2‐(2′‐Pyridyl)‐4,6‐diphenylphosphinine versus 2‐(2′‐Pyridyl)‐4,6‐diphenylpyridine: Synthesis,Characterization, and Reactivity of Cationic RhIII and IrIII Complexes Based on Aromatic Phosphorus Heterocycles

The bidentate P,N hybrid ligand 1 allows access for the first time to novel cationic phosphinine‐based Rh^III and Ir^III complexes, broadening significantly the scope of low‐coordinate aromatic phosphorus heterocycles for potential applications. The coordination chemistry of 1 towards Rh^III and Ir^III was investigated and compared with the analogous 2,2′‐bipyridine derivative, 2‐(2′‐pyridyl)‐4,6‐diphenylpyridine ( 2 ), which showed significant differences. The molecular structures of [RhCl(Cp*)( 1 )]Cl and [IrCl(Cp*)( 1 )]Cl (Cp*=pentamethylcyclopentadienyl) were determined by means of X‐ray diffraction and confirm the mononuclear nature of the λ³‐phosphinine–Rh^III and Ir^III complexes. In contrast, a different reactivity and coordination behavior was found for the nitrogen analogue 2 , especially towards Rh^III as a bimetallic ion pair [RhCl(Cp*)( 2 )]⁺[RhCl₃(Cp*)]^? is formed rather than a mononuclear coordination compound. [RhCl(Cp*)( 1 )]Cl and [IrCl(Cp*)( 1 )]Cl react with water regio‐ and diastereoselectively at the external P?C double bond, leading exclusively to the anti‐addition products [MCl(Cp*)( 1 H ? OH)]Cl as confirmed by X‐ray crystal‐structure determination.     

3‐Bromo‐2‐Pyrone: An Alternative and Convenient Route to Functionalized Phosphinines

The facile access to 3‐bromo‐2‐pyrone allows the preparation of 6‐bromo‐2‐trimethylsilyl‐phosphinine by a [4+2] cycloaddition with Me₃Si‐C≡P for the first time. The regioselectivity of this reaction could be verified by means of single crystal X‐ray diffraction of the corresponding W⁰ complex. In the presence of ZnBr₂ and dppp (1,3‐bis(diphenylphosphino)propane) as a bidentate ligand, the bromo‐phosphinine quantitatively undergoes a Negishi cross‐coupling reaction with PhLi that selectively leads to 6‐phenyl‐2‐trimethylsilyl‐phosphinine. This heterocycle could again be characterized by means of X‐ray diffraction as a W⁰ complex. These results describe a new and convenient route to 2,6‐disubstituted phosphinines that makes use of readily available starting materials.     

Pyridyl‐Functionalised 3H‐1,2,3,4‐Triazaphospholes: Synthesis,Coordination Chemistry and Photophysical Properties of Low‐Coordinate Phosphorus Compounds

Novel conjugated, pyridyl‐functionalised triazaphospholes with either tBu or SiMe₃ substituents at the 5‐position of the N₃PC heterocycle have been prepared by a [3+2] cycloaddition reaction and compared with structurally related, triazole‐based systems. Photoexcitation of the 2‐pyridyl‐substituted triazaphosphole gives rise to a significant fluorescence emission with a quantum yield of up to 12 %. In contrast, the all‐nitrogen triazole analogue shows no emission at all. DFT calculations indicate that the 2‐pyridyl substituted systems have a more rigid and planar structure than their 3‐ and 4‐pyridyl isomers. Time‐dependent (TD) DFT calculations show that only the 2‐pyridyl‐substituted triazaphosphole exhibits similar planar geometry, with matching conformational arrangements in the lowest energy excited state and the ground state; this helps to explain the enhanced emission intensity. The chelating P,N‐hybrid ligand forms a Re^I complex of the type [(N^N)Re(CO)₃Br] through the coordination of nitrogen atom N² to the metal centre rather than through the phosphorus donor. Both structural and spectroscopic data indicate substantial π‐accepting character of the triazaphosphole, which is again in contrast to that of the all‐nitrogen‐containing triazoles. The synthesis and photophysical properties of a new class of phosphorus‐containing extended π systems are described.     

