A digermyne with a Ge-Ge single bond that activates dihydrogen in the solid state

Article abstract of DOI:10.1021/ja209215a

The reduction of the bulky amido-germanium-(II) chloride complex, LGeCl (L = N(SiMe₃)(Ar^*); Ar^* = C ₆H₂Me{C(H)Ph₂}₂-4,2,6), with the magnesium(I) dimer, [{(^MesNacnac)M

Full text of DOI:10.1021/ja209215a

COMMUNICATION
pubs.acs.org/JACS
A Digermyne with a GeꢀGe Single Bond That Activates Dihydrogen in
the Solid State
Jiaye Li,† Christian Schenk,† Catharina Goedecke,‡ Gernot Frenking,*,‡ and Cameron Jones*,†
†School of Chemistry, Monash University, P.O. Box 23, Clayton, Melbourne, VIC 3800, Australia
‡Fachbereich Chemie, Philipps-Universit€at Marburg, 35032, Marburg, Germany
S Supporting Information
b
Scheme 1. Syntheses of Compounds 2 and 3 (Byproducts
Omitted)
ABSTRACT: The reduction of the bulky amido-germanium-
(II) chloride complex, LGeCl (L = N(SiMe3)(Ar*); Ar* =
C6H2Me{C(H)Ph2}2-4,2,6), with the magnesium(I) dimer,
[{(MesNacnac)Mg}2] (MesNacnac = [(MesNCMe)2CH]ꢀ;
Mes = mesityl), afforded LGeGeL, which represents the first
example of a digermyne with a GeꢀGe single bond. Computa-
tional studies of the compound have highlighted significant
electronic differences between it and multiply bonded diger-
mynes. LGeGeL was shown to cleanly activate H2 in solution
or the solid state, at temperatures as low as ꢀ10 °C, to give the
mixed valence compound, LGeGe(H)2L.
he past two decades have seen major fundamental advances
T
in the chemistry of low oxidation state p-block compounds.1
Perhaps the most archetypal of such systems are the heavier
group 14 element(I) alkyne analogues (or ditetrelynes), REER
(E = Si, Ge, Sn, or Pb; R = bulky terphenyl, silyl, or aryl), which
unlike linear alkynes, have increasingly “trans-bent” REE frag-
ments as the group is descended (RSiSi ∼135° to RPbPb
∼92°).2 The results of many theoretical analyses of the bonding
in ditetrelynes have shown that their EꢀE bond orders can range
between three and one.3 The most studied systems are the
digermynes, RGeGeR, which invariably have short (ca. 2.2ꢀ2.3 Å)
GeꢀGe multiple bonds, and can exhibit significant singlet biradi-
caloid character.2,4 Their resultant high reactivity toward small
molecules and unsaturated substrates has led to digermynes be-
ing described as main group mimics of transition metal com-
plexes.1a,d This reactivity is epitomized by the landmark study of
Power et al. who, in 2005, demonstrated for the first time that
H2 can be activated by a main group metal complex, Ar0GeGeAr0
(Ar0 = C6H3(C6H3Pri2-2,6)2-2,6), at room temperature and atmo-
spheric pressure.5 The hydrogenation of Ar0GeGeAr0 yielded
mixtures of Ar0(H)GeGe(H)Ar0, Ar0(H)2GeGe(H)2Ar0, and
Ar0GeH3, the compositions of which were shown to be depen-
dent upon the reaction stoichiometry.5 Since 2005, a variety of
stable carbenes, main group based Frustrated Lewis Pairs, and
other low oxidation p-block state systems have been shown to
activate H2 at or near ambient temperature.6
multiple bonding by offering the possibility of N lone pair f Ge
π-interactions in the complex. Here, we report the synthesis,
characterization, and bonding analysis of the first amido-sub-
stituted digermyne, which possesses a GeꢀGe single bond, and
which cleanly activates H2 in solution or the solid state at
atmospheric pressure and temperatures as low as ꢀ10 °C.
