Molybdenum and tungsten η2-alkyne-1-thio complexes acting as sulfur donors in homoleptic werner type complexes with nickel(II) and palladium(II)

Article abstract of DOI:10.1021/ic052178y

Facile access to the η²-alkyne-1-thio complexes [Tp‰M(CO)₂{η²-(BnS)CC(S)}] (Tp′ = hydrotris(3,5-dimethylpyrazolyl)-borate; Bn = benzyl; M = Mo, W) by reductive removal of one benzyl group in the corresponding bis(benzylthio)-acetylene complexes, [Tp′M(CO)₂{η²-(BnS)CC(SBn)}](PF ₆), has been thoroughly investigated. Experimental evidence of the intermediates, [Tp′M(CO)₂{η²-(BnS)CC(SBn)}] (M = Mo, W), and the fate of the cleaved benzyl group by isolation of the byproduct, [Tp′W(CO){C(O)Bn}{η²-(BnS)CC(SBn)}], is provided. Neutral η²-alkyne-1-thio complexes [Tp′M(CO)₂{η ²-(BnS)CC(S)}] bearing a free terminal sulfur atom have been established as monodentate ligands L in homoleptic pentanuclear [M′L ₄]²⁺ complexes with nickel(II) and palladium(II). Comparison of the NMR and IR spectroscopic as well as cyclovoltammetric data of the heterobimetallic complexes with the free thio-alkyne complexes reveals a strong electronic coupling of the redox-active η²-CC-bound metal centers and the sulfur-coordinated metal ion.

Full text of DOI:10.1021/ic052178y

Inorg. Chem. 2006, 45, 4791−4800
Molybdenum and Tungsten
η
2-Alkyne-1-thio Complexes Acting as Sulfur
Donors in Homoleptic Werner Type Complexes with Nickel(II) and
Palladium(II)
Wolfram W. Seidel,* Maria D. Ibarra Arias, Markus Schaffrath, Mareike C. Jahnke,
Alexander Hepp, and Tania Pape
Institut fu¨r Anorganische und Analytische Chemie, Westfa¨lische Wilhelms-UniVersita¨t Mu¨nster,
Corrensstrasse 30/36, 48149 Mu¨nster, Germany
Received December 22, 2005
Facile access to the
borate; Bn benzyl; M
acetylene complexes, [Tp
of the intermediates, [Tp
isolation of the byproduct, [Tp
[Tp
M(CO)2{η2-(BnS)CC(S)
L in homoleptic pentanuclear [M
η
2-alkyne-1-thio complexes [Tp
Mo, W) by reductive removal of one benzyl group in the corresponding bis(benzylthio)-
M(CO)2{η2-(BnS)CC(SBn)
](PF6), has been thoroughly investigated. Experimental evidence
M(CO)2{η2-(BnS)CC(SBn)
] (M Mo, W), and the fate of the cleaved benzyl group by
W(CO) ], is provided. Neutral
C(O)Bn}{η2-(BnS)CC(SBn) 2-alkyne-1-thio complexes
′ }] (Tp′ ) hydrotris(3,5-dimethylpyrazolyl)-
M(CO)2{η2-(BnS)CC(S)
)
)
′
′
}
}
)
′
{
}
η
′
}
] bearing a free terminal sulfur atom have been established as monodentate ligands
+
′
L4]2 complexes with nickel(II) and palladium(II). Comparison of the NMR and IR
spectroscopic as well as cyclovoltammetric data of the heterobimetallic complexes with the free thio-alkyne complexes
reveals a strong electronic coupling of the redox-active
metal ion.
η
2-CC-bound metal centers and the sulfur-coordinated
Introduction
complexes because of the high degree of metal discrimination
of the two different donors.
Cooperative phenomena between two or more electroni-
cally related metal ions are attracting enormous attention in
current coordination chemical research. The continuing
interest can be attributed to the fact that metallo-enzyme
activity is frequently based on bimetallic reactivity patterns.
The profound understanding of electronic and magnetic
exchange interactions through or across different bridging
ligands is fundamental to the search for cooperative reactiv-
ity. Our approach is directed toward linkage of organome-
tallic complex centers with predominantly sulfur-coordinated
metal ions. This can be achieved by the use of either alkyne-
1-thiolato or alkyne-1,2-dithiolato ligands, which can bind
one metal center by the triple bond and the other by
coordination through sulfur donor atoms. In contrast to the
more-rigid sulfide bridging of two metal centers, the η2-CC-
η1-S linkage by an alkyne-1-thiolato unit leads to bimetallic
complexes with more structural flexibility. Additionally, the
approach establishes a general method for combining several
organometallic components with a variety of Werner type
First reports on alkyne-1-thiolato ligands indicated the
tendency of sulfur to govern the coordination behavior even
in the formation of dinuclear complexes.1-3 Bis(alkyne-
thiolato)titanocene complexes, for example, have been shown
to bind Ni(0) at the sulfur atoms and not at the triple bond.4
Additionally, some investigations disclosed the propensity
of alkyne-sulfur bonds to cleave in the coordination sphere
of metal complexes.5-7 Therefore, our approach is directed
at the preceding formation of stable four-electron-donor
alkyne complexes with bis(benzylthio)acetylene and subse-
quent reductive removal of the benzyl groups in the complex.
(1) Weigand, W. Z. Naturforsch. 1991, 46b, 1333-1337.
(2) Weigand, W.; Weisha¨upl, M.; Robl, C. Z. Naturforsch. 1996, 51b,
501-505.
(3) Ara, I.; Delgado, E.; Fornies, J.; Hernandes, E.; Lalinde, E.; Mansilla,
N.; Moreno, M. T. J. Chem. Soc., Dalton Trans. 1998, 3199-3208.
(4) Sugiyama, H.; Hayashi, Y.; Kawaguchi, H.; Tatsumi, K. Inorg. Chem.
1998, 37, 6773-6779.
(5) Weigand, W.; Robl, C. Chem. Ber. 1993, 126, 1807-1809.
(6) Alcalde, M. I.; Carty, A. J.; Chi, Y.; Delgado, E.; Donnadieu, B.;
Hernandez, E.; Dallmann, K.; Sanches-Nieves, J. J. Chem. Soc., Dalton
Trans. 2001, 2502-2507.
(7) Sunada, Y.; Hayashi, Y.; Kawaguchi, H.; Tatsumi, K. Inorg. Chem.
2001, 40, 7072-7078.
* To whom correspondence should be addressed. E-mail:
seidelww@uni-muenster.de.
10.1021/ic052178y CCC: $33.50
Published on Web 05/17/2006
© 2006 American Chemical Society
Inorganic Chemistry, Vol. 45, No. 12, 2006 4791

Seidel et al.
Chart 1
standard methods and freshly distilled prior to use. PhCH2-
SCCSCH2Ph,15 [Tp′Mo(CO)3],16 [Tp′W(CO)3]17 (Tp′ ) Hydrotris-
(3,5-dimethylpyrazolyl)borate), [Ni(CH3CN)6](BF4)2,18 and C8K19
were prepared according to literature methods. [Pd(CH3CN)4](BF4)2
was purchased from Aldrich. 1H and 13C NMR spectra were
recorded on Bruker AC 200, Bruker Avance 400, and Varian Unity
plus 600 NMR spectrometers. Elemental analyses were performed
on a Vario EL III CHNS elemental analyzer. ESI mass spectra were
obtained using a QUATTRO LCZ (Waters-Micromass). Infrared
spectra were recorded on a Bruker Vector 22 and UV-vis spectra
on a Cary 50 Varian spectrophotometer. Cyclic voltammograms
were acquired on an ECO/Metrohm PGSTAT 30 potentiostat using
a glassy carbon working electrode, a Ag/0.01 M AgNO3/CH3CN
reference electrode, (NBu4)(PF6) as the supporting electrolyte, and
ferrocene as the internal standard.
