Synthesis and Electronic Structures of Heavy Lanthanide

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Synthesis and Electronic Structures of Heavy Lanthanide Metallocenium Cations Conrad A. P. Goodwin, Daniel Reta, Fabrizio Ortu, Nicholas F. Chilton, and David P. Mills J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b11535 • Publication Date (Web): 28 Nov 2017 Downloaded from http://pubs.acs.org on November 29, 2017

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Journal of the American Chemical Society

Synthesis and Electronic Structures of Heavy Lanthanide Metallocenium Cations Conrad A. P. Goodwin, Daniel Reta, Fabrizio Ortu, Nicholas F. Chilton,* and David P. Mills* School of Chemistry, The University of Manchester, Oxford Road, Manchester, M13 9PL, UK

Abstract Since the origin of 60 K magnetic hysteresis in the dysprosocenium complex [Dy(Cpttt)2][B(C6F5)4] (Cpttt = C5H2tBu3-1,2,4, 1-Dy) remains mysterious, we envisaged that analysis of a series of [Ln(Cpttt)2]+ (Ln = lanthanide) cations could shed light on these properties. Herein we report the synthesis and physical characterization of a family of isolated [Ln(Cpttt)2]+ cations (1-Ln; Ln = Gd, Ho, Er, Tm, Yb, Lu), synthesized by halide abstraction of [Ln(Cpttt)2(Cl)], (2-Ln; Ln = Gd, Ho, Er, Tm, Yb, Lu). Complexes within the two families 1-Ln and 2-Ln are isostructural, and display pseudo-linear and pseudo-trigonal crystal fields, respectively. This results in archetypal electronic structures, determined with CASSCF-SO calculations and confirmed with SQUID magnetometry and EPR spectroscopy, showing easy-axis or easy-plane magnetic anisotropy depending on the choice of Ln ion. Study of their magnetic relaxation dynamics reveal that 1-Ho also exhibits an anomalously low Raman exponent similar to 1-Dy, both being distinct from the larger and more regular Raman exponents for 2-Dy, 2-Er and 2-Yb. This suggests that low Raman exponents arise from the unique spin-phonon coupling of isolated [Ln(Cpttt)2]+ cations. Crucially, this highlights a direct connection between ligand coordination modes and spin-phonon coupling, and therefore we propose that the exclusive presence of multi-hapto ligands in 1-Dy is the origin of its remarkable magnetic properties. Controlling the spin-phonon coupling through ligand design thus appears vital for realizing the nextgeneration of high-temperature single-molecule magnets.

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Introduction Metallocenes [M(CpR)2] (CpR = substituted cyclopentadienyl, C5R5) are one of the most celebrated families of compounds in organometallic chemistry.1 The bis-η5-CpR motif imparts unique physicochemical properties on metal complexes, as exemplified by ferrocene, [Fe(Cp)2], which has opened up countless new research fields since its discovery over 65 years ago and has found widespread applications in fine chemical synthesis, electrochemistry, homogenous catalysis and polymeric materials.2 The subsequent preparation of an isostructural series of 3d metallocenes [M(Cp)2] (M = V, Cr, Mn, Fe, Co, Ni) with various dn (n = 3-8) configurations provided landmark opportunities for the physical properties and electronic structures of these iconic compounds to be probed and rationalized.1 Following initial studies of the d-block, parallel (D5d) and nonparallel (C2v, bent) neutral metallocenes were synthesized for the s-, p- and f-block elements.1 Lanthanide (Ln) metallocenium cations [Ln(CpR)2]+ were proposed by Birmingham and Wilkinson in 19563 and many solvated derivatives have been reported, but complexes with no equatorial interactions were elusive until the dysprosocenium complex, [Dy(Cpttt)2][B(C6F5)4] (Cpttt = C5H2tBu3-1,2,4, 1-Dy) was reported in 2017.4 Complex 1-Dy is a single-molecule magnet (SMM) that exhibits magnetic hysteresis up to 60 K along with a crossover from conventional Orbach magnetic relaxation to an anomalous Raman-type process at this temperature; this unprecedented behavior is not yet fully understood.4 The first monometallic Ln SMM, a TbIII sandwich complex [N(nBu)4][Tb(phthalocyanine)2], was reported in 2003,5 and since this discovery controlling the electronic structure of Ln SMMs has been a primary goal. For sandwich-type Ln SMMs, initial studies of homo- and heteroleptic bis-COTR (COTR = substituted cyclooctatetraenyl) complexes showed that COTR ligands present an equatorial crystal field (CF) potential, yielding more favorable electronic structures and SMM properties for ErIII over DyIII,6-10 and more recently, point-like charges on a single axis have been shown to generate a strong axial CF yielding excellent SMM characteristics for DyIII.11-15 It is intuitive that shrinking the size of the organic sandwich ligand from C8 COTR down to C5 CpR and approaching a more point-like axial charge

distribution

would

result

in

large

magnetic

anisotropy

for

1-Dy.

However,

[Dy(tBuO)2(pyridine)5][B(Ph)4]14 has approximately the same electronic structure as 1-Dy, but only 2 ACS Paragon Plus Environment

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shows magnetic hysteresis up to 4 K; therefore the electronic structure is clearly not the only consideration and the unique spin-phonon coupling in 1-Dy must be intrinsic to its remarkable properties. We envisaged that the characterization of a series of [Ln(Cpttt)2]+ cations could provide valuable insights into the intriguing magnetic properties of 1-Dy, as well as providing a systematic study of a 4f-metallocenium family. For the lanthanides, bulky CpR ring R-substituents are essential to thwart decomposition, occlusion, aggregation and solvation of [Ln(CpR)2] complexes,16,17 but these have been limited to Ln with the most readily accessible +2 oxidation states (Sm, Eu, Tm, Yb).18-28 Aside from the isolation of 1-Dy from [Dy(Cpttt)2(Cl)] (2-Dy), significant milestones towards these ambitious

targets

have

included

[Sm{C5Me4(SiMe2CH2CH=CH2)}2][BPh4],29

[Sc(Cp*)2{µ-

(C6F5)2B(C6F5)2}] (Cp* = C5Me5),30 [{Ln(Cp*)2}2{µ-(C6F5)2B(C6F5)2}2] (Ln = Pr, Nd),31 [Ln(Cp*)2{µ-(Ph)2BPh2}] (Ln = Sm,32 Nd,32 Gd,33 Dy,34,35 Tb,34,35 Ho,36 Er,36 Tm,32 Y33) and [Ln(Cp*)2(NH3)(L){µ-(Ph)2BPh2}] (L = NH3 or THF).37 Herein, we report the synthesis of [Ln(Cpttt)2][B(C6F5)4] (1-Ln; Ln = Gd, Ho, Er, Tm, Yb, Lu) by halide abstraction of the parent [Ln(Cpttt)2(Cl)] (2-Ln) with [H(SiEt3)2][B(C6F5)4],38 showing that this approach is transferable to a range of heavy Ln. We analyze the properties of 1-Ln and 2-Ln by a range of physical and computational techniques to define the characteristic features of a structurally analogous series of [Ln(Cpttt)2]+ cations.

Results and discussion Synthesis and NMR spectroscopy Following previous methodologies for the syntheses of 1-Dy,4 2-Dy,4 [Y(Cpttt)2][B(C6F5)4] (1Y),4a and [Y(Cpttt)2(Cl)] (2-Y),4a the Ln metallocenium complexes [Ln(Cpttt)2][B(C6F5)4] (1-Ln; Ln = Gd, Ho, Er, Tm, Yb, Lu) were prepared by halide abstraction of [Ln(Cpttt)2(Cl)] (2-Ln; Ln = Gd, Ho, Er, Tm, Yb, Lu) with [H(SiEt3)2][B(C6F5)4]38 in benzene under an inert atmosphere (Ar) (Scheme 1). [H(SiEt3)2][B(C6F5)4] was selected as a reagent for Ln metallocenium cation formation as: i) its solubility in benzene allows these reactions to be performed in the absence of coordinating solvents; and, ii) these reactions are thermodynamically driven by both the large enthalpy of Si–Cl bond 3 ACS Paragon Plus Environment


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formation and the positive entropy associated with the formation of a molecule of HSiEt3.38 Complexes 1-Ln were all crystallized from dichloromethane (DCM) layered with hexane, and became more temperature-sensitive as soon as DCM was added, thus these manipulations were performed in ice baths. The facile decomposition of crystalline 1-Ln at room temperature (~15 mins) necessitates storage at –25 °C; they are stable for > 3 months in these conditions. Elemental analysis results obtained for 1-Ln and 2-Ln were generally in excellent agreement with expected values, though low carbon values, likely due to carbide formation from incomplete combustion, were consistently obtained for some complexes. Microanalysis of air-sensitive organometallics is known to be problematic;39 the temperature-sensitivity of 1-Ln is also likely a contributing factor, but as these outliers are part of a continuous series and the other analytical data obtained are consistent (see below) we are confident of the bulk purity of all 1-Ln. The precursors 2-Ln were synthesized directly from the parent LnCl3 and two equivalents of KCpttt by analogous methods to those reported in the literature for 2-Y,4 2-Dy4 and [Dy(Cpttt)2(Br)],40 except for 2-Yb, which was synthesized by the oxidation of [Yb(Cpttt)2]21 (3) with tBuCl (Scheme 1). Several crystals of [Yb(Cpttt)(THF)(µ-I)]2 (4) were obtained during the synthesis of 3 from YbI2(THF)2 and NaCpttt, as identified by single crystal X-ray diffraction (XRD, See Supplementary Information). The reaction of YbCl3 with 2 eq. KCpttt in refluxing THF gave a bright purple reaction mixture, which turned brown following reflux in toluene. This brown color is characteristic of the YbII complex 3, therefore we postulate that reduction of YbIII to YbII occurred, but no products could be isolated to confirm this hypothesis. It is noteworthy that whilst the reactions of LnCl3 (Ln = Pr, Nd) with two equivalents of NaCpttt to give [Ln(Cpttt)2(Cl)] (Ln = Pr, Nd) proceed at room temperature,41 the greater Lewis acidity of the smaller late LnIII ions has previously made the installation of two Cpttt ligands problematic. For example, the reaction of YbCl3 with two equivalents of NaCpttt in DME was found to give the alkoxide [Yb(Cpttt)(Cl)(µ-OCH2CH2OMe)]2 by solvent cleavage,41 whilst no products could be isolated from the reaction of TmCl3 with Na/KCpttt in refluxing toluene.42 Interestingly, despite multiple attempts, we were unable to synthesize 2-Tb from TbCl3 and MCpttt (M = Li, Na, K) using identical reaction conditions to those used for the rest of the series. Oily residues containing CptttH were consistently obtained from these reaction mixtures, even though numerous 4 ACS Paragon Plus Environment

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batches of TbCl3 from various suppliers were investigated; thus 2-Tb and 1-Tb remain elusive following these procedures. Wide variations in crystalline yields (18–63 %) were observed for 2-Ln, depending upon the identity of Ln. Although these yields do not follow any obvious trend, such variations for structurally analogous series of f-element complexes have previously been ascribed to differing metal:ligand size ratios.43

Scheme 1. Synthesis of the Ln complexes 1-Ln and 2-Ln; the syntheses of 1-Dy and 2-Dy have been previously reported in reference 4, and [Yb(Cpttt)2] (3) in reference 21.

