Synthesis and characterization of gold (I) complexes of bis(diphenylphosphino)-acetonitrile.

Overview

• Polymorphism and its Occurrence in Gold Based Complexes
• Phosphine Ligands: Their Importance in the Coordination Chemistry of Transition Elements
• Monodentate Phosphine Ligands
• Bidentate Phosphine Ligands: Insights into the Chemistry of bis(diphenylphosphino)-
• Bis(diphenylphosphino)acetonitrile (dppm-CN)
• Gold bis(phosphine) complexes
• Luminescence in Gold(I) Phosphine Complexes
• Techniques for Characterization of Gold Phosphine Complexes
• Nuclear Magnetic Resonance (NMR) Spectroscopy
• Single Crystal X-Ray Crystallography
• Luminescence

The smallest members of the bis(phosphine) ligand family are the bis(dialkyl and diarylphosphino)methanes, (R2P-CH2-PR2) and within the bis(diarylphosphine)alkane family dppm stands out as one of the best studied systems. [38] The chemistry in dppm is mostly inspired. Derivatives of [R2P-(CH2)n-PR2] systems, such as the [R2P-X-PR2] class (X = NH), have also received much attention over the years with the use of CH3(CH2)2N( CH2PPh2 )2 for the production of dendrimer catalysts is an example.[56]. Until recently, the preparation of the dppm-CN ligand involved the use of toxic materials such as (CN), the chemistry of dppm-CN was rejuvenated by Erker and co-workers with facile preparation starting with common acetonitrile[34,63].

Gold complexes with bis(phosphine) ligands are part of the most used systems in a variety of fields, especially catalysis.

Figure 1.2. Two trinuclear gold(I) complexes that each posses polymorphic forms with different luminescence properties

Goals of the Study

Leinen in Crystal Engineering: From Molecules and Crystals to Materials (Eds.: D. Braga, F. Grepioni, A. G. Orpen) Kluwer Academic, Dordrecht, 1999, f. Sheng (redaktorët), Dukuritë Nanoscale: Basic Science to Device Applications-Lection Notes in Nanoscale Science and Technology Vol 2, Springer Science & Business Media, Nju Jork, 2008, f.

General

Solvents, Gases and Reagents

Instrumentation

• Melting Points
• Elemental Analysis
• Nuclear Magnetic Resonance (NMR)
• Infrared (IR)
• X-ray crystallography
• Luminescence Studies

31P{1H} spectra were obtained on a Bruker 400 MHz spectrometer, with chemical shifts reported relative to an 85% H3PO4 in D2O external standard solution. A homogeneous potassium bromide (KBr) pellet was used for the analysis of some compounds obtained in powder form. An FT-IR Perkin Elmer Spectrometer, Model 100 equipped with Universal ATR Sampling Accessory was used to analyze compounds obtained as powders when KBr discs were not used, (w = weak, m = medium, s = strong, vs = very strong, br = broad).

Data were collected using a Bruker CCD (charge-coupled device) based diffractometer equipped with an Oxford Cryostream low-temperature apparatus operating at 173 K. Data were measured using omega and phi scans of 0.5° per frame for 30 s. The total number of images was based on results from the COSMO program [8] where the expected redundancy was 4.0 and the completeness was 100% to 0.83 Å. Cell parameters were obtained using APEX II software[9] and refined using SAINT on all observed reflections. The structures were solved by direct methods using the SHELXS-97 program and refined by the least-squares method on F2, SHELXL-97, which are included in SHELXTL-PC V 6.10.[12]

All non-hydrogen atoms are refined anisotropically and hydrogens are calculated by geometric methods and refined as a riding model. The Flack[13] parameter was used to determine chirality of the studied crystal, the value should be close to zero, a value of one is the other enantiomer, and a value of 0.5 is racemic. An emission spectrum of 3 was recorded using a Photon Technologies Int. PTI) fluorescence spectrometer controlled by PTI's Felix32© Version 1.1 Software.[14].

