Heterobimetallic complexes stabilized by the P2N2 macrocyclic ligand system: synthesis and reactivity of a rhodium-copper system that activates molecular hydrogen.

The synthesis and reactivity of the bimetallic rhodium-copper complex, Rh(COE)[P2N2]Cu, which is stabilized by the P2N2 macrocycle, is reported. In the solid state, the rhodium and copper centers are on opposite sides of the macrocyclic ring with the Cu(I) in a linear environment and the Rh(I) in a square planar array. However, in solution a very symmetrical structure is suggested on the basis of the 1H NMR data, which is consistent with at least two separate fluxional processes, rotation of the cyclooctene unit and movement of the Rh(I) unit between the two amido donors. Addition of H2 to Rh(COE)[P2N2]Cu results in the formation of ([P2N2H]RhH(µ-H)2Cu)2via hydrogenation of the coordinated cyclooctene unit, oxidative addition of H2 to the rhodium center and hydrogenolysis of the copper amido unit. Monitoring the reaction of H2 by NMR spectroscopy indicated the formation of a number of intermediates which suggests hydrogenolysis of the copper amido linkage occurs to generate CuH in some form, along with Rh(COE)[P2N2H], which is converted to Rh(H)2[P2N2H] by hydrogenation of the cyclooctene, which then recombines with the CuH present to generate the final product. Deuteration studies indicate that there is considerable H/D scrambling in the cyclooctane produced that we attribute to reversible beta-elimination, migratory insertion steps.

Forays into rhodium macrocyclic chemistry stabilized by a P2N2 donor set. Activation of dihydrogen and benzene.

The reaction of the dilithium diamido-diphosphine macrocycle, Li2[N(SiMe2CH2P(Ph)CH2SiMe2)2N] (Li2[P2N2]) with [Rh(COD)Cl]2 generates the dirhodium macrocyclic compound, [P2N2][Rh(COD)]2 (where COD = η4-1,5-cyclooctadiene), wherein both rhodium-COD units are syn to each other and have square planar geometries. While this dirhodium derivative does react with H2, no clean products could be isolated. Upon reaction of Li2[P2N2] with [Rh(COE)2Cl]2 (where COE is η2-cyclooctene), the dilithium-dihodium derivative ([Rh(COE)][P2N2]Li)2(dioxane) forms, which was characterized by single-crystal X-ray analysis and NMR spectroscopy. The cyclooctene derivative reacts with dihydrogen in benzene to generate the dilithium-dirhodium-dihydride complex ([Rh(H)2][P2N2]Li)2(dioxane); also formed is the dilithium-dirhodium-phenylhydride complex ([Rh(C6H5)H][P2N2]Li)2(dioxane) via oxidative addition of a C-H bond of the solvent. The phenyl-hydride is eventually converted to the dihydride derivative via further reaction with H2. This process is complicated by adventitious H2O, which leads to the isolation of the amine-dihydride, Rh[P2N2H](H)2; drying of the H2 eliminates this side product. Nevertheless, careful addition of H2O to ([Rh(COE)][P2N2]Li)2(dioxane) results in protonation of one of the amido units and the formation of the rhodium-amine cyclooctene derivative, Rh[P2N2H](COE), which upon reaction with H2 generates the aforementioned amine-dihydride, Rh[P2N2H](H)2. The mechanism by which dihydrogen and C-H bonds of benzene are activated likely involves initial dissociation of cyclooctene from the 18-electron centers in ([Rh(COE)][P2N2]Li)2(dioxane), followed by H-H and C-H bond activation. The ability of one of the amido units of the P2N2 macrocycle to be protonated is a potentially useful proton storage mechanism and is of interest in other bond activation processes.

Discovery and properties of a new indigoid structure type based on dimeric cis-indigos.

Reactions of indigo with quinones in the presence of sodium hydride leads unexpectedly to products containing two indigo subunits; one indigo is featured in a cis configuration and fused via its indole nitrogen atoms to a second indigo at the central C-C bond of the latter. Structural, optical, and redox properties of the new compounds are reported.

Synthesis and redox chemistry of Pd(ii) complexes of a pincer verdazyl ligand.

