Homoleptic complexes of cobalt(0) and nickel(0,I) with 1,1′- bis(diphenylphosphino)ferrocene (dppf): Synthesis and characterization

Article abstract of DOI:10.1021/ic061185z

Reduction of Co(dppf)Cl₂ with 2 equiv of sodium naphthalenide in THF, in the presence of dppf, affords the homoleptic complex Co(dppf) ₂, 1, isolated in 65% yield as a brick red solid, extremely air sensitive. In solution, under inert atmosphere, 1 slowly decomposes into Co and dppf, following a first-order kinetic law (t_1/2 = 21 h at 22°C). Similarly to the Rh and Ir congeners, 1 undergoes a one-electron reversible reduction to [Co(dppf)₂]^-. Attempts to obtain this d ¹⁰ species by chemical as well as electrochemical reduction of 1 lead to the hydride HCo(dppf)₂, 2, as the only product that can be isolated. Reduction of Ni(dppf)Cl₂ with sodium in the presence of dppf and catalytic amounts of naphthalene affords Ni(dppf)₂, 3, isolated in 60% yield as a yellow air stable solid. The stoichiometric oxidation of 3 with [FeCp₂]PF₆ forms the d⁹ complex [Ni(dppf)₂]PF₆, 4, which represents the second example of a structurally characterized Ni(I) complex stabilized by phosphines. A single-crystal X-ray analysis shows for the metal a distorted tetrahedral environment with a dihedral angle defined by the planes containing the atoms P(1), Ni, P(2) and P(3), Ni, P(4) of 78.2° and remarkably long Ni-P bond distances (2.342(3)-2.394(3) A). The EPR spectroscopic properties of 1 (at 106 K in THF) and 4 (at 7 K in 2-methyl-THF) have been examined and g tensor values measured (1, g_x = 2.008, g_y = 2.182, g_z = 2.326; 4, g_x = 2.098, g_y = 2.113, g_z = 2.332). A linear dependence between the hyperfine constants and the Ni-P bond distances has been evidenced. Finally, the change with time of the EPR spectrum of 4 indicates that it very slowly releases dppf.

Full text of DOI:10.1021/ic061185z

Inorg. Chem. 2006, 45, 10321−10328
Homoleptic Complexes of Cobalt(0) and Nickel(0,I) with
1,1 -Bis(diphenylphosphino)ferrocene (dppf): Synthesis and
′
Characterization
Giuseppe Pilloni,† Antonio Toffoletti,† Giuliano Bandoli,‡ and Bruno Longato*,†,§
Dipartimento di Scienze Chimiche, UniVersita’ di PadoVa, Via Marzolo 1, 35131 PadoVa, Italy,
Dipartimento di Scienze Farmaceutiche, UniVersita’ di PadoVa, Via Marzolo 5, 35131 PadoVa,
Italy, and Istituto di Scienze e Tecnologie Molecolari, CNR, Via Marzolo 1, 35131 PadoVa, Italy
Received June 28, 2006
Reduction of Co(dppf)Cl2 with 2 equiv of sodium naphthalenide in THF, in the presence of dppf, affords the homoleptic
complex Co(dppf)2, 1, isolated in 65% yield as a brick red solid, extremely air sensitive. In solution, under inert
atmosphere, 1 slowly decomposes into Co and dppf, following a first-order kinetic law (t1
) 21 h at 22 °C).
/
2
Similarly to the Rh and Ir congeners, 1 undergoes a one-electron reversible reduction to [Co(dppf)2]-. Attempts to
obtain this d10 species by chemical as well as electrochemical reduction of 1 lead to the hydride HCo(dppf)2, 2, as
the only product that can be isolated. Reduction of Ni(dppf)Cl2 with sodium in the presence of dppf and catalytic
amounts of naphthalene affords Ni(dppf)2, 3, isolated in 60% yield as a yellow air stable solid. The stoichiometric
oxidation of 3 with [FeCp2]PF6 forms the d9 complex [Ni(dppf)2]PF6, 4, which represents the second example of a
structurally characterized Ni(I) complex stabilized by phosphines. A single-crystal X-ray analysis shows for the
metal a distorted tetrahedral environment with a dihedral angle defined by the planes containing the atoms P(1),
Ni, P(2) and P(3), Ni, P(4) of 78.2
spectroscopic properties of 1 (at 106 K in THF) and 4 (at 7 K in 2-methyl-THF) have been examined and g tensor
values measured (1, gx 2.008, gy 2.182, gz 2.326; 4, gx 2.098, gy 2.113, gz 2.332). A linear
dependence between the hyperfine constants and the Ni P bond distances has been evidenced. Finally, the change
with time of the EPR spectrum of 4 indicates that it very slowly releases dppf.
° and remarkably long Ni−P bond distances (2.342(3)−2.394(3) Å). The EPR
)
)
)
)
)
)
−
Introduction
combined analysis of the X-ray and EPR data for the d9
species M(dppf)2, (M ) Rh and Ir) evidenced a linear
dependence of the metal-phosphorus bond distances and the
coupling constants of the unpaired electron with the phos-
phorus nuclei. This observation suggests the possibility to
correlate phosphorus coupling with the metal-phosphorus
bond length in similar complexes.
