The homoleptic diamagnetic complexes M(mer-L)(2), M = Cr, Mo,W (1a,b, 2a,b, and 4a,b), were obtained by reacting the hexacarbonyls M(CO)(6) with the tridentate ligands 2-[(2-N-arylamino)phenylazo]pyridine (HL = NH(4)C(5)N=NC(6)H(4)N(H)C(6)H(4)(H) (HL(a)) or NH(4)C(5)N=NC(6)H(4)N(H)C(6)H(4)(CH(3)) (HL(b))) in refluxing n-octane. In the case of M = Mo, the dinuclear compounds [Mo(L)(pap)](2)(mu-O) (3a,b) (pap = 2-(phenylazo)pyridine), were obtained as second products in moist solvent. X-ray diffraction analysis for Cr(L(b))(2) (1b), Mo(L(a))(2) (2a), and W(L(a))(2) (4a) reveals considerably distorted-octahedral structures with trans-positioned azo-N atoms and cis-positioned 2-pyridyl-N and anilido nitrogen atoms. Whereas the N(azo)-M-N(azo) angle is larger than 170 degrees, the other two trans angles are smaller, at about 155 degrees (M = Cr, 1b) or 146 degrees (M = Mo, W; 2a, 4a), due to the overarching bite of the mer-tridentate ligands. The bonds from M to the neutral 2-pyridyl-N atoms are distinctly longer by more than 0.08 A than those to the anilido or azo nitrogen atoms, reflecting negative charge on the latter. The N-N bond distances vary between 1.339(2) A for 1b and 1.373(3) A for 4a, clearly indicating the azo radical anion oxidation state. Considering the additional negative charge on anilido-N, the mononuclear complexes are thus formulated as M(IV)(L*(2-))(2). The diamagnetism of the complexes as shown by magnetic susceptibility and (1)H NMR experiments is believed to result from spin-spin coupling between the trans-positioned azo radical functions, resulting in a singlet diradical situation. The experimental structures are well reproduced by density functional theory calculations, which also support the overall electronic structure indicated. The dinuclear 3a with N-N distances of 1.348(10) A for L(a) and 1.340(9) A for pap is also formulated as an azo anion radical-containing molybdenum(IV) species, i.e., [Mo(IV)(L*(2-))(pap*-)](2)(mu-O). All compounds can be reversibly reduced; the Cr complexes 1a,b are also reversibly oxidized in two steps. Electron paramagnetic resonance spectroscopy indicates metal-centered spin for 1a+ and 1a- and g approximately 2 signals for 2a-, 3a+, 3a-, and 4a-. Spectroelectrochemistry in the UV-vis-NIR region showed small changes for the reduction of 2a, 3a, and 4a but extensive spectral changes for the reduction and oxidation of 1a.
In an unusual reaction of the polyacid, ammoniumheptamolybdate tetrahydrate ((NH(4))(6)[Mo(7)O(24)].4H(2)O), and the ligand, 2-[(arylamino)phenylazo]pyridine (general abbreviation HL), in the presence of PPh(3) afforded the brown oxo free molybdenum complexes of type [Mo(L)(2)] in high yields (ca. 80%). The reaction occurs smoothly in ethanol. It is slow on a steam bath (25 h) but is complete in about an hour in a microwave oven. X-ray structures of two representatives are reported. In these complexes the ligand acted as a tridentate ligand using its pyridyl(N), azo(N), and the deprotonated amine(N), respectively. The geometry is meridional, and the relative orientations within the coordinated pairs of nitrogens are cis, trans, and cis, respectively. Bond length data of the coordinated ligands are consistent with a Mo(IV)[L.](2) ([L.](2-) = azo dianion radical formed by one electron reduction of the deprotonated anionic ligand, [L](-)) description. For example, the N-N lengths (1.349(5)-1.357(2) A) in these complexes are appreciably longer than that (1.246(3) A) in the uncoordinated and protonated salt of a representative ligand, [H(2)L(d)]ClO(4). The N-N lengths, however, correspond well with metal complexes of the ligand containing azo ion radical. The complexes are diamagnetic and showed highly resolved (1)H NMR and (13)C NMR spectra. The two coordinated ligands in these are magnetically equivalent, and resonances for only one ligand were observed in their spectra. Diamagnetism in the present molybdenum complexes is attributed to strong antiferromagnetic coupling between Mo(IV)(4d(2)) and the two planar radical [L.](2-) ligands. The complexes display multiple redox responses. The ESR spectrum of electrogenerated [1a](-) showed a characteristic spectrum for Mo(III) with weak hyperfine lines due to the presence of molybdenum isotopes having nonzero nuclear spin. Visible range multiple charge transfer transitions in these complexes are ascribed to ligand-to-metal transitions.
In einer f r~e r e n Mitteilung2) ist die Ordnung der Reaktionsgeschwindigkeit zwischen Mercurichlorid und Kaliumoxalat im Dunkeln festgestellt worden. Es wurde gcfunden, daB die Dunkelreaktion sowohl in bezug auf das Mercmichlorid wie in bezug auf das Oxalat monomolekular ist. Demnach ist die Gesamtreaktion im Dunkeln bimolekular -In der vorliegenden Mitteilung wird eine Untersuchung derselben Realrtion im Sonnenlicht beschrieben. Auch in diesem Falle benutzte man OSTWALDS Isolationsverfahren, d. h. ein groBer UberschuB von Ammoniumoxalat wurde mit einer Mercurichloridlosung von bestimmter Konzentration gemischt . Das Mercurichlorid, das sich als Produlit der chemischen Realction 2HgC1,+ (NH4),C,O4 = 2NH4C1 + 2HgC1 bildete, wurde schnell abfiltriert und das Filter ausgewaschen. Zn einem bestimmten Volumen des unveriinderten TvIercurichlorides setzte man einen UberschuB einer Losung von Kaliumjodid von bekanntem Gehalt und titrierte den UberschuB des Jodides gegen eine Mercurichloridlosung. Es ist bereits beobaohtet worden, daB Kohlendioxyd die chemische Reektion zwischen Mercurichlorid und Ammoniumoxalat im Dunkeln merklich beschleunigt (DHAR, 1. c.). Auch im Licht wirkt Kohlendioxyd als Beschleuniger ; deswegen wurde in allen Fallen die Reaktionsmischung mit Kohlendioxyd gesattigt, bevor sie dem Sonnenlichte ausgesetzt wurde. Es ist augenscheinlicb, da8 die GroBe des chemischen Umsatzes der Lichtintensitat proportional sein wird. Geringe Unterschiede der Lichtintensitat verursachen merkliche Unterschiede in den aus dernselben Gemisch erhaltenen Mercurichlorid. Die Realition ist auch sehr empfindlich gegen Spuren von Verunreinigungen, wie spkter gezeigt werden wird. Sehr unbedeutende Mengen von Eisensalzen konnen 1) Am dem Mmuskript ins Deutsche iibertragen von I. KOPPEL -Berlin. ?) DEAR, Journ. C h m . SOC. 111 (1917), 750.
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