On Haptotropic Rearrangements of Diphosphene and Diarsene Ligands in Titanium Complexes.

Dipnictenes of the type RE=ER (E=P, As, Sb, Bi) are the isovalence electronic heavier analogs of alkenes. Although diphosphenes and dipnictenes in general show a variety of binding modes in metal complexes, little is known about haptotropic shift reactions involving these ligands. Herein, we report an unprecedented η2 to η1 rearrangement of the dipnictene ligands in titanocene complexes of the type Cp2Ti(Pn2Ar2) (Pn=P, As; Ar=2,4,6-Me3-C6H2, Mes; 2,6-iPr2-C6H3, Dip; 2,4,6-iPr3-C6H2, Tip), initiated by Lewis basic ligands (L=MeCN, PMe3, AdNC, CO). In the presence of L the dipnictene ligand changes its hapticity from η2 to η1 and complexes of the general form Cp2Ti(L)(Pn2Ar2) with a succinctly different electronic structure are obtained. Electronically, the new complexes are best described as biradicaloids with antiferromagnetically coupled (via a π-bond) [Cp2TiIII]⋅+ and [Pn2Ar2]⋅- fragments. However, the biradical character of these systems is affected by the electronic features of the co-ligand and significantly decreases moving from PMe3/MeCN (σ-donors) to CNAd/CO (σ-donors/π-acceptors).

Curved graphene nanoribbons derived from tetrahydropyrene-based polyphenylenes via one-pot K-region oxidation and Scholl cyclization.

Precise synthesis of graphene nanoribbons (GNRs) is of great interest to chemists and materials scientists because of their unique opto-electronic properties and potential applications in carbon-based nanoelectronics and spintronics. In addition to the tunable edge structure and width, introducing curvature in GNRs is a powerful structural feature for their chemi-physical property modification. Here, we report an efficient solution synthesis of the first pyrene-based GNR (PyGNR) with curved geometry via one-pot K-region oxidation and Scholl cyclization of its corresponding well-soluble tetrahydropyrene-based polyphenylene precursor. The efficient A2B2-type Suzuki polymerization and subsequent Scholl reaction furnishes up to ∼35 nm long curved GNRs bearing cove- and armchair-edges. The construction of model compound 1, as a cutout of PyGNR, from a tetrahydropyrene-based oligophenylene precursor proves the concept and efficiency of the one-pot K-region oxidation and Scholl cyclization, which is clearly revealed by single crystal X-ray diffraction analysis. The structure and optical properties of PyGNR are investigated by Raman, FT-IR, solid-state NMR, STM and UV-Vis analysis with the support of DFT calculations. PyGNR exhibits a narrow optical bandgap of ∼1.4 eV derived from a Tauc plot, qualifying as a low-bandgap GNR. Moreover, THz spectroscopy on PyGNR estimates its macroscopic charge mobility µ as ∼3.6 cm2 V-1 s-1, outperforming several other curved GNRs reported via conventional Scholl reaction.

Highly Tunable 4-Phosphoryl Pyrazolone Receptors for Selective Rare-Earth Separation.

Highly selective rare-earth separation has become increasingly important due to the indispensable role of these elements in various cutting-edge technologies including clean energy. However, the similar physicochemical properties of rare-earth elements (REEs) render their separation very challenging, and the development of new selective receptors for these elements is potentially of very considerable economic and environmental importance. Herein, we report the development of a series of 4-phosphoryl pyrazolone receptors for the selective separation of trivalent lanthanum, europium, and ytterbium as the representatives of light, middle, and heavy REEs, respectively. X-ray crystallography studies were employed to obtain solid-state structures across 11 of the resulting complexes, allowing comparative structure-function relationships to be probed, including the effect of lanthanide contraction that occurs along the series from lanthanum to europium to ytterbium and which potentially provides a basis for REE ion separation. In addition, the influence of ligand structure and lipophilicity on lanthanide binding and selectivity was systematically investigated via n-octanol/water distribution and liquid-liquid extraction (LLE) studies. Corresponding stoichiometry relationships between solid and solution states were well established using slope analyses. The results provide new insights into some fundamental lanthanide coordination chemistry from a separation perspective and establish 4-phosphoryl pyrazolone derivatives as potential practical extraction reagents for the selective separation of REEs in the future.

