Enhanced Fe-centered redox flexibility in Fe–Ti heterobimetallic complexes

Previously, we reported the synthesis of Ti[N(o-(NCH2P(iPr)2)C6H4)3] and the Fe–Ti complex, FeTi[N(o-(NCH2P(iPr)2)C6H4)3], abbreviated as TiL (1), and FeTiL (2), respectively. Herein, we describe the synthesis and characterization of the complete redox families of the monometallic Ti and Fe–Ti compounds. Cyclic voltammetry studies on FeTiL reveal both reduction and oxidation processes at −2.16 and −1.36 V (versus Fc/Fc+), respectively. Two isostructural redox members, [FeTiL]+ and [FeTiL]− (2ox and 2red, respectively) were synthesized and characterized, along with BrFeTiL (2-Br) and the monometallic [TiL]+ complex (1ox). The solid-state structures of the [FeTiL]+/0/– series feature short metal–metal bonds, ranging from 1.94–2.38 Å, which are all shorter than the sum of the Ti and Fe single-bond metallic radii (cf. 2.49 Å). To elucidate the bonding and electronic structures, the complexes were characterized with a host of spectroscopic methods, including NMR, EPR, and 57Fe Mössbauer, as well as Ti and Fe K-edge X-ray absorption spectroscopy (XAS). These studies, along with hybrid density functional theory (DFT) and time-dependent DFT calculations, suggest that the redox processes in the isostructural [FeTiL]+,0,– series are primarily Fe-based and that the polarized Fe–Ti π-bonds play a role in delocalizing some of the additional electron density from Fe to Ti (net 13%)

Room-Temperature Distance Measurements of Immobilized Spin-Labeled Protein by DEER/PELDOR

Nitroxide spin labels are used for double electron-electron resonance (DEER) measurements of distances between sites in biomolecules. Rotation of gem-dimethyls in commonly used nitroxides causes spin echo dephasing times (Tm) to be too short to perform DEER measurements at temperatures between ∼80 and 295 K, even in immobilized samples. A spirocyclohexyl spin label has been prepared that has longer Tm between 80 and 295 K in immobilized samples than conventional labels. Two of the spirocyclohexyl labels were attached to sites on T4 lysozyme introduced by site-directed spin labeling. Interspin distances up to ∼4 nm were measured by DEER at temperatures up to 160 K in water/glycerol glasses. In a glassy trehalose matrix the Tm for the doubly labeled T4 lysozyme was long enough to measure an interspin distance of 3.2 nm at 295 K, which could not be measured for the same protein labeled with the conventional 1-oxyl-2,2,5,5-tetramethyl-3-pyrroline-3- (methyl)methanethio-sulfonate label

Room-Temperature Distance Measurements of Immobilized Spin-Labeled Protein by DEER/PELDOR

Nitroxide spin labels are used for double electron-electron resonance (DEER) measurements of distances between sites in biomolecules. Rotation of gem-dimethyls in commonly used nitroxides causes spin echo dephasing times (Tm) to be too short to perform DEER measurements at temperatures between ∼80 and 295 K, even in immobilized samples. A spirocyclohexyl spin label has been prepared that has longer Tm between 80 and 295 K in immobilized samples than conventional labels. Two of the spirocyclohexyl labels were attached to sites on T4 lysozyme introduced by site-directed spin labeling. Interspin distances up to ∼4 nm were measured by DEER at temperatures up to 160 K in water/glycerol glasses. In a glassy trehalose matrix the Tm for the doubly labeled T4 lysozyme was long enough to measure an interspin distance of 3.2 nm at 295 K, which could not be measured for the same protein labeled with the conventional 1-oxyl-2,2,5,5-tetramethyl-3-pyrroline-3- (methyl)methanethio-sulfonate label

Rapid-Scan EPR of Immobilized Nitroxides

X-band electron paramagnetic resonance spectra of immobilized nitroxides were obtained by rapid scan at 293 K. Scan widths were 155 G with 13.4 kHz scan frequency for 14N-perdeuterated tempone and for T4 lysozyme doubly spin labeled with an iodoacetamide spirocyclohexyl nitroxide and 100 G with 20.9 kHz scan frequency for 15N-perdeuterated tempone. These wide scans were made possible by modifications to our rapid-scan driver, scan coils made of Litz wire, and the placement of highly conducting aluminum plates on the poles of a Bruker 10 magnet to reduce resistive losses in the magnet pole faces. For the same data acquisition time, the signal-to-noise for the rapid-scan absorption spectra was about an order of magnitude higher than for continuous wave first-derivative spectra recorded with modulation amplitudes that do not broaden the lineshapes

