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Physicochemical Properties of Near-Linear Lanthanide(II) Bis(silylamide) Complexes (Ln = Sm, Eu, Tm, Yb) Conrad A. P. Goodwin,† Nicholas F. Chilton,† Gianni F. Vettese,† Eufemio Moreno Pineda,† Iain F. Crowe,‡ Joseph W. Ziller,§ Richard E. P. Winpenny,† William J. Evans,§ and David P. Mills*,† †
School of Chemistry, The University of Manchester, Oxford Road, Manchester M13 9PL, U.K. Photon Science Institute and School of Electrical and Electronic Engineering, The University of Manchester, Oxford Road, Manchester M13 9PL, U.K. § Department of Chemistry, University of California, Irvine, California 92697-2025, United States ‡
S Supporting Information *
ABSTRACT: Following our report of the first near-linear lanthanide (Ln) complex, [Sm(N††)2] (1), herein we present the synthesis of [Ln(N††)2] [N†† = {N(SiiPr3)2}; Ln = Eu (2), Tm (3), Yb (4)], thus achieving approximate uniaxial geometries for a series of “traditional” LnII ions. Experimental evidence, together with calculations performed on a model of 4, indicates that dispersion forces are important for stabilization of the nearlinear geometries of 1−4. The isolation of 3 under a dinitrogen atmosphere is noteworthy, given that “[Tm(N″)(μ-N″)]2” (N″ = {N(SiMe3)2}) has not previously been structurally authenticated and reacts rapidly with N2(g) to give [{Tm(N″)2}2(μ-η2:η2-N2)]. Complexes 1−4 have been characterized as appropriate by singlecrystal X-ray diffraction, magnetic measurements, electrochemistry, multinuclear NMR, electron paramagnetic resonance (EPR), and electronic spectroscopy, along with computational methods for 3 and 4. The remarkable geometries of monomeric 1−4 lead to interesting physical properties, which complement and contrast with comparatively well understood dimeric [Ln(N″)(μ-N″)]2 complexes. EPR spectroscopy of 3 shows that the near-linear geometry stabilizes mJ states with oblate spheroid electron density distributions, validating our previous suggestions. Cyclic voltammetry experiments carried out on 1−4 did not yield LnII reduction potentials, so a reactivity study of 1 was performed with selected substrates in order to benchmark the SmIII → SmII couple. The separate reactions of 1 with 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO), azobenzene, and benzophenone gave crystals of [Sm(N††)2(TEMPO)] (5), [Sm(N††)2(N2Ph2)] (6), and [Sm(N††){μ-OPhC(C6H5)CPh2O-κO,O′}]2 (7), respectively. The isolation of 5−7 shows that the SmII center in 1 is still accessible despite having two bulky N†† moieties and that the N-donor atoms are able to deviate further from linearity or ligand scrambling occurs in order to accommodate another ligand in the SmIII coordination spheres of the products.
