1,3 Dichloro- and 1,3 dibromotetra-n-butyldistannoxane mixtures: Ligand redistribution and fluxional dynamics in distannoxanes

Article abstract of DOI:10.1016/S0022-328X(00)00815-9

The halogen redistribution reaction in the binary [ⁿBu₂SnCl]₂O/[ⁿBu ₂SnBr]₂O system is examined by ¹¹⁹Sn- and ¹³C-NMR spectroscopy. Binary mixtures of [ⁿBu₂SnCl]₂O and [ⁿBu₂SnBr]₂O reach equilibrium rapidly at room temperature. The reactant dimers are found to be in equilibrium with all five possible mixed distannoxane dimers in the equimolar mixture. These mixed distannoxane dimers differ in the ratio of Cl and Br as well as the relative positioning of the halogens. The mechanism responsible for the rapid formation of the mixed Cl:Br distannoxane dimers is found to proceed via bimolecular collisions producing a four-centered transition state, which in turn undergoes a concerted exchange of the halogens. The equilibrium concentrations of the reactant and product dimers are well represented by a statistical distribution, indicating that Cl and Br exhibit equivalent donor abilities. At 298 K, the NMR spectral data are consistent with time-averaged structures arising from rapidly interconverting rigid ladder pairs. Lowering the temperature to 173 K failed to freeze out this fluxional process. A reversible configurational rearrangement is also observed in which rotation about the oxygen-exocyclic tin bond results in the mutual exchange of halogens associated with the same exocyclic tin atom.

Full text of DOI:10.1016/S0022-328X(00)00815-9

www.elsevier.nl/locate/jorganchem
Journal of Organometallic Chemistry 620 (2001) 211–226
1,3 Dichloro- and 1,3 dibromotetra-n-butyldistannoxane mixtures:
ligand redistribution and fluxional dynamics in distannoxanes
David L. Tierney a,1, Peter J. Moehs a, Dennis L. Hasha b,*
a Department of Chemistry, Saginaw Valley State Uni6ersity, Uni6ersity Center, MI 48710, USA
b Department of Chemistry, Uni6ersity of Missouri-Rolla, 142 Schrenk Hall, Rolla, MO 65409-0010, USA
Received 7 July 2000; received in revised form 11 October 2000
Abstract
The halogen redistribution reaction in the binary [nBu2SnCl]2O/[nBu2SnBr]2O system is examined by 119Sn- and 13C-NMR
spectroscopy. Binary mixtures of [nBu2SnCl]2O and [nBu2SnBr]2O reach equilibrium rapidly at room temperature. The reactant
dimers are found to be in equilibrium with all five possible mixed distannoxane dimers in the equimolar mixture. These mixed
distannoxane dimers differ in the ratio of Cl and Br as well as the relative positioning of the halogens. The mechanism responsible
for the rapid formation of the mixed Cl:Br distannoxane dimers is found to proceed via bimolecular collisions producing a
four-centered transition state, which in turn undergoes a concerted exchange of the halogens. The equilibrium concentrations of
the reactant and product dimers are well represented by a statistical distribution, indicating that Cl and Br exhibit equivalent
donor abilities. At 298 K, the NMR spectral data are consistent with time-averaged structures arising from rapidly interconverting
rigid ladder pairs. Lowering the temperature to 173 K failed to freeze out this fluxional process. A reversible configurational
rearrangement is also observed in which rotation about the oxygenꢀexocyclic tin bond results in the mutual exchange of halogens
associated with the same exocyclic tin atom. © 2001 Elsevier Science B.V. All rights reserved.
Keywords: Tin; Distannoxane; Rearrangement; Redistribution; NMR
measurements [16–19], IR [19,20] and 119Sn-NMR
[7c,7f,9,12,21–32] spectroscopy have demonstrated that
1. Introduction
this important class of compounds exists as dimers in
nonpolar solvents. In 1963, Okawara and Wada [17]
proposed that in solution the tetraorganodistannoxanes
are represented by a dimeric ladder structure. Subse-
quent X-ray analyses have shown that the ladder struc-
ture is the predominant structure adopted in the solid
state [9,28–37]. The tin, oxygen and bridging X ligands
of the ladder structure are essentially coplanar. Each tin
possesses two pendent organic groups; however, for the
purpose of clarity these have been omitted. The ladder
Tetraorganodistannoxanes have been used to cata-
lyze the polymerizations of ethylene oxide [1], ethylene
terephthalate [2], urethanes [3] and butyrolactone [4].
Tetraorganodistannoxanes have also been instrumental
in cross-linking room temperature-vulcanized (RTV)
silicone elastomers [5] and catalyzing chemical modifi-
cations [6] of immiscible polymer blends resulting in
improved compatibility [7]. In addition, these important
catalytic compounds have been shown to exhibit in
vitro activity against leukemia [8] and various cancer
tumor cell lines [9], as well as in the control of phyto-
pathogenic fungi and bacteria [10]. Structurally, vapor-
phase osmometry studies [11–16], cyroscopic
* Corresponding author. Fax: +1-573-3416033.
E-mail address: dhasha@umr.edu (D.L. Hasha).
1 Present address: Department of Chemistry, University of New
Mexico, Albuquerque, NM 87131, USA.
0022-328X/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S0022-328X(00)00815-9