1

H NMR spectra were recorded from –550 to +400 ppm for 1-Ln and 2-Ln (Table 1).

Significant paramagnetic broadening of the NMR signals are a characteristic feature of strongly anisotropic magnetic ions; the Cpttt ring protons were not observed for any paramagnetic species apart from 2-Dy4 and only diamagnetic impurities were observed in the 1H NMR spectra of 1-Gd, 2-Gd, 1Dy,4 2-Er and 2-Tm. This phenomenon also results in broad signals for tBu group protons in 2-Dy,4 1-Ho, 1-Er, 1-Tm, 1-Yb, 2-Ho and 2-Yb. Three peaks were observed in the 1H NMR spectra of diamagnetic 1-Lu and 2-Lu in a 18:36:4 ratio; these correspond to two sets of tBu groups and the Cpttt ring protons, respectively. Paramagnetism also precludes the assignment of all 13C{1H} NMR spectra 5 ACS Paragon Plus Environment


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except for diamagnetic 1-Lu and 2-Lu; e.g. for 1-Lu the four tBu group resonances (two quaternary, two methyl) were seen between 32.46–35.73 ppm and the Cpttt ring carbons were located at 116.96 (C–H), 141.07 (C–tBu) and 146.81 (C–tBu) ppm. The 1H and 13C{1H} NMR spectra of 1-Lu and 2-Lu are similar to those previously reported for 1-Y and 2-Y.4a

Complex

δ 1H / ppm {C5H2(tBu)3}– (FWHM / Hz)

δ 1H / ppm {C5H2(tBu)3}–

Complex

δ 1H / ppm {C5H2(tBu)3}– (FWHM / Hz)

δ 1H / ppm {C5H2(tBu)3}–

1-Gd

–

–

2-Gd

–

–

1-Dy4

–

–

2-Dy4

1-Ho

186.09 (~2,200)

–

2-Ho

1-Er

65.06 (~250)

–

2-Er

–

–

1-Tm

74.03 (~160)

–

2-Tm

–

–

–

2-Yb

92.31 (~4,000)

–

6.42, 2H

2-Lu

1.21, 18H; 1.53, 36 H

6.70, 2H

1-Yb 1-Lu

20.51, (~280), 36H; 25.45, (~20), 18H 1.39, 18H; 1.51, 36H

–44.0, (~12,000), 18H; 13.26, (~600), 36H –205.45, (~1,400), 18H; –58.44 (~1,850), 36H

7.29, (~ 300), 4H –

Table 1. 1H NMR spectral chemical shifts of 1-Ln and 2-Ln in CD2Cl2.

No NMR signals were observed for the [B(C6F5)4]– anion in the 13C NMR spectrum of 1-Ln due to extensive coupling with

19

F, but these anions were assigned by

11

B{1H} and

19

F{1H} NMR

spectroscopy in all cases (see Supplementary Information). It is noteworthy that the paramagnetism of the [Ln(Cpttt)2]+ cations in 1-Ln instigate broad and paramagnetically shifted signals in the 11B and 19F NMR spectra of the [B(C6F5)4]– anions, with wide variations in chemical shifts [e.g. δB range from – 12.12 (1-Er) to –29.04 (1-Dy)]; the extent of paramagnetic shifts in these spectra can be emphasized by comparison with diamagnetic 1-Lu (δB = –16.64). In the case of 1-Gd the meta-19F resonance was broadened to such an extent that it could not be unequivocally assigned. Solution phase magnetic


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moments were quantified by the Evans method (Table S7),44 and are in good agreement with solidstate measurements and theoretical values (see below).

Solid-state structural characterization The solid state structures of 1-Ln, 2-Ln, 3 and 4 were determined by single crystal XRD (1Gd•CH2Cl2 is depicted in Figure 1 and 2-Gd in Figure 2; selected bond distances and angles of 1-Ln compiled in Table 2). Whilst DCM is the predominant lattice solvent found in crystals of 1-Ln, crystalline modifications with variable lattice solvation have also been obtained (see Supplementary Information). The [Ln(Cpttt)2]+ cations in 1-Ln all exhibit remarkably consistent geometrical features to 1-Dy,4 despite the significant variation in LnIII size across the series (6-coordinate ionic radii for GdIII and LuIII are 0.938 and 0.861 Å, respectively),45 thus they are discussed together for brevity. The cations adopt bent metallocene arrangements in all cases, as is typical of most LnII metallocenes,18-28 and a small range of Cpcentroid···Ln···Cpcentroid angles are observed from 150.2(2)° for 1-Gd•CH2Cl2 to 155.11(6)° for 1-Tm•C6H14. These values are closer to linearity than the precursors 2-Ln [range Cpcentroid···Ln···Cpcentroid = 146.07(10)° (2-Gd) – 147.36(11)° (2-Lu)] but are more bent than the less Lewis acidic LnII analogues, e.g. for 3, Cpcentroid···Yb···Cpcentroid = 164.54(5)°. Smaller Cpcentroid···Ln···Cpcentroid angles were typically seen for the larger LnIII ions for both series, but the analysis of several polymorphs indicated that lattice solvent polarity and crystal packing effects are more significant factors than ionic radius; e.g. Cpcentroid···Ln···Cpcentroid = 152.00(7)° for 1-Gd•C6H6 and 152.89(14)° 1-Lu•C6H14. Each Ln center in 1-Ln (Ln = Gd, Dy,4 Ho, Er, Tm, Yb) is coordinated by two nearly eclipsed η5-Cpttt rings, with the mean Ln···Cpcentroid distances decreasing regularly with LnIII ionic radii [2.355(7) Å for 1-Gd•CH2Cl2 and 2.272(7) Å for 1-Yb•C6H14]; 1-Lu•C6H14 is anomalous as its η5Cpttt rings are not eclipsed [mean Lu···Cpcentroid = 2.246(4) Å]. The eclipsed conformations are unusual and may result from a favorable packing arrangement of the 18 Me groups. We consistently observe two equatorial electrostatic Ln···C interactions with carbon atoms from tBu groups, labeled C(7) and C(24) in Figure 1, giving two equatorial Ln···C contacts with C···Ln···C angles ranging from 152.9(3)° (1-Gd•CH2Cl2) to 130.76(11)° (1-Tm•C6H14); again 1-Lu•C6H14 is an outlier 7 ACS Paragon Plus Environment


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[C···Lu···C = 92.9(2)°]. This leads to an approximately trans-arrangement of two H atoms for all 1Ln [Ln···H = 2.450 and 2.461 Å (1-Gd•CH2Cl2), 2.297 and 2.491 Å (1-Yb•C6H14); H···Ln···H = 176.25° (1-Gd•CH2Cl2), 168.44° (1-Yb•C6H14)] except for 1-Lu•C6H14, which exhibits three short Lu···H distances [2.423, 2.530 and 2.645 Å]. The minor structural variations exhibited by 1Lu•C6H14 is manifested in the crystallographic symmetry; this crystallizes in the P21/c space group, whilst all other 1-Ln crystallized in P–1 regardless of the lattice solvation (though on one occasion we also obtained a P1 polymorph for 1-Er•CH2Cl2). The [B(C6F5)4]– anions in 1-Ln are unremarkable; of most importance no Ln···F equatorial interactions are observed, thus the Ln metallocenium cations, [Ln(Cpttt)2]+, in 1-Ln can be considered to be isolated and directly comparable to dysprosocenium, [Dy(Cpttt)2]+.4 In accordance with the single crystal XRD study the solid state ATR-IR spectra of 1-Ln (See Supplementary Information) all have essentially identical features to those previously reported for 1-Dy and 1-Y;4 therefore we conclude that the molecular vibrational modes of 1-Ln are commensurate with one another. The two equatorial electrostatic Ln···C interactions in 1-Ln did not result in a lowered C–H stretching frequency that could be detected by ATR-IR spectroscopy.


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Figure 1. Molecular structure of [Gd(Cpttt)2][B(C6F5)4]•CH2Cl2 (1-Gd•CH2Cl2). Displacement ellipsoids set at 30 % probability level, solvent of crystallization (CH2Cl2) and hydrogen atoms are omitted for clarity.

Figure 2. Molecular structure of [Gd(Cpttt)2(Cl)] (2-Gd). Displacement ellipsoids set at 30 % probability level, and hydrogen atoms are omitted for clarity.


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Complex

Cpcentroid···Ln / Å

Cpcentroid···Ln···Cpcentroid / °

Ln···C / Å

C(7)···Ln···C(24) / °

1-Gd•CH2Cl2

2.364(5), 2.345(5)

150.2(2)

2.973(11), 2.959(11)

152.9(3)

1-Gd•C6H6

2.357(2), 2.360(2)

152.00(7)

2.983(5), 2.911(5)

144.1(2)

1-Dy•CH2Cl24

2.318(2), 2.314(2)

152.56(7)

2.971(5), 2.956(5)

149.95(11)

1-Ho•CH2Cl2

2.302(2), 2.304(2)

153.07(7)

2.950(5), 2.967(5)

148.7(2)

1-Er•CH2Cl2

2.294(5), 2.300(6)

153.2(2)

2.911(10), 2.954(10)

146.4(3)

1-Er•C6H6

2.2997(10), 2.2967(10)

153.75(4)

2.867(2), 2.982(2)

140.52(6)

1-Tm•C6H14

2.277(2), 2.275(2)

155.11(6)

2.882(4), 2.932(4)

130.76(11)

1-Yb•C6H14

2.274(5), 2.270(5)

154.5(2)

2.952(10), 2.985(11)

147.2(3)

1-Yb•C6H6

2.280(4), 2.268(4)

154.95(14)

2.882(8), 3.005(8)

139.2(2)

1-Lu•C6H14

2.251(3), 2.240(3)

152.89(14)

2.805(8), 3.069(9)

92.9(2)

Table 2. Selected bond distances and angles of [Ln(Cpttt)2]+ cations in 1-Ln.

