Pseudooctahedral mononuclear cobat(II) complex [Co(abpt)2(tcm)2] (1), where abpt = 4-amino-3,5-bis(2-pyridyl)-1,2,4-triazole and tcm = tricyanomethanide anion, shows field-induced slow relaxation of magnetization with U = 86.2 K and large axial and rhombic single-ion zero-field-splitting parameters, D = +48(2) cm(-1) and E/D = 0.27(2) (D = +53.7 cm(-1) and E/D = 0.29 from ab initio CASSCF/NEVPT2 calculations), thus presenting a new example of a field-induced single-ion magnet with transversal magnetic anisotropy.

The organometallic first-row transition-metal complexes [M(2,2'-bipy)(mes)2] (M = Cr (1), Mn (2), Co (4), Ni (5); 2,2'-bipy = 2,2'-bipyridine; mes = 2,4,6-Me3C6H2) were reacted with potassium and a suitable alkali-metal sequestering agent to yield salts of the anionic species [M(2,2'-bipy)(mes)2](-). The neutral parent compounds and their corresponding anionic congeners were characterized by single-crystal X-ray diffraction in [Cr(2,2'-bipy)(mes)2]·1.5C6H6, [Mn(2,2'-bipy)(mes)2], [Co(2,2'-bipy)(mes)2]·THF, [Ni(2,2'-bipy)(mes)2], [K(dibenzo-18-crown-6)·THF][Cr(2,2'-bipy)(mes)2]·2THF, [K(18-crown-6)][Mn(2,2'-bipy)(mes)2]·2THF, [K(18-crown-6)][Mn(2,2'-bipy)(mes)2]·0.67py·0.67tol, [K(2,2,2-crypt)][Co(2,2'-bipy)(mes)2], and [K(2,2,2-crypt)][Ni(2,2'-bipy)(mes)2]. These species, along with the previously reported neutral and anionic iron complexes [Fe(2,2'-bipy)(mes)2](0/-) (3/3(-)), form a homologous series of compounds which allow for an in-depth study of the interactions between metals and ligands. Single-crystal X-ray diffraction data, DFT calculations, and various spectroscopic and magnetic measurements indicate that the anionic complexes (1(-)-5(-)) can be best formulated as M(II) complexes of the 2,2'-bipyridyl radical anion. These findings complement recent studies which indicate that bond metric data from single-crystal X-ray diffraction may be employed as an important diagnostic tool in determining the oxidation states of bipyridyl ligands in transition-metal complexes.

A series of first-row transition metal complexes with 15-membered pyridine-based macrocycle (3,12,18-triaza-6,9-dioxabicyclo[12.3.1]octadeca-1(18),14,16-triene = L) was prepared ([M(II)(L)Cl2], where M = Mn, Co, Ni, Zn (1, 3, 4, 6); [Fe(III)(L)Cl2]Cl (2), [Cu(II)(L)Cl]Cl (5)) and thoroughly characterized. Depending on the complexated metal atom, the coordination number varies from 7 (Mn, Fe, Co), through 5 + 2 for Ni and 4 + 1 for Cu, to 5 for Zn accompanied by changes in the coordination geometry from the pentagonal bipyramid (1-4) to the square pyramid (5 and 6). Along the series, the metal-oxygen distances were prolonged in such manner that their bonding character was investigated, apart from X-ray structural analysis, also by ab initio calculations (Mayer's bond order, electron localization function), which confirmed that, in 4 and 5, two and one oxygen donor atoms are semicoordinated, respectively, and one and two oxygen atoms are uncoordinated in 5, and 6, respectively. On the basis of the temperature variable magnetic susceptibility measurements, 1 and 2 behave as expected for 3d(5) high-spin configuration with negligible zero-field splitting (ZFS). On the other hand, a large axial ZFS (D(Co) ≈ 40 cm(-1), D(Ni) ≈ -6.0 cm(-1)) was found for 3 and 4, and rhombic ZFS (E/D ≈ 0.15) for 4. Antiferromagnetic exchange coupling was observed for 4 and 5 (J(Ni) = -0.48 cm(-1), and J(Cu) = -2.43 cm(-1), respectively). The obtained results correlate well with ab initio calculations of ZFS parameters as well as J-values, which indicate that the antiferromagnetic exchange is mediated by hydrogen bonds. The complexes were also investigated by cyclic voltammetry in water or acetonitrile. A quasi-reversible couple Mn(II)/Mn(III) at 1.13/0.97 V, an almost reversible couple Fe(II)/Fe(III) at 0.51/0.25 V, and a one-step/multistep reduction/oxidation of Cu(II) complex 5 at -0.33 V/0.06-0.61 V were detected.

Addition of potassium metal and 2,2,2-crypt (4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane) to a tetrahydrofuran (THF) solution of Fe(2,2'-bipyridine)(mes)(2) (1; mes = 2,4,6-Me(3)C(6)H(2)) yielded the anionic complex [Fe(2,2'-bipyridine)(mes)(2)](-) which was isolated as [K(2,2,2-crypt)][Fe(2,2'-bipyridine)(mes)(2)] (2) alongside the side-product [K(2,2,2-crypt)][Fe(mes)(3)] x C(6)H(12) (3). A compositionally pure sample of 2 was obtained by dissolving a mixture of 2 and 3 in dry pyridine and layering the resulting solution with toluene. Solid state magnetic susceptibility measurements on 1 reveal Curie-Weiss paramagnetic behavior with a molar magnetic moment of 5.12(1) mu(B) between 20 and 300 K, a value which is in line with the expected iron(II) spin-only value of 4.90 mu(B). The magnetic measurements carried out on 2 reveal more complex temperature dependent behavior consistent with intramolecular antiferromagnetic coupling (J = -46 cm(-1)) between the unpaired electrons of the iron(II) ion (S(Fe) = 2) and a pi* orbital of the bipyridyl radical (S(bipy) = 1/2). Structural data, Mossbauer and electron paramagnetic resonance (EPR) spectroscopic measurements, and density functional theory (DFT) calculations are all consistent with this model of the electronic structure. To the best of our knowledge, species 2 represents the first crystallographically characterized transition metal complex of the 2,2'-bipyridyl ligand for which magnetic, spectroscopic, and computational data indicate the presence of an unpaired electron in the pi* antibonding orbital.

A one-pot synthetic procedure yields the octanuclear Fe(III) complexes Fe(8)(micro(4-)O)(4)(micro-pz(*))(12)X(40, where X = Cl and pz(*) = pyrazolate anion (pz = C(3)H(3)N(2)-) (1), 4-Cl-pz (2), and 4-Me-pz (3) or X = Br and pz(*) = pz (4). The crystal structures of complexes 1-4, determined by X-ray diffraction, show an Fe(4)O(4)-cubane core encapsulated in a shell composed of four interwoven Fe(micro-pz(*))(3)X units. Complexes 1-4 have been characterized by 1H NMR, infrared, and Raman spectroscopies. Mössbauer spectroscopic analysis distinguishes the cubane and outer Fe(III) centers by their different isomer shift and quadrupole splitting values. Electrochemical analyses by cyclic voltammetry show four consecutive, closely spaced, reversible reduction processes for each of the four complexes. Magnetic susceptibility studies, corroborated by density functional theory calculations, reveal weak antiferromagnetic coupling among the four cubane Fe centers and strong antiferromagnetic coupling between cubane and outer Fe atoms of 1. The structural similarity between the antiferromagnetic Fe(8)(micro(4-)O)(4) core of 1-4 and the antiferromagnetic units contained in the minerals ferrihydrite and maghemite is demonstrated by X-ray and Mössbauer data.