Preferred coordination sites for metal fragments in σ,π-bimetallic complexes. Detailed mechanistic insight from heteroatom, bridging ligand, solvent, temperature, and pressure effects on the irreversible exchange of coordination sites

4056

Organometallics 1997, 16, 4056-4070

P r efer r ed Coor d in a tion Sites for Meta l F r a gm en ts in

σ,π-Bim eta llic Com p lexes. Deta iled Mech a n istic In sigh t

fr om Heter oa tom , Br id gin g Liga n d , Solven t,

Tem p er a tu r e, a n d P r essu r e Effects on th e Ir r ever sible

Exch a n ge of Coor d in a tion Sites¹

Thomas A. Waldbach,^†Rudi van Eldik,*^,‡Petrus H. van Rooyen,^§and

Simon Lotz*^,§

Department of Chemistry, University of Pretoria, Pretoria 0002, South Africa, and

Institute for Inorganic Chemistry, University of Erlangen-Nu¨rnberg, Egerlandstrasse 1,

91058 Erlangen, Germany

Received J une 11, 1997^X

New heterobimetallic complexes with σ,π-bridging thiophene and selenophene ligands,

(η¹:η⁵-XCRCHCHCMn(CO)₅)Cr(CO)₃(X ) S, R ) H (1), R ) Me (2); X ) Se, R ) H (3)), were

synthesized from (η⁵-XCRCHCHCLi)Cr(CO)₃and Mn(CO)₅Hal (Hal ) Cl^-, Br^-, CF₃SO₃^-).

The complexes 1-3 irreversibly convert at 0 °C in acetone into the complexes (η¹:η⁵-

XCRCHCHCCr(CO)₅)Mn(CO)₃(X ) S, R ) H (4), R ) Me (5); X ) Se, R ) H (6)) by

exchanging coordination sites. The σ,π- exchange of coordination sites is a first-order process

and rate constants for the reaction of 2 are (3.8 ( 0.1) × 10^-5and (1.1 ( 0.1) × 10^-5s^-1in

acetone and cyclohexane at 15 °C, respectively. This reaction shows no significant pressure

dependence. The activation entropies for the exchange process are -16 ( 6 and -30 ( 11

J /(mol K) for 1 and 2, respectively. The kinetic data suggest an intramolecular exchange

mechanism involving bridging carbonyls without any direct involvement of the solvent. It

is suggested that in the activated complex the metal centers are η¹-bonded to the C2 of the

thienyl ligand and that the free coordination sites are occupied by two bridging carbonyls.

The bimetallic complexes (η¹:η⁵-XCRCHCHCC(O)Mn(CO)₅)Cr(CO)₃(X ) S, R ) H (7), R )

Me (8); X ) Se, R ) H (9)) were also isolated from the reaction mixtures and could be obtained

in higher yields by working under a CO atmosphere. The inserted carbonyl in the bridging

ligand inhibits the metal fragments from exchanging coordination positions. Excess BuLi

leads to the formation of the trimetallic five-membered ring complexes (µ-Hal){µ-(η¹:η¹:η⁵-

SCRCHCHCC(O)Mn(CO)₄)Cr(CO)₃}Mn(CO)₄(R ) H, Hal ) Cl (10); R ) Me, Hal ) Br (11)).

The lithiated thiophene precursor exclusively attacks a carbonyl of Re(CO)₅Br to give a

bimetallic acylate, which after subsequent alkylation with Et₃OBF₄affords the bimetallic

rhenium carbene complex (η¹:η⁵-SCHCHCHCC(OEt)Re(CO)₄Cl)Cr(CO)₃(12). The target

complex (η¹:η⁵-SCHCHCHCRe(CO)₅)Cr(CO)₃(13), which could not be converted and did not

insert a carbonyl, was obtained from Re(CO)₅CF₃SO₃and the lithiated precursor. The

structures of 7 and 10 were confirmed by single-crystal X-ray diffraction studies.

In tr od u ction

such as benzene, thiophene, and their derivatives,

linked by σ- and π-coordination to two or more different

metal fragments.³

The activation of simple organic molecules by more

than one transition metal constitutes an area of re-

search which has increased in importance over the last

few years.²Organometallic compounds that incorporate

two or more metal sites in close proximity provide access

to reaction pathways not available to mononuclear

systems as a result of cooperative electronic and/or steric

effects. Of particular interest in our laboratories are

studies relating to the activation of organic substrates,

The coordination chemistry of thiophene and thiophene

derivatives is of great importance not only for model

studies of the adsorption processes involved during the

catalytic hydrodesulfurization of petroleum products⁴

but also because of the versatility of thiophene as a

ligand⁵and the fascinating coordination chemistry⁶it

displays. Recently we have added to this impressive list

an irreversible exchange of coordination sites by metal

fragments in bimetallic⁷and trimetallic¹complexes with

σ,π-bridged thiophene ligands (Scheme 1). We are now

* To whom correspondence should be addressed.

^†Present address: Institute for Organic and Macromolecular Chem-

istry, University of J ena, Humboldtstrasse 10, 07743 J ena, Germany.

^‡University of Erlangen-Nu¨rnberg.

^§University of Pretoria.

(3) (a) Lotz, S.; Schindehutte, M.; van Rooyen, P. H. Organometallics

1992, 11, 629. (b) van Rooyen, P. H.; Schindehutte, M.; Lotz, S.

Organometallics 1992, 11, 1104. (c) van Rooyen, P. H.; Schindehutte,

M.; Lotz, S. Inorg. Chim. Acta 1993, 208, 207. (d) Meyer, R.; van

Rooyen, P. H.; Schindehutte, M.; Lotz, S. Inorg. Chem. 1994, 33, 3605.

(4) Lead review articles: (a) Angelici, R. J . Acc. Chem. Res. 1988,

21, 387. (b) Sanchez-Delgado, R. A. J . Mol. Catal. 1994, 86, 287. (c)

Bianchini, C.; Meli, A. J . Chem. Soc., Dalton Trans. 1996, 801.

^XAbstract published in Advance ACS Abstracts, August 1, 1997.

(1) π-Heteroatom Complexes. 3. Part 2: Waldbach, T. A.; van

Rooyen, P. H.; Lotz, S. Organometallics 1993, 12, 4250.

(2) For recent reviews, see: (a) Beck, W.; Niemer, B.; Wieser, M.

Angew. Chem., Int. Ed. Engl. 1993, 32, 923. (b) Lotz, S.; van Rooyen,

P. H.; Meyer, R. Adv. Organomet. Chem. 1995, 37, 219. (c) Fornie´s, J .;

Lalinde, E. J . Chem. Soc., Dalton Trans. 1996, 2587.

Preferred Coordination Sites for Metal Fragments

Organometallics, Vol. 16, No. 19, 1997 4057

Sch em e 1

Sch em e 2

convinced that this result is far more general and

demonstrates a new concept dealing with the presence

of preferred coordination sites (σ vs π) around a common

ligand in σ,π-heterobimetallic complexes.

Fluxional behavior in solution or the changing of

coordination modes from σ to π by metal fragments,

particularly for σ,π-bridged acetylide ligands, was previ-

ously observed by Erker⁸and Fornie´s⁹and their co-

workers for σ,π-bridged acetylide ligands, (see A and B

in Scheme 2). Erker showed that by reacting [M(R′Cp)₂-

Ch a r t 1

(5) Theoretical studies: (a) Harris, S. Organometallics 1994, 13,

2628. (b) Rincon, L.; Terra, J .; Guenzburger, D.; Sanchez-Delgado, R.

A. Organometallics 1995, 14, 1292. For recent reviews, see: (c) Angelici,

R. J . Coord. Chem. Rev. 1990, 105, 61. (d) Rauchfuss, T. B. Prog. Inorg.

Chem. 1991, 39, 259. (e) Sadimenko, A. P.; Garnovskii, A. D.; Retta,

N. Coord. Chem. Rev. 1993, 126, 237.

(CtCR)₂] (M ) Zr, Hf, R ) Ph, Me, R′ ) H, Me, Bu^t)

with zirconocenes “(RCp)₂Zr” (R ) H, Me, Bu^t) bimetallic

complexes were isolated with bridging σ,π-acetylide

ligands, in which one σ-acetylide ligand had migrated

from the one metal center to the other. Similarly,

Fornie´s reported that dianionic complexes cis-[Pt(X)₂-

(CtCR)₂]¹⁰(X ) C₆F₅, CCR, R ) Rh, Bu^t) reacted with

cis-[Pt(C₆F₆)₂(THF)₂] to afford products of type B. It is

also reasonable to assume that acetylide migrations

proceed through the µ-η¹:η¹-CtCR intermediate C. In

fact, intermediates of this type can be stable and have

been isolated. The X-ray structure determination of

[Cp₂Ti(µ-CtCBu^t)₂Pt(C₆F₅)₂] reveals that although the

acetylide ligands were initially σ-bonded to the Ti-

center, the asymmetric µ-σ-alkynyl ligands which bridge

both metals are now tilted in such a way that the

σ-bonding orbitals point more to the Pt atom in the final

product.¹⁰In another example, the dimetalated ethyne

(CO)₅ResCtCsRe(CO)₅reacts with (PPh₃)₂PtC₂H₄to

give the σ,σ,π-bridged acetylide complex PtRe₂(µ-η¹:η¹:

η²-CtC)(CO)₉(PPh₃)₂with the Pt fragment not in a

π-bonded but in a σ-bonded position (Chart 1).¹¹

(6) Deprotonation and nucleophilic additions: (a) Luo, S.; Rauchfuss,

T. B.; Wilson, S. R. J . Am. Chem. Soc. 1992, 114, 8515. (b) Huckett, S.

C.; Sauer, N. N.; Angelici, R. J . Organometallics 1987, 6, 591. (c)

Huckett, S. C.; Angelici, R. J . Organometallics 1988, 7, 1491. (d)

Skaugset, A. E.; Rauchfuss, T. B.; Wilson, S. R. J . Am. Chem. Soc.

1992, 114, 8526. (e) Chen, J .; Young, V. G.; Angelici, R. J . Organome-

tallics 1996, 15, 325. (f) Robertson, M. J .; White, C. J .; Angelici, R. J .

J . Am. Chem. Soc. 1994, 116, 5190. (g) White, C. J .; Angelici, R. J .

Organometallics 1994, 13, 5132. (h) Bianchini, C.; J imenez, M. V.; Meli,

A.; Moneti, F.; Vizza, F. J . Organomet. Chem. 1995, 504, 27. (i)

Partridge, M. G.; Field, L. D.; Messerle, B. A. Organometallics 1996,

15, 872. (j) Niemer, B.; Breimair, J .; Vo¨lkel, T.; Wagner, B.; Polborn,

K.; Beck, W. Chem. Ber. 1991, 124, 2237. Desulfurization: (k) Ogilvy,

A. E.; Skaugset, A. E.; Rauchfuss, T. B. Organometallics 1989, 8, 2739.

(l) Luo, S. F.; Ogilvy, A. E.; Rauchfuss, T. B.; Rheingold, A. C.; Wilson,

S. R. Organometallics 1991, 10, 1002. (m) Kaesz, H. D.; King, R. B.;

Manuel, T. A.; Nichols, L. D.; Stone, F. G. A. J . Am. Chem. Soc. 1960,

82, 4750. (n) Ogilvy, A. E.; Draganjac, M.; Rauchfuss, T. B.; Wilson, S.

R. Organometallics 1988, 7, 1171. (o) King, M.; Holt, E. M.; Radnia,

P.; McKennis, J . S. Organometallics 1982, 1, 1718. CS bond cleavage:

(p) Hachgenei, J . W.; Angelici, R. J . Angew. Chem., Int. Ed. Engl. 1987,

26, 909. (q) Dailey, K. M. K.; Rauchfuss, T. B.; Rheingold, A. L.; Yap,

G. P. A. J . Am. Chem. Soc. 1995, 117, 6396. (r) Bianchini, C.; Frediani,

P.; Herrera, V.; J imenez, M. V.; Meli, A.; Rincon, L.; Sanchez-Delgado,

R. A.; Vizza, F. J . Am. Chem. Soc. 1995, 117, 4333. (s) Bianchini, C.;

J imenez, M. V.; Meli, A.; Vizza, F. Organometallics 1995, 14, 3196. (t)

Bianchini, C.; J imenez, M. V.; Meli, A.; Vizza, F. Organometallics 1995,

14, 4858. (u) Bacchi, A.; Bianchini, C.; Herrera, V.; J imenez, M. V.;

Mealli, C.; Meli, A.; Moneti, S.; Peruzzini, M.; Sanchez-Delgado, R. A.;

Vizza, F. J . Chem. Soc., Chem. Commun. 1995, 921. (v) Dong, L.;

Duckett, S. B.; Ohman, K. F.; J ones, W. D. J . Am. Chem. Soc. 1992,

114, 151. (w) Chen, J .; Daniels, L. M.; Angelici, R. J . Organometallics

1996, 15, 1223. Metal insertions: (x) Chen, J .; Daniels, L. M.; Angelici,

R. J . J . Am. Chem. Soc. 1990, 112, 199. (y) J ones, W. D.; Dong, L. J .

Am. Chem. Soc. 1991, 113, 559. (z) J ones, W. D.; Chin, R. M.; Crane,

T. D.; Baruch, D. M. Organometallics 1994, 13, 4448. (aa) Selnau, H.

E.; Merola, J . S. Organometallics 1993, 12, 1583. (bb) Bianchini, C.;

Meli, A.; Peruzzini, M.; Vizza, F.; Frediani, P.; Herrera, V.; Sanchez-

Delgado, R. A. J . Am. Chem. Soc. 1993, 115, 2731. (cc) Garcia, J . J .;

Mann, B. E.; Adams, H.; Bailey, N. A.; Maitlis, P. M. J . Am. Chem.

