Designing Cobalt(II) Complex for Chemoselective Synthesis of 2-Aryl-3-Formyl Indoles from Amino Alcohols and Alcohols†.

An air-stable, inexpensive, and isolable cobalt(II) complex (C1) of N-((1-methyl-1H-imidazol-2-yl)methyl)-2-(phenylselanyl)ethan amine (L1) was synthesized and characterized. The complex was used to catalyze a one-pot cascade reaction between 2-(2-aminophenyl)ethanols and benzyl alcohol derivatives. Interestingly, 2-aryl-3-formylindole derivatives were formed instead of N-alkylated or C-3 alkylated indoles. A broad substrate scope can be activated using this protocol with only 5.0 mol % catalyst loading to achieve up to 87 % yield of 2-aryl-3-formylindole derivatives. The mechanistic studies suggested that the reaction proceeds through tandem imine formation followed by cyclization.

Twisted and Disconnected Chains: Flexible Linear Tetracuprous Arrays and a Decanuclear CuI Cluster as Blue- and Green/Yellow-Light Emitters.

Defined arrays of transition metal ions embedded in tailored polydentate ligand scaffolds allow for a systematic design of their physical properties. Such molecular strings of closed-shell transition metal centers are particularly interesting for Group 11 metal ions in the oxidation state +1 if they undergo metallophilic d10···d10 contact interactions since these clusters are oftentimes efficient photoluminescence (PL) emitters. Copper is particularly attractive as a sustainable earth-abundant coinage metal source and because of the ability of several CuI complexes to serve as powerful thermally activated delayed fluorescence (TADF) emitters in molecular/organic light-emitting devices (OLEDs). Our combined synthetic, crystallographic, photophysical, and computational study describes a straight tetracuprous array possessing a centrally disconnected CuI2···CuI2 chain and a continuous helically bent CuI4 complex. This molecular helix undergoes a facile rearrangement in diethyl ether solution, yielding an unprecedented nanosized CuI10 cluster (2.9 × 2.0 nm) upon crystallization. All three clusters show either bright blue phosphorescence, TADF, or green/yellow multiband phosphorescence with quantum yields between 6.5 and 67%, which is persistent under hydrostatic pressure up to 30 kbar. Temperature-dependent PL investigations in combination with time-dependent density-functional theory (TD-DFT) calculations and void space analyses of the crystal packings complement a comprehensive correlation between the molecular structures and photoluminescence properties.

Molecules that Turn Themselves Inside-Out: Tuning in/out Equilibria and Homeomorphic Isomerization in Macrobicyclic Dibridgehead Diphosphines P((CH2 )n )3 P Newly Accessible by Earth-Abundant-Metal Templates.

Photolyses of trans-Fe(CO)3 (P((CH2 )n )3 P) (n=10 (a), 12 (b), 14 (c), 16 (d), 18 (e)) in the presence of PMe3 provide the first economical and scalable route to macrobicyclic dibridgehead diphosphines P((CH2 )n )3 P (1). These are isolated as mixtures of in,in/out,out isomers that equilibrate with degenerate in,out/out,in isomers at 150 °C via pyramidal inversion at phosphorus. For the entire series, VT 31 P NMR data establish or bound Keq , rates, and activation parameters for a variety of phenomena, many of which involve homeomorphic isomerizations, topological processes by which certain molecules can turn themselves inside out (e. g., in,in⇌out,out). This provides the first detailed mapping of such trends in homologous series of aliphatic bicyclic compounds XE((CH2 )n )3 EX with any type of bridgehead. Isomeric diborane adducts 1 a,d ⋅ 2BH3 are also characterized. Crystal structures of out,out-1 a and in,in-1 a ⋅ 2BH3 aid isomer assignments and reveal unusual cage conformations.

Enhancing the Resistive Switching Behavior of WORM Memory Devices Using D-π-A Based Ester-Flanked Quinolines.

