Single-crystal to cingle-crystal addition of H2to [Ir(iPr-PONOP)(propene)][BArF4] and comparison between solid-state and solution reactivity

The EPSRC (EP/M024210/2, EP/T019867/1), SCG Chemicals, The Clarendon Trust, The Leverhulme Trust (RPG-2020-184), Diamond Light Source for funding (PhD studentship to AM).The reactivity of the Ir(I) PONOP pincer complex [Ir(iPr-PONOP)(η2-propene)][BArF4], 6, [iPr-PONOP = 2,6-(iPr2PO)2C6H3N, ArF= 3,5-(CF3)2C6H3] was studied in solution and the solid state, both experimentally, using molecular density functional theory (DFT) and periodic-DFT computational methods, as well as in situ single-crystal to single-crystal (SC-SC) techniques. Complex 6 is synthesized in solution from sequential addition of H2and propene, and then the application of vacuum, to [Ir(iPr-PONOP)(η2-COD)][BArF4], 1, a reaction manifold that proceeds via the Ir(III) dihydrogen/dihydride complex [Ir(iPr-PONOP)(H2)H2][BArF4], 2, and the Ir(III) dihydride propene complex [Ir(iPr-PONOP)(η2-propene)H2][BArF4], 7, respectively. In solution (CD2Cl2) 6 undergoes rapid reaction with H2to form dihydride 7 and then a slow (3 d) onward reaction to give dihydrogen/dihydride 2 and propane. DFT calculations on the molecular cation in solution support this slow, but productive, reaction, with a calculated barrier to rate-limiting propene migratory insertion of 24.8 kcal/mol. In the solid state single-crystals of 6 also form complex 7 on addition of H2in an SC-SC reaction, but unlike in solution the onward reaction (i.e., insertion) does not occur, as confirmed by labeling studies using D2. The solid-state structure of 7 reveals that, on addition of H2to 6, the PONOP ligand moves by 90° within a cavity of [BArF4]-anions rather than the alkene moving. Periodic DFT calculations support the higher barrier to insertion in the solid state (ΔG‡= 26.0 kcal/mol), demonstrating that the single-crystal environment gates onward reactivity compared to solution. H2addition to 6 to form 7 is reversible in both solution and the solid state, but in the latter crystallinity is lost. A rare example of a sigma amine-borane pincer complex, [Ir(iPr-PONOP)H2(η1-H3B·NMe3)][BArF4], 5, is also reported as part of these studies.Peer reviewe

Room Temperature Acceptorless Alkane Dehydrogenation from Molecular σ-Alkane Complexes

The non-oxidative catalytic dehydrogenation of light alkanes via C-H activation is a highly endothermic process that generally requires high temperatures and/or a sacrificial hydrogen acceptor to overcome unfavorable thermodynamics. This is complicated by alkanes being such poor ligands, meaning that binding at metal centers prior to C-H activation is disfavored. We demonstrate that by biasing the pre-equilibrium of alkane binding, by using solid-state molecular organometallic chemistry (SMOM-chem), well-defined isobutane and cyclohexane σ-complexes, [Rh(Cy2PCH2CH2PCy2)(η: η-(H3C)CH(CH3)2][BArF4] and [Rh(Cy2PCH2CH2PCy2)(η: η-C6H12)][BArF4] can be prepared by simple hydrogenation in a solid/gas single-crystal to single-crystal transformation of precursor alkene complexes. Solid-gas H/D exchange with D2 occurs at all C-H bonds in both alkane complexes, pointing to a variety of low energy fluxional processes that occur for the bound alkane ligands in the solid-state. These are probed by variable temperature solid-state nuclear magnetic resonance experiments and periodic density functional theory (DFT) calculations. These alkane σ-complexes undergo spontaneous acceptorless dehydrogenation at 298 K to reform the corresponding isobutene and cyclohexadiene complexes, by simple application of vacuum or Ar-flow to remove H2. These processes can be followed temporally, and modeled using classical chemical, or Johnson-Mehl-Avrami-Kologoromov, kinetics. When per-deuteration is coupled with dehydrogenation of cyclohexane to cyclohexadiene, this allows for two successive KIEs to be determined [kH/kD = 3.6(5) and 10.8(6)], showing that the rate-determining steps involve C-H activation. Periodic DFT calculations predict overall barriers of 20.6 and 24.4 kcal/mol for the two dehydrogenation steps, in good agreement with the values determined experimentally. The calculations also identify significant C-H bond elongation in both rate-limiting transition states and suggest that the large kH/kD for the second dehydrogenation results from a pre-equilibrium involving C-H oxidative cleavage and a subsequent rate-limiting β-H transfer step

A series of crystallographically characterized linear and branched σ-alkane complexes of rhodium : from propane to 3-methylpentane

