Brian S. Dolinar

Heterogeneous H-Bonding in a Foldamer Helix

Structural characterization of new α/γ-peptide foldamers containing the cyclically constrained γ-amino acid I is described. Crystallographic and 2D NMR analysis shows that γ residue I promotes the formation of a 12/10-helical secondary structure in α/γ-peptides. This helix contains two different types of internal H-bond, and the data show that the 12-atom C═O(i) → H-N(i+3) H-bond is more favorable than the 10-atom C═O(i) → H-N(i-1) H-bond. Several foldamer helices featuring topologically distinct H-bonds have been discovered, but our findings are the first to show that such H-bonds may differ in their favorability.

Structurally Diverse Diazafluorene-Ligated Palladium(II) Complexes and Their Implications for Aerobic Oxidation Reactions

4,5-Diazafluoren-9-one (DAF) has been identified as a highly effective ligand in a number of Pd-catalyzed oxidation reactions, but the mechanistic basis for its utility has not been elucidated. Here, we present the complex coordination chemistry of DAF and palladium(II) carboxylate salts. Multiple complexes among an equilibrating mixture of species have been characterized by (1)H and (15)N NMR spectroscopy and X-ray crystallography. These complexes include monomeric and dimeric Pd(II) species, with monodentate (κ(1)), bidentate (κ(2)), and bridging (μ:κ(1):κ(1)) DAF coordination modes. Titration studies of DAF and Pd(OAc)2 reveal the formation of two dimeric DAF/Pd(OAc)2 complexes at low [DAF] and four monomeric species at higher [DAF]. The dimeric complexes feature two bridging acetate ligands together with either a bridging or nonbridging (κ(1)) DAF ligand coordinated to each Pd(II) center. The monomeric structures consist of three isomeric Pd(κ(1)-DAF)2(OAc)2 complexes, together with Pd(κ(2)-DAF)(OAc)2 in which the DAF exhibits a traditional bidentate coordination mode. Replacing DAF with the structurally related, but more-electron-rich derivative 9,9-dimethyl-4,5-diazafluorene (Me2DAF) simplifies the equilibrium mixture to two complexes: a dimeric species in which the Me2DAF bridges the two Pd centers and a monomeric species with a traditional κ(2)-Me2DAF coordination mode. The use of DAF in combination with other carboxylate ligands (CF3CO2(-) or tBuCO2(-)) also results in a simplified collection of equilibrating Pd(II)-DAF complexes. Collectively, the results highlight the ability of DAF to equilibrate rapidly among multiple coordination modes, and provide valuable insights into the utility of DAF as a ligand in Pd-catalyzed oxidation reactions.

Lanthanide Triangles Supported by Radical Bridging Ligands

The first examples of metallacycles containing rare earth ions bridged by radicals are reported. The molecular triangles [Ln3(hfac)6(bptz•-)3] (Ln = DyIII, YIII; hfac = 1,1,1,5,5,5-hexafluoro-2,4-pentanedionate; bptz = 3,6-bis(2-pyridyl)-1,2,4,5-tetrazine) consist of lanthanide ions bridged by bptz radical anion (bptz•-) ligands. Magnetic susceptibility measurements and CASSCF calculations performed on [Dy3(hfac)6(bptz•-)3] reveal the presence of antiferromagnetic coupling between the DyIII centers and the bptz•- ligands, with J = -6.62 cm-1.

Lewis acid enhanced axial ligation of [Mo2]4+ complexes.

We report here the syntheses, X-ray crystal structures, electrochemistry, and density functional theory (DFT) single-point calculations of three new complexes: tetrakis(monothiosuccinimidato)dimolybdenum(II) [Mo2(SNO5)4, 1a], tetrakis(6-thioxo-2-piperidinonato)dimolybdenum(II) [Mo2(SNO6)4, 1b], and chlorotetrakis(monothiosuccinimidato)pyridinelithiumdimolybdenum(II) [pyLiMo2(SNO5)4Cl, 2-py]. X-ray crystallography shows unusually short axial Mo2-Cl bond lengths in 2-py, 2.6533(6) Å, and dimeric 2-dim, 2.644(1) Å, which we propose result from an increased Lewis acidity of the Mo2 unit in the presence of the proximal Li(+) ion. When 2-py is dissolved in MeCN, the lithium reversibly dissociates, forming an equilibrium mixture of (MeCNLiMo2(SNO5)4Cl) (2-MeCN) and [Li(MeCN)4](+)[Mo2(SNO5)4Cl](-) (3). Cyclic voltammetry was used to determine the equilibrium lithium binding constant (room temperature, K(eq) = 95 ± 1). From analysis of the temperature dependence of the equilibrium constant, thermodynamic parameters for the formation of 2-MeCN from 3 (ΔH° = -6.96 ± 0.93 kJ mol(-1) and ΔS° = 13.9 ± 3.5 J mol(-1) K(-1)) were extracted. DFT calculations indicate that Li(+) affects the Mo-Cl bond length through polarization of metal-metal bonding/antibonding molecular orbitals when lithium and chloride are added to the dimolybdenum core.

