Exploring structural trends for complexes of Me2E(OSO 2CF3)2 (E = Si, Ge, Sn) with pyridine derivatives

Article abstract of DOI:10.1039/c4cc01109k

The coordination of Me₂E(OTf)₂ (E = Si, Ge, Sn) acceptors by dmap or 2,2′-bipy furnishes two series of complexes which exhibit distinct structural trends that correlate with the covalent radii of the tetrael elements, and which contrast complexes of these ligands with EX ₄ (X = Cl or Br). the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc01109k

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Exploring structural trends for complexes of
Me E(OSO CF ) (E = Si, Ge, Sn) with pyridine
derivatives†
2
2
3 2
Cite this: Chem. Commun., 2014,
0, 7979
5
Received 11th February 2014,
Accepted 4th June 2014
a
a
b
Alasdair P. M. Robertson, Jordan N. Friedmann, Hilary A. Jenkins and
Neil Burford*
a
DOI: 10.1039/c4cc01109k
www.rsc.org/chemcomm
The coordination of Me
2
E(OTf )
2
(E = Si, Ge, Sn) acceptors by dmap or
0
2
,2 -bipy furnishes two series of complexes which exhibit distinct struc-
tural trends that correlate with the covalent radii of the tetrael elements,
and which contrast complexes of these ligands with EX (X = Cl or Br).
4
The formation of donor–acceptor complexes has been applied
extensively to describe the chemistry of the transition metals and
1
the group 13 elements. Heavier p-block elements also engage an
2 2
Scheme 1 Synthesis of dmap and bipy complexes of Me E(OTf) (E = Si,
array of ligand types, but coordination chemistry of the lighter
p-block elements is apparently limited by atomic size and
acceptor orbital availability. Consequently, there are significant
differences in the classification of the structure and bonding for
Ge or Sn).
Two series of complexes with generic formulae Me
2
E(dmap)
2
-
19
the group 14 elements. For example, a vast array of coordination (OTf) and Me E(bipy)(OTf) [E = Si, Ge, Sn; dmap = 4-(dimethyl-
2 2 2
2
0
complexes has been established for compounds of tin, while amino)pyridine, bipy = 2,2 -bipyridine], were prepared by a combination
compounds involving silicon or carbon as acceptors are rare. of halide abstraction and donor coordination steps. For E = Si or Ge, the
4 2 2
Nevertheless, silicon, germanium and tin-tetrahalides (EX ; E = Si, complexes were prepared in high yield through treatment of Me ECl
Ge, Sn; X = Cl or Br) are all sufficiently Lewis acidic to form with AgOTf, to generate Me E(OTf) in situ, and subsequently
2
2
0
isolable octahedral complexes of the general formula trans-EX L
with dmap or 2,2 -bipy at ꢀ30 1C (Scheme 1a). In contrast,
E = Si, Ge, Sn; L = PMe or pyridine) with monodentate donors, Me SnCl was found to be unreactive towards AgOTf at ambient
Interestingly, temperature, thus the tin complexes were accessed via halide
despite previous reports of complexes of the related acceptor abstraction from the in situ generated adducts Me Sn(dmap) Cl
4
2
3–8
(
2
2
3
9–11
while chelating ligands give cis configurations.
2
2
2
1
2,13
11
Me
for Me
2
SnCl
SiCl
2
,
analogous compounds have not been observed and Me
2
Sn(bipy)Cl
2
in MeCN (Scheme 1b).
2
2
or Me GeCl . Recognizing that exchange of halide All three dmap complexes were isolated as colourless crystalline
2
2
substituents at a potential tetrael acceptor centre for weakly- solids, and characterised by multinuclear NMR spectroscopy, elemen-
coordinating anions can facilitate coordination of neutral tal microanalysis and X-ray crystallography. In the solid-state, Me Si(d-
2
3,14–18
2+
ligands,
we have studied complexes of Me E(OTf) (E = Si, map) (OTf) contains a distinct [Me Si(dmap) ] moiety in the
2 2 2 2 2 2
Ge, Sn; OTf = OSO CF , trifluoromethanesulfonyl) with pyridine asymmetric unit, along with two triflate anions and two molecules
2
3
2 2
derivative ligands. Herein we describe structural trends which of CH Cl (Fig. 1a). The silicon centre adopts a distorted tetrahedral
evolve the coordination chemistry of the tetrael elements, and geometry [105.3 (N–Si–N); 115.71 (C–Si–C); Average: 109.41] with the
2
0
have implications for the p-block elements in general.
