Monomeric Sn(ii) and Ge(ii) hydrides supported by a tridentate pincer-based ligand

Article abstract of DOI:10.1039/c2cc31214j

Herein we report the syntheses of terminal Sn(ii) (3) and Ge(ii) (4) hydrides from the corresponding chloride precursors [{2,6-iPr₂C ₆H₃NCMe}₂C₆H₃MCl] (M = Sn (1), Ge (2)) using [K{B(sec-Bu)₃}H] as a hydrogenating agent. Combination of steric shielding and intramolecular N → M interactions resulted in the protection of M(ii)-H bonds.

Full text of DOI:10.1039/c2cc31214j

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COMMUNICATION
Monomeric Sn(II) and Ge(II) hydrides supported by a tridentate
pincer-based ligandwz
Shabana Khan,a Prinson P. Samuel,a Reent Michel,a Johannes M. Dieterich,b
Ricardo A. Mata,*b Jean-Philippe Demers,c Adam Lange,c Herbert W. Roesky*a and
Dietmar Stalke*a
Received 17th February 2012, Accepted 12th March 2012
DOI: 10.1039/c2cc31214j
Herein we report the syntheses of terminal Sn(II) (3) and
Ge(II) (4) hydrides from the corresponding chloride precursors
[{2,6-iPr2C6H3NCMe}2C6H3MCl] (M = Sn (1), Ge (2)) using
[K{B(sec-Bu)3}H] as a hydrogenating agent. Combination of
steric shielding and intramolecular N - M interactions resulted
in the protection of M(II)–H bonds.
be due to the weaker nature of the Sn–H bond relative to the
Ge–H bond.8 Apart from these, Rivard et al. recently described
the stabilization of the parent GeH2 and SnH2 complexes using
a N-heterocyclic carbene (NHC) as a donor and a BH3 or metal
carbonyl complex as an acceptor (D–F).9 These hydrides are
coordinatively saturated because the lone pair of electrons at Ge
and Sn effectively takes part in the coordination (Chart 1).
However, in an insertion reaction of F with benzaldehyde, the
C: - Sn bond readily cleaves indicating the lability of the NHC
donor.9b
Main group metal hydrides are extremely important in chemical
synthesis. They are notably used as precursors for the preparation
of other metal hydrides, as reducing agents for a big variety of
inorganic and organic substrates and function as potential
feedstocks for hydrogen storage.1,2 Hydrides are considered as
promising candidates for the synthesis of new clusters and
nanoparticles by controlled thermolysis.3 By taking advantage
of sterically bulky terphenyl ligands Power et al. isolated
the first Sn(II) hydride [{2,6-Trip2C6H3Sn(m-H)}2, Trip =
2,4,6-iPr3C6H2]. However, this compound is dimeric with bridging
hydrogen atoms.4 The first dimeric structurally characterized
Ge(II) hydride [ArGeH]2 (Ar = 2,6-Dipp2C6H3, Dipp =
2,6-iPr2C6H3) was reported by the same group.5 Moreover in
2006, the first structurally characterized terminal Sn(II) and
Ge(II) hydrides [{HC(CMeNAr)2}MH, Ar = 2,6-iPr2C6H3,
M = Sn (A) and Ge (B)] were outlined, but compound A
exhibited weak intermolecular Sn–Hꢀ ꢀ ꢀSn contacts.6 In 2011,
Jones et al. reported the synthesis of an analogous Ge(II)
hydride [{HC(CMeNMes)2}GeH, (Mes = 2,4,6-Me3C6H2)
(C)] stabilized by a similar b-diketiminato backbone.7 Further
attempts to prepare the corresponding tin(II) hydride complex
were not successful.7 The instability of the tin analogue could
New monomeric Sn(II) hydride could be achieved by
increasing the coordination around the Sn(II) center. Very
recently, we have reported a pincer supported bis-stannylene
([{2,6-iPr2C6H3NCMe}2C6H3Sn]2) with a Sn(I)–Sn(I) bond by
reducing [{2,6-iPr2C6H3NCMe}2C6H3SnCl] (1) with KC8.10
So et al. reported the analogous bis-germylene compound
using a comparable ligand.11 Moreover, Jurkschat et al.
outlined the synthesis of [{2,6-(Me2NCH2)2C6H3}Sn(H)W(CO)5],
a pincer based Sn(II) hydride stabilized in the coordination sphere
of W(CO)5.12 This encouraged us to prepare the hitherto elusive
terminal Sn(II) hydride.
