Chelated O2BOH2- or Chelated O2CO2-: An Isoelectronic Conundrum.

The species originally reported as [Co(bipy)2O2BOH]·[B5O6(OH)4]·H3BO3·H3O·H2O, a Co(II) complex containing a chelated O2BOH2- ligand, is shown to be [Co(bipy)2O2CO]·[B5O6(OH)4]·H3BO3·2H2O, a Co(III) complex containing a chelated O2CO2- ligand. This was confirmed by 1H and 13C NMR, MS, IR, and an X-ray crystal structure.

Does H3O+ Really Act as a Ligand in the Solid State?

The evidence for the existence of metal complexes containing H3O+ as a ligand in the solid state is examined. Each of the 68 examples in the Cambridge Structural Database in which H3O+ is bound to a transition metal, lanthanoid, actinoid, or main group metal ion is detailed and critically appraised. It is concluded that none of the reported examples of complexes containing coordinated H3O+ have been unequivocally characterized and that they result from either curation errors or misinterpretations of the crystallographic data. These conclusions are supported by computational techniques, which show that three purported H3O+ complexes based on the 1,4,7,10,13,16,21,24-octa-azabicyclo(8.8.8)hexacosane azacryptand skeleton are better described as aqua complexes, with protonation occurring at the amine ligand.

Multifrequency cw-EPR and DFT Studies of an Apparent Compressed Octahedral Cu(II) Complex.

The syntheses and single-crystal X-ray structures of the mononuclear complexes [Cu(bmet)](ClO4)2·H2O, [Cu(bmet)]Br2·2MeCN, and [Zn(bmet)](ClO4)2·H2O (bmet = N,N'-bis(2,2'-bipyridin-6-ylmethyl)ethane-1,2-diamine) are described. All three complexes feature a central metal ion bound to all six N atoms of the bmet ligand, which displays a meridional-facial-facial-meridional (mffm) configuration. The three complexes show one N-M-N axis to be significantly shorter than the others in agreement with an apparent compressed octahedral geometry. The X-ray structures of a single crystal of [Cu(bmet)](ClO4)2·0.375H2O resolved from data recorded at different temperatures display no remarkable structural modifications. However, they all display both as a powder and, in solution, an axial g1 > g2 ≳ g3 > g(e) electron paramagnetic resonance (EPR) pattern at low temperature, which is indicative of tetragonally elongated octahedra, while at room temperature the Q-band EPR spectra display a more rhombic g1 ≳ g2 > g3 > g(e) pattern. The fully density functional theory optimized structure of the Cu(II) complexes displays significant structural modifications only along one N(imine)-M-N(amine) axis resulting in an elongated octahedral structure. Furthermore, the EPR parameters predicted from this structure are comparable to those determined experimentally from the axial EPR signal recorded at low temperature, consistent with the unpaired electron residing mainly in the {3d(x(2)-y(2))} orbital. The structural and electronic properties of [Cu(bmet)](2+) are different from those in other previously described dynamic Jahn-Teller systems. We propose that these data can be rationalized by a dynamic Jahn-Teller effect perturbed by the strain of the hexadentate bmet ligand.

Synthesis, Characterization, and Photocatalytic H2-Evolving Activity of a Family of [Co(N4Py)(X)](n+) Complexes in Aqueous Solution.

