Novel isolated,L‐amino acid‐ligated rhodium catalysts that induce highly helix‐sense‐selective polymerization of an achiral 3,4,5‐trisubstituted phenylacetylene

Two novel chiral well‐defined rhodium complexes, Rh(cod)(L‐Phe) (cod = 1,5‐cyclooctadiene, Phe = phenylalanine) and Rh(cod)(L‐Val) (Val = valine) were synthesized, isolated by recrystallization, and characterized. The helix‐sense‐selective polymerization (HSSP) of an achiral 3,4,5‐trisubstituted phenylacetylene, p‐dodecyloxy‐m,m‐dihydroxyphenylacetylene (DoDHPA) was examined by using the two Rh complexes as catalysts. These catalysts provided high molecular weight polymers (M_w 28 × 10⁴?45 × 10⁴) in about 40%–85% yields. The resulting polymers exhibited a bisignated CD signal at about 300 nm and a broad signal around 470 nm, indicating that they have preferential one‐handed helical structure. The present catalysts achieved larger molar ellipticity up to [θ]₃₁₀ = 13.0 × 10⁴ deg cm²/dmol than those with binary chiral catalytic systems, [Rh(cod)Cl]₂/(L‐phenylalaninol), [Rh(cod)Cl]₂/(L‐valinol), and [Rh(nbd)Cl]₂/(R)‐PEA. All these results manifest that the present, well‐defined Rh complexes serve as excellent catalysts for the HSSP of DoDHPA. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 2346–2351     

Polymerization of substituted acetylenes by various rhodium catalysts: Comparison of catalyst activity and effect of additives

This research deals with comparison of the activity of various Rh catalysts in the polymerization of monosubstituted acetylenes and the effect of various amines used in conjunction with [Rh(nbd)Cl]₂ in the polymerization of phenylacetylene. A zwitterionic Rh complex, Rh⁺(nbd)[(η⁶‐C₆H₅)B^?(C₆H₅)₃] ( 3 ), was able to polymerize phenylacetylene ( 5a ), t‐butylacetylene ( 5b ), N‐propargylhexanamide ( 5c ) and n‐hexyl propiolate ( 5d ), and displayed higher activity than the other catalysts examined, that is [Rh(nbd)Cl]₂ ( 1 ), [Rh(cod)(O‐o‐cresol)]₂ ( 2 ), and Rh‐vinyl complex ( 4 ). Monomers 5a and 5c polymerized virtually quantitatively or in fair yields with all these catalysts, while monomer 5b was polymerizable only with catalyts 3 and 4 . Monomer 5d did not polymerize in high yields with these Rh complexes. The catalytic activity tended to decrease in the order of 3 > 4 > 2 > 1 . Although polymerization of 5a did not proceed at all in toluene with [Rh(nbd)Cl]₂ alone, it smoothly polymerized in the presence of various amines as cocatalysts. The polymerization rate as well as the molecular weight of polymer depended on the basicity and steric bulkiness of amines. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 4530–4536, 2005     

Polyacetylene‐Type Networks Prepared by Coordination Polymerization of Diethynylarenes: New Type of Microporous Organic Polymers

Microporous organic polymers (MOP) of a new type have been synthesised in high yields by a simple coordination polymerization of 1,3‐diethynylbenzene, 1,4‐diethynylbenzene and 4,4′‐diethynylbiphenyl catalysed by [Rh(cod)acac] and [Rh(nbd)acac] complexes. The new MOPs are non‐swellable polyacetylene‐type conjugated networks consisting of ethynylaryl‐substituted polyene main chains that are crosslinked by arylene linkers. Prepared MOP samples have a mole fraction of branching units (by ¹³C CP/MAS NMR) from 0.30 to 0.47 and exhibit the BET (Brunaer‐Emmett‐Teller) surface up to 809 m² g⁻¹ and hydrogen uptake up to 0.69 wt% (77 K, H₂ pressure 750 torr).     

