Synthesis, structure and redox properties of bis(cyclopentadienyl) dithiolene complexes of molybdenum and tungsten

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PAPER

Synthesis, structure and redox properties of bis(cyclopentadienyl)dithiolene

complexes of molybdenum and tungsten†

Alexandra L. Whalley,^aAlexander J. Blake,^aDavid Collison,^bE. Stephen Davies,^aHelen J. Disley,^a

Madeleine Helliwell,^bFrank E. Mabbs,^bJonathan McMaster,^aClaire Wilson^aand C. David Garner*^a

Received 14th April 2011, Accepted 2nd June 2011

DOI: 10.1039/c1dt10663e

The compounds [Cp₂M(S₂C₂(H)R)] (M = Mo or W; R = phenyl, pyridin-2-yl, pyridin-3-yl, pyridin-4-yl

or quinoxalin-2-yl) and [Cp₂Mo(S₂C₂(Me)(pyridin-2-yl)] have been prepared by a facile and general

route for the synthesis of dithiolene complexes, viz. the reaction of [Cp₂MCl₂] (M = Mo or W) with the

dithiolene pro-ligand generated by reacting the corresponding 4-(R)-1,3-dithiol-2-one with CsOH.

These Mo compounds were reported previously (Hsu et al., Inorg. Chem. 1996, 35, 4743); however, the

preparative method employed herein is more versatile and generates the compounds in good yield and

all of the W compounds are new. Electrochemical investigations have shown that each compound

undergoes a diffusion controlled one-electron oxidation (OX^I) and a one-electron reduction (RED^I)

process; each redox change occurs at a more positive potential for a Mo compound than for its W

counterpart. The mono-cations generated by chemical or electrochemical oxidation are stable and the

structures of both components of the [Cp₂Mo(S₂C₂(H)R)]⁺/[Cp₂Mo(S₂C₂(H)R)] (R = Ph or

pyridin-3-yl) redox couples have been determined by X-ray crystallography. For each redox related pair,

the changes in the Mo–S, S–C and C–C bond lengths of the {MoSCCS} moiety are generally consistent

with OX^Iinvolving the loss of an electron from a p-orbital that is Mo–S and C–S antibonding and C–C

bonding in character. These results have been interpreted successfully within the framework provided

by DFT calculations accomplished for [Cp₂M(S₂C₂(H)Ph)]ⁿ(M = Mo or W; n = +1, 0 or -1). The

HOMO of the neutral compounds is derived mainly from the dithiolene p₃orbital (65%); therefore,

OX^Iis essentially a dithiolene-based process. The similarity of the potentials for OX^I(ca. 30 mV) for

analogous Mo and W compounds is consistent with this interpretation and the EPR spectra of each of

the Mo cations show that the unpaired electron is coupled to the dithiolene proton but relatively weakly

to ^95,97Mo. The DFT calculations indicate that the unpaired electron is more localised on the metal in

the mono-anions than in the mono-cations. In agreement with this, the EPR spectrum of each of the

Mo-containing mono-anions manifests a larger ^95,97Mo coupling (A_iso) than observed for the

corresponding mono-cation and RED^Ifor a W compound is signiﬁcantly (ca. 300 mV) more negative

than that of its Mo counterpart. [Cp₂W(S₂C₂(H)(quinoxalin-2-yl))] is anomalous; RED^Ioccurs at a

potential ca. 230 mV more positive than expected from that of its Mo counterpart and the EPR

spectrum of the mono-anion is typical of an organic radical. DFT calculations indicate that these

properties arise because the electron is added to a quinoxalin-2-yl p-orbital.

structures, and strategically valuable applications, including their

Introduction

potential use as tunable near-IR dyes and conducting materials.^1–3

Transition metal complexes containing one, two or three dithiolene

The dithiolene group also plays a vital role in the majority of

ligands have attracted a sustained interest since the 1960s, given

living systems as the component of the molybdopterin (MPT) co-

their versatile redox behaviour, novel electronic and molecular

factor that coordinates the metal at the catalytic centre of Mo-

and W-oxotransferase enzymes⁴and is essential for their catalytic

^aSchool of Chemistry, The University of Nottingham, University Park, Not-

action.⁵Therefore, the synthesis^2,6of new dithiolene complexes

tingham, UK, NG7 2RD. E-mail: dave.garner@nottingham.ac.uk; Fax: +44

represents valuable new chemistry and investigations of their

electronic structure,⁷properties³and reactions.⁸

Metal dithiolene complexes with 1 : 1, 1 : 2 or 1 : 3 metal:ligand

stoichiometry have been investigated extensively;^1,2complexes with

a 1 : 1 stoichiometry allow the effects of a redox change localised

0115 951 3563; Tel: +44 0115 951 4188

^bSchool of Chemistry, The University of Manchester, Oxford Road, Manch-

ester, UK, M13 9PL

† Electronic supplementary information (ESI) available. CCDC reference

numbers 822493–822498. For ESI and crystallographic data in CIF or

other electronic format see DOI: 10.1039/c1dt10663e

This journal is

The Royal Society of Chemistry 2011

Dalton Trans., 2011, 40, 10457–10472 | 10457

©

on a single metalladithiolene moiety to be studied.³In this latter

respect, the h -cyclopentadienyl group restricts the coordination

that contain one of the dithiolene ligands S₂C₂(H)R (R =

phenyl, pyridin-2-yl, pyridin-3-yl, pyridin-4-yl or quinoxalin-2-

yl). These investigations have led to the development of a new

synthetic route to these [Cp₂M(S₂C₂(H)R)] (M = Mo or W)

compounds and their characterisation by ¹H and ¹³C NMR spec-

troscopies. Electrochemical and associated UV/vis/NIR spectro-

electrochemical studies have demonstrated the redox versatility

of these compounds and the structures of both constituents of

[Cp₂Mo(S₂C₂(H)R)]⁺/[Cp₂Mo(S₂C₂(H)R)] (R = Ph and pyridin-3-

yl) redox couples have been determined by X-ray crystallography.

X-band Electron Paramagnetic Resonance (EPR) spectra have

been recorded for the mono-cations and mono-anions and DFT

calculations have provided valuable insights into the electronic

structure of selected neutral compounds and the corresponding

mono-cation and mono-anion.

5

sites available for dithiolene binding and compounds such as

[CpM(dithiolene)] (M = Co, Rh, Ir, or Ni) and [Cp₂M(dithiolene)]

(Ti, V, Mo, W, Zr, Hf, or Nb) have been isolated.³Also, we

have reported the synthesis of a range of [CpCo(dithiolene)]

compounds,^9–12the reversible redox chemistry of which is limited

to a one-electron reduction. In contrast, [Cp₂M(dithiolene)] (M =

Mo or W) compounds can undergo two reversible oxidations and

some of the mono-cationic products have been characterised by

X-ray crystallography.^13–17The information obtained shows that

the one-electron oxidation of a [Cp₂M(dithiolene)] (M = Mo

or W) compound leads to a decrease in the length of the C–

S bonds and an increase in the length of the C–C bond within

the metal-dithiolene unit (i.e. the dithiolene moiety possesses

more dithioketonic character in the mono-cation than in the

neutral compound); these changes are consistent with the results

of Density Functional Theory (DFT) calculations which indicate

that the frontier molecular orbital is essentially localised on the

dithiolene.¹⁷

Results and discussion

Synthesis

An important aspect of the structure of [Cp₂M(dithiolene)]

compounds is the extent of the folding of the {MSCCS} metalla-

cycle along the S–S axis (q); compounds with a d⁰conﬁguration

are folded to a signiﬁcant extent (40^◦< q < 50^◦) whereas those

with a d²conﬁguration are essentially planar (q ~ 0^◦). Lauher

and Hoffmann¹⁸used extended Hu¨ckel calculations and explained

these observations in terms of metal-dithiolene p-bonding and the

d-orbital occupancy of the metal. The results of subsequent studies

of the electronic structure of these compounds, including gas-

phase photoelectron spectroscopic studies and DFT calculations

accomplished for [Cp₂Ti(bdt)] and [Cp₂Mo(bdt)] (bdt = 1,2-

benzenedithiolate),¹⁹are consistent with this view and highlight

the interaction of metal in-plane and sulfur p orbitals. An

interesting reﬁnement of this situation is the observed variation in

the value of q for the d¹complex, [Cp₂Mo(dmit)]⁺(dmit = 2-thioxo-

1,3-dithiole-4,5-dithiolate) with the nature of the anion: q ~ 0^◦for

the [PF₆]^-, [AsF₆]^-and [SbF₆]^-salts, 23.2^◦for the [BF₄]^-salt and

30.5^◦for the Br^-salt.¹⁶DFT calculations show that the variation

in the value of q observed for [Cp₂Mo(dmit)]⁺is a consequence of

the very low energy barrier (<1 kcal mol^-1) for folding along the

S–S axis of this complex.¹⁷

The compounds 1 to 10, [Cp₂M(S₂C₂(H)R] (M = Mo or W; R =

phenyl, pyridin-2-yl, pyridin-3-yl, pyridin-4-yl or quinoxalin-2-yl)

have been synthesised in good (Mo; 44 to 71%) or moderate (W; 15

to 35%) yield by hydrolysis of the appropriate 4-(R)-1,3-dithiol-2-

one with CsOH to release the corresponding ene-1,2-dithiolate

for metathesis with [Cp₂MCl₂] (M = Mo or W) (Scheme 1).

Compound 11, [Cp₂Mo(S₂C₂(Me)(pyridin-2-yl)], was synthesised

in an analogous manner using 4-(pyridin-2-yl)-5-(methyl)-l,3-

dithiol-2-one. Given that the hydrolysed ligand precursors are

sensitive to decomposition, the lower yields for W compounds

may reﬂect the longer reaction times that are required for their

formation. The synthesis of these [Cp₂W(S₂C₂(H)R] compounds

has not been reported previously and, although the majority of

[Cp₂Mo(S₂C₂(H)R] were reported by Hsu et al.,²¹the synthetic

procedure described herein generally leads to higher yields.

NMR Spectroscopy

The ¹H and ¹³C NMR spectra recorded for 1 to 10 are consistent

with the structure of [Cp₂Mo(S₂C₂(H)R] compounds depicted in

Scheme 1, as conﬁrmed by our X-ray crystallographic studies of

3, 7 and 9 (vide infra) and that of 5 accomplished by Hsu et

Following the synthesis and structural characterisation of

20

1

[Cp₂Mo(S₂C₂(C(O)Me)(quinoxalin-2-yl))] Hsu et al. reported

al.²¹The H NMR spectra of 1 to 10 at ambient temperature in

5

the synthesis of [Cp₂Mo(S₂C₂(R)Ar)] (R = H, Ar = quinoxalin-2-

yl, pyridin-2, 3 or 4-yl, Ph, 4-chlorophenyl or pyrenyl; R = Me,

Ar = quinoxalin-2-yl; or Ar = R = Ph).²¹UV/vis, ¹H NMR and IR

properties of these compounds were recorded and electrochemical

studies showed that each undergoes a reversible one-electron oxi-

dation, the potential of which depends upon the nature of the Ar

and R groups. Also, Hsu et al. showed that [Cp₂Mo(S₂C₂(R)Ar)]

(R = H, Ar = quinoxalin-2-yl, pyridin-2, 3, or 4-yl; R = Me,

Ar = quinoxalin-2-yl) undergo a reversible protonation. X-ray

crystallographic studies showed that [Cp₂Mo(S₂C₂(quinoxalin-2-

yl))] possesses a planar metallo-1,2-enedithiolate group whereas

CDCl₃contain a single, sharp resonance for the h -Cp protons

(at 5.22–5.35 ppm, cf. 5.25–5.29 ppm for 1 to 5²¹) consistent

with the absence of processes involving either the bending of the

dithiolene ligand about the S–S axis or a dynamic inversion.²⁴The

1

chemical shifts for the dithiolene H resonance (6.30–7.82 ppm

for 1 to 10) are consistent with the dithiolene ligand having ene-

1,2-dithiolate character,^25,26in agreement with the results of our

DFT calculations (vide infra) and theoretical studies of related

1

compounds.^17,18The relative chemical shift of the dithiolene H

NMR resonance for each [Cp₂M(S₂C₂(H)R] (M = Mo or W)

series correlates with the nature of the other dithiolene substituent

as: R = quinoxalin-2-yl > pyridin-2-yl > pyridin-4-yl > pyridin-

3-yl > phenyl. This sequence corresponds to that observed for

in its protonated derivative (as the [BF₄]^-salt) the Mo is 0.344(10)

20

˚

A out of the plane of the ene-1,2-dithiolate unit.

