Uranyl templates

Scheme 1-4 Boron and uranyl templates direct the formation of sub- and superphthalocyanines respectively (12 and 13). Removing the template results in the formation of the normal phthalocyanine (15).

Invariably hydrolytic instability of lanthanide Por complexes, particularly that observed for the larger lanthanide elements, negatively influences their prospective application in terms of biomedicine (Sect. 7.4). As a response to this problem, larger porphyrin-like or expanded porphyrins , the so-called texaphyrins (Tx), have been examined by Sessler et al. [246]. The motivation, that expanded systems better accommodate larger ions, was previously demonstrated in a uranyl superphthalocyanine (SPc) complex [247], This SPc-complex contains an expanded, cyclic five-subunit pentakis(2-iminoisoindoline) which is formed by a template reaction of o-dicyanobenzene with anhydrous uranyl chloride. The uranium is displaced by only 0.02 A from the mean N5-plane. 

Reinhoudt and co-workers (101-105) have reported a series of Schiff base macrocyclic polyether ligand complexes prepared via barium cation-templated Schiff base condensation of the appropriate polyether dialdehyde with a diamine, in the presence of a transition metal or uranyl acetate, followed by removal of the Ba2+ template cation on subsequent addition of guanidinium sulfate (Scheme 19). The copperdl) and nickeldl) complexes (62) and (63) exhibit reversible redox couples... 

Template reactions2 represent an extensive and important class of chemical transformations in which a metal ion serves as the hub for the coordinative cyclization of organic ligands. The method can lead either to enhanced yields over direct cyclizations or to the formation of otherwise inaccessible macrocycles. It is in this latter context that the uranyl ion acts, as a result of its large ionic radius and propensity toward pentagonal bipyramidal coordination, to form expanded macrocycles. 

Figure 51 Phosphate coordination geometries and cocrystallized template from the structure-directed preparation of a layered uranyl phosphate (Francis, Drewitt et al. Chem. Commun. 1998, 279-280).

Figure 52 Coordination environment (a) about uranyl and within and (b) between uranyl phosphate layers in the first organically templated open-framework uranium phosphate, [Et2NH2]2[(U02)5(P04)4] (Danis,...

Template reactions of 1,2-dicyanobenzenes with uranyl salts lead to the synthesis of super-phthalocyanines (Scheme 54).237,23s jjjg uranyl ion can be displaced from the macrocycle by a range of metal salts, but a ring contraction occurs to yield normal phthalocyanine complexes (Scheme... 

Figure 7 The uranophane sheet anion topology (a) and the uranophane-type sheet (b) found in some diamine-templated layered uranyl vanadates (c), in structure of (U02)3(V0 )2.5H20 where layers are connected by UO7 pentagonal bipyramids (d), in diamine-templated 3-D uranyl vanadates (e), in uranyl divanadate U2V2O11 (f). The occupation of the sqnares of the uranophane anion topology by the VO5 entities results in 2-D layers (g) that are linked into a 3-D framework inUVOj.

In diamine-templated compounds such as diaminobutane- (H2dbu) and diaminopropane-(H2dpr) templated uranyl vanadates, the aa/aa isomer allows sharing of two opposite equatorial edges of the interlayer UO7 bipyramids with four VO4 tetrahedra of the adjacent layers, so one interlayer uranyl ion is linked to four vanadate tetrahedra (Fig. 7e). Thus, the stmctural notation of these... 

Thus, the U V ratio plays a key role in the type of polymerization and on the obtained 2-D or 3-D structural architectures. However, the role of the counter ion is not obvious. It can be suggested that, as already observed for other uranyl oxoanion compounds, the counter ions have no template effects but rather act as space fillers within the uranium vanadate substructure. 

Most of the templates diseussed in this chapter are kinetic templates. Some of the early examples of metal cation templated macrocyclization studied by Curtis and Busch [7,9] (Schemes 1-1 and 1-2) are thought to operate under thermodynamie eontrol. The clearest evidence for a thermodynamic template effect comes when the template-free product is not stable under the reaction conditions. For example, treatment of 1,2-dicyanoben-zene 11 with boron trichloride or uranyl chloride results in the formation of subphthalo-... 