Spectrophotometric studies on complex formation in dilute benzene solutions of [{Rh(diene)(μ–Cl)}2]–PR3–SnCl2 systems

There is no correlation between the stability of complexes formed in dilute solutions of [{Rh(diene)(–Cl)}2]–PR3–SnCl2 systems [with ligand combinations cod+PPh3, nbd+PPh3, cod+PPhMe2 and cod+ PCy3 (Cy=cyclohexyl)] and the possibility of isolating them as solids. In general, high dilution favours the formation of pentacoordinate complexes that decompose upon attempted crystallization. [RhCl(cod)(PPhMe2)2], [Rh(SnCl3)(cod)(PPh3)2], [Rh(SnCl3)(cod)(PCy3)2], [Rh(SnCl3)(cod)(PPhMe2)2] and [Rh(SnCl3)(nbd)(PPh3)2] have been identified in solution, but only the last two, previously known, have been isolated in the solid state. The steric properties of the coordinated phosphine seem to be the most important factor in determining the stabilities of [RhCl(diene)(PR3)2] complexes, whilst in the case of [Rh(SnCl3)(diene)(PR3)2] complexes the steric properties of the phosphine and the diene appear to have similar importance.     

Selective oxygenation of amphiphilic thiacalix[3]pyridine Rh(I) diene complexes in both water and organic solvents

Thiacalix[3]pyridine (Py3S3) reacted with [Rh(diene)(mu-Cl)]2(diene = 1,5-cyclooctadiene (cod), 2,5-norbornadiene (nbd)) to give amphiphilic trigonal bipyramidal complexes, [Rh(Py3S3)(diene)]Cl. Sulfur bridges of the Py3S3 ligand in these complexes were selectively oxygenated by m-chloroperoxybenzoic acid in dichloromethane to give sulfinylcalix[3]pyridine complexes, [Rh(Py3(SO)3)(diene)]+, in which all three oxygen atoms of the SO groups occupy the equatorial positions. Structures of the complexes were analysed by X-ray crystallography and the oxidation reaction was investigated using 1H NMR spectroscopy and electrospray ionisation mass spectrometry showing that the oxygenation of the sulfur atoms in the ligand proceeded stepwise and further oxygenation of the SO moiety occurred only for the nbd complex having the smaller diene ligand resulting in [Rh(Py3(SO)2(SO2))(nbd)]+. On the other hand, the oxidation of [Rh(Py3S3)(cod)]+ by H2O2 in water did not result in oxygenation of the sulfur bridges but the cod ligand is hydroxygenated to give 1,4,5,6-eta4-2-hydroxycycloocta-4-ene-1,6-di-yl.     

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 .     

Catalytic polymerization of phenylacetylene with dimeric [Rh(OMe)(cod)]2 complex in ionic liquids

Dimeric rhodium(I) complex [Rh(OMe)(cod)]₂ was found to be an active catalyst of phenylacetylene polymerization to poly(phenylacetylene) (PPA) in ionic liquids containing imidazolium or pyridinium cations. The highest yield of PPA (92%) was obtained in 1‐butyl‐4‐methylpyridinium tetrafluoroborate as reaction medium. The yield of PPA in imidazolium ionic liquids containing BF₄^? or PF₆^? anions increased to 83–99% when Et₃N or cycloocta‐1,5‐diene were added as co‐catalysts. In 1‐methyl‐3‐octylimidazolium chloride (MOI · Cl) polymerization rate was much lower than in other ionic liquids, although the highest M_w (72 400) was obtained. Spectroscopic studies confirmed that [Rh(OMe)(cod)]₂ reacted with MOI · Cl forming new carbene Rh(I) complex, which can participate in the polymerization process. Copyright © 2006 John Wiley & Sons, Ltd.     

Synthesis and asymmetric polymerization of (S)‐N‐maleoyl‐L‐leucine propargyl ester

A kind of N‐substituted maleimide (RMI), chiral (S)‐N‐maleoyl‐L ‐leucine propargyl ester ((S)‐PLMI) with a specific rotation of [α]₄₃₅ = ?27.5° was successfully synthesized from maleic anhydride, L ‐leucine, and propargyl alcohol. (S)‐PLMI was polymerized by three polymerization methods to obtain the corresponding optically active polymers. Asymmetric anionic, radical, and transition‐metal‐catalyzed polymerizations were carried out using organometal/chiral ligands, 2,2′‐azobisisobutyronitrile (AIBN) and (bicyclo [2,2,1]hepta‐2,5‐diene) chloro rhodium (I) dimer ([Rh(nbd) Cl]₂), respectively. Poly((S)‐PLMI) obtained by [Rh(nbd)Cl]₂ in DMF showed the highest specific rotation of ?280.6°. Chiroptical properties and structures of the polymers obtained were investigated by GPC, CD, IR, and NMR measurements. Two types of poly((S)‐PLMI)‐bonded‐silica gels as the chiral stationary phase (CSP) were prepared for high‐performance liquid chromatography (HPLC). Their optical resolution abilities were also elucidated. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 3722–3738, 2007     