1

The aim of this study is to compare and contrast the nature

and properties, notably the redox characteristics and electronic

structure, of analogous compounds of molybdenum and tungsten

the H NMR resonance for [MO(dithiolene)₂]^2-(M = Mo or W)

complexes involving the same set of dithiolene ligands. This order

has been rationalised on the basis of inductive effects and the

10458 | Dalton Trans., 2011, 40, 10457–10472

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©

Scheme 1

relative ability of the R group to stabilise a resonance form in

The values of l_maxand e for the absorptions observed for these

mono-cations and mono-anions are summarised in Table 2 and

the spectroscopic proﬁles for the conversion of [1] to [1]⁺and that

of [1] to [1]^-are presented in Fig. 2 and 3, respectively; changes

observed in the spectroscopic proﬁles for other electrochemical

conversions are shown in Fig. S3–S11.

which positive charge is localised at the carbon atom bound to the

dithiolene proton.²²The chemical shift of the dithiolene proton of

each of the W compounds is ca. 0.4 ppm more shielded than

that of the analogous Mo compound and possesses a pair of

3

satellite peaks with J values of 9.6–10.6 Hz that are attributed

to the coupling of the dithiolene proton with ¹⁸³W (abundance

14.6%). The ³J values observed are comparable to those reported

for [W{S₂C₂(H)(C₆H₄OMe-p)}₃] (J = 10.5 Hz)²⁷and [WO(qedt)₂]^2-

(J = 10.2 Hz).²³

Our electrochemical studies show that the [Cp₂M(dithio-

lene)]⁺/[Cp₂M(dithiolene)] couple (OX^I) of each of the

compounds 1 to 10 is reversible over the scan rates of 20

to 300 mVs^-1, in agreement with the cyclic voltammetric

studies of 1 to 5 in CH₂Cl₂accomplished by Hsu et al.²¹Also,

spectroelectrochemical studies show that OX^Iis chemically

reversible for 1 to 3, 5 to 8, and 10 since, for each compound,

oxidation and subsequent reduction regenerated the original

spectrum. However, for 4 and 9 this redox cycle produced a

UV/vis spectrum with the same proﬁle as that of the parent

compound, but with a slight variation in the band intensities.

For a given metal, the potential of the ﬁrst oxidation (OX^I, see

Table 1) varies with the nature of the dithiolene substituent as:

R = quinoxalin-2-yl > pyridin-4-yl > pyridin-3-yl > pyridin-2-yl

> phenyl; this sequence corresponds to that previously observed

for OX^Iof [MO(dithiolene)₂]^2-(M = Mo or W) complexes of these

dithiolene ligands.^22,23The E_1/2values for OX^Iof 1 to 5 and 6 to

10 in dmf (Table 1) show a linear correlation with the Hammett

Electrochemical and spectroelectrochemical investigations

Cyclic voltammetric studies of compounds 1 to 10, at ambient

temperature in dmf containing [ⁿBu₄N][BF₄] (0.2 M) as the

supporting electrolyte, have shown that each compound can be

oxidised and reduced (Table 1). As a typical example, the cyclic

voltammetric response for 1 is shown in Fig. 1; Fig. S1 and S2

show cyclic voltammograms recorded for 2 and 6, respectively.†

The changes in the UV/vis spectroscopic proﬁles were recorded

for the oxidation of each of the compounds 1 to 10 to their mono-

cation (OX^I) and the reduction of each of the compounds 1 to

5 and 10 to their mono-anion (RED^I). Also, our NIR spectra

were recorded for the formation of selected pairs of analogous

mono-cations, viz. [1]⁺and [6]⁺, [3]⁺and [8]⁺, and [5]⁺and [10]⁺.

Table 1 Cyclic voltammetric data for compounds 1 to 10^a

Complex

E_1/2OX^I/V

E_pOX^II/V

a

E_1/2OX^III/V

E_1/2RED^I/V

E_pRED^II/V

c

E_pRED^III/V

c

DE (Fc⁺/Fc)/V

1

2

-0.33 (0.07)

-0.28 (0.07)

-0.27 (0.07)

-0.23 (0.07)

-0.20 (0.07)

-0.35 (0.07)

-0.31 (0.07)

-0.30 (0.07)

-0.26 (0.07)

-0.24 (0.07)

0.34

0.39

0.37

0.44

0.21 (0.08)

0.29

0.36

—

-2.16 (0.07)

-2.14 (0.07)

-2.13 (0.07)

-2.12 (0.07)

-2.10 (0.07)

-2.45 (0.08)

-2.44 (0.08)

-2.43 (0.08)

-2.42 (0.07)

-2.17 (0.07)

—

(0.07)

(0.08)

(0.07)

-0.12 (0.07)

-0.18 (0.08)

-0.06 (0.07)

-0.17 (0.07)

—

-0.17 (0.07)

-0.22 (0.07)

-0.11 (0.07)

-0.22 (0.07)

-2.63

-2.61

-2.59

-2.29

—

-2.65

-2.64

-2.62

-2.48

-2.79

-2.78

-2.74

—

-2.81

-2.69

3

4

5

6

7

8

9

10

0.35

^aPotentials (E_1/2= (E_p+ E_p)/2) in V are quoted to the nearest 0.01 V. Data reported at 0.1 V s^-1for 1 mM test solutions in dmf containing 0.2 M

a

c

a

c

a

[ⁿBu₄N][BF₄] as the supporting electrolyte. The anodic/cathodic peak separation (DE = E_p- E_p) is given in brackets where applicable. OX^IIare E_p

values except for 6. RED^IIand RED^IIIare E_p^cvalues. OX^IIIresults from the application of a potential > OX^II. DE for the Fc⁺/Fc was used as the internal

standard.

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©

Table 2 Bands observed in the UV/vis/NIR spectra recorded for 1 to 10, [1]⁺to [10]⁺, [1]^-to [5]^-and [10]^-at 273 K in dmf containing [ⁿBu₄N][BF₄]

(0.2 M) using an optically transparent electrode

[M] l_max/nm (e ¥ 10^-3/mol^-1dm³cm^-1)

[M]⁺l_max/nm (e ¥ 10^-3/mol^-1dm³cm^-1)

[M]^-l_max/nm (e ¥ 10^-3/mol^-1dm³cm^-1)

1

364 (7.3), 547 (1.8)

383 (5.0), 529 sh (1.5)

375 (5.4), 519 (1.5)

387 (7.4), 426 sh (4.9), 524 sh (2.2)

327 (15.4), 517 (8.9)

336 (6.0), 390 sh (4.6)

332 (5.5), 409 (4.8)

348 (5.1), 397 (4.9)

356 (4.8), 418 (6.6)

325 (15.5), 510 (8.3)

418 sh (4.4), 447 (4.8), 484 sh (4.1), 657 (1.5), 1149 (1.1)

434 (3.5), 647 (1.0)

412 sh (3.3), 446 (3.6), 653 (1.1), 1142 (1.3)

406 sh (4.2), 442 (4.4), 633 (1.6)

321 (11.7), 370 sh (5.9), 474 (7.2), 652 (1.9), 1142 (1.1)

365 (3.5), 461 (4.1), 625 (1.6), 1046 (1.4)

370 sh (3.6), 454 (3.2), 623 (1.2)

361 sh (3.4), 454 (3.4), 625 (1.2), 1030 (1.2)

358 sh (3.8), 445 (3.6), 616 (1.5)

291 (19.0), 349 (8.6), 427 sh (3.7), 571 (3.2)

300 (16.7), 364 (6.8), 445 sh (3.5), 565 (2.6)

294 (15.3), 361 (6.3), 429 sh (3.3), 557 (2.7)

301(18.9), 377 (7.7), 444 (5.0), 559 (4.1)

346 (16.7), 559 (6.8)

2

3

4

5

6

7

8

9

10

322 (10.0), 370 (7.2), 463 (5.7), 636 (2.0), 1046 (1.7)

321 (17.7), 451 (5.5), 586(4.8), ~650 sh (3.0)

Fig. 2 Changes in the UV/vis/NIR spectroscopic proﬁle observed for the

electrochemical conversion of [Cp₂Mo(sdt)] ([1]) to [1]⁺in dmf containing

[ⁿBu₄N][BF₄] (0.2 M) at 273 K.

Fig. 1 Cyclic voltammetric response of [Cp₂Mo(sdt)] (1) (1 mmol) in dmf

containing [ⁿBu₄N][BF₄] (0.2 M) at ambient temperature and a scan rate

of 0.1 V s^-1. The dashed line shows expansion of the potential range to

include OX^II.

parameter (s_p) of R²⁸with quinoxalin-2-yl being the most electron

withdrawing of the substituents (s_pca. 0.55). Each W compound is

slightly (ca. 30 mV) more easily oxidised than its Mo counterpart;

similar observations have been made for other [Cp₂M(dithiolene)]

compounds (M = Mo or W, dithiolene = dmit (2-thioxo-1,3-

dithiole-4,5-dithiolate), dmid (2-oxo-1,3-dithiole-4,5-dithiolate

(-2)) or dddt (5,6-dihydro-1,4-dithiine-2,3-dithiolate)).^13–15

Protonation of compounds 2 to 5 and 7 to 10 in dmf by the

addition of >4 equivalents of p-toluene sulfonic acid, results in

a positive shift in the value of E_1/2for OX^I. The magnitude of

this shift is similar for the Mo and W series of compounds (0.18,

0.11, 0.15 and 0.06 V for 2 to 5, respectively; 0.18, 0.10, 0.15 and

0.04 V for 7 to 10, respectively) and correlates with the pK_aof R,

as observed by Hsu et al. for 2 to 5 in MeCN.²¹

Fig. 3 Changes in the UV/vis spectroscopic proﬁle observed for the

electrochemical conversion of [Cp₂Mo(sdt)] ([1]) to [1]^-in dmf containing

[ⁿBu₄N][BF₄] (0.2 M) at 273 K.

