Synthesis, Structures, and properties of 1,2,4,5-benzenetetrathiolate linked group 10 metal complexes

Inorg. Chem. 2009, 48, 10591–10607 10591

DOI: 10.1021/ic901257s

Synthesis, Structures, and Properties of 1,2,4,5-Benzenetetrathiolate Linked Group

10 Metal Complexes

†

‡

,§

ꢀ

Kuppuswamy Arumugam, Mohamed C. Shaw, P. Chandrasekaran, Dino Villagran, Thomas G. Gray,*

Joel T. Mague,^†and James P. Donahue*^,†

^†Department of Chemistry, Tulane University, 6400 Freret Street, New Orleans, Louisiana 70118-5698,

^‡Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge,

Massachusetts 02139, and ^§Department of Chemistry, Case Western Reserve University, 10900 Euclid Avenue,

Cleveland, Ohio

Received June 30, 2009

Dimetallic compounds [(P-P)M(S₂C₆H₂S₂)M(P-P)] (M=Ni, Pd; P-P=chelating bis(phosphine), 3a-3f) are prepared

n

from OdCS₂C₆H₂S₂CdO or Bu₂SnS₂C₆H₂S₂SnⁿBu₂, which are protected forms of 1,2,4,5-benzenetetrathiolate.

Selective monodeprotections of OdCS₂C₆H₂S₂CdO or ⁿBu₂SnS₂C₆H₂S₂SnⁿBu₂lead to [(P-P)Ni(S₂C₆-

H₂S₂CdO)] or [(P-P)Ni(S₂C₆H₂S₂SnⁿBu₂)]; the former is used to prepare trimetallic compounds [(dcpe)-

Ni(S₂C₆H₂S₂)M(S₂C₆H₂S₂)Ni(dcpe)] (M = Ni (6a) or Pt (6b); dcpe = 1,2-bis(dicyclohexylphosphino)ethane).

Compounds 3a-3f are redox active and display two oxidation processes, of which the first is generally reversible.

Dinickel compound [(dcpe)Ni(S₂C₆H₂S₂)Ni(dcpe)] (3d) reveals two reversible oxidation waves with ΔE_1/2=0.66 V,

corresponding to K_cof 1.6 ꢀ 10¹¹for the mixed valence species. Electrochemical behavior is unstable to repeated

scanning in the presence of [Bu₄N][PF₆] electrolyte but indefinitely stable with Na[BArF₂₄] (BArF₂₄= tetrakis(3,

5-bis(trifluoromethyl)phenyl)borate), suggesting that the radical cation generated by oxidation is vulnerable to reaction

with PF₆^-. Chemical oxidation of 3d with [Cp₂Fe][BArF₂₄] leads to formation of [3d][BArF₂₄]. Structural identification

of [3d][BArF₂₄] reveals appreciable shortening and lengthening of C-S and C-C bond distances, respectively, within

the tetrathioarene fragment compared to charge-neutral 3d, indicating this to be the redox active moiety. Attempted

oxidation of [(dppb)Ni(S₂C₆H₂S₂)Ni(dppb)] (3c) (dppb = 1,2-bis(diphenylphosphino)benzene) with AgBArF₂₄

produces [[(dppb)Ni(S₂C₆H₂S₂)Ni(dppb)]₂(μ-Ag₂)][BArF₂₄]₂, [4c][BArF₂₄]₂, in which no redox chemistry has

occurred. Crystal structures of bis(disulfide)-linked compounds [(P-P)Ni(S₂C₆H₂(μ-S₂)₂C₆H₂S₂)Ni(P-P)] are

reported. Near IR spectroscopy upon cationic [3d]^þand neutral 6a reveals multiple intense absorptions in

the 950-1400 nm region. Time-dependent density functional theory (DFT) calculations on a 6a model compound

indicate that these absorptions are transitions between ligand-based π-type orbitals that have significant contributions

from the sulfur p orbitals.

Introduction

complexes with a dithiolene-type connecting ligand, most

with tetrathiooxalate (tto^2-), have been described.^5-13These

species are still planar, or nearly so, and offer the possibility

Metallodithiolene complexes have been the subject of

extensive study because of the possible applications arising

from their conducting, magnetic, and optical properties.

Much of this work has dealt with planar monometallic

complexes of the Group 10 metals, which in the crystalline

state typically form stacking arrangements that enable electro-

nic delocalization.^1-4More recently, a variety of dimetallic

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Reynolds, J. R. Inorg. Chem. 1996, 35, 4420–4426.

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*To whom correspondence should be addressed. E-mail: donahue@

tulane.edu.

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r

2009 American Chemical Society

Published on Web 10/15/2009

pubs.acs.org/IC

10592 Inorganic Chemistry, Vol. 48, No. 22, 2009

Arumugam et al.

of forming favorable columnar stacks with individual units

that are more redox active and have greater electronic

delocalization than their monometallic analogues. The pro-

cedures by which such dimetallic species have been prepared

are variable and include the reaction of M^2þ(M=Ni or Cu),

tto^2-, and a suitable monodithiolenecapping ligand or ligand

precursor in a 2:1:2 ratio,^5-8by oxidative^9-11or reductive¹²

decomposition of one of two dithiolene ligands in a metal

bis(dithiolene) complex, and by mixing of a metal bis-

Selected leading results of this work have been communi-

cated.²²

Experimental Section

Literature procedures were employed for the syntheses of

1,2-bis(diphenylphosphino)benzene (dppb),²³1,3,5,7-tetra-

thia-s-indacene-2,6-dione,^{24 n}Bu₂SnS₂C₆H₂S₂SnⁿBu₂,²⁵[(P-

P)NiX₂] (P-P=1,2-bis(diphenylphosphino)ethane, dppe; 1,

2-bis(diphenylphosphino)ethylene, dppee; 1,2-bis(dicyclohex-

ylphosphino)ethane, dcpe; X = halide),²⁶[(cod)PdCl₂] (cod =

1,5-cyclooctadiene),²⁷Na[BArF₂₄]²⁸(BArF₂₄=tetrakis(3,5-

bis(trifluoromethyl)phenyl)borate), Ag[BArF₂₄],²⁹and

[Cp₂Fe][BArF₂₄].³⁰All other reagents were purchased from

commercial sources and used as received (MCl₂(M=Ni, Pd,

Pt), 10% LiOMe in MeOH, 21% NaOEt in EtOH, AgPF₆,

I₂). Solventswereeitherdriedwithasystemofdryingcolumns

from the Glass Contour Company (CH₂Cl₂, n-pentane, hex-

anes, Et₂O, THF, C₆H₆, toluene) or freshly distilled accord-

ing to standard procedures³¹(MeOH, CH₃CN, 1,

2-dichloroethane). Silica columns were run in the open air

using 60-230 μm silica (Dynamic Adsorbents). All reactions

and manipulations were conducted under an atmosphere of

N₂unless indicated otherwise.

(dithiolene) anion with 1/2 equiv of tto^{2- 13}

The last ap-

.

proach has also yielded two interesting examples of planar,

trinickel tetrathiooxalate-linked species. Metallodithiolene

polymers have also been described and generally observed

to have electrical conductivities of 10⁰-10²Ω^-1cm^{-1 14-19}

.

Presumably, conductivity in these materials is mediated

through the covalent bonds of an extended metal dithiolene

chain rather than through any stacking arrangement.

Because of their insolubility, such metallodithiolene poly-

mers are not readily subject to methods of physical char-

acterization other than X-ray scattering and X-ray

absorption.^18,19

A relatively unexplored avenue in metallodithiolene

materials research is the idea of developing a well-defined

set of related, soluble, metallodithiolene oligomers of increas-

ing complexity whose properties should trend in clear ways.

For instance, the redox potentials measured in a set of

metallodithiolene oligomers should converge toward a limit-

ing value characteristic of the polymer. The rationale for this

“oligomer approach” has been well articulated in the context

of organic systems.²⁰A methodology by which such a family

of metallodithiolene oligomers might be prepared is via the

monodeprotection of a doubly protected bis(dithiolene)

ligand. The feasibility of this principle was demonstrated by

Purrington, Bereman, and co-workers, who described the

preparation of dimetallic compounds, including heterodime-

tallic compounds, with the butadienetetrathiolate connecting

ligand.²¹We have applied this methodology to 1,3,5,7-tetra-

thia-s-indacene-2,6-dione, a dione protected form of 1,2,4,

5-benzenetetrathiolate, because it is a bis(dithiolene) ligand

better suited to promote electronic delocalization. Using

various chelating diphosphine ligands as capping end

groups to obviate uncontrolled polymer growth, we have

applied this selective deprotection protocol to the prepara-

tion of dimetallic benzenetetrathiolate complexes of Ni and

Pd and extended it to the deliberate synthesis of “linear”

trimetallic complexes as well, including an example of a

mixed-metal species. The syntheses, structures, electrochem-

istry, and absorption spectra of these compounds, interpreted

with the aid of DFT calculations, are described herein.

Syntheses

[(dppe)Ni(S₂C₆H₂S₂CO)] (2a). A procedure analogous

to that described for 2d was employed on a scale of 0.100 g

OCS₂C₆H₂S₂CO (0.38 mmol). Compound 2a was puri-

fied on a silica column eluted with 1:1 CH₂Cl₂/hexanes

and obtained as orange-red block crystals by evaporation

of the eluant. Yield: 0.145 g, 54%. R_f(9:1 CH₂Cl₂/

hexanes): 0.21. ¹H NMR (δ, ppm in CD₂Cl₂):

7.75-7.80 (m, 8H, aromatic CH), 7.54-7.57 (m, 4H,

aromatic CH), 7.47-7.51 (m, 8H, aromatic CH), 7.39

(s, 2H, aromatic CH), 2.40 (d, 4H, Ph₂PCH₂CH₂PPh₂).

¹³C NMR (δ, ppm in CD₂Cl₂): 191.8, 150.9, 150.8, 133.7,

131.7, 129.1, 124.4, 120.7, 27.6. ³¹P NMR (δ, ppm in

CD₂Cl₂): 59.92. Absorption spectrum (CH₂Cl₂), λ_maxin

nm (ε_M): 278 (43400), 282 (43200), 344 (sh, 9640), 354

(10600), 521 (307). MS (MALDI-TOF): 710 (M þ Na^þ).

HRMS (MALDI-TOF) monoisotopic m/z: 685.9672

(calcd for C₃₃H₂₆NiOP₂S₄(M^þ) 685.9689). Anal. Calcd

for C₃₃H₂₆NiOP₂S₄: C, 57.65; H, 3.81; S, 18.66. Found:

C, 57.47; H, 3.93; S, 21.18.

[(dppee)Ni(S₂C₆H₂S₂CO)] (2b). A procedure analo-

gous to that described for 2d was employed on a scale

of 0.100 g OCS₂C₆H₂S₂CO (0.38 mmol). Compound 2b

was purified on a silica column eluted with 1:1 CH₂Cl₂/

hexanes and obtained as red-orange block crystals by

evaporation of the eluant. Yield: 0.145 g, 55%. R_f(9:1

1

CH₂Cl₂/hexanes): 0.26. H NMR (δ, ppm in CD₂Cl₂):

::

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Article

Inorganic Chemistry, Vol. 48, No. 22, 2009 10593

7.73-7.79 (m, 8H, aromatic CH), 7.53-7.57 (m, 4H,

aromatic CH), 7.46-7.50 (m, 8H, aromatic CH), 7.43

(s, 2H, aromatic CH), 7.35 (dd, 2H, vinylic CH). ¹³C

(CH₂Cl₂), λ_maxin nm (ε_M): 260 (33300), 286 (57000), 344

(sh, 9280), 352 (10200), 494 (660). IR (cm^-1, KBr): 3437

(br), 2922 (s), 2845 (m), 1635 (s, CdO), 1503 (m), 1439 (m),

1263 (m), 1087 (m). MS (MALDI-TOF): 684 (M - CO)^þ.

HRMS (MALDI-TOF) monoisotopic m/z: 710.1548

(calcd for C₃₃H₅₀NiOP₂S₄(M^þ) 710.1567). Anal. Calcd

for C₃₃H₅₀NiOP₂S₄: C, 55.70; H, 7.08; S, 18.02. Found: C,

54.90; H, 6.98; S, 18.63.

[(dcpe)Pd(S₂C₆H₂S₂CO)] (2e). A procedure analogous

to that described for 2d was followed. Compound 2e was

only identified by X-ray crystallography (see Supporting

Information).

NMR (δ, ppm in CD₂Cl₂): 133.4, 131.8, 129.2, 120.7. ³¹

P

NMR (δ, ppm in CD₂Cl₂): 67.0. Absorption spectrum

(CH₂Cl₂), λ_maxin nm (ε_M): 276 (40500), 296 (34800), 334

(sh, 9990), 352 (10200), 536 (338). MS (MALDI-TOF):

708 (M þ Na^þ). HRMS (MALDI-TOF) monoisotopic

m/z: 683.9550 (calcd for C₃₃H₂₄NiOP₂S₄(M^þ) 683.9532).

Anal. Calcd for C₃₃H₂₄NiOP₂S₄: C, 57.82; H, 3.53.

Found: C, 56.53; H, 3.80.

