OR Ligands Research Articles

A hemiaminal-ether structure stabilized by lanthanide complexes with an imidazole-based Schiff base ligand.

Imidazole-based lanthanide complexes bearing a hemiaminal-ether pendant, [Ln(L'-OR)(NO3)2](NO3)·solvent (Ln = Tb and Gd; L'-OR = hemiaminal-ether ligand, 1,9-bis(1H-imidazole-4-yl)-5-[(R-1H-imidazole-4-yl)alkyl]-2,5,8-triazanon-1,8-dien; R = Me and Et; solvent = MeOH, EtOH, and H2O; abbreviated as LnL'-OR), were unexpectedly obtained from the reaction of diethylenetriamine with 4-imidazolecarbaldehyde in the presence of lanthanide nitrate in alcohol. These LnL'-OR structures were also obtained by treating dipodal imidazole-based lanthanide complexes [Ln(HL)(NO3)2(MeOH)](NO3)·MeOH (HL = bis{[2-{(imidazole-4-yl)methylidene}amino]ethyl}amine; abbreviated as LnHL) independently with (i) 4-imidazolecarbaldehyde, (ii) HNO3, or (iii) NO in alcohol. The results indicate that the hemiaminal-ether structure is stabilized by the lanthanide complex. The LnL'-OR structures were confirmed by X-ray analysis and cold electrospray ionization mass spectrometry in solution. The photophysical properties indicate that the L'-OR ligands can sensitize the Tb3+ luminescence. A significant spectral change was observed by the ligand component exchange. The exchange of the ligand component of the non-luminescent imidazole-based Tb complex, [Tb(MeL)(NO3)2](NO3) (MeL = bis{[2-{(1-methylimidazole-2-yl)methylidene}amino]ethyl}amine; abbreviated as TbMeL), results in a significant spectral change.

Computational study of C(sp3)-O bond formation at a PdIV centre.

This report describes a computational study of C(sp3)-OR bond formation from PdIV complexes of general structure PdIV(CH2CMe2-o-C6H4-C,C')(F)(OR)(bpy-N,N') (bpy = 2,2'-bipyridine). Dissociation of -OR from the different octahedral PdIV starting materials results in a common square-pyramidal PdIV cation. An SN2-type attack by -OR (-OR = phenoxide, acetate, difluoroacetate, and nitrate) then leads to C(sp3)-OR bond formation. In contrast, when -OR = triflate, concerted C(sp3)-C(sp2) bond-forming reductive elimination takes place, and the calculations indicate this outcome is the result of thermodynamic rather than kinetic control. The energy requirements for the dissociation and SN2 steps with different -OR follow opposing trends. The SN2 transition states exhibit "PdCO" angles in a tight range of 151.5 to 153.0°, resulting from steric interactions between the oxygen atom and the gem-dimethyl group of the ligand. Conformational effects for various OR ligands and isomerisation of the complexes were also examined as components of the solution dynamics in these systems. In all cases, the trends observed computationally agree with those observed experimentally.

2-(2-Hydroxy-4-methoxybenzoyl)benzoic acid derivatives of Group 4 metal alkoxides

Continued exploration of the coordination behavior of derivatives of 2-benzophenone-based ligands with metal alkoxides ([M(OR)4]) was undertaken from the reaction of 2-(2-hydroxy-4-methoxybenzoyl)benzoic acid (H2-OBzA) with a series of Group 4 precursors. The products of these reactions were identified as: [(OR)2Ti(μ-(c,c-OBzA))]2 (OR = OCHMe2 (OPri; 1 •2tol); OCMe3 (OBut; 2 •THF); OCH2CMe3 (ONep; 3)), [[(OPri)3Ti(μ-OPri)Ti(OPri)2]2(μ-(μc,μ-OBzA))2]2 (4), [(ONep)3Zr(μ-ONep)2Zr(ONep)2]2(μ-(c,μ-OBzA)2) (5 •tol), [(py)(OBut)3Zr]2(μ-(c,c-OBzA)) (6), [(OBut)2Hf(μ-OBut)]2(μ-(c,η1-OBzA)) (7) where ‘c’ = chelating or η2; ‘μ’ = bridging or η1,η1(O,O’); and μc = bridging chelating or η1,η1(O,O’); η2 : η1. The metal centers for each of these compounds adopt a pseudo-octahedral geometry employing the OBzA ligand in numerous binding modes. The different functional oxygens (carboxylate, hydroxyl, and carbonyl) were employed in a variety of coordination modes for 1–7. The complexity of these OBzA-modified compounds is driven by a combination of the coordination behavior of the OBzA moieties, the size of the metal cation, and the pendant chain of the OR ligand. Solution NMR indicates a complex structure exists in solution that was considered to be consistent with the solid-state structure.

