L. I. Denisovich

Electrochemical generation of 19- and 20-electron rhodocenium complexes and their properties

Abstract The electrochemical reduction of the rhodocenium salts [Rh(η 5 -C 5 Me 5 )(η 5 -L)] + PF 6 − ( 2 + , L  C 5 H 5 ; 3 + L  C 5 Me 5 ; 4 + L  C 9 H 7 ) has been studied by cyclic voltammetry. All three complexes ( 2 + - 4 + ) are reduced in two one-electron processes, first to the 19-electron radicals ( 2 - 4 ) and then to the 20-electron anions ( 2 − - 4 − ). When complex 2 + was reduced in bulk (NaHg in THF) the dinuclear complex, [(η 5 -C 5 H 5 )Rh(μ-η 4 : η 4 -C 5 Me 5 -C 5 H 5 )Rh(η 5 -C 5 Me 5 )] ( 6a ), obtained by dimerization of 2 , was isolated as sole product. Analogous reduction of 3 + gave the η 4 -pentamethylcyclopentadiene complex, [Rh(η 5 -C 5 Me 5 )(η 4 -C 5 Me 5 H)] ( 7 ). Reduction of 4 + leads to formation of a stable radical 4 .

Reduction of ruthenium arenecyclopentadienyl complexes reactions induced by electron transfer

Abstract Ruthenium arenecyclopentadienyl complexes [Ru(η5-C5R5)(η6-arene)]+ (1, R = H, arene = C6H6; 2, R = Me, arene = C6H6; 3, R = H; arene = C6H3Me3; 4, R = Me, arene = C6H3Me3; 5, R = H, arene = C6Me6; 6, R = Me, arene = C6Me6; 7, R = Me, arene = C10H8) and [Ru(η5-C9H7)(η6-C6H6)]+ (8) have been studied by cyclic voltammetry; the complexes are capable of both reduction and oxidation. The reduction peak potential values for 1–6 become more negative by about 31 mV for each Me-group at the arene ring and 61 mV for each Me-group at the cyclopentadienyl ring. Reduction of naphthalene complex 7 proceeds by two one-electron processes; the first one is reversible and the second one is irreversible. Two reversible reduction peaks were observed for indenyl complex 8. The following reactions occur on reduction of benzene complexes 1, 2 and 8 with sodium amalgam in tetrahydrofuran (THF): hydrogen atom addition to and decoordination of benzene ligand as well as dimerization of ligand-to-ligand type. Mesitylene compounds 3 and 4 form dimers [(η5-C5R5)Ru(μ-η5:η5-Me3H3C6C6H3Me3)Ru(η5-C5R5)] (14, R = H; 15, R = Me) in both chemical and electrochemical reduction processes. Reaction of [Ru(η5-C5H5)(η6-C6Me6)]+ (5) with sodium amalgam in THF leads to the dimer [(η5-C5H5)Ru(μ-η5:η5-Me6C6C6Me6)Ru(η5-C5H5)] (16) as the major product; products of H-atom addition to both hexamethylbenzene and cyclopentadienyl ligands, [Ru(η5-C5H5)(η5-C6Me6H)] (17) and [Ru(η4-C5H6)(η6-C6Me6)] (18), are also formed in low yields. In the case of permethylated 6 only H-atom addition to hexamethylbenzene was observed and the mixture of endo-H and exo-H isomers [Ru(η5-C5H5)(η5-C6Me6H)] (19a,b) was isolated. Reduction of 7 gives [Ru(η5-C5Me5)(η5-C10H9)] (20). The modes of reaction of 19-electron radicals formed by reduction of 1–8 depend on electronic and steric properties of ligands.

Redox-induced activation of CH bonds in 1,2,3,4,5-pentamethylcyclopentadiene platinum complexes

The electrochemical behaviour of the platinum complexes [Pt(η4C5Me5H)(η5C5H5)] +BF4− (1+) and [Pt(η4-diene)(η5C5Me5)]+BF4− (diene = C5Me5H (2+) or 1,3-cyclohexadiene; (3+) 1,5-cyclooctadiene(4+)) has been studied by cyclic voltammetry. The complexes 1+−4+ are capable of both oxidation and reduction. The bulk electrolysis of 2+ at the potential of its reduction gives a mixture of twe neutral isomeric complexes 5a and 5b. These are assigned structures with η3-allylic and of σ, π-bonding methylene cyclopentenyl Me3C5H(CH2) ligand. This is the proposed result from methyl CH bond breakage in pentamethylcyclopentadiene ligand of an intermediate 19-electron complex [Pt(η4C5Me5H) (η5C5Me5)]. (2.). The same mixture of isomers 5a and 5b forms in interaction of 2+ and tBuOK in tetrahydrofuran. The preparative electrochemical oxidation of 2+ proceeds with CH bond scission at the pentamethylcyclopentadiene sp3-hydridized carbon in an intermediate 17-electron dication radical [Pt(η4C5Me5H)(η5C5Me5)].2+ (2.2+) to give decamethylplatinacene dication [Pt(η5C5Me5)2]2+(BF4−)2 (72+). The one-electron reduction of 72+ regenerates 2+.

