A Rhodium‐Mediated Contraction of Benzene to Cyclopentadiene: Transformations of Rhodium(III) m‐Benziporphyrin

The contraction of benzene is one of an exclusive group of reactions where the cleavage of aromatic structure is of fundamental importance. Rhodium(III) m‐benziporphyrin undergoes an unprecedented transformation of the built‐in m‐phenylene in which a perimeter carbon atom is extruded to form rhodium(III) 21‐carbaporphyrin, stabilizing the formyl‐unit‐substituted rhodacyclopropane motif.     

Novel half‐sandwich η5‐Cp *–rhodium(III) and η5‐Cp *–ruthenium(II) complexes bearing bis(phosphino)amine ligands and their use in the transfer hydrogenation of aromatic ketones

Two new half‐sandwich η⁵‐Cp*–rhodium(III) and η⁵‐Cp*–ruthenium(II) complexes have been prepared from corresponding bis(phosphino)amine ligands, thiophene‐2‐(N,N‐bis(diphenylphosphino)methylamine) or furfuryl‐2‐(N,N‐bis(diphenylphosphino)amine). Structures of the new complexes have been elucidated by multinuclear one‐ and two‐dimensional NMR spectroscopy, elemental analysis and IR spectroscopy. These Cp*–rhodium(III) and Cp*‐ruthenium(II) complexes bearing bis(phosphino)amine ligands were successfully applied to transfer hydrogenation of various ketones by 2‐propanol. Copyright © 2013 John Wiley & Sons, Ltd.     

Chlorido(dimethyl 2,2′‐bipyridine‐4,4′‐dicarboxylate‐κ2N,N′)(η5‐pentamethylcyclopentadienyl)rhodium(III) chloride 1‐hydroxypyrrolidine‐2,5‐dione disolvate

The title complex, [Rh(C₁₀H₁₅)Cl(C₁₄H₁₂N₂O₄)]Cl·2C₄H₅NO₃, has been synthesized by a substitution reaction of the precursor [bis(2,5‐dioxopyrrolidin‐1‐yl) 2,2′‐bipyridine‐4,4′‐dicarboxylate]chlorido(pentamethylcyclopentadienyl)rhodium(III) chloride with NaOCH₃. The Rh^III cation is located in an RhC₅N₂Cl eight‐coordinated environment. In the crystal, 1‐hydroxypyrrolidine‐2,5‐dione (NHS) solvent molecules form strong hydrogen bonds with the Cl⁻ counter‐anions in the lattice and weak hydrogen bonds with the pentamethylcyclopentadienyl (Cp*) ligands. Hydrogen bonding between the Cp* ligands, the NHS solvent molecules and the Cl⁻ counter‐anions form links in a V‐shaped chain of Rh^III complex cations along the c axis. Weak hydrogen bonds between the dimethyl 2,2′‐bipyridine‐4,4′‐dicarboxylate ligands and the Cl⁻ counter‐anions connect the components into a supramolecular three‐dimensional network. The synthetic route to the dimethyl 2,2′‐bipyridine‐4,4′‐dicarboxylate‐containing rhodium complex from the [bis(2,5‐dioxopyrrolidin‐1‐yl) 2,2′‐bipyridine‐4,4′‐dicarboxylate]rhodium(III) precursor may be applied to link Rh catalysts to the surface of electrodes.     

Oxidative Olefination of Anilides with Unactivated Alkenes Catalyzed by an (Electron‐Deficient η5‐Cyclopentadienyl)Rhodium(III) Complex Under Ambient Conditions

The oxidative olefination of sp² C?H bonds of anilides with both activated and unactivated alkenes using an (electron‐deficient η⁵‐cyclopentadienyl)rhodium(III) complex is reported. In contrast to reactions using this electron‐deficient rhodium(III) catalyst, [Cp*RhCl₂]₂ showed no activity against olefination with unactivated alkenes. In addition, the deuterium kinetic isotope effect (DKIE) study revealed that the C?H bond cleavage step is thought to be the turnover‐limiting step.     

