Carenium-Calkyl bond making and breaking: Key process in the platinum-mediated Caryl-Calkyl bond formation. Analogies to organic electrophilic aromatic substitution

DOI: 10.1021/ja003685b

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Journal of the American Chemical Society p. 7233 - 7246 (2001)

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Albrecht

Spek Spek

Van Koten

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Article abstract of DOI:10.1021/ja003685b

The reaction of cationic platinum aqua complexes 2 [Pt(C₆H₂ (CH₂NMe₂}₂-E-4)(OH₂)](X′) (X′ = SO₃CF₃, BF₄) with alkyl halides RX gave various air-stable areni

Full text of DOI:10.1021/ja003685b

J. Am. Chem. Soc. 2001, 123, 7233-7246

7233

C_arenium-C_alkylBond Making and Breaking: Key Process in the

Platinum-Mediated C_aryl-C_alkylBond Formation. Analogies to Organic

Electrophilic Aromatic Substitution

Martin Albrecht,^†Anthony L. Spek,^‡,§and Gerard van Koten^†,*

Contribution from the Debye Institute, Department of Metal-Mediated Synthesis, Utrecht UniVersity,

and BijVoet Center for Biomolecular Research, Crystal and Structural Chemistry, Utrecht UniVersity,

Padualaan 8, 3584 CH Utrecht, The Netherlands

ReceiVed October 16, 2000

Abstract: The reaction of cationic platinum aqua complexes 2 [Pt(C₆H₂{CH₂NMe₂}₂-E-4)(OH₂)](X′) (X′ )

SO₃CF₃, BF₄) with alkyl halides RX gave various air-stable arenium complexes 3-5 containing a new C-C

bond (R ) Me, 3; Et, 4; Bn, 5). Electron-releasing oxo-substituents on the aromatic ligand (E ) e.g., OH, b;

OMe, c) enhance the reactivity of the aqua complex 2 and were essential for arenium formation from alkyl

halides different from MeX. This process is initiated by oxidative addition of alkyl halides to the platinum(II)

center of 2, which affords (alkyl)(aryl) platinum(IV) complexes (e.g., 9, alkyl ) benzyl) as intermediates.

Spectroscopic analyses provided direct evidence for a subsequent reversible 1,2-sigmatropic shift of the alkyl

group along the Pt-C_arylbond, which is identical to repetitive C_arenium-C_alkylbond making and breaking and

concerted metal reduction and oxidation. Temperature-dependent NMR spectroscopy revealed ∆H° ) -1.3

(( 0.1) kJ mol^-1, ∆S° ) +3.8 (( 0.2) J mol^-1K^-1, and ∆G°₂₉₈) -2.4 (( 0.1) kJ mol^-1for the formation

of the arenium complex 5b from 9 involving the migration of a benzyl group. The arenium complexes were

transformed to cyclohexadiene-type addition products 7 or to demetalated alkyl-substituted arenes, 8, thus

completing the platinum-mediated formation of a sp²-sp³C-C bond which is analogous to the aromatic

substitution of a [PtX]⁺unit by an alkyl cation R⁺. The formation of related trimethylsilyl arenium complexes

6 suggests arenium complexes as key intermediates, not only in (metal-mediated) sp²-sp³C-C bond making

and breaking but also in silyl-directed cyclometalation.

Introduction

Scheme 1

The selective cleavage and formation of carbon-carbon

bonds, in particular of sp²-sp³C-C bonds, is of fundamental

importance both to bulk chemical syntheses (oil chemistry,

cracking processes) and to fine chemistry (natural product

synthesis, pharmaceuticals).¹The availability of powerful

catalytic systems for these processes is crucial, in particular

because of the high bond dissociation energy of such bonds

(ca. 400 kJ mol^-1).²Appropriate catalysts are expected to

provide access to selective and efficient bond cleavage and

formation procedures which tolerate a large diversity of

functional groups. Hence, considerable efforts have been devoted

to the metal-mediated catalytic activation of C-C bonds in

solution.³Obviously, an appropriate understanding of the

mechanism of these reactions is essential for the design of better

catalysts.

Formally, this reaction is analogous to Friedel-Crafts alkylation

of arenes, a special case of electrophilic aromatic substitution

(Scheme 1).⁴The generally accepted mechanism includes

trapping of the electrophile by an aromatic system under

formation of a reactive encounter complex (π-complex). This

species subsequently rearranges to a σ-complex (arenium,

Wheland intermediate),⁵which contains an activated C-H bond.

In the final step, this activated proton is abstracted by a (weak)

A rational approach toward the mechanistic elucidation of

the C_aryl-C_alkylbond cleavage processes relies on the examina-

tion of the microscopic reverse, viz. C-C bond formation.

* To whom correspondence should be addressed. Fax: +31-30-2523615.

E-mail: g.vankoten@chem.uu.nl.

(3) (a) Noyori, R.; Kumagai, Y.; Umeda, I.; Takaya, H. J. Am. Chem.

Soc. 1972, 94, 4018. (b) Huffman, M. A.; Liebeskind, L. S. J. Am. Chem.

Soc. 1991, 113, 2771. (c) Perthuisot, C.; Jones, W. D. J. Am. Chem. Soc.

1994, 116, 3647. (d) Edelbach, B. L.; Lachicotte, R. J.; Jones, W. D. J.

Am. Chem. Soc. 1998, 120, 2843. (e) Rybtchinski, B.; Milstein, D. Angew.

Chem. 1999, 111, 918; Angew. Chem. Int. Ed. 1999, 38, 870 and references

therein. (f) Jia, C.; Piao, D.; Oyamada, J.; Lu, W.; Kitamura, T.; Fujiwara,

Y. Science 2000, 287, 1992.

^§To whom correspondence pertaining to crystallographic studies should

be addressed. E-mail: a.l.spek@chem.uu.nl.

^†Debye Institute.

^‡Bijvoet Center.

(1) (a) Crabtree, R. H. Chem. ReV. 1985, 85, 245. (b) ActiVation of

UnreactiVe Bonds and Organic Synthesis; Murai, S., Ed.; Springer: Berlin,

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(2) Benson, S. Thermochemical Kinetics; Wiley: New York, 1976; p

309.

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Marcel Dekker: New York, 1984.

Published on Web 07/06/2001

7234 J. Am. Chem. Soc., Vol. 123, No. 30, 2001

Albrecht et al.

Scheme 2

base, thus affording a C-substituted benzene derivative contain-

ing a new carbon-carbon bond. Alternatively, a nucleophile

can add to the arenium ion, thus giving 1,2- or 1,4-disubstituted

cyclohexadiene addition products (Scheme 1).⁶

stability and reactivity of arenium (σ-complexes) key intermedi-

ates in electrophilic aromatic substitution chemistry.^15,16Cyanide-

mediated removal of the metal in 3a has been demonstrated to

result in the formation of an alkyl-substituted arene¹⁰(i.e., the

product of C_aryl-C_alkylbond formation) thus identifying the

arenium complex (i.e., C_arenium-C_alkylbond formation) as a

potential intermediate of metal-mediated sp²-sp³C-C bond

making and breaking. Moreover, such arenium ions also

represent frozen intermediates of electrophilic metalation reac-

tions of arenes, which involve metal-mediated C_aryl-E bond

activation (E ) H, C, Si).¹⁷An elegant theoretical investigation

suggested that proceeding to metal-stabilized arenium formation,

an (aryl)(alkyl) metal complex with a formally oxidized metal

center would be formed.^10dThe more labile (alkyl) component

subsequently migrates from the metal to the (aryl) ipso-carbon

thus inducing arenium formation. Variation of the alkyl halide

(RX) substrates for reaction with 2a did, however, result in a

cascade of reactions which slowly afford the corresponding

alcohol (ROH) and an unusual zwitterionic platinum(II) dimer

(Scheme 2).¹⁸The proposed (alkyl)(aryl) metal complexes have

been isolated in related rhodium chemistry where the corre-

sponding arenium intermediates obviously are not sufficiently

stable to be detected.¹⁹

These reactivities underline the importance of the arenium

intermediate as precursors for substituted arenes:⁷its control

allows the selective determination of the product formation and

may be directed toward both, C-C bond cleavage (i.e., release

of the alkyl unit) or toward C-C bond formation (i.e., release

of a proton) or toward the addition product. Therefore, the

characterization and isolation of arenium ions attracted particular

interest and resulted in the spectroscopic identification of the

very reactive benzenium ion C₆H₇⁺, the simplest member of

the arenium family.⁸Due to the sensitivity of this benzenium

ion, its isolation has not been successful so far. In contrast,

arenium ions with an enhanced stability, imposed by electron-

releasing substituents such as in C₆Me₇⁺, have been isolated

and fully characterized, including the X-ray structure determi-

nation.⁹

Among the first air-stable arenium ions was the metal-

stabilized arenium 3a, which has been prepared from MeI and

the aromatic precursor 2a (Scheme 2), a cationic platinum

solVato complex containing the N,C,N-terdentate coordinating

pincer ligand [C₆H₃(CH₂NMe₂)₂-2,6]^-(abbreviated as NCN).^10,11

Metal complexes of such “pincer” class ligands, that is,

potentially terdentate chelating aryl-carbanions of the NCN-¹²

or PCP-type,^3e,13are known for their outstanding properties in

stabilizing unusual reaction intermediates and thus may provide

insight into mechanistic details of such reactions.¹⁴

Here, we present a successful approach to circumvent this

restricted reactivity of arylplatinum complexes toward higher

alkyl halide reactants, which identifies such arenium ion

formation as an essential and general process in metal-mediated

C_aryl-C_alkylbond activation. This approach relies on the

introduction of electron-releasing substituents as activators on

the aromatic ring (in this case on the NCN ligand), which is a

classical concept derived from electrophilic aromatic substitution

theory.⁴These modifications allow for arenium formation (and

hence C_aryl-C_alkylbond making) with various alkyl halides,

which points to the existence of strong similarities between the

metal-mediated sp²-sp³C-C bond activation process and

electrophilic aromatic substitution reactions. Moreover, direct

evidence for the intimate steps of this reversible C-C bond-

making and -breaking process has been obtained by using the

cyclometalated NCN-“pincer” ligand. Preliminary results of

Due to its stability toward oxygen (i.e., air) and moderate

heat, complex 3a is an easily accessible model to study the

(5) (a) Olah, G. A. Acc. Chem. Res. 1971, 4, 250. (b) Ridd, J. H. Acc.

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J. N.; Noltes, J. G.; Spek, A. L.; Ubbels, H. J. C. J. Am. Chem. Soc. 1982,

104, 6609. (c) Terheijden, J.; van Koten, G.; Vinke, I. C.; Spek, A. L. J.

Am. Chem. Soc. 1985, 107, 2891. For an early computational study of the

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(d) Ortiz, J. V.; Havlas, Z.; Hoffmann, R. HelV. Chim. Acta 1984, 67, 1.

(11) The term “arenium” is preferred to “arenonium”, as used by us earlier

[see ref 10]. For a discussion of this nomenclature, see: Olah, G. A. J. Am.

Chem. Soc. 1972, 94, 808.

(12) (a) van Koten, G. Pure Appl. Chem. 1989, 61, 1681. (b) Rietveld,

M. P. H.; Grove, D. M.; van Koten, G. New J. Chem. 1997, 21, 751.

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1020. (b) Rimmel, H.; Venanzi, L. M. J. Organomet. Chem. 1983, 259,

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Organometallics 1983, 2, 1447. (d) Karlen, T.; Dani, P.; Grove, D. M.;

Steenwinkel, P.; van Koten, G. Organometallics 1996, 15, 5687.

(14) (a) Gossage, R. A.; Ryabov, A. D.; Spek, A. L.; Stufkens, D. J.;

van Beek, J. A. M.; van Eldik, R.; van Koten, G. J. Am. Chem. Soc. 1999,

121, 2488. (b) Dani, P.; Karlen, T.; Gossage, R. A.; Kooijman, H.; Spek,

A. L.; van Koten, G. J. Am. Chem. Soc. 1997, 119, 11317. (c) van der

Boom, M. E.; Higgitt, C. L.; Milstein, D. Organometallics 1999, 18, 2413.

(d) Albrecht, M.; Dani, P.; Lutz, M.; Spek, A. L.; van Koten, G. J. Am.

Chem. Soc. 2000, 122, 11822. (e) Vigalok, A.; Milstein, D. J. Am. Chem.

Soc. 1997, 119, 7873. (f) Vigalok, A.; Shimon, L. J. W.; Milstein, D. J.

Am. Chem. Soc. 1998, 120, 477.

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1982, 1, 1366.

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(b) Wheland, G. W. J. Am. Chem. Soc. 1942, 64, 900. (c) Farcasiu, D. Acc.

Chem. Res. 1982, 15, 46.