Compounds 1 to 10 undergo a second oxidation (OX^II, Table 1)

that is chemically irreversible in dmf at ambient temperature; each

W compound is more easily oxidised (by ca. 100 mV) than its Mo

counterpart (Table 1). For 2 to 5 and 7 to 10 signiﬁcant changes

were observed in the shape of the voltammogram generated when

the potential was held at, or slightly above, OX^II(that for 2 is

shown in Fig. S1†). These changes involved the depletion of the

OX^Iredox couple and the generation of a new redox couple

(OX^III) with an E_1/2value slightly more positive than that of

10460 | Dalton Trans., 2011, 40, 10457–10472

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©

OX^Ifor the parent compound (Table 1). These electrochemical

results are consistent with a mechanism for the formation of OX^III

from OX^IIthat involves electron transfer coupled to a chemical

reaction, as reported for the oxidation of [Cp₂Mo(SR)₂] (R =

Me or Ph).²⁹We have not investigated the nature of any of the

products formed. In contrast, scanning through OX^IIfor 1 and

6 did not signiﬁcantly perturb the current response for OX^Iin

the reverse sweep of the cyclic voltammogram (for 1 see Fig. 1

and for 6 see Fig. S2†). For 6, at a scan rate between 100 and

300 mV s^-1, a return (reduction) wave was associated with the

second oxidation suggesting an electrochemical reversibility for

OX^IIof this compound, i.e. the existence of a [6]²⁺/[6]⁺couple, as

reported for [Cp₂M(dithiolene)] (M = Mo or W; dithiolene = dmit,

dmid, dddt)^13,14and [Cp₂Mo(S₂C₂S(CH₂CH₂O)₃CH₂CH₂S)].³⁰

However, spectroelectrochemical studies showed that [6]²⁺is

chemically unstable. Similar observations of a return (reduction)

wave associated with OX^IIwere noted in the cyclic voltammograms

recorded for 1, 7 and 10, but each of these oxidation products was

more transient than [6]²⁺and was only observed at fast scan rates.

Compounds 1 to 10 are capable of undergoing at least one

reduction process (Table 1 and e.g. Fig. 1), as observed for

[Cp₂Mo(dithiolene)] (dithiolene = dmid or dmit),¹³with RED^I

being electrochemically reversible in each case. UV/vis spectro-

electrochemical studies showed that RED^Iis chemically reversible

for 1 to 5 and 10, since one-electron reduction and subsequent

re-oxidation regenerated the original spectrum, but this was not

the case not for 6, 7 and 8. For 1 to 9, the E_1/2value of the

ﬁrst reduction (RED^I) correlates with the Hammett parameter,

s_p, of the dithiolene substituent (R).²⁸However, 10 is reduced at a

potential that is signiﬁcantly less negative than expected, given the

electron withdrawing nature of the quinoxalinyl substituent and

the value of RED^Ifor 5.

[Cp₂Mo(dithiolene)] compounds.^20,31–33Oxidation of 1 and 3 to

the mono-cation results in a decrease in the length of the Mo–S

˚

bond (by 0.013–0.041 A) and S–C bonds (by 0.042–0.080 A);

oxidation leads to an increase in the length of the C–C bond of 1

˚

(by ca. 0.43 A) and 3 (by ca. 0.34 A).

The {MoSCCS} metallacycle of 1 and 3 is planar with a

˚

mean deviation of the atoms from the plane of <0.01 A. The

{WSCCS} metallacycle of 9 is folded by 7.72(8)^◦about the S–S

axis, which is close to the value of 8.1^◦in [Cp₂W(bdt)] (bdt =

1,2-benzenedithiolate).³²The lack of a major fold about the S–

S axis for these compounds is consistent with the structures of

other [Cp₂Mo(dithiolene)] compounds^{13,17,20,32–34}and theoretical

considerations of the electronic structure of [Cp₂M(dithiolene)]

(M = Mo or W) compounds¹⁸that (formally) involve a metal centre

with a d²conﬁguration. In each cation, the {MSCCS} metallacycle

is folded at th_◦e S–S axis, by 7.79(16)^◦for [1]⁺, 23.73(17)^◦for [3]⁺,

and 29.03(3) for [8]⁺.

The substitution of W for Mo in [Cp₂M(3-pedt)]⁺(compounds

[3]⁺and [8]⁺, respectively) does not result in major change in

the bond distances and interbond angles within the {MSCCS}

metallacycle.

EPR spectroscopic studies

The EPR spectra of [1]⁺to [11]⁺were recorded at X-band

frequency. These cations were produced by in situ electrochemical

oxidation of the parent compound in dmf containing [ⁿBu₄N][BF₄]

(0.2 M) or by oxidation of the corresponding parent compound

in CH₂Cl₂solution by ca. one equivalent of [Cp₂Fe][BF₄]. Details

of the EPR spectra recorded at 273 K in dmf and 210 and 77 K

in CH₂Cl₂are summarised in Table 4 and illustrated in Fig. 5 to 8

and S12 to S16.†

Each of the compounds 2 to 5 and 7 to 10 is capable of

undergoing one or two further reductions (Table 1); however, in

no case was an associated return wave observed and the products

of these reductions were not investigated further.

The EPR spectrum of each chemically generated Mo-containing

cation ([1]⁺to [4]⁺and [11]⁺) in CH₂Cl₂at 273 K was an unresolved

singlet. Optimum resolution of the ﬂuid solution EPR spectra for

these cations was obtained in CH₂Cl₂:dmf (4 : 1 v/v) at ca. 210 K

(e.g. see Fig. 5(a)); at this temperature six ^95,97Mo (I = 5/2) satellites

were observed superimposed on the ^{92,94,96,98,100}Mo (I = 0) signal.

Also manifest in the EPR spectra of [1]⁺to [4]⁺in CH₂Cl₂:dmf

(4 : 1 v/v) at ca. 210 K was a further splitting that is attributed

to the coupling of the unpaired electron spin with the magnetic

moment of a unique proton, presumably the dithiolene proton.

Samples of [1]⁺, [3]⁺, and [5]⁺, prepared by in situ electrochemical

oxidation of the parent compound in dmf containing [ⁿBu₄N][BF₄]

(0.2 M), each gave a well resolved ﬂuid solution EPR spectrum at

273 K; metal and ligand (dithiolene) hyperﬁne structure was

evident in the spectra of [1]⁺(see Fig. S12†), [3]⁺and [5]⁺, and

this was consistent with the EPR spectra recorded for samples

generated by chemical oxidation. In [11]⁺the dithiolene proton

of [2]⁺is replaced by a methyl group; in addition to the coupling

with ^95,97Mo; the EPR spectrum of [11]⁺[see Fig. 5(b)] manifests

hyperﬁne splitting consistent with the coupling of the unpaired

electron spin with the magnetic moment of three equivalent

protons, presumably those of the methyl group.

Structural studies

The facile one-electron oxidation of the [Cp₂M(dithiolene)]

compounds

1 to 10 achieved electrochemically has been

achieved chemically by oxidation of the parent compound with

[Cp₂Fe][BF₄], thereby providing a convenient route to products

such as [1][BF₄], [3][BF₄] and [8][BF₄]. The structures of 1,

[1][BF₄], 3, [3][BF₄], [8][BF₄] and 9 have been determined by X-ray

crystallography (Fig. 4). In each case, the geometry at the metal

centre can be considered to approximate to that of a distorted

tetrahedron, with the vertices comprising the centroids of the two

Cp rings and the two dithiolene sulfur atoms. The {MSCCS}

metallacycle is not coplanar with the R group of the dithiolene,

but there is a shift toward co-planarity upon oxidation.³¹

The bond lengths and interbond angles of the {MoSCCS}

metallacycle of [Cp₂Mo(dithiolene)]ⁿ(n = 0 or +1) are summarised

in Table 3 where it is evident that the asymmetric nature of each

dithiolene ligand can give rise to different Mo–S and S–C bond

lengths within each metallacycle. A similar effect is noted for

W-containing [8][BF₄] and 9. For 1, 3 and 9, the lengths of

the M–S, S–C and C–C bonds and the values of the interbond

angles are similar to those of other structurally characterised

For the W-containing cations, whether generated electrochemi-

cally in dmf ([6]⁺and [10]⁺) or chemically in CH₂Cl₂([6]⁺to [9]⁺), at

273 K each EPR spectrum comprises two ¹⁸³W (I = 1/2) satellites

superimposed on a single line ^{180,182,184,186}W (I = 0), e.g. Fig. 6. The

EPR spectrum recorded for each of the cations [1]⁺(Fig. S13†)

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Fig. 4 Molecular structures of [Cp₂Mo(sdt)] (1), [1][BF₄], [Cp₂Mo(3-pedt)] (3), [3][BF₄], [Cp₂W(3-pedt)][BF₄] ([8][BF₄]) and [Cp₂W(3-pedt)] (9); hydrogen

atoms and anions are omitted for clarity.

to [4]⁺, [6]⁺(Fig. S14†) to [9]⁺and [11]⁺, prepared by chemical

oxidation in CH₂Cl₂:dmf (4 : l, v/v), as a frozen (77 K) solution

was rhombic and showed no clearly resolved metal hyperﬁne

structure. Within experimental error, the g_ave(~g_iso) value observed

for each cation was the same for the frozen and the ﬂuid solutions,

implying that no major structural changes occur between these

phases.

The in situ electrochemical production of [1]^-, [3]^-and [5]^-in

dmf at 273 K allowed their EPR spectra to be recorded; details

of these spectra are given in Table 4 and the spectra of [1]^-and

[3]^-are shown in Fig. 7 and S15,† respectively. Each spectrum

comprises six ^95,97Mo (I = 5/2) satellites superimposed on the single

line ^{92,94,96,98,100}Mo (I = 0) with A_isoca. 40 G. No ligand hyperﬁne

coupling was observed but this may be concealed since the line

10462 | Dalton Trans., 2011, 40, 10457–10472

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©

◦

˚

Table 3 Selected bond lengths (A) and interbond angles ( ) for 1, [1][BF₄], 3, [3][BF₄], [8][BF₄] and 9

Complex

1

1[BF₄]

3

3[BF₄]

8[BF₄]

9

˚

Bond lengths/A

M–S(1)

M–S(2)

S(1)–C(1)

S(2)–C(2)

2.4402(7)

2.4396(7)

1.746(3)

1.771(3)

1.327(3)

2.413(2)

2.425(2)

1.666(5)

1.714(5)

1.370(7)

2.4506(15)

2.4375(15)

1.728(6)

1.771(5)

1.339(8)

2.4097(12)

2.4240(12)

1.688(5)

1.730(4)

1.367(7)

2.397(3)

2.404(3)

1.694(10)

1.726(9)

1.353(13)

2.4446(7)

2.4326(8)

1.743(3)

1.769(3)

1.355(4)

C(1)–C(2)

Interbond angles/^◦

S(1)–M–S(2)

M–S(1)–C(1)

M–S(2)–C(2)

S(1)–C(1)–C(2)

S(2)–C(2)–C(1)

82.29(2)

106.06(9)

106.85(8)

124.2(2)

120.5(2)

81.77(4)

82.30(5)

105.8(2)

106.9(2)

125.0(4)

119.9(4)

81.41(4)

81.34(9)

103.3(3)

103.2(4)

123.3(8)

119.3(7)

82.13(2)

106.42(9)

107.19(10)

123.2(2)

120.3(2)

106.14(17)

107.07(17)

125.3(4)

104.08(16)

104.42(16)

124.4(4)

119.0(4)

118.6(3)

Table 4 EPR spectroscopic data recorded for cations and anions generated from 1 to 11

Frozen solution data^a,b

Fluid solution data^a,c

g_iso

Complex

g_x

g_y

g_z

g_ave

A_iso/¥ 10^-4cm^-1

a_iso/¥ 10^-4cm^-1

1⁺

1.994 (15)

—

1.992 (15)

1.992 (12.5)

—

1.993 (11)

—

2.012 (10)

—

2.010 (10)

2.010 (9)

—

2.010 (9)

—

2.027 (8.5)

—

2.026 (10)

2.027 (9)

