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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Homoleptic Imidophosphorane Stabilization of Tetravalent Cerium Natalie T. Rice,†,§ Jing Su,‡,§ Thaige P. Gompa,† Dominic R. Russo,† Joshua Telser,∥ Lukas Palatinus,# John Bacsa,† Ping Yang,*,‡ Enrique R. Batista,*,‡ and Henry S. La Pierre*,†,⊥

Inorg. Chem. Downloaded from pubs.acs.org by UNIV OF LOUISIANA AT LAFAYETTE on 04/04/19. For personal use only.

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School of Chemistry and Biochemistry and ⊥Nuclear and Radiological Engineering Program, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, United States ‡ Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States ∥ Department of Biological, Physical and Health Sciences, Roosevelt University, Chicago, Illinois 60605, United States # Institute of Physics, Czech Academy of Sciences, Na Slovance 2, 182 21 Prague 6, Czechia S Supporting Information *

ABSTRACT: The homoleptic complexes of cerium with the tris(piperidinyl)imidophosphorane ligand, [NP(pip)3]−, present the most negative Ce3+/4+ redox couple known (4.0 V shift from the Ce3+/4+ couple in 1 M HClO4(aq)] is established through reactivity studies. Spectroscopic studies (UV−vis, electron paramagnetic resonance, and Ce L3-edge X-ray absorption spectroscopy), in conjunction with density functional theory studies, reveal the dominant covalent metal−ligand interactions underlying the observed redox chemistry and the dependence of the redox potential on the binding of potassium in the inner coordination sphere.

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INTRODUCTION The redox chemistry of lanthanide1−11 and actinide12−33 molecular complexes has emerged as a powerful process with which to interrogate the role of the metal valence f and d orbitals in metal−ligand bonding. Cerium redox chemistry is particularly attractive for probing the role of ligand bonding on the metal valence electronic structure because of the neardegeneracy of the 4f and 5d orbitals and the availability of three formal oxidation states (2+, 3+, and 4+) in solution.4,6,34 The 3+/4+ redox couple is crucial to understanding the behavior of cerium materials that are of broad industrial importance.35−38 In molecular complexes, this couple is remarkably sensitive to the ligand coordination sphere and, prior to this study, spanned the range of +1.87 to −1.59 V versus normal hydrogen electrode (NHE; across aqueous and organic media and a range of electrolytes).34 A recent report quantified the oxidation-state dependence of 4f-orbital participation in covalent bonding: specifically, significant 4f covalent character in Ce−Cl bonds is found in the tetravalent [CeCl6]2− but is nearly absent in its trivalent congener, [CeCl6]3−.39 This measurement by Cl K-edge X-ray absorption spectroscopy (XAS) mirrors other reports indicating an increase in the 4f covalency in the lanthanides with increased oxidation state.40−64 However, recent synthetic studies have also indicated a significant dependence of the observed redox behavior (both thermodynamics and kinetics) of cerium complexes on electrostatic interactions and the identity of the supporting cations.65−67 Thus, the observed redox chemistry of cerium complexes is dependent on both covalent interactions between the metal and ligand (i.e., © XXXX American Chemical Society

energy, radial extent, and symmetry of ligand orbitals with respect to the metal valence orbitals) and electrostatic interactions between the complex and supporting ions. Ceria (CeO2) and the related cerium binary, CeN, illustrate the complexities of the electronic structure of cerium materials in the tetravalent and trivalent states, respectively. In ceria and other tetravalent lanthanide dioxides, the ground state is best described as multiconfigurational with mixing between 4fn5d0L and 4fn+15d0L states (where n = number of f electrons in the tetravalent ground state and L is a ligand hole).68 This ligand hole represents a ligand (O 2p) to metal 4f charge transfer. However, in the trivalent lanthanide sesquioxides (Ln2O3), the covalency is driven exclusively by O 2p to Ln 5d charge transfer. For the early lanthanides with a small 4f/5d gap (Ce, Pr, and Nd), the metal−ligand covalency is increased due to 4f/5d hybridization.69 This unusual electronic structure in ceria (i.e., stabilization of the tetravalent state via the metal− ligand covalency) and the metal−ligand covalency in early lanthanide trivalent sesquioxides (driven by metal valence orbital hybridization) belie the unique electronic structure in the formally trivalent cerium in CeN. These phenomena lead to the destabilization of f states in CeN to the extent that they hybridize with the conduction band.70,71 Our hypothesis is that molecular analogues of these materials can be designed in order to elucidate the role of covalent bonding to control the relative stability of trivalent and tetravalent cerium in weakfield complexes and materials. Received: February 7, 2019

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DOI: 10.1021/acs.inorgchem.9b00368 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Herein, the synthetic chemistry of dialkylamide-substituted imidophosphorane complexes is presented. Imidophosphoranes, [(R2N)3PN]−, which are isoelectronic to siloxides and cyclopentadienides (as 1σ and 2π donors), provide steric control through the substituents at phosphorus and bind metals through a single donor atom. They differ from siloxides in their degree of zwitterionic character and the energy and radial extent of the donor atom (N 2p vs O 2p; Scheme 1A).

inclusion of [2.2.2]cryptand during the reduction of 2−Ce gave 3−Ce−K222, with a chelated outer-sphere potassium ion, in 62% yield. The solution and solid-state structural features of these cerium complexes were established by 1H, 31P, and 13C NMR and single-crystal X-ray diffraction (SC-XRD). Relevant bond lengths and angles for the complexes described below are included in Table 1. The molecular structure of 2−Ce is shown

Scheme 1. (A) Resonance Form of Imidophosphoranes and (B) Synthetic Procedures for the Formation of 3−Ce− K(Et2O) and 3−Ce−K222 from 2−Ce

Table 1. Relevant Bond Lengths and Angles for Compounds 2−Ce, 3−Ce−K(DME)2, and 3−Ce−K222 compound metric

2−Ce

3−Ce−K(DME)2

3−Ce−K222

avg. Ce−N (Å)

2.20(2)

2.323(3)

avg. Nimido−P (Å) avg. Ce−N−P (deg) avg. N−Ce−N (deg)

1.42(4)

potassium-bound: 2.37(1), 2.31(1) potassium-bound: 1.529(6), 1.528(7) potassium-bound: 141.4(4), 174.1(4) 109.4(3)

166.9(1) 109.4(6)

1.530(3) 150.8(3) 109.5(1)

in Figure 1A. It crystallized in the R3 space group with whole molecule disorder and a commensurate modulated supercell on the 3-fold axis. Despite these challenges, the structure is sufficiently resolved to present in detail. The structure is pseudotetrahedral with a τ4 index of 0.91 and N−Ce−N bond angles of 111.8(6)° and 107.0(6)°.88 The Ce−N bond lengths average 2.20(2) Å, three of which are equivalent at 2.17(3) Å with Ce−N−P angles of 153.7(15)° and P−Nimido bond lengths of 1.48(2) Å. The inequivalent Ce−N5 bond length is 2.27(6) Å with a Ce−N5−P1 angle of 180° and lies on the 3fold axis with a P1−N5 bond length of 1.42(6) Å. The average Ce−N bond length in 2−Ce is slightly shorter than those observed in homoleptic cerium amide complexes: [Ce(N(iPr2)4], 2.224(1) Å; [Ce(N(SiHMe2)2)4], 2.247(6) Å.89,90 The complex 3−Ce−K(Et2O) was initially crystallographically characterized as the bis(diethyl ether) adduct 3−Ce− K(Et2O)2 (see Figure S18 and Tables S1 and S9). This structure suffers from a modulated superstructure with weak superlattice reflections that are insufficient to resolve disorder in the three molecules in the asymmetric unit. Therefore, it was further structurally characterized as its bis(1,2-dimethoxyethane) adduct, 3−Ce−K(DME)2, via crystallization from DME. The cerium ion in the molecular structure is also pseudotetrahedral with a τ4 index of 0.93 and N−C−N bond angles ranging from 96.5(3)° to 115.0(2)° (Figure 1B). The structure features an inner-sphere potassium atom that is bridged by two of the imido nitrogen atoms and further bound by two coordinating DME molecules. The two equivalent Ce− N bond lengths for the potassium-bound ligands are 2.37(1) Å with a Ce−N−P angle of 141.4(4)° and a P−Nimido bond length of 1.529(6) Å. The equivalent Ce−N bond lengths for the terminal ligands are 2.31(1) Å with a Ce−N−P angle of 174.1(4)°. The P−N bond length in this case is 1.528(7) Å, very comparable to that of the potassium-bound ligands and notably longer than that in 2−Ce. Both types of Ce−N bonds in 3−Ce−K(DME)2 lengthen in comparison to 2−Ce, which is consistent with a decrease in the oxidation state (the ionic radius of Ce3+ is 1.01 Å in comparison to that of Ce4+, 0.87 Å).91 The coordination geometry of the Ce3+ ion in 3−Ce−K222 is nearly tetrahedral with a τ4 index of 0.95 and N−Ce−N

In contrast to the few previous studies of imidophosphorane chemistry employing alkyl or aryl substituents,72−86 dialkylamide substituents (piperidine in this case) amplify this zwitterionic character by stabilizing the P cationic character and terminal nitrogen N2− character (Scheme 1A). Spectroscopic, reactivity, and theoretical studies of homoleptic trivalent and tetravalent complexes of cerium supported by the tris(piperidinyl)imidophosphorane ligand, [NP(pip)3]− [pip = piperidinyl, a six-membered ring, N(C5H10)], indicate that covalent interactions between the metal and ligand govern the most significant stabilization of tetravalent cerium observed to date.34,87 The binding of a potassium ion in the inner coordination sphere of the trivalent complex contributes significantly to the thermodynamic driving force for complex oxidation.

