Crystal structures and stereodynamics of neutral hexacoordinate silicon chelates: Use of an optically active ligand for assignment of an intramolecular ligand exchange process

Article abstract of DOI:10.1021/ja9730021

Crystal structures determined for several neutral hexacoordinate bis(N → Si) chelates revealed in all cases near octahedral geometries with the nitrogen ligands in relative trans and the monodentate ligands in cis positions. To assign the two consecutive

Full text of DOI:10.1021/ja9730021

J. Am. Chem. Soc. 1998, 120, 4209-4214
4209
Crystal Structures and Stereodynamics of Neutral Hexacoordinate
Silicon Chelates: Use of an Optically Active Ligand for Assignment
of an Intramolecular Ligand Exchange Process
Daniel Kost,*,† Inna Kalikhman,*,† Sonia Krivonos,† Dietmar Stalke,‡ and Thomas Kottke‡
Contribution from the Department of Chemistry, Ben Gurion UniVersity of the NegeV,
Beer-SheVa 84105, Israel, and Institut fu¨r Anorganische Chemie der UniVersita¨t Wu¨rzburg,
D-97074 Wu¨rzburg, Germany
ReceiVed August 26, 1997
Abstract: Crystal structures determined for several neutral hexacoordinate bis(NfSi) chelates revealed in all
cases near octahedral geometries with the nitrogen ligands in relative trans and the monodentate ligands in cis
positions. To assign the two consecutive intramolecular ligand-site exchange processes (reported earlier), a
bis-chelate was prepared containing a chiral carbon center in each of the chelate rings (14). By means of a
phase sensitive NOESY NMR spectrum it was possible to conclude that the lower-barrier process involved
exchange of diastereotopic groups only within each diastereomer, and not between them, resulting in the
assignment of this process to the direct nondissociative interchange of the monodentate ligands (X, Y). The
Si-Cl bond lengths were found to inversely correlate with the lower of the activation barriers. Dissociation-
recombination of the NfSi dative bond was also observed by the high-temperature NMR spectra. Lack of
correlation between Si-N distances in the crystals and activation barriers led to the conclusion that Si-N
dissociation was not involved in the measured rate processes, but followed, at slightly higher temperature, the
epimerization at the silicon center by exchange of the two oxygen sites.
The structure and chemistry of hexacoordinate silicon chelates
have been the focus of a number of recent studies.1-5 However,
only a few structural and stereodynamic studies have been
reported, and they have been described as being still in an
“embryonic stage”.3 Within the few known crystal structures
of neutral bis(NfSi) chelates, the structural diversity is rather
large: most of the reported crystal structures feature a “bicapped
tetrahedral” geometry (1-4, Figure 1), in which the basic
tetrahedral structure around silicon is retained, with two donor
nitrogens coordinated opposite two faces of the tetrahedron.6-9
By contrast, crystal structures of nearly octahedral bis(NfSi)
complexes have also been reported recently (5,10 6,11 7,11 8,11
912). The factors responsible for the solid-state structures of
these complexes are not yet understood. In the present paper
we report two new crystal structures of neutral dinitrogen-
chelated silicon complexes, 10, 11, and discuss their geometries.
Hexacoordinate silicon complexes undergo a variety of inter-
13 and intramolecular14 ligand-site exchange reactions, observ-
able by NMR spectroscopy. In a previous publication we
analyzed the stereodynamic behavior of a series of hexacoor-
dinate bis(NfSi) complexes 8-11 and, on the basis of their
multinuclear NMR spectra, concluded that they underwent two
consecutive intramolecular ligand-site exchange reactions on the
NMR time scale.15-17 These two proposed processes were
assigned to the interchange of adjacent ligands (“1,2-shift”), via
a bicapped tetrahedron transition state or intermediate (Figure
2). One process involves the interchange of the two monoden-
tate ligands (X,Y)-1,2-shift, while in the other process exchange
of the two oxygens takes place. On the basis of the reported
results it was not possible to individually assign each exchange
process to one or the other observed barrier. Utilizing a chiral
† Ben Gurion University.
‡ Universita¨t Wu¨rzburg.
(1) Tandura, St. N.; Alekseev, N. V.; Voronkov, M. G. Top. Curr. Chem.
1986, 131, 99.
(2) Corriu, R. J. P.; Young, J. C. In The Chemistry of Organic Silicon
Compounds; Patai, S., Rappoport, Z., Eds.; Wiely: Chichester, U.K., 1989;
p 1241.
(3) Holmes, R. R. Chem. ReV. 1996, 96, 927.
(4) Chuit, C.; Corriu, R. J. P.; Reye, C.; Young, J. C. Chem. ReV. 1993,
93, 1371.
