Synthesis and molecular structure of [Rh(η⁵-C₅H₅)(coe)(PtBu₂H)] (coe=cis-cyclooctene)

1 Introduction

Recently we described the synthesis and crystal structure of the olefinic complex [Rh(η⁵-Cp)(coe)₂] (1, coe=cis-cyclooctene) which was obtained by reacting [{Rh(μ-Cl)(coe)₂}₂] with thallium cyclopentadienide in THF [1]. In earlier work we have shown, that rhodium complexes containing the labile coe ligand are suitable precursors for introducing e.g. secondary phosphane ligands [2]. In these cases, the square-planar complexes exhibited a 16-electron configuration with a vacant orbital available to the incoming ligand. In contrast, in light of the well-known parent compound [Rh(η⁵-Cp)(C₂H₄)₂], Cramer showed a long time ago that this 18-electron complex is inert to substitution by ligands such as phosphanes, CO and cyanide [3]. The olefinic ligand in the latter species is, however, readily displaced under mild conditions by ligands exhibiting a Lewis-acidic character whereas the metal can be considered as the donor in these reactions [3], [4]. Later it has been reported that substitution reactions of one olefinic ligand by several basic donor ligands can be achieved mainly under photolytic conditions [5]. The complex [Rh(η⁵-Cp)(C₂H₄)(PPh₃)] was first reported in 1971, prepared however from [Rh(acac)(C₂H₄)(PPh₃)] (acac=acetylacetonato) by replacing the anionic ligand using sodium cyclopentadienide [6]. Later on that compound was obtained as the photolytic and as the thermal reaction product, respectively, from [Rh(η⁵-Cp)(C₂H₄)₂] in the presence of PPh₃ [5]. In this context we were interested in substitutions of the coe ligands in our complex 1 by secondary phosphanes which we intend to use for further functionalization reactions. Herein we report on first results in this field.

2 Experimental section

3 Results and discussion

First attempts to react [Rh(η⁵-Cp)(coe)₂] (1) with the secondary phosphane PtBu₂H in refluxing THF over longer periods confirmed (by ³¹P{¹H} NMR investigations) the reports from the literature that the olefinic ligands showed some inertness to substitution under these conditions as in the case of [Rh(η⁵-Cp)(C₂H₄)₂]. Reactions of the latter with phosphanes were initiated at higher temperatures in the range of 115–130°C, e.g. [5], and therefore we changed to toluene as the solvent for the thermolysis over 30 min. At the boiling point of the solvent, a quick color change to yellow and, finally, to orange was observed. After the workup the title compound [Rh(η⁵-Cp)(coe)(PtBu₂H)] (2), was obtained in good yield according to the reaction shown in Eq. (1):

Complex 2 was characterized by elemental analysis and by NMR spectroscopy (¹H, ³¹P and ¹³C, see Experimental section). The observed data from the ³¹P{¹H} NMR spectrum, especially the coupling constant J_RhP, correspond very well with the literature data reported for closely related species, e.g. [Rh(η⁵-Cp)(C₂H₄)(PMe₃)] (δ=4.4, d, J_RhP=200.0 Hz, C₆D₆) and [Rh(η⁵-Cp)(C₂H₄)(PPh₃)] (δ=59.5, d, J_RhP=210.0 Hz, C₆D₆) [9]. During our investigation we obtained crystals of 2 suitable for an X-ray diffraction study. The crystals belong to the monoclinic space group P2₁/n with four molecules in the unit cell. A view of the molecule of 2 is shown in Fig. 1, and selected bond lengths and bond angles are given in the caption.

Figure 1:

Molecular structure of 2 in the crystal (the less-occupied part of the disordered section in the 8-membered ring is not depicted). Displacement ellipsoids are drawn at the 50% probability level. Selected bond lengths (Å) and angles (deg): Rh1–P1 2.2211(5), Rh1–C1 2.1199(17), Rh1–C2 2.1156(18), Rh1–C9 2.3022(19), Rh1–C10 2.2988(19), Rh1–C11 2.2819(19), Rh1–C12 2.2942(18), Rh1–C13 2.2347(18), P1–C14 1.882(2), P1–C18 1.881(2), P1–H1 1.29(2), C1–C2 1.418(3); Rh1–P1–C14 116.554(6), Rh1–P1–C18 115.56(6), Rh1–C1–C2 70.28(10), Rh1–C2–C1 70.61(10), P1–Rh1–C1 87.92(5), P1–Rh1–C2, 90.58(5), P1–Rh1–C9 116.59(5), P1–Rh1–C10 151.52(6), P1–Rh1–C11 154.80(6), P1–Rh1–C12 119.19(5), P1–Rh1–C13 101.30(5).

The coordination sphere around the rhodium atom in 2 can be best described as a two-legged piano-stool structure considering the mid-point of the bond C1–C2 as one coordination site. The central bond lengths agree well with the corresponding ones as reported for [Rh(η⁵-Cp)(C₂H₄)(PPh₃)] [9]. For the latter, the following data were found (atom numbering as in ref. [9]): R1–P1 2.2330(11), Rh1–C6 2.097(4), Rh1–C7 2.109(5) (ethene), C6–C7 1.402(6) and Rh1–C(Cp) 2.233(4)–2.294(4) Å as well as C6–Rh1–C7 38.94(17), C6–Rh1–P1 91.82, C7–Rh1–P1 94.38(13)°. Similarly, as was found for the molecular structure of [Rh(η⁵-Cp)(C₂H₄)(PPh₃)], one can define also for 2 an approximate mirror plane passing through the rhodium, the P1 atom and the midpoint of the C1–C2 bond of the coordinated coe ligand.

In summary, we report the synthesis of [Rh(η⁵-Cp)(coe)(PtBu₂H)] (2) by the thermolytic reaction of [Rh(η⁵-Cp)(coe)₂] (1) with the secondary phosphane in refluxing toluene. The molecular structure of 2 in the solid has been confirmed by a single-crystal X-ray diffraction study. It should be noted that compound 2 shows an inert behavior in a prolonged thermolytic treatment even at higher temperatures. Furthermore, we attempted to prepare complexes closely related to 2 using the phosphanes PR₂H (R=Ph, o-tolyl) under similar reaction conditions. In contrast to the clean synthetic protocol of 2, as described herein, we observed a complicated reaction behaviour using the latter two phosphanes. On this subject we will report in the near future.