Rapid-Scan EPR of Immobilized Nitroxides

X-band electron paramagnetic resonance spectra of immobilized nitroxides were obtained by rapid scan at 293 K. Scan widths were 155 G with 13.4 kHz scan frequency for 14N-perdeuterated tempone and for T4 lysozyme doubly spin labeled with an iodoacetamide spirocyclohexyl nitroxide and 100 G with 20.9 kHz scan frequency for 15N-perdeuterated tempone. These wide scans were made possible by modifications to our rapid-scan driver, scan coils made of Litz wire, and the placement of highly conducting aluminum plates on the poles of a Bruker 10 magnet to reduce resistive losses in the magnet pole faces. For the same data acquisition time, the signal-to-noise for the rapid-scan absorption spectra was about an order of magnitude higher than for continuous wave first-derivative spectra recorded with modulation amplitudes that do not broaden the lineshapes

Australian clinical practice guidelines for the diagnosis and management of Barrett's esophagus and early esophageal adenocarcinoma

Author version made available following 12 month embargo from date of publication according to publisher copyright policy.Barrett's esophagus (BE), a common condition, is the only known precursor to esophageal adenocarcinoma (EAC). There is uncertainty about the best way to manage BE as most people with BE never develop EAC and most patients diagnosed with EAC have no preceding diagnosis of BE. Moreover, there have been recent advances in knowledge and practice about the management of BE and early EAC. To aid clinical decision making in this rapidly moving field, Cancer Council Australia convened an expert working party to identify pertinent clinical questions. The questions covered a wide range of topics including endoscopic and histological definitions of BE and early EAC; prevalence, incidence, natural history, and risk factors for BE; and methods for managing BE and early EAC. The latter considered modification of lifestyle factors; screening and surveillance strategies; and medical, endoscopic, and surgical interventions. To answer each question, the working party systematically reviewed the literature and developed a set of recommendations through consensus. Evidence underpinning each recommendation was rated according to quality and applicability

Configuring Bonds between First-Row Transition Metals

ConspectusAlfred Werner, who pioneered the field of coordination chemistry, envisioned coordination complexes as a single, transition metal atom at the epicenter of a vast ligand space. The idea that the locus of a coordination complex could be shared by multiple metals held together with covalent bonds would eventually lead to the discovery of the quadruple and quintuple bond, which have no analogues outside of the transition metal block. Metal–metal bonding can be classified into homometallic and heterometallic groups. Although the former is dominant, the latter is arguably more intriguing because of the inherently larger chemical space in which metal–metal bonding can be explored.In 2013, Lu and Thomas independently reported the isolation of heterometallic multiple bonds with exclusively first-row transition metals. Structural and theoretical data supported triply bonded Fe–Cr and Fe–V cores. This Account describes our continued efforts to configure bonds between first-row transition metals from titanium to copper. Double-decker ligands, or binucleating platforms that brace two transition metals in proximity, have enabled the modular synthesis of diverse metal–metal complexes. The resulting complexes are also ideal for investigating the effects of an “ancillary” metal on the properties and reactivities of an “active” metal center.A total of 38 bimetallic complexes have been compiled comprising 18 unique metal–metal pairings. Twenty-one of these bimetallics are strictly isostructural, allowing for a systematic comparison of metal–metal bonding. The nature of the chemical bond between first-row metals is remarkably variable and depends on two primary factors: the total d-electron count, and the metals’ relative d-orbital energies. Showcasing the range of covalent bonding are a quintuply bonded (d-d)¹⁰ Mn–Cr heterobimetallic and the singly bonded late–late pairings, e.g., Fe–Co, which adopt unusually high spin states.A long-term goal is to rationally tailor the properties and reactivities of the bimetallic complexes. In some cases, synergistic redox and magnetic properties were found that are different from the expected sum of the individual metals. Intermetal charge transfer was shown in a Co–M series, for M = Mn to Cu, where the transition energy decreases as M is varied across the first-row period. The potential of using metal–metal complexes for multielectron reduction of small-molecules is addressed by N₂ binding studies and a mechanistic study of a dicobalt catalyst in reductive silylation of N₂ to N­(SiMe₃)₃. Finally, metal-ion exchange reactions with metal–metal complexes can be selective under appropriate reaction conditions, providing an alternative synthetic route to metal–metal species

Bimetallic Cobalt–Dinitrogen Complexes: Impact of the Supporting Metal on N₂ Activation