We have recently demonstrated that the organometallic
ligand 1,1′-bis(diphenylphosphino)ferrocene) (dppf) is able
to stabilize iridium and rhodium atoms in their oxidation
states +I, 0, and -I, and the characterization of the
complexes [M(dppf)2]+, [M(dppf)2]0, and [M(dppf)2]- (M )
Ir and Rh) has been described.1,2 The X-ray structure analysis
of these redox-related complexes provided the opportunity
to study the structural changes of the {MP4}n moiety on
changing the charge of the complex (n ) +1, 0, -1). The
In this paper we report the synthesis and characterization
of related cobalt and nickel complexes. The neutral Co(0)
compound, Co(dppf)2 (1), and Ni(0) analogue, Ni(dppf)2 (3),
have been prepared by reduction of the precursors M(dppf)-
Cl2, (M ) Co, Ni), while the paramagnetic Ni derivative,
[Ni(dppf)2]PF6 (4), has been obtained by chemical oxidation
of 3 with the ferrocenium salt [FeCp2]PF6. The molecular
structure of 4 has been obtained by single-crystal X-ray
analysis and represents a rare example among the homoleptic
phosphine complexes of Ni(I) chemically characterized.3 To
the best of our knowledge, only one of them, i.e., [Ni-
* To whom correspondence should be addressed. E-mail: bruno.longato@
unipd.it. Phone: +39-049-8275197. Fax: +39-049-8275161.
† Dipartimento di Scienze Chimiche, Universita’ di Padova.
‡ Dipartimento di Scienze Farmaceutiche, Universita’ di Padova.
§ Istituto di Scienze e Tecnologie Molecolari, CNR.
(1) Longato, B.; Riello, L.; Bandoli, G.; Pilloni, G. Inorg. Chem. 1999,
38, 2818.
(2) Longato, B.; Coppo, R.; Pilloni, G.; Corvaja, C.; Toffoletti, A.; Bandoli,
G. J. Organomet. Chem. 2001, 637-639, 710.
10.1021/ic061185z CCC: $33.50
Published on Web 11/16/2006
© 2006 American Chemical Society
Inorganic Chemistry, Vol. 45, No. 25, 2006 10321

Pilloni et al.
through an appropriate salt bridge. In all cases silver/0.1 mol dm-3
silver perchlorate in acetonitrile, separated from the test solution
by 0.2 mol dm-3 TBAP in THF solution sandwiched between two
fritted disks, was used as the reference electrode. Compensation
for iR drop was achieved by positive feedback. Ferrocene was added
at the end of each experiment as the internal reference. All potentials
(PMe3)4]+, has been structurally authenticated.3a A detailed
study of 4 by electron paramagnetic resonance spectroscopy,
in 2-methyltetrahydrofuran at 7 K, confirms the linear
dependence of the metal-phosphorus bond distances and the
coupling constant of the unpaired electron with the phos-
phorus nuclei, previously evidenced for the Rh and Ir
analogues.2
In addition, we show that the isolation of the anionic
complex [Co(dppf)2]- is prevented by abstraction of a proton
from the medium to give the hydride HCo(dppf)2, 2, which
has also been isolated. Moreover, different synthetic strate-
gies applied to the synthesis of the homoleptic cationic d8
species, [Co(dppf)2]+ and [Ni(dppf)2]2+, proved to be unsuc-
cessful.
are referred to the ferrocenium/ferrocene redox couple (E1/2
)
+0.080 V relative to the actual Ag/AgClO4 reference electrode and
+0.535 V vs aqueous SCE under the present experimental
conditions).
Preparation of the Complexes. Co(dppf)2 (1). To a suspension
of Co(dppf)Cl2 (1.121 g, 1.64 mmol) and dppf (0.908 g, 1.64 mmol)
in THF (10 mL) was added 10.0 mL of a 0.33 mol dm-3 THF
solution of sodium naphthalenide (2 equiv). The mixture was stirred
at room temperature for 30 min and then filtered. Addition of hexane
to the filtrate afforded a precipitate which was collected by filtration,
washed with hexane, and carefully dried under vacuum to give
1.251 g of a brick red solid (yield 65%). Anal. Calcd for C68H56P4-
Fe2Co: C, 69.94; H, 4.83. Found: C, 70.38; H, 5.12. Visible-
NIR spectrum in THF: λmax 856 nm (ꢀ = 528 M-1 cm-1); 1395
nm (ꢀ = 1432 M-1 cm-1).
Experimental Section
General Procedures and Materials. All reactions and manipu-
lations of solutions were performed under a dinitrogen atmosphere
of a Braun MB150 drybox. Anhydrous tetrahydrofuran (THF),
2-methyltetrahydrofuran (Me-THF), toluene, and hexane, purchased
from Aldrich, were further purified by distillation over Na/
benzophenone. 1,1′-Bis(diphenylphosphino)ferrocene, dppf, was
purchased from Aldrich and used as received. Electrochemical grade
tetrabutylammonium perchlorate, TBAP, was obtained from Fluka
and used without further purification after drying in a vacuum at
60 °C. High-purity argon, further purified from oxygen by passage
over reduced copper at 450 °C, was used in the electrochemical
experiments. M(dppf)Cl2 (M ) Ni, Co) were prepared as previously
reported.4
Apparatus. 1H and 31P{1H}NMR spectra were obtained at 298
K on a Bruker Avance 300 MHz spectrometer. The external
reference was H3PO4 (85% w/w in H2O) for 31P spectra. 1H
chemical shifts are referred to the residual peak(s) of the deuterated
solvents used (Aldrich). IR and electronic spectra were obtained
using a Nicolet 5SXC-FTIR and a Cary 5E spectrometer, respec-
tively. EPR spectra from room temperature to 106 K were recorded
with a Bruker ER 200 D X-band spectrometer equipped with a
liquid-nitrogen-temperature controller, whereas low-temperature
spectra (7 K) were recorded with a Bruker ECS 106 X-band
spectrometer equipped with a helium flow cryostat (Oxford ESR
900). Solutions ca. 10-3 mol dm-3 of [Ni(dppf)2]PF6 (in Me-THF)
and Co(dppf)2 (in THF) were prepared under strict conditions of
oxygen and humidity absence. The samples were placed in EPR
quartz tubes connected to a vacuum line, and after several freeze-
pump-thaw cycles, the tubes were sealed under vacuum.
