Triple Bonds Between Iron and Heavier Group 15 Elements in AFe(CO)3− (A=As,Sb, Bi) Complexes

Heteronuclear transition‐metal–main‐group‐element carbonyl complexes of AsFe(CO)₃⁻, SbFe(CO)₃⁻, and BiFe(CO)₃⁻ were produced by a laser vaporization supersonic ion source in the gas phase, and were studied by mass‐selected IR photodissociation spectroscopy and advanced quantum chemistry methods. These complexes have C_3v structures with all of the carbonyl ligands bonded on the iron center, and feature covalent triple bonds between bare Group 15 elements and Fe(CO)₃⁻. Chemical bonding analyses on the whole series of AFe(CO)₃⁻ (A=N, P, As, Sb, Bi, Mc) complexes indicate that the valence orbitals involved in the triple bonds are hybridized 3d and 4p atomic orbitals of iron, leading to an unusual (dp–p) type of transition‐metal–main‐group‐element multiple bonding. The σ‐type three‐orbital interaction between Fe 3d/4p and Group 15 np valence orbitals plays an important role in the bonding and stability of the heavier AFe(CO)₃⁻ (A=As, Sb, Bi) complexes.     

Magnesium(II) Complexes with High Emission: The Distinct Charge‐Transfer Process from Transition Metal

Generally, the first‐row transition‐metal complexes are notorious in luminescence materials because of their metal‐ligand charge transfer in emission process. Herein, we rationally used magnesium instead the first‐row transition metal to coordinate with 2‐(anthracen‐9‐yl)‐1H‐imidazo[4,5‐f][1,10]phenanthroline (AIP) in the construction of luminescent complexes. Further investigation revealed AIP could work as detector for quantitative determination of Mg²⁺ cation. Comparing to other divalent cations, this fluorescence sensor exhibited high selectivity for the quantitative determination of Mg²⁺ with the low limit of detection (5 × 10^–7 m ). Through X‐ray single crystal diffraction, the crystal structures of [Mg(AIP₂)(NO₃)₂ · (H₂O)₄] ( 1 ), [Mn(AIP)(NO₃) · EtOH] ( 2 ), and [Co₂(AIP)₂Cl₄ · (MeOH)₂] ( 3 ) were observed in various arrangements. The theory calculations based on crystal structures indicated the Mg^II complex undergoes distinct charge‐transfer process from other transition‐metals based compounds, in which charge‐transfer excited‐state lifetimes were deactivated rapidly through metal‐to‐ligand charge‐transfer (MLCT) process. This study provided insight into construction of luminescence compounds by using d⁰ metals in main groups instead of transition metals.     

Octacarbonyl Anion Complexes of Group Three Transition Metals [TM(CO)8]− (TM=Sc,Y, La) and the 18‐Electron Rule

We report the gas‐phase synthesis of stable 20‐electron carbonyl anion complexes of group 3 transition metals, TM(CO)₈⁻ (TM=Sc, Y, La), which are studied by mass‐selected infrared (IR) photodissociation spectroscopy. The experimentally observed species, which are the first octacarbonyl anionic complexes of a TM, are identified by comparison of the measured and calculated IR spectra. Quantum chemical calculations show that the molecules have a cubic (O_h) equilibrium geometry and a singlet (¹A_1g) electronic ground state. The 20‐electron systems TM(CO)₈⁻ are energetically stable toward loss of one CO ligand, yielding the 18‐electron complexes TM(CO)₇⁻ in the ¹A₁ electronic ground state; these exhibit a capped octahedral structure with C_3v symmetry. Analysis of the electronic structure of TM(CO)₈⁻ reveals that there is one occupied valence molecular orbital with a_2u symmetry, which is formed only by ligand orbitals without a contribution from the metal atomic orbitals. The adducts of TM(CO)₈⁻ fulfill the 18‐electron rule when only those valence electrons that occupy metal–ligand bonding orbitals are considered.     