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Soc., Chem. Commun. 1989, 1562.

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117, 9774. (b) Rybtchinski, B.; Vigalok, A.; Ben-David, Y.; Milstein, D.

J. Am. Chem. Soc. 1996, 118, 12406.

C_arenium-C_alkylBond Making and Breaking

J. Am. Chem. Soc., Vol. 123, No. 30, 2001 7235

Scheme 3

these studies have been communicated recently.²⁰Extended

investigations now provide a general understanding for the

mechanism of homogeneous metal-mediated C_aryl-C_alkylbond

making and breaking. Extension of our studies to transition

metal-mediated C_aryl-Si (and C_aryl-H) bond activation showed

that the developed mechanistic concept is of broad validity.

Figure 1. Resonance structures of the π-complex A and the arenium

complexes B-D containing containing an activating substituent (E )

OH, OMe).

indicated by a new CH₃resonance at 2.70 ppm. In addition,

two distinct singlets for the NCH₃protons (at 3.14 and 2.82

ppm, respectively) and two AB doublets for the benzylic

ArCH₂N protons (centered at 4.94 and 3.63 ppm) have been

observed. This remarkably large chemical shift difference of

1.3 ppm together with the presence of vicinal ¹H-¹H couplings

(²J ) 13.4 Hz) implies a reduced symmetry of the molecular

geometry and a fixed conformation of the ligand unit. Obviously,

dynamic puckering processes as established for neutral aryl-

platinum complexes of the type [PtX(NCN-E-4)] are absent.^10b,21

Further indications for a lower symmetry arises from examina-

Results

Reaction of chloroplatinum complexes of the type [PtCl-

(NCN-E-4)] (E ) H, OR), 1a-d, with a silver salt (trifluo-

romethane sulfonate (OTf) or BF₄) in wet acetone results in

quantitative abstraction of the chloride ligand and precipitation

as silver chloride.^10b,21The open coordination site on the metal

is filled by a water molecule thus yielding the cationic

arylplatinum(II) aqua complexes [Pt(NCN-E-4)(OH₂)]⁺with

either OTf or BF₄as anions (Scheme 3).²²These complexes

2a-d were isolated as colorless solids in more than 85% yield.

Coordinated water appears in their ¹H NMR spectra as a broad

singlet at approximately 2.8 ppm. Interestingly, 2b possesses

hydrophilic groups at either end of the molecule, and conse-

quently, this complex represents a water-soluble organoplatinum

species. The hydroxyl group appears at 7.99 ppm and the

coordinated water as a broad peak centered at 2.83 ppm (acetone

tion of the coupling constants due to H-¹⁹⁵Pt interactions. In

3b, the coupling constants of the two chemically inequivalent

benzylic protons vary considerably (29.4 vs 38.6 Hz), which is

presumably induced by different dihedral angles of the Pt-N-

C-H skeleton. Furthermore, the signals due to the aromatic

protons shift from ca. 6.5 ppm (aqua-complexes 2b and 2c)

downfield (to 7.07 and 7.28 ppm in 3b and 3c, respectively)

upon C-C bond formation, suggesting a decreased electron

density of the aryl fragment.

solution). In the H NMR spectra of all the aqua-complexes,

Principally, two different types of structures must be con-

sidered for these complexes: first, π-complexes, which contain

a platinum center that is η¹- or η³-coordinated to the arene unit

(A, Figure 1; a classical η⁶-coordination mode is probably not

accessible due to the strongly chelating amino groups), and

second, σ-complexes (arenium), which comprise a Pt-C σ-bond

(B, Figure 1). Due to the continuum of π- and σ-complexes,²³

either term often represents an approximative description of the

effective structure only. On the basis of various structural

information (vide infra) the herein reported complexes unam-

biguously show a η¹-coordination mode and have been assigned

to be arenium structures rather than π-complexes and are

therefore referred to as arenium ions.²⁴

A more detailed insight into the electronic nature of the

methyl arenium cations 3a-c is gained by ¹³C {¹H} NMR

spectroscopy. In 3a, where para substituents are lacking, the

ipso-carbon is shifted upfield to 102 ppm (from ca. 140 ppm in

1a or 2a), indicating a considerable change of the hybridization

from sp²toward sp³. Furthermore, all of the other aryl-carbons

show resonance signals between 135 and 145 ppm and support

a structure containing an unperturbed aromatic system for this

C₅fragment of the ring, comprising a delocalized positive charge

the signals due to coordinated water or phenolic hydroxyl groups

disappear on addition of D₂O. In addition, ¹H-¹⁹⁵Pt couplings

of the NMe₂and the CH₂N protons of the ligands are observed,

the coupling constants of the latter being substantially larger

than those found for the corresponding neutral complexes (50-

52 Hz in 2 vs 40-46 Hz in 1).

Methyl Arenium Complexes. The aqua-complexes 2a-d

undergo a characteristic color change to dark red when treated

with MeI (λ_maxca. 460 nm in acetone). This color is indicative

for the formation of a cationic arenium complex (Scheme 3).¹⁰

The methyl arenium species 3a-c are obtained quantitatively

and analytically pure by evaporation of all volatiles in vacuo

and subsequent recrystallization of the product from a CH₂Cl₂

solution by slow diffusion of pentane. Despite of their ionic

character, these complexes are highly soluble in acetone and

CH₂Cl₂, to a lesser extent as well in EtOAc and CHCl₃.

However, they do not dissolve in alkane solvents and benzene

or toluene.

The ¹H NMR spectra of these cationic complexes are

characteristic: in 3b, for example, C-C bond formation is

(20) Albrecht, M.; Gossage, R. A.; Spek, A. L.; van Koten, G. J. Am.

Chem. Soc. 1999, 121, 11898.

(21) (a) Terheijden, J.; van Koten, G.; Muller, F.; Grove, D. M.; Vrieze,

K.; Nielsen, E.; Stam, C. H. J. Organomet. Chem. 1986, 315, 401. (b)

Albrecht, M.; Gossage, R. A.; Lutz, M.; Spek, A. L.; van Koten, G. Chem.

Eur. J. 2000, 6, 1431.

(22) Schmu¨lling, M.; Grove, D. M.; van Koten, G.; van Eldik, R.;

Veldman, N.; Spek, A. L. Organometallics 1996, 15, 1384.

(23) (a) Reed, C. A. Acc. Chem. Res. 1998, 31, 325. (b) Hubig, S. M.;

Kochi, J. K. J. Org. Chem. 2000, 65, 6807.

(24) For a more detailed discussion concerning the σ- and π-bond

contribution in the platinum-stabilized structures, see Supporting Informa-

tion; for a general discussion, see ref 23 and Hubig, S. M.; Lindeman, S.

V.; Kochi, J. K. Coord. Chem. ReV. 2000, 200-202, 831.

7236 J. Am. Chem. Soc., Vol. 123, No. 30, 2001

Albrecht et al.

(Figure 1, structure B). This contrasts with the findings from

measurements on 3b and 3c, which both contain an oxo-

substituent on the aryl ring. In these complexes, the resonance

of C_ipsois highfield-shifted (δ_Cbetween 80 and 85 ppm) and

appears in a region typical for (substituted) alkanes rather than

for aryl carbons. This demonstrates a much higher degree of

sp³hybridization of this carbon when compared to C_ipsoin 3a

(cf. B-D in Figure 1). Moreover, the signals due to the para-

and ortho-carbons in 3b are remarkably deshielded (δ_C) 172.1

and 155.7, respectively), whereas the resonances of C_metaappears

at lower frequency (δ_C) 117.4). These chemical shift values

strongly resemble those found in related (but very unstable)

methoxy-substituted benzenium ions.²⁵On the basis of these

data, a structure in solution is surmised which is characterized

by a partial localization of the positive charge at the para and

ortho positions of the arenium system in 3b (Figure 1, structures

C, D). Similar results are obtained from spectroscopic analyses

of the methyl arenium 3c, and hence, both of these oxo-

substituted arenium ions have significant R,â-unsaturated

ketone-type character. Such a model corroborates the chemical

shift values of the oxo-bound aryl-carbons, which appear at a

frequency typical for ketones.

Figure 2. Perspective view (ORTEP, 50% probability) of the methyl

arenium cation 3c. Hydrogen atoms and the OTf^-anion were omitted

for clarity.

Table 1. Selected Bond Length (Å) and Angles (deg) for Arenium

Complexes 3c, 5b,^aand 5c

3c (X ) I)

5b (X ) Br)

5c (X ) Br)

bond lengths

Pt-X

2.6247(8)

2.100(8)

2.4383(5)

2.090(4)

2.100(4)

2.106(4)

1.538(7)

1.445(6)

1.376(7)

1.402(7)

1.396(7)

1.370(7)

1.436(7)

1.327(6)

2.4304(9)

2.079(5)

2.071(6)

2.129(6)

1.535(8)

1.453(8)

1.375(8)

1.386(8)

1.415(8)

1.354(8)

1.453(8)

1.350(7)

Pt-N1

Pt-N2

Pt-C1

C1-C13

C1-C2

C2-C3

C3-C4

C4-C5

C5-C6

C6-C1

C4-O

2.103(8)

2.127(8)

A different reactivity pattern was established for the cationic

1.541(12)

1.443(13)

1.365(13)

1.381(14)

1.399(14)

1.384(12)

1.440(13)

1.339(11)

aqua-complex 2d containing a siloxy substitutent on the pincer

ligand (E ) OSiMe₂Bu) in the presence of MeI. Formation of

an arenium species is strongly indicated by the diagnostic dark

red color of the reaction solution. The isolated product revealed

a signal pattern in the ¹H NMR spectrum which is characteristic

for arenium complexes (inequivalent NMe₂and CH₂N protons,

vide supra). Surprisingly, however, the resonances due to the

bond angles

SiMe₂Bu group were entirely absent. Further analyses (UV-

Pt-C1- C13

C2-C1-C13

C6-C1-C13

C6-C1-C2

C1-Pt-X

103.9(5)

112.2(3)

107.2(4)

120.3(5)

119.7(5)

114.8(5)

174.60(15)

93.40(14)

93.32(17)

87.4(2)

vis, ¹³C {¹H} NMR) confirmed cleavage of the Si-O bond and

formation of 3b (containing an unsubstituted phenol) rather than

the corresponding O-silylated arenium cation. Activation of the

Si-O bond is presumably supported by the constructive overlap

of the leaving group properties of the silyl unit and of keto-

enol tautomerization of both the arenium product and acetone,

which has been used as solvent (Figure 1). This emphasizes

indirectly the significance of a ketone-like arenium in the oxo-

substituted systems rather than a charge-delocalized cation as

found in 3a (vide supra).

120.2(8)

118.3(8)

115.0(7)

169.9(2)

94.7(2)

94.1(2)

86.0(3)

170.7(3)

117.4(4)

119.4(4)

114.8(4)

177.97(14)

92.91(12)

93.68(12)

86.22(16)

87.19(16)

173.41(17)

N1-Pt-X

N2-Pt-X

N1-Pt-C1

N2-Pt-C1

N1-Pt-N2

85.9(2)

173.3(2)

torsion angles

C1-C2-C3-C4

C2-C1-C6-C5

C2-C3-C4-O

Pt-C1-C6-C5

C13-C1-C2-C3

Pt-C1-C2-C3

-2.2(13)

1.6(7)

10.9(6)

1.4(10)

13.2(9)

14.3(12)

Solid-State Structure of [PtI(NCN-Me-1-OMe-4)][OTf] 3c.

The precise molecular geometry of the arenium 3c has been

unequivocally determined by single-crystal X-ray studies. Dark

red plates of 3c were grown by slow diffusion of pentane into

a CHCl₃solution containing 3c. The molecular structure of 3c

(Figure 2) displays characteristics similar to those found for

related arenium species. The platinum center is in a slightly

distorted square-planar geometry with the nitrogen donors in

mutual trans position (N1-Pt-N2 170.7(3)°, Table 1). The

fourth coordination site of the platinum center is occupied by

iodide, originating from the activation of the CH₃-I bond. A

tetrahedral environment around the ipso-carbon is indicated by

the pertinent angles of the four substituents varying between

103.9(5)° and 120.2(8)°. This suggests sp³rather than sp²

hybridization at this carbon. Further evidence for such a model

was taken from the Pt-C1 bond length (2.127(8) Å), which is

substantially longer than in arylplatinum(II) complexes contain-

ing a fully sp²hybridized metal-bound carbon (typically in the

range of 1.90 to 1.96 Å). The C₆ring shows a boatlike

conformation (torsion angle C2-C1-C6-C5 is 14.3(12)°) and

forms an angle of 112.4° with the platinum coordination plane,

-169.0(8)

113.5(8)

-174.2(2)

106.9(4)

136.3(5)

-105.5(4)

-175.7(7)

106.6(6)

138.7(9)

141.6(6)

-111.5(8)

-107.3(6)

^aFrom ref 20.

thus preventing the postulation of any type of agostic interac-

tion²⁶or (η³or exocyclic η²) π-bonding.^14f,23Remarkably, the

bond lengths between C2, C3, C4, and C5 are all in the range

of 1.36-1.40 Å and deviate only slightly form the typical C-C

bond length in unperturbed aromatic systems (1.38 Å). The

bonds to the ipso-carbon are considerably stretched (1.44 Å),

however, which is in good agreement with an arenium structure.

