Cyclic polyamidato dianions as bridges between Mo(2)(4+) units: synthesis,crystal structures,electrochemistry, absorption spectra,and electronic structures

Compounds in which quadruply bonded Mo(2)(4+) units, Mo(2)(DAniF)(3) (DAniF = N,N'-di-p-anisylformamidinate), are linked by cyclic diamidate anions have been synthesized and characterized by X-ray crystallography and spectroscopic methods. As identified by the diamidate linker, these compounds are 4,6-dioxypyrimidinate (2), 2,3-dioxypyrazinate (3), 2,3-dioxyquinoxalinate (4), 2,3-dioxy-5,6-dicyanopyrazinate (5), and cyanurate (6). With uracilate, a dinuclear unlinked 1:1 adduct is formed, Mo(2)(DAniF)(3)(uracilate) (1). The cyclic voltammograms of 3-5 reveal significantly larger DeltaE(1/2) values (258 mV-308 mV) than that of the oxalate linked analogue (212 mV), which is indicative of greater charge delocalization in the mixed valent Mo(2)(4+)/Mo(2)(5+) species and hence greater communication between the two Mo(2) units. DeltaE(1/2) for 2 is substantially lower than those for 3-5. This difference is attributed to the meta disposition of the two amidate groups in 4,6-dioxypyrimidinate as compared to their ortho arrangement in the pyrazinate-type linkers. The absorption spectra of the linked compounds 3-5 are more complex than those of the analogous polyunsaturated dicarboxylate linked compounds and reveal at least two significant absorption bands within the region 420-550 nm. Compound 2 also has two bands but with significantly lower intensity. Time dependent DFT calculations upon 2 and 3 indicate rather different electronic structures for these two structural isomers. The two bands for 3 have delta --> pi character, and the pi type orbitals have substantial contributions from the Mo(2) units as well as from the diamidate linker. The excitations observed in 2 are mainly metal based. The differences between the electronic spectra of 2 and 3 are consistent with the electrochemistry in underscoring the profound physical effect of changing the symmetry of the diamidate linker.     

Uniquely strong electronic communication between [Mo2] units linked by dioxolene dianions

Unprecedented strong electronic communication has been found in dimolybdenum pairs containing quadruply bonded Mo2(DAniF)3(+) (DAniF = N,N'-di-p-anisylformamidinate) units linked by dioxolene (C6X2O4(2-)) anions. The neutral compounds [Mo2(DAniF)3]2(C6X2O4) (1, X = H; 2, X = Cl; 3, X = NO2) and the singly oxidized products {[Mo2(DAniF)3]2(C6X2O4)Mo2}PF6 (4, X = H; 5, X = Cl) have been synthesized and characterized by X-ray crystallography and spectroscopic methods. Unusually short Mo-O distances (approximately 2.05 A for 1 and 2, and approximately 2.01 A for 4 and 5) are implicated in the remarkably strong interaction between [Mo2] units via the linkers. This leads to an extensive charge delocalization in the mixed-valence species, which is mediated by the dioxolene linker, as revealed by the large deltaE(1/2) values (763, 795, and 816 mV for 1, 2, and 3, respectively). Additional evidence for the strong electronic coupling is provided by UV-vis, NIR, and EPR spectroscopies and DFT calculations.     

Polyunsaturated dicarboxylate tethers connecting dimolybdenum redox and chromophoric centers: absorption spectra and electronic structures

