Lewis base induced tuning of the Ge-Ge bond order in a digermyne

Article abstract of DOI:10.1039/b612202g

The reaction of the digermyne Ar′GeGeAr′ (Ar′ = C₆H₃-2,6(C₆H₃-2,6-Pr ⁱ₂)₂; Ge-Ge = 2.2850(6) A) with mesityl isocyanide affords the bis adduct [Ar′GeGeAr′(CNMes)₂] which results in the conversion of a Ge-Ge multiple bond to a long Ge-Ge single bond (= 2.6626(8) A). The Royal Society of Chemistry.

Full text of DOI:10.1039/b612202g

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Lewis base induced tuning of the Ge–Ge bond order in a ‘‘digermyne’’
Geoffrey H. Spikes and Philip P. Power*
Received (in Berkeley, CA, USA) 23rd August 2006, Accepted 28th September 2006
First published as an Advance Article on the web 19th October 2006
DOI: 10.1039/b612202g
16
3
˚
˚
(2.3432 A) increased slightly relative to that in 1 (2.2850(6) A).
This is consistent with the retention of the GeGe multiple bond as
illustrated in 2 (Scheme 1). This behavior can be accounted for by
reference to Fig. 1, which shows that there is a change in the
frontier orbitals for the heavier derivatives with bent geometry.18,19
The new arrangement differs from the familiar 2p(HOMO)-
2p*(LUMO) pattern in alkynes as a result of a second order Jahn–
Teller mixing of a s*-orbital and the in-plane p-orbital (both of
which have bu symmetry in the C2h point group) to afford n+ and
n2 non-bonding orbitals. This occurs because of weaker E–E
bonding and the resultant smaller energy separation, and hence a
stronger interaction, between the molecular energy levels. The
HOMO remains a p-orbital (au symmetry) whereas the LUMO is
an n+ non-bonding orbital of ag symmetry. Thus, the Lewis base
tBuNC interacts with the unoccupied ag(n+) orbital. This
interaction stabilizes n+ but diminishes the overlap between the
two Ge centers slightly.{
The reaction of the ‘‘digermyne’’ Ar9GeGeAr9 (Ar9 = C6H3–
2,6(C6H3–2,6-Pri2)2; Ge–Ge
=
2.2850(6) A) with mesityl
˚
isocyanide affords the bis adduct [Ar9GeGeAr9(CNMes)2]
which results in the conversion of a Ge–Ge multiple bond to
˚
a long Ge–Ge single bond (= 2.6626(8) A).
Stable homonuclear alkyne analogues of all the heavier group
14 elements have now been isolated and characterized.1–8 They
have the general formula REER (E = Si, Ge, Sn, Pb; R = bulky
aryl or silyl ligand) and display strongly trans-bent geometries and
bond orders that are less than three because of increasing lone pair
character at the group 14 element as the group is descended. For
the heaviest element, lead, the element–element bond is essentially
single, as shown in C,1,9 where A represents a triple bond, as found
in acetylene and its derivatives. For the silicon, germanium and tin
analogues, the bonding lies somewhere between these two
extremes, as illustrated by the canonical form B.10–12
According to Fig. 1, the reaction with the second Lewis base
molecule should result in addition to the next available Ge–Ge p*
level, leading to substantial lengthening of the Ge–Ge bond. The
synthesis of such an adduct has not been reported for any heavier
alkyne analogues and the addition of excess tBuNC to
Ar9GeGeAr9 does not result in the isolation of a product
incorporating more than one isonitrile unit.16 We now report that
the reaction of Ar9GeGeAr9 with MesNC (Mes = C6H2–2,4,6-
Me3) yields the bis adduct 3 (Scheme 1), which is characterized by
a very long Ge–Ge bond.
A consequence of the bent geometry is that electron density is
removed from the bonding region so that the EE unit becomes
increasingly electron poor in contrast to the electron rich alkynes.
We have shown that Ar9GeGeAr9 (1, Ar9 = C6H3–2,6(C6H3–2,6-
Pri2)2, Scheme 1) displays high reactivity toward a variety of
electron rich unsaturated molecules and many of these reactions
have led to complete cleavage of the Ge–Ge bond.13–17 Uniquely,
however, the reaction of tBuNC with Ar9GeGeAr9 afforded a 1 : 1
adduct, Ar9GeGeAr9?CNBut (2), in which the isonitrile was
coordinated to one of the Ge centers.16
The coordination of the tBuNC donor occurred in the plane of
the C(ipso)GeGeC(ipso) core and the GeGe distance in 2
t
Scheme 1 Reactivity of Ar9GeGeAr9 toward BuNC or MesNC.
