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.     

Metal-metal bonding in mixed valence Ni2(5+) complexes and spectroscopic evidence for a Ni2(6+) species

Dinickel(II) complexes of the ligands N,N'-di-p-anisylformamidinate (DAniF) and N,N',N'-triphenylguanidinate (TPG) have been synthesized and crystallographically characterized, along with their one-electron-oxidized analogues. In both systems, the Ni-Ni distances become shorter by approximately 0.1 A upon oxidation, in accord with the proposal that the resulting Ni2(5+) complexes are appropriately described as having one electron removed from a metal-based sigma orbital and an overall Ni-Ni bond order of 1/2. Although conventional DFT calculations on the model compounds Ni2(HNCHNH)4 and [Ni2(HNCHNH)4]+ appear to predict that the lowest energy state of the latter species would have one unpaired electron in an essentially ligand-based orbital. A single-point calculation of Ni2(DAniF)4 employing the geometry of its crystal structure with the full ligand included reveals a reversal of the previously predicted order of the HOMO and HOMO-1, and suggests that the unpaired electron in [Ni2(DAniF)4]+ is in a metal-based orbital of sigma symmetry. This is verified by the axial EPR spectrum of the compound in solution. The compound Ni2(DAniF)4 shows an unexpectedly rich cyclic voltammogram with four stepwise reversible oxidation waves. Coulometric experiments show that the doubly oxidized species has a significant lifetime at -25 degrees C, and by spectroelectrochemistry, its UV-vis spectrum was recorded. We propose that this species contains a Ni2(6+) core with a single Ni-Ni sigma bond.     

Dinuclear palladium(ii) complexes with bridging amidate ligands

New homonuclear dimeric Pd(ii) complexes have been synthesized by the reaction of Pd(en)(2+) or Pd(bipy)(2+) (where en = ethylenediamine and bipy = 2,2'-bipyridine) units with acetamide or by the Pd(ii) mediated hydrolysis of CH(3)CN. In these dimers the two metal centers are bridged by either two amidates or by the combination of one hydroxo group and one amidate ligand. The crystal structures of complexes {[Pd(bipy)](2)(micro-1,3-CH(3)CONH)(2)}(NO(3))(2).H(2)O.1/2(CH(3))(2)CO.1/2CH(3)CN () and {[Pd(bipy)](2)(micro-1,3-CH(3)CONH)(2)}(OTf)(2) () showed intrametallic Pd-Pd distances of 2.8480(8) A () and 2.8384(7) A (), respectively, in accordance with the accepted values for a strong Pd-Pd interaction. The presence of pi[dot dot dot]pi interactions between the bipyridine ligands on the di-micro-amidate complexes of Pd(bipy)(2+) shortens the distance between the two Pd centers and allows the formation of the metal-metal interaction. By contrast, the crystal structure of complex {[Pd(en)](2)(micro-1,3-CH(3)CONH)(2)}(OTf)(2).H(2)O (), (where OTf = triflate) where there is no pi[dot dot dot]pi interaction between the ligands on the metal centers, is also reported, and no Pd-Pd interaction is observed. Additionally, one of the complexes, {[Pd(en)](2)(micro-OH)(micro-CH(3)CONH)}(NO(3))(2) (), presents an interesting hydrogen bonded 3-D network formed by nitrate ions and water molecules. All complexes have been characterized by infrared and (1)H NMR spectroscopy.     

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.     

Synthesis, X-ray crystal structure and X-band EPR of [Cu(H(2)O)(2)(imH)(4)] x Sq single crystals

The copper salt [Cu(H(2)O)(2)(imH)(4)] x Sq has been synthesized at room temperature. Crystal structure of the [Cu(H(2)O)(2)(imH)(4)] x Sq (Sq is squarete dianion (C(4)O(4)(2-)) and imH is imidazole (C(3)H(4)N(2))) complexes has been investigated by single-crystal X-ray diffraction analyses and the environment of copper ion has been identified by EPR. The single crystal is triclinic with the space group P1. The unit cell dimensions of the crystals are a=9.317 Angstrom, b=9.958 Angstrom, c=12.130 Angstrom, alpha=69.99 degrees , beta=76.61 degrees and gamma=78.13 degrees . The unit cell contains two molecules. The Cu(II) atom has an octahedral arrangement in which the Cu(II) ion lies on the inversion canter and is coordinated by four imidazole ligands with the equatorial plane and two water molecules with the octahedral axial. The complex shows a normal magnetic moment and the single crystal EPR spectra consist of two sets of four hyperfine lines of copper. The ground wave function of the hole of the Cu(2+) is an admixture of d(x(2) - y(2)) and d(z(2)) states.     

