Chiral Rhodium(I) and Iridium(I) Amino−Olefin Complexes: p<i>K</i>_a, N−H Bond Dissociation Energy, and Catalytic Transfer Hydrogenation

The chiral tetrachelating amino−olefins (R,R)-N,N‘-bis(5H-dibenzo[a,d]cyclohepten-5-yl)-1,2-diaminocyclohexane ((R,R)-trop2dach) and (S,S)-N,N‘-bis(5H-dibenzo[a,d]cyclohepten-5-yl)-1,2-diphenyl-1,2-ethylenediamine ((S,S)-trop2dpen) were prepared and used as ligands in the complexes (R,R)-[Rh(trop2dach)]OTf and (S,S)-[Rh(trop2dpen)]OTf (OTf- = CF3SO3-). Quasi-reversible reductions, d8-[RhI(trop2diamine)]+ + e- → d9-[Rh0(trop2diamine)] and d9-[Rh0(trop2diamine)] + e- → d10-[Rh-I(trop2diamine)]-, at rather negative potentials (trop2diamine = trop2dach, E1/21 = −1.83 V, E1/22 = −2.27 V; trop2diamine = trop2dpen, E1/21 = −1.78 V, E1/22 = −2.24 V; vs Fc+/Fc) indicate the donor capacity of the amine functions. One NH group in (R,R)-[Rh(trop2dach)]OTf (pKa = 15.7(2), NH bond dissociation energy (BDE) 317(2) kJ mol-1) is easily deprotonated to give the neutral amide (R,R)-[Rh(trop2dach-H)], which is quasi-reversibly oxidized at E° = −0.34 V (vs Fc/Fc+) to the radical cation (R,R)-[Rh(trop2dach-H)]•+. The pKa and BDE values of the NH group in these 16-electron complexes are lower than in related pentacoordinated 18-electron complexes. X-ray diffraction analyses show that the rhodium complexes have distorted-square-planar structures. The Rh−N bond shortens by about 7% upon deprotonation. The rhodium complexes are inactive as catalysts for transfer and direct hydrogenation of ketones. However, the distorted-trigonal-bipyramidal iridium complex (S,S)-[IrCl(CO)(trop2dpen)], where the amino−olefin ligand serves as a tridentate ligand, serves as a chiral precursor to an active phosphane-free catalyst in the transfer hydrogenation of acetophenone with 2-propanol, and the R isomer of 1-phenylethanol was obtained in 82% ee (>98% conversion when acetophenone:KOtBu:cat = 1:0.1:0.01, T = 80 °C, reaction time 1 h)

Chiral Olefins as Steering Ligands: Syntheses of <i>C</i>₁-Symmetric Dibenzo[<i>a</i>,<i>e</i>]cyclooctenes (^Rdbcot)

A simple three-step synthesis of chiral dibenzo[a,e]cyclooctenes (dbcot) starting from commercially available dibenzosuberenone was developed. These compounds give highly stable and robust rhodium(I) and iridium(I) diene complexes of the type [M(Rdbcot)(L1)(L2)] (M = Rh, Ir; L1, L2 = MeCN, Cl, diamine, chloride). The complex [Rh((R)-Phdbcot)((R)-(+)-1,1‘-binaphthyl-2,2‘-diamine)]+OTf- could be obtained in enantiomerically pure form and catalyzes the enantioselective 1,2-addition of PhB(OH)2 to α,β-unsaturated ketones with good activity and acceptable enantiomeric excess (62%). The iridium complex [Ir(Phdbcot)(MeCN)2]+OTf- catalyzes the hydrogenation of dimethylitaconate with good activity, while the rhodium complexes are almost inactive. Likewise, the complex [Ir(Phdbcot)(H2NCH2CH2NH2)]+OTf- serves as a rather efficient catalyst precursor with an activity 4 orders of magnitude higher than for the analogous rhodium complex. These experiments further establish the use of dienes as steering ligands in catalysis

Chiral Rhodium(I) and Iridium(I) Amino−Olefin Complexes: p<i>K</i>_a, N−H Bond Dissociation Energy, and Catalytic Transfer Hydrogenation

