Christine M. Beavers

The Structural Chemistry of Metallocorroles: Combined X-ray Crystallography and Quantum Chemistry Studies Afford Unique Insights

Although they share some superficial structural similarities with porphyrins, corroles, trianionic ligands with contracted cores, give rise to fundamentally different transition metal complexes in comparison with the dianionic porphyrins. Many metallocorroles are formally high-valent, although a good fraction of them are also noninnocent, with significant corrole radical character. These electronic-structural characteristics result in a variety of fascinating spectroscopic behavior, including highly characteristic, paramagnetically shifted NMR spectra and textbook cases of charge-transfer spectra. Although our early research on corroles focused on spectroscopy, we soon learned that the geometric structures of metallocorroles provide a fascinating window into their electronic-structural characteristics. Thus, we used X-ray structure determinations and quantum chemical studies, chiefly using DFT, to obtain a comprehensive understanding of metallocorrole geometric and electronic structures. This Account describes our studies of the structural chemistry of metallocorroles. At first blush, the planar or mildly domed structure of metallocorroles might appear somewhat uninteresting particularly when compared to metalloporphyrins. Metalloporphyrins, especially sterically hindered ones, are routinely ruffled or saddled, but the missing meso carbon apparently makes the corrole skeleton much more resistant to nonplanar distortions. Ruffling, where the pyrrole rings are alternately twisted about the M-N bonds, is energetically impossible for metallocorroles. Saddling is also uncommon; thus, a number of sterically hindered, fully substituted metallocorroles exhibit almost perfectly planar macrocycle cores. Against this backdrop, copper corroles stand out as an important exception. As a result of an energetically favorable Cu(d(x2-y2))-corrole(π) orbital interaction, copper corroles, even sterically unhindered ones, are inherently saddled. Sterically hindered substituents accentuate this effect, sometimes dramatically. Thus, a crystal structure of a copper β-octakis(trifluoromethyl)-meso-triarylcorrole complex exhibits nearly orthogonal, adjacent pyrrole rings. Intriguingly, the formally isoelectronic silver and gold corroles are much less saddled than their copper congeners because the high orbital energy of the valence d(x2-y2) orbital discourages overlap with the corrole π orbital. A crystal structure of a gold β-octakis(trifluoromethyl)-meso-triarylcorrole complex exhibits a perfectly planar corrole core, which translates to a difference of 85° in the saddling dihedral angles between analogous copper and gold complexes. Gratifyingly, electrochemical, spectroscopic, and quantum chemical studies provide a coherent, theoretical underpinning for these fascinating structural phenomena. With the development of facile one-pot syntheses of corrole macrocycles in the last 10-15 years, corroles are now almost as readily accessible as porphyrins. Like porphyrins, corroles are promising building blocks for supramolecular constructs such as liquid crystals and metal-organic frameworks. However, because of their symmetry properties, corrole-based supramolecular constructs will probably differ substantially from porphyrin-based ones. We are particularly interested in exploiting the inherently saddled, chiral architectures of copper corroles to create novel oriented materials such as chiral liquid crystals. We trust that the fundamental structural principles uncovered in this Account will prove useful as we explore these fascinating avenues.

M2@C79N (M = Y, Tb): Isolation and Characterization of Stable Endohedral Metallofullerenes Exhibiting M-M Bonding Interactions Inside Aza[80]Fullerene Cages

Y2@C79N and Tb2@C79N have been prepared by conducting the Kratschmer-Huffman electric-arc process under 20 Torr of N2 and 280 Torr of He with metal oxide-doped graphite rods. These new heterofullerenes were separated from the resulting mixture of empty cage fullerenes and endohedral fullerenes by chemical separation and a two-stage chromatographic process. Crystallographic data for Tb2@C79N x Ni(OEP) x 2 C6H6 demonstrate the presence of an 80-atom cage with idealized I(h) symmetry and two, widely separated Tb atoms inside with a Tb-Tb separation of 3.9020(10) A for the major terbium sites. The EPR spectrum of the odd-electron Y2@C79N indicates that the spin density largely resides on the two equivalent yttrium ions. Computational studies on Y2@C79N suggest that the nitrogen atom resides at a 665 ring junction in the equator on the fullerene cage and that the unpaired electron is localized in a bonding orbital between the two yttrium ions of this stable radical. Thus, the Tb-Tb bond length of the single-electron bond is an exceedingly long metal-metal bond.

