Hypercoordination in triphenyl oxinates of the group 14 elements

Article abstract of DOI:10.1016/j.jorganchem.2009.11.027

The triphenyl oxinates of the group 14 elements (M = Si, Ge, Sn, and Pb) contain the 8-hydroxyquinoline ligand (HOx), which can function in either a bidentate or monodentate fashion. The compounds Ph₃MO_x were prepared by reaction of the triphenylmetal chloride with HOx in the presence of an HCl scavenger triethylamine or, sodium acetate, and in the case of lead, with the sodium salt of 8-hydroxyquinoline. The interaction of the nitrogen with the central atom was studied through the use of the NMR chemical shifts of the central metal atom and the ¹⁵N atom of the ligand. The chemical shifts of the central metal provided evidence that the triphenylgermanium and silicon oxinates are uncoordinated while the triphenyltin and lead oxinates are five-coordinate. These conclusions are confirmed by molecular modeling, ¹⁵N chemical shifts and the metal-¹³C one bond coupling constants at the ipso carbon. The NMR data provides evidence that the strength of the interaction of the nitrogen with the metal increases from silicon and germanium to lead. Two peaks in the 5-coordinate region of the ²⁰⁷Pb NMR spectra can be rationalized with the postulate that strong interaction with lead produces two geometrical isomers. Two peaks were also present in the 5-coordinate region of the ¹¹⁹Sn NMR spectra at low temperatures indicating a rapid exchange between the two geometrical isomers at room temperature.

Full text of DOI:10.1016/j.jorganchem.2009.11.027

Journal of Organometallic Chemistry 695 (2010) 518–523
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Hypercoordination in triphenyl oxinates of the group 14 elements
a
b
b
b
Claude H. Yoder a, , Allison K. Griffith , Alaina S. DeToma , Cameron J. Gettel , Charles D. Schaeffer Jr.
*
a Department of Chemistry, Franklin and Marshall College, P.O. Box 3003, Lancaster, PA 17604-3003, USA
b Department of Chemistry, 1 Alpha Drive, Elizabethtown College, Elizabethtown, PA 17022-2298, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 4 August 2009
Received in revised form 17 November 2009
Accepted 18 November 2009
Available online 24 November 2009
The triphenyl oxinates of the group 14 elements (M = Si, Ge, Sn, and Pb) contain the 8-hydroxyquinoline
ligand (HOx), which can function in either a bidentate or monodentate fashion. The compounds Ph3MOx
were prepared by reaction of the triphenylmetal chloride with HOx in the presence of an HCl scavenger
triethylamine or, sodium acetate, and in the case of lead, with the sodium salt of 8-hydroxyquinoline. The
interaction of the nitrogen with the central atom was studied through the use of the NMR chemical shifts
of the central metal atom and the 15N atom of the ligand. The chemical shifts of the central metal pro-
vided evidence that the triphenylgermanium and silicon oxinates are uncoordinated while the triphen-
yltin and lead oxinates are five-coordinate. These conclusions are confirmed by molecular modeling,
15N chemical shifts and the metal–13C one bond coupling constants at the ipso carbon. The NMR data pro-
vides evidence that the strength of the interaction of the nitrogen with the metal increases from silicon
and germanium to lead. Two peaks in the 5-coordinate region of the 207Pb NMR spectra can be rational-
ized with the postulate that strong interaction with lead produces two geometrical isomers. Two peaks
were also present in the 5-coordinate region of the 119Sn NMR spectra at low temperatures indicating a
rapid exchange between the two geometrical isomers at room temperature.
Keywords:
8-Hydroxyquinoline
Lewis acidity
Group 14 oxinates
Triphenylsilicon oxinate
Triphenylgermanium oxinate
Triphenyltin oxinate
Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
coordinating solvents. Kawakami and Okawara [7] also proposed
the chelated structure on the basis of the shift in UV peak at
The potentially bidentate ligand 8-hydroxyquinoline has been
widely used to study hypercoordination in a variety of organome-
tallic compounds, particularly those of tin. This ligand is ideal be-
cause of the relatively acidic phenolic proton that can usually be
removed in nucleophilic substitution reactions with metal halides
and because of the pyridine-like nitrogen with its sp2 hybridized
lone pair of electrons in the plane of the aromatic ring. As shown
in Fig. 1 for a 1–1 complex of the ligand to a trialky- or triaryl
tin, the oxygen and nitrogen are conformationally constrained in
the cis conformation and form a 5-membered ring that includes
the metal atom.
