Evaluating Electron Transfer Reactivity of Rare-Earth Metal(II) Complexes Using EPR Spectroscopy

Complexes Using EPR Spectroscopy

Article

Evaluating Electron Transfer Reactivity of Rare-Earth Metal(II)

Samuel A. Moehring and William J. Evans*

Cite This: https://dx.doi.org/10.1021/acs.organomet.9b00837

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sı Supporting Information

ABSTRACT: To evaluate the relative reducing capacities of rare-earth metal complexes of

Sc(II), Y(II), and complexes of the lanthanide metals in their +2 oxidation state, a series of

III

1−

reactions of trivalent Ln A compounds with divalent [Ln′ A′ ] complexes has been

examined, where Ln = Sc, Y, or a lanthanide and A is C H SiMe (Cp′), C H (SiMe )

3 2

tet

(

Cp″), C Me H (Cp ), N(SiMe ) (NR ), 2,6- Bu -C H O (OAr), or 2,6- Bu -4-Me-

3 2

C H O (OAr′). The speciﬁc combinations were chosen to allow evaluation by EPR

1−

spectroscopy of the Ln(II) complex. The [Ln Cp′ ] complexes of Y(II), La(II), and Lu(II)

III

have similar reducing abilities in that they all reduce Ln Cp′ complexes of the other metals

in this group. However, these Y(II), La(II), and Lu(II) complexes all are stronger reductants

1−

III

than [Gd Cp′ ] , which cannot reduce Ln Cp′ complexes of Y, La, and Lu. These results do not apply to all ligand sets, since

1−

III

1−

[

Gd (NR ) ] can reduce Y (NR ) to [Y (NR ) ] . The amide and aryloxide complexes of Y and Sc are similar in that

2 3

1−

III

1−

III

1−

III

Y (NR ) ] reduces Sc (NR ) and [Y (OAr′) ] reduces Sc (OAr′) . Both [Y (NR ) ] and [Y (OAr′) ] reduce Y Cp′ .

2 3

tet 1−

1−

La Cp ₃] has reductive capacity similar to that of [La Cp′ ] , and both are stronger reductants than [La Cp″ ] . None of the

III

1−

Ln I complexes of Sm, Tm, Dy, and Nd can reduce Ln A complexes of Y and La to [Ln A ] . In the “same-metal-diﬀerent-

ligands” reactions, multiple EPR signals are found, suggesting that ligand exchange occurs alongside the electron transfer reactivity.

INTRODUCTION

■

Redox chemistry is one of the two most fundamental types of

reactivity, along with acid−base chemistry. Inherent in the

utilization of redox reactions is the availability of diﬀerent

oxidation states and the redox potentials that interconnect them.

In the ﬁeld of lanthanide chemistry, new opportunities have

arisen with the discovery that stable molecular complexes of +2

III

To ﬁll this gap in experimental data, Ln A complexes that

ions could be isolated not only for Eu, Yb, Sm, Tm, Dy, and Nd,

but also for all the rest of the lanthanides except radioactive

are known to form new Ln(II) ions (A = anion such as

cyclopentadienyl, amide, or aryloxide) were reacted with

Ln A ] complexes involving either diﬀerent metals or

diﬀerent ligands to evaluate their comparative reduction

chemistry. To accomplish this comparison, Ln(II) systems

were selected that allow EPR spectroscopy to be used to

interrogate the systems. This is most readily accomplished with

−6

Pm.

Examples with silylcyclopentadienyl ligands are shown

1−

[

in eq 1. Surprisingly, these new Ln(II) ions made by reduction of

f Ln(III) ions had properties consistent with 4f 5d electron

n+1

conﬁgurations rather than the 4f conﬁgurations of Eu(II),

Yb(II), Sm(II), Tm(II), Dy(II), and Nd(II). Although these

recently discovered +2 oxidation states are being found in an

,4−6,19

−16

La, Gd, Lu, Y, and Sc.

It was found that EPR spectroscopy

increasing number of coordination environments,

informa-

even allows characterization when complicated mixtures of

products are formed. The comparative reaction data are

presented along with a discussion of the implications for rare-

earth reduction chemistry.

tion on the redox potentials of these ions has been elusive. The

highly reducing species can react with supporting electrolytes

and in some cases with THF solvent. For example, reduction of

III

Ln Cp (Cp = C H Me; Ln = La, Pr) according to eq 1,

Me 1−

presumably to form [Ln Cp ₃] complexes, causes ring

2−

opening of THF to form products containing [O(CH ) ]

Received: December 9, 2019

dianions. Since the new Ln(II) complexes can be generated

with K and Na (eq 1), the redox potentials must be less negative

than −2.7 V vs SHE. However, the relative reactivities of these

species and the dependence of their redox potentials on the

speciﬁc metal and the ligand remain unknown.

XXXX American Chemical Society

Organometallics XXXX, XXX, XXX−XXX

Organometallics

Article

RESULTS

on reduction with traditional Ln(II) ions of Sm, Tm, Dy, and

Nd.

