Photoelectron Spectroscopy of the Bis(dithiolene) Anions [M(mnt)2]n- (M = Fe − Zn; n = 1, 2): Changes in Electronic Structure with Variation of Metal Center and with Oxidation

Characterizing Excited States of a Copper-Based Molecular Qubit Candidate with Correlated Electronic Structure Methods

Molecular spins have a variety of potential advantages as qubits for quantum computation, such as tunability and well-understood design pathways through organometallic synthesis. Organometallic and heavy-metal-based molecular spin qubits can also exhibit rich electronic structures due to ligand field interactions and electron correlation. These features make consistent and reliable modeling of these species a considerable challenge for contemporary electronic structure techniques. Here, we elucidate the electronic structure of a Cu(II) complex analogous to a recently proposed room-temperature molecular spin qubit. Using active space methods to describe the electron correlation, we show the nuanced interaction between the metal d orbitals and ligand σ and π orbitals makes these systems challenging to model, both in terms of the delocalized spin density and the excited state ordering. We show that predicting the correct spin delocalization requires special consideration of the Cu d orbitals and that the excited state spectrum for the Cu(III) complex also requires the explicit inclusion of the π orbitals in the active space. These interactions are rather common in molecular spin qubit motifs and may play an important role in spin-decoherence processes. Our results may lend insight into future studies of the orbital interactions and electron delocalization of similar complexes.

A mixed valence zinc dithiolene system with spectator metal and reactor ligands

Neutral complexes of zinc with N,N'-diisopropylpiperazine-2,3-dithione ( ⁱ Pr₂Dt⁰) and N,N'-dimethylpiperazine-2,3-dithione (Me₂Dt⁰) with chloride or maleonitriledithiolate (mnt^2-) as coligands have been synthesized and characterized. The molecular structures of these zinc complexes have been determined using single crystal X-ray diffractometry. Complexes recrystallize in monoclinic P type systems with zinc adopting a distorted tetrahedral geometry. Two zinc complexes with mixed-valent dithiolene ligands exhibit ligand-to-ligand charge transfer bands. Optimized geometries, molecular vibrations and electronic structures of charge-transfer complexes were calculated using density functional theory (B3LYP/6-311G+(d,p) level). Redox orbitals are shown to be almost exclusively ligand in nature, with a HOMO based heavily on the electron-rich maleonitriledithiolate ligand, and a LUMO comprised mostly of the electron-deficient dithione ligand. Charge transfer is thus believed to proceed from dithiolate HOMO to dithione LUMO, showing ligand-to-ligand redox interplay across a d¹⁰ metal.

Cobalt-dithiolene complexes for the photocatalytic and electrocatalytic reduction of protons in aqueous solutions

Artificial photosynthesis (AP) is a promising method of converting solar energy into fuel (H(2)). Harnessing solar energy to generate H(2) from H(+) is a crucial process in systems for artificial photosynthesis. Widespread application of a device for AP would rely on the use of platinum-free catalysts due to the scarcity of noble metals. Here we report a series of cobalt dithiolene complexes that are exceptionally active for the catalytic reduction of protons in aqueous solvent mixtures. All catalysts perform visible-light-driven reduction of protons from water when paired with Ru(bpy)(3)(2+) as the photosensitizer and ascorbic acid as the sacrificial donor. Photocatalysts with electron withdrawing groups exhibit the highest activity with turnovers up to 9,000 with respect to catalyst. The same complexes are also active electrocatalysts in 11 acetonitrile/water. The electrocatalytic mechanism is proposed to be ECEC, where the Co dithiolene catalysts undergo rapid protonation once they are reduced to CoL(2)(2-). Subsequent reduction and reaction with H(+) lead to H(2) formation. Cobalt dithiolene complexes thus represent a new group of active catalysts for the reduction of protons.