Is the Isolated Pentagon Rule Merely a Suggestion for Endohedral Fullerenes? The Structure of a Second Egg-Shaped Endohedral Fullerene—Gd3N@Cs(39663)-C82

The structure of Gd3N@Cs(39663)-C82 has been determined through single crystal X-ray diffraction on Gd3N@Cs(39663)-C82.NiII(OEP).2(C6H6) The carbon cage has a distinct egg shape because of the presence of a single pair of fused pentagons at one apex of the molecule. Although 9 IPR structures are available to the C82 cage, one of the 39709 isomeric structures that do not conform to the IPR was found in Gd3N@Cs(39663)-C82. The egg-shaped structure of Gd3N@Cs(39663)-C82 is similar to that observed previously for M3N@Cs(51365)-C84 (M = Gd, Tm, Tb). As noted for other non-IPR endohedral fullerenes, one Gd atom in Gd3N@Cs(39663)-C82 is nestled within the fold of the fused pentagons.

Near-Infrared Light Activated Release of Nitric Oxide from Designed Photoactive Manganese Nitrosyls: Strategy, Design, and Potential as NO Donors

Two new manganese complexes derived from the pentadentate ligand N,N-bis(2-pyridylmethyl)amine-N-ethyl-2-quinoline-2-carboxamide, PaPy2QH, where H is dissociable proton), namely, [Mn(PaPy2Q)(NO)]ClO4 (2) and [Mn(PaPy2Q)(OH)]ClO4 (3), have been synthesized and structurally characterized. The Mn(III) complex [Mn(PaPy2Q)(OH)]ClO4 (3), though insensitive to dioxygen, reacts with nitric oxide (NO) to afford the nitrosyl complex [Mn(PaPy2Q)(NO)]ClO4 (2) via reductive nitrosylation. This diamagnetic {Mn-NO}6 nitrosyl exhibits nuNO at 1725 cm-1 and is highly soluble in water, with lambdamax at 500 and 670 nm. Exposure of solutions of 2 to near-infrared (NIR) light (810 nm, 4 mW) results in bleaching of the maroon solution and detection of free NO by an NO-sensitive electrode. The quantum yield of 2 (Phi = 0.694 +/- 0.010, lambdairr = 550 nm, H2O) is much enhanced over the first generation {Mn-NO}6 nitrosyl derived from analogous polypyridine ligand, namely, [Mn(PaPy3)(NO)]ClO4 (1, Phi = 0.385 +/- 0.010, lambdairr = 550 nm, H2O), reported by this group in a previous account. Although quite active in the visible range (500-600 nm), 1 exhibits very little photoactivity under NIR light. Both 1 and 2 have been incorporated into sol-gel (SG) matrices to obtain nitrosyl-polymer composites 1.SG and 2.SG. The NO-donating capacities of the polyurethane-coated hybrid materials 1.HM and 2.HM have been determined. 2.HM has been used to transfer NO to reduced myoglobin with 780 nm light. The various strategies for synthesizing photosensitive metal nitrosyls have been discussed to establish the merits of the present approach. The results of the present study confirm that proper ligand design is a very effective way to isolate photoactive manganese nitrosyls that could be used to deliver NO to biological targets under the control of NIR light.

Large Metal Ions in a Relatively Small Fullerene Cage: The Structure of Gd3N@C2(22010)-C78 Departs from the Isolated Pentagon Rule

An isomerically pure sample of Gd(3)N@C(78) has been extracted from the carbon soot formed in the electric-arc generation of fullerenes using hollow graphite rods packed with Gd(2)O(3) and graphite powder under an atmosphere of helium and dinitrogen. Purification has been achieved by chromatographic methods and the product has been characterized by mass spectrometry, UV/vis absorption spectroscopy, and cyclic voltammetry. Although a number of endohedral fullerenes have been found to utilize the D(3h)(5)-C(78) cage, comparison of the spectroscopic and electrochemical properties of the previously characterized Sc(3)N@D(3h)(5)-C(78) with those of Gd(3)N@C(78) reveals significant differences that indicate that these two endohedrals do not possess the same cage structure. A single crystal X-ray diffraction study indicates that the fullerene cage does not follow the isolated pentagon rule (IPR) but has two equivalent sites where two pentagons abut. The endohedral has been identified as Gd(3)N@C(2)(22010)-C(78). Two of the gadolinium atoms of the planar Gd(3)N unit are located within the pentalene folds formed by the adjacent pentagons. The third gadolinium atom resides at the center of a hexagonal face of the fullerene.

New egg-shaped fullerenes: non-isolated pentagon structures of Tm3N@Cs(51365)-C84 and Gd3N@Cs(51365)-C84

Although there are 51 568 non-IPR and 24 IPR structures for C84, the egg-shaped endohedral fullerenes Tm3N@C(s)(51 365)-C84 and Gd3N@C(s)(51 365)-C84 utilize the same non-IPR cage structure as found initially for Tb3N@C(s)(51 365)-C84.

