Learning about steric effects in NHC complexes from a 1D silver coordination polymer with Frechet dendrons

Article abstract of DOI:10.1021/om401060u

The complex (NHC)AgBr, which bears G1 poly(benzyl ether) dendritic substituents in the NHC ligand, forms a 1D coordination polymer based on unusual zigzag -Ag-halide- chains. The asymmetric distribution of the volume buried by the NHC ligand in the Ag coordination sphere is important to explain the formation of this structure. A V_bur eccentricity parameter has been introduced and used in conjunction with the %V_bur descriptor to analyze the X-ray structures of 100 (NHC)AgX complexes.

Full text of DOI:10.1021/om401060u

Note
pubs.acs.org/Organometallics
Learning about Steric Effects in NHC Complexes from a 1D Silver
́
Coordination Polymer with Frechet Dendrons
Alba M. Ortiz, Pilar Gomez-Sal, Juan C. Flores,* and Ernesto de Jesus*
́
́
́
́
Departamento de Quımica Organ
́
ica y Quımica Inorgan
́
ica, Universidad de Alcala,
́
́
28871 Alcala de Henares, Spain
S
* Supporting Information
ABSTRACT: The complex (NHC)AgBr, which bears G1
poly(benzyl ether) dendritic substituents in the NHC ligand,
forms a 1D coordination polymer based on unusual zigzag
−Ag−halide− chains. The asymmetric distribution of the
volume buried by the NHC ligand in the Ag coordination
sphere is important to explain the formation of this structure.
A Vbur eccentricity parameter has been introduced and used in
conjunction with the %Vbur descriptor to analyze the X-ray
structures of 100 (NHC)AgX complexes.
Scheme 1. Synthesis of the NHC Silver Complexes
n extensive array of structural topologies in which the metal
Adisplays a wide range of coordination numbers and
geometries are known for silver(I) complexes.1 This coordinative
flexibility is in part due to the lack of stereochemical preference
arising from the d10 configuration and weak nature of the bond
between this soft acceptor and most donors.2 The plasticity of
Ag(I) is of interest for crystal engineering and subsequent
potential applications3 because weak interactions acquire a
greater influence, and factors such as small ligand variations can
drastically modify the supramolecular assembly process.4 In this
regard, N-heterocyclic carbene ligands (NHC) are appealing due
to their versatile functionalization and the synthetic availability
and air and water stability of their Ag(I) complexes. This has led
to a plethora of silver NHC complexes5−7 with a wide diversity of
supramolecular motifs, ranging from discrete arrangements to
extended staircase structures, aesthetic cages, or cubane-type
clusters.8
As a consequence of our interest in the structural chemistry of
metallodendrimers,9,10 this report describes the unusual zigzag
−Ag−halide− chain formed in the solid state by the (NHC)AgBr
dendritic complex 4. The importance of the asymmetric
distribution of the volume buried by the NHC ligand will be
outlined, leading to the introduction of a Vbur eccentricity
parameter that might be a useful addition to the %Vbur descriptor.
Silver(I) NHC complexes 4−6, which have previously been
utilized as “non-isolated” intermediates for carbene trans-
metalation to Rh(I),11 were obtained using Lin’s standard
procedure (Scheme 1).12 Preparation of the imidazolium
precursors 1−3 was also optimized using a stepwise protocol
that simplified the purification steps and significantly increased
the overall yields. Full synthetic and characterization details are
given in the Supporting Information.
