Characterization of a Monomeric, Homoleptic, Solvent-Free Samarium

Oct 18, 2018 - The two-coordinate samarium(II) bis(aryloxide) complex, which has a bent O−Sm−O [111.08(9)°] core geometry, has been synthesized v...
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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Characterization of a Monomeric, Homoleptic, Solvent-Free Samarium Bis(aryloxide) Pei Zhao, Qihao Zhu, James C. Fettinger, and Philip Power* Department of Chemistry, University of California, Davis, California 95616, United States

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however, no homoleptic monomeric bis(aryloxide) or bis(alkyloxide) lanthanide complexes have been reported. Herein we report synthesis of the homoleptic monomeric solvent-free bis(aryloxide) lanthanide complex Sm(OAriPr6)2 [AriPr6 = −C6H3-2,6-(C6H2-2,4,6-iPr3)2], as well as its structural, spectroscopic, and magnetic characterization. The reaction of Sm{N(SiMe3)2}2(THF)218 with the highly sterically hindered aryloxide ligand HOAriPr619 in hexanes at room temperature resulted in a color change from purple to dark-brown. Followed by refluxing the solution overnight to drive the reaction to completion, the product Sm(OAriPr6)2 (1) was obtained, after workup, as dark-brown crystals in moderate yield (46%). (Scheme 1) Compound 1 is solvent-free with only

ABSTRACT: The reaction of Sm[N(SiMe3)2]2(THF)2 with 2 equiv of the bulky aryloxide ligand HOAriPr6 [AriPr6 = −C6H3-2,6-(C6H2-2,4,6-iPr3)2] afforded the first monomeric homoleptic solvent-free bis(aryloxide) lanthanide complex Sm(OAriPr6)2. The complex was characterized by crystallography and UV−visible and IR spectroscopy and magnetically by Evans’ method. The O−Sm−O angle is highly bent at 111.08(9)°. The samarium ion in Sm(OAriPr6)2 also displays close contacts with the flanking aryl rings of the terphenyloxide ligands. The complex is paramagnetic at room temperature with a magnetic moment of 3.51 μB.

Scheme 1. Synthetic Route to 1

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wide range of low-coordinate lanthanide complexes has been investigated because of their magnetic properties and possible behavior as single-molecule magnets (SMMs).1−9 Recent calculations by Winpenny, Mills, Chilton, and coworkers predicted that a two-coordinate dysprosium(III) complex possesses an unprecedentedly high-spin reversal barrier (Ueff = ca. 1800 cm−1).10 A further computational study by Chilton showed that a two-coordinate dysprosium(III) complex can achieve large Ueff even in the absence of linear/near-linear coordination and the weak agostic-type interactions within the compound have negligible effects on the Ueff value.11 The same study pointed out that the coordination of any donor molecules to the lanthanide ion would result in a significant decrease in the Ueff value.11 However, two-coordinate solvent-free lanthanide complexes are extremely rare due to the synthetic difficulty caused by the large atomic radii of lanthanide elements, which tend to favor higher coordination numbers.12 Three twocoordinate lanthanide bis(alkyl) compounds, M{C(SiMe3)3}2 (M = Yb,13 Eu,14 Sm15), and four two-coordinate lanthanide bis(amido) compounds, M{N(SiiPr3)2}2 (M = Sm, Eu, Tm, Yb),10,16 have been structurally characterized to date. Two decades ago Eaborn, Smith, and co-workers reported the first two-coordinate lanthanide complex, Yb{C(SiMe3)3}2, with a nonlinear C−Yb−C angle of 137.0(4)°.13 More recently, the first near-linear two-coordinate lanthanide complex Sm{N(SiiPr3)2}2 [N−Sm−N = 175.52(18)°] was reported by Winpenny and co-workers.10 Interestingly, agostic-type interactions exist in all of the reported two-coordinate lanthanide complexes. In 2018, Gao and co-workers reported a series of low-coordinate lanthanide complexes, [(ArO)Ln(OAr′)] (Ar = 2,6-Dipp2C6H3, Dipp = 2,6-diisopropylphenyl, Ar′ = 6-Dipp-2(2′-iPr-6′-CHMe(CH2−)C6H3)C6H3O−; Ln = Tb, Dy, Ho, Er, Tm), with support of the bulky aryloxide ligand.17 Currently, © XXXX American Chemical Society