[(dppb)Ni(S₂C₆H₂S₂CO)] (2c). A procedure analogous

to that described for 2d was employed on a scale of 0.100 g

OCS₂C₆H₂S₂CO (0.38 mmol). Compound 2c was puri-

fied on a silica column eluted with a CH₂Cl₂-hexanes

mixture in which the CH₂Cl₂/hexanes ratio was incre-

mentally varied from 0.0 to 1.0 and collected as the

leading fraction. Further purification by crystallization

was accomplished by diffusion of hexanes vapor into a

1,2-dichloroethane solution (solvated orange plates) or

by diffusion of n-pentane into a CH₂Cl₂solution

(unsolvated orange columns). Yield: 0.165 g, 58%. R_f

(9:1 CH₂Cl₂/hexanes): 0.23; R_f(2:3 EtOAc/hexanes):

0.50. ¹H NMR (δ, ppm in CD₂Cl₂): 7.61-7.66 (m, 10H,

aromatic CH); 7.46-7.50 (m, 6H, aromatic CH);

7.36-7.41 (m, 10H, aromatic CH). ³¹P NMR (δ, ppm

in CDCl₃): 57.82. Absorption spectrum (CH₂Cl₂), λ_maxin

nm (ε_M): 278 (47700), 292 (46400), 340 (sh, 12500), 374

(sh, 7060), 520 (640). IR (cm^-1, KBr): 1640 (s, CdO),

1433 (m), 1097 (s), 755 (s), 745 (m), 709 (s), 692 (s), 548 (s),

534 (s), 521 (s). MS (MALDI-TOF): 757 (M þ Na^þ).

HRMS (MALDI-TOF) monoisotopic m/z: 733.9677

(calcd for C₃₇H₂₆NiOP₂S₄(M^þ) 733.9688). Anal. Calcd

[(dppe)Ni(S₂C₆H₂S₂)Ni(dppe)] (3a) (Rxn III, Scheme 1).

A procedure analogous to that described for the prepara-

tion of 3d by Rxn III was employed on a scale of 0.100 g of

OCS₂C₆H₂S₂CO (0.38 mmol), 0.76 mL of LiOMe in

MeOH (0.066 g, 1.74 mmol), and 30 mL of 1:1 THF:

MeOH. After 12 h of stirring at ambient temperature

under N₂, a dark green precipitate was isolated by filtra-

tion and washed with 3 ꢀ 5 mL Et₂O. Yield: 0.353 g, 82%.

NMR characterization. ³¹P NMR (δ, ppm in CD₂Cl₂):

57.74 (s). Absorption spectrum (CH₂Cl₂), λ_maxin nm

(ε_M): 232 (59300), 266 (61000), ∼273 (sh, 58600), 300

(74300), ∼352 (sh, 17700), ∼600 (1200). MS (MALDI-

TOF): 1115.8 (M^þ). HRMS (MALDI-TOF) monoiso-

topic m/z: 1114.0450 (calcd for C₅₈H₅₀Ni₂P₄S₄(M^þ)

1114.0446). Anal. Calcd for C₅₈H₅₀Ni₂P₄S₄: C, 62.39;

H, 4.51. Found: C, 59.76; H, 4.47.

[(dppe)Ni(S₂C₆H₂S₂)Ni(dppe)] (3a) (Rxn VI, Scheme 1).

A 25 mL Schlenk flask charged with a stir bar,

ⁿBu₂SnS₂C₆H₂S₂SnⁿBu₂(0.100 g, 0.15 mmol), [(dppe)-

NiCl₂] (0.158 g, 0.339 mmol), and 1,2-dichloroethane

(20 mL) was fitted to a reflux condenser leading to a

Schlenk line and refluxed under N₂for 12 h. During this

time, the initial orange suspension was converted to a

dark green precipitate. The mixture was cooled, and the

dark precipitate was collected by filtration, washed with

3 ꢀ 5 mL MeOH, followed by 3 ꢀ 5 mL of Et₂O, and dried

under vacuum. Yield: 0.136 g, 81%.

[(dppee)Ni(S₂C₆H₂S₂)Ni(dppee)] (3b) (Rxn VI, Scheme 1).

The same procedure and scale as described in the preced-

ing paragraph for the synthesis of 3a by Rxn VI were

employed. Yield: 0.140 g, 84%. The solubility of 3b was

too sparing to permit ¹H and ¹³C NMR characterization.

³¹P NMR (δ, ppm in CD₂Cl₂): 65.51 (s). Absorption

spectrum (CH₂Cl₂), λ_maxin nm (ε_M): 234 (70800), 270

(51200), 276 (51400), 302 (58000), ∼363 (sh, 15000), ∼621

(∼1220). MS (MALDI-TOF): 1111.8 (M^þ). HRMS

(MALDI-TOF) monoisotopic m/z: 1110.0159 (calcd for

C₅₈H₄₆Ni₂P₄S₄(M^þ) 1110.0134). Anal. Calcd for

C₅₈H₄₆Ni₂P₄S₄: C, 62.62; H, 4.17. Found: C, 61.15; H,

4.08.

The solubility of 3a was too sparing to permit ¹H and ¹³

C

for [(dppb)Ni(S₂C₆H₂S₂CO)] 1/2(ClCH₂CH₂Cl), C₃₈-

H₂₈ClOP₂S₄Ni: C, 58.14; H, 3.60; P, 7.89. Found: C,

58.79; H, 3.71; P, 8.22.

3

[(dcpe)Ni(S₂C₆H₂S₂CO)] (2d). A 100 mL Schlenk flask

with stir bar was charged with 1,3,5,7-tetrathia-s-inda-

cene-2,6-dione (0.250 g, 0.97 mmol), 50 mL of 1:1 THF:

MeOH, and LiOMe solution (0.47 mL, 0.041 g, 1.1

mmol). The resulting mixture was stirred for 1 h, during

which time the insoluble 1,3,5,7-tetrathia-s-indacene-2,

6-dione completely dissolved in the THF-MeOH mixture.

To this homogeneous mixture was then added [(dcpe)-

NiBr₂] (0.434 g, 0.67 mmol) as a single portion under

nitrogen flow. An immediate color change from pale

yellow to orange-red was observed. This mixture was

stirred for 12 h at 25 °C, during which time a suspension

of LiBr formed in the solution. The solution was filtered

via filter cannula, and the solvent was removed under

reduced pressure. The red solid residue was purified on a

silica column eluted with 3:7 CH₂Cl₂/hexanes and col-

lected as the first visible fraction. Compound 2d readily

crystallized from the eluant as fine, pale orange, flat

needle crystals. Yield: 0.301 g, 62%. R_f(9:1 CH₂Cl₂/

[(dppb)Ni(S₂C₆H₂S₂)Ni(dppb)] (3c) (Rxn VI, Scheme 1).

The same procedure and scale as described in the preced-

ing paragraphs for the synthesis of 3a and 3b by Rxn VI

were employed. Yield: 0.145 g, 80%. ³¹P NMR (δ, ppm in

CD₂Cl₂): 57.14 (s). Absorption spectrum (CH₂Cl₂),

λ_maxin nm (ε_M): 232 (60500), 270 (sh, 39000), 278

(39400), 310 (51500), 408 (7880), 610 (∼1340). MS

(MALDI-TOF): 1212.0 (M^þ). HRMS (MALDI-TOF)

monoisotopic m/z: 1210.0441 (calcd for C₆₆H₅₀Ni₂P₄S₄

(M^þ) 1210.0447).

1

hexanes): 0.45. H NMR (δ, ppm in CD₂Cl₂): 7.53 (s,

2H, aromatic CH), 2.20 (d, 4H, aliphatic CH), 2.09 (m,

4H, aliphatic CH), 1.91 (d, 4H, aliphatic CH), 1.82 (d, 8H,

aliphatic CH), 1.70 (d, 4H, aliphatic CH), 1.55-1.20

(multiple overlapped signals, 24H, aliphatic CH); ¹³C

NMR (δ, ppm in CD₂Cl₂): 191.0, 149.5, 123.0, 119.5,

35.1, 34.9 (d), 28.0, 27.8, 26.2 (m), 25.9, 22.0, 21.5 (d); ³¹P

NMR (δ, ppm in CD₂Cl₂) 78.94. Absorption spectrum

10594 Inorganic Chemistry, Vol. 48, No. 22, 2009

Arumugam et al.

Scheme 1. Summary of Syntheses Reported and Definition of Numbering System by Which the Compounds Are Identified

[(dcpe)Ni(S₂C₆H₂S₂)Ni(dcpe)] (3d) (Rxn III, Scheme 1).

A 50 mL Schlenk flask with stir bar was charged with

1,3,5,7-tetrathia-s-indacene-2,6-dione (0.079 g, 0.31

mmol), LiOMe, (0.54 mL, 0.047 g, 1.2 mmol) and 40

mL of dry 1:1 THF/MeOH. The resulting mixture was

stirred under N₂for 1 h, during which time the insoluble

1,2,5,7-tetrathia-s-indacene-2,6-dione completely dis-

solved. To the resulting mixture was added [(dcpe)-

NiBr₂] (0.353 g, 0.55 mmol) all at once as a solid under N₂

flow. An immediate change in the solution color from pale

yellow to dark red was observed. This reaction mixture was

stirred for 12 h at ambient temperature. This mixture was

filtered via filter cannula to remove suspended LiBr, and the

solvent was removed under reduced pressure. The resulting

dark red solid residue was purified on a silica column eluted

with a mixture of CH₂Cl₂/hexanes in which the CH₂Cl₂/

hexanes ratio was incrementally varied from 0.0 to 1.0.

Elution was continued with EtOAc/CH₂Cl₂mixtures in

which the EtOAc/CH₂Cl₂ratio was similarly varied. The

title compound, [(dcpe)Ni(S₂C₆H₂S₂)Ni(dcpe)], readily crys-

tallized from the eluant as fine, dark red plate crystals. Yield:

0.232 g, 72%. R_f(CH₂Cl₂): 0.13 (with trailing). ¹H NMR (δ,

ppm in CD₂Cl₂): 7.35 (br s, 2H, aromatic CH), 2.26 (d, 8H,

aliphatic CH), 2.08 (m, 8H, aliphatic CH), 1.86-1.79 (m,

32H, aliphatic CH), 1.69 (d, 8H, aliphatic CH), 1.56-1.21

(multiple overlapped signals, 40H, aliphatic CH). ¹³C NMR

(δ, ppm in CD₂Cl₂): 36.2, 29.5, 29.2, 27.6, 27.5, 26.6, 23.2 (t).

³¹P NMR (δ, ppm in CD₂Cl₂): 77.23. Absorption spectrum

(CH₂Cl₂), λ_maxin nm (ε_M): 256 (44000), 296 (83400), 360 (sh,

21000), 366 (22300), 546 (970). MS (MALDI-TOF): 1164

(M^þ). HRMS (MALDI-TOF) monoisotopic m/z:

1162.4226 (calcd for C₅₈H₉₈Ni₂P₄S₄(M^þ) 1162.4203). Anal.

Calcd for C₅₈H₉₈Ni₂P₄S₄: C, 59.80; H, 8.48; S, 11.01. Found:

C, 59.26; H, 8.68; S, 11.20.

(0.012 g, 0.176 mmol), and 20 mL of dry 1:1 THF/EtOH.

The resulting mixture was stirred for 1 h, during which

time the pale orange solution turned dark red in color. To

this solution mixture was added [(dcpe)NiBr₂] (0.045 g,

0.07 mmol) as a single portion under N₂flow, which

occasioned an immediate change in the solution color

from dark red to dark brown. The reaction mixture was

stirred for 12 h at ambient temperature and then filtered

to remove suspended LiBr. The solvent was removed

from the filtrate under reduced pressure. The resulting

dark red solid residue was purified as described in the

preceding paragraph. Yield: 0.040 g, 49%.

[(dcpe)PdCl₂]. A 50 mL Schlenk flask was charged with

a stir bar, 1,2-bis(dicyclohexylphosphino)ethane (dcpe)

(0.226 g, 0.535 mmol) and 10 mL of dry CH₂Cl₂. A

solution of [(cod)PdCl₂] (0.153 g, 0.536 mmol) in 20 mL

of CH₂Cl₂was slowly transferred to the dcpe solution via

cannula with constant stirring. The resulting pale yellow

reaction mixture was stirred at ambient temperature for 4

h. The solvent was removed under reduced pressure, and

the pale yellow solid residue was washed with 2 ꢀ 10 mL

of pentane and dried under vacuum for a day. Yield: 0.296

g, 92%.

[(dcpe)Ni(S₂C₆H₂S₂)Pd(dcpe)] (3e) (Rxn II, Scheme 1).

A 25 mL Schlenk flask with stir bar was charged with

[(dcpe)Ni(S₂C₆H₂S₂CO)] (0.050 g, 0.07 mmol), NaOEt,

(0.012 g, 0.18 mmol), and 20 mL of dry 1:1 THF/EtOH.

The resulting mixture was stirred for 1 h, during which

time the pale orange solution became dark red in color.

To this reaction mixture was added solid [(dcpe)PdCl₂]

(0.042 g, 0.07 mmol) in one portion under N₂flow, which

induced an immediate change in the solution color from a

dark red to a dark brown. The flask contents were stirred

for 12 h at ambient temperature, during which time the

formation of a NaCl suspension was noted. The reaction

solution was filtered, and the solvent was removed from

the filtrate under reduced pressure. The resulting dark red

[(dcpe)Ni(S₂C₆H₂S₂)Ni(dcpe)] (3d) (Rxn II, Scheme 1).

