Calixarene-monophosphines as supramolecular chelators

Abstract

The calix[4]arenes 5-diphenylphosphino-17-R1-11,23-diR2-25,26,27,28-tetrapropoxycalix[4]arene (1, R1 = R2 = Br; 2, R1 = Br, R2 = H; 3, R1 = R2 = p-tolyl; 4, R1 = p-tolyl, R2 = H; 5, R1 = R2 = H; 20, R1 = p-tolyl, R2 = H), all bearing a diphenylphosphino group attached to the calixarene upper rim, have been synthesised starting from 5,11,17,23-tetrabromo-25,26,27,28-tetrapropoxycalix[4]arene. Reaction of 1-5 with [RuCl2(p-cymene)]2 leads quantitatively to monophosphine complexes, [RuCl2(p-cymene)L], in which the endo-oriented ruthenium atom unit sits inside the cone delineated by the four phenoxy rings. The particular orientation of the P-Ru vector appears to result from pi-pi interactions between the p-cymene ligand and two aromatic cavity walls. Overall, in each case the calixarene end of the ligand behaves as a supramolecular receptor towards the Ru(p-cymene) fragment. Formation of complexes with exo-oriented P-M bonds were observed in the complexes cis-[PtCl2 x 1(2)] and in [RuCl2(p-cymene) x 20]. As shown by a variable temperature study carried out on [RuCl2(p-cymene) x 3], in which the ruthenium is embedded in an expanded cone, the p-cymene ring undergoes a fast oscillation about the Ru-arene bond which can be frozen out at low temperatures. Reaction of [RuCl2(p-cymene) x 3] with AgBF4 in CH3CN results in an intra-cavity reaction, with one chloride ligand being replaced by acetonitrile.

Cavity‐Shaped Ligands: Calix[4]arene‐Based Monophosphanes for Fast Suzuki–Miyaura Cross‐Coupling

“…[20,32] A similar supramolecular interaction may, in principle, also occur in transient [Pd 0 A C H T U N G T R E N N U N G (ArX)L] complexes obtained from the calixarenyl phosphanes 6-10 (Scheme 7). Although this hypothesis remains speculative, it is consistent with the observation that subtle size modifications of the lower-rim substituents do improve the catalytic outcome, probably as a result of an induced shape modification of the calixarene receptor, which may then operate as a better receptor.…”

“…A striking feature of these ligands concerns their ability to behave as supramolecular chelators towards metal-arene moieties. [19,20] …”

Resorcin[4]arene‐Derived Mono‐ and Diphosphines in Suzuki Cross‐Coupling

“…[2,3] While ligands of this class have found numerous applications in separation science and sensing technology, [4][5][6][7][8] catalytic properties, surprisingly, have been little studied. [9,10] This is in stark contrast with podands obtained from the related calix [4]arene cavities, [11] in particular their phosphane derivatives, [11][12][13][14][15][16][17][18][19][20] which have already led to a rich transition metal chemistry. [2,21] We have recently described the synthesis of the first mono-and diphosphines built on a resorcinarene cavitand (1-3, Scheme 1) and shown that these are suitable for complexation of transition metals, altering neither the cavity shape nor its receptor properties.…”

Crystal structure of 5-(imidazol-1-yl)-25,26,27,28-tetrabenzyloxy- calix[4]arene (cone), C59H50N2O4

“…The calixarene skeleton adopts atypical pinched cone conformation [1,2], with dihedral angles between opposing phenoxy rings of 109.8°and 25.3°. The two distal rings with the smaller interplanar angle have their upper part (i.e.the Catoms in para to theoxygen atoms) pushed towards the calixarene axis (d(C1-C15 = 4.08 Å).…”