Person:
Powers, Tamara Michelle

Profile Picture

Email Address

AA Acceptance Date

Birth Date

Research Projects

Organizational Units

Job Title

Last Name

Powers

First Name

Tamara Michelle

Name

Powers, Tamara Michelle

Filters

2013 - 20164

Settings

Author's Publications: search.filters.isAuthorOfPublication.853e87e3-f3aa-48b7-97b6-19e38b58948e

Search Results

Now showing 1 - 4 of 4
  • Thumbnail Image
    Publication
    Maximizing Electron Exchange in a [Fe3] Cluster
    (American Chemical Society (ACS), 2016) Hernández Sánchez, Raúl; Bartholomew, Amymarie; Powers, Tamara Michelle; Ménard, Gabriel; Betley, Theodore
    The one-electron reduction of (tbsL)Fe3(thf)1 furnishes [M][(tbsL)Fe3] ([M]+ = [(18-C-6)K(thf)2]+ (1, 76%) or [(crypt-222)K]+ (2, 54%)). Upon reduction, the ligand tbsL6– rearranges around the triiron core to adopt an almost ideal C3-symmetry. Accompanying the (tbsL) ligand rearrangement, the THF bound to the neutral starting material is expelled, and the Fe–Fe distances within the trinuclear cluster contract by ∼0.13 Å in 1. Variable-temperature magnetic susceptibility data indicates a well-isolated S = 11/2 spin ground state that persists to room temperature. Slow magnetic relaxation is observed at low temperature as evidenced by the out-of-phase (χM″) component of the alternating current (ac) magnetic susceptibility data and by the appearance of hyperfine splitting in the zero-field 57Fe Mössbauer spectra at 4.2 K. Analysis of the ac magnetic susceptibility yields an effective spin reversal barrier (Ueff) of 22.6(2) cm–1, nearly matching the theoretical barrier of 38.7 cm–1 calculated from the axial zero-field splitting parameter (D = −1.29 cm–1) extracted from the reduced magnetization data. A polycrystalline sample of 1 displays three sextets in the Mössbauer spectrum at 4.2 K (Hext = 0) which converge to a single six-line pattern in a frozen 2-MeTHF glass sample, indicating a unique iron environment and thus strong electron delocalization. The spin ground state and ligand rearrangement are discussed within the framework of a fully delocalized cluster exhibiting strong double and direct exchange interactions.
  • Thumbnail Image
    Publication
    Testing the Polynuclear Hypothesis: Multielectron Reduction of Small Molecules by Triiron Reaction Sites
    (American Chemical Society (ACS), 2013) Powers, Tamara Michelle; Betley, Theodore
    High-spin trinuclear iron complex (tbsL)Fe3(thf) ([tbsL]6– = [1,3,5-C6H9(NC6H4-o-NSitBuMe2)3]6–) (S = 6) facilitates 2 and 4e– reduction of NxHy type substrates to yield imido and nitrido products. Reaction of hydrazine or phenylhydrazine with (tbsL)Fe3(thf) yields triiron μ3-imido cluster (tbsL)Fe3(μ3-NH) and ammonia or aniline, respectively. (tbsL)Fe3(μ3-NH) has a similar zero-field 57Fe Mössbauer spectrum compared to previously reported [(tbsL)Fe3(μ3-N)]NBu4, and can be directly synthesized by protonation of the anionic triiron nitrido with lutidinium tetraphenylborate. Deprotonation of the triiron parent imido (tbsL)Fe3(μ3-NH) with lithium bis(trimethylsilyl)amide results in regeneration of the triiron nitrido complex capped with a thf-solvated Li cation [(tbsL)Fe3(μ3-N)]Li(thf)3. The lithium capped nitrido, structurally similar to the pseudo C3-symmetric triiron nitride with a tetrabutylammonium countercation, is rigorously C3-symmetric featuring intracore distances of Fe–Fe 2.4802(5) Å, Fe–N(nitride) 1.877(2) Å, and N(nitride)–Li 1.990(6) Å. A similar 2e– reduction of 1,2-diphenylhydrazine by (tbsL)Fe3(thf) affords (tbsL)Fe3(μ3-NPh) and aniline. The solid state structure of (tbsL)Fe3(μ3-NPh) is similar to the series of μ3-nitrido and -imido triiron complexes synthesized in this work with average Fe–Nimido and Fe–Fe bond lengths of 1.941(6) and 2.530(1) Å, respectively. Reductive N═N bond cleavage of azobenzene is also achieved in the presence of (tbsL)Fe3(thf) to yield triiron bis-imido complex (tbsL)Fe3(μ3-NPh)(μ2-NPh), which has been structurally characterized. Ligand redox participation has been ruled out, and therefore, charge balance indicates that the bis-imido cluster has undergone a 4e– metal based oxidation resulting in an (FeIV)(FeIII)2 formulation. Cyclic voltammograms of the series of triiron clusters presented herein demonstrate that oxidation states up to (FeIV)(FeIII)2 (in the case of [(tbsL)Fe3(μ3-N)]NBu4) are electrochemically accessible. These results highlight the efficacy of high-spin, polynuclear reaction sites to cooperatively mediate small molecule activation.
  • Thumbnail Image
    Publication
    Synthesis of Open-Shell, Bimetallic Mn/Fe Trinuclear Clusters
