A combined kinetico-mechanistic and computational study on the competitive formation of seven- versus five-membered platinacycles; the relevance of spectator halide ligands

The metalation reactions between [Pt2(4-MeC6H4)4(μ-SEt2)2] and 2-X,6-FC6H3CHvNCH2CH2NMe2 (X = Br, Cl) have been studied. In all cases, seven-membered platinacycles are formed in a process that involves an initial reductive elimination from cyclometallated PtIV intermediate compounds, [PtX(4-CH3C6H4)2(ArCHvNCH2CH2NMe2)] (X = Br, Cl), followed by isomerization of the resulting PtII complexes and a final cyclometallation step. For the process with X = Br, the final seven-membered platinacycle and two intermediates, isolated under the conditions implemented from parallel kinetic studies, have been characterized by XRD. Contrary to previous results for the parent non-fluorinated imine 2-BrC6H4CHvNCH2CH2NMe2 the presence of a fluoro substituent prevents the formation of the more stable five-membered platinacycle. Temperature and pressure dependent kinetico-mechanistic and DFT studies indicate that the final cyclometallation step is strongly influenced by the nature of the spectator halido ligand, the overall reaction being much faster for X = Cl. The same DFT study conducted on the previously studied systems with imine 2-BrC6H4CHvNCH2CH2NMe2 indicates that, when possible, fivemembered platinacycles are kinetically preferred for X = Br, while the presence of Cl as a spectator halido ligand leads to a preferential faster formation of seven-membered analogues

Magnetic behavior of heterometallic wheels having a [MnIV6M2O9]10+ core with M = Ca2+ and Sr2+

Two new heterometallic MnIV−M2+ compounds with formula [Mn6M2O9(4-tBuC6H4COO)10(4-tBuC6H4COOH)5] (M = Ca2+ (1), Sr2+ (2)) have been crystallized. The core of both compounds consists of a planar Mn6 ring, where the MnIV ions are alternatively bridged by (μ3-O)2(μ-RCOO) and (μ4-O)(μ-RCOO)2 ligands, and the two alkaline earth ions are located to both sides of the wheel, linked to the oxo bridges, generating three fused [Mn2M2O4]4+ cuboids. These compounds show a net antiferromagnetic behavior, more important for 2 (Sr2+) than for 1 (Ca2+). The fitting of the experimental data was performed with the support of DFT calculations, considering four different exchange pathways: two between adjacent MnIV ions (J1 and J2) and two between nonadjacent MnIV ions (J3 and J4). The results of the analysis show that J1 and J2 are of the opposite sign, the ferromagnetic contribution corresponding to the [Mn2(μ4- O)(μ-RCOO)2]4+ unit (J2). The influence of the M2+ ions in the magnetic behavior is analyzed for 1 and 2 and for three hypothetical models with the structural parameters of 1 containing Mg2+, Sr2+ or without the M2+ ions. In spite of the diamagnetic character of the alkaline earth ions, their influence on the magnetic behavior has been evidenced and correlated with their polarizing effect. Moreover, the magnetic interactions between nonadjacent ions are non-negligible

Diarylplatinum(II) Scaffolds for Kinetic and Mechanistic Studies on the Formation of Platinacycles via an Oxidative Addition/Reductive Elimination/Oxidative Addition Sequence

Oxidative addition and reductive elimination reactions are fundamental steps in processes related to synthetic chemistry involving organometallic compounds. In these reactions a metal in two available oxidation states (generally differing in two units) is needed, platinum centers being a very good example. The relative inertness of diamagnetic PtII and PtIV organometallic species (having, respectively, d8 square-planar or d6 octahedral arrangements), enables an easy monitoring of time-resolved reactivity, including its posterior kinetic analysis. Specifically, imine ligands containing C-X bonds have been observed to oxidatively add to {PtII(Aryl)2} moieties, which sequentially undergo C-C reductive elimination and C-H bond activation on the new ligand formed. These new species have been found to contain mostly seven-membered metallacycles, despite the obvious thermodynamic preference for five-membered cycles, which are found only in some rather specific instances. The kinetic preference of the complexes obtained has been studied from a kinetico-mechanistic perspective, that included obtaining thermal- and pressure-derived activation parameters, and a dramatic influence on the spectator halido ligands, and the substituents on the aryl groups has been established. To complete this kinetic and mechanism (kinetico-mechanistic) study, theoretical calculations have also been conducted to model the data collected and propose both the elementary steps and the factors determining the specificity of the full process

Intra- vs Intermolecular Aurophilic Contacts in Dinuclear Gold(I) Compounds: Impact on the Population of the Triplet Excited State