Steady-state emission spectra were recorded using PTI's XenoFlashTM 300 Hz pulsed light source and gated emission scans with a delay of 95 µs, an integration window time of 100 µs and 50 pulses per channel (shots). The excitation wavelength was 250 nm for the sample; with the scattered light removed by means of a suitable wavelength bandpass filter.

Preparation of gold(I) and copper(I) starting materials

• Preparation of H[AuCl 4 ]·3H 2 O
• Preparation of [AuCl(tht)]
• Preparation of [AuC 6 F 5 (tht)]
• Preparation of [AuClPPh 3 ]
• Preparation of [Cu(CH 3 CN)][PF 6 ]
• Preparation of [(ClAu) 2 ( dppm)]
• Preparation of [(ClAu) 2 (dppe)]
• Preparation of [(ClAu) 2 (dpph)]
• Preparation of [(ClAu) 2 (dppb)]

The intensities (peak heights) of the emission maxima were recorded at room temperature and the spectrum is uncorrected from the instrument response. To this solution it was added dropwise and the solution formed a yellow precipitate [AuCl3(tht)]. The addition of this was stopped and the white precipitate was collected by filtration and the product was washed with portions of cold EtOH (3 x 5 mL).

A Schlenk flask equipped with a magnetic stir bar was charged with Et 2 O (35 mL) and the flask was cooled to -78°C in a dry ice/acetone bath. To the cooled flask, C6F5Br (0.95 mL, 7.5 mmol) was slowly added using an airtight syringe and the mixture was stirred for 5 minutes. To the white precipitate formed, 5 drops of deionized water were added and the mixture was filtered over anhydrous MgSO4.

All solvent was removed under reduced pressure and the product was obtained as a free-flowing white powder. The gold(I) solution was added to the PPh3 solution in one portion and the final mixture was stirred at room temperature for a period of 30 minutes. The product was collected by filtration and the product was washed with portions of cold Et2O (2 x 3 ml).

The gold(I) mixture was poured into the ligand solution in one portion and the mixture was stirred at room temperature for a period of 15 minutes. All the solvent and it was removed and the product further dried by adding and removing Et2O under reduced pressure.

Preparation of complex [(ClAu){PPh 2 (OH)}], (1)

Preparation of the dppm-CN ligand (2)

Preparation of the [(ClAu) 2 (dppm-CN)] complex (3)

Preparation of the [Au 2 {(PPh 2 ) 2 CCN}] complex (5)

Single crystals were obtained by slow diffusion of hexane vapor into a saturated solution of 4 in dichloromethane. Single crystals were obtained by slow diffusion of hexane vapor into a saturated solution of 5 in dichloromethane.

After the reaction period, all the solvents were removed and the product (6), obtained as a free-flowing off-white powder, was isolated. 10.SAINT V 7.34 Software for the Integration of CCD Detector System Bruker Analytical X-ray Systems, Madison, WI, 2008.

Introduction and Goals of Study

The development of dppm-CN (Chapter 1, Section 1.1.3) as a versatile ligand in coordination chemistry requires further investigation,[7] and this includes reactivity studies of the lithiated dppm-CN towards alkylation and the formation of various phosphor- ylides (Scheme 3.1). Development of the dppm-CN ligand by Erker and co-workers through the formation of phosphorous ylides. As mentioned before, the chemistry of the ligand dppm-CN has been rejuvenated through a new synthetic route reported by Erker and co-workers.[8] This study reports the first use of the ligand in molecular gold(I) complexes and an investigation into its structure and luminescent properties.

In this study, the dppm-CN ligand was synthesized by modifying a procedure involving double deprotonation of acetonitrile and changing reaction times and conditions. The ligand generated open and cyclic dinuclear gold(I) complexes, whereby the role of the coordination state of dppm-CN in the bridging state opened the possibility to explore new potential reactivity trends of the cyano functional group.

