Different Coordination Modes of a Tripod Phosphine in Gold(I) and Silver(I) Complexes

The following gold(I) and silver(I) complexes of the tritertiary phosphine 1,1,1- tris(diphenylphosphinomethyl)ethane, tripod , have been synthesised: Au3(tripod)X3 [X = Cl(1), Br(2), I(3)]; [Au3(tripod)2Cl2]Cl (4); Au(tripod)X [X = Br(5), I(6)]; Ag3(tripod) (NO3)4 (7), Ag(tripod)NO3 (8). They were characterized by X-ray diffraction (complexes 2, 3 and 4), 31P NMR spectroscopy, electrospray and FAB mass spectrometry and infrared spectroscopy. Complexes 2 and 3 show a linear coordination geometry for Au(I), with relatively short Au-P bond distances. Complex 3 has a Au•••Au intramolecular distance of 3.326 A ° , while complex 2 had a short Au•••Au intermolecular interaction of 3.048 A ° . Complexes 4-6 were found by 31P NMR spectroscopy studies to contain a mixture of species in solution, one of which crystallised as [Au3(tripod|)2Cl2]Cl which was shown by X-ray diffraction to contain both tetrahedral and linear Au(I), the first example of a Au(I) complex containing such a mixture of geometries. The reaction of [Au3 (tripod)Cl3] (1) with tripod led successfully to the formation of [Au3(tripod|)2Cl2]+ and [Au3(tripod)2Cl3]+ and [Au3(tripod|)3Cl]2+. The silver(I) complexes, 7 and 8 appear to contain linear and tetrahedral Ag(I), respectively.


INTRODUCTION
In recent years considerable research has been focused on the chemistry of diphosphines, whereas the chemistry of triphosphines is less well developed Transition-metal complexes of 1,1,1,tris(diphenylphosphinomethyl)ethane, tripod, in which tripod acts as a tridentate chelating ligand, with facoctahedral geometry are known, 3 and five-coordinate complexes with square-pyramidal 4' or trigonalbipyramidal 8 geometries have also been reported.
There is only one example of tripod acting as a tridentate-bridging ligand in which the ligand bridges three chlorogold(I) fragments and there is a Au..Au intramolecular interaction. Our interest in metal phosphine complexes arises from the antiarthritic activity of the linear Au(1) complex auranofin and anticancer activity oftetrahedral Au(I) and Ag(I) diphosphine complexes. : 2 We report here the preparation, characterisation and properties of some Au(I) and Ag(I) tripod complexes including a highly unusual Au(I) complex containing both linear and tetrahedral Au(I) centers.

Materials and Methods
The complexes were prepared using 1,1,1,-tris(diphenylphosphinomethyl)ethane and AuI from Strem Chemicals, AgNO3 from Analema, 2,2 thiodiethanol from Aldrich and metallic gold from Sociedad Espafiola temperature device operating at 220 K. Crystals of 2 and 4 diffracted very weakly and so Cu-K radiation was used for data collection on account of its intensity advantage over Mo-Kc radiation, which was used for 3. All three datasets were collected in co-0 mode, those for 2 and 3 with on-line profile-fitting. 26 A numerical absorption correction was applied for 2, the crystal dimensions having been optimised against a set of Wscans; 27 all data for which either the incident or diffracted beams made an angle of less than with the lamina face, (001), were omitted. Absorption corrections for 3 and 4 were based purely on l/-scans. 28 All structures were solved by Patterson methods (DIRDIF) 29 and completed by iterative cycles of least-squares refinement and difference syntheses (CRYSTALS 3 for 2; SHELXTS for 3 and 4).
All the analyses were complicated by the effects of weak diffraction and disorder. In 2 one phenyl group is disordered over two orientations, and this was modelled with two intersecting rigid hexagons. Disordered lattice solvent, assumed to be CH2CI2, was treated as described by van der Sluis and Spek, r and corresponds to 222 e/cell, which amounts to 1.33CH2C12 per formula unit. All full-weight non-H atoms were refined anisotropically, with H-atoms in calculated positions. In 3 and 4 all the phenyl groups (two of which are disordered in each structure) were refined as rigid bodies, and the light atoms refined isotropically. Hatoms were again placed in calculated positions, although no attempt was made to place H-atoms on water molecules. In 4 charge balance requires two CI per fomula unit; one of these was refined as full weight, the other was disordered over two sites.

