Lanthanide complexes with a new luminescent iminophosphonamide ligand bearing phenylbenzothiazole substituents.

A new iminophosphonamine Ph2P(HNPbt)(NPbt) (1, HL) bearing chromophore 2-(phen-2'-yl)-1,3-benzothiazole (Pbt) substituents was synthesized and introduced into lanthanide complexes. It was found that salt metathesis reactions between KL (2) generated in situ and LnCl3 lead to the formation of tris-iminophosphonamide complexes [LnL2]L (Ln = Y (3), Sm (4), Gd (5), Dy (6)), regardless of the 2/LnCl3 ratio. Compounds 3-6 consist of a cationic fragment [LnL2]+, where the lanthanide atom is surrounded by two rigidly κ4-coordinated ligands, and an L- anion residing in the outer coordination sphere. Iminophosphonamine 1 shows a rare excitation wavelength-dependent two-band luminescence in the solid state. For compounds containing the deprotonated form, namely potassium salt KL and complexes of Gd and Dy, a single-band luminescence with the color changing from turquoise to orange was observed. The Sm complex reveals a set of a few narrow well-resolved bands corresponding to the f-f transitions against the background of the outer-sphere ligand's emission.

Pd(II)- and Pt(II)-Assisted P-C Activation/Cyclization Reactions with a Luminescent α-Aminophosphine.

There is unceasing interest toward transformations of phosphine derivatives, which are facilitated by transition metals. We report a facile Pd(II)- and Pt(II)-assisted P-C bond cleavage in a luminescent 2-phenylbenzothiazole-based α-methylaminophosphine (PCN, 1). Specifically, reactions between 1 and [M(COD)Cl2] (M = Pd, Pt; COD = cycloocta-1,5-diene) in different solvents (methylene chloride, acetonitrile, pyridine, toluene) resulted in the formation of PPh2-, captured either as a bridging ligand in binuclear complexes with a {M2(PPh2)2} moiety or as an adduct to COD in [Pt2(PPh2COD)2Cl2]. The heterocyclic part transforms to annulated c-CN+ species with a 1,2-dihydroquinazoline cycle formed. In the presence of pyridine as a base, annulated form c-CN+ destabilizes and undergoes reverse cyclization transforming to deprotonated CN form. Quantum-chemical density functional theory (DFT) calculations predict that a crucial step in the reactions involves proton transfer from the N atom of the amino group of PCN to a neighboring molecule. A combination of high photophysical sensitivity of c-CN+ toward its immediate environment and rich structural capabilities in assembling (c-CN)22+ pairs in different crystal packings in a family of phases with the general formula (c-CN)2[M2(PPh2)2Cl4] allows one to fine-tune the luminescence properties of the latter. The results were rationalized as a variation of π-π intercationic spacings, which tunes the degree of excited-state charge transfer between c-CN+ cations. As a result, compounds with relatively short interplanar π-π-separation between the cations show a stronger charge-transfer-mediated bathochromic shift.

Chromium-Lanthanide Complexes Containing the Cr═P═Cr Fragment: Synthesis, Characterization, and Computational Study.

Heterometallic complexes [Cp*2Ln(µ-isoCO)2{Cr2(µ-P)Cp2(CO)2}] [Ln = Yb (1), Sm (2)] were obtained in reactions of [Cp*2Ln(thf)2] (Ln = Sm, Yb) with [{CpCr(CO)2}2(µ,η2:2-P2)] (4). An analogous yttrium compound [Cp*2Y(µ-isoCO)2{Cr2(µ-P)Cp2(CO)2}] (3) was synthesized using a three-component reaction between [Cp*2Y(BPh4)], 4, and KC8. Compounds 1-3 were isolated as solvent-free crystalline phases; in the case of 2, the 2·0.5C7H8 solvate was also obtained. The structures of all crystalline phases were determined by single-crystal X-ray diffraction analysis. All compounds contain a unique {((CO)2CpCr═P═CrCp(CO)2)}- unit, which is linked to Ln3+ ions through CO ligands in the isocarbonyl mode. Compounds 1 and 3 have a molecular structure, while compound 2 contains polymeric chains of triangular [Cp*2Sm(µ-isoCO)2{Cr2(µ-P)Cp2(CO)2}] units linked by µ-isoCO-ligands. 31P NMR studies demonstrated similar dramatic downfield shifts for complexes 1-3. To realize the electronic structure of 1-3 and to elucidate the nature of the high downfield chemical 31P shift, quantum chemical calculations were performed both for 1-3 and for related Cr- and Fe-phosphido complexes. Calculations show that the anomalously high downfield chemical shifts for 1-3 are due to the anisotropic effect of the Cr═P double bonds.

