Syntheses, Structural Characterizations, and Heterocumulene Metathesis Studies of New Monomeric Bis(triorganosilylamido)tin(II) Derivatives

Article abstract of DOI:10.1021/om990526w

The syntheses and solid-state structures of two new monomeric bis(triorganosilyl)amido stannylenes, bis[N-trimethylsilyl-N-2,6-diisopropylphenylamido]tin(II) (1) and bis[N,N-bis(dimethylphenylsilyl)amido]tin(II) (2), are reported. Heterocumulene metathesis studies reveal that while 1 is inert toward carbon dioxide and isocyanates, 2 undergoes controlled metathesis with these reagents, including a range of para-substituted arylisocyanates, X-C₆H₄NCO (X = OCH₃, CH₃, F, Cl, and CF₃), that allow for a Hammett investigation of the electronic factors that influence this process. This latter study shows that the rate of tin(II)-mediated heterocumulene metathesis is enhanced by an increased nucleophilicity of the heterocumulene.

Full text of DOI:10.1021/om990526w

Organometallics 1999, 18, 4437-4441
4437
Syn th eses, Str u ctu r a l Ch a r a cter iza tion s, a n d
Heter ocu m u len e Meta th esis Stu d ies of New Mon om er ic
Bis(tr ior ga n osilyla m id o)tin (II) Der iva tives
J ason R. Babcock,‡ Louise Liable-Sands,† Arnold L. Rheingold,† and
Lawrence R. Sita*,‡,¥
Department of Chemistry, University of Chicago, Chicago, Ilinois 60637, and
Department of Chemistry, University of Delaware, Newark, Delaware 19716
Received J uly 8, 1999
The syntheses and solid-state structures of two new monomeric bis(triorganosilyl)amido
stannylenes, bis[N-trimethylsilyl-N-2,6-diisopropylphenylamido]tin(II) (1) and bis[N,N-bis-
(dimethylphenylsilyl)amido]tin(II) (2), are reported. Heterocumulene metathesis studies
reveal that while 1 is inert toward carbon dioxide and isocyanates, 2 undergoes controlled
metathesis with these reagents, including a range of para-substituted arylisocyanates,
X-C6H4NCO (X ) OCH3, CH3, F, Cl, and CF3), that allow for a Hammett investigation of
the electronic factors that influence this process. This latter study shows that the rate of
tin(II)-mediated heterocumulene metathesis is enhanced by an increased nucleophilicity of
the heterocumulene.
Ch a r t 1
In tr od u ction
Recently, we have been developing heterocumulene
metathesis, a process involving metathetical exchange
between a metal triorganosilylamide, M[NR(SiR′3)]2 (M
) Ge, Sn, Pb, Zn, and Cd, R ) alkyl, aryl, or SiR′3) and
a heterocumulene, XdCdE (X ) O or NR′′, E ) O, S,
and Se), as a tool for the high-yield synthesis of metal
triorganosilylchalcogenolates, M(ESiR′3)2, and of sym-
metric and unsymmetric carbodiimides, RNdCdNR′′,
under mild conditions.1 During the course of these
studies, it was necessary to extend the current database
of structurally characterized bis(triorganosilylamido)-
tin(II) derivatives, Sn[NR(SiR′3)]2 (R ) alkyl, aryl, or
SiR′3),2 to elucidate some of the structure/property
relationships that govern the tin(II)-mediated heterocu-
mulene metathesis process. Herein, we report the
syntheses and solid-state structures of two new mono-
meric stannylenes, bis[N-trimethylsilyl-N-2,6-diisopro-
pylphenylamido]tin(II) (1) and bis[N,N-bis(dimethylphe-
nylsilyl)amido]tin(II) (2) (Chart 1). Heterocumulene
metathesis studies reveal that while 1 is inert toward
carbon dioxide and isocyanates, 2 undergoes controlled
metathesis with these reagents, including a range of
para-substituted arylisocyanates, X-C6H4NCO (X )
OCH3, CH3, F, Cl, and CF3), that allow for a Hammett
investigation of the electronic factors that influence this
process. This latter study shows that the rate of tin(II)-
mediated heterocumulene metathesis is enhanced by an
increased nucleophilicity of the heterocumulene.