The resulting metal-stabilized alkynethiolates can finally be
reacted with thiophilic transition-metal ions. Connor and
Angelici demonstrated that the coordination behavior of bis-
(methylthio)acetylene is determined by the triple bond and
that heterobimetallic complexes are accessible by subsequent
sulfide coordination.8,9 However, depending on the particular
metal center, the sulfide function can compete as a donor
and the alkyne-sulfur bond is likewise sensitive to bond
cleavage.10
[Tp′Mo(CO)2{η2-(BnS)CC(SBn)}](PF6) 1a. A solution of 6.92
g (14.5 mmol) of [Tp′Mo(CO)3] and 3.92 g (14.5 mmol) of bis-
(benzylthio)acetylene in 150 mL of dichloromethane was treated
with 4.8 g (14.5 mmol) of [(C5H5)2Fe](PF6). CO evolution was
observed while the solution turned dark green. After 3 h, the solvent
was removed in vacuo, and the residue was washed three times
with 50 mL of diethyl ether. The residue was dissolved again in
60 mL of dichloromethane and filtered through a Celite pad. 1a
was crystallized by diffusion of diethyl ether into a solution of the
complex in dichloromethane. Yield: 8.7 g (10 mmol, 69%), olive
Alkyne complexes with a terminal sulfur atom have been
synthesized in W(II) and Mo(II) complexes by coupling of
either intermediate carbyne and CS ligands11,12 or carbyne
and CO ligands and subsequent reaction with Lawesson’s
reagent.13 The η2-CC binding in the resulting thio-alkyne
complexes is best described by superposition of two reso-
nance forms underlining either the thioketenyl character
(Chart I, form A) or the rather zwitterionic alkyne complex
form (Chart I, form B). However, the nucleophilicity at the
sulfur atom, which is particularly evident in form B, has not
been explored with regard to coordination chemistry.
We have recently reported on a heterodinuclear complex
of [Tp′W(CO)2{η2-(BnS)CC(S)}] (Tp′ ) hydrotris(3,5-
dimethylpyrazolyl)borate, Bn ) benzyl hereafter) with Cu-
(I) that exhibits the η2-CC -η1-S bridging mode of an alkyne-
1-thio ligand.14 Herein, we report on cationic bis(benzylthio)-
acetylene complexes and neutral η2-CC-alkyne-1-thio com-
plexes with the [Tp′M(CO)2] fragment (M ) MoII, WII),
comprising a thorough investigation of the conversion
reaction and disclosure of the unexpected fate of the cleaved
benzyl group. Finally, we give an account of the coordination
behavior of [Tp′M(CO)2{η2-(BnS)CC(S)}] (M ) Mo, W)
with Ni(II) and Pd(II). We present molecular structures in
the solid state at each level of the synthetic scheme, NMR
analyses regarding the dynamic behavior of the η2-CC-
alkyne-1-thio complexes in solution, and electrochemical
studies with the heterobimetallic pentanuclear [Tp′M(CO)2-
{η2-(BnS)CC(S)}]4M′(BF4)2 complexes (M ) Mo, W; M′
) Ni, Pd).
1
crystals. H NMR (CDCl3): δ 7.41-7.28 (m, 8 H, Ph-H), 6.88
(m, 2 H, Ph-H), 6.03 (s, 3 H, CH(CCH3)2), 4.91 (s, 2 H, SCH2),
3.11 (s, 2 H, SCH2), 2.62 (3 H, CH3), 2.51 (6 H, CH3), 2.43 (3 H,
CH3), 1.65 (s, 6 H, CH3). 13C NMR (CDCl3): δ 227.0 (MoCSCH2),
215.4 (CO), 205.8 (MoCSCH2), 153.5, 151.3, 149.0, 147.0 (CCH3),
133.9, 132.3 (Ph-Cipso), 129.1, 129.0, 128.8, 128.7, 128.6 (Ph-C,
one resonance is not resolved), 109.3, 107.7 (CH(CCH3)2), 44.8,
42.1 (SCH2), 15.6, 14.1, 13.0, 12.5 (CH3). IR (KBr, cm-1): ν 2051,
2000 (CO), 842 (PF6).
[Tp′W(CO)2{η2-(BnS)CC(SBn)}](PF6) 1b. Tungsten complex
1b was synthesized by a method analogous to that used for 1a using
8.2 g (14.5 mmol) of [Tp′W(CO)3]. Yield: 8.9 g (9.34 mmol, 64%),
1
green crystals. H NMR (CDCl3): δ 7.42-7.25 (m, 8 H, Ph-H),
6.87 (m, 2 H, Ph-H), 6.13 (s, 3 H, CH(CCH3)2), 4.87 (s, 2 H, SCH2),
3.11 (s, 2 H, SCH2), 2.65 (s, 3 H, CH3), 2.52 (s, 6 H, CH3), 2.35
(s, 3 H, CH3), 1.80 (s, 6 H, CH3). 13C NMR (CDCl3): δ 219.0
(1JWC ) 50 Hz, WCSCH2), 213.8 (1JWC ) 137 Hz, WCO), 193.6
(WCSCH2), 154.8, 152.5, 149.8, 147.2 (CCH3), 134.2, 132.7 (Ph-
Cipso), 129.0, 129.0, 128.8, 128.7, 128.6, 128.5 (Ph-C), 110.0, 108.2
(CH(CCH3)2), 43.6, 41.6 (SCH2), 16.2, 14.8, 13.0, 12.4 (CH3). IR
(KBr, cm-1): ν 2041, 1980 (CO), 840 (PF6).
[Tp′Mo(CO)2{η2-(BnS)CC(S)}] 3a. At -78 °C, 510 mg (3.8
mmol) of C8K was added to a solution of 2.77 g (3.2 mmol) of 1a
in 100 mL of THF. While the solution was stirred for one minute,
the color turned from green to red; the formation of 2a was checked
by IR spectroscopy (CO absorptions at 1887 and 1970 cm-1). The
solution was heated to 40 °C for approximately 3 days until the
disappearance of 2a was complete, as indicated by IR spectroscopy.
The solvent was removed in vacuo, and the product was purified
by column chromatography using toluene as eluent. Yellow-brown
Experimental Section
General Procedures. All operations were carried out under a
dry argon atmosphere using standard Schlenk and glovebox
techniques. All solvents were dried and saturated with argon by
(8) Connor, J. A.; Hudson, G. A. J. Organomet. Chem. 1978, 160, 159-
164.
(9) Miller, D. C.; Angelici, R. J. J. Organomet. Chem. 1990, 394, 235-
249.
(15) Klein, T. R.; Bergemann, M.; Yehia, N. A. M.; Fangha¨nel, E. J. Org.
Chem. 1998, 63, 4626-4631.
(10) Miller, D. C.; Angelici, R. J. Organometallics 1991, 10, 79-89.
(11) Mayr, A.; Lee, T.-Y. Angew. Chem. 1993, 105, 1835-1837; Angew.
Chem., Int. Ed. 1993, 32, 1726-1728.
(16) Shiu, K.-B.; Lee, L.-Y. J. Organomet. Chem. 1988, 348, 357-360.
(17) Philipp, C. C.; White, P. S.; Templeton, J. L. Inorg. Chem. 1992, 31,
3825-3830.
(12) Lee, T.-Y.; Mayr, A. J. Am. Chem. Soc. 1994, 116, 10300-10301.
(13) Hill, A. F.; Malget, J. M.; White, A. J. P.; Williams, D. J. J. Chem.
Soc., Chem. Commun. 1996, 721-722.
(18) Heintz, R. A.; Smith, J. A.; Szalay, P. S.; Weisgerber, A.; Dunbar, K.
R. Inorg. Synth. 2002, 33, 75-79.
(19) Cotton, F. A.; Hillard, E. A.; Murillo, C. A.; Wang, X. Inorg. Chem.
2003, 42, 6063-6070.
(14) Seidel, W. W.; Ibarra Arias, M. D.; Schaffrath, M.; Bergander, K.
Dalton Trans. 2004, 2053-2054.