Soc. 1995, 117, 2179. (dd) Chen, J .; Young, V. G.; Angelici, R. J .

Organometallics 1996, 15, 1414. Isomerizations: (ee) Choi, M.-G.;

Angelici, R. J . Organometallics 1992, 11, 3328. (ff) Choi, M.-G.;

Robertson, M. J .; Angelici, R. J . J . Am. Chem. Soc. 1991, 113, 4006.

Carbon-carbon couplings: (gg) Chin, R. M.; J ones, W. D. Angew.

Chem., Int. Ed. Engl. 1992, 32, 710. (hh) Chen, J .; Angelici, R. J .

Organometallics 1992, 11, 992.

Substitution processes represented by the irreversible

changing of σ- and π-coordination positions in bimetallic

complexes with bridging acetylide ligands were de-

scribed as acetylide migrations⁸or alkynyl transfer

processes.⁹By comparison, we described the multiple

substitution processes whereby two metal fragments

exchange σ and π coordination sites around a thiophene

ring as a metal exchange process.^1,7In both cases the

terminology was determined by the number of coordina-

tion positions involved during the conversion. As more

examples are reported, irreversible metal exchange

processes or irreversible acetylide migrations seems to

be both the result of the same driving force, i.e. the

existence of a kinetically favorable pathway for the

changing of coordination sites leading to products of far

greater thermodynamic stability. Therefore, we now

refer to this observation as preferred coordination sites

for metal fragments in σ,π-bimetallic complexes.

(7) Waldbach, T. A.; van Rooyen, P. H.; Lotz, S. Angew. Chem., Int.

Ed. Engl. 1993, 32, 710.

(8) (a) Erker, G.; Fro¨mberg, W.; Benn, R.; Mynott, R.; Angermund,

K.; Kru¨ger, C. Organometallics 1989, 8, 911. (b) Erker, G.; Fro¨mberg,

W.; Mynott, R.; Gabor, B.; Kru¨ger, C. Angew. Chem., Int. Ed. Engl.

1986, 25, 463.

(9) (a) Fornie´s, J .; Lalinde, E. J . Chem. Soc., Dalton Trans. 1996,

2587. (b) Fornie´s, J .; Gomez-Saso, M. A.; Lalinde, E.; Martinez, F.;

Moreno, M. T.; Organometallics 1992, 11, 2873. (c) Fornie´s, J .; Lalinde,

E.; Martin, A.; Moreno, M. T. J . Chem. Soc., Dalton Trans. 1994, 135.

(d) Berenguer, J . R.; Fornies, J .; Lalinde, E.; Martin, A. Angew. Chem.,

Int. Ed. Engl. 1994, 33, 2083.

In preliminary communications^1,7we reported the

first examples of σ,π-complexes exhibiting metal ex-

(10) Berenguer, J . R.; Falvello, L. R.; Fornie´s, J .; Lalinde, E.;

Thomas, M. Organometallics 1993, 12, 6.

(11) Weidmann, T.; Weinrich, V.; Wagner, B.; Robl, C.; Beck, W.

Chem. Ber. 1991, 124, 1363.

4058 Organometallics, Vol. 16, No. 19, 1997

Waldbach et al.

ing at 70 eV. Elemental analyses were obtained from the

Analytical Division (PCMT) of the Council for Scientific and

Industrial Research, Pretoria, South Africa.

change processes. This unusual reaction, where six

bonds are broken and six new bonds are formed,

prompted us to perform a detailed kinetic investigation

of the metal exchange process as a function of several

chemical and physical variables in order to gain insight

into the underlying reaction mechanism. In this paper,

the synthesis and characterization of several new het-

erobimetallic complexes with σ,π-bridged thienyl and

selenyl ligands are reported and the metal exchange

process is investigated. Aspects associated with the

metalation of (π-selenophene)Cr(CO)₃and the reactivity

of the metalated precursors used in the synthesis of

novel σ,π-bimetallic compounds are also discussed. The

insertion of a carbonyl ligand into the σ-bond of σ,π-

complexes and reaction conditions utilizing 3 equiv of

butyllithium and excess Mn(CO)₅Hal gave stable σ,π-

complexes with modified bridging ligands and trime-

tallic complexes with novel five-membered rings con-

sisting of a 1,2-bridging CO unit, two manganese

fragments, and a µ-halogen ligand. The X-ray struc-

tures of (η¹:η⁵-SCRCHCHCC(O)Mn(CO)₅)Cr(CO)₃and

{µ-Cl){µ-(η¹:η⁵:η¹-SCRCHCHCC(O)Mn(CO)₄)Cr(CO)₃}-

Mn(CO)₄are described and give insight into structural

properties of these carbonyl-inserted products.

The kinetics of the metal fragment exchange reactions were

followed at ambient pressure in the thermostated cell com-

partment (quartz sample cell) of a Shimadzu 2100 spectro-

photometer and on a Bruker Avance DRX 400 MHz WB

spectrometer. For kinetic experiments at elevated pressure

(up to 150 MPa), a thermostated ((0.1 °C) high-pressure cell²⁰

built into the cell compartment of the spectrophotometer and

a

specially designed optical cell equipped with a Teflon

pressure transmitting tube²¹was employed. Reported rate

constants are the average of at least three kinetic runs.

II. Syn t h esis of (π-Th iop h en e)Cr (CO)₃a n d (π-Sele-

n op h en e)Cr (CO)₃. The synthesis of the tricarbonyl com-

plexes of chromium is illustrated by the general method

described below.

A suspension of 7.37 g (39.4 mmol) of Cr(NH₃)₃(CO)₃, 197

mmol of ligand (thiophene, selenophene, and thiophene de-

rivatives), and 130 mmol of BF₃‚Et₂O (40 mL) was stirred at

room temperature for 14 h, during which time the solution

slowly changed color from yellow to red. A further 80 mL of

ether was added (for selenophene, dichloromethane was used

instead of ether), the mixture was cooled to -40 °C, and 50

mL of deoxygenated water was added dropwise over a period

of 30 min. The reaction flask was removed from the cold and

stirred at room temperature until all the ice had melted. The

ether phase was decanted; the water phase was extracted with

30 mL portions of ether until the ether was almost colorless.

The combined ether fractions were filtered through silica gel

and evaporated to dryness. The residue was dried for 2 h

under vacuum. The solid was washed 13 times with 30 mL of

petroleum ether, dissolved in 120 mL of dichloromethane, and

filtered through silica gel. The solution was concentrated to

30 mL, whereupon some precipitate separated. The addition

of 60 mL of petroleum ether and the subsequent concentration

of the mixture to 30 mL afforded red crystals of the desired

compound. The solution was decanted, the crystals washed

with petroleum ether (2 × 30 mL), and the product dried under

reduced pressure. Yields were higher than 70% based on the

mass of Cr(NH₃)₃(CO)₃. This method avoids separation by

column chromatography. When 3-bromothiophene was used,

however, a mixture of (η⁵-3-bromothiophene)Cr(CO)₃and (η⁵-

thiophene)Cr(CO)₃was obtained and the desired product could

be isolated after separation by column chromatography (1/1

dichloromethane/petroleum ether mixture) in 40% yield.

Exp er im en ta l Section

I. Gen er a l P r oced u r es. Unless otherwise stated, all

reactions and manipulations were conducted under a nitrogen

atmosphere by using standard Schlenk and vacuum line

techniques.¹²Thiophene was purified as described previ-

ously.¹³Literature procedures¹⁴were used for the preparation

of (π-thiophene)Cr(CO)₃and complexes with π-bonded thiophene

derivatives (2-methylthiophene, 3-bromothiophene, 2,2′-

bithiophene, 2-lithiothiophene). A modified version of the

reported procedure¹⁵was used for the synthesis of (π-sele-

nophene)Cr(CO)₃. Literature preparations were used for Mn-

(CO)₅Hal (Hal ) Br^-, Cl^-),¹⁶Mn(CO)₅CF₃SO₃,¹⁷Re(CO)₅CF₃-

SO₃,¹⁷and Cr(NH₃)₃(CO)₃.¹⁸Solvents were freshly distilled

from the appropriate drying agents prior to use.¹⁹Column

chromatography was performed on silica gel (0.063-0.200 mm)

and the column cooled by recycling (-20 °C) 2-propanol

through the column jacket. For the kinetic measurements

acetone, THF, acetonitrile (all Roth, HPLC grade), cyclohexane

(Aldrich, water free), and ¹³CO (Aldrich) were used as received.

NMR spectra were recorded at various temperatures from

III. P r ep a r a tion of a THF Solu tion of Lith ia ted (π-

Th iop h en e)Cr (CO)₃. A THF solution (30 mL) of 0.60 g (2.7

mmol) of (π-thiophene)Cr(CO)₃was cooled to -90 °C and

treated with 1.8 mL (2.88 mmol) of n-BuLi (1.6 M solution in

hexane). During the addition the color changed from orange

to yellow. The mixture was stirred at -70 °C for 10 min,

cooled to -90 °C, and used for subsequent reactions.

IV. Rea ction of Lith ia ted (π-Th iop h en e)Cr (CO)₃w ith

Mn (CO)₅Ha l (Ha l ) Cl, Br ). (a) A solution of 2.7 mmol of

(η⁵-LiC₄H₃S)Cr(CO)₃in 30 mL of THF was cooled to -90 °C

and treated with a cold (-90 °C) THF solution (50 mL) of 2.7

of mmol Mn(CO)₅Hal. The brown solution was stirred and

warmed to room temperature after 2 h. After the mixture was

stirred at room temperature for a further 1 h, the solvent was

removed under vacuum and the remaining residue chromato-

graphed on silica gel (3 cm × 30 cm). A number of bands

developed on elution with a 1/1 dichloromethane/petroleum

ether solution and were collected. The first yellow band gave

Mn₂(CO)₁₀(2%), the following yellow fraction gave (η⁵-

SCHCHCHCCr(CO)₅)Mn(CO)₃(4; (118 mg, 10.5%), a red

-20 to +20 °C on

a Bruker AC-300 spectrometer with

reference to the deuterium signal of the solvent employed. ¹H

NMR and ¹³C NMR spectra were measured at 300.135 and

75.469 MHz, respectively, unless specified otherwise. NMR

solvents were degassed by several freeze-pump-thaw cycles,

and NMR sample tubes were sealed under nitrogen. Infrared

spectra were recorded as liquid solutions on a Bomem Mich-

elson-100 FT spectrophotometer, and frequencies (cm^-1) were

assigned relative to a polystyrene standard. Mass spectra

were recorded on a Perkin-Elmer RMU-6H instrument operat-

(12) Shriver, D. F.; Drezdzon, M. A. The Manipulation of Air-

Sensitive Compounds, 2nd ed.; Wiley: New York, 1980.

(13) Spies, G. H.; Angelici, R. J . Organometallics 1987, 6, 1897.

(14) (a) Fischer, E. O.; O¨ fele, K. Chem. Ber. 1958, 91, 2395. (b) Novi,

M.; Guanti, G.; Dell’Erba, C. J . Heterocycl. Chem. 1975, 12, 1055. (c)

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N.; Kursanov, D. N. J . Organomet. Chem. 1983, 244, C21.

(15) White, C. J .; Angelici, R. J .; Choi, M-G. Organometallics 1995,

14, 332.

(16) Quick, M. H.; Angelici, R. J . Inorg. Synth. 1990, 28, 155.

(17) Schmidt, S. P.; Nitschke, J .; Trogler, W. Inorg. Synth. 1989,

26, 115.

(20) Spitzer, M.; Ga¨rtig, F.; van Eldik, R. Rev. Sci. Instrum. 1988,

(18) Rausch, M. D.; Moser, G. A.; Zaiko, E. J .; Lipman, A. L., J r. J .

Organomet. Chem. 1970, 23, 185.

(19) Perrin, D. D.; Armarego, W. L. F.; Perrin, D. R. Purification of

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(21) van Eldik, R. In Inorganic High Pressure Chemistry: Kinetics

and Mechanisms; van Eldik, R., Ed.; Elsevier: Amsterdam, 1986, p

11.

Preferred Coordination Sites for Metal Fragments

Organometallics, Vol. 16, No. 19, 1997 4059

fraction yielded (π-thiophene)Cr(CO)₃, and the next darker

fraction afforded (η⁵-SCHCHCHCC(O)Mn(CO)₅)Cr(CO)₃(7; 60

mg, 5.1%).

4: C₁₂H₃O₈SCrMn (414.12). Anal. Found (calcd): C, 34.71

(34.80); H, 0.72 (0.74). MS (m/ z (fragment, I %)): 414 (M⁺,

17), 386 (M⁺- CO, 4), 358 (M⁺- 2CO, 8), 330 (M⁺- 3CO,

10), 302 (M⁺- 4CO, 21), 274 (M⁺- 5CO, 59), 246 (M⁺- 6CO,

31), 218 (M⁺- 7CO, 61), 190 (M⁺- 8CO, 79).

SC(Me)CHCHCMn(CO)₅)Cr(CO)₃(2; 60 mg, 6.5%), and (η⁵-SC-

(Me)CHCHCC(O)Mn(CO)₅)Cr(CO)₃(8; 20 mg, 2.1%).

2: C₁₃H₅O₈SMnCr (428.15). Anal. Found (calcd): C, 36.39

(36.47); H, 1.16 (1.18).

8: C₁₄H₅O₉SMnCr (456.19). Anal. Found (calcd): C, 36.69

(36.86); H, 1.05 (1.09).

11: C₁₇H₅BrO₁₂SMn₂Cr (675.06). Properties and spectro-

scopic data were comparable to those of 10.

