Open-Chain Ligands

Crown ethers and eryptands have a eentral cavity where cations of opportune dimensions are held by chelation, enabling formation of complexes with other species. The stability of the complexes thus obtained depends on radii and electronic configurations as well as on the topology of the ligand. Open-chain PEGs behave in a similar way although the stability of their complexes is much lower. 

Complexes with Open Pentadienyl Ligands. Open-chain analogues of the Cp ligand have only received limited attention. Compoimds 21 (87) and 22 (88) have been featured in patent examples as moderately active polyethylene catalysts after activation hy [Cp2Fe][B(C6F5)4] and MAO, respectively. 

For trinuclear cluster complexes, open (chain) or closed (cycHc) stmctures are possible. Which cluster depends for the most part on the number of valence electrons, 50 in the former and 48 in the latter. The 48-valence electron complex Os2(CO)22 is observed in the cycHc stmcture (7). The molecule possesses a triangular arrangement of osmium atoms with four terminal CO ligands coordinated in a i j -octahedral array about each osmium atom. The molecule Ru (00) 2 is also cycHc and is isomorphous with the osmium analogue. 

Frensch and Vdgtle have recently appended three crown ether units to the cyclo-triveratrylene unit . Note that Hyatt had previously prepared the open-chained relatives of this structure (see Sect. 7.3 and Eq. 7.6). Whereas Hyatt prepared the cyclo-triveratrylene skeleton and then appended polyethyleneoxy arms to it, Frensch and Vogtle conducted the condensation reaction (formaldehyde/HCl) on the preformed benzocrown. Thus benzo-15-crown-5 was converted into the corresponding tris-crown (IS) (mp 203.5—205.5°) in 4% yield. The yield was somewhat higher for the condensation of benzo-18-crown-6, but in both cases, yield ranges were observed. These species formed 1 3 (ligand/salt) complexes with sodium and potassium ions. 

Open-chained ligands having aromatic terminal groups.323... 

Open-chained ligands containing the 2,6-pyridyl subunit.341... 

Conjugated dienes sucb as butadiene and its open-chain analogues can act as 17 ligands the complexes are u.sunlly prepared from melal carbonyl complexes by direct replacement of 2CO by the diene. Isomerization or rearrangement of the diene may occur a.s indicated schematically below ... 

Complexes of open chain tetradentate ligands containing heavy donor atoms. C. A. McAuliffe, Adv. Inorg. Chem. Radiochem., 1975,17,165-187 (58). 

Complexes of Open-Chain Tetradenate Ligands Containing Heavy Donor Atoms... 

The observed pattern of C—C and C—N distances indicates delocalization of n-electrons within the six-membered P-diiminate rings. Open-chain tetraaza ligand complexes [AuL j, in which condensation of only one P-diketonate has occurred. 

Electrogenerated monovalent Co complexes of the well-known open chain N202 Schiff base ligands salen (8), salphen (9), and their substituted derivatives undergo oxidative additions with alkyl halides. Reactions of the complex with substrates within the series RBr (R = Pr, Bu, t-Bu) proceed at different rates. The reaction occurs by an inner-sphere alkyl-bridged electron transfer, with a Co1- R+- X-transition state, which is sensitive to distortions of the complex in different configurations.124... 

Mononuclear Complexes with Open-chain Ligands Containing Mixed Donor Atoms 346... 

Di- and oligonuclear complexes with open-chain alkoxo- or phenolato-based dinucleating ligands 427... 

With open-chain chelate arms at the heterocycle, however, coordination of additional coligands may be accompanied by detachment of some ligand side arms, e.g., in (830) or (831).422,2032,2072... 

Bis(ethylenediamine)gold(III) chloride reacts with /3-diketonates in aqueous base via Schiff base condensation to form complexes of gold(III) with a 14-membered macrocyclic tetraaza ligand such as (292).1711-1713 The X-ray structure showed the cation to be nearly planar. Delocalization of -electrons within the six-membered /3-diketonate rings was indicated by the observed pattern of C—C and C—N distances. Open-chain tetraaza ligand complexes in which condensation of only one /3-diketonate has occurred, can be isolated as intermediates in this reaction. They may be used for further condensation with a different /3-diketonate. Oxidation of this complex with trityl tetrafluoroborate introduces a double bond in position C2C3.1712,1714... 

Open chain polyamine ligands have been widely studied. Often the coordination of zinc is compared with other first row transition metals and factors, such as behavior across a pH range, studied. The protonation patterns and stability constants are of particular interest. Octahedral zinc tris(ethylenediamine) structures have been characterized by X-ray diffraction with a number of different counter anions.94 The X-ray structure of zinc tris(ethylenediamine) with fluoride counter ions reveals extensive hydrogen bonding.95... 

Complexation of Cd with a series of polyamine macrocycles, but also related open-chain polyamines, comprising or attached to the 2,2 -bipyridine (bipy) and 1,10-phenanthroline (phen) moieties, has been studied by combined UV/vis spectrometry and potentiometry.24 Formation constants and distribution diagrams of the species present have been evaluated. As a result the thermodynamic stabilities, i.e., the formation constants, are lower for the bipy- and phen-contain-ing ligands than those for Cd complexes with aliphatic oligoaza macrocycles containing the same number of N donors. The probable reason is loss of flexibility of the ligands caused by the size and stiffness of the inserted heteroaromatic moieties. 

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