M.J. Macazaga

Alkynyl cobalt complexes. An electrochemical study

Abstract The preparation and characterisation of the complexes Co2(CO)5(PMe3)(μ-η2-Me3SiC2CCSiMe3) (2) and Co2(CO)4(PMe3)2(μ-η2-Me3SiC2CCSiMe3) (3) are described. A comparative electrochemical study of the complexes Co2(CO)6−nLn(μ-η2-Me3SiC2CCSiMe3) (n=0 (1); n=1, L=PMe3 (2); n=2, L=PMe3 (3), PPh2Me (4), dppa (5), dppm (6)) is presented by means of the cyclic and square-wave voltammetry techniques. Substitution of CO by phosphine ligands transforms the Co2C2 redox centre from a readily reducible to an easily oxidisable centre and contributes to the stabilisation of the Co–Co bond increasing the lifetime of the radical cations and anions.

SYNTHESIS AND CHARACTERIZATION OF NEW TRANSITION METAL DIYNYL COMPLEXES

The reaction of (η 5 -C 5 H 5 )Mo(CO)(dppe)Cl with LiCCCCSiMe 3 yielded (η 5 -C 5 H 5 )(CO)(dppe)MoCCCCSiMe 3 ( 1b ) and, as a by-product (η 5 -C 5 H 5 )Mo(CO)(dppe)Br ( 1a ). Treatment of 1b with 0.2 equivalents of tetrabutylammonium fluoride or (η 5 -C 5 H 5 )Mo(CO)(dppe)Cl with HCCCCH gave the terminal butadiyne complex (η 5 -C 5 H 5 )(CO)(dppe)MoCCCCH ( 2 ). Complex 2 was deprotonated with sec -BuLi or lithium diisopropylamide, and the resulting anion (η 5 -C 5 H 5 )(CO)(dppe)MoCCCCLi ( 3 ) was trapped with Me 3 SiCl to regenerate 1b . The synthesis of Co 2 (CO) 4 L 2 (μ-η 2 -Me 3 SiC 2 CCSiMe 3 ) (L 2 =dppa 4 , 2PPh 2 Me 5 ) compounds can be achieved by two methods: from Co 2 (CO) 6 (μ-dppa) by reaction with Me 3 SiCCCCSiMe 3 in 1:1 ratio to yield 4 , or from Co 2 (CO) 6 (μ-η 2 -Me 3 SiC 2 CCSiMe 3 ) by reaction with dppa (1:1 ratio) and PPh 2 Me (1:2 ratio) to yield 4 and 5 , respectively. When the Co 2 (CO) 4 (μ-dppa)(μ-η 2 -Me 3 SiC 2 CCSiMe 3 ) complex was treated with more Co 2 (CO) 6 (μ-dppa) the green di-substituted complex [Co 2 (CO) 4 (μ-dppa)] 2 (μ-η 2 :μ-η 2 -Me 3 SiC 2 C 2 SiMe 3 ) ( 6 ) was obtained. Desilylation of 4 with Bu 4 NF gave Co 2 (CO) 4 (μ-dppa)(μ-η 2 -Me 3 SiC 2 CCH) ( 7 ). All compounds synthesized have been characterized by analytical and spectroscopic data (IR, 1 H-, 31 P-, 13 C-NMR, MS). In addition, compounds 1a and 4 were characterized by X-ray structure analysis.

Tetracyanoethylene complexes of monomethylcyclopentadienylcobalt derivatives

Abstract The complexes (MeCp)Co(L)TCNE (L = CO, PPh 3 , P(OMe) 3 , py; MeCp = methylcyclopentadienyl; TCNE = tetracyanoethylene; py = pyridine) have been made. The TCNE acts as a one-electron oxidizing agent, and is σ-bonded to cobalt through a nitrile nitrogen. All the compounds have been characterized by elemental analysis, magnetic susceptibility measurements, and IR, electronic, 1 H NMR, and ESR spectroscopy.

Electrochemical study of methylfulvalene and methylcyclopentadiene molybdenum complexes

Abstract The product of the synthesis of ( η 5 : η 5 -(C 5 H 3 Me) 2 )Mo 2 (CO) 6 ( 1 ) is reported as a mixture of six stereoisomers, the ratio of which has been unambigously assigned using homonuclear two dimensional correlation spectroscopy (COSY and NOESY). The reaction of Li 2 [( η 5 : η 5 -(C 5 H 3 Me) 2 )Mo 2 (CO) 6 ] ( 2 ) with IMe yields ( η 5 : η 5 -(C 5 H 3 Me) 2 )Mo 2 (CO) 6 Me 2 ( 3 ) and that of 1 with I 2 yields ( η 5 : η 5 -(C 5 H 3 Me) 2 )Mo 2 (CO) 6 I 2 ( 4 ). The electrochemical behaviour of 1 , 3 , and 4 is reported and compared with analogous complexes where other substituents are present on the fulvalene rings. Electronic communication through the fulvalene ligand seems to take place in 4 . The related compounds [( η 5 -C 5 H 4 R)Mo(CO) 3 ] 2 (R=Me ( 5 ), H ( 9 )), ( η 5 -C 5 H 4 R)Mo(CO) 3 Me (R=Me ( 7 ), H ( 10 )) and ( η 5 -C 5 H 4 R)Mo(CO) 3 I (R=Me ( 8 ), H ( 11 )) have been synthesized in order to compare their electrochemical behaviour with the fulvalene analogous.