The chiral tetrachelating amino−olefins (R,R)-N,N‘-bis(5H-dibenzo[a,d]cyclohepten-5-yl)-1,2-diaminocyclohexane ((R,R)-trop2dach) and (S,S)-N,N‘-bis(5H-dibenzo[a,d]cyclohepten-5-yl)-1,2-diphenyl-1,2-ethylenediamine ((S,S)-trop2dpen) were prepared and used as ligands in the complexes (R,R)-[Rh(trop2dach)]OTf and (S,S)-[Rh(trop2dpen)]OTf (OTf- = CF3SO3-). Quasi-reversible reductions, d8-[RhI(trop2diamine)]+ + e- → d9-[Rh0(trop2diamine)] and d9-[Rh0(trop2diamine)] + e- → d10-[Rh-I(trop2diamine)]-, at rather negative potentials (trop2diamine = trop2dach, E1/21 = −1.83 V, E1/22 = −2.27 V; trop2diamine = trop2dpen, E1/21 = −1.78 V, E1/22 = −2.24 V; vs Fc+/Fc) indicate the donor capacity of the amine functions. One NH group in (R,R)-[Rh(trop2dach)]OTf (pKa = 15.7(2), NH bond dissociation energy (BDE) 317(2) kJ mol-1) is easily deprotonated to give the neutral amide (R,R)-[Rh(trop2dach-H)], which is quasi-reversibly oxidized at E° = −0.34 V (vs Fc/Fc+) to the radical cation (R,R)-[Rh(trop2dach-H)]•+. The pKa and BDE values of the NH group in these 16-electron complexes are lower than in related pentacoordinated 18-electron complexes. X-ray diffraction analyses show that the rhodium complexes have distorted-square-planar structures. The Rh−N bond shortens by about 7% upon deprotonation. The rhodium complexes are inactive as catalysts for transfer and direct hydrogenation of ketones. However, the distorted-trigonal-bipyramidal iridium complex (S,S)-[IrCl(CO)(trop2dpen)], where the amino−olefin ligand serves as a tridentate ligand, serves as a chiral precursor to an active phosphane-free catalyst in the transfer hydrogenation of acetophenone with 2-propanol, and the R isomer of 1-phenylethanol was obtained in 82% ee (>98% conversion when acetophenone:KOtBu:cat = 1:0.1:0.01, T = 80 °C, reaction time 1 h)

New Insight into Hydrogallation Reactions: Facile Synthesis of a Gallium-Bridged [3,3,3]-Cyclophane

Treatment of 1,3,5-tris(3,3-dimethyl-1-butynyl)benzene, C6H3(C⋮C−CMe3)3, with di(neopentyl)gallium hydride, HGa(CH2CMe3)2, resulted in the addition of one Ga−H bond to each C⋮C triple bond (hydrogallation). Spontaneous condensation by the release of tri(neopentyl)gallium yielded a [3,3,3]-cyclophane derivative (1) with three tricoordinated Ga atoms in bridging positions

Comparative Studies of Substitution Reactions of Rhenium(I) Dicarbonyl−Nitrosyl and Tricarbonyl Complexes in Aqueous Media

The ligand substitution behavior of [ReBr3(CO)3](NEt4)2 (1) and [ReBr3(CO)2(NO)]NEt4 (2) in aqueous media was compared. Ligand exchange reactions were performed with multidentate chelating systems such as picolylaminediacetic acid (L1; N,N‘,O,O‘), nitrilotriacetic acid (L2; N,O,O‘,O‘ ‘), iminodiacetic acid (L3; N,O,O‘), and bis(2-pyridyl)methane (L4; N,N‘). The products of the substitution reactions were isolated and characterized by means of IR, NMR, MS, and X-ray structure analysis. NMR and crystallographic analyses confirmed the formation of single structural isomers in all cases with a ligand-to-metal ratio of 1:1. With ligands L1 and L2 and precursor 1 the tridentately coordinated complexes [Re(L1)(CO)3] (7) and [Re(L2)(CO)3]2- (8) were formed. With precursor 2 the same ligands unexpectedly coordinated tetradentately after displacing a CO ligand, yielding complexes [Re(L1)(CO)(NO)] (3) and [Re(L2)(CO)(NO)]- (4). In both complexes NO was found to be coordinated trans to the carboxylate group. Time-dependent IR spectra of the reaction of 2 with ligand L1 and L2 confirmed the loss of one CO during the reaction. The product of the reaction of 2 with L3 was identified as the neutral complex [Re(L3)(CO)2(NO)] (5), again, with the nitrosyl coordinated trans to the carboxylate. With 1, ligand L3 formed the anionic complex [Re(L3)(CO)3]- (9). Finally the reactions with L4 yielded the complexes [ReBr(L4)(CO)2(NO)]Br (6) and [ReBr(L4)(CO)3] (10), in which bromide was found to be coordinated trans to the NO and CO, respectively. The X-ray structures of 3, 5−7, and 10 are discussed:  3, monoclinic P21/n, with a = 14.607(1) Å, b = 8.057(1) Å, c = 24.721(1) Å, β = 107.117(5)°, and Z = 4; 5, triclinic P1̄, with a = 6.909(1) Å, b = 9.882(1) Å, c = 14.283(1) Å, α = 89.246(9)°, β = 89.420(9)°, γ = 86.196(9)°, and Z = 4; 6, triclinic P1̄, with a = 9.823(1) Å, b = 10.094(1) Å, c = 12.534(1) Å, α = 108.679(9)°, β = 111.992(9)°, γ = 95.426(10)°, and Z = 2; 7, orthorhombic Pbca, with a = 14.567(1) Å, b = 13.145(1) Å, c = 14.865(1) Å, and Z = 8; 10, monoclinic P21/c, with a = 12.749(1) Å, b = 13.302(1) Å, c = 9.011(1) Å, β = 107.195(2)°, and Z = 4