We were only able to obtain suitable crystals for X-ray
diffraction studies in the case of the G1 complex (4). The
difficulties associated with the growth of single crystals from large
dendrimers is well known and, therefore, it is not surprising that
the few X-ray structures reported for metal complexes containing
Frec
́
het dendrons correspond to generations G111,13 and G2.10
Complex 4 crystallizes as 1D zigzag chains which are formed by
the assembly of (NHC)AgBr units by halide bridges to give a
planar three-coordinate silver environment (Figure 1). The C2
axis defined by the Ag−CNHC bond renders the −(AgBr)n− chain
symmetric, with identical Ag−Br distances (2.662 Å) and Br−
Ag−Br and Ag−Br−Ag angles (106.5°). The monomers are
Received: October 31, 2013
Published: January 10, 2014
© 2014 American Chemical Society
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dx.doi.org/10.1021/om401060u | Organometallics 2014, 33, 600−603

Organometallics
Note
Figure 1. (a) ORTEP diagram showing the silver coordination environment in 4 (H atoms omitted for clarity). (b) View of a single 1D chain along the
crystallographic b axis. (c) View along the a axis. Selected bond lengths (Å) and angles (deg): Ag−Br, 2.662(1); Ag−Br(a), 2.662(1); Ag−C(2),
2.152(6); N−C(2), 1.372(5); N−C(1), 1.500(5); C(3)−C(3a), 1.350(10); C(2)−Ag−Br, 126.75(2); Br−Ag−Br(a), 106.50(3); Ag−Br−Ag(a),
106.50(3); Ag−C(2)−N, 128.0(2); N−C(2)−N(a), 104.1(5); C(2)−N−C(1), 122.5(4).
propagated along the crystallographic a axis at a constant
distance of 4.27 Å. This dense stacking of NHC ligands is
sterically possible because the dendrons are fully extended (Ph−
C−O−Ar torsion angles of 174−175°) and the NHC rings are
rotated around the perpendicular to the propagating axis a (N−
C(2)−Ag−Br torsion angle of 52.9°).10,14 The Ag−CNHC
distance of 2.15 Å is longer than that observed in [(IBz)AgBr]2
(2.10 Å)15 and in most (NHC)Ag(halide) complexes (usually
2.08−2.10 Å).5 The Ag−Br distance (2.662 Å) is also longer than
in [(NHC)AgBr] monomers (2.40−2.43 Å)16 and intermediate
between the short (2.4−2.5 Å) and long Ag−Br distances (2.8−
3.1 Å) found in [(NHC)Ag(μ-Br)]2 dimers.15,16
large slippage moves the C6 rings away from the distances more
favorable for attractive interactions between aromatic rings.
Second, and more importantly, interactions between small π
rings are not necessarily stronger than those between saturated
fragments of the same number of C atoms.22,23 In other words,
the geometrical existence of stacked aromatic rings does not
necessarily involve an abnormal added stabilization that will
inevitably force a given structural arrangement.
Consequently, we focused our attention on the silver
coordination sphere. Figure 2 shows a structural classification
Although single-stranded Ag(I) coordination polymers are
common, they are typically accessible by means of bidentate
ligands bridging the metal atoms,1 whereas those exclusively
enchained by halide bridges are rare.17 Indeed, the complex
(phen)AgCl is the only precedent in the Cambridge Structural
Database (CSD) of an unsupported zigzag −(Ag(halide))n−
chain.18 Both polymeric structures display π-stacked aromatic
rings (phen or Bz) in a parallel-displaced disposition of close
geometric parameters. These parameters are compared in Table
1 with those reported by Janiak on the basis of the most frequent
Table 1. Comparison of Geometric Parameters (Å) for the
Parallel-Displaced π Stacking of Aromatic Rings
a
b
c
dcent−cent
dinterplanar
dslip
4
4.27
4.28
3.8
3.41−3.82
3.37
1.91−2.57
2.64
(phen)AgCl18
CSD database
Figure 2. Structural arrangements of (NHC)Ag(halide) units in the
solid state. Halide-bridged dimers are usually asymmetric. Double chains
are constituted of dimers stacked by formation of a third X bridge. The
single-chain structure corresponds to 4.