Coordination Chemistry of N,N,N′,N′-Tetrakis(3,5-substituted benzyl-2-oxide)-2,2′-(ethylenedioxy)diethanamine Modified Group 4 Metal Alkoxides

The coordination behavior of a set of (ethylenedioxy)diethanamine-based tetraphenol ligands with a series of Group 4 metal alkoxides ([M(OR)(4)]) was determined. The ligands were synthesized from a modified Mannich reaction and fully characterized as N,N,N',N'-tetrakis(3,5-tert-butyl-benzyl-2-hydroxy)-2,2'-(ethylenedioxy)diethanamine, termed H(4)-OEA-DBP(4) (1), and N,N,N',N'-tetrakis(3,5-chloro-benzyl-2-hydroxy)-2,2'-(ethylenedioxy)diethanamine, termed H(4)-OEA-DCP(4) (2). The reaction of 1 with a set of [M(OR)(4)] [M = Ti, Zr, or Hf; OR = iso-propoxide (OPr(i)), neo-pentoxide (ONep), or tert-butoxide (OBu(t))] precursors led to the isolation of [(OPr(i))(2)Ti](2)(μ-OEA-DBP(4)) (3), [(ONep)(2)Ti](2)(μ-OEA-DBP(4)) (4), and [(OBu(t))(2)M](2)(μ-OEA-DBP(4)) where M = Ti (5), Zr (6), or Hf (7). In addition, the [(ONep)(2)Ti](2)(μ-OEA-DCP(4)) (4a) derivative was isolated from the reaction of 2 and [Ti(ONep)(4)], demonstrating the similarity of coordination behavior between the two OEA-R(4) ligands. For 3-7, the metal center adopts a slightly distorted octahedral geometry by binding the two O atoms of the phenoxide moiety, as well as one N and one O atom from the OEA moieties, while retaining two of the original OR ligands. Solution NMR demonstrates inequivalent protons for the majority of the bound OEA ligands, which argues for retention of structure in solution. The synthesis and characterization of these compounds are presented in detail.

Electrochemical, Computational, and Photophysical Characterization of New Luminescent Dirhenium–Pyridazine Complexes Containing Bridging OR or SR Anions

A series of [Re(2)(μ-ER)(2)(CO)(6)(μ-pydz)] complexes have been synthesized (E = S, R = C(6)H(5), 2; E = O, R = C(6)F(5), 3; C(6)H(5), 4; CH(3), and 5; H, 6), starting either from [Re(CO)(5)O(3)SCF(3)] (for 2 and 4), [Re(2)(μ-OR)(3)(CO)(6)](-) (for 3 and 5), or [Re(4)(μ(3)-OH)(4)(CO)(12)] (for 6). Single-crystal diffractometric analysis showed that the two μ-phenolato derivatives (3 and 4) possess an idealized C(2) symmetry, while the μ-benzenethiolato derivative (2) is asymmetrical, because of the different conformation adopted by the phenyl groups. A combined density functional and time-dependent density functional study of the geometry and electronic structure of the complexes showed that the lowest unoccupied molecular orbital (LUMO) and LUMO+1 are the two lowest-lying π* orbitals of pyridazine, whereas the highest occupied molecular orbitals (HOMOs) are mainly constituted by the "t(2g)" set of the Re atoms, with a strong Re-(μ-E) π* character. The absorption spectra have been satisfactorily simulated, by computing the lowest singlet excitation energies. All the complexes exhibit one reversible monoelectronic reduction centered on the pyridazine ligand (ranging from -1.35 V to -1.53 V vs Fc(+)|Fc). The benzenethiolato derivative 2 exhibits one reversible two-electron oxidation (at 0.47 V), whereas the OR derivatives show two close monoelectronic oxidation peaks (ranging from 0.85 V to 1.35 V for the first peak). The thioderivative 2 exhibits a very small electrochemical energy gap (1.9 eV, vs 2.38-2.70 eV for the OR derivatives), and it does not show any photoluminescence. The complexes containing OR ligands show from moderate to poor photoluminescence, in the range of 608-708 nm, with quantum yields decreasing (ranging from 5.5% to 0.07%) and lifetimes decreasing (ranging from 550 ns to 9 ns) (3 > 4 > 6 ≈ 5) with increasing emission wavelength. The best emitting properties, which are closely comparable to those of the dichloro complex (1), are exhibited by the pentafluorophenolato derivative (3).

