Matthew P. Shores

Photooxidizing Chromium Catalysts for Promoting Radical Cation Cycloadditions

The photooxidizing capabilities of selected Cr(III) complexes for promoting radical cation cycloadditions are described. These complexes have sufficiently long-lived excited states to oxidize electron-rich alkenes, thereby initiating [4+2] processes. These metal species augment the spectrum of catalysts explored in photoredox systems, as they feature unique properties that can result in differential reactivity from the more commonly employed ruthenium or iridium catalysts.

High-Spin Square-Planar CoII and FeII Complexes and Reasons for Their Electronic Structure†

More than half a century of intense investigation in coordination compounds has laid a firm foundation for our understanding of the ligand fields in transition-metal complexes. Complexes of the heavier 4d and 5d metals are generally low spin, whereas the spin of 3d metal complexes can be high or low, depending on ligand characteristics. The number and type of donor atoms, ligand substituents, and the presence or absence of chelate rings all influence metal spin states. A combination of data-mining and detailed computational study have quantified recently these empirical observations. In spite of such variety, there are still some types of metal complexes that are rarely observed. The stereospinomers of high-spin, square-planar complexes, for example, are extremely rare because the large separation of the dx2 y2 orbital from the rest of the d-manifold favors low-spin electron configurations for d with n> 4, and four-coordinate compounds are rare for d systems which could have all four low-lying d orbitals half filled. Known d examples include multiple Cr species, a Mn species, and one Nb complex. The rarity of this geometry and spin-state combination is demonstrated by only a handful of examples with late 3d metals. An interesting {NiO2N2} d 8 system is known whose high-spin examples are subtly dependent on ligand substitution. Until the structure was confirmed as tetrameric with octahedral coordination at the Co center, [Co(acac)2] was postulated to have square-planar geometry based on magnetic and spectroscopic data that differed from tetrahedral complexes. A search of the Cambridge Structural Database (V 5.32) for Fe and Co complexes in a fourcoordinate environment with t4 parameter [11] < 0.25 and magnetic susceptibility data revealed five high-spin d Fe complexes with macrocyclic or chelating N4 [12, 13] and O4 [14,15] coordination, and three high-spin d Co complexes 16,17] with varied ligand systems. A complete description of the CSD search and results can be found in the Supporting Information. Herein we report a unique pair of high-spin, square-planar {MO4} species. Our group has prepared several families of homoleptic fluorinated aryloxide and alkoxide complexes of 3d metals. We have extensively investigated the high-spin aryloxide compounds [M(OAr)4] 2 , M = Fe, Co, Ni, and Cu and [M(OAr)5] 2 , M = Fe, in which OAr = OC6F5 or 3,5OC6H3(CF3)2, as well as the high-spin alkoxide compounds [M(OC4F9)3] 1 , M = Fe, Co, Cu and [M(OC4F9)4] 2 , M = Co, Ni. Spectroscopic and computational work have shown that these fluorinated ligands are medium field ligands, on par with OH and F , and stronger than NCO . The electron-withdrawing power of extensively fluorinated ligands reduces the p-donor character of the O atom, such that bridging is not observed and mononuclear species are readily prepared. More recently, we have begun studies of the chelating perfluoropinacolate ligand, ddfp . Magnetic susceptibility and elemental analysis data were reported for K2[M(ddfp)2], (M = Mn, Ni, Cu) for which square-planar geometry was proposed. An octahedral bis-H2O adduct, (Me4N)2[Co(OH2)2(ddfp)2] has been proposed based on elemental analysis data. Despite the relative ease in making the [M(ddfp)2] 2 complexes with first-row transition metals, no examples of M = Co or Fe have been published. We now report a highspin, square-planar Co complex, {K(DME)2}2[Co(ddfp)2] (1), and the analogous high-spin, square-planar Fe complex {K(DME)2}2[Fe(ddfp)2] (2). We also provide a discussion of three other square-planar {MO4} species from the recent literature whose composition and spin-state characteristics clarify the ligand requirements for the highly unusual highspin, square-planar combination in late row 3d metals. Compound 1 has been prepared as pale pink crystals as shown in Equation (1), and is stable in an inert atmosphere and in various organic solvents, but yields a brown oil upon prolonged exposure to air. Iron-containing 2, and the Zn derivative, {K(DME)2}2[Zn(ddfp)2] (3), were similarly prepared as purple-pink, and colorless crystals, respectively. No [*] S. A. Cantalupo, Prof. Dr. L. H. Doerrer Department of Chemistry, Boston University 590 Commonwealth Avenue, Boston, MA 02215 (USA) E-mail: doerrer@bu.edu

Ambient temperature anion-dependent spin state switching observed in “mostly low spin” heteroleptic iron(II) diimine complexes

