Reactivity of a Stable Phosphinonitrene towards Small Molecules

The room‐temperature stable phosphinonitrene 1 undergoes a thermal rearrangement into heterocycle 2 through a process involving a nitrene insertion into a CH bond. In the presence of acetonitrile, a nitrene–acetonitrile adduct has been isolated; then it first rearranges into a ketenimine and subsequently into a rare example of diazaphosphete. Compound 1 also splits water, carbon dioxide, carbon disulfide, and elemental sulfur, although it reacts with white phosphorus, leading to a P₅N cluster formally resulting from the insertion of the PN moiety into a P?P edge of the P₄ tetrahedron.     

Molybdenum Tricarbonyl Complexes Supported by Linear PNP Ligands: Influence of P- and N-Substituents on Relative Stability,Stereoisomerism and on the Activation of Small Molecules

Series of linear tridentate PN^PhP^R-ligands (R=Me, Et, Pln, Ph, Cyp, ⁱPr, Cy, ^tBu) and molybdenum tricarbonyl complexes [Mo(CO)₃PN^PhP^R] (R=Ph, Et, Cyp, ⁱPr, Cy,) were synthesized and characterized using NMR-, IR-, and Raman spectroscopy as well as X-ray crystallography. The influence of the different phosphine donor groups of the PN^PhP^R ligands on the bonding and activation of CO ligands is investigated. Importantly, all complexes are found to adopt a fac geometry, both in solution and in the solid state. This is in contrast to analogous complexes supported by PN^HP ligands. DFT calculations reveal that the phenyl ring at the central amine function is the cause of the preferred geometry, hindering isomerization to a mer geometry.     

Lanthanide Complexes with Multidentate Oxime Ligands as Single‐Molecule Magnets and Atmospheric Carbon Dioxide Fixation Systems

The synthesis, structure, and magnetic properties of five lanthanide complexes with multidentate oxime ligands are described. Complexes 1 and 2 ( 1 : [La₂(pop)₂(acac)₄(CH₃OH)], 2 : [Dy₂(pop)(acac)₅]) are synthesized from the 2‐hydroxyimino‐N‐[1‐(2‐pyridyl)ethylidene]propanohydrazone (Hpop) ligand, while 3 , 4 , and 5 ( 3 : [Dy₂(naphthsaoH)₂(acac)₄H(OH)]?0.85 CH₃CN?1.58 H₂O; 4 : [Tb₂(naphthsaoH)₂(acac)₄H(OH)]?0.52 CH₃CN?1.71 H₂O; 5 : [La₆(CO₃)₂(naphthsao)₅ (naphthsaoH)_0.5(acac)₈(CO₃)_0.5(CH₃OH)_2.76H_5.5(H₂O)_1.24]?2.39 CH₃CN?0.12 H₂O) contain 1‐(1‐hydroxynaphthalen‐2‐yl)‐ethanone oxime (naphthsaoH₂). In 1 – 4 , dinuclear [Ln₂] complexes crystallize, whereas hexanuclear La^III complex 5 is formed after fixation of atmospheric carbon dioxide. Dy^III‐based complexes 2 and 3 display single‐molecule‐magnet properties with energy barriers of 27 and 98 K, respectively. The presence of a broad and unsymmetrical relaxation mode observed in the ac susceptibility data for 3 suggest two different dynamics of the magnetization which might be a consequence of independent relaxation processes of the two different Dy³⁺ ions.     

Imidazolin‐2‐ylidenaminophosphines as Highly Electron‐Rich Ligands for Transition‐Metal Catalysts

A variety of chemical transformations benefit from the use of strong electron‐donating ancillary ligands, such as alkylphosphines or N‐heterocyclic carbenes when electron‐rich metal centers are required. Herein, we describe a facile and highly modular access to monodentate and bidentate imidazolin‐2‐ylidenamino‐substituted phosphines. Evaluation of the phosphine’s electronic properties substantiate that the formal replacement of alkyl or aryl groups by imidazolin‐2‐ylidenamino groups dramatically enhance their donor ability beyond that of alkylphosphines and even N‐heterocyclic carbenes. The new phosphines have been coordinated onto palladium(II) centers, and the beneficial effect of the novel substitution patterns has been explored by using the corresponding complexes in the palladium‐catalyzed Suzuki–Miyaura cross‐coupling reaction of non‐activated aryl chloride substrates.     