Among the most frequently studied are the aryltin oxinates. X-
ray diffraction studies demonstrated 6-coordination for diphenyl-
tin dioxinates [1] and 7-coordination for aryltin trioxinates [2].
The structure of triphenyltin oxinate has been a subject of some
debate. Four-coordinate structures were proposed by Roncucci
et al. [3] and Clark et al. [4] on the basis of electronic and 119Sn
NMR spectra in CDCl3, while Lycka et al. [5,6] used 1H, 13C, 15N,
and 119Sn NMR analyses to support 5-coordination citing upfield
shifts of both 119Sn and 15N as evidence for coordination in non-
320 nm. More recently, Szorcsik et al. [8] also postulated the coor-
dinated structure based on Mössbauer and IR shifts.
There are no studies of triphenylgermanium-, silicon, and lead
oxinates, but Wada and Okawara [9] reported that dimethyl- and
trimethylsilyl oxinates as well as dimethyl- and trimethylgermyl
oxinates are four coordinate primarily on the basis of the shift in
the 320 nm band in the UV spectrum of the oxinate, but that diphe-
nylsilyl- and -germyldioxinate are six coordinate [9]. Wagler et al.
[10] suggested four coordinate structures for diphenyl- and
dimethylsilyl oxinates both in solution and in the solid state based
on 29Si shifts. Faraglia et al. [11] concluded that diphenyllead diox-
inate is chelated in benzene, but not in ethanol based on UV
spectra.
The objectives of the present study are to determine the extent
of hypercoordination in the triphenyl oxinates of the group 14
elements in a non-coordinating solvent. The Ph3MX system is of
particular interest because the presence of three phenyl groups
around the central atom should produce a very weakly Lewis
acidic site unless X is quite electronegative, as, for example, when
X = Cl. Hence, the extent of hypercoordination in triphenyl oxi-
nates of the group fourteen elements have been investigated with
NMR spectroscopy because of the well-known dependence of
chemical shifts and coupling constants on electron density and
geometry.
* Corresponding author. Tel.: +1 717 291 3806; fax: +1 717 291 4343.
E-mail address: claude.yoder@fandm.edu (C.H. Yoder).
0022-328X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2009.11.027

C.H. Yoder et al. / Journal of Organometallic Chemistry 695 (2010) 518–523
519
disk. The solvent was removed in vacuo (0.76 g, 53%), m.p. 67–
70 °C.
O
Sn
R
R
2.4. Molecular modeling
N
Geometry optimizations were performed with GAUSSIAN 03 [14]
using the DFT method using the B3PW91 functional. Basis sets
were 6-31G for triphenylgermanium oxinate, 3-21G for triphenyl-
tin oxinate, and LanL2DZ for the triphenyllead oxinate (due to the
larger atomic numbers of tin and lead which exceed the ranges of
the 6-31G and 3-21G basis sets).
R
Fig. 1. Triorganotin oxinate in the cis conformation.
2. Experimental
3. Results
2.1. Materials and reagents
All of the metal oxinates were synthesized by nucleophilic sub-
stitution of the oxinate at the M–Cl bond in Ph3MCl. For the triphe-
nylsilicon and germanium oxinates triethylamine was used as an
HCl scavenger. For the preparation of the triphenyltin oxinate, so-
dium acetate and ammonia were used to pick up HCl [12], while
triphenyllead oxinate was synthesized using the sodium salt of
8-hydroxyquinoline [13]. The triphenylsilicon naphthoxide used
as a comparison for the silicon chemical shift was prepared with
the sodium salt of 1-naphthol.
