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We report an investigation of complexes of the type M(2)(dmp)(4) (M = Mo, Cr; dmp = 2,6-dimethoxyphenyl) using resonance Raman (RR) spectroscopy, Cr isotopic substitution, and density functional theory (DFT) calculations. Assignment of the Mo-Mo stretching vibration in the Mo(2) species is straightforward, as evidenced by a single resonance-enhanced band at 424 cm(-1), consistent with an essentially unmixed metal-metal stretch, and overtones of this vibration. On the other hand, the Cr(2) congener has no obvious metal-metal stretching mode near 650-700 cm(-1), where empirical predictions based on the Cr-Cr distance as well as DFT calculations suggest that this vibration should appear if unmixed. Instead, three bands are observed at 345, 363, and 387 cm(-1) that (a) have relative RR intensities that are sensitive to the Raman excitation frequency, (b) exhibit overtones and combinations in the RR spectra, and (c) shift in frequency upon isotopic substitution ((50)Cr and (54)Cr). DFT calculations are used to model the vibrational data for the Mo(2) and Cr(2) systems. Both the DFT results and empirical predictions are in good agreement with experimental observations in the Mo(2) complex, but both, while mutually consistent, differ radically from experiment in the Cr(2) complex. Our experimental and theoretical results, especially the Cr isotope shifts, clearly demonstrate that the potential energy of the Cr-Cr stretching coordinate is distributed among several normal modes having both Cr-Cr and Cr-ligand character. The general significance of these results in interpreting spectroscopic observations in terms of the nature of metal-metal multiple bonding is discussed.

A dimer of dimers containing two quadruply bonded [Mo(2)(DAniF)(3)](+) units (DAniF = N,N'-di(p-anisyl)formamidinate) linked by the S-donor linker, dimethyldithiooxamidate was synthesized, structurally characterized, and electronic communication was probed. The core of [Mo(2)(DAniF)(3)](2)(C(2)S(2)N(2)Me(2)), 1, formed by the Mo(2)NSC(2)SNMo(2) atoms shows two fused but non planar six-membered rings, which differs from that of the beta form of dimethyloxamidate analogue that has a heteronaphthalene-type structure (Cotton, F. A.; Liu, C. Y.; Murillo, C. A.; Villagran, D.; Wang, X. J. Am. Chem. Soc. 2004, 126, 14822). For these two analogous compounds electronic coupling between the two [Mo(2)] units, as determined by electrochemical measurements, diminishes considerably upon replacement of O-donor by S-donor atoms (DeltaE(1/2) = 531 mV and 440 mV, respectively). This suggests that the non planar conformation of the linker in 1 hampers a pathway leading to pi conjugation. Density functional theory (DFT) calculations show that the highest occupied molecular orbitals HOMO-HOMO-1 energy gap of 0.12 eV for 1 is much smaller than that of 0.61 eV for the O-donor analogue, which is consistent with the electrochemical data.

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)).

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.

Structures of compounds having two dimolybdenum units Mo2(DAniF)3+ (DAniF = N,N'-di-p-anisylformamidinate) connected by unsubstituted oxamidate (1) and dithiooxamidate (2) linkers are isomorphous, and the cores of the molecules are planar because of two intramolecular hydrogen bonds within the linkers. Molecular mechanics calculations show a barrier of rotation along the C-C bond of approximately 10 kcal x mol(-1), which suggests that planar conformations are also expected in solution. Changing the two oxygen atoms in the linker of 1 to sulfur atoms results in a significant enhancement of the electronic coupling between the dimetal units (Delta(E1/2) = 204 mV for 1 and 407 mV for 2). The electronic spectrum of 2 shows an intense low energy (600 nm) metal-to-ligand charge transfer (MLCT) band, whereas that for 1 shows only a weak absorption band at 460 nm. DFT calculations on models 1' and 2', in which the anisyl groups were replaced by hydrogen atoms, show that the energy of the pi* orbital of the linker is much lower for 2'. This allows dpi-ddelta interactions from the electrons in the delta orbitals of the Mo2 unit to the sulfur atom that in turn facilitates an electron hopping pathway.