A 25 mL Schlenk flask with stir bar was charged with

[(dcpe)Ni(S₂C₆H₂S₂CO)] (0.050 g, 0.07 mmol), NaOEt

Article

Inorganic Chemistry, Vol. 48, No. 22, 2009 10595

solid residue was purified on a silica column eluted with a

CH₂Cl₂/hexanes mixture, followed by a CH₂Cl₂/EtOAc

mixture, as described for 3d. Heterodimetallic [(dcpe)-

Ni(S₂C₆H₂S₂)Pd(dcpe)] readily crystallized from the

eluant as fine, dark red crystals. Yield: 0.040 g, 47%. R_f

(CH₂Cl₂): 0.14 (with trailing). ¹H NMR (δ, ppm in

CD₂Cl₂): 7.25 (br s, 2H, aromatic CH), 2.25 (d, 4H,

aliphatic CH), 2.10 (m, 12H, aliphatic CH), 1.97 (d, 4H,

aliphatic CH), 1.86-1.79 (m, 24H, aliphatic CH), 1.68 (d,

8H, aliphatic CH), 1.57-1.21 (multiple overlapped sig-

nals, 44H, aliphatic CH). ¹³C NMR (δ, ppm in CD₂Cl₂):

110.2, 36.5 (m), 29.5, 29.2, 29.1, 27.6 (m), 27.3 (m), 26.6,

26.4, 23.7 (t), 23.2 (t). ³¹P NMR (δ, ppm in CD₂Cl₂):

78.32, 76.88. Absorption spectrum (CH₂Cl₂), λ_maxin nm

(ε_M): 284 (69800), 354 (sh, 15200), 364 (17600), 514 (830).

MS (MALDI-TOF): 1212 (M^þ).

[(dcpe)Pd(S₂C₆H₂S₂)Pd(dcpe)] (3f) (Rxn VI, Scheme 1).

A procedure similar to that described for the synthesis of

3a by Rxn VI was used but on a scale employing 0.055 g

(0.082 mmol) of ⁿBu₂SnS₂C₆H₂S₂SnⁿBu₂and with

[(dcpe)PdCl₂] (0.100 g, 0.167 mmol) and CH₂Cl₂

(25 mL) used in place of [(dppe)NiCl₂] and 1,2-dichloro-

ethane, respectively. This reaction mixture was heated to

50 °C for 3 h, cooled to room temperature, and taken to

dryness under reduced pressure. The residual solid was

washed with MeOH (3 ꢀ 10 mL) and Et₂O (3 ꢀ 10 mL)

and then dried under vacuum for 24 h. Compound 3f was

obtained as a brown solid and recrystallized as orange

blocks by diffusion of pentane vapor into a 1,2-dichloro-

ethane solution. Yield 0.043 g, 41%. ¹H NMR (δ, ppm in

CD₂Cl₂): 7.18 (s, 2H, aromatic CH), 2.12 (br, 16H,

aliphatic CH), 1.97 (m, 8H, aliphatic CH), 1.82 (br,

∼20H, aliphatic CH), 1.68 (m, 8H, aliphatic CH), 1.55

(d, ∼8H, aliphatic CH), 1.51 (m, ∼12H, aliphatic CH),

1.36-1.22 (m, 24H, aliphatic CH). ³¹P NMR (δ, ppm in

CD₂Cl₂): 76.78. Absorption spectrum (CH₂Cl₂), λ_maxin

nm (ε_M): 278 (109000), 350 (sh, 26300), 362 (28300). MS

(MALDI-TOF): 1260 (M^þ).

The solid residue was extracted with 2 ꢀ 5 mL portions of

THF, and the extracts were filtered through Celite. The

solvent was removed from the filtrate under reduced

pressure to yield [[(dppb)Ni(S₂C₆H₂S₂)Ni(dppb)]₂-

(μ-Ag₂)][BArF₂₄]₂. Yield: 0.140 g, 78%. ³¹P NMR (δ,

ppm in CD₂Cl₂): 60.70 (s), 57.84 (s). Absorption spectrum

(CH₂Cl₂), λ_maxin nm (ε_M): 261 (80000), 297 (130000), 353

(29000), 489 (3000).

[(dcpe)Ni(S₂C₆H₂S₂SnⁿBu₂)], 5d. A 200 mL Schlenk

flask with stir bar was charged with ⁿBu₂SnS₂C₆H₂S₂Sn^n-

Bu₂(0.286 g, 0.428 mmol) and 100 mL of dry CH₂Cl₂at

25 °C. To a separate 100 mL Schlenk flask containing

[(dcpe)NiCl₂] (0.250 g, 0.453 mmol) under N₂was added

50 mL of dry CH₂Cl₂. The latter solution was added to the

former solution via syringe pump at a rate of 5 mL/h over

period of 10 h, during which time the slightly yellow

colored solution slowly assumed an orangish hue. After

the addition was complete, the resulting mixture was

allowed to stir for a further 2 h at 25 °C and then was

taken to dryness under reduced pressure. The dark red

residue was further purified by flash chromatography on

a silica column eluted with 4:6 CH₂Cl₂/hexanes to yield

[(dcpe)NiS₂C₆H₂S₂SnⁿBu₂]. Prolonged exposure on the

silica column leads to decomposition. Yield: 0.141 g,

1

36%. R_f(4:6 CH₂Cl₂/hexanes): 0.27. H NMR (δ, ppm

in CD₂Cl₂): 7.42 (s, 2H, aromatic CH), 2.20 (d, 4H,

aliphatic CH), 2.10 (br, 4H, aliphatic CH), 1.89-1.78

(m, 14H, aliphatic CH), 1.69 (d, 6H, aliphatic CH),

1.60-1.26 (multiple overlapped signals, 32H, aliphatic

CH), 0.90 (t, 6H, -CH₃). ¹³C NMR (δ, ppm in CD₂Cl₂):

130.0, 127.2, 36.1 (t), 29.2, 29.0, 28.1, 27.2, 26.8, 26.2,

23.0, 21.3, 13.5. ³¹P NMR (δ, ppm in CD₂Cl₂): 78.47.

Absorption spectrum (CH₂Cl₂), λ_maxin nm (ε_M): 262

(30800), 292 (54700), 346 (8110), 358 (8060), ∼534.

HRMS (MALDI-TOF) monoisotopic m/z: 912.2055

(calcd for C₄₀H₆₈NiPS₄Sn (M^þ) 912.2043). Anal. Calcd

for C₄₀H₆₈NiP₂S₄Sn: C, 52.42; H, 7.48; P, 6.76. Found: C,

52.61; H, 7.29; P, 6.83.

[(dcpe)Ni(S₂C₆H₂S₂)Ni(dcpe)][BArF₂₄], [3d][BArF₂₄].

A 25 mL Schlenk flask with stir bar was charged with

[(dcpe)Ni(S₂C₆H₂S₂)Ni(dcpe)] (0.050 g, 0.043 mmol),

[Cp₂Fe][BArF₂₄] (0.045 g, 0.043 mmol), and 10 mL of

dry THF at 25 °C. This mixture was stirred at 25 °C for

12 h and then was taken to dryness under reduced

pressure. The resulting solid residue was washed with

copious amounts of hexanes and then extracted with 2 ꢀ

5 mL portions of THF. These THF extracts were filtered

through Celite and taken to dryness under reduced

pressure to yield [(dcpe)Ni(S₂C₆H₂S₂)Ni(dcpe)][BArF₂₄].

Yield: 0.061 g, 70%. Absorption spectrum (CH₂Cl₂), λ_max

in nm: ∼319 (sh), ∼ 386 (sh), 542 (max), ∼610 (sh), ∼680

(sh), ∼980 (sh), 1127 (max).

[(dcpe)Ni(S₂C₆H₂S₂)Ni(S₂C₆H₂S₂)Ni(dcpe)], (6a). A

50 mL Schlenk flask with stir bar was charged with

[(dcpe)Ni(S₂C₆H₂S₂CO)] (0.200 g, 0.28 mmol), NaOEt

(0.0478 g, 0.70 mmol), and dry THF/EtOH (20/20 mL).

The resulting mixture was stirred under N₂for 1 h, during

which time the pale orange solution assumed a dark red

color. To this reaction mixture was added solid NiBr₂

(0.031 g, 0.14 mmol) all at once under an outward flow

of N₂. An immediate change in solution color from dark

red to dark brown was observed. This mixture was stirred

for 12 h at ambient temperature. After removal of the

solvent under reduced pressure, the solid residue was

extracted with dry CH₃CN, and the extracts were filtered

through a paper filter cannula to remove the NaBr

byproduct. Solid I₂(0.071 g, 0.28 mmol) was added to

the CH₃CN filtrate, which induced the formation of a

brown precipitate. This heterogeneous mixture was

further stirred for 2 h. The brown precipitate was isolated

from the solution by filtration and dried under reduced

pressure. Yield: 0.075 g, 37%. ³¹P NMR (δ, ppm in

(CD₃)₂NC(O)D): 79.74. Absorption spectrum (DMF),

λ_maxin nm (ε_M): 526 (1700), 689 (870), 1159 (16000),

∼1408 (9700). MS (MALDI-TOF): 1426 (M^þ). HRMS

(MALDI-TOF) monoisotopic m/z: 1422.2581 (calcd for

C₆₄H₁₀₀Ni₃P₄S₈(M^þ) 1422.2595). Anal. Calcd for

[[(dppb)Ni(S₂C₆H₂S₂)Ni(dppb)]₂(μ-Ag₂)][BArF₂₄]₂, [4c]-

[BArF₂₄]₂. A 25 mL Schlenk flask with stir bar was

charged with [(dppb)Ni(S₂C₆H₂S₂)Ni(dppb)] (0.050 g,

0.041 mmol), AgBArF₂₄(0.040 g, 0.041 mmol), and

10 mL of dry CH₂Cl₂at 25 °C. This mixture was stirred

at 25 °C for 12 h, by the end of which time the insoluble

[(dppb)Ni(S₂C₆H₂S₂)Ni(dppb)] completely went into so-

lution with an attendant change in color from green to

dark red. The homogeneous reaction solution was taken

to dryness under reduced pressure, and the resulting solid

residue was washed with copious amounts of hexanes.

10596 Inorganic Chemistry, Vol. 48, No. 22, 2009

Arumugam et al.

[(dcpe)Ni(S₂C₆H₂S₂)Ni(S₂C₆H₂S₂)Ni(dcpe)] 2DMF, C₇₀-

H₁₁₄N₂Ni₃O₂P₄S₈: C, 53.48; H, 7.31; N, 1.78. Found: C,

50.46; H, 6.86, N, 1.44.

(triphos=1,1,1-tris(diphenylphosphanomethyl)ethane), re-

ported by Huttner et al.³³The arene tetrathiolate-linked

anions [[(Cl₄C₆S₂)Ni]₂(S₂C₆X₂S₂)]^2-(X = H, F, Cl),

[(F₄C₆S₂)Ni((S₂C₆F₂S₂)Ni)_n(S₂C₆F₄)]^(nþ1)-(n = 4 or 5)

and [(Cl₄C₆S₂)Ni((S₂C₆Cl₂S₂)Ni)₈(S₂C₆Cl₄)]^10-have been

characterized by UV-vis-near IR spectroscopy, but the

samples employed in this study apparently consist of mix-

tures of species related by dynamic solution equilibrium

3

[(dcpe)Ni(S₂C₆H₂S₂)Pt(S₂C₆H₂S₂)Ni(dcpe)] (6b). A

procedure analogous to that used for the synthesis of

compound 6a above was followed but with PtCl₂used in

place of NiBr₂. The reaction scale employed 0.150 g

(0.210 mmol) of [(dcpe)Ni(S₂C₆H₂S₂CO)], 0.028 g

(0.105 mmol) of PtCl₂, and 0.027 g (0.106 mmol) of I₂.

Yield: 0.071 g, 43%. ³¹P NMR (ppm in (CD₃)₂NC(O)D):

79.22. Absorption spectrum (DMF), λ_maxin nm (ε_M): 507

(560), 1177 (12400). MS (MALDI-TOF): 1562 (M^þ).

[(dppb)Ni(S₂C₆H₂(S-S)₂C₆H₂S₂)Ni(dppb)] 7c, [(dcpe)-

Ni(S₂C₆H₂(S-S)₂C₆H₂S₂)Ni(dcpe)] 7d. Compounds 7c

and 7d were obtained in trace amounts as adventitious

byproducts attending the syntheses of compound 3c

and 3d, respectively, and were only identified by X-ray

crystallography. Deliberate, optimized preparations for

7c and 7d were not obtained.

processes.³⁴More recently, Nomura and Fourmigue have

ꢀ

described the synthesis and properties of Cp*Co-

(S₂C₆H₂S₂)CoCp*.²⁵

Preparations of benzenetetrathiol/thiolate were first re-

ported by Testaferri and Wudl,^35,36but a later, more facile

procedure described by Bechgaard²⁴has made this ligand

more accessible. Because 1,2,4,5-benzenetetrathiolate is

inherently very electron rich and prone to aerial oxidation,

its most convenient form for handling and storage is the

air-stable bis(1,3-dithiol-2-one) form, 1,3,5,7-tetrathia-s-

indacene-2,6-dione, 1 (Scheme 1). From thence, base

hydrolysis (LiOMe or NaOEt) in THF/MeOH mixtures

readily liberates one or both of the ene-1,2-dithiolate

moieties. Monodeprotection of this compound appears

to be optimized relative to full deprotection when a sub-

stoichiometric 1 equiv of alkoxide is used.

