Perturbation of the Charge Density between Two Bridged Mo₂ Centers: The Remote Substituent Effects

A series of terephthalate-bridged dimolybdenum dimers with various formamidinate ancillary ligands, denoted as [Mo₂(ArNCHNAr)₃]₂­(μ-O₂CC₆H₄CO₂) (Ar = <i>p</i>-XC₆H₄, with X = OCH₃ (<b>1</b>), CH₃ (<b>2</b>), F (<b>3</b>), Cl (<b>4</b>), OCF₃ (<b>5</b>), and CF₃ (<b>6</b>)), has been synthesized and studied in terms of substituent effects on electron delocalization between the two dimetal sites. X-ray structural analyses show that these complexes share the same molecular scaffold with the <i>para</i>-substituents (X) being about 8 Å away from the Mo₂ center. It is found that the remote substituents have the capability to tune the electronic properties of the complexes. For the series <b>1</b> to <b>6</b>, the metal–metal bond distances (<i>d</i>_Mo–Mo) decrease slightly and continuously; the potential separations (Δ<i>E</i>_1/2) for the two successive one-electron oxidations decrease constantly, and the metal to ligand transition energies (λ_max) increase in order. More interestingly, the two types of methine protons, H_∥ on the horizontal and H_⊥ on the vertical ligands with respect to the plane defined by the Mo–Mo bond vectors and bridging ligand, display separate resonant signals δ_∥ and δ_⊥ in the NMR spectra. The displacements of the chemical shifts, Δδ_∥–⊥ = δ_∥ – δ_⊥, are getting smaller as the substituents vary from electron-donating to -withdrawing. These results show that the peripheral groups on the [Mo₂] units function to fine-tune the metal–metal interactions crossing the bridging ligand. The experimental parameters, Δ<i>E</i>_1/2, λ_max, and Δδ_∥–⊥, which are linearly related with the Hammett constants (σ_X) of the X groups, can be used to probe the charge density on the two [Mo₂] units and the electronic delocalization between them

Perturbation of the Charge Density between Two Bridged Mo₂ Centers: The Remote Substituent Effects

A series of terephthalate-bridged dimolybdenum dimers with various formamidinate ancillary ligands, denoted as [Mo₂(ArNCHNAr)₃]₂­(μ-O₂CC₆H₄CO₂) (Ar = <i>p</i>-XC₆H₄, with X = OCH₃ (<b>1</b>), CH₃ (<b>2</b>), F (<b>3</b>), Cl (<b>4</b>), OCF₃ (<b>5</b>), and CF₃ (<b>6</b>)), has been synthesized and studied in terms of substituent effects on electron delocalization between the two dimetal sites. X-ray structural analyses show that these complexes share the same molecular scaffold with the <i>para</i>-substituents (X) being about 8 Å away from the Mo₂ center. It is found that the remote substituents have the capability to tune the electronic properties of the complexes. For the series <b>1</b> to <b>6</b>, the metal–metal bond distances (<i>d</i>_Mo–Mo) decrease slightly and continuously; the potential separations (Δ<i>E</i>_1/2) for the two successive one-electron oxidations decrease constantly, and the metal to ligand transition energies (λ_max) increase in order. More interestingly, the two types of methine protons, H_∥ on the horizontal and H_⊥ on the vertical ligands with respect to the plane defined by the Mo–Mo bond vectors and bridging ligand, display separate resonant signals δ_∥ and δ_⊥ in the NMR spectra. The displacements of the chemical shifts, Δδ_∥–⊥ = δ_∥ – δ_⊥, are getting smaller as the substituents vary from electron-donating to -withdrawing. These results show that the peripheral groups on the [Mo₂] units function to fine-tune the metal–metal interactions crossing the bridging ligand. The experimental parameters, Δ<i>E</i>_1/2, λ_max, and Δδ_∥–⊥, which are linearly related with the Hammett constants (σ_X) of the X groups, can be used to probe the charge density on the two [Mo₂] units and the electronic delocalization between them

Electronic Coupling and Electron Transfer between Two Dimolybdenum Units Spaced by a Biphenylene Group