Applications of high-field (W-band) EPR to M-M bonded units (M = Cr,Mo): the first confirmed oxidation of a Cr2 4+ paddlewheel complex to a stable isostructural Cr2 5+ product

The EPR spectra of [Cr(2)[(PhN)(2)CN(CH(2))(4)](4)]PF(6) and [Mo(2)(TiPB)(4)]PF(6) (TiPB = anion of 2,4,6-triisopropylbenzoic acid) at W-band are shown to have g values significantly lower than 2.00 and exhibit parallel and perpendicular components (not resolved at X-band). Therefore the unpaired electrons of the M(2)(5+) units must reside on metal-based (not ligand-based) orbitals. Thus, the chromium compound must be considered as the first confirmed oxidation product of a Cr(2)(4+) paddlewheel complex comparable to the Mo(2)(5+) compounds.     

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.     

Formation of [PtPd(2)(mu(3)-X)(2)(P-P)(dppmX)(2)](2+) (X = S, Se; P-P = dppe, 2 x PPh(3)) aggregates through activation of the chalcogen-rich [PtX(4)] ring by a Pd(I)-Pd(I) bond

Redox addition of the Pd-Pd bond in [Pd(2)Cl(2)(dppm)(2)] across S-S or Se-Se bond in [Pt(X(4)-kappa(2)X(1),X(4))(P-P)] (X = S, Se; P-P = dppe or 2 x PPh(3); dppm = bis(diphenylphosphino)methane, dppe = bis(diphenylphosphino)ethane) leads to the isolation of [PtPd(2)(mu(3)-X)(2)(P-P)(dppmX-kappa(2)X,P(4))(2)](2+) and represents an atom-economy process that converts chalcogen-rich complexes to heterometallic chalcogenide aggregates. Activation of the [PtX(4)] ring is achieved by tetrachalcogenide reduction and dual oxidation of palladium and phosphine.     

EPR and ENDOR analysis of Fe3+ impurity centers in fluoroelpasolite lattices

Fe(3+) ions in hexagonal and cubic fluoroelpasolite crystals (A(1)(2)B(I)M(III)F(6)) have been investigated in a combined Electron Paramagnetic Resonance (EPR) and Electron Nuclear Double Resonance (ENDOR) study. A detailed analysis of the ENDOR spectra for the nearest (19)F and (23)Na shells in X (9.5 GHz) and Q band (34 GHz) allowed the complex EPR spectra to be disentangled and to determine the spin Hamiltonian parameters for the various S = 5/2 Fe(3+) centres. W-band (95 GHz) EPR measurements as a function of temperature were performed to provide unambiguous evidence about the absolute signs of the Zero Field Splitting (ZFS) and SuperHyperFine (SHF) parameters for Fe(3+) in Cs(2)NaAlF(6) as already determined from the ENDOR work. It could be concluded that all principal (19)F hyperfine values were positive, in agreement with earlier assignments in the literature for related systems. A comparative analysis of the (19)F SHF data for Fe(3+) at a perfectly octahedral site in the cubic crystal, and at two slightly trigonally distorted environments in the hexagonal crystals, indicates that the metal-to-ligand distance changes upon doping. The obtained set of parameters concerning one defect in various analogous environments can furthermore be used to test different methods of theoretical calculations for ZFS and SHF values.     

Pyridine-2-thionate as a versatile ligand in Pd(II) and Pt(II) chemistry: the presence of three different co-ordination modes in [Pd2(micro2-S,N-C5H4SN)(micro2-kappa2S-C5H4SN)(micro2-dppm)(S-C5H4SN)2

