Publications by authors named "Tanya Mikulas"

6 Publications

  • Page 1 of 1

Structures and Properties of the Products of the Reaction of Lanthanide Atoms with H2O: Dominance of the +II Oxidation State.

J Phys Chem A 2016 Feb 28;120(5):793-804. Epub 2016 Jan 28.

Department of Chemistry, The University of Alabama , Shelby Hall, Tuscaloosa, Alabama 35487-0336, United States.

The reactions of lanthanides with H2O have been studied using density functional theory with the B3LYP functional. H2O forms an initial Lewis acid-base complex with the lanthanides exothermically with interaction energies from -2 to -20 kcal/mol. For most of the Ln, formation of HLnOH is more exothermic than formation of H2LnO, HLnO + H, and LnOH + H. The reactions to produce HLnOH are exothermic from -25 to -75 kcal/mol. The formation of LnO + H2 for La and Ce is slightly more exothermic than formation of HLnOH and is less or equally exothermic for the rest of the lanthanides. The Ln in HLnOH and LnOH are in the formal +II and +I oxidation states, respectively. The Ln in H2LnO is mostly in the +III formal oxidation state with either Ln-O(-)/Ln-H(-) or Ln-(H2)(-)/Ln=O(2-) bonding interactions. A few of the H2LnO have the Ln in the +IV or mixed +III/+IV formal oxidation states with Ln=O(2-)/Ln-H(-) bonding interactions. The Ln in HLnO are generally in the +III oxidation state with the exception of Yb in the +II state. The orbital populations calculated within the natural bond orbital (NBO) analysis are consistent with the oxidation states and reaction energies. The more exothermic reactions to produce HLnOH are always associated with more backbonding from the O(H) and H characterized by more population in the 6s and 5d in Ln and the formation of a stronger Ln-O(H) bond. Overall, the calculations are consistent with the experiments in terms of reaction energies and vibrational frequencies.
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http://dx.doi.org/10.1021/acs.jpca.5b11215DOI Listing
February 2016

The Synthesis, Characterization and Dehydrogenation of Sigma-Complexes of BN-Cyclohexanes.

Chemistry 2016 Jan 25;22(1):310-22. Epub 2015 Nov 25.

Department of Chemistry, University of Oxford, Mansfield Road, Oxford, OX1 3TA (UK).

The coordination chemistry of the 1,2-BN-cyclohexanes 2,2-R2 -1,2-B,N-C4 H10 (R2 =HH, MeH, Me2 ) with Ir and Rh metal fragments has been studied. This led to the solution (NMR spectroscopy) and solid-state (X-ray diffraction) characterization of [Ir(PCy3 )2 (H)2 (η(2) η(2) -H2 BNR2 C4 H8 )][BAr(F) 4 ] (NR2 =NH2 , NMeH) and [Rh(iPr2 PCH2 CH2 CH2 PiPr2 )(η(2) η(2) -H2 BNR2 C4 H8 )][BAr(F) 4 ] (NR2 =NH2 , NMeH, NMe2 ). For NR2 =NH2 subsequent metal-promoted, dehydrocoupling shows the eventual formation of the cyclic tricyclic borazine [BNC4 H8 ]3 , via amino-borane and, tentatively characterized using DFT/GIAO chemical shift calculations, cycloborazane intermediates. For NR2 =NMeH the final product is the cyclic amino-borane HBNMeC4 H8 . The mechanism of dehydrogenation of 2,2-H,Me-1,2-B,N-C4 H10 using the {Rh(iPr2 PCH2 CH2 CH2 PiPr2 )}(+) catalyst has been probed. Catalytic experiments indicate the rapid formation of a dimeric species, [Rh2 (iPr2 PCH2 CH2 CH2 PiPr2 )2 H5 ][BAr(F) 4 ]. Using the initial rate method starting from this dimer, a first-order relationship to [amine-borane], but half-order to [Rh] is established, which is suggested to be due to a rapid dimer-monomer equilibrium operating.
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http://dx.doi.org/10.1002/chem.201502986DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4818983PMC
January 2016

Bis-BN cyclohexane: a remarkably kinetically stable chemical hydrogen storage material.

J Am Chem Soc 2015 Jan 23;137(1):134-7. Epub 2014 Dec 23.

Department of Chemistry, Boston College , Chestnut Hill, Massachusetts 02467-3860, United States.

