Synthesis and structural characterization of lanthanum and cerium substituted cyclopentadienyl borohydride complexes

Accepted Manuscript Synthesis and structural characterization of lanthanum and cerium substituted cyclopentadienyl borohydride complexes Fabrizio Ortu, Daniel Packer, Jingjing Liu, Matthew Burton, Alasdair Formanuik, David P. Mills PII:

S0022-328X(17)30521-1

DOI:

10.1016/j.jorganchem.2017.09.010

Reference:

JOM 20085

To appear in:

Journal of Organometallic Chemistry

Received Date: 21 July 2017 Revised Date:

1 September 2017

Accepted Date: 4 September 2017

Please cite this article as: F. Ortu, D. Packer, J. Liu, M. Burton, A. Formanuik, D.P. Mills, Synthesis and structural characterization of lanthanum and cerium substituted cyclopentadienyl borohydride complexes, Journal of Organometallic Chemistry (2017), doi: 10.1016/j.jorganchem.2017.09.010. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT - Submission to Journal of Organometallic Chemistry, Lanthanide and Actinide Complexes Issue Synthesis

and

structural

characterization

of

lanthanum

and

cerium

substituted

cyclopentadienyl borohydride complexes

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Fabrizio Ortu, Daniel Packer, Jingjing Liu, Matthew Burton, Alasdair Formanuik, and David P. Mills*

School of Chemistry, The University of Manchester, Oxford Road, Manchester, M13 9PL, U.K.

[email protected] (F. Ortu)

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E-mail addresses:

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*Corresponding author.

[email protected] (D. Packer) [email protected] (J. Liu)

[email protected] (M. Burton) [email protected] (A. Formanuik)

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[email protected] (D.P. Mills)

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Keywords: lanthanide, cyclopentadienyl, borohydride.

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ACCEPTED MANUSCRIPT ABSTRACT The lanthanide borohydrides [Ln(BH4)3(THF)n] [Ln = La, n = 4 (1); Ln = Ce, n = 3.5] were treated with the substituted cyclopentadienyl potassium transfer agents KCptt (Cptt = C5H3tBu2-1,3), KCpttt (Cpttt = C5H2tBu3-1,2,4) and KCp′′′ (Cp′′′ = C5H2(SiMe3)3-1,2,4) in various reaction to

yield

[La(Cpttt)(BH4)2(THF)2]

the (3),

heteroleptic

lanthanide

[La(Cpttt)(µ-BH4)2] 6

(4),

[La(Cptt)2(µ-BH4)] 2

(2),

[Ce(Cpttt)(BH4)(µ-BH4)(THF)] 2

(5),

complexes

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conditions

[La(Cpttt)2(BH4)] (6) and [Ln(Cp′′′)2(BH4)(THF)] (Ln = La, 7; Ce, 8) by salt metathesis protocols.

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NMR and FTIR spectroscopies, and elemental analyses.

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Complexes 1-8 have been variously characterized by single crystal X-ray diffraction, multi-nuclear

1. Introduction

Convenient routes to molecular lanthanide borohydrides, [Ln(BH4)3(THF)n] (Ln = La-Lu; n

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= 0-3), from LnCl3 and MBH4 (M = Li, Na) have been known since the 1970s [1]. The superior

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solubility of these reagents over their parent LnCl3 precursors in THF has proven advantageous in the synthesis of a wide range of organolanthanide complexes [2]. As well as exhibiting distinctive

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and interesting chemical features, borohydride groups can be treated as pseudo-halides as they are readily displaced by group 1 ligand transfer agents by salt metathesis protocols [1,2]. This attribute

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gives heteroleptic organolanthanide borohydride complexes useful synthetic functionality, as other ligands may be later installed by these methods to generate more sophisticated molecular systems. Further to this, lanthanide borohydride chemistry has flourished even more rapidly over the last 15 years as applications are being sought for these complexes in organic synthesis and as polymerization catalysts [2(a)]. Recently, our group has been investigating synthetic routes to heteroleptic lanthanide complexes that contain two bulky substituted cyclopentadienyl ligands, [Ln(CpR)2(X)] (CpR = Cptt , 2

ACCEPTED MANUSCRIPT C5H3tBu2-1,3; Cpttt, C5H2tBu3-1,2,4; Cp′′, C5H3(SiMe3)2-1,3; Cp′′′ = C5H2(SiMe3)3-1,2,4; X = anionic co-ligand) [3,4], and we have utilized some of these precursors to generate more complex species [5]. During the course of our investigations, we found that the direct synthesis of [Ln(CpR)2(X)] from KCpR and the lanthanide triiodide THF solvates, [Ln(I)3(THF)4] (Ln = La, Ce)

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[6], were unsuitable for CpR ligands with significant steric demands under the conditions employed; e.g. for Cp′′′ the mono-ring complex, [Ce(Cp′′′)(I)2(THF)2], was isolated [3]. We envisaged that [Ln(BH4)3(THF)n] (Ln = La, Ce), KCpttt and KCp′′′ would be suitable precursors for the synthesis

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of di-substituted Ln(III) cyclopentadienyl complexes, as similar procedures have previously been reported for the direct synthesis of [Ln(C5Me4R)2(BH4)(THF)] (Ln = Y, Sm, Lu; R = H, Me, Et, iPr) [Ln(C5HiPr4)2(BH4)(THF)]