—

2.027 (8.5)

—

2.011

—

2.009

2.010

—

2.010

—

2.011 (2.5)

2.012 (1.5)

1.931 (10.0)

2.006 (2.75)

2.008 (2.4)

1.929 (9.5)

2.007 (2.2)

2.012 (2.1)

1.929 (15.0)

2.010 (2.65)

1.994 (5.5)

1.995 (4.8)

1.995 (5.5)

1.994 (6.0)

1.995 (6.0)

1.999 (6.1)

2.004^e

7.2

7.6

36.1

7.6

35.8

8.4

10.1

36.4

6.6

21.1

23.4

22.4

22.3

22.8

25.2

—

3.5

3.7

—

3.1

3.2

—

3.1

2.7

—

2.9

—

1^+d

1^-d

2⁺

3⁺

3^-d

4⁺

5^+d

5^-d

11⁺

6⁺

—

1.991(13)

1.967 (33)

—

1.975 (17)

1.971 (26)

1.974 (27)

—

2.009 (11)

1.990 (20)

—

1.995 (6.5)

1.991 (11.2)

1.993 (11)

—

2.027 (10)

2.022 (27)

—

2.025 (17.7)

2.024 (25)

2.027 (23)

—

2.009

1.993

—

1.998

1.995

1.998

—

6^+d

7⁺

8⁺

9⁺

10^+d

10^-d

—

^aValues in brackets are linewidths, in Gauss, used for the simulation of the experimental spectra; estimated errors are 0.001 and 0.1 ¥ 10^-4cm^-1, for

g-values and hyperﬁne couplings, respectively; samples prepared by chemical oxidation with ca. one equivalent of [Cp₂Fe][BF₄] in CH₂Cl₂unless stated

otherwise. ^bAs a frozen glass in CH₂Cl₂:dmf (4 : l, v/v) at 77 K. ^cIn CH₂Cl₂at 210 K, unless stated otherwise. ^dElectrochemically generated in dmf

containing [ⁿBu₄N][BF₄] (0.2 M) at 273 K; frozen glass data at 77 K not recorded. ^eAt 268 K, extensive hyperﬁne splittings were observed.

width required for simulation of these spectra (e.g. 15 G for [5]^-)

is larger than that required for simulation of the EPR spectra of

the corresponding cations (e.g. 2.1 G for [5]⁺). Repeated attempts

to generate [1]^-by coulometry in order to permit EPR studies at

77 K failed to yield a spectrum that could be simulated satisfacto-

rily.

Electrochemically generated [10]^-possesses an EPR spectrum

that is consistent with the presence of an organic radical, possibly

located on the quinoxalinyl moiety (Table 4; Fig. 8);^35,36this

contrasts with the Mo analogue [5]^-that possesses an EPR

spectrum which manifests coupling of the unpaired electron with

^95,97Mo (see Fig. S16†). The results of the DFT calculations

(vide infra) provide an explanation of this signiﬁcant difference

between these closely related anions. Our attempts to simulate the

experimental spectrum were unsuccessful.

Selected bond lengths and interbond angles obtained from the

DFT calculations for [1]^zand [6]^z(z = 0, +1 or -1) are presented

in Table 5.

The theoretical studies reproduce magnitude of the bond lengths

and the interbond angles within the {MSCCS} metallacycle

of 1 and [1]⁺and the sense of the asymmetry observed by

crystallography. Also, the magnitude of the decrease in the length

of the C–S bonds and the increase in the length of the C–C bond

that are observed by crystallography upon oxidation of 1 to [1]⁺

is reproduced. However, in contrast to the slight decrease in the

length of the Mo–S bonds observed upon oxidation, the DFT

calculations predict a slight increase in the length of these bonds.

This discrepancy parallels the corresponding observations for

[Cp₂M(dithiolene)] [M = Mo or W; dithiolene = dmit or dsit (= 2-

thioxo-1,3-dithiole-4,5-dithiolate)].¹⁷Similar trends in the changes

in the lengths of the M–S, C–S and C–C bonds are also observed

in the calculated geometries of 6 and [6]⁺for which there are no

experimentally determined X-ray crystallographic structures. The

geometries calculated for [1]^-and [6]^-involve an elongation of the

M–S bonds and small changes in the length of the C–S and C–C

bonds relative to those for 1 and 6, respectively (Table 5). This is

DFT Calculations

We have undertaken DFT calculations for [1]^zand [6]^z(z = 0, +1

or -1), and 5 and 10 to provide a qualitative description of the

structure and bonding within these compounds.

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Fig. 5 (a) X-band EPR spectra of [Cp₂Mo(sdt)]⁺([1]⁺): lower trace,

spectrum recorded for [1]⁺(1 mM) in CH₂Cl₂:dmf (4 : 1 v/v) at 210 K, [1]⁺

was generated by the oxidation of 1 with ca. 1 equivalent of [Cp₂Fe][BF₄];

upper trace, simulation of the experimental spectrum using the parameters

given in Table 4 and a Lorentzian line shape. (b) X-band EPR spectra of

[Cp₂Mo(S₂C₂(Me)(pyridin-2-yl))] ([11]⁺): lower trace, spectrum recorded

for [11]⁺(1 mM) in CH₂Cl₂:dmf (4 : 1 v/v) at 210 K, [11]⁺was generated

by the oxidation of 11 with ca. 1 equivalent of [Cp₂Fe][BF₄]; upper trace,

simulation of the experimental spectrum using the parameters given in

Table 4 and a 80% Gaussian and 20% Lorentzian line shape.

Fig. 7 X-band EPR spectra of [Cp₂Mo(sdt)]^-([1]^-): lower trace, spectrum

recorded for electrochemically generated [1]^-(1 mM) in dmf containing

[ⁿBu₄N][BF₄] (0.2 M) at 273 K; upper trace, simulation of the experimental

spectrum using the parameters given in Table 4 and a Lorentzian line shape.

* indicates a small quantity of [1]⁺generated at the secondary electrode

during the in situ generation of [1]^-.

Fig. 8 X-band EPR spectra of electrochemically generated [Cp₂W(qedt)]^-

([10]^-) in dmf containing [ⁿBu₄N][BF₄] (0.2 M) at 273 K.

The signiﬁcant difference between the theoretical (14.45^◦) and

experimental (7.79(16)^◦) values of q in [1]⁺may be associated with

a low energy barrier to folding about the S–S axis as has been

established for other [Cp₂M(dithiolene)]⁺(d¹) complexes.^13–17

The composition of the frontier orbitals of 1 and 6 are

summarised in Table 6 and 7, respectively, and illustrated for

1 in Fig. 9. The HOMO is primarily (65.0%) the dithiolene p₃

orbital that has C–C p-bonding and C–S antibonding character.

Similar results have been obtained by DFT calculations for

[Cp₂M(dmit)].¹⁷The LUMO of 1 is composed of Mo d_yz(51.3%)

and Cp₂(27.9%) orbitals with a small (19.5%) contribution from

Fig. 6 X-band EPR spectra of [Cp₂W(sdt)]⁺([6]⁺): lower trace, spectrum

recorded for electrochemically generated [6]⁺(1 mM) in dmf containing

[ⁿBu₄N][BF₄] (0.2 M) at 273 K; upper trace, simulation of the experimental

spectrum using the parameters given in Table 4 and a Lorentzian line shape.

consistent with a SOMO (Singly Occupied Molecular Orbital) in

[1]^-and [6]^-that is p-antibonding with respect to the M–S bonds

(vide infra). The values calculated for the metalladithiolene fold

angle (q) in 1 and [1]⁺reproduce the trend in the experimentally

determined geometries in which the MoS₂and S₂C₂planes are

essentially co-planar in 1 and are signiﬁcantly folded in [1]⁺.

10464 | Dalton Trans., 2011, 40, 10457–10472

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Table 5 Selected bond lengths and interbond angles obtained from the DFT calculations for [1]^zand [6]^z(z = 0, +1 or -1)^a

Complex

1

1⁺

1^-

6

6⁺

6^-

˚

Bond lengths/A

M–S(1)

M–S(2)

S(1)–C(1)

S(2)–C(2)

2.406 [2.4402(7)]

2.404 [2.4396(7)]

1.747 [1.746(3)]

1.782 [1.771(3)]

1.357 [1.327(3)]

2.462 [2.4135(13)]

2.466 [2.4252(14)]

1.709 [1.666(5)]

1.744 [1.714(5)]

1.389 [1.370(7)]

2.430

2.425

1.749

1.800

1.362

2.436

2.428

1.759

1.794

1.353

2.457

2.461

1.709

1.747

1.391

2.456

2.440

1.746

1.799

1.365

C(1)–C(2)

Bond angles/^◦

S(1)–M–S(2)

M–S(1)–C(1)

M–S(2)–C(2)

S(1)–C(1)–C(2)

S(2)–C(2)–C(1)

q/^◦

81.7 [82.29(2)]

106.1 [106.07(8)]

109.2 [106.85(8)]

123.0 [124.2(2)]

118.0 [120.5(2)]

0.02 [0.0]

81.3 [81.77(4)]

81.9

82.5

81.8

82.0

106.7 [106.14(17)]

106.9 [107.07(17)]

124.7 [125.3(4)]

119.0 [119.0(4)]

14.45 [7.79(16)]

107.7

108.7

123.8

117.9

0.56

106.6

107.9

124.1

118.9

0.25

106.4

107.7

124.9

119.0

4.07

106.2

107.6

124.4

118.4

10.93

^aData in square brackets are experimental values obtained from crystallographic studies of 1 and [1][BF₄].

Table 6 The nature, energy and composition of the frontier orbitals obtained from the DFT calculations for the geometry optimised structure of 1

Orbital

Energy/eV

% Contribution

Designation

Nature

Cp₂

Mo

S₂C₂

Ph

105

104

103

102

101

100

99

98

97

96

95

LUMO+5

LUMO+4

LUMO+3

LUMO+2

LUMO+1

LUMO

Ph

-0.81

-1.21

-1.43

-1.60

-2.04

-2.73

-3.99

-4.39

-5.31

-5.86

-5.92

-6.01

-6.53

-6.75

-6.96

-7.45

2.4

4.4

0.9

6.3

1.1

95.6

57.7

0.8

4.7

3.2

1.0

8.1

0.8

27.3

40.1

64.0

36.5

3.7

21.2

0.3

S₂C₂p₄+ Ph + Mo d_xy

31.6

14.5

25.7

6.7

2

²-y²

Mo d_x+ d_z+ S₂C₂s₁

28.6

33.2

39.8

27.9

20.2

11.5

11.2

12.5

10.0

16.3

13.2

42.3

65.6

58.4

56.2

36.3

50.1

51.3

6.7

76.9

1.9

6.2

2.2

4.2

12.9

19.3

23.8

29.4

Mo d_xz+ Cp₂+ S₂C₂s₂

Mo d_xy+ Cp₂

Mo d_yz+ S₂C₂p₃+ Cp₂

19.5

65.0

10.8

59.6

41.2

23.8

43.0

70.2

17.2

10.2

11.6

HOMO

S₂C₂p₃+Mo d_yz

2

HOMO-1

HOMO-2

HOMO-3

HOMO-4

HOMO-5

HOMO-6

HOMO-7

HOMO-8

HOMO-9

Mo d_z

S₂C₂p₂+ Ph

Ph + S₂C₂p₂

S₂C₂s₂+ Mo d_xz

94

93

92

91

2

S₂C₂s₁+ Mo d_z

Cp₂+ Mo d_yz

Cp₂+ Mo d_xz

90

0.6

the dithiolene p₃-orbital. LUMO+1, LUMO+2, and LUMO+3

each have a similar composition to the LUMO and involve the

Mo d_xy, Mo d_xzand Mo d_x2-y2orbitals, respectively. LUMO+4

is mainly (57.7%), and LUMO+5 virtually exclusively (95.6%),

phenyl p* in character. The bonding in 6 is very similar to that in

1; the major differences being that the LUMO is higher in energy

in 6 (-1.70 eV, Table 6) relative to 1 (-2.73 eV, Table 7).

of redox potentials for OX^Ifor 1 and 6 and the other pairs of

related compounds (Table 1), i.e. OX^Ifor a W compound is ca.

