Synthesis and characterization of a trigonal bipyramidal supramolecular cage based upon rhodium and platinum metal centers

The reaction of 4-ethynyl-pyridine with tert-butyl lithium followed by its addition to (Me3tacn)RhCl3 affords the facial octahedral complex (Me3tacn)Rh(CCPy)3, condensation of which with the square planar complex cis-(DCPE)Pt(NO3)2 results in a self-assembled trigonal bipyramidal cage with Rh(III) and Pt(II) atoms occupying the vertices.     

Efficient aziridination of olefins catalyzed by mixed-valent dirhodium(II,III) caprolactamate

[reaction: see text] A mild, efficient, and selective aziridination of olefins catalyzed by dirhodium(II) caprolactamate [Rh(2)(cap)(4).2CH(3)CN] is described. Use of p-toluenesulfonamide (TsNH(2)), N-bromosuccinimide (NBS), and potassium carbonate readily affords aziridines in isolated yields of up to 95% under extremely mild conditions with as little as 0.01 mol % Rh(2)(cap)(4). Aziridine formation occurs through Rh(2)(5+)-catalyzed aminobromination and subsequent base-induced ring closure. An X-ray crystal structure of a Rh(2)(5+) halide complex, formed from the reaction between Rh(2)(cap)(4) and N-chlorosuccinimide, has been obtained.     

The first fullerene-metal sandwich complex: an unusually strong electronic communication between two C(60) cages

Reaction of Rh(6)(CO)(9)(dppm)(2)(mu(3)-eta(2),eta(2),eta(2)-C(60)) (1) with C(60) in refluxing chlorobenzene followed by treatment with CNR (R = CH(2)C(6)H(5)) at room temperature affords the first fullerene-metal sandwich complex Rh(6)(CO)(5)(dppm)(2)(CNR)(mu(3)-eta(2),eta(2),eta(2)-C(60))(2) (2). Compound 2 has been characterized by an X-ray diffraction study. Electrochemical study of 2 reveals six well-separated reversible redox couples localized at C(60) cages due to a strong electronic communication between the two C(60) centers via the Rh(6) cluster spacer.     

Dihydrogen complexes of rhodium: [RhH2(H2)x (PR3)2]+ (R = Cy, iPr; x = 1, 2)

Addition of H2 (4 atm at 298 K) to [Rh(nbd)(PR3)2][BAr(F)4] [R = Cy, iPr] affords Rh(III) dihydride/dihydrogen complexes. For R = Cy, complex 1a results, which has been shown by low-temperature NMR experiments to be the bis-dihydrogen/bis-hydride complex [Rh(H)2(eta2-H2)2(PCy3)2][BAr(F)4]. An X-ray diffraction study on 1a confirmed the {Rh(PCy3)2} core structure, but due to a poor data set, the hydrogen ligands were not located. DFT calculations at the B3LYP/DZVP level support the formulation as a Rh(III) dihydride/dihydrogen complex with cis hydride ligands. For R = iPr, the equivalent species, [Rh(H)2(eta2-H2)2(P iPr3)2][BAr(F)4] 2a, is formed, along with another complex that was spectroscopically identified as the mono-dihydrogen, bis-hydride solvent complex [Rh(H)2(eta2-H2)(CD2Cl2)(P iPr3)2][BAr(F)4] 2b. The analogous complex with PCy3 ligands, [Rh(H)2(eta2-H2)(CD2Cl2)(PCy3)2][BAr(F)4] 1b, can be observed by reducing the H2 pressure to 2 atm (at 298 K). Under vacuum, the dihydrogen ligands are lost in these complexes to form the spectroscopically characterized species, tentatively identified as the bis hydrides [Rh(H)2(L)2(PR3)2][BAr(F)4] (1c R = Cy; 2c R = iPr; L = CD2Cl2 or agostic interaction). Exposure of 1c or 2c to a H2 atmosphere regenerates the dihydrogen/bis-hydride complexes, while adding acetonitrile affords the bis-hydride MeCN adduct complexes [Rh(H)2(NCMe)2(PR3)2][BAr(F)4]. The dihydrogen complexes lose [HPR3][BAr(F)4] at or just above ambient temperature, suggested to be by heterolytic splitting of coordinated H2, to ultimately afford the dicationic cluster compounds of the type [Rh6(PR3)6(mu-H)12][BAr(F)4]2 in moderate yield.     