Hydrogenolysis of Palladium(II) Hydroxide, Phenoxide, and Alkoxide Complexes

A series of pincer ((tBu)PCP)Pd(II)-OR complexes ((tBu)PCP = 2,6-bis(CH(2)P(t)Bu(2))C(6)H(3), R = H, CH(3), C(6)H(5), CH(2)C(CH(3))(3), CH(2)CH(2)F, CH(2)CHF(2), CH(2)CF(3)) were synthesized to explore the generality of hydrogenolysis reactions of palladium-oxygen bonds. Hydrogenolysis of the Pd hydroxide complex to generate the Pd hydride complex and water was shown to be inhibited by formation of a water-bridged, hydrogen-bonded Pd(II) hydroxide dimer. The Pd alkoxide and aryloxide complexes exhibited more diverse reactivity. Depending on the characteristics of the -OR ligand (steric bulk, electron-donating ability, and/or the presence of β-hydrogen atoms), hydrogenolysis was complicated by hydrolysis by adventitious water, a lack of reactivity with hydrogen, or a competing dissociative β-hydride abstraction reaction pathway. Full selectivity for hydrogenolysis was observed with the partially fluorinated Pd(II) 2-fluoroethoxide complex. The wide range of Pd-OR substrates examined helps to clarify the variety of reaction pathways available to late-transition-metal alkoxides as well as the conditions necessary to tune the reactivity to hydrogenolysis, hydrolysis, or dissociative β-hydride abstraction.

Methyleneimidazoline Complexes of Iridium, Rhodium, and Palladium from Selective C(sp3)H Bond Activation

Palladium, iridium, and rhodium complexes of 2-methyleneimidazolines have been synthesized by selective phosphine-assisted activation of the 2-methyl C-H bonds in 2-methylimidazolium compounds. Metallacycles of various sizes were obtained in the reaction of phosphine-tethered 2-methylimidazolium compounds and [{M(cod)X}(2)] (M = Rh or Ir; cod = 1,5-cyclooctadiene; X = alkoxyl or Cl). Representative complexes were characterized by X-ray crystallography. The selectivity for aliphatic C(sp(3))H versus aromatic C(sp(2))H activation could be adjusted by means of the steric bulk of the OR ligand, whereby a bulky OR group favors activation of the 2-methyl C(sp(3))-H bond. Experimental results confirmed that a methyl C-H activation product (a seven-membered iridacycle) is the kinetic product, while the aryl C-H activation product (a six-membered iridacycle) is the thermodynamic product.