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RESULTS AND DISCUSSION Synthesis and Characterization. The novel ligand tris(piperidinyl)imidophosphorane, [NP(pip)3]−, is prepared as the potassium salt K[NP(pip)3] (1−K) in a tetrameric cluster (Figure S14). The redox pair of homoleptic imidophosophorane complexes [Ce(NP(pip)3)4] (2−Ce) and [(Et2O)KCe(NP(pip)3)4], 3−Ce−K(Et2O), were prepared by sequential reactions: initial salt metathesis and in situ oxidation to give 2−Ce, followed by reduction to give 3−Ce− K(Et2O). Specifically, the addition of 4 equiv of 1−K to a mixture of 1 equiv of CeI3(THF)4 (THF = tetrahydrofuran) and 0.5 equiv of iodine in diethyl ether afforded the tetrahomoleptic, pseudotetrahedral Ce4+ compound 2−Ce in 71% yield. Reduction of 2−Ce in diethyl ether with KC8 proceeded within 10 min to give the Ce3+ compound 3−Ce− K(Et2O) in 68% yield with an inner-sphere potassium ion. The B

DOI: 10.1021/acs.inorgchem.9b00368 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. (A) Molecular structure of 2−Ce with thermal ellipsoids shown at 50% probability. Carbon and hydrogen atoms are omitted for clarity. (B) Molecular structure of 3−Ce−K(DME)2 with thermal ellipsoids shown at 50% probability. Carbon and hydrogen atoms as well as two DME molecules coordinated to potassium are omitted for clarity. (C) Molecular structure of 3−Ce−K222 with thermal ellipsoids shown at 50% probability. Hydrogen and carbon atoms on the core structure and one diethyl ether molecule in the unit are omitted for clarity.

cence is observed, implying a low barrier to intramolecular potassium exchange within the complex. Reactivity Studies. The solution redox behaviors of 2−Ce, 3−Ce−K(Et2O), and 3−Ce−K222 were surveyed with a spectrum of single-electron oxidants and reductants (Table 2

angles ranging from 106.88(9) to 113.55(11)° (Figure 1C). With the potassium ion in the outer sphere, all Ce−N bonds are unique and span from 2.29(3) to 2.339(2) Å [2.323(3) Å avg.], with Ce−N−P bond angles ranging from 142.79(15) to 160.1(2)° [150.8(3)° avg.], falling between the two different Ce−N−P angles of 3−Ce−K(DME)2. Additionally, the Ce−N bond lengths for this compound are overall slightly shorter than those in the potassium-bound complex but close to the Ce−N bond for the unbound ligands in 3−Ce−K(DME)2. The P−N bond lengths range from 1.522(2) to 1.535(3) Å, very similar to the bond lengths observed in 3−Ce−K(DME)2. The closest analogous complexes in the literature to 3−Ce− K(DME)2 and 3−Ce−K222 are neutral and anionic Ce3+ homoleptic amide complexes. The distorted trigonal-planar, homoleptic bis(trimethylsilyl)amide complex [Ce(N(SiMe3)2)3] has an average Ce−N bond length of 2.320(3) Å, while a pseudotetrahedral cerate complex, [LiCe(N(iPr2))4], has a Ce−N average bond length of 2.378(5).89,92 These Ce−N bond lengths are similar to those observed in 3− Ce−K(DME)2 and 3−Ce−K222. The average Ce−N bond length of the charge-paired salt, Na[Ce(N(SiMe3)2)4], is 2.442(6) Å, which is longer than that found in 3−Ce−K222.93 However, both redox pairs for [LiCe(N(i-Pr2)4] and 3−Ce− K222 shorten to a similar degree upon oxidation to the tetravalent state. Solution spectroscopic characterization of 2−Ce, 3−Ce− K(Et2O), and 3−Ce−K222 agree with that of the structures determined in the solid state. The high symmetry of the complexes in the solid state is most clearly seen in solution in the 31P NMR spectra. For 2−Ce, a single 31P NMR shift at −12.30 ppm is observed as expected for a tetrahomoleptic, formally closed-shell, monometallic compound.53 The 31P NMR spectrum for 3−Ce-K222 exhibits a single downfield resonance at 99.79 ppm. The observed single shift is consistent with that expected for the paramagnetic, tetrahomoleptic, monometallic complex. While 3−Ce−K(Et2O) has distinct coordination environments for ligands in the inner coordination sphere, so that two 31P resonances in solution would be expected, only a single 31P resonance is observed at 85.77 ppm at ambient temperature. Variable-temperature 31P NMR studies from +25 to −75 °C (Figure S10) evince a shift in the resonance to higher field and a broadening of the signal. However, within this temperature range, no peak decoales-

Table 2. Experimental Reduction Potentials and Comparison to Relevant Literature Compounds compound

potentiala

event type and solvent

Ce (8 M HClO4) 2−Ce [Ce(NP(NMe2)3)4] 3−Ce−K(Et2O) [K(DME)2Ce(NP(NMe2)3)4] [Ce(pyNO)4] Ce[O2C6H4]4 [Ce(L)(OtBu)2]

1.87b −2.30 < x < −2.47c −2.42d −2.64 < x < −3.10c −2.99d −1.95, −2.09e −1.00f −2.39g

E1/2 Epc in THF E1/2 Epa in THF E1/2 E1/2, Epc in DCM E1/2 in 5 M NaOH Epc in THF

a Potential in volts versus Fc/Fc+ unless otherwise noted. bPotential versus NHE. Reference 103. cPotential experimentally determined in this work. dPotential theoretically calculated in this work. eReference 87. fReference 105. gReference 106.

Figure 2. Chemical bounding of reduction potentials for 2−Ce, 3− Ce−K(Et2O), and 3−Ce−K222. C

DOI: 10.1021/acs.inorgchem.9b00368 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry and Figure 2).94 The reactions of the neutral tetrahomoleptic, formally tetravalent 2−Ce complex with KC8 [−2.94 V vs NHE in water (H2O) for K0], Na0 [−3.04 V vs ferrocene/ ferrocenium (Fc/Fc+) in THF and glyme], Na[anthracenide] (−2.47 V vs Fc/Fc+ in glyme), and Na[benzophenone] (−2.30 V vs Fc/Fc+ in THF) in THF-d8 were followed by 31P NMR.94−97 The outer-sphere reduction was found to lie between −2.47 and −2.30 V vs Fc/Fc+: no reduction of 2−Ce with Na[benzophenone] was observed. The oxidation potential of 3−Ce−K(Et2O) in THF was bracketed by reactions with trityl chloride [−0.11 V vs Fc/Fc+ in acetonitrile (MeCN)], benzophenone, anthracene, 2,6-ditert-butylanthracene [−2.64 V vs Fc/Fc+ in THF],98 and naphthalene [−3.10 V vs Fc/Fc+ in THF] in THF-d8.99,100 The complex 3−Ce−K(Et2O) was oxidized cleanly by 2,6-ditert-butylanthracene, but no reaction with naphthalene was observed. Therefore, the oxidation potential for 3−Ce− K(Et2O) lies between −3.10 and −2.64 V versus Fc/Fc+. This significant asymmetry in the redox properties of the 2− Ce/3−Ce−K(Et2O) redox pair suggests that the binding of potassium in the inner coordination sphere of 3−Ce−K(Et2O) contributes significantly to the thermodynamic driving force for electron transfer from 3−Ce−K(Et2O) to the oxidant. To gain further experimental insight into this observed dependency of the redox couple on the binding of potassium in the inner coordination sphere in the trivalent state, oxidation of the trivalent cerium complex with a sequestered outersphere supporting cation, 3−Ce−K222, was examined. The complex 3−Ce−K222 reduces benzophenone, cleanly giving 2−Ce. If the outer-sphere ion were to yield a formally reversible couple to the reduction of 2−Ce, it would be expected to have no reaction with either 2,6-di-tertbutylanthracene or anthracene. In practice, some redox activity with both reagents is observed: 20% of 2-Ce is formed in the 1:1 reaction with anthracene along with some of the protonated ligand [HNP(pip)3] and with 77% of unreacted 3−Ce−K222 (similar results are observed with 2,6-di-tertbutylanthracene). These results suggest that the reduction potential of the trivalent ion in 3−Ce−K(Et2O) is attenuated by sequestration of the potassium ion (a result supported by the theoretical analysis, vide infra). However, inclusion of the cryptand also changes the thermodynamic stability of the products and introduces the possibility of competing mechanisms, as indicated by the formation of [HNP(pip)3]. Protonlysis or oxidation of 3−Ce−K222 by adventitious H2O or O2 cannot be ruled out. Direct, quantitative measurement of the redox properties of these complexes via electrochemical methods at a glassycarbon electrode in low-polarity organic solvents compatible with the reaction profile of our ligand and complexes proved intractable. A range of electrolytes, solvents, and temperatures were examined including [nBu4N][PF6], [nBu4N][BF4], [nPr4N][B(C6F5)4], and [nPr4N][B(C6H3(3,5-bis-CF3))4] salts in THF and [K][B(C6F5)4] and [Na][B(C6H3(3,5-bisCF3))4] in diethyl ether at room temperature and −35 °C.101 In all cases, no redox events associated with the complexes or ligands were observed and 1H and/or 31P NMR of the electrolyte/analyte solutions reveal complex disproportionation and decomposition in the presence of the electrolyte. Table 2 summarizes the redox properties of the complexes along with their predicted properties (vide infra) and those of related cerium complexes. As has been noted by other investigators, the Ce3+/4+ redox couple is extremely sensitive

