First examples of AuI-X-AgI halonium cations (X = Cl and Br)

Article abstract of DOI:10.1021/ja035064z

Novel {[(μ-PAnP)(AuX)₂]₂Ag}⁺SbF₆^- halonium ions (X = Cl, Br; PAnP = 9,10-bis(diphenylphosphino)anthracene) were synthesized from the reactions between (μ-PAnP)(AuX)₂ and 1/2 mol equiv of AgSbF₆. The compounds feature an unprecedented distorted Au₄X₄ dodecahedron which encapsulates a silver(I) ion at its center. The halonium ions are stabilized by collective actions of metallophilic Au-Ag, aromatic π-π, and Ag-X interactions. Copyright

Full text of DOI:10.1021/ja035064z

Published on Web 06/20/2003
First Examples of AuI-X-AgI Halonium Cations (X ) Cl and Br)
Ke Zhang, Janardhana Prabhavathy, John H. K. Yip,* Lip Lin Koh, Geok Kheng Tan, and
Jagadese J. Vittal
Department of Chemistry, The National UniVersity of Singapore, 10 Kent Ridge Crescent, 119260, Singapore
Received March 9, 2003; E-mail: chmyiphk@nus.edu.sg
Silver(I) ion has strong affinity for soft halides (X′ ) Cl-, Br-,
I-). This property has long been harnessed to remove the poor
leaving halide ligand from a metal complex [M - X′]n+ (eq 1,
Y ) anion).1
[M - X′]n+ + AgY T [M](n+1)+Y- + AgX′V
(1)
The stability and spontaneous precipitation of AgX′ from the
solution drive the equilibrium toward the complex [M](n+1)+Y-
which would react readily with an incoming ligand.1 This simple
synthetic strategy has been used to generate the (R3PAu)+ ion from
R3PAuCl (R ) aryl or alkyl).2 The ion, a powerful aurating agent,
has been used extensively in synthesizing metal clusters, espe-
cially polygold(I) complexes.3 An example related to the present
study is the formation of the novel [digold(I)]halonium cations
Figure 1. ORTEP plot of 1 in crystals of 1‚1/2Et2O (thermal ellipsoid )
50%). All the H atoms and Et2O are removed for clarity.
Scheme 1
[(R3PAu)2X′]Y (Y ) ClO4-,4 BF4- 5a SbF6- 5b,c) from the coordina-
,
tion of (R3PAu)+ to the halide in R3PAuX′. Interestingly, recent
-
studies showed that when SbF6 is the counterion, the halonium
crystallize in the tetragonal space group I4/m. X-ray crystal analysis
revealed that 3 (Figure 2) and 4 (SI) are isostructural, being
composed of helical coordination polymers of (µ-PAnP)(AuX)2 and
AgI ions. The SbF6- ions have no contact with the polymer chains.
Notably, the (µ-PAnP)(AuX)2 molecules undergo pronounced
conformational changes in forming compounds 3 and 4: the
anthracenyl rings are slightly curved away from the Au atoms, and
the P-Au-X units are tilted toward one end of the anthracenyl
ring (Figure 2b). The dihedral angles between the lateral benzene
rings in 3 and 4 are 18.8° and 17.9°, respectively. Because of the
distortion, the two P-Au-X units are drawn closer and the Au-
Au distances are shortened to 7.737(2) and 7.768(1) Å in 3 and 4,
respectively. The large structural changes suggest that the anthra-
cenyl backbone of the ligand is highly flexible. The conformational
changes facilitates π-π interactions between two (µ-PAnP)(AuX)2
molecules whose anthracenyl rings overlap partially with separations
typical for aromatic π-π stacking (3.627(2) Å (3) and 3.618(2) Å
(4)).9Intriguingly, the Au and X atoms of four neighboring
(µ-PAnP)(AuX)2 molecules constitute the vertexes of a distorted
deltahedral dodecahedron (Figure 2c). Encapsulated at the center
of the polyhedron is a silver(I) ion. Because of the different Au-X
distances, the symmetry of the dodecahedron degenerates from the
maximum D2d to S4. The Au4X4 dodecahedron can be also called
bisdiphenoids as it comprises two interpenetrating Au4 and X4
tetrahedra. The four halides coordinate weakly to the central AgI
ion, showing long Ag-X bond distances (Ag-Cl ) 2.624(2) Å,
Ag-Br ) 2.7137(1) Å). The compounds feature the first examples
of (AuI-X)4AgI halonium ions which are nonclassical as the halides
bear a fractional formal charge of +1/4. The AgX4 tetrahedra are
distorted with Cl-Ag-Cl angles of 115.00(5)° and 98.90(9)° and
Br-Ag-Br angles of 114.18(3)° and 100.42(5)°. On the other hand,
the distortion in the AgAu4 tetrahedra is more severe: the Au-
Ag-Au angles in 3 and 4 are 126.385(9)°, 79.2569(2)° and
