Synthesis, structures, and redox properties of mixed-sandwich complexes of cyclopentadienyl and hydrotris(pyrazolyl)borate ligands with first-row transition metals

Organometallics 2002, 21, 3123-3138

3123

Syn th esis, Str u ctu r es, a n d Red ox P r op er ties of

Mixed -Sa n d w ich Com p lexes of Cyclop en ta d ien yl a n d

Hyd r otr is(p yr a zolyl)bor a te Liga n d s w ith F ir st-Row

Tr a n sition Meta ls

Tim J . Brunker, Andrew R. Cowley, and Dermot O’Hare*

Inorganic Chemistry Laboratory, University of Oxford, South Parks Road,

Oxford OX1 3QR, U.K.

Received J anuary 7, 2002

A series of [MCp^RTp]ⁿ⁺(Cp^R) Cp*(pentamethylcyclopentadienyl) or Cp (cyclopentadienyl),

Tp ) hydrotris(pyrazolyl)borate) complexes have been synthesized by reaction of a suitable

MCp^Rprecursor with KTp (Cp^R) Cp*; n ) 0, M ) Cr, Fe, Co, Ni; n ) 1, M ) Cr, Co, Ni; Cp^R

) Cp; n ) 0, M ) V, Co, Ni; n ) 1, M ) V, Co). Oxidation with [FeCp₂]⁺salts or reduction

with CoCp₂where appropriate provided easy access to the corresponding M(III) or M(II)

species. All of the complexes studied showed reversible M(III)/M(II) redox couples. Similarly

[MCp^RTpm]ⁿ⁺complexes have also been isolated (Tpm ) hydrotris(pyrazolyl)methane; Cp^R

) Cp*, n ) 1, M ) Fe; Cp^R) Cp, n ) 1 or 2, M ) Co). Analytical, NMR, IR, and mass

spectroscopic data are consistent with the formulation of these species as mixed-sandwich

complexes. Oxidation of VCpTp in MeCN solution yields [VCpTp(MeCN)]⁺, whereas similar

reaction in CH₂Cl₂solution yields [VCpTp]⁺. [VCpTp]⁺reacts with σ-donor ligands L, where

L ) CN^tBu and PMe₃, to form [VCpTpL]⁺species, but is unreactive when L ) CO, indicating

little π-base character to the V center. Crystal structure determinations were performed for

various complexes: CoCp*Tp is unique in displaying κ²-Tp and η⁵-Cp* coordination, whereas

[VCpTp]⁺, [CoCp*Tp]⁺, NiCp*Tp, and [CoCpTpm]²⁺all display mixed-sandwich structures

with κ³-Tp binding. The factors determining the relative conformations of the Cp^Rand Tp

ligands are discussed. The structures of [VCpTp(MeCN)]⁺and [VCpTp(PMe₃)]⁺were also

determined and show that considerable distortion of the sandwich moiety has occurred to

accommodate coordination of the extra ligand. For the MCp^RTp complexes a dependence of

ν_B-Hon Tp hapticity, ancillary ligand, and oxidation state is observed from IR spectroscopic

data in the solid state. IR data obtained in solution suggest that CoCp*Tp is in conformational

1

equilibrium, with a κ³-Tp structure also present. Some of the factors affecting H and ¹³C

NMR shifts are discussed and compared with relevant homoleptic analogues. Electrochemical

data reveal, in general, that MCp₂and MCp*₂species are more electron rich, although

comparisons are best confined to a per metal basis.

In tr od u ction

abilities of Tp, Cp, and their methylated analogues (Tp*

) hydrotris(3,5-dimethylpyrazolyl)borate, and Cp* )

pentamethylcyclopentadienyl) have been a matter of

some debate. Pertinent data have been recently sum-

marized and the conclusion has been drawn that no

consistent trend was observable across the transition

series and that donor ability is a function of the metal,

its oxidation state, and the ancillary ligands.⁵As each

ligand forms a series of simple sandwich complexes with

most divalent first-row transition metals, ML₂(MCp₂

is known for M ) V-Ni, MCp*₂is known for M ) Ti-

Ni,⁶and MTp₂is known for M ) Ti-Zn⁷), we found it

Hydrotris(pyrazolyl)borate (Tp) ligands have often

been described as analogues to cyclopentadienyl (Cp)

ligands, in that they are both monoanionic, six-electron

donors which facially coordinate to a metal.¹Thus Cp

is often replaced by Tp in organo-transition metal

systems as a way of modifying the reactivity of the metal

center by changing its coordination environment and

electronic properties.²Tp is the more sterically demand-

ing ligand;^3,4however, the relative electron-donating

(1) Trofimenko, S. Chem. Rev. 1993, 93, 943-980.

(2) For some recent leading references see (a) Ruba, E.; Simanko,

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384. (b) Tellers, D. M.; Bergman, R. G. Organometallics 2001, 20,

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metallics 1998, 17, 5384-5389. (d) Gutierrez-Puebla, E.; Monge, A.;

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VCH: Weinheim, 1992.

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2001, 2482-2483. M ) V and Cr: Dapporto, P.; Mani, F.; Mealli, C.

Inorg. Chem. 1978, 17, 1323-1329. M ) Fe - Zn: Trofimenko, S.

J . Am. Chem. Soc. 1967, 89, 3170-3177.

(3) Maitlis, P. M. Chem. Soc. Rev. 1981, 1-48.

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Inorg. Chem. 1994, 33, 3666-3676.

Publication on Web 06/18/2002

3124 Organometallics, Vol. 21, No. 15, 2002

Brunker et al.

somewhat surprising that relatively few mixed-sand-

wich MCp^RTp species are known (Cp^R) Cp or Cp*).

The synthesis and study of such complexes should

provide further insight into the effects of replacing Cp^R

with Tp at a transition metal center by direct compari-

son with the homoleptic sandwich analogues.

η⁸-COT and κ³-Tp bonding.²¹A series of trivalent

lanthanide Tp′ sandwich complexes have also been

prepared with COT as ancillary ligand, where Tp′ ) Tp

and Tp*.^22-24

In this paper we report the systematic investigation

of the synthesis of such complexes with 3d metals and

their structural characterization and redox properties.

We also report the synthesis of several mixed-sandwich

complexes with hydrotris(pyrazolyl)methane (Tpm).

Some of this work has appeared in preliminary form.^25,26

Previous studies of MCp^RTp complexes have been

mainly confined to the case where M ) Ru. RuCpTp was

first synthesized by reaction of KTp with RuCp(COD)-

Cl (COD ) 1,5-cyclooctadiene).^8,9Similarly, reaction of

[RuCp^R(CH₃CN)₃]⁺with a variety of poly(pyrazolyl)-

borate ligands gave a series of Ru(II) complexes (RuCpTp,

RuCp*Tp, RuCp[B(pz)₄], and RuCpTp*), and the oxi-

Syn th esis

In this work, we utilized the previously successful

synthetic strategy of reacting Cp^RM half-sandwich

complexes^27,28containing labile ancillary ligands with

the appropriate Tp anion. For Cp^R) Cp*, the avail-

ability of M(II) starting materials allowed ready access

to the MCp*Tp species (where M ) Cr (1), Fe (2), Co

(3), and Ni (4)), as shown in Scheme 1. Only in the case

where M ) Fe did we find serious contamination of the

product with species formed by ligand redistribution,

FeTp₂and FeCp*₂. A mixture of FeTp₂and 2 was

isolated after recrystallization from pentane; the almost

total insolubility of FeTp₂in acetone allows the two

components to be separated; however this is of limited

use, as 2 is somewhat labile in solution, with subsequent

crops of crystals containing FeTp₂also. We hoped that

a cationic species might be more stable; thus reaction

of [FeCp*(MeCN)]⁺with Tpm in MeCN solution gave

8⁺[PF₆]^-, which could be isolated as a green solid by

precipitation with Et₂O. 8⁺is also unstable in solution,

with attempts to grow crystals leading to extensive

decomposition. [CoCp*Cl]₂was found to be preferable

to CoCp*(acac) as the starting material for the synthesis

of 3, as we found some contamination of the product

with CoTp₂in the latter case also. As yet, attempts to

synthesize a MCp*Tp complex with Mn and V have been

unsuccessful, maybe in part due to a lack of a suitable

starting material. Reaction of MnCl₂with 1 equiv each

of LiCp* and KTp gave only the homoleptic species.

Similarly, reaction of [Cp*VCl₂]₃^29,30with KTp or TlTp,

or VTpCl₂(THF)¹⁴with Cp*SnBu₃,³¹gave VTp₂as the

only isolable product. Formation of the mixed-sandwich

complex by displacement of a Cp* ring from VCp*₂³²was

also attempted: no reaction was observed to occur even

after prolonged reflux and only starting material was

recovered from the reaction mixture.

dized derivative [RuCpTp*]⁺was also isolated.¹⁰

A

preliminary communication has reported the synthesis

of [Cp^RCoTp′]⁺from CoCp^R(CO)I₂(Cp^R) Cp, Cp*; Tp′

) Tp, B(pz)₄) and [RhCp*Tp′]⁺from [RhCp*Cl₂]₂(Tp′

) Tp and B(pz)₄). This did not include any experimental

details or characterizing data, however.¹¹The authors

did note their failure to prepare FeCpTp and NiCpTp

from FeCp(CO)₂Cl and NiCp(PPh₃)Cl, respectively.

Several other references to attempted preparations of

FeCpTp have also appeared; in each case the isolated

products were FeCp₂and FeTp₂.^10,12The crystal struc-

ture of [RhCp*Tp]⁺[PF₆]^-and preparative details for

this and its B(pz)₄analogue were subsequently reported

in a full paper.¹³Some mixed Cp^R/Tp complexes of Ti

and V were synthesized by Manzer,¹⁴including VCpTp,

TiCpTpCl₂, TiCpTpCl, and TiCp₂Tp′ (Tp′ ) Tp, B(pz)₄,

and H₂B(pz)₂). Another related complex containing Cp,

Tp, and ancillary CO ligands is the fluxional species Mo-

(κ²-Tp)(η⁵-Cp)(CO)₂.¹⁵

There has been some interest in preparing sandwich

complexes with other carbocyclic aromatic ligands and

Tp. The preparation of [Ru(C₆H₆)Tp′]⁺(Tp′ ) Tp and

B(pz)₄) and the cyclobutadienyl complexes [Co(Ph₄C₄)-

Tp]⁺was also the subject of previous reports,¹¹and the

crystal structure of [Ru(η⁶-C₆H₆)(B(pz)₄]⁺was subse-

quently described.^13,16Several other Tp-containing spe-

cies with a variety of substituted arenes have been

isolated more recently, including [Ru(C₆H₆)Tp*]⁺,^17,18

and the reactivity of [Ru(C₆H₆)Tp]⁺(and other substi-

tuted-arene analogues) with nucleophiles has also been

studied.^19,20Mixed cyclooctatetraene (COT)/Tp′ (Tp′ )

Tp and Tp*) sandwich complexes of Ti(III) have been

reported, and the X-ray crystal structure of the Tp

complex has been determined: the Ti center displays

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Mixed-Sandwich Complexes

Organometallics, Vol. 21, No. 15, 2002 3125

Sch em e 1. Syn th esis of MCp *Tp a n d MCp *Tp m Com p lexes

Sch em e 2. Syn th esis a n d Oxid a tion Ch em istr y of VCp Tp

Oxidation of MCp*Tp to the corresponding M(III)

species (where M ) Cr, Co, and Ni) was achieved by

reaction with [FeCp₂]⁺[PF₆]^-. For M ) Cr and Ni, the

reaction was performed in THF solution, from which the

[PF₆]^-salts of 1⁺and 4⁺precipitated. The oxidation of

3 to 3⁺[PF₆]^-was carried out in MeCN; however use of

this solvent in the oxidation of 4 did not lead to 4⁺[PF₆]^-,

but a light brown solid whose identity is unclear (vide

infra).

We followed Manzer’s route for the synthesis of

VCpTp, 5, by the reaction of VCp₂Cl with KTp in THF,

as shown in Scheme 2. Adopting a slight modification

to the workup procedure enabled isolation of clean 5 in

57% yield. This is an unusual reaction involving reduc-

tion of V(III) and displacement of a Cp^-ligand; however

we were unable to identify the dark purple oily byprod-

uct that was also formed. Degradation of Tp ligands in

reaction with V(III) species has been observed in several

instances, and such a process may be occurring here.³³

Reaction of VCp₂with 1 equiv of KTp in THF was also

found to produce 5, although this approach is less

satisfactory, as varying amounts of VTp₂were also

isolated. Cp displacement reactions are well known for

VCp₂;^34,35presumably Tp is a sufficiently strong ligand

to displace both rings. The affinity of V(II) for N donor

ligands has also previously been noted.³⁶

Oxidation of 5 in MeCN with [FeCp₂]⁺[PF₆]^-gave a

blue-green crystalline solid identified as [VCpTp(Me-

CN)]⁺[PF₆]^-, 5(MeCN)⁺[PF₆]^-, the 16-electron mono-

(33) Etienne, M. Coord. Chem. Rev. 1996, 156, 201-236.

(34) J onas, K.; Wiskamp, V. Z. Naturforsch. B 1983, 38b, 1113-

1121.

(35) J onas, K. Angew. Chem., Int. Ed. Engl. 1985, 24, 295-311.

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S.; Smeets, W. J . J .; Spek, A. L. Inorg. Chem. 1990, 29, 1302-1306.

3126 Organometallics, Vol. 21, No. 15, 2002

Sch em e 3. Syn th esis of MCp Tp a n d MCp Tp m Com p lexes of Co a n d Ni

Brunker et al.