Three symmetrical dimolybdenum dimers bridged by 4,4′-biphenyldicarboxylate and the partially and fully thiolated derivatives have been synthesized and studied with respect to electronic coupling and intramolecular electron transfer. As generally denoted by [Mo₂]–(ph)₂–[Mo₂], the complexes are differentiated by the [Mo₂] units but have a biphenylene spacer in common, where [Mo₂] = [Mo₂(DAniF)₃(EE′C)] with auxiliary ligands DAniF (<i>N</i>,<i>N</i>′-di­(<i>p</i>-anisyl)­formamidinate) and donor atoms E and E′ (O or S). The radical cations {[Mo₂]–(ph)₂–[Mo₂]}⁺, prepared by one-electron oxidation of the corresponding neutral precursor, exhibit a characteristic intervalence (IV) charge transfer absorbance in the near-IR spectra. The electronic coupling matrix elements (<i>H</i>_ab) calculated from the Mulliken–Hush expression vary in the range of 245–415 cm^–1 depending on the number of sulfur atoms in the [Mo₂] units. These parameters are also calculated by CNS superexchange formalism, in which only the electron-hopping pathway is taken into account because of the lack of ligand to metal charge transfer absorptions in the spectra. The results show remarkable alignment between the two different methods. Thus, the mixed-valence complexes are assigned to weakly coupled Class II in terms of Robin–Day’s classification. Under the Marcus–Hush theoretical framework, the adiabatic electron transfer rate constants (<i>k</i>_et) are optically determined in the range of 10⁹ – 10¹¹ s^–1. The fastest electron transfer is observed in the fully thiolated species. In comparison with the reported (ph)₁ series, a relatively small attenuation factor, ca. β­(<i>H</i>_ab) 0.17, is also estimated for the tetrathiolated system. Therefore, the introduction of sulfur atoms along the charge transfer axis efficiently enhances the electronic coupling and facilitates the electron transfer between the two dimetal centers

Spectroscopic Study of δ Electron Transfer between Two Covalently Bonded Dimolybdenum Units via a Conjugated Bridge: Adequate Complex Models to Test the Existing Theories for Electronic Coupling

Three symmetrical and one unsymmetrical dimolybdenum dimers, namely, [Mo₂(DAniF)₃]₂(E₂CC₆H₄CE₂) (DAniF = <i>N</i>,<i>N</i>′-di­(<i>p</i>-anisyl)­formamidinate and E = O or S), are structurally and electronically closely related. The mixed-valence cation radicals display well-defined metal to ligand (ML), ligand to metal (LM), and metal to metal (MM) charge transfer absorption bands. Successive thiolations of the complexes result in steady increases of the electronic coupling between the two [Mo₂] units. The electronic coupling matrix elements (<i>H</i>_ab) calculated from the Hush model fall in the range of 600–900 cm^–1, which are remarkably consistent with the results from the CNS superexchange formalism. Spectroscopic analyses suggest that the intramolecular electron transfer occurs by electron-hopping and hole-hopping in concert. The rate constants (<i>k</i>_et) are estimated in the range of 10¹¹–10¹² s^–1 for the symmetrical analogues and 10⁷ s^–1 for the unsymmetrical species. The ultrafast electron transfer in such a weakly coupled system (<i>H</i>_ab < 1000 cm^–1) is attributed to the d­(δ)–p­(π) conjugation between the dimetal centers and the bridge

