A ditetrylyne as a π-electron donor: Synthesis and characterization of [AgAr′GeGeAr′]+SbF6- and [Ag 2Ar′GeGe(F)Ar′]+SbF6- (Ar′ = C6H3-2,6(C6H3-2,6- Pri2)2)

Article abstract of DOI:10.1021/ja1051236

The reaction of a digermyne Ar′GeGeAr′ (Ar′ = C ₆H₃-2,6-(C₆H₃-2,6-Pr ⁱ₂)₂) with AgSbF₆ at-40 °C forms the complex (AgAr′GeGeAr′)⁺SbF₆^-, which provides the first example of a heavier group 14 element alkyne analogue behaving as a π donor to a transition metal. The cation (AgAr′ GeGeAr′)⁺ is best described as a hybrid of a π-complex and a σ-metallacyclopropene structure.

Full text of DOI:10.1021/ja1051236

Published on Web 09/01/2010
A Ditetrylyne as a π-Electron Donor: Synthesis and Characterization of
-
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[AgAr′GeGeAr′]+SbF6 and [Ag2Ar′GeGe(F)Ar′]+SbF6
(Ar′ ) C6H3-2,6(C6H3-2,6-Pri2)2)
Xinping Wang, Yang Peng, Marilyn M. Olmstead, Ha˚kon Hope, and Philip P. Power*
Department of Chemistry, UniVersity of California, DaVis, One Shields AVenue, DaVis, California 95616
Received June 11, 2010; E-mail: pppower@ucdavis.edu
Scheme 1. Reactions of 1 with AgSbF6 at -40 and 25 °C
Abstract: The reaction of a digermyne Ar′GeGeAr′ (Ar′ ) C6H3-
2,6-(C6H3-2,6-Pri2)2) with AgSbF6 at -40 °C forms the complex
(AgAr′GeGeAr′)+SbF6-, which provides the first example of a
heavier group 14 element alkyne analogue behaving as a π donor
to a transition metal. The cation (AgAr′GeGeAr′)+ is best de-
scribed as a hybrid of a π-complex and a σ-metallacyclopropene
structure.
Several alkyne analogues (REER, E ) Si, Ge, Sn, Pb; R ) aryl
or silyl ligand) of heavier group 14 elements have been isolated
and characterized.1 Calculations on the Ge and Sn species show
that their bonding can be represented by a or b.2 In valence bond
terms, there is an approximate E-E double bond and a nonbonded,
The structure of 2·SbF6 consists of well-separated cations and
anions. The cation (Figure 1) is characterized by a C2 axis through
the silver atom and midpoint of the Ge-Ge bond. The C-Ge-Ge-C
core of 2 has a trans configuration (Ge-Ge-C ) 130.4(3)°) featuring
a near-planar Ge2{C(ipso)}2 array. The silver bridges the two Ge atoms
in a symmetric fashion. The Ag-Ge bonds (2.6501(14) Å) are ca.
0.2 Å longer than the Ag-Ge(II) bonds in complexes Ag[Ge(OC6HPh4-
2,3,5,6)3(AgOSO2CF3)]·4C6H6, HB{3,5(CF3)2Pz}3]AgGeCl[(Me)2-
ATI] and [HB{3,5-(CF3)2Pz}3]AgGe-(OSO2CF3)[(Me)2ATI] ((Me)2-
ATI ) N-methyl-2-methylamino-troponiminate), consistent with their
bridging character.6 In addition, silver is coordinated by four C atoms
from two flanking rings with Ag-C distances of 2.636(10) and
2.680(10) Å that are considerably longer than Ag-arene bonds (∼2.4
Å) in Ag(η2-PhMe)2(Al(OCH(CF3)2)4.7 The Ge-Ge distance (2.303(2)
Å) is within the range (2.2060(7)-2.307(1) Å) for digermynes1b,8,9
and is slightly longer than the 2.2850(6) Å of the Ge-Ge bond in
uncomplexed 1.1b The closest previously known structure to 2 is that
of NaAr′GeGeAr′10 (cf. very similar effective radii of four coordinate
resonating lone pair at an E. It is also possible to represent the
Ag+ (1.00 Å) and Na+ (0.99 Å)).11 In that compound, however, the
structure as the singlet diradicaloid form c.2 Their Lewis acidic
Na· · ·Ge distances (3.1391(15) and 3.1596(15) Å) are ca. 0.5 Å longer
although the closest Na-C distances are 0.15-0.2 Å longer than
Ag-C in 2. These distances suggest a significantly stronger Ge-M
character has been supported by reactions of REER (E ) Si, Ge,
Sn) with R′NC: (R′ ) But, Mes, SiMe3) that led to the formation
of adducts shown by d or e.3 In contrast, complexes such as f or g
demonstrating Lewis basic properties and species such as h, in
which REER acts as a π electron donor resembling the alkyne
congener i, have not been described.4 We now report that the
reaction of Ar′GeGeAr′ (1, Ar′ ) C6H3-2,6-(C6H3-2,6-Pri2)21b with
AgSbF6 at -40 °C (Scheme 1) leads to a metal cation REER adduct
(AgAr′GeGeAr′)+SbF6- (2·SbF6) with an SbF6- counteranion, the
first example of a heavier group 14 element alkyne analogue-
transition-metal π-complex.5
Reaction of AgSbF6 and 1 equiv of 1 in chlorobenzene at -40
°C yielded, after workup, green crystals of compound 2·SbF6 in
Figure 1. Thermal ellipsoid (50%) drawing of 2. H atoms and Pri groups
23% yield (Scheme 1). The UV-vis spectrum of 2·SbF6 in C6H5Cl
are not shown. Selected bond distances (Å) and angles (deg): Ag1-C9
displays an absorption maximum at λmax ) 412 nm (ε ) 640), which
2.636(10), Ag1-Ge1 2.6501(14), Ag1-C8 2.680(10), Ge1-C1 1.972(9),
Ge1-Ge1′ 2.303(2), Ge1-Ag1-Ge1′ 51.51(5), C1-Ge1-Ge1′ 130.4(3),
C1-Ge1-Ag1 108.6(3), Ge1-Ge1′-Ag1 64.24(2).
differs considerably from that of uncomplexed 1 (λmax ) 501 nm
(ε ) 7500), 371 nm (34 000)).1b
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13150 J. AM. CHEM. SOC. 2010, 132, 13150–13151
10.1021/ja1051236  2010 American Chemical Society