Chelation versus Binucleation: Metal Complex Formation with the Hexadentate all‐cis‐N1,N2‐Bis(2,4,6‐trihydroxy‐3,5‐diaminocyclohexyl)ethane‐1,2‐diamine

The hexadentate ligand all‐cis‐N¹,N²‐bis(2,4,6‐trihydroxy‐3,5‐diaminocyclohexyl)ethane‐1,2‐diamine (L^e) was synthesized in five steps with an overall yield of 39 % by using [Ni(taci)₂]SO₄?4 H₂O as starting material (taci=1,3,5‐triamino‐1,3,5‐trideoxy‐cis‐inositol). Crystal structures of [Na_0.5(H₆L^e)](BiCl₆)₂Cl_0.5?4 H₂O ( 1 ), [Ni(L^e)]‐ Cl₂?5 H₂O ( 2 ), [Cu(L^e)](ClO₄)₂?H₂O ( 3 ), [Zn(L^e)]CO₃?7 H₂O ( 4 ), [Co(L^e)](ClO₄)₃ ( 5 c ), and [Ga(H_?2L^e)]‐ NO₃?2 H₂O ( 6 ) are reported. The Na complex 1 exhibited a chain structure with the Na⁺ cations bonded to three hydroxy groups of one taci subunit of the fully protonated H₆(L^e)⁶⁺ ligand. In 2 , 3 , 4 , and 5 c , a mononuclear hexaamine coordination was found. In the Ga complex 6 , a mononuclear hexadentate coordination was also observed, but the metal binding occurred through four amino groups and two alkoxo groups of the doubly deprotonated H_?2(L^e)^2?. The steric strain within the molecular framework of various M(L^e) isomers was analyzed by means of molecular mechanics calculations. The formation of complexes of L^e with Mn^II, Cu^II, Zn^II, and Cd^II was investigated in aqueous solution by using potentiometric and spectrophotometric titration experiments. Extended equilibrium systems comprising a large number of species were observed, such as [M(L^e)]²⁺, protonated complexes [MH_z(L^e)]^2+z and oligonuclear aggregates. The pK_a values of H₆(L^e)⁶⁺ (25 °C, μ=0.10 m ) were found to be 2.99, 5.63, 6.72, 7.38, 8.37, and 9.07, and the determined formation constants (log β) of [M(L^e)]²⁺ were 6.13(3) (Mn^II), 20.11(2) (Cu^II), 13.60(2) (Zn^II), and 10.43(2) (Cd^II). The redox potentials (vs. NHE) of the [M(L^e)]^3+/2+ couples were elucidated for Co (?0.38 V) and Ni (+0.90 V) by cyclic voltammetry.     

Simple Synthesis of Functionalized 2‐Phosphanaphthalenes

A simple synthesis of sodium 2‐phosphanaphthalene‐3‐olate ( 1 ) based on the extrusion of N₂ from phthalazine using Na[OCP] is reported. This heterocycle can be readily functionalized at the negatively charged oxygen center using a variety of electrophilic substrates. The coordination chemistry of both 1 and its neutral derivatives was explored, revealing their facile use as P‐donor ligands for late‐transition‐metal complexes.     

Pd(II) and Pt(II) complexes of 2-(2'-pyridyl)-4,6-diphenylphosphinine: synthesis, structure, and reactivity

The coordination chemistry of the bidentate P,N hybrid ligand 2-(2'-pyridyl)-4,6-diphenylphosphinine (1) towards Pd(II) and Pt(II) has been investigated. The molecular structures of the complexes [PdCl(2)(1)] and [PtCl(2)(1)] were determined by X-ray diffraction, representing the first crystallographically characterized λ(3)-phosphinine-Pd(II) and -Pt(II) complexes. Both complexes reacted with methanol at the P=C double bond at an elevated temperature, leading to the corresponding products [MCl(2)(1H·OCH(3))]. The molecular structure of [PdCl(2)(1H·OCH(3))] was determined crystallographically and revealed that the reaction with methanol proceeds selectively by syn addition and exclusively to one of the P=C double bonds. Strikingly, the reaction of [PdCl(2)(1H·OCH(3))] with the chelating diphosphine DPEphos at room temperature in CH(2)Cl(2) led quantitatively to [PdCl(2)(DPEphos)] and phosphinine 1 by elimination of CH(3)OH and rearomatization of the phosphorus heterocycle.     

