An unsymmetric oxo/imido-bridged germanium-centered singlet diradicaloid

Article abstract of DOI:10.1021/ja906053y

(Chemical Equation Presented) Reaction of the digermyne Ar′GeGeAr′ (1) with ONC₆H₄-2-CH₃ affords a unique example of an unsymmetric singlet diradicaloid, the oxo/imido-bridged species Ar′Ge(μ-O)(μ-NC₆H ₄

Full text of DOI:10.1021/ja906053y

Published on Web 09/16/2009
An Unsymmetric Oxo/Imido-Bridged Germanium-Centered Singlet
Diradicaloid
Xinping Wang, Yang Peng, Marilyn M. Olmstead, James C. Fettinger, and Philip P. Power*
Department of Chemistry, UniVersity of California, One Shields AVenue, DaVis, California 95616
Received July 28, 2009; E-mail: pppower@ucdavis.edu
The isolation and characterization of stable singlet diradicaloid
derivatives of the main-group elements are of high current interest,
and several different classes of diradicaloids are now known.1
Among the most prominent are those featuring four-membered-
ring skeletons, as shown in Scheme 1.2 These rings incorporate N
of interligand angles ) 318.6°) and that at N is trigonal-planar (sum
of interligand angles ) 359.7°). The two Ar′ rings are disposed in
a trans fashion across the four-membered GeNGeO ring. The Ge-N
bond lengths [1.866(8) and 1.888(8) Å] are within the range found
in dimeric Ge(IV) imides (1.70-1.88 Å),4 while the Ge-O bond
lengths [1.786(7) and 1.817(7) Å] are shorter than that in 1,3-
cyclodigermoxane [R2Ge(µ-O)]2 [Ge-O ) 1.857(3) Å, R ) 2,6-
diethylphenyl]5 but similar to the average Ge-O bond length
[1.805(9) Å] found in the compound [{(Me3Si)2N}2Ge(µ-O)]2.5 The
long N · · ·O distance (2.466 Å) clearly demonstrates a NdO double-
bond cleavage of the nitrosoarene. The Ge · · · Ge separation
[2.7280(16) Å] is ∼0.3 Å longer than a normal Ge-Ge single bond
(average 2.44 Å),6 indicating essentially no Ge-Ge bonding. The
long Ge · · · Ge separation is consistent with the singlet diradical
character of 2. Its normal 1H and 13C NMR signals indicate that it
has a singlet ground state. Compound 2 is extremely air- and
moisture-sensitive and, when exposed to the atmosphere, rapidly
changes to a white powder that was shown to be a bisoxo-bridged
germanium amide hydride (Scheme 2; for structural details, see
the Supporting Information).
Scheme 1
or P atoms as well as B or a group-14 element. Elegant experimental
and theoretical work by Bertrand and Scholler3a,b and theoretical
studies by various groups3 showed that the singlet diradical
character of the B-P species (II in Scheme 1) can be regulated
by altering the substitutents on the B or P atoms. In addition,
calculations suggested that changing the atoms within the rings
could also have a considerable effect on the molecular energy levels
and hence the extent of diradical character. This has not been
supported by experiments, however.3a,c We showed earlier that the
reaction of Ar′GeGeAr′ [1; Ar′ ) C6H3-2,6(C6H3-2,6-Pri2)2] with
N3SiMe3 afforded the purple diradicaloid Ar′Ge(µ-NSiMe3)2GeAr′
in good yield with N2 elimination.2d The structural data suggested
that the bulky aryl ligand Ar′ would be sufficiently large to stabilize
rings in which the imido substituents are replaced by smaller atoms
or groups. We now report that treatment of 1 with ONC6H4-2-CH3
affords an unsymmetric singlet diradicaloid, the oxo/imido-bridged
species Ar′Ge(µ-O)(µ-NC6H4-2-CH3)GeAr′ (2), which was char-
acterized by spectroscopy, X-ray crystallography, and density
functional theory (DFT) computations. The synthesis of Ar′Ge-
(µ-O)2GeAr′ was also investigated.
Figure 1. (a) Thermal ellipsoid (50%) plot of 2. H atoms and flanking
rings are not shown. Selected bond distances (Å) and angles (deg) for 2:
Ge1-N1, 1.888(8); Ge1-O1, 1.817(7); Ge1-C1, 2.032(9); Ge1· · ·Ge2,
2.7280(16); N1-C61, 1.410(11); N1-Ge1-O1, 83.4(3); Ge1-N1-Ge2,
93.2(3); Ge1-O1-Ge2, 98.4(3); N1-Ge1-C1, 118.3(3); O1-Ge1-C1,
111.0(3). (b) Representation of the HOMO of MeGe(µ-O,µ-NH)GeMe from
DFT calculations. Color code for atoms: gray, Ge; red, N; purple, O; yellow,
C; blue, H.
To further understand 2, we performed DFT calculations7 at the
UB3LYP/6-31G* level on the model compound MeGe(µ-NH)-
(µ-O)GeMe (2′), in which Ar′ was replaced by a smaller methyl
group and C6H4-2-CH3 by H (for details, see the Supporting
Information). 2′ was found to be an energy minimum whose
geometrical parameters compare well to those found in the X-ray
structure of 2. Inspection of the frontier Kohn-Sham orbitals
(Figure 1b) shows that the HOMO corresponds mainly to a
nonbonding combination centered on the Ge atoms with a minor
component at the N centers. This HOMO orbital also has a weak
Ge-C component. The calculated HOMO-LUMO gap (55.3 kcal/
mol) for 2′ is close to the energy difference of 51.60 kcal/mol
suggested by the experimental UV-vis absorption of 2.
