Steric and Electronic Effects on the Configurational Stability of Residual Chiral Phosphorus‐Centered Three‐Bladed Propellers: Tris‐aryl Phosphanes

A series of tris‐aryl phosphanes, structurally designed to exist as residual enantiomers or diastereoisomers, bearing substituents differing in size and electronic properties on the aryl rings, were synthesized and characterized. Their electronic properties were evaluated on the basis of their electrochemical oxidation potential determined by voltammetry. The configurational stability of residual phosphanes, evaluated by dynamic HPLC on a chiral stationary phase or/and by dynamic ¹H and ³¹P NMR spectroscopy, was found to be rather modest (barriers of about 18–20 kcal mol^?1), much lower than that shown by the corresponding phosphane oxides (barriers of about 25–29 kcal mol^?1). For the first time, the residual antipodes of a tris‐aryl phosphane were isolated in enantiopure state and the absolute configuration assigned to them by single‐crystal anomalous X‐ray diffraction analysis. In this case, the racemization barrier could be calculated also by CD signal decay kinetics. A detailed computational investigation was carried out to clarify the helix reversal mechanism. Calculations indicated that the low configurational stability of tris‐aryl phosphanes can be attributed to an unexpectedly easy phosphorus pyramidal inversion which, depending upon the substituents present on the blades, can occur even on the most stable of the four conformers constituting a single residual stereoisomer.     

Strongly Absorbing π–π* States in Heteroleptic Dipyrrin/2,2′‐Bipyridine Ruthenium Complexes: Excited‐State Dynamics from Resonance Raman Spectroscopy

A recently reported new class of ruthenium complexes containing 2,2′‐bipyridine and a dipyrrin ligand in the coordination sphere exhibit both strong metal‐to‐ligand charge‐transfer (MLCT) and π–π* transitions. Quantitative analysis of the resonance Raman scattering intensities and absorption spectra reveals only weak electronic interactions between these states despite direct coordination of the bipyridyl and dipyrrin ligands to the central ruthenium atom. On the basis of DFT calculations and time‐dependent DFT (TD‐DFT), we propose that the electronic excited states closely resemble “pure” MLCT and π–π* states. Resonance Raman intensity analysis demonstrates that a large amplitude transannular torsional motion provides a mechanism for relaxation on the π–π* excited‐state surface. We assert that this result is generally applicable to a range of dipyrrin complexes such as boron–dipyrrin and metallodipyrrin systems. Despite the large torsional distortion between the phenyl ring and the dipyrromethene plane, π–π* excitation extends out onto the phenyl ring which may have important consequences in solar‐energy‐conversion applications of ruthenium–dipyrrin complexes.     

Synthesis and Structural Characterization of Four Zinc Complexes Containing 3,5‐Dimethylpyrazole and Carboxylate Ligands

Four new zinc(II) complexes Zn₂(μ‐dmpz)₂(Hdmpz)₂(L1)₂ ( 1 ) (Hdmpz = 3,5‐dimethylpyrazole, HL1 = 2‐methyl‐2‐phenoxypropanoic acid), Zn(Hdmpz)₂(L2)₂ ( 2 ) [HL2 = 2‐hydroxy‐5‐(phenyldiazenyl)benzoic acid], Zn₂(μ‐dmpz)₂(Hdmpz)₂(L3)₂ ( 3 ) [HL3 = 3,4‐(methylenedioxy)benzoic acid], and Zn₂(μ‐dmpz)₂(Hdmpz)₂(L4)₂ ( 4 ) [HL4 = 3‐(4‐methoxyphenyl)acrylic acid] were prepared and structurally characterized by different techniques including elemental analysis, IR spectroscopy, and single‐crystal X‐ray diffraction analysis. The X‐ray studies suggested that all these complexes except compound 2 are centrosymmetric dinuclear complexes with a tetrahedral arrangement around each zinc ion, whereas compound 2 is a mononuclear complex. The pyrazole ligand is coordinated in both terminal as well as a bridging fashion in the dinuclear moiety, whereas the pyrazole ligand in compound 2 is coordinated only in monodentate terminal fashion with its neutral nitrogen group. In all four complexes the carboxylate functions behave as monodentate ligands. All complexes show intramolecular hydrogen bonding of N–H ··· O between N–H of pyrazole and nonbonded oxygen atom of carboxylate. Furthermore, rich intermolecular weak interactions such as classical hydrogen bonds, C–H ··· O, C–H ··· N, C–H ··· π, and CH₃–π interactions exist and complexes 1 – 4 display a set of 3D superamolecular frameworks. In addition, the four compounds are thermally stable below 150 °C.     

