each, tBu), 2.23 (s, 3H, NCH3), 7.23 (d, 2 Hz, 2H, ArH), 7.34 (m, 5H,
ArH, m-, p-Ph), 7.50 (m, 2H, o-Ph). 13C{1H} NMR: δ 29.69, 31.77
(C(CH3)3), 35.36, 35.91 (C(CH3)3), 48.66 (NCH3), 117.08, 122.84,
128.39, 129.85, 130.80, 135.70, 137.99, 138.23, 144.36, 147.23 (CArO).
29Si{1H} NMR: δ −62.16. IR (Nujol mull, cm−1): 3076 (w), 1482 (s),
on a Varian Inova 500 spectrometer. Chemical shifts for H, 13C{1H},
1
and 29Si{1H} spectra are reported in ppm downfield of TMS, and
119Sn{1H} chemical shifts are reported in ppm downfield of Me4Sn.
Infrared spectra were recorded as neat solids (except as otherwise
noted) between NaCl plates. ESI mass spectra were obtained using a
Bruker micrOTOF-II mass spectrometer, and peaks reported are the
mass number of the most intense peak of isotope envelopes. Samples
were injected as dichloromethane or acetonitrile solutions, preceded
and followed by methanol. In all cases, the observed isotope patterns
were in good agreement with calculated ones.
Pb(ONOQ)2. The procedure is adapted from that of McGarvey and
co-workers.11 Into a 50 mL Erlenmeyer flask were weighed 0.45 g of
3,5-di-tert-butyl-1,2-benzoquinone (2.0 mmol), 0.45 g of 3,5-di-tert-
butylcatechol (2.0 mmol), and 0.25 g of lead(II) fluoride (1.0 mmol).
After 40 mL of absolute ethanol and a stirbar were added, to the
stirred suspension of white solids in a brown solution was added 4 mL
of concentrated aqueous ammonia. The solution rapidly turned dark
green and was stirred overnight. The green slurry was suction-filtered
through a glass frit, the precipitate was washed with 5 mL of ethanol,
and the filtrate and wash were discarded. The solid was then washed
with 3 × 3 mL of CHCl3 to extract the brownish green lead complex;
insoluble white lead salts were left behind on the frit and discarded.
The compound was isolated on evaporation of the chloroform filtrate;
yield 0.81 g (77%). NMR data do not agree with those in the litera-
ture. 1H NMR: δ 1.22, 1.23 (s, 36H each, tBu), 6.91, 7.19 (d, 2 Hz, 4H
each, ArH). 13C{1H} NMR: δ 29.65, 30.50 (C(CH3)3), 34.86, 35.46
(C(CH3)3), 119.13, 131.62, 143.26, 144.84, 146.17, 177.36 (CArO).
Anal. Calcd for C56H80N2O4Pb: C, 63.91; H, 7.66; N, 2.66. Found: C,
64.17; H, 7.34; N, 2.81.
1409 (m), 1376 (m), 1360 (m), 1258 (s), 1237 (m), 1213 (w), 1196
(w), 1164 (w), 1119 (m), 1057 (m), 1025 (w), 947 (s), 886 (m), 792
(w), 775 (m), 739 (m), 722 (m), 657 (w), 620 (w). ESI-MS: 542.3464
(M+ − Cl, calcd 542.3449). The analytical sample was purified by crys-
tallization from chloroform/hexane and analyzed as a hemichloroform
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Organometallics
Article
solvate. Anal. Calcd for C35H48ClNO2Si·0.5CHCl3: C, 66.83; H, 7.66; N,
2.20. Found: C, 66.77; H, 7.07; N, 2.20.
chloride (0.1332 g, 0.3455 mmol, 2.0 equiv) was added to 4 mL
of a chloroform solution of 0.1818 g of Pb(ONOQ)2 (0.1727 mmol).
The dark green solution changed to brown within 2 min of stirring.
The reaction mixture was stirred overnight, and lead chloride was
removed by filtration. The mother liquor was cooled to −20 °C to
furnish microcrystalline aggregates of Ph2Sn(ON[Ph]O) (0.1614 g,
2,4,8,10-Tetra-tert-butyl-6,12-dimethyl-6-phenyl-1H-
dibenzo[d,g][1.3.6.2]dioxaazasilocine, MePhSi(ON[Me]O). After
reaction as described above (0.1118 g of Pb(ONOQ)2 (0.1062 mmol),
2 mL of CHCl3, PhMe2SiCl (38.5 μL, 2.05 equiv), 18 h) and removal
of lead chloride by filtration, the red-brown filtrate was layered with
3 mL of methanol and stored at −36 °C for 3 days. The white
crystalline solid was filtered and washed with 2 × 1 mL of CH3OH to
yield 0.0664 g of MePhSi(ON[Me]O) (56%). The NMR shows an
8.7:1 ratio of isomers; only peaks due to the major isomer are reported
below. 1H NMR: δ 0.40 (s, 3H, SiCH3), 1.22, 1.42 (s, 18H each, tBu),
1.99 (s, 3H, NCH3), 7.10 (d, 2 Hz, 2H, ArH), 7.24 (m, 3H, m-, p-Ph),
7.27 (d, 2 Hz, 2H, ArH), 7.40 (m, 2H, o-Ph). 13C{1H} NMR: δ 2.85
(SiCH3), 29.83, 31.75 (C(CH3)3), 34.70, 35.33 C(CH3)3), 45.92
(NCH3), 119.11, 121.57, 127.49, 127.93, 131.27, 137.84, 137.95,
139.84, 143.15, 148.15 (CArO). 29Si{1H} NMR: δ −32.34. IR (Nujol
mull, cm−1): 3068 (m), 3052 (m), 1580 (w), 1421 (m), 1388 (m),
1372 (m), 1352 (s), 1266 (s), 1241 (m), 1176 (m), 1127 (s), 1074
(m), 1025 (w), 976 (m), 947 (s), 886 (s), 874 (s), 804 (s), 784 (m),
767 (w), 739 (w), 710 (s), 694 (s), 641 (w), 604 (w), 620 (w). ESI-
MS: 558.3757 (M + H+, calcd 558.3762). Anal. Calcd for
C36H51NO2Si: C, 77.50; H, 9.21; N, 2.51. Found: C, 77.42; H, 9.28;
N, 2.42.
