Base-assisted synthesis of 4-pyridinate gold(I)metallaligands; study of their use in Self-Assembly reactions.

The synthesis of di- and tritopic gold(I) metallaligands of the type [(Au4-py) 2 (µ 2 -diphosphane)] (diphosphane = bis(diphenylphosphanyl)isopropane or dppip ( 1 ), 1,2-bis(diphenylphosphanyl)ethane or dppe ( 2) , 1,3-bis(diphenylphosphanyl)propane or dppp ( 3 ), 1,4-bis(diphenylphosphanyl)butane or dppb ( 4 )), and [(Au4-py) 3 (µ 3 -triphosphane)] (triphosphane = 1,1,1-tris(diphenylphosphanylmethyl)ethane metallaligands in good yields. The photophysical properties of both the metallaligands and the corresponding assemblies have been investigated.


Introduction
The use of organometallic gold(I) complexes as building blocks has become a very useful strategy for the assembly of different species with specific structures such as macrocycles, helicates, cages, catenanes, rigid-rod oligomers or polymers among others. 1 Both the linear coordination and the tendency to establish aurophilic interactions between gold centers 2 are the two main factors for the use of gold(I) complexes in the formation of supramolecular structures by self-assembly that often display interesting photoluminescence from metaland/or ligand-based excited states.
Pyridine is one of the most versatile ligands in coordination chemistry; it forms stable complexes with most transition metal cations and has been used as a donor fragment in the coordination-driven self-assembly of a wealth of architectures. 3 The fact that gold(I) prefers phosphorus, sulfur or carbon donor ligands due to its soft character opens the possibility of designing gold(I) metallaligands containing dangling pyridine rings appropriate to coordinate with other transition metal moieties in self-assembly reactions. Effectively, several classes of gold(I) metallaligands bearing terminal pyridine rings as a appended unit of other P-, S-or Cbonded fragments have been described, including pyridine phosphanes, 4 pyridine thiolates, 5 pyridine carbenes, 6 pyridine acetylenes, 7 and tetrafluorobenzenepyridine 8 (Chart 1). Many of them have been successfully used as building blocks in the formation of a wide range of supramolecular assemblies, as it has been recently comprehensively reviewed. 1b

Scheme 1. Synthesis of the 4-pyridinate gold(I) metallaligands
All the synthesized compounds were characterized by 31 P{ 1 H} and 1 H NMR spectroscopy, high-resolution mass spectrometry (ESI+), IR spectroscopy and elemental analysis. In addition, the structures of 2, 3 and 5 were determined by X-ray crystallography (see below).
As expected by the chemical equivalence of the phosphorus atoms, the 31 P{ 1 H} NMR spectra show only one signal whose chemical shift appears slightly downfield shifted (5-10 ppm) with respect to that of the initial chlorido complexes. The 1 H NMR spectra show the signals due to both α and β protons of the pyridine rings and those attributable to the phosphane ligands.

Reaction with dppm
In contrast with the phosphanes described above, when the dppm derivative [(AuCl) 2 (µ 2dppm)] was treated with 4-pyridylboronic acid and Cs 2 CO 3 in isopropanol under the aforementioned conditions, a complex mixture of compounds was obtained (as observed in both the 31 P{ 1 H} and 1 H NMR spectra). After scanning new reaction conditions, we found that an increase in temperature gave rise to a mixture where a main product could be observed. After workup, the new tetranuclear [(Au4-py) 2 (CH) 2 (µ 2 -Au(PPh 2 ) 2 )] (I) compound, where each methylene group in the dppm is singly deprotonate and coordinate to a Au(I)-pyridine moiety, was serendipitously isolated but in a very low yield. (Scheme 2) Unfortunately, despite numerous efforts trying to improve the reaction conditions, we were not able to obtain a better yield of the named tetragold compound.

Scheme 2. Synthesis of the tetranuclear dppm derivative (I)
So the base-assisted transmetallation, although generally reliable, can be inefficient for Brönsted acidic substrates (as reported by other groups), 9 precluding the obtention of the targeted compounds. In our case, the acidity of the methylene protons in the carbon chain of dppm seems to be the responsible of the unexpected alternative reaction.
Alternatively, a more rational route was explored starting from the preformed tetranuclear [(AuCl) 2 (CH) 2 (µ 2 -Au(PPh 2 ) 2 )]. 17 We attempted to introduce in the latter the 4-pyridine fragment by a base-assisted transmetallation with 4-pyridylboronic acid or by the classical reaction with 4-pyridyllithium. Again, the resulting reactions were not clean and the desired product was only obtained in a very low yield. This fact effectively ruled out the study of the participation of the compound [(Au4-py) 2 (CH) 2 (µ 2 -Au(PPh 2 ) 2 ] (I) as a ditopic metallaligand in self-assembly processes, despite its interest as a tetragold building block.