All solvents were dried over Drierite and stored over Linde 4A
molecular sieves. Reactions using triphenylchlorosilane and tri-
phenylchlorogermane were performed under a positive pressure
of argon.
2.2. NMR analyses
The NMR spectra of all four compounds were obtained as
approximately 10% solutions in CDCl3 for proton and carbon
NMR analysis. Saturated solutions were used for nitrogen, silicon,
germanium and tin NMR analyses. A minimal amount of Cr(acac)3
was used as a relaxation reagent for the nitrogen resonance. NMR
spectra were obtained on a Varian INOVA 500 MHz spectrometer.
The following transmitter frequencies were used: hydrogen
(500 MHz), carbon (125.6 MHz), silicon (99.3 MHz), tin
(186.3 MHz), lead (104.4 MHz), and nitrogen (50.6 MHz). The delay
time for the observation of 15N was 1 s with typically 35,000
transients.
The triphenyl oxinates were analyzed using 1H, 13C, 15N, 29Si,
119Sn, and 207Pb NMR spectroscopy. The numbering used for the
carbon and proton assignments are shown in Fig. 2. Peak assign-
ments (Table 2) were based on the shifts of the starting materials,
peak intensities, and metal–carbon coupling constants of the phe-
nyl groups (Table 3).
In general, the chemical shifts of protons 3, C, and D are very
similar and not easily distinguishable. As a result, the resonances
7
O
8
2.3. Synthesis
M
D
6
A
9
3
B
The triphenylsilicon and germanium oxinates were prepared by
combining 8-hydroxyquinoline (0.25 g, 1.7 mmol) and triethyl-
amine (0.24 mL, 1.7 mmol). THF and toluene were used as solvents
for the triphenylsilicon and -germanium oxinates, respectively. The
triphenylmetal chloride (1.7 mmol) was dissolved in the appropri-
ate solvent and slowly added to the stirred solution. The reaction
was heated to approximately 65 °C for 1/2 h, and filtered by suc-
tion, after which solvent was removed from the filtrate in vacuo
yielding, for triphenylsilicon oxinate, a light yellow solid (0.52 g,
76%), m.p. 57–59 °C, and for triphenylgermanium oxinate, a light
yellow solid (0.62 g, 94%), m.p. 61–62 °C.
C
5
3
N
10
2
4
Fig. 2. Carbon and proton numbering scheme for the Ph3M oxinates. M = Si, Ge, Sn
and Pb.
Table 1
Central atom (M = Si, Ge, Sn and Pb) and 15N chemical shifts for triphenylmetal
oxinates in CDCl3 unless noted.
Triphenyltin oxinate was prepared by following previously re-
ported methods [12]. The yellow solid was isolated by suction fil-
tration (0.35 g, 35%), m.p. 140–144 °C. Lit. [3] m.p. 145–146.5 °C.
Triphenyllead oxinate was prepared as previously reported [13].
M.p. 99–102 °C. Lit. [11] m.p. 108–111 °C. Anal. Calc. for
C27H28NOPb: C, 55.66; H, 3.63. Found: C, 55.07; H, 3.84%.
Triphenylsilicon naphthoxide was prepared with sodium naph-
thoxide, which was obtained by dropwise addition of sodium eth-
oxide (0.27 mL, 3.4 mmol) in absolute ethanol to an alcoholic
solution of naphthol (0.5 g, 3.5 mmol). The solution was allowed
to stir for 15 h and the solvent was then removed in vacuo. Triphe-
nylchlorosilane (1.02 g, 3.5 mmol) was dissolved in 20 mL of THF
and added dropwise under argon to the resulting product also dis-
solved in 20 mL of THF. The endothermic reaction was heated
gently at 65 °C for 2 h and then allowed to stir for 15 h at room
temperature. The solution was vacuum filtered and the resulting
Compound
M Shift (ppm)a
15N Shift (ppm)b,c
Ph3SiOx
Ph3Sinaphthoxide
Ph3GeOx
À12.6
À12.4
À99.3
À98.4
Ph3SnOx
Ph3SnOx
Ph3SnOx
À190.5
À120.7, À118.6g
d
À259.6
e
À189.3
Ph3Snnaphthoxide
Ph3PbOx
Methoxyquinoline
À92.4f
À158.9, À187.2
À107.5, À244.5
À84.0g
a
Silicon shifts referenced to TMS. Tin shifts referenced to (CH3)4Sn. Lead shifts
referenced to (CH3)4Pb.
b
Nitrogen shifts referenced to nitromethane.