A series of palladium(ii) complexes containing a redox-active, tridentate verdazyl ligand of general formula (verdazyl)PdL (L = Cl, CH3CN) are synthesized. The tetrazine core of tridentate verdazyl ligand 5 is flanked by two pyridyl groups, creating a geometry in which the ancillary ligand L is bound trans to the verdazyl ring in the square planar metal complexes. Pd(ii) complexes were isolated with the verdazyl ligand in either its neutral radical charge state (6: L = CH3CN, 12: L = Cl) or its closed-shell monoanionic charge state (10: L = CH3CN, 9: L = Cl). The charge state of the ligand was determined using X-ray crystallography and NMR, EPR, and IR spectroscopy. The cyclic voltammograms of radical complexes 6 and 12 each contain a reversible one-electron reduction wave and an irreversible one-electron oxidation wave. The complexes can be chemically interconverted between radical ligand (6, 12) and reduced, closed-shell anion (9, 10) using decamethylferrocene as the reductant and a mixture of 2,3-dichloro-5,6-dicyano-1,4-benzoquinone and fluoroboric acid as the oxidant.

Diastereomerically Differentiated Excited State Behavior in Ruthenium(II) Hexafluoroacetylacetonate Complexes of Diphenyl Thioindigo Diimine.

Mono- and diruthenium hexafluoroacetylacetonate (hfac) complexes of the thioindigo-N,N'-diphenyldiimine chelating ligand 3 have been prepared. The thioindigo diimine ligand binds to ruthenium in a bidentate fashion in the mononuclear compound 2 and serves as a bidentate chelating bridging ligand in the diruthenium complexes 1a and 1b. Compound 2 was isolated as a racemic mixture while the diruthenium complexes were isolated as the meso (ΔΛ) 1a and rac (ΔΔ and ΛΛ) 1b diastereomers. In-depth structural characterization of the compounds was performed, including X-ray crystallography, 1H, 13C, and 19F nuclear magnetic resonance (NMR) spectroscopy, and 2D NMR correlation experiments. Electrochemical properties were evaluated utilizing cyclic voltammetry. Ground state optical properties of the complexes were examined using UV-visible spectroscopy and spectroelectrochemistry. The excited state dynamics of the series were investigated by ultrafast transient absorption spectroscopy. Variable temperature NMR experiments demonstrated that the rac diruthenium compound 1b undergoes conformational exchange with a rate constant of 8700 s-1 at 298 K, a behavior that is not observed in the meso diastereomer 1a. The series of complexes possess metal-to-ligand charge transfer (MLCT) absorption bands in the near-infrared (λmax 689-783 nm). The compounds do not display photoluminescence in room temperature solution-phase experiments or in experiments at 77 K. Transient absorption spectroscopy measurements revealed excited states with picosecond lifetimes for 1a, 1b, and 2, and spectroelectrochemical experiments confirmed assignment of the transient species as arising from MLCT transitions. Unexpectedly, the transient absorption measurements revealed disparate time constants for the excited state decay of diastereomers 1a and 1b.

Synthesis and redox reactions of bis(verdazyl)palladium complexes.

The synthesis and ligand-centered redox chemistry of palladium complexes bearing two potentially bidentate verdazyl ligands is explored. Reaction of 1,5-diisopropyl-3-pyridin-2-yl-6-oxoverdazyl radical 1 with Pd(NCMe)4·2BF4 gives a complex containing two coordinated verdazyl radicals. The structure of this complex consists of one verdazyl bound to Pd in a bidentate mode and the second verdazyl bound in a monodentate fashion through the pyridine substituent; the fourth coordination site is occupied by a solvent molecule (acetonitrile (3) or dimethyl sulfoxide (4)). Two-electron reduction of this complex with decamethylferrocene affords a bis(verdazyl) palladium complex (5) in which both verdazyls have been reduced to their anionic state and are both bound to Pd in bidentate manner. Complex 5 can be independently synthesized by a redox reaction between 1 and Pd2(dba)3. Reduced complex 5 can be re-oxidized to 3 or 4 with AgBF4; in contrast, oxidation with PhICl2 leads to ligand dissociation, ultimately giving radical 1 and a mono(verdazyl)dichloropalladium complex 2. One-electron oxidation using PhICl2 produces a formally "mixed valent" (in ligand) bis(verdazyl)chloropalladium complex (6) with one bidentate verdazyl anion ligand and one monodentate (pyridine-bound) verdazyl radical. Attempted protonation of the verdazyl ligands in complex 5 leads to complete ligand dissociation and protonation of both the tetrazine and pyridine moieties; deprotonation regenerates 5. Subsequent air oxidation of the tetrazane/pyridinium cation (formed as a tetrachloropalladate salt) leads to re-coordination of the verdazyl ligands to give 6 initially, but ultimately produces a combination of free radical 1 and 2.

Classical and non-classical redox reactions of Pd(II) complexes containing redox-active ligands.

Reactivity studies of a Pd(II)-verdazyl complex reveal novel ligand-centred reduction processes which trigger pseudo-reductive elimination at Pd. Reaction of the complex with water induces a ligand-centred redox disproportionation. The reduced verdazyl ligands can also be reversibly protonated.