Acyclic dialkylstannylene and -plumbylene compounds that are monomeric in the solid state

Article abstract of DOI:10.1021/om900614q

The new phosphine-borane adduct (Me₂PhSi)CH₂P(BH ₃)Me₂ is prepared by the reaction between Me ₂PhSiCl and in situ-generated Me₂P(BH₃)CH ₂Li; the adduct undergoes clean

Full text of DOI:10.1021/om900614q

Organometallics 2009, 28, 5661–5668 5661
DOI: 10.1021/om900614q
Acyclic Dialkylstannylene and -Plumbylene Compounds That Are
Monomeric in the Solid State
Keith Izod,* Corinne Wills, William Clegg, and Ross W. Harrington
Main Group Chemistry Laboratories, School of Chemistry, Bedson Building, Newcastle University,
Newcastle upon Tyne, NE1 7RU, U.K.
Received July 14, 2009
The new phosphine-borane adduct (Me2PhSi)CH2P(BH3)Me2 is prepared by the reaction between
Me2PhSiCl and in situ-generated Me2P(BH3)CH2Li; the adduct undergoes clean deprotonation on
treatment with n-BuLi to give the phosphine-borane-stabilized carbanion complex [(Me2PhSi)-
{Me2P(BH3)}CH]Li. The reaction between 2 equiv of [(Me2PhSi){Me2P(BH3)}CH]Li and either
Cp2Sn or Cp2Pb gives the acyclic dialkylstannylene and -plumbylene compounds rac-[(Me2PhSi)-
{Me2P(BH3)}CH]2E [E = Sn (13), Pb (14)]. Similarly, the reaction between 2 equiv of [(Me3Si)-
{Me2P(BH3)}CH]Li and either Cp2Sn or Cp2Pb yields rac-[(Me3Si){Me2P(BH3)}CH]2E [E=Sn (15),
Pb (16)]. X-ray crystallography reveals that compounds 13-16 crystallize as discrete monomers that
are stabilized by two agostic-type B-H E contacts in each case; multielement NMR spectroscopy
3 3 3
and UV/visible spectroscopy indicate that these agostic-type contacts are preserved in solution. DFT
calculations reveal that these B-H E contacts stabilize compounds 13-16 by between 38.0 and
3 3 3
43.7 kcal mol-1. Calculations suggest that the dimerization of 15, which is isoelectronic with the
archetypal dialkylstannylene {(Me3Si)2CH}2Sn, to the corresponding distannene [(Me3Si){Me2-
P(BH3)}CH]2SndSn[CH{P(BH3)Me2}(SiMe3)]2 (152) is disfavored by some 30.5 kcal mol-1
.
Introduction
carbenes. Of these compounds the heavier group 14 carbene
analogues (tetrylenes) hold a special fascination, as these
compounds are potentially useful ligands for transition
metal centers.1 Although much progress has been made in
this field over the past decade, especially in the synthesis of
diaminotetrylenes, (R2N)2E (E = Si, Ge, Sn, Pb), and their
cyclic counterparts, the N-heterocyclic tetrylenes,2 the num-
ber of hydrocarbyl-substituted tetrylenes remains relatively
low. In particular, although the chemistry of diaryltetry-
lenes, Ar2E, has become reasonably well established over the
There is a great deal of current interest in the chemistry of
heavier group 14 element analogues of alkenes, alkynes, and
*Corresponding author. E-mail: k.j.izod@ncl.ac.uk.
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2004, 689, 4475. (d) Weidenbruch, M. Eur. J. Inorg. Chem. 1999, 373.
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Compounds; Rappoport, Z., Ed.; Wiley: New York, 2002; Vol. 2, pp 283-357.
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2009 American Chemical Society
Published on Web 09/09/2009
pubs.acs.org/Organometallics

5662 Organometallics, Vol. 28, No. 19, 2009
Izod et al.
past few years,3 few dialkyltetrylenes, (R3C)2E, have been
isolated.1,4-10 Such diaryl- and dialkyltetrylenes do not
benefit from stabilization by pπ-pπ interactions between
the group 14 center and adjacent heteroatoms (cf. diamino-
carbenes and their heavier group 14 analogues), but are
typically stabilized solely by the presence of sterically de-
manding substituents.
Scheme 1
Results and Discussion
The archetypal dialkylstannylene and -plumbylene,
{(Me3Si)2CH}2E [E = Sn (1), Pb (2)], were first reported by
Lappert and co-workers in the 1970s.4 However, whereas 1 is
monomeric in the gas phase, this compound dimerizes to the
corresponding distannene (12) in the solid state; in solution
compounds 1 and 12 are in dynamic equilibrium.4b Com-
pound 2 also crystallizes with a dimeric structure (22),
The new phosphine-borane adduct (Me2PhSi)CH2P(BH3)-
Me2 was prepared by the reaction between in situ-generated
Me2P(BH3)CH2Li and Me2PhSiCl and was obtained as
colorless crystals in good yield after crystallization from cold
methylcyclohexane. Treatment of (Me2PhSi)CH2P(BH3)Me2
with n-BuLi in THF leads to quantitative deprotonation, as
1
determined by 31P{1H} and H NMR spectroscopy. The
although the long Pb Pb distance suggests that the plum-
3 3 3
bylene subunits are only weakly bonded.1e The few known
monomeric dialkyltetrylenes (3-8) are stabilized toward
dimerization in the solid state by the steric bulk of the alkyl
substituents.
lithium salt [(Me2PhSi){Me2P(BH3)}CH]Li is isolated as a
colorless, viscous oil that is soluble in ether and toluene, but
that we have been unable to isolate in a state suitable for
detailed analysis; however, the identity of this compound is
confirmed by its ready metathesis reactions (see below).
The reaction between either Cp2Sn14 or Cp2Pb12,15 and
2 equiv of [(Me2PhSi){Me2P(BH3)}CH]Li in toluene cleanly
gives the corresponding dialkyltetrylenes rac-[(Me2PhSi)-
{Me2P(BH3)}CH]2E [E = Sn (13), Pb (14)] (Scheme 1).
Similarly, the reaction between either Cp2Sn or Cp2Pb and
2 equiv of the known alkyllithium [(Me3Si){Me2P(BH3)}-
CH]Li16 in toluene yields rac-[(Me3Si){Me2P(BH3)}CH]2E
[E = Sn (15), Pb (16)]. Compounds 13-16 are isolated in
good to excellent yields as yellow, air- and moisture-sensitive
crystals that decompose to the free phosphine-borane and
elemental Sn or Pb on exposure to ambient light for extended
periods or on heating above ca. 50 °C. Once isolated,
compounds 13 and 14 exhibit limited solubility in hydro-
carbon solvents, but are soluble in ethereal and halocarbon
solvents; in contrast, compounds 15 and 16 are soluble in
most common aprotic solvents, including light petroleum
and toluene.