The absorption spectra of a series of compounds of the type [Mo(2)(DAniF)(3)](O(2)CXCO(2))[Mo(2)(DAniF)(3)] (DAniF = N,N '-di-p-anisylformamidinate) have been measured and revealed a strong dependence of the electronic transitions and, therefore, the colors upon the chemical nature of the dicarboxylate linker. The more intense colors and lower energy absorptions are observed with those compounds having unsaturated dicarboxylate linkers. Static and time-dependent DFT calculations were undertaken to identify the electronic excitations responsible for the observed colors. For those compounds with chemically unsaturated and fully conjugated dicarboxylate linkers (oxalate, 6; fumarate, 8; acetylene dicarboxylate, 9; cis,cis-muconate, 11; trans,trans-muconate, 12; tamuate, 13; texate, 14; terephthalate, 15), the lowest energy absorptions are Mo(2)(4+) delta --> dicarboxylate pi metal-to-ligand charge transfer transitions. Those compounds with chemically saturated linkers (succinate, 20) have delta --> delta transitions as their lowest energy absorptions with essentially independent and noninteracting Mo(2)(4+) chromophores.     

Enhancement in electronic communication upon replacement of Mo-O by Mo-S bonds in tetranuclear clusters of the type [Mo2]2(mu-E-E)2 (E = O or S)

A tetranuclear cluster containing two quadruply bonded cis-Mo2(DAniF)2 units (DAniF = N,N'-di-p-anisylformamidinate) linked by four hydroxide groups (1) was obtained by hydrolysis of [Mo2(cis-DAniF)2](mu-OCH3)4. Analogous compounds linked by two bidentate bridges (o-O2C6H4 for 2, o-O2C10H6 for 3, and o-S2C6H4 for 4) were synthesized by direct assembly of the corner species precursor [Mo2(cis-DAniF)2(NCCH3)4](BF4)2 and the respective protonated ligands. All four compounds were characterized by X-ray crystallography. Cyclic voltammograms of the O-linked compound 2 and the S analogue 4 show two reversible one-electron-oxidation processes with potential separations (DeltaE(1/2)) of 474 and 776 mV, respectively. The large increase of about 300 mV in DeltaE(1/2) for the S analogue relative to that of the O compound is consistent with a large increase in electronic communication. This enhancement occurs despite the increase of ca. 0.45 A in nonbonding separation between the midpoints of the Mo2 units, which changes from 3.266 A in 2 to 3.72 A in 4, and the increase of ca. 0.4 A in M-E distances as E changes from O to S. Density functional theory calculations show that the increase in electronic communication between the metal centers in 4 is due to a superexchange pathway involving d and p orbitals in the linker E atoms that is less important in 2.     

Strong electronic interaction between two dimolybdenum units linked by a tetraazatetracene

The large rigid dianion fluoflavinate, C(14)H(8)N(4)(2)(-), consisting of four fused and planar six-membered rings with four nitrogen donor atoms, has been used to link two metal-to-metal bonded and redox-active Mo(2)(n)()(+) units which are each locally bridged by three additional groups, collectively denoted [Mo(2)]. In 1, the [Mo(2)] units are Mo(2)(DAniF)(3) (DAniF = N,N'-di-p-anisylformamidinate), and in 5, they are trans-Mo(2)(DAniF)(2)(O(2)CCH(3)) groups. These [Mo(2)](fluoflavinate)[Mo(2)] compounds show three reversible one-electron oxidation steps, one more than all other [Mo(2)](linker)[Mo(2)] species known to date. The first two redox processes are metal-based, and the third one has been assigned to a ligand oxidation by comparison to that of paddlewheel compound 4 which contains only one dimolybdenum unit with a monoanionic fluoflavinate ligand. Chemical oxidations of 1 produce the singly- and doubly-oxidized species 2 and 3, respectively. All compounds have been characterized by X-ray crystallography and, as appropriate, by various techniques such as NMR, EPR, near-IR, and UV-vis. The fluoflavinate ligand strongly mediates electronic communication between the dimetal units, and the mixed valence species 2 can be described as electronically delocalized. Calculations at the DFT level using a variety of functionals support such an assignment and indicate that a strong transition in the NIR for the singly oxidized species can be assigned to the HOMO-1 to SOMO transition.     