VI. P r ep a r a tion of (η⁵:η⁵-Bith iop h en e)Cr ₂(CO)₆(14). A

7.37 g (39.4 mmol) amount of Cr(NH₃)₃(CO)₃, 3.24 g (19.5

mmol) of 2,2′-bithiophene, and 130 mmol of BF₃‚Et₂O were

mixed in 40 mL of ether. The suspension was stirred for 14 h

at room temperature, and the solution changed color from

yellow to red with the precipitation of a red solid. An

additional 80 mL of ether was added to the mixture, which

was cooled to -40 °C. Deoxygenated water (50 mL) was added

dropwise over 30 min. The reaction flask was removed from

the cold and stirred at room temperature until all the ice

melted. The mixture was filtered, and the filtrate, which

yielded (η⁵-bithiophene)Cr(CO)₃, was handled according to the

procedures described in section II. The precipitate was washed

(4 × 30 mL) with water, ethanol, and ether before it was

dissolved in acetone and filtered through silica gel. The

solvent was removed from the filtrate under vacuum to give

the red-brown product (η⁵:η⁵-bithiophene)Cr₂(CO)₆(14; 3.33 g,

39%). Analytically pure crystals of 14 could be obtained by

recrystallization from dichloromethane.

7: C₁₃H₃O₉SCrMn (442.13). Anal. Found (calcd): C, 35.27

(35.32); H, 0.66 (0.68). MS (m/ z (fragment, I %)): 414 (M⁺

CO, 4), 386 (M⁺- 2CO, 1), 358 (M⁺- 3CO, 3), 330 (M⁺

4CO, 3), 302 (M⁺- 5CO, 4), 274 (M⁺- 6CO, 13), 246 (M⁺

7CO, 9), 218 (M⁺- 8CO, 17), 190 (M⁺- 9CO, 27).

-

(b) The same method was used as described in (a) with the

only exception being that the temperature of the reaction

solution was slowly raised to -10 °C, after which volatiles were

immediately removed. Chromatography of the residue again

yielded the products described in (a) with an additional orange

band which eluted immediately after (π-thiophene)Cr(CO)₃and

afforded (η⁵-SCHCHCHCMn(CO)₅)Cr(CO)₃(1; 101 mg, 9.0%).

1: C₁₂H₃O₈SMnCr (414.12). Anal. Found (calcd): C, 34.65

(34.80); H, 0.71 (0.74). MS (m/ z (fragment, I %)): 413 (M⁺

-

H, 35), 385 (M⁺- H - CO, 7), 357 (M⁺- H - 2CO, 15), 329

(M⁺- H - 3CO, 14), 301 (M⁺- H - 4CO, 25), 273 (M⁺- H

- 5CO, 66), 245 (M⁺- H - 6CO, 31), 217 (M⁺- H - 7CO,

55), 190 (M⁺- 8CO, 75).

(c) Higher yields of 1 were obtained with the same method

as in (b) with the exception that the Mn(CO)₅CF₃SO₃complex

was dissolved at -90 °C in 40 mL of THF (cooled before to

-90 °C). Yields of 1 increased by more than 100%.

(d) Improved yields of 7 (134 mg, 11.1%) were obtained by

repeating the method described in (a) under a CO atmosphere.

Yields increased by ca. 120% for 7 but were much lower for 1

and 4.

14: C₁₄H₆S₂O₆Cr₂(438.31). Anal. Found (calcd): C, 37.97

(38.36); H, 1.32 (1.38). MS (m/ z (fragment, I %)): 437 (M⁺

-

H, 9), 409 (M⁺- H - CO, 1), 381 (M⁺- H - 2CO, 1), 353 (M⁺

- H - 3CO, 5), 325 (M⁺- H - 4CO, 4), 297 (M⁺- H - 5CO,

9), 269 (M⁺- H - 6CO, 39), 166 (C₈H₆S₂⁺, 27).

VII. Rea ction of (η⁵-SCHCHCHCLi)Cr (CO)₃w ith Re-

(CO)₅Cl a n d Re(CO)₅O₃SCF ₃. (a) To a cooled (-90 °C)

solution containing 2.7 mmol of (η⁵-SCHCHCHCLi)Cr(CO)₃,

prepared according to section III, a THF solution containing

0.98 g (2.7 mmol) of Re(CO)₅Cl was added. The color of the

solution immediately changed from yellow to black. The

temperature of the solution was slowly raised to 0 °C and the

volatile material removed under vacuum. The remaining

black residue contained a salt which was dissolved in 40 mL

of dichloromethane. The solution was cooled to -60 °C and

treated with 0.54 g of Et₃OBF₄in 20 mL of dichloromethane.

The color of the solution changed from brown to violet when

the temperature was raised to room temperature. The solvent

was removed by reduced pressure and the residue chromato-

graphed by using a 1:1 mixture of hexane and dichloromethane

as eluant. The product isolated from the column gave (η⁵-

SCHCHCHCC(OEt)Re(CO)₄Cl)Cr(CO)₃(12; 2.05 g, 75%).

12: C₁₄H₈O₈SReCr (609.92). Anal. Found (calcd): C, 27.70

(27.57); H, 1.28 (1.32). IR (hexane, cm^-1): Cr(CO)₃, 1949 (A₁,

s), 1938, 1931 (E, vs); Re(CO)₄, 2104 (A₁, m), 2027 (B₁, s), 2007

(B₂, s), 1995 (A₁, m). ¹H NMR (-20 °C, CDCl₃): δ 6.89 (d, J )

3.13, 1H), 6.20 (d, J ) 3.21, 1H) 5.83 (s, 1H), 5.04 (q, 2H), 1.53

(t, 3H). ¹³C NMR (-20 °C, CDCl₃): δ Cr(CO)₃232, Re(CO)₄

183, 185, and 189, thienyl 89, 93, 99, and 107, OEt 78 (CH₂)

and 15 (CH₃). See Tables 3-5 for more data.

(e) To a cooled (-90 °C) THF solution (30 mL) of 2.7 mmol

of (η⁵-LiC₄H₃S)Cr(CO)₃was added, in small portions, 0.52 g

(2.7 mmol) of CuI. The suspension was warmed slowly to -50

°C and stirred for 10 min. The mixture darkened and was

cooled to -90 °C, after which it was treated with 0.63 g (2.7

mmol) of Mn(CO)₅Cl, which was dissolved in 50 mL of THF.

After removal of volatile materials the residue was chromato-

graphed as above and gave Mn₂(CO)₁₀, 2,2′-bithiophene, (π-

thiophene)Cr(CO)₃, and 1 (68 mg, 6%).

(f) A 0.60 g (2.7 mmol) amount of (π-thiophene)Cr(CO)₃was

dissolved in 30 mL of THF; this solution was cooled to -90 °C

and treated with excess 5.1 mL (8.2 mmol) of n-BuLi (1.6 M

solution in hexane). The mixture turned yellow and was

stirred for 10 min at -70 °C. The mixture was cooled to -90

°C and a THF solution (150 mL) of 1.26 g (8.2 mmol)

Mn(CO)₅Cl was added dropwise. The mixture turned red-

brown, and the temperature was slowly raised to -10 °C. The

solvent was removed under vacuum and the residue chro-

matographed on silica gel as before. The fractions were

collected and yielded Mn₂(CO)₁₀, the black compound (µ-Cl)-

{µ-(η¹:η¹:η⁵-SCRCHCHCC(O)Mn(CO)₄)Cr(CO)₃}Mn(CO)₄(10;

123 mg, 7.3%), which was recrystallized from dichloromethane

and hexane, and unreacted (π-thiophene)Cr(CO)₃.

(b) To a cooled (-90 °C) THF solution (60 mL) of 1.2 g (2.5

mmol) of Re(CO)₅CF₃SO₃was added a cold solution of 2.5 mmol

of (η⁵-SCHCHCHCLi)Cr(CO)₃(see section III). The mixture

was slowly heated to room temperature and stirred for an

additional 1 h. The solvent was removed under reduced

pressure and the residue chromatographed using a 1:1 mixture

of hexane and dichloromethane as eluant. Fractions were

collected, affording (η⁵-thiophene)Cr(CO)₃and the orange

compound (η⁵-SCHCHCHCRe(CO)₅)Cr(CO)₃(13; 50 mg, 3.6%).

13: C₁₂H₃CrO₈SReCr (545.41) Anal.Found (calcd): C, 25.55

(26.13); H, 0.60 (0.56).

10: C₁₆H₃ClO₁₂SMn₂Cr (616.58). Anal. Found (calcd): C,

31.29 (31.17); H, 0.50 (0.48).

V. Rea ction of (η⁵-SC(Me)CHCHCLi)Cr (CO)₅w ith Mn -

(CO)₅Br . To a solution of 0.50 g (2.1 mmol) of (η⁵-SC(Me)-

CHCHCH)Cr(CO)₃in 30 mL of THF at -90 °C was added 1.47

mL (2.5 mmol) of n-BuLi dropwise. The mixture was warmed

to -80 °C, stirred for 10 min, and cooled to -90 °C, and a cold

(-90 °C) THF solution (40 mL) of 0.37 g (1.3 mmol) of Mn-

(CO)₅Br was added. The temperature of the mixture was

slowly brought to -10 °C and the solvent removed under

reduced pressure. The residue was chromatographed on silica

gel, and the fractions were collected. Products obtained from

VIII. Meta la tion of (η⁵-SeCHCHCHCH)Cr (CO)₃a n d

Su bsequ en t Rea ction s w ith Mn (CO)₅Br . (a) A 100 mg

(0.37 mmol) amount of (π-selenophene)Cr(CO)₃was dissolved

in 20 mL of THF and the solution cooled to -90 °C. A hexane

the reaction were Mn₂(CO)₁₀

,

(µ-Br){µ-(η¹:η¹:η⁵-SC(Me)-

CHCHCC(O)Mn(CO)₄)Cr(CO)₃}Mn(CO)₄(11; 7 mg, 0.5%),

some unreacted precursor, (η⁵-SC(Me)CHCHCH)Cr(CO)₃, (η⁵-

4060 Organometallics, Vol. 16, No. 19, 1997

Waldbach et al.

solution (10 mL) containing 0.23 mL (0.37 mmol) of n-BuLi

was added, whereupon the solution immediately changed from

red to dark brown. After the mixture was stirred in the cold

for 10 min, 1.0 mL (7.9 mmol) of Me₃SiCl was added dropwise

and the solution was removed from the cold and stirred for 2

h; the residue was chromatographed, but no silylated product

or starting compound could be collected.

added. The mixture was warmed slowly until -20 °C, after

which the solvent was removed under vacuum. Separation

by column chromatography on silica gel gave Mn₂(CO)₁₀,

unreacted (--selenophene)Cr(CO)₃, and 9.

IX. Con ver sion of 1, 2, a n d 3 in to 4, 5, a n d 6, Resp ec-

tively. The conversion of 1-3 into 4-6, respectively, was

studied under controlled reaction conditions. The complexes

1-3 (10-20 mg) were dissolved at -20 °C in d₆-acetone and

(b) A solution of 100 mg (0.37 mmol), (π-selenophene)Cr-

(CO)₃was prepared as before and slowly added to a cold (-90

°C) THF solution (15 mL) containing 0.37 mmol of lithium

diisopropylamide (LDA). The mixture was stirred at -50 °C

for 10 min and cooled again to -90 °C, and 1 mL of Me₃SiCl

was added. The mixture was removed from the cold and

stirred until it reached room temperature, after which the

solvent was removed under vacuum. The solvent was removed

under vacuum, the residue was chromatographed, and the

fractions that were collected gave (η⁵-SeCHCHCHCSiMe₃)-

Cr(CO)₃(15; 95.2 mg, 75%) and unreacted (η⁵-SeCHCHCH-

CH)Cr(CO)₃.

1

sealed under nitrogen in a NMR tube. The H and ¹³C NMR

spectra were recorded at -20 °C once a week, and the mixture

was stored at 0 °C for a period of 3 weeks. During this time

the solution gradually changed from orange to yellow and the

conversions from 1-3 into 4-6 were completed quantitatively

with little decomposition.

X. Attem p ted Con ver sion of 13. A sample of 13 pre-

pared and treated the same as under section X changed color

from orange to yellowish green. The NMR spectra of this

product indicated that no conversion took place. Instead, the

Cr(CO)₃fragment was displaced from the σ,π-heterobimetallic

complex, affording Re(2-thienyl)(CO)₅(17).

15: C₁₀H₁₂O₃SeSiCr (339.25). Anal. Found (calcd): C,

35.20 (35.40); H, 3.47 (3.57).

17: C₉H₃O₅SRe (409.4). Anal. Found (calcd): C, 26.88

(26.40); H, 0.85 (0.73). IR (CH₂Cl₂, cm^-1): 2138 (A₁, m), 2079

(B, vw), 2028 (E, vs), 1993(A₁, s). ¹H NMR (-20 °C, CDCl₃):

δ (ppm) 7.44 (d, H5, 1H, J ) 5.0 Hz), 7.15 (d, H3, 1H, J )

2.9 Hz), 6.98 (dd, H4, 1H, ³J ) 3.2, ⁴J ) 0.8 Hz). ¹³C NMR

(-20 °C, CD₃C(O)CD₃): δ (ppm) Re(CO)₅183 (cis), 182 (trans);

thienyl 124, 128, 129, 139.

