Facile central-element exchange in neutral hexacoordinate germanium and silicon complexes; Synthesis and characterization of germanium complexes

Article abstract of DOI:10.1039/c0dt00625d

Neutral hexacoordinate germanium complexes with hydrazido chelating ligands have been synthesized and characterized. Facile exchange of central element between silicon and germanium in these complexes is demonstrated, following given selectivity constrain

Full text of DOI:10.1039/c0dt00625d

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Facile central-element exchange in neutral hexacoordinate germanium and
silicon complexes; synthesis and characterization of germanium complexes†
Shiri Yakubovich, Inna Kalikhman and Daniel Kost*
Received 9th June 2010, Accepted 25th July 2010
DOI: 10.1039/c0dt00625d
Neutral hexacoordinate germanium complexes with hy-
drazido chelating ligands have been synthesized and char-
acterized. Facile exchange of central element between silicon
and germanium in these complexes is demonstrated, follow-
ing given selectivity constraints.
characterized by 1H and 13C NMR spectroscopy, elemental
analysis and by single crystal X-ray diffraction analysis. The
molecular structures of 1 and 2 in the solid state are depicted
in Fig. 1 and 2, respectively, and selected bond lengths and angles
are listed in Table 1.
We have recently shown that complete and rapid exchange
of ligands takes place between neutral hexacoordinate silicon
complexes and their differently substituted precursors:1,2 formal
exchange of monodentate ligands between a complex and a
trichlorosilane (eqn (1)), following a well defined “priority list”;
exchange of bidentate chelating ligands between complexes and
a trimethylsilyl-hydrazide precursor (eqn (2)) as well as between
differently substituted complexes (eqn (3)).
(4)
(1)
(2)
(3)
Fig. 1 ORTEP representation of the molecular structure of 1, depicted
at the 50% probability level and omitting hydrogen atoms.
We now find that even the central element in these complexes
can readily be replaced by a different one, namely the silicon is
replaced by germanium and germanium by silicon, obeying certain
selectivity constraints. This is quite a remarkable observation in
view of the many bonds which must be cleaved, and others which
are formed during the exchange.
Hexacoordinate germanium complexes3 (1 and 2) were prepared
like their silicon analogues, from the O-trimethylsilylated hy-
drazide (3) and methylgermanium trichloride (4) and germanium
tetrachloride (5), respectively (eqn (4)).4 Products 1 and 2 were
From Fig. 1 and 2, and the data in Table 1, it is evident that
the germanium complexes are hexacoordinate and their geometry
around the central atom is a distorted octahedron. Like in the
silicon analogues,2f,2g the nitrogen ligands in 1 and 2 are positioned
trans to each other, while the oxygen pair, as well as the pair of
monodentate ligands, possess cis positions. Further examination
of the data in Table 1 reveals substantial similarity between the
dichloro-germanium (2) and -silicon (6) complexes: all of the
bonds to germanium are, as one might expect, slightly longer
than the corresponding bonds in the silicon complex (2 vs. 6). It
may be worth noting that while the Ge–N and Ge–Cl bonds are
Department of Chemistry, Ben Gurion University of the Negev, Beer
Sheva, 84105, Israel. E-mail: kostd@bgu.ac.il; Fax: 97286472943;
Tel: 97286461192
† CCDC reference numbers 780239 and 780240. For crystallographic data
in CIF or other electronic format see DOI: 10.1039/c0dt00625d
˚
ca. 0.08 A longer than those to silicon, the corresponding Ge–O
˚
bond elongation (0.15 A), relative to Si–O, is almost twice as large.
This is undoubtedly a manifestation of the special strength of the
Si–O bond, relative to the Ge–O bond.6
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Dalton Trans., 2010, 39, 9241–9244 | 9241
©

