Persistent radicals of trivalent tin and lead

Article abstract of DOI:10.1021/ic801198p

In this report we present synthetic, crystallographic, and new electron paramagnetic resonance (EPR) spectroscopic work that shows that the synthetic route leading to the recently reported, first persistent plumbyl radical ?PbEbt₃ (Ebt = ethylbis(trimethylsilyl)silyl), that is, the oxidation of the related PbEbt₃-anion, was easily extended to the synthesis of other persistent molecular mononuclear radicals of lead and tin. At first, various novel solvates of homoleptic potassium metallates KSnHyp ₃ (4a), KPbHyp₃ (3a), KSnEbt₃ (4b), KPblDt ₃ (3c), and KSnlbt₃ (4c) (Hyp = tris(trimethylsilyl)silyl, lbt = isopropylbis(trimethylsilyl)silyl), as well as some heteroleptic metallates, such as [Li(OEt₂)₂][SnⁿBuHyp ₂] (3d), [Li(OEt₂)₂][PbⁿBuHyp ₂] (4d), [Li(thf)₄][PbPhHyp₂] (3e), and [K(thf)₇][PbHyp₂{N(SiMe₃)₂}] (3f), were synthesized and crystallographically characterized. Through oxidation by tin(II) and lead(II) bis(trimethylsilyl)amides or the related 2,6-di-tert-butylphenoxides, they had been oxidized to yield in most cases the corresponding radicals. Five novel persistent homoleptically substituted radicals, that is, ?SnHyp₃ (2a), ?PbHyp₃ (1a), ?SnEbt₃ (2b), ?Snlbt₃ (2c), and ?Pblbt ₃ (1c), had been characterized by EPR spectroscopy. The stannyl radicals 2a and 2c as well as the plumbyl radical 1c were isolated as intensely colored crystalline compounds and had been characterized by X-ray diffraction. Persistent heteroleptically substituted radicals such as ?PbHyp ₂Ph (1e) or ?PbHyp₂Et (1g) had also been generated, and some selected EPR data are given for comparison. The plumbyl radicals ?PbR₃ exhibit a clean monomolecular decay leading to the release of a temperature-dependent stationary concentration of branched silyl radicals. They may thus serve as tunable sources of these reactive species that may be utilized as reagents for mild radical silylations and/or as initiators for radical polymerizations. We present EPR-spectroscopic investigations for the new tin- and lead-containing compounds giving detailed insights into their electronic and geometric structure in solution, as well as structural studies on the crystalline state of the radicals, some of their anionic precursors, and some side-products.

Full text of DOI:10.1021/ic801198p

Inorg. Chem. 2008, 47, 9965-9978
Persistent Radicals of Trivalent Tin and Lead
Marco Becker,† Christoph Fo¨rster,† Christian Franzen,† Johannes Hartrath,† Enzio Kirsten,†
Jo¨rn Knuth,† Karl W. Klinkhammer,*,† Ajay Sharma,‡ and Dariush Hinderberger*,‡
Institut fu¨r Anorganische and Analytische Chemie, Johannes Gutenberg-UniVersita¨t Mainz,
Duesbergweg 10-14, 55128 Mainz, Germany, and Max-Planck-Institut fu¨r Polymerforschung,
Ackermannweg 10, 55128 Mainz, Germany
Received June 28, 2008
In this report we present synthetic, crystallographic, and new electron paramagnetic resonance (EPR) spectroscopic
work that shows that the synthetic route leading to the recently reported, first persistent plumbyl radical •PbEbt3
(Ebt ) ethylbis(trimethylsilyl)silyl), that is, the oxidation of the related PbEbt3-anion, was easily extended to the
synthesis of other persistent molecular mononuclear radicals of lead and tin. At first, various novel solvates of
homoleptic potassium metallates KSnHyp3 (4a), KPbHyp3 (3a), KSnEbt3 (4b), KPbIbt3 (3c), and KSnIbt3 (4c) (Hyp
) tris(trimethylsilyl)silyl, Ibt ) isopropylbis(trimethylsilyl)silyl), as well as some heteroleptic metallates, such as
[Li(OEt2)2][SnnBuHyp2] (3d), [Li(OEt2)2][PbnBuHyp2] (4d), [Li(thf)4][PbPhHyp2] (3e), and [K(thf)7][PbHyp2{N(SiMe3)2}]
(3f), were synthesized and crystallographically characterized. Through oxidation by tin(II) and lead(II) bis(trimeth-
ylsilyl)amides or the related 2,6-di-tert-butylphenoxides, they had been oxidized to yield in most cases the
corresponding radicals. Five novel persistent homoleptically substituted radicals, that is, •SnHyp3 (2a), •PbHyp3
(1a), •SnEbt3 (2b), •SnIbt3 (2c), and •PbIbt3 (1c), had been characterized by EPR spectroscopy. The stannyl radicals
2a and 2c as well as the plumbyl radical 1c were isolated as intensely colored crystalline compounds and had
been characterized by X-ray diffraction. Persistent heteroleptically substituted radicals such as •PbHyp2Ph (1e) or
•PbHyp2Et (1g) had also been generated, and some selected EPR data are given for comparison. The plumbyl
radicals •PbR3 exhibit a clean monomolecular decay leading to the release of a temperature-dependent stationary
concentration of branched silyl radicals. They may thus serve as tunable sources of these reactive species that
may be utilized as reagents for mild radical silylations and/or as initiators for radical polymerizations. We present
EPR-spectroscopic investigations for the new tin- and lead-containing compounds giving detailed insights into their
electronic and geometric structure in solution, as well as structural studies on the crystalline state of the radicals,
some of their anionic precursors, and some side-products.
Introduction
and stannyl (E ) Sn) radicals.2 However, persistent or stable
mononuclear radicals of all heavier tetrels3 but the heaviest
congener lead have been known for more than 30 years.4
Starting with the pioneering work of Lappert and co-workers
Recently, we reported the synthesis of the first stable
plumbyl radical •PbEbt3 (1b, Ebt ) ethylbis(trimethylsilyl)-
silyl).1 It had been obtained through oxidation of the silyl-
substituted potassium plumbanide KPbEbt3 (3b) by molecular
lead(II) salts. Analogous reactions had been utilized shortly
before by Sekiguchi et al. in the synthesis of the homologous
series •E(SitBu2Me)3 of silyl (E ) Si), germyl (E ) Ge),
(2) (a) Sekiguchi, A.; Fukawa, T.; Nakamoto, M.; Lee, V. Ya.; Ichinohe,
M. J. Am. Chem. Soc. 2002, 124, 9865. (b) Sekiguchi, A.; Fukawa,
T.; Lee, V. Ya.; Nakamoto, M. J. Am. Chem. Soc. 2003, 125, 9250.
(c) Chrostowska, A.; Dargelos, A.; Graciaa, A.; Bayle`re, P.; Lee, V.
Ya.; Nakamoto, M.; Sekiguchi, A. Organometallics 2008, 27, 2915.
(d) For a recent review, see Inoue, S.; Ichinohe, M.; Sekiguchi, A.
Organometallics 2008, 27, 1358.
* To whom correspondence should be addressed. E-mail: hinderberger@
mpip-mainz.mpg.de (D.H.), klink@uni-mainz.de (K.W.K.).
(3) The term “tetrels” had been suggested by the IUPAC as common name
for the elements of group 14. To avoid terms such as “anions of the
elements of group 14”, it will be used together with derived
denominations for derivatives such as tetryls (ER3), tetrelanes (ER4),
or tetrelanides (MER3) throughout this article.
†
Johannes Gutenberg-Universita¨t Mainz.
Max-Planck-Institut fu¨r Polymerforschung.
‡
(1) (a) Fo¨rster, C.; Klinkhammer, K. W.; Tumanskii, B.; Kru¨ger, H.-J.;
Kelm, H. Angew. Chem. 2007, 119, 1174–1177. (b) Angew. Chem.,
Int. Ed. 2007, 46, 1156-1159.
(4) For recent reviews, see (a) Power, P. P. Chem. ReV. 2003, 103, 789.
(b) Lee, V. Y.; Sekiguchi, A. Eur. J. Inorg. Chem. 2005, 1209.
10.1021/ic801198p CCC: $40.75  2008 American Chemical Society
Inorganic Chemistry, Vol. 47, No. 21, 2008 9965
Published on Web 09/30/2008