Fig. 1 Selected orbital interactions in the C2h symmetric trans-bent
REER molecule. The opposite phases for s- and p-orbitals are indicated
by black and gray shading respectively.
Department of Chemistry, University of California, Davis, California,
95616, USA. E-mail: pppower@ucdavis.edu; Fax: +1-530-752-8995;
Tel: +1-530-752-6913
This journal is ß The Royal Society of Chemistry 2007
Chem. Commun., 2007, 85–87 | 85

View Article Online
The complex 3 was obtained as dark red crystals by the addition
of two equivalents of MesNC to a solution of Ar9GeGeAr9 in
hexane.{ The X-ray crystal structure§ of 3 is illustrated in Fig. 2
where it can be seen that each germanium is complexed by a
MesNC: donor. As a result, the germaniums are pyramidally
coordinated with interligand angular sums at Ge1 = 293.2(2)u and
Ge2 = 292.5(2)u. The Ge–C distances to both the MesNC: and Ar9
ligands are very similar and lie in the narrow range 1.996(5)–
bond and of the flanking rings of the terphenyl ligands. At
elevated temperature in d8-toluene the spectrum simplifies to
two isopropyl environments, indicated by four doublets at 1.05,
1.14, 1.27 and 1.50 ppm, and two septets at 2.73 and 3.18 ppm
(CH groups). The IR spectrum displays a CN stretching
absorption at 2113 cm21
.
˚
The ca. 0.38 A lengthening of the GeGe bond upon
coordination of 1 by two MesNC molecules is the most notable
feature of the structure of 3. It stands in contrast to the slight
increase observed upon coordination of a single isonitrile in 2 and
is indicative of significantly greater Ge–Ge bonding changes. It
seems probable that, unlike tBuNC, the addition of two
equivalents of MesNC can occur because the MesNC has a flat,
two-dimensional shape and the mesityl rings can be oriented
parallel to each other without undue steric repulsion. Thus, the
steric congestion between the isonitriles can be minimized for the
MesNC groups by parallel ring orientation but this cannot occur
in the case of tBuNC because of the steric pressure of the bulky tBu
substituents. In addition, the mesityl substituents are partially
eclipsed since the two isonitriles complex on the same side of the
molecule with only a 55u torsion angle between them. This also
results in a decrease in the 180u torsion angle between the two large
Ar9 substituents in the precursor, Ar9GeGeAr9, to 104.4u in 3. It is
notable that the coordination of both isonitriles occurs perpendi-
cularly to the coordination plane at each germanium in the
Ar9GeGeAr9 unit. Furthermore, the GeGeC(ipso) bending angles
in the Ar9GeGeAr9 unit have decreased by ca. 23u between 1 and
3. Thus, the Ar9GeGeAr9 moiety in 3 is much more strongly trans-
bent than it is in free Ar9GeGeAr9. In effect, the Ge–Ge bond is
now a single one and complexation of the MesNC donors takes
place perpendicularly to the germanium coordination planes. The
˚
˚
2.033(4) A. The Ge1–Ge2 bond length is 2.6626(8) A. There is a
torsion angle of 104.4u in the C1–Ge1–Ge2–C31 unit of the
Ar9GeGeAr9 array and the torsion angle involving the two
isonitrile units, i.e. C61–Ge1–Ge2–C71, is 55u. The N1–C61 and
N2–C71 distances within the isonitrile donors are 1.145(6) and
˚
1.157(6) A, with bending angles of 159.5(5)u at the ligating carbons
of each ligand.
The Ge1–C61 and Ge2–C71 bonds subtend to almost
right angles (average 88.3u) with respect to the Ge–Ge bond.
Inspection of Fig. 2 shows that the mesityl rings of the MesNC
ligands are essentially parallel to each other. The ring planes are
˚
separated by ca. 3.5 A. However, the two rings have staggered
geometries with respect to each other and the p-interactions appear
to be weak.
At ambient temperature the 1H NMR spectrum of 3 is
complicated but shows only one mesityl environment with singlet
resonances at 1.99, 2.13 and 6.48 ppm. The isopropyl region
consists of three overlapping methyl environments at 1.14 ppm
and one further downfield at 1.50 ppm, with a similarly
complicated region corresponding to the methine environments
at 2.79–3.27 ppm, suggesting hindered rotation around the Ge–Ge
˚
Ge–Ge bond is much longer than the ca. 2.44 A distance normally
seen in elemental germanium20 or digermanes,21 R3GeGeR3. The
Ge–Ge bond length probably is also increased because the bond is
now formed by head-to-head overlap of essentially 4p-orbitals
rather than hybrid orbitals as in R3GeGeR3 species.