4792 Inorganic Chemistry, Vol. 45, No. 12, 2006

η2-Alkyne-1-thio Complexes as Sulfur Donors
crystals of 3a were obtained by slow evaporation of dichlo-
romethane from a dichloromethane/n-hexane solution. Yield: 0.9
g (1.43 mmol, 45%). Anal. Calcd for C26H29BN6O2S2Mo: C, 49.69;
15.1, 12.8, 12.7, 12.2 (CH3). IR (KBr, cm-1): ν 1899 (s, CO), 1576
(m, BnCO). IR (THF, cm-1): ν 1923 (s, CO), 1574 (m, BnCO).
[{Tp′(CO)2Mo-η2-(BnS)CC(S)}4Ni](BF4)2 5a. Under strict anaer-
obic conditions, 150 mg of 3a (0.24 mmol) was added to a solution
of 29 mg (0.06 mmol) of [Ni(CH3CN)6](BF4)2 in 15 mL of
dichloromethane. The light blue solution of [Ni(CH3CN)6](BF4)2
turned red immediately, and the complex formation was checked
by IR spectroscopy. After 1 h of being stirred, the solution was
reduced in volume and complex 5a was isolated by gas-phase
diffusion of diethyl ether in a dichloromethane solution of 5a.
Yield: 155 mg (93%), red solid. 1H NMR (CDCl3): δ 7.31 (m, 12
H, Ph-H), 6.90 (m, 8 H, Ph-H), 5.96 (s, 4 H, CH(CCH3)2), 5.90 (s,
8 H, CH(CCH3)2), 4.84 (s, br, 4 H, BH), 3.21 (s, 8 H, SCH2), 2.55
(s, 12 H, CH3), 2.48 (s, 24 H, CH3), 2.38 (s, 12 H, CH3), 1.53 (s,
24 H, CH3). 13C NMR (CDCl3): δ 238.3 (MoCSCH2), 219.5 (CO),
219.2 (MoCSNi), 153.5, 151.4, 147.8, 146.2 (CCH3), 134.2, 129.1,
129.0, 128.3 (Ph-C), 108.7, 107.2 (CH(CCH3)2), 41.2 (SCH2), 15.5,
14.2, 13.0, 12.7 (CH3). IR (CH2Cl2, cm-1): ν 2038, 1976 (s, CO).
[{Tp′(CO)2W-η2-(BnS)CC(S)}4Ni](BF4)2 5b. The correspond-
ing tungsten complex 5b was synthesized by a method analogous
to that used for 5a using 173 mg (0.24 mmol) of 3b. Yield: 165
1
H, 4.65; N, 13.37. Found: C, 49.43; H, 4.75; N, 13.20. H NMR
(CDCl3, 213 K): δ (two isomers) 7.41 (m, 1 H, Ph-H), 7.40-7.24
(m, 3 H, Ph-H), 6.99 (m, 1 H, Ph-H), 5.93 (s, 2 H, CH(CCH3)2),
5.82 (s, 1 H, CH(CCH3)2), 5.16 (approximately 1 H, 52% SCH2),
4.75 (s, br, 1 H, BH), 3.26 (approximately 1 H, 48% SCH2), 2.69
(s, approximately 1.5 H, CH3), 2.65 (s, approximately 1.5 H, CH3),
2.47 (s, 3 H, CH3), 2.42 (s, 3 H, CH3), 2.37 (s, approximately 1.5
H, CH3), 2.34 (s, approximately 1.5 H, CH3), 1.74 (s, 3 H, CH3),
1.58 (s, 3 H, CH3). 13C NMR (CDCl3, 213 K): δ (two isomers)
255.5, 251.6 (MoCSCH2), 227.5, 226.3 (CO), 223.7, 222.5 (MoCS),
154.7, 154.5, 151.4, 150.7, 147.0, 146.8, 145.6, 145.2 (CCH3),
137.5, 135.2, 130.1, 129.7, 129.4, 129.2, 128.3, 127.6 (Ph-C), 108.6,
108.5, 107.2, 107.0 (CH(CCH3)2), 41.2, 38.7 (SCH2), 16.1, 16.0,
15.0, 14.9, 14.8, 14.7, 13.8, 12.8 (CCH3). IR (KBr, cm-1): ν 2013,
1952 (s, CO). IR (CH2Cl2, cm-1): ν 2012, 1941 (s, CO).
[Tp′W(CO)2{η2-(BnS)CC(S)}] 3b and [Tp′W(CO){C(O)Bn}-
{η2-(BnS)CC(SBn)}] 4b. At -78 °C a solution of 3.05 g (3.2
mmol) 1b in 100 mL THF was treated with 510 mg (3.8 mmol) of
C8K. While the solution was stirred for one minute at -78 °C, the
color of the solution turned from green to red, and the formation
of 2b was checked by IR spectroscopy (CO absorptions at 1865
and 1974 cm-1). The solution was warmed to ambient temperature.
After 2 h, the disappearance of 2b was complete, as indicated by
IR spectroscopy. The solvent was removed in vacuo, and the
product was purified by column chromatography using toluene/
THF gradient elution. The less-polar, red product 4b was eluted
first and crystallized from toluene/n-hexane. Yield: 720 mg (0.8
mmol, 25%). Yellow-brown crystals of 3b were obtained by slow
evaporation of dichloromethane from dichloromethane/n-hexane
solution. Yield: 1.1 g (1.54 mmol, 48%).
1
mg (88%), red solid. H NMR (CDCl3): δ 7.25 (m, 12 H, Ph-H),
6.86 (m, 8 H, Ph-H), 6.00 (s, 4 H, CH(CCH3)2), 5.91 (s, 8 H,
CH(CCH3)2), 4.77 (s, br, 4 H, BH), 3.16 (s, 8 H, SCH2), 2.56 (s,
12 H, CH3), 2.47 (s, 24 H, CH3), 2.35 (s, 12 H, CH3), 1.65 (s, 24
H, CH3). 13C NMR (CDCl3): δ 227.0 (WCSCH2), 217.3 (CO),
207.4 (WCSNi), 154.8, 152.6, 148.2, 146.0 (CCH3), 134.7, 128.7,
128.6, 127.8 (Ph-C), 109.1, 107.4 (CH(CCH3)2), 40.9 (SCH2), 15.9,
14.9, 12.9, 12.5 (CH3). IR (CH2Cl2, cm-1): ν 2027, 1955 (s, CO).
[{Tp′(CO)2Mo-η2-(BnS)CC(S)}4Pd](BF4)2 6a. Under strict
anaerobic conditions, 19 mg (0.05 mmol) of [Pd(CH3CN)4](BF4)2
was added to a solution of 123 mg (0.2 mmol) of 3a in 15 mL of
dichloromethane. Immediately, the yellow-green solution of 3a
turned red, and the complex formation was checked by IR
spectroscopy. After 12 h of being stirred, the solution was reduced
in volume and complex 6a was crystallized by gas-phase diffusion
of diethyl ether in a dichloromethane solution of 6a. Yield: 122
mg (88%), red crystals. 1H NMR (CDCl3): δ 7.31 (m, 4 H, Ph-H),
7.27 (m, 8 H, Ph-H), 6.82 (m, 8 H, Ph-H), 5.95 (s, 4 H,
CH(CCH3)2), 5.87 (s, 8 H, CH(CCH3)2), 5.03 (s, 0.32 H, SCH2),
4.83 (s, br, 4 H, BH), 3.13 (s, 7.68 H, SCH2), 2.57 (s, 12 H, CH3),
[Tp′W(CO)2{η2-(BnS)CC(S)}] 3b. Anal. Calcd. for C26H29-
BN6O2S2W: C, 43.59; H, 4.08; N, 11.73. Found: C, 43.42; H,
1
4.15; N, 11.79. H NMR (CDCl3, 213 K): δ (two isomers) 7.42
(m, 1 H, Ph-H), 7.28 (m, 3 H, Ph-H), 6.95 (m, 1 H, Ph-H), 6.00 (s,
2 H, CH(CCH3)2), 5.88 (s, 1 H, CH(CCH3)2), 5.24 (s, approximately
1 H, 52% SCH2), 4.6 (s, br, 1 H, BH), 3.22 (s, approximately 1 H,
48% SCH2), 2.69 (s, approximately 1.5 H, CH3), 2.65 (s, ap-
proximately 1.5 H, CH3), 2.50 (s, 3 H, CH3), 2.46 (s, 3 H, CH3),
2.38 (s, approximately 1.5 H, CH3), 2.35 (s, approximately 1.5 H,
CH3), 1.88 (s, 3 H, CH3), 1.73 (s, 3 H, CH3). 13C NMR (CDCl3,
213 K): δ 237.6, 232.7 (WCSCH2), 220.7, 219.3 (WCO), 214.3,
213.8 (WCS), 154.9, 154.7, 151.8, 151.0, 146.6, 146.4, 144.9, 144.5
(CCH3), 137.1, 134.8, 129.3, 128.8, 128.5, 128.4, 127.4, 126.9 (Ph-
C), 108.3, 108.2, 106.7, 106.6 (CH(CCH3)2), 39.7, 36.2 (SCH2),
15.9, 15.8, 14.9, 14.7, 13.1, 13.0, 12.8, 12.7 (CH3). IR (KBr, cm-1):
ν 1995, 1924 (s, CO). IR (CH2Cl2, cm-1): ν 2004, 1925 (s, CO).