Dropwise addition of a toluene solution of ONC6H4-2-CH3 to 1
equiv of 1 in toluene at ∼0 °C yielded, after workup, purple-black
crystals of compound 2 (Scheme 2) in 68% yield. The UV-vis
spectrum of 2 in n-hexane shows a strong absorption maximum at
λmax ) 554 nm (ε ) 2600 M-1 cm-1). The deep-purple color of 2
disappeared when the compound was heated to 186 °C in a sealed
capillary tube.
Scheme 2
A proposed reaction pathway for the formation of 2 is presented
in Scheme 3. The initial step is the interaction between the frontier
orbitals of the digermyne 1 and ONC6H4-2-CH3, which gives an
The structure of 2 has a planar GeNGeO core arrangement
(Figure 1a). The coordination geometry at Ge is pyramidal (sum
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C O M M U N I C A T I O N S
intermediate 3 in which ONC6H4-2-CH3 is σ-bonded through N to
one Ge atom in the C(ipso)GeGeC(ipso) plane.8 The intramolecular
donor-acceptor interaction leads to the [2 + 2] cycloaddition
species 5, which transforms to 6 because of weakening of the
GedGe double bond by the electronegative substituents.8 The
structure of 6 is related to that of Ar′Ge(Ph)NN(Ph)GeAr′, which
is formed from the reaction of 1 with PhNdNPh and also has no
Ge-Ge bond.9 This allows rotation around the N-O bond to give
intermediate 7, in which each Ge atom weakly coordinates both to
N and O atoms.8 Cleavage of the N-O bond affords the singlet
diradicaloid 2.
contain planar Ge2O2 rings.5 The structure is in agreement with
that calculated by DFT methods (see the Supporting Information).
Heavier chalcogenide derivatives RM(µ-Ch)2(η1,η1:µ2-Ch2)MR
[R ) C6H3-2,6-(C6H2-2,4,6-Pri3)2, M ) Sn, Ch ) Se; R )
(Me3Si)3C, M ) Si, Ch ) S or Se) with related geometries have
been reported.11 It is possible that this reaction proceeds via the
sequence shown in Scheme 4, during which cleavage of the O-O
bond to give the singlet diradicaloid Ar′Ge(µ-O)2GeAr′ and
cycloaddition of 1 equiv of triplet diradical O2 occur.
In summary, we have described the first example of a reaction
of a nitrosoarene with a multiple bonded heavier main-group
compound to give a rare example of a singlet diradicaloid
incorporating oxygen. The reaction demonstrates an unexpected
NdO double-bond cleavage and sharply differs from the reaction
of alkynes with nitrosoarenes, which usually affords hydroxyindoles
or methoxyindoles.12 The UV-vis absorption for 2 is ∼3.3 kcal/
mol less than that of the bisimido-bridged analogue IV (Scheme
1), which is in agreement with theoretically predicted 3.6 kcal/mol.7,13
Future work will involve reactivity studies of 2 and reactions of 1
with other nitrosoarenes.
Scheme 3
Acknowledgment. We thank the National Science Foundation
(CHE-0641020) for support of this work.
Supporting Information Available: Crystallographic data for 2 and
8 (CIF), experimental details, and DFT calculations. This material is
available free of charge via the Internet at http://pubs.acs.org.
DFT calculations7 on simple models predicted that successive
replacement of NH with isoelectronic O decreases the HOMO-
LOMO gap by 8.1 kcal/mol and affords a shorter Ge · · · Ge
separation (Table S1 in the Supporting Information). This increases
the diradicaloid character and the isolation difficulty.10 Nontheless,
we investigated the synthesis of the oxo-bridged diradicaloid
Ar′Ge(µ-O)2GeAr′ by the reaction of 1 with O2.
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Scheme 4
Exposure of 1 in toluene to O2 at room temperature provided a
unique species {Ar′Ge(µ-O)2(η1,η1:µ2-O2)GeAr′} (8), in which 2
equiv of O2 was included. The single-crystal X-ray structure of 8
(Figure 2) clearly shows that the two Ge atoms are connected by
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Figure 2. Thermal ellipsoid (50%) plot of 8. H atoms and flanking rings
are not shown. Selected bond distances (Å) and angles (deg): Ge1-O1,
1.792(5); Ge2-O2, 1.812 (5); Ge1-O3, 1.811(6); Ge2-O4, 1.825(5);
O3-O4, 1.528(7); Ge1-C1, 1.911(6); O1-Ge1-O2, 87.1(2); Ge1-O1-Ge2,
84.2(2).
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(13) Molecular orbital calculations show that the HOMO-LUMO gaps for the
model compounds MeGe(µ-NSiH3)(µ-O)GeMe (53.2 kcal/mol) and MeGe(µ-
NC6H5)(µ-O)GeMe (53.1 kcal/mol) are similar, indicating similar effects
of the substituents SiMe3 and C6H4-2-CH3 on the diradical character. For
details, see the Supporting Information.
two µ2-oxo groups and an η1,η1:µ2 O1-O2-coordinated peroxo
bridge. Two terphenyl ligands complete the distorted tetrahedral
environment at each Ge. The Ge1 · · · Ge2 distance [2.4127(10) Å]
is quite short, although there is no Ge-Ge bond. The four-
membered Ge2O2 ring is folded (GeOGeO dihedral angle ) 148.6°),
in contrast to the related Ge(IV) {R2Ge(µ-O)}2 complexes that
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