Quadridentate bridging EO(4)(2-) (E = S, Mo, W) ligands and their role as electronic bridges

Three compounds containing two quadruply bonded Mo(2)(DAniF)(3) (DAniF = N,N'-di-p-anisylformamidinate) units linked by tetrahedral EO(4)(2-) anions (E = S, Mo, W) have been prepared and characterized by crystallography and NMR. The linkers in these [Mo(2)(DAniF)(3)](2)(mu-EO(4)) compounds hold the Mo(2) units in an approximately perpendicular orientation and mediate strong electrochemical communication between them. Each of the three compounds shows two quasireversible (mu-SO(4)) or fully reversible (mu-MoO(4), mu-WO(4)) features in its cyclic voltammogram corresponding to successive oxidation of each of its Mo(2) units. The DeltaE(1/2) values are the largest thus far measured for Mo(2)-X-Mo(2) bridged complexes and may be sufficiently large to permit isolation of the singly oxidized species.     

Connecting pairs of dimetal units to form molecular loops

Four compounds consisting of molecular loops formed from two quadruply bonded Mo2(DAniF)2 (DAniF = N,N'-di-p-anisylformamidinate) units linked by two dicarboxylate anions have been prepared in high purity and essentially quantitative yields. These compounds have been characterized by crystallography and NMR spectroscopy and display electrochemical behavior dependent on the nature of the dicarboxylate anion. However, the electronic communication between the two Mo2(4+) units is not strong. As denoted by the dicarboxylate linkers, the compounds are malonate, 1, 1,4-phenylendiacetate, 2, homophthalate, 3, and trans-cyclopentane-1,2-dicarboxylate, 4.     

The first designed syntheses of bis-dimetal molecules in which the bridges are diamidate ligands

The first deliberate syntheses of molecules in which pairs of quadruply bonded Mo2 units are bridged by N,N'-diarylterephthaloyldiamidate (aryl = Ph, m-CF3Ph) ligands are described. The addition of neutral N,N'-diarylterephthaloyldiamide to 2 equiv of [Mo2(DAniF)3(MeCN)2]+ (DAniF = N,N'-di-p-anisylformamidinate) followed by the introduction of excess H3CO- in MeCN results in the formation of (DAniF)3Mo2[(C6H5)NC(O)C6H4(O)CN(C6H5)]Mo2(DAniF)3 (1) and (DAniF)3Mo2[[(m-CF3)C6H5]NC(O)C6H4(O)CN[(m-CF3)C6H5]]Mo2(DAniF)3 (2). The DeltaE1/2 for the oxidation of each Mo2 unit is greater for these terephthaloyldiamidate-bridged molecules (approximately 100 mV) than for the analogous terephthalate-bridged compound (approximately 60 mV). Variation in the nature of the substituents on the diamidate nitrogen atoms offers a means to fine-tune the oxidation potentials of the Mo2 units.     

Very large difference in electronic communication of dimetal species with heterobiphenylene and heteroanthracene units

Two neutral compounds having [Mo2] units linked by squarate dianions, [Mo2(DAniF)3]2(mu4-C4O4) (DAniF = N,N'-di(p-anisyl)formamidinate) (1) and [Mo2(DmCF3F)3]2(mu4-C4O4) (DmCF3F = N,N'-di(m-trifluoromethylphenyl)formamidinate) (2), as well as the singly oxidized compound {[Mo2(DmCF3F)3]2(mu4-C4O4)}SbF6 (3) and the doubly oxidized species {[Mo2(DAniF)3]2(mu4-C4O4)}(TFPB)2 (TFPB = [B(3,5-(CF3)2C6H3)4]-) (4), were synthesized and structurally characterized. Electrochemical measurements of the two neutral species showed only very weak electronic interactions between the two dimolybdenum units linked by the squarate anion in contrast to what was observed in dioxolene analogues having C6 instead of C4 rings (J. Am. Chem. Soc. 2006, 128, 3281) which led to differences in comproportionation constants of over 108. In the squarate species, the pi electrons are localized within the carbonyl and dimetal units in the heterometallic six-membered Mo2O2C2 rings to minimize the antiaromaticity in the central C4 square. The oxidized species 3 and 4 are electronically localized in the time scale of the physical measurements. Calculations at the DFT level suggested that the energy mismatch of the frontier orbitals of the linker and dimetal units contributes to the weak communication between the Mo2 units. For the doubly oxidized complex 4, DFT calculations gave a J value of -130 cm(-1) which suggests that the two unpaired electrons are only weakly antiferromagnetically coupled, as shown by magnetic studies (J = -121 cm(-1)).     