Mono-μ-hydrido complexes of pentamethylcyclopentadienylcobalt

Abstract The complex (η5-C5Me5)Co(CO)2 reacts with Cl2 and with Br2 to give [(η5-C5Me5)CoCl(μ-Cl)]2 and (η5-C5Me5)Co(CO)Br2, respectively. The latter was converted into the dimeric species [(η5-C5Me5CoBr(μ-Br)]2. The reaction of dimeric pentamethylcyclopentadienylcobalt complexes [(η5-C5Me5)CoX(μ-X)]2 (X = Cl, Br, I) with potassium hydroxide gives the mono-μ-hydrido complexes [(η5-C5Me5)CoX]2(μ-H)(μ-X).

Oxidative addition reactions TO Cp′Co(CO)L (Cp′ = C5H4CH3; L = CO, PPh3 and tetracyanoethylene) with XCN (X = Br, I)

Abstract Oxidative addition of XCN (X = Br, I) to Cp′Co(CO)L (L = CO, PPh 3 ) leads to the formation of Cp′CoL(CN)X. The complexes C′ p CoTCNE(L) do not react with XCN.

[Co4(CO)12] derivatives with bis(diphenylphosphino) amine, an electrochemical study

Abstract [CO 4 (CO) 12 ] ( A ) reacts with bis(diphenylphosphino)amine (dppa) to give [Co 4 (CO) 10 (dppa)] ( B ) or [Co 4 (CO) 8 (dppa) 2 ] ( C ) when the reactant ratios are 1:1 or 1:2, respectively. The complexes have been characterized by elemental analysis, IR, 1 H and 31 P NMR spectroscopy. The electrochemical behaviour of the three compounds has been studied. The reduction voltammetric waves and the first oxidation of C are reversible, although other waves were also observed which were attributed to the fragmentation induced by electron transfer. Using the electrochemical and UV-visible data, it was possible to construct MO diagrams and to locate the HOMO, LUMO and other MOs of the metal core.

Stabilization of 17-electron metal-centered species. Electrochemical study of [WX(CO)3(η5-C5H5)] (X=Cl, Br, I) and [WI(CO)2(PCy3)(η5-C5H5)]

The electrochemical study of [WX(CO)3(η5-C5H5)] (X=Cl, Br, I) (1–3) is reported. The reductions follow ECE mechanisms, yielding the anion [W(CO)3(η5-C5H5)]− (7−). The stability of 1–3 towards reduction increases with the increasing electronegativity and decreasing size of the halide. 7− reoxidizes to the unstable 17-electron radical [W(CO)3(η5-C5H5)] (7), which readily dimerizes. The oxidations of 1–3 follow EC mechanisms, leading to the cations [WX(CO)3(η5-C5H5)]+, which are very unstable and readily decompose. [WI(CO)2(PCy3)(η5-C5H5)] has been prepared and characterized as a mixture of cis (4) and trans (5) isomers (ratio cis:trans 95:5). The electrochemical reduction of the cis isomer (4) is also an ECE process, but takes place at a potential significantly more negative than 3. An anion [W(CO)2(PCy3)(η5-C5H5)]− (8−) is formed which reoxidizes to 8. This new 17-electron radical is considerably more stable than 7 due to the presence of the bulky PCy3 ligand. A similar effect is observed in the oxidation of 4, where the 17-electron product 4+ is significantly more stable than the analogue 3+.

Reactions of (η5-C5H4CO2CH3)Co(CO)I2 with polydentate phosphines

The reactions of (η5-C5H4CO2CH3)Co(CO)I2 with polydentate phosphines such as dppm (bis(diphenylphosphino)methane), dppe (bis(diphenylphosphino)ethane), dppa (bis(diphenylphosphino)amine), t-dppv (trans-1,2-vinylenebis(diphenylphosphine)), dpmp [bis((diphenylphosphino)methyl)phenylphosphine] and Ph2Ppy (2-(diphenylphosphino)pyridine) yield the neutral species (η5-C5H4CO2CH3)CoLI2 (L = t-dppv and Ph2Ppy), when the ligand behaves as monodentate, and monovalent cations [(η5-C5H4CO2CH3)Co(P P)I]+I− (P P = dppm, dppe, dppa and dpmp), when it behaves as bidentate chelate. All the compounds have been characterized by elemental analysis, conductivity measurements and IR, electronic, and 1H and 31P NMR spectroscopy.

Oxidative addition reactions of pseudohalogens to (η5-C5Me5CoL2 (L = CO OR C2H4)

Abstract The oxidative addition of the pseudohalogens (SCN) 2 and S(CN) 2 to the complexes (η 5 -C 5 Me 5 CoL 2 (L = CO or C 2 H 2 ) give the complexes (η 5 -C 5 Me 5 )-Co(L)(NCS) 2 and (η 5 -C 5 Me 5 )Co(L)(NCS)(CN), respectively. All of the compounds have been characterized by elemental analysis and IR, electronic and 1 H NMR spectroscopy.