Preparation of compounds
Au3(tripod)Cl3, 1. A solution 23 of [Au(thiodiglycol)Cl] (0.25 g of Au, 0.127 mmol) in MeOH (10 ml), was added dropwise to a solution of tripod (0.2642 g, 0.423 mmol) in CH2C12 (25 ml 24 (0.1 g of Au, 0.508 mmol) in Et20 (30 ml), was added dropwise. The solution was stirred for 24 h at ambient temperature, the volume was reduced and H20 (30 ml) was added to give a white solid. The solid was filtered off, washed with water, and dried in vacuo. Suitable crystals for X-ray diffraction were obtained by recrystallization from CH2CI.,/MeOH. (Found: C, 33 [Au3(tripod)2Cl2]Cl, 4. To a solution of tripod (0.1585 g, 0.254 mmol) in CHC13 (15 ml) a solution 23 of [Au(thiodiglycol)Cl] (0.05 g of Au, 0.254 mmol) in MeOH (5 ml), was added dropwise. The resultant solution was stirred for 24 h. Afterwards the volume was reduced and H20 (30 ml) was added to afford a white solid. This was filtered off, washed with water and recrystallized from CHzClz/Et20. Crystals suitable for X-ray diffraction were obtained from the same solvents. (Found: C, 51 34 Au...Au contacts are commonly found in crystal structures of Au(I) complexes 35 and the short distances found for 2 suggest that the interactions are relatively strong. Such interactions in polymeric Au(I) 36 41 phosphine complexes often give rise to luminiscence behaviour, but this has yet to be investigated for 2.
The iodo-complex 3 has a crystal structure (Figure 3, Table I) similar to the analogous chloride complex but intermolecular interactions are absent. The Au-P distances (2.229-2.261 ) are within the expected range, as are the Au-I bond lengths (2.548-2.554 A). 33 Again, two arms of the ligand are crossed 42 orthogonally" wth Au-Au-P angles of 86.0 and 84.5 , as a consequence of an Auo,,Au intramolecular interaction of 3.326 ,.
Different Coordination Modes of a Tripod Phosphine in Gold(l) and Silver(I) Complexes Table I. Selected bond distances (A) and angles () for 2, 3 and 4.