MeCN adduct of 5⁺. Oxidation of 5 was performed in

the absence of a coordinating solvent by reaction with

[FeCp₂]⁺[BAr′₄]^-(Ar′ ) 3,5-bis(trifluoromethyl)phenyl)

in CH₂Cl₂. [BAr′₄]^-is well known as a noncoordinating

counterion,³⁷and the solubility it confers on metalloce-

nium salts has recently been demonstrated.³⁸[FeCp₂]⁺[B-

Ar′₄]^-is soluble in both Et₂O and CH₂Cl₂, allowing its

use as a homogeneous oxidant in these solvents. A deep

pink solid was isolated and identified as the unsolvated

14-electron V(III) species [VCpTp]⁺[BAr′₄]^-, 5⁺[BAr′₄]^-.

Oxidation of VCpTp in acetone solution with [FeCp₂]⁺[P-

F₆]^-initially gave a green-blue solid, presumably

[VCpTp(acetone)]⁺[PF₆]^-, which on drying in vacuo

slowly turned orange-brown. The IR spectrum of this

solid closely resembled that of 5⁺. Redissolution of this

solid in acetone regenerated the blue-green solution.

Similarly, addition of THF to solid [VCpTp]⁺[PF₆]^-

turned the solid from red-brown to green: removal of

the solvent in vacuo regenerated the red-brown solid.

Addition of Et₂O to a solution of 5⁺[BAr′₄]^-in CD₂Cl₂

caused no color change, and the resonances of the cation

immediate color change from dark red to blue-green and

the isolation of a light blue solid after workup. Elemen-

tal analysis data were consistent with the formation of

the adducts [VCpTp(CNBu^t)]⁺[BAr′₄]^-, 5(CNBu^t)⁺[B-

Ar′₄]^-, and [VCpTp(PMe₃)]⁺[BAr′₄]^-, 5(PMe₃)⁺[BAr′₄]^-.

Attempts to bind CO to 5⁺were unsuccessful however.

Oxidation of 5 in acetone under a CO atmosphere

yielded a brown solid after removal of solvent, whose

IR spectrum contained no CO stretches and was similar

to that of 5⁺. Treatment of 5⁺[BAr′₄]^-in CH₂Cl₂with

CO under elevated pressure (5 bar CO pressure in a

Fischer-Porter bottle) again yielded only starting mater-

ial.

For the synthesis of mixed Cp/Tp Co complexes, we

first synthesized the Co(III) species by the reaction of

CoCp(CO)I₂with KTp in THF (Scheme 3). A dark purple

precipitate, [CoCpTp]⁺[I]^-, 6⁺[I]^-, was isolated in almost

quantitative yield. The reduction of 6⁺[I]^-with CoCp₂

in THF then proceeds smoothly to give good yields of

CoCpTp, 6. The success of this route prompted us to

attempt to synthesize [CoCpTp*]⁺by the analogous

reaction with KTp*. However, instead of the putative

mixed-sandwich complex, we isolated CoTp*I.³⁹In this

reaction the starting material has been reduced from

Co(III) to Co(II) in conjunction with displacement of

Cp^-, CO, and I^-. No other Co-containing product was

identified, so it is assumed that a corresponding organic-

based oxidation is occurring. The Tpm analogue [CoCp-

Tpm]²⁺[I]^-₂, 9²⁺[I]^-₂, was formed on reaction of CoCp-

(CO)I₂with Tpm in THF. Reduction of 9²⁺(as its [PF₆]^-

1

remained unchanged in the H NMR spectrum, indicat-

ing no adduct formation in this case.

In light of this behavior, further investigations into

the coordinating properties of 5⁺were conducted. Thus

oxidation of 5 with [FeCp₂]⁺[BAr′₄]^-in CH₂Cl₂followed

by treatment with excess CN^tBu or PMe₃led to an

(37) Brookhart, M.; Grant, B.; Volpe, A. F., J r. Organometallics 1992,

11, 3920-3922.

(38) Chavez, I.; Alvarez-Carena, A.; Molins, E.; Roig, A.; Maniuk-

iewicz, W.; Arancibia, A.; Arancibia, V.; Brand, H.; Manriquez, J . M.

J . Organomet. Chem. 2000, 601, 126-132.

(39) Brunker, T. J .; O’Hare, D. Unpublished results.

Mixed-Sandwich Complexes

Organometallics, Vol. 21, No. 15, 2002 3127

salt) with CoCp₂in THF lead to the formation of a

yellow solid. Analysis and H NMR spectroscopy of this

can be attributed to coordinated MeCN. All these

observations support the notion that 4⁺is labile in

coordinating solvents and that [TpNi(solvent)_x] adducts

are readily formed. [NiCp₂]⁺is also of low stability in

solution.⁴³

1

material were consistent with a mixture of the desired

product [CoCpTpm]⁺[PF₆]^-, 9⁺[PF₆]^-, and [CoCp₂]⁺[P-

F₆]^-; however we were unable to further purify this

mixture by fractional crystallization. Further attempts

to reduce 9²⁺with the anionic complex [MnCp*₂]^{- 40}

(thus leaving a neutral byproduct) also resulted in the

isolation of a similar mixture, leading us to conclude

that 9⁺[PF₆]^-is labile in solution, forming [CoCp₂]⁺as

a decomposition product.

For the synthesis of NiCpTp, 7, we turned to

[NiCp(COD)]⁺[BF₄]^-(COD ) 1,5-cyclooctadiene) as a

NiCp source;⁴¹the reaction of NiCpCl(PPh₃) with KTp

has been reported to not give the desired mixed-

sandwich product.¹¹Thus reaction with KTp in acetone

solution gave, after recrystallization from pentane, 7 as

well as quantities of NiTp₂and NiCp₂. This reaction is

capricious; repetition under the same conditions gave

these products in varying ratios, and in some reactions

no 7 was observed at all. 7 and NiCp₂can be separated

by sublimation of NiCp₂; nevertheless only small amounts

of 7 could be isolated at any one time. We found that

use of other solvents (MeCN, THF, and nitromethane)

gave NiTp₂as the only isolable species. ([NiCp(COD)]⁺[B-

F₄]^-is insoluble in THF, unstable in MeCN, but stable

and soluble in nitromethane.)

The [I]^-salts of 6⁺and 9²⁺were found to be much

less soluble than those with [PF₆]^-counterions; anion

exchange could be readily performed by simple meta-

thesis reactions. 9²⁺[I]^-is soluble only in H₂O but

2

slowly decomposes in aqueous solution; however ex-

change of [I]^-for [PF₆]^-renders 9²⁺soluble and stable

in polar organic solvents. A noticeable feature of the

solid [I]^-salts of 6⁺and 9²⁺is that they are considerably

darker in color than their [PF₆]^-analogues. This may

be due to an I^-to cation/dication charge-transfer transi-

tion in the solid state. Similar behavior has been seen

for [CoCp₂]⁺[I]^-, which shows an [I]^-to metal charge-

transfer band in solution.⁴⁴The electronic spectra of

both species in solution were not counterion dependent

and were superimposable, so this charge-transfer pro-

cess is presumably confined to the solid.⁴⁵The Tpm

complexes are distinctly more soluble than their Tp

analogues; for example [FeCp*Tpm]⁺[PF₆]^-is much

more soluble in CH₂Cl₂than [CoCp*Tp]⁺[PF₆]^-.

Ch a r a cter iza tion

Table 1 summarizes the basic characterizing data for

all compounds. The identity of the products was initially

established by elemental analysis and mass spectro-

scopy. IR spectroscopy provides a convenient method of

identifying the Tp complexes, particularly the paramag-

netic species, with characteristic absorptions for ν_B-H

and many strong absorptions in the fingerprint region.

¹H and ¹³C NMR spectroscopy of the diamagnetic

complexes (data are given in Tables 3 and 4) showed

resonance patterns consistent with three equivalent

pyrazolyl rings and unrestricted rotation of the Cp^Rring

on the NMR time scale. The spectra of the [PF₆]^-and

[I]^-salts of 6⁺and 9²⁺are essentially identical, sug-

gesting that the I^-ligand is not participating in the

coordination sphere of the metal in the latter case. At

room temperature in solution, 2 displays a ¹H NMR

spectrum typical of a paramagnetic species (see Table

5); however in the solid-state this complex is diamag-

netic. The solution spectrum shows a marked temper-

ature dependence which we have interpreted as arising

from a spin equilibrium between S ) 0 and S ) 2

electronic configurations in solution. In contrast, 8⁺is

diamagnetic in solution at room temperature, an ob-

servation that again illustrates the subtle dependence

of spin states in Fe(II) complexes on structure.

Attempts to form MCpTp complexes (where M ) Cr

and Mn) by reaction of KTp with CrCp₂and MnCpCl-

(TMEDA) (TMEDA ) N,N,N′,N′-tetramethylethylene-

diamine) both resulted in the isolation of MTp₂only. In

contrast to the reaction with VCp₂, the second Cp

displacement from CrCp₂appears to be faster than the

first.

P r op er ties

The neutral M(II) Cp^R/Tp complexes are soluble in

hydrocarbon, aromatic, ethereal, halogenated, and polar

solvents (such as acetone and MeCN). 2 is most unstable

in CH₂Cl₂solution, with decomposition to unidentified

products complete within several hours. As solids, 2, 4,

and 7 are stable in air for short periods, but in solution

all the M(II) species should be treated as air-sensitive.

The neutral complexes do not sublime easily. The

[M(III)]⁺[PF₆]^-salts are most soluble in MeCN or

acetone and are generally air-stable as solids and in

solution. 4⁺[PF₆]^-is unstable in solution, and its

1

decomposition was followed by H NMR spectroscopy

of a solution in acetone-d₆, whereby the initial set of

resonances attributed to the product are slowly replaced

by a second set of paramagnetically shifted peaks.⁴²The

electrospray mass spectrum of 4⁺contained a low-

intensity peak corresponding to the cation as well as

various fragmentation peaks, of which the two of highest

intensity correspond to the fragments [NiTp + MeOH]⁺

and [NiTp]⁺. (MeOH is utilized as the carrier solvent

in the spectrometer.) The IR spectrum of the product of

oxidation of 4 in MeCN solution contained absorptions

at 2480 cm^-1(ν_B-H) and at 2318 and 2291 cm^-1, which

SQUID magnetometry was used to determine the spin

state of the paramagnetic complexes in the solid state,

and the observed moments at 300 K are given in Table

1. We have previously noted the difference in spin states

between 3 (low spin) and 6 (high spin)²⁵and the high-

spin state of 1.²⁶The other complexes display moments

consistent with the expected ground states, although

also worthy of note is the S ) 1 ground state of the V(III)

species, 5⁺. A more detailed report on the magnetic

(40) Robbins, J . L.; Edelstein, N. M.; Cooper, S. R.; Smart, J . C. J .

Am. Chem. Soc. 1979, 101, 3853-3857.

(41) Salzer, A.; Court, T. L.; Werner, H. J . Organomet. Chem. 1973,

54, 325-330.

(43) Gribble, J . D.; Wherland, S. Inorg. Chem. 1990, 29, 1130-1132.

(44) Bockman, T. M.; Chang, H.-R.; Drickamer, H. G.; Kochi, J . K.

J . Phys. Chem. 1990, 94, 8483-8493.

(45) Brunker, T. J .; Green, J . C.; Barlow, S.; O’Hare, D. Manuscript

in preparation.

(42) The resonances of the paramagnetic decomposition species are

δ ) 57, 43, and -10; integration 6:6:1 (and a set of resonances

attributable to a diamagnetic species).

3128 Organometallics, Vol. 21, No. 15, 2002

Ta ble 1. Ba sic Ch a r a cter izin g Da ta for MCp ^RTp a n d MCp ^RTp m Com p lexes

Brunker et al.

anal. found (calc)

H

b

compound

color

C

N

IR data (cm^-1

2457

)

MS^c(m/z)

µ_eff(µ_B)^f

CrCp*Tp,1

dark purple

mauve

blue-green

red-brown

57.07(57.02)

41.70(41.86)

56.03(56.47)

55.85(56.04)

6.16(6.30)

4.70(4.62)

6.17(6.24)

6.16(6.19)

21.01(21.00)

14.90(15.41)

21.12(20.80)

20.06(20.64)

400

5.02

3.73

g

[CrCp*Tp]⁺[PF₆]^-, 1⁺

FeCp*Tp, 2

2519, 839[PF₆]

2463

400^d

336^e

407

CoCp*Tp, 3

(2448), 2435,

(2404)

1.85

[CoCp*Tp]⁺[PF₆]^-, 3⁺

NiCp*Tp, 4

dark blue

41.38(41.33)

56.33(56.08)

41.03(41.35)

50.91(51.10)

46.20(46.34)

37.49(37.31)

4.68(4.56)

6.18(6.19)

4.69(4.57)

4.59(4.59)

2.57(2.28)

3.57(3.52)

15.25(15.22)

20.65(20.65)

14.73(15.23)

25.57(25.54)

7.05(7.05)

2505, 838[PF₆]

2460

407^d

406

golden brown

black/brown

mid-green

deep pink

pale blue

2.93

1.91

3.69

2.81

2.78

[NiCp*Tp]⁺[PF₆]^-, 4⁺

VCpTp, 5

2496

2509

2518

2497; 2312

[MeCN],

2286[MeCN]

2501, 2212

[CN^tBu]

2538

406^d

329

[VCpTp]⁺[BAr′₄]^-, 5⁺

[VCpTp(MeCN)]⁺[PF₆]^-,

5(MeCN)⁺

19.06(19.04)

[VCpTp(CN^tBu)]⁺[BAr′₄]^-, pale blue-green 47.88(48.03)

5(CN^tBu)⁺

3.21(2.84)

2.62(2.86)

7.76 (7.69)

6.72(6.63)

[VCpTp(PMe₃)]⁺[BAr′₄]^-,

5(PMe₃)⁺

pale blue-gray

46.45(46.40)

CoCpTp, 6

yellow-green

purple

light green

green

purple

yellow

49.83(49.89)

36.22(36.24)

50.30(49.92)

42.41(43.66)

28.96(28.68)

4.53(4.49)

3.39(3.26)

4.93(4.49)

4.98(4.58)

2.48(2.41)

24.79(24.93)

18.09(18.11)

25.00(24.95)

15.71(15.27)

13.89(13.38)

2494

2501

2472

839[PF₆]

337

5.80

[CoCpTp]⁺[I]^-, 6⁺

NiCpTp, 7

337^d

337(M⁺+ H)

[FeCp*Tpm]⁺[PF₆]^-, 8⁺

[CoCpTpm]²⁺[PF₆]^-₂, 9²⁺

[CoCpTpm]⁺[PF₆]^-, 9⁺

338, 169^d

36.61(37.06)^a2.77(3.11)^a14.54(14.52)^a839[PF₆]

a

b

Complex was not isolated free of [CoCp₂]⁺; calculated analysis is for composition [CoCp₂PF₆]₁[CoCpTpmPF₆]_3.5

.

