Mossbauer and computational study of an N2-bridged diiron diketiminate complex:: Parallel alignment of the iron spins by direct antiferromagnetic exchange with activated dinitrogen

This work reports Mossbauer and DFT studies of the diiron-N-2 complex (LFeNNFeLMe)-Fe-Me (L = beta-diketiminate), 1a. Complex 1a, formally diiron(1), has a system spin S = 3 with an isolated M-S = +/- 3 quasi-doublet as a ground state; the M-S = +/- 2 doublet is > 100 cm(-1) higher in energy. Complex 1a exhibits at 4.2 K a large, positive magnetic hyperfine field, B-int = + 68.1 T, and an effective g value of 16 +/- 2 along the easy magnetization axis of the ground doublet; this value is significantly larger than the spin- only value ( g) 12). These results have been rationalized by DFT calculations, which show that each Fe site donates significant electron density into the pi* orbitals of dinitrogen, resulting in a configuration best described as two high-spin Fe-II (S-a = S-b = 2) bridged by triplet N-2(2-) (S-c = 1). In this description the minority spin electron of each iron is accommodated by two nonbonding, closely spaced 3d orbitals, z(2) and yz(z is perpendicular to the diketiminate planes, x is along the Fe center dot center dot center dot Fe vector). Spin-orbit coupling between these orbital states generates a large unquenched orbital momentum along the iron-iron vector. The S = 3 ground state of 1a results from strong antiferromagnetic direct exchange couplings of the Fe spins (S-a = S-b = 2) to the N-2(2-) spin (S-c = 1) and can be formulated as vertical bar((S-a,S-b)S-ab = 4, S-c = 1), S = 3); H = J(S-a+ S-b)center dot S-c with J approximate to 3500 cm(-1).