30 mV less than its Mo counterpart. Furthermore, the sense of

this difference in redox potentials is consistent with the results of

the DFT calculations.

In respect of the EPR spectra recorded for [1]⁺(Fig. 5(a), S12

and S13†) the observation of a relatively small hyperﬁne coupling

to ^95,97Mo (I = 5/2) and the presence of coupling to a unique

proton (I = 1/2) is consistent with the nature of the SOMO of this

cation [Fig. 10(c)], with the unique proton being the dithiolene

hydrogen. These considerations are applicable to [2]⁺, [3]⁺, [4]⁺

and [5]⁺, since their EPR spectra closely resemble that of [1]⁺, and

to [11]⁺, the EPR spectrum of which [Fig. 5(b)] manifests coupling

of the unpaired electron both to ^95,97Mo and three protons (i.e.

the CH₃substituent of the dithiolene). Similar observations are

made in respect of the EPR spectra recorded for the W complexes

[6]⁺to [10]⁺, although coupling of the unpaired electron to the

dithiolene proton was not observed in the experimental spectra

(e.g. Fig. 6 and S14†), possibly due to the greater linewidths of these

spectra.

The results of the DFT calculations performed for the [1]^z

and [6]^z(z = +1, 0, or -1) complexes provide insight into the

nature of the EPR spectra of [1]^zand [6]^z(z = +1 or -1) and

the corresponding ions of related species. Fig. 10 illustrates the

nature, and indicates the energy, of the HOMOs of 1 and 6 and

the SOMOs of [1]⁺, [6]⁺, [1]^-and [6]^-. The SOMO of [1]⁺and [6]⁺

is primarily dithiolene p₃-based (66.6% and 70.8%, respectively)

2

with a minor contribution from the metal d_x

and d_zorbitals

-y

(12.6% and 3.6%, respectively). Given the nature of the HOMO

of 1 and 6 (Table 6) and the SOMO of [1]⁺and [6]⁺(Fig. 10),

OX^Ifor all of the compounds investigated herein is considered

to be primarily a dithiolene-based process. This is consistent with

the potential of OX^Iof both the Mo (1 to 5) and W (6 to 10)

groups of compounds varying with the nature of the dithiolene

substituent (Table 1). Also, the similarity in the energies of 1 and

6 and [1]⁺and [6]⁺(Fig. 10) explains the similarity in the value

The DFT calculations accomplished for 1 and 6 indicate that the

LUMO is signiﬁcantly higher in energy in 6 than in 1 (Tables 6 and

7). This is consistent with the value of RED^Ifor a W compound

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©

Table 7 The nature, energy and composition of the frontier orbitals obtained from the DFT calculations for the geometry optimised structure of 6

Orbital

Energy/eV

% Contribution

Designation

Nature

Cp₂

W

S₂C₂

Ph

121

120

119

118

117

116

115

114

113

112

111

110

109

108

107

106

LUMO+5

LUMO+4

LUMO+3

LUMO+2

LUMO+1

LUMO

Ph

-0.70

-0.96

-1.24

-1.40

-1.57

-1.70

-3.75

-4.42

-5.12

-5.79

-5.98

-6.51

-6.83

-7.00

-7.51

2.1

0.7

1.4

95.9

21.8

33.0

17.9

3.2

0.5

8.3

S₂C₂p₄+ Ph + W d_xy

21.8

33.0

27.6

40.8

36.7

12.9

19.8

10.7

3.2

28.7

37.0

37.5

36.2

51.5

2.9

69.0

1.9

2.2

2.3

7.7

14.8

14.4

22.3

24.9

13.6

20.7

17.0

19.8

11.3

76.0

10.3

63.7

14.6

32.9

63.1

68.4

24.1

8.0

2

²-y²

W d_x+ d_z+ S₂C₂s₁

W d_xy+ Cp₂

W d_xz+ Cp₂+ S₂C₂s₂

W d_yz+ S₂C₂p₃+ Cp₂

S₂C₂p₃+ W d_yz

2

HOMO

²-y²

HOMO-1

HOMO-2

HOMO-3

HOMO-4

HOMO-5

HOMO-6

HOMO-7

HOMO-8

HOMO-9

W d_x+ d_z

1.0

S₂C₂p₂+ Ph

Ph + S₂C₂p₂

S₂C₂s₂+ W d_xz

S₂C₂s₁+ W d_z

Cp₂+ W d_yz

23.7

80.1

55.3

12.9

4.0

20.7

0.6

9.5

16.3

12.8

40.8

68.8

67.4

2

Cp₂+ W d_xz

Cp₂+ W d_x+ d_z

²-y²

2

4.8

2.9

Table 8 Crystallographic data for 1, [1][BF₄], 3, [3][BF₄], [8][BF₄] and 9

Complex

1

[1][BF₄]

3

[3][BF₄]

[8][BF₄]

9

Formula

M

C₁₈H₁₆S₂Mo

392.37

Red

C₁₈H₁₆S₂MoBF₄

479.18

C₁₇H₁₅NS₂Mo

393.37

Purple

C₁₇H₁₆NS₂MoBF₄C₁₇H₁₅NS₂WBF₄C₁₇H₁₅NS₂W

481.18

568.08

Brown

481.27

Colour

Dark red

Monoclinic

P2₁/c (no. 14)

6.7275(5)

14.5860(10)

18.3070(10)

98.2730(10)

1777.7(2)

4

150

1.790

956

10.11 (Mo-Ka)

Bruker SMART

Dark red

Monoclinic

P2₁/n (no. 14)

6.9390(10)

14.514(2)

17.696(3)

101.018(3)

1749.4(5)

4

Deep red

Monoclinc

P2₁/n (no. 14)

10.5703(14)

11.879(2)

11.992(2)

97.136(2)

1494.1(4)

4

Crystal system

Space group

Monoclinic

P2₁/c (no. 14)

8.8648(8)

15.5930(10)

10.8010

90.990(2)

1492.8(2)

4

150

1.746

792

11.5 (Mo-Ka)

Bruker SMART

Monoclinic

P2₁/a (no. 14)

10.732(2)

16.083(2)

8.660(2)

90.16(2)

1494.6(4)

4

Monoclinic

P2₁/n (no. 14)

6.9700(9)

14.514(2)

17.689(2)

100.907(10)

1757.1(4)

4

˚

a/A

˚

b/A

˚

c/A

b (^◦)

3

˚

U/A

Z

T/K

296

1.748

792.00

150

1.827

203(2)

2.147

150

2.140

D_c/g cm^-3

F(000)

960

1084

920

m/cm^-1

98.15 (Cu-Ka)

Rigaku AFC5R Bruker SMART

10.3 (Mo-Ka)

68.53 (Mo-Ka) 80.02 (Mo-Ka)

Enraf-Nonius

Diffractometer

Bruker SMART

CCD area detector CCD area detector

0.27 ¥ 0.18 ¥ 0.05 0.20 ¥ 0.20 ¥ 0.12 0.10 ¥ 0.12 ¥ 0.35 0.54 ¥ 0.14 ¥ 0.10 0.25 ¥ 0.15 ¥ 0.1 0.3 ¥ 0.1 ¥ 0.1

CCD area detector CAD-4

CCD area detector

Crystal dimensions/mm

˚

l/A

0.71073

57.5

w

9524

3471

0.71073

57.48

w

15612

4176

1.54178

156.0

w-2q

0.71073

57.5

w

11124

4270

0.71073

49.94

W

0.71073

57.2

w

15505

3608

2q_max(^◦)

Scan type

Total data

Unique data

3276

3343

3107

3082

‘Observed’ data, N_o[I > 2s(I)] 2774

3265

217

0.048

0.129

1.064

1.040, -0.939

2284

191

0.046

0.045

2.47

2717

246

0.046

0.112

0.967

0.86, -1.02

2147

234

0.040

0.070

1.027

0.918, -0.671

3275

190

0.019

0.48

1.019

0.948, -0.907

Least square variables, N_v

190

R

R_w

S

0.028

0.062

0.96

-3

˚

Difference map features/e A

0.65, -0.54

0.78, -1.22

(6 to 9) being signiﬁcantly (ca. 300 mV) more negative (Table 1)

than that of its Mo counterpart (1 to 4).

to [5]^-) manifests a larger ^95,97Mo coupling (A_iso) than is observed

for the corresponding mono-cation. Also, in contrast to the EPR

spectra of [1]⁺(Fig. 5(a)) to [5]⁺, the EPR spectra of [1]^-(Fig. 7)

to [5]^-do not manifest coupling to the dithiolene hydrogen.

For 10, two aspects of the reduction are surprising. First, the

value of RED^Iis signiﬁcantly less negative than expected from the

values of RED^Ifor 5 and compounds 6 to 9 (Table 1). Second, the

EPR spectrum of [5]^-is consistent with the information presented

above, i.e. the unpaired electron is located in an orbital with a

signiﬁcant Mo contribution, by contrast the EPR spectrum [10]^-

(Fig. 8) is typical of an organic radical. The energies and the

The SOMOs of both [1]^-[Fig. 10(e)] and [6]^-[Fig. 10(f)] possess

2

signiﬁcant metal character involving the Mo d_yzand W d_x

and

-y

d_yzorbitals (the total Mo and W contributions are 47.1% and

53.6%, respectively), together with contributions from the two

Cp groups (18.7% and 24.9%, respectively) and the dithiolene

p₃orbital (30.3% and 17.6%, respectively). Thus, the unpaired

electron is more localised on the metal in mono-anions than in the

mono-cations. In agreement with these results, the EPR spectrum

of each of the molybdenum containing mono-anions ([1]^-, Fig. 7,

10466 | Dalton Trans., 2011, 40, 10457–10472

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Fig. 11 Pictorial representations and energies of the Kohn–Sham frontier

orbitals of: (a) [Cp₂Mo(qedt)] (5) and (b) [Cp₂W(qedt)] (10).

of RED^Ifor 10 is less negative by ca. 230 mV than expected and

the EPR spectrum of [10]^-is characteristic of an organic radical,

∑

-

i.e. {quinoxalinyl} .

Experimental

General

Quinoxaline, 2-, 3- and 4-acetylpyridine and bromoacetophenone

were obtained from Lancaster Synthesis Ltd. [Cp₂MCl₂] (M =

Mo or W) were obtained from Strem Chemicals. Anhydrous

methanol and N,N-dimethylformamide (dmf) were purchased

from Aldrich and Fluka, respectively. All other solvents were dried

by standard techniques. Column chromatography was performed

on alumina (Brockmann Grade II). The pro-ligands R-1,3-

dithiol-2-one (R = phenyl, pyridin-2-yl, pyridin-3-yl, pyridin-4-yl

or quinoxalin-2-yl)^9,37,38and [Cp₂Fe][BF₄]³⁹were synthesised by

published procedures. The pro-ligand 4-(pyridin-2-yl)-5-(methyl)-

1,3-dithiol-2-one was synthesised by the method of Dinsmore et

al.⁹All reactions were performed under an atmosphere of argon

using standard Schlenk-line techniques.

