In-plane Coordination Research Articles

Bipyridine-Confined Silver Single-Atom Catalysts Facilitate In-Plane C-O Coupling for Propylene Electrooxidation.

The electrooxidation of propylene presents a promising route for the production of 1,2-propylene glycol (PG) under ambient conditions. However, the C-O coupling process remains a challenge owing to the high energy barrier. In this work, we developed a highly efficient electrocatalyst of bipyridine-confined Ag single atoms on UiO-bpy substrates (Ag SAs/UiO-bpy), which exposed two in-plane coordination vacancies during reaction for the co-adsorption of key intermediates. Detailed structure and electronic property analyses demonstrate that CH3CHCH2OH* and *OH could stably co-adsorb in a square planar configuration, which then accelerates the charge transfer between them. The combination of stable co-adsorption and efficient charge transfer facilitates the C-O coupling process, thus significantly lowering its energy barrier. At 2.4 V versus a reversible hydrogen electrode, Ag SAs/UiO-bpy achieved a record-high activity of 61.9 gPG m-2 h-1. Our work not only presents a robust electrocatalyst but also advances a new perspective on catalyst design for propylene electrooxidation.

Chemical Bonding and Dynamic Structural Fluxionality of a Boron-Based Na5B7 Sandwich Cluster.

Doping alkali metals into boron clusters can effectively compensate for the intrinsic electron deficiency of boron and lead to interesting boron-based binary clusters, owing to the small electronegativity of the former elements. We report on the computational design of a three-layered sandwich cluster, Na5B7, on the basis of global-minimum (GM) searches and electronic structure calculations. It is shown that the Na5B7 cluster can be described as a charge-transfer complex: [Na4]2+[B7]3-[Na]+. In this sandwich cluster, the [B7]3- core assumes a molecular wheel in shape and features in-plane hexagonal coordination. The magic 6π/6σ double aromaticity underlies the stability of the [B7]3- molecular wheel, following the (4n + 2) Hückel rule. The tetrahedral Na4 ligand in the sandwich has a [Na4]2+ charge-state, which is the simplest example of three-dimensional aromaticity, spherical aromaticity, or superatom. Its 2σ electron counting renders σ aromaticity for the ligand. Overall, the sandwich cluster has three-fold 6π/6σ/2σ aromaticity. Molecular dynamics simulation shows that the sandwich cluster is dynamically fluxional even at room temperature, with a negligible energy barrier for intramolecular twisting between the B7 wheel and the Na4 ligand. The Na5B7 cluster offers a new example for dynamic structural fluxionality in molecular systems.

Cover Feature: On‐Surface‐Assembled Single‐Layer Metal‐Organic Frameworks with Extended Conjugation (ChemPlusChem 5/2023)

The cover feature image shows the construction of single-layer metal-organic frameworks with extended conjugation (SL-cMOFs) by on-surface coordination assembly. SL-cMOFs exhibit electronic bands that lead to high conductivity or non-trivial quantum phases. On-surface coordination assembly, in which molecular linkers and metal atoms assemble on atomic flat substrates via in-plane coordination, offers a unique approach for preparing SL-cMOFs. Experimental characterizations of the geometries and conjugated band structures of SL-cMOFs have been achieved by surface techniques such as scanning tunneling microscopy and spectroscopy. More information can be found in the Review by Jing Liu and Nian Lin.

17 O Hyperfine Spectroscopy Reveals Hydration Structure of Nitroxide Radicals in Aqueous Solutions.

The hydration structure of nitroxide radicals in aqueous solutions is elucidated by advanced 17 O hyperfine (hf) spectroscopy with support of quantum chemical calculations and MD simulations. A piperidine and a pyrrolidine-based nitroxide radical are compared and show clear differences in the preferred directionality of H-bond formation. We demonstrate that these scenarios are best represented in 17 O hf spectra, where in-plane coordination over -type H-bonding leads to little spin density transfer on the water oxygen and small hf couplings, whereas -type perpendicular coordination generates much larger hf couplings. Quantitative analysis of the spectra based on MD simulations and DFT predicted hf parameters is consistent with a distribution of close solvating water molecules, in which directionality is influenced by subtle steric effects of the ring and the methyl group substituents.

