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.