Spectra recorded as

d

KBr disks, and unless otherwise indicated, data refer to ν_B-H

.

^cAll EI mass spectra unless otherwise indicated. Spectra measured by

ES in MeOH or MeCN. ^eM⁺- pzH. ^fAs measured by SQUID magnetometry at 300 K. This compound is diamagnetic as a solid at 300

g

K, but paramagnetic at room temperature in solution.

Ta ble 2. P oten tia ls (m V) for M(III)/M(II) Red ox Cou p les vs [F eCp ₂]⁺/F eCp ₂for Hom olp etic a n d

Mixed -Liga n d Com p lexes of Tp a n d Cp /Cp * ^a

V

Cr

Fe

Co

Ni

complex

MTp₂

data

-820 (MeCN)^c62

ref

data

ref

data

-260

(CH₂Cl₂)

ref

this work -500^e

(CH₂Cl₂)

data

ref

data

ref

99

b

this work

MTpCp

-760 (CH₂Cl₂) this work

-950 (MeCN)

-590

this work -420

(CH₂Cl₂)

this work

(CH₂Cl₂)

MTpCp*

MTpmCp

MCp₂

-1590^f

this work

-890

this work -385

(CH₂Cl₂)

this work

(THF)

(CH₂Cl₂)

-295^g

(CH₂Cl₂)

-1350

-1110 (THF)^c

100

32

-960

32

0

76

-420

76

(MeCN)^d

-1450

(CH₂Cl₂)^d

-1970

(CH₂Cl₂)^d

-1220

MCp*₂

a

-570

76

(MeCN)^d

(CH₂Cl₂)^c

(CH₂Cl₂)^d

b

^aCV is reported to be complex with no reversible one-electron waves.³²No redox events observed in solvent window. ^cValue reported

vs SCE, corrected to [FeCp₂]⁺/FeCp₂using the value quoted for this couple in appropriate solvent/electrolyte combination quoted in ref

d

101. Values are corrected to [FeCp₂]⁺/FeCp₂using the author’s own quoted value for this couple in the same solvent/electrolyte combination.

^eValue is measured using Ag/AgCl reference electrode; couple is corrected to SCE using the authors’ own correction (E(Ag/AgCl) - E(SCE)

) 44 mV in aqueous solution and thence to [FeCp₂]⁺/FeCp₂as in footnote c. The value is essentially identical to that reported by Trofimenko

for the same couple in MeCN (-490 mV, after correction).⁸⁶^fThis couple is quasi-reversible, as indicated by a peak-to-peak separation

g

of 110 mV, which is greater than that of the [FeCp₂]⁺/FeCp₂couple (75 mV) under identical conditions. We also observe an irreversible

event at -1480 mV in the same solvent attributed to a Co(II)/Co(I) process.

properties and electronic structures of these complexes

is in preparation.⁴⁵

mum symmetry, in this case a mirror plane: these are

the “staggered” and “eclipsed” geometries illustrated in

Figure 1. RuCpTp and 6 adopt the former, while

[RhCp*Tp]⁺adopts the latter conformation. (Three-fold

rotational symmetry about the molecular axis requires

disorder of the Cp^Rring.) The eclipsed conformation

involves direct overlap of a pyrazolyl (pz) group with a

ring substituent (either H or Me), whereas a staggered

arrangement places two pz rings in close proximity to

substituents. We have determined the structures of

several of these complexes, and details of the refine-

ments are given in Table 6. Table 7 compares relevant

structural parameters for the simple mixed-sandwich

species described in this work and also those of 1 and

6.

Molecu la r Str u ctu r es of MCp ^RTp Sp ecies. Prior

to our investigations the structures of RuCpTp and

[RhCp*Tp]⁺[PF₆]^-had both been reported and show

regular η⁵-Cp^Rand κ³-Tp ligation to the metal center.^10,13

We have since communicated the structures of CoCpTp,

6, and CrCp*Tp, 1.^25,26As a consequence of a high-spin

d⁴ground state, 1 exhibits J ahn-Teller distortion,

resulting in one long and two short Cr-N bonds and

considerable variation in the Cr-C bond lengths. 6

displays regular geometry however, but with bond

lengths consistent with a high-spin Co(II) center. For

η⁵-Cp/κ³-Tp mixed-sandwich complexes two possible

ideal conformations can be adopted that preserve maxi-

Mixed-Sandwich Complexes

Organometallics, Vol. 21, No. 15, 2002 3129

Ta ble 3. ¹H a n d ¹¹B{¹H} NMR Sp ectr oscop ic Da ta for 18-Electr on Com p lexes (cou p lin g con sta n ts in Hz

given in p a r en th eses)

compound

solvent

D₂O

(CD₃)₂CO

CD₃CN

(CD₃)₂CO

CD₂Cl₂

(CD₃)₂CO

CD₃CN

pzH₃

pzH₄

pzH₅

CpH

Cp*H

Tpm CH

¹¹B

[CoTp₂]⁺[PF₆]^{- a}

[CoCp*Tp]⁺[PF₆]^-

[CoCpTp]⁺[I]^-

6.67 (2.2)

8.82 (2.20)

8.62 (2.20)

9.37 (2.44)

9.07

6.17

6.56

6.54

6.94

6.58

8.04 (2.5)

7.89 (2.44)

7.85 (2.44)

8.57 (2.93)

8.12

-13.8^e

-6.22^f

-6.12^f

1.55

6.42

7.12

[CoCpTpm]²⁺[PF₆]^-

[FeCp*Tpm]⁺[PF₆]^-²

[CoCp*₂]⁺[PF₆]^{- b}

[CoCp₂]⁺[PF₆]^{- c}

[CoCp*Cp]⁺[PF₆]^{- d}

9.33

8.3

1.47

1.78

5.95

5.20

2.02

a

b

d

Data from ref 50. Data from ref 32. ^cData from ref 102. Data from ref 75. ^{e 11}B NMR data from ref 103 measured in (CD₃)₂SO and

referenced to B(OMe)₃(δ ) 0). ^fReferenced to BF₃‚Et₂O.

Ta ble 4. ¹³C{¹H} NMR Sp ectr oscop ic Da ta for 18-Electr on Com p lexes

compound

solvent

pzC₃

pzC₄

pzC₅

CpC

Cp*Me

Cp* quat

Tpm CH

[CoTp₂]⁺[PF₆]^{- a}

[CoCp*Tp]⁺[PF₆]^-

[CoCpTp]⁺[I]^-

D₂O

(CD₃)₂CO

CD₃CN

(CD₃)₂CO

CD₂Cl₂

(CD₃)₂CO

CD₃CN

145.8

150.0

152.2

147.9

110.0

108.8

109.5

110.8

109.2

140.0

138.8

139.9

138.3

134.3

11.4

96.9

90.5

91.7

[CoCpTpm]²⁺[PF₆]^-

[FeCp*Tpm]⁺[PF₆]^-²

[CoCp*₂]⁺[PF₆]^{- b}

[CoCp₂]⁺[PF₆]^{- c}

[CoCp*Cp]⁺[PF₆]^{- d}

74.3

72.0

10.4

6.3

74.4

93.4

85.7

87.5

11.0

98.8

a

b

d

Data from ref 50. Data from ref 32. ^cData from ref 102. Data from ref 75.

highly bulky species [Co(1,2,3,4-i-PrCp)₂]⁺(2.09(1) Å)

however.⁵³The close proximity of Me groups C15 and

C17 to the pz rings containing N3 and N5 results in a

compression of the N3-Co-N5 angle as compared to

N1-Co-N5 and N1-Co-N3.

By comparison the 18-electron Cp-containing complex

9

²⁺[PF₆]^-adopts an eclipsed conformation; two views

2

F igu r e 1. Possible ideal staggered and eclipsed geometries

for MCp^RTp complexes (R ) Me, H).

of the structure of the dication are shown in Figure 4.

The average Co-N bond length (1.961(2) Å) is somewhat

shorter than that in 3⁺, as is the average Co-C distance

(2.061(2) Å), consistent with a dicationic species. The

Co-C bond lengths are longer than those observed in a

variety of [CoCp₂]⁺salts (Co-C 2.018(5), 2.019(5)

Å).^54-56We are not aware of any other structures of Tpm

complexes of Co(III); however the average Co-N bond

length is, as expected, substantially shorter than that

A view of the structure of 3 is given in Figure 2. Given

that all the other examples of these complexes adopt a

κ³-Tp binding mode, we were somewhat surprised to

discover that in this case the Tp ligand is actually κ².

The molecule sits on a crystallographic mirror plane,

which contains the unbound pz ring, B, Co, and C9. The

bond lengths around Co are consistent with a low-spin

species and much shorter than the corresponding dis-

tances in high-spin 6 (see Table 7). The average Co-C

bond length (2.078(2) Å) is similar to those observed for

a variety of low-spin Co(II)Cp^Rcomplexes,²⁵specifically

CoCp₂(2.096(8) Å)⁴⁶and CoCp*(acac) (2.089(5) Å).⁴⁷The

Co-N bond length (1.931(2) Å) is considerably shorter

than that in CoTp₂(2.129(7) Å), which is high-spin

also.⁴⁸Although the B-H bond of the Tp ligand points

toward the metal center, the Co‚‚‚H-B distance (3.082

Å) is not indicative of any interaction.

observed in the Co(II) complex [Co(Tpm)₂]²⁺[NO₃]^-

2

(2.115(3) Å).⁵⁷The structure of the dication is quite

regular, as indicated by almost equal N-Co-N and

N-C-N angles.

The structure of 4 contains two independent mol-

ecules in the asymmetric unit, and a view of one

molecule is shown in Figure 5. The structural param-

eters for both molecules are similar. The Ni-N bond

lengths (2.089(3) Å averaged over both molecules) are

identical to those in NiTp₂(average 2.093(2) Å).⁵⁸The

Oxidation of 3 leads to the 18-electron species 3⁺[PF₆]^-,

which adopts a κ³-Tp structure and a staggered confor-

mation. Two views of the cation are shown in Figure 3.

The average Co-N bond length (2.016(2) Å) is somewhat

longer than that in crystallographically characterized

[CoTp₂]⁺cations, 1.918(6)⁴⁹and 1.927(3) Å,⁵⁰and the

average Co-C bond length (2.113(3) Å) is longer than

observed in comparison to [CoCp*₂]⁺species with a

variety of counterions (average Co-C 2.051(6) Å).^51,52

This distance is similar (at the 3σ level) to that in the

(49) Curnow, O. J .; Nicholson, B. K. J . Organomet. Chem. 1984, 267,

257-263.

(50) Hayashi, A.; Nakajima, K.; Nonoyama, M. Polyhedron 1997,

16, 4087-4095.

(51) Miller, J . S.; Calabrese, J . C.; Harlow, R. L.; Dixon, D. A.; Zhang,

J . H.; Reiff, W. M.; Chittipeddi, S.; Selover, M. A.; Epstein, A. J . J .

Am. Chem. Soc. 1990, 112, 5496-5506, and references therein.

(52) Dixon, D. A.; Miller, J . S. J . Am. Chem. Soc. 1987, 109, 3656-

3664.

(53) Burkey, D. J .; Hays, M. L.; Duderstadt, R. E.; Hanusa, T. P.

Organometallics 1997, 16, 1465-1475.

(54) Meng, X.; Waterworth, S.; Sabat, M.; Grimes, R. N. Inorg. Chem.

1993, 32, 3188-3192.

(55) J agg, P. N.; Kelly, P. F.; Rzepa, H. S.; Williams, D. J .; Woollins,

J . D.; Wylie, W. J . Chem. Soc., Chem. Commun. 1991, 942-944.

(56) Silvestru, A.; Breunig, H. J .; Ebert, K. H.; Kaller, R. J .

Organomet. Chem. 1995, 501, 117-121.

(57) Astley, T.; Gulbis, J . M.; Hitchman, M. A.; Tiekink, E. R. J .

Chem. Soc., Dalton Trans. 1993, 509-515.

(46) Bunder, W.; Weiss, E. J . Organomet. Chem. 1975, 92, 65-68.

(47) Smith, M. E.; Andersen, R. A. J . Am. Chem. Soc. 1996, 118,

11119-11128.