All electrochemical experiments were performed in anhydrous
deoxygenated THF solutions with 0.2 mol dm-3 TBAP as the
supporting electrolyte, using a conventional three-electrode liquid-
jacketed cell. Cyclic voltammetry (CV) measurements were per-
formed with an Amel 551 potentiostat modulated by an Amel 566
function generator. The recording device was an Amel model 863
X-Y recorder. The working electrode was a planar platinum
microelectrode (ca. 0.3 mm2) surrounded by a platinum spiral
counter electrode. Controlled potential electrolyses were performed
with an Amel 552 potentiostat linked to an Amel 731 digital
integrator. The working electrode was platinum gauze (ca. 100 cm2),
and the counter electrode was external, the connection being made
HCo(dppf)2 (2). To a suspension of Co(dppf)Cl2 (0.692 g, 1.01
mmol) and dppf (0.561 g, 1.01 mmol) in THF (12 mL) was added
13.3 mL of sodium amalgam (0.41 mol dm-3, 5.46 mmol), which
was then stirred at room temperature for 30 min. The solution was
separated from the amalgam with a syringe and filtered. Addition
of hexane to the filtrate afforded a precipitate which was collected
by filtration, washed with hexane, and dried under vacuum to give
0.892 g of a red solid (yield 75%). Anal. Calcd for C68H57P4Fe2-
Co: C, 69.88; H, 4.92. Found: C, 70.77; H, 5.49. IR spectrum (in
1
KBr): ν (CoH) ) 1962 cm-1 (w). H NMR (δ, ppm, in C6D6):
7.8-6.8 (complex multiplet, 40 H, Ph); 4.5-3.8 (c.m. 16 H, Cp);
2
-19.5 (apparent quintet, JHP 35 Hz, 1 H, HCo). 31P{1H} NMR
(in C6D6): δ 38.7 (broad singlet).
Ni(dppf)2 (3). To a suspension of Ni(dppf)Cl2 (993 mg, 1.45
mmol) and dppf (807 mg, 1.45 mmol) in toluene (40 mL) was
dropwise added 20 mL of a 0.146 mol dm-3 THF solution of
sodium naphthalenide (2.92 mmol). The reaction mixture was stirred
at room temperature for 30 min and then evaporated to dryness.
The residue was dissolved in C6H6 and filtered to eliminate small
amounts of a dark solid. Addition of hexane afforded a yellow
powdered solid which was recovered by filtration, washed with
hexane, and dried under vacuum to give 1.1 g of Ni(dppf)2 (yield
60%). Anal. Calcd for C68H56P4Fe2Ni: C, 69.96; H, 4.83. Found:
C, 69.06; H, 4.79. 1H NMR (δ, ppm, in C6D6): 7.77 (broad singlet,
16 H, Ph); 6.90 (br s, 24 H, Ph); 4.24 (br s, 8 H, Cp); 3.96 (br s,
8 H, Cp). 31P NMR (in C6D6): δ 14.98 s. Yellow-orange crystals
of 3, obtained from a benzene solution by layering hexane, were
not appropriate for a X-ray determination.
[Ni(dppf)2]PF6 (4). A mixture of Ni(dppf)2 (1.034 g, 0.88 mmol)
and [Fe(C5H5)2]PF6 (282 mg, 0.85 mmol) was dissolved in THF
(20 mL) and stirred at room temperature for 12 h. The resulting
yellow-green precipitate was recovered by filtration, washed with
hexane, and dried under vacuum to give 1.01 g of 4 (yield 90%).
Purification of the crude product by dissolution in acetone/THF
(2:1) and precipitation by addition of hexane afforded green
microcrystals. Anal. Calcd for C68H56P5F6Fe2Ni: C, 62.20; H, 4.30.
Found: C, 61.48; H, 4.82. 31P NMR (in acetone-d6): δ -144.3
(septuplet, JPF ) 710 Hz, PF6-). Visible-NIR spectrum in THF:
1
λmax ca. 1600 nm (ꢀ ) 118 M-1 cm-1).
(3) (a) Gleizes, A.; Dartiguenave, M.; Dartiguenave, Y.; Galy, J. J. Am.
Chem. Soc. 1977, 99, 5187. (b) Holah, D. G.; Hughes, A. N.; Hui, B.
C.; C.-T. Kan, C.-T. Can. J. Chem. 1978, 56, 2552.
(4) Corain, B.; Longato, B.; Favero, G.; Ajo`, D.; Pilloni, G.; Russo, U.;
Kreissl, F. R. Inorg. Chim. Acta 1989, 157, 259.
X-ray Data Collection and Processing. Suitable black crystals
of the d9 species, analyzing as [Ni(dppf)2]PF6‚2C6H5CH3, were
obtained by layering hexane into a toluene/THF (9:1 v/v) solution
10322 Inorganic Chemistry, Vol. 45, No. 25, 2006

Homoleptic Complexes of Co(0) and Ni(0,I) with dppf
Table 1. Crystallographic Data for [Ni(dppf)2](PF6)‚2C7H8
Results and Discussion
chem formula
fw
cryst system
space group
unit cell dimens
a (Å)
C82H72F6Fe2NiP5
1496.7
orthorhombic
Pbca (No. 61)
Synthesis and Characterization of the Co(0) Complex,
Co(dppf)2 (1). Addition of the stoichiometric amount of
sodium naphthalenide (NaNp) to a THF suspension of Co-
(dppf)Cl2, in the presence of dppf, forms immediately a deep-
red solution of 1, according to eq 1:
24.210(4)
21.279(5)
27.345(6)
14087(5)
8
1.411
8.46
ω-2θ
45
9199
5803
767
0.079
0.139
1.135
b (Å)
c (Å)
V (Å3)
Co(dppf)Cl2 + dppf + 2NaNp f
Co(dppf)2 + 2NaCl + 2Np (1)
Z
F
calc (g cm-3
)
µ(Mo KR) (mm-1
scan type
)
Separation of the sodium chloride followed by the rapid
precipitation with hexane allowed the isolation of 1 as a brick
red solid, very air sensitive, which has been characterized
by elemental analysis, vis-NIR, and EPR spectra. In THF,
a fresh solution of 1 displayed two absorption bands at λmax
856 and 1395 nm, whose intensities slowly decreased with
time due to decomposition into dppf and metallic cobalt.