An alternative explanation for the formation of 3 is based on the
valence bond description of the more strongly bent skeleton as a
bis(germylene) structure, which is illustrated by C. In this model,
each germanium carries a lone pair in the molecular plane as well
as an empty 4p-orbital perpendicular to that plane. The latter are
thus available for interaction with two molecules of a Lewis base
such as MesNC:. This view is also consistent with recent
computational work,11,22 where it was shown that the greater
bending of the skeleton eventually leads to two non-bonding, lone
pair orbitals, n+ and n2, with the Ge centers being connected by a
single s-bond as in C. The LUMO and LUMO + 1 are now p and
p* levels (see ref. 11 and 22) which can interact with the two
isonitrile donors to form complex 3 in which the germaniums are
connected by a simple s-bond, as argued above.
In summary, it has been shown that the ‘‘digermyne’’
Ar9GeGeAr9 can form the complex 3 with two Lewis base
isonitriles. The structural parameters of 3, the coordination
mode of the isonitriles and the fundamental differences in the
isonitrile bonding between 2 and 3 have provided experimental
insight into the orbital arrangements in these species and
Ar9GeGeAr9.
Fig. 2 Thermal ellipsoid (30%) drawing of 3, H atoms are not shown.
˚
Selected bond lengths (A) and angles (u): Ge1–Ge2 2.6626(8), Ge1–C1
2.033(4), Ge1–C61 2.026(5), Ge2–C31 2.023(4), Ge2–C71 1.996(5), C61–
N1 1.145(6), N1–C62 1.402(6), Ge2–C71 1.996(5), C71–N2 1.157(6), N2–
C72 1.395(6); C1–Ge1–C61 100.09(18), Ge2–Ge1–C61 88.64(14), C1–
Ge1–Ge2 104.45(12), C31–Ge2–C71 99.34(19), Ge1–Ge2–C71 88.00(14),
C31–Ge2–Ge1 105.14(13), Ge1–C61–N1 159.5(4), C61–N1–C62 173.9(5),
Ge2–C71–N2 159.4(4), C71–N2–C72 174.5(5).
We thank the National Science Foundation for support of
this work.
86 | Chem. Commun., 2007, 85–87
This journal is ß The Royal Society of Chemistry 2007

View Article Online
4 N. Wiberg, M. Niedermayer, G. Fischer, H. No¨th and M. Suter,
Eur. J. Inorg. Chem., 2002, 1066.
Notes and references
5 L. Pu, A. D. Phillips, A. F. Richards, M. Stender, R. S. Simons,
M. M. Olmstead and P. P. Power, J. Am. Chem. Soc., 2003, 125, 11626.
6 N. Wiberg, S. K. Vasisht, G. Fischer and P. Mayer, Z. Anorg. Allg.
Chem., 2004, 630, 1823.
7 A. Sekiguchi, R. Kinjo and M. Ichinohe, Science, 2004, 305, 1755.
8 Y. Sugiyama, T. Sasamori, Y. Hosoi, Y. Furukawa, N. Takagi,
S. Nagase and N. Tokitoh, J. Am. Chem. Soc., 2006, 128, 1023.
9 Y. Chen, M. Hartmann, M. Diedenhofen and G. Frenking, Angew.
Chem., Int. Ed., 2001, 40, 2051.
10 N. Takagi and S. Nagase, Organometallics, 2001, 20, 5498.
11 Y. Jung, M. Brynda, P. P. Power and M. J. Head-Gordon, J. Am.
Chem. Soc., 2006, 128, 7185.
12 A. J. Bridgeman and L. R. Ireland, Polyhedron, 2001, 20, 2841.
13 C. Cui, M. M. Olmstead and P. P. Power, J. Am. Chem. Soc., 2004, 126,
5062.
14 C. Cui, M. Brynda, M. M. Olmstead and P. P. Power, J. Am. Chem.
Soc., 2004, 126, 6510.
15 C. Cui, M. M. Olmstead, J. C. Fettinger, G. H. Spikes and P. P. Power,
J. Am. Chem. Soc., 2005, 127, 17530.
16 G. H. Spikes, J. C. Fettinger and P. P. Power, J. Am. Chem. Soc., 2005,
127, 12232.
17 G. H. Spikes, Y. Peng, J. C. Fettinger, J. Steiner and P. P. Power, Chem.
Commun., 2005, 6041.
