Structure carbon bridging

Isoporphycene is of special interest because this structural type should exist in two stereoisomeric forms (E)- and (Z)-15, respectively, which arc possible due to the thrcc-carbon bridge linking two of the pyrrole rings. 

The dipyrrylacelylenedicarbaldehyde 18, which already contains, along with the acetylene moiety, two of the two-carbon bridges of the final macrotetracycle, can be prepared by a Heck-type coupling of 5-iodopyrrole-2-carbaldehydes with acetylene.8a,b The main conjugation pathway with 22n electrons is already present in the acetylene-cumulate systems 19. The expected planarity of this chromophore has been confirmed by X-ray structure analysis.8a b... 

One-electron oxidation of the vinylidene complex transforms it from an Fe=C axially symmetric Fe(ll) carbene to an Fe(lll) complex where the vinylidene carbon bridges between iron and a pyrrole nitrogen. Cobalt and nickel porphyrin carbene complexes adopt this latter structure, with the carbene fragment formally inserted into the metal-nitrogen bond. The difference between the two types of metalloporphyrin carbene, and the conversion of one type to the other by oxidation in the case of iron, has been considered in a theoretical study. The comparison is especially interesting for the iron(ll) and cobalt(lll) carbene complexes Fe(Por)CR2 and Co(Por)(CR2) which both contain metal centers yet adopt... 

Triply bridging carbonates between three zinc centers have been identified in nine different X-ray structures deposited in the CSD 458,461,465-467 For example, a binuclear ft-OH zinc complex with a tetradentate /V-donor ligand absorbs atmospheric carbon dioxide to a triply bridged carbonate.468 Examples are also known where the metal atoms are in varying coordination environments. The complex cation [Zn3(bipyridine)6(/U3-C03)(H20)2]4+ contains one penta- and two hexacoordinate zinc centers.469 A tetrapodal compartmental ligand forms a tetrameric complex with zinc that contains the carbonate bridging between three of the four zinc centers.470... 

During the investigations leading to the determination of these structures, another compound, containing a carbonate-bridged di-nuclear anion, was obtained (217), whose structure is shown in Fig. 

Fig. 22. Molecular structure of the carbonate-bridged 3-methyl salicylate complex anion [Be2(0H)(C03)(C7H6 0C02)2]3. Reproduced with permission from Ref. 217). Copyright 1993, Wiley-VCH Verlag GmbH. 

Additional studies have been reported on the electron diffraction of both monomeric and dimeric AlMe3 in the gas phase, and a variety of other spectroscopic studies have appeared which all support structure IV (7). Several of the most important parameters for this structure and related carbon-bridged species are collected in Table II. 

Scott and Hashemi have examined the effect of constraining the C(l)-C(7) distance in the homotropenylium ring by linking these two atoms by a three carbon bridge (termed a molecular caliper ), 40112. While the structure of 40 has not been determined, the C(1 )-C(7)... 

Figure 2 summarizes the structural parameters of the vinylidene or propadienylidene ligands when bonded to various metal clusters. It can be seen that the C—C double bond is lengthened to ca. 1.4 A as a result of the interaction with the cluster the a-carbon bridges three or four metal atoms, with M-C distances consistent with normal single bonds. 

In Section 2.2.1-2.2.3, a brief discussion on the chemistry, properties and structure of contracted, isomeric, and inverted porphyrins has been presented. Here, a brief sketch of the structural diversity in expanded porphyrins is given. Expanded porphyrins represent those synthetic macrocycles in which pyrrole or heterocyclic rings possess connectivity through mcso-carbon bridges. The conjugation of the porphyrin has been... 

The structural mutation of our 29 by variation of the macrocycle (via its heteroatoms and carbon bridges), the lumophore, and the spacer is obviously a rich vein to mine for luminescent PET sensors. Kubo s laboratory (formerly at Kyushu University, now at Kanagawa University, Japan) has systematically explored this line [81-84], 33 has a straight swap of 1-pyrenyl for 9-anthryl [81]. 34 to 36 [81-83] use two nitrogens in the crown ether instead of the one in 29,... 

Amitriptyline hydrochloride and imipramine hydrochloride are similar in structure, with the exception of the nitrogen in the center ring, and belong to the family of phenothiazine compounds. Finally, the two-carbon bridge linking the aromatic rings may be ethyl (-CH2CH2-) or ethylene (-CH=CH-). 

Table 13.6.1. Structural parameters found in some carbon-bridged organoalu-minum compounds...

---