Electronic Structure of Ru2(II,II) Oxypyridinates: Synthetic, Structural, and Theoretical Insights into Axial Ligand Binding

Reduction of (4,0)-Ru2(chp)4Cl (1) (chp = 6-chloro-2-oxypyridinate) with Zn or FeCl2 yields a series of axial ligand adducts of the Ru2(II,II) species Ru2(chp)4(L), with L = tetrahydrofuran (2), dimethyl sulfoxide (DMSO; 3), PPh3 (4), pyridine (5), or MeCN (6). Zn reduction in noncoordinating solvents such as toluene or CH2Cl2 leads to the dimeric species [Ru2(chp)4]2 (7) or [Ru2(chp)4]2(ZnCl2) (8), whereas addition of strongly σ-donating ligands such as CO causes cleavage of the Ru-Ru bond. Density functional theory (DFT) models of these complexes, the axially free species, and the axial adducts of several other potential ligands (H2O, NH3, CH2Cl2, S-bound DMSO, N2, and CO) indicate that these compounds can be divided into three distinct categories, based on their Ru-Ru bond length and electronic structure. Compounds 2, 3, 5, 6, 7, and 8, the hypothetical axially free species, and adducts of H2O and NH3 fit in Category 1 with a (δ*)(2)(π*)(2) ground state, as indicated by their electronic spectra, magnetic properties, and Ru-Ru bond distances. However, compound 4 and the CH2Cl2 adduct (Category 2) show a pseudo-Jahn-Teller distortion and spectroscopic signs of δ*/π* orbital mixing suggestive of a new electronic ground state intermediate between the (δ*)(2)(π*)(2) and (δ*)(1)(π*)(3) configurations. Category 3 consists of the hypothetical adducts of N2, S-bound DMSO, and CO, all of which are predicted to have a (δ*)(1)(π*)(3) configuration. Electronic spectra were recorded and assigned using time-dependent DFT, allowing assignment of a band in the 10,000-13,000 cm(-1) range as the δ → π* transition. The axial ligands π-acid character heavily influences the δ*-π* gap, and thereby the ground-state electronic configuration, but not the axial ligand binding strength, which is dictated more by the σ-donor character of the ligands. Thus, this work greatly expands the number of axial ligand adducts known for Ru2(II,II) complexes supported by N,O-donor ligands and provides a predictive theoretical framework for their stability and electronic structures.

Enforcing Ising-like magnetic anisotropy via trigonal distortion in the design of a W(V)–Co(II) cyanide single-chain magnet

A new octacyanotungstate(V) based single chain magnet {[(Tpm)Co(DMF)W(CN)8]2[Co(DMF)4]·2DMF}n (1, Tpm = 1,1,1-trispyrazoylmethane), with an effective barrier of 39.7(3) cm−1 is reported. The Ising-like magnetic anisotropy of the chain originates from the nearly parallel local orientations of the Co(II) ions with the source of the uniaxial magnetic anisotropy being a trigonal distortion of the octahedral environment with the fac-tridentate capping Tpm ligand.

Putting a New Spin on Supramolecular Metallacycles: Co3 Triangle and Co4 Square Bearing Tetrazine-Based Radicals as Bridges

The synthesis of two new radical-bridged compounds [Co3(bptz)3(dbm)3]·2toluene (1) and [Co4(bptz)4(dbm)4]·4MeCN (2) (bptz = 3,6-bis(pyridyl)-1,2,4,5-tetrazine; dbm = 1,3-diphenyl-1,3-propanedionate) is reported. The presence of the ligand-centered radical has been confirmed by X-ray crystallography and SQUID magnetometry. These complexes are the first metallacycles bearing nitrogen heterocyclic radicals as bridges. Magnetic studies reveal strong antiferromagnetic metal···radical coupling with coupling constants of J = -67.5 and -66.8 cm-1 for 1 and 2, respectively. DFT calculations further support the strong antiferromagnetic coupling between CoII ions and bptz radicals and confirm S = 3 and S = 4 spin ground states for 1 and 2, respectively.

Synthesis and Characterization of a Binuclear Copper(II) Naphthoisoamethyrin Complex Displaying Weak Antiferromagnetic Coupling

The reaction between a naphthylbipyrrole-containing hexaphyrin-type expanded porphyrin and copper acetate affords a bench-stable dicopper(II) complex. UV-vis spectroscopy, cyclic voltammetry, and X-ray crystallographic analysis measurements provide support for the conclusion that this complex displays aromatic features. A weak antiferromagnetic exchange interaction between the binuclear copper(II) ions is evidenced by variable-temperature electron paramagnetic resonance and by fitting of the bulk magnetic susceptibility to a dimer model, yielding J = -5.1 cm-1.

Positional and compositional disorder in a ruthenium(II) piano-stool complex.

In (η⁶-p-cymene)(difluorophosphinato-κO){2-[(1H-pyrazol-1-yl)methyl-κN²]pyridine-κN}ruthenium(II) 0.85-hexafluorophosphate 0.15-tetrafluoroborate, [Ru(PO₂F₂)(C₁₀H₁₄)(C₉H₉N₃)](PF₆)0.85(BF₄)0.15, (I), the [PO₂F₂]⁻ ligand exhibits positional disorder due to one F atom and one O atom sharing the same two positions related by a mirror reflection across the O-P-F plane. The correct composition of this coordinated anion was successfully determined to be [PO₂F₂]⁻ by refining the complex with various tetrahedral anions in which terminal atoms have similar atomic form factors. The noncoordinated counter-ion is compositionally disordered between [PF₆]⁻ and [BF₄]⁻. The difficulty in determining the correct composition of this anion illustrates the importance of a crystallographer remaining impartial and open to encountering unexpected moieties in the process of elucidating a structure.

K3[Mo2(SNO5)4Cl]3[Mo2(SNO5)4]: the first example of a heterometallic extended metal atom node (HEMAN)

We present the synthesis, structure, and electrochemistry of K3[Mo2(SNO5)4Cl]3[Mo2(SNO5)4] (1, HSNO5 = monothiosuccinimide), the first example of a heterometallic extended metal atom node (HEMAN). The HEMAN consists of two perpendicular, intersecting lines of metal atoms formed by three [Mo2(SNO5)4Cl]- units and one [Mo2(SNO5)4] unit tethered together by K+ ions.