We thank the EPSRC (EP/M024210, and the UK National Crystallography Service), the Leverhulme Trust (RPG-2015-447), and SGC Chemicals for funding, T. M. Boyd (York) for experimental assistance and useful discussions, and Dr. M. Chadwick (Imperial College) for the initial synthesis of [1-isoprene][BAr ] . This work used the ARCHER UK National Supercomputing Service ( http://www.archer.ac.uk ) and the Cirrus UK National Tier-2 HPC Service at the EPCC ( http://www.cirrus.ac.uk ) funded by the University of Edinburgh and the EPSRC (EP/P020267/1).Using solid-state molecular organometallic (SMOM) techniques, in particular solid/gas single-crystal to single-crystal reactivity, a series of σ-alkane complexes of the general formula [Rh(Cy2PCH2CH2PCy2)(ηn:ηm-alkane)][BArF4] have been prepared (alkane = propane, 2-methylbutane, hexane, 3-methylpentane; ArF = 3,5-(CF3)2C6H3). These new complexes have been characterized using single crystal X-ray diffraction, solid-state NMR spectroscopy and DFT computational techniques and present a variety of Rh(I)···H-C binding motifs at the metal coordination site: 1,2-η2:η2 (2-methylbutane), 1,3-η2:η2 (propane), 2,4-η2:η2 (hexane), and 1,4-η1:η2 (3-methylpentane). For the linear alkanes propane and hexane, some additional Rh(I)···H-C interactions with the geminal C-H bonds are also evident. The stability of these complexes with respect to alkane loss in the solid state varies with the identity of the alkane: from propane that decomposes rapidly at 295 K to 2-methylbutane that is stable and instead undergoes an acceptorless dehydrogenation to form a bound alkene complex. In each case the alkane sits in a binding pocket defined by the {Rh(Cy2PCH2CH2PCy2)}+ fragment and the surrounding array of [BArF4]- anions. For the propane complex, a small alkane binding energy, driven in part by a lack of stabilizing short contacts with the surrounding anions, correlates with the fleeting stability of this species. 2-Methylbutane forms more short contacts within the binding pocket, and as a result the complex is considerably more stable. However, the complex of the larger 3-methylpentane ligand shows lower stability. Empirically, there therefore appears to be an optimal fit between the size and shape of the alkane and overall stability. Such observations are related to guest/host interactions in solution supramolecular chemistry and the holistic role of 1°, 2°, and 3° environments in metalloenzymes.Peer reviewe

Selectivity of Rh⋅⋅⋅H−C Binding in a σ-Alkane Complex Controlled by the Secondary Microenvironment in the Solid State

By using single-crystal to single-crystal solid-state molecular organometallic (SMOM) techniques, the σ -alkane complex [Rh( t Bu 2 PCH 2 CH 2 CH 2 P t Bu 2 )( η 2 , η 2 -C 7 H 12 )][BAr F 4 ] (Ar F = 3,5-(CF 3 ) 2 C 6 H 3 ) is synthesized and structurally characterized, in which the alkane (norbornane) binds through two exo -C-H···Rh interactions. In contrast, the bis-cyclohexyl phosphine analogue shows endo -alkane binding. Comparison of the two systems, supported by periodic DFT calculations, NCI plots and Hirshfeld surface analyses, trace this different regioselectivity to subtle changes in the local microenvironment surrounding the alkane ligand. A tertiary periodic structure supporting a secondary microenvironment that controls binding at the metal site has parallels with enzymes. The new σ -alkane complex is also a catalyst for solid/gas 1-butene isomerization, and catalyst resting states are identified for this

A Series of Crystallographically Characterized Linear and Branched s- Alkane Complexes of Rhodium: From Propane to 3-Methylpentane

Using solid-state molecular organometallic (SMOM) techniques, in particular solid/gas single-crystal to single-crystal reactivity, a series of σ-alkane complexes of the general formula [Rh(Cy2PCH2CH2PCy2)(ηn:ηm-alkane)][BArF4] have been prepared (alkane = propane, 2-methylbutane, hexane, 3-methylpentane; ArF = 3,5-(CF3)2C6H3). These new complexes have been characterized using single crystal X-ray diffraction, solid-state NMR spectroscopy and DFT computational techniques and present a variety of Rh(I)···H–C binding motifs at the metal coordination site: 1,2-η2:η2 (2-methylbutane), 1,3-η2:η2 (propane), 2,4-η2:η2 (hexane), and 1,4-η1:η2 (3-methylpentane). For the linear alkanes propane and hexane, some additional Rh(I)···H–C interactions with the geminal C–H bonds are also evident. The stability of these complexes with respect to alkane loss in the solid state varies with the identity of the alkane: from propane that decomposes rapidly at 295 K to 2-methylbutane that is stable and instead undergoes an acceptorless dehydrogenation to form a bound alkene complex. In each case the alkane sits in a binding pocket defined by the {Rh(Cy2PCH2CH2PCy2)}+ fragment and the surrounding array of [BArF4]− anions. For the propane complex, a small alkane binding energy, driven in part by a lack of stabilizing short contacts with the surrounding anions, correlates with the fleeting stability of this species. 2-Methylbutane forms more short contacts within the binding pocket, and as a result the complex is considerably more stable. However, the complex of the larger 3-methylpentane ligand shows lower stability. Empirically, there therefore appears to be an optimal fit between the size and shape of the alkane and overall stability. Such observations are related to guest/host interactions in solution supramolecular chemistry and the holistic role of 1°, 2°, and 3° environments in metalloenzymes