Table 1 Selected crystallographic bond lengths and angles for 1 and 2, and for the silicon analogue 6 for comparison
1
2
6a
CCDC No. 780239
CCDC No. 780240
CCDC No. SOJQAJ
˚
Bond lengths/A
Ge–N
Ge–N
Ge–Cl
2.141(3)
2.147(3)
2.2899(9)
1.975(3)
1.977(2)
1.952(2)
Ge–N
Ge–N
Ge–Cl
Ge–Cl
Ge–O
Ge–O
2.085(5)
2.106(5)
2.2234(19)
2.2226(19)
1.934(4)
1.928(4)
Si–N
Si–N
Si–Cl
Si–Cl
Si–O
Si–O
2.011(2)
2.013(2)
2.141(1)
2.142(1)
1.775(1)
1.777(1)
Ge–C
Ge–O
Ge–O
Bond angles (◦)
N–Ge–N
Cl–Ge–N
C–Ge–N
O–Ge–N
158.50(12)
N–Ge–N
Cl–Ge–N
164.5(2)
N–Si–N
Cl–Si–N
168.73(7)
98.23(8); 93.06(8)
95.88(13); 101.31(13)
86.99(10); 78.93(10)
79.10(10); 83.40(10)
84.24(10)
87.38(8); 169.11(7)
93.94(13); 174.57(12)
92.20(15); 98.29(15)
92.31(15); 98.31(16)
87.7(2); 81.17(19)
80.9(2); 87.4(2)
85.9(2)
90.14(15); 172.49(14)
90.19(15); 172.26(14)
92.16(5); 94.09(5)
90.84(5); 98.28(5)
87.37(7);83.22(7)
82.91(6); 90.60(7)
88.28(7)
89.53(5); 174.95(5)
90.40(6); 173.64(5)
O–Ge–N
O–Si–N
O–Ge–O
Cl–Ge–O
C–Ge–O
O–Ge–O
Cl–Ge–O
O–Si–O
Cl–Si–O
a Taken from ref. 5; data for only one of two unique molecules are cited.
spectrum (Fig. 3) the Si–Me singlet of 7a at 0.64 ppm is converted
to the MeSiCl3 signal at 1.11 ppm, and the NMe resonances at 2.75
and 2.94 ppm are transformed to those of 2, at 3.04 and 3.15 ppm.
(5)
When methyltrichlorogermane (4) is used instead of 5, no
exchange takes place. The ability of germanium to replace silicon
in its complex depends on the electronegativity of the ligands
attached to germanium, and to those attached to silicon, in
analogy with the “ligand priority list” reported previously for
the exchange reaction shown in eqn (1).1 Thus, GeCl4 produces
dichloro-complexes, which take priority over the monodentate
ligands in the starting silicon complex: Me and Cl in 7a, or Ph and
Cl in 8c. As a result, the lower-priority ligands with the attached
central silicon are replaced by the chloro-ligands and germanium.
In line with this observation, when equally substituted silicon
and germanium are concerned, i.e., when the two monodentate
ligands attached to the silicon complex are the same as the ones
that would enter the expected germanium complex, if exchange
took place (Z = X in eqn (5)), no intermolecular exchange is
observed. In other words, when equally substituted, and in the
absence of any ligand priority driving force, silicon takes priority
over germanium, and is not replaced from its complex. Conversely,
germanium is readily and quantitatively replaced by silicon in the
reverse reaction, when both are equally substituted (Z = X in
eqn (5), reverse direction), confirming again the priority of silicon
over germanium in the formation of these neutral hexacoordinate
complexes. Thus, SiCl4 replaces germanium in any one of its
Fig. 2 ORTEP representation of the molecular structure of 2, depicted
at the 50% probability level and omitting hydrogen atoms.
When a silicon complex (7, 8) is allowed to react with excess
GeCl4 (5) in chloroform solution for 1 h at boiling temperature,
or for two days at room temperature, nearly all of the silicon
complex has disappeared, and a new compound, identified as the
germanium complex, is formed (eqn (5)). This is evident from
the disappearance of the high-field signal characteristic of the
hexacoordinate silicon compound (-124.4 ppm) in the reaction of
7a with 5, and its conversion to the signal (12.6 ppm) assigned
to MeSiCl3 in the 29Si NMR spectrum. Likewise, in the 1H NMR
9242 | Dalton Trans., 2010, 39, 9241–9244
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Fig. 3 1H NMR spectrum of the reaction (eqn (5)) between 7a and GeCl4 (5) in CDCl3 solution, after partial conversion, featuring both silicon and
germanium complexes. The signal at 2.58 ppm belongs to residual 3.
complexes, any ligand Z (reverse eqn (5)), and similarly XSiCl3
replaces germanium as long as Z = X.
germanium), followed by O–Si cleavage and complete transport of
the bidentate ligand from silicon to the neighboring element. This
initial process is then followed by cleavage and transfer of all the
bidentate ligands from one molecule to its neighbor and vice versa.
At no point along this exchange is a silicon carbon or germanium
carbon bond ever cleaved.
It is evident that the ligand electronegativities play a major role
in the exchange of silicon and germanium. The extent to which
silicon takes intrinsic priority over germanium can only be partly
assessed: silicon replaces germanium as long as they are equally
substituted. Can a less electronegative ligand X be attached to
silicon, without loss of its power to replace germanium? When
PhSiCl3 (X = Ph) was allowed to react with 2, so that the
germanium ligands (two chloro ligands) have priority over the
silicon ligands (chloro and phenyl), no trace of exchange could
be detected (reverse eqn (5)). Likewise, HSiCl3 caused no central-
element exchange with the dichloro-germanium complex (2, Z =
Cl, X = H in reverse eqn (5)). This means that ligand priority
dominates the reaction, and hence that the central-element priority
is of lesser importance.
Acknowledgements
Financial support by the Israel Science Foundation, grant No.
ISF-242/09, is gratefully acknowledged.
References
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The limits of priority were further probed in the forward
reaction: germanium replacing silicon. It takes GeCl4 to replace
silicon substituted by Me, Cl (Z = Cl, eqn (5)). An attempt to
lower the ligand priority, by using PhGeCl3 (9) to replace SiMeCl
from 7a, led to no exchange, despite the fact that the germanium
ligand priorities (Ph, Cl in 9) were greater than the silicon ligand
priorities (Me, Cl in 7a).
From the results presented in this paper, combined with previous
results on ligand exchange in silicon compounds,1 it appears that
the mechanism of central-element exchange is similar to that of
the ligand exchange reaction. This is supported by the observation
that silicon-germanium exchange is essentially dominated by the
same ligand priority order, just as the ligand exchange reactions
described previously. Apparently the bidentate, chelate forming
ligands, are capable of rapid bond cleavage and transfer from one
central element to the other, thereby effecting complete exchange
between complexes. It is likely that the dative N → Si bonds are
first to cleave and attack a neighboring “heavy” element (silicon or
3 For different hexacoordinate germanium compounds see: (a) S. D.
Pastor, V. Huang, D. NabiRahni, S. A. Koch and H.-F. Hsu, Inorg.
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The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 9241–9244 | 9243
©

