Copper complexes of mono- and ditopic [(methylthio)methyl]borates: Missing links and linked systems en route to copper enzyme models

DOI: 10.1002/chem.200800588

Copper Complexes of Mono- and Ditopic [(Methylthio)methyl]borates:

Missing Links and Linked Systems En Route to Copper Enzyme Models

Kai Ruth, Sandor Tüllmann, Hannes Vitze, Michael Bolte, Hans-Wolfram Lerner,

Max C. Holthausen,* and Matthias Wagner*^[a]

ꢀ

Abstract: TMEDA-free (TMEDA:

tetramethylethylenediamine) LiCH₂-

SMe is a suitable reagent for the selec-

tive introduction of (methylthio)methyl

groups into PhBBr₂and its p-silylated

poly(pyrazol-1-yl)borate

p-[pz₃B

possesses a square-pyramidal (N₄S)

C₆H₄Bpz

₂(CH₂SMe)]^2ꢀ([10]^2ꢀ) that

ligand sphere (S-atom in the axial posi-

tion). The remarkable redox properties

of K[Cu(5)₂] and [Cu(5)₂] have been

assessed by cyclic voltammetry and

quantum chemical calculations. The re-

action of K[Cu(5)₂] with dry air leads

to the Cu^IIspecies [Cu(5)₂] and to a

tetranuclear Cu^IIcomplex featuring

[PhB(O)pz₂]^2ꢀligands. Addition of

CuCl to K₂[10] gives the complex

K₃[Cu(10)₂] containing two ligand mol-

ecules per Cu^Icenter. The Cu^Iion

binds to both heteroscorpionate moiet-

ies and thereby establishes a coordina-

tion environment similar to that of the

Cu^Iion in K[Cu(5)₂].

ꢀ

G

mimics the two ligation sites of the

copper enzymes peptidylglycine a-hy-

droxylating monooxygenase and dopa-

mine b-monooxygenase. Treatment of

the monotopic tripod [5]^ꢀwith CuCl

and CuBr₂results in the formation of

complexes K[Cu(5)₂] and [Cu(5)₂]. An

ꢀ

derivative Me₃Si C₆H₄BBr₂. The re-

ꢀ

sulting compounds, R* C₆H₄B(Br)-

(CH₂SMe) (R*=H: 2; R*=SiMe₃: 7)

and PhB(CH₂SMe)₂(3), form cyclic

ACHTREUNG

ꢀ

dimers through B S adduct bonds in

solution and in the solid state. Com-

pounds 2 and 3 have successfully been

used for preparing the (N₂S) scorpio-

X-ray

crystallography

study

of

K[Cu(5)₂] revealed a tetrahedral (N₂S₂)

coordination environment for the Cu^I

ion, whereas the Cu^IIion of [Cu(5)₂]

nate [PhBpz

pyrazol-1-yl) and the (NS₂) scorpionate

[PhBpz respectively.

(CH₂SMe)₂]^ꢀ,

R

Keywords: borates

·

copper

·

ACHTREUNG

metalloenzymes

tripodal ligands

· N,S ligands

Compound 7 proved to be an excellent

building block for the heteroditopic

Introduction

to the fact that introduction of appropriate substituents, R¹,

into the 3-positions of the pyrazolyl rings allows extensive

control over the steric demand of the ligands, and thus over

their ability to kinetically stabilize reactive complex frag-

ments. By contrast, an adjustment of the ligand field

strengths of tris(pyrazol-1-yl)borates is much harder to ach-

ieve, because electronic substituent effects of R¹–R³on the

donor properties of the pyrazolyl rings turned out to be

rather modest.

One way to alter the donor/acceptor properties of scorpi-

onate ligands over a wider range is to replace the pyrazolyl

rings by phosphorus- (B^[3–5]) or sulfur-containing groups

(C,^[6,7]D;^[8–12]Figure 1). Similar to the parent scorpionates

A, ligands B–D provide a monoanionic, tridentate, face-cap-

ping coordination mode, but they differ from A with regard

to the softness of their donor sets. Moreover, in the thioimi-

dazolyl derivatives C, the boron atom is separated from the

donor atoms by three bonds as opposed to two bonds in A.

Very recently, even hybrid scorpionates of A–D have

become available, which provide nitrogen and phosphorus

For several decades now, tris(pyrazol-1-yl)borate (scorpio-

nate) ligands A are widely used in bioinorganic and organo-

metallic chemistry (Figure 1).^[1,2]Part of this success is owed

[a] Dipl.-Chem. K. Ruth, Dipl.-Chem. S. Tüllmann,

Dipl.-Chem. H. Vitze, Dr. M. Bolte, Dr. H.-W. Lerner,

Prof. Dr. M. C. Holthausen, Prof. Dr. M. Wagner

Institut für Anorganische und Analytische Chemie

Johann Wolfgang Goethe-Universität Frankfurt am Main

Max-von-Laue-Straße 7, 60438 Frankfurt (Germany)

Fax : (+49)69-798-29260

E-mail: Max.Holthausen@chemie.uni-frankfurt.de

Matthias.Wagner@chemie.uni-frankfurt.de

Supporting information for this article is available on the WWW

under http://dx.doi.org/10.1002/chem.200800588: Crystallographic

characterization of 3 and 7; view of the polymeric structure of K[5];

cyclic voltammograms of K[5] and K[11]; illustration of the sterically

hindered interconversion of [Cu^I(N₄)^tet^ꢀand [Cu^II(N₄)^spl]; comparison

]

of calculated structures with X-ray data; atomic coordinates and total

energies of all optimized structures.

6754

ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Chem. Eur. J. 2008, 14, 6754 – 6770

FULL PAPER

Figure 1. Tridentate facially coordinating borate ligands A–D (substitu-

ents R¹–R³on two pyrazolyl rings of A not shown).

((N₂P),^[13,14](NP₂)^[15]

) or nitrogen and sulfur donors

((N₂S),^[16](NS₂)^[17–22]). These derivatives are most interesting,

because they allow a smooth modulation of the coordination

environment between the extremes (N₃) and (P₃)/(S₃). Com-

plete homogeneous series of closely related ligands facilitate

an evaluation of the influence of gradual changes in the

donor set of tripods on the chemical properties of the coor-

dinated metal ion. This has been impressively demonstrated

by Vahrenkamp et al., who compared the biomimetic thio-

Scheme 1. Synthesis of the 1,4-dimethyl-1,4-dithionia-2,5-diboratacyclo-

hexanes 2/3 and of the (N₂S) scorpionate salts Li[5]/K[5]. i) 2: n=2, 3:

n=4; C₆H₆, 68C!RT, ii) C₆H₆, 68C!RT, iii) toluene, reflux.

late-alkylating activity of tetrahedral (tet) zinc complexes

ꢀ

supported by [HBpz_x(thioimidazolyl)_3ꢀx

]

(pz=pyrazol-1-yl)

ligands upon variation of x from x=0 to 3.^[23–26]For our own

ligand systems consisting of two identical scorpionate units

that are bridged by organometallic or aromatic linkers. Re-

placement of pyrazolyl rings by thioether moieties allows to

extend the research to heteroditopic scorpionate ligands in

which the two donor sites are no longer the same so that the

two coordinated metal centers will show different reactivity.

Our choice of the first heteroditopic target scorpionate

for bioinorganic research projects was guided by the idea

that it should be similar to the ligand environment of natu-

rally occurring dinuclear metalloproteins for which the

added value of the second metal ion is already proven. In

this respect, the two copper-containing enzymes dopamine

b-monooxygenase (DbM) and peptidylglycine a-hydroxylat-

ing monooxygenase (PHM) appeared to be particularly at-

tractive, because each of them contains two copper ions sep-

arated by approximately 11 , one with three histidine-imi-

dazolyl ligands (Cu_H) and the other bonded to two imida-

zoles and a methionine-thioether chain (Cu_M).^[36–38]DbM

research on bioinorganic model systems with mixed-donor

ꢀ

scorpionates [RBpz

A

]

we prefer (methylthio)-

methyl- over thioimidazolyl groups, because different sub-

ꢀ

stituents linked to the same boron center through B N and

ꢀ

B C bonds are less likely to scramble than substituents that

are all bonded through sp²-hybridized nitrogen atoms. Un-

fortunately, in the case of the thioether scorpionates [RBpz_x-

ꢀ

A

]

the homogeneous series is incomplete, be-

cause derivatives [RBpz

ACHTREUNG

therefore necessary to develop a facile route to these miss-

ing members, and a high-yield synthesis of the derivative

[PhBpz

₂(CH₂SMe)]^ꢀwill be described in this paper ([5]^ꢀ;

R

Scheme 1).

ꢀ

The potential of scorpionates [RBpz ] goes

_x(CH₂SR¹)_3ꢀx

A

far beyond the preparation of mononuclear complexes, be-

cause they can be tied to other ligands through their non-co-

ordinating substituent R. In the past, our group has spent

considerable effort developing ditopic scorpionates which

we have used for the assembly of coordination poly-

mers,^[27,28]multiple-decker sandwich complexes,^[29–31]metal-

lo-macrocycles^[32]and dinuclear complexes with cooperating

metal ions.^[33–35]So far, the main focus was on homoditopic

ꢀ

and PHM catalyze stereospecific hydroxylations of C H

bonds in the biosynthetic pathways of norepinephrine and

a-amidated peptides, respectively. Cu_Mis the presumed site

of dioxygen binding, whereas the role of Cu_His merely that

of an electron source.^[37,39]

Chem. Eur. J. 2008, 14, 6754 – 6770

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www.chemeurj.org

6755

M. Wagner, M. C. Holthausen et al.

The coordination spheres of Cu_Hand Cu_Mcan be nicely

our aim of developing a general route to heteroditopic thio-

ether scorpionates, it was now inevitable to elucidate in

detail the chemical and structural properties not only of the

ligands themselves, but also of key intermediates.

modeled by [RBpz₃]^ꢀand [RBpz₂(CH₂SMe)]^ꢀligands, re-

A

spectively.^[40]Connection of the two fragments by a p-phen-

ylene linker would result in the heteroditopic scorpionate

[10]^2ꢀ(Scheme 2), which, in its straight conformation, sup-

Syntheses: The lithium and potassium (N₂S) scorpionates

Li[5] and K[5] are accessible through a three-step sequence

starting from PhBBr₂^[42](Scheme 1).

For the selective introduction of one or two (methylthio)-

methyl groups in the first reaction step we employed donor-

free LiCH₂SMe,^[43]because, in our hands, it gave far better

results than the corresponding TMEDA-adduct (TMEDA:

tetramethylethylenediamine). The improvement in terms of

selectivity and yield of 2 and 3 was so significant that it out-

weighed the hazards associated with working with donor-

free LiCH₂SMe (the compound violently explodes upon air

contact^[43]). One advantage of the TMEDA-free reagent lies

in its poor solubility in the reaction solvent (C₆H₆) which

guarantees low stationary concentrations of the nucleophile

and makes it easier to control whether mono- (2) or disub-

stitution (3) takes place. Moreover, removal of LiBr during

workup is greatly facilitated if TMEDA is absent. According

to X-ray crystallography and NMR spectroscopy, both 2 and

3 form dimers in the solid state as well as in solution. Irre-

spective of that, addition of Me₂NSiMe₃to 2 resulted in the

clean formation of the monomeric aminoborane 4. The

borate salts Li[5] and K[5] were finally obtained by transa-

mination of 4 with a mixture of pyrazole and alkali metal

pyrazolide. Another hybrid ligand, Riordanꢁs^[18](NS₂) scor-

pionate Li

N

lithium pyrazolide (Lipz).

For the synthesis of the heteroditopic scorpionates Li₂[10]

and K₂[10] the p-silylated borylbenzene 6^[44,45]was chosen

,

as starting material (Scheme 2). Introduction of the thio-

A

lylated analog (7) of 2. Similar to 2, 7 exists as dimer in solu-

tion and in the solid state (see the Supporting Information).

This is of critical importance for the entire synthesis con-

Scheme 2. Synthesis of the heteroditopic scorpionate ligands Li₂[10] and

K₂[10]. i) C₆H₆, 68C!RT, ii) C₆H₆, RT, iii) toluene, reflux.

ꢀ

cept, because B S adduct formation protects the thioether

groups from BBr₃attack in the subsequent reaction step,

thereby allowing the clean conversion of 7 into 8 via silicon/

boron exchange. Treatment of 8 with 6 equiv of Me₂NSiMe₃

led to the replacement of all bromide substituents by dime-

thylamino groups and consequently to the break-up of the

1,4-dithionia-2,5-diboratacyclohexane ring. The resulting

monomer 9 was transformed into the heteroditopic scorpio-

nates Li₂[10] and K₂[10] by reaction with Hpz/Mpz (6:4;

M=Li, K) in toluene at reflux temperature.

