Solid state13C NMR spectra of metal carbonyl clusters

The solid state¹³C NMR spectra of four¹³CO enriched carbonyl clusters having a tri-iron metallic core have been analyzed to provide structural and dynamic information. In Fe₃(CO)₁₂ (1), the high temperature spectra suggest the occurrence of large amplitude motions of the CO groups around their position at the vertexes of the coordination polyhedron in addition to the motion involving the Fe₃-triangle previously detected in the VT-¹³C MAS spectra.¹³C and³¹P NMR data of Fe₃(CO)₁₁PPh₃ (2) indicates the presence of one molecule in the asymmetric unit in apparent disagreement with the previously reported X-ray data. Furthermore, we show that structural information can be obtained from the chemical shift tensor components readily available from the analysis of the spinning sideband manifold.     

Natural abundance 17O NMR spectroscopy of heteroaromatic Nitro compounds

The ¹⁷O chemical shift data, at natural abundance, for selected nitroquinolines, nitroindoles, nitroindazoles and nitrothiophenes are reported. In the absence of a peri or a lone-pair repulsion effect, the nitroquinolines' chemical shifts differ little from those of their carbocyclic analogs. However, the signal for 5-nitroquinoline, 2 , is deshielded by 25 ppm compared to 6-nitroquinoline, 1 , and the ¹⁷O nucleus in 8-nitroquinoline, 3 , is deshielded by 49 ppm compared to that in 1 . Both these shifts are attributed to rotation of the nitro group from the plane of the heteroaromatic ring arising from peri hydrogen interaction and lone pair repulsion, respectively. The signals for nitro groups on electron excessive ring systems (e.g., indoles and thiophenes) are shielded relative to corresponding ones in electron deficient heterocyclic ring system analogs. The chemical shifts for the π-excessive systems are interpreted in terms of electronic effects.     

Vibrational spectra of bridging carbonyl groups in transition metal carbonyl clusters

The spherical harmonic model (SHM), previously used for the analysis of the terminal nu(CO) vibrations of transition metal carbonyl clusters, is applied to the corresponding bridging CO modes. The model is applicable, although the spectra show a greater sensitivity to the molecular geometry than is the case for their terminal counterparts. The reasons for this sensitivity are discussed. When both micro(2) and micro(3) CO groups are present in a molecule, a spectral distinction may not be apparent.     

17O NMR spectra of alpha-diesters

17O NMR spectra of title compounds were measured at natural abundance in acetonitrile solutions. Intercarbonyl dihedral angles have been estimated by molecular mechanics, which show invariance except in one case. Because of this invariance, contrary to other alpha-dicarbonyl compounds, a correlation between chemical shifts and dihedral intercarbonyl angles could not be developed. Spectroscopic and computational results allowed us to evaluate other conformational features.     

Natural abundance 17O NMR study of 2- and 4-substituted benzoyl chlorides

Natural abundance ¹⁷O NMR chemical shift data for 17 ortho and para benzoyl chlorides recorded in acetonitrile at 75°C are reported. ¹⁷O NMR data for the para substituted benzoyl chlorides are correlated with ¹⁷O NMR data for similarly substituted acetophenones and methyl benzoates. The ¹⁷O NMR signals for ortho isomers are downfield (ca 30 ppm) from their para isomers; the downfield shifts are consistent with torsion angle change. The ¹⁷O NMR data for the para isomers gave good correlations with σ⁺ constants and with dual substituent parameters (DSP).     

Arsido iron carbonyl clusters

Summary The XAsFe₄(CO)₁₄ (X = Cl, Br) clusters have been prepared and characterised and a new synthesis under mild conditions has been found for As₂Fe₃(CO)₉. The latter complex has at least two isomeric structures in solution.A preliminary account of this work was given in a lecture.The terms pnicogen and pnictide have been introduced as group names for the group Va family of elements analogous to the use of chalcogen and chalcogenid as group names for the group VIa elements.     

