Studies of structural effects on the half-wave potentials of mononuclear and dinuclear nickel(II) diphosphine/dithiolate complexes

Article abstract of DOI:10.1021/ic061740x

Two series of mononuclear Ni(II) complexes of the formula (PNP)Ni(dithiolate) where PNP = R₂PCH₂N(CH ₃)CH₂-PR₂, R = Et and Ph, have been synthesized containing dithiolate ligands that vary from five- to seven-membered chelate rings. Two series of dinuclear Ni(II) complexes of the formula {[(diphosphine)Ni]₂(dithiolate)}(X)₂ (X = BF₄ or PF₆) have been synthesized in which the chelate ring size of the dithiolate and diphosphine ligands have been systematically varied. The structures of the alkylated mononuclear complex, [(PNP^Et)Ni(SC ₂H₄SMe)]OTf, and the dinuclear complex, [(dppeNi) ₂(SC₃H₆S)](BF₄)₂, have been determined by X-ray diffraction studies. The complexes have been studied by cyclic voltammetry to determine how the half-wave potentials of the Ni(II/I) couples vary with chelate ring size of the ligands. For the mononuclear complexes, this potential becomes more positive as the natural bite angle of the dithiolate ligand increases. However, the potentials of the Ni(II/I) couples of the dinuclear complexes do not show a dependence on the chelate ring size of the ligands. Other aspects of the reduction chemistry of these complexes have been explored.

Full text of DOI:10.1021/ic061740x

Inorg. Chem. 2007, 46, 1268−1276
Studies of Structural Effects on the Half-Wave Potentials of
Mononuclear and Dinuclear Nickel(II) Diphosphine/Dithiolate Complexes
Kendra Redin, Aaron D. Wilson, Rachel Newell, M. Rakowski DuBois, and Daniel L. DuBois*
Contribution from the Department of Chemistry and Biochemistry, UniVersity of Colorado,
Boulder, Colorado 80309, and the DiVision of Chemical Sciences, Pacific Northwest National
Laboratory, Richland, Washington 99352
Received September 13, 2006
Two series of mononuclear Ni(II) complexes of the formula (PNP)Ni(dithiolate) where PNP
PR2, R Et and Ph, have been synthesized containing dithiolate ligands that vary from five- to seven-membered
chelate rings. Two series of dinuclear Ni(II) complexes of the formula [(diphosphine)Ni]2(dithiolate) (X)2 (X BF4
) R2PCH2N(CH3)CH2-
)
{
}
)
or PF6) have been synthesized in which the chelate ring size of the dithiolate and diphosphine ligands have been
systematically varied. The structures of the alkylated mononuclear complex, [(PNPEt)Ni(SC2H4SMe)]OTf, and the
dinuclear complex, [(dppeNi)2(SC3H6S)](BF4)2, have been determined by X-ray diffraction studies. The complexes
have been studied by cyclic voltammetry to determine how the half-wave potentials of the Ni(II/I) couples vary with
chelate ring size of the ligands. For the mononuclear complexes, this potential becomes more positive as the
natural bite angle of the dithiolate ligand increases. However, the potentials of the Ni(II/I) couples of the dinuclear
complexes do not show a dependence on the chelate ring size of the ligands. Other aspects of the reduction
chemistry of these complexes have been explored.
Introduction
shown to exhibit a linear correlation with the potentials of
the Ni(II/I) couples.2 Consequently, as the natural bite angles
of the ligands increase, the hydride acceptor abilities of these
complexes increase, and large bite angles favor the heterolytic
activation of hydrogen. These systematic changes are related
to a tetrahedral distortion of the square planar nickel(II)
complexes as ligand bite angles increase.
We were interested in determining whether similar rela-
tionships between ligand bite angles and half-wave potentials
hold for Ni(II) complexes with other types of chelating
ligands. For example, the replacement of one diphosphine
ligand with a dithiolate results in significant changes in both
the steric and electronic features of the nickel complexes.
Steric interactions of the phosphorus substituents will be
minimized, while the π interactions with the metal ion may
be significantly increased with thiolate coordination.6 As a
result, it is not clear whether the relationships observed
between chelate bite size and half-wave potentials will hold
for square planar Ni complexes that include dithiolate
ligands. In this paper we report a new series of Ni(PNP)-
(dithiolate) complexes and systematic studies on how the
potentials of the Ni(II/I) couples vary with chelate bite angles
Previous studies in our laboratories on simple synthetic
nickel bis(diphosphine) complexes have focused on a
fundamental understanding of the factors controlling the half-
wave potentials of these complexes and the thermodynamic
properties of the metal-hydrogen bonds in their correspond-
ing hydride complexes.1-5 A significant finding in these
studies is that the potential of the Ni(II/I) couple becomes
more positive as the chelate bite size or natural bite angle
of the ligand increases,1 and this systematic variation can
therefore be used to control the potential of this couple.
Similarly, the hydride-acceptor abilities of the [Ni(diphos-
phine)2]2+ complexes, as defined by the free energy of the
2+
+
reaction NiLn + H- f HNiLn in solution, have been
* To whom correspondence should be addressed. E-mail: daniel.dubois@
pnl.gov.
(1) Berning, D.; Noll, B.; DuBois, D. J. Am. Chem. Soc. 1999, 121,
11432-11447.
(2) Berning, D.; Miedaner, A.; Curtis, C. J. Noll, B. C.; Rakowski DuBois,
M.; DuBois, D. L. Organometallics 2001, 20, 1832-1839.
(3) Curtis, C.; Miedaner, A.; Raebiger, J.; DuBois, D. L. Organometallics
2004, 23, 511-516.
(4) Curtis, C.; Miedaner, A.; Ellis, W. W.; DuBois, D. L. J. Am. Chem.
Soc. 2002, 124, 1918-1925.
(5) Miedaner, A.; Haltiwanger, R. C.; DuBois, D. L. Inorg. Chem. 1991,
30, 417.
(6) Ashby, M. T.; Enemark, J. H.; Lichtenberger, D. L. Inorg. Chem. 1988,
27, 191-197.
1268 Inorganic Chemistry, Vol. 46, No. 4, 2007
10.1021/ic061740x CCC: $37.00
© 2007 American Chemical Society
Published on Web 01/24/2007

Studies of Structural Effects of Half-WaWe Potentials
of the sulfur donor ligands. In addition, new dinuclear nickel
complexes with bridging dithiolate ligands of the formulas
[(diphosphine)Ni]2(µ-dithiolate)2+ have also been synthe-
sized. The chelate bite sizes of both the diphosphine and
dithiolate ligands in this series have been systematically
varied with the objective of determining if ligand bite angles
can be used to control electrochemical and chemical reduc-
tion processes.
This fundamental study of nickel complexes with thiolate
donors is prompted in part by the composition of important
nickel enzymes. For example, a major class of the hydro-
genase enzymes contains a dinuclear Ni-Fe active site in
which the nickel ion is coordinated by terminal and bridging
cysteine ligands.7 Both the A-cluster and C-cluster active
sites in bifunctional CO dehydrogenase/acetyl CoA synthase
enzymes contain dinuclear or cluster complexes in which
nickel is coordinated by cysteinyl sulfur donors.8,9 In the
enzymes, the coordination geometry about the nickel atom
in the active sites appears to be controlled by the steric
constraints imposed by the protein matrix of the enzymes.
Distortions from planar or tetrahedral nickel geometries may
influence the reduction potentials and reactivities of the active
sites. However, a clear relationship between structural dis-
tortions and redox potentials has not been established for
nickel thiolate structures.
