Iridium Dihydroxybipyridine Complexes Show That Ligand Deprotonation Dramatically Speeds Rates of Catalytic Water Oxidation

We report highly active iridium precatalysts, [Cp*Ir­(N,N)­Cl]­Cl (1–4), for water oxidation that are supported by recently designed dihydroxybipyridine (dhbp) ligands. These ligands can readily be deprotonated in situ to alter the electronic properties at the metal; thus, these catalyst precursors have switchable properties that are pH-dependent. The pKa values in water of the iridium complexes are 4.6(1) and 4.4(2) with (N,N) = 6,6′-dhbp and 4,4′-dhbp, respectively, as measured by UV–vis spectroscopy. For homogeneous water oxidation catalysis, the sacrificial oxidant NaIO4 was found to be superior (relative to CAN) and allowed for catalysis to occur at higher pH values. With NaIO4 as the oxidant at pH 5.6, water oxidation occurred most rapidly with (N,N) = 4,4′-dhbp, and activity decreased in the order 4,4′-dhbp (3) > 6,6′-dhbp (2) ≫ 4,4′-dimethoxybipyridine (4) > bipy (1). Furthermore, initial rate studies at pH 3–6 showed that the rate enhancement with dhbp complexes at high pH is due to ligand deprotonation rather than the pH alone accelerating water oxidation. Thus, the protic groups in dhbp improve the catalytic activity by tuning the complexes’ electronic properties upon deprotonation. Mechanistic studies show that the rate law is first-order in an iridium precatalyst, and dynamic light scattering studies indicate that catalysis appears to be homogeneous. It appears that a higher pH facilitates oxidation of precatalysts 2 and 3 and their [B­(ArF)4]− salt analogues 5 and 6. Both 2 and 5 were crystallographically characterized.

本工作报道了一系列用于水氧化反应的高活性铱预催化剂[Cp*Ir(N,N)Cl]Cl（1~4），其配体为新近设计的二羟基联吡啶（dihydroxybipyridine，dhbp）。该类配体可在原位轻松发生去质子化，从而调控金属中心的电子性质，因此此类催化剂前驱体具备pH依赖的可调变特性。通过紫外-可见（UV–vis）光谱表征测得，当(N,N)分别为6,6′-二羟基联吡啶（6,6′-dhbp）和4,4′-二羟基联吡啶（4,4′-dhbp）时，该铱配合物在水中的pKa值分别为4.6(1)和4.4(2)。在均相水氧化催化体系中，牺牲氧化剂高碘酸钠（NaIO4）的催化效果优于硝酸铈铵（CAN），且可使催化反应在更高pH条件下进行。在pH 5.6且以NaIO4为氧化剂的条件下，当(N,N)为4,4′-dhbp时水氧化反应速率最快，催化活性按如下顺序递减：4,4′-dhbp（3）> 6,6′-dhbp（2）≫ 4,4′-二甲氧基联吡啶（4）> 联吡啶（bipy，1）。此外，在pH 3~6范围内的初始速率研究表明，dhbp配合物在高pH条件下的速率提升源于配体的去质子化，而非单纯的pH升高加速水氧化反应。因此，dhbp配体中的质子性基团可通过去质子化调控配合物的电子性质，从而提升催化活性。机理研究表明，该反应的速率方程对铱预催化剂呈一级动力学；动态光散射（dynamic light scattering）研究则显示，催化过程为均相反应。更高的pH环境可促进预催化剂2、3及其对应[B(ArF)4]−盐类似物5、6的氧化过程。配合物2与5均经单晶X射线衍射表征。