d
3.6
1.3
e
ab initio calc
3.6
3.2
1.6
b
a
Distance between centroids of adjacent rings in the π stack. Distance
c
d
between the ring planes. Ring slippage. Most frequent parameters
found in structures containing almost parallel pyridine rings after
of (NHC)Ag(halide) complexes that is mostly based on the
reviews of Garrison5 and Lin,6 omitting the cases in which
(NHC)2Ag+ ions are formed. To analyze more exactly at which
point steric effects and structural types are linked, we calculated
the percent buried volume (%Vbur)24,25 for the NHC ligands in
around 100 reported X-ray structures containing monodentate
NHC ligands. The resulting distribution of %Vbur, which is
represented in Figure 3, outlines the limits imposed by the steric
crowding of the NHC ligand on the tendency of the AgX
moieties to condense by formation of halide bridges or
argentophilic interactions. In this context, the combination of a
very high %Vbur (42.2) with a chain structure in 4 is unusual.
Indeed, this is possible because the distribution of the NHC-
buried volume around the metal center is highly polarized in two
of the three spatial dimensions (corresponding to the crystallo-
searches in the CSD database.19 Equilibrium distances in benzene
e
dimers in parallel-displaced configurations, as determined by ab initio
calculations.21
values found in the CSD database for π-stacked pyridine
complexes19,20 and with those determined by Sinnokrot et al.
based on ab initio calculations on benzene dimers.21 The
distances between ring centroids within the chains (4.3 Å) are
imposed by the geometry of the halide bridges, whereas ligand
rotation with respect to the perpendicular to the chain axis results
in short interplanar distances (3.4−3.8 Å) and large parallel
displacements between rings in the stacks (1.9−2.6 Å). The
temptation to assign a determinant role in the solid-state
organization of 4 to π−π interactions must be resisted. First, the
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Note
Figure 3. Distribution of percent buried volumes for NHC ligands by
structural type of the (NHC)Ag(halide) complex. %Vbur values were
calculated at an M−NHC distance of 2.28 Å.
graphic b and c axes, Figure 1c), thereby leaving free space for
propagation of the chain in the a direction.26
To overcome the limitations of %Vbur as an average property,
2D contour maps are becoming popular as a powerful tool for
detailed analysis of the impact of Vbur asymmetry on limited sets
of complexes.27 However, the simplicity of a single parameter for
measuring the steric asymmetry, especially that originating
around the metal−CNHC bond, might be practical for under-
standing structural or chemical trends in large sets of complexes.
As such, herein we propose a definition for such a parameter
based on the well-established %Vbur and conveniently calculated
using the SambVca application.25 This parameter is especially
pertinent for NHC ligands because of their noncylindrical
(planar) core, although the methodology is also applicable to
phosphanes and other common ligands. We start by considering
an ellipsoid with three independent dimensions (a, b, and c) and
a spheroid, which represents a special case of an ellipsoid with
symmetry around the c axis (a = b, Figure 4). When an ellipsoid is
Figure 5. (a) Buried volume eccentricities for a set of representative
NHC ligands, determined as described in the text. (b) Space-filling
representation of selected NHC ligands.
above which steric factors prevent the formation of condensed
structures appears at the top (Figure 6).29 The positive slope of
Figure 4. Overlap ratio (S) between an ellipsoid and the same ellipsoid
rotated by 90° around the c axis. This value is equal to 1 for a spheroid
and decreases with the asymmetry of the a and b dimensions, in
accordance with the displayed formula.
Figure 6. Distribution of the structures reported for (NHC)AgX
complexes with symmetrically substituted NHC ligands in the 2D map
of percent buried volumes and eccentricities.
rotated around the c axis, the overlap between the original and
rotated ellipsoids is determined by the relative length of the
semiprincipal axes a and b. The overlap coefficient is S = 1 for a
spheroid S and decreases as the eccentricity of the ellipsoid
increases. Considering the ellipsoid as an acceptable analogy, we
calculated the buried volume overlap for NHC ligands in
(NHC)AgX complexes by considering the metal−NHC bond as
the axis for the 90° rotation (see the Supporting Information for
details). The results for a selection of NHC ligands are given in
Figure 5 in the form of a/b values, calculated using the formula
shown in Figure 4, as this gives a more workable scale of values
than the direct use of S.28
the borderline means that condensed structures are compatible
with ligands of low steric demand or with ligands of higher steric
demand when this demand is directionally focused (as for 4).
