Ligand substitution and growth reactions with Ru 3 Se 2 nido clusters. Coordination isomerism in...

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Inorganica Chimica Acta 358 (2005) 161–172

Ligand substitution and growth reactions with Ru3Se2 nidoclusters. Coordination isomerism in

[WRu3(l4-Se)2(l-CO)4(CO)7(L)] (L = phosphine ligand) andelectrochemical behaviour of the Ru3MSe2 core clusters

(M = Ru, W) in solution

Daniele Belletti a, Claudia Graiff a, Roberto Pattacini a, Giovanni Predieri a,Antonio Tiripicchio a,*, Fabrizia Fabrizi de Biani b, Piero Zanello b

a Dipartimento di Chimica Generale ed Inorganica, Chimica Analitica, Chimica Fisica, Universita di Parma,

Viale delle Scienze 17A, I-43100 Parma, Italyb Dipartimento di Chimica, Universita di Siena, Via Aldo Moro, I-53100 Siena, Italy

Received 12 July 2004; accepted 23 July 2004

Available online 28 August 2004

Abstract

The monosubstituted selenido-carbonyl open-triangular nido cluster [Ru3(l3-Se)2(CO)8(L)] (L = trispyrrolidinylphosphine, tpnp)

reacts with trisparamethoxyphenylphosphine (t4mpp) and a,a 0-bis(diphenylphosphinemethyl)benzene (dpmb), giving, respectively,

the di- and tri-substituted [Ru3(l3-Se)2(CO)6(tpnp)(dpmb)] and [Ru3(l3-Se)2(CO)7(tpnp)(t4mpp)] clusters. Moreover, clusters

[Ru3(l3-Se)2(CO)8(L)] (L = tpnp, t4mpp, diphenylmethoxyphosphine (dpxp)) show a reactivity towards [W(CO)3(MeCN)3], leading

to the corresponding bimetallic closo clusters [WRu3(l4-Se)2(l-CO)4(CO)7(L)]. The formation of two coordination isomers, due to

the position of L on different Ru atoms, confirms the presence in solution of two [Ru3(l3-Se)2(CO)8(L)] isomers before the attack of

the W(CO)3 fragment. Finally, electrochemical measurements show that the substitution of a Ru atom with a W atom in the closo

Ru4Se2 cluster core triggers significant changes in the redox activity of the cluster core, which is further modulated by the electronic

effects of the peripheral phosphines.

� 2004 Elsevier B.V. All rights reserved.

Keywords: Ruthenium; Tungsten; Selenium; Cluster compounds; Crystal structure; Voltammetry

1. Introduction

Transition-metal chalcogenido clusters chemistry is

an attracting research field [1–3], owning their potential

application as catalysts, electrocatalysts and as precur-

sors for low gap semiconductors [3–6].

0020-1693/$ - see front matter � 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.ica.2004.07.027

* Corresponding author. Tel.: +39 0521 905418; fax: +39 0521

905557.

E-mail address: [email protected] (A. Tiripicchio).

Regarding the selenido clusters, we have carried out

systematic investigations on the selenium transfer reac-

tions by tertiary phosphine and diphosphine selenides,

towards iron and ruthenium carbonyl clusters [7–9]

obtaining a variety of phosphine-substituted selenido

clusters as shown in Scheme 1.

Following our interest on selenido cluster chemistry,we reacted phosphine selenides with bimetallic clusters

with the aim of setting up convenient synthetic routes

to phosphine-substituted mixed-metal selenido clusters.

Two different synthesis strategies have been developed:

mailto:[email protected]

M

M M

R3PSeSe

M

M

M

Se

M

M

M

Se

R3PSe

Se

M

M

Se M

Se

M

M

M

Se

-M

M = FeM = Ru

M= Ru, Fe

L1

L2L3

L3

L2

L1

L1

L2

L2 = R3P, CO

L2, L3 = R3P, CO

L2, L3 = R3P, COL2 = R3P, COL1

L2

L1 = R3P

L1 = R3P

L1 = R3P L1 = R3P

+M

Scheme 1. Carbonyls omitted for clarity.

162 D. Belletti et al. / Inorganica Chimica Acta 358 (2005) 161–172

(a) reactions between preformed bimetallic clusters andtertiary phosphine selenides and (b) cluster growth reac-

tions from preformed chalcogenide carbonyl metal clus-

ters and suitable organometallic fragments such as

M(CO)3 (M = Mo or W) [9–12].

This paper deals with the reactivity of the monosub-

stituted clusters [Ru3(l3-Se)2(CO)8(L)] (L = trispyrrolid-

inylphosphine (tpnp), diphenylmethoxy phosphine

(dpxp), trisparamethoxyphenylphosphine (t4mpp)) ob-tained by reaction between [Ru3(CO)12] and the corre-

sponding phosphine selenides. These monosubstituted

selenido nido clusters can undergo further carbonyl sub-

stitution with mono- and diphosphines giving di- and

tri-substituted selenido clusters and react with the

W(CO)3 fragment giving the corresponding closo clus-

ters [WRu3(l4-Se)2(l-CO)4(CO)7(L)]. The ligand can

be coordinated to the ruthenium atom bound to tung-sten or to the ruthenium opposite to the tungsten atom

(two isomers).

Finally, electrochemical measurements have been car-

ried out in order to investigate about the electronic ef-

fects played of the substitution of a Ru atom with a

W atom in the Ru4Se2 core, of the substitution by differ-

ent phosphine ligands and of different position of the

phosphine in the two isomers [WRu3(l4-Se)2(l-CO)4(CO)7(tpnp)].

