A new tetrathiafulvalene-salphen uranyl complex has been prepared. The system was designed to study the electronic coupling between actinides and a redox active ligand framework. Theoretical and experimental methods - including DFT calculations, single crystal X-ray analysis, cyclic voltammetry, NMR and IR spectroscopies - were used to characterize this new uranyl complex.Office of Basic Energy Sciences, U. S. Department of Energy (DOE) DE-FG02-01ER15186Ministry of Education, Science and TechnologyHeavy Element Chemistry Program by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, U.S. Department of EnergyGlenn T. Seaborg InstituteNational Nuclear Security Administration of U.S. Department of Energy DE-AC52-06NA25396Chemistr

Barker, Beau J.

Batista, Enrique R.

Bejger, Christopher

Boland, Kevin S.

Kozimor, Stosh A.

Scott, Brian L.

Sessler, Jonathan L.

Tian, Yong-Hui

Dalton Transactions

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DaltonTransactionsCOMMUNICATIONCite this: Dalton Trans., 2013, 42, 6716Received 14th March 2013,Accepted 15th March 2013DOI: 10.1039/c3dt50698cwww.rsc.org/daltonSynthesis and characterization of a tetrathiafulvalene-salphen actinide complex†Christopher Bejger,a Yong-Hui Tian,b Beau J. Barker,b Kevin S. Boland,bBrian L. Scott,b Enrique R. Batista,*b Stosh A. Kozimor*b and Jonathan L. Sessler*a,cA new tetrathiafulvalene-salphen uranyl complex has been pre-pared. The system was designed to study the electronic couplingbetween actinides and a redox active ligand framework. Theoreti-cal and experimental methods – including DFT calculations, singlecrystal X-ray analysis, cyclic voltammetry, NMR and IR spectro-scopies – were used to characterize this new uranyl complex.A large number of complexes based on organic donor tetra-thiafulvalene (TTF) ligands and 3d transition metal centers areknown.1 Complementing studies of these systems are recentefforts to develop TTF-containing complexes of the lanthanidecations.2 To the best of our knowledge this work has not beenextended to the actinide series.Early actinides are typically characterized by more radiallyextended and accessible f orbitals compared to the lantha-nides. Accordingly, substitution of a lanthanide ion for an acti-nide in a redox active ligand framework offers an opportunityto enhance electronic communication between the ligand andthe coordinated metal.3 Here, we report a uranyl complex (1)based on a TTF-functionalized N,N′-phenylenebis(salicylidene-imine) analogue (i.e., TTFsalphenH2). Although complex 1 con-tains a diamagnetic ion, characterizing the interactionsbetween UO22+ and the TTF-ligand represents a crucial firststep towards subsequent experiments involving paramagneticactinyls. Complex 1 was analyzed using a combination oftheoretical, electrochemical, and spectroscopic methods.Taken in concert, these studies support the notion that com-munication between an actinyl cation and a redox activeligand can be achieved. They thus set the stage for moretechnically challenging studies involving related species suchas the PuO22+ cation.The present strategy for obtaining actinide complexes basedon TTF-functionalized ligands involves coordinating a uranylcation to an N,N′-phenylenebis(salicylideneimine) salphen-type ligand, specifically the known TTF-salphen dianion(TTFsalphen2−).4 The TTF part of the TTFsalphen2− dianion isredox active and capable of two reversible