Syntheses,Crystal Structures and Thermal Behavior of Five New Complexes Containing 2, 4, 6‐Trifluorobenzoate as Ligand

Five new complexes containing 2, 4, 6‐trifluorobenzoateas ligand have been synthesized and structurally characterized, namely Li(C₆F₃H₂COO)(H₂O) (P2₁, Z = 2, 1 ),Cs(C₆F₃H₂COO)(C₆F₃H₂COOH) (P2₁/c, Z = 4, 2 ),Cu(C₆F₃H₂COO)₂(H₂O)₂ (P$\bar{1}$ , Z = 1, 3 ), Cu(C₆F₃H₂COO)₂(MeOH) (P2₁/c. Z = 4, 4 ) and Ag(C₆F₃H₂COO)(H₂O) (C2/c, Z = 8, 5 ). 1 – 3 and 5 are coordination polymers forming strands ( 1 , 3 , 5 ) or corrugated layers ( 2 ). In 1 and 2 the benzoate ligand acts as a bridging ligand, whereas in 3 and 5 the benzoate ligand is not bridging and the molecular units are interconnected by bridging water molecules. In 4 and 5, dimeric Cu₂ and Ag₂ units, respectively, are formed with short M ··· M contacts. The dimeric units in 4 resemble the well‐known paddlewheel structural motif. In 5 these dimeric units are further connected by bridging water molecules, whereas in 4 only very weak F ··· H interactions connect the dimeric units. DTA/TG experiments on 1 , 3 and 4 reveal that in a first step solvent molecules (H₂O, MeOH) are unquestionably released. In 1 – 5 the torsion angles of the carboxylate group with respect to the aromatic ring deviate significantly from zero. These results are in very good agreement with the results of quantum chemical calculations of free 2, 4, 6‐trifluorobenzoic acid and its dimer at the DFT and RI‐MP2 level of theory.     

Effects of Epoxidation and Nitration on Ballistic Properties of FOX‐7 – A DFT Study

1, 1‐Diamino‐2, 2‐dinitroethylene (FOX‐7) has received increasing attention since it was industrialized in the late 1990s. It has lower sensitivity and comparable performance to RDX. This paper presents ballistic properties of FOX‐7, its mono and dinitro derivatives and their epoxide derivatives computationally. The structures were optimized at the B3LYP/6‐31G(d, p) level and the bond lengths were calculated. The calculated data for FOX‐7 are compatible with the literature one. We have investigated the bond dissociation energies of the molecules. Mulliken electro negativities (χ_M) and chemical hardness (η) were reviewed using Frontier Molecular Orbitals at HF/6‐31G(d, p)//B3LYP/6‐31G(d, p) theoretical level. The detonation performance analyses were done using empirical Kamlet‐Jacobs equations. Additionally, power index values were calculated. All the compounds considered in the present article are powerful candidates for high energy materials.     

Cationic Heteroleptic Cyclometalated Iridium Complexes with 1‐Pyridylimidazo[1,5‐α]pyridine Ligands: Exploitation of an Efficient Intersystem Crossing

Luminescent ligands in Ir^III cyclometalated complexes. The photophysical and photochemical properties of Ir‐cyclometalated complexes containing luminescent ligands are evaluated (see figure). Significant admixture between Ir and ligand orbitals induces an efficient intersystem crossing. Photochemical reactions performed in the presence of oxygen lead to new Ir‐cyclometalated complexes containing N(amido) groups directly bound to Ir.

