Trinuclear and Tetranuclear Ruthenium Carbonyl Nitrosyls: Oxidation of a Carbonyl Ligand by an Adjacent Nitrosyl Ligand.

Trinuclear and tetranuclear ruthenium carbonyls of the types Ru3(CO)n(NO)2, Ru3(N)(CO)n(NO), Ru3(N)2(CO)n, Ru3(N)(CO)n(NCO), Ru3(CO)n(NCO)(NO), Ru4(N)(CO)n(NO), Ru4(N)(CO)n(NCO), and Ru4(N)2(CO)n related to species observed experimentally in the chemistry of Ru3(CO)10(µ-NO)2 have been investigated using density functional theory. In all cases, the experimentally observed structures have been found to be low-energy structures. The low-energy trinuclear structures typically have a central strongly bent Ru-Ru-Ru chain with terminal CO groups and bridging nitrosyl, isocyanate, and/or nitride ligands across the end of the chain. The low-energy tetranuclear structures typically have a central Ru4N unit with terminal CO groups and a non-bonded pair of ruthenium atoms bridged by a nitrosyl or isocyanate group.

Tris(Butadiene) Compounds versus Butadiene Oligomerization in Second-Row Transition Metal Chemistry: Effects of Increased Ligand Fields.

The geometries, energetics, and preferred spin states of the second-row transition metal tris(butadiene) complexes (C4H6)3M (M = Zr-Pd) and their isomers, including the experimentally known very stable molybdenum derivative (C4H6)3Mo, have been examined by density functional theory. Such low-energy structures are found to have low-spin singlet and doublet spin states in contrast to the corresponding derivatives of the first-row transition metals. The three butadiene ligands in the lowest-energy (C4H6)3M structures of the late second-row transition metals couple to form a C12H18 ligand that binds to the central metal atom as a hexahapto ligand for M = Pd but as an octahapto ligand for M = Rh and Ru. However, the lowest-energy (C4H6)3M structures of the early transition metals have three separate tetrahapto butadiene ligands for M = Zr, Nb, and Mo or two tetrahapto butadiene ligands and one dihapto butadiene ligand for M = Tc. The low energy of the experimentally known singlet (C4H6)3Mo structure contrasts with the very high energy of its experimentally unknown singlet chromium (C4H6)3Cr analog relative to quintet (C12H18)Cr isomers with an open-chain C12H18 ligand.

[Prediction of PM10 Concentration in Dry Bulk Ports Using a Combined Deep Learning Model Considering Feature Meteorological Factors].

Accurate prediction of PM10 concentration is important for effectively managing PM10 exposure and mitigating health and economic risks posed to humans in dry bulk ports. However, accurately capturing the time-series nonlinear variation characteristics of PM10 concentration is challenging owing to the specific intensity of port operation activities and the influence of meteorological factors. To address such challenges, a novel combined deep learning model （CLAF） was proposed, merging cascaded convolutional neural networks （CNN）, long short-term memory （LSTM）, and an attention mechanism （AM）. This integrated model aimed to forecast hourly PM10 concentration in dry bulk ports. First, using the random forest characteristic importance algorithm, the distinct meteorological factors were identified among the selected five meteorological factors. These selected factors were incorporated into the prediction model along with the PM10 concentration. Subsequently, the CNN layer was employed to extract high-dimensional time-varying features from the input variables, while the LSTM layer captured sequential features and long-term dependencies. In the AM layer, different weights were assigned to the output components of the LSTM layer to amplify the effects of important information. Finally, three evaluation metrics were applied to compare the performance of the CLAF model with three basic models and three commonly used prediction models. Real-case data was collected and used in this study. Comparison results demonstrated that considering the meteorological factors could improve the prediction accuracy and fitting performance of PM10 concentration in ports. The CLAF model reduced the mean absolute error statistic by 13.92%-56.9%, decreased the mean square error statistic by 45.99%-81.02%, and improved the goodness-of-fit statistic by 3.2%-15.5%.

Molybdenum-molybdenum multiple bonding in homoleptic molybdenum carbonyls: comparison with their chromium analogues.