With monitoring of the intensities of these bands, both in
the absence and in the presence of added dppf, a first-order
kinetic law has been observed (t1/2 ) 21 h, at 22 °C).
As expected for a homoleptic zerovalent cobalt phosphine
complex,6 1 undergoes an essentially reversible one-electron
reduction to Co(-I) species and two discrete oxidation
processes to Co(I) and Co(II) derivatives. Representative
cyclic voltammograms are shown in Figure 1a,b.
An inspection of the voltammetric profile reveals that the
reduction product [Co(dppf)2]-, unlike the Rh and Ir
analogues,1,2 is unstable in the CV time scale. In fact,
electrochemical reduction of 1, as well as chemical reduction
of Co(dppf)Cl2 with an excess of Na amalgam in the presence
of dppf, gave a deep red solution of the hydride HCo(dppf)2,
2. Complex 2 has been characterized by elemental analysis
and IR and NMR spectra. In the IR spectrum the Co-H
stretching is observed as a single weak absorption at 1962
cm-1. In the 1H NMR spectrum the hydride resonance occurs
as a quintet (2JHP 35 Hz) at δ -19.5 ppm indicative of a
pentacoordinate metal center having two chelated, magneti-
cally equivalent, dppf ligands. Accordingly, in the proton-
decoupled 31P NMR spectrum the phosphines exhibit a single
resonance at δ 37.8.
2θmax (deg)
measd reflcns
obsd reflcns [I > 2σ(I)]
refined params
R(Fo)a
2
wR(Fo )b
goodness-of-fit
2
2
2 2
a R ) Σ||Fo| - |Fc||/Σ|Fo|. b wR ) [Σw(|Fo | - |Fc |)2]/Σw|Fo | ]1/2
.
Table 2. Selected Bond Lengths and Contact Distances (Å) and Angles
(deg) for [Ni(dppf)2](PF6)‚2C7H8
Ni-P(1)
2.368(3)
2.342(3)
2.394(3)
2.376(3)
1.84(1)
1.80(1)
1.82(1)
1.84(1)
Fe(1)-X(1)a
Fe(1)-X(2)a
Fe(2)-X(3)a
Fe(2)-X(4)a
P(1)···P(2)
1.636(4)
1.646(4)
1.629(4)
1.645(5)
3.586(7)
3.668(7)
3.877(8)
Ni-P(2)
Ni-P(3)
Ni-P(4)
P(1)-C(1)
P(2)-C(6)
P(3)-C(11)
P(4)-C(16)
P(3)···P(4)
P(1)···P(4)
P(2)···P(3)
3.750(7)
130.2(6)
125.0(6)
126.1(6)
130.7(6)
109.9(5)
111.2(4)
P(1)-Ni-P(2)
P(3)-Ni-P(4)
Ni-P(1)-C(1)
Ni-P(2)-C(6)
Ni-P(3)-C(11)
Ni-P(4)-C(16)
99.2(1)
Fe(1)-C(1)-P(1)
Fe(1)-C(6)-P(2)
Fe(2)-C(11)-P(3)
Fe(2)-C(16)-P(4)
C(1)-Fe(1)-C(6)
C(11)-Fe(2)-C(16)
100.5(1)
118.3(4)
116.6(4)
115.2(4)
117.1(4)
a Distances of Fe atom from the centroid (X) of Cp rings.
of 4. Data collection used a STOE four-circle diffractometer, and
many of the details of the structure analysis are listed in Table 1.
The selected crystal was coated with mineral oil and sealed in a
glass capillary tube. The crystal was slightly sensitive toward X-rays
and decomposed after prolonged irradiation. The decay in intensity
was up to 10% during data collection. Scans round the diffraction
vector of several strong reflections showed some variation in
intensity, but owing to the decay, absorption correction was not
applied.
As far as the oxidation pattern of 1 is concerned, attempts
to ascertain the viability of the homoleptic complexes [Co-
(dppf)2]+ and [Co(dppf)2]2+, closely related to the well-
known 1,2-bis(diphenylphosphino)ethane (dppe) derivatives,7
by controlled potential electrolyses proved to be unsuccessful
in that the resulting spent anolytes exhibited electrochemical
features quite different from those observed in precursor 1.
A similar failure resulted in the chemical oxidation of
complex 1.
The structure was solved by heavy-atom Patterson methods and
expanded using Fourier techniques to locate all non-H atoms.
During refinement, performed on F2, it became apparent that the
PF6 anion was suffering from high thermal parameters and a
Fourier-difference map revealed several small maxima in the
vicinity of the toluene molecules, indicating positional disorder.
However, efforts to identify alternative sites for these atoms were
not successful and hence the toluene molecules were treated as rigid
groups. As a result of this severe disorder, the diffraction data were
of rather poor quality; the diffracting ability of the sample fell off
rapidly with increasing Bragg angle (observed reflections only 63%
of the total), and presumably the unresolved disorder is responsible
for the high R values. In any case, the final difference map was
featureless, the largest peak being of 0.64 e Å-3, around the toluene
molecules. Selected bond lengths and angles are listed in Table 2.