212
D.L. Tierney et al. / Journal of Organometallic Chemistry 620 (2001) 211–226
2. Experimental
structure contains two distinct pentacoordinated tin
environments, designated as endocyclic, Sn(1) and
Sn(1%), and exocyclic, Sn(2) and Sn(2%). The endocyclic
tin atoms are located within the four-membered ring
containing the two oxygens and attached to these ring
oxygens are the exocyclic tin atoms. The geometries of
the endocyclic and exocyclic tin atoms are distorted
cis-R2SnXYZ trigonal bipyramids.
2.1. Chemicals
Di-n-butyltin dichloride and di-n-butyltin oxide were
obtained from Strem Chemicals and used without fur-
ther purification. Di-n-butyltin dibromide was prepared
according to literature methods [43,44] from concen-
trated HBr and di-n-butyltin oxide. All of the solvents
used in this study were newly acquired and possessed
one of the following purity designations: NMR, HPLC
or analytical grade.
In solution, the 119Sn-NMR spectra of monofunc-
tional tetraorganodistannoxanes, {[R2SnX]2O}2, con-
tain two resonances with chemical shifts consistent with
pentacoordinated tin environments, −60 to −235
ppm [7c,9,21–32]. With few exceptions, the mixed te-
traorganodistannes, {YR2SnOSnR2X}2 (Y=Cl, Br, I,
NCS; Y=OH, alkoxy, phenoxy) [22,23], studied thus
far, also yield two tin resonances in the same spectral
region. The ladder structure for monofunctional te-
traorganodistannoxanes (X=Y) has been observed to
undergo a dynamic process [28,30] involving intercon-
verting ladder structures. This fluxional process desig-
nated the intradimeric distannoxane process [27], yields
time-averaged NMR spectra. The observed chemical
shifts and scalar couplings of {[R2SnX]2O}2, are simple
averages of the individual ladder values. For mixed
distannoxanes, {YR2SnOSnR2X}2, this fluxional pro-
cess has been suggested to be inoperative, resulting in a
single frozen dimer with the better donor ligand occu-
pying the bridging sites [27].
2.2. Preparation of [nBu2SnCl]2O
A solution containing 30.4 g di-n-butyltin dichloride
and 100 g methylene dichloride was added to 2.0 l of
deionized water and mixed vigorously. The extent of
the reaction was monitored by continuous measurement
of the pH in the aqueous layer. When the pH ap-
proached the theoretical end-point, 1.30, signaling the
completion of the reaction, the organic layer was sepa-
rated and dried over sodium sulfate. The solid product
obtained from solvent evaporation was then recrystal-
lized from t-amyl alcohol. This procedure afforded
1,3-dichlorotetra-n-butyldistannoxane in 93% yield;
m.p.: 111–113°C. NMR (1.0 M in CDCl3) 119Sn, −
92.0, −140.6 ppm; (aCH2bCH2gCH2dCH3) 13C (from 13C
2D INADEQUATE) 32.8 (a-endo), 32.2 (a-exo), 27.3
(b-exo), 27.1 (b-endo), 26.5 (g-endo), 26.4 (g-exo), 13.6
The exceptions to the ubiquitous two-line
119Sn-NMR spectra are the [nBu2SnCl]2O/[nBu2Sn(O2-
CCH3)]2O [24] and [nBu2SnF]2O/[nBu2Sn(O2CR)]2O,
(d-endo), 13.5 ppm (d-exo); J(119Sn(endo), C)=594,
1J(119Sn(exo), aC)=569, 2J(119Sn(endo), bC)=34.4,
2J(119Sn(exo), bC)=30.7, 3J(119Sn(endo), gC)=119,
1
a
t
(R=CH3, Bu and Ph) [25], binary systems. In the
g
1
1
3J(119Sn(exo), C)=107 Hz; H (from 2D H–13C het-
eronuclear correlation (HETCOR)) 1.83 (b-endo), 1.80
(b-exo), 1.78 (a-endo), 1.76 (a-exo), 1.43 (g-endo), 1.39
equimolar mixture of [nBu2SnCl]2O and [nBu2Sn-
(O2CCH3)]2O, the 18 reported 119Sn resonances were
interpreted using ladder structures with the acetoxy
ligand preferentially occupying the bridging sites [24].
Using ladder structures, Jain and coworkers [25] iden-
tified similar F-bridged products in the binary mixtures
(g-exo),
0.96
(d-endo),
0.95
ppm
(d-exo),
2J(119Sn(endo), aH)=76, 2J(119Sn(exo), aH)=74,
3J(119Sn(endo), H)=81, J(119Sn(exo), H)=101 Hz.
Anal. Calc. for [nBu2SnCl]2O: C, 34.73; H, 6.56. Found:
C, 34.52; H, 6.46%.
b
3
b
t
of [nBu2SnF]2O/[nBu2Sn(O2CR)]2O, (R=CH3, Bu and
Ph). Gross suggested that formation of the mixed dis-
tannoxane dimers takes place by dissociation of the
dimers and subsequent ligand exchange among
monomeric species [24]. However, for metal complexes,
it is generally believed that ligand redistribution occurs
via bimolecular collisions producing a four-centered
transition state, which in turn undergoes a concerted
swap of the ligands [38–41]. In the case of tin(IV)
halides, R2SnX2, a dissociative mechanism, in which
labile halides catalyze the exchange, is responsible for
the redistribution of the halogens [42].
2.3. Preparation of [nBu2SnBr]2O
Synthesis of [nBu2SnBr]2O was carried out by the
aqueous hydrolysis of di-n-butyltin dibromide using the
same procedure described for the synthesis of
[nBu2SnCl]2O with a yield of 72%; m.p.: 108–110°C.
NMR (1.0 M in CDCl3) 119Sn, −83.1, −132.2 ppm;
(aCHb2CHg2CH2dCH3) 13C (from 13C 2D INADE-
QUATE) 36.2 (a-endo), 36.0 (a-exo), 27.7 (b-exo), 27.2
(b-endo), 26.5 (g-endo), 26.3 (g-exo), 13.6 (d-endo), 13.5
In this paper, 119Sn- and 13C-NMR spectroscopy is
employed to examine the reaction products, fluxional
dynamics and halogen redistribution mechanism for the
[nBu2SnCl]2O/[nBu2SnBr]2O binary system.
1
1
ppm (d-exo); J(119Sn(endo), aC)=595, J(119Sn(exo),
aC)=547, 2J(119Sn(endo), bC)=33.9, 2J(119Sn(exo),
bC)=31.0, 3J(119Sn(endo), gC)=139, 3J(119Sn(exo),
1
1
gC)=105 Hz; H (from 2D H–13C heteronuclear cor-

D.L. Tierney et al. / Journal of Organometallic Chemistry 620 (2001) 211–226
213
relation (HETCOR)) 1.93 (a-endo) 1.87 (a-exo), 1.85
(b-endo), 1.81 (b-exo), 1.41 (g-endo), 1.39 (g-exo), 0.95
(d-endo), 0.94 ppm (d-exo), 2J(119Sn(endo), aH)=89,
2J(119Sn(exo), aH)=73, 3J(119Sn(endo), bH)=85,
3J(119Sn(exo), bH)=105 Hz. Anal. Calc. for
[nBu2SnBr]2O: C, 29.92; H, 5.65. Found: C, 30.51; H,
5.78%.
observed integrated volumes of the two-dimensional
spectra to the kinetic parameters of the rate matrix, L.
The off-diagonal elements of the rate matrix, Lji, are
given by the rate constants, −kij, associated with mag-
netization transfer from site i to j. The 2D volume
matrix, I, is related to L by
ln(P−1I) ln J
L=
=
,
~
m
~
m
2.4. Binary mixtures
where J is the normalized volume matrix and P is the
volume matrix at ~m=0. The elements of the diagonal
matrix P are proportional to the equilibrium popula-
tions. The off-diagonal elements of L, which are the
off-diagonal elements of ln J, are readily calculated via
matrix diagonalization of J [47–51]. For a given tem-
perature, up to eight spectra were obtained; one at
~m=0 and the others at mixing times centered near the
optimum (0.5~m(opt) to 2~m(opt); ~m(opt):1/{(1/T1)+
kij+kji} [52,53]). The reported rate constants are mean
values, kij=Žkij, with variance given by |2(kij)=
_m(kij(~m)−kij)2/(N−1), where N is the number of
spectra acquired with ~m\0. The variance, |2(kij), is
found to be significantly larger than the variance in the
mean determined from rms noise of the individual
experiments, 1/S(1/|2(kij)). The reported errors in the
rate constants, as well as the activation parameters
represent an estimate of the 95% confidence level.
Binary mixtures of [nBu2SnCl]2O and [nBu2SnBr]2O,
with approximate mol ratios 10:1, 3:1, 1:1, 1:3 and 1:10
([nBu2SnCl]2O:[nBu2SnBr]2O), were prepared by dissolv-
ing the appropriate amount of distannoxanes in 0.5 ml
benzene-d6. The desired quantities of [nBu2SnCl]2O and
[nBu2SnBr]2O were determined by constraining the total
tin concentration of each of the solutions to [Sn]T=2.0
M.
2.5. NMR spectra
149.21 MHz 119Sn MAS-NMR solid-state spectra
were acquired with high power proton decoupling at
various spin rates between 3.5 and 7.0 kHz, using a
Bruker MSL400 NMR spectrometer. The solid-state
spectra were referenced using external (CH3)4Sn
(TMT=0.0 ppm). The principal elements of the shield-
ing tensors, |11, |22, and |33, are extracted using HBA
[45]. The HBA computer program analyzes the intensi-
ties of the spinning sidebands using the method of
Herzfeld and Berger [46]. The accuracy of liso, d (and
n), s (and p) corresponds to 90.05 ppm (digital
resolution), 910 ppm and 90.05, respectively. 111.96
MHz 119Sn-, 75.51 MHz 13C- and 300.14 MHz 1H-
NMR spectra of solutions containing [nBu2SnCl]2O and
[nBu2SnBr]2O were obtained using Varian INOVA 300
and 400 NMR spectrometers. The 119Sn-NMR spectra
were referenced with internal tetramethyltin (TMT=
0.0 ppm); while tetramethylsilane (TMS=0.0 ppm) was
3. Results and discussion
3.1. 119Sn-NMR spectrum
The 149.21 MHz 119Sn MAS-NMR solid-state spec-
tra of [nBu2SnCl]2O and [nBu2SnBr]2O (Fig. 1) are
dominated by chemical shift anisotropy (CSA). Both
spectra consist of two spinning sideband manifolds that
trace out extremely large asymmetric CSA patterns,
approximately 1000 ppm in breadth. Each CSA pattern
is comprised of a single isotropic resonance flanked by
spinning sidebands that are separated by the sample
rotation frequency. By varying the speed of sample
rotation, the isotropic chemical shifts associated with
the two tin environments are readily identified:
{[nBu2SnCl]2O}2, liso= −76.8 and −140.2 ppm, and
{[nBu2SnBr]2O}2, liso= −56.0 and −134.4 ppm. The
observed isotropic chemical shifts are consistent with
the pentacoordinated tin environments of the dimeric
ladder structure. The principal elements of the shielding
tensors, |11, |22, and |33, as well as their derivative
functions, d and s [54] (or alternatively, n and p) [55],
for {[nBu2SnCl]2O}2 and {[nBu2SnBr]2O}2 are not only
comparable to the values reported for the pentacoordi-
nated tins of {[(CH3)2Sn(O2CR)]2O}2 (R=CH3, p-tolu-
ene, C6F5, t-butyl) but also the hexacoordinated tins of
{[(CH3)2Sn(O2CR)]2O}2 (R=CH3 and t-butyl) [32]
1
used to reference the 13C- and H-NMR spectra. Quan-
titation of the tin spectra were assured by using a
recycle time of 1.5 s which is significantly longer than
the observed spin-lattice relaxation times, 80–130 ms.
The 119Sn 1D and 2D INADEQUATE experiments
were optimized for J(119Sn,119Sn)=60 Hz.
The 13C 2D chemical exchange (EXSY) experiment
was conducted at 328 K, on the 1:1 [nBu2SnCl]2O/
[nBu2SnBr]2O mixture in benzene-d6 with the mixing
period set to 0.5 and 1 s. Using the same experimental
conditions, 13C EXSY spectra were also obtained on
the same solution after the addition of 23, 46, 78 and
190 mg of iodine ([I2]=0.03–0.25 M). 119Sn EXSY
experiments were performed on the equimolar
[nBu2SnCl]2O/[nBu2SnBr]2O mixture in two solvents,
benzene-d6 and CDCl3, as a function of temperature,
298–348 K. Rate constants are obtained by fitting the