Water Stability and Luminescence of Lanthanide Complexes of Tripodal Ligands Derived from 1,4,7‐Triazacyclononane: Pyridinecarboxamide versus Pyridinecarboxylate Donors

A series of europium(III) and terbium(III) complexes of three 1,4,7‐triazacyclononane‐based pyridine containing ligands were synthesized. The three ligands differ from each other in the substitution of the pyridine pendant arm, namely they have a carboxylic acid, an ethylamide, or an ethyl ester substituent, i.e., these ligands are 6,6′,6″‐[1,4,7‐triazacyclononane‐1,4,7‐triyltris(methylene)]tris[pyridine‐2‐carboxylic acid] (H₃tpatcn), ‐tris[pyridine‐2‐carboxamide] (tpatcnam), and ‐tris[pyridine‐2‐carboxylic acid] triethyl ester (tpatcnes) respectively. The quantum yields of both the europium(III) and terbium(III) emission, upon ligand excitation, were highly dependent upon ligand substitution, with a ca. 50‐fold decrease for the carboxamide derivative in comparison to the picolinic acid (=pyridine‐2‐carboxylic acid) based ligand. Detailed analysis of the radiative rate constants and the energy of the triplet states for the three ligand systems revealed a less efficient energy transfer for the carboxamide‐based systems. The stability of the three ligand systems in H₂O was investigated. Although hydrolysis of the ethyl ester occurred in H₂O for the [Ln(tpatcnes)](OTf)₃ complexes, the tripositive [Ln(tpatcnam)](OTf)₃ complexes and the neutral [Ln(tpatcn)] complexes showed high stability in H₂O which makes them suitable for application in biological media. The [Tb(tpatcn)] complex formed easily in H₂O and was thermodynamically stable at physiological pH (pTb 14.9), whereas the [Ln(tpatcnam)](OTf)₃ complexes showed a very high kinetic stability in H₂O, and once prepared in organic solvents, remained undissociated in H₂O.     

Robust and Electron‐Rich cis‐Palladium(II) Complexes with Phosphine and Carbene Ligands as Catalytic Precursors in Suzuki Coupling Reactions

A new imidazolinium ligand precursor [L²H]Cl ( 2 ) was prepared in 86 % yield. Compared with its imidazolium counterpart, [L¹H]Cl ( 1 ), 2 is very sensitive to moisture and can undergo ring‐opening reactions very readily. Palladium complexes with the ring‐opened products from imidazolinium salts were isolated and characterized by X‐ray crystallography. Theoretical studies confirmed that the imidazolinium salt has a higher propensity for the ring‐opening reaction than the imidazolium counterpart. New mixed phosphine/carbene palladium complexes, cis‐[PdCl₂(L)(PR₃)] (L=L¹ and L²; R=Ph, Cy), were successfully prepared. These complexes are highly robust as revealed by variable‐temperature NMR spectroscopic studies and thermal gravimetric analysis. The structural and electronic properties of the new complexes on varying the carbene group (imidazol‐2‐ylidene group (unsaturated carbene) vs. imidazolin‐2‐ylidene (saturated carbene)) and the phosphine group (PPh₃ vs. PCy₃) were studied in detail by X‐ray crystallography, X‐ray photoelectron spectroscopy, and theoretical calculations. The catalytic study reveals that cis‐[PdCl₂(L²)(PCy₃)] is a competent Pd^II precatalyst for Suzuki coupling reactions, in which unreactive aryl chlorides can be applied as substrates.     