The reduction of the bulky monomeric amido-germanium(II)
chloride, LGeCl 1 (L = N(SiMe3)(Ar*); Ar* = C6H2Me{C-
(H)Ph2}2-4,2,6),9 with half an equivalent of the magnesium(I)
dimer, [{(MesNacnac)Mg}2] (MesNacnac = [(MesNCMe)2
CH]ꢀ; Mes = mesityl),10 afforded a moderate isolated yield
(55%) of 2 as an extremely air sensitive pink-orange solid after
recrystallization from diethyl ether (Scheme 1). It is of note that
attempts toprepare2 viathereductionof 1 with alkalimetals, KC8,
or sodium naphthalide, gave, at best, only low yields of 2, and
generally led to the deposition of elemental germanium. Solutions
of 2 in aromatic solvents are red-purple in color (toluene: λmax
[nm] (ε [L molꢀ1 cmꢀ1]): 531 (588), 383 (3235)) and exhibit
NMR spectra consistent with the proposed formulation of the
compound.
The molecular structure of 2 (Figure 1) reveals the compound
to be a trans-bent dimer with NꢀGeꢀGe angles (100.09(6)°)
that are markedly more acute than the CꢀGeꢀGe angles in
all previously reported digermynes (range: 123.6ꢀ138.7°2).
Although digermynes containing GeꢀGe single bonds are
unknown,7 they are attractive synthetic targets both for funda-
mental reasons, and because their reactivity could differ signifi-
cantly to that of their well studied multiply bonded counterparts.
We reasoned such systems could be accessed by incorporation of
very bulky monodentate amide ligands,8 which would not only
provide kinetic protection, but could also circumvent GeꢀGe
Received: September 30, 2011
Published: October 25, 2011
r
2011 American Chemical Society
18622
dx.doi.org/10.1021/ja209215a J. Am. Chem. Soc. 2011, 133, 18622–18625
|

Journal of the American Chemical Society
COMMUNICATION
Figure 1. Thermal ellipsoid plot (20% probability surface) of the
molecular structure of [LGeGeL] (2); hydrogen atoms are omitted
for clarity. Selected bond lengths (Å) and angles (°): Ge(1)ꢀN(1)
1.872(2), Ge(1)ꢀGe(1)0 2.7093(7), Si(1)ꢀN(1) 1.762(2), N(1)ꢀ
C(1) 1.445(3), N(1)ꢀGe(1)ꢀGe(1)0 100.09(6), C(1)ꢀN(1)ꢀSi(1)
118.50(16), C(1)ꢀN(1)ꢀGe(1) 110.12(15), Si(1)ꢀN(1)ꢀGe(1)
131.38(11). Symmetry operation: 0 ꢀx + 1/2, ꢀy + 1, ꢀz + 1.
Figure 2. (a) LUMO (ꢀ2.934 eV), (b) HOMO (ꢀ3.557 eV), (c)
HOMO-6 (ꢀ5.844 eV), and (d) HOMO-23 (ꢀ6.623 eV) of 2.
Given the emerging “transition metal-like” reactivity of multi-
ply bonded digermynes,1a,d reactions of 2 with dihydrogen were
examined. When toluene or d6-benzene solutions of the com-
pound were exposed to an excess of H2 at 1 atm and 20 °C, the
orange monohydrogenation product, 3, was formed in essentially
quantitative yield in less than 20 min (Scheme 1). Similar
reactions carried out at 0 and ꢀ10 °C were complete after 1
and 2 h, respectively. Remarkably, when finely ground crystalline
samples of 2 were placed under 1 atm of H2 at 20 °C, compound 3
was also formed in yields of >95% after 1 h. Similar solid-state
reactions carried out at ꢀ10 °C led to formation of 3 in yields of
ca. 60% after 1 h. In contrast to Ar0(H)GeGe(H)Ar0, solutions of
compound 3 do not react with further equivalents of H2 up to
100 °C.15 It is of note, however, that mixtures of the formal
double hydrogenation product, L(H)2GeGe(H)2L 4, and 3 were
obtained from the reactions of LGeCl with either Li[HBBus3] or
K[HBEt3].16 Considering the thermal stability and reluctance to
hydrogenation of 3, the germanium(III) hydride, 4, probably
results from the disproportionation of an unstable monomeric
intermediate, LGeIIH, in these reactions. The other product, 3,
can easily be envisaged as ultimately resulting from the dimeriza-
tion of LGeIIH (vide infra).