■

Methods. Each reaction described below involves reaction of

]¹⁻with Ln′ Cp′

III

1−

Reactions of [Ln Cp′

(Cp′

(THF) (La :

a THF solution of a [Ln A ] complex with a THF solution of

1−

III

SiMe

). Treatment of colorless La Cp′

Ln A , where either the metal or ligand of the Ln(III) reagent

1− 89 II

f 5d , S = 0) with maroon [Y Cp′ ] ( Y : S = / , I = / ,

1−

diﬀers from that of the Ln(II) reagent. Some of the [Ln A ]

00% abundance) in THF (reaction 2) gave a maroon mixture

complexes utilized in this study were too reactive to be isolated

and were identiﬁed only through EPR spectroscopy. In order to

maintain a consistent experimental approach for both isolable

1−

with an EPR spectrum containing signals for both [La Cp′ ]

139 II

(

Y Cp ′ ] (Figure 1 ). Hence, Y(II) reduced La(III), but the

La : 4f 5d , S = / , I = / , 99.9% abundance) and

2 2

2,4

1−

1−

and nonisolable compounds, the “reductant” [Ln A ] species

was generated by passing a THF solution (∼10 mg/mL; 10−20

III

mM) of Ln A through a pipet packed with KC that had been

chilled to −35 °C. The resulting intensely colored solution was

dripped onto a THF solution (10−20 mM, equimolar with the

III

“

reductant” species) of a “substrate” Ln A compound (where

Ln or A was diﬀerent) containing 1 equiv of 2.2.2-cryptand

crypt). In each case, the mixture was then loaded into a chilled

(

EPR tube and quickly frozen. This method allowed the unstable

Ln(II) compounds to be quickly generated and trapped in the

frozen solution. The whole process took no longer than 90 s.

Crypt was included to stabilize any Ln(II) products generated,

since it has been seen in this study and in others that thermal

stability of Ln(II) complexes can be enhanced by the presence of

crypt. Crypt could not be included in the reductant solution,

however, because passing a THF solution of only crypt through

Figure 1. Room-temperature X-band EPR spectrum (red solid) and

simulated spectrum (black dashed) of the products of reaction 2 in the

forward direction at (top) ×1 and (bottom) ×10. Signals are present at

1−

139

KC produces a deep-blue solution of an electride species that

g = 1.99, A( Y) = 37.1 G ([Y Cp′ ] ) and g = 1.97, A( La) = 153.4

20,21

1−

can act as a reductant itself (Figure S42 ).

G ([La Cp′ ] ).

The reactions with DyI and NdI were carried out with slight

modiﬁcations of the procedure. Soluble DyI (THF) and

reaction did not form La(II) exclusively. Consistent with this,

III

NdI (THF) were ﬁrst extracted from base-free DyI and

treatment of yellow Y Cp′ (Y : 4d , S = 0) with maroon

II 1− II 1−

NdI with THF at −35 °C. These solutions were then reacted

[La Cp′ ] formed [Y Cp′ ] , i.e., La(II) can reduce Y(III).

with the substrate Ln(III) compounds under the same

These results (reaction 2) indicate that the reduction potentials

III

conditions as in the other experiments, i.e., at −35 °C in THF.

of La Cp′ (THF) and Y Cp′ are very similar.

Since DyI (THF) and NdI (THF) are known to decompose

1−

III

1−

[Y Cp′ ] + La Cp′ (THF) V Y Cp′ + [La Cp′ ]

in THF over hours at room temperature, the reactions were

done quickly while the colors indicative of DyI (THF) and

(

NdI (THF) were still present.

The four combinations of Ln(II) reagents of Y and Lu with

III

1−

Reduction of a Ln′ A′ “substrate” by a [Ln A ]

Ln′(III) substrates of La and Lu gave similar results (reactions 3

III

“

reductant” (where Ln and Ln′ are the two metals and A and

and 4; Figure 2). Hence, La Cp′

(THF), Y Cp′

, and

III

A′ are the ligands involved in the electron transfer experiment)

Lu Cp′

(Lu : 4f , S = 0) all have similar reduction potentials.

1−

was counted as a successful reaction only when the [Ln′ A′ ]

This was somewhat surprising since the half-lives of the

species could be detected by EPR spectroscopy, i.e., loss of the

complexes (measured by UV−vis spectroscopy) are quite

175

1−

diﬀerent: [K(crypt)][Lu Cp ′ ] ( Lu : 4f 5d , S = / , I =

EPR signal of the [Ln A ] reactant was not suﬃcient. The

reactions in this paper are written to show when a trans-

formation occurred in the forward direction. Since equilibrium

constants for these systems are not known, only reactions in the

forward direction are reported. All of the EPR spectra used for

this study can be found in the Supporting Information.

