Monica Soler

Quantum phase interference (Berry phase) in single-molecule magnets of [Mn12]2−

Magnetization measurements of molecular clusters [Mn12]2− with a spin ground state of S=10 show resonance tunneling at avoided energy level crossings. The observed oscillations of the tunnel probability as a function of the magnetic field applied along the hard anisotropy axis are due to topological quantum phase interference of two tunnel paths of opposite windings. [Mn12]2− is therefore the second molecular cluster exhibiting quantum phase interference.

Hexameric Palladium(II) Terpyridyl Metallomacrocycles: Assembly with 4,4′-Bipyridine and Characterization by TWIM Mass Spectrometry†

The construction of 2D or 3D materials using supramolecular chemistry principles has become an intriguing area of research. In particular, terpyridine-based building blocks have played a pivotal role in the construction of dimers, triangles, trigonal prisms, squares, 7–9] pentagons, and hexagons, based on their planar tridentate coordination mode and their facile potential for modification. Terpyridine (terpy) and its metal complexes are also of interest because of their well-known photophysical and electronic properties. Thus, the use of terpyridine for the construction of materials and molecular architectures with increasing complexity will continue to mature. However, the difficulty of obtaining single crystals suitable for X-ray structure determination means that other reliable techniques are essential for the characterization of macromolecular structures. Electrospray ionization (ESI) mass spectrometry has been applied in the identification and characterization under mild ionization conditions. In particular, the cold-spray (CSI) technique reported by Fujita and co-workers, and the Fourier transform mass spectrometry (FTMS) technique developed by Schalley and coworkers are the most prominent ESI-based methods. The work of Piguet and co-workers is also notable. Unfortunately, the signals that correspond to different charge states are superimposed and only a few isotope patterns of different charge states can be deconvoluted. Recently, traveling wave ion mobility mass spectrometry (TWIM-MS), a variant of ion mobility mass spectrometry (IM-MS), has been successfully applied to the detection and characterization of supramolecules. Ion-mobility-based separation enhances the resolving power of mass spectrometry by adding shapeand charge-dependent dispersion, which reduces isomer superposition and can deconvolute the isotope patterns of different charge states. Notably, isomeric linear and cyclic structures have been separated based on their different drift time in the ion mobility device. Herein, we report the 4,4’-bipyridine (bpy) assisted assembly of a hexagonal, dodeca Pd terpyridyl based macrocycle, and its characterization by NMR and TWIMMS. A recent example of the use of terpyridine and its Pd coordination for the formation of a metallocyclic rectangle was reported by Bosnich and co-workers, whereby two cofacially oriented, Pd terpyridine MeCN adducts were dimerized upon the addition of 4,4’-bipyridine. Our synthetic efforts began with an improved preparation of 1,3-bis(2,2’:6’,2’’-terpyridin-4’-yl)-5-tert-butylbenzene (1), which was isolated in 60% yield and exhibited identical H and C NMR spectra to that of the initially reported ligand. Ligand 1 was prepared by the reduction of commercially available 5-tert-butylbenzene-1,3-dicarboxylic acid with BH3·THF, followed by selective oxidation with pyridinium chlorochromate (PCC) and subsequent grinding with 2-acetylpyridine (4.05 equivalents) and NaOH to give an orange solid, which was then added to NH4OH and EtOH and heated at reflux for 24 hours. This ligand, which has coordination sites that are positioned 120 degrees apart, was treated with [Pd(MeCN)4](BF4)2 in dry MeCN to give [(1,3-bis(2,2’:6’,2’’-terpyridin-4’-yl)-5-tert-butylbenzene)Pd2(MeCN)2](BF4)4 2 in nearly quantitative yield (Scheme 1). Ligand 1 was initially insoluble in MeCN, but was readily solubilized when treated with [Pd(MeCN)4](BF4)2 in MeCN. The H NMR data confirmed the formation of adduct 2, with signals at d= 8.68 (s, 3’,5’-terpyH), 8.63-8.58 (m, 6,6’’-terpyH and 3,3’’-terpyH), 8.49 (t, 4,4’’-terpyH), and 7.89 ppm (t, 5,5’’terpyH); see Figure 1. A downfield shift of the tert-butyl singlet peak (from d= 1.51 to 1.57 ppm, Dd= 0.06 ppm) and the IR absorptions observed at 2334 and 2304 cm 1 assigned to the C N stretch also support the formation of a Pd adduct 2. [*] Dr. S. Perera, Dr. X. Li, Dr. M. Soler, A. Schultz, Prof. Dr. C. Wesdemiotis, Dr. C. N. Moorefield, Prof. Dr. G. R. Newkome Department of Polymer Science, Department of Chemistry, The University of Akron 302 Buchtel Common, Akron, OH 44325 (USA) Fax: (+1)330-972-2368 E-mail: wesdemiotis@uakron.edu newkome@uakron.edu Homepage: http://www.dendrimers.com [] These authors contributed equally to this work.

