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.     

Sterically Demanding AgI and CuI N-Heterocyclic Carbene Complexes: Synthesis,Structures, Steric Parameters,and Catalytic Activity

The synthesis and full characterization of new air-stable Ag^I and Cu^I complexes bearing structurally bulky expanded-ring N-heterocyclic carbene (erNHC) ligands is presented. The condensation of protonated NHC salts with Ag₂O afforded a collection of Ag^I complexes, and their first use as ligand transfer reagents led to novel isostructural Cu^I or Au^I complexes. In situ deprotonation of the NHC salts in the presence of a copper(I) source, provides a library of new Cu^I complexes. The solid-state structures feature large N-C_NHC-N angles (118–128°) and almost identical angles between the aryl groups on the nitrogen atoms and the plane of the N-C-N unit of the carbene (i.e. torsion angles close to 0°). Among the steric parameters, the percent buried volume (%V_bur) values span easily in the 50–57 % range, and that one of (9-Dipp)CuBr complex (%V_bur=57.5) overcomes to other known erNHC–metal complexes reported to date. Preliminary catalytic experiments in the copper-catalyzed coupling between N-tosylhydrazone and phenylacetylene, afforded 76–93 % product at the 0.5–2.5 mol % catalyst loading, proving the stability of Cu^I erNHC complexes at elevated temperatures (100 °C).     

Palladium complexes of bis(acyclic diaminocarbene) ligands with chiral N-substituents and 8-membered chelate rings

Reaction of cis-dichloridobis(p-trifluoromethylphenylisocyanide)palladium(II) with N,N′-bis[(R)-1-phenylethyl]-1,3-diaminopropane afforded an enantiomerically pure, C₁-symmetric bis(acyclic diaminocarbene)PdCl₂ complex in 41% yield. The X-ray crystal structure of the complex revealed that three of the four carbene nitrogens are twisted out of conjugation with the carbene units, apparently as a result of steric interactions between one phenyl group and the propylene backbone of the chelate. A similar reaction with N,N′-bis[(R)-1-(1-naphthyl)ethyl]-1,3-diaminopropane did not lead to an isolable bis(carbene) complex, instead forming significant amounts of bis(ammonium) salt as a decomposition product. However, reaction of the same palladium isocyanide precursor with a mixture of all diastereomers of N,N′-bis[1-(1-naphthyl)ethyl]-1,3-diaminopropane provided an achiral, C_s-symmetric palladium bis(acyclic diaminocarbene) complex derived exclusively from the (R,S) diamine in 20% yield. An X-ray structure showed that the (R,S) stereochemistry allows the bulky naphthyl groups to adopt an orientation that avoids steric interactions with the backbone that likely lead to the instability of the homochiral analogue. The two palladium carbene complexes catalyzed the aza-Claisen rearrangement of an allylic imidate to an allylic amide in 24–34% yield, with an enantiomeric excess of 8% ee for the [(R)-1-phenylethyl]-substituted complex.     

BIS(DITHIOSQUARAMIDE) LIGANDS AND THEIR COMPLEXES WITH DIVALENT NICKEL: THERMOCHROMIC BEHAVIOUR AND UNANTICIPATED FORMATION OF 2:2 SPECIES

Abstract

Reaction of two equivalents of N-mono- or di-substituted 3-amino-4-(n-butoxy)-3-cyclobutene-1,2-diones with a 1,2-diaminoethane gave N-mono- or di-substituted 1,2-bis((2-amino-1-cyclobutene-3,4-dione)amino)-ethane derivatives (bis(squaramides)). Reaction of the bis(squaramides) with excess P₄S₁₀ gave the analogous tetrathio derivatives (bis(dithiosquaramides), LH₂) of formula (NR¹R²)C₄S₂(NHCH₂CH₂NH)-C₄S₂(NR¹R²) (R¹=n-Bu, R²=H; R¹=R²=Et, n-Bu). The new bis(dithiosquaramide) ligands were characterized by elemental analysis, IR, ¹H NMR, ¹³C NMR, electronic, and mass spectroscopic methods. The complexes of these ligands with nickel(II) were prepared, isolated and characterized. The isolated complexes are neutral 2:2 species of formula Ni₂L₂, as evidenced by results from mass spectrometry, and they exhibit thermochromic behaviour in pyridine solution. Additional spectroscopic data (IR, NMR) are consistent with the ligands being coordinated only through sulfur donor atoms and a structure for the complexes is proposed.     