The structure was solved and refined using the SHELXTL NT suite
of programs.5
Synthesis of the Ni(0), Ni(dppf)2 (3), and Ni(I), [Ni-
(dppf)2]PF6 (4), Complexes. Following the procedure
described for the Co analogue, the Ni(0) complex Ni(dppf)2,
(5) Sheldrick, G. M. SHELXTL NT, ver. 5.10; Bruker AXS Inc.: Madison,
WI, 1997.
(6) Pilloni, G.; Zotti, G.; Martelli, M. J. Electroanal. Chem. 1974, 50,
295.
(7) Nobile, C. F.; Rossi, M.; Sacco, A. Inorg. Chim. Acta 1971, 5, 698
and ref. therein.
Inorganic Chemistry, Vol. 45, No. 25, 2006 10323

Pilloni et al.
Figure 1. Cyclic voltammograms of a fresh solution 3.0 mM of Co(dppf)2, 1, in THF, TBAP 0.2 M, at 25 °C, with scan rate 100 mV/s, vs ferrocenium/
ferrocene reference: (a) oxidation mode; (b) reduction mode.
3, has been isolated in fairly good yield. The reduction of
the Ni(II) complex is accompanied by the formation of small
amounts of black solid, likely Ni in colloidal form. This side
reaction can be partially avoided by reducing Ni(dppf)Cl2
with Na in the presence of catalytic amounts of naphthalene.
The crystals obtained by vapor diffusion of hexane into a
saturated solution of 3 were not appropriate for a X-ray
diffraction study. The compound appears to be air stable in
the solid phase while in benzene or toluene it is indefinitely
stable only under inert atmosphere. Moreover, the complex
is unstable in chlorinated solvents. For instance, addition of
CHCl3 (a few percent) to a benzene solution of 3 determines
the immediate formation of a precipitate which was not
further investigated. The solid is likely the product of an
oxidative addition of CHCl3 to complex 3, in line with
previous observations on other Ni(0) phosphine complexes.8
Exhaustive electrolysis at potentials past the anodic peak
results in the removal of one electron/molecule of depolarizer
to give a greenish solution with a voltammetric reduction
profile virtually complementary to the oxidation pattern of
the precursor, thus raising in evidence the chemical revers-
ibility of the electron-transfer process. In the anodic scan of
the voltage, as expected for a Ni(I) derivative, an oxidation
peak is displayed. However, this signal occurs at far more
anodic potential values (Ep ) -0.25 V) when compared with
that of the d10 parent and its profile is that anticipated for a
totally irreversible charge-transfer reaction. This pattern
appears to be in striking contrast with that exhibited by the
strictly related dppe complex10 and it questions the viability
of the cationic d8 species [Ni(dppf)2]2+ (see below).
As anticipated on the basis of the electrochemical analysis,
complex 4 can be conveniently prepared by oxidation of 3
with the ferrocenium ion, according to eq 2:
1
The H NMR spectrum of 3 shows, in addition to the
resonances due to the phenyl groups, two unresolved
multiplets, attributable to the cyclopentadienyl protons, at δ
) 4.24 and 3.96 ppm. The corresponding 31P NMR spectrum
is characterized by a sharp singlet at δ 15.02, in agreement
with the presence of chemically and magnetically equivalent
dppf molecules bonded to a d10 metal center. The coordina-
tion chemical shift of the dppf ligand (∆δ ) δcomplex - δdppf
) 33.01 ppm) is in line with the values found in other Ni(0)
diphosphine complexes.9
CV tests in THF show that the d10 species undergoes a
fully reversible one-electron oxidation step centered at E1/2
) -1.175 V, as mean value of the potentials for positive
and negative peak currents, to give the species [Ni(dppf)2]+,
4. A typical voltammogram is shown in Figure 2.
Ni(dppf)2 + [Fe(C5H5)2]PF6 f [Ni(dppf)2]PF6 + Fe(C5H5)2
(2)
In THF the reaction occurs in ca. 12 h and the moderate
solubility of 4 allows its isolation by a simple filtration of
the green solid separated. The NIR spectrum of 4 in THF
exhibits a maximum at λ ) 1600 nm with ꢀ ) 118 M-1
cm-1, in good agreement with the value reported for [Ni-
(PPh3)4]+.3b
Control experiments confirm the electrochemical sugges-
tions on the instability of the homoleptic cationic d8 species,
[Ni(dppf)2]2+. As a matter of fact, addition of 1 equiv of the
ferrocenium salt to a THF solution of 4 gives rise to the
decomposition of the oxidation product as inferred by the
(8) Tsou, T. T.; Kochi, J. K. J. Am. Chem. Soc. 1979, 101, 6319 and
reference therein.
(9) Garrou, P. E. Chem. ReV. 1981, 81, 229.
(10) Martelli, M.; Pilloni, G.; Zotti, G.; Daolio, S. Inorg. Chim. Acta 1974,
11, 155.
10324 Inorganic Chemistry, Vol. 45, No. 25, 2006

Homoleptic Complexes of Co(0) and Ni(0,I) with dppf
Figure 2. Cyclic voltammogram for oxidation of a solution 9.0 mM of Ni(dppf)2, in THF, TBAP 0.2 M, at 25 °C, with scan rate 200 mV/s, vs ferrocenium/
ferrocene reference.
presence in the 31P NMR spectrum of the crude reaction
product of the only resonance at -17.9 ppm, distinctive of
uncoordinated dppf ligand.
X-ray Structure of [Ni(dppf)2]PF6. The ORTEP11 rep-
resentation and the numbering scheme of the complex are
illustrated in Figure 3, and the relevant bond distances and
angles are reported in Table 2.
The Ni atom is four-coordinate and the geometry around
the metal is essentially tetrahedral, the dihedral angle between
the P(1)-Ni-P(2) and P(3)-Ni-P(4) planes being 78.2°.