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approximately uniaxial Ln ligand fields preceded the isolation of 1.3,7 A linear arrangement of ligands should optimize the singlemolecule-magnet (SMM) properties for LnII 4f8 or 4f9 ions (e.g., TbII) by stabilizing the largest mJ state, while the opposite effect should be observed for ions such as TmII.3f LnII chemistry is currently undergoing a renaissance, with the 2+ oxidation state now known for all of the Ln save radioactive Pm.9 Exciting trends are now being discovered in the electronic structures of novel LnII systems,10 and it follows that expanding this area with new ligands and bonding motifs is essential to open up new vistas for future exploration. The molecular chemistry of the “traditional” LnII ions (SmII, EuII, and YbII) has been dominated by substituted Cp ligands,11 but complexes
INTRODUCTION
Lanthanide (Ln) complexes are characterized by their highly flexible ligand environments, with absolute geometric control only achieved by meticulous ligand design and selection.1 Welldefined Ln coordination spheres are vitally important for the optimization of desired optical2 and magnetic properties3 and catalytic processes.4 The use of sterically demanding ligands to engender unusual Ln geometries has been constant since the trigonal-pyramidal LnIII complexes, [Ln(N″)3] [N″ = {N(SiMe3)2}], were reported in seminal work by Bradley and coworkers in the 1960s.5 Of most relevance here, the LnII complexes [Ln{C(SiMe3)3}2] (Ln = Sm, Eu, Yb)6 provided landmark examples of bent geometries, and some of us recently disclosed the first near-linear f-element complex, [Sm(N††)2] (1; N†† = {N(SiiPr3)2}),7 as part of our investigations into early-metal bulky silylamide chemistry.8 Predictions of some of the potentially remarkable properties of Ln complexes with © XXXX American Chemical Society
Special Issue: New Trends and Applications for Lanthanides Received: March 31, 2016
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DOI: 10.1021/acs.inorgchem.6b00808 Inorg. Chem. XXXX, XXX, XXX−XXX
Forum Article
Inorganic Chemistry
[Tm(N††)2] (3). A scintillation vial was charged with TmI2 (0.312 g, 0.74 mmol) and benzene (10 mL). A solution of [K(N††)] (0.489 g, 1.33 mmol) in benzene (3 mL) was added rapidly, followed by 5 drops of THF. After 16 h, the volatiles of the dark-purple reaction mixture were removed in vacuo, and the product was extracted with hexane (5 mL). Storage at −35 °C for 1 h gave 3 as dark-purple crystals (0.369 g, 67%). Anal. Calcd for C36H84N2Si4Tm: C, 52.33; H, 10.25; N, 3.39. Found: C, 51.66; H, 10.67; N, 3.15. 1H NMR (C6D6, 500 MHz): δ 86.99 (br, ν1/2 ≈ 2630 Hz, 12 H, CH(CH3)2), 8.43 (s, ν1/2 ≈ 280 Hz, 72 H, CH(CH3)2). 29Si{1H} NMR (C6D6, 99 MHz): δ −167.04 (s). FTIR (KBr pellet, cm−1) v′ 2940 (s), 2864 (s), 2722 (m), 2604 (w), 1735 (w), 1461 (m), 1388 (w), 1365 (w), 1364 (w), 1287 (v.w), 1219 (m), 1214 (m), 1043 (m), 994 (m), 942 (m), 881 (m), 695 (m), 653 (m). [Yb(N††)2] (4). A scintillation vial was charged with [YbI2(THF)2] (0.422 g, 0.74 mmol) and toluene (10 mL). A solution of [K(N††)] (0.489 g, 1.33 mmol) in toluene (3 mL) was added rapidly. After 16 h, the volatiles of the orange reaction mixture were removed in vacuo, and the product was extracted with hexane (5 mL). Storage at −35 °C for 1 h gave 4 as orange-red crystals (0.371 g, 67%). Anal. Calcd for C36H84N2Si4Yb: C, 52.07; H, 10.19; N, 3.37. Found: C, 50.95; H, 10.54; N, 3.17. 