Redox-Dependent Metal-Metal Bonding in Trinuclear Metal Chains: Probing the Transition from Covalent Bonding to Exchange Coupling.

The synthesis and physical properties of two new cationic tri-metallic chains, [(PEt3 )3 RuCl3 M'Cl3 Ru(PEt3 )3 ]1+ , M'=Rh and Ir are reported. These are isostructural with a previously reported 17-electron all-ruthenium analogue, but replacing a d5 RuIII ion in the central position with d6 RhIII /IrIII has a significant impact on the nature of the metal-metal interactions. All three materials have been characterized electrochemically at the 18-, 17- and 16-electron levels. X-ray crystallography and spectroelectrochemistry, complemented by electronic structure analysis at the DFT and CASPT2 levels, indicate that whilst the presence of a RuIII ion in the center of the chain allows multi-center covalent bonding to develop, a closed-shell RhIII /IrIII ion pushes the system towards the exchange-coupled limit, where the outer Ru centers are only weakly interacting. This family of three isostructural compounds reveals how changes in metal composition can have subtle effects on physical properties of systems that lie close to the localized/delocalized borderline.

Synthesis, electrochemistry, and bioactivity of the cyanobacterial calothrixins and related quinones.

The calothrixins are quinone-based natural products isolated from Calothrix cyanobacteria which show potent antiproliferative properties against several cancer cell lines. Preliminary mechanistic studies suggest that the biological mode of action of the calothrixins may be linked to their ability to undergo redox cycling. In this study we compare the bioactivities of the calothrixins with those of structurally related quinones in order to identify the structural features in the calothrixins essential for biological activity. In particular, the reduction potentials of the calothrixins and some related quinones were measured electrochemically. Our studies indicate that while there is no direct correlation between the reduction potentials and biological activities of the studied compounds, in all cases quinones with EC(50) values <1.6 microM undergo reduction to their respective semiquinones readily, with their E(1/2) values being more positive than -0.5 V versus the standard hydrogen electrode (SHE).

Organometallic complexes for nonlinear optics. 30.1 electrochromic linear and nonlinear optical properties of alkynylbis(diphosphine)ruthenium complexes.

A combination of cyclic voltammetry, UV-vis-NIR spectroelectrochemistry, time-dependent density functional theory (TD-DFT), and Z-scan measurements employing a modified optically transparent thin-layer electrochemical (OTTLE) cell has been used to identify and assign intense transitions of metal alkynyl complexes at technologically important wavelengths in the oxidized state and to utilize these transitions to demonstrate a facile electrochromic switching of optical nonlinearity. Cyclic voltammetric data for the ruthenium(II) complexes trans-[RuXY(dppe)(2)] [dppe = 1,2-bis(diphenylphosphino)ethane, X = Cl, Y = Cl (1), Ctbd1;CPh (2), 4-Ctbd1;CC(6)H(4)Ctbd1;CPh (3); X = Ctbd1;CPh, Y = Ctbd1;CPh (4), 4-Ctbd1;CC(6)H(4)Ctbd1;CPh (5)] show a quasi-reversible oxidation at 0.50-0.60 V (with respect to ferrocene/ferrocenium 0.56 V), which is assigned to the Ru(II/III) couple. The ruthenium(III) complex cations trans-[RuXY(dppe)(2)](+) were obtained by the in situ oxidation of complexes 1-5 using an OTTLE cell. The UV-vis-NIR optical spectra of 1(+)-5(+) contain a low-energy band in the near-IR region ( approximately 8000-16 000 cm(-)(1)), in contrast to 1-5, which are optically transparent at wavelengths < 22 000 cm(-)(1). TD-DFT calculations have been applied to model systems trans-[RuXY(PH(3))(4)] [X = Cl, Y = Cl, Ctbd1;CPh, or 4-Ctbd1;CC(6)H(4)Ctbd1;CPh; X = Ctbd1;CPh, Y = Ctbd1;CPh or 4-Ctbd1;CC(6)H(4)Ctbd1;CPh] to rationalize the optical spectra of 1-5 and 1(+)-5(+). The important low-energy bands in the electronic spectra of 1(+)-5(+) are assigned to the promotion of an electron from either a chloride p orbital or an ethynyl p orbital to the partially occupied HOMO. These absorption bands have been utilized to demonstrate a facile switching of cubic nonlinear optical (NLO) properties at 12 500 cm(-)(1) (corresponding to the wavelength of maximum transmission in biological materials such as tissue) using the OTTLE cell, the first electrochromic switching of molecular nonlinear refraction and absorption, and the first switching of optical nonlinearity using an electrochemical cell.

Remarkable homology in the electronic spectra of the mixed-valence cation and anion radicals of a conjugated bis(porphyrinyl)butadiyne.