to the coordination environment and media dielectric.34,102 While comparisons between different media (aqueous and organic) are not necessarily directly reflective of changes in the coordination environment, they are useful for demonstrating the wide range of known redox potentials. The Ce3+/4+ standard potential in 1 M HClO4 is 1.70 V versus NHE (1.87 V in 8 M HClO4).103 For the sake of comparison, this latter value is 1.30 V versus Fc/Fc+ in H2O.94 The aqueous redox chemistry of cerium is bounded at the other extreme by the octadentate catecholate complex [Ce(O2C6H4)4], which has the potential of −1.00 V versus Fc/Fc+ in 5 M NaOH.104 In contrast, in organic media, cerium ammonium nitrate has the most positive potential for the Ce3+/4+ couple at 0.62 V versus Fc/Fc+ in MeCN.105 Significantly stronger thermodynamic reductants have been established for cerium. The most negative reversible Ce3+/4+ redox couple to date is that observed by Schelter and coworkers using a pyridine nitroxide ligand to yield [Ce(pyNO)4], which has an observed reduction potential of −1.95 versus Fc/Fc+ in dichloromethane (DCM; Epc = −2.09 V vs Fc/Fc+).87 Like many of the benchmark systems including [Ce(O2C6H4)4], [Ce(pyNO)4] is eight-coordinate. Spectroscopic evidence and computational modeling indicate that the significant stabilization of the tetravalent state in this complex is based on the covalent overlap of filled ligand orbitals with the empty metal valence orbitals. Diaconescu and co-workers have used a heteroleptic ligand system, which included iminophosphoranes in the coordination sphere, and have demonstrated an irreversible reduction potential with an Epc of −2.39 V versus Fc/Fc+ in THF.106 The 3−Ce−K(Et2O) compound reported here has the most negative reduction potential in the literature, a minimum of 0.69 V more negative than E1/2 for a reversible couple for cerium and 0.25 V more negative than an irreversible couple for cerium.87,103 As established in the following section, the reduction potential of 3−Ce−K(Et2O) likely lies significantly more negative, theoretically predicted at −2.99 V versus Fc/Fc+. The redox properties of lower-coordinate complexes of cerium, particularly with nitrogen donors, have not been explored experimentally or computationally. While reference data exist for the comparison of our structural data to tetrahomoleptic amides for the Ce3+/4+ systems reported here, relevant redox data have not been published.89,90,92,93 By changing to zwitterionic ligands with single nitrogen donors, the complexes reported here have a substantially greater stabilization of the tetravalent center than has been observed using oxygen donors. Imidophosphoranes with nitrogen donors, which are hard donor atoms with ionization potentials lying between carbon and oxygen donors, lead to localized charge directly proximate to the metal center. Because imidophosphoranes are 1σ and 2π donors with respect to the metal−ligand bond, in pseudo-Td coordination environments, the complexes have the correct symmetry and ligand orbital energy to effectively covalently bond with tetravalent lanthanides.39 This analysis is supported by the following computational and spectroscopic studies. Electronic Structure and Predicted Redox Properties. Density functional theory (DFT) calculations were carried out on Ce3+/4+ complexes supported by the [NP(NMe2)3]− ligand, which is a truncated form of [NP(pip)3]− with similar electronic properties, to gain insight into the electronic structures and redox properties of 2−Ce, 3−Ce−K(DME)2, and 3−Ce−K222. The computed average bond distances of D

DOI: 10.1021/acs.inorgchem.9b00368 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry [Ce(NP(NMe2)3)4]0/− and [(DME)2KCe(NP(NMe2)3)4]+/0 compounds, as well as the experimental crystal structure data, are summarized in Table S12. The computed structural metrics are in reasonable agreement with the XRD data, with Ce−N, K−N, and K−O bond distances within 2%, 3%, and 1% of the experiment, respectively, providing validity for the theoretical modeling. Calculated oxidation potentials of the Ce3+ complexes are shown in Table 3 (see the Theoretical Calculations section for

Table 4. Mulliken Charge (q) on a Cerium Atom in the [Ce(NP(NMe2)3)4]0/− and [Ce(pyNO)4]0/− Compounds compound [Ce(NP(NMe2)3)4] [Ce(NP(NMe2)3)4]− [Ce(pyNO)4] [Ce(pyNO)4]−

This analysis is supported by the valence molecular energy level diagram in Figure 3 and the percentage of the metal component in the main bonding orbitals in Figure S25. As mentioned before, [NP(NMe2)3]− is a one σ and two π nitrogen-donor ligand. A tetrahedral arrangement of four [NP(NMe2)3]− ligands around a central Ce4+/Ce3+ center generates four symmetry-adapted linear combinations (SALCs) of N 2p atomic orbitals that have σ symmetry and

Table 3. Calculated Oxidation Potentials in Volts of Ce3+ Complexes in THF versus Fc0/+a half-reactions

calcd

[Ce(NP(NMe2)3)4]− → [Ce(NP(NMe2)3)4]

−2.42

[K(DME)2Ce(NP(NMe2)3)4] → [K(DME)2] [Ce(NP(NMe2)3)4]+ [K(DME)2Ce(NP(NMe2)3)4] → [Ce(NP(NMe2)3)4] + K(DME)2+ [Ce(pyNO)4]− → [Ce(pyNO)4]

−2.09 −2.99 −2.08

q(Ce) 1.22 1.20 1.88 1.78

exptl −2.47 < x < −2.30 −3.10 < x < −2.64 −1.95

a

The calculated absolute redox potential of Fc0/+ is 5.436 V.

computational details). The predicted potential of the [Ce(NP(NMe2)3)4]−/[Ce(NP(NMe2)3)4] couple is −2.42 V versus Fc/Fc+. If the binding of K(DME)2+ is considered for both members of the redox couple (i.e., [(DME)2KCe(NP(NMe2)3)4]/[(DME)2KCe(NP(NMe2)3)4]+), the value shifts to −2.09 V. This positive shift is the result of K(DME)2+ cation association to the neutral [Ce(NP(NMe2)3)4], which is thermodynamically unfavored (calculated to have a Gibbs free energy of +20.6 kcal/mol). Therefore, the dissociation will happen spontaneously for [(DME)2KCe(NP(NMe2)3)4]+ to produce [Ce(NP(NMe2)3)4] and K(DME)2+ in a THF solution. Accounting for this spontaneous dissociation results in an oxidation potential for [(DME)2KCe(NP(NMe2)3)4]/ [Ce(NP(NMe 2 ) 3 ) 4 ] of −2.99 V versus Fc/Fc + . This accompanying counterion/ligand dissociation in the redox process has also been observed in transition-metal complexes.107,108 The predicted potential of the [Ce(NP(NMe2)3)4]−/ [Ce(NP(NMe2)3)4] couple is 0.34 V more negative than the reported result of the [Ce(pyNO)4]−/[Ce(pyNO)4] couple.87 To understand differences in the ligand coordination and the effect on redox properties, the thermodynamics of ligand replacement reactions were calculated with the results shown in Table S13. The Gibbs free energy of [NP(NMe2)3]− replacing [pyNO]− is 1.3 kcal/mol in Ce4+ complexes and as high as 9.1 kcal/mol in the Ce3+ complexes. This comparison suggests that these two ligands show a similar stabilization ability for the Ce4+ ion but that the [pyNO]− ligand stabilizes Ce3+ much more than [NP(NMe2)3]−. This difference in stability is probably due to the much stronger ionic interaction of PyNO− with the metal ion in a bidentate coordination fashion, in contrast to the monodentate coordination of [NP(NMe2)3]−. The covalent interactions between the ligands and metal ion can be inferred from the Mulliken charge analysis in Table 4. These results show that the charge on the cerium center is more positive in [pyNO]− complexes than [NP(NMe2)3]− complexes by 0.6−0.7e for both Ce4+ and Ce3+ oxidation states. This change in the effective charge at the metal suggests that electron donation from [NP(NMe2)3]− to the metal center is greater than that from [pyNO]−.

