New multidentate potential ionophors of ether-amide type

The syntheses of acyclic compounds featuring ether-amide groups and different terminal substituents are presented. Three series of new multidentate potential ligands were obtained.

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Neue mehrzähnige potentielle Ionophore des Ether-Amid-Typs

Zusammenfassung Es wird die Synthese acyclischer Verbindungen mit Ether-Amid-Gruppen und verschiedenen endständigen Substituenten beschrieben. Es wurden drei Verbindungsreihen erhalten, die neue potentielle mehrzähnige Liganden repräsentieren.

     

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.     

Functionalised Tris(pyrazolyl)methane Ligands and Re(CO)3 Complexes Thereof

The synthesis of new tripodal nitrogen ligands derived from tris(pyrazolyl)methane (Tpm^R, R = H, tBu, Ph in 3‐position) is described. After deprotonation of the parent tris(pyrazolyl)methane Tpm^R, the carbanion reacts readily with ethylene oxide to yield the 3,3,3‐tris(3′‐substituted pyrazolyl)propanol ligands[(3‐Rpz)₃CCH₂CH₂OH, R = H, tBu, Ph, 1a – c ]. These ligands can be easily derivatised at the alcohol function. Microwave‐assisted reactions of these ligands and [Re(CO)₅Br] yields the complex [( 1a )Re(CO)₃]Br ( 4 ) in the case of ligand 1a , whereas in the case of the substituted ligands 1b and 1c degradation was observed. The degradation products are identified as [(Hpz^R)₂Re(CO)₃Br] [R = tBu ( 7b ), Ph ( 7c )]. These complexes were also prepared directly from [Re(CO)₅Br] and the corresponding pyrazoles by microwave‐assisted synthesis. The Re(CO)₃ complexes 4 and [( 1a )Re(CO)₃]OTf ( 5 ) are water‐soluble. The structures of 5· H₂O and [{(pz)₃CCH₂CH₃}Re(CO)₃]OTf · 1.5H₂O · ¹/₂CH₃CN ( 6· 1.5H₂O · ¹/₂CH₃CN) as well as the structure of 7b have been elucidated by X‐ray crystallography.     

Binding H2, N2, H-, and BH3 to transition-metal sulfur sites: synthesis and properties of [RuL(PR3)(N2Me2S2)] Complexes (L=eta2-H2, H-, BH3; R=Cy, iPr)

The reactions of [Ru(N₂)(PR₃)(‘N₂Me₂S₂’)] [‘N₂Me₂S₂’=1,2‐ethanediamine‐N,N′‐dimethyl‐N,N′‐bis(2‐benzenethiolate)(2?)] [ 1 a (R=iPr), 1 b (R=Cy)] and [μ‐N₂{Ru(N₂)(PiPr₃)(‘N₂Me₂S₂’)}₂] ( 1 c ) with H₂, NaBH₄, and NBu₄BH₄, intended to reduce the N₂ ligands, led to substitution of N₂ and formation of the new complexes [Ru(H₂)(PR₃)(‘N₂Me₂S₂’)] [ 2 a (R=iPr), 2 b (R=Cy)], [Ru(BH₃)(PR₃)(‘N₂Me₂S₂’)] [ 3 a (R=iPr), 3 b (R=Cy)], and [Ru(H)(PR₃)(‘N₂Me₂S₂’)]^? [ 4 a (R=iPr), 4 b (R=Cy)]. The BH₃ and hydride complexes 3 a , 3 b , 4 a , and 4 b were obtained subsequently by rational synthesis from 1 a or 1 b and BH₃?THF or LiBEt₃H. The primary step in all reactions probably is the dissociation of N₂ from the N₂ complexes to give coordinatively unsaturated [Ru(PR₃)(‘N₂Me₂S₂’)] fragments that add H₂, BH₄^?, BH₃, or H^?. All complexes were completely characterized by elemental analysis and common spectroscopic methods. The molecular structures of [Ru(H₂)(PR₃)(‘N₂Me₂S₂’)] [ 2 a (R=iPr), 2 b (R=Cy)], [Ru(BH₃)(PiPr₃)(‘N₂Me₂S₂’)] ( 3 a ), [Li(THF)₂][Ru(H)(PiPr₃)(‘N₂Me₂S₂’)] ([Li(THF)₂]‐ 4 a ), and NBu₄[Ru(H)(PCy₃)(‘N₂Me₂S₂’)] (NBu₄‐ 4 b ) were determined by X‐ray crystal structure analysis. Measurements of the NMR relaxation time T₁ corroborated the η² bonding mode of the H₂ ligands in 2 a (T₁=35 ms) and 2 b (T₁=21 ms). The H,D coupling constants of the analogous HD complexes HD‐ 2 a (¹J(H,D)=26.0 Hz) and HD‐ 2 b (¹J(H,D)=25.9 Hz) enabled calculation of the H? D distances, which agreed with the values found by X‐ray crystal structure analysis ( 2 a : 92 pm (X‐ray) versus 98 pm (calculated), 2 b : 99 versus 98 pm). The BH₃ entities in 3 a and 3 b bind to one thiolate donor of the [Ru(PR₃)(‘N₂Me₂S₂’)] fragment and through a B‐H‐Ru bond to the Ru center. The hydride complex anions 4 a and 4 b are extremely Brønsted basic and are instantanously protonated to give the η²‐H₂ complexes 2 a and 2 b .     

