A joint experimental and computational study on the electronic communication in diethynylaryl-bridged (η5-C5H 5)Fe(η2-dppe) and (η5-C 5H5)-Fe(CO)2 units

FULL PAPER

DOI: 10.1002/ejic.200600024

A Joint Experimental and Computational Study on the Electronic

Communication in Diethynylaryl-Bridged (η⁵-C₅H₅)Fe(η²-dppe) and (η⁵-C₅H₅)-

Fe(CO)₂Units

Laura Medei,^[a]Laura Orian,^[b]Oleg V. Semeikin,^[c]Mikhail G. Peterleitner,^[c]

Nikolai A. Ustynyuk,^[c]Saverio Santi,*^[b]Christian Durante,^[b]Antonella Ricci,^[d]and

Claudio Lo Sterzo*^[d]

Keywords: Mixed-valence compounds / Iron / Electron transfer / Electronic communication / Ethynylaryl bridging ligand

A family of bimetallic complexes [Cp(CO)₂Fe–CϵC–Ar–

CϵC–Fe(CO)₂Cp] {Cp = C₅H₅; 6a–g: Ar = C₄H₂S (a), 3-

(C₄H₉)-C₄HS (b), 3-(C₁₆H₃₃)-C₄HS (c), C₆H₄(d), 2,5-bis-

(OC₄H₉)-C₆H₂(e), 2,5-bis(OC₈H₁₇)-C₆H₂(f), (C₆H₄)₂(g)} was

prepared by the three-step Pd-catalysed extended one-pot

(EOP) synthetic protocol from Bu₃Sn–CϵCH, X–Ar–X (X = I,

Br) and Cp(CO)₂FeI. Complexes 6a,d,g were then exposed

to ultraviolet irradiation in the presence of an equivalent

amount of 1,2-bis(diphenylphosphanyl)ethane (dppe) to form

the corresponding bimetallic complexes [Cp(dppe)Fe–CϵC–

Ar–CϵC–Fe(dppe)Cp] (7a,d,g). Compounds 6a–g and 7a,d,g

were characterised by cyclic voltammetry (CV). The most

significant electrochemical information comes from the oxi-

dation of the dppe derivatives. The ∆E° separations between

the subsequent reversible waves suggest that the efficiency

of the metal–metal electronic coupling decreases in the order

7a Ͼ 7d Ͼ 7g. Complexes 7a and 7g were also chemically

oxidised with [Fe(Cp*)]₂[BF₄] {Cp*

= C₅(CH₃)₅} and

[Fe(Cp)]₂[BF₄] respectively, and the near infrared (NIR) spec-

tra of the mixed-valence species 7a⁺and 7g⁺were recorded.

A strong intervalence transition (IT) band was observed only

for the radical cation 7a⁺. While this finding confirms the

existence of an electronic interaction between the two ter-

mini when a 2,5-thiophene group is present in the spacer,

the NIR spectrum of 7g⁺reveals a reduced efficiency in con-

veying electrons when the C₄H₂S moiety is replaced by a

4,4Ј-biphenyl. In order to rationalise and quantify the extent

of electronic communication, ruled by geometrical and elec-

tronic factors, density functional computational results on se-

lected [Cp (PH₃)₂Fe] and [Cp(CO)₂Fe] binuclear model com-

plexes are reported.

Germany, 2006)

tron transfer along rod-like unsaturated molecular chains

(eventually) incorporating metal centres.^[2]The possibilities

of extended delocalisation through the π networks can

hardly influence physical properties such as polarisability,

conductivity, and so forth, making these complexes suitable

as active components in optoelectronic devices.^[3]

We have recently disclosed a new one-pot route to poly-

(acetylide)s and poly(metallaacetylide)s based on palla-

dium-catalysed carbon–carbon and metal–carbon bond for-

mation (Scheme 1).^[4]

By this procedure, named extended one-pot (EOP), a

great variety of organic co- and homopolymers can be

straightforwardly obtained with few steps and very good

yields, with no need to isolate intermediate compounds (see

path a₁and path b in Scheme 1). Moreover, taking advan-

tage of a new performance of Pd catalysts we discovered,

allowing the formation of metal–carbon bonds, also orga-

nometallic oligomers and polymers can be conveniently ac-

cessed by this procedure^[4a,4b,5](path a₂in Scheme 1).

With such versatile synthetic methodology at our dis-

posal, we pursued our study by aiming for the valuation of

the electrochemical and optical properties of some spacers

Introduction

Highly ethynylated organic or organometallic com-

pounds are currently providing new opportunities in pro-

jecting materials to be used in electronics, optics and ceram-

ics.^[1]

The attention devoted to such compounds arises from

unique chemical and physical properties due to facile elec-

[a] Istituto CNR di Metodologie Chimiche (IMC-CNR), Sezione

Meccanismi di Reazione Dipartimento di Chimica, Box 34,

Roma 62, Università “La Sapienza”,

Piazzale Aldo Moro 5, 00185 Roma, Italy

[b] Dipartimento di Scienze Chimiche, Università degli Studi di Pa-

dova,

Via Marzolo 1, 35131 Padova, Italy

E-mail: saverio.santi@unipd.it

[c] A. N. Nesmeyanov Institute of Organoelement Compounds

(INEOS), Russian Academy of Science,

Vavilov Street 28, Moscow 119991, Russian Federation

[d] Dipartimento di Scienze degli Alimenti, Facoltà di Agraria,

Università degli Studi di Teramo,

Via Carlo R. Lerici 1, 64023 Mosciano Sant’Angelo (Teramo),

Italy

E-mail: closterzo@unite.it

Supporting information for this article is available on the

WWW under http://ww w .eurjic.org or from the author.

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Electronic Communication in Diethynylaryl-Bridged Units

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Scheme 1.

commonly used as building blocks in the preparation of

Several compounds with different metallic termini, that

opto- and electroactive polymers. To achieve this purpose, is iron and rhenium, bridged by unsaturated all-carbon

insertion of different spacers between two iron centres chains (C₄to C₂₀),^[6a–6j], or iron, ruthenium and osmium,

[CpFe(dppe)-] was planned, in order to exploit the elec- bridged by σ-acetylides intercalated by different aryl

tronic interaction between the metallic nuclei in a given re- cores,^[6k–6m]have been prepared and evaluated for their

dox state and the bridging ligand.

physical properties by other authors. Elegant studies on the

Actually, depending on both the redox state of the mole- possibility of tuning the electronic communication by

cule and its structure, electron transfer and magnetic ex- lengthening the polyalkynyl bridging ligand and/or by

change can occur along the intermetallic bridge.

changing the metallic termini have been presented by

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Lapinte,^[7]Gladysz^[8]and Bruce^[9]and their co-workers. In oxidised and their products were studied by near infrared

a few cases aryl cores have been intercalated in the hydro- (NIR) spectroscopy. A quenching of the NIR absorption

carbon spacer.^{[7g,7h,7k,7l,7o,7p,7s]}

band, which is characteristic of mixed-valence species, oc-

In this paper new binuclear Fe^IIcompounds incorporat- curs when the thiophenyl core is replaced by the 4,4Ј-bi-

ing aromatic groups are presented. Aromatic groups to be phenyl moiety.

introduced between iron centres have been chosen in order

Theoretical calculations, based on state-of-the-art den-

to keep the rigid-rod geometry of the carbon chain, to in- sity functional theory (DFT) methods, were carried out on

crease the chemical stability of the compounds and to main- selected (PH₃)₂and (CO)₂model derivatives and provided

tain a high level of electronic coupling. The availability of (i) reliable structures for the title complexes, for which crys-

compounds that differ for the aryl core in their bridge and tallographic data are not available yet, (ii) a detailed de-

the metal ancillary ligands allows discussion of the effects scription of the electronic structure of the neutral and radi-

of structural and electronic molecular features on the extent cal cation species, and (iii) prediction and interpretation of

of metal–metal communication. Particularly interesting are the experimental NIR bands based on the model radical

the effects on the metal–metal interaction resulting from (i) cations.

the replacement of a thiophenyl core by a phenyl core, a

structural change in the bridge which does not greatly affect

Results and Discussion

the intermetallic distance, (ii) the insertion of a 4,4Ј-bi-

phenyl core, which significantly increases the length of the

hydrocarbon linker and (iii) the presence of different ancil-

lary ligands, that is CO and dppe.