The C-O bond length (C4-O 1.34 Å) relates well with those

found for other similar structures and is too long for a ketone-

type bond (typically around 1.24 Å).^25,27

Arenium Complexes from Activation of RX (R not Me).

Earlier studies related to oxidative addition of alkyl halides to

(26) (a) Brookhart, M.; Green, M. L. H.; Wong, L. L. Prog. Inorg. Chem.

1988, 36, 1. (b) Crabtree, R.; Hamilton, D. G. AdV. Organomet. Chem.

1988, 28, 299. (c) Dani, P.; Karlen, T.; Gossage, R. A.; Smeets, W. J. J.;

Spek, A. L.; van Koten, G. J. Am. Chem. Soc. 1997, 119, 11317. (d) Vigalok,

A.; Uzan, O.; Shimon, L. J. W.; Ben-David, Y.; Martin, J. M. L.; Milstein,

D. J. Am. Chem. Soc. 1998, 120, 12539.

(25) Olah, G. A.; Porter, R. D.; Jeuell, C. L.; White, A. M. J. Am. Chem.

Soc. 1972, 94, 2044.

C_arenium-C_alkylBond Making and Breaking

J. Am. Chem. Soc., Vol. 123, No. 30, 2001 7237

Scheme 4

as a singlet at 4.46 ppm. Similar to the corresponding methyl

arenium complex 3b, the meta-carbons are considerably shielded

(δ_C) 118.4), but not the ortho- and para-carbons (δ_C) 157.5,

_ortho; 172.5, C_para), and C_ipsohas strong sp³character (δ_C

)

79.8). The formation of the chloride analogue of this benzyl

arenium complex, viz. 5a, was much slower and did not proceed

quantitatively. The spectroscopic characteristics are strongly

related to those found for the analogous bromide complex 5b.

Similar reactivities were observed for the methoxy-substituted

arylplatinum complex 2c. In the presence of BnBr, a consecutive

color change to purple and finally to orange indicated a reaction

that is strongly related to the one starting from 2b. Spectroscopic

analysis of the product revealed essentially the same NMR

resonances for 5c and 5b, containing a methoxy- and a hydroxy-

substituent, respectively. The most characteristic difference in

the ¹H NMR spectrum is the presence of a singlet at δ_H) 4.21,

assigned to the MeO-group in 5c. Notably, this resonance is at

significantly lower field than would be expected for methyl aryl

ether signals (e.g, δ_H) 3.73 ppm in 2c), pointing to a strong

electron-deficiency on the aryloxy unit. This is in agreement

with the proposed charge distribution in such platinum-stabilized

arenium systems.

arylplatinum(II) complexes containing NCN-type pincer ligands

have shown that arenium formation takes place solely when

the electrophile is a methyl cation. However, when larger alkyl

halides RX such as ethyl (Et), isopropyl (ⁱPr), or benzyl (Bn)

were used, complex 2a undergoes a cascade of reactions to form

eventually the corresponding alcohol ROH along with an

unprecedented zwitterionic dimetallic Pt(II) dication (Scheme

2).¹⁸In contrast to 2a, the oxo-substituted aqua-complexes 2b-d

possess a sufficiently activated arene system to react with

various alkyl halides.

Any of the cationic aqua-complexes 2 is inert toward the

presence of excess of aryl iodides (I-C₆H₄-R′-4), irrespective

on the nature of the substituent R′. Variation of the electronic

influence of R′ from strongly withdrawing (R′ ) CHO) to

neutral (R′ ) H) or releasing (R′ ) OMe) did not induce any

For example, addition of EtI to 2b in acetone solution gave,

after 24 h, the corresponding ethyl arenium complex 4a, which

was isolated after repeated dissolution in acetone and precipita-

tion with Et₂O in good yields (Scheme 4). The spectral data

are similar to those discussed for the methyl arenium analogue

3b, except that the signal due to the C_ipso-bound methyl group

in the ¹H NMR located at 2.70 ppm was absent, and instead, a

quartet and a triplet (assigned to the ethyl group) appeared at

3.53 and 0.64 ppm, respectively. A similar reaction using excess

of EtBr as alkylating agent gave 4b but proceeded much more

slowly and was not complete even after 15 days of reaction,

while arenium formation of 4a with the corresponding alkyl

iodide was complete within 24 h. Consequently, formation of

EtOH and the bromide analogue of the zwitterionic dimer shown

in Scheme 2 became a strongly competitive process. This was

reaction. Also, addition of bulkier alkyl halides (e.g., PrI or

^tBuI) to 2b or 2c did not cause formation of any stable arenium

product. Instead of the characteristic red colored solution, a dark

brown reaction mixture was obtained. Analyses (¹H NMR

spectroscopy) showed the presence of the ligand moiety which

lacked any platinum satellites thus indicating Pt-N bond fission.

Moreover, two singlets were found in the aryl region at similar

positions as those of the free ligand precursor. This is in

accordance with platinum-carbon bond cleavage and protona-

tion of C_ipso. Hence, a cascade of reactions involving oxidative

addition and reductive elimination sequences is likely to have

occurred, leading to the protonated metal-free ligand NCHN-E

and most probably to a PtI₄-type fragment. These reaction

products are in agreement with the observed dark brown color

obtained during the process, which indicate the presence of

molecular I₂, which is anticipated to origin from the added alkyl

halide.

most obvious in the H NMR spectrum of the crude reaction

mixture, which revealed the dimeric species (doublets at 4.55

ppm and at 3.09 ppm, for the benzylic and the methyl protons,

respectively, of the CH₂NHMe₂substituent¹⁸) as the predomi-

nant product. Increase in reaction temperature did neither

significantly accelerate the reaction nor change the product

distribution.

When the aqua complex 2b was treated with benzyl halides

(BnX; X ) Cl, Br) the corresponding benzyl arenium complexes

5 were obtained (5a, X ) Cl, 5b X ) Br). The reaction with

BnBr proceeded much faster than with BnCl and was complete

within 1 h. Hence, addition of BnBr to a colorless solution of

2 caused a rapid color change of the solution to dark purple

(within several minutes) and then slowly to orange. After 1 h,

UV-vis photospectroscopy indicated the reaction to be com-

plete. The pertinent NMR and UV-vis data strongly resemble

those of related alkyl arenium complexes (3b, 4b) and are

consistent with the formation of a benzyl arenium ion. Most

characteristically, the benzylic protons of the Bn group appear

Using silyl chlorides (e.g., Me₃SiCl) as reagents with the aqua

complexes 2a-d resulted in the quantitative formation of the

neutral arylplatinum complexes 1a-d rather than arenium

formation. This reaction pathway was successfully suppressed,

however, when the neutral complex 1 and a more electrophilic

silyl-cation were used as starting materials. Hence, treatment

of the iodo-analogue of complex 1d, that is [PtI(NCN-OSiMe₂-

Bu)],^21bwith excess Me₃SiOTf in anhydrous CH₂Cl₂afforded

an immediate color change of the reaction solution from color-

less to dark red (λ_max) 465 nm), indicative for arenium systems

comprising a metal-bound iodide.^10cSimilarly, the H NMR

spectrum of this solution showed a signal pattern characteristic

for arenium systems: two AB doublets appeared at 4.25 and

3.40 ppm and also, two inequivalent NMe groups were observed

at 3.04 and 2.72 ppm, respectively. All these results are in good

agreement with the formation of a new Si-C bond as postulated

in the arenium complex 6a (Scheme 5). Unambiguous evidence

for the presence of a trimethylsilyl arenium ion was obtained

from ²⁹Si NMR spectroscopy. The resonance signal at 78.9 ppm

is in excellent agreement with the calculated values for related

(27) (a) Davies, P. J.; Veldman, N.; Grove, D. M.; Spek, A. L.; Lutz, B.

T. G.; van Koten, G. Angew. Chem. 1996, 108, 2078; Angew. Chem., Int.

Ed. Engl. 1996, 35, 1959. (b) Vagedes, D.; Fro¨hlich, R.; Erker, G. Angew.

Chem. 1999, 111, 3561; Angew. Chem., Int. Ed. Engl. 1999, 38, 3362.

7238 J. Am. Chem. Soc., Vol. 123, No. 30, 2001

Albrecht et al.

the square-planar platinum(II) center is ligated by the terdentate

chelating pincer ligand and by bromide, which is located in

mutual trans position to the metal-bound carbon. The arenium

ring is considerably tilted with respect to the metal coordination

plane (105° in both structures, Table 1). The geometry at the

ipso-carbon has significant tetrahedral character with all the bond

angles with vertex C1 close to 109°, suggesting considerable

sp³-type hybridization at C_ipso. The bonds to C6 and C2 are

significantly stretched (1.44-1.45 Å) when compared to the

idealized C-C distance in conjugated aromatic systems (cf. 1.38

Å in benzene), and the Pt-C1 distance is also relatively long

(2.10 Å).^10b,21Intriguingly, the bond lengths of C2-C3 and C5-

C6 are substantially shorter (1.35-1.38 Å) than those of C3-

C4 and C4-C5 (1.39-1.42 Å), indicating partial location of

the double bonds between C2-C3 and C5-C6, respectively.

On the basis of these C-C and the regular C4-O bond

distances, the positive charge in the cationic benzyl arenium

systems can be tentatively located to a large extent on C4. These

solid-state results strongly corroborate an arenium structure as

deduced from NMR spectroscopy of these complexes in solution

(vide supra) and deemphasize the relevance of π-complexes for

these structures (cf. Figure 1). The presence of the phenolic

proton (and therefore the absence of a ketone-type structure) in

5b was confirmed indirectly. In the BF₄anion, one B-F bond

is significantly longer than the others and points to the expected

position of the phenolic hydrogen. Hence, hydrogen-bond-

mediated self-assembly of the anion and the cation of 5b occurs

in the solid state. The large potential of the phenolic hydroxyl

group in hydrogen bonding has some precedents from crystal

engineering with related platinum(II) complexes containing

functionalized pincer ligands.²⁹

Reactivity toward Lewis Bases. Aqueous NaX solutions (X

) Cl, Br, I) were shown previously to induce C-C bond

cleavage in the platinum-stabilized arenium complex 3a with

concomitant re-aromatization, thus forming the neutral aryl-

platinum(II) complex 1a.^10b,cRemarkably, the reactivity of

arenium systems containing an oxo-functionality differs con-

siderably. When the methyl arenium 3b is dissolved in a

minimum amount of THF and then treated with H₂O, an

immediate color change from dark red to yellow is observed,

irrespective of whether NaX salts are present or not. Toluene

extraction gives a single organoplatinum complex 7 as a yellow

solid, which exhibits the characteristic NMR-spectroscopic

properties of 2,5-cyclohexadienones (Scheme 6).¹⁵For example,

the protons attached to the cyclohexadiene system give rise to

a singlet at a field typical for olefinic protons (δ_H) 5.89, toluene

solution). Also, the ¹³C NMR spectrum of 7 is in agreement

with the presence of an R,â-unsaturated carbonyl system

(δ_CdO) 186.3, δ_C) 162.1 and 119.6). Obviously, H₂O is a

sufficiently strong base to abstract the proton, hence implying

a pK_aof the phenolic proton in 3b which is negative (cf. pK_aof

H₃O⁺). Complex 7 is also obtained quantitatively when the

arenium 3b is reacted with an organic base under anhydrous

conditions (NEt₃or NEtⁱPr₂).

Figure 3. Perspective view (ORTEP, 50% probability) of the benzyl

arenium cations 5b (a) and 5c (b). Hydrogen atoms, the anions (5b:

BF₄^-; 5c: OTf^-), and disordered solvent molecules (5c) were omitted

for clarity.

Scheme 5

silyl arenium ions and rules out the presence of a silyl cation

(vide infra).²⁸The proposed complex 6a is highly reactive and

unstable in the solid state. Any attempts to isolate 6a, for

example by precipitation or by crystallization under strictly

anhydrous conditions, yielded the neutral arylplatinum complex

1d (as its iodide analogue) together with some unidentified

brown residue. When the chloride complex 1b was used, an

immediate evolution of a dark yellow color was noted (λ_max

)

435 nm), typical for platinum-stabilized arenium systems

containing a metal-bound chloride ligand.^10cSpectroscopic

measurements supported the formation of the trimethylsilyl

arenium complex 6b. However, also solid 6b decomposed

rapidly and could therefore neither be isolated nor purified.