The copper complexes K[11] and 12 (Scheme 3) were pre-

pared by the reaction of K[5] with 1.3 equiv of CuCl and

0.5 equiv of CuBr₂, respectively. Toluene was chosen as the

reaction medium, which led to heterogeneous conditions

and thus to reaction times of several days. The solubility of

K[5] in toluene is higher than the solubility of CuCl, thus

promoting the formation of complexes of stoichiometry

K[Cu(5)₂] (i.e., K[11]), even though the ratio of the starting

materials was roughly 1:1. Samples prepared in solvents

ports a metal–metal distance of about 11 as it has been

found in DbM and PHM.

Given this background, we have prepared the monotopic

scorpionate [PhBpz

₂(CH₂SMe)]^ꢀ([5]^ꢀ) and characterized its

T

Cu^Iand Cu^IIcomplexes. Based on these results, we have fur-

ther developed this chemistry to the point where a targeted

synthesis of the first heteroditopic scorpionate ligand p-

ꢀ

[pz₃B C₆H₄Bpz

₂(CH₂SMe)]^2ꢀ([10]^2ꢀ) was possible and we

Cu^Iions.

Results and Discussion

So far, (S₃) and (NS₂) borate ligands have been prepared by

using efficient one-pot protocols.^{[9–11,18,19,41]}With regard to

6756

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Chem. Eur. J. 2008, 14, 6754 – 6770

Copper Scorpionate Complexes

FULL PAPER

Spectroscopy: The ¹¹B NMR spectrum of 2 shows one reso-

nance at 2.5 ppm testifying to the presence of four-coordi-

nated boron nuclei.^[46]This result can best be explained by

ꢀ

assuming that a B S adduct is present in C₆D₆solution.

Owing to the self-complementarity of PhB(Br)CH₂SMe and

entropy reasons, the structure of this adduct is most likely

that of a cyclic dimer (PhB(Br)CH₂SMe)₂(Scheme 1). The

BCH₂protons of 2 are not magnetically equivalent, but give

rise to two doublets (each integrating to 2H) in the

¹H NMR spectrum, which is in accord with the presence of

chiral boron centers in (PhB(Br)CH₂SMe)₂(d(¹H)=2.64,

2.08 ppm; ²J

carbon atoms lead to only one resonance (at d=34.9 ppm)

(H,H)=12.7 Hz). The corresponding BCH₂

1

that is considerably broadened, owing to unresolved J(B,C)

A

coupling and fast quadrupolar relaxation of the boron

nuclei. Only one set of phenyl and methyl signals is ob-

1

served in the H and ¹³C NMR spectra of 2, thus ruling out

the presence of more than one of the possible isomers re-

sulting from different distributions of the substituents over

the axial/equatorial positions of the 1,4-dithionia-2,5-dibora-

tacyclohexane ring.

Similar to 2, the triorganylborane 3 reveals one ¹¹B reso-

nance at d=ꢀ4.7 ppm and is therefore also likely to exist as

B S adduct in solution. In contrast to 2, the ¹H and

ꢀ

¹³C NMR spectra of 3 are rather complex and difficult to

assign because of many broadened and overlapping signals.

If we again assume a 1,4-dithionia-2,5-diboratacyclohexane

structure, one part of the problem clearly arises from the

two different CH₂SMe fragments present in the molecule. In

addition, different isomers may exist in parallel or slowly

transform into each other in C₆D₆solution. We therefore

ꢀ

added pyridine to the NMR tube in order to cleave the B S

adducts and to form PhB

A

As a result, the ¹¹B NMR signal was shifted downfield to

0.0 ppm and the H NMR spectrum simplified considerably

Scheme 3. Synthesis of the mononuclear Cu^Iand Cu^IIcomplexes K[11]

and 12, the tetranuclear species 13, and of the dinuclear K⁺/Cu^Icomplex

14; the molecular structures are drawn according to the results of the X-

ray crystal structure analyses. i) +CuCl, toluene, RT, ii) +CuBr₂, toluene,

RT, iii) C₆H₆, RT, iv) +CuCl, THF, RT.

1

and showed a better resolution. All in all, we observe three

resonances for the phenyl ring, three for the coordinated

pyridine molecule, and one signal for the SMe groups. The

corresponding integral ratio of 5:5:6 gives conclusive evi-

dence for the proposed structure. The BCH₂protons are

other than toluene did not provide single crystalline-

still inequivalent (d(¹H)=2.55, 2.44 ppm; ²J

ACHTREUNG

A

11.9 Hz), because the boron atom of PhB

prochiral.

ACHTREUNG

complexes of the (N₂S) scorpionate ligand [5]^ꢀcan be re-

garded as mimics of the O₂-activating copper site in the hy-

droxylating enzymes DbM and PHM. We therefore investi-

gated the behavior of K[11] towards dry molecular oxygen

in C₆H₆solution at room temperature. From the paramag-

netic reaction mixture, crystals were grown of the Cu^IIcom-

plex 12 (major component) and of a degradation product 13

containing [PhBpz₂(O)]^2ꢀligands (minor component;

The ¹¹B NMR spectrum of the aminoborane 4 is charac-

terized by one signal at d=40.6 ppm that lies in a range typ-

ical of three-coordinated boron atoms.^[46]N B p-bonding

apparently reduces the Lewis acidity of the boron center to

ꢀ

such an extent that no B S adducts are formed anymore.

This view is supported by the observation of two singlets in

the ¹H NMR spectrum that are assignable to the NMe₂

ꢀ

group and point towards a hindered rotation about the B N

ꢀ

Scheme 3). The oxidative cleavage of the B C bond indi-

bond as a result of significant double bond character

cates activation of O₂by K[11] in a manner reminiscent of

related enzyme-mediated processes. The heterodinuclear

complex 14 (Scheme 3) was synthesized from K₂[10] and

CuCl in THF (stoichiometric ratio=1:3).

(d(¹H)=2.64, 2.47 ppm; d(¹³C)=41.0, 39.7 ppm). The proton

C

resonance of the BCH₂fragment appears as one singlet at

d(¹H)=2.22 ppm.

In line with the proposed borate structure of Li[5], its ¹¹B

resonance possesses a chemical shift value of ꢀ0.8 ppm. Pyr-

Chem. Eur. J. 2008, 14, 6754 – 6770

ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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6757

M. Wagner, M. C. Holthausen et al.

azolyl resonances are observed at d(¹H)=8.18, 7.45,

6.14 ppm and d

(¹³C)=139.3, 135.8, 102.9 ppm. The integral

spectra of [PhBpz₃]^ꢀ[47]and [5]^ꢀ. Resonances for the p-phen-

ylene bridge are observed at d(¹H)=6.42–6.30 (Li₂[10])/

ACHTREUNG

ratio of the Ph, pz, and CH₂SMe signals in the proton spec-

trum of Li[5] fits to a (methylthio)methyl-bis(pyrazol-1-yl)-

borate ion. All key NMR data of K[5] are similar to those

of Li[5] and therefore do not merit a detailed discussion.

The NMR data of 7 closely resemble those of 2, apart

from the fact that the proton resonances of the aromatic

ring now give a well-resolved AA’BB’ pattern (d(¹H)=7.94,

7.02–6.89 ppm (K₂[10]) and at d

133.6, 133.0 ppm (K₂[10]).

A

The room temperature ¹H and ¹³C NMR spectra of the

Cu^Icomplex K[11] are characterized by broad and ill-de-

fined signals. Much sharper and interpretable resonances

are, however, obtained if the sample is heated to +508C.

This clearly indicates motional broadening rather than para-

magnetic Cu^IIimpurities to be responsible for the poor reso-

lution of the room temperature spectra of K[11]. At +508C,

only one set of signals is observed for the pyrazolyl protons

and all BCH₂protons have the same chemical shift value.

These observations are not compatible with a static molecu-

lar structure, but rather point towards association/dissocia-

tion equilibria in solution. We note, however, that the NMR

data of K[11] at elevated temperature are still decisively dif-

ferent from those of the free ligand [5]^ꢀ. Owing to their par-

amagnetic nature, no interpretable NMR data of the Cu^II

complexes 12 and 13 were obtained. In the case of the Cu^I

complex 14, the yield of single crystalline material was very

low so that the entire sample was consumed in the selection

of an appropriate specimen for X-ray crystallography.

7.59 ppm; d

U

(¹³C)=ꢀ1.0 ppm. The successful re-

placement of SiMe₃by BBr₂in 8 is evident from the disap-

pearance of the SiMe₃signals accompanied by a downfield

shift of p-phenylene resonances, owing to the p-electron

withdrawing nature of the dibromoboryl substituent

(d(¹H)=8.16, 7.74 ppm; d(¹³C)=137.5, 132.9 ppm). Some-

what surprisingly, no ¹¹B NMR signal was detectable for the

BBr₂group in 8 ([D₈]toluene; four-coordinated boron atom:

d

(¹¹B)=2.2 ppm). This problem is solved after amination,

E

because 9 gives rise to two ¹¹B NMR signals with an integral

ratio of 1:1 and chemical shift values of 40.7 and 33.1 ppm.

Characteristically, the four methyl groups of the B

fragment are all equivalent at room temperature (d(¹H)=

2.64 ppm (12H); d

(¹³C)=41.3 ppm), whereas those of the B-

(NMe₂)(CH₂SMe) moiety appear as two singlets with

d(¹H)=2.69 (3H)/2.57 ppm (3H) and (¹³C)=41.2/

A

ACHTREUNG

A

N

X-Ray crystal structure analyses: The compounds 2, 3,

d

A

(Li[5])₂, K[5], 7, K₂[10]

N

39.8 ppm. This well-known feature is usually attributed to

were characterized by X-ray crystallography. Key crystallo-

graphic data are compiled in Tables 1 and 2 and Table 1S (3,

7) in the Supporting Information).

ꢀ

resonance saturation and thus to a lower degree of N B p-

bonding in the B(NMe₂)₂case.

A

For the heteroditopic scorpionates Li₂[10] and K₂[10] we

note NMR spectra that are merely a superposition of the

Intermediate 2 crystallizes from pentane/C₆H₆with two

crystallographically independent molecules in the asymmet-

Table 1. Crystallographic data for 2, (Li[5])₂, K[5] and K₂[10](thf)₂.

G

2

(Li[5])₂

K[5]

K₂[10](thf)₂

A

formula

fw

C₁₆H₂₀B₂Br₂S₂

457.88

C₂₈H₃₂B₂Li₂N₈S₂

580.24

C₁₄H₁₆BKN₄S

322.28

C₃₁H₄₀B₂K₂N₁₀O₂S·C₄H₈O

788.71

color, shape

crystal size [mm]

T [K]

colorless, plate

0.310.260.12

173(2)

colorless, block

0.420.370.35

173(2)

colorless, needle

0.230.130.12

173(2)

colorless, block

0.230.210.18

173(2)

radiation []

crystal system

space group

a []

b []

c []

MoK_a0.71073

triclinic

P1

MoK_a0.71073

monoclinic

P2₁/c

8.4318(4)

10.3095(5)

17.7212(8)

90

97.510(3)

90

1527.25(12)

2

MoK_a0.71073

orthorhombic

P2₁2₁2₁

9.2358(7)

12.7378(11)

13.8257(13)

90

MoK_a0.71073

triclinic

P1

¯

6.7169(7)

10.1354(12)

21.012(2)

77.436(9)

81.911(8)

82.653(9)

1375.3(3)

3

11.6438(14)

12.6677(16)

15.2056(17)

70.376(9)

79.682(9)

86.565(10)

2078.4(4)

2

a [8]

b [8]

90

g [8]

V [³]

1626.5(2)

4

1.316

Z

D_calcd[gcm^ꢀ3

]

1.659

1.262

1.260

F

A

684

608

672

832

m

A

]

4.641

0.207

0.452

0.324

no. of reflns collected

10646

38150

10736

17462

no. of indep reflns (R_int

)

4931 (0.0475)

4065

4931/0/298

1.013

4254 (0.0348)

4067

4254/0/191

1.096

3037 (0.0718)

2374

3037/0/190

1.017

7748 (0.1079)

4272

7748/114/478

1.003

no. of reflns obsd (I>2s(I))

no. of data/restraints/params

GOF on F²

R1, wR2 (I>2s(I))

R1, wR2 (all data)

largest diff peak and hole (e

0.0419, 0.1002

0.0542, 0.1055

0.825/ꢀ1.264

0.0606, 0.1704

0.0626, 0.1722

0.992/ꢀ0.655

0.0627, 0.1394

0.0857, 0.1506

0.533/ꢀ0.434

0.1001, 0.2601

0.1576, 0.3059

0.708/ꢀ0.371

ꢀ3

)