Towards Structural‐Functional Mimics of Acetylene Hydratase: Reversible Activation of Acetylene using a Biomimetic Tungsten Complex

The synthesis and characterization of a biomimetic system that can reversibly bind acetylene (ethyne) is reported. The system has been designed to mimic catalytic intermediates of the tungstoenzyme acetylene hydratase. The thiophenyloxazoline ligand S‐Phoz (2‐(4′,4′‐dimethyloxazolin‐2′‐yl)thiophenolate) is used to generate a bioinspired donor environment around the W center, facilitating the stabilization of W–acetylene adducts. The featured complexes [W(C₂H₂)(CO)(S‐Phoz)₂] ( 2 ) and [WO(C₂H₂)(S‐Phoz)₂] ( 3 ) are extremely rare from a synthetic and structural point of view as very little is known about W–C₂H₂ adducts. Upon exposure to visible light, 3 can release C₂H₂ from its coordination sphere to yield the 14‐electron species [WO(S‐Phoz)₂] ( 4 ). Under light‐exclusion 4 re‐activates C₂H₂ making this the first fully characterized system for the reversible activation of acetylene.     

Small Molecule Activation by POCOP‐Nickel Complexes

This contribution describes the reactivities of CO₂, CO, O₂, and ArNC with the pincer‐type complexes [(κ^P,κ^C,κ^P′‐POCOP)NiX] (POCOP=(R₂POCH₂)₂CH; R=iPr; X=OSiMe₃, NArH; Ar=2,6‐iPr₂C₆H₃). Reaction of the amido derivative with CO₂ and CO leads to a simple insertion into the Ni?N bond to give stable carbamate and carbamoyl derivatives, respectively, the pincer ligand backbone remaining intact in both cases. In contrast, the analogous reactions with the siloxide derivative produced kinetically labile insertion products that either revert to the starting material (in the case of CO₂) or react further to give the mixed‐valent, dinickel species [(POCOP)Ni^II{μ,κ^O,κ^P,κ^P′‐OCOCH(CH₂CH₂OPR₂)₂}Ni⁰(CO)₂]. The zero‐valent center in the latter compound is ligated by a new ligand arising from transformation of the POCOP ligand backbone. The carbonylation and carboxylation of the siloxido derivative also produced minor quantities of a side‐product identified as the trinickel species, [{(η³‐allyl)Ni(μ^O,κ^P‐R₂PO)₂}₂Ni], arising from total dismantling of the POCOP ligand. Similar reactivities were observed with isonitrile, ArNC: reaction with the siloxido derivative resulted in a complex sequence of steps involving initial insertion, a 1,3‐hydrogen shift, and an Arbuzov rearrangement to give [Ni(CNAr)₄] and a methacrylamide based on fragments of the POCOP ligand. Oxygenation of the amido and siloxido derivatives led to the phosphinate derivative, [(POCOP)Ni(OP(O)R₂)], arising from oxidative transformation of the original ligand frame; the reaction with the Ni‐NHAr derivative also gave ArHNP(O)R₂ through a complex N?P bond‐forming reaction.     

Hafnium Trialkyls Stabilized by Bulky,Electron‐Rich Aminopyridinates

A series of hafnium aminopyridinates were synthesized and spectroscopically analyzed. In addition, selected examples of these hafnium complexes were characterized by X‐ray single crystal structure analysis. The aminopyridinato ligands used here carry an additional dialkylamine substituent to enhance the electron donating ability of the ligands. Structural data and low temperature NMR spectroscopic investigations are indicative of the increased donating ability of the ligands. Ethylene polymerization studies revealed a rather low catalytic activity most likely due to catalyst instability. Details of the deactivation reaction were investigated by NMR spectroscopy.     

New Vistas in N‐Heterocyclic Silylene (NHSi) Transition‐Metal Coordination Chemistry: Syntheses,Structures and Reactivity towards Activation of Small Molecules

This account is a review on the synthesis and transition‐metal coordination chemistry of N‐heterocyclic silylenes (NHSi’s) over the last 20 years till the present time (2012). Recently, fascinating and novel synthetic methods have been developed to access transition‐metal–NHSi complexes as an emerging class of compounds with a wealth of intriguing reactivity patterns. The striking influence of coordinating NHSi’s to transition‐metal complex fragments affording different reactivities to the “free” NHSi is a connecting theme (“leitmotif”) throughout the review, and highlights the potential of these compounds which lie at the interface of contemporary main‐group and classical organometallic chemistry towards new molecular catalysts for small‐molecule activation.     