Two series of dinuclear gold(I) complexes that contain two Au–chromophore units (chromophore = dibenzofurane or dimethylfluorene) connected through a diphosphane bridge that differs in the flexibility and length (diphosphane = dppb for 1,4-bis(diphenylphosphino)butane, DPEphos for bis[(2-diphenylphosphino)phenyl]ether, xanthphos for 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene, and BiPheP for 2,2′-bis(diphenylphosphino)-1,1′-biphenyl) have been synthesized and structurally characterized. Their photophysical properties have been carefully investigated, paying attention to the role of the presence, or absence, of aurophilic contacts and their nature (intra- or intermolecular character). This analysis was permitted due to the X-ray crystallographic determination of all of the structures of the compounds discussed herein. The quantum yields of the triplet population, ϕT, have been calculated by nanosecond-laser flash photolysis measurements, and we could determine the main role of the character of the aurophilic contacts in the resulting ϕT, being especially favored in the presence of intermolecular contacts. Time-dependent density functional theory (TD-DFT) calculations support the absorption and emission assignments and the shorter distance between S1 and the closest triplet excited state energy in the case of the compounds with a higher triplet-state population

Direct and Asymmetric Aldol Reactions of N-Azidoacetyl-1,3-thiazolidine-2-thione Catalyzed by Chiral Nickel(II) Complexes. A New Approach to the Synthesis of -Hydroxy--Amino Acids

A direct and asymmetric triisopropylsilyltrifluoromethanesulfonate (TIPSOTf) mediated aldol reaction of N-azidoacetyl-1,3-thiazolidine-2-thione with aromatic aldehydes catalyzed by a chiral nickel(II)-Tol-BINAP complex has been developed (BINAP=2,2'-bis(diphenylphosphino)-1,1'-binaphthyl). The catalytic protocol gives the corresponding anti α-azido-β-silyloxy adducts with outstanding stereocontrol and in high yields. Theoretical calculations account for the stereochemical outcome of the reaction and lay the foundations for a mechanistic model. In turn, the easy removal of the thiazolidinethione yields a wide array of enantiomerically pure derivatives in a straightforward and efficient manner. Such a noteworthy character of the heterocyclic scaffold together with the appropriate manipulation of the azido group open a new route to the synthesis of di- and tripeptide blocks containing a β-aryl-β-hydroxy-α-amino acid

Direct and asymmetric nickel(II)-catalyzed construction of carbon−carbon bonds from N‑acyl thiazinanethiones

A wide array of new N-acyl thiazinanethiones are employed in a number of direct and enantioselective carbon-carbon bond forming reactions catalyzed by nickel(II) complexes. The electrophilic species are mostly prepared in situ from ortho esters, methyl ethers, acetals, and ketals, which makes the overall process highly efficient and experimentally straightforward. Theoretical calculations indicate that the reactions pro-ceed through an open transition state in a SN1-like mechanism. The utility of this novel procedure has been demonstrated by the asymmetric preparation of syntheti-cally useful intermediates and the total synthesis of pe-peromin D

Synthesis, characterization, solution behavior and theoretical studies of Pd(II) Allyl Complexes with 2-Phenyl-3H-indoles as ligands

The study of the reactivity of three 2-phenyl-3H-indole ligands of general formulae C8H3N-2-(C6H4-4-R1)-3-NOMe-5-R2 (1) [with R1 = H, R2 = OMe (a); R1 = R2 = H (b) or R1 = Cl, R2 = H (c)] with [Pd(η3-1-R3C3H4)(μ-Cl)]2 (R3 = H or Ph) has allowed us to isolate two sets of new Pd(II)-allyl complexes of general formulae [Pd(η3-1-R3C3H4)(1)Cl] {R3 = H (2) or Ph (3)}. Compounds 2a-2c and 3a-3c were characterized by elemental analyses, mass spectrometry and IR spectroscopy. The crystal structures of 2a, 3a and 3b were also determined by X-ray diffraction. 1H-NMR studies reveal the coexistence of two (for 2a-2c) or three (for 3a-3c) isomeric forms in CD2Cl2 solutions at 182 K. Additional studies on the catalytic activity of mixtures containing [Pd(η3-C3H5)(μ-Cl)]2 and the parent ligand (1a-1c) in the allylic alkylation of (E)-3-phenyl-2-propenyl (cinnamyl) acetate with sodium diethyl 2-methylmalonate as well as the stoichiometric reaction between compounds 3a and 3c with the nucleophile reveal that in both cases the formation of the linear trans- derivative is strongly preferred over the branched product. Computational studies at a DFT level on compound 3a allowed us to compare the relative stability of their isomeric forms present in solution and to explain the regioselectivity of the catalytic and stoichiometric processes

