Isolation and characterization of polydentate ligands derived from the reaction of pentaerythrityltetrabromide and ethylenediamine

Reaction of pentaerythrityltetrabromide with ethylenediamine yields three product amines. These have been isolated by separation of the corresponding Ni2+ complexes. The free amine hydrochlorides have been obtained by acid hydrolysis of the Ni2+ species and have been characterized by nuclear magnetic resonance and mass spectroscopy. The crystal structures have been determined for the hexahydrochloride salt of polyamine ligand L1, [H6(L1)]Cl6•4H2O (L1 = C11H28N6, 6,6-bis(4-amino-2-azabutyl)-1,4-diazacycloheptane)(C2/c, a = 15.975) Å, b = 10.723 (1) Å, c = 14.686(1) Å, β = 99.40(1)°, Z = 4), and for the octahydrochloride salt of L2, [H2(L2)Cl8•2H2O (L2 = 5,5-bis(4-amino-2-azabutyl)-1,9-diamino-3,7-diazanonane) (Pccn, a = 12.834(2) Å, b = 16.691(1) Å, c = 14.167(1) Å, Z = 4). Both crystals exhibit strong N—H … Cl,O,N bonding, although in the latter compound the waters of crystallization do not participate in the apparent hydrogen bonding interaction. Both these species are of interest since they represent the precursors to square planar Ni2+ complexes that incorporate the MN4 chromophore. Ring closure of both the free amines L1 and L2 by reaction with diethyl malonate leads to cyclized products that are related to the cyclam ([14-aneN4) tetraaza macrocylic ligands. Keywords: pentaerythrityl, amine, spiro ligand hydrochlorides.

Electron transfer reactions of nickel complexes of 1,4,7-triazacyclodecane

The preparation of [Ni(1,4,7-triazacyciodecane)2]3+, (Ni(10-aneN3)23+) is described. The existing procedure has been modified leading to good yields of the ligand trihydrochloride. The nickel(II) analogue (reported previously) is oxidised in a facile manner, either by Co3+aq in acidic aqueous media or by NO+ in CH3CN. Since the octahedral NiN6, chromophore is retained upon electron transfer, outer sphere reactions both of the Ni(II) and Ni(III) species have been studied. Rates of oxidation by various nickel(III) macrocycles have been measured and details are provided. Electrochemical oxidation of the Ni(II) complex is consistent with E0(Ni(10-aneN3)23+/2+) = 0.997 V (vs. NHE). The data have been used in a Marcus correlation, leading to the self-exchange rate k11 for the couple (Ni(10-aneN3)23+/2+) = (2 ± 1) × 104 M−1 s−1. This value is compared with other data derived using octahedral Ni(II)/Ni(III) centres. The oxidation of the Ni(II) complex by Co(III)aq has been studied in both protonated and deuterated media. There is no evidence for any proton transfer (from the N—H) being coupled to the electron transfer step. The observed rate constant for the reaction of Co3+ with Ni(II)(10-aneN3)22+ (550 M−1 s−1) may be compared with the calculated outer sphere rate (270 M−1 s−1). An estimate of k11 (CoOH2+/+) ~ 3 M−1 s−1 for the CoOH2+/+ exchange is discussed.

Kinetics and mechanism of the oxidation of benzenediols and ascorbic acid by bis(1,4,7-triazacyciononane)nickel(III) in aqueous perchlorate media

The reactions of the outer-sphere electron transfer reagent, Ni(9-aneN3)23+, (bis(1,4,7-triazacyclononane)nickel(III) ion) with ascorbic acid, hydroquinone, catechol, and resorcinol have been investigated. The absence of any proton related equilibria with the oxidant provides a means of ascribing the observed inverse hydrogen ion dependences to reactions of the dissociated ascorbate or quinolate ions, (HA−). The data are consistent with the rate-determining one-electron transfer reactions:[Formula: see text]followed by rapid oxidation of the radical ions formed. In the reaction with ascorbic acid, k1 ~ 0 and k2 (T = 25° C) = 5.2 × 106 M−1 s−1 (ΔH≠ = 10.1 ± 2.5 kcal mol−1, ΔS≠ = 5.7 ± 5.1 cal mol−1 K−1). For hydroquinone, catechol, and resorcinol, k1 = 2.9 × 103, 2.8 × 102, and ~0 M−1 s−1and k2 = 6.9 × 109, 4.1 × 109, and 2.8 × 108 M−1 s−1, respectively. These data have been combined with those from other similar reactions leading, by use of a Marcus correlation, to self-exchange rate constants for the HAsc−/HAsc• couple of 3.5 × 105 M−1 s−1 and for the H2Quin0/+ and H2cat0/+ systems of 5 × 107 and 2 × 107 M−1 s−1, respectively. The importance of the effect of bond-reorganisation on electron transfer is discussed.

