Platinum corroles

Platinum has been inserted into corroles for the first time and three oxidized Pt(IV)(corrole˙(2-))ArAr' complexes have been structurally characterized. The Soret maxima of these complexes exhibit an unusually strong dependence on the meso-aryl substituents on the corrole, indicating aryl → corrole˙(2-) charge transfer character in these transitions.

As trianionic ligands with contracted N 4 cores, corroles sustain a great deal of unique coordination chemistry relative to porphyrins. 1 Within this area, heavy element corroles are of particular interest as optical sensors, near-IR dyes, phosphors, and organic light emitting diodes. 2 The size mismatch between the corrole N 4 cores and the large ionic radii of the lower oxidation states of the 5d elements, however, poses formidable challenges for metal insertion.As of today, Hf, 3 W, 4 Re, 5 Ir, 6 and Au, 7 corroles have been synthesized, whereas others such as osmium and platinum corroles are still to be reported.Here we present unambiguous evidence, including multiple singlecrystal X-ray analyses, for the formation of platinum corroles.
Insertion of Pt into corroles proved to be extraordinarily challenging.A large number of commercially available Pt precursors, each tested with a wide selection of solvents, failed to yield isolable Pt corrole derivatives.In the end, microwave irradiation of the commercially unavailable tetranuclear platinum acetate complex [Pt(OAc) 2 ] 4 Á2HOAc 8 in benzonitrile at 140-150 1C for 2 hours led to low but reproducible yields (B6%) of diamagnetic Pt(IV) corroles.Notably, aerobic conditions were critical to the success of the reaction; strict exclusion of oxygen did not result in Pt-containing products.Based on MALDI-TOF mass spectrometry and 1 H NMR spectroscopy, the products could be formulated as Pt{T( p-X-Ph)C}(o/m/p-C 6 H 4 CN)(PhCN), where {T( p-X-Ph)C} 3À is a meso-triarylcorrole trianion with aryl para substituents X = CF 3 , H, CH 3 , and OCH 3 and the axial benzonitrile-derived aryl ligand may be bound through the o-, m-, or p-carbon, relative to the CN group (Fig. 1).Thus, C-H activation of benzonitrile resulting in a Pt(IV)-aryl center has been critical to the synthesis of stable platinum corroles.Electrospray ionization mass spectrometric studies indicated the N-bound benzonitrile ligand in these Pt(IV) complexes to be labile.Preliminary X-ray analysis of the putative Pt{T( p-CF 3 -Ph)C}(m-C 6 H 4 CN)(PhCN) complex indicated extensive disorder involving benzonitrilederived ligands; despite some evidence of five-coordinate Pt{T( p-X-Ph)C}(o/m/p-C 6 H 4 CN), much of the material consisted of various regioisomers of Pt{T( p-X-Ph)C}(o/m/p-C 6 H 4 CN) 2 , which formally correspond to a Pt(V) oxidation state.Thus, a second C-H activation, involving the N-coordinated benzonitrile, had taken place during crystallization.Air-stable, oxidized platinum complexes Pt{T( p-X-Ph)C}(o/m/p-C 6 H 4 CN)( p-C 6 H 4 CH 3 ) could be more reliably obtained by treating the Pt(IV) complexes with an aryl-Grignard reagent (Fig. 1).Because of the low yields, only the m-C 6 H 4 CN regioisomers were fully characterized for all the corroles, whereas the p-C 6 H 4 CN isomer could be characterized for only one of the corroles.Fortunately, three oxidized Pt corroles yielded X-ray quality crystals, affording full structural characterization.
X-band EPR spectra were for the oxidized Pt corroles in the solid state (Fig. 2), in solution (2 : 1 CH 2 Cl 2 : toluene) at RT, and in a frozen glass at 73 K obtained from the same solution.The complexes all gave strong signals with g-values B2.00, consistent with the ligand radical formulation Pt(corrole 2À )ArAr 0 .The spectra could be simulated with fairly narrow linewidths (Lorentzian, fwhh = 3 G) and a slight g-anisotropy, but notably without any 195 Pt hyperfine coupling. 9pon dissolution in a mixture of CH 2 Cl 2 and toluene at RT, the spectra changed profoundly (Fig. 3); the bandwidth became nearly twice as large, with concomitant appearance of fully resolved hyperfine coupling to 195 Pt.Freezing of the solution at 73 K had only a slight effect on the spectrum (Fig. 3).The much narrower signals in the solid state may be interpreted as an unusually well-behaved example of ''exchange narrowing''. 10This is in agreement with the crystal structures, which indicate partial corrole p-stacking with relatively short interplanar distances of about 3.4 Å.The observed g-value range of 1.997-2.011agrees with literature values for corrole-based radical complexes. 11lthough well-resolved in solution, the hyperfine coupling to Pt is too small for the radical to be Pt-centered.1 and a representative thermal ellipsoid plot is shown in Fig. 4. As for many metallocorroles, 12,13 the corrole macrocycles in all three complexes are almost perfectly planar.The short Pt-N distances of approximately 1.95 Å are consistent with a Pt(IV) oxidation state and the sterically constrained nature of the corrole N 4 core, while the axial Pt-C distances of about 2.1 Å are typical for unconstrained Pt IV -C bond distances.