3 4
The single-crystal X-ray structures of complexes 2 and 3 confmn that they contain Au:P:halide in a 1:1:1 ratio. However, complex 4, crystallised from CHzClz/Et20 solution as [Au3(tripod)2Cl2]Cl, in which Au(I) exhibits both tetrahedral and linear geometries ( Figure 4; Table I). This complex appears to be the first example of a cation in which tripod acts both as a bidentate chelating and monodentate bridging ligand and which has been characterised by X-ray crystallography. In the literature there are reports of mixed-valence [44][45][46][47] Au(I)/Au(III) complexes with this ligand 43 and other pnospnlnes, n which Au(l) and Au(III) have coordination numbers of two and four, respectively. However, to date no examples of Au(I) complexes with this phosphine, showing both geometries for Au(I) in the same compound, have been reported. The central Au(1) is tetrahedral (Figure 4) being bound to P(1) and P(2) of one tripod ligand and to P(1) and P(3) of another tripod. Au-P distances for tetrahedral Au(I) are ca. 2.4 A (Table I) [4][5][6] show several broad peaks, suggesting that ligand exchange processes are occurring, similar to those reported for related complexes. 43"48 The downfield shift from 15.18 to 20.41 ppm, from the chloro to the iodo complex 1-3, is probably due to an increase in the metal-to-ligand back-bonding.
The 3p NMR spectrum of [Au3(tripod)2Cl2]Cl in CD2C12 at room temperature (Table II) shows a sharp singlet resonance at 26.00 ppm which is assigned to oxidised phosphorus of the ligand, and three broad signals, at 16.40, 49"2"90 and. -9.96 pp which are assigned on the basis of known shifts of related Au(I)phosphlne complexes, assuming that P signals are shifted upfield when the coordination number increases. Thus, the signal at 16.40 ppm corresponds to P coordinated to linear Au(I) and the signals at 9.96 and-2.90 ppm to P in tetrahedral environment. The broadening suggest that some exchange processes are occurring in solution. 43 On cooling to -90C, the broad peak at 16.40 ppm give rise to two singlets (19.61 and 15.35 ppm) and the peak at -2.90 ppm at ambient temperature also gave rise to two broad peaks (-0.18 and -4.35 ppm). The broad peak at -9.96 ppm remains broad, and shifts ca. 3 ppm to high field when the temperature is lowered.
3p NMR spectra in CD2C12 of complexes 5 and 6 (Table II) have two broad peaks at-3.48 and -10.16 ppm (5), and -3.71 and-9.81 ppm (6) also suggesting the presence of some exchange processes. The spectra of 5 and 6 at -90C show the appearance of new peaks in the region of 30 to 5 ppm, which are associated with P coordinated to linear Au(I). The two broad P resonances (0 to -15 ppm) associated with P in tetrahedral environment also give rise to several broad peaks indicating the presence of different species in exchange in solution. It seems that bromide and iodide also form complexes containing linear and tetrahedral Au(I), giving rise to complicated equilibrium in solution.

Silver complexes
The 31p NMR spectra of silver(I) compounds (7-8) 31 show the typical pair of doublets due to coupling of P to both 7Ag (51.82% abundance) and gAg (48.18% s0 abundance) and were well resolved at low temperatures (Table II).
The unresolved doublet of doublets (ambient temperature) at 8 -8 ppm for complex 7 (Table II)  Hz, which is consistent with each Ag being coordinated 5 to only one P, and nitrate acting as a monodentate ligand. The equivalence of the three -CH2-groups in solution is also evident from H NMR measurements.
The 3p NMR spectrum at ambient temperature for 8 shows (Table II) a doublet of doublets at 7 ppm with coupling constants IJ(3P-I7/I9Ag)= 487/558 Hz corresponding to Ag(I) coordinated to two 31p atoms ( Figure 6). Another broad peak appears at-12.3 ppm in CDCI3 solutions that may be due to tetrahedral Ag(l), 52 with nitrate acting mainly as a bidentate ligand in a polynuclear complex allowing facile ligand 45 redistribution. At lower temperatures, or in CDzCI2 solutions, the broad peak disappears. The complex Au3(tripod)Cl3, 1, was titrated with 0.5, 1, 1.5 and 2 mol equivalents of tripod and the course of the reaction was followed by 31p NMR spectroscopy ( Figure 5). The alp spectrum shows that when mol. equiv, of tripod is added, complex I was converted to [Au3(tripod)2CI2] i.e. complex 4. When 2 mol. equiv. of tripod were added, the product was [Au3(tripod)3Cl]2+.

IR Spectroscopy
The IR data show the presence of terminal Au-X bonds in complexes 1-6 and the coordinated nitrate in compounds [7][8] (Table II).

Electron Spray Mass Spectrometry (ESMS)
Ambient temperature ESMS can reveal the individual components of a system in which redistribution and exchange of phosphine ligands is fast on the NMR time scale. Thus ESMS spectra were recorded for complexes [4][5][6] (Table III)    Thus, based on 31p NMR spectrum, two possible structures with two different "AuP4" environments can be proposed (