(Ln

=

Nd,

Sm)

[8],

[Tm(Cpttt)2(BH4)]

[9]

and

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[7],

[Ln(C5Me4R)2(BH4)(THF)] (Ln = Sc, Y; R = H, Me) [10]. Germane to this, the solid state structures of [Dy(Cpttt)2(BH4)] and [Dy(Cpttt)2(μ-BH4)K] have also been disclosed [11], indicating that [Ln(CpR)2(BH4)] (Ln = La, Ce; CpR = Cpttt, Cp′′′) synthetic targets are feasible. Herein, we report an alternative synthesis of [Ln(BH4)3(THF)n] from [Ln(I)3(THF)4] and KBH4 for La and Ce. These

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precursors were allowed to react with KCpR (CpR = Cptt, Cpttt, Cp′′′) in a variety of conditions to

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2. Experimental

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afford a series of mono- and di-substituted heteroleptic organolanthanide borohydride complexes.

2.1 General methods

All syntheses and manipulations were conducted under argon with rigorous exclusion of oxygen and water using Schlenk line and glove box techniques. Toluene, THF and hexane were dried by refluxing over potassium; toluene and hexane were stored over potassium mirrors and THF was stored over activated 4 Å molecular sieves. All solvents were degassed before use. For NMR spectroscopy C6D6 was dried by refluxing over K and was vacuum transferred and degassed by 3

ACCEPTED MANUSCRIPT three freeze-pump-thaw cycles before use. [Ln(I)3(THF)4] (Ln = La, Ce) [6], KCptt [9], KCpttt [9] and KCp′′′ [12] were prepared according to literature methods and KBH4 was used as received. 1H (400 MHz), 13C{1H} (100 MHz),

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B{1H} (128 MHz) and

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Si{1H} (79 MHz) NMR spectra were

obtained on an Avance III 400 MHz spectrometer 298 K. These were referenced to the solvent 13

C,

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Si), or H3BO3/D2O (11B). Fourier transform infrared spectra

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used, or to external TMS (1H,

were recorded as Nujol mulls in KBr discs using a Shimadzu IRAffinity-1S spectrometer. Elemental analyses were performed by Mrs Anne Davies and Mr Martin Jennings at The University

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of Manchester, U.K.

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2.2 Synthesis

[La(BH4)3(THF)4] (1). Prepared by a modification of literature procedures for the synthesis of [La(BH4)3(THF)3] [13]. THF (40 mL) was added to an ampule containing a mixture of [La(I)3(THF)4] (3.232 g, 4 mmol) and KBH4 (2.158 g, 40 mmol). The mixture was refluxed for 5 days, forming a white suspension. The mixture was allowed to settle for 2 hours at 50 °C and then

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filtered and stored at –25 °C, affording 1 as colorless crystals (1.578 g, 84 %). The crystals were

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dried in vacuo for 1 hour at 10–2 mbar. Elemental microanalysis results indicate desolvation may have occurred as they are in best agreement with a [La(BH4)3(THF)3.5] formulation. Anal. calcd (%)

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for C14H40B3LaO3.5: C, 38.58; H, 9.25. Found (%): C, 38.16; H, 9.37. 1H NMR (C6D6, 298 K): δ = 1.25 (s, 14H, THF-CH2), 1.90 (br q, 12H, 1JBH = 81 Hz, BH4), 3.82 (s, 14H, THF-OCH2). 11B{1H}

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NMR (C6D6, 298 K): δ = –18.85 (BH4). 13C{1H} NMR (C6D6, 298 K): δ = 25.67 (THF-CH2), 71.93 (THF-OCH2). FTIR (Nujol, cm−1) ): ̃ = 2440 (br, s, B–H str.), 2325 (br, s, B–H str.), 2217 (br, s w/ shoulder, B–H str.), 1340 (m), 1297 (w), 1262 (m), 1151 (br, s), 1096 (br, s), 1037 (w), 1014 (s), 956 (w), 919 (s), 858 (br, s w/ shoulder), 721 (m), 668 (m). [Ce(BH4)3(THF)3.5]. Prepared by a modification of the literature procedure [14]. THF (30 mL) was added to an ampule containing a mixture of [Ce(I)3(THF)4] (1.619 g, 2 mmol) and KBH4 (1.079 g, 20 mmol). The mixture was refluxed for 5 days, forming a white suspension. The mixture 4

ACCEPTED MANUSCRIPT was allowed to settle for 2 hours at 50 °C and then filtered and stored at –25 °C, affording [Ce(BH4)2(THF)5][Ce(BH4)4(THF)2] as colorless crystals (0.767 g, 88 %). Elemental microanalysis results indicate desolvation may have occurred as they are in best agreement with a [Ce(BH4)3(THF)3] formulation. Anal. calcd (%) for C12H36B3CeO3: C, 35.95; H, 9.05. Found (%):

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C, 35.38; H, 8.72. [La(Cptt)2(µ-BH4)] 2 (2). THF (10 mL) was added to a pre-cooled mixture (–30 °C) of KCptt (0.432 g, 2 mmol) and [La(BH4)3(THF)4] (0.472 g, 1 mmol) with stirring. The reaction mixture was