Kinetics and mechanisms of the oxidation of ascorbic acid and benzene diols by nickel(III)tetraazamacrocycles in aqueous perchloric acid

The kinetics of reaction of ascorbic acid, hydroquinone, and catechol (H2A) with nickel(III) macrocycles (NiL3+) (L = cyclam, meso-(5,12)-7,7,14,14-Me6-14-ane-1,4,8,11-N4 (tet-a), and rac-(5,14)-7,7,12,12-Me6-14-ane-1,4,8,11-N4 (tet-c):[Formula: see text]have been investigated in aqueous perchloric acid solutions using the stopped-flow technique. The data are consistent with a rate-determining one-electron transfer reactions:[Formula: see text]followed by a rapid oxidation of the radical formed. In the reaction with ascorbic acid, for Ni(cyclam)3+, k1 = 250 M−1 s−1 (25 °C), k4Ka = 680 s−1; Ni(tet-c)3+, k1 = 2.52 × 103 M−1 s−1, k4Ka = 1.06 × 104 s−1; and Ni(tet-a)3+, k1 = 2.85 × 103 M−1 s−1 (21.95 °C), k4Ka = 1.26 × 104 s−1. With catechol, k1 = 6.98 × 102 M−1 s−1, 1.73 × 104 M−1 s−1, and 3.3 × 104 M−1 s−1 respectively in reactions with Ni(cyclam)3+, Ni(tet-c)3+, and Ni(tet-a)3+. With hydroquinone, k1 = 1.09 × 104 M−1s−1 (Ni(cyclam)3+) and 2.49 × 105 M−1 s−1 (20.9 °C) (Ni(tet-c)3+). The reactions are considered to take place via an outer-sphere mechanism and rate constants are discussed in terms of the Marcus cross correlation. Use has been made of predicted rate constants to identify reaction pathways in the hydrogen-ion dependent systems.

The oxidation of a nickel(II) oxime complex by polypyridine iron(III) ions in acidic aqueous media. Formation of a Ni(IV) species

The kinetics of the oxidation of a nickel(II) oxime complex by a series of iron(III) phenanthroline and bipyridine cations, Fe(LL)33+, have been investigated. The overall reaction[Formula: see text]is considered to take place via two one-electron steps, the first (rate-determining) process yielding Ni(III)(H2L)3+ being followed by very rapid oxidation to the nickel(IV) product. The second order rate constants at 25 °C in 0.50 M p-toluenesulphonic acid are: Fe(dmbpy)33+, 1.32 × 103 M−1 s−1; Fe(dmphen)33+, 3.73 × 103 M−1 s−1; Fe(bpy)33+, 3.20 × 104 M−1 s−1; Fe(mphen)33+, 6.83 × 104 M−1 s−1; Fe(phen)33+, 9.16 × 104 M−1 s−1; and Fe(cphen)33+, 5.6 × 105 M−1 s−1. The reactions are outer-sphere in character. The rate of the corresponding cross reaction of nickel(II) species with Ru(bipy)33+ (k12 = 1.1 × 106 M−1 s−1) has also been measured. Activation parameters have been incorporated in a Marcus theory correlation to yield information (k11 ~ 3 × 103 M−1 s−1) for the NiII(H2L)2+/NiIII(H2L)3+ electron exchange reaction. This value is compared with exchange parameters for NiII/NiIII tetraaza-macrocycle and other similar systems.

Kinetics and mechanisms of the oxidation of a nickel(II) oxime complex by cobalt(III) aquo ions in acidic aqueous media. Formation of a Ni(IV) complex

The kinetics of the oxidation of a nickel(II) oxime complex to the corresponding nickel(IV) species by aquo cobalt(III) has been investigated in aqueous CF3SO3H at various acidities (0.063–0.263 M) and temperatures (10.5–30.6 °C). The inverse dependence of the rate constants on [H+] is attributed to a reaction pathway[Formula: see text]with k2 = 1.75 × 104 M−1 s−1, ΔH≠ = 6.2 ± 0.5 kcal mol−1, ΔS≠ = −29.6 ± 8.0 cal deg mol−1 (Kh = 2 × 10−3 M for the hydrolysis constant for cobalt(III)). The self-exchange parameter for the NiII/NiIII-oxime couple (k11 = 2.3 × 103 M−1 s−1, ΔG≠ = 10.4 kcalmol−1) has been estimated from Marcus theory correlations. Comparisons are made between this value and the parameters determined for the Ni(II)/(III) tetra-aza macrocycle complexes.

Nickel(III)–tris(di-imine) complexes: a series of strong one-electron oxidants

The complexes [Ni(III)(LL)3](ClO4)3 (LL = phen, bipy, and substituted derivatives) have been isolated as products of the electrolysis in acetonitrile of the corresponding nickel(II) species. Electron spin resonance measurements for all nickel(III) species in frozen anhydrous CH3CN and in frozen 4.0 M HClO4 solutions (77 K) show a single near symmetric line shape (g ≈ 2.10) with fine structures appearing only in the presence of mixed solvent systems. In a CH3CN/5% H2O glass (77 K) quintets in the [Formula: see text] and [Formula: see text] regions are observed [Formula: see text]consistent with Jahn–Teller distortion expected for a low-spin d7 system. Ultraviolet–visible spectra are also reported. Electrode potentials for the Ni(LL)33+/2+ couples in CH3CN (0.1 M NaClO4) lie in the range 1.51 V (Ni(4,4′-Me2bipy)33+) to 1.82 V (Ni(5-NO2-phen)33+) and the powerful oxidizing nature of these species has been confirmed in reaction with both organic and inorganic substrates.