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Solution phase optical properties The UV-vis-NIR spectra of ca. 1 mM solutions (See Supplementary Information for details) of 1-Ln and 2-Ln in DCM were obtained at room temperature (vis-NIR spectra of 1-Ln compiled in Figure 3 with λ < 400 nm excluded for clarity; see Supplementary Information for full UV-vis-NIR spectra of 1-Ln and 2-Ln). With the exception of 1-Yb and 2-Yb, dilute solutions of 1-Ln or 2-Ln in DCM are pale yellow. The visible spectra of 1-Ln are dominated by intense bands tailing in from the UV region; a ca. 2 mM THF solution of “[K(15-crown-5)2][Cpttt]” prepared in situ from KCpttt and 2 eq. 15-crown-5 (which should contain isolated [Cpttt]– anions by analogy with [K(15-crown5)2][Cp]46) shows a band with λ < 380 nm due to π-π* excitations (Figure S118), implying that the features in 1-Ln with λ > 400 nm are due to LMCT. Weak absorptions (ε < 200 mol–1 dm3 cm–1) corresponding to f-f transitions were observed for 1-Ho, 1-Er and 1-Tm: 1-Ho exhibits a broad feature at max = 21,400 cm-1 (ε = 70 mol–1 dm3 cm–1), which likely owes to a series of 5I8 → 5F7/2,5/2,3/2 transitions; 1-Er shows absorptions at max = 20,400 (ε = 80 mol–1 dm3 cm–1), 19,900 (ε = 90 mol–1 dm3 cm–1), 18,900 (ε = 190 mol–1 dm3 cm–1) and 15,000 cm-1 (ε = 50 mol–1 dm3 cm–1), which likely arise from the 4I15/2 → 4F7/2, 2H11/2, 4S3/2, and 4F9/2 transitions, respectively; and 1-Tm shows clear absorptions at max = 14,950 (ε = 80 mol–1 dm3 cm–1) and 12,550 cm-1 (ε = 70 mol–1 dm3 cm–1), owing to the 3H6 → 3F2,3 and 3F4 transitions, respectively. These absorptions are relatively intense for f-f transitions because they are nearly all spin-allowed and presumably have significant vibronic coupling. We observe similar spectra for the parent 2-Ln (Figures S109–S115, S117). In contrast to the other 1-Ln and 2-Ln, dilute DCM solutions of 1-Yb and 2-Yb are blue and purple, respectively. Both species exhibit broad absorptions spanning the majority of the visible region [max/cm–1 (ε/mol–1 dm3 cm–1): 16,300 (380) and 13,800 (420), 1-Yb; 18,400 (210), 2-Yb]. These absorptions are attributed to ligand-to-metal charge transfer (LMCT) from Cpttt, as such bands have previously been seen for solutions of [Yb(Cp)3]47 and [Yb(Cp*)2(X)]+ cations (X = 2,2′bipyridine, bipy; 1,10-phenanthroline, phen).48,49 Furthermore, for 1-Yb we also observe two broad and relatively strong absorptions in the NIR region [max/cm–1 (ε/mol–1 dm3 cm–1): 9,800 (150); 9,050 (190)], which we assign to the 2F7/2 → 2F5/2 transitions, split by the significant CF interaction (see



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below). We observe similarly strong f-f transitions for 2-Yb in the NIR region, however in this case we observe three sharp absorptions [max/cm–1 (ε/mol–1 dm3 cm–1): 11,450 (30), 10,740 (40), 9,850 (220)], which directly correspond to the approximately mJ = ±5/2, ±3/2 and ±1/2 excited CF states of the 2F7/2 → 2F5/2 transition, respectively (see below); such sharp features have been seen before for [Yb(Cp*)2(X)]+ cations (X = bipy, phen).48,49

Figure 3. Room temperature vis-NIR spectra of 1-Ln (ca. 1 mM in DCM) from 6,000–25,000 cm–1.

Solid-state electronic structure and magnetic properties The first example of a dysprosocenium cation in 1-Dy was shown to have remarkable magnetic properties.4 Therefore we have studied the electronic structure and magnetic properties of the series of heavy 4f analogues 1-Ln reported here. However, the precursor complexes 2-Ln also possess a distinct structural motif that would be expected to yield characteristically different electronic structures to 1-Ln, and thus we examine both series herein. The title complexes 1-Ln have a pseudo-linear arrangement of charge, compared to the precursor complexes 2-Ln which have three charged ligands in a pseudo-trigonal planar arrangement; for the 1-Ln structures we define the CF quantization axis as the average Ln···Cptttcentroid vector, and for 2-Ln structures as the normal to the plane formed by the two Cpttt centroids and the Cl– ion (Figures 4k and l; Figures S140–S144 e and f). 12 ACS Paragon Plus Environment

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Following the electrostatic principles for predicting magnetic anisotropy,50 we would expect the pseudo-linear environment of 1-Ln to stabilize projections (mJ) of the total angular momentum (J) possessing oblate-spheroid electron density distributions along the quantization axis, while the pseudo-trigonal planar environment of 2-Ln should stabilize prolate-spheroid mJ projection along the quantization axis; the former is clearly true owing to the remarkable magnetic anisotropy of 1-Dy.4 Therefore, we would expect to observe the largest mJ projections as the ground states for 1-Ho, 2-Er, 2-Tm and 2-Yb, and the smallest mJ projections as the ground states for 2-Dy, 2-Ho, 1-Er, 1-Tm and 1-Yb. However, this simple oblate vs. prolate argument considers only the signs of the quadrupolar terms in the multipole decomposition of the mJ electron densities, and so is less robust for HoIII and ErIII which have the smallest magnitude quadrupolar terms of the heavy Ln.51 This leads to an inherent sensitivity of the electronic structure for HoIII and ErIII, rendering low symmetry perturbations (i.e. the lack of true C3 and D∞ symmetry) and even subtle effects such as covalency important for determining the CF. Nevertheless, the simple electrostatic argument does a remarkably good job at predicting the electronic structure in most cases (Figures 4 and S140–S144), which have been determined by complete active space self-consistent field spin-orbit (CASSCF-SO) calculations using MOLCAS 8.052 using the unoptimized XRD structures (see Supplementary Information).

Figure 4. Electronic structure of a) 1-Dy, b) 2-Dy, c) 1-Ho, d) 2-Ho, e) 1-Er, f) 2-Er, g) 1-Tm, h) 2Tm, i) 1-Yb and j) 2-Yb, calculated with CASSCF-SO, and shown in the presence of a 0.1 T DC field



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along the quantization axis. CF quantization axes for k) 1-Yb and l) 2-Yb are representative of all 1Ln and 2-Ln, respectively; tBu groups are truncated and H atoms are omitted for clarity. is the expectation value of Jz, a way to represent mixed mJ states, and is proportional to the magnetic moment.

The clearest example is for YbIII, where the largest mJ states have prolate electron density distributions, and the smallest mJ states have oblate distributions; thus we observe an mJ = ±1/2 ground state for 1-Yb and an mJ = ±7/2 ground state for 2-Yb (Figure 4i and j; Tables S25 and S45). Indeed, the electrostatic contribution to the CF for 1-Yb and 2-Yb is so dominant that the simple electrostatic arguments hold for all the mJ states, resulting in the characteristic double-well potential of easy-axis magnetic anisotropy for 2-Yb and the inverse double-well potential of easy-plane magnetic anisotropy for 1-Yb (Figure 4i and j). Indeed, the strong CF interaction for both 1-Yb and 2Yb is directly visible in the splitting of the excited 2F5/2 term by optical spectroscopy (Figures S107 and S114). The three states of this excited term are most clearly observed in the optical spectrum of 2Yb, which shows absorption bands at 9,850, 10,740 and 11,450 cm–1, in excellent agreement with the splitting predicted by CASSCF-SO of 10,380, 11,000 and 11,310 cm–1 for the mJ = ±5/2, ±3/2 and ±1/2 states. While the electronic structures for 1-Dy and 2-Dy seem to be clear-cut and the opposite to 1Yb and 2-Yb as expected from electrostatic arguments (Figure 4a and b), the ground Kramers doublet for 2-Dy has effective g-values of 0.02, 0.04 and 19.50 (Table S29), where the eigenvector for the large g-value points along the Cpttt···Cpttt direction. This is conventionally indicative of an easy-axis mJ = ±15/2 state, however, when the CF is quantized along the pseudo-trigonal axis the ground doublet is 56% mJ = ±1/2 and 30% mJ = ∓3/2 while the most energetic doublet is mJ = ±15/2 with gvalues of 0.00, 0.00 and 19.87 (Table S29, where the eigenvector for the large g-value is aligned with the quantization axis). The presence of two doublets with orthogonal g ~ 20 components is a common observation for DyIII in low-symmetry environments,53 and indicates that the CF is not well-defined as either axial or equatorial, where the Cpttt rings have a stronger contribution in determining the CF ground state than the “soft” Cl– anion; our results are consistent with the report of Guo et al.4b 14 ACS Paragon Plus Environment

