General formation of trigonal-prismatic [Ag6X 5(dppf)3]+ (X = Cl, Br, I) through an unusual ligand migration from NiX2(dppf) to AgOTf

Article abstract of DOI:10.1039/b707218j

Use of NiX₂(dppf) (X = Cl, Br, I) as a ligand transfer reagent to AgOTf results in the trigonal prismatic [Ag₆X₅(dppf) ₃]OTf complexes. Similar reactions with the dppe analogues give at least 4 different types of dppe-bridged coordination polymers. The Royal Society of Chemistry.

Full text of DOI:10.1039/b707218j

Chemical Communications
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Number 41 | 7 November 2007 | Pages 4177–4300
ISSN 1359-7345
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T. S. Andy Hor et al.
General formation of trigonal-prismatic
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FEATURE ARTICLE
[Ag X (dppf) ] (X = Cl, Br, I) through
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Brian E. Mann and Roberto Motterlini an unusual ligand migration from
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NiX (dppf) to AgOTf
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General formation of trigonal-prismatic [Ag6X5(dppf)3]+ (X = Cl, Br, I)
through an unusual ligand migration from NiX2(dppf) to AgOTf{
Peili Teo, Lip Lin Koh and T. S. Andy Hor*
Received (in Cambridge, UK) 14th May 2007, Accepted 1st June 2007
First published as an Advance Article on the web 20th June 2007
DOI: 10.1039/b707218j
(m/z = 1502.3), [Ag4Cl3(dppf)2]+ (m/z = 1646.3) etc. with matching
Use of NiX2(dppf) (X = Cl, Br, I) as a ligand transfer reagent to
isotopic patterns.
AgOTf results in the trigonal prismatic [Ag6X5(dppf)3]OTf
complexes. Similar reactions with the dppe analogues give at
least 4 different types of dppe-bridged coordination polymers.
Single-crystal X-ray crystallographic analysis shows a cage
aggregate [Ag6Cl5(dppf)3](OTf), 1 of six silver atoms forming a
3
˚
trigonal prismatic core (vol. y18.6 A ). The prism is longitudinally
We have been interested in the coordination1 and catalytic2
chemistry of dppf for two decades. Our earlier work on the
coinage metals, especially Ag+, revealed a large variety of mono-,
di- and polynuclear structures that can be assembled from dppf.3
The structural outcome is generally hard to predict. This is
understandable given the numerous permutations arising from the
flexible metal geometry (linear u trigonal planar u tetrahedral),
dppf bonding (unidentate u bridging u chelating) and coordina-
tion state of the supporting anion (halide, pseudohalide etc.)
(uncoordinated u terminal u bridging u chelating u capping).
Our current understanding of [AgX(dppf)]n complexes is that they
generally prefer to give dimeric structures.4 Several variations are
known within the dinuclear framework, depending on the
interplay of the coordination modes of dppf and X2. These
complexes are commonly prepared from direct addition of AgX to
dppf (or Ag+ + X2 + dppf) using stoichiometric conditions of 1 : 1.
In this communication, we report a new synthetic route for dppf
complexes of silver halides, from which we isolated a general class
of Ag6 trigonal prismatic structures that are unprecedented and
unexpected. These compounds are formed under a diphosphine-
limiting conditions created by the dppf-carrier of Ni(II). When the
ferrocene-based dppf is replaced by alkyl-chain-based dppe,
different types of coordination polymers are isolated. This study
hence demonstrated a unique difference between dppf and dppe
and their roles in determining the outcome of the Ag(I) assemblies.
Attempts to carry out a ‘‘standard’’ metathesis reaction between
NiCl2(dppf) with AgOTf (OTf = triflate, CF3SO3) did not result in
Ni(OTf)2(dppf) or AgCl but an unexpected [Ag6Cl5(dppf)3][OTf],
1. The formation of this complex supports the high propensity of
Ag+ not just for chloride but also for phosphine. Similar attempts
on RuCl2(bpy) by AgBF4 in the presence of dppf also led to the
isolation of dimeric [AgCl(dppf)]2.5
compressed such that the vertical edges are y14% shorter than the
triangular sides. Each of the two (top and bottom) trigonal faces
is capped by a m3-Cl, whereas each rectangular face at the side is
capped by m4-Cl (Fig 1). Each vertical edge of the prism is
overheaded by a dppf. These give an overall aggregate framework
with an yC3h symmetry. Such arrangement enables all the Ag(I)
centers to be chemically identical with each of the [AgCl3P] moiety
adopting a stable 18-e tetrahedral configuration. This formulation
ignores any active Ag–Ag interaction despite the relatively short
˚
metal contacts (av. 3.10(7) A). As expected, the Ag–m4-Cl bonds
˚
(av. 2.6740 A) are significantly longer than the Ag–m3-Cl bonds (av.
