Chi-Keung Li

Low-spin iron(III) complexes of a tetradentate bis-amide ligand and X-ray crystal structure of trans-[Fe(bpc)(1-MeIm)2]ClO4 (H2bpc=4, 5-dichloro-1, 2-bis(2-pyridinecarboxamido)benzene; 1-MeIm=1-methylimidazole)

A series of low-spin iron(III) complexes [Fe(bpc)(L) 2 ] + (H 2 bpc=4,5-dichloro-1,2-bis(2-pyridinecarboxamido)- benzene; LBu 3 P, Im, 1-MeIm, t Bupy) have been synthesized. These iron complexes display reversible one-electron oxidation and reduction couples. The stable one-electron-oxidized species [Fe(bpc)(Bu 3 P) 2 ] 2+ has been generated electrochemically. The complex trans -[Fe(bpc)(1-MeIm) 2 ](ClO 4 ) has been characterized by X-ray crystallography: space group P ; a =8.425(1), b =9.512(1), c =18.413(19) A; α=96.91(4), β=95.83(4), γ=90.44(1)°; V =1457(2) A 3 ; Z =2.

Synthesis, structure, reactivity and electrochemistry of cis-dioxoruthenium-(VI) and -(V) complexes containing N,N,N′,N′,3,6-hexamethyl-3,6-diazaoctane-1,8-diamine

The complexes cis-[RuVILO2]2+, cis-[RuVLO2]+ and cis-[RuIIL(MeCN)2]2+(L =N,N,N′,N′,3,6-hexamethyl-3,6-diazaoctane-1,8-diamine) have been prepared and their structures determined. The two RuO bonds in cis-[RuVILO2]2+ are equivalent [1.795(9)A] and the O–Ru–O angle is 112.0(4)°. In cis-[RuVLO2]+ the two RuO distances are 1.751(3) and 1.756(4)A, and the O–Ru–O angle is 115.1(2)°. The N(MeCN)–Ru–N(MeCN) angle in cis-[RuIIL(MeCN)2]2+ is 86.1(2)°. The cyclic voltammogram of cis-[RuVILO2]2+ in acetonitrile exhibits a reversible one-electron RuVI–RuV couple at 0.53 V vs. Ag–AgNO3(0.1 mol dm–3). In aqueous solutions, proton-coupled electron-transfer redox couples are observed. This complex is capable of oxidising a wide variety of organic substrates including 2,3-dimethylbutane and adamantane. Oxidation of saturated alkanes occurred preferentially at the tertiary C–H bond.

Ligand C-C bond cleavage on ruthenium : observation of a reversible ruthenium(V)-imido/ruthenium(II)-amine couple and X-ray crystal structure of [Ru(bpy)2(NH=CMe2)2](PF6)2 (bpy = 2,2'-bipyridine)

A reversible three-proton three-electron couple has been observed for the electrochemical oxidation of cis-[Ru(bpy)2(L)]2+(L = 2,3-dimethyl-2,3-diaminobutane) in aqueous acidic solution; controlled-potential electrochemical oxidation of cis-[Ru(bpy)2(L)]2+ led to C–C bond cleavage in L and formation of cis-[Ru(bpy)2(NHCMe2)2]2+ whose structure has been determined by X-ray crystallography.

The synthesis and reactivity of the heptanuclear dianion [Os7(CO)20]2− including the synthesis and structural characterizations of the compounds [Os7(CO)20(AuPEt)32] and [Os8(CO)20(η6-C6H6]

Reduction of the heptaosmium cluster [Os7(CO)21] With [Et4N][NH4) gives the cluster dianion [Os7(CO)20]2−,1, in high yield. The reaction of the dianion with [AuPR3Cl] (R=Et or Ph) in the presence of TlPF6 forms [Os7((CO)20(AuPR3)2] [R=Et (2a);R = Ph(2b)] in 80% yield, while the corresponding reaction with (Os(C6H6)(CH3CN)3]2+ gives [Os8(CO)20 (η6-C6H6)] (3) in reasonable yield (ca. 30%). The dianion,1, and the clusters2 and3 have been fully characterized by bout spectroscopic and crystallographic methods. The crystal structure of the [Ph4P]+ salt of1 shows that the metals in the anion adopt a capped octahedral geometry, with all twenty carbonyl ligands in terminal sites. The metal core geometry in2a is best described as a tricapped octahedron, and is based on the structure of the dianion1 with two adjacent octahedral faces capped by the Au atoms of the two AuPEt3 groups. In a similar fashion, the geometry of3 is related to that of1 with the addition of an Os(C6H6) unit capped to a triangular face, to give a bicapped octahedral framework.