Hydrodefluorination of pentafluoropyridine at rhodium using dihydrogen: detection of unusual rhodium hydrido complexes

The pentafluoropyridyl complex [Rh(4-C5NF4)(PEt3)3] (3) reacts with H2 to give initially the dihydrido complex cis-mer-[Rh(H)2(4-C5NF4)(PEt3)3] (6). Within a few hours 2,3,5,6-tetrafluoropyridine as well as two rhodium(III) complexes mer-[Rh(H)3(PEt3)3] (mer-) and fac-[Rh(H)3(PEt3)3] (fac-) are formed. A catalytic C-F activation process for the formation of 2,3,5,6-tetrafluoropyridine starting from pentafluoropyridine and dihydrogen using 3 as a catalyst has been developed. Reaction of [RhH(PEt3)3] (1) with hydrogen affords fac-[Rh(H)3(PEt3)3] (fac-7) and mer-[Rh(H)3(PEt3)3] (mer-7) in a ratio of 1 : 7.25 at 193 K. The latter complex represents the first mononuclear rhodium compound bearing trans-hydrides.     

A mechanistic investigation of oxidative addition of methyl iodide to [Tp*Rh(CO)(L)

Reaction of methyl iodide with square planar [kappa(2)-Tp*Rh(CO)(PMe(3))] 1a (Tp* = HB(3,5-Me(2)pz)(3)) at room temperature affords [kappa(3)-Tp*Rh(CO)(PMe(3))(Me)]I 2a, which was fully characterized by spectroscopy and X-ray crystallography. The pseudooctahedral geometry of cationic 2a, which contains a kappa(3)-coordinated Tp* ligand, indicates a reaction mechanism in which nucleophilic attack by Rh on MeI is accompanied by coordination of the pendant pyrazolyl group. In solution 2a transforms slowly into a neutral (acetyl)(iodo) rhodium complex [kappa(3)-Tp*Rh(PMe(3))(COMe)I] 3a, for which an X-ray crystal structure is also reported. Kinetic studies on the reactions of [kappa(2)-Tp*Rh(CO)(L)] (L = PMe(3), PMe(2)Ph, PMePh(2), PPh(3), CO)] with MeI show second-order behavior with large negative activation entropies, consistent with an S(N)2 mechanism. The second-order rate constants correlate well with phosphine basicity. For L = CO, reaction with MeI gives an acetyl complex, [kappa(3)-Tp*Rh(CO)(COMe)I]. The bis(pyrazolyl)borate complexes [kappa(2)-Bp*Rh(CO)(L)] (L = PPh(3), CO) are much less reactive toward MeI than the Tp* analogues, indicating the importance of the third pyrazolyl group and the accessibility of a kappa(3) coordination mode. The results strengthen the evidence in favor of an S(N)2 mechanism for oxidative addition of MeI to square planar d(8) transition metal complexes.     

Key factors determining the course of methyl iodide oxidative addition to diamidonaphthalene-bridged diiridium(I) and dirhodium(I) complexes