(5) Kost, D.; Kalikhman, I. In The Chemistry of Organic Silicon
Compounds; Apeloig, Y., Rappoport, Z., Eds.; Wiley: Chichester, U.K.,
in press.
(6) Breliere, C.; Carre, F.; Corriu, R. J. P.; Poirier, M.; Royo, G.;
Zwecker, J. Organometallics 1989, 8, 1831.
(7) Carre, F.; Cerveau, G.; Chuit, C.; Corriu, R. J. P.; Reye, C. New J.
Chem. 1992, 16, 63.
(8) Carre, F.; Chuit, C.; Corriu, R. J. P.; Mehdi, A.; Reye, C. Organo-
metallics 1995, 14, 2754.
(9) Belzner, J.; Shar, D. In Organosilicon Chemistry II; Auner, N., Weis,
J., Eds.; VCH: Weinheim, 1996; pp 459-465.
(10) Klebe, G.; Tran Qui, D. Acta Crystallogr. 1984, C40, 476.
(11) Kalikhman, I.; Krivonos S.; Stalke, D.; Kottke, T.; Kost, D.
Organometallics 1997,16, 3255.
(12) Mozzhukhin, A. O.; Antipin, M. Yu.; Struchkov, Yu. T.; Gostevskii,
B. A.; Kalikhman, I. D.; Pestunovich, V. A.; Voronkov, M. G. Metalloorg.
Khim. 1992, 5, 658; Chem. Abstr. 1992, 117, 234095w.
(13) Farnham, W. B.; Whitney, J. F. J. Am. Chem. Soc. 1984, 106, 3992.
(14) (a) Breliere, C.; Corriu, R. J. P.; Royo, G.; Zwecker, J. Organo-
metallics 1989, 8, 1834. (b) Breliere, C.; Carre´, F.; Corriu, R. J. P.; Douglas,
W. E.; Poirier, M.; Royo, G.; Wong Chi Man, M. Organometallics 1992,
11, 1586. (c) Carre´, F.; Chuit, C.; Corriu, R. J. P.; Fanta, A.; Mehdi, A.;
Reye, C. Organometallics 1995, 14, 194.
(15) Kalikhman, I.; Kost, D. J. Chem. Soc., Chem. Commun. 1995, 1253.
(16) Kost, D.; Kalikhman, I.; Raban, M. J. Am. Chem. Soc. 1995, 117,
11512.
(17) Kost, D.; Krivonos, S.; Kalikhman, I. In Organosilicon Chemistry
III; Auner, N., Weis, J., Eds.; VCH-Wiley: Weinheim, 1997; pp 435-
445.
S0002-7863(97)03002-3 CCC: $15.00 © 1998 American Chemical Society
Published on Web 04/18/1998

4210 J. Am. Chem. Soc., Vol. 120, No. 17, 1998
Kost et al.
Figure 3. X-ray crystal structures for 10 (left) and 11 (right). Only
one of the two unique molecules in the asymmetric unit of each crystal
is shown. Hydrogen atoms have been omitted for clarity.
geometry reported previously for members of this series, and
hence the octahedron is a characteristic geometry for this series,
in contrast to 1-4 with the bicapped tetrahedral structure. The
question now arises as to which complexes form octahedral and
which tetrahedral molecular structures in the crystal. It appears
that in 1-4, the tetrahedral complexes, the number of elec-
tronegative ligands attached to silicon is smaller than in the
octahedral complexes (5-11). A maximum of one Si-C bond
is allowed in the octahedral complexes. When the number is
greater than one, bicapped tetrahedral geometries are obtained
in the solid state.
Tables 1 and 2 list various bond lengths and angles (res-
pectively) extracted from the X-ray crystallographic determina-
tions. Examination of the silicon-nitrogen distances confirms
the previous observations that octahedral complexes are char-
acterized by relatively short Si-N bond lengths (∼1.95-2.2 Å
in intramolecular neutral chelates3,4,12), while in the bicapped
tetrahedral structures the Si-N distances are greater than 2.5
Å.3,4,6-9
From Table 2 it is evident that the near-octahedral geometries
are characterized by near 90° bond angles between the bonds
connecting silicon to the various cis ligand pairs (O, N, X, Y).
This is in contrast to the near 109° measured in bicapped
tetrahedral complexes.6-9
Figure 1. Hexacoordinate silicon chelates, and the corresponding Si-N
bond lengths (Å). Literature references are shown where appropriate.