(c) A solution of 500 mg (1.87 mmol) of (π-selenophene)Cr-

(CO)₃in 50 mL of THF was slowly added to a THF solution

(20 mL) containing 1.9 mmol of LDA at -90 °C. The mixture

was warmed to -50 °C, stirred for 10 min, and cooled to -90

°C. First 3.50 mL (5.6 mmol) of n-BuLi was added, the same

temperature changes as above were repeated, and 3.0 mL (23.6

mmol) of Me₃SiCl was added slowly. The mixture was stirred

until it reached room temperature, after which the solvent was

removed under vacuum. Chromatography of the residue

afforded a red fraction, which after recrystallization from

dichloromethane and hexane gave red metallic crystals of (η⁵-

2,3,5-tris(trimethylsilyl)selenophene)Cr(CO)₃(16; 606.7 mg,

70%).

3

XI. Kin etic Measu r em en ts. Solutions of 1 and 2, whether

prepared from either dried or nondried solvents such as

acetone and dichloromethane, revealed no significant differ-

ence in the observed kinetics, and solvents were therefore used

as received. In essence, no differences in the general kinetic

behavior were observed when the metal exchange processes

of 1 and 2 into 4 and 5, respectively, were studied. Since

complex 2 was found to be more stable than 1, our kinetic

16: C₁₆H₂₈O₃Si₃SeCr (463.47). Anal. Found (calcd): C,

41.19 (41.47); H, 6.21 (6.13).

(d) A solution of 620 mg (2.25 mmol) of (π-selenophene)Cr-

(CO)₃in 50 mL of THF was added slowly to a solution of 2.5

mmol of LDA in 20 mL of THF at -90 °C. The mixture was

warmed to -50 °C and stirred for 10 min. before it was again

cooled to -90 °C, and a cold (-90 °C) THF solution (50 mL) of

617 mg of Mn(CO)₅Br was added. The temperature of the

mixture was slowly raised to -20 °C, and the solvent was

removed under reduced pressure. Chromatography of the

residue and collection of the different fractions afforded Mn₂-

(CO)₁₀, unreacted (π-selenophene)Cr(CO)₃, the orange com-

pound (η⁵-SeCHCHCHCMn(CO)₅)Cr(CO)₃3; 60 mg, 5.2%), and

the black compound (η⁵-SeCHCHCHCC(O)Mn(CO)₅)Cr(CO)₃

9; 20 mg, 1.8%).

investigations focused on 2 and its converted product 5. A

typical kinetic run involved the following procedure. Stock

solutions of a fixed concentration of 1 or 2 were prepared in

different solvents. The progress of the reaction was monitored

by the increase and decrease of characteristic MLCT bands in

the UV-vis spectra over the range 325-600 nm. The tem-

perature dependence of the conversion was studied in acetone

over the range 10-30 °C and the concentration (20 °C) and

pressure dependence (18.7 °C) of 2 in acetone over the ranges

(2-42) × 10^-5M and 10-150 MPa, respectively. OLIS

KINFIT²²software was employed to calculate k_obsvalues from

absorbance vs time traces by fitting single- or double-

exponential functions through the data points.

3: C₁₂H₃O₈SeMnCr (461.05) Anal. Found (calcd): C, 31.11

(31.26); H, 0.55 (0.59).

In a control experiment the ¹³C NMR spectrum of 2 in d₆-

acetone was recorded repeatedly over the period required for

complete site exchange.

9: C₁₃H₃O₉SeMnCr (489.06) Anal. Found (calcd): C, 31.78

(31.93); H, 0.58 (0.61).

XII. X-r a y St r u ct u r e Det er m in a t ion of (η¹:η⁵-

SCHCHCHCC(O)Mn (CO)₅)Cr (CO)₃(7) a n d (µ-Cl){µ-(η¹:η¹:

η⁵-SCRCHCHCC(O)Mn (CO)₄)Cr (CO)₃}Mn (CO)₄(10). Data

Collection a n d P r ocessin g. Crystal parameters and ex-

perimental conditions are listed in Table 1. All data were

collected on an Enraf-Nonius CAD4 diffractometer by using

graphite-monochromatized Mo KR (λ ) 0.7107 Å) radiation at

room temperature. Accurate unit cell parameters were ob-

tained by least-squares methods from the position of 25

centered reflections for each crystal (θ > 14° for 7, θ > 15° for

10). The zone collected for 7 was h (0, +7), k (-8, +8) and l

(-21, 21) and for 10 was h (-10, +10), k (-12, +12), and l (0,

20). Both data sets were collected in the ω-2θ mode. Three

standard reflections were monitored every 200 reflections.

Reflections were corrected for Lorentz-polarization and ab-

sorption effects.

(e) A solution of 6.75 mmol of LDA in 20 mL of THF was

treated with a solution of 600 mg (2.25 mmol) of (π-sele-

nophene)Cr(CO)₃in 50 mL of THF at -90 °C. The mixture

was warmed to -50 °C, stirred for 10 min. and again cooled

to -90 °C before a cold (-90 °C) THF solution (70 mL) of Mn-

(CO)₅Br was added. The temperature of the mixture was

raised to -20 °C and the solvent removed under vacuum.

Chromatography on silica gel and the collection of fractions

afforded Mn₂(CO)₁₀, unreacted (π-selenophene)Cr(CO)₃, 3, and

an impure mixture that contained 9.

(f) A solution of 600 mg (2.25 mmol) of (π-selenophene)Cr-

(CO)₃in 50 mL of THF was slowly added to 2.5 mmol of LDA

in 20 mL of THF at -90 °C. The mixture was warmed to -50

°C, stirred for 10 min, and cooled to -90 °C again before 4.21

mL (6.7 mmol) of n-BuLi was added. The temperature of the

mixture was raised to -50 °C, the mixture was stirred for 10

min and again cooled to -90 °C, and a cold THF solution (-90

°C) of 1.23 g (4.5 mmol) of Mn(CO)₅Br in 70 mL of THF was

(22) OLIS KINFIT, Bogart, GA.

Preferred Coordination Sites for Metal Fragments

Organometallics, Vol. 16, No. 19, 1997 4061

Ta ble 1. Cr ysta l a n d Da ta Collection P a r a m eter s

for 7 a n d 10

Ta ble 2. Sp ectr oscop ic Da ta for (π-r in g)Cr (CO)₃

Com p lexes

7

10

IR^aν(CO) (cm^-1

)

¹H NMR^b(δ, ppm; J , Hz)

formula

fw

space group

a, Å

b, Å

c, Å

C₁₃H₃O₉SCrMn

442.16

C₁₆H₃O₁₂ClSCrMn₂

616.58

complex assignt

assignt

14^c

15

A₁

E

1964 (s)

H5, H5′ 6.14 (d, 2H, J 3.5)

H3, H3′ 6.09 (d, 2H, J 3.7)

H4, H4′ 5.99 (t, 2H)

H5

3

P1h (No. 2)

6.162(1)

7.017(1)

18.194(2)

83.67(1)

86.32(1)

85.75(1)

778.5(2)

2

P1h (No. 2)

8.006(1)

9.651(2)

16.062(3)

72.74(2)

83.57(2)

68.06(1)

1099.4(3)

2

3

1882 (vs)

1961 (s)

1864,

3

A₁

E

6.70 (dd, 1H, J 3.5,

⁴J 0.6, J _H,Se38.4)

2

R, deg

â, deg

γ, deg

V, Å³

Z

3

1884 (vs) H3

H4

6.23 (d, 1H, J 3.2)

3

6.20 (t, 1H, J 3.5)

16

A₁

E

1952 (s)

1856,

1879 (vs) SiMe

SiMe

0.27 (s, 9H)

6.26 (s, 1H)

0.28 (s, 9H)

0.36 (s, 9H)

0.40 (s, 9H)

H4

d

_calcd, g/cm³

1.89

1.77

cryst size, mm

appearance

0.07 × 0.25 × 0.50 0.12 × 0.25 × 0.37

purple

dark red

(parallelepiped)

µ(Mo KR), cm^-1

scan angle

(ω + 0.34 tan θ), deg

data collection range,

2θ, deg

a

b

CH₂Cl₂. CD₃COCD₃, -10 °C. ^cCD₃COCD₃, -20 °C.

16.1

0.66

18.1

0.52

this method which was developed by O¨ fele,^14eas no

column separation is necessary and yields of final

products are high (over 70%). Hermann et al.²⁸were

successful in isolating only (η⁵-2,2′-bithiophene)Cr(CO)₃,

whereas we succeeded in isolating (η⁵:η⁵-2,2′-bithiophene)-

{Cr(CO)₃}₂(14) from Cr(NH₃)₃(CO)₃. Complex 14 is

poorly soluble in dichloromethane and precipitates as

a red solid in ether. Purification of 14, mainly from

unreacted Cr(NH₃)₃(CO)₃, was achieved by extraction

with cold acetone and column chromatography with

dichloromethane. The Cr(CO)₃fragment is very labile

and is easily displaced to give (η⁵-bithiophene)Cr(CO)₃

3-25

3-27

no. of data collected

total no. of unique

data with I > 2σ(I)

no. of params refined

transmission factors:

max, min (ψ scans)

R^a

2744

1860

4034

2971

217

1.000, 0.836

299

1.000, 0.961

0.1036

0.0701

0.03

0.0592

0.0431

0.02

b

R_w

largest positional

shift/esd, final cycle

largest peak, e/Å³

1.57^c

0.52

1

and ultimately bithiophene. In the H NMR spectrum

a

b

2

R ) ∑||F_o| - |F_c||/∑|F_o|. Rw ) [∑w(|F_o| - |F_c|)²/∑w|F_o| ]^1/2; w

) 1/σ²(|F_o|). ^c1.1 Å from Mn.

of 14 three resonances are observed for the six protons,

and in the ¹³C NMR spectrum four resonances for the

eight carbon atoms are observed, indicating both rings

to be electronically equivalent. The chemical shifts of

the resonances are shifted upfield compared to free

bithiophene, as is expected for π-bonded heteroatom ring

systems.²⁹

Str u ctu r a l An a lysis a n d Refin em en t. The structures

were solved by standard Patterson methods.²³Refinement was

by full-matrix least-squares methods.²⁴The non-hydrogen

atoms were refined anisotropically (excluding C2 and O8 in

7), and the hydrogen atoms were placed in idealized positions

with common isotropic thermal parameters, which were also

refined. Atomic scattering factors were taken from the

literature.²⁵Perspective views of the molecules were prepared

with ORTEP.²⁶

Meta la tion . Deprotonation of the thiophene com-

plexes at the 2-position is achieved with n-BuLi in THF

at low (-70 °C) temperatures.^14dThe metalation of (η⁵-

selenophene)Cr(CO)₃under similar reaction conditions

with n-BuLi, however, led to decomposition. When we

employed the less nucleophilic LDA, a clear orange

solution was obtained, which, on the addition of Me₃-

SiCl, changed to red. The NMR and IR spectral data

of the red product conformed to a formulation of (η⁵-

SeCHCHCHC(SiMe₃))Cr(CO)₃(15) for the product, prov-

ing that the monolithiation of the chromium tricarbonyl

selenophene complex was achieved. The reaction of the

π-selenophene complex with 1 equiv of LDA, followed

by 3 equiv of n-BuLi and subsequent treatment with

excess Me₃SiCl, afforded the trisubstituted trimethyl-

silyl complex (η⁵-2,3,5-SeC(SiMe₃)CHC(SiMe₃)C(SiMe₃))-

Cr(CO)₃(16). A progressive increase of electron density

on the selenyl ring when protons were substituted by

trimethylsilyl groups was evident from the lowering of

carbonyl frequencies for the two bands of the Cr(CO)₃

unit in the infrared spectra on going from (π-thiophene)-

Cr(CO)₃to 15 to 16. The coupling of the proton at the

5-position to the ⁷⁷Se nucleus gives the two-bond

Resu lts a n d Discu ssion

Syn th esis a n d Ch a r a cter iza tion . The complexes

1-17 were characterized by elemental analysis, mass

spectra, and IR spectra (Tables 2 and 3) and ¹H and

¹³C NMR spectroscopy (Tables 2, 4, and 5). Spectral

assignments are based on fixed numbers assigned to

ring protons and carbon atoms of the heteroaromatic

ring with the lower number 2 allocated to the substitu-

ent containing the metal fragment.

η⁵-Heter oa tom -Rin g Com p lexes. Chromium tri-

carbonyl thiophene and selenophene complexes were

prepared in a two-step procedure first by converting Cr-

(CO)₆into Cr(CO)₃(NH₃)₃and second by treating the

latter with BF₃‚Et₂O in the presence of excess thiophene

or selenophene. Unlike Angelici and Sanger,²⁷we favor

(23) Sheldrick, G. M. SHELX86: A Program for the Solution of

Crystal Structures; University of Go¨ttingen, Go¨ttingen, Germany,

1986.

(24) Sheldrick, G. M. SHELX76: Program for Crystal Structure

Determination; University of Cambridge, Cambridge, U.K., 1976.

(25) International Tables for X-Ray Crystallography; Kynoch

Press: Birmingham, U.K., 1974; Vol. 4. Cromer, D. T.; Liberman, D.

J . Chem. Phys., 4, 1970, 1891.

2

coupling constant J _H,Se) 38.4 Hz for 15 and makes

the assignment of the resonance of the doublet of

doublets at δ 6.70 ppm for H(5) unambiguous. This

value is smaller than J _H,Se) 47 Hz found for free

2

(26) J ohnson, C. K. ORTEP; Report ORNL-3794; Oak Ridge Na-

tional Laboratory: Oak Ridge, TN, 1965.

(27) Sanger, M. J .; Angelici, R. J . Organometallics 1994, 13, 1821.

(28) Hermann, W. A.; Andrejewski, D. Chem. Ber. 1986, 119, 878.

(29) Mangini, A.; Taddei, F. Inorg. Chim. Acta 1968, 2, 12.

4062 Organometallics, Vol. 16, No. 19, 1997

Ta ble 3. In fr a r ed Absor p tion s (cm ^-1) for th e Com p lexes in CH₂Cl₂

Waldbach et al.