The synthesis of dppe-substituted bimetallic compounds

7a–g (Scheme 2) was first attempted using the EOP syn-

thetic strategy outlined in path a₂of Scheme 1; however, all

the attempts to couple CpFe(dppe)I (4) with the different

in situ generated Bu₃Sn–CϵC–Ar–CϵC–SnBu₃(3a–g) in

the presence of Pd failed (step 3 of path a₂). The failure in

the formation of the Fe–CϵC bond was probably due to

the strong electron-donating effect of dppe groups on the

metal, which decreases the electrophilicity of the Fe–I bond,

a basic requirement for oxidative addition to the Pd cata-

lyst, thus the disclosure of the coupling process leading

from 4 and 3a–g to 7a–g.^[7j,11]

As direct EOP formation of 7a–g proved not to be pos-

sible, we decided first to form the (CO)-substituted bimetal-

lic compounds 6a–g, then to subject these compounds to

photolytic substitution in the presence of dppe as reported

for similar compounds.^[7d,12]

The physical–chemical study was performed by electro-

chemical (CV) and spectroscopic techniques [IR, near infra-

red (NIR), UV/Vis] on selected synthesised dppe complexes.

Cyclic voltammetry (CV) results give information on the

stability of the oxidation products but are not sufficient to

determine the extent of electronic coupling between the

metal termini. Actually, in a typical redox process involving

symmetric bimetallic complexes, the separation between the

two subsequent reversible waves, in the chemical and elec-

trochemical sense, ∆E°, allows a quantitative estimate of the

thermodynamic stability of the intermediate oxidation state

with respect to disproportionation (i.e., the compro-

portionation constant K_c).^[10]∆E° is only a qualitative indi-

cator of an interaction between the two sites. According to

the classification proposed by Robin and Day,^[10a]∆E° val-

ues close to zero are characteristic of noninteracting metal

sites, that is of class I complexes. Small ∆E° separations

correspond to a weak interaction between the metals and

this situation is described by a mixed-valence state involving

Synthesis of (CO)-Substituted Bimetallic Compounds 6a–g

In Scheme 2 the EOP route to the bimetallic compounds

trapped-valence systems (class II). Finally, larger ∆E° val- 6a–g and the subsequent conversion into the dppe deriva-

ues correspond to a stabilisation of the mixed-valence com- tives 7a–g are outlined. The EOP transformations were per-

plexes involving the totally delocalised class III systems. Al- formed in dioxane solvent using 2–4% of Pd⁰catalyst (sin-

though the larger the conjugation along the intermetallic gle addition at the beginning of step 1). While step 1 and

bridge is, the easier the transfer of electronic information step 2 were rapidly accomplished (20–30 min overall time)

between metal centres is, the greater ∆E° tends to be, the with quantitative formation of 3a–g, the accomplishment of

more CV separations are influenced by other factors, in step 3 was revealed to be a critical event because of the

particular the medium (solvent and support electrolyte), delicate nature of the bimetallic complexes 6a–g. Although

which can modify the electrostatic interaction in the dicat- the reaction temperature was lowered to 50–80 °C, with

ions.^[10n]Thus, the CV analysis should always be integrated consequent prolongation of reaction times, monitoring of

1

with spectroscopic investigation, in particular in the NIR the reaction progress by H NMR revealed that formation

region where mixed-valence species typically absorb.

of the expected product was always accompanied by un-

While CV provides no information on the thermo- identified side products and a small amount of starting ma-

dynamic stability in the carbonylated species because of the terials always remained.

irreversible nature of their waves, the dppe derivatives are

Isolation of products 6a–g was not straightforward.

studied in detail. In particular, the two dppe-substituted Chromatographic separation proved to be always detrimen-

compounds, including the thiophenyl core and the 4,4Ј-bi- tal, therefore products had to be isolated and purified by

phenyl core respectively, exhibit very different reversible repeated crystallisation (see Experimental Section), which

electrochemical behaviour. Thus they were also chemically affected the yields of recovered products.

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Electronic Communication in Diethynylaryl-Bridged Units

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Synthesis of (dppe)-Substituted Bimetallic Compounds

7a,d,g

dissolution of residues in toluene (or benzene), which al-

lowed free dppe to be removed. By this method, products

could be isolated and obtained in a satisfactory pure state,

but the numerous cycles of precipitation/dissolution caused

the yields to be drastically decreased.

Complexes 6a,d,g were subjected to ultraviolet irradia-

tion (Scheme 2) to obtain the corresponding dppe-substi-

tuted bimetallic complexes. All reactions were carried out

in distilled benzene using an equivalent amount per iron

centre of 1,2-bis(diphenylphosphanyl)ethane, which readily

replaces all the carbonyl groups.

Cyclic Voltammetry (CV)

Concerning the recovery of products in a pure form,

The CV at the platinum electrode, at scan rate 0.1 Vs^–1,

chromatographic purification proved always too aggressive, of a 2-m solution of complex 7a in CH₂Cl₂in the presence

and isolation of complexes 7a,d,g was achieved only by re- of 0.1  nBu₄NPF₆exhibits two reversible (in the chemical

peated cycles of precipitation from hexane and following and electrochemical sense) one-electron waves at E_pa

=

Scheme 2.

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–

Table 1. Cyclic voltammetric data for [Cp(dppe)Fe–CϵC–Ar–CϵC–Fe(dppe)Cp]⁺[BF₄].^[a]

[a] Solvent CH₂Cl₂/0.2  nBu₄NBF₄; potential scan rate 0.1 Vs^–1at a 0.5-mm diameter platinum disk electrode; T = 20 °C. [b] Volts vs.

SCE. [c] Data are in mV.

–0.16 V and 0.14 V versus saturated calomel electrode ation of a 4,4Ј-biphenyldiyl spacer drastically reduces the

(SCE), current ratio (i_pa/i_pc) = 1 (Figure 1, a). These two electronic communication between the metal sites.

waves correspond to the formation of 7a⁺and 7a²⁺. The

We have also measured by CV the CO-substituted pre-

anodic and cathodic peak separations (E_pa–E_pc) are 56 and cursors 6a–g. Such compounds are unstable in redox cycles,

70 mV at scan rate 0.1 Vs^–1(Table 1).

as expected from the absence of the bidentate phosphane

ligands, which guarantee the reversibility of the process for

7a,d,g. As a consequence, the possibility of obtaining analo-

gous information about their electrical behaviour and the

metal–metal electronic interaction is compromised. The

CVs of all (CO)-substituted complexes (except for 6c, this

compound is very weakly soluble in most of the commonly

used solvents) are characterised in the anodic scan by two

irreversible waves. The CV in Figure 2 (a) is representative

of the anodic scan of complexes 6a,b and 6d–g; in Table 2

the values of E_pare reported. The first waves correspond

to the formation of [Cp(CO)₂Fe–CϵC–Ar–CϵC–Fe-

(CO)₂Cp]⁺in the range of E_p= 0.84–1.15 V; potentials as

high as expected on the basis of the electron-poor nature of

the CpFe(CO)₂moiety. Concerning the second oxidation

waves, they cannot be definitely assigned to the formation

of [Cp(CO)₂Fe–CϵC–Ar–CϵC–Fe(CO)₂Cp]²⁺because of

the irreversible nature of the preceding peaks. It is easily

seen from Figure 2 that the height of the second oxidation

peak is noticeably smaller than that of the first oxidation

peak. The CVs at higher scan rates up to 100 Vs^–1did not

Figure 1. Cyclic voltammetric scan at a 0.5-mm diameter platinum

disk electrode, T = 20 °C. Solvent CH₂Cl₂/0.1  nBu₄NBF₄; scan

rate of 0.1 Vs^–1; (a) solid line, 1 m 7a; (b) dashed line, 1 m 7g.

