Stannylplumbylenes: Bonding between tetravalent tin and divalent lead

Article abstract of DOI:10.1039/c2cc30687e

Depending on stoichiometry, reactions of the mixed valence Sn(0)/Sn(iii) compound Sn(SnAr₃)₂ (1) (Ar = C₆H ₄(OⁱPr)₂-2,6) with the likewise substituted plumbylene PbAr₂ (3) afforded either the homoleptic distannylplumbylene Pb(SnAr₃)₂ (4) or the heteroleptic arylstannylplumbylene Pb(Ar)SnAr₃ (5), a valence isomer of a stannaplumbene. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2cc30687e

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Stannylplumbylenes: bonding between tetravalent tin and divalent leadwz
¨
Christian Drost,* Peter Lonnecke and Joachim Sieler
Received 31st January 2012, Accepted 22nd February 2012
DOI: 10.1039/c2cc30687e
Depending on stoichiometry, reactions of the mixed valence
Sn(0)/Sn(III) compound Sn(SnAr3)2 (1) (Ar = C6H4(OiPr)2-2,6)
with the likewise substituted plumbylene PbAr2 (3) afforded
either the homoleptic distannylplumbylene Pb(SnAr3)2 (4) or the
heteroleptic arylstannylplumbylene Pb(Ar)SnAr3 (5), a valence
isomer of a stannaplumbene.
tin compounds with direct metal to metal bonding, but led to
13
homoleptic stannylenes SnR2,12 distannenes (SnR2)2 or cyclo-
tristannanes (SnR2)3,14 depending on steric bulk of R, respectively.
Furthermore, compound 1—being monomeric and V-shaped in
solid state—in solution appears to reversibly disintegrate into
arylstannylstannylene Sn(Ar)SnAr3 2. This was corroborated by
119Sn NMR solution studies of 1 in C6D6, revealing two signals
in a 1 : 1 signal ratio at d ꢀ14 and 1296 ppm only with
119Sn–119Sn coupling constants (1JSnSn = 9011 Hz).
Mixed valence homo- or heterometallic compounds of the
heavier group 14 elements with direct metal to metal bonding
received growing attention in recent years.1 Compounds of
type RM–MR3 (M = Ge, Sn; R = organic group) have long
been anticipated as transient valence isomers of dimetallenes
and initially been proven by trapping reactions.2 Only a few
stable homobimetallic compounds have been prepared to date
by either (i) combination of organic groups with high or low/high
steric requirements [Ar0GeGe(tBu)3 (Ar0 = C6H3Mes2-2,6;
Mes = C6H2Me3-2,4,6); Ar*SnSn(Ph)2Ar*, Ar*SnSn(Me)2Ar*
The interconversion between the mixed valence compounds
Sn(SnAr3)2 1 and Sn(Ar)SnAr3 2 presumably occurs via an
invertible aryl rearrangement and elimination/addition process
of a transient stannylene [SnAr2], which has not been detected
so far. However, pertinent results to this work involving a
phenyl group migration have been obtained by Power et al.;
they described a reversible valence isomer equilibrium between
the stannylene Sn(Ph)Ar* and the valence isomer of its dimer,
the stannylstannylene Ar*SnSn(Ph)2Ar* in solution.4c Aryl
migration as a part of ligand exchange between two different
homoleptic stannylenes has also been exploited in the synthesis
of heteroleptic stannylenes.15 Furthermore, our results parallel
observations from Masamune and Sita, who established an
interconversion between the cyclic trimer c-(SnTrip2)3 I in
solid state and the corresponding distannene (SnTrip2)2 II in
solution,14b considering Sn(SnAr3)2 1 and Sn(Ar)SnAr3 2 as
valence isomer analogues of I and II, respectively (Scheme 1).