(PNPEt)(S2CNEt2)]BF4 (1a), Ni(PNPEt)(SC2H4S) (2a), Ni-
(PNPEt)(SC3H6S) (3a), and Ni(PNPEt)(SCH2(C6H4)CH2S)
(4a) in which the size of the metal-sulfur chelate ring varies
from four to seven atoms. A second analogous series of
complexes of the formula Ni(diphosphine)(dithiolate) (1b-
4b) was prepared using the phenyl-substituted PNP ligand,
Ph2PCH2NMeCH2PPh2, PNPPh. The syntheses of each of the
dithiolate complexes followed a similar route in which [Ni-
(CH3CN)6](BF4)2 was reacted with an equivalent of PNPEt
or PNPPh and an equivalent of the dithiol in the presence of
NEt3, e.g. eq 1 for 2.
[Ni(CH3CN)6]2+ + PNP + HSC2H4SH + 2NEt3 S
Ni(PNP)(SC2H4S) + 2[HNEt3]+ (1)
Complexes were isolated as orange or red crystalline solids
and characterized by spectroscopic techniques, including 1H
and 31P NMR, and electrospray mass spectroscopy. Each of
the complexes shows a singlet in the 31P NMR spectrum be-
tween 9 and 15 ppm, and in most cases m/z values that corres-
pond to the parent ions are observed in the mass spectra.
Attempts were made to synthesize related nickel com-
plexes with thioether ligands for comparative purposes. A
thiolate/thioether complex, [(PNPEt)Ni(MeSC2H4S)]OTf (5),
was synthesized by the addition of methyl triflate to the
corresponding dithiolate complex. In this reaction, the
methylation of the nitrogen base is competitive and ∼20%
of the N-methylated cation (6) is observed in the NMR
spectrum, eq 2. The latter cation was found to react with
Results and Discussion
Syntheses of Mononuclear Ni Diphosphine/Dithiolate
Complexes. Many nickel complexes with mixed phosphine/
thiolate ligation have been synthesized previously.10 In this
work, a series of related (PNPEt)Ni(II) complexes (where
PNPEt ) Et2PCH2N(CH3)CH2PEt2) have been synthesized
that incorporate anionic sulfur chelates. These include Ni-
(7) (a) Higuchi, Y.; Yagi, T.; Yasuoka, N. Structure 1997, 167, 1671-
1680. (b) Ogata, H.; Mizoguchi, Y.; Mizuno, N.; Miki, K.; Adachi,
S.; Yasuoka, N.; Yagi, T.; Yamauchi, O. Hirota, S.; Higuchi, Y. J.
Am. Chem. Soc. 2002, 124, 11628-11635. (c) Volbeda, A.; Garcin,
E.; Piras, C.; de Lacey, A.; Fernandez, V. M.; Hatchikian, E. C.; Frey,
M.; Fontecilla-Camps, J. J. Am. Chem. Soc. 1996, 118, 12969-12996.
(8) (a) Doukov, T. L.; Iverson, T. M.; Seravalli, J.; Ragsdale, S. W.;
Drennan, C. L. Science 2002, 298, 567. (b) Darnault, C.; Volbeda,
A.; Kim, E. J.; Legrand, P.; Vernede, X.; Lindahl, P. A. ; Fontecilla-
Camps, J. C. Nat. Struct. Biol. 2003, 10, 271-279.
(9) (a) Dobbek, H.; Svetlitchnyi, V.; Gremer, L.; Huber, R.; Meyer, O.
Science, 2001, 293, 1281-1285. (b) Dobbek, H.; Svetlitchnyi, V.; Liss,
J.; Meyer, O. J. Am. Chem. Soc. 2004, 126, 5382-5387. (c) Doukov,
T. I.; Iverson, T. M.; Seravalli, J.; Ragsdale, S. W.; Drennan, C. L.
Science 2002, 298, 567-572. (d) Volbeda, A.; Fontecilla-Camps, J.
C. Dalton Trans. 2005, 3443-3450.
(10) Autissier, V.; Clegg, W.; Harrington, R. W.; Henderson, R. A. Inorg.
Chem. 2004, 43, 3098. (b) Autissier, V.; Zarza, P. M.; Petrou, A.;
Henderson, R. A.; Harrington, R. W.; Clegg, W. C.; Inorg. Chem.
2004, 43, 3106. (c) Lee, C. M.; Chen, C. H.; Ke, S. C.; Lee, G. H.;
Liaw, W. F. J. Am. Chem. Soc. 2004, 126, 8406. (d) Martins, M. A.
C.; Silva, R. M.; Huffman, J. C. Polyhedron 2002, 21, 421. (e) Tenorio,
M. J.; Puerta, M. C.; Valerga, P. J. Chem. Soc., Dalton Trans. 1996,
1935. (f) Hsiao, Y. M.; Chojnacki, S. S.; Hinton, P.; Reibenspies, J.
H.; Darensbourg, M. Y. Organometallics 1993, 12, 870. (g) Bow-
maker, G. A.; Boyd, P. D. W.; Campbell, G. K. Inorg. Chem. 1982,
21, 2403. (h) Rauchfuss, T. B.; Roundhill, D. M. J. Am. Chem. Soc.
1975, 97, 3386. (i) Darkwa, J. Inorg. Chim Acta 1997, 257, 137. (j)
Pfeiffer, E.; Pasquier, M. L.; Marty, W. HelV. Chim. Acta 1984, 67,
654. (k) Schmidt, M.; Hoffmann, G. G. J. Organomet. Chem. 1977,
124, C5.
tetramethylguanidine to reform the neutral starting material,
which could then be readily separated from the desired
cationic thiolate/thioether complex (5) by toluene extraction.
The 31P NMR spectrum of the cationic thioether complex,
isolated by this route, showed a single AB pattern at 4.7
and 17.2 ppm.
An X-ray diffraction study was carried out on a single
crystal of 5. A perspective drawing of the cation structure is
shown in Figure 1, and selected bond distances and angles
are given in Table 1.
The structural parameters of the PNPEt ligand, which
adopts a chair conformation, are similar to those observed
previously for [Ni(PNBuP)2]2+ and [Ni(PNMeP)(dmpm)]2+
(where PNBuP ) Et2PCH2N(Bu)CH2PEt2, PNMeP ) Et2-
PCH2N(Me)CH2PEt2 , and dmpm ) Me2PCH2PMe2).11 The
structure of 5 shows relatively little distortion from a square
planar geometry. The bite angle of the diphosphine ligand
(11) Curtis, C. J.; Miedaner, A.; Ciancanelli, R.; Ellis, W. W.; Noll, B. C.;
DuBois, M. R.; DuBois, D. L. Inorg. Chem. 2003, 42, 216.
Inorganic Chemistry, Vol. 46, No. 4, 2007 1269

Redin et al.
Figure 1. Drawing of the cation of [Ni(PNP)(SC2H2SCH3)]OTf, 5,
showing the atom numbering scheme. Thermal ellipsoids are shown at 50%
probability level.
where the backbone of the dithiolate ligand has been varied
from two to four carbons (dithiolate ) 1,2-SC2H4S, 1,3-
SC3H6S, o-SCH2C6H4CH2S), respectively.