the aryloxide ligands coordinated to the samarium ion. In contrast, the reaction of Sm{N(SiMe3)2}2(THF)2 with the phenol HOC6H2-2,6-tBu2-4-Me afforded the product Sm(OC6H2-2,6-tBu2-4-Me)2(THF)3, in which the samarium atom is coordinated by three THF molecules regardless of whether the reaction was carried out in hexanes or THF.20−22 The related terphenyloxide ligand HOAriPr4 (AriPr4 = -C6H3-2,6(C6H2-2,6-iPr2)2),19 which is only slightly less sterically hindered than HOAriPr6, was also reacted with Sm{N(SiMe3)2}2(THF)2 under the same reaction conditions. Although the same color change was observed during the reaction, no crystalline materials could be isolated. The structure of 1 (Figure 1) was determined by X-ray crystallography. Selected bond lengths and angles are listed in the caption of Figure 1. Complex 1 features a monomeric bis(aryloxide) structure in which two oxygen atoms are bonded to the samarium ion. The two Sm−O bond lengths are slightly different from each other at 2.287(2) Å (Sm1−O1) and 2.311(3) Å (Sm1−O2). In addition to bonding to two oxygen atoms, the samarium ion also displays relatively close π interactions with one flanking aryl ring on each of the ligands. The samarium···carbon distances span the range of 3.060−3.445 Å for Sm1 and C7−C12 and 3.051−3.494 Å for Sm1 and C47− C52. The samarium···centroid (CNT= centroid) distances are Received: September 20, 2018

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DOI: 10.1021/acs.inorgchem.8b02677 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

(OAriPr4)229 [AriPr4 = −C6H3-2,6-(C6H2-2,6-iPr2)2], which are strictly linear, despite the fact that the aryloxide ligand in 1 (−OAriPr6) is more sterically hindered than −OAriPr4. This narrower angle is permitted by the much larger size of the samarium ion30 and lower steric congestion, thereby allowing stronger Sm···C π interactions, which provide the driving force for bending to occur readily. The C1−O1−Sm1 [139.1(2)°] and C41−O2−Sm1 [137.5(2)°] angles are significantly narrower than the corresponding C−O−Sm angles (167.0− 175.2°) in the THF-coordinated compound Sm(OC6H22,6- t Bu 2 -4-Me) 2 (THF) 3 21,22 or Sm(OC 6 H 2 -2,6- t Bu 2 -4OMe)2(THF)3.23 The narrower C−O−Sm angles are probably due to the Sm···flanking ring interactions. The CNT1−Sm1− CNT2 angle is 137.17(5)°. Thus, the coordination environment of the samarium ion may be considered to be distorted tetrahedral. The central phenyl rings of the two ligands are oriented and almost perpendicular to each other with a torsion angle of 84.82(11)°. Thus, can be contrasted with the central phenyl rings of in Fe(OAriPr4)228 and Co(OAriPr4)2,29 which are coplanar and have linear coordination31 at the metals. The magnetic moment of 1 was determined by Evans’ method32,33 in C6D6, which showed that 1 is paramagnetic at room temperature with a magnetic moment of 3.51 μB, which is similar to the magnetic moment (3.62 μB) observed in the twocoordinate complex Sm{N(SiiPr3)2}2.10 This result is consistent with the fact that Sm2+(f6) has the ground state 7F0, with a zero magnetic moment, and the observed moment is a result of the population of low-lying excited states at room temperature and Zeeman effects.34 In conclusion, the first monomeric homoleptic solvent-free lanthanide bis(aryloxide) complex Sm(OAriPr6)2 (1) was synthesized and structurally characterized. It features a bent samarium coordination geometry with an O−Sm−O angle of 111.08(9)°. The samarium(II) ion also displays Sm···C π interactions with two flanking aryl rings from the ligands. This complex has a magnetic moment of 3.51 μB at room temperature due to the mixing of low-lying paramagnetic excited states and/ or Zeeman effects. Investigation of the reactivity of 1 toward small molecules and the synthesis of other lanthanide aryloxides are underway.