The introduction of in situ generated Li₂[S₂C₆H₂S₂CdO]

to 1 equiv of [(P-P)MX₂] (M=Ni, Pd; X=halide; P-P=

1,2-bis(diphenylphosphino)ethane, dppe; 1,2-bis(diphen-

ylphosphino)ethylene, dppee; 1,2-bis(diphenylphos-

phino)benzene, dppb; 1,2-bis(dicyclohexylphosphino)-

ethane, dcpe) leads to formation of the corresponding

mononickel and monopalladium bis(phosphine) dithio-

lene compounds, [(P-P)M(S₂C₆H₂S₂CdO)] (2a-2e),

through halide displacement reactions (Rxn I,

Scheme 1). These compounds are generally air-stable,

readily soluble in polar organic solvents, and easily

purified by column chromatography on silica. The im-

mediate purpose served by the chelating bis(phosphine)

ligand is prevention of the formation of undesired metal

dithiolene polymer. However, since the identity of the

phosphine substituents and the nature of the chelate

backbone could modulate the solubility, redox potentials,

and so forth of the metal complexes that are formed,³⁷a

variety of chelating bis(phosphines) have been studied for

effect. Nickel dithiolene bis(phosphine) complexes were

first prepared by Schrauzer.³⁸A variety of additional

examples have appeared since then, the most significant

corpus of work being that contributed by Bowmaker and

co-workers.^39-41

Physical Methods and Calculations

UV-vis spectra (molar absorptivities reported in M^-1

cm^-1) were obtained at ambient temperature with a Hewlett-

Packard 8452A diode array spectrometer, while IR spectra

were taken as pressed KBr pellets with a Thermo Nicolet

Nexus 670 FTIR instrument in absorption mode. All NMR

spectra were recorded at 25 °C either with a Varian Unity

Inova spectrometer operating at 400, 100.5, and 161.8 MHz

for ¹H, ¹³C, and ³¹P, respectively, or with a Bruker AVANCE

300 spectrometer operating at 121.5 MHz for ³¹P. Spectra

were referenced to the solvent residual for ¹H and ¹³C and to

external 85% H₃PO₄for ³¹P. Mass spectra (MALDI-TOF)

were obtained with either an ABI Voyager-DE STR instru-

ment or a Bruker Autoflex III instrument. Electrochemical

measurements were made with a CHI620C electroanalyzer

workstation using a Ag/AgCl reference electrode, a platinum

disk working electrode, Pt wire as auxiliary electrode, and

Bu₄NPF₆or Na[BArF₂₄] as the supporting electrolyte. The

Cp₂Fe^þ/Cp₂Fe couple occurred at þ0.54 mV (by CV) in

0.10 M [Bu₄N][PF₆] in CH₂Cl₂, at þ0.30 V (by CV) in 0.01 M

Na[BArF₂₄] in 5:4:1 CH₂Cl₂/anisole/THF, and at þ0.20 V

(by DPV) in 0.01 M Na[BArF₂₄] in CH₂Cl₂. Elemental

analyses were performed by Canadian Microanalytical of

Delta, British Columbia or by Midwest Microlab, LLC of

Indianapolis, IN. All details regarding crystal growth, X-ray

diffraction data collection, crystal structure solution and

refinement, and computational work are deferred to the

Supporting Information.

Results and Discussion

(33) Heinze, K.; Huttner, G.; Zsolnai, L. Z. Naturforsch., B: Chem. Sci.

1999, 54, 1147–1154.

(34) Campbell, J.; Jackson, D. A.; Stark, W. M.; Watson, A. A. Dyes

Pigm. 1991, 15, 15–22.

(35) Maiolo, F.; Testaferri, L.; Tiecco, M.; Tingoli, M. J. Org. Chem.

1981, 46, 3070–3073.

(36) Dirk, C. W.; Cox, S. D.; Wellman, D. E.; Wudl, F. J. Org. Chem.

1985, 50, 2395–2397.

(37) Lobana, T. S.; Gratzel, M.; Nazeeruddin, M. K.; Vlachopolous, N.

Trans. Met. Chem. 1996, 21, 551–552.

(38) Mayweg, V. P.; Schrauzer, G. N. Chem. Commun. 1966, 640–641.

(39) Bowmaker, G. A.; Boyd, P. D. W.; Campbell, G. K. Inorg. Chem.

1982, 21, 2403–2412.

Syntheses. The 1,2,4,5-benzenetetrathiolate bis(dithio-

lene) ligand, like tto^2-, is expected to support extended,

delocalized electronic structures in its metallocomplexes.

Although polymeric [Ni(S₂C₆H₂S₂)]_xwas prepared and

studied in 1986,¹⁶the first well-defined dimetal complexes

with the 1,2,4,5-benzenetetrathiolate connecting ligand

were L₂M(S₂C₆H₂S₂)ML₂(L = Cp or η⁵-C₅H₄(SiMe₃);

::

M = Ti; Zr or Hf), described by Kopf and

co-workers,³²and [(triphos)Co(S₂C₆H₂S₂)Co(triphos)]^2þ

(40) Bowmaker, G. A.; Boyd, P. D. W.; Campbell, G. K. Inorg. Chem.

1983, 22, 1208–1213.

(41) Bowmaker, G. A.; Williams, J. P. J. Chem. Soc., Dalton Trans. 1993,

3593–3600.

::

(32) (a) Kopf, H.; Balz, H. J. Organomet. Chem. 1990, 387, 77–81. (b) Balz,

::

H; Kopf, H.; Pickardt, J. J. Organomet. Chem. 1991, 417, 397–406.

Article

The dinickel compounds, 3a-3d, are readily prepared

Inorganic Chemistry, Vol. 48, No. 22, 2009 10597

vapors. Apart from the [[(edo)Ni(tto)]₂Ni]^2-and

[[(dddt)Ni(tto)]₂Ni]^2-dianions (edo(2-) = 5,6-dihydro-

1,4-dioxine-2,3-dithiolate; dddt(2-) = 5,6-dihydro-1,4-

dithine-2,3-dithiolate) recently isolated and characterized

by Kato and co-workers,¹³compounds 6a and 6b are the

only well-defined metal dithiolene compounds consisting

of three metal atoms linked via bridging bis(dithiolene)

ligands.

either by full deprotection of 1,3,5,7-tetrathia-s-indacene-

2,6-dione (Rxn III, Scheme 1), followed by addition of

2 eq [(P-P)NiX₂] (X=halide), or by starting with isolated

samples of the corresponding compound 2, deprotecting

the peripheral 1,3-dithiol-2-one group, and then adding a

second equiv of [(P-P)NiX₂], (Rxn II, Scheme 1). Yet

another way to prepare compounds 3 proceeds through

di-n-butyl tin protected forms of benzenetetrathiolate

(Rxns V and VI, Scheme 1), the doubly protected version

of which was recently described by Nomura and Four-

Elemental analysis data for compounds 2a-2d, 3a-3d,

6a, and 6b that are consistent with bulk sample purity

have been difficult to obtain despite their characterization

by spectroscopic means and by X-ray crystallography.

We are inclined to the view, however, that the sample

purities for these compounds may be better than sug-

gested by the analytical data. One complication afflicting

some of these compounds, 6a for example, is their co-

crystallization with multiple solvent molecules whose

number and identity are unclear because of their highly

disordered nature. Thus, these solvent molecules are

impossible to account for analytically. Adding further

to the challenge is the fact that compounds 2 and 6 are

centrosymmetric and amply substituted with phenyl or

cyclohexyl groups, which greatly diminishes their inher-

ent solubility in common organic solvents. This poor

solubility makes the complete redissolution, drying, and

recovery of a crystalline sample of adequate mass very

difficult to achieve. An additional problem that may be

obscuring the actual purity of some of these new com-

pounds is an interference by phosphorus with sulfur

analyses, a complicating issue with sulfur analyses that

has been noted in the analytical literature.⁴⁵In the case of

compound 2b, for example, carbon and hydrogen analyze

well, as expected for the spectroscopic purity that was

indicated for the compound, but sulfur analyzes for a

somewhat higher value than calculated. It is plausible that

this discrepancy can be attributed to an interfering effect

by phosphorus.

ꢀ ²⁵

migue. A set of reactions analogous to I-III in

Scheme 1, but using instead di-n-butyl tin as protecting

group for the dithiolene chelate, can be applied for the

synthesis of dinickel compounds 3 (Reactions IV-VI,

respectively). Tin dithiolene compounds have been em-

ployed by others in transmetalation reactions^42,43but

have underappreciated utility for the preparation of

metallodithiolene compounds. In general, the reactions

which make use of the di-n-butyl tin protected ligand in

Scheme 1 are cleaner and produce better yields than those

involving the base hydrolysis of 1,3-dithiol-2-ones.

Use of isolated mononickel compounds 2a-2d or 5d

affords the possibility of preparing asymmetric dimetal

complexes by introduction of a [(P-P)MX₂] compound

differing in the identity of M and/or the chelating diphos-

phine. Thus, for example, the mixed nickel-palladium

complex 3e was synthesized by Rxn II. For reference

purposes, dipalladium compound 3f was also prepared by

Rxn VI. The mixed metal composition of 3e was shown

not only by a MALDI-TOF mass spectrum displaying the

parent ion peak, with correct isotopic distribution profile,

but also by the observation of more complex NMR

spectra that reflect the two distinct halves of the molecule.

Compounds 3 are air-stable in the solid form but, as

might be anticipated from their higher symmetry, suffer

the limitation of much diminished solubility in common

organic solvents. The closest analogues to compounds 3

that have been prepared are the butadienetetrathiolate-

linked compound [(dppe)Ni(S₂C₂(H)-(H)C₂S₂)Ni(dppe)]

reported by Purrington and Bereman²¹and [(Ph₃P)₂Pt-

(S₂(TTF)S₂)Pt(PPh₃)₂] (TTF=tetrathiafulvalene).⁴⁴

Reactions II and V by which compounds 3 are prepared

essentially treat metal compounds 2 and 5 merely as

protected “ligands,” a point which suggests that new,

higher nuclearity compounds could be prepared by sim-

ple addition of two or three ligand equivalents to a

suitable metal halide. Thus, for instance, the introduction

of 2 equiv of [(dcpe)Ni(S₂C₆H₂S₂)]^2-to NiCl₂or to PtCl₂,

followed by addition of 1 equiv of I₂, leads to precipita-

tion of the trimetallic compounds [(dcpe)Ni(S₂C₆H₂S₂)]₂Ni

(6a) or [(dcpe)Ni(S₂C₆H₂S₂)]₂Pt (6b). Compounds 6a and

6b are appreciably darker in color than the mono- and

dimetallic compounds, consistent with the formation of a

greater extended delocalized electronic structure.

Although sparing in their solubility in common organic

solvents, 6a and 6b are sufficiently soluble in N,N-di-

methylformamide (DMF) as to crystallize readily in the

form of large prisms upon diffusion of ethereal solvent

Structures. The structures of 2a-2d have been deter-

mined (Table 1) and observed to have the square planar

coordination geometries expected for a d⁸metal in the

strong ligand field environment presented by dithiolene

and chelating bis(phosphine) ligands. Previous structural

studies of this molecule type have been limited to com-

pounds with the 1,2-bis(diphenylphosphino)ethane^46-53

or 1,2-bis(diphenylphosphino)methane⁵⁴ligands. Ther-

mal ellipsoid plots of 2c and 2d are displayed in Figure 1;

those of 2a, 2b, and 2e, which are atom-labeled in a

(45) Ma, T. S.; Rittner, R. C. Modern Organic Elemental Analysis; Marcel

Dekker: New York, 1979; pp 211-212.

(46) Velazquez, C. S.; Baumann, T. F.; Olmstead, M. M.; Hope, H.;

Barrett, A. G. M.; Hoffman, B. M. J. Am. Chem. Soc. 1993, 115, 9997–10003.

(47) Tian, Z.-Q.; Donahue, J. P.; Holm, R. H. Inorg. Chem. 1995, 34,

5567–5572.

(48) Kaiwar, S. P.; Hsu, J. K.; Liable-Sands, L. M.; Rheingold, A. L.;

Pilato, R. S. Inorg. Chem. 1997, 36, 4234–4240.

(49) Chesney, A.; Bryce, M. R.; Batsanov, A. S.; Howard, J. A. K. Chem.

Commun. 1997, 2293–2294.

(50) Darkwa, J. Inorg. Chim. Acta 1997, 257, 137–141.

(51) Landis, K. G.; Hunter, A. D.; Wagner, T. R.; Curtin, L. S.; Filler, F.

L.; Jansen-Varnum, S. A. Inorg. Chim. Acta 1998, 282, 155–162.

(52) Nihei, M.; Kurihara, M.; Mizutani, J.; Nishihara, H. J. Am. Chem.

Soc. 2003, 125, 2964–2973.

ꢀ

(53) Ni, C.-L.; Li, Y.-Z.; Ni, Z.-P.; Dang, D.-B.; Meng, Q.-J. Acta

Crystallogr., Sect. E. 2004, 60, m410–m412.

(54) Kang, B.-S.; Chen, Z.-N.; Gao, H.-R.; Zhou, Z.-Y.; Wu, B.-M.;

Mak, T. M. C.; Lin, Z. Huaxue Xuebao 1998, 56, 58–67.