Solid-State Structures of Benzyl Arenium Complexes 5b

and 5c. The molecular structures of 5b (Figure 3a)²⁰and 5c

(Figure 3b) are similar in many respects. In both complexes,

Surprisingly, an identical product 7 is formed exclusively,

when the methoxy-substituted arenium 3c is subjected to basic

aqueous solutions. This may be explained by nucleophilic attack

of OH^-ions at C4 of the arenium system, followed by cleavage

of the in situ formed hemiketal and elimination of MeOH

(Scheme 6). This hypothesis is strengthened by the results from

experiments performed in ¹⁷O-enriched H₂O. The ¹⁷O NMR

spectrum clearly reveals a broad signal at +450 ppm, which is

(28) (a) Olah, G. A.; Rasul, G. R.; Prakash, G. K. S. J. Organomet. Chem.

1996, 521, 271. (b) Lambert, J. B.; Zhao, Y. Angew. Chem. 1997, 109,

389; Angew. Chem., Int. Ed. Engl. 1997, 36, 400. (c) Lambert, J. B.; Kania,

L.; Zhang, S. Chem. ReV. 1995, 95, 1191.

(29) (a) Albrecht, M.; Lutz, M.; Spek, A. L.; van Koten, G. Nature 2000,

406, 970. (b) Braga, D.; Grepioni, F.; Desiraju, G. R. Chem. ReV. 1998,

98, 1375.

C_arenium-C_alkylBond Making and Breaking

J. Am. Chem. Soc., Vol. 123, No. 30, 2001 7239

Scheme 6

indicative for the presence of ¹⁷O-labeled cyclic ketones.³⁰The

absence of any product raising from nucleophilic attack at C2

provides another (indirect) evidence for arenium systems in the

complexes 3b-c, which display a strong charge localization

particularly on C4.¹⁵

In acidic environment, the ketone 7 was immediately proto-

nated to give quantitatively arenium 3b as was demonstrated

by the typical red color of the product solution and by the

pertinent NMR spectroscopic data. As assumed for such a

ketone-to-enol transformation, the ¹⁷O-labeled oxygen is more

shielded in the enol situation, clearly reflected by a downfield

shift of the ¹⁷O NMR resonance to +300 ppm after addition of

HBF₄or HClO₄to 7. Hence, the transformation of arenium 3b

to the neutral platinum complex 7 is fully reversible.

Irreversible, however, are reactions with CN^-ions as nu-

cleophiles, which lead to quantitative Pt-C bond cleavage and

formation of an organic benzene derivative (Scheme 6).

Platinum abstraction occurs presumably in the form of [Pt(CN)₄]^2-

and corresponds to the last sequence of the aromatic substitution

of a platinum substituent (formally PtX⁺) by a carbocation (Me⁺,

8a; Bn⁺, 8b).

Figure 4. Overlapping absorption spectra (UV-vis spectroscopy, 2

min interval, acetone solution) showing the consecutive color changes

(a) for the formation of the purple intermediate and (b) for the

subsequent direct transformation to the orange arenium complex 5b.

Identification of Intermediates Preceding Arenium For-

mation. The reactions of platinum aqua complexes with RX

have been investigated in more detail by time-dependent

spectroscopy. In particular, the reaction of 2b with a large excess

of BnBr in acetone is illustrative, since two sequential color

changes have been observed (from colorless to purple and then

to orange, vide supra). Monitoring of the arenium formation

by in situ UV-vis spectroscopy provides valuable information

on the course of the reaction. The initial color change from

colorless to purple is reflected by the evolution of a broad

absorption band located at 505 nm (purple solution, Figure

4a).The absorption band at this wavelength has reached its

maximum 14 min after mixing of the reactants and subsequently

starts to decrease. Simultaneously, a new absorption maximum

at 430 nm is generated (orange solution, Figure 4b). The

overlapping absorption spectra of this second color change

clearly show the presence of an isosbestic point at 472 nm.

Analysis of the time-dependent concentration of the purple

intermediate, by its absorption at λ_max, has resulted in a reaction

profile which is characteristic for sequential transformations of

the type A f B f C (Figure 5). The best theoretical

approximation of the measured data points is based on rates

for k_B) 0.215 min^-1and for k_C) 0.0148 min^-1(where k_Bis

Figure 5. Time-dependent absorbance of the reaction solution (acetone)

during the formation of 5b, monitored at λ ) 505 nm, (λ_maxof the

purple intermediate). The solid line represents the calculated time-

dependent concentration of the intermediate during a consecutive

reaction from 2b to 5b (for reaction rates, see Table 2).

the observed rate constant for the formation of the intermediate

and k_Cthe one for the formation of the arenium product).

Qualitatively similar results have been obtained from measure-

ments using BnBr and the methoxy-substituted aqua complex

2c (Table 2).

No colored intermediates have been detected in the reaction

of 2b with MeI, and only the evolution of the dark red color of

the arenium 3b has been observed. The overlapping absorption

spectra reveal a gradual, although not linear increase of the

absorption band at λ_max) 438 nm without displaying any

(30) Kintzinger, J.-P. In Oxygen-17 and Silicon-29 NMR, Basic Principles

and Progress; Diehl, P., Fluck, E., Kosfeld, R., Eds; Springer: Berlin, 1981;

Vol. 17.

7240 J. Am. Chem. Soc., Vol. 123, No. 30, 2001

Albrecht et al.

Table 2. Rate Constants for the Oxidative Addition (Formation of

the (Alkyl)(Aryl) Platinum Species), and the Subsequent

1,2-Sigmatropic Shift (Arenium Formation)

oxidative addition

_obs/min^-1∆G^q₂₉₂/kJ mol^-1k_obs/min^-1∆G^q₂₉₂/kJ mol^-1

sigmatropic shift

starting

compounds

2b + MeI

2b + BnBr

2c + BnBr

0.980

0.215

0.305

10.0

13.6

12.8

0.0225

0.0148

0.0199

19.1

20.1

19.4

Figure 7. Limiting resonance structures for the postulated (alkyl)-

(aryl) platinum complexes 9 and 10, comprising a platinum(II) center

and a cationic alkyl group (E), or an oxidized platinum(IV) center

including a formally anionic alkyl ligand (F).

the Bn group in 9, the notation of a platinum(IV) complex (i.e.,

F) is favored.³⁴Moreover, the π-electron delocalization in the

benzyl group is increased upon binding to platinum, which is

in agreement with all spectroscopic properties of the observed

intermediate (cf. the strong purple color of 9). However, the

postulated intermediates containing metal-bound ethyl or methyl

groups such as 10 are expected to be colorless, as there is

no π-electron delocalization along the alkyl fragment. Indeed

these compounds are inactive in the diagnostic UV-vis region,

and no intermediate was observed spectroscopically. Attempts

to detect these intermediates by NMR spectroscopy also

failed, probably because of a low effective concentration of 10

with respect to reactant and product (cf. Table 2), paired with

a presumably large overlap of the characteristic resonance

signals of the proposed intermediates and the formed arenium

products.

Prolonged reaction time caused a gradual decrease of the set

of signals assigned to 9 and concomitant formation of a new

set of resonances, identical to the one of the final product 5b

(vide supra). The resonances of 9 do not disappear completely,

however, because an equilibrium situation between complexes

9 and 5b is reached (cf. isosbestic point in UV-vis spectra).

The population of 5b is strongly temperature-dependent and

increases upon cooling. The benzylic protons of 5b and 9

are diagnostic and were used to determine the equilibrium

constant K (eq 1) at different temperatures. Following equations

(1) and (2),

Figure 6. Time-dependent absorbance of the reaction solution (acetone)

during the formation of 3b, monitored at λ ) 438 nm, (λ_maxof the

arenium complex 3b). The solid line represents the calculated time-

dependent concentration of 3b formed by a consecutive reaction from

2b via an intermediate (for reaction rates, see Table 2).

isosbestic point. The interpretation of the observed changes in

absorbance (Figure 6) by means of sequential reaction kinetics

affords an optimal correlation. This implies the postulation of

a (not detected) intermediate, which has probably a structure

similar to the one observed in the reaction of 2b with BnBr

(vide supra). Detailed analysis of the kinetic data reveals that

the first step is dependent on both the alkyl group and the

activating substituent on the aromatic system. The subsequent

reaction of the intermediate to the arenium product is an order

of magnitude slower and seems to be dominated by the nature

of the alkyl group, since the activation energy ∆G^qfor this

process is smaller for benzyl groups than for methyl groups

(Table 2).

Structural information on the purple intermediate has been

obtained by in situ NMR spectroscopy of the reaction mixture.

After addition of BnBr to a solution of 2b (acetone-d₆), a

significant broadening of the signal due to coordinated water is

noted (δ_H) 2.85; ∆w_1/2) 25 Hz). After a few minutes, a set

of signals appears which corresponds to the purple intermediate

and which is characterized by diastereotopic NMe₂(δ_H) 3.12

and 2.83, respectively) and well-resolved CH₂N groups (AB

K ) [5b]/[9]

(1)

(2)

ln(K) ) -∆G°/RT ) ∆S°/R - ∆H°/RT

the thermodynamic parameters were calculated as ∆H° ) -1.3

(( 0.1) kJ mol^-1, ∆S° ) +3.8 (( 0.2) J mol^-1K^-1and ∆G°₂₉₈

doublet resonances at δ_H) 5.06 and 3.58, J_HH) 13.2 Hz)

(32) The platinum center in complexes of the general type [PtX(NCN)]

(X ) halide) is a Lewis base, see, e.g.: (a) Reference 21b. (b) Muijsers, J.

C.; Niemantsverdriet, J. W.; Wehman-Ooyevaar, I. C. M.; Grove, D. M.;

van Koten, G. Inorg. Chem. 1992, 31, 2655.

resulting from a reduced symmetry in the intermediate complex.

The signal due to the benzylic protons of the Bn group appears

as a singlet at 4.61 ppm (²J_PtH) 24 Hz). A structure which is

in full agreement with the observed spectroscopic data (NMR,

UV-vis) comprises an (aryl)(alkyl) platinum complex contain-

ing the pincer ligand with an aryl unit that displays still aromatic

character and a metal-bound alkyl group (benzyl in 9, methyl

in 10; Figure 7).³¹

(33) The relevance of similar resonance structures was also addressed

in earlier studies on Pt(IV)⁺-H/Pt(II)-H⁺(hydride vs proton) species, which

showed that the actual structure is strongly determined by the nature of the

extra (sixth) ligand trans to the Pt-H bond: (a) Wehman-Ooyevaar, I. C.

M.; Grove, D. M.; Kooijman, H.; van der Sluis, P.; Spek, A. L.; van Koten,

G. J. Am. Chem. Soc. 1992, 114, 9916. (b) Wehman-Ooyevaar, I. C. M.;

Grove, D. M.; de Vaal, P.; Dedieu, A.; van Koten, G. Inorg. Chem. 1992,

31, 5484. Note the related discussion concerning the catalytic cycle in Pd-

mediated Heck chemistry, especially with respect to the existence of Pd-

(IV) intermediates, which may or may not be involved, see, e.g.: (c)

Mitchell, T. N. In Metal-Catalyzed Cross-coupling Reactions; Diederich,

F., Stang, P. J., Eds.; Wiley-VCH: Weinheim, 1998; p 167. (d) Reetz, M.

T.; Westermann, E. Angew. Chem. 2000, 112, 170; Angew. Chem., Int. Ed.

2000, 39, 165. (e) Shaw, B. L. New J. Chem. 1998, 77. (f) Ohff, M.; Ohff,

A.; van der Boom, M. E.; Milstein, D. J. Am. Chem. Soc. 1997, 119, 11687.

(g) Amatore, C.; Jutland, A. Acc. Chem. Res. 2000, 33, 314. (h) Beletskaya,

I. P.; Cheprakov, A. V. Chem. ReV. 2000, 100, 3009.

Two limiting resonance structures can be envisaged for these

intermediates, comprising (i) a cationic alkyl group R (a Lewis

acid), which is bonded to the nucleophilic and hence Lewis basic

platinum(II) center (structure E, Figure 7),³²or (ii) a carbanion

bound to an oxidized cationic platinum(IV) site as the product

of an oxidative addition (structure F).³³On the basis of the

relatively small coupling constants of the benzylic protons of

(31) Vigalok, A.; Rybtchinski, B.; Shimon, L. J. W.; Ben-David, Y.;

Milstein, D. Organometallics 1999, 18, 895.

(34) Terheijden, J.; van Koten, G.; de Booys, J. L.; Ubbels, H. J. C.;

Stam, C. H. Organometallics 1983, 2, 1882.