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Similar electronic structures to 1-Yb and 2-Yb are observed for 1-Er and 2-Er, respectively (as ErIII has a similar profile of mJ electron density distributions to YbIII), but the mixing between the smallest mJ states is more complicated owing to the less dominant nature of electrostatics in determining the CF (Figures 4e and f, and S143; Tables S17 and S37). On the other hand, all states for the non-Kramers species (1-Ho, 2-Ho, 1-Tm and 2-Tm) have an expectation of Jz (, a measure of the effective mJ state and proportional to the magnetic moment) of zero (Figures S142 and S144a and b; Tables S13, S21, S33 and S41); although the CF can split the J multiplet of a non-Kramers ion into singlets, both 1-Ln and 2-Ln have relatively strong pseudo-symmetric electrostatic potentials and so we may expect to observe a number of pairs of pseudo-doublets with nearly degenerate energies. This is exemplified by 1-Tm, where the CF wavefunction shows an mJ = 0 ground state followed by pairs of pseudo-doublets with increasing |mJ| (Table S21; note the similarity to 1-Yb, however here the nonKramers nature of TmIII allows the two opposing projections of each mJ pair to be mixed together directly by the CF, resulting in = 0 for all states. Application of a small magnetic field along the quantization axis is able to break the mixing within the pseudo-doublets when the Zeeman energy is larger than the intra-pseudo-doublet splitting in zero-field, and thus we can resolve the magnetic character of the two highest-energy pseudo-doublets for 1-Tm with a 0.1 T field (Figures 4g and S144d; Table S23). A similar situation is true for 1-Ho, 2-Ho and 2-Tm, which also have remarkably symmetric wavefunction compositions with equal components of both +mJ and –mJ (Tables S13, S33 and S41), and application of a small magnetic field along the quantization axis reveals the magnetic character of their states (Figures 4c, d, and h, and S142 and S144 c and d; Tables S15, S35 and S43). Under these conditions, we observe that the largest mJ components are dominant in the ground states for 1-Ho and 2-Tm, and in the most excited states for 2-Ho and 1-Tm, exactly as predicted by simple electrostatic arguments (Figure 4). The preceding discussion is not appropriate for GdIII which differs from the other LnIII ions in that is has an exactly half-filled 4f shell with no orbital angular momentum in the ground state and can be considered “spin-only” S = 7/2. In this case, the magnetic anisotropy is often well-described by an = − ⁄3 + ⁄2 + , where D effective zero-field splitting (ZFS) Hamiltonian



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and E are the axial and rhombic ZFS parameters, respectively. ZFS arises from the CF splitting of excited orbitally-degenerate states mixing into the ground state (i.e. “spin-only” is an approximation), and can therefore be calculated with a method explicitly considering these excited states such as CASSCF-SO. Unsurprisingly, given the strong axial potential generated by the trans-bis-Cpttt motif of 1-Ln, 1-Gd is predicted to have a strongly axial ZFS with D = –0.1937 cm-1 and |E| = 0.0148 cm–1 (|E/D| = 0.076). Conversely, 2-Gd is predicted to have a ZFS approaching the rhombic limit (|E/D| = 1/3) with D = +0.0920 cm–1 and |E| = 0.0239 cm–1 (|E/D| = 0.26). The magnetic properties predicted on the basis of these CASSCF-SO-calculated electronic structures are generally in excellent agreement with the experimental magnetic data of polycrystalline 1-Ln and 2-Ln (Figures 5 and S119, see Supplementary Information); we note that the discrepancy between the CASSCF-SO calculation and the experimental χMT data for 1-Dy is due to a small inaccuracy in sample mass.4 On cooling, the χMT products show a gradual and steady decrease for 1Ho, 1-Er, 1-Tm, 1-Yb, 2-Dy (we note that our present data differs from our original report and is now consistent with Guo et al.4b), 2-Ho, 2-Er and 2-Tm due to depopulation of excited CF states, while χMT is more-or-less temperature independent for 1-Gd, 2-Gd and 2-Yb due to well isolated S = 7/2 ground states for GdIII and a large CF splitting for 2-Yb; 1-Dy is an outlier which shows a precipitous drop at low temperatures due to magnetic blocking. The largest difference in χMT behavior between the pairs of 1-Ln and 2-Ln is observed for 1-Tm vs. 2-Tm, the former of which shows a much more pronounced drop at lower temperatures as a consequence of the inverse double-well potential with an mJ = 0 ground state (Figure 4g; Table S21). The low temperature isothermal magnetization data for all compounds are generally in excellent agreement with that calculated by CASSCF-SO (Figure S119), indicating that the ground states are well predicted by the calculations. Notably, the experimental superposition of the 2 and 4 K isotherms for 1-Tm (Figure S119), which is direct evidence of an isolated mJ = 0 ground state, is reproduced by the CASSCF-SO calculations.


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Figure 5. Temperature dependence of the molar magnetic susceptibility χMT products measured under a 0.1 T DC field for a) 1-Ln and b) 2-Ln complexes. Circles and solid lines are the experimental and CASSCF-SO calculated values, respectively. Experimental data for 1-Dy taken from reference 4a.

Further experimental proof for the electronic structures of the Kramers ions can be obtained from electron paramagnetic resonance (EPR) spectroscopy. In the absence of rigorously high symmetry environments, the ground doublets of Kramers ions should be detectable by EPR, providing spin-lattice relaxation is not too fast; this can usually be achieved by cooling to cryogenic temperatures. Notable exceptions are for 1-Dy which has a pure mJ = ±15/2 ground state that should not give an EPR signal and 2-Dy which is EPR silent. The Q-band EPR spectra (ca. 34 GHz) of 2-Er and 2-Yb at 10 K (Figures S121 and S123) show only one broad absorption-like feature at low field, characteristic of a single large g-value, with some hyperfine structure visible for 2-Yb. Simulations of these spectra show that the g-values are 17.5 and 7.7 for 2-Er and 2-Yb, respectively, in excellent



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agreement with the CASSCF-SO predictions of 17.59 and 7.98 (Tables S37 and S45), and directly indicative of maximal mJ = ±15/2 and ±7/2 ground states, respectively, as predicted by electrostatic arguments. The 10 K Q-band spectrum for 1-Er, much like 2-Er, shows only a single broad absorption-like feature at low field, with a g-value of 13.88 (Figure S120), which is reasonable agreement with the CASSCF-SO prediction of 14.36. However, CASSCF-SO predicts a second gvalue within the experimental range of 2.14 that we do not observe; given that CASSCF-SO predicts the ground state to be very mixed with a close-lying excited state at ca. 19 cm–1 (Table S17), it is likely that the calculations do not catch the subtlety of the electronic structure perfectly. The 10 K Qband spectra for 1-Yb (Figure S122) is quite different from the other three previously discussed, showing two clear features: one structured absorption-like resonance at low field and a second broad derivative-type feature at ca. 0.9 T. This is indicative of one large and one intermediate g-value, with hyperfine structure visible on the low-field feature. Simulation of the spectrum gives g2 = 2.65 and g3 = 5.71, in good agreement with CASSCF-SO (Table S17, g1 = 1.09, g2 = 3.50 and g3 = 5.49; g1 not observed experimentally), where the hyperfine coupling is well reproduced using the empirical expressions based on the effective g-values described by Denning et al.54 These g-values are indicative of a rhombic Kramers doublet ground state, dominated by a mJ = ±1/2 component (mJ = ±1/2 has g1 = 1.14, g2 = g3 = 4.57), as expected from electrostatic considerations. EPR spectra for GdIII are dominated by ZFS of the S = 7/2 state, leading to highly-detailed, fingerprint-like spectra. The Q-band spectrum for 1-Gd is directly indicative of a huge ZFS, spanning the entire field range (Figure 6b). In order to accurately model this highly featured Q-band spectrum, we have also recorded the X-band spectrum for 1-Gd (ca. 9.4 GHz, Figure 6a). Both these spectra can be simultaneously modeled very well with D = ±0.3347 cm–1, |E| = 0.01629 cm–1 (|E/D| = 0.049) and g = 1.95, which also provide good simulations of the low-temperature magnetization data (Figure S127), and is in good agreement with CASSCF-SO. We are unable to determine the sign of D from these data (Figure S124), but given the good agreement with CASSCF-SO, we suggest that D < 0. This very large magnitude of D is, to our knowledge, second largest for a 4f7 ion and only smaller than a near-linear EuII species [Eu(N{SiiPr3}2)2] with D = +0.575 cm–1,55 and is a direct correlation to the large magnetic anisotropy of 1-Dy.4 The spread of the Q-band spectrum for 2-Gd is much smaller 18 ACS Paragon Plus Environment

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than that of 1-Gd, indicating a significantly smaller ZFS as expected due to the competing axial and equatorial contributions to the CF (Figure S125). Likewise, modelling the Q- and X-band spectra of 2Gd simultaneously allows us to precisely measure the ZFS, finding that |D| = 0.08724 cm–1, |E| = 0.02079 cm–1 (approaching the rhombic limit with |E/D| = 0.24) and g = 1.95 provide the best agreement with our data, and also agree well with the low-temperature magnetization data (Figure S127). Again we are unable to determine the sign of D (Figure S126), but due to the good agreement with CASSCF-SO, we suggest that D > 0 in this case.

Figure 6. a) X-band (9.37559 GHz) and b) Q-band (34.065 GHz) EPR for 1-Gd at 10 K. Simulation parameters: S = 7/2, g = 1.95, D = –0.3347 cm–1, |E| = 0.01629 cm–1, linewidth (X-band) = 1.2 GHz, linewidth (Q-band) = 1.0 GHz.

