I^-/I₃ ^- Redox-Assisted Synthesis and Properties of Low Dimensional, Mixed-Valent Gold Iodide Perovskite Derivatives

We report a set of three new mixed-valent Au-I Au-III iodides: (ClPy)(3)[AuI2](2)[AuI4] [1], (BrPy)(3)[AuI2](2)[AuI4] [2], and (ClPy)(2)[AuI2][AuI4] [3], as well as three new monovalent Au-III iodides: (XPy)(2)[AuI4][I-3] (Py = 4-X-pyridinium X = Cl, Br, and I) [4-6]. Two of these mixed-valent compounds (1 and 2) incorporate both monovalent Au-IAu-I (aurophilic bonding) and mixed-valent (AuI)-I-I(AuI)-I-III couples (Au-I halogen bonding), to the best of our knowledge an unprecedented structural feature. These same two mixed-valent compounds also exhibit a rare low-dimensional molecular architecture with respect to second sphere AuI interactions, namely, 1D chains of AuI interactions, extending along a single crystallographic axis. All compounds were synthesized with the assistance of the I-/I-3(-) redox couple and tacit manipulation of the polyiodide content during synthesis. Air-free synthesis was used to influence the redox process of I-3(-)/I-, resulting in better selection for mixed-valent products. Compounds 1-2 and 4-5 exhibit a characteristically narrow bandgap (1.04-1.25 eV), as measured via diffuse reflectance spectroscopy (DRS). Computational analyses were used to rationalize the specific assembly modes of [AuI2](-) and [AuI4](-) species, and they show that the Au-II interaction type is favored over the Au-IIII.