Different reactivity of the heavier group 14 element alkyne analogues Ar′MMAr′ (M = Ge, Sn; Ar′ = C6H3-2, 6(C6H3-2,6-Pri2)2) with R2NO

Article abstract of DOI:10.1039/b513189h

The reactions of the digermanium and ditin alkyne analogues Ar′MMAr′ (M = Ge or Sn) with R₂NO, (R₂NO = Me₂C(CH₂)₃CMe₂NO or N₂O), result in complete MM bond cleavage to afford the

Full text of DOI:10.1039/b513189h

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Different reactivity of the heavier group 14 element alkyne analogues
Ar9MMAr9 (M = Ge, Sn; Ar9 = C6H3-2,6(C6H3-2,6-Pri2)2) with
R2NO{
Geoffrey H. Spikes, Yang Peng, James C. Fettinger, Jochen Steiner and Philip P. Power*
Received (in Berkeley, CA, USA) 16th September 2005, Accepted 6th October 2005
First published as an Advance Article on the web 9th November 2005
DOI: 10.1039/b513189h
The reactions of the digermanium and ditin alkyne analogues
Ar9MMAr9 (M
= Ge or Sn) with R2NO, (R2NO =
Me2C(CH2)3CMe2NO or N2O), result in complete MM bond
cleavage to afford the germylene :Ge(Ar9)ONR2 or the
germanium(II) or tin(II) hydroxides {M(Ar9)(m–OH)}2.
Stable heavier group 14 alkyne congeners RMMR are now known
for all the heavier elements.1 Their reactivity has been little
explored but preliminary results suggest that the germanium
derivative Ar9GeGeAr9 (1) is considerably more reactive than its
tin counterpart Ar9SnSnAr9 (2).2 This is counterintuitive on the
basis of the bond energies, bond polarities and steric effects. It has
been proposed that the main reason for this difference is the
greater diradical character of Ar9GeGeAr9.3 In this paper we show
that the germanium compound, 1, is not only more reactive
towards TEMPO (tetramethylpiperidineoxide) than its tin analo-
gue, 2, but that different types of products are obtained. A similar
difference in the reactivity of 1 and 2 is observed upon treatment
with N2O but in this case congeneric products are obtained.
Upon the addition of 2 equiv. of TEMPO to a stirred solution
of 1 in hexane at ca. 25 uC, a rapid color change from dark red to
pale yellow was observed. Reduction of the solution volume and
subsequent recrystallization at ca. 218 uC led to to the isolation of
pale yellow crystals of the germylene :Ge(Ar9)TEMPO, 3, in high
yield. For the corresponding reaction of 2 with 2 equiv. of
TEMPO in toluene at ca. 25 uC the dark green solution slowly
faded to a pale red color overnight. Recrystallization from a
minimum volume of toluene at ca. 218 uC gave the tin(II)
hydroxide {Sn(Ar9)(m–OH)}2, 4, as colorless crystals in high yield.
When the reaction was repeated, and the solvent removed after
overnight stirring, analysis by 1H NMR spectroscopy revealed the
presence of 4 and 2,2,6,6-tetramethylpiperidine.
Scheme 1 Reactions of 1 and 2 with TEMPO and N2O to give 3–5.
Compounds 3, 4 and 5 were characterized by 1H and 13C NMR,
IR and UV spectroscopy as well as by X-ray crystallography.{ The
electronic spectrum of 3 demonstrated a strong absorption at
374 nm. No O–H or N–H absorptions were observed in the IR
1
spectrum. At room temperature in C6D6 the H NMR spectrum
showed the expected pattern for the Ar9 ligand as well as three
broad resonances at 0.41, 0.90 and 1.31 ppm due to the bound
TEMPO. A variable temperature 1H NMR study demonstrated a
sharpening of these resonances at elevated temperature indicating
restricted rotation around the N–O bond. The X-ray crystal
structure (Fig. 1) showed that the Ge–Ge bond has been
completely cleaved by the addition of a TEMPO to each Ge
center to afford two monomeric :Ge(Ar9)TEMPO germylene
units. The coordination at Ge is V-shaped with an interligand
The addition of 2 equiv. of N2O to a stirred solution of 1 at ca.
25 uC led to a rapid fading (ca. 1 min) of the red color to colorless
with concomitant evolution of N2. Upon workup the hydroxide,
{Ge(Ar9)(m–OH)}2, 5, was isolated in high yield as pale yellow
crystals by recrystallisation from a minimum volume of toluene at
ca. 218 uC (Scheme 1). The corresponding reaction with 2
resulted in slow evolution of N2 and bleaching of the color over a
period of several hours to yield the analogous tin(II)
hydroxide{Sn(Ar9)(m–OH)}2, 4.