Polyunsaturated dicarboxylate tethers connecting dimolybdenum redox and chromophoric centers: syntheses,structures, and electrochemistry

Compounds with two quadruply bonded Mo(2)(4+) units, Mo(2)(DAniF)(3) (DAniF = N,N '-di-p-anisylformamidinate), linked by unsaturated dicarboxylate dianions of various lengths have been prepared and their spectroscopic and electrochemical properties studied. As identified by the dicarboxylate linkers, these compounds are maleate (7), allene-1,3-dicarboxylate (10), cis,cis-muconate (11), trans,trans-muconate (12), octa-2,4,6-trans,trans,trans-hexatriene-1,8-dioate (tamuate, 13), and deca-2,4,6,8-trans,trans,trans,trans-octatetraene-1,10-dioate (texate, 14). The latter three molecules complete the five-membered (all trans) series [Mo(2)(DAniF)(3)](O(2)C(CH=CH)(n)CO(2))[Mo(2)(DAniF)(3)] (n = 0-4). Several unsymmetrical paddlewheel compounds of the type Mo(2)(DAniF)(3)(O(2)CX) (X = C triple bond CH (3), CH=CH(2) (4), (E)-CH=CH-CH=CH(2) (5)) have also been prepared for comparison to the molecules in which there are linked Mo(2) units. The precursors [Mo(2)(DAniF)(3)(MeCN)(2)](BPh(4)), [1]BPh(4), and Mo(2)(DAniF)(3)Cl(MeCN) (2) have also been isolated and characterized. The structures of all new molecules have been established by X-ray crystallography, including the methyl esters of various carboxylates used as ligands. All of the linked molecules have been examined by cyclic and differential pulse voltammetry, and deltaE(1/2) values, the separation between successive Mo(2)(4+)/Mo(2)(5+) oxidations, have been determined. Those compounds with highly unsaturated, fully conjugated linkers demonstrate electrochemical communication from end-to-end that is more persistent over distance than is accounted for by an electrostatic interaction alone, implying that the pi system of these dicarboxylate linkers is mediating communication. In the series [Mo(2)(DAniF)(3)](O(2)C(CH=CH)(n)CO(2))[Mo(2)(DAniF)(3)] (n = 0-4), the first oxidation potential shifts progressively to less positive values due to an increasing contribution of the polyolefinic alpha,omega-dicarboxylate to the molecular orbital undergoing oxidation. This first oxidation potential approaches a limiting value of 63 mV (vs Ag/AgCl) as n becomes infinitely long. Compound 11 can be photoisomerized to 12 in a process that is affected by the presence of the Mo(2)(4+) units, as the analogous rearrangement of dimethyl cis,cis-muconate is faster.     

A rare dimer of dimers having four hydride linkers joining two quadruply bonded dimolybdenum units

Hydride anions, H-, have been found to cause the assembly of dimolybdenum units [Mo2(cis-DAniF)2]2+, DAniF = N,N'-di(p-anisyl)formamidinate, forming a tetranuclear complex [Mo2(cis-DAniF)2]2(mu-H)4 (1) with an Mo4H4 core that may be described as an elongated tetrahedron in which the H atoms are along the four long edges of such tetrahedron and the Mo2 units are along the short edges. The two quadruply bonded dimolybdenum units, separated by only 2.718 A, are essentially orthogonal. This gives the shortest [Mo2]...[Mo2] distance known for complexes with multiple dimolybdenum units. DFT calculations indicate that the energy of a cuboidal isomer is only 3.8 kcal/mol above that of 1, but such an isomer has not been observed.     

