Protic N‐Heterocyclic Carbene Versus Pyrazole: Rigorous Comparison of Proton‐ and Electron‐Donating Abilities in a Pincer‐Type Framework

Evaluation of the acidity of proton‐responsive ligands such as protic N‐heterocyclic carbenes (NHCs) bearing an NH‐wingtip provides a key to understanding the metal–ligand cooperation in enzymatic and artificial catalysis. Here, we design a CNN pincer‐type ruthenium complex 2 bearing protic NHC and isoelectronic pyrazole units in a symmetrical skeleton, to compare their acidities and electron‐donating abilities. The synthesis is achieved by direct C?H metalation of 2‐(imidazol‐1‐yl)‐6‐(pyrazol‐3‐yl)pyridine with [RuCl₂(PPh₃)₃]. ¹⁵N‐Labeling experiments confirm that deprotonation of 2 occurs first at the pyrazole side, indicating clearly that the protic pyrazole is more acidic than the NHC group. The electrochemical measurements as well as derivatization to carbonyl complexes demonstrate that the protic NHC is more electron‐donating than pyrazole in both protonated and deprotonated forms.     

Luminescent Dinuclear Bis‐Cyclometalated Gold(III) Alkynyls and Their Solvent‐Dependent Morphologies through Supramolecular Self‐Assembly

A series of luminescent bis‐cyclometalated gold(III) complexes containing bridging alkynyl ligands of different natures has been synthesised and characterised. The photophysical properties of the complexes have been investigated through electronic absorption spectroscopy and emission studies. The vibronic emission bands are found to originate from the triplet intraligand (IL) π–π* excited states of the bis‐cyclometalating ligands with some mixing of ³IL π–π* character of the alkynyl ligands. The electrochemical study of a nonsymmetric dinuclear complex shows two successive reduction processes originating from the reductions of the two different cyclometalating ligands. The complexes are found to undergo supramolecular self‐assembly processes driven by π–π stacking and hydrophobic/hydrophilic interactions to give honeycomb nanostructures, as revealed from the SEM images. Solvent‐dependent morphological transformations have also been observed, which have been studied by SEM and ¹H NMR spectroscopy.     

Concurrent Stabilization of π‐Donor and π‐Acceptor Ligands in Aromatized and Dearomatized Pincer [(PNN)Re(CO)(O)2] Complexes

Aromatized cationic [(PNN)Re(π acid)(O)₂]⁺ ( 1 ) and dearomatized neutral [(PNN*)Re(π acid)(O)₂] ( 2 ) complexes (where π acid=CO ( a ), tBuNC ( b ), or (2,6‐Me₂)PhNC ( c )), possessing both π‐donor and π‐acceptor ligands, have been synthesized and fully characterized. Reaction of [(PNN)Re(O)₂]⁺ ( 4 ) with lithiumhexamethyldisilazide (LiHMDS) yield the dearomatized [(PNN*)Re(O)₂] ( 3 ). Complexes 1 and 2 are prepared from the reaction of 4 and 3 , respectively, with CO or isocyanides. Single‐crystal X‐ray structures of 1 a and 1 b show the expected trans‐dioxo structure, in which the oxo ligands occupy the axial positions and the π‐acidic ligand occupies the equatorial plane in an overall octahedral geometry about the rhenium(V) center. DFT studies revealed the stability of complexes 1 and 2 arises from a π‐backbonding interaction between the d_xy orbital of rhenium, the π orbital of the oxo ligands, and the π* orbital of CO/isocyanide.     

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.     

Boryl‐Functionalized σ‐Alkynyl and Vinylidene Rhodium Complexes: Synthesis and Electronic Properties

The synthesis, reactivity, and properties of boryl‐functionalized σ‐alkynyl and vinylidene rhodium complexes such as trans‐[RhCl(?C?CHBMes₂)(PiPr₃)₂] and trans‐[Rh(C?CBMes₂)(IMe)(PiPr₃)₂] are reported. An equilibrium was found to exist between rhodium vinylidene complexes and the corresponding hydrido σ‐alkynyl complexes in solution. The complex trans‐[Rh(C?CBMes₂)(IMe)(PiPr₃)₂] (IMe=1,3‐dimethylimidazol‐2‐ylidene) was found to exhibit solvatochromism and can be quasireversibly oxidized and reduced electrochemically. Density functional calculations were performed to determine the reaction mechanism and to help rationalize the photophysical properties of trans‐[Rh(C?CBMes₂)(IMe)(PiPr₃)₂].     