PtII Complexes with 6‐(5‐Trifluoromethyl‐Pyrazol‐3‐yl)‐2,2′‐Bipyridine Terdentate Chelating Ligands: Synthesis,Characterization, and Luminescent Properties

A series of Pt^II complexes Pt(fpbpy)Cl ( 1 ), Pt(fpbpy)(OAc) ( 2 ), Pt(fpbpy)(NHCOMe) ( 3 ), Pt(fpbpy)(NHCOEt) ( 4 ), and [Pt(fpbpy)(NCMe)](BF₄) ( 5 ) with deprotonated 6‐(5‐trifluoromethyl‐pyrazol‐3‐yl)‐2,2′‐bipyridine terdentate ligand are prepared, among which 1 is converted to complexes 2 – 5 by a simple ligand substitution. Alternatively, acetamide complex 3 is prepared by hydrolysis of acetonitrile complex 5 , while the back conversion from 3 to 1 is regulated by the addition of HCl solution, showing the reaction sequence 1 → 5 → 3 → 1 . Multilayer OLED devices are successfully fabricated by using triphenyl‐(4‐(9‐phenyl‐9H‐fluoren‐9‐yl)phenyl) silane (TPSi‐F) as host material and with doping concentrations of 1 varying from 7 to 100 %. The electroluminescence showed a substantial red‐shifting versus the normal photoluminescence detected in solution. Moreover, at a doping concentration of 28 %, the device showed a saturated red luminescence with a maximum external quantum yield of 8.5 % at 20 mA cm^?2 and a peak luminescence of 47 543 cd m^?2 at 18.5 V.     

Photochemistry of RuII 4,4′‐Bi‐1,2,3‐triazolyl (btz) Complexes: Crystallographic Characterization of the Photoreactive Ligand‐Loss Intermediate trans‐[Ru(bpy)(κ2‐btz)(κ1‐btz)(NCMe)]2+

We report the unprecedented observation and unequivocal crystallographic characterization of the meta‐stable ligand loss intermediate solvento complex trans‐[Ru(bpy)(κ²‐btz)(κ¹‐btz)(NCMe)]²⁺ ( 1 a ) that contains a monodentate chelate ligand. This and analogous complexes can be observed during the photolysis reactions of a family of complexes of the form [Ru($\widehat{NN}$ )(btz)₂]²⁺ ( 1 a – d : btz=1,1′‐dibenzyl‐4,4′‐bi‐1,2,3‐triazolyl; $\widehat{NN}$ =a) 2,2′‐bipyridyl (bpy), b) 4,4′‐dimethyl‐2,2′‐bipyridyl (dmbpy), c) 4,4′‐dimethoxy‐2,2′‐bipyridyl (dmeobpy), d) 1,10‐phenanthroline (phen)). In acetonitrile solutions, 1 a – d eventually convert to the bis‐solvento complexes trans‐[Ru($\widehat{NN}$ )(btz)(NCMe)₂]²⁺ ( 3 a – d ) along with one equivalent of free btz, in a process in which the remaining coordinated bidentate ligands undergo a new rearrangement such that they become coplanar. X‐ray crystal structure of 3 a and 3 d confirmed the co‐planar arrangement of the $\widehat{NN}$ and btz ligands and the trans coordination of two solvent molecules. These conversions proceed via the observed intermediate complexes 2 a – d , which are formed quantitatively from 1 a – d in a matter of minutes and to which they slowly revert back on being left to stand in the dark over several days. The remarkably long lifetime of the intermediate complexes (>12 h at 40 °C) allowed the isolation of 2 a in the solid state, and the complex to be crystallographically characterized. Similarly to the structures adopted by complexes 3 a and d , the bpy and κ²‐btz ligands in 2 a coordinate in a square‐planar fashion with the second monodentate btz ligand coordinated trans to an acetonitrile ligand.     