Direct conversion of white phosphorus to versatile phosphorus transfer reagents via oxidative onioation.

The main feedstock for the value-added phosphorus chemicals used in industry and research is white phosphorus (P4), from which the key intermediate for forming P(III) compounds is PCl3. Owing to its high reactivity, syntheses based on PCl3 are often accompanied by product mixtures and laborious work-up procedures, so an alternative process to form a viable P(III) transfer reagent is desirable. Our concept of oxidative onioation, where white phosphorus is selectively converted into triflate salts of versatile P1 transfer reagents such as [P(LN)3][OTf]3 (LN is a cationic, N-based substituent; that is, 4-dimethylaminopyridinio), provides a convenient alternative for the implementation of P-O, P-N and P-C bonds while circumventing the use of PCl3. We use p-block element compounds of type RnE (for example, Ph3As or PhI) to access weak adducts between nitrogen Lewis bases LN and the corresponding dications [RnELN]2+. The proposed equilibrium between [RnELN]2+ + LN and [RnE(LN)2]2+ allows for the complete oxidative onioation of all six P-P bonds in P4 to yield highly reactive and versatile trications [P(LN)3]3+.

Insights at the molecular level into the formation of oxo-bridged trinuclear uranyl complexes.

Reaction of 1,3,4,6-tetra-O-acetyl-N-(2-hydroxy)-naphthylidene glucosamine (HL(Ac)) with uranyl acetate in ethanol leads to formation of dinuclear [(UO2)2(L)2] (1). In a second step 1 is quantitatively transferred into the trinuclear oxo-bridged complex [(UO2)3(µ3-O)(L)3]2- (22-) via deprotonation and coordination of a water molecule. This transformation was followed by NMR and UV/Vis spectroscopy and it proved possible to selectively introduce 18O into the µ3-bridge.

4-Phosphoryl Pyrazolones for Highly Selective Lithium Separation from Alkali Metal Ions.

Effective receptors for the separation of Li+ from a mixture with other alkali metal ions under mild conditions remains an important challenge that could benefit from new approaches. In this study, it is demonstrated that the 4-phosphoryl pyrazolones, HL2 -HL4 , in the presence of the typical industrial organophosphorus co-ligands tributylphosphine oxide (TBPO), tributylphosphate (TBP) and trioctylphosphine oxide (TOPO), are able to selectively recognise and extract lithium ions from aqueous solution. Structural investigations in solution as well as in the solid state reveal the existence of a series of multinuclear Li+ complexes that include dimers (TBPO, TBP) as well as rarely observed trimers (TOPO) and represent the first clear evidence for the synergistic role of the co-ligands in the extraction process. Our findings are supported by detailed NMR, MS and extraction studies. Liquid-liquid extraction in the presence of TOPO revealed an unprecedented high Li+ extraction efficiency (78 %) for HL4 compared to the use of the industrially employed acylpyrazolone HL1 (15 %) and benzoyl-1,1,1-trifluoroacetone (52 %) extractants. In addition, a high selectivity for Li+ over Na+ , K+ and Cs+ under mild conditions (pH ∼8.2) confirms that HL2 -HL4 represent a new class of ligands that are very effective extractants for use in lithium separation.

Polymorphic Phosphorescence from Separable Aggregates with Unique Photophysical Properties.

Platinum complexes aggregate into polymorphs with different intermolecular interactions leading to different photophysical properties. Strong intermolecular interactions stabilize the aggregate to such an extent that the polymorphs can be separated directly by column chromatography. Solid-state structures as well as quantum-chemical calculations confirmed the effect of the interactions on the photophysical properties.

Bifunctional Fluorophosphonium Triflates as Intramolecular Frustrated Lewis Pairs: Reversible CO2 Sequestration and Binding of Carbonyls, Nitriles and Acetylenes.

Electrophilic fluorophosphonium triflates bearing pyridyl (3[OTf]) or imidazolyl (4[OTf])-substituents act as intramolecular frustrated Lewis pairs (FLPs) and reversibly form 1 : 1 adducts with CO2 (5+ and 6+ ). An unusual and labile spirocyclic tetrahedral intermediate (72+ ) is observed in CO2 -pressurized (0.5-2.0 bar) solutions of cation 4+ at low temperatures, as demonstrated by variable-temperature NMR studies, which were confirmed crystallographically. In addition, cations 3+ and 4+ actively bind carbonyls, nitriles and acetylenes by 1,3-dipolar cycloaddition, as shown by selected examples.

