Geometries of third-row transition-metal complexes from density-functional theory

A set of 41 metal-ligand bond distances in 25 third-row transition-metal complexes, for which precise structural data are known in the gas phase, is used to assess optimized and zero-point averaged geometries obtained from DFT computations with various exchange-correlation functionals and basis sets. For a given functional (except LSDA) Stuttgart-type quasi-relativistic effective core potentials and an all-electron scalar relativistic approach (ZORA) tend to produce very similar geometries. In contrast to the lighter congeners, LSDA affords reasonably accurate geometries of 5d-metal complexes, as it is among the functionals with the lowest mean and standard deviations from experiment. For this set the ranking of some other popular density functionals, ordered according to decreasing standard deviation, is BLYP > VSXC > BP86 approximate to BPW91 approximate to TPSS approximate to B3LYP approximate to PBE > TPSSh > B3PW91 approximate to B3P86 approximate to PBE hybrid. In this case hybrid functionals are superior to their nonhybrid variants. In addition, we have reinvestigated the previous test sets for 3d- (Buhl M.; Kabrede, H. J. Chem. Theory Comput. 2006, 2, 1282-1290) and 4d- (Waller, M. P.; Buhl, M. J. Comput. Chem. 2007,28,1531-1537) transition-metal complexes using all-electron scalar relativistic DFT calculations in addition to the published nonrelativistic and ECP results. For this combined test set comprising first-, second-, and third-row metal complexes, B3P86 and PBE hybrid are indicated to perform best. A remarkably consistent standard deviation of around 2 pm in metal-ligand bond distances is achieved over the entire set of d-block elements.PostprintPeer reviewe

Design of CO2 hydrogenation catalysts based on phosphane/borane frustrated Lewis pairs and xanthene-derived scaffolds

UK Catalysis Hub is kindly thanked for resources and support provided via our membership of the UK Catalysis Hub Consortium and funded by EPSRC grant: EP/R026939/1, EP/R026815/1, EP/R026645/1, EP/R027129/1, and EP/M013219/1. J.W.M.C. acknowledges financial support from FAPERJ (grants E-26/203.001/2017, E-26/010.101118/2018, and E-26010.001424/2019) and CNPq (grants 309080/2015-0 and 434955/2018-3). M.B. thanks the School of Chemistry in St Andrews and EaStCHEM for support.New naphtho[2,1,8,7-klmn]xanthene and benzo[kl]xanthene-based intramolecular phosphane–borane frustrated Lewis pairs (FLPs) were investigated in catalyzed H2 activation and CO2 hydrogenation processes. According to DFT predictions at the B3LYP-D3 level, the presence of rigid scaffolds and increased P···B distances in the investigated FLPs lead to a remarkable drop in the energy barrier for CO2 hydrogenation (by up to 19.2 kcal mol−1, compared to the parent dimethylxanthene-based FLP). Furthermore, the energy differences between the transition states for H2 activation and CO2 hydrogenation are significantly reduced, making both processes feasible under relatively mild experimental conditions.Publisher PDFPeer reviewe

Sterically restricted tin phosphines, stabilized by weak intramolecular donor-acceptor interactions