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allowed to slowly warm to room temperature and then stirred for a further 16 hours, forming a white suspension. The mixture was allowed to settle for 2 hours and filtered. Volatiles were

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removed in vacuo to afford a white solid. Recrystallization from toluene (2 mL) at –25 °C gave colorless crystals of 2 (0.158 g, 31 %). Anal. calcd (%) for C26H46BLa: C, 61.43; H, 9.12. Found (%): C, 61.03; H, 9.04. 1H NMR (C6D6, 298 K): δ = 1.09 (br q, 8H, 1JBH = 81 Hz, BH4), 1.37 (s, 72H, C(CH3)3), 6.26 (br s, 4H, Cp-CH), 6.33 (m, 8H, Cp-CH). 13C{1H} NMR (C6D6, 298 K): δ =

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32.88 (C(CH3)3), 33.39 (C(CH3)3), 61.77 (C(CH3)), 108.97 (Cp-CH), 111.00 (Cp-CH) 146.36 (Cp-

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CtBu). 11B{1H} NMR (C6D6, 298 K): δ = −22.49 (s, BH4). FTIR (Nujol, cm−1): ̃ = 2430 (m, B–H str.), 2266 (br, s w/shoulder, B–H str.), 2206 (m, B–H str.), 1250 (m), 1099 (m), 1022 (w), 808 (s).

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[La(Cpttt)(BH4)2(THF)2] (3) and [La(Cpttt)(-BH4)2] 6 (4). THF (10 mL) was added to a pre-

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cooled (–30 °C) mixture of KCpttt (0.545 g, 2 mmol) and [La(BH4)3(THF)4] (0.472 g, 1 mmol) with stirring. The reaction mixture was allowed to slowly warm to room temperature and then stirred for a further 16 hours. The yellow mixture was allowed to settle for 2 hours and filtered. Volatiles were removed in vacuo to afford an orange solid. Recrystallization from toluene (2 mL) at –25 °C gave colorless crystals of 3 (0.235 g, 43 %). A small quantity of crystals were obtained from the supernatant liquid; these were identified as 4 by single crystal XRD, but no further data could be obtained. Data for 3: Anal. calcd (%) for C25H53B2LaO2: C, 54.94; H, 9.77. Found (%): C, 54.66; H, 9.57. 1H NMR (C6D6, 298 K): δ = 1.24 (br s, 6H, THF-CH2), 1.37 (s, 9H, C(CH3)3), 1.54 (s, 18H, 5

ACCEPTED MANUSCRIPT C(CH3)3), 1.70 (br, 8H, BH4) 3.70 (s, 6H, THF-OCH2), 6.41 (s, 2H, Cp-CH). 13C{1H} NMR (C6D6, 298 K): δ = 25.55 (THF-CH2), 30.96 (C(CH3)3), 32.47 (C(CH3)3), 34.48 (C(CH3)3), 71.95 (THFOCH2), 114.10 (Cp-CH), 137.48 (Cp-CtBu), 140.34 (Cp-CtBu), 141.78 (Cp-CtBu) ppm.

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B{1H}

NMR (C6D6, 298 K): δ = –18.96 (s, BH4), –15.87 (s, BH4). FTIR (Nujol, cm−1): ̃ = 2422 (m, B–H

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str.), 2301 (m, B–H str.), 1240 (br, m), 1100 (m), 866 (w), 675 (w). [Ce(Cpttt)(BH4)(-BH4)(THF)] 2 (5). THF (10 mL) was added to a pre-cooled (−30 °C) mixture of KCpttt (0.294 g, 1.08 mmol) and [Ce(BH4)3(THF)3.5] (0.236 g, 0.54 mmol) with stirring.

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The reaction mixture was allowed to slowly warm to room temperature and stirred for a further 16 hours. The orange mixture was allowed to settle for 2 hours and filtered. Volatiles were removed in

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vacuo to afford a tacky orange solid. Recrystallization from hexane (2 mL) at –25 °C gave yellow crystals of 5 (0.160 g, 62 %). Anal. calcd (%) for C36H90B2Ce2O2: C, 54.82; H, 9.75. Found (%): C, 54.55; H, 10.07. 1H NMR (C6D6, 298 K): δ = −13.45 (br, v1/2 = 79 Hz, BH4), −3.63 (br, v1/2 = 21 Hz, 18H, C(CH3)3), −1.82 (br, v1/2 = 24 Hz, 9H, C(CH3)3), −1.22 (br s, v1/2 = 71 Hz, 8H, THF), −0.61

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(br s, v1/2 = 18 Hz, 2H, Cp-CH), 0.54 (br , v1/2 = 25 Hz, 2H, Cp-CH), 2.47 (br, v1/2 = 29 Hz, 8H,

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THF), 25.48 (br, v1/2 = 792 Hz, 10H, BH4), 28.08 (br, v1/2 = 132 Hz, BH4). The paramagnetism of 5 precluded the integration of the BH4 signals in the 1H NMR spectrum and assignment of the C{1H} NMR spectrum.