two N–Si bonds averaging 1.80 Å [SCR = 1.87 Å]. The interion Si–O
distances are greater than the sum of the covalent radii for Si and
O [Si–O = 3.476(3) and 3.819(3) Å; SCR = 1.79 Å], and we
a
Department of Chemistry, University of Victoria, P.O. Box 3065, Stn. CSC,
2
+
Victoria, BC, Canada. E-mail: nburford@uvic.ca
interpret the compound as a salt containing an Me Si moiety
stabilised by two dmap ligands. In contrast, the solid-state
structure of Me Sn(dmap) (OTf) evidences a hexacoordinate Sn
2 2 2
centre with two cis-configured dmap ligands (N–Sn–N = 87.91), two
cis-configured triflate substituents (O–Sn–O = 100.51) and two
trans-configured methyl substituents (C–Sn–C = 158.61) (Fig. 1c).
2
b
Analytical X-ray Diffraction Facility, McMaster University, 1280 Main Street West,
Hamilton, Ontario, Canada
†
Electronic supplementary information (ESI) available: Details of the synthesis
and characterisation of all six described complexes, and further crystallographic
parameters. CCDC 977497–977502. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c4cc01109k
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0
Fig. 2 Observed solid-state configurations for derivatives of (a) Me
and (b) EX
2
EL
2
X
2
4 2
L .
derivatives is in marked contrast to the essentially invariant distorted
octahedral structures of the series trans-ECl , for E = Si, Ge or Sn,
Furthermore, the observed cis-configuration
4 2
L
3–8
3
L = pyridine or PMe .
of the ligands in all three complexes (Fig. 2a) contrasts the trans-
configuration reported for derivatives of EX4L2 in the solid-state
(
Fig. 2b), and is consistent with that proposed for the solid-state
12,13
structure of Me
2
SnCl
2
(pyridine)
2
based on IR spectroscopy.
The
apparent preference for cis-configuration of the ligands at these
acceptors may reflect the minimization of steric effects within the
complexes, but the trans relationship between the ligands and the
N
triflate anions also implicates a possible S 2 anion displacement.
The corresponding bipy complexes of generic formula Me E-
2
(
bipy)(OTf ) (E = Si, Ge, Sn) were obtained by analogous procedures
2
2 2 2
to derivatives of Me E(dmap) (OTf ) (Scheme 1), and were charac-
terised by multinuclear NMR spectroscopy, elemental microanalysis
and X-ray crystallography. In the solid-state, the silicon centre
2
of [Me Si(bipy)(OTf )][OTf ] adopts a distorted trigonal bipyramidal
. All hydrogen atoms and solvent molecules geometry, with the two nitrogen atoms of the bipy ligand in an
equatorial and an axial site, respectively (Fig. 3a). An oxygen centre of
Fig. 1 Solid-state structures of (a) [Me
and (c) Me Sn(dmap) (OTf)
are omitted for clarity.
2 2 2 2 2 2
Si(dmap) ][OTf ] , (b) [Me Ge(dmap) ][OTf]
2
2
2
a triflate substituent occupies the second axial position and the two
Both triflate anions strongly coordinate to the Sn centre, which
exhibits a distorted octahedral geometry with relatively long Sn–OTf
bonds [2.420(1) Å; SCR = 2.03 Å]. The degree of anion coordination in
methyl groups occupy the other equatorial sites. The Si–O bond
1.924(2) Å] is significantly shorter than the interaction with the
second triflate anion [4.051(2) Å; SCR = 1.79 Å], and contrasts the
dicationic formulation of [Me Si(dmap) ][OTf ] , presumably reflecting
[
the solid-state observed for [Me
in character to those of the four-coordinate cation observed in
Me Si(dmap) ][OTf ] and the six-coordinate neutral complex
2 2 2
Ge(dmap) ][OTf ] is intermediate
2
2
2
the lesser steric demands of the essentially planar bipy ligand. The
[
2
2
2
structure of [Me Ge(bipy)(OTf )][OTf ] (Fig. 3b) is very similar to that of
2
Me Sn(dmap) (OTf) . The geometry at the germanium centre is best
2
2
2
the silicon derivative, with the germanium centre again trigonal
approximated as tetrahedral, but long interactions with two triflate
anions [Ge–O = 2.824(2) and 3.297(2) Å; SCR = 1.84 Å] impart distortion
towards an octahedral geometry [N–Ge–N = 93.71; C–Ge–C = 126.01]
bipyramidal, although the Ge–OTf bond is longer at 2.495(1) Å, a
difference that cannot be explained solely on the basis of the greater
covalent radius of Ge. The interaction with the second triflate anion
(
Fig. 1b). As such, the three structures demonstrate differing degrees
of triflate displacement from the Me E(OTf) acceptors, and
conceptually can be considered as snapshots of the reaction profile
for two in-tandem S 2-type processes: [R EX + 2L - R E(L) ][X] .