ν(CO)

M(CO)₃fragment

M(CO)₅fragment

B₂

(1)

(2)

complex

Mn-C(O)

A₁

E

A₁

E

A₁

1

2

3

7

1947 (m)

1943 (m)

1947 (m)

1974 (m)

1859 (s)

1854 (s)

1858 (s)

1884 (s)

1908 (s)

1880 (s)

1903 (s)

1881 (s)

1910 (s)

1856 (s)

2131 (w)

2130 (w)

2125 (w)

2075 (vw)

2074 (vw)

2069 (vw)

2040 (vs)

2039 (vs)

2031 (vs)

2021 (s)

2020 (s)

2019 (s)

2026 (s)

1563 (vw)

1566 (vw)

1560 (vw)

8

9

1970 (m)

1975 (m)

1946 (m)

2124 (w)

2146 (w)

ν(CO)

2069 (vw)

2078 (vw)

2031 (vs)

2036 (vs)

2025 (s)

2026 (s)

2011 (s)

13

M(CO)₃fragment

cis-M(CO)₄fragment

(1)

(2)

complex

Mn-C(O)

A₁

E

A₁

B₁

B₂

A₁

12^a

1942 (m)

(1949) (m)

1974 (m)

1910 (s)

2106 (m)

(2104) (m)

2005 (vs)

(2027) (s)

2005 (vs)

(2007) (s)

1990 (s)

(1995) (s)

(1938, 1931) (s)

1890 (s)

1913

1886 (s)

1909

10^b

11^b

n.o.

2117 (w), 2091 (m), 2042 (vs), 2013 (s), 1946 (s)

1972 (m)

2114 (w), 2087 (m), 2040 (vs), 2031 (s), 1947 (s)

4^b

5^b

6^b

2066 (m), 2045 (s), 1993 (s), 1978 (s), 1929 (vs)

2064 (m), 2043 (s), 1990 (s), 1973 (s), 1927 (vs)

2064 (m), 2042 (s), 1991 (s), 1973 (s), 1928 (vs)

a

b

Values in parentheses are for data recorded in hexane. Assignments not possible due to overlapping of bands.

Ta ble 4. ¹H NMR^aCh em ica l Sh ifts (δ) a n d Cou p lin g Con sta n ts (Hz) for th e Com p lexes in CD₃C(O)CD₃

thiophene

complex

other

1

6.16 (dd, H5)

5.94 (dd, H3)

5.98 (dd, H4)

4

3

³J 3.5, J 0.7

³J 3.1, J 0.8

³J 3.5, J 3.1

2

3

5.87 (d, H3 or H4)

5.80 (d, H3 or H4)

2.31 (s, CH₃)

³J 3.0

6.63 (dd, H5)

6.23 (dd, H3)

6.22 (t, H4)

³J 3.3

4

2

4

³J 3.6, J 0.9, J _H,Se33.5

7.11 (d, H5)

³J 3.6

³J 3.5, J 0.9

4^b

5^b

6^b

7^c

6.50 (d, H3)

6.78 (t, H4)

³J 2.9

³J 3.2

6.54 (d, H3 or H4)

6.36 (d, H3 or H4)

³J 2.8

2.54 (s, CH₃)

2.31 (s, CH₃)

³J 2.8

7.52 (d, H5)

6.83 (d, H3)

7.05 (t, H4)

³J 3.4

2

³J 3.8, J _H,Se28.3

³J 3.1

6.51 (d, H3 or H5)

6.16 (d, H3 or H5)

³J 3.1

6.07 (t, H4)

³J 3.7

³J 3.6

8^d

9^b

10^e

12

13

6.47 (sbr, H3 or H4)

5.91 (d, H3 or H4)

³J 3.2

6.65 (d, H3 or H5)

³J 3.7

6.54 (d, H3 or H5)

³J 3.9

6.32 (t, H4)

³J 3.9

6.46 (d, H3 or H5)

5.84 (d, H3 or H5)

5.58 (t, H4)

³J 3.6

³J 3.5

³J 3.7

7.11 (d, H3 or H5)

³J 3.1

6.97 (d, H3 or H5)

³J 3.2

6.32 (t, H4) br

5.32 (m, CH₂)

1.51 (t, CH₃)

6.11 (dd, H3 or H5)

6.09 (dd, H3 or H5)

5.89 (dd, H4)

4

3

³J 3.6, J 0.5

³J 3.1, J 0.5

³J 3.1, J 3.6

a

Assignments for H5 of 1 and 4 were made by comparison with 3 and 6, respectively. All data were recorded at -20 °C unless otherwise

noted. 10 °C. ^c5 °C. 0 °C. ^e0 °C (CDCl₃).

b

d

selenophene³⁰but larger than the reported value of 18.8

Hz for (π-selenophene)Cr(CO)₃.¹⁵Three single reso-

nances were observed for the SiMe₃substituents in 16.

The chemical shifts of the selenyl carbons in the ¹³C

NMR spectra move downfield with decreasing positive

charge on the ring, owing to the higher degree of

substitution with SiMe₃but the carbonyl carbon reso-

nances stay unaffected.

Syn th esis of σ,π-Heter obim eta llic Com p lexes.

Although π-coordinated thiophene complexes with two

or more metal fragments bonded to the sulfur atom have

been reported,³¹we are aware of only two other types

of synthesized heterobimetallic σ,π-complexes with η¹:

η⁵-bridging thiophene ligands, one class having thienyl

32

ligands, (η¹:η⁵-SCHCHCHCFeCp(CO)₂)Cr(CO)₃and

(η¹:η⁵-SCHCHCHCPt(L₂)(C₄H₃S))Cr(CO)₃(L₂) diphos,

The reaction of monolithiated (η⁵:η⁵-bithiophene){Cr-

(CO)₃}₂(14) from n-BuLi or LDA with Mn(CO)₅Hal

leads to excessive decomposition.

(31) (a) Chen, J .; Angelici, R. J . Organometallics 1990, 9, 879. (b)

Choi, M.-G.; Angelici, R. J . Organometallics 1991, 10, 2436. (c) Chen,

J .; Angelici, R. J . Appl. Organomet. Chem. 1992, 6, 479. (d) Chen, J .;

Young, V. G.; Angelici, R. J . Organometallics 1996, 15, 2727.

(32) Kolobova, N. E.; Goncharenko, L. V. Bull. Acad. Sci. SSR, Div.

Chem. Sci. 1979, 843.

(30) Simonin, M. P.; Pouet, M. J .; Cense, J . M.; Paulmier, C. Org.

Magn. Reson. 1976, 8, 508.

Preferred Coordination Sites for Metal Fragments

Organometallics, Vol. 16, No. 19, 1997 4063

Ta ble 5. ¹³C NMR Ch em ica l Sh ifts (δ) for th e Com p lexes in CD₃C(O)CD₃a t -20 °C

complex

thiophene

M(CO)₅

Cr(CO)₃

Mn(CO)₃

MnC(O)

other

1

2

3

96, 97, 109, 113

98, 109, 111, 116

98, 101, 109, 121

102, 103, 109, 164

101, 109, 124, 159

103, 108, 110, 177

91, 94, 95, 118

90, 95, 110, 117

93, 95, 96, 126

89, 91, 94, 113

94, 96, 102

208

236

15 (CH₃)

4^a

5^b

6^a

7^c

8^b

9^a

10^d

12

13

218 (cis), 222 (trans)

208

209

220

234

232

233

236

245

246

n.o.

16 (CH₃)

n.o.

79.9 (OCH₂), 15 (CH₃), 187, 186 (Re(CO)₄)

91, 96, 98, 109

218 (cis), 222 (trans)

a

b

d

10 °C. 0 °C. ^c5 °C. 5 °C (CDCl₃).

Sch em e 3

Whereas the utilization of phenylcopper instead of

phenyllithium was very successful in increasing the

yield of (η⁵-C₅H₅)Re(CO)(NO)Ph,³⁵the reaction of the

lithiated precursor with CuI and Mn(CO)₅Cl afforded

2,2′-bithiophene, (η⁵-2,2′-bithiophene)Cr(CO)₃, and (in

a very low yield) 1. Yields of 1 could be doubled by

treating Mn(CO)₅Br with silver triflate prior to the

reaction with the lithiated precursors. Irrespective of

whether the reactions were carried out under normal

conditions or in the dark, the yield of Mn₂(CO)₁₀was

unaffected. Furthermore, the fact that no products

containing bithiophene or biselenophene were found in

the reactions seems to exclude a radical mechanism for

the metal exchange.

2 PMe₃, 2 CO),³³and the other having bridging thie-

nylcarbene ligands, (η¹:η⁵-SCHCHCHCC(OR)M(CO)₅)-

Cr(CO)₃(M ) W, Cr; R ) Et, O(CH₂)₄OEt).³³Beck and

co-workers³⁴synthesized the bimetallic complex (η¹:η⁴-

SCHCHCHCH{Re(CO)₅})Mn(CO)₃from [Re(CO)₅]^-and

[(η⁵-SCHCHCHCH)Mn(CO)₃]⁺and the trimetallic com-

plex {(η¹:η⁴-SCHCHCHCH)Mn(CO)₃}₂Os(CO)₄from

[Os(CO)₄]^2-and the cationic manganese precursors.

The utilization of lithiated arene complexes of chro-

mium in reactions of complexes with halogen ligands

has been well-established in our laboratories and was

successfully employed to synthesize a large range of σ,π-

bimetallic arene complexes.³The reaction of equimolar

solutions of the lithiated thiophene and selenophene

precursors with Mn(CO)₅Hal (Hal ) Br^-, Cl^-) afforded

Mn₂(CO)₁₀, 4-6, and 7-9 in low yields.

Careful repetition of the synthesis without raising

temperature over -10 °C and swift chromatography

enabled the isolation and characterization of 1-3 in low

yields. When crystals of 1-3 were dissolved in polar

solvents such as THF and acetone and the temperature

raised to 0 °C, they spontaneously and irreversibly

underwent a metal exchange process (eq 1) to afford the

The conversions of 1-3 into 4-6 are accompanied by

dramatic spectral changes as shown by the UV-vis, IR,

and NMR spectra. The IR spectra display two totally

different patterns for the ν(CO) bands in the IR spectra

on exchanging the metal carbonyl fragments from σ- to

π-environments. Typically, π-bonded thienyl or selenyl

complexes of chromium reported in the literature²⁷

display two bands for the asymmetric vibrations (E) of

the Cr(CO)₃fragment. This was also observed in the

spectra of 4-9 recorded in dichloromethane, but not for

1-3, where these bands overlap. Uncertainty exists in

the assignment of some of the bands to the vibrational

modes of the Cr(CO)₅and Mn(CO)₃moieties of 4-6.

Replacing a π-bonded thienyl by a selenyl ring or a Mn-

(CO)₅by a Re(CO)₅fragment does not affect the posi-

tions of the Cr(CO)₃frequencies. A small but significant

lowering of these carbonyl frequencies is observed in 2

compared to 1 by the introduction of a methyl substitu-

ent in the 5-position. In contrast, a pronounced lower-

ing of ν(CO) bands of the Cr(CO)₃fragment occurs on

replacing the electron-withdrawing substituent R )

C(O)Mn(CO)₅in 7 and 9 by R ) H as found in the

reference complexes. This trend of a lowering in the

ν(CO) value of the Cr(CO)₃vibrations (ca. 20 cm^-1) is

continued by replacement of R ) H by Mn(CO)₅in 1

and 3. Therefore, the σ-bond is polarized by the metal

fragment to the extent that the positive pole of the bond

resides on the metal and the negative charge on the

carbon atom of the ring.

{Cr(η⁵-XC₄H₃), Mn(η¹-XC₄H₃)} f

{Cr(η¹-XC₄H₃), Mn(η⁵-XC₄H₃)} (1)

thermodynamically favored products 4-6. The site

exchange was practically quantitative, with only some

minor decomposition occurring during the conversion.

Two minor side products, (2-methyl-5-thienyl)Mn(CO)₅

and Cr(CO)₆, could be identified more clearly in the

¹³CO experiment (see further discussion).

The ν(CO) bands of the acyl groups of 7-9 are

observed between 1560 and 1570 cm^-1, indicating a

lowering of the C-O bond order of the acyl group when

compared to values of 1593 and 1607 cm^-1recorded for

the analogous benzene complexes {η¹:η⁶-C₆H₄RC(O)Mn-

(CO)₅}Cr(CO)₃(R ) H, F).^3aThis is consistent with the

(33) Lotz, S. Unpublished results.

(34) (a) Niemer, B.; Weidmann, T.; Beck. W. Z. Naturforsch. 1992,

B47, 509. (b) Beck, W.; Niemer, B.; Wagner, B. Angew. Chem., Int.

Ed. Engl. 1989, 28, 1705.

(35) Sweet, J . R.; Graham, W. A. G. J . Organomet. Chem. 1983, 241,

45.

4064 Organometallics, Vol. 16, No. 19, 1997

Waldbach et al.

Sch em e 4

delocalization of π-electron density from either the

heteroatom ring or the σ-bonded metal fragment to the

acyl oxygen. Support of less electron density on the

rings of 7-9 compared to the case for the (π-ring)Cr-

(CO)₃precursors²⁷is manifested in the higher wave-

numbers (frequencies for the A₁and E bands increased

by ca. 8 and 20 cm^-1, respectively) for the Cr(CO)₃

fragment.