Table 1. Selected Bond Distances (Å), Bond Angles (deg), and
Dihedral Angles (deg) for [Ni(PNPEt)(SC2H4SCH3)](OTf), 5
A slightly different procedure was used to prepare the
series of related propanedithiolate-bridged nickel dimers with
a variation in chelating diphosphine ligands. The reaction
of 2 equiv of Ni(diphosphine)Cl213,14 with 1,3-propanedithiol
and excess triethylamine in chloroform led to the formation
of [(µ-SC3H6S)Ni2(diphosphine)2]2+ complexes (10-12),-
which were isolated as the PF6 salts [diphosphine ) dppp
(1,3-bis(diphenylphosphino)propane), 10; dmpp (1,3-bis-
(dimethylphosphino)propane), 11; and dppx (R,R′-bis(diphe-
Bond Distances
Ni(1)-P(1)
Ni(1)-P(2)
Ni(1)-S(1)
Ni(1)-S(2)
Ni(1)‚‚‚N(1)
2.183(2)
2.179(2)
2.188(2)
2.208(2)
3.781(10)
S(1)-C(1)
S(2)-C(2)
S(2)-C(3)
C(1)-C(2)
1.845(8)
1.815(8)
1.801(8)
1.552(1)
Bond Angles
91.93(8)
90.45(8)
91.66(8)
87.77(8)
P(1)-Ni(1)-P(2)
S(1)-Ni(1)-S(2)
P(1)-Ni(1)-S(2)
P(2)-Ni(1)-S(1)
Ni(1)-S(1)-C(1) 106.55(33)
Ni(1)-S(2)-C(2) 103.98(28)
Ni(1)-S(2)-C(3) 101.38(31)
C(2)-S(2)-C(3) 102.82(43)
15.58(9)
P(1)-Ni(1)-P(2)‚‚‚S(1)-Ni(1)-S(2)
is slightly larger than that of the thiolate/thioether ligand,
but both bite angles are near 90o. The dihedral angle between
the two planes defined by the P-Ni-P and S-Ni-S atoms
is 15.6o. In the structure of [Ni(PNBuP)2]2+ a significantly
larger dihedral angle between the two chelating ligands was
observed (34.1o) as a result of steric interactions between
ethyl substituents on all four phosphorus donors.
Synthesis of Dinuclear Complexes [(µ-dithiolate)Ni2-
(diphosphine)2]2+. Few examples of the bis(thiolate) bridged
dinuclear nickel structure with phosphine ligands have been
reported previously.10i,12 The neutral bis(sulfido) bridged
dimer [(dippe)Ni(µ-S)]2 has been synthesized and structurally
characterized, and double alkylation with methyl triflate
formed the bis(thiolate) derivative, [(dippe)Ni(µ-SMe)]2-
(OTf)2,12 where dippe ) bis(diisopropylphosphino)ethane].
However, electrochemical studies of this derivative were not
pursued. It is interesting that attempts to react the neutral
sulfido-bridged dimer with 1,3-dichloropropane to form a
dithiolate-bridged derivative were reported to be unsuccess-
ful, resulting instead in the formation of the mononuclear
complexes (dippe)Ni(S2C3H6) and (dippe)NiCl2.12b
nylphosphino)o-xylene), 12), eq 4. Together with the dppe
complex 8, discussed above, this series of complexes
provides a comparison of diphosphine ligands with five-, six-,
and seven-membered chelate rings.
1
Characterization of all of the new compounds by H and
31P NMR spectroscopy, mass spectroscopy, and elemental
analyses support their formulation. For example, the ESI
mass spectrum of [(µ-SC2H4S)Ni2(dppe)2][BF4]2 showed a
strong pattern centered at m/z ) 1091 that corresponds to
the loss of one BF4 ion from the dinuclear complex, and a
single resonance in the 31P NMR spectrum was observed at
59.8 ppm in CDCl3. In the room-temperature 31P NMR
spectrum of the product with the xylenedithiolate bridge, 9,
two phosphorus singlets were observed, suggesting that the
rate of inversion for the backbone of the dithiolate ligand is
slow on the NMR time scale. Variable-temperature NMR
studies indicated that coalescence of the two peaks occurred
at 38 °C, and the coalescence temperature was used to
estimate a ∆G( of 19 kcal/mol for this inversion process.15
In the spectra of [(µ-SC3H6S)Ni2((dppe)2]2+, we were unable
to detect evidence for inequivalent diphosphine ligands that
In this work, a series of dithiolate-bridged dimers have
been successfully synthesized by reaction of (dppe)Ni-
(dithiolate)10g,h,k (where dppe ) 1,2-bis(diphenylphosphino-
ethane) with [Ni(CH3CN)6](BF4)2 in the presence of 1 equiv
of dppe, carried out at room temperature in acetonitrile
solution (e.g., eq 3 for complex 8). The resulting red products
are formulated as [(µ-dithiolate)Ni2(dppe)2](BF4)2 (7-9)
(13) Van Hecke, G. R.; Horrocks, W. D. Inorg. Chem. 1966, 5, 1968. (b)
Booth, G.; Chatt, L. J. Chem. Soc. 1965, 3238.
(14) (a) Brown, M. D.; Levason, W.; Reid, G.; Watts, R. Polyhedron 2005,
24, 75. (b) Camalli, M.; Caruso, F.; Chaloupka, S.; Leber, E. M.;
Rimml, H.; Venanzi, L. M. HelV. Chim. Acta 1990, 73, 2263.
(15) Williams, D. H.; Fleming, I. Spectroscopic Methods in Organic
Chemistry, 5th ed.; McGraw-Hill: London, 1995; pp 102-105.
(12) (a) Vicic, D. A.; Jones, W. D. J. Am. Chem. Soc. 1999, 121, 7606.
(b) Oster, S. S.; Lachicotte, R. J.; Jones, W. D. Inorg. Chim. Acta
2002, 330, 118.
1270 Inorganic Chemistry, Vol. 46, No. 4, 2007

Studies of Structural Effects of Half-WaWe Potentials
Table 3. Electrochemical Data for Nickel Diphosphine/Dithiolate
Complexes
Mononuclear Complexes
E1/2a (∆Ep, mV)b
E1/2a (∆Ep, mV)b
complex
PNPEt, Ni(II/I)
PNPPh, Ni(II/I)
[Ni(PNP)(S2CNEt2)]BF4
Ni(PNP)(SC2H4S)
Ni(PNP)(SC3H6S)
Ni(PNP)(SCH2C6H4CH2S)
[Ni(PNP)(MeSC2H4S)]OTf
-1.64 (113)
-2.34 (110)
-2.26 (131)
-2.02 (93)
-1.47c (81)
-1.43b (110)
-2.02 (74)
-1.94 (77)
-1.75 (96)
Dinuclear Complexes
E1/2a (∆Ep, mV)b
E1/2a (∆Ep, mV)b
dithiolate bridge variation
Ni(II/I)
Ni(I/0)
[Ni(dppe)]2(SC2H4S)2+, 7
[Ni(dppe)]2(SC3H6S)2+, 8
[Ni(dppe)]2(S-o-xylyl-S)2+, 9
-1.02 (100)
-1.18 (100)
-1.12 (90)
-1.26 (80)
-1.08 (irr)
-1.30 (irr)
-2.10 (90)
-2.47 (105)
-2.02 (75)
-2.16 (70)
-2.04 (80)
-2.44 (irr)
Figure 2. Perspective drawing of [(µ-SC3H6S)Ni2(dppe)2]2+, 8. Thermal
ellipsoids are shown at the 50% probability level.