Preparation,spectroscopic investigation and antibacterial activity of some organomercury(II) and organotin(IV) dithio complexes

Phenylmercuric acetate, triphenyltin chloride and dibutyltin chloride react with alkali‐metal or ammonium salts of some 1,1‐ and 1,2‐dithio ligands in appropriate molar ratios to yield a series of organometallic dithio complexes of the type [PhHgX] (X = butylxanthate (Buxant⁻), cyclohexylxanthate (Cyxant⁻), benzylxanthate (Bzxant⁻) or pyrrolidin‐1‐yldithiocarbamate (Pdtc⁻); [(PhHg)₂X] (X = isomaleonitriledithiolate (i‐MNT²⁻) or 1‐ethoxycarbonyl‐1‐cyanoethylene‐2,2‐dithiolate (ecda²⁻); Ph₃SnX (X = Buxant⁻ or Pdtc⁻); [(Ph₃Sn)₂(i‐MNT)] and [Bu₂SnMNT] (MNT²⁻ = maleonitriledithiolate). These complexes have been characterized by elemental analysis, molar conductance measurements, IR, FT‐Raman, ¹H and ¹³C NMR and fast atom bombardment (FAB) mass spectra. Cyclic dimeric structures for phenylmercuryxanthates and monomeric structures for the remaining complexes are suggested. Antibacterial activities of the complexes and parent ligands have been screened against some well‐known pathogenic bacteria. Organomercury dithiolates have been found to be more potential antibacterial than organotin complexes. Copyright © 2000 John Wiley & Sons, Ltd.     

Mechanism and Kinetics of the Reaction of Nitrosamines with OH Radical: A Theoretical Study

The reaction of nitrosodimethylamine, nitrosoazetidine, nitrosopyrrolidine, and nitrosopiperidine with the hydroxyl radical has been studied using electronic structure calculations in gas and aqueous phases. The rate constant was calculated using variational transition state theory. The reactions are initiated by H‐atom abstraction from the αC─H group of nitrosamines and leads to the formation of alkyl radical intermediate. In the subsequent reactions, the initially formed alkyl radical intermediate reacts with O₂ forming a peroxy radical. The reaction of peroxy radical with other atmospheric oxidants, such as HO₂ and NO radicals, is studied. The structures of the reactive species were optimized by using the density functional theory methods, such as M06‐2X, MPW1K, and BHandHLYP, and hybrid methods G3B3. The single‐point energy calculations were also performed at CCSD(T)/6‐311+G(d,p)// M062X/6‐311+G(d,p) level. The calculated thermodynamical parameters show that the reactions corresponding to the formation of intermediates and products are highly exothermic. We have calculated the rate constant for the initial H‐atom abstraction and subsequent favorable secondary reactions using canonical variational transition state theory over the temperature range of 150–400 K. The calculated rate constant for initial H‐atom abstraction reaction is ∼3 × 10⁻¹² cm³ molecule⁻¹ s⁻¹ and is in agreement with the previous experimental results. The calculated thermochemical data and rate constants show that the reaction profile and kinetics of the reactions are less dependent on the number of methyl groups present in the nitrosoamines. Furthermore, it has been found that the atmospheric lifetime of nitrosamines is around 5 days in the normal atmospheric OH concentration.     

Preparation and Characterization of Uranium–Iron Triple‐Bonded UFe(CO)3− and OUFe(CO)3− Complexes

We report the preparation of UFe(CO)₃⁻ and OUFe(CO)₃⁻ complexes using a laser‐vaporization supersonic ion source in the gas phase. These compounds were mass‐selected and characterized by infrared photodissociation spectroscopy and state‐of‐the‐art quantum chemical studies. There are unprecedented triple bonds between U 6d/5f and Fe 3d orbitals, featuring one covalent σ bond and two Fe‐to‐U dative π bonds in both complexes. The uranium and iron elements are found to exist in unique formal U(I or III) and Fe(−II) oxidation states, respectively. These findings suggest that there may exist a whole family of stable df–d multiple‐bonded f‐element‐transition‐metal compounds that have not been fully recognized to date.     