Another question of interest relates to the relative orientation
of the two nitrogen ligands: in all of the compounds of the
series 6-11, the nitrogens are trans relative to each other.11,12
However, in 5, in which the ligand environment around silicon
is identical to that in 6-11, the nitrogen ligands occupy cis
positions.10 Comparison with the analogous intermolecular
adducts, SiF4‚2NH3 (12),18 SiF4‚2pyridine (13),19 shows that the
latter adducts have the nitrogen ligands in trans positions, while
the four fluoro ligands lie in an equatorial plane. It may be
concluded that in the absence of steric constraints the nitrogen
ligands prefer the trans geometry, and apparently in the single
structure 5 in which they are cis to each other, the strained
chelates force a cis geometry. This conclusion is also supported
by the observation that the Si-N distances in 6-11 (Table 1)
are very close to those found in the strain-free 12 and 13, 1.89518
and 1.93519 Å, respectively.
(b) Stereodynamic Analysis. As mentioned earlier, two
intramolecular ligand-site exchange reactions (shown in Figure
2) were proposed in a previous study for compounds of the
type 6-11.15,16 These two exchange processes were each
observed by coalescence of signals due to the diastereotopic
N-methyl groups in the NMR spectra, and in most cases were
well resolved and separated from each other on the temperature
scale. In the lower of the two processes interconversion of the
two chelate rings is observed, while in the second one complete
Figure 2. Ligand exchange mechanism by 1,2-shift of adjacent ligands,
via a bicapped-tetrahedron intermediate or transition state: upper part,
monodentate ligand (X-Y) interchange; lower part, oxygen-oxygen
interchange.
carbon ligand, we have been able in the present study to
discriminate between these two exchange reactions, and to
definitely assign one of them.
Results and Discussion
(a) Crystallographic Structures. The preparation of com-
plexes of the series 6-11 was described previously.11,16 Figure
3 depicts the single-crystal X-ray structures for compounds 10
and 11. These structures have the same near-octahedral
(18) Plitzko, C.; Meyer, G. Z. Anorg. Allg. Chem. 1996, 622, 1646.
(19) Bain, V. A.; Killean, R. C. G.; Webster, M. Acta Crystallogr., Sect.
B, 1969, 25, 156.

Neutral Hexacoordinate Silicon Chelates
J. Am. Chem. Soc., Vol. 120, No. 17, 1998 4211
Table 1. Selected Crystallographic Bond Lengths for Hexacoordinate Silicon Complexes (Å)
compd
R
X
Y
Si-Na
Si-Nb
Si-Ob
Si-Oa
Si-Cl, Si-F
1.638(2)
1.615(2)
2.184(2)
2.197(4)
2.187(2)
2.189(2)
2.141(1); 2.142(1)
2.147(1); 2.138(1)
Si-C
ref
6
7
8
CF3
CF3
CF3
Ph
Ph
F
H
Me
Ph
F
F
Cl
Cl
Cl
2.029(3)
1.962(2)
2.002(3)
2.015(7)
2.042(4)
2.042(4)
2.011(2)
2.010(2)
2.021(3)
1.962(2)
2.002(3)
2.036(6)
2.046(4)
2.045(4)
2.013(2)
2.017(2)
1.817(2)
1.782(2)
1.772(3)
1.780(6)
1.784(3)
1.790(3)
1.775(1)
1.774(1)
1.826(3)
1.782(2)
1.772(3)
1.771(6)
1.808(3)
1.804(3)
1.777(1)
1.775(1)
1.911(3)
11
11
11
12
9
2.089(8)
1.908(4)
1.921(4)
10c
CF3
this work
11c
CF3
Cl
Cl
this work
a Chelate trans to Ph. b Chelate trans to Cl, F. c For 10 and 11 geometric parameters for both molecules in the asymmetric unit are listed.
Table 2. Selected Bond Angles (deg) for Complexes 6-11
compd
N-Si-N′
F(Cl)-Si-N
O-Si-N
O-Si-O
F(Cl)-Si-O
6
163.62(8)
91.23(12); 96.85(12)
85.29(10); 80.88(9)
82.42(10); 87.67(12)
89.24(8); 83.85(9)
83.42(11); 89.41(11)
81.5(3)
81.2(2); 84.6(2)
82.4(2); 90.6(2)
83.0(2); 85.7(2)
82.4(2); 89.3(2)
87.37(7); 83.22(7)
82.91(6); 90.60(7)
83.09(6); 90.18(7)
83.33(6); 91.38(7)
85.16(9)
87.40(8); 170.58(7)
7
8
170.49(14)
170.1(2)
170.7(3)
164.7(2)
91.12(9); 95.39(8)
95.20(10); 90.64(9)
91.8(2)
86.95(9)
87.7(2)
86.7(3)
89.89(9); 174.09(9)
82.41(13)
86.3(2)
9
10a
91.84(12); 93.09(12)
85.20(14)
86.31(11); 170.13(13)
166.2(2)
168.73(7)
171.59(7)
90.38(12); 95.52(13)
85.08(14)
88.28(7)
88.13(7)
86.58(11); 169.71(13)
11a
92.16(5); 94.09(5)
90.84(5); 98.28(5)
91.33(5); 94.67(5)
91.75(5); 93.67(5)
89.53(5); 174.95(5)
90.40(6); 173.64(5)
88.91(6); 173.64(15)
88.91(5); 174.26(5)
89.78(5); 173.99(5)
a For 10 and 11 geometric parameters for both molecules in the asymmetric unit are listed.
