Cationic rare-earth metal hydrides

Accepted Manuscript Cationic Rare-Earth Metal Hydrides Jun Okuda PII: DOI: Reference:

S0010-8545(16)30336-8 http://dx.doi.org/10.1016/j.ccr.2016.09.009 CCR 112311

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Coordination Chemistry Reviews

Received Date: Revised Date: Accepted Date:

20 August 2016 18 September 2016 18 September 2016

Please cite this article as: J. Okuda, Cationic Rare-Earth Metal Hydrides, Coordination Chemistry Reviews (2016), doi: http://dx.doi.org/10.1016/j.ccr.2016.09.009

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RevisedCationic

Rare-Earth Metal Hydrides

Jun Okudaa,* a

Institute of Inorganic Chemistry, RWTH Aachen University, Landoltweg 1, 52056 Aachen, Germany E-mail: [email protected] Tel: (+49) 241 8094645

Contents 1. Introduction 2. Scope 3. Solvent-Stabilized Hydride Cations 3.1

Divalent Hydride Cations of the Type [(Ll)LnH]+

3.2

Trivalent Dihydride Cations of the Type [(Ll)LnH2]+

4. Ligand-Supported Dihydride Cations of the Type [(Ll)LnH2]+ 4.1

Hydride Cations Containing Me6TREN Ligands (l = 4)

4.2

Hydride Cations Containing Me4TACD Ligands (l = 4)

5. Ligand-Supported Hydride Cations of the Type [(LlX)LnH2]+ 5.1

Hydride Cations Containing a Benzamidinate Ligand (l = 1)

5.2

Hydride Cations Containing an Aminopyridinate Ligand (l = 1)

5.3

Hydride Cations Containing a PNP-Type Ligand (l = 2)

5.4

Cationic Half-Sandwich Hydride Complexes [CpLnH]+ (l = 2)

5.5

Hydride Cations Containing a NNNX-Type Ligand (X = C, N; l = 3)

6. Hydride Cations Containing a Dianionic Ligand 7. Conclusion Acknowledgment References

Abstract A survey of the literature on cationic rare-earth metal hydride complexes is presented. This overview follows systematics based on the nature of supporting ligands and provides comparisons between neutral and cationic hydrides with respect to their structure and reactivity.

Keywords Rare-earth metals, hydrides, cationic complexes

1. Introduction Since lanthanide-based interstitial alloys such as LaNi5H6 have been studied as hydrogen storage materials,[1] search for molecular models for these solid state systems has been initiated in order to understand the molecular mechanism of dihydrogen activation during uptake and release processes. Eventually the relationship between interstitial hydrides of the lanthanides of the type [LnHn] (n = 2,3) and molecular hydride complexes within a defined coordination sphere would lead to concepts for new materials based on lanthanide hydrides.[2] Lanthanide hydride complexes have been known since the 1980s.[3] As is generally the case in organometallic chemistry of early transition metals, the introduction of cyclopentadienyl scaffolds has helped to develop this area tremendously. In particular, metallocene or bis(η5-cyclopentadienyl) metal units allowed the straightforward synthesis of molecular hydrides of the lanthanides, mostly by σ-bond metathesis of the corresponding hydrocarbyl complexes.[4] Following the introduction of non-bis(cyclopentadienyl) ligand scaffolds for group 4 metal based polymerization catalysts (“post-metallocene catalysts”),[5] new types of supporting ligands are increasingly being applied to rare-earth metals allowing to expand their structural and reactivity patterns.[6] Electronic and steric control of the relatively electropositive and large rare-earth metal centers is not trivial, as intramolecular ligand fluxionality and intermolecular ligand scrambling often preclude stable and inert molecules with a defined ligand sphere.[7] As the first step, mono(cyclopentadienyl) complexes were shown to open up the ligand sphere but this comes at the cost of aggregation.[8a] With non-cyclopentadienyl ligands aggregation becomes even more prevalent, as steric demand diminishes. Therefore the introduction of cationic charges can be expected to reduce the nuclearity of rare-earth metal hydride fragments, at the same time to result in increase of Lewis acidity and electrophilicity.[9]