Table 2. Selected Bond Distances and Angles for
[(µ-SC3H6S)Ni2(dppe)2](BF4)2, 8
Diphosphine Variation
-1.12 (90)
[Ni(dppe)]2(SC3H6S)2+, 8
-2.02 (75)
-2.16 (70)
-1.92 (115)
-2.35 (irr)
-2.54 (irr)
-1.75 (160)
-1.99 (irr)
-2.14 (irr)
-1.26 (80)
distances, Å
Ni(1)-P(1)
Ni(1)-P(2)
Ni(1)-S(1)
Ni(1)-S(2)
Ni(2)-P(3)
Ni(2)-P(4)
Ni(2)-S(1)
angles, deg
P(1)-Ni(1)-P(2)
P(1)-Ni(1)-S(1)
P(2)-Ni(1)-S(1)
P (1)-Ni(1)-S(2)
P(2)-Ni(1)-S(2)
Ni(1)-S(1)-Ni(2)
Ni(1)-S(2)-Ni(2)
[Ni(dppp)]2(SC3H6S)2+, 10
-1.17 (95)
2.175 (2)
2.164(2)
2.221 (2)
2.217 (2)
2.182 (2)
2.181(2)
2.228(2)
86.44(7)
91.11(7)
169.89(8)
176.86(8)
95.85(7)
80.37(6)
80.57(6)
-1.29 (140)
[Ni(dppx)]2(SC3H6S)2+, 12
-1.13 (140)
-1.51 (irr)
a E1/2 ) (Epc + Epa)/2 reported vs the ferrocene/ferrocenium couple. All
experiments were performed at a glassy carbon electrode in 0.3 M Bu4NBF4/
CH3CN at scan rates of 100 or 200 mV/s. Values of ipc/ipa = 1 unless
otherwise noted. b Each value of ∆Ep is similar to that of the ferrocene/
would result from a rigid orientation of the propanedithiolate
bridge. A single 31P resonance was observed near 60 ppm
in the low-temperature 31P NMR spectra at temperatures
down to -80 °C in CD2Cl2. These ligand differences are
similar to those reported for [(CO)3Fe]2(µ-dithiolate) com-
plexes in which inversion of the ligand backbone of the
xylenedithiolate complex was not observed by NMR spec-
troscopy while fluxional behavior was seen for the analogous
propanedithiolate complex.16
c
ferrocenium couple in the same solution. ipc/ipa ≈ 3.
X-ray Diffraction Study of [(µ-SC3H6S)Ni2(dppe)2]-
(BF4)2. Single crystals of [(µ-SC3H6S)Ni2(dppe)2](BF4)2, 8,
were obtained by slow ether diffusion into an acetonitrile
solution, and an X-ray diffraction study was carried out. The
structure confirms that the dication contains two square
planar nickel centers symmetrically bridged by an η2-
propanedithiolate ligand. A perspective drawing of the
dication is shown in Figure 2, and selected bond distances
and angles are included in Table 2. The two dithiolate sulfurs
lie above the plane of the Ni-Ni vector. The Ni-S distances
are all very similar at 2.22 Å and the Ni-S-Ni angles, which
are identical within error, average 80.5°. The dihedral angle
between each nickel square plane is 128.45 (9)°, and the
Ni-Ni distance is 2.871(1) Å.
Figure 3. Cyclic voltammogram of Ni(PNPEt)(SC3H6S) (3 × 10-3 M)
recorded at 200 mV/s in CH3CN/Bu4NBF4. Initial scan is negative, starting
at -0.8 V. (The sample contains a small amount of decamethylferrocene
which shows a reversible wave at -0.51 V.)
Electrochemical Studies. a. Mononuclear Complexes.
The electrochemical data for the new nickel complexes are
summarized in Table 3. In the series of mononuclear
complexes, each of the dithiolate and dithiocarbamate
derivatives displays a reversible reduction wave and an
irreversible oxidation, as shown in the cyclic voltammogram
(CV) for Ni(PNPEt)(SC3H6S) in Figure 3. (Additional poorly
defined waves are observed at more negative potentials.) A
In the previously reported neutral bis(sulfido) bridged
dimer [(dippe)Ni(µ-S)]212b, the dihedral angle between the
nickel square planes increases to 140.2° and the Ni-Ni
distance is 2.941(2) Å. Rationales for the nonplanar nature
of the M2(µ-S)2 core have been discussed.12b,17
coulometric experiment on Ni(PNPEt/SC H4S) carried out at
2
a potential negative of the first reduction wave (-2.7 V)
resulted in the passage of 1.05 F per mole of complex,
confirming that the wave corresponds to a one-electron
process, and this is assigned to the Ni(II/I) couple. In this
work we have focused our attention on the half-wave
potentials assigned to the Ni(II/I) couples because these are
the potentials that have been shown previously to be most
(16) Lyon, E. J.; Georgakaki, I. O.; Reibenspies, J. H.; Darensbourg, M.
Y. J. Am. Chem. Soc. 2001, 123, 3268-3278.
(17) Capdevila, M.; Clegg, W.; Gonza´lez-Duarte, P.; Jarid, A.; Lledo´s, A.
Inorg. Chem. 1996, 35, 490.
Inorganic Chemistry, Vol. 46, No. 4, 2007 1271

Redin et al.
affected by chelate bite size in the [Ni(diphosphine)2]2+
complexes. For the series of neutral PNPEt/dithiolate deriva-
tives the E1/2 values for the (II/I) couples are all more negative
than -2.0 V vs Fc0/+. As expected, the anionic thiolate
donors make these values much more negative than the Ni-
(II/I) half-wave potential for the hydrogen-activating com-
plex, [Ni(PNPEt)2]2+, observed at -0.64 V vs Fc.
Comparison of the data for the PNPPh derivatives shows
the effect of the phosphine substituents on the half-wave
potentials. Each of the waves assigned to the Ni(II/I) couples
is ca. 0.3 V more positive than that of the analogous PNPEt
complexes. As indicated in Table 3, the cationic dithiocar-
bamate and thiolate/thioether complexes are significantly
easier to reduce than the neutral derivatives. For example,
the E1/2 value for the first reduction wave for [Ni(PNPEt)-
(MeSC2H4S)]OTf was found to be -1.47 V vs Fc+/0, more
than 0.5 V positive of the corresponding dithiolate complex.
In both series of mononuclear dithiolate complexes, the
E1/2 values shift to more positive values by ca. 80 mV as
the size of the dithiolate chelate ring increases from five- to
six-membered rings and by ca. 200 mV in going from six-
to seven-membered rings. For the bis(diphosphine) com-
plexes, a similar effect of increasing the chelate bite angle
on the Ni(II/I) couple was observed for the systematic
variation of a single chelate ring.2,5 The effect was attributed,
through both theoretical and structural studies, to a tetrahedral
distortion of these complexes that results from an increasingly
large dihedral angle between the two planes defined by the
two phosphorus atoms of each diphosphine ligand and the
metal. This distortion reduces the antibonding overlap
Figure 4. Cyclic voltammogram of 8 (3 × 10-3 M) recorded at a glassy
carbon electrode at a scan rate of 1000 mV/s in CH3CN/Et4NBF4. Initial
scan direction is negative. The wave at 0.0 V corresponds to the ferrocene/
ferrocenium couple used as an internal standard.
the rapid decomposition of the reduced dimer. Further
evidence for this decomposition is discussed below. Half-
wave potentials for the other dinuclear complexes at scan
rates of 100 mV/s are included in Table 3.
In the dinuclear complexes studied here, no systematic
trend has been observed for the Ni(II/I) couples as the size
of the dithiolate chelate is increased or as the chelate bite
angle of the diphosphine ligand is varied (Dinuclear Com-
plexes, Table 3). One possibility for the failure to observe a
positive shift in the Ni(II/I) half-wave potential as the number
of atoms bridging the dithiolate ligand increases is that the
presence of the four-membered Ni2S2 ring controls the
S-Ni-S bond angles and minimizes steric interactions
between the bridging dithiolate ligand and the two diphos-
phine ligands. As a result, changing the number of bridging
atoms in the dithiolate ligand has little effect on the geometry
of the dimer and on the (II/I) or (I/0) couples. A similar
insensitivity (as determined by CO stretching frequencies)
to the nature of the bridging dithiolate ligands has been
reported previously for [(CO)3Fe]2(µ-dithiolate) complexes.16
The Ni(II/I) couples of the dinuclear complexes are also not
affected in a systematic way by an increase in the natural
bite angle of the diphosphine ligands. This is tentatively
attributed to a minimal steric interaction of the substituents
on the diphosphine ligand with the dithiolate bridge.
