Roger D. Sommer

Syntheses of tris(pyrazolyl)methane ligands and {[tris(pyrazolyl)methane]Mn(CO)3}SO3CF3 complexes: comparison of ligand donor properties

Abstract The known ligands HC(pz)3, HC(3,5-Me2pz)3, HC(3-Phpz)3, and HC(3-tBupz)3 and the new ligand HC(3-iPrpz)3 (pz=pyrazolyl ring) are prepared in CHCl3–H2O using the appropriate pyrazole, an excess of Na2CO3, and tetra-n-butylammonium bromide as the phase transfer catalyst. Using these conditions, good yields of the ligands are consistently obtained. The new ligand PhC(pz)2py (py=pyridyl ring) is prepared in the CoCl2 catalyzed condensation reaction of (pz)2SO and Ph(py)CO. The reaction of HC(pz)3, KOtBu and para-formaldehyde followed by quenching with water yields HOCH2C(pz)3. All of these ligands, except HC(3-tBupz)3, react with [Mn(CO)5]SO3CF3, prepared in situ from Mn(CO)5Br and Ag(SO3CF3), to yield the respective [(ligand)Mn(CO)3]SO3CF3 complex. The carbonyl stretching frequencies and 13C-NMR trends of these complexes indicate that the donor abilities of all of the ligands are fairly similar. The solid state structure of {[HC(3-iPrpz)3]Mn(CO)3}+ shows the HC(3-iPrpz)3 ligand is tridentate with the iso-propyl groups rotated away from the Mn(CO)3 core of the cation relieving any possible steric congestion.

Cobalt single-molecule magnet

A cobalt molecule that functions as a single-molecule magnet, [Co4(hmp)4(MeOH)4Cl4], where hmp− is the anion of hydroxymethylpyridine, is reported. The core of the molecule consists of four Co(II) cations and four hmp− oxygen atom ions at the corners of a cube. Variable-field and variable-temperature magnetization data have been analyzed to establish that the molecule has a S=6 ground state with considerable negative magnetoanisotropy. Single-ion zero-field interactions (DSz2) at each cobalt ion are the origin of the negative magnetoanisotropy. A single crystal of the compound was studied by means of a micro-superconducting quantum interference device magnetometer in the range of 0.040–1.0 K. Hysteresis was found in the magnetization versus magnetic field response of this single crystal.

Solvate Structures and Spectroscopic Characterization of LiTFSI Electrolytes

A Raman spectroscopic evaluation of numerous crystalline solvates with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI or LiN(SO2CF3)2) has been conducted over a wide temperature range. Four new crystalline solvate structures-(PHEN)3:LiTFSI, (2,9-DMPHEN)2:LiTFSI, (G3)1:LiTFSI and (2,6-DMPy)1/2:LiTFSI with phenanthroline, 2,9-dimethyl[1,10]phenanthroline, triglyme, and 2,6-dimethylpyridine, respectively-have been determined to aid in this study. The spectroscopic data have been correlated with varying modes of TFSI(-)···Li(+) cation coordination within the solvate structures to create an electrolyte characterization tool to facilitate the Raman band deconvolution assignments for the determination of ionic association interactions within electrolytes containing LiTFSI. It is found, however, that significant difficulties may be encountered when identifying the distributions of specific forms of TFSI(-) anion coordination present in liquid electrolyte mixtures due to the wide range of TFSI(-)···Li(+) cation interactions possible and the overlap of the corresponding spectroscopic data signatures.

