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 [Mn-IV(Schiff base)(mu-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[S-1 . S-2]) and the Mn-O-Mn angle of the Mn-IV(mu-O)](2) core. The new and unique structures of [Mn-IV(3,5-di-t-busalpn)(mu-O)](2) (2) and [Mn-IV(mu-3,5-di-Cl-salpn)(mu-O)](2) (3) exhibit unexpected structures compared with those of [Mn-IV(salpn)(mu-O)](2) (1) (H-2(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-C-2 axis through the oxo bridges of the [Mn-IV(mu-O)](2) core. The Mn-Mn distance of 2 is 2.7753(8) Angstrom and that of 3 is 2.7430(8) Angstrom. The 3,5-di-Cl-salpn(2-) 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 degrees. The data imply that a ferromagnetically coupled [Mn-IV(mu-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 S-2 state. (C) 2000 Elsevier Science S.A. All rights reserved.