Much attention has been devoted to increase dimensionality to one, two- and/or three-dimensional network complex systems with the aim of understanding the mechanism of magnetic interactions and enhancing bulk magnetic properties. In this connection, the bridging ligands which can mediate magnetic couplings between paramagnetic metal centers have played a crucial role in improvement of molecular structure. Our research is directed toward the use of three kinds of bridging ligands such as bpe (=1,2-bis(4-pyridyl)ethane), tp (=terephthalate) and azide. The treatment of Mn(II) ion with bpe ligand and dipotassium tp produced a novel three-dimensional complex, $[Mn(bpe)(H_{2}O)_{4}]_{n}(tp)_{n}(bpe)_{2n}$ ($bf{1}) whose solid-state structure is spread out via hydrogen bonds of packing bpe ligands and tp counter dianion. The use of sodium azide as another bridge instead of tp caused two dimensional sheet, $[Mn(bpe)(N_{3})_{2}]_{n}$ ($bf{4}) in which the azide group covalently links Mn(bpe) chain via end-to-end mode.
A useful way to fabricate higher dimensional compounds is to use both one-dimensional polymers and additional bridging ligands. From this background a precursor compound was prepared by the reaction of Mn(II) ion with bpe (=1,2-bis(4-pyridyl)ethane) ligand, giving a one-dimensional bpe-bridged complex, $[Mn(bpe)(H_{2}O)_{4}]_{n}(ClO_{4})_{2n}(bpe)_{4n}(H_{2}O)_{n}$ ($bf{2}$) which has four water molecules around Mn(II) ion. To improve the molecular dimensionality, the treatment of this one-dimensional precursor with another bridging ligand, tp (=terephthalate) was attempted, affording a novel three-dimensional network, $[Mn(bpe)_{1.5}(tp)(H_{2}O)]_{n}(H_{2}O)_{n}$ ($bf{3}$) whose molecular structure reveals that tp ligands with two kinds of binding modes of unidentate and bridging form two-dimensional sheets which are linked via bpe ligands, leading to three-dimensional network.
In order to explore the conformational availability of the long and flexible bpe ligand further, we attempted to devise new systems by adopting charged capping ligand, SCN-. The comparison with this bpe systems was made by applying 4,4-dipyridyl (=dipy) with the short distance between nitrogen atoms and trans-1,2-bis(4-pyridyl)ethylene (=tbp) with a structural rigidity. Herein we present syntheses, structures and magnetic behaviors of a 1D chain compound, $[Mn(bpe)_2(SCN)_2]_n$ ($bf{5$), and a 2D sheet compound, $[Mn(bpe)(H_2O)_2(SCN)_2]_n(bpe)_n(H_2O)_n$ ($bf{6}$), and a 2D compound, $[Mn(dipy)(H_2O)_2(SCN)_2]_n(dipy)_n$ ($bf{7}$), and a mononuclear compound, $[Mn(tbp)_2(H_2O)_2(SCN)_2]$ ($bf{8}$), and $[Co(bpe)_3(H_2O)_2]_n(ClO_4)_{2n}(bpe)_{0.5n}(H_2O)_n$ ($bf{9}$).
The magnetic behavior of low-dimensional compounds has been investigated actively with the aim of understanding magnetic mechanism and magnetostructural relationships. In order to extend this study, the reactivity of the copper(II) dimer, $[(tmen)_2Cu_2(ox)(H_2O)_2](ClO_4)_2$, which can be used as building block by replacing labile water molecules with bridging ligands such as azide and dicarboxylates showing versatile bridging modes has been explored in our group. The reaction of the copper precursor with sodium azide in aqueous solution yielded a novel hexameric compound, $[Cu_6(tmen)_6(ox)_3(N_3)_2(H_2O)_2](ClO_4)_4(H_2O)_2$ ($bf{12a}$), which shows alternating antiferromagnetic systems through oxalato and azido ligands. The 1:1 reaction of the copper precursor with dipotassium terephthalate $(= K_2tp)$ in an aqueous solution afforded zigzag one-dimensional products, $[Cu(tmen)(ox)]_n(H_2O)_{4n}$ ($bf{10}$) and $[Cu(tmen)(tp)(H_2O)_2]_n$ ($bf{11}$), both of which show ferromagnetic interactions. To compare magnetic natures, linear one-dimensional compound $[Cu(pyrazole)(tp)]_n$ ($bf{13}$) was prepared, demonstrating the moderate antiferromagnetic interaction through tp bridge.