Funding: Engineering and Physical Sciences Research Council (EPSRC)Four related sterically restricted pen-substituted acenaphthenes have been prepared containing mixed tin phosphorus moieties in the proximal 5,6-positions (Acenap[SnR3][(PPr2)-Pr-i]; Acenap = acenaphthene-5,6-diyl; R-3 = Ph-3 (1), Ph2Cl (2), Me2Cl (3), Bu2Cl (4)). The degree of intramolecular P-Sn bonding within the series was investigated by X-ray crystallography, solution and solid-state NMR spectroscopy, and density functional theory (DFT/B3LYP/SBKJC/PCM) calculations. All members of the series adopt a conformation such that the phosphorus lone pair is located directly opposite the tin center, promoting an intramolecular donor acceptor P -> Sn type interaction. The extent of covalent bonding between Sn and P is found to be much greater in triorganotin chlorides 2-4 in comparison with the triphenyl derivative 1. Coordination of a highly electronegative chlorine atom naturally increases the Lewis acidity of the tin center, enhancing the Ip(P)-sigma*(Sn-Y) donor acceptor 3c-4e type interaction, as indicated by conspicuously short Sn-P peri distances and significant (1)J(P-31,Sn-119) spin spin coupling constants (SSCCs) in the range 740-754 Hz. Evidence supporting the presence of this interaction was also found in solid-state NMR spectra of some of the compounds which exhibit an indirect spin spin coupling on the same order of magnitude as observed in solution. DFT calculations confirm the increased covalent bonding between P and Sn in 2-4, with notable WBIs of ca. 0.35 obtained, in comparison to 0.1 in 1.PostprintPeer reviewe

Pandemic Pedagogy: Elements of Online Supportive Course Design

The purpose of this study was to identify which course design elements students perceive as supporting an easier transition to emergency remote teaching due to COVID-19, as well as to use those items to develop the Online Supportive Course Design (OSCD) measure. By asking students to rate their course with the easiest transition and hardest transition to emergency remote teaching, this study identified which structural elements were most important for supporting students during the transition. Using exploratory and confirmatory factor analyses, a seven-item measure was developed to operationalize OSCD, and initial validity was established by examining the relationships between OSCD, autonomy support, and teacher competence. Finally, practical implications for university faculty and areas for future research are discussed

Unveiling the mechanism of N‐methylation of indole with dimethylcarbonate using either DABCO or DBU as catalyst

B.R.V.F., P.H.V. and M.N.E. acknowledge the São Paulo State Science Foundation (FAPESP) and the Brazilian National Science Council (CNPq) for financial support. L.A.Z. thanks CNPQ for his PhD scholarship (142476/2018-8), R.A.C. thanks SDumont and CESUP. M.B. thanks the School of Chemistry and EaStCHEM for support and for access to a computer cluster maintained by Dr. H. Früchtl.Depending on the catalyst used, N‐methylation of indole with dimethylcarbonate (DMC) ‐ an environmentally friendly alkylation agent – yields different products. With 1,4‐diazabicyclo[2.2.2]octane (DABCO), the reaction forms only N‐methylated indole but with or 1,8‐diazabicyclo[5.4.0]undec‐7‐ene (DBU), both N‐methylated and N‐methoxycarbonylated indole are formed. Using direct ESI(+)‐MS monitoring to collect actual snapshots of the changing ionic composition of the reaction solution, we report on the interception and characterization of key intermediates for such reactions. Although a mechanism has been proposed with methoxycarbonylated base as the key intermediate for both DBU and DABCO, the ESI(+)‐MS data as well as B3LYP‐D3/6‐311+G** calculations suggest that the reaction of DMC with indole under either DABCO or DBU catalysis follow contrasting mechanisms.PostprintPeer reviewe

Constrained phosphine chalcogenide selenoethers supported by peri-substitution

A series of phosphorus and selenium peri-substituted acenaphthene species with the phosphino group oxidized by O, S, and Se has been isolated and fully characterized, including by single-crystal X-ray diffraction. The P(V) and Se(II) systems showed fluxional behavior in solution due to the presence of two major rotamers, as evidenced with solution NMR spectroscopy. Using Variable-Temperature NMR (VT NMR) and supported by DFT (Density Functional Theory) calculations and solid-state NMR, the major rotamers in the solid and in solution were identified. All compounds showed a loss of the through-space JPSe coupling observed in the unoxidized P(III) and Se(II) systems due to the sequestration of the lone pair of the phosphine, which has been previously identified as the major contributor to the coupling pathway.Publisher PDFPeer reviewe

Isothiourea-catalyzed enantioselective Michael addition of malonates to α,β-unsaturated aryl esters