1H NMR (C6D6, 500 MHz): δ 1.27 (d, 3JHH = 7.54 Hz, 48 H, CH(CH3)2), 1.17 (m, 27 H, CH(CH3)2 and CH(CH3)2), 0.95, (m, 9 H, CH(CH3)2). 13C{1H} NMR (C6D6, 100 MHz): δ 20.51, 19.36, 18.50, 14.04. 29Si{1H} NMR (C6D6, 80 MHz): δ −8.81 (s). 171 Yb{1H} NMR (C6D6, 87.5 MHz): 36.52 (s). FTIR (KBr pellet, cm−1) v′ 2941 (s), 2861 (s), 2721 (m), 2609 (w), 1741 (vw), 1733 (vw), 1463 (s), 1388 (m), 1374 (m), 1364 (m), 1291 (w), 1234 (m), 1210 (w), 1159 (vw), 1065 (m), 1043 (s), 1000 (m), 943 (m), 915 (m), 882 (s), 694 (s), 654 (s). [Sm(N†† )2 (TEMPO)] (5). 2,2,6,6-Tetramethylpiperidine-1-oxyl (TEMPO; 0.156 g, 1.00 mmol) in hexane (10 mL) was added dropwise to a precooled solution (−78 °C) of 1 (0.808 g, 1.00 mmol) in hexane (10 mL). The reaction mixture immediately turned dark red and was allowed to warm to room temperature. Concentration to approximately 10 mL, filtration, and storage at −25 °C gave 5 as red crystals. Multiple crops were obtained (0.432 g, 45%). Anal. Calcd for C27H102N3OSi4Sm: C, 56.07; H, 10.66; N, 4.36. Found: C, 55.60; H, 10.48; N, 4.32. μeff = 1.98 μB (Evans method). 1H NMR (C6D6, 400 MHz): δ −5.21 (br, ν1/2 = 132 Hz, 12 H, CH(CH3)2), −2.68 (br, ν1/2 = 15 Hz, 2 H, p-CH2), 0.36 (s, ν1/2 = 44 Hz, 72 H, CH(CH3)2), 2.97 (s, 12 H, C5H6NMe4), 3.36 (t, 3JHH = 5.72 Hz, 4 H, m-CH2). 13C{1H} NMR (C6D6, 100 MHz): δ 14.04, 17.29, 19.35, 19.77, 20.02, 30.08, 44.62. FTIR (Nujol, cm−1): ν 2367 (w), 1302 (w), 1260 (m), 1242 (w), 1206 (w), 1153 (w), 1130 (w), 1078 (w), 1063 (w), 1011 (m), 991 (w), 939 (s), 878 (s), 795 (m), 694 (s), 658 (m), 629 (s). [Sm(N††)2(N2Ph2)] (6). Ph2N2 (0.091 g, 0.50 mmol) in toluene (5 mL) was added dropwise to a precooled (−78 °C) solution of 1 (0.808 g, 1.00 mmol) in toluene (5 mL). The dark-blue reaction mixture was allowed to warm to room temperature and was stirred for 14 h. Volatiles were removed in vacuo, and the product was extracted into hot hexane (8 mL). Storage at room temperature gave 6 as blue/ purple dichroic crystals. Multiple crops were obtained (0.301 g, 61% with respect to Ph2N2). Anal. Calcd for C48H94N4Si4Sm: C, 58.23; H, 9.57; N, 5.66. Found: C, 57.92; H, 9.59; N, 5.50. μeff = 2.45 μB (Evans method). 1H NMR (C6D6, 400 MHz): δ 74.20 (br s, 4 H, ν1/2 = 80 Hz, NPh-m-CH), 1.37 (br s, 84 H, ν1/2 = 29 Hz, CH(CH3)2), −4.48 (br s, 12 H, ν1/2 = 76 Hz, CH(CH3)2), −148.90 (br s, 4 H, ν1/2 = 225 Hz, NPh-o-CH), −155.51 (br s, 2 H, ν1/2 = 375 Hz, NPh-p-CH). 13 C{1H} NMR (C6D6, 100 MHz): δ 24.90 (CH(CH3)2), 19.34 (CH(CH3)2). 29Si{1H} NMR (C6D6, 80 MHz): δ −6.56 (s). FTIR (Nujol, cm−1) v′ 2359 (s), 2342 (s), 1261 (w), 1244 (w), 1213 (w), 1163 (w), 1153 (w), 1099 (w), 1061 (w), 1011 (w), 993 (w), 934 (m), 885 (w), 876 (w), 847 (w), 791 (w), 772 (w), 754 (w), 694 (m), 667 (w), 658 (w), 627 (w). [Sm(N††){μ-OPhC(C6H5)CPh2O-κO,O′}]2 (7). A solution of Ph2CO (0.182 g, 1.0 mmol) in toluene (10 mL) was added dropwise to a solution of 1 (0.808 g, 1.0 mmol) in toluene (5 mL). The purple solution was allowed to stir for 4 h. Removal of volatiles in vacuo gave a red solid, which was extracted with 10 mL of hot hexane. Storage of
featuring N-pincer ligands,12 hydrotris(pyrazolyl)borates (Tp),13 and monophospholyls14 have also been widely studied. Most structurally characterized LnIIL2 complexes reported to date exhibit L−M−L bending angles of