Bearing in mind that 1 is subject to disintegrative processes
in solution, we reacted 1 with the diarylplumbylene PbAr2 3 in
different stoichiometries. Whereas the reaction of 1 with 3 in a
2 : 3 ratio afforded the dark green crystalline distannylplumbylene
Pb(SnAr3)2 4, the purple arylstannylplumbylene Pb(Ar)SnAr3
5 was isolated from a reaction in a 1 : 3 ratio. Starting from either
5 or 4, both compounds may also be interconverted by addition
of 1 or 3 in corresponding amounts, respectively (Scheme 2).y
Formally, the generation of 5 (or 4) can be rationalized
by the reaction of a transient stannylene [SnAr2] with the
plumbylene PbAr2 3 (or 5), yielding the corresponding
i
(Ar* = C6H3Trip2-2,6; Trip = C6H2 Pr3-2,4,6)],3,4 (ii) incorpo-
rating the M–M unit into stabilizing organic ring structures
(M = Sn,5a Pb5b), or (iii) employing additional coordination of the
divalent metal atoms.6 Heterobimetallic or homo-/heterotrimetallic
compounds are even scarce and include Ar0SnGe(tBu)3 or
3
Sn(SnPh2Ar*)2,7 apart from the hypersilylated bisgermyl-
stannylene and -plumbylene M[Ge(SiMe3)3]2 (M = Sn, Pb),8
or N-heterocyclic carbene stabilized analogues, e.g. [{(Me)-
C(iPr)N}2C]Sn[Sn(SiMe3)3]2.9 In contrast, dicoordinated
plumbylenes with Sn–Pb bonds have not been described so
far. Moreover, for organometallic compounds structurally
authenticated bonding between tin and lead in general is
limited to few hexaarylstannylplumbanes (e.g. Ph3SnPbPh3).10
Recently, we described the unforeseen isolation of the brownish-
red distannylstannylene Sn(SnAr3)2 1 [Ar = C6H3(OiPr)2-2,6],
which we obtained in a straightforward salt metathesis reaction
of SnCl2 with two equivalents of LiAr in Et2O and crystallisation
from hexane.11 The formation of 1 was surprising and relevant,
since the employment of only one type of aryl ligand in
comparable reactions did thitherto not yield mixed valence
Universitat Leipzig, Institut fur Anorganische Chemie, Johannisallee 29,
¨
¨
04103 Leipzig, Germany. E-mail: cdrost@uni-leipzig.de;
Fax: +49 341 9736199; Tel: +49 341 9736130
w This communication is dedicated to Professor Reinhard Kirmse on
the occasion of his 65th birthday.
z Electronic supplementary information (ESI) available: Experimental
details, analytical and crystal data with structure refinement for 3. CCDC
864029 (3), 864030 (4) and 864028 (5). For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/c2cc30687e
Scheme 1
c
3778 Chem. Commun., 2012, 48, 3778–3780
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Scheme 2
Scheme 3
Fig. 1 Thermal ellipsoid plot (40% probability surface) of 4 (hydrogen
atoms and methyl groups are omitted for clarity). Selected bond
lengths (A) and angles (1): Pb(1)–Sn(1) 2.9283(4), Pb(1)–Sn(2) 2.9483(6),
Sn(1)–Pb(1)–Sn(2) 114.26(1), Cipso–Sn(1)/Sn(2)–Pb(1) 97.65(8)–123.65(8).
stannylene-plumbylene adduct, which suffers aryl migration from
lead to tin to yield the mixed valence compounds (Scheme 3 for 5).
It is noteworthy to mention that attempts to synthesize 4 via
the conventional salt metathesis route with two equivalents of
lithium stannate Ar3SnLi11 and PbCl2 failed and resulted in
the formation of elemental lead and Ar3SnCl.
Sn(1)–Pb(1)–Sn(2) skeleton and an interligand angle of 114.26(1)1.