18 R. S. Grev, Adv. Organomet. Chem., 1991, 33, 125.
19 T. L. Allen, W. H. Fink and P. P. Power, J. Chem. Soc., Dalton Trans.,
2000, 407.
20 A. F. Wells, Structural Inorganic Chemistry, Clarendon, Oxford, 5th
edn, 1984, p. 1279.
21 (a) M. Weidenbruch, F. Grimm, M. Herrndorf and A. J. Schafer,
J. Organomet. Chem., 1998, 341, 335; (b) N. Fukaya, M. Ichinohe and
A. Sekiguchi, Angew. Chem., Int. Ed., 2000, 39, 3881; (c)
M. Weidenbruch, A. Hagedorn, K. Peters and H. G. von Schnering,
Chem. Ber., 1996, 129, 401; (d) M. Weidenbruch, A. Hagedorn, K. Peters
and H. G. von Schnering, Angew. Chem., Int. Ed. Engl., 1995, 34, 1085.
22 M. Lein, A. Krapp and G. Frenking, J. Am. Chem. Soc., 2005, 127,
6290.
{ Factors that may contribute to the lengthening of the Ge–Ge bond in 2
include an increase in the coordination number at one of the germaniums.
t
In addition, the coordination of BuNC: results in a ‘‘freezing out’’ of the
resonating lone pair (cf. structure B) at the uncomplexed Ge center. This
leads to a dramatic reduction in the Ge–Ge–C bending angle,15 which
probably increases the 4p-character of the Ge–Ge s-bond, which lengthens
as a result.
{ All manipulations were carried out under anaerobic and anhydrous
conditions. To a solution of Ar9GeGeAr9 (1, Ar9 = C6H3–2,6(C6H3–2,6-
Pri2)2, 0.300 g, 0.320 mmol) in n-hexane (50 mL) was added MesNC
(0.093 g, 0.640 mmol) in n-hexane (20 mL) at ambient temperature and the
mixture was stirred at room temperature for 6 h. The resulting dark red/
purple solution was filtered, concentrated and stored at ca. 218 uC
overnight to give dark red/purple crystals of 3 (0.253 g, 64.2%). Mp: 77–
79 uC. 1H NMR (C6D6, 300.08 MHz): d 1.14 (m, 36H, CHMe2), 1.50 (m,
12H, CHMe2), 1.99 (s, 6H, Mes), 2.13 (s, 12H, Mes), 2.90 (m, 4H,
CHMe2), 3.23 (sept, 4H, J = 6.6 Hz, CHMe2), 6.48 (s, 4H, Mes), 7.24 (m,
18H, Ar–H). 13C NMR (C6D6, 100.52 MHz): d 18.6 (CMN), 20.9 (Mes),
23.6 (CHMe2), 24.2 (CHMe2), 25.9 (CHMe2), 26.1 (CHMe2), 31.0
(CHMe2), 31.2 (CHMe2), 123.05, 123.25, 123.62, 123.90, 124.03, 128.64,
128.76, 129.09, 129.25, 129.58, 139.01, 141.81, 141.96, 142.13, 142.23,
144.89, 145.48, 145.90, 146.19, 146.64, 146.89, 147.70, 147.82, 151.80,
196.06 (unsaturated carbon). IR (KBr, Nujol): 2113 (m, CN stretch) cm21
.
§ Crystal data for 3 at T = 90(2) K with MoKa (l = 0.71073 A):
C80H96Ge2N2, M = 1230.77, monoclinic, space group P21/n, a =
˚
˚
˚
˚
13.141(1) A, b = 21.235(2) A, c = 24.860(2) A, b = 97.467(2)u, U =
6878.1(11) s3, Z = 4, m = 0.918 mm21, Rint = 0.190, R1 = 0.0611 for 12 431
(I . 2s(I)) reflections, wR2 = 0.1478 (all data). CCDC 618885. For
crystallographic data in CIF or other electronic format see DOI: 10.1039/
b612202g
1 L. Pu, B. Twamley and P. P. Power, J. Am. Chem. Soc., 2000, 122, 3524.
2 A. D. Phillips, R. J. Wright, M. M. Olmstead and P. P. Power, J. Am.
Chem. Soc., 2002, 124, 5930.
3 M. Stender, A. D. Phillips, R. J. Wright and P. P. Power, Angew.
Chem., Int. Ed., 2002, 41, 1785.
This journal is ß The Royal Society of Chemistry 2007
Chem. Commun., 2007, 85–87 | 87