Fluorescence vs. Phosphorescence: Which Scenario Is Preferable in Au(I) Complexes with Benzothiadiazoles?

The photoluminescence of Au(I) complexes is generally characterized by long radiative lifetimes owing to the large spin-orbital coupling constant of the Au(I) ion. Herein, we report three brightly emissive Au(I) coordination compounds, 1, 2a, and 2b, that reveal unexpectedly short emission lifetimes of 10-20 ns. Polymorphs 2a and 2b exclusively exhibit fluorescence, which is quite rare for Au(I) compounds, while compound 1 reveals fluorescence as the major radiative pathway, and a minor contribution of a microsecond-scale component. The fluorescent behaviour for 1-2 is rationalized by means of quantum chemical (TD)-DFT calculations, which reveal the following: (1) S0-S1 and S0-T1 transitions mainly exhibit an intraligand nature. (2) The calculated spin-orbital coupling (SOC) between the states is small, which is a consequence of overall small metal contribution to the frontier orbitals. (3) The T1 state features much lower energy than the S1 state (by ca. 7000 cm-1), which hinders the SOC between the states. Thus, the S1 state decays in the form of fluorescence, rather than couples with T1. In the specific case of complex 1, the potential energy surfaces for the S1 and T2 states intersect, while the vibrationally resolved S1-S0 and T2-S0 calculated radiative transitions show substantial overlap. Thus, the microsecond-scale component for complex 1 can stem from the coupling between the S1 and T2 states.

Structural Diversity and Multielectron Reduction Reactivity of Samarium(II) Iodido-ß-diketiminate Complexes Dependent on Tetrahydrofuran Content.

The molecular structures of complexes [Sm(Nacnac)I(thf)n] (Nacnac = HC(C(Me)Ndipp)2-, dipp = 2,6-diisopropylphenyl, thf = tetrahydrofuran) depending on the number of thf ligands are studied. The complete removal of thf from a known complex [Sm(Nacnac)I(thf)2] leads to a tetranuclear product [Sm(Nacnac)I]4 (4). The partial removal of thf results in mixtures of dinuclear [Sm2(Nacnac)2I2(thf)] (2), trinuclear [Sm3(Nacnac)3I3(thf)] (3), and tetranuclear [Sm4(Nacnac)4I4(thf)2] (4*) complexes and 4, depending on the conditions. The reaction of solvent-free SmI2 with 1 equiv of K(Nacnac) results mainly in [Sm(Nacnac)2] (1), while the interaction of 4 with certain amounts of thf allows obtaining pure 2 and 3 (with the admixture of 4*). Complex 4* is the exact dimer of 2, and both compounds are stable in solutions. Reactions with 3 and 4 as reductants are studied. 4 is oxidized by I2 to stoichiometrically yield two products, mixed-valent tetranuclear [Sm4(Nacnac)4I5] (5) and binuclear [Sm(Nacnac)I2]2 (6) complexes. In the reaction of 4 with nBu3PTe, a trinuclear complex [Sm3(Nacnac)3(µ-I)3(µ3-E)2] (8, E = I or Te) is formed in small amounts, with the formation of 6 as the second product. 3 serves as a two-electron reductant in the reaction with nBu3PTe to yield a trinuclear complex [Sm3(Nacnac)3I3(µ-Te2)] (7). Complexes 2, 4, 4*, 5, 6, and 8 possess a unique flat SmxIy core of heavy atoms, which is assumed to be a consequence of the Nacnac ligand geometry.