Natural abundance 17O NMR spectroscopy of heterocyclic N-oxides and Di N-oxides. Structural effects

The ¹⁷O chemical shift data for a series of azine N-oxides, diazine N-oxides and di-N-oxides at natural abundance are reported. Isomeric methyl substituted quinoline N-oxides exhibited chemical shifts which are interpreted in terms of electronic and compressional effects. The ¹⁷O chemical shift for 8-methylquinoline N-oxide (370 ppm) is deshielded by 25 ppm more than predicted, based upon electronic considerations. The ¹⁷O chemical shift for the N-oxide of 8-hydroxyquinoline (289 ppm) is substantially shielded as a result of intramolecular hydrogen bonding. The relative ¹⁷O chemical shifts for diazine N-oxides of pyrazine, pyridazine and pyrimidine follow predictions based on back donation considerations. Because of solubility limitations, spectra of only two N,N′-dioxides were obtained. The chemical shift of benzopyrazine di N-oxide in acetonitrile was shielded by 18 ppm compared to that of its mono N-oxide.     

Electron-counting procedures in clusters of the iron triad

Until recently all known high-nuclearity carbonyl clusters of the iron triad had metal core structures based on an octahedron with formulations corresponding to seven skeletal electron pairs (S = 7), in agreement with the predictions of polyhedral skeletal electron counting procedures. The isolation of the monoanion [HOs₈(CO)₂₂]^?, which also has S = 7, first broke this pattern, as it has a fused tetrahedral core geometry. The extension of skeletal electron counting procedures to include multiple “capping” of incomplete polyhedra permits a simple rationalisation of this structure as a tetra-capped arachno-octahedral. This approach has allowed the development of a matrix correlation for the main structural types observed in this triad. Structural relationships between the first members of a new class of high-nuclearity clusters based on the fusion of two are also discussed.     

The nu(CO) vibrational spectra of planar transition metal carbonyl clusters

The nu(CO) vibrational spectra of planar transition cluster carbonyls containing M(CO)(4) groups are studied. It is possible to anticipate qualitatively, both for the infrared and Raman, the band intensity changes associated with increasing metallic nature of the cluster. These enable a unification of the band patterns shown by the species reported. As for (idealized) spherical clusters, the spherical harmonic model (SHM), suitably modified, becomes of more general applicability as cluster size increases, although for smaller species the tensor harmonic model (THM) makes a contribution.     

Characterization of oxide supported metal carbonyl clusters

The chemisorption of [Ma₃(CO)_{1 2}] on silica (M = Ru and Os) and alumina (M = Os) has been studied by vibrational and X-ray absorption spectroscopies making close comparisons with model compounds. The results indicate that the first chemisorption species observed has the form [M₃H(CO)₁₀(O---O)]; the bridging hydride was observed directly for the silica systems as evidenced by the M-H-M bending vibration in the i.r. Also consistent with this structure are the EXAFS analysis of the Ru/SiOz material. This indicated an essentially equilateral ruthenium triangle and coordination to oxygen. The published low frequency Raman data on the Os/Al2Oa product was shown to match most closely with that of model compounds with a bidentate oxygen donor ligand (acac or O2CR). The tethered cluster [O_s3H₂(CO)₉(PPh₂C₂H₄SIL)] was found to be a relatively short lived species on a silica surface. Under ambient conditions it reacts further and the i.r., EXAFS and ³¹P NMR data of this species suggest that the two osmium atoms not coordinated to the tethering phosphine become involved with a bidentate site from the surface.     

17O NMR spectra of some butyltin(IV) acetates

The ¹⁷O NMR spectra of ¹⁷O isotope‐enriched tributyltin(IV) acetate (1) and dibutyltin(IV) diacetate (2) were recorded in various solvents over wide temperature ranges. Only a single ¹⁷O signal was observed for both oxygen atoms of the —COOSn—groups under these experimental conditions, in both non‐coordinating and coordinating solvents. The ¹⁷O NMR spectra of tert‐butyl acetate (3) were obtained for comparison. Copyright © 2002 John Wiley & Sons, Ltd.     

Gas-phase IR spectroscopy of anionic iron carbonyl clusters

The first gas-phase vibrational spectra are presented for several anionic iron carbonyl clusters, ranging in size from Fe(CO)4- to Fe5(CO)14- in the CO-stretching region (1600-2100 cm-1). The experimental spectra provide some immediate structural information about the clusters in the form of low-wavenumber (1750-1850 cm-1) bands marking the presence of bridging carbonyl ligands (mu2-COs). Supporting DFT calculations are presented for the smaller clusters (<3 Fe atoms) and give good agreement with the experimental data, allowing structural assignments for these cases. The Fe2(CO)7- spectrum suggests a structure lacking bridging carbonyl ligands, in agreement with the DFT results. For the case of Fe2(CO)8-, there are two possible structures based on the calculations, both with and without bridging carbonyls. The presence of a low-frequency band ( approximately 1770 cm-1) in the experimental spectrum conclusively demonstrates the existence of the bridged form. The ramifications of these data for metal-metal bonding in the clusters are also considered.     