X-ray crystallography
Crystals of compounds 2, 3 and 5 suitable for X-ray diffraction studies were obtained from slow diffusion of diethylether into dichloromethane solutions and their molecular structures are shown in Figure 1; crystal data can be found in Supporting Information. Some of the most relevant features of the determined structures are indicated below. An analogous arrangement is found in the X-ray structure of the related compound [(Au4-F-C 6 H 4 ) 2 (µ 2 -dppp)] 24 (d(Au···Au)= 2.997 Å, Au1-P1-P1a-Au1a = -57°).
In the crystal lattice, compound 3 forms rows of molecules following the b-axis, where C-H···π(ring) weak interactions between hydrogen atoms of the alkyl chain of the dppp and the pyridine rings of adjacent molecules can be observed (see Figure S48). 8 The molecular structure of compound 5 (Figure 1) features the association of the trinuclear derivatives into dimers, so that all the gold(I) atoms are involved in aurophilic interactions, ( Figure S49) with an intramolecular Au1···Au2 distance (3.2044(14) Å) that is larger than the intermolecular one (d(Au3···Au3') = 3.1717(17) Å). The C 2 symmetry of the dimers (C 2 axis passing through the middle point of Au3···Au3' intermolecular bond) makes them chiral and a racemic mixture is found in the crystal. This arrangement differs from that exhibited by other organometallic compounds of gold(I) that contain 1,1,1-tris(diphenylphosphinomethyl), 7b, 8, 25 where a pair of branches of the triphosphane are near enough to allow aurophillic contacts  Table 1.
The molecular structure of compound I was also established by single-crystal X-ray diffraction (Crystal Data can be found in Supporting Information). The crystal lattice is composed of cyclic [(Au4-py) 2 (CH) 2 (µ 2 -Au(PPh 2 ) 2 ] molecules with the eight-membered Au 2 P 4 C 2 ring in a chair conformation ( Figure 2) and dichloromethane solvent molecules.
According with the general trend observed in other compounds containing this type of structure, [18][19][20]26 there is a short intramolecular Au···Au contact (2.9763(3) Å) whose value is very similar to that found in the closely related [(AuL) 2 (CH) 2 (µ 2 -Au(PPh 2 ) 2 ] (L = acac 2.969 Å, 18 C 6 F 5 2.917 Å 19 and {(PBRe(CO 3 )(NO 3 )} 2.982 Å. 20 Both the methylene protons of dppm and the nitrogen atoms of the pyridine groups are again involved in weak C-H···N py interactions (2.45-2.50 Å) giving rise to a 2D layer structure disposed in a parallel way to the ac plane of the crystal packing ( Figure S51). Selected bond lengths and angles are indicated in Table 1.

Self-assembly studies
In our previous studies where the tetrafluorobenzenepyridine ditopic metallaligands Scheme 3. Self-assembly of [2+2] metallamacrocycles As indicated above, the most insoluble [(Au4-py) 2 (µ 2 -dppb)] (4) did not produce the selective formation of discrete aggregates as indicated by the complex 31 P{ 1 H} NMR spectra of the reaction solutions. The insolubility of compound 4 could be due to the formation of polymeric species that preclude the pyridine coordination to the acceptor compounds.
After the adequate workup, the desired metallamacrocycles could be isolated as solids and characterized. Interestingly, the 1 H NMR spectra do not show the expected downfield shift of α-pyridine protons, characteristic of the coordination of pyridine rings to metal centers. 13 This is most probably due to the fact that the deshielding associated to electron donation opposes with the upfield shift due to the anisotropic effect exerted by the phenyl rings of the dppp ligands attached to either palladium or platinum centers in the assembly. In parallel, a 13 significant upfield shift was observed for the β-pyridine protons (≈ 0.5 ppm), in agreement with the influence of the ring currents of the close phenyl rings.
High resolution ESI(+)-MS spectrometry allowed to establish the stoichiometry of the selfassembled species. As can be seen in the Supporting Information Section, all the studied metallamacrocycles present peak series that are in agreement with the exclusive formation of [2+2] architectures. For most of the compounds, several charged aggregates are obtained by subsequent loss of triflate anions, being their isotopic distribution in agreement with the theoretical one.
To explore the possibility of obtaining more complex architectures, such as metallacages, as those that have been selectively built by combination of tripodal ligands and cis-blocked Pd II or Pt II square-planar complexes, 27 the self-assembly reaction between the tritopic gold(I) metallaligands [(AuC 6 F 4 4-py) 3 (µ 3 -triphosphane)] (triphos (5), triphosph (6))   Although we have not been able to isolate a crystal suitable for X-ray analyses, Spartan calculations 28 were undertaken in order to optimise the proposed BPT structure of the synthesized cages. A representation of the minimized structure of the cationic part of compound 6 2 A 3 is depicted in the Supporting Information (Fig. S52).