2.47 (s, 24 H, CH3), 2.36 (s, 12 H, CH3), 1.55 (s, 24 H, CH3). 13
C
NMR (CDCl3): δ (major isomer) 237.5 (MoCSCH2), 220.2 (CO),
218.4 (MoCSPd), 153.2, 150.8, 147.3, 145.5 (CCH3), 134.1, 128.7,
128.6, 127.9 (Ph-C), 108.2, 106.8 (CH(CCH3)2), 40.6 (SCH2), 15.6,
15.3, 13.2, 12.9 (CH3). IR (CH2Cl2, cm-1): ν 2039, 1976 (s, CO).
[{Tp′(CO)2W-η2-(BnS)CC(S)}4Pd](BF4)2 6b. The correspond-
ing tungsten complex 6b was synthesized by a method analogous
to 6a using 143 mg (0.2 mmol) 3b. Yield: 142 mg (90%), red
1
solid. H NMR (CDCl3): δ 7.25 (m, 4 H, Ph-H), 7.20 (m, 8 H,
[Tp′W(CO){C(O)Bn}{η2-(BnS)CC(SBn)}] 4b. Anal. Calcd. for
Ph-H), 6.77 (m, 8 H, Ph-H), 6.00 (s, 4 H, CH(CCH3)2), 5.90 (s, 8
H, CH(CCH3)2), 4.88 (s, 0.24 H, SCH2), 4.76 (s, br, 4 H, BH),
3.08 (s, 7.76 H, SCH2), 2.59 (s, 12 H, CH3), 2.47 (s, 24 H, CH3),
2.35 (s, 12 H, CH3), 1.71 (s, 24 H, CH3). 13C NMR (CDCl3): δ
(major isomer) 227.2 (WCSCH2), 217.3 (CO), 209.1 (WCSPd),
154.8, 153.0, 148.4, 146.3 (CCH3), 135.2, 128.9, 128.8, 128.0 (Ph-
C), 109.3, 107.6 (CH(CCH3)2), 40.4 (SCH2), 16.2, 15.1, 13.2, 12.8
(CH3). IR (CH2Cl2, cm-1): ν 2027, 1955 (s, CO).
C40H43BN6O2S2W: C, 53.46; H, 4.82; N, 9.35. Found: C, 53.60;
1
H, 5.03; N, 9.23. H NMR (C6D6): δ 7.66 (d, 2 H, Ph-H), 7.39-
6.80 (m, 13 H, Ph-H), 5.68 (s, 1 H, CH), 5.49 (s, 1 H, CH), 5.39
2
(s, 1 H, CH), 4.84 (d, 1 H, CHH, J ) 12.7 Hz), 4.72 (d, 1 H,
2
2
CHH, J ) 12.7 Hz), 4.15 (d, 1 H, CHH, J ) 13.3 Hz), 3.56 (d,
1 H, CHH, 2J ) 13.3 Hz), 3.84 (d, 1 H, CHH, 2J ) 12.1 Hz), 3.15
(d, 1 H, CHH, J ) 12.1 Hz), 2.40 (s, 3 H, CH3), 2.32 (s, 3 H,
2
CH3), 2.11 (s, 3 H, CH3), 2.07 (s, 3 H, CH3), 1.93 (s, 3 H, CH3),
1.86 (s, 3 H, CH3). 13C NMR (CDCl3): δ 285.6 (WC(O)Bn), 239.4
(WCSCH2), 205.2 (CO), 196.5 (WCSCH2), 153.5, 153.3, 150.3,
144.3, 137.5, 136.4 (CCH3), 130.2-124.8 (12 Ph-C), 107.8, 107.2,
106.7 (CH), 70.9 (C(O)CH2Ph), 40.8, 40.6 (SCH2Ph), 16.3, 16.2,
X-ray Crystallography. Crystallographic data and structure
refinement parameters of complexes 1b, 3a, 4b, and 6a are
summarized in the Supporting Information. Single crystals suitable
for X-ray analysis were coated in hydrocarbon oil and mounted on
a glass fiber. X-ray intensities were measured at 153 K (1b, 4b)
Inorganic Chemistry, Vol. 45, No. 12, 2006 4793

Seidel et al.
with SHELXS-9720 by direct methods and were refined with
SHELXL-97,21 using anisotropic thermal parameters for all non-
hydrogen atoms. The hydrogen atoms were included in the structure
factor calculations at idealized positions. The PF6 anion in 1b is
badly resolved and probably disordered. To obtain a stable
refinement, we treated it as a fixed fragment. The F atoms share a
common isotropic temperature factor.