one-electron oxi-dations, while the salphen portion of the ligand provides atetradentate binding site extensively used in actinide sciencefor stabilizing multiple uranium oxidation states (U4+, UO21+,UO22+).3f,5 Complex 1 was thus deemed attractive as a platformfor examining electronic coupling effects involving actinideelements and redox active ligands.The synthesis of complex 1 is shown in Scheme 1. Briefly,reacting bis(propylthio)-TTF-benzo-o-diamine6 with salicyl-aldehyde and UO2(NO3)2·6H2O in ethanol provided complex 1as a red solid in 73% yield.‡ The 1H NMR spectrum of thisnew product was consistent with the uranyl center being com-plexed by the TTFsalphen2− ligand (see ESI†).4 Complex 1 (as itsmethanol complex) was unambiguously characterized in thesolid-state via an X-ray crystallographic analysis. Suitable crys-tals for the analysis were obtained by slow evaporation of asolution of complex 1 in a 1 : 1 mixture of CH2Cl2–MeOH(Fig. 1).†The ligand geometry around the uranium ion in 1·MeOHis best described as pentagonal bipyramidal. It resemblesprevious structures of TTF-unsubstituted complexes, i.e.Scheme 1 Synthesis of complex 1·MeOH.†Electronic supplementary information (ESI) available: Details of synthetic,experimental, CV studies, 1H-NMR studies, and X-ray crystallography. CCDC913827. For ESI and crystallographic data in CIF or other electronic format seeDOI: 10.1039/c3dt50698caDepartment of Chemistry and Biochemistry and Institute for Cellular and MolecularBiology, The University of Texas at Austin, 105 E. 24th Street – Stop A5300, Austin,Texas 78712-1224, USA. E-mail: Sessler@cm.utexas.edubLos Alamos National Laboratory, Los Alamos, New Mexico 87545, USA.E-mail: Stosh@lanl.gov, erb@lanl.govcDepartment of Chemistry, Yonsei University, Seoul 120-749, Korea6716 | Dalton Trans., 2013, 42, 6716–6719 This journal is © The Royal Society of Chemistry 2013Published on 09 April 2013. Downloaded by University of Texas Libraries on 18/08/2016 18:12:18. View Article OnlineView Journal  | View Issue(salphen)UO2(L) (L = neutral ligand) reported by Bandoli et al.and Ikeda et al.5 The uranyl U–O distances in 1·MeOH are1.780(2) and 1.776(2) Å, and consistent with previous uranylcomplexes.5 The axial uranyl U–O distances in 1·MeOH aremuch shorter than the average equatorial U–O(salphen) dis-tance (2.281 Å; σ = 0.056 Å). The equatorial U–N(imine) dis-tances in (TTFsalphen)UO2(HOMe) are 2.544(3) and 2.531(3) Å.In this regard, both equatorial U–O and U–N distances incomplex 1 are similar to unfunctionalized uranyl salphen com-plexes previously reported.5a,bThe extended structure of 1·MeOH is unique in comparisonto other uranyl salphen complexes. To the best of our knowl-edge, previous solid state structures of uranyl salphen com-plexes that contained a solvent molecule occupying the fifthsite in the uranyl equatorial plane were monomeric and devoidof any intermolecular interactions. In contrast, the extendedstructure of 1·MeOH is best described as a one dimensionalchain (Fig. 1). For example, the short 2.615(3) Å (HOMe)H⋯O(TTFsalphen)2− distance seen in the extended structure is con-sistent with hydrogen-bonding interactions between themethanol ligands and the salphen phenoxide subunits presentin adjacent 1·MeOH complexes. The TTF subunits of theTTFsalphen2− ligands were also seen to stack with eclipsedoverlapping π-faces (Fig. 1). The shortest observed distancebetween