Bis(silylene)-Stabilized Monovalent Nitrogen Complexes

The first series of bis(silylene)-stabilized nitrogen(I) compounds is described. Starting from the 1,2-bis(N-heterocyclic silylenyl) 1,2-dicarba-closo-dedocaborane(12) scaffold 1 , [1,2-(LSi)₂C₂B₁₀H₁₀; L=PhC(N^tBu)₂], reaction with adamantyl azide (AdN₃) affords the terminal N-μ₂-bridged zwitterionic carborane-1,2-bis(silylium) AdN₃ adduct 2 with an open-cage dianionic nido-C₂B₁₀ cluster core. Remarkably, upon one-electron reduction of 2 with C₈K and liberation of N₂ and adamantane, the two silylene subunits are regenerated to furnish the isolable bis(silylene)-stabilized N^I complex as an anion of 3 with the nido-C₂B₁₀ cluster cage. On the other hand, one-electron oxidation of 2 with silver(I) yields the monocationic bis(silylene) N^I complex 4 with the closo-C₂B₁₀ cluster core. Moreover, the corresponding neutral N^I radical complex 5 results from single-electron transfer from 3 to 4 .     

Dinuclear cobalt(II) complexes with double phosphate ester bridges and tetradentate ligands having anisole or quinoline appendages

Phosphate esters provide a rigid and stable polymeric backbone in nucleic acids. Metal complexes with phosphate ester groups have been synthesized as structural and spectroscopic models of phosphate‐containing enzymes. Dinucleating ligands are used extensively to synthesize model complexes since they provide the support required to stabilize such complexes. The crystal structures of two dinuclear Co^II complexes, namely bis(μ‐diphenyl phosphato‐κ²O :O ′)bis({2‐methoxy‐N ,N‐bis[(pyridin‐2‐yl)methyl]aniline‐κ⁴N ,N ′,N ′′,O }cobalt(II)) bis(perchlorate), [Co(C₁₂H₁₀O₄P)₂(C₁₉H₁₉N₃O)₂](ClO₄)₂, and bis(μ‐diphenyl phosphato‐κ²O :O ′)bis({N ,N‐bis[(pyridin‐2‐yl)methyl]quinolin‐8‐amine‐κ⁴N ,N ′,N ′′,O }cobalt(II)) bis(perchlorate), [Co(C₁₂H₁₀O₄P)₂(C₂₁H₁₈N₄)₂](ClO₄)₂, with tetradentate 2‐methoxy‐N ,N‐bis[(pyridin‐2‐yl)methyl]aniline (L ¹) and N ,N‐bis[(pyridin‐2‐yl)methyl]quinolin‐8‐amine (L ²) ligands are reported. The complexes have similar structures, with distorted octahedral geometries around the metal centres. Both are centrosymmetric (Z ′ = 0.5), with the Co^II centres doubly bridged by diphenyl phosphate ester groups. A number of aromatic–aromatic interactions are present and differ between the two complexes as the anisole group in L ¹ is replaced by a quinoline group in L ². A detailed study of these interactions is presented.     

