Bernhard Metz

Chromium Sandwich Complexes of Polycyclic Aromatic Hydrocarbons: Triphenylene and Fluoranthene as η6 Ligands

The sandwich complexes bis(η6-triphenylene)chromium (12) and bis(η6-fluoranthene)chromium (13) have been prepared by means of metal atom/ligand vapor cocondensation. Whereas for triphenylene exclusive coordination to the peripheral rings is observed, the situation is more complicated for fluoranthene. According to NMR evidence initial metal coordination to the benzene (B) as well as to the naphthalene (N) section of the fluoranthene ligand occurs, leading to the isomers 13(I) (η6-B, η6-B), 13(II) (η6-B, η6-N) and 13(III) (η6-N, η6-N). Since the substitutional lability of the chromium–naphthalene bond largely exceeds that of the chromium–benzene bond, the isomer distribution depends on the workup conditions; 13(I) is clearly the most stable isomer. Crystal structure determinations performed for the salts [12][BPh4] and [13][I] point to the preference for syn orientation of the polycyclic aromatic hydrocarbons and to a minute metal slippage in the peripheral direction. The triphenylene complex 12 features the electrochemically reversible redox couples 12 (+/0, metal-centered), 12 (0/–, ligand-centered) and 12 (–/2–, ligand-centered), the latter displaying a redox splitting of 300 mV. Conversely, for the fluoranthene complex 13, secondary reduction 13 (–/2–) is irreversible. This finding is consistent with the larger redox splitting of ca. 480 mV which indicates more extensive interligand interaction in the dianion 132–, thereby favoring metal–ligand cleavage. While the radical cations 12+· and 13+· are amenable to EPR study, the radical anions 12–· and 13–· are too unstable. Instead, the radical anions of the free ligands are observed by EPR upon electrochemical reduction. In the case of 12, the temporary existence of the radical anion 12–· is indicated, however.

Metall-π-Komplexe von benzolderivaten XXXVIX. Di[1,4-bis(diphenylphosphano)-η6 benzol]chrom: Ein baustein für polynukleare bimetallkomplexe?

Abstract Nucleophilic substitution at bis(1,4-dichloro-η 6 -benzene)chromium by means of lithiumdiphenylphosphide affords di[1,4-bis(diphenylphosphano)η 6 -benzene]chromium ( 5 ). Two para -positioned organometallic diphos units and a central metal atom featuring variable charge and magnetism, render the complex 5 a versatile potential bridging ligand in oligomeric materials. Here the bifunctional chelating nature of 5 is exploited for the synthesis of the hetera-metallocyclophane [2,2,11,11-tetracarbony-1,1,3,3,10,10,12,12-octaphenyl-2,11-dinickela(0)-1,3,10,12-tetraphospha- η 12 -[3.3]paracyclophane]chromium ( 6 ). According to 1 H, 13 C, and 31 P NMR spectroscopic evidence the bridges -PPh 2 -Ni(CO) 2 -PPh 2 - in 6 in the range − 80 T 2 -Ni(CO) 2 -PPh 2 - bridged complex 2 which was prepared from bis(diphenylphosphano- η 6 -benzene)chromium ( 1 ) and Ni(CO) 4 . Cyclic voltammetry uncovers a rich redox chemistry for the trinuclear complex 6 which, however, is reversible only for the first oxidation step 6 +/0 . The anodic peak potentials which follow this first, chromium-centered, oxidation are assigned to the couples Ni +/0 , Cr 2+/1 , and Ni 2+/1+ . The reduction 6 0/− occurs quasi reversibly; the relative ease, with which the stage 6 − is reached, points to electron delocalization over both Ni(CO) 2 units. The radical cations 1 + , 2 + , 5 + and 6 + are also accessible through chemical oxidation, they are subjected to EPR spectroscopic study. The hyperfine interaction a ( 31 P), observed for 2 + and 5 + testifies to spin delocalization into the ligand periphery.

Metal π complexes of benzene derivatives: XLIII. Intramolecular interactions between bis(benzene) chromium and cymantrene units in di- and tri-nuclear species containing PMe2 spacers, as studied by CV and EPR

Abstract Taking advantage of the ability of R2P-substituted derivatives of bis(benzene)chromium and bis(benzene)vanadium to act as organometallic ligands, we have prepared the di- and tri-nuclear complexes [(η)6-C6H6)(Me2P-η6-C6H5)Cr](η5-MeC5H4)(CO)2Mn) (3), μ(Me2P-η6-C6H5)2CrI(η5-MeC5H4)(CO)2Mn]2 (7), [(Me2P-η6-C6H5)2Cr](η5-MeC5H4)(CO)Mn (9) and [(Me2P-η6-C6H5)2V] (η5-MeC5H4)(CO)Mn (10). The structures of 7 and 9 were determined by X-ray diffraction studies. Intermetallic communication in the binuclear complexes is reflected in small shifts of their redox potentials relative to those of the corresponding mononuclear component. Redox splitting for the oxidation of the terminal MeCpMn(CO)2 units of 7+. is not resolved. The EPR spectra of 3+., 7+. and 9+. show, in addition to strong 53Cr hyperfine coupling, a small interaction caused by 55Mn. Whereas the monoradical monocations 3+., 7+. and 9+. can be isolated as hexafluorophosphates, the dications 3+.+. and 9+.+. and the trication 7+.+.+.; although formed reversibly as indicated by cyclic voltammetry, could not be isolated on a preparative scale. In the case of 9+.+. the biradical character was demonstrated by the observation of a ΔMs = 2 transition in the EPR spectrum. Detailed analysis of the ΔMs = 1 region was hampered by substantial decomposition of the dication even at low temperature.