the ligand planes is 3.581(4) Å. However, due to thehead-to-tail arrangement of the individual ligands, no shortS⋯S intermolecular contacts were observed.Electrochemical analyses of 1·MeOH and controls werecarried out using cyclic voltammetry (CV). Fig. 2 compares theCV of 1·MeOH with a similar (salphen)UO2(DMSO) (in 10%DMSO/CH2Cl2) – which does not contain a redox activesalphen ligand – and the free-ligand, TTFsalphenH2 (inCH2Cl2). In the voltammogram of the free ligand, two waveswere observed with E1/2 values of 0.124 V and 0.501 V andwhose peak potential separations (Ep) are 0.115 V and 0.124 V,respectively. Hence, these waves were attributed to successiveand quasireversible TTF/TTF˙+ and TTF˙+/TTF2+ oxidations. Asexpected based on previous reports,5b the CV of (salphen)-UO2(DMSO) contained a single wave at −1.536 V (Ep = 0.080 V)attributable to the quasireversible UO22+/UO21+ one electronreduction. An additional weak uncoupled oxidation peak wasalso observed near −1.1 V that has previously been attributedto the dissociation of a coordinated DMSO ligand from theelectrochemical reduction product (salphen)UO2(DMSO).5bThe CV of 1·MeOH displayed four redox processes, Fig. 2.When scanned in the negative direction, a quasireversiblesingle electron redox process was observed at −1.496 V (Ep =0.092 V) that was attributed to the single electron UO22+/UO21+redox process. For comparison, these values were slightlycathodically shifted in comparison to those for (salphen)-UO2(DMSO), a finding that reflects the presence of a moreextended π-system in the case of TTFsalphen2− system. Asobserved for (salphen)UO2(DMSO), a weak uncoupled oxi-dation peak near −1.1 V was also observed, which likely arosefrom the dissociation of coordinated DMSO.5b When scannedin the positive direction, a quasireversible single-electron oxi-dation at 0.116 V (Ep = 0.85 V) and a second non-reversiblesingle-electron oxidation process at 0.377 V (Ep = 0.124 V) wereFig. 1 Top, A thermal ellipsoid plot of 1·MeOH with the thermal ellipsoidsdrawn at the 35% probability level. Bottom, A thermal ellipsoid plot showingthe extended structure of 1·MeOH (U, green; S, yellow; O, red; C, grey; and H,white). All hydrogen atoms positions were idealized.Fig. 2 Cyclic voltamagram of TTFsalphenH2 (top, CH2Cl2), (salphen)UO2(EtOH)(middle, DMSO (10%)/CH2Cl2), and 1·MeOH (bottom, DMSO (10%)/CH2Cl2).The measurements were performed using a glassy carbon working and a Ptcounter electrode at a scan rate of 50 mV s−1 with TBA·PF6 as the supportingelectrolyte. Potentials are referenced to (C5H5)2Fe/(C5H5)2Fe1+.Dalton Transactions CommunicationThis journal is © The Royal Society of Chemistry 2013 Dalton Trans., 2013, 42, 6716–6719 | 6717Published on 09 April 2013. Downloaded by University of Texas Libraries on 18/08/2016 18:12:18. View Article Onlineobserved. These waves, based on their similarity to the freeligand, were assigned to a quasireversible oxidation of the TTFmoiety in TTFsalphen2− to the corresponding radical cation fol-lowed by a second non-reversible oxidation to the dication.Overall, comparisons between 1·MeOH with TTFsalphenH2 and(salphen)UO2(DMSO), provide support for the notion thatcoordination of TTFsalphen2− to UO22+ subltly affects theUO22+/UO21+ and TTF/TTF˙+ redox potentionals while chan-ging the reversibility of the TTF˙+/TTF2+ couple.To understand better the electrochemical behavior of1·MeOH in comparison