All four compounds 13-16 are isolated solely as the rac
isomer; there is no evidence from 31P{1H} NMR spectra of
the crude reaction solutions for the formation of the alter-
native meso diastereomers. This contrasts markedly with the
cyclic compounds 9 and 10,11,12 which are isolated as mix-
tures of the rac and meso diastereomers in ratios of 1:1 and
3:2, respectively; for 11, which contains a seven-membered
ring, there is evidence for the formation of the meso diaster-
eomer in up to 25% yield, but there is no evidence for the
formation of the meso diastereomer of the corresponding
dialkylplumbylene 12.13 Since racemization of the chiral
carbanions in both [(Me2PhSi){Me2P(BH3)}CH]Li and
[(Me3Si){Me2P(BH3)}CH]Li is likely to be rapid in solution,
we attribute the selective formation of the rac isomers of 13-
16 to the stabilization afforded to these compounds by the
We recently isolated a series of new cyclic dialkyltetrylenes
(9-12), which exhibit unusual agostic-type B-H E inter-
3 3 3
actions [E=Sn, Pb], and showed that such interactions may
potentially serve as an alternative method for the stabiliza-
tion of dialkylstannylenes and -plumbylenes.11-13 However,
although 9-12 are monomeric in the solid state, stabilization
of these compounds is afforded by a combination of agostic-
type interactions and the steric bulk of the ligands; for these
compounds it is not possible to separate the two distinct
stabilization effects. The question therefore remains as to
whether such agostic-type interactions alone are sufficiently
stabilizing that the dimerization of dialkyltetrylenes to the
corresponding tetraalkylditetrenes, R2EdER2, is inhibited.
In order to address this issue, we have now prepared direct
isoelectronic analogues of 1 and 2 containing phosphine-
borane substituents and show that these are indeed mono-
meric in the solid state.
(6) Kira, M.; Yauchibara, R.; Hirano, R.; Kabuto, C.; Sakurai, H.
J. Am. Chem. Soc. 1991, 113, 7785.
(7) Kira, M.; Ishida, S.; Iwamoto, T.; Kabuto, C. J. Am. Chem. Soc.
two agostic-type B-H E interactions in each case (see
3 3 3
below).
1999, 121, 9722.
Single crystals of 13 and 14 suitable for X-ray crystal-
lography were obtained from cold toluene/THF or cold diethyl
ether/THF, respectively; single crystals of 15 and 16 were
(8) Kira, M.; Ishida, S.; Iwamoto, T.; Ichinohe, M.; Kabuto, C.;
Ignatovich, L.; Sakurai, H. Chem. Lett. 1999, 263.
(9) Eaborn, C.; Hill, M. S.; Hitchcock, P. B.; Patel, D.; Smith, J. D.;
Zhang, S. Organometallics 2000, 19, 49.
(10) Eaborn, C.; Ganicz, T.; Hitchcock, P. B.; Smith, J. D.; Sozerli,
S. E. Organometallics 1997, 16, 5621.
(14) Fischer, E. O.; Grubert, H. Z. Naturforsch. B 1956, 11, 423.
(15) (a) Fischer, E. O.; Grubert, H. Z. Anorg. Allg. Chem. 1956, 286,
237. (b) Armstrong, D. R.; Davidson, M. G.; Moncrieff, D.; Russell, C. A.;
Stourton, C.; Steiner, A.; Stalke, D.; Wright, D. S. Organometallics 1997, 16,
3340.
(11) Izod, K.; McFarlane, W.; Tyson, B. V.; Carr, I.; Clegg, W.;
Harrington, R. W. Organometallics 2006, 25, 1135.
(12) Izod, K.; McFarlane, W.; Wills, C.; Clegg, W.; Harrington,
R. W. Organometallics 2008, 27, 4386.
(13) Izod, K.; Wills, C.; Clegg, W.; Harrington, R. W. Organometal-
(16) Izod, K.; Bowman, L. J.; Wills, C.; Clegg, W.; Harrington, R. W.
lics 2009, 28, 2211.
Dalton Trans. 2009, 3340.

Article
Organometallics, Vol. 28, No. 19, 2009 5663
Figure 1. Molecular structures of (a) rac-[(Me2PhSi){Me2P(BH3)}CH]2Sn (13) and (b) rac-[(Me2PhSi){Me2P(BH3)}CH]2Pb (14) with
40% probability ellipsoids and with methyl and aryl H atoms omitted for clarity.
˚
and 16 are 7.165 and 6.661 A, respectively; in no case is the
obtained by crystallization from cold diethyl ether. Com-
pounds 13 and 14 adopt similar, but not identical, structures,
whereas compounds 15 and 16 are both isostructural and
isomorphous. The molecular structures of 13 and 14 are
shown in Figure 1, and the molecular structure of 15 is shown
in Figure 2; selected bond lengths and angles for 13 and 14
and for 15 and 16 are given in Tables 1 and 2, respectively.
The structures of 13 and 14 differ solely in the orientation
of one of the two Me2PhSi groups. Whereas in 14 both of
these groups are oriented such that the phenyl rings are at the
extremes of the molecule, in 13 one of the Me2PhSi groups is
rotated by approximately 120°, such that the phenyl ring in
this group lies proximal to the tin atom.
vector between the two group 14 elements consistent with an
E
E bonding interaction (see Supporting Information for
3 3 3
crystal packing diagrams). The Sn Sn and Pb Pb dis-
3 3 3
3 3 3
tances in 13-16 are substantially longer than the corresponding
distances of 2.768(2) and 4.129 A in dimeric 12 and 22, res-
˚
pectively.4,1e This clearly suggests that the two agostic-type
B-H Sn interactions observed in each of compounds 13-16
3 3 3
are sufficiently stabilizing that dimerization to the correspond-
ing distannene or diplumbene is inhibited; this suggestion is
supported by DFT calculations (see below).