Strong electronic communication by direct metal-metal interaction in molecules with halide-bridged dimolybdenum pairs

Reactions of [cis-Mo2(DAniF)2(NCCH3)4](BF4)2, DAniF = N,N'-di-p-anisylformamidinate, with an excess of anhydrous Bu(n)4NX (X = Cl, Br, I), produced the halide-bridged tetranuclear clusters, [cis-Mo2(DAniF)2]2(mu-X)4, X = Cl (1), Br (2), and I (3). All three compounds show two reversible one-electron oxidation processes with potential separations (DeltaE(1/2)) between the two oxidation processes of 540, 499, and 440 mV, respectively. These DeltaE(1/2) values show that the strength of the electronic coupling between the dimetal units decreases as the Mo2...Mo2 distance increases from 1 to 2, and then to 3. The structures, EPR spectra, and near-IR (NIR) spectra of the corresponding mixed-valence species (1-PF6, 2-PF6, and 3-PF6) indicate that the clusters are electronically delocalized. Calculations at the DFT level indicate that the strong electronic communication is principally due to a direct overlap between the delta orbitals from the adjacent dimetal units.     

Transition from a nonbonding to a bonding interaction in a tetranuclear [Mo2]2(mu-OR)4 cluster

Tetranuclear Mo4 clusters with two quadruply bonded Mo2(4+) units, [Mo2(cis-DAniF)2] (DAniF = N,N'-di-p-anisylformamidinate), linked by alkoxides (OCH3 for 1 and OC2H5 for 4) have been prepared. The nonbonding separation between the midpoints of the quadruply bonded units, ca. 3.24 A, is the shortest among compounds having two linked Mo2(4+) units. Electrochemical measurements show two redox waves for each compound with large DeltaE(1/2) values (554 and 587 mV for 1 and 4, respectively) that correspond to K(C) values on the order of 10(9). The large electronic communication is attributed to the short separation between dinuclear units that favor direct delta-to-delta orbital interactions between the two dimetal centers. Compound 1 was chemically oxidized using stoichiometric amounts of ferrocenium salts to a one-electron oxidation product 2 (in which the counteranion is PF6-) and a two-electron oxidation product 3 (which contains two BF4- anions). Upon oxidation there are significant decreases in the distance between the two [Mo2] units to 3.100 A and then to 2.945 A. The mixed-valence species 2 shows two broad absorption bands at 5900 and 7900 cm(-1) in the NIR region which are assigned to the HOMO-1 --> SOMO and HOMO-2 --> SOMO transitions. Compound 3 is fluxional in solution, as shown by variable-temperature 1H NMR spectra. The sharp signals in the NMR spectrum at -50 degrees C and the lack of an EPR signal suggest that this species is diamagnetic and that a four-center, two-electron bond is formed in the cyclometallic Mo4 cluster. To a first-order approximation, an average bond order of 0.25 is assigned to the bonding interaction between the two Mo atoms along the long edges of the rectangle defined by the four Mo atoms.     

Modifying electronic communication in dimolybdenum units by linkage isomers of bridged oxamidate dianions

Reactions of Mo(2)(O(2)CCH(3))(DAniF)(3), DAniF = N,N'-di-p-anisylformamidinate, with oxamidate dianions [ArNC(O)C(O)NAr](2-), Ar = C(6)H(5) and p-anisyl, give pairs of isomeric compounds where the [Mo(2)] units are bridged by the oxamidate anions. For the alpha isomers, the C-C unit of the dianion is nearly perpendicular to the Mo-Mo bonds, and these are essentially perpendicular to each other. For the beta isomers, the corresponding C-C unit and the Mo-Mo bonds are essentially parallel to each other. Each type of isomer is stable in solution. The electronic communication as measured by the DeltaE(1/2) for the oxidation of each of the Mo(2) units is significantly better for the beta isomers. This is supported also by the appearance of what is conventionally called an intervalence charge-transfer band in the near infrared region upon oxidation of the beta isomers but not the alpha isomers. Molecular mechanics and DFT calculations help explain the relative conformations in the alpha isomers and the relative energy differences between the alpha and beta isomers.     