Donor-Acceptor systems are highly appreciated in the field of organic memory devices due to their efficient charge transport within the systems. In this work, we have designed and synthesized a D-π-A system constituting ester-flanked quinolines and functionalized triarylamines (TAA) through a single-step cross-coupling reaction to fabricate memory devices via Write-Once Read-Many times (WORM) non-volatile memory. Structure-property relationships are reconnoitered for these conjugated D-π-A systems through a series of UV, fluorescence, XRD, DFT, and memory characterizations. The UV and CV data show efficient charge transfer with intramolecular charge transfer occurring at 407-417 nm and a short band gap of 2.56-2.65 eV. An enhancement in the resistive switching behavior of the memory devices is observed for the compounds with simple TAA-quinoline and tert-butylphenyl substituted TAA and fluorophenyl substituted quinoline due to balanced charge distribution in the compounds. This enhanced switching induces an on/off ratio of 103 by generating a highly ordered arrangement in the thin films. The HOMO, LUMO levels, and the ESP images together estimate a charge transfer and charge trapping as the plausible mechanism for the solution-processable WORM memory devices. The longer retention time (103  s) and lower threshold voltages (-1.21--2.12 V) of the devices makes them intriguing compounds for memory applications.

Rapid Assembly of Functionalized 2H-Chromenes and 1,2-Dihydroquinolines via Microwave-Assisted Secondary Amine-Catalyzed Cascade Annulation of 2-O/N-Propargylarylaldehydes with 2,6-Dialkylphenols.

An efficient, secondary amine-catalyzed cascade annulation of 2-O/N-propargylarylaldehydes with 2,6-dialkylphenols was established to access biologically relevant functionalized 2H-chromenes and 1,2-dihydroquinolines tethered with a synthetically useful p-quinone methide scaffold in high yields under microwave irradiation and conventional heating conditions. The microwave-assisted strategy was convenient, clean, rapid, and high yielding in which the reactions were completed in just 15 min, and the yields obtained were up to 95%. This highly atom-economical domino process constructed two new C-C double bonds and a six-membered O/N-heterocyclic ring in a single synthetic operation. Its mechanism process was rationalized as involving sequential iminium ion formation, nucleophilic addition, and intramolecular annulation steps. Furthermore, the synthesized 2H-chromene derivatives were transformed into valuable indeno[2,1-c]chromenes, 5H-indeno[2,1-c]quinolines, and oxireno[2,3-c]chromene via a palladium-catalyzed double C-H bond activation process and epoxidation, respectively.

Diversity-Oriented Synthesis of Benzo[f][1,4]oxazepine-, 2H-Chromene-, and 1,2-Dihydroquinoline-Fused Polycyclic Nitrogen Heterocycles under Microwave-Assisted Conditions.

An efficient, diversity-oriented synthesis of oxazepino[5,4-b]quinazolin-9-ones, 6H-chromeno[4,3-b]quinolines, and dibenzo[b,h][1,6]naphthyridines was established involving a substrate-based approach under microwave-assisted and conventional heating conditions in high yields (up to 88%). The CuBr2-catalyzed, chemoselective cascade annulation of O-propargylated 2-hydroxybenzaldehydes and 2-aminobenzamides delivered oxazepino[5,4-b]quinazolin-9-ones involving a 6-exo-trig cyclization-air oxidation-1,3-proton shift-7-exo-dig cyclization sequence. This one-pot process showed excellent atom economy (-H2O) and constructed two new heterocyclic rings (six- and seven-membered) and three new C-N bonds in a single synthetic operation. On the other side of diversification, the reaction between O/N-propargylated 2-hydroxy/aminobenzaldehydes and 2-aminobenzyl alcohols delivered 6H-chromeno[4,3-b]quinolines and dibenzo[b,h][1,6]naphthyridines involving sequential imine formation-[4 + 2] hetero-Diels-Alder reaction-aromatization steps. The influence of microwave assistance was superior to conventional heating, where the reactions were clean, rapid, and completed in 15 min, and the conventional heating required a longer reaction time at a relatively elevated temperature.