Overview. The logic of the order of presentation of the

3−

following sections is described here. Since the (Cp′ ) ligand

set (Cp′ = C H SiMe ) is known to stabilize all of the Ln(II)

ions except small Sc(II) and radioactive Pm(II), this ligand set

was chosen for the initial investigations in which the ligand set

was kept constant and the metal was varied. Metal variation was

3−

next studied with complexes of the [(NR ) ] ligand set (R =

SiMe ) since several are available and this provides a comparison

−

3−

of the ligand sets (Cp′ ) versus [(NR ) ] . Ligand variation is

2 3

Figure 2. Room-temperature X-band EPR spectrum (red solid) and

simulated spectrum (black dashed) of the products of reaction 4 in the

reverse direction at (top) ×1 and (bottom) ×50. Signals are present at g

subsequently described with yttrium as the sole metal since a

wide variety of Y(II) compounds have been identiﬁed by EPR

spectroscopy. After the yttrium section is a section discussing

only scandium complexes for a comparison of Y versus Sc. La

chemistry is then described last, since it shows the most ligand

exchange. Ligand exchange is then discussed with a ﬁnal section

139

1−

175

1.97, A( La) = 153.5 G ([La Cp′ ] ); g = 1.97, A( Lu) = 425.3 G

175 II 1− 176 176 II 1−

(

[ Lu Cp′ ] ); and g = 1.97, A( Lu) = 300.3 G ([ Lu Cp′ ] ).

The large Lu hyperﬁne coupling, which is not ﬁrst-order, has been

discussed previously.

Organometallics XXXX, XXX, XXX−XXX

Organometallics

3 ) does not occur (Figure 3 ), as no signals consistent with

Article

⁷/₂, 97.4% abundance; ¹⁷⁶Lu : 4f 5d , S = / , I = 7, 2.6%

1− 45 II

[Sc (NR ) ] ( Sc : 3d , S = / ; I = / , 100% abundance)

2 3

abundance) has a ﬁrst-order t₁of 19 min, and [K(crypt)]-

can be used to detect successful reductions, as they are the only

Ln(II) compounds in this ligand set with known EPR spectra at

77 K and room temperature.

While [Y (NR ) ] can reduce Sc (NR ) (Sc : 3d , S = 0)

to [Sc (NR ) ] (reaction 12), the reverse reaction (reaction

[

Y Cp′ ] has a second-order t of 2.3 h at 3 mM, while

1/2

,26

K(crypt)][La Cp′ ] appears to have a half-life of over 30 h.

1−

III

1−

III

1−

2 3

1−

2 3

[

Lu Cp′ ] + Y Cp′ V Lu Cp′ + [Y Cp′ ]

(3)

(4)

2 3

1−

III

[

Lu Cp′ ] + La Cp′ (THF) V Lu Cp′

1−

[La Cp′ ]

In contrast to these reactions that proceed from either

1−

155

direction, [Gd Cp′ ] (Gd : 4f 5d , S = 4; Gd, I = / ,

4.8%; Gd, I = / , 15.6%; hyperﬁne splitting is not observed

1−

in the EPR spectrum of [Gd Cp′ ] ) does not reduce

III

La Cp′ (THF), Y Cp′ , or Lu Cp′ . Consistent with this,

III

1−

Gd Cp′ (Gd : 4f , S = / ) is reduced by [La Cp′ ] ,

1−

[

Y Cp′ ] , and [Lu Cp′ ] (reactions 5−7). It is not obvious

1−

why this was the case. The second-order t₁of [Gd Cp′ ] at 3

mM is 89 h.

1−

III

1−

[

Y Cp′ ] + Gd Cp′ → Y Cp′ + [Gd Cp′ ]

(5)

(6)

Figure 3. Room-temperature X-band EPR spectrum (red solid) and

simulated spectrum (black dashed) of the products of reaction 13. A

1−

III

1−

[

La Cp′ ] + Gd Cp′ → La Cp′ (THF)

signal is present at g = 1.98, A( Sc) = 214.9 G ([Sc (NR ) ] ).

2 3

1−

[Gd Cp′ ]

1−

[

Y (NR ) ] (g = 1.97, A( Y) = 110.5 G) are observed.

2 3

1−

III

1−

Therefore, [Y (NR

)

]

is a stronger reductant than

[

Lu Cp′ ] + Gd Cp′ → Lu Cp′ + [Gd Cp′ ]

1−

[

Sc (NR ) ] .

2 3

(7)

1−

III

The 4f 5d Tb(II) complex [Tb Cp′ ] (¹⁵⁹Tb : 4f 5d , S =

, I = / , 100% abundance) was also studied as a reductant

since it has an electron conﬁguration close to that of 4f 5d

1−

[Y (NR ) ] + Sc (NR ) → Y (NR )

2 3

⁷/₂

1−

[Sc (NR ) ]

(12)

(13)

2 3

1−

III

Gd(II) in [Gd Cp′ ] . Tb Cp′ (Tb : 4f , S = 3) was not used

1−

III

[

Sc (NR ) ] + Y (NR ) → Sc (NR )

1−

2 3

as a substrate since the anticipated [Tb Cp′ ] product would

1−

not be identiﬁable by X-band EPR spectroscopy at either 77 K or

+ [Y (NR ) ]

1−

room temperature. Thus, only reactions in which [Tb Cp′ ]

]¹⁻and [Tb (NR

]¹⁻can reduce the

1−

Both [Gd (NR

)

acted as a reducing agent were examined. [Tb Cp′ ] reduces

1−

Y(III) and Sc(III) compounds (reaction 14 and Figure S20 ), so

these two compounds are either as reducing or more reducing

all of the substrates that are unreactive with [Gd Cp′ ] ,

III

namely, La Cp′ (THF), Y Cp′ , and Lu Cp′ , as well as

1−

III

than [Y (NR

)