The study on the tp compounds was undertaken and focused on the copper systems, but the systems involving the other paramagnetic centers are limited up to date. We attempted to proceed the exploration of the tp system including nickel(II) ions, and herein report the syntheses, structures, and magnetic behaviors of a dinuclear compounds $[Ni_2(pmdtn)_2(tp)(H_2O)_2](ClO_4)_2(H_2O)_4$ ($bf{15}$), $[Ni_2(pmdtn)_2(tp)(N_3)_2](H_2O)_4$ ($bf{16}$) and a 1D chain compound $[Ni(pyrazole)_4(tp)]_n$ ($bf{17}$). Also, the synthesis and characterization of compound $[Ni_2(pmdtn)_2(ox)(H_2O)_2](ClO_4)_2(H_2O)_2$ ($bf{14}$) is presented
By employing the versatile coordination ability of the terephthalate (=tp) ligand, three-dimensional manganese(II) complex $[Mn(tp)(5-methylpyrazole)_2]_n$ ($bf{18}$), and one-dimensional complexes $[Mn(tp)(4-methylpyrazole)_3(H_2O)]_n$ ($bf{19}$), $[Mn(tp)(4-methylpyrazole)_4]_n$ ($bf{20}$) were synthesized and characterized. The structure of $bf{18}$ shows that the terephthalate group acts as a tetrakis(monodetate) ligand which is bound on each side to two Mn(II) ions through the carboxylato oxygens in a syn-syn conformation yielding three-dimensional network. The crystal structures of $bf{19}$ and $bf{20}$ reveal that the Mn(II) ions are bridged sequentially by tp dianions in a bis(monodentate) fashion. The magnetic behavior of three compounds, measured in the range from 300 K to 1.8 K, was explained by a modified chain model, showing the presence of the weak antiferromagnetic interactions through tp. The azido ligand has been used as a good building motif to form discrete polynuclear complexes in either end-to-end (antiferromagnetic interaction) or end-on (ferromagnetic interaction) mode. The reaction of Ni(II) ion with four equiv of 3(5)-methylpyrazole and two equiv of sodium azide yielded a monomeric compound, $[Ni(5-methylpyrazole)_4(N_3)_2]$ ($bf{23}$) with end-on mode while the use of one equiv of sodium azide instead of two gave one-dimensional chain compound, $[M(5-methylpyrazole)_4(N_3)]_n(H_2O)_n(ClO_4)_n$ [M = Ni ($bf{22a}$), Co ($bf{24a}$)], exhibiting a ferromagnetic interaction which are the first examples of single bridged, end-to-end type azido system. From the field-cooled magnetization (FCM), the critical temperature was measured around 1.8 K or slightly lower and further confirmed by field dependence of the compound. The dehydrated compounds of $bf{22a}$ and $bf{24a}$ act as magnets at 2.3 K for $bf{22b}$ and at 2 K for $bf{24b}$.
Crystal engineering in high-dimensional systems is the important aspect of solid state supramolecular chemistry leading to the construction of extended inorganic network. In this respect, copper(II) ion surrounded by tridentate ligands and ferrocyanide with multi-binding ability are employed to give two-dimensional compounds $[Fe(CN)_6{Cu(L)}_3]_n(X)_{2n}(H_2O)_{pn}$ (L = tridentate ligands; $X = (ClO_4)^-, (CF_3SO_3)^-, (PF_6)^-, (BF_4)^-; p = 2 - 4)$ ($bf{25}$, $bf{26}$). In the case of $(ClO_4)^-,$ the molecular structure indicates that the coordination sphere around ferrocyanide consists of six copper ons. The paramagnetic copper(II) ions display blended magnetic behaviors of antiferromagnetic and ferromagentic couplings via diamagnetic ferrocyanide bridge. Since the variation of capping ligands led to new systems with a structural big change, attempts to employ different capping ligands were made. In this line, a pentanuclear compound $[Fe(CN)_6{Cu(dmen)_2}4](ClO_4)_4$ ($bf{27}$) (dmen = N,N-dimethylethylenediamine) was synthesized. The blended magnetism is present in $bf{27}$.