6758

www.chemeurj.org

ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Chem. Eur. J. 2008, 14, 6754 – 6770

Copper Scorpionate Complexes

FULL PAPER

Table 2. Crystallographic data for (K[11])₂, 12, 13 and 14

(K[11])₂

A

12

13

14(thf)₂

A

formula

fw

C₅₆H₆₄B₄Cu₂K₂N₁₆S₄·2C₆H₆

1494.21

C₂₈H₃₂B₂CuN₈S₂

629.90

C₃₆H₃₄B₂Cu₄N₁₆O₂

998.57

C₅₄H₆₄B₄CuK₃N₂₀O₂S₂·2C₄H₈O

1457.66

color, shape

crystal size [mm]

T [K]

colorless, block

0.220.210.17

173(2)

light violet, plate

0.180.130.07

173(2)

dark blue, block

0.240.230.19

173(2)

colorless, plate

0.120.110.03

173(2)

radiation []

crystal system

space group

a []

b []

c []

MoK_a0.71073

triclinic

P1

MoK_a0.71073

triclinic

P1

MoK_a0.71073

monoclinic

P2₁/n

13.0380(9)

11.8504(11)

13.0596(9)

90

103.699(5)

90

1960.4(3)

2

MoK_a0.71073

monoclinic

C2/c

28.393(4)

15.3601(14)

18.695(3)

90

117.551(10)

90

7228.7(17)

4

¯

10.3367(6)

11.6603(7)

31.508(2)

81.959(5)

81.978(5)

81.510(5)

3691.6(4)

2

9.9336(15)

10.2087(13)

16.917(3)

90.134(12)

98.521(13)

114.819(10)

1535.9(4)

2

a [8]

b [8]

g [8]

V [³]

Z

D_calcd[gcm^ꢀ3

]

1.344

1.362

1.692

1.339

F

A

1552

654

1008

3048

m

A

]

0.854

0.880

2.198

0.592

no. of reflns collected

44320

12355

12130

20283

no. of indep reflns (R_int

)

14312 (0.0798)

10089

14312/0/865

1.143

5395 (0.0857)

2622

5395/0/370

0.947

3617 (0.0476)

3052

3617/0/271

1.017

6372 (0.1449)

2410

6372/0/435

0.870

no. of reflns obsd (I>2s(I))

no. of data/restraints/params

GOF on F²

R1, wR2 (I>2s(I))

R1, wR2 (all data)

largest diff peak and hole (e

0.0969, 0.2352

0.1242, 0.2434

1.207/ꢀ0.687

0.0809, 0.1709

0.1692, 0.2075

1.133/ꢀ0.618

0.0336, 0.0815

0.0446, 0.0857

0.363/ꢀ0.524

0.0824, 0.1259

0.2194, 0.1691

0.909/ꢀ0.383

ꢀ3

)

ric unit. One has C₁symmetry, the other possesses an inver-

sion center. All key structural parameters of the two mole-

cules are the same within the margins of experimental error.

Thus, only the C_isymmetric structure will be discussed here

(Figure 2).

in the solid state. Both bromide substituents occupy axial

positions, whereas the sterically more demanding phenyl

rings as well as the methyl groups are located at equatorial

positions. The only other structurally characterized example

of a 1,4-dithionia-2,5-diboratacyclohexane was reported by

As already deduced from its NMR spectra, compound 2 is

indeed a 1,4-dithionia-2,5-diboratacyclohexane derivative.

The six-membered heterocycle adopts a chair conformation

Nçth, who prepared the compound (BH

thermolysis of the amine adduct Me₃N·BH₂

According to X-ray crystallography, (BH₂(CH₂SMe))₂also

ACHTREUNG

AHCTREUNG

prefers a chair conformation with two equatorial methyl

substituents in the crystal lattice. NMR spectroscopy, howev-

er, indicated the presence of two conformational isomers,

trans-(eq,eq) and trans-(ax,ax), in solution. The comparison

of 2 with (BH₂(CH₂SMe))₂revealed two differences in their

G

ꢀ

molecular structures: First, the B S adduct bonds of 2 are

significantly longer (2.000(5) vs. 1.951(2) ) and second,

ꢀ ꢀ

the S B C and B S C bond angles are the same in 2

#

ꢀ

(S(3) B(3) C(3 )=105.5(3)8, B(3) S(3) C(3)=104.5(2)8)

but deviate from each other by 5.88 in (BH₂(CH₂SMe))₂

A

ꢀ

(108.1(1)8, 102.3(1)8). The B S bond in 3 is elongated even

ꢀ

further to a value of 2.029(3) , although the B S bond in 7

is 1.972(10) long (see the Supporting Information,

Figures 1aS and 1bS).

Figure 2. Molecular structure of 2 in the solid state; thermal ellipsoids at

the 50% probability level. Selected bond lengths [], bond angles [8],

The lithium scorpionate complex Li[5] crystallizes in the

form of centrosymmetric dimers (Li[5])₂(Figure 3).

Each Li⁺ion has a distorted tetrahedral ligand environ-

ment consisting of two pyrazolyl rings of one borate ligand

together with one pyrazolyl ring and the (methylthio)methyl

group of the other ligand. Two of the four pyrazolyl donors

within the aggregate adopt bridging positions between Li⁺

ions. The entire structure is strongly reminiscent of the mo-

#

ꢀ

torsion angles [8], and dihedral angles [8]: B(3) S(3) 2.000(5), B(3) C(3 )

ꢀ

1.641(6), B(3) C(31) 1.610(6), B(3) Br(3) 2.065(5), S(3) C(3) 1.812(4),

S(3) C(37) 1.811(5); S(3) B(3) C(3 ) 105.5(3), B(3) S(3) C(3) 104.5(2),

#

ꢀ

B(3) S(3) C(37) 104.9(2), C(3) S(3) C(37) 102.1(2); B(3) S(3) C(3)

B(3^#) 66.1(4), C(3 ) B(3) S(3) C(37) ꢀ168.5(3), C(31) B(3) S(3)

#

ꢀ

C(37) 70.3(3), Br(3) B(3) S(3) C(37) ꢀ50.8(3); C(3^#)B(3)S(3)//

C(3)S(3)C(3^#)S(3^#) 58.3. Symmetry transformation used to generate

equivalent atoms: #: ꢀx+1, ꢀy+1, ꢀz+2.

ꢀ

Chem. Eur. J. 2008, 14, 6754 – 6770

ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemeurj.org

6759

M. Wagner, M. C. Holthausen et al.

Figure 3. Molecular structure of (Li[5])₂in the solid state; thermal ellip-

soids at the 50% probability level. Selected bond lengths [], bond

Figure 4. Section of the polymeric structure of K[5] in the solid state;

thermal ellipsoids at the 50% probability level. The H atoms have been

omitted for clarity. Selected bond lengths [], bond angles [8], and tor-

ꢀ

angles [8], and torsion angles [8]: Li(1) N(12) 1.960(4), Li(1) N(22)

#

(22^#) 2.160(4), Li(1) S(1 ) 2.498(3), B(1) N(11)

ꢀ

2.115(4), Li(1) N

ꢀ

sion angles [8]: K(1) N

G

ACHTREUNG

ꢀ

1.579(2), B(1) N(21) 1.586(2), B(1) C(1) 1.641(3), B(1) C(31) 1.626(3),

S(1) C(1) 1.804(2), S(1) C(2) 1.815(2), Li(1)···Li(1 ) 2.624(7); N(12)

ꢀ

G

N

#

ꢀ

N(11) B(1) N(21) 106.0(3), C(1) B(1) C(31) 112.3(4), N

(12 A) K(1)

#

ꢀ

Li(1) N(22) 96.5(2),

N(22 ) Li(1) S(1 ) 88.8(1), Li(1) N(22) Li(1 )

G

ꢀ

N(22 A) 69.0(1); C(32) C(31) B(1) C(1) 85.4(5). Symmetry transforma-

75.7(2), N(22) Li(1) N

(22^#) 104.3(2), N(11) B(1) N(21) 107.7(1),

ꢀ

tion used to generate equivalent atoms: A: xꢀ = , ꢀy+¹= , ꢀz.

1

2

ꢀ

N(21) B(1) C(1) 109.7(1); B(1) N(11) N(12) Li(1) 16.3(3), B(1)

#

ꢀ

N(21) N(22) Li(1) ꢀ33.2(2), B(1) N(21) N(22) Li(1 ) 54.4(2), B(1)

#

ꢀ

C(1) S(1) Li(1 ) 83.3(2). Symmetry transformation used to generate

ꢀ

centroids (COG) amount to K(1) COG(pzN(11))=3.000

A

equivalent atoms: #: ꢀx+1, ꢀy+2, ꢀz+1.

ꢀ

and K(1) COG

A

The potassium salt of the heteroditopic scorpionate K₂[10]

crystallizes from THF together with three solvent molecules.

lecular framework of the related dimer (Li

A

Two of them coordinate the two K⁺ions (K₂[10]

ACHTREUNG

from the fact that in the latter case ferrocenyl substituents

(Fc) and pyrazolyl rings are occupying the positions of the

ACHTREUNG

phenyl and (methylthio)methyl groups, respectively.^[28]Im-

tablishes a polymeric structure in the solid state. A section

of this polymer is shown in Figure 5.

Again similar to K[5], the phenylene ring of K₂[10](thf)₂

takes part in potassium coordination so that an inverse sand-

wich complex is created in which two K⁺ions are bonded to

the opposite sides of the same p-electron system

#

ꢀ

portant bond lengths of (Li[5])₂are Li(1) S(1 )=

ꢀ

2.498(3) , Li(1) N(12)=1.960(4) , and Li(1) N(22)=

G

2.115(4) /Li(1) N

(22^#)=2.160(4) . Comparable Li S

ꢀ

bond lengths have been reported for the dimeric compound

ꢀ

(Li

(TMEDA)

U

(K(1)···COG(Ar)=2.988 ,

K(2)···COG(Ar)=3.045 ;

(ladder-like structure; Li S=2.531(5) ^[50]), and the infinite

K(1)···COG(Ar)···K(2)=164.68). In addition to the phenyl-

ene ring and one THF ligand, potassium ion K(1) is coordi-

nated to the pyrazolyl ring pzN(11) of the heteroscorpionate

fragment and to pzN(31) of the homoscorpionate moiety (s-

modes; K(1) N(12)=2.856(5) , K(1) N(32)=2.910(6) ).

K(2) binds to pzN(51) (s-mode; K(2) N(52)=3.389(5) )

and to the thioether chain (K(2) S(1)=3.363(2) ). The K

S interaction is remarkable, because (methylthio)methyl co-

ordination does not occur in the monotopic derivative K[5].

Most interestingly, trans-chelation of the K⁺ions involving

both scorpionate fragments is preferred over cis-chelation

by either the tris(pyrazol-1-yl)borate or the [(methylthio)-

methyl]borate ligand alone. The pyrazolyl rings pzN(21) and

pzN(41) are not engaged in the coordination of K(1) or

K(2), but bind to potassium ions of adjacent complexes (s-

modes). Additional short intermolecular contacts are estab-

lished between these potassium ions and the p-faces of

pzN(11)/pzN(51).

ꢀ

network of solvent-free (methylthio)methyllithium (Li S=

2.443(13) /2.662(12) ^[43]). The phenomenon of considera-

bly shorter Li N bonds to the h -coordinating pyrazolyl

rings compared to bonds between Li⁺and m-coordinating

ꢀ

pyrazolyl donors has also been observed for (Li

A

The solid state structure of the K⁺complex K[5] is funda-

mentally different from that of the Li⁺complex (Li[5])₂:

First, K[5] is polymeric rather than dimeric in the crystal lat-

tice (Figure 4, Figure 1cS in the Supporting Information).

Second, the (methylthio)methyl chains are merely dangling

side groups, not coordinated to K⁺ions in K[5]. Third, p-in-

teractions involving both the phenyl and pyrazolyl rings con-

tribute to the coordinative saturation of the K⁺centers.