Treatment of compounds 1 and 2, respectively, with
K[B(sec-Bu)3H] in THF at 0 1C followed by recrystallization in
toluene afforded dark reddish-orange colored crystals of 3 and 4
a Institut fur Anorganische Chemie, Georg-August-Universitat,
Tammannstrasse 4, 37077 Go¨ttingen, Germany.
¨
¨
Chart 1 Monomeric Lewis base coordinate Sn(II) and Ge(II) hydrides
A–C; Lewis acid and Lewis base coordinate Sn(II) and Ge(II) hydrides D–F.
E-mail: hroesky@gwdg.de, dstalke@chemie.uni-goettingen.de
b Institut fur Physikalische Chemie, Georg-August-Universitat,
Tammannstrasse 6, 37077 Gottingen, Germany.
¨
¨
¨
E-mail: rmata@gwdg.de
c Solid-State NMR, Max-Planck-Institut fur Biophysikalische Chemie,
Am Faßberg 11, 37077 Gottingen, Germany
¨
¨
w This paper is dedicated to Professor Anton Meller on the occasion of
his 80th birthday.
z Electronic supplementary information (ESI) available: Full synthetic,
spectroscopic, X-ray crystallographic and theoretical details for 3 and 4.
CCDC 863378 (3) and 863377 (4). For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c2cc31214j
Scheme 1 Syntheses of 3 and 4.
c
4890 Chem. Commun., 2012, 48, 4890–4892
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(Scheme 1). Conclusive evidence for the formation of the mono-
meric hydrides 3 and 4 was obtained by X-ray crystallography
(Fig. 1), NMR and IR spectroscopy, and theoretical calculations.
The stabilization of 3 and 4 may be occurring by increasing the
electron density at the empty p orbital of the metal centre from
the adjacent nitrogen donors. The most noticeable feature
of 3 is the absence of any kind of intermolecular interactions
(Sn–Sn or Sn–H) in the crystal packing confirming an exclusively
monomeric nature of Sn(II) hydride. However, the position of the
MH (M = Sn and Ge) hydrogen atom cannot be determined
reliably. Therefore, the corresponding bond distance and angle
are not discussed herein. In both cases the MH moiety is
disordered over two positions resulting in a superposition of
the electron density of the M–H bond and the Ge and Sn lone
pairs, respectively (for more details see ESIz).
shift of 3 compared to that of 4 is similar to those found in A
and B and presumably due to the less electron transfer from
the nitrogen atoms to the Sn–H bond than that within the
Ge–H bond.6a The 119Sn NMR spectrum of 3 exhibits a singlet
(d ꢁ114.27 ppm) with a coupling constant of 1JSn–H = 112.9 Hz.
The chemical shift is high field shifted when compared with A
(d ꢁ4.45 ppm) and the precursor 4 (d ꢁ20 ppm). The Sn–H
coupling constant is in line with that of A (1JSn–H = 45 Hz) and
with several dimeric tin(II) hydrides (1JSn–H = 87–95 Hz)
reported previously.16 In all the cases, Sn centres carry a lone
pair of electrons. However the value of the coupling constant
is significantly smaller than that of F (1JSn–H = 1158 Hz) and
[{2,6-(Me2NCH2)2C6H3}Sn(H)W(CO)5] (1302 Hz)12 where the
lone pair at the Sn centre coordinates to a Lewis acid. This
indicates that the coupling constant is strongly influenced by the
nature of surrounding ligands at the Sn centre. The solid-state
CPMAS NMR of 3 (See ESIz, Fig. S6), which shows a single
119Sn resonance at ꢁ119.4 ppm, supports the assignment of a
monomeric structure for 3 both in solution and in the solid state.
In the IR spectra of 3 and 4, strong absorptions were observed
at 1826 cmꢁ1 and 1985 cmꢁ1 which are tentatively assigned to
the Sn–H and Ge–H stretching frequencies, respectively. The
corresponding deuterated complex LGeD (5) was prepared by
reacting 2 with LiAlD4 and yielded the isotopically shifted Ge–D
(1462 cmꢁ1) IR band and thereby supported the initially assigned
Ge–H stretching vibration of 4. The wave numbers match well
with those of respective four-coordinate M–H species by Rivard
et al. (D–F),9 Power et al.,17 and our results18 but more than those
reported for A (1733 cmꢁ1),6a B (1849 cmꢁ1),6a and C (1722 cmꢁ1).7
In the EI-MS spectra of 3 and 4, each molecular ion was observed
as the most abundant peak with highest relative intensity at
m/z 599 and 553, respectively. The corresponding m/z for 5
was found at 555.