2,4,8,10-Tetra-tert-butyl-6-chloro-6,12-diphenyl-1H-
dibenzo[d,g][1.3.6.2]dioxaazasilocine, ClPhSi(ON[Ph]O). After
reaction and filtration of lead chloride as described above (0.2014 g
of Pb(ONOQ)2 (0.2140 mmol), 4 mL of CHCl3, 81.0 μL of Ph2SiCl2
(0.384 mmol, 2.0 equiv), 18 h), the filtrate was evaporated to dryness,
redissolved in a minimum amount of CHCl3, and layered with
acetonitrile. Crystals of the product were isolated by filtration after
3 days, washed with 2 × 2 mL of acetonitrile, and vacuum-dried to give
0.1034 g of ClPhSi(ON[Ph]O) (43%). 1H NMR: δ 1.35, 1.66 (s, 18H
each, tBu), 6.05 (d, 8 Hz, 2H, o-NPh), 6.61 (t, 7 Hz, 1H, p-NPh), 6.78
(t, 8 Hz, 2H, m-NPh), 6.96 (t, 8 Hz, 2H, m-SiPh), 7.15 (tt, 8, 1 Hz, 1H,
p-SiPh), 7.21 (d, 7 Hz, 2H, o-SiPh), 7.31 (d, 2 Hz, 2H, ArH), 7.42 (d,
2 Hz, 2H, ArH). 13C{1H} NMR: δ 29.52, 31.28 (C(CH3)3), 34.51,
35.11 (C(CH3)3), 117.48, 120.13, 122.43, 122.77, 127.51, 127.75,
129.52, 130.84, 131.08, 135.23, 138.81, 145.27, 148.60, 149.73.
29Si{1H} NMR: δ −49.86. IR (evaporated film, cm−1): 3068 (w),
2958 (s), 2868 (s), 1593 (m), 1572 (m), 1474 (s), 1454 (m), 1446
(m), 1413 (w), 1388 (s), 1360 (s), 1307 (m), 1245 (s), 1204 (w),
1155 (w), 1123 (s), 1082 (m), 963 (s), 935 (s), 898 (s), 792 (m), 759
(w), 730 (m) 690 (m), 583 (w). ESI-MS: 604.3631 (M+ − Cl, calcd
604.3605). Anal. Calcd for C40H50ClNO2Si: C, 75.02; H, 7.87; N, 2.19.
Found: C, 74.66; H, 7.56; N, 2.22.
2,4,8,10-Tetra-tert-butyl-6,6,12-triphenyl-12H-dibenzo[d,g]-
[1.3.6.2]dioxazasilocine, Ph2Si(ON[Ph]O). A solution of 0.2074 g
of Pb(ONOQ)2 (0.1979 mmol) and 0.1162 g of triphenylchlorosilane
(0.3940 mmol, 1.99 equiv) in 5 mL of CHCl3 in a 20 mL scintillation
vial sealed with a Teflon-lined cap was stirred and heated for 4 days in
a 70 °C oil bath. After filtration of lead chloride, the solution was
layered with acetonitrile and allowed to stand for 3 days. Crystals of
the product were filtered to give 0.0975 g of Ph2Si(ON[Ph]O) (36%).
1H NMR: δ 1.25, 1.42 (s, 18H each, tBu), 5.87 (d, 8 Hz, 2H, o-NPh),
6.53 (tt, 7, 2 Hz, 1H, p-NPh), 6.66 (t, 8 Hz, 2H, m-NPh), 6.85 (t,
8 Hz, 2H, m-SiPh), 7.05 (tt, 8, 1 Hz, 1H, p-SiPh), 7.14 (d, 8 Hz, 2H,
o-SiPh), 7.23 (d, 2 Hz, 2H, ArH), 7.28 (d, 2 Hz, 2H, ArH), 7.29 (t,
8 Hz, 2H, m-SiPh), 7.36 (tt, 7, 1.6 Hz, 1H, p-SiPh), 7.71 (d, 8 Hz, 2H,
o-SiPh). 13C{1H} NMR: δ 29.78, 31.39 (C(CH3)3), 34.48, 35.16
(C(CH3)3), 116.50, 118.48, 122.39, 123.69, 127.31, 127.38, 127.65,
128.79, 129.38, 131.10, 132.14, 134.65, 135.68, 136.87, 139.11, 144.49,
149.43, 150.59. 29Si{1H} NMR: δ −38.74. IR (cm−1): 3101 (w), 3076
(m), 2929 (s), 2864 (s), 2708 (w), 1593 (s), 1446 (s), 1311 (m), 1237
(m), 1127 (m), 1041 (m), 927 (m), 845 (w), 780 (w), 681 (w), 600
(w). ESI-MS: 681.3985 (M+, calcd 681.3997). Anal. Calcd for
C46H55NO2Si: C, 81.01; H, 8.13; N, 2.05. Found: C, 77.79; H, 8.11;
N, 1.83.