Hinged Bipodal Furoylthiourea-Based Ru(II)-Arene Complexes: Effect of (ortho, meta, or para)-Substitution on Coordination and Anticancer Activity.

We set out to design and synthesize bipodal ligands with the phenyl group as the spacer and varied the substitution on the spacer between ortho (L1), meta (L2), and para (L3). The respective ligands and complexes containing either p-cymene (PL1-PL3) or benzene (BL1-BL3) as the arene unit were synthesized and characterized successfully. The influence of the ligands due to substitution change on their coordination behavior was quite minimal; however, the differences were seen in the anticancer activity of the complexes. DFT studies revealed the structural variations between the three different substitutions, which was further confirmed by single-crystal X-ray diffraction studies. The anticancer activity of the complexes could be correlated with their rate of hydrolysis and their lipophilicity index as determined by UV-visible spectroscopy. The cell death mechanism of the active complexes was deduced to be apoptotic via staining assays, flow cytometry, and Western blot analysis.

Development of Nonclassical Photoprecursors for Rh2 Nitrenes.

Characterization of reactive intermediates in C-H functionalization is challenging due to the fleeting lifetimes of these species. Synthetic photochemistry provides a strategy to generate post-turnover-limiting-step intermediates in catalysis under cryogenic conditions that enable characterization. We have a long-standing interest in the structure and reactivity of Rh2 nitrene intermediates, which are implicated as transient intermediates in Rh2-catalyzed C-H amination. Previously, we demonstrated that Rh2 complexes bearing organic azide ligands can serve as solid-state and in crystallo photoprecursors in the synthesis of transient Rh2 nitrenoids. Complementary solution-phase experiments have not been available due to the weak binding of most organic azides to Rh2 complexes. Furthermore, the volatility of the N2 that is evolved during in crystallo nitrene synthesis from these precursors has prevented the in crystallo observation of C-H functionalization from lattice-confined nitrenes. Motivated by these challenges, here we describe the synthesis and photochemistry of nonclassical nitrene precursors based on sulfilimine ligands. Sulfilimines bind to Rh2 carboxylate complexes more tightly than the corresponding azides, which has enabled the full solid-state and solution-phase characterization of these new complexes. The higher binding affinity of sulfilimine ligands as compared with organic azides has enabled both solution-phase and solid-state nitrene photochemistry. Cryogenic photochemical studies of Rh2 sulfilimine complexes confined within polystyrene thin films demonstrate that sulfilimine photochemistry can be accomplished at low temperature but that C-H amination is rapid at temperatures compatible with N═S photoactivation. The potential of these structures to serve as platforms for multistep in crystallo cascades is discussed.

Spin diffusion in the Phosphorus-31 NMR relaxation in a layered crystalline α-Sn(IV) phosphate contaminated by paramagnetic impurities.

The study of a layered crystalline Sn(IV) phosphate by solid-state NMR has demonstrated that the 31P T1 relaxation of phosphate groups, dependent on spinning rate is completely controlled by the limited spin diffusion to paramagnetic ions found by EPR. The spin-diffusion constant, D(SD), was estimated as 2.04 10-14 cm2s-1. The conclusion was supported by the 31P T1 time measurements in zirconium phosphate 1-1, also showing paramagnetic ions and in diamagnetic compound (NH4)2HPO4.

A High-Performance Single-Molecule Magnet Utilizing Dianionic Aminoborolide Ligands.

The first example of a homoleptic f-block borolide sandwich complex is presented and shown to be a high-performance single-molecule magnet (SMM). The bis(borolide) complex [K(2.2.2)][[1-(piperidino)-2,3,4,5-tetraphenylborolyl]2Dy] (1) features an unusual example of an anionic Ln3+ metallocene that supports short metal-ligand bonds and a high degree of linearity around the central Dy3+ ion, resulting in comparatively large barriers to magnetization reversal (Ueff = 1600 cm-1 for the most linear orientation) and, importantly, a high blocking temperature (TB, defined as T(τ100s)) of 66 K. These metrics put complex 1 among the very best performing SMMs reported to date and highlight the potential of dianionic borolide ligands to increase ligand field axiality, compared to monoanionic cyclic ligands, to ultimately maximize magnetic anisotropy in f-block-based SMMs.