] . This leads to the following ranking of these

Gd Cp′ (reactions 8−11). This gives a ranking of these

1−

[Ln (NR

)

]

complexes from most reducing to least

[

Ln Cp′ ] complexes, from most reducing to least reducing,

reducing: Tb(II) ≈ Gd(II) ≳ Y(II) > Sc(II). The Gd(II) versus

as Tb(II) ≳ Y(II) ≈ La(II) ≈ Lu(II) > Gd(II). Tb(II) could be

more reducing than Y(II), La(II), and Lu(II), but it is not

known whether these three ions would be able to reduce Tb(III)

to Tb(II) in this ligand set.

−

Y(II) results with the [(NR ) ] ligand set, where Gd(II) is

−

comparable to Y(II), are opposite to those with the (Cp′ )

ligand set, where Y(II) is a stronger reductant than Gd(II). This

indicates that the order of the reducing abilities of the new

Ln(II) ions is not the same in every ligand environment. To

investigate this hypothesis further, experiments comparing the

same Ln(II) in diﬀerent ligand sets were carried out.

1−

III

1−

[

Tb Cp′ ] + Y Cp′ → Tb Cp′ + [Y Cp′ ]

(8)

1−

III

[

Tb Cp′ ] + La Cp′ (THF) → Tb Cp′

1−

III

[La Cp′ ]

(9)

[Gd (NR ) ] + Y (NR ) → Gd (NR )

2 3

1−

III

1−

+ [Y (NR ) ]

(14)

[

Tb Cp′ ] + Lu Cp′ → Tb Cp′ + [Lu Cp′ ]

1−

III

(

10)

11)

Reactions of [Y A ] with Y A′ Complexes (A, A′ =

Anion). The investigation of diﬀerent ligand sets on the same

metal was initially conducted with yttrium as the metal since a

wide variety of Y(II) compounds have been identiﬁed by EPR

spectroscopy. The Y(II) compounds investigated display a range

1−

III

1−

Tb Cp′ ] + Gd Cp′ → Tb Cp′ + [Gd Cp′ ]

1−

III

Reactions of [Ln (NR ) ] with Ln′ (NR ) Complexes

2 3

−

1−

(

R = SiMe ). The [(NR ) ] ligand set (R = SiMe ) is known to

of hyperﬁne coupling constants: [Y Cp′ ] , A( Y) = 36.6 G;

2 3

II 1− 89 II 1− 89

allow the isolation of complexes of Ln(II) ions for Sc, Y, Nd, Gd,

Tb, Ho, and Er as well as the traditional divalent ions of Eu, Yb,

Sm, and Tm.

[Y (NR ) ] , A( Y) = 110 G; and “[Y (OAr) ] ”, A( Y) =

2 3 3

156 G (OAr = 2,6- Bu C H O). It should be noted that the

,9,12,23−25

Reactions of amide complexes were

aryloxide complex has not been characterized by X-ray

crystallography. Since the A value has been shown to increase

with the amount of metal contribution to the singly occupied

studied to determine whether the trend in the reducing abilities

of the metals that was established with Cp′ would change with

1−

diﬀerent ligands. For these studies, only [Y (NR ) ] and

molecular orbital (SOMO), it was hypothesized that a higher

Organometallics XXXX, XXX, XXX−XXX

Organometallics

,13

Article

A value (i.e., more metal character in the SOMO) might increase

291 G). The Sc(II) amide and aryloxide mutually reduce one

1−

III

the reducing power of the resulting Y(II) compound.

another, i.e., [Sc (NR ) ]

reduces Sc (OAr′) and

1−

III

[

Y Cp′ ] is the weakest reductant of the compounds

[Sc (OAr′) ] reduces Sc (NR ) (reaction 21). A similar

2 3

studied with yttrium, as it cannot reduce any of the other Y(III)

situation was observed with yttrium and these ligand sets.

III

compounds (reactions 15 and 16). Consistent with this, Y Cp′

1−

III

1−

[

Sc (OAr′) ] + Sc (NR ) V [Sc (NR ) ]

can be reduced by all of the other Y(II) species (reactions 17 and

2 3

1−

III

1−

8). [Y (NR ) ] reduces Y (OAr) to [Y (OAr) ]

III

Sc (OAr′)

(21)

(

reaction 19), but the reverse reaction occurs as well (reaction

0). Therefore, the ordering of ligand sets, in terms of the

1−

III

Reactions of [La A ] with La A′ Complexes. La(II)

−

reducing ability they confer on Y(II) compounds, is (OAr)

≈

compounds were also investigated with diﬀerent ligands since

−

3−

1−

[

(NR ) ] > (Cp′)₃

several options are known: [La Cp″ ] (Cp″ = C H (SiMe ) )

3 2

1−

139

1−

III

(A( La) = 133.5 G), [La Cp′

]

(A( La) = 154 G), and

[

Y Cp′ ] + Y (OAr) → unknown EPR‐active species

tet 1−

tet

139

[

(

La Cp

]

(Cp = C Me H) (A( La) = 291 G).