1
t
60%). H NMR: δ 1.32, 1.57 (s, 18H each, Bu), 6.29 (d, 8 Hz, 2H,
o-Ph), 6.79 (m, 3H, m-, p-Ph), 7.04 (d, 8 Hz, 2H, o-Ph), 7.11 (t, 8 Hz,
2H, m-Ph), 7.23 (tt, 8 Hz, 1 Hz, 1H, p-Ph), 7.31 (d, 2 Hz, 2H, ArH),
7.36 (d, 2 Hz, 2H, ArH), 7.43 (m, 3H, Ph), 7.80 (m, 2H, Ph). 13C{1H}
NMR: δ 29.92, 31.81 (C(CH3)3), 34.61, 35.63 (C(CH3)3), 120.77,
121.74, 122.48, 122.53, 127.83, 128.44, 128.68, 129.37, 129.58, 134.16,
134.40, 135.95, 137.95, 138.32, 140.31, 144.37, 149.47, 153.43.
119Sn{1H} NMR: δ −204.06. IR (Nujol mull, cm−1): 3067 (w),
3046 (w), 1597 (w), 1565 (w), 1411 (m), 1387 (m), 1378 (m), 1361
(m), 1298 (s), 1260 (s), 1238 (s), 1200 (m), 1190 (m), 1155 (w),
1127 (m), 1078 (m), 1032 (m), 1003 (m), 956 (w), 915 (w), 897 (w),
883 (w), 866 (w), 839 (s), 780 (m), 768 (s), 726 (s), 685 (s), 676
(m), 656 (w), 641 (w), 551 (m), 534 (m). ESI-MS: 796.3186 (M +
Na+, calcd 796.3157). Anal. Calcd for C46H55NO2Sn: C, 71.51; H,
7.17; N, 1.81. Found: C, 71.67; H, 7.40; N, 1.66.
X-ray Crystallography. Crystals of Me2Si(ON[Me]O) and
MePhSi(ON[Me]O) were grown by layering a concentrated dichloro-
methane solution of each complex with methanol, while crystals of
ClMeSi(ON[Me]O)·2C6H6 were grown from a concentrated solution
of the compound in benzene. Crystals of ClPhSi(ON[Me]O) and
Ph2Si(ON[Ph]O) were grown by layering concentrated solutions of
the complex in CDCl3 with hexane and acetonitrile, respectively.
Crystals of Pb(ONOQ)2·2CHCl3 were grown by slow evaporation of a
solution of the complex in chloroform. Crystals of (ONOSQ)SiMe2
deposited from the crude reaction mixture of Pb(ONOQ)2 with
Me2SiCl2 in CH2Cl2, after filtration to remove PbCl2, at −20 °C.
Crystals of (ON[Me]O)Si(Me)(μ-O)Si(Me)(ON[Me]O)·2C6H6 de-
posited from a crude sample of ClMeSi(ON[Me]O) dissolved in C6H6
and layered with CH3CN. Crystals of (ONOSQ)2Si were formed when
the reaction mixture of MeSiCl3 with Pb(ONOQ)2, after filtration and
concentration to dryness, was dissolved in CDCl3 and layered with
hexane.
Crystals were placed in inert oil before being transferred to the cold
N2 stream of a Bruker Apex II CCD diffractometer. Data were re-
duced, correcting for absorption, using the program SADABS. The
structures were solved using direct methods, except for that of
Pb(ONOQ)2, which was solved using a Patterson map. All non-
hydrogen atoms not apparent from the initial solutions were found on
difference Fourier maps, and all heavy atoms were refined anisotropi-
cally. The TWIN command was used to address a 27% racemic twin in
the refinement of ClMeSi(ON[Me]O)·2C6H6.
The tert-butyl group centered on C150 on in MePhSi(ON[Me]O)
was disordered in two different orientations, as were the tert-butyl
groups in Me2Si(ON[Me]O) centered on C150 and C250, the tert-
butyl group centered on C15 in Pb(ONOQ)2·2CHCl3, and the group
centered on C280 in Si(ONOSQ)2. Disordered tert-butyl groups were
modeled by constraining the thermal parameters of the methyl carbons
to be equal to those of the carbons opposite them in the other
orientation and allowing the occupancy of the two orientations to
refine. Hydrogens on the disordered methyl groups were placed in
calculated positions, while all other hydrogen atoms were found on
difference Fourier maps and refined isotropically, except for those in
ClMeSi(ON[Me]O)·2C6H6 and (ON[Me]O)Si(Me)(μ-O)Si(Me)-
(ON[Me]O)·2C6H6, which were all placed in calculated positions.
Calculations used SHELXTL (Bruker AXS),20 with scattering factors
and anomalous dispersion terms taken from the literature.21 Further
details about the individual structures are given in Tables S1A and S1B
(Supporting Information).
