Homotrinuclear ruthenium(ii) and rhodium(i) complexes of redox-active tris(ferrocenyl)arene-based tris-phosphanes

Abstract

Homotrinuclear complexes of the C ₃-symmetric tris(ferrocenyl)arene-based tris-phosphanes 1a-d with ruthenium(ii) ([1a-d(Ru)₃]) and rhodium(i) ([1a-d(Rh)₃]) were prepared and fully characterised. Complexes [1a-d(Ru)₃] and [1a-d(Rh)₃] are electrochemically active. The nature of the arene core in 1a-d ranging from benzene, 1,3,5-trifluorobenzene and mesitylene to s-triazine allows to fine-tune the exact oxidation potentials for tailoring the electrochemical response. With a BAr^F ₄ ^--based supporting electrolyte, a distinct separation of the three iron-centred oxidations of the ligand backbone is observable. Under these conditions, these oxidations are mostly reversible but, especially for the third oxidation, already show signs of irreversibility. In general, while the coordinated metal complex fragment does not strongly alter the electrochemical response of the arene-trisferrocenyl core 1a-d, there are observable differences. Rhodium(i) complexes are oxidised at slightly higher potentials than ruthenium(ii) complexes. In both cases, individual oxidation states for the C₆H₃(CH₂)₃-based ligand (1d) are difficult to address and the C₃N₃-based ligand (1c) shows the most complicated and least reversible electrochemistry with severely broadened third oxidations and reduced reversibility in cyclic voltammetry. The most well-suited system for potential applications in redox-switchable catalysis, in all cases, is the C₆H₃-based ligand (1a), showing entirely reversible and well-separated redox events.

Scheme 1. Preparation of ruthenium(ii) ([1a–d(Ru)₃]) and rhodium(i) ([1a–d(Rh)₃]) complexes from tris-phosphanes 1a–d and (I) [{RuCl₂(p-cym)}₂] or [{RhCl(1,5-cod)}₂] in CH₂Cl₂ at r.t.

Fig. 1. Molecular structure of the homotrinuclear rhodium(i) complex [1a(Rh)₃] with part of the atom-numbering scheme and the ³¹P{¹H} NMR signal in THF-d₈ solution, featuring the characteristic ¹J coupling between ³¹P and ¹⁰³Rh. Thermal ellipsoids are set at the 50% probability level. For clarity, the phenyl rings and 1,5-cyclooctadiene ligands are depicted in wireframe style, and co-crystallised solvent and hydrogen atoms have been omitted.

Fig. 2. Partial cyclic voltammograms (iron-centred oxidations only) of ruthenium (left) and rhodium (right) complexes [1a–d(M)₃] at ca. 1 mmol L⁻¹ in 0.1 mol per L (ⁿBu₄N)BAr^F₄/CH₂Cl₂ (scan rate: 100 mV s⁻¹, working electrode: glassy carbon, counter electrode: platinum wire). The 2^nd of three cycles is shown for all compounds, recorded currents are shown normalised for easier comparison. Scanning direction as indicated. The rhodium-centred oxidation event for [1d(Rh₃)] is marked with an asterisk (*). For full voltammograms, see ESI (Section 5).