214
D.L. Tierney et al. / Journal of Organometallic Chemistry 620 (2001) 211–226
(Table 1). The span, d=|33−|11, defines the overall
breadth of the CSA pattern; while the skew, s=
3(|iso−|22)/d, is a measure of the axial anisotropy.
When s= 91, the symmetry of the chemical shielding
tensor is described by prolate or oblate ellipsoids, re-
spectively; while axial symmetry in the chemical shield-
ing tensor is absent when s=0 [54]. The spans
associated with the chemical shielding tensors for the
two tin sites of {[nBu2SnCl]2O}2 and {[nBu2SnBr]2O}2
define the range for the tetraorganodistannoxanes stud-
ied thus far, 995–1447 ppm. For the three unambigu-
than the van der Waal radii sum [33]. For example, in
[(CH3)2SnCl]2O the internuclear distance between the
endocyclic tin atom and proximal nonbridging Cl is
,
3.409 A; while the exocyclic tin atom and bridging Cl
,
belonging to an adjacent dimer are 3.339 A apart
[32,34]. These additional long-range interactions, which
suggest a possible increase in coordination number
from 5 to 6, may explain the similarity in the CSA
patterns of the pentacoordinated and hexacoordinated
tin atoms found in tetraorganodistannoxanes.
In noncoordinating solvents, [nBu2SnCl]2O and
[nBu2SnBr]2O produce 119Sn resonances with chemical
shifts comparable to those observed for the solids:
ously
assigned
distannoxanes,
{[nBu2SnCl]2O}2,
{[nBu2SnBr]2O}2 and {[(CH3)2Sn(O2CtBu)]2O}2, it is ob-
served that d (endo)\d (exo) by 88–320 ppm. The
skew values for the tetraorganodistannoxanes are ob-
served to be positive, s=0.16, 0.33–0.50. Smaller val-
ues of s, yet still positive, are found for the
pentacoordinated tin atoms of polymeric R2SnO,
s((CH3)2SnO)=0.08 [56,57] and s(nBu2SnO)=0.13
[56–58], indicating a near total lack of axial symmetry.
The polymeric R2SnO lattice, which consists of inter-
connected R2SnOSnR2O four-membered rings, pos-
sesses tin atoms with nearly ideal cis-R2SnX3
geometries [59,60]. The CSA patterns due to the nearly
ideal R2SnO geometries exhibit significantly smaller
spans compared to the tetraorganodistannoxanes;
d((CH3)2SnO)=762 ppm [57,58] and d(nBu2SnO)=
737 ppm [56–58]. The increased span and larger skew
of the CSA patterns associated with the tetraorgan-
odistannoxanes are due to distortions from ideal cis-
R2SnX3 trigonal bipyramidal geometry. These
distortions may be due to the constraints imposed by
the fused ring system as well as possible weak interac-
tions between the tin and ligand atoms at distances less
{[nBu2SnCl]2O}2,
−89.6
and
−142.3
ppm;
{[nBu2SnBr]2O}2, −82.7 and −133.7 ppm (0.25 M in
benzene-d6). Similar chemical shifts for both phases
indicate that the solution and solid-state structures are
closely
related
[27,31,32],
confirming
that
{[nBu2SnCl]2O}2 and {[nBu2SnBr]2O}2 are well repre-
sented by the dimeric ladder structure of the solid state.
Based on the relative number of halogens attached to
the endo- and exocyclic tins of {[R2SnX]2O}2 (X=Cl,
Br) and {YR2SnOSnR2OR%}2 (Y=Cl, Br; R%=H,
phenyl), Otera et al. [22] assigned the high frequency
resonance to the exocyclic tin environment. The tin
resonances of {[nBu2SnCl]2O}2 and {[nBu2SnBr]2O}2 ex-
hibit satellites due to tin–carbon and tin–tin couplings
(Fig. 2). The exo- and endocyclic tin resonances for
both {[nBu2SnCl]2O}2 and {[nBu2SnBr]2O}2 possess an
intense satellite pair with a common splitting of 72 and
78 Hz, respectively. These satellites are due to unre-
solved 119Snꢀ119Sn and 119Snꢀ117Sn couplings between
the exo- and endocyclic tins, 2J(exo,endo), and are
equal to previously reported values [23,27]. Although a
single 2J(endo,exo) is observed for {[nBu2SnCl]2O}2,
{[nBu2SnBr]2O}2 and {[nBu2SnX]2O}2 (X=OR, O2CR)
[7c,7f,9,23,27,30–32], ladder structures are expected to
possess two possible exo–endocyclic tin couplings,
2J(1,2)
and
2J(1,2%).
Deconvolution
of
the
{[nBu2SnCl]2O}2 and {[nBu2SnBr]2O}2 spectra reveals
that the integrated intensity of these satellites relative to
the main line, are twice the expected sum of the natural
abundance of 119Sn and 117Sn. This observation indi-
cates that 2J(1,2) and 2J(1,2%) are either fortuitously
coincidental [23] or equivalent due to averaging caused
by the intradimeric distannoxane process [27]. For
{[nBu2SnBr]2O}2 weaker satellite pairs with different
splittings 34.5 Hz (exo) and 42.4 Hz (endo) are ob-
served; while satellites with similar splittings for
{[nBu2SnCl]2O}2 are observed only with the assistance
of resolution enhancement, 30.9 (exo) and 21.4 Hz
(endo). The 119Sn INADEQUATE experiment, which
identifies 119Snꢀ119Sn pairs, is unable to detect these
weaker satellites indicating that the splittings arise from
119Snꢀ117Sn couplings involving identical tin environ-
Fig. 1. 149.21 MHz 119Sn MAS-NMR spectra of {[nBu2SnCl]2O}2
(top) and {[nBu2SnBr]2O}2 (bottom) obtained at a spinning rate of
4.5 kHz. Recycle time, 30 s; 64 transients.