The course of methyl iodide oxidative addition to various nucleophilic complexes, [Ir2(mu-1,8-(NH)2naphth)(CO)2(PiPr3)2] (1), [IrRh(mu-1,8-(NH)2naphth)(CO)2(PiPr3)2] (2), and [Rh2(mu-1,8-(NH)2naphth)(CO)2(PR3)2] (R = iPr, 3; Ph, 4; p-tolyl, 5; Me, 6), has been investigated. The CH3I addition to complex 1 readily affords the diiridium(II) complex [Ir2(mu-1,8-(NH)2naphth)I(CH3)(CO)2(PiPr3)2] (7), which undergoes slow rearrangement to give a thermodynamically stable stereoisomer, 8. The reaction of the Ir-Rh complex 2 gives the ionic compound [IrRh(mu-1,8-(NH)2naphth)(CH3)(CO)2(PiPr3)2]I (10). The dirhodium compounds, 3-5, undergo one-center additions to yield acyl complexes of the formula (Rh2(mu-1,8-(NH)2naphth)I(COCH3)(CO)(PR3)2] (R = iPr, 12; Ph, 13; p-tolyl, 14). The structure of 12 has been determined by X-ray diffraction. Further reactions of these Rh(III)-Rh(I) acyl derivatives with CH3I are productive only for the p-tolylphosphine derivative, which affords the bis-acyl complex [Rh2(mu-1,8-(NH)2naphth)(CH3CO)2I2(P(p-tolyl)3)2] (15). The reaction of the PMe3 derivative, 6, allows the isolation of the bis-methyl complex [Rh2(mu-1,8-(NH)2naphth)(mu-I)(CH3)2(CO)2(PMe3)2]I (16a), which emanates from a double one-center addition. Upon reaction with methyl triflate, the starting materials, 1, 2, 3, and 6, give the isostructural cationic methyl complexes 9, 11, 17, and 18, respectively. The behavior of these cationic methyl compounds toward CH3I, CH3OSO2CF3, and tetrabutylamonium iodide is consistent with the role of these species as intermediates in the SN2 addition of CH3I. Compounds 18 and 17 react with an excess of methyl triflate to give [Rh2(mu-1,8-(NH)2naphth)(mu-OSO2CF3)(CH3)2(CO)2(PMe3)2][CF3SO3] (19) and [Rh2(mu-1,8-(NH)2naphth)(OSO2CF3)(COCH3)(CH3)(CO)(PiPr3)2][CF3SO3] (20), respectively. Upon treatment with acetonitrile, complexes 17 and 18 give the isostructural cationic acyl complexes [Rh2(mu-1,8-(NH)2naphth)(COCH3)(NCCH3)(CO)(PR3)2][CF3SO3] (R = iPr, 21; Me, 22). A kinetic study of the reaction leading to 21 shows that formation of these complexes involves a slow insertion step followed by the fast coordination of the acetonitrile. The variety of reactions found in this system can be rationalized in terms of three alternative reaction pathways, which are determined by the effectiveness of the interactions between the two metal centers of the dinuclear complex and by the steric constraints due to the phosphine ligands.     

Insight into the photoinduced ligand exchange reaction pathway of cis-[Rh2(μ-O2CCH3)2(CH3CN)6]2+ with a DNA model chelate

