Organometallic Intermediates in the Synthesis of Photoluminescent Zirconium and Hafnium Complexes with Pyridine Dipyrrolide Ligands.

The two commercially available zirconium complexes tetrakis(dimethylamido)zirconium, Zr(NMe2)4, and tetrabenzylzirconium, ZrBn4, were investigated for their utility as starting materials in the synthesis of bis(pyridine dipyrrolide)zirconium photosensitizers, Zr(PDP)2. Reaction with one equivalent of the ligand precursor 2,6-bis(5-methyl-3-phenyl-1H-pyrrol-2-yl)pyridine, H2MePDPPh, resulted in the isolation and structural characterization of the complexes (MePDPPh)Zr(NMe2)2thf and (MePDPPh)ZrBn2, which could be converted to the desired photosensitizer Zr(MePDPPh)2 upon addition of a second equivalent of H2MePDPPh. Using the more sterically encumbered ligand precursor 2,6-bis(5-(2,4,6-trimethylphenyl)-3-phenyl-1H-pyrrol-2-yl)pyridine, H2MesPDPPh, only ZrBn4 yielded the desired bis-ligand complex Zr(MesPDPPh)2. Careful monitoring of the reaction at different temperatures revealed the importance of the organometallic intermediate (cyclo-MesPDPPh)ZrBn, which was characterized by X-ray diffraction analysis and 1H NMR spectroscopy and shown to contain a cyclometalated MesPDPPh unit. Taking inspiration from the results for zirconium, syntheses for two hafnium photosensitizers, Hf(MePDPPh)2 and Hf(MesPDPPh)2, were established and shown to proceed through similar intermediates starting from tetrabenzylhafnium, HfBn4. Initial studies of the photophysical properties of the photoluminescent hafnium complexes indicate similar optical properties compared to their zirconium analogues.

Aerobic Oxidation Reactivity of Well-Defined Cobalt(II) and Cobalt(III) Aminophenol Complexes.

This paper describes the synthesis and reactivity studies of three cobalt complexes bearing aminophenol-derived ligands without nitrogen substitution: CoII(tBu2APH)2(tBu2AP)2 (1), CoIII2(tBu2APH)2(tBu2AP)2(µ-tBu2BAP)2 (2), and CoIII(tBu2AP)3 (3), where tBu2APH = 2-amino-4,6-di-tert-butylphenol, tBu2AP = 2-amino-4,6-di-tert-butylphenolate, and µ-tBu2BAP = bridging 2-amido-4,6-di-tert-butylphenolate. Stoichiometric reactivity studies of these well-defined complexes demonstrate the catalytic competency of both cobalt(II) and cobalt(III) complexes in the aerobic oxidative cyclization of tBu2APH with tert-butylisonitrile. Reactions with O2 reveal the aerobic oxidation of the cobalt(II) complex 1 to generate the cobalt(III) species 2 and 3. UV-visible time-course studies and electron paramagnetic resonance spectroscopy indicate that this oxidation proceeds through a ligand-based radical intermediate. These studies represent the first example of well-defined cobalt aminophenol complexes that participate in catalytic aerobic oxidation reactions and highlight a key role for a ligand radical in the oxidation sequence.

Photochemical synthesis of a zirconium cyclobutadienyl complex.

Photolysis of (MePMPMe)2ZrBn2 (MePMPMe = 3,5-dimethyl-2-(2-pyridyl)pyrrolide) in the presence of diphenylacetylene yields the first η4-cyclobutadienyl zirconium complex, (MePMPMe)2Zr(η4-C4Ph4), through formal [2+2] cycloaddition of two alkynes at a putative low-valent zirconium intermediate. This unique reactivity expands the scope of alkyne coupling reactions at low-valent zirconium centers that traditionally produce zirconacyclopentadienes.

A Co8 metallacycle stabilized by double anion-π interactions.

Self-assembly reactions of CoII ions in the presence of the 2,2-bipyrimidine (bpym) ligand produced both a dinuclear and an octanuclear cation with the nuclearity being governed by hydrogen-bonding versus anion-π interactions between the anions and the ligands.

Hard versus soft: zero-field dinuclear Dy(iii) oxygen bridged SMM and theoretical predictions of the sulfur and selenium analogues.

Two dinuclear lanthanide complexes (Gd and Dy) were prepared and characterized by X-ray, magnetic and computational methods. The Dy analogue shows SMM behavior with an energy barrier of 98.4 K in the absence of an applied dc field. Theoretical calculations were performed on model complexes which support the hypothesis that the energy barrier will increase if the soft-donor atoms S and Se are used in lieu of an O donor.

Hexagonal Bipyramidal Dy(III) Complexes as a Structural Archetype for Single-Molecule Magnets.

Single-molecule magnets (SMMs), are regarded as excellent nanomaterials for high-density information storage and quantum computing. The local symmetry of the crystal field for the metal ion plays an important role in pursuing a high-performance SMM. Herein, two highly stable distorted hexagonal bipyramidal (quasi- D6 h) Dy complexes exhibiting slow relaxation of the magnetization are reported. A hexagonal bipyramidal Dy model complex with 18-crown-6 was also designed to study the relationship between magnetic anisotropy and symmetry. The combined experimental and theoretical results indicate that quantum tunneling is highly dependent on the local symmetries of the crystal field. The magnetic anisotropy becomes much stronger when the symmetry is closer to a standard D6 h geometry. These results support the conclusion that the hexagonal bipyramidal geometry is a viable one for the design of new classes of SMMs.