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B{1H} NMR (C6D6, 298 K): δ = 33.15 (br, BH4), 65.94 (br, BH4). μeff

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(Evans method, C6D6, 298 K): 2.33 μB. FTIR (Nujol, cm−1): ̃ = 2449 (m, B–H str.), 2411 (m, B–H

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str.), 2299 (br, m, B–H str.), 1238 (m), 1165 (s), 1016 (s), 860 (w). [La(Cpttt)2(BH4)] (6). THF (10 mL) was added to a mixture of KCpttt (0.545 g, 2 mmol) and [La(BH4)3(THF)4] (0.472 g, 1 mmol) with stirring. The reaction mixture was refluxed for 16 hours. The reaction mixture was cooled to room temperature and allowed to settle for 2 hours. The resulting yellow mixture was filtered. Volatiles were removed in vacuo to afford a white solid. Recrystallization from toluene (2 mL) at –25 °C gave colorless crystals of 6 (0.415 g, 67 %). Anal. calcd (%) for C34H62BLa: C, 65.79; H, 10.07. Found (%): C, 62.50; H, 10.02. 1H NMR (C6D6, 298 6

ACCEPTED MANUSCRIPT K): δ = 1.19 (s, 18H, C(CH3)3) 1.50 (s, 36H, C(CH3)3), 1.72 (br q, 4H, 1JBH = 76 Hz, BH4), 6.51 (s, 4H, Cp-CH). 13C{1H} NMR (C6D6, 298 K): δ = 31.21 (C(CH3)3), 32.99 (C(CH3)3), 34.87 (C(CH3)), 34.96 (C(CH3)3), 114.46 (Cp-CH), 137.71 (Cp-CtBu), 139.92 (Cp-CtBu). 11B{1H} NMR (C6D6, 298 K): δ = −16.80 (BH4). FTIR (Nujol, cm-1): ̃ = 2479 (m, B–H str.), 2374 (w, B–H str.), 2299 (m, B–

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H str.), 1238 (s), 1163 (s), 821 (s), 800 (s). [La(Cp′′′)2(BH4)(THF)] (7). THF (20 mL) was added to a pre-cooled (–78 °C) mixture of [La(BH4)3(THF)4] (0.331 g, 0.7 mmol) and KCp′′′ (0.449 g, 1.4 mmol). The solution was allowed to

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warm slowly to room temperature and stirred for 16 hours, forming a white suspension. The pale yellow mixture was allowed to settle for 2 hours and then filtered. Volatiles were removed in vacuo

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to afford a white solid. Recrystallization from toluene (2 mL) at –25 °C gave colorless crystals of 7 (0.473 g, 0.6 mmol, 86 %). Anal. calcd (%) for C32H70BLaOSi6: C, 48.71; H, 8.95. Found (%): C, 48.39; H, 9.30. 1H NMR (C6D6, 298 K): δ = 0.35 (s, 18H, Si(CH3)3), 0.51 (s, 36H, Si(CH3)3), 1.27 (br q, 4H, 1JBH = 72 Hz, BH4), 1.27 (br m, 4H, THF-CH2), 3.73 (br m, 4H, THF-OCH2), 7.08 (s,

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4H, Cp-CH). 13C{1H} NMR (C6D6, 298 K): δ = 2.70 (Si(CH3)3), 2.94 (Si(CH3)3), 25.69 (THF-CH2), 11

B{1H} NMR

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73.09 (THF-OCH2), 130.27 (Cp-CSiMe3), 135.92 (Cp-CH), 138.49 (Cp-CSiMe3). (C6D6, 298 K): δ = –17.03 (BH4).

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Si{1H} NMR (C6D6, 298 K): δ = –10.51 (Si(CH3)3), –9.57

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(Si(CH3)3). FTIR (Nujol, cm-1): ̃ = 2439 (m, B–H str.), 2279 (m, B–H str.), 2215 (m, B–H str.), 2112 (s, B–H str.), 1250 (s), 1174 (m), 1095 (s), 833 (br, s) cm–1.

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[Ce(Cp′′′)2(BH4)(THF)] (8). THF (30 mL) was added to a pre-cooled (–78 °C) mixture of [Ce(BH4)(THF)3.5] (1.311 g, 3 mmol) and KCp′′′ (1.924 g, 6 mmol) with stirring. The yellow reaction mixture was allowed to warm slowly to room temperature and stirred for 16 hours, forming an orange suspension. The mixture was allowed to settle for 4 hours and then filtered. Volatiles were removed in vacuo to afford a yellow solid. Recrystallization from toluene (3 mL) at –25 °C gave bright yellow crystals of 8 (1.48 g, 62 %). 1H NMR (C6D6, 298 K): δ = –10.68 (br, v1/2 = 204 Hz, BH4), –3.99 (s, 4H, v1/2 = 65 Hz, THF-CH2), –2.90 (s, 18H, v1/2 = 29 Hz, Si(CH3)3), –0.74 (br, 7

ACCEPTED MANUSCRIPT 2H, v1/2 = 16 Hz, Cp-CH), 0.30 (br, 40H, v1/2 = 140 Hz, Si(CH3)3 + THF-OCH2), 2.13 (br, 2H, v1/2 = 16 Hz, Cp-CH), 7.97 (br, v1/2 = 426 Hz, BH4), 18.02 (br, v1/2 = 1046 Hz, BH4).