Lanthanide Compounds with Fluorinated OC6F5 Ligands: Homo- and Heterovalent Complexes of Eu(II) and Eu(III)

The fluorinated phenoxide OC6F5 forms the stable Eu(II) and Eu(III) derivatives (DME)2Eu(mu-OC6F5)3Eu(mu-OC6F5)3Eu(DME)2 and (DME)2Eu(OC6F5)3, as well as the heterovalent product (DME)2Eu(mu-OC6F5)3Eu(DME)(OC6F5)2, in redox reactions of Eu with HOC6F5 or in proton-transfer reactions of HOC6F5 with Eu(SPh)2. The divalent complex crystallizes as a trimer with three bridging phenoxides bridging each pair of metals, with the terminal metals coordinating DME and the central metal ion encapsulated totally by O(C6F5) and dative fluoride interactions. The trivalent compound is monomeric with terminal phenoxide ligands and no Eu-F interactions. The heterovalent compound has clearly localized metal valence states and coordination features that mimic the homovalent species with the terminal OC6F5 bound to the Eu(III) ion, three bridging OR ligands spanning the Eu(II) and Eu(III) ions, and dative Eu(II)-F bonds. At elevated temperatures, these compounds decompose to give a mixture of solid-state fluoride phases.

Controlled Synthesis of a Structurally Characterized Family of Sterically Constrained Heterocyclic Alkoxy-Modified Titanium Alkoxides

The reaction of [Ti(mu-ONep)(ONep)3]2 (ONep = OCH2C(CH3)3) with a series of heterocyclic methanol derivatives [tetrahydrofurfuryl alcohol (H-OTHF), thiophene methanol (H-OTPM), or 2-pyridylcarbinol (H-OPy)-collectively termed H-OR*], led to the isolation of a novel family of OR*-substituted titanium alkoxide precursors. Independent of the initial stoichiometry for the H-OTHF reaction, a monosubstituted, dinuclear species was isolated as [(ONep)3Ti(muc-OTHF)]2 (1). For 1, each Ti was octahedrally (Oh) bound by three terminal ONep ligands, one bidentate bridging OTHF ligand (muc-OTHF), and an oxygen from the other muc-OTHF ligand. For the OTPM derivatives, the product was identified as [(ONep)3Ti(mu-OTPM)]2 (2). For this ligand, the soft S atom does not bind to the Ti but the O atom does act as a bridge between the two trigonal bipyramidal bound Ti metal centers. The OPy system yielded (OPy)2Ti(OR)2 independent of the OR and the stoichiometry used [OR = ONep (3), OCHMe2 (4), OCMe3 (5)]. For 3-5, the two OPy ligands chelate to the Oh-bound Ti metal center with two terminal OR ligands. Compounds 1-5 were fully characterized using a variety of analytical techniques. An initial investigation of the proposed chemical stability of the '(OPy)2Ti' moiety of 3-5 to alcoholysis exchange pathways involving (i) alkyl alcohols, (ii) aryl alcohols, (iii) substituted phenols, (iv) H-OR* derivatives, and (v) silanols proved successful through the isolation of a novel family of structurally characterized (OPy)2Ti(OR')2 (7-24) compounds.

A Re(CO)5Fragment Bonded to a Polyhedral Oligomeric Hydroxysilsesquioxane as a Model which Gives Further Evidence to the Existence of an Unexpected Re(CO)5Surface Species

Reaction of Re(CO)5O3SCF3 with (c-C6H11)7Si8O12O-Li+ at 273 K under a CO atmosphere affords the [Re(CO)5OR] (R = (c-C6H11)7Si8O12) derivative (1). 1 is the first example of a rhenium pentacarbonyl bearing an OR ligand (R = alkyl, aryl, or silyl) stable enough to be characterized, and it represents also the first molecular model of the surface [Re(CO)5OSi] species formed by reductive carbonylation of silica-supported [Re(CO)3OH]4. At room temperature, 1 loses one carbonyl ligand and dimerizes to afford {Re(CO)4[(mu-O)O12Si8(c-C6H11)7]}2 (2), which has been characterized by X-ray diffraction and is the first reported example of a rhenium tetracarbonyl mu-oxo-bridged dimer of the type [Re(CO)4(mu-OR)]2.