Reactions of Nickel Complexes with Hydrogen. We
wished to determine if the correlation between the Ni(II/I)
couple and the hydride acceptor ability observed for [Ni-
(diphosphine)2]2+ complexes extends to planar Ni(II) com-
plexes with one diphosphine and one dithiolate chelate. To
probe the ability of the (PNP)nickel(dithiolate) complexes
to activate hydrogen by heterolytic cleavage, they were
treated with hydrogen (0.82 atm) in the presence of tetram-
ethylguanadine at room temperature. No reactions were
observed for the series of Ni(PNPEt)(dithiolate) or Ni(PNPPh)-
(dithiolate) complexes, which show Ni(II/I) half-wave po-
tentials over the range of -1.75 to -2.34 V. The dithiocar-
bamate complex, [Ni(PNPPh)(S2CNEt2)]BF4 (E1/2 ) -1.43
V vs Fc), reacted in acetonitrile with H2 (0.82 atm) in the
presence of tetramethylguanidine over a period of several
days (eq 5). No reaction was observed with this base in the
absence of hydrogen or with hydrogen and the weaker base,
triethylamine. Although the NMR spectrum of the reaction
solution showed only resonances for the starting complex
2
2
between the σ orbitals of the phosphine ligands and the dx -y
orbital of the metal for the LUMO of the complexes. As a
result, the energy of the LUMO drops rapidly in energy as
the natural bite angle of the ligand increases and the reduction
potential becomes less cathodic. Similar effects appear to
be in play for these mononuclear dithiolate complexes. The
trend in half-wave potentials for these derivatives is consis-
tent with the greater accessibility of a tetrahedrally distorted
geometry for the reduced Ni product as the chelate ring size
increases. Similar structural distortions imposed by the
protein matrix in nickel-containing enzymes may also
significantly affect the potentials of the Ni(II/I) couples and
provide a mechanism in addition to charge and substituent
effects for controlling ease of reduction.
b. Dinuclear Complexes. Within the series of dinuclear
complexes, the CVs of all the complexes show similar
features. For example, the CV for 8 at a scan rate of 1.0 V/s
shows a pair of closely spaced reversible waves at -1.12
and -1.27 V vs Fc+/0 that are attributed to the Ni(II/I)
couples of electronically coupled metal ions, Figure 4. The
waves show slightly less reversible character at a scan rate
of 100 or 200 mV/s. Additional reductions near -2 V were
also observed, and when the scan direction is reversed at
-2.6 V, new anodic waves are observed with Ep at -0.68
and -0.86 V (see Figure S1 for CVs over an expanded range
of potentials). The peak potentials for the new anodic waves
are very similar to the potentials reported for the oxidation
of Ni(dppe)2,1 suggesting that this monomer is formed in
1272 Inorganic Chemistry, Vol. 46, No. 4, 2007

Studies of Structural Effects of Half-WaWe Potentials
(slightly broadened), over a period of about a week a mixture
of yellow and red crystalline precipitates was observed. The
reaction was found to involve metal ion reduction, as well
as ligand exchange, and the yellow crystals were identified
by an X-ray diffraction study as Ni(PNPPh)2, 13. Structural
data on 13 are included in the Supporting Information. The
formation of a Ni(0) product may involve metal hydride
formation followed by deprotonation, but no hydride inter-
mediate was detected by NMR spectroscopy.
structure, and mixtures of mononuclear products, including
dppeNi(SC3H6S), dppeNi(CN)2,19 and (dppe)2Ni, were ob-
served by 31P NMR spectroscopy.
Comparison of Redox Properties for Several Nickel
Dimers. A number of unsymmetrical nickel dimers with
bridging thiolate ligands have been reported recently as
models for a portion of the A-cluster active site in bifunc-
tional CO dehydrogenase/acetyl CoA synthase enzymes.8
These dimers involve a tetradentate N2S2 nickel complex in
which the terminal thiolate ligands bridge to a second nickel-
(II) or Ni(0) center.20-25 In several cases, the second nickel-
(II) has been stabilized with the bidentate ligand bis-
(diphenylphosphino)ethane, dppe, e.g., structures 14-17.20-23
[(Et2NCS2)Ni(PNPPh)]+ + H2 + TMG f Ni0(PNPPh)2 +
other products (5)
The thiolate/thioether derivative [Ni(PNP)(MeSC2H4S)]BF4
(E1/2 ) -1.45 V vs Fc) also reacted with hydrogen in the
presence of tetramethylguanidine. However, a complex
mixture of products was observed in the 31P NMR spectra,
and this reaction was not pursued.
Reactions of the Dinuclear Nickel Complexes. The
complexes of the formulas [diphosphine)Ni]2(µ-dithiolate)]2+
have modest half-wave potentials and we were interested in
determining whether this feature permitted reactivity with
hydrogen. However, the complexes were not stable in the
presence of the strong base tetramethylguanadine, and no
reactions with hydrogen (0.8 atm) were observed in the
presence of triethylamine. Other aspects of the reduction
chemistry of these dimers have also been investigated. The
reaction of 8 with 2-4 equiv of PPNBH4 (PPN ) bis-
(triphenylphosphine)iminium), LiAlH4, or sodium amalgam
in THF was monitored by 31P NMR spectroscopy. The
reaction proceeded rapidly to completion, and the formation
of a single new product was observed by 31P NMR
spectroscopy with a resonance at 44.2 ppm. No evidence
for formation of a nickel hydride was observed in the
corresponding 1H NMR spectrum. The product was identified
Electrochemical studies of 14 and 15 have revealed a
reversible one-electron reduction process attributed to the
nickel(dppe) portion of the dimer with additional reductions
at more cathodic potentials.20-21 For the dicationic unsym-
metrical dimer 14, E1/2 was reported to be -0.47 V vs SCE
in CH2Cl2, or ∼ -0.9 vs ferrocene.26 Although differences
in solvent and reference electrode make comparisons with
previous work approximate, the Ni(II/I) half-wave potential
is clearly less negative than those observed for the dicationic
series of symmetrical dimers reported here (-1.2 vs Fc). A
similar trend is observed when the half-wave potential of
the neutral unsymmetrical dimer 15 is compared with the
symmetrical dimers. With similar ligand environments, a
neutral complex should be significantly more difficult to
1
as the known complex Ni(dppe)2 by comparison with an
independently prepared sample of the monomer and by
further protonation of the reduced product to form the
previously reported hydride complex [HNi(dppe)2]+.2 Ni-
(dppe)2 was also the major product observed in the reduction
reaction of [(dppeNi)2(µ-SC2H4S)](PF6)2, 7, with sodium
amalgam, although 31P NMR resonances for other unidenti-
fied products were also present in this case. These results
indicate that the dimeric complexes undergo a decomposition
reaction under these reducing conditions, e.g., eq 6. Although
(19) Rigo, P.; Corain, B.; Turco, A. Inorg. Chem. 1968, 7, 1623.
(20) Wang, Q.; Blake, A. J.; Davies, E. S.; McInnes, E. J. L.; Wilson, C.;
Schroder, M. Chem. Commun. 2003, 3012.
(21) Krishnan, R.; Riordan, C. G. J. Am. Chem. Soc. 2004, 126, 4484.
(22) Rao, P. V.; Bhaduri, S.; Jiang, J.; Holm, R. H. Inorg. Chem. 2004,
43, 5833.
(23) Harrop, T. C.; Olmstead, M. M.; Mascharak, P. K. J. Am. Chem. Soc.
2004, 126, 14714.
(24) Linck, R. C.; Spahn, C. W.; Rauchfuss, T. B.; Wilson, S. R. J. Am.
Chem. Soc. 2003, 125, 8700.