coalescence of N-methyl signals takes place. However, since
both of the processes shown in Figure 2 involve interconversion
of the chelate rings, assignment of each barrier to one or the
other reaction was not possible. To reach such an assignment,
complex 14, in which two chiral centers have been incorporated
in the chelate rings, was prepared from (R)-(-)-2-phenylpro-
pionic acid according to the sequence shown in eq 1.
15.7 ( 0.1 kcal mol-1; higher process, ∆ν ) 110 Hz, Tc2
355 K, ∆G2* ) 17.0 kcal mol-1).20
)
We now examine the two rate processes, (Ph,Cl) and (O,O)
exchange in 14, in terms of their NMR-spectral consequences:
the former effects interchange between the two chelate rings,
such that the ring that was in a trans position relative to Cl
becomes trans to the phenyl group, while that trans to phenyl
is now opposite Cl. This process is a topomerization, by which
diastereotopic groups exchange roles within the same diaste-
reomer. It does not affect the chirality at any one of the chiral
centers, and therefore does not exchange between diastereomers.
The (O,O) exchange, on the other hand, interchanges the two
chelate rings and in addition inverts the configuration at the
silicon center: R-R-R a R-S-R. Thus, this process involves
exchange between the diastereomers and consequent coalescence
of signals from one stereoisomer with those of the other.
It follows that if coalescence of signal pairs within each
stereoisomer is observed during the lower energy exchange
process, this process constitutes a (Ph,Cl) exchange. Con-
versely, if the lower of the two processes exchanges signals
belonging to one of the diastereomers with those of the other,
then (O,O) exchange is observed. The NMR consequences of
the two possible alternative pathways are shown schematically
in Figure 5.21
14 has a total of three chiral centers, one at silicon and two
at carbons, and may exist in two diastereomeric forms: R-R-R
and R-S-R, differing in the configuration at the silicon center.
Indeed the low-temperature 1H NMR spectrum (Figure 4) clearly
shows two unequally populated isomers: the four N-methyl
groups in each diastereomer give rise to four singlets (labeled
1 and 2, respectively), with relative signal intensities between
the two groups of ca. 4:3. The same intensity distribution is
found in the C-methyl signals (two doublets for each diastere-
omer) and the Me-CH signals (two quartets for each diaste-
reomer). The coalescence phenomena of the latter signals were
best resolved, and served for the evaluation of rate constants
and activation free energies for the two exchange processes (in
toluene-d8, lower process for diastereomer 14-1, ∆ν ) 165 Hz,
Tc1 ) 335 K, ∆G1* ) 15.8 ( 0.1 kcal mol-1; lower process
for diastereomer 14-2, ∆ν ) 11 Hz, Tc1 ) 300 K, ∆G1* )
These expectations are borne out in the phase-sensitive 2D-
NOESY experiment shown in Figure 6. The experiment was
run in toluene-d8 solution at 250 K, an intermediate temperature
(below the coalescence temperatures) at which the higher of
the two rate processes is too slow to be effective, while the
(20) We reported previously16 on the substantial solvent dependence of
the N-methyl exchange barriers in analogous complexes. This was also tested
for 14: the lower barrier measured in CD2Cl2, Tc1 ) 223 K, ∆ν1 ) 200
Hz, ∆G1* ) 10.1 kcal mol-1; the higher process measured in nitrobenzene-
d5 solution, Tc2 ) 350 K, ∆ν2 ) 77 Hz, ∆G2* ) 17.0 kcal mol-1
.
(21) For a similar case, in which two consecutive rate processes were
resolved by their different NMR consequences, see: Kost, D.; Egozy, H.
J. Org. Chem. 1989, 54, 4909. Kost, D.; Zeichner, A.; Sprecher, M. S. J.
Chem. Soc., Perkin Trans. 2 1980, 317.

4212 J. Am. Chem. Soc., Vol. 120, No. 17, 1998
Kost et al.
Figure 4. 1H NMR spectrum of 14 at the slow exchange limit temperature, 250 K, in toluene-d8 solution, featuring two diastereomers. Only the
high-field portion is shown (the quintet at 2.1 ppm is due to the solvent).