A more detailed inspection of the bonding situation re-

veals that each K⁺ion interacts with one scorpionate ligand

ꢀ

through its two pyrazolyl nitrogen atoms (s-mode; K(1) N-

ꢀ

A

N

phenyl ring (p-mode; shortest contact: K(1) C

(31A)=

Compound K[11] contains two scorpionate ligands per

Cu^Iion. The monoanionic charge of the complex is balanced

by a K⁺ion. Electrostatic attraction and the need for K⁺

solvation lead to the formation of dimers (K[11])₂in the

3.054(4) ). A second borate ion functions as a chelating

ligand using the p-faces of both pyrazolyl rings for K⁺coor-

dination; the distances between the metal ion and the ring

6760

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Chem. Eur. J. 2008, 14, 6754 – 6770

Copper Scorpionate Complexes

FULL PAPER

Figure 6. Molecular structure of one of the two crystallographically inde-

pendent dimers (K[11])₂in the solid state; thermal ellipsoids at the 50%

probability level. The H atoms have been omitted for clarity. Selected

Figure 5. Section of the polymeric structure of K₂[10](thf)₂in the solid

A

state; thermal ellipsoids at the 50% probability level. For clarity, the H

atoms have been omitted; the coordinated THF molecules are represent-

ed as sticks. Selected bond lengths [], bond angles [8], and torsion

ꢀ

bond lengths [], bond angles [8], and torsion angles [8]: Cu(1) N(12)

ꢀ

2.054(7), Cu(1) N(42) 2.058(7), Cu(1) S(1) 2.251(2), Cu(1) S(2)

2.267(2), K(1) N(22) 2.851(7), K(1) N

(22^#) 2.821(7), K(1) N(52)

A

ꢀ

angles [8]: B(1) C(1) 1.640(9), B(1) C(61) 1.625(8), B(1) N(11)

ꢀ

2.757(7), K(1) N(12) 3.072(7); N(12) Cu(1) N(42) 108.5(3), N(12)

ꢀ

1.578(7), B(1) N(21) 1.563(9), B(2) C(64) 1.618(8), B(2) N(31)

ꢀ

Cu(1) S(1) 102.2(2), N(12) Cu(1) S(2) 104.5(2), N(42) Cu(1) S(1)

ꢀ

1.566(8), B(2) N(41) 1.571(8), B(2) N(51) 1.564(7), K(1) N(12)

2.856(5), K(1) N(32) 2.910(6), K(1) O(91) 3.003(7), K(2) N(52)

3.389(5), K(2) O(81) 2.761(6), K(2) S(1) 3.363(2), K(1)···COG(C₆H₄)

ꢀ

111.5(2), N(42) Cu(1) S(2) 101.3(2), S(1) Cu(1) S(2) 127.7(1), C(1)

ꢀ

S(1) Cu(1) 104.0(3), C(2) S(2) Cu(1) 101.9(3), B(1)···Cu(1)···B(2)

ꢀ

ACHTREUNG

ꢀ

140.9(2); B(1) N(11) N(12) Cu(1) ꢀ4.2(11), B(1) C(1) S(1) Cu(1)

ꢀ

2.988, K(2)···COG(C₆H₄) 3.045; C(1) B(1) C(61) 112.4(5), C(1) B(1)

R

ꢀ

48.2(6), B(2) N(41) N(42) Cu(1) 17.4(10), B(2) C(2) S(2) Cu(1)

54.9(7). Symmetry transformation used to generate equivalent atoms: #:

ꢀx+2, ꢀy+1, ꢀz.

ꢀ

N(11) 108.0(5), C(1) B(1) N(21) 109.4(5), N(12) K(1) N(32) 151.5(2),

ꢀ

S(1) K(2) N(52) 156.4(1); B(1) N(11) N(12) K(1) ꢀ3.3(9), B(1)

ꢀ

C(1) S(1) K(2) 46.4(6), B(2) N(31) N(32) K(1) 24.5(8), B(2) N(51)

ꢀ

N(52) K(2) 26.8(7).

ꢀ

Cu N bond lengths in the range from 1.95 to 2.05 and

[55,56]

ꢀ

solid state; two crystallographically independent, centrosym-

metric dimers constitute the asymmetric unit of the crystal

lattice. Since both dimers have similar bond lengths and

angles, only one of them will be described here (Figure 6).

Each Cu^Icenter is coordinated by two thioether and two

pyrazolyl groups (pzN(11), pzN(41)) in a distorted tetrahe-

Cu S bond lengths between 2.21 and 2.29 ,

which

agree nicely with the corresponding bond lengths in (K[11])₂

(Cu(1) N(12)=2.054(7) ,

ꢀ

Cu(1) N(42)=2.058(7) ,

ꢀ

Cu(1) S(1)=2.251(2) , Cu(1) S(2)=2.267(2) ).

The ligand/metal ratio of 2:1 present in the Cu^Icomplex

(K[11])₂is retained in the Cu^IIcomplex 12 (Figure 7). How-

ꢀ

dral fashion (smallest/largest bond angle: N(42) Cu(1)

ꢀ

S(2)=101.3(2)8

/ S(1) Cu(1) S(2)=127.7(1)8). The two

other pyrazolyl rings (pzN(21), pzN(51)) act as donors to-

ꢀ

wards K(1) (K(1) N(22)=2.851(7) , K(1) N(52)=

2.757(7) ). The ligand sphere of K(1) is further completed

by the p-faces of pzN(11) and pzN(41). Importantly,

pzN(21) occupies a bridging position between K(1) and the

symmetry-related ion K(1^#) and thus links the two halves of

the dimer together (K(1 ) N(22)=2.821(7) ). Copper ions

#

ꢀ

surrounded by an (N₂S₂) ligand sphere as in (K[11])₂occur

naturally in so-called “blue copper proteins” that are func-

tional in the reversible electron transfer: Cu^IQCu^II+

e^ꢀ.^[38,51–53]Crystal structures of these proteins show a very ir-

regular tetrahedral coordination with two sulfur atoms from

methionine and cysteinate and two histidine nitrogen

atoms.^[54]Numerous Cu^Iand Cu^IIcomplexes of mixed N-

and S-donor ligands have been investigated as mimics of

blue copper proteins to understand their intricate spectro-

scopical, electrochemical and structural features. We have

selected representative examples with imine- and thioether-

containing macrocyclic or thiosalen-type ligands for a struc-

tural comparison with (K[11])₂. These compounds exhibit

Figure 7. Molecular structure of 12 in the solid state; thermal ellipsoids at

the 50% probability level. The H atoms have been omitted for clarity.

Selected bond lengths [], bond angles [8], and torsion angles [8]:

ꢀ

Cu(1) N(12) 2.009(7), Cu(1) N(22) 1.999(6), Cu(1) N(42) 1.980(7),

ꢀ

Cu(1) N(52) 1.986(6), Cu(1) S(1) 2.805(3); N(12) Cu(1) N(22) 88.4(2),

ꢀ

N(12) Cu(1) N(42) 172.5(3), N(12) Cu(1) N(52) 90.0(3), N(42)

ꢀ

Cu(1) N(52) 88.4(3), N(12) Cu(1) S(1) 83.1(2), N(22) Cu(1) S(1)

ꢀ

90.1(2), C(1) S(1) Cu(1) 98.7(3), B(1)···Cu(1)···B(2) 168.1(1); B(1)

ꢀ

N(11) N(12) Cu(1) ꢀ1.5(9), B(1) N(21) N(22) Cu(1) -1.8(10), B(1)

ꢀ

C(1) S(1) Cu(1) ꢀ15.7(8), B(2) N(41) N(42) Cu(1) ꢀ0.9(9), B(2)

ꢀ

N(51) N(52) Cu(1) 5.6(9).

Chem. Eur. J. 2008, 14, 6754 – 6770

ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemeurj.org

6761

M. Wagner, M. C. Holthausen et al.

ever, we observe a distinctly different coordination pattern

with four pyrazolyl donors and one (methylthio)methyl

group spacedly disposed at the corners of a slightly distorted

square pyramid (trigonality index t^[57]=0.05).

structure of 12 appears to be sufficiently more favorable

than the alternative (N₂S₂) coordination to outweigh the

high degree of ligand redistribution that is required for the

transition from K[11] to 12.

Most importantly, the sulfur atom occupies the axial posi-

tion. This feature makes compound 12 rather unique among

After treatment of the Cu^Icomplex K[11] with dry air,

two products, the mononuclear Cu^IIcomplex 12 and the tet-

ranuclear Cu^IIspecies 13 (Figure 8) could be isolated.

the few structurally characterized [Cu^II

N

complexes, which are either square pyramidal (spy), but

have the sulfur atom in an equatorial position,^[58]or adopt a

trigonal bipyramidal conformation^[58,59](again with the

sulfur donor in an equatorial position). Thus, compound 12

fills a gap in the series of blue copper protein model sys-

tems, because an axially coordinated thioether donor in

trigonal (bi)pyramidal active site geometries is suggested to

confer highly positive Cu^I/Cu^IIredox potentials.^[58,60]As a

specific example of blue copper proteins, we are considering

the dimensions of the copper site in poplar Cu^II-plastocya-

nin, which has the methionine sulfur atom located at a long

but yet bonding distance of 2.82 to the Cu^IIion.^[54]This

ꢀ

value compares nicely to the Cu(1) S(1) bond length in

ꢀ

complex 12 (2.805(3) ). In contrast, a much shorter Cu S

bond (2.372(3) ) is observed in the square-pyramidal (t=

0.26) [Cu^II

A

(SR₂)]²⁺complex [Cu

G

ACHTREUNG

Figure 8. Molecular structure of 13 in the solid state; thermal ellipsoids at

the 50% probability level. The H atoms have been omitted for clarity.

Selected bond lengths [], bond angles [8], and torsion angles [8]:

pyridyl-N-(2’-methylthiophenyl)methyleneimine; 2,9-dmp=

2,9-dimethyl-1,10-phenanthroline).^[58]The pronounced dif-

ꢀ

Cu(1) O(1) 1.920(2), Cu(1) N(12) 1.980(3), Cu(1) N(31) 1.965(2),

ꢀ

ference in Cu S bond lengths between 12 and [Cu

(pmtpm)-

ꢀ

Cu(1) N(41) 1.976(2), Cu(2) O(1) 1.924(2), Cu(2) N(22) 1.980(3),

(2,9-dmp)]

N

#

ꢀ

Cu(2) N(32) 1.982(3), Cu(2) N(42 ) 1.973(2), B(1) O(1) 1.443(4), B(1)

A

tortion generally exhibited by five- and six-coordinated d⁹-

ꢀ

C(1) 1.626(4), B(1) N(11) 1.595(4), B(1) N(21) 1.578(4); Cu(1) O(1)

ꢀ

complexes. With regard to the Cu S distance, 12 is, there-

Cu(2) 111.4(1), Cu(1) O(1) B(1) 117.1(2), Cu(2) O(1) B(1) 118.4(2),

fore, better compared to the Cu^IIcomplex [Cu(L)]

(NO₃)₂

ꢀ

O(1) Cu(1) N(12) 84.5(1), O(1) Cu(1) N(31) 86.9(1), O(1) Cu(1)

ꢀ

N(41) 178.9(1), N(12) Cu(1) N(31) 170.0(1), N(12) Cu(1) N(41)

(L=4,7-bis(3-aminopropyl)-1-thia-4,7-diazacyclononane)

possessing four sp³-nitrogen atoms at the base of a square

pyramid and a thioether moiety at the axial position. How-

ꢀ

94.5(1), N(31) Cu(1) N(41) 94.1(1), O(1) Cu(2) N(22) 84.3(1), O(1)

#

ꢀ

Cu(2) N(32) 86.2(1), O(1) Cu(2) N(42 ) 177.8(1), N(22) Cu(2) N(32)

A

ꢀ

170.4(1), N(22) Cu(2) N

(42^#) 94.0(1), N(32) Cu(2) N(42^#) 95.6(1);

G

ꢀ

B(1) N(11) N(12) Cu(1) ꢀ13.2(3), B(1) N(21) N(22) Cu(2) 2.3(3),

ever, the bond distance Cu S=2.561(1) of the latter com-

#

ꢀ

pound^[61]is still 0.244 shorter than in 12 which may be due

to the short (methylthio)methyl tether of the scorpionate

ligand that prevents a closer approach of the sulfur atom.

This interpretation is supported by a view at the Cu–S

vector in 12, which deviates by 10.78 from the normal to the

least-squares plane (r.m.s. deviation 0.038 ) through Cu(1)

Cu(1) N(31) N(32) Cu(2) -0.4(2), Cu(1) N(41) N(42) Cu(2 ) 17.7(3).

Symmetry transformation used to generate equivalent atoms: #: ꢀx+1,

ꢀy+1, ꢀz+1.