Unexpected Isolation of a Pentameric Metallomacrocycle from the FeII‐Mediated Complexation of 120° Juxtaposed 2,2′:6′,2′′‐Terpyridine Ligands

Self-assembly of specially designed building blocks has been widely employed in the construction of rigid supramolecular structures. Towards this goal, many successful strategies have been developed for the synthesis of metallomacrocycles, such as for trigonal, rectangular, hexagonal, heptagonal, and octagonal structures. In contrast, only a limited number of pentagonal shapes have been reported in the literature. Accordingly, building blocks require a suitable angle to generate five-sided motifs. For example, a carbazole unit that introduced a 1058 angle between two terpyridine ligands was readily self-assembled to give metallopentacycles. Tetragonal and trigonal metallomacrocycles formed by 608 juxtaposed bis(terpyridine) ligands possessing bendable alkyne spacers have been documented. However, the formation of unexpected products by using rigid linkers is rare. Herein, we report the isolation and characterization of a unique pentameric macrocycle, along with its anticipated hexameric homologue, generated from an Fe-mediated complexation of functionalized bis(terpyridine) ligands with a 1208 angle relative to the coordination sites. Reaction of 1 (Scheme 1) with 1.05 equivalents of FeCl2·4H2O in MeOH at 25 8C for 24 h gave the pentamer [Fe5(1)5 ACHTUNGTRENNUNG(NO3)10] (2a ; 4.3%) and the hexamer [Fe6(1)6ACHTUNGTRENNUNG(NO3)12] (2b ; 11.9%), which were isolated by gradient column chromatography eluting with H2O/MeCN/sat. KNO3(aq) (from 1:20:1 to 1:15:1; v/v/v). The H NMR spectrum of pentamer 2a revealed sharp singlets at 8.22 and 8.75 ppm for 2,6-ArHs and 4-ArH respectively, supporting the presence of the symmetric macrocycle, in contrast to linear oligomers that exhibit more complicated patterns. Other supportive data (H NMR spectroscopy) included an expected upfield shift for 6,6’’-tpyHs (d= 7.41 ppm, Dd= 1.34 ppm) and a downfield shift for 3’,5’tpyHs (d=9.65 ppm, Dd=+0.86 ppm) relative to the corresponding peaks in the uncomplexed ligand 1 (Figure 1). The H NMR spectrum of hexamer 2b exhibited a similar pattern, but the peaks of terpyridine part showed a slight downfield shift relative to the corresponding peaks in 2a presumably due to the additional metal centers in 2b resulting in added electron deficiency. In comparison with 2b, notably the 4-ArH peak of 2a has an upfield shift (Dd= 0.2 ppm) due to the enhanced crowded inner space. This proved to be a key discernable feature in recognizing, isolating and characterizing the otherwise remarkably similar fiveand sixmembered ring structures. The 2D COSY NMR spectra of the bis(terpyridine) ligand 1 and the self-assembled macrocycle 2a also ensured the proper assignments. Notably, at [a] Y.-T. Chan, Dr. M. Soler, Prof. G. R. Newkome Departments of Polymer Science and Chemistry The University of Akron, Akron, OH 44325-4717 (USA) Fax: (+1)330-972-2413 E-mail : newkome@uakron.edu [b] Dr. C. N. Moorefield, Prof. G. R. Newkome Maurice Morton Institute for Polymer Science The University of Akron, Akron, OH 44325 (USA) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.200902988. Scheme 1. Unexpected and predetermined products 2a and 2b obtained by self-assembly of 1. a) FeCl2·4H2O, MeOH, 25 8C, 24 h (each Fe II macrocycle isolated as the polyNO3 salt; see Experimental Section).

Single-molecule magnets: control by a single solvent molecule of Jahn-Teller isomerism in [Mn12O12(O2CCH2But)16(H2O)4]

Faster- and slower-relaxing versions of the title Mn12 compound have been obtained in pure forms that crystallize in the same space group and differ only in the identity of one lattice solvent molecule; solvent loss causes isomerization from the faster- to the slower-relaxing form.

Synthesis, crystal structure and magnetic properties of a new ferromagnetic nickel(II) dimer derived from a hexadentate Schiff base ligand

Abstract A new end-on bis(μ-azido) nickel(II) complex of the formula [Ni2(L1)(N3)4] (1) (L1=hexadentate Schiff base ligand made from the reaction of 2 moles of 2-benzoyl pyridine and 1 mole of triethylenetetramine) has been prepared and characterized by single crystal X-ray diffraction studies. Each nickel center is coordinated in a distorted octahedral (NiN6) fashion consisting of three nitrogen atoms from the hexadentate ligand moiety, two nitrogen atoms from two end-on azido ligands and the last one from another terminal azido ion. The variable temperature magnetic susceptibility of compound 1 shows that the nickel centers are ferromagnetically coupled. The NiNNi bridge angles are very similar [98.36(8)° and 97.66(9)°] and the Ni⋯Ni separation is 3.155(1) A. The magnetic exchange coupling constant J=21.8 cm−1 is consistent with the structure and the value has been compared with some similar reported compounds.