Table 1. EPR Parameters of Molybdenum(I) Complexes
giso Aiso g1 (A1) g2 (A2)
1.994 28.0 1.980 (41) 2.009 (18) 2.029 (33)
g3 (A3)
2a
[Tp′Mo(PhCCPh)(CO)2]24
[(η5-C5Me5)Mo(CO)-
(PhCCPh)2]26
2.007 30.7 1.977
1.991 27.5
2.009
2.036
[(η5-C5Me5)Mo(PhCCPh)2]26 2.027
2.006 (10) 2.024 (-) 2.048 (14)
[Tp′Mo(NO)Cl(NC5H5)]25
1.97 48.6
[(η6-C6H3 Bu3)(η7-C7H7)-
1.9710 (55) 2.0024 (10)
t
27
Mo](PF6)2
Results and Discussion
Table 2. Bonding Parameters in Thio-Alkyne Complexes, Selected
Bonds (Å) and Angles (deg)
Bis(benzylthio)acetylene Complexes. In the search for
metal complex moieties with a strong alkyne binding ability
in order to obtain robust complexes that can survive the
aimed conversion reactions at the sulfur atoms, we found
the Tp′M(CO)2 complex fragments (M ) Mo, W) particu-
larly useful. Cationic alkyne complexes [Tp′M(CO)2{η2-
(BnS)CC(SBn)}](PF6) (M ) Mo for 1a, M ) W for 1b)
have been synthesized by the straightforward method of
Templeton22 via oxidation of the 17-electron radical [Tp′M-
(CO)3] (M ) Mo, W) with ferrocenium salts in the presence
of bis(benzylthio)acetylene. Crystallization of the complexes
from CH2Cl2/Et2O yielded air-stable emerald 1a and green
1b. The 13C NMR spectra of both compounds show alkyne
resonances in the range of 194-227 ppm, which are typical
for four-electron-donor alkyne complexes. Small changes of
the benzylic CH2 13C resonances of 1a/1b compared with
the free alkyne rule out coordination of the sulfide groups
at the metal center. Evidence of neither intermediate coor-
dination of the sulfide groups in solution nor corresponding
byproducts could be observed (as described, for example,10
for [(η5-C5H5)Ru(PMe3)2(MeSCCSMe)]+). IR spectroscopy
indicates the fast and exclusive appearance of two carbonyl
bands for one Cs symmetric complex species, displaying
somewhat lower frequencies for 1b (2041, 1980 cm-1)
because of the stronger π-back-bonding of tungsten. The
molecular structure of 1b (Figure 1) shows a roughly
1b
3a
4b
M-C1
2.064(10)
2.060(10)
2.143(4)
2.020(3)
2.044(3)
2.024(3)
2.165(3)
1.974(3)
1.339(4)
1.711(3)
1.701(3)
M-C2
M-C17
M-C25
C1-C2
C1-S1
C2-S2
C1-C2-S2
C2-C1-S1
2.026(4)
1.332(5)
1.650(4)
1.678(4)
132.9(3)
146.3(3)
1.313(14)
1.701(11)
1.706(11)
139.1(9)
141.7(8)
134.8(2)
141.4(3)
Table 3. CO Stretching Frequencies and Peak Potentials of
Thio-Alkyne Complexes
ν (CO, cm-1
)
Ep,c vs [Fc/Fc+]
-1.54
Ep,a vs [Fc/Fc+]
3a
2013, 1942
0.55
[Ni(3a)4](BF4)2 5a 2038, 1976
[Pd(3a)4](BF4)2 6a 2039, 1976
-0.87, -1.56
-1.07, -1.37, -1.53
-1.69
3b
2004, 1925
0.53
[Ni(3b)4](BF4)2 5b 2027, 1955
[Pd(3b)4](BF4)2 6b 2027, 1955
-1.05, -1.71
-1.29, -1.62
and at 173 K (3a, 6a) by using a Bruker AXS Apex system
equipped with a rotating anode. All data were measured with Mo
KR radiation (λ ) 0.71073 Å). Data collection, cell refinement,
data reduction, and integration, as well as absorption correction,
were performed with the Bruker AXS program packages SMART,
SAINT, and SADABS. Crystal and space group symmetries were
determined using the XPREP program. For further crystal and data
collection details, see Table 4. All crystal structures were solved
Table 4. Crystallographic Data
1b
3a
4b
6a
formula
fw
cryst syst
space group
a (Å)
C33H36BF6N6O2PS2W
952.43
hexagonal
R3h
21.64(2)
21.64(2)
46.42(6)
90.00
C33H37BMoN6O2S2
720.56
triclinic
P1h
9.9460(10)
11.8320(11)
15.2025(15)
88.173(2)
78.319(2)
79.183(2)
1720.8(3)
2
1.391
0.541
0.71073
173(2)
13 552
5960
C47H50BN6O2S2W
989.71
monoclinic
P21
11.0754(5)
17.8315(8)
11.7455(6)
90
108.4900(10)
90
2199.89(18)
2
C111H133B6F8Mo4N24O9PdS8
2910.91
triclinic
P1h
17.3601(13)
18.2711(13)
25.0020(18)
93.194(2)
109.147(2)
115.6360(10)
6570.0(8)
2
1.471
0.706
0.71073
173(2)
53 040
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
90.00
120.00
18825(34)
18
1.512
2.962
0.71073
153
38 137
5477
F
calcd (g cm-3
)
1.494
2.766
µ (mm-1
MoKR (Å)
T (K)
)
λ
0.71073
153(1)
25 485
12 324
2.48
no. of reflns collected
no. of unique reflns
Rint
23 137
9.20
12.54
4.62
no. of obsd reflns
F(000)
4006
8496
5.46
16.80
4926
744
4.85
13.12
11 744
1002
2.67
12 371
2966
6.73
16.24
a
R1
b
wR2
6.25
GOF
0.997
0.935
1.007
0.959
no. of params
448
460
539
1596
a Final R [F > 2σ(F)]. b R indices (all data).
4794 Inorganic Chemistry, Vol. 45, No. 12, 2006

η2-Alkyne-1-thio Complexes as Sulfur Donors
Figure 2. Cyclic voltammograms of 1a (broken line) and 1b (solid line)
measured in CH2Cl2.
Figure 1. Molecular structure of the cation in 1b. Hydrogens have been
omitted for clarity.
Scheme 1. Synthesis of the Thio-Alkyne Complexes 3a/3b and 4b
octahedral complex considering the alkyne as occupying a
single coordination site. The M-C and C-C bond lengths
are within the range of corresponding d4 alkyne complexes
bearing carbon substituents.22 The alkyne C-C bond lengths
of 1.317 Å and the strong bendback of the sulfur substituents
from the C-C vector by 37 and 42° are consistent with a
four-electron-donor alkyne complex. The alkyne SCCS plane
lies between the carbonyl ligands approximately on the
symmetry plane of the molecule. Fluxional behavior has not
been observed in the room temperature 1H NMR spectra of
1a/1b. To evaluate the specific donor strength of sulfide-
substituted alkynes by its influence on the CO frequencies,
we used the approximation of Cotton and Kraihanzel.23
A
3b14 disclosed the intended cleavage of one benzyl group in
1a/1b and the identity of 3a/3b as thio-alkyne complexes
(Figure 5). This reactivity is remarkable with regard to the
stability of the corresponding tolane complex [Tp′Mo(CO)2-
(η2-PhCCPh)], which was described by Connelly and Or-
pen.24
comparison of the CO force constants of 1b (16.33 N cm-1)
with the corresponding tungsten complexes with 2-butine22
(16.39 N cm-1) and with diphenylacetylene22 (16.45 N cm-1)
led to the finding that the nucleophilicity of the alkyne ligand
increases with the substitution of phenyl by methyl groups
and, to a similar extent, the substitution of methyl by
benzylthio groups.
To get insights into the reaction mechanism, we monitored
the reduction process by IR spectroscopy using the CO
stretching frequencies as a probe (Figure 3). Immediately
after addition of C8K to tungsten complex 1b, a red reaction
solution formed; at -78 °C, the solution displayed two
markedly low-energy-shifted CO bands at 1959 and 1861
cm-1. These bands are assigned to the intermediate complex
2b with an electron-rich metal center (Scheme 1). In the
course of 1 h at ambient temperature, the conversion of 2b
to 3b is indicated by the appearance of a new pair of bands
at 1996 and 1926 cm-1. The CO stretching frequencies of
3b reveal a loss of electron density at the metal compared
to the intermediate 2b. These observations, supported by the
cyclic voltammograms of 1a/1b, point to a mechanism in
Reduction of the Bis(benzylthio)acetylene Complexes.
Reductive removal of benzyl groups is a standard reaction
in thiol protection chemistry. However, the alkyne sulfide
functions in 1a/1b raise the question of whether the carbon
sulfur bonds are selectively cleaved at the sp or at the sp3
position. According to the cyclic voltammograms, 1a and
1b undergo two reduction steps (Figure 2). The first reduction
is indicated by a reversible wave at -0.80 and -0.97 V,
respectively, and the second step appears less reversible at
-1.65 and -1.74 V, respectively (peak potentials referenced
against ferrocene/ferrocenium hereafter). Tungsten complex
1b is more difficult to reduce than molybdenum congener
1a. The first step at moderate potential is assigned to the
reduction of the metal center because of its reversibility. To
confirm this perception, we carried out reduction of 1a/1b
in preparative scale in THF solution by the addition of 1
equiv of potassium graphite (Scheme 1). The products 3a/
3b, which could be crystallized from red-brown toluene
(20) Sheldrick, G. M. SHELXS-97: Program for Crystal Structure Solution;
Universita¨t Go¨ttingen: Go¨ttingen, Germany, 1986-1997.
(21) Sheldrick, G. M. SHELXL-97: Program for Crystal Structure Refine-
ment; Universita¨t Go¨ttingen: Go¨ttingen, Germany, 1997.
(22) Feng, S. G.; Philipp, C. C.; Gamble, A. S.; White, P. S.; Templeton,
J. L. Organometallics 1991, 10, 3504-3512.
(23) Cotton, F. A.; Kraihanzel, C. S. J. Am. Chem. Soc. 1962, 84, 4432-
4438.