5 4

(

15)

tet 1−

III

La Cp

]

can reduce La Cp′ (THF) (reaction 22), and

1−

III tet

1−

III

1−

La Cp′ ] can do the reverse reaction to reduce La Cp

[

Y Cp′ ] + Y (NR ) → Y Cp′ + [Y (NR ) ]

2 3

tet 1−

III

reaction 23). [La Cp ₃] can also reduce La Cp″ (reaction

(

16)

1−

III

tet

4), but [La Cp″ ] cannot reduce either La Cp

1−

III

1−

III

1−

III

[

Y (OAr) ] + Y Cp′ → Y (OAr) + [Y Cp′ ]

La Cp′

(THF). [La Cp′

]

can also reduce La Cp″

1−

III

(

reaction 25), but [La Cp″ ] cannot reduce La Cp′ (THF).

other EPR‐active species

(17)

Thus, the ordering of reducing strengths for ligand sets with

tet 3−

La(II), from most reducing to least reducing, is (Cp )

(Cp′)₃> (Cp″)₃

≈

1−

III

1−

[

Y (NR ) ] + Y Cp′ → Y (NR ) + [Y Cp′ ]

3−

2 3

(18)

[

La Cp ]¹⁻+ La Cp′ (THF) →

tet

III

1−

III

[

Y (NR ) ] + Y (OAr) → Y (NR )

2 3

III

tet

1−

La Cp + [La Cp′ ] + La‐A + La‐B

1−

(22)

[Y (OAr) ] + other EPR‐active species

(19)

1−

III

tet

III

1−

III

1−

III

[La Cp′ ] + La Cp → La Cp′ (THF)

[

Y (OAr) ] + Y (NR ) → [Y (NR ) ] + Y

2 3

[La Cp ]¹⁻+ La‐A + La‐B

tet

(OAr)₃

(20)

(23)

(24)

(25)

Some of these yttrium reactions led to additional EPR signals

that cannot be attributed to known Y(II) compounds. For

example, the room-temperature EPR spectrum of reaction 15

tet

III

tet

1−

[

La Cp ]¹⁻+ La Cp″ → La Cp + [La Cp″ ]

other La(II) species

Figure 4 ) contains a variety of signals attributable to Y(II)

1−

III

La Cp′ ] + La Cp″ → La Cp′ (THF)

1−

[La Cp″ ] + La‐C + La‐D

In all of the above reactions with lanthanum, multiple species

were present. This was previously observed for the reactions of

yttrium complexes (vide supra) but not in the case of scandium,

as no unidentiﬁed EPR signals were found in the Sc spectra. The

1−

EPR spectrum of reaction 22 contains signals for [La Cp′ ] (g

1.97, A( La) = 153.4 G) and two other unknown La(II)

species labeled as La-A with g = 1.96, A ( La) = 186.5 G and

La-B with g = 1.96, A ( La) = 229.8 G (Figure 5) and

assigned as La(II) complexes on the basis of the eight-line

patterns. The EPR spectrum of reaction 23 (the reverse of

Figure 4. Room-temperature X-band EPR spectrum (red solid) and

simulated spectrum (black dashed) of the products of reaction 15.

Signals are present at g = 2.00 (likely electride, labeled with a black

arrow) and g = 2.00, A = 92.8 G (the highest A value that could be

simulated for these data, an unknown Y(II) species).

1−

139

reaction 22) contains signals for [La Cp′ ] (g = 1.97, A( La)

tet 1−

139

153.5 G), [La Cp

]

(g = 1.95, A( La) = 290.0 G), and

La-A and La-B (Figure 6). Multiple signals were also seen for

reaction 25. The EPR spectrum from reaction 25 contains

1−

139

signals for its target compound [La Cp″ ] (g = 1.96, A( La)

complexes (by their doublet pattern), but none that can be

assigned to known Y(II) compounds (the large central single

= 133.6 G) as well as two unknown La(II) species labeled as La-

C with g = 1.96, A ( La) = 144.8 G and La-D with g = 1.96,

14,20,21

139

III

line at g = 2.00 probably arises from electride

in the

A ( La) = 149.8 G. Treatment of La Cp′ (THF) with

II 1−

sample). The appearance of multiple Ln(II) species was

observed in experiments with other metals (vide infra) and

will be described in more detail in those sections.

[La Cp″ ] , which does not result in the formation of the target

1−

compound [La Cp′ ] , still contains the same two unknown

La(II) species as reaction 25, i.e., La-C and La-D.

1−

III

Reactions of [Sc A ]

with Sc A′ Complexes.

The EPR parameters from these reactions are listed in Table

1. The La(II) species observed here with A values diﬀering from

those of known compounds could be heteroleptic La(II)

compounds formed by ligand exchange in the course of the

Comparison of diﬀerent ligands on the same metal was also

1−

carried out with the Sc(II) complexes [Sc (NR ) ] (A = 214

1−

G) and [Sc (OAr′) ] (OAr′ = 2,6- Bu -4-Me-C H O) (A =

Organometallics XXXX, XXX, XXX−XXX

Organometallics

). This result supports the hypothesis that these extra species

Article

patterns are easier to simulate and are less prone to obfuscation

by other signals, as is the case with the two-line patterns of Y(II).

