Nickel cyclam complex

Table 6.1 summarizes the thermodynamic parameters relating to the macrocyclic effect for the high-spin Ni(n) complexes of four tetraaza-macrocyclic ligands and their open-chain analogues (the open-chain derivative which yields the most stable nickel complex was used in each case) (Micheloni, Paoletti Sabatini, 1983). Clearly, the enthalpy and entropy terms make substantially different contributions to complex stability along the series. Thus, the small macrocyclic effect which occurs for the first complex results from a favourable entropy term which overrides an unfavourable enthalpy term. Similar trends are apparent for the next two systems but, for these, entropy terms are larger and a more pronounced macrocyclic effect is evident. For the fourth (cyclam) system, the considerable macrocyclic effect is a reflection of both a favourable entropy term and a favourable enthalpy term. 

In addition to the charge control over the reaction discussed above, there is also a marked element of conformational control over alkylation reactions. This is seen clearly in the methylation of the nickel(n) complex of the tetraaza macrocyclic ligand, cyclam (Fig. 5-32). Reaction of the nickel complex with methylating agents allows the formation of a A, A V",A "-tetramethylcyclam complex. In this product, each of the four nitrogen atoms is four-co-ordinate and tetrahedral, and specific configurations are associated with each. Of the four methyl groups in the product, two are oriented above the square plane about the nickel, and two below it. 

The assembly of 4-ferf-butylbenzoic acid and cyclam (A, Figure 2) [54] has a planar arrangement of the macrocycle s N4 donor set with a tram disposition of the carboxylates very similar to that observed in the structure of the nickel complex, [Ni(cyclam)(benzoate)2], (B, Figure 2).[55] Formation of the nickel complex from the assembly and extraction of the nickel ion can then be represented by the pH-swing equilibrium... 

Syntheses of 5,12-dioxocyclams containing the quinoxaline nucleus have utilized the ring opening of both rings of 3,3,6-trimethyl-l-azapenam 148. Treatment of 148 with acid gave the cyclam (Equation 16) after reduction of the imine. Unfortunately, this and related cyclams and their nickel complexes showed little useful biological activity in their interaction with DNA <2003JOC4179>. 

Cyclam is often desired as a ligand for other metal ions. The free base may be obtained in good yields by the following procedure without isolating the nickel complex. 

Like [Ni(cyclam)], the above dinuclear nickel complex exists in aqueous solution as a mixture of low-spin, four-coordinated and high-spin, six-coordinated species, with the equilibrium between the two being dependent on the ionic strength. In the presence of excess per-... 

Tetraheterodecalin podands, their linkers, and resulting macrocycles A hoard of constitutionally and stereochemically dynamic systems 13IJC45. Thermodynamics and kinetics of CO2, CO, and H binding to the metal centre of CO2 reduction catalysts, in particular, cobalt and nickel complexes of 1,4,8,11-tetraazacyclotetradecane (cyclam) and its derivatives 12CSR2036. 

Macrocycle Vila readily forms the doubly deprotonated nickel complex upon addition of Ni(OAc)2. The methylene chloride soluble complex was unreactive under reaction conditions similar to those employed in the nickel cyclam/PhIO system. However, oxidation of - 8-methylstyrene occurred readily when the hypochlorite phase transfer conditions were used. Epoxidation represented about 50% of the reaction pathway with the remainder yielding a mixture of benzaldehyde and other over-oxidation products. 

Cobalt(III) sepulchrate (l)8 and tetrazamacrocyclic complexes of cobalt(II) (2)9 and nickel(II) (3) (6)9-11 catalyze the electroreduction of water to dihydrogen, at potentials ranging from - 0.7 V (complex (1)) to — 1.5 V (complexes (4)-(6)) vs. SCE in aqueous electrolytes, with current efficiencies as high as 95% for complex (4).9 It is noteworthy that the binuclear nickel biscyclam complex (6) is 10 times more active (at pH 7) than the mononuclear nickel cyclam complex (5). This behavior tends to indicate that some cooperativity between the two metal centers occurs in complex (6), as depicted in the possible reaction (Scheme 3) involving a dihydride intermediate.11... 

It is well-established that electroreduced nickel(I) complexes of cyclam and a variety of substituted cyclams add oxidatively to alkyl halides to give alkylnickel(III) complexes in organic solvents,251,276 the lifetime of the carbon-nickel bond governing the overall behavior of the system. However, it was shown that [Ni (tmc)]+ (one-electron reduced form of complex (17) tmc= 1,4,8,11-teramethyl 1,4,8,11-tetraazacyclotetradecane) reacts with alkyl chlorides in aqueous alkaline solution in a one-electron process.277,278... 

Nickel compounds can also be used as radical generators in electroreductive processes, according to previous work on Ni-cyclam and related complexes [64], Three research groups have reinvestigated the process and extended its use... 

Nickel-cyclam and related complexes can also be used though previous reports indicated that the turnover of Ni/(tet a) in acetonitrile is low [85]. The process has now been reinvestigated to show that Ni/(tet a) can been used in catalytic conditions (2%) in DMF containing NH4CIO4 as proton source to perform the alkylation of unsaturated esters, ketones, or nitriles (Table 9) [86]. Yields are good if the terminal carbon of the double bond is not substituted (Ri = H). 

However, the redox potentials of the Ni(II) complexes of the aza-cyclam (3b-3g) containing carboxamide or sulfonamide functional groups are reported to be influenced by the nature of the functional group. In particular, the amide fragment controls the reduction potential for the Nim/Nin and NiI1/NiI redox couples, which may be attributed to the it interaction between the nickel ion and the amido group 14). 

Ni(II) complexes of cyclam and oxocyclam derivatives catalyze the epoxidation of cyclohexene and various aryl-substituted alkenes with PhIO and NaOCl as oxidants, respectively. In the epoxidation catalyzed by the Ni(II) cyclam complex using PhIO as a terminal oxidant, the high-valent nickel- complexes (e.g., LNiin-0, LNi=0, LNiin-0-... 

The oxidation of nickel(II) complexes of cyclam and Me6cyclam by NO+ in acetonitrile solutions has been studied by Barefield and Busch.3049... 

Coordinated secondary amines can also be alkylated, but only after deprotonation by a strong base generates a suitable nucleophile. Work on rhodium(III) complexes of ethylenediamine12 has been extended to nickel(II) complexes of various fully saturated macrocycles such as cyclam (Scheme l).13,14 The methylated cyclam complex is kinetically inert, unlike the isomer with all four methyl groups on the same side of the ring, which is obtained on reaction of the preformed tetramethyl cyclam with nickel ions. 

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