Heterometallic d8–d10 Coupling of Rh(I) and M(0) (M=Pd,Pt) in a Sandwich Framework of π-Conjugated Ligands

A heterometallic M−M′ bond formation is a key to construct atomically precise bimetallic clusters and materials. However, it is sometimes not straightforward to construct a heterometallic M−M′ bond through conventional methods including redox condensation. Here, we found that a sandwich framework of π-conjugated unsaturated hydrocarbon ligands provides a unique coordination environment that facilitates unusual coupling of d⁸ Rh^I and d¹⁰ M⁰ (M=Pd, Pt). The molecular orbital analysis showed that the electron-accepting ability of the sandwich framework through back-donation allows the formation of a dσ-type Rh−Pd bond in a (d–d)¹⁸ electron system.     

New functional inorganic polymers containing phosphorus

The C=C bond plays numerous roles in polymer science. This moiety is used as a precursor to polymers by addition polymerization and has been incorporated into π-conjugated polymers. The addition polymerization reaction has been extended to P=C bonds and the first example of a poly(methylenephosphine) has been prepared. The new macromolecule is of moderate molecular weight (ca. 10⁴ g/mol) and the oxidized polymers are air-stable. Poly(p-phenylenephosphaalkene), the first π-conjugated polymer containing P=C bonds in the backbone, has been prepared. The UV/Vis spectrum of this polymer shows a red shift in λ_max when compared with molecular model systems.     

Electron-Rich Diruthenium Complexes with π-Extended Alkenyl Ligands and Their F4TCNQ Charge-Transfer Salts**

The synthesis of dinuclear ruthenium alkenyl complexes with {Ru(CO)(PⁱPr₃)₂(L)} entities (L=Cl⁻ in complexes Ru₂-3 and Ru₂-7 ; L=acetylacetonate (acac⁻) in complexes Ru₂-4 and Ru₂-8 ) and with π-conjugated 2,7-divinylphenanthrenediyl ( Ru₂-3 , Ru₂-4 ) or 5,8-divinylquinoxalinediyl ( Ru₂-7 , Ru₂-8 ) as bridging ligands are reported. The bridging ligands are laterally π-extended by anellating a pyrene ( Ru₂-7 , Ru₂-8 ) or a 6,7-benzoquinoxaline ( Ru₂-3 , Ru₂-4 ) π-perimeter. This was done with the hope that the open π-faces of the electron-rich complexes will foster association with planar electron acceptors via π-stacking. The dinuclear complexes were subjected to cyclic and square-wave voltammetry and were characterized in all accessible redox states by IR, UV/Vis/NIR and, where applicable, by EPR spectroscopy. These studies signified the one-electron oxidized forms of divinylphenylene-bridged complexes Ru₂-7 , Ru₂-8 as intrinsically delocalized mixed-valent species, and those of complexes Ru₂-3 and Ru₂-4 with the longer divinylphenanthrenediyl linker as partially localized on the IR, yet delocalized on the EPR timescale. The more electron-rich acac⁻ congeners formed non-conductive 1 : 1 charge-transfer (CT) salts on treatment with the F₄TCNQ electron acceptor. All spectroscopic techniques confirmed the presence of pairs of complex radical cations and F₄TCNQ^.− radical anions in these CT salts, but produced no firm evidence for the relevance of π-stacking to their formation and properties.     

Heteroselenometallic cluster compounds with tetraselenometalates

The coordination cluster compounds of tetraselenomolybdate and tetraselenotungstate anions with metal ions are reviewed. New heteroselenometallic cluster compounds are primarily of interest regarding their structures and reactivities, and their potential as non-linear optical (NLO) materials. This article focuses from a synthetic and structural point of view on coordination cluster compounds with tetraseleno-molybdate and -tungstate anions as polydentate ligands. A comprehensive survey is presented of the heteroselenometallic clusters known to date according to the number of center [MSe₄]²⁻ (M=Mo, W) anions. Representative spectroscopic and NLO properties of these clusters are also discussed.     

Quantification of the trans influence in d8 square planar and d6 octahedral complexes: a database study

A systematic search of the Cambridge structural database was undertaken to quantify the trans influence in square planar and octahedral transition metal compounds. For square planar geometry, d⁸ metal centers were studied, while octahedral searches focused on low-spin d⁶ complexes. Two probe ligands (PL) were used to measure the effect of the trans ligand (TL), chloride, and triphenylphosphine (PPh₃). For the TLs O=CX₂, NR₃, pyridine, and Cl^? (X?=?any non-metal, R?=?H or hydrocarbon), the effects on the metal–probe ligand (M–PL) distance were statistically equal and were taken as essentially no trans influence. The other ligands studied showed significant decrease in the mean M–PL bond order, relative to the above ligands: SR₂?=?0.941; S=CX₂?=?0.887; PPh₃?=?0.825; phenyl?=?0.743; CR₃?=?0.719; hydride?=?0.685. Some variation in the trans influence is observed, based on the geometry of the metal center and the PL. In general, electron-donating, σ-bonding ligands lead to a larger trans influence, but π-bonding effects can also be important, particularly when the probe ligand also has π-bonding properties.     