2D conjugated metal-organic framework as a proton-electron dual conductor

<h2>Summary</h2> Herein, we report a Zn-based dual-conductive metal-organic framework (MOF), Zn-HHTP-H₂O, that simultaneously conducts electrons and protons. We employed an octahedral Zn node with axial H₂O ligands to induce proton transport while in-plane HHTP coordination provides d-π conjugation for electron transport. The axial H₂O as a weak binding ligand served as functionalizable sites through post-synthetic ligand substitution. Zn-HHTP-H₂O shows 4.5 × 10⁻² and 1.6 × 10⁻⁵ S cm⁻¹ of electrical and proton conductivities, respectively, at 343 K and 95% RH. Upon post-synthetic urea treatment of Zn-HHTP-H₂O, the proton conductivity was 10 times higher than the pristine MOF, while electrical conductivity remained in the same order. We believe that our system provides a chemically intriguing platform, valuable for the further development of dual-conducting materials.

Comparing Cyanophenyl and Pyridyl Ligands in the Formation of Porphyrin-Based Metal-Organic Coordination Networks.

In recent studies, porphyrin derivatives have been frequently used as building blocks for the fabrication of metal–organic coordination networks (MOCNs) on metal surfaces under ultrahigh vacuum conditions (UHV). The porphyrin core can host a variety of 3d transition metals, which are usually incorporated in solution. However, the replacement of a pre-existing metal atom in the porphyrin core by a different metallic species has been rarely reported under UHV. Herein, we studied the influence of cyanophenyl and pyridyl functional endgroups in the self-assembly of structurally different porphyrin-based MOCNs by the deposition of Fe atoms on tetracyanophenyl (Co-TCNPP) and tetrapyridyl-functionalized (Zn-TPPyP) porphyrins on Au(111) by means of scanning tunneling microscopy (STM). A comparative analysis of the influence of the cyano and pyridyl endgroups on the formation of different in-plane coordination motifs is performed. Each porphyrin derivative formed two structurally different Fe-coordinated MOCNs stabilized by three- and fourfold in-plane coordination nodes, respectively. Interestingly, the codeposited Fe atoms did not only bind to the functional endgroups but also reacted with the porphyrin core of the Zn-substituted porphyrin (Zn-TPyP), i.e., an atom exchange reaction took place in the porphyrin core where the codeposited Fe atoms replaced the Zn atoms. This was evidenced by the appearance of molecules with an enhanced (centered) STM contrast compared with the appearance of Zn-TPyP, which suggested the formation of a new molecular species, i.e., Fe-TPPyP. Furthermore, the porphyrin core of the Co-substituted porphyrin (Co-TCNPP) displayed an off-centered STM contrast after the deposition of Fe atoms, which was attributed to the binding of the Fe atoms on the top site of the Co-substituted porphyrin core. In summary, the deposition of metal atoms onto organic layers can steer the formation of structurally different MOCNs and may replace pre-existing metal atoms contained in the porphyrin core.

Atomic Undercoordination in Ag Islands on Ru(0001) Grown via Size-Selected Cluster Deposition: An Experimental and Theoretical High-Resolution Core-Level Photoemission Study.

The possibility of depositing precisely mass-selected Ag clusters (Ag1, Ag3, and Ag7) on Ru(0001) was instrumental in determining the importance of the in-plane coordination number (CN) and allowed us to establish a linear dependence of the Ag 3d5/2 core-level shift on CN. The fast cluster surface diffusion at room temperature, caused by the low interaction between silver and ruthenium, leads to the formation of islands with a low degree of ordering, as evidenced by the high density of low-coordinated atomic configurations, in particular CN = 4 and 5. On the contrary, islands formed upon Ag7 deposition show a higher density of atoms with CN = 6, thus indicating the formation of islands with a close-packed atomic arrangement. This combined experimental and theoretical approach, when applied to clusters of different elements, offers the perspective to reveal nonequivalent local configurations in two-dimensional (2D) materials grown using different building blocks, with potential implications in understanding electronic and reactivity properties at the atomic level.

Thermodynamic stability of borophene, B2O3 and other B1−xOx sheets

The recent discovery of borophene, a two-dimensional allotrope of boron, raises many questions about its structure and its chemical and physical properties. Boron has a high chemical affinity to oxygen but little is known about the oxidation behaviour of borophene. Here we use first principles calculations to study the phase diagram of free-standing, two-dimensional B1−xOx for compositions ranging from x = 0 to x = 0.6, which correspond to borophene and sheets, respectively. Our results indicate that no stable compounds except borophene and sheets exist. Intermediate compositions are heterogeneous mixtures of borophene and . Other hypothetical crystals such as are unfavorable and some of them underwent spontaneous disproportionation into borophene and . It is also shown that oxidizing borophene inside the flakes is thermodynamically unfavorable over forming at the edges. All findings can be rationalized by oxygen’s preference of two-fold coordination which is incompatible with higher in-plane coordination numbers preferred by boron. These results agree well with recent experiments and pave the way to better understand the process of oxidation of borophene and other two-dimensional materials.

