Molecular and electronic structure of nitridochromium(V) complexes with macrocyclic amine ligands

Photolysis of red cis- and trans-[Cr-III(cyclam)(N-3)(2)](ClO4) (1a, 2a) (cyclam = 1,4,8,11-tetraazacyclotelradecane) in the solid state or aqueous solution by light affords the nitridochromium(V) complexes of cis- or trans-[Cr-V-(N)(cyclam)(N-3)](ClO4) (3a) and dinuclear [{trans-[Cr-V(N)(cyclam)]}(2)(mu-N-3)](ClO4)(3). 3.5H(2)O (4a), respectively. 4a was converted to the bis(tetraphenylborate)-azide-2C(2)H(5)OH salt (4b). Crystallization of 4a from acetonitrile-after addition of AgClO4 and removal of AgN3-yields octahedral trans-[Cr-V(N)(cyclam)(NCCH3)](ClO4)(2) (5a). The crystal structures of 1a, 2b (which is the tetraphenylborate salt of 2a), 4a,b, 5a,b have been determined by single-crystal X-ray crystallography. All nitridochromium(V) compounds contain the trans-[Cr-V(N)(cyclam)](2+) fragment with an N=Cr-V moiety (1.56 +/- 0.01 Angstrom). The electronic spectra of d(1) configurated complexes display three low-intensity d-d bands which have been assigned in C-4v symmetry as (xy) ((xy,yz) B-2(2) --> E-2, (xy)--> (x(2) - y(2)) B-2(2) --> B-2(1), and (xy) --> (z(2)) B-2(2) --> (2)A(1), where the molecular z-axis coincides with the N=Cr vector. Ligand-field parameters within the angular overlap model (AOM) of e(sigma)(cyclam) = 7300-8600, e(sigma)(ax) = 22 000, e(sigma)(ax) = 21300-25000 cm(-1) have been determined which reproduce the experimental electronic spectra well. Temperature dependent magnetic susceptibility measurements and X-band EPR spectroscopy on solid samples of 4a containing a linear Cr-V-N=N=N-Cr-V moiety revealed a weak intramolecular antiferromagnetic exchange coupling which is nearly absent in 4b containing a bent azido bridge. A mechanism for this unexpected coupling in 4a is proposed.