Becker et al.
who synthesized radicals such as •E[N(SiMe3)2]3 or •E[CH-
(SiMe3)2]3,5 such radicals have been the subject of several
papers in the last three decades. In the 1990s a revival of
radical chemistry of group 14 had been observed. Several
oligonuclear radicals with varying degree of delocalization
of the unpaired electron had been prepared since then.6
Our early electron paramagnetic resonance (EPR) mea-
surements on the mononuclear plumbyl radical 1b in solution
revealed strong, broad, and structureless resonances for 1b
at a g value markedly larger than for all related radicals of
the lighter congeners in group 14.1 A second signal was
present at even higher g-values. It was at first attributed to
a second, lead-centered radical species. The expected satellite
pattern according to hyperfine-coupling within the isotopomer
207PbEbt3 could not readily be found, however. Therefore,
we at first proposed a very large hyperfine-coupling constant
for this isotopomer lying outside the range of the employed
spectrometer. In this report we present a detailed analysis
of new EPR data of 1b which reveals that the second signal
does not derive from a further radical species but is part of
the low-field manifold of the 207Pb hyperfine splitting pattern.
Additionally, we present synthetic and analytic work that
shows that the synthetic route mentioned above, that is, the
oxidation of appropriate tetrel anions, may easily be extended
to the synthesis of further related persistent homoleptic
molecular radicals of lead and tin: •PbHyp3 (1a), •PbIbt3 (1c),
•SnHyp3 (2a), •SnEbt3 (2b), and •SnIbt3 (2c) (Hyp )
Si(SiMe3)3, Ibt ) isopropylbis(trimethylsilyl)silyl), and by
slight modifications may also give access to related hetero-
leptic species such as SnHyp2Ph and PbHyp2Ph. We fur-
thermore present detailed continuous wave EPR (CW EPR)
and low-temperature, echo-detected EPR data on the four
selected persistent molecular tetryl radicals 1a, 1b, 2a, and
2c, (see Figure 1) which allow the characterization of the
structure and dynamics of these molecules in fluid and frozen
solution.
Figure 1. Molecular structures of the four persistent or stable lead- and
tin based radicals that were characterized in detail using EPR spectroscopy.
were stored under Ar over 3 Å molecular sieves. All other reagents
not mentioned in the following were synthesized by procedures
according to the literature or purchased and used without further
purification.
Sn(OC6H3 Bu2-2,6)2 and Pb(OC6H3 Bu2-2,6)2.11 To a 350 mL
n-pentane solution of Sn[N(SiMe3)2]2 (16.66 g, 37.9 mmol) or
Pb[N(SiMe3)2]2 (20.01 g, 37.9 mmol) was added 2,6-di-tert-
butylphenol (15.64 g, 75.8 mmol), and the resulting yellow (Sn)
or orange (Pb) suspension was stirred overnight. The solid was
separated by filtration through a sintered glass filter (G4), washed
three times with 20 mL of toluene and subsequently dried in
dynamic vacuum. The products are obtained as bright yellow or
orange powder in yields of 92 and 94%, respectively and are only
sparingly soluble in organic solvents.
t
t
t
Sn(OC6H3 Bu2-2,6)2. Anal. Calcd for C28H42O2Sn: C, 63.53; H,
1
8.00. Found: C, 63.64; H, 8.12. H NMR (C6D6): δ 7.48 (d, 4H),
6.74 (t, 2H), 1.62 (s, 18H).
t
Pb(OC6H3 Bu2-2,6)2. Anal. Calcd for C28H42O2Pb: C, 54.43; H,
1
Experimental Section
6.85. Found: C, 54.56; H, 6.96. H NMR (C6D6): δ 7.36 (d, 4H),
6.90 (t, 2H), 1.58 (s, 18H).
Syntheses. Preparation of Compounds. Reactions were per-
formed under an argon atmosphere using standard Schlenk lines,
and the resulting air-sensitive compounds were manipulated under
nitrogen atmosphere within a drybox. Syntheses of the potassium
salts KHyp, KEbt, and KIbt, as well as those of the molecular Sn(II)
and Pb(II) precursors Sn[N(SiMe3)2]2, Pb[N(SiMe3)2]2, SnHyp2 (5),
and PbHyp2 (6), were carried out as reported previously.7-10 Diethyl
ether and pentane were distilled from LiAlH4. Toluene and
tetrahydrofuran were distilled from Na/benzophenone. All solvents
[Li(thf)4][PhHyp2Pb] (3e). To a stirred yellow 10 mL di-n-
butylether solution of phenyllithium (1.91 mmol) at 0 °C was added
quickly a green 6 mL solution of 6 (1.34 g, 1.91 mmol) in diethyl
(7) Harris, D. H.; Lappert, M. F. J. Chem. Soc., Chem. Commun. 1974,
895.
(8) Marschner, C. Eur. J. Inorg. Chem. 1998, 221.
(9) Kayser, C.; Fischer, R.; Baumgartner, J.; Marschner, C. Organome-
tallics 2002, 21, 1023.
(10) Klinkhammer, K. W.; Schwarz, W. Angew. Chem., Int. Ed. 1995, 34,
1334.
t
(11) Sn(OC6H3 Bu2-2,6) was synthesized before in moderate yield by
2
t
(5) See, for example, (a) Davidson, P. J.; Hudson, A.; Lappert, M. F.;
Lednor, P. W. J. Chem. Soc., Chem. Commun. 1973, 829. (b) Cotton,
J. D.; Cundy, C. S.; Harris, D. H.; Hudson, A.; Lappert, M. F.; Lednor,
P. W. J. Chem. Soc., Chem. Commun. 1974, 651. (c) Lappert, M. F.;
Lednor, P. W. AdV. Organomet. Chem. 1976, 14, 345. (d) Hudson,
A.; Lappert, M. F.; Lednor, P. W. J. Chem. Soc., Dalton Trans. 1976,
2369.
(6) See, for example, (a) Olmstead, M. M.; Pu, L.; Simons, R. S.; Power,
P. P. Chem. Commun. 1997, 1595. (b) Olmstead, M. M.; Simons, R. S.;
Power, P. P. J. Am. Chem. Soc. 1997, 119, 11705. (c) Pu, L.; Haubrich,
S. T.; Power, P. P. J. Organomet. Chem. 1999, 582, 100.
reaction of SnI4 and KOC6H3 Bu2-2,6 (detailed procedure not given)
and had been characterized crystallographically: Barnhart, D. M.;
Clark, D. L.; Watkin, J. G. Acta Cryst. C 1994, 50, 702. Related tetrel
aryloxides with an additional 4-Me substituent had also been described
before. However, the better solubility of the corresponding alkali metal
aryloxides in organic solvents, make them less suitable as oxidizers
in the synthesis of tetryl radicals. For preparation and properties see:
C¸ etinkaya, B.; Gu¨mru¨kc¸u¨, I.; Lappert, M. F.; Atwood, J. L.; Rogers,
R. D.; Zaworotko, M. J. J. Am. Chem. Soc. 1980, 102, 2086. C¸ etinkaya,
B.; Gu¨mru¨kc¸u¨, I.; Lappert, M. F.; Atwood, J. L.; Shakir, R. J. Am.
Chem. Soc. 1980, 102, 2088.
9966 Inorganic Chemistry, Vol. 47, No. 21, 2008

Persistent Radicals of TriWalent Tin and Lead
ether (prepared at -60 °C). The resulting orange solution was
evaporated to dryness, and the obtained solid was redissolved in 2
mL of tetrahydrofuran. After storing at -60 °C for 18 h crystalline
yellow plumbanide 3e was isolated in 88% yield. 1H NMR (C6D6):
δ 8.75 (d, 2H), 7.12 (t, 2H), 7.01 (t, 1H), 0.50 (d, 4H); 13C NMR
(C6D6): 123.3, 127.8, 147.5, 5.8; 29Si NMR (C6D6): -122.9 (SiSi3),
-4.8 (SiMe3).
CH3), 4.0 (SiMe3 [1JSiC 41.1 Hz]); 29Si NMR (C6D6): -78.2 (SiSi3
[1JPbSi 1378 Hz]), -9.6 (SiMe3 [2JPbSi 20.4 Hz]); 207Pb-NMR (C6D6):
-1119.8.
3c: K(OEt2)2Pb[SiiPr(SiMe3)2]3: 67% yield; 1H NMR (C6D6): δ
3.14 (q, 8H, OCH2CH3), 1.48 (d, 18H), 1.04 (sept, 3H), 1.01 (t,
12H, OCH2CH3), 0.51 (s, 18H, SiMe3); 29Si NMR (C6D6): -79.3
(SiSi3 [1JPbSi 1378 Hz]), -9.3 (SiMe3).
n
n
Li(thf)2SnHyp2 Bu (4d) and Li(thf)2PbHyp2 Bu (3d). A 0.5
mL portion of a 2.0 M solution of n-butyllithium in n-hexane was
added at -60 °C to 10 mL of diethyl ether. Under intense stirring,
a precooled 5 mL dark brown (respectively blue) solution of 5 in
n-pentane (0.71 g, 1 mmol) (respectively 6 (0.65 g, 1 mmol) was
quickly added to this mixture, and an immediate color change to
yellow (respectively orange) was observed. After evaporating to
dryness the resulting solid was recrystallized from n-pentane. The
lithium stannanide 4d (respectively plumbanide 3d) was obtained
as a pale yellow (respectively orange) crystalline material in 65%
(respectively 86%) yield. 4d: 1H NMR (C6D6): δ 3.26 (q, 8H,
OCH2CH3), 1.81 (m, 2H), 1.62 (m, 2H), 1.56 (m, 2H), 1.12 (t, 3H,
CH3 (nBu)), 0.92 (t, 12H, OCH2CH3), 0.51 (s, 54H, Hyp); 13C NMR
(C6D6): 65.2 (Et2O), 39.0, 30.1, 14.5 (Et2O), 14.1, 5.9 (Hyp); 29Si
NMR (C6D6): -142.1 (SiSi3), -7.2 (SiMe3 [1JSiC 42.1 Hz]); 119Sn-
4b: K(OEt2)3Sn[SiEt(SiMe3)2]3: 85% yield; 1H NMR (C6D6): δ
3.14 (q, 12H, OCH2CH3), 1.22 (q, 6H), 1.03 (t, 9H), 1.02 (t, 18H,
OCH2CH3), 0.41 (s, 18H, SiMe3); 29Si NMR (C6D6): -76.1 (SiSi3
[1JPbSi 1378 Hz]), -11.0 (SiMe3); 119Sn-NMR (C6D6): -702.6.
4c: K(OEt2)2Sn[SiiPr(SiMe3)2]3: 79% yield; 1H NMR (C6D6): δ
3.16 (q, 8H, OCH2CH3), 1.44 (d, 18H), 1.67 (sept, 3H), 1.01 (t,
12H, OCH2CH3), 0.50 (s, 18H, SiMe3); 13C NMR (C6D6): 65.8
(OCH2CH3), 25.6 (PbCH2CH3), 15.4 (OCH2CH3), 4.7 (SiMe3); 29Si
NMR (C6D6): -63.5 (SiSi3), -10.7 (SiMe3).
(OEt2)2KPbHyp3 (3a′) and [K(thf)6][PbHyp3] (3a′′). Solvent-
free KHyp (1.43 g, 5 mmol) was suspended in 10 mL of n-pentane
and cooled to -60 °C. Then 10 mL of diethyl ether was added. A
cooled (-60 °C) 15 mL n-pentane solution of 6 (3.56 g, 5 mmol)
was slowly added to the KHyp suspension, and a color change to
red-violet as well as precipitation of a violet solid was observed.
The reaction mixture was stirred for 2 h, and the cold solution
evaporated to dryness. The obtained solid was recrystallized from
n-pentane. 3a′ was isolated as red-violet platelets in 89% yield.
Recrystallization from tetrahydrofuran or direct synthesis in this
solvent yielded the thf-solvate 3a′′ as violet rod-shaped crystals in
86% yield.
7
NMR (C6D6): -345 [1JLiSn 534 Hz]; Li NMR (C6D6): 0.54. 3d:
1H NMR (C6D6): δ 3.22 (q, 8H, OCH2CH3), 2.22 (m, 2H), 2.05
(m, 2H), 1.60 (m, 2H), 1.15 (t, 3H), 0.94 (t, 12H, OCH2CH3), 0.55
(s, 54H, Hyp); 13C NMR (C6D6): 65.1 (Et2O), 40.2, 32.4, 14.9
(Et2O), 13.9, 6.9 (Hyp); 29Si NMR (C6D6): -142.6 (SiSi3), -5.4
(SiMe3 [1JSiC 42.1 Hz]);7Li NMR (C6D6): 1.36.
General Procedure for the Generation of Heteroleptic
Stannyl and Plumbyl Radicals •EHyp2R. A solution of 2 mmol
of the stannylene 5 or the plumbylene 6 in 10 mL of n-pentane,
respectively, was added to about 15 mL solution of 1.1 equiv of
the appropriate hydrocarbyllithium at -80 °C. This had been freshly
prepared by mixing the commercial available alkane or ether
solution (approximately 1-2 mol/L) with 10 mL of precooled
tetrahydrofuran. Under intense stirring, the solution was allowed
to warm to room temperature and kept at 20 °C for 1 h. After
filtering out the precipitate, the resulting solution was treated with
0.55 equiv of solid Pb[OC6H3tBu2-2,6]2, giving a deeply colored
solution of the desired radical, which could be identified by its EPR
signals. 1H NMR spectra recorded on this solution generally show
the presence of varying amounts of diamagnetic byproducts.
General Procedure for the Synthesis of the Homoleptic
Tetrelanides KE[SiR(SiMe3)2]3 3b, 3c, 4b, and 4c. An equimolar
mixture of the appropriate silane SiR(SiMe3)3 (20 mmol) and KOtBu
(20 mmol) was dissolved in 100 mL of tetrahydrofuran and stirred
for 12 h. The resulting yellow or orange solution was evaporated
to dryness and heated in dynamic vacuum to 30-40 °C for 1d.
The residue was then dissolved in diethyl ether, cooled to -60 °C,
and a suspension of E(OAr*)2 (6.67 mmol) in 100 mL of diethyl
ether was added slowly during 3 h. The yellow to red reaction
mixture was stirred for an hour at -30 °C and afterward filtered at
-50 °C. The volume of the filtrate was reduced to about 10 mL at
-50 °C, and 20 mL of n-pentane was added. After 2 d at -60 °C
the tetrelanide had precipitated as a yellow to red crystalline material
(67-85% overall yield). Additional amounts of the tetrelanide may
be obtained from the mother liquor by repeated concentration and
cooling.
1
3a′: H NMR (C6D6): δ 3.21 (q, 8H, OCH2CH3), 1.05 (t, 12H,
OCH2CH3), 0.57 (s, 18H, SiMe3); 13C NMR (C6D6): 65.9 (OCH2-
CH3), 14.2 (OCH2CH3), 4.6 (SiMe3); 29Si NMR (C6D6): -129.9
(SiSi3), -9.5 (SiMe3).
[K(thf)6][SnHyp3] (4a′′) and [K(thf)4(C6H6)2][K(SnHyp3)2]
(4a′′′). To a 20 mL diethyl ether solution of KHyp (0.315 g, 1.1
mmol) was slowly added a 10 mL diethyl ether solution of 5 (0.713
g, 1.1 mmol) at -60 °C. The brown solution was stirred for 2 h at
-60 °C and further 15 min at room temperature. After concentration
in vacuum to a volume of 5 mL, tetrahydrofurane (2 mL) was added.
Storing at -60 °C for 2 d afforded needle-shaped orange crystals
of 4a′′ in 65% yield.
Thorough drying of 4a′′ for 12 h at 30 °C and subsequent
recrystallization from a saturated benzene solution (25°/6 °C) finally
gave orange platelets of the potassiate 4a′′′.
4a′′: 1H NMR (C6D6): δ 0.55 (s, 81H, SiMe3); 13C NMR (C6D6):
7.2 (SiMe3); 29Si NMR (C6D6): -134.1 (SiSi3), -7.2 (SiMe3).
4′′′: 1H NMR (C6D6): δ 0.53 (s, 81H, SiMe3); 13C NMR (C6D6):
6.9 (SiMe3); 29Si -NMR (C6D6): -132.3 (SiSi3), -7.5 (SiMe3).
[K(thf)7][PbHyp2{N(SiMe3)2}] (3f). Twelve milliliters of a
solution of KN(SiMe3)2 (0.31 g, 1.54 mmol) in a 5:1 mixture of
tetrahydrofuran and diethyl ether was added to a green 5 mL diethyl
ether solution of 6 (1.10 g, 1.54 mmol) at -60 °C, resulting in an
immediate color change to red-brown. After 2 h the solution was
warmed to room temperature and stirred for an additional 15 min.
After solvent reduction to 5 mL and storing at -60 °C for 2 d, 3f
had been obtained as red platelets (85% yield), which, however,
proved not suitable for X-ray analysis. A suitable red single
crystalline block was finally obtained by recrystallization from
toluene.1H NMR (C6D6): δ 3.52 (m, thf), 1.40 (m, thf), 0.41 (s,
Hyp), 0.17 (s, N(SiMe3)2); 13C NMR (C6D6): 68.3 (thf), 25.8 (thf),
3b: K(OEt2)2Pb[SiEt(SiMe3)2]3: 72% yield; 1H NMR (C6D6): δ
3.16 (q, 8H, OCH2CH3), 1.44 (m, 6H), 1.35 (t, 9H), 1.00 (t, 12H,
OCH2CH3), 0.48 (s, 18H, SiMe3); 13C NMR (C6D6): 65.8
(OCH2CH3), 17.8 (PbCH2CH3), 15.4 (OCH2CH3), 11.2 (PbCH2-
Inorganic Chemistry, Vol. 47, No. 21, 2008 9967