Light-Driven Hydrogen Evolution by BODIPY-Sensitized Cobaloxime Catalysts

We report four photocatalytically active cobaloxime complexes for light-driven hydrogen evolution. The cobaloxime catalysts are sensitized by different meso-pyridyl boron dipyrromethene (BODIPY) chromophores, bearing either two bromo- or iodo-substituents on the BODIPY core. The pyridine linker between the BODIPY and cobaloxime is further modified by a methyl substituent on the pyridine, influencing the stability and electronic properties of the cobaloxime catalyst and thus the photocatalytic efficiency of each system. Four cobaloxime catalyst complexes and three novel BODIPY chromophores are synthesized and characterized by absorption, fluorescence, infrared (IR) and nuclear magnetic resonance (NMR) spectroscopy, mass spectrometry, and electrochemistry. Crystal structures for the BODIPY-cobaloxime complexes 2 and 3 are presented. In contrast to the photocatalytically inactive, nonhalogenated reference complex 1, the four newly reported molecules are active for photocatalytic hydrogen evolution, with a maximum turnover number (TON) of 30.9 mol equiv of H2 per catalyst for the meso-methylpyridyl 2,6-diiodo BODIPY-sensitized cobaloxime complex 5. We conclude that accessing the photoexcited triplet state of the BODIPY chromophore by introducing heavy atoms (i.e., bromine or iodine) is necessary for efficient electron transfer in this system, enabling catalytic hydrogen generation. In addition, relatively electron-donating pyridyl linkers improve the stability of the complex, increasing the overall TON for hydrogen production.

An effective synthesis of alkyl β-cyano-α,γ-diketones using chlorosulfonylisocyanate and a representative Cu(II) complex

Abstract Chlorosulfonylisocyanate, a source of electrophilic cyanide equivalent, was used for the synthesis of three β-cyano-α,γ-diketones. The reactions proceeded in a straightforward fashion and the desired organic compounds were isolated in high-yield and excellent purity. The ligands then yielded homoleptic Cu(II) coordination complexes stabilized by intermolecular contacts. This is illustrated in the X-ray crystal structure of bis(4-cyano-2,2,6,6-tetramethyl-3,5-heptanedionato)copper(II).

Oxyfunctionalization with Cp*IrIII(NHC)(Me)(Cl) with O2: Identification of a Rare Bimetallic IrIV μ-Oxo Intermediate

Methanol formation from [Cp*Ir(III)(NHC)Me(CD2Cl2)](+) occurs quantitatively at room temperature with air (O2) as the oxidant and ethanol as a proton source. A rare example of a diiridium bimetallic complex, [(Cp*Ir(NHC)Me)2(μ-O)][(BAr(F)4)2], 3, was isolated and shown to be an intermediate in this reaction. The electronic absorption spectrum of 3 features a broad observation at ∼660 nm, which is primarily responsible for its blue color. In addition, 3 is diamagnetic and can be characterized by NMR spectroscopy. Complex 3 was also characterized by X-ray crystallography and contains an Ir(IV)-O-Ir(IV) core in which two d(5) Ir(IV) centers are bridged by an oxo ligand. DFT and MCSCF calculations reveal several important features of the electronic structure of 3, most notably, that the μ-oxo bridge facilitates communication between the two Ir centers, and σ/π mixing yields a nonlinear arrangement of the μ-oxo core (Ir-O-Ir ∼ 150°) to facilitate oxygen atom transfer. The formation of 3 results from an Ir oxo/oxyl intermediate that may be described by two competing bonding models, which are close in energy and have formal Ir-O bond orders of 2 but differ markedly in their electronic structures. The radical traps TEMPO and 1,4-cyclohexadiene do not inhibit the formation of 3; however, methanol formation from 3 is inhibited by TEMPO. Isotope labeling studies confirmed the origin of the methyl group in the methanol product is the iridium-methyl bond in the [Cp*Ir(NHC)Me(CD2Cl2)][BAr(F)4] starting material. Isolation of the diiridium-containing product [(Cp*Ir(NHC)Cl)2][(BAr(F)4)2], 4, in high yields at the end of the reaction suggests that the Cp* and NHC ligands remain bound to the iridium and are not significantly degraded under reaction conditions.

Superexchange Contributions to Distance Dependence of Electron Transfer/Transport: Exchange and Electronic Coupling in Oligo(para-Phenylene)- and Oligo(2,5-Thiophene)-Bridged Donor–Bridge–Acceptor Biradical Complexes

The preparation and characterization of three new donor-bridge-acceptor biradical complexes are described. Using variable-temperature magnetic susceptibility, EPR hyperfine coupling constants, and the results of X-ray crystal structures, we evaluate both exchange and electronic couplings as a function of bridge length for two quintessential molecular bridges: oligo(para-phenylene), β = 0.39 Å(-1) and oligo(2,5-thiophene), β = 0.22 Å(-1). This report represents the first direct comparison of exchange/electronic couplings and distance attenuation parameters (β) for these bridges. The work provides a direct measurement of superexchange contributions to β, with no contribution from incoherent hopping. The different β values determined for oligo(para-phenylene) and oligo(2,5-thiophene) are due primarily to the D-B energy gap, Δ, rather than bridge-bridge electronic couplings, H(BB). This is supported by the fact that the H(BB) values extracted from the experimental data for oligo(para-phenylene) (H(BB) = 11,400 cm(-1)) and oligo(2,5-thiophene) (12,300 cm(-1)) differ by <10%. The results presented here offer unique insight into the intrinsic molecular factors that govern H(DA) and β, which are important for understanding the electronic origin of electron transfer and electron transport mediated by molecular bridges.