A series of [Co(III)(N4Py)(X)](ClO4)n (X = Cl(-), Br(-), OH(-), N3(-), NCS(-)-κN, n = 2: X = OH2, NCMe, DMSO-κO, n = 3) complexes containing the tetrapyridyl N5 ligand N4Py (N4Py = 1,1-di(pyridin-2-yl)-N,N-bis(pyridin-2-ylmethyl)methanamine) has been prepared and fully characterized by infrared (IR), UV-visible, and NMR spectroscopies, high-resolution electrospray ionization mass spectrometry (HRESI-MS), elemental analysis, X-ray crystallography, and electrochemistry. The reduced Co(II) and Co(I) species of these complexes have been also generated by bulk electrolyses in MeCN and characterized by UV-visible and EPR spectroscopies. All tested complexes are catalysts for the photocatalytic production of H2 from water at pH 4.0 in the presence of ascorbic acid/ascorbate, using [Ru(bpy)3](2+) as a photosensitizer, and all display similar H2-evolving activities. Detailed mechanistic studies show that while the complexes retain the monodentate X ligand upon electrochemical reduction to Co(II) species in MeCN solution, in aqueous solution, upon reduction by ascorbate (photocatalytic conditions), [Co(II)(N4Py)(HA)](+) is formed in all cases and is the precursor to the Co(I) species which presumably reacts with a proton. These results are in accordance with the fact that the H2-evolving activity does not depend on the chemical nature of the monodentate ligand and differ from those previously reported for similar complexes. The catalytic activity of this series of complexes in terms of turnover number versus catalyst (TONCat) was also found to be dependent on the catalyst concentration, with the highest value of 230 TONCat at 5 × 10(-6) M. As revealed by nanosecond transient absorption spectroscopy measurements, the first electron-transfer steps of the photocatalytic mechanism involve a reductive quenching of the excited state of [Ru(bpy)3](2+) by ascorbate followed by an electron transfer from [Ru(II)(bpy)2(bpy(•-))](+) to the [Co(II)(N4Py)(HA)](+) catalyst. The reduced catalyst then enters into the H2-evolution cycle.

Five-coordinate [Pt(II)(bipyridine)2(phosphine)](n+) complexes: long-lived intermediates in ligand substitution reactions of [Pt(bipyridine)2](2+) with phosphine ligands.

The reaction of [Pt(N-N)2](2+) [N-N = 2,2'-bipyridine (bpy) or 4,4'-dimethyl-2,2'-bipyridine (4,4'-Me2bpy)] with phosphine ligands [PPh3 or PPh(PhSO3)2(2-)] in aqueous or methanolic solutions was studied by multinuclear ((1)H, (13)C, (31)P, and (195)Pt) NMR spectroscopy, X-ray crystallography, UV-visible spectroscopy, and high-resolution mass spectrometry. NMR spectra of solutions containing equimolar amounts of [Pt(N-N)2](2+) and phosphine ligand give evidence for rapid formation of long-lived, 5-coordinate [Pt(II)(N-N)2(phosphine)](n+) complexes. In the presence of excess phosphine ligand, these intermediates undergo much slower entry of a second phosphine ligand and loss of a bpy ligand to give [Pt(II)(N-N)(phosphine)2](n+) as the final product. The coordination of a phosphine ligand to the Pt(II) ion in the intermediate [Pt(N-N)2(phosphine)](n+) complexes is supported by the observation of (31)P-(195)Pt coupling in the (31)P NMR spectra. The 5-coordinate nature of [Pt(bpy)2{PPh(PhSO3)2}] is confirmed by X-ray crystallography. X-ray crystal structural analysis shows that the Pt(II) ion in [Pt(bpy)2{PPh(PhSO3)2}]·5.5H2O displays a distorted square pyramidal geometry, with one bpy ligand bound asymmetrically. These results provide strong support for the widely accepted associative ligand substitution mechanism for square planar Pt(II) complexes. X-ray structural characterization of the distorted square planar complex [Pt(bpy)(PPh3)2](ClO4)2 confirms this as the final product of the reaction of [Pt(bpy)2](2+) with PPh3 in CD3OD. The results of density functional calculations on [Pt(bpy)2](2+), [Pt(bpy)2(phosphine)](n+), and [Pt(bpy)(phosphine)2](n+) indicate that the bonding energy follows the trend of [Pt(bpy)(phosphine)2](n+) > [Pt(bpy)2(phosphine)](n+) > [Pt(bpy)2](2+) for stability and that the formation reactions of [Pt(bpy)2(phosphine)](n+) from [Pt(bpy)2](2+) and [Pt(bpy)(phosphine)2](n+) from [Pt(bpy)2(phosphine)](n+) are energetically favorable. These calculations suggest that the driving force for the formation of [Pt(bpy)(phosphine)2](n+) from [Pt(bpy)2](2+) is the formation of a more energetically favorable product.

Co(III) complexes of the pentadentate NHC ligand PY4Im: carbene-induced trans influences and the non-disappearing 13C NMR peak.