The pi-radical cation and anion of the dizinc complex of a bis(triarylporphyrinyl)butadiyne, 1+ and 1-, respectively, display remarkably similar near-IR signatures, with intense bands near 1000 and 2500 nm, as predicted by the appropriate frontier-orbital model for inter-porphyrin coupling across the conjugated bridge.

Mixed-metal cluster chemistry. 19. Crystallographic, spectroscopic, electrochemical, spectroelectrochemical, and theoretical studies of systematically varied tetrahedral group 6-iridium clusters.

A systematically varied series of tetrahedral clusters involving ligand and core metal variation has been examined using crystallography, Raman spectroscopy, cyclic voltammetry, UV-vis-NIR and IR spectroelectrochemistry, and approximate density functional theory, to assess cluster rearrangement to accommodate steric crowding, the utility of metal-metal stretching vibrations in mixed-metal cluster characterization, and the possibility of tuning cluster electronic structure by systematic modification of composition, and to identify cluster species resultant upon electrochemical oxidation or reduction. The 60-electron tetrahedral clusters MIr(3)(CO)(11-x)(PMe(3))(x)(eta(5)-Cp) [M = Mo, x = 0, Cp = C(5)H(4)Me (5), C(5)HMe(4) (6), C(5)Me(5) (7); M = W, Cp = C(5)H(4)Me, x = 1 (13), x = 2 (14)] and M(2)Ir(2)(CO)(10-x)(PMe(3))(x)(eta(5)-Cp) [M = Mo, x = 0, Cp = C(5)H(4)Me (8), C(5)HMe(4) (9), C(5)Me(5) (10); M = W, Cp = C(5)H(4)Me, x = 1 (15), x = 2 (16)] have been prepared. Structural studies of 7, 10, and 13 have been undertaken; these clusters are among the most sterically encumbered, compensating by core bond lengthening and unsymmetrical carbonyl dispositions (semi-bridging, semi-face-capping). Raman spectra for 5, 8, WIr(3)(CO)(11)(eta(5)-C(5)H(4)Me) (11), and W(2)Ir(2)(CO)(10)(eta(5)-C(5)H(4)Me)(2) (12), together with the spectrum of Ir(4)(CO)(12), have been obtained, the first Raman spectra for mixed-metal clusters. Minimal mode-mixing permits correlation between A(1) frequencies and cluster core bond strength, frequencies for the A(1) breathing mode decreasing on progressive group 6 metal incorporation, and consistent with the trend in metal-metal distances [Ir-Ir < M-Ir < M-M]. Cyclic voltammetric scans for 5-15, MoIr(3)(CO)(11)(eta(5)-C(5)H(5)) (1), and Mo(2)Ir(2)(CO)(10)(eta(5)-C(5)H(5))(2) (3) have been collected. The [MIr(3)] clusters show irreversible one-electron reduction at potentials which become negative on cyclopentadienyl alkyl introduction, replacement of molybdenum by tungsten, and replacement of carbonyl by phosphine. These clusters show two irreversible one-electron oxidation processes, the easier of which tracks with the above structural modifications; a third irreversible oxidation process is accessible for the bis-phosphine cluster 14. The [M(2)Ir(2)] clusters show irreversible two-electron reduction processes; the tungsten-containing clusters and phosphine-containing clusters are again more difficult to reduce than their molybdenum-containing or carbonyl-containing analogues. These clusters show two one-electron oxidation processes, the easier of which is reversible/quasi-reversible, and the more difficult of which is irreversible; the former occur at potentials which increase on cyclopentadienyl alkyl removal, replacement of tungsten by molybdenum, and replacement of phosphine by carbonyl. The reversible one-electron oxidation of 12 has been probed by UV-vis-NIR and IR spectroelectrochemistry. The former reveals that 12(+) has a low-energy band at 8000 cm(-1), a spectrally transparent region for 12, and the latter reveals that 12(+) exists in solution with an all-terminal carbonyl geometry, in contrast to 12 for which an isomer with bridging carbonyls is apparent in solution. Approximate density functional calculations (including ZORA scalar relativistic corrections) have been undertaken on the various charge states of W(2)Ir(2)(CO)(10)(eta(5)-C(5)H(5))(2) (4). The calculations suggest that two-electron reduction is accompanied by W-W cleavage, whereas one-electron oxidation proceeds with retention of the tetrahedral core geometry. The calculations also suggest that the low-energy NIR band of 12(+) arises from a sigma(W-W) --> sigma*(W-W) transition.

Steric and Electronic Effects in the First Homoleptic Imino Ether Complex: Synthesis and X-ray Crystallographic Determination of [Pt(NHC(OEt)Et)(4)](CF(3)SO(3))(2).

