Rac Configuration Research Articles

Configurational Isomerization of Dinuclear Iridium and Rhodium Complexes with a Series of NPPN Ligands, 2-PyCH2(Ph)P(CH2)nP(Ph)CH2-2-Py (Py = Pyridyl, n = 2–4)

New heterodonor NPPN tetradentate ligands, 2-PyCH2(Ph)P(CH2)nP(Ph)CH2-2-Py (meso- and rac-Ln; n = 2–4, Py = pyridyl), were prepared and reacted with [Cp*MCl2]2 (M = Ir, Rh; Cp* is pentamethylcyclopentadienyl) in the presence of NH4BF4 to afford a series of dinuclear complexes [(Cp*MCl)2(meso-Ln)](BF4)2 (M = Ir, n = 2 (2a), 3 (3a), 4 (4a); M = Rh, n = 2 (2c), 3 (3c), 4 (4c)) and [(Cp*MCl)2(rac-Ln)](BF4)2 (M = Ir, n = 2 (2b), 3 (3b), 4 (4b); M = Rh, n = 2 (2d), 3 (3d), 4 (4d)), which were characterized by IR, 1H and 31P{1H} NMR, and ESI mass spectroscopic techniques and X-ray crystallography. The configurations around the two metal centers were controlled by the configuration of the coordinated P atoms so as to avoid repulsive interaction between the phenyl group on P and the chloride ligand, resulting in the formation of stereospecific isomers; a meso configuration of the metal centers is induced from meso-Ln (abbreviated as meso-P2/meso-M2), and in contrast, a rac configuration is induced from rac-Ln (rac-P2/rac-M2). Furthermore, inversion of metal centers for the Ir2 complexes occurred in DMSO at higher temperatures (60–100 °C), generating equilibrium mixtures of minor diastereomers (meso-P2/rac-M2 or rac-P2/meso-M2) in low ratios together with the major isomers (meso-P2/meso-M2 or rac-P2/rac-M2). The equilibrium constants, K = [minor isomer]/[major isomer], varied appreciably depending on the lengths of the methylene chains as well as configurations of the NPPN ligands; the overall propensity for the K values was observed to be L2 < L3 < L4 and meso-Ln < rac-Ln, while rac-L3, rac-L4, and meso-L4 showed almost identical equilibrium constants, presumably resulting from no steric influence between the two metal centers.

Valence‐State Analysis through Spectroelectrochemistry in a Series of Quinonoid‐Bridged Diruthenium Complexes [(acac)2Ru(μ‐L)Ru(acac)2]n (n=+2, +1, 0, −1, −2)

The quinonoid ligand-bridged diruthenium compounds [(acac)(2)Ru(mu-L(2-))Ru(acac)(2)] (acac(-)=acetylacetonato=2,4-pentanedionato; L(2-)=2,5-dioxido-1,4-benzoquinone, 1; 3,6-dichloro-2,5-dioxido-1,4-benzoquinone, 2; 5,8-dioxido-1,4-naphthoquinone, 3; 2,3-dichloro-5,8-dioxido-1,4-naphthoquinone, 4; 1,5-dioxido-9,10-anthraquinone, 5; and 1,5-diimido-9,10-anthraquinone, 6) were prepared and characterized analytically. The crystal structure analysis of 5 in the rac configuration reveals two tris(2,4-pentanedionato)ruthenium moieties with an extended anthracenedione-derived bis(ketoenolate) pi-conjugated bridging ligand. The weakly antiferromagnetically coupled {Ru(III)(mu-L(2-))Ru(III)} configuration in 1-6 exhibits complicated overall magnetic and EPR responses. The simultaneous presence of highly redox-active quinonoid-bridging ligands and of two ruthenium centers capable of adopting the oxidation states +2, +3, and +4 creates a large variety of possible oxidation state combinations. Accordingly, the complexes 1-6 exhibit two reversible one-electron oxidation steps and at least two reversible reduction processes. Shifts to positive potentials were observed on introduction of Cl substituents (1-->2, 3-->4) or through replacement of NH by O (6-->5). The ligand-to-metal charge transfer (LMCT) absorptions in the visible region of the neutral molecules become more intense and shifted to lower energies on stepwise reduction with two electrons. On oxidation, the para-substituted systems 1-4 exhibit monocation intermediates with intervalence charge transfer (IVCT) transitions of Ru(III)Ru(IV) mixed-valent species. In contrast, the differently substituted systems 5 and 6 show no such near infrared (NIR) absorption. While the first reduction steps are thus assigned to largely ligand-centered processes, the oxidation appears to involve metal-ligand delocalized molecular orbitals with variable degrees of mixing.