Compound 3 crystallizes in the monoclinic space group
Pc.13 The molecular structure of 3 reveals that the Sn atom
is four-coordinated and exhibits a distorted seesaw geometry
considering the lone pair of electrons. The flanking aromatic
rings are nearly perpendicular to the central ring of the ligand.
The two nitrogen atoms are coordinated intra-molecularly to
the Sn atom with bond lengths of 2.4538(13) and 2.4664(14) A,
respectively, which are consistent with those for intra-molecularly
coordinated Sn–N bonds.14 Similarly, the Sn–C bond length of 3
(2.1665(16) A) is in good accordance with the Sn–C bond
distances in [{2,6-iPr2C6H3NCMe}2C6H3Sn]2 (2.1575(18) and
2.1880(19) A)10 and [{2,6-(Me2NCH2)2C6H3}Sn]2 (2.180(11)
and 2.193(10) A).15 Compound 4 crystallizes in the monoclinic
space group C2/c13 and adopts an isostructural motif as its Sn–H
congener 3. The two intra-molecularly coordinated Ge–N bond
lengths in 4 (2.2722(15) and 2.2746(15) A) are in line with that of
[{2,6-iPr2C6H3NC(H)}2C6H3GeCl] (2.247(3) A).11 The Ge1–C1
bond length (1.9483(18) A) is shorter compared to that of
[{2,6-iPr2C6H3NC(H)}2C6H3GeCl] (2.028(3) A).11
In order to draw a clearer picture on the Sn–H and Ge–H
bonds, quantum chemical calculations were carried out on 3
and 4. Their structures were optimized at the BP86/def2-SVP
(with effective core potentials for the Sn and Ge atoms) level of
theory.19 Frequency calculations were used to confirm the
structures as true minima. Taking into account the bulkiness of
both compounds, one could expect that dispersion interactions
play a significant role in their structures. Therefore, we have also
carried out optimizations with the same functional and basis set,
but including dispersion corrections, as proposed by Grimme
(BP86-D/def2-SVP).20 Both sets of calculations agree closely in
the local geometry of the M–H (M = Sn and Ge) bond. The
M–H distances are 1.792 and 1.787 A for M = Sn, using BP86
and BP86-D, respectively. However, the computed stretching
frequencies (1621/1646 cmꢁ1) are somewhat lower than the
experimentally measured value of 1826 cmꢁ1. In the case of
Ge, the distances are 1.588 (BP86) and 1.583 A (BP86-D) with a
The constitutions of 3 and 4 derived from single crystal
X-ray diffraction studies were further confirmed by spectro-
chemical analyses. In 3 a sharp singlet appears at d 10.59 ppm
1
with J(119Sn–1H) = 112 Hz which is shifted to the high field
relative to that of A (d 13.96 ppm) but to low field relative to that
of [{2,6-Trip2C6H3Sn(m-H)}2] (d 7.87 ppm).4 The corresponding
resonance for 4 appears at d 6.69 ppm for the hydrogen attached
to germanium (Ge–H) which is at lower field than those reported
for [ArGeH]2 (Ar = 2,6-Dipp2C6H3, Dipp = 2,6-iPr2C6H3)
(d 3.48 ppm) and Ar(H)2GeGeArꢀPMe3 (Ar = 2,6-Dipp2C6H3,
Dipp = 2,6-iPr2C6H3) (d 3.81 ppm)5 but shifted to higher field
than that reported for B (d 8.08 ppm).6a The low field chemical
stretching frequency of 1882/1918 cmꢁ1
.
The most significant difference between the DFT results
with and without dispersion corrections is the conformation of
the isopropyl groups. In the case of BP86-D/def2-SVP, for
both 3 and 4, one of these groups has a methyl moiety pointing
towards the metal bonded hydrogen atom. The experimentally
derived structures, as well as the computed BP86/def2-SVP
geometries, show the less steric hydrogen in this position. This
is somewhat unexpected since there should be little energy gain
Fig. 1 Molecular structures of 3 and 4. Anisotropic displacement
parameters are depicted at the 50% probability level. Hydrogen atoms
except the ones attached to Sn1 and Ge1, lattice toluene molecules and
the disordered Sn–H and Ge–H moieties are not shown for clarity. The
Sn1–H1 and Ge1–H1 distances were set to the theoretical data.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 4890–4892 4891

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in such an interaction. Instead, one would expect all methyl groups
to point outward, conserving the symmetry and avoiding steric
contacts. In order to better understand this occurrence, DF-MP2/
cc-pVTZ (cc-pVTZ-PP in the case of Sn and Ge)21 single points
were computed on the DFT optimized geometries. The results
show that, in fact, the BP86-D optimized structures are higher in
energy. The BP86/def2-SVP structures are 2.9 and 0.2 kcal molꢁ1
lower in energy for structures 3 and 4, respectively, than the
BP86-D analogues. The torsion of the isopropyl group seems,
therefore, to be an artifact of the dispersion correction terms.