We previously showed that [Rh(2)(O(2)CCH(3))(2)(CH(3)CN)(6)](2+) binds to dsDNA only upon irradiation with visible light and that photolysis results in a 34-fold enhancement of its cytotoxicity toward Hs-27 human skin fibroblasts, making it potentially useful for photodynamic therapy (PDT). With the goal of gaining further insight on the photoinduced binding of DNA to the complex, we investigated by NMR spectroscopy the mechanism by which 2,2'-bipyridine (bpy), a model for biologically relevant bidentate nitrogen donor ligands, binds to [Rh(2)(O(2)CCH(3))(2)(CH(3)CN)(6)](2+) upon irradiation in D(2)O. The photochemical results are compared to the reactivity in the dark in D(2)O and CD(3)CN. The photolysis of [Rh(2)(O(2)CCH(3))(2)(CH(3)CN)(6)](2+) with equimolar bpy solutions in D(2)O with visible light affords [Rh(2)(O(2)CCH(3))(2)(eq/eq-bpy)(CH(3)CN)(2)(D(2)O(ax))(2)](2+) (eq/eq) with the reaction reaching completion in ~8 h. Only vestiges of eq/eq are observed at the same time in the dark, however, and the reaction is ~20 times slower. Conversely, the dark reaction of [Rh(2)(O(2)CCH(3))(2)(CH(3)CN)(6)](2+) with an equimolar amount of bpy in CD(3)CN affords [Rh(2)(O(2)CCH(3))(2)(η(1)-bpy(ax))(CH(3)CN)(5)](2+) (η(1)-bpy(ax)), which remains present even after 5 days of reaction. The photolysis results in D(2)O are consistent with the exchange of one equiv CH(3)CNeq for solvent, and the resulting species quickly reacting with bpy to generate eq/eq; the initial eq ligand dissociation is assisted by absorption of a photon, thus greatly enhancing the reaction rate. The photolytic reaction of [Rh(2)(O(2)CCH(3))(2)(CH(3)CN)(6)](2+):bpy in a 1:2 ratio in D(2)O affords the eq/eq and (eq/eq)(2) adducts. The observed differences in the reactivity in D(2)O vs CD(3)CN are explained by the relative ease of substitution of eq D(2)O vs CD(3)CN by the incoming bpy molecule. These results clearly highlight the importance of dissociation of an eq CH(3)CN molecule from the dirhodium core to attain high reactivity and underscore the importance of light for the reactivity of these compounds, which is essential for PDT agents.     

Carbenerhodium complexes of the half-sandwich-type: synthesis, substitution, and addition reactions

A series of carbenerhodium(I) complexes of the general composition [(eta5-C5H5)Rh(=CRR')(L)] (2a-2i) with R = R'= aryl and L = SbiPr3 or PR3 has been prepared from the square-planar precursors trans-[RhCl(=CRR')(L)2] and NaC5H5 in excellent yields. Reaction of the triisopropylsibane derivative 2a. which contains a rather labile Rh-Sb bond, with CO, PMe3, and CNR (R = Me, CH2Ph, tBu) leads to the displacement of the SbiPr3 ligand and affords the substitution products [(eta5-C5H5)Rh(=CPh2)(L)] (3-7). In contrast, treatment of the triisopropylphosphane compound 2c with CO and CNtBu leads to the cleavage of the Rh=CPh2 bond and gives besides [(eta5-C5H5)Rh(PiPr3)(L)] (10, 12) by metal-assisted C-C coupling diphenylketene Ph2C=C=O (11) or the corresponding imine Ph2C=C=NtBu (13). While the reaction of 2a, c with C2H4 yields [(eta5-C5H5)Rh(C2H4)(L)] (14, 15) and the trisubstituted olefin Ph2C=CHCH3 (16), treatment of 2a, c with RN3 leads to the cleavage of both the Rh-EiPr3 and Rh=CPh2 bonds and gives the chelate complexes [(eta5-C5H5)Rh(kappa2-RNNNNR)] (19, 20). The substitution products 3 (L=CO) and 4 (L= PMe3) react with an equimolar amount of sulfur or selenium by addition of the chalcogen to the Rh=CPh2 bond to generate the complexes [(eta5-C5H5)Rh(kappa2-ECPh2)(L)] (21-24) with thio- or selenobenzophenone as ligand. Similarly, treatment of 3 with CuCl affords the unusual 1:2 adduct [(eta5-C5H5)(CO)Rh(mu-CPh2)(CuCl)2] (25), which reacts with NaC5H5 to form [(eta5-C5H5)(CO)Rh(muCPh2)Cu(eta5-C5H5)] (26). The molecular structures of 3 and 22 have been determined by X-ray crystallography.     

Asymmetric hydrogenation of enamides with Rh-BisP and Rh-miniPHOS catalysts. scope, limitations, and mechanism.