The way into the bridge: a new bonding mode of tertiary phosphanes, arsanes, and stibanes

Until recently, tertiary phosphanes, arsanes, and stibanes were considered to bind to transition-metal centers only in a terminal coordination mode. Investigations on the reactivity of square-planar trans-[RhCl(=CRR')(L)(2)] compounds revealed that compounds in which L=SbiPr(3) can be converted upon heating into dinuclear complexes [Rh(2)Cl(2)(micro-CRR')(2)(micro-SbiPr(3))] with the carbene and stibane ligands in bridging positions. Although attempts to replace the stibane in these complexes with a tertiary arsane or phosphane failed, substitution of the chloro ligands for acetylacetonates followed by bridge-ligand exchange allowed the preparation of the phosphane- and arsane-bridged compounds [Rh(2)(acac)(2)(micro-CRR')(2)(micro-PR(3))] and [Rh(2)(acac)(2)(micro-CRR')(2)(micro-AsMe(3))]. The acac ligands can be replaced by anionic Lewis bases to give either monomeric [Rh(2)X(2)(micro-CRR')(2)(micro-ER(3))] or dimeric chain-like [XRh(micro-CRR')(2)(micro-ER(3))Rh(micro-X)(2)Rh(micro-CRR')(2)(micro-ER(3))RhX] molecules.     

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.     

Sulfinato Complexes

Sulfinatometal complexes exhibit unusual features in their structure, preparation, and use, and have attracted rapidly growing interest. Their structures are determined by the ability of the RSO group both to act as a monodentate ligand (sulfinato-S or sulfinato-O) and to add to coordination centers as a bidentate ligand, either intramolecularly or intermolecularly, via the two oxygens atoms (sulfinato-O,O′) or via the sulfur atom and one of the oxygens (sulfinato-O,S).     

Aryldimethylsilyl Modified Hypersilanes

Aryldimethylsilyl‐substituted hypersilanes ArSiMe₂Hyp [Hyp =Si(SiMe₃)₃; Ar = Mes ( 3a ), Bph ( 3b ), Mph ( 3c ), Pph ( 3d ), Tph ( 3e ); Dpp ( 3f ); with Mes = 2,4,6‐Me₃C₆H₂, Bph = 2‐PhC₆H₄, Mph = 2‐MesC₆H₄, Pph = 2′,3′,4′,5′,6′‐pentamethylbiphenyl2‐yl, Tph = 2′,4′,6′‐triisopropylbiphenyl‐2‐yl and Dpp = 2,6‐Ph₂C₆H₃] have been synthesized by a multi‐step reaction starting from the readily available starting materials ArI and (thf)_xKHyp. For this purpose the aryl iodides were first converted into the lithium aryls by lithium‐halogen exchange with n‐butyl lithium. A salt‐metathesis reaction with SiMe₂Cl₂ gave the corresponding arylchlorodimethylsilanes ( 2a – e ) in excellent yields. Finally, reaction of 2a – e with (thf)_xKHyp afforded the hypersilanes 3a – e in yields up to 86 %. Bis(aryldimethylsilyl)‐substituted silanes (ArSiMe₂)₂Si(SiMe₃)₂ (Ar = Mph ( 4c ), Tph ( 4e )) were obtained by the reaction of 3c or 3e with KOtBu and the corresponding chlorosilanes 2c or 2e . All new compounds have been characterized by melting point, elemental analysis, ¹H, ¹³C, and ²⁹Si NMR spectroscopy and for selected species by IR spectroscopy or mass spectrometry. In addition, the solid‐state structures of silanes 3a – d , 3f , and 4c have been investigated by single crystal X‐ray diffraction.     