Saccharified Uranyl Ions: Self-Assembly of UO2 2+ into Trinuclear Anionic Complexes by the Coordination of Glucosamine-Derived Schiff Bases.

The reaction of UO2 (OAc)2 ⋅ 2H2 O with the biologically inspired ligand 2-salicylidene glucosamine (H2 L1 ) results in the formation of the anionic trinuclear uranyl complex [(UO2 )3 (µ3 -O)(L1 )3 ]2- (12- ), which was isolated in good yield as its Cs-salt, [Cs]2 1. Recrystallization of [Cs]2 1 in the presence of 18-crown-6 led to formation of a neutral ion pair of type [M(18-crown-6)]2 1, which was also obtained for the alkali metal ions Rb+ and K+ (M=Cs, Rb, K). The related ligand, 2-(2-hydroxy-1-naphthylidene) glucosamine (H2 L2 ) in a similar procedure with Cs+ gave the corresponding complex [Cs(18-crown-6)]2 [(UO2 )3 (µ3 -O)(L2 )3 ([Cs(18-crown-6)]2 2). From X-ray investigations, the [(UO2 )3 O(Ln )3 ]2- anion (n=1, 2) in each complex is a discrete trinuclear uranyl species that coordinates to the alkali metal ion via three uranyl oxygen atoms. The coordination behavior of H2 L1 and H2 L2 towards UO2 2+ was investigated by NMR, UV/Vis spectroscopy and mass spectrometry, revealing the in situ formation of the 12- and 22- dianions in solution.

Coordination of trivalent lanthanum and cerium, and tetravalent cerium and actinides (An = Th(IV), U(IV), Np(IV)) by a 4-phosphoryl 1H-pyrazol-5-olate ligand in solution and the solid state.

Structural investigations of three actinide(iv) 4-phosphoryl 1H-pyrazol-5-olate complexes (An = Th(iv), U(iv), Np(iv)) and their cerium(iv) analogue display the same metal coordination in the solid state. The mononuclear complexes show the metal centre in a square antiprismatic coordination geometry composed by the two O-donor atoms of four deprotonated ligands. Detailed solid state analysis of the U(iv) complex shows that dependent on the solvent used altered arrangements are observable, resulting in a change in the coordination polyhedron of the U(iv) metal centre to bi-capped trigonal prismatic. Further, single crystal analyses of the La(iii) and Ce(iii) complexes show that the ligand can also act as a neutral ligand by protonation of the pyrazolyl moiety. All complexes were comprehensively characterized by NMR, IR and Raman spectroscopy. A single resonance in each of the 31P NMR spectra for the La(iii), Ce(iii), Ce(iv), Th(iv) and Np(iv) complex indicates the formation of highly symmetric complex species in solution. Extended X-ray absorption fine structure (EXAFS) investigations provide evidence for the same local structure of the U(iv) and Np(iv) complex in toluene solution, confirming the observations made in the solid state.

Coordination Chemistry and Methylation of Mixed-Substituted Tetraphosphetanes (RP-PtBu)2 (R=Ph, Py).

Invited for the cover of this issue is the group of Jan J. Weigand at Technische Universität Dresden. The image depicts a puzzle illustrating the coinage metal cations and the methyl group, from which the three-dimensional structure of a multiply methylated tetraphosphetane arises. Read the full text of the article at 10.1002/chem.202001360.

Iron-Gallium and Cobalt-Gallium Tetraphosphido Complexes.