(25) Golden, M. L.; Rampersad, M. V.; Reibenspies, J. H.; Darensbourg,
M. Y. Chem. Commun. 2003, 1824.
(26) In these comparisons, E1/2 for the ferrocene/ferrocenium couple is
estimated to be +0.40 V vs SCE (Connelly, N. G.; Geiger, W. E.
Chem. ReV. 1996, 96, 879) and +0.54 V vs NHE (Faulkner, L. R.;
Bard, A. J. Electrochemical Methods; John Wiley and Sons: New
York, 2001; p 811).
nickel thiolate complexes were not identified in eqs 5 or 6,
polynuclear homoleptic nickel(II) complexes with ethane-
and propanedithiolate ligands have been characterized previ-
ously.18 The reaction of 8 with tetrabutylammonium cyanide
in acetonitrile also resulted in disruption of the dinuclear
(18) Tremel, W.; Kriege, M.; Krebs, B.; Henkel, G. Inorg. Chem. 1988,
27, 3886.
Inorganic Chemistry, Vol. 46, No. 4, 2007 1273

Redin et al.
Concluding Remarks. Our studies have shown that the
half-wave potentials of the Ni(II/I) couples for mononuclear
complexes Ni(diphosphine)(dithiolate) shift to more positive
values as the chelate ring size of the dithiolate is increased.
This presumably occurs through structural distortions similar
to those observed previously for [Ni(diphosphine)2]2+ com-
plexes. In contrast, the dinuclear nickel complexes with
terminal diphosphines and a bridging dithiolate do not show
a systematic change in half-wave potentials of the Ni(II/I)
couple as the chelate ring size of dithiolate or diphosphine
is varied. This does not imply that the tetrahedral distortions
imposed by the protein structure on the dinuclear clusters in
enzymes do not result in changes in the Ni(II/I) couple.
However, attempts to mimic this effect by extending concepts
regarding ligand bite angles observed for synthetic mono-
nuclear complexes to synthetic dinuclear complexes will
likely not be productive.
Figure 5. Cyclic voltammograms for the reduction of 8 under an
atmosphere of nitrogen (red scan) and under an atmosphere of carbon
monoxide (blue scan) at a scan rate of 1.0 V/s in CH3CN/Et4NBF4.
reduce than a dication (e.g., see Table 3). However, in this
case the E1/2 for the neutral complex (-0.53 V vs NHE in
CH3CN,21 or ∼ -1.1 vs ferrocene)26 is in fact similar to the
values for the symmetrical dications (-1.2 V vs ferrocene).
The potential for 15 occurs at a much less negative value
than expected relative to 8. The overall effect of the N2S2Ni
“ligand” in 14 and 15 appears to be a withdrawal of electron
density from the Ni(dppe) center and a stabilization of the
reduced products.
Experimental Section
Instrumentation. 1H, 13C, and 31P NMR spectra were recorded
1
on a Varian Inova 400 MHz spectrometer. H and 13C chemical
In the study of 15, the Ni(dppe) center in the presence of
CO was proposed to undergo an ECE electrochemical
process in which reduction to Ni(I) is followed by CO
coordination and further reduction to Ni(0).20 The one-
electron reduction product of 17, formed from reaction with
NaBH4, also reacted with CO to form a (dppe)Ni-CO adduct
(vCO ) 1997 cm-1).23 We wished to determine whether a
symmetrical {(µ-dithiolate)[Ni(dppe)]2}2+ showed similar
reactivity with carbon monoxide. No reaction of 8 with CO
(1 atm) was detected by 31P NMR spectroscopy, but
electrochemical studies indicate that the reduced form of 8
does react under these conditions. When the cathodic scan
of the CV of the complex is repeated under an atmosphere
of CO, a new irreversible reduction wave with increased
current intensity occurs with Ep at -1.08 V, and upon
reversing the scan direction, irreversible oxidation waves are
observed near 0 V, Figure 5. The change in the CV under
CO is characteristic of an ECE mechanism involving
formation and reduction of a CO adduct.21 The changes in
the CV of 8 are readily reversed when the solution is purged
with nitrogen.
Unfortunately, the CO adduct does not appear to be stable
on the time scale of a chemical reduction. When the reaction
of 8 with LiAlH4 in THF was carried out in the presence of
1 atm of CO over a period of e1 h, we observed the
formation of a new product with a singlet at 46.9 ppm in
the 31P NMR spectrum. However, the same product is
observed when CO is bubbled into a THF solution of Ni-
(dppe)2 and when [Ni(dppe)2]2+ is reduced in the presence
of CO. The new resonance is therefore assigned to a
mononuclear product, tentatively identified as (dppe)Ni(CO)2,
which has been previously reported.27 Thus, under reducing
conditions, both the mononuclear complexes and the di-
nuclear complexes studied in this work tend to form the
thermodynamically stable Ni(0) complexes with either CO
or phosphine ligands.
shifts are reported in ppm and are referenced to residual solvent
protons. 31P NMR spectra were proton-decoupled, and chemical
shifts in ppm are referenced to external phosphoric acid. Hetero-
nuclear multiple quantum coherence (HMQC), field gradient
heteronuclear single quantum coherence (gHSQC), and 1H-1H
correlation spectroscopy (COSY) NMR experiments were carried
1
out on the Inova instrument in order to assign 13C and H shifts.
Electrospray ionization (ESI) mass spectra were collected using
an HP 59987A electrospray with an HP 5989B mass spectrometer.
Electronic spectra were recorded on an Agilent 8453 UV-vis
spectrometer. CVs were obtained at 21 ( 2 °C using a Cypress
Systems model CS-1200 computer-aided electrolysis system with
a three-electrode arrangement. The working electrode was a glassy
carbon disk with a 2 mm diameter, and the counter electrode was
a glassy carbon rod with a 3 mm diameter. The pseudo-reference
electrode was a silver chloride-coated silver wire. Either ferrocene
(Cp2Fe) or decamethylferrocene (Cp*2Fe) (E1/2 ) 0.51 vs [Cp2Fe]0/+
)
was added to the solution as an internal standard after recording
the CV of each nickel complex, and all reported potentials are
referenced to the [Cp2Fe]0/+ couple. CVs were recorded at a scan
rate of 100 or 200 mV/s, unless otherwise indicated, under a
nitrogen atmosphere in 0.3 M Et4NBF4/acetonitrile solutions.
Elemental analyses were performed by Desert Analytics Laboratory,
Tucson, AZ.
Methods and Materials. All reactions were carried out using
standard Schlenk or glove box techniques under a nitrogen
atmosphere. Solvents were dried by conventional methods and
distilled prior to use. Dithiol and most diphosphine reagents were
purchased from commercial suppliers and used without purification.
PNPEt,11 [Ni(CH3CN)6](BF4)2,28 (dppp)NiCl2,29 (dppe)NiCl2,30 (dp-
px)NiCl2,31 (dppe)Ni(SC3H6S),10k (dppe)Ni(SC2H4S),10h and (dppe)-
Ni(SCH2C6H4CH2S)10g were synthesized by published procedures.
Controlled-Potential Coulometry. A cell containing two com-
partments separated by a Nafion membrane was used for this
(28) Hathaway, B. J.; Holah, D. G.; Underhill, A. E. J. Chem. Soc. 1962,
2444.
(29) Van Hecke, G. R.; Horrocks, W. D. Inorg. Chem. 1966, 5, 1968.
(30) Booth, G.; Chatt, J. J. Chem Soc. 1965, 3238.
(31) (a) Brown, M. D.; Levason, W.; Reid, G.; Watts, R. Polyhedron 2005,
24, 75. (b) Camalli, M.; Caruso, F.; Chaloupka, S.; Leber, E. M.;
Rimml, H.; Venanzi, L. M. HelV. Chim. Acta 1990, 73, 2264.