2. Scope This review summarizes and classifies cationic rare-earth metal hydrides, a relatively recent addition to the growing body of molecular rare-earth metal hydrides. The latter has been reviewed several times in the literature.[4,6] Rare-earth metal hydride complexes containing cyclopentadienyl (Cp) ligands have been reviewed in the past.[4,8b,10] An overview on non-Cp rare-earth metal hydrides appeared in 2008,[6] followed by a review on rare-earth metal alkyl and hydrido complexes with guanidinato and aminopyridinato ligands[11] and a comprehensive review in 2015.[12] Only in 2006 was systematic access to ligand-supported rare-earth metal hydrido cations reported.[13] These cationic complexes were obtained by the protonation of the metalhydride bonds in the corresponding neutral hydride compounds (using weak Brønsted acids 2

such as [NEt3H][BPh4]), by X-type ligand (hydride, alkyl) abstraction using a strong Lewis acid such as trityl Ph3C+, or by hydrogenolysis of cationic rare-earth metal alkyl precursors using H2 or PhSiH3 (σ-bond metathesis).[6,9,12] Figure 1 compiles the possible complex units for lanthanide hydride cations. a) [(Ll)LnH2 ] +

[(Ll)LnH] unknown

[(LlX)LnH]

b) [(Ll)LnH3] +

[(Ll)LnH2] l = 4: + [(Me4TACD)LnH2]

[(LlX)LnH2] +

2+

[(Ll)LnH] unknown

[(LlX)LnH] l = 4: + [(Me5 TRENCH2 )LnH] + [(NN)LnH(THF)3]

[(LlX)2LnH]

Figure 1. Possible types of cationic hydride complexes of rare-earth metals with representative examples: a) divalent, b) trivalent metal center; L: two-electron ligands; l: number of L ligands; X: one-electron ligands.

Cluster-type hydride cations can be formally considered as being composed of neutral dihydride units [(LlX)LnH2] and the cationic fragment [(LlX)LnH]+: [(LlX)n+1Lnn+1H2n+1]+ = n [(LlX)LnH2]·[(LlX)LnH]+.

3. Solvent-Stabilized Hydride Cations 3.1 Divalent Hydride Cation of the Type [(Ll)LnH]+ Although reports on THF-soluble ytterbium dihydrides of the formula [(Ll)YbH2] exist,[14] systematic studies on divalent hydride compounds have been conducted only rarely. Thus cationic hydride compounds of the type [(Ll)YbH]+ does not seem to have been reported so far. With the current surge in the fascinating chemistry of divalent lanthanide complexes,[15] molecular rare-earth metal hydrides with metal centers having oxidation state +2 will offer a new area of study.

3.2. Trivalent Dihydride Cation of the Type [(Ll)LnH2]+

3

Although a neutral trihydride complex that only contains a neutral ligand set [(Ll)LnH3] remains elusive,[16] cationic derivatives of the type [(Ll)LnH2]+ have become known with solvent molecules such as L = THF.[17] Pronounced tendency to form aggregates makes the study of these systems most tedious.[17d] In principle, σ-bond metathesis of a dialkyl cation [(Ll)LnR2]+ (R = CH3, CH2SiMe3, C3H5, C6H3CH2NMe2) using H2 or silane should give [(Ll)LnH2]+. However, despite decade long efforts conclusive understanding of the course of the hydrogenolysis or the product formation and structure could not be obtained.[17] All indirect evidence unambiguously proves formation of what appears to be a tetranuclear tetracationic aggregate of the type [{LnH2(Ll)}4]4+, at least in the case of yttrium (Scheme 1). Depending on the solvent and the anion, 1H NMR spectra show a diagnostic quintet at around δ 5 ppm with 1JYH = 12.5 Hz. Degradation of this aggregate by NNNN-type azamacrocycle Me3TACDH and Me4TACD to give structurally defined dinuclear tetrahydride dications further point at the existence of this tetranuclear aggregate (see chapter 4.2). Also, reaction of this species with benzophenone gave a NMR spectroscopically and crystallographically fully characterized insertion product [(Ll)Y(OCHPh2)2]+.[17d]

Ph Ph

H

+

O

1/n [YH2(THF)3]n[A]n

Ph2CO, triglyme

O

THF, 25 °C

O

A = Al(CH2SiMe3)4

O Y

THF

[A]–

O O

Ph H Ph

Scheme 1. Formation and reaction with benzophenone of [{YH2(THF)3}4]4+.