Solid-state (15)N CPMAS NMR and computational analysis of ligand hapticity in rhodium(η-diene) poly(pyrazolyl)borate complexes

Novel [Rh(η-diene)Tp(x)] complexes of sterically encumbered Tp(x) ligands (Tp(x) = Tp(4Bo), diene = cod, 1; nbd, 2; Tp(x) = Tp(4Bo,5Me), diene = cod, 3; nbd, 4; Tp(x) = Tp(a,3Me), diene = cod, 5; nbd, 6; Tp(x) = Tp(a*,3Me), diene = cod, 7; nbd, 8) have been prepared by treatment of [Rh(η-diene)(μ-Cl)](2) with TlTp(x) (Tp(x) in general, in detail: Tp(4Bo) = hydrotris(indazol-1-yl)borate, Tp(4Bo,5Me) = hydrotris(5-methyl-indazol-1-yl)borate, Tp(a,3Me) = hydrotris(3-methyl-2H-benz[g]-4,5-dihydroindazol-2-y1)borate, Tp(a*,3Me) = hydrotris(3-methyl-2H-benz[g]indazol-2-yl)borate), and characterized by analytical and spectral data (IR, (1)H, (11)B, and (13)C NMR solution). The structures adopted by [Rh(nbd)Tp(4Bo)] 2, [Rh(cod)Tp(4Bo,5Me)] 3, [Rh(nbd)Tp(a,3Me)] 6, [Rh(nbd)Tp(a*,3Me)] 8, and [Rh(nbd)Tp(a*,3Me*)] 8* (incorporating a borotropomeric ligand), have been investigated. Low steric hindrance between the ligands in 2 and 3 permits κ(3) coordination of the pyrazolylborate while the high steric encumbrance present in 6, 8, and 8* results in κ(2) ligands. The coordination modes of the ligands to the metal have also been established by (15)N CPMAS studies of selected ligands and their corresponding Rh complexes. These spectroscopic data are in agreement with the (15)N chemical shifts obtained by using quantum-chemical methods to assist reliable assignments of the experimental values, affording new insights into the extraction of structural information concerning the hapticity (κ(2) or κ(3)) of the poly(pyrazolyl)borate ligands to the Rh metal.     

The influence of the ligands on the catalytic activity of a series of RhI complexes in reactions with phenylacetylene: Synthesis of stereoregular poly(phenyl) acetylene

The catalytic activity of a series of [Rh L-L chel]X complexes, in which we have varied the unsaturated ligand [L-L = cis, cis-cycloocta 1,5-diene(cod) or 2,5-norbornadiene(nbd) the nitrogen chelating ligand [chel = 2,2′-bipyridine(bipy), 2,2′-dipyridylamine(dipyam), 2,2′-bipyrazine (bipz), 4,4′-dimethyl-2,2′-bipyridine (4,4′-Me₂bipy)] and the counter ion [X = PF₆, ClO₄, BPh₄], has been examined in reactions with phyenylacetylene (PA). The catalytic behaviour of the [Rh(cod)Cl₂],tmeda (tmeda = N,N,N′,N′tetramethylethylendiamine), [Rh(cod)Cl₂],teda] (teda = triethylendiamine), of the dimer [Rh(cod)Cl]₂, and the use of NaOH as cocatalyst in different reaction conditions was also examined. The influence of the ligands on the catalytic activity of these Rh^I complexes is discussed. ¹H and ¹³C NMR spectra have shown that highly stereoregular polyphenylacetilene can be obtained. Conditions for homogeneous doping of PPA, to obtain materials whose conductivity varies over 10–11 magnitude orders, are proposed. The stability of the doped polymers is also discussed.     