Three-dimensional docking of alcohols to ketones: an experimental benchmark based on acetophenone solvation energy balances.

The two hydrogen bond solvation sites exhibited by the carbonyl group in acetophenone are influenced by alkylation of the methyl group in both the acetophenone and in the prototype solvent methanol, largely due to London dispersion forces. Phenyl docking and alkyl docking preferences can be realized at will by appropriate substitution. In particular, cyclopropylation helps to stabilize the opposite phenyl docking site. In all cases, the energy gap is small enough to allow for a simultaneous detection even under low temperature conditions. This density functional prediction is checked experimentally by jet FTIR spectroscopy and largely confirmed. A spurious out-of-plane solvation preference predicted for cyclopropylphenylketone with tert-butyl alcohol by B3LYP-D3 calculations is not confirmed experimentally. It is unlikely that this discrepancy is due to zero-point energy effects. Instead, the second most stable alkyl-side solvation motif predicted with a more in-plane coordination is found in the jet expansion. Overall, the ability of carbonyl solvation balances to benchmark subtle electronic structure effects for non-covalent interactions without major nuclear motion corrections is supported.

Complex Magnetic Exchange Coupling between Co Nanostructures and Ni(111) across Epitaxial Graphene.

We report on the magnetic coupling between isolated Co atoms as well as small Co islands and Ni(111) mediated by an epitaxial graphene layer. X-ray magnetic circular dichroism and scanning tunneling microscopy combined with density functional theory calculations reveal that Co atoms occupy two distinct adsorption sites, with different magnetic coupling to the underlying Ni(111) surface. We further report a transition from an antiferromagnetic to a ferromagnetic coupling with increasing Co cluster size. Our results highlight the extreme sensitivity of the exchange interaction mediated by graphene to the adsorption site and to the in-plane coordination of the magnetic atoms.

π-Rich σ(2)P-heterocycles: bent η(1)-P- and μ(2)-P-coordinated 1,3-benzazaphosphole copper(I) halide complexes.

The reaction of 1-neopentyl-1,3-benzazaphosphole 1 with CuCl, CuBr, or Cu(SMe2)Br in THF at room temperature provides sparingly soluble [Cu7(μ(2)-L6)(μ(2)-X7)](+)[CuX2](-) cluster complexes 2a,b (L indicates coordinated 1, a X = Cl, b X = Br), with loosely bound THF, in high yields. The conversions proceed via transient THF-soluble labile [(L2CuX)2] complexes. Separation before complete conversion, combined with suitable conditions for crystallization, allowed these intermediates to be trapped. Depending on the reactant ratios, crystals of the clusters or of dimeric L2CuX complexes were formed. The crystal structure analyses of 2a·4THF and the dimers 3b [{Cu(η(1)-L)2(μ(2)-Br)}2], 4b [{Cu(μ(2)-L)(η(1)-L)(κBr)}2], 5a·2MeOH, and 5b·2MeOH [{Cu(μ(2)-L)(η(1)-L)(κX···HOMe)}2] generally display μ(2)-P- and/or tilted η(1)-P-coordination, contrasting with the preference for the η(1)-P in-plane coordination mode of phosphinine ligands in their copper(I) halide complexes. DFT studies of geometry-optimized monomers LCuBr, L(CuBr)2, L2CuBr, and the dimers 3b and 4b, calculated at the ωB97xD/cc-PVDZ level, suggest that weak competing interactions with the solvent THF and the entropy factor of the dimerization result in lability and a subtle balance between the different complexes in solution, whereas the particular coordination observed in the crystals is attributable to conservation of the delocalized π-system in the ligand. The HOMO of 4b is composed of Cu d orbitals and the π-type HOMO of the bridging ligand. Interestingly, despite the rather short Cu···Cu interatomic separation (2.726 Ǻ), no bond critical point could be located in 4b, indicating the absence of weak cuprophilic interactions in this compound.