All nitrogen shifts obtained using Cr(acac)3 as a relaxation reagent.
In DMSO-d6.
In toluene-d8.
Lit. [10].
Lit. [15].
c
d
e
f
g
filtrate was then filtered with a syringe fitted with a 0.45 lm filter

520
C.H. Yoder et al. / Journal of Organometallic Chemistry 695 (2010) 518–523
Table 2
13C NMR shifts (ppm).
Compounda
2
3
4
5
6
7
8
9
10
Ph3SiOx
147.8
147.9
144.7
144.3
147.8
149.2
121.8
121.8
121.7
120.8
121.6
121.7
136.1
136.1
138.4
137.5
136.1
135.9
117.9
117.8
114.3
114.3
117.8
107.5
127.7
127.7
130.0
134.1
127.6
126.7
110.1
110.0
114.0
112.8
110.4
119.6
152.2
152.2
156.4
158.6
152.3
155.4
138.3
138.3
138.8
137.2
138.2
140.0
128.5
128.5
129.2
130.0
128.5
127.7
Ph3GeOx
Ph3SnOx
Ph3PbOx
8-HOQb
8-MeOQb,c
Compound
Ph3SiOx
A
B
C
D
135.2
135.1
134.7
145.4
137.4
163.2
135.0
135.0
134.0
136.5
136.5
136.1
130.1
130.1
130.5
128.6
129.1
129.7
127.9
127.9
128.6
129.0
130.4
128.9
Ph3SiONpb
Ph3GeOx
Ph3SnOx
Ph3SnONpb,d
Ph3PbOx
a
All samples in CDCl3.
Q = quinoline, Np = naphthoxide.
Lit. [20].
Lit. [10].
b
c
d
of protons 3, C, and D are frequently listed as a range, as having the
same shifts, or with fewer significant figures.
the synthesis described in Section 2. In one preparation, using
the same synthesis on a larger scale (2 mmol increased to 6 mmol),
the product contained two peaks in the 119Sn NMR spectrum. The
peak at À190.4 ppm is in the pentacoordinate region and was ob-
served in the 119Sn spectra of the products of all syntheses of the
triphenyltin oxinate. The other peak at À393.7 ppm, which is sig-
nificantly smaller than the peak at À190.4 ppm, is clearly in the
hexacoordinate region [1,5]. Linden et al. previously used single
crystal X-ray diffraction to identify this hexacoordinate compound
as diphenyltin dioxinate [1].
4. Discussion
The chemical shift of the Lewis acidic central atom has fre-
quently been used as the primary evidence for hypercoordination,
with a low frequency (upfield) shift as an indication of an increase
in electron density [15,16]. For the heavier group 14 atoms,
changes in shifts of at least 100 ppm are commonly observed for
structural changes from 4- to 5-coordinate [4]. Upon coordination,
the increase in electron density at the central atom is also accom-
panied by the loss of electron density at the nitrogen atom, which
has been shown to produce a low frequency (upfield) shift in the
15N resonance in similar systems [6,17]. For example, the 15N
chemical shift of the pyridinium ion, in which the lone pair on
the nitrogen is coordinated to a proton, is 100 ppm upfield from
that of pyridine [18].
The 119Sn spectrum at À40 °C contained two incompletely re-
solved peaks at À179.2 and À179.3 ppm with a half height width
of 17 Hz. At room temperature, the half height width was 7.5 Hz.
At À60 °C, the peaks had baseline resolution at À188.6 and
À189.9 ppm. The presence of two peaks at lower temperature
can be attributed to two geometrical isomers (see below). The
presence of only one peak at room temperature can be attributed
to rapid exchange between the two isomers.