The formation of 13 is remarkable because not only have

pyrazolide ions been released, but the (methylthio)methyl

group has been replaced by an oxygen atom in the course of

the reaction. Given the fact that we have taken all precau-

tions to exclude moisture from the reaction mixture, the in-

troduction of the oxygen atom into the scorpionate ligand is

most likely the result of a Cu^I-mediated O₂activation. This

hypothesis is confirmed by the fact, that exposure of a tolu-

ene solution of the Cu^IIspecies 12 to air and moisture over

a period of several weeks did not yield any indication for

the formation of 13. Again, we note that oxidative cleavage

ꢀ

and its four N ligands. The four Cu N bond lengths range

ꢀ

from Cu(1) N(42)=1.980(7) to Cu(1) N(12)=2.009(7)

and are thus in good agreement with the corresponding

values in the related five-coordinate Cu^IIcomplexes.^[58,59]

The molecular structures of K[11] and 12 are distinctly dif-

ferent. Since these differences can easily be explained by ap-

plying basic rules for interactions between hard and soft

Lewis acids and bases, it is interesting to note that Meyer

et al. have published a Cu^IIcomplex in which the metal ion

is surrounded by two pyrazolide-N- and two thioether-S-

atoms in a distorted square-planar (dspl) cis-(N₂S₂) coordi-

nation sphere.^[62]On principle, it would be possible to estab-

lish a similar ligand environment also for [Cu^II(5)₂], simply

by twisting the orthogonal planes S(1)/Cu(1)/N(12) and

S(2)/Cu(1)/N(42) of (K[11])₂into a coplanar arrangement

upon oxidation. However, the experimentally determined

ꢀ

of the B C-bond is reminiscent of the C H activation ach-

ieved by the enzymes DbM and PHM. Once pyrazolylbo-

rates [PhBpz₂(OH)]^ꢀor [PhBpz₂(O)]^2ꢀhave been formed,

further degradation of these molecules with liberation of

pyrazolide can easily occur because the three-coordinated

boron atoms of the resulting fragments PhBpz(OH)/

A

ꢀ

A

6762

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Chem. Eur. J. 2008, 14, 6754 – 6770

Copper Scorpionate Complexes

FULL PAPER

Each half of the centrosymmetric molecule of 13 consists

bonds: 2.048(7) in (K[11])₂, 2.023(7) in 14(thf)₂). The

A

of one [PhBpz₂(O)]^2ꢀligand that symmetrically bridges two

tris(pyrazol-1-yl)borate functionalities of the two ligands

bind jointly to the potassium ion K(2) with bond lengths of

2.840(8) . Additional short contacts are established be-

Cu^IIions by its oxygen atom (Cu(1) O(1)=1.920(2) ,

ꢀ

[63]

ꢀ

Cu(2) O(1)=1.924(2) ).

The pyrazolyl nitrogen atom

ꢀ

N(12) binds to Cu(1); N(22), which is part of the second

tween K(2) and the sulfur atoms (K(2) S(1)=3.385(3) )

ꢀ

pyrazolyl ring, coordinates Cu(2) (Cu(1) N(12)=Cu(2)

N(22)=1.980(3) ). In addition to O(1), Cu(1) and Cu(2)

as well as the phenylene rings (K(2) COG

A

COG=ring centroid). Two more potassium ions, K(1) and

the symmetry-related K(1^#), coordinate to the metalloma-

crocyclic core to maintain charge neutrality. K(1) binds to

one THF molecule, one pyrazolyl ring of a heteroscorpio-

nate fragment, one pyrazolyl ring of the corresponding ho-

moscorpionate moiety, and to the phenylene spacer. Each

repeat unit is connected to the neighboring unit through

ꢀ

are connected by a pyrazolide ion with Cu(1) N(31) and

ꢀ

Cu(2) N(32) bond lengths of 1.965(2) and 1.982(3) , re-

spectively. The square planar coordination sphere of each

Cu^IIion is completed by a second pyrazolide ligand that

ꢀ

links the two halves of the tetranuclear aggregate (Cu(1)

#

[64]

ꢀ

N(41)=1.976(2) , Cu(2 ) N(42) 1.973(2) ).

Complex 14 crystallized from a mixture of ligand K₂[10]

pz(N(41)) and pz(N(51)), which bind to K(1*) (*: ꢀx+³= ,

2

and 3.3 equiv of CuCl in THF with one THF ligand bonded

ꢀy+³= , ꢀz+1) in a s- and p-mode, respectively. Interest-

2

to each K⁺ion (14

A

ingly, a striking similarity is obvious between the coordina-

tion polymers in the solid state; a view of the repeat unit,

which possesses a C₂axis running through Cu(1) and K(2),

is provided in Figure 9.

tion spheres of K(1)/K(2) in the potassium salt K₂[10]

A

and in the Cu^Icomplex 14

AHCTREUNG

Electrochemical investigations: Compounds K[5], K[11],

and 12 were investigated by cyclic voltammetry (CH₂Cl₂,

[NBu₄][B

(C₆F₅)₄] (0.05m), versus FcH/FcH⁺; see the Sup-

G

porting Information for cyclic voltammograms of K[5] in

Figure 2aS and K[11] in Figure 2bS). The K⁺salt of the free

ligand (K[5]) is electrochemically inactive in the potential

range between 0.75 V and ꢀ1.75 V; in the more anodic

regime, an irreversible oxidation process takes place.

In the potential range between 1.30 V and ꢀ2.10 V, the

cyclic voltammogram (CV) of the Cu^Icomplex K[11] shows

three irreversible redox events with peak potentials of E_paꢁ

ꢀ0.71 V, 0.27 V, and E_pcꢁꢀ1.49 V.

Cu^IIcomplex 12 reveals two irreversible redox events at

E_paꢁꢀ0.79 V and E_pcꢁ-1.65 V (Figure 10). In more oxidiz-

ing conditions, 12 undergoes a further irreversible transition

Figure 9. Molecular structure of 14(thf)₂in the solid state. For clarity, the

U

H atoms have been omitted and atoms are represented as balls. Selected

ꢀ

bond lengths [], bond angles [8], and torsion angles [8]: Cu(1) S(1)

ꢀ

2.361(2), Cu(1) N(12) 2.023(7), K(1) N(22) 2.969(8), K(1) N(52)

ꢀ

2.828(7), K(1) O(71) 2.731(6), K(2) N(32) 2.840(8), K(2) S(1) 3.385(3),

#

ꢀ

K(1) COG

G

U

#

ꢀ

128.2(1), S(1) Cu(1) N(12) 98.8(2), S(1) Cu(1) N(12 ) 109.6(2), N(12)

U

#

ꢀ

Cu(1) N

(12 ) 112.0(4), N(22) K(1) N(52) 160.0(2), N(22) K(1) O(71)

ꢀ

86.6(2), N(52) K(1) O(71) 95.4(2); B(1) N(11) N(12) Cu(1) ꢀ0.7(11),

ꢀ

B(1) C(1) S(1) Cu(1) 47.3(6), B(1) N(21) N(22) K(1) 36.9(9), B(2)

ꢀ

N(31) N(32) K(2) -21.3(11), B(2) N(51) N(52) K(1) ꢀ21.1(11). Sym-

metry transformation used to generate equivalent atoms: #: ꢀx+1, y,

Figure 10. Cyclic voltammogram of 12 vs. FcH/FcH⁺; CH₂Cl₂, [NBu₄][B-

ꢀz+¹= .

N

2

In this intricate structure, the Cu^Iion is placed between

two scorpionates into a tetrahedral (N₂S₂) ligand environ-

ment. We observe the same key structural features as in the

Cu^Icomplex (K[11])₂(Figure 6) of the monotopic scorpio-

that is also observed in the CV of K[5] and therefore attrib-

uted to a ligand-centered process.

The determination of peak potentials of K[11] is associat-

ed with estimated error margins of ꢂ 0.05 V. Moreover, E_pc

and E_paof 12 were found to be somewhat concentration de-

pendent. Given this background, we attribute the redox

nate [5]^ꢀ(mean values of the Cu S bonds: 2.264(2) in

ꢀ

(K[11])₂, 2.361(2) in 14

A

Chem. Eur. J. 2008, 14, 6754 – 6770

ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemeurj.org

6763

M. Wagner, M. C. Holthausen et al.

waves E_paꢁꢀ0.71 V/E_pcꢁꢀ1.49 V (CV of the Cu^Icomplex)

and E_paꢁꢀ0.79 V/E_pcꢁꢀ1.65 V (CV of the Cu^IIcomplex)

to the same electron transfer reaction, which in both sam-

ples occurs between the species [NBu₄]⁺[11]^ꢀQ12. The

highly cathodic value of E_pcis likely owed to the fact that

the Cu^IIion is coordinated by two negatively charged li-

gands. Moreover, provided that the molecular structure of

12 in solution resembles its solid state structure, a square-

pyramidal (N₄S) donor set is not well-suited to stabilize the

Cu^Istate. It is therefore plausible that the initially created

species undergoes ligand rearrangement to form a tetrahe-

dral Cu^Icomplex. As a consequence, the redox event be-

comes electrochemically irreversible. The associated return

peak corresponds to the transformation of the tetrahedral

Cu^Icomplex into the corresponding Cu^IIcompound that

subsequently relaxes to the original structure.

sulfur-donor ligands.^[66]We have, therefore, included the

three tetrahedral species [Cu^I(N₄)^tet]^ꢀ, [Cu^I(N₃S)^tet]^ꢀ, and

A

[Cu^I(N₂S₂)^tet]^ꢀinto our DFT study (Figure 11). Cu^IIions, in

G

turn, have a significantly greater tendency to bind to amine

The electrochemical properties of our complex stand in

stark contrast to a related system in which the copper ion is

surrounded by two chelating 1-methyl-2-((methylthio)-

Figure 11. Relative energies (DE in kcalmol^ꢀ1obtained at the BP86/

SDD(Cu)TZVP(+COSMO:CH₂Cl₂)//BP86/SDD(Cu)TZVP level) be-

tween different coordination polyhedra of [11]^ꢀ(top row) and 12

(bottom row).

methyl)-1H-benzimidazole (mmb) ligands. [Cu

N

undergoes an electrochemically reversible oxidation at

0.31 V vs. FcH/FcH⁺.^[65]In accordance with that is the ap-

parent similarity between the molecular structures of [Cu^I-

A

(mmb)₂

A

than to thioether ligands.^[66]Our calculations on the Cu^II

firmed by X-ray crystallography for the solid state and by

complex 12 were therefore restricted to the two most plausi-

DFT calculations for the gas phase.^[65]

ble isomers, the square-pyramidal structure [Cu^II(N₄S)^spy

]

G

The redox event at E_paꢁ0.27 V in the voltammogram of

K[11] was not detected in the CV of 12. We suggest as an

explanation that a cation exchange equilibrium leads to the

coexistence of K[11] and [NBu₄]⁺[11]^ꢀand that these two

species have different oxidation potentials, i.e., 0.27 V and

ꢀ0.71 V, respectively. The more anodic redox potential of

K[11] with respect to [NBu₄]⁺[11]^ꢀcan then be rationalized

by assuming that the former exists as contact ion pairs in

CH₂Cl₂solution, whereas less tightly associated ions are

present in solutions of the latter. We note in this context

that the Cu^I/Cu^IIredox couple covers a broad range of stan-

dard oxidation potentials (exceeding variations of 1 V) de-

pending on the particular ligand environment, the solvent,

and on the nature of the counter ions (see p. 418 of

ref. [51b]).

(see the X-ray crystal structure analysis) and its square-

planar congener [Cu^II(N₄)^spl] (Figure 11). Calculated relative

energies (DE in kcalmol^ꢀ1) with respect to the most stable

isomer are compiled in Figure 11.

First we note a pleasing agreement between the experi-

mentally determined bond lengths and angles of K[11] and

12 with the corresponding calculated values for K

(N₂S₂)^tet] and [Cu^II(N₄S)^spy], respectively (see the Supporting

Information).

The Cu^Icomplex [Cu^I(N₄)^tet]^ꢀrepresents the most stable

A

ACHTREUNG

structure of all three anionic isomers. The [Cu^I(N₃S)^tet]^ꢀ

U

isomer is slightly less stable (DE=0.4 kcalmol^ꢀ1). Replace-

ment of a second pyrazolyl ligand by a (methylthio)methyl

group leads to a further increase in energy by 2.6 kcalmol^ꢀ1

([Cu^I(N₂S₂)^tet]^ꢀ), even though one might have anticipated

R

the Cu^Iion to bind preferably to the softer (methylthio)-

methyl donors. At this point we note, however, that the

energy differences obtained here might fall within the typi-

cal error margins of the method employed,^[67]and thus we

refrain from providing any definitive statement with regard

to the most stable coordination isomer of the free anion. We

would rather see these results as an indication for the pres-

ence of a set of flexible isomers of [11]^ꢀin solution, which is

in accord with the NMR spectroscopic results on K[11] (see

above). In the crystal lattice, we observed the seemingly

Quantum chemical calculations: Density functional theory

(DFT) at the BP86 level including the COSMO continuum

model (solvent CH₂Cl₂) was employed in order to scrutinize

the hypothesis that a transition from the contact ion pair

K[11] to the largely solvent separated system [NBu₄]⁺[11]^ꢀ

can indeed cause a shift in the corresponding Cu^I/Cu^IIredox

potential of almost 1 V. Furthermore, we provide a theoreti-

cal assessment of the surprisingly cathodic potential re-

quired for the reduction of 12. Details of the theoretical

strategies are given in the section Experimental and Compu-

tational Methods below.

least stable [Cu^I(N₂S₂)^tet]^ꢀcoordination mode for the com-

U

plex anion of K[11]. A closer inspection of the solid state

structure suggests, however, a counterion effect as the likely

explanation, because the K⁺cation in crystalline K[11] is p-

coordinated to two pyrazolyl rings,^{[30,31,47,68]}one from each

scorpionate ligand. Such a type of molecular pocket is only

As one cannot necessarily expect the solid-state structures

of [11]^ꢀand 12 to prevail in solution, we also considered a

selected number of reasonable structural isomers. Cu^Iions

are known to possess a high affinity both to nitrogen- and to

6764

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Chem. Eur. J. 2008, 14, 6754 – 6770

Copper Scorpionate Complexes

FULL PAPER

available in the [Cu^I

A

into electrochemical potentials by division by the Faraday

constant F (403305.5 Ccal^ꢀ1).