Several single crystals were subjected to X-ray diffraction study.[17b] Due to severe disorder and low quality data set, only the tetranuclear framework could be verified. It is obvious that packing of an ion quintuplet AB4 is relatively difficult. DFT calculations on [{YH2(THF)3}4]4+ confirmed a stable cluster structure of closo-type with delocalized molecular orbitals. It features six µ2- and two µ3-hydride ligands.[17d] Solution dynamics of this tetranuclear aggregate remains somewhat puzzling. Once the tetranuclear aggregate has been formed in solution, no interconversion into species with 4

different nuclearity, detectable in solution by 1H NMR spectroscopy, seems to occur on the NMR time scale.[17d]

4. Ligand-Supported Dihydride Cations of the Type [(Ll )LnH2]+ 4.1 Hydride Cations Containing Me6TREN Ligands (l = 4) Tetradentate tripodal ligand Me6TREN (tris{2-(dimethylamino)ethyl}amine) was utilized to generate cationic dihydrido complexes starting from the cationic bis(alkyl) precursors [(Me6TREN)Ln(CH2SiMe3)2][A] (A = [B{C6H3-3,5-(CF3)2}4]). This dialkyl cation was thermally unstable, ultimately generating [(Me5TREN-CH2)Lu(CH2SiMe3)][A] as result of C–H bond activation at one of the six methyl groups of Me6TREN (Scheme 1).[18] Upon reaction with PhSiH3, [(Me5TREN-CH2)Lu(CH2SiMe3)][A] gave the dinuclear dicationic dihydride complex, [(Me5TREN-CH2)Lu2H2][A]2. In the 1H NMR spectrum two hydride resonances at δ 11.65 and 11.69 ppm were recorded due to the presence of two diastereomers (meso and rac). The analogous yttrium dihydride dication shows two doublets centered at δ 5.62 with 1JYH = 19.7 Hz. This dihydride dication added H2 to afford the dicationic trihydride, [(Me5TRENCH2)Lu(µ2H)3Lu(Me6TREN)][A]2 (1H NMR: δ 8.27 ppm). This hydrido complex and the isostructural yttrium

analogue,

[(Me5TRENCH2)Y(µ2-H)3Y(Me6TREN)][A]2,

were

obtained

by

hydrogenolysis of the adducts generated in situ [(Me6TREN)Ln(CH2SiMe3)2][A]. Attempts to prepare a tetrahydride by further hydrogenolysis under restoration of the second Me6TREN ligand remained unsuccessful.

Scheme 1. Dicationic dinuclear hydrido complexes with a neutral Me6TREN and metalated Me5TRENCH2 ligands.

The Lu···Lu distance of 3.2775 Å in [(Me5TRENCH2)Lu(µ2-H)3Lu(Me6TREN)][A]2 is shorter by 0.24 Å compared with Lu···Lu distance of 3.5147 Å in the dihydride. According to DFT 5

calculations a LUMO localized along the Lu···Lu axis in the dihydride [(Me5TRENCH2)Lu2H2][A]2 facilitates the σ-bond metathesis reaction that leads to the trihydride complex. Reactions of the di- and trihydrido complexes with benzophenone resulted in quantitative conversion of the hydrides and metal-bonded carbon atoms to the corresponding alkoxides. Analogous gadolinium and dysprosium hydrido complexes were shown to be single molecule magnets.[19]