Charge Transfer Properties of Triarylamine Integrated Dimolybdenum Dyads

Three quadruply bonded dimolybdenum complexes equipped with a triarylamine pendant, [(DAniF)₃Mo₂(μ-O₂CC₆H₄N­(C₆H₄CH₃)₂] (DAniF = <i>N,N</i>′-di­(<i>p</i>-anisyl)­formamidinate; [<b>OO–ph–N</b>]), [(DAniF)₃Mo₂(μ-OSCC₆H₄N­(C₆H₄CH₃)₂] ([<b>OS–ph–N</b>]), and [(DAniF)₃Mo₂(μ-S₂CC₆H₄N­(C₆H₄CH₃)₂] ([<b>SS–ph–N</b>]), have been synthesized and characterized by single crystal X-ray diffraction. In electrochemical measurements, the redox couple for the organic amine group becomes irreversible, reflecting the substantially strong electronic interaction between the dimetal center and organic redox site. The potential difference for the two successive redox events, ca. Δ<i>E</i>_1/2(<i>E</i>_1/2(2)­(N/N^<b>•</b>+) – <i>E</i>_1/2(1)­(Mo₂^IV/V)), falls in the range of 0.5–0.8 V as estimated from the differential pulse voltammograms. For the monocation radicals [<b>OO–ph–N</b>]⁺, [<b>OS–ph–N</b>]⁺, and [<b>SS–ph–N</b>]⁺, obtained by chemical oxidation of the neutral precursor, a broad ligand (amine) to metal (Mo₂) charge transfer (LMCT) absorption band is observed in the near-IR region. Interestingly, analogous to the intervalence charge transfer (IVCT) bands for mixed-valence complexes, the LMCT absorption bands, which are solvent dependent, decrease in energy and bandwidth as the electronic coupling between the two redox sites increases in an order of increasing S content in the chelating group. The electronic coupling matrix elements (<i>H</i>_ab) are determined by optical analyses from the generalized Mulliken–Hush (GMH) theory, falling in the range of 400–800 cm^–1 in CH₂Cl₂. These results indicate that in these radical cations the charge is localized. Time-dependent DFT calculations show that the frontier molecular orbitals for these asymmetrical donor–acceptor systems have unbalanced distribution of electron density, and the LMCT bands arise from an electronic transition from the pendant ligand-based to metal-based molecular orbitals, corresponding to donor (N)–acceptor (Mo₂) charge transfer

Electronic Coupling between Two Covalently Bonded Dimolybdenum Units Bridged by a Naphthalene Group

Using 2,6-naphthalenedicarboxylate and its thiolated derivatives as bridging ligands, three Mo₂ dimers of the type [Mo₂(DAniF)₃]­(E₂CC₁₀H₆CE₂)­[Mo₂(DAniF)₃] (DAniF = <i>N</i>,<i>N</i>′-di-<i>p</i>-anisylformamidinate; E = O, S) have been synthesized and characterized by X-ray diffraction. These compounds can be generally formulated as [Mo₂]–naph–[Mo₂], where the complex unit [Mo₂] ([Mo₂(DAniF)₃(μ-E₂C)]) functions as an electron donor (acceptor) and the naphthalene (naph) group is the bridge. The mixed-valence (MV) complexes, generated by one-electron oxidation of the neutral precursors, display weak, very broad intervalence charge-transfer absorption bands in the near-to-mid-IR regions. The electronic coupling matrix elements for the MV complexes, <i>H</i>_ab = 390–570 cm^–1, are calculated from the Mulliken–Hush equation, which fall between those for the phenyl (ph) and biphenyl (biph) analogues reported previously. The three series consisting of three complexes with the same [Mo₂] units exhibit exponential decay of <i>H</i>_ab as the bridge changes from ph to biph via naph, with decay factors of 0.21–0.17 Å^–1. Therefore, it is evidenced that while the extent of the bridge conjugacy varies, the electronic coupling between the two [Mo₂] units is dominated by the Mo₂···Mo₂ separation. The absorption band energies for metal-to-ligand charge transfer are in the middle of those for the ph and biph analogues, which is consistent with variation of the HOMO–LUMO energy gaps for the complex series. These results indicate that the interplay of the bridge length and conjugacy is to affect the enegy for charge transfer crossing the intervening moiety, in accordance with a superechange mechanism

Abnormally Long-Range Diamagnetic Anisotropy Induced by Cyclic d_δ–p_π π Conjugation within a Six-Membered Dimolybdenum/Chalcogen Ring