Oxidative Cleavage of Cellobiose by Lytic Polysaccharide Monooxygenase (LPMO)-Inspired Copper Complexes

Correction published on October 23, 2020 https://doi.org/10.1021/acsomega.0c04910The potentially tridentate ligand bis[(1-methyl-2-benzimidazolyl)ethyl]amine (2BB) was employed to prepare copper complexes [(2BB)CuI]OTf and [(2BB)CuII(H2O)2](OTf)2 as bioinspired models of lytic polysaccharide copper-dependent monooxygenase (LPMO) enzymes. Solid-state characterization of [(2BB)CuI]OTf revealed a Cu(I) center with a T-shaped coordination environment and metric parameters in the range of those observed in reduced LPMOs. Solution characterization of [(2BB)CuII(H2O)2](OTf)2 indicates that [(2BB)CuII(H2O)2]2+ is the main species from pH 4 to 7.5; above pH 7.5, the hydroxo-bridged species [{(2BB)CuII(H2O)x}2(μ-OH)2]2+ is also present, on the basis of cyclic voltammetry and mass spectrometry. These observations imply that deprotonation of the central amine of Cu(II)-coordinated 2BB is precluded, and by extension, amine deprotonation in the histidine brace of LPMOs appears unlikely at neutral pH. The complexes [(2BB)CuI]OTf and [(2BB)CuII(H2O)2](OTf)2 act as precursors for the oxidative degradation of cellobiose as a cellulose model substrate. Spectroscopic and reactivity studies indicate that a dicopper(II) side-on peroxide complex generated from [(2BB)CuI]OTf/O2 or [(2BB)CuII(H2O)2](OTf)2/H2O2/NEt3 oxidizes cellobiose both in acetonitrile and aqueous phosphate buffer solutions, as evidenced from product analysis by high-performance liquid chromatography-mass spectrometry. The mixture of [(2BB)CuII(H2O)2](OTf)2/H2O2/NEt3 results in more extensive cellobiose degradation. Likewise, the use of both [(2BB)CuI]OTf and [(2BB)CuII(H2O)2](OTf)2 with KO2 afforded cellobiose oxidation products. In all cases, a common Cu(II) complex formulated as [(2BB)CuII(OH)(H2O)]+ was detected by mass spectrometry as the final form of the complex

Direct and enantioselective aldol reactions catalyzed by chiral nickel(II) complexes

A direct and asymmetric aldol reaction of N-acylthiazinanethiones with aromatic aldehydes catalyzed by chiralnickel(II) complexes is reported. The reaction gives thecorresponding O-TIPS-protected anti-aldol adducts in highyields and with remarkable stereocontrol and atom economy.Furthermore, the straightforward removal of the achiralscaffold provides enantiomerically pure intermediates ofsynthetic interest, which involve precursors for anti-a-amino-b-hydroxy anda,b-dihydroxy carboxylic derivatives. Theoret-ical calculations explain the observed high stereocontrol

Synthesis, characterization, crystal structures and computational studies on novel cyrhetrenyl hydrazones

The synthesis of novel cyrhetrenyl hydrazones of general formula [Re{(η5-C5H4)single bondC(R1) = NNHR2}(CO)3] {with R1 = H and R2 = 4-NO2single bondC6H4 (4a), C6H5 (4b) or H (4c) or R1 = Me and R2 = 4-NO2single bondC6H4 (5a), C6H5 (5b) or H (5c)} is described. Compounds 4a-4c and 5a-5c were characterized by mass spectrometry and IR spectroscopy. 1H and 13C{1H} NMR studies revealed that 4a-4c and 5a-5c adopt the anti-(E) configuration in solution. X-ray crystal structures of compounds 4a and 5c confirmed the trans-arrangement of the cyrhetrenyl 'Re(η5-C5H4)(CO)3' and the -NHR2 moieties and the existence of strong hydrogen bonds involving the single bondNHsingle bond unit. Molecular Orbital calculations at a DFT level have also been carried out in order to rationalize the influence of the nature of the substituent R3 of [R3CH = NNH(4-NO2single bondC6H4)] (R3 = ferrocenyl, (3a), cyrhetrenyl (4a), phenyl (6a) or cymantrenyl (7a) on the electronic delocalization, the nucleophilicity of the imine carbon, the polarizability and hyperpolarizability of these compounds, and computational studies using time-dependent density functional (TD-DFT) calculations have also been carried out in order to assign the bands detected in their electronic spectra and to explain the effect produced by the solvent