Modification of alkoxo ligands of BINOL–Ti ladder: Isolation and X-ray crystallographic analysis

The stable binaphthol–titanium ladder complexes have been successfully prepared by using bulky alkoxo ligands. From the secondary OR ligand (cyclohexyloxo, 2,4-dimethyl-3-pentyloxo or 2-adamantyloxo) and terially OR ligand ( tert-butyloxo, 1-adamantyloxo), partial hydrolysis proceeded to give the μ 3-oxo titanium complexes. The use of [Ti(BINOLato)(OEt) 2] n made it possible to prepare the Ti(BINOLato)(OR) 2 complexes using alcohols (ROH) of high boiling point (R = cyclohexyl, 2-adamantyl, 1-adamantyl). X-ray analyses of [( R)-1,1′-bi-2-naphtholato]bis(O-2,4-dimethyl-3-pentyloxo)titanium and [( R)-3,3′-dimethyl-1,1′-bi-2-naphtholato]bis(2-adamantyloxo)titanium showed a good agreement with the estimated ladder complexes. The catalytic activity of BINOL–Ti catalyst analogues, obtained by partial hydrolysis of Ti(BINOLato)(OR) 2 with wet MS 4A was studied in asymmetric glyoxylate-ene reaction by two methods. Moderate to good chemical yields and enantioselectivities were obtained.

Cadmium Amido Alkoxide and Alkoxide Precursors for the Synthesis of Nanocrystalline CdE (E = S, Se, Te)

The synthesis and characterization of a family of alternative precursors for the production of CdE nanoparticles (E = S, Se, and Te) is reported. The reaction of Cd(NR2)2 where NR2 = N(SiMe3)2 with n HOR led to the isolation of the following: n = 1 [Cd(mu-OCH2CMe3)(NR2)(py)]2 (1, py = pyridine), Cd[(mu-OC6H3(Me)(2)-2,6)2Cd(NR2)(py)]2 (2), [Cd(mu-OC6H3(CHMe2)(2)-2,6)(NR2)(py)]2 (3), [Cd(mu-OC6H3(CMe3)(2)-2,6)(NR2)(py)]2 (4), [Cd(mu-OC6H2(NH2)(3)-2,4,6)(NR2)(py)]2 (5), and n = 2 [Cd(mu-OC6H3(Me)(2)-2,6)(OC6H3(Me)(2)-2,6)(py)2]2 (6), and [Cd(mu-OC6H3(CMe3)(2)-2,6)(OC6H3(CMe3)(2)-2,6)(THF)]2 (7). For all but 2, the X-ray crystal structures were solved as discrete dinuclear units bridged by alkoxide ligands and either terminal -NR2 or -OR ligands depending on the stoichiometry of the initial reaction. For 2, a trinuclear species was isolated using four mu-OR and two terminal -NR2 ligands. The coordination of the Cd metal center varied from 3 to 5 where the higher coordination numbers were achieved by binding Lewis basic solvents for the less sterically demanding ligands. These complexes were further characterized in solution by 1H, 13C, and 113Cd NMR along with solid-state 113Cd NMR spectroscopy. The utility of these complexes as "alternative precursors" for the controlled preparation of nanocrystalline CdS, CdSe, and CdTe was explored. To synthesize CdE nanocrystals, select species from this family of compounds were individually heated in a coordinating solvent (trioctylphosphine oxide) and then injected with the appropriate chalcogenide stock solution. Transmission electron spectroscopy and UV-vis spectroscopy were used to characterize the resultant particles.

4,6-Dihydroxypyrimidine: a selective bridging ligand for controlled Group IV metal alkoxide structures

The generation of controlled metal alkoxide structures has been explored using 4,6-dihydroxypyrimidine (DHP-H2) as a selective bridging ligand. DHP-H2 was reacted with Ti(OPri)4 in pyridine (py). Crystallization from this solution led to the isolation of [(OPri)2(μ-OPri)Ti(μ-DHP)Ti(OPri)3(py)]2 (1). The two dinuclear subunits of 1 possess a bridging DHP ligand bound through both the O and N of the ring, forming an M–O–C–N ring on each metal center. The first Ti possesses three terminal OR ligands and one coordinated pyridine solvent molecule. The second Ti is connected to a μ-OPri group from the other subunit, forming a tetranuclear species. In contrast, recrystallization of 1 from toluene provided the solvent-free, cyclic adduct {[(OPri)2(μ-OPri)Ti]2(μ-DHP)}2 (2). An identical reaction using more sterically hindering alkoxy precursors yielded products of the general structure [(py)(RO)3M]2(μ4-DHP), where M = Ti, OR = OCH2CMe3 (3), OPri/OCMe3 (4); M = Zr, OR = OCMe3 (5). Each metal center adopts an octahedral geometry, with one py, three terminal alkoxide ligands, and one bridging DHP. Solid-state 13C MAS NMR studies indicate the bulk materials are consistent with the crystal structures and solution 1H NMR is consistent with retention of the structures in solution for 2–5.

