Two Rhodium(III) Ions Confined in a [18]Porphyrin Frame: 5,10,15,20‐Tetraaryl‐21,23‐Dirhodaporphyrin

Abstract

Tetraaryl‐21,23‐dirhodaporphyrin and a series of related monorhodaporphyrins have been obtained by tellurium‐to‐rhodium exchange in a reaction of tetraaryl‐21,23‐ditelluraporphyrin with [RhCl(CO)2]2. These organometallic metallaporphyrins contain rhodium(III) centers embedded in rhodacyclopentadiene rings, incorporated within the porphyrin frames. The skeletons of 21,23‐dirhodaporphyrin and 21‐rhoda‐23‐telluraporphyrin are strongly deformed in‐plane from the rectangular shape typical for porphyrins, due to rhodium(III) coordination preferences, the large size of the two core atoms, and the porphyrin skeleton constrains. These two metallaporphyrins exhibit fluxional behavior, as studied by 1H NMR and DFT, involving the in‐plane motion and the switch of the rhodium center(s) between two nitrogen donors. A side product detected in the reaction mixture, 21‐oxa‐23‐rhodaporphyrin, results from tellurium‐to‐oxygen exchange, occurring in parallel to the tellurium‐to‐rhodium exchange. The reaction paths and mechanisms have been analyzed. The title 21,23‐dirhodaporphyrin contains a bridged bimetallic unit, Rh2Cl2, in the center of the macrocycle, with two rhodium(III) ions lying approximately in the plane of the porphyrinoid skeleton. The geometry of the implanted Rh2Cl2 unit is affected by macrocyclic constrains.

Metalation of Tellurophene: Reactivity of 21,23-Ditelluraporphodimethene toward Palladium(II), Platinum(II), and Rhodium(I)

“…Taking into account several analogies between 21,23ditelluraporphodimethene and 21,23-ditelluraporphyrin metalation products observed for palladium, platinum, and rhodium, as depicted above and reported previously, 4,18,19 we postulate that the reaction path leading from 1 to 21-metalla-23porphodimethenes is similar to that recognized in our former studies for 21,23-ditelluraporphyrin. Thus, the tellurium-tometal exchange commences with a side-on complex formation, then follows an insertion of a metal ion into the C−Te bond, with the formation of a reactive species including a six-membered TeMC 4 ring, and finally an extrusion of tellurium occurs to obtain a metallaporphyrinoid.…”

“…For 2 and 3 (spectra C and D), the β-protons of metallacyclopentadiene rings, involved in the π-conjugation, have relatively large chemical shifts (7.9–7.4 ppm in CDCl 3 ) and due to the presence of spin active nuclei, 103 Rh ( I = 1/2, 100%) and 195 Pt ( I = 1/2, 33.8%), show patterns which are diagnostic for these structures. The rhodacyclopentadiene protons in 2 give characteristic doublets of doublets ( 3 J HH = 3.9 Hz, 3 J RhH = 1.2–1.4 Hz), best resolved in the spectrum measured in deuterated benzene at 350 K. These relatively small coupling constants are similar to those observed for aromatic rhodaporphyrins ( 3 J HH 4.6–5.2 Hz; 3 J RhH 0.7–2.3 Hz) …”

“…Similar values, however, were already observed for other rhodaporphyrins and are also recognized by characteristic 13 C−H one-bond coupling constants, 1 J RhC of 27 and 31 Hz, characteristic of 103 Rh− 13 C constants for sp 2 carbons, similar to those observed for other porphyrinoids (23)(24)(25)(26), and are much smaller than those in the case of C�O axial ligands (J ∼ 70 Hz), which give similar chemical shifts. 4,30 ■ CONCLUSIONS 21,23-ditelluraporphodimethene may serve as a neutral bidendate ligand, binding a metal ion to two tellurium donors in a side-on fashion. Alternatively, the metalation reaction may follow the substitution path, leading to the transformation of the tellurophene ring into a metallacyclopentadiene unit.…”

“…200 kJ·mol –1 ), as compared to other heteroatom–carbon bonds, C–Se (234 kJ·mol –1 ), C–S (272 kJ·mol –1 ), and C–N (305 kJ·mol –1 ). Thus, under oxidative conditions, the tellurophene ring embedded in 21-telluraporphyrin was capable of transforming into the furan ring, to form a 21-oxaporphyrin. , A similar transformation, promoted by the presence of a rhodium salt, has been documented in low yield for 21,23-ditelluraporphyrin . The transformation of a series of aromatic telluraporphyrins, including expanded ditelluraporphyrins, has led to annulene-porphyrin hybrids, in an acid-promoted extrusion of tellurium atom(s), resulting in the formation of the butadiene unit from the tellurophene ring, while the macrocyclic integrity has been conserved. − …”

“…For the same reason, a typical metal ion binding mode observed for telluraporphyrins has been a side-on complex formation; however, their collection is still limited. 4,8,9,18,19 Thus, in crystallographically characterized palladium(II) complexes with 21-tellura-23-vacataporphyrin 18 and tellura-m-benziporphyrin, 9 both acting as neutral ligands, the metal ion binds to two porphyrin donors (Te and N), and the palladium(II) square planar coordination sphere is practically perpendicular to the porphyrin plane defined by four meso-carbons, keeping the metal ion far from the cavity. An entirely different reactivity toward metal ions, observed uniquely for porphyrins containing the tellurophene unit, follows a substitution path, that is, a heteroatom-to-metal exchange.…”