˚
angle of 100.15(6)u. The Ge–O bond, 1.804(2) A, in 1 is similar to
˚
the 1.802(8) and 1.812(7) A in the aryloxide Ge (OAr)2 (Ar =
C6H2-2,6-But-4-Me)4a and is slightly shorter than those in
t
Ge(OCBu 3)2 (1.896(6) and 1.832(11)A). The long N–O bond,
4b
˚
˚
1.476(2)A, and pyramidal geometry of the nitrogen atom (SuN =
Department of Chemistry, University of California Davis, One Shields
Ave., Davis, CA 95616, USA. E-mail: pppower@ucdavis.edu;
Fax: 001 530 752 8995; Tel: 001 530 752 6913
334.0(3)u) indicates that the TEMPO ligand is fully reduced and is
comparable to the only previously reported example of a
germanium TEMPO derivative, the germanium(IV) species
{ Dedicated to the memory of Ian Rothwell
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Fig. 1 Thermal ellipsoid (50%) drawing of 3, H atoms are not shown.
˚
Fig. 2 Thermal ellipsoid (50%) drawing of 4, H atoms (except OH) are
˚
Selected bond lengths (A) and angles (u): Ge1–O1 1.804(2), Ge1–C1
2.033(2), O1–N1 1.476(2), O1–Ge1–C1 100.15(6), N1–O1–Ge1 106.57(9),
not shown. Selected bond lengths (A) and angles (u): Sn1–O1 2.145(2),
Sn1–O19 2.149(1), Sn1–C1 2.227(2), O1–Sn1–O19 72.27(6), O1–Sn1–C1
O1–N1–C35 107.52(12), O1–N1–C31 107.0(1), C35–N1–C31 119.4(1).
95.33(6), O19–Sn1–C1 94.60(6), Sn1–O1–Sn19 107.73(6).
GeC{(SiMe3)2(CH2)2C(SiMe3)2}(TEMPO)2 (average Ge–O
˚
=
gave strong absorptions at 290 and 316 nm while the IR spectrum
displayed an absorption at 3540 cm21 corresponding to the O–H
1
stretching mode. The H NMR spectrum showed the expected
˚
1.825(2)A; average N–O = 1.482(4)A) which was synthesised
by the reaction of TEMPO with the germylene
:GeC(SiMe3)2(CH2)2C(SiMe3)2.5
pattern for the Ar9 ligand as well as a resonance at 0.91 ppm that
was assigned to the hydroxide protons.
The X-ray crystal structure of the tin derivative 4 shows that the
Sn–Sn bond in the precursor 2 has been cleaved. However the
product does not have a structure like that of 3. Instead a hydroxy-
bridged dimer is obtained. It represents the first example of a
structurally characterised molecular tin(II) hydroxide. A number of
tin(II) hydroxide salts have been isolated in which tin has a high
coordination number6 but there are relatively few examples of
structurally characterized molecular species possessing Sn(II)–O
The reaction of Ar9GeGeAr9, 1, with TEMPO can be
postulated as a two step oxidation of the ‘digermyne’ unit via an
interaction of the SOMO of TEMPO with the LUMO of 1 (which
is the n+ combination).11 Steric crowding, and the introduction of
an electronegative ligand, favours the dissociation of the dimer into
two monomeric germylene molecules12 which can be isolated as
the stable species 3. The corresponding reaction of TEMPO with
Ar9SnSnAr9, 2, proceeds more slowly than that of its germanium
counterpart. A tin analogue of 3 could not be isolated from this
reaction mixture. The isolation of the hydroxide 4 may be
˚
bonds. The Sn–O distances in 4 (average Sn–O = 2.143(3)A)
are similar to those observed in bridged tin(II) alkoxides (average
Sn–O = 2.147 A).4,7 The Sn2O2 core is planar with an acute angle,
˚
72.27(6)u, at Sn and a wider angle, 107.73(6)u, at O (Fig. 2). The
electronic spectrum of 4 exhibited a strong absorption at 319 nm
while the IR spectrum displayed an absorption at 3480 cm21
corresponding to the O–H stretching mode. The 1H NMR
spectrum showed the expected pattern for the Ar9 ligand as well as
a resonance at 20.86 ppm that was assigned to the hydroxide
hydrogens. A 119Sn NMR signal was found at 216.4 ppm which is
similar to that previously observed for Sn(N(SiMe3)2)(OAr),
227 ppm, and downfield from that observed for Sn(OAr)2,
2193 ppm, (Ar = C6H2-2,6-But-4-Me).7a The germanium(II)
hydroxide 5 is precedented only by the recently reported
monomeric LGeOH (L = {N(Ar)C(Me)CHC(Me)N(Ar)}; Ar =
C6H3-2,6-Pri2).8 Its structure is very similar to that of its tin
analogue 4 and has significantly longer Ge–O distances (average
˚
˚
Ge–O = 1.975(2) A) than that reported for LGeOH, 1.828(1)A, as
a result of the bridging nature of the bonding.9 It is similar to the
˚
average Ge–O distance, 1.988A, found previously for bridged
germanium(II) alkoxides.7c,7f–g,10 The Ge2O2 core is planar
although it is less rhomboid (internal angles 75.34(6)u, at Ge
and, 104.66(6)u, at O) than that in 4. The electronic spectrum of 5
Scheme 2 Possible mechanism for the formation of 3 and 4.