˚
The Sn-C distances of 2.296(5) and 2.308(5) A in 13 and
2.3149(16) and 2.2864(16) A in 15 are similar to the corre-
˚
Compounds 13-16 crystallize as discrete dialkyltetrylenes.
sponding distances in the few previously reported dialkyl-
stannylenes; for example, the Sn-C distances in 7 are 2.284(3)
The shortest Sn Sn distances in 13 and 15 are 4.401 and 6.685
˚
3 3 3
A, respectively, whereas the shortest Pb Pb distances in 14
9
˚
and 2.286(3) A, and the Sn-C distances in rac-9are 2.2984(14)
3 3 3

5664 Organometallics, Vol. 28, No. 19, 2009
Izod et al.
between the isoelectronic pairs 15/1 and 16/2 may, at least in
part, be attributed to the different orientations of the alkyl
ligands in the two systems: in the gas phase structures of 1
and 2 the ligands adopt a syn,anti-configuration, which
minimizes steric interactions between the bulky substituents,
whereas in 15 and 16 (and also in 13 and 14) the ligands adopt
a syn,syn-configuration.
The adoption of a syn,syn-configuration in 13-16 is a
consequence of the two short, agostic-type B-H E con-
3 3 3
tacts in each compound, one to each of the two BH3 groups,
which essentially lock the two alkyl substituents into this
configuration. The B-H E distances of 2.41(8) and
Figure 2. Molecular structure of rac-[(Me3Si){Me2P(BH3)}CH]2-
Sn (15) with 40% probability ellipsoids and with methyl H
atoms omitted for clarity.
3 3 3
˚
2.35(6) A (13), 2.48(2) and 2.50(2) A (14), 2.38(2) and
˚
˚
2.29(2) A (15), and 2.51(3) and 2.36(2) A (16) lie well within
˚
the sum of the van der Waals radii of H and either Sn or Pb
˚
(3.37 and 3.22 A, respectively). The B-H Sn distances in
3 3 3
˚
Table 1. Selected Bond Lengths (A) and Angles (deg) for 13
(E=Sn) and 14 (E = Pb)
13 and 15 compare with B-H Sn distances ranging from
3 3 3
2.25(4) to 2.32(2) A in 9 and 11,11,13 whereas the B-H Pb
˚
3 3 3
13
14
distances in 14 and 16 compare with B-H Pb distances
3 3 3
12,13
˚
ranging from 2.25(4) to 2.59(6) A in 10 and 12.
To the
best of our knowledge, outside of our own studies the only
E-C(1)
2.296(5)
2.308(5)
1.797(5)
1.794(5)
1.875(5)
1.885(5)
1.913(6)
1.905(7)
2.35(6)
2.363(2)
2.424(2)
1.786(2)
1.783(2)
1.862(2)
1.858(2)
1.905(3)
1.902(3)
2.48(2)
E-C(12)
C(1)-P(1)
C(12)-P(2)
C(1)-Si(1)
C(12)-Si(2)
P(1)-B(1)
P(2)-B(2)
example of a short B-H Sn contact is found in the anion
3 3 3
of [10-endo-(SnPh3)-10-μ-H-7,8-nido-C2B9H10][trans-Ir(CO)-
(PPh3)2(MeCN)], in which the tin(IV) center is directly
bonded to the boron atom of a carborane cage [Sn
H
3 3 3
2.349 A].18 Similarly, outside of our own studies we are aware
˚
E
E
H(1A)
H(1B)
C(1)-E-C(12)
of only one other short B-H Pb contact, which is found in
3 3 3
3 3 3
3 3 3
the tris(2-mercaptoimidazolyl)borate complex (TmPh)2Pb
[TmPh = HB(2-S,3-PhC3N2)3]; in this compound one of the
TmPh ligands adopts an inverted η4-coordination mode,
binding the lead atom through its three S-donors and the
2.41(8)
99.60(17)
2.50(2)
97.42(8)
˚
Table 2. Selected Bond Lengths (A) and Angles (deg) for 15
(E=Sn) and 16 (E = Pb)
19
˚
central B-H group, and the Pb H distance is 2.39 A.
3 3 3
15
16
In addition to the B-H E contacts observed in 13-16,
there are relatively short distances between the tin or lead
centers and the methine hydrogen atoms of each ligand
[C-H Sn 2.76 and 2.79 A (13), 2.77 and 2.77 A (15);
C-H Pb 2.84 and 2.89 A (14), 2.86 and 2.86 A (16)];
3 3 3
E-C(1)
2.3149(16)
2.2864(16)
1.7877(16)
1.7919(18)
1.8777(18)
1.8778(19)
1.909(3)
1.918(2)
2.38(2)
2.29(2)
98.26(6)
2.408(2)
2.383(2)
1.783(2)
1.792(2)
1.879(2)
1.876(2)
1.916(3)
1.921(3)
2.51(3)
E-C(7)
˚
˚
C(1)-P(1)
C(7)-P(2)
C(1)-Si(1)
C(7)-Si(2)
P(1)-B(1)
P(2)-B(2)
3 3 3
3 3 3
˚
˚
however, DFT studies (see below) indicate that these are a
consequence solely of the ligand conformation and are not
due to any significant C-H E interactions.
3 3 3
1
The H, 13C{1H}, 31P{1H}, and 11B{1H} NMR spectra
E
E
H(1A)
H(2A)
C(1)-E-C(7)
3 3 3
3 3 3
2.36(2)
97.51(8)
of 13-16 are as expected and reveal no evidence for dyna-
mic behavior in solution (cf. 9 and 10). The 119Sn chemical
shifts of 13 and 15 (375 and 377 ppm, respectively) and
the 207Pb chemical shifts of 14 and 16 (3931 and 3902 ppm,
respectively) are to significantly higher field than the cor-
responding shifts of the monomeric forms of 1 and 2 (2328
and 9112 ppm, respectively)4 and of the related cyclic
dialkyltetrylenes 6 (2323),6 7 (2299),9 and 8 (10050 ppm),10
but are similar to those of rac-9 (578),11 rac-10 (4580),12
rac-11 (320), and rac-12 (5920 ppm),13 consistent with the
and 2.3046(14) A.11 Similarly, the Pb-C distances of 2.363(2)
and 2.424(2) A in 14 and 2.408(2) and 2.383(2) A in 16 are
comparable to the corresponding distances in 8 [2.397(6) and
˚
˚
˚
10
2.411(5) A] and rac-10 [2.390(4) and 2.402(4) A].
12
˚
˚
The C-Sn-C angles of 99.60(17)° and 98.26(6)° in 13 and
15, respectively, are similar to the C-Sn-C angle observed
in the gas phase structure of 1 [97(2)°];4 however, the C-
Pb-C angles of 97.42(8)° and 97.51(8)° in 14 and 16, res-
pectively, are substantially narrower than the C-Pb-C angle
in the corresponding gas phase structure of 2 [103.6°],1e,17
although in the loosely dimeric solid state structure of 22 the
C-Pb-C angle is 93.6°.1e The differences in C-E-C angles
B-H E contacts observed in the solid state persisting in
3 3 3
solution.