(27) Sacco, A.; Mastrorilli, P. J. Chem. Soc., Dalton. Trans. 1994, 2761.
1274 Inorganic Chemistry, Vol. 46, No. 4, 2007

Studies of Structural Effects of Half-WaWe Potentials
experiment. Reticulated vitreous carbon (1 cm diameter × 2.5 cm
length) was used as the working electrode. A glass tube fitted with
a Vycor frit at the bottom, containing 0.3 M Bu4NBF4 solution
and a silver wire was also inserted into the working half of the
cell. In the other half of the cell a platinum wire in a solution of
ferrocene/ferrocenium was used as the counter electrode. The
working half of the cell was typically filled with 10 mL of 0.3 M
Bu4NBF4 in acetonitrile. [Ni(PNP)(SC2H4S)] (0.093 mmol) was
added to the solution. Controlled-potential coulometry was per-
formed at -2.70 V versus the ferrocene/ferrocenium reference
electrode. A total of 9.48 C of charge was passed before the current
decayed to background levels. This corresponds to 1.05 F per mole
of catalyst, and it supports a one-electron process for the first
reduction wave.
vacuum. Yield: 0.12 g, 0.20 mmol, 20%. 1H NMR (CD3CN): 7.79
(m, meta-aromatics); 7.44 (apparent tr, para-aromatics); 7.37
(apparent tr, ortho-aromatics); 3.22 (apparent tr, PCH2N); 2.32 (m,
NCH3); 2.19 (m, SCH2). 31P NMR (CD3CN): 9.53 (s). Anal. Calcd
for C30H33NNiP2S2: C, 60.83; H, 5.62; N, 2.36. Found: C, 60.59;
H, 5.86; N, 2.20. CV (CH3CN): E1/2, V vs ferrocene (∆Ep, mV):
-1.94 (77). Oxidation, Ep, -0.11 (irrev).
Syntheses of Dinuclear Complexes. The syntheses of the
dinuclear nickel complexes follow two routes, and the procedures
and characterization data are given for two representative examples.
The characterization data for the remaining complexes are given
in the Supporting Information.
Synthesis of [(µ-SC3H6S)Ni2(dppe)2](BF4)2. Solid 1,2-bis-
(diphenylphosphino)ethane (0.092 g, 0.23 mmol) was added to a
solution of [Ni(CH3CN)6.5](BF4) (0.115 g, 0.230 mmol) in aceto-
nitrile (10 mL). Solid Ni(dppe)(SC3H6S) (0.13 g, 0.23 mmol) was
then added to the resulting yellow solution. The dark red solution
was stirred at room temperature for 3 h and then dried in vacuum.
An acetonitrile solution of the resulting red oil yielded red crystals
by ether diffusion. Yield: 0.214 g, 78%. Anal. Calcd for
C55H54P4S2B2F8Ni2: C, 55.32; H, 4.56. Found: C, 55.12; H, 4.38.
1H and 13C resonances are assigned on the basis of gCOSY, gHSQC,
Synthesis of (PNPPh). A round-bottom flask was charged with
diphenylphosphine (10.40 g, 56.4 mmol) and degassed aqueous
formaldehyde (37 wt %, 4.40 mL, 58.7 mmol) in ethanol (20 mL).
After the solution was stirred for 30 min, CH3NH3Cl (1.80 g, 26.6
mmol) in ethanol /water (8 mL/4 mL) was added. The mixture was
stirred at room temperature for 3 h, and the solvent was removed
in vacuo. The product was extracted with diethyl ether (3 × 25
mL). Removal of the solvent from the combined extracts produced
a viscous oil. Yield: 10.66 g, 24.9 mmol, 94%. The product (10.66
g, 24.9 mmol) was dissolved in CD3CN (3.55 g) to form a standard
1
and gHMQC experiments. H NMR (20 °C, CDCl3): 7.62-7.42
(40H, m, PC6H5); 2.76 (8H, broad m, PCH2CH2); 2.23 (2H, broad
s, SCH2CH2CH2); 2.14 (4H, broad s, SCH2CH2CH2). 31P NMR
(CDCl3): 60.01 (s). 13C NMR (CDCl3): 133.5-126.7 ppm (PC6H5);
36.0 (s, SCH2CH2CH2); 32.3 (s, SCH2CH2CH2); 27.7 (m, PCH2CH2).
ESI+ (CH3OH): m/z 1107 {[(µ-SC3H6S)Ni2(dppe)2](BF4)}+; m/z
509 [(µ-SC3H6S)Ni2(dppe)2]2+. CV at 1000 mV/s (CH3CN): E1/2
or Ep if irreversible, V vs ferrocene (∆Ep, mV): -1.12 (74); -1.27
(80); -2.20 (80); -2.51 (irrev); -2.66 (irrev). Oxidations: 0.73
(270). UV-vis, (CH3CN) λmax, nm (ꢀ, M-1 cm-1): 281 (46 900);
351 (23 400).
[(µ-SC3H6S)Ni2(dppx)2](PF6)2 Solutions of 1,3-propanedithiol
(12 µL, 0.12 mmol) in chloroform (5 mL) and triethylamine (0.26
mL, 1.8 mmol) in chloroform (5 mL) were added to a solution of
Ni(dppx)Cl2 (0.150 g, 0.248 mmol) in chloroform (15 mL). The
resulting brown solution was stirred at room temperature for 1 h,
and the volume was reduced to 10 mL. A solution of NaPF6 (0.13
g, 0.25 mmol) in 1:1 ethanol/water (15 mL) was added, and the
resulting orange precipitate was filtered. Yield: 0.073 g, 40%. Anal.
Calcd for C67H62P6S2F12Ni2: C, 55.02; H, 4.27. Found: C, 54.84;
H, 4.48. 1H and 13C resonances are assigned on the basis of gHMQC
experiments. 1H NMR (CD3CN): 7.93-7.46 (40H, m, PC6H5); 6.64
(4H, m, PCH2CCH); 6.08 (4H, m, PCH2CCHCH); 4.53 (4H, pseudo
d, PCH2); 3.45 (4H, broad m, PCH2); 1.71 (2H, broad m, SCH2CH2-
CH2); 0.04 (4H, broad m, SCH2CH2CH2). 31P NMR (CD3CN):
11.01 (s). 13C NMR (CD3CN): 136.3-127.2 (PC6H5); 131.7 (s,
PCH2CCHCH); 128.3 (s, PCH2CCH); 37.4 (s, SCH2CH2CH2); 35.4
(s, SCH2CH2CH2); 34.22 (s, PCH2C6H4CH2). ESI+ (CH3CN): m/z
1318 {[(µ-SC3H6S)Ni2(dppx)2](PF6)}+; m/z 584 [(µ-SC3H6S)Ni2-
(dppx)2]2+. CV (CH3CN): E1/2 or Ep if irreversible, V vs ferrocene
(∆Ep, mV): -1.13 (140); -1.51 (irrev); -1.75 (160); -1.99 (irrev);
-2.14 (irrev). Oxidations: 0.75 (irrev); 0.92 (irrev); 1.4 (irrev).
UV-vis (CH3CN), λmax, nm (ꢀ, M-1 cm-1): 304 (67 000); 395
(40 000).
1
solution of 75 wt %. H NMR (CD3CN): 7.32 and 7.40 (two m,
2
aromatics); 3.43 (d, JPH ) 4.0 Hz, PCH2N); 2.58 (s, NCH3). 31P
NMR (CD3CN): -26.66 (s).
Syntheses of Mononuclear Complexes. The syntheses of the
mononuclear nickel complexes are all very similar, and the
procedures and characterization data are given for only two
representative examples. Spectroscopic characterization data for the
remaining complexes are given in the Supporting Information.