Assessing the Influence of Mutation on GTPase Transition States by Using X‐ray Crystallography, 19F NMR,and DFT Approaches

We report X‐ray crystallographic and ¹⁹F NMR studies of the G‐protein RhoA complexed with MgF₃⁻, GDP, and RhoGAP, which has the mutation Arg85′Ala. When combined with DFT calculations, these data permit the identification of changes in transition state (TS) properties. The X‐ray data show how Tyr34 maintains solvent exclusion and the core H‐bond network in the active site by relocating to replace the missing Arg85′ sidechain. The ¹⁹F NMR data show deshielding effects that indicate the main function of Arg85′ is electronic polarization of the transferring phosphoryl group, primarily mediated by H‐bonding to O^3G and thence to P^G. DFT calculations identify electron‐density redistribution and pinpoint why the TS for guanosine 5′‐triphosphate (GTP) hydrolysis is higher in energy when RhoA is complexed with RhoGAP_Arg85′Ala relative to wild‐type (WT) RhoGAP. This study demonstrates that ¹⁹F NMR measurements, in combination with X‐ray crystallography and DFT calculations, can reliably dissect the response of small GTPases to site‐specific modifications.     

Assessing the Influence of Mutation on GTPase Transition States by Using X‐ray Crystallography, 19F NMR,and DFT Approaches

We report X‐ray crystallographic and ¹⁹F NMR studies of the G‐protein RhoA complexed with MgF₃⁻, GDP, and RhoGAP, which has the mutation Arg85′Ala. When combined with DFT calculations, these data permit the identification of changes in transition state (TS) properties. The X‐ray data show how Tyr34 maintains solvent exclusion and the core H‐bond network in the active site by relocating to replace the missing Arg85′ sidechain. The ¹⁹F NMR data show deshielding effects that indicate the main function of Arg85′ is electronic polarization of the transferring phosphoryl group, primarily mediated by H‐bonding to O^3G and thence to P^G. DFT calculations identify electron‐density redistribution and pinpoint why the TS for guanosine 5′‐triphosphate (GTP) hydrolysis is higher in energy when RhoA is complexed with RhoGAP_Arg85′Ala relative to wild‐type (WT) RhoGAP. This study demonstrates that ¹⁹F NMR measurements, in combination with X‐ray crystallography and DFT calculations, can reliably dissect the response of small GTPases to site‐specific modifications.     

Dispersion Forces,Disproportionation, and Stable High‐Valent Late Transition Metal Alkyls

The transition metal tetra‐ and trinorbornyl bromide complexes, M(nor)₄ (M=Fe, Co, Ni) and Ni(nor)₃Br (nor=1‐bicyclo[2.2.1]hept‐1‐yl) and their homolytic fragmentations were studied computationally using hybrid density functional theory (DFT) at the B3PW91 and B3PW91‐D3 dispersion‐corrected levels. Experimental structures were well replicated; the dispersion correction resulted in shortened M−C bond lengths for the stable complexes, and it was found that Fe(nor)₄ receives a remarkable 45.9 kcal mol⁻¹ stabilization from the dispersion effects whereas the tetragonalized Co(nor)₄ shows stabilization of 38.3 kcal mol⁻¹. Ni(nor)₄ was calculated to be highly tetragonalized with long Ni−C bonds, providing a rationale for its current synthetic inaccessibility. Isodesmic exchange evaluation for Fe(nor)₄ confirmed that dispersion force attraction between norbornyl substituents is fundamental to the stability of these species.     

Tridentate Complexes of Palladium(II) and Platinum(II) Bearing bis‐Aryloxide Triazole Ligands: A Joint Experimental and Theoretical Investigation

A novel class of palladium(II) and platinum(II) complexes bearing tridentate bis‐aryloxide triazole ligands was prepared by using straightforward and high‐yielding synthetic routes. The complexes were fully characterized and the molecular structures of four derivatives were unambigously determined by single‐crystal X‐ray diffractometric analyses. For the most promising luminescent Pt^II derivatives, further experimental investigations were carried out to characterize their photophysical features and to ascertain the nature of the emitting excited state by means of electronic absorption, steady‐state, and time‐resolved emission techniques in different conditions. In degassed fluid solution the complexes displayed broad and featureless photoluminescence with λ_em=522–585 nm, excited‐state lifetime up to few microseconds and quantum yield (PLQY) up to 17 %, depending on the nature of both ancillary ligand and substituent on the tridentate ligand. Computational investigation using density functional theory and time‐dependent DFT were performed to gain insight into the electronic processes responsible for optical transitions and structure–photoluminescence relationship. Jointly, experimental and theoretical characterization indicated that the radiative transition arises from an excited state with admixed triplet‐manifold metal‐to‐ligand charge transfer and ligand‐centered (³MLCT/³LC) character. We elucidated the modulation of the photophysical properties upon variation of substituents for this new family of complexes.     