RESULTS AND DISCUSSION
■
Silylation of Pb(ONOQ)2 To Form Products of Silicon
to Nitrogen Migration. Stannylated compounds of the ONO
ligand have been prepared by reaction of Zn(ONOQ)2 with
2,4,8,10-Tetra-tert-butyl-6,6,12-triphenyl-12H-dibenzo[d,g]-
[1.3.6.2]dioxaazastannocine, Ph2Sn(ON[Ph]O). Triphenyltin
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chlorostannanes.8 We anticipated that the lead complex
Pb(ONOQ)2 would be an even more efficient source of
[ONOQ]−, on the basis of the insolubility of lead(II) chloride,
and have therefore explored its chemistry. The lead complex is
prepared in good yield by self-assembly from PbF2, 3,5-di-tert-
butylcatechol, 3,5-di-tert-butyl-1,2-benzoquinone, and aqueous
ammonia as described by McGarvey and co-workers,11 except
that the crude product must be extracted into chloroform in
order to separate it from inorganic lead salts that coprecipitate
from ethanol.
compounds containing unalkylated ONO ligands; this has been
confirmed by the crystallization of Me2Si(ONOSQ)22 from the
reaction of Me2SiCl2 (Figure S2, Supporting Information) and
23
the crystallization of Si(ONOSQ)2 from the reaction with
MeSiCl3 (Figure S3, Supporting Information). Products lacking
Si−Cl bonds are air- and moisture-stable solids, while products
retaining one or more Si−Cl bonds are susceptible to hydro-
lysis (see, for example, the structure of (ON[Me]O)SiMe-
(μ-O)SiMe(ON[Me]O) formed by hydrolysis of ClMeSi(ON-
[Me]O); Figure S4, Supporting Information).
Reaction rates correlate with the reactivity of the chlorosilanes
as silylating agents. Reactions of phenyldimethylsilyl chlo-
ride take several hours to go to completion, and those of
trimethylsilyl chloride and phenylmethyldichlorosilane are
complete in less than 1 h, while dimethyldichlorosilane and
methyltrichlorosilane react immediately at room temperature.
The least reactive chlorosilane, triphenylsilyl chloride, requires
several days of heating at 70 °C to react completely with
Pb(ONOQ)2. The lead complex is more reactive than its zinc
analogue Zn(ONOQ)2. For example, Pb(ONOQ)2 reacts
completely with PhMe2SiCl in CDCl3 in about 4 h at room
temperature, while silylation of Zn(ONOQ)2 is only about 10%
complete over 20 h under the same conditions. The reactions
are thermal rather than photochemical, as they proceed equally
well in ambient light or in the dark.
1
We find that the H and 13C NMR spectra of Pb(ONOQ)2
do not agree with those reported in the literature.11 In
1
particular, while the H NMR spectrum was reported to be
symmetrical, the 13C NMR spectrum was reported to show two
different environments for the ONO ligand. In contrast, we
1
observe symmetrical H and 13C NMR spectra in CDCl3 or
CD2Cl2 that are invariant down to −70 °C in the latter solvent.
In order to confirm that we had prepared the same compound,
we redetermined its crystal structure. While crystals were grown
in the same way as described in the literature, we obtained
a chloroform solvate rather than an unsolvated compound
(Table S1B, Supporting Information), but the molecular
structure is in excellent agreement with what was previously
determined (Table S3 and Figure S1, Supporting Information).
In particular, the structure is quite asymmetric in the solid state,
with one oxygen of each ligand forming a short bond to lead
(2.255(5) Å average) and the other oxygen forming a much
longer bond (2.773(2) and 2.822(2) Å). Overall, the structure
most resembles a sawhorse geometry with the two strongly
bonded oxygens apical and the two nitrogens roughly 90° from
each other and the two oxygens. This geometry is expected for
divalent lead with a stereochemically active lone pair.
Presumably the asymmetry is retained in solution, but the
NMR spectra indicate that the structure is highly fluxional and
the two ligand environments are averaged on the NMR time
scale.
Multinuclear NMR spectroscopy provides strong support for
the formulation of the products as containing five-coordinate
silicon with a reduced, N-alkylated tridentate ON[R]O ligand.
Thus, for example, the 1H and 13C NMR spectra of the product
of reaction with Me3SiCl show three distinct chemical shifts for
the methyl groups, with two methyl groups showing upfield
shifts characteristic of Si-CH3 groups (1H δ 0.34, 0.46; 13C δ
0.71, 2.81 ppm) and the third methyl group resonating more
downfield (1H δ 2.98, 13C δ 45.62 ppm), consistent with its
formulation as an N-CH3 group. The reduction in the ligand
can be assessed by the chemical shift of the aromatic carbon
attached to oxygen,8 which is at δ 148.32 ppm in Me2Si(ON-
[Me]O), typical of that observed for an aryloxide and far upfield of
the δ 177.36 observed for the quinonoid ligand in Pb(ONOQ)2.