(18) Kim, J.; Kim, S.; Do, Y. J. Chem. Soc., Chem. Commun. 1992,
938.
(19) Bridgewater, B. M.; Parkin, G. Inorg. Chem. Commun. 2000, 3,
534.
(17) Lappert, M. F. Main Group Met. Chem. 1994, 17, 183.

Article
This proposal is supported by the UV-visible spectra of
Organometallics, Vol. 28, No. 19, 2009 5665
Table 3. Comparison of Selected Calculated and Experimental
Bond Lengths and Angles for 13-16
solutions of 13-16, which exhibit absorption maxima be-
tween 324 and 370 nm [cf. 546 and 610 nm for 7 and 8,
respectively], which may be assigned to the n f p transition
in each case; the significant blue shift for 13-16 is consistent
with retention of the agostic-type interactions in solution.
The solid state infrared spectra of 13-16 exhibit absorptions
in the range 2079-2170 cm-1, which may be attributed to
B-H stretching vibrations of the H atoms involved in the
˚
˚
H (A)
˚
E-C (A)
E
B-H (A)
C-E-C (deg)
3 3 3
15(expt) 2.3149(16) 2.38(2) [2.31] 1.07(2)-1.17(2) 98.26(6)
2.2864(16) 2.29(2) [2.25]
15(calc) 2.3503
2.3134
2.38
2.31
1.21-1.25
99.56
13(expt) 2.296(5)
2.308(5)
2.35(6) [2.31] 1.01(9)-1.17(6) 99.60(17)
2.41(8) [2.26]
13a
2.3102
2.3545
2.3096
2.3588
2.33
2.33
2.38
2.30
1.21-1.25
1.20-1.25
99.16
99.44
agostic-type B-H E interactions.
3 3 3
DFT Calculations. The nature of the bonding and the
E
13b
degree of stabilization afforded by the short B-H
3 3 3
contacts in 13-16 was probed using DFT calculations.
Geometries were optimized using the B3LYP hybrid func-
tional20 with a Lanl2dz basis set21 on Sn and Pb and a
6-31G(d,p) basis set22 on all other atoms. The location of
minima was confirmed by the absence of imaginary vibra-
tional frequencies. For the phenyl-substituted compounds 13
and 14 geometries were optimized for both of the possible
conformations of each compound; we designate the confor-
mation corresponding to the solid state structure of 14, in
which both Me2PhSi groups are oriented such that the
phenyl rings are at the periphery of the molecule, as con-
former a, whereas the alternative conformation, in which one
of the Me2PhSi groups is rotated through ca. 120°, corre-
sponding to the solid state structure of 13, is designated as
conformer b. Calculations reveal that the difference in energy
16(expt) 2.383(2)
2.408(2)
2.36(2) [2.36] 0.99(2)-1.23(2) 97.51(8)
2.51(3) [2.44]
16(calc) 2.4423
2.3979
2.40
2.43
1.21-1.24
99.13
14(expt) 2.363(2)
2.424(2)
2.48(2) [2.43] 1.03(2)-1.18(2) 97.42(8)
2.50(2) [2.47]
14a
2.3947
2.4476
2.4537
2.3928
2.41
2.41
2.39
2.43
1.21-1.24
1.21-1.25
99.05
98.65
14b
Table 4. Selected NBO Energies for 13-16
13a
13b
15
14a
14b
16
E(2)a/kcal mol-1
EDELb/kcal mol-1
20.8
18.7
43.7
20.2
18.9
42.6
20.4
18.3
42.9
18.4
16.4
38.4
17.3
17.2
38.0
17.4
17.0
38.0
between conformers 13a and 13b is only 1.4 kcal mol-1
,
a E(2) energy from NBO calculations for the elements corresponding
to B-H E delocalizations. b Energy difference between the ground
state structure and the structure in which the elements responsible for the
whereas the difference in energy between conformers 14a and
14b is just 1.3 kcal mol-1; in each case the more symmetrical a
conformer is the lowest energy minimum.
3 3 3
principal B-H E delocalizations have been deleted (see text).
3 3 3
Natural bond orbital analyses23 of 13-16 reveal that the
HOMO and LUMO in each case consist of an essentially
s-type lone pair on the tin or lead center and a vacant tin or
lead p orbital lying perpendicular to the C-E-C plane,
The calculated minimum-energy structures of 13-16 cor-
relate extremely well with those determined by X-ray crystal-
lography (Table 3); in each case the calculated C-E distances
˚
are overestimated by approximately 0.02-0.06 A, although
there is a close correspondence between the calculated and
experimental C-E-C angles. The B-H E contacts ob-
respectively. The short B-H E contacts in 13-16 are
3 3 3
3 3 3
associated with significant delocalization of electron density
from one of the B-H σ-bonds in each BH3 group into the
vacant p orbital on the tin or lead atom. The E(2) energy
associated with this interaction, calculated via second-order
perturbation theory, provides a measure of the degree of
stabilization afforded by these contacts (Table 4). For the tin
compounds 13a, 13b, and 15 the E(2) energies fall in a narrow
range from 18.3 to 20.8 kcal mol-1, whereas for the plumby-
lenes 14a, 14b, and 16 the calculated E(2) energies lie in the
range 16.4 to 18.4 kcal mol-1. The overall stabilization
served crystallographically are replicated extremely well in
the calculated structures; in each calculated structure there
are two short B-H E contacts, one to each BH3 group.