127.168(1)°, 77.977(2)°, respectively. The shortest Au-Au dis-
tances in the dodecahedra, 4.105(2) Å (3) and 4.115(2) Å (4), are
2+
ion would dimerize into {[(PPh3PAu)2X′]2}2 via aurophilic
attractions.5b,c However, there are notable exceptions where silver
halides are found incorporated into the final clusters,6 i.e., [(p-
tol3P)12Au18Ag20Cl14],6a and interestingly, both AgCl and (PPh3-
Au)+ ions, produced from the reaction of AgNO3 with PPh3AuCl,
are assembled in the cluster [Pt3(µ3-S)(AuPPh3)(µ3-AgCl)(µ-Ph2-
PCH2PPh2)3]+.6c Uso´n et al.7 demonstrated that the reactions
between organoplatinum halides and AgI ion give rise to complexes
which contain [Pt-X-Ag] linkage (X ) Cl, Br). Remarkably, the
structures of the complexes implicate the existence of Pt f Ag
bonding. In this communication, we report a new reaction between
AgI and phosphinegold(I) halides: instead of precipitating AgX,
the reactions of (µ-PAnP)(AuX)2 (PAnP ) 9,10-bis(diphenylphos-
phino)anthracene,8 X ) Cl and Br) with 1/2 mol equiv of AgSbF6
were found to give rise to unprecedented AuI-X-AgI halonium
cations (Scheme 1).
Reacting PAnP with 2 mol equiv of Me2SAuX gives the
binuclear complexes (µ-PAnP)(AuX)2 (X ) Cl, 1; ) Br, 2). The
X-ray crystal structures of 1‚1/2Et2O (Figure 1) and 2‚Et2O (SI)
show a PAnP coordinating to two linear syn-oriented P-Au-X
groups (av P-Au-Cl angle ) 177.65(6)°, av P-Au-Br angle )
177.43(4)°). The Au-P and Au-X bond lengths of the compounds
are typical for the phosphinegold(I) halides. The two Au atoms
are widely separated by 9.154(2) Å (1) and 9.091(4) Å (2). To
relieve the steric repulsion with the phenyl rings, the anthracenyl
rings in 1 and 2 are slightly curved toward the Au-X units. The
dihedral angles between the two lateral benzene rings are 23.0°
and 24.2° for 1 and 2, respectively.
While reacting 1 and 2 with 2 mol equiv of AgSbF6 leads to
spontaneous precipitation of AgX and formation of [(µ-PAnP)-
Au2]2+ ion, slow diffusions between a CH2Cl2 solution of (µ-PAnP)-
(AuX)2 and a THF solution of 1/2 mol equiv of AgSbF6 give yellow
crystals of {[(µ-PAnP)(AuX)2]2Ag}+SbF6-‚2CH2Cl2 (X ) Cl, 3‚
2CH2Cl2; X ) Br, 4‚2CH2Cl2) as products. Both compounds
9
8452
J. AM. CHEM. SOC. 2003, 125, 8452-8453
10.1021/ja035064z CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
Figure 2. (a) ORTEP plot of 3‚2CH2Cl2 (thermal ellipsoid ) 50%). All H atoms, solvent and anions are not shown. (b) Compound 3 viewed along c-axis.
All phenyl rings are not shown. (c) Diagram showing the AgAu4Cl4 dodecahedron; the solid lines indicate bonding interactions and the broken lines show
the faces of the dodecahedron.
too long for aurophilic interactions. The Au-X bonds (X ) Cl,
2.313(2) Å; ) Br, 2.4171(1) Å) are basically unaffected by the
Ag-X coordination as they are only slightly longer than the
corresponding ones in 1 (2.2975(2) Å) and 2 (2.3924(7) Å). As
the coordinated halides are known as poor electron donors and AgI
ion as a strong electron acceptor, the halonium ions can be regarded
as strong Lewis acid and weak Lewis base adducts.10 While the
Au-P bonds of 3 and 4 (2.241(2) Å) have the same length, the
Au-X and Ag-X bonds are elongated significantly as X is changed
from Cl to Br (Au-Cl ) 2.313(2) Å, Au-Br ) 2.4171(1) Å, Ag-
Cl ) 2.624(2) Å, Ag-Br ) 2.7137(1) Å). Surprisingly the Au-
Ag distance does not show a proportional increase: the unusually
acute Au-Cl-Ag angle of 81.08(7)° leads to the Au-Ag separation
of 3.2180(4) Å in 3, and further compressed Au-Br-Au of 79.04-
(4)° in 4 keeps a very similar Au-Ag of 3.2701(5) Å. Notably,
phopshines and anions on the structures and stability of the
halonium cations are now underway in our laboratory.
Acknowledgment. We thank the National University of Sin-
gapore for financial support.
Supporting Information Available: Experimental procedures and
X-ray crystal structures of 2‚Et2O and 4‚2CH2Cl2 (PDF). Crystal data
and CIF files of 1‚1/2Et2O, 2‚Et2O, 3‚2CH2Cl2, and 4‚2CH2Cl2. This
material is available free of charge via the Internet at http://pubs.acs.org
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