A method is described for determining the formal oxidation-reduction potentials of cuprous-cupric complexes. It is applied to a large number of copper complexes, mostly with 1, 10-phenanthrolines, bipyridyls, and other nitrogenous bases. For a series of complexes with differently substituted phenanthrolines and bipyridyls the redox potentials are not simply related to the acid dissociation constants of the ligands. The effect of change of co-ordination number of the ions upon the redox potentials is described, and the causes of changes in co-ordination number are discussed.THIS paper describes the determination of the oxidation-reduction potentials of a number of copper complexes. As in a previous paper on ferrous-ferric couples1 the potentials measured are formal mid-point potentials, E j . They refer to reversible potentials in a solution of equal concentrations of the oxidised and the reduced form of the element, here cupric and cuprous ions respectively, at some fixed pH and ionic strength. No account has been taken of activity coefficients. (E'f is the measured potential at an undefined degree of formation of the two complexes CuIL, and CuIIL,; Ef* refers to equal degrees of formation of the two complexes; EfO is the standard potential, traditionally defined.) From the variations of potential with pH, the degree of complex formation is found so that the potentials, Ef*, for equal co-ordination of the two ions can be calculated. EXPERIMENTALThe procedure for determination of ferrous-ferric couples 1 was modified for cuprous-cupric couples. In the earlier work ferrous and ferric ions were introduced into an acid solution and in a chloride medium. Before the addition of the solid ligand to the solution (from a glass basket) all oxygen was removed from the cell dead-space. Cuprous ions disproportionate in aqueous solution and can only be stabilised by complex formation. I n the present series of experiments therefore we made up cupric ion and ligand solutions separately and these were first put into the titration ce1l.l The final cupric-ion concentration was 1-75 x 1 0 -4 ~; the ligand concentration depended on the system being investigated but was always a t least 3.5 times the total copper concentration. The cuprous ion, as a weighed amount of cuprous chloride, was placed in a short-necked thin-glass bulb which floated on the surface of the cupric ion-ligand solution and was kept vertical by a glass rod which passed through an air-tight gland in the cell-cap. Pushing the rod enabled the bulb to be broken on the bottom of the titration cell. Before de-oxygenation of the cell and its contents the cupric-ligand solution was adjusted to a high pH, if necessary by the addition of sodium hydroxide. It was found advisable to bring the pH of the solution to about one unit above the acid dissociation constant, pK,, of the ligand so that when the cuprous ion was added to the solution it was converted into its complex and did not disproportionate. The solution and dead-space of the cell were swept free from oxygen as des...

Hydrogenolysis reactions of so-called lignin model dimers using a Ru-xantphos catalyst are presented (xantphos = 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene). For example, of some nine models studied, the alcohol, 2-(2-methoxyphenoxy)-1-phenylethanol (1), with 5 mol% Ru(H)(2)(CO)(PPh(3))(xantphos) (18) in toluene-d(8) at 135 °C for 20 h under N(2), gives in ~95% yield the C-O cleavage hydrogenolysis products, acetophenone (14) and guaiacol (17), and a small amount (<5%) of the ketone, 2-(2-methoxyphenoxy)-1-phenylethanone (4), as observed by (1)H NMR spectroscopy. The in situ Ru(H)(2)(CO)(PPh(3))(3)/xantphos system gives similar findings, confirming a recent report (J. M. Nichols et al., J. Am. Chem. Soc., 2010, 132, 12554). The active catalyst is formulated 'for convenience' as 'Ru(CO)(xantphos)'. The hydrogenolysis mechanism proceeds by initial dehydrogenation to give the ketone 4, which then undergoes hydrogenolysis of the C-O bond to give 14 and 17. Hydrogenolysis of 4 to 14 and 17 also occurs using the Ru catalyst under 1 atm H(2); in contrast, use of 3-hydroxy-2-(2-methoxyphenoxy)-1-phenyl-1-propanone (7), for example, where the CH(2) of 4 has been changed to CHCH(2)OH, gives a low yield (≤15%) of hydrogenolysis products. Similarly, the diol substrate, 2-(2-methoxyphenoxy)-1-phenyl-1,3-propanediol (9), gives low yields of hydrogenolysis products. These low yields are due to formation of the catalytically inactive complexes Ru(CO)(xantphos)[C(O)C(OC(6)H(4)OMe)=C(Ph)O] (20) and/or Ru(CO)(xantphos)[C(O)CH=C(Ph)O] (21), where the organic fragments result from dehydrogenation of CH(2)OH moieties in 7 and 9. Trace amounts of Ru(CO)(xantphos)(OC(6)H(4)O), a catecholate complex, are isolated from the reaction of 18 with 1. Improved syntheses of 18 and lignin models are also presented.

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Optimized syntheses of 5,15-diphenylporphyrin (DPP, 1) and its dipyrrolic precursors are described. A novel procedure for the synthesis of dipyrromethane (2), prepared by hydrodesulfurization of the corresponding di-2-pyrrolylthione (8), is presented, as well as an improved method to isolate large quantities of 5-phenyldipyrromethane (4), prepared by the acid-catalysed condensation of pyrrole with benzaldehyde. These dipyrromethanes are key reagents in two high-yield (2+2)-type syntheses of DPP. 5-Phenyldipyrromethane was formylated to provide 1-formyl- (11) and 1,9-diformyl-5-phenyldipyrromethane (12), and reduction of 11 provided the corresponding hydroxymethyl compound 13. These compounds (11-13), however, were much less efficient precursors to DPP. Two polypyrrolic compounds, 1,1,2,2-dipyrrolylethane (9) and 5,10-diphenyltripyrrane (10), potentially useful for the synthesis of porphyrinic macrocycles, were isolated as side-products during the dipyrromethane and 5-phenyldipyrromethane syntheses.

The Rh(I) carbene precursors [RhCl(COE)(NHC)] 2 , where the N-heterocyclic carbene is 1,3-bis(2,6diisopropylphenyl)imidazol-2-ylidene (IPr) or 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes), were used to synthesize the RhCl(NHC)(P-N) complexes 4 (NHC ) IPr) and 5 (NHC ) IMes), where P-N is P,N-chelated o-(diphenylphosphino)-N,N-dimethylaniline, and the corresponding cis-RhCl(NHC)-(PPh 3 ) 2 complexes 6 and 7. The synthesis of 4 surprisingly requires the reaction to be carried out under a hydrogen atmosphere and occurs via the intermediate dihydride RhCl(H) 2 (IPr)(P-N) (3). Complexes 4-7 in benzene readily undergo irreversible oxidative addition of O 2 to form the corresponding Rh(III) peroxide complexes 9-12. For comparative purposes, RhCl(PPh 3 )(P-N) (8) was synthesized from RhCl-(PPh 3 ) 3 , and this also added O 2 to form a peroxo complex (13). All of the complexes were generally characterized by elemental analysis and 1 H, 31 P{ 1 H}, and 13 C{ 1 H} NMR and IR spectroscopies and, in the cases of 9, 10, and 13, by X-ray crystallography.