Non-Heme FeII Diastereomeric Complexes Bearing a Hexadentate Ligand: Unexpected Consequences for the Spin State and Catalytic Oxidation Properties

The reactivity and selectivity of non-heme Fe^II complexes as oxidation catalysts can be substantially modified by alteration of the ligand backbone or introduction of various substituents. In comparison with the hexadentate ligand N,N,N′,N′-tetrakis(pyridin-2-ylmethyl)ethane-1,2-diamine (TPEN), N,N′-bis[1-(pyridin-2-yl)ethyl]-N,N′-bis(pyridin-2-ylmethyl)ethane-1,2-diamine (_2MeL₆²) has a methyl group on two of the four picolyl positions. Fe^II complexation by _2MeL₆² yields two diastereomeric complexes with very similar structures, which only differ in the axial/equatorial positions occupied by the methylated pyridyl groups. In solution, these two isomers exhibit different magnetic behaviors. Whereas one isomer exhibits temperature-dependent spin-state conversion between the S=0 and S=2 states, the other is more reluctant towards this spin-state equilibrium and is essentially diamagnetic at room temperature. Their catalytic properties for the oxidation of anisole by H₂O₂ are very different and correlate with their magnetic properties, which reflect their lability/inertness. These different properties most likely depend on the different steric constraints of the methylated pyridyl groups in the two complexes.     

Systematische Studie zu den Koordinationseigenschaften des Guanidin‐Liganden Bis(tetramethylguanidino)propan mit den Metallen Mangan,Cobalt, Nickel,Zink, Cadmium,Quecksilber und Silber

The transition metal complexes with the ligand 1,3‐bis(N,N,N′,N′‐tetramethylguanidino)propane (btmgp), [Mn(btmgp)Br₂] ( 1 ), [Co(btmgp)Cl₂] ( 2 ), [Ni(btmgp)I₂] ( 3 ), [Zn(btmgp)Cl₂] ( 4 ), [Zn(btmgp)(O₂CCH₃)₂] ( 5 ), [Cd(btmgp)Cl₂] ( 6 ), [Hg(btmgp)Cl₂] ( 7 ) and [Ag₂(btmgp)₂][ClO₄]₂·2MeCN ( 8 ), were prepared and characterised for the first time. The stoichiometric reaction of the corresponding water‐free metal salts with the ligand btmgp in dry MeCN or THF resulted in the straightforward formation of the mononuclear complexes 1 – 7 and the binuclear complex 8 . In complexes with M^II the metal ion shows a distorted tetrahedral coordination whereas in 8 , the coordination of the M^I ion is almost linear. The coordination behavior of btmgp and resulting structural parameters of the corresponding complexes were discussed in an comparative approach together with already described complexes of btmgp and the bisguanidine ligand N¹,N²‐bis(1,3‐dimethylimidazolidin‐2‐ylidene)‐ethane‐1,2‐diamine (DMEG₂e), respectively.     

Transition-Metal Complexes with Bidentate Ligands Spanning trans-Positions. Part XVII. Some platinum(II) complexes with anionic ligands such as methoxide,formate, carboxylate,hydride, and borohydride,and the X-ray crystal structure of [PtHCl( )] (PP = 2,11-bis(diphenylphosphinomethyl)benzo[c]phenanthrene)

The complexes trans-[PtXY( 2 ] (X = H or Me; Y = OMe, OCHO, CO₂H, and BH₄; 2 = 2,11-bis{bis[3-(trifluoromethyl)phenyl]phosphinomethyl}benzo[c]phenanthrene) were prepared, and their decompositions to trans[PtHX( 2 )] were studied. Some binuclear hydrido-bridged complexes, e.g.[( 2 )HPt(μ-H)PtH( 2 )]⁺, were also obtained. The preparation of complexes trans-[PtHX( 28 )₂] (X = H or Me, 28 = bis[3-(trifluoromethyl)phenyl]benzylphosphine) is also reported. The X-ray crystal structure of trans-[PtHCl 1 )] ( 1 = 2,11-bis(diphenylphosphinomethyl)benzo[c]phenanthrene) was carried out.     