2. Experimental

2.1. General remarks

The starting reagents [Ru3(CO)12], elemental selenium,tris-pyrrolidinylphosphine (tpnp), diphenylmethoxy-

phosphine (dpxp), trisparamethoxyphenylphosphine

(t4mpp), a,a 0-bis(diphenylphosphinemethyl)benzene

(dpmb), Me3NO, were pure commercial products

(Aldrich and Fluka) and were used as received. Com-pounds [Ru3(l3-Se)2(CO)8(t4mpp)] [13], [Ru4(l4-Se)2(l-CO)4(CO)6(tpp)2] [8], [WRu3(l4-Se)2(l-CO)4(CO)6(tpp)2], [WRu3(l4-Se)2(l-CO)4(CO)6(dpmp)2], [WRu3(l4-Se)2(l-CO)4(CO)6(t4mpp)2], [Ru3(l3-Se)2(CO)8-

(dpxp)] [12] and W(CO)3(CH3CN)3 [14] were prepared

according to the procedure described in the literature.

The solvents (C. Erba) were dried and distilled by stand-

ard techniques before use. TLC were carried out on sil-ica (Merck, 60 PF254, 20 · 20 cm glass plates). After the

chromatographic runs, products were extracted with

dichlorometane and recrystallized from methanol–

dichloromethane mixtures. New compounds were iden-

tified by comparison of their spectroscopic data with

those of known compounds. Purity was checked by ele-

mental analysis and/or by 1H and 31P NMR spectros-

copy. Elemental (C, H, N) analyses were performedwith a Carlo Erba EA 1108 automated analyzer. IR

spectra (KBr discs or CH2Cl2 solutions) were recorded

on Nicolet 5PC and Nexus FT spectrometers. 1H (300

MHz) and 31P (81.0 MHz, 85%-H3PO4 as external refer-

ence) NMR spectra (CDCl3 solutions) were recorded on

Bruker instruments, AC 300 (1H) and CXP 200 (31P).

2.2. Synthesis of tpnpSe

An excess of elemental selenium (250 mg, 3.16 mmol)

was added to a toluene solution (100 ml) of tpnp (500

mg, 2.07 mmol), and then stirred at 90 �C for 2 h; the

solution was dried in vacuo and the residue extracted

with dichloromethane. The white powder was recog-

nized as the pure phosphine selenide. Yield: 95%. Anal.

Calc. for C12N3H24PSe: C, 45.00; N, 13.12; H, 7.50.Found: C, 44.57; N, 13.01; H, 7.48%. FT-IR (KBr)

(cm�1): 534 s (mP = Se). 1H NMR (CDCl3-d1): d 1.72

(m, 12H, Hb), 3.04 (m, 12H, Ha).31P NMR (CDCl3-

d1): d93.0 (s, with 77Se satellites, 1J(P,Se) 718 Hz).

D. Belletti et al. / Inorganica Chimica Acta 358 (2005) 161–172 163

2.3. Reaction of tpnpSe with [Ru3(CO)12]. Synthesis of

[Ru3(l3-Se)2(CO)8(tpnp)] (1)

[Ru3(CO)12] (300 mg, 0.47 mmol) was reacted with

tpnpSe (300 mg, 0.94 mmol) in toluene at 70 �C for 2

h in the presence of Me3NO (35 mg, 0.47 mmol) untilthe solution turned red. The mixture was evaporated

to dryness and the residue was redissolved in a small

amount of dichloromethane. TLC separation on silica,

using 2:1 dichloromethane/hexane as eluent mixture,

yielded a red–orange band together with some minor

decomposition. The band contained cluster [Ru3(l3-Se)2(CO)8(tpnp)] (1) which was purified by recrystalliza-

tion by evaporation from a MeOH/CH2Cl2 mixture(Yield: 45%).

Compound 1: FT-IR (CH2Cl2) m(CO) (cm�1): 2071m,

2037vs, 2020s, 1998s, 1967m. 31P{1H} NMR (CDCl3,

298 K): d 105.9 (s). 1H NMR (CDCl3, 298 K): d 1.90

(m, 12H, Hb), 3.26 (m, 12H, Ha). Anal. Calc. for

C20H24O8N3PRu3Se2: C, 25.93; N, 4.54; H, 2.61. Found:

C, 25.88; N, 4.41; H, 2.48%.

2.4. Reaction of 1 with dpmb. Synthesis of [Ru3(l3-Se)2(CO)6(tpnp)(dpmb)] (2)

Compound 1 (100 mg, 0.11 mmol) and dpmb (51 mg,

0.11 mmol) were stirred in dry toluene for 1.5 h at 90 �Cunder N2. The solution turned from orange to brilliant

red. The solvent was removed in vacuo and the residue

was dissolved in CH2Cl2 (10 cm3). The product wasidentified as [Ru3(l3-Se)2(CO)6(tpnp)(dpmb)] (2, Yield:

95%). Crystals suitable for X-ray analysis were obtained

by layering methanol on a dichloromethane solution of

2.

Compound 2: FT-IR (CH2Cl2) m(CO) (cm�1): 2026vs,

1981s, 1961vs, 1922m. 31P{1H} NMR (CDCl3, 298 K): d21.3 (s), 116.0 (s). 1H NMR (CDCl3, 298 K): d 1.90 (m,

12H, Hb), 3.27 (m, 12H, Ha), 3.72 (dd, 2Ha, CH2,2J(H,P) = 13.5 Hz, 2J(H,H) = 4.2 Hz), 4.26 (dd, 2Hb,

CH2,2J(H,P) = 13.5 Hz, 2J(H,H) = 6.5 Hz), 6.10 (m,

2Ha), 6.93 (m, 2Hb), 7.25–7.80 (m, 20H, 4Ph). Anal.

