Electronic Desymmetrization of Cu(μ-E) Clusters (E = S, Se) Induced by Edge-to-Face π-Stacking Interactions in the Second Coordination Sphere.

A pair of cyclophane-encapsulated [Cu(μ-E)] complexes (E = S and Se) were characterized by resonant X-ray diffraction anomalous fine structure (DAFS), revealing unexpected polarization among the three Cu sites attributed to long-range effects of π-stacking interactions with cocrystallized benzene molecules. The resonant K-edge energies of individual Cu sites within the cluster molecules were found to vary as a function of distance from the cocrystallized benzene. This pattern was interpreted in the context of T-shaped, edge-to-face π-stacking with the assistance of theoretical charge density difference calculations.

Isocyanide Substituent Influences Reductive Elimination versus Migratory Insertion in Reaction with an [Fe(μ-H)] Complex.

Iron hydrides are proposed reactive intermediates for N and CO conversion in industrial and biological processes. Here, we report a reactivity study of a low-coordinate di(μ-hydrido)diiron(II) complex, Fe(μ-H), where is a bis(β-diketiminate) cyclophane, with isocyanides, which have electronic structures related to N and CO. The reaction outcome is influenced by the isocyanide substituent, with 2,6-xylyl isocyanide leading to H loss, to form a bis(μ-1,1-isocyanide)diiron(I) complex, whereas all of the other tested isocyanides insert into the Fe-H bond to give (μ-1,2-iminoformyl) complexes.

Ligand Noninnocence in β-Diketiminate and β-Diketimine Copper Complexes.

Metal-ligand cooperative systems have a long precedent in catalysis, with the classification depending on the site of substrate bond cleavage and formation and on redox state changes. Recently, our group reported the participation of a β-diketiminate ligand in chemical bonding to heterocumulenes such as CO and CS by tricopper complexes, leading to cooperative catalysis. Herein, we report the reactivity of these copper clusters, [CuE] (E = S, Se; = tris(β-diketiminate) cyclophane ligand), toward other electrophiles, alkyl halides and Brønsted acids.

Metal-Tuned Ligand Reactivity Enables CX (X = O, S) Homocoupling with Spectator Cu Centers.

Ligand non-innocence is ubiquitous in catalysis with ligands in synthetic complexes contributing as electron reservoirs or co-sites for substrate activation. The latter chemical non-innocence is manifested in H storage or relay at sites beyond the metal primary coordination sphere. Reaction of a competent CO-to-oxalate reduction catalyst, namely, [K(THF)](CuS), where is a tris(β-diketiminate) cyclophane, with CS affords tetrathiooxalate at long reaction times or at high CS concentrations, where otherwise an equilibrium is established between the starting species and a complex-CS adduct in which the CS is bound to the C atom on the ligand backbone.

Metal Site-Specific Electrostatic Field Effects on a Tricopper(I) Cluster Probed by Resonant Diffraction Anomalous Fine Structure (DAFS).

Studies of multinuclear metal complexes are greatly enhanced by resonant diffraction measurements, which probe X-ray absorption profiles of crystallographically independent metal sites within a cluster. In particular, X-ray diffraction anomalous fine structure (DAFS) analysis provides data that can be interpreted akin to site-specific XANES, allowing for differences in metal K-edge resonances to be deconvoluted even for different metal sites within a homometallic system. Despite the prevalence of Cu-containing clusters in biology and energy science, DAFS has yet to be used to analyze multicopper complexes of any type until now.

Partial Deoxygenative CO Homocoupling by a Diiron Complex.

One route to address climate change is converting carbon dioxide to synthetic carbon-neutral fuels. Whereas carbon dioxide to CO conversion has precedent in homo- and heterogeneous catalysis, deoxygenative coupling of CO to products with C-C bonds-as in liquid fuels-remains challenging. Here, we report coupling of two CO molecules by a diiron complex.

Dinitrogen Coordination to a High-Spin Diiron(I/II) Species.

Dinitrogen coordination to iron centers underpins industrial and biological fixation in the Haber-Bosch process and by the FeM cofactors in the nitrogenase enzymes. The latter employ local high-spin metal centers; however, iron-dinitrogen coordination chemistry remains dominated by low-valent states, contrasting the enzyme systems. Here, we report a high-spin mixed-valent cis-(μ-1,2-dinitrogen)diiron(I/II) complex [(FeBr) (μ-N )L ] (2), where [L ] is a bis(β-diketiminate) cyclophane.

Access to Metal Centers and Fluxional Hydride Coordination Integral for CO Insertion into [Fe(μ-H)] Clusters.