Pentamethylcyclopentadienyl Halfsandwich Complexes of Vanadium and Tantalum with Amido‐, Imido‐ Nitrido‐ and Azido Ligands

Lithium bis(trimethylsilyl)amide, LiN(SiMe₃)₂, reacts with Cp*V(O)Cl₂ and Cp*TaCl₄ to give trimethylsilylimido complexes such as [Cp*V(NSiMe₃)(μ‐NSiMe₃)]₂ ( 7 ) and Cp*Ta(Cl)(NSiMe₃)[N(SiMe₃)₂] ( 19 ), respectively. Substitution of the chloro ligand in 19 by anionic groups leads to complexes with 3 different N‐containing ligands, Cp*Ta(X)(NSiMe₃)[N(SiMe₃)₂] (X = N₃ ( 20 ) or NPEt₃ ( 21 )). Complex 7 is air‐ and moisture‐sensitive, and several derivatives containing oxo and trimethylsiloxy ligands have been identified. Trimethylsilyl azide, Me₃Si‐N₃, is able to replace the oxygen‐containing ligands for azido ligands. The two complete series of bis(azido)‐bridged complexes, [Cp*VCl_n(N₃)_2‐n(μ‐N₃)]₂ (n = 2, 1, 0) and [Cp*TaCl_n(N₃)_3‐n(μ‐N₃)]₂(n = 3, 2, 1, 0), are accessible from the reactions of Cp*VCl₃ and Cp*TaCl₄, respectively, with trimethylsilyl azide. A bis(nitrido)‐bridged azido‐vanadium complex, [Cp*V(N₃)(μ‐N)]₂ ( 18 ), has also been obtained and structurally characterized.     

The Activation of Sulfur Hexafluoride at Highly Reduced Low‐Coordinate Nickel Dinitrogen Complexes

The greenhouse gas sulfur hexafluoride is the common standard example in the literature of a very inert inorganic small molecule that is even stable against O₂ in an electric discharge. However, a reduced β‐diketiminate nickel species proved to be capable of converting SF₆ into sulfide and fluoride compounds at ambient standard conditions. The fluoride product complex features an unprecedented [NiF]⁺ unit, where the Ni atom is only three‐coordinate, while the sulfide product exhibits a rare almost linear [Ni(μ‐S)Ni]²⁺ moiety. The reaction was monitored applying ¹H NMR, IR and EPR spectroscopic techniques resulting in the identification of an intermediate nickel complex that gave insight into the mechanism of the eight‐electron reduction of SF₆.     

Activation of Sulfur Dioxide by Bis[N,N′‐diisopropylbenzamidinato(−)]silicon(II): Synthesis of Neutral Six‐Coordinate Silicon(IV) Complexes with Chelating O,O′‐Sulfito or O,O′‐Dithionito Ligands

The neutral six‐coordinate silicon(IV) complexes 2 and 3 (mixture of cis‐ 3 and trans‐ 3 ) were synthesized by reaction of the donor‐stabilized silylene bis[N,N′‐diisopropylbenzamidinato(?)]silicon(II) ( 1 ) with SO₂. Compounds 2 and 3 are the first silicon(IV) complexes with chelating sulfito or dithionito ligands, and 3 is even the first molecular compound with a chelating dithionito ligand. Compounds 2 and 3 were structurally characterized by crystal structure analyses and multinuclear NMR spectroscopic studies in the solid state and in solution.     

Crystal‐Field Tuning of Photoluminescence in Two‐Dimensional Materials with Embedded Lanthanide Ions

Lanthanide (Ln) group elements have been attracting considerable attention owing to the distinct optical properties. The crystal‐field surroundings of Ln ions in the host materials can determine their energy level splitting, which is of vital importance to tailor their optical properties. 2D MoS₂ single crystals were utilized as the host material to embed Eu³⁺ and energy‐level splitting was achieved for tuning its photoluminescence (PL). The high anisotropy of the 2D host materials makes them distort the degenerate orbitals of the Ln ions more efficiently than the symmetrical bulk host materials. A significant red‐shift of the PL peak for Eu³⁺ was observed. The strategy for tailoring the energy level splitting of Ln ions by the highly designable 2D material crystal field provides a new method to extend their optical properties.     

