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Clifford P Kubiak

Publications and source records attributed to Clifford P Kubiak.

15 recordsLinked to original sources
PubMed2006

Charge gating and electronic delocalization over a denderimeric assembly of trinuclear ruthenium clusters.

A zeroth-order dendrimer was formed using a tridentate bridging ligand, 2,4,6-tri-4-pyridyl-S-triazine, and the redox-active trinuclear ruthenium cluster Ru3O(OAc)6(CO)(py)(H2O). The electronic properties of this dendrimer were probed using cyclic voltammetry. IR spectroelectrochemistry was performed at both low (-30 degrees C) and room temperature. The IR spectroelectrochemical response at -30 degrees C was straightforward, but at room temperature, the dendrimer exhibits an unusual and complex series of electronic behaviors, including intramolecular cluster-to-bridging-ligand charge transfer, gated electron transfer, and dynamic exchange on the IR time scale.

Dendrimers↗
PubMed2006

Tuning the electronic communication and rates of intramolecular electron transfer of dimers of trinuclear ruthenium clusters: bridging and ancillary ligand effects.

Ten new bridged dimers of oxo-centered triruthenium clusters with CO and 4-(dimethylamino)pyridine (dmap), pyridine (py), or 4-cyanopyridine (cpy) as terminal ligands and pyrazine-d(4) (d(4)-pz), 2,5-dimethylpyrazine (dmpz), 2-methylpyrazine (mpz), and 2-chloropyrazine (clpz) as bridging ligands were prepared. The carbonyl stretching frequency, nu(CO), was used as a probe for infrared spectroelectrochemical measurements. In the neutral and doubly reduced states, a single band was observed for each of the dimers, with a shift in frequency due to the oxidation state of the triruthenium clusters. In the singly reduced state, a range of nu(CO) line shapes was observed, depending on the nature of the ligands, from two bands centered at the frequencies of the bands of the neutral and doubly reduced species to one broad band at the average of these two frequencies. By synthesizing new combinations of bridging and ancillary ligands, electronic communication between two bridged triruthenium clusters was effectively tuned, and electron-transfer rates were estimated by IR spectral line-shape analysis. In dimers bridged by the asymmetric ligand mpz, it was possible through selective isotope labeling of one CO ligand to observe "mixed-valence isomers," the two alternate charge distributions of a mixed-valence complex.

Journal Article↗
PubMed2005

Intervalence-resonant Raman spectroscopy of strongly coupled mixed-valence cluster dimers of ruthenium.

Resonance Raman spectroelectrochemistry (RR-SEC) at -20 degrees C has been performed on the pyrazine-bridged dimer of mu-oxo-centered trinuclear ruthenium-acetate "clusters"--[(dmap)(CO)(mu-OAc)6(mu3-O)Ru3(mu-L(b))Ru3(mu3-O)(mu-OAc)6(CO)(dmap)]n (where dmap = 4-(dimethylamino)pyridine and L(b) = pyrazine-h4 and pyrazine-d4)-in three oxidation states: n = 0, -1, and -2. In the one-electron reduced, "mixed-valent" state (overall -1 charge and a single odd electron; formal oxidation states [II, II, III]-[III, III, II] on the metal centers), the Raman excitation at 800 nm is resonant with a cluster-to-cluster intervalence charge-transfer (IVCT) band. Under these conditions, scattering enhancement is observed for all four totally symmetric vibrational modes of the bridging pyrazine ligand (nu8a, nu9a, nu1, and nu6a) in the investigated spectral range (100-2000 cm(-1)), and there is no evidence of activity in non-totally symmetric vibrations. Resonantly enhanced Raman peaks related to peripheral pyridyl (dmap) ligand modes and low-frequency features arising from the trigonal Ru3O cluster core and the cluster[Ru]-[N]ligand vibrations were also observed in the spectra of the intermediate-valence (n = -1) cluster dimer. The vibrational assignments and interpretations proposed in this work were reinforced by observation of characteristic isotopic frequency shifts accompanying deuteration of the bridging pyrazine. The results reveal that the fully symmetric (A(g)) vibrational motions of the organic bridge are coupled to the nominally metal cluster-to-metal cluster fast intramolecular electron transfer (ET) and provide validation of the near-delocalized description according to a predicted three-site/three-state (e.g., metal-bridge-metal) vibronic coupling model, in which the important role of the bridging ligand in mediating electronic communication and delocalization between charge centers is explicitly considered. Further compelling evidence supporting an extended five-state model, which incorporates the peripheral cluster-bound pyridyl ligands, is also presented.

Journal Article↗
PubMed2005

Mixed valence isomers.

The infrared spectroscopic observation of mixed valence isomers, the two alternate charge distributions of a mixed valence complex, is reported. Asymmetry induced by the 2-methylpyrazine bridging ligand was used to energetically differentiate the two states, and isotopic labeling of CO was used to spectroscopically observe the two states. Infrared line shape analysis was used to determine rate constants for electron transfer of 6.5 x 1011 s-1 and equilibrium constants of 2.2 for the mixed valence isomers.