Co(III) complexes of the N-heterocyclic carbene ligand PY4Im (PY4Im = (1,3-bis(bis(2-pyridyl)methyl)imidazol-2-ylidene)) having the general formula [(PY4Im)Co(X)](ClO4)n (X = NCMe; n = 3: OH-, N3-, NCS-, ONO-, F-; n = 2: O2CO2-, n = 1; (N3-)3, n = 0) were prepared and structurally characterised. X-ray structural data are consistent with the presence of a trans influence due to the coordinated carbene carbon, and this is also supported by computational results. 13C NMR spectra of the complexes did not display peaks corresponding to the carbene carbon, except in the case of the [(PY4Im)Co(O2CO)]+ cation, where a peak at δ = 170.21 ppm was observed. However, HMBC spectra allowed indirect determination of the chemical shifts of the carbene carbon in the remaining complexes, owing to the geometry of the PY4Im ligand. Calculated 13C chemical shifts for the complexes showed very good agreement with the experimental values for all but the carbene carbon atoms in all cases.

Heteroleptic Cu(I) bis-diimine complexes of 6,6'-dimesityl-2,2'-bipyridine: a structural, theoretical and spectroscopic study.

A series of heteroleptic Cu(I) complexes containing 6,6'-dimesityl-2,2'-bipyridine and phenanthroline-, bipyridine-, and biquinoline-based ligands is studied. The HETPHEN strategy is utilized to synthesize the heteroleptic complexes, which are stable in solution. The X-ray crystal structures of the complexes are presented; the solid-state four-coordinate Cu(I) geometries are quantified by using the τ4 parameter. A feature of the crystal structures is the intramolecular π-stacking between the mesityl ring(s) and the diimine ligand; the phen-based complexes exhibit stacking between the phen ligand and one of the mesityl rings, creating a "Pac-Man" motif. On the other hand, the bpy-based complexes show different types of packing interaction, with both mesityl rings "clamping down" on the bpy based ligand to give π-stacking. Cyclic voltammetry is used to examine the redox chemistry of the complexes. The most positive potentials for the oxidation process are observed for the complexes with bulky substituents ortho to the coordination nitrogens atoms, i.e., 2,9-dimethyl-1,10-phenanthroline and 6,6'-dibromo-2,2'-bipyridine. The Cu(I) MLCT transitions of the complexes are investigated by resonance Raman spectroscopy in concert with TD-DFT calculations. The resonance Raman spectra of complexes containing substituted biquinolines are straightforward, in that vibrational bands of the biquinoline-based ligand are selectively enhanced over bpy(Mes)2 bands. This is consistent with the purple color of the complexes, due to the lower energy of the biquinoline-based LUMO compared to the bpy(Mes)2 LUMO. All the phen- and bpy-based complexes show enhancement of bpy(Mes)2 bands.

Synthesis, characterization, and photophysics of oxadiazole- and diphenylaniline-substituted Re(I) and Cu(I) complexes.

Transition-metal complexes of the types [Re(CO)(3)Cl(NN)], [Re(CO)(3)py(NN)](+), and [Cu(PPh(3))(2)(NN)](+), where NN = 4,4'-bis(5-phenyl-1,3,4-oxadiazol-2-yl)-2,2'-bipyridine (OX) and 4,4'-bis(N,N-diphenyl-4-[ethen-1-yl]-aniline)-2,2'-bipyridine (DPA), have been synthesized and characterized. Crystal structures for [Re(CO)(3)Cl(DPA)] and [Cu(PPh(3))(2)(OX)]BF(4) are presented. The crystal structure of the rhenium complex shows a trans arrangement of the ethylene groups, in agreement with density functional theory calculations. The structure of the copper complex displays the planar aromatic nature of the bpy-oxadiazole ligand. Density functional theory modeling of the complexes was supported by comparison of calculated and experimental normalized Raman spectra; the mean absolute deviations of the complexes were <10 cm(-1). The Franck-Condon state was investigated using UV-vis and resonance Raman spectroscopic as well as density functional theory computational techniques. It was shown that the lowest energy absorption peaks are metal to ligand charge transfer and ligand-centered charge transfer for the oxadiazole- and diphenylaniline-substituted bipyridine ligands, respectively. The lowest energy excited states were characterized using transient emission and absorption spectroscopic techniques in conjunction with density functional theory calculations. These showed that the DPA complexes had ligand-centered nonemissive "dark" states with lifetimes ranging from 300 to 2000 ns.