Halbsandwich‐Komplexe von Ruthenium(II), Rhodium(III) und Iridium(III) mit Tripeptidestern aus α‐, β‐ und γ‐Aminosäuren als Liganden. — Peptidsynthese und Cyclisierung zu Cyclotripeptiden am Metallzentrum

Metal Complexes with Biological Important Ligands. CXLII. Half Sandwich Complexes of Ruthenium(II), Rhodium(III), and Iridium(III) with Tripeptide Esters from α‐, β‐, and γ‐Amino Acids as Ligands. — Peptide Synthesis and Cyclization to Cyclotripeptides at Metal Centers Halfsandwich complexes of ruthenium, rhodium and iridium with deprotonated N, N', N"‐tripeptide ester ligands were obtained from chloro bridged compounds and tripeptide methyl esters ( 1—6 ) or by peptide synthesis at a metal centre ( 9—15 ). For the peptide synthesis at the complex (C₆Me₆)Ru coordinated dipeptide methyl esters from glycine and β‐alanine or γ‐amino butyric acid were elongated by an a‐amino acid methylester. The tripeptide ester Ru(η⁶‐C₆Me₆) complexes with chiral amino acid components and an “asymmetric” metal atom are formed with high diastereoselectivity. The tripeptide esters Gly‐Gly‐β‐AlaOMe, Val‐Gly‐β‐AlaOMe and Phe‐Gly‐β‐AlaOMe can be condensated at the (C₆Me₆)Ru complex with sodium methanolate to give triple deprotonated cyclic tripeptides.     

Rhodium(III)‐Catalyzed [3+2]/[5+2] Annulation of 4‐Aryl 1,2,3‐Triazoles with Internal Alkynes through Dual C(sp2)H Functionalization

A rhodium(III)‐catalyzed [3+2]/[5+2] annulation of 4‐aryl 1‐tosyl‐1,2,3‐triazoles with internal alkynes is presented. This transformation provides straightforward access to indeno[1,7‐cd]azepine architectures through a sequence involving the formation of a rhodium(III) azavinyl carbene, dual C(sp²)? H functionalization, and [3+2]/[5+2] annulation.     

3‐Deoxy‐β‐d‐ribo‐hexopyranose (3‐deoxy‐β‐d‐glucopyranose)

The β‐pyranose form, (III), of 3‐deoxy‐d ‐ribo‐hexose (3‐deoxy‐d ‐glucose), C₆H₁₂O₅, crystallizes from water at 298 K in a slightly distorted ⁴C₁ chair conformation. Structural analyses of (III), β‐d ‐glucopyranose, (IV), and 2‐deoxy‐β‐d ‐arabino‐hexopyranose (2‐deoxy‐β‐d ‐glucopyranose), (V), show significantly different C—O bond torsions involving the anomeric carbon, with the H—C—O—H torsion angle approaching an eclipsed conformation in (III) (−10.9°) compared with 32.8 and 32.5° in (IV) and (V), respectively. Ring carbon deoxygenation significantly affects the endo‐ and exocyclic C—C and C—O bond lengths throughout the pyranose ring, with longer bonds generally observed in the monodeoxygenated species (III) and (V) compared with (IV). These structural changes are attributed to differences in exocyclic C—O bond conformations and/or hydrogen‐bonding patterns superimposed on the direct (intrinsic) effect of monodeoxygenation. The exocyclic hydroxymethyl conformation in (III) (gt) differs from that observed in (IV) and (V) (gg).     

η5‐(3)‐1‐Methyl‐1,2‐dicarbollyl‐η5‐2′,5′‐dimethylpyrrolylcobalt(III)

In the title compound, (η⁵‐2,5‐di­methyl­pyrrolyl)[(7,8,9,10,11‐η)‐7‐methyl‐7,8‐dicarba‐nido‐undecaborato]­cobalt(III), [3‐Co{η⁵‐[2,5‐(CH₃)₂‐NC₄H₂]}‐1‐CH₃‐1,2‐C₂B₉H₁₀] or [Co(C₃H₁₃B₉)(C₆H₈N)], the Co^III atom is sandwiched between the pentagonal faces of the pyrrolyl and dicarbollide ligands, resulting in a neutral mol­ecule. The C—C distance in the dicarbollide cage is 1.649 (3) Å.     

