Ni 12 tetracubane cores with slow relaxation of magnetization and efficient charge utilization for photocatalytic hydrogen evolution.

We report two Ni 12 multicubane topologies enclosed in the polyanions [Ni 12 (OH) 9 (WO 4 ) 3 (PO 4 )(B-α-PW 9 O 34 ) 3 ] 21- {Ni 12 W 30 } and [Ni 12 (OH) 9 (HPO 4 ) 3 (PO 4 )(B-α-PW 9 O 34 )(A-α-PW 9 O 34 ) 2 ] 21- {Ni 12 W 27 } that magnetically behave as Ni 12 units clearly distinguishing them from typical Ni 4 cubanes as shown by magnetic studies together with high field and frequency electron paramagnetic resonance (HFEPR). Beyond the unprecedented static properties, {Ni 12 W 30 } shows the unusual coexistence of slow relaxation of the magnetization and a diamagnetic ground state ( S = 0), providing the unique opportunity of studying the essentially elusive magnetic relaxation behavior in excited states. The cubane-topology dependent activity of {Ni 12 W 30 } and {Ni 12 W 27 } as homogeneous HER photocatalysts unveils the structural key features significant for the design of photocatalysts with efficient charge utilization exemplified by high quantum yields (QY) of 10.42% and 8.36% for {Ni 12 W 30 } and {Ni 12 W 27 }, respectively.