Nucleation of Antiferromagnetically Coupled Chromium Dihalides: from Small Clusters to the Solid State

The nucleation of chromium dihalide clusters is investigated by studying clusters of the form CrnX2n (n ≤ 4, X = F, Cl, Br, and I) for different spin states and the corresponding low temperature solid-state modifications using density functional theory. Using both wave function based (coupled cluster) and density functional theory, we predict that in all cases the ground state of the CrX2 monomer is a bent 5B2 state arising from a weakly Renner−Teller distorted 5Πg state of the linear CrX2 unit. These quintet units can form antiferromagnetically coupled, two-dimensional chains with chromium being bridged by two halides and a nucleation growth pattern that resembles the structural motif found for the solid state. Deviations from this two-dimensional chain growth are only found for the trimers and tetramers of CrBr2 and CrI2, where a “triangular” three-dimensional geometry takes slight precedence over the planar ribbon motif. We find that each single CrX2 unit adds an almost constant amount of energy between 45 and 50 kcal/mol to the cluster growth. This is in accordance with the calculated sublimation energies for the solid state which gave 58 kcal/mol for CrF2, and between 41 and 46 kcal/mol for CrCl2, CrBr2, and CrI2. The large deviation of the calculated from the experimental sublimation energy for CrF2 is due to the high electronegativity of fluorine ligand, which substantially increases the ionic interactions, resulting in a much more tightly packed solid-state structure, which is not so well described by spin-broken density functional theory. In accordance with this, CrF2 shows an unusually large bulk modulus (395 kbar) compared to the heavier halides CrCl2 (82 kbar), CrBr2 (40 kbar), and CrI2 (18 kbar)

Extending the Range of Neutral N-Donor Ligands Available for Metal Catalysts: <i>N</i>-[1-Alkylpyridin-4(1<i>H</i>)-ylidene]amides in Palladium-Catalyzed Cross-Coupling Reactions

<i>N</i>-[1-Alkylpyridin-4(1<i>H</i>)-ylidene]amides (PYAs) are a new class of easily prepared, neutral N-donor ligands that share some features in common with N-heterocyclic carbenes. They are strongly electron-donating toward metal centers, and a palladium(II) complex of one of these ligands has been shown to successfully catalyze both the Heck–Mizoroki and Suzuki–Miyaura cross-coupling reactions

Extending the Range of Neutral N-Donor Ligands Available for Metal Catalysts: <i>N</i>-[1-Alkylpyridin-4(1<i>H</i>)-ylidene]amides in Palladium-Catalyzed Cross-Coupling Reactions

<i>N</i>-[1-Alkylpyridin-4(1<i>H</i>)-ylidene]amides (PYAs) are a new class of easily prepared, neutral N-donor ligands that share some features in common with N-heterocyclic carbenes. They are strongly electron-donating toward metal centers, and a palladium(II) complex of one of these ligands has been shown to successfully catalyze both the Heck–Mizoroki and Suzuki–Miyaura cross-coupling reactions

Heptane Coordination to an Iron(II) Porphyrin

Heptane Coordination to an Iron(II) Porphyri

Supramolecular Fullerene-Porphyrin Chemistry. Fullerene Complexation by Metalated “Jaws Porphyrin” Hosts

Porphyrins and fullerenes are spontaneously attracted to each other. This new supramolecular recognition element is explored in discrete, soluble, coordinatively linked porphyrin and metalloporphyrin dimers. Jawlike clefts in these bis-porphyrins are effective hosts for fullerene guests. X-ray structures of the Cu complex with C60 and free-base complexes with C70 and a pyrrolidine-derivatized C60 have been obtained. The electron-rich 6:6 ring-juncture bonds of C60 show unusually close approach to the porphyrin or metalloporphyrin plane. Binding constants in toluene solution increase in the order Fe(II) < Pd(II) < Zn(II) -1. Unexpectedly, the free-base porphyrin binds C60 more strongly than the metalated porphyrins. This is ascribed to electrostatic forces, enhancing the largely van der Waals forces of the π−π interaction. The ordering with metals is ascribed to a subtle interplay of solvation and weak interaction forces. Conflicting opinions on the relative importance of van der Waals forces, charge transfer, electrostatic attraction, and coordinate bonding are addressed. The supramolecular design principles arising from these studies have potential applications in the preparation of photophysical devices, molecular magnets, molecular conductors, and porous metal-organic frameworks

Tapes, Sheets, and Prisms. Identification of the Weak C−F Interactions that Steer Fullerene−Porphyrin Cocrystallization