The binuclear molybdenum carbonyls Mo(2)(CO)(n) (n = 11, 10, 9, 8) have been studied by density functional theory using the BP86 and MPW1PW91 functionals. The lowest energy Mo(2)(CO)(11) structure is a singly bridged singlet structure with a Mo-Mo single bond. This structure is essentially thermoneutral toward dissociation into Mo(CO)(6) + Mo(CO)(5), suggesting limited viability similar to the analogous Cr(2)(CO)(11). The lowest energy Mo(2)(CO)(10) structure is a doubly semibridged singlet structure with a Mo═Mo double bond. This structure is essentially thermoneutral toward disproportionation into Mo(2)(CO)(11) + Mo(2)(CO)(9), suggesting limited viability. The lowest energy Mo(2)(CO)(9) structure has three semibridging CO groups and a Mo≡Mo triple bond analogous to the lowest energy Cr(2)(CO)(9) structure. This structure appears to be viable toward CO dissociation, disproportionation into Mo(2)(CO)(10) + Mo(2)(CO)(8), and fragmentation into Mo(CO)(5) + Mo(CO)(4) and thus appears to be a possible synthetic objective. The lowest energy Mo(2)(CO)(8) structure has one semibridging CO group and a Mo≡Mo triple bond similar to that in the lowest energy Mo(2)(CO)(9) structure. This differs from the lowest energy Cr(2)(CO)(8) structure, which is a triply bridged structure. A higher energy unbridged D(2d) Mo(2)(CO)(8) structure was found with a very short Mo-Mo distance of 2.6 Å. This interesting structure has two degenerate imaginary vibrational frequencies. Following the corresponding normal modes leads to a Mo(2)(CO)(8) structure, lying ~5 kcal/mol above the global minimum, with two four-electron donor bridging CO groups and a Mo═Mo distance suggesting a formal double bond. All of the triplet Mo(2)(CO)(n) (n = 10, 9, 8) structures were found to be relatively high energy structures, lying at least 22 kcal/mol above the corresponding global minimum. The singlet-triplet splittings for the Mo(2)(CO)(n) (n = 10, 9, 8) structures are significantly higher than those of the Cr(2)(CO)(n) analogues. The Mo-Mo Wiberg bond indices confirm our assigned bond orders based on predicted bond distances.

Time-Frequency Analysis of Particulate Matter (PM10) Concentration in Dry Bulk Ports Using the Hilbert-Huang Transform.

To analyze the time-frequency characteristics of the particulate matter (PM10) concentration, data series measured at dry bulk ports were used to determine the contribution of various factors during different periods to the PM10 concentration level so as to support the formulation of air quality improvement plans around port areas. In this study, the Hilbert-Huang transform (HHT) method was used to analyze the time-frequency characteristics of the PM10 concentration data series measured at three different sites at the Xinglong Port of Zhenjiang, China, over three months. The HHT method consists of two main stages, namely, empirical mode decomposition (EMD) and Hilbert spectrum analysis (HSA), where the EMD technique is used to pre-process the HSA in order to determine the intrinsic mode function (IMF) components of the raw data series. The results show that the periods of the IMF components exhibit significant differences, and the short-period IMF component provides a modest contribution to all IMF components. Using HSA technology for these IMF components, we discovered that the variations in the amplitude of the PM10 concentration over time and frequency are discrete, and the range of this variation is mainly concentrated in the low-frequency band. We inferred that long-term influencing factors determine the PM10 concentration level in the port, and short-term influencing factors determine the difference in concentration data at different sites. Therefore, when formulating PM10 emission mitigation strategies, targeted measures must be implemented according to the period of the different influencing factors. The results of this study can help guide recommendations for port authorities when formulating the optimal layout of measurement devices.

The crucial roles of guest water in a biocompatible coordination network in the catalytic ring-opening polymerization of cyclic esters: a new mechanistic perspective.

The ring-opening polymerization (ROP) of cyclic esters/carbonates is a crucial approach for the synthesis of biocompatible and biodegradable polyesters. Even though numerous efficient ROP catalysts have been well established, their toxicity heavily limits the biomedical applications of polyester products. To solve the toxicity issues relating to ROP catalysts, we report herein a biocompatible coordination network, CZU-1, consisting of Zn4(µ4-O)(COO)6 secondary building units (SBUs), biomedicine-relevant organic linkers and guest water, which demonstrates high potential for use in the catalytic ROP synthesis of biomedicine-applicable polyesters. Both experimental and computational results reveal that the guest water in CZU-1 plays crucial roles in the activation of the Zn4(µ4-O)(COO)6 SBUs by generating µ4-OH Brønsted acid centers and Zn-OH Lewis acid centers, having a synergistic effect on the catalytic ROP of cyclic esters. Different to the mechanism reported in the literature, we propose a new reaction pathway for the catalytic ROP reaction, which has been confirmed using density functional theory (DFT) calculations, in situ diffuse reflectance IR Fourier transform spectroscopy (DRIFTS), and matrix-assisted laser desorption/ionization time-of-flight mass spectroscopy (MALDI-TOF MS). Additionally, the hydroxyl end groups allow the polyester products to be easily post-modified with different functional moieties to tune their properties for practical applications. We particularly expect that the proposed catalytic ROP mechanism and the developed catalyst design principle will be generally applicable for the controlled synthesis of biomedicine-applicable polymeric materials.