In addition, the GeꢀGe separation (2.7093(7)Å) in the com-
pound is consistent with a very long single bond (sum of two Ge
covalent radii = 2.44 Å),4 which is more than 0.4 Å longer than
those of other multiply bonded digermynes.2 It is noteworthy
that terphenyl substituted, singly bonded distannynes and di-
plumbynes related to 2 have been reported.11 The differences
between 2 and other digermynes could result from the fact that
its NSiCGe and Ge2N2 least-squares planes are essentially
coplanar (dihedral angle: 5.2°), thereby potentially allowing
N f Ge π-interactions,12 which in turn would discourage
GeꢀGe multiple bond formation. In this respect, compound 2
could be viewed as an unprecedented two-coordinate 1,2-bis-
(germylene), that is, a heavier analogue of as yet unknown 1,2-
dicarbenes. There are no significant arylꢀGe contacts (closest:
3.34 Å, cf. 3.03 Å in LGeCl9) in the compound.
To shed light on the electronic structure of 2, DFT calculations
were carried out on the full molecule in the gas phase (RI-BP86/def2-
SVP). Its optimized geometry is very similar to that in the crystal
structure of the compound, but with slightly overestimated Geꢀ
Ge (2.764 Å) and NꢀGe (1.909 Å; NꢀGeꢀGe angle: 100.1°)
bonds (see Supporting Information for full details). An analysis of the
frontier orbitals of the molecule (see Figure 2) revealed that the
GeꢀGe single bond is associated with the HOMO, and is of very high
p-character (92.8%). This is likely a significant factor behind the
length of this bond13 (cf. the GeꢀGe distance in H3GeGeH3: 2.403
Å14). The LUMO has considerable GeꢀGe π-bonding character,
while the highest energy orbital with significant Ge lone pair character
is the HOMO-23, which lies some 3.06 eV below the HOMO (N.B.
the HOMOꢀLUMO gap is very narrow at 0.62 eV, cf. 2.10 eV for
MeGeGeMe1a). As expected, several MOs (e.g., HOMO-6) exhibit
significant NꢀGe π-bonding character which is seemingly the basis
of the electronic differences between 2 and the aforementioned
multiply bonded aryl substituted digermynes. It is of note that a
previous theoretical study on the parent digermyne, HGeGeH, has
shown that its singly bonded form (GeꢀGe 2.720 Å, HꢀGeꢀGe
90.6°) is 15.3 kcal/mol higher in energy than the multiply bonded
isomer (GeꢀGe 2.211 Å, HꢀGeꢀGe 124.0°).3b In contrast, in
the current study, the multiply bonded isomer of 2, viz., 2a
(GeꢀGe 2.393 Å, NꢀGeꢀGe 119.4°), was calculated to lie ΔG =
10.0 kcal/mol higher in energy than 2.
Both compounds 3 and 4 display two GeꢀH stretching bands in
their infrared spectra (3:ν1990, 2031 cmꢀ1;4:ν2030, 2076 cmꢀ1),
which lie in the expected region for terminal tetravalent germanium
hydrides.17 In their 1H NMR spectra, hydride resonances appear at
δ 6.13 ppm (3) and δ 4.91 ppm (4); however, that for 3 is very
broad at 20 °C, as are the silyl and methine resonances for the
compound. Upon warming a d8-THF solution of 3 to 70 °C, these
resonances sharpen, yielding a spectrum consistent with a com-
pound having chemically equivalent amide ligands. In contrast,
cooling the same solution to ꢀ50 °C produced a resolved spectrum
with one hydride resonance and two sets of amide resonances. It
appears that a fluxional process is occurring at higher temperatures
which is rapid on the NMR time scale. This could involve an
equilibrium between the mixed valence compound, 3, and its sym-
metrical germanium(II) isomer, L(H)GeGe(H)L 5, which inter-
convert via intramolecular hydride migration. A similar equilibrium
has been proposed between Ar0(H)GeGe(H)Ar0 and Ar0(H)2Ge
GeAr0,1a evidence for which comes from the addition of PMe3 to a
solution of Ar0(H)GeGe(H)Ar0, which leads to the base stabilized
mixed valence compound, Ar0(H)2GeGeAr0 PMe3.18
3
18623
dx.doi.org/10.1021/ja209215a |J. Am. Chem. Soc. 2011, 133, 18622–18625

Journal of the American Chemical Society
COMMUNICATION
toward H2, could well count against this proposal. We are
currently employing experimental and computational techniques
to explore the mechanism, kinetics, and potential reversibility of
the hydrogenation of 2. In addition, studies of the use of this
reactive molecule for the “transition metal-like” activation of
other small molecules are in hand. These investigations will form
the basis of future publications.