C_arenium-C_alkylBond Making and Breaking

J. Am. Chem. Soc., Vol. 123, No. 30, 2001 7241

Scheme 7

Scheme 8

) -2.4 (( 0.1) kJ mol^-1. Note the small entropy factor, which

is typical for intramolecular processes. Remarkably, no indica-

tion was found for a similar equilibrium during the migration

of the methyl group from 10 to form 3b, presumably owing to

a higher stability of the arenium product relative to the (alkyl)-

(aryl) platinum intermediate, that is ∆G° is much larger than

for the products from a reversible 1,2-shift of the benzyl group

from 5b to 9 along the Pt-C bond.

Discussion

So far, accessibility of metal-stabilized arenium complexes

and hence the formation of a new C-C bond in Pt(NCN) and

related complexes has been restricted to methyl arenium ions.

Only recently, stable ethyl and benzyl analogues have been

reported.^20,31The introduction of electron-releasing groups as

activators on the aromatic ring broadens the scope of this

reaction significantly and provides access to various arenium

systems containing, for example, ethyl, benzyl, or silyl groups.

This identifies the arenium formation as a general process in

metal-catalyzed C_aryl-C_alkylbond making and breaking, similar

to Friedel-Crafts alkylations.³⁵Full control on such species is

therefore highly desirable and requires (i) accessibility and

stabilization (e.g., isolation) of arenium intermediates and (ii)

the ability to tune their reactivity, thus determining the further

reaction profile and hence product selectivity (Scheme 1).

Controlling the Arenium Formation. Activation of the

Alkyl halide (RX). The fact that the platinum complex 2a is

prone to form selectively arenium ions with MeX suggested an

S_N2-type reaction at the halide-bound carbon. For these reac-

tions, a high sensitivity of the reaction rate to steric bulk at the

carbon atom is expected, and this is confirmed by the reactivity

of the activated platinum complexes 2b-d toward various RX

(MeX > BnX > EtX). No arenium product at all has been

observed when aryl halides PhX or secondary or tertiary alkyl

halides such as ⁱPrX or ^tBuX have been used. These latter results

are expected in S_N2-type nucleophilic substitutions, since highly

branched alkyl halides are known to follow almost exclusively

ionic pathways via prior heterolytic C-X bond cleavage (S_N1-

type reactions).³⁶Aryl halides, in contrast, generally do not

follow nucleophilic substitution pathways, irrespective of whether

they contain electron-withdrawing or -releasing substituents.

Moreover, in all the experiments leading to arenium formation,

the halide X behaves as a classical leaving group (cf. the faster

rate of the reaction of 2b with BnBr compared to that of the

reaction with BnCl), thus supporting a nucleophilic substitution

reaction as initial step of arenium formation (Scheme 7). Owing

to the orbital situation on the metal center in 2 (d_zempty, d_xz

filled), the nucleophilic attack of the platinum center most

presumably occurs “side-on” (Scheme 7) and not in a classical

conformation with the leaving group and the nucleophile in the

two apical positions of a trigonal bipyramidal carbon center.^10d

This yields an (alkyl)(aryl) platinum complex (e.g. 9, 10 in

Scheme 7) as an intermediate, which may or may not contain

a coordinated solvent molecule (H₂O).

Reversible 1,2-Shift of the Organic Ligand R along the

Pt-C Bond. Arenium formation is completed by a second

reaction sequence, that is, a 1,2-sigmatropic shift of the metal-

bound alkyl group to the aromatic carbon (Scheme 7). This is

equal to the intimate step of reversible C-C bond making and

breaking. The isosbestic point observed in the overlapping UV-

vis spectra of this reaction indicates an equilibrium situation

and hence a reversible process. In addition, the isosbestic point

excludes any intermediate during this reaction (on the UV-vis

time scale), which identifies this sigmatropic shift as a direct

transformation of a C-C bond cleaved species to a C-C bond

formed product and vice versa. Formally, the migrating species

is a carbocation (a metal-generated electrophile), which reacts

in a highly regioselective electrophilic substitution reaction with

the aromatic system of the pincer ligand (Scheme 8). The

cationic nature of the migrating group is essential to ensure

conservation of orbital symmetry³⁷and was explained by

(35) Taylor, R. Electrophilic Aromatic Substitution; Wiley: New York,

1990.

hybridization of the metal-centered d_z(empty in a d⁶Pt(IV)

(36) The observed products resulting from Pt-C bond cleavage may be

a consequence of the oxidizing environment due to released I^-. A cascade

of oxidative addition and reductive elimination cycles may then afford

inorganic salts such as [PtI₄]^2-along with noncoordinating pincer ligand.

complex) with a filled p_zorbital (hybridization not shown in

(37) Woodward, R. B.; Hoffmann, R. The ConserVation of Orbital

Symmetry; VCH: Weinheim, 1970.

7242 J. Am. Chem. Soc., Vol. 123, No. 30, 2001

Albrecht et al.

Figure 8. Energy profile of the reaction progress coordinate of the reaction of the platinum aqua complex 2 with an alkyl halide RX to form either

a new C-C bond or the corresponding alcohol ROH (energy values belong to the reaction of 2b with BnBr and are taken from Table 2 (∆G^q) and

from temperature-dependent NMR experiments (∆G°), vide supra). Notably, for the reaction of 2a (lacking aromatic oxo-substituents) with RX (R

* Me), the activation energy for the 1,2-sigmatropic shift is much higher than 19 kJ mol^-1and consequently, the reaction trajectory toward the

formation of ROH is a slow but favored process.

Scheme 8) during the sigmatropic shift process.^10dTilting of

the phenyl ring with respect to the Pt-C_ipsobond presumably

occurs synchronously with the 1,2-sigmatropic migration of the

alkyl group, similar to the established process for, for example,

the pinacol rearrangement. Owing to the resonance-stabilization

in benzyl groups, the transition state of the sigmatropic shift

(Scheme 8) is expected to be more easily accessible with Bn⁺

than with Me⁺ions, which is also reflected in the corresponding

rates of this migration process (Table 2).

state, d⁶vs d,⁸PCP vs NCN ligand skeleton) directs whether

agostic structure and arenium complexes are intermediates or

transition states in metal-mediated C-C bond making and

breaking.

Activated Aromatic Systems. According to the reactivity

of the platinum solVato complexes 2a-d, the presence of

electron-releasing oxo-substituents on the aromatic ring of the

pincer ligand affects the formation of alkyl arenium ions

significantly. Such substituents are known to activate the

aromatic system and also to stabilize the positive charge in the

arenium product. These influences are particularly well-

documented in electrophilic aromatic substitution theory.⁴

This analogy to electrophilic aromatic substitution provides

also a rationale for the earlier observed formation of ROH and

a zwitterionic platinum dimer, when 2a is exposed to alkyl

halides RX (where R * Me, vide supra). Notably, this process

is very slow and therefore was attributed to a secondary reaction

pathway which becomes obviously competitive when the

primary, much faster process (arenium formation) is disfavored.

This may be caused either by a too large activation energy for

the formation of the (alkyl)(aryl) platinum complex or for the

arenium product (Figure 8). In particular, the 1,2-sigmatrophic

shift leading to arenium complexes is expected to be very

sensitively tuned by the electronic properties of the aryl unit.

In the absence of activating oxo-substituents (cf. 2a), the energy

barrier for the 1,2-sigmatropic migration is probably too high,

and instead, the backward reaction becomes important, that is,

reductive elimination of RX from the (alkyl)(aryl) platinum

complex and irreversible formation of ROH. Formation of ROH

is most likely a metal-mediated process comprising solvent

coordination on the (alkyl)(aryl) platinum(IV) species to give

an octahedral d⁶complex. Reductive elimination is accompanied

by the release of one equivalent of H⁺, which slowly protonates

the nitrogen donor sites of the pincer ligand, thus ultimately

leading to the zwitterionic dimeric complex as depicted in

Scheme 2.

These results provide for the first time direct evidence for a

reVersible migration of a benzyl cation along a metal-carbon

bond: the spectroscopic observations visualize the reaction

trajectory for a reductive elimination process (i.e., C_arenium

C_alkylbond formation) taking place at a platinum center, and in

particular, also demonstrate the microscopic reverse, that is that

for a metal-mediated C-C bond activation and cleavage process.

In either reaction direction, the formation of the arenium ion is

crucial. Notably, these findings seem to contrast with P,C,P-

terdentate coordinating PCP-pincer complexes with Rh(I), which

were shown to mediate both C_aryl-C_alkyland C_alkyl-H bond

cleavage.³⁸With these systems, arenium complexes such as 3-5

are less easily accessible than with platinum. Theoretical

considerations predict that, irrespective on the substituents on

the phosphine donors, a π-complex is an initial intermediate of

the C-C bond activation process. Ab initio calculations have

emphasized the relevance of agostic C_aryl-C_alkylinteractions,

and such structures have indeed been isolated and proposed as

arrested intermediates. Although similar agostic bonding has

not been observed in Pt(NCN)-type complexes thus far, it is

noteworthy that the transition state of the 1,2-sigmatropic

shift possesses a strongly related geometry (Scheme 8, Figure

8). Obviously, the different electronic configuration of the

rhodium and platinum center in these complexes (oxidation

(38) (a) Sundermann, A.; Uzan, O.; Milstein, D.; Martin, J. M. J. J. Am.

Chem. Soc. 2000, 122, 7095. (b) Rybtchinski, B.; Vigalok, A.; Ben-David,

Y.; Milstein, D. J. Am. Chem. Soc. 1996, 118, 12406.

C_arenium-C_alkylBond Making and Breaking

J. Am. Chem. Soc., Vol. 123, No. 30, 2001 7243

Controlling the Reactivity of Arenium Ions. Trapping the

Arenium Intermediate by Nucleophiles: Addition Reaction.

For the oxo-substituted arenium species 3b, 3c, 4, and 5, an

R,â-unsaturated ketone-type structure has been deduced (vide

supra). Therefore, the reactivity of these complexes was probed

with various nucleophiles to form the corresponding neutral

cyclohexadiene-type addition products. In contrast to the

established reactivity of 3a (lacking an oxo-substituent on the

arenium moiety), all of the used nucleophiles undergo selective

1,4-disubstitution, leading to cyclohexa-2,5-diene products

exclusively (Scheme 6). No 1,2-disubstituted cyclohexa-2,4-

diene products (Scheme 1) were detected.¹⁵The hydroxyl-

substituted arenium ions undergo a proton abstraction reaction

rather than a nucleophilic substitution, mediated by organic

(NEt₃) or weak inorganic bases (H₂O), thus forming a cyclo-

hexa-2,5-dien-4-one product. Interestingly, nucleophilic attack

by H₂O may occur either at the phenolic proton of 3b, or on

the oxygen-bound carbon, thus initially producing a geminal

diol, which subsequently dehydrates spontaneously (cf. Scheme

6).

Scheme 9

for this palladation reaction which is the microscopic reverse

of the formation of the silyl arenium 6a and related alkyl

arenium ions and hence involves as a key step the electrophilic

aromatic substitution of the silyl substituent Me₃Si⁺by a

palladium unit PdL⁺.⁴¹Similarly, metal-mediated C_aryl-H bond

activation is assumed to follow a similar reaction trajectory as

outlined in Figure 8 (with R ) H).

Completion of C-C Bond Formation: Formal Substitu-

tion of [PtX]⁺by [R]⁺. The substitution reaction was completed

by addition of CN^-ions, which induced irreversible Pt-C bond

cleavage in the arenium ions 3-5 (Scheme 7). The platinum

fragment was probably released as an inorganic salt, for

example, [Pt(CN)₄]^2-, with concomitant re-aromatization of the

aryl system. The full microscopic reverse of this C-C bond

formation process, that is the cleavage of a tolyl C_aryl-C_alkyl

bond in 8 to from 2 is not accessible so far in NCN pincer

chemistry, but ample evidence for this reaction has been given

by using corresponding PCP pincer-type ligands instead of

NCN.^3e,19,31This difference in reactivity presumably originates

from the different M-L bond strength of metals to amines and

phosphines, which is stronger for the latter (L ) PR₃) compared

to amines (L ) NR₃). Therefore, replacement of a ligand in the

metal precursor, which is in this case essential for subsequent

C-C bond activation and cleavage, is generally achieved by

phosphine ligands but not by amines.^10a,14d

Silylated Arenium Complexes: Intermediates in Trans-

metalation Reactions. The color and especially the chemical

shift values from ²⁹Si NMR of the complex 6a provide good

evidence for the presence of a silylated arenium complex.

Alternative structures comprising either silicenium ions, Me₃-

Si⁺, or siloxonium ions, (R₃Si)₂OR⁺, may be excluded.^23a,28,39

This is particularly relevant in the context of recent (trans)pal-

ladation studies using silylated NCN pincer ligands.