An Antimony(III) Fluoride Oxalate with Large Birefringence

Birefringent materials play a key role in modulating the polarization of light and thus in optical communication as well as in laser techniques and science. Designing new, excellent birefringent materials remains a challenge. In this work, we designed and synthesized the first antimony(III) fluoride oxalate birefringent material, KSb₂C₂O₄F₅, by a combination of delocalized π-conjugated [C₂O₄]²⁻ groups, stereochemical active Sb³⁺ cations, and the most electronegative element, fluorine. The [C₂O₄]²⁻ groups are not in an optimal arrangement in the crystal structure of KSb₂C₂O₄F₅; nonetheless, KSb₂C₂O₄F₅ exhibits a large birefringence (Δn=0.170 at 546 nm) that is even better than that of the well-known commercial birefringent material α-BaB₂O₄, even though the latter features an optimal arrangement of π-conjugated [B₃O₆]³⁻ groups. Based on first-principles calculations, this prominent birefringence should be attributed to the alliance of planar π-conjugated [C₂O₄]²⁻ anions, highly distorted SbO₂F₂ and SbOF₃ polyhedra with a stereochemically active lone pair. The combination of lone-pair electrons and π-conjugated systems boosts the birefringence to a large extent and will help the development of high-performance birefringent materials.     

Controlled Expansion of a Strong‐Field Iron Nitride Cluster: Multi‐Site Ligand Substitution as a Strategy for Activating Interstitial Nitride Nucleophilicity

Multimetallic clusters have long been investigated as molecular surrogates for reactive sites on metal surfaces. In the case of the μ₄‐nitrido cluster [Fe₄(μ₄‐N)(CO)₁₂]^?, this analogy is limited owing to the electron‐withdrawing effect of carbonyl ligands on the iron nitride core. Described here is the synthesis and reactivity of [Fe₄(μ₄‐N)(CO)₈(CNAr^Mes2)₄]^?, an electron‐rich analogue of [Fe₄(μ₄‐N)(CO)₁₂]^?, where the interstitial nitride displays significant nucleophilicity. This characteristic enables rational expansion with main‐group and transition‐metal centers to yield unsaturated sites. The resulting clusters display surface‐like reactivity through coordination‐sphere‐dependent atom rearrangement and metal–metal cooperativity.     

Synthesis of a new diarylethene diradical which has extended π-conjugated chains from the 2,5-position of one thiophene ring

A diarylethene diradical having a new switching unit for intramolecular magnetic interaction was synthesized. The photoswitching unit has an extended π-conjugated chain in one aryl unit, and two nitronyl nitroxide radical are placed at both ends of the π-conjugated chain. The diarylethene moiety is located in the middle of the chain. This diarylethene is designed to change the hybrid orbital from sp² to sp³ at the 2-position of the thiophene ring when this diarylethene undergoes a photochromic reaction. But the new diradical compound did not undergo photocyclic reaction upon irradiation with UV light. The photochemical behavior is perturbed by a resonant quinoid structure which stabilizes the open-ring isomer.     

Enhanced nonlinear optical properties of porphyrin with an extended π-conjugated bridge

In this work, a series of molecules with an extended π-conjugated bridge have been theoretically designed based on porphyrin, where -(CH=CH)_n- (n = 1–4, 8, 12) chain is served as an extended π-conjugated bridge. It is found that all molecules exhibit large energy gaps in the range of 3.484–4.151 eV for porphyrin-(CH=CH)_n-NH₂, and 3.624–4.250 for porphyrin-(CH=CH)_n-NO₂. The maximum absorption wavelengths of all molecules show a red shift trend with increasing -(CH=CH)_n- length, which leads to small transition energy. It is observed that long chain brings these molecules the large first hyperpolarizability, which are 1.04 × 10⁵ au for porphyrin-(CH=CH)₁₂-NH₂, 1.26×10⁵ au for porphyrin-(CH=CH)₁₂-NO₂. Moreover, compared with -(CH=CH)_n-NH₂ with the same chain length, -(CH=CH)_n-NO₂ chain can achieve larger nonlinear optical response. It is hoped that the research in this paper can provide a new strategy for the experimental design of nonlinear optical materials.