3 3 3
The calculated and experimental B-H Sn distances in 15
3 3 3
˚
[2.38/2.31 and 2.38(2)/2.29(2) A, respectively] are identical
within experimental error, whereas the calculated
˚
B-H Pb distances in 16 [2.40 and 2.43 A] are close to
3 3 3
those determined crystallographically [2.36(2) and 2.51(3)
˚
A]. There is a similarly close correspondence between the
calculated and experimental B-H E distances in the
energy afforded by the B-H E contacts may be calculated
3 3 3
3 3 3
phenyl-substituted stannylenes 13 and 13b and plumbylenes
14 and 14a. It should be noted that bond lengths to H atoms
are generally underestimated by X-ray crystallography; if all
by selective deletion of the elements associated with the
agostic-type interactions and recalculation of the energy of
the system.24 This method yields stabilization energies
(EDEL) between 42.6 and 43.7 kcal mol-1 for the three
stannylenes and between 38.0 and 38.4 kcal mol-1 for the
three plumbylenes. The values of E(2) and EDEL for 13-16
are in keeping with the corresponding energies calculated for
the cyclic stannylenes and plumbylenes 9-12. As expected,
˚
the B-H bond lengths in these structures are reset to 1.23 A
(consistent with the calculated results), without changing
their directions, the adjusted E H distances, given in
3 3 3
square brackets in Table 3, give an even better agreement
overall with the calculated distances.
stabilization via agostic-type B-H E interactions in the
3 3 3
(20) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Stephens, P. J.;
Devlin, F. J.; Chablowski, C. F.; Frisch, M. J. J. Phys. Chem. 1994, 98,
11623. (c) Hertwig, R. H.; Koch, W. Chem. Phys. Lett. 1997, 268, 345.
(21) (a) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270. (b) Wadt,
W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284. (c) Hay, P. J.; Wadt, W. R.
J. Chem. Phys. 1985, 82, 299.
(22) (a) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28,
213. (b) Francl, M. M.; Petro, W. J.; Hehre, W. J.; Binkley, J. S.; Gordon,
M. S.; DeFrees, D. J.; Pople, J. A. J. Chem. Phys. 1982, 77, 3654.
(23) (a) Carpenter, J. E.; Weinhold, F. J. Mol. Struct. (THEOCHEM)
1988, 169, 41. (b) Carpenter, J. E. Ph.D. thesis, University of Wisconsin,
Madison, WI, 1987. (c) Foster, J. P.; Weinhold, F. J. Am. Chem. Soc. 1980, 102,
7211. (d) Reed, A. E.; Weinhold, F. J. Chem. Phys. 1983, 78, 4066. (e) Reed,
A. E.; Weinhold, F. J. Chem. Phys. 1983, 1736. (f) Reed, A. E.; Weinstock,
R. B.; Weinhold, F. J. Chem. Phys. 1985, 83, 735. (g) Reed, A. E.; Curtiss,
L. A.; Weinhold, F. Chem. Rev. 1988, 88, 899.
(24) Wong, M. W. Chem. Phys. Lett. 1996, 256, 391.

5666 Organometallics, Vol. 28, No. 19, 2009
Izod et al.
a
Scheme 2. Dimerization of 15 to 152
a Optimized geometry of 152 shown with H atoms omitted for clarity.
plumbylenes 14 and 16 is less than for the corresponding
stannylenes 13 and 15, consistent with poorer orbital overlap
in the latter case due to the increased size and diffuse nature
of the 6p orbital on lead compared to the 5p orbital on tin.
One of the key questions we set out to address through the
Conclusions
In contrast to the archetypal dialkyltetrylenes 1 and 2, the
readily accessible isoelectronic dialkyltetrylenes 15 and 16
crystallize as discrete monomers, with no close Sn Sn or
3 3 3
Pb Pb contacts. The closely related compounds 13 and 14
3 3 3
synthesis of 13-16 was whether agostic-type B-H
E
3 3 3
similarly crystallize as discrete monomers. Each of the
compounds 13-16 is stabilized by two short, agostic-type
interactions were sufficiently stabilizing to prevent the di-
merization of dialkyltetrylenes to the corresponding tetra-
alkylditetrenes. The isoelectronic nature of 15 and 1 makes
these ideal compounds for comparison in this regard;
whereas 1 dimerizes to the distannene 12 in the solid state,
compound 15 is isolated as a dialkylstannylene. In order to
gain further insight into the relative energies of the mono-
meric and dimeric forms of 15, we have calculated the energy
of the dimerization reaction shown in Scheme 2.
B-H E contacts, one to each of the two BH3 groups in
each case. The B-H E contacts appear to be preserved in
solution and significantly affect the spectroscopic properties
of these compounds. DFT calculations suggest that the
3 3 3
3 3 3
B-H E contacts stabilize compounds 13-16 by between
3 3 3
38.0 and 43.7 kcal mol-1. There is no evidence for the
dimerization of these species in solution, and DFT calcula-
tions reveal that the dimerization of 15 to 152 is significantly
disfavored.
Initial attempts to locate a local minimum-energy geometry
for 152 failed: irrespective of the starting geometry, calcula-
tions consistently yielded a pair of monomers in which the
Experimental Section
B-H Sn contacts had re-established themselves and in
3 3 3
which there was no evidence for any Sn Sn interaction.
3 3 3
General Considerations. All manipulations were carried out
using standard Schlenk techniques under an atmosphere of dry
nitrogen or in a nitrogen-filled glovebox. Toluene, THF, and
diethyl ether were distilled under nitrogen from sodium, potas-
sium, or sodium/potassium alloy, respectively, and were stored
However, a minimum energy geometry for152 (Scheme 2) was
successfully located by initially constraining the Sn-Sn dis-
tance in the dimer to the value observed in 12 (2.768 A) and
˚
then reoptimizing the resulting geometry in the absence of any
constraints. There are only marginal differences between the
overall structures of the constrained and unconstrained di-
mers, with the exception of the Sn-Sn distance, which
˚
over either a potassium film or activated 4 A molecular sieves, as
appropriate. Deuterated toluene and THF were distilled under
nitrogen from potassium, and CDCl3 was distilled under nitro-
gen from CaH2; NMR solvents were deoxygenated by three
˚
increased to 2.902 A in the latter, clearly suggesting a reduc-
˚
freeze-pump-thaw cycles and were stored over activated 4 A
tion in the Sn-Sn bond order in 152 compared to 12. Con-
sistent with this, the Wiberg bond index of 1.28 for the SndSn
bond in 152 is somewhat lower than in a traditional double
bond; for comparison, the bond order for the model complex
H2SndSnH2 has been calculated as 1.46.25
In all other respects, the optimized geometry of 152 is very
similar to the solid state structure of 12 and has the typical
trans-bent motif common to distannenes. The bending angle
of 34.1° in 152 (i.e., the angle by which the C-Sn-C planes
deviate from the Sn-Sn vector) compares with a correspond-
ing angle of 32.2° in 12.4 It is notable that in 152 the four
borane groups are oriented away from the tin atoms and so
molecular sieves. The compounds Me3P(BH3),26 {Me2P(BH3)}-
(SiMe3)CH2,16 Cp2Sn,14 and Cp2Pb12,15 were prepared by pre-
viously published procedures; n-BuLi was purchased as a 2.5 M
solution in hexanes.