Deacetylated nimbin analog N2 fortifies alloxan-induced pancreatic ß-cell damage in insulin-resistant zebrafish larvae by upregulating phosphoenolpyruvate carboxykinase (PEPCK) and insulin levels.

This study aims to evaluate the protective behaviour of N2, a semi-natural analog of nimbin, for its anti-diabetic efficacy against alloxan-induced oxidative damage and ß-cell dysfunction in in-vivo zebrafish larvae. A 500 µM of alloxan was exposed to zebrafish larvae for 24 h to induce oxidative stress in the pancreatic ß-cells and co-exposed with N2 to study the protection of N2 by inhibiting ROS by DCFH-DA, DHE and NDA staining along with Cellular damage, apoptosis and lipid peroxidation. The zebrafish was further exposed to 500 µM alloxan for 72 h to induce ß-cell destruction along with depleted glucose uptake and co-exposed to N2 to study the protective mechanism. Glucose levels were estimated, and PCR was used to verify the mRNA expression of phosphoenolpyruvate carboxykinase (PEPCK) and insulin. Alloxan induced (24 h) oxidative stress in the pancreatic ß-cells in which N2's co-exposure inhibited ROS by eliminating O-2 radicals and restoring the glutathione levels, thus preventing cellular damage and lipid peroxidation. The zebrafish exposed to 500 µM alloxan for 72 h was observed with ß-cell destruction along with depleted glucose uptake when stained with 2NBDG, wherein N2 was able to protect the pancreatic ß-cells from oxidative damage, promoted high glucose uptake and reduced glucose levels. N2 stimulated insulin production and downregulated PEPCK by inhibiting gluconeogenesis, attenuating post-prandial hyperglycemia. N2 may contribute to anti-oxidant protection against alloxan-induced ß-cell damage and anti-hyperglycemic activity, restoring insulin function and suppressing PEPCK expression.

High Mobility and ON/OFF Ratio of Solution-Processable p-Channel OFETs from Arylacetylene End-Capped Alkoxyphenanthrenes.

New arylacetylene end-capped alkoxyphenanthrenes were synthesized and demonstrated as the best active layer for solution-processable p-channel organic field-effect transistors. The alkoxy chain embedded compounds exhibited enhanced solubility and induced non-covalent interactions resulting in effective molecular packing. The 'Lewis soft' heteroatoms direct the most stable conformation with dihedral angles possible for molecular interactions, and energy levels. DFT studies supported the fine-tuning of FMOs, with high HOMO levels ∼-5.2 eV ensuring a low barrier for charge injection. OFET devices exhibited a maximum charge carrier mobility up to 1.30 cm2 /Vs with the highest ON/OFF ratio of 107 . The strong π-π interactions and the crystallinity of the films are well supported by GIXRD and SEM analysis.

Microwave-Assisted Copper(II)-Catalyzed Cascade Cyclization of 2-Propargylamino/Oxy-Arylaldehydes and O-Phenylenediamines: Access to Densely Functionalized Benzo[f]Imidazo[1,2-d][1,4]Oxazepines and Benzo[f]Imidazo[1,2-d][1,4]Diazepines.

