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Herein we report heteroleptic Co( ii ) diimine complexes [Co(H 2 bip) 2 Cl 2 ] ( 1 ), [Co(H 2 bip) 2 Br 2 ] ( 2 ), [Co(H 2 bip) 3 ]Br 2 ·1MeOH ( 3 ) and [Co(H 2 bip) 2 (Me 2 bpy)]Br 2 ·(MeCN) 0.5 ·(H 2 O) 0.25 ( 4 ) (H 2 bip = 2,2′-bi-1,4,5,6-tetrahydropyrimidine, bpy = 2,2′-dipyridyl, Me 2 bpy = 4,4′-Me-2,2′-dipyridyl), purposefully prepared to enable a systematic study of magnetic property changes arising from the increase of overall ligand field from σ/π-donor chlorido ( 1 ) to π-acceptor 4,4′Me-2,2′bpy ( 4 ). The presence of axial and rhombic anisotropy ( D and E ) of these compounds is sufficient to allow 1–4 to show field-induced slow relaxation of magnetization. Interestingly, we found as the effective ligand field is increased in the series, rhombicity ( E / D ) decreases, and the magnetic relaxation profile changes significantly, where relaxation of magnetization at a specific temperature becomes gradually faster. We performed mechanistic analyses of the temperature dependence of magnetic relaxation times considering Orbach relaxation processes, Raman-like relaxation and quantum tunnelling of magnetization (QTM). The effective energy barrier of the Orbach relaxation process ( U eff ) is largest in compound 1 (19.2 cm −1 ) and gradually decreases in the order 1 > 2 > 3 > 4 giving a minimum value in compound 4 (8.3 cm −1 ), where the Raman-like mechanism showed the possibility of different types of phonon activity below and above ∼2.5 K. As a precursor of 1 , the tetrahedral complex [Co(H 2 bip)Cl 2 ] ( 1a ) was also synthesized and structurally and magnetically characterized: this compound exhibits slow relaxation of magnetization under an applied dc field (1800 Oe) with a record slow relaxation time of 3.39 s at 1.8 K. 

Phenochalcogenazines such as phenoxazines and phenothiazines have been widely employed as photoredox catalysts (PCs) in small molecule and polymer synthesis. However, the effect of the chalcogenide in these catalysts has not been fully investigated. In this work, a series of four phenochalcogenazines is synthesized to understand how the chalcogenide impacts catalyst properties and performance. Increasing the size of the chalcogenide is found to distort the PC structure, ultimately impacting the properties of each PC. For example, larger chalcogenides destabilize the PC radical cation, possibly resulting in catalyst degradation. In addition, PCs with larger chalcogenides experience increased reorganization during electron transfer, leading to slower electron transfer. Ultimately, catalyst performance is evaluated in organocatalyzed atom transfer radical polymerization and a photooxidation reaction for C(sp²)−N coupling. Results from these experiments highlight that a balance of PC properties is most beneficial for catalysis, including a long‐lived excited state, a stable radical cation, and a low reorganization energy.