Resu lts a n d Discu ssion
Compound 1 was obtained as an orange crystalline
solid in a 93% yield by reaction of lithium N-trimeth-
ylsilyl-N-2,6-diisopropylamide with tin(II) dichloride in
diethyl ether.3 Analytically pure material and single
crystals were produced through recrystallization from
toluene at -35 °C. 1H NMR (400 MHz) spectra of 1
taken at 298 and 343 K indicated that substantial steric
barriers to bond rotations exist in this compound due
to two sharp resonances being observed in a 1:1 ratio
‡ University of Chicago.
† University of Delaware.
¥ Present address: University of Maryland, College Park, MD 20742.
(1) (a) Sita, L. R.; Babcock, J . R.; Xi, R. J . Am. Chem. Soc. 1996,
118, 10912-10913. (b) Sita, L. R.; Xi, R.; Yap, G. P. A.; Liable-Sands,
L. M.; Rheingold, A. L. J . Am. Chem. Soc. 1997, 119, 756-760. (c)
Babcock, J . R.; Zehner, R. W.; Sita, L. R. Chem. Mater. 1998, 10, 147-
149. (d) Babcock, J . R.; Sita, L. R. J . Am. Chem. Soc. 1998, 120, 5585-
5586. (e) Xi, R.; Sita, L. R. Inorg. Chim. Acta 1998, 270, 118-122. (f)
Weinert, C. S.; Guzei, I. A.; Rheingold, A. L.; Sita, L. R. Organometallics
1998, 17, 498-500. (f) Babcock, J . R.; Incarvito, C.; Rheingold, A. L.;
Sita, L. R. Organometallics, in press.
(2) (a) Veith, M. Z. Naturforsch. 1978, 33B, 7-12. (b) Fjeldberg, T.;
Hope, H.; Lappert, M. F.; Power, P. P.; Thorne, A. J . J . Chem. Soc.,
Chem. Commun. 1983, 639-641 (c) Braunschweig, H.; Gehrhus, B.;
Hitchcock, P. B.; Lappert, M. F. Z. Anorg. Allg. Chem. 1995, 621, 1922-
1928. (d) Westerhausen, M.; Greul, J .; Hausen, H.-D.; Schwarz, W. Z.
Anorg. Allg. Chem. 1996, 622, 1295-1305. (e) Braunschweig, H.; Drost,
C.; Hitchcock, P. B.; Lappert, M. F.; Pierssens, L. J .-M. Angew. Chem.,
Int. Ed. Engl. 1997, 36, 261-263.
(3) For prior use of the N-trimethylsilyl-N-2,6-diisopropylphenyl-
amido ligand, see: (a) Murugavel, R.; Chandrasekhar, V.; Voigt, A.;
Roesky, H. W.; Schmidt, H.-G.; Noltemeyer, M. Organometallics 1995,
14, 5298-5301. (b) Voigt, A.; Murugavel, R.; Roesky, H. W. Organo-
metallics 1996, 15, 5097-5101. (c) Waezsada, S. D.; Liu, F.-Q.; Murphy,
E. F.; Roesky, H. W.; Teichert, M.; Uson, I.; Schmidt, H.-G.; Albers,
T.; Parisini, E.; Noltemeyer, M. Organometallics 1997, 16, 1260-1264.
10.1021/om990526w CCC: $18.00 © 1999 American Chemical Society
Publication on Web 09/23/1999

4438 Organometallics, Vol. 18, No. 21, 1999
Babcock et al.
