Factors Dictating Carbene Formation at (PNP)Ir

The mechanistic subtleties involved with the interaction of an amido/bis(phosphine)-supported (PNP)Ir fragment with a series of linear and cyclic ethers have been investigated using density functional theory. Our analysis has revealed the factors dictating reaction direction toward either an iridium-supported carbene or a vinyl ether adduct. The (PNP)Ir structure will allow carbene formation only from accessible carbons α to the ethereal oxygen, such that d electron back-donation from the metal to the carbene ligand is possible. Should these conditions be unavailable, the main competing pathway to form vinyl ether can occur, but only if the (PNP)Ir framework does not sterically interfere with the reacting ether. In situations where steric hindrance prevents unimpeded access to both pathways, the reaction may progress to the initial C−H activation but no further. Our mechanistic analysis is density functional independent and whenever possible confirmed experimentally by trapping intermediate species experimentally. We have also highlighted an interesting systematic error present in the DFT analysis of reactions where steric environment alters considerably within a reaction

Activation and Cleavage of the N-O Bond in Dinuclear Mixed-Metal Nitrosyl Systems and Comparative Analysis of Carbon Monoxide, Dinitrogen, and Nitric Oxide Activation

The activation and scission of the N–O bond in nitric oxide using dinuclear mixed-metal species, comprising transition elements with d3 and d2 configurations and trisamide ligand systems, have been investigated by means of density functional calculations. The [Cr(III)–V(III)] system is analyzed in detail and, for comparative purposes, the [Mo(III)–Nb(III)], [W(III)–Ta(III)], and (mixed-row) [Mo(III)–V(III)] systems are also considered. The overall reaction and individual intermediate steps are favourable for all systems, including the case where first row (Cr and V) metals are exclusively involved, a result that has not been observed for the related dinitrogen and carbon monoxide systems. In contrast to the cleavage of dinitrogen by three-coordinate Mo amide complexes where the dinuclear intermediate possesses a linear [Mo–NN–Mo] core, the [M–NO–M′] core must undergo significant bending in order to stabilize the dinuclear species sufficiently for the reaction to proceed beyond the formation of the nitrosyl encounter complex. A comparative bonding analysis of nitric oxide, dinitrogen and carbon monoxide activation is also presented. The overall results indicate that the π interactions are the dominant factor in the bonding across the [M–L1L2–M′] (L1L2 = N–O, N–N, C–O) moiety and, consequently, the activation of the L1–L2 bond. These trends arise from the fact that the energy gaps between the π orbitals on the metal and small molecule fragments are much more favourable than for the corresponding σ orbitals. The π energy gaps decrease in the order [NO \u3c N2 \u3c CO] and consequently, for each individual π orbital interaction, the back donation between the metal and small molecule increases in the order [CO \u3c N2 \u3c NO]. These results are in accord with previous findings suggesting that optimization of the π interactions plays a central role in increasing the ability of these transition metal systems to activate and cleave small molecule bonds

Ligand Rotation in [Ar(R)N]\u3csub\u3e3\u3c/sub\u3eM-N\u3csub\u3e2\u3c/sub\u3e-M′[N(R)Ar]\u3csub\u3e3\u3c/sub\u3e (M, M′ = Mo\u3csup\u3eIII\u3c/sup\u3e, Nb\u3csup\u3eIII\u3c/sup\u3e; R = \u3csup\u3ei\u3c/sup\u3ePr and \u3csup\u3et\u3c/sup\u3eBu) Dimers

Earlier calculations on the model N2-bridged dimer (µ-N2)-{Mo[NH2]3}2 revealed that ligand rotation away from a trigonal arrangement around the metal centres was energetically favourable resulting in a reversal of the singlet and triplet energies such that the singlet state was stabilized 13 kJ mol−1 below the D3d triplet structure. These calculations, however, ignored the steric bulk of the amide ligands N(R)Ar (R = iPr and tBu, Ar = 3,5-C6H3Me2) which may prevent or limit the extent of ligand rotation. In order to investigate the consequences of steric crowding, density functional calculations using QM/MM techniques have been performed on the MoIIIMoIII and MoIIINbIII intermediate dimer complexes (µ-N2)-{Mo[N(R)Ar]3}2 and [Ar(R)N]3Mo-(µ-N2)-Nb[N(R)Ar]3 formed when threecoordinate Mo[N(R)Ar]3 and Nb[N(R)Ar]3 react with dinitrogen. The calculations indicate that ligand rotation away from a trigonal arrangement is energetically favourable for all of the ligands investigated and that the distortion is largely electronic in origin. However, the steric constraints of the bulky amide groups do play a role in determining the final orientation of the ligands, in particular, whether the ligands are rotated at one or both metal centres of the dimer. Analogous to the model system, QM/MM calculations predict a singlet ground state for the (µ-N2)-{Mo[N(R)Ar]3}2 dimers, a result which is seemingly at odds with the experimental triplet ground state found for the related (µ-N2)-{Mo[N(tBu)Ph]3}2 system. However, QM/MM calculations on the (µ-N2)-{Mo[N(tBu)Ph]3}2 dimer reveal that the singlet–triplet gap is nearly 20 kJ mol−1 smaller and therefore this complex is expected to exhibit very different magnetic behaviour to the (µ-N2)-{Mo[N(R)Ar]3}2 system