Similarly the CV of complex 7d in the same conditions

is characterised by two reversible one-electron waves in the

oxidation scan from 1.5 to –1.5 V (E_pa= 0.12 V and 0.34 V

vs. SCE) and shows a wave separation ∆E° of 0.22 V and

peak separations (E_pa–E_pc) of 60 mV (Table 1). On the con-

trary, the CV of complex 7g (Figure 1, b) reveals only one

reversible wave at E_pa= 0.08 V with (E_pa–E_pc) = 70 mV and

current ratio (i_pa/i_pc) = 1 (Table 1).

The low values of the oxidation potentials show that

these binuclear complexes are electron-rich compounds.

More interesting the quite large wave separations found for

complexes 7a and 7d (∆E° = 0.29 V and 0.22 V, respec-

tively) reveal a significant degree of metal–metal interaction

when the 2,5-thiophene and 1,4-phenylene groups are pres-

ent in the diethynylaryl spacer. The presence of a single

Figure 2. Cyclic voltammetric scan of 1 m 6e at a 0.5-mm dia-

meter platinum disk electrode, T = 20 °C. Solvent CH₂Cl₂/0.1 

nBu₄NBF₄; (a) solid line, scan rate of 0.1 Vs^–1; (b) dashed line,

wave in the CV of complex 7g suggests that the intercal- scan rate 0.5 V.

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allow unequivocal analysis of the variation of the intensity another absorption mode at lower energy.^[13]These results

of the second waves because of the intrinsic slowness of the reveal that substitution of the Cp* with the Cp groups in

corresponding electron transfer. However, by increasing the the metal sites renders more efficient the electronic coupling

scan rate from 0.1 to 0.5 Vs^–1, the intensity of this wave, between the metal groups, the difference presumably com-

i×v^–1/2(v = scan rate), decreases (Figure 2, b). One can sug- ing from the metal/carbon ratio in the frontier orbitals be-

gest that the second oxidation process does not correspond cause of the presence of the methyl groups.

to the oxidation of the radical cation of the neutral ethyn-

The NIR spectrum of 7a⁺(Figure 3) displays a band

ylaryl binuclear complexes, but rather indicates the oxi- with a maximum at 5640 cm^–1and a shoulder at 6500 cm^–1.

dation of other intermediates formed at the potential of the

first wave. In this context, the separation between the two

subsequent waves does not furnish sound information on

the electronic interaction between the CpFe(CO)₂units.

Table 2. Cyclic voltammetric data for [Cp(CO)₂Fe–CϵC–Ar–

CϵC–Fe(CO)₂Cp]⁺[BF₄].^[a]

–

Figure 3. NIR spectra in CH₂Cl₂at 20 °C of (a) 3.41 m 7a⁺and

(b) 7a²⁺; optical path, b = 0.100 cm.

[a] Solvent CH₂Cl₂/0.2  nBu₄NBF₄; potential scan rate 0.1 Vs^–1

at a 0.5-mm diameter platinum disk electrode; T = 20 °C. [b] Volts

vs. SCE.

The bands in this spectral region are considered diagnos-

tic for an intervalence transfer process.^[10]A detailed theo-

retical overview on the presence of two or more bands, as

previously discussed for the closely related Cp* deriva-

tives^[7h]and other mixed-valence complexes,^[8a]is given by

IR, UV/Vis, NIR Analysis

The mixed-valence radical cation 7a⁺was generated at Meyer et al.^[1l]As shown in Figure 4, the deconvolution of

room temperature in CH₂Cl₂by chemical oxidation of a the spectrum has been satisfactorily obtained by consider-

3.4-m wine-coloured solution of the neutral complex with ing three Gaussian components (5500, 6500, 7900 cm^–1)

1

equiv. of decamethylferrocenium tetrafluoroborate, and the tail of the UV/Vis absorption. The effect of the

[Fe(Cp*)]₂[BF₄] {Cp* = C₅(CH₃)₅}. The resulting solution polarity and ionic strength of the medium, which is gen-

was deep-blue coloured. erally employed to distinguish between class II and class III

The IR analysis of 7a⁺shows a single vibration mode for behaviour, was tested. The spectra of 7a⁺in CH₃CN (ε =

the CϵC stretches (1981 cm^–1), at an intermediate fre- 37.5) and CH₂Cl₂(ε = 10.7) are almost identical, that is

quency between those found for 7a (2031 cm^–1) and 7a²⁺no variation of the band energy and shape was observed.

(1929 cm^–1). Averaging of the vibration modes of the triple Moreover, there is no dependence of the band energy and

bonds is expected in homobimetallic class III complexes shape on ionic strength up to an electrolyte concentration

containing a polyalkynyl bridge, as the unpaired electron is of 0.2  in [nBu₄N][BF₄].^[6i,7h,10]The analysis of the bands

fully delocalised and the resulting structure is symmetrical. gives a bandwidth for the lower energy band I (Figure 4),

Conversely, for the previously reported [Cp*(dppe) Fe– ∆ν = 1000 cm^–1, which is much below the theoretical

˜

1/2

CϵC–Ar–CϵC–Fe(dppe)Cp*] analogue (Ar = 2,5-C₄H₂S), value (3630 cm^–1) predicted by the Hush equation for a

Lapinte and co-workers^[7h]found that the two metal sites localised mixed-valence (class II) symmetric system,^[14]

are not equivalent on the basis of the presence of two sepa- ∆ν = (2310ν )^1/2cm^–1, where ν is the frequency of the

˜

c

1/2

c

rate CϵC vibration modes in the IR spectrum (1983 and maximum. By analogy with the data reported by Lapinte

1927 cm^–1), predicting that the unpaired electron is partially and co-workers on the Cp* analogues,^[7h]band I of 7a⁺

localised on the IR time scale (10^–13s). Alternatively, in this could be an intervalence transfer (IT) transition and 7a⁺is

latter case, the lower energy band could not be a conse- assigned to class III, at least on the timescale of the NIR

quence of a symmetry breaking associated with the localis- spectroscopy (10^–14s). The value of the H_abparameter,^[14,15]

ation of the odd electron. Possibly this band could be due calculated using the formula H_ab(cm^–1) = ν_max/2, is

˜

to a Fermi coupling or to a harmonic combination with 2750 cm^–1.

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The couple 7a/7a⁺was also studied in the UV/Vis spec-

troscopic region. The solution of 7a in CH₂Cl₂presents a

very strong absorption at λ = 410 nm, a wavelength quite

similar to that reported for the Cp* analogue (418 nm), and

attributed to a metal-to-ligand charge transfer (MLCT)

transition.^[7h]The spectrum of the deep-blue solution of 7a⁺

is totally different from its precursor 7a. In fact, two new

strong bands appears at 612 and 765 nm, which are respon-

sible for the deep-blue colour of the mixed-valence species.

They might be ascribed to LMCT transitions, similarly to

what Lapinte and co-workers previously proposed for the

absorptions at 588 and 653 nm of the Cp* mixed-valence

analogous species.^[7h]In that case, the intense MLCT transi-

tion observed for the neutral complex is still present in the

spectrum of the monocation at a slightly red-shifted wave-

length (423 nm), indicating that the characteristic absorp-

tions of both Fe^IIand Fe^IIImetal groups coexist (trapped

valence, class II). On the contrary, in the UV/Vis spectrum

of 7a⁺the MLCT band, which is characteristic of a Fe^II

species, is absent, clearly showing that in 7a⁺the valence is

delocalised on the very fast UV/Vis timescale (10^–15s).

Figure 4. Gaussian deconvolution of the NIR bands of 7a⁺(dashed

line, experimental curve; solid lines, deconvoluted bands I, II, III;

bold line, the sum of I, II, III components).

The addition either in CH₃CN or CH₂Cl₂of a second

equivalent of ferrocenium, whose oxidation potential is suf-

ficiently high to oxidise the second metal group forming a

green solution of 7a²⁺, causes the disappearance of the

bands of the monocation and the appearance of a less in-

tense absorption at 13100 cm^–1(Figure 3), likely because of

a ligand-to-metal charge transfer (LMCT) transition. Un-

der the same condition, we have found that Cp₂Fe⁺and

Cp*₂Fe⁺show bands at 13600 and 12900 cm^–1respectively.