1
solutions, displayed puzzling H NMR spectra with broad
(24) Bartlett, I. M.; Connelly, N. G.; Orpen, A. G.; Quayle, M. J.; Rankin,
J. C. J. Chem. Soc., Chem. Commun. 1996, 2583-2584.
benzyl resonances. The X-ray structure analysis of 3a and
Inorganic Chemistry, Vol. 45, No. 12, 2006 4795

Seidel et al.
electron density to the thio-alkyne unit, which is evident by
the medium CO frequencies of 3a/3b compared to those of
1a/1b and 2a/2b, respectively. The cleavage of the benzyl
group proceeds much slower with the molybdenum system,
which is surprising at a first glance because of the usually
higher reactivity of molybdenum complexes. However, the
cleavage is presumably promoted by the higher electron
density at tungsten.
The first metal-directed reduction step could be unequivo-
cally proven by EPR spectroscopy (Figure 4). The room-
temperature EPR spectrum of 2a displays an isotropic signal
with a clearly resolved hyperfine coupling to I ) 5/2 isotopes
(95Mo (15.9%) and 97Mo (9.6%)). This observation points
to Mo(I) intermediate 2a, in which the unpaired electron is
predominantly localized at the metal. In a frozen solution, a
rhombic g-tensor is apparent from the low-temperature
spectrum at X-band frequency. In addition, a simulation of
the spectrum provides hyperfine coupling values with
pronounced anisotropy. The average of the three anisotropic
A values agrees well with the isotropic value Aiso observed
in the room-temperature spectrum. The hyperfine coupling
constants, which are summarized and related to literature data
in Table 1, are rather small compared to innocent paramag-
netic molybdenum complexes. Comprising Mo(I) d5 com-
plexes, the A values of 2a are distinctively smaller compared
to those of [Tp′Mo(NO)Cl(pyridine)]25 but higher compared
to those of alkyne complex [Cp*Mo(η2-PhCCPh)2]26 with a
predominantly ligand-based SOMO. This observation can be
rationalized with some delocalization of the unpaired electron
into the coordinated alkyne, which is reflected by the
uncovered reactivity. The accordance of the g and A values
of [Tp′Mo(CO)2(η2-PhCCPh)]24 with 2a is notable, the latter
of which is furnished with just the weak S-Cbenzyl bond.
With the intention of increasing the yields of products 3a/
3b, the mother liquors of crystallization attempts were
subjected to chromatographic purification. Thereby, we
isolated a byproduct 4b in a surprisingly high yield that is
comparable to the yield of 3b. Tungsten complex 4b could
be isolated in substance and fully characterized, whereas
corresponding molybdenum complex 4a turned out to be
unstable. The 1H NMR spectrum of diamagnetic compound
4b displays six nonequivalent CHH protons that represent
three different benzyl groups in a chiral metal complex. The
identity of 4b could be resolved by X-ray structure analysis
(Figure 6). The molecular structure of 4b disclosed the fate
of the benzyl radical in the cleavage reaction, because the
tungsten center is coordinated beside the Tp′ and one CO
ligand by the still-intact alkyne and one acyl group. The
cleaved benzyl radicals do not simply recombine but attack,
preferably, another molecule of 2b in terms of an alternative
combination of radicals.
Figure 3. CO stretching frequencies of 1b (broken line), 2b (dotted line),
and 3b (solid line).
Figure 4. EPR spectra of 2a in a frozen THF solution (measured, solid
line; calculated, broken line) and at room temperature (inset).
Further mechanistic evidence was sought from kinetic
investigations on the basis of UV-vis spectroscopic moni-
toring of the cleavage reaction with molybdenum complex
Figure 5. Molecular structure of 3a. Hydrogens have been omitted for
clarity.
which the metal center is reduced primarily in a fast step
and the benzyl group is cleaved in a subsequent slower
process. Thereby, the obvious removal of a benzyl radical
at one sulfide function involves a transfer of metal-based
(25) McWhinnie, S. L.; Jones, C. J.; McCleverty, J. A.; Collison, D.; Mabbs,
F. E. J. Chem. Soc., Chem. Commun. 1990, 940-942.
(26) Connelly, N. G.; Geiger, W. E.; Lovelace, S. R.; Metz, B.; Paget, T.
J.; Winter, R. Organometallics 1999, 18, 3201-3207.
4796 Inorganic Chemistry, Vol. 45, No. 12, 2006

η2-Alkyne-1-thio Complexes as Sulfur Donors
With regard to the rotation of the η2-C,C-bound ligand at
the metal center, 3a/3b behave in solution like typical alkyne
1
complexes.30 In the H NMR spectrum of 3a at -60 °C,
two CH2 resonances at 5.20 and 3.26 ppm exhibit comparable
intensity. The high-field-shifted signal is assigned to the
isomer found in the crystal, because NOE experiments
disclose proximity of the CH2 protons to the pyrazole pocket.
A double signal set is consistently observed in the 13C NMR
spectrum at -60 °C, and the correct assignment of the alkyne
carbon resonances for both rotational isomers has been done
by correlation of the PhCH2 resonance pair with the low-
field-shifted alkyne C pair in a gHMBC spectrum. The free
activation enthalpies ∆G*298 of the rotation between the syn
and the anti positions of the terminal sulfur regarding the
carbonyl ligands have been determined to be 60.5 kJ/mol
(3a) and 58.3 kJ/mol (3b) by line-shape analysis of the
Figure 6. Molecular structure of 4b. Hydrogens have been omitted for
clarity.
1
variable temperature H NMR spectra (see the Supporting
Information, Figures S1 and S2).
2a. The observation of isosbestic points led to the assumption
of a bimolecular mechanism. However, an initial rate vs start
concentration correlation was found to be linear, which does
not support a bimolecular mechanism. The first-order kinetics
for the overall consumption of starting material 2a can be
rationalized by the assumption of parallel reactions with a
steady-state concentration of the benzyl radical. The radical
attack is so fast relative to the comparatively slow cleavage
reaction (the half-life of 2a at room-temperature falls in the
dimension of days) that the sum reaction 2 (2a) f 3a + 4a,
becomes apparent by the observation of isosbestic points in
the UV-vis monitoring.
Molecular Structures of 3a and 4b. The overall complex
structure of 3a (Figure 5) resembles the structure of corre-
sponding cationic tungsten complex 1b. Considering the thio-
alkyne ligand as occupying a single coordination site, 3a
exhibits a roughly octahedral complex polyhedron. The thio-
alkyne moiety in 3a is coordinated in a η2-CC fashion with
a free terminal sulfur atom, as already found in corresponding
complexes obtained by C-C bond formation in the com-
plex.12,13 The difference of the C1-C2 bond lengths between
1b and 3a (1.332 Å; see Table 2) is within the margin of
error. The SCCS plane likewise lies approximately on the
symmetry plane of the molecule. The terminal sulfur atom
adopts a syn position with respect to the carbonyl ligands.
The direct comparison of the Csp-S bond lengths to the
sulfide group (1.678 Å) and to the terminal sulfur atom
(1.650 Å) in an individual molecule of 3a discloses little
double-bond character for the C1-S1 bond when applying
the bond-length criteria. However, according to resonance
structure A, the thioketenyl character is evident by the
distinctly shorter Mo-C2 bond (2.020 Å) compared with
that of Mo-C1 (2.143 Å). Interestingly, stable thioketenes
form either η1-S or η2-SC complexes.28,29
The molecular structure of 4b (Figure 6) is interesting
because of its correlation to 1b. The tungsten(II) center in
4b bears one acyl ligand in substitution for one CO, and
complex 4b is consequently neutral in contrast to cationic
1b. Considering the alkyne ligand again as occupying a single
coordination site, 4b likewise shows a roughly octahedral
structure. However, the C2S2 alkyne plane does not bisect
the C3 symmetric Tp′ ligand; it is aligned with the residual
CO ligand. Compared with those of 1b, the W-C1 and
W-C2 bonds (2.024 and 2.044 Å, respectively) are some-
what shorter and the C1-C2 distance (1.339 Å) is slightly
longer. The W-N bond lengths increase in the order W-N5,
W-N3, and W-N1, which correspond to the trans position
of CO, acyl function, and alkyne ligand, respectively. The
W-C17 bond of the acyl group displays the longest tungsten
carbon bond in 4b (2.165 Å), which falls at the shorter limit
for tungsten η1-acyl complexes.31-33 Comparison of 4b with
related alkyne complex [Tp′Mo(CO)2(η2-PhCCPh)]24 is
instructive, because the reduction of [Tp′Mo(CO)2(η2-PhC-
CPh)]+ by one electron induces a decrease in the C-C bond
length and an elongation of the metal-carbon bonds. The
opposite is observed for the cationic 1b and neutral 4b pair,
because in this case, the electron density at molybdenum
increases without a change in the oxidation state. With regard
to Tp compounds of group VI metals, predominantly η2-
acyl complexes with distinctively shorter M-C bonds are
known.34-36 The η2-alkyne-η1-acyl binding mode of 4b has
already been found in [Tp′W(CO){ η2-MeCCPh}{C(O)-
Bu}].37 Corresponding molybdenum complexes have solely
(30) Templeton, J. L. AdV. Organomet. Chem. 1989, 29, 44-61.