Ta ble 2. Selected Str u ctu r a l P a r a m eter s for
Com p ou n d s 1 a n d 2
Ch a r t 2
compound 1 compound 2
Bond Lengths (Å)
Sn-N(1)
2.095(3)
Sn-N(1)
2.133(2)
2.124(2)
1.729(2)
1.724(2)
1.729(2)
1.726(2)
Sn-N(2)
2.093(3)
1.457(5)
1.449(5)
1.737(3)
1.737(3)
Sn-N(2)
N(1)-C(6)
N(2)-C(21)
N(1)-Si(2)
N(2)-Si(1)
N(1)-Si(1)
N(1)-Si(2)
N(2)-Si(3)
N(3)-Si(4)
Ta ble 1. Cr ysta l Str u ctu r e a n d Refin em en t Da ta
for Com p ou n d s 1 a n d 2
Bond Angles (deg)
111.04(11) N(1)-Sn-N(2)
N(1)-Sn-N(2)
C(6)-N(1)-Sn
C(6)-N(1)-Si(2)
Si(2)-N(1)-Sn
C(21)-N(2)-Sn
101.51(7)
111.90(9)
122.51(9)
1
2
103.5(2)
116.5(2)
139.0(2)
106.3(2)
Si(1)-N(1)-Sn
Si(2)-N(1)-Sn
formula
Mw
temp (K)
wavelength (Å)
space group
a (Å)
C
30H52N2Si2Sn
C32H44N2Si4Sn
687.74
244(2)
615.61
241(2)
0.71073
Si(1)-N(1)-Si(2) 124.70(10)
Si(3)-N(2)-Sn
Si(4)-N(2)-Sn
Si(3)-N(1)-Si(4) 121.58(10)
111.91(9)
124.84(9)
0.71073
C(21)-N(2)-Si(1) 119.1(2)
Si(1)-N(2)-Sn 134.2(2)
monoclinic, P2(1)/n triclinic, P1h
14.960(1)
9.795(2)
24.208(2)
90
104.29(1)
90
3437.7(5)
4
1.190
9.530(2)
9.609(2)
19.315(3)
100.505(13)
93.77(2)
92.85(2)
1731.8(5)
2
1.319
0.899
712
0.50 × 0.30 × 0.20
2.15-30.00
11799
b (Å)
c (Å)
chiral conformation in solution, which gives rise to
magnetically inequivalent isopropyl groups within the
same aryl substituent. The two tin-nitrogen bond
lengths, while somewhat elongated, are not particularly
informative with respect to the nature of these steric
interactions, as they both fall within the range observed
for other sterically stabilized monomeric bis(triorga-
nosilylamido)tin(II) derivatives.2 That these strong non-
bonded interactions are present in 1, however, is most
apparent in the unusually large N(1)-Sn-N(2) angle
of 111.04(11)°, which is similar in magnitude to the
largest value for this parameter yet found, which is
111.3(1)°.2d Further evidence for a strong steric repul-
sion between the two trimethylsilyl groups is provided
by the large Si-N-Sn bond angles of 139.0(2)° and
134.2(2)°. Curiously, though, despite this steric conges-
tion, the nitrogen atoms remain trigonal planar [cf., sum
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
Dcalcd (mg/m3)
abs coeff (mm-1
F(000)
)
0.832
1296
crystal size, mm3
0.30 × 0.30 × 0.30
θ range (deg)
no. of reflns collected 5806
2.25-22.50.
no. of ind reflns
4486
10110
(Rint ) 0.0341)
(Rint ) 0.0286)
goodness of fit on F2
final R indices
[I > 2σ(I)]
1.219
1.056
R1 ) 0.0361,
wR2 ) 0.0872
R1 ) 0.0344,
wR2 ) 0.0789
for magnetically nonequivalent methyls of the isopropyl
fragments of the aryl groups at both temperatures.
Evidence that 1 is monomeric in solution was obtained
by a 119Sn NMR (186 MHz, 298 K) spectrum that
displayed a single resonance at 440 ppm. Interestingly,
while this chemical shift is sandwiched between those
of the known monomeric Sn[N(SiMe3)Ar]2 (Ar ) aryl)
related structures, the cyclic bis(stannylene) 3 (δ 505
ppm)2e and the more ring-constrained cyclic monostan-
nylene 4 (δ 269 ppm) (Chart 2),2c it is found a few
hundred ppm upfield from the more closely related
derivative where Ar ) 2,6-dimethylphenyl (δ 655 ppm).4
Support for the monomeric nature of 1 in the solid state
was provided by a crystallographic analysis.5 Table 1
provides crystal structure and refinement data for this
compound, while Table 2 lists selected bond lengths and
bond angles. As can be seen in Figure 1, the tin atom
in 1 resides at the bottom of a pocket formed by the two
2,6-diisopropylphenyl groups, which are both being
projected in the same direction. Importantly, in the solid
state, 1 adopts a chiral conformation that is dictated
by a twist in the Si(1)-N(2)-Sn-N(1)-Si(2) backbone
[cf. the torsional angles: Si(1)-N(2)-Sn-N(1), 31.8°,
and Si(2)-N(1)-Sn-N(2), 32.3°]. Further, in this con-
formation, the two aryl groups are virturally identical
due to the pseudo-C2 axis that is present. Accordingly,
it is likely that strong steric repulsions between the two
trimethylsilyl groups, coupled with a barrier to rotation
about the nitrogen-carbon bonds, also enforce this
of the bond angles about each nitrogen atom, ∑θN(1)
359.6° and ∑θN(2) ) 359.0°, respectively].