Incorporating two quadruply bonded dimolybdenum units [Mo₂(DAniF)_³]⁺ (ancillary ligand DAniF = <i>N</i>,<i>N</i>′-di-<i>p</i>-anisylformamidinate) with two hydroselenides (SeH^–) gave rise to [Mo₂(DAniF)₃]₂(μ-SeH)₂ (<b>1</b>). With the molecular scaffold remaining unchanged, aerobic oxidation of <b>1</b>, followed by autodeprotonation, generated [Mo₂(DAniF)₃]₂(μ-Se)₂ (<b>2</b>). The two complexes share a common cyclic six-membered Mo₂/Se core, but compound <b>2</b> is distinct from <b>1</b> by having structural, electronic, and magnetic properties that correspond with aromaticity. Importantly, the aromatic behaviors for this non-carbon system are ascribable to the bonding analogy between the δ component in a Mo–Mo quadruple bond and the π component in a C–C double bond. Cyclic π delocalization via d_δ–p_π conjugation within the central unit, which involves six π electrons with one electron from each of the Mo₂ units and two electrons from each of the bridging atoms, has been confirmed in a previous work on the oxygen- and sulfur-bridged analogues (Fang, W.; et al. <i>Chem.Eur. J.</i> <b>2011</b>, <i>17</i>, 10288). Of the three members in this family, compound <b>2</b> exhibits an enhanced aromaticity because of the selenium bridges. The remote in-plane and out-of-plane <i>methine</i> (ArNC<i>H</i>NAr) protons resonate at chemical shifts (δ) 9.42 and 7.84 ppm, respectively. This NMR displacement, Δδ = 1.58 ppm, is larger than that for the oxygen-bridged (1.30 ppm) and sulfur-bridged (1.49 ppm) derivatives. The abnormally long-range shielding effects and the large diamagnetic anisotropy for this complex system can be rationalized by the induced ring currents circulating the Mo₂/chalcogen core. By employment of the McConnell equation {Δσ = Δχ­[(l – 3 cos 2θ)/3<i>R</i>³<i>N</i>]}, the magnetic anisotropy (Δχ = χ_⊥ – χ_||) is estimated to be −414 ppm cgs, which is dramatically larger than −62.9 ppm cgs for benzene, the paradigm of aromaticity. In addition, it is found that the magnitude of Δχ is linearly related to the radius of the bridging atoms, with the selenium analogue having the largest value. This aromaticity sequence is in agreement with that for the chalcogen-containing aromatic family, e.g., furan < thiophene < selenophene

Optical Determination of Electron Transfer Dynamics and Kinetics for Asymmetrical [Mo₂]–ph–[Mo₂] Systems

We report the study, in terms of electronic coupling (EC) and electron transfer (ET), on three asymmetrical Mo₂ dimers [Mo₂(DAniF)₃]₂[μ-(NH)­OCC₆H₄CO₂] ([<b>NO–ph–OO</b>]) (DAniF = <i>N</i>,<i>N</i>′-di­(<i>p</i>-anisyl)­formamidinate), [Mo₂(DAniF)₃]₂[μ-(NH)₂CC₆H₄CO₂] ([<b>NN–ph–OO</b>]), and [Mo₂(DAniF)₃]₂[μ-(NH)₂CC₆H₄C­(NH)­O] ([<b>NN–ph–NO</b>]), which are closely related to the three symmetrical analogues [<b>OO–ph–OO</b>], [<b>NO–ph–NO</b>], and [<b>NN–ph–NN</b>] reported earlier. The mixed-valence (MV) complexes [<b>NO–ph–OO</b>]⁺, [<b>NN–ph–OO</b>]⁺, and [<b>NN–ph–NO</b>]⁺ exhibit metal to ligand and ligand to metal charge transfer bands, along with an intervalence (IV) transition absorption in the Near-IR region. The free energy change (Δ<i>G</i>°) for ET is determined by comparing the redox potential splitting (Δ<i>E</i>_1/2) and IV transition energy (<i>E</i>_IT) with the data for the symmetrical species. The reorganization energy (λ) is estimated from the Hush model (Δν_1/2 = [16ln(2)<i>λRT</i>]^1/2). Significantly, electrochemical and optical analyses verify <i>E</i>_IT = Δ<i>G</i>° + λ, the core energetic relationship underlying the semiclassical theories. With the electronic coupling parameters calculated from the method suggested by Creutz, Newton and Sutin (<i>H</i>_MM′ = ∼ 500 cm^–1), the adiabatic ET rate constants <i>k</i>_et (<i>f</i>) are determined to be ∼10¹⁰ s^–1 for [<b>NO–ph–OO</b>]⁺ and [<b>NN–ph–NO</b>]⁺, smaller than <i>k</i>_et (<i>r</i>) for the backward reaction and <i>k</i>_et for the symmetrical analogues by 1 order of magnitude, and ∼10⁹ s^–1 for [<b>NN–ph–OO</b>]⁺. This work illustrates that the redox asymmetry in D–B–A systems controls the ET rate and direction