Molecular Environment Effects That Modulate the Photophysical Properties of Novel 1,3-Phosphinoamines Based on 2,1,3-Benzothiadiazole.

We report synthesis, crystal structure, and photophysical properties of novel 1,3-phosphinoamines based on 4-amino-2,1,3-benzothiadiazole (NH2-btd): Ph2PCH(Ph)NH-btd (1) and Ph2P(E)CH(Ph)NH-btd, (E = O (2α and 2ß·thf), S (3), Se (4)). Chalcogenides 2-4 exhibit bright emissions with a major band at 519-536 nm and a minor band at 840 nm. According to TD-DFT calculations, the first band is attributed to fluorescence, while the second band corresponds to phosphorescence. In the solid state, room temperature quantum yield reaches 93% in the case of the sulphide. The compounds under study feature effects of the molecular environment on the luminescent properties, which manifest themselves in fluorosolvatochromism as well as in a luminescent response to changes in crystal packing and in contributions to aggregation effects. Specifically, transformation of solid 2ß·thf to solvate-free 2ß either by aging or by grinding causes crystal packing changes, and, as a result, a hypsochromic shift of the emission band. Polystyrene films doped with 2 reveal a bathochromic shift upon increasing the mass fraction from 0.2 to 3.3%, which is caused by molecular aggregation effects.

Unexpectedly Long Lifetime of the Excited State of Benzothiadiazole Derivative and Its Adducts with Lewis Acids.

We report a study of photoluminescent properties of 4-bromo-7-(3-pyridylamino)-2,1,3-benzothiadiazole (Py-btd) and its novel Lewis adducts: (PyH-btd)2(ZnCl4) and [Cu2Cl2(Py-btd)2{PPO}2]·2C7H8 (PPO = tetraphenyldiphosphine monoxide), whose crystal structure was determined by X-ray diffraction analysis. Py-btd exhibits a lifetime of 9 microseconds indicating its phosphorescent nature, which is rare for purely organic compounds. This phenomenon arises from the heavy atom effect: the presence of a bromine atom in Py-btd promotes mixing of the singlet and triplet states to allow efficient singlet-to-triplet intersystem crossing. The Lewis adducts also feature a microsecond lifetime while emitting in a higher energy range than free Py-btd, which opens up the possibility to color-tune luminescence of benzothiadiazole derivatives.

d/f-Polypnictides Derived by Non-Classical Ln2+ Compounds: Synthesis, Small Molecule Activation and Optical Properties.

Reduction chemistry induced by divalent lanthanides has been primarily focused on samarium so far. In light of the rich physical properties of the lanthanides, this limitation to one element is a drawback. Since molecular divalent compounds of almost all lanthanides have been available for some time, we used one known and two new non-classical reducing agents of the early lanthanides to establish a sophisticated reduction chemistry. As a result, six new d/f-polyphosphides or d/f-polyarsenides, [K(18-crown-6)] [Cp''2 Ln(E5 )FeCp*] (Ln=La, Ce, Nd; E=P, As) were obtained. Their reactivity was studied by activation of P4 , resulting in a selective expansion of the P5 rings. The obtained compounds [K(18-crown-6)] [Cp''2 Ln(P7 )FeCp*] (Ln=La, Nd) are the first examples of an activation of P4 by a f-element-polypnictide complex. Additionally, the first systematic femtosecond (fs)-spectroscopy investigations of d/f-polypnictides are presented to showcase the advantages of having access to a broader series of lanthanide compounds.

Synthesis of Unprecedented 4d/4f-Polypnictogens.