CO insertion into tri(μ-hydrido)triiron(II) clusters ligated by a tris(β-diketiminate) cyclophane is demonstrated to be balanced by sterics for CO approach and hydride accessibility. Time-resolved NMR and UV-vis spectra for this reaction for a complex in which methoxy groups border the pocket of the hydride donor (FeH, ) result in a decreased activation barrier and increased kinetic isotope effect consistent with the reduced sterics. For the ethyl congener FeH (), no correlation is found between rate and reaction solvent or added Lewis acids, implying CO coordination to an Fe center in the mechanism.

Dinitrogen Insertion and Cleavage by a Metal-Metal Bonded Tricobalt(I) Cluster.

Reduction of a tricobalt(II) tri(bromide) cluster supported by a tris(β-diketiminate) cyclophane results in halide loss, ligand compression, and metal-metal bond formation to yield a 48-electron Co cluster, Co (). Upon reaction of with dinitrogen, all metal-metal bonds are broken, steric conflicts are relaxed, and dinitrogen is incorporated within the internal cavity to yield a formally (μ-η:η:η-dinitrogen)tricobalt(I) complex, . Broken symmetry DFT calculations (PBE0/def2-tzvp/D3) support an N-N bond order of 2.

Cleavage of cluster iron-sulfide bonds in cyclophane-coordinated FeS complexes.

Reaction of the tri(μ-sulfido)triiron(iii) tris(β-diketiminate) cyclophane complex, FeSL (1), or of the di(μ-sulfido)diiron(iii) complex FeSHL (5), with the related tri(bromide)triiron(ii) complex FeBrL (2) results in electron and ligand redistribution to yield the mixed-ligand multiiron complexes, including FeBrSL (3) and FeBrSHL (4). The cleavage and redistribution observed in these complexes is reminiscent of necessary Fe-S bond cleavage for substrate activation in nitrogenase enzymes, and provides a new perspective on the lability of Fe-S bonds in FeS clusters.

Synthetic Factors Governing Access to Tris(β-diketimine) Cyclophanes versus Tripodal Tri-β-aminoenones.

Tris(β-diketimine) cyclophanes are an important ligand class for investigating cooperative multimetallic interactions of bioinorganic clusters. Discussed herein are the synthetic factors governing access to tris(β-diketimine) cyclophanes versus tripodal tri-β-aminoenones. Cyclophanes bearing Me, Et, and MeO cap substituents and β-Me, Et, or Ph arm substituents are obtained, and a modified condensation method produced α-Me β-Me cyclophane.

Evaluating Metal Ion Identity on Catalytic Silylation of Dinitrogen Using a Series of Trimetallic Complexes.

We report catalytic silylation of dinitrogen to tris(trimethylsilyl)amine by a series of trinuclear first row transition metal complexes (M = Cr, Mn, Fe, Co, Ni) housed in our tris(β-diketiminate) cyclophane ( ). Yields are expectedly dependent on metal ion type ranging from 14 to 199 equiv NH /complex after protonolysis for the Mn to Co congeners, respectively. For the series of complexes, the number of turnovers trend observed is Co > Fe > Cr > Ni > Mn, consistent with prior reports of greater efficacy of Co over Fe in other ligand systems for this reaction.

Activation of Dinitrogen by Polynuclear Metal Complexes.

Activation of dinitrogen plays an important role in daily anthropogenic life, and the processes by which this fixation occurs have been a longstanding and significant research focus within the community. One of the major fields of dinitrogen activation research is the use of multimetallic compounds to reduce and/or activate N into a more useful nitrogen-atom source, such as ammonia. Here we report a comprehensive review of multimetallic-dinitrogen complexes and their utility toward N activation, beginning with the -block metals from Group 4 to Group 11, then extending to Group 13 (which is exclusively populated by B complexes), and finally the rare-earth and actinide species.

Carbon Dioxide Insertion into Bridging Iron Hydrides: Kinetic and Mechanistic Studies.

The reduction of CO to formic acid by transition metal hydrides is a potential pathway to access reactive C1 compounds. To date, no kinetic study has been reported for insertion of a bridging hydride in a weak-field ligated complex into CO; such centers have relevance to metalloenzymes that catalyze this reaction. Herein, we report the kinetic study of the reaction of a tri(-hydride)triiron(II/II/II) cluster supported by a tris(β-diketimine) cyclophane () with CO monitored by H-NMR and temperature-controlled UV-vis spectroscopy.

Vitamin B12 for the treatment of vasoplegia in cardiac surgery and liver transplantation: a narrative review of cases and potential biochemical mechanisms.

Purpose: Hydroxocobalamin, or vitamin B12 (V-B12), is frequently used to treat smoke inhalation and cyanide poisoning. Recent reports have also described its use to treat vasoplegia in cardiac surgery and liver transplantation. This narrative review discusses this "off-label" indication for V-B12, focusing on the potential biochemical mechanisms of its actions.

Isolation of chloride- and hydride-bridged tri-iron and -zinc clusters in a tris(β-oxo-δ-diimine) cyclophane ligand.