Also the oxidation of 7g was monitored by NIR spec-

troscopy (Figure 5). The oxidation with 1 equiv. of ferro-

cenium gives a green solution whose spectrum in this region

presents a very weak band at about 6000 cm^–1and a more

intense band at 12500 cm^–1, which could be assigned to the

IT process of 7g⁺and to an LMCT transition of the dicat-

ion 7g²⁺, respectively, indicating that the mixed-valence

state is not stable with respect to the disproportionation to

the 7g and 7g²⁺states, in agreement with the presence of a

single oxidation wave in the CV of complex 7g.

Density Functional Theory (DFT) Analysis

In order to gain insight on the factors ruling the extent

of electronic communication and possibly to deconvolute

the effects due to the structural parameters (i.e., metal–

metal distance) from those related to the peculiar electronic

features (i.e., the nature of the bridge and the nature of the

ancillary ligands), we investigated systematically selected

model complexes by DFT calculations.^[16]

DFT optimisations were performed at B3LYP/

LANL2DZ,6-31G** level of theory without any restriction

in symmetry and tight conditions (see Computational

Details) for neutral and oxidised PH₃analogues of 7a^0/+1

,

7d^0/+1and 7g^0/+1, which differ in the nature of the conju-

gated spacer. A transoid conformation of the metal centres

is found to be more stable for PH₃-7a and PH₃-7d. Instead,

in the fully optimised PH₃-7g the phenyl rings are oriented

in distorted cisoid conformation.^[17]It is worth noticing that

the orientation of the metal fragments is such that the plane

of the aromatic cores bisects the dihedral angle P–Fe–P.

Consequently the conformation calculated for PH₃-7g is di-

rectly related to the torsion angle between the two phenyl

rings. The calculated geometries of the neutral complexes

are shown in Figure 6 (I–III). Significant interatomic dis-

tances and bond angles are reported in Table 3. In several

cases the crystallographic data of structurally similar com-

pounds are indicated and the good agreement with the

analogous computed parameters enhances our confidence

in the quality of the optimised geometries. A distinct alter-

nation of single and multiple bonds is predicted in the three

complexes. The average bond length of the alkyne units of

the linkers is 1.232 Å, a value that nicely compares to the

triple bond length in acetylene, that is 1.212 Å. The local-

ised double bonds of the rings have an average value of

1.386 Å, which is also comparable to the ethylene double

bond length (1.320 Å).

Figure 5. NIR spectra in CH₂Cl₂at 20 °C of 3.4 m 7g oxidised

with 1 equiv. of ferrocenium.

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tylides is well documented.^[18]A qualitative fragment analy-

sis reveals that the cationic [Cp(PH₃)₂Fe]⁺fragments bond

by strong σ-type interactions between unoccupied high-ly-

ing metallic frontier orbitals and low-lying occupied orbit-

als of the bridge (electron donation from the bridge to the

metal fragments). Metal–carbon π-type interactions occur

between occupied metal frontier orbitals and unoccupied

carbon levels (weak π-type back-donation). The Kohn–

Sham HOMOs are mainly localised on the metal centres

and the ethynyl groups, and, to a less extent, on the aro-

matic rings. They result from out-of-phase combination of

metal dπ orbitals and out-of-plane π orbitals of the linker;

they exhibit anti bonding character between adjacent mul-

tiple bonds. The LUMO and LUMO+1 are almost degener-

ate and localised on the metal fragments. The topologies

of Kohn–Sham frontier orbitals of PH₃-7d are shown in

Figure 7.

Figure 7. Top view of the contour plot of the frontier Kohn–Sham

MOs of PH₃-7d. Contour values are 0.035, 0.04 and 0.05 (e/

bohr³)^1/2

.

A rather large energy gap separates the occupied and

empty orbital sets in PH₃-7a, PH₃-7d and PH₃-7g (see Fig-

ure 8). The HOMO–LUMO gap in PH₃-7d is larger by

about 0.2 eV than that of PH₃-7a (3.50 vs. 3.30 eV), and

this is mainly due to the fact that the energy of HOMO of

PH₃-7d is lower than that of HOMO of PH₃-7a. The

HOMO–LUMO gap of PH₃-7g is 3.69 eV.

Figure 6. Calculated molecular structures of neutral model com-

plexes: PH₃-7a (I), PH₃-7d (II), PH₃-7g (III), 6d (IV) and CH₃O-

6e (V). Heteroatoms are labelled for sake of clarity.

The ground-state electronic structures of PH₃-7a, PH₃-

7d (the electronic structure of this complex was previously

reported by Lapinte et al.^[7o]) and PH₃-7g exhibit similar

features.

Figure 8. Frontier MOs for complexes PH₃-7a, PH₃-7d, PH₃-7g, 6d

and CH₃O-6e. HOMOs are indicated by double occupancy arrows;

bold lines indicate almost degenerate energy levels.

Several authors have reported their theoretical studies on

polyalkynyl- and alkynylaryl-bridged bimetallic complexes;

Upon oxidation no significant conformational changes

thus the bonding between group 8 metal centres and ace- are observed. Both in PH₃-7a⁺and PH₃-7d⁺the metal units

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Table 3. Selected interatomic distances [Å] and angles [°] for the neutral and oxidised complexes PH₃-7a^0/+1, PH₃-7d^0/+1and PH₃-7g^0/+1

FULL PAPER

.

[a] Average values. [b] X-ray data of (η⁵-C₅Me₅)(η²-dppe)Fe–CϵC–C₆H₅.^[7j][c] Single C_β–C bond in the ethynyl bridge. [d] Average crys-

tallographic values of [Fe(η⁵-C₅Me₅)(η²-dppe)(CϵC)]₃(µ-1,3,5-C₆H₃).^[7o]. [e] Triple C_αϵC_βbond in the ethynyl bridge. [f] Torsion angle

between the two rings, around the bond C_d–C_dЈ. [g] By indicating the centroids of the Cp rings as Q and QЈ, a torsion angle γ can be

defined as Q–Fe1–Fe2–QЈ, so that γ = 0 corresponds to a cis conformation of the metal termini and γ = 180 indicates a trans conformation.

are still oriented trans with respect to the bridging ligand, orbitals. The HOMOs of the cations PH₃-7d⁺and PH₃-7g⁺

while in PH₃-7g⁺the cisoid conformation is still more are characterised by an in-phase contribution of metal dπ

stable. Upon going from the neutral to the corresponding orbitals disposed in an anti bonding fashion with respect to

oxidised species, the Fe–C_αbond and the bond between C_βthe out-of-plane pπ orbitals of the linker. While the extent

and the adjacent carbon atom of the aryl core shorten, of delocalisation is large in PH₃-7a⁺, the contribution of

while the alkyne bond C_αϵC_βelongates. The cation PH₃- p(π) carbon orbitals of the phenyls to HOMO is poorer

7a⁺is generated by removal of one electron from the both in PH₃-7d⁺and PH₃-7g⁺. The Kohn–Sham HOMOs

HOMO of the neutral parent compound. In PH₃-7d⁺and of PH₃-7a⁺, PH₃-7d⁺and PH₃-7g⁺are shown in Figure 9.

PH₃-7g⁺there is a rearrangement of the energy levels, be-

The effect of replacing the ancillary ligands PH₃with CO

cause their HOMOs (118β and 138β) correspond to groups was investigated theoretically on the couple 6d^0/+1

HOMOs–1 of the neutral parent complexes. The trend in by direct comparison with PH₃-7d^0/+1. In addition, the in-

stability, based on the HOMO–LUMO gap, is inverted in fluence of alkoxy groups on the phenyl core was explored

the series of cations, PH₃-7a⁺being the most stable, as ex- by calculating the electronic structures of the methoxy ana-

pected on the basis of the lower resonance energy of the logues of 6e^0/+1, to be compared with those of 6d^0/+1. The

thiophenyl unit compared to that of the phenyl unit, be- full geometry optimisations of 6d^0/+1and of CH₃O-6e^0/+1

cause of the mixing of sulfur d orbitals with carbon pπ were calculated at B3LYP/LANL2DZ,6-31G** level of

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Electronic Communication in Diethynylaryl-Bridged Units

FULL PAPER

theory. The neutral species are shown in Figure 6 (IV, V)

and their significant structural data are reported in Table 4.