MB thanks EaStCHEM and the School of Chemistry for support.An enantioselective Michael addition of malonates to α,β-unsaturated para-nitrophenyl esters was achieved using the Lewis basic isothiourea HyperBTM, giving excellent levels of product enantioselectivity (up to >99:1 enantiomeric ratio) in good yields and with complete regioselectivity (>20:1 regioselectivity ratio) in the presence of alternative (phenyl ketone and ethyl ester) Michael acceptors. Density functional theory calculations indicate that N-acylation is rate-limiting. This constitutes a rare example of a highly enantioselective addition of simple, readily available malonates to α,β-unsaturated esters.Publisher PDFPeer reviewe

Structure-directing effects in (110)-layered hybrid perovskites containing two distinct organic moieties

We acknowledge support from the University of St Andrews, the China Scholarship Council (studentship to YYG) and the Leverhulme trust (RPG-2018-065).The hybrid perovskites (ImH)(GuH)PbBr4 and (TzH)(GuH)PbBr4 (ImH+ = imidazolium, GuH+ = guanidinium, TzH+ = 1,2,4-triazolium) both adopt (110)-oriented layer structures. However, the GuH+ cation adopts differing crystallographic sites in the two structures (intra-layer versus inter-layer); this is discussed in terms of the sizes of the organic cations and their hydrogen-bonding preferences.PostprintPeer reviewe

Formation of metallacarboxylic acids through Hieber base reaction. A density functional theory study

We thank EaStCHEM and the School of Chemistry for support.Using density functional theory (B97-D/ECP2/PCM//RI-BP86/ECP1 level), we have studied the effects of ligand variation on OH− uptake by transition-metal carbonyls (Hieber base reaction), i.e., LnM(CO) + OH− → [LnM(CO2H)]−, M = Fe, Ru, Os, L = CO, PMe3, PF3, py, bipy, Cl, H. The viability of this step depends notably on the nature of the co-ligands, and a large span of driving forces is predicted, ranging from ΔG = −144 kJ/mol to +122 kJ/mol. Based on evaluation of atomic charges from natural population analysis, it is the ability of the co-ligands to delocalize the additional negative charge (through their π-acidity) that is the key factor affecting the driving force for OH− uptake. Implications for the design of new catalysts for water gas shift reaction are discussed.Publisher PDFPeer reviewe

Understanding a hydroformylation catalyst that produces branched aldehydes from alkyl alkenes

The authors thank the EPSRC for funding (EP/M003868/1).This paper reports experimental and computational studies on the mechanism of a rhodium-catalysed hydroformylation that is selective for branched aldehyde products from unbiased alkene substrates. This highly unusual selectivity relies on a phospholane-phosphite ligand prosaically called BOBPHOS. Kinetic studies using in situ high pressure IR (HPIR) and the reaction progress kinetic analysis methodology suggested two steps in the catalytic cycle were involved as turnover determining. Negative order in CO and positive orders in alkene and H2 were found and the effect of hydrogen and carbon monoxide partial pressures on selectivity were measured. Labeling studies found rhodium hydride addition to the alkene to be largely irreversible. Detailed spectroscopic HPIR and NMR characterization of activated rhodium-hydrido dicarbonyl species were carried out. In the absence of H2, reaction of the rhodium-hydrido dicarbonyl with allylbenzene allowed further detailed spectroscopic characterization of four- and five-coordinate rhodium-acyl species. Under single-turnover conditions the ratios of branched to linear acyl species were preserved in the final ratios of aldehyde products. Theoretical investigations uncovered unexpected stabilizing CH-π interactions between the ligand and substrate which influenced the high branched selectivity by causing potentially low energy pathways to become unproductive. Energy span and degree of TOF control analysis strongly support experimental observations and mechanistic rationale. A three-dimensional quadrant model was built to represent the structural origins of regio- and enantioselectivity.PostprintPeer reviewe