Figure 3. Valence molecular orbital energy levels of [Ce(NP(NMe2)3)4]. Bonding orbitals are in red, with the cerium atomic orbital compositions in Figure S25. The Z direction is labeled by the arrow. E

DOI: 10.1021/acs.inorgchem.9b00368 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry eight SALCs of π symmetry with respect to the Ce−N axes, respectively. Symmetry-allowed mixing between these N 2p SALCs and Ce 4f and 5d orbitals generates a manifold of Ce− N σ- and π-bonding orbitals. The three σ (i.e., HOMO−20, −21, and −22) and two π (i.e., HOMO−10 and −11) bonding orbitals of the Ce 5d−N 2p type and one π-bonding interaction of the Ce 4f−N 2p type (i.e., HOMO) in [Ce(NP(NMe2)3)4] are shown in Figure 3. The bonding picture of [Ce(NP(NMe2)3)4]− is similar to the Ce4+ complex except the Ce 4f−N 2p type π-bonding interaction is missing, as indicated in Figure S25. The main covalent bonding interaction is between N 2p orbitals of [NP(NMe2)3]− and Ce 5d orbitals for both [Ce(NP(NMe2)3)4] and [Ce(NP(NMe2)3)4]−, with the Ce 5d contribution no more than 12.3% and 8.5% in the Ce4+ and Ce3+ complexes, respectively. In the tetravalent [Ce(NP(NMe2)3)4] complex, one Ce 4f orbital shows a small orbital mixing with N 2p in HOMO at about 5%. The trends of these 4f and 5d covalent interactions are in line with the observed changes in the covalency for triand tetravalent lanthanides established spectroscopically. Spectroscopy and Magnetism. The electronic absorption spectra of the three yellow compounds, 2−Ce, 3−Ce− K(Et2O), and 3−Ce−K222, in THF are consistent with the expected oxidation states and the strong covalent bonding indicated by the theoretical model (Figures 4A and S20−S22). The absorption spectrum of 2−Ce shows a broad transition at 335 nm (3.70 eV) with a molar absorptivity of 11000 M−1 cm−1, consistent with that of ligand-to-metal charge transfer (LMCT) and the assignment of a Ce4+ ion. This high-energy LMCT is distinct from the LMCT absorptions typically observed for nitrogen-donor-supported complexes, which are usually more strongly colored dark blue or purple, indicating LMCT transitions at the lower energies of the visible spectrum.87,89,109,110 Complexes of oxygen-donor ligands, such as catecholates, are red, indicating a slightly higherenergy LMCT transition.104 In contrast to these compounds, the yellow 2−Ce has a higher-energy LMCT despite nitrogen having a lower ionization energy than oxygen, indicating a significant orbital overlap (covalency) and larger energy gap between the filled ligand and empty metal valence orbitals. On the basis of the forgoing theoretical analysis, we suggest that this high-energy LMCT may be related to the unique charge localization in the [NP(pip)3]− ligand. For 3−Ce−K(Et2O), the LMCT is shifted to a higher energy in the UV, and a lower-intensity absorption appears at 366 nm (3.39 eV) with a molar absorptivity of 600 M−1 cm−1, consistent with an f−d transition as expected for a Ce3+, 4f1 ion. For the outer-sphere complex 3−Ce−K222, this f−d transition is shifted to a slightly higher energy (351 nm and 3.53 eV) and gains intensity (molar absorptivity of 1115 M−1 cm−1). This change in intensity of the f−d absorption is likely related to the removal of the potassium ion and the accompanied change in local symmetry of the molecular ion from C2 to pseudo-Td, hence affecting the electronic structure and transition dipole moment.111 The trivalent complex 3−Ce−K(Et2O) was evaluated by Xband electron paramagentic resonance (EPR) and directcurrent magnetic susceptibility measurements to confirm the spin localization at the metal and to evaluate the nature of the metal-valence ground state. The EPR spectrum of 3−Ce− K(Et2O) recorded at 3.6 K in toluene is shown in Figure 4B. The observed spectrum is completely attenuated at 20 K. This attenuation is attributed to spin−lattice relaxation at higher

Figure 4. (A) Molar absorptivity plots for 2−Ce (purple) and 3− Ce−K(Et2O) (pink) in THF. (B). X-band (9.364 GHz) EPR spectrum of 3−Ce−K(Et2O) in toluene glass at 3.6 K (inset: χT vs T plot of 3−Ce−K(Et2O)).

temperatures (consistent with sharp room temperature 31P and H NMR spectra), which strongly suggests that the unpaired electron is cerium-based rather than ligand-based.112 In the 3.6 K spectrum, two broad features are resolved at g ≈ 3.6 and 1.1. A third feature is expected for the rhombic spectrum at g ≈ 0 (vide infra) and is out of the spectrometer magnetic-field range. These observations give an average giso > 1.56, which is reduced from the expected value (ge = 2.0023) for a pure S = 1 /2 system due to spin−orbit coupling (SOC; vide infra). The line widths for the observed features are quite large (greater than 2.5 GHz) and prevent effective simulation of the spectrum.113 However, the observed μeff at 4 K (1.85 μB) is in good agreement with that predicted from the EPR spectrum [4(μeff)2 = gz2 + gx2 + gy2] of 1.88 μB (with gy = 0), which implies that that the ground state is observed in the EPR experiment.51 There are relatively few EPR studies of molecular f1 ions (particularly low-symmetry systems), and most have focused on U 5+ ions, 114−124 with limited comparisons to Ce3+ ions49,51 and no comparisons to Pa4+. Following the analysis of Gourier and Andersen on the EPR spectra of U5+ and Ce3+ in the higher symmetry complexes, [U(C7H7)2]− and [Ce(COT)2]−, respectively, analysis can be built from the free ion but is limited by the lack of high axial 1

F

DOI: 10.1021/acs.inorgchem.9b00368 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry symmetry.51,119 The Ce3+ free-ion term is 2F, and SOC splits this state into two levels with J = 5/2 (ground state) and J = 7/2 (excited state). The crystal field lifts the degeneracy of these J states and splits the ground state into three Kramers doublets: |J, MJ⟩ = |5/2, ±1/2⟩, |5/2, ±3/2⟩, and |5/2, ±5/2⟩. Because, at best, 3−Ce−K(Et2O) could be considered C2 in solution, mixing between the MJ states of ΔMJ = ±2 is expected and evident in the observed spectrum. Mixing with the excited-state manifold is also possible (i.e., |7/2, ±1/2⟩). Studies on highersymmetry complexes are necessary to see more clearly the effect of the imidophosphorane ligand field on the splitting of Ce3+ free-ion terms. Because the 2F-ion excited state, 2F7/2, is not populated at room temperature,51,125 the observed Ueff = 2.44 μB (0.60 χT) at 300 K agrees well with the expected moment for J = 5/2 of 2.54 μB and is in agreement with that observed for other Ce3+ systems (generally 1.88−2.60 μB).126−128 The plot of χ−1 versus T (Figure S24) deviates from linearity below about 175 K and reflects the thermal depopulation of the higher-lying MJ Kramers doublets. As seen in the χT versus T plot (Figure 4B, inset), there is a precipitous drop below 10 K related to the further depopulation of either thermally excited crystal-field states or intermolecular antiferromagnetic interactions.51,87,129 Ce L3-Edge X-ray Absorption Near-Edge Spectroscopy (XANES). In order to spectroscopically interrogate the covalent basis of the observed redox behavior of 2−Ce and 3− Ce−K(Et2O), cerium L3-edge XANES was employed. The L3edge probes electric-dipole-allowed transitions from Ce 2p orbitals to unoccupied states with Ce 5d character (i.e., 2p64fn5d0 → 2p54fn5d1, where n corresponds to the number of f electrons in the ground state). The transition is sensitive to both the ground state 4f occupation and the effective nuclear charge on the cerium ion (i.e., differences in charge donation from the ligand field). Figure 5 compares the curve-fit analysis of the backgroundsubtracted and normalized spectra of 2−Ce and 3−Ce− K(Et2O) recorded at 15 K. The absorption spectrum of 3− Ce−K(Et2O) is typical for that of a Ce3+ ion in that the spectrum presents a single white-line feature (Figure 5A). The inflection point of the rising edge is 5722.7 eV. In comparison, the inflection point is shifted to lower energy than Ce3+ complexes such as [CeCl6]3− at 5724.5 eV.39 This shift to lower energy reflects the decreased effective nuclear charge at cerium in 3−Ce−K(Et2O) in comparison to [CeCl6]3−. Furthermore, this shift is consistent with the Mulliken analysis for the model complexes, which indicate substantially increased charge donation to the metal center from the imidophosphorane ligands in comparison to other ligand systems (even those with higher coordination numbers). In the spectrum of the tetravalent complex 2−Ce, the inflection point of the rising edge is 1.7 eV higher, at 5723.6 eV, than that of 3−Ce−K(Et2O), indicating an increase in the oxidation state. Similarly, [CeCl6]2− (formally Ce4+) has a rising edge inflection point at 5724.7 eV, 1.6 eV higher than that of its Ce3+ counterpart. The overall shape of the absorption spectrum of 2−Ce is similar to that of other Ce4+ compounds in that it exhibits a white-line doublet, as shown in Figure 5B.39,53,68,87,130−133 The origin of this feature is the subject of significant debate and is thought to be a consequence of either a multiconfigurational ground state or a final state effect. Following with the former argument, the multiconfigurational ground state consists of both a 4f1L openshell singlet [Ce3+ character with a ligand hole (L),