Calc. for C50H52O6N3P3Ru3Se2: C, 44.65; N, 3.12; H,

3.90. Found: C, 44.60; N, 3.00; H, 3.88%.

2.5. Reaction of 1 with t4mpp. Synthesis of [Ru3(l3-Se)2(CO)7(tpnp)(t4mpp)] (3)

Compound 1 (50 mg, 0.054 mmol) and t4mpp (19 mg,

0.054 mmol) were stirred in dry toluene for 1.5 h at 90

�C under N2. The resulting brilliant red solution was

evaporated to dryness and the residue was dissolved in

CH2Cl2 (10 cm3). The product was identified as

[Ru3(l3-Se)2(CO)7(tpnp)(t4mpp)] (3, Yield: 87%).

Compound 3: FT-IR (CH2Cl2) m(CO) (cm�1): 2037vs,2003vs, 1962m, 1935w sh. 31P{1H} NMR (CDCl3, 298

K): d 113.2 (s), 111.4 (s), 109.3 (s), 43.81 (s), 41.67 (s),

40.4 (s) (9:1:1:1:1:9). Anal. Calc. for C40H45O7N3-

P2Ru3Se2: C, 39.94; N, 3.49; H, 3.77. Found: C, 39.92;

N, 3.41; H, 3.77%.

2.6. Reaction of 1 with [W(CO)3(CH3CN)3]. Synthesis

of [WRu3(l4-Se)2(l-CO)4(CO)7(tpnp)] (isomers 4and 5)

Compound 1 (80 mg, 0.086 mmol) and

[W(CO)3(CH3CN)3] (67 mg, 0.172 mmol) were stirred

in dry CH2Cl2 for 1 h at room temperature under N2.

The resulting dark solution was evaporated to dryness

and the residue was dissolved in CH2Cl2 (10 cm3).

TLC separation on silica, using 1:1 dichloromethane/hexane as eluent mixture, yielded a brown band and a

reddish brown, in order of eluition, along with the two

starting reagents and some decomposition. The two

bands contained clusters 4 (brown, yield 35%) and 5

(reddish brown, yield 15%), respectively. Crystals suita-

ble for X-ray analysis were obtained by layering metha-

nol on dichloromethane solutions of the two

compounds. Clusters 4 and 5 were recognized as twocoordination isomers of [WRu3(l4-Se)2(l-CO)4(CO)7(tpnp)] (see Scheme 4).

Compound 4: FT-IR (CH2Cl2) m(CO) (cm�1): 2061w,

2030vs, 2001s, 1984m sh, 1911w, 1868w, 1829w. 31P{1H}

NMR (CDCl3, 298 K): d 105.0 (s). 1H NMR (CDCl3,

298 K): d 1.90 (m, 12H, Hb), 3.17 (m, 12H, Ha). Anal.

Calc. for C23H24O11N3PWRu3Se2: C, 23.13; N, 3.52;

H, 2.03. Found: C, 23.10; N, 3.50; H, 2.00%.Compound 5: FT-IR (CH2Cl2) m(CO) (cm�1): 2065m,

2048w, 2033s, 2012vs, 1994s, 1979m, 1908w, 1857w,

1835w. 31P{1H} NMR (CDCl3, 298 K): d 96.3 (s). 1H

NMR (CDCl3, 298 K): d 1.88 (m, 12H, Hb), 3.09 (m,

12H, Ha). Anal. Calc. for C23H24O11N3PWRu3Se2: C,

23.13; N, 3.52; H, 2.03. Found: C, 23.08; N, 3.47; H, 1.98%.

2.7. Reaction of [Ru3(l3-Se)2(CO)8(t4mpp)] with

[W(CO)3(CH3CN)3]. Synthesis of [WRu3(l4-Se)2(l-CO)4(CO)7(t4mpp)] (6)

Cluster [Ru3(l3-Se)2(CO)8(t4mpp)] (80 mg, 0.077

mmol) and [W(CO)3(CH3CN)3] (60 mg, 0.155 mmol)

were stirred in dry CH2Cl2 for 1.5 h at room tempera-

ture under N2. The resulting dark solution was evapo-

rated to dryness and the residue was dissolved inCH2Cl2 (10 cm3). A reddish brown band (containing

cluster 6, yield 45%) was separated and purified by

TLC on silica, using CH2Cl2–hexane (1:1) as eluent mix-

ture. Crystals suitable for X-ray analysis were obtained

by layering methanol on a dichlorometane solution of 6.

Compound 6: FT-IR (CH2Cl2) m(CO) (cm�1): 2067m,

2036vs, 2016vs, 1998vs, 1910m, 1866w, 1843w. 31P{1H}

NMR (CDCl3, 298 K): d 40.0 (s). Anal. Calc. forC32H21O14PWRu3Se2: C, 29.44; H, 1.62. Found: C,

29.40; H, 1.58%.


2.8. Reaction of [Ru3(l3-Se)2(CO)8(dpxp)] with

[W(CO)3(CH3CN)3]. Synthesis of [WRu3(l4-Se)2(l-CO)4(CO)7(dpxp)] (7)

Cluster [Ru3(l3-Se)2(CO)8(dpxp)] (50 mg, 0.055

mmol) and [W(CO)3(CH3CN)3] (42 mg, 0.11 mmol)were stirred in dry CH2Cl2 for 1.5 h at room tempera-

ture under N2. The resulting dark solution was evapo-

rated to dryness and the residue was dissolved in

CH2Cl2 (10 cm3). The product (cluster 7, Yield: 55%)

was separated and purified by TLC on silica, using

CH2Cl2–hexane (1:1) as eluent mixture. Crystals suitable

for X-ray analysis were obtained by layering methanol

on a dichlorometane solution of 7.Compound 7: FT-IR (CH2Cl2) m(CO) (cm�1): 2068m,

2036vs, 2024s, 2001vs, 1915m, 1870w, 1837w. 31P{1H}

NMR (CDCl3, 298 K): d 128 (s). 1H NMR (CDCl3,

298 K): d 3.34 (d, OCH3,3J(H,P) = 12.5 Hz), 3.27 (d,

OCH3,3J(H,P) = 12.5 Hz), 7.10–7.80 (m, 10H, 2Ph).