Both the patterns and positions of the chemical shifts

of the thienyl and selenyl protons in the ¹H NMR

spectra of the unconverted complexes 1, 3, and 13 and

the converted complexes 4 and 6 are totally different

and represent two characteristic sets of signals. The

spectra exhibit resonances for 1, 3, and 13 over a much

narrower range (0.20-0.40 ppm), which are shifted

upfield by ca. 1 ppm compared to 4 and 6. The chemical

shifts of H(4) and H(3) occur close together with H(5)

shifted downfield by 0.20-0.40 ppm for 1 and 3 and

resemble those displayed by Cp₂M(2-thienyl)₂(M ) Zr,

Ti),³⁶(η¹:η⁵-SCHCHCHC{FeCp(CO)₂})Cr(CO)₃,³²and

Cp(NO)(Ph₃P)Re(2-thienyl).^6fThe resonances of 1 are

shifted upfield compared to those of Cp₂M(2-thienyl)₂

(M ) Zr, δ 7.45, 7.01, and 6.99) but downfield compared

to those of (η¹:η⁵-SCHCHCHC{FeCp(CO)₂})Cr(CO)₃(δ

5.73, 5.45, and 5.34). However, the thienyl resonances

of 4 and 6 are more evenly spread, with features similar

to those recorded for the allylic compound (η¹:η⁴-

SCHCHCHCH{Re(CO)₅})Mn(CO)₃,^34afor which the reso-

nances of H(5)-H(3) (δ 5.28, 4.77, and 4.22) are shifted

upfield. The chemical shifts for H(5) of 3 and 6 are

clearly identifiable from the two-bond coupling with

selenium, and the corresponding resonances for H(5) of

1 and 4 have been assigned accordingly by comparison

with values reported in the literature.^15,30In the ¹H

NMR spectra of 1, 3, and 13 the H(3) and H(5)

resonances appear as doublets of doublets by displaying

in addition to the three-bond coupling a four-bond

Sch em e 5

Sch em e 6^a

a

Legend: (i) 3 equiv of BuLi and excess Mn(CO)₅Hal.

°C), all the signals were resolved. Resonances for 7-9

observed at δ 245 ppm were assigned to the chemical

shift of the carbonyl-carbon of the acyl group. From the

spectroscopic data it was evident that the complexes had

planar η¹:η⁵rings and that irreversible conversions of

1-3 into 4-6 occurred whereby the metal fragments

exchanged coordination positions. It was also clear that,

whereas the Mn(CO)₅substituents increased the charge

on the heterocyclic ring, the charge was decreased by

acyl-metal fragments.

Conclusions drawn from the NMR data of the elec-

tronic properties of the thienyl and selenyl rings of 1-9

as affected by the nature of the σ-bonded substituents

were supported by the IR data and are illustrated by

the different resonance structures in Schemes 4 and 5.

Carbonyl-inserted products 7-9 were observed in all

of the reactions but did not form during the metal

exchange processes. Thus, the formation of carbonyl-

inserted products 7-9 presented an alternative, com-

petitive pathway. The yields of 7-9 were markedly

improved when the synthesis was carried out under a

CO atmosphere. Stabilization of complexes with metal-

alkyl σ-bonds by carbonyl insertion to afford σ-bonded

acyl analogs is well-known in the chemistry of alkyl

complexes of cobalt and manganese carbonyls³⁸and was

also reported for (η¹:η⁶-C₆H₄RC(O)Mn(CO)₃L₂)Cr(CO)₃

(L ) CO, P(OMe)₃, L₂) diphos, tmeda).^3a,cThe com-

plexes 4-9 are stable in solution under an inert

atmosphere.

4

coupling with a constant of J ) 0.7-0.9 Hz. The two

three-bond coupling constants for H(4) of the thienyl and

selenyl rings are the same, with the exception of those

observed for 1 and 13, and appear as a triplet in the

1

spectra. The chemical shifts in the H NMR spectra of

7 and 9 are further downfield, revealing less electron

density on the thienyl and selenyl rings when compared

to 1 and 3, which can be ascribed to the electron-

withdrawing properties of the acyl substituent. The

same is found when 5 is compared with 2. The broad

signals of lower intensity are associated with Mn(CO)₅

at δ 208 ppm, and the chemical shifts for Cr(CO)₃at δ

236 in 1-3 are absent in the ¹³C NMR spectra of 4-6.

Instead, a broad signal for Mn(CO)₃at δ 220 and two

resonances of different intensity at δ 218 (cis-CO) and

222 (trans-CO) are observed. The positions of these

signals are consistent with values reported for com-

plexes which contain the same metal carbonyl frag-

ments.³⁷Notable is the relatively high value for the ipso

carbon of the thienyl and selenyl ring σ-bonded to

chromium (δ (ppm) 164 (4), 159 (5), and 177 (6)), which

is indicative of some carbene character in the σ-bond.

Initially the spectrum of 6 was recorded at -20 °C ,but

only resonances of Mn(CO)₃, Cr(CO)₅, and the C(2)

carbon of the selenyl ring were observed. When the

measurement was repeated at higher temperature (10

Temperature plays an important role during the

lithiation of π-coordinated thiophene. Treatment of the

chromium thiophene precursor with 3 equiv of n-BuLi

at -70 °C afforded the monolithiated intermediates,

which subsequently reacted with 3 equiv of Mn(CO)₅Hal

to give the novel complexes 10 and 11. The complexes

1, 2, 4, and 5 did not form under these conditions. The

black crystals of 10 and 11 are less polar than (π-

(36) Erker, G.; Petrenz, R.; Kru¨ger, C.; Lutz, F.; Weiss, A.; Werner,

S. Organometallics 1992, 11, 1646.

(37) Mann, B. E. Adv. Organomet. Chem. 1974, 12, 135.

(38) Calderazzo, F. Angew. Chem., Int. Ed. Engl. 1977, 16, 299.

Preferred Coordination Sites for Metal Fragments

Organometallics, Vol. 16, No. 19, 1997 4065

Sch em e 7

Sch em e 8^a

thiophene)Cr(CO)₃and are very soluble in dichlo-

romethane. To explore a foundation for understanding

the formation of 10 and 11, two possible reaction

pathways were considered. The trimetallic carbene

moiety X can be regarded as a resonance form of an

O-metalated acylate accompanied by a halogen elimina-

tion and subsequent carbonyl substitution at a second

Mn(CO)₅Hal. Heterobimetallic Fischer carbene com-

plexes are readily synthesized by metalation of acylate

intermediates at the oxygen and have been reported in

the literature.³⁹The resonance form Y is the result of

the acyl group of 7 or 8 and the halo ligand of a second

Mn(CO)₅Hal group substituting carbonyl ligands.

Whereas assignments of bands to ν(CO) vibrations of

the Cr(CO)₃unit were made, it was impossible to assign

bands for the vibrations of the two different Mn(CO)₄

units, which displayed only five bands due to overlap-

ping. The vibration associated with the bridging acyl

carbonyl was not observed. The ν(CO) vibrations for Cr-

(CO)₃are unaffected by the formation of a five-

membered ring with a second Mn(CO)₄moiety and are

practically the same as those observed for 7 and 8.

Furthermore, this also applies for the chemical shifts

of the protons of the thienyl ring, which are shifted

slightly downfield compared to those for 7 and 8. Final

confirmation of the structures of 10 and 11 was obtained

from a single-crystal X-ray diffraction study of 10. Even

though the structural units MnsHalsMn, cis-Mn-

(CO)₄L₂, MsOdC, and (π-thiophene)Cr(CO)₃are well-

represented in the literature,⁴⁰we are unaware of any

reported examples of all of them present in a single

compound. The mechanism for the formation of 10 and

11 is complex in nature, and among the factors to be

considered in proposing such a mechanism are the large

excess of butyllithium required, the lithiated thienyl

chromium precursor not affording the target compound

when reacted consecutively with 2 equiv of Mn(CO)₅Hal,

and the fact that no substitution reaction was observed

when 7 or 8 was treated with Mn(CO)₅Hal, even under

forcing reaction conditions.

a

Legend: (i) Re(CO)₅Cl, Et₃OBF₄; (ii) Re(CO)₅OTf.

corresponding hydroxy(methyl)carbene complex.⁴²An-

gelici and co-workers⁴³synthesized a series of carbene

complexes of rhenium, [Re(carbene)(CO)₄X], which con-

tain five-membered cyclic carbene ligands by a halide-

catalyzed reaction of coordinated carbonyls and thio-

carbonyls with three-membered heterocycles. The

conversion of anionic [Re(CO)₅]^-by alkyl halides or the

decarboxylation of the acyl complexes RC(O)Re(CO)₅(R

) alkyl, aryl) are general methods⁴⁴for the synthesis

of alkyl complexes of rhenium carbonyls. Therefore, it

is anticipated that the reaction of Na[Re(CO)₅] with (η⁵-

2-bromothiophene)Cr(CO)₃should lead to the target

compound (η⁵-SCHCHCHC{Re(CO)₅})Cr(CO)₃(13). The

electrophilicity of Re was increased by exchanging the

chloro ligand for a triflate ion, and subsequent reaction

with the lithiated precursor afforded 13 in low yield.

Experiments to facilitate the metal exchange process

{η¹-Re, η⁵-Cr} f {η¹-Cr, η⁵-Re} according to conditions

used for the corresponding Mn-Cr conversions failed,

and no metal exchange was observed. Instead, the

labile Re-thienyl ligand was substituted by solvent

molecules, giving Re{thienyl}(CO)₅(17), which was

isolated and identified by IR and ¹H and ¹³C NMR

spectra. The Re results focused our attention on the

importance of relative bond strengths of metal-thienyl

σ-bonds and the lability of metal carbonyl ligands of Re

compared to Mn. A carbonyl ligand is more easily

displaced by PPh₃in MeMn(CO)₅compared to MeRe-

(CO)₅,⁴⁵and the energy for the Mn-Me σ-bond dissocia-

tion is lower compared to the Re-Me bond.⁴⁶Therefore,

a stronger Re-thienyl σ-bond and less labile carbonyl

ligands could account for the bimetallic complex, with

the Re fragment being inert toward a metal site

exchange. However, Angelici and co-workers⁴⁷synthe-

sized [(η⁵-thiophene)Mn(CO)₃]⁺from Mn(CO)₅CF₃SO₃

In contrast, nucleophilic attack of (η⁵-SCHCHCHC-

Li)Cr(CO)₃occurs at a cis carbonyl of Re(CO)₅Cl, which

is consistent with reactions of organolithium reagents

with Re(CO)₅Cl.⁴¹The resulting salt was alkylated to

the corresponding bimetallic carbene complex 12, which

is stable in air for short periods of time but loses the

Cr(CO)₃fragment in polar solvents. Protonation of the

acetyl anion cis-[Re(CO)₄{C(O)Me}Cl]^-afforded the

(42) (a) Darst, K. P.; Lukehart, C. M. J . Organomet. Chem. 1979,

171, 65. (b) Darst, K. P.; Lenhert, P. G.; Lukehart, C. M.; Warfield, L.

T. J . Organomet. Chem. 1980, 195, 317.

(43) (a) Singh, M. M.; Angelici, R. J . Inorg. Chem. 1984, 23, 2699.

(b) Singh, M. M.; Angelici, R. J . Inorg. Chim. Acta 1985, 100, 57. (c)

Miessler, G. L.; Kim, S.; J acobson, R. A.; Angelici, R. J . Inorg. Chem.

1987, 26, 1690. (d) Wang, S.-J .; Angelici, R. J . J . Organomet. Chem.

1988, 352, 157.

(44) (a) Warner, K. E.; Norton, J . R. Organometallics 1985, 4, 2151.

(b) Hieber, W.; Braun, G.; Beck, W. Chem. Ber. 1960, 93, 901. (c) Casey,

C. P.; Baltusis, L. M. J . Am. Chem. Soc. 1982, 104, 6347.

(45) (a) Kraihanzel, C. S.; Maples, P. K. J . Am. Chem. Soc. 1965,

87, 5267. (b) Kinney, R. J . M.; Kaesz, H. D. J . Am. Chem. Soc. 1975,

97, 3066.

(46) (a) Brown, D. L. S.; Connor, J . A.; Skinner, H. A. J . Organomet.

Chem. 1974, 81, 403. (b) Mu¨ller, H. J .; Beck, W. J . Organomet. Chem.

1987, 330, C13. (c) Hendra, P. J .; Qurashi, M. M. J . Chem. Soc. A 1968,

2963.

(39) (a) Raubenheimer, H. G.; Fischer, E. O. J . Organomet. Chem.

1975, 91, C23. (b) Anslyn, E. V.; Santarsiero, B. D.; Grubbs, R. H.

Organometallics 1988, 7, 2137. (c) Balzer, B. L.; Cazanoue, M.; Sabat,

M.; Finn, M. G. Organometallics 1992, 11, 1759. (d) Sabat, M.; Gross,

M. F.; Fin, M. G. Organometallics 1992, 11, 745. (e) Petz, W. J .

Organomet. Chem. 1995, 456, 85.

(40) (a) Abel, E. W.; Wilkinson, G. J . Chem. Soc. 1959, 1501. (b)

Bannister, W. D.; Booth, B. L.; Green, M.; Haszeldine, R. N. J . Chem.

Soc. A 1969, 698. (c) Booth, B. L.; Green, M.; Haszeldine, R. N.;

Woffenden, N. P. J . Chem. Soc. A 1969, 920. (d) Cooney, J . M.;

Gommans, L. H. P.; Main, L.; Nicholson, B. K. J . Organomet. Chem.

1988, 349, 197. (e) Adams, R. D.; Chen, G.; Chen, L.; Wu, W.; Yin, J .

Organometallics 1993, 12, 3431.

(47) (a) Lesch, D. A.; Richardson, R. A., J r.; J acobson, R. A.; Angelici,

R. J . J . Am. Chem. Soc. 1984, 106, 2901. (b) Singer, H. J . Organomet.

Chem. 1967, 9, 135.

(41) Lukehart, C. M. Acc. Chem. Res. 1981, 14, 109.