Electronic Coupling between Two Covalently Bonded Dimolybdenum Units Bridged by a Naphthalene Group

Using 2,6-naphthalenedicarboxylate and its thiolated derivatives as bridging ligands, three Mo₂ dimers of the type [Mo₂(DAniF)₃]­(E₂CC₁₀H₆CE₂)­[Mo₂(DAniF)₃] (DAniF = <i>N</i>,<i>N</i>′-di-<i>p</i>-anisylformamidinate; E = O, S) have been synthesized and characterized by X-ray diffraction. These compounds can be generally formulated as [Mo₂]–naph–[Mo₂], where the complex unit [Mo₂] ([Mo₂(DAniF)₃(μ-E₂C)]) functions as an electron donor (acceptor) and the naphthalene (naph) group is the bridge. The mixed-valence (MV) complexes, generated by one-electron oxidation of the neutral precursors, display weak, very broad intervalence charge-transfer absorption bands in the near-to-mid-IR regions. The electronic coupling matrix elements for the MV complexes, <i>H</i>_ab = 390–570 cm^–1, are calculated from the Mulliken–Hush equation, which fall between those for the phenyl (ph) and biphenyl (biph) analogues reported previously. The three series consisting of three complexes with the same [Mo₂] units exhibit exponential decay of <i>H</i>_ab as the bridge changes from ph to biph via naph, with decay factors of 0.21–0.17 Å^–1. Therefore, it is evidenced that while the extent of the bridge conjugacy varies, the electronic coupling between the two [Mo₂] units is dominated by the Mo₂···Mo₂ separation. The absorption band energies for metal-to-ligand charge transfer are in the middle of those for the ph and biph analogues, which is consistent with variation of the HOMO–LUMO energy gaps for the complex series. These results indicate that the interplay of the bridge length and conjugacy is to affect the enegy for charge transfer crossing the intervening moiety, in accordance with a superechange mechanism

Electronic Coupling in [Mo₂]–Bridge–[Mo₂] Systems with Twisted Bridges

In order to evaluate the impact of bridge conformation on electronic coupling in donor–bridge–acceptor triad systems, two Mo₂ dimers, [Mo₂(DAniF)₃]₂[μ-1,4-{C­(O)­NH}₂-Naph] (<b>1</b>, DAniF = <i>N</i>,<i>N</i>′-di­(<i>p</i>-anisyl)­formamidinate and Naph = naphthalenyl) and [Mo₂(DAniF)₃]₂[μ-1,4-(CS₂)₂-2,5-Me₂C₆H₂] (<b>2</b>), have been synthesized and structurally characterized. These two compounds feature a large dihedral angle (>60°) between the central aromatic ring and the plane defined by the Mo–Mo bond vectors, which is distinct from the previously reported phenylene bridged analogues [Mo₂(DAniF)₃]₂[μ-1,4-{C­(O)­NH}₂-ph] (<b>I</b>) and [Mo₂(DAniF)₃]₂[μ-1,4-(CS₂)₂-C₆H₄] (<b>II</b>), respectively. Unusual optical behaviors are observed for the mixed-valence (MV) species (<b>1</b>⁺ and <b>2</b>⁺), generated by single-electron oxidation. While <b>2</b>⁺ exhibits a weak intervalence charge transfer (IVCT) absorption band in the near-IR region, the IVCT band is absent in the spectrum of <b>1</b>⁺, which is quite different from what observed for <b>I</b>⁺ and <b>II</b>⁺. Optical analyses, based on superexchange formalism and Hush model, indicate that, in terms of Robin–Day classification, mixed-valence species <b>1</b>⁺ belongs to the electronically uncoupled Class I and complex <b>2</b>⁺, with <i>H</i>_ab = 220 cm^–1, is assigned to the weakly coupled Class II. Together with <b>I</b>⁺ and <b>II</b>⁺, the four MV complexes complete a transition from Class I to Class II–III borderline as a result of manipulating the geometric topology of the bridge. Given the structural and electronic features for the molecular systems, the impacts of electrostatic interaction (through-space) and electron resonance (through-bond) on electronic coupling are discussed