Isolation and crystal structure of a water-soluble iridium hydride: A robust and highly active catalyst for acid-catalyzed transfer hydrogenations of carbonyl compounds in acidic media
Isolation and crystal structure of a water-soluble iridium hydride: A robust and highly active catalyst for acid-catalyzed transfer hydrogenations of carbonyl compounds in acidic media
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DOI：
10.1021/ja0288237
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发表时间：
2003-04-09
影响因子：
15
通讯作者：
Fukuzumi, S
中科院分区：
文献类型：
--
作者：
Abura, T;Ogo, S;Fukuzumi, S
本文报道了一种水溶性氢化物配合物[Cp*Ir-III（bpy）H]（+）（1，Cp* = eta（5）-C5 Me 5，bpy = 2，2 '-bipyridine）的分离和结构测定。催化剂1是由预催化剂[Cp*Ir-III（bpy）（OH 2）]（2+）（2）与氢供体HCOOX（X = H或Na）在H2O中在受控条件（2.0 < pH < 6.0，25 ℃）下反应合成的，所述受控条件避免了1的配位体在低于约pH的情况下质子化。1.0以及在pH约为2以上的水配体的去质子化。6.0（2的pK（a）值= 6.6）。X射线衍射分析表明，配合物1为扭曲的八面体构型，Ir原子与一个η（5）-Cp*、一个双齿bpy和一个占据键位的末端双配位体配位。1的分离使我们能够研究1在酸性介质中的稳健能力和1在化学计量和催化条件下与羰基化合物反应的还原能力。酸催化的转移氢化的速率极大地依赖于溶液的pH、反应温度和HCOOH的浓度。pH对转移氢化速率的影响通过1的pH依赖性形成和质子对羰基化合物的活化过程来合理化。在pH 2.0-3.0的酸催化的转移氢化的高周转率归因于不仅对质子活化的羰基1的亲核性，但也对1的质子特征的C2 H2O配体，抑制C2 H2O配体的质子化。
This paper reports the isolation and structural determination of a water-soluble hydride complex [Cp*Ir-III(bpy)H](+) (1, Cp* = eta(5)-C5Me5, bpy = 2,2'-bipyridine) that serves as a robust and highly active catalyst for acid-catalyzed transfer hydrogenations of carbonyl compounds at pH 2.0-3.0 at 70 degreesC. The catalyst 1 was synthesized from the reaction of a precatalyst [Cp*Ir-III (bpy) (OH2)](2+) (2) with hydrogen donors HCOOX (X = H or Na) in H2O under controlled conditions (2.0 < pH < 6.0, 25 degreesC) which avoid protonation of the hydrido ligand of 1 below pH ca. 1.0 and deprotonation of the aqua ligand of 2 above pH ca. 6.0 (pK(a) value of 2 = 6.6). X-ray analysis shows that complex 1 adopts a distorted octahedral geometry with the Ir atom coordinated by one eta(5)-Cp*, one bidentate bpy, and one terminal hydrido ligand that occupies a bond position. The isolation of 1 allowed us to investigate the robust ability of 1 in acidic media and reducing ability of 1 in the reaction with carbonyl compounds under both stoichiometric and catalytic conditions. The rate of the acid-catalyzed transfer hydrogenation is drastically dependent on pH of the solution, reaction temperature, and concentration of HCOOH. The effect of pH on the rate of the transfer hydrogenation is rationalized by the pH-dependent formation of 1 and activation process of the carbonyl compounds by protons. High turnover frequencies of the acid-catalyzed transfer hydrogenations at pH 2.0-3.0 are ascribed not only to nucleophilicity of 1 toward the carbonyl groups activated by protons but also to a protonic character of the hydrido ligand of 1 that inhibits the protonation of the hydrido ligand.