Spelling pronunciation and visual preview both facilitate learning to spell irregular word

Spelling pronunciations are hypothesized to be helpful in building up relatively stable phonologically underpinned orthographic representations, particularly for learning words with irregular phoneme-grapheme correspondences. In a four-week computer-based training, the efficacy of spelling pronunciations and previewing the spelling patterns on learning to spell loan words in Dutch, originating from French and English, was examined in skilled and less skilled spellers with varying ages. Reading skills were taken into account. Overall, compared to normal pronunciation, spelling pronunciation facilitated the learning of the correct spelling of irregular words, but it appeared to be no more effective than previewing. Differences between training conditions appeared to fade with older spellers. Less skilled young spellers seemed to profit more from visual examination of the word as compared to practice with spelling pronunciations. The findings appear to indicate that spelling pronunciation and allowing a preview can both be effective ways to learn correct spellings of orthographically unpredictable words, irrespective of age or spelling ability. Copyright © 2006 by The International Dyslexia Association®

Detecting functional magnetic resonance imaging activation in white matter: Interhemispheric transfer across the corpus callosum

<p>Abstract</p> <p>Background</p> <p>It is generally believed that activation in functional magnetic resonance imaging (fMRI) is restricted to gray matter. Despite this, a number of studies have reported white matter activation, particularly when the corpus callosum is targeted using interhemispheric transfer tasks. These findings suggest that fMRI signals may not be neatly confined to gray matter tissue. In the current experiment, 4 T fMRI was employed to evaluate whether it is possible to detect white matter activation. We used an interhemispheric transfer task modelled after neurological studies of callosal disconnection. It was hypothesized that white matter activation could be detected using fMRI.</p> <p>Results</p> <p>Both group and individual data were considered. At liberal statistical thresholds (p < 0.005, uncorrected), group level activation was detected in the isthmus of the corpus callosum. This region connects the superior parietal cortices, which have been implicated previously in interhemispheric transfer. At the individual level, five of the 24 subjects (21%) had activation clusters that were located primarily within the corpus callosum. Consistent with the group results, the clusters of all five subjects were located in posterior callosal regions. The signal time courses for these clusters were comparable to those observed for task related gray matter activation.</p> <p>Conclusion</p> <p>The findings support the idea that, despite the inherent challenges, fMRI activation can be detected in the corpus callosum at the individual level. Future work is needed to determine whether the detection of this activation can be improved by utilizing higher spatial resolution, optimizing acquisition parameters, and analyzing the data with tissue specific models of the hemodynamic response. The ability to detect white matter fMRI activation expands the scope of basic and clinical brain mapping research, and provides a new approach for understanding brain connectivity.</p

Prognostic model to predict postoperative acute kidney injury in patients undergoing major gastrointestinal surgery based on a national prospective observational cohort study.

Background: Acute illness, existing co-morbidities and surgical stress response can all contribute to postoperative acute kidney injury (AKI) in patients undergoing major gastrointestinal surgery. The aim of this study was prospectively to develop a pragmatic prognostic model to stratify patients according to risk of developing AKI after major gastrointestinal surgery. Methods: This prospective multicentre cohort study included consecutive adults undergoing elective or emergency gastrointestinal resection, liver resection or stoma reversal in 2-week blocks over a continuous 3-month period. The primary outcome was the rate of AKI within 7 days of surgery. Bootstrap stability was used to select clinically plausible risk factors into the model. Internal model validation was carried out by bootstrap validation. Results: A total of 4544 patients were included across 173 centres in the UK and Ireland. The overall rate of AKI was 14·2 per cent (646 of 4544) and the 30-day mortality rate was 1·8 per cent (84 of 4544). Stage 1 AKI was significantly associated with 30-day mortality (unadjusted odds ratio 7·61, 95 per cent c.i. 4·49 to 12·90; P < 0·001), with increasing odds of death with each AKI stage. Six variables were selected for inclusion in the prognostic model: age, sex, ASA grade, preoperative estimated glomerular filtration rate, planned open surgery and preoperative use of either an angiotensin-converting enzyme inhibitor or an angiotensin receptor blocker. Internal validation demonstrated good model discrimination (c-statistic 0·65). Discussion: Following major gastrointestinal surgery, AKI occurred in one in seven patients. This preoperative prognostic model identified patients at high risk of postoperative AKI. Validation in an independent data set is required to ensure generalizability