Perfluorinated polycyclic aromatic hydrocarbons: anthracene, phenanthrene, pyrene, tetracene, chrysene, and triphenylene.

The properties of perfluoroanthracene (1-C(14)F(10)), perfluorophenanthrene (2-C(14)F(10)), perfluoropyrene (C(16)F(10)), perfluorotetracene (1-C(18)F(12)), perfluorochrysene (2-C(18)F(12)), and perfluorotriphenylene (3-C(18)F(12)) and their radical anions have been studied using density functional theory (DFT). Three measures of neutral-anion energy separations reported in this work are the adiabatic electron affinity (EA(ad)), the vertical electron affinity (EA(vert)), and the vertical detachment energy (VDE). The vibrational frequencies of these perfluoro PAHs and their radical anions are also examined. The predicted adiabatic electron affinities (DZP++ B3LYP) are: 1.84 eV, 1-C(14)F(10); 1.41 eV, 2-C(14)F(10); 1.72 eV, C(16)F(10); 2.39 eV, 1-C(18)F(12); 1.83 eV (C(i) symmetry) and 1.88 eV (C(2) symmetry), 2-C(18)F(12); and 1.69 eV, 3-C(18)F(12). The perfluorotetracene is clearly the most effective electron captor. Perfluorophenanthrene, perfluoropyrene, perfluorochrysene, and perfluorotriphenylene, as well as their radical anions deviate from planarity. For example, the nonplanar perfluorochrysene structures are predicted to lie 7-13 kcal/mol below the pertinent C(2h) stationary points.

Recombinant virus-like particles presenting IL-33 successfully modify the tumor microenvironment and facilitate antitumor immunity in a model of breast cancer.

Recently, interleukin (IL)-33 has been closely associated with a variety of clinical cancers. IL-33 presents both protumorigenic, and less frequently, antitumorigenic functions depending on disease conditions. IL-33 signaling appears to be a possible target for the treatment of applicable tumor diseases. This study aimed to develop an effective approach to intervene in IL-33 functioning in tumors and reveal the immunotherapeutic potential of anti-IL-33 active immunization. Recombinant truncated hepatitis B virus core antigen (HBcAg), presenting mature IL-33 molecules on the surface of virus-like particles (VLPs), was prepared and used to immunize BALB/c mice in a model of murine 4T1 breast cancer. The immunization was performed through either a preventive or therapeutic strategy in two separate studies. Anti-IL-33 immunization with VLPs elicited a persistent and highly titrated specific antibody response and significantly suppressed orthotopic tumor growth in the preventive study and lung metastasis in both studies. The underlying mechanisms might include promoting tumor-specific Th1 and CTL-mediated cellular responses and the expression of the effector molecule interferon-γ (IFN-γ), suppressing T-helper type 2 (Th2) responses, and significantly reducing the infiltration of immunosuppressive Treg (regulatory T) cells and myeloid-derived suppressor cells (MDSCs) into tumor tissues in the immunized mice. In conclusion, anti-IL-33 active immunization employing recombinant VLPs as an antigen delivery platform effectively modified the tumor microenvironment and promoted antitumor immunity, indicating the potential of this approach as a new and promising immunotherapeutic strategy for the treatment of cancers where IL-33 plays a definite protumorigenic role. STATEMENT OF SIGNIFICANCE: Interleukin (IL)-33 is closely associated with a variety of clinical cancers. IL-33 signaling appears to be a possible target for the treatment of applicable tumor diseases. Recombinant truncated hepatitis B virus core antigen (HBcAg), presenting mature IL-33 molecules on the surface of virus-like particles (VLPs), was prepared and used to immunize BALB/c mice in a model of murine 4T1 breast cancer. The immunization was performed through either a preventive or therapeutic strategy in two separate studies. Anti-IL-33 immunization with VLPs elicited a persistent and highly titrated specific antibody response and significantly suppressed orthotopic tumor growth and lung metastasis in both studies. Furthermore, anti-IL-33 active immunization employing recombinant VLPs as an antigen delivery platform effectively modified the tumor microenvironment and promoted antitumor immunity, indicating its potential as a new and promising immunotherapeutic strategy for the treatment of cancers where IL-33 plays a definite protumorigenic role.