Journal Article↗
PubMed2004

Structural, spectroscopic, and electrochemical studies of the complexes [Ni2(mu-CNR)(CNR)2(mu-dppm)2](n+) (n = 0, 1, 2): unusual examples of nickel(0)-nickel(I) and nickel(0)-nickel(II) mixed valency.

Reaction of Ni(COD)(2) (COD = cyclooctadiene) with dppm (dppm = bis(diphenylphosphino) methane) followed by addition of alkyl or aryl isocyanides yields the class of nickel(0) dimers Ni(2)(mu-CNR)(CNR)(2)(mu-dppm)(2) (R = CH(3) (1), n-C(4)H(9) (2), CH(2)C(6)H(5) (3), i-C(3)H(7) (4), C(6)H(11) (5), t-C(4)H(9) (6), p-IC(6)H(4) (7), 2,6-(CH(3))(2)C(6)H(3) (8)). The cyclic voltammograms of the dimers exhibit two sequential single electron oxidations to the +1 and +2 forms. Specular reflectance infrared spectroelectrochemical (IRSEC) measurements demonstrate reversible interconversions between the neutral Ni(0) dimers and their +1 and +2 forms. Bulk samples of the +2 forms are prepared by chemical oxidation using [FeCp(2)][PF(6)], while the +1 forms are prepared by the comproportionation of neutral and +2 forms. The neutral complexes 6 and 8 were characterized by X-ray diffraction as symmetric, locally tetrahedral binuclear Ni(0) complexes. The +2 forms of these complexes, 6(2+) and 8(2+), have asymmetric structures with one locally square planar and one locally tetrahedral metal center, evidence for a Ni(II)-Ni(0) mixed valence state. The X-ray structural characterization of 6(+) is symmetrical and qualitatively similar to that of the neutral complex 6. The +1 forms all exhibit intense near IR electronic absorptions that are assigned as intervalence charge transfer (IVCT) bands. On the basis of structural, spectroscopic, and electrochemical data, the +1 forms of the complexes, 1(+)-8(+), are assigned as Robin-Day class III, fully delocalized Ni(+0.5)-Ni(+0.5) mixed valence complexes.

Journal Article↗
PubMed2003

Electron transfer and dynamic infrared-band coalescence: it looks like dynamic NMR spectroscopy, but a billion times faster.

Broadening and coalescence of infrared bands can occur due to chemical exchange processes occurring on very fast, femtosecond-to-picosecond timescales. One such fast process of recent investigation is intramolecular electron transfer in transition-metal complexes with strong communication between electron-donor and -acceptor sites. The observation of partial coalescence of metal-carbonyl stretching bands in hexanuclear ruthenium mixed-valence complexes due to electron-transfer rates on the order of 10(11)-10(12) s(-1) is chronicled here. Several important advances have been made with the aid of dynamic infrared-band coalescence in these complexes, including the observation of dynamic solvent relaxation effects on electron-transfer rates, the determination of the equilibrium constant between charge-transfer isomers, and a reconsideration of the theory of electron transfer and delocalization in bridged, near-delocalized electron-transfer systems.

Journal Article↗
PubMed2003

Intervalence involvement of bridging ligand vibrations in hexaruthenium mixed-valence clusters probed by resonance Raman spectroscopy.

Resonance Raman spectroscopy, performed using spectroelectrochemistry and with excitation in the intervalence bands of three pyrazine-bridged, mixed-valence dimers of trinuclear ruthenium clusters, shows resonant enhancement of symmetric bridging ligand modes. The resonant enhancements and frequency shifts of these bridging ligand modes are observed as a function of varying electronic communication between charge sites, and they show that a three-state vibronic model which explicitly includes the participation of the bridging ligand is needed to explain the spectroscopic behavior of these near-delocalized complexes.

Journal Article↗
PubMed2003

Hydrocarbation chemistry proceeding from nickel carbenes.

Nickel carbene complex 2 [Ni(triphos)C(H)N(H)xylyl]2+(BF4-)2 reacts with alkenes quantitatively and regiospecifically to give the anti-Markovnikov hydrocarbation products. X-ray crystallography shows significant iminium alkyl character of the hydrocarbation products, similar to that observed in parent carbene 2. Mechanistic studies suggest the importance of a "hydride" pathway over "alkene" (metallocycle formation or carbocation) pathways.

Journal Article↗
PubMed2003

Infrared activity of symmetric bridging ligand modes in pyrazine-bridged hexaruthenium mixed-valence clusters.