4.2. Hydride Cations Containing Me4TACD Ligands (l = 4) Tetradentate NNNN-type macrocycle Me4TACD (1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane) was shown to support cationic lutetium alkyl and hydrido complexes (Scheme ).[17c,20] As for the Me6TREN ligand, the methyl substituents of the Me4TACD ligand underwent C–H bond activation (metalation). Using phenylsilane as hydride source, a triply hydride bridged bimetallic Me4TACD complex (1H NMR: δ 8.92 ppm) was obtained which contains a metalated methyl group (Lu···Lu distance of 3.1747(5) Å). This metal–carbon bond adds dihydrogen under mild conditions to give a quadruply hydride bridged dinuclear rare-earth metal complex (1H NMR: δ 8.99 ppm) with a short Lu···Lu distance of only 2.9270(6) Å. Most remarkably, a reverse C–H bond activation toward the trihydride is achieved by heating the tetrahydride complex under vacuum (even in the crystalline state). Mechanistic investigations using DFT calculations, kinetic studies, and isotope labelling experiments indicate that the addition and release processes involve several steps including possibly highly labile dihydrogen adducts. Hydrogenolysis of the trihydride and the alkyl complexes in the solid state and in a single-crystal to single-crystal transformation may shed light on hydrogen storage mechanisms involved in bulk materials containing metal–carbon bonds.[20]

Scheme 3. Dicationic dinuclear hydrido complexes with a neutral Me4TACD and metalated Me3TACDCH2 ligand showing reversible hydrogen addition and elimination. 6

The analogous dinuclear quadruply hydride bridged yttrium complex [(Me4TACD)2Y2H4] [A]2

was

prepared

from

the

metalated

1

mono(trimethylsilylmethyl)

complex

by

1

hydrogenolysis ( H NMR: δ 6.58 ppm, t, JYH = 22.9 Hz) and crystallographically shown to exhibit a Y···Y distance of 3.0615(9) Å, the shortest Y···Y distance reported in the literature (Scheme 4).[17d] This hydride cation was also umambiguously obtained by reacting the solvent-stabilized dihydride cation with Me4TACD.[17c,d]

Scheme 4. Synthesis of the dicationic dinuclear hydrido complex [(Me4TACD)2Y2H4]2+ of yttrium with a Me4TACD ligand.

5.

Ligand-Supported Hydride Cations of the Type [(LlX)LnH2]+

5.1

Hydride Cations Containing a Benzamidinate Ligand (l = 1)

A monohydride cation containing a bulky benzamidinate ligand was reported as the first cationic terminal hydrido rare-earth complex [(NN)LnH(THF)3][BPh4] (NN = PhC(NC6H3-2,6i

Pr2)2).[21]

The

high

pressure

[(NN)Ln(CH2SiMe3)(THF)3]BPh4

hydrogenolysis

generated

in

situ

of from

the the

cationic reaction

alkyl of

[(NN)Ln(CH2SiMe3)2(THF)] with [NEt3H][BPh4], gave [(NN)LnH(THF)3][BPh4] (Scheme 5). The 1H NMR spectrum of the yttrium complex exhibited a characteristic doublet at δ 7.50 ppm for the terminal hydrido ligand with a large Y–H coupling constant of 1JYH = 73.4 Hz, which is comparable to those found in [(η5-C5Me5)2YH(THF)] (1JYH = 81.7 Hz),[22a] [(Ind*)2YH(THF)]

(1JYH

=

82.0

Hz,

Ind*

=

heptamethylindenyl),[22b]

and

[(η5-

C5Me4SiMe3)2YH(THF)] (1JYH = 74.8 Hz).[22c] The lutetium cation showed a signal at δ 12.28 ppm Recrystallization of these complexes from chlorobenzene gave dimeric species featuring two bridging hydrido ligands [(NN)Ln(µ-H)(THF)2]2[BPh4]2 (1H NMR for Ln = Y: δ 7.55 ppm, t, 1JYH = 29.0 Hz; for Ln = Lu: δ 12.52 ppm), which underwent dissociation into their mononuclear compounds upon dissolution in THF (Scheme ). When the lutetium hydride cation was treated with one equiv. of [NEt3H][BPh4], the mononuclear dicationic complex [(NN)Ln(THF)4](BPh4)2 was formed.[21]

7

Scheme 5. Preparation of cationic monohydrido complexes with amidinato ligands and their solvent-dependent dimerization.