Rationalizing the Different Products in the Reaction of N\u3csub\u3e2\u3c/sub\u3e with Three-coordinate MoL\u3csub\u3e3\u3c/sub\u3e Complexes

The reaction of N2 with three-coordinate MoL3 complexes is known to give rise to different products, N–MoL3, L3Mo–N–MoL3 or Mo2L6, depending on the nature of the ligand L. The energetics of the different reaction pathways are compared for L = NH2, NMe2, N(iPr)Ar and N(tBu)Ar (Ar = 3,5-C6H3Me2) using density functional methods in order to rationalize the experimental results. Overall, the exothermicity of each reaction pathway decreases as the ligand size increases, largely due to the increased steric crowding in the products compared to reactants. In the absence of steric strain, the formation of the metal–metal bonded dimer, Mo2L6, is the most exothermic pathway but this reaction shows the greatest sensitivity to ligand size varying from significantly exothermic, −403 kJ mol−1 for L = NMe2, to endothermic, +78 kJ mol−1 for L = N(tBu)Ar. For all four ligands, formation of N–MoL3via cleavage of the N2 bridged dimer intermediate, L3Mo–N–N–MoL3, is strongly exothermic. However, in the presence of excess reactant MoL3, formation of the single atom-bridged complex L3Mo–N–MoL3 from N–MoL3 + MoL3 is both thermodynamically and kinetically favoured for L = NMe2 and N(iPr)Ar, in agreement with experiment. In the case of L = N(tBu)Ar, the greater steric bulk of the tBu group results in a much less exothermic reaction and a calculated barrier of 66 kJ mol−1 to formation of the L3Mo–N–MoL3 dimer. Consequently, for this ligand, the energetically and kinetically favoured product, consistent with the experimental data, is the nitride complex L3Mo–N

Borane-ctalyzed stereoselective C–H insertion, cyclopropanation, and ring-opening reactions

Lewis acidic boranes have been shown to be effective metal-free catalysts for highly selective reactions of donor-acceptor diazo compounds to a range of substrates. The reactions of α-aryl α-diazoesters with nitrogen heterocycles indole or pyrrole selectively generate C3 and C2 C–H insertion products, respectively, in good to excellent yields even when using unprotected indoles. Alternatively, benzofuran, indene, and alkene substrates give exclusively cyclopropanated products with α-aryl α-diazoesters, whereas the reactions with furans lead to ring-opening. Comprehensive theoretical calculations have been used to explain the differing reactivities and high selectivities of these reactions. Overall, this work demonstrates the selective metal-free catalytic reactions of α-aryl α-diazoesters with (hetero)cycles and alkenes. This simple, mild reaction protocol represents an alternative to the commonly used precious metal systems and may provide future applications in the generation of biologically active compounds

Proton supplier role of binuclear gold complexes in promoting hydrofunctionalisation of nonactivated alkenes

Density functional theory (DFT) was used to investigate PR 3 AuOTf-catalyzed hydrofunctionalisation of nonactivated alkenes using acetic acid and phenol where OTf = triflate (CF 3 SO 3- ). The gold(i) complex itself is found to be unlikely to operate as the π-activator due to its relatively low electrophilicity. Instead, the concurrent coordination of two gold(i) complexes to a nucleophile (PhOH or AcOH) enhances the acidity of the latter\u27s proton and causes the ensuing binuclear complex to serve as a strong proton supplier for activating the alkene π-bonds. Alternatively, the binuclear complex is also susceptible to produce a hidden HOTf. This hidden acid is accessible for hydrofunctionalization to occur but it is not in sufficient concentration to decompose the final product

