Cycloaddition reactions of metalloaromatic complexes of iridium and rhodium: a mechanistic DFT investigation

One-pot Syntheses of Benzo- and Benzofuran-fused Iridaoxabenzenes via CH Bond Activations of Alkyl-bridged Diphenol Derivatives

One-pot syntheses of new π-extended metallaaromatic compounds have been developed by utilizing Ir-mediated CH bond activation of ethylene- or ethylidene-bridged diphenol derivatives. Depending on the bridging alkyl groups, two types of iridaoxabenzenes, both of which are doubly fused with benzo and benzofuran units, have been obtained. Studies on their structures and electronic characters indicate that both complexes have an aromatic character on the iridaoxacycles, and their π-conjugated systems are fully delocalized over the whole molecular skeletons. These novel metallaaromatic complexes exhibited some reactivities which are distinct from those reported for the non-fused metallaaromatic compounds.

Hafnium(IV) Chemistry with Imide-Dioxime and Catecholate-Oxime Ligands: Unique {Hf5} and Metalloaromatic {Hf6}-Oxo Clusters Exhibiting Fluorescence

Hafnium(IV) molecular species have gained increasing attention due to their numerous applications ranging from high-resolution nanolithography, heterogeneous catalysis, and electronics to the design of molecule-based building blocks in metal-organic frameworks (MOFs), with applications in gas separation, sorption, luminescence sensing, and interim storage of radioactive waste. Despite great potential, their chemistry is relatively underdeveloped. Here, we use strong chelators (2Z-6Z)-piperidine-2,6-dione (H₃pidiox) and 2,3-dihydroxybenzaldehyde oxime (H₃dihybo) to synthesize the first ever reported pentanuclear {Hf₅/H₃pidiox} and hexanuclear {Hf₆/H₃dihybo} clusters (HfOCs). The {Hf₆} clusters adopt unique core structures [Hf₆^IV(μ₃-O)₂(μ-O)₃] with a trigonal-prismatic arrangement of the six hafnium atoms and have been characterized via single-crystal X-ray diffraction analysis, UV-vis spectroscopy in the solid state, NMR, fluorescence spectroscopy, and high-resolution mass spectrometry in solution. One-dimensional (1D) and two-dimensional (2D) ¹H NMR and mass spectroscopies reveal the exceptional thermodynamic stability of the HfOCs in solution. Interestingly, the conjunction of the oxime group with the catechol resulted in the remarkable reduction of the clusters' band gap, below 2.51 eV. Another prominent feature is the occurrence of pronounced metalloaromaticity of the triangular {Hf₃} metallic component revealed by its NICS_zz scan curve calculated by means of density functional theory (DFT). The NICS_zz(1) value of -44.6 ppm is considerably higher than the -29.7 ppm found at the same level of theory for the benzene ring. Finally, we investigated the luminescence properties of the clusters where 1 emits light in the violet region despite the lack of fluorescence of the free H₃pidiox ligand, whereas the {Hf₆} 3 shifts the violet-emitting light of the H₃dihybo to lower energy. DFT calculations show that this fluorescence behavior stems from ligand-centered molecular orbital transitions and that Hf^IV coordination has a modulating effect on the photophysics of these HfOCs. This work not only represents a significant milestone in the construction of stable low-band-gap multinuclear Hf^IV clusters with unique structural features and metal-centered aromaticity but also reveals the potential of Hf(IV) molecule-based materials with applications in sensing, catalysis, and electronic devices.

Novel Quinazolinone–Isoxazoline Hybrids: Synthesis, Spectroscopic Characterization, and DFT Mechanistic Study

Quinazolinone and isoxazoline systems have attracted much attention due to their interesting pharmacological properties. The association of these two pharmacophores in a single hybrid structure can boost the biological activity or bring a new one. Inspired by this new paradigm, in the present work we report the synthesis and spectroscopic characterization of new quinazolinone–isoxazoline hybrids. The target compounds were obtained via 1,3-dipolar cycloaddition reactions of arylnitriloxides and N-allylquinazolinone. The synthesized compounds were characterized using spectroscopic techniques such as IR, 1D NMR (1H and 13C), 2D NMR (COSY and HSQC), and high-resolution mass spectrometry (HRMS). The spectral data show that this reaction leads only to the 3,5-disubstituted isoxazoline regioisomer, and that the observed regiochemistry is not affected by the nature of the substituents in the phenyl ring of the dipole. In addition, a theoretical study was performed using density functional theory (DFT) to support the experimental results in regard to the regiochemistry of the studied reactions. The computational mechanistic study was in good agreement with the experimental data.

