含有立體阻障吡啶-碳烯配位基之過渡金屬錯合物的合成,性質及催化研究

Synthesis, Properties and Catalysis of Transition Metal Complexes with Bulky Pyridine-Carbene Ligand

王朝鈺(Chao-yu Wang)

指導教授 ： 劉緒宗

國立臺灣大學/理學院/化學研究所/博士(2006年)

https://doi.org/10.6342/NTU.2006.00815

摘要

本篇研究以含有立體障礙取代基之吡啶-碳烯配位基之過渡金屬錯合物為主題，探討它們的配位化學性質與這類配位基系統對催化反應的影響。
以6-溴吡啶-2-甲醛(2)與2,4,6-三甲基苯基硼酸(3)進行Suzuki-Miyaura偶合反應，再進行官能基轉換及咪唑的取代反應，可以得到吡啶-咪唑鹽6a與6b。此鹽類與氧化銀作用生成碳烯-銀錯合物7a、7b。藉由X光單晶繞射分析，可以確認它們的化學結構，而兩錯合物均可做為後續碳烯轉移的起始試劑。
以[Rh(COD)Cl]2、[Ir(COD)Cl]2以及[Pd(C3H5)Cl]2分別與7a、7b反應可以得到單核的錯合物8、9、10，四氟硼酸銀與此系列錯合物反應，則得到吡啶-碳烯螯合的錯合物11、12、13。
螯合錯合物的吡啶配位，可以容易被σ-予體，如鹵離子、疊氮陰離子、膦試劑，置換；但是銠與銥金屬螫合物與π-酸試劑，如一氧化碳、異腈，則發生COD被置的產物14a與16；與三甲基亞磷酸酯反應時，COD及吡啶均會被取代而得到單芽的三(磷酸酯)銠-碳烯錯合物18；雙膦化合物則會取代銠金屬上全部的配位基。立體障礙的取代基阻礙了銠錯合物對鹵化烷類的氧化加成反應，造成11a對碘甲烷及碘化乙腈均無氧化加成活性；立障也造成膦試劑的配位強度下降，含膦錯合物17a與17b在空氣下，很快氧化生成膦氧化物與11a。常溫下11a對氫氣沒有活性，但較高溫下會分解生成起始吡啶-咪唑鹽。
單芽或雙芽的鈀-碳烯錯合物在鹼性條件下會生成鈀金屬的奈米顆粒，與鹽酸反應生成丙烯與咪唑陽離子；由於吡啶對鈀中心的配位較弱，三苯基膦的加入會發生置換反應，生成錯合物21a與21b，由於三苯基膦與吡啶-碳烯配位基的立體效應減慢交換速率，室溫下氫譜呈現兩物種的混合，其相對濃度比隨溫度變化而改變，平衡之ΔG 約為78 kJ/mol。
吡啶-咪唑配位基的立障與配位強度影響其錯合物的催化能力。銠錯合物11a對氫甲醯化反應有高活性；配合添加三苯基膦，支鏈醛的百分比亦增加。苯乙烯衍生物進行氫甲醯化反應結果較佳，脂肪烯類則在產率及選擇性的控制效果均不佳。在鄰炔基苯胺及鄰炔基酚的環化反應中，吡啶受取代基立障影響，配位強度變弱，使其易於解離而空出配位空間，使催化反應易於進行；但咪唑側取代基的立障增加時，中心金屬週圍的空間被遮蔽的程度也增加，會減低催化反應的活性。
碳烯強的α-予體性質使金屬中心電子密度增加，可以有效催化芳香硝化物的還原。藉由控制鹼溶液的濃度與催化劑濃度，來有效控制還原產物為苯胺(aniline)或偶氮二苯(azobenzene)。芳香環電子密度較高的硝基化合物較難被還原，反應後僅能得到苯胺為唯一產物；若芳香環電子密度較低，硝化物可以在較低鹼濃度或較低試劑濃度下還原為偶氮二苯。
鈀錯合物10及13對Suzuki-Miyaura偶合反應與Mizoroki-Heck反應有催化活性，但是若以氯芳類或具有立體障礙的溴芳類進行反應時效率不佳。

關鍵字

吡啶 ； 碳烯 ； 過渡金屬 ； 催化 ； 銠 ； 銥 ； 鈀

並列摘要

Coordination of pyridine-imidazole-2-ylidene ligands (pyN^C-R) [R = Me, Mes], processing bulky substituents, toward rhodium (I) and iridium (I) metal ions has been investigated. The complexes [(C-pyN^C-R)M(COD)Cl] (8, M=Rh; 9, M=Ir) were prepared via trans- metallation from corresponding silver carbene complexes. Upon chloride abstraction, the chelation of pyN^C-R becomes feasible and results the formation of [(C,N-pyN^C-R)M(COD)]BF4 (11a, M= Rh, R= Mes; 11b, M= Rh, R = Me; 12a, M= Ir, R= Mes; 12b, M= Ir, R = Me). Furthermore, the palladium (II) complex [(C-pyN^C-Mes)Pd(C3H5)Cl] (10) was prepared by a similar method. Treatment of complex 10 with AgBF4 afforded a chelation species [(C,N-pyN^C-Mes)Pd(C3H5)]BF4 (13).
The coordinated COD of 3 and 4 can be replaced by carbon monoxide to yield the corresponding dicarbonyl species [(C,N-pyN^C-R) M(CO)2]BF4 (14, M= Rh; 15, M= Ir). Ligand substitution of 11a by phosphines and azide causes the de-complexation of pyridinyl-nitrogen donor to generate the corresponding carbene rhodium complexes [(C-pyN^C-Mes)Rh(COD)PR3] (17a, R = Ph, 17b, R = Et) and [(C-pyN^C-R)Rh(COD)N3] (19), respectively, indicating the labile nature of steric-hindered pyridinyl-nitrogen donor. Three equivalent of trimethylphosphite, a π-acid donor, can replace both COD and pyridinyl-nitrogen donor to form complex [(C-pyN^C-R)Rh{P(OMe)3}3] BF4 (18, R = Mes), and excess of bisphosphines replace all ligands to give [Rh(P^P)2]BF4. Palladium complexe 13 decomposes via the reaction with KOtBu to form Pd nanoparticles. The π-allyl donor was decomposed into propene when complex 13 was treated with concentrated HCl.
Complexes 11a and 17a are active catalysts for the hydroformylation of styrene derivatives with a chemoselectivity favored the branched ones. Substituted anilines and phenols containing ortho-ethynyl groups undergo cyclization in the presence of carbene iridium complexes to yield the indole or benzofuran, respectively. Hydrogen transfer reductions of nitroarenes catalyzed by iridium complexes have been investigated. Complex 12b was found to be the best catalyst for reduction of nitroarenes. Meanwhile, the diazo-compounds can be obtained as the exclusive product by manipulation through changing the concentration of base and substrates. Suzuki-Miyaura and Mizoroki-Heck reactions catalyzed by complex 13 were also examined. The activity of 13 appears to be less active as compared to those of diaminocarbene Pd(II) complexes reported in the literatures.

並列關鍵字

pyridine ； carbene ； transition metal ； catalysis ； rhodium ； iridium ； palladium

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