To gain some insight into the anomalous magnetic relaxation dynamics of 1-Dy,4 we have studied the dynamics of the heavy 4f analogues with easy-axis ground states, viz 1-Ho, 2-Dy, 2-Er, 2Tm and 2-Yb. Alternating current (AC) susceptibility measurements performed in zero direct current (DC) field showed no appreciable out-of-phase component for 2-Dy (consistent with the report of Guo et al.4b), 2-Er, 2-Tm and 2-Yb; only for 1-Ho was there indication of slow magnetic relaxation 19 ACS Paragon Plus Environment


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(Figures 7 and S128). However, application of a DC field of 0.1 T is able to reveal slow dynamics for 2-Dy, 2-Er and 2-Yb (Figures S131-S136; 1-Ho is unchanged in a 0.1 T field, Figures S129 and S130), but not for 2-Tm; the latter is presumably due to the inability of the small DC field to break the mixing between the mJ projections of the ground pseudo-doublet. Fitting these AC data with the modified Debye model yields the temperature dependence of the magnetic relaxation rate (Figures S135 and S136). Magnetic relaxation is very fast for both 2-Er and 2-Yb only being observable below 4 and 7 K, respectively, and appears to show a power-law dependence on temperature (Figure S137), characteristic of a two-phonon Raman relaxation mechanism (τ-1 = CTn);56 fitting these data yield C = 0.015 s–1 K-n and n = 8.5 for 2-Er and C = 0.017 s–1 K-n and n = 7.1 for 2-Yb, where the exponents are in the range n ~ 7-9 as expected for Raman relaxation for Kramers ions.56 Conversely, for 1-Ho and 2Dy plots of ln(τ) vs. 1/T appear to show a linear slope above 5 K and 6 K, respectively (Figures S138 and S139); conventionally, this is suggestive of relaxation via the Orbach mechanism (τ–1 = τ0–1exp[Ueff/kT]), however fitting the linear regions give Ueff = 23 cm–1 (τ0 = 1.1×10–5 s) for 1-Ho and Ueff = 39 cm–1 (τ0 = 5.1×10–6 s) for 2-Dy, which are much smaller than the first excited CF states of 64 cm–1 and 221 cm–1, respectively, (Tables S13 and S29) and the values of τ0 are orders of magnitude too large for real Orbach processes. Alternatively, log-log plots of τ-1 vs. T are linear and thus a two-phonon Raman mechanism is much more likely in both cases, where best fits give C = 3.4 s–1 K–n and n = 2.9 for 1Ho, and C = 0.0023 s–1 K–n and n = 5.3 for 2-Dy. The value of n for 1-Ho is much lower than expected for a non-Kramers ion (n ~ 7),56 and differs greatly from the values observed for 2-Dy, 2-Er and 2-Yb, however, it is approaching the regime observed for 1-Dy (n = 2.151).4 The value of n for 2Dy is lower than expected for a Kramers ion (n ~ 7-9),56 but it is much larger than that observed for 1Dy and approaching those observed for 2-Er and 2-Yb. These observations suggest that a low Raman exponent is characteristic of 1-Ln, and is an effect that is either switched off or overcome by a more efficient conventional Raman process (i.e. the exponent returns towards a traditionally-expected value) upon coordination of a single Cl– ion in 2-Ln. The smaller-than-expected exponent for 2-Dy likely arises due to the more dominant contribution of the two Cpttt ligands over the Cl– ion in determining the CF ground state in this case (i.e. the ground Kramers doublet shows easy-axis anisotropy along the Cpttt···Cpttt direction like 1-Dy). 20 ACS Paragon Plus Environment

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Figure 7. In-phase (left) and out-of-phase (right) AC susceptibilities for 1-Ho in zero static DC field. 2 K (purple) to 16 K (red) in steps of 1 K. Solid lines are fits to the generalized Debye model, giving 0.05 ≤ α ≤ 0.58.

Low Raman exponents have been suggested to arise from an acoustic-optic two-phonon process,57 which in this case appears to be intrinsic to the constrained vibrational modes of the transbis-Cpttt motif, and a feature which disappears with the addition of an equatorial ligand. Therefore, our results show that a coordination environment constructed entirely from multi-hapto donor ligands is the origin of a characteristic spin-phonon coupling, which leads to an anomalously-low Raman exponent in 1-Ln, and that the anomalous results for 1-Dy are not an outlier. This reinforces the connection between molecular structure and magnetic relaxation dynamics, and indicates that spinphonon coupling may be controlled through ligand design. It appears that the unique spin-phonon coupling in 1-Ln is crucial for observing magnetic hysteresis at high temperatures in 1-Dy, compared to

complexes

containing

ligands

with

localized

donor

atoms,

such

as

[Dy(tBuO)2(pyridine)5][B(Ph)4].14 Therefore, the exclusive employment of multi-hapto ligands, such as arenes and other π-donors, may be useful as a design strategy to realize better performance in the next generation of high-temperature SMMs.

Conclusions 21 ACS Paragon Plus Environment


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We have presented a family of heavy 4f-metallocenium cations, which are all structurally analogous in the solid state. Through a systematic study of the electronic structures, spectroscopic and magnetic properties of 1-Ln, we have highlighted the remarkably large magnetic anisotropy that stems from the [Ln(Cpttt)2]+ motif. Although complexes 1-Ln and 2-Ln have formally low point group symmetry (C1), the strong crystal fields of the respective pseudo-linear (1-Ln) and pseudo-trigonal planar (2-Ln) geometries are dominated by electrostatic contributions, leading to archetypal axial electronic structures with easy-axis type magnetic anisotropy for 1-Dy, 1-Ho, 2-Er, 2-Tm and 2-Yb and easy-plane type magnetic anisotropy for 2-Ho, 1-Er, 1-Tm and 1-Yb; 2-Dy is an outlier that shows an easy-axis ground state despite the pseudo-trigonal planar nature of the complex due to competing contributions to the crystal field. Out of the species with easy-axis magnetic anisotropy, only 2-Tm does not exhibit slow magnetic relaxation, while 2-Dy, 2-Er and 2-Yb exhibit magnetic relaxation at low temperatures via a conventional two-phonon Raman process. 1-Ho, on the other hand, shows Raman-type magnetic relaxation with a small exponent of n = 2.9, similar to that observed for 1-Dy of n = 2.151, providing further evidence that anomalous Raman relaxation is characteristic of isolated 4f-metallocenium cations 1-Ln. These results support our hypothesis that the presence of only multi-hapto donor ligands in 1-Dy is key to its remarkable properties, and thus strengthens the proposed connection between molecular structure and spin-phonon coupling. We envisage that the isolation and analysis of novel Ln metallocenium cations with modified structural features based on the [Ln(Cpttt)2]+ framework presented herein will help to disentangle this relationship further in the future, leading ultimately to improved high-temperature SMMs.

Experimental Materials and methods All manipulations were conducted under argon with rigorous exclusion of oxygen and water using Schlenk line and glove box techniques. Toluene, benzene and hexane were dried by refluxing over potassium and were stored over potassium mirrors. Dichloromethane (DCM) was dried over CaH2 and stored over or 4 Å molecular sieves. All solvents were degassed before use. For NMR spectroscopy C6D6 was dried by refluxing over K and CD2Cl2 was dried by refluxing over CaH2. Both 22 ACS Paragon Plus Environment

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NMR solvents were vacuum transferred and degassed by three freeze-pump-thaw cycles before use. Anhydrous LnCl3 were purchased from Alfa Aesar and were used as received. KCpttt,23 [H(SiEt3)2][B(C6F5)4],38 1-Dy,4 2-Dy,4 and [Yb(Cpttt)2] (3)21 were prepared according to literature methods. 1H (400 MHz),

13

C{1H} (100 MHz and 125 MHz),

13

C{19F} (126 MHz),

11

B{1H} (128

MHz) and 19F{1H} (376 MHz) NMR spectra were obtained on an Avance III 400 MHz or 500 MHz spectrometer at 298 K. These were referenced to the solvent used, or to external TMS (1H,

13

C),

H3BO3/D2O (11B) or C7H5F3/CDCl3 (19F). UV-Vis-NIR spectroscopy was performed on samples in Youngs tap appended 10 mm pathlength quartz cuvettes on an Agilent Technologies Cary Series UVVis-NIR Spectrophotometer from 175–3300 nm. ATR-Fourier Transform infrared (ATR-FTIR) spectra were recorded as microcrystalline powders using a Bruker Tensor 27 spectrometer. General synthetic procedures for 1-Ln and 2-Ln are outlined below; full details are given in the Supplementary Information.

General synthesis of 1-Ln: In a typical procedure, benzene (15 mL) was added to a Schlenk vessel containing an intimate mixture of 2-Ln and [H(Et3Si)2][B(C6F5)4] at either 4 °C or room temperature. The starting materials rapidly dissolved and a precipitate formed within 10 mins (color dependent on 2-Ln; no precipitate was observed for 1-Lu). The reaction mixture was stirred for 16 hours and volatiles were removed in vacuo. The resultant oily solid was washed with hexane (2 x 10 mL) and dried in vacuo (crude yields typically 70–90%). The powder was cooled to 4 °C, dissolved in DCM (typically 0.8–1.0 mL), layered with 1-2 equivalents of hexane and stored at –23 °C overnight to afford crystals of 1-Ln. 1-Gd: Bright yellow crystals (0.6066 g, 75 %). Anal calcd (%) for C58H58BF20Gd·0.5CH2Cl2: C, 51.05; H, 4.36; Found: C, 51.19; H, 4.36. χMT product = 8.17 cm3 mol–1 K (Evans method). The paramagnetism of 1-Gd precluded assignment of its 1H and

13

C{1H} NMR spectra.

11

B{1H} NMR

(CD2Cl2, 128 MHz, 298 K): δ = –16.71 (s). 19F{1H} NMR (CD2Cl2, 376 MHz, 298 K): δ = –163.9 (br, p-F), –133.4 (br, o-F); the m-F environment could not be confidently assigned, though this is likely the broad feature between –170 and –190 ppm. FTIR (ATR, microcrystalline): = 2972 (w), 2923 (w), 2880 (w), 1644 (w), 1562 (v. w), 1513 (m), 1460 (s), 1405 (w), 1369 (w), 1348 (v. w), 1277 (w), 23 ACS Paragon Plus Environment


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1240 (w), 1203 (w), 1167 (w), 1085 (s), 1032 (br. w), 977 (s), 930 (w), 918 (w), 871 (w), 846 (w), 832 (w), 804 (w), 775 (m), 757 (m), 749 (w), 728 (w), 710 (w), 683 (m), 661 (m), 612 (w), 608 (w), 598 (w), 573 (w), 555 (w), 534 (v. w), 512 (v. w), 494 (v. w), 477 (w), 439 (m), 408 (w) cm–1. 1-Ho:

Bright

salmon-orange

crystals

(0.3906

g,

53

%).

Anal

calcd

(%)

for

C58H58BF20Ho·0.5CH2Cl2: C, 50.77; H, 4.33; Found: C, 49.69; H, 4.04. χMT product = 12.90 cm3 mol– 1

K (Evans method). 1H NMR (CD2Cl2, 400 MHz, 298 K): δ = –186.09 (br, v1/2 ~ 2,200 Hz, C(CH3)3);

no other signals observed. The paramagnetism of 1-Ho precluded assignment of its spectrum.

