具異原子之三牙基及其過渡金屬錯合物之合成、結構鑑定 與醇氧化催化反應之研究

Synthesis, Structural Characterization and Catalytic Alcohol Oxidation Reactions of Transition Metal Complexes with Tridentate Ligands Containing Hetero-Functional Donor Groups

李虹儀(Horng Yi Li)

指導教授 ： 陳竹亭

國立臺灣大學/理學院/化學研究所/博士(2007年)

https://doi.org/10.6342/NTU.2007.02812

摘要

本論文主題為探討具有弱配位能力之異原子(如硫或是氧等原子)三牙配位基，其合成方式、與過渡金屬間的配位化學及所合成之配位化合物於醇氧化催化反應之研究。
由於本篇實驗所使用之氮氮硫或是氮氮氧的三牙配位基具有一定程度的不穩定性(labile)，以至當此類型配位基與不同過渡金屬錯合時，隨著使用之金屬鹽起始物的不同而展現多元的幾何配位結果。金屬鹽原有之陰離子與三牙基之間相互競爭，因此所生成之錯合物的幾何結構隨著陰離子配位能力的不同而有所改變。論文中利用幾種常見之金屬鹽起始物，並利用模版效應(template effect)與配位基起始物一起反應，如此，可解決配位基本身無法反應完全之合成問題並且簡化錯合物之純化過程。
於論文中，數種不同幾何型態的晶體皆被解析出來並且比較其不同。由晶體結構上可以得知，此類配位基可取代極弱之配位基(如過氯酸根(perchlorate))形成雙三牙配基螯合之穩定結構， 但無法擠下稍具配位能力之陰離子(如硝酸根(nitrate))，僅能形成單三牙配基與陰離子共同配位之金屬錯合物，若是配位能力稍強之陰離子(如醋酸根(acetate))，則無法得到錯合物。
另外，於磁性探討時發現，當配位基上的氮原子由砒啶(pyridine)換為喹啉(quinoline)時，其電子組態由低自旋組態轉為高自旋組態，由此顯示，換為喹啉取代基之配位基相對而言為弱場配基(weak filed ligand)。而由電化學的資料亦可加以佐證，含有砒啶取代基之鈷錯合物1b’，其氧化電位相對較低，較容易氧化，反之，含有喹啉取代基之鈷錯合物1b則顯示高氧化電位(~1 V)，此結論與此類化合物於空氣中呈現高穩定性之性質亦相吻合。
本篇所合成之鈷錯合物更進一歩進行醇氧化之催化反應研究，結果發現所合成之鈷金屬錯合物須於高當量(~17 當量)之過氧化氫或是TBHP(tert-butyl hydrogen peroxide)的存在下，且反應溫度必須高於攝氏60度才能達到較為理想的活性，而於此條件下，所得到的產物以酸為多數，選擇性略低。同時亦做了其它的反應變因，結果發現溫度與氧化劑的當量在兩種不同氧化劑系統裡，對活性及選擇性的影響呈現截然不同的效果，推測可能因為反應途徑不同所造成。

關鍵字

鈷 ； 異原子 ； 硫醚 ； 三牙基 ； 醇氧化催化

並列摘要

The synthesis, structural characterization, and catalytic oxidation reactions of mononuclear transition metal compounds ligated by hetero-functional donor groups are reported in this thesis. A series of divalent metal complexes bearing with Schiff-base ligand L1 were prepared in one-pot synthesis with mixing carboxaldehyde, thioether/ether and metal salts in methanol. And the Schiff-base ligand (L1) serves as a tridentate ligand with their NN’X (X = S, O) sets donors. Their structures are characterized by FAB-MS, UV-vis, IR, and X-ray crystallography.

In this system, it is interesting to find that different metal salts may lead to different geometry for synthesized complexes. In the case of Co(ClO4)2 6H2O, a six-coordinate complex containing two identical tridentate ligands in the form of [(L1)2CoII](ClO4)2 is obtained. The reactions using the nitrate or chloride salt yield the complexes containing only one tridentate ligand. In a similar reaction using hydrated zinc triflate, two zinc complexes with or without coordinating water are acquired. Although the chelation effect of the multidentates is considered to the dominant for their complexes, the thioether and imine donors of the studied ligands have been found relatively labile.
Magnetic studies from SQUID find that ligand L1 bearing quinoline group reveals a weak field characteristic. And for this reason, all of the bond lengths are comparably longer than that in pyridine analogue. Also all of the metal complexes with ligand L1 bearing quinoline groups show high-spin configuration and air resistance.
In order to further investigate the extent of stabilization of CoII/CoIII state toward oxidation, cyclic voltammetric studies were performed. Each complex exhibits a quasi-reversible cyclic voltammetric response in CH2Cl2, corresponding to the CoII/CoIII redox process. The E1/2 value of complex 1a-1c [(L1)2CoII](ClO4)2 (X = S, R1=Me, Et, Bn,) is significantly higher (~1.0 V) than that for six-coordinate complexes of pyridine ligand appending thioether donor. These studies reveal that incorporation of quinoline group will enhance the lability of ligand, but increase the stability toward oxidation.
Complex 1 and 2 show moderate catalytic activity toward alcohol oxidation in the presence of 17 equivalent of hydrogen peroxide. And the activity will increase dramatically when the temperature is over 60 ℃. Reaction conditions are also varied, but limited changes of activity and aldehyde selectivity were observed.

並列關鍵字

Cobalt ； Hetero-functional donor ； Thioether ； Tridentate ； Catalytic alcohol oxidation

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