A series of tetranuclear compounds consisting of two {Mo2[(m-CF3C6H4)NC(H)N(m-CF3C6H4)]3}n+ moieties linked by two OH- or two O2- ions has been characterized. Abbreviating the dimolybdenum plus three spectator bridging ligands as [Mo2], the following three compounds have been made-[Mo2](mu-OH)2[Mo2] (1), [Mo2](mu-O)2[Mo2] (2), and {[Mo2](mu-O)2[Mo2]}SbF6 (3). Compound 1, which is diamagnetic and contains quadruply bonded Mo2(4+) units, is converted to diamagnetic 2 by oxidation with O2. Compound 2, which has Mo2(5+) units, is oxidized by NOSbF6 to 3 which has a rare Mo2(5.5+) core and an odd electron delocalized over the two dimolybdenum units.

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.

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.

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.

Research has been conducted on the dimolybdenum tetracarboxylato cations. The preparation and structural characterization of these compounds are described.

Research has been conducted on [Mo (sub)2](CH (sub)3 O) (sub)2 M(CH (sub)3 O) (sub)2 [Mo (sub)2] compounds, where M is zinc or cobalt. The authors report the occurrence of tetramethoxyzincate ion as a bridging ligand in the compounds, and discuss the preparation method via the replacement of tetrahedrally coordinated Zn (super)2+ ion by a tetrahedral Co (super)2+ species.

Research has been conducted on chromium compounds. The authors report EPR spectra of these compounds at W-band which exhibit perpendicular and parallel components, and they consider these compounds to be confirmed oxidation product of a Cr (sub)2 (super)4+ paddlewheel complex.

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(μ-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 Å, 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. [ABSTRACT FROM AUTHOR]

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 (C6X2O42-) 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 = CI) have been synthesized and characterized by X-ray crystallography and spectroscopic methods. Unusually short Mo-O distances (∼2.05 Å for 1 and 2, and ∼2.01 Å 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 ΔE1/2 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. [ABSTRACT FROM AUTHOR]

Abstract: A new molecular pair composed of two quadruply bonded units (DAniF=(p-MeOC6H4)NC(H)N(p-MeOC6H4)) linked by a trishomobarrelene-1,5-dicarboxylate dianion has been prepared. This compound, [Mo2(DAniF)3]2(O2CC11H12CO2) (1), has been characterized by X-ray crystallography and by 1H NMR and UV–Vis spectroscopy. There is only weak electronic coupling between the two Mo2 units according to electrochemical measurements. A comparison with other dicarboxylate-linked molecular pairs is presented. [Copyright &y& Elsevier]

A novel asymmetric, edge-sharing bioctahedral complex with formamidinato ligands (η²-DAniF)Mo(µ-DAniF)2(µ-O,Cl)MoCl2, 1, (DAniF=N,N'-di-p-anisylformamidinate) has been isolated as a byproduct from the preparation of the dimolybdenum(II,III) compound Mo2(DAniF)3Cl2. An X-ray crystallographic study shows that the structure consists of edge-sharing bioctahedra, with the shared edge defined by the vector joining the: µ-O and µ-Cl ligands. Compound 1 is best formulated as a mixed-valent MoIII → MoV compound, the Mo(V) ion being that with the two terminal Cl- ligands. The cyclic voltammogram of 1 shows a reversible reduction at -0.472 V and a reversible oxidation at 1.028 V vs. the Ag/AgCl reference electrode, while the absorption spectrum of 1 reveals an intense low energy absorption at 523 nm (∈max = 12 500) which is attributed to an intervalence charge transfer transition. Crystallographic data for 1 are as follows: a= 13.387(1) Å, b=15.500(2) Å, c=21.855(2) Å, β=98.786(2)°, V= 4481.8(8) ų, P21/c, R1 (wR2) = 0.082 (0.177). KEY WORDS: Edge-sharing bioctahedra; dimolybdenum mixed-valence compound; intervalence charge transfer. [ABSTRACT FROM AUTHOR]