The Ni-P bond lengths range between 2.342(3) and 2.394-
(3) Å, with an average of 2.370(3) Å. A search in the
Cambridge Crystallographic Database (version 5.27 of Nov
2005 + 1 update)12 for Ni(I) complexes (coordination number
) 4) shows that the mean Ni-P bond length calculated over
14 structures is 2.253 Å. The same search shows that the
longest Ni-P distance found in 4 is also the fifth longest
Ni-P distance recorded so far for tetracoordinated nickel.
A more appropriate comparison should be limited to the
Ni complexes containing the dppf ligand: about 10 Ni(II)
structures have been reported so far. In Ni(dppf)Cl2 and
Ni(dppf)Br214 the Ni environment is tetrahedral and the mean
Ni-P distance is 2.290(9) Å. In the remaining structures,
the metal is bound to another bidentate ligand and the
geometry around the metal becomes square planar, with a
mean Ni-P bond length of 2.211(3) Å.15-20 This contraction
of the Ni-P bond length is consistent with the one observed
13
(13) Butler, J. R.; Cullen, W. R.; Kim, T.-J.; Rettig, S. J.; Trotter, J.
Organometallics 1985, 4, 972.
(14) Casellato, U.; Ajo`, D.; Valle, G.; Corain, B.; Longato, B.; Graziani,
R. J. Crystallogr. Spectrosc. Res. 1988, 18, 583.
(15) Pastorek, R.; Tra´vn´ıe`ek, Z.; Maarek, J.; Dastych, D.; Z. Sindela´, Z.
Polyhedron 2000, 19, 1713.
(16) Kamenicek, J.; Pastorek, R.; Sykora, J.; Kratochvil, B. Acta UniV.
Palacki. Olomuc. 2000, 39, 45.
(17) Pastorek, R.; Kamenicek, J.; Sindela´ø Z.; Zak, Z. Pol. J. Chem. 2001,
65, 363.
(18) Pastorek, R.; Kamenicek, J.; Pavlicek, M.; Husarek, J.; Sindela´, Z.;
Zak, Z. J. Coord. Chem. 2002, 55, 1301.
(11) Johnson, C. K. ORTEP; Report ORNL-5138; Oak Ridge National
Laboratory: Oak Ridge, TN, 1976.
(19) Pawlas, J.; Nakao, Y.; Kawasutra, M.; Hartwig, J. F. J. Am. Chem.
Soc. 2002, 124, 3669.
(20) Arulsamy, N.; Bohle, D. S.; Imonigie, J. A.; Levine, S. Angew. Chem.,
Int. Ed. 2002, 41, 2371.
(12) Allen, F. H. Acta Crystallogr. B 58 2002, 380-388 [Cambridge
Structural Database (version 5.27 of Nov 2005 + 2 updates)].
Inorganic Chemistry, Vol. 45, No. 25, 2006 10325

Pilloni et al.
Figure 4. Experimental (solid line) and calculated (dashed line) EPR
spectra of Co(dppf)2 ca. 2 × 10-3 M in THF at 106 K.
The broad signal consistent with a d9 metal center (S )
1/2) has a scarcely resolved hyperfine structure so that it
has been reproduced taking into account only the anisotropy
of the Zeeman interaction. The simulation has been obtained
with the g tensor values, gx ) 2.008, gy ) 2.182, and gz )
2.326, similar to the elements recently reported for the
complex Co(troppph)2 (troppph ) tropylindene phosphane).25
As far as the Ni(I) derivative is concerned, at room
temperature a freshly prepared solution of [Ni(dppf)2]PF6 (ca.
10-3 M in Me-THF) does not show any EPR signal, similarly
to Co(0) complex 1. This is not surprising since the absence
of EPR signal in liquid solution has been already reported
for other phosphine complexes of Ni(I).10,26 The failure to
observe the EPR spectrum is attributed to the fast electron
spin relaxation of the Ni(I) unpaired electron. The same
sample was examined at low temperature (7 K) and gave
the EPR spectrum shown in Figure 5 (solid line) along with
a spectrum (dashed line A) simulated using the parameters
reported in Table 4. The spectrum is almost axial (gx ≈ gy)
and indicates the presence of four not equivalent 31P nuclei
coupled with the unpaired electron of the Ni(I) species, as
supported by the five-component hyperfine structure present
both in the gz and the gx, gy features. The spectrum calculation
included the anisotropy of the g tensor and only the isotropic
contribution to the hyperfine coupling with the four 31P
nuclear spins, following a procedure previously applied for
strictly related complexes.2
The calculated spectrum reproduces quite satisfactory the
experimental one, despite the approximation used, with g
tensor values falling in the range 2.098-2.332. The average
value for 4 (2.181), although similar to that found in 1
(2.172), is significantly larger than those of other diphosphine
Ni(I) complexes (for instance in [Ni(dppe)2]+ g ) 2.07).26,27
Concerning the hyperfine constants a, the comparison with
the same type of complexes (a ) ca. 168 MHz) reveals that
our values are quite smaller (average a ) ca. 104 MHz).
We believe that such discrepancy is related to the unusually
long Ni(I)-P bonds found in 4.