Anal. Calc. for C24H13O12PWRu3Se2: C, 24.65; H,

1.12. Found: C, 24.60; H, 1.10%.

2.9. Crystal structure determination of 2 Æ2CH3OH, 4,5 Æ1/2CH2Cl2, 6 ÆCH2Cl2 and 7

The intensity data were collected at room tempera-

ture on a Philips PW 1100 single-crystal diffractometer

(2 Æ2CH3OH, 4 and 5 Æ1/2CH2Cl2) and on a Bruker

AXS Smart 1000 single crystal diffractometer (6 ÆCH2Cl2and 7) equipped with an area detector, using a graphitemonochromated Mo Ka radiation (k = 0.71073 A).

Crystallographic and experimental details for the struc-

tures are summarized in Table 1. A correction for

absorption was made for 2 Æ2CH3OH, 4 and 5 Æ1/2CH2Cl2 [maximum and minimum value for the trans-

mission coefficient was 1.000 and 0.703 (2 Æ2CH3OH),

1.000 and 0.618 (4) and 1.000 and 0.813 (5 Æ1/2CH2Cl2)]

[15]. For 6 ÆCH2Cl2 and 7 the raw frame data were proc-essed using SAINT and SADABS to yield the reflection

data file and the Bruker software was used for the

absorption correction [maximum and minimum trans-

mission value was 1.000 and 0.580 (6 ÆCH2Cl2) and

1.000 and 0.629 (7) [16]. The structures were solved by

Patterson and Fourier methods and refined by full-ma-

trix least-squares procedures (based on Fo2) (SHELXSHELX-

97) [17] first with isotropic thermal parameters and thenwith anisotropic thermal parameters in the last cycles of

refinement for all the non-hydrogen atoms except for the

carbon and oxygen atoms of the solvent molecules in

2 Æ2CH3OH. The hydrogen atoms were introduced into

the geometrically calculated positions and refined on

the corresponding parent atoms, except the hydrogen

atoms of the methanol molecules in 2 Æ2CH3OH, and

of the dichloromethane molecule in 5 Æ1/2CH2Cl2.The supplementary material for the structures in-

cludes lists of atomic coordinates for the non-H atoms,

of calculated coordinates for the hydrogen atoms, of

anisotropic thermal parameters and complete lists of

bond lengths and angles. CCDC Reference Nos.

236568, 236569, 236570, 236571 and 236572, for

2 Æ2CH3OH, 4, 5 Æ1/2CH2Cl2, 6 ÆCH2Cl2, and 7, respec-

tively. Copies of this information be obtained free ofcharge from The Director, CCDC, 12 Union Road,

Cambridge, CB2 1EZ, UK (fax: +44-1223-336033; e-

mail: [email protected] or http://www.ccdc.cam.

ac.uk).

2.10. Electrochemistry

Materials and apparatus for electrochemistry havebeen described elsewhere [18]. All the potential values

are referred to the saturated calomel electrode (SCE).

Under the present experimental conditions, the one-elec-

tron oxidation of ferrocene occurs at E0 0 = +0.39 V.

3. Results and discussion

The new tertiary phosphine selenide tpnpSe has been

obtained in quantitative yield by selenization of the par-

ent phosphine with elemental Se. It exhibits the charac-

teristic P = Se IR stretching band at 534 cm�1 consistent

with the values observed in the literature [12,13]. 77Se

satellites are present in the 31P NMR spectrum, the1J(P,Se) being 718 Hz as normally found in similar spe-

cies [19]. The reaction between [Ru3(CO)12] and tpnpSein hot toluene and in the presence of Me3NO gave clus-

ter [Ru3(l3-Se)2(CO)8(tpnp)] (1) as unique product. It

could be described as a square pyramid with two ruthe-

nium and two selenium atoms alternating on the basal

plane and the third ruthenium atom at the apex of the

pyramid. The phosphine coordinates one ruthenium

atom at the base of the pyramid. The monosubstituted

cluster 1 shows reactivity at 90 �C towards different ter-tiary mono- and di-phosphines giving the correspondent

di- or tri-mixed substituted open-triangular nido clusters

of the same family. We have reacted compound 1 with

an equimolar amount of diphosphine dpmb and of

monophosphine t4mpp, obtaining clusters 2 and 3 (see

Scheme 2) as unique products in quantitative yields.

In the 31P NMR spectrum of 2 two sharp singlets at d116.0 and 21.3 due to the P atoms of the tpnp and dpmbligands, respectively, are present. In the 1H NMR spec-

trum the methylenic protons (Ha and Hb) of the pyrro-

lidinyl rings resonate as two unresolved multiplets at d1.9 and 3.27. The signals due to the diasterotopic methy-

lenic protons of the dpmb ligand are two doublets of

doublets at d 4.26 and 3.72 (2J(H,H) = 13.5 Hz,2J(H,31P) = 4.2 and 6.5 Hz, respectively). The aromatic

xylenyl protons resonate as unresolved multiplets at d6.10 and 6.93. The signals due to the phenyl protons

are present in the range 7.30–7.80 ppm. The structure

http://www.ccdc.cam.ac.uk

http://www.ccdc.cam.ac.uk

Table 1

Crystal data and structure refinement for 2 Æ2CH3OH, 4, 5 Æ1/2CH2Cl2, 6 ÆCH2Cl2 and 7

2 Æ2CH3OH 4 5 Æ1/2CH2Cl2 6 ÆCH2Cl2 7

Formula Ru3Se2P3O6N3C50H522CH3OH WRu3Se2PO11N3C23H24 WRu3Se2PO11N3C23H241/2CH2Cl2 WRu3Se2PO14C32H21CH2Cl2 WRu3Se2PO12C24H13