1H, 11B{1H}, 13C{1H}, 31P{1H}, 119Sn{1H}, and 207Pb{1H}
NMR spectra were recorded on a JEOL Eclipse500 spectro-
meter operating at 500.16, 160.35, 125.65, 202.35, 186.50, and
104.32 MHz, respectively; chemical shifts are quoted in ppm
relative to tetramethylsilane (1H and 13C), external BF3(OEt2)
(11B), external 85% H3PO4 (31P), external Me4Sn (119Sn),
or external Me4Pb (207Pb), as appropriate. The positions of
the BH3 signals in the 1H NMR spectra and JPH for these
signals were determined using a selective H{11B} experiment.
1
there are no short B-H Sn contacts in the dimer.
3 3 3
Infrared spectra were recorded as neat powders on a Varian 800
FTIR spectrometer; UV-visible spectra were recorded as
1.0 mM solutions in matched quartz cells on a Hitachi
F4500 spectrophotometer. Elemental analyses were obtained
by the Elemental Analysis Service of London Metropolitan
University.
In keeping with the stabilizing nature of the B-H
E
3 3 3
contacts observed in 15, the energy for the dimerization
reaction shown in Scheme 2 is calculated to be þ30.5 kcal
mol-1. This clearly suggests that the energy gained on
formation of the SndSn bond in the dimer is insufficient to
(Me2PhSi)CH2P(BH3)Me2. To a cold (0 °C) solution of
Me3P(BH3) (1.90 g, 21.1 mmol) in THF (30 mL) was added
n-BuLi (8.45 mL, 21.1 mmol). The resulting solution was allowed
compensate for the loss of the two B-H Sn interactions in
each monomer unit and, thus, that these interactions provide
a substantial barrier toward dimerization.
3 3 3
(26) (a) CrysAlisPro; Oxford Diffraction Ltd.: Oxford, UK, 2008.
(b) Sheldrick, G. M. Acta Crystallogr., Sect. A 2008, 64, 112.
(25) Lendvay, G. Chem. Phys. Lett. 1991, 181, 88.

Article
Organometallics, Vol. 28, No. 19, 2009 5667
Table 5. Crystallographic Data for 13, 14, 15, and 16:
13
14
15
16
formula
Mw
C22H42B2P2Si2Sn
565.0
C22H42B2P2PbSi2
653.5
C12H38B2P2Si2Sn
440.9
C12H38B2P2PbSi2
529.4
cryst size/mm
cryst syst
space group
0.30 ꢀ 0.20 ꢀ 0.20
monoclinic
P21/c
11.8589(5)
18.8499(9)
13.0329(6)
101.717(4)
0.42 ꢀ 0.20 ꢀ 0.10
monoclinic
C2/c
30.4056(6)
12.4203(2)
16.7601(4)
112.894(3)
0.40 ꢀ 0.30 ꢀ 0.30
monoclinic
P21/c
6.68543(14)
21.6435(3)
15.9783(4)
100.905(2)
2270.24(7)
4
0.30 ꢀ 0.20 ꢀ 0.20
monoclinic
P21/c
6.66086(15)
21.7767(5)
15.9405(3)
100.237(2)
2275.40(8)
4
˚
a /A
˚
b /A
˚
c /A
β/deg
3
˚
V/A
2852.7(2)
4
1.100
5830.8(2)
8
5.986
Z
μ/mm-1
1.361
7.650
transmn coeff range
reflns measd
unique reflns
Rint
0.735-0.810
16 839
5569
0.045
4372
294
0.052
0.144
1.086
0.188-0.590
27 608
7256
0.033
5387
294
0.021
0.035
0.900
0.612-0.686
24 088
5661
0.032
4520
206
0.023
0.049
1.021
0.207-0.310
16 219
5515
0.025
4405
206
0.019
0.033
0.929
reflns with F2 >2σ
refined params
R (on F, F2 >2σ)a
Rw (on F2, all data)a
goodness of fita
-3
˚
max., min. electron density/e A
1.99, -1.28
P
0.84, -0.64
P
0.51, -0.46
0.86, -0.72
P
P
P
a Conventional R =
Fo| - |Fc
/
|Fo|; Rw = [ w(Fo2 - Fc2)2/ w(Fo2)2]1/2; S = [ w(Fo2 - Fc2)2/(no. data - no. params)]1/2 for all data.
to attain room temperature and was stirred for 3 h, then added
to a cold (-78 °C) solution of Me2PhSiCl (3.61 g, 21.1 mmol) in
THF (10 mL). This mixture was allowed to attain room tem-
perature and was stirred for 16 h. Water (50 mL) was added, the
organic layer was extracted into diethyl ether (3 ꢀ 30 mL), the
combined organic extracts were dried over MgSO4 and filtered,
and the solvent was removed in vacuo from the filtrate to give
(Me2PhSi)CH2P(BH3)Me2 as a white solid, which was puri-
fied by crystallization from cold (-30 °C) methylcyclohexane
(20 mL). Isolated yield: 2.55 g, 54%. Anal. Calcd for
C11H22BPSi: C 58.94, H 9.89. Found: C 59.04, H 10.03. 1H{11B}
NMR (CDCl3,25°C): δ0.49 (s, 6H, SiMe2),0.58(d,2JPH=16.1Hz,
1411 (m), 1254 (m), 1096 (s), 922 (s), 829 (s), 809 (s), 731 (s),
687 (s).
rac-[{Me2P(BH3)}(Me2PhSi)CH]2Pb (14). To a solution of
(Me2PhSi)CH2P(BH3)Me2 (0.51 g, 2.28 mmol) in THF (20 mL)
was added n-BuLi (0.91 mL, 2.28 mmol). The resulting solution
was stirred for 1 h, and then solvent was removed in vacuo to
yield a colorless oil. This oil was dissolved in toluene (20 mL)
and added to a solution of freshly sublimed Cp2Pb (0.38 g, 1.14
mmol) in toluene (20 mL). The solution was stirred for 30 min,
then filtered. The solvent was removed in vacuo from the filtrate
to yield a yellow solid, which was dissolved in diethyl ether/THF
(15 mL of a 1:1 mixture) and cooled to -30 °C for 16 h to yield 14
as yellow blocks. Isolated yield: 0.54 g, 72%. Anal. Calcd for
C22H42B2P2Si2Pb: C 40.43, H 6.48. Found: C 40.50, H 6.51.