)
Compound 2 was prepared in a manner similar to 1
in a 75% yield starting with commercially available 1,3-
diphenyl-1,1,3,3-tetramethyldisilazane. Once more, a
119Sn NMR spectrum revealed that 2 was monomeric
in solution by exhibiting a single resonance at 501 ppm.
1
However, in contrast to 1, a H NMR spectrum did not
reveal any diagnostic pattern of resonances that might
reveal the nature of the conformation that 2 adopts in
solution. Crystallographic analysis, on the other hand,
showed that, as with 1, the tin atom of 2 resides within
a pocket formed by the cooperative arrangement of the
phenyl groups and that the molecule assumes a chiral
conformation in the solid state, as can be seen in Figure
2 (see Tables 1 and 2 for crystal structure and refine-
ment data and selected structural parameters, respec-
tively).5 Given the expected strong steric interactions
between the substituents, it is likely that this conforma-
tion is retained in solution as well. In contrast to the
structure of 1, however, where nonbonded interactions
were revealed in abnormal bond angles, compound 2
appears to favor bond length distortions as a way to
accommodate steric strain. More specifically, inspection
of Table 2 shows that the tin-nitrogen bond lengths
found for 2, Sn-N(1), 2.133(2) Å, and Sn-N(2), 2.124-
(2) Å, are substantially longer than those found in other
sterically stabilized bis(triorganosilylamido)tin(II) de-
rivatives.2 The bond angles in this compound, however,
all fall within normal ranges. For instance, the N(1)-
(4) Babcock, J . R. Ph.D. Dissertation, University of Chicago, 1998.
(5) Detailed information is provided in the Supporting Information.

Bis(triorganosilylamido)tin(II) Derivatives
Organometallics, Vol. 18, No. 21, 1999 4439
F igu r e 2. ORTEP (30% thermal ellipsoids) representation
of the molecular structure of 2. Hydrogen atoms have been
omitted for the sake of clarity.
with CO2 (30 psi) to produce a mixture of dimethylphen-
ylsilylisocyanate, PhMe2SiNCO (5), and 1,3-bis(dimeth-
ylphenylsilyl)carbodiimide, PhMe2SiNdCdNSiMe2Ph
(6), in a 4.4:1 ratio, according to Scheme 1. The identity
of 5 was established through its synthesis by a known
route,6 while 6, which has also been previously reported,
was isolated in pure form (86% yield) through a pre-
parative-scale reaction of 5 with 0.51 equiv of 2, as
shown in Scheme 1. Finally, the tin(II) coproduct of
these heterocumulene metathesis reactions was deter-
mined to be the expected dimer, bis[bis(dimethylphen-
ylsilanolato)tin(II)] (7), on the basis of the similarity
between the observed 119Sn NMR (186 MHz, benzene-
2
d6) data of this compound [δ -232, J (119Sn-117Sn) )
128 Hz] and the known dimer, bis[bis(trimethylsilano-
lato)tin(II), [cf., δ -220].1a Attempts to obtain analyti-
cally pure 7 failed, however, due to its poor crystallinity
and thermal instability.
F igu r e 1. (a) ORTEP (30% thermal ellipsoids) representa-
tion of the molecular structure of 1. Hydrogen atoms have
been omitted for the sake of clarity. (b) Space-filling (van
der Waals surface) representation of the molecular struc-
ture of 1.
The retarding effects of steric interactions on the
reactivity of 2 with isocyanates were found to be pro-
nounced, and to our advantage, in exploring the elec-
tronic factors that govern this process. Thus, while bis-
[N,N-bis(trimethylsilyl)amido)]tin(II), Sn[N(SiMe3)3]2,2b
was previously found to rapidly react in solution with
tBuNCO even at -78 °C,1c no reaction between 2 and
this heterocumulene occurred at this temperature. Upon
warming a well-mixed solution of these reagents to -10
°C, however, clean metathesis occurred to produce the
corresponding carbodiimide at such a rate that the
progress of the reaction could be conveniently monitored
Sn-N(2) bond angle of 2 is only 101.51°, and the
nitrogen atoms are both trigonal planar with no unusual
bond angle contribution being made to the sum of the
angles [cf., sum of the bond angles about each nitrogen
atom, ∑θN(1) ) 359.1° and ∑θN(2) ) 358.3°, respectively.