D.L. Tierney et al. / Journal of Organometallic Chemistry 620 (2001) 211–226
215
Table 1
119Sn solid-state-NMR spectral data of tetraorganodistannoxanes
a
c
d
Compound
Assignment
CN
liso (ppm) b
|
(ppm) b
|
(ppm) b
|
(ppm) b d (ppm) c
33
s
n (ppm) d
p
11
22
[nBu2SnCl]2O e
[nBu2SnBr]2O e
Exocyclic
Endocyclic
Exocyclic
Endocyclic
5
5
5
5
5
5
5
5
5
6
5
5
6
6
−76.8
−140.2
−56.0
−134.4
−177
−219
−195
−233
−184
−267
−178
−209
−287
−298
−409
−464
−384
−369
−280
−331
−335
−249
−349
−184
−316
−334
−140
−155
−79
−99
−59
−11
4
59
82
112
140
74
718
983
611
783
807
929
838
836
760
911
795
812
944
995
1127
1447
995
0.41 641
0.50 843
0.35 555
0.38 649
0.48 630
0.38 710
0.29 643
0.33 603
0.48 576
0.38 644
0.33 617
0.16 603
0.64 657
0.64 697
0.51
0.43
0.59
0.55
0.45
0.55
0.65
0.60
0.49h
0.60h
0.60
0.80
0.30
0.30
1152
1087
1260
1173
1085
1087
1260
1111
1146
1084
1150
[Me2Sn(O2CR)]2O f Endo (or exo)g
R=C6F5
[Me2Sn(O2CR)]2O f Endo (or exo)g
R=p-toluene Exo (or endo)
[Me2Sn(O2CR)]2O f Exocyclic
R=tBu
Endocyclic
[Me2Sn(O2CR)]2O f Endo (or exo)g
Exo (or endo)
55
149
57
R=CH3
Exo (or endo)
Endo (or exo)
Exo (or endo)
54
a CN, coordination number.
b liso=−|iso=−(|11+|22+|33)/3; |33]|22]|11
.
c Convention proposed by Mason [54]. d(span)=|33−|11; s(skew)=3(|iso−|22)/d. s corresponds to the parameter z used in the
Herzfeld–Berger analysis; while d is related to the Herzfeld–Berger v parameter (d=vwrot/wo, where wrot is the spinning rate and wo is the
spectrometer frequency in MHz) [46].
d Haeberlen–Mehring [55] convention: ꢀ|ZZ−|isoꢀ]ꢀ|XX−|isoꢀ]ꢀ|YY−|isoꢀ; n(anisotropy)=|11−|isoB0 (for |ZZ=|11) and =|33−|iso\0
(for |ZZ=|33); p(asymmetry)=(|22−|33)/n (for nB0) and p=(|22−|11)/n (for n\0). For the chemical shielding tensors presented here
|
ZZ=|33, |YY=|22 and |XX=|11; n=d+(|11−|iso) and p={|iso−(|11+ds/3)}/{d+(|11−|iso)}.
e This work.
f Ref. [32].
g Tentative assignments.
h The asymmetries, p, reported for [Me2Sn(O2CtBu)]2O in Ref. [32] do not correspond to the experimentally determined values of the shielding
tensor. The p-values presented here are recalculated from the principal elements of the shielding tensor in Ref. [32].
ments. This observation confirms that in solution
{[nBu2SnCl]2O}2 and {[nBu2SnBr]2O}2 each possess a
pair of chemically equivalent tin atoms for both endo-
and exocyclic sites. These 119Snꢀ117Sn couplings, which
are designated 4J(exo,exo) and 2J(endo,endo), fall in the
range previously reported for {[nBu2SnCl]2O}2 [27],
{nBu2Sn(O2CCH3)]2O}2 [27] and {ClnBuSnOSnnBu-
range of 370–480 Hz for tetracoordinated tin and
900–970 Hz for hexacoordinated tin. The magnitudes
of 1J(119Sn,13C) reported for several distannoxane
dimers are intermediate indicating pentacoordinated tin
environments, 510–820 Hz [7c,f,9,27,29,31,32]. For
{[nBu2SnCl]2O}2 and {[nBu2SnBr]2O}2, the magnitudes
1
of J(119Sn,13C) associated with the endo- and exocyclic
4
OH}2 [61]. The observed magnitudes of J(exo,exo) are
sites fall towards the low end of the distannoxane
range: 1J(119Sn(endo),13C) is 601 and 602 Hz for
{[nBu2SnCl]2O}2 and {[nBu2SnBr]2O}2, respectively;
while the corresponding exocyclic couplings are slightly
4
similar to the four-bond coupling, J(119Sn,117Sn)=44
Hz, detected in the sterically congested (neophyl3-
Sn)2CO3 (neophyl=C6H5(CH3)2CCH2ꢀ) [62]. The
small couplings between endocyclic tins which operate
through the Sn(1)OSn(1%)O constrained four-membered
ring, can be explained using a simple Fermi contact
model [63]. In this model, the magnitude of the cou-
pling constant is dependent on the s character of the
tinꢀoxygen bond. Since the SnꢀOꢀSn bond angle in the
constrained four-membered ring is approximately 105°
[9,28–37], the tinꢀoxygen bond would be expected to
contain insufficient s character to yield a sizeable cou-
pling constant. Still weaker couplings to the a and g
carbons as well as satellites arising from coupling to
two tin nuclei are also observed.
1
smaller, J(119Sn(exo),13C)=574 and 552 Hz. The mag-
1
nitude of J(119Sn,13C) is linearly related to the CꢀSnꢀC
bond angle, q [67–69]. Using the relationship developed
specifically for di-n-butyltin(IV) compounds by
Holecˇek and Lycˇka [67], values of
q
for
{[nBu2SnCl]2O}2 are 135° (endo) and 132° (exo); while
for {[nBu2SnBr]2O}2 q values are 135° (endo) and 130°
(exo). Although the correlation yields angles with an
uncertainty of 94° [68], the relationship indicates that
q (exo) decreases slightly on going from Cl to the larger
Br ligand; while q (endo) remains virtually unchanged.
Changing the solvent to a 2:1 mixture (by volume) of
tetrachloroethylene and benzene-d6, results in an in-
crease in 1J(119Sn,13C) associated with both sites of
{[nBu2SnCl]2O}2: 1J(119Sn(endo),13C)=605 Hz and
1J(119Sn(exo),13C)=577 Hz [27]; while changing the
The spin–spin coupling responsible for the a-carbon
1
satellites, J(119Sn,13C), is diagnostic of tin coordination
[64–69]. Mitchell [64] has reported that for dialkyltin
1
compounds, the magnitude of J(119Sn,13C) falls in the