A series of 4d/4f-polyarsenides, -polyarsines and -polystibines was obtained by reduction of the Mo-pnictide precursor complexes [{Cpt Mo(CO)2 }2 (µ,η2:2 -E2 )] (E=As, Sb; Cpt =tBu substituted cyclopentadienyl) with two different divalent samarocenes [Cp*2 Sm] and [(CpMe4nPr )2 Sm]. For the reductive conversion of the Mo-stibide only one product was isolated, featuring a planar tetrastibacyclobutadiene moiety as an unprecedented ligand for organometallic compounds. For the corresponding Mo-arsenide a tetraarsacyclobutadiene and a second species with a side-on coordinated As2 2- anion was isolated. The latter can be considered as reaction intermediate for the formation of the tetraarsacyclobutadiene.

Europium and ytterbium complexes with o-iminoquinonato ligands: synthesis, structure, and magnetic behavior.

Complexes of divalent ytterbium (1) and europium (2) with a dianionic o-amidophenolate ligand were prepared by both the direct reduction of 4,6-di-tert-butyl-N-(2,6-diisopropylphenyl)-o-iminobenzoquinone (dpp-IQ) and the salt metathesis reaction of potassium o-amidophenolate with LnI2 (Ln = Yb, Eu). Oxidation of o-amidophenolates 1, 2 with one equivalent of dpp-IQ as well as the salt metathesis reaction of potassium o-iminosemiquinolate with LnI2 afforded ligand mixed-valent o-iminosemiquinonato-amidophenolato complexes of trivalent ytterbium (3) and europium (4). All novel complexes 1-4 were fully characterized, including the solid state structures of 1 and 2 determined by single crystal X-ray diffraction. The magnetic properties of paramagnetic 2-4 were examined.

Samarium Polyarsenides Derived from Nanoscale Arsenic.

Zintl phases of arsenic and molecular compounds containing Zintl-type polyarsenide ions are of fundamental interest in basic and applied sciences. Unfortunately, the most obvious and reactive arsenic source for the preparation of defined molecular polyarsenide compounds, yellow arsenic As4 , is very inconvenient to prepare and neither storable in pure form nor easy to handle. Herein, we present the synthesis and reactivity of elemental As0 nanoparticles (As0 Nano , d=7.2±1.8 nm), which were successfully utilized as a reactive arsenic source in reductive f-element chemistry. Starting from [Cp*2 Sm] (Cp*=η5 -C5 Me5 ), the samarium polyarsenide complexes [(Cp*2 Sm)2 (µ-η2 :η2 -As2 )] and [(Cp*2 Sm)4 As8 ] were obtained from As0 nano , thereby generating the largest molecular polyarsenide of the f-elements and circumventing the use of As4 in preparative chemistry.

Radical Anions, Radical-Anion Salts, and Anionic Complexes of 2,1,3-Benzochalcogenadiazoles.

By means of cyclic voltammetry (CV) and DFT calculations, it was found that the electron-acceptor ability of 2,1,3-benzochalcogenadiazoles 1-3 (chalcogen: S, Se, and Te, respectively) increases with increasing atomic number of the chalcogen. This trend is nontrivial, since it contradicts the electronegativity and atomic electron affinity of the chalcogens. In contrast to radical anions (RAs) [1].- and [2].- , RA [3].- was not detected by EPR spectroscopy under CV conditions. Chemical reduction of 1-3 was performed and new thermally stable RA salts [K(THF)]+ [2].- (8) and [K(18-crown-6)]+ [2].- (9) were isolated in addition to known salt [K(THF)]+ [1].- (7). On contact with air, RAs [1].- and [2].- underwent fast decomposition in solution with the formation of anions [ECN]- , which were isolated in the form of salts [K(18-crown-6)]+ [ECN]- (10, E=S; 11, E=Se). In the case of 3, RA [3].- was detected by EPR spectroscopy as the first representative of tellurium-nitrogen π-heterocyclic RAs but not isolated. Instead, salt [K(18-crown-6)]+ 2 [3-Te2 ]2- (12) featuring a new anionic complex with coordinate Te-Te bond was obtained. On contact with air, salt 12 transformed into salt [K(18-crown-6)]+ 2 [3-Te4 -3]2- (13) containing an anionic complex with two coordinate Te-Te bonds. The structures of 8-13 were confirmed by XRD, and the nature of the Te-Te coordinate bond in [3-Te2 ]2- and [3-Te4 -3]2- was studied by DFT calculations and QTAIM analysis.