We report the synthesis, characterization, and representative anion-binding studies on a series of heteroleptic FeII diimine complexes that have been designed to show anion-triggered spin state switching at ambient temperatures. Starting with [(H2bip)2FeBr2] (1), ligand substitution affords [(H2bip)2Fe(NN)]2+ complexes, where (NN) = pipi (2), bpy (3), and phen (4). In the solid state, the tetraphenylborate and bromide salts of the complexes display different thermally induced spin crossover properties, with spin transition temperatures above 395 K. The solid state magnetic properties depend on the FeII ligand field parameters and anion–cation interactions as well as solvent and packing effects. In dichloromethane solution, the weakly bound tetraphenylborate salts show spin state diminution, amplified chemical shift responses toward the diamagnetic 1H NMR spectral window, and visible colour changes upon titration of bromide anions (as nBu4N+ salts). Both 1H NMR- and electronic absorption-monitored titration studies unequivocally link anion binding to a high spin → low spin transition for the minority species in solution. Further, the complexes show pronounced air stability, in contrast to the homoleptic parent complex [Fe(H2bip)3]2+. Tuning the FeII ligand field by judicious ligand substitution toward “mostly low spin” species thus increases both the environmental stability and operating temperature of the complexes, and provides evidence that spin state switching may serve as a viable signalling event for chemical sensing in solution.

Uncovering the Roles of Oxygen in Cr(III) Photoredox Catalysis

A combined experimental and theoretical investigation aims to elucidate the necessary roles of oxygen in photoredox catalysis of radical cation based Diels-Alder cycloadditions mediated by the first-row transition metal complex [Cr(Ph2phen)3](3+), where Ph2phen = bathophenanthroline. We employ a diverse array of techniques, including catalysis screening, electrochemistry, time-resolved spectroscopy, and computational analyses of reaction thermodynamics. Our key finding is that oxygen acts as a renewable energy and electron shuttle following photoexcitation of the Cr(III) catalyst. First, oxygen quenches the excited Cr(3+)* complex; this energy transfer process protects the catalyst from decomposition while preserving a synthetically useful 13 μs excited state and produces singlet oxygen. Second, singlet oxygen returns the reduced catalyst to the Cr(III) ground state, forming superoxide. Third, the superoxide species reduces the Diels-Alder cycloadduct radical cation to the final product and reforms oxygen. We compare the results of these studies with those from cycloadditions mediated by related Ru(II)-containing complexes and find that the distinct reaction pathways are likely part of a unified mechanistic framework where the photophysical and photochemical properties of the catalyst species lead to oxygen-mediated photocatalysis for the Cr-containing complex but radical chain initiation for the Ru congener. These results provide insight into how oxygen can participate as a sustainable reagent in photocatalysis.

Multielectron C–O Bond Activation Mediated by a Family of Reduced Uranium Complexes

A family of cyclopentadienyl uranium complexes supported by the redox-active pyridine(diimine) ligand, (Mes)PDI(Me) ((Mes)PDI(Me) = 2,6-((Mes)N═CMe)2-C5H3N, Mes = 2,4,6-trimethylphenyl), has been synthesized. Using either Cp* or Cp(P) (Cp* = 1,2,3,4,5-pentamethylcyclopentadienide, Cp(P) = 1-(7,7-dimethylbenzyl)cyclopentadienide), uranium complexes of the type Cp(X)UI2((Mes)PDI(Me)) (1-Cp(X); X = * or P), Cp(X)UI((Mes)PDI(Me)) (2-Cp(X)), and Cp(X)U((Mes)PDI(Me))(THF)n (3-Cp(X); *, n = 1; P, n = 0) were isolated and characterized. The series was generated via ligand centered reduction events; thus the extent of (Mes)PDI(Me) reduction varies in each case, but the uranium(IV) oxidation state is maintained. Treating 2-Cp(X), which has a doubly reduced (Mes)PDI(Me), with furfural results in radical coupling between the substrate and (Mes)PDI(Me), leading to C-C bond formation to form Cp(X)UI((Mes)PDI(Me)-CHOC4H3O) (4-Cp(X)). Exposure of 3-Cp* and 3-Cp(P), which contain a triply reduced (Mes)PDI(Me) ligand, to benzaldehyde and benzophenone, respectively, results in the corresponding pinacolate complexes Cp*U(O2C2Ph2H2)((Mes)PDI(Me)) (5-Cp*) and Cp(P)U(O2C2Ph4)((Mes)PDI(Me)) (5-Cp(P)). The reducing equivalents required for this coupling are derived solely from the redox-active ligand, rather than the uranium center. Complexes 1-5 have been characterized by (1)H NMR and electronic absorption spectroscopies, and SQUID magnetometry was employed to confirm the mono(anionic) [(Mes)PDI(Me)](-) ligand in 1-Cp(P) and 5-Cp(P). Structural parameters of complexes 1-Cp(P), 2-Cp(X), 4-Cp*, and 5-Cp(X) have been elucidated by X-ray crystallography.

Synthesis of terminal uranium(IV) disulfido and diselenido compounds by activation of elemental sulfur and selenium.