Condensed structures prevail below the borderline, but crystal
packing or other weak forces can determine the formation of
monomers due to the stereochemical nonrigidity of silver(I).
In summary, the singular zigzag-chain arrangement found in 4
can be understood in steric terms when the asymmetry of the
ligand is considered. The combined use of %Vbur and the
eccentricity parameter proposed herein is useful for under-
standing the solid-state structures of silver NHC complexes. This
approach can be convenient in other contexts in which the shape
and orientation of the NHC N substituents are considered to be
determining factors. The Vbur eccentricity is sensitive to them and
When the structures of (NHC)AgX complexes with sym-
metrically substituted NHC ligands are placed in a 2D map of
percent buried volume and eccentricity values, a borderline
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Note
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complements the %Vbur value. For instance, the IAd ligand has a
%Vbur value intermediate between those of bulky IMes and IPr
ligands (Figure 5b) but its Vbur eccentricity is much higher. This
is in agreement with the differences found in the reactivity of
their metal complexes, which arise from localization of the most
hindered region around the adamantyl group.30
ASSOCIATED CONTENT
* Supporting Information
■
́
(14) For a study on the structural features of Frechet-type dendrons in
S
single crystals, see: Stadler, A.-M. Cryst. Growth Des. 2010, 10, 5050−
5065.
Text, figures, tables, and a CIF file giving general and synthetic
procedures, characterization data, crystallographic data for 4,
calculations and Vbur eccentricity values for (NHC)Ag(halide)
structures. This material is available free of charge via the Internet
at http://pubs.acs.org.
(15) Berding, J.; Kooijman, H.; Spek, A. L.; Bouwman, E. J. Organomet.
Chem. 2009, 694, 2217−2221.
(16) See the Supporting Information for references.
(17) Healy, P.; Kildea, J.; White, A. Aust. J. Chem. 1988, 41, 417−418.
́
Wolper, C.; Polo Bastardes, M. D.; Dix, I.; Kratzert, D.; Jones, P. G. Z.
̈
AUTHOR INFORMATION
Corresponding Author
*E-mail: ernesto.dejesus@uah.es (E.d.J.); juanc.flores@uah.es
(J.C.F.).
Naturforsch., B: Chem. Sci. 2010, 65, 647−673.
■
(18) Odoko, M.; Wang, Y.; Okabe, N. Acta Crystallogr., Sect. E 2004,
60, m1522−m1524.
(19) Janiak, C. J. Chem. Soc., Dalton Trans. 2000, 3885−3896.
(20) We obtained values similar to those reported by Janiak in CSD
searches for structures with benzyl groups either not substituted or
substituted with oxygen at the meta ring positions (as for the aryl rings of
the dendron in 4; see the Supporting Information for details).
(21) Sinnokrot, M. O.; Sherrill, C. D. J. Phys. Chem. A 2006, 110,
10656−10668.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was supported by the Ministerio de Economia y
Competitividad (Projects CTQ2011-24096 and CSD2006-
■
́
(22) The term “π−π stacking” should be used as a geometrical
descriptor of the interaction mode in unsaturated molecules and to
understand π−π interactions as a special type of electron correlation
(dispersion) effect that can only act in large unsaturated systems when
they are in the stacked orientation: Grimme, S. Angew. Chem., Int. Ed.
2008, 47, 3430−3434. See also: Wheeler, S. E. Acc. Chem. Res. 2012, 46,
1029−1038.
́
00015). A.M.O. is grateful to the Ministerio de Educacion for a
FPU Fellowship. Dedicated to Professor A. Laguna on the
occasion of his 65th birthday.
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