Efficient photocatalytic hydrogen production in water using a cobalt(III) tetraaza-macrocyclic catalyst: electrochemical generation of the low-valent Co(I) species and its reactivity toward proton reduction.

A very efficient homogeneous system for visible-light driven hydrogen production in water is reported. This comprises the [Co(CR)Cl2](+) cobalt(III) tetraaza-macrocyclic complex (Cat1) as a noble metal-free catalyst, [Ru(bpy)3]Cl2 as a photosensitizer and ascorbate/ascorbic acid as a sacrificial electron donor and buffer. This system gives up to 1000 turnovers at pH 4.0 versus the catalyst with a relatively low photosensitizer/catalyst ratio (10/1) and a high concentration of catalyst (1 × 10(−4) M), thus producing a significant amount of H2 (12.3 mL for 5 mL of solution). It also exhibits long-term stability (more than 20 hours). The efficiency of Cat1 has been compared under the same experimental conditions to those of three other H2-evolving catalysts, which are known to operate in water, [Co{(DO)(DOH)pn}Br2] (Cat2), [Co(dmbpy)3]Cl2 (Cat3) and [Rh(dmbpy)2Cl2]Cl (Cat4). These comparative studies show that Cat4, although based on a noble metal, is about four times less active, while Cat2 and Cat3 produce more than one hundred times less hydrogen than Cat1. The low-valent CoI form of Cat1 has been successfully electrogenerated in CH3CN. Its high stability can be related to the high catalytic performance of the Cat1 system. We have also shown that in acidic aqueous solution (photocatalytic conditions) reduction at a slightly more negative potential than the Co(II)/Co(I) couple is needed to ensure efficient catalysis; this reduction is performed by the photogenerated [Ru(II)(bpy)2(bpy(˙−))](+) species.

Complete family of mono-, bi-, and trinuclear Re(I)(CO)3Cl complexes of the bridging polypyridyl ligand 2,3,8,9,14,15-hexamethyl-5,6,11,12,17,18-hexaazatrinapthalene: syn/anti isomer separation, characterization, and photophysics.

The syn and anti isomers of the bi- and trinuclear Re(CO)(3)Cl complexes of 2,3,8,9,14,15-hexamethyl-5,6,11,12,17,18-hexaazatrinapthalene (HATN-Me(6)) are reported. The isomers are characterized by (1)H NMR spectroscopy and X-ray crystallography. The formation of the binuclear complex from the reaction of HATN-Me(6) with 2 equiv of Re(CO)(5)Cl in chloroform results in a 1:1 ratio of the syn and anti isomers. However, synthesis of the trinuclear complex from the reaction of HATN-Me(6) with 3 equiv of Re(CO)(5)Cl in chloroform produces only the anti isomer. syn-{(Re(CO)(3)Cl)(3)(µ-HATN-Me(6))} can be synthesized by reacting 1 equiv of Re(CO)(5)Cl with syn-{(Re(CO)(3)Cl)(2)(µ-HATN-Me(6))} in refluxing toluene. The product is isolated by subsequent chromatography. The X-ray crystal structures of syn-{(Re(CO)(3)Cl)(2)(µ-HATN-Me(6))} and anti-{(Re(CO)(3)Cl)(3)(µ-HATN-Me(6))} are presented both showing severe distortions of the HATN ligand unit and intermolecular π stacking. The complexes show intense absorptions in the visible region, comprising strong π → π* and metal-to-ligand charge-transfer (MLCT) transitions, which are modeled using time-dependent density functional theory (TD-DFT). The energy of the MLCT absorption decreases from mono- to bi- to trinuclear complexes. The first reduction potentials of the complexes become more positive upon binding of subsequent Re(CO)(3)Cl fragments, consistent with changes in the energy of the MLCT bands and lowering of the energy of relevant lowest unoccupied molecular orbitals, and this is supported by TD-DFT. The nature of the excited states of all of the complexes is also studied using both resonance Raman and picosecond time-resolved IR spectroscopy, where it is shown that MLCT excitation results in the oxidation of one rhenium center. The patterns of the shifts in the carbonyl bands upon excitation reveal that the MLCT state is localized on one rhenium center on the IR time scale.

Effect of sulfur-based substituents on the electronic properties of Re(I) dppz complexes.