Synthesis of a novel one-handed helical poly(phenylacetylene) bearing poly(l-lactide) side chains

The living ring-opening polymerization of l-lactide was carried out by using organocatalyst to synthesize the molecular weight controlled poly(l-lactide) with an phenylacetylenyl end group (HCCPLLA), then the homopolymerization of HCCPLLA was performed by using two different rhodium catalysts. Low molecular weight poly-PLLA₆-1 (M_w,SEC-MALLS = 46,700) was synthesized by using Rh(nbd)BPh₄ as the catalyst, and higher molecular weight poly-PLLA₆-2 (M_w,SEC-MALLS = 471,000) was synthesized by using the [Rh(nbd)Cl]₂/Et₃N catalyst system. Then high molecular weight poly-PLLA₂₀, poly-PLLA₂₉, and poly-PLLA₆₈ were successfully synthesized by using the [Rh(nbd)Cl]₂/Et₃N catalyst system. The α values of the poly-PLLAs using [Rh(nbd)Cl]₂/Et₃N catalyst system were all in the range of 0.6–0.8, this means that these polymers possess linear flexible chain. It is concluded that [Rh(nbd)Cl]₂/Et₃N was more suitable for the synthesis of the cylindrical polymer brush, poly-PLLA with high molecular weight. The analyses of the CD spectra indicated that poly-PLLA possesses a predominantly one-handed helical conformation, temperature and solvents had significant influences on the helical structure of poly-PLLA.     

Manganese‐based transition metal complexes as new catalysts for olefin polymerizations

Three manganese complexes, Mn(acac)₃ (acac = acetylacetonate), Cp₂Mn (Cp = cyclopentadienyl), and Mn(salen)Cl [salen = 1,2‐cyclohexanediamino‐N,N′‐bis(3,5‐dit‐butyl‐salicylidene)], were used for ethylene and propylene polymerizations. These complexes, in combination with an alkylaluminum cocatalyst such as methylaluminoxane (MAO) or diethyl aluminum chloride (AlEt₂Cl), could promote ethylene polymerizations that yielded extremely high molecular weight linear polymers, but were inactive for propylene polymerizations. The counterparts supported on MgCl₂ showed activities even for propylene polymerizations and had remarkably enhanced activities for ethylene polymerizations. In the presence of an electron donor such as ethylbenzoate, the MgCl₂‐supported manganese‐based catalysts yielded a highly isotactic polypropylene with a high molecular weight. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 3733–3738, 2001     

Rhodium‐catalyzed transfer hydrogenation with aminophosphines and analysis of electrical characteristics of rhodium(I) complex/n‐Si heterojunctions

A series of novel neutral mononuclear rhodium(I) complexes of the P―NH ligands have been prepared starting from [Rh(cod)Cl]₂ complex. Structural elucidation of the complexes was carried out by elemental analysis, IR and multinuclear NMR spectroscopic data. The complexes were applied to the transfer hydrogenation of acetophenone derivatives to 1‐phenylethanol derivatives in the presence of 2‐propanol as the hydrogen source. Catalytic studies showed that all complexes are also excellent catalyst precursors for transfer hydrogenation of aryl alkyl ketones in 0.1 m iso‐PrOH solution. In particular, [Rh(cod)(PPh₂NH―C₆H₄―4‐CH(CH₃)₂)Cl] acts as an excellent catalyst, giving the corresponding alcohols in excellent conversion up to 99% (turnover frequency ≤ 588 h^?1). Furthermore, rhodium(I) complexes have been used in the formation of organic–inorganic heterojunction by forming their thin films on n‐Si and evaporating Au on the films. It has been seen that the structures have rectifying properties. Their electrical properties have been analyzed with the help of current–voltage measurements. Finally, it has been shown that the complexes can be used in the fabrication of temperature and light sensors. Copyright © 2014 John Wiley & Sons, Ltd.     

New Rhodium Complexes Coordinated by Bidentate Imine Ligands with Benzothiazolyl Functionalities