(42) Abel, E. W.; Jenkins, C. R. J. Chem. Soc. A 1967, 1344–1346.

ꢀ

(43) Uson, R.; Vicente, J.; Oro, J. Inorg. Chim. Acta 1981, 52, 29–34.

(44) McCullough, R. D.; Belot, J. A. Chem. Mater. 1994, 6, 1396–1403.

10598 Inorganic Chemistry, Vol. 48, No. 22, 2009

Arumugam et al.

Table 1. Crystal and Refinement Data for Compounds 2a-d, 3ab, 3d-f, [3d]^þ, [4c]^2þ, 5d, 6ab, 7d

2a

2b

2c

2d

3a

solvent

formula

fw

none

₃₃H₂₆NiOP₂S₄

687.43

none

C₃₃H₂₄NiOP₂S₄

685.41

1/2DCE

C₃₃H₂₈ClNiOP₂S₄

784.94

none

C₃₃H₅₀NiOP₂S₄

711.62

none

C₅₈H₅₀Ni₂P₄S₄

1116.52

C

xtl system

space grp

color, habit

a, A

b, A

c, A

R, deg.

β, deg.

γ, deg.

V, A³

orthorhombic

P2₁2₁2₁

orange plate

11.155(3)

12.849(3)

21.133(5)

90

orthorhombic

P2₁2₁2₁

red block

9.872(2)

15.547(3)

19.629(4)

90

monoclinic

P2₁/c

monoclinic

P2₁/c

monoclinic

P2₁/n

orange plate

9.048(2)

21.139(5)

18.511(4)

90

95.679(3)

90

3523(1)

orange plate

12.065(2)

17.391(3)

16.307(3)

90

102.639(3)

90

3338.6(9)

100

orange-brown block

11.320(3)

15.354(4)

14.681(4)

90

3029(1)

100

90

3013(1)

100

103.202(5)

90

2484(1)

T, K

Z

100

4

100

2

4

R1, wR2^a

GoF

0.0686, 0.1616

1.107

0.0267, 0.0628

1.027

0.0333, 0.0749

1.029

0.0460, 0.1065

1.036

0.0454, 0.1040

1.059

3b

3d

3e

3f

[3d][BArF₂₄]

b

solvent

formula

fw

xtl system

space grp

color, habit

a, A

b, A

c, A

R, deg.

β, deg.

γ, deg.

V, A³

none

C₅₈H₄₆Ni₂P₄S₄

1112.49

monoclinic

P2₁/n

∼3(C₆H₁₄

)

2.5DCE^c

C_60.5H₁₀₃Cl_2.5NiP₄PdS₄

1336.28

4C₆H₅Cl 2DCE^c

C₆H₅Cl

3

C₅₈H₉₈Ni₂P₄S₄

1164.90

monoclinic

Pc

C₈₆H₁₂₆Cl₈P₄Pd₂S₄

1908.39

tetragonal

P4₂/n

orange column

29.076(3)

11.430(1)

90

C₉₆H₁₁₅BClF₂₄Ni₂P₄S₄

2140.68

monoclinic

C2/c

triclinic

P1

orange-brown needle

10.971(2)

15.318(3)

15.200(3)

90

red plate

11.115(2)

35.667(6)

17.500(3)

90

99.111(4)

90

6851(2)

100

red parallelpipeds

12.054(1)

25.100(2)

25.131(2)

88.735(1)

82.764(1)

85.975(1)

7524(1)

orange plate

32.915(9)

14.480(4)

51.18(1)

90

105.047(4)

90

23550(10)

100

100.976(4)

90

2507.5(8)

100

90

9664(2)

100

T, K

Z

100

4

2

4

8

R1, wR2^a

GoF

0.0616, 0.1397

1.030

0.0810, 0.1589

1.005

0.0631, 0.1789

1.074

0.0587, 0.1391

1.092

0.0626, 0.1746

0.971

compound

5d

6a

6b

7d

[4c][BArF₂₄]₂

b

solvent

formula

fw

xtl system

space grp

color, habit

a, A

b, A

c, A

R, deg.

β, deg.

γ, deg.

V, A³

1/2DCE^c

C₄₁H₇₀ClNiP₂S₄Sn

966.07

∼3DME^b,d

C₆₄H₁₀₀Ni₃P₄S₈

1425.93

rhombohedral

R3

brown-black block

44.592(2)

41.023(3)

90

∼3CH₃C(OCH₃)₂CH₃

2-3CH₂Cl₂

4CHCl₃

C₂₀₀H₁₂₈Ag₂B₂Cl₁₂F₄₈Ni₄P₈S₈

C₆₄H₁₀₀Ni₂P₄PtS₈

1562.31

rhombohedral

C₆₄H₁₀₀Ni₂P₄S₈

1367.22

monoclinic

P2₁/n

yellow plate

11.593(2)

15.536(3)

21.865(4)

90

4844.86

triclinic

P1

orange plate

11.682(4)

19.518(6)

23.550(7)

86.004(4)

77.999(4)

87.417

monoclinic

P2₁/c

R3

brown-black block

45.13(1)

40.63(2)

90

orange column

38.367(3)

10.3214(9)

48.842(4)

90

97.497(3)

90

19176(3)

100

16

90

120

70643(6)

100

18

90

120

71670(40)

100

18

94.771(2)

90

3925(1)

100

5237(3)

100

T, K

Z

2

1

R1, wR2^a

GoF

0.0806, 0.1687

1.086

0.0714, 0.1225

1.029

0.0755, 0.0998

1.026

0.0750, 0.1947

1.004

0.0754, 0.1930

1.039

P

^aR1 = ||F_o| - |F_c||/ |F_o|. wR2 = { w(F_o- F_c²)²/ w(F_o²)²}^1/2; w = 1/[σ²(F_o²) þ (xP)²], where P = (F_oþ 2F_c²)/3. ^bSolvent was highly

2

disordered and removed with the use of SQUEEZE in PLATON. ^cDCE = 1,2-dichloroethane. ^dDME = 1,2-dimethoxyethane.

fashion strictly analogous to 2c and 2d, are available in

Supporting Information. Ni-S bond lengths range from

2.1481(6)-2.179(2) A, while Ni-P bond lengths span a

broader range from 2.1530(7)-2.1987(7) A. The longest

Ni-P bond lengths (2.1847(7), 2.1987(7) A) occur in 2d

and are likely due to the greater closeness to idealized

square planar geometry in this compound compared to

2a-2c, as measured by the angle, θ, between the S(1)-Ni-

(1)-S(2) and P(1)-Ni(1)-P(2) planes (1.6°, Supporting

Information, Table S1) and by the mean atom deviation,

δ_m, from the S₂NiP₂plane (0.012 A, Supporting Informa-

tion, Table S1). A consequence of the small tetrahedral

distortion at Ni in 2d is a greater trans influence exerted

upon the phosphine ligand by the thiolate sulfur atoms of

the dithiolene ligand.

Structural identification of arenetetrathiolate-linked

dimetal compounds 3abdef has also been accomplished

(Table 1, Figure 1). The closest analogues to these com-

pounds that have been crystallographically characterized

are [(η⁵-C₅H₄(SiMe₃))M(S₂C₆H₂S₂)M(η⁵-C₅H₄(SiMe₃))]

Article

Inorganic Chemistry, Vol. 48, No. 22, 2009 10599

Figure 1. Structures of mononickel compounds 2c, 2d and dimetal compounds 3a, 3b, 3d, 3e, 3f, and [3d]^1þ. Thermal ellipsoid plots are drawn at the 50%

probability level. H atoms are omitted for clarity.

(M = Ti, Zr, Hf)^32band Cp*Co(S₂C₆H₂S₂)CoCp*.²⁵

Compounds 3a and 3b crystallize on inversion centers in

monoclinic space group P2₁/n with similar unit cell

dimensions. Compound 3b differs from 3a by revealing

somewhat greater tetrahedralization at the nickel center,

manifested by θ and δ_mvalues of 16.9° and 0.181 A as

compared to 4.9° and 0.053 A for 3a (Table 2). The

structure of 3d was solved in the non-centric space group

Pc with two independent molecules in the asymmetric

unit, each of which reveals a concave, end-to-end bending

that precludes a symmetry center (Supporting Informa-

tion, Figure S1). This saddle-shaped structure is appar-

ently due to crystal packing effects, as opposed to

stereoelectronic factors through which a distorted geo-

metry ends up at lower energy than a more symmetric

planar configuration. As seen in Supporting Information,

10600 Inorganic Chemistry, Vol. 48, No. 22, 2009

Arumugam et al.

Table 2. Selected Averaged^aBond Distances (A) and Angles (deg) for Compounds 3abdef and [3d]^þ

3a

3b

3d^b

3e^b

3f

[3d]^þ

M-S^c

2.1686[6]

2.1920[6]

1.758[1]

1.398(3)^d

8.518

2.158[1]

2.154[1]

1.757[4]

1.401(7)^d

8.447

2.172[1]

2.178[1]

1.789[4]

1.387[6]

8.489

91.38[5]

88.45[6]

90.20[4]

175.51[5]

0.114

2.2358[5]

2.2253[5]

1.758[2]

1.403[4]

8.505

91.06[3]

87.73[3]

90.70[2]

175.36[2]

0.087

2.3105[7]

2.2775[7]

1.765[3]

1.397(6)^d

8.759

2.1695[5]

2.1728[5]

1.722[2]

1.432[4]

8.412

92.02[3]

88.61[3]

90.37[2]

170.92[2]

0.145

M-P^c

S-C

chelate C-C

intermetal dist., A

S-M-S

91.58(3)^d

87.07(3)^d

90.76[2]

175.79[2]

0.053

92.76(6)^d

87.50(6)^d

91.10[4]

167.64[5]

0.181

88.88(4)^d

86.40(4)^d

92.35[3]

178.55[3]

0.006

P-M-P

P-M-S_cis

P-M-S_trans

e,f

δ_m, A, S₂MP₂

f,g

δ_m, A, S₂M⁰P₂

0.025

11.3

2.4

26.1

0.042

7.9

5.2

28.7

0.126

13.3

11.8

13.5

θ, deg.^h

θ⁰, deg.ⁱ

τ, deg.^j

j, deg.^k

4.9

16.9

1.1

0

4.0

0

4.3

0

5.6

18.9

3.9

9.3

^aUncertainties for averaged values are computed according to the general formula for error propagation in a function of multiple variables as

described by Taylor, J. R. An Introduction to Error Analysis; University Science Books: Sausalito, CA, 1997; pp 73-77. ^bData are for one of two

independent molecules in the asymmetric unit. ^cM = Ni for 3a, 3b, 3d, [3d]^þ; M = Pd for 3f; for 3e, M = both Ni and Pd, which are disordered over the

metal positions in a ∼4:6 ratio. ^dNon-averaged values. ^eM = Ni(1) for 3a, 3b, 3d, [3d]^þ; M = Pd(1) for 3f; for 3e, M = the metal atom position

predominantly occupied by Ni(1), as illustrated in Figure 1. ^fMean atom deviation from S₂MP₂or S₂M⁰P₂plane. ^gM⁰= Ni(2) for 3d, [3d]^þ; for 3e, M⁰= the

metal atom position predominantly occupied by Pd(1), as illustrated in Figure 1. ^hAngle between the S(1)-M-S(2) and P(1)-M-P(2) planes. ⁱAngle between the

S(3)-M⁰-S(4) and P(3)-M⁰-P(4) planes. ^jAngle between mean S₂MP₂planes. ^kEnd-to-end molecular twist, defined as absolute value of the P(1)-M(1)-M-

(1A)-P(2A) or P(1)-M(1)-M⁰-P(3) torsion angle.

mization of the two ends. Crystallization of dipalladium

compound 3f occurred on an inversion center in tetra-

gonal space group P4₂/n. This compound reveals almost

perfect planarity from end-to-end with virtually no tetra-

hedralization at the metal atoms (Table 2), a contrast to

the dinickel compounds.

Observations noted in the electrochemistry studies of

compounds 3a-d, for example, a pronounced sensitivity

toward supporting electrolytes with small, potentially

coordinating anions, suggested that the redox processes

seen in these compounds are oxidations primarily loca-

lized on the arenetetrathiolate ligand that bridges the

Figure 2. Possible formulations of metal and ligand oxidation states in

compounds 3 (a and b); possible oxidation states of dithiolene ligands

metal centers. The usefulness of crystallography in iden-

tifying ligand-based, as opposed to metal-based, redox

events^55-60thus made the isolation and structural study

of a cationic form of one of these dimetallic compounds a

particularly desirable objective. Use of Ag[BArF₂₄] in

CH₂Cl₂as one-electron chemical oxidant of compounds 3

resulted in an immediate and noticeable change in color,

implying that oxidation occurred. However, the forma-

tion of a diamagnetic species was suggested by ³¹P NMR

spectroscopy, an observation inconsistent with the un-

paired spin of the intended product. When compound 3c

was subjected to this oxidation by Ag[BArF₂₄], orange

plate crystals serendipitously formed in the NMR tube

that were of a quality suitable for X-ray diffraction.

Crystallographic identification of this compound re-

vealed it to be an unanticipated tetra-nickel species in

which two molecules of 3c are brought together in an

(c-e).