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B{1H} NMR

(C6D6, 298 K): δ = 16.82 (BH4). The paramagnetism of 8 precluded the integration of the BH4 signals in the 1H NMR spectrum and assignment of the

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C{1H} and

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Si{1H} NMR spectra. µeff

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(Evans method, C6D6, 298 K): 2.83 µB. FTIR (Nujol, cm-1): ̃ = 2449 (s, B–H str.), 2358 (br, m w/ shoulder, B–H str.), 2220, (w, B–H str.), 2119 (m, B–H str.), 1247 (s), 1092 (s), 752 (m) cm-1. 2.3 X-ray crystallography

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The crystal data for 1-8 is compiled in the Supporting Information (Tables S1-S2). Crystals

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of 1, 2, 4, 5, 7 and 8 were examined using an Agilent Supernova diffractometer, equipped with CCD area detector and mirror-monochromated Mo Kα radiation (λ = 0.71073 Å). Crystals of 3 and 6 were examined using an Oxford Diffraction Xcalibur diffractometer, equipped with CCD area detector and mirror-monochromated Mo Kα radiation (λ = 0.71073 Å). Cell parameters were refined from the observed positions of all strong reflections in each data set. A Gaussian grid face-

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indexed (1, 7 and 8), multi-scan (2, 4-6) or analytical (3) absorption correction was applied [15].The

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structure was solved by direct methods using SHELXS [16] and the dataset was refined by fullmatrix least-squares on all unique F2 values, with anisotropic displacement parameters for all non-

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hydrogen atoms, and with constrained riding hydrogen geometries; Uiso(H) was set at 1.2 (1.5 for methyl and borohydride groups) times Ueq of the parent atom. The largest features in final

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difference syntheses were close to heavy atoms and were of no chemical significance. CrysAlisPro [15] was used for control and integration, and SHELX [16,17] and OLEX2 [18] were employed for structure solution and refinement. ORTEP-3 [19] and POV-Ray [20] were employed for molecular graphics. CCDC 1562025-1562032 contains the supplementary crystal data for this article. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

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ACCEPTED MANUSCRIPT 3. Results and discussion 3.1 Synthesis and spectroscopic characterization The lanthanide borohydrides [Ln(BH4)3(THF)n] [Ln = La, n = 4 (1); Ln = Ce, n = 3.5] were

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synthesized from the parent molecular triodides [Ln(I)3(THF)4] (Ln = La, Ce) [6] and excess KBH4 in very good yields (84-88 %) (Scheme 1). This is a modification of the most commonly utilized literature procedures, where LnCl3 and LiBH4 or NaBH4 starting materials are typically employed to form [Ln(BH4)3(THF)3] [1,2,13,14]. Given that [Ln(BH4)3(THF)n] precursors are well-known to

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exhibit variable degrees of solvation [1,2], we purified the products by recrystallization from THF

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and analyzed their composition by single crystal XRD to ensure the correct formulations were used for subsequent reaction steps. Interestingly, for La we obtained the molecular structure of the tetrasubstituted THF complex 1 instead of [La(BH4)3(THF)3] [13], whereas for Ce crystals of the known ion-pair complex [Ce(BH4)2(THF)5][Ce(BH4)4(THF)2] [14], hereafter referred to as [Ce(BH4)3(THF)3.5], were identified. The structure of the corresponding La ion-pair complex,

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[La(BH4)2(THF)5][La(BH4)4(THF)2], has also been reported previously [21]. It is noteworthy that

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Li et al. reported that [La(BH4)3(THF)3] and [La(BH4)3(THF)3.5] can both be synthesized from LaCl3 and NaBH4, with the product formed dependent upon the temperature of the reaction mixture

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[22]. To the best of our knowledge the solid state structure of 1 has not been reported previously, nor has any f-element tris-borohydride with four coordinated THF molecules [1,2], thus we attribute

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its formation to the different reaction conditions employed herein. We dried crystals of 1 in vacuo for 1 hour at 10–2 mbar, and the elemental microanalysis results and integrals in the 1H NMR spectrum of 1 indicated that desolvation had occurred, as these data were in agreement with a [La(BH4)3(THF)3.5] formulation [22]. However, the FTIR spectrum of 1 exhibited vibrations at 2440, 2325 and 2217 cm–1, indicative of bidentate and tridentate BH4 binding modes [1,2], that differed to those reported for [La(BH4)3(THF)3.5] ( ̃ = 2436, 2290 and 2224 cm–1) [22]. Given this ambiguity we arbitrarily set a 3 BH4 : 4 THF formulation for subsequent reactions of 1. 9

ACCEPTED MANUSCRIPT

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Scheme 1. Synthesis of [Ln(BH4)3(THF)n] [Ln = La, n = 4 (1); Ln = Ce, n = 3.5].