Chem., 1997, 36, 5966–5968; (b) A. Biller, C. Burschka, M. Penka and
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(CDCl3, 300 K): d 18.2 (GeCH3), 48.9, 49.4, 49.7, 50.6 (NCH3), 117.3
1 1
(q, J(F–C) = 278 Hz, CF3), 117.4 (q, J(F–C) = 280 Hz, CF3), 157.4 (q,
2
2J(F–C) = 36.5 Hz, C N), 158.2 (q, J(F–C) = 36.5 Hz, C N). Anal.
calcd. for C9H15ClF6GeN4O2: C, 24.95; H, 3.49; N, 12.93. Found: C,
25.20; H, 3.39; N, 12.81. Bis(N-(dimethylamino)trifluoroacetimidato-
N,O)dichlorogermanium(IV) (2): 2 was prepared as described for 1, using
5 instead of 4. Yield: 0.63 g, 95%. Mp: 118 ◦C (dec). 1H NMR (CDCl3,
300 K): d 3.11, 3.17 (2 s, 12H, N(CH3)). 13C NMR (CDCl3, 300 K): d
50.6, 50.7 (NCH3), 117.0 (q, 1J(F–C) = 278 Hz, CF3), 157.1 (q, 2J(F–C) = 38
Hz, C N). Anal. calcd. for C8H12Cl2F6GeN4O2: C, 21.18; H, 2.67; N,
12.35. Found: C, 21.37; H, 2.82; N, 12.17.
4 Synthesis:
Bis(N-(dimethylamino)trifluoroacetimidato-N,O)chloro-
(methyl)germanium(IV) (1): A mixture of 0.70 g (3.1 mmol) of 3 and
0.31 g (1.6 mmol) of 4 in 5 mL of chloroform was stirred at room
temperature for 30 min. The volatiles were removed under reduced
pressure (0.1 mmHg) and the residue was washed with 5 mL of
n-hexane. A single crystal for X-ray analysis was grown from n-hexane.
Yield: 0.65 g (94%). Mp, 105 ◦C (dec). 1H NMR (CDCl3, 300 K): d
1.26 (s, 3H, GeCH3), 2.87, 2.89, 2.97, 3.09 (4 s, 12H, NCH3). 13C NMR
5 D. Kost, I. Kalikhman, S. Krivonos, D. Stalke and T. Kottke, J. Am.
Chem. Soc., 1998, 120, 4209–4214.
6 T. L. Cottrell, The Strengths of Chemical Bonds, 2nd ed., Butterworths,
London, 1958.
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