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We present here a first systematic study of substituent effects in metallocorroles, based on electronic absorption, resonance Raman (RR), and infrared (IR) spectroscopic studies and electrochemical measurements on 10 copper(III) meso-triarylcorroles, Cu(III)[beta-Y(8)TArC], where the beta-substituent Y = H or Br and the meso-aryl group Ar = C(6)F(5) or p-X-C(6)H(4) and X = CF(3), H, CH(3), and OCH(3). The results afford a number of significant inisights. (1) The RR (and IR) results show that at least two and possibly more high-frequency bands in the 1400-1550 cm(-1) region exhibit significant frequency downshifts on beta-octabromination and, thus, qualify as structure-sensitive marker bands. DFT geometry optimizations indicate that the saddled conformation should be clearly preferred for the beta-octabromo-meso-triarylcorrole derivatives studied and that beta-octabromination results in expansion of a number of skeletal bond distances of the corrole macrocycle, consistent with observed frequency downshifts. (2) Electrochemical measurements on planar Cu(III)[TArC] derivatives have shown that the para substituents on the meso-aryl groups exert a strong influence on the half-wave potentials for oxidation (rho(ox) = DeltaE(1/2ox)/Delta(3sigma) = 95 mV), suggesting that oxidation involves removal of an electron from the corrole "b(1)" HOMO, which has significant amplitudes at the meso postions and crudely resembles a porphyrin a(2u) HOMO in shape. In contrast, the Hammett rho(ox) is much lower for the nonplanar Cu(III)[Br(8)TArC] derivatives and we suggest that this ultimately results from a b(1)-to-a(2) HOMO reversal which in turn stems from a metal (d(x2-y2)-corrole ("b(1)") orbital interaction that becomes symmetry-allowed under a saddle distortion of the corrole macrocycle. In contrast to what has been observed for metallotetraphenylporphyrins, beta-octabromination dramatically raises the half-wave potential for one-electron oxidation of the triarylcorrole derivatives studied. This appears to be due to the fact that both the "a(2)" and "b(1)" HOMOs of a corrole (in C(2v) notation) have significantly higher amplitudes at the beta positions, compared to a porphyrin a(2u) HOMO. Thus, although many metallocorroles are significantly more easily oxidizable than analogous metalloporphyrins, certain beta-octahalogeno-meso-triarylcorrole derivatives can indeed be extremely electron deficient and oxidation resistant and may, therefore, find use as rugged catalysts or reagents under highly oxidizing conditions. (3) Finally, the Soret absorption maxima of high-valent metallotriarylcorroles exhibit a uniquely sensitive dependence on the substituents on the meso-aryl groups. Thus, on going from Cu(III)[T(p-CF(3)-P)C] (T(p-CF(3)-P)C = meso-tris((p-trifluoromethyl)phenyl)corrolato) to Cu(III)[T(p-OM-P)C] (T(p-OM-P)C = meso-tris(p-methoxyphenyl)corrolato), the Soret maximum red shifts by 26 nm, from 407 to 433 nm. Similarly, on going from Cu(III)[Br(8)T(p-CF(3)-P)C] (Br(8)T(p-CF(3)-P)C = beta-octabromo-meso-tris((p-trifluoromethyl)...

Electrochemical and spectroelectrochemical studies are presented for nine copper corroles with electron-withdrawing or electron-donating substituents on the three meso-phenyl rings of the compounds or on the eight β-pyrrole positions of the macrocycle. Up to three reversible oxidations can be seen for each Cu (III) corrole in CH 2 Cl 2 containing 0.1 M TBAP. Unlike the case of ( OEC ) Cu , no dimer is electrochemically detected upon the first oxidation of these compounds at room temperature. However, a dimer forms at low temperature (< -50°C) for compounds having strong electron-donating groups. Two reductions are observed for all nine corroles at low temperature in CH 2 Cl 2, 0.1 M TBAP, but only one reduction is detected at room temperature for four of these compounds which have weak electron-withdrawing or electron-donating groups. The neutral, reduced or oxidized Cu corroles were also characterized by thin-layer UV-visible spectroelectrochemistry and ESR. The resulting data indicates that eight of the nine neutral complexes contain a Cu (III) center while only one complex, [ Br 8( C 6 F 5)3 Cor ] Cu , exists in its Cu (II) form in CH 2 Cl 2 containing 0.2 M TBAP.

