Mahua Menon

SYNTHESIS AND X-RAY CRYSTAL STRUCTURE OF AN OXORHENIUM(V) COMPLEX WITH A DOUBLE SCHIFF BASE

Abstract The reaction of the double Schiff bases, RL, formed from 2,6-diformyl-4-methyl-phenol and p-RC6H4NH2 (R=H, Me, OMe, Cl) with ReOCl3(PPh3)2 afforded red coloured complexes of the type RevOCl3(RL). The X-ray structure of the R = Me complex (crystal data: ReOCl3(MeL)·CH2Cl2, triclinic, space group P I , a = 8.736(2), b=10.632(3), c=10.575(5)A, a=104.64(2)°, β=93.22(2)°, γ = 110.57(2)°, V = 1376.6(7) A , Z =2, R = 3.06%, R w = 3.51% ) revealed meridional chloro coordination in an ReOCl3(ON) coordination sphere. The RL ligand binds in the bidentate ON mode, the second imine function occurring in the protonated from (NH…O hydrogen bonding). IR and 1H NMR data indicate that the solid-state structure is substantially retained in solution. The complexes display a quasi-reversible ReVIO/ReVO couple whole potential depends on the R substituent.

Synthesis, spectroscopic characterisation, electron-transfer propertiesand crystal structure of[RuII(bipy)2(2-SC5H4N)]ClO4 (bipy = 2,2′-bipyridine)

Two new ruthenium(II) mixed-ligand tris-chelated complexes of the type [Ru(bipy) 2 L]ClO 4 , (bipy = 2,2′-bipyridine; L = pyridine-2-thiolate 1 or pyridin-2-olate 2) have been synthesized. The complexes are essentially diamagnetic and behave as 1∶1 electrolytes in acetonitrile solution. They display two metal-to-ligand charge-transfer (m.l.c.t.) transitions near 500 and 340 nm respectively along with intraligand transitions in the UV region. Both exhibit room-temperature emission from the highest-energy (m.l.c.t.) band. At room temperature the lifetime of the excited states for the thiolato (1) and phenolato (2) complexes are 100 and 90 ns respectively. The geometry of the complexes in solution has been assessed by high-resolution 1 H NMR spectroscopy. The molecular structure of complex 1 in the solid state has been determined by single-crystal X-ray diffraction. It shows the expected pseudo-octahedral geometry with considerable strain due to the presence of the sterically hindered ligand L 1 . In acetonitrile solution the complexes show quasi-reversible ruthenium(II)–ruthenium(III) oxidation couples at 0.54 and 0.64 V versus saturated calomel electrode and quasi-reversible ruthenium(III)–ruthenium(IV) oxidations at 1.41 and 1.03 V respectively. Two reversible reductions are observed near -1.6 and -1.9 V for each complex due to electron transfer to the co-ordinated bipy units. The trivalent analogues of 1 and 2 are unstable at room temperature but can be generated in solution by coulometric oxidation at 263 K as evidenced by EPR spectroscopy.

Pyridine-2-carboxamide complexes of arylimidorhenium(VI)derived from 2-pyridylmethyleneamine complexes ofarylimidorhenium(V) via oxygen-atomtransfer

Complexes of type [Re V L(NC 6 H 4 Y-p)Cl 3 ] (Y = H or Cl) have been synthesized where L is the Schiff base formed from pyridine-2-carbaldehyde and the aniline p-XC 6 H 4 NH 2 (X = Me or Cl). Treatment with aqueous nitric acid in acetonitrile converts them into [Re VI L′(NC 6 H 4 Y-p)Cl 3 ], where L′ is a monoanionic pyridine-2-carboxamide. The latter complexes display hyperfine-split six-line solution EPR spectra at room temperature. The crystal structures of [ReL(NC 6 H 4 Cl)Cl 3 ] and [ReL′(NC 6 H 4 Cl)Cl 3 ] (X = Me in both cases) revealed the presence of severely distorted and meridionally configured ReCl 3 N 3 co-ordination spheres. The effective metal radius decreases only marginally upon metal oxidation and the imide fragment is approximately linear and triple bonded, ReN–C, in both cases. The rhenium(VI)–rhenium(V) reduction potentials in the two types of complex are ≈1.0 and ≈0.2 V respectively.