˚
2.6372 A). One other related Ag6 cage is found in [Ag6(SC6H5)6-
(PPh3)6].6 Unlike 1, this electronically neutral aggregate takes up an
antiprismatic structure with S6 symmetry with the thiolato edge-
bridging each of the trigonal sides and a significantly longer
…
˚
Ag Ag separation of 3.875A (av.). Other bigger Ag12 and Ag20
cages assembled by pyrazine-2-carboxamide ligands are known.7
Use of NiBr2(dppf) as substrate in the above reaction similarly
results in [Ag6Br5(dppf)3][OTf], 2. X-ray crystallographic analysis
suggested it to be isostructural to 1. The volume of the prism
3
˚
expands to y19.9 A in order to accommodate the larger halide.
Attempts to prepare NiI2(dppf) were unsuccessful. We therefore
used a direct method from the the addition reaction between AgI
and dppf (2:1), followed by exchange with triflate or PF6. The 31
P
NMR spectrum of the yellow product shows characteristic Ag–P
resonance peaks at d = 211.5, 28.9, with Ag–P couplings. X-Ray
The 31P NMR spectrum of 1 at 223 K revealed that all
phosphines are chemically equivalent with the expected couplings
to 107/109Ag. The ESI-MS spectrum shows molecular fragments
corresponding to [Ag(dppf)]+ (m/z = 661.2), [Ag2Cl(dppf)]+
(m/z = 804.8), [Ag2Cl(dppf)2]+ (m/z = 1359.4), [Ag3Cl2(dppf)2]+
Department of Chemistry, National University of Singapore, 3 Science
Drive 3, S117543, Singapore. E-mail: andyhor@nus.edu.sg;
g0404218@nus.edu.sg
{ Electronic supplementary information (ESI) available: Experimental
procedures, analytical data and crystal data. See DOI: 10.1039/b707218j
Fig. 1 ORTEP diagram of 1, showing the cation. (Probability level of
thermal ellipsoid 50%.)
This journal is ß The Royal Society of Chemistry 2007
Chem. Commun., 2007, 4221–4223 | 4221

2
crystallographic analysis of the PF6 salt confirmed another
isostructural derivative [Ag6I5(dppf)3][PF6], 3. The prismatic
3
volume (20.0 A ) is similar to that of the bromide analogue. The
˚
Ag6 core however is more significantly distorted from a regular
prism, as evident from the irregular Ag…Ag distances. The
distortion appears to be needed to accommodate the larger iodide
caps, which also create steric conflicts with the bulky dppf. This
complex is significantly more insoluble, thus hampering efforts in
purification or attempts to produce better crystals for analysis.
The three Ag6 complexes 1–3 are monocationic and isostruc-
tural.{ Their general formation suggested that the hexanuclear
trigonal prismatic form is the thermodynamically most stable
entity under the stoichiometry [Ag2X2n-1(dppf)]n. Attempted
synthesis of 1 or 2 directly from AgCl or AgBr with dppf under
stoichiometric control did not yield the desired products. Instead,
the known dinuclear [Ag2X2(dppf)2] complexes were obtained. The
high affinity of Ag+ towards free dppf ligand appears to be
overwhelming and drives towards the thermodynamic Ag2
product, even when dppf is in deficient. Use of [NiII(dppf)] as a
diphosphine source overcomes this problem by creating a hurdle
for Ag-dppf interaction since the dppf is bound to Ni(II). There is
no inhibition however for Ag(I) to capture the halide because the
latter has free basic lone pairs. Effectively, the transfer agent
promotes a slow nucleation process that ensures an effective
stoichiometric excess of Ag+ over dppf throughout the course of
reaction, thereby pushing towards the Ag6 product.
Fig. 3 ORTEP diagram of 4, showing the polymeric coordination chain
(hydrogen atoms omitted for clarity). (Probability level of thermal ellipsoid
50%.)
The assembly is expected to be phosphine dependent, although
the product outcome is largely unpredictable. This system provides
a model to study the influence of a diphosphine. A diphosphine
with a much shorter skeletal backbone (e.g. dppe) may not be
suitable for edge-bridging a prism. Indeed, X-ray crystallographic
analysis of the product 4{ from a similar reaction between
NiI2(dppe) and Ag+ revealed a coordination polymeric chain with
[Ag2(dppe)3](OTf)2 as the stoichiometric unit. This bridge-only
assembly comprises two repeating and intercalating entities of
doubly-bridging and single-bridging dppe (Fig 3) The nearby
Scheme 1 A schematic representation of the products formed from the
ligand migration reaction from NiX2(P–P) (P–P = dppf, dppe) to AgOTf.
columnar channels. 31P NMR spectrum of 4 shows the two
inequivalent sets of phosphines (d = 3.3 and 4.5 ppm) in the doubly
and singly bridging modes. Two sets of singlet are also recorded in
the Au analogue.
