Unsaturated hydrocarbon bridges

Bridges that will be surveyed include saturated hydrocarbon bridges, unsaturated conjugated hydrocarbon bridges, and DNA. This review is not intended to be exhaustive, because many extensive reviews have already been published in this area [6-14]. 

Fiereby, Vq refers to the maximal electronic coupling element and p is the decay coefficient factor (damping factor), which depends primarily on the nature of the bridging molecule. From the linear plot of In ETmax versus R the p value is obtained as 0.60 A [47]. This p value is located within the boundaries of nonadiabatic ET reactions for saturated hydrocarbon bridges (0.8-1.0 A ) and unsaturated phenylene bridges (0.4 A ) [1-4,54,55]. 

The reactivity of osmium with terminal alkynes can be used to prepare bimetallic complexes. When an osmium hydride reacts with a terminal aUcyne that is part of a ruthenium complex, the product is a Ru-Os bimetallic complex with a hydrocarbon bridge (Scheme 16). Another bimetallic complex, [Cp(PPh3 )20s=C=C=CH-C=C-0s(PPh3)2Cp]+, resulted from the reaction between [CpOs(PPh3)2]+ with HC=CCH(OH)C=CH followed by alumina. The unsaturated C5 bridging group is V-shaped. 

Finally, it is instructive to compare the P value of about 0.4 A for photoinduced ET through the unsaturated bridges of 48(1) and 48(3) with the value of about 0.85 (0.95 bond" ) for ET through saturated hydrocarbon bridges (Eq. 23). 

TT Ceccon, A., Santi, S., Orian, L., Bisello, A., Electronic communication in heterobinuclear organometallic complexes through unsaturated hydrocarbon bridges . Coord. Chem. Rev. 2004, 248, 683-724. 

These are quite comparable to distance dependences of charge transfer rates for molecules in homogeneous solution from, for example, 2nm for unsaturated hydrocarbon bridges to 10nm for saturated hydrocarbon bridges. 

Addition of anionic Fischer carbene complexes, of their CS2 adducts and of an anionic thiocarbyne complex to cationic organometallic Lewis acids and to coordinated, unsaturated hydrocarbons gives novel hydrocarbon bridged heterodimetallic compounds. Oxidative addition of halocarbyne complexes to zerovalent platinum and palladium complexes provides a synthesis of carbido bridged complexes. 

Nucleophilic attack on coordinated unsaturated hydrocarbons is one of the fundamental and particularly well studied reactions in Organometallic Chemistry. The addition of carbonylme-tallates instead of common nucleophiles provides a directed synthesis of hydrocarbon bridged complexes. Carbonylmetallates (particularly Re(CO)5", Os(CO)4 ") add to 7C-bonded olefin, acetylene, allyl, diene, trimethylenemethane, dienyl, benzene, triene and cycloheptatrienyl ligands in cationic complexes and give hydrocarbon bridged bi- and trimetallic, homo- or he-teronuclear complexes [1]. 

A direct connection between the allene group and another unsaturated functional group is no prerequisite for interesting chemical behavior, as illustrated for hydrocarbons 28-31 in Scheme 5.3, in which the allene unit and the above four substituents are connected either by a methylene group or by an ethano bridge, as for allenes 32-35. 

Tetraborane(lO) reacts, as does diborane, readily with unsaturated hydrocarbons. Alkenes give access to B2,B4-alkylene bridged B4H8(RCH-CHR) organo-boranes [37] while alkynes, allenes and l-ene-3-ynes lead to a variety of carba-boranes [38-40]. 

Cholesterol as well as unsaturated or saturated hydrocarbon chains are used as lipophilic lipid anchors. Although Cl 8-hydrocarbon chains (oleoyl or oleyl unit) are only used in unsaturated compounds, structural variations with C14-, C16-, or even Cl 8-hydrocarbon chains in saturated compounds are known (27). The lipophilic units are linked with a parent structure (usually glycerol) via ether (e.g., DOTMA) or ester bridges (e.g., DOTAP). Ester bridges are often used to create the linkage to avoid cytotoxicity, because ether bonds are more difficult to break down biologically than ester bonds (58). Substances that are easy to decompose and are therefore often used as a spacer are carbamate units (29) [e.g., 3p-[A-(A, A -dimethylaminoethyl)carbamoyl]-cholesterol (DC-Chol)], amide units, or phosphate esters. However, a direct correlation between toxicity and the... 

The structure of the ozonides of the simplest unsaturated cyclic hydrocarbons can be elucidated based on the rotational spectra of the FOZ and its isotopomers, such as species singly enriched with H, or 0 or multiply enriched with 0. Thus, the FOZ of cyclobutene has structure 293, of Cj symmetry, with the 00 bridge slightly longer than that of the FOZ of ethylene, to accommodate some ring tension . The microwave spectra of the FOZ of cyclopentene point to a ground state of structure 294, of Cj symmetry, with the OO bridge in the endo conformation . ... 

Polymeric hydrocarbon elastomers, such as natural rubber, are cross-linked or vulcanized by the use of sulfur, which reacts with the carbon of the unsaturated bonds in polymer molecules to form a bridge between two molecules so that one polymer molecule is covalently bonded to a second polymer molecule (6). 

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