1
Figure 5. Two schematic routes for the H NMR spectral changes
associated with N-methyl exchange in 14 upon an increase of
temperature: route A, exchange within each diastereomer; route B,
exchange between diastereomers.
Figure 6. Phase-sensitive NOESY spectrum of 14 (N-methyl region)
in toluene-d8 at 250 K. Positive (exchange) cross-signals are drawn by
solid lines, negative (NOE) cross-signals by dotted lines. Exchange
(and NOE) cross-peaks connect only signals within the same diaste-
reomer: 1 with 1 and 2 with 2.
faster process occurs at a significant rate. Negative (NOE) cross
signals connect pairs of geminal N-methyl groups, which are
near each other in space. The positive cross signals in Figure
6 connect pairs of interchanging signals. Clearly only signals
belonging to the same diastereomer interchange in this rate
process: signals of the high-intensity isomer (labeled 1) have
positive cross-signals only with signals of the same isomer, and
those labeled 2 exchange only with others labeled 2. It is thus
concluded unequivocally that the rate process observed at lower
temperature, with the lower activation barrier, represents 1,2-
shift of the adjacent Ph,Cl ligands.
Support for the assignment of (Ph,Cl) exchange to the lower
of the two exchange processes comes from an analysis of the
X-ray crystallographic data. The activation barriers for this
process for the chloro complexes (8-10) in toluene-d8 solutions
were reported (listed in order of increasing barrier):16 9 (X )
Me), 13.7; 10 (X ) Ph), 15.9; 8 (X ) H), 17.5 kcal mol-1. The
order of increasing barriers for 8-10 corresponds roughly to
the decrease in Si-Cl bond length in the crystal structures (Table
1). A reasonable inverse linear relationship is obtained (∆G1*
) -274dSi-Cl +616, R ) 0.977). Such a correlation of bond
lengths with activation energies generally suggests that dis-
sociation of the bond is involved in the rate-determining part
of the reaction sequence. However, earlier evidence (persistence
of Si-F coupling constants in an analogous fluoro complex at
elevated temperatures)16 ruled out the possibility of intermo-
lecular ligand exchange. The Si-Cl bond-length dependence
of the barrier may also be consistent with a nondissociatiVe (X,-
Cl)-1,2-shift, assuming that the twist of the two ligands involves

Neutral Hexacoordinate Silicon Chelates
J. Am. Chem. Soc., Vol. 120, No. 17, 1998 4213
Table 3. Crystal Data of 10 and 11
elongation of the Si-ligand bonds, and hence that lower barriers
should be expected in the case of longer ground-state Si-Cl
bonds.
10
11
formula
fw
C14H17ClF6N4O2Si C8H12Cl2F6N4O2Si
If indeed, as the data suggest, Si-Cl bond elongation
accompanies the (X,Cl) exchange, then the distinction between
a nondissociative vs an intramolecular-dissociative process
becomes ill-defined: a Cl atom (or anion) may dissociate to a
relatively large distance from silicon, followed by recombina-
tion, or it may partially dissociate (i.e., without complete
cleavage of the bond at any point along the reaction coordinate),
during the (X,Cl)-shift. The result in both cases is the same,
and in fact, these two mechanisms represent the two ends of a
mechanistic continuum. This mechanistic interpretation is in
agreement with the observation of the Si-Cl bond length
correlation with the barrier, as well as with the intense solvent
dependence of the barriers.16
450.86
0.4 × 0.3 × 0.3
P21/c
409.21
crystal size (mm3)
space group
a (Å)
0.5 × 0.5 × 0.4
P1h
9.135(2)
26.969(3)
15.818(4)
90
9.606(2)
13.463(3)
13.725(3)
110.112(12)
104.037(13)
96.07(2)
1581.7(6)
4
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
101.884(8)
90
3813.4(11)
8
Z
temp (K)
173(2)
1.571
0.338
173(2)
1.718
0.561
824
F
calc (Mg m-3
)
µ (mm-1
F(000)
)
1840
2θ range (deg)
no. of reflections collected
6-45
5385
6-50
6334
The second process, with the higher activation energy, is
compatible with (O,O) exchange. However, at the fast exchange
limit temperature for 14, if only these two, (Ph,Cl) and (O,O),
exchanges take place, the geminal N-methyl groups should
remain diastereotopic and give rise to two singlets, as a result
of the adjacent carbon chirality. In fact, a broad singlet is
observed in toluene-d8 solution up to 380 K. The geminal
N-methyls can only become truly equivalent if Si-N dissocia-
tion, followed by fast rotation about the NN bond and
recombination, become fast on the NMR time scale. This result
leaves several open options: the observation that above
coalescence the signal is slow to narrow, relative to other signals,
suggests that either (a) a gradual change in the static chemical
shifts with eventual accidental equivalence occurs, above the
coalescence due to (O,O) exchange, or (b) a second rate process,
Si-N cleavage, follows (O,O) exchange. A third possibility
(c) might be a direct Si-N dative bond cleavage which brings
about complete equivalence of the N-methyl groups, even
without the (O,O) exchange mechanism.