Metallaaromatic Chemistry: History and Development

Since the prediction of the existence of metallabenzenes in 1979, metallaaromatic chemistry has developed rapidly, due to its importance in both experimental and theoretical fields. Now six major types of metallaromatic compounds, metallabenzenes, metallabenzynes, heterometallaaromatics, dianion metalloles, metallapentalenes and metallapentalynes (also termed carbolongs), and spiro metalloles, have been reported and extensively studied. Their parent organic analogues may be aromatic, non-aromatic, or even anti-aromatic. These unique systems not only enrich the large family of aromatics, but they also broaden our understanding and extend the concept of aromaticity. This review provides a comprehensive overview of metallaaromatic chemistry. We have focused on not only the six major classes of metallaaromatics, including the main-group-metal-based metallaaromatics, but also other types, such as metallacyclobutadienes and metallacyclopropenes. The structures, synthetic methods, and reactivities are described, their applications are covered, and the challenges and future prospects of the area are discussed. The criteria commonly used to judge the aromaticity of metallaaromatics are presented.

Recent Advances in Metallaaromatic Chemistry

Metallaaromatics can be broadly defined as aromatic compounds in which one of the ring atoms is a transition metal. The metallabenzenes are one important class of these compounds that has undergone extensive study recently. Closely related species such as fused-ring metallabenzenes, heterometallabenzenes, π-coordinated metallabenzenes and metallabenzynes have also attracted considerable attention. Although many metallaaromatics can be considered as metalla-analogues of classic organic aromatic compounds, this is not always the case. Recent seminal studies have shown that metallapentalenes and metallapentalynes, which are metalla-analogues of the anti-aromatic compounds pentalene and pentalyne, are in fact aromatic and highly stable. Very unusual spiro-metallaaromatic compounds have also recently been isolated. In this concepts article, key features of all these intriguing metallaaromatic compounds are discussed with reference to the structural, spectroscopic, reactivity and theoretical studies that have been undertaken. These compounds continue to generate much interest, not only because of the contributions they make to fundamental chemical understanding, but also because of the promise of possible practical applications.

Preparation and characterization of terdentate [C,N,N] acetophenone and acetylpyridine hydrazone platinacycles: a DFT insight into the reaction mechanism

The reaction of N-ortho-chlorophenyl-substituted acetylpyridine hydrazones (a and d) with K₂[PtCl₄] (n-butanol/water, 100 °C) gave mononuclear complexes 1a and 1d with the ligands as [N,N] bidentate. In contrast, the reaction of N-phenyl or N-meta-chlorophenyl hydrazones (b and c, respectively) under analogous reaction conditions gave the cycloplatinated species 2b and 2c with the ligand as [C,N,N] terdentate. The treatment of the mononuclear complexes 1a and 1d with NaOAc (n-butanol, 100 °C) gave the corresponding cycloplatinated complexes 2a and 2d. Acetophenone hydrazone platinacycle 2e was prepared in a similar fashion and its reaction with tertiary mono- and triphosphines gave mono- or trinuclear species depending on the reaction conditions. The X-ray crystal structures of some of these complexes showed interesting π-π slipped stacking interactions between metallacyclic rings which, according to NCI analyses, showed an aromatic character. With an aim to rationalize the different reactivities shown by acetylpyridine hydrazones and the precise role of the acetate anion, the energy profiles for the three main steps of cycloplatination (iminoplatinum complex formation, chelation and cyclometallation) have been determined by using the DFT (M06) methods. Calculations indicate that the cycloplatination of 1b proceeds via electrophilic substitution, involving the direct replacement of the chloride anion at the Pt(ii) centre with the N-phenyl moiety as the rate-determining step, to give an agostic intermediate 5b+ that, subsequently, leads to the elimination of a proton as hydrogen chloride. When present as an "external" base, acetate enters the coordination sphere around the Pt(ii) centre and facilitates hydrazone N-H deprotonation and electrophilic C-H activation through a dissociative route, leading to a Wheland-type σ-complex intermediate 9ac.

Back to basics: identification of reaction intermediates in the mechanism of a classic ligand substitution reaction on Vaska's complex

The mechanism of methylation of Vaska's complextrans-[ClIr(CO)(PPh₃)₂] by trimethylgallium was studied and the identification of the spectroscopically detected intermediates was achieved with the aid of computational methods.

Metalated nitriles: SNi' cyclizations with a propargylic electrophile

A rare 5-exo-dig SNi' cyclization with magnesiated and lithiated nitriles affords a cis-fused hydrindane bearing an exocyclic allene. The cyclization of the dilithiated nitrile pits a stereoelectronic preference for a trans-hydrindane against a cyclization through a less strained transition structure to the corresponding cis-hydrindane. Computational modeling suggests that the dilithiated nitrile cyclizes to a cis-hydrindane because the preferred transition structure positions the lithium cation in a cone of electron density that bridges the nitrile-bearing carbon, an alkoxide, and an electron-rich alkyne functionality.