Combining Topological and Steric Constraints for the Preparation of Heteroleptic Copper(I) Complexes

Heteroleptic copper(I) complexes have been prepared from a macrocyclic ligand incorporating a 2,9‐diphenyl‐1,10‐phenanthroline subunit ( M30 ) and two bis‐phosphines, namely bis[(2‐diphenylphosphino)phenyl] ether (POP) and 1,3‐bis(diphenylphosphino)propane (dppp). In both cases, the diphenylphosphino moieties of the PP ligand are too bulky to pass through the 30‐membered ring of M30 during the coordination process, hence the formation of C_2v‐symmetrical pseudo‐rotaxanes is prevented. When POP is used, X‐ray crystal structure analysis shows the formation of a highly distorted [Cu( M30 )(POP)]⁺ complex in which the POP ligand is only partially threaded through the M30 unit. This compound is poorly stable as the Cu^I cation is not in a favorable coordination environment due to steric constraints. By contrast, in the case of dppp, the bis‐phosphine ligand undergoes both steric and topological constraints and adopts a nonchelating coordination mode to generate [Cu₂( M30 )₂(μ‐dppp)](BF₄)₂. This compound exhibits metal‐to‐ligand charge transfer (MLCT) emission characterized by a very large Stokes’ shift (≈200 nm) that is not attributed to a dramatic structural distortion between the ground and the emitting states but to very weak MLCT absorption transitions at longer wavelengths. Accordingly, [Cu₂( M30 )₂(μ‐dppp)](BF₄)₂ shows unusually high luminescence quantum yields for Cu^I complexes, both in solution and in the solid state (0.5 and 7 %, respectively).     

Benign synthesis of quinolinecarboxamide ligands,H2bqbenzo and H2bqb and their Pd(II) complexes: X-ray crystal structure,electrochemical and antibacterial studies

Two carboxamide ligands, H₂bqbenzo {3,4-bis(2-quinolinecarboxamido)benzophenone} and H₂bqb {N,N′-bis[(2-quinolinecarboxamide)-1,2-benzene]}, have been prepared using tetrabutylammonium bromide as an environmentally benign reaction medium. Two new Pd(II) complexes, [Pd^II(bqbenzo)] (1) and [Pd^II(bqb)] (2), have been synthesized, characterized, and their structures determined by single crystal X-ray diffraction. The di-anionic ligands, bqbenzo^2? and bqb^2?, are coordinated via two N_amide atoms and the nitrogens of the two quinoline rings, with Pd?N_amide < Pd–N_quinoline bond lengths. The geometry around palladium(II) in both complexes is distorted square planar. The electrochemical behaviors of the ligands and their Pd(II) complexes have been investigated by cyclic voltammetry in DMF. An irreversible Pd^II/I reduction is observed at ?1.06 V for 1 and at ?1.177 V for 2, indicating the influence of the R substituent on the central phenyl ring of carboxamide ligands on the Pd^II/I reduction potential. The ligands and palladium complexes were also screened for in vitro antibacterial activity. The Pd(II) complexes show strong biological activity against S.typhi and E.coli as Gram ?ve and B.cereus and S.aureus as Gram +ve bacteria comparable to the antibiotic penicillin. The antibacterial results also reveal that coordination of Pd(II) significantly improves the activity.     

Three zinc(II) and cadmium(II) complexes containing 4‐amino‐3,5‐bis(pyridin‐2‐yl)‐1,2,4‐triazole and polynitrile ligands: synthesis,molecular and supramolecular structures,and photoluminescence properties