With [Ln(BH4)3(THF)n] for Ln = La and Ce in hand, we first performed the reaction of 1 with two equivalents of KCptt in THF to give [La(Cptt)2(µ-BH4)]2 (2) in 31 % yield (Scheme 2). This demonstrated that our methodology was appropriate, as the previously reported homologs

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[Ln(Cptt)2(μ-BH4)]2 (Ln = Ce, Sm) were synthesized by different routes [23], although the poor crystalline yield of 2 indicated that other products had formed in this reaction. We extended these

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procedures to perform the 2:1 reactions of KCpttt with [Ln(BH4)3(THF)n] in THF, but we were only able to isolate mono-ring complexes; for Ln = La, [La(Cpttt)(BH4)2(THF)2] (3) was the major product (43 %), along with a trace amount of hexameric [{La(Cpttt)(-BH4)2}6] (4), whereas for Ln = Ce, dimeric [Ce(Cpttt)(BH4)(µ-BH4)(THF)]2 (5) was isolated in fair yield (62 %) (Scheme 2). We

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did not obtain a sufficient amount of crystalline 4 to characterize this complex further. We

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considered that monosubstitution had occurred as these reactions were sluggish, thus we repeated the reaction of 1 with two equivalents of KCpttt in THF and refluxed the reaction mixture for 16

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hours to furnish [La(Cpttt)2(BH4)] (6) in 67 % yield (Scheme 2). Finally, we separately reacted 1 and

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[Ce(BH4)3(THF)3.5] with two equivalents of KCp′′′ in THF to afford [Ln(Cp′′′)2(BH4)(THF)] (Ln = La, 7; Ce, 8) in fair-good yields (62-86 %) (Scheme 2). Interestingly, these reaction mixtures did not require heating to proceed to completion, in contrast to the Cpttt reactions, which we attribute to the greater ability of the silyl groups in Cp′′′ to bend away from the lanthanide centers in the solid state (see below). It is noteworthy that refluxing a THF mixture of [Ce(I)3(THF)4] with two equivalents of KCp′′′ gave only the mono-ring complex, [Ce(Cp′′′)(I)2(THF)2] [3], and the La complex [La(Cp′′′)2(I)(py)] was previously reported by Giesbrecht et al. to be only a minor product

10

ACCEPTED MANUSCRIPT from the reaction of [La(I)3(THF)4] and KCp′′′, with the mono-ring complex [La(Cp′′′)(I)2(py)3] the

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major product [24].

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[Ce(BH4)3(THF)3.5].

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Scheme 2. Synthesis of 2-3 and 5-8 from KCpR (CpR = Cptt, Cpttt, Cp′′′) and 1 or

Elemental microanalyses obtained for 2, 3, 5 and 7 were close to predicted values, though

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low carbon values were reproducibly obtained for 6, which we attribute to carbide formation. The reasons behind this discrepancy are unclear, but it has been acknowledged that elemental analysis experiments can be capricious for air-sensitive organometallic complexes [25]. Microanalysis services typically collect data using a standard set of conditions and a number of parameters could potentially be adjusted to optimize combustion of 6 (e.g. the furnace temperature). However, the 1H NMR spectrum of 6 indicated that there are negligible protic impurities (< 5 %), thus we are confident of its formulation. Despite repeated attempts we were unable to obtain elemental analysis results for 8 that were in acceptable agreement to calculated values, but as this complex is 11

ACCEPTED MANUSCRIPT analogous with fully characterized diamagnetic 7, the crystalline sample appeared to be homogenous, and only one species was observed in the 11B{1H} NMR spectrum, we also report the supporting data for 8 herein. The anticipated signals and integrals were seen in the 1H NMR spectra of diamagnetic 2, 3,

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6 and 7 for the CpR ring and substituent protons, and the 13C{1H} NMR spectra of these complexes were also as expected and comparable to previously reported La Cptt, Cpttt and Cp′′′ complexes [3,24]. The BH4 groups were observed between 1.09 (2) and 1.72 (6) ppm in the 1H NMR spectra as

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broad signals, which resolved to quartets for 2, 6 and 7, with 1JBH = 81, 76 and 72 Hz, respectively; these coupling constants are typical for lanthanide borohydrides [1,2]. The BH4 groups were also

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seen in the 11B{1H} NMR spectra [δB: –22.49 (2); –18.96 and –15.87 (3); –16.80 (6); –17.03 (7)], and additionally for 7 two signals were observed in the

29

Si{1H} NMR spectrum at 10.51 and –

9.57 ppm. We propose that the presence of two signals in the 11B{1H} NMR spectrum of 3 is due to the presence of trace amounts of 4 in solution, with only the major signal (δB: –18.96) tentatively

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assigned to 3. The observation of some additional signals in the 1H and 13C{1H} NMR spectra of 3

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supports this hypothesis.

Complexes 5 and 8 are paramagnetic and thus exhibit significantly broadened signals in

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their 1H NMR spectra over a 40-50 ppm range. The signals were assigned by integration and line shape rather than chemical shift, although the BH4 signals were broadened to such an extent that

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integrals could not be reliably extracted. Two signals were observed in the 11B{1H} NMR spectrum of 5 (δB: 33.15 and 65.94) due to terminal and bridging BH4 groups, whilst only one signal was observed for monomeric 8 (δB: 16.82). The paramagnetism of 5 and 8 precluded assignment of their 13

C{1H} NMR spectra, and in the case of 8 no signal was observed in the 29Si{1H} NMR spectrum.