The asymmetric hydrogenation of aryl- and alkyl-substituted enamides catalyzed by Rh-BisP complex affords optically active amides with very high ee values. The Rh-MiniPHOS catalyst gives somewhat less satisfactory results. Hydrogenation of the aryl-substituted enamides with (S,S)-BisP-Rh catalyst gives R-amides, whereas the t-Bu- and 1-adamantyl-substituted enamides give S-products with 99% ee. Reaction of [Rh(BisP)(CD(3)OD)(2)]BF(4) (11) with CH(2)=C(C(6)H(5))NHCOCH(3) (5) gives two diastereomers of the catalyst-substrate complex (12a,b), which interconvert reversibly by both intra- and intermolecular pathways as shown by EXSY data. Only one isomer in equilibrium with solvate complex 11 was detected for each of the catalyst-substrate complexes 17 and 18 obtained from CH(2)=C(t-Bu)NHCOCH(3) (6) or CH(2)=C(1-adamantyl)NHCOCH(3) (7). Hydrogenation of these equilibrium mixtures at -100 degrees C gave monohydride intermediates 19 and 20, respectively. In these monohydrides the Rh atom is bound to the beta-carbon. A new effect of the significant decrease of ee was found for the asymmetric hydrogenation of CH(2)=C(C(6)H(4)OCH(3)-o)NHCOCH(3) (21), when H(2) was substituted for HD or D(2).     

Encapsulation versus Tethering in the Incorporation of C5H5Rh(CO)2 into Hybrid Inorganic-Organic Cubane {Si8O12} Copolymers

Copolymerization of cubane octavinylsilsesquioxane, {Si8O12}(CH=CH2)8, with the cubane silses-quioxane hydrides, {Si8O12}H8 and {Si8O12}(OSiMe2H)8 in the presence of C5H5Rh(CO)2 affords hybrid inorganic-organic copolymers incorporating monocarbonyl {C5H5Rh(CO)} species. These are of two types, one containing Rh(III) and the other Rh(I). The Rh(III) species appears to be that formed initially, and can be converted into the Rh(I) species by photolysis. BET measurements show that a significant increase in surface area occurs when the copolymerization is carried out in the presence of the rhodium complex.     

Ortho-Metallation reactions V. Reactions of rhodium complexes with azobenzene

Hydrated rhodium(III) chloride reacts with azobenzene (HAzb) affording RhCl₃(PhNH₂)₂ and the dimeric [(Azb)₂RhCl]₂. The latter reacts with donor ligands to give (Azb)₂RhCl(L), (L=PPh₃, tetrahydrofuran). With [Rh(CO)₂Cl]₂, azobenzene affords an unusual Rh^I---Rh^III complex, [(Azb)₂RhCl₂Rh(CO)₂], which can also be obtained from [Rh(CO)₂Cl]₂ and [(Azb)₂RhCl]₂. These complexes contain the ortho-metallated (phenylazo)phenyl-2C,N′ ligand, and their spectroscopic properties are summarised.     

Remarkable synthesis of 2-(Z)-6-(E)-4H-[1,4]-thiazepin-5-ones by zwitteronic rhodium-catalyzed chemo- and regioselective cyclohyrocarbonylative ring expansion of acetylenic thiazoles.

Cyclohydrocarbonylative ring expansion of acetylenic thiazoles in the presence of CO, H(2), and catalytic quantities of the zwitterionic rhodium complex (eta(6)-C(6)H(5)BPh(3))(-)Rh(+)(1,5-COD) and triphenyl phosphite affords thiazepinones in 61 to 90% yields. This novel transformation of a 5- to a 7-membered heterocycle is readily applied to acetylenic thiazoles containing hydro, alkyl, alkyl halide, vinyl, and benzo substitutents in positions 4 and 5 of the thiazole ring in addition to alkyl-, ether-, ester-, vinyl-, and aryl-substituted alkynes at position 2.     

