Roman Boča

Simple Mononuclear Cobalt(II) Complex: A Single-Molecule Magnet Showing Two Slow Relaxation Processes

Complex [Co(PPh3)2Br2] possesses intermediate magnetic anisotropy, D/hc = -13 cm(-1). It displays superparamagnetic behavior either in the absence of the magnetic field or in fields of Bdc = 0.05, 0.1, 0.15, and 0.2 T. At Bdc = 0.1 T, the barrier to spin reversal U/kB = 37 K and the extrapolated relaxation time τ0 = 9.4 × 10(-11) s confirm its classification as a single-molecule magnet. At Bdc = 0.2 T, two relaxation processes are evidenced.

Bonding properties of thiocyanate groups in copper(II) and copper(I) complexes

Abstract The bonding properties of thiocyanate groups of over one hundred copper species, studied by X-ray analysis were analysed. It is shown that the bonding properties of thiocyanate groups in heterogeneous copper thiocyanate complexes depend on the size of the positive charge on the copper atom.

Heterometallic CoIII4FeIII2 Schiff Base Complex: Structure, Electron Paramagnetic Resonance, and Alkane Oxidation Catalytic Activity

The heterometallic complex [Co(4)Fe(2)OSae(8)]·4DMF·H(2)O (1) was synthesized by one-pot reaction of cobalt powder with iron chloride in a dimethylformamide solution of salicylidene-2-ethanolamine (H(2)Sae) and characterized by single crystal X-ray diffraction analysis, magnetic measurements, high frequency electron paramagnetic resonance (HF-EPR), and Mössbauer spectroscopies. The exchange coupling in the Fe(III)-Fe(III) pair is of antiferromagnetic behavior with J/hc = -190 cm(-1). The HF-EPR spectra reveal an unusual pattern with a hardly detectable triplet signal of the Fe(III) dimer. The magnitude of D (ca. 13.9 cm(-1)) was found to be much larger than in related dimers. The catalytic investigations disclosed an outstanding activity of 1 toward oxidation of cycloalkanes with hydrogen peroxide, under mild conditions. The most efficient system showed a turnover number (TON) of 3.57 × 10(3) with the concomitant overall yield of 26% for cyclohexane, and 2.28 × 10(3)/46%, respectively, for cyclooctane. A remarkable turnover frequency (TOF) of 1.12 × 10(4) h(-1) (the highest initial rate W(0) = 3.5 × 10(-4) M s(-1)) was achieved in oxidation of cyclohexane. Kinetic experiments and selectivity parameters led to the conclusion that hydroxyl radicals are active (attacking C-H bonds) species. Kinetic and electrospray ionization mass spectrometry (ESI-MS) data allowed us to assume that the trinuclear heterometallic particle [Co(2)Fe(Sae)(4)](+), originated from 1 in solution, could be responsible for efficient generation of hydroxyl radicals from hydrogen peroxide.

Single-molecule magnetism in a pentacoordinate cobalt(II) complex supported by an antenna ligand.

Pentacoordinate complex [CoL(3)Cl2] with a tridentate antenna-like ligand L(3) forms a dimer held by short π-π stacking with head-to-head contacts at 3.4 Å. The direct-current (dc) magnetic susceptibility and magnetization data confirm weak ferromagnetic interaction and a large-magnetic anisotropy, D/hc = 150 cm(-1) and E/hc = 11.6 cm(-1). The system shows superparamagnetic behavior at low temperature that depends upon the applied magnetic field. At Bdc = 0.2 T, a low-frequency peak at the out-of-phase susceptibility is seen (ν ∼ 0.3 Hz), whereas the onset of the second peak appears at ν > 1500 Hz, indicating the existence of two slow relaxation processes.

Magnetostructural D correlation in nickel(II) complexes: reinvestigation of the zero-field splitting.

The magnetostructural D correlation interrelates the zero-field-splitting parameter D withdrawn from the magnetic data with the structural tetragonality parameter D(str). This correlation allows the quantitative prediction that D < 0 occurs for the tetragonally compressed nickel(II) complexes.

Zero-Field Splitting in Pseudotetrahedral Co(II) Complexes: a Magnetic, High-Frequency and -Field EPR, and Computational Study

Six pseudotetrahedral cobalt(II) complexes of the type [CoL2Cl2], with L = heterocyclic N-donor ligand, have been studied in parallel by magnetometry, and high-frequency and -field electron paramagnetic resonance (HFEPR). HFEPR powder spectra were recorded in a 50 GHz < ν < 700 GHz range in a 17 T superconducting and 25 T resistive magnet, which allowed constructing of resonance field vs frequency diagrams from which the fitting procedure yielded the S = 3/2 spin ground state Hamiltonian parameters. The sign of the axial anisotropy parameter D was determined unambiguously; the values range between -8 and +11 cm(-1) for the given series of complexes. These data agree well with magnetometric analysis. Finally, quantum chemical ab initio calculations were performed on the whole series of complexes to probe the relationship between the magnetic anisotropy, electronic, and geometric structure.