The full optimisations converged in both neutral complexes

with a trans conformation of the metal groups. As for the

PH₃neutral derivatives, the calculations predict a distinct

alternation of adjacent single and multiple bonds (see

Table 4). In particular, a contraction of the alkyne bond

length C_αϵC_βis observed compared to the PH₃-substituted

species, which is a consequence of the electron density loss

of the metal (1.225 vs. 1.232 Å).

Another effect of replacing the phosphanes with car-

bonyl groups is a reduction in energy of the frontier orbit-

als, as expected on the basis of the electron-poorer nature

of the metal centres in the carbonylated complexes due to

backbonding from the occupied metal orbitals to the empty

π* of the carbonyl ligands (Figure 8). The HOMOs of 6d

and CH₃O-6e, resulting from out-of-phase combination of

metal dπ orbitals and out-of-plane π orbitals of the linker,

are strongly localised on the bridge, with minor metal con-

tribution, and show anti bonding character between adja-

cent multiple bonds. The LUMOs and LUMOs+1 are al-

most degenerate and localised on the end-capping termini.

The HOMO–LUMO gap of CH₃O-6e (3.17 eV) is smaller

that that of 6d (3.39 eV) and this is mainly the result of the

destabilisation of the HOMO of the former complex,

caused by the presence of the methoxy substituents (Fig-

ure 8). The variation in bond lengths occurring upon oxi-

dation is reported in Table 4. No rearrangements of the

frontier electronic levels are observed: the electron is re-

moved from the HOMO of the neutral parent compound,

as found for PH₃-7a.

Figure 9. Kohn–Sham HOMOs of PH₃-7a⁺, PH₃-7d⁺, PH₃-7g⁺,

6d⁺and CH₃O-6e⁺. Surface values are 0.04 (e/bohr³)^1/2

.

Table 4. Selected interatomic distances [Å] and angles [°] for the neutral and oxidised complexes 6d^0/+1and CH₃O-6e^0/+1

.

[a] Average values. [b] Single C_β–C bond in the ethynyl bridge. [c] Triple C_αϵC_βbond in the ethynyl bridge. [d] By indicating the centroids

of the Cp rings as Q and QЈ, a torsion angle γ can be defined as Q–Fe1–Fe2–QЈ, so that γ = 0 corresponds to a cis conformation of the

metal termini and γ = 180 indicates a trans conformation.

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Table 5. Mulliken spin densities.

[a] Alkyne carbon atom linked to iron. [b] Alkyne carbon atom linked to the aromatic ring. [c] Carbon atom of the aromatic core; the

labelling is reported in the schemes above.

The extent of delocalisation of the unpaired electron in tational Details). The lowest excitation energy computed for

the radical cations was estimated on the basis of Mulliken PH₃-7a⁺in the NIR region is 8560 cm^–1(0.47), with an er-

spin densities, which are reported in Table 5.

ror comparable to those reported in a similar analysis.^[20a]

The distribution of the spin density is symmetric in all It corresponds to the lowest energy deconvoluted experi-

the radical cations, although symmetry was not taken into mental absorption at 5500 cm^–1. The detailed composition

account in the calculations. The spin density is well delocal- is shown in Table 6. Its most relevant components involve

ised on the iron centres and on the C_αatoms in PH₃-7a⁺the couples of levels 120α–121α and 119β–120β, that is the

and in PH₃-7d⁺. In PH₃-7a⁺the spin density is distributed highest occupied α/β levels and the lowest unoccupied α/β

also on the carbon atoms adjacent to S in the thiophenyl levels respectively (pictures of the couples of MOs involved

core, while in both PH₃-7d⁺and PH₃-7g⁺the spin density in the calculated excitation energies are given in the Sup-

values on the phenyl rings are rather small. In 6d⁺and porting Information), which have significant contributions

CH₃O-6e⁺the significant decrease of the spin density on from d orbitals of both iron centres and p(π) carbon orbit-

the metals as compared to PH₃-7d⁺is accompanied by its als of the bridge.

increase on the two α carbons and on two carbon atoms of

the rings (see Table 5).

In the case of PH₃-7d⁺, an absorption is computed at

6859 cm^–1(f = 0.62) corresponding mainly to the HOMO

The extent of delocalisation is in favour of a stronger (118β)–LUMO (119β) transition. The experimental spec-

metal–metal interaction,^[7m]and decreases in the PH₃deriv- trum of the Cp* analogue of 7d⁺exhibits a single band at

atives in the order PH₃-7a⁺Ͼ PH₃-7d⁺Ͼ PH₃-7g⁺, in 4960 cm^–1.^[7l]Also in this case the couple of orbitals in-

agreement with the trend based on the experiments.

volved in the monoelectronic transition are delocalised all

To investigate the presence and the nature of the NIR over the molecular backbone.

absorptions of these model di-iron cations we have carried

By TDDFT calculations we also found an allowed low-

out time-dependent density functional theory (TDDFT) energy absorption for PH₃-7g⁺at 5216 cm^–1(f = 0.83) as-

calculations,^[19]which have been recently employed with signed mainly to HOMO (138β)–LUMO (139β) and

success for mixed-valence organometallic species.^[20]The HOMO–1 (137β)–LUMO (139β) transitions. This exci-

level of theory is B3LYP/LANL2DZ,6-31+G* (see Compu- tation energy in the NIR region corresponds to the weak

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Electronic Communication in Diethynylaryl-Bridged Units

Table 6. NIR vertical excitation energies.

FULL PAPER

PH₃-7a⁺

PH₃-7d⁺

PH₃-7g⁺

6d⁺

CH₃O-6e⁺

HOMO–LUMO gap 1.525 eV^[a]

1.452 eV^[b]

0.886 eV^[c]

1.474 eV^[d]

1.451 eV^[e]

Excitation energy

[nm]

1168

(8562)

1458

(6859)

1917

(5216)

871

(11481)

809

(12361)

[cm^–1]

Oscillator strength

0.47

120α Ǟ 121α

(0.296)

0.62

119α Ǟ 122α

(0.258)

0.83

139α Ǟ 140α

(0.239)

0.52

111α Ǟ 112α

(0.277)

0.39

126α Ǟ 128α

(0.101)

Composition^[f]

108β Ǟ 120β

(–0.104)

116β Ǟ 123β

(–0.161)

107β Ǟ 119β

(–0.107)

111β Ǟ 119β

(–0.160)

132β Ǟ 139β

(0.200)

135β Ǟ 140β

(–0.149)

111α Ǟ 116α

(0.133)

108β Ǟ 111β

(–0.174)

127α Ǟ 128α

(–0.300)

126β Ǟ 127β

(0.935)

119β Ǟ 120β

(1.006)

115β Ǟ 122β

(–0.171)

118β Ǟ 119β

(0.996)

138β Ǟ 139β

(0.865)

110β Ǟ 111β

(0.904)

[a] The HOMO is 120α, the LUMO is 120β. [b] The HOMO is 118β, the LUMO is 119β. [c] The HOMO is 138β, the LUMO is 139β.

[d] The HOMO is 111α, the LUMO is 111β. [e] The HOMO is 127α, the LUMO is 127β. [f] The most relevant monoelectronic transitions

are indicated; values in brackets are the CI coefficients.

absorption experimentally detected for 7g⁺around of such organometallic assembly (7a,d,g) proved to be a

6000 cm^–1. It should be noted that in PH₃-7d⁺and PH₃-7g⁺useful manifold to study the electrical and optical proper-

the strongest NIR absorptions have an important contri- ties of the conjugated spacer and the interaction between

bution from the HOMO–LUMO transition and are thus the metal centres, and thus, in perspective, the properties of

shifted toward lower frequencies, as compared to that of polymeric materials incorporating such building blocks in

PH₃-7a⁺. Most importantly, no charge transfer character the backbone.

between the metal termini is observed, as reported recently

for other organometallic binuclear systems,^[20a]but these

NIR absorptions are better interpreted as allowed elec-

tronic transitions between filled and empty delocalised

frontier levels. The oscillator strengths increase with the

length of the alkynylaryl bridge, as already noticed in pre-

vious theoretical studies.^[20c,20d]

TDDFT calculations on the carbonylated model cations

6d⁺and CH₃O-6e⁺do not reveal any low-energy transition.

