Theoretical Investigations On Structures And Stabilities For Several Important Complexes

Coordinate chemistry is one of the most important parts of modern chemistry. Complexes are widely applied to our life, industry and biological science, and recently have been developed rapidly. Complexes are associated with inorganic compounds, organic compounds, cluster chemistry, coordinate catalyzer and molecular biology. In this thesis, quantum chemical investigations on the design a series of stable complexes as well as their structures and stabilities have been carried out. Our calculations provide theoretical supports for future experimental study on the detection and synthesis, and enrich the coordinate chemistry.The main results are summarized as follows:1. We investigated the electronic spins and counterions influences for the kinetic stability of sandwich-like complexes [N₄MN₄]^q (M= Ti, V, Cr, Fe, Co, Ni) based on the energetic all-nitrogen 6Ï€-aromatic species N₄^2-, which are reported previously. We studied the stability of the charged [N₄TiN₄]^2- and [N₄TiN₄TiN₄]^2- systems as well as the neutral [N₄TiN₄]Li2 system in both singlet and triplet electronic states at DFT level. We found that the ground state structures of di-deckered [N₄TiN₄]^2- and [N₄TiN₄]Li2 as well as the tri-deckered [N₄TiN₄TiN₄]^2- are all in triplet state, rather than the previously reported singlet ones. Therefore, the N₄^2- and Ti2+-based sandwich-like complexes should be in high spins and may have potential use for new paramagnetic materials. Moreover, our calculations indicated that although the counterions can induce the electronic stabilization, they on the other hand can lead to the considerable kinetic destabilization of the N₄^2-—based sandwich-like complexes since the counterions can structurally destroy the perfectness of aromatic N₄^2-. Thus, in study of the sandwich-like complexes, the effect of counterions can not be neglected for assessment of the kinetic stability.2. We made the first successful assembly-design of the long escaped N₃^-based compounds, i.e., [N₃NiN₃]^2-, [N₃M(CO)₂N₃]^q ((M,q)=(Fe, 0) (Mn, -1)), [N₃M(CO)₃]^q ((M,q)=(Co, 0) (Fe, -1)), and [N₃MCp]^q ((M,q)=(Ni, 0) (Co, -1)), at the density functional level. The conversion and dissociation of them need to overcome considerable barriers kinetically. The detailed structural, charge distribution and orbital analysis consistently reveal a triplet polynitrogen unit, cyclic-3N₃^-, rather than another simplest trinitrogen unit cyclic-1N₃+. The two unpaired spins within the naked cyclic-3N₃^- have effectively participated in the bonding interaction with the central transition metal atoms (here M is Ni, Fe, Co and Mn). Moreover the possible experimental routes of N₃Co(CO)₃ were proposed. The diradical-like polynitrogen ring, cyclic-3N₃^-, would add to the polynitrogen family as a novel building block.3. We studied a type of hetero-decked sandwich-like structures [N₃MN₅]^q containing two odd-membered all-nitrogen rings (N₃ and N₅) on the hypersurface of [N₈M]^q [(M,q)=(Ni,0), (Co,-1), (Fe,-2)]. At the B3LYP/6-311+G(d) level, the new isomers are energetically more stable than the previously reported homo-decked sandwich-like isomers [N₄MN₄]^q based on the even-membered all-nitrogen ring N₄^2-. In particular, theη³-η² (η³-η¹) isomers of [N₃MN₅]^q [(M,q)=(Ni,0), (Co,-1), (Fe,-2)] possess considerable kinetic stability for laboratory characterization. The bond length and natural charge analysis of [N₃MN₅]^q [(M,q)=(Ni,0), (Co,-1), (Fe,-2)] indicate that each complex possesses the smallest triplet all-nitrogen ring 3N₃^-.4. We performed the density functional theory investigations on a series of experimentally long-knownη³-C₃R₃^- assembled compounds, e.g., C3Ph3NiCp, (C₃Ph₃)Co(CO)₃ and (C3Ph3)NiCl(py)2, in which theη3-C₃R₃ unit was generally conceived as a singlet 2Ï€-aromatic C₃R₃⁺ have been carried out. The structural, bonding and natural charge analysis definitively reveals the existence of a negative diradical-like unit 3C₃R₃^- rather than the generally accepted 1C₃R₃⁺. The two unpaired electrons within the nakedη³-³C₃R₃^- have effectively participated in the bonding interaction with the central transition metal atoms, resulting in the eventual diamagnetism. Moreover, we for the first time designed various half and fully sandwich-likeη³-C₃R₃^-based complexes containing alkali and alkaline-earth metals. Interestingly, their intrinsic di-radical or tetra-radical characters allow their potential use as paramagnetic materials. Finally, we showed that the model unit ³C₃H₃^- in both free and assembled form is kinetically stable against ring-opening, and when H is replaced byÏ€-type substitutes, ³C₃R₃^- becomes more stable than singlet. Therefore, for the simplest triplet 4nÏ€aromatic 3C₃R₃^-, we have for the first time proven its long existence in transition metal assembled complexes and have predicted its existence in main-group metal assembled complexes.5. We systematically investigated the thermodynamic and kinetic stability of Zn(CO)₃ towards CO-extrusion at the BP86, B3PW91, BPW91, PBEPBE, BH&HLYP, B3LYP, MP2, MP4SDQ, QCISD, CCSD and CASPT2 levels as well as the Born-Oppenheimer molecular dynamic (BOMD) simulation. All these calculations consistently reveal that the 18e Zn(0) complex Zn(CO)₃ is neither a genuine minimum point nor kinetically stable with negligibly low barriers. In particular, Zn(CO)₃ is thermodynamically quite unstable with respect to the fragments ¹Zn+3CO by around 40 kcal/mol at all the three sophisticated correlation levels, i.e., MP4SDQ, QCISD and CCSD. We thus concluded that the tricarbonyl Zn(0) complex, Zn(CO)₃, should not exist even for spectroscopic characterization. Interestingly, our extensive structural search predicts that two triplet di-zinc carbonyls, i.e., ³(CO)ZnZn and ³(CO)₂ZnZn, have noticeable kinetic stability(10.41 and 8.11 kcal/mol at the CCSD level) against the respective CO- and Zn-extrusion, which can be compared with the value 8.70 kcal/mol for the already detected ³Zn(CO)₂. Our designed ³(CO)ZnZn and ³(CO)₂ZnZn together with the experimentally known ³ZnCO and ³Zn(CO)₂ are formally associated with the zinc (0)"spin-based zinc carbonyls"and should be considered as remarkable, since most of the known zinc complexes usually contain +2 or +1 oxidation state Zn.