A family of robust metal complexes of [Mn(L)X₂] and (LH)₂[MnX₄] type (X = Cl, Br, I) have been synthesized based on xanthene-derived bisphosphine dioxides (L = XantphosO₂ or t-Bu-XantphosO₂). At ambient temperature, the designed compounds exhibit strong green to yellow phosphorescence with quantum yields as high as 90%. Remarkably, complex [Mn(XantphosO₂)Br₂] reveals polymorphic luminescence, which is a rare phenomenon for Mn compounds. Two polymorphs of this compound, crystallizing in P and P2₁/n space groups, display green and yellow luminescence, respectively, with the emission quantum yield being 33 and 50%. Moreover, the “yellow” polymorph features bright triboluminescence upon grinding its crystals. An in-depth photophysical study evidences that the observed phosphorescence is due to the ligand-sensitized ⁴T₁ → ⁶A₁ transitions in Mn²⁺ ions.

A mononuclear complex, [FeL₂](BF₄)₂·xH₂O (1^LS·xH₂O), where L is 4-(3,5-dimethyl-1H-pyrazol-1-yl)-2-(pyridin-2-yl)-6-methylpyrimidine, can be converted into several phases showing different spin crossover regimes. In the first heating 1^LS·xH₂O loses water molecules and converts into a mixture of two high spin phases, 1^A/HS and 1^C/HS. Further cycling produces the low spin phase 1^A/LS. The transition 1^A/LS ↔ 1^A/HS is accompanied by a 130 K wide hysteresis loop (T_c↑ = 490 K, T_c↓ = 360 K). Annealing the complex 1^LS·xH₂O at 500 K yields a high spin phase 1^C/HS. The phase 1^C/HS undergoes spin conversion to the corresponding low spin phase 1^C/LS on cooling, T_1/2 ≈ 320 K. Dehydration of 1^LS·xH₂O at 370 or 400 K yields a certain low spin phase, 1^X/LS, which irreversibly transforms into a high spin phase 1^B/HS, which, in turn, reversibly transforms to the low spin phase 1^B/LS on cooling, T_1/2 ≈ 320 K.