The reaction of copper beta-octabromo- meso-triarylcorrole derivatives with methyl 2,2-difluoro-2-(fluorosulfonyl)acetate has provided four beta-octakis(trifluoromethyl)corrole complexes, Cu[(CF 3) 8T( p-XP)C] (X = F, H, Me, OMe), in moderate yields. The new complexes present a conglomeration of remarkable substituent effects, both steric and electronic. DFT (OLYP/TZP) geometry optimization of Cu[(CF 3) 8TPC] (i.e., X = H) indicates a sterically hindered, strongly saddled geometry, with numerous short F...F nonbonded contacts of 2.5-2.9 A and certain beta carbons displaced by over 1.5 A relative to the mean corrole plane. The CF 3 groups generally appear as quartets in the (19)F NMR spectra, with unexpectedly large (5) J FF coupling constants of about 14 Hz, apparently a manifestation of the highly crowded structure. The eight CF 3 groups together exert a powerful influence on the redox potentials of the copper corrole core. Thus, the E 1/2ox of Cu[(CF 3) 8TPC] (1.4 V vs saturated calomel electrode) is a full half of a volt above that of Cu(TPC) (0.9 V) and a quarter of a volt above that of Cu(Br 8TPC) (1.14 V). Intriguingly, the beta CF 3 groups also greatly intensify the influence of the meso aryl substituents on the redox potentials, relative to the other Cu[Y 8T( p-XP)C] series, where Y = H, F, and Br. The Cu[(CF 3) 8T( p-XP)C] complexes also exhibit the most red-shifted optical spectra of any series of metallocorroles synthesized to date. Thus, between Cu(TPC) and Cu[(CF 3) 8T( p-MeO-P)C], the Soret maximum shifts by nearly 100 nm. The observed red-shifts are attributed in part to charge-transfer transitions of the Soret region and in part to the extreme nonplanar distortions.

DFT calculations have underscored the importance of the tripodal supporting ligand in tuning the spin-state energetics of pseudotetrahedral transition metal imido complexes. In particular, we have focused on Co(III)-imido complexes, where our best estimate (OLYP) of the singlet-triplet splitting varies from 0.75 eV for a trisphosphine complex (1) and 0.3 eV for a tris(N-heteroyclic-carbene) complex (2) to essentially 0.0 eV for a hydrotris(pyrazolyl)borate (3) complex. The experimentally studied analogues of 1, 2, and 3 all exhibit S = 0 ground states; however, the experimental analogue of 3 exhibits spin-crossover behavior due to a low-lying S = 1 state. Interestingly, whereas all the pure functionals examined successfully predict nearly equienergetic singlet end triplet states for 3, the hybrid functionals B3LYP and O3LYP exhibit a clear (and incorrect) preference for the S = 2 state. In addition, we have also carried out an exploratory survey of Cr(III), Mn(III), and Fe(III) imido complexes with trisphosphine and hydrotris(pyrazolyl)borate (Tp) supporting ligands. Among the more interesting predictions of this study is that an FeIII(Tp)(imido) species should exhibit a high-spin S = 5/2 ground state, which would be unique for an iron-imido complex.

We have carried out a broad survey of tetraphenylporphyrin derivatives in relation to their possible hyperporphyrin character. The majority of the free-base tetraphenylporphyrins studied, i.e., TArPH2; Ar = p-X−C6H4, where X = CH3, H, F, CF3, and NO2, when dissolved in trifluoroacetic acid (i.e. when centrally diprotonated), exhibit red-shifted “hyperporphyrin” spectra. The “hyper” features are attributable to phenyl-to-porphyrin charge-transfer transitions. However, certain free-base tetraphenylporphyrins with extremely electron-deficient phenyl groups, such as TPFPPH2, do not exhibit hyperporphyrin spectra in trifluoroacetic acid solution. Certain anionic tetraphenylporphyrin derivatives such as T(p-OH−P)PH2 or Ni[T(p-OH−P)P] dissolved in methanolic Bu4NOH also qualify as hyperporphyrins. The hyper transitions in these cases involve charge transfer from anionic phenolate substituents to the neutral porphyrin core. This study also presents a first systematic resonance Raman spectroscopic exploration of hyperporphyrins. Comparison of the Soret-resonant Raman spectra of various normal, hyper-, and hypso- tetraphenylporphyrin derivatives indicates that the former two categories generally exhibit a more intense ν1 band, which is the fully symmetric Cmeso−Cphenyl stretching vibration, relative to hypsoporphyrins such as square-planar nickel tetraarylporphyrins. We have also reinvestigated recent reports of large red shifts observed for the electronic spectra of saddled porphyrins in polar solvents, an effect attributed to increased N−H···solvent hydrogen bonding in polar solvents. Interestingly, we find that such solvent-induced red shifts are observed for the relatively electron-deficient porphyrin Br8TPPH2, Cl8TPPH2, and OETNPH2 but not for the relatively electron-rich OETPPH2. Resonance Raman spectra of these saddled porphyrins in different solvents reveal little shift in the high-frequency marker bands, which is consistent with little change in macrocycle conformation with solvent polarity. The observed solvent-induced red shifts in the electronic spectra therefore appear to reflect a largely electronic (as opposed to conformational) effect of N−H···solvent hydrogen bonding in polar solvents. Finally, we also present a chronological summary of the controversial question as to whether nonplanar deformations are actually responsible for the red-shifted electronic spectra of the majority of nonplanar porphyrins.

DFT calculations reveal the existence of metastable side-on {CuNO}10 and {CuNO}11 species relevant to the type 2 copper site of nitrite reductase (CuNIR). Side-on NO coordination seems especially favorable in energy terms for the {CuNO}11 species. The {CuNO}11 geometry parameters also seem to be in better agreement with those reported for a crystallographically characterized CuNIR intermediate, relative to the {CuNO}10 parameters.

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