An important indicator of hypercoordination in the 13C spectra
is the chemical shift of the ipso carbons (carbon A). A high fre-
quency (downfield) shift of carbon A from that of tetracoordinate
tin compounds has been previously reported in other trigonal
bipyramidal triorganotin complexes [5,19,20]. An increase in the
coupling constant of carbon A, 1J (M, 13C), where M = Si, Sn, Pb, is
also observed in the change from four to five coordination [15]. A
general trend between the coordination number of the central
atom and the coupling constant has been previously presented
[21].
Tin spectra were also obtained in DMSO-d6 and acetone-d6 as
solvents. In acetone-d6, the tin shifts are quite similar to those in
CDCl3, whereas in DMSO-d6, the pentacoordinate peak moves up-
field considerably, presumably indicating increased coordination
at the tin atom with the DMSO.
The 15N NMR shift (Table 1) also indicates that triphenyltin oxi-
nate is a hypercoordinate compound. The 15N resonance of tri-
phenyltin oxinate (À120.7 ppm) shifts upfield by 40 ppm from
methoxyquinoline, indicating a loss of electron density from the
nitrogen atom [15].
The 13C NMR spectrum (Table 2) of triphenyltin oxinate shows a
downfield shift of the ipso carbon (carbon A) of approximately
4.1. Triphenyltin oxinate
The 119Sn chemical shift of triphenyltin oxinate (À190.5 ppm) is
shifted upfield from that of the four-coordinate compound, tri-
phenyltin naphthoxide (À92.4 ppm) by approximately 100 ppm
[6]. The tin chemical shift of triphenyltin naphthoxide was used
for comparison because of the similarity of the structure of the
naphthoxide to the triphenyltin oxinate but with the absence of
a potentially coordinating nitrogen atom [6]. The upfield shift is
consistent with 5-coordination. Five coordinate tin shifts appear
in the region from À90 to À330 ppm [15]. The concentration inde-
pendence of the tin chemical shift ( 0.02 ppm for three different
concentrations) eliminated intermolecular coordination as the
cause of the upfield tin shift.
Table 3
Central atom–phenyl carbon coupling constants J(M, 13C), M = Si, Sn, Pb, (Hz).
Compound
1J
2J
3J
4Ja
Ph3SiOx
80
80
78
634, 664
589, 615
797
624
549
14
n.o.
12
47
49
48
91
91
87
56
55
55
63
65
70
106
105
111
n.o.
44
12
n.o.
14
15
n.o.
n.o.
24
Ph3SiCl
Ph3SiClb
Ph3SnOx
Ph3SnCl
Ph3SnClÁDMSO
Ph3PbOx
Ph3PbCl
Ph3PbClÁDMSO
743
a
A single tin peak was observed in the tin NMR spectra for tri-
phenyltin oxinate in CDCl3 obtained in three preparations using
n.o = Not observed.
In DMSO.
b

C.H. Yoder et al. / Journal of Organometallic Chemistry 695 (2010) 518–523
521
Table 4
1H NMR shifts (ppm).a
Compound
2
3
4
5
6
7
B
C
D
Ph3SiOx
Ph3GeOx
Ph3SnOx
Ph3PbOx
8-HOQ
8.75
8.78
8.21
8.22
8.81
7.4
7.40–7.50
7.3
7.24
7.38
8.15
8.14
8.16
8.10
8.11
7.18
7.19
7.18
7.18
7.32
7.45
7.40–7.50
7.51
7.56
7.46
7.33
7.33
7.25
7.04
7.25
7.63
7.64
7.54
7.68
–
7.43
7.40–7.50
7.34
7.45
–
7.37
7.40–7.50
7.34
7.31
–
a
All in CDCl3.
10 ppm compared to the starting material, also indicative of hyper-
coordination [5,19,20]. An increase in the tin–carbon one-bond
coupling constant of the ipso carbon was observed relative to that
of the starting material (Table 3). The 1J (Sn, 13C) value (663.8 Hz) is
between the value for tetracoordinate triphenylchlorotin
(614.8 Hz) and that of its pentacoordinate complex in DMSO
(797.4 Hz). Triphenyltin compounds with axial–equatorial chela-
tion have been reported to have 1J (Sn, 13C) values in a range of
600–660 Hz [19]. The 1J (Sn, 13C) values of DMSO complexes with
triphenyltin derivatives lie in the range of 750–850 Hz [19].