We have also performed a NBO analysis of the two compounds
at the BP86/def2-SVP level of theory.22 The Sn–H bond is built
from a Sn hybrid orbital with 89.3% p-character. This value is
close to the one reported for compound A (87.7%) and higher
than that of F (65.1%), which again relates to the small JSn–H
coupling constant measured. The p-character of the hybrid is
similar for Ge–H in compound 4 (85.8%). The lone pair of Sn and
Ge is predominantly of s-character (79.4% (Sn) 70.9% (Ge)). In
the dominant Lewis structures, the metals are only covalently
bound to the hydrogen and the ring carbon. However, the
second-order perturbation energy analysis of the NBOs shows
a significant donation from each N lone pair to a p-orbital
of the metal. The values are about 50 kcal molꢁ1 for Ge
(per Ge–N contact) and 35 kcal molꢁ1 for Sn.
T.-K. Sham and J. G. C. Veinot, J. Phys. Chem. C, 2008, 112,
14247–14254.
4 B. E. Eichler and P. P. Power, J. Am. Chem. Soc., 2000, 122,
8785–8786.
5 A. F. Richards, A. D. Phillips, M. M. Olmstead and P. P. Power,
J. Am. Chem. Soc., 2003, 125, 3204–3205.
6 (a) L. W. Pineda, V. Jancik, K. Starke, R. B. Oswald and
H. W. Roesky, Angew. Chem., 2006, 118, 2664–2667
(Angew. Chem., Int. Ed., 2006, 45, 2602–2605); (b) A. Jana,
H. W. Roesky, C. Schulzke and A. Doring, Angew. Chem., 2009,
¨
121, 1126–1129 (Angew. Chem., Int. Ed., 2009, 48, 1106–1109);
(c) A. Jana, D. Ghoshal, H. W. Roesky, I. Objartel and D. Stalke,
J. Am. Chem. Soc., 2009, 131, 1288–1293.
7 S. L. Choong, W. D. Woodul, C. Schenk, A. Stasch,
A. F. Richards and C. Jones, Organometallics, 2011, 30,
5543–5550.
8 G. Leroy, D. Riffi Temsamani and C. Wilante, THEOCHEM,
1994, 306, 21–39.
9 (a) S. M. I. Al-Rafia, A. C. Malcolm, R. Mcdonald,
M. J. Ferguson and E. Rivard, Angew. Chem., 2011, 123,
8504–8507 (Angew. Chem., Int. Ed., 2011, 50, 8354–8357);
(b) S. M. I. Al-Rafia, A. C. Malcolm, S. K. Liew,
M. J. Ferguson and E. Rivard, J. Am. Chem. Soc., 2011, 133,
777–779; (c) K. C. Thimer, S. M. I. Al-Rafia, M. J. Ferguson,
R. McDonald and E. Rivard, Chem. Commun., 2009, 7119–7121.
10 S. Khan, R. Michel, J. M. Dieterich, R. A. Mata, H. W. Roesky,
J.-P. Demers, A. Lange and D. Stalke, J. Am. Chem. Soc., 2011,
133, 17889–17894.
11 S.-P. Chia, H.-X. Yeong and C.-W. So, Inorg. Chem., 2012, 51,
1002–1010.
12 R. Jambor, S. H-Pawlis, M. Schurmann and K. Jurkschat, Eur. J.
Inorg. Chem., 2011, 344–348.
¨
While Dostal et al. exploited the pincer-based NCN ligand
´
to isolate the first monomeric stibinidene and bismuthinidene
derivatives,23 we have shown that such a ligand is also useful
to isolate monomeric Sn(II) and Ge(II) hydrides. This is the
first example of a Sn(II) hydride where no Sn–H intermolecular
interaction is present. The monomeric structures of 3 and 4 are
mainly stabilized by donating electron density to the empty p
orbital of the central metal atoms as supported by theoretical
calculations. The chemistry of these new hydrides will be
explored and published in due course.
13 (a) T. Kottke and D. Stalke, J. Appl. Crystallogr., 1993, 26,
615–619; (b) D. Stalke, Chem. Soc. Rev., 1998, 27, 171–178.
14 (a) F. E. Hahn, L. Wittenbecher, M. Kuhn, T. Lugger and
¨
R. Frohlich, J. Organomet. Chem., 2001, 617–618, 629–634;
¨
¨
(b) K. Jurkschat, S. van Dreumel, G. Dyson, D. Dakternieks,
T. J. Bastow, M. E. Smith and M. Drager, Polyhedron, 1992, 11,
2747–2755.