Identification of delta- and mu-type opioid receptors on human and murine dendritic cells

The purpose of this study was to evaluate μ- and δ-opioid receptors (OR) on human and murine dendritic cells (DC). Expression of μ- and δ-OR mRNA on DC was demonstrated by RT-PCR. The immunocytochemical and Western blot analyses revealed the expression of OR protein in DC. Radioreceptor assay demonstrated the specific saturated temperature-dependent binding of [ 3H]-labeled opioid ligand on DC and B max=2.8±0.3 fmol/10 6 cells and K D=4.8±1.0 nM were calculated by a Scatchard analysis. Finally, OR ligands DADLE and DAGO dose-dependently modulated the capacity of DC to induce T cell proliferation in an MLR assay. Importantly, expression of functional OR on DC was significantly increased upon TNF-α-induced DC maturation. Thus, these data suggest a new mechanism of opioid-dependent neuroendocrine immunomodulation.

Studies of reductive elimination reactions to form carbon-oxygen bonds from Pt(IV) complexes.

The platinum(IV) complexes fac-L(2)PtMe(3)(OR) (L(2) = bis(diphenylphosphino)ethane, o-bis(diphenylphosphino)benzene, R = carboxyl, aryl; L = PMe(3), R = aryl) undergo reductive elimination reactions to form carbon-oxygen bonds and/or carbon-carbon bonds. The carbon-oxygen reductive elimination reaction produces either methyl esters or methyl aryl ethers (anisoles) and L(2)PtMe(2), while the carbon-carbon reductive elimination reaction affords ethane and L(2)PtMe(OR). Choice of reaction conditions allows the selection of either type of coupling over the other. A detailed mechanistic study of the reductive elimination reactions supports dissociation of the OR(-) ligand as the initial step for the C-O bond formation reaction. This is followed by a nucleophilic attack of OR(-) upon a methyl group bound to the Pt(IV) cation to produce the products MeOR and L(2)PtMe(2). C-C reductive elimination proceeds from L(2)PtMe(3)(OR) by initial L (L = PMe(3)) or OR(-) (L(2) = dppe, dppbz) dissociation, followed by C-C coupling from the resulting five-coordinate intermediate. Our studies demonstrate that both C-C and C-O reductive elimination reactions from Pt(IV) are more facile in polar solvents, in the presence of Lewis acids, and for OR(-) groups that contain electron withdrawing substituents.

Molecular structure of Te(OMe)4.ClTe(OMe)3, a model for ligand exchange between Te(IV) centers.

ClTe(OiPr)3 could be prepared from stoichiometric amounts of TeCl4 and Te(OiPr)4, a reaction that requires the exchange of ligands between different Te centers. Ligand redistribution between telluranes was studied, and rapid exchange of -Cl and -OR (R = Me, iPr) ligands in solutions of several binary mixtures of Te(OMe)4, Te(OiPr)4, ClTe(OMe)3, and ClTe(OMe)3, and ClTe(OiPr)3 was established by multinuclear NMR spectroscopy. The solid-state structure of Te(OMe)4.ClTe(OMe)3, the first structurally characterized adduct between different telluranes, was investigated by single-crystal X-ray diffraction. It exhibits a very short Te-O...Te bridge between the two Te centers and additional Te...O and Te...Cl contacts between different adduct molecules. Selected structural parameters of Te(OMe)4.ClTe(OMe)3 are Te1-Cl1 274.6(3), Te1-O13 191.0(5), Te1-O12 194.9(6), Te1-O11 200.9(7), Te1...O24 226.8(5), Te1...O11a 314.2(8), Te2-O21 191.6(5), Te2-O22 198.7(5), Te2-O23 190.1(5), Te2-O24 225.3(5), Te2...O13 307.8(6), and Te2...O22b 269.2(6) pm and Te2-O24...Te1 126.1(2) degrees. Ab initio (MP2/LANL2DZP) geometry optimization of the model compound Te(OH)4.ClTe(OH)3 revealed that the central Te-O...Te bridge is less symmetric and hence weaker in the isolated adduct molecule than in the solid state. The stability of Te(OMe)4.ClTe(OMe)3 toward decomposition in Te(OMe)4 and ClTe(OMe)3 is attributed to the strengths of the short Te-O...Te bridge between Te(OMe)4 and ClTe(OMe)3. On the basis of the molecular structures of Te(OH)4.ClTe(OH)3 and Te(OMe)4.ClTe(OMe)3, a mechanism for the exchange of -OR groups between Te(IV) centers is proposed.