(31) Adams, H.; Bailey, N. A.; Tattershall, C. E.; Winter, M. J. J. Chem.
Soc., Chem. Commun. 1991, 912-914.
(32) Chen, J.-D.; Wu, C.-K.; Wu, I.-Y.; Huang, B.-C.; Lin, Y.-C.; Wang,
Y. J. Organomet. Chem. 1993, 454, 173-181.
(33) Sakaba, H.; Tsukamoto, M.; Kabuto, C.; Horino, H. Chem. Lett. 2000,
1404-1405.
(27) Beddoes, R. L.; Elwell, M. S.; Mabbs, F. E.; McInnes, E. J. L.; Roberts,
A.; Sarwar, M. F.; Whiteley, M. W. J. Chem. Soc., Dalton Trans.
2000, 4669-4676.
(34) Curtis, M. D.; Shiu, K.-B.; Butler, W. M. J. Am. Chem. Soc. 1986,
108, 1550-1561.
(35) Rusik, C. A.; Collins, M. A.; Gamble, A. S.; Tonker, T. L.; Templeton,
J. L. J. Am. Chem. Soc. 1989, 111, 2550-2560.
(28) Wormsba¨cher, D.; Edelmann, F.; Behrens, U. Chem. Ber. 1982, 115,
1332-1338.
(36) Stone, K. C.; Onwuzurike, A.; White, P. S.; Templeton, J. L.
Organometallics 2004, 23, 4255-4264.
(29) Werner, H.; Kolb, O.; Schubert, U.; Ackermann, K. Chem. Ber. 1985,
118, 873-879.
(37) Feng, S. G.; Templeton, J. L. Organometallics 1992, 11, 2168-2175.
Inorganic Chemistry, Vol. 45, No. 12, 2006 4797

Seidel et al.
Scheme 2. Synthesis of the Heterobimetallic Complexes of 3a/3b
with Ni(II) and Pd(II)
been discussed as intermediates in the C-C bond formation
between alkynes and η2-bound acyl groups in the complex.35
This reactivity, investigated by Templeton, provides a
rationale for the difficulties in isolating the molybdenum
analogue of 4b.
Complex Formation of 3a/3b with Ni(II) and Pd(II).
To explore the coordination behavior of thio-alkyne com-
plexes, we reacted 3a/3b with [Ni(CH3CN)6](BF4)2 and
[Pd(CH3CN)4](BF4)2 in CH2Cl2 (Scheme 2). The immediate
complex formation is indicated by a distinct increase in the
CO stretching frequencies in the IR spectrum (∆ν(3a) ) 34
and 25 cm-1, ∆ν(3b) ) 30 and 23 cm-1). Independent from
the equivalents of 3a/3b used in the synthesis, the tetraco-
ordinated complexes [Ni(3a)4](BF4)2 5a, [Ni(3b)4](BF4)2 5b,
[Pd(3a)4](BF4)2 6a, and [Pd(3b)4](BF4)2 6b are obtained after
crystallization from n-hexane/CH2Cl2, according to ESI-mass
spectroscopy. 13C NMR spectroscopy discloses that the
sulfide groups do not take part in the coordination. Regarding
the rotational isomerism in the alkyne complex, the isomer
with the terminal sulfur atom in proximity to the CO ligands
prevails either absolutely (5a/5b) or extremely (6a/6b), as
indicated by 1H NMR spectroscopy. The effect of the sulfur
coordination with nickel and palladium at the tungsten and
molybdenum alkyne moiety, respectively, can be estimated
by comparison of the CO frequencies in the IR spectra (Table
3) and the alkyne 13C NMR shifts. The CO vibrations at
molybdenum and tungsten show approximately 70% com-
pensation of the original low-frequency shift from cationic
1a/1b to neutral 3a/3b. The increase in the CO frequencies
reveals an enhanced positive partial charge at the group VI
metal centers, which is affected by the sulfur coordination
with nickel and palladium.
The difference between the two alkyne carbon resonances
in the 13C NMR spectra changes distinctly from 1a (∆δ )
11.6) to 3a (∆δ ) 30.4) and finally to 5a (∆δ ) 21.1).
Generally, this observation corroborates the thioketenyl
complex character of 3a. Interestingly, the high-field shift
upon coordination of 3a at nickel is larger for C2 (∆δ )
15.2) than for C1 (∆δ ) 3.9). The stronger effect on C2,
which is only indirectly affected by the coordination of nickel
at the sulfur attached to C1, can be understood by the
decrease in the Mo-C2 double-bond character. The π-back-
bonding of molybdenum in 3a is primarily directed at C2,
which is reflected by the double bond in thioketenyl complex
resonance form A. This effect is reduced in 5a, because the
electron-withdrawing effect of the LEWIS acid Ni2+ causes
a more-balanced ratio of π-back-bonding to both C1 and C2.
The reduced π-back-bonding to C2 in turn decreases the
σ-donation, which is indicated by the observed high-field
shift in the 13C NMR spectrum. These changes are less-
Figure 7. Molecular structure of the cation in 6a. Hydrogen atoms and
methyl and phenyl groups have been omitted for clarity. Selected bonds
(Å) and angles (deg): Pd-S1A 2.334(2), Pd-S1B 2.313(2), Pd-S1C 2.347-
(2), Pd-S1D 2.320(2), Mo1A-C1A 2.074(7), Mo1A-C2A 2.041(7),
C1A-C2A 1.341(9), C1A-S1A 1.659(8), C2A-S2A 1.647(7); S1A-Pd-
S1C 90.41(6), S1A-Pd-S1D 90.27(5), S1B-Pd-S1C 90.02(6), S1B-
Pd-S1D 90.03(5), S1A-Pd-S1B 171.32(5), S1C-Pd-S1D 175.18(5),
Pd-S1A-C1A 110.0(3), S1A-C1A-C2A 145.3(6).
pronounced for the tungsten congeners, possibly because of
the stronger π-back-bonding capacity of tungsten.
The identity of [Pd(3a)4](BF4)2 6a has been unambiguously
confirmed by X-ray structure analysis (Figure 7). The entire
complex cation is crystallographically independent, rendering
the variable number comparatively high. The central pal-
ladium is distorted square planar coordinated with Pd-S1
bonds ranging from 2.313 to 2.347 Å, which are typical for
Pd(II) complexes with neutral thioketo donors.38-41 The
molybdenum complex moieties in trans position are tilted
to the same side of the central square unit, obviously
releasing the sterical crowding. Whereas the Mo-C2 bonds
(average 2.023 Å) are comparable with those of the metal-
laligand 3a, the Mo-C1 bonds (average 2.078 Å) are
remarkably shorter and resemble instead the corresponding
value in 1b. The carbon-sulfur bonds (C1-S1 average,
1.661 Å; C2-S2 average, 1.667 Å) are equal within the
margin of error. Tetracoordinated complexes of Pd(II) with
neutral thioketo ligands such as thiourea derivatives or
pyridinium-2-thione show frequently weak coordination of
the counterions at the apical positions.38-41 In 6a, one of the
palladium sulfide distances is notably shorter (Pd-S2C,
3.543 Å) than the other three, which leads to the longest
Pd-S1 bond (Pd-S1C, 2.347 Å) and the smallest Pd-S1-
C1 angle (Pd-S1C-C1C, 103.7°). However, even the Pd-
S2C bond length just coincides with the sum of the van der
Waals radii.42
(38) Berta, D. A.; Spofford, W. A.; Boldrini, P.; Amma, E. L. Inorg. Chem.