4066 Organometallics, Vol. 16, No. 19, 1997

Waldbach et al.

Sch em e 9^a

a

Legend: (i) BuLi, Mn(CO)₅Hal; (ii) THF; (iii) HX, L.

and thiophene, but the analogous Re complex has not

yet been reported.

Ligand exchange reactions from the coordination

sphere of a metal are the most common of all reaction

types. In contrast, the displacement of one metal atom

by another in a fixed arrangement of ligands is unprec-

edented in the literature for simple organometallic

compounds. The exchange of metal atoms, each from

their own ligand environment to the site of the other,

involves the breaking and making of many bonds. After

the irreversible metal exchange conversion was realized,

the substitution of the chromium metal atom in (π-

thiophene)Cr(CO)₃by a manganese metal atom to give

[(π-thiophene)Mn(CO)₃]⁺became an attractive possibil-

ity. The stability of 4, however, is remarkable, and

initial efforts to displace the σ-bonded Cr(CO)₅fragment

by protonation of the thienyl group failed. Treatment

of 4 with HBF₄under a CO atmosphere and at elevated

temperatures afforded an unstable intermediate which

F igu r e 1. Repetitive scan spectra recorded for the conver-

sion of 2 at 15 °C: (a, top) experimental conditions [2] )

2.8 × 10^-4M, ∆t ) 1500 s, solvent acetone; (b, bottom)

experimental conditions [2] ) 2.4 × 10^-4M, ∆t ) 3600 s,

solvent cyclohexane.

displayed an additional carbonyl frequency (1658 cm^-1

)

Ta ble 6. Ra te P a r a m eter s for th e Exch a n ge

in the infrared spectrum but immediately converted

back to 4 during workup procedures. Further work in

this area has been initiated.

Rea ction of 2 in Va r iou s Solven ts a t 15 °C^a

solvent

DN

ꢀ

10⁶k_obs, s^-1

Kin etic Stu d ies. The metal exchange reaction of

1-3 into 4-6 can be studied by several spectroscopic

techniques, as conversions are associated with major

spectral changes. The UV-vis spectra of 1 or 2 in

acetone display maxima at 402 and 338 nm or 404 and

338 nm and a minimum at 365 or 363 nm, respectively.

Typical examples are given in Figure 1. The exchange

of coordination sites during the conversion of 1-3 into

4-6 are accompanied by characteristic changes in the

spectra, as is indicated by a decrease in the intensity of

the band at 402 nm and a small increase of the intensity

of the band at 338 nm for 1 and increases at 338 and

363 nm with a decrease at 404 nm for 2. The spectra

exhibit two isosbestic points at 507 and 377 nm for 1

and at 533 and 397 nm for 2. The largest spectroscopic

change was found at 402 nm, and kinetic measurements

were performed at this wavelength. The absorbance-

time traces could be fitted to a single-exponential

function, i.e. a first-order process, under all experimen-

tal conditions.

dioxane

tetrahydrofuran

acetonitrile

dimethyl sulfoxide

dichloromethane

cyclohexane

14.8

2.21

7.32

dec

36.3 ( 0.6

dec

14.1

29.8

36.2

49

8.9

2.02

20.7

dec

25.9 ( 0.8

11 ( 1

37.6 ( 0.5

acetone

17

a

DN ) donor number; ꢀ ) solvent dielectric constant.

Ta ble 7. Kin etic Da ta for th e In flu en ce of th e

Con cen tr a tion of 2 on th e Exch a n ge Rea ction in

Aceton e a t 20 °C

10⁵[2], M

10⁵k_obs, s^-1

10⁵[2], M

10⁵k_obs, s^-1

3.2

6.8

13.7

8.65 ( 0.13

7.16 ( 0.04

7.67 ( 0.04

29.8

41.8

7.55 ( 0.03

7.20 ( 0.03

The changes in absorbance in the spectrum of 2 during

the conversion in cyclohexane at 413 nm (decrease) and

359 nm (increase) are sufficient in magnitude to allow

the determination of k_obsat the two different wave-

lengths. In cyclohexane, however, the reaction is ac-

companied by a slower decomposition process, which

affected the final absorbance value and the calculated

rate constants. At 359 nm the infinity absorbance was

Table 6 lists rate constants for the metal exchange

process of 2 in different solvents, and Table 7 lists data

for acetone at different concentrations of 2. Measure-

ments for the conversion of 2 in THF or dichloromethane

result practically in no change in the position of the

bands, whereas a bathochromic shift is observed for the

spectrum in cyclohexane compared to that in acetone.

reached too soon and the rate constant (1.24 × 10^-5s^-1

)

was too high as compared to the data at 413 nm, where

the infinity value was not reached and the rate constant

(1.02 × 10^-5s^-1) was too low. Therefore, the true value

Preferred Coordination Sites for Metal Fragments

Organometallics, Vol. 16, No. 19, 1997 4067

Ta ble 8. Tem p er a tu r e Dep en d en ce for th e Meta l

F r a gm en t Exch a n ge Rea ction s of 1 a n d 2 in

Aceton e

temp, °C

10⁶k_obs(1), s^-1

10⁵k_obs(2), s^-1

10

5.48 ( 0.04

11.30 ( 0.01

22.90 ( 0.01

41.90 ( 0.09

2.20 ( 0.02

3.76 ( 0.03

7.53 ( 0.05

12.83 ( 0.06

26.48 ( 0.16

15

20

25

30

∆H^q, kJ /mol

∆S^q, J /Kmol

93 ( 2

-16 ( 6

86 ( 3

-30 ( 11

must lie somewhere in between. Fortunately, the

decomposition reaction is an order of magnitude slower

than the metal exchange process, such that for a given

time scale the data could be fitted to a first-order

process. The rate constants in Table 6 indicate that

solvent polarity plays no important role in the metal

exchange reaction, such that changes in electrostriction

or the direct participation of coordinated solvent mol-

ecules in the transition state can be ruled out. Fur-

thermore, the observed first-order rate constant is

independent of the employed complex concentration.

F igu r e 2. Effect of pressure on the reaction of 2. Experi-

mental conditions: [2] ) (1.5-1.7) × 10^-4M; solvent

acetone; temperature 18.7 °C.

1

The reaction of 2 was also followed kinetically with H

NMR in d₆-acetone (k_obs) (1.06 ( 0.06) × 10^-5s^-1at

10 °C) and in d₂-dichloromethane (k_obs) (1.46 ( 0.09)

× 10^-5s^-1at 20 °C). The formation of any intermediate

species could not be observed; the spectra only showed

the formation of small quantities of side products such

as (2-methyl-5-thienyl)Mn(CO)₅and Cr(CO)₆.

The temperature dependence of the rate constants for

the conversions of 1 and 2 was used to determine the

thermal activation parameters (see Table 8). Only a

narrow temperature range could be used, as excessive

decomposition occurred at 30 °C for 1 and 35 °C for 2,

and temperatures lower than 10 °C did not lead to

experimentally significant amounts of converted prod-

ucts 4 and 5, respectively. Activation enthalpies for the

site-exchange reactions of 1 and 2 were found to be 93

( 2 and 86 ( 3 kJ /mol, respectively, which are charac-

teristic for the cleavage of covalent bonds. The activa-

tion entropies are small and negative, almost zero

within the experimental error limits, in both cases and

point to an intramolecular exchange process.

Further investigations into the reaction mechanism

were conducted on 2 because of its greater stability as

compared to 1. In order to establish whether an

addition or dissociation of ligands is important in the

rate-determining step of the reaction, the volume of

activation for the exchange reaction of 2 in acetone was

determined. The measurements showed an increased

deviation from the isosbestic point with increasing

pressure during the conversion of 2 to 5, which could

be attributed to an acceleration of the decomposition

reaction at higher pressures. The kinetic data showed

that the reaction in acetone is independent of pressure

within the experimental error limits and that the

activation volume is approximately zero (-0.6 ( 0.5 cm³

mol^-1) for the site-exchange process (Figure 2). In light

of this observation and the fact that the metal exchange

process is independent of concentration, we can exclude

the possibility of an intermolecular arene displacement

route, for the reaction of which an intermediate of the

type A in Chart 2 is representative. Since the kinetic

data reveal no significant actvation entropy and activa-

tion volume, it is concluded that an intramolecular

mechanism is operative. Furthermore, the reactions

F igu r e 3. ¹³C NMR spectra recorded after the completion

of the metal exchange reaction of 2 in d₆-acetone in the

presence of ¹³CO (a) and in the absence of ¹³CO (b) (see

Results and Discussion). Experimental conditions: [2] )

4.0 × 10^-2M; temperature 20 °C.

afford first-order rate constants and are practically

solvent-independent, thus excluding a dominant role by

coordinating solvent molecules during the site-exchange

process. A solvent-dependent rollover type of mecha-

4068 Organometallics, Vol. 16, No. 19, 1997

Waldbach et al.

Ta ble 9. Selected Bon d Dista n ces (Å) a n d An gles

(d eg) for 7 a n d 10

Ch a r t 2

7

10

Distances

Cr-C(O)^a

Cr-C(7)

1.83(2)

1.853(8)

2.168(6)

2.188(7)

2.223(8)

2.154(7)

2.354(2)

1.790(7)

1.750(7)

1.403(8)

1.450(8)

1.399(9)

1.365(10)

1.275(7)

2.058(7)

1.867(9)

1.856(8)

1.784(8)

2.391(2)

1.893(8)

1.802(8)

2.360(2)

2.020(4)

2.16(1)

2.18(1)

2.23(2)

2.14(2)

2.369(5)

1.75(1)

1.73(1)

1.37(2)

1.54(2)

1.42(2)

1.37(2)

1.25(2)

2.07(1)

1.91(2)

1.84(2)

Cr-C(6)

Cr-C(5)

Cr-C(4)

Cr-S

S-C(7)

S-C(4)

C(7)-C(6)

C(7)-C(8)

C(6)-C(5)

C(5)-C(4)

C(8)-O(4)

C(8)-Mn

Mn-C(O)^b

Mn-C(11)

Mn(1)-C(9)

Mn(1)-Cl

Mn(2)-C(O)^b

Mn(2)-C(15)

Mn(2)-Cl

Mn(2)-O(4)

Sch em e 10

nism, of which an important intermediate B is shown

in Chart 2, is therefore unlikely. On the basis of the

kinetic data, the formation of an intermediate with

bridging carbonyls in the transition state and with both

metal centers attached to the carbon in the 2-position

in a η¹:η¹manner, similar to the bridging carbon of the

acetylide ligands in [Cp₂Ti(µ-CCBu^t)₂Pt(C₆F₅)₂],¹⁰is

proposed (Scheme 10). A planar thienyl ligand will

imply coordinative unsaturated metal fragments,

whereas coordination of the thienyl sulfur will probably

result in the destruction of the ring aromaticity and

cause a deviation in the planarity of the ring. Coordi-

nation to the sulfur atom of π-coordinated thiophene by

a second metal fragment invariably leads to the distor-

tion of the ring and the formal changing of the coordina-

tion mode from η⁵to µ₂-η⁴(S).^31b

Angles

S-C(7)-C(6)

114.3(10)

114.6(9)

113.2(10)

110.6(12)

124.2(8)

112.9(10)

131.1(11)

112.5(12)

122.6(9)

88.5(6)

109.2(5)

117.3(5)

112.7(6)

114.2(7)

125.5(5)

112.3(6)

133.0(7)

113.0(7)

122.2(5)

90.2(3)

S-C(7)-C(8)

S-C(4)-C(5)

C(7)-C(6)-C(5)

C(7)-C(8)-Mn

C(7)-C(8)-O(4)

C(6)-C(7)-C(8)

C(6)-C(5)-C(4)

O(4)-C(8)-Mn

C(4)-S-C(7)

C(8)-Mn(1)-Cl

C(8)-O(4)-Mn(2)

Mn(1)-Cl-Mn(2)

O(4)-Mn(2)-Cl

89.6(2)

134.7(4)

106.4(1)

86.9(1)

a

b

Average of M-C(carbonyl) distances. Average of M-C(car-

bonyl) distances for carbonyls trans to other carbonyls.

A control experiment was set up to monitor the

progress of the site-exchange process of 2 by ¹³C NMR

spectroscopy and to determine whether free CO is

formed at some stage during the reaction. For this

purpose a solution of 2 in acetone was treated with

Mn(CO)₅, a possible product in the presence of free CO,

was found. The inclusion of ¹³CO in Cr(CO)₆is probably

responsible for the interesting coupling observed at 212

ppm. The additional quartet of signals could be due to

¹³C-⁵³Cr coupling (J ) 26.5 Hz), which to our knowl-

edge has not been reported in the literature. From a

comparison of the spectra (Figure 3) it can be seen that

in the presence of ¹³CO both of the side products (2-

methyl-5-thienyl)Mn(CO)₅and Cr(CO)₆are formed to

a larger extent than in the absence of ¹³CO. The

intensity ratio of the signals for Mn(¹³CO), Cr(cis¹³CO),

and Cr(trans ¹³CO) is, however, within the experimental

error limits (10-20%) the same in both NMR spectra.

If free CO is formed during the site-exchange process,

¹³CO should in the described experiments bind to the

Cr fragment and cause a significant change in the signal

ratio. Since this is not the case, we conclude that no

significant release of coordinated CO occurs during the

exchange process, which is in line with all the other

arguments presented in favor of an intramolecular

process.

1

¹³CO and H and ¹³C NMR spectra were recorded at the

start and end of the site-exchange process (Figure 3a).

