Metallo-squares

Figure 2.3 Solvent free supramolecular self assembly of a metallo-square.

It is worth mentioning that the reaction of 58 with the achiral diphosphine ligands [Pd (c -Et3P)2][OTf 2 and [Pt(cri-Et3P)2][OTf]2 under the above self-assembly conditions provided a diastereomeric mixture of products. In theory, six possible isomers could be formed (Eigure 5.36). In this example the chirality of the metallo-squares arises from the chiral conformation of the iodonium moiety in the assembly and the restricted rotation of the pyridine ligands around the M—N bond. 

Lippert has shown that the assembly between platimrai(II) centres and purine bases can be controlled by specific anions. In analogy to the hydrogen-bonded tetrads that guanine bases can form, this group synthesised a metallo-square in which the purine bases are interconnected by coordination to plat-inum(II) centres rather than hydrogen bonding interactions (see Fig. 2) [26]. 

Fig. 2 Schematic representation of a guanine quartet with a cationic guest and b metallo-square 9 based on platinum(II) and methylpurine with an anionic guest...

Fig.13 X-ray crystal structure of metallo-square [ Ni4(bptz)4(CH3CN)8 BF4](BF4)7...

The metallo-square hauvin s mechanism is still operating, as demonstrated by the isolation of metallacyclobutadiene complexes formed by cycloaddition of alkynes to alkylidyne complexes. The metallacyclobutadiene complexes themselves can also serve as alkyne metathesis catalysts confirming their intermediacy in the catalytic reactions starting from the alkylidyne complexes. Interestingly, they do not react readily with alkenes, rendering alkyne metathesis selective in the presence of olefmic bonds. ... 

The Schrock complex W H - u NAr O- - u, previously mentioned section. as an alkene metathesis catalyst, also initiates the polymerization of acetylene in toluene in the presence of quinuclidine with metallacyclobutene formation followed by opening of the metallacyclobutene, which again confirms the metallo-square mechanism. This polymerization can be controlled if a limited amount of acetylene 3 to 13 equivalents is introduced. The polyenes are formed... 

For the known nickel sites in biological systems, four-coordinate square planar, five-coordinate, and six-coordinate octahedral geometries are found.1840-1846 In general, the flexible coordination geometry of nickel causes its coordination properties in metallo-biomolecules to be critically influenced by the protein structure. 

A Simple Molecular Orbital Description of Square-Planar Metallo-bis(dithiolene) Bonding / 143... 

Square-planar metallo(diimine)(dithiolene) complexes generally display intense, solvatochromatic absorptions in the visible region of the spectrum that are not found in the corresponding metallo-bis(dithiolene) or metallo-bis (diimine) complexes. Futhermore, the LLCT transition energy does not vary appreciably as a function of the metal ion. Extended Hiickel calculations on Ni, Pt, and Zn metallo(diimine)(dithiolene) complexes indicate that the HOMO is comprised almost entirely of dithiolene orbital character (Figure 2), while the LUMO was found to possess essentially all diimine n orbital character (112, 252, 268). In stark contrast to the spectra of square-planar Ni and Pt metallo (diimine)(dithiolene) complexes, the psuedo-tetrahedral complexes of Zn possess extremely weak LLCT transitions. Now, it is of interest to discuss the differences in LLCT intensity as a function of geometry from a MO point of view. This discussion should help to explain important orientation-dependent differences in photoinduced electron delocalization and charge separation. 

Figure 24 displays the high energy (E > 25,000 cm-1) region of the room temperature electronic absorption spectrum for Zn(bpy)(tdt), where bpy = 2,2 -bipyridine. The LLCT transition occurs at 22,470 cm-1 (445 nm) with very weak absorption intensity (e = 72 M 1cm 1). The origin of the weak LLCT is a function of the symmetry of this psuedo-tetrahedral complex. A MO diagram for Zn(bpy)(tdt), derived from extended Hiickel calculations, is presented in Fig. 25. Irrespective of whether the metallo(diimine)(dithiolene) complex is square-planar or psuedo-tetrahedral, the point symmetry is C2V, and all intermediate geometries possess C2 symmetry. When the dithiolene and diimine planes are orthogonal (psuedo-tetrahedral geometry) the HOMO — LUMO transition represents a b2 —> b one-electron promotion and is electric dipole forbidden. However, the HOMO —> LUMO transition in a square-planar...

Herein, we will discuss in detail the bonding descriptions of two common types of metallo-bis(dithiolenes) four-coordinate square-planar complexes with Ni, Pt, or Pd as the central metal ion, and five-coordinate square-pyramidal Mo and W complexes that possess a strong 7t-donor axial oxo ligand. These are arguably the most intensely studied and best understood of the metallo-bis(dithiolenes). A detailed understanding of their electronic structure and spectroscopy will certainly provide much needed insight into the bonding descriptions of new and more complex metallo-bis(dithiolene) complexes. 

There have been a considerable number of MO calculations performed on four-coordinate square-planar metallo-bis(dithiolene) complexes at various levels of theory (283, 327, 328, 379-384). The most important differences in the results of these various calculations rests in the energy level ordering of the valence MOs as well as the degree of metal-sulfur covalency. This point is important as the nature of the MO scheme has greatly affected how the results of both ground and excited-state spectroscopic studies on very similar... 

The ability of the dithiolene ligand to promote in-plane spin delocalization in metallo-bis(dithiolenes) has been determined using [Cu(mnt)2]2, which possesses an unpaired electron in an in-plane b g (d ) orbital (390). This electronic structure allows a detailed comparison with the out-of-plane 7t-type spin delocalization present in square-planar nickel bis(dithiolenes). Interestingly, the measured spin delocalization onto the dithiolene ligands in [Cu(mnt)2]2 is remarkably different from that determined for [Ni(mnt)2]1-and [Ni(tds)2]1. The ESEEM studies on the [Cu(mnt)2]2 ion doped in a diamagnetic [Ni(mnt)2]2 host have determined that 50% of the unpaired spin... 

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