The proton chemical shifts of triphenyltin oxinate (Table 4) are
in general agreement with previously reported values. Although
the shifts are approximately 0.2 ppm lower than those reported
by Lycka et al. [5], the same values (within 0.02 ppm) were ob-
tained for compounds produced in four different preparations.
This value was obtained by applying a correction of approximately
200 ppm for the substitution of three phenyl groups for three
methyl groups (trimethyllead thiomethoxide and triphenyllead
thiomethoxide have shifts of 239 and 42 ppm, respectively) to
the chemical shift of trimethyllead methoxide, which has a lead
shift of 331 ppm [22]. Although this shift is only approximate,
the lead shifts are at sufficiently low frequency to indicate 5-
coordination.
Three geometrical isomers can be postulated for triphenyllead
oxinate (and the other oxinates): the oxygen atom may be in an ax-
ial position and the coordinating nitrogen atom may be in the
equatorial position, the oxygen may be equatorial and the nitrogen
axial, or both atoms could be in equatorial positions, seen in Fig. 3.
The presence of two 207Pb peaks at room temperature can be
attributed to two geometric isomers that have a relatively slow
rate of exchange. The slower rate of exchange in the lead oxinate
relative to the tin oxinate may be a reflection of a stronger interac-
tion between nitrogen and the lead. Only one set of peaks is pres-
ent in the 13C NMR of triphenyllead oxinate because of the smaller
expected difference between the 13C shifts of the isomers in com-
parison to the difference in shifts (in Hz) for 207Pb and 15N.
The 15N spectrum contains two peaks, also presumably due to
the presence of two geometrical isomers. Both peaks are upfield
of methoxyquinoline. In particular the peak at À244.5 ppm is
shifted the farthest upfield of any of the four oxinates, indicating
the largest loss of electron density and strongest hypercoordina-
tion. A similar difference in the shifts of axial vs. equatorial atoms
is seen for the 19F nuclei in the fluorophosphoranes. For example,
in (CH3)2PF3 the axial fluorine has a shift of 74 ppm while the equa-
torial fluorine has a resonance at À9.8 ppm [23]. When expressed
in Hertz this difference is considerably larger than the difference
between the two 15N shifts. Thus, the large difference between
the two nitrogen resonances appears to be reasonable for struc-
tural changes from axial to equatorial nitrogen atoms.
4.2. Triphenyllead oxinate
The 207Pb NMR spectrum of triphenyllead oxinate contained
two peaks at À158.9 and À187.2 ppm. The chemical shifts of the
two peaks in the lead spectrum are relatively close and conse-
quently likely represent similarly coordinated lead atoms. These
values are approximately 250 ppm to lower frequency than our
estimated value of the shift of triphenyllead methoxide (80 ppm).
O
R
Pb
R
R
N
N
R
N
Pb
R
R
Pb
R
O
R
O
R
In the 13C NMR spectrum, the ipso carbon resonance of
triphenyllead oxinate (163.2 ppm) is shifted downfield almost
30 ppm from the starting material. Even more indicative of strong
Fig. 3. Three possible structures for triphenyllead oxinate. The geometric isomers
contain oxygen axial and nitrogen equatorial, oxygen equatorial and nitrogen axial,
or both oxygen and nitrogen equatorial.
Fig. 4. Optimized conformations of the Ph3MOx compounds using the DFT method through the B3PW91 functional.

522
C.H. Yoder et al. / Journal of Organometallic Chemistry 695 (2010) 518–523
Table 5
Parameters of optimized structures using the DFT method through the B3PW91 functional.