Rare stakes: Terminal uranium(IV) disulfido and diselenido compounds, Tp*2U(E2) (E=S, Se), were synthesized by the activation of elemental chalcogens. Structural, spectroscopic, computational and magnetic studies of these species establish their tetravalency and highly polarized U-E bonds.

Antiferromagnetic coupling across a tetrametallic unit through noncovalent interactions

Three paramagnetic heterobimetallic lantern complexes of the form [PtM(tba)4(OH2)] (M = Fe, 1; Co, 2; Ni, 3; tba = thiobenzoate) have been prepared in a single-step, bench-top procedure. In all three cases, a lantern structure with Pt–M bonding is observed in solution and in the solid state. Compound 1 is a monomer whereas 3 exists as a dimer in the solid state via a Pt⋯Pt metallophilic interaction. Compound 2 has been characterized in forms with (2a, purple) and without (2b, yellow) Pt⋯Pt metallophilic interactions. The dimers 2a (J = −10 cm−1, based on the spin Hamiltonian Ĥ = −2J(SA·SB)) and 3 (J = −60 cm−1) exhibit antiferromagnetic coupling between the two first-row metal ions in the solid state via a Pt⋯Pt non-covalent metallophilic interaction. The electronic structure of C4v [PtM(tba)4], C2 [PtM(tba)4(OH2)], (M = Fe, Co, Ni) and D2 symmetry [PtM(tba)4(OH2)]2 M = Co, Ni, units have been studied with DFT calculations, confirming the relative spin-state energies observed and the antiferromagnetic exchange pathway through four dz2 orbitals. The compounds 2a and 3 are the first examples of antiferromagnetic coupling through an unbridged M⋯M contact.

Thiocyanate-Ligated Heterobimetallic {PtM} Lantern Complexes Including a Ferromagnetically Coupled 1D Coordination Polymer

A series of heterobimetallic lantern complexes with the central unit {PtM(SAc)4(NCS)} have been prepared and thoroughly characterized. The {Na(15C5)}[PtM(SAc)4(NCS)] series, 1 (Co), 2 (Ni), 3 (Zn), are discrete compounds in the solid state, whereas the {Na(12C4)2)}[PtM(SAc)4(NCS)] series, 4 (Co), 5 (Ni), 6 (Zn), and 7 (Mn), are ion-separated species. Compound 7 is the first {PtMn} lantern of any bridging ligand (carboxylate, amide, etc.). Monomeric 1-7 have M(2+), necessitating counter cations that have been prepared as {(15C5)Na}(+) and {(12C4)2Na}(+) variants, none of which form extended structures. In contrast, neutral [PtCr(tba)4(NCS)]∞ 8 forms a coordination polymer of {PtCr}(+) units linked by (NCS)(-) in a zigzag chain. All eight compounds have been thoroughly characterized and analyzed in comparison to a previously reported family of compounds. Crystal structures are presented for compounds 1-6 and 8, and solution magnetic susceptibility measurements are presented for compounds 1, 2, 4, 5, and 7. Further structural analysis of dimerized {PtM} units reinforces the empirical observation that greater charge density along the Pt-M vector leads to more Pt···Pt interactions in the solid state. Four structural classes, one new, of {MPt}···{PtM} units are presented. Solid state magnetic characterization of 8 reveals a ferromagnetic interaction in the {PtCr(NCS)} chain between the Cr centers of J/kB = 1.7(4) K.

A cobalt(II) bis(salicylate)-based ionic liquid that shows thermoresponsive and selective water coordination

A metal-containing ionic liquid (MCIL) has been prepared in which the [Co(II)(salicylate)2](2-) anion is able to selectively coordinate two water molecules with a visible colour change, even in the presence of alcohols. Upon moderate heating or placement in vacuo, the hydrated MCIL undergoes reversible thermochromism by releasing the bound water molecules.

Trivalent Uranium Phenylchalcogenide Complexes: Exploring the Bonding and Reactivity with CS2 in the Tp*2UEPh Series (E = O, S, Se, Te)

The trivalent uranium phenylchalcogenide series, Tp*2UEPh (Tp* = hydrotris(3,5-dimethylpyrazolyl)borate, E = O (1), S (2), Se (3), Te (4)), has been synthesized to investigate the nature of the U-E bond. All compounds have been characterized by (1)H NMR, infrared and electronic absorption spectroscopies, and in the case of 4, X-ray crystallography. Compound 4 was also studied by SQUID magnetometry. Computational studies establish Mulliken spin densities for the uranium centers ranging from 3.005 to 3.027 (B3LYP), consistent for uranium-chalcogenide bonds that are primarily ionic in nature, with a small covalent contribution. The reactivity of 2-4 toward carbon disulfide was also investigated and showed reversible CS2 insertion into the U(III)-E bond, forming Tp*2U(κ(2)-S2CEPh) (E = S (5), Se (6), Te (7)). Compound 5 was characterized crystallographically.