The experiment was also repeated under identical

conditions in the absence of ¹³CO (Figure 3b). The ¹³C

NMR spectra in the presence of ¹³CO showed, in

addition to the signals associated with the formation of

8, signals at 212, 210, 185, 139, 128, 127, 126, and 15

ppm. The signal at 185 ppm can be assigned to ¹³C-

labeled CO,⁴⁸whereas those between 15 and 139 ppm

can be assigned to the 2-methyl-5-thienyl fragment. The

broad signal at 210 ppm and the signal at 212 ppm can

be assigned to the Mn(CO)₅and Cr(CO)₆moieties,

respectively.⁴⁹The presence of Cr(CO)₆and Mn(CO)₅

is supported by the IR spectra recorded in hexane

following the experiment: the bands at 2000 cm^-1and

between 2100 and 2130 cm^-1are associated with these

fractions, respectively. The bands between 2100 and

2130 cm^-1for the Mn(CO)₅fragment suggest the

incorporation of ¹³CO. Important to note is that no

evidence for the formation of (2-methyl-5-acylthienyl)-

Str u ctu r a l Ch a r a cter iza tion of 7 a n d 10. The

structures of complexes 7 and 10 are represented in

parts a and b of Figure 4, respectively, and selected bond

distances and angles are given in Table 9. The atom-

numbering schemes for the molecular structures 7 and

10 differ from those used in the assignments of the

(48) Kalinowski, H.-O.; Berger, S.; Braun, S. ¹³C NMR Spectroscopy;

Georg Thieme Verlag: Stuttgart, Germany 1984.

(49) Mann, B. E. Adv. Organomet. Chem. 1974, 12, 135.

Preferred Coordination Sites for Metal Fragments

Organometallics, Vol. 16, No. 19, 1997 4069

Ta ble 10. Su m m a r y of Selected X-r a y Da ta for Com p lexes w ith σ,π-Br id ged Th iop h en e Liga n d s

complex

R¹

bond distance (Å)

C2C3 C3C4 C4C5 C2S

C5S M¹C2 M¹C3 M¹C4 M¹C5

1.370 1.424 1.370 1.714 1.714

bond angle (deg)^b

no. M¹

T^a

X

R²

a

b

c

d

e

ref

111.5 112.5 112.5 111.5 92.2 47

1

4

7

Cr

Mn

Cr CO

H

Mn(CO)₅

Cr(CO)₅

Mn(CO)₅

1.390 1.428 1.371 1.764 1.723 2.289 2.228 2.218 2.198 107.6 115.5 111.8 111.7 93.3

1.485 1.472 1.399 1.768 1.729 2.299 2.252 2.150 2.131 109.7 109.4 115.3 111.0 94.5

7

1.37

1.42

1.37

1.748 1.735 2.164 2.183 2.23

2.14

114.3 110.6 112.5 113.2 88.5

10 Cr CO

Mn₂(CO)₈Cl 1.403 1.399 1.365 1.790 1.750 2.168 2.188 2.223 2.154 109.2 114.2 113.0 112.7 90.2

1.391 1.427 1.388 1.768 1.767 2.168 2.178 2.227 2.299 109.1 113.3 115.7 107.9 93.8

1.421 1.425 1.385 1.768 1.763 2.304 2.151 2.136 2.23 101.9 117.6 116.6 103.9 99.9

Cr CO Mn(CO)₅Mn(CO)₅

Mn

1

Mn(CO)₅Cr(CO)₅

a

b

T ) thiophene. Bond angles a-e are defined as follows:

are not entirely planar and the larger sulfur lies above

the C(7)-C(6)-C(5)-C(4) plane and away from the

chromium by 0.14(3) Å for 7 and 0.16(1) Å for 10. The

bond lengths of the bridging thiophene ligands in 7 and

10 display shorter C(7)-C(6) and C(5)-C(4) distances

compared to the C(6)-C(5) distance and C-S distances

which are longer than those of free thiophene.⁵²The

sulfur atom of the ring is trans to one of the carbonyl

ligands, which is a common feature of η⁵-bonded

thiophene carbonyl systems and was also observed for

the disorderly structure of (π-thiophene)Cr(CO)₅.⁵¹The

most remarkable feature of structures 7 and, to a lesser

extent, 10 is an acyl unit which is in the same plane as

the thiophene ring. This is manifested in C(6)-C(7)-

C(8)-O(4) torsion angles of 0.6 (7), 18.6 (10), and 3.7°

for the trimetallic complex (η¹:η¹:η⁵-C₄H₂{Mn(CO)₅}-

C(O)Mn(CO)₅)Cr(CO)₃.¹The planar acyl unit is also

shown by the angles between the planes described by

the thiophene ring and C(7)-C(8)-O(4)-Mn are only

4.3° for 7, 14.4 and 14.7° for 10, and 1.5° for the

trimetallic complex. In contrast, the acyl deviates by

34° for (η¹:η⁶-C₆H₅C(O)Mn(CO)₅)Cr(CO)₃,^3a39° for cis-

[(η¹:η⁶-C₆H₅C(O)Mn{P(OMe)₃}(CO)₄)Cr(CO)₃],^3cand 89°

for [NEt₄][(η¹:η⁶-o-C₆H₄(Cl)C(O)Fe(CO)₄)Cr(CO)₃].⁵³The

planar nature of the bridging ligand facilitates π-elec-

tron distribution from the thiophene ring and the

manganese to the oxygen of the inserted carbonyl.

Spectroscopic data support the removal of electron

density from the thienyl ring, as was illustrated by the

resonance structure shown in Scheme 5. The C-O bond

distance of the acyl group is 1.254 Å for 7, which is 0.051

Å longer than the C-O distance of MeC(O)Mn{P(OPh)₃}₂-

(CO)₂⁵⁴but 0.061 Å shorter than the C-O(carbene) bond

in MnCp(CO)₂{C(OEt)Ph}.⁵⁵The Mn-C(acyl) distances

of 2.066 Å for 7 and 2.058 Å for 10 are shorter than the

corresponding distance of 2.103 Å for (η¹:η⁶-C₆H₅C(O)-

Mn(CO)₅)Cr(CO)₃,^3aand 2.080 Å for cis-[η¹:η⁶-C₆H₅C-

(O)Mn{P(OMe)₃}(CO)₄)Cr(CO)₃]^3cand significantly

shorter than the value of 2.207 Å recorded for the Mn-

C(sp²) distance in Mn{C(O)CH₂CH₂Ph}(dppm)(CO)₂.⁵⁶

By comparison, the Mn-C(O) bond length of 10 is

shorter than the corresponding distance for 7, indicating

F igu r e 4. ORTEP drawings of 7 (a, top) and 10 (b,

bottom).

carbon atoms and protons in the NMR data. The Cr-C

distances are practically the same and together with the

Cr-S distances of 2.369(5) Å for 7 and 2.354(2) Å for

10, which are significantly shorter than the typical

Cr-S single bond (ca. 2.50 Å; the Cr-S distance in [Cr-

{S(Et)CH₂CH₂Ph}(CO)₅] is 2.459(2) Å⁵⁰), are comparable

to or slightly longer than the average value of 2.32(2) Å

reported for Cr(η⁵-thiophene)(CO)₃.⁵¹The data clearly

indicate that all five atoms of the thiophene ring are

bonded to the chromium in an η⁵fashion. As is expected

for η⁵-bonded thiophene ligands, the rings in 7 and 10

(52) Harshbarger, W. R.; Bauer, S. H. Acta Crystallogr. 1970, B26,

1010.

(53) Heppert, J . A.; Thomas-Miller, M. E.; Swepston, P. N.; Extine,

M. W. J . Chem. Soc., Chem. Commun. 1988, 8, 1199.

(54) Berke, H.; Weiler, G.; Huttner, G.; Orama, O. Chem. Ber. 1987,

120, 297.

(50) Raubenheimer, H. G.; Boeyens, J . C. A.; Lotz, S. J . Organomet.

Chem. 1976, 112, 145.

(51) Bailey, M. F.; Dahl, L. F. Inorg. Chem. 1965, 4, 1306.

(55) Schubert, U. Organometallics 1982, 1, 1085.

(56) Carriedo, G. A.; Parra Soto, J . B.; Riera, V.; Solans, X.;

Miravitlles, C. J . Organomet. Chem. 1985, 297, 193.

4070 Organometallics, Vol. 16, No. 19, 1997

Waldbach et al.

more carbene character, but is significantly longer than

the Mn-C bond length of 1.96 Å for the carbene complex

cis-[Mn{COCH₂CH₂O}(CO)₄Cl].⁵⁷The C-O distance of

the bridging acyl group of 10 is longer than the corre-

sponding distance in 7 due to the coordination of the

oxygen to a second manganese fragment. Whereas the

Mn-Hal-Mn distances are the same for Mn₂(CO)₈Br₂,⁵⁸

a significant difference of 0.031 Å was found for the

Mn-Cl distances in the chloro bridge of 10. Again the

structural data support the spectroscopic data, and

contributions of both resonance structures X and Y in

Scheme 7 are evident. Selected bond distances and

angles relating to σ,π-bridged thiophenic ligands in

bimetallic and trimetallic complexes are summarized in

Table 10. Comparison of all bond distances in the frame

of the bridging thiophene ring, which, if added and

taken as a qualitative measure for the electron density

on the ring, indicates the shortest distance (highest

density) for free thiophene,⁵²which increases (lower

density) after π-coordination to Cr(CO)₃,⁵¹decreases

only slightly for a metal fragment in the 2-position (1),⁷

and gives the longest overall distance for an allylic-type

thienyl ring (4).⁷This trend supports conclusions drawn

from the NMR and infrared data. The structures of

both free thiophene⁵²and 1⁷(where the thienyl ring is

bonded to Cr(CO)₃in a η⁵fashion) afford C-C bond

distances in the thiophene ring that follow the sequence

of short (C(2)-C(3) ) 1.370 and 1.390 Å, respectively)-

long (C(3)-C(4) ) 1.424 and 1,428 Å, respectively)-

short (C(4)-C(5) ) 1.370 and 1.371 Å, respectively).

This ordering changes to long (1.485 Å)-long (1.472 Å)-

short (1.399 Å) from C(2) to C(5) for 4,⁷displaying allylic

character for the ring of the converted products on

bonding to Mn(CO)₃in an η⁵fashion. The latter is

supported by some carbene character in the σ-bond, as

is illustrated by the last two resonance structures in

Scheme 4. Given the standard deviations of 7, there

are no significant differences in the C-C distances of

the thiophene ring. Interestingly, an inductive effect

is also evident in the σ-bond of the bimetallic complexes,

where the largest angle of the ring is displayed at the

ipso carbon with an electron-withdrawing acyl substitu-

ent in the 2-position and the smallest angle with an

electron-donating metal fragment. However, this ob-

servation is more difficult to apply to the trimetallic

complexes.

and Mn fragments bonded to thiophene and selenophene

in this manner can exchange coordination sites. Some

analogy exists with the behavior of certain bimetallic

acetylide complexes reported in the literature.^8,9Kinetic

studies showed that the exchange process follows first-

order kinetics, is independent of the solvent, exhibits

an almost zero activation entropy, and is characterized

by a zero volume of activation. On the basis of these

and other observations on the site-exchange reaction,

an intramolecular metal exchange mechanism is pos-

tulated in which the first step is rate-determining and

involves bridging carbonyl ligands. Furthermore, the

metal exchange process is a clean, almost quantitative

process and no side, competitive reactions, such as a

carbonyl insertion into the metal-2-methyl-5-thienyl

σ-bond, were observed. From our results and results

reported for site occupation by metal fragments in

bimetallic acetylide chemistry,^8-11it seems reasonable

to conclude that these are but the first examples of a

far more general concept dealing with preferred coor-

dination sites for metal fragments in σ,π-bimetallic

complexes. In such compounds preferred coordination

sites may exist and, when a favorable reaction pathway

is present, a new type of reaction, i.e. an irreversible

site exchange of metal fragments, occurs. In the process

coordination sites are exchanged to give a converted

thermodynamically favored bimetallic complex with the

previously σ-bonded metal in the π-position and the

π-bonded metal fragment in the σ-position in the

converted product. In all the examples of metal ex-

change reactions observed so far, the product is always

less polar than the starting material, as observed in the

chromatographic separation. The metal exchange reac-

tion was only observed in systems where an sp²- or sp-

hybridized carbon atom is simultaneously involved in

σ and π metal-carbon bonding.

Ack n ow led gm en t. This research was financially

supported by a grant (S.L.) and a postdoctoral stipen-

dium (T.A.W.) from the Foundation of Research and

Development in Pretoria, the Deutsche Forschungsge-

meinschaft, and the Fonds der Chemischen Industrie.

Su p p or tin g In for m a tion Ava ila ble: Figures giving in-

frared and ¹H and ¹³C NMR spectra for 1 and 4 and X-ray

structural information, including tables of atomic coordinates,

bond lengths, bond angles, anisotropic thermal parameters,

and coordinates of hydrogen atoms for 7 and 10 and ORTEP

drawings of the structures (12 pages). Ordering information

is given on any current masthead page.

Con clu sion s. Two metal fragments, bonded directly

to an aromatic ring in a σ,π-fashion, are in electronic

contact via π-resonance effects. It was found that Cr

(57) Green, M.; Moss, J . R.; Nowell, I. W.; Stone, F. G. A. J . Chem.

Soc., Chem. Commun. 1972, 1339.

(58) Dahl, L. F.; Wei, C. H. Acta Crystallogr. 1963, 16, 611.

OM970494H

Article Doi

Preferred coordination sites for metal fragments in σ,π-bimetallic complexes. Detailed mechanistic insight from heteroatom, bridging ligand, solvent, temperature, and pressure effects on the irreversible exchange of coordination sites

DOI: 10.1021/om970494h

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Article abstract of DOI:10.1021/om970494h

Full text of DOI:10.1021/om970494h

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