The Pb(1)–Sn(1)/Sn(2) distances amount to 2.9283(4)/2.9483(6) A
and the geometries at the two tetra-coordinated Sn(1)/Sn(2)
atoms are heavily distorted with Cipso–Sn(1)/Sn(2)–Pb(1) angles
ranging from 97.65(8)1 to 123.65(8)1. A peculiar feature—similar
to the structural array of the tritin analogue 1—is the distorted
transoid arrangement of two oxygen atoms in proximity to the
central lead atom with Pb(1)ꢁ ꢁꢁO(6)/O(7) separations of 2.968(2)
and 2.965(2) A [O(6)/O(7)–Pb(1)–Sn(1)/Sn(2): 66.82(4)/66.31(4)1;
O(6)–Pb(1)–O(7): 178.57(6)1].
Heteroleptic plumbylene 5 is also monomeric in solid state
(Fig. 2) with a more relaxed Sn(1)–Pb(1)–C(37) interligand angle of
102.94(12)1 and a slightly diminished Sn(1)–Pb(1) bond length of
2.8784(4) A compared with 4. In contrast, the Sn–Pb bond lengths
determined for stannylplumbanes Ph3SnPbPh3 are significantly
shorter with 2.809(2) and 2.848(2) for two independent molecules,
respectively.10 The three innerligand angles [Cipso–Sn(1)–Pb(1)]
at the tetracoordinated tin atom Sn(1) vary from 99.87(11)1
to 125.60(12)1. In 5 only one intramolecular Pb(1)ꢁ ꢁ ꢁO(1)
separation becomes apparent with 2.876(3) A.
Compounds 3–5 have been characterised by multinuclear
NMR-spectroscopy. Each of their 207Pb NMR spectra exhibit
signals at d = 5859 (3), 7853 (4) and 6905 ppm (5); the
119Sn NMR spectra of the latter two compounds comprise
resonances at d = 1273 (4) and 1270 ppm (5) relating to the
tetravalent tin atoms, respectively. While for 5 an 119Sn–207Pb
coupling (1JSnPb = 19971 Hz) is revealed, for 4 the 119Sn–207Pb
coupling (1JSnPb = 22560 Hz) and additional 117Sn–119Sn
satellites (2JSnSn = 3254 Hz) are observed, confirming its
1
structural integrity in solution. These JSnPb coupling values
are markedly larger than those determined for hitherto isolated
stannylplumbanes, e.g. Ph3SnPbPh3 (1JSnPb = 3469 Hz).10 The
119Sn NMR stannyl group signals for 4 and 5 are located
strikingly downfield (d = ca. 1270 ppm) and in sharp contrast
to the corresponding 119Sn NMR shift of Sn(Ar)SnAr3
2
(d = ꢀ14 ppm), which is indicative of considerably deshielded
The observed PbꢁꢁꢁO distances for 4 and 5 are clearly within the
sum of the van der Waals radii (3.54 A)19 but exceed the sum of
their covalent radii (2.14 A)19 and also go beyond comparable
lead-bonded tetracoordinated tin centres.
The 207Pb NMR resonance for 3 at d = 5859 ppm is in
agreement with the generally observed range of 207Pb NMR
chemical shifts for organolead(II) compounds, although at the upper
end but still downfield of PbMes*[CH2CMe2C6H3(tBu)2-2,5]
(5067 ppm; Mes* = C6H2(tBu)3-2,4,6),16 or Pb[C6H3(NMe2)2-2,6]2
(3919 ppm),17 with the latter reflecting a higher than two-
coordinate Pb(II) environment. Substitution of each aryl-ligand in
3 by SnAr3-groups leads to a stepwise (ca. 1000 ppm) downfield
shifting of the 207Pb NMR signal from d = 5859 (3) to 6905 (5)
and 7853 ppm (4). The latter two values may be compared
with the 207Pb NMR shift of the related arylsilylplumbylene
Pb(R0)Ar* [R0 = Si(SiMe3)3],18 which is located more down-
field at d = 10745 ppm, but consistent with d = 7545 ppm for
Pb(R0)(C6HMe3-4,5,6-tBu-2), being dimeric in solid state.16
Single crystals of complexes 4 and 5 suitable for X-ray diffrac-
tion were obtained from benzene (4) and hexane (5) solutions;
their molecular structures are provided in Fig. 1 (4) and 2 (5) with
selected bond lengths and bond angles given in the figure
caption.z The molecular structure of the distannylplumbylene 4
(Fig. 1) shows it to be mononuclear with a V-shaped
Fig. 2 Thermal ellipsoid plot (40% probability surface) of 5 (hydrogen
atoms and methyl groups are omitted for clarity). Selected bond
lengths (A) and angles (1): Pb(1)–Sn(1) 2.8784(4), Pb(1)–Cipso 2.285(4),
Sn(1)–Pb(1)–Cipso 102.94(12), C0ipso–Sn(1)–Pb(1) 99.87(11)–125.60(12).