Synergy effects of Polyinosinic-polycytidylic acid, CpG oligodeoxynucleotide, and cationic peptides to adjuvant HPV E7 epitope vaccine through preventive and therapeutic immunization in a TC-1 grafted mouse model.

Cross-talk by pattern recognition receptors may facilitate the maturation of dendritic cells and fine tune the immune response. Thus, the inclusion of ligands agonistic to multiple receptors in a vaccine formula may be an effective strategy to elicit robust antitumor cellular immunity. We tested the adjuvant effects and possible synergy of CpG (CpG oligodeoxynucleotide), Poly I:C (polyinosinic-polycytidylic acid) and the cationic peptide Cramp (cathelicidin-related antimicrobial peptide) formulated in a DOTAP (1,2-dioleoyl-3-trimethylammonium-propane) liposomal HPV E7 epitope vaccine on a TC-1 grafted mouse model. The vaccine formulations were administered both preventively and therapeutically. Based on our results, both CpG and Poly I:C-adjuvanted vaccines abolished tumor development in a preventive trial and significantly suppressed tumor growth in a therapeutic trial. Increased interferon (IFN)-γ expression and potent memory T cells in splenocytes as well as elevated CD8+IFN-γ+ cells in both spleen and tumor tissue indicated an elevated E744-62-specific cellular immune response. Although synergistic effects were detected between CpG and Poly I:C, their adjuvant effects were not enhanced further when combined with Cramp. Because the enhancement of tumor antigen-specific cellular immune responses is vital for the clearance of infected and cancerous cells, our results contribute a potential adjuvant combination for cancer vaccines.

Combined immunization against TGF-ß1 enhances HPV16 E7-specific vaccine-elicited antitumour immunity in mice with grafted TC-1 tumours.

Therapeutic vaccine appears to be a potential approach for the treatment of human papillomavirus (HPV)-associated tumours, but its efficacy can be dampened by immunosuppressive factors such as transforming growth factor (TGF)-ß1. We sought to investigate whether active immunity against TGF-ß1 enhances the anti-tumour immunity elicited by an HPV16 E7-specific vaccine that we developed previously. In this study, virus-like particles of hepatitis B virus core antigen were used as vaccine carriers to deliver either TGF-ß1 B cell epitopes or E7 cytotoxic T-lymphocyte epitope. The combination of preventive immunization against TGF-ß1 and therapeutic immunization with the E7 vaccine significantly reduced the growth of grafted TC-1 tumours in C57 mice, showing better efficacy than immunization with only one of the vaccines. The improved efficacy of combined immunization is evidenced by elevated IFN-γ and decreased IL-4 and TGF-ß1 levels in cultured splenocytes, increased E7-specific IFN-γ-expressing splenocytes, and increased numbers of CD4+IFN-γ+ and CD8+IFN-γ+ cells and decreased numbers of Treg (CD4+Foxp3+) cells in the spleen and tumours. The results strongly indicate that targeting TGF-ß1 through active immunization might be a potent approach to enhancing antigen-specific therapeutic vaccine-induced anti-tumour immune efficacy and providing a combined strategy for effective cancer immunotherapy.

Exogenous murine antimicrobial peptide CRAMP significantly exacerbates Ovalbumin-induced airway inflammation but ameliorates oxazolone-induced intestinal colitis in BALB/c mice.