Benzene and Its Derivatives as Bridging Ligands in Transition-Metal Complexes

Transition-metal complexes in which two or more metal atoms are bridged by one or more arene ligands led a shadowy existence in comparison to the extensive class of mononuclear arene complexes. Arene bridges can occur in a variety of coordination modes and with almost all of the transition–metal elements of the periodic table. Nowhere else are found so many forms of distorted and bent arene rings. The binuclear compounds can be divided into two classes: adducts which show relatively weak metal–arene bonding and complexes which show strong arene–metal interaction. Most of the adducts are in equilibrium with mononuclear complexes in solution or are only stable in the solid state (often as polymers). In both classes syn and anti coordination occurs; their geometries show a wide variation between the extreme cases of η¹ : η¹-bridge and η⁶ : η⁶-triple-decker structure. Metal surfaces with chemisorbed arenes can be seen as a form of multinuclear arene–metal complexes. On transition-metal surfaces, benzene can be bonded to one, two, or four surface atoms. Molecular clusters with face-capping arene ligands that are bonded to three metal atoms have until now mainly been limited to two classes. The arenes bound to {(CO)₃M}₃ (M = Ru, Os) or (CpCo)₃ clusters as μ₃-η² : η² : η² ligands show only a weak trigonal distortion towards a Kekulé structure. Detailed investigations of the molecular structure and ligand dynamics of [(CpCo)₃(μ₃-arene)] complexes considerably help the understanding of the bonding of arenes to metal clusters and to metal surfaces.     

Nitrosyl derivatives of the unsaturated dihydrides [Mn2(μ-H)2(CO)6(μ-L2)] (L2 = Ph2PCH2PPh2, (EtO)2POP(OEt)2)

The title dimanganese complexes react with NO (5% in N₂) at room temperature to give as major products the corresponding hexanitrosyl derivatives [Mn₂(NO)₆(μ-L₂)] in moderate yields, and they react rapidly with NO₂ to give the corresponding hydride derivatives [Mn₂(μ-H)(μ-NO₂)(CO)₆(μ-L₂)], these having a nitrite ligand bridging the dimetal centre through the N and O atoms. The dppm-bridged dihydride also reacts selectively at 273 K with (PPN)NO₂ to give first the nitro derivative (PPN)[Mn₂(μ-H)(H)(NO₂)(CO)₆(μ-dppm)], which then transforms into the nitrosyl complex (PPN)[Mn₂(μ-CO)(CO)₅(NO)(μ-dppm)] at room temperature or above (dppm = Ph₂PCH₂PPh₂; PPN⁺ = [N(PPh₃)₂]⁺). The latter anion reacts with (NH₄)PF₆ to give the hydride-bridged nitrosyl complex [Mn₂(μ-H)(μ-NO)(CO)₆(μ-dppm)] and with [AuCl(PPh₃)] to give the trinuclear cluster [AuMn₂(μ-NO)(CO)₆(μ-dppm)(PPh₃)] (Mn-Au = ca. 2.68 Å; Mn-Mn = 2.879(2) Å). Both products are derived from the addition of the added electrophile at the intermetallic bond and rearrangement of the nitrosyl ligand into a bridging position. In contrast, methylation of the anion with CF₃SO₃Me takes place at the nitrosyl ligand to yield the unstable methoxylimide derivative [Mn₂(μ-NOMe)(CO)₆(μ-dppm)]. Analogous reactions at the nitrosyl ligand take place upon the addition of HBF₄·OEt₂ to the nitrosyl-bridged hydrides [Mn₂(μ-H)(μ-NO)(CO)_n(μ-dppm)_m] (n = 6, m = 1; n = 4, m = 2) to give the corresponding hydroxylimide derivatives [Mn₂(μ-H)(μ-NOH)(CO)_n(μ-dppm)_m]BF₄, which were also thermally unstable and could not be isolated nor fully characterized.     

RP-Bridged Metal Carbonyl Clusters: Synthesis,Properties, and Reactions

Metal carbonyl clusters possess a complicated chemistry that is only beginning to be understood. One of the main current goals in this area is thus an understanding of their reactivity. This article describes the syntheses and reactions of clusters that contain metal carbonyl fragments bridged by a main-group element. But what is the sense of making such clusters still more complicated by the incorporation of main-group elements? The example of μ₃-bridged carbonyl clusters will serve to show that the main-group element plays an important role in the study of reaction paths; it holds the metal carbonyl fragments together even when the bonds between them are broken in the course of a reaction. Trinuclear μ₃-bridged clusters prove to be small enough to allow the analysis of typical cluster reactions (such as the reversible breaking of metal-metal bonds) in terms of single reaction steps. They are also large enough to provide surprises by their multifaceted reactivity. It will be shown that a detailed study of trinuclear RX-bridged metal carbonyl clusters (X ? N, P, As, Sb, Bi)—a very small part of carbonyl cluster chemistry—can lead to a better understanding of the general reaction principles involved.     

