Syntheses, Structures And Magnetic Properties Of Complexes Based On Planar Molecules

In recent years, molecule-based magnetic materials have been an important focal research field in chemistry, physics and material science because of their potential applications in such as high density magnetic storage, quantum computation and spintronic devices. This research field contains:single molecule magnets, chiral magnets, molecule-based photomagnets, ferromagnetic molecular conductors, magnetic superconductors and so on. The use of molecules or molecular assemblies for information processing is one of the most appealing perspectives in molecular chemistry. However, no matter how rich the field of molecular magnetic materials is, getting the higher Tc is the most basic and important issue. Using exchange interaction to synthesize higher Tc molecule-based magnets has reached a bottleneck. According to relevant references, ion-pair complexes formed by high-conjugated small molecules and metal ions are expected to make a breakthrough of the challenge in the synthesis of high Tc molecule-based magnets, and the mechanism of these high Tc magnets is ambiguous. Therefore, we would like to use high-conjugated molecular to design and synthesize molecular magnetic materials and then study its magnetic mechanism. In addition, organic and metal-organic radicals could bear one or more unpaired electrons in their π-system arising from the overlap between 2p atomic orbitals, they have rapidly been recognised as interesting building units for molecule-based magnetic materials. They can also exhibit exotic physical properties, such as ferromagnetic interaction by π-π stacking and spin-Peierls transition. In the synthesis process of the planar conjugated complexes, we found metal dithiolene complexes as counteranions could perturb the cooperativity of spin-crossover in a certain degree, so that they can achieve the industrialized purposes. One of the most spectacular examples of molecular bistability is the spin-crossover phenomenon. The complexes with spin-crossover may exhibit a transition between a low-spin (LS) and a high-spin (HS) state by applying external stimuli (temperature, light irradiation or pressure). Therefore, these complexes could be used in many molecular or electronic devices such as memory devices, thermal or optical switches and pressure sensors.In this work, we used many planar π-system molecules to synthesis molecule-based magnetic materials. The planar π-system molecules are [M(mnt)2]n-(mnt = maleonitriledithiolate, M = Ni, Pt, Cu), [Ni(dmit)2]- (dmit = 2-thioxo-1,3-dithiole-4,5-dithiolato), TCNQ and TTF. We obtained a series of novel complexes (ion-pair complexes and charge-transfer complexes) containing these planar molecules. The preparation, structures and magnetic properties for all complexes were investigated in details.The content includes three parts as follow:Part Ⅰ. In this part, we used the conjugated 2,2’,2"-terpyridine reacting with other conjugated molecules such as [M(mnt)2]- for getting complexes with π-π stacking interactions by in situ reaction, and then studying the magnetic mechanism. At first, we synthesized an ion-pair complex, [Mn(terpy)2][Cu(mnt)2] (1). The magnetic analysis shows 1 exhibits antiferromagnetic interaction. When the diamagnetic [Ru(terpy)2]2+ was selected as the cation, the isomorphic complex 2 was obtained. Contrarily, when the counteranion was changed into diamagnetic [Ni(mnt)2]2-, another isomorphic complex 3 was obtained. By comparing the magnetic properties of three complexes, we concluded that the antiferromagnetic interaction in complex 1 mainly results from the coupling between the anions. Furthermore, unexpectedly, we got complex 4, which showed a 1D antiferromagnetic chain.Part Ⅱ. In this part, TCNQ and TTF as the building units react with different coordination complexes. Finally, three complexes (ion-pair and charge-transfer complexes) were obtained. Particularly, complex 5 is an unpredictable production. We illustrated the possible reaction mechanism. Structural and magnetic investigations show that 5 exhibit one dimensional antiferromagnetic chain. TCNQ- anions in complex 6 undergo dimension, so 6 is open-shell diamagnetic. Complex 7 is a charge-transfer complex. The structure consists of alternating stacks of dimerised TTF+ cations and [Fe(bpy)(CN)4]-anions and they are linked together by short S···S contacts. Within the organic stacks, two dimerised TTF+cations are arranged in a slipped face-to-face mode with short intra-dimer and long inter-dimer S···S distances. Strong antiferromagnetic exchange was found in 7.Part Ⅲ. We used [M(mnt)2]2-as counteranions for tuning spin-crossover phenomenon of mononuclear MnⅢ. The reaction of [Mn(5-Br-sal-N-1,5,8,12)]C104 with TBA2[Ni(mnt)2], TBA2[Pt(mnt)2] and TBA[Ni(dmit)2] respectively lead to the formation of three ion-pair complexes. [Mn(5-Br-sal-N-1,5,8,12)]2[Ni(mnt)2] (8) and [Mn(5-Br-sal-N-1,5,8,12)]2[Pt(mnt)2] (9) are isomorphic and show axial compression of the octahedral coordination environment of MnⅢ ions. With the temperature increasing the equatorial metal-ligand bond lengths show significant elongation, but the axial bond lengths remain unchanged. Complex 10, [Mn(5-Br-sal-N-1,5,8,12)] [Ni(dmit)2]·CH3CN, contains π-π, p-π and H-bonds weak interactions. Magnetic investigation shows the spin-crossover phenomena for 8 and 9, and the T1/2 has been increased 230 K comparing with the reactant complex. However, no spin-crossover was observed in complex 10, and theoretical calculations show that there are weak antiferromagnetic couplings mediated through π-π interactions.