A theoretical study on the alkene insertion step in Rh-Yanphos catalyzed hydroformylation

This paper studied the mechanism of the alkene insertion elementary step in the asymmetric hydroformylation (AHF) catalyzed by RhH(CO)2[(R,S)-Yanphos] using four alkene substrates (CH2CHPh, CH2CHPh(p)Me, CH2CHC(O)OCH3 and CH2CHOC(O)Ph, abbreviated as A1–A4). Interestingly, the equatorial vertical coordination mode (A mode) with respect to the Rh center was found for A1 and A2 but not for A3 and A4, although the equatorial in-plane coordination mode (E mode) was found for A1–A4. The relative energy of the E mode of the η2-intermediates is lower than that of the A mode. In the alkene insertion step, Path 1 is more favorable than Path 2 for this system. As for A1 and A2, there could be a transformation between 2eq and 2ax.

Anion binding to a tetracopper resorcinarene-based complex

UV–vis and electron paramagnetic resonance (EPR) spectroscopic studies have been carried out on the multinuclear copper(II) complex Cu4BpaRes. The copper atoms are in a tetragonal distorted geometry with nitrogens from bispicolylamine (Bpa) coordinating in the equatorial plane and water molecules or anions completing four coordination sites. The interaction of anions with this polynuclear copper complex in aqueous solution supports the formation of different complex species, which depend on the type and the concentration of the anions. In the presence of excess anions, frozen solution EPR parameters show the formation of species in which the in-plane coordination is characterised by the presence of three nitrogen atoms coming from the ligand and a donor atom from the specific anion. For the bidentate anion ligands and especially for malonate, UV–vis titrations indicate the formation of a 1:4 (Cu4BpaRes:anion) species. EPR experiments support the formation of such a species and indicate that the four copper centres are equivalent and reach penta-coordination via the coordination of both oxygens from the bidentate ligand.

Direct proof of mesoscopic misfit in nanoscale islands by x-ray absorption spectroscopy

Polarization-dependent extended x-ray-absorption fine-structure measurements above the Co-K absorption edge were carried out to experimentally prove the theoretically predicted contraction of interatomic distances in nanoscale Co islands ($\ensuremath{\oslash}\ensuremath{\approx}1$ nm) on Cu(001). Within the Co islands of thickness one monolayer (ML) which were deposited at 160 K substrate temperature, we find an in-plane Co-Co distance of $2.45\ifmmode\pm\else\textpm\fi{}0.02$ \AA{}, significantly shorter than in bulk Co (2.51 \AA{}) and in close agreement with theoretical predictions. The effective in-plane coordination number ${N}_{\mathrm{Co}}^{*}(\ensuremath{\parallel})$ is equal to $3.2\ifmmode\pm\else\textpm\fi{}0.5$ and $5.0\ifmmode\pm\else\textpm\fi{}0.7$ (${N}^{*}=6$ for an infinite island) for samples covered by 0.3 and 0.7 MLs of Co, respectively. The low value for the 0.3 ML sample is related to the finite island size and to about 20$%$ of the intermixing between the adsorbate and substrate atoms. The experimental results are supported by molecular dynamics calculations.

Different geometrical arrangements in carboxylate coordination polymers of flexible dicarboxylic acid

Dicarboxylate coordination polymers ( 1– 5) of Mn(II), Ni(II), Cu(II), Zn(II) and Cd(II), respectively, derived from (7-carboxymethoxy-naphthalen-2-yloxy)-acetic acid ( L 1 H 2 ) are synthesized and characterized. Depending on the coordination sites around the metal centers and coordination mode of the ligand, dimensionality of these polymers varies. The dicarboxylates adopt three spatial orientations: in-plane linear coordination, out-of-plane cis coordination and out-of-plane trans coordination mode. Both the cis and trans out-of-plane coordination modes are found to exist only if the ancillary ligand pyridine is coordinated to the metal ion. When the aquoligand coordinates the in-plane linear coordination mode of L 1 predominates. The coordination polymers 4 and 5 show photoluminescence in solution. The dicarboxylate of (5-carboxymethoxy-naphthalen-1-yloxy)-acetic acid ( L 2 H 2 ) does not form coordination polymer under ambient conditions, but prefers to remain as uncoordinated anion providing hydrophobic confinement to hexa-aquometal(II) cation. Compound 3 crystallizes in P2 1 space group and it shows broadband ultra-violet fluorescence centered at 352.9 nm on focusing 632.8 nm He:Ne laser.

Adenine containing architectures from silver supported dimeric units

This communication reports an unusual side-on coordination of Ag(I) ions with C-8 substituted -N9-propyladenine. Crystal structure investigation shows the influence of C-8 substituent over silver–adenine framework, where the bulky 8-dimethylamino group restricts the ring imino nitrogens (μ-N1, N3, N7) for in-plane coordination, leading to the formation of unprecedented metallaquartets.