A cyanide-bridged wheel featuring a seven-coordinate Mo(iii) center.

A new cyclic molecule incorporating [MoIII(CN)7]4- has been characterized by single crystal X-ray methods, SQUID magnetometry and theoretical calculations. The wheel molecule [MoIII(CN)7]6[Ni(L)]12(H2O)6 exhibits ferromagnetic Mo-Ni coupling which did not exist for the previously reported octacyanometallate analogue [MoIV(CN)8]6[Ni(L)]12(H2O)6. These results indicate that known supramolecular architectures incorporating octacyanometallates can be used as platforms for making new molecules incorporating seven-coordinate cyanide precursors.

Tunable Rh2(II,II) Light Absorbers as Excited-State Electron Donors and Acceptors Accessible with Red/Near-Infrared Irradiation.

A series of dirhodium(II,II) paddlewheeel complexes of the type cis-[Rh2(µ-DTolF)2(µ-L)2][BF4]2, where DTolF = N,N'-di( p-tolyl)formamidinate and L = 1,8-naphthyridine (np), 2-(pyridin-2-yl)-1,8-naphthyridine (pynp), 2-(quinolin-2-yl)-1,8-naphthyridine (qnnp), and 2-(1,8-naphthyridin-2-yl)quinoxaline (qxnp), were synthesized and characterized. These molecules feature new tridentate ligands that concomitantly bridge the dirhodium core and cap the axial positions. The complexes absorb light strongly throughout the ultraviolet/visible range and into the near-infrared region and exhibit relatively long-lived triplet excited-state lifetimes. Both the singlet and triplet excited states exhibit metal/ligand-to-ligand charge transfer (ML-LCT) in nature as determined by transient absorption spectroscopy and spectroelectrochemistry measurements. When irradiated with low-energy light, these black dyes are capable of undergoing reversible bimolecular electron transfer both to the electron acceptor methyl viologen and from the electron donor p-phenylenediamine. Photoinduced charge transfer in the latter was inaccessible with previous Rh2(II,II) complexes. These results underscore the fact that the excited state of this class of molecules can be readily tuned for electron-transfer reactions upon simple synthetic modification and highlight their potential as excellent candidates for p- and n-type semiconductor applications and for improved harvesting of low-energy light to drive useful photochemical reactions.

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.

A chiral diamine: practical implications of a three-stereoisomer cocrystallization.

A brief comparison of seven straightforward methods for molecular crystal-volume estimation revealed that their precisions are comparable. A chiral diamine, N2,N3-bis[2,6-bis(propan-2-yl)phenyl]butane-2,3-diamine, C28H44N2, has been used to illustrate the application of the methods. Three stereoisomers of the diamine cocrystallize in the centrosymmetric space group P21/c with Z' = 1.5. The molecules occupying general positions are RR and SS, whereas that residing on an inversion center is meso. This is one of only ten examples of three stereoisomers with two asymmetric atoms cocrystallizing together reported to the Cambridge Structural Database (CSD). The conformations of the SS/RR and meso molecules differ considerably and lead to statistically significantly different C(asymmetric)-C(asymmetric) bond lengths in the diastereomers. An advanced Python script-based CSD searching technique for chiral compounds is presented.

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.

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.

Strong Ferromagnetic Exchange Coupling Mediated by a Bridging Tetrazine Radical in a Dinuclear Nickel Complex.

The radical bridged compound [(Ni(TPMA))2-µ-bmtz•-](BF4)3·3CH3CN (bmtz = 3,6-bis(2'-pyrimidyl)-1,2,4,5-tetrazine, TPMA = tris(2-pyridylmethyl)amine) exhibits strong ferromagnetic exchange between the S = 1 NiII centers and the bridging S = 1/2 bmtz radical with J = 96 ± 5 cm-1 (-2JNi-radSNiSrad). DFT calculations support the existence of strong ferromagnetic exchange.

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.

An air stable radical-bridged dysprosium single molecule magnet and its neutral counterpart: redox switching of magnetic relaxation dynamics.

A pair of related dinuclear lanthanide compounds, viz., [Dy(tmhd)3]2(bptz) (1) and {Cp2Co}{[Dy(tmhd)3]2(bptz)} (2) (tmhd = 2,2,6,6-tetramethyl-3,5-heptane dionate; bptz = 3,6-bis(2-pyridyl)-1,2,4,5-tetrazine) are reported. These molecules represent the first study of rare earth ions in which an organic ligand bridged species is isolated in both the neutral and radical anion forms. Both compounds exhibit out-of-phase signals below 4 K. The radical compound is highly air stable, with minimal changes in magnetic behavior after exposure to the atmosphere for two weeks.

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.

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.

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 ligand's 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.

AEE-active cyclic tetraphenylsilole derivatives with ∼100% solid-state fluorescence quantum efficiency.

Two new strongly AEE active (I/I0 ≈ 94) tetraphenylsilole-containing cyclosiloxanes with cyan emissions (λem = 500 nm) and ∼100% solid state fluorescence quantum yields are reported. The intra- and intermolecular C-Hπ interactions in the crystal play a major role in the observed high solid state fluorescence quantum yields.

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.