Two compounds of a new type, [Mo[sub 2]](CH[sub 3]O)[sub 2]M(CH[sub 3]O)[sub 2][MO[sub 2]] where [Mo[sub 2]] is an abbreviation for Mo[sub 2][(p-MeOC[sub 6]H[sub 4])NCHN(p-MeOC[sub 6]H[sub 4])][sub 3] and M = Zn (1) and Co (2), are reported. Discrete [M(OR)[sub 4]][sup 2-] ions, either as such or in the μ[sub 2],η[sup 4] role, have not heretofore been described. In these compounds they have distorted tetrahedral structures and bridge two [Mo[sub 2]] groups in much the same way as did SO[sub 4, sup 2-], MoO[sub 4, sup 2-], and WO[sub 4, sup 2-] ions in other recently reported compounds (Cotton, F. A.; Bonahue, J. P.; Murillo, C. A. Inorg. Chem. 2001, 40, 2229). The ¹H NMR spectrum of 1 and the visible spectrum and magnetic properties of 2 are consistent with these structures. The M(OCH[sub 3])[sub 4] bridges are moderately effective in coupling the two [Mo[sub 2]] redox centers. Compounds 1 and 2 may also be viewed as having Zn[sup II] and Co[sup II] centers tetrahedrally coordinated by the bidentate ligand [Mo[sub 2][(p-MeOC[sub 6]H[sub 4])NCHN(p-MeOC[sub 6]H[sub 4])][sub 3](OMe)[sub 2]][sup -]. From that point of view they may be compared with Zn(DPM)[sub 2] and Co(DPM)[sub 2] (3), where DPM is the anion of dipivaloylmethane. For purposes of comparison, 3 has been fully characterized structurally, spectroscopically, and magnetically. Close analogies between 2 and 3 are shown to exist. [ABSTRACT FROM AUTHOR]

Examines the characterization of dimolybdenum paddlewheel compounds with electronic configuration. Increase on Mo-Mo bond length; Stabilization of higher oxidation numbers of the molybdenum atoms; Modification of reduction-oxidation reactions potentials.

Determines the structures of molecules containing molybdenum (Mo) to Mo quadruple bonds. Causes of the deviation of the bond lengths; Estimation of the normal range of Mo-Mo lengths; Variation of the Mo-Mo distance.

Describes the novel behavior of paddlewheel complex of molybdenum(II) upon oxidation. Oxidative scission of a molybdenum-molybdenum quadruple bond; Edge-sharing bioctahedral complexes; Factors favoring the bioctahedral coordination environment; Two-electron oxidation.

Examines relative stabilities on rotation conformational isomers of Mo2Cl4(R-py)4 (R-py = substituted pyridine molecules), Mo = molybdenum and Cl = chlorine. Factors affecting the quadruple bond rotation; Analysis on conformation types of Mo2Cl4(R-py); Adoption of density functional theory; Calculation on energy variation of molecules.

Studies the chemical reaction of molybdenum (III) complex with diethylamine at room temperature. Methodology of the study; Results of the experiment; Conclusions.

Examines the structural characterization of the first cyclotetramolybdenum diynes with bidentate phosphines. Synthesis by cycloaddition of K4Mo2Cl8 in methanol; Reaction with dppm or dmpm ligand; Bond lengths of the planar rectangular Mo4.

Examines the formation of a neutral triangular supramolecule by molybdenum[sub 2][sup 4+] units. Use of mononuclear metal-containing units to produce polygon types; Investigation of dirhodium molecular square with oxalate linkers; Application of absorption corrections.