J. Selbin

Electronic structure, spectra and magnetic properties of oxycations—IV ligation effects on the infra-red spectrum of the vanadyl ion

Abstract Vanadium-oxygen multiple bond stretching frequencies have been tabulated for fifty-one oxovanadium(IV) and three oxovanadium(V) compounds. The spread of the values may be summarized by v = 985 ± 50 cm − , which corresponds to a force constant spread, k = 7·0 ± 0·7 mdyne/ A . The frequency shifts are used to establish several short ligand series, and they are interpreted in terms of σ and π contributions to the vanadium-ligand bond. It is concluded that frequency decreases are directly and primarily caused by decreased pπ - dπ donation from oxygen to metal and to increased electrostatic repulsion of the vanadium and oxygen species. The effect may be written Δ v = − σ(L → M) − π ∥ (L → M) − π(L → M) where it is estimated that the relative importance of terms is 8:2:1, and it is assumed that the M → L donation is negligible for the d1 vanadium(IV).

Synthesis of homodinuclear macrocyclic complexes of lanthanides and phenolic schiff bases

Abstract Successful syntheses of the first examples of homodinuclear macrocyclic lanthanide complexes are reported. The complexes were obtained as compounds of the 2:2 Schiff base formed by condensing 2,6-diformyl-p-cresol and triethylenetetramine (L7) by a template procedure using lanthanide nitrates and perchlorates. When reactant methanolic solutions were concentrated the complexes were deposited as yellow or orange microcrystalline precipitates, Ln2L7(NO3)4sigma; nH2O or Ln2L7(NO3)4tau; x(OH)x, x = 1 or 2, whereas solutions diluted three times deposited complexes as flaky off-white crystalline precipitates of light lanthanides. The orange Ln2L7(NO3)2(OH)2 complexes can be converted in quantitative yield to the off-white flaky form of Ln2L7(NO3)4sigma; nH2O by refluxing them in methanolic solution containing triethylenetetramine and a three-fold excess of Ln(NO3)3. The complexes were characterized by elemental analysis, fast atom bombardment mass spectrometry, UV-Vis and infrared spectroscopy and thermogravimetry. Interesting and mostly new polyatomic oxo clusters, e.g. Ln2O3+, Ln3O4+, Ln4O6+, Ln5O7+, were dominant in the mass spectra but are treated in detail elsewhere.

Electronic Structure, Spectra, and Magnetic Properties of Oxycations. V. The Electronic Spectra of Some Vanadyl Complexes

The observation of four low‐intensity absorption bands below 20 000 cm—1 for the C2v molecule vanadyl bisacetylacetonate is verified and four such bands are reported for C4v vanadyl species. The likelihood of these four bands representing the whole of the d—d series and the plausibility of understanding the splitting of the crystal field levels in the C4v molecules in terms of a spin—orbit coupling mechanism is discussed. The time course of the oxidation V(iv) to V(v) has been followed spectrally; the results identify the d—d series of transitions and suggest that the ∼25 000 cm—1 transition is charge transfer in nature. The paper should be regarded as a critique of two alternative energy‐level schemes of the vanadyl ion, that scheme which is proposed herein being heavily favored by the arguments presented.

Cyclometallation. Palladium 2-arylpyridine complexes

Abstract A number of new cyclometallated compounds of palladium with 2-arylpyridines have been synthesized and characterized. Palladium acetate has been proven to be a more useful starting material than Li2PdCl4, since the resulting acetato-bridged dimers, [Pd(OAc)(2-arylpyridine)]2, unlike the chloro-bridged dimers, are conveniently soluble in common organic solvents. The effect of varying substituents on the aryl nucleus supports the concept that after initial N-complexation, the 2-position of the aryl nucleus undergoes electrophilic attack by the palladium atom. Detailed NMR studies of the soluble acetato compounds showed that the 6-heteroaryl and “ortho” to the Pd-C bond protons in the complexes are shifted (≈0.75 and 0.5 ppm, respectively) upfield from the ligand position. These shielding effects are believed to be due primarily to through-space interactions of overlying aromatic rings and secondarily to through-bond (Pd-to-ligand) effects. Both acetato- and chloro-bridged dimers will react with certain ligands to give mononuclear species containing the intact cyclometallated ligand.

Magnetic and luminescence characteristics of dinuclear complexes of lanthanides and a phenolic schiff base macrocyclic ligand

Abstract The magnetic and luminescence characteristics of trimorphic homodinuclear macrocyclic complexes of lanthanides and a 2:2 phenolate Schiffs base L, derived from 2,6-diformyl- p -cresol and triethylenetetramine were determined. The complexes of Pr 3+ exhibit non-Curie-Weiss temperature dependent magnetic susceptibilities for which satisfactory fits to an axial relationship depends on crystal field splitting and a weak binuclear Pr 3+ Pr 3+ antiferromagnetic interaction. The exchange interaction parameters are z J ′ = −2.2, −4.4 and −7.0 cm −1 for ‘off-white’ Pr 2 L(NO 3 ) 4 ·2H 2 O, ‘yellow’ Pr 2 L(NO 3 ) 4 , and ‘orange’ Pr 2 L(NO 3 ) 2 (OH) 2 , respectively. In contrast, magnetic susceptibilities of the Ln 2 L(NO 3 ) 3 (OH) complexes (Ln = Dy, Ho) follow Curie-Weiss behavior over the entire temperature range (6 K to 300 K). The complexes of closed shell ions La 3+ , Lu 3+ , Y 3+ and those of the half filled shell ion Gd 3+ exhibit a strong ligand fluorescence in the 450 nm to 650 nm range with decay times at 77 K of 5–8 ns for Ln≠Gd or 2–4 ns for Ln = Gd. The complexes of Gd 3+ also exhibit a phosphorescence at 600 nm (decay time ∼ 200 μs). The complexes containing Ce 3+ , Eu 3+ , Tb 3+ and Er 3+ show very weak ligand luminescence indicative of effective quenching processes. Sensitized emission from the lanthanide ion is observed only with the Eu 3+ complexes ( 5 D o → 7 F j transitions). The emission lifetimes are on the order of 250 μs in the pure Eu 3+ complexes. The emission decay curves from dilute samples of Eu 3+ in ‘off-white’ La 2 L(NO 3 ) 4 n H 2 O show a noticeable rise time as well as a biphasic decay (fast component ∼ 400 μs; slow component ∼ 2500 μs). The luminescing states of L and Eu 3+ have a common excitation spectrum which is similar to the electronic absorption spectrum of L indicating that ligand-to-metal ion energy transfer processes are dominant. Overall the result if this study suggest that the spectral properties of the complexes are determined by the coordination mode of the lanthanide ions to the Schiff base portion of macrocyclic ligand.