Rare‐Earth‐Metal–Hydrocarbyl Complexes Bearing Linked Cyclopentadienyl or Fluorenyl Ligands: Synthesis,Catalyzed Styrene Polymerization,and Structure–Reactivity Relationship

A series of rare‐earth‐metal–hydrocarbyl complexes bearing N‐type functionalized cyclopentadienyl (Cp) and fluorenyl (Flu) ligands were facilely synthesized. Treatment of [Y(CH₂SiMe₃)₃(thf)₂] with equimolar amount of the electron‐donating aminophenyl‐Cp ligand C₅Me₄H‐C₆H₄‐o‐NMe₂ afforded the corresponding binuclear monoalkyl complex [({C₅Me₄‐C₆H₄‐o‐NMe(μ‐CH₂)}Y{CH₂SiMe₃})₂] ( 1 a ) via alkyl abstraction and C? H activation of the NMe₂ group. The lutetium bis(allyl) complex [(C₅Me₄‐C₆H₄‐o‐NMe₂)Lu(η³‐C₃H₅)₂] ( 2 b ), which contained an electron‐donating aminophenyl‐Cp ligand, was isolated from the sequential metathesis reactions of LuCl₃ with (C₅Me₄‐C₆H₄‐o‐NMe₂)Li (1 equiv) and C₃H₅MgCl (2 equiv). Following a similar procedure, the yttrium‐ and scandium–bis(allyl) complexes, [(C₅Me₄‐C₅H₄N)Ln(η³‐C₃H₅)₂] (Ln=Y ( 3 a ), Sc ( 3 b )), which also contained electron‐withdrawing pyridyl‐Cp ligands, were also obtained selectively. Deprotonation of the bulky pyridyl‐Flu ligand (C₁₃H₉‐C₅H₄N) by [Ln(CH₂SiMe₃)₃(thf)₂] generated the rare‐earth‐metal–dialkyl complexes, [(η³‐C₁₃H₈‐C₅H₄N)Ln(CH₂SiMe₃)₂(thf)] (Ln=Y ( 4 a ), Sc ( 4 b ), Lu ( 4 c )), in which an unusual asymmetric η³‐allyl bonding mode of Flu moiety was observed. Switching to the bidentate yttrium–trisalkyl complex [Y(CH₂C₆H₄‐o‐NMe₂)₃], the same reaction conditions afforded the corresponding yttrium bis(aminobenzyl) complex [(η³‐C₁₃H₈‐C₅H₄N)Y(CH₂C₆H₄‐o‐NMe₂)₂] ( 5 ). Complexes 1 – 5 were fully characterized by ¹H and ¹³C NMR and X‐ray spectroscopy, and by elemental analysis. In the presence of both [Ph₃C][B(C₆F₅)₄] and AliBu₃, the electron‐donating aminophenyl‐Cp‐based complexes 1 and 2 did not show any activity towards styrene polymerization. In striking contrast, upon activation with [Ph₃C][B(C₆F₅)₄] only, the electron‐withdrawing pyridyl‐Cp‐based complexes 3 , in particular scandium complex 3 b , exhibited outstanding activitiy to give perfectly syndiotactic (rrrr >99 %) polystyrene, whereas their bulky pyridyl‐Flu analogues ( 4 and 5 ) in combination with [Ph₃C][B(C₆F₅)₄] and AliBu₃ displayed much‐lower activity to afford syndiotactic‐enriched polystyrene.     

An Odd‐Electron Complex [Ruk(NOm)(Qn)(terpy)]2+ with Two Prototypical Non‐Innocent Ligands

Six combinations of oxidation states are conceivable for the paramagnetic title complex. Single‐crystal X‐ray diffraction, spectroscopic analysis (IR, EPR at conventional and high frequency), and DFT calculations establish that it is the iminosemiquinone radical structure that is formed: [Ru^k(NO^m)(Qⁿ)(terpy)]²⁺ (k=2+, m=1+, n=1?).