Synthesis and electrochemistry of nickel and cobalt complexes of mixed thia–aza crown ethers: single-crystal structures of [Ni([18]aneN2S4)][PF6]2·0.33H2O and [CO([18]aneN2S4)][PF6]3·3H2O ([18]aneN2S4= 1,4,10,13-tetrathia-7,16-diazacyclooctadecane)

The mixed thia–aza donor macrocyclic complexes [M([18]aneN2S4)]2+(M = Ni or Co; [18]aneN2S4= 1,4,10,13-tetrathia-7,1 6-diazacyclooctadecane) and [Ni(Me2[18]aneN2S4)]2+(Me2[18]aneN2S4= 7,16-dimethyl-1,4,10,13-tetrathia-7,16-diazacyclooctadecane) were prepared by reaction of M(NO3)2·6H2O with the appropriate crown ether in aqueous ethanol. The complex [Ni([18]aneN4S4)][PF6]2·0.33H2O crystallises in the orthorhombic space group Pcab with a= 12.8260(20), b= 18.083(3), c= 22.200(4)A and Z= 8. The structure shows the NiII encapsulated in a near-octahedral geometry, with the macrocycle adopting a rac configuration. Both diastereoisomeric forms of rac-[Ni([18]aneN2S4)]2+ are present in the crystal structure in approximately 1 : 1 ratio. Aerial oxidation of [Co([18]aneN2S4)][PF6]2 in water afforded the corresponding cobalt (III) complex: [Co([18]aneN2S4)][PF6]3·3H2O crystallises in the monoclinic space group P21/n with a= 11.5485(3), b= 13.9779(2), c= 19.1378(4)A, β= 106.561 (2)° and Z= 4. The structure shows the CoIII bound to all six macrocyclic donors in a distorted-octahedral geometry. The ligand adopts a rac configuration around the metal centre. In both complexes there is a small but significant tetrahedral distortion of the four sulfur donors out of the best S4 co-ordination plane. The complex [Co([18]aneN2S4)][PF6]3·3H2O exhibits an extensive hydrogen-bonding network involving the macrocyclic cation, PF6– anions and H2O solvent molecules. Electrochemical data, electronic and ESR spectroscopic data for the complexes are also discussed in the context of their donor sets and ligand configurations and conformations.

Silver macrocyclic complexes: synthesis, crystal structures and redox properties of [Ag([18]aneN2S4)]PF6and [Ag(Me2[18]aneN2S4)]BPh4([18]aneN2S4= 1,4,10,13-tetrathia-7,16-diazacyclooctadecane, Me2[18]aneN2S4= 7,16-dimethyl-1,4,10,13-tetrathia-7,16-diazacyclooctadecane)

Reaction of AgNO3 with one molar equivalent of [18]aneN2S4(1,4,10,13-tetrathia-7,16-diazacyclooctadecane) or Me2[18]aneN2S4(7,16-dimethyl analogue) in refluxing MeOH–H2O affords the complex cations [Ag([18]aneN2S4)]+ and [Ag(Me2[18]aneN2S4)]+ respectively. [Ag([18]aneN2S4)]PF6 crystallises in the orthorhombic space group Pcab with a= 16.7327(25), b= 17.5023(11), c= 14.7031(6)Å and Z= 8. The metal ion is bound to a highly distorted octahedral arrangement of the six macrocyclic donor atoms [Ag–S 2.630(4), 2.664(4), 2.719(4), 2.774(4)Å; Ag–N 2.533(10), 2.817(15)Å]. The complex cation adopts a rac configuration in which the two S–N–S linkages are bound meridionally to the metal ion. [Ag(Me2[18]aneN2S4)]BPh4 crystallises in the monoclinic space group P21/n with a= 10.7529(15), b= 19.0571(28), c= 18.9360(22)Å, β= 106.035(10)° and Z= 4. The cation is disordered and shows two different configurations. The major [70.8(8)%] component shows an unusual kite-based pyramidal geometry with AgI bound via a periplanar arrangement of four S-donor atoms [Ag–S(1) 2.583(4), Ag–S(4), Ag–S(4) 2.819(3), Ag–S(10) 2.663(4), Ag–S(13) 2.673(4)Å] and one apical N-donor atom [Ag–N(7) 2.517(11)Å]. The second N-donor atom does not interact with the metal centre [Ag ⋯ N(16) 3.684(11)Å]. In view of the asymmetric bond length distribution, the overall stereochemistry may alternatively be described as tetrahedral through S(1), N(7), S(10) and S(13), with S(4) interacting more weakly at long range. Although this complex is severely distorted, it is clear that unlike in [Ag([18]aneN2S4)]+, the S(4)–N(7)–S(10) linkage in [Ag(Me2[18]aneN2S4)]+ binds facilally to the AgI centre to give a distorted meso isomer. The minor [29.2(8)%] component differs only in the orientation of the S(1), N(16′) and S(13) linkage. Although the C–C and C–S bond lengths in this region were constrained, it is clear that N(16′) is directed towards, and is co-ordinated to the Ag1 centre [Ag–N(16′) 2.778(10)Å], giving an octahedral complex in a genuine meso configuration. [Ag([18]aneN2S4)]+ shows a reversible AgI–AgII couple at E½=+0.65 V and a quasi-reversible AgI–Ag0 couple at E½=–0.74 V while [Ag(Me2[18]aneN2S4)]+ shows an irreversible AgI–AgII couple at Epa=+0.78 V and an irreversible AgI–Ag0 couple at Epc=–0.78 V (all potentials vs. ferrocene–ferrocenium).