Tetra(pentafluorophenyl)porphyrin (H2TPFPP) has been cocrystallized with C60 from arene solvents to give H2TPFPP·C60, 1, H2TPFPP·C60·8Benzene, 3, and 3H2TPFPP·2C60·6Toluene, 4. Their X-ray structures have been determined to identify the supramolecular interactions that lead to tape, sheet, and prismatic packing motifs. In addition to close fullerene−porphyrin π−π interactions, attractive C−F···H−C interactions are important in connecting the C6F5 groups of one porphyrin to the pyrrole positions of a neighbor. This interaction is also seen in the structure of the free-base porphyrin H2TPFPP·3p-Xylene, 2

Tapes, Sheets, and Prisms. Identification of the Weak C−F Interactions that Steer Fullerene−Porphyrin Cocrystallization

Tetra(pentafluorophenyl)porphyrin (H2TPFPP) has been cocrystallized with C60 from arene solvents to give H2TPFPP·C60, 1, H2TPFPP·C60·8Benzene, 3, and 3H2TPFPP·2C60·6Toluene, 4. Their X-ray structures have been determined to identify the supramolecular interactions that lead to tape, sheet, and prismatic packing motifs. In addition to close fullerene−porphyrin π−π interactions, attractive C−F···H−C interactions are important in connecting the C6F5 groups of one porphyrin to the pyrrole positions of a neighbor. This interaction is also seen in the structure of the free-base porphyrin H2TPFPP·3p-Xylene, 2

Atom Transfer Reactions of (TTP)Ti(η²-3-hexyne): Synthesis and Molecular Structure of <i>trans</i>-(TTP)Ti[OP(Oct)₃]₂

Atom and group transfer reactions were found to occur between heterocumulenes and (TTP)Ti(η2-3-hexyne), 1 (TTP = meso-5,10,15,20-tetra-p-tolylporphyrinato dianion). The imido derivatives (TTP)TiNR (R = iPr, 2; tBu, 3) were produced upon treatment of complex 1 with iPrNCNiPr, iPrNCO, or tBuNCO. Reactions between complex 1 and CS2, tBuNCS, or tBuNCSe afforded the chalcogenido complexes, (TTP)TiCh (Ch = Se, 4; S, 5). Treatment of complex 1 with 2 equiv of PEt3 yielded the bis(phosphine) complex, (TTP)Ti(PEt3)2, 6. Although (TTP)Ti(η2-3-hexyne) readily abstracts oxygen from epoxides and sulfoxides, the reaction between 1 and OP(Oct)3 did not result in oxygen atom transfer. Instead, the paramagnetic titanium(II) derivative (TTP)Ti[OP(Oct)3]2, 7, was formed. The molecular structure of complex 7 was determined by single-crystal X-ray diffraction:  Ti−O distance 2.080(2) Å and Ti−O−P angle of 138.43(10)°. Estimates of TiO, TiS, TiSe, and TiNR bond strengths are discussed

Calix[4]arene-Linked Bisporphyrin Hosts for Fullerenes: Binding Strength, Solvation Effects, and Porphyrin−Fullerene Charge Transfer Bands

A calix[4]arene scaffolding has been used to construct bisporphyrin (“jaws” porphyrin) hosts for supramolecular binding of fullerene guests. Fullerene affinities were optimized by varying the nature of the covalent linkage of the porphyrins to the calixarenes. Binding constants for C60 and C70 in toluene were explored as a function of substituents at the periphery of the porphyrin, and 3,5-di-tert-butylphenyl groups gave rise to the highest fullerene affinities (26 000 M-1 for C60). The origin of this high fullerene affinity has been traced to differential solvation effects rather than to electronic effects. Studies of binding constants as a function of solvent (toluene < benzonitrile < dichloromethane ≪ cyclohexane) correlate inversely with fullerene solubility, indicating that desolvation of the fullerene is a major factor determining the magnitude of binding constants. The energetics of fullerene binding have been determined in terms of ΔH and ΔS and are consistent with an enthalpy-driven, solvation-dependent process. A direct relationship between supramolecular binding of a fullerene guest to a bisporphyrin host and the appearance of a broad NIR absorption band have been established. The energy of this band moves in a predictable manner as a function of the electronic structure of the porphyrin, thereby establishing its origin in porphyrin-to-fullerene charge transfer

Tapes, Sheets, and Prisms. Identification of the Weak C−F Interactions that Steer Fullerene−Porphyrin Cocrystallization

Tetra(pentafluorophenyl)porphyrin (H2TPFPP) has been cocrystallized with C60 from arene solvents to give H2TPFPP·C60, 1, H2TPFPP·C60·8Benzene, 3, and 3H2TPFPP·2C60·6Toluene, 4. Their X-ray structures have been determined to identify the supramolecular interactions that lead to tape, sheet, and prismatic packing motifs. In addition to close fullerene−porphyrin π−π interactions, attractive C−F···H−C interactions are important in connecting the C6F5 groups of one porphyrin to the pyrrole positions of a neighbor. This interaction is also seen in the structure of the free-base porphyrin H2TPFPP·3p-Xylene, 2