Reports: GB3
48223-GB3 Mechanistic Studies on Organogold Complexes. Toward the Development of Novel Homogeneous Catalysts for C-O, C-N, and C-C Bond Formations
The objective of this project is the synthesis of gold(III) and gold(I) alkene complexes and there application in homogeneous catalysis. First we started to explore the synthesis of AuIII complexes with related structures to those of the known square planar platinum(II) alkene complexes [Pt(PNP)(alkene)](BF4)2 (PNP = 2,6-bis(diphenylphosphinomethyl)pyridine), containing a tridentate ligand and leave one site open for substrate activation and reaction. Since these dicationic platinum alkene complexes showed a high reactivity toward weakly basic nucleophiles, it was thought that analogous gold complexes may be very interesting to investigate in their reactivity. In the initial exploring studies to prepare gold(III) complexes of the type [AuIII(L)Cl]Cl2 different tridentate ligands such as 2,6-pyridinedicarboxylic acid, PNP, and 2,6-bis(methylthio)pyridine (SNS) were respectively reacted with AuCl3. The resulting complexes, which were characterize so far only by NMR spectroscopy, were either unstable in solution, thermally unstable, showed fluctuating structures in solution, or constitutes more complex, presumably multinuclear structures. None of these structures have been unambiguously solved yet. In contrast, the known [Au(terpy)Cl]Cl2 (terpy = terpyridine) appeared a much more stable and hence more suitable AuIII species to perform mechanistic studies on substrate activation and reactivity. Reaction with silver salts AgX (X = SO3CF3, BF4) gave in aqueous solution the corresponding species [Au(terpy)Cl]X2. X-ray single crystal diffraction analyses showed a number of short contacts between the complex cation and the counter anions displaying an expanded, distorted pseudo-octahedral coordination geometry. A particularly short contact was observed between one of the oxygen atoms of the triflate and the central gold atom [2.76 Å]. This short contact may be a useful model or indication for an incipient reaction of a substrate. Studies on C–H, N–H, and O–H activation are currently in progress using the [Au(terpy)Cl]X2 complexes.
We also started to explore the synthesis of new cationic gold(I) alkene complexes of the type [Au(L)(alkene)]BF4 containing a neutral bidentate ligand L such as bipyridine (bipy), phentanthroline (phen), and 1,2-bis(diphenylphosphino)benzene (diphos). The first step of the synthesis is the reaction of Na[AuCl4] with the ligand in the presence of excess NaBF4 giving [Au(L)Cl2]BF4 (L = bipy, phen). In case of the reaction with diphos, two different products were obtained. One of the products was identified as linear [AuICl{PhP(C6H5)P(O)Ph}] by X-ray diffraction analysis. This complex may be also a useful precursor to prepare a new gold(I) alkene complex. In the next reaction step the complexes [Au(L)Cl2]BF4 (L = bipy, phen) were hydrolyzed in the presence of either Ag2O or KOH to give [Au2(L)2(μ-O)2](BF4)2. An attempted was made to react [Au2(bipy)2(μ-O)2](BF4)2 with different alkenes (1,2-dimethylbutene, styrene) using methanol as solvent to obtained the respective gold(I) alkene complexes was not successful. The only product which was isolated in each case was [Au(L)(OMe)2]BF4 due to the reaction of the oxo-bridged gold(III) complex with methanol. Studies are in progress using different polar solvents such as acetone or acetonitrile. The reaction needs to be carried out at low temperature (usually 10-15ºC) to avoid gold(0) deposits, but this causes also a problem with insufficient solubility of the starting material and lengthen the reaction time (>10 days).
Furthermore we have been studying a few interesting reactions with dicationic platinum(II) alkene complexes. Facile vinylic deprotonation occurred with [Pt(PNP)(CH2=CHAr)](BF4)2 (Ar = Ph, naphthyl) in weakly basic solvents ROH (R = H, Me, Et). These dicationic platinum complexes are the most reactive metal complexes promoting spontaneous vinylic deprotonation, and therefore they are useful species to investigate further organic transformations. The acidity of the vinylic protons of the aromatic alkene CH2=CHAr is enhanced by approximate 43 orders of magnitude when it is coordinated in the diactionic platinum PNP complex.
Another reaction we have been investigating is the hydration of alkynes as an important industrial reaction due to its atom economic character. [Pt(PNP)(CH2=CH2)](BF4)2 was used as starting compound. It was found that at –78ºC aliphatic alkynes RC≡CR' (R/R' = H/H, H/Me, Me/Me) displace rapidly the coordinated ethylene followed by an immediate addition of water, which resulted in respective σ-complexes containing a C=O functional group. In case of acetylene the σ-aldehyde complex [Pt(PNP)(CH2CHO)]BF4 was obtained which was characterized by different NMR spectroscopic techniques and X-Ray single crystal structure analysis. In the reaction with propyne two regioisomers were formed, [Pt(PNP){CH2C(O)CH3}]BF4 and [Pt(PNP)(COCH2CH3)]BF4, in a ratio of 2:3. The latter isomer was characterized by X-Ray single crystal diffraction. In contrast to the aliphatic alkynes phenylacetylene underwent a C-H bond activation, similar to the aromatic alkenes, resulting in the σ-phenylacetylide complex [Pt(PNP)(CH≡CPh)]BF4. Platinum(II) phenylacetylide complexes are interesting compounds to investigate their photochemical properties.
The students participated in these research projects gained valuable experiences for their professional career. When the undergraduate students begin their research work in the lab they are usually very confused. Over the course of one to three semesters they showed considerable progress in their skills. They all expressed that they became more prepared for other upper level science courses or for their future graduate studies. Especially the intensive practical laboratory work made them much better understanding scientific problems, with which before they had difficulties when they studied mainly the theory only.
The results on detailed mechanistic studies of the vinylic deprotonation were presented as a poster on the 237th National ACS Meeting and were summarized in a manuscript, which has been accepted very recently to be published in Organometallics. A part of the studies on the hydration of alkynes were presented as a poster on the 236th National ACS Meeting. Further presentations on the gold and platinum projects are planned for the upcoming Regional and National ACS Meetings. The undergraduate students are involved in poster preparation and presenting their results.
