Reactivity of the 44a,b,d 45c and 48e towards H 2 and 1-hexene

H2 affinity studies were then approached for 44a, 44b, 44d, 46c and 48e to explore the potential of these complexes to act as hydrogenation catalysts. Exposing 44a, 44b, 44d, 46c and 48e to 2 bar of H₂ led in all cases to hydrogenation of the ethylene ligand generating ethane, which could be monitored by NMR spectroscopy. In any case the presumed 16e- hydride complexes 43a-e could not be detected. In the case of 44a and 44d hardly soluble red solids precipitated from the reaction solution. Their structures are yet unknown but can be assumed to be analogous to 50b. Nevertheless the reaction of H₂ with 44b, 46c and 48e furnished identifiable products. The reaction of a benzene solution of 44b under an atmosphere of H₂ afforded the hydride bridged dimer 50b after 1 day at room temperature as hardly soluble red single crystals containing 2 equivalents of benzene per molecule 50b. Due to its low solubility 50b was characterized only by an X-ray diffraction study (Figure 15), elemental analysis and IR spectroscopy.

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Figure 15: ORTEP diagram of 50b drawn at 50% probability..

Selected bond lengths [Å]: Br1-Re1 2.604, H-Re1 1.948/2.061, N1-Re1 1.754, N1-O1 1.184, P1-Re1 2.449, P2-Re1 2.430, Re1-Re1 2.847. Selected bond angels [°]: Br1-Re1-N1, H-Re1-P1, O1-N1-Re 1 1.74.8, P1-Re1-P1

In the X-ray diffraction study, the Re-H distances were found to be quite long and slightly asymmetric with bond lengths of 1.948 Å (Re₁-H₁, Re₂-H₂) and 2.061 Å (Re₂-H₁, Re₂-H₂). As the asymmetric unit comprises only one half of the molecule, the bonding situation is symmetric. The Re-Re distance was found to be 2.847 Å, which indicates a μ²-H bridge supported metal-metal bond.

Single crystals suitable for X-ray diffraction studies of the Sixantphos derivative 50e could be obtained in a similar way as for 50b. The solid state IR spectra of 50b showed a strong sharp ν(ReH) band at 1982 cm^-1 whereas the ν(ReH) band of 50e at 1990 cm^-1 was broad. The ν(NO) bands of 50b (1672 cm^-1) and 50e (1693 cm^-1) are in both cases strong and sharp. This is in contrast to the products of the reactions of 44a and 44d with H2, which displayed only broad ν(NO) bands, but no ν(ReH) bands in the IR spectra. This might indicate structures different from 50b and 50e - supposedly oligomers of 43a and 43d with higher nuclearities .

The reaction of 46c with hydrogen in benzene led immediately to the targeted complex [ReBrH(MeCN)(NO)(dpephos)] 51c, which is however unstable in solution and could therefore not be isolated. In the ¹H NMR spectrum of 51c a resonance at 1.95 ppm (dd, ²J_HP(trans) = 96.0 Hz, ²J_HP(cis) = 25.5 Hz) was attributed to a hydride ligand located in the plane formed by the two phosphorus nuclei and the rhenium center. In the ³¹P NMR spectrum a broad resonance at 18.1 ppm (m, P_cisH) and a sharp signal at -5.1 ppm (d, ²J_HP(trans) = 96.0 Hz, P_transH) were observed. Exposing a solution of 51c to an atmosphere of ethylene leads instantly to the formation of 44c and 46c. The reactions of 44a and 44d with H2 was anticipated to lead also to hydrides, which could probably also be stabilized by the

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addition of acetonitrile (Scheme 46). Indeed reacting 44a and 44d in the presence of acetonitrile with H₂ led to the formation of the acetonitrile hydride complexes 51a and 51d. Their spectroscopic features parallel those of 51c. Hydride signals appeared in the ¹H spectra of 51a at 1.44 ppm (dd,

2J_HP(trans) = 63.3 Hz, ²J_HP(cis) = 23.7 Hz) and of 51d at 2.01 ppm (dd, ²J_HP(trans) = 72.0 Hz, ²J_HP(cis) = 19.5 Hz).

The ³¹P NMR spectra of 51a and 51d consist of broad singlets at 19.5 ppm and at 25.4 ppm, assigned to the phosphine atoms cis to the rhenium hydride and doublets at -4.4 ppm (d, ²JHP(trans) = 63.3 Hz, P_trans-H) and at 2.0 ppm (d, ²J_HP(trans) = 72.0 Hz, P_trans-H) assigned to the phosphine atoms trans to the rhenium hydride. As in the case of 51c the hydride complexes 51a and 51d are not sufficiently stable to be isolated. The ethylene complexes 44a and 44d can be regenerated by exposing them to ethylene (Scheme 46). Exposing 51a-d to a mixture of H2/D2 results in isotope scrambling within minutes, which was pursued using ¹H NMR spectroscopy. Therefore we propose the existence of a trihydride complex [ReBrH₃(NO)(P∩P)+ 49a-d, which allows the efficient H2/D₂ scrambling according to the mechanism depicted in Scheme 49.

Scheme 49: Proposed mechanism for the observed HD exchange.

The olefin affinity of the 16e- complexes 43a-e was qualitatively assessed by estimating the olefin exchange/isomerization rates of 44a-d. For that purpose 1-hexene as a typical substrate was added to CDCl₃ solutions of 44a,b,d, 44/46c and 48e at room temperature. Under these conditions isomerization of 1-hexene into Z-2-hexene and the thermodynamically more favorable E-2-hexene was observed (Scheme 50).

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Scheme 50: Stepwise isomerization of 1-hexene.

The pursuit of this reaction using ¹³C NMR spectroscopy⁹² revealed that there is neither a kinetic preference for the release of E-2-hexene, nor for the release of Z-2-hexene, since both are initially formed in equal amounts. But at the end of the isomerization process the thermodynamically favored E-2-hexene is exclusively found⁹³. The thermodynamically highly favored Z- and E-3-hexenes were not formed. Quantitative NMR pursuit revealed that the 1-hexene isomerization depends on the catalysts concentration and is independent of the 1-hexene concentration, which was interpreted in terms of kinetically preferred coordination of terminal olefins to 43a-d and a rate limiting elimination of the isomerized olefin ligand. The homoxantphos catalyst 44d proved to be the fastest catalyst in the isomerization process with a TOF of ca 14 h^-1 followed by the dpephos derivatives 44c/46c with a TOF of ca 5 h^-1. The two bisphosphinoferrocene complexes 44a,b were about equally active with a TOF of ca. 0.3 h^-1. The Sixantphos complexes 48e(up) or 48e(down) were inactive in this process of 1-hexene to E-2-hexene isomerization - presumably because of the absence of a hydride ligand in these complexes. From these experiments we concluded that ethylene dissociation from 44a,b,d and 44/46c is a slow process, supporting the assumption that all the ligands are quite strongly bound to the rhenium center. The 45 fold increase in the isomerization activity of the homoxantphos derivative 44d compared to the bisphosphinoferrocene derivatives 44a and 44b also reflects diphosphine bite angle dependence. Since the 16e- hydride complexes 43a-d preferably bind terminal olefins, a high selectivity for the hydrogenation of terminal olefins can be expected.