Binucleating macrocycles

Ligand (119) yields nickel complexes of type [Ni2L](BF4)4 (low spin) and [Ni2L(NCS)4].2H20 (high spin). For each of these complexes, physical studies indicate that the macrocycle circumscribes the two nickel ions such that each ion is surrounded by four sulfur donors in a planar array. 

Dimetallic large-ring systems incorporating bridging small 

Some representative systems. A further type of binuclear species incorporates two metal ions bridged by one or two small groups within the macrocyclic ring. Such complexes differ from the previous type in that the bridges between the metal ions do not involve donor groups which are directly attached to the macrocycle - see (122) and (123). A variety of 

A feature of the metal-ion chemistry of these large ring macrocycles is thus the structural diversity which may occur from one system to the next. This diversity can result directly from small changes in the structure of the cyclic ligand and is also aided by the inherent flexibility of the large rings involved. It is clearly also influenced by the nature of the other ligands available for complex formation. 

Some typical systems. An early example of a metal derivative of this class of ligand is (129) which was obtained (Cunningham Sievers, 1973) by a 

---

A few mixed-donor binucleating macrocycles incorporating some ether oxygen functions have already been mentioned in Chapter 3. This earlier discussion is now expanded to include cyclic systems in which the donor sets consist (totally or predominantly) of ether oxygens. 

Love and coworkers have reported a series of dinuclear cobalt complexes derived from a rigid binucleating macrocycle H4L 18 as shown in Fig. 26 (150). The synthesis of the dicobalt complex [Co2(L18)] (36) was achieved by an anaerobic transamination reaction between H4L18 and [Co(thf) N(SiMe3)2 2] in THF. The unsaturated species 36 forms a bis(pyridine) adduct, 36 py2 (py — pyridine), which has a cleft-like structure reminiscent of pacman diporphyrin complexes (151,152). Both cobalt ions are square pyramidal, with Col and Co2 displaced out of the N4-basal planes by 0.17 and 0.18 A, respectively. The apical sites are occupied by pyridine nitrogen atoms that are exo and endo to the cleft. Interestingly the endo pyridine is canted and reflects the... 

Of particular interest are the cases where, by a suitable design of the complex, the factors (a) to (d) above can be eliminated so that the departure of the electrochemical response from the purely statistical behaviour arises solely from cause (e). Two examples of such complexes which have been well studied are binuclear ruthenium complexes bridged by 4,4 -bipyridyl type ligands, and binuclear complexes where L, the binucleating macrocyclic ligand, is a symmetric... 

Figure L Dijferent types of binucleating macrocycles a) large rings, b) bis-macrocycles, c) axial macrobicycles, d) lateral macrobicycles, and e) cylindrical macrobicycles.

The more elaborate open-chain precursor H2LI674 has also been prepared and used for the construction of binucleating macrocyclic ligands (38, 39]. Similar to H2LI664, the compound H2LI674 readily forms monometallic complexes by reacting with metal acetate and triethylamine in alcohol solvents (Eq. 8.10). 

Figure 24. Proposed mechanism for the oxidation of phenol to o-benzoquinone catalyzed by a dicopper complex in a binucleating macrocycle.

---