A Preliminary Study: An Evaluation and Redevelopment of Current First Year Laboratory Practices

This study consisted of an evaluation and redevelopment of first year laboratory experiments in Chemistry 1 at the School of Chemistry, University of Tasmania, with respect to the teaching styles implemented. The teaching approaches focused on were expository, guided inquiry, and problem solving and these were applied to two physical chemistry experiments. The aims of this study included investigation into the engagement and input of both students and demonstrators, the understanding achieved by students through completion of the laboratory experiment, and the enjoyment of students in participating and completing the laboratory experiment. The major outcomes of this study found that both problem solving and guided inquiry approaches had greater success than the expository approach in areas such as the engagement of students within the laboratory environment, and the deeper understanding the students gained in the chemical concepts. In addition, expository and problem solving approaches were found to have more acceptable workloads than the guided inquiry variant. The greatest contribution of this study is in providing a foundation for further investigations to be continued into this field of research

Connecting Binuclear Pd(III) and Mononuclear Pd(IV) Chemistry by Pd–Pd Bond Cleavage

Oxidation of binuclear Pd(II) complexes with PhICl$_2$ or PhI(OAc)$_2$ has previously been shown to afford binuclear Pd(III) complexes featuring a Pd–Pd bond. In contrast, oxidation of binuclear Pd(II) complexes with electrophilic trifluoromethylating (“CF$_3^+$”) reagents has been reported to afford mononuclear Pd(IV) complexes. Herein, we report experimental and computational studies of the oxidation of a binuclear Pd(II) complex with “CF$_3^+$” reagents. These studies suggest that a mononuclear Pd(IV) complex is generated by an oxidation–fragmentation sequence proceeding via fragmentation of an initially formed, formally binuclear Pd(III), intermediate. The observation that binuclear Pd(III) and mononuclear Pd(IV) complexes are accessible in the same reactions offers an opportunity for understanding the role of nuclearity in both oxidation and subsequent C–X bond-forming reactions.Chemistry and Chemical Biolog

The science threshold learning outcomes: a ‘student-friendly’ version

BACKGROUND The Science Threshold Learning Outcomes (TLOs) are a nationally agreed set of learning outcomes for graduates of bachelor level degrees in science that are increasingly being used in curriculum review, design and bench-marking at Australian universities. The Science TLOs were published in the Science Standards Statement (Jones, Yates & Kelder 2011) together with notes on their interpretation and application: this is necessarily a formal document, aimed primarily at teaching academics and those with relevant quality assurance responsibilities. Talking with students during the development of the Science TLOs indicated that students were unlikely to engage with the TLOs as thus expressed. Therefore, if the Science TLOs are to be an effective means of informing students about the learning achieved through studying science at university, a modified version was required. AIM This project aimed to develop a ‘student-friendly’ version of the Science TLOs directed at potential and current students, their parents, future employers, teachers and career advisors. APPROACH The development was carried out in collaboration with The Bookend Trust (http://www.bookendtrust.com/), a not-for-profit organisation dedicated to inspiring students to undertake positive careers in environmental science. In particular, Bookend links media and journalism students with professional scientists or science students, thus fostering effective communication about contemporary science. STAGE 1 As a scoping exercise, a Bookend scholarship student investigated students’ attitudes to the formally expressed Science TLOs. She presented four college (Year 11/12) science classes with a short survey that explored their conceptions of studying science at university and potential career paths in science. She also sought their opinions on a DL card- sized flier of the Science TLOs, and asked how the same information could be presented in a way that students would find relevant and attractive. The survey showed that a ‘student-friendly’ flier would need pictures, catchy words, bright colours, and common-use language. The students were enthusiastic about adding a QR scan code linking to a website, saying that would increase the likelihood of their picking up such a flier. The survey results informed Stage 2 of this project, particularly the design approach. STAGE 2 To embed the Science TLOs into a concrete context, Bookend summer scholarship students interviewed employers using standard questions designed to elicit responses illustrating how the Science TLOs are directly relevant to working in science. The interviews were professionally recorded on video, and the footage was edited to create short clips that were loaded onto a website. THE FINAL PRODUCT We have produced a two-sided coloured flier that provides a ‘student-friendly’ version of the Science TLOs (re-phrased as questions) and a QR code linking to a website where the employer interviews can be viewed. The ‘Student-Friendly TLOs flier’ is now freely available as a downloadable pdf from the Office for Learning and Teaching (OLT) Resource Library (http://www.olt.gov.au/resource-library) and other relevant sites. We thank the OLT for funding this project