Results and Discussion

• Synthesis and Characterization
• A New Polymorph of [(ClAu){PPh 2 (OH)}] (1)
• Open-ended and cyclic gold(I) complexes from of dppm-CN
• The open-ended [(ClAu) 2 (dppm-CN)] Complex (3)
• The open-ended [(C 6 F 5 Au) 2 (dppm-CN)] Complex (4)
• The neutral cyclic [Au 2 {(PPh 2 ) 2 CCN} 2 ] Complex (5)
• Luminescence Studies

The interaction of the Cl(1) atom is far from the O(2)-H(2) group such that there is very little or no hydrogen bonding. The formation of new gold(I) complexes depended on the preparation of the ligand 2, obtained by modifying the stoichiometric amounts used in the literature procedure.[7]. The formation of the complex [(ClAu)2dppm-CN], 3, resulted from the treatment of dppm-CN with [ClAu(tht)] in DCM (1:2) and was isolated in high yields.

Coordination of the bridging phosphine to Au(I) was evident from solution 31P NMR of 3 which exhibited a major resonance at δ = +34.8 for two equivalent P atoms. The effect of the CN group appears to alter the reactivity of complex 3 significantly compared to conventional [(XAu)2dppm]. Similar to complex 3, coordination of the bridging phosphine to Au(I) was evident from 31P solution NMR of 4 exhibiting a major resonance at δ = +42.0.

A software (Mercury© version: 2.3) generated packing arrangement diagram of the complex (4) viewed along the a-axis. The hydrogen bond is observed from the alignment between the F atoms of the C6F5 units and the phenyl rings of the dppm-CN ligand in the structure. A dinuclear Cu(I) complex (6) was prepared and spectroscopically characterized, the solid state structure of the complex remains to be confirmed followed by an exploration of the reactivity trends.

To begin studies of the luminescence properties of the newly formed gold(I) complexes of the dppm-CN ligand, a study of the [(ClAu)2(dppm-CN)] complex (3) was performed. The implication of the results obtained for complex 4 is by no means a statement that gold(I) complexes with. The cyclic gold(I) complex 5 presented in this study is very similar to the previously mentioned complexes.[16] The luminescence properties of 5 remain to be investigated, although 3 and 4 in the solid qualitative results for 5 like complexes show that the complex luminesces at room temperature when exposed to UV light in the solid state.

Novel neutral dinuclear gold(I) complexes with the dppm-CN ligand were prepared and characterized, and initial steps were taken in an investigation of luminescence properties and reactivity.

Figure 3.1. Above: Molecular structure of (1) with relevant atom labelling shown. Drawn at 50% probability level

A second polymorph of [(ClAu){PPh2(OH)}] has been structurally characterized and compared to the polymorph reported in the literature. The dppm-CN ligand was synthesized using a modified procedure and new open and closed gold(I) complexes were generated. Although Erker and colleagues have developed the ligand (chapters 1 and 3), there is interest in further development of the ligand by isolation (work in progress) of a new tetraphosphine ligand (Scheme 4.1) by means of a Thorpe self-condensation reaction for which this system is ideally suited because the nitrile contains an active α-proton. Investigations into the luminescence properties of gold(I) complexes and their luminescence properties that may arise from this expected multidentate ligand would be of interest.

The effect of the cyano group on the newly formed gold(I) complexes brings interesting properties and is of great interest for the development of a variety of bi- and heterometallic complexes based on the exploration of the reactivity of the complexes presented in this study (Scheme 4.2.) Conducting the reactions such as the oxidative addition of halogens to the formation of rare Au(II) and (more abundant) Au(III) complexes is essential, as it has been demonstrated by Nocera and co-workers that Au(III) concentrates undergo halogen photoreductive elimination reactions and are among the ideal class of compounds in the development of catalysts for renewable energy systems.[2] Whether the halogens preferentially attack the cyano nitrogen remains to be seen. The use of X-ray crystallography in ensuring the identities of the complexes in this study is essential, as it helps eliminate uncertainty in the results that can follow from spectroscopic studies such as NMR and IR.