Fig. 9 Energy level diagram and pictorial representations of the

Kohn–Sham frontier orbitals of [Cp₂Mo(sdt)] (1).

Microanalyses were conducted by the staff of the

Microanalytical Service of the Department of Chemistry,

The University of Manchester and of the School of Chemistry,

The University of Nottingham. For 2 and 10, the C analyses

differ from those expected; however, spectroscopic data support

the proposed formulation.

CI mass spectra were recorded on a Fisons VG Trio 200 or a

1

Fisons VG Autospec mass spectrometer. H NMR spectra were

recorded on a Varian Inova 300 or a Bruker DPX300 spectrometer

and 125 Hz ¹³C NMR spectra were recorded on a Varian Inova 500

spectrometer. The air-sensitive nature of the [Cp₂M(dithiolene)]

(M = Mo or W) compounds resulted in a broadening of ¹H

resonances of the dithiolene group due to the formation of a small

amount of paramagnetic [Cp₂M(dithiolene)]⁺; the addition of a

small quantity of [Cp₂Co] (as speciﬁed in the Synthesis section)

gave complete resolution of all proton environments without

affecting their chemical shifts.

Fig. 10 Pictorial representations and energies of the Kohn–Sham

HOMOs of: (a) [Cp₂Mo(sdt)] (1) and (b) [Cp₂W(sdt)] (6) and the SOMOs

of: (c) [1]⁺; (d) [6]⁺; (e) [1]^-and (f) [6]^-.

nature of the frontier MOs of 5 and 10 obtained from the DFT

calculations are illustrated in Fig. 11. Thus, the LUMO in 10 is at

a signiﬁcantly lower energy than that in 5 and is a quinoxalinyl-

based p-orbital, whereas in 5 the LUMO involves a signiﬁcant

metal contribution (cf . 1, Table 6) and the quinoxalinyl-based

orbital is the LUMO+1. These results explain why the potential

Cyclic voltammetric and coulometric studies were carried

out using an Autolab PGSTAT20 potentiostat. Standard cyclic

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©

voltammetric measurements were accomplished for the solution

of interest under an atmosphere of argon using a three-electrode

arrangement in a single compartment cell. The cell comprised a

glassy carbon working electrode, a Pt wire secondary electrode

and a saturated calomel reference electrode, chemically isolated

from the test solution via a bridge tube containing electrolyte

solution and ﬁtted with a porous vycor frit. The compound of

interest was present at a concentration of 10^-3M in dmf and

the solution contained [ⁿBu₄N][BF₄] (0.2 M) as the supporting

electrolyte. Cyclic voltammograms were recorded at ambient

temperature in the scan rate range 20–300 mV s^-1. Redox

potentials are quoted versus the [Cp₂Fe]⁺/[Cp₂Fe] couple used as

an internal reference. Compensation for internal resistance was not

applied.

The redox changes of each of the compounds 1 to 10 at 273 K in

dmf solution containing [ⁿBu₄N][BF₄] (0.2 M) as the supporting

electrolyte were monitored by UV/vis spectroelectrochemical

studies using an optically transparent electrode (OTE).⁴⁰Spectra

were recorded between 270 and 900 nm using a Perkin Elmer

Lambda 16 spectrophotometer in conjunction with a Sycopel

Scientiﬁc Ltd. DD10M potentiostat and a saturated calomel

reference electrode. Changes in the UV/vis/NIR spectra between

270 and 1800 nm for the one-electron oxidations of 1, 3, 5,

6, 8 and 10 as ca. 10^-3M solutions containing [ⁿBu₄N][BF₄]

(0.2 M) as the supporting electrolyte at 273 K were monitored

using a PerkinElmer 9 spectrophotometer in conjunction with a

Metrohm Polarecord E506 potentiostat and a Ag/Ag⁺reference

electrode. For the latter spectroelectrochemical studies the cell

was a modiﬁed quartz cuvette of optical path length 0.5 mm and

involved three-electrodes: a Pt/Rh gauze working electrode; a Pt

wire secondary electrode (held in a fritted PTFE sleeve); and a

reference electrode chemically isolated from the test solution via a

bridge tube containing the electrolyte solution and terminating

in a porous frit. The cavity was purged with dinitrogen and

temperature of the sample was controlled by ﬂowing cooled

dinitrogen across the surface of the cell.

Syntheses

[Cp₂Mo(sdt)] (sdt

=

^-SC(H)C(C₆H₅)S^-) (1). CsOH·H₂O

(227 mg, 1.35 mmol) was dissolved in anhydrous MeOH (5 cm³).

4-(Phenyl)-1,3-dithiol-2-one (125 mg, 0.64 mmol) was added and

the mixture stirred for 15 min producing a yellow/brown solution.

[Cp₂MoCl₂] (195 mg, 0.65 mmol) and MeOH (5 cm³) were then

added and the mixture stirred overnight at room temperature.

The solvent was removed under a reduced pressure to yield a

black solid. Column chromatography of the reaction mixture on

alumina gave, on elution with CH₂Cl₂, the product as a single deep

purple band which was collected and evaporated to dryness. The

black solid was dissolved in a minimum volume of CH₂Cl₂and n-

hexane was added until the product precipitated as a black/purple

solid. The solvent was removed by decantation and the product

dried under reduced pressure. Yield 169 mg, 67%. Anal. Calcd.

for C₁₈H₁₆MoS₂: C, 55.09; H, 4.10%. Found: C, 54.80; H, 4.16%.

¹H NMR (300 MHz, CDCl₃+ Cp₂Co): d 7.70 (d, [J_H–H= 8.2 Hz],

2 H, H², H⁶), 7.27 (m, 2H, H³, H⁵), 7.13 (t, [J_H–H= 7.4 Hz], 1 H,

H⁴), 6.82 (s, 1 H, H^dithiolene), 5.31 (s, 10 H, 2C₅H₅). ¹³C NMR (125

MHz, CDCl₃+ Cp₂Co): d 140.3 (C^1¢/1), 140.1 (C^1¢/1), 127.7 (C³,

C⁵), 125.9 (C², C⁶), 125.4 (C^dithiolene), 124.2 (C⁴), 98.2 (C₅H₅). Mass

Spectrum (CI): m/z 395 [Cp₂Mo(sdt)+H]⁺. IR (KBr disc): 3114

(w), 3078 (w), 3066 (w), 1637 (m), 1589 (m), 1534 (s), 1483 (m),

1437 (m), 1085 (m), 1074 (m), 1016 (s), 1002 (m), 900 (w), 808 (vs)

749 (vs), 696 (m), 546 (m) cm^-1.

1 was dissolved in CH₂Cl₂and the diffusion of n-hexane

into a CH₂Cl₂solution of 1 gave red rectangular crystals.

[Cp₂Mo(sdt)][BF₄], ([1⁺][BF₄]) was prepared by stirring 1 (9 mg,

2.29 ¥ 10^-5mol) and [Cp₂Fe][BF₄] (6 mg, 2.19 ¥ 10^-5mol) in

CH₂Cl₂(2 cm³) for 2 h. The resultant green solution was ﬁltered

then layered with toluene and the mixture yielded red crystals

upon standing at room temperature.

[Cp₂Mo(2-pedt)] (2-pedt = ^-SC(H)C((C₅H₄N)-2)S^-) (2).

2

was prepared as described for 1, from CsOH·H₂O (229 mg,

1.36 mmol), 4-(pyridin-2-yl)-1,3-dithiol-2-one (130 mg,

0.66 mmol) and [Cp₂MoCl₂] (197 mg, 0.66 mmol), and

obtained as a black solid. Column chromatography of the

reaction mixture on alumina gave, on elution with CH₂Cl₂:MeOH

(99 : 1), the product as a single deep purple band which was

collected and evaporated to dryness. The black solid was dissolved

in a minimum volume of CH₂Cl₂and n-hexane was added until

the product precipitated as a black/purple solid. The solvent was

removed by decantation and the product dried under reduced

pressure. Yield 131 mg, 50%. Anal. Calcd. for C₁₇H₁₅NMoS₂: C,

51.90; H, 3.84; N, 3.56%. Found: C, 48.80; H, 3.52; N, 3.26%. ¹H

NMR (300 MHz, CDCl₃+ Cp₂Co): d 8.42 (d, [J_H–H= 4.4 Hz],

1 H, H⁶), 7.61 (d, [J_H–H= 8.1 Hz], 1 H, H³), 7.43 (m, 1 H, H⁴),

7.34 (s, 1 H, H^dithiolene), 6.87 (m, 1 H, H⁵), 5.23 (s, 10 H, 2C₅H₅).

¹³C NMR (125 MHz, CDCl₃): d 157.3 (C^1¢/2), 148.4 (C⁶), 140.5

(C^1¢/2), 135.5 (C⁴), 130.9 (C^dithiolene), 119.7 (C^3/5), 119.5 (C^3/5), 98.1

(C₅H₅). Mass Spectrum (CI): m/z 396 [Cp₂Mo(2-pedt)+H]⁺. IR

(KBr disc): 3098 (w), 3062 (w), 1577 (s), 1562 (m), 1517 (vs), 1460

(vs), 1438 (w), 1420 (m), 1377 (w), 1364 (w), 1268 (w), 1016 (w),

1000 (w), 920 (m), 831 (s), 815 (vs), 767 (s), 730 (m), 701 (w), 671

(m), 635 (w), 607 (w), 587 (w), 599 (w) cm^-1.

Each spectroelectrochemical study involved a single potential

step. The operating potential applied to the working electrode

a

was typically >100 mV more positive than E_pfor an oxidation

experiment and >100 mV more negative than E_p^cfor a reduction.

A redox process was considered complete when consecutive

spectra were identical. The chemical reversibility of a process was

investigated by application of a potential at the working electrode

that was sufﬁcient to re-reduce an electrogenerated oxidation

product or to re-oxidise an electrogenerated reduction product.

The applied potential was typically >100 mV more negative than

c

E_pto reverse an oxidation process and >100 mV more positive

than E_p^ato reverse a reduction process. The process was considered

to be electrochemically reversible if the spectroscopic proﬁle of the

starting material was reproduced.

In situ electrochemical EPR studies were performed with a

standard quartz tube of internal diameter 3 mm into which

was inserted a modiﬁed version of the 3-electrode arrangement

described by Hartl et al.⁴¹The working, secondary and pseudo-

reference electrodes were a Pt helix, a Pt wire and a Ag wire,

respectively. X-band EPR spectra were recorded on either a Bruker

ESP300E or Bruker EMX spectrometer and simulated using

WINEPR SimFonia, Shareware version 1.25, Bru¨ker Analytische

Messtechnik GmbH.