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390 (w) cm21. UV-Vis (n-hexane): 316 nm (e = 2400), 290 (e = 2400).
˚
Crystal data for 3, 4 and 5 at 90(2) K with MoKa (l = 0.71073 A). 3:
accounted for in terms of N–O bond cleavage in the putative
:Sn(Ar9)TEMPO intermediate where the N–O bond is weakened
by the more electropositive tin. The Ar9SnO? and ?NR2 fragments
may then abstract hydrogen from the solvent to afford 4 and
HNR2.5,13–16 In contrast, upon reaction with N2O, both 1 and 2
afford the hydroxide products (4 and 5). It may be assumed that
the coordination of N2O to the metal centers in 1 and 2 affords
unstable species that contain the Ar9M–O–N2 moiety which
eliminates N2 rapidly to give the radical Ar9MO?. Hydrogen
abstraction then yields the hydroxide product. The examination of
reactions of 1 and 2 with other oxygen transfer agents is in hand.§
This work was supported by the National Science Foundation.
¯
C39H55GeNO, M = 626.43, triclinic, space group P1, Z = 2, m =
0.887 mm21, a = 10.693(1) A, b = 11.120(1) A, c = 16.042(1) A, a =
˚
˚
˚
100.976(1)u, b = 91.353(1)u c = 108.140(1)u, V = 1785(1) s3, R1 = 0.0375
for 10297 (I > 2s(I)) reflections, wR2 = 0.0942 (all data). 4?2PhMe:
¯
C44H54OSn, M = 717.56, triclinic, space group P1, Z = 2, m = 0.711 mm
21
,
˚
˚
˚
a = 11.965(5) A, b = 13.328(5) A, c = 13.730(9) A, a = 111.010(8)u, b =
˚
102.783(8)u c = 102.983(6)u, V = 1879.0(16) A, R1 = 0.0309 for 11343 (I >
2s(I)) reflections, wR2 = 0.0695 (all data). 5?2PhMe: C39H55GeO, M =
¯
˚
671.46, triclinic, space group P1, a = 11.990(1) A, b = 13.109(1) A,
˚
˚
c = 13.771(1) A, a = 110.633 (1)u, b = 103.258(1)u c = 102.685(1)u, V =
1864.4(2), R1 = 0.0383 for 10680 (I > 2s(I)) reflections, wR2 = 0.1015
(all data).
1 (a) A. Sekiguchi, R. Kinjo and M. Ichinohe, Science, 2004, 305, 1755;
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3524.
Notes and references
{ Crystallographic data for: 3 CCDC 284211. 4?2PhMe CCDC 284212.
5?2PhMe CCDC 284213. For crystallographic data in CIF or other
electronic format see DOI: 10.1039/b513189h
§ All manipulations were conducted under strict exclusion of air and
moisture in an atmosphere of dry argon or in vacuo using Schlenk line and
glovebox techniques. :Ge(Ar9)(TEMPO), 3: To a solution of Ar9GeGeAr9
(Ar9 = C6H3-2,6(C6H3-2,6-Pri2)2) (0.30 g, 0.32 mmol) in hexane (20 mL)
was added a solution of TEMPO (0.10 g, 0.64 mmol) in hexane (20 mL)
dropwise. The red solution faded to pale yellow over 2 min. Reduction of
the solvent in vacuo to incipient crystallisation and storage at 218 uC
yielded 3 as pale yellow crystals in high yield 0.26 g, 0.24 mmol, 74%). Mp:
198–200 uC. 1H NMR (C6D6, 300.08 MHz): d 0.41 (br, s, 12H), 0.90 (br, s,
2H), 1.11 (d, 12H, J = 6.9 Hz, CHMe2), 1.31 (br, s, 4H), 1.42 (d, 12H, J =
6.9 Hz, CHMe2), 3.17 (sept, 4H, J = 6.6 Hz, CHMe2), 7.14 (m, 3H, Ar–H),
7.24 (m, 6H, Ar–H). 13C NMR (C6D6, 100.52 MHz): d 17.7 (CMe2), 23.3
(CHMe2), 25.7 (CHMe2), 31.3 (CHMe2), 40.6 (CMe2), 58.1 (CH2–CH2–
CH2), 114.7, 123.8, 124.7, 129.0, 129.1, 132.1, 143.7, 147.1 (unsaturated
2 (a) C. Cui, M. M. Olmstead and P. P. Power, J. Am. Chem. Soc., 2004,
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carbon). IR (KBr, Nujol): 870 (w), 760 (s), 750 (s), 460 (m), 390 (m) cm21
.