Becker et al.
8.3 (Hyp), 7.2 (N(SiMe3)2); 29Si NMR (C6D6): -31.6 (SiSi3), -20.1
(NSi2), 7.9 (SiSi3). UV-vis (n-pentane): 238 (sh, 3.09 × 104), 303
(sh, 1.36 × 104), 341 (sh, 5.96 × 103), 455 (1.61 × 103).
Scheme 1. General Synthetic Routes Leading to Alkali Metal
Tetrelanidesa
•SnHyp3 (2a). 2a was either synthesized by oxidation of pure
stannanide 4a or preferably in a one-pot fashion without isolating
4a prior to oxidation. The latter procedure started from a 10 mL
diethyl ether solution of solvated KHyp (K(thf)0.8Hyp, 1.53 g, 4.44
t
mmol). A suspension of Sn(OC6H3 Bu2-2,6)2 (0.87 g, 1.64 mmol)
in 10 mL of diethyl ether was added to the KHyp solution in four
portions during 2 h at -60 °C. The resulting orange-red mixture
was warmed to -30 °C and stirred for another 30 min. Then again
t
10 mL of a suspension of Sn(OC6H3 Bu2-2,6)2 (0.42 g, 0.80 mmol)
in diethyl ether was added in four portions at -60 °C during 1 h.
The resulting dark brown mixture was warmed to -30 °C and
stirred for 45 min. After subsequent cooling to -60 °C it was
filtered and the filtrate was concentrated at -50 °C in a dynamic
vacuum to 10 mL. From this solution very pure 2a was isolated by
crystallization at -60 °C for 2 d in 49% yield (0.7 g 0.81 mmol).
UV-vis (n-pentane): 343 (1.46 × 103), 457 (sh, 100).
Generation of •PbHyp3 (1a). Five milliliters of an n-pentane
solution of Pb[N(SiMe3)2]2 (0.46 g, 0.88 mmol) was added to a 20
mL tetrahydrofuran solution of (OEt2)2KPbHyp3 (3a′) (2 g, 1.76
mmol) at -60 °C and stirred for 2 h. After filtration at -60 °C the
mixture was concentrated to 5 mL in a dynamic vacuum at -20
°C. By storing at -60 °C for 1 week most of the formed PbHyp2
(6) was isolated as the blue-green thf-solvate Hyp2Pb•thf (6a) and
plumbanide 3f. EPR experiments on the dark green mother liquor
prove the presence of significant amounts of 1a. By NMR, further
6, the octasilane Hyp2, and small amounts of KN(SiMe3)2 could
be detected as diamagnetic contaminations.
a
(a) homoleptic silyl-substituted tetrelanides KER3 (E ) Sn, Pb; R )
Hyp, Ebt, Ibt), i.e. the series 3a-c and 4a-c. EX2 ) E[N(SiMe3)2]2 or
preferably E(OC6H3-tBu-2,6)2 in aprotic coordinating solvents; (b) hetero-
leptic tetrelanides: dihypersilyltetrelandiyls EHyp2 are synthesized and
isolated first and then in a subsequent step are reacted with alkali metal
hydrocarbyls or other alkali metal salts MY.
CCDC 692387-692399 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif. The crystal parameters and
basic information related to data collection and structure refinement
are, however, summarized in Tables S1-S3 in the Supporting
Information.
In all of the listed compounds but hexastannane 7, all non-
hydrogen atoms were refined anisotropically while the positions
of all hydrogen atoms were generated with appropriate geometric
constraints and allowed to ride on their respective parent carbon
atoms with fixed isotropic thermal parameters. Compound 7 had
been obtained as very small crystals only (V < 10-3 mm3). Because
of weak reflection data only the tin and silicon atoms could be
refined anisotropically while the carbon atoms were refined with
isotropic thermal parameters.
EPR Spectroscopy. CW EPR spectra in fluid (T ) 293 K)
solution at X-band (∼9.4 GHz) were measured either on a Bruker
ELEXSYS 580 spectrometer with a rectangular cavity (4103TM,
Q-values typically 3000) and an Oxford open flow cryostat cooling
system or a Magnettech MiniScope MS200 benchtop CW EPR
spectrometer with a variable-temperature cooling/heating finger. The
tin and lead substances were dissolved in 1:1 mixtures of n-pentane
and n-heptane and subsequently transferred into EPR sample tubes
(quartz glass, 4 mm outer diameter tubes for the ELEXSYS 580
spectrometer, 3 mm outer diameter for the MiniScope MS200).
Echo-detected, field-swept EPR spectra were recorded with a
primary echo sequence (π/2)-τ-(π)-τ-echo while sweeping the
magnetic field using the same Bruker ELEXSYS 580 spectrometer
with a dielectric resonator (MD4EN, overcoupled to Q-values of
typically 100). The temperature was set to 20 K by cooling with
liquid helium using an Oxford cryostat and cooling system.
The samples were prepared as concentrated solutions (concentra-
tions typically >10 mM) and the sample volume was always large
enough to fill the complete resonator volume in the probehead
(>300 µL).
•SnEbt3 (2b), •SnIbt3 (2c), and •PbIbt3 (1c). Solid Sn(OC6H3tBu2-
t
2,6)2 (2.65 g, 5 mmol) or Pb(OC6H3 Bu2-2,6)2 (3.08 g, 5 mmol)
was added to a 50 mL diethyl ether solution of stannanide 4b (10
mmol), stannanide 4c (10 mmol), or plumbanide 3c (10 mmol) at
-60 °C, respectively. The reaction mixture was stirred at -60 °C
for 1 h and at -30 °C for 30 min. The resulting brown-red (2b
and 2c) or brown-green suspensions (1c) were filtered at -30 °C,
and the filtrates were concentrated to 5 mL in a dynamic vacuum.
After storing at -60 °C for 2 weeks (2c) or 2 days (1c), the
respective radicals 2c and 1c could be isolated as large orange-
yellow or small green platelets in 65% and 43% yield, while •Sn3Ebt
(2b) did not crystallize, but was isolated as an orange-yellow oil
(approximately 70% yield). Very small black crystals of the
hexaprisma-hexastannane 7 were embedded within the crystals of
2c.
X-ray Crystallographic Study. All crystals were covered with
inert oil at -50 °C and mounted to a nylon loop using an X-TEMP
2 system.12 They were then transferred to the diffractometer under
permanent cooling. X-ray diffraction experiments were carried out
on a Bruker SMART CCD diffractometer using Mo KR radiation
(λ ) 0.71073 Å). Data were collected at -80 or -100 °C with a
frame width of 0.5° either in ω or in φ. The diffraction frames
were integrated using the SAINT package and most were corrected
for absorption with MULABS.13,14 The structures were solved by
direct methods and refined by the full-matrix method based on F2
using the SHELXTL software package.15
(12) Kottke, T.; Stalke, D. J. Appl. Crystallogr. 1993, 26, 615–619.
(13) SMART Data Collection and SAINT-Plus Data Processing Software
for the SMART System (various versions); Bruker Analytical X-Ray
Instruments, Inc.: Madison, WI, 2000.
(14) Blessing, B. Acta Crystallogr. 1995, A51, 33.
(15) (a) Sheldrick, G. M. SHELXTL, Version 5.1; Bruker AXS: Madison,
WI, 1998. (b) Sheldrick, G. M. SHELXL-97; University of Go¨ttingen:
Go¨ttingen, Germany, 1997.
9968 Inorganic Chemistry, Vol. 47, No. 21, 2008