Reactions of [MCl2(L-L)], M = Pt, Pd; L-L = bis(diphenylphosphino)methane (dppm) or bis(diphenylphosphino)ethane (dppe), with NaC5H4SN in a 1 : 2 molar ratio lead to mononuclear species [M(S-C5H4SN)2(P-P)], M = Pt; L-L = dppm (1) or dppe (2) and M = Pd; L-L = dppe (3), as well as to the dinuclear [Pd2(micro2-S,N-C5H4SN)(micro2-kappa2S-C5H4SN)(micro2-dppm)(S-C5H4SN)2] (4). In contrast, reaction of [MCl2(dppm)] with NaC5H4SN in a 1 : 1 molar ratio leads to [Pd2(micro2-S,N-C5H4SN)3(micro2-dppm)]Cl (5) and trans-[Pt(S-C5H4SN)2(PPh2Me)2] (6) respectively. The latter is formed in low yield by cleavage of the dppm ligand. The dinuclear derivatives 4 and 5 present an A-frame and lantern structure, respectively. The former showing three different co-ordination modes in the same molecule with a short Pd-Pd distance of 2.9583 (9) A and the latter with three bridging S,N thionate ligands showing a shorter Pd-Pd distance of 2.7291 (13) A. Both distances could be imposed by the bridging ligands or point to some sort of metal-metal interaction.     

Direct evidence from electron paramagnetic resonance for additional configurations in uncommon paddlewheel Re2(7+) units surrounded by an unsymmetrical bicyclic guanidinate

Three rare compounds have been synthesized and structurally characterized; these species have paddlewheel structures and Re(2)(7+) cores surrounded by four bicyclic guanidinates and two axial ligands along the Re-Re axis. Each possesses a formal bond order of 3.5 and a σ(2)π(4)δ(1) electronic configuration that entails the presence of one unpaired electron for each compound. The guanidinate ligands characterized by having CH(2) entities and a central C(N)(3) unit that joins two cyclic units--one having two fused 6-membered rings (hpp) and the other having a 5- and a 6-membered ring fused together (tbn)--allowed the isolation of [Re(2)(tbn)(4)Cl(2)]PF(6), 1, [Re(2)(tbn)(4)Cl(2)]Cl, 2, and [Re(2)(hpp)(4)(O(3)SCF(3))(2)](O(3)SCF(3)), 3. Because of the larger bite angle of the tbn relative to the hpp ligand, the Re-Re bond distances in 1 and 2 (2.2691(14) and 2.2589(14) ?, respectively) are much longer than that in 3 (2.1804(8) ?). Importantly, electron paramagnetic resonance (EPR) studies at both X-band (~9.4 GHz) and W-band (112 GHz) in the solid and in frozen solution show unusually low g-values (~1.75) and the absence of zero-field splitting, providing direct evidence for the presence of one metal-based unpaired electron for both 1 and 3. These spectroscopic data suggest that the unsymmetrical 5-/6-membered ligand leads to the formation of isomers, as shown by significantly broader EPR signals for 1 than for 3, even though both compounds possess what appears to be similar ideal crystallographic axial symmetry on the X-ray time scale.     

Probing defect sites on TiO2 with [Re3(CO)12H3]: spectroscopic characterization of the surface species

Samples of the anatase phase of titania were treated under vacuum to create Ti(3+) surface-defect sites and surface O(-) and O(2) (-) species (indicated by electron paramagnetic resonance (EPR) spectra), accompanied by the disappearance of bridging surface OH groups and the formation of terminal Ti(3+)-OH groups (indicated by IR spectra). EPR spectra showed that the probe molecule [Re(3)(CO)(12)H(3)] reacted preferentially with the Ti(3+) sites, forming Ti(4+) sites with OH groups as the [Re(3)(CO)(12)H(3)] was adsorbed. Extended X-ray absorption fine structure (EXAFS) spectra showed that these clusters were deprotonated upon adsorption, with the triangular metal frame remaining intact; EPR spectra demonstrated the simultaneous removal of surface O(-) and O(2) (-) species. The data determined by the three complementary techniques form the basis of a schematic representation of the surface chemistry. According to this picture, during evacuation at 773 K, defect sites are formed on hydroxylated titania as a bridging OH group is removed, forming two neighboring Ti(3+) sites, or, when a Ti(4+)-O bond is cleaved, forming a Ti(3+) site and an O(-) species, with the Ti(4+)-OH group being converted into a Ti(3+)-OH group. When the probe molecule [Re(3)(CO)(12)H(3)] is adsorbed on a titania surface with Ti(3+) defect sites, it reacts preferentially with these sites, becoming deprotonated, removing most of the oxygen radicals, and healing the defect sites.     