11

B{1H} NMR (CD2Cl2, 128 MHz, 298 K): δ = –26.43 (s).

19

13

C{1H} NMR

F{1H} NMR (CD2Cl2, 376

MHz, 298 K): δ = –179.3 (br, m-F), –171.1 (br, p-F), –142.7 (br, o-F). FTIR (ATR, microcrystalline): = 2968 (w), 2912 (w), 2874 (w), 1643 (w), 1626 (v. w), 1603 (v. w), 1560 (w), 1513 (m), 1460 (s), 1413 (w), 1403 (w), 1395 (w), 1369 (m), 1344 (w), 1331 (w), 1277 (m), 1240 (m), 1203 (w), 1165 (w), 1085 (s), 1028 (w), 977 (s), 932 (w), 914 (w), 869 (w), 853 (m), 830 (br. w), 810 (w), 775 (m), 757 (m), 742 (w), 728 (w), 708 (w), 683 (m), 661 (m), 612 (m), 604 (w), 573 (m), 559 (v. w), 528 (v. w), 516 (v. w), 500 (v. w), 485 (sh. v. w), 475 (w), 449 (sh. v. w), 439 (w), 426 (w) cm–1. 1-Er: Bright yellow crystals (0.1753 g, 30 %). Anal calcd (%) for C58H58BF20Er: C, 53.05; H, 4.45; Found: C, 52.40; H, 4.44. χMT product = 11.68 cm3 mol–1 K (Evans method). 1H NMR (CD2Cl2, 400 MHz, 298 K): δ = 65.06 (br, v1/2 ~ 250 Hz, C(CH3)3); no other signals observed. The paramagnetism of 1-Er precluded assignment of its 13C{1H} NMR spectrum. 11B{1H} NMR (CD2Cl2, 128 MHz, 298 K): δ = –12.12 (s). 19F{1H} NMR (CD2Cl2, 376 MHz, 298 K): δ = –165.0 (br, m-F), – 161.1 (br, p-F), –129.7 (br, o-F). FTIR (ATR, microcrystalline): = 2972 (w), 2923 (w), 2878 (w), 1644 (w), 1564 (w), 1513 (m), 1460 (s), 1405, (w), 1369 (w), 1348 (v. w), 1277 (w), 1240 (w), 1203 (w), 1167 (w), 1085 (s), 1034 (w), 977 (s), 930 (w), 914 (w), 871 (w), 853 (w), 828 (w), 810 (w), 775 (m), 757 (m), 744 (w), 728 (w), 710 (w), 683 (m), 661 (m), 612 (w), 608 (w), 598 (w), 573 (w), 556 (br. w), 477 (w), 441 (m), 411 (v. w), 412 (w) cm–1. 1-Tm: Orange crystals (0.7163 g, 74 %). Anal calcd (%) for C58H58BF20Tm: C, 52.98; H, 4.45; Found: C, 50.13; H, 4.16. χMT product = 4.97 cm3 mol–1 K (Evans method). 1H NMR (CD2Cl2, 400 MHz, 298 K): δ = 74.03 (br, v1/2 ~160 Hz, C(CH3)3); no other signals observed. The paramagnetism of 1-Tm precluded assignment of its 13C{1H} NMR spectrum. 11B{1H} NMR (CD2Cl2, 128 MHz, 298 24 ACS Paragon Plus Environment

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K): δ = –12.37 (s). 19F{1H} NMR (CD2Cl2, 376 MHz, 298 K): δ = –164.3 (br, m-F), –160.7 (br, p-F), –129.5 (br, o-F). FTIR (ATR, microcrystalline): = 2968 (w), 2914 (w), 2876 (w), 1976 (w), 1643 (m), 1513 (s), 1460 (s), 1415 (w), 1403 (w), 1395 (w), 1375 (sh.w), 1369 (m), 1346 (w), 1305 (w), 1277 (m), 1240 (m), 1201 (w), 1191 (w), 1167 (w), 1150 (w), 1083 (br. s), 1028 (w), 997 (sh. w), 977 (s), 932 (w), 912 (w), 855 (w), 842 (w), 834 (w), 810 (w), 793 (w), 775 (m), 757 (m), 744 (w), 728 (w), 708 (w), 683 (m), 661 (m), 612 (w), 602 (w), 573 (w), 555 (br. v. w), 536 (v. w), 477 (w), 461 (w), 439 (w), 422 (w), 416 (w), 404 (w) cm–1. 1-Yb: Dark blue crystals (0.2742 g, 40 %). Anal calcd (%) for C58H58BF20Yb: C, 52.82; H, 4.43; Found: C, 52.82; H, 4.33. χMT product = 2.11 cm3 mol–1 K (Evans method). 1H NMR (CD2Cl2, 400 MHz, 298 K): δ = 20.51 (br, 36 H, v1/2 ~ 280 Hz, C(CH3)3), 25.45 (br, 18 H, v1/2 ~ 20 Hz, C(CH3)3); no other signals observed. The paramagnetism of 1-Yb precluded assignment of its spectrum.

11

B{1H} NMR (CD2Cl2, 128 MHz, 298 K): δ = –15.52 (s).

19

13

C{1H} NMR

F{1H} NMR (CD2Cl2, 376

MHz, 298 K): δ = –166.4 (br, m-F), –162.8 (br, p-F), –131.9 (br, o-F). FTIR (ATR, microcrystalline): = 2968 (w), 2912 (w), 2874 (w), 1643 (m), 1609 (v. w), 1564 (w), 1513 (m), 1460 (s), 1411 (w), 1401 (w), 1375 (m), 1277 (m), 1242 (m), 1201 (w), 1171 (v. w), 1148 (w), 1085 (s), 1034 (w), 975 (s), 932 (w), 909 (w), 895 (w), 875 (w), 857 (w), 842 (w), 832 (w), 812 (w), 791 (w) 775 (m), 757 (m), 742 (m), 728 (w), 708 (w), 683 (m), 661 (m), 612 (w), 606 (w), 573 (w), 556 (w), 545 (w), 498 (v. w), 477 (v. w), 469 (v. w), 459 (v. w), 439 (w), 428 (v. w), 418 (w), 410 (w) cm–1. 1-Lu: Colorless crystals (0.2917 g, 40 %). Anal calcd (%) for C58H58BF20Lu: C, 52.82; H, 4.43; Found: C, 52.82; H, 4.33. 1H NMR (CD2Cl2, 400 MHz, 298 K): δ = 1.39 (s, 18H, Cp-C(CH3)3), 1.51 (s, 36H, Cp-C(CH3)3), 6.42 (s, 4H, Cp-CH).

13

C{1H} NMR (CD2Cl2, 100 MHz, 298 K): δ = 32.46

(Cp-C(CH3)3), 33.11 (Cp-C(CH3)3), 34.14 (Cp-C(CH3)3), 35.73 (Cp-C(CH3)3), 116.96 (s, Cp-CH), 141.07 (s, Cp-C), 146.81 (s, Cp-C); [B(C6F5)4]– signals not observed.

11

B{1H} NMR (CD2Cl2, 128

MHz, 298 K): δ = –16.64 (s). 19F{1H} NMR (CD2Cl2, 376 MHz, 298 K): δ = –167.5 (s, m-F), –163.6 (t, JFF = 20.4 Hz, p-F), –133.1 (s, o-F). FTIR (ATR, microcrystalline): = 2968 (w), 2939 (sh. w), 2882 (w), 2774 (v. w), 1643 (m), 1607 (v. w), 1564 (v. w), 1511 (m), 1458 (s), 1413 (w), 1373 (m), 1348 (w), 1275 (m), 1240 (m), 1203 (w), 1195 (w), 1152 (w), 1085 (s), 1036 (w), 975 (s), 932 (w),



Page 26 of 33

910 (w), 873 (v. w), 836 (w), 773 (m), 755 (m), 728 (w), 718 (v. w), 683 (m), 661 (m), 610 (w), 573 (w), 545 (v. w), 449 (v. w), 416 (w) cm–1. General synthesis of 2-Ln (Ln = Gd, Ho, Er, Tm, Lu):

In a typical procedure, THF (30 mL)

was added to a pre-cooled (–78 °C) intimate mixture of LnCl3 (2-4 mmol) and KCpttt (4-8 mmol) in an ampoule equipped with a Teflon high-pressure stopper (Rotaflo). The reaction mixture was allowed to warm to room temperature with stirring, placed under a static vacuum and refluxed in an oil bath at 80 °C for 16 hours to give a white slurry. Volatiles were removed in vacuo and toluene (30 mL) was added. This typically resulted in significant dissolution of the solids and color changes (e.g. to pale green for 1-Tm). The ampoule was placed under a static vacuum, and refluxed in an oil bath at 130 °C for 16 hours. The reaction mixture was allowed to cool, then filtered and concentrated to ca. 3 mL. Significant amounts of precipitate formed; this solid was re-dissolved by heating the mixture briefly prior to storage at –23 °C overnight to afford crystals of 2-Ln. 2-Gd: Yellow crystals (0.8313 g, 63 %). Anal calcd (%) for C38H58ClGd: C, 61.92; H, 8.86; Found: C, 61.20; H, 8.92. χMT product = 8.15 cm3 mol–1 K (Evans method). The paramagnetism of 2Gd precluded assignment of its 1H and

13

C{1H} NMR spectra. FTIR (ATR, microcrystalline): =

3108 (w), 3025 (w), 3000 (w), 2957 (s), 2908 (m), 2874 (m), 2074 (br. w), 2048 (w), 2037 (w), 2025 (w), 2013 (w), 1976 (w), 1972 (w), 1491 (w), 1460 (s), 1389 (s), 1356 (s), 1271 (w), 1240 (s), 1222 (w), 1193 (w), 1167 (m), 1116 (w), 1028 (w), 1001 (m), 961 (m), 952 (w), 928 (w), 917 (w), 836 (m), 826 (s), 775 (m), 730 (w), 677 (s), 647 (v. w), 632 (v.w), 624 (v. w), 591 (w), 567 (m), 549 (m), 518 (v. w), 506 (v. w), 479 (w), 439 (s), 414 (w) cm–1. 2-Ho: Pale pink crystals (0.4834 g, 18 %). Anal calcd (%) for C38H58ClHo: C, 61.21; H, 8.76; Found: C, 60.73; H, 8.80. χMT product = 13.22 cm3 mol–1 K (Evans method). 1H NMR (C6D6, 400 MHz, 298 K): δ = –205.45 (br, 18 H, v1/2 ~ 1,400 Hz, C(CH3)3), –58.44 (br, 36 H, v1/2 ~ 1,850 Hz, C(CH3)3); no other signals observed. The paramagnetism of 2-Ho precluded assignment of its 13

