Formation of exceptionally weak C-C bonds by metal-templated pinacol coupling

Redox induced oxidative C-C coupling of non-innocent bis(heterocyclo)methanides

Redox driven C-C bond formation has gained recent attention over the traditional sequence of oxidative addition, insertion and reductive elimination reactions. In this regard, the transient radical mediated diverse reactivity profile of bis(heterocyclo)methanes (H-BHM: HL1-HL4) has been demonstrated as a function of varying metal ions and ligand backbones. It highlighted the following events: (a) redox induced homocoupling of deprotonated HL1 and HL4 on coordination to M(OAc)₂ precursors (M = Cu^II, Zn^II, Pd^II, Ag^I), including the effective role of molecular oxygen in the transformation process; (b) steric inhibition of C-C coupling of HL1 or HL4 on inserting the substituent at the bridged methylene centre (Ph in HL2 or CH₃ in HL3); (c) competitive C-C coupling versus oxygenation of free HL1 with varying concentrations of Pd^II(OAc)₂ as the ease of oxygenation over dimerisation of the deprotonated HL1 was corroborated by the DFT calculated lower activation barrier and greater thermodynamic stability of the former; and (d) redox non-innocence of BHMs on a coordinatively inert ruthenium platform, which in turn favored the involvement of a radical pathway for the aforestated coupling or oxygenation process. A combined structural, spectroscopic and DFT calculated transition state analysis demonstrated the mechanistic outline for the metal assisted oxidative coupling of BHMs.

Exceptionally Long Covalent CC Bonds-A Local Vibrational Mode Study

For decades one has strived to synthesize a compound with the longest covalent C−C bond applying predominantly steric hindrance and/or strain to achieve this goal. On the other hand electronic effects have been added to the repertoire, such as realized in the electron deficient ethane radical cation in its D3d form. Recently, negative hyperconjugation effects occurring in diamino-o-carborane analogs such as di-N,N-dimethylamino-o-carborane have been held responsible for their long C−C bonds. In this work we systematically analyzed CC bonding in a diverse set of 53 molecules including clamped bonds, highly sterically strained complexes such as diamondoid dimers, electron deficient species, and di-N,N-dimethylamino-o-carborane to cover the whole spectrum of possibilities for elongating a covalent C−C bond to the limit. As a quantitative intrinsic bond strength measure, we utilized local vibrational CC stretching force constants ka(CC) and related bond strength orders BSO n(CC), computed at the ωB97X-D/aug-cc-pVTZ level of theory. Our systematic study quantifies for the first time that whereas steric hindrance and/or strain definitely elongate a C−C bond, electronic effects can lead to even longer and weaker C−C bonds. Within our set of molecules the electron deficient ethane radical cation, in D3d symmetry, acquires the longest C−C bond with a length of 1.935 Å followed by di-N,N-dimethylamino-o-carborane with a bond length of 1.930 Å. However, the C−C bond in di-N,N-dimethylamino-o-carborane is the weakest with a BSO n value of 0.209 compared to 0.286 for the ethane radical cation; another example that the longer bond is not always the weaker bond. Based on our findings we provide new guidelines for the general characterization of CC bonds based on local vibrational CC stretching force constants and for future design of compounds with long C−C bonds.

Versatile Coordination and C-C Coupling of Diphosphine-Tethered Imine Ligands with Ni(II) and Ni(0)

Ligands that can adapt their coordination mode to the electronic properties of a metal center are of interest to support catalysis or small molecule activation processes. In this context, the ability of imine moieties to bind in either an η¹(N)-fashion via σ-donation of the lone pair or, less commonly, in an η²(C,N)-fashion via π-coordination is potentially attractive for the design of new metal–ligand cooperative systems. Herein, the coordination chemistry of chelating ligands with a diphosphine imine framework (PCNP) to nickel is investigated. The imine moiety binds in an η¹(N)-fashion in a Ni(II)Cl₂ complex. The uncommon η²(C,N)-interaction is obtained in Ni(0) complexes in the presence of a PPh₃ coligand. Increasing the bulk on the phosphine side-arms in the Ni(0) complexes, by substituting phenyl for o-tolyl groups, leads to a distinct binding mode in which only one of the phosphorus atoms is coordinated. In the absence of a coligand, a mixture of two different dimeric Ni(0) complexes is formed. In one of them, the imine adopts an uncommon η¹(N)η²(C,N) bridging mode of the ligand to nickel, while the second one may involve reactivity on the ligand by the formation of a new C–C bond by oxidative coupling. The latter is supported by the isolation and structural characterization of a crystalline bis-CO derivative featuring a C–C bond formed by oxidative coupling of two imine moieties.

The coordination chemistry of a series of diphosphine-imine ligands (PCNP) to Ni is investigated, with the purpose of developing systems that present metal−ligand cooperativity. The ligands bind in versatile ways, adapting to the oxidation state and coordination environment of the metal center. Additionally, a dimeric derivative undergoes oxidative C−C coupling in the presence of CO, resulting in an unusual mixed valence Ni(II)/Ni(0) dinuclear complex.

1,2-Addition of Diethylzinc to a Bis(Imidazolyl)ketone Ligand

In this study, the selective 1,2-addition of diethylzinc to the ketone functionality of BMdiPhIK [bis(1-methyl-4,5-diphenylimidazolyl)ketone] is shown. The reaction product is isolated in a dimeric form with a planar Zn2(µ-O)2-motif keeping the two monomers together. This compound can serve as a model for reactive intermediates in the catalytic alkylation of ketones with diorganozinc reagents. Hydrolysis of this binuclear zinc compound leads to isolation of the C-alkylated product in 89 % yield. A reaction pathway is proposed in which BMdiPhIK initially coordinates to diethylzinc as a bidentate bis(nitrogen) ligand. This is followed by the homolytic cleavage of the Zn-Et bond and in-cage recombination of the Et-radical and the Zn-coordinated ligand-centered radical, which is mainly localized on the carbonyl moiety of the ligand.