Systematic preparation of Mo 2 4+ building blocks for supramolecular assemblies

The preparation of additional and useful building blocks for the construction of supramolecular entities with quadruply bonded Mo(2)(4+) units has been explored, and five new mixed-ligand complexes with three types of ligands and various basicities are reported. The ligands used were the DAniF (N,N'-di-p-anisylformamidinate) anion, the acetate anion, and neutral acetonitrile molecules. The formamidinate ligands are the least labile, and the acetonitrile molecules are the most labile. This difference as well as a relatively strong trans directing influence by the formamidinate anions in ligand substitution reactions allows designed synthesis of various mixed-ligand building blocks, including rare pairs of cis and trans isomers. The new compounds are cis-Mo(2)(DAniF)(2)(O(2)CCH(3))(2) (1), trans-Mo(2)(DAniF)(2)(O(2)CCH(3))(2) (2), trans-[Mo(2)(DAniF)(2)(O(2)CCH(3))(CH(3)CN(eq)())(2)]BF(4) (3), trans-[Mo(2)(DAniF)(2)(CH(3)CN(eq)())(4)](BF(4))(2) (4), and [Mo(2)(O(2)CH(3))(CH(3)CN(eq)())(6)(CH(3)CN(ax)())](BF(4))(3) (5), where eq and ax designate equatorial and axial ligands, respectively. A comparison with some previously synthesized complexes is given along with a discussion of the overall reactivity of all compounds.     

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.     

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.     

Electronic structure description of a [Co(III)3Co(IV)O4] cluster: a model for the paramagnetic intermediate in cobalt-catalyzed water oxidation

Multifrequency electron paramagnetic resonace (EPR) spectroscopy and electronic structure calculations were performed on [Co(4)O(4)(C(5)H(5)N)(4)(CH(3)CO(2))(4)](+) (1(+)), a cobalt tetramer with total electron spin S = 1/2 and formal cobalt oxidation states III, III, III, and IV. The cuboidal arrangement of its cobalt and oxygen atoms is similar to that of proposed structures for the molecular cobaltate clusters of the cobalt-phosphate (Co-Pi) water-oxidizing catalyst. The Davies electron-nuclear double resonance (ENDOR) spectrum is well-modeled using a single class of hyperfine-coupled (59)Co nuclei with a modestly strong interaction (principal elements of the hyperfine tensor are equal to [-20(±2), 77(±1), -5(±15)] MHz). Mims (1)H ENDOR spectra of 1(+) with selectively deuterated pyridine ligands confirm that the amount of unpaired spin on the cobalt-bonding partner is significantly reduced from unity. Multifrequency (14)N ESEEM spectra (acquired at 9.5 and 34.0 GHz) indicate that four nearly equivalent nitrogen nuclei are coupled to the electron spin. Cumulatively, our EPR spectroscopic findings indicate that the unpaired spin is delocalized almost equally across the eight core atoms, a finding corroborated by results from DFT calculations. Each octahedrally coordinated cobalt ion is forced into a low-spin electron configuration by the anionic oxo and carboxylato ligands, and a fractional electron hole is localized on each metal center in a Co 3d(xz,yz)-based molecular orbital for this essentially [Co(+3.125)(4)O(4)] system. Comparing the EPR spectrum of 1(+) with that of the catalyst film allows us to draw conclusions about the electronic structure of this water-oxidation catalyst.     

High-field EPR study and crystal and molecular structure of trans-RSSR-[CrCl2(cyclam)]nX (X=ZnCl4 2-, Cl- and Cl-.4H2O.0.5HCl)

For the first time, HF-EPR (94.5 GHz) spectroscopy has been used to determine crystal field parameters in chromium(III) coordination compounds. The large zero-field splitting parameters of the dark-green photochromic trans-RSSR-[CrCl(2)(cyclam)](2)ZnCl(4), 1, the red-purple trans-RSSR-[CrCl(2)(cyclam)]Cl, 2, and the red-purple trans-RSSR-[CrCl(2)(cyclam)]Cl.4H(2)O.0.5HCl, 3, where cyclam = 1,4,8,11-tetraazacyclotetradecane, have been obtained. A full analysis of EPR spectra at 94.5 GHz of diluted complexes 1, 2 and 3 at 300 K revealed that they are extremely sensitive to D and E values. The rhombic distortion was precisely determined for each compound. For 1, g= 2.01, D=-0.305 cm(-1), E= 0.041 cm(-1) and lambda=|E/D|= 0.1396; for 2, g= 2.01; D=-0.348 cm(-1), E= 0.042 cm(-1) and lambda=|E/D|= 0.1206 and for 3, g= 1.99, D=-0.320 cm(-1), E= 0.041 cm(-1) and ambda=|E/D|= 0.1281. The EPR study at 94.5 GHz at 10 K allowed us to confirm the sign of the D value for all compounds. These data indicate that at room temperature the crystal field is mainly rhombic and as the temperature decreases, the rhombicity of the D tensor increases slightly. These found differences between 1, 2 and 3 allowed us to establish the importance of the intermolecular interactions in the solid state due to different hydrogen bonding networks in their crystalline arrangement.     