Mercury macrocyclic complexes: The synthesis of [Hg([18]aneN 2S 4)] 2+ and [Hg(Me 2[18]aneN 2S 4)] 2+. The single crystal x-ray structure of [Hg([18]aneN 2S 4)](PF 6) 2·4/3H 2O

Reaction of HgSO 4 with one molar equivalent of L{L = [18]aneN 2S 4 (1,4,10,13-tetrathia-7,16-diazacyclooctadecane) or Me 2[18]aneN 2S 4 (7,16-dimethyl-1,4,10,13-tetrathia-7,16-diazacyclooctadecane)} in refluxing MeOH-H 2O for 15 min affords a colourless solution containing the complex cation [Hg(L)] 2+. Addition of excess PF 6 − counterion gives the complex [Hg([18]aneN 2S 4)](PF 6) 2·4/3H 2O as a cream coloured solid. A single crystal X-ray structure determination shows mercury(II) bound to a severely distorted octahedral arrangement of the six macrocyclic donor atoms, HgS = 2.655(5), 2.735(4), 2.751(4), 2.639(5) Å, HgN = 2.473(11), 2.472(17) Å. The cation is in a rac configuration with the two SCH 2CH 2NCH 2CH 2S linkages bound meridionally to the metal centre.

Iron macrocyclic complexes: The synthesis and single crystal X-ray structure of [Fe([18]aneN 2S 4)](BPh 4) 2·2MeCN·1/2MeOH {[18]aneN 2S 4= 1,4,10,13-tetrathia-7,16-diazacyclooctadecane}

Reaction of FeCl 2 with one molar equivalent of [18]aneN 2S 4 in refluxing MeOH-H 2O for 15 min affords a deep blue solution containing the complex cation [Fe([18] aneN 2S 4)] 2+. Addition of excess BF 4 − or BPh 4 − counterions gives the complex as a blue solid. A single crystal X-ray structure determination of [Fe([18]aneN 2S 4)](BPh 4) 2· 2MeCN · 1/2MeOH shows iron(II) bound to all six macrocyclic donor atoms in a distorted octahedral geometry, FeN = 2.022(4), 2.037(5), FeS = 2.2578(17), 2.2588(16), 2.2673 (16), 2.2674(15) Å. The macrocycle binds to give a rac configuration with each SCH 2CH 2 NCH 2CH 2S linkage bound meridionally to the metal centre. Solutions of [Fe([18]aneN 2S 4)] (BF 4) 2 show two irreversible, overlapping oxidations at E pa = + 0.78 and +0.88 V vs Fc/Fc +.

Harmonic level reduction in arrays reflecting magnetostatic forward volume waves

Reflection of magnetostatic forward volume waves (MSFVW) by ion implanted arrays in a RAC configuration has been investigated as broad-band microwave signal processing components. Harmonics reflected by the 45° oblique incidence array greatly limit the useful bandwidth. They are mainly attributed to the ion implanted bar profile. By improving this profile, the space-harmonic levels have been lowered. Bar width adjustment has been used to reduce the level of even harmonics. A two step-shaped profile realized by two patterns has reduced the level of both amplitudes of odd and even harmonics. A level of 14 dB below the fundamental has been obtained for the second harmonic reflected by periodic gratings. The relative harmonic level reaches - 26 dB for the third one and - 18 dB for the fourth one.