(48) Churchill, M. R.; Gold, K.; Maw, C. E. Inorg. Chem. 1970, 9,

1597-1605.

3130 Organometallics, Vol. 21, No. 15, 2002

Ta ble 5. ¹H NMR Sp ectr oscop ic Da ta for P a r a m a gn etic Com p ou n d s

Brunker et al.

compound

δ (ppm)

solvent

CrCp*Tp, 1

85 (Cp*H), 16 (pzH), -12 (pzH), -50 (pzH)

95, -4, -20

C₆D₆

(CD₃)₂CO

C₆D₆

[CrCp*Tp]⁺[PF₆]^-, 1⁺

FeCp*Tp, 2

47.3 (Cp*H), 23.1 (pzH), 15.3 (pzH), -6.8 (pzH)

15, -30

CoCp*Tp, 3

NiCp*Tp, 4

45 (pzH), 41(pzH), 39 (pzH), -15 (BH), -90 (Cp*H)

54 (Cp*H), 28 (pzH), 23 (pzH), 22 (pzH), -5 (BH)

11, 9.5, -31, -90

12.5, 7.72 (s, 8H, o-CH-BAr′₄), 7.55 (s, 4H, p-CH-BAr′₄), 4.5, -29.6

-1.4

7.64 (s, 8H, o-CH-BAr′₄), 7.48 (s, 4H, p-CH-BAr′₄), 2.1, -1.9

7.73 (s, 8H, o-CH-BAr′₄), 7.56 (s, 4H, p-CH-BAr′₄), 2.2, -1.9

96.2 (CpH), 65.0 (pzH), 50.5 (pzH), -64.0 (pzH)

52 (pzH), 49 (pzH), 40 (pzH), -10 (BH), -32 (CpH)

94.0 (CpH), 84 (pzH), 48.5 (pzH), 47 (Tpm CH), -60 (pzH)

[NiCp*Tp]⁺[PF₆]^-, 4⁺

VCpTp, 5

(CD₃)₂CO

C₆D₆

[VCpTp]⁺[BAr′₄]^-, 5⁺

[VCpTp(MeCN)]⁺[PF₆]^-, 5(MeCN)⁺

[VCpTp(CN^tBu)]⁺[PF₆]^-, 5(CN^tBu)⁺

[VCpTp(PMe₃)]⁺[PF₆]^-, 5(PMe₃)⁺

CoCpTp, 6

CD₂Cl₂

CD₃CN

CD₂Cl₂

C₆D₆

NiCpTp, 7

C₆D₆

CD₂Cl₂

[CoCpTpm]⁺[PF₆]^-, 9⁺

Ta ble 6. Deta ils of Cr ysta l Str u ctu r e P a r a m eter s a n d Refin em en ts

3

3⁺[PF₆]^-

4

5⁺[BAr′₄]^-‚2CH₂Cl₂5(MeCN)⁺[PF₆]^-5(PMe₃)⁺[BAr′₄]^-

9

²⁺[PF₆]^-

2

formula

fw

size, mm

class

space gp

a, Å

b, Å

C₁₉H₂₅BCoN₆

407.19

0.3, 0.2, 0.03

ortho

Pnma (62)

11.0641(4)

11.1712(3)

16.1568(7)

90

C₁₉H₂₅BCoF₆N₆P C₁₉H₂₅BN₆Ni C₄₈H₃₁B₂C₁₄F₂₄N₆V C₁₆H₁₈BF₆N₇PV C₄₉H₃₆B₂F₂₄N₆PV C₁₅H₁₅CoF₁₂N₆P₂

552.16

0.16, 0.20, 0.30

mono

P2₁/c (14)

11.7592(2)

14.5587(3)

13.0853(2)

90

93.16(5)

90

2236.8

4

406.97

0.4, 0.6, 0.6

ortho

Pca2₁(29)

14.356(1)

19.730(1)

13.955(1)

90

3952.7

8

1326.15

0.5, 0.3, 0.2

triclinic

P1h (2)

12.5841(2)

13.7584(2)

17.3823(3)

110.282(1)

98.276(1)

99.804(1)

2713.80(7)

2

515.09

0.3, 0.3, 0.05

ortho

Pbca (61)

12.000(1)

15.632(1)

22.465(1)

90

4214.1

8

1268.37

0.4, 0.2, 0.2

mono

P2/c (13)

26.3352(5)

12.7266(2)

16.2206(2)

90

74.064(1)

90

5227.53(1)

4

628.20

0.4, 0.4, 0.15

triclinic

P1h (2)

9.9120(1)

10.8257(1)

12.0264(2)

107.630(1)

101.360(1)

111.794(1)

1069.88(2)

2

c, Å

R, deg

â, deg

γ, deg

V, Å³

Z

90

1996.97(12)

4

1.35

0.88

852

F

_calc, Mg m^-3

1.64

1.37

1.67

1.62

1.61

1.95

µ, mm^-1

0.91

1.00

0.505

0.619

0.349

1.074

F(000)

1132

1720

1356

2080

2544

624

θ range, deg

total no. of data

no. of unique data 2385

5.19-27.45

0-27.5

22 874

5337

0-26.4

28 467

7623

2.46-27.52

5.14-27.48

5.10-27.49

18 545

11267

2.22-27.49

4340

22 346

8949

8551

12376

4796

4762

[R(int))0.0347] [R(int))0.038]

[R(int))0.043] [R(int))0.0285]

[R(int))0.0443] [R(int) ) 0.0321] [R(int))0.0114]

no. obs reflns

GOF

1808^a

3708^b

1.041

7156^b

1.0479

8954^a

3278^a

1.013

7896^a

1.031

4530^a

1.230

1.024

1.028

R indices

R1)0.0363

wR2)0.0759

0.33, -0.33

R1)0.0473

wR2)0.0569

-0.68, 1.08

R1)0.0459

R1)0.0644

R1)0.0383

wR2)0.0808

0.36, -0.33

R1)0.0617

wR2)0.1435

0.99, -0.54

R1)0.0383

wR2)0.1051

0.98, -0.61

wR2)0.0397 wR2)0.1674

residuals, e Å^-3

-0.91, 1.18

1.46, -0.89

a

b

I > 2σ(I). I > 3σ(I).

Ta ble 7. Selected Bon d Len gth s a n d An gles for Bin a r y MTp (m )Cp ^RCom p lexes

4

5⁺[BAr′₄]^-

1

3

3⁺[PF₆]^-

6

9

²⁺[PF₆]^-

molecule 1

molecule 2

2

Bond Lengths (Å)

M-N

M-C

2.059(3)

2.055(3)

2.024(3)

2.292(4)

2.265(3)

2.247(3)

2.258(4)

2.287(4)

1.933

2.092(2)

2.123(2)

2.439(2)

2.372(3)

2.296(3)

2.319(3)

2.317(3)

2.399(3)

2.008

1.931(2)

2.009(2)

2.006(2)

2.034(2)

2.134(3)

2.079(3)

2.087(3)

2.137(3)

2.129(3)

1.724

2.069(2)

2.103(1)

2.268(3)

2.286(2)

2.305(2)

1.957

1.964(2)

1.962(3)

1.957(2)

2.068(2)

2.069(2)

2.054(2)

2.064(2)

2.050(2)

1.672

2.053(3)

2.085(3)

2.119(3)

2.262(3)

2.285(3)

2.269(4)

2.203(4)

2.215(3)

1.903

2.083(3)

2.077(3)

2.116(3)

2.238(3)

2.278(3)

2.288(3)

2.266(3)

2.237(3)

1.912

2.067(2)

2.079(2)

2.096(3)

1.686

1.551(2)

1.518(4)

M-Ct^a

B-N

1.534(5)

1.557(4)

1.549(4)

1.544(4)

1.540(4)

1.537(4)

1.569(4)

1.497(4)

1.531(4)

1.541(2)

1.543(3)

1.453(3)^b

1.444(3)^b

1.446(3)^b

1.538(5)

1.543(5)

1.544(5)

1.550(4)

1.532(5)

1.536(4)

Bond Angles (deg)

B-M-Ct^a

N-B-N

172.4

167.4

146.2

178.2

177.4

178.7^b

177.3

176.9

107.8(3)

108.7(3)

106.9(2)

88.46(10)

89.15(11)

90.35(10)

109.0(2)

110.0(2)

109.1(2)

88.72(9)

82.17(8)

83.06(8)

110.37(16)

106.8(2)

91.82(9)

108.2(2)

108.9(3)

105.3(3)

89.9(1)

109.26(13)

107.04(19)

88.22(5)

109.36(17)^b

109.18(17)^b

109.39(18)^b

87.30(8)

109.2(3)

108.1(3)

106.8(3)

88.41(11)

87.66(11)

85.3(1)

108.5(3)

107.2(3)

107.8(3)

88.07(11)

85.6(1)

88.3(1)

N-M-N

91.46(11)

83.75(11)

88.22(5)

86.00(7)

89.47(8)

87.67(8)

a

b

Ct ) Cp(centroid). For these parameters replace B with Tpm apical C (C15).

Ni-C bond lengths (2.254(3) Å) are significantly length-

ened compared to the sterically uncrowded systems

NiCp₂(average 2.164(7) Å)⁵⁹and bis(isodicyclopentadi-

enyl)Ni(II) (average 2.196(8) Å),⁶⁰but similar to the

sterically bulky complex Ni(C₅Ph₄H)₂(average 2.22(1)

Å).⁶¹Both molecules are in the staggered conformation,

Mixed-Sandwich Complexes

Organometallics, Vol. 21, No. 15, 2002 3131

F igu r e 2. View of structure of 3, with all H atoms omitted

for clarity (except B-H). Thermal ellipsoids are at 50%

probability, and primed atoms are generated by symmetry.

and again (as in 3⁺) there is some closure of the angle

between the pz rings in close proximity to the Cp*Me

groups.

The structure of 5⁺[BAr′₄] confirms that it is a 14-

electron sandwich complex with η⁵-Cp and κ³-Tp bond-

ing and no additional ligands. A view of the structure

of the cation is given in Figure 6. The cation is in an

eclipsed conformation: the average V-N bond length

is 2.046(3) Å, which is slightly shorter than the average

in [VTp₂]⁺, 2.073(2) Å.⁶²The average V-C bond length

is 2.270(4) Å, which is identical to that in [VCp₂-

(acetone)]⁺(2.265(4) Å).⁶³The V-Ct (Ct ) Cp centroid)

distance is shorter than those observed for the only two

crystallographically characterized 14-electron planar

metallocene species, Ti(C₅Me₄(SiMe₂Bu^t)₂(2.018(4) Å)⁶⁴

and Ti(C₅Me₄(SiMe₃))₂(2.020(2) Å).⁶⁵In comparison to

the B-V-Ct angles in 3⁺, 9²⁺, 4, and 6, this parameter

in 5⁺is distinctly nonlinear at 172.4°. The Ct-V-N

angle to N3 (132.7°) is opened considerably compared

to those to N1 and N5 (121.2° and 122.9°), which is

reflected in an inclination of the plane containing the

three ligating N atoms of the Tp ligand to the least-

squares plane of the Cp ring of 5.3(2)°. This distortion

is hard to justify on steric grounds, i.e., due to repulsions

between the C3-H bond and the pz ring that eclipses

it. 9²⁺adopts a much more regular geometry but has

much shorter M-C and M-N distances (in this instance

the angle between the two planes is 0.8° and the Ct-

Co-C angle, 178.7°).

F igu r e 3. Two views of the structure of 3⁺(anion not

shown) illustrating the staggered geometry. All H atoms

are omitted for clarity (except B-H), and thermal ellipsoids

are at 50% probability.

Molecu la r Str u ctu r es of [VCp Tp L]⁺Sp ecies. We

have also determined the structures of two adducts of

5⁺, 5(MeCN)⁺[PF₆]^-and 5(PMe₃)⁺[BAr′₄]^-; important

bond lengths and angles are detailed in Table 8. A view

of the structure of 5(MeCN)⁺[PF₆]^-is given in Figure

7. 5(MeCN)⁺contains a nine-coordinate V center with

η⁵-Cp and κ³-Tp bonding and a terminal nitrile ligand.

The angle between the least-squares plane of the Cp

ring and the plane of the three N donor atoms is now

opened to 21.7(1)°, reducing the Ct-V1-B1 angle to

161.7°. Similar ring-tilting is observed in [VCp₂L]⁺

species (where L is a unidentate donor), in which Ct-

V-Ct angles are found in the range 153.6-130.9°.⁶⁶The

V-pyrazolyl(N) bond lengths vary significantly, with

V1-N5 being 0.086(2) Å longer than the average of V1-

N1 and V1-N3, which are identical. These two short

bond lengths are significantly longer than the average

V-N distance in 5⁺. The V-N(MeCN) bond length

(58) Bandoli, G.; Clemente, D. A.; Paolucci, G.; Doretti, L. Cryst.

Struct. Commun. 1979, 8, 965-970.

(59) Seiler, P.; Dunitz, J . D. Acta Crystallogr., Sect. B 1980, B36,

2255-2260.

(60) Scroggins, W. T.; Rettig, M. F.; Wing, R. M. Inorg. Chem. 1976,

15, 1381-1390.

(61) Castellani, M. P.; Geib, S. J .; Rheingold, A. L.; Trogler, W. C.

Organometallics 1987, 6, 1703-1712.

(62) Mohan, M.; Holmes, S. M.; Butcher, R. J .; J asinski, J . P.;

Carrano, C. J . Inorg. Chem. 1992, 31, 2029-2034.

