Chapter Four Conclusion and Summary Chapter Four Conclusion and Summary 4.1 Conclusion and Summary N,S-heterocyclic carbene complexes of Pd(II) and their catalytic activities have been studied in this work. The results are presented and discussed in Chapter Two, Section 2.1 to 2.12. In Chapter Two, Part I, Section 2.1, the synthesis of A-D under solvent-free conditions is discussed. The synthesis and coordination chemistry of monocarbene and biscarbene Pd(II)-NSHC complexes as well as dinuclear Pd(II)-NSHC complexes are described in Chapter Two, Section 2.2. Complexes 2.1 and 2.2(a) are synthesized in CH3CN and DMSO and formed as biscarbene Pd(II) and dinuclear Pd(II) complexes respectively. The mononuclear and solvated monocarbene Pd(II)complexes (2.3 and 2.4) are obtained from the bridge cleavage reaction of complex 2.2 with CH3CN and DMF. These complexes are shown to be catalytically active in the Mizoroki-Heck reaction. The reaction of compound B and Pd(OAc)2 giving two major complexes cis-[PdBr2(NSHC)2] (cis-2.5) and trans-[PdBr2(NSHC)2] (trans-2.5) as well as two minor side products of monocarbene complexes [PdBr2(NSHC)] (2.6) and trans-[PdBr2(benzothiazole-ĸN)(NSHC)] (2.7) is discussed in Chapter Two, Section 2.4. The mixed dicarboxylato-bis(carbene) complex cis- [Pd(O2CCF3)2(NSHC)2] (2.8) is obtained from ligand exchange of cis-2.5 with AgO2CCF3. The catalytic activity of complexes cis-2.5, trans-2.5 and 2.8 as well as an in situ mixture of Pd(OAc)2 and compound B are studied in the Suzuki–Miyaura 209 Chapter Four Conclusion and Summary coupling of aryl bromides and activated aryl chlorides and give good conversions of diaryl products. Complexes 2.1-2.4, cis-2.5, 2.6-2.7 are structurally characterized. Chapter Two, Part II, Sections 2.4-2.8 discusses the synthesis and characterization of a range of mixed-NSHC-L Pd(II) complexes (L = phosphine, aromatic N-heterocycles, pyridyl and azole ligands (2.9-2.27)). They are obtained from bridge cleavage of dinuclear NSHC complexes of type [PdBr2(NSHC)]2 (2.2(a)2.2(b) and 2.17) complexes with different nucleophilic ligands. Complexes 2.9-2.16 are active in the Suzuki–Miyaura coupling reactions of selected aryl bromides. Complexes 2.22-2.27 catalyze sp2-sp3 cross-coupling of fluoroaryl halides with arylboronic acid giving diarylmethanes with good yields and high turnovers. Five Pd(II) complexes with N,N-benzimidazolin-2-ylidene are prepared and characterized by X-ray single-crystal diffraction analysis. These complexes are more active in the Suzuki-Miyaura coupling of selected aryl bromides than their N,S-heterocyclic carbene (NSHC) counterparts. The 13 Ccarbene NMR signals of Pd(II)-complexes benzylbenzothiazolin-2-ylidene are given in Fig. 4.1. The 13 bearing 3- Ccarbene NMR of Pd(II)- biscarbene complex 2.1 resonances at 203.8 ppm. When different donor ligands are coordinated to the Pd(II)-NSHC dinuclear complex 2.2(a), the 13 Ccarbene chemical shifts of these mix-NSHC-ligand Pd(II) complexes vary expectedly. An upfield shift is observed with coordinating solvents or aromatic N-heterocycle ligands (less donor ability co-ligands) while a downfield shift is observed for phosphine ligands (more donor ability co-ligands). 210 Chapter Four S N Conclusion and Summary P S Pd Br Br N 208.7 208.4 S P S Br N Pd Br Br N 203.8 207.7 197.5 Br N 195.9 S S N S Pd DMF N P H 191.5 S S N N 195.7 195.8 Br Pd N Br Br Pd Br N Br Pd NCCH3 Br Br 165.1 13 C NMR (ppm) Pd Br Br S N S S N Br Pd N Br N N Br Pd Br N S Pd Br Fig. 4.1 13 Br Ccarbene NMR signals of Pd(II)-complexes bearing the 3-benzylbenzothiazolin-2- ylidene ligand. Chapter Two, Part III, Section 2.10 discusses some unexpected findings on the reaction of benzothiazolium salts with Ag2O. Some oxidation and cyclization products instead of the expected benzothiazolin-2-ylidenes of Ag(I) are isolated. Single-crystal X-ray studies of the ten-membered heteromacrocycle 1,2,5,8dithiadiazecine-6,7-diones (2.34(a)) and the complex salt of 3- isopropylbenzothiazolium with polymeric [Ag2I4]n2n- dianion (2.38(a)) have been conducted. The silver-carbene transfer route may not be suitable for metal benzothiazolin-2-ylidene syntheses. The synthesis and coordination of mixed-NSHC/NNHC-cyclometallated Pt(II) complexes are presented in Chapter Two, Part III, Section 2.11. X-ray single-crystal diffraction reveals the NSHC ligand to be trans to the nitrogen in 2-phenylpyridine (2.39(a) and 2.39(b)) while the NNHC ligand is trans to the carbon in 2phenylpyridine (2.40). These compounds are essentially isostructural but differ in the 211 Chapter Four Conclusion and Summary phenyl ring orientation and significantly on the inter-planar angle between the Pt coordination and the heterocycle planes. Finally, the five- and six-membered fused-ring imidazolium, thiazolium and benzimidazolium salts (2.42 (f, g, l, n, p)) are obtained from the catalytic Ni(0)/Ni(II) reaction starting from alkenyl-substituted azolium salts. A catalytically active Ni(II)H species generated from the oxidative addition of azolium salt to Ni(0) is formed. The Pt(II)-H complex 2.43 is formed similarly from the reaction of Pt(0) and imidazolium salt (2.41(f)) and has been observed by 1H NMR spectroscopy. 212 . obtained from bridge cleavage of dinuclear NSHC complexes of type [PdBr 2 (NSHC)] 2 (2. 2(a)- 2. 2(b) and 2. 17) complexes with different nucleophilic ligands. Complexes 2. 9 -2. 16 are active in the Suzuki–Miyaura. complexes cis-[PdBr 2 (NSHC) 2 ] (cis -2. 5) and trans-[PdBr 2 (NSHC) 2 ] (trans -2. 5) as well as two minor side products of monocarbene complexes [PdBr 2 (NSHC)] (2. 6) and trans-[PdBr 2 (benzothiazole-ĸN)(NSHC)]. trans-[PdBr 2 (benzothiazole-ĸN)(NSHC)] (2. 7) is discussed in Chapter Two, Section 2. 4. The mixed dicarboxylato-bis (carbene) complex cis- [Pd(O 2 CCF 3 ) 2 (NSHC) 2 ] (2. 8) is obtained from ligand exchange of cis -2. 5 with AgO 2 CCF 3 .