Samarium Polystibides Derived from Highly Activated Nanoscale Antimony.

Zintl ions in molecular compounds are of fundamental interest for basic research and application. Two reactive antimony sources are presented that allow direct access to molecular polystibide compounds. These are Sb amalgam (Sb/Hg) and ultrasmall Sb0 nanoparticles (d=6.6±0.8 nm), which were used independently as precursors for the synthesis of the largest f-element polystibide, [(Cp*2 Sm)4 Sb8 ]. Whereas the reaction of the nanoparticles with [Cp*2 Sm] directly led to [(Cp*2 Sm)4 Sb8 ], Sm/Sb/Hg intermediates were isolated when using Sb/Hg as the precursor. These Sm/Sb/Hg intermediates [{(Cp*2 Sm)2 Sb}2 (µ-Hg)] and [{(Cp*2 Sm)3 (µ4 ,η1:2:2:2 -Sb4 )}2 Hg] were synthetically trapped and structurally characterized, giving insight in the formation mechanism of polystibide compounds.

Open Chain Polyarsenides of the Lanthanides.

The reaction of [(Cp'''Co)2 (µ,η2:2 -As2 )2 ] with the decamethylsamarocenes [Cp*2 Sm(THF)2 ] or [Cp*2 Sm], or the bis(tetramethyl-n-propyl)samarocene [(C5 Me4 (n-propyl))2 Sm] resulted in the mixed d/f polyarsenides [(Cp'''Co)2 As4 Sm(η5 -C5 Me4 R)2 ] (R=Me, n-propyl). They are the first structural representatives of open chain-like polyarsenides as ligands in the coordination sphere of lanthanides. Their formation can be explained by an intramolecular As-As coupling within the cobalt polyarsenide complex after reduction by the divalent samarium complex. Density functional theory calculations give insight into the structural change of the (As2 )2 unit in [{Cp'''Co(µ,η2:2 -As2 )}2 ] upon reduction.

Polysulfide Coordination Clusters of the Lanthanides.

The reaction of [(DippForm)2 Ln(thf)2 ] with an excess of elemental sulfur in toluene resulted in the formation of the trinuclear polysulfide coordination clusters [(DippForm)3 Ln3 S12 ] (Ln=Sm, Yb; DippForm=N,N'-bis(2,6-diisopropylphenyl)formamidinate). These are the first f element coordination clusters (Lnn Sx ) with a larger polysulfide unit (n and x>2). The formation of the coordination clusters can be rationalized by the reductive cleavage of S8 with divalent lanthanides.

Nature of Bonding in Donor-Acceptor Interactions Exemplified by Complexes of N-Heterocyclic Carbenes with 1,2,5-Telluradiazoles.

Comprehensive structural, spectroscopic, and quantum chemical analyses of new donor-acceptor complexes between N-heterocyclic carbenes and 1,2,5-telluradiazoles and a comparison with previously known complexes involving tellurenyl cations showed that the dative C-Te bonds cannot be solitarily described with only one Lewis formula. Canonical Lewis formulas that denote covalency and arrows emphasizing ionicity complement each other in varying extents. Evaluation of the relative weights of these resonance forms requires proper bonding description with a well-balanced toolbox of analytical methods. If for conciseness only, one resonance form is used, it must be the most significant one according to the analytical evaluation. If unclear, all significant resonance forms should be displayed.