no. of unique reflections (Rint) 4901 (0.035)
5577 (0.009)
10
no. of restraints
198
no. of refined parameters
goodness of fit (all data)
R1a [I > 2σ(I)]
514
1.048
0.050
0.116
0.0450; 1.8127
0.303/-0.288
423
1.033
0.035
0.072
0.0325; 1.1252
0.446/-0.381
wR2b (all data)
c
g1; g2
largest diff peak/hole (e Å-3
)
a R1 ) ∑||Fo| - |Fc||/∑|Fo|. b wR2 ) {∑[w(Fo2 - Fc2)2]/∑[w(Fo2)2]}1/2.
c w ) 1/[σ2(Fo ) + (g1P)2 + g2P]; P ) (Fo + 2Fc2)/3.
2
2
Experimental Section
X-ray Measurements of 10 and 11. The crystal data for both
structures are presented in Table 3. Crystals of 10 and 11 were mounted
on the tip of a glass fiber, coated by a drop of perfluorinated polyether
and shock-frozen in the cold nitrogen gas stream of the low-temperature
device of the diffractometer.22 Data were collected at -100 °C on an
Enraf-Nonius CAD4 four-circle diffractometer, equipped with a
homemade low-temperature device, using graphite-monochromated Mo
KR radiation (λ ) 0.710 73 Å). The structures were solved by direct
methods (SHELXS-9023) and refined to convergence by full-matrix,
least-squares iteration against F2 (SHELXL-9624) minimizing the
function w(Fo2 - Fc2) (Table 3). All non-hydrogen atoms were refined
anisotropically. The hydrogen atoms were refined on calculated
positions using a riding model. The isotropic displacement parameters
of the hydrogen atoms were fixed to equal 1.2 times (aromatic CH)
and 1.5 times (CH3 group) the value of Ueq of the preceding carbon
atom. A rotational disorder of one CF3 group in 10 was resolved and
refined with 90/10% occupation applying geometric and ADP restraints.
In addition, similarity restraints were applied to refine equivalent
geometric parameters of both molecules within the asymmetric unit
(10). Further details of the crystallographic data are contained in the
Supporting Information.
Possibility (a) may be ruled out on the basis of a comparison
with the methyl analogue of 14, 15 (R ) (()-1-phenylethyl, X
) Me, Y ) Cl): at elevated temperature the 1H NMR spectrum
of 15 featured, like that of 14, one singlet for the N-methyl
groups. It is highly unlikely that accidental equivalence would
be found in both compounds.
From Table 1 it is evident that the Si-N bond lengths do
not correlate with the barrier for the second (slower) ligand
exchange process (8, 18.0; 9, 14.8; 10, 16.3; 11, 20.7 kcal
mol-1),16 in the manner observed for the Si-Cl bonds and the
first barrier. This suggests that Si-N dissociation is not
involved in the rate-determining process measured by the
spectral changes, but rather follows the (O,O) exchange at
slightly higher temperature (option (b) above). Thus, the
measured barrier is for the (O,O) exchange, while at the fast
exchange limit temperature (after narrowing of the spectral lines)
both processes ((O,O) exchange and N-Si dissociation) have
already taken place, and hence a single resonance is observed
for the N-methyl groups.
NMR spectra were recorded on a Bruker DMX-500 spectrometer
operating at 500.13 MHz for protons, and are reported in δ (ppm)
relative to internal tetramethylsilane (TMS). The NOESY spectrum
was run in toluene-d8 solution at 250 K with a standard pulse sequence
for a phase-sensitive spectrum, using a NOE evolution time of 300
ms. Elemental analyses were performed by Mikroanalytisches Labo-
ratorium Beller, Go¨ttingen, Germany.
Bis(N-(dimethylamino)trifluoroacetimidato-N,O)fluoro(phenyl)-
silicon(IV) (6).11 A solution of 1.368 g (0.006 mol) of Me3SiOC-
In conclusion, the presence of the chiral carbon centers leads
to unequivocal NMR evidence that the first (faster) of the two
observed rate processes in 14 and analogous complexes is
exchange between adjacent monodentate ligands ((Ph,Cl)-1,2-
shift). This is supported by comparison of Si-Cl crystal-
lographic bond lengths and measured exchange barriers. At
higher temperature, epimerization of the silicon center takes
place via (O,O) exchange, followed by dissociation and
recombination of the N-Si dative bond.