The known uranium-macrocycle complexes of metal-templated origin are listed in table 22 all contain the trans-dioxouraniumCVI) ion, (uranyl), as the metal... 

It is by now well established that all lanthanide (III) ions, as well as the yttrium(III) and uranyl ions, are effective templates for the synthesis of macrocyclic complexes the scarcity of information on similar systems involving 5f-block elements other than uranium appears to be due to lack of investigation rather than to any intrinsic failure of these elements to function as templates in macrocyclic syntheses. 

The vast majority of the macrocyclic complexes of the lanthanide(III), yttrium(III) and uranyl ions obtained so far by metal-templated synthesis are of the Schiff-base type even the few known examples of simple polyamine complexes actually result from the cyclic condensation of a diamine with a (modified) carbonyl precursor. In general, the metal-templated synthesis is facilitated by the presence of oxygen-donor anions, such as nitrate, acetate, or trifluoromethylsulfonate lanthanide(III) thiocyanates have also been successfully used. With a few exceptions, the outcome of the synthesis appears to be independent of the order of addition of the reactants. No deliberate attempts have been made to investigate the detailed mechanism of these metal-templated cyclic condensation reactions. 

The synthesis of metallocycles that contain an immobilized uranyl cation follows the route depicted in Scheme 1 [ref 8 and 14]. The appropiate dialdehyde and diamine undeigo a Ba -templated macrocyclization in methanol. Upon reaction with uranyl acetate the barium salt is eliminated. The cavity thus created can be occupied by small neutral molecules, which can coordinate to the uranyl cation. The uranyl cation provides a fifth coordination site in the equatorial plane. This planar coordination environment offers the possibility to complex neutral molecules, instead of doing reactions at the activated or reduced transition metal cation. Usually, the complexes are obtained with a water molecule filling the cavity. By introducing ffnent guest molecules that can coordinate to the electrophilic center it is possible to replace the water. 

Receptor 15 has been designed for the complexation of barbiturates [ref 17]. The syndiesis was accomplished by Ba -templated macrocyclization of the appropiate dialdehyde and cis-l,2-cyclohexanediamine, followed by reaction with uranyl acetate. NMR experiments in CDCI3 revealed that intramolecular self-complexation takes place and a constant Kg if-ass for this process of IS has been estimated. The association constants of three barbiturates could be determined by NMR titrations in a mixture of CDC13 and DMSO-dg (95 5). The association constants of dimethyl, diethyl, and ethylphenyl barbituric acid are 112, 97, and 45 Imol respectively. It is obvious that a sterically more demanding barbiturate results in a lower association constant. The host 15 is able to transport barbituric... 

In contrast, the thermodynamic template effect involves a particular template species (usually a transition-metal ion) binding to a ligand that is complementary to itself, within an equilibrating mixture of products that are formed without the involvement of the template. The binding of the template thermodynamically stabilises the most complementary product (usually a macrocyclic compound). An excellent example is the preparation of phthalocyanine (2.8). Treatment of 1,2-dicyanobenzene with either boron trichloride or uranyl chloride results in two different-sized macrocycles (2.6 and 2.7, respectively) (Scheme 2.4). Macrocycles 2.6 and 2.7 are themselves only stable when the template is still present. On the removal of the template, the normal phthalocyanine (2.8) is formed, which is highly stable and forms many coordination complexes with a range of transition-metal ions. This is also a very attractive synthetic procedure for the preparation of unsymmetrical phthalocyanines. 

The uranyl ion turns out to be effective in the template synthesis of L646... 

The uranyl cation, UOj, turned out to be an effective template for constructing binuclear macrocyclic systems related to those described above [108-110,112-116]. Thus, metallomacrocycles U02(L1755)(H20) to U02(L1757)(H20) were synthesised from 1,2-benzenediamine and the corresponding aldehydes in the presence of U02(CH3C00)2-2H20 in methanol/dichloromethane (Eq. 8.44) [117]. 

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