Stabilization of Human Telomeric G‐Quadruplex and Inhibition of Telomerase Activity by Propeller‐Shaped Trinuclear PtII Complexes

Two novel propeller‐shaped, trigeminal‐ligand‐containing, flexible trinuclear Pt^II complexes, {[Pt(dien)]₃(ptp)}(NO₃)₆ ( 1 ) and {[Pt(dpa)]₃(ptp)}(NO₃)₆ ( 2 ) (dien: diethylenetriamine; dpa: bis‐(2‐pyridylmethyl)amine; ptp: 6′‐(pyridin‐3‐yl)‐3,2′:4′,3′′‐terpyridine), have been designed and synthesized, and their interactions with G‐quadruplex (G4) sequences are characterized. A combination of biophysical and biochemical assays reveals that both Pt^II complexes exhibit higher affinity for human telomeric (hTel) and c‐myc promoter G4 sequences than duplex DNA. Complex 1 binds and stabilizes hTel G4 sequence more effectively than complex 2 . Both complexes are found to induce and stabilize either antiparallel or parallel conformation of G4 structures. Molecular docking studies indicate that complex 1 binds into the large groove of the antiparallel hTel G4 structure (PDB ID: 143D) and complex 2 stacks onto the exposed G‐quartet of the parallel hTel G4 structure (PDB ID: 1KF1). Telomeric repeat amplification protocol assays demonstrate that both complexes are good telomerase inhibitors, with IC₅₀ values of (16.0±0.4) μM and (4.20±0.25) μM for 1 and 2 , respectively. Collectively, the results suggest that these propeller‐shaped flexible trinuclear Pt^II complexes are effective and selective G4 binders and good telomerase inhibitors. This work provides valuable information for the interaction between multinuclear metal complexes with G4 DNA.     

(H3O)2NaAl3F12, isostructural with A2NaAl3F12 (A = K+, Rb+, Cs+) fluorides having HTB‐type sheets

The title compound, (H₃O)₂NaAl₃F₁₂ [dihydronium sodium trialuminum(III) dodecafluoride], was obtained by solvothermal synthesis from the reaction of aluminium hydroxide, sodium hydroxide, 1,2,4‐triazole and aqueous HF in ethanol at 463 K for 48 h. The structure consists of AlF₆ octahedra organized in [AlF₄⁻]_n HTB‐type sheets (HTB is hexagonal tungsten bronze) separated by H₃O⁺ and Na⁺ cations.     

Computational study of structures and proton transfer in hydrogen‐bonded ammonia complexes using semiempirical valence‐bond approach

In this study, the seGVB method was implemented for the N H bonding system, specifically for hydrogen‐bonded ammonia complexes, and the model well reproduces the MP2 geometries and energetics. A comparison between the ammonia dimer and water dimer is given from the viewpoint of valance‐bond structures in terms of the calculated bond energies and pair–pair interactions. The linear hydrogen bond is found to be stronger than the bent bonds in both cases, with the difference in energy between the linear and cyclic structures being comparable in both cases although the NH bonds are generally weaker. The energy decomposition clearly demonstrates that the changes in electronic energy are quite different in the two cases due to the presence of an additional lone pair on the water molecule, and it is this effect which leads to the net stabilization of the cyclic structure for the ammonia dimer. Proton‐transfer profiles for hydrogen‐bonded ammonia complexes [NH₂ H NH₂]⁻ and [NH₃ H NH₃]⁺ were calculated. The barrier for proton transfer in [NH₃ H NH₃]⁺ is larger than that in [NH₂ H NH₂]⁻, but smaller than that in the protonated water dimer. The different bonding structures substantially affect the barrier to proton transfer, even though they are isoelectronic systems. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 73: 357–367, 1999     