16
(CF3)dNNMe2 (16) and 0.49 g (0.003 mol) of PhSiF3 in 2 mL of
dry toluene was allowed to react under a dry argon atmosphere. The
mixture was evacuated followed by refrigeration at -20 °C for a week.
Crystallization was effected by the addition of petroleum ether (40-
60 °C) until the solution turned turbid, followed by 24 h at -20 °C. 6
(22) Kottke, T.; Stalke, D. J. Appl. Crystallogr. 1993, 26, 615.
(23) Sheldrick, G. M. Acta Crystallogr. 1990, A46, 467.
(24) Sheldrick, G. M. Program for crystal structure refinement, Uni-
versity of Go¨ttingen, Germany, 1996.

4214 J. Am. Chem. Soc., Vol. 120, No. 17, 1998
Kost et al.
crystallized out in essentially quantitative yield, the solution was
decanted off under reduced pressure, and the crystals were dried under
low pressure, mp 89-90 °C. An analytical sample and a single crystal
°C. The mixture was allowed to warm to room temperature with stirring
for another 3 h. The precipitate was filtered off and the product was
crystallized out by addition of petroleum ether (40-60 °C). Mp: 100-
101 °C (mp of racemic compound 80-81 °C). Yield: 7.6 g (94%).
1
suitable for crystallography were taken directly from this product. H
3
NMR (CD2Cl2, 300 K): δ 2.23 (br s, 3H, NMe), 2.68 (br s, 6H, NMe),
3.10 (br s, 3H, NMe), 7.30-8.00 (m, 5H, Ph). 13C NMR (CD2Cl2,
300 K): δ 48.8 (br, NMe), 49.8 (br, NMe), 52.6 (br, NMe), 117.8 (q,
1H NMR (CDCl3): δ 1.43, 1.48 (2d, J ) 7 Hz, 3H, (E,Z) CCH3),
3
2.50 (s, 6H, NCH3), 3.47, 4.34 (2q, J ) 7 Hz, 1H, (E,Z) CH), 7.2-
7.4 (m, 5H, Ph). Anal. Calcd for C11H16N2O: C, 68.72; H, 8.39; N,
14.57. Found: C, 68.13; H, 8.27; N, 14.62.
1
2
CF3, J(C-F) ) 275 Hz), 128-145 (Ph), 157.46 (q, CdN, J(C-F)
) 38 Hz). 29Si NMR (CD2Cl2, 300 K): δ -146.2 (d, 1J(Si-F) ) 280
Hz). Anal. Calcd for C14H17F7N4O2Si: C, 38.71; H, 3.92; N, 12.90.
Found: C, 38.69; H, 4.02; N, 12.84.
(b) O-Trimethylsilylated 1,1-Dimethyl-2-((R)-2-phenylpropionyl)-
hydrazine. The hydrazide from (a) (0.0396 mol, 7.6 g) was dissolved
in 100 mL of toluene with 0.079 mol (11 mL) of triethylamine under
dry argon. To this solution was added dropwise and with stirring a
trimethylchlorosilane (0.0475 mol, 6 mL) solution in 10 mL of toluene
at ambient temperature. The reaction was refluxed for 20 h, after which
it was cooled to room temperature and filtered under argon. The filtrate
was distilled to yield 8.5 g (0.032 mol, 81% yield), collected between
66 and 68 °C (0.4 Torr). 1H NMR (CDCl3): δ 0.03, 0.22 (2s, 9H,
Bis(N-(dimethylamino)trifluoroacetimidato-N,O)difluorosilicon-
(IV) (7).11 SiF4, generated by heating of BaSiF6,25 was passed through
a solution of 2 mL of dry toluene and 1.14 g (0.005 mol) of 16 under
an argon atmosphere, until a white precipitate had separated. The
mixture was evacuated followed by refrigeration at -20 °C. Crystal-
lization was complete after a few days. The solution was decanted,
and the crystals were dried in vacuo. An analytical sample and a single
crystal were taken without further treatment. 1H NMR (CDCl3, 300
3
(E,Z) SiCH3), 1.39, 1.45 (2d, J ) 7.2 Hz, 3H, (E,Z) CCH3), 2.31,
2.33 (2s, 6H, (E,Z) NCH3), 3.63, 4.77 (2q, 3J ) 7.2 Hz, 1H, CH), 7.21-
7.34 (m, 5H, Ph). 13C NMR: δ 0.2, 0.5 (2s, (E,Z) SiCH3), 17.4, 18.2
(2s, (E,Z) CCH3), 37.6, 44.5 (2s, (E,Z) CH), 46.6, 48.5 (2s, (E,Z) NCH3),
126-128 (m, Ph), 141.5, 142.4 (2s, (E,Z) ipso-Ph), 162.2, 169.1 (2s,
(E,Z) CdN). 29Si NMR (CDCl3): δ 17.7, 20.4 ((E,Z) isomers).