[Ni(PNPEt)(S2CNEt2)](BF4). Solid [Ni(CH3CN)6.5](BF4)2 (0.67
g, 1.34 mmol) was added to a solution of PNPEt (0.31 g, 1.33 mmol)
and NaS2CNEt2 (0.30 g, 1.34 mmol) in 40 mL of CH3CN. The
reaction mixture was stirred overnight at room temperature. The
solution was filtered, and the solvent was removed from the filtrate
under vacuo. The resulting solid was dissolved in 3 mL of CH3CN
and precipitated with 20 mL of Et2O. The dark yellow microcrys-
talline solid was collected by filtration and washed with 2 × 20
mL of Et2O. Yield: 0.69 g, 1.08 mmol, 81%. Anal. Cacld for
C16H37BF4N2NiP2S2 + 1 NaBF4: C, 30.08; H, 5.84; N, 4.38. Found
1
3
C, 30.52; H, 5.55; N, 4.31. H NMR (CD3CN): 3.73 (q, JHH
)
7.2 Hz, NCH2CH3); 2.77 (m, PCH2N); 2.44 (m, NCH3); 1.81 (m,
3
3
PCH2CH3); 1.27 (dt, JPH ) 17.4 Hz; JHH ) 7.9 Hz, PCH2CH3);
1.26 (tr, JHH ) 7.2 Hz, NCH2CH3). 13C NMR (CD3CN): (As-
3
signments are supported by dept and HSQC data.) 200.8 (s, NCS);
50.8 (tr, 1JPC ) 21.8 Hz, PCH2N); 50.1 (tr, 3JPC ) 12.5 Hz, NCH3);
44.7 (s, NCH2CH3); 17.6 (tr, 1JPC ) 14.2 Hz, PCH2CH3); 11.9 (s,
NCH2CH3); 8.5 (s, PCH2CH3). 31P NMR (CD3CN): 15.5 (s). MS
[Ni(PNP)(S2CNEt2)]+ - H2 simulated max m/z 439; found max
m/z 439 with matching isotope pattern. CV (CH3CN): E1/2, V vs
ferrocene (∆Ep, mV): -1.64 (113). Oxidation, Ep, 0.08 (irrev).
(PNPPh)Ni(SC3H6S). Solid [Ni(CH3CN)6.5](BF4)2 (0.50 g, 1.0
mmol) was added to a solution of PNPPh (0.43 g, 1.00 mmol) and
HSC3H6SH (0.11 g, 1.0 mmol) in 30 mL of CH3CN, upon which
a deep red color was observed. After the solution was stirred for
20 min, NEt3 (0.25 g, 2.47 mmol) was added. The solution, which
turned a darker red, was stirred for 2 h. The solvent was removed
under vacuo until ∼2 mL of solution remained, and the resulting
precipitate was filtered and dried under vacuum. The product was
recrystallized from ∼2 mL of CH3CN. Resulting red crystals were
filtered, washed with 3 × 25 mL hexanes, and dried overnight under
Reduction of [(µ-SC3H6S)Ni2(dppe)2](BF4)2, 5. In a typical
procedure monitored by NMR spectroscopy, solid LiAlH4 (0.002
g, 0.05 mmol) was added to a solution of 5 (0.020 g, 0.017 mmol)
in THF in an NMR tube. The resulting dark greenish brown solution
was shaken for 10 min. A single product was observed by 31P NMR-
(THF): 44.2 ppm (Ni(dppe)2). On a larger scale, a flask was charged
with solid [(µ-SC3H6S)Ni2(dppe)2](BF4)2 (0.050 g, 0.042 mmol)
Inorganic Chemistry, Vol. 46, No. 4, 2007 1275

Redin et al.
Table 4. Details of Data Collection and Structure Refinement for [Ni(PNPEt)(SC2H4SMe)]OTf, 6, and [(µ-SC3H6S)Ni2(dppe)2](BF4)2·CH3CN, 8
empirical formula
fw
C57H57B2F8NNi2P4S2, 8
1235.06
C15H34F3NNiO3P2S3, 6
550.26
T, K
143(2)
143(2)
cryst syst
space group
a (Å)
b (Å)
monoclinic
P21/c
12.1836(5)
47.750(2)
10.3067(4)
90
107.914 (1)
90
5705.4(4)
4
triclinic
P1h
8.781(3)
10.217(3)
14.543(4)
71.276(7)
82.684(7)
81.593(7)
1218.0(6)
2
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
density (calcd) (mg m-3
)
1.435
0.910
1.500
1.224
abs coeff (mm-1
)
reflns measd/indep
GOF
45 702/13 090
0.951
9693/5580
1.045
R1 [I > 2σ(I)] (all data)
0.0808 (0.1783)
0.1621 (0.1885)
0.679 and -0.605
0.0978 (0.1526)
0.2568 (0.3083)
1.943 and -1.131
wR2 [I > 2σ(I)] (all data)
largest diff. peak and hole (e/Å3)
and 1% sodium/mercury amalgam (1.87 g Na/Hg, 0.084 mmol Na).
Acetonitrile (10 mL) was added, and the reaction mixture was stirred
overnight. The cloudy reddish-brown solution was removed from
the mercury and then dried in vacuum. 31P NMR (THF): 44.2 ppm
(Ni(dppe)2). The sodium/amalgam reduction was also carried out
in THF, and the same product was observed. Addition of HBF4·
Et2O to the reduced product resulted in the formation of [HNi-
tions and were included in structure factor calculations but were
not refined. Details of the data collection, structure solution, and
refinement are given in Table 4.
Acknowledgment. This work was supported by a grant
from the National Science Foundation (CHE-0240106). NMR
instrumentation used in this work was supported in part by
the National Science Foundation CRIF program (CHE-
0131003). D.L.D. acknowledges the support of the Chemical
Sciences Program of the Office of Basic Energy Sciences
of the Department of Energy. The Pacific Northwest National
Laboratory is operated by Battelle for the U.S. Department
of Energy. We thank Dr. Michael McNevin (U. of Colorado)
for help with the X-ray diffraction data.
1
(dppe)2]BF4. 31P NMR (THF): 45.48 (CD3CN): 45.72. H NMR
(CD3CN): 7.34-7.20 (40H, m, PC6H5); 2.39 (8H, m, PCH2CH2P);
-12.99 (1H, pent, NiH).
X-ray Diffraction Studies. For each structure, a crystal of
appropriate size was mounted on a glass fiber using Paratone-N
oil, transferred to a Siemens SMART diffractometer/CCD area
detector, centered in the beam (Mo KR; λ ) 0.71073 Å; graphite
monochromator), and cooled to -120 ( 10 °C by a nitrogen low-
temperature apparatus. Preliminary orientation matrix and cell
constants were determined by collection of 60 frames, followed
by spot integration and least-squares refinement. A minimum of a
hemisphere of data was collected using 0.3° ω scans. The raw data
were integrated and the unit cell parameters refined using SAINT.
Data analysis was performed using XPREP. Absorption correction
was applied using SADABS. The data were corrected for Lorentz
and polarization effects, but no correction for crystal decay was
applied. Structure solutions and refinements were performed
(SHELXTL-Plus V5.0) on F2.32 All non-hydrogen atoms were
refined anisotropically. Hydrogens were placed in idealized posi-
Supporting Information Available: Characterization data for
additional complexes, Figure S1 and X-ray structural data including
tables of crystal and refinement data, atomic positional and thermal
parameters and interatomic distances and angles for 5, 8, and 13.
This material is available free of charge via the Internet at
http://pubs.acs.org.
IC061740X
(32) Sheldrick, G. M. SHELXTL-Plus: A Program for Crystal Structure
Determination, version 5.1; Bruker AXS: Madison, WI, 1998.
1276 Inorganic Chemistry, Vol. 46, No. 4, 2007