Figure 3. Crystal structure of [Ni(dppf)2]PF6‚2C7H8, 4. Thermal ellipsoids
are drawn at the 40% probability level. The asymmetric unit of the structure
comprises one [Ni(dppf)2]+ cation balanced by the PF6- anion, along with
two molecules of the recrystallization solvent. The hydrogen atoms, the
anion, and the crystallization solvent molecules have been omitted in this
view.
comparing tetrahedral dichlorobis(triphenylphosphine)nickel-
(II) (Ni-P ) 2.320(9) Å) and square-planar trans-dichlorobis-
(triphenylphosphine)nickel(II)‚2C2H4Cl2 (Ni-P ) 2.242(3)
Å).21
As anticipated, the coordination environment of Ni(I) in
4 is almost tetrahedral and the unusually long bond distance
of 2.370 Å (average) observed is attributable to the steric
crowding of the two bulky dppf ligands, which is also likely
responsible for the moderate stability of the complex in
solution (see below). In 4 the dppf molecules assume the
synclinal staggered arrangement, the most frequent confor-
mation for this chelated ligand. The parameters describing
the ferrocenyl moieties (τ, θ, XA···Fe···XB, P···Fe···P), are
collected in Table 3, along with those of [M(dppf)2]n (M )
Rh, Ir; n ) +1, 0, -1) previously reported.22-24 With respect
to the parameters involving the metal center, there is a good
agreement with the data found for the neutral Rh(dppf)22 and
1
Ir(dppf)2 derivatives, especially those regarding the P-M-P
and γ parameters, the latter angle measuring how much the
coordination around M deviates from an ideal tetrahedron
(γ ) 90°). The value found in 4 (γ ) 78.2°) is similar to
those found for Rh(dppf)2 (75.2°) and Ir(dppf)2 (74.7°), in
line with the fact that in these complexes Ni, Rh, and Ir have
the same number of valence electrons.
EPR Study of Co(dppf)2 and [Ni(dppf)2]PF6. A freshly
prepared solution of Co(dppf)2 (ca. 2 × 10-3 M in THF)
does not give an EPR spectrum at room temperature.
However, in frozen matrix at 106 K, the same sample gives
the spectrum shown in Figure 4 along with the simulation
(dashed line).
However, some differences between experimental and
calculated spectra are visible: in the simulation A (Figure
5) the gz feature exhibits a too low intensity and in the region
(21) Corain, B.; Longato, B.; Angeletti, R.; Valle, G. Inorg. Chim. Acta
1985, 104, 15.
(22) Di Noto, V.; Valle, G.; Zarli, B.; Longato, B.; Pilloni. G.; Corain, B.
Inorg. Chim. Acta 1995, 233, 165.
(23) Casellato, U.; Corain, B.; Graziani, R.; Longato, B.; Pilloni, G. Inorg.
Chem. 1990, 29, 1193.
(24) Bandoli, G.; Dolmella, A. Coord. Chem. ReV. 2000, 209, 161.
(25) Deblon, S.; Liesum, L.; Harmer, J.; Scho¨nberg, H.; Schweiger, A.;
Gru¨tzmacher, H. Chem. Eur. J. 2002, 8, 601.
(26) Miedaner, A.; Haltiwanger, R. C.; Du Bois, D. L. Inorg. Chem. 1991,
30, 417.
(27) Bowmaker, G. A.; Boyd, P. D. W.; Campbell, G. K.; Hope, J. M.;
Martin, R. L. Inorg. Chem. 1982, 21, 1152.
10326 Inorganic Chemistry, Vol. 45, No. 25, 2006

Homoleptic Complexes of Co(0) and Ni(0,I) with dppf
Table 3. Relevant Structural Parameters for Mononuclear Neutral and Charged [M(dppf)2] Complexes Containing Two η2-Chelating dppf Ligandsa
complex
τ (deg) Θ (deg) XA···Fe···XB (deg) P···Fe···P (deg)
M-P (Å)
P-M-P (deg)
Fe···M (Å) γ (deg)
ref
[Rh(dppf)2]+ 35.4, 24.1 7.5, 2.9
176.1, 178.3
179.5, 179.1
177.8, 179.1
176.1, 179.9
179.0, 178.9
177.8, 177.7
178.5, 177.5
60.9, 59.1
64.2, 60.6
62.9, 60.0
61.1, 59.3
63.3, 59.6
62.1, 59.6
62.4, 63.8
2.312(1), 2.407(4), 2.358(3), 2.321(3) 94.0(1), 92.8(2)
2.306(1), 2.323(1), 2.312(1), 2.318(1) 102.2(1), 97.6(1) 4.32, 4.32 75.2
2.230(5), 2.250(5), 2.244(5), 2.251(5) 105.1(2), 100.2(2) 4.27, 4.23 95.0
2.317(6), 2.389(5), 2.337(5), 2.343(6) 95.0(2), 93.6(2)
2.296(3), 2.326(3), 2.301(4), 2.315(3) 102.1(1), 97.6(1) 4.37, 4.37 74.7
2.242(3), 2.256(3), 2.254(3), 2.270(3) 104.8(1), 99.8(1) 4.33, 4.28 95.0
4.45, 4.44 49.7 22
Rh(dppf)2
39.1, 18.5 2.9, 1.7
2
2
[Rh(dppf)2]- 38.7, 21.2 2.7, 2.5
[Ir(dppf)2]+ 34.8, 25.1 9.4, 4.9
4.44, 4.43 51.0 23
Ir(dppf)2
38.4, 18.7 4.4, 2.5
1
1
[Ir(dppf)2]- 38.0, 22.9 3.6, 3.4
[Ni(dppf)2]+ 25.5, 21.1 1.5, 1.9
2.368(3), 2.342(3), 2.394(3), 2.376(3) 99.2(1), 100.5(1) 4.31, 4.30 78.2 present work
a τ is the CA···XA···XB···CB torsion angle. CA is the carbon atom of Cp ring “A” bonded to the P atom; XA is the Cp ring “A” centroid. CB and XB indicate
the same positions in Cp ring “B”. Θ is the dihedral angle between the mean planes through the Cp rings. γ is the dihedral angle defined by the P(1)-
M-P(2) and the P(3)-M-P(4) planes, where P(1) and P(2) are the donor atoms of a dppf ligand and P(3) and P(4) are those of the other.
Figure 5. Experimental (solid line) EPR spectrum of a fresh sample of
[Ni(dppf)2]PF6 ca. 10-3 M in Me-THF at 7 K. Simulated spectra (calculated
with parameters of Table 4) were obtained neglecting (dashed line A) and
considering (dashed line B) the presence of the species [Ni(dppf)(Me-
THF)x]+.