The observation of two distinct Si-CH3 resonances in
Me2Si(ON[CH3]O) is strongly suggestive of a five-coordinate
silane with inequivalent axial and equatorial methyl groups. The
29Si resonance at δ −16.11 ppm is shifted by about 10 ppm
upfield of the four-coordinate analogue Me2Si(OPh)2 (δ −6.8
ppm),24 consistent with retention of a N→Si interaction in
solution. This shift is small compared to other five-coordinate
silanes with sp2-hybridized nitrogen donor atoms. For example,
Me2Si(2,6-[OCAr2]2C5H3N) resonates at δ ∼−57 ppm,25 while
Me2Si(κ3-OC6H4NC[Ph]C6H3-4-[OCH3]-2-O) has δ(29Si)
−52.4, while its crystallographically characterized κ2 isomer
resonates at −7.3 ppm.26 However, the 10 ppm upfield shift
appears to be typical of compounds with sp3-hybridized nitro-
gen donors. For example, there is crystallographic27 and NMR
evidence28 for the presence of a transannular N→Si interac-
tion in dialkylsilyl diethanolamine complexes, and Me2Si-
(OCH2CH2N[Me]CH2CH2O) resonates at δ −10.1 (com-
pared to δ −1.4 for its methiodide).29 The greater shifts for sp2
donors in comparison to sp3 is likely related to the shorter
Si−N distances in the former than in the latter, as a correlation
between δ(29Si) and the silicon−nitrogen distance in silatranes
has been observed.30 The diphenylsilyldiethanolamine Ph2Si-
([OCH2CH2]2NH) has dSiN = 2.301(6) Å,27 while imine donors
such as in Ph2Si(OC6H4NC[Ph]C6H3-4-[OCH3]-2-O)
Silylation of the oxidized ligand in Pb(ONOQ)2 with
chlorosilanes RSi(X)(Y)Cl in chloroform solution, or much
more slowly in benzene solution, results in precipitation of
lead(II) chloride and formation of diamagnetic products of five-
coordinated silicon where the ONO ligand has been modified
by migration of a methyl or phenyl group from silicon to
nitrogen (eq 2). Isolated yields are modest, due principally to
solubility losses, since in situ NMR monitoring indicates that
the migrated products are usually formed in good to excellent
yields. As the reactions proceed, the solutions change color
from the dark green of Pb(ONOQ)2 to a much lighter purple
or magenta. The isolated products are colorless, so the purple
colors are indicative of the formation of small amounts of
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a
Table 1. Bond Distances (Å) and Angles (deg) in the Five-Coordinate Products (X)(Y)Si(ON[Z]O)
Me2Si(ON[Me]O)
ClMeSi(ON[Me]
O)·2C6H6 (X = Cl,
Y = C3, Z = C1)
ClPhSi(ON[Me]O)
(X = Cl, Y = C31,
Z = C1)
MePhSi(ON[Me]O)
(X = C2, Y = C31,
Z = C1)
Ph2Si(ON[Ph]O)
(molecule 1; X = C2,
(molecule 1; X = C41,
b
b
Y = C3, Z = C1)
Y = C51, Z = C31)
Si−O1
Si−O2
Si−X
Si−Y
Si−N
N−C11
N−C21
N−Z
O1−Si−O2
O1−Si−X
O1−Si−Y
X−Si−Y
O2−Si−X
O2−Si−Y
O1−Si−N
O2−Si−N
X−Si−N
Y−Si−N
C11−N−C21
C11−N−Z
C21−N−Z
C11−N−Si
C21−N−Si
Z−N−Si
C12−O1−Si
C22−O2−Si
1.6599(15)
1.6541(17)
1.848(3)
1.651(2)
1.6467(13)
1.6520(13)
2.1176(7)
1.847(2)
1.6539(9)
1.6577(9)
1.8533(14)
1.8563(13)
2.5393(11)
1.4472(14)
1.4514(15)
1.4773(15)
120.24(5)
99.65(6)
1.6454(10)
1.6515(10)
1.8622(14)
1.8519(15)
2.9184(13)
1.4272(17)
1.4395(18)
1.4163(18)
115.44(5)
101.10(6)
116.36(6)
107.63(6)
105.22(6)
110.31(6)
66.79(4)
1.642(2)
2.1442(10)
1.856(3)
1.843(3)
2.605(2)
2.302(2)
2.3043(16)
1.460(2)
1.449(3)
1.455(3)
1.442(3)
1.463(3)
1.464(2)
1.475(3)
1.486(3)
1.485(2)
115.32(8)
100.20(10)
114.62(12)
107.59(13)
99.41(10)
116.56(11)
74.50(7)
117.67(11)
93.52(7)
124.64(7)
93.98(5)
119.25(13)
120.65(14)
91.83(7)
115.26(8)
117.24(8)
92.78(5)
113.40(5)
115.47(5)
98.21(6)
100.10(11)
81.82(9)
100.55(7)
80.94(6)
105.73(6)
75.84(4)
73.31(7)
81.89(9)
80.80(6)
75.68(4)
66.50(4)
167.29(10)
85.08(10)
115.26(16)
111.07(17)
113.74(17)
95.76(12)
98.41(12)
121.21(15)
128.66(13)
130.71(14)
169.24(6)
90.63(12)
112.37(19)
110.9(2)
167.31(5)
92.14(7)
168.53(5)
85.72(4)
157.21(5)
92.48(5)
114.50(14)
112.17(14)
110.00(14)
102.01(11)
110.00(14)
116.49(12)
125.16(12)
124.64(11)
115.03(9)
113.24(9)
111.42(9)
99.26(7)
116.88(11)
119.84(11)
118.61(11)
93.10(8)
112.1(2)
100.98(15)
101.52(15)
118.26(16)
123.67(17)
124.08(17)
98.42(7)
88.06(8)
118.33(7)
130.50(8)
129.82(7)
109.97(8)
136.66(9)
128.65(9)
a
b
X = axial substituent, Y = equatorial substituent, Z = migrated group. The crystal contains two inequivalent molecules, with data for molecule
1 given (Si = Si1, N = N1). Metrical data for molecule 2 are in Table S2.