Persistent Radicals of TriWalent Tin and Lead
Figure 2. Molecular structures of the alkali metal tetrelanides 3a, 3c, 3d, 4b, and 4d forming contact-ion pairs. Selected bond lengths and angles are given
in [Å, deg]. (Et2O)2KPbHyp3 (3a′): K1-O1 2.596(6), K1-O2a 2.683(9), Pb1-K1 3.694(2), Pb1-Si1 2.851(2), Pb1-Si2 2.809(2), Pb1-Si3 2.837(2),
Si1-Pb1-Si2 106.02(7), Si1-Pb1-Si3 109.34(7), Si2-Pb1-Si3 107.98(7), Si1-Pb1-K1 113.55(6), Si2-Pb1-K1 116.37(6), Si3-Pb1-K1 103.35(6).
(Et2O)2KPbIbt3 (3c): K1-O2 2.60(2), K1-O1 2.77(2), Pb1-K1 3.607(4), Pb1-Si1 2.705(5), Pb1-Si2 2.722(5), Pb1-Si3 2.726(6), Si1-Pb1-Si2 105.4(2),
Si1-Pb1-Si3 105.8(2), Si2-Pb1-Si3 107.9(2), Si1-Pb1-K1 110.42(15), Si2-Pb1-K1 114.04(15), Si3-Pb1-K1 112.8(2). (Et2O)2LiPbnBuHyp2 (3d):
Li1-O1 1.906(11), Li1-O2a 1.874(15), Pb1-Li1 2.954(10), Pb1-C1 2.323(5), Pb1-Si1 2.726(2), Pb1-Si2 2.741(2), Si1-Pb1-Si2 113.18(5), C1-Pb1-Si1
93.15(15), C1-Pb1-Si2 96.02(15), C1-Pb1-Li1 120.6(3), Si1-Pb1-Li1 115.4(2), Si2-Pb1-Li1 115.3(2). (Et2O)3KSnEbt3 (4b): K1-O4 2.768(3), K1-O5
2.808(3), K1-O6 2.701(4), Sn1-K1 3.7234(9), Sn1-Si1 2.6738(8), Sn1-Si2 2.6442(8), Sn1-Si3 2.6735(9), Si2-Sn1-Si3 105.67(3), Si2-Sn1-Si1
104.82(3), Si3-Sn1-Si1 100.87(3), Si2-Sn1-K1 121.74(2), Si3-Sn1-K1 113.55(2), Si1-Sn1-K1 107.90(2). (Et2O)2LiSnnBuHyp2 (4d): Li1-O1 1.909(5),
Li1-O2a 1.895(11), Sn1-Li1 3.020(5), Sn1-C1 2.233(3), Sn1-Si1 2.6544(7), Sn1-Si2 2.6668(7), Si1-Sn1-Si2 114.29(2), C1-Sn1-Si1 94.15(8),
C1-Sn1-Si2 97.68(9), C1-Sn1-Li1 119.88(12), Si1-Sn1-Li1 113.23(11), Si2-Sn1-Li1 115.19(10).
Data Analysis. All spectral simulations were performed with
home-written programs in MATLAB (The MathWorks, Inc.)
employing the EasySpin toolbox for EPR spectroscopy.16 Simula-
tions of CW EPR spectra in fluid solution were performed using a
model, which is based on the slow-motion theory and a program
developed by Schneider and Freed as implemented in EasySpin.
These simulations can account for the effect of intermediate or slow
rotational diffusion of the radical on the EPR spectra.17 Low-
temperature, powder-type EPR spectra were simulated by directly
computing the resonance fields and transition probabilities over a
triangular orientational powder grid, again as implemented in
EasySpin.
Results and Discussion
The homoleptic silyl-substituted tetrelanides KE(SiR3)3 (E
) Sn {3a-c}, Pb {4a-c}; R ) Hyp, Ebt, Ibt) are obtained in
good to excellent yields by the reaction of 3 equiv of the
proper bulky alkali metal silanides with soluble lead(II) or
tin(II) salts such as E[N(SiMe3)2]2 or preferably E(OC6H3-
tBu-2,6)211 in aprotic coordinating solvents (Scheme 1a). For
E ) Pb, the oxidation of the potassium silanide is observed
as a side reaction, giving elementary lead and metal-free
silanes. The latter become the predominant products if
inorganic lead salts such as PbCl2 are used as starting
material.
In the absence of coordinating solvents or if the anion of
the alkali metal silanide reaches a certain size, the equilibrium
(2) in Scheme 1 (top) is markedly shifted to the left side. If
very large silanide anions are introduced, plumbanides cannot
be isolated any more. We have found that (from THF
solution) the critical size which still allows for the isolation
of solvated homoleptic potassium plumbanides is reached
The corresponding magnetic parameters (g- and hyperfine (A-)
tensor elements) found from the simulations of all tin- and lead-
centered radicals are given in Table 4.
(16) Stoll, S.; Schweiger, A. J. Magn. Reson. 2006, 178, 42–55; see http://
www.easyspin.org.
(17) Schneider, D. J.; Freed, J. H. In Biological Magnetic Resonance Vol.8:
Spin Labeling - Theory and Applications; Berliner, L. J., Reuben, L. J.,
Eds.; Plenum Press: New York, 1989.
Inorganic Chemistry, Vol. 47, No. 21, 2008 9969

Becker et al.
Scheme 2. Generation of the Persistent or Stable Lead- and Tin-Based
Radicals by Oxidation of the Alkali Metal Salts (Scheme 1) with
Appropriate Oxidants (See Text for Details)a
Scheme 3. Formation and Decay of the Hypersilyl-Based Lead Radical
1a As Corroborated by Chemical and NMR Analysis
a
Oxidation is achieved by molecular tin(II) or lead(II) salts such as
separated from the much better soluble tetrelanides, E(OC6H3-
tBu2-2,6)2 for most cases is the better choice.11 For plum-
banides even the 2,6-di-tert-butylphenol itself was found to
be a suitable oxidant, though here protolysis of E-Si bonds
has been detected as a side-reaction.19
E[N(SiMe3)2]2 and E(OAr*)2 or the phenol Ar*OH (Ar* ) C6H3-tBu2-
2,6); (a) homoleptic tetryl radicals; (b) heteroleptic tetryl radicals.
for the hypersilyl anion Si(SiMe3)3 (Hyp-). While the
-
respective stannanide KSnHyp3 (4a), probably because of
the stronger Sn-Si bonds, does not dissociate to SnHyp2
and KHyp to a notable extent, solutions of the plumbanide
KPbHyp3 (3a) exhibit broad H- and 13C NMR resonances
The yields of homoleptic tetryl radicals (series 1 and 2)
obtained by oxidation of the homoleptic tetrelanides (series
3 and 4) are good in most cases but far from quantitative
(according to NMR spectra of the reaction mixtures). Several
diamagnetic compounds, most of them still unidentified, can
be detected as side-products by NMR spectroscopy and
sometimes prevent the isolation of the tetryl radical. Trihy-
persilylplumbyl •PbHyp3 (1a) is only formed in a maximum
yield of about 20% since most of the introduced plumbylene
6 was recovered as the thf-adduct Hyp2Pb•thf (6a, Figure 6)
and the heteroleptic plumbanide 3f. As mentioned above and
depicted in Scheme 3, the introduced potassium trihypersi-
lylplumbanide (3a) is most probably present in an equilib-
rium mixture with KHyp and 6. Therefore, not only 3a is
oxidized to the desired plumbyl radical 1a, but also (or
preferably) KHyp is oxidized to yield the octasilane Hyp2.
By detracting KHyp from the equilibrium, 6 is formed and
precipitates as thf-adduct 6a in the presence of tetrahydro-
furan at lower temperatures or as plumbanide 3f by reaction
with KN(SiMe3)2 which in turn has been formed as side-
product from the metathesis of Pb[N(SiMe3)3]2 with KHyp.
Further PbHyp2 (6) may result from thermal decomposition
of •PbHyp3 (1a). EPR measurements on 1a solutions indicate
a high stationary concentration of the hypersilyl radical in
solution which is in accordance with the presumed dissocia-
tion of 1a into 6 and Hyp radicals.
1
with chemical shifts markedly depending on the concentra-
tion, and in the solid-state structure of the solvate (Et2O)2-
KPbHyp3 (3a′, Figure 2) very long Pb-Si bonds are
observed. Replacing one of the nine methyl groups of the
hypersilyl substituent for a tert-butyl group completely
prevents the formation of the respective plumbanide. Instead,
unreacted Pb[Si(SitBuMe2)(SiMe3)2]2 and K[Si(SitBuMe2)-
(SiMe3)2] are detected as the predominant species in solution
18
1
by H-, 13C-, and 29Si NMR experiments.
Heteroleptic tetrelanides are also available if bis(silyl)te-
trelandiyls are synthesized and isolated first and are then in
a subsequent step reacted with alkali metal hydrocarbyls or
other alkali metal salts (Scheme 1b). Following this route,
we were able to isolate several unsymmetrically substituted
tetrelanides bearing two hypersilyl groups and an alkyl, aryl,
or amido substituent. The synthesis is obviously restricted
to systems where the respective bis(silyl)tetrelandiyl pos-
sesses a sufficient kinetic stability to allow for its isolation.
Dihypersilyltetrelandiyls, SnHyp2 (5; as the dimer 52) and
PbHyp2 (6),10 are such species of sufficient stability and can
therefore be used for the synthesis of a range of heteroleptic
hypersilyl-substituted tetrelanides such as 3d, 3e, 3f, and 4d.
However, heteroleptic derivatives comprising less demanding
groups such as ethylbis(trimethylsilyl)silyl (Ebt) are hitherto
unknown, because tetrelandiyls such as Ebt2E are not
accessible under the conditions used for the preparation of
the hypersilyl derivatives 5 and 6.
Nevertheless, two plumbyl radicals, that is, 1b and 1c, are
isolated as almost pure compounds by recrystallization from
pentane or ether/n-pentane mixtures as green-brown crystal-
line solids. Crystalline stannyl (as well as silyl and germyl)
radicals had already been reported very recently for slightly
different substitution patterns by Sekiguchi et al.2b Here,
further species could be generated (Table 4) of which
The next step to tetryl radicals would be the oxidation of
the alkali metal salts with appropriate oxidants (Scheme 2).
We found that molecular tin(II) or lead(II) salts such as
E[N(SiMe3)2]2 or E(OC6H3-tBu2-2,6)2 can be used. Since the
alkali metal salts of 2,6-di-tert-butylphenol exhibit very low
solubility in weakly polar solvents and may thus be easily
(19) At this moment we do not know whether the oxidation of 3b by 2,6-
di-tert-butylphenol proceeds via single-electron transfer or via the
intermediate formation of the plumbane HPbEbt3 and a subsequent
thermal induced Pb-H bond fission.
(18) Fo¨rster, C.; Klinkhammer, K. W., to be published.
(20) Klinkhammer, K. W. Chem. Eur. J. 1997, 3, 1418.
9970 Inorganic Chemistry, Vol. 47, No. 21, 2008