to simpler (salphen)UO2(DMSO) andTTFsalphenH2, DFT calculations were carried out on 1·MeOH,(salphen)UO2(DMSO), andTTFsalphenH2, as well as theircorresponding one- and two-electron oxidation and reductionproducts (cf. ESI†). These calculations are consistent with thenumerous theoretical studies that have been conducted onAnO22+, and the predicted structures are in good agreementwith the bond distances and angles for TTFsalphenH2,4H2salphen, (salphen)UO2(DMSO),4,5a,b and 1·MeOH.The ground state DFT calculations offer insight into thetrends observed experimentally, especially in regards to theTTF oxidation potentials for TTFsalphenH2 and 1·DMSO. Wenote that analogous comparisons cannot be made experimen-tally since a common solvent that provided reversible TTF/TTF˙+ redox processes for both compounds could not beidentified. The calculations indicate coordination of UO22+ toTTFsalphenH2 serves to shift cathodically the TTF/TTF˙+ oxi-dation potentials. A large 0.166 eV shift was calculated undervacuum, intermediate 0.063 eV shift was observed in CH2Cl2,and only a small change (0.006 eV) found in DMSO. While themagnitude of these changes were calculated to be highlydependent on the dielectric medium, the overall trend towardcathodically shifted oxidation potentials appears to result frommore appreciable delocalization of the radical cation in1·DMSO˙+ over the π-system of the salphen fragment as com-pared to the free TTFsalphenH2˙+ ligand. This conclusion wasreached through comparison of the natural orbitals corres-ponding that to the Kohn–Sham singly occupied molecularorbitals in Fig. 3.In summary, the first TTF-actinide complex, 1·MeOH, hasbeen synthesized and fully characterized. The accessibility ofthe organic, TTF-centered radical cation, as well as stablereduction to form the corresponding UO21+ complex, wasborne out via cyclic voltammetry analyses. These studiesprovide support for the conclusion that 1·MeOH exhibits redoxactivity mediated by both the organic ligand and inorganicactinyl unit. Computational studies also provide support forthe conclusion that the extended π-system of the (TTFsalphen)-UO2(DMSO) complex contributes to stabilization of both thepentavalent UO21+ cation – in comparison to (salphen)-UO2(DMSO) – as well as to stabilization of the radical cation,in comparison to the free TTFsalphenH2 ligand. We thuspropose that complexes, such as 1·MeOH, that contain a rigidredox-active organic TTF group linked to 5f metals warrantfurther study. They may allow for the creation of dual propertymaterials involving inter alia complexation and both redox-based modulation and electrochemical “sensing” of particularactinide cations. More broadly, redox-active actinyl complexes,of which the present system serves as a vanguard, couldprovide insights into the electronic and magnetic properties ofthe actinide cations.Work at UT Austin was supported by the Office of BasicEnergy Sciences, U.S. Department of Energy (DOE) (grant no.DE-FG02-01ER15186 to J.L.S.). J.L.S. also thanks the WCU(World Class University) program of Korea (R32-10217) admi-nistered by the National Research Foundation of Korea andfunded by the Ministry of Education, Science and Technology.Research at LANL was supported by the Heavy Element Chem-istry Program by the Division of Chemical Sciences, Geo-sciences, and Biosciences, Office of Basic Energy Sciences, U.S.Department of Energy and the Glenn