α-Diketones are an important class of building blocks employed in many organic synthetic reactions. However, their coordination chemistry has rarely been explored. In light of this, our earlier report on [(acac)2RuII(µ-2,2'-pyridil)RuII(acac)2] (acac = acetylacetonate) showcased the sensitivity of a diketone fragment towards oxidative C-C cleavage. Following the lead, the synthesis of similar but stable diketo fragments containing diruthenium compounds was attempted. Three diruthenium compounds with the bridge 1,2-bis(2-hydroxyphenyl)ethane-1,2-dione (L) were prepared: diastereomeric [(acac)2RuIII(µ-L2-)RuIII(acac)2], 1a(rac)/1b(meso), [(bpy)2RuII(µ-L2-)RuII(bpy)2](ClO4)2, [2](ClO4)2 and [(pap)2RuII(µ-L2-)RuII(pap)2](ClO4)2, [3](ClO4)2 with ancillary ligands of different donating/accepting characteristics. The metal is stabilised in different oxidation states in these complexes: Ru(iii) is preferred in 1a/1b when σ-donating acac is used as the co-ligand whereas electron rich Ru(ii) is preferred in [2](ClO4)2 and [3](ClO4)2 when co-ligands of moderate to strong π-accepting properties are employed. The oxidative chemistry of these systems is of particular interest with respect to the participation of varying bridging-ligands which contain phenoxide groups. On the other hand, the reduction processes primarily resulting from the metal or the ancillary ligands are noteworthy as the normally reducible 1,2-diketo- group remains unreduced. These results have been rationalised and outlined from thorough experimental and theoretical investigations. The results presented here shed light on the stability of metal coordinated α-diketones as a function of their substituents.
Unactivated olefins usually react poorly in conventional alkenylation reactions. Their introduction via C-H activation is limited to aromatic acids. Herein, we disclose a C-H functionalization protocol of aromatic amines with unactivated olefins, which shows exclusive allylic selectivity for the distal ring of the biphenyl system by exploiting a readily available cobalt(II) catalyst. The allylation proceeds smoothly involving a broad set of unbiased olefins and biaryls, giving access to the functionalization of the biphenyl scaffold.
In the recent years, there has been an emerging research interest in the domain of C-C bond-cleavage reactions. The present contribution deals with the redox-mediated dioxygen activation and C-C bond cleavage in a diruthenium complex [(acac)2 RuII (µ-L1)RuII (acac)2 ], 1 (acac=acetylacetonate) incorporating 2,2'-pyridil (L1) as the bridging ligand. The above process leads to a C-C-cleaved monomeric product [(acac)2 RuIII (pic- )], 2 (pic- =piconilate). Intriguingly, similar diastereomeric complexes [(acac)2 RuII (µ-L2)RuII (acac)2 ], meso (ΔΛ): 3 a and rac (ΔΔ/ΛΛ): 3 b, involving an analogous diimine bridge (L2=N1,N2-diphenyl-1,2-di(pyridin-2-yl)ethane-1,2-diimine), were stable towards such oxidative transformations. Electrochemical and spectroelectrochemical studies, in combination, establish the potential non-innocent feature of the 2,2'-Pyridil (L1) and its derivative (L2) both in oxidation and reduction processes. Additionally, theoretical calculations have been employed to verify the redox states and their behavior. Furthermore, transition state (TS) calculations at the M06L/6-31G*/LANL2DZ level of theory together with detailed kinetic studies outline a putative mechanism for the selective transformation of 1â2 involving the formation of an intermediate bearing peroxide linkage to complex 1.
Ligands containing the azo group are often used in various metal complexes owing to their facile one-electron reduction, which in effect extends the means of degrading environmentally harmful azo dyes. In order to probe the idea of the generally accepted ease of reduction of azo-containing compounds, we present here three different diruthenium complexes [(acac)2RuIII(µ-L2-)RuIII(acac)2] (diastereomeric 1/2), [(bpy)2RuII(µ-L2-)RuII(bpy)2](ClO4)2 ([3](ClO4)2), and [(pap)2RuII(µ-L2-)RuII(pap)2](ClO4)2 ([4](ClO4)2 ) with a bridging ligand (L2- = 1,8-bis(( E)-phenyldiazenyl)naphthalene-2,7-dioxido) that contains azo groups in addition to phenoxide-type donors. The RuIII-RuIII complexes (1/2) display interesting one-dimensional-chain effects, as revealed by temperature-dependent magnetic studies. The stability of the RuIII oxidation state in 1/2 under ambient conditions correlates well with the σ-donating acetylacetonato (acac) coligands. However, with π-accepting 2,2/-bipyridine (bpy) or phenylazopyridine (pap) the RuII state is preferably stabilized in 32+ or 42+, respectively, but there are interesting differences in their oxidative chemistry. The moderately π accepting bpy allows for the RuII to RuIII oxidation at reasonably low anodic potentials. However, for the strongly π accepting pap, no RuII to RuIII oxidation is observed within the solvent window. Instead, a phenoxide to phenoxyl radical type of oxidation based on the bridging ligand is observed. Surprisingly, the reductive chemistry of all three complexes is dominated by either the ruthenium centers or the coligands (bpy or pap), with no reductions observed on the azo function associated with the central bridging ligand (L2-). All of the above conclusions were drawn from combined structural, electrochemical, magnetic, spectroelectrochemical, and DFT investigations. Our results thus conclusively establish that the ease of reduction of an azo group in a particular compound is critically dependent on its substituents and that the noninnocence of the bridging ligands (L2-) in the dinuclear complexes can be decisively tuned by the appropriate choice of ancillary ligands.
Addressing remote C-H functionalization is a prominent challenge in the field of homogeneous catalysis. The past two decades have accounted for major developments in this domain, proclaiming efficient selectivity at the meta and para positions. Recognizing such transformations remains significant, owing to their importance in the biological and chemical industries. This focus review aims to summarize the relatively new concept of σ-C-H activation enabled by a ruthenium metal center.