Persistent Radicals of TriWalent Tin and Lead
Figure 3. Structures of the anions of the alkali metal plumbanides 3e and 3f. The counter cations have been omitted for clarity. Selected bond lengths and
angles are given in [Å, deg]. [Li(thf)4][PbPhHyp2] (3e): cation: Li-O (av.) anion: Pb1-Si1 2.7664(14), Pb1-Si2 274.81(12), Pb1-C31 2.324(4), Si1-Pb1-Si2
111.5(9), Si1-Pb1-C11 103.8(2), Si2-Pb1-C11 104.6(2). [K(thf)7]{Pb[N(SiMe3)2]Hyp2} (3f): cation: K-O1 2.861(12), K-O2 2.769(10), K1-O3 2.732(10),
K1-O4 2.755(12), K1-O5 2.753(13), K1-O6 2.846(12), K1-O7 2.755(12); anion: Pb1-Si1 2.872(3), Pb1-Si2 2.819(3), Pb1-N1 2.377(8), Si1-Pb1-Si2
111.5(9), Si1-Pb1-N1 103.8(2), Si2-Pb1-N1 104.6(2).
Figure 4. Structure of the ate-complex [K(thf)4(benzene)2][K(SnHyp3)2] (4a′′′) in the crystal. Selected bond lengths and angles are given in [Å, deg].
Cation: K2-O (av.) 2.67, K2-C013 3.215(3); anion: K1-Sn1 3.7202(1), K1-Sn1-Si1 110.85 (1), K1-Sn1-Si2 110.15(1), K1-Sn1-Si3 110.04(1),
Sn1-Si1 2.7222(4), Sn1-Si2 2.7301(4), Sn1-Si3 2.7283(4).
•SnHyp3 (2a) and •SnIbt3 (2c) are isolated in high yields as
brown and orange-yellow crystalline materials, respectively.
The plumbyl radical •PbHyp3 (1a) is formed as side-product
only but could be characterized unambiguously by EPR
experiments. •SnEbt3 (2b) had also been synthesized but
refused to crystallize from the obtained orange-yellow oily
mixture and could therefore not be separated from diamag-
netic contaminations. A notable side-product from the
synthesis of 2c is the hexastannaprismane Sn6Ibt6 (7, Figure
6). It was found in small amounts as very small black crystals
embedded into the large orange-yellow specimens of radical
5 and could be identified by X-ray diffraction. Whether it is
formed along the synthesis of 2c or through its decomposition
remains an open question.
Some heteroleptic tetryl radicals RHyp2E• could also been
generated via the oxidation of heteroleptic alkali metal salts
such as RHyp2ELi (E ) Sn, Pb; R ) Ph, alkyl). Though to
date no crystalline compound could be isolated, several
species such as PhHyp2Pb• (1e) and EtHyp2Pb• (1g) had been
identified unambiguously by their unique EPR-spectroscopic
Table 1. Structural Parameters for Selected Solvated Homoleptic
Silylsubstituted Alkali Metal Tetrelanides [Cat][E(SiR3)3] and the
Potassiate 4a′′′
E
Cat
R
M-Ea E-Sia
Si-E-Si
Sn Na(toluene)
(4a*)20 Hyp
3.07
3.72b
3.72
2.70 109.2-110.5
2.73 107.9-109.1
2.66 101.0-105.7
2.66 109.5-119.3
2.66 108.2-119.6
2.57 98.4-98.9
Sn K(C6H6)2(thf)4 (4a′′′) Hyp
Sn K(OEt2)3
Sn Li(thf)2
(4b)
23
23
Ebt
SitBu2Me 2.83
Sn Li(toluene)
Sn Li(thf)3
Sn Na(15-cr-5)
Pb K(OEt2)2
Pb K(OEt2)2
Pb K(OEt2)2
SitBu2Me 2.77
24
25
(3a′)
(3b)1
(3c)
SiMe3
SiMe3
Hyp
Ebt
2.87
3.08
3.69
3.59
3.61
2.59 98.8-99.4
2.83 106.0-109.3
2.72 100.2-109.5
2.72 105.8-107.9
Ibt
a Mean values. b In the anion.
data (Table 4). A detailed description of their synthesis, their
geometric and electronic structure and their analytical data
will be topic of a subsequent paper.
Molecular Structures. Figures 2-4 show the molecular
structures of some novel homoleptic and heteroleptic alkali
metal tetrelanides in the solid-state. They have been prepared
in either of the two ways described above, (i) by reacting a
Inorganic Chemistry, Vol. 47, No. 21, 2008 9971

Becker et al.
Table 2. Structural Parameters for Heteroleptic Silylsubstituted Alkali
tetrelanide ions are pyramidal EY3-skeletons, relatively long
E-Si bonds and small Si-E-Si or Si-E-Y angles (Tables
1 and 2). Similar structural data had been determined for
tetrelanide fragments EY3 of other alkali metal derivatives
of stannanes and plumbanes before and are generally
attributed to the presence of a lone-pair with predominant
s-character at E and E-Si or E-Y bonds with a predominant
p-character of the contributing orbital at the central
E-atom.21-25
Metal Tetrelanides [Cat][EHyp2Y]
Si-E- Si-E-
E
Cat
Y
M-E E-Sia E-X
Si
Xa
Sn (4d) Li(Et2O)2 nBu
3.02 2.65 2.23 114.3
4.19 2.68 2.24 110.2
2.95 2.73 2.32 113.2
95.9
96.4
94.5
95.6
104.1
Sn 20 Csb 13
CH2Ph
Pb (3d) Li(OEt2)2 nBu
Pb (3e) Li(OEt2)2 Ph
c
c
2.75 2.32 112.6
2.85 2.38 111.5
Pb (3f) K(thf)7
N(SiMe3)2
a Mean values. b Mixed stannanide/amide showing intramolecular co-
ordination by benzyl groups. c Solvent-separated ions.
Figure 5 displays the molecular structures of the novel
structurally characterized homoleptic stannyl and plumbyl
radicals 1c, 2a, and 2c. From etheral solutions trihypersi-
lylstannyl (2a) crystallizes together with half an equivalent
of ether. In the crystal two unique molecules are present.
Both exhibit a well ordered structure with an approximate
(noncrystallographic) C3-symmetry. The Si-Sn-Si angles
range from 116.9-119.8°; the sum of angles of 354.2° shows
that the molecules are nearly but not perfectly planar. For
the related stannyl radical •Sn(SiMetBu2)3 as well as for its
Si- and Ge-analogues Sekiguchi reported a fully planar
structure within the error of the experiment (Table 3).
The Sn-Si bonds (2.648 Å) in the stannyl radical 2a are
significantly shorter than those in the respective distannene
Sn2Hyp4 (52) (2.672 Å),10 the sodium trihypersilylstannanide
4a* (2.70 Å),20 and the potassium potassiate 4a′′′ (2.726 Å).
As one would expect, this finding may indicate a larger
contribution of Sn 5s orbitals to the Sn-Si bonds in the
radicals 2a and 2c than in the distannene (5)2 or than the
stannanide anions of 4a and4a*. While •SnIbt3 (2c) with
2.625 Å (av.) has Sn-Si bond lengths similar to those found
in the radical •Sn(SiMetBu2)3 (2.618 Å),2 the Sn-Si bonds
in 2a are slightly elongated owing to the greater bulkiness
of the hypersilyl substituent as compared to the MetBu2Si
group. The structural data of 2c are unfortunately biased by
disorder such that the Si-Sn-Si bond angles are less well
defined. The mean value for the major orientation is 118.0°.
A remarkable difference between the solid-state structure of
radical 2c and the structures of the other tin and lead radicals
is its significant deviation from C3-symmetry (which is not
reflected in the EPR spectra of the frozen solutions, however).
While in all other stannyl radicals the substituents are
positioned paddle-wheel-like, in 2c two Ibt-substituents are
oriented in such a way that their less sterically demanding
hydrocarbon-soluble tin or lead salt with 3 equiv of the
potassium salt of a branched silane or (ii) by reacting 5 or 6
with an equimolar amounts of an alkali metal salt MY of
hypersilane, of a hydrocarbon, or of an amine (Y ) SiR3;
hydrocarbyl, NR2). Most of the characterized species, for which
selected structural data are given in Tables 1 and 2, have been
isolated from diethyl ether or other coordinating solvents as
contact-ion-pairs with an M-E bond linking the solvated cation
with the stannanide or plumbanide anion (Figure 2).
For trihypersilyl substituted derivatives MEHyp3 3a and
4b such structural motifs are also found. The solvates
(toluene)NaSnHyp3 (4a*)20 and 3a•2Et2O (3a′, Figure 2)
gave reasonable structural data, whereas the related salt
4a•3Et2O (4a′) is severely disordered and no reliable
structural parameters could be derived. Solvent separated ion-
pairs are present in [Li(thf)4][PbPhHyp2] (3d) and [K(thf)7]-
[PbHyp2{N(SiMe3)2}] (3f, Figure 3) as well as in the salts
3a•6THF (3a′′) and 4a•6THF (4a′′). While 3d and 3f have
been fully characterized, the two isomorphous THF-solvates
3a′′ and 4a′′ show severe disorder or twinning in the
crystalline state, and only a rough structural picture could
be derived so far, showing that both salts consist of isolated
K(thf)6-cations and pyramidal EHyp3-anions. However, when
the crystals of 4a′′ are exposed to dynamic vacuum for
several hours and subsequently crystallized from benzene,
very large orange crystals of a well ordered phase are isolated
with the composition (thf)2(benzene)KSnHyp3 (4a′′′). An
X-ray diffraction experiment reveals an unexpected structure:
4a crystallizes as a unique potassium potassiate (Figure 4).
It comprises two novel structural motifs: a [K(thf)4-
(benzene)2]+ cation in which the two benzene molecules are
coordinating, in a unprecedented fashion, through a C-H
bond and the first structurally characterized potassiate anion,
[Hyp3Sn-K-SnHyp3]- with linearly two-coordinate potas-
sium. However, a closer look at the Ci-symmetric anion
reveals the presence of six intramolecular agostic Me· · · K
bonding interactions leading to K-C distances of 3.40-3.49
Å. An analogous treatment of the plumbanide 3a did not
lead to an analogous bis(plumbyl)potassiate but to an almost
quantitative decomposition and the formation of KHyp,
elementary lead, and the octasilane Hyp2.
i
parts, the Pr-groups, face each other, thus allowing for a
small Si2-Sn1-Si3 angle of 106.7°: As a consequence, two
Ibt-groups approach each other via their bulkiest parts,
leading to an Si1-Sn1-Si3 angle of 131.5°.
The related novel plumbyl radical •PbIbt3 (1c) is also
paddle-wheel-shaped and expectedly exhibits somewhat
longer Pb-Si bonds (2.68 Å) than the less sterically hindered
(21) Reed, D.; Stalke, D.; Wright, D. S. Angew. Chem., Int. Ed. Engl. 1991,
30, 1459.
All M-E bonds are expected to be predominantly ionic,
within the neutral contact-ion pairs of 3a′ or 4a* as well as
within the potassiate anion of 4a′′′. The M-E bond length
depends on the size of the alkali metal ion M and the tetryl
ion E, the coordination number of M, and the steric demand
of the whole anion. Common structural features for all
(22) Armstrong, D. R.; Davidson, M. G.; Moncrieff, D.; Stalke, D.; Wright,
D. S. Chem. Commun. 1992, 1413.
(23) Fukawa, T.; Nakamoto, M.; Lee, V. Ya.; Sekiguchi, A. Organome-
tallics 2004, 23, 2376.
(24) Cardin, C. J.; Cardin, D. J.; Clegg, W.; Coles, S. J.; Constantine, S. P.;
Rowe, J. R.; Teat, S. J. J. Organomet. Chem. 1999, 573, 96.
(25) Fischer, R.; Baumgartner, J.; Marschner, C.; Uhlig, F. Inorg. Chim.
Acta 2005, 358, 3174.
9972 Inorganic Chemistry, Vol. 47, No. 21, 2008