     

Ambient Chemical Fixation of CO2 Using a Robust Ag27 Cluster-Based Two-Dimensional Metal–Organic Framework

Unprecedented double S²⁻ templated Ag₂₇ clusters have been stabilized by 5,10,15,20-tetra(4-pyridyl)porphyrin (TPyP-H₂) ligands to afford a robust 2D metal–organic framework ( Ag27-MOF ). This silver cluster-assembled material serves as a highly efficient heterogeneous catalyst for the cyclization of both terminal and internal propargylamines with CO₂ under atmospheric pressure. Density functional theory (DFT) calculations illustrate that the high catalytic activity and broad substrate scope are attributable to the saddle-shaped metallic node in Ag27-MOF , which features an accessible platform with high-density silver atoms as π-Lewis acid sites for activating C≡C triple bonds. As a result, different sterically hindered alkyne substrates can be effectively activated through π-interactions with these cationic silver centers.     

Comparison of the Complexation of Open-Chain and Cyclic N2S2 Ligands with Cu+ and Cu2+

The complexation properties of the open-chain N₂S₂ ligands 1–4 are described and compared to those of analogous N₂S₂ macrocycles 5–7 . With Cu²⁺, the open-chain ligands give complexes with the stoichiometry CuL²⁺ and CuLOH⁺, the stabilities and absorption spectra of which have been determined. The ligand field exerted by these ligands is relatively constant and independent of the length of the chain. With Cu⁺, the species CuLH, CuLH²⁺, and CuL⁺ were identified and their stabilities measured. The redox potentials calculated from the equilibrium constants and measured by cyclic voltammetry agree and lie between 250 and 280 mV against SHE. The comparison between open-chain and cyclic ligands shows that (1) a macrocyclic effect is found for Cu²⁺ but not for Cu⁺, (2) the ligand-field strength is very different for the two types of ligands, and (3) the redox potentials span a larger interval for the macrocyclic than for the open-chain complexes.     

Cooperative Photocatalysis with 4-Amino-TEMPO for Selective Aerobic Oxidation of Amines over TiO2 Nanotubes

Attaching π-conjugated molecules onto TiO₂ can form surface complexes that could capture visible light. However, to make these TiO₂ surface complexes durable, integrating 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) or its analogues as a redox mediator with photocatalysis is the key to constructing selective chemical transformations. Herein, sodium 6,7-dihydroxynaphthalene-2-sulfonate (DHNS) was obtained by extending the π-conjugated system of catechol by adding a benzene ring and a substituent sodium sulfonate (−SO₃⁻Na⁺). The DHNS−TiO₂ showed the best photocatalytic activity towards the blue light-induced selective aerobic oxidation of benzylamine. Compared to TEMPO, 4-amino-2,2,6,6-tetramethylpiperidine-1-oxyl (4-amino-TEMPO) could rise above 70% in conversion of benzylamine over the DHNS−TiO₂ photocatalyst. Eventually, a wide range of amines could be selectively oxidized into imines with atmospheric O₂ by cooperative photocatalysis of DHNS−TiO₂ with 4-amino-TEMPO. Notably, superoxide (O₂^•−) is crucial in coupling the photocatalytic cycle of DHNS−TiO₂ and the redox cycle of 4-amino-TEMPO. This work underscores the design of surface ligands for semiconductors and the selection of a redox mediator in visible light photocatalysis for selective chemical transformations.     

Synthesis and physical properties of π-conjugated metallacycle polymers of cobalt and ruthenium

Recent studies on two types of π-conjugated metallacylce polymers are reviewed. Reaction of CpCo(PPh₃)₂ with conjugated diacetylenes afford poly(arylene cobaltacyclopentadienylene) and that of CpRuBr(cod) does poly(arylene ruthenacyclopentrienylene)s in ambient conditions. Regioselectivity of the former metallacycling reacion is not perfect (at most 80% of the 2,5-diaryl selectivity) but that of the latter is satisfactory (∼100% of the 2,5-diaryl selectivity) for the formation of π-conjugated structure. Electrochemical oxidation of the cobaltacyclopentadiene polymer and reduction of the ruthenacycle polymer occur facilely and quasi-reversibly by the contribution of metal d-orbitals. Physical properties in undoped (neutral) and doped (charged) sates show the behavior of electronic band structure derived from the organic π-conjugated main chain strongly coupled with the metal d-orbitals. This affords, for example, photoconductivity in the neutral form of the cobaltacylopentadiene polymer and ferromagnetic interaction in the reduced form of the ruthenacyclopentatriene polymer.     