Sc2S@Cs(10528)-C72: A Dimetallic Sulfide Endohedral Fullerene with a Non Isolated Pentagon Rule Cage

A non isolated pentagon rule metallic sulfide clusterfullerene, Sc(2)S@C(s)(10528)-C(72), has been isolated from a raw mixture of Sc(2)S@C(2n) (n = 35-50) obtained by arc-discharging graphite rods packed with Sc(2)O(3) and graphite powder under an atmosphere of SO(2) and helium. Multistage HPLC methods were utilized to isolate and purify the Sc(2)S@C(72). The purified Sc(2)S@C(s)(10528)-C(72) was characterized by mass spectrometry, UV-vis-NIR absorption spectroscopy, cyclic voltammetry, and single-crystal X-ray diffraction. The crystallographic analysis unambiguously elucidated that the C(72) fullerene cage violates the isolated pentagon rule, and the cage symmetry was assigned to C(s)(10528)-C(72). The electrochemical behavior of Sc(2)S@C(s)(10528)-C(72) shows a major difference from those of Sc(2)S@C(s)(6)-C(82) and Sc(2)S@C(3v)(8)-C(82) as well as the other metallic clusterfullerenes. Computational studies show that the Sc(2)S cluster transfers four electrons to the C(72) cage and C(s)(10528)-C(72) is the most stable cage isomer for both empty C(72)(4-) and Sc(2)S@C(72), among the many possibilities. The structural differences between the reported fullerenes with C(72) cages are discussed, and it is concluded that both the transfer of four electrons to the cage and the geometrical requirements of the encaged Sc(2)S cluster play important roles in the stabilization of the C(s)(10528)-C(72) cage.

A Family of Calix{[}4]arene-Supported {[}(Mn2Mn2II)-Mn-III] Clusters

In the cone conformation calix[4]arenes possess lower-rim polyphenolic pockets that are ideal for the complexation of various transition-metal centres. Reaction of these molecules with manganese salts in the presence of an appropriate base (and in some cases co-ligand) results in the formation of a family of calixarene-supported [Mn(III)(2)Mn(II)(2)] clusters that behave as single-molecule magnets (SMMs). Variation in the alkyl groups present at the upper-rim of the cone allows for the expression of a degree of control over the self-assembly of these SMM building blocks, whilst retaining the general magnetic properties. The presence of various different ligands around the periphery of the magnetic core has some effect over the extended self-assembly of these SMMs.

An S-Shaped [Fe4Dy2] Complex Exhibiting Slow Relaxation of Magnetization: Synthesis, Magnetism, and Crystal Structures of a Family of [Fe4Ln2][Ln] Coordination Compounds (Ln = Nd, Gd, Tb, Dy, and Ho)

A series of heterobimetallic 3d-4f cluster coordination compounds has been synthesized using a Schiff-base ligand (H(3)L), a trinuclear iron(III) precursor complex and lanthanide nitrates as reactants. Five new isostructural complexes were prepared with the ligand in 4 different forms, i.e., neutral and 3 different levels of deprotonation, and with the general formula [Fe(III)(4)Ln(III)(2)(H(2)L)(2)(HL)(2)L(2)(CH(3)OH)(2)((CH(3))(3)CCOO)(2)(NO(3))(2)][Ln(III)(NO(3))(4)(H(3)L)(CH(3)OH)].NO(3).H(2)O, where Ln is Nd, Gd, Tb, Dy, and Ho for compounds 1, 2, 3, 4, and 5, respectively. The single-crystal structures of two complexes were determined by X-ray diffraction, consisting of an original [Fe(III)(4)Ln(III)(2)](2+) moiety with a linear S-shaped [Ln-Fe(4)-Ln] core and an isolated Ln(III) ion coordinated by nitrate anions and the neutral Schiff-base ligand. The isostructural nature of all five coordination compounds is further illustrated both by XRPD and IR analysis. Magnetic properties of all five compounds have been studied and are discussed in light of magnetostructural considerations. Among these five compounds, the Dy (4) cluster shows frequency-dependent ac-susceptibility indicative of probable single-molecule magnet behavior.

Isolation of a Small Carbon Nanotube: The Surprising Appearance of D5h(1)‐C90

have become well-known; however, thecarbon soot from arc generators contains small amounts(generally less than 1%) of higher fullerenes. The isolation ofthese higher fullerenes in isomerically pure form is challeng-ing, especially since the number of isomers that follow theisolated-pentagon rule (IPR) increases as the size of thefullerene cage expands.