Persistent Radicals of TriWalent Tin and Lead
Figure 5. Molecular structures of some of the radicals in the crystal. Selected bond lengths and angles are given in [Å, deg]. •SnHyp3 2a: Sn1-Si1
2.6523(12), Sn-Si2 2.6355(13), 2.6486(13), Si1-Sn1-Si2 116.93(4), Si1-Sn1-Si3 119.80(4), Si2-Sn1-Si3 117.51(4). •SnIbt3 2c: Sn1a-Si2a 2.607(9),
Sn1a-Si1a 2.618(10), Sn1a-Si3a 2.621(7), Si2a-Sn1a-Si1a 131.5(3), Si2a-Sn1a-Si3a 106.7(3), Si1a-Sn1a-Si3a 115.7(3). •PbIbt3 1c: Pb1-Si1a 2.678(5),
Pb1-Si2 2.679(2), Pb1-Si3a 2.676(2), Si1a-Pb1-Si2 117.34(12), Si1a-Pb1-Si3a 115.9(2), Si2-Pb1-Si3a 117.1(2).
Figure 6. Molecular structures of the THF-adduct 6a and hexastannaprismane 7 in the crystal. Selected bond lengths and angles are given in [Å, deg].
Hyp2Pb•THF (6a): Pb1-O1 2.531(5), Pb-Si1 2.743(2), Pb-Si2 2.7318(15), Si1-Pb1-Si2 112.89(5), Si1-Pb1-O1 93.32(12), Si2-Pb1-O1 96.30(13).
Sn6Ibt6 (7): All methyl groups (two on each carbon and three on each silicon) as well as all remaining hydrogen atoms had been omitted for clarity.
Sn1-Sn4 2.827(10), Sn1-Sn6 2.851(10), Sn1-Sn3 2.866(9), Sn2-Sn3 2.811(10), Sn2-Sn4 2.853(9), Sn2-Sn5 2.850(7), Sn3-Si3 2.584(10), Sn3-Sn6
2.839(7), Sn4-Sn5 2.849(10), Sn5-Sn6 2.821(10), Sn1-Si1 2.566(10), Sn2-Si2 2.582(9), Sn4-Si4 2.586(9), Sn5-Si5 2.577(10), Sn6-Si6 2.594(9).
Table 3. Selected Structural Parameters for Silylsubstituted Tetryl
similar to the respective value for •PbEbt3 1b (353°) and
again prove a non-planar structure for this plumbyl radical.
Radicals ER3 (E ) Sn, Pb)
E
R
E-Sia
Si-E-Si
Σ(Si-E-Si)
Two side-products occurring during the synthesis of the
tetryl radicals 1a and 2c could also be structurally character-
ized and are depicted in Figure 6. The right-hand side of
Figure 6 displays the molecular structure of the hexastan-
naprismane 7 derived from diffraction data on very small
black crystals that were embedded into larger orange crystals
of the stannyl radical 2c. While the Sn6Si6-scaffold is very
similar compared to the related hexastannaprismane, (tBu3Si)6-
Sn6 that was structurally characterized by Wiberg some years
ago,26 the Sn-Sn and Si-Si bonds in 7 as compared to
(tBu3Si)6Sn6 are shorter by 7 and 13 pm, respectively, because
of the less bulky substituents.
Sn
Sn
Sn
Pb
Pb
Hyp
Ibt
tBu2Me
Ebt
Ibt
2.64
2.62
2.63
2.64
2.68
116.9-119.8
106.7-131.3
119.7-120.2
115.2-121.4
115.9-117.1
354
354
360
353
350
4a
4c
2b
3b
3c
a Mean values.
•PbEbt3 (1b, 2.65 Å).1 In these plumbyl radicals a trend
similar to that in the stannyl radicals is observed: the Pb-Si
bonds are significantly shorter than for silylsubstituted
plumbandiyls such as 6 (2.702 Å) and Pb[Si(SiMe3)2(SiiPr3)]2
(2.745 Å) or the related plumbanide 3a′ (2.715 Å). Unfor-
tunately, the structural parameters are again biased by
disorder. However, the derived sums of the Si-Pb-Si angles
of 350° and 355° for the two refined orientations are very
Figure 6 (left) shows the molecular structure of the
tetrahydrofuran-adduct 6a, which is formed alongside the
Inorganic Chemistry, Vol. 47, No. 21, 2008 9973