The pseudo‐square‐planar complexes [Rh(cod)(Hbbtm)]BF₄ ( 3 ), [Rh(bbte)(cod)]BF₄ ( 4 ), [Rh(CO)₂(Hbbtm)]BF₄ ( 5 ), [Rh(bbte)(CO)₂]BF₄ ( 6 ), [Rh(bbtm)(cod)] ( 7 ) and [Rh(bbtm)(CO)₂] ( 8 ) (Hbbtm=bis(benzothiazol‐2‐yl)methane=2,2′‐methylenebis[benzothiazole], bbte=bis(benzothiazol‐2‐yl)ethane=2,2′‐(ethane‐1,2‐diyl)bis[benzothiazole], and cod=cycloocta‐1,5‐diene) were synthesized and characterized. Diastereotopic protons were observed for the protons at the bridge in the ¹H‐NMR of 3 and 5 . Twisting of the ethane‐1,2‐diyl bridge in 4 and 6 effects chemical equivalence of the CH₂ groups in solution. Unusually large downfield shifts occur on coordination of the deprotonated ligand Hbbtm as the negative charge is delocalized in 7 and 8 . The NMR signals of the cod ligand in 4 could be differentiated. The X‐ray crystal structures of 3, 4 , and 6 are reported.     

Rhodium Complexes Containing O‐Bonded NHxMe2−xCHO (x=0, 1, 2): X‐Ray Structure of [Rh(PPh3)3(OCHNHMe)]ClO4

Displacement of norbornadiene (nbd; bicyclo[2.2.1]hepta‐2,5‐diene) from [Rh(PPh₃)₂(nbd)]ClO₄ by hydrogenation in the presence of PPh₃ and formamide or Me‐substituted derivatives, results in the formation of O‐bonded formamide complexes [Rh(PPh₃)₃(OCHNH_xMe_2−x)]ClO₄ (x=0, 1, 2) rather than N‐bonded derivatives. These have been characterised by spectroscopic measurements and, in the case of [Rh(PPh₃)₃(OCHNHMe)]ClO₄, by X‐ray crystallography. All undergo oxidative addition with H₂, and the rates of ligand exchange in the Rh^I and Rh^III complexes have been determined by magnetisation‐transfer measurements.     

Polymerization of phenylacetylene by iridium catalysts

Polymerization of phenylacetylene (PA) with [(cod)IrCl]₂‐based catalysts (cod: 1,5‐cyclooctadiene) was examined. The [(cod)IrCl]₂/n‐BuLi and [(cod)IrCl]₂/Ph₂C?C(Ph)Li systems induced the polymerization of PA to produce polymers with a number‐average molecular weight (M_n) of around several thousand in rather low yields. On the other hand, the catalyst composed of [(cod)IrCl]₂, norbornadiene (nbd), Ph₃P, and Ph₂C?C(Ph)Li (molar ratio of 1:1:1.1:2) produced polymer in a high yield (ca. 80%) in toluene at 0 °C. The resulting polymer showed a bimodal gel permeation chromatographic profile (M_n = 209,000 and 4300; ratio: 81/19). On the basis of these findings, the presence of two active species, that is, Ir complexes with nbd and cod, are discussed. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 1075–1080, 2002     

RhI‐Catalyzed Cyclizative Addition Reaction of 1,6‐Enyne and Sulfonyl Chloride by Carbophilic Activation

The π‐acid‐catalyzed cyclizations of 1,n‐enynes by carbophilic activation have been extensively studied and appear as highly attractive processes, yet the cases within a catalytic cycle based on redox principle are rare. Herein, we report the cyclizative addition reactions of 1,6‐enynes and sulfonyl chlorides by using a [Rh(cod)Cl/dppf] (dppf=1,1′‐bis(diphenylphosphino)ferrocene) catalyst system. The process features the involvement of oxidative addition of sulfonyl chloride to Rh^I catalyst, which generates [(dppf)(RSO₂)RhCl₂] as a π‐acid species to trigger cyclizative addition in a 6‐endo‐dig manner by carbophilic activation. Moreover, the catalytic protocol is also applicable to 1,6‐diene analogues.     

Cationic (Azulene)(diene)rhodium(I) Complexes: Spectroscopic,Dynamic, and Catalytic Properties