1H{11B} NMR (d8-THF, 25 °C): δ 0.00 (d, 2JPH =10.1 Hz, 3H,
BH3), 0.38(s, 3H, SiMe), 0.56 (s, 3H, SiMe), 0.87(d, 2JPH=13.8Hz,
2
2
3H, BH3), 1.14 (d, JPH = 14.7 Hz, 2H, CH2), 1.18 (d, JPH
=
10.6 Hz, 6H, PMe), 7.34-7.60 (m, 5H, Ph). 13C{1H} NMR
(CDCl3, 25 °C): δ -1.06 (d, JPC =1.9 Hz, SiMe2), 14.89 (CH2),
14.93 (d, JPC = 37.4 Hz, PMe2), 128.16 (m-Ph), 129.67 (p-Ph),
133.60 (o-Ph), 137.99 (d, JPC = 3.84, ipso-Ph). 11B{1H} NMR
(CDCl3, 25 °C): δ -37.3 (d, JPB = 61 Hz). 31P{1H} NMR
(CDCl3, 25 °C): δ 3.8 (q, JPB =61 Hz).
2
2
1H, CH), 1.09 (d, JPH = 10.1 Hz, 3H, PMe), 1.16 (d, JPH
=
10.5 Hz, 3H, PMe), 7.28-7.59 (m, 5H, Ph). 13C{1H} NMR (d8-
3
THF, 25 °C): δ -0.84 (SiMe), 1.35 (d, JPC = 5.8 Hz, SiMe),
rac-[{Me2P(BH3)}(Me2PhSi)CH]2Sn (13). To a solution of
(Me2PhSi)CH2P(BH3)Me2 (1.24 g, 5.53 mmol) in THF (20 mL)
was added n-BuLi (2.21 mL, 5.53 mmol). The resulting solution
was stirred for 1 h, and then solvent was removed in vacuo to
yield a colorless oil. This oil was dissolved in toluene (20 mL)
and added to a solution of freshly sublimed Cp2Sn (0.69 g, 2.77
mmol) in toluene (20 mL). This mixture was stirred for 30 min,
then filtered. The solvent was removed in vacuo from the filtrate
to yield a yellow solid, which was dissolved in toluene/THF
(30 mL of a 1:1 mixture) and cooled to -30 °C for 16 h to yield 13
as yellow blocks. Isolated yield: 1.34 g, 86%. Anal. Calcd for
C22H42B2P2Si2Sn: C 46.77, H 7.49. Found: C 46.85, H 7.53.
1H{11B} NMR (d8-THF, 25 °C): δ 0.41 (s, 3H, SiMe), 0.45 (d,
19.60 (d, 1JPC=34.5 Hz, PMe), 20.78 (d, 1JPC=35.5 Hz, PMe),
52.52 (d, 1JPC = 11.5 Hz; JPbC = 744 Hz, CH), 127.8 (m-Ph),
1
128.77 (p-Ph), 133.75 (o-Ph), 143.21 (d, 3JPC=1.9 Hz, ipso-Ph).
11B{1H} NMR (d8-THF, 25 °C): δ -40.4 (d, JPB = 77 Hz).
31P{1H} NMR (d8-THF, 25 °C): δ 6.8 (q, JPB = 77 Hz). 207Pb
NMR (d8-THF, 25 °C): δ 3931 (br s). UV/vis (1.0 mM in
toluene): λmax 370 nm (ε = 738 dm3 mol-1 cm-1). IR (solid,
cm-1): 2962 (w), 2356 (w), 2156 (w), 2104 (w), 1569 (w), 1412
(w), 1258 (m), 1088 (s), 1029 (s), 916 (m), 800 (s), 727 (m), 692
(m), 622 (m).
rac-[{Me2P(BH3)}(Me3Si)CH]2Sn (15). To a solution of
(Me3Si)CH2P(BH3)Me2 (1.06 g, 6.54 mmol) in THF (30 mL)
was added n-BuLi (2.60 mL, 6.54 mmol). This mixture was
stirred for 1 h, and then the solvent was removed in vacuo. The
resulting colorless oil was dissolved in toluene (20 mL) and
added to a solution of freshly sublimed Cp2Sn (0.81 g, 3.27
mmol) in toluene (20 mL). This solution was stirred for 30 min
and then filtered. The solvent was removed in vacuo from the
filtrate to yield a yellow solid, which was dissolved in diethyl
ether (10 mL) and cooled to -30 °C for 16 h to yield 15 as yellow
needles. Isolated yield: 1.35 g, 94%. Anal. Calcd for C12H38-
2JPH = 11.9 Hz, 3H, BH3), 0.63 (s, 3H, SiMe), 1.13 (d, 2JPH
=
10.1 Hz, 3H, PMe), 1.19 (d, 2JPH =10.5 Hz, 3H, PMe), 1.36 (d,
2JPH = 14.2 Hz, 1H, CH), 7.29-7.62 (m, 5H, Ph). 13C{1H}
NMR (d8-THF, 25 °C): δ -1.53 (SiMe), 1.14 (d, 3JPC =4.8 Hz,
SiMe), 15.47 (d, 1JPC=34.5 Hz, PMe), 17.41 (d, 1JPC=34.5 Hz,
PMe), 19.70 (d, 1JPC=11.5 Hz; 1JSnC=418 Hz, CH), 127.75 (m-
Ph), 128.81 (p-Ph), 133.77 (o-Ph), 141.48 (ipso-Ph). 11B{1H}
NMR (d8-THF, 25 °C): δ -34.5 (d, JPB = 76 Hz). 31P{1H}
NMR (d8-THF, 25 °C): δ 6.8 (q, JPB=76 Hz). 119Sn{1H} NMR
(d8-THF, 25 °C): δ 375 (br). UV/vis (1.0 mM in dichloro-
methane): λmax 324 nm (ε = 250 dm3 mol-1 cm-1). IR (solid,
cm-1): 3055 (w), 2950 (w), 2904 (w), 2565 (w), 2354 (s),
2170 (m), 2082 (m), 2039 (m), 1977 (m), 1920 (w), 1832 (w),
B2P2Si2Sn: C 32.69, H 8.69. Found: C 32.62, H 8.59. H{11B}
1
NMR (d8-toluene, 25 °C): δ 0.26 (s, 9H, SiMe3), 0.82 (d, 2JPH
=
=
9.2 Hz, 3H, BH3), 0.97 (d, 2JPH=14.7 Hz, 1H, CH), 1.05 (d, 2JPH
10.1 Hz, 3H, PMe), 1.07 (d, 2JPH=10.5 Hz, 3H, PMe). 13C{1H}
NMR (d8-toluene, 25 °C): δ 2.45 (d, 3JPC=2.9 Hz, SiMe3), 16.18

5668 Organometallics, Vol. 28, No. 19, 2009
Izod et al.