    (American Chemical Society (ACS), 2013) Powers, Tamara Michelle; Gu, Nina; Fout, Alison R.; Baldwin, Anne M.; Hernández Sánchez, Raúl; Alfonso, Denise Marie; Chen, Yu-Sheng; Zheng, Shao-Liang; Betley, Theodore
    Concomitant deprotonation and metalation of hexadentate ligand platform tbsLH6 (tbsLH6 = 1,3,5-C6H9(NHC6H4-o-NHSiMe2tBu)3) with divalent transition metal starting materials Fe2(Mes)4 (Mes = mesityl) or Mn3(Mes)6 in the presence of tetrahydrofuran (THF) resulted in isolation of homotrinuclear complexes (tbsL)Fe3(THF) and (tbsL)Mn3(THF), respectively. In the absence of coordinating solvent (THF), the deprotonation and metalation exclusively afforded dinuclear complexes of the type (tbsLH2)M2 (M = Fe or Mn). The resulting dinuclear species were utilized as synthons to prepare bimetallic trinuclear clusters. Treatment of (tbsLH2)Fe2 complex with divalent Mn source (Mn2(N(SiMe3)2)4) afforded the bimetallic complex (tbsL)Fe2Mn(THF), which established the ability of hexamine ligand tbsLH6 to support mixed metal clusters. The substitutional homogeneity of (tbsL)Fe2Mn(THF) was determined by 1H NMR, 57Fe Mössbauer, and X-ray fluorescence. Anomalous scattering measurements were critical for the unambiguous assignment of the trinuclear core composition. Heating a solution of (tbsLH2)Mn2 with a stoichiometric amount of Fe2(Mes)4 (0.5 mol equiv) affords a mixture of both (tbsL)Mn2Fe(THF) and (tbsL)Fe2Mn(THF) as a result of the thermodynamic preference for heavier metal substitution within the hexa-anilido ligand framework. These results demonstrate for the first time the assembly of mixed metal cluster synthesis in an unbiased ligand platform.
  • Thumbnail Image
    Publication
    Multi-Electron Reduction of Small Molecules by Triiron Reaction Sites
    (2013-10-15) Powers, Tamara Michelle; Betley, Theodore A; Holm, Richard; Gordon, Roy
    The observation that multi-electron activation of small molecule substrates occurs at polynuclear reaction sites, common to both metalloenzymes and heterogeneous catalysts, has led to the articulation of the polynuclear hypothesis - the idea that the expanded redox reservoir afforded by M-M interactions in polynuclear systems stabilizes multiple oxidation states and facilitates multi-electron transformations. Currently, examples of synthetic clusters that test the viability of polynuclear reaction sites towards effecting multi-electron activation of small molecule substrates are lacking. To test the polynuclear hypothesis, we targeted a system that embodies design elements common to metaloenzyme cofactors: polynuclear reaction sites that feature high-spin, coordinatively unsaturated metal centers. Metallation of tbsLH6 [tbsLH6 = 1,3,5-C6H9(NHC6H4-o-NHSiMe2tBu)3] yields high-spin trinuclear Fe(II) complex (tbsL)Fe3(THF). The filled anti-bonding orbitals in high-spin cluster (tbsL)Fe3(THF) renders ligand reorganization facile, which allows for a range of metal-substrate binding modes. The polynuclear site within the (tbsL)Fe3(THF) cluster cooperatively binds anionic donors and allows 2e- reduction of substrates including inorganic azide and hydrazines, yielding μ3-nitrido and μ3-imido products, respectively. The 4e- reductive N=N bond cleavage of azobenzene is also achieved in the presence of (tbsL)Fe3(THF) to yield Fe3 bis-imido complex ((tbsL)Fe3(μ3-NPh)(μ2-NPh), which has been structurally characterized. Cyclic voltammograms of a series of selected triiron imido and nitrido clusters suggest that oxidation states up to [Fe(IV)][Fe(III)]2 are electrochemically accessible. Addition of neutral pi-acidic molecules including tert-butylisonitrile (tBuNC) and carbon monoxode (CO) to trinuclear cluster (tbsL)Fe3(THF) led to the formation of a new series of coordination compounds, where binding to a single metal center is favored over cooperative substrate binding. Coordinated substrates are activated toward further reactivity, highlighted by the reductive coupling of isonitriles by (tbsL)Fe3(μ1-CNtBu)3 in the presence of phenylsilane. Finally, efforts to synthesize of a family of mixed Fe-Mn clusters that differ by single metal-site substitutions are presented. Substitutionally homogeneous (tbsL)Fe2Mn(THF) cluster is accessed from binuclear complex (tbsLH2 )Fe2. Attempts to synthesize similar Mn2Fe clusters results in isolation of a mixture of heterotrinuclear species. In conjunction with NMR, EPR, Mössbauer, and X-ray fluorescence spectroscopies, anomalous scattering measurements were critical for the unambiguous assignment of the metal substitution products that were synthesized.