Figure S1, the (dcpe)Ni end groups of 3d bend upward to

accommodate the steric crowding of another molecule

abutting its underside. Both Ni centers reveal some degree

of tetrahedralization, although that for Ni(1) in 3d is

appreciably greater (Table 2) than that at Ni(2). Com-

pounds 3a, 3b, and 3d reveal S-C bond lengths (S-C_ave

=

1.757[4]-1.789[4] A) that are consistent with bona fide

single bonds and confirm the description of the bridging

ligand as a fully reduced arene tetrathiolate, rather than

as a dithiolato-dithione species with corresponding Ni(I)

centers (Figure 2).

The structure of the mixed-metal compound 3e is

highly similar to that of 3d and, as would be expected a

priori, reveals a disorder of the Ni and Pd atoms over both

metal sites. When the description of the molecule is not

restricted to 1:1 distribution of each metal atom over the

two sites, the refinement optimizes at a 40:60 distribution.

In Figure 1, the Ni and Pd atoms are drawn in their

predominant positions. The position with majority occu-

pancy by Pd reveals somewhat less tetrahedralization

than the site that is predominantly Ni. A DFT computa-

tional analysis of 3d and 3e (vide infra) predicts greater

planarity at the Pd center, and this slight difference

between the two ends of the molecule is possibly the basis

by which the molecules find a way to pack themselves

with a slight ordering rather than with complete rando-

::

(55) Muresan, N.; Chlopek, K.; Weyhermuller, T.; Neese, F.; Wieghardt,

K. Inorg. Chem. 2007, 46, 5327–5337.

::

(56) Patra, A. K.; Bill, E.; Weyhermuller, T.; Stobie, K.; Bell, Z.; Ward,

M. D.; McCleverty, J. A.; Wieghardt, K. Inorg. Chem. 2006, 45, 6541–6548.

::

(57) Ray, K.; Bill, E.; Weyhermuller, T.; Wieghardt, K. J. Am. Chem. Soc.

2005, 127, 5641–5654.

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(58) Ray, K.; Weyhermuller, T.; Goossens, A.; Craje, M. W. J.; Wie-

ghardt, K. Inorg. Chem. 2003, 42, 4082–4087.

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(59) Herebian, D.; Bothe, E.; Neese, F.; Weyhermuller, T.; Wieghardt, K.

J. Am. Chem. Soc. 2003, 125, 9116–9128.

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(60) Sun, X.; Chun, H.; Hildenbrand, K.; Bothe, E.; Weyhermuller, T.;

Neese, F.; Wieghardt, K. Inorg. Chem. 2002, 41, 4295–4303.

Article

Inorganic Chemistry, Vol. 48, No. 22, 2009 10601

Figure 3. Thermal ellipsoid plot of dication [4c]^2þ, di-n-butyltin-protected compound 5d, trinuclear compounds 6a and 6b, and bis(disulfide) linked

compound 7d. Ellipsoid plots are drawn at the 50% level for [4c]^2þ, 5d, and 7d and at the 40% level for 6a and 6b. H atoms are omitted for clarity. For [4c]^2þ

and 5d (right side), phenyl and cyclohexyl groups on the phosphorus atoms are truncated for clarity.

offset face-to-face way and bridged via a Ag₂unit that

interacts with four arenethiolate sulfur atoms, two from

each of the contributing 3c halves (Figure 3, [4c]^2þ). This

hexametallic species is a dication with two corresponding

[BArF₂₄]^-counteranions. An inversion center occurs at

arenetetrathiolate unit as intended. Instead, the two Ag^þ

cations are stabilized in close proximity by thiophilic

interactions with the electron-rich reduced sulfur atoms.

The redox potential of the Ag^þ/Ag couple is well-known

to be a function of solvent donor ability.⁶¹In the presence

of soft donor groups such as those afforded by the

arenetetrathiolate ligand, it apparently is diminished

to a value that is smaller than the [3c]^þ/3c couple.

Structures analogous to [4c]^2þhave been observed with

the midpoint of the 2.996(1) A Ag Ag interatomic

3 3 3

separation such that only one silver atom and one of the

3c “halves” is crystallographically unique. The average

S-C and chelate C-C bond lengths in [4c]^2þ(1.764[4]

and 1.405[7] A, respectively, Table 3) are not significantly

different than those found in molecules 3abd (Table 2)

and indicate that electron transfer did not occur from the

similar Ag^þAg^þdistances, selected examples being

3 3 3

(61) Connelly, N. G.; Geiger, W. E. Chem. Rev. 1996, 96, 877–910.

10602 Inorganic Chemistry, Vol. 48, No. 22, 2009

Arumugam et al.

Table 3. Selected Averaged^aInteratomic Distances and Angles for [4c]^2þ

Table 4. Selected Averaged^aBond Distances (A) and Angles (deg) for Trimetallic

Compounds 6a and 6b

Ni(1)-S

Ni(1)-P

Ni(2)-S

2.155[1]

2.155[2]

2.170[1]

2.175[1]

2.9490[7]

2.441[1]

2.996(1)

1.764[4]

1.405[7]

8.439

S-Ni(1)-P_cis

91.05[6]

172.40[7]

92.80(8)

83.86(8)

91.54[6]

174.03[6]

61.05(3)

54.03(6)

54.80(6)

98.69(7)

99.00(7)

9.5

S-Ni(1)-P_trans

S(3)-Ni(2)-S(4)^b

P(3)-Ni(2)-P(4)^b

S-Ni(2)-P_cis

6a

6b

6a

6b

Ni(2)-P

Ni(2)-Ag

S-Ag

Ni_term-S^b

M-S^c

2.161[1] 2.151[2] S-Ni-S_term

2.141[1] 2.228[2] P-Ni-P

2.168[1] 2.159[2] S-Ni-P_cis

92.29[6] 92.67[9]

89.03[6] 88.3[1]

89.70[4] 89.97[8]

173.13[5] 172.51[9]

b

S-Ni(2)-P_trans

Ag(1)-Ni(2)-Ag(1A)^b

S(3)-Ni(2)-Ag(1)^b

S(4)-Ni(2)-Ag(1A)^b

S(3)-Ni(2)-Ag(1A)^b

S(4)-Ni(2)-Ag(1)^b

θ, Ni(1), deg.^d

Ni-P

Ag(1)-Ag(1A)^b

S-C

b

S-C_term

1.741[4] 1.716[6] S-Ni-P_trans

d

S-C_cent

1.730[4] 1.726[5] chelate S-M-S^c92.66[6] 89.80[8]

chelate C-C

chelate C-C_term^b1.393[6] 1.369[9] S-M-S_cis

87.37[6] 90.06[8]

174.34[6] 174.61[9]

c

Ni(1) Ni(2)

Ni(2) Ni(2A)

d

c

3 3 3

chelate C-C_cent1.395[6] 1.392[8] S-M-S_trans

5.080

0.101

0.024

Ni(1) Ni(2), A 16.5

16.6

θ, Ni(1), deg.^e

8.3

10.2

9.4

12.3

7.5

7.6

3 3 3

δ_m, Ni(1), A^c

δ_m, Ni(2), A^c

3 3 3

δ_m, Ni(1), A^f

δ_m, Ni(2), A^f

δ_m, M, A^c,e

0.090 0.133 θ, Ni(2), deg.^e

0.111 0.081 θ, M, deg.^c,f

0.086 0.052 β, deg.^g

τ, deg.^h

θ, Ni(2), deg.^d

5.5

τ, Ni(1), deg.^e

46.5

20.9

46.7

23.5

S(1)-Ni(1)-S(2)^b

P(1)-Ni(1)-P(2)^b

93.09(9)

85.44(9)

τ, Ni(2), deg.^e

7.9

0

j, deg.^f

^aUncertainties for averaged values are computed according to the

general formula for error propagation in a function of multiple variables

as described by Taylor, J. R. An Introduction to Error Analysis; Uni-

versity Science Books: Sausalito, CA, 1997, pp 73-77. ^bBond length or

angle for terminal nickel or nickel dithiolene chelate. ^cM = Ni(3) for 6a,

Pt(1) for 6b; ^dBond length or angle for central metal or metal dithiolene

chelate. ^eAngle between S₂Ni and P₂Ni or S₂M and S₂M planes. ^fMean

atom deviation from S₂NiP₂or S₄M plane. ^gAngle between S₂Ni(1)P₂

and S₂Ni(2)P₂mean planes. ^hEnd-to-end molecular twist, defined as the

P(1)-Ni(1)-Ni(2)-P(3) torsion angle.

^aUncertainties for averaged values are computed according to the

general formula for error propagation in a function of multiple variables

as described by Taylor, J. R. An Introduction to Error Analysis; Uni-

versity Science Books: Sausalito, CA, 1997; pp 73-77. ^bNon-averaged

value. ^cMean atom deviation from S₂NiP₂plane. ^dAngle between S₂Ni

and P₂Ni planes. ^eAngle between mean S₂NiP₂plane and mean S₂C₆S₂

plane. ^fAngle between mean S₂C₆S₂planes.

[[(Ph₃P)₂Pt(S₂C₁₀H₆)]₂(μ-Ag₂)]^{2þ 62}

and [[Cp*Ru(tpdt)]₂-

,

(μ-Ag₂)]^2þ(tpdt=3-thiapentane-1,5-dithiolate).⁶³

such example.²⁵The asymmetric unit for 5d is composed

of four independent molecules, two of which are com-

pletely noninteracting with anything and hold the tetra-

hedral Sn atoms approximately in the same plane as the

S₂C₆H₂S₂bridging ligand. The remaining two molecules

of 5d show a pronounced folding of the SnⁿBu₂fragment

Structural identification of [4c][BArF₂₄]₂clarified the

need for an innocent, outer-sphere electron transfer re-

agent for the preparation of [3d]^þ. Reaction of 3d with

[Cp₂Fe][BArF₂₄] as oxidizing agent resulted in immediate

darkening of color and formation of a product with

greater solubility than the charge-neutral starting com-

pound. Crystals grown by the vapor diffusion technique

were examined by X-ray diffraction and found to be the

desired monocationic species, [3d]^þ, without any perturb-

ing influences being exerted upon the tetrathiobenzene

unit. The averaged S-C and chelate C-C bond distances

for [3d]^þ(1.722[2] and 1.432[4] A, Table 2) are seen to be

shorter and longer, respectively, than the corresponding

values for 3d (1.789[4] and 1.387[6] A, Table 2) by a degree

that is significant within the resolution of the experimen-

tal data. Thus, the removal of an electron from a fully

reduced ene-1,2-dithiolate has occurred, with the result

that the S-C bonds now have partial thione character

while the C-C chelate bond lengths have some admixture

of single bond quality (d in Figure 2).

As noted earlier, compound 5d was examined as an

alternative synthon to compound 2d. Although a variety

of dithiolene ligands masked with dialkyl tin protecting

groups have been crystallographically identified,^64-665d

represents the first structural characterization of a bis-

(dithiolene) type molecule in which one dithiolene chelate

is coordinated to a transition metal while the other

remains protected with a dialkyl tin group and is available

for further chemistry. A molecule analogous to 5d,

Cp*Co(S₂C₆H₂S₂)SnⁿBu₂, has been recently described

upward along the S S interatomic axis of the two sulfur

3 3 3

atoms coordinated to tin. This different conformation is

due to a weak intermolecular interaction with the thiolate

sulfur of another molecule such that the tin atoms are

pseudo-pentacoordinate (Figure 3). These two conforma-

tions have been independently observed in different crys-

tal structures of R₂C₂S₂SnR⁰₂compounds^64-66but not

simultaneously within the same crystal structure. Selected

bond lengths and angles for 5d are presented in Support-

ing Information, Table S4.

Trimetallic compounds 6a and 6b, owing to solubility

limitations, could only be crystallized from DMF solu-

tions. The diffusion of various types of ethers into DMF

solutions readily formed large, dark, prism-shaped crys-

tals that displayed only weak diffraction because of a high

degree of solvent disorder. Both 6a and 6b solved in

rhombohedral space group R3 and are essentially iso-

morphous. To our knowledge, 6a and 6b comprise only

the second set of well characterized trimetallic molecules

in which metal atoms are linked with bis(dithiolene)

ligands. A pair of trimetallic anions with metal atoms

linked by tetrathiooxalate anions and capped with dithio-

lene end groups was recently described by Kato et al.¹³

Both molecules 6a and 6b display an appreciable end-to-

end bending of ∼46° (Table 4) in addition to a twisting or

corkscrewing of ∼20° along the 22 A length of the

molecule (Table 4). These deformations, somewhat sur-

prising in their magnitude, completely obviate any mo-

lecular center of symmetry.

ꢀ

by Nomura and Fourmigue and constitutes the only other

(62) Robertson, S. D.; Slawin, A. M. Z.; Woollins, J. D. Eur. J. Inorg.

Chem. 2007, 247–253.

(63) Shin, R. Y. C.; Tan, G. K.; Koh, L. L.; Vittal, J. J.; Goh, L. Y.;

Webster, R. D. Organometallics 2005, 24, 539–551.

(64) Allan, G. M.; Howie, R. A.; Skakle, J. M. S.; Wardell, J. L.; Wardell,

S. M. S. V. J. Organomet. Chem. 2001, 627, 189–200.

(65) Ma, C.; Han, Y.; Li, D. Polyhedron 2004, 23, 1207–1216.

Bis(disulfide)-linked compounds 7c and 7d, were iden-

tified in the course of exploring the Rxn III synthesis

for compounds 3. These compounds both crystallize

upon inversion centers and display parallel planar arene

ꢀ

(66) Chandrasekaran, P.; Arumugam, K.; Jayarathne, U.; Perez, L. M.;

Mague, J. T.; Donahue, J. P. Inorg. Chem. 2009, 48, 2103–2113.