A highly efficient microwave-assisted copper(II)-catalyzed cyclization cascade was established starting from readily accessible O/N-propargylated 2-hydroxy or 2-aminobenzaldehydes and o-phenylenediamines to synthesize densely functionalized imidazo[1,2-d][1,4]oxazepines and imidazo[1,2-d][1,4]diazepines in high yields (up to 93%). This one-pot two-step process was found to be highly atom economical (-H2O, -H2) and operationally simple and enabled the generation of two new heterocycle rings (seven- and five-membered) and three new C-N bonds in a single synthetic operation. These reactions well tolerated a variety of substituents including electron-donating and electron-withdrawing groups and furnished the desired fused heterocycles in high yields under microwave irradiation in a very short reaction time. The mechanism of the established protocol involves sequential imine formation-intramolecular cyclization-air oxidation followed by 7-exo-dig cyclization steps. A comparative study between the microwave-assisted approach and conventional heating was also performed to demonstrate the advantages of the microwave-assisted protocol in terms of high yield and shorter reaction time.

Influence of π-Endcaps on the Performance of Functionalized Quinolines for p-Channel OFETs.

This study investigates the influence of aryl and ethynyl linkers as well the effect of various pi-end-groups on the performance of the quinoline-based organic field-effect transistors. A series of new functionalized quinolines with D-π-A-π-D and A-π-A-π-A architectures are designed and synthesized via the Sonagashira cross-coupling reaction. All the new compounds are well characterized and their photophysical properties are studied. The bottom gate-top contact-organic field-effect transistors devices are fabricated using the spin-coating technique. By employing the pre and post-annealing technique, films with uniform surface coverage are obtained. The variation in the end-groups results in versatile packing arrangements which determine their good charge transport properties. The p-channel transistor behavior is observed for all the new compounds. Among the molecules studied, methoxyphenyl and thiophen-2-yl terminal functionalized with D-π-A-π-D architecture exhibit the higher p-channel transistor characteristics with hole mobilities of 1.39 and 1.33 cm2 V-1 s-1 , respectively. The good charge carrier mobilities are supported by an electron-donating methoxy group and thiophene as the end-groups with high highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO) levels, extensive π-conjugation, and better self-assembly.

Design and synthesis of heterocyclic azole based bioactive compounds: Molecular structures, quantum simulation, and mechanistic studies through docking as multi-target inhibitors of SARS-CoV-2 and cytotoxicity.

Two heterocyclic azole compounds, 3-(2,3-dihydrobenzo[d]thiazol-2-yl)-4H-chromen-4-one (SVS1) and 5-(1H-indol-3-yl)-4-methyl-2,4-dihydro-3H-1,2,4-triazole-3-thione (SVS2) were obtained unexpectedly from 2-aminothiophenol and 4-oxo-4H-chromene-3-carbaldehyde (for SVS1), and (E)-2-((1H-indol-3-yl)methylene)-N-methylhydrazine-1-carbothioamide in the presence of anhydrous FeCl3 (for SVS2), respectively. The compounds were well characterized by analytical and spectroscopic tools. The molecular structures of both the compounds were determined by single crystal X-ray diffraction (XRD) study. The results obtained from density functional theory (DFT) study revealed the molecular geometry and electron distribution of the compounds, which were correlated well with the three-dimensional structures obtained from the single crystal XRD. DMol3 was used to calculate quantum chemical parameters [chemical potential (µ), global hardness (η), global softness (σ), absolute electronegativity (χ) and electrophilicity index (ω)] of SVS1 and SVS2. Molecular docking study was performed to elucidate the binding ability of SVS1 and SVS2 with SARS-CoV-2 main protease and human angiotensin-converting enzyme-2 (ACE-2) molecular targets. Interestingly, the binding efficiency of the compounds with the molecular targets was comparable with that of remdesivir (SARS-CoV-2), chloroquine and hydroxychloroquine. SVS1 showed better docking energy than SVS2. The molecular docking study was complemented by molecular dynamics simulation study of SARS-CoV-2 main protease-SVS1 complex, which further exemplified the binding ability of SVS1 with the target. In addition, SVS1, SVS2 and cisplatin were assessed for their cytotoxicity against a panel of three human cancer cells such as HepG-2 (hepatic carcinoma), T24 (bladder) and EA.hy926 (endothelial), as well as Vero (kidney epithelial cells extracted from an African green monkey) normal cells using MTT assay. The results showed that SVS2 has significant cytotoxicity against HepG-2 and EA.hy926 cells with the IC50 values of 33.8 µM (IC50 = 49.9 µM-cisplatin and 8.6 µM-doxorubicin) and 29.2 (IC50 = 26.6 µM-cisplatin and 3.8 µM-doxorubicin), respectively.