The cyclic voltammogram of K[11] is considered first

the others. The (N₂S₂) ligand environment therefore appears

to be stabilized through p-coordination of the potassium

counter ion (the structure of this contact ion pair was

indeed shown to correspond to a minimum on the potential

energy surface). It should be emphasized that p-interactions

between K⁺and [11]^ꢀmay also be relevant in the liquid

phase as they can lead to different distributions of coordina-

tion polyhedra in solutions of K[11] as compared to samples

of [11]^ꢀgenerated by reduction of Cu^II-complex 12 under

the conditions of a CV experiment (i.e. in solutions contain-

ing non-coordinating [NBu₄]⁺-salts as supporting electro-

lytes).

(Scheme 4). In the initial step, vertical ionization of K

A

AHCTREUNG

The crystal structure of 12 reveals a square-pyramidal

ꢀ

complex with a long axial Cu S bond of 2.805(3) suggest-

ing a weakly coordinated (methylthio)methyl ligand. A very

similar five-coordinated structure was found to be a mini-

mum on the potential energy surface of 12 by reoptimiza-

tion of the crystallographically determined structural param-

ꢀ

Scheme 4. Proposed pathways for reactions taking place during electro-

chemical conversion of K[11]. Experimental data (in brackets) were

taken from the cyclic voltammogram of K[11]; notation of coordination

polyhedra: see Figure 11 and text for details.

eters ([Cu^II(N₄S)^spy]; Figure 11). Avoiding of the Cu S con-

G

ꢀ

tact by turning the CH₂SMe arm by 908 about the B C

bond and subsequent reoptimization of the structure results

in an energy increase of merely 0.7 kcalmol^ꢀ1for the result-

ing square-planar (spl) species [Cu^II(N₄)^spl].

plex [K

A

E

All in all, our calculations lead to the conclusion that the

tetrahedral Cu^Icomplex [Cu^I(N₄)^tet]^ꢀand the square-planar

Cu^IIcomplex [Cu^II(N₄)^spl] should be among the most abun-

dant isomers in solutions of [11]^ꢀand 12, respectively. With

regard to the interconversion of both species under electro-

chemical conditions it is important to note that [Cu^I(N₄)^tet]^ꢀ

cannot simply rearrange into [Cu^II(N₄)^spl] by counterrotation

an electrode potential of 0.31 V, which compares well with

the experimental peak potential of E_pa=0.27 V.

After the electron transfer, the K⁺ion dissociates from

the Cu^IIcomplex and the ligand sphere of [Cu^II(N₂S₂)^tet] re-

T

organizes to give the distorted square-planar (dspl) isomer

[Cu^II(N₄)^dspl]. Subsequent vertical reduction of [Cu^II(N₄)^dspl

]

yields the corresponding Cu^Icomplex [Cu^I(N₄)^dspl]^ꢀ. The cal-

culated potential required for this transition amounts to

ꢀ1.54 V, in good agreement with the experimental peak po-

tential of E_pc=ꢀ1.49 V taken from the CV of K[11]. In con-

trast, vertical reduction of the undistorted isomer

[Cu^II(N₄)^spl] would correspond to a highly cathodic potential

value of ꢀ2.23 V, a clear indication that the distorted square

planar Cu^IIcomplex is responsible for the redox wave at

E_pc=ꢀ1.49 V in the cyclic voltammogram of K[11] (cf. the

ꢀ

of the two N Cu N planes. This reorganization is blocked

by the pyrazolyl protons in the 3-positions of the scorpio-

nate ligands (Figure 3S in the Supporting Information).

Indeed, we located a second minimum resulting from geom-

etry optimization commencing at the tetrahedral Cu^Icoordi-

nation environment, from which the ligands arrange as

closely as possible into the square planar geometry after oxi-

dation to the Cu^IIstate. The resulting distorted square

planar (dspl) isomer [Cu^II(N₄)^dspl] is less stable than [Cu^II-

concept of the “entatic state” of enzyme active sites^[70]).

ꢀ

A

[Cu^I(N₄)^dspl

]

rearranges to the more stable tetrahedral

structure [Cu^I(N₄)^tet]^ꢀ. Oxidation of this free anion to the

neutral complex [Cu^II(N₄)^tet] takes place at a substantially

more negative potential (-0.74 V) than the oxidation of the

contact ion pair K[11] (0.31 V). The value of ꢀ0.74 V com-

pares well with the experimentally determined peak poten-

transitions and structural rearrangements taking place

during the cyclic voltammetric measurements on K[11] and

12. An important aspect of most microscopic theories of

electron transfer is the assumption that the reactants and

products share a common nuclear configuration at the

moment of the actual act of oxidation or reduction.^[69]We

thus see the experimental peak potentials arising as a conse-

quence of vertical oxidation or reduction events. According-

ly, we simulate these processes by computing the vertical

ionization potentials (IE) or electron affinities of the respec-

tive species. We relate the computed potentials to that ob-

tained for the ferrocene/ferrocenium (FcH/FcH⁺) redox

couple (computed: IE_FcH=119.2 kcalmol^ꢀ1), which serves as

internal standard also in our CV measurements. Computed

energy differences (obtained in kcalmol^ꢀ1) were converted

tial E_pa=ꢀ0.71 V for K[11]. Oxidation of [Cu^I

A

[Cu^II

G

contrast to the experimental finding and therefore again

suggests that [Cu^I(N₄)^tet]^ꢀis indeed the most stable coordi-

nation geometry for the free ion, whereas the solid state

isomer [Cu^I(N₂S₂)^tet]^ꢀis clearly negligible in solution.

As expected and outlined above, the two redox transitions

visible in the CV of the Cu^IIcomplex 12 are also observed

in the CV of K[11], which indicates that in both cases the

same species are involved in the electron transfer processes.

Following our quantum chemical interpretation, this points

Chem. Eur. J. 2008, 14, 6754 – 6770

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www.chemeurj.org

6765

M. Wagner, M. C. Holthausen et al.

to the predominance of the distorted square-planar configu-

ration [Cu^II(N₄)^dspl] in both experiments. Yet, in the case of

12, this is somewhat counterintuitive considering the fact

that we have prepared the sample by dissolving single crys-

tals evidently containing the undistorted square-planar

isomer [Cu^II(N₄)^spl]. For this species, however, a redox po-

tential of ꢀ2.23 V results from our DFT calculations, as op-

posed to a value of ꢀ1.54 V for [Cu^II(N₄)^dspl] and in disa-

greement with the experimental redox potentials of ꢀ1.49 V

and ꢀ1.65 V for K[11] and 12, respectively.

mimic the two ligation sites of the copper enzymes peptidyl-

glycine a-hydroxylating monooxygenase and dopamine b-

monooxygenase in which the cooperative action of one

[his₃Cu] and one [his₂(met)Cu] fragment leads to oxidative

N

ꢀ

C H activation (his=histidine; met=methionine). In a sys-

tematic study, we first investigated the coordination proper-

ties of the monotopic tripod [5]^ꢀtoward Cu^Iand Cu^IIions

and isolated 2:1 complexes K

[Cu^I(5)₂] (K[11]) and [Cu^II(5)₂]

E

(12). The solid state structure of K[11] revealed a tetrahe-

dral (N₂S₂) coordination environment for the Cu^Iion,

whereas the Cu^IIion of 12 possesses a square-pyramidal

(N₄S) ligand sphere. In the cyclic voltammograms of K[11]

and 12, the Cu^I/Cu^IItransition is irreversible, most likely be-

cause electron transfer is accompanied by extensive ligand

reorganization. The quantum chemical assessment of the

redox processes provided detailed insights into the underly-

ing elementary processes and the nature of the dominant

species involved.

At first sight, one could plainly see this as a large error in

the DFT description of the intricate electronic processes

behind CV measurements. Notwithstanding all appropriate

skepticism as to the uncritical use of DFT for chemical

problems involving transition metal complexes,^[71]we did

not find any hint for a source of error in the quantum chem-

ical results. And indeed, given the overall good performance

of the DFT description of CV data above, this would appear

as an unexpected failure of DFT for an otherwise innocuous

species that largely resembles all other isomers studied here.

Also, we have previously established the level of DFT em-

ployed here as a very robust means to predict trends in the

electrochemical properties of a homologous series of transi-

tion metal complexes.^[72]Rather than plainly assuming a fun-

damental DFT error, we suggest the following chemical sce-

nario as an alternative explanation: Assuming a fast equilib-

rium between the distorted and the undistorted isomers

After K[11] had been treated with dry air, Cu^IIcomplex

12 was isolated as the main product together with smaller

ꢀ

amounts of a decomposition product indicating oxidative B

C activation. So far, we have no evidence for sulfur oxida-

tion.

Addition of excess CuCl to K₂[10] led to the formation of

a compound K₃[Cu(10)₂] (14), which again contains two

ligand molecules per Cu^Icenter. The Cu^Iion binds to both

heteroscorpionate moieties and thereby establishes a coordi-

nation environment similar to that of the Cu^Iion in K[11].

By using the same synthetic approach as for [5]^ꢀand

[10]^2ꢀ, an attractive goal for future ligand development lies

in the preparation of more sterically demanding derivatives

that are able to form well-defined complexes featuring only

one scorpionate unit per copper ion. Such complexes are

not only expected to show an increased affinity toward ele-

mental oxygen but also to kinetically stabilize the primary

intermediates of the O₂-activation sequence so that they can

be unambiguously identified.^[73–77]

[Cu^II(N₄)^dspl] and [Cu^II(N₄)^spl] any amount of [Cu^II(N₄)^dspl

present will give rise to a reduction wave at about ꢀ1.5 V in

the course of the CV scan. Simultaneously, the [Cu^II(N₄)^dspl

]

consumed in this process will be replenished from the reser-

voir of the undistorted isomer [Cu^II(N₄)^spl]. Provided that

the scan rate is slow on the time scale of the equilibrium, no

undistorted [Cu^II(N₄)^spl] will be left to yield a signal at the

predicted redox potential of ꢀ2.23 V.

Conclusion

Straightforward protocols have been elaborated for prepar-

Experimental and Computational Methods

ing the (N₂S) scorpionate ligand [PhBpz₂

(CH₂SMe)]^ꢀ([5]^ꢀ)

N

ꢀ

General remarks: All reactions were carried out under a nitrogen atmos-

phere using Schlenk tube techniques. Reaction solvents were freshly dis-

tilled under argon from Na/Pb alloy (pentane, hexane) and Na/benzophe-

none (C₆H₆, toluene, Et₂O, THF) prior to use; C₆D₆was distilled under

nitrogen from Na/benzophenone and stored in a Schlenk flask. NMR:

Bruker AM 250, Avance 300, Avance 400, and DPX 250. Chemical shifts

are referenced to residual solvent signals (¹H, ¹³C{¹H}) or external

BF₃·Et₂O (¹¹B{¹H}). Abbreviations: s=singlet, d=doublet, vtr=virtual

triplet, m=multiplet, br=broad, n.o.=signal not observed, Ph=phenyl,

Ar=p-phenylene, pz=pyrazolyl ring of a bis(pyrazol-1-yl)borate unit,

pz’=pyrazolyl ring of a tris(pyrazol-1-yl)borate unit. BBr₃was stored

over mercury under a nitrogen atmosphere. SMe₂and BBr₃are commer-

and the heteroditopic poly(pyrazol-1-yl)borate p-[pz₃B

C₆H₄Bpz

₂(CH₂SMe)]^2ꢀ([10]^2ꢀ). In our hands, the key to

ꢀ

success was the use of TMEDA-free LiCH₂SMe for the in-

troduction of the sulfur donor group. With regard to the

synthesis of [10]^2ꢀit was important to recognize that the in-

termediate

B(Br)

[(methylthio)methyl]borane

p-Me₃Si-C₆H₄-

(CH₂SMe) forms dimeric B S adducts in solution. As

ꢀ

A

a result, the sulfur atom is protected from further electro-

philic attack so that BBr₃can be employed in a subsequent

silicon-boron exchange step leading to the unsymmetric

ꢀ

[42,45]

[44,45]

cially available. PhBBr_2,

were synthesized following literature procedures.

