Ruthenium protonation/deprotonation

Switching the cubic nonlinearity of ruthenium alkynyl complexes by a protmi-ation/deprotonation sequence (via a vinylidene complex) was demraistrated by fs Z-scan studies at 800 nm several years ago [41]. Recently, protic and electrochemical switching were demonstrated in the ruthenium alkynyl cruciform complex 11 for which distinct linear optical and NLO behavior were noted for the vinylidene complex and the Ru(II) and Ru(III) alkynyl complexes [42]. Because the oxidation/ reduction and protonation/deprotonation procedures are independent, this system corresponds to switching by orthogonal stimuli. 

Protons on the 8 carbon of a ruthenium allenylidene complex are acidic, and deprotonation at this position often occurs to give ene-yne derivatives. Reaction of I with cyclic propargyl alcohols of type 110, for example, did not yield the allenylidene complexes, but rather the ruthenium ene-yne products (111) were isolated [Eq. (97)] (78). Reaction of the cor-... 

Hence, the first clearcut evidence for the involvement of enol radical cations in ketone oxidation reactions was provided by Henry [109] and Littler [110,112]. From kinetic results and product studies it was concluded that in the oxidation of cyclohexanone using the outer-sphere one-electron oxidants, tris-substituted 2,2 -bipyridyl or 1,10-phenanthroline complexes of iron(III) and ruthenium(III) or sodium hexachloroiridate(IV) (IrCI), the cyclohexenol radical cation (65" ) is formed, which rapidly deprotonates to the a-carbonyl radical 66. An upper limit for the deuterium isotope effect in the oxidation step (k /kjy < 2) suggests that electron transfer from the enol to the metal complex occurs prior to the loss of the proton [109]. In the reaction with the ruthenium(III) salt, four main products were formed 2-hydroxycyclohexanone (67), cyclohexenone, cyclopen tanecarboxylic acid and 1,2-cyclohexanedione, whereas oxidation with IrCl afforded 2-chlorocyclohexanone in almost quantitative yield. Similarly, enol radical cations can be invoked in the oxidation reactions of aliphatic ketones with the substitution inert dodecatungstocobaltate(III), CoW,20 o complex [169]. Unfortunately, these results have never been linked to the general concept of inversion of stability order of enol/ketone systems (Sect. 2) and thus have never received wide attention. 

From a coordination point of view, Complex 75 and the related ruthenium complex [(ij6-p-cymene)Ru(pz)2(Hpz)], 76 (7), are comparable to protonated polypyrazolylborates RB(pz)2(pzH) (R = H or pz) (12). The formal similarity between the tris(pyrazolyl)borate anion and the deprotonated form of the iridium complex 75 suggested the preparation of heterodinuclear complexes by using 75 as a building block. Thus, heterodinuclear (n-pz)2 complexes of the formula [(C5Me5)(pz)Ir(/i-pz)2M(PPh3)] (M = Cu, 77 Ag, 78 Au, 79) were obtained (96). 

These alkynyl complexes can be protonated to afford vinylidene complexes, which can in turn be deprotonated to give the starting alkynyl complex, reactions that are spectroscopically quantitative. The tabulated data also provide the opportunity to assess the effect of this protonation, in proceeding from alkynyl complex to vinylidene derivative. One would perhaps expect that replacing the electron-rich ruthenium donor in the alkynyl complexes with a (formally) cationic ruthenium centre in the vinylidene complexes would result in a significant decrease in nonlinearity. 

The fluxional behavior of the osmium equivalents of the ruthenium(II) complexes mentioned in the final citation of the previous section has been described/ Time scales have been established for proton exchange at sulfur and at phosphorus in (26), R = Me or Et, and for deprotonation of (27) by the base DBU (1,8-diazabicyclo[5.4.0]undec-7-ene)/ ... 

For example, the pyridine-based pincer complexes 1, 4, 7, and 10 undergo smooth deprotonation to provide complexes 2, 5, 8, and 11 (Schemes 1.2 and 1.3). NMR studies of 2, 5, 8, and 11 indicate dearomatization, as the pyridine protons are shifted to lower frequency (olefinic region). Moreover, the structure of complex 2 is unequivocally corroborated by single-crystal X-ray diffraction studies [23]. Importantly, the dearomatized complexes of 2, 5, and 8 activate dihydrogen by cooperation between the rathenium center and the deprotonated phosphine arm, resulting in aromatization to quantitatively yield the ruthenium traws-dihydride complexes of 3, 6, and 9, respectively (Schemes 1.2). The magnetically equivalent... 

Polymers containing benzimidazole units in their backbones have also been used in the synthesis of coordination metallopolymers (159-162). Osmium and ruthenium coordinated polymers with bipyridine ligands have been prepared (159,160). These polymers (72, 73) possessed metal-metal interactions through their conjugated backbones. Communication between the ruthenium centers of 72 increased by deprotonating the imidazole protons (160). The osmium coordinated polymer (73) showed two reduction waves separated by 0.32 V, indicative of strong cnmmimication between the Os centers (159). 

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