…
˚
triflate is unbonded [Ag O 2.781 A (av.)]. Such assembly is rare
but its gold analogue has been reported.8 The Ag–Ag distance
The P : Ag ratio increases from 0.5 : 1.0 in 1–3 to 1.5 : 1.0 in 4
although the synthetic conditions are similar. The Ag(I) in 4 thus
shows a stronger self-selection of dppe over dppf to the extent that
it would saturate itself with phosphine donors in dppe and leaves
no room for the halides. Another major difference between 1–3
and 4 is that the former complexes are made up of tetrahedral
Ag(I) whereas the latter comprises of trigonal planar Ag(I).
In our attempts to prepare 4 directly from the addition reaction
of AgOTf with dppe under stoichiometric control, viz. Ag : dppe 2
: 3, we succeeded in controlling the stoichiometry but could not
yield the same assembly. Instead, it resulted in a similar repeating
polymeric chain except that one of the two Ag(I) atoms
˚
[6.581 A (av.)] is too large for any significant argentophilic
…
interactions. The Au analogue shows similar separation (Au Au
˚
6.596 A) and hence no aurophilic bonding. The crystal packing
diagram reveals a discrete series of coil-like coordination polymeric
chain propagating unidirectionally to create a network of
˚
coordinated to triflate [Ag–O 2.554(8) A] whereas the other was
…
˚
uncoordinated but in proximity (Ag O 2.819 A). This resulted in
a polymeric chain of alternating tetrahedral and pyramidally
distorted trigonal planar (deviations of Ag1, P1, P2 and P3 from
the least square plane of {AgP3} are +0.3758, 20.1211, 20.1250,
˚
and 20.1298 A respectively; P–Ag–P angles range 112.98u–
118.23u) structure, best represented as [Ag2(OTf)(m-dppe)3]n-
(OTf)n 5{ (Fig 4). A coordination isomer of 5, with triflates
˚
weakly associated with all Ag(I) atoms [2.645(5) and 2.653(3) A],
has been recently reported.9
Fig. 2 A schematic sketch of the trigonal prismatic framework of the Ag6
…
core of 1–3, with the Ag Ag separations (A) indicated for three complexes.
˚
4222 | Chem. Commun., 2007, 4221–4223
This journal is ß The Royal Society of Chemistry 2007

Fig. 6 ORTEP diagram of 7 (hydrogen atoms removed for clarity).
(Probability level of thermal ellipsoid 50%.)
Fig. 4 ORTEP diagram of 5 (hydrogen atoms removed for clarity).
(Probability level of thermal ellipsoid 50%.)
properties of dppf and dppe allow them to promote the formation
of polyhedral and polymeric Ag(I) respectively. Isolation of the
trigonal prismatic aggregates fuels the impetus in our search for
new Ag cages and polymers in a functional network. The use of a
diphosphine carrier complex such as Ni(II) could lead to a different
outcome compared to the use of free phosphines. In this system,
there is no evidence for Ni(II) interference in form of the formation
of heterometallic Ni(II)–Ag(I) complexes.
The authors acknowledged the National University of
Singapore (NUS) for financial support, A* STAR (Agency for
Science, Technology and Research) for a scholarship to P. Teo, as
well as G. K. Tan for assistance in X-ray crystallographic analysis.
Fig. 5 ORTEP diagram of 6 (hydrogen atoms removed for clarity).
(Probability level of thermal ellipsoid 50%.)
Notes and references
The same complex 5 is formed from the reaction between
NiBr2(dppe) and AgOTf, showing again the higher propensity of
Ag(I) for dppe. However, reaction of NiCl2(dppe) with AgOTf
yields two products [Ag2(dppe)3Cl2)]n, 6{ and [Ag2(dppe)2(m-Cl)2]n,
7{ with different stoichiometries and structures. They however co-
crystallized in an asymmetric unit of the single crystal, which is
unusual but not unprecedented.10 Unlike 4 and 5, they show halide
coordination, which is not surprising considering that in aprotic
solvent chloride is a better nucleophile. Complex 6 is similar to 4,
showing a coordination polymer chain intercalated by doubly- and
singly-bridging dppe, except that Cl takes up the 4th site of the
metal at the {Ag2(dppe)2} ring, thus achieving 18-e tetrahedral
Ag(I) (Fig 5). Complex 7, despite being short of a dppe ligand
compared to 6, also achieves saturation by forming doubly
{ Crystals of 1–3 are obtained by slow evaporation of a solution of the
compound in CHCl3. Crystals for 4–7 are obtained by slow diffusion of
hexane into a solution of the compounds in CH2Cl2. CCDC numbers of
complexes 1 to 6/7 are 647357–647362 respectively. For crystallographic
data in CIF or other electronic format see DOI: 10.1039/b707218j
1 (a) K. S. Gan and T. S. A. Hor, Ferrocene-Homogeneous Catalysis,
Organic Syntheses and Materials Science, ed. A. Togni and T. Hayashi,
VCH, Weinheim, 1995, ch.1, p.3; (b) G. Li, C. K. Lam, S. W. Chien,
T. C. W. Mak and T. S. A. Hor, J. Organomet. Chem., 2004, 690, 990;
(c) P. Teo, T. S. A. Hor, L. L. Koh and T. S. A. Hor, Inorg. Chim. Acta,
2006, 359, 3435; P. Teo, D. M. J. Foo, L. L. Koh and T. S. A. Hor,
Dalton Trans., 2004, 20, 3389; (d) P. Teo, L. L. Koh and T. S. A.