The preference for 2 to undergo bond length elongation
rather than bond angle distortion as the means by which
to accommodate steric strain may be the result of
weaker tin-nitrogen bonding in this compound relative
to 1. If so, it is possible that this difference also
manifests itself in differences in chemical reactivity
between the two compounds.
1
by H NMR spectroscopy at this temperature. Qualita-
tively, comparing the rate of this reaction with that
between 2 and trimethylsilylisocyanate, TMSNCO,
revealed the latter reaction to be significantly slower.
t
A similar decrease in reactivity on going from BuNCO
Regarding heterocumulene metathesis, in solution,
compound 1 was found to be inert toward both carbon
dioxide and tert-butylisocyanate, tBuNCO, even at
elevated temperatures up to 80 °C. A solution of 2 in
pentane, on the other hand, reacted readily at 50 °C
to TMSNCO had been noted previously in our heterocu-
mulene studies of Sn[N(SiMe3)3]2,2b and it had been
assumed, at that time, that this difference was related
to the enhanced nucleophilicity of the former reagent
relative to the latter. This argument now appears to be

4440 Organometallics, Vol. 18, No. 21, 1999
Babcock et al.
Sch em e 1
Ch a r t 3
Exp er im en ta l Section
All manipulations were performed under an inert atmo-
sphere of nitrogen using standard Schlenk and/or glovebox
techniques. All solvents were dried and distilled under nitro-
gen prior to use. Silica gel (400 mesh) was prepared for use in
the glovebox by heating at 200 °C under vacuum (10-4 Torr)
1
for 3 days. H NMR and 119Sn NMR spectra were recorded at
400 and 186 MHz, respectively, using benzene-d6 as the solvent
and tetramethyltin (0 ppm) for the external reference in the
latter. Chemical analyses were performed by Oneida Research
Services, Inc.
F igu r e 3. Hammett plot for the reaction of 2 with the
para-substituted arylisocyanates, X-C6H5NCO (X ) OCH3,
CH3, H, F, Cl, and CF3). The dashed line is the linear
regression curve fit of the data [slope (F) ) -0.64].
Syn t h esis of Bis(N-t r im et h ylsilyl-N-2,6-d iisop r op yl-
p h en yla m id o)tin (II) (1). To a solution of 1 g (4.0 mmol) of
N-trimethylsilyl-2,6-diisopropylaniline in 10 mL of diethyl
ether (Et2O) was added 1.25 mL of n-butyllithium (3.21 M in
hexanes), and the resulting mixture was stirred for 2 h. This
solution was then added dropwise by pipet to a stirred
suspension of 380 mg (2 mmol) of tin(II) dichloride in 10 mL
of Et2O. After stirring for 12 h, the reaction mixture was
filtered through a small pad of Celite on a glass frit. The
volatiles were then removed from the orange-red filtrate in
vacuo to provide the desired compound 1 as a deep orange
crystalline material that was recrystallized from pentane at
-35 °C to provide 1.14 g of pure product (93% yield). For 1:
1H NMR δ (ppm) 0.33 (s, 18H), 1.11 (d, 12H, J ) 6.9 Hz), 1.27
(d, 12H, J ) 6.9 Hz), 3.69 (sept. 4H, J ) 6.9 Hz), 7.02 (m, 2H),
7.10 (m, 4H); 119Sn NMR (298 K) δ (ppm) 440. Anal. Calcd for
justified on the basis of the results of a Hammett study7
of 2 with a range of para-substituted arylisocyanates,
X-C6H4NCO (X ) OCH3, CH3, F, Cl, CF3), which
possess both electron-donating and electron-withdraw-
ing groups. As can be seen in Figure 3, a plot of log(kX/
k0) vs σ, where kX and k0 are the rates for the substituted
and unsubstituted (i.e., X ) H) isocyanates, respectively,
and σ is the Hammett constant for group X, provides a
linear correlation with a negative slope of F ) -0.64.