Expanding a family of cobalt bimetallic complexes, we report the synthesis of the Ti­(III) metalloligand, Ti­[N­(<i>o-</i>(NCH₂P­(^<i>i</i>Pr)₂)­C₆H₄)₃] (abbreviated as TiL), and three heterobimetallics that pair cobalt with an early transition metal ion: CoTiL (<b>1</b>), K­(crypt-222)­[(N₂)­CoVL] (<b>2</b>), and K­(crypt-222)­[(N₂)­CoCrL] (<b>3</b>). The latter two complexes, along with previously reported K­(crypt-222)­[(N₂)­CoAlL] and K­(crypt-222)­[(N₂)­Co₂L], constitute an isostructural series of cobalt bimetallics that bind dinitrogen in an end-on fashion, i.e. [(N₂)­CoML]⁻. The characterization of <b>1</b>–<b>3</b> includes cyclic voltammetry, X-ray crystallography, and infrared spectroscopy. The [CoTiL]^0/– reduction potential is extremely negative at −3.20 V versus Fc⁺/Fc. In the CoML series where M is a transition metal, the reduction potentials shift anodically as M is varied across the first-row period. Among the [(N₂)­CoML]⁻ compounds, the dinitrogen ligand is weakly activated, as evidenced by N–N bond lengths between 1.110(8) and 1.135(4) Å and by N–N stretching frequencies between 1971 and 1995 cm^–1. Though changes in ν_N₂ are subtle, the extent of N₂ activation decreases across the first-row period. A correlation is found between the [CoML]^0/– reduction potentials and N₂ activation, where the more cathodic potentials correspond to lower N–N frequencies. Theoretical calculations of the [(N₂)­CoML]⁻ complexes reveal important variations in the electronic structure and Co–M interactions, which depend on the exact nature of the supporting metal ion, M

Bimetallic Cobalt–Dinitrogen Complexes: Impact of the Supporting Metal on N₂ Activation

Expanding a family of cobalt bimetallic complexes, we report the synthesis of the Ti­(III) metalloligand, Ti­[N­(<i>o-</i>(NCH₂P­(^<i>i</i>Pr)₂)­C₆H₄)₃] (abbreviated as TiL), and three heterobimetallics that pair cobalt with an early transition metal ion: CoTiL (<b>1</b>), K­(crypt-222)­[(N₂)­CoVL] (<b>2</b>), and K­(crypt-222)­[(N₂)­CoCrL] (<b>3</b>). The latter two complexes, along with previously reported K­(crypt-222)­[(N₂)­CoAlL] and K­(crypt-222)­[(N₂)­Co₂L], constitute an isostructural series of cobalt bimetallics that bind dinitrogen in an end-on fashion, i.e. [(N₂)­CoML]⁻. The characterization of <b>1</b>–<b>3</b> includes cyclic voltammetry, X-ray crystallography, and infrared spectroscopy. The [CoTiL]^0/– reduction potential is extremely negative at −3.20 V versus Fc⁺/Fc. In the CoML series where M is a transition metal, the reduction potentials shift anodically as M is varied across the first-row period. Among the [(N₂)­CoML]⁻ compounds, the dinitrogen ligand is weakly activated, as evidenced by N–N bond lengths between 1.110(8) and 1.135(4) Å and by N–N stretching frequencies between 1971 and 1995 cm^–1. Though changes in ν_N₂ are subtle, the extent of N₂ activation decreases across the first-row period. A correlation is found between the [CoML]^0/– reduction potentials and N₂ activation, where the more cathodic potentials correspond to lower N–N frequencies. Theoretical calculations of the [(N₂)­CoML]⁻ complexes reveal important variations in the electronic structure and Co–M interactions, which depend on the exact nature of the supporting metal ion, M

Heterobimetallic Complexes That Bond Vanadium to Iron, Cobalt, and Nickel

Zero-valent iron, cobalt, and nickel were installed into the metalloligand V­[N­(<i>o</i>-(NCH₂P­(ⁱPr)₂)­C₆H₄)₃] (<b>1</b>, VL), generating the heterobimetallic trio FeVL (<b>2</b>), CoVL (<b>3</b>), and NiVL (<b>4</b>), respectively. In addition, the one-electron-oxidized analogues [FeVL]­X ([<b>2</b>^<b>ox</b>]­X, where X^– = BPh₄ or PF₆) and [CoVL]­BPh₄ ([<b>3</b>^<b>ox</b>]­BPh₄) were prepared. The complexes were characterized by a host of physical methods, including cyclic voltammetry, X-ray crystallography, magnetic susceptibility, electronic absorption, NMR, electron paramagnetic resonance (EPR), and Mössbauer spectroscopies. The CoV and FeV heterobimetallic compounds have short M–V bond lengths that are consistent with M–M multiple bonding. As revealed by theoretical calculations, the M–V bond is triple in <b>2</b>, <b>2</b>^<b>ox</b>, and <b>3</b>^<b>ox</b>, double in <b>3</b>, and dative (Ni → V) in <b>4</b>. The (d–d)¹⁰ species, <b>2</b> and <b>3</b>^<b>ox</b>, are diamagnetic and exhibit large diamagnetic anisotropies of −4700 × 10^–36 m³/molecule. Complexes <b>2</b> and <b>3</b>^<b>ox</b> are also characterized by intense visible bands at 760 and 610 nm (ε > 1000 M^–1 cm^–1), respectively, which correspond to an intermetal (M → V) charge-transfer transition. Magnetic susceptibility measurements and EPR characterization establish <i>S</i> = ¹/₂ ground states for (d–d)⁹ <b>2</b>^<b>ox</b> and (d–d)¹¹ <b>3</b>, while (d–d)¹² <b>4</b> is <i>S</i> = 1 based on Evans’ method