1970, 9, 136-142.
(39) Butler, L. M.; Creighton, J. R.; Oughtred, R. E.; Raper, E. S.; Nowell,
I. E. Inorg. Chim. Acta 1983, 75, 149-154.
(40) Umakoshi, K.; Ichimura, A.; Kinoshita, I.; Ooi, S. Inorg. Chem. 1990,
29, 4005-4010.
(41) Vilar, R.; Mingos, D. M. P.; White, A. J. P.; Williams, D. J. Chem.
Commun. 1999, 229-230.
4798 Inorganic Chemistry, Vol. 45, No. 12, 2006

η2-Alkyne-1-thio Complexes as Sulfur Donors
Figure 9. UV-vis spectra of 5b in CH2Cl2 (broken line), 5b in 1% THF
in CH2Cl2 (dotted line), and [Ni(tmtu)4](BF4)2 (line).
Figure 8. Cyclic voltammograms of 3a (dotted line), 5a (broken line),
and 6a (line) measured in CH2Cl2 solution with a scan rate of 100 mV s-1
.
in the anodic part of the CVs. Metalla-ligands 3a/3b show
an oxidation wave at approximately 0.54 V that is assigned
to the disulfide formation. This electron transfer is reasonably
not observed in the CV of heteronuclear complexes 6a/6b,
if the potential sweep is started at 0 V with the oxidation.
However, the oxidation wave at 0.54 V is observed in the
reverse sweep mode, indicating the existence of free ligand
in solution after the reduction (see the Supporting Informa-
tion, Figure S13).
The redox responsivity of the group VI metals for the
coordination of the sulfur donor with nickel and palladium
has been investigated by cyclic voltammetry (Figure 8). The
metallaligands 3a/3b undergo a reversible electron transfer
at Ep,c ) -1.54 for 3a and Ep,c ) -1.69 V for 3b. The
coordination of 3a/3b to nickel and palladium leads to a
remarkable positive shift in the first reduction potentials. The
CV of 6a exemplarily displays three successive reduction
waves. The first two are irreversible; the third is quasire-
versible and represents an electron transfer at a potential
comparable to that of free metallaligand 3a. The CVs of the
nickel complexes show only one broad irreversible reduction
wave and a quasireversible signal at approximately -1.5 V.
An unequivocal assignment of the reductions seems difficult.
However, the first reduction step can presumably be at-
tributed to the group VI metal centers for the following
reasons. The peak potentials of the first reduction wave of
3a/3b, 5a/5b, and 6a/6b (measured at invariable sweep rates
of 100 mV s-1) are in a reasonable linear relationship with
the force constants of the CO ligands at molybdenum and
tungsten (see the Supporting Information, Figure S11). The
potential difference between Mo complex 3a and its W
congener 3b is even increased within both the nickel and
the palladium complexes, with nickel compounds 5a/5b
being easier to reduce. However, the irreversibility of the
first reduction process is even observed, if the potential sweep
is confined to this process (see the Supporting Information,
Figure S12). A plausible explanation for this behavior implies
a fast intramolecular electron transfer to the central metal
ion. This interpretation is corroborated by reduction experi-
ments in preparative scale. If the nickel and palladium
complexes are reduced by 2 equiv of cobaltocene in CH2-
Cl2 (5a/5b) or THF (6a/6b), the release of ligands 3a/3b is
observed, according to IR and UV-vis spectroscopic evi-
dence. Hence, the released ligands 3a/3b give rise to the
quasireversible signal in the CVs of the heterobimetallic
complexes 5a/5b and 6a/6b at -1.5 V. Additional evidence
of the ligand release after the first reduction step is uncovered
Surprisingly, nickel complexes 5a/5b turned out to be
unstable in donor solvents such as THF and CH3CN, because
these solvents compete as ligands. In contrast to 6a/6b and
1
according to H NMR and IR spectroscopic data, THF
solutions of 5a/5b contain predominantly free ligands 3a/
3b. The UV-vis spectrum of intensely red 5b is dominated
by the charge-transfer band of the tungsten complex moiety
(Figure 9). The nickel-based transition is probably obscured,
as indicated by comparison with the UV-vis spectrum of
green [Ni(tmtu)4](BF4)2 (tmtu ) tetramethylthiourea). Inter-
estingly, the release of 3b is more evident by the sharpening
of the tungsten-based charge-transfer band than by an energy
shift. Possibly the solution structure of 5b adopts a low
symmetry comparable to the solid-state structure of 6a. Such
a low-symmetry structure must be fluxional in the NMR time
scale, because the corresponding spectra display 4 equiv 3b
ligands. Alternatively, the broadening of the band in 5b can
be attributed to the electronic interaction of the four
chromophores over the central metal ion. The ESI mass
spectrum of 5b in THF solution shows a number of diverse
species such as [Ni(3b)3]2+, [Ni(3b)2(THF)]2+, and [Ni(3b)-
(THF)5]2+. We are currently exploring possibilities for
activating substrates at nickel complex ions in CH2Cl2/THF
solvent mixtures.
Conclusion
The goal of the work described in this contribution was
to access heterobimetallic complexes featuring η2-C,C-η1-S
alkyne-thiolate linkers. Our approach, comprising primary
formation of alkyne complexes with suitable acetylenedis-
ulfides, subsequent removal of the sulfur protection groups,
(42) Bondi, A. J. Phys. Chem. 1964, 68, 441-451.
Inorganic Chemistry, Vol. 45, No. 12, 2006 4799

Seidel et al.
and final complex formation with thiophilic transition-metal
ions, turned out to be generally successful. The η2-alkyne
complexes of bis(benzylthioacetylene) with d4 metal complex
fragments Tp′M(CO)2 (M ) Mo, W) are sufficiently robust
to allow necessary conversions at the sulfur atom. The
resulting thio-alkyne complexes with one terminal sulfur
donor have been proven in turn to act as metallaligands,
which form homoleptic Werner type complexes with Ni(II)
and Pd(II). The structural, electrochemical, and IR spectro-
scopic data reveal a partially zwitterionic nature of the
metallaligands in these pentanuclear complexes. This charge
transfer into the alkyne complexes is emphasized in a
resonance structure with two cationic peripheral alkyne
complex moieties and a neutral complex core. The reactivity
of the heterobimetallic complexes in donor solvents and
redox reactions discloses a high electronic flexibility of the
metallaligands. Pentanuclear heterobimetallic complexes of
type 5a/5b and 6a/6b seem promising, because the generally
redox-active alkyne complex moieties might be able to
deliver up to four electrons to a potential substrate coordi-
nated at the central metal ion (e.g., in the apical position).
In this context, further work is directed toward less-noble
central metal ions, such as Fe(II).
Acknowledgment. The Deutsche Forschungsgemein-
schaft is gratefully acknowledged for financial support of
this work. W.W.S. thanks Prof. F. E. Hahn for generous
support and Dr. E. Bill (MPI Mu¨lheim) for the EPR
measurements.
1
Supporting Information Available: Variable-temperature H
NMR spectra of 3a; Eyring plot and activation parameters for 3a
and 3b obtained by line shape analysis; kinetic data for the
conversion of 2a to 3a; NMR spectra of 5a/5b and 6a/6b; additional
CV data of 5a/5b and 6a, crystallographic information files for
1b, 3a, 4b, and 6a. This material is available free of charge via the
Internet at http://pubs.acs.org.
IC052178Y
4800 Inorganic Chemistry, Vol. 45, No. 12, 2006