A critical component for the successful development of fuel cell applications is hydrogen storage. For back-up power applications, where long storage periods under extreme temperatures are expected, the thermal stability of the storage material is particularly important. Here, we describe the development of an unusually kinetically stable chemical hydrogen storage material with a H2 storage capacity of 4.7 wt%. The compound, which is the first reported parental BN isostere of cyclohexane featuring two BN units, is thermally stable up to 150 °C both in solution and as a neat material. Yet, it can be activated to rapidly desorb H2 at room temperature in the presence of a catalyst without releasing other detectable volatile contaminants. We also disclose the isolation and characterization of two cage compounds with S4 symmetry from the H2 desorption reactions.
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http://dx.doi.org/10.1021/ja511766pDOI Listing
January 2015

Rhodium-catalyzed B-H activation of 1,2-azaborines: synthesis and characterization of BN isosteres of stilbenes.

Org Lett 2014 Jun 11;16(12):3340-3. Epub 2014 Jun 11.

Department of Chemistry, Boston College , Chestnut Hill, Massachusetts 02467, United States.

The first example of catalytic B-H activation of azaborines leading to a new family of stilbene derivatives through dehydrogenative borylation is reported. Ten 1,2-azaborine-based BN isosteres of stilbenes have been synthesized using this method, including a BN isostere of a biologically active stilbene. It is demonstrated that BN/CC isosterism in the context of stilbenes can lead to significant changes in the observed photophysical properties such as higher quantum yield and a larger Stokes shift. Direct comparative analysis of BN stilbene 3g and its carbonaceous counterpart 6g is consistent with a stronger charge-transfer character of the excited state exhibited by 3g in which the 1,2-azaborine heterocycle serves as a better electron donor than the corresponding arene.
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http://dx.doi.org/10.1021/ol501362wDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4068778PMC
June 2014

Reactions of lanthanide atoms with oxygen difluoride and the role of the Ln oxidation state.

Inorg Chem 2014 Jan 17;53(1):446-56. Epub 2013 Dec 17.

Department of Chemistry, The University of Alabama , Shelby Hall, Tuscaloosa, Alabama 35487-0336, United States.

Laser-ablated lanthanide metal atoms were condensed with OF2 in excess argon or neon at 4 K. New infrared absorption bands were observed and assigned to the oxidative addition products OLnF2 and OLnF on the basis of (18)O isotopic substitution and electronic structure calculations of the vibrational frequencies. The dominant absorptions in the 500 cm(-1) region are identified as Ln-F stretching modes, which follow the lanthanide contraction. The Ln-O stretching frequency is an important measure of the oxidation states of the Ln and oxygen and the spin state of the complex. The OCeF2, OPrF2, and OTbF2 molecules have higher frequency Ln-O stretching modes. The Ce is assigned to the IV oxidation state and the Pr and Tb are assigned to a mixed III/IV oxidation state. The remaining OLnF2 compounds have lower Ln-O stretches, and the Ln is in the III oxidation state and the O is in the -1 oxidation state. For all of the OLnF compounds, the metal is in the III oxidation state, and the Ln-F bonds are ionic. In OCeF2, OLaF, and OLuF, the bonding between the Ln and O is best described as a highly polarized σ bond and two pseudo π bonds formed by donation from the two 2p lone pairs on the O to the Ln. Bonding for the OLnF2 compounds in the III oxidation state is predicted to be fully ionic. The bonding in OLnF2 and OLnF is dominated by the oxidation state on the lanthanide and the spin state of the molecule. The observation of larger neon to argon matrix shifts for Ln-O modes in several OLnF molecules as compared to their OLnF2 analogues is indicative of more ionic character in the OLnF species, consistent with the more formal negative charge on the oxygen in OLnF.
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http://dx.doi.org/10.1021/ic402422hDOI Listing
January 2014

Boron-substituted 1,3-dihydro-1,3-azaborines: synthesis, structure, and evaluation of aromaticity.

Angew Chem Int Ed Engl 2013 Jul 7;52(29):7527-31. Epub 2013 Jun 7.

Department of Chemistry and Biochemistry, University of Oregon, Eugene, Oregon 97403-1253, USA.

Getting the family together: A general synthetic strategy based on nucleophilic substitution provided B-substituted 1,3-dihydro-1,3-azaborines (see scheme), BN isosteres of arenes with potential for application in biomedicine and materials science. Experimental structural analysis and calculations suggest that the aromaticity of the 1,3-dihydro-1,3-azaborine heterocycle is intermediate between that of benzene and that of 1,2-dihydro-1,2-azaborine.
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http://dx.doi.org/10.1002/anie.201302660DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3813462PMC
July 2013