’ ASSOCIATED CONTENT
S
Supporting Information. Details of the synthesis and
b
characterizing data for 2ꢀ4. Full details and references for the
crystallographic and computational studies. Crystallographic data in
CIF format, ORTEP diagrams for 4, [L(H)2GeGe(H)(OH)L] and
[L(H)2GeGe(H){K(OEt2)2}L]. This material is available free of
charge via the Internet at http://pubs.acs.org.
Figure 3. Thermal ellipsoid plot (20% probability surface) of the molec-
ular structure of [LGeGe(H)2L] (3); hydrogen atoms (except hydrides)
are omitted for clarity. Selected bond lengths (Å) and angles (°): Ge-
(1)ꢀGe(1A)0 2.5507(8), Ge(1)ꢀN(1) 1.8802(16), Ge(1A)0ꢀN(1)0
1.8748(16), Ge(1A)0ꢀH(1)0 1.515(19), Ge(1A)0ꢀH(2)0 1.482(18), N-
(1)ꢀGe(1)ꢀGe(1A)0 101.04(5), N(1)0ꢀGe(1A)0ꢀGe(1) 114.71(5),
H(1)0ꢀGe(1A)0ꢀH(2)0 101(2). Symmetry operation: 0 ꢀx + 1/2, ꢀy +
3/2, ꢀz + 1.
’ AUTHOR INFORMATION
Corresponding Author
cameron.jones@monash.edu; frenking@chemie.uni-marburg.de
The crystal structure of 3 is isomorphic with that of 2
(Figure 3, see Supporting Information for the structure of 4),
and confirms that the compound exists in the solid state in its
GeI/GeIII mixed valence isomeric form. This is supported by the
fact that both hydride ligands were located from difference maps
and refined isotropically. Moreover, the GeꢀGe distance
(2.5507(8) Å) in the compound is considerably shorter than
that in 2, comparable with the GeꢀGe single bond length
er than the double bond in Ar0(H)GeGe(H)Ar0 (2.3026(3) Å).5
The differing geometries about the Ge(1) and Ge(2) centers of 3
are also fully consistent with its mixed valence formulation.
It is intriguing that 2 appears to be considerably more reactive
toward monohydrogenation than Ar0GeGeAr0, yet unlike that
compound, its second hydrogenation does not proceed (N.B.
Ar0(H)GeGe(H)Ar0 readily reacts with H2 at ambient temp-
erature5). To assess if there is a thermodynamic reason behind
these differences, the first and second hydrogenation energies for
2 were calculated (RI-BP86/def2-SVP). The first hydrogenation
was found to be exothermic to give either 3 (ΔH = ꢀ18.7 kcal/
mol, ΔG = ꢀ9.6 kcal/mol) or 5 (ΔH = ꢀ15.9 kcal/mol, ΔG =
ꢀ8.6 kcal/mol), and revealed that the mixed valence system, 3, is
favored over the germanium(II) hydride, 5 by only 1.1 kcal/mol.
This is in line with the apparent equilibrium between these
isomers in solution (vide supra). The second hydrogenation to
give 4 (from 3) is also exothermic (ΔH = ꢀ16.6 kcal/mol, ΔG =
ꢀ6.5 kcal/mol) which suggests that there is a significant kinetic
barrier to this hydrogenation in practice.