Becker et al.
Table 4. EPR Data of Selected Mononuclear Tetryl Radicals ER3
E
R
giso (293 K)
aiso (293 K)/mT
g
A/mT
1aa
1ba
1ca
1ga
1e
Pb
Pb
Pb
Pb
Pb
Pb
Sn
Hyp
2.089
2.105
2.097
2.055
2.043
2.039 (77K)
2.042
(
(
(
(
(
207Pb): 84.3
207Pb): 50.6
207Pb): 79.1
207Pb): 125.8
207Pb): 124.3
[2.237, 2.213, 1.893]
[2.246, 2.245, 1.888]
(
(
207Pb): [42.0, 44.0, 175]
207Pb): [13.9, 15.3, 144.2]
SiEt(SiMe3)2
SiiPr(SiMe3)2
Hyp (2x), Et
Hyp (2x), Ph
Me
[2.226, 2.079, 1.890]
[2.255, 1.998, 1.894]
[2.106, 2.097, 1.914]
[2.074, 2.074, 1.991]
(
207Pb): [104.0, 142.0, 147.0]
24
(
(
(
207Pb): [115, 115, 324]
117Sn): [36.3, 36.3, 107.8]
119Sn): [38.0, 38.0, 112.5]
2aa
Hyp
(
(
(
(
(
(
117Sn): 59.8
119Sn): 62.4
117Sn): 39.1
119Sn): 41.6
117Sn): 37.7
119Sn): 39.8
2ba
2ca
24
Sn
Sn
Sn
Sn
Sn
SiiPr(SiMe3)2
SiEt(SiMe3)2
Me
2.055
2.053
[2.080, 2.079, 1.985]
[2.030, 2.024, 1.995]
[2.016, 2.016, 1.994]
(
(
(
(
117Sn): [18.8, 20.9, 90.2]
119Sn): [20.9, 24.0, 94.4]
117Sn): [133.2, 119.2, 206.7]
119Sn): [141.5, 126.2, 215.7]
2.016 (77K)
2.009
5a
CH(SiMe3)2
Tarb
(
(
(
(
117Sn): 169.8
119Sn): 177.6
117Sn): 175.6
119Sn): 182.7
32
2.001
33
5d
Sn
Sn
H
3800
[2.024, 2.024, 2.015]
N(SiMe3)2
1.991
(
(
(
117Sn): 317.6
119Sn): 342.6
117/119Sn): 32.9
2b
2a
32
2a
Sn
Ge
Ge
Si
SitBu2Me
SitBu2Me
Tarb
2.048
2.023
2.005
2.006
(73Ge): 2.0
(73Ge): 8.5
(29Si): 5.8
SitBu2Me
a The isotropic values giso and aiso from simulations of the solution CW EPR spectra at 293 K; g- and A-tensors from simulations of frozen solution
b
spectra. Margins of error are ∆g ) (0.0002; ∆A ) (0.1 mT. Tar ) C6HtBu-2-Me3-4,5,6.
preparation of 1a and may be independently prepared as blue-
green crystals in quantitative yields by addition of tetrahy-
drofuran to a solution of 6 in n-pentane and subsequent
crystallization at -60 °C. As expected, 6a possesses a
pronounced pyramidal Si2PbO-scaffold (sum of angles
around Pb: 302.5°). The length of the Pb-O dative bond
(2.53 Å) is somewhat longer than in the thf-adduct of the
more polar lead thiolate Pb{S[C6H2(CF3)3-2,4,6]2 (2.50 Å)27
but significantly shorter than in the extremely sterically
overcrowded thf-adduct of the molybdylplumbylene Pb[Mo(η-
C5Me5)(CO)3]2 (2.75 Å).28
EPR Spectroscopy. Figure 7 displays CW EPR spectra
of the four compounds 1a, 1b, 2a, and 2c at room
temperature (293K), while Figure 8 depicts the echo-detected,
field-swept EPR spectra at low temperature (20K). Overlaid
in red we also present spectral simulations including the
hyperfine pattern expected from the magnetically active
isotopes 207Pb (I ) 1/2, natural abundance of 22.1%) and
117Sn (I ) 1/2, natural abundance of 7.5%) as well as 119Sn
(I ) 1/2, natural abundance of 8.6%).
Room-Temperature CW EPR Spectra. When measuring
solutions of the four compounds in alkanes at room tem-
perature (293 K), the usually orientation-dependent (i.e.,
anisotropic, tensorial) magnetic interactions of the electron
spin with the magnetic field (the electron-Zeeman interaction)
and the interaction of the electron spin with magnetic nuclear
spins (the hyperfine (hf) interaction) are averaged to their
isotropic, scalar values giso and aiso.29 While the spectra of
the stannyl radicals 2a and 2c are shown in panels a and b
of Figure 7, spectra of the plumbyl radicals 1a and 1b are
presented in panels c and d of Figure 7, respectively. All
spectra in Figure 7 show the same general pattern and possess
a strong main signal from the radical centered at nonmagnetic
Pb- and Sn-isotopes, as well as peak patterns that reflect
hyperfine splitting from the magnetically active isotopes. The
hyperfine coupling constants that were extracted from the
spectral simulations are summarized in Table 4. In general,
the isotropic g-values giso can be seen as the fingerprint for
the individual radical species, and the isotropic hf coupling
constants aiso characterize the interaction between the electron
spin and the nuclear spin of the magnetically active nuclei.29
In the case of the Sn-based radicals, we find for 2a giso
)
2.042 and an aiso(117Sn) ) 59.8 mT, aiso(119Sn) ) 62.4 mT
while 2c shows significantly higher g- and lower a-values:
giso ) 2.053, aiso(117Sn) ) 37.7 mT, aiso(119Sn) ) 39.8 mT.
As it is expected, the ratio of the hf coupling constants for
the two isotopomers reflects the ratio of the respective
gyromagnetic ratios γ(119Sn)/γ(117Sn). Because of the larger
hf coupling of 2a, in this case even the characteristic splitting
pattern for the two isotopomers can be distinguished, while
for 2c the hf lines still overlap.
The lead-based radicals show g-values that deviate stronger
from the free electron g-value. For 1a we find giso ) 2.089
and aiso(207Pb) ) 84.3 mT, while 1b, as is the case for the
respective stannyl radical, shows a higher g- and a lower
a-value: giso ) 2.105, aiso(207Pb) ) 50.6 mT. When exchang-
ing the central SnIII (fifth period in the periodic table of
elements) with the heavier congener PbIII (sixth period), the
larger contribution of the spin-orbit coupling is responsible
for an even greater deviation of the plumbyl radicals’
g-values from the free electron g-value ge ) 2.0023.
(26) Wiberg, N.; Lerner, H.-W.; No¨th, H.; Ponikwar, W. Angew. Chem.,
Int. Ed. 1999, 38, 1103.
(27) Labahn, D.; Brooker, S.; Sheldrick, G. M.; Roesky, H. W. Z. Anorg.
Allg. Chem. 1992, 610, 163.
(28) Hitchcock, P. B.; Lappert, M. F.; Michalczyk, M. J. J. Chem. Soc.,
Dalton Trans. 1987, 2635.
(29) Atherton, N. M. Principles of Electron Spin Resonance; Ellis Horwood:
New York, 1993.
9974 Inorganic Chemistry, Vol. 47, No. 21, 2008

Persistent Radicals of TriWalent Tin and Lead
Figure 7. CW EPR spectra of the radicals depicted in Figure 1 in mixtures of n-pentane and n-hexane at 273 K. All experimental spectra (black) are
overlaid with spectral simulations (red). (a) •SnHyp3 (2a), the inset shows the full spectrum; the enlarged part shows the detailed hyperfine splitting pattern;
(b) •SnIbt3 (2c), the inset shows the full spectrum; the enlarged part shows the detailed hyperfine splitting pattern; (c) •PbHyp3 (1a), (d) •PbEbt3 (1b); the
respective lines marked with an asterisk are from a stationary concentration of a silyl-based radical, which for 1a is displayed in detail in (e).
Note that two heteroleptic plumbyl radicals 1e and 1g for
which we report g- and A-values in this work (spectra not
shown; for the values see Table 4) also have markedly
smaller g-values and significantly larger aiso-values as
compared to the homoleptic Pb-based radicals.
One interesting feature in Figure 7 is the asymmetric
broadening of the hyperfine lines, which in the case of the
two plumbyl radicals (see Figure 7c,d) even leads to such a
strong broadening that the high-field line is not distinguish-
able from the background signal any more. Note that the
difficulties in observing the high-field line manifolds of
plumbyl radicals have led to some confusion about the purity
of the materials in the initial report on these materials.1 This
effect can be understood when considering the modulation
of the anisotropic electron Zeeman and hf interactions by
the rotational motion. It can be accounted for by using a
Figure 8. Echo-detected, field swept EPR spectra of the radicals depicted
slow motion model for spectral simulations that is based on
in Figure 1 in mixtures of n-pentane and n-heptane at 20 K. All experimental
solving the stochastic Liouville equation (simulations shown
in Figure 7).16,29 We find that a characteristic rotational
correlation time of τc ≈ 1.3 · 10-10 s can nicely reproduce
the hf splitting and intensity pattern for the two lead-based
spectra (black) are overlaid with spectral simulations (red). The approximate
g-tensor element positions are marked with arrows, and the largest hf
coupling tensor element Azz is marked by dashed lines. (a) 2a; (b) 2c; (c)
1a; (d) 1b; the asterisk marks features from an unknown impurity or
paramagnetic side product.
Inorganic Chemistry, Vol. 47, No. 21, 2008 9975

Becker et al.
radicals, while we find significantly faster rotation with τc
≈ 6.3 × 10-11 s for the two tin-based radicals (all measured
in the same solvent mixture). This apparent slowdown in
the rotational motion when the central atom Sn is replaced
with the larger and much heavier Pb can be attributed to the
overall increased mass of the plumbyl radical molecules.
Note that in the spectra of 1a and 2a, and to a lesser degree
also in the spectra of 1b and 2c, additional narrow lines
corresponding to significant amounts of a radical species with
a g-value of 2.0058 rather close to the value of a free electron
and a characteristic hyperfine splitting pattern are observed
(marked with asterisks in Figure 7. This five-line pattern is
shown in Figure 7e and can be described as one intense
central line and two pairs of hyperfine lines. We assign this
specific EPR pattern to a silicon-based decay product of our
stannyl and plumbyl radicals, the hypersilyl ligand radical
•Si(SiMe3)3 (or •SiiPr(SiMe3)2, and •SiEt(SiMe3)2 in the case
of •SnIbt3 and •PbEbt3, respectively). The strong central line
stems from the electron spins located on the almost 96% of
Si nuclei that are not magnetic and therefore do not lead to
a hyperfine splitting. The g-value of 2.0058 and the observed
hyperfine-splittings of 6.25 mT and 0.86 mT (due to the 4.2%
29Si nuclei) are in reasonable agreement to the values of
branched silyl radicals found in literature.4,30
As can be expected, the difference of the splitting patterns
between the hypersilyl ligand radical and the ligand radicals
derived from •EIbt3 and •EEbt3 lies mainly in the intensity-
ratios of the 29Si hyperfine lines. The splittings (i.e., position
of the lines) are virtually identical. From the ratio of the
isotropic hyperfine splitting constants it is obvious that in
the hypersilyl ligand radical the electron spin is mainly
located on the one Si-atom with the largest hyperfine splitting
(Fs∼ 0.841) but is delocalized to a certain degree onto the
three Si atoms of the trimethylsilyl groups (Fs ∼ 0.053 for
each).
We can quantify the concentration of the hypersilyl radical
decay product in solutions of PbHyp3 by measuring the
double integral of the CW EPR spectral excerpt that contains
the Si-based radical signal. The double integral is a direct
measure of the number of electron spins. We find that the
concentration of the Si-based radical remains constant within
the precision of the experiment for several hours at room
temperature (or elevated temperatures) while the decay
of the Pb-based radical is apparent. This leads to the
conclusion that the concentration of the hypersilyl radical is
a stationary intermediate in the decomposition reaction of
•PbHyp3. The fact that a stationary concentration of the
hypersilyl radical builds up in a highly reproducible manner
opens up new possibilities for use of the radical as a clean
silylating agent. Note that despite the highly intense central
silyl-radical line, the overall contribution to the concentration
of electron spins (i.e., the double integral) was never more
than 1.5%.
Low-Temperature EPR Spectra. In frozen solution the
anisotropies of the g- and hf (A-) tensors of the radicals are
not averaged out any more and now dominate the spectra.
The echo-detected EPR spectra of the four radicals at 20 K
along with powder-averaged EPR spectral simulations are
presented in Figure 8. The g- and A-tensor elements are
summarized in Table 4. All spectra consist of one main
spectral component that is typical for a radical with an
approximately axial g-tensor (gxx ≈ gyy * gzz) and less intense
pattern spreading out to lower and higher magnetic field
values that stem from the hyperfine coupling to the magnetic
nuclei in the respective isotopomers. In Figure 8 we also
indicate the approximate position of the gxx/gyy and gzz tensor
elements in the isotopomers that have no magnetically active
Sn and Pb nuclei.
Our simulations clearly show that also the hf tensor for
homoleptic radicals is nearly axial (Axx ≈ Ayy * Azz) with
Azz . Axx/Ayy, and in Figure 8 the splitting from the Azz-
tensor element is marked by dashed lines. For both cases,
tin and lead, the hypersilyl compounds display the signifi-
cantly larger hyperfine coupling (aiso as well as all the
A-tensor elements) when compared with the radicals having
unsymmetrically substituted ligands (2c and 1b, respectively).
In addition, the heteroleptic lead-based radicals 1g and 1d
clearly have an orthorhombic low-temperature EPR-spectrum
(see Table 4, data not shown, gxx * gyy * gzz) and (Axx * Ayy
* Azz) with g-values smaller than those of the homoleptic
radicals and hyperfine values significantly larger than those
of the homoleptic radicals. There are only few reports in
the literature on EPR investigations of simple mononuclear
triorganylstannyl and triorganylplumbyl radicals such as
•PbMe3, •SnMe3, or •SniBu3.31 Two general trends can
nonetheless be easily recognized going from hydrogen via
organyl to silyl substitution (Table 4): (i) g-values increase
and (ii) hyperfine-coupling constants markedly decrease. It
becomes obvious that those trends have to be attributed
mainly to differences in electronic properties of the substit-
uents and only to a lesser extent to different steric demands
of the substituents (vide infra).
Electronic and Geometric Structure and the Dynam-
ics of the Radicals. The information gained from the EPR
spectra in both, fluid solution at room temperature and in
frozen solution at 20 K, together with structural data from
X-ray diffraction give unique insight into these new com-
pounds. Conclusions can be drawn not only about the
electronic structure but also about the geometric structure
and the dynamic properties of the persistent or stable heavy
main group element radicals. Some of the generated radicals,
which we have extensively characterized by EPR measure-
ments, could also be obtained as suitable crystals for X-ray
diffraction, others were obtained as microcrystalline powders
(31) (a) Bennett, J. E.; Howard, J. A. Chem. Phys. Lett. 1972, 15, 322–
324. (b) Booth, R. J.; Fieldhouse, S. A.; Symons, M. C. R. J. Chem.
Soc., Dalton Trans. 1976, 1506–1515.
(32) Della Bona, M. A.; Cassani, M. C.; Keates, J. M.; Lawless, G. A.;
Lappert, M. F.; Stu¨rmann, M.; Weidenbruch, M. J. Chem. Soc., Dalton
Trans. 1998, 1187.
(33) Morehouse, R. L.; Christiansen, J. J.; Gordy, W. J. Chem. Phys. 1966,
45, 1751.
(30) (a) Bennett, S. W.; Eaborn, C.; Hudson, A.; Jackson, R. A.; Root,
K. D. J. J. Chem. Soc. A 1970, 348–351. (b) Azinovic, D.; Bravo-
Zhivotovskii, D.; Bendikov, M.; Apeloig, Y.; Tumanskii, B.; Veprek,
S. Chem. Phys. Lett. 2003, 374, 257–263.
9976 Inorganic Chemistry, Vol. 47, No. 21, 2008