Synthesis,Structures, and Reactivities of Pincer‐Type Ruthenium Complexes Bearing Two Proton‐Responsive Pyrazole Arms

A new metal–ligand bifunctional, pincer‐type ruthenium complex [RuCl( L1‐H2 )(PPh₃)₂]Cl ( 1 ; L1‐H2 =2,6‐bis(5‐tert‐butyl‐1H‐pyrazol‐3‐yl)pyridine) featuring two proton‐delivering pyrazole arms has been synthesized. Complex 1 , derived from [RuCl₂(PPh₃)₃] with L1‐H2 , underwent reversible deprotonation with potassium carbonate to afford the pyrazolato–pyrazole complex [RuCl(L1‐H)(PPh₃)₂] ( 2 ). Further deprotonation of 1 and 2 with potassium hexamethyldisilazide in methanol resulted in the formation of the bis(pyrazolato) complex [Ru(L1)(MeOH)(PPh₃)₂] ( 3 ). Complex 3 smoothly reacted with dioxygen and dinitrogen to give the side‐on peroxo complex [Ru(L1)(O₂)(PPh₃)₂] ( 4 ) and end‐on dinitrogen complex [Ru(L1)(N₂)(PPh₃)₂] ( 5 ), respectively. On the other hand, the reaction of [RuCl₂(PPh₃)₃] with less hindered 2,6‐di(1H‐pyrazol‐3‐yl)pyridine ( L3‐H2 ) led to the formation of the dinuclear complex [{RuCl₂(PPh₃)₂}₂(μ₂‐ L3‐H2 )₂] ( 6 ), in which the pyrazole‐based ligand adopted a tautomeric form different from L1‐H2 in 1 and the central pyridine remained uncoordinated. The detailed structures of 1 , 2 , 3 , 3.MeOH , 4 , 5 , 6 were determined by X‐ray crystallography.     

Umpolung of Methylenephosphonium Ions in Their Manganese Half‐Sandwich Complexes and Application to the Synthesis of Chiral Phosphorus‐Containing Ligand Scaffolds

Half‐sandwich manganese methylenephosphonium complexes [Cp(CO)₂Mn(η²‐R₂P?C(H)Ph)]BF₄ were obtained in high yield through a straightforward reaction sequence involving a classical Fischer‐type manganese complex and a secondary phosphine as key starting materials. The addition of various nucleophiles (Nu) to these species took place regioselectively at the double‐bonded carbon center of the coordinated methylenephosphonium ligand R₂P⁺?C(H)Ph to produce the corresponding chiral phosphine complexes [Cp(CO)₂Mn(κ¹‐R₂P? C(H)(Ph)Nu)], from which the phosphines were ultimately recovered as free entities upon simple irradiation with visible light. The synthetic potential of this umpolung approach is illustrated herein by the preparation of novel chiral pincer‐type phosphine–NHC–phosphine ligand architectures.     

Gaseous Chloride Complexes with Halogen Bridges—Homo-Complexes and Hetero-Complexes

The chemistry of the gaseous halide complexes which have been observed in large numbers in recent years is discussed for the example of chlorides. Homo-complexes—dimeric or polymeric chloride molecules containing chlorine-bridge linkages [e.g.(NaCl)_n, n = 2—4; (CuCl)_n, n = 2—5; (BeCl₂)_n, n = 2—4; Pd₆Cl₁₂]—occur through broad areas of the Periodic Table. Comparison of the dissociation enthalpy of such molecules with the vaporization enthalpy of the liquid chlorides is particularly informative. Hetero-complexes are formed from different chloride molecules by linkage through Cl bridges. Not only 1:1 complexes (e.g. NaAlCl₄, TIPbCl₃, KThCl₅, CdPbCl₄, AlUCl₈) but also larger molecules (e.g. CoAl₂Cl₈, CrAl₃Cl₁₂, Cu₂UCl₆, In₂UCl₁₀) are known. Types of formulas, structures, and above all the formation enthalpies of such complexes are discussed critically. Hetero-complexes are useful in chemical transport reactions, as aids in syntheses, and in gas-chromatographic separations (lanthanoids, actinoids). They also play a part in many industrial processes (chlorination of ores) and in recently developed types of high-pressure discharge tubes.     

Alkyl,Silyl, and Phosphane Ligands—Classical Ligands in Nonclassical Bonding Modes

Alkyl, silyl, and phosphane ligands are amongst the most familiar and ubiquitous ligands in organometallic and coordination chemistry. The C, Si, and P donor atoms of these ligands are sp³‐hybridized and the ligands are related to each other by the isolobal analogy: (CR₃)−(SiR₃)−PR₃. Herein, we demonstrate that although a number of unusual observations concerning the reactivity and bonding of these ligands appears unrelated at first sight, they in fact provide offer an exiting and consistent picture that may form the basis for new paradigms. The characterization of stable complexes in which alkyl, silyl, and phosphane ligands behave as symmetrical bridges confirms that there is no inherent thermodynamic instability associated with these bonding situations, and, in fact, reactivity studies suggest that these ligands should be able to bridge between metal centers in reaction intermediates or transition states.     