Three photoluminescent complexes containing either Zn^II or Cd^II have been synthesized and their structures determined. Bis[4‐amino‐3,5‐bis(pyridin‐2‐yl)‐1,2,4‐triazole‐κ²N ¹,N ⁵]bis(dicyanamido‐κN ¹)zinc(II), [Zn(C₁₂H₁₀N₆)₂(C₂N₃)₂], (I), bis[4‐amino‐3,5‐bis(pyridin‐2‐yl)‐1,2,4‐triazole‐κ²N ¹,N ⁵]bis(dicyanamido‐κN ¹)cadmium(II), [Cd(C₁₂H₁₀N₆)₂(C₂N₃)₂], (II), and bis[4‐amino‐3,5‐bis(pyridin‐2‐yl)‐1,2,4‐triazole‐κ²N ¹,N ⁵]bis(tricyanomethanido‐κN ¹)cadmium(II), [Cd(C₁₂H₁₀N₆)₂(C₄N₃)₂], (III), all crystallize in the space group P , with the metal centres lying on centres of inversion, but neither analogues (I) and (II) nor Cd^II complexes (II) and (III) are isomorphous. A combination of N—H…N and C—H…N hydrogen bonds and π–π stacking interactions generates three‐dimensional framework structures in (I) and (II), and a sheet structure in (III). The photoluminescence spectra of (I)–(III) indicate that the energies of the π–π* transitions in the coordinated triazole ligand are modified by minor changes of the ligand geometry associated with coordination to the metal centres.     

Novel α-fluorinated cyclic phosphite and phosphinite ligands and their Rh-complexes as suitable catalysts in hydroformylation

Asymmetrical cyclic phosphite and phosphinite ligands of a novel type bearing either trifluoromethyl or pentafluorophenyl group were synthesized using >PCl or >PN< species and racemic fluorinated alcohols. The P-ligands were converted to complexes of Rh^III(L)(Cp^∗)Cl₂ type (where L = phosphite or phosphinite) and, in two instances, their stereostructures were evaluated by X-ray analysis. These complexes along with in situ systems, formed from Rh(CO)₂(acac) precursor and the corresponding ligand, were tested in the hydroformylation of styrene. Both systems provided excellent hydroformylation activities at 100 °C. Using the Rh^I in situ systems, moderate and high regioselectivities towards the branched aldehyde (2-phenyl-propanal) were obtained at 100 and 40 °C, respectively.     

Synthesis of flexible ligands for assembling two metals in close proximity. Magnetic,electrochemical and spectral properties of binuclear copper (II) complexes with different exogenous bridging motifs

Binuclear Cu^II complexes having new flexible heptadentate ligands 2,6-bis{[bis(3,3-N,N-dimethylaminopropyl)amino]methyl}-4-bromophenol [HL¹], 2,6-bis(3,3-N,N-dimethylaminopropyl)amino]methyl}-4-methylphenol [HL²], and 2,6-bis{[bis(3,3-N,N-dimethylaminopropyl)amino]methyl}-4-methoxyphenol [HL³], capable of assembling two copper ions in close proximity have been synthesized. Comparisons of the charge-transfer (CT) features, observed in electronic spectra of these complexes, are correlated with the electronic effect on the aromatic ring of the ligand systems. Cyclic voltammetry has revealed the existence of two reduction couples,

Red Phosphorescent Bis‐Cyclometalated Iridium Complexes with Fluorine‐, Phenyl‐, and Fluorophenyl‐Substituted 2‐Arylquinoline Ligands

Red phosphorescent iridium(III) complexes based on fluorine‐, phenyl‐, and fluorophenyl‐substituted 2‐arylquinoline ligands were designed and synthesized. To investigate their electrophosphorescent properties, devices were fabricated with the following structure: indium tin oxide (ITO)/4,4′,4′′‐tris[2‐naphthyl(phenyl)amino]triphenylamine (2‐TNATA)/4,4′‐bis[N‐(1‐naphthyl)‐N‐phenylamino]biphenyl (NPB)/4,4′‐bis(N‐carbazolyl)‐1,1′‐biphenyl (CBP): 8 % iridium (III) complexes/bathocuproine (BCP)/tris(8‐hydroxyquinolinato)aluminum (Alq₃)/8‐hydroxyquinoline lithium (Liq)/Al. All devices, which use these materials showed efficient red emissions. In particular, a device exhibited a saturated red emission with a maximum luminance, external quantum efficiency, and luminous efficiency of 14200 cd m^?2, 8.44 %, and 6.58 cd A^?1 at 20 mA cm^?2, respectively. The CIE (x, y) coordinates of this device are (0.67, 0.33) at 12.0 V.     