Cathelicidin has been reported to be multifunctional. The current study aimed to investigate the influences of exogenous cathelicidin-related antimicrobial peptide (CRAMP) on inflammatory responses in different disease models. In OVA-induced allergic airway inflammation, CRAMP significantly enhanced the infiltration of inflammatory cells and accumulation of proinflammatory Th2 cytokine IL-13 and IL-33 in bronchial alveolar lavage fluid (BALF), exacerbated lung tissue inflammation and airway goblet cell hyperplasia, and elevated OVA-specific IgE level in serum. In oxazolone-induced intestinal colitis, the expression levels of CRAMP and its receptor FPR2 significantly increased in comparison with those of TNBS-induced mice, vesicle and normal controls. Exogenous CRAMP significantly prevented the development of ulcerative colitis, evidenced by improved body weight regain, decreased colons weight/length ratio, elevated epithelial integrity, and ameliorated colon tissue inflammation. In addition, pro-inflammatory cytokines TNF-α, IL-1ß, IL-4 and IL-13, as well as chemokines CXCL2 and CXCL5 for neutrophils recruitment were significantly decreased in CRAMP-treated mice, and epithelial repair-related factors MUC2 and Claudin1 were increased, determined by real time-PCR and ELISAs. The results indicated that although CRAMP has pro-inflammatory effects in airway, local application of exogenous CRAMP might be a potential approach for the treatment of ulcerative colitis.

A mixed-valence metallogrid [CoCo] with an unusual electronic structure and single-ion-magnet characterization.

The reaction of the multisite coordination ligand (H2L) with Co(Ac)2·4H2O in the absence of any base affords a homometallic tetranuclear mixed-valence complex, [Co4(L)4(CH3CO2)2(CH3OH)2]·Et2O (1). This mixed-valence metallogrid [Co4(L)4(CH3CO2)2 (CH3OH)2]·Et2O (1) has been theoretically and experimentally analyzed to assign the valence and spin state in the form of trans-[Co-Co-Co-Co]. HF-EPR reveals the presence of axial anisotropy (D = -34.4 cm-1) with a significant transverse component (E = 9.5 cm-1) in the local high spin cobalt centers. Slow magnetic relaxation effects were observed in the presence of a dc field, demonstrating field-induced single ion magnetic behavior, which is associated with the unusual electronic structure of Co(ii) within the metallogrid.

A copper-based layered coordination polymer: synthesis, magnetic properties and electrochemical performance in supercapacitors.

A copper-based layered coordination polymer ([Cu(hmt)(tfbdc)(H2O)]; hmt = hexamethylenetetramine, tfbdc = 2,3,5,6-tetrafluoroterephthalate; Cu-LCP) has been synthesized, and it has been structurally and magnetically characterized. The Cu-LCP shows ferromagnetic interactions between the adjacent copper(II) ions. Density functional theory calculations on the special model of Cu-LCP support the occurrence of ferromagnetic interactions. As an electrode material for supercapacitors, Cu-LCP exhibits a high specific capacitance of 1274 F g(-1) at a current density of 1 A g(-1) in 1 M LiOH electrolyte, and the capacitance retention is about 88% after 2000 cycles.

Theoretical Study of Electronic Structures and Spectra of Chalcogenido Complexes of Molybdenum, trans-Mo(Q)(2)(PH(3))(4) (Q = O, S, Se, Te).

Electronic structures of the title complexes have been studied using quantum chemical computations by different methods. It is shown that the results of Xalpha calculations agree well with expectations from classical ligand-field theory, but both are far from being in agreement with the results given by ab initio calculations. The HOMO in the ab initio Hartree-Fock molecular orbital diagrams of all these complexes is a chalcogen p(pi) lone pair orbital rather than the metal nonbonding d(xy)() orbital previously proposed. Electronic transition energies were calculated by CASSCF and CI methods. The results suggest that in the cases when Q = S, Se, and Te the lowest energy transitions should be those from the p(pi) lone pair orbitals to the metal-chalcogen pi orbitals. The calculated and observed electronic spectra of the oxo complex are in good agreement and very different from the spectra of the other complexes, and the lowest absorptions were accordingly assigned to transitions of different origins.

Further Study of Very Close Nonbonded Cu(I)-Cu(I) Contacts. Molecular Structure of a New Compound and Density Functional Theory Calculations.

A new compound containing a Cu(I)-Cu(I) unit with a short internuclear distance, 2.453(1) Å, is reported. The question of whether such a short distance justifies the postulation of a metal-metal bond is addressed using density functional theory (DFT). The new compound is Cu(2)(hpp)(2), where hpp(-) (C(7)N(3)H(12)) is the anion derived from 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine. Cu(2)(hpp)(2) crystallizes in the monoclinic space group P2(1)/c with Z = 2 and cell dimensions a = 7.320(2) Å, b = 12.418(4) Å, c = 8.689(2) Å, and beta = 93.76(2) degrees. The density functional calculations show that the close approach of the copper atoms is predictable without involving any significant amount of covalent bonding. A DFT calculation on the Cu(3)[(p-tol)N(5)(p-tol)](3) molecule also foretells the observed short Cu-Cu separation (2.35 Å observed vs 2.38 Å calculated) but, again, does not imply the formation of Cu-Cu bonds. Very short Cu(I) to Cu(I) distances can be attributed to a combination of strong Cu-N bonding and very short (ca. 2.2 Å) bite distances for the ligands.