Figure 5. Ce L3-edge XAS experimental data (black) obtained for 3− Ce−K(Et2O) (A) and 2−Ce (B) and the pseudo-Voigt [purple (p2) and pink (p3)] and steplike functions (gray dashed line), which sum to generate the curve fit (red). The shoulder feature observed near the edge onset has little intensity in 2−Ce and is modeled with a single function [blue (p1)] that is barely visible in the baseline of the spectrum.

corresponding to p2] and a 4f0L closed-shell singlet (Ce4+ character corresponding to p3), which have 4f15d1L and 4f05d1L excited states, respectively. The 4f 1 character of the ground state shields the core hole, which results in a shift of the peak energy corresponding to the Ce3+ character to lower energy than the Ce4+ contribution.39,52,53 There is also a weak pre-edge feature that is attributable to a quadrupole-allowed 2p3/2 → 4f transition, p1 (blue trace).39 It should be noted that there are weak features that are either obscured or unresolved in this spectrum and may be resolved with higher resolution techniques.134 With these features in mind, the 2−Ce absorption edge is fit with three Voigt functions, one step function, and a Voigt function to account for the first negative extended X-ray absorption fine structure (EXAFS) oscillation. Within the restrictions of this two-peak model, the relative proportion of f1 character (nf) can be determined by the weighted relative intensity of p2 to the total weight of p2 (purple trace) and p3 (pink trace) (nf = Ap2/(Ap2 + Ap3), where A is the intensity of the peak). The value of nf is sensitive to the curve-fit methodology. There are two approaches: (1) modeling the absorption to continuum states with a steep step function at the rising edge39 and (2) employing a broader step function at the weighted average of the energies of p2 and p3.53,68 In this report, we have employed the former model, which gives a lower limit to the nf value of 2−Ce, 0.21(1) (see Supporting Information for further details). G

DOI: 10.1021/acs.inorgchem.9b00368 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

electron donation to the metal: a phenomenon supported by Mulliken charge analysis, which shows a smaller positive charge on 2−Ce than related complexes. Further synthetic and spectroscopic studies are necessary to understand ligand control of the multiconfigurational behavior and redox properties through tuning of the metal−ligand covalency and electrostatic interactions.

Recent spectroscopic and magnetic studies have explored the role of the ligand field in controlling the mixed-valence ground state in tetravalent cerium complexes.53,68 Where 2− Ce differs from other formally Ce4+ compounds is the relative distribution of the f0L (Ce4+) and f1L (Ce3+) character in the ground state. This relative difference can be immediately seen from the shape of the white-line doublet in comparison to that of other compounds. In the spectrum of 2−Ce, the intensity of the lower-energy feature of the white-line doublet is significantly decreased, leading to the lowest nf reported to date, 0.21(1). The contribution of 4f1 character is significantly less than other formally Ce4+ compounds such as cerocene [nf = 0.82(3)], ceria [nf = 0.58(3)], and [CeCl6]2− [nf = 0.51(5)].39,53,68 The nf value and energy of p2 observed for 2−Ce do not follow the loose correlation noted in previous studies between decreasing nf and increasing energy of p2. This discrepancy implies that the metal−ligand covalency is not the only property that governs the mixed-valent ground states in cerium complexes (as observed in the magnitude of nf). As noted, the covalency is a function of both orbital overlap and energy degeneracy.135,136 In the case of orbital-overlap-driven covalency, electron density accumulation at the metal increases with increased covalency. Therefore, the energy of p2 at the Ce L3-edge of Ce4+ is probably the most indicative spectroscopic feature of covalency. This value should shift to lower energy with increased covalency and correlate with an effective charge at cerium (for which Mulliken charge is an appropriate measure).53 In 2−Ce, the energy of p2 is 5728.2(1) eV, only slightly higher than the energy of p1 in 3−Ce−K(Et2O), 5725.2(1) eV, implying significant covalency in 2−Ce. The effective charge at the metal (cerium) is dependent on the ligand field.137−140 Future studies will explore ligand-donor properties that govern the nature of the mixed-valent ground state within a conserved ligand field.

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EXPERIMENTAL SECTION

General Considerations. Unless otherwise noted, all reagents were obtained from commercial suppliers, and the syntheses and manipulations were conducted under argon with exclusion of dioxygen (O2) and H2O using Schlenk techniques or in an inertatmosphere box (Vigor) under a dinitrogen (8 h) at a temperature of ca. 160 °C. Celite and molecular sieves were dried under vacuum at a temperature >250 °C for a minimum of 24 h. C6D6 was stored over 3 Å molecular sieves and then vacuum-transferred from purple sodium/benzophenone prior to use. Diethyl ether, npentane, n-hexane, benzene, toluene, tetrahydrofuran (THF), and 1,2dimethoxyethane (DME) were purged with UHP-grade argon (Airgas) and passed through columns containing Q-5 and molecular sieves in a solvent purification system (JC Meyer Solvent Systems). All solvents in the glovebox were stored in bottles over 3 Å molecular sieves. Methanol was dried by refluxing over magnesium turnings activated with iodine for 12 h, then distilled, and stored over 3 Å molecular sieves. THF adducts of lanthanide triiodide starting materials were prepared according to literature procedures.141 Tris(piperidinyl)phosphine was prepared according to a published procedure.142 Potassium benzyl was prepared according to a published procedure.143 Potassium tert-butoxide was sublimed prior to use. NMR spectra were obtained on a Bruker Advance III 400 MHz spectrometer at 298 K, unless otherwise noted. 1H, 13C, and 31P NMR chemical shifts are reported in δ, parts per million. 1H NMR are references to the residual 1H resonances of the solvent. 13C NMR are referenced to the 13C resonance of the deuterated solvent.144 The peak position is listed, followed by the peak multiplicity, integration value, and proton assignment, where applicable. The multiplicity and shape are indicated by one or more of the following abbreviations: s (singlet); d (doublet); t (triplet); q (quartet); dd (doublet of doublets); td (triplet of doublets); m (multiplet); br (broad). IR samples were taken on a Bruker ALPHA FTIR spectrometer with an attenuated-total-reflectance attachment from 400 to 4000 cm−1. The peaks are listed in wavenumber [cm−1] and intensity using the following abbreviations: vw (very weak); w (weak); m (medium); s (strong); vs (very strong); br (broad). UV−vis spectroscopy was performed in Teflon-valve-sealed quartz cuvettes with a 1 cm path length on a Shimadzu UV-3101 PC UV−vis−NIR scanning spectrophotometer between 800 and 200 nm. Elemental analyses were determined at Robertson Microlit Laboratories (Ledgewood, NJ). Crystallographic Analyses. Crystals suitable for X-ray diffraction were covered in paratone oil in a glovebox and transferred to the diffractometer in a 20 mL capped vial. Crystals were mounted on a loop with paratone oil on a Bruker D8 VENTURE diffractometer. The crystals were cooled and kept at T = 100(2) K during data collection. The structures were solved with the ShelXT structure solution program using the Intrinsic Phasing solution method and by using Olex2 as the graphical interface.145,146 The model was refined with version 2014/7 of XL using least-squares minimization.147 Disorder effects were observed for the piperidinyl groups and therefore treated with similarity restraints on the C−C and C−N bonds and on the C−C−C and C−N−C angles. Rigid-body restraints were applied on the anisotropic temperature factors of these groups. Structure Solution of 2−Ce. Solution and refinement methods were standard for the crystal structures, except for compound 2−Ce. The X-ray diffraction is weak, and the large crystals diffract to very low resolution even with the heavy cerium atoms. This implies that the

