Nm In CH Research Articles

Proton-Induced Tuning of Electrochemical and Photophysical Properties in Mononuclear and Dinuclear Ruthenium Complexes Containing 2,2'-Bis(benzimidazol-2-yl)-4,4'-bipyridine: Synthesis, Molecular Structure, and Mixed-Valence State and Excited-State Properties.

A series of mono- and dinuclear Ru(bpy)(2) complexes (bpy = 2,2'-bipyridine) containing 2,2'-bis(benzimidazol-2-yl)-4,4'-bipyridine (bbbpyH(2)) were prepared. The mononuclear complex [Ru(bpy)(2)(bbbpyH(2))](ClO(4))(2).CH(3)OH.4H(2)O was characterized by an X-ray structure determination. Crystal data are as follows: triclinic, space group P&onemacr;, a = 14.443(4) Å, b = 15.392(4) Å, c = 11.675(2)Å, alpha = 101.44(2) degrees, beta = 107.85(2) degrees, gamma = 96.36(2) degrees, V = 2380(1) Å(3), Z = 2. The coordination geometry of the ruthenium(II) ion is approximately octahedral. The dihedral angle between the two pyridyl rings in bbbpyH(2) is 9.4(3) degrees, which is close to coplanar, in the complex. Mono- and dinuclear complexes exhibit broad charge-transfer absorption bands at 420-520 nm and emission at 660-720 nm in CH(3)CN solution with lifetimes of 200-800 ns at room temperature. Transient difference absorption spectra and resonance Raman (rR) spectra were used to assign the charge-transfer bands in the 420-520 nm region and to identify the lowest excited states. Both absorption and emission spectra are sensitive to solvent and solution pH. Deprotonation of the dinuclear complex raises the energies of the pi orbitals of the bbbpyH(2) ligand, so that they become closer in energy to the pi orbitals of bpy. The intervalence band of [(bpy)(2)Ru(bbbpyH(2))Ru(bpy)(2)](5+)()()is observed at 1200 nm ( epsilon = 170 M(-)(1) cm(-)(1)) in CH(3)CN. The value of the electronic coupling matrix element, H(AB), was determined as 120 cm(-)(1). Upon deprotonation, the IT band was not observed. It is therefore concluded that a superexchange pathway occurs predominantly via the Ru(II) dpi-bbbpyH(2) pi interaction, since deprotonation decreases the interaction. The role of the intervening fragments in the bridging ligand is discussed from the viewpoint of orbital energies and their orbital mixing with Ru dpi orbitals.

Complexes of Oxygenated trans-Azoalkanes and Tetracyanoethylene. The Interaction of pi Oxygen Dipoles with an Electron Poor Alkene.

The N-oxide and N,N'-dioxide (1ao and 1ado) of trans-1,1'-azonorbornane (1) associate with TCNE in solution and in the solid state. The solution complexes are characterized by charge-transfer optical absorptions at lambda(m) 364 and 540 nm in CH(2)Cl(2) for 1ao/TCNE and 1ado/TCNE, respectively. These complexes are weakly bound, with K(f) values of 0.5-3.0 M(-)(1). Crystals of (1ao)(2)/TCNE and 1ado/TCNE are isolable, and their structures have been determined by X-ray diffraction. Local donor-acceptor (DA) interactions between the pi dipolar donors and the electron poor TCNE are found. Crystals of (1ao)(2)/TCNE are composed of discrete D-A-D triads in which the 1ao oxygen approaches one of the olefinic C atoms (C(1), 2.86 Å) of TCNE more closely than the other (C(2), 3.07 Å). The O-C(1)-C(2) angle is 87 degrees, and the azooxide molecular plane lies perpendicular to the molecular plane of TCNE. 1ado/TCNE crystals are composed of extended -D-A-D-A- strands in which both oxygen poles of the azodioxide simultaneously interact with alternate TCNE acceptors. The D-A geometry here is structurally analogous to that in the (1ao)(2)/TCNE crystal. PM3 calculations of 1:1 and 1:2 trans HN(O)NH(O)/TCNE complexes constrained to have D-A (O-C) distances of 2.88 Å, but which are otherwise geometry optimized, reproduce the DA topology observed in the crystalline samples.

Polycyclic bis(hydrazine) and bis(hydrazyl) radical cations: high and low inner-sphere reorganization energy organic intervalence compounds

Four-σ-bond-linked bis(hydrazine) radical cations s3 .+ , a3 .+ , and a8 .+ show broad visible a with λ max =512-548 nm in CH 3 CN at room temperature, attributed to Hush-type charge-transfer bands (transition energies E op =52.2-55.8 kcal/mol). The corresponding bis(hydrazyl) radical cations s2 .+ , a2 .+ , and a7 .+ show near-IR asorption with λ max =1062-1199 nm (E op =26.9-29.3 kcal/mol). The large difference in E op is caused by innersphere reorganization energy differences, which are predicted well by AM1 semiempirical molecular orbital calculations

Dinuclear europium(3+), terbium(3+) and gadolinium(3+) complexes of a branched hexaazacyclooctadecane ligand containing six 2,2'-bipyridine pendant units

