Exploring the reactivity of four-coordinate PNPCoX with access to three-coordinate spin triplet PNPCo

The compounds (PNP)CoX, where PNP is (tBu2PCH2SiMe2)2N- and X is Cl, I, N3, OAr, OSO2CF3 and N(H)Ar, are reported. Some of these show magnetic susceptibility, color, and 1H NMR evidence of being in equilibrium between a blue, tetrahedral S=3/2 state and a red, planar S=1/2 state; the equilibrium populations are influenced by subtle solvent effects (e.g., benzene and cyclohexane are different), as well as by temperature. Attempted oxidation to Co(III) with O2 occurs instead at phosphorus, giving [P(O)NP(O)]CoX species. The single O-atom transfer reagent PhI=O likewise oxidizes P. Even I2 oxidizes P to give the pendant phosphonium species (tBu2P(I)CH2SiMe2NSiMe2CH2PtBu2)CoI2 with a tetrahedral S=3/2 cobalt; the solid-state structure shows intermolecular PI...ICo interactions. Attempted alkyl metathesis of PNPCoX inevitably results in reduction, forming PNPCo, which is a spin triplet with planar T-shaped coordination geometry with no agostic interaction. Triplet PNPCo binds N2(weakly) and CO (whose low CO stretching frequency indicates strong PNP-->Co donor power), but not ethene or MeCCMe.     

C-D(0) (D(0) = pi-donor, F) Cleavage in H(2)C=CH(D(0)) by (Cp(2)ZrHCl)(n): mechanism, agostic fluorines, and a carbene of Zr(IV).

Consistent with the C-O cleavage behavior of vinyl ethers, vinyl fluoride reacts with Cp(2)ZrHCl to give Cp(2)ZrFCl and C(2)H(4) as primary products. DFT (B3PW91) calculations show this reaction to be highly exoenergetic (-55 kcal/mol), and reveal a sigma-bond metathesis mechanism to be unfavorable compared to a Zr-H addition across the C=C bond, with regiochemistry placing F on C(beta) of the resulting fluoroethyl ligand. beta-F elimination (onto Zr) then completes the reaction. There is no eta(2)-olefin intermediate on the reaction path. DFT calculations seeking the energy and structure of the two carbenes Cp(2)ZrHCl[CF(CH(3))] and Cp(2)ZrFCl[CH(CH(3))] are also described.     

Reductive Silylation Using a Bis-silylated Diaza-2,5-cyclohexadiene

1,4-Bis(trimethylsilyl)-1,4-diaza-2,5-cyclohexadiene, 1 , was tested as a reagent for the reductive silylation of various unsaturated functionalities, including N-heterocycles, quinones, and other redox-active moieties in addition to deoxygenation of main group oxides. Whereas most reactions tested are thermodynamically favorable, based on DFT calculations, a few do not occur, perhaps giving limited insight on the mechanism of this very attractive reductive process. Of note, reductive silylation reactions show a strong solvent dependence where a polar solvent facilitates conversions.     

Multi-electron Reduction Capacity and Multiple Binding Pockets in Metal–Organic Redox Assembly at Surfaces

Metal–ligand complexation at surfaces utilizing redox-active ligands has been demonstrated to produce uniform single-site metals centers in regular coordination networks. Two key design considerations are the electron storage capacity of the ligand and the metal-coordinating pockets on the ligand. In an effort to move toward greater complexity in the systems, particularly dinuclear metal centers, we designed and synthesized tetraethyltetra-aza-anthraquinone, TAAQ, which has superior electron storage capabilities and four ligating pockets in a diverging geometry. Cyclic voltammetry studies of the free ligand demonstrate its ability to undergo up to a four-electron reduction. Solution-based studies with an analogous ligand, diethyldi-aza-anthraquinone, demonstrate these redox capabilities in a molecular environment. Surface studies conducted on the Au(111) surface demonstrate TAAQ′s ability to complex with Fe. This complexation can be observed at different stoichiometric ratios of Fe:TAAQ as Fe 2p core level shifts in X-ray photoelectron spectroscopy. Scanning tunneling microscopy experiments confirmed the formation of metal–organic coordination structures. The striking feature of these structures is their irregularity, which indicates the presence of multiple local binding motifs. Density functional theory calculations confirm several energetically accessible Fe:TAAQ isomers, which accounts for the non-uniformity of the chains.     

