Abstract
Changing the aromatic core of C3-symmetric tris(ferrocenyl)arene-based tris-phosphanes has profound effects on their coordination behaviour towards gold(i). Depending on the arene (s-triazine, benzene, or trifluorobenzene), four different coordination modes can be distinguished and their preference has been rationalised using computational methods. The corresponding 1 : 1 ligand-to-metal complexes, studied by variable-temperature NMR spectroscopy, revealed fluctional behaviour in solution. Given the presence of up to three or six ferrocenylene spacers per complex, their electrochemistry was investigated. The redox-responsive nature of the complexes can be advantageously exploited in the catalytic ring-closing isomerisation of N-(2-propyn-1-yl)benzamide, where the benzene-based 2 : 3 ligand-to-metal complex has been shown to display multiple activity states depending on the degree of (reversible) oxidation in a preliminary trial.
Highlights
Conceived as small-molecule analogues of redox-switchable dendrimers,[32,33,34] we have recently reported the synthesis of a new family of tris-phosphanes 1 (Chart 1) incorporating a redox-active, C3-symmetric tris(ferrocenyl)arene backbone.[35]
Intrigued by the possibility of modifying the coordination properties of tris(ferrocenylene)arene-based tris-phosphanes via the substitution pattern of the arene, we introduced fluorine in the diphenylphosphanyl groups of 1a in form of the bis( pentafluorophenyl)phosphanyl moiety [P(PhF)2] (Chart 3)
Even though a density-functional theory (DFT)-level geometry optimisation of the corresponding C3-symmetric tricoordinate gold(I) complex cation [1aF(Au)]closed+ initially suggested the synthetic accessibility of this coordination mode, the open form [1aF(Au)]open+ was found to be energetically far more favourable by 207 kJ mol−1
Summary
Modern-day ligand design is hardly imaginable without the inclusion of ferrocene.[1,2,3] Pairing great stability with conformational flexibility, easy synthetic modifiability, the possibility for exploiting planar chirality, and reversible redox activity, ferrocene has attained the special place of perhaps the key organometallic building block.[4,5] Next to numerous applications in asymmetric catalysis,[6,7,8] ferrocene-containing ligands feature prominently in redox-switchable catalysis (RSC).[9,10,11] Based on the concept of reversibly modifying the donor properties of a redox-active ligand and in turn, the electronic nature of the coordinated metal,[12,13] initial breakthroughs were achieved by Wrighton[14] and Long.[15]. The three individual gold(I) centres are close-to-linearly coordinated by two diphenylphosphanyl groups each, and the P–Au–P bond angles (170.45(5)–172.72(5)°, cf Table S3†) are closer to the ideal 180° than those found in [1b(Au)]OTf, most likely because the 1,3,5-tris(ferrocenyl)arene-derived ligand structure itself does not impose any steric restrictions anymore.
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