Theoretical investigation into the mechanism of reductive elimination from bimetallic palladium complexes

Reductive elimination of C-Cl and C-C bonds from binuclear organopalladium complexes containing Pd-Pd bonds with overall formal oxidation state +III are explored by density functional theory for dichloromethane and acetonitrile solvent environments. An X-ray crystallographically authenticated neutral complex, [(L-C,N)ClPd(μ-O(2)CMe)](2) (L = benzo[h]quinolinyl) (I), is examined for C-Cl coupling, and the proposed cation, [(L-C,N)PhPd(1)(μ-O(2)CMe)(2)Pd(2)(L-C,N)](+) (II), examined for C-C coupling together with (L-C,N)PhPd(1)(μ-O(2)CMe)(2)Pd(2)Cl(L-C,N) (III) as a neutral analogue of II. In both polar and nonpolar solvents, reaction from III via chloride dissociation from Pd(2) to form II is predicted to be favored. Cation II undergoes Ph-C coupling at Pd(1) with concomitant Pd(1)-Pd(2) lengthening and shortening of the Pd(1)-O bond trans to the carbon atom of L; natural bond orbital analysis indicates that reductive coupling from II involves depopulation of the d(x(2)-y(2)) orbital of Pd(1) and population of the d(z(2)) orbitals of Pd(1) and Pd(2) as the Pd-Pd bond lengthens. Calculations for the symmetrical dichloro complex I indicate that a similar dissociative pathway for C-Cl coupling is competitive with a direct (nondissociative) pathway in acetonitrile, but the direct pathway is favored in dichloromethane. In contrast to the dissociative mechanism, direct coupling for I involves population of the d(x(2)-y(2)) orbital of Pd(1) with Pd(1)-O(1) lengthening, significantly less population occurs for the d(z(2)) orbital of Pd(1) than for the dissociative pathway, and d(z(2)) at Pd(2) is only marginally populated resulting in an intermediate that is formally a Pd(1)(I)-Pd(2)(III) species, (L-Cl-N,Cl)Pd(1)(μ-O(2)CMe)Pd(2)Cl(O(2)CMe)(L-C,N) that releases chloride from Pd(2) with loss of Pd(I)-Pd(III) bonding to form a Pd(II) species. A similar process is formulated for the less competitive direct pathway for C-C coupling from III, in this case involving decreased population of the d(z(2)) orbital of Pd(2) and strengthening of the Pd(I)-Pd(III) interaction in the analogous intermediate with η(2)-coordination at Pd(1) by L-Ph-N, C(1)-C(2).     

An infinite zigzag chain of alternating Cl-Pd-Pd-Cl and Mo-Mo units

Treatment of Pd(2)Cl(2)(CNC(6)H(3)Me(2)-2,6)(4) (1) with Mo(2)(O(2)CCF(3))(4) (2) in dichloromethane afforded an infinite zigzag chain [[Pd(2)Cl(2)(CNC(6)H(3)Me(2)-2,6)(4)][Mo(2)(O(2)CCF(3))(4)]](n) (3), where two metal-metal bonded dinuclear Pd-Pd and Mo-Mo units were bridged by chloro atoms. The Mo-Mo distance (2.1312(3) A) of 3 is significantly elongated compared to that of 2 (2.090(4) A) and lies in the range of that of the quadruple Mo-Mo bonded complexes. Such elongation might be attributed to the axial donation of the chloro atoms of the Pd-Pd unit to the Mo-Mo moiety.     

EPR study of Cu(2+)-doped tetraaqua-di(nicotinamide)Co(II) saccharinate single crystals

The electron paramagnetic resonance spectra of Cu(2+) impurities in [Co(nicotinamide)(2)(H(2)O)(4)](saccharinate)(2) single crystals have been studied at ambient temperature in three mutually perpendicular planes. The angular variation of the spectra shows that the Cu(2+) ion substitutes the Co(2+) site in the lattice. The EPR spectra of Cu(2+) ions are characteristic of tetragonally elongated octahedral site. The spin-Hamiltonien parameters were obtained from the single crystal EPR analysis. The ground-state wave function of Cu(2+) ion in the lattice has been constructed.