216
D.L. Tierney et al. / Journal of Organometallic Chemistry 620 (2001) 211–226
1
solvent to CDCl3 yields reduced J(119Sn,13C) values:
1
1J(119Sn(endo),13C)=594 Hz and J(119Sn(exo),13C)=
569 Hz.
The [nBu2SnCl]2O/[nBu2SnBr]2O binary system
reaches equilibrium rapidly at room temperature. The
111.96 MHz 119Sn-NMR spectrum resulting from the
addition of equimolar quantities of [nBu2SnCl]2O and
[nBu2SnBr]2O in benzene-d6, acquired at 298 K consists
of 20 resonances (Fig. 3c). These resonances fall into
two well-defined regions; one-half of the signals are
located in the spectral region assigned to exocyclic tin
atoms, −82 to −91 ppm, and the remaining reso-
nances are observed in the low frequency region at-
tributed to the endocyclic tin atoms, −129 and −149
ppm. The resonance pairs detected at −89.8, −142.6
and −82.9, −134.0 ppm indicate the presence of the
reactant dimers, {[nBu2SnCl]2O}2 and {[nBu2SnBr]2O}2,
respectively. The remaining 16 resonances can be ex-
Fig. 3. 111.96 MHz 119Sn-NMR spectra of binary [nBu2SnCl]2O/
[nBu2SnBr]2O mixtures with [Sn]T=2.0 M in benzene-d6 at 298 K.
The spectra correspond to the following compositions
([nBu2SnCl]2O:[nBu2SnBr]2O): (a) 10:1; (b) 3:1; (c) 1:1; (d) 1:3 and (e)
1:10. 4Cl={[nBu2SnCl]2O}2; 4Br={[nBu2SnBr]2O}2; I=3:1 (Cl:Br)
mixed distannoxane dimer; II=1:3 mixed distannoxane dimer and
the eight resonances associated with the 2:2 mixed distannoxane
dimers are denoted with asterisks.
plained by assuming that the products are also dimers,
which contain four tin atoms and four halogen ligands.
These product dimers differ in the ratio of Cl and Br
ligands as well as their relative positions. By varying the
molar ratio of the reactant distannoxanes, the reso-
nances due to the product dimers are readily identified.
The 119Sn-NMR spectrum of the 10:1 ([nBu2SnCl]2O/
[nBu2SnBr]2O) (Fig. 3a) contains two intense resonances
due to {[nBu2SnCl]2O}2 and four weaker resonances
with equal intensity at −87.5, −90.6, −136.8 and
−145.1 ppm. These four resonances are assigned to the
most likely product, which in a solution containing a
tenfold excess of [nBu2SnCl]2O is expected to be the 3:1
(Cl:Br) mixed distannoxane dimer, I. In a similar man-
ner the resonances associated with the 1:3 mixed distan-
noxane dimer, II, are identified from the 1:10 mixture:
−82.6, −88.5, −131.5 and −141.9 ppm (Fig. 3e).
Since the parent dimers and the 3:1 and 1:3 mixed
distannoxanes account for 12 of the 20 resonances, the
Fig. 2. Expansion of the 111.96 MHz 119Sn-NMR spectra of (a) 0.50
M [nBu2SnCl]2O and (b) 0.50 M [nBu2SnBr]2O solutions in benzene-d6
at 298 K. Resolution enhancement of the {[nBu2SnCl]2O}2 spectrum
is accomplished using a 15° phase shifted sinebell of fractional length
0.2. For the {[nBu2SnBr]2O}2 spectrum exponential apodization, 0.5
Hz linebroadening, is employed. The weak resonance denoted with an
asterisk in the spectrum of {[nBu2SnBr]2O}2 is attributed to an
unidentified impurity.

D.L. Tierney et al. / Journal of Organometallic Chemistry 620 (2001) 211–226
217
Fig. 4. 111.96 MHz 119Sn 2D INADEQUATE spectrum of the
equimolar [nBu2SnCl]2O/[nBu2SnBr]2O mixture with [Sn]T=2.0 M in
benzene-d6 at 298 K. The experiment was optimized for 60 Hz
couplings, ~DQ=4.17 ms. Lines are drawn to show the coupling
network associated with the 2:2 mixed distannoxane dimers, III (solid
line), IV (dashed line) and V (dot-dashed line).
remaining eight resonances are attributed to the 2:2
mixed distannoxane dimers. The 119Sn 2D INADE-
QUATE spectrum of the equimolar mixture (Fig. 4)
reveals that these eight signals are due to three 2:2
mixed distannoxane dimers: III, −87.8(1) and −
139.3(1) ppm; IV, −88.4(2), −129.6(1) and −148.6(1)
ppm; V, −82.1(1), −91.0(1) and −139.6(2) ppm. The
numbers given in parentheses are the relative number of
tin nuclei responsible for each resonance. Thus, the
eight 2:2 mixed distannoxane dimer resonances are
distributed among III, IV, and V as follows 2:3:3
(III:IV:V).
A total of eight possible ladder structures can be
drawn to describe the Cl:Br mixed distannoxane dimers
(Fig. 5). The ladder structures describing the 3:1, I and
I%, and 1:3 mixed distannoxane dimers, II and II%, would
each produce four tin resonances. Ladder structures IV
and V would each produce four tin resonances; while
III and III% would each contribute a pair of tin reso-
nances to the NMR spectrum. If relative donor ability
is determined solely on the basis of electronegativity,
then for the Cl:Br mixed distannoxane dimers, the Cl
ligand would be expected to preferentially occupy the
bridging sites. For the 3:1 and 1:3 mixed distannoxane
dimers, preferential Cl bridging would indicate that I
Fig. 5. Ladder structures for the mixed distannoxane dimers of the
[nBu2SnCl]2O/[nBu2SnBr]2O binary system.
and II% are preferred. Although this simple argument
would explain the number of observed resonances for
the 3:1 and 1:3 mixed distannoxane dimers, it does not
reconcile their presence in the equimolar mixture nor
does it correctly predict the observed number of 2:2
mixed distannoxane dimers. In fact, no combination of
three ladder structures can account for the observed
distribution of tin resonances due to the 2:2 mixed
distannoxane dimers.
A successful interpretation of the 119Sn-NMR spec-
tral data is achieved using time-averaged ladder struc-
tures resulting from the intradimeric distannoxane
process. Computer modeling [27] supports the view that
this fluxional process proceeds via migration of the
bridging and nonbridging bonds coupled with the cor-
related motion of the tins, oxygens and ligands (Scheme
1). The transition state is characterized by ligands lo-
cated at positions that are weighted averages of the
bridging and nonbridging sites of I and I%. Rapidly