Lanthanide(II) Complexes Supported by N,O‐Donor Tripodal Ligands: Synthesis,Structure, and Ligand‐Dependent Redox Behavior

The preparation and characterization of a series of complexes of the Yb and Eu cations in the oxidation state II and III with the tetradentate N,O‐donor tripodal ligands (tris(2‐pyridylmethyl)amine (TPA), BPA^? (HBPA=bis(2‐pyridylmethyl)(2‐hydroxybenzyl)amine), BPPA^? (HBPPA=bis(2‐pyridylmethyl)(3.5‐di‐tert‐butyl‐2‐hydroxybenzyl)amine), and MPA^2? (H₂MPA=(2‐pyridylmethyl)bis(3.5‐di‐tert‐butyl‐2‐hydroxybenzyl)amine) is reported. The X‐ray crystal structures of the heteroleptic Ln²⁺ complexes [Ln(TPA)I₂] (Ln=Eu, Yb) and [Yb(BPA)I(CH₃CN)]₂, of the Ln²⁺ homoleptic [Ln(TPA)₂]I₂ (Ln=Sm, Eu, Yb) and [Eu(BPA)₂] complexes, and of the Ln³⁺ [Eu(BPPA)₂]OTf and [Yb(MPA)₂K(dme)₂] (dme=dimethoxyethane) complexes have been determined. Cyclic voltammetry studies carried out on the bis‐ligand complexes of Eu³⁺ and Yb³⁺ show that the metal center reduction occurs at significantly lower potentials for the BPA^? ligand as compared with the TPA ligand. This suggests that the more electron‐rich character of the BPA^? ligand results in a higher reducing character of the lanthanide complexes of BPA^? compared with those of TPA. The important differences in the stability and reactivity of the investigated complexes are probably due to the observed difference in redox potential. Preliminary reactivity studies show that whereas the bis‐TPA complexes of Eu²⁺ and Yb²⁺ do not show any reactivity with heteroallenes, the [Eu(BPA)₂] complex reduces CS₂ to afford the first example of a lanthanide trithiocarbonate complex.     

Syntheses and crystal structures of neutral oxorhenium(V) complexes of N2O2-donor tripodal ligands

The neutral distorted octahedral complexes [ReOCl(L)] {H₂L = N,N-bis(2-hydroxybenzyl)-2-(2-aminoethyl)dimethylamine (H₂had); N,N-bis(2-hydroxybenzyl)-aminomethylpyridine (H₂hap); N,N-bis(2-hydroxybenzyl)-2-(2-aminoethyl)pyridine (H₂hae)} were prepared by the reaction of trans-[ReOCl₃(PPh₃)₂] with a twofold molar excess of H₂L in ethanol. X-ray structure determinations of [ReOCl(had)] (1) and [ReOCl(hap)] (2) were performed, and the structures compared. In both complexes the choride is coordinated trans to the tripodal tertiary amino nitrogen, with a phenolate oxygen trans to the oxo oxygen.     

Delocalization and Valence Tautomerism in Vanadium Tris(iminosemiquinone) Complexes

To survey the noninnocence of bis(arylimino) acenaphthene (BIAN) ligands (L) in complexes with early metals, the homoleptic vanadium complex, [V(L)₃] ( 1 ), and its monocation, [V(L)₃]PF₆ ( 2 ), were synthesized. These complexes were found to have a very rich electronic behavior, whereby 1 displays strong electronic delocalization and 2 can be observed in unprecedented valence tautomeric forms. The oxidation states of the metal and ligand components in these complexes were assigned by using spectroscopic, crystallographic, and magnetic analyses. Complex 1 was identified as [V^IV(L^red)(L^.)₂] (L^red=N,N′‐bis(3,5‐dimethylphenylamido)acenaphthylene; L^.=N,N′‐bis(3,5‐dimethylphenylimino)acenaphthenesemiquinonate). Complex 2 was determined to be [V^V(L^red)(L^.)₂]⁺ at T<150 K and [V^IV(L^.)₃]⁺ at T>150 K. Cyclic voltammetry experiments reveal six quasi‐reversible processes, thus indicating the potential of this metal–ligand combination in catalysis or materials applications.     