     

New RuII(arene) Complexes with Halogen‐Substituted Bis‐ and Tris(pyrazol‐1‐yl)borate Ligands

[RuCl(arene)(μ‐Cl)]₂ dimers were treated in a 1:2 molar ratio with sodium or thallium salts of bis‐ and tris(pyrazolyl)borate ligands [Na(Bp)], [Tl(Tp)], and [Tl(Tp^{iPr, 4Br})]. Mononuclear neutral complexes [RuCl(arene)(κ²‐Bp)] ( 1 : arene=p‐cymene (cym); 2 : arene=hexamethylbenzene (hmb); 3 : arene=benzene (bz)), [RuCl(arene)(κ²‐Tp)] ( 4 : arene=cym; 6 : arene=bz), and [RuCl(arene)(κ²‐Tp^{iPr, 4Br})] ( 7 : arene=cym, 8 : arene=hmb, 9 : arene=bz) have been always obtained with the exception of the ionic [Ru₂(hmb)₂(μ‐Cl)₃][Tp] ( 5′ ), which formed independently of the ratio of reactants and reaction conditions employed. The ionic [Ru(CH₃OH)(cym)(κ²‐Bp)][X] ( 10 : X=PF₆, 12 : X=O₃SCF₃) and the neutral [Ru(O₂CCF₃)(cym)(κ²‐Bp)] ( 11 ) have been obtained by a metathesis reaction with corresponding silver salts. All complexes 1 – 12 have been characterized by analytical and spectroscopic data (IR, ESI‐MS, ¹H and ¹³C NMR spectroscopy). The structures of the thallium and calcium derivatives of ligand Tp, [Tl(Tp)] and [Ca(dmso)₆][Tp]₂ ? 2 DMSO, of the complexes 1 , 4 , 5′ , 6 , 11 , and of the decomposition product [RuCl(cym)(Hpz^{iPr, 4Br})₂][Cl] ( 7′ ) have been confirmed by using single‐crystal X‐ray diffraction. Electrochemical studies showed that 1 – 9 and 11 undergo a single‐electron Ru^II→Ru^III oxidation at a potential, measured by cyclic voltammetry, which allows comparison of the electron‐donor characters of the bis‐ and tris(pyrazol‐1‐yl)borate and arene ligands, and to estimate, for the first time, the values of the Lever E_L ligand parameter for Bp, Tp, and Tp^{iPr, 4Br}. Theoretical calculations at the DFT level indicated that both oxidation and reduction of the Ru complexes under study are mostly metal‐centered with some involvement of the chloride ligand in the former case, and also demonstrated that the experimental isolation of the μ³‐binuclear complex 5′ (instead of the mononuclear 5 ) is accounted for by the low thermodynamic stability of the latter species due to steric reasons.     

Zinc(II) and Mercury(II) Complexes of Pyrazole‐Based NNN‐Type Ligands: Syntheses,X‐ray Structures,Thermal Analyses and DFT Studies

Monomeric zinc(II) and mercury(II) complexes containing tripodal nitrogen donor ligand 2,6‐bis(3,4,5‐trimethyl‐N‐pyrazolyl)pyridine (btmpp) were synthesized, and characterized by elemental and spectroscopic (IR, UV/Vis) analyses, TG‐DTA and single‐crystal X‐ray diffraction studies. X‐ray analyses of the complexes [Zn(btmpp)Cl₂] ( 2 ) and [Hg(btmpp)(SCN)₂] ( 3 ) showed that both structures crystallize in space group P2₁/c with a = 7.9722(6), b = 18.3084(13), c = 13.3117(9) Å and Z = 4 for 2 and a = 8.7830(3), b = 21.1489(7), c = 12.0682(4) Å and Z = 4 for 3 . Both monomeric units contain pentacoordinate metal ions in distorted square‐pyramidal arrangement. The structures of complexes 2 and 3 were also computed with DFT methods at B3LYP/LanL2DZ level and are in good agreement with the experimental values obtained from X‐ray analysis. The NPa charge distributions, HOMO–LUMO gaps, and dipole moments for 1 , 2 , and 3 were also reported. Natural bond orbital analyses were performed to reveal local charges and charge transfers in 1 , 2 , and 3 .     