We used the Evans method [26] to obtain values for the solution magnetic susceptibility of 5 (2.33 µB) and 8 (2.83 µB); these are both close to the predicted magnetic moment for a Ce(III) ion (2.54 µB) [27]. As with 1, the FTIR spectra of 2-8 show multiple signals that can be assigned to the 12

ACCEPTED MANUSCRIPT variable borohydride binding modes [1,2], which we have probed further via X-ray crystallography (see below). 3.2 Structural Characterization

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The solid state structures of 1-8 were determined by single crystal XRD (see Figures 1-6 for the molecular structures of 1, 3-6 and 8 and Table 1 for selected bond lengths and angles; additional crystallographic tables are compiled in the Supporting Information). The data for 2 is poor, thus we do not discuss the metrical parameters of this structure in detail and include it in the Supporting

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Information only, although the connectivity is clear-cut and the bulk features are similar to the

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previously reported structures of [Ln(Cptt)2(μ-BH4)]2 (Ln = Ce, Sm) [23]. The H atoms of the borohydride units of 1 and 3-8 were all identified in the difmap; the B–H distances were restrained to be approximately equal but were not fixed during refinement.

Complex 1 (Figure 1) exhibits an approximate pentagonal bipyramidal geometry in the solid

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state, with two axial BH4 units and the equatorial positions occupied by the remaining BH4 and four THF molecules. The structure of 1 is reminiscent of that of the [La(I)3(THF)4] precursor [6].

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Related to this, the structure of 1 has been refined with a minor iodide component at the two axial

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positions; the total contamination of iodide is only 3 % in the model presented, thus only the major component is depicted here. As the structures of the early lanthanide borohydride tris-THF adducts

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[Ln(BH4)3(THF)3] (Ln = La-Eu) have not previously been reported to the best of our knowledge, the metrical parameters of 1 [Ln···B range: 2.784(6)-2.904(13) Å; Ln–O range: 2.535(3)-2.608(3) Å] are compared here with those of [Y(BH4)3(THF)3] [Y···B range: 2.58(1)-2.68(2) Å; Y–O range: 2.350(6)-2.412(7) Å] [28] and [U(BH4)3(THF)3] [U···B range: 2.63(2)-2.69(1) Å; U–O range: 2.541(10)-2.579(8) Å] [29]. In agreement with the FTIR spectrum, the two axial BH4 units in the refined structure of 1 are set as κ3-bound and the equatorial BH4 is assigned a κ2-binding mode; this mixed motif was also seen in the structure of [Y(BH4)3(THF)3] [28].

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Fig. 1. Molecular structure of 1 with selective atom labeling, with displacement ellipsoids set at the 30 % probability level; hydrogen atoms (except for those in BH4 units) have been omitted for

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clarity.

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The mono-ring complex 3 is monomeric in the solid state, with a mutually trans- array of two κ3-BH4 and two THF ligands (Figure 2); this arrangement of ligands is similar to that seen

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previously for the related complex [Ce(Cp′′′)(I)2(THF)2] [3]. Whilst we could find no mono-ring Ln(III) Cpttt complexes to compare with 3, structurally characterized examples of mono-ring CpR Ln borohydride complexes are plentiful [8,30] and the geometrical features of this complex are unremarkable. We attribute the dimeric structure of the analogous mono-ring complex 5 to result from the different workup conditions employed, where displacement of a THF molecule at each Ce center creates a vacant coordination site for a bridging BH4 unit. As such, the Ce(III) ions in 5 each exhibit one terminal κ3- and one bridging µ-κ1:κ2-BH4 ligand by the refinement method used. As 14

ACCEPTED MANUSCRIPT expected the La∙∙∙Bterminal distance [2.645(6) Å] is much shorter than the La∙∙∙Bbridging [2.949(5) Å]; the terminal distance is comparable to that seen for 6 (see below), whilst the bridging distance is similar to those previously observed for [Ln(Cptt)2(μ-BH4)]2 [Ln∙∙∙Bbridging 2.93(2) Å (Ce); 2.833(6)-

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2.882(6) Å (Sm)] [23].

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Fig. 2. Molecular structure of 3 with selective atom labeling, with displacement ellipsoids set at the

clarity.

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30 % probability level; hydrogen atoms (except for those in BH4 units) have been omitted for

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Fig. 3. Molecular structure of 5 with selective atom labeling, with displacement ellipsoids set at the 30 % probability level; hydrogen atoms (except for those in BH4 units) have been omitted for

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clarity. Symmetry operation to generate equivalent atoms: i = 1–x, –y, –z.

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Complex 4 (Figure 4) crystallizes in a trigonal R–3 space group and exhibits a hexameric structure with the six La centers forming the vertices of an octahedron [La∙∙∙La∙∙∙La angles 60 and

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90° by definition; La∙∙∙La distances 5.397(2) and 5.469(2) Å]. All twelve edges are bridged by μ-

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BH4 units, thus all La centers are each bound by four BH4 groups, and all La vertices are capped with an η5-Cpttt ligand [La···Cpcent = 2.520(7) Å]. Each La center exhibits two short [2.79(3) Å mean] and two long [3.04(3) Å mean] La∙∙∙B distances, but identical La∙∙∙B∙∙∙La angles [137.4(7) and 138.0(9)°]. We have modeled μ-,κ2:κ2- binding modes for the BH4 units in the molecular structure of 4 despite this asymmetry; this formalism is for refinement purposes only as we are not fully confident in assigning precise binding modes of BH4 anions from the single crystal XRD data and FTIR spectrum obtained. The structure of 4 is best compared to a series of substituted Cpcapped Sm and Nd octahedral clusters reported by Bonnet et al. [31]. The most closely related 16

