Doubly reduced phthalocyanine complexes of V(IV), Cu(II), and Sn(II): synthesis, structure, and spin dynamics

Doubly reduced phthalocyanine complexes remain largely unexplored despite their potential relevance for molecular spin-based applications. Herein, we report the synthesis of a series of crystalline complexes, {cryptand(Cs⁺)}₂[M(Pc)^4-]^2-·xC₇H₈ (M = V^IVO, Cu^II, and Sn^II, x = 1.0-1.3), obtained via reduction of the corresponding metal phthalocyanines with cesium anthracenide in toluene. X-ray diffraction, optical spectroscopy, EPR spectroscopy, and magnetic measurements demonstrate ligand-centered two-electron reduction, affording spin-free (Pc)^4- ligands while preserving the metal oxidation states. The vanadyl and copper complexes exhibit isolated S = 1/2 spin systems, whereas the tin analogue is diamagnetic. Continuous-wave and pulsed EPR spectroscopy, supported by magnetic susceptibility data, reveal slow magnetic relaxation and quantum coherence in the paramagnetic complexes. In particular, the copper complex shows field-induced slow relaxation with relaxation times up to 0.1 s. Spin coherence was observed in magnetically diluted vanadyl and copper systems, including crystalline dispersions and frozen solutions. The key role of the sample preparation strategy in relaxation behavior was demonstrated. These findings demonstrate that doubly reduced phthalocyanine complexes can serve as robust platforms for molecular S = 1/2 spin qubits. Additionally, the straightforward reductive synthetic approach provides access to phthalocyanine-based systems that are mostly poorly soluble, opening new opportunities for the design of functional molecular materials.