Catalytic Reactivity of Molybdenum–Trihalide Complexes Bearing PCP‐Type Pincer Ligands

Molybdenum–iodide complexes bearing a PCP[1] ligand have been found to work as excellent catalysts toward ammonia formation under ambient reaction conditions among dinitrogen‐bridged dimolybdenum complexes and other molybdenum complexes bearing PNP and PCP[2] ligands.     

Lactide Polymerisation with Air‐Stable and Highly Active Zinc Complexes with Guanidine–Pyridine Hybrid Ligands

New class of air‐stable catalysts for lactide polymerisation: Guanidine–pyridine hybrid ligands (picture, left) were used to prepare a series of zinc complexes (e.g., depicted cation [ZnL₂(CF₃SO₃)]⁺, where L is the quinoline‐containing ligand with R¹=R²=R³=R⁴=Me), in which the ligand binds through two different N‐donor atoms. The zinc complexes show high activity in ring‐opening polymerisation of d,l ‐lactide (right), giving polylactide with molecular masses up to 176 000 g mol^?1 and in high yield.

     

1,4,6,9‐Tetraoxa‐5λ4‐selena‐spiro[4.4]nonane – A Combined Theoretical and Experimental Study

The symmetric spiro‐selenurane derived from ethylene glycol, 1,4,6,9‐tetraoxa‐5λ⁴‐selena‐spiro[4.4]nonane, was prepared from selenium tetrachloride and ethylene glycol and its molecular structure was determined by single crystal X‐ray diffraction. NBO analyses for the title compound and a related compound were conducted to assess the role of the stereochemical active lone pair on the selenium atom on the structure.     

New Insights into Keggin‐Type 12‐Tungstophosphoric Acid from 31P MAS NMR Analysis of Absorbed Trimethylphosphine Oxide and DFT Calculations

The acid and transport properties of the anhydrous Keggin‐type 12‐tungstophosphoric acid (H₃PW₁₂O₄₀; HPW) have been studied by solid‐state ³¹P magic‐angle spinning NMR of absorbed trimethylphosphine oxide (TMPO) in conjunction with DFT calculations. Accordingly, ³¹P NMR resonances arising from various protonated complexes, such as TMPOH⁺ and (TMPO)₂H⁺ adducts, could be unambiguously identified. It was found that thermal pretreatment of the sample at elevated temperatures (≥423 K) is a prerequisite for ensuring complete penetration of the TMPO guest probe molecule into HPW particles. Transport of the TMPO absorbate into the matrix of the HPW adsorbent was found to invoke a desorption/absorption process associated with the (TMPO)₂H⁺ adducts. Consequently, three types of protonic acid sites with distinct superacid strengths, which correspond to ³¹P chemical shifts of 92.1, 89.4, and 87.7 ppm, were observed for HPW samples loaded with less than three molecules of TMPO per Keggin unit. Together with detailed DFT calculations, these results support the scenario that the TMPOH⁺ complexes are associated with protons located at three different terminal oxygen (O_d) sites of the PW₁₂O₄₀³⁻ polyanions. Upon increasing the TMPO loading to >3.0 molecules per Keggin unit, abrupt decreases in acid strength and the corresponding structural variations were attributed to the change in secondary structure of the pseudoliquid phase of HPW in the presence of excessive guest absorbate.     

Design of Luminescent,Heteroleptic, Cyclometalated PtII and PtIV Complexes: Photophysics and Effects of the Cyclometalated Ligands