2H‐Azirines as Ligands: Structural Characterization of the First Neutral and Cationic (η5‐C5Me5)Rh(III) 2H‐Azirine Complexes

The synthesis and structural characterization of two azirine rhodium(III ) complexes are described. The stabilization, N‐coordination and phenylgroup π‐stacking of the highly reactive and strained 3‐phenyl‐2H‐azirine by transition metal coordination is observed. The reaction of the dimeric complex [(η⁵‐C₅Me₅)RhCl₂]₂ with 3‐phenyl‐2H‐azirine (az) in CH₂Cl₂ at room temperature in a 1:2 molar ratio afforded the neutral mono‐azirine complex [(η⁵‐C₅Me₅)RhCl₂(az)]. The subsequent reaction of [(η⁵‐C₅Me₅)RhCl₂]₂ with six equivalents of az and 4 equivalents of AgOTf yielded the cationic tris‐azirine complex [(η⁵‐C₅Me₅)Rh(az)₃](OTf)₂. After purification, all complexes have been fully characterized. The molecular structures of the novel rhodium(III ) complexes exhibit slightly distorted octahedral coordination geometries around the metal atoms.     

Luminescent Benzoquinolate‐Isocyanide Platinum(II) Complexes: Effect of Pt⋅⋅⋅Pt and π⋅⋅⋅π Interactions on their Photophysical Properties

The neutral compounds [Pt(bzq)(CN)(CNR)] (R=tBu ( 1 ), Xyl ( 2 ), 2‐Np ( 3 ); bzq= benzoquinolate, Xyl=2,6‐dimethylphenyl, 2‐Np=2‐napthyl) were isolated as the pure isomers with a trans‐C_bzq,CNR configuration, as confirmed by ¹³C{¹H} NMR spectroscopy in the isotopically marked [Pt(bzq)(¹³CN)(CNR)] (R=tBu ( 1′ ), Xyl ( 2′ ), 2‐Np ( 3′ )) derivatives (δ¹³C_CN≈110 ppm; ¹J(Pt,¹³C)≈1425 Hz]. By contrast, complex [Pt(bzq)(C≡CPh)(CNXyl)] ( 4 ) with a trans‐N_bzq,CNR configuration, has been selectively isolated from [Pt(bzq)Cl(CNXyl)] (trans‐N_bzq,CNR) using Sonogashira conditions. X‐ray diffraction studies reveal that while 1 adopts a columnar‐stacked chain structure with Pt–Pt distances of 3.371(1) Å and significant π???π interactions (3.262 Å), complex 2 forms dimers supported only by short Pt???Pt (3.370(1) Å) interactions. In complex 4 the packing is directed by weak bzq???Xyl and bzq???C≡E (C, N) interactions. In solid state at room temperature, compounds 1 and 2 both show a bright red emission (?=42.1 % 1 , 57.6 % 2 ). Luminescence properties in the solid state at 77 K and concentration‐dependent emission studies in CH₂Cl₂ at 298 K and at 77 K are also reported for 1 , 1·CHCl3 , 2 , 2' , 2·CHCl3 , 3 , 4 .     

Heterobimetallic Complexes: Substitution Effects in the [Tetracarbonylrhodiumbis{μ‐(diphenylphosphino)}{μ‐(η:η2‐(4‐diphenylphosphino‐κP)‐2‐methyl‐oxobutyl)}molybdenum](Mo‐Rh) System

Syntheses of the array of heterobimetallic complexes [(OC)₃M(μ‐PPh₂)₂(μ‐O‐C(CHMe(CH₂)₂PPh₂)RhL], M = Cr, Mo, W, L = ^tBuNC, are described, extending the previous study of the counterpart array for L = CO. A single crystal X‐ray structure determination is reported for the M = Mo adduct, enabling comparison with its previously reported L = CO counterpart, for which an improved redetermination is also reported. In the present complex the ^tBuNC ligand is found to be much more weakly bound (Rh‐C 2.026(5) Å) than the carbonyl group it displaces (Rh‐C 1.945(2) Å) with concomitant minor impact on the remainder of the rhodium ambience.     

