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

Abstract

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(Ar(F))4](-) salt analogues 5 and 6. Both 2 and 5 were crystallographically characterized.

Studies of the Pathways Open to Copper Water Oxidation Catalysts Containing Proximal Hydroxy Groups During Basic Electrocatalysis

“…Similarly, our studies of Ru II and Ir III complexes of 6,6′-dhbp showed pK a values of 5−7.27 and 4.6, respectively, 18,21,23 for removal of ligand protons which are typically more acidic than protons of a metal-bound aqua ligand. 21,22 The last pK a value (8.30) is tentatively assigned to the removal of 2 protons (Scheme 1, 1c goes to 1e, presumably via 1d). This value is higher or similar to previously observed values for Ru II complexes, 22,23 and this makes sense given the 2− charge associated with complete deprotonation of complex 1 to form 1e.…”

Ligand Exchange and Redox Processes in Iridium Triazolylidene Complexes Relevant to Catalytic Water Oxidation

“…[29][30][31][32] In particular iridium cyclopentadienyl complexes [Ir(Cp*)(L,L)X] + containing a chelating N,N-, C,N-, or C,Cbidentate ligand motive afforded high catalytic activity (Cp* = = C 5 Me 5 -). [7][8][9]17,19,33,34 Postreaction analyses strongly indicate oxidative degradation of the Cp* ligand 27,32,33 rather than formation of IrO x nanoparticles as a heterogeneous catalyst phase. These reactivity pattern underline the relevance of initial steps of transformation at the iridium center as a concept to understand and improve catalytic activity and to reduce the catalyst activation barrier.…”

Ruthenium (II) and iridium (III) complexes of N-heterocyclic carbene and pyridinol derived bidentate chelates: Synthesis, characterization, and reactivity

“…[44–46] Frequently, the deprotonated metal complexes of 6,6′-dhbp have an enhanced ability to catalyze various reactions including transfer hydrogenation and water oxidation. [44, 47] Ruthenium complexes of 6,6′-dhbp can allow for cancer selective cytotoxicity by low pH triggered photo-dissociation in acidic regions of cancer cells. [48] Thus, in some cases, 6,6’-dhbp ligand can be labile especially when bound to a first row transition metal (e.g.…”