[Ge–O = 3.594(1) Å] is slightly less than that in the silicon derivative,
2
2
also evidenced by the greater C–Ge–C angle [133.34(7)1], which may
partially explain the relatively long interaction with the first anion.
The structure of Me Sn(bipy)(OTf ) (Fig. 3c) is analogous to that of
N
2
2
2
2
2
2
2
Key solid-state metrical parameters for the three derivatives
of Me E(dmap) (OTf) are compared in Table 1, and highlight
2 2 2
Me Sn(dmap) (OTf) , with an octahedral geometry at tin distorted by
the constrained chelate ligand, resulting in a narrower N–Sn–N
2
2
2
distinct trends that correlate with the relative covalent radii of the
tetrael elements. The C–E–C and N–E–N angles illustrate geometries
at E ranging from tetrahedral to octahedral, with increasingly short
E–OTf interactions apparent as the covalent radii of the tetrael
increases, in line with the stronger coordination of the anions.
angle [73.18(4)1], and larger O–Sn–O angle [115.85(4)1]. The structure
of Me
structures of R
Relevant metrical parameters for the derivatives of Me
bipy)(OTf) are presented in Table 2. The difference in covalent
2
Sn(bipy)(OTf )
2
is also consistent with the reported solid-state
21
2
Sn(bipy)Cl
2
(R = Me, iPr, nBu, Bn, Ph).
2
E-
(
2
The distinct structural differences for the series of Me E(dmap) (OTf )
2
2
2
radii of silicon and tin imparts a significant variation in the
C–E–C angle consistent with the trigonal bipyramidal and octahe-
dral geometries, respectively. The variation in the N–E–N angles
Table 1 Selected solid-state parameters for Me
2
E(dmap)
2
(OTf)
2
(
bite angle) is relatively small due to the constraints imposed by
Covalent
Mean E–N
N–E–N
C–E–C Shortest
the chelating bipy ligand. The tabulated parameters also illustrate
the deviation of the germanium complex from the trends defined
by covalent radii relative to the silicon and tin analogues, and
those illustrated for the dmap adducts. Notably, the structural
E
radius (Å) bond length (Å) angle (1)
angle (1)
E–OTf (Å)
3.476(3)
Si
1.16
Ge 1.21
Sn 1.40
1.800(3)
1.933(2)
2.210(1)
105.3(1)
93.66(8) 126.02(9) 2.824(2)
87.93(4) 158.57(6) 2.420(1)
115.7(1)
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halide abstraction from Me
2
CX
2
were also unsuccessful. We postu-
late that the inductive effect of the methyl substituents that is
expected to stabilise any developing positive charge is countered
by their steric bulk, impeding nucleophilic attack.
In summary, we have prepared two series of compounds based on
2 2
Me E(OTf) (E = Si, Ge or Sn) acceptors ligated by dmap or bipy
donors. The solid-state structures implicate cationic complexes for
silicon in which one or both of the triflate anions can be regarded as
non-coordinating, and neutral six-coordinate complexes for tin,
in which both triflate anions coordinate to the Sn centre. The
germanium complexes exhibit intermediate structural features.
The distinct structural trends contrast previous observations for the
corresponding complexes of EX (X = Cl or Br), which have been
4
characterised as octahedral molecular compounds. Ongoing work in
our laboratories targets complexes of tetrael acceptors bearing higher
numbers of weakly coordinating anions, which we perceive will lead
to significant structural diversity, and novel bonding.
We thank the Natural Sciences and Engineering Council of
Canada (NSERC) for funding.