Synthesis, electrochemistry, and reactivities of trans-[RuVI(L)O2]2+ (L=N,N'-dimethyl-N,N'-bis(2-pyridylmethyl)propylenediamine

The synthesis and characterization of the ligand N,N′-dimethyl-N,N′-bis(2-pyridylmethyl)-propylenediamine (L) and the complex trans-[RuVI(L)O2]2+ are described. The complex has been characterized by u.v.–visible [dxy→dπ*(dπ*=dxz,dyz) at 380–420 nm] and i.r. [νasym(RuO2) at 860 cm–1] spectroscopy. At pH 1.0 it exhibits three reversible couples RuVI–RuIV, RuIV–RuIII, and RuIII–RuII at potentials of 0.89, 0.60, and 0.28 V respectively vs. saturated calomel electrode. The RuVI–RuIV couple splits into two reversible one-electron couples RuVI–RuV and RuV–RuIV in alkaline solutions (pH > 9). The complex is a powerful oxidant, capable of oxidizing toluene to benzaldehyde, alcohols to aldehydes/ketones, tetrahydrofuran to γ-butyrolactone, and norbornene to exo-2,3-epoxy-norbornane in high yields at room temperature. Oxidation of cis- and trans-stilbenes gave benzaldehyde and trans-stilbene oxides.

Cluster build-up using the [Os(η6-C6H6)(MeCN)3]2+ cation; synthesis and structural characterisation of some hexa- and hepta-nuclear arene-substituted clusters

Reactions of the cluster dianion [Os5(CO)15]2– with [Os(η6-C6H6)(MeCN)3]2+ and [Os(C6H5Me)(CF3SO3)2] provided [Os6(CO)15(η6-C6H6)]1 and [Os6(CO)15(η6-C6H5Me)]2, respectively, in good yield (≈45%). Reduction of the hexaosmium cluster [Os6(CO)18] with K–Ph2CO gave the cluster dianion [Os6(CO)17]2–3 in quantitative yield. When this dianion was treated with [Os(η6-C6H6)(MeCN)3]2+ the heptanuclear cluster [Os7(CO)17(η6-C6H6)]4 was obtained in fair yield, while the corresponding reaction with [Os(η6-C6H5Me)(CF3SO3)2] gave [Os7(CO)17(η6-C6H5Me)]5 in similar yield (ca. 25%). The four arene clusters have been characterised by spectroscopic techniques, and the molecular geometries of 1, 2 and 4 established by single-crystal X-ray diffraction techniques. In both 1 and 2 the metal framework geometry is best described as a bicapped tetrahedron. In 1 the η6-C6H6 ligand occupies a site on the central Os4 tetrahedron while in 2 in the η6-arene is co-ordinated to one of the capping Os atoms. The metal framework in 4 may be viewed as derived from a bicapped tetrahedron with the seventh metal capping one of the caps to give a chain of four fused tetrahedra. The η6-C6H6 ligand in 4 occupies a similar site to that found in 1.