Regularities of thermal decay of carbonyl chalcogenide metal clusters

The thermal decay of 19 individual carbonyl homo- and heterochalcogenide clusters with different M/X ratios (M = Fe, Mn, Pt, Cr, W, Mo, Re, Ru; X = S, Se, Te) was studied by differential scanning calorimetry and thermogravimetry. The process is stepwise and occurs at relatively low temperatures (100—350 °C). The general fact of incomplete removal of carbon monoxide (due to the formation of carbide and oxide impurities) during thermolysis of narbonyl chalcogenide clusters with the M : X ratio greater than 1 was elucidated. Conversely, when M : X 1 (or at any M/X ratio for clusters containing methylcyclopentadienyl groups), pure metal chalcogenides are formed.     

Ab initio MO calculations and 17O NMR at natural abundance of para-substituted acetophenones

¹⁷O NMR chemical shifts and calculated (ab initio MO theory) electron densities are reported for a series of para-substituted acetophenones, X? C₄H₆? COCH₃, where X = NH₂, OCH₃, F, Cl, CH₃, H, COCH₃, CN, NO₂. The ¹⁷O shifts are very sensitive to the para substituent and cover a range of some 51 ppm. Donors induce upfield shifts and acceptors downfield shifts. The substituent chemical shifts (SCS) correlate precisely with σ_I and σ_R⁺ using the Dual Substituent Parameter (DSP) method. The derived transmission coefficients ρ_I and ρ_R indicate that polar and resonance mechanisms contribute approximately equally to the observed substituent effects. The shifts also correlate well with calculated π-electron densities (slope = 1500 ppm per electron) confirming their electronic origin. λ values are also reported, and the role of the average excitation energy, ΔE, in determining ¹⁷O SCS values is discussed. It is concluded that variations in ΔE are minor and that the local Δ-electron density is the dominant feature controlling ¹⁷O SCS values.     

The influence of shells,electron thermodynamics,and evaporation on the abundance spectra of large sodium metal clusters

Measurements of the mass abundance spectra of sodium clusters containing up to 600 atoms are presented. The clusters are produced in a seeded supersonic expansion of Ar or Kr gas, and the spectra are obtained by a time-of-flight technique. The sawtooth features in the spectra are interpreted as evidence of a regular spherical shell structure with magic numbers,N ₀, scaling approximately with the cube root of the number of sodium atoms. Altogether twelve shell closings are observed,N ₀=2, 8, 20, 40, 58, 92, 138, 196, 260, 344, 440 and 558. There is also a pronounced odd-even staggering all the way up toN=70. The experimentally observed intensity changes for the clusters around the magic numbers are discussed in terms of the electronic free energy,F(N), calculated at finite temperature, and the second differences of the free energy Δ₂ F(N)=F(N?1)?2F(N)+F(N+1). The processes behind the non-uniform abundance distributions, and the thermodynamics of finite electron systems with non-uniform level spacings are discussed on this basis.     

The indirect location of hydride ligands in metal carbonyl clusters

A semi-empirical method of predicting the location of hydride ligands in metal carbonyl cluster compounds using X-ray structural information is described. The hydride positions thus derived are compared with those obtained from neutron diffraction experiments and predictions made for previously ambiguous cases.     

Photoabsorption spectra ofs 1-electron metal clusters

Collinear laser/molecular beam photodepletion spectroscopy has developed into a powerful tool for the determination of electronic absorption in neutral alkali metal clusters free of perturbing matrix effects. We briefly discuss the method, present data on selected Na _x and Li _y Na _x?y ,x≤20, contrast measurements with the known optical response of larger colloidal metal clusters, compare microcluster spectra with the predictions of various models and speculate on future experimental and theoretical developments in this field.     

15N and17O NMR spectra of vinyl ethers of pyridine and quinoline

Conclusions ¹⁵N (¹⁴N) and¹⁷O NMR spectra have been obtained on vinyl ethers of pyridine and quinoline, with unenriched samples. The principal factor determining the chemical shifts is p- conjugation of the unshared pairs of the heteroatom with the unsaturated fragments of the molecule; an additional contribution in the case of the¹⁵N signals comes from interaction of the nitrogen atom with a proton through space.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 4, pp. 769–773, April, 1987.