Macrocyclic Complexes Derived from Four cis-L2 Pt Corners and Four Butadiynediyl Linkers; Syntheses, Electronic Structures, and Square versus Skew Rhombus Geometries.

The dialkyl malonate derived 1,3-diphosphines R2 C(CH2 PPh2 )2 (R=a, Me; b, Et; c, n-Bu; d, n-Dec; e, Bn; f, p-tolCH2 ) are combined with (p-tol3 P)2 PtCl2 or trans-(p-tol3 P)2 Pt((C≡C)2 H)2 to give the chelates cis-(R2 C(CH2 PPh2 )2 )PtCl2 (2 a-f, 94-69 %) or cis-(R2 C(CH2 PPh2 )2 )Pt((C≡C)2 H)2 (3 a-f, 97-54 %). Complexes 3 a-d are also available from 2 a-d and excess 1,3-butadiyne in the presence of CuI (cat.) and excess HNEt2 (87-65 %). Under similar conditions, 2 and 3 react to give the title compounds [(R2 C(CH2 PPh2 )2 )[Pt(C≡C)2 ]4 (4 a-f; 89-14 % (64 % avg)), from which ammonium salts such as the co-product [H2 NEt2 ]+ Cl- are challenging to remove. Crystal structures of 4 a,b show skew rhombus as opposed to square Pt4 geometries. The NMR and IR properties of 4 a-f are similar to those of mono- or diplatinum model compounds. However, cyclic voltammetry gives only irreversible oxidations. As compared to mono-platinum or Pt(C≡C)2 Pt species, the UV-visible spectra show much more intense and red-shifted bands. Time dependent DFT calculations define the transitions and principal orbitals involved. Electrostatic potential surface maps reveal strongly negative Pt4 C16 cores that likely facilitate ammonium cation binding. Analogous electronic properties of Pt3 C12 and Pt5 C20 homologs and selected equilibria are explored computationally.

Butterfly-Like Triarylamines with High Hole Mobility and On/Off Ratio in Bottom-Gated OFETs.

Highly π-extended butterfly-shaped triarylamine dyads with aryleneethynylene spacer were constructed using an efficient synthetic route. These aryleneethynylene-bridged dyads are highly fluorescent and exhibited high HOMO levels, and low bandgaps, which are suitable for high-performance p-type OFETs. The field-effect transistors were fabricated through a solution-processable method and exhibited promising p-type performance with field-effect mobility up to 4.3 cm2 /Vs and high Ion/off of 108 under ambient conditions.

Trapping of Terminal Platinapolyynes by Copper(I) Catalyzed Click Cycloadditions; Probes of Labile Intermediates in Syntheses of Complexes with Extended sp Carbon Chains, and Crystallographic Studies.

The silylated hexatriynyl complex trans-(C6 F5 )(p-tol3 P)2 Pt(C≡C)3 SiEt3 (PtC6 TES) is converted in situ to PtC6 H (wet n-Bu4 N+ F- , THF) and cross coupled with the diyne H(C≡C)2 SiEt3 (HC4 TES; CuCl/TMEDA, O2 ) to give PtC10 TES (71 %). This sequence is repeated twice to afford PtC14 TES (65 %) and then PtC18 TES (27 %). An analogous series of reactions starting with PtC8 TES gives PtC12 TES (60 %), then PtC16 TES (43 %), and then PtC20 TES (17 %). Similar cross couplings with H(C≡C)2 Si(i-Pr)3 (HC4 TIPS) give PtC12 TIPS (68 %), PtC14 TIPS (68 %), and PtC16 TIPS (34 %). The trialkylsilyl species (up to PtC18 TES) are converted to 3+2 "click" cycloadducts or 1,4-disubstituted 1,2,3-triazoles trans-(C6 F5 )(p-tol3 P)2 Pt(C≡C)n-1 C=CHN(CH2 C6 H5 )N=N (29-92 % after workups). The most general procedure involves generating the terminal polyynes PtCx H (wet n-Bu4 N+ F- , THF) in the presence of benzyl azide in DMF and aqueous CuSO4 /ascorbic acid. All of the preceding complexes are crystallographically characterized and the structural and spectroscopic properties analyzed as a function of chain length. Two pseudopolymorphs of PtC20 TES are obtained, both of which feature molecules with parallel sp carbon chains in a pairwise head/tail packing motif with extensive sp/sp van der Waals contacts.