catena‐Poly[[bis[5‐(4‐pyridyl‐κN)‐2‐(2‐pyridylmethylsulfanyl)‐1,3,4‐oxadiazole]cadmium(II)]‐di‐μ‐thiocyanato‐κ2N:S;κ2S:N]

The title compound, [Cd(NCS)₂(C₁₃H₁₀N₄OS)₂]_n, contains SCN⁻ anions acting as end‐to‐end bridging ligands which utilize both S and N atoms to link cadmium(II) centers into one‐dimensional double chains. The multidentate 5‐(4‐pyridyl)‐2‐(2‐pyridylmethylsulfanyl)‐1,3,4‐oxadiazole ligands behave as monodentate terminal ligands, binding metal centers only through the N atoms of the 4‐pyridyl groups. Two types of eight‐membered rings are formed by two SCN⁻ anions bridging Cd^II centers, viz. planar and chair conformation, which are alternately disposed along the same chain. Finally, chains define a two‐dimensional array through two different interchain π–π stacking interactions.     

A series of silver coordination polymers constructed from flexible bis(benzimidazole) ligands and different carboxylates

In this article, eight new silver coordination polymers constructed from two structurally related ligands, 1,1′-(1,4-butanediyl)bis(2-methylbenzimidazole) (bbmb) and 1,1′-(1,4-butanediyl)bis(2-ethylbenzimedazole) (bbeb), have been synthesized: [Ag(L1)(bbmb)]·C₂H₅OH·H₂O (1), [Ag(L2)(bbmb)]·C₂H₅OH (2), [Ag(L3)(bbmb)] (3), [Ag₂(L4)(bbmb)₂]·C₂H₅OH (4), [Ag(L2)(bbeb)]·C₂H₅OH (5), [Ag(L5)(bbeb)]·CH₃OH (6), [Ag₂(L6)₂(bbeb)]·H₂O (7), and [Ag₂(L7)(bbeb)₂]·4(H₂O) (8), where L1 = benzoate anion, L2 = p-methoxybenzoate anion, L3 = 2-amino-benzoate anion, L4 = oxalate anion, L5 = cinnamate ainon, L6 = 3-amino-benzoate anion, and L7 = fumaric anion. In 1-3, 5 and 6, the bidentate N-donor ligands (bbmb and bbeb) in trans conformations bridge neighboring silver centers to form 1D single chain structures. The carboxylate anions are attached on both sides of the chains. Moreover, 1 and 3 are extended into 2D layers, while 2 and 6 are extended into 3D frameworks through π-π interactions. In 4, the bbmb ligands bridge adjacent Ag(I) centers to form -Ag-bbmb-Ag- chains, which are further connected by L4 anions to form a 2D layer. The resulting layers are extended into 3D frameworks through strong π-π interactions. In 7, the N-donor ligands (bbeb) in trans conformations bridge two silver centers to generate a [Ag₂(bbeb)]²⁺ unit. The adjacent [Ag₂(bbeb)]²⁺ units are further connected via the L6 anions to form a 1D ladder chain. Moreover, the structure of compound 7 is extended into a 3D framework through hydrogen bonding and π-π interactions. In 8, two Ag(I) cations are bridged by two bbeb ligands in cis conformations to form a [Ag₂(bbeb)₂]²⁺ ring, which are further linked by L7 anions to generate a 1D string chain. Furthermore, the hydrogen bonding and π-π interactions link L7 anions to form a 2D supramolecular sheet. Additionally, the luminescent properties of these compounds were also studied.     

Monomeric Chini‐Type Triplatinum Clusters Featuring Dianionic and Radical‐Anionic π*‐Systems

Owing to their unique topologies and abilities to self‐assemble into a variety of extended and aggregated structures, the binary platinum carbonyl clusters [Pt₃(CO)₆]_n^2? (“Chini clusters”) continue to draw significant interest. Herein, we report the isolation and structural characterization of the trinuclear electron‐transfer series [Pt₃(μ‐CO)₃(CNAr^Dipp2)₃]^n? (n=0, 1, 2), which represents a unique set of monomeric Pt₃ clusters supported by π‐acidic ligands. Spectroscopic, computational, and synthetic investigations demonstrate that the highest‐occupied molecular orbitals of the mono‐ and dianionic clusters consist of a combined π*‐framework of the CO and CNAr^Dipp2 ligands, with negligible Pt character. Accordingly, this study provides precedent for an ensemble of carbonyl and isocyanide ligands to function in a redox non‐innocent manner.     