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.     

Fine‐Tuning the Reactivity and Stability by Systematic Ligand Variations in CpCoI Complexes as Catalysts for [2+2+2] Cycloaddition Reactions

CpCo^I‐olefin‐phosphite and CpCo^I‐bisphosphite complexes were systematically prepared and their reactivity in [2+2+2] cycloaddition reactions compared with highly active [CpCo(H₂C?CHSiMe₃)₂] ( 1 ). Whereas 1 is an excellent precursor for the synthesis of [CpCo(olefin)(phosphite)] complexes ( 2 a – f ), [CpCo(phosphite)₂] complexes ( 3 a – e ) were prepared photochemically from [CpCo(cod)]. The complexes were evaluated in the cyclotrimerization reaction of diynes with nitriles yielding pyridines. For [CpCo(olefin)(phosphite)], as well as some of the [CpCo(phosphite)₂] complexes, reaction temperatures as low as 50 °C were sufficient to perform the cycloaddition reaction. A direct comparison showed that the order of reactivity for the complex ligands was olefin₂>olefin/phosphite>phosphites₂. The complexes with mixed ligands favorably combine reactivity and stability. Calculations on the ligand dissociation from [CpCo(olefin)(phosphite)] proved that the phosphite is dissociating before the olefin. [CpCo(H₂C?CHSiMe₃){P(OPh)₃}] ( 2 a ) was investigated for the co‐cyclization of diynes and nitriles and found to be an efficient catalyst at rather mild temperatures.     

Molybdenum amido complexes with single Mo-N bonds: synthesis,structure, and reactivity

Reactions of the complex [MoCl(eta(3)-C(3)H(4)-Me-2)(CO)(2)(phen)] (1) (phen=1,10-phenanthroline) with potassium arylamides were used to synthesize the amido complexes [Mo(N(R)Ar)(eta(3)-C(3)H(4)-Me-2)(CO)(2)(phen)] (R=H, Ar=Ph, 2 a; R=H, Ar=p-tolyl, 2 b; R=Me, Ar=Ph; 2 c). For 2 b the Mo-N(amido) bond length (2.105(4) A) is consistent with it being a single bond, with which the metal attains an 18-electron configuration. The reaction of 2 b with HOTf affords the amino complex [Mo(eta(3)-C(3)H(4)-Me-2)(NH(2)(p-tol))(CO)(2)(phen)]OTf (3-OTf). Treatment of 3-OTf with nBuLi or KN(SiMe(3))(2) regenerates 2 b. The new amido complexes react with CS(2), arylisothiocyanates and maleic anhydride. A single product corresponding to the formal insertion of the electrophile into the Mo-N(amido) bond is obtained in each case. For maleic anhydride, ring opening accompanied the formation of the insertion product. The reaction of 2 b with maleimide affords [Mo(eta(3)-C(3)H(4)-Me-2)[NC(O)CH=CHC(O)](CO)(2)(phen)] (7), which results from simple acid-base metathesis. The reaction of 2 b with (p-tol)NCO affords [[Mo(eta(3)-C(3)H(4)-Me-2)(CO)(2)(phen)](2)(eta(2)-MoO(4))] (8), which corresponds to oxidation of one third of the metal atoms to Mo(VI). Complex 8 was also obtained in the reactions of 2 b with CO(2) or the lactide 3,6-dimethyl-1,4-dioxane-2,5-dione. The structures of the compounds 2 b, 3-OTf, [Mo(eta(3)-C(3)H(4)-Me-2)[SC(S)(N(H)Ph)](CO)(2)(phen)] (4), [Mo(eta(3)-C(3)H(4)-Me-2)[SC(N(p-tol))(NH(p-tol))](CO)(2)(phen)] (5 a), and [Mo(eta(3)-C(3)H(4)-Me-2)[OC(O)CH=CHC(O)(NH(p-tol))](CO)(2)(phen)] (6), 7, and 8 (both the free complex and its N,N'-di(p-tolyl)urea adduct) were determined by X-ray diffraction.