Coordination, arenes

A convenient and easily accessible way to quantify hydrophobicity is the determination of the octanolAvater partition coefficient (log P) and we have determined the hydrophobicity of 13 selected ruthenium-arene complexes (71). As expected, hydrophobicity increases with an increase of the size of the coordinated arene ring, but decreases significantly when the chloride is replaced by neutral ligands such as pyridine and 4-cyanopyridine. The latter observation is somewhat counter intuitive at first inspection, but correlates with replacement of anionic chloride to yield a dicationic complex. The hydrophobicity... 

The rate of hydrolysis depends strongly on the nature of all three building blocks, i.e. the leaving group, the coordinated arene, and the chelate, and can be varied over several orders of magnitude, opening a time-window of activation. A detailed study of the aquation and the reverse, anation reactions of three [Ru(rj6-arene)Cl(en)](PF6) complexes (arene — bip (10), dha (11), and tha (12)) showed that the rates of aquation... 

Loss of Coordinated Arene. We previously stated that the arene ligand in ruthenium(II)-arene complexes is relatively inert towards displacement under physiological conditions. While this is generally true, there are a few exceptions to this rule and this type of reactivity can be used to advantage. Weakly bound arenes, for instance, can be thermally displaced, a property convenient for the synthesis of ruthenium-arene complexes that are not readily available through more common synthetic routes. This way, the reaction of a precursor dimer, [RuCl2(etb)]2 (etb, ethylbenzoate) (68), with either 3-phenyl-1-propylamine or... 

The intramolecular stabilization of germyl cation 34 by a remote aryl substituent was demonstrated by the NMR chemical shifts of the coordinated aryl ring. The chemical shift pattern found for the coordinated arene ring of 34 is characteristic for arenium ions and it closely resembles that found bissilylated arenium ions 78. 

The activity was higher with p-cymene, as coordinated arene, than with hexa-methylbenzene which leads to a more stable ruthenium-arene bond. 

The carbonyl complexes listed in Table V are of two types tricarbonyl-chromium rj6-arene 77-complexes, and pentacarbonyltungsten <7-pyridine complexes, with both complex types having relatively low y. Nonlinearities increase on arene or pyridine 77-system lengthening, and on proceeding from acceptor to donor substituent on the tricarbonylchromium-coordinated arene ring. Relative magnitudes and trends thus mirror those observed with quadratic nonlinearities of these complexes (see Ref. 1)... 

This approach, in which an arene appended to a coordinated donor atom replaces a labile ligand, is by far the most generally useful procedure and has been applied to the preparation of tethered arene complexes of Ti(IV), Cr(0), Mo(0), Mo(II), W(II), Ru(II), Rh(I), and Ir(I). The labile ligand itself is often an arene, as shown in Scheme lb, in which case the replacement is an intramolecular version of the intermolecular exchange between free and coordinated arenes, which has been studied for complexes of the type [M(CO)3(ri6-arene)] (M = Cr, Mo),26 31 [Ir(ri6-arene)(ri4-l,5-COD)]BF4,28 32 and [Ru(ri6-arene)(ri4-l,5-COD)]28 (COD = cyclooctadiene). 

Displacement of a coordinated arene by a pendant arene (Scheme lb) has been used to prepare tethered arene complexes in which the auxiliary ligand is a carbene. 

Catalyst optimization has been reported only for the [RuXtarene)(B INAP) +X series by variation of the bisphosphines, halogen, and coordinated arene ligands. The reductions of [3-dicarbonyl compounds with Ru(BINAP-type) catalysts have been reviewed.49... 

There are major challenges to be tackled relating to the chemical and biochemical mechanisms of action of organometallic complexes, for example involving the mechanism of displacement and release of coordinated arenes and cyclopentadienyl ligands which can expose several reactive coordination positions on a metal ion. Also the metabolism of coordinated arene and Cp ligands (e.g. modification by P450 enzymes in the liver) may have a major effect on the biodistribution and excretion of complexes. 

The formation of these C-N bonds is interesting, because it is not easy to create them in the case of non-coordinated arenes [40]. Very often, another form of activation is necessary [41]. The last example of a C-N bond formation is represented by the recent preparation of the indazole CT-chromium complex 55, which was obtained by cyclization of p6-2-(2 -phenylhydrazine)-l,3-dioxolane-Cr(CO)3 (54) under acidic conditions. The ipso SNAr was achieved by refluxing with 2.5 equivalents of hydrazine and one equivalent of 2-(2 -fluorophenyl)-l,3-dioxolane-Cr(CO)3 (53) for 2 days [39]. 

The donor/acceptor properties and the electronic coupling interactions determine the redistribution of electron density between the aromatic donor and the electron acceptor upon complexation. Significant changes in structure and reactivity of the coordinated arene can be rationalized in terms of spectral and thermodynamic properties within the framework of the CT formalism. This section is devoted to a consideration of the structural effects of arene coordination (in terms of donor/acceptor bond distance and type of bonding, distortion of arene planarity, expansion of the aromatic ring, and re-bond localization). 

Effect of Charge Transfer on the Structural Features of Coordinated Arenes 13.3.3.1 Expansion of the Arene Ring... 

Redistribution of electron density in CT complexes results in a modification of the chemical properties of coordinated arenes, and this effect is widely used in organometallic catalysis [2]. To demonstrate the relationship between charge transfer in arene complexes and their reactivity, we focus our attention on carbon-hydrogen bond activation, nucleophilic/ electrophilic umpolung, and the donor/acceptor properties of arenes in a wide variety of organometallic reactions. 

Modification of the Donor/Acceptor Properties of Coordinated Arene Ligands... 

Quantitative Evaluation of Arenes as Electron Donors 437 Spectral (UV/vis) Probe for the Formation of CT Complexes 438 IR Spectroscopic Studies of Charge-Transfer Complexation 442 Thermodynamics of Charge-Transfer Complexation 443 Structural Features of Arene Charge-Transfer Complexes 445 Bonding Distance of the Donor/Acceptor Dyad in Arene Complexes 446 Relationship Between Hapticity and Charge Transfer in Arene Complexes 447 Effect of Charge Transfer on the Structural Features of Coordinated Arenes 448... 

Charge-Transfer Activation of Coordinated Arenes 452 Carbon-Hydrogen Bond Activation 453 Nucleophilic/ Electrophilic Umpolung 455... 

Improved catalyst recycling was also achieved in the reduction of acetophenone with formic acid/triethyl amine azeotrope with the r 6-benzene analogue of 14 bearing an imidazolium moiety on the diamine ligand instead of on the coordinated arene (see Figure 3.4).[51l In C4C im [PF6] the stability of the catalyst was superior in repeated runs when a mixture of [RuCl2(n6-benzene)]2 and 15 was employed rather than the neutral complex RuCl(r 6-benzene)(T sDPEN). 

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