**Gas-Dependent Reversible Structural and Magnetic Transformation between Two Ladder Compounds**

**Jun Manabe 1, Kazuki Nishida 1, Xiao Zhang 1, Yuki Nakano 1, Masaru Fujibayashi 1,2, Goulven Cosquer 1,2, Katsuya Inoue 1,2,3, Seiya Shimono 4, Hiroki Ishibashi 5, Yoshiki Kubota 5, Misaki Shiga 6, Ryo Tsunashima 6, Yoko Tatewaki <sup>7</sup> and Sadafumi Nishihara 1,2,3,8,\***


Received: 26 August 2020; Accepted: 17 September 2020; Published: 19 September 2020

**Abstract:** We report reversible structural transformation that occurs in two ladder compounds: Cu2CO3(ClO4)2(NH3)6 (**1**) and Cu2CO3(ClO4)2(NH3)5(H2O) (**2**), when they are exposed to gaseous vapors. The ladder structures of both **1** and **2** consist of two Cu2<sup>+</sup> ions and one CO3 <sup>2</sup><sup>−</sup> ion. In **1**, the Cu2<sup>+</sup> ions are coordinated by three NH3 molecules on each side, while those in **2** are coordinated by three NH3 molecules on one side, and two NH3 molecules and one H2O molecule on the other side. We demonstrated reversible transformation of **1** and **2** via the exposure of **1** to H2O vapor and the exposure of **2** to NH3 vapor using a simple bench-scale method. The minor structural change observed led to a significant difference in physical properties, which we observed using several methods.

**Keywords:** magnetic property; reversible transformation; spin ladder