Tuning the Gap: Electronic Properties and Radical-Type Reactivities of Heteronuclear [1.1.1]Propellanes of Heavier Group 14 Elements

Two heteronuclear [1.1.1]propellanes of group 14, Ge2Si3Mes6 (1) and Sn2Si3Mes6 (2) (Mes = 2,4,6-Me3C6H2), were prepared by reductive coupling of Mes2SiCl2 and GeCl2·dioxane or SnCl2. Both compounds were characterized in detail, including X-ray structure analyses on single crystals. In each case it was found that the E2Si3 cluster core consists of three bridging {SiMes2} units and two ligand-free bridgehead atoms (Eb). As a result of the different size of the bridging units, the distances between the bridgehead atoms are considerably shorter (0.10 Å for 1 and 0.27 Å for 2) than in the homonuclear counterparts Ge5Mes6 and Sn5Dep6 (Dep = 2,6-Et2C6H3) known from the literature. The stronger Eb···Eb interactions in 1 and 2 were confirmed by electrochemical studies using cyclic voltammetry. UV/vis studies, together with density functional theory (DFT) calculations, further supported these findings. A correlation of the Eb···Eb distances and the singlet and triplet A2 transitions for a series of homo- and heteronuclear [1.1.1]propellanes revealed that higher 3A2 excitation wavelengths, and thus lower ΔES→T energies, are obtained either by increasing the distances between the bridgehead atoms or by arranging the involved orbitals in close spatial proximity. Reactivity studies on 1 and 2 using selected reagents showed that Me3SnH or the disulfide FcS−SFc (Fc = ferrocenyl), which are prone to radical-type reactivity, can be readily added across the bridge (the tin hydride reacts only with 1). The resulting 1,3-disubstituted bicyclo[1.1.1]pentane derivatives Me3Sn−Ge(SiMes2)3Ge−H (3) and FcS−E(SiMes2)3E−SFc (4 (E = Ge) and 5 (E = Sn)) were characterized in detail, including X-ray structures of 4 and 5. Interestingly, the homolytic S−S bond addition reactions were found to be susceptible to light. Even though the tin-containing propellane 2 turned out to be more reactive than 1, both conversions can be drastically enhanced simply by using daylight in the lab

Catalytic Allylic Alkylation and Allylic Phenolation Reactions with Ruthenium Complexes. Solid-State Structures of a Model Catalytic DMF Intermediate, [Ru(Cp*)(Cl)(η³-C₃H₅)(DMF)](PF₆), and a New Tetranuclear Salt, [Ru(Cp){Ru(Cp)(η⁶-<i>p</i>-CH₃C₆H₄CN)}₃](PF₆)₄

Results from Ru-catalyzed (i) allylic alkylation reactions for linear and branched para-substituted aryl carbonates, p-R1C6H4CHCHCH2OCO2But and p-R1C6H4CH(OCO2But)-CHCH2, with dimethyl malonate and (ii) allylic phenolation reactions using C6H5CH(OCO2But)CHCH2 and phenol compounds are presented. The possible role of the π-arene complexes [Ru(Cp*)(η6-p-XC6H4CHCHCH2OCO2But)]PF6 is discussed. Solid-state structures for [Ru(Cp*)(Cl)(η3-C3H5)(DMF)](PF6) (12) and a new tetranuclear salt, [Ru(Cp){Ru(Cp)(η6-p-CH3C6H4CN)}3](PF6)4, based on toluinitrile, are presented. Analysis of the solid-state data for the model salt 12 provides a partial understanding with respect to how several factors, e.g., the choice of solvent and the nature of the reagents themselves, might affect the regioselectivity of these reactions

Comparative Studies of Substitution Reactions of Rhenium(I) Dicarbonyl−Nitrosyl and Tricarbonyl Complexes in Aqueous Media