Intramolecular Metal⋅⋅⋅π‐Arene Interactions in Neutral and Cationic Main Group Compounds

The role of intramolecular metal???π‐arene interactions has been investigated in the solid‐state structures of a series of main group compounds supported by the bulky amide ligands, [N(^tBuAr^≠)(SiR₃)]^? (^tBuAr^≠=2,6‐(CHPh₂)₂‐4‐tBuC₆H₂, R=Me, Ph). The lithium and potassium amide salts showed different patterns of solvation and demonstrated that the SiPh₃ substituent is able to be involved in stabilizing the electrophilic metal. These group 1 metal compounds served as ligand transfer reagents to access a series of bismuth(III) halides. Chloride extraction from Bi(N{^tBuAr^≠}{SiPh₃})Cl₂ using AlCl₃ afforded the 1:1 salt [Bi(N{^tBuAr^≠}{SiPh₃})Cl][AlCl₄]. This was accompanied by a significant rearrangement of the stabilizing π‐arene contacts in the solid‐state. Attempted preparation of the corresponding tetraphenylborate salt resulted in phenyl‐transfer and generation of the neutral Bi(N{^tBuAr^≠}{SiPh₃})(Ph)Cl.     

CH⋅⋅⋅N Hydrogen‐Bonding Interaction in 7‐Azaindole:CHX3 (X=F,Cl) Complexes

The C?H???Y (Y=hydrogen‐bond acceptor) interactions are somewhat unconventional in the context of hydrogen‐bonding interactions. Typical C?H stretching frequency shifts in the hydrogen‐bond donor C?H group are not only small, that is, of the order of a few tens of cm^?1, but also bidirectional, that is, they can be red or blue shifted depending on the hydrogen‐bond acceptor. In this work we examine the C?H???N interaction in complexes of 7‐azaindole with CHCl₃ and CHF₃ that are prepared in the gas phase through supersonic jet expansion using the fluorescence depletion by infra‐red (FDIR) method. Although the hydrogen‐bond acceptor, 7‐azaindole, has multiple sites of interaction, it is found that the C?H???N hydrogen‐bonding interaction prevails over the others. The electronic excitation spectra suggest that both complexes are more stabilized in the S₁ state than in the S₀ state. The C?H stretching frequency is found to be red shifted by 82 cm^?1 in the CHCl₃ complex, which is the largest redshift reported so far in gas‐phase investigations of 1:1 haloform complexes with various substrates. In the CHF₃ complex the observed C?H frequency is blue shifted by 4 cm^?1. This is at variance with the frequency shifts that are predicted using several computational methods; these predict at best a redshift of 8.5 cm^?1. This discrepancy is analogous to that reported for the pyridine‐CHF₃ complex [W. A. Herrebout, S. M. Melikova, S. N. Delanoye, K. S. Rutkowski, D. N. Shchepkin, B. J. van der Veken, J. Phys. Chem. A­ 2005 , 109, 3038], in which the blueshift is termed a pseudo blueshift and is shown to be due to the shifting of levels caused by Fermi resonance between the overtones of the C?H bending and stretching modes. The dissociation energies, (D₀), of the CHCl₃ and CHF₃ complexes are computed (MP2/aug‐cc‐pVDZ level) as 6.46 and 5.06 kcal mol^?1, respectively.     

Dicarbonyldi‐μ‐chloro‐cis,cis‐η4‐1,5‐cyclo­octa­dienedirhodium(I)

The title compound, dicarbonyl‐1κ²C‐di‐μ‐chloro‐1:2κ⁴Cl‐[cis,cis‐2(η⁴)‐1,5‐cyclo­octa­diene]­di­rhodium(I), [Rh₂Cl₂(C₈H₁₂)(CO)₂], consists of a di­chloro‐bridged dimer of rhodium, with a non‐bonded Rh?Rh distance of 3.284 (2) Å. One Rh atom is coordinated to two carbonyl ligands, while the other Rh atom is coordinated to the cyclo­octa­diene moiety.     

