Synthesis And Properties Of Four-membered Ring Complexes Containing N,N- Or N,O-bidentate Ligands

Exploring the syntheses of novel structural complexes, studying their physical and chemical properties as well as their applications is the central motif of coordination chemistry. Cyclic complexes in coordination compounds have become a very important class of compounds. Generally, five or six-membered ring complexes are quite stable, so these types of complexes have been mostly studied. However, owing to the tension effect of four-membered ring, the related complexes show relatively low stability and less attention has been drawn to their studies. Notably, the existed studies have shown some four-membered ring complexes are also stable and possess excellent properties, and they have been widely applied in the field of catalysis and organometallic chemistry. Drawing from these contributions, the synthesis of novel complexes featuring a four-membered ring and subsequently studing their properties will not only enrich the content of the coordination chemistry, but also build up the theoretical basis for its applications.Stable four-membered ring complexes can be synthesized by design and selection of suitable ligands, central atom and experiment conditions. Amidinate and amidate ligands have similar structures. The use of amidinate and amidate as ligands in coordination chemistry has attracted much attention for several reasons: first, they can be easily synthesized in high yields from readily available starting materials; second, the electronic and steric properties of coordination complexes containing amidinate and amidate ligands can be readily tuned by altering the substituents on amidine and amide; third, binding modes to the central atom are virious.In recent years, some four-membered ring complexes with novel structure and excellent performance have been synthesized from amidinate ligand. Only few four-membered ring complexes containing amidate ligands have been synthesized. In this paper we present a systematic study on the synthesis and/or properties of four-membered ring complexes containing amidinate or amidate ligand by using combined experimental and theoretical methods. The main contents are summarized as follows:1. Based on the studies of amidinate four-membered ring silicon compouds, we explored the amidinato tricoordinate silylene on the stability and coordination ability. Reaction of [PhC（NtBu）₂SiOBu^t] and diiron nonacarbonyl[Fe₂（CO）₉] led to the formation of {[PhC（NtBu）₂SiOtBu}Fe（CO）₄, which is the first stable metal complex derived from a amidinato tricoordinate silylene ligand. This complex was characterized by elemental analyses, IR, and ¹H, ¹³C, and （²9）Si NMR methods. Furthermore, single crystal X-ray diffraction analysis was performed to confirm its geometry in which the silicon center has been shown to be four-coordinate with a distorted tetrahedral geometry. To examine the stability and bonding properties of this complex, density functional theory method with B3LYP was used to obtain the ground state structure and energy of the reaction. The calculated structural parameters were in good agreement with the crystallographic structure. NBO analysis indicated that larger electronic density can be found on Fe than on Si. The results show that four-membered ring amidinato tricoordinate silylene can be used as a stable ligand.2. The synthesis, stability and properties of a series of four-membered ring iridium complexes with amidate ligands were studied. Furthermore, the electronic effect of the substituents on the amidate ligand on the photophysical properties of these complexes has been elucidated by means of combined experimental and theoretical analysis. Thirty iridium complexes with cyclometalating ligands 2-phenylpyridine, 2-phenylquinoline and 2-phenylbenzothiazole and various amidate ancillary ligands have been synthesized. All the complexes were characterized by IR, NMR and thermogravimetric analyses. In addition, 14 complexes have been structurally characterized by X-ray crystallography. It was found that the amidate ligand binds to the Ir center in an imine/alkoxide mode via a four-membered, nearly planar ring （N-Ir-O-C）. Photopysical properties of these Ir amidate complexes were investigated, including UV-vis absorption and solution-phase emission spectra. To further understand the electronic structure of these complexes, density functional theory calculations with B3LYP method were conducted to study the ground state configuration, frontier orbital composition and energy. Additionally, TD-DFT calculations were performed to understand the UV-vis absorption behaviour of these complexes. The results show that amidate ligand can form stable four-membered ring with iridium metal. The resulting iridium amidate complexes have significant photoluminescence with emission wavelength ranging from 510 nm to 586 nm. The photophysical properties are mainly affected by the electronic properties of the cyclometalating ligand. However, it can be further tuned by the electronic effect of the substituents on amidate ligands.3. During our study on the preparation of four-membered ring iridium complexes with amidate ancillary ligands, an unexpected dinuclear iridium complex, without containing amidate ligand, has been obtained with the same start materials in different solvents and temperatures and fully characterized by NMR, IR, MS, elemental analysis and X-ray crystallographic methods. This complex is quite unprecedented and has the following structural features: first, there is a bridging phenylquinolinyl ligand supporting a fromally Ir（Ⅱ）-Ir（Ⅱ） bond via aμ-η¹:η² manner; second, a head-on dinitrogen was coordinated to one of the two Ir centers. To further understand the absorption properties of this complex, TD-DFT calculations were conducted. The unique absorption spectrum of the dinuclear iridium complex compared to mononuclear Ir complexes possibly highlights the involvement of Ir-Ir bond in excitation of the dinuclear Ir complexes.