Fig. 5 depicts key results from DFT calculations on the model complexes Pt(corrole)(Ph)(PhCN) (C s ) and Pt(corrole)Ph 2 (C 2v ).The optimized structural parameters are in excellent agreement with those observed experimentally (Table 1).To a first approximation, the spin density of Pt(corrole)Ph 2 corresponds to a corrole b 1 radical (in terms of C 2v irreps), which resembles a porphyrin a 2u -type radical, 14 i.e., the spin density is largely localized on the three meso carbons and the four nitrogens and to a lesser extent on the direct C b -C b linkage.The Pt does not carry a significant amount of spin density, thereby ruling out any degree of Pt(V) character.Closer examination of Fig. 5 indicates that the corrole carries only about two-thirds of the total molecular spin density; the remaining onethird of the spin density is evenly divided between the two formally anionic phenyl ipso carbons.Thus, the HOMO is not a pure corrole b 1 HOMO, but has a certain amount of aryl character as well.
Table 2    in CH 2 Cl 2 .Note that, whereas the first oxidation potentials vary little among the different classes of metallocorroles, the reduction potentials vary considerably.Compared with Cu corroles, the Pt(IV) and Au(III) corroles undergo reduction at considerably substantially more negative potentials.7c Thus, Pt(IV) and Au(III) corroles exhibit relatively large electrochemical ''HOMO-LUMO gaps'' (i.e., the algebraic difference between the first oxidation and potentials) -B1.4 eV for Pt(IV) and B2.2 eV for Au(III).These two metal ions appear to be strongly stabilized by the trianionic corrole ligands, which would explain the resistance to reduction.In contrast, the oxidized Pt{T( p-X-Ph)C}(m-C 6 H 4 CN)( p-C 6 H 4 CH 3 ) complexes, like Cu triarylcorroles, are readily reduced at approximately 0.1 AE 0.1 V (vs.SCE), as expected on the basis of their corrole 2À radical character. 17 number of metallocorrole families such as Cu, MnCl and FeCl 15 meso-triarylcorroles exhibit strongly substituent-sensitive electronic absorption spectra, with the Soret maximum shifting sensitively as a function of substituents on the meso-aryl groups.These have been analyzed for copper triarylcorroles with TDDFT calculations and ascribed to so-called hyper character, i.e., phenyl-to-metal charge transfer (CT) character mixing into the Soret transitions. 17For CrO, MoO, Ag and Au triarylcorroles on the other hand the Soret maxima are relatively independent of substituents on the meso-aryl groups.7c,18 Against this backdrop, the Soret maxima of Pt corroles were found to be substituent-sensitive or -insensitive, depending on the overall oxidation level of the complexes.As shown in Fig. 6 and Table 3, the Pt(IV) complexes Pt{T( p-X-Ph)C}(m-C 6 H 4 CN)-(PhCN) exhibit substituent-insensitive Soret maxima, whereas the oxidized Pt{T( p-X-Ph)C}(m-C 6 H 4 CN)( p-C 6 H 4 CH 3 ) series exhibits some of the strongest meso substituent effects observed for metallocorroles.These observations suggest that meso-aryl substituent sensitivity occurs precisely in those cases where the corrole has substantial corrole 2À character; this is the case for Cu, FeCl, MnCl and the oxidized Pt corroles.For the other metallocorroles, where the corrole is relatively innocent, the Soret maxima are substituent-insensitive.
Compared with other 5d metallocorroles such as Au and Ir corroles, which are comparatively unreactive, platinum corroles have long been of interest on account of their potential for significant axial reactivity vis-a `-vis small-molecule activation.
Here we have presented the first unambiguous proof of platinum insertion into the corrole macrocycle, including three single-crystal X-ray structures.Two series of complexes have been prepared in low yields: the six-coordinate Pt(IV) series Pt{T( p-X-Ph)C}(m-C 6 H 4 CN)(PhCN) and the oxidized series Pt{T( p-X-Ph)C}(m-C 6 H 4 CN)( p-C 6 H 4 CH 3 ).Ongoing research on Pt corroles focuses on developing higher-yielding syntheses and on detailed studies of C-H activation and other reactivity.Whether the compounds exhibit significant biological activity, particularly anticancer activity, remains an exciting question for the future.

Table 3
Soret absorption maxima (nm) of Cu, Au and Pt {T( p-X-P)C} complexes in CH 2 Cl 2