The synthesis and characterization of two heterobimetallic complexes [K([18]crown-6){(η4-C14H10)Fe(µ-η4:η2-P4)Ga(nacnac)}] (1) (C14H10 = anthracene) and [K(dme)2{(η4-C14H10)Co(µ-η4:η2-P4)Ga(nacnac)}] (2) with strongly reduced P4 units is reported. Compounds 1 and 2 are prepared by reaction of the gallium(III) complex [(nacnac)Ga(η2-P4)] (nacnac = CH[CMeN(2,6-iPr2C6H3)]2) with bis(anthracene)ferrate(1-) and -cobaltate(1-) salts. The molecular structures of 1 and 2 were determined by X-ray crystallography and feature a P4 chain which binds to the transition metal atom via all four P atoms and to the gallium atom via the terminal P atoms. Multinuclear NMR studies on 2 suggest that the molecular structure is preserved in solution.

Coordination Chemistry and Methylation of Mixed-Substituted Tetraphosphetanes (RP-PtBu)2 (R=Ph, Py).

Synthesis of mixed-substituted tetraphosphetanes (RP-PtBu)2 (R=Ph (4), Py (5); Py=2-pyridyl) is achieved from the condensation of dipyrazolylphosphanes RPpyr2 (R=Py (1), Ph (3); pyr=3,5-dimethylpyrazolyl) as P1 -building block (R-P) and tBuPH2 in an equimolar ratio. Compound 5 is of special interest since the presence of two pyridyl-substituents as well as the P4 -core allows for a rich coordination chemistry with coinage metal salts [Cu(MeCN)4 ][OTf], Ag[OTf] and in situ formed [Au(tht)][OTf] (tht=tetrahydrothiophene). Both tetraphosphetanes undergo alkylation reaction with MeOTf to give a series of tetraphosphetanium and tetraphosphetanediium triflate salts with additional methylation of the pyridyl-moiety in case of 5 resulting in interesting novel cyclic trications. Harsh reaction condition and an excess of MeOTf converts 5 into the cyclic trication [-P(Me Py)PMe2 P(Me Py)PtBu-]3+ (133+ ; Me Py=1-methylpyridiniumyl) through the elimination of isobutene. This salt undergoes a complicated rearrangement reaction involving a P-P/P-P bond metathesis to form trication [-P(MePy)3 PtBu-]3+ (173+ ) when reacted with Me2 PPMe2 .

Toward N,P-Doped π-Extended PAHs: A One-Pot Synthesis to Diannulated 1,4,2-Diazaphospholium Triflate Salts.

A novel method for the one-pot synthesis of diannulated 1,4,2-diazaphospholium triflate salts by a Me3SiOTf-mediated self-condensation of dichlorophosphaneyl aza-(poly)cyclic aromatic hydrocarbons (aza-(P)AHs; namely, pyridine, quinoline, phenanthridine, and benzo[d]thiazole) is reported. The diannulated 1,4,2-diazaphospholium triflate salts are characterized by multinuclear NMR spectroscopy, X-ray analysis, as well as their calculated NICS values, underlining their aromatic character. Quantum mechanical calculations shed light on the intermolecular reaction mechanism. Follow up chemistry, such as the halogenation reaction with XeF2 or SO2Cl2 with the dipyridinium derivative selectively yields the respective dihalo-σ4,λ5- and tetrahalo-σ5,λ6-diazaphospholium triflate salts. The dihalo-σ4,λ5-diazaphospholium triflate salt serves well as a surrogate for the introduction of the cationic 2-(1,2'-bipyridin)-1-iumyl ligand (1,2'-bipyl), the monocationic structural isomer of the prototypical 2,2'-bipyridine ligand (2,2'-bipy).

Helical Nanographenes Containing an Azulene Unit: Synthesis, Crystal Structures, and Properties.

Three unprecedented helical nanographenes (1, 2, and 3) containing an azulene unit are synthesized. The resultant helical structures are unambiguously confirmed by X-ray crystallographic analysis. The embedded azulene unit in 2 possesses a record-high twisting degree (16.1°) as a result of the contiguous steric repulsion at the helical inner rim. Structural analysis in combination with theoretical calculations reveals that these helical nanographenes manifest a global aromatic structure, while the inner azulene unit exhibits weak antiaromatic character. Furthermore, UV/Vis-spectral measurements reveal that superhelicenes 2 and 3 possess narrow energy gaps (2: 1.88 eV; 3: 2.03 eV), as corroborated by cyclic voltammetry and supported by density functional theory (DFT) calculations. The stable oxidized and reduced states of 2 and 3 are characterized by in-situ EPR/Vis-NIR spectroelectrochemistry. Our study provides a novel synthetic strategy for helical nanographenes containing azulene units as well as their associated structures and physical properties.