Preparation and Characterization of 1,3-Butadiene and Isoprene Complexes, W2(OCH2tBu)6(diene)(py), and Studies of the Selective Hydrogenation of 1,3-Dienes

In hydrocarbon solutions W2(OCH2tBu)6(py)2, 1, and 1,3-butadiene and isoprene reversibly form adducts W2(OCH2tBu)6(diene)(py), compounds 2 (diene = 1,3-butadiene) and 3 (diene = isoprene). The structures of 1, 2, and 3 are reported. For 1 there is a central O3NW⋮WNO3 core with W−W = 2.334(1) A, and each W atom is in a distorted square-based pyramidal environment with the W⋮W bond in the apical site. In 2 and 3 there is a common W2O6C4N core wherein the diene adopts a novel mode of coordination, μ,η1,η4. One W atom is coordinated to five oxygen atoms and the bridging carbon atom of the μ-diene ligand, while the other is bonded to three oxygen atoms, a pyridine nitrogen, and four carbon atoms of the diene. The structure may be viewed as being derived from a confacial bioctahedron with two bridging OR ligands and one bridging alkyl. In this view the diene can be considered as a μ-metalated π (η3-)allyl or a 2− ligand. The W−W distances are 2.471(1) A in 2 and 2.464(1) A in 3, consistent with a (WW)8+ core. T...

Mapping of reaction pathways by structure correlation methods. A study of the ligand dissociation reaction in quasi-octahedral Re(V) and Tc(V) oxo-complexes.

Intercorrelations among geometrical parameters of a molecular fragment as found in different crystal structures are called structure correlations. Such correlations are believed to represent possible reaction pathways mapping the course of chemical reactions. Pyramidal Oâ–·ML4 oxo-complexes [MRe(V) and Tc(V)] react easily with oxygenated ligands of different basicities (H2O, RO-, ArO-, RCOO-etc.) to give quasi-octahedral Oâ–·ML4OR addition compounds which are often observed in the crystalline state and a relatively large number of structural and spectroscopic data on such complexes are available. Coordination changes from square-pyramidal to quasi-octahedral caused by the approach of the sixth ligand are found to induce systematic variations in the polyhedron geometry which are found to correlate with IR ν(Mâ–·O) stretching frequencies and pKa values of the entering ligands. According to structure correlation methods, each fragment geometry was assumed to represent a point along a single reaction pathway of the dissociation reaction Oâ–·ML4–OR→Oâ–·ML4+OR of the complex associating rather similar Oâ–·ML4 acceptors with a series of OR ligands having quite different donor properties. Assuming that the ligand pKa (or related ΔG°) values can be considered as a measure of the relative thermodynamic stabilities of the complexes, a mathematical model of the reaction pathway is proposed which, on the grounds of the Marcus rate-equilibrium theory, relates activation free energies, thermodynamic stabilities, and geometrical distances from the reaction transition state. The reliability of the model is tested, aposteriori, against experimental values of energies, bond distances and quadratic vibrational force constants.