c
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Chem. Commun., 2012, 48, 3778–3780 3779

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Pbꢁ ꢁ ꢁO distances described earlier, e.g. 2.518(2)
A
A
for
for
(Rint = 0.0580), final R indices (I > 2s(I)): R1 = 0.0296, wR2 = 0.0574,
R indices (all data): R1 = 0.0575, wR2 = 0.0608. Data were collected on a
Stoe-IPDS imaging plate diffractometer (f scan mode) at 213(2) K with
Mo-Ka radiation (l = 0.71073 A). All data were solved by direct
methods and refined by full-matrix least squares on F2.23
[Pb{C6H2[P(O)OEt2]2-2,6-tBu-4}Cl]n
21
or 2.598(12)
20
[Pb{C(SiMe3)2(SiMe2OMe)}Cl]2
with the latter Pbꢁ ꢁ ꢁO
contact regarded as not being particularly strong. Nevertheless—
despite the long PbꢁꢁꢁO distances in 4 and 5, a distorted PbꢁꢁꢁO
arrangement and 207Pb NMR shifts adequate for dicoordinated
lead(II) atoms—some weak interactions can possibly not be
denied. Moreover, they may be supportive for the formation of
mixed valence compounds with only one ligand-type, provided
that three aryl groups can join together at the tetravalent tin
atoms without steric constraints.
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In conclusion, the reaction of the distannylstannylene
Sn(SnAr3)2 1 with the monomeric plumbylene PbAr2 3 afforded
the dark green distannylplumbylene Pb(SnAr3)2 4 and a purple
arylstannylplumbylene Pb(Ar)SnAr3 5 depending on stoichiometric
ratios, respectively. Their molecular structures feature unique
bonding between tetracoordinated Sn and dicoordinated Pb. We
deduce that 2,6-dialkoxyphenylene-based ligands have potential in
heavy group 14 element chemistry22 and the presented compounds
may be the forerunners of a series in mixed valence chemistry.
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y All manipulations were performed using Schlenk line techniques
under an atmosphere of dry nitrogen. Synthesis of 4: a solution of 1
(1.44 g, 2.42 mmol) in benzene (20 ml) was added to a solution of 3
(2.45 g, 1.62 mmol) in an equal amount of benzene at 0 1C. The
reaction mixture became dark green and was stirred for 30 min at 0 1C
and further 5 h at room temperature. After filtration the filtrate was
concentrated upon crystallization and set aside at room temperature and
at 6 1C to afford dark green crystals of 4. A further crop of 4 was isolated
from the mother liquid to give in total 3.6 g (93%). Characterization: mp
(nitrogen, sealed capillary): 191 1C (decomp.); NMR (C6D6, TMS, 300 K):
1H (400 MHz), d = 0.81 (d, J = 6.0 Hz, 18H, CHMe2), 0.93
(d, J = 6.0 Hz, 18H, CHMe2), 1.01 (d, J = 6.0 Hz, 18H, CHMe2),
1.18 (d, J = 6.0 Hz, 18H, CHMe2), 4.05 (q, J = 6.0 Hz, 4H, CHMe2),