1−

[

La Cp″ ] was treated with KCp′ to determine whether this

would lead to ligand exchange with the La(II) complex (reaction

6). The EPR spectrum of the product mixture contains four

1−

La(II) signals: those of the known [La Cp′ ] (g = 1.97,

II 1−

139

A( La)= 153.4 G) and [Cp″ La ] (g = 1.97, A( La) = 134

G) as well as those of La-C (g = 1.97, A ( La) = 144.6 G) and

La-D (g = 1.97, A ( La) = 149.7 G) from reaction 25 (Figure

Figure 5. Room-temperature X-band EPR spectrum (red solid, ﬁrst

panel), sum of the simulated signals (black dashed, ﬁrst panel), and

individual simulated signals (black) of the products of reaction 22.

1−

Signals are present at g = 1.97, A( La) = 153.4 G ([La Cp′ ] ,

second panel), g = 1.96, A ( La) = 186.5 G (La-A, third panel), and

g = 1.96, A ( La) = 229.8 G (La-B , fourth panel).

139

Figure 7. Room-temperature X-band EPR spectrum (red solid, ﬁrst

panel), sum of the simulated signals (black dashed, ﬁrst panel), and

individual simulated signals (black) of the products of reaction 26.

1−

Signals are present at g = 1.97, A( La) = 133.6 G ([La Cp″ ] ,

second panel), g = 1.97, A ( La) = 144.6 G (La-C, third panel), g =

.97, A_D( La) = 149.7 G (La-D, fourth panel), and g = 1.97, A( La)

153.4 G ([La Cp′ ] , ﬁfth panel).

139

1−

Figure 6. Room-temperature X-band EPR spectrum (red solid, ﬁrst

panel), sum of the simulated signals (black dashed, ﬁrst panel), and

individual simulated signals (black) of the products of reaction 23.

observed in the EPR spectra are the result of ligand exchange.

Hence, both alkali metal salts (reaction 26) and La(III)

compounds (reaction 25) can act as Cp′ ligand transfer agents.

Furthermore, if all of the EPR-detectable La(II) compounds in

reaction 26 are assumed to be tris(cyclopentadienyl) com-

pounds (N.B., this is not necessarily true, as [K(crypt)]-

1−

Signals are present at g = 1.97, A = 153.5 G ([La Cp′ ] , second panel,

magniﬁed ×5), g = 1.96, A ( La) = 186.4 G (La-A, third panel), g =

.96, A_B(¹La) = 230.0 G (La-B, fourth panel), and g = 1.96, A( La) =

139

tet 1−

90.0 G ([La Cp ] , ﬁfth panel, magniﬁed ×5).

III

[

La Cp′ ] is known), there is a maximum of four unique

species, corresponding to the number of species seen in reaction

[

experiment. Ligand exchange is well-known in Ln(III)

chemistry. Support for this assessment comes from the EPR

1−

6: [La Cp″ ] , [La Cp″ Cp′] , [La Cp″Cp′ ] , and

II 1−

La Cp′ ] . The results of reactions 22, 23, 25, and 26 are

1−

spectrum of [Y Cp″ Cp] (g = 1.99, A( Y) = 34.6 G), which

shown in Table 1.

1−

diﬀers from both that of “[Y Cp″ ] ” (g = 1.99, A( Y) = 36.1

1−

G) and that of “[Y Cp ] ” (g = 1.99, A( Y) = 42.8 G).

[La Cp″ ] + 3KCp′ → [La Cp″ ] + [La Cp′ ]

1−

[

La Cp″ ] + KA (A = Anionic Ligand): Ligand

La‐C + La‐D

(26)

Exchange Reactions. To investigate whether ligand exchange

could be the cause of these extra signals (i.e., La-A, La-B, La-C,

and La-D) in addition to those of the known compounds,

Since the signals for the putative heteroleptic La(II)

compounds also occur in reaction 25, ligand exchange

1−

III

reactions of [La Cp″ ] with potassium salts were examined.

experiments between La Cp″ and La Cp′ (THF) were

Experiments were carried out with La(II) since its eight-line

carried out in THF-d and C D to investigate by NMR

8 6 6

Table 1. Simulated Room-Temperature X-Band EPR Parameters of Reactions of La Complexes

assignment

rxn 22

rxn 23

rxn 25

reverse of rxn 25

rxn 26

1−

[

La Cp″ ]

133.6 (367.3), 1.96

144.8 (398.0), 1.96

149.8 (411.9), 1.96

133.5 (367.2), 1.96

144.8 (398.0), 1.96

149.7 (411.8), 1.97

133.6 (367.8), 1.97

144.6 (398.0), 1.97

149.7 (412.2), 1.97

153.4 (422.5), 1.97

La-C

La-D

1−

[

La Cp′ ]

153.4 (422.5), 1.97

186.5 (512.5), 1.96

229.8 (630.5), 1.96

153.4 (422.3), 1.97

186.3 (512.1), 1.96

230.0 (631.0), 1.96

290.0 (794.1), 1.96

La-A

La-B

La Cp ]¹⁻

tet

[

1,4,14

Spectra were simulated using EasySpin. Values are the A(¹³⁹La) value in

Assignments were made on the basis of published EPR parameters.