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SUMMARY AND OUTLOOK We have presented a new imidophosphorane ligand supported by dialkylamido substituents. This ligand architecture stabilizes the zwitterionic character of the ligand and enables 1σ and 2π donation to metal centers through a single-atom donor. Tetrahomoleptic neutral and anionic cerium complexes supported by [NP(pip)3)]−, 2−Ce and 3−Ce−K(Et2O), which are a formal redox pair, exhibit an extremely negatively shifted redox couple, as established through chemical reactivity studies and theoretical calculations. The oxidation potential of 3−Ce−K(Et2O) is experimentally determined to be at least 0.69 V more negative than any cerium compound reported and is calculated to be as much as 1.04 V more negative, demonstrating the thermodynamic preference for the tetravalent state and the significant driving force afforded by the elimination of the potassium cation from the inner coordination sphere on oxidation. The electronic basis of this destabilization of the f orbitals in the homoleptic, imidophosphorane complexes of Ce3+ and Ce4+ was interrogated by Ce L3-edge XANES and theoretical modeling. The white-line doublet of the XANES spectrum for 2−Ce shows a significant decrease of the Ce3+ character (smaller nf) in the ground state of the complex, demonstrating destabilization of the Ce3+ character. Additionally, peak 2 (Ce3+L character) is shifted to lower energy, suggesting a less positive charge on the cerium ion due to significant metal− ligand covalent bonding. This observation suggests significant H

DOI: 10.1021/acs.inorgchem.9b00368 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry crystals are microscopically twinned and that the molecular structures are highly disordered and poorly defined in the crystal structures. The diffraction patterns were indexed using a hexagonal cell, and the reflections established the space groups to be R3 or R3̅ with a = b = 20.671(3) Å. Weak superlattice reflections, however, were observed in the diffraction. The large superlattice cell has a = b = 42 Å (and Z′ > 1). Intrinsic Phasing solution methods failed to solve the structure. The cerium atom could be located and occupies a special position with point symmetry 3. The ligands were extremely disordered (4-fold disorder). The cerium atom resides on a centrosymmetric position, and this may be a source of the disorder. However, lowering the symmetry did not resolve the disorder, and therefore the 3-fold symmetry is real (at least in the macroscopic crystal structure). A structural model with ordered piperidinyl groups was defined by looking at the multiple positions, removing atoms that were not chemically defined, and obtaining a clean structure from the overlapping groups. Once a chemically sensible molecule had been separated out, the structure was refined with similarity restraints on the C−C and C−N bonds and on the C−C−C and C−N−C angles. Rigid-body restraints were applied on the anisotropic temperature factors of these groups. The structure was also refined as an inversion twin (which improved the structural model). The arrangement of the four CeL4 groups is almost perfectly symmetrical. This symmetry is probably the reason for the very large displacement parameters of all atoms except for the cerium atom. The Hirshfeld test fails because of the large ADPs of the carbon atoms, i.e., about 10 times more than what is usual. The supercell reflections multiply the a and b axes by two and are therefore 4-fold supercells. The 4-fold disorder occurs along the c axis. The 4-fold supercell is likely due to a small distortion of each of the CeL4 groups due to symmetry breaking. A small amount of distortion destroys the symmetry, giving an extremely disordered structure. EPR. X-band EPR measurements were performed at 9.36400 GHz on a sample of 3−Ce−K(Et2O) in a toluene glass at 3.6 and 20 K. Magnetism. Magnetic measurements were conducted using a Quantum Design MPMS-5S magnetometer. Magnetic measurements were performed by two methods. Powdered samples were loaded inside a sealed polyethylene pouch and placed into straws, which were loaded into the instrument. Alternatively, powdered samples were loaded inside gel capsules and placed into straws, which were loaded into the instrument. Diamagnetic corrections for the sample container as well as the ligand were performed using Pascal’s constants.148 Ce L3-Edge XAS Measurements. Cerium samples were prepared in an argon glovebox at Stanford Synchrotron Radiation Lightsource (SSRL) because both Ce4+ and Ce3+ complexes are air-sensitive. A mixture of the analyte and boron nitride (BN) was weighed, such that the edge jump for the absorbing atom was calculated to be at one absorption length in transmission (∼8−12 mg for the cerium samples). The samples were diluted with BN (∼10 mg), which had been dried at elevated temperature (250 °C) under vacuum (1 × 10−3 Torr) for 24 h prior to use. Samples were ground with a mortar and pestle. Solid-state sample holders for the cerium samples consisted of an aluminum plate with a 3 × 15 mm oval window and screw holes. One side of the plate was covered with 0.5 mm Kapton tape, and the sample was evenly loaded in the window. The powder was then secured by covering the sample with a second piece of 0.5 mm Kapton tape. A second layer of the compound was painted onto a third piece of Kapton tape, which was subsequently fixed to the backside of the sample holder. The sample holder was then loaded onto a sample rod, taken out of the glovebox, and transported to the beamline while submerged within a liquid-nitrogen cooling bath. Once at the beam, the rod with the sample was placed at 45° inside the Oxford liquidhelium cryostat, which was precooled at 85 K and attached to the SSRL beamline 4-3 rail. When the cryostat was closed, the system was put under vacuum and flushed with helium three times. The valve was closed, and the measurements were performed in the cryostat at 15 K. The solid-state cerium complexes were characterized by metal L3edge X-ray measurements. The XAS measurements were made at SSRL, under dedicated operating conditions (3.0 GeV, 5%, 500 mA

using continuous top-off injections) on end station 4-3. With the use of a liquid-nitrogen-cooled double-crystal Si(220) (φ = 0) monochromator that employed collimating and focusing mirrors, a single energy was selected from the incident white beam. For cerium measurements, the beam was fully tuned at 6023 eV, and harmonic rejection was achieved with a titanium foil. The horizontal slit sizes were 8 mm, and vertical slit sizes were 1 mm in all measurements. The cryostat was attached to the beamline 4-3 XAS rail (SSRL), which was equipped with three ionization chambers, through which nitrogen gas was continually flowed. One chamber was positioned before the helium beam pass and the cryostat (10 cm) to monitor the incident radiation (I0). The second chamber was positioned after the cryostat (30 cm) so that sample transmission (I1) could be evaluated against I0 and so that the absorption coefficient (μ) could be calculated as ln(I0/I1). The third chamber (I2; 30 cm) was positioned downstream from I1 so that the XANES of a calibration foil could be measured against I1. A potential of 1100 V was applied in series to the ionization chambers. A Lytle detector under argon was placed on one side of the cryostat (4 cm) to detect the fluorescence from the samples. The cerium samples were calibrated in situ to the energy of the first inflection point of the K-edge of chromium foil (5989 eV). Data were acquired in triplicate and averaged. Background subtraction and normalization (at 5800 eV) were performed in Athena.149 Curve fitting was performed in IgorPro 7.0 using a modified version of EDG_FIT.150 Derivative spectra were used as guides to determine the number and positions of the peaks, and edge features were modeled by pseudo-Voigt line shapes and an additional function consisting of arctangent and error function contributions, which was used to model the absorption threshold. Deconvoluted spectral models were performed over several energy ranges. In the spectrum of 2−Ce, three pseudo-Voigts were employed to fit the spectrum: p1 (the quadrupole-allowed 2p3/2 → 4f transition), p2 and p3 to model the double-white line feature. In the spectrum of 3−Ce−K(Et2O), a single pseudo-Voigt was employed to model the white-line feature. The area under the pseudo-Voigt functions (defined as the intensity) was calculated with the formula ph × fwhm × 1/4([π/ln(2)]1/2 + π), where ph = peak height (normalized intensity), fwhm = full-width at half-maximum height (eV), and the value 1/4([π/ln(2)]1/2 + π) ≈ 1.318 is a constant associated with the pseudo-Voigt function. The fits are shown in Figures 5 and summarized in Table S16. Relative parameter error estimates are calculated from the covariance matrix assuming normally distributed variances in the data. The absolute error in nf is about 0.01 or 5%. Theoretical Calculations. To reduce the computational cost to a manageable degree, the simplified [Ce(NP(pip)3)4] complexes were used in calculations. The idea is to reduce the size of the side groups in the ligand that are bulky and electronically unimportant to less bulky and electronically equivalent ones. In this work, we replace the pip group in the [NP(pip)3]− ligand with a −N(CH3)2 substituent and denote this simplified ligand as [NP(NMe2)3]−. Ground-state electronic structure calculations were performed on [K(DME)2]+, [NP(NMe 2 ) 3 ] − , [Ce(NP(NMe 2 ) 3 ) 4 ], [Ce(NP(NMe 2 ) 3 ) 4 ] − , [(DME)2)KCe[NP(NMe2)3)4]+, and [(DME)2)KCe[NP(NMe2)3)4] as well as the [pyNO]− ligand in the previous work using the hybrid functional B3LYP implemented in Gaussian 16.112,151−156 All of the geometries were optimized in the gas phase without any constraint. Harmonic frequency calculations were performed to obtain the corresponding thermochemical corrections and to confirm that the optimized structures were stationary points on the potential energy surface. For each molecule, a subsequent single point using the implicit CPCM solvation model with the universal force field radii and solvent excluding surface was calculated to account for the solvation effects.157−160 A relative permittivity of 7.4257 was assumed in the solvation calculations to simulate THF as the solvent experimentally used for all cases. We applied the 6-311G* basis sets for nonmetal atoms, Stuttgart energy-consistent relativistic pseudopotentials ECP28MWB and the corresponding ECP28MWB-SEG basis set for the cerium atom, and Stuttgart energy-consistent relativistic pseudopotentials ECP10MWB and the corresponding ECP10MWB basis set for the potassium atom.161−165 We calculated the redox I