A cyclophane ligand (H6L) bearing three β-oxo-δ-diimine arms and the corresponding tri-iron and -zinc complexes in which the metal ions are bridged by either chlorides, viz. Fe3Cl3(H3L) (1) and Zn3Cl3(H3L) (2), or hydrides, viz. Fe3H3(H3L) (3), Zn3H3(H3L) (4), were synthesized and characterized.

Cyclophanes as Platforms for Reactive Multimetallic Complexes.

Multimetallic cofactors supported by weak-field donors frequently function as reaction centers in metalloproteins, and many of these cofactors catalyze small molecule activation (e.g., N, O, CO) with prominent roles in geochemical element cycles or detoxification.

Catalytic Silylation of Dinitrogen by a Family of Triiron Complexes.

A series of triiron complexes supported by a tris(β-diketiminate)cyclophane ( ) catalyze the reduction of dinitrogen to tris(trimethylsilyl)amine using KC and MeSiCl. Employing FeBr affords 83 ± 7 equiv. NH /complex after protonolysis, which is a 50% yield based on reducing equivalents.

Chalcogen Impact on Covalency within Molecular [Cu(μ-E)] Clusters (E = O, S, Se): A Synthetic, Spectroscopic, and Computational Study.

Reaction of the tricopper(I)-dinitrogen tris(β-diketiminate) cyclophane, Cu(N)L, with O-atom-transfer reagents or elemental Se affords the oxido-bridged tricopper complex Cu(μ-O)L (2) or the corresponding Cu(μ-Se)L (4), respectively. For 2 and 4, incorporation of the bridging chalcogen donor was supported by electrospray ionization mass spectrometry and K-edge X-ray absorption spectroscopy (XAS) data. Cu L-edge X-ray absorption data quantify 49.

A Tricopper(I) Complex Competent for O Atom Transfer, C-H Bond Activation, and Multiple O Activation Steps.

Oxygenation of a tricopper(I) cyclophanate (1) affords reactive transients competent for C-H bond activation and O atom transfer to various substrates (including toluene, dihydroanthracene, and ethylmethylsulfide) based on H NMR, gas chromatography/mass spectrometry (MS), and electrospray ionization (ESI)/MS data. Low product yields (<1%) are determined for C-H activation substrates (e.g, toluene, ethylbenzene), which we attribute to competitive ligand oxidation.

Countercations and Solvent Influence CO Reduction to Oxalate by Chalcogen-Bridged Tricopper Cyclophanates.

One-electron reduction of CuEL (L = tris(β-diketiminate)cyclophane, and E = S, Se) affords [CuEL], which reacts with CO to yield exclusively CO (95% yield, TON = 24) and regenerate CuEL. Stopped-flow UV/visible data support an A→B mechanism under pseudo-first-order conditions ( k = 115(2) s), which is 10 larger than those for reported copper complexes. The k values are dependent on the countercation and solvent (e.

Correlating Bridging Ligand with Properties of Ligand-Templated [MnX] Clusters (X = Br, Cl, H, MeO).

Polynuclear manganese compounds have garnered interest as mimics and models of the water oxidizing complex (WOC) in photosystem II and as single molecule magnets. Molecular systems in which composition can be correlated to physical phenomena, such as magnetic exchange interactions, remain few primarily because of synthetic limitations. Here, we report the synthesis of a family of trimanganese(II) complexes of the type MnXL (X = Cl, H, and MeO) where L is a tris(β-diketiminate) cyclophane.

Reactivity of hydride bridges in a high-spin [Fe(μ-H)] cluster: reversible H/CO exchange and Fe-H/B-F bond metathesis.

The triiron trihydride complex FeH () [where is a tris(β-diketiminate)cyclophanate] reacts with CO and with BF·OEt to afford (FeCO)Fe(μ-H) () and FeF (), respectively. Variable-temperature and applied-field Mössbauer spectroscopy support the assignment of two high-spin (HS) iron(i) centers and one HS iron(ii) ion in . Preliminary studies support a CO-induced reductive elimination of H from , rather than CO trapping a species from an equilibrium mixture.

Insights into small molecule activation by multinuclear first-row transition metal cyclophanates.

The rational design of trimetallic transition metal clusters supported by a trinucleating cyclophane ligand, L(3-), and the reactivities of these complexes with dinitrogen and carbon dioxide are discussed. Emphasis is placed on the differences in the observed reactivity between these trimetallic cyclophane complexes and that of the mono- and dinuclear transition metal compounds.

A three-coordinate Fe(ii) center within a [3Fe-(μ3-S)] cluster that provides an accessible coordination site.

A μ3-sulfide bridged triiron cluster(ii,ii,iii) supported by a cyclophane ligand undergoes metal-based reduction to yield an all-ferrous species. The latter complex incorporates a three-coordinate iron center that provides an accessible coordination site to a solvent molecule.