Caution! SMe₂is flammable liquid of pungent odor; donor-free

p-Me₃Si-C₆H₄-BBr_2,

and LiCH₂SMe^[43]

building block p-Br₂B C₆H₄-B(Br)(CH₂SMe).

R

(Hetero)ditopic ligands are important for the preparation

a

of (hetero)dinuclear complexes in which an interaction be-

tween the two metal sites brings about chemical and/or

physical properties not exhibited by the mononuclear con-

stituents alone. The discorpionate [10]^2ꢀis designed to

LiCH₂SMe violently explodes upon exposure to air or upon thermolysis

under an argon atmosphere.^[43]

Synthesis of 2: C₆H₆(30 mL) was added to neat LiCH₂SMe (0.48 g;

7.05 mmol) and the resulting suspension cooled to 68C. A solution of

6766

www.chemeurj.org

ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Chem. Eur. J. 2008, 14, 6754 – 6770

Copper Scorpionate Complexes

FULL PAPER

PhBBr₂(1.75 g; 7.05 mmol) in C₆H₆(8 mL) was added dropwise with stir-

ring over a period of 45 min. The reaction mixture was allowed to warm

to RT and stirred for 12 h. The resulting colorless suspension was filtered

and the filtrate evaporated to dryness in vacuo. Recrystallization by slow

diffusion of pentane into a solution of the crude product in C₆H₆gave

colorless crystals of 2. Yield: 1.36 g (2.97 mmol, 84%). ¹H NMR

(400.1 MHz, C₆D₆): d=7.89 (d, 4H; PhH-o), 7.29 (vtr, 4H; PhH-m), 7.19

[D₈]THF): d=7.87, 7.26 (2d, 22H; pzH-3,5), 7.04–6.92, 6.90–6.84 (2

m, 3H, 2H; PhH), 5.99 (vtr, 2H; pzH-4), 2.40 (s, 2H; BCH₂), 1.94 ppm

(s, 3H; SMe); ¹³C NMR (62.9 MHz, [D₈]THF): d=138.3 (pzC-3 or 5),

134.1 (PhC), 133.7 (pzC-3 or 5), 126.7 (PhC), 125.1 (PhC-p), 102.6 (pzC-

4), 33.8 (br, BCH₂), 20.1 ppm (SMe), n.o. (PhC-i); ¹¹B NMR (96.3 MHz,

[D₈]THF): d=ꢀ1.0 ppm (h_1/2=110 Hz); elemental analysis calcd (%) for

C₁₄H₁₆BKN₄S [322.28]·0.5 H₂O [18.01] : C 50.75, H 5.17, N 16.90; found:

C 50.77, H 4.99, N 16.57. Note: K[5] is hygroscopic.

(t, 2H; PhH-p), 2.64, 2.08 (2d, ²J

(H,H)=12.7 Hz, 22H; BCH₂),

G

1.36 ppm (s, 6H; SMe); ¹³C NMR (100.6 MHz, C₆D₆): d=133.3 (PhC-o),

128.3 (PhC-p), 128.3 (PhC-m), 34.9 (br, BCH₂), 20.3 ppm (SMe), n.o.

(PhC-i); ¹¹B NMR (128.4 MHz, C₆D₆): d=2.5 ppm (h_1/2=250 Hz); ele-

mental analysis calcd (%) for C₁₆H₂₀B₂Br₂S₂[457.88]: C 42.14, H 4.48;

found: C 41.97, H 4.40.

Synthesis of 6: p-Me₃Si-C₆H₄-SiMe₃(0.99 g; 4.45 mmol) was dissolved in

C₆H₆(10 mL). Neat BBr₃(1.11 g; 4.43 mmol) was added through a sy-

ringe and the pale yellow solution stirred for 18 h. All volatiles were re-

moved in vacuo leaving behind the product as a yellow oil. Yield: 1.41 g

(4.40 mmol, 99%). ¹H NMR (400.1 MHz, C₆D₆): d=8.10, 7.34 (2d, ³J-

A

Synthesis of 3: C₆H₆(25 mL) was added to neat LiCH₂SMe (0.57 g;

8.37 mmol) and the resulting suspension cooled to 68C. A solution of

PhBBr₂(1.04 g; 4.20 mmol) in C₆H₆(15 mL) was added dropwise with

stirring over a period of 40 min. The reaction mixture was allowed to

warm to RT and stirred for 12 h. The resulting colorless suspension was

filtered and the filtrate concentrated in vacuo until a colorless microcrys-

talline precipitate formed. Pentane (40 mL) was added, the insoluble

product was collected on a frit and dried in vacuo. The crude product

was not analytically pure, but its purity was sufficiently high for further

(100.6 MHz, C₆D₆): d=150.1 (CSi), 136.8, 133.3 (ArC), ꢀ1.6 ppm (SiMe),

n.o. (CB); ¹¹B NMR (128.4 MHz, C₆D₆): d=57.5 ppm (h_1/2=285 Hz).

Synthesis of 7: C₆H₆(15 mL) was added to solvent-free LiCH₂SMe

(0.52 g; 7.64 mmol) and the resulting suspension cooled to 68C. A solu-

tion of 6 (2.53 g; 7.91 mmol) in C₆H₆(10 mL) was added dropwise with

stirring over a period of 30 min. The reaction mixture was allowed to

warm to RT and stirred for 12 h. The resulting pearl white suspension

was filtered and the filtrate evaporated to dryness in vacuo leaving

behind the product as a colorless white solid. Yield: 1.83 g (3.04 mmol,

80%). Single crystals were grown by gas-phase diffusion of hexane into a

benzene solution of 7. ¹H NMR (400.1 MHz, C₆D₆): d=7.94, 7.59 (2d,

transformation into [PhBpz

(CH₂SMe)₂]^ꢀ.^[18]Yield of the crude material:

T

approx. 65%; single crystals of 3 formed from an NMR sample (C₆D₆) at

RT. Yield of single crystalline material: 0.08 g (0.19 mmol, 9%). To facili-

³J

N

G

tate its NMR-spectroscopic characterization, 3 was transformed into the

1

2H; BCH₂), 1.40 (s, 6H; SMe), 0.27 ppm (s, 18H; SiMe); ¹³C NMR

(100.6 MHz, C₆D₆): d=140.0 (CSi), 133.4, 132.8 (ArC), 20.4 (SMe),

ꢀ1.0 ppm (SiMe), n.o. (CB, BCH₂); ¹¹B NMR (128.4 MHz, C₆D₆): d=

3.0 ppm (h_1/2=585 Hz); elemental analysis calcd (%) for C₂₂H₃₆B₂Br₂S₂Si₂

[602.25]: C 43.87, H 6.03; found: C 43.62, H 5.96.

pyridine adduct PhB

A

(d, 2H; pyH-o), 7.51 (d, 2H; PhH-o), 7.37 (vtr, 2H; PhH-m), 7.25 (t,

1H; PhH-p), 6.70–6.63 (m, 1H; pyH-p), 6.35 (vtr, 2H; pyH-m), 2.55, 2.44

(2d, ²J

(H,H)=11.9 Hz, 22H; BCH₂), 2.02 ppm (s, 6H; SMe);

R

¹³C NMR (100.6 MHz, C₆D₆): d=146.9 (pyC-o), 139.4 (pyC-p), 133.0

(PhC-o), 127.7 (PhC-m), 126.0 (PhC-p), 124.3 (pyC-m), 33.7 (br, BCH₂),

19.9 ppm (SMe), n.o. (PhC-i); ¹¹B NMR (128.4 MHz, C₆D₆): d=0.0 ppm

(h_1/2=160 Hz).

Synthesis of 8: 7 (2.10 g; 3.49 mmol) was dissolved in C₆H₆(50 mL). Neat

BBr₃(1.75 g; 6.98 mmol) was added through a syringe and the colorless

solution stirred for 18 h, whereupon a colorless precipitate formed which

was collected on a frit. The crude product was washed with C₆H₆(10 mL)

and dried in vacuo. Yield: 1.33 g (1.67 mmol, 48%). ¹H NMR

Synthesis of 4: 2 (0.34 g; 0.74 mmol) was dissolved in C₆H₆(20 mL) and

the solution cooled to 68C. Neat Me₂NSiMe₃(0.25 mL; 0.19 g;

1.62 mmol) was added through a syringe, the resulting colorless suspen-

sion was allowed to warm to RT and stirred for 12 h. All volatiles were

removed in vacuo leaving behind the product as a colorless waxy solid.

(400.1 MHz, [D₈]toluene): d=8.16, 7.74 (2d, ³J

(H,H)=8.2 Hz, 24H;

A

ArH), 2.52, 1.97 (2d, ²J

(H,H)=12.5 Hz, 22H; BCH₂), 1.35 ppm (s,

N

6H; SMe); ¹³C NMR (100.6 MHz, [D₈]toluene): d=137.5, 132.9 (ArC),

34 (very br, BCH₂), 20.3 ppm (SMe), n.o. (2CB); ¹¹B NMR

1

Yield: 0.24 g (1.23 mmol, 83%). H NMR (400.1 MHz, C₆D₆): d=7.52 (d,

2H; PhH-o), 7.28 (vtr, 2H; PhH-m), 7.20 (t, 1H; PhH-p), 2.64, 2.47 (2s,

23H; NMe), 2.22 (s, 2H; BCH₂), 1.88 ppm (s, 3H; SMe); ¹³C NMR

(100.6 MHz, C₆D₆): d=132.0 (PhC-o), 127.9, 127.9 (PhC-m,p), 41.0, 39.7

(NMe), 24.4 (br, BCH₂), 19.1 ppm (SMe), n.o. (PhC-i); ¹¹B NMR

(128.4 MHz, C₆D₆): d=40.6 ppm (h_1/2=150 Hz).

(128.4 MHz, [D₈]toluene): d=2.2 ppm (B(Br)(CH₂SMe)), n.o. (BBr₂).

A

Synthesis of 9: 8 (1.15 g; 1.44 mmol) was suspended in C₆H₆(40 mL).

Neat Me₂NSiMe₃(1.04 g; 8.87 mmol) was added through a syringe and

the resulting pale pink solution stirred for 12 h. All volatiles were re-

moved in vacuo to give the product as a pale yellow oil. Yield: 0.81 g

(2.78 mmol, 97%). ¹H NMR (250.1 MHz, C₆D₆): d=7.68–7.56 (m, 4H;

Synthesis of Li[5]:

4 (0.58 g; 3.02 mmol) was dissolved in toluene

(20 mL). To the stirred solution was added a solid mixture of Hpz

(0.21 g; 3.08 mmol) and Lipz (0.22 g; 2.95 mmol). The resulting suspen-

sion was heated to reflux for 12 h. Evaporation of the pale yellow solu-

tion in vacuo yielded a colorless solid foam. Recrystallization by gas dif-

fusion of pentane into a toluene solution of the crude product yielded

colorless crystals of (Li[5])₂. Yield: 0.79 g (2.71 mmol, 92%). ¹H NMR

(250.1 MHz, [D₈]THF): d=8.18, 7.45 (2d, 22H; pzH-3,5), 6.93–6.84,

6.47–6.37 (2m, 3H, 2H; PhH), 6.14 (vtr, 2H; pzH-4), 2.30 (s, 2H;

BCH₂), 2.06 ppm (s, 3H; SMe); ¹³C NMR (62.9 MHz, [D₈]THF): d=

139.3, 135.8 (pzC-3,5), 132.2, 126.9 (PhC), 125.1 (PhC-p), 102.9 (pzC-4),

33.6 (br, BCH₂), 20.1 ppm (SMe), n.o. (PhC-i); ¹¹B NMR (80.3 MHz,

[D₈]THF): d=ꢀ0.8 (h_1/2=125 Hz); elemental analysis calcd (%) for

C₁₄H₁₆BLiN₄S [290.12]·0.25 C₇H₈[92.14]: C 60.41, H 5.79, N 17.89; found:

C 59.85, H 6.42, N 17.19. Note: The presence of 0.25 equiv of toluene in

ArH), 2.69 (s, 3H; B

3H; B(NMe₂)(CH₂SMe)), 2.29 (s, 2H; BCH₂), 1.90 ppm (s, 3H; SMe);

¹³C NMR (62.9 MHz, C₆D₆): d=133.2, 131.5 (ArC), 41.3 (B

(NMe₂)₂),

41.2, 39.8 (B(NMe₂)(CH₂SMe)), 19.2 ppm (SMe), n.o. (2CB, BCH₂);

¹¹B NMR (128.4 MHz, C₆D₆): d=40.7 (1B,

_1/2=270 Hz, (NMe₂)-

(CH₂SMe)), 33.1 ppm (1B, h_1/2=190 Hz, B(NMe₂)₂).