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Inorganic Chemistry potential using the revised Born−Haber cycle and Fc0/Fc+ as the reference.166 The absolute oxidation potentials of [Ce3+] → [Ce4+] and Fc → Fc+ at the scalar relativistic level are listed in Table S15. The SOC correction of −0.127 eV, applied to the ground-state electronic energies of all of the trivalent cerium complexes of [NP(NMe2)]− and [pyNO]− ligands, is taken from our previous work.112 This approach has been validated by various systems including transition-metal and f-element compounds and produced satisfactory results.66,87,112,151,152,166−173 Syntheses. K4[(C5H10N)3PN]4 (1−K). The title compound was prepared in a three-step, one-flask reaction sequence. Inside a glovebox, trimethylsilyl azide (3.48 mL, 26.2 mmol) was added to a solution of tris(piperidinyl)phosphine (4.96 g, 17.4 mmol) in 40 mL of toluene inside a 100 mL Schlenk pear-shaped flask equipped with a Teflon stir bar. The flask was transferred to a Schlenk line, and the reaction mixture was stirred at reflux for 12 h. Volatiles were removed in vacuo to yield an off-white solid. To the resulting solid were added 30 mL of MeOH (40 equiv) and 3 drops of concentrated H2SO4. The reaction mixture was stirred for a further 24 h, and then the volatiles were again removed in vacuo. The flask was transferred to the glovebox, and the resulting cloudy, off-white liquid was triturated three times with n-pentane. The resulting white solid was dissolved in toluene (20 mL), then potassium benzyl (2.39 g, 18.4 mmol) was added as a solid, and the mixture was stirred for 3 h. During the course of the reaction, a beige precipitate was formed. The solid was isolated on a fine-porosity frit and washed once with 15 mL of npentane to yield an off-white solid, and the remaining volatiles were removed in vacuo (2 h at 500 mTorr). The solid was taken up in THF, filtered through a fine-porosity frit packed with Celite, and precipitated from the solution at −35 °C as a white solid (3.08 g, 52%), which was isolated by decantation, and residual THF was removed in vacuo (2 h at 400 mTorr). The amount of residual THF depended on absolute vacuum. 1H NMR (400 MHz, THF-d8): δ 2.98 (s, 4H), 1.47−1.41 (m, 6H). 31P NMR (400 MHz, THF-d8): δ −0.12. 13 C NMR (400 MHz, THF-d8): δ 47.52 (s, CH2), 28.28 (d, CH2), 26.63 (s, CH2). IR (cm−1): ν 2918 (m), 2841 (m), 2791 (m), 2718 (w), 1438 (m), 1367 (m), 1321 (m), 1220 (m), 1203 (s), 1148 (m), 1123 (w), 1111(w), 1041 (s), 1026 (s), 921 (vs), 853 (m), 831 (m), 691 (s), 673 (w), 656 (w), 557 (s), 548 (s), 470 (s). Elem anal. Found (calcd) for K4P4N16C60H120: C, 52.95 (53.54); H, 9.05 (8.99); N, 16.30 (16.65). Carbon was consistently low on multiple burns. XRD-quality crystals were grown from THF at −35 °C as the THF adduct 1−K·4THF. [Ce(NP)(pip)3)4] (2−Ce). Inside a glovebox, CeI3(THF)4 (0.47 g, 0.58 mmol) was added to a 20 mL scintillation vial charged with a glass stir bar and 4 mL of diethyl ether. I2 (0.074 g, 0.29 mmol) was added to the scintillation vial as a solid, and the reaction mixture was allowed to stir for 5 min. 1−K (0.80 g, 2.3 mmol) was added as a solid. An immediate color change and the formation of a yellow suspension were observed. The reaction mixture was stirred overnight and then filtered through a fine-porosity frit with minimal diethyl ether. The filter cake was washed with a further 60 mL of toluene. The toluene filtrate was concentrated in vacuo to afford the title compound in 71% yield (0.55 g). 1H NMR (400 MHz, C6D6): δ 3.31 (s, 4H), 1.63 (s, 6H). 31P NMR (400 MHz, C6D6): δ −12.30. 13 C NMR (400 MHz, C6D6): δ 46.71 (s, CH2), 27.65 (d, CH2), 26.12 (s, CH2). IR (cm−1): ν 2923 (m), 2844 (m), 2809 (m), 2734 (w), 1439 (w), 1371 (w), 1324 (w), 1216 (m), 1201 (w), 1119 (vs), 1053 (s), 1023 (s), 928 (vs), 852 (m), 834 (w), 809 (vw), 706 (s), 672 (vw), 562 (s), 468 (s), 422 (vw). Elem anal. Found (calcd) for CeP4N16C60H120: C, 52.95 (54.15); H, 9.11 (9.11); N, 16.47 (16.85). Carbon was consistently low on multiple burns. XRD-quality crystals were grown from toluene at −35 °C. [(Et2O)KCe(NP)(pip)3)4] [3−Ce−K(Et2O)]. Inside a glovebox, 2− Ce (0.100 g, 0.0075 mmol) was added to a 20 mL scintillation vial charged with a glass stir bar and 2 mL of diethyl ether. KC8 (0.010 g, 0.0075 mmol) was added to the scintillation vial as a solid, and the reaction mixture was allowed to stir for 10 min. An immediate color change from a bronze to black suspension was observed. The mixture was filtered through a pipet filter packed with glass filter paper and

Celite. The solvent was concentrated in vacuo, and the solution was placed into the freezer at −35 °C overnight, during which time yellow crystals formed. After decanting and drying in vacuo, the title compound was isolated as the mono ether adduct (the diethyl ether ratio is dependent on absolute vacuum; 0.062 g, 68%). 1H NMR (400 MHz, C6D6): δ 3.35 (q, 4 OCH2), 1.21 (t, 6 CH2CH3) 1.08 (s, 48 CH2), 0.95−0.93 (m, 72 CH2). 31P NMR (400 MHz, C6D6): δ 85.77 (s). 13C NMR (400 MHz, C6D6): δ 66.03 (s, 4 OCH2), 43.15 (s, CH2), 26.81 (s, CH2), 25.39 (s, CH2), 15.70 (s, CH2CH3). IR (cm−1): ν 2919 (m), 2843 (m), 2801 (m), 2727 (w), 1438 (m), 1369 (m), 1323 (m), 1176 (vs), 1123 (m), 1107 (m), 1046 (s), 1026 (s), 921 (vs), 853 (m), 832 (m), 810 (w), 700 (s), 665 (w), 561 (s), 472 (s), 430 (vw). Elem anal. Found (calcd) for KCeP4N16C64H130O: C, 50.61 (53.30); H, 8.98 (9.09); N, 15.77 (15.52). Carbon was consistently low on multiple burns. Crystallographic analysis of the ether adduct revealed significant disorder in the two bound diethyl ethers, and higher-quality refinement data were obtained from crystals grown from DME at −35 °C. This crystallization procedure resulted in the isolation of 3−Ce−K(Et2O) as the bis(DME) adduct, 3−Ce− K(DME)2. [2.2.2.cryptandK][Ce(NP(pip)3)4] (3−Ce−K222). Inside a glovebox, 2−Ce (0.100 g, 0.0752 mmol) was added to a 20 mL scintillation vial charged with a glass stir bar and 2 mL of diethyl ether. A solution of 2.2.2. cryptand (0.028 g, 0.075 mmol) in diethyl ether was added and the mixture allowed to stir for 5 min. KC8 (0.011 g, 0.083 mmol) was added to the scintillation vial as a solid, and the reaction mixture was stirred for 10 min. An immediate color change from a bronze to a black suspension was observed. The mixture was filtered through a pipet filter packed with glass filter paper and Celite. The solution was concentrated in vacuo and placed in the freezer at −35 °C overnight, during which yellow, XRD-quality crystals grew (0.081 g, 62%). 1H NMR (400 MHz, C6D6): δ 3.82 (s, 12 H, cryptand CH2), 3.43 (s, 12 H, cryptand CH2), 3.27−3.24 (m, 52 H, piperidine CH2 and OCH2), 2.46 (s, 12 H, cryptand CH2), 1.59 (s, 72 H, piperidine CH2), 1.12 (t, 6 CH2CH3). 31P NMR (400 MHz, C6D6): δ 99.79 (s). 13C NMR (400 MHz, C6D6): δ 67.26 (s), 65.93 (s), 53.92 (s), 45.50 (s), 27.90 (s), 26.43 (s). IR (cm−1): ν 2914 (m), 2840 (m), 2798 (m), 2727 (w), 1438 (m), 1355 (m), 1322 (m), 1258 (w), 1192 (vs), 1150 (m), 1105 (s), 1080 (w), 1043 (s), 1027 (s), 923 (vs), 852 (m), 830 (m), 752 (w), 696 (s), 664 (m), 559 (s), 550 (s), 479 (s), 468 (s). Elem anal. Found (calcd) for CeP4N18C82H166KO7: C, 53.19 (54.13); H, 9.22 (9.20); N, 14.29 (13.86). Carbon was consistently low on multiple burns. Crystallographic data for 1−K, 2−Ce, 3−Ce−K(DME)2, 3−Ce− K222, and 3−Ce−K(Et2O)2 are deposited as CCDC 1874748, 1874855, 1874755, 1874746, and 1895860.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b00368. Complete experimental details, NMR and UV−vis spectra, reactivity studies, computational details including Cartesian coordinates, magnetism data, XANES fitting details, and crystallographic data (PDF) Accession Codes