Selective C_aryl-Si bond cleavage is a key step in this protocol

and has been tentatively proposed to be induced by an

electrophilic attack of the palladium precursor, thus affording

a (unstable) silylated arenium intermediate (Scheme 9).⁴⁰

Unfortunately, the high reaction rate of these sequences

(presumably second-order in nucleophile) and also of the

subsequent re-aromatization step (comprising the loss of Me₃-

Si⁺) precludes experimental verification of this mechanistic

proposal. The platinum silyl arenium 6a represents a frozen

intermediate of this process and identifies the transpalladation

sequence as an electrophilic aromatic substitution of the Me₃-

Si⁺fragment by a PdL⁺moiety. This points to a mechanism

Conclusions

Platinum-mediated sp²-sp³C-C bond formation between

an aryl carbon of the NCN pincer ligand alkyl halides RX have

been established for various alkyl groups (R ) methyl, ethyl,

benzyl). Essential for the occurrence of these processes is the

presence of activating substituents on the aromatic system of

the platinum-bound NCN pincer ligand such as electron-

releasing OH or OR groups. Application of this principle

allowed furthermore for the preparation and identification of

silyl arenium ions without using a superacidic environment.

Apart from the activating effect, the aromatic oxo-substituents

also stabilize important intermediates. This provided direct

evidence for the intimate steps of this carbon-carbon bond

formation process, which involves first oxidative addition of

RX to the platinum center and formation of an (alkyl)(aryl) metal

complex. A reversible 1,2-sigmatropic shift of the alkyl group

along the platinum-carbon bond subsequently generates the

arenium product. Strong similarities between the reaction

trajectory of metal-mediated C-C bond making and breaking

and the electrophilic aromatic substitution reaction (Friedel-

Crafts alkylation) have been established, including: (i) the

electrophilic mode of action of the metal center in activating

the alkyl halide; (ii) the increase of reactivity of the arene unit

upon substitution with electron-releasing substituents (such as

OR); (iii) electrophilic attack of the alkyl cation on the arene

system with a reactivity sequence Et < Bn < Me. In both

processes, control on the stability and reactivity of the arenium

intermediate is crucial for determining the outcome of the

reaction, that is, C-C bond formation, cleavage or nucleophilic

addition (cyclohexadiene formation). Consideration of these

fundamental principles is expected to provide access to an

improved understanding of the essential parameters for the

design of efficient catalysts for C_aryl-C_alkylbond activation or

cleavage.

Experimental Section

General. Syntheses involving air-sensitive reagents (organolithium

species, Me₃SiOTf) were carried out under a dry nitrogen atmosphere

using Schlenk techniques and freshly distilled solvents (CH₂Cl₂from

(39) (a) Olah, G. A.; Rasul, G.; Prakash, G. K. S. J. Am. Chem. Soc.

1999, 121, 9615. (b) Lambert, J. B.; Zhang, S.; Stern, C. L.; Huffman, J.

C. Science 1993, 260, 1917.

(40) (a) Valk, J.-M.; Boersma, J.; van Koten, G. J. Organomet. Chem.

1994, 483, 213. (b) Steenwinkel, P.; Gossage, R. A.; van Koten, G. Chem.

Eur. J. 1998, 5, 759. (c) Steenwinkel, P.; Gossage, R. A.; Maunula, T.;

Grove, D. M.; van Koten, G. Chem. Eur. J. 1998, 5, 763.

(41) For a similar arenium intermediate as a snapshot during a trans-

metalation reaction, see: Abbenhuis, H. C. L.; Feiken, N.; Haarman, H.

F.; Grove, D. M.; Horn, E.; Kooijman, H.; Spek, A. L.; van Koten, G.

Angew. Chem. 1991, 103, 1046; Angew. Chem., Int. Ed. Engl. 1991, 30,

996.

7244 J. Am. Chem. Soc., Vol. 123, No. 30, 2001

Albrecht et al.

CaH₂, pentane from Na/benzoketyl). All NMR spectra (δ in ppm; J in

(s, 4H, J_PtH) 49.6 Hz, CH₂N), 3.73 (s, 3H, OCH₃), 3.02 (s, 12H,

³J_PtH) 36.6 Hz, NCH₃), 2.8 (br s, 2H, H₂O); Anal. Calcd for C₁₃H₂₃-

BF₄N₂O₂Pt (521.23): C, 29.96; H, 4.45; N, 5.37. Found: C, 30.01; H,

4.46; N, 5.19.

Hz; H at 300 MHz, ¹³C {H} at 75 MHz, ¹⁷O at 54.2 MHz, and ²⁹Si

{H} NMR at 59.6 MHz) were recorded at room temperature, unless

stated otherwise, and referenced to SiMe₄(¹H, ¹³C, and ²⁹Si) or residual

H₂O (δ ) 0.0 ppm; ¹⁷O). All assignments in H and ¹³C NMR are

[Pt(C₆H₂{CH₂NMe₂}₂-2,6-OSiMe₂^tBu-4)(OH₂)][CF₃SO₃] (2d). The

procedure was analogous to the one described for 2b, starting from 1d

(1.17 g, 2.1 mmol) and AgCF₃SO₃(0.55 g, 2.1 mmol) in acetone/H₂O

(10 mL, 10:1) Filtration off the precipitate formed after 30 min (Celite)

and concentration of the filtrate to ca. 1 mL followed by layering with

based either on distortionless enhancement of polarization transfer

(DEPT) experiments or on heteronuclear shift correlation spectroscopy.

UV-vis photospectroscopy was carried out on a Cary1 spectropho-

tometer at room temperature. Elemental analyses were performed by

Kolbe, Mikroanalytisches Laboratorium, Mu¨hlheim/Ruhr. The platinum

complexes 1a,^10b1b,^27a1d,^21b2a,^10b3a,^10band the ligand precursor NC-

(Br)N-OMe⁴²were prepared according to literature procedures.

[PtCl(C₆H₂{CH₂NMe₂}₂-2,6-OMe-4)] (1c). To a cooled (-78 °C)

solution of C₆H₃-Br-1-{CH₂NMe₂}₂-2,6-OMe-4 (1.21 g, 4.0 mmol) in

hexane (15 mL) was added a solution of ^tBuLi (5.3 mL, 7.9 mmol, 1.5

M in hexane). The reaction solution was allowed to warm to room

temperature within 12 h. After removal of all volatiles in vacuo, the

yellowish residue was redissolved in Et₂O (20 mL). To this solution

was added solid [PtCl₂(SEt₂)₂] (1.78 g, 4.0 mmol), and stirring was

continued for 12 h. Solid LiBr (0.87 g, 10 mmol) was added and the

mixture stirred another 12 h. All volatiles were evaporated in vacuo

and the residue suspended in pentane. The precipitated was collected

by centrifugation and washed with portions of pentane (2 × 30 mL).

Consecutive extraction of the solid with CH₂Cl₂afforded 1.75 g (3.5

mmol) of the bromide analogue of 1c, which has been redissolved in

wet acetone (10 mL, acetone/H₂O 10:1). After treatment with AgOTf

(0.91 g, 3.5 mmol) in the absence of light for 30 min and subsequent

filtration through Celite, a colorless solution was obtained. A solution

of NaCl (0.88 g, 15 mmol in 6 mL H₂O) was added and the suspension

stirred vigorously for 2 h. The precipitate was collected and recrystal-

lized from CH₂Cl₂and pentane to yield analytically pure 1c (1.52 g,

Et₂O gave colorless crystals of 2d (Yield: 1.01 g, 87%). H NMR

(acetone-d₆): δ ) 6.47 (s, 2H, C₆H₂), 4.16 (s, 4H, J_PtH) 50.4 Hz,

CH₂N), 3.01 (s, 12H, J_PtH) 37.5 Hz, NCH₃), 2.82 (br s, 2H, OH₂),

0.98 (s, 9H, CCH₃), 0.18 (s, 6H, SiCH₃); ¹³C {¹H} NMR (acetone-

d₆): δ ) 154.8 (C_para), 145.3 (C_ortho), 112.6 (³J_PtC) 43.5 Hz, C_meta),

76.5 (²J_PtC) 72.0 Hz, CH₂N), 54.1 (²J_PtC) 17.0 Hz, NCH₃), 26.0

(CCH₃), 18.3 (SiCCH₃), -4.3 (SiCH₃), C_ipsonot observed; Anal. Calcd

for C₁₉H₃₅F₃N₂O₅PtSSi (683.74): C, 33.38; H, 5.16; N, 4.10. Found:

C, 33.37; H, 5.10; N, 4.13.

[PtI(C₆H₂{Me}-1-{CH₂NMe₂}₂-2,6-OH-4)][CF₃SO₃] (3b). To a

solution of 2b (98 mg, 0.17 mmol) in acetone (3 mL) was added excess

MeI (1 mL, 16 mmol). Stirring was continued for 24 h during which

time the initially colorless solution slowly turned dark red. Evaporation

of all volatiles in vacuo afforded analytically pure 3b as a red solid

(119 mg, 100%). ¹H NMR (acetone-d₆): δ ) 7.07 (s, 2H, C₆H₂), 4.94

(d, 2H, ²J_HH) 13.4 Hz, ³J_PtH) 29.4 Hz, CH₂N, lowfield part of ABq),

3.63 (d, 2H, ²J_HH) 13.7 Hz, ³J_PtH) 38.6 Hz, CH₂N, highfield part of

ABq), 3.14 (s, 6H, J_PtH) 36.1 Hz, NCH₃Me), 2.82 (s, 6H, J_PtH

)

31.3 Hz, NMeCH₃), 2.70 (s, 3H, CCH₃); ¹³C {¹H} NMR (acetone-d₆):

δ ) 172.1 (C_para), 155.7 (²J_PtC) 27.8 Hz, C_ortho), 117.4 (³J_PtC) 33.2

Hz, C_meta), 80.4 (¹J_PtCnot resolved, C_ipso), 68.9 (²J_PtC) 47.2 Hz, CH₂N),

58.0 (²J_PtCnot resolved, NCH₃Me), 55.1 (²J_PtC) 19.1 Hz, NMeCH₃),

19.2 (²J_PtCnot resolved, CCH₃); ¹⁷O NMR (THF): δ ) 300; Anal.

Calcd for C₁₄H₂₂F₃IN₂O₄PtS (693.39): C, 24.25; H, 3.20; N, 4.04.

Found: C, 24.16; H, 3.26; N, 4.02.

84%). ¹H NMR (CDCl₃): δ ) 6.45 (s, 2H, C₆H₂), 3.99 (s, 4H, ³J_PtH

)

45.9 Hz, CH₂N), 3.73 (s, 3H, OCH₃), 3.07 (s, 12H, J_PtH) 37.5 Hz,

NCH₃); ¹³C {¹H} NMR (CDCl₃): δ ) 157.4 (C_para), 143.8 (C_ortho), 135.7

(C_ipso), 105.7 (C_meta), 77.8 (²J_PtC) 66 Hz, CH₂N), 55.6 (OCH₃), 54.4

(²J_PtC) 18 Hz, NCH₃); Anal. Calcd for C₁₃H₂₁ClN₂OPt (451.87): C,

34.56; H, 4.68; N, 6.20. Found: C, 34.41; H, 4.69; N, 6.16.

[PtI(C₆H₂{Me}-1-{CH₂NMe₂}₂-2,6-OMe-4)][CF₃SO₃] (3c). To a

solution of 2c (1.17 g, 2.0 mmol) in acetone (8 mL) was added excess

MeI (2 mL, 32 mmol). The initially colorless solution slowly turned

red. After 20 h, the solution was evaporated to dryness, and the residue

was redissolved in CH₂Cl₂(3 mL). This solution was overlayered with

pentane (15 mL) to afforded red crystals of 3c (1.33 g, 94%). ¹H NMR

[Pt(C₆H₂{CH₂NMe₂}₂-2,6-OH-4)(OH₂)]X (2b, X ) BF₄or CF₃SO₃).

To a suspension of [PtCl(C₆H₂{CH₂NMe₂}₂-2,6-OH-4)] (1b, 97 mg,

0.22 mmol) in acetone/H₂O (3 mL, 10:1) was added solid AgCF₃SO₃

(57 mg, 0.22 mmol) or AgBF₄(43 mg, 0.22 mmol). The mixture was

stirred while protected from light at room temperature for 30 min. The

suspension was filtered through Celite under nitrogen while protected

from light to yield a colorless filtrate. The solvent was removed from

the filtrate in vacuo to leave the title product as a colorless solid.