Reactions of osmapyridinium with terminal alkynes

The reactions of osmapyridinium with terminal alkynes are presented, which lead to the formation of ten-membered osmacycles and η⁴-coordinated cyclopentadiene complexes.

Functionalization of metallabenzenes through nucleophilic aromatic substitution of hydrogen

The cationic metallabenzenes [Ir(C(5)H(4){SMe-1})(κ(2)-S(2)CNEt(2))(PPh(3))(2)]PF(6) (1) and [Os(C(5)H(4){SMe-1})(CO)(2)(PPh(3))(2)][CF(3)SO(3)] (2) undergo regioselective nucleophilic aromatic substitution of hydrogen at the metallabenzene ring position γ to the metal in a two-step process that first involves treatment with appropriate nucleophiles and then oxidation. Thus, reaction between compound 1 and NaBH(4), MeLi, or NaOEt gives the corresponding neutral iridacyclohexa-1,4-diene complexes Ir(C(5)H(3){SMe-1}{H-3}{Nu-3})(κ(2)-S(2)CNEt(2))(PPh(3))(2) (Nu = H (3), Me (4), OEt (5)). Similarly, reaction between 2 and NaBH(4) or MeLi gives the corresponding osmacyclohexa-1,4-diene complexes Os(C(5)H(3){SMe-1}{H-3}{Nu-3})(CO)(2)(PPh(3))(2) (Nu = H (8), Me (9)). The metallacyclohexa-1,4-diene rings in all these compounds are rearomatized on treatment with the oxidizing agent O(2), CuCl(2), or 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). Accordingly, the cationic metallabenzene 1 or 2 is returned after reaction between 3 and DDQ/NEt(4)PF(6) or between 8 and DDQ/NaO(3)SCF(3), respectively. The substituted cationic iridabenzene [Ir(C(5)H(3){SMe-1}{Me-3})(κ(2)-S(2)CNEt(2))(PPh(3))(2)]PF(6) (6) or [Ir(C(5)H(4){SMe-1}{OEt-3})(κ(2)-S(2)CNEt(2))(PPh(3))(2)]PF(6) (7) is produced in a similar manner through reaction between 4 or 5, respectively, and DDQ/NEt(4)PF(6), and the substituted cationic osmabenzene [Os(C(5)H(3){SMe-1}{Me-3})(CO)(2)(PPh(3))(2)]Cl (10) is formed in good yield on treatment of 9 with CuCl(2). The starting cationic iridabenzene 1 is conveniently prepared by treatment of the neutral iridabenzene Ir(C(5)H(4){SMe-1})Cl(2)(PPh(3))(2) with NaS(2)CNEt(2) and NEt(4)PF(6), and the related starting cationic osmabenzene 2 is obtained by treatment of Os(C(5)H(4){S-1})(CO)(PPh(3))(2) with CF(3)SO(3)CH(3) and CO. The stepwise transformations of 1 into 6 or 7 as well as 2 into 10 provide the first examples in metallabenzene chemistry of regioselective nucleophilic aromatic substitutions of hydrogen by external nucleophiles. DFT calculations have been used to rationalize the preferred sites for nucleophilic attack at the metallabenzene rings of 1 and 2. The crystal structures of 1, 3, 6, and 7 have been obtained.

Osmabenzenes from osmacycles containing an eta2-coordinated olefin

Treatment of HC[triple bond]CC(CH3)(OH)CH=CH2 with [OsCl2(PPh3)3] in dichloromethane yielded the eta2-olefin-coordinated osmacycle [Os{CH=C(PPh3)C(=CH2)-eta2-CH=CH2}Cl2(PPh3)2] (9). Transformations of osmacycle 9 by treatment with benzonitrile under various conditions have been investigated. Reaction of 9 with excess benzonitrile at room temperature afforded the dicationic osmacycle [Os{CH=C(PPh3)C(=CH2)-eta2-CH=CH2}(PhCN)2(PPh3)2]Cl2 (11) by ligand substitution, which reacted further to the intramolecularly coordinated eta2-allene complex [Os{CH=C(PPh3)C(CH3)=(eta2-C=CH2)}(PhCN)2(PPh3)2]Cl2 (12). In contrast, heating a chloroform solution of 9 to the reflux temperature in the presence of excess benzonitrile generated osmabenzene [Os{CHC(PPh3)C(CH3)CHCH}(PhCN)2(PPh3)2]Cl2 (14). Complexes 11, 12 and 14 are in fact isomers. In the absence of excess benzonitrile, the isolated dicationic 12 and 14 readily dissociate the benzonitrile ligands in solution to produce the neutral complex [Os{CH=C(PPh3)C(CH3)=(eta2-C=CH2)}Cl2(PPh3)2] (13) and the monocationic osmabenzene [Os{CHC(PPh3)C(CH3)CHCH}Cl(PhCN)(PPh3)2]BPh4 (15), respectively. Mechanisms for the formation of osmabenzene 14 from 11 and 12 are proposed based on DFT calculations.