Crucial Roles of Two Hydrated Mg2+ Ions in Reaction Catalysis of the Pistol Ribozyme

Pistol ribozymes constitute a new class of small self‐cleaving RNAs. Crystal structures have been solved, providing three‐dimensional snapshots along the reaction coordinate of pistol phosphodiester cleavage, corresponding to the pre‐catalytic state, a vanadate mimic of the transition state, and the product. The results led to the proposed underlying chemical mechanism. Importantly, a hydrated Mg²⁺ ion remains innersphere‐coordinated to N7 of G33 in all three states, and is consistent with its likely role as acid in general acid base catalysis (δ and β catalysis). Strikingly, the new structures shed light on a second hydrated Mg²⁺ ion that approaches the scissile phosphate from its binding site in the pre‐cleavage state to reach out for water‐mediated hydrogen bonding in the cyclophosphate product. The major role of the second Mg²⁺ ion appears to be the stabilization of product conformation. This study delivers a mechanistic understanding of ribozyme‐catalyzed backbone cleavage.     

The Effect of the Spacer of Bis(biurea) Ligands on the Structure of A2L3‐type (A=anion) Phosphate Complexes

By tuning the length and rigidity of the spacer of bis(biurea) ligands L, three structural motifs of the A₂L₃ complexes (A represents anion, here orthophosphate PO₄^3?), namely helicate, mesocate, and mono‐bridged motif, have been assembled by coordination of the ligand to phosphate anion. Crystal structure analysis indicated that in the three complexes, each of the phosphate ions is coordinated by twelve hydrogen bonds from six surrounding urea groups. The anion coordination properties in solution have also been studied. The results further demonstrate the coordination behavior of phosphate ion, which shows strong tendency for coordination saturation and geometrical preference, thus allowing for the assembly of novel anion coordination‐based structures as in transition‐metal complexes.     

Designing Point Mutants to Detect Structural Coupling in a Heterotrimeric G Protein α‐subunit by NMR Spectroscopy†

To better understand the mechanism by which the activating signal is transmitted from the receptor‐interacting regions on the G protein α‐subunit (G_α) to the guanine nucleotide‐binding pocket, we generated and characterized mutant forms of G_α with alterations in switch II (Trp‐207→Phe) and the carboxyl‐terminus (Phe‐350→Ala). Previously reported bacterial expression methods for the high‐level production of a uniformly isotope‐labeled G_tα/G_i1α chimera, ChiT, were successfully used to isolate milligram quantities of ¹⁵N‐labeled mutant protein. NMR analysis showed that while the GDP/Mg²⁺‐bound state of both mutants shared an overall conformation similar to that of the GDP/Mg²⁺‐bound state of ChiT, formation of the “transition/activated” state in the presence of aluminum fluoride (AlF₄^?) revealed distinct differences between the wild‐type and mutant G_α subunits, particularly in the response of the ¹HN, ¹⁵N cross‐peak for the Trp‐254 indole in the Trp‐207→Phe mutant and the ¹HN, ¹⁵N cross‐peak for Ala‐350 in the Phe‐350→Ala mutant. Consistent with the NMR data, the F350→Ala mutant showed an increase in intrinsic fluorescence that was similar to G_tα and ChiT upon formation of the “transition/activated” state in the presence of AlF₄^?, whereas the intrinsic fluorescence of the Trp‐207→Phe mutant decreased. These results show that the substitution of key amino acid positions in G_α can effect structural changes that may compromise receptor interactions and GDP/GTP exchange.     

Aminocyclopentitol Inhibitors of α‐L‐Fucosidases

Aminocyclopentitol analogs of α‐L ‐fucose were synthesized stereoselectively from D ‐ribose. Alkyl substituents were attached at the NH₂ group to mimic the glycosidic leaving group. The resulting (alkylamino)cyclopentitols inhibited α‐L ‐fucosidases selectively with inhibition constants in the range of K_i=10⁻⁷ M . Comparisons with stereoisomers and acyclic analogs showed that this inhibition only occurs with N‐alkyl substitution and proper configuration at the cyclopentane, as expected for transition‐state‐analog‐type inhibition. These observations were supported by molecular‐modeling comparisons between inhibitor and transition state.     