P-P Condensation and P-N/P-P Bond Metathesis: Facile Synthesis of Cationic Tri- and Tetraphosphanes.

[LC R P((PhP)2 C2 H4 )][OTf] (4 a,b[OTf]) and [LC iPr P(PPh2 )2 ][OTf] (5 b[OTf]) were prepared from the reaction of imidazoliumyl-substituted dipyrazolylphosphane triflate salts [LC R P(pyr)2 ][OTf] (3 a,b[OTf]; a: R=Me, b=iPr; LC R =1,3-dialkyl-4,5-dimethylimidazol-2-yl; pyr=3,5-dimethylpyrazol-1-yl) with the secondary phosphanes PhP(H)C2 H4 P(H)Ph) and Ph2 PH. A stepwise double P-N/P-P bond metathesis to catena-tetraphosphane-2,3-diium triflate salt [(Ph2 P)2 (LC Me P)2 ][OTf]2 (7 a[OTf]2 ) is observed when reacting 3 a[OTf] with diphosphane P2 Ph4 . The coordination ability of 5 b[OTf] was probed with selected coinage metal salts [Cu(CH3 CN)4 ]OTf, AgOTf and AuCl(tht) (tht=tetrahydrothiophene). For AuCl(tht), the helical complex [{(Ph2 PPLC iPr )Au}4 ][OTf]4 (9[OTf]4 ) was unexpectedly formed as a result of a chloride-induced P-P bond cleavage. The weakly coordinating triflate anion enables the formation of the expected copper(I) and silver(I) complexes [(5 b)M(CH3 CN)3 ][OTf]2 (M=Cu, Ag) (10[OTf]2 , 11[OTf]2 ).

Formation of an imidazoliumyl-substituted [(LC)4P4]4+ tetracation and transition metal mediated fragmentation and insertion reaction (LC = NHC).

Tetracationic cyclo-tetraphosphane [(LC)4P4]4+ as triflate salt (3[OTf]4) (LC = 4,5-dimethyl-1,3-diisopropyl-imidazol-2-yl) is obtained in high yield from the reduction of [LCPCl2]+ (4[OTf]) with 1,4-bis(trimethylsilyl)-1,4-dihydropyrazine (6) and represents the first salt of the cationic cyclo-phosphane series with the general formula [L n P n ] n+. Theoretical calculations reveal the electrophilic nature of the P atoms within the P4-ring due to the influence of the imidazoliumyl-substituents. Further reduction of 3[OTf]4 with 6 affords the unexpected formation of the notricyclane P7-type cation [(LC)3P7]3+ (9[OTf]3). Selective transition metal mediated [2 + 2]-fragmentation of 3 4+ is achieved when 3[OTf]4 is reacted with Fe2(CO)9, Pd(PPh3)4 and Pt(PPh3)4 leading to the formation of the dicationic diphosphene complexes [(η2-LCP[double bond, length as m-dash]PLC)Fe(CO)4]2+ (12[OTf]2) and [(η2-LCP[double bond, length as m-dash]PLC)M(PPh3)2]2+ (13[OTf]2 for M = Pd; 14[OTf]2 for M = Pt). In contrast, the reaction of 3[OTf]4 with an excess of AuCl(tht) gives rise to the formation of the five-membered ring complex [((LC)4P4)AuCl2]3+ (15[OTf]3), where the Au(i) atom reductively inserts into a P-P bond of 3 4+.

Wave-shaped polycyclic hydrocarbons with controlled aromaticity.

Controlling the aromaticity and electronic properties of curved π-conjugated systems has been increasingly attractive for the development of novel functional materials for organic electronics. Herein, we demonstrate an efficient synthesis of two novel wave-shaped polycyclic hydrocarbons (PHs) 1 and 2 with 64 π-electrons. Among them, the wave-shaped π-conjugated carbon skeleton of 2 is unambiguously revealed by single-crystal X-ray crystallography analysis. The wave-shaped geometry is induced by steric congestion in the cove and fjord regions. Remarkably, the aromaticity of these two structural isomers can be tailored by the annulated direction of cyclopenta[b]fluorene units. Isomer 1 (Eoptg = 1.13 eV) behaves as a closed-shell compound with weakly antiaromatic feature, whereas its structural isomer 2 displays a highly stable tetraradical character (y 0 = 0.23; y 1 = 0.22; t 1/2 = 91 days) with a narrow optical energy gap of 0.96 eV. Moreover, the curved PH 2 exhibits remarkable ambipolar charge transport in solution-processed organic thin-film transistors. Our research provides a new insight into the design and synthesis of stable functional curved aromatics with multiradical characters.