Synthesis, solid-state structure, magnetic properties and Mössbauer spectral studies of {Fe[HC(3,5-Me2pz)3](H2O)3}(BF4)2

Abstract The reaction of Fe(BF4)2·6H2O and one equivalent of HC(3,5-Me2pz)3 in acetonitrile leads to the formation of {Fe[HC(3,5-Me2pz)3](H2O)3}(BF4)2. The solution phase 1H NMR spectrum is broad with chemical shifts ranging from 54 to −42 ppm, a range that is indicative of a paramagnetic high-spin complex. Magnetic moment measurements show that the complex is paramagnetic in the solid-state with μeff=4.89μB. The solid-state structure shows the N3FeO3 central core is a distorted octahedron with N–Fe–N angles averaging 84.4° and O–Fe–O angles averaging 90.0°. The average Fe–N distance is 2.18 A and the average Fe–O bond distance is 2.12 A. Mossbauer spectra, obtained at 78 and 295 K, clearly reveal that this complex is high-spin at these temperatures with hyperfine parameters typical of a pseudooctahedral iron(II) complex.

Synthesis of Unsupported d1–dx Oxido-Bridged Heterobimetallic Complexes Containing VIV: A New Direction for Metal-to-Metal Charge Transfer

Heterobimetallic complexes composed only of first-row transition metals [(TMTAA)V(IV)═O→M(II)Py5Me2](OTf)2 (TMTAA = 7,16-dihydro-6,8,15,17-tetramethyldibenzo[b,i][1,4,8,11]tetraazacyclotetradecine; Py5Me2 = 2,6-bis(1,1-bis(2-pyridyl)ethyl)pyridine; M = Mn(II), Fe(II), Co(II), Ni(II), Cu(II); OTf = trifluoromethanesulfonate) have been synthesized through a dative interaction between a terminal oxido and M(II) metal centers. This is the first series of V(IV)═O→M(II) heterobimetallic complexes containing an unsupported oxido bridge. Among these five complexes, only V(IV)═O→Fe(II) (3b) has a clear new absorption band upon formation of the dinuclear species (502 nm, ε = 1700 M(-1) cm(-1)). This feature is assigned to a metal-to-metal charge transfer (MMCT) transition from V(IV) to Fe(II), which forms a V(V)-O-Fe(I) excited state. This assignment is supported by electrochemical data, electronic absorption profiles, and resonance Raman spectroscopy and represents the first report of visible-light induced MMCT in a heterobimetallic oxido-bridged molecule where the electron originates on a d(1) metal center.

Synthesis and structure of platinum and palladium complexes of dimesitylphosphine

Abstract Treatment of Pd(tmeda)Me2 with dimesitylphosphine (PMes2H, L) gave cis-PdL2Me2 (1). trans-ML2Cl2 (M=Pd (2), Pt (3)) were prepared from a variety of starting materials. The reaction of Pt(cod)Cl2 with L gave cis-PtL2Cl2 (4), which reacted with PPh3 to yield cis-Pt(L)(PPh3)Cl2 (5). cis-PtL2(Me)(Cl) (6) was prepared from L and Pt(cod)(Me)(Cl), while reaction of L with Pt(cod)(Et)(I) gave cis-PtL2(Et)(I) (7), which isomerized to trans-PtL2(Et)(I) (8). The phosphine–borane PMes2H·BH3 (9) was made by reaction of L with BH3·SMe2. Crystal structures of 2·2CH2Cl2, 4·CH2Cl2, 5·2CH2Cl2, 6, and 9 provided 7information on the steric bulk of L (cone angle ca. 149°). Restricted rotation about the PtP and PC bonds in complexes 4–8 was studied by variable temperature NMR spectroscopy.