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 [Mo(2)(DAniF)((3))](+) (ancillary ligand DAniF = N,N'-di-p-anisylformamidinate) with two hydroselenides (SeH(-)) gave rise to [Mo(2)(DAniF)(3)](2)(μ-SeH)(2) (1). With the molecular scaffold remaining unchanged, aerobic oxidation of 1, followed by autodeprotonation, generated [Mo(2)(DAniF)(3)](2)(μ-Se)(2) (2). The two complexes share a common cyclic six-membered Mo(2)/Se core, but compound 2 is distinct from 1 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 Mo(2) 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. Chem.-Eur. J.2011, 17, 10288). Of the three members in this family, compound 2 exhibits an enhanced aromaticity because of the selenium bridges. The remote in-plane and out-of-plane methine (ArNCHNAr) 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 Mo(2)/chalcogen core. By employment of the McConnell equation {Δσ = Δχ[(l - 3 cos 2θ)/3R(3)N]}, 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.     

Tuning the Electronic Coupling and Electron Transfer in Mo2 Donor–Acceptor Systems by Variation of the Bridge Conformation

Assembling two quadruply bonded dimolybdenum units [Mo₂(DAniF)₃]⁺ (DAniF=N,N′‐di(p‐anisyl)formamidinate) with 1,4‐naphthalenedicarboxylate and its thiolated derivatives produced three complexes [{Mo₂(DAniF)₃}₂(μ‐1,4‐O₂CC₁₀H₆CO₂)], [{Mo₂(DAniF)₃}₂(μ‐1,4‐OSCC₁₀H₆COS)], and [{Mo₂(DAniF)₃}₂(μ‐1,4‐S₂CC₁₀H₆CS₂)]. In the X‐ray structures, the naphthalene bridge deviates from the plane defined by the two Mo?Mo bond vectors with the torsion angle increasing as the chelating atoms of the bridging ligand vary from O to S. The mixed‐valent species exhibit intervalence transition absorption bands with high energy and very low intensity. In comparison with the data for the phenylene analogues, the optically determined electronic coupling matrix elements (H_ab=258–345 cm^?1) are lowered by a factor of two or more, and the electron‐transfer rate constants (k_et≈10¹¹ s^?1) are reduced by about one order of magnitude. These results show that, when the electron‐transporting ability of the bridge and electron‐donating (electron‐accepting) ability of the donor (acceptor) are both variable, the former plays a dominant role in controlling the intramolecular electron transfer. DFT calculations revealed that increasing the torsion angle enlarges the HOMO–LUMO energy gap by elevating the (bridging) ligand‐based LUMO energy. Therefore, our experimental results and theoretical analyses verify the superexchange mechanism for electronic coupling and electron transfer.     

Distinguishing the Strength of Electronic Coupling for Mo2‐Containing Mixed‐Valence Compounds within the Class III Regime

Two, symmetrical, mixed‐valence (MV), complex cations—{[Mo₂(DAniF)₃]₂(μ‐oxamidate)}⁺ ( 1 ⁺) and {Mo₂(DAniF)₃]₂(μ‐dithiooxamidate)}⁺ ( 2 ⁺; DAniF=N,N′‐di(p‐anisyl)formamidinate)—are significantly differentiated in terms of electronic coupling between the two [Mo₂] units. For 1 ⁺ the intervalence (IV) charge‐transfer band in the near‐IR spectrum is truncated in half on the low‐energy side as predicted for MV compounds at the Class II–III limit (2H_ab/λ=1; for which H_ab=electronic coupling matrix element and λ=reorganization energy). In contrast, the very strongly coupled analogue 2 ⁺, as indicated by 2H_ab/λ=3.5 (> >1), exhibits a higher energy and more symmetrical IV band. As rare examples, this pair of MV species shows distinct optical behaviors for MV systems crossing the Class III region. Optical analysis and DFT calculations are carried out to elucidate the transformation from vibronic to electronic vertical transition.     