Mononuclear metal (II) complexes of a Bis(organoamido)phosphate ligand with antimicrobial activities against Escherichia coli

A new tripodal ligand [PO(NH²MePy)₃] ( L ) (²MePy = 2‐(4‐methyl pyridyl)) have been synthesized by treating phosphorous oxychloride with 2‐Amino‐4‐methylpyridine in toluene under refluxing condition. The ligand was appeared as a white solid and characterized by several standard analytical and spectroscopic techniques such as FT‐IR, NMR (¹H, ¹³C{¹H} and ³¹P{¹H}) and ESI‐MS spectroscopy. The ligand ( L ) undergone metal‐assisted hydrolysis of one P–N bond when treated it with hydrated metal nitrates, M(NO₃)₂·xH₂O (M = Zn, Cu, Co and Ni) under hydrothermal reaction condition in DMF‐H₂O (1:1). This results in the formation of four mononuclear complexes [{PO₂(NH²MePy)₂}₂M] [M = Zn ( 1 ), Cu ( 2 ), Co ( 3 ), Ni ( 4 )], where ligand ( L ) hydrolyses to a anionic bis(organoamido)phosphate, [PO₂(NH²MePy)₂]⁻. All complexes were completely characterized by various analytical techniques and their solid state molecular structures were established by single crystal X‐ray diffraction. All complexes are isostructural with a metal (II) ion situating at the centre of a distorted octahedron. Two tridentate [PO₂(NH²MePy)₂]⁻ ligands are coordinated to metal(II) ion through N‐ and O‐donor atoms, thus neutralizing the charge of the complex. Optical properties of all complexes in solid state have been studied. Moreover, antimicrobial activities of complexes 1 – 4 have been explored. To the best of our knowledge, this is the first report of such compounds investigated for their antimicrobial activities.     

Efficient hydrolysis of glucose-1-phosphate catalyzed by metallomicelles with histidine residue

Phosphate esters play an important role in genetic information transfer, cell signal transduction, energy transmission and metabolic processes of living beings. Efficient catalytic hydrolysis of phosphate esters is still an attractive and challenging problem. Here, a new 2-amino-N-dodecyl-3-(1H-imidazol-5-yl)propanamide (L²) surfactant was synthesized and its metallomicelles of La³⁺, Cu²⁺, Co²⁺, and Zn²⁺ complexes were used as mimic metalloenzymes to catalyze the hydrolysis of glucose-1-phosphate (G1P) in a buffer solution at 35°C. The metallomicelle systems can efficiently catalyze the hydrolysis of G1P. The rare-earth metallomicelle LaL² has the highest catalytic activity compared with those of the transition metal micelles CuL², CoL², and ZnL². Different association behaviors of metallomicelles and substrate G1P were proposed. The imidazole group might accelerate the hydrolysis by activating H₂O associated with the metal into a metal-OH^? group. A possible catalytic mechanism was also discussed.     

Fluoride‐Bridged {GdIII3MIII2} (M=Cr,Fe, Ga) Molecular Magnetic Refrigerants

The reaction of fac‐[M^IIIF₃(Me₃tacn)]⋅x H₂O with Gd(NO₃)₃⋅5H₂O affords a series of fluoride‐bridged, trigonal bipyramidal {Gd^III₃M^III₂} (M=Cr ( 1 ), Fe ( 2 ), Ga ( 3 )) complexes without signs of concomitant GdF₃ formation, thereby demonstrating the applicability even of labile fluoride‐complexes as precursors for 3d–4f systems. Molecular geometry enforces weak exchange interactions, which is rationalized computationally. This, in conjunction with a lightweight ligand sphere, gives rise to large magnetic entropy changes of 38.3 J kg⁻¹ K⁻¹ ( 1 ) and 33.1 J kg⁻¹ K⁻¹ ( 2 ) for the field change 7 T→0 T. Interestingly, the entropy change, and the magnetocaloric effect, are smaller in 2 than in 1 despite the larger spin ground state of the former secured by intramolecular Fe–Gd ferromagnetic interactions. This observation underlines the necessity of controlling not only the ground state but also close‐lying excited states for successful design of molecular refrigerants.