Formation of 18e− and 16e− Acyl(η3‐cyclooctenyl)rhodium(III) Complexes in the Reaction of Cationic (Cycloocta‐1,5‐diene)rhodium(I) Compounds with 2‐(Diphenylphosphino)benzaldehyde

The reaction of cationic diolefinic rhodium(I) complexes with 2‐(diphenylphosphino)benzaldehyde (pCHO) was studied. [Rh(cod)₂]ClO₄ (cod=cycloocta‐1,5‐diene) reacted with pCHO to undergo the oxidative addition of one pCHO with (1,2,3‐η)cyclooct‐2‐en‐1‐yl (η³‐C₈H₁₃) formation, and the coordination of a second pCHO molecule as (phosphino‐κP)aldehyde‐κO(σ‐coordination) chelate to give the 18e⁻ acyl(allyl)rhodium(III) species [Rh(η³‐C₈H₁₃)(pCO)(pCHO)]ClO₄ (see 1 ). Complex 1 reacted with [Rh(cod)(PR₃)₂]ClO₄ (R=aryl) derivatives 3 – 6 to give stable pentacoordinated 16e⁻ acyl[(1,2,3‐η)‐cyclooct‐2‐en‐1‐yl]rhodium(III) species [Rh(η³‐C₈H₁₃)(pCO)(PR₃)]ClO₄ 7 – 10 . The (1,2,3‐η)‐cyclooct‐2‐en‐1‐yl complexes contain cis‐positioned P‐atoms and were fully characterized by NMR, and the molecular structure of 1 was determined by X‐ray crystal diffraction. The rhodium(III) complex 1 catalyzed the hydroformylation of hex‐1‐ene and produced 98% of aldehydes (n/iso=2.6).     

Gold(III)‐Mediated Contraction of Benzene to Cyclopentadiene: From p‐Benziporphyrin to Gold(III) True Tetraarylcarbaporphyrin

The reaction of p‐benziporphyrin, sodium tetrachloroaurate(III) dihydrate, and potassium carbonate in dichloromethane yielded gold(III) 5,10,15,20‐tetraaryl‐21‐carbaporphyrin owing to the contraction of p‐phenylene to cyclopentadiene. This molecule is the very first representative of a true 5,10,15,20‐tetraaryl‐21‐carbaporphyrin complex where four trigonal donor atoms are involved in equatorial coordination. The contraction adds an unprecedented route to numerous organic transformations of aromatic compounds catalyzed by simple gold(III) compounds. p‐Benziporphyrin provided the unique environment to alter the fundamental reactivity of the benzene unit facilitating its contraction to cyclopentadiene.     

Mono‐ and Binuclear Rhodium and Platinum Complexes of 1, 3, 5‐Trimethyl‐1, 3, 5‐triaza‐2σ3λ3‐phosphorin‐4, 6‐dionyloxy‐substituted Calix[4]arenes

The reaction behaviour of 1, 3, 5‐triaza‐2σ³λ³‐phosphorin‐4, 6‐dionyloxy‐substituted calix[4]arenes towards mono‐ and binuclear rhodium and platinum complexes was investigated. Special attention was directed to structure and dynamic behaviour of the products in solution and in the solid state. Depending on the molar ratio of the reactands, the reaction of the tetrakis(triazaphosphorindionyloxy)‐substituted calix[4]arene ( 4 ) and its tert‐butyl‐derivative ( 1 ) with [(cod)RhCl]₂ yielded the mono‐ and disubstituted binuclear rhodium complexes 2 , 3 , and 5 . In all cases, a C₂‐symmetrical structure was proved in solution, apparently caused by a fast intramolecular exchange process between cone conformation and 1, 3‐alternating conformation. The X‐ray crystal structure determination of 5 confirmed [(calixarene)RhCl]₂‐coordination through two opposite phosphorus atoms with a P ⃜P separation of 345 pm. The complex displays crystallographic inversion symmetry, and the Rh₂Cl₂ core is thus exactly planar. Reaction of 1 and of the bis(triazaphosphorindionyloxy)‐bis(methoxy)‐substituted tert‐butyl‐calix‐[4]arene ( 7 ) with (cod)Rh(acac) in equimolar ratio and subsequent reaction with HBF₄ led to the expected cationic monorhodium complexes 5 and 8 , involving 1, 3‐alternating P‐Rh‐P‐coordination. The cone conformation in solution was proved by NMR spectroscopy and characteristic values of the ¹J(PRh) coupling constants in the ³¹P‐NMR‐spectra. Reaction of equimolar amounts of 4 with (cod)Rh(acac) or (nbd)Rh(acac) led, by substitution of the labile coordinated acetylacetonato and after addition of HBF₄, to the corresponding mononuclear cationic complexes 9 and 10 . Only two of the four phosphorus atoms in 9 and 10 are coordinated to the central metal atom. Displacement of either cycloocta‐1, 5‐diene or norbornadiene was not observed. For both compounds, the cone conformation was proved by NMR spectroscopy. Reaction of 4 with (cod)PtCl₂ led to the PtCl₂‐complex ( 11 ). As for all compounds mentioned above, only two phosphorus atoms of the ligand coordinate to platinum, while two phosphorus atoms remain uncoordinated (proved by δ³¹P and characteristic values of ¹J(PPt)). NMR‐spectroscopic evidence was found for the existence of the cone conformation in the cis‐configuration of 11 .     