5.2. Hydride Cations Containing an Aminopyridinate Ligand (l = 1) Protonolysis of the mixed alkyl hydrido aggregate [(Ap*)3Y3H5(CH2SiMe3)(THF)2] (Ap* = (Dipp){6-(2,4,6-triisopropylphenyl)pyridin-2-yl}amine)[23a] with one equivalent of [PhNMe2H][B(C5F5)4] led to the selective protonation of the trimethylsilylmethyl group resulting in the formation of the monocationic trinuclear hydride, [Ap*3Y3H5(THF)3][B(C5F5)4] (Scheme 6).[23b] The Y3H5 core resembles that found in [(Me-PNPiPr)3Ln3H5][BPh4] (Scheme 7). The catalytic activity of this cation toward ethylene polymerization was appreciably higher than that of the corresponding mixed alkyl hydrido complex. The 1H NMR spectrum at 190 K showed a broad multiplet at δ 4.80 ppm for the µ3-hydride and a triplet at 6.62 ppm with 1JYH = 31 Hz for the µ2-hydride.

Scheme 6. Selective protonation of the alkyl group in the mixed alkyl hydrido complex [(Ap*)3Y3H5(CH2SiMe3)(THF)2].

5.3 Hydride Cations Containing a PNP-Type Ligand (l = 2) Hydrogenolysis of the dialkyl [(Me-PNPiPr)Y(CH2SiMe3)2] (Me-PNPiPr = {4-methyl-2(iPr2P)C6H3}2N) gave the trinuclear hexahydride complex [{(Me-PNPiPr)YH2}3] [24] with fairly 8

common structure motif.[12,25] Protonolysis with [NEt3H][BPh4] led to the monocationic pentahydride [(Me-PNPiPr)3Y3H5][BPh4]. The 1H NMR spectrum showed a singlet at δ 5.33 ppm for the µ3-hydride and a triplet at δ 6.52 ppm with 1JYH = 31.7 Hz for the µ2-hydrides. The lutetium analogue showed two broad resonances at δ 8.43 and 11.85 ppm.[24] The core structure of [(Me-PNPiPr)3Ln3H5][BPh4] resembles the one found in the neutral [{(Dipp)NH(CH2)2N(Dipp)}2{(Dipp)N(CH2)}2Y3H5(THF)] [26] (Scheme 7). The Ln3H5 core is rigid even in solution with the hydride ligands giving rise to two different sets of resonances in the 1H NMR spectrum. The [BPh4]– anion does not appear to interact with the cation either in the solid state or in solution. In the case of [(η5-C5Me4SiMe3)4Y4H7][B(C6F5)4], a contact ionpair structure was found in non-coordinating solvents.[27]

Scheme

7.

Hydride

abstraction

from

[(Me-PNPiPr)LnH2]3

upon

protonolysis

with

[NEt3H][BPh4] to afford a cationic trimeric hydride complex. In an alternative route, the dialkyl precursors [(Me-PNPiPr)Ln(CH2SiMe3)2] (Ln = Lu, Y) were reacted with H2 in the presence of 0.5 equivalent of [NEt3H][BPh4] to give dinuclear complexes with an Ln2H3 core [(Me-PNPiPr)2Ln2H3(THF)2][BPh4] (Scheme ). The 1H NMR spectrum shows a triplet at δ 5.72 ppm with 1JYH = 23.1 Hz for the µ2-hydrides. The lutetium analogue shows a resonances at δ 8.45 ppm.