(63) Gambarotta, S.; Pasquali, M.; Floriani, C.; Chiesa-Villa, A.;

Guastini, C. Inorg. Chem. 1981, 20, 1-1178.

(64) Hitchcock, P. B.; Kerton, F. M.; Lawless, G. A. J . Am. Chem.

Soc. 1998, 120, 10264-10265.

(65) Horacek, M.; Kupfer, V.; Thewalt, U.; Stepnicka, P.; Polasek,

M.; Mach, K. Organometallics 1999, 18, 3572-3578.

(66) Holloway, C. E.; Melnik, M. J . Organomet. Chem. 1986, 304,

41-82.

3132 Organometallics, Vol. 21, No. 15, 2002

Brunker et al.

F igu r e 6. View of the structure of 5⁺(anion not shown)

with all H atoms omitted for clarity (except B-H). Thermal

ellipsoids are at 50% probability.

Ta ble 8. Str u ctu r a l P a r a m eter s for

5(MeCN)⁺[P F ₆]^-a n d 5(P Me₃)⁺[BAr ′₄]^-

5(MeCN)⁺

5(PMe₃)⁺

Bond Lengths (Å)

2.094(2)

2.100(2)

2.183(2)

2.271(2)

2.296(2)

2.330(2)

2.333(2)

2.290(2)

1.541(3)

1.534(3)

1.543(3)

2.165(2)

1.139(3)

V1-N1

V1-N3

V1-N5

V1-C1

V1-C2

V1-C3

V1-C4

V1-C5

B1-N2

B1-N4

B1-N6

V1-N7

N7-C15

V1-P1

2.109(2)

2.105(3)

2.188(3)

2.274(3)

2.318(4)

2.323(3)

2.296(3)

2.258(3)

1.554(4)

1.544(4)

1.546(4)

F igu r e 4. Two views of the structure of 10²⁺(anions not

shown) illustrating the eclipsed geometry. All H atoms are

omitted for clarity (except Tpm apical C-H), and thermal

ellipsoids are at 50% probability.

2.559(1)

Bond Angles (deg)

93.23(7)

N1-V1-N3

N1-V1-N5

N3-V1-N5

N1-V1-N7/P1

N3-V1-N7/P1

N5-V1-N7/P1

V1-N7-C15

N2-B1-N4

N2-B1-N6

N6-B1-N4

95.97(10)

79.44(9)

80.09(10)

78.56(7)

79.07(7)

147.75(7)

79.69(7)

81.50(7)

78.33(7)

79.74(7)

150.09(7)

173.89(17)

110.84(18)

105.89(17)

106.84(18)

111.8(3)

106.1(3)

105.7(2)

Torsion Angles (deg)

V1-N1-N2-C8

V1-N3-N4-C11

V1-N5-N6-C14

V1-N1-N2-B1

V1-N3-N4-B1

V1-N5-N6-B1

167.45

176.23

172.13

16.96

10.24

0.54

171.46

178.05

177.80

18.50

14.19

0.31

Other Parameters

1.971

V1-Ct^a(Å)

1.959

Ct-V1-B1 (deg)

ring tilt^b(deg)

Ct-V1-N1 (deg)

Ct-V1-N3 (deg)

Ct-V1-N5 (deg)

161.69

159.09

21.07(18)

134.09

129.84

107.87

21.66(10)

133.14

133.44

107.16

F igu r e 5. View of structure of 4, with all H atoms omitted

for clarity (except B-H). Thermal ellipsoids are at 50%

probability, and only one of the two independent molecules

is shown.

a

b

Ct ) Cp(centroid). Angle between least-squares plane of the

Cp ring and plane formed by the three N atoms of the Tp ligand

bonded to V.

MeCN ligand) deviates somewhat from linearity, being

173.9(2)°; this may be due to a crystal-packing effect. A

view of the cation of 5(PMe₃)⁺[BAr′₄]^-is shown in

Figure 8. The cation displays a geometry similar to that

of 5(MeCN)⁺, with an almost equal ring tilt and Ct-

(2.165(2) Å) is slightly longer than that observed in other

V(III) MeCN complexes: this distance is 2.09(1) Å in

[V₂Fv₂(MeCN)₂]²⁺(Fv ) fulvalenyl)⁶⁷and 2.087(4) Å in

[VBr₂(MeCN)₄]⁺.⁶⁸The V-N-C bond angle (to the

Mixed-Sandwich Complexes

Organometallics, Vol. 21, No. 15, 2002 3133

Ta ble 9. Con for m a tion s a n d M-Liga n d Dista n ces

in MCp ^RTp Sa n d w ich Com p lexes

complex

M-Ct (Å)

M-N (av) (Å)

conformation

[CoCpTpm]²⁺, 9²⁺

[CoCp*Tp]⁺, 3⁺

RuCpTp^a

1.672

1.724

1.777

1.820

1.909^c

1.933

1.957

2.008

1.961

2.016

2.128

2.150

2.089^c

2.046

2.092

2.108^d

E

S

E

S

E

S

[RhCp*Tp]^{+ b}

NiCp*Tp, 4

[VCpTp]⁺, 5⁺

CoCpTp, 6

CrCp*Tp, 1

a

Reference 10. Reference 13. ^cAverage of both molecules.

b

d

Average of short Cr-N bond lengths. Ct ) Cp(centroid), E )

eclipsed, S ) staggered.

graphically characterized M*(η⁵-Cp^R)(κ³-Tp/Tpm) com-

plexes. Intraligand repulsions are expected to be greater

with Cp* as ligand than with Cp, although it is unclear

which conformation should lead to least steric interac-

tion. 3⁺has the shortest M-ligand distances of Cp*-

containing complexes and is staggered, whereas [Rh-

Cp*Tp]⁺with longer M-ligand distances is eclipsed.

(The Tp ligand in this complex is significantly twisted

out of ideal C_3vsymmetry to accommodate the intrali-

gand repulsions, and short contact distances between

Tp(C) and Cp*(C) atoms between 3.29 and 3.57 Å are

observed, which are comparable to those in 3⁺). 1 and

4 have staggered conformations, although X-ray data

obtained for 4 at room temperature suggest that the Cp*

ring is disordered, implying that there is only a small

energy difference between both conformations in this

structure. Intraligand repulsions may therefore only be

significant in affecting the conformation in Cp* com-

plexes with short M-ligand distances. As these param-

eters lengthen, the exact conformation adopted may

depend on crystal-packing forces or electronic effects.

For complexes of Cp, there seems to be no real correla-

tion between the conformation adopted and the M-ligand

bonding distances. 9²⁺shows little distortion in its

structure and has the shortest M-Ct and M-N dis-

tances of all the complexes studied, yet is eclipsed. 5⁺

is also eclipsed with a marked structural distortion,

although the M-ligand distances are much longer than

those of 9²⁺. We therefore believe that the distortion in

5⁺has an electronic rather than a steric origin.

F igu r e 7. View of the structure of 5(MeCN)⁺(anion not

shown) with all H atoms on pyrazolyl and Cp rings omitted

for clarity. Thermal ellipsoids are at 50% probability.

F igu r e 8. View of the structure of 5(PMe₃)⁺(anion not

shown) with all H atoms omitted for clarity (except B-H).

Thermal ellipsoids are at 50% probability.

V-B angle, and there is no significant difference in the

V-N and V-C bond lengths in both cases. The V-P

bond (2.559(1) Å) is substantially longer than the

average in [VCp₂(PPhH₂)]⁺[BPh₄]^-(2.405(4) Å).⁶⁹In an

ideal C_3vconformation, a TpM fragment should display

M-N-N-B torsion angles of 0° and M-N-N-C5

torsion angles of 180°. These angles are listed for

5(MeCN)⁺and 5(PMe₃)⁺in Table 8 and indicate that

substantial strain in the pz rings adjacent to L is

necessary to accommodate this extra ligand.

The value of ν_B-Hfor a Tp′ ligand has recently been

used as an indication of its hapticity. Akita et al. found

that for κ²-Tp^iPr2ML_ncomplexes ν_B-Hwas found in the

range 2471-2486 cm^-1, whereas if bound in a κ³-mode

ν_B-Hwas found in the range 2527-2554 cm^{-1 70}

J ones

.

et al. have also correlated the ¹¹B NMR shifts of a range

of Tp* complexes with its hapticity: κ³-complexes had

resonances between δ -8.44 and -9.76, while κ²-

complexes had resonances between -5.90 and -6.99.⁷¹

From our studies we find that solid-state measurements

of ν_B-Hfor crystallographically characterized κ³-Tp

complexes vary between 2460 and 2522 cm^-1, whereas

the B-H region in the IR spectrum of 3 consists of a

strong absorption resolved into three bands, the most

intense of which appears at 2435 cm^-1(see Table 1).

This observation is therefore consistent with Akita’s

observation that ν_B-Hfor κ²-Tp complexes is lower in

energy than that for κ³-Tp. We have no structural evi-

Discu ssion

The crystal structures presented above establish the

monomeric, axially symmetric nature of the MCpTp

class of complexes. The only exception to κ³-Tp bonding

is found in the structure of 3. Thus replacement of Cp

with Cp* in 6, not only causes a change in the spin state

but also causes a dramatic structural change.²⁵We will

report in more detail on the effect of electronic structure

on molecular structure in these systems.⁴⁵

Table 9 summarizes the M-Ct and M-N distances

and the conformations adopted by all the crystallo-

(67) Smart, J . C.; Pinsky, B. L.; Fredrich, M. F.; Day, V. W. J . Am.

Chem. Soc. 1979, 101, 4371-4373.

(68) Cotton, F. A.; Lewis, G. E.; Schwotzer, W. Inorg. Chem. 1986,

25, 3528-3529.

(70) Akita, M.; Ohta, K.; Takahashi, Y.; Hikichi, S.; Mora-Oka, Y.

Organometallics 1997, 16, 4121-4128.

(71) Northcutt, T. O.; Lachicotte, R. J .; J ones, W. D. Organometallics

1998, 17, 5148-5152.

(69) Ho, J .; Breen, T. L.; Ozarowski, A.; Stephan, D. W. Inorg. Chem.

1994, 34, 865-870.

3134 Organometallics, Vol. 21, No. 15, 2002

Brunker et al.

dence of the Tp hapticity in 4⁺(which is formally iso-

electronic with 3); however ν_B-Hof 2496 cm^-1suggests

κ³-Tp coordination. Several other features are worth

noting: first, the range that ν_B-Hadopts is large, span-

ning some 62 cm^-1, whereas for κ³-Tp^iPr2it was only

27 cm^-1. Second ν_B-Hfor the M(III) complex is always

higher than for the corresponding M(II) complex; for 6

and 6⁺this difference is only 4 cm^-1, but for 1 this

difference is substantial. Third, for M(II) complexes,

those with Cp as ligand all have higher ν_B-Hthan those

containing Cp*. These observations suggest some de-

pendence of ν_B-Hon the oxidation state of a particular

metal and on the ancillary ligand, in contradiction to

J ones et al.’s observations that δ (¹¹B) and ν_B-Hare

almost solely dependent on hapticity.⁷¹Interestingly,

the ¹¹B chemical shifts observed for 3⁺and 6⁺(see Table

3) would be indicative of κ²-coordination from their

conclusions.

F igu r e 9. Labeling scheme for NMR assignments of

[CoCp^RTp/Tpm]ⁿ⁺species: when X ) B, n ) 1; X ) C, n )

2. R ) H or Me.

3⁺the magnitudes of ³J were found to be consistent with

3

the previously observed trends with J (H₄H₅) ) 2.44 Hz

3

and J (H₃H₄) ) 2.20 Hz. We were unable to resolve the

coupling in the spectrum of 9⁺; some slight broadening

of the resonances is observed, which is probably a

consequence of a paramagnetic decomposition product.

It is noticeable that H₃in [CoTp₂]⁺is considerably

shielded in comparison to the mixed-sandwich species.

In the bis(Tp) sandwich complex this proton is situated

between two pyrazolyl rings of the second Tp and may

experience an aromatic ring shielding current, whereas

in our cases these protons point toward the Cp^Rring.⁵⁰

The chemical shifts of H₅are much more similar, as the

environment around this proton is not significantly

different in these complexes. For the Tp-containing

species, the trend in chemical shift δ(C₃) > δ(C₅) > δ(C₄)

is the same, supporting the notion that the difference

in δ(H₃) for [CoTp₂]⁺is due to a steric influence.

Although ¹³C NMR shifts are subject to many influ-

ences, δ(C₃) and δ(Cp* q) should in some degree reflect

the charge density at the metal center, which in turn

should be a function of the ancillary ligand. Thus δ(C₃)

for 6⁺is significantly deshielded compared to δ(C₃) for

3⁺and [CoTp₂]⁺, which both occur at a similar shift,

suggesting a trend in electron-donating ability, Cp* ≈

We were interested to discover whether, in common

with other κ²-Tp′ complexes, 3 is fluxional in solution.

1

Only two broadened H NMR resonances are observed

in C₆D₆solution at room temperature, which may be a

consequence of the paramagnetism of the Co(II) center,

fluxionality, or both. The IR spectrum of 3 in THF

solution reveals a ν_B-Hband at 2462 cm^-1, with a

shoulder at ∼2430 cm^-1(the intensity ratio is ap-

proximately 2:1). Thus there is an increase in energy of

the most intense absorption of 27 cm^-1vs the solid-state

measurement, suggesting that the κ³-Tp structure is

partially adopted in solution. (This value of ν_B-His

almost identical to the solid-state values for other

MCp*Tp complexes.) For comparison the IR spectrum

of 3⁺in CH₂Cl₂solution has ν_B-Hat 2504 cm^-1

,

confirming that in this case the solid-state structure

remains intact in solution.