Chemistry of [RuL2(PPh3)2]o, +(HL = 8-hydroxyquinoline): synthesis, structure, metal redox behaviour and isomer selectivity

The reaction of [Ru(PPh3)3Cl2] with 8-hydroxyquinoline (HL) has afforded [RuL2(PPh3)2] in two isomeric forms, one red and one yellow-green, the RuN2O2P2 co-ordination sphere being cis,trans,cis and trans,trans,trans respectively. Chemical or electrochemical oxidation of either isomer affords exclusively red-brown trans,trans,trans-[RuL2(PPh3)2]+ isolated as the PF6– salt. The latter complex is one-electron paramagnetic (low-spin t25) and in frozen glass (77 K) displays a rhombic EPR spectrum. Reduction of trans,trans,trans-[RuL2(PPh3)2]PF6 by chemical or electrochemical means yields only trans,trans,trans-[RuL2(PPh3)2]. X-Ray crystallography has been performed on cis,trans,cis-[RuL2(PPh3)2] and trans,trans,trans-[RuL2(PPh3)2]PF6·CH2Cl2. The Ru–O and Ru–N bond lengths decrease upon metal oxidation but the Ru–P bond length increases by ca. 0.15 A(strong 4dπ–3dπ RuII–P back-bonding). The reduction potential (RuIII–RuII) order E½(cis,trans,cis) > E½(trans,trans,trans) corresponds to superior back-bonding in the cis- as compared to the trans-Ru(PPh3)2 motif. The stability of the observed geometry–valence combinations has been rationalised in terms of back-bonding, phosphine crowding and phenolato oxygen repulsion.

Geometrical preference of ruthenium oxidation states: metal redox and isomerisation of [Ru(S2CNEt2)2(PPh3)2]0,+

Reaction of Na[S2CNEt2] with [Ru(PPh3)3Cl2] afforded cis-[RuL2(PPh3)2](L = S2CNEt2), which upon oxidation by cerium(IV) furnished trans-[RuL2(PPh3)2]+, isolated as its PF6– salt. Both complexes are obtained in nearly quantitative yields. The trivalent complex displays one-electron paramagnetism and a rhombic frozen-solution EPR spectrum which has been analysed affording axial and rhombic distortion parameters of ≈ 8000 and ≈ 1700 cm–1 respectively. A ligand-field transition within the Kramers doublets is observed at 6450 cm–1, the predicted value being ≈ 7000 cm–1. Variable-temperature (253–303 K) voltammetric studies have revealed that electrode reactions proceed in a stereoretentive fashion [E½(cis), 0.23 and E½(trans), –0.09 V vs. saturated calomel electrode (SCE)] but the species cis-[RuL2(PPh3)2]+ and trans-[RuL2(PPh3)2] are unstable and spontaneously isomerise affording the stable trans-trivalent and cis-bivalent complexes respectively, the equilibrium concentration of the unstable isomers being very small. The rates and activation parameters of these isomerisation reactions presumably proceeding by the twist mechanism (ΔS‡ highly negative) have been determined. The factors controlling geometrical preference of oxidation states viz., metal–phosphorus back-bonding and PPh3⋯ PPh3 steric repulsion are considered in relation to the osmium analogues reported previously.

Oxidation of coordinated azomethine to amide. Synthetic and structural studies on a rhenium and a ruthenium system

The title reaction is exemplified by the high yield conversions 1→2 and 3→4 by H2O2, and the X-ray structures of 2 and 4 are reported; the species 1+ and 3+ afford 2 and 4 respectively when treated with water.