UV-Vis (n-hexane): 374 nm (e = 2300). {Sn(Ar9)(m–OH)}2, 4, Method A: To
a solution of Ar9SnSnAr9 (Ar9 = C6H3-2,6(C6H3-2,6-Pri2)2) (0.20 g,
0.19 mmol) in toluene (20 mL) was added a solution of TEMPO (0.06 g,
0.38 mmol) in toluene (20 mL) dropwise. The dark green solution faded to
pale red over 24 h. Reduction of the solvent in vacuo to incipient
crystallisation and storage at 218 uC yielded 4 as colorless crystals in high
yield (0.16 g, 0.13 mmol, 68%). Mp: 190 uC (dec). 1H NMR (C6D6,
300.08 MHz): d 20.86 (s, 2H), 1.03 (d, 24H, J = 6.9 Hz, CHMe2), 1.11 (d,
24H, J = 6.9 Hz, CHMe2), 3.00 (sept, 8H, J = 6.6 Hz, CHMe2), 7.05 (m,
6H, Ar–H), 7.12 (m, 8H, Ar–H) 7.21 (m, 4H, Ar–H). 13C NMR (C6D6,
100.52 MHz): d 23.5 (CHMe2), 26.5 (CHMe2), 31.0 (CHMe2), 119.0, 123.6,
128.9, 129.8, 139.6, 145.5, 147.1, 175.2 (unsaturated carbon). 119Sn NMR
(C6D6, 149.00 MHz): d 216.4. IR (KBr, Nujol): 3480 (w, O–H), 760 (s),
730 (s), 450 (m), 380 (w) cm21. UV-Vis (n-hexane): 319 nm (e = 2300). The
remaining solvent was removed in vacuo and the residue was identified as a
mixture of 4 and 2,2,6,6-tetramethylpiperidine by 1H NMR spectroscopy.
Method B: To a solution of Ar9SnSnAr9 (Ar9 = C6H3-2,6(C6H3-2,6-Pri2)2)
(0.20 g, 0.19 mmol) in toluene (20 mL) was added N2O (8.5 mL,
0.38 mmol). The dark green solution faded to pale yellow over 4 h. The
solvent was removed in vacuo and the residue identified as 4 by 1H NMR
spectroscopy. {Ge(Ar9)(m–OH)}2, 5, Method A: To a solution of
Ar9GeGeAr9 (Ar9 = C6H3-2,6(C6H3-2,6-Pri2)2) (0.30 g, 0.32 mmol) in
hexane (20 mL) was added N2O (14.3 mL, 0.64 mmol). The red solution
faded to pale yellow over 30 s. Reduction of the solvent in vacuo to
incipient crystallisation and storage at 218 uC yielded 5 as pale yellow
crystals in high yield 0.26 g, 0.27 mmol, 84%). Mp: 157–158 uC. 1H NMR
(C6D6, 300.08 MHz): d 0.91 (s, 2H), 1.02 (d, 24H, J = 6.9 Hz, CHMe2),
1.04 (d, 24H, J = 6.9 Hz, CHMe2), 3.00 (sept,. 8H, J = 6.6 Hz, CHMe2),
7.00 (m, 6H, Ar–H), 7.12 (m, 8H, Ar–H) 7.23 (t, 4H, Ar–H). 13C NMR
(C6D6, 100.52 MHz): d 23.5 (CHMe2), 26.2 (CHMe2), 31.1 (CHMe2),
111.0, 118.3, 124.1, 126.9, 129.0, 129.8, 146.7 (unsaturated carbon). IR
(KBr, Nujol): 3540 (w, O–H), 940 (w), 750 (s), 720 (s), 460 (m),
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˚
9 Selected bond lengths (A) and angles (u) for 5: Ge1–O1 1.974(1), Ge1–
O19 1.976(1), Ge1–C1 2.034(1), O1–Ge1–O19 75.34(6), O1–Ge1–C1
98.30(6), O19–Ge1–C1 98.31(6), Ge1–O1–Ge19 104.66(6).
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is the LUMO; T. L. Allen, W. H. Fink and P. P. Power, J. Chem. Soc.,
Dalton Trans., 2000, 407.
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Chem. Commun., 2005, 6041–6043 | 6043