Rac Diastereomers Research Articles

Synthesis and coordination chemistry of new asymmetric donor/acceptor pincer ligands, 1,3-C6H4(CH2PtBu(Rf))2 (Rf = CF3, C2F5).

Syntheses of new asymmetric pincer precursors 1,3-C6H4{CH2P(tBu,X)}2 (tBu,XPCPH; X = Cl, SiMe3, OPh) and a new class of hybrid donor/acceptor pincer ligands 1,3-C6H4{CH2P(tBu,Rf)}2 (tBu,RfPCPH; Rf = CF3, C2F5) are reported. All tBu,XPCPH compounds are obtained as mixtures of meso and rac diastereomers in varying ratios (meso : rac ∼ 4 : 1 to 3 : 2) which were used without separation. Treatment of Ru(cot)(cod) with tBu,CF3PCPH under 1 atm H2 in acetone at 20 °C produced the hydride solvate (tBu,CF3PCP)Ru(acetone)xH which was not isolated, but could be trapped as stable diene complexes (tBu,CF3PCP)Ru(L)2H (L2 = cod (1[combining low line]), nbd (2[combining low line])). Catalytic cyclooctane dehydrogenation studies demonstrate that 2[combining low line] has ∼50% the activity of (CF3PCP)Ru(cod)(H), but significantly higher catalyst stability and is able to operate at higher catalyst loading concentrations without deactivation via bimolecular decomposition.

Synthesis, Diastereomer Separation, and Optoelectronic and Structural Properties of Dinuclear Cyclometalated Iridium(III) Complexes with Bridging Diarylhydrazide Ligands

A series of diiridium complexes 13–16 bridged by diarylhydrazine ligands and cyclometalated by phenylpyridine or phenylpyrazole ligands was synthesized. In all cases the ΛΔ meso and ΛΛ/ΔΔ rac diastereomers were separated and characterized by single-crystal X-ray diffraction, revealing intramolecular π–π stacking between arenes of the bridging and cyclometalating ligands. Density functional theory (DFT) calculations show that in general the HOMOs are mainly localized on the iridium centers, the cyclometalating phenyl moieties, and the central hydrazide components of the bridging ligands, while the LUMOs are primarily localized on the N-heterocycles (pyridine or pyrazole) of the cyclometalating ligands. This series of complexes, especially with the separated diastereomers, provides an ideal opportunity to study the effects of subtle structural changes on the optoelectronic properties of diiridium systems: significant differences are observed between the rac and meso isomers in some cases. A cyclic voltammet...

Metal-Metal Bridging Using the DPPP Dye System: Electronic Configurations within Multiple Redox Series.

Redox series [LnRu(μ-DPPP)RuLn]k, H2DPPP = 2,5-dihydro-3,6-di-2-pyridylpyrrolo(3,4-c)pyrrole-1,4-dione and L = 2,4-pentanedionato (acac-), 2,2'-bipyridine (bpy), and 2-phenylazopyridine (pap), have been studied by voltammetry (CV, DPV), EPR, and UV-vis-NIR spectroelectrochemistry, supported by TD-DFT calculations. Crystal structure analysis and 1H NMR revealed oxidation states [(acac)2RuIII(μ-DPPP2-)RuIII(acac)2] and [(bpy)2RuII(μ-DPPP2-)RuII(bpy)2]2+ for the corresponding precursors, isolated as rac diastereomers. Oxidation was observed to occur mainly at the bridging ligand (DPPP2- → DPPP•-), whereas the site of reduction (DPPP, Ru, or L) depends on effects from the ancillary ligands L. The metal coordination of a derivative of the pigment forming 2,5-dihydro-pyrrolo(3,4-c)pyrrole-1,4-dione (DPP) dyes and the analysis of corresponding multistep redox series add to the previously recognized coordinative and electron transfer potential of dye molecules of the azo, indigo, anthraquinone, and formazanate type.

Isomeric Diruthenium Complexes of a Heterocyclic and Quinonoid Bridging Ligand: Valence and Spin Alternatives for the Metal/Ligand/Metal Arrangement.