Scheme 8. Hydrogenolysis of a PNP pincer dialkyl complex in the presence of a Brønsted acid. 9

These dinuclear compounds can be regarded as a combination of a neutral dihydride and a cationic monohydride species formed during the hydrogenolysis of the corresponding neutral dialkyl and the monocationic alkyl species. Formal classification would be [(L2X)2Ln2H3]+

= [(L2X)LnH2]·[(L2X)LnH]+. Alkyl complexes of rare earth metals and

ruthenium hydrides were used to synthesize heterobimetallic hydrido complexes.[27,28]

5.4 Cationic Half-Sandwich Hydride Complexes [CpLnH]+ (l = 2) Following the synthesis and structural characterization of the first example of a half-sandwich dihydride complex of yttrium,[29] an extensive series of tetranuclear octahydride rare-earth metal complexes of general formula [(η5-C5Me4SiMe3)Ln(µ-H)2]4(THF)n (Ln = Sc, Y, Gd, Dy, Ho, Er, Tm, Lu; n = 0, 1, or 2) that contain cyclopentadienyl ligand such as C5Me4SiMe3 as an ancillary ligand have been prepared and structurally characterized.[10b,c,30] These hydride clusters maintain their tetranuclear framework in solution. The reaction of these neutral polyhydrides with one equivalent of [Ph3C][B(C6F5)4] gave the corresponding cationic heptahydrido clusters [(η5-C5Me4SiMe3)4Ln4H7(THF)n][B(C6F5)4] (n = 0,1,2) (Scheme 9).[13] X-ray analysis of the THF free compound and mono(THF) adduct revealed that they have a distorted tetrahedral Y4 frame, which is bound by seven hydride ligands, one in µ4, two in µ3, and four in µ2 bonding modes. Direct interaction between the [(η5-C5Me4SiMe3)4Ln4H7(THF)n]+ cation and the [B(C6F5)4]− anion through a Y-F bond was found in the crystal of the THF free compound (n = 0). The 1H NMR spectrum of the THF free compound shows a broad signal at δ 4.62 ppm for all seven hydrides. The mono(THF) adduct exhibited a broad resonance at δ 1.45 ppm for the µ4-hydride, two quartets at δ 3.24 ppm (1JYH = 21.0 Hz) and δ 3.53 ppm (1JYH = 25.2 Hz) for the µ3-hydrides and two triplets at δ 4.98 ppm (1JYH = 25.2 Hz) and δ 5.40 ppm (1JYH = 37.5 Hz) for the µ2-hydrides. Interestingly the bis(THF) adduct has a butterfly-like [Y4] core. These hydride cluster cations catalyze the syndiospecific polymerization of styrene and regio- and stereospecific cis-1,4-polymerization of 1,3-cyclohexadiene.[13]

10

Me3Si

Me3Si

Y

Y H H

Me3Si Y H

SiMe 3 +

H

Y

-

[Ph3C] [B(C6F5)4]

Y

H

H

H

H

H

H H

Me3Si H

Y H

H

Y

Y

H [B(C6F5)4] SiMe 3

SiMe 3

Me3Si

Me3Si

Y

Y Me 3Si

THF

SiMe 3

H

H H H

Y

+

H Y

H H

Y

SiMe 3 [Ph3C] [B(C 6F5)4]

-

Me 3Si

THF

H

H H H

Y H

H

H

Y

Y

H THF

SiMe 3 [B(C 6F5) 4]

SiMe 3

SiMe 3

Scheme 9. Cationic half-sandwich hydride cluster. Hydride cations of the type [(η5-C5Me4SiMe3)LnH(THF)n][A] remain elusive so far (Scheme 10).[31]

Scheme 10. Attempt to prepare [(η5-C5Me4SiMe3)ScH(THF)n]+.

5.5 Hydride Cations Containing a NNNX-Type Ligand (X = C, N; l = 3) A dicationic dilutetium hydrido complex was obtained by using a NNNC-type macrocyclic ligand derived from meta-cyclophane, (Scheme 11).[32] The Lu···Lu distance is 3.4565(8) Å.

11

Scheme 11. Hydrogenolysis of an alkyl lutetium cation featuring a meta-cyclophane derived NNNC-type ligand to afford a dicationic dihydrido complex.