Figure 1. Solid-state molecular structure of 1 (hydrogen atoms are not shown; thermal ellipsoids are shown at 30% probability; CNT = centroid). Selected bond lengths (Å) and angles (deg.): Sm1−O1 2.287(2), Sm1−O2 2.311(3), Sm1−CNT1 2.9315(15), Sm1−CNT2 2.9457(17), Sm1−C7 3.060(3), Sm1−C8 3.226(4), Sm1−C9 3.383(4), Sm1−C10 3.445(4), Sm1−C11 3.258(4), Sm1−C12 3.094(4), Sm1−C47 3.051(4), Sm1−C48 3.214(4), Sm1−C49 3.390(5), Sm1−C50 3.494(4), Sm1−C51 3.288(4), Sm1−C52 3.098(4); O1−Sm1−O2 111.08(9), C1−O1−Sm1 139.1(2), C41− O2−Sm1 137.5(2), CNT1−Sm1−CNT2 137.17(5).

2.9315(15) Å (Sm1···CNT1) and 2.9457(17) Å (Sm1··· CNT2), respectively. The Sm−O bond lengths in 1 are close to, but slightly shorter than, the Sm−O bond lengths [2.331(11) and 2.347(13) Å] in the THF-coordinated samarium(II) complex Sm(OC6H2-2,6-tBu2-4-Me)2(THF)3,21,22 and they are almost identical with the Sm−O distances [2.299(7) and 2.305(7) Å] in Sm(OC6H2-2,6-tBu2-4-OMe)2(THF)3.23 In comparison to the Sm−O bond lengths [2.129−2.186 Å] in a structurally related samarium(III) homoleptic aryloxide compound SmIII{OC6H3-2,6-(C6H5)2}3,24 the Sm−O bond lengths in 1 are significantly longer because of the lower oxidation state of samarium(II) in 1. The samarium−phenyl π interactions in samarium(II) complexes Sm{η6-bis(Me3Si-fluorene-AlMe3)},25 Sm{η6-bis(Me3Si-fluorene-AlEt3)},25 (C5Me5 )Sm(μ-η 6:η1Ph)2BPh2,26 (C5Me5)Sm(μ-η6:η1-Ph)2BPh2(N2Ph2),26 and {(κ1-N,η6-Piso)Sm(THF)(μ-I)2Sm(κ1-N,η6-Piso)} (Piso− = {(DippN)2CBut}−; Dipp = 2,6-diisopropylphenyl)27 display samarium···centroid distances of 2.602−2.754 Å.25−27 However, the samarium···centroid distances in 1 are 0.2−0.3 Å longer than those, implying that π interactions are weaker. The samarium··· carbon distances in 1 (3.051−3.494 Å) are also comparable to the agostic-type interaction distances (Sm···C) in the samarium dialkyl complex Sm{C(SiMe3)3}2 (2.984−3.281 Å)15 and the samarium bis(amide) complex Sm{N(SiiPr3)2}2 (3.082−3.224 Å).10 The metal coordination in 1 is strongly bent with an O1− Sm1−O2 angle of 111.08(9)°. This angle is in sharp contrast to the oxygen−metal−oxygen angles in the structurally related iron and cobalt bis(aryloxide) species Fe(OAriPr4)228 and Co-

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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.8b02677. Full details on the synthesis, experimental setup, and characterization of the described compounds (PDF) Accession Codes

CCDC 1866123 contains 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_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: + 44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

James C. Fettinger: 0000-0002-6428-4909 Philip Power: 0000-0002-6262-3209 B

DOI: 10.1021/acs.inorgchem.8b02677 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the National Science Foundation for financial support (Grants CHE-1565501) and for the dual-source X-ray diffractometer (Grant 0840444).

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DOI: 10.1021/acs.inorgchem.8b02677 Inorg. Chem. XXXX, XXX, XXX−XXX