5,7,12,14-Tetraazapentacene-6,13-quinone (L) reacts with 2 equiv of [Ru(acac)2(CH3CN)2] to form two linkage isomeric bis(chelate) compounds, [{RuII(acac)2}2(μ-L)], blue 1, with 5,6;12,13 coordination and violet 2 with 5,6;13,14 coordination. The linkage isomers could be separated, structurally characterized in crystals as rac diastereomers (ΔΔ/ΛΛ), and studied by voltammetry (CV, DPV), EPR, and UV-vis-NIR spectroelectrochemistry (meso-1, rac-2). DFT and TD-DFT calculations support the structural and spectroscopic results and suggest a slight energy preference (ΔE = 263 cm-1) for the rac-isomer 1 as compared to 2. Starting from the RuII-(μ-L0)-RuII configurations of 1 and 2 with low-lying metal-to-ligand charge transfer (MLCT) absorptions, the compounds undergo two reversible one-electron oxidation steps with open-shell intermediates 1+ (Kc = 4 × 104) and 2+ (Kc = 6 × 105). Both monocations display metal-centered spin according to EPR, but the DFT-calculated spin densities suggest a RuIII(μ-L•-)RuIII three-spin situation with opposite spin density at the bridging ligand for the meso form of 1+, estimated to lie 1887 cm-1 lower in energy than rac-1+, which is calculated with a Class II mixed-valent situation RuIII-(μ-L0)-RuII. A three-spin arrangement RuIII-(μ-L•-)-RuIII with negative spin density at one metal site is suggested by DFT for rac-2+ which is more stable by ΔE = 890 cm-1 than rac-1+. Reduction of 1 or 2 (Kc = 107-108) occurs mainly at the central bridging ligand with notable contributions (30%) from the metals in 1- and 2-. The mixed-valent RuIII(μ-L)RuII versus radical-bridged RuIII(μ-L•-)RuIII alternative is discussed comprehensively in comparison with related valence-ambiguous cases.

1,2-Bis(2,6-diisopropylphenyl)diphosphane: a structural, spectroscopic, and computational investigation

The title compound Dipp(H)P–P(H)Dipp where Dipp = 2,6-diisopropylyphenyl has been prepared starting from DippPH2 via DippPHLi reacting 2:1 with BrCH2CH2Br. A high-resolution single-crystal X-ray diffraction study at 153 ± 2 K provides an accurate description of the structure in the solid state, only the fourth dihydro diorganophosphane to have been so characterized. The Raman spectrum of the solid shows an intense, broad peak for P–H stretching at 2314 cm−1 and two peaks at 460 and 450 cm−1 for P–P stretching, assignments confirmed by RHF/6-31G+(3d,2p) frequency calculations undertaken on the anti conformers. 31P, 31P{1H }, 1H{31P}, and 13C{1H } solution NMR spectra in CDCl3 are consistent with the presence of 58% meso and 42% rac diastereomers. Analysis of their AA′XX′ spin systems enabled calculation of 1JPP′ values of (–)189 and (–)205 Hz, respectively. A B3LYP/6-31G(d,p) computational analysis of the energetics of rotation about the P–P bond in both diastereomers shows that the anti and two gauche conformers are likely to be populated approximately equally in each. The resulting average ϕ angles (torsions of the phosphorus lone pairs) are 110° for meso and 103° for rac; the smaller ϕ angle fits with the larger 1JPP′ value for the rac isomer.

Palladium(II) complexes supported by a bidentate bis(secondary)phosphine linked by pyridine

A series of complexes of the type (PNP-H2)PdX2 (X=Cl, Br, I) have been synthesized, where PNP-H2 is a bis(secondary)phosphine ligand linked by a pyridine, 2,6-(2′-(Ph(H)P)(C6H4))2(C5H3N). Due to chirality at phosphorus, the parent ligand exists as a mixture of nearly equivalent rac and meso diastereomers non-interconverting at room temperature. When ligated to Pd(II) halides, however, the diastereomeric ratio is dependent upon the halide. The chloro, bromo, and iodo complexes have been characterized crystallographically. Conformationally similar meso diastereomers of each dihalide are roughly Cs symmetric in the solid state, while the rac diastereomers (identified only for X=Br, I) show substantially different solid-state conformations.

From Functional Phospholide Ions to Bifunctional 1,1′-Diphosphaferrocenes

Both the 2-ethoxycarbonyl and the 2-benzoyl phospholides (1 and 2) react with FeCl2 in the presence of ZnCl2 to give the corresponding 2,2′-difunctional 1,1′-diphosphaferrocenes as mixtures of meso and rac diastereomers. Stable rotamers are also detected by 1H and 13C NMR spectroscopy. Under the same conditions, the 2,5-bis(ethoxycarbonyl)phospholide (3) gives the tetrafunctional 1,1′-biphospholyl (6). The diphosphaferrocenes and the 1,1′-biphospholyl are probably formed from a common type of intermediates with a bis(η1-phospholyl)iron structure. DFT computations indicate that this kind of intermediate is pyramidal at P and only weakly aromatic.