Aromatic iridacycles

The first example of a family of related aromatic metallacycles has been synthesized, and the physical and chemical properties of its individual members have been investigated. The metallacyclic rings in these compounds are, in general, constructed from pentadienyl- or heteropentadienyliridium precursors via C-H bond activation processes. The molecules display the structural and spectroscopic features of aromaticity, including ring planarity, pi-bond delocalization, and diamagnetic ring current effects. The reactivity profile of the molecules is complex, reflecting the simultaneous presence of an aromatic ring and a reactive metal center.

Recent Advances in Metallabenzene Chemistry

Research into aromatic metallacycles, though discussed in the literature over the last quarter century, has undergone a major expansion since 2000. A wide variety of new metallabenzenes, encompassing new synthetic methods and new metal centers, is now available. New aromatic metallacycle topologies (iridanaphthalene, osmabenzynes) have been isolated and characterized. The first metallabenzene valence isomers (iridabenzvalenes, rhodabenzvalenes) and constitutional isomers (isoosmabenzenes) are now known. This review discusses the synthesis, chemistry, and physical properties of these intriguing aromatic compounds.

Metallabenzenes and metallabenzenoids

Metallabenzenes are analogues of benzene in which one of the benzene CH groups has been formally replaced by a transition metal and its ancillary ligands. This article summarises some of the important recent highlights of metallabenzene chemistry including the synthesis of the first platinabenzene, the synthesis of the first metallanaphthalene (an iridanaphthalene) and the development of new synthetic routes to metallabenzenes. The results of a number of theoretical studies that provide important insights into the nature and reactivity of metallabenzenes are discussed and in the concluding section some of the immediate goals and challenges in this field are identified.

Generalized Anomeric Interpretation of the “High-Energy” N−P Bond in N-Methyl-N‘-phosphorylguanidine: Importance of Reinforcing Stereoelectronic Effects in “High-Energy” Phosphoester Bonds

Electronic structure calculations have been performed on a model N-phosphorylguanidine, or phosphagen, to understand the stereoelectronic factors contributing to the lability of the "high-energy" N-P bond. The lability of the N-P bond is central to the physiological role of phosphagens involving phosphoryl transfer reactions important in cellular energy buffering and metabolism. Eight protonated forms of N-methyl-N'-phosphorylguanidine have been energy minimized at levels of theory ranging up to B3LYP/6-311++G(d,p) and MP2/6-311++G(d,p) to investigate the correlation between protonation state and N-P bond length. Selected forms have also been minimized using the CCSD/6-311++G(d,p) and QCISD/6-311++G(d,p) levels of theory. Bulk solvation energies using the polarized continuum model (PCM) with B3LYP/6-311++G(d,p) test the influence of the surroundings on computed structures and energies. The N-P bond length depends on the overall protonation state where increased protonation at the phosphoryl group or deprotonation at the unsubstituted N'' nitrogen results in shorter, stronger N-P bonds. Natural bond orbital analysis shows that the protonation state affects the N-P bond length by altering the magnitude of stabilizing n(O) --> sigma*(N-P) stereoelectronic interactions and to a lesser extent the sigma(N-P) --> sigma*(C-N'') and sigma(N-P) --> sigma*(C-N) interactions. The computations do not provide evidence of a competition between the phosphoryl and guanidinium groups for the same lone pair on the bridging nitrogen, as previously suggested by opposing resonance theory. The computed n(O) --> sigma*(N-P) anomeric effect provides a novel explanation of "high-energy" N-P bond lability. This offers new mechanistic insight into phosphoryl transfer reactions involving both phosphagens and other biochemically important "high-energy" phosphoester bonds.

Metallabenzene versus Cp Complex Formation: A DFT Investigation

One common reaction that metallabenzene complexes undergo is the formation of cyclopentadienyl (Cp) complexes. Density functional theory (DFT) was used here to investigate the reaction mechanism. It was found that the reaction can proceed via a carbene migratory insertion class of C-C coupling. Cp complexes are found to be thermodynamically favored, except for the case of (C5H5Ir)(PH3)2Cl2 (1j) where the metallabenzene was favored. Isolation a rhodiabenzene of the type (C5H5Rh)(PH3)2Cl2 (1m) and a palladiabenzene, such as (C5H5Pd)Cp (1p), may be possible.