Analysis for X ) SO₃CF₃: yield 110 mg, 84%. ¹H NMR (acetone-d₆):

δ ) 7.90 (s, 1H, Ar-OH), 6.45 (s, 2H, ⁴J_PtH) 8.9 Hz, C₆H₂), 4.12 (s,

(acetone-d₆): δ ) 7.28 (s, 2H, C₆H₂), 5.01 (d, 2H, J_HH) 13.2 Hz,

³J_PtH) 25.4 Hz, CH₂N, lowfield part of ABq), 4.21 (s, 3H, OCH₃),

3.77 (d, 2H, ²J_HH) 13.2 Hz, ³J_PtH) 41.0 Hz, CH₂N, highfield part of

ABq), 3.15 (s, 6H, J_PtH) 36.1 Hz, NCH₃Me), 2.84 (s, 3H, J_PtH

)

30.4 Hz, CCH₃), 2.81 (s, 6H, J_PtH) 30.4 Hz, NMeCH₃); ¹³C {¹H}

NMR (acetone-d₆): δ ) 171.0 (C_para), 152.2 (²J_PtC) 25.1 Hz, C_ortho),

116.2 (³J_PtC) 38.2 Hz, C_meta), 84.8 (¹J_PtC) 1100 Hz, C_ipso), 68.8 (²J_PtC

) 51.1 Hz, CH₂N), 58.8 (OCH₃), 58.3 (²J_PtC) 13.1 Hz, NCH₃Me),

55.1 (²J_PtC) 16.9 Hz, NMeCH₃), 18.9 (²J_PtC) 75.2 Hz, CCH₃); Anal.

Calcd for C₁₅H₂₄F₃IN₂O₄PtS (707.42): C, 25.47; H, 3.42; N, 3.96.

Found: C, 25.35; H, 3.35; N, 3.84.

4H, J_PtH) 50.5 Hz, CH₂N), 3.01 (s, 12H, J_PtH) 33.0 Hz, NCH₃),

2.6-3.1 (br s, 2H, H₂O); ¹³C {¹H} NMR (acetone-d₆): δ ) 156.8

(C_para), 145.2 (²J_PtCnot resolved, C_ortho), 108.1 (³J_PtC) 42.7 Hz, C_meta),

76.6 (²J_PtC) 70.9 Hz, CH₂N), 54.1 (²J_PtC) 19.6 Hz, NCH₃), C_ipsonot

observed; Anal. Calcd for C₁₃H₂₁F₃N₂O₅PtS (569.47): C, 27.42; H,

3.72; N, 4.92. Found: C, 27.58; H, 3.68; N, 4.96. Analysis for X )

Synthesis of [PtI(C₆H₂{Et}-1-{CH₂NMe₂}₂-2,6-OH-4)][BF₄] (4a).

To a stirred solution of 2b (0.63 g, 1.24 mmol) in acetone (15 mL)

was poured excess EtI (5 mL, 60 mmol). After stirring over 16 h, the

solution had become dark red, and a white precipitate had formed. The

precipitate was removed by filtration (Celite), and the filtrate was

evaporated to dryness. The crude product was dissolved in CH₂Cl₂(5

mL) and precipiated with pentane (50 mL). Repetition of this procedure

BF₄(recrystallized from THF at -20 °C): Yield 108 mg (96%). H

NMR (acetone-d₆): δ ) 7.99 (s, 1H, Ar-OH), 6.47 (s, 2H, J_PtH) 9

Hz, C₆H₂), 4.17 (s, 4H, ³J_PtH) 51.6 Hz, CH₂N), 2.99 (s, 12H, ³J_PtH

)

37.8 Hz, NCH₃), 2.83 (br s, 2H, H₂O); ¹³C {¹H} NMR (acetone-d₆):

δ ) 156.7 (C_para), 145.1 (²J_PtCnot resolved, C_ortho), 108.2 (³J_PtC) 40

Hz, C_meta), 76.6 (²J_PtC) 72 Hz, CH₂N), 54.3 (²J_PtC) not resolved,

NCH₃), C_ipsonot observed; Anal. Calcd for C₁₂H₂₁BF₄N₂O₂Pt × 0.5

THF (543.26): C, 30.95; H, 4.64; N, 5.16. Found: C, 30.55; H, 5.08;

N, 5.34.

afforded 4a as a red oily solid (0.32 g, 40%). H NMR (acetone-d₆):

δ ) 7.12 (s, 2H, C₆H₂), 4.94 (d, 2H, ²J_HH) 13.2 Hz, CH₂N, lowfield

part of ABq), 3.58 (d, 2H, J_HH) 13.7 Hz, CH₂N, highfield part of

ABq), 3.53 (q, 2H, J_HH) 7.2 Hz, CCH₂CH₃), 3.12 (s, 6H, J_PtH

)

[Pt(C₆H₂{CH₂NMe₂}₂-2,6-OMe-4)(OH₂)][CF₃SO₃] (2c). The pro-

cedure was analogous to the preparation of 2b. Reaction of 1c (0.90 g,

2.0 mmol) with AgCF₃SO₃(0.51 g, 2.0 mmol) in acetone/H₂O (5 mL,

10:1) yielded, after filtration through Celite, 2c as a white solid (1.01

g, 87%). Analytically pure 2c was obtained by recrystallization from

THF at -20 °C. ¹H NMR (acetone-d₆): δ ) 6.54 (s, 2H, C₆H₂), 4.19

36.0 Hz, NCH₃Me), 2.81 (s, 6H, J_PtH) 28.5 Hz, NMeCH₃), 0.64 (t,

3H, ³J_HH) 7.2 Hz, CCH₂CH₃); ¹³C {¹H} NMR (acetone-d₆): δ ) 173.2

(C_para), 156.5 (C_ortho), 119.0 (C_meta), 86.4 (C_ipso), 69.0 (CH₂N), 58.1

(NCH₃Me), 55.4 (NMeCH₃), 28.9 (CCH₂CH₃), 16.0 (CCH₂CH₃); Anal.

Calcd for C₁₄H₂₄BF₄IN₂OPt (645.15): C, 26.06; H, 3.75; N, 4.34.

Found: C, 25.89; H, 3.82; N, 4.25.

[PtBr(C₆H₂{Et}-1-{CH₂NMe₂}₂-2,6-OH-4)][BF₄] (4b). The pro-

cedure was analogous to the preparation of 4a, starting from 2b (0.63

g, 1.24 mmol) in acetone (15 mL) and EtBr (5 mL, 70 mmol). After

(42) van de Kuil, L. A.; Luitjes, J.; Grove, D. M.; Zwikker, J. W.; van

der Linden, J. G. M.; Roelofsen, A. M.; Jenneskens, L. W.; Drenth, W.;

van Koten, G. Organometallics 1994, 13, 468.

C_arenium-C_alkylBond Making and Breaking

J. Am. Chem. Soc., Vol. 123, No. 30, 2001 7245

stirring over 24 h, the formed suspension was filtered (Celite) and the

red filtrate was evaporated to dryness. Analysis of the crude product

showed ca. 50% unreacted 2c. Acetone (10 mL) and additional EtBr

(5 mL) were added, and the reaction was left for 7 days, during which

again a precipitate formed. This was removed and the residual solution

treated with pentane. Repeated dissolving in CH₂Cl₂(5 mL) and

precipitating with pentane (30 mL) afforded 4a as a red solid which

was still contaminated with significant amounts of 2c. ¹H NMR

showed spectroscopic properties identical to those of the platinum(II)

starting material. However, carrying out the same reaction in CD₂Cl₂

allowed for spectroscopic analysis of 6a. ¹H NMR (CD₂Cl₂): δ ) 7.09

(s, 2H, C₆H₂), 4.25 (d, 2H, J_HH) 12.9 Hz, CH₂N, lowfield part of

ABq), 3.40 (d, 2H, J_HH) 12.8 Hz, CH₂N, highfield part of ABq),

3.04 (s, 6H, ³J_PtHnot resolved, NCH₃Me), 2.72 (s, 6H, ³J_PtHnot resolved,

NMeCH₃), 1.00 (s, 9H, CCH₃), 0.69 (s, 9H, Me₃Si), signal of Me₂Si

not resolved due to overlap with the resonances of residual Me₃SiOTf;

(acetone-d₆): δ ) 7.12 (s, 2H, C₆H₂), 4.93 (d, 2H, J_HH) 13.2 Hz,

²⁹Si NMR (CD₂Cl₂): δ ) 78.9 (Me₃Si-arenium), 9.64 (Me₂BuSi-O).

CH₂N, lowfield part of ABq), 3.62 (d, 2H, J_HH) 12.9 Hz, CH₂N,

[PtCl(C₆H₂{Me₃Si}-1-{CH₂NMe₂}₂-2,6-OSiMe₂Bu-4)][CF₃SO₃]

highfield part of ABq), 3.46 (q, 2H, J_HH) 7.2 Hz, CCH₂CH₃), 3.11

(6b). To a solution of 2b (0.06 g, 0.1 mmol) in dry CD₂Cl₂(0.7 mL)

(s, 6H, J_PtHnot resolved, NCH₃Me), 2.77 (s, 6H, J_PtHnot resolved,

was added excess Me₃SiOTf (0.1 mL, 0.5 mmol). Immediately, the

NMeCH₃), 0.63 (t, 3H, J_HH) 7.4 Hz, CCH₂CH₃).

solution turned dark orange. H NMR (CD₂Cl₂): δ ) 7.05 (br s, 2H,

C₆H₂), 3.99 (br s, 4H, CH₂N), 3.00 (br s, 6H, ³J_PtHnot resolved, NCH₃-

Me), 2.77 (br s, 6H, ³J_PtHnot resolved, NMeCH₃), 1.01 (s, 9H, CCH₃),

0.69 (s, 9H, Me₃Si), signal of Me₂Si not resolved due to overlap with

the resonances of residual Me₃SiOTf.

[PtCl(C₆H₂{CH₂C₆H₅}-1-{CH₂NMe₂}₂-2,6-OH-4)][BF₄] (5a). To

a stirred solution of 2b (0.35 g, 0.69 mmol) in acetone (10 mL) was

poured BnCl (4 mL, 35 mmol) which caused a slow colorization to

orange. After 12 h, the formed precipitate was removed by filtration

and the filtrate evaporated in vacuo. The crude product was precipitated

from CHCl₃/Et₂O to yield 5a as a yellow solid (0.28 g, 67%). ¹H NMR

(acetone-d₆): δ ) 7.1-6.8 (m, 5H, C₆H₅), 6.96 (s, 2H, C₆H₂), 5.10 (d,

[PtI(C₆H₂{Me}-1-{CH₂NMe₂}₂-2,6-O-4)] (7). Procedure A. A

solution of 3b (0.70 g, 1 mmol) in THF (5 mL) was treated with excess

NEt₃(1.4 mL, 10 mmol), which immediately caused a color change of

the solution from dark red to yellow. Toluene (20 mL) was added and

the suspension filtered. Evaporation of the filtrate gave 7 as a yellow

solid (0.53 g, 98%). Procedure B. Aqueous NaOH (4 M, 0.5 mL, 2

mmol) was added to a solution of 3c (0.71 g, 1 mmol) in THF (5 mL).

The reaction mixture became immediatlely yellow and was stirred for

30 min. After addition of NaOH (1 M, 15 mL) and toluene (20 mL),

the organic phase was separated and the aqueous layer extracted with

toluene (2 × 15 mL). The combined organic layers were dried over

Na₂SO₄and filtered, and the solvent was removed under reduced

pressure. Slow precipitation from CH₂Cl₂/pentane afforded 7 as a

2H, J_HH) 13.2 Hz, J_PtH) 48.8 Hz, CH₂N, lowfield part of ABq),

4.77 (s, 2H, ³J_PtHnot resolved, CCH₂Ar), 3.71 (d, 2H, ²J_HH) 13.4 Hz,

³J_PtH) 39.7 Hz, CH₂N, highfield part of ABq), 3.06 (s, 6H, J_PtH

)

27.9 Hz, NCH₃Me), 2.78 (s, 6H, ³J_PtH) 29.6 Hz, NMeCH₃); ¹³C {¹H}

NMR (acetone-d₆): δ ) 173.1 (C_para), 161.5 (C_ortho), 129.2, 127.8, 127.5

(all C₆H₅), 118.7 (C_meta), C_ipsonot observed, 68.9 (CH₂N), 53.7 (NCH₃-

Me), 51.9 (NMeCH₃), 39.6 (CCH₂Ar); Anal. Calcd for C₁₉H₂₆BClF₄N₂-

OPt (615.78): C, 37.06; H, 4.26; N, 4.55. Found: C, 38.14; H, 4.55;

N, 4.50.

