Syntheses, structure, and reactivity of cobalt and nickel complexes of 1,4,7-trithiacyclodecane: self-exchange rates for Co(S6)2+/3+ couples from cobalt-59 NMR spectroscopy

Field-induced single-ion magnet based on a quasi-octahedral Co(II) complex with mixed sulfur-oxygen coordination environment

Synthesis and characterization of structure and magnetic properties of the quasi-octahedral complex (pipH₂)[Co(TDA)₂] 2H₂O (I), (pipH₂²⁺ = piperazine dication, TDA^2- = thiodiacetic anion) are described. X-ray diffraction studies reveal the first coordination sphere of the Co(II) ion, consisting of two chelating tridentate TDA ligands with a mixed sulfur-oxygen strongly elongated octahedral coordination environment. SQUID magnetometry, frequency-domain Fourier-transform (FD-FT) THz-EPR spectroscopy, and high-level ab initio SA-CASSCF/NEVPT2 quantum chemical calculations reveal a strong "easy-plane" type magnetic anisotropy (D ≈ +54 cm^-1) of complex I. The complex shows field-induced slow relaxation of magnetization at an applied DC field of 1000 Oe.

Fast Low-Spin Cobalt Complex Redox Shuttles for Dye-Sensitized Solar Cells

A low-spin cobalt(II) complex, cobalt bis(trithiacyclononane), [Co(ttcn)2](3+/2+), was investigated for use as a redox shuttle in dye-sensitized solar cells, DSSCs. This unique cobalt complex redox shuttle is stable, transparent, and easy to synthesize from commercial ligands and has attractive energetic and kinetic features for use in DSSCs. Initial results indicate that the overall performance is limited by recombination. Variation of the sensitizer and deposition of an ultrathin coating of alumina on nanoparticle-based TiO2 DSSC photoanodes reduced recombination, which resulted in significantly improved quantum yields. The photovoltaic behavior was compared to the current record efficiency cobalt tris-bipyridine, [Co(bpy)3](3+/2+), redox shuttle and produced similar results. Further use of high extinction organic sensitizers with only ∼200 mV of driving force for regeneration was examined, which produced efficiencies of over 2%; importantly, regeneration is not rate-limiting in this system, thus demonstrating the promise of using such fast redox shuttles.

The synthesis and characterization of the pendant-armed ligand N,N′-bis(2′-pyridylmethyl)-1,7-dithia-4,11-diazacyclotetradecane (L4) and crystal structures of L4 and the copper(II) complex [Cu(L4)](ClO4)2 — Crystal structure of the nickel(II) complex of N-(2′-pyridylmethyl)-1,4,7-trithia-11-azacyclotetradecane (L2), [Ni(L2)(CH3CN)](ClO4)2·CH3CN

The synthesis and characterization of the mixed donor ligand N,N′-bis(2′-pyridylmethyl)-1,7-dithia-4,11-diazacyclotetradecane (L4) containing two pendant pyridine arms is described. The macrocycle formation involves reaction of N-tosylbis(2-bromoethyl)amine with N-tosylbis(3-mercaptopropyl)amine at high dilution followed by detosylation at the nitrogen atoms using LiAlH4 in view of the presence of the thioether groups. An X-ray study of the ligand shows the sulphur atoms lie exo to the macrocyclic ring. The nickel(II) and copper(II) complexes of L4 have been characterized and the X-ray structure of the Cu(II) ion exhibits a five-coordinate copper ion despite the presence of six available donor atoms. In the case of ligand N-(2-pyridylmethyl)-1,4,7-trithia-11-azacyclotetradecane (L2), with a single pendant N donor, the Ni(II) complex is distorted octahedral with a molecule of acetonitrile in the sixth site. Details of UV–vis, ESR spectra, and electrochemical studies on the complexes are presented.Key words: synthesis, pendant-armed, mixed donor S,N macrocycles, copper, nickel.

New low-spin Co(II) complexes with novel tripodal 1,1,1-tris(dimethylphosphinomethyl)ethane ligand: electron transfer kinetics and spectroscopic characterization of Co(II)P6 and Co(II)P3S3 ions in aqueous solution

Substitution-inert Co(II) complexes with tripodal 1,1,1-tris(dimethylphosphinomethyl)ethane were produced in aqueous solution by the controlled-potential electrolysis. EPR studies indicated that the Co(II)P6 and Co(II)P3S3 species are in the low-spin t62ge1g state. The analyses of the absorption spectra of the Co(II) species in solution indicated significant Jahn–Teller distortion in Co(II)P6 and Co(II)P3S3 ions as expected for the low-spin d7 electronic configuration, while less distortion was observed for the Co(II)S6 ion. Kinetic studies of the redox reactions involving these Co(III/II) species confirmed that the electron self-exchange reactions for the Co(III/II)P6 and Co(III/II)P3S3 couples are relatively fast (kex ~ 1 × 104 dm3 mol–1 s–1), which is consistent with the results for other low-spin – low-spin Co(III/II) couples. It was concluded that the nephelauxetic effect of the P-donor atom stabilizes the low-spin state in Co(II), and that the rather rapid electron exchange rates for these Co(III/II) couples were explained by the lack of the spin reorganization process required for the ordinary low-spin – high-spin Co(III/II) couples.Key words: cobalt(III/II), spin state, EPR study, kinetics, electron transfer.