When a solution of solvent-stabilized hydride cluster [{YH2(THF)3}4]4+[A]-4 was treated with the mono(protic) amide-triamine proligand (Me3TACD)H dihydrogen gas evolution was observed to give the dinuclear dihydride dication [(Me3TACD)2Y2H2(THF)2]2+[A]-2. The 1H NMR spectrum of the reaction mixture showed a broad triplet at δ 5.24 ppm (1JYH = 22.3 Hz). This

cation

was

also

obtained

by

reacting

the

cationic

alkyl

[(Me3TACD)Y(CH2SiMe3)(THF)]+[A]- with dihydrogen (1 bar) or phenylsilane in diethyl ether (Scheme 12). Single crystal X-ray diffraction shows a dinuclear C2-symmetrical cationic part in the solid state. Each seven-coordinate yttrium center is surrounded by four nitrogen atoms of the Me3TACD ligand, two bridging hydride ligands and an additional THF molecule in the apical position trans to the amide function, resulting in pentagonal bipyramidal coordination geometry around each metal center.[17c,d] The 1H NMR spectrum shows a triplet at δ 5.30 ppm with 1JYH = 22.6 Hz for the µ2-hydrides.

Scheme 12. Dicationic dihydride complex containing a Me3TACD ligand.

6. Hydride Cations Containing a Dianionic Ligand A scandium hydride ion pair supported by the dianionic NNNN-type macrocyclic ligand 1,7-Me2TACD was shown to consist of a trimetallic dihydride cation and a trimetallic tetrahydride anion (Scheme 13).[33] The cationic part is similar to that in the monocationic calcium hydrido complex [(1,4,7-Me3TACD)3Ca3H2][A] exhibiting two µ3-bridging hydrides (1H NMR: δ 2.16 ppm).[34] The anion is chain-like featuring four µ2-bridging hydrides (1H NMR: δ 4.37 ppm).

12

Scheme 13. Scandium hydride ion pair stabilized by dianionic Me2TACD ligands. Higher reactivity was observed for the anion which was doubly protonated by trimethylammonium borate or oxidized by diphenyldisulfide. In both cases, a cation with the C3-symmetric core structure was obtained, charge-balanced by a borate or thiophenolate anion, respectively. Reaction with ethylene was unexpectedly accompanied by cluster rearrangement to give a dinuclear complex featuring one bridging hydrido ligand and a terminal ethyl group. The latter seems to show a dynamic β-agostic interaction.[33]

7. Conclusion Cationic alkyl complexes of rare-earth metals are now well established and have gained importance as polymerization catalysts.[9,10] Although synthetic methods have become routine and the cationic hydride complexes are thermodynamically favored, they are less common. This may be due to the presence of sterically accessible, polarizable hydride ligands which can lead to complicated structures. Despite the cationic charge, aggregation cannot be prevented in many cases, with some structural motifs analogous to those of the related neutral hydride clusters.[12] So far, only one example of cationic hydride with a terminal hydride ligand has been reported.[21] The nucleophilicity of hydride ligands at Lewis acidic lanthanide centers is retained, although general reactivity patterns remain still to be explored. Not directly related to the topic covered here, it is worth noting that cationic tetrahydridoborates of the lanthanides are also known.[35]

Acknowledgment Financial support by the Fonds der Chemischen Industrie, the Deutsche Forschungsgemeinschaft through the International Research Training Group “Selectivity in Chemo- and 13

Biocatalysis (SeleCa)”, and Cluster of Excellence “Tailor Made Fuel from Biomass” is gratefully acknowledged. I thank all coworkers engaged in this project. This work is dedicated to Prof. G. E. Herberich on the occasion of his 80th birthday.

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Highlight

This review summarizes and classifies cationic rare-earth metal hydrides, a relatively recent addition to the growing body of molecular rare-earth metal hydrides. The latter has been reviewed several times in the literature. The introduction of cationic charges can be expected to reduce the nuclearity of rare-earth metal hydride fragments, at the same time to result in increase of Lewis acidity and electrophilicity.

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