A new family of mixed-valence dinuclear rhodium complexes containing the two metal centers in different stereochemical environments

A series of dinuclear chelate complexes of the general composition [Rh2(kappa2-L)2(mu-CR2)2(mu-SbiPr3)] (R = Ph, p-Tol; L = CF3CO2-, acac-, acac-f3-) and [Rh2Cl(kappa2-L)(mu-CR2)2(mu-SbiPr3)] (R = Ph, p-Tol; L = acac-, acac-f3-) has been prepared by replacement of the chloro ligands in the precursors [Rh2Cl2(mu-CR2)2(mu-SbiPr3)] by anionic chelates. The lability of the SbiPr3 bridge in the rhodium dimers is illustrated by the reactions of [Rh2(kappa2-acac)2(mu-CR2)2(mu-SbiPr3)] (7, 8) with Lewis bases such as CO, CNtBu, and SbEt3 which lead to the formation of the substitution products [Rh2(kappa2-acac)2(mu-CR2)2(mu-L')] (13-16) in excellent yields. Treatment of 7 and 8 with sterically demanding tertiary phosphanes PR3 (R3 = iPr3, iPr2Ph, iPrPh2, Ph3) affords the mixed-valence Rh0-RhII complexes [(kappa2-acac)2Rh(mu-CPh2)2Rh(PR3)] (21-24) and [(kappa2-acac)2Rh(mu-C(p-Tol)2]2Rh(PiPr3)] (25) for which there is no precedence. The terminal PiPr3 ligand of 21 is easily displaced by alkynes, CNtBu, and CO to give, by preserving the [(kappa2-acac)2Rh(mu-CPh2)2Rh] molecular core, the related dinuclear compounds 26-31 in which the coordination number of the Rh0 center is 3, 4, or 5. The molecular structures of [Rh2Cl(kappa2-acac)(mu-CPh2)2(mu-SbiPr3)] (5), [Rh2(kappa2-acac)2(mu-CPh2)2(mu-CO)] (13), [(kappa2-acac)2Rh(mu-CPh2)2Rh(PiPr3)] (21), and [(kappa2-acac)2Rh(mu-CPh2)2Rh(CNtBu)2] (30) have been determined crystallographically.     

Rhodium macrocyclic complexes: The synthesis and single crystal X-ray structure of [Rh([18]aneN2S4)](PF6)3 · 3H2O ([18]aneN2S4 = 1,4,10,13-tetrathia-7,16-diazacyclooctadecane)

Reaction of[Rh(H₂O)₆]³⁺ with one molar equivalent of [18]aneN₂S₄ in refluxing MeOH-H₂O (1 : 1 v/v) for 12 h affords an orange solution from which the complex [Rh([18]aneN₂S₄)](PF₆)₃ can be isolated upon addition of NH₄PF₆. A single crystal X-ray structure determination shows a distorted octahedral geometry at rhodium(III) involving the four thioether and two aza-donors of the macrocycle. The complex cation adopts a rac-configuration via meridional coordination of the two SCH₂CH₂NCH₂CH₂S linkages.     

Rh(I)L]-catalyzed cyclotetramerization of 1,3-butadiene: a theoretical investigation of alternative mechanistic paths for the generation of the [Rh(III)(octadienediyl)(PR3)]+ complex

A detailed theoretical investigation of alternative mechanistic paths for the formation of the [Rh(III)(octadienediyl)(PiPr3)]+ complex is presented, employing a gradient-corrected density functional theory (DFT) method (BP86). This process represents most likely the first step in the recently reported [Rh(I)L]-catalyzed cyclotetramerization of butadiene (M. Bosch, M. S. Brookhart, K. Ilg and H. Werner, Angew. Chem., Int. Ed., 2000, 39, 2304). The favorable route for oxidative addition under C-C-bond formation starts from the prevalent [Rh(I)(butadiene)2(PiPr3)]+ form of the active catalyst through oxidative coupling between two cis-eta4-butadienes. This affords the [Rh(III)(bis-eta3-anti-octadienediyl)(PiPr3)]+ compound as the kinetic coupling product that consecutively undergoes transformation into the thermodynamically favorable bis-eta3-syn-octadienediyl-Rh(III) isomer via facile allylic conversions occurring in the octadienediyl framework. The computationally predicted energy profile is almost in quantitative agreement with the experimentally determined kinetics and allows a consistent rationalization of the experimental observations.     