ACCEPTED MANUSCRIPT examples, [Ln(C5Me4nPr)(μ-BH4)2]6 (Ln = Sm, Nd), also contain twelve μ-BH4 edges but they crystallize in the C2/c space group and thus contain two geometrically distinct hexamers. The octahedra that are most comparable to 4 have similar mean Ln∙∙∙Ln (Ln = Sm, 5.217 Å; Nd, 5.255 Å), Ln∙∙∙Cpcent (Ln = Sm, 2.39 Å; Nd, 2.42 Å) and Ln∙∙∙B (Ln = Sm, 2.66 and 2.98 Å; Nd, 2.69 and

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2.99 Å) distances [31]. Interestingly, in contrast to 4, [Ln(C5Me4nPr)(μ-BH4)2]6 (Ln = Sm, Nd)

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exhibit two sets of Ln∙∙∙B∙∙∙Ln angles (Ln = Sm, 132.2 and 138.8°; Nd, 131.6 and 138.4°) [31].

Fig. 4. Molecular structure of 4, with displacement ellipsoids set at the 30 % probability level; hydrogen atoms have been omitted for clarity. Symmetry operations to generate equivalent atoms: i = –y, +x–y, +z; ii = +y–x, –x, +z.

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ACCEPTED MANUSCRIPT Finally, complexes 6-8 have similar bulk features, so are therefore discussed together [Figures 5 (6) and 7 (8); complex 7 is structurally analogous to 8, so is featured in the Supporting Information]. The La (6,7) and Ce (8) centers each exhibit two CpR rings in typical bent motifs [Cpcent···Ln···Cpcent: 141.57(6) Å (6); 133.14(12) Å (7); 132.92(12) Å (8)], with equatorially

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coordinated κ3-BH4 ligands. For 7-8 an additional molecule of THF completes the Ln coordination spheres; this can be attributed to the greater deviation from linearity of the Cp cent···Ln···Cpcent angles in these complexes. The structure of 6 is analogous to that of [Tm(Cpttt)2(BH4)]

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[Cpcent···Tm···Cpcent: 148.98(5)°] [9] and [Dy(Cpttt)2(BH4)] [Cpcent···Dy···Cpcent: 149.01(4)°] [11], whilst 7-8 are similar to that of the related bis-ring complex [Ce(Cp′′′)2(I)(THF)]

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[Cpcent···Ce···Cpcent: 133.77(6) Å] [3].

Fig. 5. Molecular structure of 6 with selective atom labeling, with displacement ellipsoids set at the 30 % probability level; hydrogen atoms (except for those in BH4 units) have been omitted for clarity.

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Fig. 6. Molecular structure of 8 with selective atom labeling, with displacement ellipsoids set at the

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30 % probability level; hydrogen atoms (except for those in BH4 units) have been omitted for

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4. Conclusions

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clarity.

We have reported the structure of a tetrakis-THF adduct of an f-element borohydride, 1. We have utilized La and Ce molecular borohydrides as starting materials for the direct synthesis of bisCpttt and bis-Cp′′′ La and Ce borohydride complexes, following similar methodologies to those employed by Jaroschik et al. for the smaller Tm(III) ion [9]. As with lanthanide trihalide precursors, mono-ring complexes of Cpttt are isolated when the reaction conditions are modified. Given the synthetic utility of BH4 groups, we anticipate that the heteroleptic mono-ring (3-5) and di-

19

ACCEPTED MANUSCRIPT substituted (2, 6-8) complexes characterized herein can be utilized in future to generate more complex structures.

The authors declare no competing financial interest.

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Acknowledgements

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Conflict of interest

Funding: This work was supported by the Engineering and Physical Sciences Research Council

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(Grant number EP/L014416/1; Nuclear FiRST studentship for A.F.), the China Scholarship Council (studentship for J.L.) and the University of Manchester. Additional research data supporting this publication are available from Mendeley Data at doi:10.17632/x5563zzr4g.1. Appendix A. Supplementary data

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Supplementary data related to this article can be found, in the online version, at doi:

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Ln···Cpcent 2.784(6)-

3

4

5

2.558(2)

2.520(7)

2.509(2)

2.710(4)2.645(6)

2.904(13)

6

7

2.585(2)

2.607(4)

2.724(5)

8 2.574(3)2.579(4)

2.684(5)

2.667(11)

2.641(13)

141.57(6)

133.14(12)

132.92(12)

2.555(6)

2.538(7)

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Ln···Bterminal

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1

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Table 1. Selected bond lengths (Å) and angles (°) for 1 and 3-8 (Ln = La, Ce).

2.786(14)Ln···Bbridging

2.949(5)

3.07(2)

2.535(3)-

2.583(2)-

2.608(3)

2.591(2)

2.519(2)

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Ln–O

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Cpcent···Ln···Cpcent

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ACCEPTED MANUSCRIPT Lanthanide borohydrides reacted with a series of potassium cyclopentadienyls.

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Facile disubstitution occurs for KC5H3tBu2-1,3 and KC5H2(SiMe3)3-1,2,4.

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For KC5H2tBu3-1,2,4 mono-ring complexes were isolated at room temperature.

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Forcing conditions facilitate disubstitution by KC5H2tBu3-1,2,4.

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