New [Rh^I(η⁵‐azulene)(η⁴‐diene)][BF₄] complex salts 3 – 5 (diene=8,9,10‐trinorborna‐2,5‐diene (nbd) and (1Z,5Z)‐cycloocta‐1,5‐diene (cod)) were synthesized according to a known procedure (Scheme 1). All of these complexes show dynamic behavior of the diene ligand at room temperature. In the case of the [Rh^I(η⁵‐azulene)(cod)]⁺ complex salts 3 and [Rh^I(η⁵‐guaiazulene)(nbd)]⁺ complex salt 4a (guaiazulene=7‐isopropyl‐1,4‐dimethylazulene), the coalescence temperature of the ¹H‐NMR signals of the olefinic H‐atoms was determined. The free energy of activation (ΔG; Table 1) for the intramolecular movement of the diene ligands exhibits a distinct dependency on the HOMO/LUMO properties of the coordinated azulene ligand. The DFT (density‐functional theory) calculated ΔG values for the internal diene rotation are in good to excellent agreement with the observed ones in CD₂Cl₂ as solvent (Table 2). Moreover, the ΔG values can also be estimated in good approximation from the position of the longest‐wavelength, azulene‐centered UV/VIS absorption band of the complex salts (Table 2). These cationic Rh^I complexes are stable and air‐resistant and can be used, e.g., as precursor complexes in situ in the presence of (M)‐6,7‐bis[(diphenylphosphino)methyl]‐8,12‐diphenylbenzo[a]heptalene for asymmetric hydrogenation of (Z)‐α‐(acetamido)cinnamic acid with ee values of up to 68% (Table 4).     

Synthesis and Characterization of Heterobimetallic Tantalum–Rhodium and Tantalum–Iridium Complexes Connected by a Tantalacyclopentadiene Fragment

We report the synthesis of heterobimetallic Ta–Rh and Ta–Ir complexes bridged by a 2,5‐di‐tert‐butyltantalacyclopentadiene fragment. A mononuclear 2,5‐di‐tert‐butyltantalacyclopentadiene complex 2 was prepared by the reaction of (η²‐Me₃SiC≡CSiMe₃)TaCl₃(dme) ( 1 ) with excess amounts of 3,3‐dimethylbut‐1‐yne in the presence of AlCl₃. The tantalacyclopentadiene moiety of complex 2 served as a η⁴‐diene unit for coordinating the Rh and Ir centers; treatment of 2 with [M(μ‐Cl)(cod)]₂ (M = Rh and Ir; cod = cycloocta‐1,5‐diene) in toluene gave TaRh(μ‐C₄H₂^tBu₂)Cl₄(cod) ( 3 ) and [TaIr(μ‐C₄H₂^tBu₂)Cl₄]₂ ( 5 ), respectively. The X‐Ray diffraction study of 3 revealed a dative bond from an electron‐rich Rh toward an electron‐deficient Ta. Upon dissolving 3 in THF, [(thf)TaRh(μ‐C₄H₂^tBu₂)Cl₃]₂(μ‐Cl)₂ ( 4 ) was isolated together with free cycloocta‐1,5‐diene. When complex 5 was treated with 1,2‐bis‐(diphenylphosphino)ethane (dppe), a monomeric Ta–Ir complex, TaIr(μ‐C₄H₂^tBu₂)Cl₄(dppe) ( 6 ), was isolated. Ta–Rh and Ta–Ir heterobimetallic complexes 3 and 6 were reduced by a two‐electron process upon reaction with 2,3,5,6‐tetramethyl‐1,4‐bis(trimethylsilyl)‐1,4‐dihydropyrazine ( 7a : Si‐Me₄‐DHP) or 2,5‐dimethyl‐1,4‐bis(trimethylsilyl)‐1,4‐dihydropyrazine ( 7b : Si‐Me₂‐DHP) to afford the corresponding complexes TaM(μ‐C₄H₂^tBu₂)Cl₂(L) ( 8 : M = Rh, L = cod; 9 : M = Ir, L = dppe), where the metallacycle moiety was assigned to have a tantalacyclopentadiene fragment with a large contribution of a tantalacyclopentatriene canonical form.     

Rhodium-catalyzed addition of arylboron compounds to nitriles, ketones, and imines

The rhodium-catalyzed addition reactions of sodium tetraphenylborate and arylboronic acids to nitriles, ketones, and imines were examined. The reaction of nitriles could be carried out efficiently in the presence of a catalyst system of [RhCl(cod)]₂-dppp and H₂O to give the corresponding monoarylated products selectively. Although unactivated ketones and imines are known to be poor electrophiles for rhodium-catalyzed arylation, the phenylation of them with use of sodium tetraphenylborate proceeded smoothly in the presence of [RhCl(cod)]₂ and Rh(acac)(cod) as catalysts, respectively. The addition of NH₄Cl was found to be crucial to effectively conduct the reaction of ketones and imines.