Synthesis and Reactivity of Nickel(II) Complexes of the Pyridine-Based Phosphorus-Containing Macrocycle 6-Phenyl-15-aza-6-phospha-3,9- dithiabicyclo[9,3,1]pentadeca-1(15),11,13-triene

The reactivity of the phosphomacrocycle 6-phenyl-15-aza-6-phospha-3,9-dithiabicyclo[9,3,1]pentadeca-1(15), 11,13-triene (L) toward different Ni(II) salts has been studied. The reaction of L with 1 molar equiv of Ni(II) perchlorate in acetonitrile solution gives the complex [Ni(L)(CH(3)CN)(2)](ClO(4))(2) (1), which crystallizes in the monoclinic space group P2(1)/c (No. 14) with a = 12.704(2) Å, b = 13.538(2) Å, c = 17.377(2) Å, beta = 107.554(8) degrees, and Z = 4. The Ni(II) atom in 1 shows an octahedral environment defined by the four donor atoms of L and two acetonitrile molecules in a cis disposition. The reaction of L with Ni(II) perchlorate in 2:1 molar ratio yields the green complex [Ni(Lox)(2)](ClO(4))(2) (2) in which the phosphine groups of the ligand L are in their phosphine-oxide form (Lox). The same complex is also obtained by adding more ligand L to an acetonitrile solution of 1. Complex 2 crystallizes in the triclinic space group P&onemacr; (No. 2) with a = 10.564(1) Å, b = 11.975(2) Å, c = 9.124(2) Å, alpha = 109.79(1) degrees, beta = 95.67(2) degrees, gamma = 109.19(1) degrees, and Z = 1. The octahedral coordinated Ni(II) atom in 2 lies on a crystallographic center of symmetry, and it is surrounded by two pyridine nitrogen atoms, two thioether sulfur atoms, and two phosphine-oxide oxygen atoms provided by two facially coordinated Lox ligands. The reaction of L with Ni(II) chloride and Ni(II) thiocyanate yields the complexes [NiCl(2)(L)] (3) and [Ni(NCS)(2)(L)] (4), respectively, which are also obtained by adding Me(4)NCl or KSCN to an acetonitrile solution of 1. Complex 4 crystallizes in the monoclinic space group P2(1)/n (No. 14), with a = 11.139(2) Å, b = 13.270(2) Å, c = 14.885(1) Å, beta = 92.34(1) degrees, and Z = 4, and its structure is similar to that of complex 1, simply replacing the two acetonitrile molecules by two SCN(-) anions. The reactions of the complex 1 with different chelating (en, bipy, o-phen) and nonchelating (py) N-donor ligands have also been carried out. In all cases the two acetonitrile molecules of 1 are easly replaced by the N-donor ligands, but the macrocycle L molecule remains coordinated to the Ni(II) atom. The crystal structure of the complex [Ni(L)(py)(2)](ClO(4))(2) (5) is also elucidated. Crystal data: monoclinic space group P2(1)/c (No. 14) a = 13.063(2) Å, b = 16.615(3) Å, c = 14.973(3) Å, beta = 91.76(2) degrees, Z = 4.

Preparations for 1,4,7,11,14,17-hexa-thiacycloeicosane, 1,4,7-trithiecan-9-ol, 1,11 -dioxa-4,8,14,18-tetrathia-cycloeicosane, 1 -oxa-4,8-dithiacyclo-decane, and some complexes of iron, cobalt, nickel, and palladium

Preparations are reported for the title ligands (L1–L4, respectively) and for [Co(L1)][ClO4]2, [Ni(L1)][ClO4]2, [{Pd(η3-C3H5)}2(L1)][ClO4]2, [Co(L2)2][ClO4]2, [Ni(L2)2][ClO4]2, [Co(L3)][ClO4]2, and [Ni(L3)2][ClO4]2. Clarification of earlier reports of L2 and some of its complexes has been achieved by X-ray crystallographic characterization. Crystal data for orthorhombic [Ni(L1)][ClO4]2: space group Pna21 (No. 33); a = 13.270(4), b = 16.564(4), c = 11.348(4) Å; Z = 4; R = 0.047, Rw = 0.040 for 1474 reflections. For monoclinic [Ni(L2)2][ClO4]2: space group P21/c (No. 14); a = 7.617(3), b = 18.034(3), c = 9.098(2) Å, β = 91.58(2)°; Z = 2; R = 0.046, Rw = 0.039 for 1487 reflections. For tetragonal [Co(L3)][ClO4]2: space group [Formula: see text] (No. 121); a = 13.840(5), c = 14.902(4) Å; Z = 4; R = 0.054, Rw = 0.058 for 974 reflections. The new complexes and previously reported [Fe(L1)][ClO4]2have been examined by cyclic voltammetry, NMR, ESR, electronic spectroscopy, and magnetic susceptibility measurements. The influence of L2's extra-cyclic hydroxyl, of changes in M—S lengths, and of O for S substitution on structural, magnetic, redox, and spectroscopic properties has been studied. Keywords: X-ray structures, electrochemistry, magnetic phenomena.