Syntheses, Characterization And Reactivities Of Poly(N-heterocyclic Carbene)Borate Metal Complexes

N-Heterocyclic carbenes (NHCs) metal complexes have attracted increasing attention in coordination chemistry and homogeneous catalysis since NHC firmly bonds to metal center to form strong NHC-metal bonds (NHCs) due to its strong a-donor capacity. Poly(pyrazolyl)borate ligands (Bp and Tp) are the most widely used ligands in coordination chemistry and organometallic chemistry. Poly(N-heterocyclic carbene)borate have similar framework of poly(pyrazolyl)borate and strong a-donor nature of N-heterocyclic carbene, which have attracted considerable research attention. Some metal complexes such as tris(N-heterocyclic carbene)borate iron compounds have been extensively studied. It is surprising that no rhodium, iridium, ruthenium and osmium complex with poly(N-heterocyclic carbene)borate ligands have been reported yet.The research presented in this thesis focused on the synthesis, characterization and reactivities of bis(N-heterocyclic carbenes)borate rhodium(iridium) and tris(N-heterocyclic carbenes)borate ruthenium(osmium) complexes. This thesis contains the following four sections:1. Rhodium(I) and iridium(I) carbonyl complexes with bis(N-heterocyclic carbene)borate ligand and the B-H functionalization reaction. Treatment of bis(N-heterocyclic carbene)borate ligand precursor H2B(tBuImH)2+I-with lithium diisopropylamine (LDA) afforded free carbenes in situ, which was then reacted with [Ir(CO)2Cl]2or [Rh(CO)2(acac)] to yield iridium(I) and rhodium(I) carbonyl complexes [H2B(ImtBu)2]Ir(CO)2(1) and [H2B(ImtBu)2]Rh(CO)2(2). Reactions of1and2with [FeCp2][PF6], AgPF6or AgBF4unexpectedly yielded the B-F products [F2B(ImtBu)2]Ir(CO)2(3) and [F2B(IntBu)2]Rh(CO)2(4), respectively. Reaction of complexes1or2with pyridine or1-methyl-imidazole in the presence of [FeCp2][PF6] generated complexes [HB(ImtBu)2(Py)]Ir(CO)2+PF6-(5),[HB(ImtBu)2(Py)]Rh(CO)2+PF6-(6) and [HB(ImtBu)2(ImMe)]Ir(CO)2+PFe6-(7). These complexes have been characterized by1H,11B,13C and19F NMR spectroscopies, ESI-MS, infrared spectroscopy and elemental analyses. The molecular structures of1-4and7have been established by single-crystal X-ray diffraction studies, confirming that the coordination sphere is square planar.The electron-donating properties of [H2B(ImtBu)2] and [F2B(ImtBu)2] ligands have been evaluated by IR specta. The results show that [H2B(ImtBu)2] and [F2B(ImtBu)2] ligands exhibit stronger electron-donating capacity than the analogous isoelectronic ligands bis(pyrazolyl)borate and bis(NHC)methylene.2. Synthesis and structural characterization of rhodium(I) and iridium(I) carbonyl phosphine complexes with bis(N-heterocyclic carbene)borate ligandsComplexes2-4reacted with PR3(R=Ph, Cy) in diethyl ether solution to yield the one carbonyl substituted products [H2B(ImtBu)2]Rh(CO)(PPh3)(8),[F2B(ImtBu)2]Rh(CO)(PPh3)(9),[F2B(ImtBu)2Ir(CO)(PPh3)(10),[H2B(ImtBu)2]Rh(CO)(PCy3)(11),[F2B(ImtBu)2]Rh(CO)(PCy3)(12) and [F2B(ImtBu)2]Ir(CO)(PCy3)(13) in good yields (>95%). These complexes have been characterized by1H,11B,13C,31P and19F NMR spectroscopies, infrared spectroscopy and elemental analyses. The molecular structures of complexes9-13have been established by single-crystal X-ray diffraction studies, which confirm that the coordination sphere is square planar.IR stretching value of the CO ligands, the value of JRh-P and electrochemical data have been rationalized by the electronic effects of the bis(N-heterocyclic carbene)borate and phosphine ligands. The results show that the a-donor capacity of bis(N-heterocyclic carbene)borate is stronger than the analogous isoelectronic ligands bis(pyrazolyl)borate and acetylacetonato.3. B-H fluorination of iridium(I) and rhodium(I) cyclooctadiene complexes with bis(N-heterocyclic carbene)borate ligand. Isolation of unusual rhodium(III) complex stabilized with a pair of agostic interactions.Treatment of bis(N-heterocyclic carbene)borate ligand precursor H2B(’BuImH)2+I-with lithium diisopropylamine (LDA) afforded free carbenes in situ, which was then reacted with [M(COD)Cl]2(M=Ir, Rh) to yield rhodium(I) and iridium(I)complexes [H2B(ImtBu)2]M(COD)(M=Ir,14; M=Rh,15). The coordinated COD was replaced by carbon monoxide to yield the corresponding carbonyl species [H2B(ImtBu)2]M(CO)2(M=Ir,1; M=Rh,2). Reaction of14with [FeCp2][PF6] yielded the B-F product [F2B(ImtBu)2Ir(COD)(16). Reaction of15with [FeCp2][PF6] in the presence of KI generated a Rh(III) species [F2B(ImtBu)2]RhI2(17). Reaction of14with [FeCp2][PF6] in the presence of KI generated complexes [F2B(H-ImtBu)(ImtBu)]Ir(COD)I (18) and [(H-ImtBu)2]Ir(COD)PF6(19).These complexes have been characterized by1H,11B,13C and19F NMR spectroscopies and elemental analyses. The molecular structures of15-19have been established by single-crystal X-ray diffraction studies, which confirm that the coordination sphere is square planar for15-16and18-19. Complex17exhibits a slightly distorted octahedral configuration around the rhodium atom, which is six-coordinated by two carbene carbons, two iodides and a pair of agostic interactions utilizing the methyl groups of t-butyl substituents.4. Syntheses, structures and electrochemical properties of homoleptic ruthenium(Ⅲ) and osmium(Ⅲ) complexes bearing two tris(N-heterocyclic carbene)borate ligandsTreatment of tris(N-heterocyclic carbene)borate ligand PhB(MeImH)3-2(OTf) with lithium diisopropylamine (LDA) afforded free carbenes in situ, which was then reacted with [Ru(COD)Cl2]n or (Bu4N)2OsCl6to yield six-coordinate homoleptic ruthenium(Ⅲ) and osmium(Ⅲ) complexes [PhB(ImMe)3]2M(OTf)(M=Ru,20; M=Os,21). They were characterized by elemental analyses, ESI-MS and single-crystal X-ray diffraction. X-ray diffraction shows that both complexes display octahedral configuration, in which the metal center is coordinated by two tridentate N-heterocyclic carbene ligands.Electrochemical properties of20and21have been studied by cyclic voltammetry (CV) measurements, which reveal two reversible redox waves at0.60,-0.63V and0.35,-0.86V for20and21, respectively. The results show that the tris(N-heterocyclic carbene)borate ligand exhibit stronger electron-donating capacity than Tp, Cp and Cp*.The binding energies of Ru3d5/2and Os4f7/2are280.1and50.3eV in20and21, respectively, which are close to those of other Ru(Ⅱ) or Os(Ⅱ) complexes. The low electron binding energies indicate that the metal centers are highly electron-rich due to the very strong electron-donating nature of the tris(carbene)borate ligand, in good agreement with the conclusions drawn from the electrochemical studies.