Notes and references
1
2
3
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4
5
6
Fig. 3 Solid-state structures of (a) [Me
2
Si(bipy)(OTf)][OTf ], (b) [Me
2
Ge-
2 2
(bipy)(OTf )][OTf ] and (c) Me Sn(bipy)(OTf) . All hydrogen atoms are omitted
7
for clarity.
8
9
R. Hulme, G. J. Leigh and I. R. Beattie, J. Chem. Soc., 1960, 366–371.
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Table 2 Selected solid-state parameters for Me
Covalent Mean E–N N–E–N
2
E(bipy)(OTf)
2
1
1
0 I. R. Beattie, T. Gilson, M. Webster and G. P. McQuillan, J. Chem.
Soc., 1964, 238–244.
1 G. W. A. Fowles, D. A. Rice and B. A. Walton, Spectrochim. Acta, Part
A, 1969, 25, 1035–1036.
C–E–C
Shortest
E–OTf (Å)
E
radius (Å) bond length (Å) angle (1) angle (1)
Si
1.16
1.918(2)
1.968(1)
2.255(1)
81.8(1)
81.81(5)
73.18(4)
128.5(1)
133.34(7) 2.495(1)
159.42(6) 2.336(1)
1.924(2)
12 J. P. Clark and C. J. Wilkins, J. Chem. Soc. A, 1966, 871–873.
13 L. A. Aslanov, V. M. Ionov, V. M. Attiya, A. B. Permin and
V. S. Petrosyan, J. Struct. Chem., 1978, 19, 166–169.
Ge 1.21
Sn 1.40
1
1
1
4 E. MacDonald, L. Doyle, N. Burford, U. Werner-Zwanziger and
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differences observed between derivatives of Me
again contrast the uniform geometries reported for the corres-
2 2
E(bipy)(OTf)
ponding adducts of EX (X = Cl or Br), for which cis-octahedral
4
17 P. Farina, T. Latter, W. Levason and G. Reid, Dalton Trans., 2013, 42,
complexes, EX (L–L), are exclusively formed.
4714–4724.
4
1
1
8 J. L. Dutton and P. J. Ragogna, Coord. Chem. Rev., 2011, 255, 1414–1425.
9 The extension of this study to Me PbX was not attempted due to the
relative instability of this class of compound towards disproportio-
nation to yield Pb(II) products.
0 SCR values are calculated from the mean single-bond covalent radii
reported in P. Pyykk ¨o and M. Atsumi, Chem. – Eur. J., 2009, 15,
Attempts to prepare the corresponding carbon derivatives
2
2
[
Me
ESI†). Nevertheless, compounds of the form [H
can be formed via the activation of haloalkanes, including
C(dmap) ][X] (X = Cl or I), and exhibit a tetrahedral geometry
2 2 2 2 2
C(dmap) ][OTf ] and [Me C(bipy)][OTf ] were unsuccessful (see
4,22–24
2 2 2
C(Donor) ][X]
2
[H
2
2
2
186–197. These values are provided as a qualitative guide, recogniz-
at carbon, in-line with our experimentally observed trends for Si, Ge
and Sn. Remarkably, [H C(dmap) ][Cl] has been reported to form
ing that heteroatomic bonds have a significant ionic component.
1 E. R. T. Tiekink, V. J. Hall, M. A. Buntine and J. Hook, Z. Kristallogr.,
2
2
2
2
2
2
2
2000, 215, 23–33.
spontaneously under ambient conditions upon dissolution of dmap
2 A. B. Rudine, M. G. Walter and C. C. Wamser, J. Org. Chem., 2010, 75,
4292–4295.
3 J. S. Driscoll, D. W. Grisley, Jr., J. V. Pustinger, J. E. Harris and
C. N. Matthews, J. Org. Chem., 1964, 29, 2427–2431.
4 S. Munavalli, E. J. Poziomek and C. S. Day, Acta Crystallogr., Sect. C:
Cryst. Struct. Commun., 1988, 44, 272–275.
22
in CH
heating of a solution of dmap in Me
upon thermolysis of a toluene solution of Me
2
Cl
2
.
In contrast, no such reaction is observed upon the
CCl over 100 h at 75 1C, nor
CX (X = Cl or Br) and
2
2
2
2
dmap at over 42 h 100 1C. Attempts to facilitate the interaction by
This journal is ©The Royal Society of Chemistry 2014
Chem. Commun., 2014, 50, 7979--7981 | 7981