Article

Inorganic Chemistry, Vol. 48, No. 22, 2009 10603

tetrathiolate groups that are separated by 2.06 A, the

length of the disulfide bonds. This configuration imposes

a chairlike conformation upon the central S₂C₂S₂C₂ring

(Figure 3). Various examples of purely organic bis-

(disulfide)-linked bis(arenes) have been reported,^67-72

but compounds 7 appear to be the only instances of

metal-containing molecules of this type. The intermetal

distance in 7d is 14.6 A, while the total end-to-end length

of the molecule is ∼21 A. In principle, compounds 7 could

serve as yet another form of protected nickel dithiolene

complex, analogous to compounds 2 and to 5d, but with

four reducing equivalents being required to liberate the

two halves as fully reduced ene-1,2-dithiolates. Selected

structural parameters for compound 7c and 7d are avail-

able in the Supporting Information, Table S6.

Electrochemistry. Mononickel mixed dithiolene and

bis(phosphine) complexes have been studied in some

detail and generally found to sustain a reversible Ni(II)

f Ni(I) reduction and, in the particular case of the

phenyl-substituted dithiolene ligand, a reversible oxida-

tion assigned as an ene-1,2-dithiolate to thioketyl ligand-

based oxidation.⁴⁰The less electron-rich S₂C₂(CN)₂and

tdt (tdt = toluene-3,4-dithiolate) ligands were not ob-

served to support reversible oxidation in [(P-P)Ni-

(S₂C₂R₂)] complexes.³⁹The known electron richness of

the tetrathioarene unit,^73,74manifested by the oxidation

waves at þ1.17 V and þ1.68 V that it reveals protected as

the tetra(isopropylthio)benzene (Figure 4), suggests at

the outset the possibility for reversible anodic electro-

chemistry. Cyclic voltammetry upon mononickel com-

pound 2d with [Bu₄N][PF₆] electrolyte reveals a single

reversible oxidation wave at 0.82 V, a potential 0.35 V less

oxidizing than the first feature observed for

1,2,4,5-(ⁱPrS)₄C₆H₂(Figure 4). If this oxidation in 2d is

indeed occurring at the tetrathioarene moiety, this shift in

potential indicates that the electron-withdrawing nature

of the carbonyl protecting group at the other end of the

arene ring is more than offset by an electron-donating

character of the (dcpe)Ni fragment. Differential pulse

voltammograms for 2a-2d are presented in Figure 5.

Somewhat surprisingly in light of the reversibility seen for

2d, compounds 2a-2c display no reversible behavior

in their cyclic voltammograms.

Figure 4. Cyclic voltammetry for l,2,4,5-(ⁱPrS)₄C₆H₂, [(dcpe)Ni-

(S₂C₆H₂S₂CO)] and [(dcpe)Ni(S₂C₆H₂S₂)Ni(dcpe)]. The solvent was

CH₂Cl₂for scans with [Bu₄N][PF₆] electrolyte and 5:4:1 CH₂Cl₂/ani-

sole/THF for scans with Na[BArF₂₄].

The dinickel compounds 3a-3d reveal increased redox

activity at milder potentials than for the corresponding

compounds 2a-d. Compound 3d, for example, shows

two fully reversible oxidations at 0.18 and 0.84 V (vs

Ag/AgCl) in CH₂Cl₂with [Bu₄N][PF₆] as supporting

Figure 5. Overlay differential pulse voltammograms for compounds

2a-2d and for the first oxidation wave for compounds 3a-3d. For

compounds 2, scans were run in CH₂Cl₂with 0.01 M Na[BArF₂₄]; only

theoxidationfor 2d is reversible. Forcompounds3, scans were run in 5:4:1

CH₂Cl₂/anisole/THF with Na[BArF₂₄].

(67) Lakshmikantham, M. V.; Raasch, M. S.; Cava, M. P.; Bott, S. G.;

Atwood, J. L. J. Org. Chem. 1987, 52, 1874–1877.

electrolyte (Figure 4, Table 5). The ΔE_1/2value of 0.66

V corresponds to a comproportionation constant, K_c, of

1.6 ꢀ 10¹¹for the [3d]^2þþ 3d T 2[3d]^þequilibrium.⁷⁵

Comproportionation constants of this magnitude are

attributed to a high degree of charge delocalization within

the mixed valence species. An assignment of the arene

tetrathiolate ligand as the redox active moiety is consis-

tent with this K_cvalue because the hole created by ligand

oxidation is distributed among the four sulfur atoms,

which are chemically indistinguishable.

(68) Chivers, T.; Parvez, M.; Vargas-Baca, I.; Schatte, G. Can. J. Chem.

1998, 76, 1093–1101.

(69) Ogawa, S.; Sugawara, M.; Kawai, Y.; Niizuma, S.; Kimura, T.; Sato,

R. Tetrahedron Lett. 1999, 40, 9101–9106.

(70) Kimura, T.; Mizusawa, S.; Yoneshima, A.; Ito, S.; Tsujimura, K.;

Yamashita, T.; Kawai, Y.; Ogawa, S.; Sato, R. Bull. Chem. Soc. Jpn. 2002,

75, 2647–2653.

(71) Yamamoto, T.; Ogawa, S.; Sugawara, M.; Kawai, Y.; Sato, R. Bull.

Chem. Soc. Jpn. 2006, 79, 460–467.

(72) Yearley, E. J.; Lippert, E. L.; Mitchell, D. J.; Pinkerton, A. A. Acta

Crystallogr., Section C. 2007, 63, o576–o577.

(73) Alberti, A.; Pedulli, G. F.; Tiecco, M.; Testaferri, L.; Tingoli, M.

J. Chem. Soc., Perkin Trans. II 1984, 975–979.

(74) Nabeshima, T.; Iwata, S.; Furukawa, N.; Morihashi, K.; Kikuchi, O.

Chem. Lett. 1988, 1325–1328.

(75) Richardson, D. E.; Taube, H. Inorg. Chem. 1981, 20, 1278–1285.

10604 Inorganic Chemistry, Vol. 48, No. 22, 2009

Arumugam et al.

Table 5. Electrochemical Data for Compounds 2a-d and 3a-d

solvent system for compounds 3a-d with Na[BArF₂₄] is

5:4:1 CH₂Cl₂/anisole/THF, conditions under which the

Fc^þ/Fc couple occurs at 0.30 V. Only 3d shows a second

oxidation that appears reversible and is stable to repeated

scanning. Figure 5 shows overlaid DPV for the first

oxidations in compounds 3a-d and reveals a similar

range in oxidation potentials as seen for the series of

mononickel compounds 2a-d. The mildest oxidation

potential is found for 3d, the compound with the most

electron-rich chelating diphosphine, while slightly stron-

ger oxidizing potentials are required to transform com-

pounds 3a-3c to their corresponding cations. Con-

sidering the appreciable difference in basicity between

dcpe and dppb, the relatively narrow range of 0.14 V in

potentials suggests that metal-based Ni^2þ/Ni^3þoxidations

do not occur. Were the Ni^2þcenters the site of oxidation,

they might be expected to reveal greater sensitivity to the

identity of the phosphine ligand.³⁷

Calculations and Absorption Spectra. To better under-

stand the deformations from idealized D_2hpoint group

symmetry in compounds 3 and 6, as well as to better see

the composition of the redox active and spectroscopically

active MOs, DFT calculations were undertaken using

simplified models for compounds 3 and 6 in which

monodentate phosphines, PH₃or PMe₃, replace the

chelating bis(phosphine) ligands. These calculations in-

dicate that the idealized D_2hgeometry for compounds 3 is

a second order saddle point on the energy surface of the

molecule. The molecule has an A_usymmetric vibration of

imaginary frequency, which carries it to a lower symmetry

D₂geometry, in which point group the highest occupied

molecular orbital (HOMO, (b_2g) and lowest unoccupied

molecular orbital (LUMO) (b_2u) both reduce to the b₂

symmetry species; as a consequence, configuration inter-

action occurs, and the HOMO is lowered in energy. An

imaginary frequency vibration of B_3gsymmetry similarly

carries the molecule to the C_2hpoint group, a symmetry in

which both the HOMO and LUMOþ1 belong to the b_g

irreducible representation and thus undergo a completely

analogous configuration interaction (Figure 6, (a)). Both

the D₂and C_2hgeometries are energy minima and are

lower in energy by 2.0 and 2.8 kcal/mol, respectively, than

the D_2hgeometry. These symmetry lowerings to D₂and

C_2h, which are instances of the second-order Jahn-Teller

effect,⁸¹are produced by localized tetrahedralizing dis-

tortions at the nickel centers and are to be distinguished

from the end-to-end folding observed in the structure of

3d, a distortion not reproduced by the calculations and

attributed instead to crystal packing (vide supra). Images

(b) in Figure 6 clarify the distinction between the D₂and

C_2hgeometries; the end-on view of the model compound

along the Ni-Ni axis shows the P-Ni-P planes to be

canted in opposite directions with respect to the S₂C₆H₂S₂

plane in D₂, while in C_2h, the P-Ni-P planes are canted

in the same direction, one eclipsing the other.

2a

2b

2c

2d

a

b

0 f 1þ

0.82 V

0.33 V

0.39 V^c

0.41 V^c

0.43 V^c

3a

3b

3c

3d

a

d

e

0 f 1þ

þ0.18 V

-0.23 V

-0.21 V

þ0.84 V

þ0.52 V

þ0.51 V

-0.14 V

-0.04 V

-0.11 V

-0.07 V

a

d

e

1þ f 2þ

0.55 V^c

-

0.68 V^c

f

^aData obtained by CV in CH₂Cl₂with 0.10 M Bu₄NPF₆electrolyte.

^bData obtained by DPV in CH₂Cl₂with 0.01M Na[BArF₂₄]. ^cIrrever-

sible. ^dData obtained by CV in 5:4:1 CH₂Cl₂:anisole:THF with 0.01 M

Na[BArF₂₄] electrolyte. ^eData obtained by DPV in 5:4:1 CH₂Cl₂:

anisole:THF with 0.10 M Bu₄NPF₆electrolyte. The Fc^þ/Fc couple

occurred at þ0.54 V (by CV) in CH₂Cl₂with 0.10 M Bu₄NPF₆

supporting electrolyte, at þ0.20 V (by DPV) in CH₂Cl₂with 0.01 M

Na[BArF₂₄], and at þ0.30 V (by CV) in 5:4:1 CH₂Cl₂:anisole:THF with

0.01 M Na[BArF₂₄] supporting electrolyte. ^fA consistent, reliable value

for a second oxidation could not be obtained under the same conditions

as used for 3a, 3c, and 3d.

Repeated scanning over a period of minutes leads to

appreciable degradation in the voltammogram of 3d and

eventually complete loss of observable current. This

deterioration in the voltammogram is not accompanied

by any change in the color or aspect of the analyte

solution. The foregoing observations are indicative of

sample degradation at the working electrode surface and

consequent passivation of the electrode. Consistent with

these observations, a quick removal and cleaning of the

electrode surface restores the voltammetry to its initial,

reversible-looking character.

Reactivity between solution-generated radical cations

and small electrolyte anions that can act as nucleophiles,

such as PF₆^-, BF₄^-, and ClO₄^-, has been noted by others

and circumvented by use of large, weakly coordinating

anions, such as Na[BArF₂₄], as electrolyte medium.^76-80

When this particular salt is used as supporting electrolye

in the voltammetry of 3d, the oxidation waves are shifted

some 0.30 V in the cathodic direction (Figure 4) and

rendered well-behaved and stable toward an indefinite

number of scans. A lack of solubility of compounds 3a-d

and the Na[BArF₂₄] electrolyte in a common solvent has

necessitated the use of a mixed solvent system. While

Na[BArF₂₄] is poorly soluble in CH₂Cl₂, it displays good

solubility in ethereal solvents such as anisole; the inverse

is true for compounds 3a-d. The use of modest amounts

of THF (e10%) was observed empirically to improve the

reversibility of the first oxidation wave in compounds

3a-d, probably because of a stabilizing donor effect

exerted by the THF solvent upon the radical monocation

that is generated by the first oxidation. Thus, an optimal

Geometry optimization of the nickel-palladium

bimetallic [(H₃P)₂Ni(S₂C₆H₂S₂)Pd(PH₃)₂] compound

(Figure 7, 3e⁰) converged to an energy minimum with a

nonplanar structure that closely approaches C₂symme-

try. The coordination geometry about palladium is nearly

perfectly planar, as is common for Pd^II. The geometry at

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E.; Mann, T. F.; Mann, D. R.; Gosser, D. K.; Doeff, M. M.; Bond, A. M.

J. Am. Chem. Soc. 1988, 110, 2109–2116.

(77) Hill, M. G.; Lamanna, W. M.; Mann, K. R. Inorg. Chem. 1991, 30,

4687–4690.

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250.

(79) Camire, N.; Nafady, A.; Geiger, W. E. J. Am. Chem. Soc. 2002, 124,

7260–7261.

ꢁ

(80) Barriere, F.; Geiger, W. E. J. Am. Chem. Soc. 2006, 128, 3980–3989.

(81) Bersuker, I. B. Chem. Rev. 2001, 101, 1067–1114.