Formula weight 1409.07 1194.40 1236.86 1390.36 1169.29

Crystal system monoclinic monoclinic triclinic triclinic monoclinic

Space group C2/m P21/n P�1 P�1 P21/c

a (A) 21.956(5) 12.182(5) 16.040(5) 12.464(4) 9.857(3)

b (A) 15.687(3) 12.120(5) 13.181(4) 13.146(5) 22.938(5)

c (A) 17.037(4) 22.160(5) 9.680(3) 14.474(5) 13.790(4)

a (�) 90 90 68.06(3) 95.68(4) 90

b (�) 99.09(5) 92.82(5) 73.56(3) 103.95(5) 96.87(5)

c (�) 90 90 66.62(3) 98.94(4) 90

V (A3) 5794(1) 3268(1) 1720(1) 2250(1) 3095(1)

Dcalc (g cm�3) 4, 1.615 4, 2.428 2, 2.387 2, 2.052 4, 2.509

F(000) 2808 2240 1162 1312 2168

Crystal size (cm) 0.12 · 0.15 · 0.11 0.17 · 0.21 · 0.15 0.11 · 0.12 · 0.20 0.22 · 0.11 · 0.20 0.15 · 0.21 · 0.19

l (cm�1) 21.62 72.03 69.20 53.64 76.01

Reflections collected 8713 9525 8303 7491 33580

Reflections unique 8713 9525 8303 6467 7091

Reflections observed [I > 2r(I)] 5088 6288 4185 4239 4910

Parameters 386 398 443 512 391

R indices [I > 2r(I)] R1 = 0.0454, wR2 = 0.1149 R1 = 0.0479, wR2 = 0.0835 R1 = 0.0819, wR2 = 0.2083 R1 = 0.0647, wR2 = 0.1667 R1 = 0.0494, wR2 = 0.1518

R indices (all data) R1 = 0.0935, wR2 = 0.1324 R1 = 0.0945, wR2 = 0.0972 R1 = 0.1496, wR2 = 0.2389 R1 = 0.0923, wR2 = 0.1854 R1 = 0.0782, wR2 = 0.1678

R1 =P

jjFoj � jFcjj/P

jFojwR2 = [P

[w(Fo2 � Fc

2)2]/P

[w(Fo2)2]]1/2.

D.Belletti

etal./Inorganica

Chim

icaActa

358(2005)161–172

165

Se

Ru

Ru

Ru

Se

L1

Se

Ru

Ru

Ru

SeL2

L2

L1L2Se

Ru

Ru

Ru

Se

L1

1 L1 = tpnp [Ru3(µ3-Se)2(CO)8(t4mpp)] [Ru3(µ3-Se)2(CO)8(dpxp)]

2 L1 = tpnp, L2 = dpmb 3 L1 = tpnp, L2 = t4mpp

Scheme 2. (Carbonyls omitted for clarity).


of 2 has been determined by X-ray diffraction methods.

In the crystals of 2 Æ2CH3OH molecules of [Ru3(l3-Se)2(-CO)6(tpnp)(dpmb)] and of methanol are present. A view

of the structure of 2 is shown in Fig. 1 together with the

atomic numbering system. Selected bond distances and

angles are given in Table 2. The compound 2 shows

Fig. 1. View (ORTEP, 30% probability level) of the molecular

structure of 2 with the atom numbering scheme.

Table 2

Selected bond distances (A) and angles (�) for compound 2

Bond distances

Ru(1)–Ru(3) 2.941(1) Ru(2)–Ru(3) 2.788(1)

Ru(1)–Se(1) 2.494(1) Ru(2)–Se(1) 2.519(1)

Ru(3)–Se(1) 2.535(1) Ru(1)–P(1) 2.343(1)

P(2)–N(2) 1.661(6) Ru(2)–P(2) 2.313(2)

P(2)–N(1) 1.666(4)

Bond angles

Ru(2)–Ru(3)–Ru(1) 84.49(5) Ru(1)–Se(1)–Ru(3) 71.57(3)

Se(1)–Ru(1)–Se(1 0) 79.58(4) Se(1)–Ru(2)–Se(10) 78.63(4)

Se(1)–Ru(3)–Se(1 0) 78.03(3) P(1)–Ru(1)–P(10) 96.99(8)

P(1)–Ru(1)–Se(1 0) 164.08(4) P(2)–Ru(2)–Ru(3) 155.54(5)

Symmetry transformation used to generate equivalent atoms: 0 x,�y,z.

the well-known pyramidal Ru3Se2 nido core. The phos-

phine ligands coordinate the ruthenium atoms at the

base of the pyramid, the phosphorus atoms substituting

three carbonyl groups. The dpmb ligand chelates the

Ru1 atom with the two P atoms substituting two carbo-

nyl groups in pseudoequatorial positions and the P2atom of the tpnp ligand substitutes one carbonyl group

in a pseudoaxial position.

The cluster has an imposed Cs symmetry with the

mirror plane passing through the three ruthenium

atoms, the P2 and N2 atoms of the tpnp ligand and

two carbonyl groups C1,O1 and C2,O2. The C atoms

of the pyrrolidinyl ring defined by N1,C21,C22,C23,

C24 have been found disordered and distributed intwo positions with occupancy factor 0.6 and 0.4, respec-

tively. Also the other pyrrolidinyl ring has been found

disordered above and below the symmetry plane. The

Ru1–P1 bond distance, 2.343(1) A, is in good agreement

with those found in a related compound of formula

[Ru3(l3-Se)2(CO)7(P,P-dpmb)] [20] (2.350(3) and

2.349(3) A). While the Ru2–P2 bond length is shorter

(2.313(2) A). The chelation of the diphosphine on Ru1produces a lengthening of the side of the cluster

[7,20,21]: the bond distance Ru1–Ru3 is 2.941(1) A,

while the Ru2–Ru3 one is 2.788(1) A . The chelating

phosphine forms with Ru1 a seven-membered ring

which presents a boat conformation, with the Ru1, C6

and C6 0 atoms deviating on the same side of the mean

plane passing through P1,P1 0,C5,C5 0.