Hydrogen-induced Sulfur Vacancies on the MoS2 Basal Plane Studied by Ambient Pressure XPS and DFT Calculations

Sulfur vacancy on an MoS₂ basal plane plays a crucial role in device performance and catalytic activity; thus, an understanding of the electronic states of sulfur vacancies is still an important issue. We investigate the electronic states on an MoS₂ basal plane by ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) and density functional theory calculations while heating the system in hydrogen. The AP-XPS results show a decrease in the intensity ratio of S 2p to Mo 3d, indicating that sulfur vacancies are formed. Furthermore, low-energy components are observed in Mo 3d and S 2p spectra. To understand the changes in the electronic states induced by sulfur vacancy formation at the atomic scale, we calculate the core-level binding energies for the model vacancy surfaces. The calculated shifts for Mo 3d and S 2p with the formation of sulfur vacancy are consistent with the experimentally observed binding energy shifts. Mulliken charge analysis indicates that this is caused by an increase in the electronic density associated with the Mo and S atoms around the sulfur vacancy as compared to the pristine surface. The present investigation provides a guideline for sulfur vacancy engineering.     

Molecular pairs and a propeller containing quadruply bonded dimolybdenum units linked by polyamidate ligands

Two molecular pairs [Mo2(DAniF)3]2[N,N'-diethylterephthalamidate] (1) and [Mo2(DAniF)3]2{1,3-C6H4[C(O)NP]} (2) where DAniF = N,N'-di-p-anisylformamidinate) and the propeller ([Mo2(DAniF)3]3{1,3,5-C6H3[C(O)NPh]3} (3)) have been prepared in good yield and high purity by directly combining Mo2(DAniF)3(O2CCH3) with the corresponding polyamidates. Electrochemical measurements of these complexes show unresolved redox waves, which indicate that the dimetal centers are only electronically weakly coupled. Compound 1 was chemically oxidized by ferrocenium tetrafluoroborate to the two-electron oxidation product 4, where one electron was removed from each of the [Mo2] units. The hyperfine coupling in the EPR spectrum (A = 22 x 10-4 cm-1 ) suggests that 4 is an electron-trapped species with one electron residing on each of the two dimolybdenum units, as suggested also by electrochemical measurements.     

An unusual trigonal D3 assembly composed of molybdate anions and multiply bonded dimolybdenum units

Oxidation of the molybdate-linked pair having two quadruply bonded Mo(2)(4+) units, [Mo(2)(DAniF)(3)](2)(micro(2)-MoO(4)), (DAniF = N,N'-di-(p-anisyl)formamidinate) leads to the formation of a species consisting of three oxidized Mo(2)(5+) units connected by two micro(3)-MoO(4)(2-) dianions, {[Mo(2)(DAniF)(3)](3)(micro(3)-MoO(4))(2)}(2+). This cation displays overall D(3) point group symmetry due to a slight twisting of the three Mo(2)(5+) units about the threefold symmetry axis. This distortion removes all mirror symmetry but preserves all C(2) axes orthogonal to the unique C(3) axis. Cyclic voltammetry of {[Mo(2)(DAniF)(3)](3)(micro(3)-MoO(4))(2)}(2+) in CH(2)Cl(2) reveals three reversible one-electron redox processes, corresponding to successive reduction of each of the three Mo(2)(5+) units, with DeltaE(1/2) separations of 0.36 V and 0.41 V.