Compound
O–M–C1 angle
(°)
O–M–C2 angle
(°)
O–M–C3 angle
(°)
C1–M–C2 angle
(°)
C1–M–C3 angle
(°)
C2–M–C3 angle
(°)
N–M–C3 angle
(°)
N–M distance
(Å)
Ph3GeOx
Ph3SnOx
Ph3PbOx
116.5
153.0
125.0
111.7
107.2
110.6
97.9
87.4
88.6
112.7
86.4
115.3
107.8
110.7
104.3
108.9
109.0
107.7
159.3
152.8
155.0
2.95
2.36
2.61
coordination to lead, the 1J (207Pb, 13C) coupling constant of the
ipso carbon in triphenyllead oxinate (624.0 Hz) increased drasti-
cally from that of the starting material, triphenyllead chloride
(548.5 Hz).
triphenylsilicon oxinate can be assumed to be tetracoordinate.
Because thecarbon,proton, and15Nshiftsofthetriphenylgermanium
oxinate are similar to those of the silicon analog, it likely also has a
tetracoordinate structure. Using the same criteria, triphenyltin and
lead oxinates can be assumed to be five-coordinate. However, the
larger upfield shift of the central atom and nitrogen atom, the larger
downfield shift of the ipso carbon, and the larger increase in the cou-
pling constant of carbon A indicate that the triphenyllead oxinate
has a stronger lead–nitrogen interaction than the tin–nitrogeninter-
action in the triphenyltin oxinate. Molecular modeling also supports
distorted trigonal bipyramidal structures for the triphenyltin and -
lead oxinates.
Both triphenyltin- and triphenyllead oxinates exhibit two geo-
metrical isomers – presumably one in which a nitrogen atom is ax-
ial and the oxygen equatorial and one in which the nitrogen atom
is equatorial and the oxygen atom is axial. At room temperature
this was observable for the triphenyllead oxinate while the tin iso-
mers were observed only at low temperatures. Thus, the Lewis
acidity of the central atoms increases down the group fourteen ele-
ments. Although no clear distinction can be made between the sil-
icon and germanium analogs, these two compounds exhibit little
or no intramolecular Lewis acidity.
4.3. Triphenylsilicon oxinate and triphenylgermanium oxinate
The 29Si chemical shift of triphenylsilicon oxinate (À12.6 ppm)
is very similar to that of triphenylsilicon naphthoxide
(À12.4 ppm), which is a good indication that the silicon atom is
not hypercoordinate. The 73Ge resonance in triphenylgermanium
oxinate could not be observed presumably due to the rapid quad-
rupolar relaxation of the germanium atom [24].
The 15N NMR shifts of both the triphenylsilicon and germanium
oxinates (À99.3 and À98.4 ppm, respectively) are both approxi-
mately 20 ppm upfield from the reference, methoxyquinoline. Be-
cause methoxyquinoline does not contain analogous phenyl
groups and a central atom, the reason for this upfield shift is some-
what unclear. However, because the change in 15N shift of triphen-
yltin oxinate is twice that of the silicon and germanium analogs,
the shift appears to indicate a smaller extent of hypercoordination
for the silicon and germanium homologs.
The proton and 13C chemical shifts of the triphenylsilicon and
germanium oxinates are very similar. In these compounds the
chemical shifts of the ipso carbons are not shifted downfield from
those of the starting materials. Moreover, the coupling constants 1J
(Si, 13C) for the phenyl carbons of triphenylsilicon oxinate (79.7 Hz)
are very similar to those of the starting material triphenylchloros-
ilane (80.2 Hz). Both the silicon and germanium oxinates can be
presumed to be tetracoordinate by virtue of the similar carbon
chemical shifts, no downfield shift of the ipso carbon and similar
carbon–silicon coupling constants of triphenylsilicon oxinate com-
pared to those of the starting material [5,19,20].
Acknowledgements
The authors are indebted to Lucille and William Hackman for
providing the Hackman research endowment at Franklin & Mar-
shall College and to the National Science Foundation for an MRI
grant supporting the acquisition of a Varian INOVA 500-MHz
NMR spectrometer. The authors also acknowledge the support of
the E. Jane Valas Fund (to A.S. D. and C.J. G.).
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