The ligand substitution behavior of [ReBr3(CO)3](NEt4)2 (1) and [ReBr3(CO)2(NO)]NEt4 (2) in aqueous media was compared. Ligand exchange reactions were performed with multidentate chelating systems such as picolylaminediacetic acid (L1; N,N‘,O,O‘), nitrilotriacetic acid (L2; N,O,O‘,O‘ ‘), iminodiacetic acid (L3; N,O,O‘), and bis(2-pyridyl)methane (L4; N,N‘). The products of the substitution reactions were isolated and characterized by means of IR, NMR, MS, and X-ray structure analysis. NMR and crystallographic analyses confirmed the formation of single structural isomers in all cases with a ligand-to-metal ratio of 1:1. With ligands L1 and L2 and precursor 1 the tridentately coordinated complexes [Re(L1)(CO)3] (7) and [Re(L2)(CO)3]2- (8) were formed. With precursor 2 the same ligands unexpectedly coordinated tetradentately after displacing a CO ligand, yielding complexes [Re(L1)(CO)(NO)] (3) and [Re(L2)(CO)(NO)]- (4). In both complexes NO was found to be coordinated trans to the carboxylate group. Time-dependent IR spectra of the reaction of 2 with ligand L1 and L2 confirmed the loss of one CO during the reaction. The product of the reaction of 2 with L3 was identified as the neutral complex [Re(L3)(CO)2(NO)] (5), again, with the nitrosyl coordinated trans to the carboxylate. With 1, ligand L3 formed the anionic complex [Re(L3)(CO)3]- (9). Finally the reactions with L4 yielded the complexes [ReBr(L4)(CO)2(NO)]Br (6) and [ReBr(L4)(CO)3] (10), in which bromide was found to be coordinated trans to the NO and CO, respectively. The X-ray structures of 3, 5−7, and 10 are discussed:  3, monoclinic P21/n, with a = 14.607(1) Å, b = 8.057(1) Å, c = 24.721(1) Å, β = 107.117(5)°, and Z = 4; 5, triclinic P1̄, with a = 6.909(1) Å, b = 9.882(1) Å, c = 14.283(1) Å, α = 89.246(9)°, β = 89.420(9)°, γ = 86.196(9)°, and Z = 4; 6, triclinic P1̄, with a = 9.823(1) Å, b = 10.094(1) Å, c = 12.534(1) Å, α = 108.679(9)°, β = 111.992(9)°, γ = 95.426(10)°, and Z = 2; 7, orthorhombic Pbca, with a = 14.567(1) Å, b = 13.145(1) Å, c = 14.865(1) Å, and Z = 8; 10, monoclinic P21/c, with a = 12.749(1) Å, b = 13.302(1) Å, c = 9.011(1) Å, β = 107.195(2)°, and Z = 4

Radical-Type Reactivity of Metalla[1.1.1]propellanes of Group 14: Syntheses, Structures, and Reduction Chemistry of Transition Metal-Terminated Bicyclo[1.1.1]pentastannanes

Two selected metalla[1.1.1]propellanes of group 14, Sn5Dep6 (1) and Ge5Mes6 (2) (Dep = 2,6-Et2C6H3, Mes = 2,4,6-Me3C6H2), were reacted with [FeCp(CO)2]2 and [RuCp(CO)2]2. While 2 did not show any reaction with [FeCp(CO)2]2 or did not lead to any isolable product in the case of [RuCp(CO)2]2, the tin propellane 1 quantitatively (monitored by 1H NMR) afforded the first transition metal-terminated bicyclo[1.1.1]pentastannanes, [{MCp(CO)2}2{μ-Sn5Dep6}] (M = Fe (3), Ru = (4)), in 68% (3) and 66% (4) isolated yield. This behavior confirms the radical-type reactivity of metalla[1.1.1]propellanes, in this case 1. The title compounds 3 and 4 have been characterized in detail using various methods. X-ray structure analyses corroborated that the two ligand-free bridgehead tin atoms within the Sn5 scaffold are bonded to the transition metal fragments. Electrochemical studies revealed the reduction chemistry of 3 and 4 in THF solutions to be very interesting. A detailed preparative and electrochemical study, including the isolation and characterization of the monosubstituted cobaltocenium salt [CoCp*2]+[{FeCp(CO)2}{Sn5Dep6}]− (5), elucidated a complex redox cycle consisting of a cascade of bond-breaking and bond-making processes

Synthesis of a Pentasilapropellane. Exploring the Nature of a Stretched Silicon−Silicon Bond in a Nonclassical Molecule

We report on the successful synthesis of Si5Mes6 (Mes = 2,4,6-trimethylphenyl), which consists of an archetypal [1.1.1] cluster core featuring two ligand-free, “inverted tetrahedral” bridgehead silicon atoms. The separation between the bridgehead Si atoms is much longer, and the bond strength much weaker, than usually observed for a regular Si−Si single bond. A detailed analysis of the electronic characteristics of Si5Mes6 reveals a low-lying excited triplet state, indicative of some biradical(oid) character. Reactivity studies provide evidence for both closed-shell and radical-type reactivity, confirming the unusual nature of the stretched silicon−silicon bond in this “nonclassical” molecule