218
D.L. Tierney et al. / Journal of Organometallic Chemistry 620 (2001) 211–226
associated with the 1:3 mixed distannoxane dimer,
IIlII%, and the centrosymmetric 2:2 mixed dimer,
IIIlIII%, would yield four and two tin resonances,
respectively. When this intradimeric distannoxane pro-
cess occurs in IV and V, different ladder structures are
not produced. However, this fluxional process leads to
the mutual exchange of the exocyclic tins of IV and
mutual exchange of the endocyclic tins of V. In the
limit of rapid interconversion, the time-averaged struc-
ture for IV yields one exo- and two endocyclic tin
resonances; while the time-averaged structure for V
produces one endo- and two exocyclic tin signals. The
16 tin resonances that would be produced by the time-
averaged mixed distannoxane dimers agree with the
number of observed resonances. In addition, the pre-
dicted number of resonances for each of the time-aver-
aged structures is in agreement with the observed
distribution of resonances among the mixed distannox-
ane dimers. The time-averaged structures for the Cl:Br
mixed distannoxane dimers, resulting from the rapid
interconversion of ladder structures, are represented
pictorially in Fig. 6.
interconverting ladder structures, IlI%, would produce
two endocyclic and two exocyclic tin resonances. Simi-
larly, rapid interconversion of the ladder structures
Scheme 1.
Describing the distannoxane dimers as time-averaged
ladder structures permits the straightforward assign-
ment of the 119Sn-NMR spectra of the binary
[nBu2SnCl]2O/[nBu2SnBr]2O system. Assignment of reso-
nances associated with the 3:1 mixed distannoxane
dimer, I, is accomplished by comparison with
{[nBu2SnCl]2O}2 (4Cl). The Sn(2%) environment of I,
I(2%), which closely approximates the exocyclic tin sites
of 4Cl, is assigned to the resonance, located at −90.6
ppm. It follows that the −87.5 ppm resonance is due
to I(2). The chemical shift of I(1) is expected to be
between the chemical shifts observed for the endocyclic
tins of 4Cl and 4Br. Therefore, the resonances located
at −136.8 and −145.1 ppm are assigned to I(1) and
I(1%), respectively. Similarly, comparison of the shifts of
II and 4Br leads to the assignment of the 1:3 mixed
distannoxane dimer. Using the chemical shift differ-
ences observed between I and II the resonances of the
2:2 mixed distannoxane dimers are readily assigned
(Table 2).
Attempts to freeze out the intradimeric distannoxane
process and obtain individual ladder structures were
unsuccessful. The low temperature behavior of the
119Sn-NMR
spectrum
of
the
[nBu2SnCl]2O/
[nBu2SnBr]2O binary system ([Sn]T=2.0 M) using a 1:3
CHCl3:CFCl3 solvent system, is shown in Fig. 7. As the
temperature is lowered, the linewidths of the tin reso-
nances increase; however, a slight preferential broaden-
ing is observed for the endocyclic tins. The observed
linewidths of the mixed distannoxane dimers do not
broaden significantly compared to the resonances of the
parent dimers, 4Cl and 4Br. Therefore, the observed
broadening of the resonances as the temperature is
lowered is due to shortening of the spin–spin relaxation
Fig. 6. Pictorial representation of the time-averaged dimeric distan-
noxane structures resulting from the intradimeric distannoxane pro-
cess, i.e. rapid interconverting ladder pairs.

D.L. Tierney et al. / Journal of Organometallic Chemistry 620 (2001) 211–226
219
Table 2
119Sn-NMR spectral parameters of distannoxane dimers present in binary mixtures of [nBu2SnCl]2O and [nBu2SnBr]2O
c
n
Dimer
119Sn (l ppm) a
ꢀ J(119Sn, 119Sn)ꢀ (Hz) b
Sn(1)
Sn(1%)
Sn(2)
Sn(2%)
2J(1,1%)
2J(1,2)
2J(1,2%)
2J(1%,2)
2J(1%,2%)
4J(2,2%)
4Cl
4Br
I
−142.6
−134.0
−136.8
−131.5
−139.6
−129.6
−139.3
(−142.6) d
(−134.0)
−145.1
−141.9
(−139.6)
−148.6
−89.8
−82.9
−87.5
−82.6
−87.8
−88.4
−82.1
(−89.8)
(−82.9)
−90.6
22.4 e
44.4 e
28.9
73.3
80.0
93.6
79.4
96.3
90.9
78.5
(73.3)
(80.0)
70.0
94.1
56.3
(73.3)
(80.0)
58.4
80.9
(56.3)
60.1
(73.3)
(80.0)
76.7
32.5 e
36.1 e
33.9
II
−88.5
40.8
58.0
35.8
f
III
IV
V
(−87.8)
(−88.4)
−91.0
35.0 e
35.5 e
35.0 e
(96.3)
(60.1)
(76.1)
f
(90.9)
76.1
(−139.3)
(78.5)
34.2
a The chemical shift scale is referenced using an internal shift standard, TMT=0.0 ppm. The shifts are reported for the equimolar
[nBu2SnCl]2O/[nBu2SnBr]2O mixture in benzene-d6 at 298 K with [Sn]T=2.0 M. Variations in the chemical shifts due to composition are less than
0.02 ppm.
b Unless specified the 119Snꢀ119Sn couplings were determined from the 119Sn 1D INADEQUATE spectrum acquired on the binary mixtures. The
sign of the couplings was not determined. The uncertainty in nJ(119Sn,119Sn) is estimated to be 90.4 Hz.
c The time-averaged structures and the numbering scheme are depicted in Fig. 6.
d Chemical shifts and couplings in parentheses are due to symmetry-related environments.
e nJ(119Sn,119Sn)=nJ(119Sn,117Sn)[g(119Sn)/g(117Sn)].
f Unobservable coupling between magnetically equivalent nuclei, in which nJ(119Sn,117Sn) is not detected due to spectral overlap.
times, T2, and not the result of slowing the intradimeric
distannoxane process.
Integration of the 119Sn-NMR spectrum of the
equimolar mixture reveals that the concentrations of
the 2:2 mixed distannoxane dimers are equivalent. This
result indicates that Cl and Br exhibit nearly identical
donor abilities towards tin. The lack of preference for
the bridging sites implies that positional ordering of the
halogens within the mixed Cl–Br distannoxane dimers
does not exist. More importantly, this observation sug-
gests that the redistribution reaction results in a ran-
dom distribution of the mixed distannoxane dimers. In
Fig. 8, the observed concentrations of the distannoxane
dimers, as a function of initial [nBu2SnCl]2O concentra-
tion, are compared to the concentrations predicted by a
statistical distribution. From the figure, it is clear that
the observed concentrations of the distannoxane dimers
are well represented by a random distribution of the
halogens. A statistical redistribution of the halogens
dictates that the concentrations of the individual ladder
structures of an interconverting pair, are also equal.
Consequently the observed 119Sn and 13C chemical
shifts as well as the 119Snꢀ119Sn scalar couplings are
expected to be simple averages of the corresponding
ladder values.
3.2. 13C spectrum
Assignment of the 13C-NMR spectrum of the
[nBu2SnCl]2O/[nBu2SnBr]2O binary system is achieved
using the same method employed to assign the 119Sn-
NMR spectrum, i.e. varying the relative concentrations
Fig. 7. 111.96 MHz 119Sn-NMR spectrum of the equimolar
of the reactant distannoxanes. The a-carbon region,
32–38 ppm, of the 75.51 MHz 13C-NMR spectrum of
the equimolar mixture in benzene-d6 is shown in Fig. 9.
[nBu2SnCl]2O/[nBu2SnBr]2O mixture with [Sn]T=2.0 M as a function
of temperature. The solvent system is a 1:3 ratio of CHCl3:CFCl3 by
volume.