Zinn in der Peripherie von Bis(aren)metall-Komplexen des Vanadiums und Chroms

Durch Metallatom-Ligand-Cokondensation sowie uber naschemische Verfahren (Lithiierung und Folgereaktion) wurden als erste organostannylsubstituierte Bis(aren)metall-Komplexe die Verbindungen (R3Sn-η6-C6H5)2M dargestellt: 15 (R = Me, M = V), 16 (R = Ph, M = V), 13 (R = Me, M = Cr), 17 (R = Ph, M = Cr). Trotz der sperrigen Natur des Ph3Sn-Substituenten weicht die Geometrie der zentralen Sandwicheinheit in 17 nur unwesentlich von der des unsubstituierten Grundkorpers (C6H6)2Cr (2) ab. Die trikline Elementarzelle von 17 (Raumgruppe P1; a = 9.414(4), b = 9.877(5), c = 11.012(13) A; α = 83.51(7), β = 87.95(7), γ = 72.67(4)°) enthalt ein unabhangiges Molekul. Organostannylgruppen storen die Elektronenstruktur der Bis(aren)metall-Einheit offenbar nur geringfugig, denn die EPR-Spektren der M(d5) Spezies lassen keine Abweichungen von axialer Symmetrie erkennen. Die Potentiale fur die reversible Oxidation der Me3Sn-substituierten Komplexe 13 und 15 weichen nur wenig (anodische Verschiebung ≤ 20 mV) von denen der Grundkorper 1 und 2 ab, die Reduktion ist in beiden Fallen irreversibel. Grosere anodische Verschiebungen weisen die Ph3Sn-Derivate 16 und 17 auf; auch fur diese sind nur die Redoxpaare 0/+ reversibel. Der Resistenz der Neutralkomplexe in protischen Medien steht die uberaus leicht erfolgende Hydrodestannylierung der Komplexkationen gegenuber. Durch Ummetallierung wird aus 13 (Li-η6-C6H5)2Cr hilfsbasenfrei und streng 1,1′-dimetalliert gewonnen. Metal π Complexes of Benzene Derivatives. 53 [1] Tin in the Periphery of Bis(arene)metal Complexes of Vanadium and Chromium By means of metal-atom ligand-vapor cocondensation as well as via wet chemical methods (lithiation and follow-up reaction) the first organostannyl substituted bis(arene)metal complexes (R3Sn-η6-C6H5)2M have been prepared: 15 (R = Me, M = V), 16 (R = Ph, M = V), 13 (R = Me, M = V), 17 (R = Ph, M = Cr). Despite the bulkiness of the Ph3Sn groups the geometry of the central sandwich unit in 17 deviates only marginally from that of the parent complex (C6H6)2Cr (2). The triclinic unit cell of 17 (space group: P1; a = 9.414(4), b = 9.877(5), c = 11.012(13) A; α = 83.51(7), β = 87.95(7), γ = 72.67(4)°) contains one independent molecule. Perturbation of the electronic structure of the bis(arene)metal unit by organostannyl groups appears to be minute because EPR spectra of the M(d5) species fail to reveal deviations from axial symmetry. The potentials for reversible oxidation of the Me3Sn-substituted complexes 13 and 15 differ insignificantly (anodic shifts ≤ 20 mV) from those of the parent species 1 and 2; reductions are irreversible in both cases. More sizeable anodic shifts are observed for the Ph3Sn-derivatives 16 and 17; here as well, only the redox pairs 0/+ are reversible. The resistance of the neutral complexes to protic media contrasts to ready hydrodestannylation of the complex cations. By way of metal exchange, employing n-butyl lithium, 13 affords (Li-η6-C6H5)2Cr strictly 1,1′-disubstituted and devoid of auxiliary base.

Metall-π-Komplexe von Benzolderivaten, XLa Selen in der Peripherie des Bis(benzol)chroms: Darstellung, Strukturdynamik und Redoxverhalten eines neuen Hetera-[3]-chromocyclophans / Metal-π Complexes of Benzene Derivatives, XLa Selenium in the Periphery of Bis(benzene)chromium: Synthesis, Structural Dynamics, and Redox Behavior of a New Hetera-[3]-chromocyclophane

Lithiation of bis(benzene)chromium and subsequent reaction with Ph2Se2 or Se and CH3I, respectively, afforded the complexes (PhSe-η6-C6H5)2Cr (6) and (MeSe-η6-C6H5)2Cr (8), the latter being convertible into interannularly bridged (MeSe-η6-C6H5)2Cr[M(CO)4] (9-11, M = Cr, Mo, W). According to 1H NMR spectroscopy, 9 undergoes both bridge inversion and reversal, the former displaying spectral coalescence at 283 K (ΔGc≠ = 55.9 kJ mol-1) whereas the latter is still rapid at 200 K. Substitution of H for –SeMe and addition of a Cr(CO)4 unit effect anodic shifts of the redox potential E1/2(+/0) of 95 mV/SeMe and 230 mV/Cr(CO)4. Despite electron donation from the bis(arene)metal backbone 8 is a fairly weak ligand, cleavage of the interannular bridge occuring with the half life τ1/2 = 41.4 min (9→8) and τ1/2 = 5.8 s (9+•→8+) as inferred from cyclic voltammetry in DME at 293 K.