Sterically‐Directed Consecutive and Size‐Selective Self‐Assembly of Palladium Diphosphane Complexes with an Ar‐BIAN Ligand: Unexpected Formation of Pentameric and Hexameric Aggregates

The coordination properties of N,N′‐bis[4‐(4‐pyridyl)phenyl]acenaphthenequinonediimine (L¹) and N,N′‐bis[4‐(2‐pyridyl)phenyl]acenaphthenequinonediimine (L²) were investigated in self‐assembly with palladium diphosphane complexes [Pd(P^P)(H₂O)₂](OTf)₂ (OTf=triflate) by using various analytical techniques, including multinuclear (¹H, ¹⁵N, and ³¹P) NMR spectroscopy and mass spectrometry (P^P=dppp, dppf, dppe; dppp=bis(diphenylphosphanyl)propane, dppf= bis(diphenylphosphanyl)ferrocene, and dppe=bis(diphenylphosphanyl)ethane). Beside the expected trimeric and tetrameric species, the interaction of an equimolar mixture of [Pd(dppp)]²⁺ ions and L¹ also generates pentameric aggregates. Due to the E/Z isomerism of L¹, a dimeric product was also observed. In all of these species, which correspond to the general formula [Pd(dppp)L¹]_n(OTf)_2n (n=2–5), the L¹ ligand is coordinated to the Pd center only through the terminal pyridyl groups. Introduction of a second equivalent of the [Pd(dppp)]²⁺ tecton results in coordination to the internal, sterically more encumbered chelating site and induces enhancement of the higher nuclearity components. The presence of higher‐order aggregates (n=5, 6), which were unexpected for the interaction of cis‐protected palladium corners with linear ditopic bridging ligands, has been demonstrated both by mass‐spectrometric and DOSY NMR spectroscopic analysis. The sequential coordination of the [Pd(dppp)]²⁺ ion is attributed to the dissimilar steric properties of the two coordination sites. In the self‐assembled species formed in a 1:1:1 mixture of [Pd(dppp)]²⁺/[Pd(dppe)]²⁺/L¹, the sterically more demanding [Pd(dppp)]²⁺ tectons are attached selectively to the pyridyl groups, whereas the more hindered imino nitrogen atoms coordinate the less bulky dppe complexes, thus resulting in a sterically directed, size‐selective sorting of the metal tectons. The propensity of the new ligands to incorporate hydrogen‐bonded solvent molecules at the chelating site was confirmed by X‐ray diffraction studies.     

New N-heterocyclic carbene mercury(II) complexes: Close mercury-arene interaction

Mononuclear mercury complexes (1, 2, and 3) bearing bis-N-heterocyclic carbene (NHC) ligands of the form [(NHC)₂-μ-Hg]⁺² have been prepared and structurally characterised. The complexes were derived from three bis-imidazolium salts as precursors to NHC; either 1,3-bis(N-methylimidazolium-1-ylmethyl)benzene bis(hexafluorophosphate) (I·2PF₆), 1,3-bis(N-butylimidazolium-1-ylmethyl)benzene bis(hexafluorophosphate) (II·2PF₆) or 3,5-bis(N-butylimidazolium-1-ylmethyl)toluene bis(hexafluorophosphate) (III·2PF₆) treated with mercury(II) acetate. Interestingly X-ray crystal structure analysis revealed a close interaction between the Hg metal centre with one carbon atom of the aryl linker in addition to coordination with two NHCs.