(c) 14. The complex was prepared directly in an NMR tube, from
0.25 g of product (b) and 0.1 g of PhSiCl3, in the appropriate deuterated
solvent. Attempts to prepare 14 (and its racemic mixture) on a larger
scale did not result in crystallization or isolation of pure complex.
However, 14 is identified by analogy of its NMR spectrum with those
of other complexes in the series. 1H NMR (CDCl3): δ 1.44, 1.51 (2d,
3J ) 7 Hz, 6H, (RRR) and (RSR) CCH3), 2.34, 2.36, 2.75, 2.77 (4s,
12H, (RRR) and (RSR) NCH3), 3.47, 3.70 (2 br s, 2H, (RRR) and (RSR)
CH), 7.0-7.8 (m, 15H, Ph). 13C NMR (CDCl3): δ 17.2 (s, CCH3),
41.85, 41.97 (2s, (RRR) and (RSR) CH), 51.14, 51.29, 51.36 (3s, NCH3),
127-143 (m, Ph), 169.79, 169.93 (2s, CdN). 29Si NMR (CDCl3): δ
130.5, 131.6 ((RRR) and (RSR) isomers).
4
4
K): δ 2.88 (t, J(F-H) ) 1.57 Hz, 6H, NCH3), 2.95 (t, J(F-H) )
0.71 Hz, 6H, NCH3). 13C NMR (CDCl3, 300 K): δ 50.04 (t, 3J(F-C)
) 1.3 Hz, NCH3), 50.25 (t, 3J(F-C) ) 5.4 Hz, NCH3), 116.8 (q, 1J(F-
2
4
C) ) 276.7 Hz, CF3), 157.7 (qt, J(F-C) ) 38.6 Hz, J(F-C) ) 1.1
Hz, CdN). 29Si NMR (CD2Cl2, 300 K): δ -159.6 (t, J(Si-F) )
1
205.7 Hz). Anal. Calcd for C8H12F8N4O2Si: C, 25.54; H, 3.21; N,
14.89. Found: C, 25.52; H, 3.11; N, 14.81.
Bis(N-(dimethylamino)trifluoroacetimidato-N,O)chloro(phenyl)-
silicon(IV) (10). The synthesis was described previously.16 10 was
crystallized for X-ray crystallography from a toluene solution at -18
°C (mp 129-131 °C). Anal. Calcd for C14H17ClF6N4O2Si: C, 37.30;
H, 3.80; N, 12.43. Found: C, 36.56; H, 3.91; N, 12.55.
Bis(N-(dimethylamino)trifluoroacetimidato-N,O)dichlorosilicon-
(IV) (11). The synthesis was described previously.16 11 was crystal-
lized for X-ray crystallography from a toluene solution at -18 °C. Anal.
Calcd for C8H12Cl2F6N4O2Si: C, 23.48; H, 2.96; N, 13.69. Found: C,
23.68; H, 3.02; N, 13.70.
Bis(N-(dimethylamino)-(R)-2-phenylpropionimidato-N,O)chloro-
(phenyl)silicon(IV) (14). (a) (R)-2-Phenyl-2′,2′-dimethylpropiono-
hydrazide (1,1-Dimethyl-2-((R)-2-phenylpropionyl)hydrazine). The
hydrazide was prepared from 0.043 mol (6.42 g) of (R)-(-)-2-
phenylpropionic acid (Aldrich) by refluxing with 0.086 mol (6.2 mL)
of thionyl chloride for 1.5 h, followed by removal of excess SOCl2
under reduced pressure and distillation of the acid chloride, collecting
7.0 g (97%) at 106-107 °C (23 Torr). The chloride was dissolved in
10 mL of CCl4 and added dropwise with stirring to an ice cold solution
of 0.063 mol (3.8 g) of 1,1-dimethylhydrazine and 0.084 mol (8.5 g)
of triethylamine in 80 mL of CCl4, keeping the temperature below 4
Acknowledgment. We thank the Israeli Ministry of Sciences
and Arts and the Israel Science Foundation (administered by
the Israel Academy of Sciences and Humanities) for financial
support.
Supporting Information Available: Tables of crystal data,
fractional coordinates and U values, bond distances and angles,
anisotropic displacement parameters, and hydrogen atom co-
ordinates and completely labeled figures of structures 10 and
11 (21 pages, print/PDF). See any current masthead page for
ordering information and Web access instructions.
(25) MacDiarmid, A. G. In PreparatiVe Inorganic Reactoins; Jolly, W.
L., Ed.; Interscience: New York, 1964; Vol. 1, p 185.
JA9730021