In general, the paramagnetic complexes showed rela-

1

tively well-resolved H NMR spectra, and the data are

Tp > Cp. δ(Cp* q) follows the order [CoCpCp*]⁺> 3⁺

>

presented in Table 5. In many cases the resonances were

assigned by inspection of the relative intensities. Al-

though we have not performed a detailed analysis of the

relative contributions of dipolar and contact shifts, it is

possible to make comparison between isoelectronic

species in cases where we have been unable to obtain

susceptibility or structural data. Thus, the spectrum of

9⁺is similar to that of isoelectronic 6 and therefore

suggests the presence of a high-spin Co(II) center in this

case also.⁷²A distinctive pattern of three overlapping

Tp resonances is observed in a similar chemical shift

range for both NiCpTp and NiCp*Tp, also suggesting

similar coordination and spin states of the Ni centers.

[CoCp*₂]⁺, which suggests a similar ordering in relative

electron-donating ability of the three ligands, Cp* > Tp

> Cp. Due to the complexity of the factors involved,

these trends are at best tentative. As expected, δ(Cp*

q) for 9⁺is much more shielded than for the Co

complexes, reflecting the trend previously observed in

18-electron MCp*₂species.³²

A comparison of electrochemical data for the homo-

leptic and mixed-sandwich complexes is provided in

Table 2. For the metallocenes it is well-established that

permethylation of the rings causes a substantial ca-

thodic shift of the M(III)/M(II) couple: for M ) Co, this

shift is equivalent to about 300 mV per ring,⁷⁵whereas

for M ) Ni this shift is slightly increased.⁷⁶Both Co(II)

compounds 3 and 6 are much less easily oxidized than

CoCp₂, suggesting that Tp is a worse electron donor

than Cp. This is in contrast to the ¹³C NMR data

mentioned above; however we note that the NMR data

refer solely to the Co(III) oxidation state, whereas

electrochemical data reflect the relative stability of two

NMR spectroscopic data for the diamagnetic species

are given in Tables 3 and 4, and a labeling scheme is

given in Figure 9. An NOE experiment allowed total

assignment of the ¹H NMR spectrum of 3⁺. Selective

irradiation of the Cp*H signal caused enhancement to

the resonance at 8.82 ppm only, allowing this to be

1

assigned as H₃. H-¹³C correlation experiments (gH-

SQC) were used to assign the ¹³C NMR spectra. A

general observation in Tp complexes is that ³J (H₄H₅)

(73) Bortolin, M.; Bucher, U. E.; Ruegger, H.; Venanzi, L. M.;

Albinati, A.; Lianza, F.; Trofimenko, S. Organometallics 1992, 11,

2514-2521.

>

³J (H₃H₄), and this assignment has also been con-

firmed by NOE experiments in several cases.^73,74For

(74) J alon, F. A.; Otero, A.; Rodriguez, A. J . Chem. Soc., Dalton

Trans. 1995, 1629-1633.

(72) An EPR spectrum of this species at 100 K is consistent with a

(75) Barlow, S. Inorg. Chem. 2001, 40, 7047-7053.

(76) Koelle, U.; Khouzami, F. Angew. Chem., Int. Ed. Engl. 1980,

19, 640-641.

species with

a single unpaired electron however. No spectrum is

observed at room temperature; see ref 45.

Mixed-Sandwich Complexes

Organometallics, Vol. 21, No. 15, 2002 3135

adjacent oxidation states. We also observe a -300 mV

shift between the 6⁺/6 and 3⁺/3 couples; however

drawing an analogy with the CoCp*₂and CoCp₂couples

is probably not warranted. The differences in spin states

and structures of the reduced species 3 and 6 introduce

additional factors such as differences in exchange and

reorganization energies, which complicate the situation

considerably. For Ni, this trend is not followed, as,

somewhat surprisingly, 7 is slightly harder to oxidize

than 4, although the potentials for both are similar to

that of NiCp₂. For both Ni species we also observed

second redox events occurring at +585 mV (4, irrevers-

ible) and +440 mV (7, quasi-reversible) in CH₂Cl₂, which

we attribute to M(IV)/M(III) processes (all couples are

quoted vs [FeCp₂]⁺/FeCp₂). Both NiCp₂and NiCp*₂

by oxidation of VCp₂with [FeCp₂]⁺[BPh₄]^-in toluene,

although it was described as extremely air-sensitive and

characterized by its subsequent reaction with CO or

Cl^-.⁸⁰One-electron oxidation in the presence of coordi-

nating anions leads to neutral 16-electron complexes

VCp^R₂X, e.g., where X ) Cl or Br.^81,82We assume that

the enhanced steric bulk of Tp over Cp or Cp* enables

the isolation of both stable 14- and 16-electron species

for the mixed species. The cone angles for Tp, Cp, and

Cp* have been given as 199°,⁴150°, and 182°,³respec-

tively (alternative values of 110° and 142° have been

calculated for Cp and Cp*).⁸³We investigated the

thermal stability of 5(MeCN)⁺[PF₆]^-and found that

MeCN was liberated by heating a sample at 60 °C/10^-2

mbar, as indicated by the characteristic color change

from blue to orange. The IR spectrum of the product

was consistent with the desolvated cation.

Both [VCp*₂]⁺and [VCp₂]⁺react with CO and iso-

cyanides to give 18-electron, diamagnetic cations of

formula [VCp^R₂L₂]⁺.^32,82,84[VCp₂]⁺also forms diamag-

netic [VCp₂L₂]⁺adducts with L ) PhPH₂(whose crystal

structure has been determined)⁶⁹and L₂) dppe (diphen-

ylphosphinoethane);⁷⁸in addition, mixed [VCp₂(CO)L]⁺

adducts (where L ) PEt₃, PBuⁿ₃, and pyridine) were also

synthesized and found to be diamagnetic.⁷⁸The failure

of 5⁺to bind CO can be rationalized by consideration of

the IR absorption of the isocyanide ligand in 5(CN^tBu)⁺,

which is to higher freqency (2212 cm^-1) than that of free

CN^tBu (2136 cm^-1). This indicates that there is little

Mfligand π-back-bonding character to the M-C bond.⁶

Thus 5⁺is a poor π-base and is therefore unable to

stabilize a M-CO bond, CO being a stronger π-acceptor

but a weaker σ-donor than isocyanide. In contrast, both

[VCp₂]⁺and [VCp*₂]⁺form bis(isocyanide) and bis(CO)

adducts; the isocyanide stretching frequencies in these

cases are lower than those of the free ligands, indicating

the increased π-basicity of the V center in these spe-

cies.^78,825(CNBu^t)⁺is thermally unstable at room tem-

perature, whereas 5(PMe₃)⁺loses PMe₃only on heating

to 140 °C/10^-2mbar. 5(PMe₃)⁺is light-sensitive how-

ever, decomposing to a brown oil under normal labora-

tory lighting over several days. The thermal decompo-

sition of both these adducts again yields orange solids

whose IR spectra are the same as that for 5⁺[BAr′₄]^-.

Thus 5⁺forms stable adducts with strong σ-donor

ligands, although these can be decoordinated thermally.

display fully reversible [NiCp^{R 2+}/[NiCp₂]⁺couples,^32,77

]

2

and [NiCp*₂]²⁺has been chemically isolated and char-

acterized.³²In general the metallocenes are more elec-

tron-rich, although 1 is easier to oxidize even than

CrCp*₂. We can attribute this to the high-spin state of

the Cr(II) species and thus the substantial stabilization

afforded on generation of a stable Cr(III) center. The

stability of all the M(III) species is in accord with the

preference of the hard N donor ligand for the higher

oxidation state. Fewer data are available for comparison

with MTp₂complexes, although we note that the Co(II)-

Cp^RTp complexes are easier to oxidize than CoTp₂, but

oxidation of 5 (in CH₂Cl₂) is harder than for VTp₂. Thus

comparison of the electron-donating ability of Cp, Cp*,

and Tp should be confined to a per metal basis, as has

previously been demonstrated by comparisons of other

relevant data.⁵

A further comment is called for on the electrochemical

behavior of 5 in MeCN solution. The cyclic voltammo-

gram of 5(MeCN)⁺recorded in MeCN solution revealed

a reversible redox process attributable to the V(III)/V(II)

couple at the same half-wave potential as was observed

in the CV of 5 in the same solvent. The cathodic shift of

-190 mV of this wave in MeCN to that observed in CH₂-

Cl₂can be explained by the stabilizing effect of MeCN

coordination on oxidation. However the similar revers-

ibility of the waves in all three cases implies that MeCN

addition/desolvation to the cation is fast on the electro-

chemical time scale.

The isolation of both 5⁺and 5(MeCN)⁺as stable

complexes highlights the intermediate behavior of 5 as

compared to the homoleptic sandwich complexes. VTp₂

is oxidized in MeCN solution, but an MeCN solvent

adduct is not formed; only the 14-electron six-coordinate

cation is isolated.⁶²Oxidation of VCp*₂in MeCN solu-

tion yields the 16-electron MeCN adduct [VCp*₂-

(MeCN)]⁺, and in other coordinating solvents [VCp*₂S]⁺;

when the oxidation was performed in THF, the product

causes polymerization of the solvent.³²Attempts to

desolvate these cations by heating in vacuo led to their

decomposition to intractable materials. Oxidation of

VCp₂in acetone led to the isolation of [VCp₂(acetone)]⁺,

containing an η¹-O-coordinated acetone molecule as

shown by a single-crystal X-ray structure.⁶³Similar

behavior with other donor solvents has been reported,⁷⁸

including the isolation of [VCp₂(THF)]⁺.⁷⁹A recent

report has described the isolation of “naked” [VCp₂]⁺,

As mentioned previously, our attempts to synthesize

MnCpTp, MnCp*Tp, and CrCpTp have failed thus far.

The observation of a tendency to high-spin behavior in

mixed Cp^R/Tp complexes, illustrated by 1, 2, and 6, as

opposed to the lower-spin configurations adopted by

their metallocene counterparts, suggests that these

complexes, if isolable, would be high spin also. Presum-

(78) Fachinetti, G.; Del Nero, S.; Floriani, C. J . Chem. Soc., Dalton

Trans. 1976, 1046-1049.

(79) Choukron, R.; Douziech, B.; Pan, C.; Dahan, F.; Cassoux, P.

Organometallics 1995, 14, 4471-4473.

(80) Calderazzo, F.; Ferri, I.; Pampaloni, G.; Englert, U. Organo-

metallics 1999, 18, 2452-2458.

(81) De Liefde Meijer, H. J .; J anssen, M. J .; Van der Kerk, G. J . M.

Recl. Trav. Chim. Pays-Bas 1961, 80, 831-845.

(82) Gambarotta, S.; Floriani, C.; Chiesi-Villa, A.; Guastini, C. Inorg.

Chem. 1984, 23, 1739-1747.

(83) Davies, C. E.; Gardiner, I. M.; Green, J . C.; Green, M. L. H.;

Grebenik, P. D.; Mtetwa, V. S. B.; Prout, K. J . Chem. Soc., Dalton

Trans. 1985, 669-683.

(77) Holloway, J . D. L.; Geiger, W. E. J . Am. Chem. Soc. 1979, 101,

2038-2044.

(84) Calderazzo, F.; Bacciarelli, S. Inorg. Chem. 1963, 2, 721-723.

3136 Organometallics, Vol. 21, No. 15, 2002

Brunker et al.

ably the lability associated with high-spin d⁵(and d⁴)

centers results in the ligand disproportionation reac-

tions observed. We assume our failure to isolate [Co-

Tp*Cp]⁺is a result of unfavorable steric interactions

between the 3-Me groups on the Tp* ligand and the Cp

ring in either a reaction intermediate or the product.

Such species are stable with 4d metals however, as the

successful isolation of RuCpTp* and [RuCpTp*]⁺has

been reported.¹⁰The synthesis of RuCp*Tp* failed

presumably due to the increased steric bulk of Cp* over

Cp.¹⁰The smaller radii of first-row transition metals

may prevent isolation of mixed Tp*/Cp complexes

altogether.

the redox couple was judged by comparison with the behavior

of the [FeCp₂]⁺/FeCp₂couple under the same conditions.

Magnetic measurements were performed as described previ-

ously.⁸⁵KTp and KTp*,⁸⁶[CrCp*Cl]₂,⁸⁷[FeCp₂]⁺[PF₆]^-, [Fe-

Cp*(MeCN)₃]⁺[PF₆]^-,⁸⁸[CoCp*Cl]₂,⁸⁹NiCp*(acac),⁹⁰VCp₂Cl,⁹¹

[FeCp₂]⁺[BAr′₄]^-,³⁸CoCp(CO)I₂,⁹²CoCp₂,⁹³[NiCp(COD)]⁺-

[BF₄]^-,⁴¹and Tpm⁹⁴were prepared by published procedures.

Syn th esis of Cr Cp *Tp , 1. [CrCp*Cl]₂(0.53 g, 1.19 mmol)

was dissolved in THF (20 mL), and a solution of KTp (0.60 g,

2.38 mmol) in THF (10 mL) was added dropwise. The reaction

mixture was stirred for 1.5 h at room temperature, and then

all volatiles were removed in vacuo. The residues were

extracted with pentane (40 mL) to yield a dark green solution.

This solution was reduced in volume by ca. 5 mL, whereupon

some pale solids precipitated. The solution was filtered to

remove this solid and then cooled to -35 °C to yield dark

purple crystals of 1. Crystals suitable for single-crystal X-ray

diffraction were grown by slow cooling of a saturated pentane

solution to -35 °C. Yield: 0.35 g (0.87 mmol, 36.7%).