Gauss (MHz), followed by the g value.

Organometallics XXXX, XXX, XXX−XXX

Organometallics

Article

1−

spectroscopy whether ligand exchange occurs in the La(III)

[Y Cp′ ] , the amide and aryloxide compounds also have

state. New Cp″ and Cp′ environments were observed at room

reduction potentials more negative than −2.8 V vs Fc /Fc.

1−

temperature (in the THF-d experiment, one new environment

The ﬁnding that [La Cp″ ] is the weakest reductant studied

III

each for Cp″ and Cp′; in the C D experiment, two new

here may explain the fact that the reduction of La Cp″ can be

−

environments each for Cp″ and Cp′) that were distinct from

carried out under N without formation of an (N ) complex.

III

tet

III

La Cp″ , La Cp′ (THF), KCp″, and KCp′ (Figures S39 and

In contrast, reductions of La (Cp ) and La (C Me ) (BPh )

complexes under N all form reduced dinitrogen complexes

containing [La (μ-η :η -N )] moieties. It may be the case

that La(II) complexes of Cp″ do not have enough reducing

capacity to activate N in this fashion. This is consistent with

ligand choice for Ln(II) complexes being an important

parameter in determining small-molecule activation chemistry.

This study shows that NR and OAr ligands give complexes

that are stronger reductants than Cp′ for Y. This roughly follows

the trend of the hyperﬁne coupling constant values observed in

3 5 5 2 4

S40 , respectively). When the THF-d solution was reduced

using KC both in the presence and in the absence of crypt, La-C

III

2 2

and La-D were observed.

1−

Early attempts to synthesize [La Cp″ ] by reduction of

La Cp″ in dimethoxyethane (DME) led to the isolation of

III

[

La Cp″ (μ-OMe)] via cleavage of OMe groups from DME,

1− 30

presumably by [La Cp″ ] . The reported EPR spectrum of a

1−

DME solution of [La Cp″ ] at 295 K shows multiple species:

1−

139

one that was assigned to [La Cp″ ] (g = 1.97, A( La) =

34.1 G), which was eventually isolated, and another that was

1−

139

the EPR spectra of the [Y A ] complexes, i.e., A = OAr (156

suggested to arise from La Cp″ (DME) (g = 1.97, A( La) =

G), NR (110 G), and Cp′ (36.6 G). Similarly, the La studies

45.1 G). This second species was never crystallographically

tet

show that Cp and Cp′ are more reducing than Cp″, following

authenticated. It seems possible that the second EPR signal

1−

the trend of the A values for the [La A ] complexes, i.e., A =

could arise from a heteroleptic “[La Cp″ (OMe)] ” complex

tet

III

Cp (291 G), Cp′ (154 G), and Cp″ (133 G). Interestingly, this

formed by reduction of the [La Cp″ (μ-OMe)] decomposi-

tion product of this reaction. The reaction of KOMe with

study shows that the relative reducing capacity of one metal

versus another depends on the ligand. For example, Y(II) is

more reducing than Gd(II) with Cp′ ligands, whereas the

1−

[

La Cp″ ] was examined to determine whether such a

complex could be accessed by ligand exchange. The EPR

spectrum of reaction 27 indicates the presence of two La(II)

reverse is true with NR ligands.

1−

139

species: one assignable to [La Cp″ ] (g = 1.97, A( La) =

In the course of this study, EPR signals for complexes beyond

the known [La A ] and [Y A ] complexes were observed

1−

33.6 G) and another species that is a near match for the second

3 3

1−

species seen in the spectrum of [La Cp″ ] in DME (g = 1.96,

that are consistent with the presence of numerous heteroleptic

Ln(II) complexes. Ligand exchange is common in the chemistry

of trivalent lanthanide complexes and evidently can also occur

with 4d Y(II) and 5d La(II). Reactions 25 and 26 clearly show

that ligand exchange can occur between Ln(II) compounds and

Ln(III) or alkali metal compounds as ligand transfer agents.

Lastly, the ﬁndings from reaction 27 suggest that the ﬁrst

A( La) = 144.8 G).

1−

[

La Cp″ ] + 3KOMe → [La Cp″ ] + other La(II)

species

(27)

1−

[

Ln A ] versus Traditional Ln(II) Compounds. The

1−

report of [La Cp″ ] also reported the ﬁrst EPR spectrum of

reducing capacities of the traditional Ln(II) ions Sm(II),

Tm(II), Dy(II), and Nd(II) to make the new Ln(II) ions of La

and Y were also investigated. This oﬀered a chance to bracket the

electrochemical potentials more precisely since a few complexes

of the traditional ions have already been electrochemically

1−

“[La Cp″

(OMe)] ”, since the parameters observed in that

report nearly match those seen for reaction 27. While

1−

“[La Cp″ (OMe)] ” has not been crystallographically authen-

ticated here and a hyperﬁne coupling constant is not a unique

identiﬁer of a compound, this is a plausible explanation and

speaks to the diversity of Ln(II) compounds possible in

heteroleptic ligand environments as well as the power of EPR

spectroscopy to detect them.