Nucleophilic additions to pseudohalides in the coordination sphere of transition metal ions and coligand isomerism

Abstract Coordinated pyrazoles exhibit a nucleophilic addition to the cyanate ligand inside the transition metal coordination sphere and new chelate complexes are formed; the new anionic ligand of the carbamoylpyrazolate type can be released. The conditions for such an addition were studied and established by selecting various central atoms and different N -donor heterocyclic ligands. Some non-linear pseudohalides, namely N(CN) 2 − , C(CN) 3 − and ONC(CN) 2 − , are also capable of such reactions but the addition proceeds under different conditions due to different electronic and steric requirements. A new type of isomerism—coligand isomerism—has been discovered: one isomer is the usual complex of the [MX 2 (pzt) 2 ] type (pzt, pyrazole-type ligand) whereas in the second isomer [M(pzt·X) 2 ] a new anionic chelating ligand formed by nucleophilic addition is present. Another type of nucleophilic addition, reaction of methanol with the N(CN) 2 − specifically in the coordination sphere of Cu II leads to the formation of a six-membered metallacycle. Nucleophilic addition of methanol to the ONC(CN) 2 − anion in the transition metal coordination sphere results in formation of a five-membered metallacycle. All nucleophilic additions studied have a common feature in a coordinative activation of the carbon β -site of the pseudohalides changing its hybridization and providing an unsaturated valency for a nucleophilic attack.

Heterometallic Cu/Co and Cu/Co/Zn complexes bearing rare asymmetric tetranuclear cores: synthesis, structures, and magnetic and catalytic properties toward the peroxidative oxidation of cycloalkanes.

The three novel heterometallic complexes [CuCo(III)Co(II)(2)(MeDea)(3)Cl(3)(CH(3)OH)(0.55)(H(2)O)(0.45)](H(2)O)(0.45) (1), [CuCo(III)Zn(2)(MeDea)(3)Cl(3)(CH(3)OH)(0.74)(H(2)O)(0.26)](H(2)O)(0.26) (2), and [CuCo(III)Zn(2)(MeDea)(3)Cl(3)(DMF)] (3) have been prepared using a one-pot reaction of copper powder with cobalt chloride (1) and zinc nitrate (2, 3) in a methanol (1, 2) or dimethylformamide (3) solution of N-methyldiethanolamine. A search of the Cambridge Structural Database shows that the tetranuclear asymmetric cores M(4)(μ(3)-X)(μ-X)(5) of 1-3 represent an extremely rare case of M(4)X(6) arrays. The magnetic investigations of 1 disclose antiferromagnetic coupling in a Co(II)-Cu(II)-Co(II) exchange fragment with J(Co-Cu)/hc = -4.76 cm(-1), J(Co-Co)/hc = -2.76 cm(-1), and D(Co)/hc = +34.3 cm(-1). Compounds 1-3 act as precursors for the mild peroxidative oxidation of cyclohexane to cyclohexanol and cyclohexanone with overall yields up to 23%. The synthetic and structural features as well as the thermogravimetric behavior and electrospray ionization mass spectrometry data are discussed.

A heptanuclear Fe(II)–Fe(III)6 system with twelve unpaired electrons

Abstract The pentadentate ligand 5 LH2 = saldptn = N,N′-bis(1-hydroxy-2-benzyliden)-1,7-diamino-4-azaheptane has been prepared by a Schiff base condensation between 1,7-diamino-4-azaheptane and the corresponding salicyaldehyde. Its complexation with Fe(III) gave the high-spin (S=5/2) complex of [Fe III ( 5 L)Cl]. This precursor was combined with [Fe(CN)6]4− and a blue heptanuclear complex [FeII{(CN)Fe III ( 5 L)}6]Cl2 resulted. This system belongs to the class of high-spin molecules possessing twelve unpaired electrons (S=6) as proven by the magnetic susceptibility measurements and Mossbauer spectra.

Spectroscopic and magnetic properties and structure of a five-coordinate, O2-binding cobalt(II) Schiff base complex and of the copper(II) analogue

Abstract The cobalt(II) complex CoL and the copper(II) complex CuL of the pentadentate O 2 N 2 N′ Salen-type Schiff base ligand H 2 L = N, N ′-bis(3-t-butyl-2-hydroxy-5-methylbenzyliden)-1, 7-diamino-4-methyl-4-azaheptane were prepared, characterized and subjected to single crystal X-ray analysis. The {MO 2 N 2 N′} coordination core forms a trigonal bipyramid differing in the degree of distortion, as expressed by the O-M-O angles 138.62(8)° [O-Co-O] and 155.21(7)° [O-Cu-O]. The distances between the metal and the aliphatic tertiary nitrogen atoms N′ are markedly different, 2.123(2) [Co-N′] and 2.300(2)A[Cu-N′]. In contrast to normal Salen complexes the imine nitrogen atoms are located trans to each other. Solution room temperature electron spectra show a broad absorption in the near-IR range at 6350 (CoL) and 8200 (CuL) cm −1 . The temperature dependence of the effective magnetic moment for high-spin CoL ( S =3/2) shows a maximum at 50 K, which is reproduced by the following set of magnetic parameters, reflecting the zero-field splitting; g 1 = 1.68, g 1 = 1.89, g = 3.21, D/hc = −38.9cm −1 , E/hc = 1.7cm −1 , α 110 = 0, zJ/hc = −1.00cm −1 . Variable temperature electron paramagnetic resonance experiments prove that CoL, dissolved in toluene, N, N -dimethylformamide or pyridine, binds dioxygen reversibly.