C{1H} NMR spectra. FTIR (ATR, microcrystalline): = 3112 (v. w), 3021 (w), 2957 (s), 2908 (m),

2872 (m), 1829 (v. w), 1691 (br.v. w), 1491 (w), 1460 (s), 1389 (s), 1358 (s), 1271 (w), 1240 (s), 1224 (w), 1207 (w), 1197 (w), 1167 (m), 1116 (w), 1026 (w), 1001 (m), 961 (m), 950 (w), 928 (w), 916 (w), 867 (w), 842 (m), 830 (s), 777 (s), 763 (w), 746 (sh. w), 730 (w), 716 (v. w), 677 (br. s), 655 26 ACS Paragon Plus Environment

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(w), 591 (m), 567 (m), 549 (m), 526 (w), 512 (w), 502 (w), 487 (br. w), 477 (w), 469 (w), 461 (w), 439 (s), 422 (w) cm–1. 2-Er: Salmon-orange crystals (0.4869 g, 36 %). Anal calcd (%) for C38H58ClEr: C, 60.99; H, 8.73; Found: C, 60.59; H, 8.87. χMT product = 11.05 cm3 mol–1 K (Evans method). The paramagnetism of 2-Er precluded assignment of its 1H and

13

C{1H} NMR spectra. FTIR (ATR,

microcrystalline): = 3024 (w), 3000 (w), 2957 (s), 2910 (m), 2872 (m), 2074 (w), 2048 (w), 2046 (w), 2013 (w), 1978 (w), 1970 (sh. w), 1937 (v. w), 1823 (v. w), 1623 (v. w), 1491 (w), 1464 (s), 1389 (s), 1358 (s), 1268 (w), 1240 (s), 1195 (w), 1167 (m), 1114 (w), 1026 (w), 1001 (m), 961 (m), 952 (w), 926 (br. w), 869 (w), 830 (s), 783 (m), 732 (w), 677 (s), 592 (w), 567 (m), 549 (m), 520 (v. w), 504 (v. w), 492 (v. w), 477 (w), 438 (s), 416 (w) cm–1. 2-Tm: Pale green crystals (1.0642 g, 40 %). Anal calcd (%) for C38H58ClTm: C, 60.84; H, 8.71; Found: C, 60.51; H, 8.86. χMT product = 6.18 cm3 mol–1 K (Evans method). The paramagnetism of 2Tm precluded assignment of its 1H and

13

C{1H} NMR spectra. FTIR (ATR, microcrystalline): =

3029 (w), 3008 (w), 2982 (w), 2955 (s), 2921 (w), 2906 (m), 2876 (m), 2839 (br. w), 1487 (m), 1462 (m), 1393 (m), 1362 (m), 1273 (w), 1240 (s), 1201 (m), 1169 (m), 1114 (w), 1026 (w), 1003 (m), 989 (br. w), 961 (m), 952 (br. w), 922 (w), 887 (v. w), 848 (s), 838 (sh. m), 828 (s), 818 (sh. w), 812 (w), 775 (w), 730 (m), 685 (s), 675 (s), 642 (w), 622 (w), 606 (w), 591 (m), 571 (w), 555 (m), 532 (w), 522 (br. w), 510 (w), 500 (w), 492 (w), 477 (w), 467 (w), 445 (s), 428 (w), 418 (w), 406 (m) cm–1. 2-Lu: Colorless crystals (0.6759 g, 29 %). Anal calcd (%) for C38H58ClLu: C, 60.30; H, 8.63; Found: C, 58.34; H, 8.59. 1H NMR (C6D6, 400 MHz, 298 K): δ = 1.21 (s, 18H, C(CH3)3), 1.53 (s, 36H, C(CH3)3), 6.70 (s, 4H, Cp-CH).

13

C{1H} NMR (C6D6, 100 MHz, 298 K): δ = 31.73 (Cp-

C(CH3)3), 33.53 (Cp-C(CH3)3), 34.46 (Cp-C(CH3)3), 35.10 (Cp-C(CH3)3), 136.17 (s, Cp-C), 137.76 (s, Cp-C); Cp-CH could not be located and is presumably masked by the solvent residual signal. FTIR (ATR, microcrystalline): = 3025 (w), 3002 (sh. w), 2957 (s), 2910 (m), 2872 (m), 2657 (br. w), 2323 (w), 2292 (w), 1650 (w), 1630 (br. w), 1519 (w), 1466 (m), 1391 (m), 1358 (s), 1311 (br. w), 1271 (w), 1240 (s), 1203 (m), 1167 (m), 1112 (m), 1097 (w), 1073 (br. w), 1026 (w), 1001 (m), 987 (m), 963 (m), 924 (br. w), 867 (w), 834 (s), 785 (m), 759 (w), 740 (v. w), 718 (br. w), 679 (br. s), 655



Page 28 of 33

(w), 616 (w), 610 (v. w), 593 (w), 561 (sh. w), 545 (s), 518 (w), 498 (w), 483 (w), 475 (w), 441 (s), 414 (m) cm–1. Synthesis of 2-Yb: A solution of tBuCl (1.3 mL, 11.8 mmol) in toluene (15 mL) was added to a pre-cooled (–78 °C) solution of [Yb(Cpttt)2] (3) in toluene (10 mL), forming a dark green/brown reaction mixture. This color persisted upon warming to room temperature, and a dark purple reaction mixture formed upon stirring for 16 hours. The volatiles were removed in vacuo and the resultant purple powder was extracted into toluene (10 mL), filtered, and concentrated to 3 mL. Storage at 4 °C gave purple crystals of 2-Yb (1.3647 g, 73 %). Anal calcd (%) for C38H58ClYb: C, 60.47; H, 8.66; Found: C, 60.19; H, 8.82. χMT product = 2.04 cm3 mol–1 K (Evans method). 1H NMR (C6D6, 400 MHz, 298 K): δ = 92.31 (br s, v1/2 ~ 4,000 Hz, C(CH3)3); no other signals observed. The paramagnetism of 2-Yb precluded assignment of its

13

C{1H} NMR spectrum. FTIR (ATR,

microcrystalline): = 3027 (w), 3002 (w), 2957 (br. s), 2910 (m), 2872 (m), 1630 (w), 1491 (w), 1479 (w), 1464 (m), 1389 (m), 1358 (s), 1271 (w), 1240 (s), 1226 (w), 1207 (w), 1195 (w), 1163 (m), 1114 (m), 1028 (w), 1001 (m), 961 (m), 950 (w), 926 (br. w), 869 (w), 832 (s), 785 (s), 734 (br. v. w), 712 (br. v. w), 679 (s), 657 (w), 644 (v. w), 632 (v. w), 614 (w), 591 (w), 595 (w), 547 (m), 516 (br. w), 477 (br. w), 439 (s), 408 (w) cm–1.

Associated Content Supporting Information Supporting experimental and computational data for this article are available free of charge via the Internet at http://pubs.acs.org.

Author Information Corresponding Authors [email protected]; [email protected] Notes The authors declare no competing financial interest.


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Acknowledgements We acknowledge funding from the Engineering and Physical Sciences Research Council (Doctoral Prize Fellowship to C.A.P.G. and EP/P002560/1 for F.O. and D.R.), the Ramsay Memorial Fellowships Trust (fellowship to N.F.C.) and The University of Manchester. We thank the EPSRC UK National Electron Paramagnetic Resonance Service for access to the EPR facility and the SQUID magnetometer, and the University of Manchester for access to the Computational Shared Facility. We also thank Mr A. Brookfield for assistance with EPR spectroscopy, Drs. G.F.S. Whitehead and I.J. Vitorica-Yrezabal for assistance with single crystal XRD, and Profs. E.J.L. McInnes, R.E.P. Winpenny and S.T. Liddle, and the referees, for constructive feedback on the manuscript. Research data

files

supporting

this

publication

are

available

from

Mendeley

Data

at

doi:10.17632/9btz3nh68b.1.

References 1. (a) Metallocenes: Synthesis Reactivity Applications; Togni, A.; Halterman, R. L., Eds.; WileyVCH, Weinheim, 1998; (b) Metallocenes: An Introduction to Sandwich Complexes; Long, N. J. Wiley-Blackwell, London, 1998. 2. (a) Astruc, D. Eur. J. Inorg. Chem. 2017, 6, and references cited therein; (b) Heinze, K.; Lang, H. Organometallics 2013, 32, 5623, and references cited therein. 3. Birmingham, J. M.; Wilkinson, G. J. Am. Chem. Soc. 1956, 78, 42. 4. (a) Goodwin, C. A. P.; Ortu, F.; Reta, D.; Chilton, N. F.; Mills, D. P. Nature 2017, 548, 439; (b) Guo, F.-S.; Day, B. M.; Chen, Y.-C.; Tong, M.-L.; Mansikkamäki, A.; Layfield, R. A. Angew. Chem. Int. Ed. 2017, 56, 11445. 5. Ishikawa, N.; Sugita, M.; Ishikawa, T.; Koshihara, S.; Kaizu, Y. J. Am. Chem. Soc. 2003, 125, 8694. 6. Jeletic, M. S.; Lin, P. H.; Le Roy, J. J.; Korobkov, I.; Gorelsky, S. I.; Murugesu, M. J. Am. Chem. Soc. 2011, 133, 19286. 7. Jiang, S.-D.; Wang, B.-W.; Sun, H.-L.; Wang, Z.-M.; Gao, S. J. Am. Chem. Soc. 2011, 133, 4730.