(d, 1JPC=34.5 Hz, PMe), 18.29 (d, 1JPC=34.5 Hz, PMe), 20.75
(d, 1JPC =10.6 Hz, 1JSnC =417.5 Hz, CH). 11B{1H} NMR (d8-
toluene, 25 °C): δ -34.2 (d, JPB =79 Hz). 31P{1H} (d8-toluene,
DFT Calculations. Geometry optimizations on the gas-phase
molecules were performed with the Gaussian03 suite of pro-
grams (revision E.02)27 via the UK National Grid Service or the
UK National Service for Computational Chemistry Software
(http://www.nsccs.ac.uk). Optimizations were performed using
the B3LYP hybrid functional20 with a Lanl2dz effective core
potential basis set21 for Sn and Pb and a 6-31G(d,p) all-electron
basis set22 on the remaining atoms [default parameters were used
throughout]. The dimerization energy for 15a was corrected for
basis set superposition error using the counterpoise method.28
The location of minima was confirmed by the absence of
imaginary vibrational frequencies in each case. Natural bond
orbital analyses were performed using the NBO 3.0 module of
Gaussian03;23 the stabilization energy associated with the
2
25 °C): δ 5.9 (q, JPB = 79 Hz; JSnP = ca. 200 Hz). 119Sn{1H}
NMR (d8-toluene, 25 °C): δ 377 (br s). UV/vis (1.0 mM in
methylcyclohexane): λmax 316 nm (ε = 1848 dm3 mol-1 cm-1).
IR (solid, cm-1): 2936 (m), 2363 (m), 2079 (w), 1415 (w), 1250
(m), 1067 (m), 924 (m), 834 (s), 758 (m), 668 (m), 572 (m), 547 (m).
rac-[{Me2P(BH3)}(Me3Si)CH]2Pb (16). To a solution of
(Me3Si)CH2P(BH3)Me2 (0.89 g, 5.49 mmol) in THF (30 mL)
was added n-BuLi (2.20 mL, 5.49 mmol). This mixture was
stirred for 1 h and solvent was removed in vacuo. The resulting
colorless oil was dissolved in toluene (20 mL) and added to a
solution of freshly sublimed Cp2Pb (0.93 g, 2.75 mmol) in
toluene (20 mL). The solution was stirred for 30 min and then
filtered. The solvent was removed in vacuo from the filtrate to
yield a yellow solid, which was dissolved in diethyl ether (15 mL)
and cooled to -30 °C for 16 h to yield 16 as yellow needles.
Isolated yield: 1.10 g, 76%. Anal. Calcd for C12H38B2P2PbSi2: C
27.23, H 7.24. Found: C 27.33, H 7.20. 1H{11B} NMR (d8-
toluene, 25 °C): δ 0.21 (s, 9H, SiMe3), 0.33 (d, 2JPH=9.7 Hz, 3H,
BH3), 0.50 (d, 2JPH=13.8 Hz, 1H, CH), 1.01 (d, 2JPH=10.1 Hz,
3H, PMe), 1.07 (d, 2JPH = 10.1 Hz, 3H, PMe). 13C{1H} NMR
(d8-toluene, 25 °C): δ 3.33 (d, 3JPC = 2.9 Hz, SiMe3), 20.85 (d,
1JPC=34.5 Hz, PMe), 21.72 (d, 1JPC=33.6 Hz, PMe), 53.24 (d,
B-H E interactions was calculated using the NBODel rou-
tine, in which the elements affording this interaction were
3 3 3
selectively deleted.24
Acknowledgment. The authors are grateful to the
EPSRC for support and acknowledge the use of the UK
National Grid Service in carrying out this work.
Supporting Information Available: For 13, 14, 15, and 16
details of structure determination, atomic coordinates, bond
lengths and angles, and displacement parameters in CIF format;
for 13, 14, and 15 crystal packing diagrams. For 13a, 13b, 14a,
14b, 15, 16, and 152 details of DFT calculations, final atomic
coordinates, and energies. This material is available free of
charge via the Internet at http://pubs.acs.org. Observed and
calculated structure factor details are available from the authors
upon request.
1
1JPC = 12.5 Hz, JPbC = 746 Hz, CH). 11B{1H} NMR (d8-
toluene, 25 °C): δ -40.3 (d, JPB = 79 Hz, JPbB = ca. 100 Hz).
31P{1H} (d8-toluene, 25 °C): δ 6.2 (q, JPB = 79 Hz, 2JPbP = ca.
200 Hz). 207Pb{1H} NMR (d8-toluene, 25 °C): δ 3902 (br s). UV/
vis (1.0 mM in methylcyclohexane): λmax 360 nm (ε = 958 dm3
mol-1 cm-1). IR (solid cm-1): 2959 (w), 2360 (w), 1611 (w), 1407
(w), 1381 (w), 1255 (m), 1018 (s), 926 (m), 791 (s), 690 (m), 604
(w), 570 (w), 534 (m).
(27) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.;
Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.;
Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson,
G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.;
Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai,
H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken,
V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev,
O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.;
Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakr-
zewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.;
Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.;
Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov,
B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.;
Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.;
Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.;
Gonzalez, C.; Pople, J. A. Gaussian 03, Revision E.02; Gaussian, Inc.:
Wallingford, CT, 2004.
Crystal Structure Determinations of 13, 14, 15, and 16. Mea-
surements were made at 150 K on an Oxford Diffraction Gemini
A Ultra diffractometer using graphite-monochromated Mo KR
˚
radiation (λ=0.71073 A). Cell parameters were refined from the
observed positions of all strong reflections. Intensities were
corrected semiempirically for absorption, based on symmetry-
equivalent and repeated reflections. The structures were solved
by direct methods and refined on F2 values for all unique
data. Further details are given in Table 5. All non-hydrogen
atoms were refined anisotropically, and C-bound H atoms
were constrained with a riding model, while B-bound H atoms
were freely refined; U(H) was set at 1.2 (1.5 for methyl groups)
times Ueq for the parent atom. Programs were Oxford Diffrac-
tion CrysAlisPro for data collection and processing and
SHELXTL for structure solution, refinement, and molecular
graphics.26
(28) (a) Simon, S.; Duran, M.; Dannenberg, J. J. J. Chem. Phys. 1996,
105, 11024. (b) Boys, S. F.; Bernardi, F. Mol. Phys. 1970, 19, 553.