220
D.L. Tierney et al. / Journal of Organometallic Chemistry 620 (2001) 211–226
when the metal center is tin, there is substantial experi-
mental evidence supporting a halide-catalyzed exchange
mechanism [42,70–73]. In this step-wise complex for-
mation process [38], initial dissociation of the dimers in
which halide and cationic tin species are liberated, is
followed by rapid reassociation of the cationic tin spe-
cies with a different halide (Scheme 3). In the case of
distannoxanes it has been suggested that ligand redistri-
bution occurs via a monomer/dimer mechanism, which
relies on the establishment of equilibria between the
fluxional dimers and their constituent monomers [24].
In order to explain the reaction products of the
[nBu2SnCl]2O/[nBu2SnBr]2O binary system the mono–
dimer mechanism must account for the formation of
the mixed monomer intermediate, ClnBu2SnOSnnBu2Br.
Using non-fluxional ladder structures, the mixed
monomer can be formed only if the reactant monomers,
[nBu2SnCl]2O and [nBu2SnBr]2O, undergo intermolecu-
lar ligand exchange. The combined processes define the
multi-step mechanism suggested by Gross [24]. The
fluxional description of the distannoxane dimers yields
the mixed monomer without resorting to the incorpora-
tion of an additional step (Scheme 4). An important
feature of both the bimolecular mechanism of Scheme 2
Fig. 8. The observed room temperature concentrations of the distan-
noxane dimers in the binary mixtures as a function of initial concen-
tration of {[nBu2SnCl]2O}2, Xo4Cl: ×, X4Cl; +, X4Br; ꢀ, XI; 2, XII;
ꢁ, X2:2=XIII+XIV+XV; XIII=XIV=XV. The curves represent the
concentrations predicted by a statistical distribution of the halogens.
The identity of endo- and exocyclic a-carbon environ-
ments are confirmed by correspondence to the tin sites
using 1J(119Sn,13C) (Table 3). At 298 K, both endocyclic
and exocyclic a-carbon resonances of III and IV as well
as the endocyclic a-carbon resonances of I and II
exhibit increased linewidth compared to the other a-
carbon resonances. Raising the temperature to 323 K
results in chemical shift changes and the preferential
broadening of the endocyclic a-carbon resonances of I,
II, III and IV, while the exocyclic a-carbon resonances
of III and IV narrow. At 348 K the endocyclic a-carbon
resonances of I, II, III, and IV have sufficiently broad-
ened, nearly disappearing into the baseline. This behav-
ior indicates that an additional fluxional process is
occurring in I, II, III and IV. Further discussion of this
fluxional process is deferred to the following sections.
3.3. Redistribution mechanism
It is generally assumed that ligand redistribution
reactions involving metal centers proceed via an inter-
molecular ligand exchange process resulting from bi-
molecular collisions in which a four-centered transition
state is formed and subsequently undergoes an inter-
change of the ligands (Scheme 2) [38–41]. However,
Fig. 9. The a-carbon region of the 75.51 MHz 13C-NMR spectrum of
the equimolar [nBu2SnCl]2O/[nBu2SnBr]2O mixture with [Sn]T=2.0
M in benzene-d6 as a function of temperature.

D.L. Tierney et al. / Journal of Organometallic Chemistry 620 (2001) 211–226
221
Table 3
13C-NMR spectral parameters of distannoxane dimers present in binary mixtures of [nBu2SnCl]2O and [nBu2SnBr]2O a
b
Dimer
13C (l ppm)
a-C(1)
1J(119Sn, 13C) (Hz)
a-C(1%)
a-C(2)
a-C(2%)
1J(1,C)
1J(1%,C)
1J(2,C)
1J(2%,C)
4Cl
4Br
I
33.63
37.09
34.87
36.62
35.38
36.22
35.38
(33.63) c
(37.09)
34.01
35.73
(35.38)
34.49
32.84
36.74
34.65
36.66
34.75
34.73
36.57
(32.84)
(36.74)
32.92
601
602
600
597
601
594
601
(601)
(602)
606
607
(601)
610
574
552
562
552
562
562
553
(574)
(552)
574
II
34.82
562
III
IV
V
(34.75)
(34.73)
33.00
(562)
(562)
574
(35.38)
(601)
a The chemical shift scale is referenced using an internal shift standard, TMS=0.0 ppm. The shifts are reported for the equimolar
[nBu2SnCl]2O/[nBu2SnBr]2O mixture in C6D6 with [Sn]T=2.0 M. Variations in the chemical shifts due to composition are less than 0.05 ppm.
Uncertainty in 1J(119Sn,13C) is estimated to be 91 Hz.
b The time-averaged structures and the numbering scheme are depicted in Fig. 6.
c Chemical shifts and couplings in parentheses are due to symmetry-related environments.
To distinguish between the bimolecular and the
halide-catalyzed mechanisms, 13C EXSY spectra were
obtained after the addition of iodine. If the operative
and the halide-catalyzed mechanism of Scheme 3 are
single halogen exchanges; whereas, the monomer/dimer
mechanism is characterized by the scrambling of endo-
and exocyclic tin environments. It must be noted that
none of the transient intermediates associated with
either the halide-catalyzed process or the monomer/
dimer mechanism are detected in the NMR spectrum.
The operative ligand redistribution mechanism is de-
termined with the aid of 13C exchange spectroscopy.
The a-carbon region of the 13C EXSY spectrum of the
equimolar [nBu2SnCl]2O/[nBu2SnBr]2O mixture ([Sn]T=
2.0 M) acquired using a mixing time of 1 s is shown in
Fig. 10. The intermolecular exchange crosspeaks, which
are connected by solid lines in the figure, are consistent
with the replacement of the ligands one halogen at a
time and is indicative of both the bimolecular and the
halide catalyzed mechanisms. Crosspeaks between
endo- and exocyclic a-carbon environments are not
observed confirming that the integrity of the tin sites is
maintained during the mixing time of the experiment,
~m=1 s. The mixing time used to obtain the 13C EXSY
spectrum presented in Fig. 10 is sufficiently long pro-
ducing crosspeaks consistent with the replacement of
two halogens in a single step. These crosspeaks are
absent at shorter mixing times, ~m=0.5 s, indicating
they are produced by an indirect intermolecular ex-
change, i.e. consecutive single halogen exchanges. Not
all of the possible crosspeaks that are dictated by the
intermolecular exchange of the halogens are observed.
The inability to detect a given crosspeak associated
with this process is due to spectral overlap, insufficient
concentration of a given dimer and/or the increased
linewidths of endocyclic a-carbons of I, II, III and IV.
The remaining crosspeaks in the 13C EXSY spectrum
arise from the mutual exchange of the endocyclic sites
of I and II confirming the presence of an additional
intramolecular exchange process.
Scheme 2.
Scheme 3.

222
D.L. Tierney et al. / Journal of Organometallic Chemistry 620 (2001) 211–226
Scheme 4.
process for the redistribution of the halogens is the
halide-catalyzed mechanism (Scheme 3), it is expected
that the addition of iodine would impede the inter-
molecular exchange by scavenging the halides via for-
mation of complex ions, I2Br− and I2Cl−. Halogen
exchange in the Me2SnCl2/Me2SnBr2 binary system
([Cl]T+[Br]T=2.0 M) is significantly retarded at 0.03
M I2, slowing the exchange rates by nearly two orders
of magnitude [42]. Reduction in the exchange rate
would lead to intermolecular exchange crosspeaks with
diminished intensities. The presence of halide impurities
accelerates the intermolecular exchange of the halogens
in dialkyl tin(IV) dihalides [42,74]. Addition of 3.1
mmol of nBu4NBr to a 1 ml solution containing an
equimolar mixture of Me2SnCl2/Me2SnBr2 increases the
The three crosspeaks observed at (−139.3, −148.6),
(−129.6, −139.3), and (−87.8, −88.4) and the asso-
ciated symmetrical crosspeaks are evidence for the in-
terconversion of III and IV. The remaining crosspeak
pair, (−129.6, −148.6) and (−148.6, −129.6), is due
to a two-step exchange process that results in the
mutual exchange of the endocyclic sites of IV. This
intramolecular exchange process, which occurs via rota-
tion of the oxygenꢀexocyclic tin bond results in the
exchange rate by
a factor of 30 [42]. In the
[nBu2SnCl]2O/[nBu2SnBr]2O binary mixture, it is not
known whether or not halide impurities are present.
However, it is anticipated that at 0.25 M I2 excess
iodine is present and halide ions resulting from possible
impurities would be in the form of complex ions, I2X−.
With the addition of 23 and 190 mg of iodine ([I2]=
0.03 and 0.25 M, respectively), the intensities of
the crosspeaks arising from the intermolecular
halogen exchange remained unchanged. Thus, in the
[nBu2SnCl]2O/[nBu2SnBr]2O binary system, intermolecu-
lar halogen exchange occurs via the bimolecular mech-
anism (Scheme 2).
3.4. Intramolecular halogen exchange
A more complete picture of the intramolecular ex-
change process responsible for the broadening of the
endocyclic a-carbon resonances, is obtained from the
119Sn EXSY spectrum of the equimolar [nBu2SnCl]2O/
[nBu2SnBr]2O mixture. The 119Sn EXSY spectrum (Fig.
11) exhibits six crosspeak pairs. The crosspeaks located
at (−136.8, −145.1) and (−131.5, −141.9) as well as
their symmetrical partners, are due to the mutual ex-
change of the endocyclic tins of I and II, respectively.
Fig. 10. The a-carbon region of the 75.51 MHz 13C EXSY spectrum
of the equimolar [nBu2SnCl]2O/[nBu2SnBr]2O mixture with [Sn]T=2.0
M in benzene-d6 at 328 K, acquired with ~mix=1.0 s. Intermolecular
exchange crosspeaks are connected with solid lines, while dashed lines
connect crosspeaks arising from an indirect (two-step) intermolecular
halogen exchange. Crosspeaks resulting from an intramolecular pro-
cess are connected with dot–dash lines, see text.