Imino ethers, NH=C(OR')R, represent an interesting alkoxy-substituted class of otherwise elusive monodentate imine ligands. We have discovered that 4-fold addition of EtOH to [Pt(N&tbd1;CEt)(4)](2+) proceeds smoothly under basic conditions to yield stable, colorless [Pt(NH=C(OEt)Et)(4)](2+)(CF(3)SO(3)(-))(2) (1), which is the firsthomoleptic imino ether complex reported for any metal ion. The monodentate imino ether ligands are all equivalent in solution, according to (1)H NMR, and prove to have the E isomeric form with cis NH/COEt groups. The IR-active C-N stretching vibration moves from 2330 cm(-1) in the nitrile complex to 1633 cm(-1) in 1, and nu(NH) emerges at 3250 cm(-1). The (195)Pt NMR shift for 1 is found to be -2450 ppm, lying between that for [Pt(NCEt)(4)](2+) at -2409 ppm and that for [Pt(NH(3))(4)](2+) at -2580 ppm (all relative to [PtCl(6)](2-)). Single-crystal X-ray diffraction establishes that 1 crystallizes in the monoclinic space group P2(1)/c, with a = 9.026(2) Å, b = 14.838 (1) Å, c = 13.946 (1) Å, beta = 106.328(2) degrees, and Z = 2. The metal ion lies at a center of inversion and displays almost ideal square-planar PtN(4) coordination with mean Pt-N distances of 2.016(4) and N-Pt-N angles of 90 +/- 0.6 degrees. Within the two crystallographically independent NH=C(OEt)Et ligands, the mean N=C distance is 1.283 Å and there is structural evidence of partial pi-delocalization of O lone pairs into the NCO moiety. Nonetheless, according to both structural and (195)Pt NMR data, the imino ether ligand behaves as innocent Nsigma donor toward Pt(2+), with a trans influence similar to that of Cl(-). Due to their extended, asymmetric planar nature, such E-NH=C(OR')R ligands have exceptional steric requirements which control the coordination geometry and prevent unhindered rotation. These requirements are dominated by alkyl tail-to-tail (R/R) clashes that can be accommodated only by systematic twisting of the ligands about their respective Pt-N bonds. The present study reveals that 1 has the Pt(HHTT) C(2)(h)() conformation, where H (head up) and T (tail up) represent mutual NH-up and NH-down orientations of the imino ether ligands, with trans-ligand pairs cooperatively rotated by about 20 degrees with respect to the PtN(4) plane. The alternative Pt(HTHT) conformer of 1, with counterrotated trans ligands, is predicted to exist independently, possibly in the chiral D(2) form, in contrast to unfavored Pt(HHHH).

Valence-Dependent Metal-Metal Bonding and Optical Spectra in Confacial Bioctahedral [Re(2)Cl(9)](z)()(-) (z = 1, 2, 3). Crystallographic and Computational Characterization of [Re(2)Cl(9)](-) and [Re(2)Cl(9)](2)(-).

The geometric structure of the confacial bioctahedral [Re(2)Cl(9)](z)()(-) anion has been determined by single-crystal X-ray diffraction in two distinct oxidation states, Re(IV)(2) and Re(III)Re(IV). [Bu(4)N][Re(2)Cl(9)] crystallizes in the monoclinic space group P2(1)/m [a/Å = 10.6363(3), b/Å = 11.420(1), c/Å = 13.612(1), beta/deg = 111.18(1), Z = 2], while [Et(4)N](2)[Re(2)Cl(9)] crystallizes in the orthorhombic space group Pnma [a/Å = 15.82(1), b/Å = 8.55(2), c/Å = 22.52(3), Z = 4]. The Re-Re separation contracts from 2.704(1) Å in [Bu(4)N][Re(2)Cl(9)] to 2.473(4) Å in [Et(4)N](2)[Re(2)Cl(9)] (or, equivalently, from 2.725 to 2.481 Å after standard corrections for thermal motions), while the formal metal-metal bond order falls from 3.0 to 2.5. SCF-Xalpha-SW molecular orbital calculations show that, despite the {d(3)d(3)} configuration, the single sigma bond in [Re(2)Cl(9)](-) dominates the observed structural properties. For [Re(2)Cl(9)](2)(-), the 0.23 Å contraction in Re-Re is attributed jointly to radial expansion of the Re 5d orbitals and to diminished metal-metal electrostatic repulsion, which act in concert to make both sigma and delta(pi) bonding more important in the reduced species. Computed transition energies and oscillator strengths for the two structurally defined anions permit rational analysis of their ultraviolet spectra, which involve both sigma --> sigma and halide-to-metal change-transfer absorptions. The intense sigma --> sigma band progresses from 31 000 cm(-)(1) in [Re(2)Cl(9)](-) to 36 400 cm(-)(1) in [Re(2)Cl(9)](2)(-), according to the present assignments. For electrogenerated, highly reactive [Re(2)Cl(9)](3)(-) (where conventional X-ray structural information is unlikely to become available), the dominant absorption band advances to 40 000 cm(-)(1), suggesting further strengthening of the metal-metal sigma bond in the Re(III)(2) species.