A useful synthetic strategy to construct the magnetic ordering systems with higher dimensionality is to use a precursor as a building block. The requirement of the building precursor is that there are binding sites more than two. In this line, we prepared many precursor compounds which have labile leaving groups like water molecules and the potential of coordination to the other metal ion. Herein we report the syntheses, characterization, and structures of those compounds, listed as follows: $[Ni(pyrazole)_4(N_3)_2] ($bf{28}$), $[Ni(pyrazole)_4(SCN)_2]$ ($bf{29}$), $[Mn(5-methylpyrazole)_4(N_3)_2]$ ($bf{30}$), $[Co(4-methylpyrimidine)_2(SCN)_2(H_2O)_2]$ ($bf{31}$), $[Cu(bpm)_2(H_2O)](ClO_4)_2(H_2O)_2$ ($bf{32}$), $[Cu(bpme)_2(H_2O)_2](ClO_4)_2(H_2O)_2$ ($bf{33}$). $[Co(bpm)_2(H_2O)_2](ClO_4)_2(H_2O)_2$ ($bf{34}$), and $[Mn(bpm)_2(H_2O)_2](ClO_4)_2(H_2O)_2$ ($bf{35}$), where bpm stands for 2,2-bipyrimidine and bpme, bis(2-pyrimidine)ether.
자기적 상호작용의 메카니즘 이해와 포괄적인 자성을 도모하기위해 일차원, 이차원 및 삼차원 화합물에 대한 연구가 진행되어 왔다. 상자기적 금속간 자기적 상호작용을 결정하는 다리리간드는 이와 같은 맥락에서 중요하다. 본 연구는 bpe (=1,2-bis(4-pyridyl)ethane), tp (=terephthalate) 및 azide 리간드를 이용한다. 밍간 이온을 bpe와 tp 존재하에서 반응시키면 삼차원 화합물 $[Mn(bpe)(H_2O)_4]_n(tp)_n(bpe)_{2n}$ ($bf{1}$)이 생성된다. tp대신 azide를 적용하면 이차원 화합물 $[Mn(bpe)(N_3)_2]_n$ ($bf{4}$)이 얻어진다.
고차원성 화합물을 제조하기위한 유용한 방법은 이차원 고분자와 다리리간드를 이용하는 것이다. 이런 배경에서 망간 이온과 bpe리간드를 반응시켜 합성된 일차원 화합물 $[Mn(bpe)(H_2O)_4]_n(ClO_4)_{2n}(bpe)_{4n}(H_2O)_n$ ($bf{2}$)이 또 하나의 다리리간드인 tp로 처리되면 공유결합에 의한 삼차원 화합물 $[Mn(bpe)_{1.5}(tp)(H_2O)]_n(H_2O)_n$ ($bf{3}$)이 생성된다.
bpe 다리리간드의 유연성을 4,4-dipyridyl (=dipy)와 trans-1,2-bis(4-pyridyl)ethylene (=tbp)과의 비교실험을 통해 탐구하였다. bpe리간드의 경우 그 유연성으로 인해 두 종류의 화합물$[Mn(bpe)_2(SCN)_2]_n$ ($bf{5}$)와 $[Mn(bpe)(H_2O)_2(SCN)_2]_n(bpe)_n(H_2O)_n$($bf{6}$)이 생성되었다. 금속 이온이 코발트인 경우는 다양한 bpe형태이성질체가 동시에 관찰되었다. dipy는 한 종류의 생성물이 형성된 반면 tbp는 구조적 제약으로 인해 단핵 화합물이 안정적으로 분리되었다.
저차원 화합물의 자기적 거동은 자기적 메카니즘과 자성-구조 상관성을 이해하려는 목적으로 연구되어 왔다. 이런 관점에서 구리 선구 물질 $[(tmen)_2Cu_2(ox)(H_2O)_2](ClO_4)_2$ 을 합성하고 tp와 반응시키면 zigzag 일차원 화합물인 $[Cu(tmen)(ox)]_n(H_2O)_{4n}$ ($bf{10}$)와 $[Cu(tmen)(tp)(H_2O)_2]_n$ ($bf{11}$)이 생성되었다. 두 화합물은 강자기적 상호작용을 나타내었다. 구리 선구 물질에 azide를 처리하면 육핵 화합물$[Cu_6(tmen)_6(ox)_3(N_3)_2(H_2O)_2](ClO_4)_4(H_2O)_2$ ($bf{12a}$)이 안정적으로 합성되었다. 구조에 따른 자성의 변화를 조사하기위해 선형 일차원 화합물 $[Cu(pyrazole)(tp)]_n$ ($bf{13a}$)를 합성하였고 $bf{11}$과는 달리 반강자기적 상호작용을 보였다.