Di‐μ‐methanolato‐bis[(η4‐tetrafluorobenzobarrelene)rhodium(I)]

The versatile synthetic precursor methanolate‐bridged title rhodium complex, [Rh₂(CH₃O)₂(C₁₂H₆F₄)₂] or [Rh(μ‐OCH₃)(tfbb)]₂ [tfbb = tetrafluorobenzobarrelene or 3,4,5,6‐tetrafluorotricyclo[6.2.2.0^2,7]dodeca‐2(7),3,5,9,11‐pentaene], has been structurally characterized. The asymmetric unit contains half a molecule that can be expanded via a twofold axis. The title compound has been shown to be a dinuclear rhodium complex where each metal centre is coordinated by two O atoms from two bridging methanolate groups and by the olefinic bonds of a tfbb ligand. Comparison of the bite angles of tfbb, norbornadiene (nbd) and cyclooctadiene (cod) olefins in their η⁴‐coordination to rhodium reveals similarities between the tfbb and nbd ligands, which are much more rigid than cod. The short distance found between the distorted square‐planar metal centres [2.8351 (4) Å] has been related to the syn conformation of the folded core `RhORhO' ring.     

Hydrogen‐transfer reduction of α,β‐unsaturated carbonyl compounds catalyzed by naphthyridine‐functionalized N‐heterocyclic carbene complexes

Substitution of silver complex of 2‐chloro‐7‐(mesitylimidazolylidenylmethyl)naphthyridine (NpNHC) with palladium(II), rhodium(I) and iridium(I) metal precursors provided [Pd(C ,N ‐NpNHC)(η³‐allyl)](BF₄) ( 5 ), RhCl(COD)(C ‐NpNHC) ( 6a ) and IrCl(COD)(C ‐NpNHC) ( 6b ), respectively. Abstraction of chloride from 6a and 6b with AgBF₄ provided the chelation complexes [Rh(COD)(C ,N ‐NpNHC)](BF₄) ( 7a ) and Ir(COD)(C ,N ‐NpNHC)(BF₄) ( 7b ), respectively. All complexes were characterized using NMR and elemental analyses and the structural details of 5 and 6a were further confirmed using X‐ray crystallography. In catalytic activity studies, complex 5 was found to be an effective catalyst in the hydrogen‐transfer reduction of α,β‐unsaturated carbonyl compounds into the corresponding saturated carbonyl compounds.     

An Inverted‐Sandwich Diuranium μ‐η5:η5‐Cyclo‐P5 Complex Supported by U‐P5 δ‐Bonding

Reaction of [U(Tren^TIPS)] [ 1 , Tren^TIPS=N(CH₂CH₂NSiiPr₃)₃] with 0.25 equivalents of P₄ reproducibly affords the unprecedented actinide inverted sandwich cyclo‐P₅ complex [{U(Tren^TIPS)}₂(μ‐η⁵:η⁵‐cyclo‐P₅)] ( 2 ). All prior examples of cyclo‐P₅ are stabilized by d‐block metals, so 2 shows that cyclo‐P₅ does not require d‐block ions to be prepared. Although cyclo‐P₅ is isolobal to cyclopentadienyl, which usually bonds to metals via σ‐ and π‐interactions with minimal δ‐bonding, theoretical calculations suggest the principal bonding in the U(P₅)U unit is polarized δ‐bonding. Surprisingly, the characterization data are overall consistent with charge transfer from uranium to the cyclo‐P₅ unit to give a cyclo‐P₅ charge state that approximates to a dianionic formulation. This is ascribed to the larger size and superior acceptor character of cyclo‐P₅ compared to cyclopentadienyl, the strongly reducing nature of uranium(III), and the availability of uranium δ‐symmetry 5f orbitals.