(dSiN = 2.107(1) Å)26 and Ph2Si(OC6H4C[Me]=NCH2CH2O)
(dSiN = 2.070(2) Å)31 show much shorter Si−N distances despite
having six-membered rings, and the five-membered-ring com-
pound Me2Si(2,6-[OC(4-BrC6H4)2]2NC5H3) has a very short
Si−pyridine distance of 1.950(8) Å.25
of the hydrolysis product (ON[Me]O)SiMe(μ-O)SiMe(ON-
[Me]O) (Table S5 and Figure S4, Supporting Information)
have been determined by single-crystal X-ray diffraction. The
structures confirm the connectivity and geometry inferred from
spectroscopic data. In all cases, the silicon adopts an approx-
imately trigonal-bipyramidal geometry with the two aryloxide
oxygens in equatorial positions and the methylated (or phenylated)
amine nitrogen occupying one of the axial positions. While the
unmodified ONO ligand is rather rigid and is only known to adopt
a mer geometry in its complexes, alkylation at nitrogen forces the
ligand out of planarity and favors a fac-type geometry, with
O−Si−O angles in the range of 115−125°, close to the 120°
expected for an ideal trigonal bipyramid. The same basic structure
was observed for the tin analogue Cl(Bu)Sn(ON[Bu]O) formed
from reaction of Bu2SnCl2 with Zn(ONOQ)2.18
The major difference between the structures is how tightly
the amine nitrogen is bonded to silicon. Among compounds
with identical ON[Me]O chelates, the silicon−nitrogen distances
increase with trans groups in the order trans-Cl (Si−N =
2.302(2), 2.3042(16) Å) < CH3 (Si−N = 2.605(2), 2.617(2),
2.5393(11) Å) ≈ μ-O (Si−N = 2.585(2), 2.675(2) Å). Silicon−
nitrogen binding in Ph2Si(ON[Ph]O) is even weaker (Si−N =
2.7516(12), 2.9184(13) Å for the two crystallographically inde-
pendent molecules), presumably because of the lowered Lewis
basicity of nitrogen on substitution of methyl for phenyl. The con-
siderable variation in bond distances between nominally equivalent
bonds in the crystallographically inequivalent molecules of
Ph2Si(ON[Ph]O) and the chemically equivalent but crystallo-
graphically distinct silicons in (ON[Me]O)Si(Me)(μ-O)Si(Me)-
(ON[Me]O)) suggests that the silicon−nitrogen interaction is
When both phenyl and methyl groups are present in the
silane (PhMe2SiCl, PhMeSiCl2), only methyl migration is ob-
served on reaction with Pb(ONOQ)2, as judged by the appearance
of the characteristic NCH3 signals in the 1H NMR spectrum. In
situ monitoring of reactions shows no signs of products in
which phenyl migration has taken place. If, however, silanes
containing only phenyl groups are used (Ph3SiCl, Ph2SiCl2),
then migration of phenyl groups is observed. For example,
Ph3SiCl reacts with Pb(ONOQ)2 to give Ph2Si(ON[Ph]O), in
which three different phenyl environments (NPh, equatorial
SiPh, and axial SiPh) are observed by 1H and 13C NMR
spectroscopy. Exclusive observation of methyl in preference to
phenyl migration is extremely unusual. Migrations from silicon often
take place with exclusive32 or moderate33 preference for phenyl over
methyl migration, and cases of moderate preference for methyl over
phenyl migration have been observed,34 but we know of no other
cases where thermal 1,2-methyl migration is strongly favored over
phenyl migration. Selective photochemical 1,3-migration of alkyl
groups from silicon in preference to phenyl groups has been
reported, although photochemical migration of phenyl groups is
observed if no alkyl groups are present.35 Thermal 1,3-migrations of
alkyl groups to coordinated ligands have also been documented.36
Structures and Stereochemical Preferences of (X)(Y)-
Si(ON[R]O). The solid-state structures of five of the products of
silylation and migration (Table 1 and Figure 2), as well as that
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Figure 2. Thermal ellipsoid plots of (a) Me2Si(ON[Me]O), (b) Cl(Me)Si(ON[Me]O)·2C6H6, (c) Cl(Ph)Si(ON[Me]O), (d) Me(Ph)Si(ON-
[Me]O), and (e) Ph2Si(ON[Ph]O). Hydrogen atoms, minor orientations of disordered tert-butyl groups, and solvent molecules are omitted for
clarity. In (a) and (e), only one of the two crystallographically inequivalent molecules is depicted.
rather weak and the potential energy surface correspondingly
shallow at these long distances and that small differences should
therefore not be overinterpreted. In contrast, the much shorter
distance trans to Cl is indicative of stronger donation of the
amine to the Si−Cl σ* orbital and is correspondingly consistent
in both Cl(Me)Si(ON[Me]O) and Cl(Ph)Si(ON[Me]O). The
degree of silicon−nitrogen interaction is also reflected in the
degree of pyramidalization at nitrogen, where the chloro com-
pounds show strongly pyramidalized nitrogen atoms (sum of
C−N−C angles 335.4, 336.7°) while Ph2Si(ON[Ph]O) shows a
nearly planar nitrogen atom (sum of C−N−C angles 355.3,
347.9°).