[PtBr(C₆H₂{CH₂C₆H₅}-1-{CH₂NMe₂}₂-2,6-OH-4)][BF₄] (5b). Ad-

dition of BnBr (3 mL, 25 mmol) to 2b (0.45 g, 0.89 mmol) in acetone

(10 mL) afforded a purple solution within 15 min. Analysis of this

intermediate 9: ¹H NMR (acetone-d₆): δ ) 7.15-6.90 (m, 5H, C₆H₅),

microcrystalline yellow solid (0.77 g, 71%). H NMR (CDCl₃): δ )

6.07 (s, 2H, ⁴J_PtH) 16 Hz, C₆H₂), 4.22 (s, 2H, ²J_HH) 13.5 Hz, lowfield

part of ABq), 3.05 (s, 6H, J_PtH) 38.8 Hz, NMeCH₃), 2.94 (s, 6H,

³J_PtH) 40.5 Hz, NCH₃Me), 2.81 (s, 2H, ²J_HH) 13.2 Hz, ³J_PtH) 54.2

Hz, CH₂N, highfield part of ABq), 1.54 (s, 3H, ³J_PtH) 51.2 Hz, CCH₃);

¹³C {¹H} NMR (CDCl₃): δ ) 187.5 (C_para), 163.0 (C_ortho), 119.7 (³J_PtC

) 24.3 Hz, C_meta), 70.4 (CH₂N), 56.2 (²J_PtC) 12 Hz, NMeCH₃), 55.0

(²J_PtC) 21 Hz, NMeCH₃), 49.1 (C_ipso), 23.5 (²J_PtC) 97 Hz, CCH₃);

¹⁷O NMR (THF): δ ) 450; Anal. Calcd for C₁₃H₂₁IN₂OPt (543.31):

C, 28.42; H, 4.95; N, 5.10. Found: C, 28.55; H, 5.05; N, 5.13.

(C₆H₂{Bn}-4-{CH₂NMe₂}₂-3,5-OH-1) (8b). A solution of 5b (15.4

mg, 23 µmol) in CH₂Cl₂(5 mL) was vigorously stirred with an aqueous

solution of KCN (0.5 M, 6 mL, 3 mmol) for 4 h. Aqueous NaOH (2

M, 5 mL) was added, and the product was extracted with CH₂Cl₂(3 ×

5 mL). The combined organic fractions were washed with brine (10

mL), dried over MgSO₄, and evaporated to dryness to afford a yellowish

oil (6.5 mg, 94%).¹H NMR (CDCl₃): δ ) 7.22-6.95 (m, 5H, C₆H₅),

6.84 (s, 2H, C₆H₂), 4.24 (s, 2H, CCH₂Ar), 3.25 (s, 4H, CH₂N), 2.18 (s,

12H, NCH₃); OH not observed; ¹³C {¹H} NMR (CDCl₃): δ ) 154.3,

141.1, 139.0, 129.8, 128.4, 127.9, 125.7, 116.3 (all C_aryl), 61.6 (CH₂N),

45.3 (NMe₂), 32.4 (CH₂).

6.80 (s, 2H, C₆H₂), 5.04 (d, 2H, J_HH) 13.2 Hz, CH₂N, lowfield part

of ABq), 4.61 (s, 2H, J_PtH) 24 Hz, CCH₂Ar), 3.58 (d, 2H, J_HH

)

13.5 Hz, CH₂N, highfield part of ABq), 3.11 (s, 6H, J_PtH) 32.6 Hz,

NCH₃Me), 2.83 (s, 6H, J_PtH) 28.5 Hz, NMeCH₃). Within 1 h, the

color of the reaction solution changed to orange, and after additional

1 h of stirring, the volatiles were removed by evaporation under reduced

pressure. The crude product was isolated by repetitive precipitation of

a CH₂Cl₂solution with pentane and was finally recrystallized from

CHCl₃/Et₂O to yield 5b as an orange solid (0.48 g, 81%). H NMR

(acetone-d₆): δ ) 7.1-6.8 (m, 5H, C₆H₅), 6.44 (s, 2H, C₆H₂), 5.11 (d,

2H, J_HH) 13.2 Hz, CH₂N, lowfield part of ABq), 4.85 (s, 2H, J_PtH

) 30 Hz, CCH₂Ar), 3.72 (d, 2H, J_HH) 13.7 Hz, CH₂N, highfield

part of ABq), 3.10 (s, 6H, J_PtHnot resolved, NCH₃Me), 2.78 (s, 6H,

³J_PtH) 37.2 Hz, NMeCH₃); ¹³C {¹H} NMR (acetone-d₆): δ ) 172.5

(C_para), 157.5 (C_ortho), 129.3, 129.1, 127.6, 127.5 (all C₆H₅), 118.4 (C_meta),

79.8 (C_ipso), 68.2 (CH₂N), 55.0 (NCH₃Me), 52.9 (NMeCH₃), 39.3

(CCH₂Ar); Anal. Calcd for C₁₉H₂₆BBtF₄N₂OPt (660.23): C, 34.57; H,

3.97; N, 4.24. Found: C, 34.72; H, 4.03; N, 4.29.

(C₆H₂{Me}-4-{CH₂NMe₂}₂-3,5-OH-1) (8a). The procedure was

similar to that of 8b, starting from a solution of 3c (0.07 g, 0.1 mmol)

in CH₂Cl₂(5 mL) and aqueous KCN (0.5 M, 6 mL, 3 mmol), giving

the title product as a yellowish oil (13 mg, 60%).

[PtBr(C₆H₂{CH₂C₆H₅}-1-{CH₂NMe₂}₂-2,6-OMe-4)][SO₃CF₃] (5c).

Addition of BnBr (1 mL, 8 mmol) to 2b (0.19 g, 0.36 mmol) in acetone

(4 mL) afforded a purple solution within 10 min, which subsequently

turned orange (1.5 h). The volatiles were removed in vacuo, and the

product was purified by repetitive crystallization from acetone/pentane

solution, which yielded 5c as orange crystals (0.16 g, 65%). ¹H NMR

(acetone-d₆): δ ) 7.29 (s, 2H, C₆H₂), 7.15-7.12 (m, 3H, C₆H₅), 6.89-

6.85 (m, 2H, C₆H₅), 5.17 (d, 2H, ²J_HH) 13.3 Hz, CH₂N, lowfield part

of ABq), 4.97 (s, 2H, ³J_PtHnot resolved, CCH₂Ar), 4.21 (s, 3H, OMe),

¹H NMR (CDCl₃): δ ) 6.15 (s, 2H, C₆H₂), 3.50 (s, 4H, CH₂N),

2.22 (s, 12H, NCH₃), 1.72 (s, 3H, ArCH₃); OH not observed.

Reaction of [Pt(C₆H₂{CH₂NMe₂}₂-2,6-OH-4)(OH₂)][CF₃SO₃] (2a)

with HCl. A solution of 2a (40 mg, 0.07 mmol) in acetone-d₆(2 mL)

was treated with freshly prepared gaseous HCl. The color of the mixture

turned orange, and a white precipitate formed, which was removed by

filtration. Analysis of the orange solution: ¹H NMR (acetone-d₆): δ

3.84 (d, 2H, J_HH) 13.4 Hz, CH₂N, highfield part of ABq), 3.12 (s,

6H, ³J_PtH) 29.4 Hz, NCH₃Me), 2.79 (s, 6H, ³J_PtH) 24.2 Hz, NMeCH₃);

¹³C {¹H} NMR (acetone-d₆): δ ) 171.1 (C_para), 154.2 (C_ortho), 140.5,

129.5, 127.7, 127.6 (all C₆H₅), 117.1 (C_meta), 83.4 (C_ipso), 69.3 (CH₂N),

59.0 (OMe), 55.2 (NCH₃Me), 53.0 (NMeCH₃), 39.0 (CCH₂Ar).

[PtI(C₆H₂{Me₃Si}-1-{CH₂NMe₂}₂-2,6-OSiMe₂^tBu-4)][CF₃SO₃] (6a).

) 5.92 (s, 2H, C₆H₂), 4.41 (s, 2H, J_PtH) 30.3 Hz, CH₂N), 4.00 (s,

2H, J_PtH) 45.9 Hz, CH₂N), 3.02 (s, 6H, J_PtH) 29.5 Hz, NMe₂),

3.00 (s, 6H, ³J_PtH) 38.7 Hz, NMe₂). Evaporation of the orange filtrate

afforded a dark oily residue. The H NMR data of this product are

identical with those of the starting material 2a.

To a solution of [PtI(C₆H₂{CH₂NMe₂}₂-2,6-OSiMe₂Bu-4)] (0.11 g, 0.2

mmol) in dry CH₂Cl₂(3 mL) was added excess Me₃SiOTf (0.8 mL, 4

mmol). Immediately, the solution turned dark red. When this solution

was layered with dry pentane, a white crystalline solid was obtained

together with some decomposition products. The crystalline material

Structure Determination and Refinement of 3c and 5c. X-ray

data were collected on an Enraf-Nonius CAD4-T diffractometer with

rotating anode (Mo KR radiation, λ ) 0.71073 Å). Crystal data and

details on data collection and refinement are given in Table 3. The

7246 J. Am. Chem. Soc., Vol. 123, No. 30, 2001

Albrecht et al.

Table 3. Crystallographic Data for 3c and 5c

hydrogen atoms were refined at calculated positions riding on their

carrier atoms. Weights were optimized in the final refinement cycles.

The unit cell of 5c contains two solvent accessible voids of 311 Å³

each. Disordered solvent molecules in the voids were taken into account

by back Fourier transformation (PLATON/SQUEEZE).⁴⁵Neutral atomic

scattering factors and anomalous dispersion corrections were taken from

the International Tables of Crystallography. All other calculations,

graphical illustrations, and checking for higher symmetry were per-

formed with the PLATON package.⁴⁵

color, shape

empirical formula

formula weight

radiation/Å

T/K

red cube

C₁₅H₂₄F₃IN₂O₄PtS

707.41

Mo KR (monochr.) 0.71073 Mo KR (monochr.) 0.71073

150

orange cube

C₂₁H₂₈BrF₃N₂O₄PtS^a

736.50^a

150

crystal system

space group

unit cell dimensions

a/Å

b/Å

c/Å

monoclinic

P2₁/c (No. 14)

monoclinic

P2₁/c (No. 14)

10.9657(12)

15.9029(14)

11.9313(11)

93.304(8)

2077.2(3)

12.0016(16)

17.7331(10)

15.008(2)

115.263(10)

2888.6(6)

Acknowledgment. We gratefully acknowledge Dr. P. J.

Davies (Utrecht University) for preparing complexes 2b and

2d, and Dr. U. Frey (Universite´ de Lausanne) for performing

the ¹⁷O NMR measurements. This work was supported in part

(A.L.S.) by the Dutch Council for Scientific Research (CW-

NWO).

â/deg

V/Å³

D_calc/g cm^-3

µ/mm^-1(Mo KR)

crystal size/mm

(sinθ/λ)_max/Å^-1

total, unique reflcns

R_int

2.262

8.391

1.694^a

6.358^a

0.4 × 0.2 × 0.1

0.48 × 0.38 × 0.38

0.65

Supporting Information Available: Listing of the temper-

ature-dependent equilibrium constants of 5b and 9 (PDF) and

X-ray structural information on 3c and 5c (CIF). This material

is available free of charge via the Internet at http://pubs.acs.org.

3984, 3644

0.059

7108, 6579

0.034

transmission range

0.171-0.643

0.362-0.776

303, 0

0.0395, 0.0962, 1.01

σ²(F_o²) + (0.0525P)²

-0.93 < 1.12

parameters, restraints 250, 0

R₁^b), wR₂), S

0.0400, 0.0959, 1.046

σ²(F_o²) + (0.0494P)²

-0.77 < 0.83

w^-1

JA003685B

resid. density/e Å^-3

(43) Beurskens, P. T.; Admiraal, G.; Beurskens, G.; Bosman, W. P.;

Garcia-Granda, S.; Gould, R. O.; Smits, J. M. M.; Smykalla, C. The DIRDIF

program system, Technical report of the Crystallography Laboratory;

University of Nijmegen: The Netherlands, 1997.

^aExcluding disordered solvent molecules in the voids. ^bR₁) Σ||F_o|

- |F_c||/Σ|F_o|, for all I > 4σ(I). ^cwR₂) [Σ[w(F_o²- F_c²)²]/Σ[w(F_o)²]]^1/2

^dP ) (max(F_o,0) + 2F_c²)/3.

(44) Sheldrick, G. M. SHELXL-97, Program for crystal structure refine-

ment; University of Go¨ttingen: Germany, 1997.

(45) Spek, A. L. PLATON, A Multipurpose Crystallographic Tool;

Utrecht University: The Netherlands, 2000.

structures were solved with Patterson methods (DIRDIF-97⁴³) and

refined against F²of all reflections (SHELXL-97⁴⁴). Non-hydrogen

atoms were refined freely with anisotropic displacement parameters;

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