Hor, Inorg. Chem., 2003, 42, 7290; (e) P. Teo, L. L. Koh and
T. S. A. Hor, Chem. Commun., 2007, 2225.
2 (a) Z. Weng, S. Teo and T. S. A. Hor, Organometallics, 2006, 25, 4878;
(b) Z. Weng, S. Teo, L. L. Koh and T. S. A. Hor, Chem. Commun.,
2006, 1319; (c) Z. Weng, S. Teo, L. L. Koh and T. S. A. Hor, Angew.
Chem., Int. Ed., 2005, 44, 7560; (d) Z. Weng, S. Teo, L. L. Koh and
T. S. A. Hor, Organometallics, 2004, 23, 4342; (e) Y. Xie, G. K. Tan,
Y. K. Yan, J. J. Vittal, S. C. Ng and T. S. A. Hor, J. Chem. Soc., Dalton
Trans., 1999, 773; (f) T. S. A. Hor and L. T. Phang, J. Organomet.
Chem., 1990, 381, 121.
3 (a) X. L. Lu, W. K. Leong, T. S. A. Hor and L. Y. Goh, J. Organomet.
Chem., 2004, 689, 1746; (b) S. P. Neo, T. S. A. Hor, Z. Y. Zhou and
T. C. W. Mak, J. Organomet. Chem., 1994, 464, 113.
4 C. D. Nicola, Effendy, C. Pettinari, B. W. Skelton, N. Somers and
A. W. White, Inorg. Chim. Acta, 2005, 358, 695.
5 K. Yang, S. G. Bott and M. G. Richmond, J. Chem. Crystall., 1995, 25,
263.
6 R. Noguchi, A. Hara, A. Sugie and K. Nomiya, Inorg. Chem. Commun.,
2006, 9, 60.
…
bridging chloride, which also shortens the Ag Ag contact to
˚
3.362(2) A (Fig 6). These two forms are not strictly constitutional
isomers but they are energetically similar. Their comparable chain
structure and stability helps them to complement in the crystal
packing such that they are aligned parallel to the a axis of the unit
cell. Their isolation and co-crystallization suggest similar demands
between a bridging dppe and doubly-bridging chloride. Complexes
6 and 7 cannot be separated in pure form. They are also likely to
be interconverting as a dynamic mixture in solution, since only a
pair of broad doublets (d = 4.9, 6.2) is observed in the 31P NMR
spectrum at rt There is no 31P-NMR evidence of free dppe in
solution.
7 X. L. Zhao and T. C. W. Mak, Organometallics, 2005, 24, 4497.
8 M. C. Brandys and R. J. Puddephatt, J. Am. Chem. Soc., 2001, 123,
4839.
9 Effendy, C. D. Nicola, C. Pettinari, A. Pizzanioca, B. W. Skelton,
N. Somers and A. H. White, Inorg. Chim. Acta, 2006, 359, 64.
10 (a) A. Lenartsson and M. Hakansson, New J. Chem., 2007, 31, 344; (b)
R. W. Salfrank, H. Maid, F. Hampel and K. Peters, Eur. J. Inorg.
Chem., 1999, 1859; (c) P. L. C. Davies, L. R. Hanton and W. Henderson,
J. Chem. Soc., Dalton Trans., 2001, 2749.
Complexes 1–3 are trigonal prismatic aggregates whereas
complexes 4–7 are dppe-bridging Ag(I) coordination polymers
differed by the number of bridging dppe, and secondary
association of (weakly) coordinative ligands such as chloride or
triflate. This system highlights the complexity and unpredictability
of Ag(I) assemblies because of the numerous parameters that could
influence the outcome of the aggregates. The different skeletal
This journal is ß The Royal Society of Chemistry 2007
Chem. Commun., 2007, 4221–4223 | 4223