Neutral pentafluorophenyl benzoquinolinyl Pt^II [Pt(bzq)(HC^N−κN)(C₆F₅)] ( 1 a – g ) complexes, bearing nonmetalated N-heterocyclic HC^N ligands [HC^N=2,5-diphenyl-1,3,4-oxadiazole (Hoxd) a , 2-(2,4-difluorophenyl)pyridine (dfppy) b , 2-phenylbenzo[d]thiazole (pbt) c , 2-(4-bromophenyl)benzo[d]thiazole (Br-pbt) d , 2-phenylquinoline (pq) e , 2-thienylpyridine (thpy) f , 1-(2-pyridyl)pyrene (pypy) g ], and heteroleptic bis(cyclometalated) Pt^IV fac-[Pt(bzq)(C^N)(C₆F₅)Cl] ( 2 b – g , bzq: benzo[h]quinolinyl) derivatives, generated by oxidation of 1 b – g with PhICl₂, are reported. The oxidation reaction of 1 a evolved with formation of the bimetallic Pt^IV complex syn-[Pt(bzq)(C₆F₅)Cl(μ-OH)]₂ 3 . The crystal structures of 1 a,d,f , 2 b,d,e and 3 were corroborated by X-ray crystallography. A comparative study of the absorption and photoluminescence properties of the two series of complexes Pt^II ( 1 ) and Pt^IV ( 2 ), supported by time-dependent DFT calculations (TD-DFT), is presented. The low-lying transitions (absorption and emission) of Pt^II complexes 1 a – e [solution and polystyrene (PS) films] were assigned to the IL/MLCT mixture located on the cyclometalated Pt(bzq) unit, with minor IL′/ML′CT/LL′CT contributions involving the non-metalated ligand. Complex 1 g , bearing the more delocalized pyridyl pyrene (Hpypy) as an ancillary ligand, shows dual ¹ππ* and ³ππ* (Hpypy) emission in fluid CH₂Cl₂ and dual ³IL/³MLCT [Pt(bzq)] and [³ππ*, Hpypy] phosphorescence at 77 K. Upon oxidation, Pt^IV complexes 2 b – f display (solution, PS) ligand-based phosphorescence that arises from the bzq in 2 b (³LC) or from the second C^N ligand in 2 c – f (³L′C) with some ³LL′CT in 2 f . Despite metalation of the pyrenyl group, 2 g exhibits dual emission ¹ππ*/³ππ* located on the pypy chromophore.     

Imino‐Bridged Bisphosphaalkenes (2,4‐Diphospha‐3‐azapentadienes)

Deprotonation of aminophosphaalkenes (RMe₂Si)₂C?PN(H)(R′) (R=Me, iPr; R′=tBu, 1‐adamantyl (1‐Ada), 2,4,6‐tBu₃C₆H₂ (Mes*)) followed by reactions of the corresponding Li salts Li[(RMe₂Si)₂C?P(M)(R′)] with one equivalent of the corresponding P‐chlorophosphaalkenes (RMe₂Si)₂C?PCl provides bisphosphaalkenes (2,4‐diphospha‐3‐azapentadienes) [(RMe₂Si)₂C?P]₂NR′. The thermally unstable tert‐butyliminobisphosphaalkene [(Me₃Si)₂C?P]₂NtBu ( 4 a ) undergoes isomerisation reactions by Me₃Si‐group migration that lead to mixtures of four‐membered heterocyles, but in the presence of an excess amount of (Me₃Si)₂C?PCl, 4 a furnishes an azatriphosphabicyclohexene C₃(SiMe₃)₅P₃NtBu ( 5 ) that gave red single crystals. Compound 5 contains a diphosphirane ring condensed with an azatriphospholene system that exhibits an endocylic P?C double bond and an exocyclic ylidic P⁽⁺⁾? C^(?)(SiMe₃)₂ unit. Using the bulkier iPrMe₂Si substituents at three‐coordinated carbon leads to slightly enhanced thermal stability of 2,4‐diphospha‐3‐azapentadienes [(iPrMe₂Si)₂C?P]₂NR′ (R′=tBu: 4 b ; R′=1‐Ada: 8 ). According to a low‐temperature crystal‐structure determination, 8 adopts a non‐planar structure with two distinctly differently oriented P?C sites, but ³¹P NMR spectra in solution exhibit singlet signals. ³¹P NMR spectra also reveal that bulky Mes* groups (Mes*=2,4,6‐tBu₃C₆H₂) at the central imino function lead to mixtures of symmetric and unsymmetric rotamers, thus implying hindered rotation around the P? N bonds in persistent compounds [(RMe₂Si)₂C?P]₂NMes* ( 11 a , 11 b ). DFT calculations for the parent molecule [(H₃Si)₂C?P]₂NCH₃ suggest that the non‐planar distortion of compound 8 will have steric grounds.     