Formic Acid Modified Ti(OCHMe(2))(4). Syntheses, Characterization, and X-ray Structures of Ti(4)(&mgr;(4)-O)(&mgr;-O)(OFc)(2)(&mgr;-OR)(4)(OR)(6) and Ti(6)(&mgr;(3)-O)(6)(OFc)(6)(OR)(6) (OFc = O(2)CH; OR = OCHMe(2)).

Novel structural arrangements for carboxylic acid modified titanium alkoxides were obtained from stoichiometric reactions between titanium isopropoxide (Ti(OPr(i))(4)) and formic acid (HOFc). The 1:1 reaction in toluene forms the tetranuclear species Ti(4)O(2)(OFc)(2)(OPr(i))(10), 1. The structure of 1 was solved in the triclinic space group P&onemacr; with a = 13.034(3) Å, b = 13.467(2) Å, c = 13.996(2) Å, alpha = 98.79(1) degrees, beta = 104.68(2) degrees, and gamma = 97.82(2) degrees for Z = 2. The general structure of 1 resembles two face-shared [Ti-O](4) cubes with one set of mirror-related titanium atoms removed. The oxygen atoms are represented by one &mgr;(4)-O, one &mgr;-O, and four &mgr;-OR ligands, with the remaining sites filled by two OFc ligands and six terminal alkoxides. Increasing the stoichiometry to 1:2 (Ti/HOFc) leads to the isolation of Ti(6)O(6)(OFc)(6)(OR)(6), 2. The structure of 2 was solved in the monoclinic space group P2(1)/c with a = 8.968(2) Å, b = 26.520(4) Å, c = 20.046(2) Å, and beta = 93.19(1) degrees for Z = 4. The structure of 2 consists of two offset six-membered [Ti-(&mgr;(3)-O)](3) rings joined through Ti-O bonds. The OFc ligands are arranged externally around the central hexagon-prism, oscillating between the top and bottom rings. Compound 1 adopts a very symmetrical arrangement in solution due to the labile OFc ligands and was found to undergo "aging" by a trans-esterification mechanism, the rate of which is enhanced by heating. Compound 2 maintains its solid-state structure in solution.

A study of the photochemical reaction between W 2(OCH 2 tBu) 6P 4. Characterization of the phosphidocluster W 4 (P) 2 (OCH 2 tBu) 10

The photochemical reaction between W 2 (OCH 2 tBu) 6 and P 4 has been studied by NMRspectroscopy in toluene- d 8 and those reactions have been complemented by studies on the benchtop. A complex sequence of reactions is implicated by the formation of a variety of products, oneof which has been observed previously in the thermal reaction between W 2 (OCH 2 tBu) 6L 2 whereL = py and HNMe 2 and P 4, namely W 3 ( μ 3-P) (OCH 2 tBu) 9, while others must arise fromdegradation of OR ligands. A new compound W 4 ( μ 3-P) 2 ( μ-OCH 2 tBu) 4 (OCH 2 tBu) 6 has beenisolated and characterized. Crystal data at −165°C : a = 19.955 (4) , b = 25.222 (3) , c = 12.775 (2) Å, Z = 4 and space group Pcab. The centrosymmetric structure consists of adistorted rhombus of metal atoms having two different W — W edge distances 2.6938 (8) and2.7981 (8) Å and a bonding W — W distance of 2.692 (1) Å between the back-bone W atomsof the planar W 4 butterfly. The outer or wing-tip tungsten atoms are in a square based pyramidalenvironment with respect to the PWO 4 moiety with the W — P bond in the apical position. The O 4 basal plane involves two terminal and two bridging OR groups. The internal or back-bonetungsten atoms are also five coordinate, P 2WO 3, where two bridging OR groups occupy axial andthe two W — P bonds and one W — OR terminal group occupy equatorial sites of a trigonalbipyramid. The cluster geometry is compared with those seen for W 3 ( μ 3-X) (OR) 9, where X = CCH 3 and P, W 4 (O iPr) 12 and W 4 (OEt) 16. The latter contains a central centrosymmetric W 16+ 4cluster as does the present compound and the presence of the distorted rhomboidal geometryobserved in both is proposed to arise from a second order Jahn-Teller distortion.