A

G

R

N

ACHTREUNG

G

Synthesis of Li₂[10]: 9 (0.56 g; 1.92 mmol) was dissolved in toluene

(30 mL). To the stirred solution was added a solid mixture of Hpz

(0.39 g; 5.73 mmol) and Lipz (0.28 g; 3.80 mmol). The resulting suspen-

sion was heated to reflux for 11 h, cooled to RT and stirred for 1 h. The

product was isolated by filtration as a pale cream-colored microcrystal-

line solid. Yield: 0.84 g (1.65 mmol, 86%). ¹H NMR (400.1 MHz,

[D₈]THF): d=8.00 (d, 2H; pzH-3 or 5), 7.44 (d, 3H; pzH’-3 or 5), 7.42

(d, 2H; pzH-3 or 5), 7.00 (d, 3H; pzH’-3 or 5), 6.42–6.30 (m, 4H; ArH),

6.07 (vtr, 2H; pzH-4), 5.99 (vtr, 3H; pzH’-4), 2.26 (s, 2H; BCH₂),

2.03 ppm (s, 3H; SMe); ¹³C NMR (100.6 MHz, [D₈]THF): d=139.7

(pzC’-3 or 5), 139.1 (pzC-3 or 5), 136.2 (pzC’-3 or 5), 135.7 (pzC-3 or 5),

132.3, 131.1 (ArC), 103.0 (pzC’-4), 102.6 (pzC-4), 34 (br, BCH₂),

20.0 ppm (SMe), n.o. (2CB); ¹¹B NMR (128.4 MHz, [D₈]THF): d=4.1

(h_1/2=400 Hz), 1.6 ppm (h_1/2=280 Hz); elemental analysis calcd (%) for

C₂₃H₂₄B₂Li₂N₁₀S [508.07]·0.5 H₂O [18.02]: C 53.42, H 4.87, N 27.08;

found: C 53.32, H 4.98, N 27.32.

1

the bulk material was proven by H NMR spectroscopy.

Synthesis of K[5]: 4 (0.14 g; 0.72 mmol) was dissolved in toluene (6 mL).

To the stirred solution was added

a solid mixture of Hpz (0.05 g;

0.73 mmol) and Kpz (0.08 g; 0.75 mmol). The resulting suspension was

heated to reflux for 3 h, cooled to RT and stirred for 12 h. The product

was isolated by filtration as a colorless microcrystalline solid. Colorless

single crystals of K [5] were obtained by slow diffusion of Et₂O into its

THF solution. Yield: 0.20 g (0.62 mmol, 86%). ¹H NMR (300.0 MHz,

Chem. Eur. J. 2008, 14, 6754 – 6770

ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemeurj.org

6767

M. Wagner, M. C. Holthausen et al.

Synthesis of K₂[10]:

9

(0.81 g; 2.78 mmol) was dissolved in toluene

These data can be obtained free of charge from The Cambridge Crystal-

lographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

(40 mL). To the stirred solution was added a solid mixture of Hpz

(0.60 g; 8.81 mmol) and Kpz (0.59 g; 5.56 mmol). The resulting suspen-

sion was heated to reflux for 24 h. The product was isolated by filtration

as a colorless microcrystalline solid. Colorless single crystals of K₂[10]-

Electrochemical measurements: All measurements were performed by

using an EG&G Princeton Applied Research 263 A potentiostat with a

platinum disc working electrode. Carefully dried (CaH₂) and degassed

A

CH₂Cl₂was used as solvent and [NBu₄][B(C₆F₅)₄] as supporting electro-

A

(2.53 mmol, 91%). ¹H NMR (300.0 MHz, [D₈]THF): d=7.57 (d, 2H;

pzH-3 or 5), 7.38 (d, 3H; pzH’-3 or 5), 7.23 (d, 2H; pzH-3 or 5), 7.16 (d,

3H; pzH’-3 or 5), 7.02–6.89 (m, 4H; ArH), 6.01 (m, 3H; pzH’-4), 5.96

(m, 2H; pzH-4), 2.50 (s, 2H; BCH₂), 1.98 ppm (s, 3H; SMe); ¹³C NMR

(62.9 MHz, [D₈]THF): d=139.5 (pzC’-3 or 5), 138.3 (pzC-3 or 5), 134.8

(pzC’-3 or 5), 133.6 (ArC), 133.0 (pzC-3 or 5; ArC), 103.3 (pzC’-4), 102.7

(pzC-4), 33 (very br, BCH₂), 19.9 ppm (SMe), n.o. (2CB); ¹¹B NMR

(96.3 MHz, [D₈]THF): d=3.8 (h_1/2=470 Hz), 1.5 ppm (h_1/2=420 Hz); ele-

mental analysis calcd (%) for C₂₃H₂₄B₂K₂N₁₀S [572.39]: C 48.26, H 4.23,

N 24.47; found: C 47.62, H 4.16, N 23.81.

lyte (0.05m). All potential values are referenced against the FcH/FcH⁺

couple.

Quantum chemical calculations: Quantum chemical calculations were

carried out employing the program Turbomole 5.9.^[81]The BP86 function-

al^[82,83]was used in combination with the TZVPbasis set of Ahlrichs

et al.^[84]for all atoms except Cu, for which we employed the relativistic

10-electron core potential/basis set combination developed by Dolg

et al.^[85]For improved efficiency we made use of the resolution of identity

(RI) approximation^[86–90]and the TZV-J coulomb fitting basis set was

used (ecp-10-mdf-J for Cu). Subsequent single point energy calculations

were performed using the COSMO continuum model^[91](solvent CH₂Cl₂,

dielectric constant at room temperature e=9.03) to account for solvation

effects.

Synthesis of K[11]: K[5] (0.15 g; 0.47 mmol) and CuCl (0.06 g;

0.61 mmol) were suspended in toluene (25 mL). The reaction mixture

was stirred for 2 d at RT After centrifugation, the pale green supernatant

was evaporated to dryness in vacuo to give a pale green solid. Colorless

single crystals were obtained by slow diffusion of pentane into a C₆H₆so-

lution of K[11]. Yield: 0.08 g (0.06 mmol, 26%). ¹H NMR (250.1 MHz,

[D₈]THF, 508C): d=7.35 (n.r., 4H; pzH-3 or 5), 7.03–6.90 (m, 10H, pzH-

3 or 5; PhH-m,p), 6.71 (n.r., 4H; PhH-o), 6.04 (n.r., 4H; pzH-4), 2.54

(n.r., 4H; BCH₂), 2.11 ppm (s, 6H; SMe); ¹³C NMR (62.9 MHz,

[D₈]THF, 508C): d=140.0 (br, pzC-3 or 5), 135.7 (br, pzC-3 or 5), 133.0

(br, PhC-o), 127.4 (br, PhC-m), 125.9 (br, PhC-p), 103.6 (br, pzC-4), 34.0

(br, BCH₂), 22.8 ppm (SMe); ¹¹B NMR (80.3 MHz, [D₈]THF, 508C): d=

ꢀ0.8 ppm (h_1/2=200 Hz); elemental analysis calcd (%) for

C₂₈H₃₂B₂CuKN₈S₂[669.01]·C₆H₆[78.11]: C 54.66, H 5.13, N 14.99; found:

C 54.50, H 5.13, N 14.91.

Acknowledgements

M.W. is grateful to the Deutsche Forschungsgemeinschaft (DFG) and the

Fonds der Chemischen Industrie (FCI) for financial support. K.R. wishes

to thank the Degussa Stiftung for a Ph.D. grant. Computer time provided

by the CSC Frankfurt and the HLR Darmstadt is acknowledged.

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Chem. Commun. 1996 , 197 5 –1 976.

Synthesis of 12: K[5] (0.35 g; 1.09 mmol) and CuBr₂(0.13 g; 0.58 mmol)

were suspended in toluene (50 mL). The reaction mixture was stirred at

RT for 4 d. After centrifugation, the emerald green supernatant was

evaporated to dryness in vacuo to give a purple microcrystalline solid.

Dark purple single crystals were obtained by slow diffusion of hexane

into a toluene solution of 12. Yield 0.20 g (0.32 mmol, 59%). UV/vis (tol-

uene): l_max

(e)=615 (70), 390 (550), 319 nm

(shoulder;

750 mol^ꢀ1dm³cm^ꢀ1); elemental analysis calcd (%) for C₂₈H₃₂B₂CuN₈S₂

[629.90] :C 53.39, H 5.12, N 17.79; found: C 53.31, H 5.15, N 17.78.

Reaction of K[11] with O₂: Compressed air (dried over P₄O₁₀) was bub-

bled through a colorless solution of K[11] (0.29 g; 0.22 mmol) in C₆H₆

(10 mL) at RT over a period of 8 h. From the green paramagnetic reac-

tion mixture, crystals were grown of the Cu^IIcomplex 12 (major compo-

nent) and the degradation product 13. Few single crystals of 13 were iso-

lated by crystal picking.

[7] J. Reglinski, M. Garner, I. D. Cassidy, P. A. Slavin, M. D. Spicer,

D. R. Armstrong, J. Chem. Soc. Dalton Trans. 1999 , 211 9 –2 126.

[8] P. Ge, B. S. Haggerty, A. L. Rheingold, C. G. Riordan, J. Am. Chem.

Soc. 1994 , 116 , 840 6 –8 407.

[9] C. Ohrenberg, P. Ge, P. Schebler, C. G. Riordan, G. P. A. Yap, A. L.

Rheingold, Inorg. Chem. 1996 , 35 , 74 9 –7 54.

[10] C. Ohrenberg, C. G. Riordan, L. Liable-Sands, A. L. Rheingold,

Coord. Chem. Rev. 1998 , 174 , 30 1 –3 11.

[11] P. J. Schebler, C. G. Riordan, I. A. Guzei, A. L. Rheingold, Inorg.

Chem. 1998 , 37 , 475 4 –4 755.

[12] C. Ohrenberg, L. M. Liable-Sands, A. L. Rheingold, C. G. Riordan,

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[13] M. A. Casado, V. Hack, J. A. Camerano, M. A. Ciriano, C. Tejel,

L. A. Oro, Inorg. Chem. 2005 , 44 , 912 2 –9 124.

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128 , 495 6 –4 957.

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8214.

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5681.

Synthesis of 14: K₂[10] (0.02 g; 0.03 mmol) and CuCl (0.01 g; 0.10 mmol)

were suspended in THF (20 mL). The resulting reaction mixture was

stirred at RT for 7 d. After centrifugation, the pale green supernatant

was evaporated to dryness in vacuo to give a pale green solid. Few color-

less single crystals of 14(thf)₂were obtained by slow evaporation of a

A

THF solution.

Crystal structure analyses: Crystals of 2, (Li[5])₂, K[5], K₂[10]

A

(K[11])₂, 12, 13, and 14(thf)₂were measured by means of a STOE IPDS-

ACHTREUNG

II diffractometer with graphite-monochromated MoK_aradiation. An em-

pirical absorption correction with program PLATON^[78]was performed

for all structures. The structures were solved by direct methods^[79]and re-

fined with full-matrix least-squares on F²using the program

SHELXL97.^[80]Hydrogen atoms were placed on ideal positions and re-

fined with fixed isotropic displacement parameters using a riding model.

For K[5], the absolute structure has been determined: Flack x-parameter

0.2(1). K₂[10]

dinated THF; (K[11])₂and 14

ACHTREUNG

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gold, Chem. Commun. 1999 , 15 9 –1 60.

AHCTREUNG

lents of non-coordinated benzene and THF, respectively. CCDC 675967

(2), 675968 (3), 675969 ((Li[5])₂), 675970 (K[5]), 677107 (7), 675971

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(K₂[10]

A

(14(thf)₂) contain the supplementary crystallographic data for this paper.

ACHTREUNG

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Chem. Eur. J. 2008, 14, 6754 – 6770

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Received: March 31, 2008

Published online: June 20, 2008

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Article Doi

Copper complexes of mono- and ditopic [(methylthio)methyl]borates: Missing links and linked systems en route to copper enzyme models

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