A series of sulfur-substituted dppz-based ligands and their Re(I)(CO)(3)Cl complexes are reported. The sulfur-substituted ligands and complexes show interesting electronic properties atypical of dppz-type systems. Substitution of dppz with thiocyanate (SCN) groups results in behavior typical of an electron withdrawing group. However, substitution of dppz with the electron donating trithiocarbonate (S(2)CS) or deca-alkylthioether (Sdec) groups confer intraligand charge-transfer (ICT) from the S adduct to the phenazine lowest unoccupied molecular orbital (LUMO). Upon complexation of the substituted dppz ligand to Re(CO)(3)Cl this ICT red-shifts and increases in intensity. Analysis of these observations using density functional theory (DFT) calculations and resonance Raman spectroscopy reveals that these transitions are a mixture of metal-to-ligand charge-transfer (MLCT) and S --> phenazine ICT in nature. The synthesized compounds are also characterized using (1)H NMR spectroscopy, IR spectroscopy, and electrochemistry. Single-crystal X-ray analysis was performed on dppz(SCN)(2) (C(20)H(18)N(6)S(2) a = 8.780 A, b = 9.792 A, c = 10.400 A, alpha = 95.95 degrees , beta = 112.13 degrees , gamma = 95.38 degrees , triclinic, P1, Z = 2, R1 = 0.0306, wR2 = 0.0829.

Five-coordinate transition metal complexes and the value of τ5: observations and caveats.

The τ5 parameter, first proposed by Addison and coworkers, is the principal measure of the geometry of five-coordinate transition metal complexes, with τ5 = 0 said to describe a perfect square pyramidal geometry and τ5 = 1 a perfect trigonal pyramidal geometry. Therefore, the geometries of all five-coordinate complexes are assumed to lie on a continuum between these two extremes. Herein we show that there are a significant number of examples of transition metal complexes having τ5 > 1, leading to an equatorially distorted trigonal bipyramidal geometry with the transition metal ion lying out of the plane of the equatorial donor atoms. We also show that complexes having τ5 = 0 and displaying perfect square pyramidal geometry are very much the exception, and that the majority of complexes for which τ5 = 0 have the metal ion sitting above the mean plane of the donor atoms in the square plane, in a basally distorted square pyramidal geometry. Density functional theory computations on a number of these complexes show that the structural distortions are inherent features of the complexes, and not merely the result of intermolecular interactions.

An unusual stable mononuclear Mn(III) bis-terpyridine complex exhibiting Jahn-Teller compression: electrochemical synthesis, physical characterisation and theoretical study.

The mononuclear manganese bis-terpyridine complex [Mn(tolyl-terpy)(2)](X)(3) (1(X)(3); X=BF(4), ClO(4), PF(6); tolyl-terpy=4'-(4-methylphenyl)-2,2':6',2"-terpyridine), containing Mn in the unusual +III oxidation state, has been isolated and characterised. The 1(3+) ion is a rare example of a mononuclear Mn(III) complex stabilised solely by neutral N ligands. Complex 1(3+) is obtained by electrochemical oxidation of the corresponding Mn(II) compound 1(2+) in anhydrous acetonitrile. Under these conditions the cyclic voltammogram of 1(2+) exhibits not only the well-known Mn(II)/Mn(III) oxidation at E(1/2)=+0.91 V versus Ag/Ag(+) (+1.21 V vs. SCE) but also a second metal-based oxidation process corresponding to Mn(III)/Mn(IV) at E(1/2)=+1.63 V (+1.93 V vs. SCE). Single crystals of 1(PF(6))(3)2 CH(3)CN were obtained by an electrocrystallisation procedure. X-ray analysis unambiguously revealed its tetragonally compressed octahedral geometry and high-spin character. The electronic properties of 1(3+) were investigated in detail by magnetic measurements and theoretical calculations, from which a D value of +4.82 cm(-1) was precisely determined. Density functional and complete active space self consistent field ab initio calculations both correctly predict a positive sign of D, in agreement with the compressed tetragonal distortion observed in the X-ray structure of 1(PF(6))(3)2 CH(3)CN. The different contributions to D were calculated, and the results show that 1) the spin-orbit coupling part (+2.593 cm(-1)) is predominant compared to the spin-spin interaction (+1.075 cm(-1)) and 2) the excited triplet states make the dominant contribution to the total D value.