Reduction of iridocenium salts [Ir(η5-C5Me5)(η5-L)]+ (L = C5H5, C5Me5, C9H7); ligand-to-ligand dimerisation induced by electron transfer

Redox properties of iridium complexes [Ir(η5-C5Me5)(η5-L)]+ (1+, L = C5H5; 2+, L = C5Me5; 3+, L = C9H7) were studied by cyclic voltammetry (CV). All three complexes can be reduced to 19-electron radicals 1–3. The stability and reactivity of these radicals depend on the electronic and steric properties of the ligands. The mixture of dimers [(η5-C5Me5)Ir(μ-η4:η4-C5H5C5Me5)Ir(η5-C5H5)] (4a) and [(η5-C5H5)Ir(μ-η4:η4-C5Me5C5Me5)Ir(η5-C5H5)] (4b) was formed as a result of reduction of 1+ with NaHg in THF. Both chemical and electrochemical reduction of 2+ gave the dimer [(η5-C5Me5)Ir(μ-η4:η4-C5Me5C5Me5)Ir(η5-C5Me5)] (5) in low yield. Reduction of 3+ gave the tetranuclear complex [(η5-C5Me5)Ir(μ-η4:η5-C9H7) Ir(μ-η4:η4-C5Me5C5Me5) Ir(μ-η5:η4-C9H7)Ir(η5-C5Me5)] (6).

Exohedral mono- and bimetallic hydride complexes of rhodium and iridium with C60 and C70: syntheses and electrochemical properties

Abstract Syntheses of mono- and bimetallic hydride complexes of rhodium and iridium with fullerenes C 60 and C 70 are described and the results of their cyclic voltammetry (CV) studies are presented. The metal-containing [HM(CO)(PPh 3 ) 2 ] moiety (M=Rh and Ir) coordinates with high stereo- and regioselectivity to the 1,2 bond of these fullerenes to form the only isomer in both cases. The attachment of the second [HM(CO)(PPh 3 ) 2 ] moiety results in a complex mixture of positional and geometric isomers. Due to a high rate of formation, metallofullerene complexes may be generated in situ in the electrochemical cell. The electrochemical behavior (their redox potentials and stability of the oxidized and reduced forms) of the studied C 60 and C 70 organometallic derivatives with the same metal-containing fragment is identical.

Bis(η5-pentamethylcyclopentadienyl)-and(η5cyclopentadienyl) (η5-pentamethylcyclopentadienyl)-platinium dications: Pt(IV) metallocenes

Abstract Reaction of [Pt2(η5-C5Me5)2(η-Br)3]3+(Br−)3 with C5R5H (R = H,Me) in the presence of AgBF4 gives the first platinocenium dications, [Pt(η5-C5Me5)(η5-C5R5)]2+(BF4− )2. On electrochemical reduction, [pt(η5-C5Me5)2]2+ yields [Pt(η4-C5Me5H)(η2-C5Me5)]+ BF4−. kw]Cyclopentadienyl; Metallocenes; Platinum; Electrochemistry

Electrochemical substitution of hydrogen in ferrocene

Abstract Interaction of radicals R. formed in anode decarboxylation of carboxylate anions with the ferricinium cation generated at the same electrode leads to alkylferrocene derivatives. A similar reaction of the ferricinium cation with radicals formed from dibasic acids or their semiesters yields esters of ferrocenyl- containing carboxylic acids. Monosubstituted ferrocene, particularly those containing electron donor substituents, undergo further substitution to give di- and polysubstituted products. The procedures are described that lead to predominant isolation of either mono- or poly-substituted products. The results of qualitative and quantitative analyses of the polymethylation products show that orienting effects by the substituents are the same for radical substitution in the ferricinium cation and for electrophilic substitution in ferrocene.

Electrochemical reductive carbon-to-carbon coupling of σ-ethynyl complexes of transition metals

The electrochemical properties of σ-ethynyl complexes of chromium subgroup metals were studied by cyclic voltammetry and preparative-scale electrolysis. The redox cycle of C5H5(CO)3CrC=CPh was shown to give the bis-carbyne complex (η5-C5H5)(CO)2Cr≡C-C(Ph)=C(Ph)-C-Cr(CO)2(η5-C5H5) formedvia the reductive Cβ−Cβ coupling of ethynyl moieties. The influence of the nature of the metal atom and the ligand environment on the course of this reaction was considered.

Redox properties of rhodium diene-cyclopentadienyl complexes

The electrochemical behavior of rhodium sandwich complexes containing η4-pentamethylcyclopentadiene or tetramethylfulvene fragments has been studied by cyclic voltammetry. The complexes undergo one-electron oxidation to give unstable 17-electron radical cations which are converted into rhodocenium salts as a result of elimination or uptake of hydrogen or C-C bond cleavage.

Oxidation of theexo- andendo-phenylcyclohexadienyl iron complexes Fe(η5-6-PhC6H6) (η5- C5H5)

The roe of oxidatively induced homolyhc scission a the C(sp3)-H bonds in the iron phenykychhexadienyl complexes Fe(η5-6-PhC6H6)(η5-C5H5) (1) depends on the spatial orientation of the Ph substitutent. In the case of the (1endo+) radical cation this process, resulting in the cationic biphenyl complex (Fe(η6 -C6H5C6H5)(η5-C5H5)]+ (2+), is fast and proceeds for several minutes. In the case of the more stable radical cation (1exo+) the formation of 2+ is slow and takes tens minutes to complete.