title: N-heterocyclic carbene-based Pd and Ni complexes and their applications to the activation of alkylnitriles
Creators: name: Wierenga, TS
Publisher: name: University of Tasmania
Year: 2019

description: α-Cyanocarbanion complexes are an interesting class of complexes that contain a nitrile ligand featuring a deprotonated α-carbon. Chapter 1 gives an overview of the various synthetic methods, bonding modes and applications that have been established in the literature. Of particular interest is the role these complexes have as intermediates in the formation of various different compounds including the synthetically versatile β-hydroxynitriles. One example of a catalyst that can be used to synthesise a range of β-hydroxynitriles from the corresponding aldehydes under base free conditions is the POCOP pincer complex [{2,6-(iPr\(_2\)PO)\(_2\)C\(_6\)H\(_3\)}Ni(CH\(_2\)CN)]. DFT calculations outlined in Chapter 2 indicated some of the key features of the complex in relation to its catalytic performance. These were that the –CH\(_2\)CN ligand isomerises to yield the active N-bound catalyst in solution, the trans donation effect of the phenyl ring lowers the energy of the transition state of the reaction and the rate determining transition state involves a 5 coordinate intermediate and therefore the steric hindrance of the phosphine substituent influences the performance. Chapter 3 describes a series of bis(NHC) palladium(II) complexes (NHC = N-heterocyclic carbene) of the type [{(RIm)\(_2\)CH\(_2\)}Pd(NCMe)\(_2\)][PF\(_6\)]\(_2\) with different N-substituents which were reacted with NaOH in CH\(_3\)CN. Two different products were formed under similar conditions. [{(RIm)\(_2\)CH\(_2\)}Pd(CH\(_2\)CN)\(_2\)] was observed for both the R = Me and 2,4,6-trimethyphenyl (Mes) substituent while [{([RIm]\(_2\)CH\(_2\))Pd(μ–CH\(_2\)CN)}\(_2\)][PF\(_6\)]\(_2\) was observed for both the R = Mes and 2,6-diisopropylphenyl substituent. The two initial products that were not formed under these conditions could be synthesised under modified conditions. Complexes of the type [{(MesIm)\(_2\)CH\(_2\)}Pd(NCR)\(_2\)][PF\(_6\)]\(_2\) where R = CH\(_2\)CH\(_3\) and CH\(_2\)CH\(_2\)CH\(_3\) were also synthesised, however, subsequent conversion to the corresponding [{(RIm)\(_2\)CH\(_2\)}Pd(CHRCN)\(_2\)] was not successful. In Chapter 4 a range of NHC-containing pincer complexes were synthesised due to their close structural similarity to the successful POCOP complex and therefore may result in successful catalytic applications under similar reaction conditions. These complexes contain two NHC ligands bridged by either a pyridine or phenyl ring. [CNC\(^{iPr}\)Pd(CH\(_2\)CN)][SbF\(_6\)] was synthesised from the corresponding NCMe complex following the same method used for the bis(NHC) complexes. [C^C^C\(^{Me}\)Pd(NCMe)][SbF\(_6\)] was prepared from the previously reported Br complex. However attempts to form the corresponding –CH\(_2\)CN complex were not fruitful. A number of alternative synthetic approaches were attempted for the desired non methylene linked phenyl based complex using various ligand modifications. Initially reactions were based on the pro-ligands of the type [1,3-{(R)ImH}\(_2\)Ph]X\(_2\) where R = CH\(_3\) and X = I or R = nBu and X = Br were reacted in various approaches, however, due to the difficulty of the three C–H metallations an alternative pathway was pursued. This strategy investigated the preparation of a phenyl–OTf based pro-ligand, however, due to difficulties in synthesising the pro-ligand an alternative approach was needed. A series of pro-ligands of the type [2,6-(RImH)\(_2\)1-BrPh]X\(_2\) where R = CH\(_3\) and X = I or R = nBu, Ad and X = Br were synthesised from 2-bromo-1,3-difluorobenzene. The desired [CCC\(^{nBu}\)PdBr] was then successfully prepared from the corresponding pro-ligand in good yield. This provided an alternative synthetic pathway to that previously developed for these complexes, although the yield was lower, the lack of a zirconium transfer reagent and the use of air-stable starting materials and higher atom efficiency make this method advantageous to previous literature methods. The catalytically relevant –NCMe derivative was synthesised, however, the attempted formation of the –CH\(_2\)CN ligated complex was not successful. [CCC\(^{nBu}\)NiCl] was also synthesised from the corresponding pro-ligand which then was reacted further to form the catalytically relevant NiNCMe complex. The catalytic studies described in Chapter 5 explored the C–C coupling of benzaldehyde and CH3CN to yield 3-hydroxy-3-phenylpropionitrile. Various bis(NHC) and NHC containing pincer –CH\(_2\)CN complexes were tested, however, no reaction was observed. For the pincer complexes in which the –CH\(_2\)CN complex could not be succesfully synthesised, the –NCMe complex was used as a pre-catalyst in solution with a catalytic amount of NaOH to prepare the required complex in situ. However, the yield of the product was identical to the background reaction performed in the absence of the transition metal catalyst.