Tethered heavy dicarbene analogues: Synthesis and structure of ditetryldiyl ethers (Ar′E)2(μ-O) (E = Ge, Sn; Ar′ = C 6H3-2,6-(C6H3-2,6- iPr2)2)

Article abstract of DOI:10.1021/om2004018

Ditetryldiyl ethers (Ar′E)₂(μ-O) (E = Ge, Sn; Ar′ = C₆H₃-2,6-(C₆H₃-2,6- ⁱPr₂)₂) which contain a pair of two-coordinate Sn or Ge atoms were successfully prepared by the reaction of the corresponding dimetallylene (Ar′E)₂ with 1 equiv of pyridine N-oxide. The tin oxide was found to cocrystallize with a hydroxide impurity: {Ar′Sn(μ-OH)}₂. Structural determination revealed a slightly bent oxide E-O-E core (E = Ge, 154.8(1)°; E = Sn, 154.7(3)°) and a trans orientation of the Ar′ groups in each complex. The compounds were also characterized by NMR and electronic spectroscopy.

Full text of DOI:10.1021/om2004018

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pubs.acs.org/Organometallics
Tethered Heavy Dicarbene Analogues: Synthesis and Structure of
Ditetryldiyl Ethers (Ar0E)2(μ-O) (E = Ge, Sn; Ar0 =C6H3-2,6-(C6H3-2,6-iPr2)2)
Owen T. Summerscales, Marilyn M. Olmstead, and Philip P. Power*
Department of Chemistry, University of California, Davis, California 95616, United States
S Supporting Information
b
ABSTRACT: Ditetryldiyl ethers (Ar0E)2(μ-O) (E = Ge, Sn; Ar0 = C6H3-2,6-(C6H3-
2,6-iPr2)2) which contain a pair of two-coordinate Sn or Ge atoms were successfully
prepared by the reaction of the corresponding dimetallylene (Ar0E)2 with 1 equiv of
pyridine N-oxide. The tin oxide was found to cocrystallize with a hydroxide impurity:
{Ar0Sn(μ-OH)}2. Structural determination revealed a slightly bent oxide EꢀOꢀE
core (E = Ge, 154.8(1)°; E = Sn, 154.7(3)°) and a trans orientation of the Ar0 groups
in each complex. The compounds were also characterized by NMR and electronic
spectroscopy.
Scheme 1. Formation of Ditetryldiyl Ethers from
Dimetallynes
he amphoteric character of singlet carbenes and their
heavier group 14 element analogues has generated a
T
wealth of chemistry for these interesting species1,2 and their
transition-metal derivatives.3 For the heavier element deriva-
tives (“metallylenes”) the two-coordinate centers are character-
ized by the presence of a nonbonded pair and by a vacant p
orbital and are typically stabilized by bulky ligands which
prevent dimerization to dimetallenes.2,4 Molecules containing
two metallylene centers are rare but can be prepared using electro-
negative bridging ligands: e.g., imido-bridged {(Mes*NH)Ge}2-
(μ-NMes*)5 and (Ar0Sn)2(μ-NSiMe3) (Ar0 = C6H3-2,6-(C6H3-
2,6-iPr2)2), the latter also being characterized by X-ray
crystallography.6 They can also be prepared using longer chains
of atoms connecting the metallylenes.7ꢀ9 In addition, base-
stabilized derivatives in which the heavy atom is three- rather
than two-coordinate have been recently reported with mon-
atomic bridging ligands: cf. {(L2)E}2(μ-X) (E = Si, Ge; X = O, S;
L2 = amidinate).10,11 The basic character of these compounds
was demonstrated by the successful synthesis of a chelated
nickel derivative. Bridged dimetallylenes of the general formula
{(R)E}2(μ-X) (E = group 14 element; X = bridging atom; R =
monodentate ligand) have not been reported, however. They are
of interest not only from a fundamental perspective but also as
chelating ligands in transition-metal catalysis.