Article

Inorganic Chemistry, Vol. 48, No. 22, 2009 10605

Figure 8. Composition ofthe HOMOand LUMO for model compound

[(PH₃)₂Ni(S₂C₆H₂S₂)Ni(PH₃)₂in both the D_2hand D₂point groups.

Contour surfaces are drawn at the 0.03 level.

Figure 6. (a) Illustration of model compound [(PH₃)₂Ni(S₂C₆-

H₂S₂)Ni(PH₃)₂] used in DFT calculations for compounds 3 and the

molecular vibrations that result in lower symmetry through configuration

interaction. (b) End-on views of the D₂and C_2hsymmetric compounds for

emphasis of their difference.

consequences of ligand-field strength, model complexes

were optimized with terminal trimethylphosphine ligands

on Ni and Pd. Structural depictions and optimized co-

ordinates are deposited as Supporting Information. A D₂-

symmetric local minimum was located for [(Me₃P)₂-

Ni(S₂C₆H₂S₂)Ni(PMe₃)₂]. In this structure, the P-Ni-P

planes are canted by 36.0° relative to the S-Ni-S planes,

compared to 27.0° in the PH₃-terminated model. A

similar result is obtained in the bimetallic model complex

[(Me₃P)₂Ni(S₂C₆H₂S₂)Pd(PMe₃)₂]. Again, the palladium

site is planar, but nickel adopts a geometry that is inter-

mediate between planar and tetrahedral. Here, the

S-S-P-P dihedral angle is 35.9°, compared to 26.8° in

3e⁰. The corresponding dihedral angle at the palladium

center is 0.95°.

The HOMO and LUMO for the [(PH₃)₂Ni-

(S₂C₆H₂S₂)Ni(PH₃)₂] model compound are shown in

both D_2hand D₂point groups and are seen to have

essentially the same composition in both symmetries

(Figure 8). The calculations unambiguously show the

HOMO to be primarily constituted of the bridging tetra-

thioarene ligand. Furthermore, the nature of the HOMO

is antibonding with respect to the C-S bond and bonding

with respect to the chelate C-C bond. Removal of an

electron from this MO would shorten and lengthen,

respectively, the C-S and C-C bond lengths. These

changes have been observed in the crystallography of 3d

and [3d]^þ(Table 3) and validate the use of

[(PH₃)₂Ni(S₂C₆H₂S₂)Ni(PH₃)₂] as a model compound

to determine the essential aspects of the electronic struc-

ture of compounds 3.

Figure 7. Model compound used for calculations on mixed metal

compound 3e.

nickel is distorted as in the dinickel complexes. The Ni

coordination geometry is again intermediate between

square planar and tetrahedral. The P-P-S-S dihedral

angle at nickel is 26.8°; the opposite angle at palladium is

1.1°. Geometry optimization in full C_2vsymmetry (that is,

with all non-H atoms rigidly coplanar) converges to a

first-order saddle point with an A₂vibration of imaginary

frequency. This normal mode is effectively localized at

nickel, and carries the metal center out of a planar

configuration toward a tetrahedral structure.

In planar (C_2v) 3e⁰, the HOMO has b₁symmetry and the

LUMO is b₂. The imaginary-frequency A₂vibration

degrades the overall symmetry to C₂, wherein both orbi-

tals transform as b. In planar 3e⁰, the HOMO-LUMO

separation is 0.896 eV, a small energy gap that favors

orbital mixing by configuration interaction. Planar 3e⁰

also undergoes a second-order Jahn-Teller distortion

that drives it toward a nonplanar geometry. The two

metal atoms contribute equally to the electron density of

the HOMO in the planar structure: 4.5% and 4.6% for Ni

and Pd, respectively. The LUMO has greater nickel

character: 28.3% Ni compared to 7.4% Pd. Upon dis-

torting to C₂symmetry, the HOMO gains Ni character at

the expense of the LUMO. The HOMO has 8.7% Ni and

3.8% Pd electron density; the LUMO, 19.1 and 13.9%,

respectively. Distortion from (planar) C_2vto (nonplanar)

Monocation [3d]^þwas generated in solution from 3d by

controlled potential electrolysis, and its UV-vis-near IR

spectrum recorded. Figure 9 displays the absorption

spectra for 3d and [3d]^þfor the 300-1400 nm region.

The oxidation of 3d results in the appearance of new,

broad absorptions at ∼980 and 1127 nm. These spectral

C₂symmetry stabilizes 3e⁰by 1.8 kcal mol^-1

.

That the planar f nonplanar distortion localizes at

nickel manifests the weaker ligand-field of the first-row

metal and the predominance of a low-spin (square-planar

and diamagnetic) ground state for palladium(II).⁸²The

phosphorus ligands in model complex 3e⁰are PH₃, and

the parent phosphine is one of the least nucleophilic

phosphine ligands.⁸³To investigate the structural

features contrast with the absorption spectrum of [4c]^2þ

,

which shows no absorption at energy lower than 500 nm

as would be expected for a mixed-valence species.

Although time-dependent DFT (TD-DFT) calculations

for [3d]^þwere not undertaken owing to the difficulties of

attaining convergence with open shell systems, the quali-

tative appearance of the HOMO for 3d (Figure 8) sug-

gests that at least one of the low energy transitions for

[3d]^þis π f π* in nature and involves a singly occupied

(82) Figgis, B. N.; Hitchman, M. A. Ligand Field Theory and Its Applica-

tions; Wiley-VCH: New York, 2000.

(83) Tolman, C. A. Chem. Rev. 1977, 77, 313–348.

10606 Inorganic Chemistry, Vol. 48, No. 22, 2009

Arumugam et al.

Figure 12. Analogy between mononuclear nickel bis(dithiolene) com-

plexes, with two oxidized ligands (a) and 6b, with two oxidized dithio-lene

chelates (b). Fully reduced thiolate sulfur is represented as S^-.

composed of the central bis(dithiolene) portion of the

molecule. By analogy to neutral mononuclear nickel bis-

(dithiolene) compounds, which have two thienyl radical

monoanionic ligands with paired spins, these dithiolene

ligands coordinating the center metal atom in 6a and 6b

are where the dithiolene radical character should pre-

dominate (Figure 12). The HOMO-1 donor orbital is a

π type orbital to which each of the eight sulfur atoms of

the molecule appear to contribute equally, suggesting that

this HOMO-1 f LUMO transition has S^-f S^•. char-

acter. Thus, the electronic absorption spectroscopy of 6a

is more closely related to [3d]^þthan to neutral 3d, which

contains only fully reduced thiolate-type sulfur ligands.

Summary and Conclusions. Much of the synthetic work

reported here involving the di- and trimetallic compounds

with the tetrathioarene unit as bridging ligand hinges

upon the controlled deprotection of one of two protected

dithiolene chelates in a bis(dithiolene)-type ligand. Both

carbonyl and di-n-butyl tin protecting groups have been

employed, although the latter is preferred for the cleaner

products that result with ensuing reactions. Trimetallic

compounds 6a and 6b are the first bis(dithiolene)-linked

trimetallic compounds produced by deliberate synthesis

and structurally identified by X-ray crystallography.

The success of the synthetic methods reported here

demonstrates the possibility that they can be applied

another iteration for the preparation of metallodithiolene

species with metal nuclearity greater than three. The

known nickel bis(dithiolene) complex [Ni(S₂C₆H₂S₂-

CdO)₂]⁸⁴is a metal-expanded form of 1,3,5,7-tetrathia-

s-indacene-2,6-dione (Scheme 1) and, in principle, subject

to a controlled monodeprotection protocol. Subsequent

steps analogous to those illustrated in Scheme 1 would

lead to a penta-nickel product molecule that is similar to

6a but expanded by two [Ni(S₂C₆H₂S₂)] units. However, a

successful extension of this chemistry may require the use

of tetrathioarenes and/or chelating bis(phosphine) end

groups that are functionalized with more soluble substit-

uents. Two advantages are inherent to this method of

synthesis and warrant emphasis. First, the approach is

convergent in nature since separate, well-defined building

blocks are taken together to produce larger, more com-

plex assemblies. Second, the method of synthesis is such

as to allow the incorporation of metals other than Ni,

which may enable some degree of control over the proper-

ties of the resulting material. For example, branched

structures can be imagined by the inclusion of Group 6

Figure 9. UV-vis near IR spectra (CH₂CI₂) for 3d (blue) and electro-

generated [3d]^þ(red).

Figure 10. UV-vis and near IR spectra in DMF solution for com-

pounds 6a (red) and 6b (blue).

Figure 11. One of the TD-DFT calculated electronic transitions occur-

ring in the near IR region. Contour surfaces for the model compound

[[(PH₃)₂Ni(S₂C₆H₂S₂)]₂Ni] are drawn at the 0.03 level.

molecular orbital (SOMO) acceptor orbital with signifi-

cant admixture of sulfur p orbitals.

Compounds 6a and 6b also display low energy transi-

tions in the near IR in DMF solution (Figure 10), a

contrast to charge-neutral compounds 3. These absorp-

tions are somewhat less broad and more intense than

those observed for [3d]^þ(Figure 9). Being a closed shell

compound, 6a was amenable to TD-DFT calculations. A

model compound with terminal PH₃ligands in place of

the chelating bis(phosphine) was chosen. Again, the

frontier MOs are ligand-based (Figure 11). One of the

allowed transitions is a HOMO-1 to LUMO excitation

that is calculated to occur at 0.99 eV (∼1250 nm). The

LUMO acceptor orbital is π* in nature and largely

(84) Mizuno, M. Synth. Met. 1993, 55-57, 2154–2157.

Article

Inorganic Chemistry, Vol. 48, No. 22, 2009 10607

metals like Mo or W, which prefer trigonal prismatic

coordination geometries with dithiolene ligands.

S₂C₆H₂S₂ligand. Another, very useful diagnostic for

distinguishing metal-based versus ligand-based mixed

valency in oxidized forms of these tetrathioarene com-

pounds is near IR spectroscopy, which is an energy region

in which transition(s) are operative to a low-lying π*

acceptor orbital containing an unpaired spin delocalized

throughout the tetrathiobenzene bridging ligand.

Continuing work by our groups will extend these

exploratory syntheses and physical studies to more com-

plex, higher nuclearity metallodithiolene systems.

As shown in our crystallographic studies, the tetra-

thioarene-linked di- and trimetallic compounds are not

strictly rigid but instead able to tolerate appreciable

distortions from planarity. Compounds 3d and 3e show

modest end-to-end folding in the crystalline state, which

is attributed to crystal packing effects, while trimetallic

compounds 6a and 6b reveal a surprising degree of

twisting (20°) and end-to-end bending (46°), which shows

they are by no means rigid, rod-like molecules but more

like flexible dumbbells. These deformation types ob-

served in the crystalline state are undoubtedly sampled

to some extent in solution as well and are a consideration

to bear in mind in understanding their solution spectra.

Our collective electrochemical data for compounds

2a-2d and 3a-3d are consistent and clear in pointing

toward the tetrathioarene unit as the redox active entity

within these molecules. Key observations include the

Acknowledgment. The Petroleum Research Fund is

thanked for support to J.P.D. (Grant 45685-G3) and

T.G.G. (Grant 42312-G3). Support from the National

Science Foundation (Grant CHE-0749086 to T.G.G.;

Grant CHE-0845829 to J.P.D.) is gratefully acknowl-

edged. T.G.G. is an Alfred Sloan Foundation Fellow.

The Louisiana Board of Regent is thanked for the sup-

port with which Tulane University’s CCD diffractometer

was obtained (Grant LEQSF-(2002-03)-ENH-TR-67),

and Tulane University is thanked for its ongoing support

of the X-ray diffraction laboratory. Prof. Daniel G.

Nocera is thanked for access to a near IR spectropho-

tometer. Prof. Bill Geiger of the University of Vermont is

thanked for helpful discussion regarding aspects of the

electrochemical studies reported here.

-

sensitivity of compounds 3 toward the PF₆electrolyte

anion when cyclic voltammetry in the anodic direction is

performed and, in contrast, the well-behaved reversible

behavior observed when an electrolyte with the weakly

coordinating BArF₂₄^-anion is employed. To the best of

our knowledge, previous transition metal systems

involving the use of the 1,2,4,5-benzenetetrathiolate

bridging ligand have not displayed redox processes that

were attributed to the tetrathioarene unit. Clearly, the

work presented here emphasizes that the possibility for

ligand-based redox chemistry in any tetrathioarene-

bridged dimetal system must be carefully examined.

The foregoing interpretation regarding ligand redox

noninnocence in compounds 3 is confirmed by structural

data for [3d^þ] and by the DFT-calculated electronic

structure of a model for compounds 3, which reveals the

HOMO to be constituted primarily of the bridging

Supporting Information Available: Details pertaining to

crystal growth, X-ray diffraction data collection, structure

solution and refinement; selected structural parameters for all

compounds (Tables S1-S6); full crystallographic details for

all structurally characterized compounds in CIF format;

thermal ellipsoid plots of all compounds with full atomic

labeling; computational details; atomic coordinates for all

geometry optimized model compounds (Tables S7-S12). This

material is available free of charge via the Internet at http://

pubs.acs.org.

Article Doi

Synthesis, Structures, and properties of 1,2,4,5-benzenetetrathiolate linked group 10 metal complexes

DOI: 10.1021/ic901257s

Source and publish data:

Authors:

Article abstract of DOI:10.1021/ic901257s

Full text of DOI:10.1021/ic901257s

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