D.L. Tierney et al. / Journal of Organometallic Chemistry 620 (2001) 211–226
223
only responsible for the mutual exchange of the endo-
cyclic sites of I and II, but it also leads to the intercon-
version of III and IV. When rotation of the
oxygenꢀexocyclic tin bond results in the interchange of
like halogens, as in 4Cl, 4Br, I, II and V, the NMR
spectra are unaffected.
Analysis of the 119Sn EXSY spectrum yields the rate
constants for the oxygenꢀexocyclic tin bond rotation.
The rate constants are a measure of the rate of magne-
tization transfer from one tin site to another. In the
case of the mutual exchange of the endocyclic tins of I
and II, the observed rate constants are a direct measure
of the rate of rotation about an oxygenꢀexocyclic tin
bond. However, the observed rate of magnetization
transfer between the exocyclic tins of III and IV, is
twice the rate of oxygenꢀexocyclic tin bond rotation.
The factor of two is a result of multiple magnetization
transfer pathways, i.e. rotation about either oxygen-ex-
ocyclic tin in III and IV bond leads to interconversion.
The measured rates of the oxygenꢀexocyclic tin bond
rotation are presented in Table 4. It is observed that the
rate of bond rotation is nearly three times faster in
chloroform-d than in benzene-d6. At room temperature
the rate constants for dimers III and IV, which are
rotational isomers of each other, are equal. As the
temperature is increased a divergence in the rate of
bond rotation for III and IV is observed, such that
k(IV)\k(III). This divergence indicates that at ele-
vated temperatures, there is a slight shift in the equi-
librium favoring isomer III. At 348 K, in benzene-d6,
the concentration ratio between III and IV is found to
be [III]/[IV]=1.1290.06. However, due to spectral
overlap, this equilibrium shift could not be detected by
integration of the quantitative spectrum. In benzene-d6
the Gibbs free energy of activation at 298 K, DG†298, is
approximately 70.8 kJ mol−1; while DG2†98 is observed
to be approximately 2.7 kJ mol−1 lower in chloroform-
d. In the same solvent and over the narrow temperature
range studied, EA and DH† are approximately equal for
each dimer. However, for each dimer the activation
parameters, EA and DH†, are smaller in chloroform-d
compared to those obtained in benzene-d6, indicating
that the activation barrier for oxygenꢀexocyclic tin
bond rotation is lower in chloroform-d than in benzene-
d6.
Fig. 11. 111.96 MHz 119Sn EXSY spectrum of the equimolar
[nBu2SnCl]2O/[nBu2SnBr]2O mixture with [Sn]T=2.0 M in benzene-d6
at 318 K, acquired with ~mix=75 ms. Crosspeaks connected with
solid lines are due to the reversible configurational rearrangement, i.e.
rotation of the exocyclic tin–oxygen bond. An indirect (two-step)
exocyclic tin–oxygen bond rotation gives rise to the exchange of the
endocyclic tins of IV (dashed line).
Scheme 5.
interchange of the halogens directly bound to the same
exocyclic tin (Scheme 5). This fluxional process is the
reversible analog to the configurational rearrangement
proposed by Michel and coworkers [27]. Initially, Sn* is
in a Sn(1) environment with different pendent halogens.
Rotation of the oxygenꢀexocyclic tin bond swaps the
halogens of Sn(2), causing the environments of Sn(1)
and Sn(1%) to interchange. This reversible process is not
4. Conclusions
The dimeric ladder structure, which the tetraorgan-
odistannoxanes are known to adopt in the solid phase,
is retained in noncoordinating solvents. Dimers of
[R2SnXY]2O in which the X and Y ligands are the same
or exhibit similar donor abilities toward tin, rapidly
rearrange in solution by an intradimeric process. This

224
D.L. Tierney et al. / Journal of Organometallic Chemistry 620 (2001) 211–226
Table 4
Rate of oxygenꢀexocyclic tin bond rotation in mixed Cl:Br distannoxane dimers a
b
Temperature (K)
Rate constant (s−1
)
Solvent:benzene-d6
Solvent:CDCl3
k(I)
k(I)
2.2690.09
k(II)
k(III)
2.4590.10
5.8490.28
12.290.9
25.491.5
45.392.4
78.392.4
k(IV)
2.5590.07
k(II)
7.7390.65
14.991.1
31.091.9
63.292.9
10998
k(III)
k(IV)
298
308
318
328
338
348
2.6290.12
6.9390.22
13.790.9
25.991.5
57.892.7
10497
6.7090.40
6.1090.21 14.091.0
7.5190.58 7.5090.57
5.8390.40
12.990.7
25.791.5
47.592.6
86.293.7
14.591.1
26.791.8
46.192.2
79.193.8
15.491.2
30.291.8
52.492.7
91.894.7
12.990.8
27.891.6
48.192.7
87.893.8
28.791.8
55.992.8
95.294.4
EA (kJ mol−1
DH† (kJ mol−1
DS† (J mol−1
DG†298 (kJ mol−1
)
5794
5394
−57913
71.090.1
5995
5795
−46914
70.690.1
5394
5094
−66914
70.890.2
5594
5294
−59914
70.790.1
5394
5194
−58914
68.390.2
5494
5295
−53913
67.990.2
4794
4594
−78914
68.090.2
4994
4794
−70914
68.090.2
)
)
)
a Rate constants are determined for solutions with [Sn]T=2.0 M.
b Reported errors are estimates of the 95% confidence level.
fluxional process involves interconverting ladder struc-
ture that proceeds via migration of the bridging and
nonbridging bonds coupled with motion of the atoms
that define the ladder structure. The ladder structure is
also capable of undergoing an additional intramolecu-
lar fluxional process, involving rotation about the oxy-
genꢀexocyclic tin bond. When either ligand exhibits
significantly higher donor ability towards tin, these
dynamic processes do not occur resulting in a frozen
dimer with the better donor ligand occupying the bridg-
ing sites. The difference in donor abilities also accounts
for the composition of binary mixtures containing dis-
similar monofunctional distannoxanes, [R2SnX]2O and
[R2SnY]2O. Depending on the relative donor abilities of
the X and Y ligands, the equilibrium composition of
the equimolar mixture ranges from a statistical distribu-
tion of X:Y mixed distannoxane products to a single
2:2 mixed dimer in which the bridging sites are occu-
pied by the better donor ligand.
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Acknowledgements
The authors wish to acknowledge the contribution of
Robert L. Henry and Joseph A. Grappin in the prelim-
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also wish to thank Dr Tony Bilecki and Bruker Instru-
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