Engineering of the XY Magnetic Layered System with Adeninium Cations: Monocrystalline Angle-Resolved Studies of Nonlinear Magnetic Susceptibility

An original example of modular crystal engineering involving molecular magnetic {Cu^II[W^V(CN)₈]}⁻ bilayers and adeninium cations (AdeH⁺) toward the new layered molecular magnet (AdeH)­{Cu^II[W^V(CN)₈]}·2H₂O (1) is presented. 1 crystallizes within the monoclinic C2 space group (a = 41.3174(12), b = 7.0727(3), c = 7.3180(2) Å, β = 93.119(3)°, and V = 2135 ų). The bilayer topology is based on a stereochemical matching between the square pyramidal shape of Cu^II moiety and the bicapped trigonal prismatic shape of [W^V(μ-CN)₅(CN)₃], and the separation between bilayers is significantly increased (by ∼50%; from ca. 9.5 to ca. 14.5 Å) compared to several former analogues in this family. This was achieved via a unique combination of (i) a 1D ribbonlike hydrogen bond system {AdeH⁺···H₂O···AdeH⁺···}_∞ exploiting planar water-assisted Hoogsteen···Sugar synthons with (ii) parallel 1D π–π stacks {AdeH⁺···AdeH⁺}_∞. In-plane 2D XY magnetism is characterized by a T_c close to 33 K, H_c,in‑plane = 60 Oe, and H_{c,out‑of‑plane} = 750 Oe, high values of in-plane γ critical exponents (γ_b = 2.34(6) for H||b and γ_c = 2.16(5) for H||c), and a Berezinskii–Kosterlitz–Thouless (BKT) topological phase transition, deduced from crystal-orientation-dependent scaling analysis. The obtained values of in-plane ν critical exponents, ν_b = 0.48(5) for H||b and ν_c = 0.49(3) for H||c, confirm the BKT transition (ν_BKT = 0.5). Full-range angle-resolved monocrystalline magnetic measurements supported by dedicated calculations indicated the occurrence of nonlinear susceptibility performance within the easy plane in a magnetically ordered state. We refer the occurrence of this phenomenon to spontaneous resolution in the C2 space group, a tandem not observed in studies on previous analogues and rarely reported in the field of molecular materials. The above magneto-supramolecular strategy may provide a novel means for the design of 2D molecular magnetic networks and help to uncover the inherent phenomena.