’ ACKNOWLEDGMENT
C.J. thanks the Australian Research Council (DP0665057)
and the US Air Force Asian Office of Aerospace Research and
Development for financial support. G.F. acknowledges the
Deutsche Forschungsgemeinschaft (grant FR641/25-1). C.S.
thanks the Alexander von Humboldt Foundation for a Feodor-
Lynen Fellowship. The EPSRC Mass Spectrometry Service at
Swansea University is also thanked. Part of this research was
undertaken on the MX1 beamline at the Australian Synchrotron,
Victoria, Australia. Dr. Andreas Stasch is acknowledged for the
refinement of the X-ray crystal structure of compound 2. This
paper is dedicated to Professor Matthias Driess on the occasion
of his 50th birthday.
in Ar0(H)2GeGeAr0 PMe3 (2.5304(7) Å),18 yet significantly long-
3
’ REFERENCES
(1) Selected recent reviews: (a) Power, P. P. Acc. Chem. Res. 2011,
44, 627. (b) Asay, M.; Jones, C.; Driess, M. Chem. Rev. 2011, 111, 354.
(c) Yao, S.; Xiong, Y.; Driess, M. Organometallics 2011, 30, 1748.
(d) Power, P. P. Nature 2010, 463, 171. (e) Fischer, R. C.; Power,
P. P. Chem. Rev. 2010, 110, 3877. (f) Schn€ockel, H. Chem. Rev. 2010,
110, 4125. (g) Lee, V. Y.; Sekiguchi, A. Organometallic Compounds of
Low-Coordinate Si, Ge, Sn and Pb; Wiley: Chichester, 2010.
(2) Recent reviews on ditetrelyne chemistry: (a) Sekiguchi, A. Pure
Appl. Chem. 2008, 80, 447. (b) Power, P. P. Organometallics 2007,
26, 4362. (c) Rivard, E.; Power, P. P. Inorg. Chem. 2007, 46, 10047.
(3) See for example (a) Takagi, N.; Nagase, S. Organometallics 2007,
26, 3627. (b) Lein, M.; Krapp, A.; Frenking, G. J. Am. Chem. Soc. 2005,
127, 6290. (c) Bridgeman, A. J.; Ireland, L. R. Polyhedron 2001, 20, 2841.
(d) Chen, Y.; Hartmann, M.; Diedenhofen, M.; Frenking, G. Angew.
Chem., Int. Ed. 2001, 40, 2052. See, also, relevant theoretical studies cited
in ref 2.
At present we can only speculate on the mechanism of
hydrogenation of 2, though this clearly must be different to that
proposed for multiply bonded Ar0GeGeAr0, which involves
interaction of its frontier π-bonding HOMO and nonbonding
LUMO (n+ combination) with the H2 σ*- and σ-orbitals,
respectively.1d Instead, it could be similar to the mechanism
(4) The first report of a stable digermyne appeared in 2002: Stender,
M.; Phillips, A. D.; Wright, R. J.; Power, P. P. Angew. Chem., Int. Ed. 2002,
41, 1785.
(5) Spikes, G. H.; Fettinger, J. C.; Power, P. P. J. Am. Chem. Soc.
2005, 127, 12232.
proposed for the hydrogenation of the germylene, Ar# Ge: (Ar#
2
= C6H3(C6H2Me3-2,4,6)2-2,6), which proceeds only at 65 °C to
give Ar# GeH2. This mechanism is thought to entail an initial
2
(6) See for example: (a) Martin, D.; Soleilhavoup, M.; Bertrand, G.
Chem. Sci. 2011, 2, 389. (b) Stephan, D. W.; Erker, G. Angew. Chem., Int.
Ed. 2010, 49, 46. (c) Ref 1a.