In the former an absorption is predicted at 11481 cm^–1(f =

0.52), while in the latter the calculated excitation energy is

12361 cm^–1(f = 0.39). In both cases the transitions involve

the highest occupied α/β levels and the lowest unoccupied

α/β levels, as for PH₃-6a⁺. Also in these species no metal–

metal charge transfer character is observed.

On the basis of the electrochemical and optical proper-

ties of complexes 7a,d,g, a correlation between the extent

of the metal–metal electronic interaction and the nature of

the diethynylaryl bridge can be pointed out. In fact, the

∆E° separations between the two subsequent reversible

waves increase in the order 7a Ͼ 7d Ͼ 7g, as does the effi-

ciency of the metal–metal electronic coupling. The pre-

viously reported [Cp*(dppe)Fe–CϵC–Ar–CϵC–Fe(dppe)-

Cp*] analogue (Ar = 2,5-C₄H₂S) displayed a somewhat

more pronounced ∆E° (0.35 V) than 7a. This indicates a

higher thermodynamic stability of the corresponding

monocationic intermediate with respect to disproportiona-

tion, probably due to the electron-donating effect of the

methyl substituents of Cp*, which stabilise the positive

charge on the metal groups. A similar trend can be estab-

lished on the basis of the spectroscopic results for the

mixed-valence species 7a⁺, 7d⁺and 7g⁺. In the case of 7a⁺,

the IR data, the shape and the solvent effect of the NIR

Conclusions

The combination of the EOP synthetic protocol and the band and the UV/Vis spectrum indicate unambiguously

Pd-catalysed metal–carbon bond formation procedure of- that the cation is a delocalised (class III) mixed-valence sys-

fers suitable synthetic access to a large family of bimetallic tem on the timescale of 10^–15s. The comparison with the

complexes (6a–g) characterised by the presence of different analogous

2,5-bis{[η⁵-C₅(CH₃)₅]Fe(η²-dppe)(σ¹-CϵC)}-

conjugated (bis-ethynyl)aromatic spacers bridging two iron C₄H₂S, which is valence-trapped on the IR timescale, high-

centres. Our EOP synthetic route represents a complemen- lights the important effect of replacing Cp* with Cp in the

tary approach to alkynylaryl-bridged bimetallic complexes metal termini. On the basis of its electrochemical behaviour

with respect to Lapinte’s method.^[7q]In our procedure the and the spectroscopic data reported in the literature, 7d⁺

straightforward formation of the bis(tributylethynyltin)aro- can also be assigned to class III. The analysis of 7g⁺is less

matic derivatives 3a–g and their direct in situ use to form clear-cut, because of the low stability of this mixed-valence

complexes 6a–g prevent the formation and isolation of the cation in solution. PH₃-7g⁺has a certain extent of delocalis-

corresponding bis(trimethylsilylethynyl) derivatives as re- ation, but the metal–metal interaction is hampered with re-

quired by Lapinte’s procedure.^[7u]

spect to PH₃-7d⁺because of the increase of the distance

Although substitution of carbonyls with dppe is conve- and the greater flexibility of the bridge. DFT calculations

niently performed only in some cases, the dppe derivatives support the experimental findings and allow deconvolution

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S. Santi, C. Lo Sterzo et al.

FULL PAPER

was dissolved in a small amount of CH₂Cl₂and then precipitated

with pentane. After filtration and repeated washings with pentane,

a dark brown powder (0.533 g, 74% yield) was obtained. IR

of geometric effects, such as the intermetallic distance (PH₃-

7d⁺vs. PH₃-7g⁺), from electronic effects, that is the different

nature of the incorporated aryl core (PH₃-7a⁺vs. PH₃-7d⁺),

on the extent of electronic delocalisation. On the basis of

the computational results on the carbonylated model cation

6d⁺a less efficient electron delocalisation is observed with

respect to the corresponding PH₃analogue (6d⁺vs. 7d⁺). In

addition, in the presence of the methoxy substituents on the

aryl core in CH₃O-6e⁺the spin density decreases further on

the iron centres and moves to the phenyl ring (CH₃O-6e⁺

vs. 6d⁺).

TDDFT calculations predict that NIR absorptions,

which are a sort of fingerprint of mixed-valence species, oc-

cur only in PH₃-7a⁺, PH₃-7d⁺and PH₃-7g⁺, and that they

do not exhibit dominant charge transfer character between

the metal termini, but are better described as allowed elec-

tronic transitions between filled and empty frontier levels.

Finally the efficiency of the dialkynyl–thiophenyl unit in

conveying electron information is further recognised and

associated to the large extent of delocalisation in the radical

cations bearing this aryl core.^[6l,6m]Work is in progress to

employ this functional unit, intercalated also by different

metal centres, in larger molecular wires.

(CH Cl ): 2095 (w) ν

, 2044 (s), 2000 (s) ν

cm^–1. ¹H NMR

˜

2

CϵC

CO

(CDCl₃): δ = 6.60 (2 H, thioph.), 5.02 (10 H, Cp) ppm. Spectro-

scopic data correspond to the reported data.^[4]

2,5-Bis[(η⁵-C₅H₅)Fe(CO)₂(σ¹-CϵC)]-3-(C₄H₉)-C₄HS (6b): This

complex was prepared as described for 6a, from Bu₃SnCϵCH

(0.837 g, 2.55 mmol), 2,5-diiodo-3-butylthiophene (0.500 g,

1.28 mmol) and Pd(PPh₃)₄(30 mg, 0.03 mmol) in dioxane (30 mL),

LDA (1.5 mL, 2 ) and (η⁵-C₅H₅)Fe(CO)₂I (0.799 g, 2.55 mmol).

Step 3 was accomplished by stirring at 70 °C for 10 h. After re-

peated precipitation cycles from CH₂Cl₂/pentane the product

(0.352 g, 51% yield) was recovered as described for 6a as a coffee-

brown powder. IR (CH Cl ): 2095 (w) ν

, 2042 (s), 1997 (s) ν

˜

CO

˜

2

CϵC

cm^–1. H NMR (C₆D₆): δ = 7.73 (thioph.), 4.04 (Cp), 2.85, 1.70–

0.80 (C₄H₉) ppm. ¹³C NMR ([D₇]DMF): δ = 213.2, 141.9, 131.3,

128.4, 107.3, 89.5, 85.9, 71.7, 66.3, 31.8, 21.7, 13.1 ppm.

C₂₆H₂₀Fe₂O₄S (540.20): calcd. C 57.81, H 3.73; found C 57.60, H

3.83.

1

2,5-Bis[(η⁵-C₅H₅)Fe(CO)₂(σ¹-CϵC)]-3-(C₁₆H₃₃)-C₄HS (6c): This

complex was prepared as described for 6a, from Bu₃SnCϵCH

(0.586 g, 1.79 mmol), 2,5-diiodo-3-hexadecylthiophene (0.500 g,

0.89 mmol) and Pd(PPh₃)₄(16 mg, 0.01 mmol) in dioxane (30 mL),

LDA (1.1 mL, 2 ) and (η⁵-C₅H₅)Fe(CO)₂I (0.559 g, 1.78 mmol).

Step 3 was accomplished by stirring at 65 °C for 23 h. After re-

peated precipitation cycles from CH₂Cl₂/methanol the product

(0.09 g, 15% yield) was recovered as described for 6a as a coffee-

Experimental Section

brown powder. IR (CH Cl ): 2096 (w) ν

, 2042 (s), 1997 (s) ν

˜

CO

˜

2

CϵC

General: All manipulations were carried out under argon using

Schlenk techniques. Dioxane and benzene were dried and distilled

respectively over Na and Na/K alloy, and argon-saturated prior to

use. The following chemicals were prepared according to the pub-

lished procedure: 1,4-diiodo-2,5-dioctyloxybenzene,^[4]1,4-diiodo-

2,5-dibutoxybenzene,^[4]2,5-diiodo-3-hexadecylthiophene^[4]and 2,5-

diiodo-3-butylthiophene.^[4]Other chemicals were used as received.