The dinuclear Eu[sup 3+], Tb[sup 3+], and Gd[sup 3+] complexes of a branched hexaazacyclooctadecane ligand containing six 2,2[prime]-bipyridine (bpy) pendant units (1) have been prepared and characterized as chloride salts. Elemental analysis and FAB[sup +] mass spectrometry show unambiguously that in the solid state two lanthanide ions are complexes by one molecule of 1. In CH[sub 3]CN and alcohol (ethanol/methanol, 5:1 v/v) solution, 1 exhibits a bpy-centered absorption band at 285 nm and a bpy-centered phosphorescence (77 K, [lambda] = 435 nm, [tau] = 1 s). The [Gd[sub 2](1)]Cl[sub 6] complex shows an absorption band at 305 nm and a phosphorescence band at 455 nm with [tau] = 1.8 ms (at 77 K) in CH[sub 3]CN solution, whereas in alcohol solution it displays an absorption band at 286 nm and two phosphorescence bands ([lambda] = 435 nm, [tau] = 1.2 s; [lambda] = 455 nm, [tau] = 3.9 ms). The [Eu[sub 2](1)]Cl[sub 6] and [Tb[sub 2](1)]Cl[sub 6] complexes show an absorption band at 306 nm in CH[sub 3]CN and at 285 in alcohol and H[sub 2]O. In all the solvents used the metal-centered luminescence, characteristic of the Eu[sup 3+] and Tb[sup 3+] metal ions, can be observed. The excitation spectramore » (recorded at the wavelength of the metal centered emission) of the Eu[sup 3+] and Tb[sup 3+] complexes show a maximum at 306 nm in all solvents, which coincides with the absorption maximum in CH[sub 3]CN but not in alcohol and H[sub 2]O solution. The results obtained indicate that in CH[sub 3]CN solution an average of almost five bpy units per molecule of 1 are coordinated to the two metal ions in the [M[sub 2](1)]Cl[sub 6] complexes (M = Eu[sup 3+], Tb[sup 3+], Gd[sup 3+]), whereas in alcohol and water most of the pendant bpy branches are not coordinated. 29 refs., 5 figs., 2 tabs.« less

Direct studies of 1,1-diazenes. Syntheses, infrared and electronic spectra, and kinetics of the thermal decomposition of N-(2,2,6,6-tetramethylpiperidyl)nitrene and N-(2,2,5,5-tetramethylpyrrolidyl)nitrene

The syntheses, direct spectroscopic observation, and kinetics of thermal decomposition of the persistent 1,1-diazenes, N-(2,2,6,6-tetramethylpiperidyl)nitrene (4) and N-(2,2,5,5-tetramethylpyrrolidyl)nitrene (5) are reported. The electronic absorption spectrum of 4 at -78 °C reveals a structured absorption for the n,π* transition: λmax= 543 nm, λ_(0,0) = 620 nm, and emax = 18 ± 3 in Et_2O; λmax= 541 nm and λ_(0,0) = 610 nm in CH_2Cl_2; λmax= 526 nm and λ_(0,0) = 592 nm in i-PrOH. The infrared spectrum of 4 shows a strong absorption at 1595 cm^(-1)(R_2^(14) = ^(14)N stretch) and provides evidence that 1,1-diazene 4 has considerable N=N double-bond character in the ground state. The infrared spectrum of 5 shows a strong absorption at 1638 cm^(-1)(R_2^(14) = ^(14)N stretch). The unimolecular rate of thermal decomposition of 4 is sensitive to solvent, the rate increasing with decreasing solvent polarity (k_(re1) = 1.0, 1.7, 4.8 in THF, Et_2O, and hexane, respectively). The activation parameters for the unimolecular fragmentation of 1,1-diazene 4 are as follows: log A = 11.6 • 0.5 and E_a= 16.9 ± 0.7 kcal mo1^(-1) in hexane; log A = 13.7 ± 0.3 and E_a = 20.0 ± 0.4 kcal mo1^(-1) in Et_2O; log A = 13.6 ± 0.3 and E_a = 20.l ± 0.4 kcal mol^(-1) in THF. The activation parameters for the bimolecular dimerization of 4 are log A = 3.8 ± 0.7 and E_a = 6.4 ± 0.9 kcal mol^(-1) in CDC1_3. The unimolecular rate of thermal decomposition of 5 is sensitive to solvent, the rate increasing with decreasing solvent polarity, k_(rel) = 1.0, 2.4, and 5.1 for THF, Et_2O, and hexane, respectively. The activation parameters for the unimolecular fragmentation of 1,1-diazene 5 are as follows: log A = 10.9 ± 0.3 and E_a = 16.8 ± 0.5 kcal mo1^(-1) in hexane; log A = 12.4 ± 0.4 and E_a = 19.0 ± 0.6 kcal mo1^(-1) in Et_2O; log A= 12.1 ± 0.3 and E_a = 19.l ± 0.4 kcal mo1^(-1) in THF. At -41.1 °C the bimolecular rate constant for the dimerization of 5 is 8.5 X 10^(-5) L/(mol s), 90 times slower than that found for 4. The change from a six-membered to a five-membered ring 1,1-diazene causes a shift to higher energy for the n,π'* transition and a shift to increased wavenumber (cm^(-1)) for the N=N stretching frequency, not unlike that of the isoelectronic ketones, tetramethylcyclohexanone and tetramethylcyclopentanone. Similar E_a values for the unimolecular thermal fragmentation of 4 and 5 may indicate the strain energy difference between 4 and 5 is also small. An approximate value of 30.5 kcal mo1^(-1) for the heat of formation of the 1,1-diazene 5 is estimated, indicating the 1,1-diazene 5 has a higher heat of formation than the cis-1,2-diazene isomer by 20 kcal mo1^(-1).