CCDC 1874746, 1874748, 1874755, 1874855, and 1895860 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_request@ccdc. cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033. J

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Theoretical Comparison of Traditional vs Recently Discovered Ln2+ Ions in the [K(2.2.2-cryptand)][(C5H4SiMe3)3Ln] Complexes: The Variable Nature of Dy2+ and Nd2+. J. Am. Chem. Soc. 2015, 137, 369− 382. (5) Meihaus, K. R.; Fieser, M. E.; Corbey, J. F.; Evans, W. J.; Long, J. R. Record High Single-Ion Magnetic Moments Through 4fn5d1 Electron Configurations in the Divalent Lanthanide Complexes [(C5H4SiMe3)3Ln]−. J. Am. Chem. Soc. 2015, 137, 9855−9860. (6) Kotyk, C. M.; Fieser, M. E.; Palumbo, C. T.; Ziller, J. W.; Darago, L. E.; Long, J. R.; Furche, F.; Evans, W. J. Isolation of +2 rare earth metal ions with three anionic carbocyclic rings: bimetallic bis(cyclopentadienyl) reduced arene complexes of La2+ and Ce2+ are four electron reductants. Chem. Sci. 2015, 6, 7267−7273. (7) Fieser, M. E.; Ferrier, M. G.; Su, J.; Batista, E.; Cary, S. K.; Engle, J. W.; Evans, W. J.; Lezama Pacheco, J. S.; Kozimor, S. A.; Olson, A. C.; Ryan, A. J.; Stein, B. W.; Wagner, G. L.; Woen, D. H.; Vitova, T.; Yang, P. Evaluating the electronic structure of formal LnII ions in LnII(C5H4SiMe3)31− using XANES spectroscopy and DFT calculations. Chem. Sci. 2017, 8, 6076−6091. (8) Jenkins, T. F.; Woen, D. H.; Mohanam, L. N.; Ziller, J. W.; Furche, F.; Evans, W. J. Tetramethylcyclopentadienyl Ligands Allow Isolation of Ln(II) Ions across the Lanthanide Series in [K(2.2.2cryptand)][(C5Me4H)3Ln] Complexes. Organometallics 2018, 37, 3863−3873. (9) Palumbo, C. T.; Halter, D. P.; Voora, V. K.; Chen, G. P.; Chan, A. K.; Fieser, M. E.; Ziller, J. W.; Hieringer, W.; Furche, F.; Meyer, K.; Evans, W. J. Metal versus Ligand Reduction in Ln3+ Complexes of a Mesitylene-Anchored Tris(Aryloxide) Ligand. Inorg. Chem. 2018, 57, 2823−2833. (10) Zhang, Q.; Hu, S.-X.; Qu, H.; Su, J.; Wang, G.; Lu, J.-B.; Chen, M.; Zhou, M.; Li, J. Pentavalent Lanthanide Compounds: Formation and Characterization of Praseodymium(V) Oxides. Angew. Chem., Int. Ed. 2016, 55, 6896−6900. (11) Xémard, M.; Jaoul, A.; Cordier, M.; Molton, F.; Cador, O.; Le Guennic, B.; Duboc, C.; Maury, O.; Clavaguéra, C.; Nocton, G. Divalent Thulium Triflate: A Structural and Spectroscopic Study. Angew. Chem., Int. Ed. 2017, 56, 4266−4271. (12) MacDonald, M. R.; Fieser, M. E.; Bates, J. E.; Ziller, J. W.; Furche, F.; Evans, W. J. Identification of the +2 Oxidation State for Uranium in a Crystalline Molecular Complex, [K(2.2.2-Cryptand)][(C5H4SiMe3)3U]. J. Am. Chem. Soc. 2013, 135, 13310−13313. (13) Langeslay, R. R.; Fieser, M. E.; Ziller, J. W.; Furche, F.; Evans, W. J. Synthesis, structure, and reactivity of crystalline molecular complexes of the {[C5H3(SiMe3)2]3Th}1− anion containing thorium in the formal +2 oxidation state. Chem. Sci. 2015, 6, 517−521. (14) Windorff, C. J.; MacDonald, M. R.; Meihaus, K. R.; Ziller, J. W.; Long, J. R.; Evans, W. J. Expanding the Chemistry of Molecular U2+ Complexes: Synthesis, Characterization, and Reactivity of the {[C5H3(SiMe3)2]3U}− Anion. Chem. - Eur. J. 2016, 22, 772−782. (15) Langeslay, R. R.; Fieser, M. E.; Ziller, J. W.; Furche, F.; Evans, W. J. Expanding Thorium Hydride Chemistry Through Th2+, Including the Synthesis of a Mixed-Valent Th4+/Th3+ Hydride Complex. J. Am. Chem. Soc. 2016, 138, 4036−4045. (16) Langeslay, R. R.; Chen, G. P.; Windorff, C. J.; Chan, A. K.; Ziller, J. W.; Furche, F.; Evans, W. J. Synthesis, Structure, and Reactivity of the Sterically Crowded Th3+ Complex (C5Me5)3Th Including Formation of the Thorium Carbonyl, [(C5Me5)3Th(CO)][BPh4]. J. Am. Chem. Soc. 2017, 139, 3387−3398. (17) Windorff, C. J.; Chen, G. P.; Cross, J. N.; Evans, W. J.; Furche, F.; Gaunt, A. J.; Janicke, M. T.; Kozimor, S. A.; Scott, B. L. Identification of the Formal +2 Oxidation State of Plutonium: Synthesis and Characterization of {PuII[C5H3(SiMe3)2]3}−. J. Am. Chem. Soc. 2017, 139, 3970−3973. (18) Su, J.; Windorff, C. J.; Batista, E. R.; Evans, W. J.; Gaunt, A. J.; Janicke, M. T.; Kozimor, S. A.; Scott, B. L.; Woen, D. H.; Yang, P. Identification of the Formal +2 Oxidation State of Neptunium: Synthesis and Structural Characterization of {NpII[C5H3(SiMe3)2]3}1−. J. Am. Chem. Soc. 2018, 140, 7425−7428.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Jing Su: 0000-0002-6895-2150 Joshua Telser: 0000-0003-3307-2556 Ping Yang: 0000-0003-4726-2860 Enrique R. Batista: 0000-0002-3074-4022 Henry S. La Pierre: 0000-0002-0895-0655 Author Contributions §

N.T.R. and J.S. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Experimental studies were supported by start-up funds provided by the Georgia Institute of Technology. We are grateful for Dr. Jordan DeGayner’s assistance in confirming the SQUID results and for Dr. Joseph Sadighi’s donation of 2,6di-tert-butylanthracene. SC-XRD experiments were performed at the Georgia Institute of Technology SC-XRD facility directed by Dr. John Bacsa and established with funding from the Georgia Institute of Technology. Prof. Brian M. Hoffman, Northwestern University, provided access to EPR spectrometers, which are supported by the NSF (Grant MCB1118613). Use of the SSRL, SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, under Contract DE-AC02-76SF00515. The SSRL Structural Molecular Biology Program is supported by the U.S., DOE, Office of Biological and Environmental Research, and by the National Institutes of Health, National Institute of General Medical Sciences (including P41GM103393). J.S., E.R.B., and P.Y. were supported by the Heavy Element Chemistry Program sponsored by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, U.S. DOE, at Los Alamos National Laboratory (LANL). LANL is operated by Triad National Security, LLC, for the National Nuclear Security Administration of the U.S. DOE (Contract 89233218CNA000001). Theoretical research was performed using EMSL (grid.436923.9), a DOE Office of Science User Facility sponsored by the Office of Biological and Environmental Research. L.P. acknowledges the support by the project LO1603 under the Ministry of Education, Youth and Sports National sustainability programme I of Czech Republic.

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