Ceyhan Kayran

Activation parameters in flash photolysis studies of Mo(CO) 6

Abstract Reported is a combined time-resolved optical (TRO) and infrared (TRIR) spectroscopic investigation of the flash photolysis of Mo(CO)6 in cyclohexane solution. TRIR studies using 308 nm excitation led to transient bleaching of the strong νCO band at 1987 cm−1 of Mo(CO)6 and appearance of new bands at 1931 and 1964 cm−1 attributed to Mo(CO)5(Sol). Using a high pressure/variable temperature flow cell, the kinetics of back reaction with CO (kCO) to regenerate the hexacarbonyl was studied over the PCO range 1–20 atm and at five temperatures. These data gave kCO=4.6±0.2×106 M−1 s−1 (298 K) and the activation parameters Δ H CO ‡ =32.6±3 kJ/mol and Δ S CO ‡ =−7.3±11 J mol−1 K−1 from which an interchange mechanism was proposed. The analogous species seen in the TRO experiment displayed a transient absorbance at 420 nm and analogous kinetics properties although at lower PCO self-trapping with Mo(CO)6 (to give Mo2(CO)11) is a competitive process. The Mo(CO)5(Sol) transient could also be trapped by nPrBr (kRBr=5.3±0.7×107 M−1 s−1).

Photocatalytic hydrosilylation of conjugated dienes with triethylsilane in the presence of Cr(CO)5L (L = CO, P(CH3)3, P(OCH3)3)

Abstract The effect of donor ligands on the chromium carbonyl photocatalysed hydrosilylation of 1,3-butadiene, 2-methyl-1,3-butadiene, 2,3-dimethyl-1,3-butadiene, trans-1,3-pentadiene with triethylsilane was studied. Photocatalytic hydrosilylation of dienes conducted in the presence of Cr(CO)5L (L = P(CH3)3 or P(OCH3)3) yields the cis-1,4-adducts, 1-triethylsilyl-2-butene derivatives, as the main products; these have been isolated by distillation or preparative GC and fully characterised by NMR spectroscopy. As with the photocatalytic hydrosilylation of dienes in the presence of Cr(CO)6, we propose a mechanism which involves the initial conversion of Cr(CO)5L into fac-Cr(CO)3L(η4-1,3-diene) followed by a further photolytic CO substitution by triethylsilane, forming a Cr(CO)2(H)(SiEt3)L(η4-1,3-diene) intermediate. The addition of hydride to diene occurs reversibly to form an η3-enyl ligand prior to the irreversible silyl transfer to the organic moiety. The 1,4-hydrosilylation adduct is then replaced by new substrates to complete the catalytic cycle. Cr(CO)5L complexes form active species which are much more stable than the one generated from Cr(CO)6 in the absence of any donor ligands. Thus, introduction of an equimolar amount of donor ligand (trimethylphosphine or trimethylphosphite) increases the life of the catalyst. This leads to the achievement of larger turn-over numbers (80 or 70) compared with the value of 30 for just Cr(CO)6. Cr(CO)5L (L = P(C6H5)3, P(C6H11)3 or C5H5N) proved inactive in the attempted photocatalytic hydrosilylation of dienes.

Retardation effect of trimethyl phosphite on chelate ring closure of 2,2′-bipyridine (bipy) in cis-[W(CO)4{P(OMe)3}(bipy)]

In order to study the effect of a donor ligand on the chelate ring-closure reaction of a diimine ligand co-ordinated in monodentate fashion, cis-[W(CO)4{P(OMe)3}(bipy)] was synthesised from [W(CO)5{P(OMe)3}] and 2,2′-bipyridine (bipy) by using a newly developed method and characterized by means of IR and NMR spectroscopies. The complex was found to undergo neither cis→trans isomerization nor chelation thermally up to 150 °C. However the ring-closure reaction occurs photochemically. Thus irradiation of the complex yielded fac-[W(CO)3{P(OMe)3}(bipy)] which was also isolated and characterized. The extraordinary retardation effect of trimethyl phosphite on the ring-closure reaction of the 2,2′-bipyridine ligand is attributed to the reduction in substitution lability of the CO groups in the complex upon introduction of the phosphite donor ligand.

Gehinderte ligandbewegungen in übergangsmetallkomplexen: XLI. Das dynamische Verhalten von 1,2-Bis(dimethylphosphino)ethan-dicarbonyl-(η4 -dien)-wolfram(O)☆

Abstract 1,2-Bis(dimethylphosphino)ethane-dicarbonyl-η4-diene-tungsten(0) complexes (diene = 1,3-butadiene (1), E-1,3-pentadiene (2), E,E-2,4-hexadiene (3) and E-2-methyl-1,3-pentadiene (4)) were synthesized by UV irradiation of 1,2-bis(dimethylphosphino)ethane-tetracarbonyltungsten(0) and the corresponding dienes in toluene. The constitutions of these complexes were determined by IR and NMR spectroscope (1H, 13C, 31P). The spectral data indicate pseudo-octahedral structures for the complexes. When the dienes are assigned to the positions b and c of the coordination octahedron, the 1,2-bis(dimethylphosphino)ethane occupies a and d (e), the two carbonyls e (d) and f positions. 1 and 3 show temperature-dependent NMR signals according to hindered ligand movements with barriers of activation of 50 and 59 kJ mol−1. In contrast, as a consequence of very different isomer populations, no ligand movement is observed for 2 and 4 up to 353 K.