Coordination modes in zirconium β-diketiminate complexes

Abstract Reaction of β-diketiminate lithium salts, nacnacRLi(THF) (R = 2,6-xylyl (1a), R-CH(Me)Ph (1b)), with ZrCl4 or ZrCl4·THF yielded the mono-diketiminate complexes nacnacRZrCl3(THF), 2a and 2b. Complex 2a did not react further with a second equivalent of 1a, but with CpLi or IndLi to form pseudo-tetrahedral complexes 3 and 4. No product could be isolated from reaction of 2b with CpLi. Coordination of the diketiminate ligand in 2a is best described as a distorted κ2-, in-plane coordination. Complexes 3 and 4, however, display a haptotropic shift of the diketiminate ligand and its coordination is best described as κ2,η2-coordination. Although not previously described, analyses of available structures in the literature reveal that non-symmetric κ2,η2-coordination is an independent coordination mode, which exists next to the previously described “η5-like” coordination.

A new three-dimensional cobalt(II) coordination polymer with a 4-connected CdSO4-like topology

The title cobalt(II) coordination polymer, poly[[diaquacobalt(II)]-mu4-3,3'-(p-phenylene)diacrylato], [Co(C12H8O4)(H2O)2]n, was obtained by reaction of Co(NO3)(2).6H2O and 3,3'-(p-phenylene)diacrylic acid (H2L) under hydrothermal conditions. Each Co(II) cation sits on a centre of inversion and is hexacoordinated by six O-atom donors in an octahedral geometry. The Co(II) centres are connected by four centrosymmetric L2- anions, resulting in a three-dimensional framework structure. The coordinated water molecules and carboxylate O atoms form hydrogen-bond interactions, stabilizing the structure of the three-dimensional framework. Topologically, the framework represents a rare example of the three-dimensional 4-connected CdSO4 network type. The metal cations and the organic ligand both show in-plane coordination with respect to the extended structure.

Chlorido-bridged polymeric and dinuclear copper(II) complexes with tridentate Schiff base: Synthesis, crystal structure and magnetic properties

Two copper(II) complexes, [Cu(qsal)Cl](DMF) ( 1) and [Cu 2(qsalBr) 2Cl 2](DMF) ( 2), with tridentate Schiff base ligands, 8-(salicylideneamino)quinoline (Hqsal) and 8-(5-bromo-salicylideneamino)quinoline (HqsalBr), respectively, were synthesised and structurally characterized. Each copper(II) ion in the two complexes is in a distorted square pyramidal N 2OCl 2 environment. Complex 1 exists as a polymeric species via equatorial–apical chloride bridges, whereas 2 is a di-chlorido-bridged dinuclear complex, where each bridging chloride simultaneously occupies an in-plane coordination site on one copper(II) ion and an apical site on the other copper(II) ion. Variable-temperature magnetical susceptibility measurements on the two complexes in the temperature range 2–300 K indicate the occurrence of intrachain ferromagnetic ( J = +6.58 cm −1) and intramolecular antiferromagnetical ( J = −6.91 cm −1) interactions.

Rhodium Complexes with Pincer Diphosphite Ligands. Unusual Olefin in-Plane Coordination in Square-Planar Compounds

A family of rhodium complexes bearing pincer diphosphite (PCP) and neutral L (L = PPh3, CO, CNXy, C2H4) ligands has been prepared and characterized. Reactions between Rh(Cl)(PPh3)3 and the diphosphites 2 lead to the chloro hydrides Rh(H)(Cl)(PCP)(PPh3) (4), which can be deprotonated to yield the complexes Rh(PCP)(PPh3) (5). In the latter, the PPh3 ligand is labile and can be exchanged by CO, CNXy, and C2H4 to give derivatives 6−8. Most noteworthy is the fact that ethylene complexes 8a,b show a remarkable in-plane (ip) conformation in the solid state. Solution NMR studies for these complexes show fast olefin rotation in all the temperature ranges, while solution and solid-state 13C{1H} NMR experiments give practically superimposable spectra, also indicating the prevalence of the ip conformer in solution. Complementary detailed DFT calculations performed with models of compounds 8 indicate that the olefin conformational preference is due to a dual combination of steric effects arising from the reduction of the P−Rh−P angle from 180°, caused by pincer ligand chelation and by the nature of the phosphite substituents, which in the case of t-Bu groups perfectly outline a cavity for an ip coordination of the olefin.