This critical review surveys factors involved in the convergent self-assembly of square [n x n] (n = 2-5) polymetallic grids, using ligands with encoded coordination information appropriate to the metal ion's coordination algorithm. While entropic factors drive these reactions, other effects, including metal ion CFSE, ligand conformational character, electrostatic factors etc., can lead preferentially to non-grid oligomers. It will be of interest to the general supramolecular community, particularly coordination chemists attempting to create predetermined architectures, with applications oriented molecular based properties (86 references).

A diamine-bis(phenolate) chromium(III) complex, {CrCl[O(2)NN'](BuBu)}(2) catalyzes the copolymerization of cyclohexene oxide with carbon dioxide. The synthesis of this metal complex is straightforward, and it can be obtained in high yields. This catalyst incorporates a tripodal amine-bis(phenolate) ligand, which differs from the salen or salan ligands typically used with Cr and Co complexes that have been employed as catalysts for the synthesis of such polycarbonates. The catalyst reported herein yields low molecular weight polymers with narrow polydispersities. Structural and spectroscopic details of this complex along with its copolymerization activity for cyclohexene oxide and carbon dioxide are presented.

Dedicated to Professor Reginald Mitchell on the occasion of his 65th birthdayCyclacenes, for example, [12]cyclacene (1; Figure 1), and cyclophenacenes, for example, cyclo [12]phenacene (2), are aromatic belts that map onto the surfaces of various fullerenes and single-walled carbon nanotubes (SWCNTs).[1] Indeed, they can be viewed as the shortest possible open-ended (uncapped) zig-zag and armchair single-walled carbon nanotubes, respectively.[1e] Although interest in the synthesis of aromatic belts, especially cyclacenes, predates the discovery of the fullerenes [2] and SWCNTs [3] by decades, [4] the discovery of SWCNTs and their homology to aromatic belts has engendered more intense synthetic and theoretical interest, not only in cyclacenes [5] and cyclophenacenes, [6] but also in related aromatic/conjugated belts. [7] Of particular interest to us are the Vögtle belts, [8] for example, 3, which are armchair SWCNT segments having a pyrenoid or rylenoid motif, depending upon ones perspective.A recurring theme in the numerous unsuccessful attempts to synthesize aromatic belts is the inability to aromatize partially saturated belt precursors. The interplay of two major energetic factors, strain and aromaticity, is at the heart of the problem.[9] The generation of a nonplanar aromatic system from a non-or partially aromatic precursor is inherently disfavored by the build-up of strain, but favored by the aromatic stabilization energy (ASE) of the aromatic system being formed. Increasing the distortion from planarity causes the strain energy to become larger and the ASE to become smaller. As a result, strain ultimately becomes dominant and aromatization, by standard methods such as the elimination of water, can become unfavorable. For example, the addition of water (2 equiv) to Schlüters "double-stranded cycle" has been calculated to be exothermic by 42.2 kcal mol À1 .[7b]Therefore, if any approach to aromatic belts is ultimately going to be successful it will not only have to target a relatively stable structural motif, but also rely upon methodology that is capable of generating highly nonplanar aromatic systems under relatively mild conditions. The valence isomerization/dehydrogenation (VID) reaction of [2.2]metacyclophane-1,9-diene (4) to give pyrene 6 is such a method (Scheme 1 a), [10] and it has been used for the synthesis of [n](2,7)pyrenophanes containing nonplanar

Poly-(n)-topic ligands with a linear arrangement of coordination pockets self-assemble, generally in high yield, to produce square [n x n] grid complexes. Oligomeric, non-grid intermediates, identified by structural studies, have shown alternative construction pathways, but have also indicated possible mechanistic routes to grid assembly. Various factors are considered critical to grid formation, including reaction pH, metal ion identity, CFSE, and metal ion redox behaviour. Ultimately the design of the ligand is pivotal to successful grid self-assembly, and the size of the [n x n] grid is largely limited only by the synthetic limitations of the ligands themselves.

Polytopic hydrazone-based ligands are discussed in the context of the design attributes of the ligand and the power of self-assembly as a methodology for the synthesis of polymetallic systems with specific and predetermined organization of the metal centers in a closely spaced bridged arrangement. Magnetic exchange coupling occurs as a result of the close proximity of the metal ions. Homometallic, heterometallic, and mixed-spin-state [n x n] (n = 2-5) square grids are highlighted and discussed in terms of their structural and magnetic properties. Antiferromagnetic, ferromagnetic, and ferrimagnetic examples are described.

New self-assembled grids and clusters are reported, with square [2 6 2] M 4 (M = Mn(II) 4 , Cu(II) 4 ), trigonal-bipyramidal Mn(II) 5 , and square [3 6 3] M 9 (M = Mn(II), Cu(II)) examples. These are based on a series of ditopic and tritopic hydrazone ligands involving pyridine, pyrimidine and imidazole end groups. In all cases the metal centres are bridged by hydrazone oxygen atoms with large (>125u) bridge angles, leading to antiferromagnetic exchange for all the Mn systems (J = 22 to 25 cm 21 ), which results in S = 0 (Mn 4 ), and S = 5/2 (Mn 5 , Mn 9 ) ground states. The copper systems have a 90u alternation of the Jahn-Teller axes within the Cu 4 and Cu 8 grid rings (Cu 9 ), which leads to magnetic orbital orthogonality, and dominant ferromagnetic coupling. For the Cu 9 grid antiferromagnetic exchange between the ring and the central copper leads to a S = 7/2 ground state, while for the Cu 4 grids S = 4/2 ground states are observed. The magnetic data have been treated using isotropic exchange models in the cases of the Cu 4 and Cu 9 grids, and the Mn 5 clusters. However due to the enormity of a fully isotropic calculation a simplified model is used for the Mn 9 grid, in which the outer Mn 8 ring is treated as the equivalent of an isolated magnetic chain, with no coupling to the central metal ion.

The first X-ray structural determinations of pristine fullerene C(60), cocrystallized 1:1 with corannulene and with its pentaalkyl-substituted derivative, 1,3,5,7,9-penta-tert-butyl-corannulene, have now been achieved.

A series of lithium complexes were prepared from 2(N-piperazinyl-N′-methyl)-2-methylene-4-R′-6-Rphenols ([ONN] RR′ ) and characterized through elemental analysis, 1 H and 13 C{ 1 H} NMR spectroscopy, and X-ray crystallography. Treatment of the ligands with n-butyllithium afforded {Li [ONN] RR′ } 3 [R = Me, R′ = t Bu, (1); R = R′ = t Bu (2); R = R′ = t Am, (3), t Am = C(CH 3 ) 2 CH 2 CH 3 ], with trimetallic structures in the solid-state as shown by single-crystal X-ray diffraction. The reactivity of these complexes in the ring-opening polymerization of ε-caprolactone (ε-CL), as well as the influences of monomer concentration, monomer/Li molar ratio, polymerization temperature and time, was studied. Rates of polymerization were first order with respect to both monomer and lithium concentrations, and activation energies for the reactions were determined. MALDI-TOF MS analysis revealed that transesterification had occurred during the polymerization.