Con clu sion

We have synthesized a series of stable mixed-

sandwich complexes of Cp^Rand Tp with a range of first-

row transition metals. Electrochemical, structural, and

spectroscopic data indicate that these compounds can

be considered as intermediate between metallocene and

MTp₂complexes. As well as having a tendency to high-

spin electronic configurations, these complexes show a

variety of gross and subtle distortions from ideal

geometry in their molecular structures. Thus replace-

ment of a Cp^Rligand in a metallocene for Tp has a

considerable impact on the electronic structure and

properties of the metal center. We intend to report

further studies on the electronic structures and bonding

in these complexes in due course.

Syn th esis of [Cr Cp *Tp ]⁺[P F ₆]^-, 1⁺[P F ₆]^-. CrCp*Tp (0.20

g, 0.50 mmol) was dissolved in THF (10 mL), and a suspension

of [FeCp₂]⁺[PF₆]^-(0.16 g, 0.48 mmol) in THF (15 mL) was

added dropwise. During the addition a dark solid was seen to

precipitate, and the solution turned light brown. The reaction

mixture was stirred for 1 h and allowed to settle, and the

resultant purple precipitate was isolated by filtration. It was

then washed with Et₂O (2 × 5 mL) and dried in vacuo. The

solids were redissolved in the minimum volume of MeCN (20

mL), stirred, and reprecipitated by the dropwise addition of

Et₂O (100 mL) to this solution. The precipitate was dried in

vacuo to yield analytically pure solid. Yield: 0.19 g (0.35 mmol,

73.3% based on [FeCp₂]⁺).

Syn th esis of F eCp *Tp , 2. [FeCp*(MeCN)₃]⁺[PF₆]^-(0.43

g, 0.93 mmol) was dissolved, and MeCN (15 mL) and a solution

of KTp (0.23 g, 0.93 mmol) in MeCN (10 mL) were added

dropwise with stirring. The reaction mixture turned dark

green by the end of the addition and was stirred for a further

40 min. All volatiles were then removed in vacuo, and the

residue was extracted with pentane (total 30 mL). The blue-

green solution was reduced slightly in volume and cooled to

-30 °C for 12 h to yield a mixture of 2 as fine blue-green

needles and FeTp₂as pale pink blocks. Analytically pure

samples were obtained by manual separation of the crystals:

pure solutions of 2 were obtained by addition of the minimum

volume of precooled (-40 °C) acetone necessary to dissolve 2

and filtration while maintaining this temperature. Total yield

of 2 and FeTp₂: 0.10 g.

Exp er im en ta l Deta ils

Unless stated otherwise, all reactions were performed under

an inert atmosphere of N₂using standard Schlenk techniques

or in a Vacuum Atmospheres glovebox. Where necessary,

solvents were dried by reflux over the appropriate drying

agent: sodium-potassium alloy (pentane), potassium (THF),

sodium (toluene), sodium-benzophenone (Et₂O), calcium hy-

dride (MeCN, CH₂Cl₂), and anhydrous calcium sulfate, Drierite

(acetone). Solvents for NMR spectroscopy of oxygen- or water-

sensitive materials were dried by reflux over the appropriate

drying agent, potassium (C₆D₆, C₆D₅CD₃, and C₄D₈O) and

calcium hydride (CDCl₃, CD₂Cl₂, and CD₃CN), and purified by

trap-to-trap distillation. (CD₃)₂CO was stored over activated

4 Å molecular sieves under N₂.

NMR spectra were recorded using a Varian Unity Plus 500

MHz spectrometer or a Varian Mercury VX-Works 300 MHz

spectrometer. Spectra were referenced via the residual protio-

solvent peak, and chemical shifts (δ) are quoted in ppm relative

to tetramethylsilane at 0 ppm. Oxygen- or water-sensitive

samples were prepared using dried solvents under a N₂

atmosphere in a glovebox and were sealed in tubes fitted with

Young’s type concentric stopcocks. Fourier transform infrared

spectra were recorded using a Perkin-Elmer FT1710 spec-

trometer as KBr disks or as Nujol mulls or thin films between

KBr plates. Analyses were performed by the Analytical

Department, Inorganic Chemistry Laboratory, Oxford, and

mass spectra by the Mass Spectrometry Service, Department

of Chemistry, Oxford. Cyclic voltammograms were recorded

using a glassy-carbon working electrode and with platinum

wire auxiliary and pseudo-reference electrodes. Measurements

were made on deoxygenated solutions ca. 5 × 10^-4M in sample

and 0.1 M in [ⁿBu₄N]⁺[PF₆]^-as supporting electrolyte. Solvents

(THF, CH₂Cl₂, or MeCN) were freshly distilled before use.

Measurements on air-sensitive samples were made under a

N₂atmosphere in a specially constructed cell with a sidearm

fitted with a Rotaflo tap, and solutions of the sample and

supporting electrolyte were transferred via cannula into the

cell. Potentials were referenced to the [FeCp₂]⁺/FeCp₂couple

at 0 mV by addition of FeCp₂to the cell. The reversibility of

Syn th esis of CoCp *Tp , 3. [CoCp*Cl]₂(0.20 g, 0.44 mmol)

was dissolved in THF (10 mL), and a solution of KTp (0.21 g,

0.87 mmol) in THF (10 mL) was added dropwise with stirring.

Over the course of the addition the deep brown solution

lightened to a red-orange color. The reaction mixture was

stirred for 1 h, and then all volatiles were removed in vacuo

and the sticky residue was dried thoroughly. The resultant

red solid was extracted with pentane (3 × 15 mL), and the

solution was reduced in volume by ∼5 mL. A small amount of

(85) Brunker, T. J .; Hascall, T.; Cowley, A. R.; Rees, L. H.; O’Hare,

D. Inorg. Chem. 2001, 40, 3170-3176.

(86) Trofimenko, S. J . Am. Chem. Soc. 1967, 89, 3170-3177.

(87) Heinitz, R. A.; Ostrander, R. L.; Rheingold, A. L.; Theopold, K.

H. J . Am. Chem. Soc. 1994, 116, 11387-11396.

(88) Catheline, D.; Astruc, D. Organometallics 1984, 3, 1094-1100.

(89) Koelle, U.; Fuss, B.; Belting, M.; Raabe, E. Organometallics

1986, 5, 980-987.

(90) Bunel, E. E.; Valle, L.; Manriquez, J . M. Organometallics 1985,

4, 1680-1682.

(91) Manzer, L. E. Inorg. Synth. 1990, 28, 260-263.

(92) King, R. B. Inorg. Chem. 1966, 5, 82-87.

(93) Wilkinson, G.; Cotton, F. A.; Birmingham, J . M. J . Inorg. Nucl.

Chem. 1956, 2, 95-113.

(94) Reger, D. L.; Grattan, T. C.; Brown, K. J .; Little, C. A.; Lamba,

J . J . S.; Rheingold, A. L.; Sommer, R. D. J . Organomet. Chem. 2000,

607, 120-128.

Mixed-Sandwich Complexes

Organometallics, Vol. 21, No. 15, 2002 3137

yellow solid precipitated which did not redissolve on warming,

so the solution was refiltered and cooled to -30 °C for 24 h to

yield 3 as a red-brown crystalline solid. Further concentration

and cooling of the supernatant to -30 °C yielded a further

crop of solid. Single crystals suitable for X-ray diffraction were

grown by slow-cooling a pentane solution to -30 °C. Yield:

0.15 g (0.37 mmol, 42.3%).

Syn th esis of [CoCp *Tp ]⁺[P F ₆]^-, 3⁺[P F ₆]^-. 3 (0.13 g, 0.32

mmol) was dissolved in dry MeCN (20 mL) to give a red-brown

solution, and a solution of [FeCp₂]⁺[PF₆]^-(0.10 g, 0.30 mmol)

in MeCN (7 mL) was added dropwise with stirring. Upon

addition the solution darkened instantly, and on completion

was dark blue-purple in color. The reaction was stirred for 1

h, and then all volatiles were removed in vacuo to yield a deep

purple solid. This was washed with Et₂O (5 × 5 mL) to remove

FeCp₂and unreacted 3, and the residue was dried in vacuo.

Redissolution of this solid in dry acetone (20 mL) and precipi-

tation with pentane (∼50 mL) gave analytically pure 3⁺[PF₆]^-

as an air-stable blue-purple powder. Single crystals were

grown by slow evaporation of a CH₂Cl₂solution. Yield: 0.12 g

(0.21 mmol, 70.2% based on [FeCp₂]⁺).

toluene (30 mL) added, yielding a blue-green microcrystalline

precipitate, which was isolated by filtration, washed with

pentane (5 mL), and dried in vacuo. Further addition of toluene

to the supernatant and cooling to -30 °C produced a second

crop of solid. Single crystals suitable for X-ray diffraction were

grown by layering a concentrated MeCN solution with Et₂O.

Yield: 0.20 g (0.39 mmol, 64.9% based on ferrocenium).

Syn th esis of [VCp Tp ]⁺[BAr ′₄]^-, 5⁺[BAr ′₄]^-. 5 (0.15 g, 0.46

mmol) and [FeCp₂]⁺[BAr′₄]^-(0.50 g, 0.46 mmol) were placed

in a Schlenk vessel under N₂, and CH₂Cl₂(15 mL) was added

with stirring. All the solids instantly dissolved, forming a dark

red solution. The reaction mixture was stirred for 15 min, and

then all volatiles were removed in vacuo. The residues were

washed with pentane (4 × 10 mL) to remove FeCp₂, and the

red-brown residue was dried in vacuo. The solids were then

redissolved in CH₂Cl₂(10 mL), and the solution was filtered

into an ampule and layered with pentane (50 mL). Dark red

crystals suitable for single-crystal X-ray diffraction were

isolated after 24 h; these were found to be of composition

[CpVTp]⁺[BAr′₄]^-‚2CH₂Cl₂. Crystals dried thoroughly in vacuo

analyzed for 5⁺[BAr′₄]^-with no solvent of crystallization.

Yield: 0.43 g (0.36 mmol, 78.2%).

Syn th esis of NiCp *Tp , 4. NiCp*(acac) (0.30 g, 1.04 mmol)

was dissolved in THF (15 mL), and a solution of KTp (0.26 g,

1.04 mmol) in THF (15 mL) was added dropwise. During the

addition a white solid was seen to precipitate and an orange-

brown solution formed. The reaction mixture was stirred for

1 h and then stripped to dryness in vacuo. The residues were

extracted with pentane (3 × 20 mL) to give a golden-brown

solution. Concentration of this solution by ca. 15 mL and

cooling to -35 °C for 12 h yielded golden-brown microcrystals

of 4. Further concentration and cooling yielded further crops

of solid. Single crystals suitable for X-ray diffraction were

grown by slow cooling of a concentrated pentane solution to

-35 °C. Yield: 0.30 g from 3 crops (0.74 mmol, 70.8%).

Syn th esis of [NiCp *Tp ]⁺[P F ₆]^-, 4⁺[P F ₆]^-. [FeCp₂]⁺[PF₆]^-

(0.19 g, 0.58 mmol) was stirred as a suspension in CH₂Cl₂(15

mL), and a solution of 4 (0.25 g, 0.61 mmol) in CH₂Cl₂(10 mL)

was added dropwise. Over the course of the addition a dark

solution formed, from which a dark solid precipitated on

continued stirring. The reaction mixture was stirred for a

further hour and the precipitate isolated by filtration as a black

powder. This was washed with Et₂O (2 × 10 mL) and dried in

vacuo to give analytically pure 4⁺[PF₆]^-. Further product could

be isolated from the CH₂Cl₂supernatant by addition of Et₂O

(100 mL) dropwise with stirring. The resultant black-brown

solid was isolated by filtration and dried in vacuo. Elemental

analysis of this material was close to that calculated for

4⁺[PF₆]^-, and its IR spectrum was similar to that of the first

sample. Yield: 0.19 g combined yield (0.34 mmol, 56.1%).

Syn th esis of VCp Tp , 5. This preparation is a modification

of a literature procedure.¹⁴VCp₂Cl (1.50 g, 6.93 mmol) was

dissolved in THF (25 mL), and a solution of KTp (1.73 g, 6.93

mmol) in THF (20 mL) was added dropwise. During the

addition the dark blue solution gradually turned dark green.

The reaction mixture was stirred at room temperature for 2

h, and then all volatiles were removed and the residue was

thoroughly dried in vacuo. The green-purple solid residue was

extracted with Et₂O (2 × 25 mL) to give a dark solution. The

solution was reduced in volume by 15 mL and cooled to -35

°C for 24 h to yield a dark green crystalline solid. This solid

was washed with precooled Et₂O (5 mL) at -80 °C to remove

a purple oily contaminant and dried in vacuo to give analyti-

cally pure 5. Yield (in 3 crops): 1.30 g (4.00 mmol, 56.8%).

Syn th esis of [VCp Tp (MeCN)]⁺[P F ₆]^-, 5(MeCN)⁺[P F ₆]^-.

A solution of [FeCp₂]⁺[PF₆]^-(0.20 g, 0.60 mmol) in MeCN (10

mL) was added dropwise to a solution of 5 (0.20 g, 0.61 mmol)

in MeCN (15 mL), yielding a dark green solution. The reaction

mixture was stirred for 1 h, and then all volatiles were re-

moved in vacuo. The residues were washed with pentane (5 ×

10 mL), and the resultant blue-green solid was dried in vacuo.