This negative value for F is indicative of a reaction that
is facilitated by electron-donating substituents, which
we infer is related to the nucleophilicity of the isocyan-
ate. On the basis of this result, it is likely that the rate-
determining step in tin(II)-mediated heterocumulene
metathesis is initial coordination of the isocyanate (or
CO2) to the electron-deficient divalent tin center. How-
ever, this simple assumption is complicated by the
results of our previous mechanistic study, which re-
vealed that the rate-determining step might involve the
breakup of intermediate monometathesis dimers, such
as 8 (Chart 3), that possess tricoordinate tin(II) centers.1e
In an attempt to differentiate between these two
processes, we are now pursuing a kinetic study of the
monomeric monoamide stannylene 9 that has been
reported by Lappert and co-workers.8 The results of this
investigation will be reported in due course.
C
30H52N2Si2Sn: C, 58.53; H, 8.51; N, 4.55. Found: C, 57.98;
H, 8.56; N, 4.35.
Syn th esis of Bis[N,N-bis(d im eth ylp h en ylsilyl)a m id o)-
tin (II) (2). To a solution of 4.04 g (14.1 mmol) of N,N′-bis-
(dimethylphenylsilyl)amine in 100 mL of Et2O, cooled to 0°,
was added 5.66 mL of n-butyllithium (2.5 M in hexanes), and
the reaction mixture was warmed to room temperature and
stirred for 2 h. The resulting solution was then added to a
stirred suspension of 1.48 g (7.8 mmol) of tin(II) dichloride in
(6) Schott, G.; Kelling, H.; Ahrens, R. Z. Chem. 1974, 14, 487-488.
(7) March, J . Advanced Organic Chemistry, 4th ed.; J ohn Wiley &
Sons: New York, 1992.
(8) Braunschweig, H.; Chorley, R. W.; Hitchcock, P. B.; Lappert, M.
F. J . Chem. Soc., Chem. Commun. 1992, 1311-1313.

Bis(triorganosilylamido)tin(II) Derivatives
Organometallics, Vol. 18, No. 21, 1999 4441
75 mL of Et2O, cooled to 0 °C. After warming to room
temperature and stirring for 2 h, the volatiles were removed
in vacuo and the crude product taken up in pentane and
filtered through a 1 × 5 pad of Celite on a glass frit. After
concentrating the filtrate in vacuo, 3.65 g (75% yield) of pure
2 were obtained as orange cubes through crystallization at -35
°C. For 2: 1H NMR δ (ppm) 0.44 (s, 24H), 7.17 (m, 12H), 7.50
(m, 8H); 119Sn NMR (298 K) δ (ppm) 501. Anal. Calcd for
Gen er a l P r oced u r e for NMR Kin etic Stu d y of 2 w ith
Isocya n a tes. A NMR tube containing a solution of 21 mg
(0.030 mmol) of 2 and 18 mg (0.095 mmol) of 1,4-di-tert-
butylbenzene in 0.3 mL of toluene-d8 was first cooled to 205
t
K, 0.2 mL of a stock solution (154 mg mL-1) of BuNCO in
toluene-d8 was added, and the mixture was well-shaken at this
temperatue. The NMR tube was then placed into a precooled
(243 K) NMR probe, which was brought to 263 K, at which
time, NMR spectra (1 scan acquisition) were taken at mea-
sured time intervals.
C
32H44N2Si4Sn: C, 55.88; H, 6.45; N, 4.07. Found: C, 55.31;
H, 6.47; N, 4.11.
Syn th esis of 1,3-Bis(dim eth ylph en ylsilyl)car bodiim ide
(6). A solution of 789 mg (1.15 mmol) of 2 and 400 mg (2.25
mmol) of 5 in 50 mL of pentane was refluxed for 2 h, and then
the volatiles were removed in vacuo after filtering the reaction
mixture through a short pad of silica gel in the glovebox. Pure
6 (600 mg, 86% yield) was obtained as a colorless oil through
bulb-to-bulb distillation of the crude product at 60 °C/0.01 Torr.
For 6: 1H NMR δ 0.30 (s, 12H), 7.18 (m, 8H), 7.54 (m, 2H).
Su p p or tin g In for m a tion Ava ila ble: Details regarding
the crystallographic analyses of 1 and 2, including complete
listings of atomic positions, isotropic and anisotropic temper-
ature factors, and bond distances and bond angles. This
material is available free of charge via the Internet at
http://pubs.acs.org.
OM990526W