T. Seaborg InstituteGraduate and Student Fellowships (CB). LANL is operated byLos Alamos National Security, LLC, for the National NuclearSecurity Administration of U.S. Department of Energy undercontract DE-AC52-06NA25396.Notes and references‡Selected data for 1: 1H NMR (pyridine-d5): δ 9.62 (s, NvCH, 2H), 7.84 (d, J = 8.6Hz, 2H), 7.72 (s, Ar-H, 2H), 7.64 (t, J = 7.5, 7.59 Hz, 2H), 7.29 (d, J = 8.3 Hz, 2H),6.79 (t, J = 7.3, 7.48 Hz, 2H), 2.82 (t, J = 7.1 Hz, –S–CH2CH2CH3, 4H), 1.61 (m, J =7.10, 7.47 Hz, –S–CH2CH2CH3, 4H), 0.92 (t, J = 7.4 Hz, –S–CH2CH2CH3, 6H)ppm; 13C NMR (pyridine-d5): δ 171.21, 166.62, 146.32, 137.08, 136.41, 128.25,124.96, 121.36, 118.12, 114.13, 112.16, 111.05, 38.33, 23.39, 13.10 ppm. Calc. (%)C 39.56; H, 3.21; N, 2.97; S, 20.45. Exp. (%) C, 39.18; H, 3.34; N, 2.96; S, 20.01.1 (a) D. Lorcy, N. Bellec, M. Fourmigue and N. Avarvari, Coord.Chem. Rev., 2009, 35, 1398; (b) M. Shatruk and L. Ray, DaltonTrans., 2010, 39, 11105; (c) F. Pointillart, S. Golhen,O. Candor and L. Ouahab, Dalton Trans., 2013, 42, 149.Fig. 3 The natural orbitals corresponding to the SOMOs for TTFsalphenH2˙+(top) and 1·MeOH˙+ (bottom); electron density cutoff = 0.02 (a.u.).Communication Dalton Transactions6718 | Dalton Trans., 2013, 42, 6716–6719 This journal is © The Royal Society of Chemistry 2013Published on 09 April 2013. Downloaded by University of Texas Libraries on 18/08/2016 18:12:18. View Article Online2 (a) S. Faulkner, B. P. Burton-Pye, T. Khan, L. R. Martin,S. D. Wray and P. J. Skabara, Chem. Commun., 2002, 1668;(b) S. J. A. Pope, B. P. Burton-Dye, R. Berridge, T. Khan,P. J. Skabara and S. Faulkner, Dalton Trans., 2006, 2907;(c) G. Cosquer, F. Pointillart, Y. Le Gal, S. Golhen, O. Cadorand L. Ouahab, Chem.–Eur. J., 2011, 17, 12502;(d) F. Pointillart, Y. Le Gal, S. Golhen, O. Cador andL. Ouahab, Chem.–Eur. J., 2011, 17, 10397.3 For examples of actinides bound to redox-active ligands see:(a) T. Arliguie, M. Fourmiguè and M. Ephritikhine, Organo-metallics, 2000, 19, 109; (b) M. J. Monreal andP. L. Diaconescu, J. Am. Chem. Soc., 2010, 132, 7676;(c) P. L. Diaconescu, P. L. Arnold, T. A. Baker, D. J. Mindiolaand C. C. Cummins, J. Am. Chem. Soc., 2000, 122, 6108;(d) S. J. Kraft, P. E. Fanwick and S. C. Bart, Inorg. Chem.,2010, 49, 1103; (e) E. J. Schelter, R. L. Wu, B. L. Scott,J. D. Thompson, T. Cantat, K. D. John, E. R. Batista,D. E. Morris and J. L. Kiplinger, Inorg. Chem., 2010, 49, 924;(f ) C. Camp, V. Mougel, P. Horeglad, J. Pécaut andM. Mazzanti, J. Am. Chem. Soc., 2010, 132, 17374.4 C. Jia, S. X. Liu, T. D. Keene, A. Neels, V. Mereacre,A. K. Powell and S. Decurtins, Inorg. Chem., 2008, 48, 3452.5 (a) G. Bandoli, D. A. Clemente, U. Croatto, M. Vidali andP. A. Vigato, J. Chem. Soc. D, 1971, 1330; (b) K. Mizuoka,S.-Y. Kim, M. Hasegawa, T. Hoshi, G. Uchiyama andY. Ikeda, Inorg. Chem., 2003, 42, 1031; (c) K. Takao andY. Ikeda, Inorg. Chem., 2007, 46, 1550.6 C. Jia, S. X. Liu, C. Tanner, C. Leiggener, A. Neels,L. Sanguinet, E. Levillian, S. Leutwyler and S. Decurtins,Chem.–Eur. J., 2007, 13, 3804.Dalton Transactions CommunicationThis journal is © The Royal Society of Chemistry 2013 Dalton Trans., 2013, 42, 6716–6719 | 6719Published on 09 April 2013. Downloaded by University of Texas Libraries on 18/08/2016 18:12:18. View Article Online

Synthesis and Characterization of A Tetrathiafulvalene-Salphen Actinide Complex

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