Tunable Anticancer Activity of Furoylthiourea-Based RuII -Arene Complexes and Their Mechanism of Action.

Fourteen new RuII -arene (p-cymene/benzene) complexes (C1-C14) have been synthesized by varying the N-terminal substituent in the furoylthiourea ligand and satisfactorily characterized by using analytical and spectroscopic techniques. Electrostatic potential maps predicted that the electronic effect of the substituents was mostly localized, with some influence seen on the labile chloride ligands. The structure-activity relationships of the Ru-p-cymene and Ru-benzene complexes showed opposite trends. All the complexes were found to be highly toxic towards IMR-32 cancer cells, with C5 (Ru-p-cymene complex containing C6 H2 (CH3 )3 as N-terminal substituent) and C13 (Ru-benzene complex containing C6 H4 (CF3 ) as N-terminal substituent) showing the highest activity among each set of complexes, and hence they were chosen for further study. These complexes showed different behavior in aqueous solutions, and were also found to catalytically oxidize glutathione. They also promoted cell death by apoptosis and cell cycle arrest. Furthermore, the complexes showed good binding ability with the receptors Pim-1 kinase and vascular endothelial growth factor receptor 2, commonly overexpressed in cancer cells.

Toward Frameworks with Multiple Aligned and Interactive Fe(CO)3 Rotators: Syntheses and Structures of Diiron Complexes Linked by Two trans-Diaxial α,ω-Diphosphine Ligands Ar2P(CH2)nPAr2.

Reactions of (η4-benzylideneacetone)Fe(CO)3 and the α,ω-diphosphines Ar2P(CH2)nPAr2 afford the trigonal bipyramidal diiron tetraphosphorus complexes trans,trans-(CO)3Fe[Ar2P(CH2)nPAr2]2Fe(CO)3 (n/Ar = 3/Ph 3, 4/Ph 4a, 4/p-tol 4b; 56-19%). Crystal structures establish essentially parallel P-Fe-P axes, iron-iron distances of 5.894(9)-5.782(1) Å (3) and 6.403(1)-6.466(1) Å (4a,b), and van der Waals radii of 4.45 Å for the Fe(CO)3 rotators, the planes of which are offset by 0.029-1.665 Å. Analogous reactions of Ph2P(CH2)6PPh2 yield the square pyramidal monoiron complex trans-(CO)3Fe[Ph2P(CH2)6PPh2] (6', 31%), a rare case where a diphosphine spans trans basal positions (∠P-Fe-P 147.4(2)°). Both 3 and 6' exhibit two CO 13C NMR signals at room temperature, indicating slow exchange on the NMR time scale, which in the former could entail Fe(CO)3/Fe(CO)3 gearing. Under analogous conditions, 4a,b exhibit one signal. Previously reported adducts of Fe(CO)3 and Ph2P(CH2)nPPh2 are surveyed (1:1, n = 1-5; 2:2, n = 5), and the IR νC≡O band patterns and energies of all complexes analyzed with the aid of DFT calculations. The diiron complexes are preferred thermodynamically. Attention is given to limiting types of Fe(CO)3/Fe(CO)3 interactions in the diiron complexes.