The Role of 2,6‐Diaminopyridine Ligands in the Isolation of an Unprecedented,Low‐Valent Tin Complex

Stabilization of the central atom in an oxidation state of zero through coordination of neutral ligands is a common bonding motif in transition‐metal chemistry. However, the stabilization of main‐group elements in an oxidation state of zero by neutral ligands is rare. Herein, we report that the transamination reaction of the DAMPY ligand system (DAMPY=2,6‐[ArNH‐CH₂]₂(NC₅H₃) (Ar=C₆H₃‐2,6‐iPr₂)) with Sn[N(SiMe₃)₂]₂ produces the DIMPYSn complex (DIMPY=(2,6‐[ArN?CH]₂(NC₅H₃)) with the Sn atom in a formal oxidation state of zero. This is the first example of a tin compound stabilized in a formal oxidation state of zero by only one donor molecule. Furthermore, three related low‐valent Sn^II complexes, including a [DIMPYSn^IICl]⁺[SnCl₃]^? ion pair, a bisstannylene DAMPY{Sn^II[N(SiMe₃)₂]₂}₂, and the enamine complex MeDIMPYSn^II, were isolated. Experimental results and the conclusions drawn are also supported by theoretical studies at the density functional level of theory and ¹¹⁹Sn Mössbauer spectroscopy.     

DFT Mechanistic Study on Diene Metathesis Catalyzed by Ru‐Based Grubbs–Hoveyda‐Type Carbenes: The Key Role of π‐Electron Density Delocalization in the Hoveyda Ligand

The catalytic activity and catalyst recovery of two heterogenized ruthenium‐based precatalysts ( H and NO₂(4) ) in diene ring‐closing metathesis have been studied by means of density functional calculations at the B3LYP level of theory. For comparison and rationalization of the key factors that lead to higher activities and higher catalyst recoveries, four other Grubbs–Hoveyda complexes have also been investigated. The full catalytic cycle (catalyst formation, propagation, and precatalyst regeneration) has been considered. DFT calculations suggest that either for the homogeneous and heterogenized systems the activity of the catalysts mainly depends on the ability of the precursor to generate the propagating carbene. This ability does not correlate with the traditionally identified key factor, the Ru???O interaction strength. In contrast, precatalysts with lower alkoxy‐dissociation energy barriers and lower stabilities compared with the propagating carbene also present larger C1? C2 bond length (i.e., lower π character of the C? C bond that exists between the metal–carbene (Ru?C) and the phenyl ring of the Hoveyda ligand). Catalyst recovery, regardless of whether a release–return mechanism occurs or not, is also mainly determined by the π delocalization. Therefore, future Grubbs–Hoveyda‐type catalyst development should be based on fine‐tuning the π‐electron density of the phenyl moiety, with the subsequent effect on the metalloaromaticity of the ruthenafurane ring, rather than considering the modification of the Ru???O interaction.     

On the Solvent‐free Structure of a Jäger‐type Cobalt(II) N2O2‐Chelate Complex

A combined synchrotron X‐ray and density functional theory (DFT) study on the structure of a Jäger‐type N₂O₂ chelate complex was carried out. The ethoxy‐substituted bis(3‐oxo‐enaminato)cobalt(II) complex ( 1 ) was an original sample from the laboratory of the late Professor Ernst‐G. Jäger (University of Jena, Germany). Single‐crystal X‐ray analysis revealed essentially flat molecules of 1 , which are unsolvated and coordinatively unsaturated. The DFT calculations on the isolated molecule predict a planar structure for the non‐hydrogen atoms, which is a local minimum on the energy surface. The crystal packing is achieved through off‐set stacking (staircase arrangement), resulting in a herringbone pattern in the space group P2₁2₁2₁. The structure of 1 is compared to known structures of related bis(3‐oxo‐enaminato)cobalt(II) complexes ( 2 – 4 ). Original bulk material of 1 was investigated by scanning electron microscopy (SEM), powder X‐ray diffraction (PXRD), melting point determination, and infrared (IR) spectroscopy.