Photophysical Studies
Absorption and emission spectra of all the compounds were recorded in dichloromethane solution at room temperature; the obtained data are listed in table 2. The UV-vis spectra of the gold(I) compounds 1-6 in CH 2 Cl 2 solution ( Figure S53 above) show two different regions. The higher energy region (265-275 nm) contains intense poorly resolved absorption bands, which are characteristic of allowed IL phosphane transitions, 12b, 29 while the less intense and more defined band at ca. 300 nm could be assigned to a IL and/or metal-perturbed IL transitions; nevertheless, a contribution of weak-gold-gold interactions could not be discarded. 21, 29b In fact compound I, which presents the closest Au···Au contact (see above) shows an intense and well-defined band at 300 nm that has been assigned to a IL transitions perturbed by intramolecular Au-Au interactions in related compounds. 30 These bands are not significantly affected by the coordination to either the palladium or the platinum moieties. As expected, higher molar extinction coefficients are observed in the case of metallamacrocycles ( Figure S54 probably pyridyl rings. In this respect, the emission maxima of the compounds studied in this work closely resemble those of the chlorido [(AuCl) n (µ n -phosphane)] precursors. 31,32 As a whole, the experimental data is in agreement with the fact that the observed emission raises from the gold metallaligands and the formation of the metallamacrocycles does not affect significantly the luminescent properties of the former.

Conclusions
Herein, we have reported the use of base-assisted transmetallation reaction for the facile and high yielding syntheses of a series of di-and tritopic gold(I) metallaligands with terminal 4pyridyl groups. The X-ray structural determination of the synthesized compounds revealed that the establishment of aurophilic interactions strongly determines the spatial arrangement within the molecules in the different species.
The base-assisted transmetallation has been proved to fail for the dppm derivative due to the Bronsted acidity of the methylene protons of the carbon chain. In this particular case, the cyclic tetranuclear [(Au4-py) 2 (CH) 2 (µ 2 -Au(PPh 2 ) 2 )] (I) compound has been obtained through an unknown mechanism of reorganization.

X-Ray Structure Determination
Data for I, 2, 3 and 5 were collected at 123.0 K on a dual source Rigaku Oxford SuperNova diffractometer equipped with an Atlas detector using mirror-monochromated Cu-Kα radiation (λ = 1.54184 Å). All the data collection and reduction were done using the program CrysAlisPro 35 and the intensities were corrected for absorption using the analytical numeric face-index absorption correction method. 36 The structures were solved with intrinsic phasing method (SHELXT) 37 and refined by full-matrix least squares on F 2 using the OLEX2, 38 which utilizes the SHELXL module. 39 In all the structures, anisotropic displacement parameters were assigned to the non-hydrogen atoms. The hydrogen atoms were introduced at the ideal positions using riding models with U eq (H) of 1.5U eq (parent) for the terminal methyl groups and of 1.2U eq (parent) for others. Restraint (ISOR and/or SIMU) commands were used where appropriate to suppress the alerts for large displacement parameter in checkcif for 1 and 5. (s, C α py), 136.9 (s, C β py), 136.0, 134.6 (m, C q , py+Ph), 132. 8, 130.6, 130.5, 128.8, 128.6 (s, CH, Ph), 25.0 (t, 1 J(C-P) = 9.7 Hz, CH , dppm