A third isolated oxidation state for the family of [Mn12O12(O2CR)16(H2O)4] compounds. Synthesis, characterization and single-molecule magnetism properties of (PPh4)2[Mn12O12(O2CR)16(H2O)x] (x=3 or 4)

Abstract The synthesis and characterization are reported of the two-electron reduced version of the [Mn12O12(O2CR)16(H2O)4] family of single-molecule magnets (SMMs). Reduction of the neutral complexes with 2 equiv. of PPh4I gives the (PPh4)2[Mn12O12(O2CR)16(H2O)x] (x=3 or 4). The crystal structure shows the overall Mn12 structure is maintained, with a 2Mn(II), 6Mn(III), 4Mn(IV) trapped oxidation state description. Magnetic characterization indicates a S=10 ground state and retention of SMM behavior.

Magnetization tunneling in an enneanuclear manganese cage

The enneanuclear mixed-valent manganese cage [Mn 9 O 7 (OAc) 1 1 (thme)(py) 3 (H 2 O) 2 ] 1 {Mn 9 } possesses an S = 17/2 ground state as a result of an antiferromagnetic interaction between three ferromagnetically coupled Mn I V ions and a wheel of four Mn I I I and two Mn I I ions. AC magnetization measurements show a frequency-dependent out-of-phase signal at 3.4 K and 997 Hz, indicative of single-molecule magnetism behaviour. Data obtained from varying the frequency of oscillation of the AC field gives an effective energy barrier (U e f f ) for the reversal of magnetization of 27 K. Magnetic measurements of single crystals of 1 at low temperature show time- and temperature-dependent hysteresis loops which contain steps at regular intervals of field. DC and AC relaxation measurements show both the temperature-dependent and temperature-independent regions, the latter being definitive evidence of magnetization tunneling in the lowest energy zero-field split component of the ground state.

New example of Jahn-Teller isomerism in [Mn12O12(O2CR)16(H2O)4] complexes

Abstract The isolation and characterization of a new pair of Jahn-Teller isomers of [Mn 12 O 12 (O 2 CCH 2 Bu t ) 16 (H 2 O) 4 ] ( 3 ) are reported: complex 3 ·CH 2 Cl 2 ·MeNO 2 ( 3a ) and 3 ·CH 2 Cl 2 ·MeCN ( 3b ). These Jahn-Teller isomers have been crystallized in the same triclinic space group, and differ only in one abnormally oriented Jahn-Teller axis in 3a and in the identity of one solvent molecule of crystallization. Magnetic data show much faster magnetization relaxation for 3a (the low-temperature, LT, form) compared with 3b (the high-temperature, HT, form), exhibiting a peak in the out-of-phase magnetic susceptibility in the 2–4 and 5–7 K ranges, respectively. Magnetization vs. DC field scans on aligned crystals of 3a and 3b display hysteresis with coercivities that vary with temperature, as expected for single-molecule magnets. The hysteresis loops also exhibit the step features that are the signature of quantum tunneling of magnetization.

Single-molecule magnetism behavior of [Mn12O12(O2CR)16(H2O)4]2− salts

Abstract (PPh 4 ) 2 [Mn 12 O 12 (O 2 CCHCl 2 ) 16 (H 2 O) 4 ] ( 3 ) has been prepared by the two-electron reduction of [Mn 12 O 12 (O 2 CCHCl 2 ) 16 (H 2 O) 4 ] ( 2 ) using iodide. Crystallization from CH 2 Cl 2 /hexanes yields a mixture of two crystal forms, 3 ·4CH 2 Cl 2 ·H 2 O ( 3a ) and 3 6CH 2 Cl 2 ( 3b ), which are triclinic and monoclinic, respectively. They are both trapped valence 2Mn(II), 6Mn(III), 4Mn(IV). DC magnetization data for dried, unsolvated 3 in 1.80–4.00 K and 10–70 kG ranges were fit to give S =10, D =−0.28 cm −1 , g =2.00. Frequency-dependent out-of-phase ( χ ″ M ) signals in AC susceptibility studies on crystalline sample of 3a and 3b combined with DC relaxation decay data were fit to the Arrhenius equation to give an effective energy barrier of U eff =18.5 and 30.3 K, respectively. Magnetization vs. DC field sweeps on single crystals of 3a and 3b gave hysteresis loops containing steps due to quantum tunneling of magnetization (QTM). The step separations yielded ∣ D ∣/ g values of 0.087 and 0.14 cm −1 , and consequently U =20 and 39 K (for g =2) for 3a and 3b , respectively, suggesting that the differences in U eff are primarily caused by changes to D . This work demonstrates the sensitivity of the magnetic properties of [Mn 12 ] 2− single-molecule magnets to subtle differences in their environment.

A new class of single-molecule magnet: (Mn9O7(OAc)11(thme)(py)3(H2O)2) with an S = 17/2 ground state

The reaction of [Mn3O(OAc)6(py)3] with 1,1,1-tris(hydroxymethyl)ethane (H3thme) gives the Mn(IV)3Mn(III)4Mn(II)2 complex [Mn9O7(OAc)11(thme)(py)3(H2O)2], which has an S = 17/2 ground state and displays strong out-of-phase signals in ac susceptibility studies that establish it as a new class of single-molecule magnet.