A fully symmetric (A(g)) vibrational mode of pyrazine is observed in the infrared spectrum of four pyrazine-bridged hexaruthenium mixed-valence complexes with varying degrees of electronic coupling between clusters. Deuteration of the bridging pyrazine ligand and the accompanying shift in frequency confirm the assignment of this mode. Previous observation of infrared line coalescence in the carbonyl stretching region assigns all of these complexes to Robin-Day class II (partial localization of charge) on the picosecond time scale. The infrared activity of the fully symmetric bridging ligand mode could provide a complementary assignment of these complexes to class II on a faster, femtosecond time scale. However, the extinction coefficient for this band is much greater than that observed in similar asymmetric, non-mixed-valence complexes and suggests that its strong IR activity is due to vibronic enhancement rather than electronic asymmetry.

Journal Article↗
PubMed2002

Inter- and intramolecular pi-stacking interactions in cis-bis[1-(9-anthracene)]phosphirane complexes of platinum(II).

The bis[1-(9-anthracene)phosphirane]dithiolatoplatinum(II) complexes, Pt[1-(9-anthracene)phosphirane](2)(dithiolate), where dithiolate = 1,1-dimethoxycarbonyl-ethylene-2,2-dithiolate (dmdt) (2), 1,1-diethoxycarbonyl-ethylene-2,2-dithiolate (dedt) (3), 1-ethoxycarbonyl-1-cyano-ethylene-2, 2-dithiolate (ecdt) (4), and 1,1-dicyano-ethylene-2,2-dithiolate (dcdt) (5), were prepared from cis-dichlorobis[1-(9-anthracene)phosphirane]platinum(II) (1). Complexes 3 and 5 were characterized by X-ray crystallography and were found to have vastly different crystal and molecular structures. The crystal and molecular structure of 3 is dominated by intramolecular pi-stacking between the anthracene rings of the cis-bis(anthracene)phosphiranes with a ring...ring separation of 3.48(6) A. The molecular structure of 5 does not exhibit an intramolecular interaction between the anthracene rings. Instead, the crystal structure of 5shows significant intermolecular pi-stacking between the anthracene rings of the phosphirane ligands of adjacent molecules packed in the crystal lattice. The intermolecular stacking interaction results in a ring...ring separation of 3.33(4) A. Complexes 2-5 were found to emit at 530 nm at low temperatures in the solid state. Complex 5 emits strongly in fluid THF or benzene solution at 430 nm.

Journal Article↗
PubMed2002

Tin-capped trinuclear nickel clusters: redox isomerism between micro(3)-stannyl and micro(3)-stannylene clusters of the class [Ni(3)(dppm)(3)(micro3-I)(micro3-SnCl(x)](n+) (x = 2, n = 1; x = 3, n = 0).

The reaction of Ni(3)(dppm)(3)(micro(3)-I)(2)) with sodium trichlorostannate affords the first tin-capped nickel cluster Ni(3)(dppm)(3)(micro(3)-I)(micro(3)-SnCl(3) (1). A site of coordinative unsaturation at tin can be introduced by the reaction of 1 with Tl[PF(6)] yielding the stannylene-capped cluster [Ni(3)(dppm)(3)(micro(3)-I)(micro(3)-SnCl(2)](+) (2). Clusters 1 and 2 were characterized by 31P NMR, X-ray diffraction, and cyclic voltammetry (CV). Clusters 1 and 2 exhibit single electron redox chemistries, [Ni(3)(dppm)(3)(micro(3)-I)(micro(3)-SnCl3](0/*-), [Ni(3)(dppm)(3)(micro(3)-I)(micro(3)-SnCl(2)](+/0), that together comprise a redox equilibrium. Thus, electrochemical reduction of 1 produces first the 49e- cluster radical anion [Ni(3)(dppm)(3)(micro(3)-I)(micro(3)-SnCl(3)](*-) which then yields the reduced form of 2, [Ni(3)(dppm)(3)(micro(3)-I)(micro(3)-SnCl(2)], upon chloride dissociation.

Journal Article↗
PubMed2002

Solvent dynamical control of electron-transfer rates in mixed-valence complexes observed by infrared spectral line shape coalescence.

Rate constants for intramolecular electron transfer within the intervalence charge transfer (-1) states of the complexes [{Ru3O(OAc)6(L)(CO)}2(mu-pz)] (where L= 4-(dimethylamino)pyridine (1), pyridine (2), 3-cyanopyridine (3), or 4-cyanopyridine (4) and pz = pyrazine) were determined by coalescence of infrared (IR) vibrational spectral line shapes in seven solvents. The electron-transfer times (kET-1) show a strong correlation with solvent relaxation times determined in separate ultrafast time-resolved fluorescence experiments. The best comparison is found with the parameter t1e, which is ascribed to inertial solvent relaxation. The IR spectra of these mixed-valence complexes are thus a steady-state spectral probe of ultrafast, dynamic solvent relaxation processes which are otherwise only accessible using laser-pumped, ultrafast time-resolved measurements.

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