Trinuclear terpyridine frustrated spin system with a Mn(IV)3O4 core: synthesis, physical characterization, and quantum chemical modeling of its magnetic properties.

The trinuclear oxo bridged manganese cluster, [Mn(IV)(3)O(4)(terpy)(terpyO(2))(2)(H(2)O)](S(2)O(8))(2) (5) (terpy = 2,2':2'',6'-terpyridine and terpyO(2) = 2,2':2'',6'-terpyridine 1,1''-dioxide), was isolated in an acidic aqueous medium from the reaction of MnSO(4), terpy, and oxone as chemical oxidant. The terpyO(2) ligands were generated in situ during the synthesis by partial oxidation of terpy. The complex crystallizes in the monoclinic space group P21/n with a = 14.251(5) A, b = 15.245(5) A, c = 24.672(5) A, alpha = 90.000(5) degrees, beta = 92.045(5) degrees, gamma = 90.000(5) degrees, and Z = 4. The triangular {Mn(IV)(3)O(4)}(4+) core observed in this complex is built up of a basal Mn(mu-O)(2)Mn unit where each Mn ion is linked to an apical Mn ion via mono(mu-O) bridges. The facial coordination of the two tridentate terpyO(2) ligands to the Mn(mu-O)(2)Mn unit allows the formation of the triangular core. 5 is also the first structurally characterized Mn complex with polypyridinyl N-oxide ligands. The variable-temperature magnetic susceptibility data for this complex, in the range of 10-300 K, are consistent with an S = 1/2 ground state and were fit using the spin Hamiltonian H(eff) with S(1) = S(2) = S(3) = 3/2, J(a) = -37 (+/-0.5) and J(b) = -53 (+/-1) cm(-1), where J(a) and J(b) are exchange constants through the mono-mu-oxo and the di-mu-oxo bridges, respectively. The doublet ground spin state of 5 is confirmed by EPR spectroscopic measurements. Density functional theory (DFT) calculations based on the broken symmetry approach reproduce the magnetic properties of 5 very well (calculated values: J(a) = -39.4 and J(b) = -55.9 cm(-1)), thus confirming the capability of this quantum chemical method for predicting the magnetic behavior of clusters involving more than two metal ions. The nature of the ground spin state of the magnetic {Mn(IV)(3)O(4)}(4+) core and the role of ancillary ligands on the magnitude of J are also discussed.

Trinuclear copper(I) complex containing 3,4,9,10,15,16-hexamethyl-1,6,7,12,13,18-hexaazatrinaphthylene: a structural, spectroscopic, and computational study.

The compound [(Cu(PPh(3))(2))(3)(HATNMe(6))](BF(4))(3) has been synthesized and characterized by X-ray crystallography, resonance Raman spectroscopy, and density functional theory (DFT) calculations. The X-ray structure of solvated [(Cu(PPh(3))(2))(3)(HATNMe(6))](BF(4))(3) [rhombohedral, R3, a = b = 21.6404(4) A, c = 53.188(3) A, alpha = beta = 90 degrees, gamma = 120 degrees] shows that the HATNMe(6) ligand is very slightly twisted. The electronic absorption spectrum of the complex in chloroform shows two bands in the visible region attributed to ligand-centered (LC) and metal-to-ligand charge-transfer (MLCT) transitions, respectively. Time-dependent DFT calculations show good agreement with experiment, with two MLCT and one LC transition predicted in the visible region (641, 540, and 500 nm). Resonance Raman spectra of the complex using discrete excitation energies between 647 and 406 nm showed a variation in enhancement patterns consistent with at least two distinct transitions. The absolute Raman cross sections have been evaluated and, through a wavepacket analysis, the amount of distortion along each vibrational mode across the Franck-Condon surface is established from the calculated dimensionless displacement (Delta) values as well as other electronic parameters. The pattern of Delta values shows good agreement with the observed calculated modes, with the MLCT transition, showing much larger Delta values for outer ring modes such as nu(93) and nu(205) than in the LC transition. This is consistent with the molecular orbitals involved in the two transitions; the donor orbitals for the LC transition have similar outer-ring bonding characteristics compared to the MLCT transition, which has no donor orbital bonding characteristics on the ligand because the donor molecular orbitals are dpi orbitals.