A Zwitterionic Phosphonio‐1, 2, 4‐Diazaphospholide and Neutral 1, 2, 4‐Diazaphosphole — A Comparative Study of Molecular Structures and Co‐ordination Properties

The bis‐phosphonio‐1, 2, 4‐diazaphospholide salt ( 1 [Cl]) reacts with complex boron hydrides under selective extrusion of one PPh₃ moiety to give borane adducts of a novel zwitterionic phosphonio‐1, 2, 4‐diazaphospholide. Both the Et₃B adduct 2b and the free zwitterionic heterocycle 3 , which was liberated by further reaction of 2b with NEt₃, were characterized by spectroscopic data and 2b , as well, by a single crystal X‐ray diffraction study. The comparison of the structural data with those of a neutral 1, 2, 4‐diazaphosphole and a lithium‐1, 2, 4‐diazaphospholide which was formed by deprotonation of the parent 1, 2, 4‐diazaphosphole 4a discloses trends in endocyclic bonding distances which can be rationalized in terms of a charge dependent shift in the π‐electron distribution. First studies of the co‐ordination properties reveal for both 2b and 4a a marked preference to bind two M(CO)₅‐fragments (M = Cr, W) via the lone‐pairs of the phosphorus and one nitrogen atom; mononuclear complexes with P‐co‐ordinated heterocycles are formed as intermediates. A single crystal X‐ray diffraction study of the dinuclear complex [Cr₂(CO)₁₀(μ₂‐C₂H₃N₂P‐κP, κN)] ( 10a ) together with spectroscopic studies (including ¹⁸³W NMR studies of tungsten complexes) suggests that M→L back donation is more efficient for P‐ than for N‐bound metal fragments. No evidence for π‐co‐ordination of the 1, 2, 4‐diazaphosphole ring to a Cr(CO)₃ fragment was obtained.     

Facile Synthesis and Properties of 2‐λ5‐Phosphaquinolines and 2‐λ5‐Phosphaquinolin‐2‐ones

Treatment of 2‐ethynylanilines with P(OPh)₃ gives either 2,2‐diphenoxy‐2‐λ⁵‐phosphaquinolines or 2‐phenoxy‐2‐λ⁵‐phosphaquinolin‐2‐ones under transition‐metal‐free conditions. This reaction offers access to an underexplored heterocycle, which opens up the study of the fundamental nature of the N?P^V double bond and its potential for delocalization within a cyclic π‐electron system. This heterocycle can serve as a carbostyril mimic, with application as a bioisostere for pharmaceuticals based on the 2‐quinolinone scaffold. It also holds promise as a new fluorophore, since initial screening reveals quantum yields upwards of 40 %, Stokes shifts of 50–150 nm, and emission wavelengths of 380–540 nm. The phosphaquinolin‐2‐ones possess one of the strongest solution‐state dimerization constants for a D–A system (130 M ^?1) owing to the close proximity of a strong acceptor (P?O) and a strong donor (phosphonamidate N? H), which suggests that they might hold promise as new hydrogen‐bonding hosts for optoelectronic sensing.     

Coordination Chemistry of N‐Heterocyclic Nitrenium‐Based Ligands

Comprehensive studies on the coordination properties of tridentate nitrenium‐based ligands are presented. N‐heterocyclic nitrenium ions demonstrate general and versatile binding abilities to various transition metals, as exemplified by the synthesis and characterization of Rh^I, Rh^III, Mo⁰, Ru⁰, Ru^II, Pd^II, Pt^II, Pt^IV, and Ag^I complexes based on these unusual ligands. Formation of nitrenium–metal bonds is unambiguously confirmed both in solution by selective ¹⁵N‐labeling experiments and in the solid state by X‐ray crystallography. The generality of N‐heterocyclic nitrenium as a ligand is also validated by a systematic DFT study of its affinity towards all second‐row transition and post‐transition metals (Y–Cd) in terms of the corresponding bond‐dissociation energies.     