Thermal Catalytic Hydrosilylation of Conjugated Dienes with Triethylsilane in the Presence of Tricarbonyl(o-xylene)Metal (Metal = Cr, Mo, W) Complexes

The thermal catalytic hydrosilylation of 1,3-butadiene (1), trans-2-methyl-1,3-pentadiene (2), 2,3-dimethyl-1,3-butadiene (3), and isoprene (4), with triethylsilane were studied in the presence of M(CO)3 (o-xylene) (M = Cr, Mo, W) complexes in polar and nonpolar solvents such as tetrahydrofuran, hexane and toluene. Mo(CO)3 (o-xylene) was found to be the only active catalyst for the hydrosilylation of 3 with triethylsilane, which gave 1-triethylsilyl-2,3-dimethyl- 2-butene (3a). as hydrosilylated product in tetrahydrofuran. The product was identified by means of 1H and 13C-NMR and GLC. The same catalyst, Mo(CO)3 (o-xylene), decomposed to Mo(CO)6 without giving hydrosilylated products of 1, 2 and 4 in tetrahydrofuran. M(CO)3(o-xylene) (M = Cr, Mo, W) complexes were found to be stable for about 6 h in hexane and toluene, and showed no catalytic activity for the hydrosilylation of 1, 2, 3 and 4.

Photo-Induced Chromiumcarbonyl Catalyzed Hydrosilylation of Conjugated Dienes with Triethylsilane: The Solvent Effect

Photocatalytic hydrosilylation of conjugated dienes (1,3-butadiene, 2-methyl-1,3-butadiene, 2,3- dimethyl-1,3-butadiene, trans-1,3-pentadiene) with triethylsilane was studied by using Cr(CO)5L (L = CO, P(CH3)3, P(OCH3)3, P(C6H5)3, P(C6H11)3, NC5H5) in two very different solvents, toluene and tetrahydrofuran, for comparison with the results found in n-hexane. In toluene, the photocatalytic hydrosilylation yields the same products as those in n-hexane, with the exception of trans- 1,3-pentadiene which gives cis-1-triethylsilyl-2-pentene as the sole product. However, each of the precursor complexes shows different catalytic activities in toluene and n-hexane. The hydrosilylation of 1,3-butadiene in toluene is, in general, significantly faster than that in n-hexane. By using Cr(CO)6, Cr(CO)5[P(CH3)3] or Cr(CO)5[P(OCH3)3] in toluene, the conversion of triethylsilane increases almost linearly as the reaction proceeds, indicating the stability of the active catalyst throughout the reaction, similar to that in n-hexane. While no hydrosilylation of 1,3-butadiene could be achieved with Cr(CO)5[P(C6H5)3] or Cr(CO)5(NC5H5) in n-hexane, the same precursor complexes appear to be active in toluene, though the conversion occurs at much lower rate compared to that obtained using Cr(CO)5[P(CH3)3] or Cr(CO)5[P(OCH3)3]. The precursor complex Cr(CO)5[P(C6H11)3] shows catalytic activity neither in toluene nor in n-hexane. No photocatalytic hydrosilylation of 1,3-butadiene with triethylsilane was observed in tetrahydrofuran by using any of the precursor complexes. The relative reactivity of conjugated dienes in the hydrosilylation was investigated by using triethylsilane in the presence of Cr(CO)5[P(OCH3)3] as catalyst in toluene, and the same reactivity order was obtained as in n-hexane solution: 1,3-butadiene > 3-methyl-1,3-butadiene > 2,3-dimethyl-1,3-butadiene > trans-1,3-pentadiene. For all of the dienes, one obtains higher conversion to hydrosilylated product in toluene than in n-hexane.

Spectroelectrochemical Investigation of Pentacarbonyl(pyrazine)metal(0) (Metal = Cr, Mo, W) Complexes of Group 6 Elements

The electrochemical behaviour of pentacarbonyl(pyrazine)metal(0) complexes of the group 6 elements was studied by cyclic voltammetry in dichloromethane-(n-Bu)4NBF4 solventelectrolyte couple at -20°C vs. Ag/Ag+ or SCE reference electrode. Constant potential electrolyses of the complexes were carried out at their first oxidation peak potentials and monitored in situ by UV-Vis spectrometry. Electrolysis of W(CO)5pz produces [W(CO)5pz]+ and a similar electrochemical mechanism is expected both for Cr(CO)5pz and Mo(CO)5pz complexes. In situ low temperature constant current ESR electrolysis also confirmed the production of [W(CO)5pz]+ after the electron transfer.

Notizen: Synthesis and NM R Study of (η6-1,4-diphenyl-1,3-butadiene)tricarbonylchromium(0)

(η6-1,4-Diphenyl-1,3-butadiene)tricarbonylchromium(0) has been synthesized in high yield by using an improved procedure, from tris(acetonitrile)chromium(0) and 1,4-diphenyl-1,3-butadiene. The IR and NMR spectroscopic data show that the organic ligand is bonded to the Cr(CO)3 moiety through only one of the two phenyl rings.