Solvent effects on the absorption spectrum of a vanadyl complex: A new sensitive parameter for ranking ligands

Abstract The effects of thirteen pure solvents on the optical absorption spectrum of vanadyl bisacetyl-acetonate have been observed in the 8000−3500 A region. Three low intensity bands allow the calculation of the three parameters Dq, Ds , and Dt . As a solvent molecule adds to the sixth position on the vanadium, it alters slightly the strong axial perturbation of the vanadyl oxygen. This effect is most sensitively recorded in a new parameter, the difference between the first and second bands since the first band is red-shifted and the second band is blue-shifted. The solvents are ranked as follows by this parameter: H 2 O ⪢ MeOH > pyridine > n-propylamine > EtOH > DMSO > DMF ⪢ CH 3 CN > CH 3 NO 2 > CHCl 3 > CS 2 ∼ C 6 H 6 > CCl 4 .

V. Molecular structures of acetate-bridged dimers of a 2-arylpyridine and a 2,6-diarylpyridine cyclometallated by palladium(II)

Abstract The crystal and molecular structures of dimeric cyclopalladated acetato(OAc)-bridged complexes of a 2-arylpyridine ( 1 ) and a 2,6-diarylpyridine ( 2 ), [Pd( 1 )OAc] 2 ( 3 ) and [Pd( 2 )OAc] 2 ( 4 ) have been determined by X-ray diffraction. In each. two planar acetates, nearly perpendicular to each other, cis -bridge two Pd II atoms, while the other two square planar sites on each metal are occupied by the cyclometallated aryl-substituted pyridine. The two metal coordination planes are in near parallel registry with one another, giving a “boat” molecule shape of exact ( 3 or near ( 4 ) two-fold symmetry. The non-bonding Pd⋯Pd distance (δ) strongly correlates with the angle (α) subtended by the metal coordination planes, which in turn seems related to the steric requirements of the non-bridging ligands. In 3 , with the planar nitrophenylpyridine ligand, α 25.7° and δ 2.822(6) A, the shortest Pd⋯Pd distance yet observed in dimers of this type. The PdC lengths, 1.94 A in 3 and 1.944 A in 4 , are significantly shorter than the calculated length, 2.05 A, suggesting partial multiple bond character in these shortest PdC (aryl) bonds so far observed. PdN bond lengths are near expected single bond lengths. Failure of trans -dimetallation to occur in 4 , analogous to that recently observed with certain 2,6-dialkylpyridines or to the tridentate behavior of terpyridine in [Pd terpy X] + compounds, appears due to a combination of the shortened PdC bond in the first-formed metallated ring and the larger constraining angles (near 120° for sp 2 carbons) in the remaining (unmetallated) ring.

Spectral studies of β-ketoenolate complexes of oxovanadium(IV)☆

Abstract Spectra in the 8000–30,000 cm−1 region for six β-ketoenolate complexes of VO2+ have been recorded in (a) reflectance, (b) KBr pellets and nujol mulls at both 298 and 77°K, (c) many non-coordinating solvents, and (d) several coordinating solvents. The first vapor phase spectra ever obtained are reported for two of the complexes. The data are analyzed and a new totally empirical ordering of the vanadium d-orbitals is presented. Earlier theoretical models have failed to take account of the important angle between the vanadyl oxygen and the equatorial ligand atoms, which has been assumed to be 90°, but which actually never is that low.

Complexes of oxovanadium(IV)

Abstract The preparation and partial characterization of a number of new complexes of the oxovanadium(IV) ion are reported. Cationic, neutral and anionic complexes in which the vanadium(IV) is either five- or six-co-ordinated are included. Conductivity studies have generally allowed the assignment of groups to the first co-ordination sphere and the presence of the vanadium-oxygen multiple bond in each compound has been established by infrared spectra.

Ligational effects in vanadyl complexes

Abstract Results of a study of the shift in the vanadium-oxygen multiple bond stretching frequency of VO(acac) 2 as a function of (a) solvent, (b) neutral ligand and (c) anionic ligand, are reported and discussed. Ligands were found to fall into three rather distinct groups. Those producing a shift of 48 ± 3 cm −1 include a number of primary amines, pyridine, ammonia and the moderately strong field anions NCS − and CH 3 COO − . Those producing no shift include ligands which owe much of their ligational strength to metal to ligand π-bonding, such as diphenylsulphide and triphenylphosphine, and weak field anions such as Cl − , ClO 4 − and NO 3 − . Certain ligands, such as tetrahydrofuran and p -dioxane, produce smaller shifts of 23 ± 1 cm −1 and the adducts which they form with the vanadyl complex have relatively low formation constants. Frequency shifts caused by mass effects alone have no relevance in these studies.