The Linear Tetraphos Ligand prP4 (1,1,4,8,11,11-Hexaphenyl-1,4,8,11-tetraphosphaundecane, Ph2PCH2CH2P(Ph)CH2CH2CH2P(Ph)CH2CH2PPh2): Synthesis and Characterization of cis- and trans-[Fe(NCS)2(prP4)]†

The stereochemistry of prP4 (1,1,4,8,11,11-hexaphenyl-1,4,8,11-tetraphosphaundecane) and its coordination properties in mononuclear complexes are explored. The free ligand is obtained as a mixture of meso and rac diastereomers from the reaction of dilithium 1,3-bis(phenylphosphido)propane−tetrakis(tetrahydrofuran) (“Lippp”) with 2 equiv of 1-chloro-2-(diphenylphosphino)ethane. Reaction of prP4 with FeCl2 and KSCN leads to cis- and trans-[Fe(NCS)2(prP4)] complexes of the meso and the rac ligands; the composition of this mixture has been fully analyzed. The iron isothiocyanato complexes cis-α- and trans-[Fe(NCS)2(rac-prP4)] have been characterized by X-ray structure determinations. The 31P NMR spectrum of the free tetraphos ligand shows an AA‘XX‘ coupling scheme with three groups of signals centered at −20.6 and −20.7 ppm and at −12.4 ppm, which belong to the rac and meso isomers, respectively. 31P NMR spectra of the cis and trans iron isothiocyanato complexes of rac-prP4 exhibit AA‘XX‘ patterns as well, which can be reproduced by simulation. trans-[Fe(NCS)2(meso-prP4)] is identified by its 31P NMR spectrum, which is similar to that of its rac counterpart. On the basis of NMR spectroscopy it is found that the cis-α complex of the rac ligand converts into the trans-rac complex with a half-life of approximately 2 days. Infrared and Raman spectra of the cis-α and trans isomers of [Fe(NCS)2(rac-prP4)] reveal characteristic differences in the region of the C−N stretching vibrations.

Mono- and Bimetalated Rhenium and Manganese Carbonyl Derivatives of 1,2-Bis(indenyl)ethane

Heating a mixture of M2(CO)10 (M = Re, Mn) with 1,2-bis(indenyl)ethane in Decalin at ca. 180 °C provided both monometalated M(CO)3{(η5-C9H6)C2H4C9H7)} (1, M = Re; 3, M = Mn) and bimetalated (M(CO)3)2{μ-(η5-C9H6)C2H4(η5-C9H6)} (2, M = Re; 4, M = Mn) products, which were separated by chromatography, isolated as pure solids, and characterized by a combination of analytical and spectroscopic methods. The bimetalated compounds 2 and 4 were formed as a mixture of meso and rac diastereomers; the rac form of 2 crystallized selectively, and its structure was determined by X-ray crystallography. UV photolysis of 1 and 3 (or 4) yielded the compounds M(CO)2{(η5-C9H6)C2H4(η2-C9H7)} (5, M = Re; 6, M = Mn), in which a carbonyl ligand is replaced by the η2 coordination of the pendant indene group. UV photolysis of 2 gave primarily the rearranged isomer Re2(CO)6{μ-(η1:η2-C9H6)C2H4(η5-C9H6)} (7), in which one indenyl group bridges the two Re centers as an η1:η2-vinyl moiety. The molecular structures of compounds 6 and 7 al...

Solution- and solid-state NMR study of intermediate η3-allyl-cobalt tricarbonyl type complexes in 3-methyl-1,2-butadiene polymerization

Abstract 1H- and 13C-NMR of η3-allyl-cobalt tricarbonyl complexes {Co(CO)3}2{(η3-Me2CCCH2)2CO} (1), {Co(CO)3}2{(η3-Me2CCCH2)3CO} (2) and {Co(CO)3}2{(η3-Me2CCCH2)4CO} (3) were studied. The X-ray structures obtained for 1 and 2 suggested that the 1,2-polymer chain would grow in a helical manner. Due to the coordination of the Co(CO)3 units to the chain-closing prochiral dimethylallylic planes and/or to the hindered rotation of the dimethylallylic units conjugated to some extent with the ketonic carbonyl several diastereomers and even more rotamers can be envisaged. In solution, however, only two symmetric forms in different ratio could be detected in all cases studied so far. We have undertaken a detailed NMR study on these compounds both in solution- and solid-state, in order to assign the resonances unambiguously and understand clearly the nature of the isomers present in solutions. The results obtained suggest the presence of meso and rac diastereomers due to the coordination of the Co(CO)3 units to the prochiral allyl planes. These dimethylallylic groups rotate relatively free around the ketonic carbonyl.