Square-planar rhodium(I) complexes partnered with [arachno-6-SB(9)H(12)]- : a route toward the synthesis of new rhodathiaboranes and organometallic/thiaborane salts

Treatment of [RhCl(eta4-diene)]2 (diene = nbd, cod) with the N-heterocyclic ligands 2,2'-bipyridine (bpy), 4,4'-dimethyl-2,2'-bipyridine (Me2bpy), 1,10-phenanthroline (phen), and pyridine (py) followed by addition of Cs[arachno-6-SB9H12] affords the corresponding salts, [Rh(eta4-diene)(L2)][SB9H12] [diene = cod, L2 = bpy (1), Me2bpy (3), phen (5), (py)2 (7); diene = nbd, L2 = bpy (2), Me2bpy (4), phen (6), (py)2 (8)]. These compounds are characterized by NMR spectroscopy and mass spectrometry, and in addition, the cod-Rh species 1 and 3 are studied by X-ray diffraction analysis. These saltlike reagents are stable in the solid state, but in solution the rhodium(I) cations, [Rh(eta4-diene)(L2)]+, react with the polyhedral anion [SB9H12]- leading to a chemistry that is controlled by the d8 transition element chelates. The nbd-Rh(I) complexes react faster than the cod-Rh(I) counterparts, leading, depending on the conditions, to the synthesis of new rhodathiaboranes of general formulas [8,8-(L2)-nido-8,7-RhSB9H10] [L2 = bpy (9), Me2bpy (10), phen (11), (py)2 (12)] and [8,8-(L2)-8-(L')-nido-8,7-RhSB9H10] [L' = PPh3, L2 = bpy (13), Me2bpy (14), phen (15); L' = NCCH3, L2 = bpy (16), Me2bpy (17), phen (18)]. Compound 13 is characterized by X-ray diffraction analysis confirming the 11-vertex nido-structure of the rhodathiaborane analogues 14-18. In dichloromethane, 1 and 3 yield mixtures that contain the 11-vertex rhodathiaboranes 9 and 10 together with new species. In contrast, the cod-Rh(I) reagent 5 affords a single compound, which is proposed to be an organometallic rhodium complex bound exo-polyhedrally to the thiaborane cage. In the presence of H2(g) and stoichiometric amounts of PPh3, the cod-Rh(I) reagents, 1, 3, and 5, afford the salts [Rh(H)2(L2)(PPh3)2][SB9H12] [L2 = bpy (19), Me2bpy (20), phen (21)]. Similarly, in an atmosphere of CO(g) and in the presence of PPh3, compounds 1-6 afford [Rh(L2)(PPh3)2(CO)][SB9H12] (L2 = bpy (22), Me2bpy (23), phen (24)]. The structures of 19 and 24 are studied by X-ray diffraction analysis. The five-coordinate complexes [Rh(L2)(PPh3)2(CO)]+ undergo PPh3 exchange in a process that is characterized as dissociative. The observed differences in the reactivity of the nbd-Rh(I) salts versus the cod-Rh(I) analogues are rationalized on the basis of the higher kinetic lability of the nbd ligand and its faster hydrogenation relative to the cod diene.     

Influence of the solvent and metal center on supramolecular chirality induction with bisporphyrin tweezer receptors. Strong metal modulation of effective molarity values