{Ar0E(μ-OH)}2,15 as reported previously. Use of TEMPO (BDE
101.0 kcal/mol) gave the spin-trapped molecule Ar0Ge(TEMPO)
or, in the case of tin, the hydroxide {Ar0Sn(μ-OH)}2.15
We report here the stoichiometric use of an oxygen transfer
reagent of intermediate strength, pyridine N-oxide (BDE 63.3
kcal/mol). Reaction with Ar0GeGeAr0 in toluene successfully
gave 1 in 42% yield as a pure compound (Scheme 1).16 Similar
reaction with Ar0SnSnAr0 led to a mixture of the desired
bridging oxide 2 and {Ar0Sn(μꢀOH)}2 (3). Screening against
the oxygen transfer reagents Me3NO, Me2SO and Me3PO gave
the same mixture in a similar ratio for Me3NO and Me2SO but
no reaction for Me3PO. The hydroxide is speculated to form
from a putative {Ar0Sn•(μ-O)}2 singlet diradicaloid ꢀ the
product of overoxidation ꢀ which presumably CꢀH activates
solvent or ligand (Scheme 2). Slowing the rate of addition,
changing the concentration of reagents or the temperature of
Toward this objective, we have successfully prepared the
ditetryldiyl ethers (Ar0E)2(μ-O) (E = Ge (1), Sn (2)), which
contain a pair of two-coordinate Sn or Ge centers.
12
Oxidation of the digermyne Ar0GeGeAr0 or distannyne
Ar0SnSnAr013 using an excess of O2 or N2O (bond dissociation
energy (BDE) 39.0 kcal/mol)14 led to overoxidation and
isolation of the hydroxides {Ar0Ge(OH)}2(μ-O2)(μ-O)6 and
Received: May 17, 2011
Published: June 07, 2011
r
2011 American Chemical Society
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Scheme 2. Formation of μ-Oxo (2) and μ-Hydroxide Species (3) from Distannyne
reaction did not remove the impurity. The reaction was carried
out under strict anhydrous conditions, and it is not likely that
the hydroxide arose from an H2O contaminant - no evidence
was found for {Ar0Ge(μꢀOH)}2 in the parallel reaction with
1 using the same reagents. Changing solvent from toluene to an
aliphatic solvent (methylcyclohexane) did not change the
propensity for hydroxide formation. Furthermore, the use of
d8-toluene did not alter the integral value of the hydroxide proton
1
in the H NMR spectrum but it is conceivable that CꢀH
activation occurred with a sacrificial equivalent of the terphenyl
ligand. The CꢀH activation of terphenyl ligands has been
previously demonstrated in a digermanium species which also
appears to occur via diradicaloid intermediate.6 It is currently
unclear why the Sn reaction is more easily overoxidized than that
of the Ge compound, although the larger size of Sn in addition to
its lower reactivity may allow the coordination of two equiva-
lents of pyridine N-oxide before oxygen transfer occurs.
Although 2 could not be isolated in a pure form, reliable data
regarding its structure and chemical properties could be ob-
tained nonetheless.
The digermyldiyl ether 1 was isolated as a green air-sensitive
Figure 1. Thermal ellipsoid (50%) plot of 1. H atoms are not shown.
Selected bond lengths (Å) and bond angles (deg) for 1: Ge(1)ꢀO(1) =
Ge(1)ꢀO(1)ꢀGe(1)0 = 154.75(12), O(1)ꢀGe(1)ꢀC(1) = 109.98(8).
solid which displays limited solubility in hydrocarbon solvents.16
Structural determination using X-ray methods revealed a pair of
two-coordinate Ge centers bridged by a single oxygen atom,
which was found to be disordered equally over two positions
related by a mirror plane (Figure 1).17 The terphenyl ligands are
disposed in a trans-bent geometry through an angle of 110.0(1)°
for O1ꢀGe1ꢀC1. The central GeꢀOꢀGe moiety is found to
have a bending angle of 154.8(2)°, in contrast to that found in the
bulky imido analogue (Ar0Ge)2(μ-NAd) at 111.38(9)°.18 The
GeꢀO distance of 1.760(2) Å is similar to that found in divalent
{(L2)Ge}2(μ-O) (1.766(5) Å)11 but is much shorter than those
found for (Ar0GeOH)2 (1.976(6) and 1.974(6) Å), consistent
with a bridging oxo rather than hydroxo unit. The internuclear
1.760(2), Ge(1)ꢀC(1) = 2.0215(13), Ge(1) Ge(1)0 = 3.868(3);
3 3 3
Ge Ge separation is nonbonding at 3.868(3) Å and is much
3 3 3
longer than that in the more strongly bent hydroxide (Ar0Ge-
OH)2 (3.127(4) Å).