4.19 (q, J = 6.0 Hz, 8H, CHMe2), 6.45 (d, J = 8.0 Hz, 6Hmeta), 6.57
(d, J = 8.0 Hz, 6Hmeta), 7.08 (t, J = 8.0 Hz, 6Hpara); 13C{1H} (100.6 MHz),
d = 21.0, 21.7, 22.8, 23.7 (CH2Me), 68.2, 70.0 (CH2Me), 104.6, 105.0, 145.9,
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22560 Hz, 2JSnSn = 3254 Hz, 2Sn); 207Pb{1H} (83.7 MHz), d = 7853;
EI-MS: m/z (%): 699 (8) [Ar3Sn]+, 504 (41) [Ar2Sn]+. Elemental
analysis calcd (%) for C72H102O12PbSn2: C 53.91, H 6.41; found (%):
C 54.18, H 6.53. Synthesis of 5: crystalline 5 was prepared from 1 and 3
in a 3:1 stoichiometry as described above for the synthesis of 4. After
filtration and removal of benzene the crystalline residue was recrystallized
from hexane to get purple 5 in a total of 89% yield. Characterization: mp
(nitrogen, sealed capillary): 134 1C (decomp.); NMR (C6D6, TMS, 300 K):
1H (400 MHz), d = 0.95 (d, J = 6.0 Hz, 36H, CHMe2), 1.02 (d, J =
6.0 Hz, 12H, CHMe2), 4.19 (q, J = 6.0 Hz, 8H, CHMe2), 6.52 (d, J =
8.0 Hz, 6Hmeta), 6.03 (d, J = 8.0 Hz, 2Hmeta), 7.08 (t, J = 8.0 Hz,
3Hpara); 7.15 (t, J = 8.0 Hz, 1Hpara); 13C{1H} (100.6 MHz), d = 21.9,
22.2 (CH2Me), 68.8, 69.0 (CH2Me), 104.5, 109.2, 145.1 (CipsoSn),
161.4, 163.7, 212.9 (CipsoPb) (phenyl); 119Sn{1H} (149.2 MHz),
d = 1270 (1JSnPb = 19971 Hz); 207Pb{1H} (83.7 MHz), d = 6905;
EI-MS: m/z (%): 699 (62) [Ar3Sn]+. Elemental analysis calcd (%) for
C48H68O8PbSn: C 52.46, H 6.24; found (%): C 52.63, H 6.38.
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z Crystallographic data for 4: empirical formula C90H111O12PbSn2,
%
FW = 1829.36, crystal system triclinic, space group P1, a = 12.272(2),
18 S. Hino, M. Olmstead, A. D. Phillips, R. J. Wright and P. P. Power,
Inorg. Chem., 2004, 43, 7346.
b = 15.377(3), c = 25.439(4) A, a = 96.850(3)1, b = 102.309(3)1, g =
109.506(3)1, V = 4325.5(12) A3, Z = 2, rcalcd = 1.405 Mg mꢀ3, 53 958
reflections collected, 22 639 independent reflections (Rint = 0.0284),
final R indices (I > 2s(I)): R1 = 0.0301, wR2 = 0.0706, R indices (all data):
R1 = 0.0490, wR2 = 0.0801. Data were collected on a Siemens CCD
diffractometer (SMART) at 208(2) K with Mo-Ka radiation (l =
0.71073 A). Crystallographic data for 5: empirical formula C51H75O8PbSn,
FW = 1141.99, crystal system monoclinic, space group P2(1)/n,
a = 26.9800(17), b = 12.5178(6), c = 31.719(2) pm, a = 901,
19 B. Cordero, V. Go
´
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b = 97.596(8)1, g = 901, V = 10618.3(11) A3, Z = 8, rcalcd
=
1.429 Mg mꢀ3, 61 579 reflections collected, 18 514 independent reflections
crystal structure analysis, University of Gottingen, Germany, 1997.
¨
c
3780 Chem. Commun., 2012, 48, 3778–3780
This journal is The Royal Society of Chemistry 2012