In compound 3, two different phosphines (tpnp andt4mpp) coordinate two ruthenium atoms at the base of

the pyramid. In the 31P NMR spectrum of 3 (Fig. 2)

two signal ranges can be observed, the former due to

the chemical shift typical for the phosphorous atom of

tpnp (three signals in the range 100–120 ppm, integral

ratio 9:1:1) and the latter for the t4mpp (three signals

in the range 30–50 ppm, integral rate 1:1:9). We suggest

that the same kind of fluxional motion observed forclusters of this family [12] takes place in solution – con-

sisting in the reversible migration of a metal–metal bond

from a side of the open triangle to the basal plane –

resulting in the presence of isomer species with the two

Fig. 2. 1P NMR spectrum (CDCl3) of compound 3.




different P atoms coordinated on two non-linked (isomer

a, broad signals at 113.2 and 40.4 ppm, integral rate 9:9;

due to the rotation of the Ru(CO)2P systems) and on

two linked Ru atoms (isomers b and b 0, signals at111.4, 109.3, 41.7, 43.8 ppm; integral rate 1:1:1:1). See

Scheme 3.

The nido square-pyramidal Ru3Se2 clusters present

seven skeletal electron pairs, in accordance with the

requirements of the skeletal electron pair (s.e.p.) theory.

As a consequence they could be prone to add a zero

s.e.p.�s fragment, such as M(CO)3 (M = Mo or W), giv-

ing the closo MRu3Se2 clusters [12]. We reacted clusters1, [Ru3(l3-Se)2(CO)8(t4mpp)] and [Ru3(l3-Se)2(CO)8(dpxp)] with a light excess of W(CO)3(CH3CN)3in dichloromethane. The reactions take place in a few

hours at room temperature, giving, as unique isolable

products the isomers [WRu3(l4-Se)2(l-CO)4(CO)7(tpnp)] (4 and 5) from 1 and [WRu3(l4-Se)2(l-CO)4(CO)7(t4mpp)], [WRu3(l4-Se)2(l-CO)4(CO)7(dpxp)] (6

and 7), from the other two reactants. The IR spectraof 4 and 6 in the carbonyl region are super imposable,

as well as those of 5 and 7 suggesting a similar structure

for each couple of compounds. The structures of the

four clusters have been fully elucidated by X-ray diffrac-

tion analysis. Views of the molecular structure of 4, 5, 6

Ru

Ru RuSe

Se

R1

R2

Ru

Ru

R1

isomer a i

Scheme 3

and 7 are shown in Figs. 3–6, respectively, selected bond

distances and angles are collected in Table 3.

The four compounds belong to the family of M4Se2closo clusters with seven s.e.p.�s. They can be describedas distorted octahedra in which the four metal atoms

lie on the basal plane and the two selenium at the apices.

Ru

Se

Se

R2

Ru

Ru Ru

Se

Se

R1 R2

somer b isomer b'

.








The Ru–Ru bond distances span from 2.783(6) to

2.884(2) A and the W–Ru bond distances from

2.870(3) to 3.021(2) A, in the range of those found in

the comparable di-substituted clusters [13]. Four car-

bonyls asymmetrically bridge the Ru–Ru and Ru–Wedges. The structures of the four compounds are very

similar, the only difference consisting in the coordina-

tion position of the phosphine ligand. In 4 and 6 the

phosphine substitutes a carbonyl group of the Ru atom

bound to the W while in 5 and 7 the phosphine is bound

to a Ru atom opposite to the W atom, so that 4 and 5

are isomers (see Scheme 4). The P–Ru bond distances

involving the phosphine ligand range from 2.304(3) Afor 5 to 2.397(4) A for 6.

A possible explanation for the formation of two iso-

mers could derive from the fluxional behaviour observed

for the Ru3(l3-Se)2 clusters. In previous reports, regard-

ing the di-substituted clusters Ru3(l3-Se)2(CO)7(L)2[13,22], we have suggested that a fluxional motion takes

place in solution consisting of the reversible migration of

a metal–metal bond from a side of the open triangle to

the basal plane resulting in the presence of two isomer

species (a and b in Scheme 3), well evidenced by the rel-evant NMR spectra. In the case of the monosubstituted

derivatives, a similar fluxional behaviour is suggested by

the 31P peak broadening in the NMR spectrum, even at

low temperatures. The reactivity towards the W(CO)3fragment provides further evidence for the proposed dy-

namic behaviour. When the phosphine ligand is local-

ized on the basal plane, the W fragment can enter in

the position adjacent to the ruthenium atom bearingthe phosphine ligand; when the phosphine ligand is

localized on the pyramid vertex, the available position

for the W fragment is opposite to the Ru–P vertex (see

Scheme 5). We can reasonably exclude that the isomeri-

zation process occurs by P ligand migration from a

coordination site to another one, considering that this

process is rarely observed and no isomerization takes

place if one of the two isomers is redissolved.

4. Electrochemistry

We have studied the electrochemical behaviour of the

Ru4Se2 and Ru3WSe2 clusters, with one or two phos-

phine ligands coordinated.

Fig. 7 shows the cyclic voltammetric behaviour of[Ru4(l4-Se)2(CO)9(tpp)2] in dichloromethane solution.