1,32

characterized.

Reactions between the Sm(II) compounds

34 35

Sm I (THF) , Sm (C Me ) (THF) , Sm (C Me ) , and

5 2

Sm Cp″ (THF) and the trivalent lanthanide complexes

III

La Cp″ and Y Cp′ did not yield any La(II) or Y(II)

products. Hence, Sm(II) is less reducing than any of the new

La(II) or Y(II) ions. Eu(II) and Yb(II) complexes were not

investigated, as they are known to be weaker reductants than

CONCLUSIONS

■

Sm(II). TmI (DME) , DyI , and NdI₂also failed to reduce

Electron transfer and ligand exchange in Ln(II) compounds

were observed using EPR spectroscopy. In compounds

III

either La Cp″ or Y Cp′ to La(II) or Y(II) products,

1−

respectively, at −35 °C in THF.

[Ln Cp′ ] , the ordering of metals from most reducing to

least reducing is Tb(II) ≳ Y(II) ≈ La(II) ≈ Lu(II) > Gd(II). In

1−

DISCUSSION

compounds [Ln (NR

)

] , the order is Tb(II) ≈ Gd(II) ≳

■

III/II

0/1−

Y(II) > Sc(II), with Gd(II) becoming a stronger reductant than

Y(II) by a change of ligand set. When [Ln A ] compounds are

Since the redox couple for [La Cp″ ]

has been measured

1−

at −2.8 V vs Fc /Fc (THF, 0.2 M [NBu ][PF ]), it is possible

compared by changing the identity of the anion (A), no clear

rule determines which ligand sets are more or less reducing, but a

loose correlation between the magnitude of the hyperﬁne

coupling constant in the EPR spectrum and strong reducing

ability is noted. Extensive ligand exchange is seen alongside

electron transfer in these experiments. The observation of these

ligand exchange products in the EPR spectra suggests that many

heteroleptic Ln(II) compounds are accessible and await the

development of targeted syntheses and full characterization.

to rank the reduction potentials of other complexes relative to

tet 1−

this couple using the results of this study. [La Cp

]

and

1−

[

La Cp′ ] are stronger reductants than [La Cp″ ] , so the

reduction potentials of their corresponding La(III) complexes

are more negative than −2.8 V vs Fc /Fc. Since the potentials of

1−

[

Ln Cp′ ] (Ln = Y, Lu) are similar to that of [La Cp′ ] , the

corresponding Ln(III) complexes can also be estimated to have

reduction potentials more negative than −2.8 V vs Fc /Fc. Since

1−

[

Y (NR ) ] and [Y (OAr) ] are stronger reductants than

2 3 3

Organometallics XXXX, XXX, XXX−XXX

Organometallics

(1) Hitchcock, P. B.; Lappert, M. F.; Maron, L.; Protchenko, A. V.

Article

EXPERIMENTAL SECTION

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ASSOCIATED CONTENT

sı Supporting Information

The Supporting Information is available free of charge at

https://pubs.acs.org/doi/10.1021/acs.organomet.9b00837.

■

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EPR and NMR spectra (PDF)

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AUTHOR INFORMATION

Corresponding Author

William J. Evans − Department of Chemistry, University of

California, Irvine, California 92697-2025, United States;

orcid.org/0000-0002-0651-418X ; Email: wevans@uci.edu

■

(

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Ziller, J. W.; Furche, F.; Long, J. R.; Evans, W. J. Synthesis, Structure,

and Magnetism of Tris(amide) {Ln[N(SiMe ) ] } Complexes of the

2 3

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Arratia-Perez, R.; Ziller, J. W.; Evans, W. J. Rare-Earth Metal(II)

(

Author

-Leiva, M. J.; Paez-Hernandez, D.;

́ ́ ́

Samuel A. Moehring − Department of Chemistry, University of

California, Irvine, California 92697-2025, United States;

orcid.org/0000-0002-6619-7737

Aryloxides: Structure, Synthesis, and EPR Spectroscopy of [K(2.2.2-

cryptand)][Sc(OC H t Bu -2,6-Me-4) ]. Chem. - Eur. J. 2018, 24,

(

8059−18067.

Complete contact information is available at:

https://pubs.acs.org/10.1021/acs.organomet.9b00837

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cryptand)][(C Me H) Ln] Complexes. Organometallics 2018, 37,

15) Angadol, M. A.; Woen, D. H.; Windorff, C. J.; Ziller, J. W.; Evans,

Notes

The authors declare no competing ﬁnancial interest.

863−3873.

ACKNOWLEDGMENTS

W. J. Tert-Butyl(cyclopentadienyl) Ligands Will Stabilize Nontradi-

■

tional +2 Rare-Earth Metal Ions. Organometallics 2019, 38 (5), 1151−

We thank the U.S. National Science Foundation for support of

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L. Lee for their generous assistance with EPR spectroscopy,

Professor A. F. Heyduk for helpful conversations, and Dr.

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