In the crystal chosen for studies, the tin analogue 2 cocrys-
tallized with hydroxide 3.19 Both 2 and 3 have identical ligand
spatial coordinates (hence their ready cocrystallization), showing
a similar trans-bent geometry but with different mutually perpen-
dicular tinꢀoxygen core moieties (Figure 2). The central unit
was refined best in two parts: 63% [Sn(μ-OH)2Sn] in a geometry
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Figure 2. Thermal ellipsoid (50%) plots of cocrystallized 2 and 3. H atoms are not shown. Selected bond lengths (Å) and bond angles (deg): for 2,
Sn(2)ꢀO(2) = 1.957(6), Sn(2)ꢀC(1) = 2.2348(15), Sn Sn = 3.894(1), Sn(2)ꢀO(2)ꢀSn(2)0 = 154.7(3); for 3, Sn(1)ꢀO(1) = 2.1354(14),
3 3 3
3 3 3
Sn(1)ꢀO(1)0 = 2.1360(13), Sn(1)ꢀC(1) = 2.2200(11), Sn Sn = 3.433(1), Sn(1)ꢀO(1)ꢀSn(1)0 = 107.0(2).
already reported and 37% [Sn(μ-O)Sn] in a geometry isostruc-
tural with that found in the Ge congener 1, with an O atom
disordered over two positions. As with 1, we find the SnꢀXꢀSn
angle to be less bent in (Ar0Sn)2(μ-X) for X = O (2: 154.7(3)°)
than for bulky X = NSiMe3 (105.4(3)°)6 or for hydroxide 3
equiv of Me3NO or Me2SO to solutions of 1 in methylcyclo-
hexane gave a colorless, intractable mixture of products, pre-
sumably due to the expected high reactivity of the anticipated
diradical. This is in contrast to the case for the highly colored
diimido {Ar0Ge•(μ-NSiMe3)}2 and oxo/imido (Ar0Ge•)2(μ-O)-
(μ-NDipp) (Dipp = C6H3-2,6-iPr2) singlet diradicaloids, which
are found to be sufficiently stable to isolate in pure form at room
temperature.22,23 DFT calculations performed on simple model
compounds have shown that replacement of an imido with an
oxo group decreases the HOMOꢀLUMO gap by 8.1 kcal/mol,
in turn increasing the triplet diradicaloid character and also the
instability of the molecule.23 Attempts to reverse the oxidation
process by reacting 2 with phosphines or alkali metals were
unsuccessful.
(107.0(2)°).13 Likewise, the Sn Sn distance is longer in 2
3 3 3
(3.894(1) Å) than in 3 (3.433(1) Å). The tinꢀoxo distance
SnꢀO = 1.957(6) Å in 2 is much shorter than that in the
hydroxide 3 (2.135(1) Å), which is also consistent with the ca.
0.2 Å difference between oxo and hydroxo germanium analogues.
The 1H NMR spectra of 1 and 2 show the expected signals for
Ar0 and in the case of 2 showed signals consistent with the
hydroxide impurity 3 in ca. 20% proportion (although this figure
varied slightly between different batches of crystalline samples).
The more tightly bonded Ge compound shows diastereotopic
isopropyl resonances, whereas the Sn analogue does not display
thismagneticinequivalence, presumablyforsteric reasons. A signal
in the 119Sn{1H} NMR spectrum was found for 2 at 903.7 ppm,
comparable with that of (Ar0Sn)2(μ-NSiMe3) at 907.1 ppm,6 and
is significantly downfield of the hydroxide impurity 3 at
381.6 ppm.13 A distinctive absorption is observed in the UVꢀ
visible spectra for the n f p transition at 450 nm (ε = 870) for 1
and at a slightly lower energy of 460 nm (ε = 1150) for 2.
The formation of the tin products is proposed to occur via
Scheme 2. The first step is very likely the formation of a 1:1
adduct in which the pyridine oxide behaves as a Lewis base donor
It has been generally accepted that isolation of stable metally-
lenes requires bulky ligands for kinetic stabilization. These results
demonstrate that bridging oxo derivatives may be synthesized for
dimetallylenes that maintain a two-coordinate group 14 element
atom when used in combination with bulky terminal coligands.
’ ASSOCIATED CONTENT
1
S
Supporting Information. Figures giving the H NMR
b
spectra of 1 and the mixture of 2 and 3 and CIF files giving the
crystallographic data for 1ꢀ3. This material is available free of
charge via the Internet at http://pubs.acs.org.
to the n LUMO of the distannyne in the C(Ar0)SnSnC(Ar0)
þ
plane. The formation of such a complex is precedented by the
synthesis ofisonitrileadductsofthedigermenes6 and distannynes.20
The formation of related adducts is known for disilynes, digermynes,
and distannynes upon treatment with isonitriles.6,21 The initial
Ar0SnSn(pyrO)Ar0 can then form 2 by pyridine elimination with
rearrangement of the unsymmetric monoxide species. 2 may
then react with another 1 equiv of pyrO to afford the diradicaloid
{Ar0Sn•(μ-O)}2, which may also form via the bis pyridine oxide
adduct {Ar0Sn(pyrO)}2. The diradicaloid {Ar0Sn•(μ-O)}2 may
then abstract H from the solvent to give 3.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: pppower@ucdavis.edu.