It undergoes a (first) irreversible oxidation and two

subsequent reductions, these latter displaying features

Table 3

Selected bond distances (A) and angles (�) for the compounds 4, 5, 6 and 7

4 5 6 7

Bond distances

Ru(1)–Ru(2) 2.884(2) 2.801(2) 2.880(2) 2.799(1)

Ru(2)–Ru(3) 2.783(6) 2.796(2) 2.844(2) 2.807(1)

Ru(1)–W(1) 2.972(7) 2.907(2) 3.021(2) 2.875(1)

Ru(3)–W(1) 2.870(3) 2.914(2) 2.926(2) 2.941(1)

W(1)–Se(1) 2.724(11) 2.693(2) 2.754(2) 2.678(1)

W(1)–Se(2) 2.692(3) 2.704(2) 2.738(2) 2.667(2)

Ru(1)–Se(1) 2.552(5) 2.569(2) 2.605(2) 2.587(1)

Ru(1)–Se(2) 2.632(3) 2.578(2) 2.644(2) 2.590(2)

Ru(2)–Se(1) 2.587(2) 2.655(2) 2.633(2) 2.618(2)

Ru(2)–Se(2) 2.561(10) 2.627(2) 2.636(2) 2.627(1)

Ru(3)–Se(1) 2.587(3) 2.554(2) 2.666(2) 2.556(2)

Ru(3)–Se(2) 2.621(6) 2.573(2) 2.658(2) 2.583(1)

Ru(1)–P(1) 2.343(3) 2.397(4)

Ru(2)–P(1) 2.371(4) 2.304(3)

Bond angles

Ru(1)–Ru(2)–Ru(3) 90.13(15) 88.40(6) 90.52(5) 89.53(4)

Ru(2)–Ru(3)–W(1) 93.56(16) 93.68(5) 92.94(5) 91.69(4)

Ru(3)–W(1)–Ru(1) 86.75(16) 84.18(5) 86.26(5) 85.51(4)

W(1)–Ru(1)–Ru(2) 89.43(15) 93.73(5) 90.27(5) 93.27(4)

W

Se

Ru

Ru

Ru

SeL

Se

RuW

Ru

Ru

Se

L

4 L = tpnp 6 L = t4mpp

5 L = tpnp 7 L = dpmp

Scheme 4.


of chemical reversibility in the cyclic voltammetric

time scale. The analysis of the cyclic voltammetric re-

sponse of the first reduction with scan rate varying

from 0.02 to 1.00 V s�1 reveals that it is complicated

Ru

Ru RuSe

Se

R

W

W(CO)3(CH3CN)3

CH2Cl2, r.t.

Scheme 5

by slow chemical reactions. The ipa/ipc ratio, which is

0.2 at the lowest scan rate, progressively increases to

0.8 at the highest scan rate [23]. Even if controlled

potential coulometric tests in correspondence of the

first reduction (Ew = �1.1 V) consumed 1.2 electrons

per molecule, it is assigned as a one-electron step, in

which, as commonly happens in the case of processes

occurring at rather negative potential values, traces ofdioxygen can cause partial reoxidation of the reduced

species.

Based on the relative peak heights, also the second

reduction is assigned as a one-electron process. Follow-

ing the same criterion, the oxidation process, which does

not display any return peak even at scan rates of

10 V s�1, is assumed as two-electrons step. Since the

Ru

Ru Ru

Se

Se

R

W

W(CO)3(CH3CN)3

CH2Cl2, r.t.

.

Fig. 7. Cyclic voltammetric profile recorded at a platinum electrode in

a CH2Cl2 solution of [Ru4(l4-Se)2(CO)9(tpp)2] (�0.7·10�3 mol dm�3).

[NBu4][PF6] (0.2 mol dm�3) supporting electrolyte. Scan rate 0.05

V s�1.

Fig. 8. Cyclic voltammetric profiles recorded at a platinum electrode

in a CH2Cl2 solution of [WRu3(l4-Se)2(l-CO)4(CO)6(tpp)2]

(�0.5·10�3 mol dm�3). [NBu4][PF6] (0.2 mol dm�3) supporting elec-

trolyte. Scan rates: (a) 0.2; (b) 5.12 V s�1.


oxidation of the PPh3 ligand (not shown in Fig. 7),

is observable at more positive potential values (see Table

4), the abovementioned two-electron removal is assigned

as cluster centred.Regarding the heteronuclear WRu3 complexes, the

cyclic voltammetric pattern of [WRu3(l4-Se)2(l-CO)4(CO)6(tpp)2] (in dichloromethane solution) is shown in

Fig. 8. At low scan rates (see Fig. 8(a)) it displays either

an irreversible oxidation or an irreversible reduction. As

previously pointed out, some electrode adsorption proc-

esses affect the reduction step, in such a way that it has

been impossible to measure coulometrically the numberof electrons involved. Since we are keen to assign it as a

two-electron process, it follows that, based on the rela-

tive peak-heights, also the above-mentioned oxidation

process is a two-electron step. This conclusion is sup-

ported analysing the cyclic voltammetric behaviour in

the latter process. In fact, at scan rates increasing from

0.02 to 10.2 V s�1, the current function ipa Æv�1/2 progres-

sively decreases by about 50% for each 10-fold increase,up to reach and maintain a constant value at scan rates

higher than 2.00 V s�1.

Table 4

Formal electrode potentials (V, vs. SCE) and peak-to-peak separations (mV)

Complex Ep(ox)a E0 0

(2+/+) DEpb

[Ru4(l4-Se)2(CO)9(tpp)2] +1.70b,c +0.88b,c

[WRu3(l4-Se)2(l-CO)4(CO)6(tpp)2] +1.60b,c d

[WRu3(l4-Se)2(l-CO)4(CO)6(dpmp)2] +1.6b,cf d

[WRu3(l4-Se)2(l-CO)4(CO)6(t4mpp)2] >1.6 +0.93 100e

[WRu3(l4-Se)2(l-CO)4(CO)7(tpnp)] (4) +1.69b,c �+1.2b,c,f

[WRu3(l4-Se)2(l-CO)4(CO)7(tpnp)] (5) +1.70b,c + 1.35b,c

a Oxidation process of the phosphine ligand.b Measured at 0.1 V s�1.c Peak-potential value for irreversible process.d Undetected.e Measured at 2.00 V s�1.f Ill-defined.