tp 화합물에 대한 연구는 주로 구리계에 집중되어 있었다. 따라서 본 연구에서는 니켈을 함유하는 반응계를 도입하여 두 종류의 이핵 화합물$[Ni_2(pmdtn)_2(tp)(H_2O)_2](ClO_4)_2(H_2O)_4$ ($bf{15}$)와 $[Ni_2(pmdtn)_2(tp)(N_3)_2](H_2O)_4$ ($bf{16}$)을 합성하여 자기적 성질을 비교하였다. 킬레이트 능력이 없는 모자리간드를 적용하여 일차원 화합물 $[Ni(pyrazole)_4(tp)]_n$ ($bf{17}$)도 얻을 수 있었다.
tp 리간드의 다양한 배위 능력을 이용하여 망간계에 대한 연구를 진행하였다. 이때 모자리간드의 미묘한 변화가 구조에 미치는 영향을 주목하여 탐색하였다. 3(5)-methylpyrazole의 경우 삼차원 화합물 $[Mn(tp)(5-methylpyrazole)_2]_n$ ($bf{18}$)이 얻어지는 반면 같은 실험 조건에서 4-methylpyrazole을 이용하면 일차원 화합물 $[Mn(tp)(4-methylpyrazole)_3(H_2O)]_n$ ($bf{19}$)과 $[Mn(tp)(4-methylpyrazole)_4]_n$ ($bf{20}$)이 생성된다. 위의 화합물들은 tp리간드를 경유하여 모두 약한 반강자기적 상호작용을 하였다.
azide리간드는 end-to-end 양식으로 결합하면 반강자기적으로 배열하고 end-on 결합 양식에서는 강자기적 상호작용을 한다고 보고되었다. 그러나 본 연구에서 비킬레이트 리간드인 3(5)-methylpyrazole를 이용하여 얻은 일차원 화합물 $[M(5-methylpyrazole)_4(N_3)]_n(H_2O)_n(ClO_4)_n$ [M = Ni ($bf{22a}$), Co ($bf{24a}$)]은 놀랍게도 강자기적으로 배열됨을 관찰하였다. 그 이유는 azide에 연결된 금속이온간의 큰 비틀림 각이 존재하기 때문인 것 같다. 탈수과정을 통해 일차원 사슬간의 결정수를 제거하면 니켈의 경우 2.3 K, 코발트의 경우 2 K에서 자발적 자화를 보였다.
확장된 무기 그물망 구조를 갖는 계를 만들기 위해 본 연구에서는 ferricyanide 와 ferrocyanide를 이용하였다. 세자리 결합 모자리간드를 적용할 경우 벌집 모양의 구조를 갖는 이차원 화합물 $[Fe(CN)_6{Cu(L)}_3]_n(X)_(2n)(H_2O)_(pn)$ (L = tridentate ligands; $X = (ClO_4)^-, (CF_3SO_3)^-, (PF_6)^-, (BF_4)^-; p = 2 - 4)$ ($bf{25}$, $bf{26}$)를 얻을 수 있었다. 두자리 결합의 경우 오핵 화합물 $[Fe(CN)_6{Cu(dmen)_2}4](ClO_4)_4$($bf{27}$)이 생성되었다. ferricyanide를 사용하면 수용액내에서 환원되어 ferrocyanide가 되는 것을 관찰하였다. 산자기적 구리 이온들은 반자기적 리간드ferrocyanide를 통해 강자기적 및 반강자기적 상호작용을 하였다.
고차원성 물질을 얻기 위한 선구 물질로서 좋은 이탈기를 함유하거나 또 다른 금속에 결합할 수 있는 여러 가지 일핵 화합물을 합성하고 특성을 규명하였다. 그 예는 다음과 같다. $[Ni(pyrazole)_4(N_3)_2]$ ($bf{28}$), $[Ni(pyrazole)_4(SCN)_2]$ ($bf{29}$), $[Mn(5-methylpyrazole)_4(N_3)_2]$ ($bf{30}$), $[Co(4-methylpyrimidine)_2(SCN)_2(H_2O)_2]$ ($bf{31}$), $[Cu(bpm)_2(H_2O)](ClO_4)_2(H_2O)_2$ ($bf{32}$), $[Cu(bpme)_2(H_2O)_2](ClO_4)_2(H_2O)_2$ ($bf{33}$). $[Co(bpm)_2(H_2O)_2](ClO_4)_2(H_2O)_2$ ($bf{34}$), 그리고 $[Mn(bpm)_2(H_2O)_2](ClO_4)_2(H_2O)_2$ ($bf{35}$).