Persistent Radicals of TriWalent Tin and Lead
pyramidalization can be approximated when inspecting the
A-tensor elements. All four investigated tetryl radicals have
an A-tensor with an Azz-value that is much larger than the
respective x-and y-values Axx and Ayy, which are (almost)
equal, that is, the tensor is axial as depicted in Figure 9.
The hypersilyl compounds 1a and 2a do not only have a
larger aiso but also the A-tensor elements along all three
directions are larger than in the respective other radical (e.g.,
for the plumbyl radicals: 1a, [42.0 44.0 175] mT vs 1b, [13.9
15.3 144.2] mT). This is fully consistent with a larger
pyramidal deviation from planarity in the case of the
hypersilyl tetryl radicals also in frozen solution.
The most pronounced pyramidalization can be found for
the Pb-based heteroleptic radicals 1e and 1g, which is due
to the size difference for the individual ligands (two
hypersilyl ligand and one ethyl or phenyl ligand, respec-
tively). This is expected and leads to a much stronger s-type
character for the orbital bearing the unpaired electron, which
in turn leads to a much larger isotropic contribution to the
hyperfine coupling (see Table 4). Also note that the g-values
for 1e and 1g are much closer to the g-value reported for
PbMe3.
At room temperature and if light is excluded, the stannyl
radicals 2a-2c show only a very slow decomposition.
Although small amounts of branched silyl radicals are always
detected in solution, their decay rates could not be determined
so far; half-life times of at least several weeks at room
temperature can, however, be estimated. All plumbyl radicals
synthesized show half-life times of a few hours at room
temperature. While in the case of trihypersilyl plumbyl 1a
the corresponding plumbandiyl 6 and the octasilane Hyp2
are detected as main products, the less hindered 1b cleanly
decomposes within a few days giving elementary lead and
PbEbt4. No intermediate and almost no side-products are
found. Most probably, the corresponding plumbandiyl PbEbt2
is the first intermediate, since the rate-determining step is
first order in 1b and the activation barrier had been measured
to be about 100 KJ mol-1.1 This would be fully in accordance
with a Pb-Si bond fission as the first and rate-determining
step. The generated Ebt radical which could be detected by
EPR to be present in a stationary concentration may then
quickly react with remaining 1b to produce the plumbane
PbEbt4. The facts that hardly any side-products are observed
and that the rate for the formation of PbEbt4 is equal to the
rate of the decomposition of 1b within the experimental
errors leads to the conclusion that the plumbandiyl PbEbt2
is not stable under these conditions, but disproportionates
very quickly to give further amount of elementary Pb and
PbEbt4. Oligonuclear plumbanes or diplumbenes are very
probable intermediates and trapping experiments are in our
hands.
Figure 9. Scheme of the proposed axial g-or A-tensor frame (right-hand
side) and main contribution to the SOMO bearing the unpaired electron
(left-hand side).
or as viscous oils only (vide supra). As mentioned before,
the structural data derived for crystalline species are unfor-
tunately frequently biased by disordering in the solid state.
Nevertheless, the diffraction data revealed that all silyl
substituted tetryl radicals exhibit a nearly but not fully planar
structure, which is also consistent with the nearly axial g-
and A-tensors found for the radicals and is schematically
displayed in Figure 9. Surprisingly, among the tin-based
radicals the most pronounced pyramidalization is found for
trihypersilyltin (2a) although the hypersilyl group is the
bulkiest substituent used and should therefore lead to less
pyramidalization. Unfortunately, because of its lower stability
we were not able to get crystals for its lead analogue 1a.
The EPR data (in particular the larger isotropic hf coupling
constants) for both 1a and 2a indicate a higher degree of
pyramidalization than for the other plumbyl and stannyl
radicals.
As pointed out by Sekiguchi et al. crowding is not the only
reason for the almost planar structures of silyl substituted group
14 radicals. Hyperconjugation, that is, delocalization of spin
density into the antibonding σ*(Si-Si)-orbitals, also has to be
taken into account. Detailed quantum-chemical analyses are
lacking, however, and are on the agenda in our groups. A further
typical structural feature of all tetryl radicals ER3 is the
comparably short E-Si bond found in the solid-state (vide
supra) being markedly shorter than that determined for the
related tetrelandiyls ER2 and close to those observed in stress-
free tetrelanes ER4. This can be explained by assuming a
significantly higher contribution of the contracted 6s orbital of
lead in bonding to the silyl substituents than in the related
plumbandiyls, in which this orbital predominantly contributes
to the lone-pair.
The EPR measurements can add further information about
the geometric structure of the radicals. As displayed in Figure
9, a fully planar group 14 radical should have a 5p (tin) or
6p (lead) singly occupied molecular orbital (SOMO) bearing
the unpaired electron and the overall hybridization should
be sp2, that is, the three ligands should be bound via such
sp2 hybrid orbitals. In this ideal sp2-hybridization case, except
for minor spin polarization effects (polarization of the
electrons in the closed shells),22 the isotropic hf coupling
should be zero (or have only small values), since the main
contribution to aiso comes from unpaired spin density in s-
(σ-)type orbitals and the unpaired electron resides in a pz-
orbital. Two factors lead to a significant aiso in fluid solution
for our tin and lead tetryl radicals: (i) an intrinsic pyrami-
dalization, probably as an effect of the different ligands, and
(ii) the structure is dynamic and even a perfectly planar
molecule will oscillate along the molecular z-direction such
that a dynamic pyramidalization takes place. The static
Conclusions and Outlook
We were able to synthesize a series of homoleptic and
heteroleptic alkali metal stannanides and plumbanides, and
via subsequent oxidation got access to further persistent
mononuclear tin and lead radicals, respectively, showing
either a homoleptic or a heteroleptic substitution pattern.
Inorganic Chemistry, Vol. 47, No. 21, 2008 9977

Becker et al.
While the homoleptic tetryl radicals 1b, 1c, 2a, and 2c could
be isolated as crystalline compounds, others such as the
homoleptic 1a and 2b or the heteroleptic species 1e and 1g
could only be characterized by EPR experiments. EPR-
spectroscopic studies enable us to draw conclusions about
the electronic and geometric structure as well as the dynamics
of these novel, molecular persistent tin and lead tetryl
radicals, even for those cases where the radical is not stable
or clean enough to be isolated as a pure compound. As
indicated in the X-ray diffraction results on some of the
radicals, the structures are best described as almost planar
with varying small pyramidal distortion (flexible pyramidal).
Interestingly, we find that the bulkiest ligand, hypersilyl,
shows the largest deviation from planarity for both cases,
stannyl and plumbyl radicals. This may possibly be due either
to differences in ligand-ligand interactions among the C3-
symmetric hypersilyl groups compared with Ebt and Ibt
groups having approximate Cs symmetry or to a different
extent of hyperconjugation with the lone-pair on E. For a
more detailed discussion of the radical structure, more
sophisticated pulse EPR methods are necessary. Currently,
we study the electronic structure of the radicals by measuring
the hyperfine coupling to the ligand protons and silicon nuclei
by means of pulse electron-nuclear double resonance
(ENDOR) and methods based on electron spin echo envelope
modulation (ESEEM, HYSCORE). These methods give
detailed information about small hyperfine couplings between
ligands and nuclei and should help in elucidating not only
the spin density distribution from the central main group
element onto the ligand but also the fine details of distortion
from planarity.
The clean decay of the •PbHyp3 (1a) and •PbEbt3 (1b)
radicals in particular is of interest since its first step seems
to deliver a stationary concentration of a well defined
branched silyl radical, which may be useful in synthetic
approaches as a clean silylating agent. Efforts toward the
preparation of polymer-supported silyl substituted tetrelanides
and respective tetryl radicals and thus precluding a contami-
nation by soluble tin or lead containing side products are
currently underway.
Acknowledgment. The authors thank Christian Bauer for
technical support, Bernd Mathiasch for NMR measurements,
and Hans Wolfgang Spiess for continuing support. This work
is dedicated to the memory of Professor Nils Wiberg.
Supporting Information Available: Tables with crystal and
structure refinement parameters. This material is available free of
charge via the Internet at http://pubs.acs.org.
IC801198P
9978 Inorganic Chemistry, Vol. 47, No. 21, 2008