Construction of alkyl-substituted pentaphosphido ligands in the coordination sphere of cobalt.

Rare mono- and diorganopentaphosphido cobalt complexes are accessible by P-P condensation using the unprecedented, reactive cobalt-gallium tetraphosphido complex [K(dme)2{(MesBIAN)Co(µ-η4:η2-P4)Ga(nacnac)}] (2). Compound 2 was prepared in good yield by reaction of [K(Et2O){(MesBIAN)Co(η4-1,5-cod)}] [1, BIAN = bis(mesitylimino)acenaphthene diimine, cod = 1,5-cyclooctadiene] with [Ga(nacnac)(η2-P4)] (nacnac = CH[CMeN(2,6-iPr2C6H3)]2). Reactions with R2PCl (R = iPr, tBu, and Cy) selectively afford [(MesBIAN)Co(cyclo-P5R2)] (3a-c), which feature η4-coordinated 1,1-diorganopentaphosphido ligands. The mechanism of formation of these species has been studied by 31P{1H} NMR spectroscopy and DFT calculations. In the case of 3a (R = iPr), it was possible to identify the intermediate [(MesBIAN)Co(µ-η4:η2-P5iPr2)Ga(nacnac)] (4) by single-crystal X-ray diffraction. A related, monosubstituted organopentaphosphido cobalt complex [(MesBIAN)Co(µ-η4:η1-P5 tBu)GaCl(nacnac)] (5) was isolated by reacting dichloroalkylphosphane tBuPCl2 with 2. Heterobimetallic complexes such as 2 thus may enable the targeted construction of a range of new metal-coordinated polyphosphorus frameworks by P-P condensation.

Controlled scrambling reactions to polyphosphanes via bond metathesis reactions.

Triphosphanes R'2PP(R)PR'2 (9a,c: R = Py; 9b R = BTz), 1,3-diphenyl-2-pyridyl-triphospholane 9d and pentaphospholanes (RP)5 (13: R = Py; 18: R = BTz) are obtained in high yield of up to 98% from the reaction of dipyrazolylphosphanes RPpyr2 (5: R = Py; 6: R = BTz; pyr = 1,3-dimethylpyrazolyl) and the respective secondary phosphane (R'2PH, R' = Cy (9a,b), t Bu (9c); PhPH(CH2)2PHPh (9d)). The formation of derivatives 9a-d proceeds via a condensation reaction while the formation of 13 and 18 can only be explained by a selective scrambling reaction. We realized that the reaction outcome is strongly solvent dependent as outlined by the controlled scrambling reaction pathway towards pentaphospholane 13. In our further investigations to apply these compounds as ligands we first confined ourselves to the coordination chemistry of triphosphane 9a with respect to coinage metal salts and discussed the observation of different syn- and anti-isomeric metal complexes based on NMR and X-ray analyses as well as quantum chemical calculations. Methylation reactions of 9a with MeOTf yield triphosphan-1-ium Cy2MePP(Py)PCy2 + (10 +) and triphosphane-1,3-diium Cy2MePP(Py)PMeCy2 2+ (11 2+) cations as triflate salts. Salt 11[OTf]2 reacts with pentaphospholane 13 in an unprecedented chain growth reaction to give the tetraphosphane-1,4-diium triflate salt Cy2MePP(Py)P(Py)PMeCy2 2+ (19[OTf]2) via a P-P/P-P bond metathesis reaction. The latter salt is unstable in solution and rearranges via a rare [1,2]-migration of the Cy2MeP-group followed by the elimination of the triphosph-2-en-1-ium cation [Cy2MePPPMeCy2]+ (20 +) to yield a novel 1,4,2-diazaphospholium salt (21[OTf]).