Preparation, Molecular and Electronic Structures, and Magnetic Properties of Face-Sharing Bioctahedral Titanium(III) Compounds: [PPh(4)][Ti(2)(&mgr;-Cl)(3)Cl(4)(PR(3))(2)].

Reduction of TiCl(4) with 1 equiv of HSnBu(3) followed by addition of [PPh(4)]Cl and then PR(3) leads to two new dinuclear titanium(III) compounds, [PPh(4)][Ti(2)(&mgr;-Cl)(3)Cl(4)(PR(3))(2)] (R = Et and R(3) = Me(2)Ph), both of which contain an anion with the face-sharing bioctahedral type structure. Their crystal structures are reported. [PPh(4)][Ti(2)(&mgr;-Cl)(3)Cl(4)(PEt(3))(2)].2CH(2)Cl(2) crystallized in the triclinic space group P&onemacr;. Cell dimensions: a = 12.461(1) Å, b = 20.301(8) Å, c = 11.507(5) Å, alpha = 91.44 degrees, beta = 113.27(1) degrees, gamma = 104.27(2) degrees, and Z = 2. The distance between titanium atoms is 3.031(2) Å. [PPh(4)][Ti(2)(&mgr;-Cl)(3)Cl(4)(PMe(2)Ph)(2)].CH(2)Cl(2) also crystallized in the triclinic space group P&onemacr; with cell dimensitions a = 11.635(4) Å, b = 19.544(3) Å, c = 11.480(3) Å, alpha = 100.69(2) degrees, beta = 109.70(1) degrees, gamma = 95.08(2) degrees, and Z = 2. The distance between titanium atoms in this compound is 2.942(1) Å. Variable temperature magnetic susceptibilities were measured for [PPh(4)][Ti(2)(&mgr;-Cl)(3)Cl(4)(PEt(3))(2)]. Electronic structure calculations were carried out for a model ion, [Ti(2)(&mgr;-Cl)(3)Cl(4)(PH(3))(2)](-), and another well-known anion, [Ti(2)(&mgr;-Cl)(3)Cl(6)](3)(-), by employing an ab initio configuration interaction method. The results of the calculations reveal that the metal-metal interaction in these Ti(III) face-sharing compounds can be best described by strong antiferromagnetic coulping that leads to a singlet ground state and a thermally accessible triplet first excited state. Accordingly the measured magnetic data were satisfactorily fitted to a spin-only formula.

How Small Can a Catenane Be?

Catenanes are playing an increasingly important role in supramolecular chemistry. In attempting to identify the minimum number of carbon atoms in a viable catenane, the B3LYP, BP86, M06-2X, MM3, and MM4 methods were applied to study representative [2]catenane models, which consist of two mechanically interlocked saturated n-cycloalkanes ([CnH2n]2). The structures, energy variations, and electron density differences vary nearly monotonically from n = 18 to 11. For example, the B3LYP/DZP++ dissociation energies [CnH2n]2 → 2CnH2n are 101, 121, 159, 191, 222, 252, 290, and 323 kcal/mol from n = 18 to 11, respectively. However, there is much variation among the energetic predictions with the B3LYP, BP86, M06-2X, MM3, and MM4 methods. The distances of the longest C-C single bond in each catenane are 1.593 (n = 18), 1.604 (n = 17), 1.631 (n = 16), 1.640 (n = 15), 1.667 (n = 14), 1.669 (n = 13), 1.680 (n = 12), and 1.689 Š(n = 11). These results display something of a shoulder in the vicinity of n = 14. This may suggest that [C15H30]2 is the smallest catenane that will resist fragmentation under specified laboratory conditions.

Heptahapticity in binuclear (cycloheptatrienyl)molybdenum carbonyl derivatives: the interplay between ring hapticity/planarity and metal-metal multiple bonding.