A synthetic, structural, spectroscopic and DFT study of Re(I), Cu(I), Ru(II) and Ir(III) complexes containing functionalised dipyrido[3,2-a:2',3'-c]phenazine (dppz).

The ligands 11-cyanodipyrido[3,2-a:2',3'-c]phenazine and 2-(11-dipyrido[3,2-a:2',3'-c]phenazine)-5-phenyl-1,3,4-oxadiazole have been coordinated to Re(I), Cu(I), Ru(II) and Ir(III) metal centres. Single-crystal X-ray analyses were performed on fac-chlorotricarbonyl(11-cyanodipyrido[3,2-a:2',3'-c]phenazine)rhenium (C(22)H(9)ClN(5)O(3)Re, a=6.509(5), b=12.403(5), c=13.907(5) A, alpha=96.88(5), beta=92.41(5), gamma=92.13(5) degrees , triclinic, P1, Z=2) and bis-2,2'-bipyridyl(2-(11-dipyrido[3,2-a:2',3'-c]phenazine)-5-phenyl-1,3,4-oxadiazole)ruthenium triflate2 CH(3)CN (C(52)H(36)F(6)N(12)O(8)RuS(2), a=10.601(5), b=12.420(5), c=20.066(5) A, alpha=92.846(5), beta=96.493(5), gamma=103.720(5) degrees , triclinic, P1, Z=2). The ground- and excited-state properties of the ligands and complexes have been investigated with a range of techniques, including electrochemistry, absorption and emission spectroscopy, spectroelectrochemistry and excited-state lifetime studies. Spectroscopic, time-resolved and DFT studies reveal that the ligand-centred (LC) transitions and their resultant excited states play an important role in the photophysical properties of the complexes. Evidence for the presence of lower-lying metal-to-ligand charge-transfer transitions is obtained from resonance Raman spectroscopy, but nanosecond transient Raman experiments suggest that once excited, the (3)LC state is populated.

A New Tetracycle from Dimerization of the N-Methylpyridazinium Ion in Aqueous Solution.

In a one-pot synthesis at room temperature the N-methylpyridazinium ion (1) dimerizes stereospecifically and with 100 % conversion in a series of OH- -catalyzed processes to give a new tetraazafluorene (2). Four of the individual steps can be identified directly and monitored by 1 H NMR spectroscopy.

Low symmetry pyrazole-based tripodal tetraamine ligands: metal complexes and ligand decomposition reactions.

The new low symmetry pyrazole-based tripodal tetraamine ligands 2-(1H-pyrazol-1-yl)-N,N-bis(1H-pyrazol-1-ylmethyl)ethanamine (bmpz) and 2-(1H-pyrazol-1-yl)-N-[2-(1H-pyrazol-1-yl)ethyl]-N-(1H-pyrazol-1-ylmethyl)ethanamine (bepz) have been prepared and characterised, as have metal complexes containing these ligands. X-ray crystal structures of [Co(bmpz)Cl](2)[CoCl(4)]·H(2)O, [Co(bmpz)MeCN](ClO(4))(2)·0.13H(2)O, [Zn(bmpz)MeCN](ClO(4))(2)·0.15H(2)O, [Zn(bepz)OH(2)](ClO(4))(2)·0.5H(2)O and [(Co(bepz)Cl)(2)]Cl(2)·6H(2)O confirm coordination of the intact tripodal ligands to the metal ions through all four N atoms. However, attempts to make Cu(2+) complexes containing bmpz and bepz gave, respectively, [Cu(7)Cl(2)]·0.2H(2)O and [Cu(8)Cl(2)] (7 = 1-(1H-pyrazol-1-yl)-N-(1H-pyrazol-1-ylmethyl)ethanamine, 8 = 2-(1H-pyrazol-1-yl)-N-[2-(1H-pyrazol-1-yl)ethyl]ethanamine), complexes containing the tridentate ligands 7 and 8 which are formed by loss of a pyrazolylmethyl arm from the appropriate tripodal ligand. This decomposition reaction occurs in protic solvents both in the presence and absence of metal ions, and is ascribed to the presence of an aminal functionality in the tripodal ligands. A possible mechanism for the decomposition, based on NMR and ESMS data, is suggested.