Syntheses,Characterization and X‐ray Structure of the First Silver(I) Complexes with 4‐Amino‐3‐thioxo‐3,4‐dihydro‐1,2,4‐triazin‐5(2H)‐one

Reaction of thiocarbohydrazide with glyoxolic acid monohydrate led to 4‐amino‐3‐thioxo‐3,4‐dihydro‐1,2,4‐triazin‐5(2H)‐one (AHTTO, 1 ). Treatment of 1 with AgNO₃ and PPh₃ gave thecomplexes [(PPh₃)₂Ag₂(μ‐N,S‐AHTTO)₂](NO₃)₂ ( 2 ) and [(PPh₃)₂Ag(AHTTO)]NO₃ · MeOH ( 3 ) was obtained under different conditions. All the compounds have been characterized by elemental analyses, IR spectroscopy and X‐ray diffraction studies.     

Between Adamantane and Atrane: Intrabridgehead Interactions in the Cage‐Like Phosphane Related to a Novel Tris(homoadamantane) Ring System

Herein, we report unusual four‐center interactions in the novel cage‐like phosphane, 1,4,7‐triaza‐9‐phosphatricyclo[5.3.2.1^4,9]tridecane (CAP). This water‐soluble ligand, the first example of a tris(homoadamantane) ring system, can be considered a macrocyclic homologue of the well‐known PTA (1,3,5‐triaza‐7‐phosphaadamantane). However, ³¹P NMR spectroscopic anomalies of CAP follow those typical for the bi‐/tricyclic atrane systems. Another atrane‐like feature of CAP is the ability of one nitrogen atom to undergo out–in pyramidal inversion. The latter is associated with a substantial decrease in the intracage N?N and P?N distances. Analysis of electron density distribution [molecular electrostatic potential (MESP) and atoms‐in‐molecules (AIM) approaches] suggests that the P and N atoms in the pyramidally inverted CAP derivatives are involved in interactions resulting in accumulation of electron density at the center of the phosphane cage. The latter can reliably explain the stereoelectronic and NMR anomalies of the new ligand. The semi‐flexible CAP cage populates the structural niche between the rigid adamantine skeleton of PTA and flexible atrane systems and can be regarded as an alternative to PTA in aqueous coordination chemistry.     

Synthesis,Coordination Chemistry and Bonding of Strong N‐Donor Ligands Incorporating the 1H‐Pyridin‐(2E)‐Ylidene (PYE) Motif

A range of N‐donor ligands based on the 1H‐pyridin‐(2E)‐ylidene (PYE) motif have been prepared, including achiral and chiral examples. The ligands incorporate one to three PYE groups that coordinate to a metal through the exocyclic nitrogen atom of each PYE moiety, and the resulting metal complexes have been characterised by methods including single‐crystal X‐ray diffraction and NMR spectroscopy to examine metal–ligand bonding and ligand dynamics. Upon coordination of a PYE ligand to a proton or metal‐complex fragment, the solid‐state structures, NMR spectroscopy and DFT studies indicate that charge redistribution occurs within the PYE heterocyclic ring to give a contribution from a pyridinium–amido‐type resonance structure. Additional IR spectroscopy and computational studies suggest that PYE ligands are strong donor ligands. NMR spectroscopy shows that for metal complexes there is restricted motion about the exocyclic C? N bond, which projects the heterocyclic N‐substituent in the vicinity of the metal atom causing restricted motion in chelating‐ligand derivatives. Solid‐state structures and DFT calculations also show significant steric congestion and secondary metal–ligand interactions between the metal and ligand C? H bonds.