DFT studies on some properties of maleonitriledithiolate complexes [M(mnt)2]2- (M=Ni, Pd, Pt and Zn, Cd, Hg)

The structures and some molecular properties of the complexes M(mnt)(2)(2-) (M=Ni, Pd, Pt and Zn, Cd, Hg; mnt(2-)=deprotonated maleonitriledithiolate) have been studied by using density functional theory (DFT) B3LYP/LanL2DZ level of theory. Computed binding energies show that the sequences of binding strengths are Ni<Pd<Pt and Cd<Hg<Zn. The natural bonding orbitals analyses show that Ni, Pd and Pt gain 1.40e, 1.62e and 1.72e, respectively to their ns, [Formula: see text] and [Formula: see text] orbitals from both ligand mnt(2-) and (n-1)d(yz) of metal ions, while Zn, Cd and Hg complexes gain electrons to their ns orbitals. The absorption spectra of these complexes were obtained by using time-dependent density functional theory associated with polarized continuum model. Comparison of the absorption spectra in acetonitrile solution with those in gas phase show that the solvatochromic effect made the lowest energy absorption red shift by 31, 34 and 44 nm for d(8) metal complexes Ni(mnt)(2)(2-), Pd(mnt)(2)(2-) and Pt(mnt)(2)(2-), respectively, while blue shift by 28, 44, 25 nm for d(10) metal complexes Zn(mnt)(2)(2-), Cd(mnt)(2)(2-) and Hg(mnt)(2)(2-), respectively. The calculated results reproduced the experimental data with the deviations less than 5% for Ni-S stretching vibrational frequencies and less than 3% for other vibrational modes.

Development of a low-temperature photoelectron spectroscopy instrument using an electrospray ion source and a cryogenically controlled ion trap

The ability to control ion temperatures is critical for gas phase spectroscopy and has been a challenge in chemical physics. A low-temperature photoelectron spectroscopy instrument has been developed for the investigation of complex anions in the gas phase, including multiply charged anions, solvated species, and biological molecules. The new apparatus consists of an electrospray ionization source, a three dimensional (3D) Paul trap for ion accumulation and cooling, a time-of-flight mass spectrometer, and a magnetic-bottle photoelectron analyzer. A key feature of the new instrument is the capability to cool and tune ion temperatures from 10 to 350 K in the 3D Paul trap, which is attached to the cold head of a closed cycle helium refrigerator. Ion cooling is accomplished in the Paul trap via collisions with a background gas and has been demonstrated by observation of complete elimination of vibrational hot bands in photoelectron spectra of various anions ranging from small molecules to complex species. Further evidence of ion cooling is shown by the observation of H2-physisorbed anions at low temperatures. Cold anions result in better resolved photoelectron spectra due to the elimination of vibrational hot bands and yield more accurate energetic and spectroscopic information. Temperature-dependent studies are made possible for weakly bonded molecular and solvated clusters, allowing thermodynamic information to be obtained.

Electronic Structure of Nitric Oxide Adducts of Bis(diaryl-1,2-dithiolene)iron Compounds: Four-Membered Electron-Transfer Series [Fe(NO)(L)2]z (z = 1+, 0, 1−, 2−)

Four members of the electron-transfer series [Fe(NO)(S(2)C(2)R(2))2]z (z = 1+, 0, 1-, 2-) have been isolated as solid materials (R = p-tolyl): [1a](BF4), [1a]0, [Co(Cp)2][1a], and [Co(Cp)2]2[1a]. In addition, complexes [2a]0 (R = 4,4-diphenyl), [3a]0 (R = p-methoxyphenyl), [Et(4)N][4a] (R = phenyl), and [PPh(4)][5a] (R = -CN) have been synthesized and the members of each of their electron-transfer series electrochemically generated in CH(2)Cl(2) solution. All species have been characterized electro- and magnetochemically. Their electronic, Mössbauer, and electron paramagnetic resonance spectra as well as their infrared spectra have been recorded in order to elucidate the electronic structure of each member of the electron-transfer series. It is shown that the monocationic, neutral, and monoanionic species possess an {FeNO}6 (S = 0) moiety where the redox chemistry is sulfur ligand-based, (L)2-(L*)1-: [Fe(NO)(L*)2]+ (S = 0), [Fe(NO)(L*)(L)]0 <--> [Fe(NO)(L)(L*)]0 (S = 1/2), [Fe(NO)(L)2]- (S = 0). Further one-electron reduction generates a dianion with an {FeNO}7 (S = 1/2) unit and two fully reduced, diamagnetic dianions L2-: [Fe(NO)(L)2]2- (S = 1/2).