C O M M U N I C A T I O N S
interaction for the silver species especially in view of the fact that, in
NaAr′GeGeAr′, the “Ar′GeGeAr′” unit formally carries the negative
charge.10
F1 are disordered with respect to a crystallographic inversion center,
and each atom was refined at half-occupancy at the two disordered
sites (see Supporting Information). The Ge-Ag distances (avg.
2.5310(11), 2.7415(12) Å) are unequal with the shorter bond distance
to the fluorinated Ge. The Ge-F distance (1.795(5) Å) is close to that
in compounds (e.g., 1.805(17) Å in [{HC-(CMeNAr)2}GeF] (Ar )
2,6-iPr2C6H3)).14 The Ge-Ge distance (2.4861(12) Å) may be
compared with the 2.44 Å of a single Ge-Ge bond.15 4 can be viewed
as an anionic digermyne fluoride adduct that is π-bonded to two Ag+
ions.16 Its formation and possible resonance structures of the
Ar′GeGe(F)Ar′ monoanion are shown in Scheme 2.
To further understand 2, we performed DFT calculations12 for
the model complex cation Ag(MeGeGeMe)+ (3). This was found
to have an energy minimum whose geometrical parameters compare
well with those found in the X-ray structure of 2. The calculated
HOMO-LUMO gap (71.0 kcal/mol) for 3 is close to the energy
difference of 69.4 kcal/mol suggested by the experimental UV/vis
absorption at 412 nm. Natural bond orbital (NBO) calculations
Scheme 2
In summary, it has been shown that digermyne Ar′GeGeAr′ (1)
reacts with Ag+ at low temperature to form the side-on complex 2.
Its geometry, the Ge-Ge bond lengthening, and calculated electron
densities indicate that the digermyne 1 is acting as a π electron
donor. The exploration of its chemistry is in hand.
reveal that 0.43 electrons are donated from the digermyne
MeGeGeMe to Ag+. According to the calculated second-order
perturbation interaction energies, the strongest orbital interaction
(54.0 kcal/mol) in 3 (and thus in 2) is between the π orbital
(HOMO) of the Ge-Ge bond and the empty 5s or 5p orbitals of
the Ag+ cation, shown above as j and k. The back-donation (l)
from the 4dxz orbital of Ag+ into the empty π* orbital (LUMO+1)
of Ge-Ge (l) is relatively weak (5.5 kcal/mol). Complex 2 is best
described as a hybrid of a π-complex m and a σ-metallacyclopro-
pene structure n shown below.
Acknowledgment. We thank the National Science Foundation
(CHE-0948417) for support of this research.
Supporting Information Available: CIF for 2 and 4, experimental
data, and DFT calculations. This material is available free of charge
via the Internet at http://pubs.acs.org.
References
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(F)Ar′]SbF6 (4·SbF6) in 30% yield were obtained upon cooling
(Scheme 1). The reaction of 1 with AgSbF6 at room temperature also
gave 4·SbF6 in a similar yield. The mechanism of formation of 4 is
unclear, but an abstraction of F from SbF6- to form what may be a
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Figure 2. Thermal ellipsoid (50%) drawing of 4 (only one of the disordered
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