A series of coordination polymers constructed from flexible bis(benzimidazole)ether ligands and different carboxylates

Six new coordination polymers constructed from two structurally related ligands, 2,2′-bis(2-methylbenzimidazole) ether (L1) and 2,2′-bis(2-ethylbenzimidazole)ether (L2), have been synthesized. They are [Cu(L1)(bz)₂] (1), [Cu(L2)(bz)₂] (2), [Zn₂(L1)(m-bdc)₂] (3), [Cd₂(L2)(m-bdc)₂(H₂O)]₂·H₂O (4), [Zn(L1)(OH-bdc)-(H₂O)] (5) and [Zn₂(L₂)(btca)] (6), where Hbz = benzoic acid, m-H₂bdc = 1,3-benzenedicarboxylic acid, OH-H₂bdc = 5-hydroxyisophthalic acid, and H₄btca = 1,2,4,5-benzenetetracarboxylic acid. In 1 and 2, the bidentate N-donor ligands (L1 and L2) bridge neighboring metal centers to form 1D single chains. The bz anions are attached on both sides of the chains. In 3 and 4, the N-donor ligands (L1 and L2) in cis conformations bridge two metal centers to generate a [M₂(L1)]⁴⁺ unit (M = Zn(II) and Cd(II)). The adjacent [M₂(L1)]⁴⁺ units are further linked via the dicarboxylate anions to form 1D double chain structures. In 5, the Zn(II) cations are bridged by OH-m-bdc anions to form an infinite polymeric chain. The L1 ligands are attached on one side of the chain in a monodentate mode. In 6, two Zn(II) cations are bridged by two L2 ligands to form a [ZnL2]₂ ⁴⁺ ring, which is further linked by btca anions to generate a 2D layer. The luminescent properties of the ligands and 3–6 in the solid state at room temperature were also studied.     

Design of the Ionic Organic Nonlinear Optical Material NH4[LiC3H(CH3)O4] with Ultrawide Band Gap and Moderate Birefringence

Ionic organic crystals containing organic planar π-conjugated units has become one of the hot spots as nonlinear optical (NLO) materials. However, although this type of ionic organic NLO crystals commonly have remarkable second harmonic generation (SHG) responses, they also suffer from overlarge birefringences and relatively small band gaps that be hardly beyond 6.2 eV. Herein, a flexible π-conjugated [C₃H(CH₃)O₄]²⁻ unit was theoretically revealed, showing great potential for designing NLO crystals with balanced optical properties. Accordingly, through the reasonable NLO-favourable layered design, a new ionic organic material, NH₄[LiC₃H(CH₃)O₄], was successfully obtained. As expected, it achieves not only a large SHG effect (4×KDP), but also a suitable birefringence (0.06@546 nm) and an ultrawide band gap (>6.5 eV). This study provides a new flexible π-conjugated NLO-active unit, contributing to design more ionic organic NLO materials with excellent balanced optical properties.     

MÖSsbauer Spectroscopic Study of Fecl3 Complexes of Poly(N-Methyl-2,5-Pyrrolylene), Poly(2,5-Thienylene), and Poly(3-Methyl-2,5-Thienylene) Prepared by Ni-Catalyzed Polycondensation

Reaction of FeCl₃ with poly(N-methyl-2,5-pyrrolylene) (PNMPy), poly(2,5-thienylene) (PTh), and poly(3-methyl-2,5-thienylene) (P3MeTh) caused reduction of FeCl₃ to afford Fe²⁺ species. Variable temperature Mössbauer spectra of the reaction systems indicated formation of FeCl₂ and FeCl^? ₄. The latter is regarded as a counter-anion for the cation delocalized along the π-conjugated polymer chain.     

Structural and electronic properties of small platinum metallorganic complexes

A theoretical first-principles study of Pt _n (ligand) _m (n = 1–3) metallorganic complexes is performed, by varying the number of metal atoms and the nature and number of organic coordinate ligands (specifically, vinylic and arylic ligands). For each system, the nature of the bonding, the structure and the energetics of the metal/organic-species interaction are analyzed to derive information on the growth of coated metal clusters in solution. It is found that two régimes can be distinguished: a “coordinatively saturated” régime, in which the ratio among the number of ligands and the number of metal atoms is high and a ligand/organic π-interaction mode is preferred, and a “coordinatively unsaturated” régime, in which the ligand/metal ratio is low and a ligand/organic σ-interaction mode is preferred. Reactive channels, such as oxidative insertion of Pt into C–H bonds with the corresponding formation of platinum hydride species, can be opened in the latter régime.