Theoretical studies on the binuclear (cycloheptatrienyl)molybdenum carbonyl complexes [(C(7)H(7))(2)Mo(2)(CO)(n)] (n=6, 5, 4, 3, 2, 1, 0) indicate structures with fully bonded heptahapto eta(7)-C(7)H(7) rings and four or fewer carbonyl groups to be energetically competitive. This is in striking contrast to the corresponding chromium derivatives. The lowest energy of such a structure for [(eta(7)-C(7)H(7))(2)Mo(2)(CO)(4)] is a singlet unbridged structure with a formal Mo--Mo single bond with a length of approximately 3.2 A. A higher-energy pentahapto structure [(eta(5)-C(7)H(7))(2)Mo(2)(CO)(4)] is also predicted with a formal Mo [triple bond]Mo triple bond with a length of approximately 2.56 A. Low-energy structures are predicted for [(C(7)H(7))(2)Mo(2)(CO)(3)] with two heptahapto eta(7)-C(7)H(7) rings, either one or two bridging carbonyl groups, and formal Mo=Mo double bonds with a length of approximately 2.8 A. However, the global minimum for [(C(7)H(7))(2)Mo(2)(CO)(3)] is a [(eta(7)-C(7)H(7))(eta(5)-C(7)H(7))Mo(2)(CO)(3)] structure with a formal Mo[triple bond]Mo triple bond with a length of approximately 2.53 A. The lowest-energy structures for [(C(7)H(7))(2)Mo(2)(CO)(2)] and [(C(7)H(7))(2)Mo(2)(CO)] have heptahapto eta(7)-C(7)H(7) rings and predicted metal-metal bond lengths of approximately 2.54 and 2.31 A, respectively, consistent with the formal triple and quadruple bonds, respectively, needed to give both metal atoms the favored 18-electron configuration. The lowest-energy structures for the carbonyl-richer systems [(C(7)H(7))(2)Mo(2)(CO)(n)] (n=6, 5) contain one trihapto eta(3)-C(7)H(7) ring and one pentahapto eta(5)-C(7)H(7) ring.

Binuclear homoleptic rhodium carbonyls: structures, energetics, and vibrational spectra.

The binuclear homoleptic rhodium carbonyls Rh2(CO)n (n = 8, 7, 6, 5) have been examined theoretically. Three energetically low-lying equilibrium structures of Rh2(CO)8 were found, i.e., one doubly bridged C2v singlet structure and two unbridged singlet structures with D(3d) and D(2d) symmetry. The doubly bridged structure is the global minimum predicted to lie 3.4 kcal/mol below the D(2d) structure and 6.4 kcal/mol below the D(3d) structure. For Rh2(CO)7 the global minimum is either a singlet C2v unbridged structure or a singlet doubly bridged C(s) structure within 1.8 kcal/mol depending upon the theoretical method. For Rh2(CO)6, the global minimum is either a singlet doubly bridged D(2) structure or a singlet unbridged D(2d) structure depending upon the method. Triplet structures for Rh2(CO)7 and Rh2(CO)6 are predicted to be of high energies relative to the low energy singlet structures. For Rh2(CO)5 the C2v singlet singly bridged structure lies below the C2 or C2v triplet structures.

Homoleptic Carbonyls of the Second-Row Transition Metals: Evaluation of Hartree-Fock and Density Functional Theory Methods.

The homoleptic mono- and multinuclear carbonyls for Mo, Tc, Ru, and Rh, namely, Mo(CO)6, Ru(CO)5, Tc2(CO)10, Ru3(CO)12, Rh4(CO)12, and Rh6(CO)16, are investigated theoretically by the Hartree-Fock method and three density functional theory (DFT) methods, i.e., BP86, B3LYP, and MPW1PW91, along with the SDD ECP basis sets. The results predicted by all the methods are basically in agreement with each other. The MPW1PW91 and BP86 methods predict geometric parameters and vibrational spectra, respectively, closest to the experimental values. For Ru3(CO)12 the relative energies of the D3h isomer with only terminal CO groups and the C2v isomer with two bridging CO groups are within 3 kcal/mol of each other with the lower energy isomer depending upon the computational method used. For Rh4(CO)12 the global minimum is predicted to have C3v symmetry, with three bridging and nine terminal carbonyls, in accord with experiment. The Rh6(CO)16 structure has Td symmetry and satisfies the Wade-Mingos rules for an octahedral cluster. Using the MPW1PW91 method the Rh-Rh distances in Rh4(CO)12 are found to be 2.692 Å and 2.750 Å and those in Rh6(CO)16 to be 2.785 Å.