As shown in Fig. 8(b), under high scan rates condi-

tions, the oxidation tends to become reversible (at the

highest scan rate, the current ratio is about 0.6) roughly

halving its peak-height with respect to the reduction

process. In conclusion, while the reduction process

undergoes a single two-electron reduction coupled with

fast chemical complications, the oxidation process

undergoes a ECE mechanism (E, electron transfer; C,chemical complication), in which the rate of the inter-

posed chemical complication lies in the time scale of cyc-

lic voltammetry and the two electron transfers proceed

at the same potential values. A quite similar behaviour

is exhibited by [WRu3(l4-Se)2(l-CO)4(CO)6(dpmp)2].

As depicted in Fig. 9, a different redox path was

found for [WRu3(l4-Se)2(l-CO)4(CO)6(t4mpp)2].

It undergoes two sequential oxidations and, appar-ently, two sequential reductions. In reality, the most

cathodic process tends to decrease at increasing scan

rates, indicating that it arises from the side-product gen-

erated at the first reduction. The other cathodic process

maintains its irreversibility also at high scan rate, as, on

the other hand, the first anodic process does. The second

for the redox processes exhibited by the selenido clusters under study

E0 0(+/0) DEp

b E0 0(�/0) DEp

b E0 0(�/2�) DEp

b

+0.88b,c �0.91 61 �1.26 95

+1.02 84e �1.04b,c �1.04b,c

+0.98 96e �1.14b,c �1.14b,c

+0.60b,c �1.15b,c �1.15b,c

+1.09b,c �0.95b,c �0.95b,c

+ 1.10b,c �0.93b,c �0.93b,c


in a CH2Cl2 solution of [WRu3(l4-Se)2(l-CO)4(CO)6(t4mpp)2]

(�0.4 · 10�3 mol dm�3). [NBu4][PF6] (0.2 mol dm�3) supporting

electrolyte. Scan rates: (a) 0.2; (a) 10.2 V s�1.


oxidation process displays some features of chemical

reversibility. It is noteworthy that either the 1:1 peak-

current ratio of the two oxidation steps does not vary

with scan rate, or the peak height of each oxidation step

is roughly halved with respect to the main cathodic

process.

On this basis, we believe that the reduction process

follows the simultaneous EirrEirr mechanism already de-scribed for the preceding WRu3 clusters, whereas the

oxidation path involves two separated one-electron

transfers, the first of which is complicated by fast chem-

ical reactions, whereas the second is complicated by slow

chemical reactions.

Finally, Fig. 10, illustrates the cyclic voltammetric

behaviour of the two isomers 4 and 5.

In both complexes, the cathodic process involves theusual irreversible two-electron process, whereas the ano-

dic process involves two sequential irreversible one-elec-

tron oxidations (the third most anodic process is


in a CH2Cl2 solution of: (a) 4 (�0.4 · 10�3 mol dm�3); (b) 5

(�0.4 · 10�3 mol dm�3). [NBu4][PF6] (0.2 mol dm�3) supporting

electrolyte. Scan rate 0.2 V s�1.

assigned to the oxidation of the phosphine), which how-

ever are sufficiently separated in 5 and partially over-

lapped in 4. In the latter case no significant resolution

was gained even in square wave voltammetry.

The formal electrode potentials of the different redox

processes exhibited by the studied complexes are listedin Table 4. Passing from Ru4Se2 to Ru3WSe2 not only

the flexibility of the cluster core to add electrons is com-

promised (as indicated by passage from two separated

one-electron reductions, with features of chemical

reversibility, to an irreversible two-electron reduction),

but also the electronic interaction between the redox

centres accepting the electrons varies (disappearing in

[WRu3Se2]�). Based on the separation of the redox pot-

entials, the mixed-valent [Ru4Se2]� would belong to the

delocalized Robin-Day Class III, Kcom = 8 · 105,

whereas [WRu3Se2]� would belong to the localized Rob-

in-Day Class I [23].

The substitution of a Ruthenium atom with Tungsten

affects the ability of the clusters to lose electrons,

increasing the intramolecular electronic mobility, which

depends on the electronic effects of the different periph-eral phosphines. Assuming that Ru3MSe2 clusters

(M = Ru, W), differently from [Ru4(l4-Se)2(CO)9(tpp)2],

undergo two (separated or simultaneous) one-electron

oxidations, [WRu3(l4-Se)2(l-CO)4(CO)6(tpp)2] and

[WRu3(l4-Se)2(l-CO)4(CO)6(dpmp)2] are able to resist

for short times to a first one-electron removal without

framework destruction (in both cases we were unable

to detect the second cluster centred one-electronoxidation). Compounds [WRu3(l4-Se)2(l-CO)4(CO)6(t4mpp)2], 4 and 5 exhibit two separate one-electron

additions, which, even if generally affected by fast chem-

ical complications, indicate that, at very short times, the

internal redox centres of the corresponding monocations

are somewhat electronically communicating.

Acknowledgements

Financial support from Ministero dell� Istruzione,

dell�Universita e della Ricerca Scientifica e Tecnologica

(Cofin 2000, PRIN 2003) is gratefully acknowledged.

The facilities of the Centro Interfacolta di Misure ‘‘G.

Casnati’’ (Universita di Parma) were used to record

the NMR and mass spectra.

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Ligand substitution and growth reactions with Ru 3 Se 2 nido clusters. Coordination isomerism in...

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