In general, the silicon−nitrogen distances observed here are
longer than those previously observed for analogous silicon
compounds with dative N→Si interactions. For example,
crystallographically characterized silatranes with apical chlo-
rides, N[CH2CH2O]3SiCl (Si−N = 2.026(10) Å)37 and
N[CH2C6H2-3-tBu2-5-Me-2-O]3SiCl (Si−N = 2.046(2) Å),38
show Si−N distances over 0.25 Å shorter than those observed
in Cl(R)Si(ON[Me]O). Some of this difference may be
accounted for by the fact that the silatranes are tricyclic rather
than bicyclic, but even bicyclic diethanolamine derivatives can
show significantly shorter Si−N distances than the ON[R]O
compounds, though no strictly analogous compounds are
known. For example, F(Ph)Si([OCH2CH2]2NCH3) has
d(Si−N) = 2.1751(7) Å,39 0.13 Å shorter than the corresponding
distance in Cl(Ph)Si(ON[Me]O). However, the Si−N distance
in Ph2Si([OCH2CH2]2NCH3) (2.68(1) Å)40 is comparable to
those in R2Si(ON[Me]O), and bulkier Ar2Si([OCH2CH2]2NCH3)
show Si−N distances greater than 2.9 Å.41 It appears that
the greater basicity of the trialkylamine donor in the di-
ethanolamine ligand is counterbalanced by torsional strain
suffered in the boat−boat conformation of the saturated di-
oxasilocane ring needed for strong nitrogen-to-silicon dona-
tion. Indeed, attenuation of the nitrogen basicity, as in
N-aryldiethanolamines, results in adoption of a crown−crown
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conformation and scission of the Si−N interaction, with
d(Si−N) > 3.0 Å.40,42
flexible saturated silocane ring. For example, dibenzodioxasilo-
cine (CH2[C6H2-3,5-tBu2-2-O]2)SiMe2, identical with Me2Si-
(ON[Me]O) except for a noncoordinating CH2 group
replacing the NMe group, shows a barrier to ring inversion
of 13.9 kcal/mol.47
Phenyl Migration from Tin to Nitrogen. Analogous
migrations of alkyl groups from tin to nitrogen were reported
upon reaction of R2SnCl2 with Zn(ONOQ)2 to form Cl(R)-
Sn(ON[R]O) (R = Me, Bu).18 However, aryl groups were not
observed to migrate in a similar fashion. In particular, Ph2SnCl2
was found to lose a phenyl group entirely to give octahedral
(ONOQ)Sn(Ph)Cl2, which could also be prepared from the
reaction of PhSnCl3 with Zn(ONOQ)2. BuSnCl3 also was
reported to give crystallographically characterized (ONOQ)Sn-
(Bu)Cl2,12 rather than a product of migration.
In compounds Cl(R)Si(ON[Me]O), the chlorine is apical in
the crystallographically characterized product. This is in
agreement with the well-known preference for more electro-
negative groups to preferentially adopt the apical positions in
trigonal-bipyramidal compounds of the main-group elements
due to the greater ionic character of the apical bonds.43 The
chloro compounds are also observed in solution to form only
one stereoisomer at equilibrium, presumably that with axial
chlorine, as is observed crystallographically. In the case of
Cl(Ph)Si(ON[Me]O), NMR data support this assignment, on
the basis of the upfield shift of the N−CH3 resonance (δ 2.23),
which can be attributed to the location of the methyl group in
the shielding cone of the equatorial phenyl group (compare to
δ 2.98 and 3.02 in Me2Si- and Cl(Me)Si(ON[Me]O),
respectively, where no Si-Ph group is present). A similar
upfield shift of the NCH3 resonance has been observed in
X(Ph)Si([OCH2CH2]2NCH3) with equatorial phenyl groups at
low temperature.44
We find that phenyl migration can take place from tin to
nitrogen, with Ph3SnCl reacting with Pb(ONOQ)2 to give
Ph2Sn(ON[Ph]O) (eq 3). Migration and consequent reduction
In the solid state, Me(Ph)Si(ON[Me]O) has an apical
methyl group and an equatorial phenyl group. In solution, this
compound shows two stereoisomers at equilibrium in a 8.7:1
ratio. The major isomer in solution appears to have an
equatorial phenyl, as observed in the crystal, judging from the
upfield shift of its methyl group (δ NCH3 1.99 major, δ 3.04
minor). An apical preference for the methyl group is
unexpected, given the greater electronegativity of sp2 carbons
over sp3 carbons, but the literature suggests that these
preferences are modest and may thus be subject to subtle
aspects of ligand structure. For example, in the solid state,
(CH2CHCH2)(Ph)Si(OC6H4NC[Ph]C6H3-4-[OCH3]-2-
O) has the sp3-hybridized carbon of the η1-allyl group apical in
preference to the phenyl group. The solid-state structure of
(Ph)(Me)Si(2,6-[OC(4-FC6H4)2CH2]2NC5H3) has an apical
phenyl group, and it shows a 4:1 preference for this isomer in
solution (at −60 °C).45 The triethanolamine complex (Me)-
(Ph)Si([OCH2CH2]2NCH3) shows a 1.5:1 preference (at −90 °C)
for an axial methyl group.44
of the ONOQ ligand is demonstrated by the presence of three
inequivalent phenyl groups in Ph2Sn(ON[Ph]O), by the
upfield shift of the OCAr resonances in the 13C NMR spectrum,
and by the fact that the product is colorless.
Thus, the major factor in distinguishing whether stannanes
react with sources of [ONOQ]− by simple transmetalation or
transmetalation/migration appears to be the number of
ancillary chlorides present, not whether the migrating group
is aryl or alkyl. Ph3SnCl, Bu2SnCl2, and Me2SnCl2 all undergo
migration, while PhSnCl3 and BuSnCl3 undergo only trans-
metalation. It is interesting to note that CH3SiCl3, in contrast
to RSnCl3, does react with migration of the methyl group to
nitrogen (as gauged, for example, by the NMe resonance at δ
The two isomers of Me(Ph)Si(ON[Me]O) equilibrate
within 15 min after dissolution of the solid in CDCl3. This
equilibration is not fast on the NMR time scale at room
temperature, however, as the two isomers give separate sharp
1
3.16 in the H NMR and δ 50.09 in the 13C NMR spectrum).