The First Lanthanide Complexes with a Redox-Active Sulfur Diimide Ligand: Synthesis and Characterization of [LnCp*2 (RN=)2 S], Ln=Sm, Eu, Yb; R=SiMe3.

The first lanthanide complexes with a redox-active sulfur diimide ligand, [LnCp*2 (Me3 SiN=)2 S] (Ln=Sm, Eu, Yb; Cp*=η5 -C5 Me5 ), are reported. The complexes were synthesized by using [LnCp*2 (THF)2 ] and (Me3 SiN=)2 S and have been thoroughly characterized by single-crystal X-ray diffraction, EPR spectroscopy, UV/Vis/NIR electronic absorption spectroscopy and SQUID magnetometry. The results, as interpreted by CASSCF/SOC-RASSI calculations providing a non-perturbative treatment of spin-orbit coupling, indicate that these paramagnetic complexes are best described as Ln3+ and [(Me3 SiN=)2 S]-. adducts. As such, these complexes contain the first isolated and structurally characterized acyclic [(RN=)2 S]-. radical anions.

Sterically induced reductive linkage of iron polypnictides with bulky lanthanide complexes by ring-opening of THF.

Reduction of [Cp*Fe(η5-E5)] (E = P, As) with divalent lanthanide reagents usually leads to reduction of [Cp*Fe(η5-E5)] followed by a Ln-E bond formation. In contrast, by using the sterically encumbered reagent [(DippForm)2Sm(thf)2] (DippForm = {(2,6-iPr2C6H3)NC(H)[double bond, length as m-dash]N(2,6-iPr2C6H3)}-), ring-opening of thf and reduction of the polypnictide is observed. This leads to two new 3d/4f polyphosphide or polyarsenide complexes [(DippForm)2Sm(Cp*Fe)E5{(CH2)4O}{(DippForm)2Sm(thf)}], in which [(DippForm)2Sm(thf)2] and [Cp*Fe(η5-E5)] are linked by a ring-opened thf molecule and no Ln-E bond formation is observed.

Samarocene oxide: from an undesired decomposition product to a new reagent.

Samarocene oxide [Cp*2Sm-O-SmCp*2] is mostly considered as an undesired decomposition product of the well-established but highly air-sensitive samarocene, [Cp*2Sm(thf)2]. [Cp*2Sm-O-SmCp*2] is often formed by accidental exposure of [Cp*2Sm(thf)2] to air or wet solvents. We show here that [Cp*2Sm-O-SmCp*2] acts as a mild oxide base and thus is a valuable synthetic equivalent for "O(2-)". The reaction of [Cp*2Sm-O-SmCp*2] with the inorganic and organic anhydrides COS, CS2, SO2, SO3, Re2O7, and (PhC(O))2O at room temperature resulted in each case in an O(2-) insertion. The corresponding products were isolated as their samarium complexes.

Molecular Polyarsenides of the Rare-Earth Elements.

Reduction of [Cp*Fe(η(5)-As5)] with [Cp''2Sm(thf)] (Cp''=η(5)-1,3-(tBu)2C5H3) under various conditions led to [(Cp''2Sm)(µ,η(4):η(4)-As4)(Cp*Fe)] and [(Cp''2Sm)2As7(Cp*Fe)]. Both compounds are the first polyarsenides of the rare-earth metals. [(Cp''2Sm)(µ,η(4):η(4)-As4)(Cp*Fe)] is also the first d/f-triple decker sandwich complex with a purely inorganic planar middle deck. The central As4(2-) unit is isolobal with the 6π-aromatic cyclobutadiene dianion (CH)4(2-). [(Cp''2Sm)2As7(Cp*Fe)] contains an As7(3-) cage, which has a norbornadiene-like structure with two short As-As bonds in the scaffold. DFT calculations confirm all the structural observations. The As-As bond order inside the cyclo As4 ligand in [(Cp''2Sm)(µ,η(4):η(4)-As4)(Cp*Fe)] was estimated to be in between an As-As single bond and a formally aromatic As4(2-) system.