[Cp₂Mo(3-pedt)] (3-pedt = ^-SC(H)C((C₅H₄N)-3)S^-) (3). 3 was

prepared as described for 1, from CsOH·H₂O (255 mg, 1.50 mmol),

10468 | Dalton Trans., 2011, 40, 10457–10472

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©

4-(pyridin-3-yl)-1,3-dithiol-2-one (142 mg, 0.72 mmol) and

[Cp₂MoCl₂] (201 mg, 0.67 mmol) and puriﬁed as described for

2 to give a black/purple solid. Yield 155 mg, 58%. Anal. Calcd.

for C₁₇H₁₅NMoS₂: C, 51.90; H, 3.84; N, 3.56%. Found: C, 51.67;

on elution with CH₂Cl₂, the product as a single orange band

which was collected and evaporated to dryness. The red solid

was dissolved in a minimum volume of CH₂Cl₂and n-hexane

added until the product precipitated as a red solid. The solvent

was removed by decantation and the product dried under reduced

pressure. Yield 56 mg, 20%. Anal. Calcd. for C₁₈H₁₆WS₂: C, 45.01;

H, 3.36%. Found: C, 45.23; H, 3.12%. ¹H NMR (300 MHz,

CDCl₃+ Cp₂Co): d 7.60 (d, [J_H–H= 8.3 Hz], 2 H, H², H⁶), 7.15

(m, 2 H, H³, H⁵), 7.01 (t, [J_H–H= 6.8 Hz], 1 H, H⁴), 6.31 (s, 1 H,

H^dithiolene), 5.18 (s, 10 H, 2C₅H₅). ¹³C NMR (125 MHz, CDCl₃+

Cp₂Co): d 143.8 (C^1¢/1), 139.0 (C^1¢/1), 127.8 (C³, C⁵), 126.5 (C², C⁶),

126.3 (C^dithiolene), 125.4 (C⁴), 94.8 (C₅H₅). Mass Spectrum (CI); m/z

481 [Cp₂W(sdt)+H]⁺. IR (KBr disc): 3118 (m), 3084 (m), 1589 (s),

1570 (m), 1536 (vs), 1483 (s), 1432 (m), 1384 (s), 1068 (m), 1012

(s), 999 (s), 916 (w), 899 (m), 832 (vs), 808 (vs), 779 (m), 749 (vs),

696 (s), 653 (m), 588 (w) cm^-1.

H, 3.83; N, 3.54%. ¹H NMR (300 MHz, CDCl₃): d 8.80 (d, [J_H–H

=

1.7 Hz], 1 H, H²), 8.23 (dd, [J_H–H= 1.3, 4.6 Hz], 1 H, H⁶), 7.84

(m, 1 H, H⁴), 6.87 (m, 1 H, H⁵), 6.84 (s, 1 H, H^dithiolene), 5.22 (s,

10 H, 2C₅H₅). ¹³C NMR (125 MHz, CDCl₃): d 147.1 (C²), 146.0

(C⁶), 136.8 (C^1¢/3), 132.7 (C⁴), 126.5 (C^dithiolene), 122.6 (C⁵), 98.2

(C₅H₅). Mass Spectrum (CI): m/z 396 [Cp₂Mo(3-pedt)+H]⁺. IR

(KBr disc): 1560 (m), 1532 (vs), 1508 (m), 1474 (m), 1438 (m), 1402

(m), 1098 (m), 1087 (m), 1066 (m), 1039 (m), 1021 (s), 1005 (m),

908 (w), 828 (s), 816 (vs), 801 (m), 766 (s), 668 (w), 550 (w) cm^-1.

Purple needle-shaped crystals of 3 were grown by the slow

diffusion of n-hexane into a solution of 3 in CDCl₃. [Cp₂Mo(3-

pedt)][BF₄] ([3⁺][BF₄]) was prepared by stirring 3 (9 mg, 2.29 ¥

10^-5mol) and [Cp₂Fe][BF₄] (5 mg, 1.83 ¥ 10^-5mol) in CH₂Cl₂for

2 h. The resultant green solution was ﬁltered then layered with

toluene and the mixture yielded red crystals upon standing at

room temperature.

[Cp₂W(2-pedt)] (7). 7 was prepared and puriﬁed as described

for 6, from CsOH·H₂O (203 mg, 1.23 mmol), 4-(pyridin-2-yl)-

1,3-dithiol-2-one (121 mg, 0.61 mmol) and [Cp₂WCl₂] (236 mg,

0.61 mmol) and obtained as a red/brown solid. Yield 52 mg,

17%. Anal. Calcd for C₁₇H₁₅WNS₂: C, 42.42; H, 3.14; N, 2.91%.

Found: C, 42.07; H, 3.31; N, 2.82%. ¹H NMR (300 MHz, CDCl₃+

Cp₂Co): d 8.42 (d, [J_H–H= 4.3 Hz], 1 H, H⁶), 7.63 (d, [J_H–H= 7.9

Hz], 1 H, H³), 7.42 (m, 1 H, H⁴), 6.97 (s, 1 H, H^dithiolene), 6.85 (m,

1 H, H⁵), 5.17 (s, 10 H, 2C₅H₅).¹³C NMR (125 MHz, CDCl₃): d

156.2 (C^1¢/2), 148.6 (C⁶), 144.4 (C^1¢/2), 135.6 (C⁴), 133.1 (C^dithiolene),

120.3 (C^3/5), 119.5 (^C3/5), 94.6 (C₅H₅). Mass spectrum (CI); m/z

482 [Cp₂W(2-pedt)+H]⁺. IR (KBr): 3105 (w), 3094 (w), 3083 (w),

1576 (s), 1519 (vs), 1459 (s), 1421 (s), 1147 (w), 1113 (w), 1087 (m),

1050 (m), 1014 (m), 997 (m), 922 (m), 834 (s), 814 (s), 767 (s), 669

(s) cm^-1.

[Cp₂Mo(4-pedt)] (4-pedt = ^-SC(H)C((C₅H₄N)-4)S^-) (4). 4 was

prepared as described for 1, from CsOH·H₂O (230 mg, 1.36 mmol),

4-(pyridin-4-yl)-1,3-dithiol-2-one (132 mg, 0.67 mmol) and

[Cp₂MoCl₂] (120 mg, 0.40 mmol) and puriﬁed as described for

2 to give a black/purple solid. Yield 113 mg, 71%. Anal. Calcd.

for C₁₇H₁₅NMoS₂: C, 51.90; H, 3.84; N, 3.56%. Found: C, 50.76;

H, 3.86; N, 3.25%. ¹H NMR (300 MHz, CDCl₃): d 8.32 (d, [J_H–H

=

4.8 Hz], 2 H, H², H⁶), 7.45 (d, [J_H–H= 4.8 Hz], 2 H, H³, H⁵), 7.07

(s, 1 H, H^dithiolene), 5.22 (s, 10 H, 2C₅H₅). ¹³C NMR (125 MHz,

CDCl₃): d 148.9 (C², C⁶), 130.7 (C^dithiolene), 120.1 (C³, C⁵), 98.2

(C₅H₅). Mass Spectrum (CI): m/z 396 [Cp₂Mo(4-pedt)+H]⁺.IR

(KBR disc): 2954 (m), 1599 (m), 1585 (vs), 1541 (m), 1513 (s),

1260 (m), 1209 (w), 1015 (m), 988 (m), 918 (m), 832 (vs), 812 (m),

771 (s), 730 (m), 702 (w), 673 (w), 652 (m), 607 (m), 556 (w) cm^-1.

[Cp₂W(3-pedt)] (8). 8 was prepared and puriﬁed as described

for 6, from CsOH·H₂O (208 mg, 1.23 mmol), 4-(pyridin-3-yl)-

1,3-dithiol-2-one (121 mg, 0.61 mmol) and [Cp₂WCl₂] (238 mg,

0.61 mmol), and obtained as an orange solid. Yield 75 mg, 25%.

Anal. Calcd for C₁₇H₁₅NWS₂: C, 42.42; H, 3.14; N, 2.91. Found:

[Cp₂Mo(qedt)] (qedt = ^-SC(H)C(C₆H₄N₂C₂H)S^-) (5). 5 was

prepared and puriﬁed as described for 1, from CsOH·H₂O

(83 mg, 0.49 mmol), 4-(quinoxalin-2-yl)-1,3-dithiol-2-one (61 mg,

0.25 mmol) and [Cp₂MoCl₂] (83 mg, 0.28 mmol), and was

obtained as a purple solid. Yield 48 mg, 44%. Anal. Calcd. for

C₂₀H₁₆N₂MoS₂: C, 54.05; H, 3.63; N, 6.30%. Found: C, 53.15; H,

3.53; N, 6.86%. ¹H NMR (300 MHz, CDCl₃): d 9.27 (s, 1 H, H³),

8.09 (d, [J_H–H= 8.2 Hz], 1 H, H^5/8), 7.96 (d, [J_H–H= 8.1 Hz], 1

H, H^5/8), 7.82 (s, 1 H, H^dithiolene), 7.63 (m, 1 H, H^6/7), 7.56 (m, 1

H, H^6/7), 5.35 (s, 10 H, 2C₅H₅). Mass Spectrum (CI): m/z 446

[Cp₂Mo(qedt)+H]⁺.

1

C, 42.08; H, 3.28; N, 2.92. H NMR (300 MHz, CDCl₃): d 8.80

(d, [J_H–H= 2.3 Hz], 1 H, H²), 8.20 (dd, [J_H–H= 1.5, 4.6 Hz], 1 H,

H⁶), 7.85 (m, 1 H, H⁴), 7.06 (m, 1 H, H⁵), 6.38 (s, 1 H, H^dithiolene),

5.16 (s, 10 H, 2C₅H₅). ¹³C NMR (125 MHz, CDCl₃): d 147.8 (C²),

146.1 (C⁶), 140.8 (C^1¢/3), 134.9 (C^1¢/3), 133.2 (C⁴), 128.8 (C^dithiolene),

122.6 (C⁵), 94.8 (C₅H₅). Mass spectrum (CI): m/z 482 [Cp₂W(3-

pedt)+H]⁺. IR (KBr): 3109 (m), 3082 (m), 1577 (w), 1560 (m),

1531 (s), 1473 (m), 1433 (m), 1403 (m), 1125 (w), 1019 (m), 1001

(m), 833 (s), 815 (s), 799 (m), 768 (s), 708 (m), 552 (w) cm^-1.

[Cp₂W(3-pedt)][BF₄] ([8⁺][BF₄]) was prepared by stirring 8

(6 mg, 1.25 ¥ 10^-5mol) and [Cp₂Fe][BF₄] (4 mg, 1.47 ¥ 10^-5mol)

in CH₂Cl₂for 2 h. The resultant green solution was ﬁltered then

layered with toluene and the mixture yielded red crystals upon

standing at room temperature.

[Cp₂W(sdt)] (6). CsOH·H₂O (185 mg, 1.10 mmol) was dis-

solved in anhydrous MeOH (5 cm³). 4-(Phenyl)-1,3-dithiol-2-one

(115 mg, 0.59 mmol) was added and the mixture stirred for 15

min producing a yellow/brown solution. [Cp₂WCl₂] (225 mg,

0.58 mmol) and CH₂Cl₂(15 cm³) were then added and the mixture

stirred overnight at room temperature. The solvent was removed

under a reduced pressure to yield a brown solid. The solid was

dissolved in CH₂Cl₂and the solution ﬁltered through alumina;

the alumina was washed with CH₂Cl₂:MeOH (98 : 2) (3 ¥ 5 cm³)

to remove any absorbed product, the washings and the ﬁltrate

combined and the solvent removed under a reduced pressure

to give a red solid. Column chromatography on alumina gave,

[Cp₂W(4-pedt)] (9). 9 was prepared and puriﬁed as described

for 6, from CsOH·H₂O (218 mg, 1.30 mmol), 4-(pyridin-4-yl)-

1,3-dithiol-2-one (126 mg, 0.65 mmol) and [Cp₂WCl₂] (251 mg,

0.65 mmol) and obtained as an orange solid. Yield 111 mg, 35%.