Routine NMR spectra were recorded with a AC-300 P Bruker spec-

trometer (300, 121 and 75 MHz respectively for ¹H, ³¹P, ¹³C).

Chemical shifts are given in parts per million (ppm) relative to tet-

ramethylsilane (TMS) for ¹H NMR and ¹³C NMR spectra and

85% H₃PO₄for ³¹P NMR spectroscopy. Cyclic voltammograms

[except the ones of complexes 6d,g and 7a,d, performed by Dr.

Mikhail G. Peterleitner of Nesmeyanov Institute of Organoelement

Compounds (INEOS), Russian Academy of Science, Moscow] were

recorded with an AMEL 500 potentiostat. Transmittance FTIR

spectra were recorded with a Nicolet FT 510 instrument in the

solvent subtraction mode, using a 0.1-mm CaF₂cell. NIR spectra

were recorded with a Bruker Equinox55 FTIR spectrometer

equipped with an optical fibre for NIR measurements on liquids.

Elemental analyses were performed by the Servizio Microanalisi of

the Dipartimento di Chimica of Università di Roma “La Sapi-

enza”.

cm^–1. ¹H NMR (C₆D₆): δ = 7.52 (thioph.), 4.08 (Cp), 1.70–0.80

(C₁₆H₃₃) ppm. ¹³C NMR (CDCl₃): δ = 249.6, 230.2, 158.8, 121.6,

90.6, 85.3, 31.9, 29.7, 29.4, 22.7, 22.3, 14.1 ppm. C₃₈H₄₄Fe₂O₄S

(708.52): calcd. C 64.42, H 6.26; found C 64.30, H 6.15.

1,4-Bis[(η⁵-C₅H₅)Fe(CO)₂(σ¹-CϵC)]C₆H₄(6d): This complex was

prepared as described for 6a, from Bu₃SnCϵCH (0.985 g,

3.00 mmol), 1,4-diiodobenzene (0.500 g, 1.5 mmol) and Pd(PPh₃)₄

(35 mg, 0.03 mmol) in dioxane (25 mL), LDA (1.8 mL, 2 ) and

(η⁵-C₅H₅)Fe(CO)₂I (0.940 g, 3.00 mmol). Step 3 was accomplished

by stirring at 60 °C for 6 h. After repeated precipitation cycles from

CH₂Cl₂/pentane the product (0.406 g, 57% yield) was recovered as

described for 6a as a dark yellow powder. IR (CH₂Cl₂): 2107 (w)

ν

, 2043 (s), 1996 (s) ν_COcm^–1. ¹H NMR (C₆D₆): δ = 7.56 (ben-

˜

CϵC

zene), 4.06 (Cp) ppm. ¹³C NMR ([D₇]DMF): δ = 213.6, 131.7–

124.9, 115.6, 91.1, 85.9 ppm. C₂₄H₁₄Fe₂O₄(478.07): calcd. C 60.30,

H 2.95; found C 60.53, H 2.91.

1,4-Bis[(η⁵-C₅H₅)Fe(CO)₂(σ¹-CϵC)]-2,5-bis(OC₄H₉)C₆H₂

(6e):

This complex was prepared as described for 6a, from Bu₃SnCϵCH

(0.692 g, 2.11 mmol), 1,4-diiodo-2,5-dibutoxybenzene (0.500 g,

1.05 mmol) and Pd(PPh₃)₄(25 mg, 0.02 mmol) in dioxane (25 mL),

LDA (1.3 mL, 2 ) and (η⁵-C₅H₅)Fe(CO)₂I (0.661 g, 2.11 mmol).

Step 3 was accomplished by stirring at 50 °C for 7 h. After repeated

precipitation cycles from CH₂Cl₂/hexane the product (0.320 g, 49%

yield) was recovered as described for 6a as a coffee-brown powder.

2,5-Bis[(η⁵-C₅H₅)Fe(CO)₂(σ¹-CϵC)]C₄H₂S (6a). Step 1: In

a

Schlenk tube Bu₃SnCϵCH (0.962 g, 3.05 mmol), 2,5-diiodothio-

phene (0.500 g, 1.49 mmol) and Pd(PPh₃)₄(40 mg, 0.035 mmol)

were stirred in dioxane (30 mL) at 110 °C for 20 min.

IR (CH Cl ): 2106 (w) ν

˜

CϵC

, 2041 (s), 1994 (s) ν_COcm^–1. ¹H NMR

˜

2

(C₆D₆): δ = 7.72 (benzene), 4.19 (Cp), 3.75 (OCH₂), 1.62–0.92

[OCH₂(CH₂)₂CH₃] ppm. ¹³C NMR ([D₇]DMF): δ = 213.6, 130.4,

116.8, 86.5, 68.0, 19.1, 13.3 ppm. C₃₂H₃₀Fe₂O₆(622.28): calcd. C

61.77, H 4.86; found C 61.90, H 4.81.

Step 2: The brown solution was then cooled (ca. 0 °C) and LDA

(1.8 mL, 2 ⁾ was slowly added.

Step 3: This mixture was left for about 30 min at room temperature, 1,4-Bis[(η⁵-C₅H₅)Fe(CO)₂(σ¹-CϵC)]-2,5-bis(OC₈H₁₇)C₆H₂

(6f):

This complex was prepared as described for 6a, from Bu₃SnCϵCH

(0.896 g, 2.73 mmol), 1,4-diiodo-2,5-dioctyloxybenzene (0.800 g,

(η⁵-C₅H₅)Fe(CO)₂I (0.914 g, 3.01 mmol) was added and the re-

sulting dark solution was heated (50 °C) for 7 h, then filtered

through a thin Celite layer and dried under vacuum. The residue

1.36 mmol) and Pd(PPh₃)₄(32 mg, 0.03 mmol) in dioxane (25 mL),

2594

www.eurjic.org

Eur. J. Inorg. Chem. 2006, 2582–2597

Electronic Communication in Diethynylaryl-Bridged Units

LDA (1.7 mL, 2 ) and (η⁵-C₅H₅)Fe(CO)₂I (0.855 g, 2.73 mmol). [{Fe(η⁵-C₅H₅)(η²-dppe)(CϵC)}₂{2,5-C₄H₂S}][BF₄] (7a⁺): [Fe(η⁵-

FULL PAPER

Step 3 was accomplished by stirring at 80 °C for 12 h. After re-

peated precipitation cycles from CH₂Cl₂/methanol the product

(0.521 g, 52% yield) was recovered as described for 6a as a brown

C₅Me₅)₂][BF₄] (0.0033 g, 0.0081 mmol, 0.95 equiv.) was added at

room temperature to a stirred solution of [{Fe(η⁵-C₅H₅)(η²-

dppe)(CϵC)}₂{2,5-C₄H₂S}] (7a) (0.01 g, 0.0085 mmol) in CH₂Cl₂

powder. IR (CH Cl ): 2106 (w) ν

, 2042 (s), 1996 (s) ν cm^–1. (10 mL). The wine-coloured mixture immediately became blue and

˜

CO

˜

2

CϵC

¹H NMR (C₆D₆):

δ = 4.20 (Cp), 3.70 (OCH₂), 1.50–0.90 was stirred at room temperature for 1.5 h. The solution was filtered

[OCH₂(CH₂)₆CH₃] ppm. ¹³C NMR (CDCl₃): δ = 212.5, 154.0–

110.0, 85.4, 69.2, 29.4–22.6, 14.1 ppm. C₄₀H₄₆Fe₂O₆(734.50):

calcd. C 65.41, H 6.31; found C 65.32, H 6.27.

through Celite, reduced in vacuo to about 2 mL, and hexane

(15 mL) was added, allowing precipitation of a blue solid. After

decantation of supernatant pale yellow liquid, the precipitate was

washed several times with 5 mL of hexane, and dried under vacuum

yielding the pure [{Fe(η⁵-C₅H₅)(η²-dppe)(CϵC)}₂{2,5-C₄H₂S}]-

[BF₄] as blue microcrystals (0.0102 g, 95%). IR (CH₂Cl₂): 2059 (s)