1
signals; similarly, the two SiMe resonances in the H and 13C
This result suggests that, in this system, the silicon−carbon
bond is actually more reactive than the tin−carbon bond. An
investigation into the details of the mechanism, and the
magnitude and origin of this difference in reactivity, is currently
in progress.
NMR spectra of Me2Si(ON[Me]O) are separate and sharp as
well. Axial/equatorial exchange of the two groups on silicon
cannot be achieved in these compounds through Berry
pseudorotations because the substituent on nitrogen renders
the two faces of the SiONO bicycle inequivalent, but either a
turnstile rotation28b or dissociation of the Si−N bond followed
by inversion of the boat−boat conformation of the eight-
membered ring would suffice to exchange the positions of the
substituents on silicon. We strongly favor the latter mechanism
on the basis of the much slower exchange rates shown by the
ON[R]O complexes in comparison to diethanolamine
complexes. Barriers to exchange in RR′Si([OCH2CH2]2NMe)
are in the range of 9−13 kcal/mol,28,44 while the lack of
exchange in the NMR spectra of (X)(Y)Si(ON[R]O) at
CONCLUSIONS
■
Methyl- and phenyl-substituted chlorosilanes react readily with
Pb(ONOQ)2 to form five-coordinate silanes ligated to
tridentate, dianionic, N-substituted bis(aryloxy)amines. The
aminebis(phenoxide) ligands result from two-electron reduc-
tion and alkyl or aryl group migration from reactant silane. The
products adopt folded structures with approximately trigonal-
bipyramidal geometry at silicon, with apical nitrogen and
equatorial oxygens. Chlorine adopts the other apical position in
strong preference to alkyl or aryl, with methyl having a modest
preference for the apical site over phenyl. Analogous reactions
of alkylstannanes had been previously reported, and we find
that arylstannanes can also undergo tin to nitrogen migration.
ambient temperatures implies ΔG⧧ > 17 kcal/mol.46
A
nondissociative mechanism would likely occur at similar rates
in the two classes of compounds, while the greatly enhanced
rigidity of the benzannelated silocine ring would substantially
enhance the barrier to inversion in comparison to the more
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Article
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Two features of these migration reactions are particularly
unusual. First, methyl migrates from silicon in exclusive pre-
ference to phenyl. Second, migration may in some circum-
stances be more facile for silicon than it is for tin. In particular,
CH3SiCl3 reacts via methyl migration, where RSnCl3 species
give stable (ONOQ)Sn(R)Cl2 products.
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ASSOCIATED CONTENT
* Supporting Information
■
S
CIF files giving crystallographic information on Me2Si(ON[Me]O),
Cl(Me)Si(ON[Me]O)·2C6H6, Cl(Ph)Si(ON[Me]O), Me-
(Ph)Si(ON[Me]O), Ph2Si(ON[Ph]O), Pb(ONOQ)2·2CHCl3,
Me2Si(ONOSQ), Si(ONOSQ)2, and (ON[Me]O)Si(Me)-
(μ-O)Si(Me)(ON[Me]O)·2C6H6 and a PDF file giving a
summary of crystallographic details for all structures, thermal
ellipsoid plots and selected metrical data for the last four of
these structures, and selected metrical data for the second
crystallographically unique molecules in Me2Si(ON[Me]O)
and Ph2Si(ON[Ph]O). This material is available free of charge
via the Internet at http://pubs.acs.org.
(18) Camacho-Camacho, C.; Mijangos, E.; Castillo-Ramos, M. E.;
́
Esparza-Ruiz, A.; Vasquez-Badillo, A.; Noth, H.; Flores-Parra, A.;
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Contreras, R. J. Organomet. Chem. 2010, 695, 833−840.
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(21) International Tables for Crystallography; Kluwer Academic:
Dordrecht, The Netherlands, 1992; Vol. C.
(22) This compound has been generated in solution and
characterized by EPR spectroscopy: Piskunov, A. V.; Sukhoshkina,
O. Y.; Smolyaninov, I. V. Russ. J. Gen. Chem. 2010, 80, 790−799.
(23) Keller, K. A.; Connor, D. M.; Lever, J. G. U.S. Pat. 2007/
0117893, May 24, 2007.
AUTHOR INFORMATION
Corresponding Author
*E-mail: Seth.N.Brown.114@nd.edu.
Notes
■
(24) Schraml, J.; Chvalosky, V.; Magi, M.; Lippmaa, E. Collect. Czech.
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Chem. Commun. 1981, 46, 377−390.
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(25) Gomez, E.; Santes, V.; de la Luz, V.; Farfan, N. J. Organomet.
Chem. 2001, 622, 54−60.
The authors declare no competing financial interest.
(26) Wagler, J.; Brendler, E. Z. Naturforsch., B 2007, 62B, 225−234.
(27) Daly, J. J.; Sanz, F. J. Chem. Soc., Dalton Trans. 1974, 2051−
2054.
ACKNOWLEDGMENTS
■
(28) (a) Liepins, E.; Popelis, J.; Birgele, I.; Urtane, I.; Zelchan, G.;
Lukevics, E. J. Organomet. Chem. 1980, 201, 113−121. (b) Pestunovich,
V. A.; Lazareva, N. F.; Albanov, A. I.; Kozyreva, O. B.; Volkova, V. V.;
Gusel’nikov, L. E. ARKIVOC 2006, 116−125.
We thank Dr. Allen Oliver for his assistance with the X-ray
crystallography. This work was supported by the National
Science Foundation (CHE-1112356). Additional support from
the Notre Dame Sustainable Energy Initiative is also gratefully
acknowledged.
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