A cis‐Divacant Octahedral and Mononuclear Iron(IV) Imide

A rare, low‐spin Fe^IV imide complex [(pyrr₂py)Fe?NAd] (pyrr₂py^2?=bis(pyrrolyl)pyridine; Ad=1‐adamantyl) confined to a cis‐divacant octahedral geometry, was prepared by reduction of N₃Ad by the Fe^II precursor [(pyrr₂py)Fe(OEt₂)]. The imide complex is low‐spin with temperature‐independent paramagnetism. In comparison to an authentic Fe^III complex, such as [(pyrr₂py)FeCl], the pyrr₂py^2? ligand is virtually redox innocent.     

Condensation of 2-Pyridylmethyllithium Nucleophiles and Pyridine Electrophiles as a Convenient Synthetic Route to Polydentate Chelating N-Donor Ligands

Condensation of 2-pyridylmethyllithium or (6-methyl-2-pyridyl)methyllithium nucleophiles and pyridine, 2-picoline, or 4-tert-butylpyridine as electrophiles leads to new polydentate N-donor ligands, methyl-, tert-butyl-substituted tripyridinedimethanes, or to tripyridinedimethane itself, in good or high yields. Depending on the reagent ratio, solvent used, and reaction conditions, the corresponding intermediate dipyridinemethanes can be minor by-product or major products of the condensation. In contrast to 2-pyridylmethyllithium, lithiated 2-isopropylpyridine does not react with pyridine electrophiles. Vice versa, nucleophilic substitution at the C(2)-pyridine carbon of 2,2-bis(2-pyridyl)propane with 2-pyridylmethyllithium takes place to produce products of condensation of 2-isopropylpyridine and dipyridylmethyllithium. DFT calculations of the Gibbs free energies of reactions combined with pK _a values of the CH-acids involved help to explain the reactivity observed.     

Spin state, structure, and reactivity of terminal oxo and dioxygen complexes of the (PNP)Rh moiety

[Rh(III)H{(tBu(2)PCH(2)SiMe(2)NSiMe(2)CH(2)PtBu{CMe(2)CH(2)})}], ([RhH(PNP*)]), reacts with O(2) in the time taken to mix the reagents to form a 1:1 eta(2)-O(2) adduct, for which O--O bond length is discussed with reference to the reducing power of [RhH(PNP*)]; DFT calculations faithfully replicate the observed O-O distance, and are used to understand the oxidation state of this coordinated O(2). The reactivity of [Rh(O(2))(PNP)] towards H(2), CO, N(2), and O(2) is tested and compared to the associated DFT reaction energies. Three different reagents effect single oxygen atom transfer to [RhH(PNP*)]. The resulting [RhO(PNP)], characterized at and above -60 degrees C and by DFT calculations, is a ground-state triplet, is nonplanar, and reacts, above about +15 degrees C, with its own tBu C--H bond, to cleanly form a diamagnetic complex, [Rh(OH){N(SiMe(2)CH(2)PtBu(2))(SiMe(2)CH(2)PtBu{CMe(2)CH(2)})}].     

Coupling of terminal alkynes by RuHXL2 (X = Cl or N(SiMe3)2, L = PiPr3)

The compounds RuL₂HX, where L = PⁱPr₃ and X = Cl or N(SiMe₃)₂, are catalyst precursors for dimerization of terminal alkynes to enynes and also to cumulenes at 23 °C; selectivity among these products is X-dependent, but not high. Conversion of Ru species onto the catalytic cycle was undetectably small, so alternative approaches to understanding the catalytic mechanism were employed: stoichiometric reactions, independent synthesis of candidate intermediates, and trapping with CO. These show the intermediacy of vinylidenes and vinyl compounds, and reveal conversion of cumulenes to the thermodynamically more stable enynes.