Synthesis, electrochemistry and X-ray crystal structure of cis-[RuIIIL1(Cl)(H2O)][ClO4]2·2H2O [L1=N,N′-dimethyl-N,N′-bis(2-pyridylmethyl)ethylenediamine]; electrochemical oxidation of alcohols and tetrahydrofuran by cis-[RuVL1(Cl)O]2+

Reaction of K2[RuCl5(H2O)] with N,N′-dimethyl-N,N′-bis(2-pyridylmethyl)ethylenediamine (L1) in ethanol gave cis-[RuIIIL1Cl2]+ and then cis-[RuIIIL1(Cl)(H2O)]2+ upon reaction with AgI in water. The structure of cis-[RuIIIL1(Cl)(H2O)][ClO4]2·2H2O has been established by X-ray crystallography: space group Pbca, a= 10.863(1), b= 21.548(2), c= 21.912(2)A. The measured Ru-Cl and Ru-OH2 distances are 2.307(4)A and 2.070(9)A respectively. A cyclic voltammogram of cis-[RuIIIL1(Cl)-(H2O)]2+ in 0.1 mol dm–3 CF3CO2H shows three reversible/quasi-reversible couples at 1.29, 0.93 and 0.23 V vs. saturated calomel electrode assigned to the couples RuV-RuIV, RuIV-RuIII and RuIII-RuII. The complex cis-[RuIIIL1(Cl)(H2O)]2+ is an active catalyst for the electrochemical oxidation of alcohols and tetrahydrofuran. The second-order rate constant for the oxidation of benzyl alcohol by electrochemically generated cis-[RuVL1(Cl)O]2+ is estimated to be 8.4 × 104 dm3 mol–1 s–1.

Synthesis, reactivities and electrochemistry of trans-dioxoruthenium(VI) complexes of π-aromatic diimines

The synthesis and electrochemistry of trans-[RuIII(L)2(OH)(H2O)][CIO4]2[L = 1,10-phenanthroline (phen) or 5,5′-dimethyl-2,2′-bipyridine(dmbipy)] are described. Oxidation of trans-[RuIII(L)2(OH)(H2O)]2+ by CeIV in water gave trans-[RuVI(L)2O2]2+ which were isolated as the yellow perchlorate salts. The complex trans-[RuVI(dmbipy)2O2][CIO4]2 is a powerful oxidant with E°(RuVI–RuIV)= 1.0 V vs. saturated calomel electrode at pH 1.0. In acetonitrile or acetone, it oxidizes alcohols to ketones/aldehydes, tetrahydrofuran to γ-butyrolactone, alkenes to epoxides and saturated or aromatic hydrocarbons to alcohols/ketones. Oxidation of saturated alkanes occurs preferentially at the tertiary C–H bond. In the presence of CCl4, cyclohexane is oxidized to cyclohexyl chloride instead of cyclohexanone. The mechanisms of alcohol and cyclohexane oxidation were investigated by kinetic experiments and by isotope labelling studies.

Synthesis and X-ray structure of the octaosmium carbido cluster Os8C(CO)22

Abstract Solid state vacuum pyrolysis of Os 3 H 3 (CO) 9 (μ 3 -COMe) ( 1 ) gives the dark purple octanuclear carbido cluster Os 8 C(CO) 22 ( 2 ) in ca . 15% yield. An X-ray analysis of 2 shows that the carbido-carbon is encapsulated in a trigonal prismatic cavity within the metal core.

Mixed alkyne-arene-substituted carbonyl clusters; the crystal and molecular structure of OS4(CO)9(η6-C6H6)(MeCCMe)

Reaction of the alkyne-substituted cluster anion [Os3(CO)9(R1CCR2)]2− with the cation [Os(η6-C6H6)(CH3CN)]2+ affords the neutral tetranuclear cluster Os4(CO)9(η6-C6H6)(R1CCR2) (R1=R2=Me (1a); R1=Me, R2=Ph (1b); R1=R2=Ph (1c)). The structure of 1a has been confirmed by a single-crystal X-ray analysis. The metals adopt a ‘butterfly’ geometry with the alkylenic CC vector parallel to the ‘hinge’ vector of the ‘butterfly’, and the η6-C6H6 ligand occupies a terminal site on one of the ‘wingtip’ Os atoms. Complexes 1a and 1b may also be prepared by the reaction of the activated arene cluster Os4H2(CO)9(η6-C6H6)(CH3CN) with MeCCMe and MeCCPh, respectively. Under similar reaction conditions, with PhCCPh, the arene and acetonitrile ligands are substituted by the alkyne to give the known complex Os4H2(CO)9(PhCCPh)2 (2c).