We describe the synthesis of a bisporphyrin tweezer receptor 1·H(4) and its metalation with Zn(II) and Rh(III) cations. We report the thermodynamic characterization of the supramolecular chirality induction process that takes place when the metalated bisporphyrin receptors coordinate to enantiopure 1,2-diaminocyclohexane in two different solvents, toluene and dichloromethane. We also performed a thorough study of several simpler systems that were used as models for the thermodynamic characterization of the more complex bisporphyrin systems. The initial complexation of the chiral diamine with the bisporphyrins produces a 1:1 sandwich complex that opens up to yield a simple 1:2 complex in the presence of excess diamine. The CD spectra associated with the 1:1 and 1:2 complexes of both metalloporphyrins, 1·Zn(2) and 1·Rh(2), display bisignate Cotton effects when the chirogenesis process is studied in toluene solutions. On the contrary, in dichloromethane solutions, only 1·Zn(2) yields CD-active 1:1 and 1:2 complexes, while the 1:2 complex of 1·Rh(2) is CD-silent. In both solvents, porphyrin 1·Zn(2) features a stoichiometrically controlled chirality inversion process, which is the sign of the Cotton effect of the 1:1 complex is opposite to that of the 1:2 complex. In contrast, porphyrin 1·Rh(2) affords 1:1 and 1:2 complexes in toluene solutions with the same sign for their CD couplets. Interestingly, in both solvents, the signs of the CD couplets associated with the 1:1 sandwich complexes of 1·Zn(2) and 1·Rh(2) are opposite. The amplitudes of the CD couplets are higher for 1·Zn(2) than for 1·Rh(2). This observation is in agreement with 1·Rh(2) having a smaller extinction coefficient than 1·Zn(2). We performed DFT-based calculations and assigned molecular structures to the 1:1 and 1:2 complexes that explain the observed signs for their CD couplets. Unexpectedly, the quantification of the thermodynamic stability of the two metallobisporphyrin/diamine 1:1 sandwich complexes revealed the existence of interplay between effective molarity values (EM) and the strength of the intermolecular interaction (K(m); N···Zn or N···Rh) used in their assembly. The EM for the N···Rh(III) intramolecular interaction is 3 orders of magnitude smaller than that for the N···Zn(II) interaction, both of which are embedded in the same scaffold of the 1·M(2) bisporphyrin receptor.     

Synthesis and X-ray structures of rhodium complexes with new chiral biaryl-based NHC-ligands

A new series of chiral NHC–rhodium complexes has been prepared from the reactions between [Rh(COD)Cl]₂, NaOAc, KI and dibenzimidazolium salt 4a or monobenzimidazolium salts 4b–d, which are derived from chiral 2,2′-diamino-6,6′-dimethyl-1,1′-biphenyl, 2,2′-diamino-1,1′-binaphthyl or 6,6′-dimethyl-2-amino-2′-hydroxy-1,1′-biphenyl. The steric and electronic effects of the ligand play an important role in the complex formation. For example, treatment of chiral monobenzimidazolium salt 4b (with a NMe₂ group) with 0.5 equiv of [Rh(COD)Cl]₂ in the presence of NaOAc and KI in CH₃CN at reflux gives a chiral Rh(I) complex 5b, while chiral monobenzimidazolium salt 4d (with a MeO group) affords a racemic Rh(I) complex 5d. Under similar reaction conditions, treatment of dibenzimidazolium salt 4a with 0.5 equiv of [Rh(COD)Cl]₂ in the presence of NaOAc and KI gives a racemic Rh(III) complex 5a, while the dibenzimidazolium salt [C₂₀H₁₂(C₇H₅N₂Me)₂]I₂ derived from chiral 2,2′-diamino-1,1′-binaphthyl affords a chiral Rh(III) complex [C₂₀H₁₂(C₇H₄N₂Me)₂]RhI₂(OAc). All compounds have been characterized by various spectroscopic techniques, and elemental analyses. The solid-state structures of the rhodium complexes have been further confirmed by X-ray diffraction analyses.     

Some catalytic properties of Rh(diphos)(η-BPh4)

The covalent complex Rh(diphos)(η-BPh₄) (I) reacts with CO in polar solvents to afford the cationic dicarbonyl cis-[Rh(diphos)(CO)₂](BPh₄). I is an effective catalyst for methylacetylene oligomerization and allene polymerization. In the presence of CO₂ and methylacetylene, I affords 4,6-dimethyl-2-pyrone.