Controlled aggregation of ME8(n-) binary anions (M = Cr, Mo; E = As, Sb) into one-dimensional arrays: structures, magnetism and spectroscopy.

Abstract

The [ME(8)](n)()(-) ions where M = Cr, Mo; E = As, Sb; n = 2, 3 have been prepared from the corresponding E(7)(3)(-) Zintl ions and M(naphthalene)(2) precursors. The complexes and their [A(crypt)](+) salts (A = Na, K) are formed in 20-45% crystalline yields and have been characterized by UV-vis spectroscopy, EPR, cyclic voltammetry, magnetic susceptibility, electrospray mass spectrometry (ESI-MS) and single-crystal X-ray diffraction. The structures are defined by crown-like cyclo-E(8) rings that are centered by transition metals. MoAs(8)(2)(-) (2) is a 16 e(-) diamagnetic complex whereas MoSb(8)(3)(-) (5) and the CrAs(8)(3)(-) salts (3 and 4) are 17 e(-) paramagnetic complexes. The ESI-MS spectra show free and alkali-complexed ME(8)(n)()(-) ions. The K(+) salt of CrAs(8)(3)(-) (4) crystallizes in a one-dimensional chain structure of [KCrAs(8)](2)(-) repeat units whereas the Na(+) salt (3) as well as 2 and 5 crystallize in "free ion" structures. The Cr atoms in 3 and 4 are formally d(1) Cr(5+) centers that show EPR signals at g = 2.001 with small As hyperfine interactions of 3.6 G. The susceptibility of the [KCrAs(8)](2)(-) salt 4 was modeled as a 1D Heisenberg antiferromagnet with a small -J/k(B) of 3K arising from antiferromagnetic couplings of the d(1) centers whereas 3 shows Curie-Weiss behavior. The electrochemical studies show metal-based oxidations for 3-5 but a ligand based oxidation for 2. The electronic spectra are interpreted in terms of the molecular orbital analysis of Li and Wu. The differences in formal oxidation states of the metals is described in terms of a Zintl-Klemm formalism involving E(8)(8)(-) rings that are isoelectronic to S(8). The factors governing the formation of 1D chains versus free ions are presented.