Anal. Calcd for C₁₇H₁₅NWS₂: C, 42.42; H, 3.14; N, 2.91%. Found:

C, 41.88; H, 3.14; N, 2.90%. ¹H NMR (300 MHz, CDCl₃+ Cp₂Co):

d 8.31 (m, 2 H, H², H⁶), 7.46 (m, 2 H, H³, H⁵), 6.67 (s, 1 H, H^dithiolene),

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©

5.19 (s, 10 H, 2C₅H₅). ¹³C NMR (75 MHz, CDCl₃): d 148.7 (C²,

C⁶), 141.7 (C^1¢/4), 133.6 (C^dithiolene), 120.8 (C³, C⁵), 94.7 (C₅H₅). Mass

spectrum (CI): m/z 482 [Cp₂W(4-pedt)+H]⁺. IR (KBr): 3091 (w),

3079 (w), 3065 (w), 3057 (w), 1587 (vs), 1543 (s), 1516 (s), 1408

(w), 1210 (w), 1013 (w), 989 (m), 917 (w), 838 (s), 814 (m), 803

(m), 773 (m), 732 (w), 651 (w), 609 (w), 558 (w) cm^-1.

reﬁned using a riding model. For [1][BF₄], one Cp ring was

disordered over two sites, the major and minor occupancies

being determined as 0.65 and 0.35, respectively; the disorder

components were constrained to be regular pentagons. The F

atoms were each disordered over two sites, the occupancies of

which were constrained to sum to unity and the bonded and

non-bonded distances of both fragments were constrained at

chemically sensible values. The non-hydrogen atoms were reﬁned

anisotropically, with the exception of the disordered Cp ring and

the minor [BF₄]^-site. Hydrogen atoms were included at calculated

positions and reﬁned using a riding model. For 3, all of the non-

hydrogen atoms were reﬁned anisotropically. Hydrogen atoms

were included at calculated positions but not reﬁned.

For [3][BF₄], one Cp ring was disordered over two sites, each

of occupancy 0.5. The F atoms were each disordered over two

sites; the occupancy of one [BF₄]^-group was set at 0.6 and the

other at 0.4 and the bonded and non-bonded distances of both

fragments were constrained at chemically sensible values. The non-

hydrogen atoms were reﬁned anisotropically, with the exception of

the disordered Cp ring and the minor [BF₄]^-site. Hydrogen atoms

were included at calculated positions and reﬁned using a riding

model. All of the non-hydrogen atoms were reﬁned anisotropically;

hydrogen atoms were included at calculated positions and reﬁned

using a riding model.

Deep red crystals of 9 were grown by the slow diffusion of n-

hexane into a solution of 9 in CH₂Cl₂.

[Cp₂W(qedt)] (10). 10 was prepared as described for 6, from

CsOH·H₂O (132 mg, 0.79 mmol), 4-(quinoxalin-2-yl)-1,3-dithiol-

2-one (96 mg, 0.39 mmol) and [Cp₂WCl₂] (139 mg, 0.36 mmol)

and obtained as a red/brown solid. Column chromatography on

alumina gave, on elution with CH₂Cl₂, the product as a single

orange band which was collected and evaporated to dryness. The

deep red solid was dissolved in a minimum volume of CH₂Cl₂

and n-hexane added until the product precipitated as a deep red

solid. The solvent was removed by decantation and the product

dried under a reduced pressure. Yield 28 mg, 15%. Anal. Calcd for

C₂₀H₁₆N₂WS₂: C, 45.12; H, 3.03; N, 5.26%. Found: C, 43.32; H,

3.04; N, 4.94%. ¹H NMR (300 MHz, CDCl₃): d 9.29 (s, 1 H, H²),

8.10 (m, 1 H, H^5/8), 7.96 (m, 1 H H^5/8), 7.64 (m, 1 H, H^6/7), 7.56

(m, 1 H, H^6/7), 7.44 (s, 1 H, H^dithiolene), 5.32 (s, 10 H, 2C₅H₅). Mass

spectrum (CI): m/z 532 [Cp₂W(qedt)+H]⁺.

For [8][BF₄] hydrogen atoms were included at calculated posi-

tions. The Cp ring C13 to C17 was disordered over two sets of

sites, each with occupancy 0.5 and constrained to be a regular

pentagon. The two disorder components were constrained to be

regular pentagons. The atoms F3 and F4 were also each disordered

over two sites, the occupancies of which were constrained to sum to

unity. All non-hydrogen atoms were reﬁned anisotropically, except

those of the disordered Cp ring.

[Cp₂Mo(S₂C₂(Me)(pyridin-2-yl))] (11). CsOH·H₂O (147 mg,

0.87 mmol) was dissolved in anhydrous MeOH (5 cm³). 4-(Pyridin-

2-yl)-5-(methyl)-l,3-dithiol-2-one (91 mg, 0.43 mmol) was added

and the mixture stirred for 15 min and a brown solution was

obtained. [Cp₂MoCl₂] (121 mg, 0.40 mmol) was then added and

the mixture stirred overnight at room temperature. The solvent

was then removed under a reduced pressure to yield a black

solid. The solid was dissolved in CH₂Cl₂and ﬁltered through

alumina; the alumina was washed with CH₂Cl₂:MeOH (98 : 2) (3 ¥

5 cm³) the washings combined and the solvent removed under a

reduced pressure to give a brown solid. Column chromatography

on alumina of the reaction mixture gave, on elution with CH₂Cl₂,

the product as a single deep purple band which was collected

and evaporated to dryness. The black solid was dissolved in

a minimum volume of CH₂Cl₂and n-hexane added until the

product precipitated as a brown solid. The solvent was removed by

decantation and the product dried under a reduced pressure. Yield

72 mg, 41%. Anal. Calcd. for C₂₀H₁₆MoN₂S₂: C, 53.07; H, 4.21; N,

3.44% Found: C, 54.08; H, 4.89; N, 3.29%. ¹H NMR (300 MHz,

CDCl₃); d 8.59 (m, 1 H, H⁶), 7.58 (m, 1 H, H³), 7.40 (m, 1 H, H⁴),

7.02 (m, 1 H, H⁵), 5.29 (s, 10 H, 2C₅H₅) 2.25 (s, 3 H, CH₃). Mass

Spectrum (CI); m/z = 409 [Cp₂Mo(S₂C₂(Me)(pyridin-2-yl)+H)]⁺.

DFT Calculations

Restricted and unrestricted DFT calculations were performed

using the Amsterdam Density Functional (ADF) suite version

2009.01.^44,45The DFT calculations employed a Slater-type orbital

(STO) all-electron triple-z-plus one polarization function basis set

from the ZORA/TZP database of the ADF suite for all atoms.

The scalar relativistic (SR) approach was used within the ZORA

Hamiltonian for the inclusion of relativistic effects and the local

density approximation (LDA) with the correlation potential due

to Vosko et al.⁴⁶was used in all of the DFT calculations. Gradient

corrections were performed using the functionals of Becke⁴⁷and

Perdew (BP).⁴⁸Models of [1]^z, [6]^z, 5 and 10 (z = 0, 1+ or 1-)

in C₁symmetry were constructed using geometrical data from

the X-ray crystal structures of the available complexes. The co-

ordinate frame employed in the calculations is shown in Fig. 9.

In each model the z-axis bisects the S–M–S angle and the x-

axis lies in the MS₂plane (M = Mo, W). The model geometries

were optimized at the ZORA SR level. For the unrestricted

calculations pictorial representations of the MOs were generated

using MOLEKEL⁴⁹and fragment contributions to the MOs were

obtained using Mulliken Population Analysis as implemented in

the AOmix package.^50,51

Crystallography

Crystals containing each component of the [1]⁺/1, [3]⁺/3 redox

couples; and [8]⁺and 9 were grown as described above. The details

of the unit cell, data collection and reﬁnement for the structures

of 1, [1][BF₄], 3, [3][BF₄], [8][BF₄] and 9 are given in Table 8.

The structures were solved by either direct methods (1, [1][BF₄],

[3][BF₄] and [8][BF₄]) or by heavy atom Patterson methods (3 and

9) using SHELXS97⁴²and reﬁned by direct and difference Fourier

methods using SHELXL97⁴²or teXsan⁴³(for 3 only).

Summary and conclusions

The procedure reported previously by Davies et al.,^22,23i.e. the re-

action of [Cp₂MCl₂] (M = Mo or W) with the pro-ligand produced

For 1, all of the non-hydrogen atoms were reﬁned anisotropi-

cally; hydrogen atoms were included at calculated positions and

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©

by treating the corresponding R-1,3-dithiol-2-one with CsOH

(Scheme 1), provides a convenient synthesis of [Cp₂M(S₂C₂(H)R)]

(M = Mo or W; R = phenyl, pyridin-2-yl, pyridin-3-yl, pyridin-

4-yl or quinoxalin-2-yl) and [Cp₂Mo(S₂C₂(Me)(pyridin-2-yl))].

The availability of analogous Mo compounds with a variety of

substituents on the dithiolene group and their W counterparts has

allowed informed comparisons to be made concerning their nature

and properties.

Electrochemical and associated UV/vis/NIR spectroelec-

trochemical investigations have shown that each of these

[Cp₂M(S₂C₂(H)R)] compounds in dmf has a rich redox chemistry;

thus, each compound undergoes two, and in some cases three,

oxidations and reductions. For each compound the ﬁrst oxidation

(OX^I) is both electrochemically and chemically reversible and the

one-electron reduction (RED^I) is electrochemically reversible; the

latter process is chemically reversible for the Mo but not for the W

compounds. Each redox change occurs at a more positive potential

for a Mo compound than for its W counterpart; typically, D ª 30

and 300 mV for OX^Iand RED^I, respectively.

DFT calculations have provided valuable information con-

cerning the electronic structure of the parent compounds and

their mono-cations and mono-anions. OX^Iinvolves the removal

of an electron from an orbital that is primarily dithiolene C–

C p-bonding and C–S p-antibonding in character. This infor-

mation is consistent with the EPR spectra of the Mo mono-

cations, in that each manifests coupling of the unpaired electron

with the dithiolene H atom (or for [11]⁺[Cp₂Mo(S₂C₂(Me)(2-

pedt))]⁺the three H atoms on the Me group) and a relatively

small ^95,97Mo hyperﬁne coupling. Also, the results of crystal-

lographic studies accomplished for both components of the

[Cp₂Mo(S₂C₂(H)R)]⁺/[Cp₂Mo(S₂C₂(H)R)] (R = Ph or pyridin-3-

yl) redox couples show changes in the length of the Mo–S, S–C and

C–C bonds of the {MoSCCS} moiety that are generally consistent

with OX^Iinvolving the loss of an electron from an orbital that is

Mo–S and C–S antibonding and C–C bonding in character.

The DFT calculations indicate that the unpaired electron is

more localised on the metal in mono-anions than in the mono-

cations. The EPR spectrum of the each Mo containing mono-

anion manifests a larger ^95,97Mo coupling than observed for the

corresponding mono-cation. Also, each redox change occurs at

a more positive potential for a Mo compound than for its W

counterpart; typically, D ª 30 and 300 mV for OX^Iand RED^I,

respectively.

we consider that these three points are relevant to the quest

of understanding the role of this cofactor in the catalyses

accomplished at the Mo (and W) centres of the oxotransferase

enzymes.

Acknowledgements

We thank the EPSRC for ﬁnancial support, including the facilities

and expertise provided by the EPSRC EPR National Service, and

Professor Lesley Yellowlees of Edinburgh University for access to

NIR spectroelectrochemical facilities.

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10472 | Dalton Trans., 2011, 40, 10457–10472

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Synthesis, structure and redox properties of bis(cyclopentadienyl) dithiolene complexes of molybdenum and tungsten

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