¨
15 R. Jambor, B. Kasna, K. N. Kirschner, M. Schurmann and
´
¨
K. Jurkschat, Angew. Chem., 2008, 120, 1674–1677 (Angew. Chem.,
Int. Ed., 2008, 47, 1650–1653).
16 E. Rivard, R. C. Fischer, R. Wolf, Y. Peng, W. A. Merrill,
N. D. Schley, Z. Zhu, L. Pu, J. C. Fettinger, S. J. Teat,
I. Nowik, R. H. Herber, N. Takagi, S. Nagase and P. P. Power,
J. Am. Chem. Soc., 2007, 129, 16197–16208.
17 G. H. Spikes, J. C. Fettinger and P. P. Power, J. Am. Chem. Soc.,
2005, 127, 12232–12233.
18 Y. Ding, H. Hao, H. W. Roesky, M. Noltemeyer and
H.-G. Schmidt, Organometallics, 2001, 20, 4806–4811.
Support from the Deutsche Forschungsgemeinschaft
(DFG RO 224/55-3 (H. W. R.), Priority Program 1178 (D. S.),
Emmy Noether Fellowship (A. L.), Deutscher Akademischer
Austausch Dienst (research fellowship to S. K.) and the NSERC
of Canada (scholarship to J.-P. D.) are acknowledged.
19 (a) A. D. Becke, Phys. Rev. A: At., Mol., Opt. Phys., 1988, 38,
3098–3100; (b) J. P. Perdew, Phys. Rev. B, 1986, 33, 8822–8824;
(c) F. Weigend and R. Ahlrichs, Phys. Chem. Chem. Phys., 2005, 7,
3297–3305; (d) B. Metz, H. Stoll and M. Dolg, J. Chem. Phys., 2000,
113, 2563–2569; (e) ORCA, an ab initio, DFT and semiempirical
electronic structure package, version 2.8, 2010.
20 S. Grimme, J. Comput. Chem., 2006, 27, 1787–1799.
21 (a) H.-J. Werner, F. R. Manby and P. J. Knowles, J. Chem. Phys.,
2003, 118, 8149–8160; (b) T. H. Jr. Dunning, J. Chem. Phys., 1989,
90, 1007–1023; (c) K. A. Peterson, J. Chem. Phys., 2003, 119,
11099–11112; (d) H.-J. Werner, P. J. Knowles, R. Lindh,
Notes and references
1 (a) T. Hiyama and T. Kusumoto, Comprehensive Organic Synthesis,
Pergamon, Oxford, UK., 1991, vol. 8, pp. 763–792; (b) G. F. Bradley
and S. R. Stobart, J. Chem. Soc., Dalton Trans., 1974, 264–269.
2 For reviews see: (a) S. K. Mandal and H. W. Roesky, Acc. Chem.
Res., 2012, 45, 298–307; (b) S. S. Sen, S. Khan, P. P. Samuel and
H. W. Roesky, Chem. Sci., 2012, 3, 659–682; (c) S. K. Mandal and
H. W. Roesky, Chem. Commun., 2010, 46, 6016–6041;
(d) E. Rivard and P. P. Power, Dalton Trans., 2008, 4336–4343;
(e) S. Aldridge and A. J. Downs, Chem. Rev., 2001, 101,
3305–3365; (f) C. Jones, Chem. Commun., 2001, 2293–2298;
(g) A. Staubitz, A. P. M. Robertson and I. Manners, Chem.
Rev., 2010, 110, 4079–4124.
F. R. Manby and M. Schutz, MOLPRO, version 2010.1, a package
¨
of ab initio programs, 2010, http://www.molpro.net.
22 J. P. Foster and F. Weinhold, J. Am. Chem. Soc., 1980, 102,
7211–7218.
3 (a) C.-F. Wang, S.-Y. Xie, S.-C. Lin, X. Cheng, X.-H. Zhang,
R.-B. Huang and L.-S. Zheng, Chem. Commun., 2004, 1766–1767;
(b) C. M. Hessel, E. J. Henderson, J. A. Kelly, R. G. Cavell,
23 P. Simon, F. De Proft, R. Jambor, A. Ruzicka and L. Dostal,
´
Angew. Chem., 2010, 120, 1674–1677 (Angew. Chem., Int. Ed.,
2010, 49, 5468–5471).
c
4892 Chem. Commun., 2012, 48, 4890–4892
This journal is The Royal Society of Chemistry 2012