The solid was redissolved in MeCN (10 mL) and filtered and

Syn th esis of [VCp Tp (CN^tBu )]⁺[BAr ′₄]^-, 5(CN^tBu )⁺[B-

Ar ′₄]^-. 5 (0.12 g, 0.35 mmol) and [FeCp₂]⁺[BAr′₄]^-(0.35 g, 0.33

mmol) were dissolved in CH₂Cl₂(15 mL) with stirring, forming

a red solution. This was stirred for 15 min, and then CNBu^t

(0.1 mL) was added by syringe, instantaneously generating a

green solution. The reaction mixture was stirred for a further

5 min, and then all volatiles were removed in vacuo. FeCp₂

was removed by washing with pentane (4 × 10 mL), to yield

a blue-green powder, which was dried in vacuo. The solids were

redissolved in CH₂Cl₂(10 mL), the solution was filtered, and

the product precipitated by the dropwise addition of pentane

(30 mL) while stirring. The blue-green microcrystalline pre-

cipitate was isolated by filtration and dried in vacuo. Yield:

0.25 g (0.19 mmol, 55.0% based on [FeCp₂]⁺).

Syn th esis of [VCpTp(P Me₃)]⁺[BAr ′₄]^-, 5(P Me₃)⁺[BAr ′₄]^-.

5 (0.10 g, 0.30 mmol) and [FeCp₂]⁺[BAr′₄]^-(0.31 g, 0.30 mmol)

were dissolved in CH₂Cl₂(15 mL) with stirring, forming a red

solution. This solution was stirred for 15 min and then cooled

to -78 °C. The Schlenk was placed under partial vacuum; a

small amount of PMe₃was then vacuum transferred into the

reaction mixture, causing the solution to immediately assume

a dark green coloration. The solution was allowed to warm to

room temperature under N₂, and then all volatiles were

removed in vacuo. FeCp₂was removed by washing with

pentane (3 × 15 mL) and the resultant green-blue solid dried

in vacuo. This solid was redissolved in CH₂Cl₂(8 mL) and

filtered, and the product was precipitated by dropwise addition

of pentane (25 mL). It was isolated by filtration, washed with

pentane (5 mL), and dried in vacuo to yield a light gray-blue

microcrystalline solid. Single crystals were grown by layering

of a concentrated CH₂Cl₂solution with pentane. Yield: 0.27 g

(0.21 mmol, 72.0% based on [FeCp₂]⁺).

Syn th esis of [CoCp Tp ]⁺[I]^-, 6⁺[I]^-. A solution of KTp

(0.62 g, 2.46 mmol) in THF (15 mL) was added dropwise to a

deep purple solution of CoCp(CO)I₂(1.00 g, 2.46 mmol) in THF

(15 mL). The resulting dark suspension was stirred for 15 h,

and then the solids were collected on a glass sintered frit. The

solids were washed with deionized H₂O (2 × 15 mL), THF (1

× 3 mL), and Et₂O (1 × 5 mL) and then dried thoroughly in

vacuo. Additional purification was achieved by dissolving the

deep purple solids in the minimum volume of MeNO₂(60 mL)

and precipitating the product by dropwise addition of Et₂O to

this solution, with stirring. The product was isolated by

filtration as an air-stable purple microcrystalline powder and

dried in vacuo. Yield: 1.12 g (2.42 mmol, 98.1%).

Syn th esis of [CoCp Tp ]⁺[P F ₆]^-, 6⁺[P F ₆]^-. 6⁺[I]^-(0.50 g,

1.08 mmol) and NaPF₆(0.20 g, 1.200 mmol) were placed in a

thick-walled ampule under N₂, and a degassed mixture of H₂O

and MeOH (4:1, 50 mL) was added. The ampule was partially

3138 Organometallics, Vol. 21, No. 15, 2002

Brunker et al.

evacuated and heated at 70 °C with stirring for 3 h. The purple

insolubles were collected on a glass sintered frit and washed

with deionized H₂O (20 mL) and then Et₂O (15 mL). The

powder was then dried in vacuo, dissolved in MeCN (30 mL),

and filtered, and Et₂O (70 mL) was added dropwise with

stirring. The resultant purple powder was isolated by filtration

and dried in vacuo. Yield: 0.35 g (0.73 mmol, 67.5%).

Syn th esis of CoCp Tp , 6. 6⁺[I]^-(0.28 g, 0.61 mmol) was

stirred as a suspension in THF (15 mL), and a solution of

CoCp₂(0.11 mg, 0.59 mmol) in THF (8 mL) was added

dropwise with stirring. Immediately a yellow precipitate

formed, and after the addition was complete the reaction was

stirred for a further 30 min, yielding a dark yellow solution

and a yellow precipitate of [CoCp₂]⁺[I]^-. The solution was

filtered off and the solids were washed with a further portion

of THF (3 mL). The filtrate and washings were combined and

stripped to dryness in vacuo. The dirty yellow residue was

extracted with pentane (35 mL), concentrated slightly, and

cooled to -30 °C overnight to yield yellow-green microcrystals

of 6. Single crystals were grown by slow cooling a saturated

pentane solution to -30 °C. Yield: 0.17 g (0.50 mmol, 82.6%).

Syn th esis of NiCp Tp , 7. [NiCp(1,5-cyclooctadiene)]⁺[BF₄]^-

(0.25 g, 0.78 mmol) was dissolved in dry acetone and a solution

of KTp (0.20 g, 0.78 mmol) added dropwise. The reaction

mixture was stirred for 1 h, during which time a lot of white

precipitate formed. All volatiles were then removed in vacuo,

and the dirty green residue was extracted with pentane (20

mL). The mid-green solution was reduced slightly in volume

and cooled to -35 °C. The first crop isolated contained a few

pale green needles of 7 and some pink crystalline material

identified as NiTp₂by EA and IR. Further concentration and

cooling yielded further quantities of 7, although these crops

were found to be contaminated with NiCp₂also. 7 can be

separated from NiCp₂by sublimation of the metallocene at

50 °C/10^-2mbar onto a coldfinger. It must be noted that on

each occasion this reaction was performed, differing amounts

of each product were formed despite attempting to reproduce

the same conditions as previously.

a microcrystalline purple solid, washed with pentane (10 mL),

and dried in vacuo. Single crystals of 9²⁺[PF₆]^-suitable for

2

X-ray crystallography were gradually deposited from the H₂O

supernatant. Yield: 1.07 g (1.70 mmol, 83.9%).

Red u ction of 9²⁺[P F ₆]^-₂. 9²⁺[PF₆]^-(0.32 g, 0.50 mmol)

2

was stirred as a suspension in THF (20 mL) and a solution of

CoCp₂(0.10 g, 0.53 mmol) in THF (10 mL) added dropwise.

During the course of the addition the purple solids dissolved

and a yellow precipitate formed. The reaction mixture was

stirred for a further 20 min and was then filtered. The solids

were washed with THF (5 mL), and the washings and filtrate

were combined and stripped to dryness in vacuo. The dirty

yellow residue was then washed with Et₂O (2 × 10 mL) and

the resulting yellow solid dried in vacuo. It was extracted with

CH₂Cl₂(20 mL) and stirred, and pentane (30 mL) was added

1

to precipitate the product as a yellow solid. H NMR spectro-

scopy and elemental analysis revealed this solid to be a

mixture of [Cp*CoTpm]⁺[PF₆]^-, 9⁺[PF₆]^-, and [CoCp₂]⁺.

Cr ystal Str u ctu r e Deter m in ation s. Crystals were mount-

ed on glass fibers using perfluoropolyether oil, transferred to

a goniometer head on the diffractometer, and cooled rapidly

to 150(2) K in a stream of cold N₂using an Oxford Cryostream

600 series. Data collections were performed using an Enraf-

Nonius DIP2000 image plate diffractometer (4) or an Enraf-

Nonius FR590 Kappa CCD (3, 3⁺[PF₆]^-, 5⁺[BAr′₄]^-, 5(MeCN)⁺-

[PF₆]^-, 5(PMe₃)⁺[BAr′₄]^-, 8, 9²⁺[PF₆]^-₂) both utilizing Mo KR

X-ray radiation (λ ) 0.71073 Å). The data were processed using

the programs DENZO and SCALEPACK.⁹⁵Structures were

solved using the direct-methods program SIR-92⁹⁶or SHELXS⁹⁷

and refined using full-matrix least-squares refinement on all

F data using the CRYSTALS program suite⁹⁸(4) or using all

F²data using SHELX-97 (3, 3⁺[PF₆]^-, 5⁺[BAr′₄]^-, 5(MeCN)⁺-

[PF₆]^-, 5(PMe₃)⁺[BAr′₄]^-, 8, 9²⁺[PF₆]^-₂).⁹⁷In general, all non-

hydrogen atoms were refined anisotropically and hydrogen

atoms were included in calculated positions with isotropic

thermal parameters. 3 crystallized in space group Pnma and

with half a molecule in the asymmetric unit astride a mirror

plane. 5⁺[BAr′₄]^-crystallized in space group P1h with a cation,

an anion, and two molecules of CH₂Cl₂in the asymmetric unit.

5(PMe₃)⁺[BAr′₄]^-crystallized in the monoclinic space group

P2/c with a cation and two-half anions in the asymmetric unit.

Each B atom of the two half [BAr′₄]^-anions sits on a special

position corresponding to a 2-fold rotation axis.

Syn th esis of [F eCp *Tp m ]⁺[P F ₆]^-, 8⁺[P F ₆]^-. [FeCp*(Me-

CN)₃]⁺[PF₆]^-(0.50 g, 1.16 mmol) was dissolved in MeCN (10

mL), and a solution of Tpm (0.25 g, 1.16 mmol) in MeCN (10

mL) was added dropwise with stirring. Over the addition the

dark purple solution turned dirty green in color. The reaction

mixture was then stirred for a further 15 min, and then the

volume of the solution reduced by about one-third. Et₂O (100

mL) was added dropwise with stirring to precipitate a dark

green powder. This was isolated by filtration, washed with

Et₂O (10 mL), and dried in vacuo. Yield: 0.31 g (0.78 mmol,

67%).

Ack n ow led gm en t. We thank Dr. Stephen Barlow

for useful discussions, Dr. Tony Hascall for crystal-

lographic assistance, and Simon J ones and Victoria

Beck for generous donations of some reagents. We also

acknowledge the EPSRC for financial support (T.J .B.).

Syn th esis of [CoCp Tp m ]²⁺[I]^-₂, 9²⁺[I]^-₂. CoCp(CO)I₂

(2.00 g, 4.93 mmol) and Tpm (1.06 g, 4.93 mmol) were placed

in a Schlenk vessel, and THF (40 mL) was added. The reaction

mixture was stirred for 1.5 h at ambient temperature, and the

resultant dark precipitate was then collected on a glass-

sintered frit. The solids were washed with Et₂O (∼30 mL) until

the washings were colorless, and the dark purple solid was

dried in vacuo. The product was shown to be essentially pure

by ¹H NMR spectroscopy (D₂O): further purification was

OM0200149

(95) Otinowski, Z.; Minor, W. Processing of X-ray Diffraction Data

Collected in Oscillation Mode, Methods Enzymol. 1997, 276, 307-326;

Carter C. W., Sweet, R. M., Eds.; Academic Press: New York.

(96) Altomare, A.; Cascarano, G.; Giacovazzo, G.; Guagliardi, A.;

Burla, M. C.; Polidori, G.; Camalli, M. J . Appl. Crystallogr. 1994, 27,

343-350.

-

achieved by metathesis to the PF₆salt. Yield: 2.74 g (4.63

(97) Sheldrick, G. M. SHELXS; 1998.

mmol, 93.9%).

(98) Watkin, D. J .; Prout, C. K.; Carruthers, J . R.; Betteridge, P.

W. CRYSTALS issue 10; Chemical Crystallography Laboratory: Ox-

ford, UK, 1996.

(99) J aniak, C.; Scharmann, T. G.; Green, J . C.; Parkin, R. P. G.;

Kolm, M. J .; Riedel, E.; Mickler, W.; Elguero, J .; Claramunt, R. M.;

Sanz, D. Chem. Eur. J . 1996, 2, 992-1000.

(100) Connelly, N. G.; Davies, J . D. J . Organomet. Chem. 1972, 38,

385-390.

(101) Connelly, N. G.; Geiger, W. E. Chem. Rev. 1996, 96, 877-910.

(102) Materikova, R. B.; Babin, V. N.; Lyatifov, I. R.; Salimov, R.

M.; Fedin, E. I.; Petrovskii, P. V. J . Organomet. Chem. 1981, 259-

267.

(103) Fujihara, T.; Schonherr, T.; Kaizaki, S. Inorg. Chim. Acta

1996, 249, 135-141.

Syn th esis of [CoCp Tp m ]²⁺[P F ₆]^-₂, 9²⁺[P F ₆]^-₂. 9²⁺[I]^-

2

(1.20 g, 2.03 mmol) was dissolved in H₂O (170 mL), and the

solution degassed with N₂and then added dropwise to a

stirred, degassed solution of NaPF₆(1.00 g, 5.95 mmol) in H₂O

(10 mL). During the addition a purple precipitate formed,

which was then allowed to settle. The supernatnant was

decanted, and the solids were washed with H₂O (15 mL) and

then dried thoroughly in vacuo. The residue was redissolved

in acetone (20 mL) and filtered. This solution was vigorously

stirred, and the product was precipitated by dropwise addition

of pentane (∼30 mL). 9²⁺[PF₆]^-was isolated by filtration as

2

Article Doi

Synthesis, structures, and redox properties of mixed-sandwich complexes of cyclopentadienyl and hydrotris(pyrazolyl)borate ligands with first-row transition metals

DOI: 10.1021/om0200149

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