interaction of the H2 σ-orbital with the empty 4p orbital at Ge,
with concomitant back-donation from the Ge lone pair into the
H2 σ*-orbital.19 Nevertheless, the relatively low lying Ge-lone
pairs calculated for 2, in combination with its very high reactivity
(7) A handful of dimers containing three-coordinate GeIꢀGeI bonds
have been reported. See for example: (a) Jones, C.; Bonyhady, S. J.;
18624
dx.doi.org/10.1021/ja209215a |J. Am. Chem. Soc. 2011, 133, 18622–18625

Journal of the American Chemical Society
COMMUNICATION
Holzmann, N.; Frenking, G.; Stasch, A. Inorg. Chem. [Online early
access]. DOI: 10.1021/ic200682p. (b) Wang, W.; Inoue, S.; Yao, S.;
Driess, M. Chem. Commun. 2009, 2661. (c) Leung, W.-P.; Chiu, W.-K.;
Chong, K.-H.; Mak, C. W. Chem. Commun. 2009, 6822. (d) Nagendran,
S.;Sen, S. S.;Roesky, H. W.; Koley, D.;Grubm€uller, H.; Pal, A.; Herbst-Irmer,
R. Organometallics 2008, 27, 5459. (e) Green, S. P.; Jones, C.; Junk, P. C.;
Lippert, K.-A.; Stasch, A. Chem. Commun. 2006, 3978.
(8) Lappert, M. F.; Power, P. P.; Protchenko, A. V.; Seeber, A. L.
Metal Amide Chemistry; Wiley-VCH: Weinheim, 2009.
(9) Li, J.; Stasch, A.; Schenk., C.; Jones, C. Dalton Trans. 2011, 40,
10448.
(10) Magnesium(I) dimers are emerging as selective reducing agents
in inorganic synthesis: (a) Woodul, W. D.; Carter, E.; M€uller, R.;
Richards, A. F.; Stasch, A.; Kaupp, M.; Murphy, D. M.; Driess, M.;
Jones, C. J. Am. Chem. Soc. 2011, 133, 10074. (b) Stasch, A.; Jones, C.
Dalton Trans. 2011, 40, 5659. (c) Bonyhady, S. J.; Jones, C.; Nembenna,
S.; Stasch, A.; Edwards, A. J.; McIntyre, G. J. Chem.—Eur. J. 2010, 16,
938.
(11) See for example Peng, Y.; Fischer, R. C.; Merrill, W. A.; Fischer,
J.; Pu, L.; Ellis, B. D.; Fettinger, J. C.; Herber, R. H.; Power, P. P. Chem.
Sci. 2010, 1, 461.
(12) Similar stabilizing interactions have been proposed for N-
heterocyclic germylenes. See Mizuhata, Y.; Sasamori, T.; Tokitoh, N.
Chem. Rev. 2009, 109, 3479.
(13) The lone-pair orbitals at the Ge atoms accumulate high
s-character which was calculated to be 86.9%. This leaves little percen-
tage s-contribution for the GeꢀGe bonding orbital. It is well known that
heavier main-group elements are much less prone to s/p hybridization
than elements of the first octal row. This comes from the poor overlap
between the valence s and p orbitals of the former atoms: Kutzelnigg, W.
Angew. Chem., Int. Ed. Engl. 1984, 23, 272.
(14) Beagley, B.; Monaghan, J. J. Trans. Faraday Soc. 1970, 66, 2745.
(15) A very bulky distannyne has been reported to react with excess
H2 to give the related mixed valence compound, [Ar00SnSn(H)2Ar00]
(Ar00 = C6HPri2(C6H2Pri3-2,4,6)2-3,5,2,6). Peng, Y.; Brynda, M.; Ellis,
B. D.; Fettinger, J. C.; Rivard, E.; Power, P. P. Chem. Commun.
2008, 6042.
(16) Two other very low yielding byproduct were obtained from
reactions with K[HBEt3], viz., [L(H)2GeGe(H)(OH)L] and [L(H)2-
GeGe(H){K(OEt2)2}L]. Their formation and X-ray crystal structures
are discussed in the Supporting Information.
(17) Rivard, E.; Power, P. P. Dalton Trans. 2008, 4336.
(18) Richards, A. F.; Phillips, A. D.; Olmstead, M. M.; Power, P. P.
J. Am. Chem. Soc. 2003, 125, 3204.
(19) Peng, Y.; Guo, J.-D.; Ellis, B. D.; Zhu, Z.; Fettinger, J. C.;
Nagase, S.; Power, P. P. J. Am. Chem. Soc. 2009, 131, 16272.
18625
dx.doi.org/10.1021/ja209215a |J. Am. Chem. Soc. 2011, 133, 18622–18625