4,4Ј-Bis[(η⁵-C₅H₅)Fe(CO)₂(σ¹-CϵC)](C₆H₄)₂(6g): This complex

was prepared as described for 6a, from Bu₃SnCϵCH (1.031 g,

3.14 mmol), 4,4Ј-dibromobiphenyl (0.500 g, 1.57 mmol) and

Pd(PPh₃)₄(37 mg, 0.03 mmol) in dioxane (30 mL), LDA (1.9 mL,

2 ) and (η⁵-C₅H₅)Fe(CO)₂I (0.984 g, 3.14 mmol). Step 3 was ac-

complished by stirring at 80 °C for 9 h. After repeated precipitation

cycles from CH₂Cl₂/pentane the product (0.728 g, 84% yield) was

recovered as described for 6a as a dark orange powder. IR

ν

cm^–1. C₇₀H₆₀BF₄Fe₂P₄S (1255.70): calcd. C 66.96, H 4.82;

˜

CϵC

found C 66.64, H 4.63.

Computational Details: Full geometry optimisations with tight con-

ditions were performed at B3LYP level of theory, as implemented

in Gaussian03.^[21]The hybrid functional is Becke’s three-parameter

nonlocal exchange functional^[22]with the local correlation func-

tional of Lee, Yang and Parr et al.^[23]The choice of this functional

is motivated by the successful results obtained in geometry calcula-

tions of both neutral and charged similar complexes^[7p]and by its

performance reported in very recent studies on the electronic coup-

ling in mixed-valence organometallic complexes.^[20a,24]In both

spin-restricted and spin-unrestricted calculations the 6-31G** basis

set was used for S, P, O, C and H and the standard LANL2DZ-

ECP was employed for Fe (B3LYP/LANL2DZ, 6-31G**).^[25]This

choice is suitable for accurate results at an acceptable computa-

tional expenditure. In charged species the total spin values do not

show significant spin contamination for doublet ion states.

TDDFT excitation energies were calculated at B3LYP level of

theory; standard LANL2DZ-ECP basis set was used for Fe and 6-

31+G* was employed for all the other atoms (B3LYP/LANL2DZ,

6-31+G*).^[25]

(CH Cl ): 2106 (w) ν

, 2041 (s), 1994 (s) ν

cm^–1. ¹H NMR

CO

˜

2

CϵC

(C₆D₆): δ = 7.66–7.01 (biph), 4.10 (Cp) ppm. ¹³C NMR ([D₇]-

DMF): δ = 213.6, 136.2–125.5, 115.3, 92.5, 86.0 ppm. C₃₀H₁₈Fe₂O₄

(554.16): calcd. C 65.02, H 3.27; found C 65.15, H 3.29.

2,5-Bis[(η⁵-C₅H₅)Fe(dppe)(σ¹-CϵC)]C₄H₂S (7a):

A rust colour

mixture of 2,5-bis[(η⁵-C₅H₅)Fe(CO)₂(σ¹-CϵC)]C₄H₂S (6a)

(0.247 g, 0.51 mmol) and 1,2-bis(diphenylphosphanyl)ethane

(0.406 g, 1.02 mmol) in benzene (200 mL) was exposed to ultravio-

let irradiation for 2 h at room temperature. The dark brown reac-

tion mixture was then filtered and dried under vacuum. The black

residue was washed with hexane, dissolved in toluene and precipi-

tated with hexane. This treatment was repeated three times to iso-

late a crystalline black powder (0.137 g, 23% yield). IR (CH₂Cl₂):

2052 (w) ν

cm^–1. ¹H NMR (C₆D₆): δ = 7.96, 740–6.90, 4.21

˜

CϵC

(Cp), 2.50–0.40 ppm. ¹³C NMR (C₆D₆): δ = 142.7–128.0, 114.2,

99.7, 81.7, 30.2, 28.6 ppm. ³¹P NMR (C₆D₆): δ = 106.9 ppm.

C₇₀H₆₀Fe₂P₄S (1168.90): calcd. C 71.93, H 5.17; found C 72.05, H

5.09.

Supporting Information (see footnote on the first page of this arti-

cle): UV/Vis spectra of 7a and 7a⁺; B3LYP/LANL2DZ,6-31G**-

optimised geometries of the studied model complexes, and pictures

of MOs involved in the NIR transitions.

1,4-Bis[(η⁵-C₅H₅)Fe(dppe)(σ¹-CϵC)]C₆H₄(7d): A yellow-orange

mixture of 1,4-bis[(η⁵-C₅H₅)Fe(CO)₂(σ¹-CϵC)]C₆H₄(6d) (0.200 g,

0.42 mmol) and 1,2-bis(diphenylphosphanyl)ethane (0.345 g,

0.42 mmol) in benzene (200 mL) was exposed to ultraviolet irradia-

tion for 3.5 h at room temperature. The dark red reaction mixture

was then filtered and dried under vacuum. The residue was washed

with hexane, dissolved in toluene and precipitated with hexane.

This treatment was repeated ten times, and the pure complex

Acknowledgments

This work was supported by the Consiglio Nazionale delle Ricer-

che (CNR, Roma) within the CNR/RAS (Russian Academy of Sci-

ence, Moscow) bilateral agreement for financial support and by the

Ministero dell’Istruzione, dell’Università e della Ricerca (MIUR)

within the PRIN 2003 research project. We are grateful to Patrizia

Gentili (Università “La Sapienza”, Roma) for her very useful coop-

eration. CINECA (Consorzio Interuniversitario di Calcolo del

Nord-Est, Casalecchio di Reno, Italia) is greatly acknowledged for

the generous allocation of their computational facilities.

(0.056 g, 12% yield) was finally recovered as a crystalline black

1

powder. IR (CH Cl ): 2065 (w) ν

cm^–1. H NMR (C₆D₆): δ =

˜

2

CϵC

7.98, 7.69, 6.99, 4.28 (Cp), 2.68–0.90 ppm. ³¹P NMR (C₆D₆): δ =

107.1 ppm. C₇₂H₆₂Fe₂P₄(1162.87): calcd. C 74.37, H 5.37; found

C 74.64, H 5.81.

4,4Ј-Bis[(η⁵-C₅H₅)Fe(dppe)(σ¹-CϵC)](C₆H₄)₂(7g):

A brick-red

mixture of 4,4Ј-bis[(η⁵-C₅H₅)Fe(CO)₂(σ¹-CϵC)](C₆H₄)₂(6d)

(0.150 g, 0.26 mmol) and 1,2-bis(diphenylphosphanyl)ethane

(0.222 g, 0.54 mmol) in benzene (200 mL) was exposed to ultravio-

let irradiation for 2 h at room temperature. The yellow-orange reac-

tion mixture was then filtered and dried under vacuum. The light-

orange residue was washed with hexane, dissolved in toluene and

precipitated with hexane. This treatment was repeated ten times,

and the pure product (0.060 g, 19% yield) was finally recovered as

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As previously described,

the orientation of the FeCp(PH₃)₂

fragments relative to the plane of the aromatic core has a negli-

gible influence on the structural and electronic features of these

complexes. In a conformational study on PH₃-7a at the em-

2596

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Eur. J. Inorg. Chem. 2006, 2582–2597

Electronic Communication in Diethynylaryl-Bridged Units

FULL PAPER

ployed level of theory we found no significant energy difference

between the conformers, i.e., less than 0.3 kcalmol^–1. In ad-

dition, a full geometry optimisation was carried out on the

model complex dHpe-7a (dHpe = PH₂CH₂CH₂PH₂) in order

to investigate the effect of replacing a chelating ligand with two

PH₃ligands. The results, included in the Supporting Infor-

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A joint experimental and computational study on the electronic communication in diethynylaryl-bridged (η5-C5H 5)Fe(η2-dppe) and (η5-C 5H5)-Fe(CO)2 units

DOI: 10.1002/ejic.200600024

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