Page 30 of 33

8. Le Roy, J. J.; Jeletic, M.; Gorelsky, S. I.; Korobkov, I.; Ungur, L.; Chibotaru, L. F.; Murugesu, M. J. Am. Chem. Soc. 2013, 135, 3502. 9. Meihaus, K. R.; Long, J. R. J. Am. Chem. Soc. 2013, 135, 17952. 10. Le Roy, J. J.; Ungur, L.; Korobkov, I.; Chibotaru, L. F.; Murugesu, M. J. Am. Chem. Soc. 2014, 136, 8003. 11. Gregson, M., Chilton, N. F., Ariciu, A.-M., Tuna, F., Crowe, I. F., Lewis, W., Blake, A. J., Collison, D., McInnes, E. J. L., Winpenny, R. E. P., Liddle, S. T. Chem. Sci. 2016, 7, 155. 12. Chen, Y.-C.; Liu, J.-L.; Ungur, L.; Liu, J.; Li, Q.-W.; Wang, L.-F.; Ni, Z.-P.; Chibotaru, L. F.; Chen, X.-M.; Tong, M.-L. J. Am. Chem. Soc. 2016, 138, 2829. 13. Liu, J.; Chen, Y.-C.; Liu, J.-L.; Vieru, V.; Ungur, L.; Jia, J.-H.; Chibotaru, L. F.; Lan, Y.; Wernsdorfer, W.; Gao, S.; Chen, X.-M.; Tong, M.-L. J. Am. Chem. Soc. 2016, 138, 5441. 14. Ding, Y.-S.; Chilton, N. F.; Winpenny, R. E. P.; Zheng, Y.-Z. Angew. Chem. Int. Ed. 2016, 55, 16071. 15. Harriman, K. L. M.; Brosmer, J. L.; Ungur, L.; Diaconescu, P. L.; Murugesu, M. J. Am. Chem. Soc. 2017, 39, 1420. 16. The Rare Earth Elements: Fundamentals and Applications, Atwood, D. A. Ed.; John Wiley & Sons Ltd: Hoboken, NJ, 2012. 17. Mills, D. P.; Liddle, S. T. Ligand Design in Modern Lanthanide Chemistry, in Ligand Design in Metal Chemistry: Reactivity and Catalysis, Stradiotto, M.; Lundgren, R. J. Eds.; John Wiley & Sons Ltd.: Hoboken, NJ, 2016; pp 330–363. 18. Evans, W. J.; Hughes, L. A.; Hanusa, T. P. J. Am. Chem. Soc. 1984, 106, 4270. 19. Evans, W. J.; Hughes, L. A.; Hanusa, T. P. Organometallics 1986, 5, 1285. 20. Schultz, M.; Burns, C. J.; Schwartz, D. J.; Andersen, R. A. Organometallics 2000, 19, 781. 21. Sitzmann, H.; Dezember, T.; Schmitt, O.; Weber, F.; Wolmershäuser, G.; Ruck, M. Z. Anorg. Allg. Chem. 2000, 626, 2241. 22. Visseaux, M.; Barbier-Baudry, D.; Blacque, O.; Hafid, A.; Richard, P.; Weber, F. New J. Chem. 2000, 24, 939.


Page 31 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


23. Weber, F. A.; Sitzmann, H.; Schultz, M.; Sofield, C. D.; Andersen, R. A. Organometallics 2002, 21, 3139. 24. Nocton, G.; Ricard, L. Dalton Trans. 2014, 43, 4380. 25. Turcitu, D.; Nief, F. Ricard, L. Chem. Eur. J. 2003, 9, 4916. 26. (a) Jaroschik, F.; Nief, F.; Ricard, L. Chem. Commun. 2006, 426; (b) Jaroschik, F.; Nief, F.; Le Goff, X.-F.; Ricard, L. Organometallics 2007, 26, 3552. 27. (a) Deacon, G. B.; Forsyth, C. M.; Jaroschik, F.; Junk, P. C.; Kay, D. L.; Maschmeyer, T.; Masters, A. F.; Wang, J.; Field, L. D. Organometallics 2008, 27, 4772; (b) Kelly, R. P.; Bell, T. D. M.; Cox, R. P.; Daniels, D. P.; Deacon, G. B.; Jaroschik, F.; Junk, P. C.; Le Goff, X. F.; Lemercier, G.; Martinez, A.; Wang, J.; Werner, D. Organometallics 2015, 34, 5624. 28. (a) Ruspic, C.; Moss, J. R.; Schürmann, M.; Harder, S. Angew. Chem. Int. Ed. 2008, 47, 2121; (b) Harder, S.; Naglav, D.; Ruspic, C.; Wickleder, C.; Adlung, M.; Hermes, W.; Eul, M.; Pöttgen, R.; Rego, D. B.; Poineau, F.; Czerwinski, K. R.; Herber, R. H.; Nowik, I. Chem. Eur. J. 2013, 19, 12272. 29. Evans, W. J.; Perotti, J. M.; Brady, J. C.; Ziller, J. W. J. Am. Chem. Soc. 2003, 125, 5204. 30. Berkefeld, A.; Piers, W. E.; Parvez, M.; Castro, L.; Maron, L.; Eisenstein, O. J. Am. Chem. Soc. 2014, 134, 10843. 31. Kaita, S.; Hou, Z.; Nishiura, M.; Doi, Y.; Kurazumi, J.; Horiuchi, A. C.; Wakatsuki, Y. Macromol. Rapid Commun. 2003, 24, 179. 32. Evans, W. J.; Seibel, C. A.; Ziller, J. W. J. Am. Chem. Soc. 1998, 120, 6745. 33. Evans, W. J.; Davis, B. L.; Champagne, T. M.; Ziller, J. W. Proc. Natl. Acad. Sci. USA 2006, 103, 12678. 34. Demir, S.; Zadrozny, J. M.; Nippe, M.; Long, J. R. J. Am. Chem. Soc. 2012, 134, 18546. 35. Demir, S.; Zadrozny, J. M.; Long, J. R. Chem. Eur. J. 2014, 20, 9524. 36. Demir,

S.;

Meihaus.

K.

R.;

Long,

J.

R.

J.

Organomet.

Chem.

DOI:10.1016/j.jorganchem.2017.10.035. 37. Demir, S.; Boshart, M. D.; Corbey, J. F.; Woen, D. H.; Gonzalez, M. I.; Ziller, J. W.; Meihaus, K. R.; Long, J. R.; Evans, W. J. Inorg. Chem. DOI:10.1021/acs.inorgchem.7b02390. 31 ACS Paragon Plus Environment


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38. (a) Stoyanov, E. S.; Stoyanova, I. V.; Reed, C. A. Chem. Eur. J. 2008, 14, 7880; (b) Lambert, J. B.; Zhang, S.; Ciro, S. M. Organometallics 1994, 13, 2430. 39. Gabbai, F. P.; Chirik, P. J.; Fogg, D. E.; Meyer, K.; Mindiola, D. J.; Schafer, L. L.; You, S.-L. Organometallics 2016, 35, 3255. 40. Jaroschik, F.; Nief, F.; Le Goff, X.-F.; Ricard, L. Organometallics 2007, 26, 1123. 41. Walter, M. D.; Bentz, D.; Weber, F.; Schmitt, O.; Wolmershaüser, G.; Sitzmann, H. New J. Chem. 2007, 31, 305. 42. Jaroschik, F. Ph.D. Thesis; École Polytechnique X, Palaiseau, France, 2007. 43. Wooles, A. J.; Mills, D. P.; Lewis, W.; Blake, A. J.; Liddle, S. T. Dalton Trans. 2010, 39, 500; (b) Evans, W. J.; Lee, D. S.; Rego, D. B.; Perotti, J. M.; Kozimor, S. A.; Moore, E. K.; Ziller, J. W. J. Am. Chem. Soc. 2004, 126, 14574. 44. Sur, S. K. J. Magn. Reson. 1989, 82, 169. 45. Shannon, R. D. Acta Cryst. Sect. A 1976, 32, 751. 46. Cole, M. L.; Jones, C.; Junk, P. C. J. Chem. Soc., Dalton Trans. 2002, 896. 47. Schlesener, C. J.; Ellis, A. B. Organometallics 1983, 2, 529. 48. L Schultz, M.; Boncella, J. M.; Berg, D. J.; Tilley, T. D.; Andersen, R. A. Organometallics 2002, 21, 460. 49. Da Re, R. E.; Keuhl, C. J.; Brown, M. G.; Rocha, R. C.; Bauer, E. D.; John, K. D.; Morris, D. E.; Shreve, A. P.; Sarrao, J. L. Inorg. Chem. 2003, 42, 5551. 50. Rinehart, J. D.; Long, J. R. Chem. Sci. 2011, 2, 2078. 51. Sievers, J. Z. Phys. B 1982, 45, 289. 52. Aquilante, F.; Autschbach, J.; Carlson, R. K.; Chibotaru, L. F.; Delcey, M. G.; De Vico, L.; Fernandez. Galván, I.; Ferré, N.; Frutos, L. M.; Gagliardi, L.; Garavelli, M.; Giussani, A.; Hoyer, C. E.; Manni, G.; Lischka, H.; Ma, D.; Malmqvist, P. Å.; Müller, T.; Nenov, A.; Olivucci, M.; Pedersen, T. B.; Peng, D.; Plasser, F.; Pritchard, B.; Reiher, M.; Rivalta, I.; Schapiro, I.; SegarraMartí, J.; Stenrup, M.; Truhlar, D. G.; Ungur, L.; Valentini, A.; Vancoillie, S.; Veryazov, V.; Vysotskiy, V. P.; Weingart, O.; Zapata, F.; Lindh, R. J. Comput. Chem. 2016, 37, 506. 53. Ungur, L.; Chibotaru, L. F. Phys. Chem. Chem. Phys. 2011, 13, 20086. 32 ACS Paragon Plus Environment

Page 33 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


54. Denning, R. G.; Harmer, J.; Green, J. C.; Irwin, M. J. Am. Chem. Soc. 2011, 133, 20644. 55. Goodwin, C. A., P.; Chilton, N. F. Vettese, G. F.; Moreno Pineda, E.; Crowe, I. F.; Ziller, J. W.; Winpenny, R. E. P.; Evans, W. J.; Mills, D. P. Inorg. Chem., 2016, 55, 10057. 56. Electron Paramagnetic Resonance of Transition Ions; Abragam, A.; Bleaney, B. Oxford University Press, Oxford, 1970. 57. Shrivastava, K. N. Phys. Stat. Sol. B 1983, 117, 437.

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