η²-SO₂ Linkage Photoisomer of an Osmium Coordination Complex

Abstract

We report the discovery of an η²-SO₂ linkage photoisomer in the osmium pentaammine coordination complex, [Os(NH₃)₅(SO₂)][Os(NH₃)₅(HSO₃)]Cl₄ (1). Its dark- and light-induced crystal structures are determined via synchrotron X-ray crystallography, at 100 K, where the photoinduced state is metastable in a single crystal that has been stimulated by 505 nm light for 2.5 h. The SO₂ photoisomer in the [Os(NH₃)₅(SO₂)]²⁺ cation contrasts starkly with the photoinactivity of the HSO₃ ligand in its companion [Os(NH₃)₅(HSO₃)]⁺ cation within the crystallographic asymmetric unit of this single crystal. Panchromatic optical absorption characteristics of this single crystal are revealed in both dark- and light-induced states, using concerted absorption spectroscopy and optical microscopy. Its absorption halves across most of its visible spectrum, upon exposure to 505 nm light. The SO₂ ligand seems to be responsible for this photoinduced bleaching effect, judging from a comparison of the dark- and light-induced crystal structures of 1. The SO₂ photoisomerism is found to be thermally reversible, and so 1 presents a rare example of an osmium-based solid-state optical switch. Such switching in an osmium complex is significant because bottom-row transition metals stand to offer linkage photoisomerism with the greatest photoconversion levels and thermal stability. The demonstration of η²-SO₂ bonding in this complex also represents a fundamental contribution to osmium coordination chemistry.