Fluorinated Isocyanides—More than Ligands with Unusual Properties

The substitution of hydrogen by fluorine in organic compounds usually results in drastic changes in their properties. For isocyanides, for which fluorinated examples have only recently become available in preparative quantities, this substitution leads to a significantly increased reactivity and a tendency to polymerize, which, on one hand, makes their handling more difficult. On the other hand, this high reactivity makes the fluorinated isocyanides useful building blocks for the synthesis of compounds like N-trimethylformamide. Energetically favorable π^* orbitals bestow excellent π-acceptor properties towards low-valent transition metal complexes, especially on the ligand trifluoromethyl isocyanide. The pronounced tendency of this ligand to bridge two metal atoms enables the formation of structural types that are not accessible with other π-acceptor ligands. Thus it was possible to prepare [(Os₃(CO)₁₁(μ₂-CNCF₃)₂] (a) derivative of the hypothetical [Os(CO)₁₃]) which may be considered as a model for an associative mechanism of ligand substitution at carbonyl clusters. In contrast to the well-studied chemistry of trifluoromethyl isocyanide, that of the few other known fluorinated isocyanides is only now receiving attention. In particular the only recently synthesized trifluorovinyl isocyanide promises a rich chemistry as a result of its difunctionality.     

Cobalt(II) Tri‐tert‐butoxysilanethiolates with Bidentate Spacer Ligands

A new series of Co^II tri‐tert‐butoxysilanethiolate complexes with bidentate N,N′‐ligands (L) such as pyrazine, quinoxaline and 4,4′‐bipy was obtained: for pyrazine and quinoxaline the complexes are binuclear {[Co{SSi(tBuO)₃}₂]₂(μ‐L)} with metal atoms linked by an adequate heterocyclic base L. The use of 4,4′‐bipy resulted in a coordination polymer [Co{μ‐SSi(tBuO)₃}{SSi(tBuO)₃}(μ‐4,4′‐bipy)]_n and two polymorphic forms of {[Co{SSi(tBuO)₃}₂]₂(μ‐4,4′‐bipy)}. Pyridyl rings in one polymorph form a torsion angle of 0.57°, whereas a rotation about the linking C–C bond of 4,4′‐bipy in second polymorph is significant and results in a torsion angle of 72.4°. Complexes were analysed and characterised using elemental analysis, solid state IR and UV/Vis spectroscopy, and single‐crystal X‐ray analysis.     

Structure and Reactivity of a Unique Y‐Shaped Tricoordinate Bis(silyl)platinum(II)–NHC Complex

Y not? A unique, three‐coordinate Y‐shaped bis(silyl)platinum(II) complex was isolated and characterized (see structure; C light gray, N blue, Si pink, Pt dark gray). DFT studies on a model system shed light on the nature of this unusual coordination mode for platinum(II).

     

A Symmetrically Bridging Triarylsilyl Ligand in a Dinuclear Rhodium Complex: Synthesis and Structure of [LRh(H)(µ-Cl)(µ-SiAr2)(µ-SiAr3)Rh(H)L] (Ar=Ph,p-FC6H4; L=PiPr3)

Simply warming solutions of the dinuclear complexes 1 provides novel complexes 2 [Eq. (a); Ar = Ph, p-FC₆H₄; L = PiPr₃], which contain a bridging triarylsilyl ligand and show unusual thermal stability. Compounds with this type of symmetric structure are of interest as models of the transition state for the migration of tertiary silyl groups in a bimetallic framework.     

Syntheses,Structures, and Photoluminescent and Magnetic Properties of Four Novel Complexes Constructed from 2,4,5‐Tri(4‐pyridyl)‐imidazole and Benzene‐1,3,5‐tricarboxylic Acid

Four novel 2D complexes M₂(Hptim)₂(HBTC)₂ [M = Co ( 1 ), Cd ( 2 ), Zn ( 3 ), Mn ( 4 ); Hptim = 2,4,5‐tri(4‐pyridyl)‐imidazole; HBTC^2– = Benzene‐1,3,5‐tricarboxylic acid] were synthesized under solvothermal conditions. The four complexes are isomorphous and present a unique structure with a 1D ladder of [Co₂(HBTC)₂]_n. The 2D network structure of 1 is achieved through bridging Hptim groups, which coordinate to the metal atoms of two adjacent 1D [Co₂(HBTC)₂]_n ladders. Magnetic measurements reveal that dominant antiferromagnetic coupling was observed in compounds 1 and 4 . Compounds 2 and 3 both exhibit strong fluorescent emissions in the solid state and may be suitable candidates for fluorescent materials.     

Synthesis and spectroscopy of anthracene-containing linear and ‘T’-shaped π-conjugated ligands

A range of new π-conjugated ethynyl- and diethynyl-benzene ligands has been synthesised and their spectroscopic characterisation carried out, most notably via IR and ¹H NMR. X-ray crystal structures were obtained for three of these ligands and one unusual ruthenium complex. Both the 4-ethynyl- and 2,5-diethynyl-benzene cores of these compounds have been functionalised through organic transformations by addition of an 9-anthracenyl. This has been attached via a range of linker moieties that vary in both their length and degree of π-conjugation. This has given rise to two groups of compounds with either a linear, e.g., 9-(2-(4-ethynylphenyl)ethynediyl)anthracene and 9-(2-(4-ethynylphenyl)ethyl)anthracene, or ‘T’-shaped morphologies, e.g., 9-(2-(2,5-diethylnylphenyl)ethyl)anthracene.     

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.