’ ACKNOWLEDGMENT
We are grateful to the U.S. Department of Energy (No. DE-
FG02-07ER4675-03) for support of this work and to Dr. Y. Peng
for useful discussions.
In principle, the diradical bridging dioxide species {Ar0Ge•-
(μ-O)}2 should be accessible from the monoxide 1 by addition of
another 1 equiv of oxidizing agent. However, addition of 0.9
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Chem. Commun. 2005, 6041. Numerous attempts at stoichrometric
oxidation of Ar‡SnSnAr‡ or Ar*SnSnAr* (Ar* = C6H3-2,6(C6H2-2,4,
6-Pri3)2) by O2 or N2O with rigorous exclusion of air and moisture
afforded {ArSn(μ-OH)}2 (Ar = Ar‡, Ar*) products which displayed
hydroxide-bridged structures similar to that of 3 as well as some variation
in SnꢀO bond lengths.
(16) All reactions were performed under anaerobic and anhydrous
conditions. 1: to a solution of (Ar0Ge)2 (0.303 g, 0.32 mmol) in 30 mL of
toluene was added dropwise a solution of pyridine N-oxide (0.026 g,
0.32 mmol) at ꢀ78 °C, resulting in a color change from dark red to
orange with a green precipitate upon warming. Heating the solution to
100 °C under static vacuum enabled the dissolution of the green
precipitate, and upon cooling to room temperature overnight, green
crystals of the product 1 were obtained (0.130 g, 0.13 mmol, 42% yield).
Mp: 180 °C dec. 1H NMR (300 MHz, C6D6, 298 K): δ 1.03 (d, 24H,
3
3
o-CH(CH3)2, JHH = 6.9 Hz), 1.20 (d, 24H, o-CH(CH3)2, JHH
=
6.9 Hz), 2.89 (sept, 8H, CH(CH3)2, 3JHH = 6.9 Hz), 7.05ꢀ7.49 ppm (m,
18H, m-C6H3, p-C6H3,m-Dipp, and p-Dipp, Dipp = C6H3-2,6-Pri2).
13C{1H} NMR (C6D6, 100.6 MHz, 298 K): δ 23.7 (CH(CH3)2), 25.4
(CH(CH3)2), 30.8 (CH(CH3)2), 123.6 (m-Dipp), 129.0 (p-C6H3),
129.2 (o-Dipp), 137.2 (m-C6H3), 142.7 (p-Dipp), 146.9 (i-Dipp), 169.1
(o-C6H3). UV: λmax (ε) 450 nm (870 molꢀ1 L cmꢀ1). 2 and 3: to a
solution of (Ar0Sn)2 (0.283 g, 0.27 mmol) in 30 mL of toluene was
added dropwise a solution of pyridine N-oxide (0.022 g, 0.27 mmol)
at ꢀ78 °C, resulting in a color change from dark green to orange with a
red precipitate upon warming. Heating the solution to 100 °C under
static vacuum enabled the dissolution of the red precipitate, and upon
cooling to room temperature overnight, orange crystals of a mixture of 2
and 3 were obtained (0.152 g). 1H NMR (300 MHz, C6D6, 298 K): (2
3
only) δ 1.04 (d, 48H, o-CH(CH3)2, JHH = 6.9 Hz), 3.09 (sept, 8H,
CH(CH3)2, 3JHH = 6.9 Hz), 7.05ꢀ7.49 ppm (m, 18H, m-C6H3, p-C6H3,
m-Dipp, and p-Dipp). 13C{1H} NMR (C6D6, 100.6 MHz, 298 K):
δ 23.8 (CH(CH3)2), 25.8 (CH(CH3)2), 30.6 (CH(CH3)2), 123.8
(m-Dipp), 129.1 (p-C6H3), 130.2 (o-Dipp), 137.8 (m-C6H3), 143.9
(p-Dipp), 146.9 (i-Dipp), 147.4 (o-C6H3). 119Sn{1H} NMR (C6D6, 100.6
MHz, 298 K): δ 903.7 ppm. UV: λmax (ε) 460 nm (1150 molꢀ1 L cmꢀ1).
(17) Crystal data for 1 at 90(2) K with Cu KR radiation
(λ = 0.710 73 Å): triclinic, space group P1, Z = 2, a = 9.1539(3) Å,
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