A magneto-structural correlation between the Heisenberg constant, J, and the MnOMn angle in [MnIV(μ-O)]2 dimers

In our ongoing studies of high valent manganese dimers of the general formula [MnIV(Schiff base)(μ-O)]2 as models for the oxygen evolving complex (OEC) of photosystem II (PSII), we now report several new Mn complexes that allow us to define a magneto-structural correlation between the Heisenberg exchange constant, J, from the Heisenberg exchange equation (H=−2J[S1 · S2]) and the MnOMn angle of the [MnIV(μ-O)]2 core. The new and unique structures of [MnIV(3,5-di-t-busalpn)(μ-O)]2 (2) and [MnIV(μ-3,5-di-Cl-salpn)(μ-O)]2 (3) exhibit unexpected structures compared with those of [MnIV(salpn)(μ-O)]2 (1) (H2(salpn)=N,N′-bis(salicylideneimino)-1,3-diaminopropane). The structure of 2, unlike that of 1, does not have a molecular inversion center due to a ‘rotation’ of one ligand. Instead, there exists a pseudo-C2 axis through the oxo bridges of the [MnIV(μ-O)]2 core. The MnMn distance of 2 is 2.7753(8) Å and that of 3 is 2.7430(8) Å. The 3,5-di-Cl-salpn2− ligand of 3 adopts an unexpected bridging mode between the manganese ions. The magneto-structural correlation is valid for these new Schiff base complexes and other non-Schiff base containing compounds. The J values range over 110 cm−1 versus a MnOMn range of 3.9°. The data imply that a ferromagnetically coupled [MnIV(μ-O)]2 would be unlikely to occur in the OEC, wherein such may be invoked to generate an S=5/2 ground state for the g=4.1 EPR signal observed in the S2 state.