Table 4. Hyperfine Coupling Constants (MHz) and g Valuesa for
[Ni(dppf)2]PF6 at 7 K
fresh soln
about 1 month later
param
P(1)b
106
2.098
2.113
2.332
P(2)
128
P(3)
87
P(4)
96
Pa
Pb
Figure 6. EPR spectra of a [Ni(dppf)2]PF6 solution (ca. 10-3 M in Me-
THF) about 1 month after the preparation of the sample: (a) at 7 K; (b) at
295 K. The simulated spectra, calculated with the parameters reported in
Table 4, are shown with dashed lines. The weak signal (asterisk) is due to
the standard used for g factor calibration.
a
165
188
gx
gy
gz
2.012
2.235
2.395
a Estimated errors of the coupling constants a are (1 MHz and of g
elements are (0.001. b The P nuclei labels are the same used in Figure 3
and Table 2. Regarding the assignment of the hyperfine constants to the
four P nuclei, see the last paragraph of the EPR discussion section.
between 330 and 350 mT two weak peaks are lacking. These
disagreements have been observed in different samples and
become more evident after several days at room temperature,
suggesting the presence of a second paramagnetic species,
tentatively formulated as [Ni(dppf)(Me-THF)x]+, resulting
from a slow and irreversible decomposition of the [Ni-
(dppf)2]+ cation. Accordingly, the spectrum of the sample
left in the sealed EPR tube at room temperature for several
weeks, and recorded again at 7 K, is shown in Figure 6a
(solid line). The simulation (Figure 6a, dashed line) has been
calculated by taking into account the hyperfine interaction
with only two nonequivalent 31P nuclei with the hyperfine
constants reported in Table 4. This result strongly suggests
that the initial complex [Ni(dppf)2]+ decomposes quantita-
tively but with a rate slow enough to allow its crystallization
as a pure compound. It is worth to note that the solution
containing the decomposition product gives an EPR signal
also at 295 K, which is depicted in Figure 6b (solid line). It
displays a well-resolved hyperfine structure caused by
coupling to two 31P nuclei with aP ) 182 MHz and with g
) 2.179.
Figure 7. Correlation between the EPR coupling constants a and Ni(I)-P
bond lengths in [Ni(dppf)2]PF6.
A much better simulation of the initial EPR spectrum of
4 (Figure 5, solid line) was obtained on considering the
presence of a few percent of the decomposition product [Ni-
(dppf)(Me-THF)x]+, as shown in Figure 5 (dashed line B).
The combined analysis of the X-ray and EPR data for the
d9 species [Ni(dppf)2]+ evidences a linear correlation of the
metal-phosphorus bond distances with the hyperfine cou-
pling to the 31P nuclei (Figure 7). Such correlation allows
us to assign each hyperfine constant a to a specific 31P
nucleus, as shown in Table 4.
A similar dependence was previously reported for the d9
analogues M(dppf)2 (M ) Rh, Ir)2 and suggested for Rh-
Inorganic Chemistry, Vol. 45, No. 25, 2006 10327

Pilloni et al.
(dppe)2.28 The correlation found in this work allows one to
estimate the nickel-phosphorus bond distance in the de-
composition product [Ni(dppf)(Me-THF)x]+. Using the aver-
age of the aP values for such species at 7 K (176 MHz), the
Ni(I)-P bond length would be about 2.28 Å, a value much
closer to the average distance found in the 14 crystal
structures containing Ni(I)-P bonds (2.253 Å).
Finally, the whole of the chemical and electrochemical
observations points convincingly to the thermodynamic
instability of [Ni(dppf)2]2+, despite the well-known stability
of other square-planar homoleptic Ni(II) diphosphine com-
plexes.26 Since the conversion from an essentially tetrahedral
geometry of the NiP4 moiety found in 4 toward a planar
arrangement of the ligands is expected to occur with a
decrease of the Ni-P bond distances,21 the observed instabil-
ity of the [Ni(dppf)2]2+ cation can be attributed to the
crowding of the bulky dppf ligands which prevents such
transformation of the Ni coordination geometry.
Concluding Remarks
The uncommon ability of the organometallic ligand dppf
in sustaining the Ir and Rh atoms in the +1, 0, and -1
oxidation states, recently reported by us, has been now
confirmed, even though only partially, for Co and Ni.
Compound 1 represents a rare example of zerovalent cobalt
complex, but unlike its Rh and Ir analogues, Co(dppf)2 is
only moderately stable in solution. This finding contrasts
with the stability shown by the series of compounds
M(troppph)2 (M(0) ) Co, Rh, Ir; troppph ) tropylindene
phosphane) recently described.25 The difference between the
Co and the congeners appears more evident when the stability
of the d10 species is considered. While the anions [Rh(dppf)2]-
and [Ir(dppf)2]- are indefinitely stable in THF solution,
[Co(dppf)2]- is rapidly quenched by the solvent to give the
hydride HCo(dppf)2.
Acknowledgment. The Ministero dell’Universita’ e della
Ricerca Scientifica e Tecnologica (PRIN 2004: Pharmaco-
logical and Diagnostic Properties of Metal Complexes) is
gratefully acknowledged. We thank Prof. C. Corvaja for
helpful discussions and Dr. A. Brunetta for his valuable help
in the synthesis of the Ni(I) complex.
Supporting Information Available: Crystallographic data in
CIF form. This material is available free of charge via the Internet
at http://pubs.acs.org. Crystallographic data for the structure reported
in this paper have also been deposited at the Cambridge Crystal-
lographic Data Center as supplementary publication Nos. Copies
of the data can be obtained free of charge upon application to
CCDC, 12 Union Road, Cambridge, CB2 1EZ U.K. Fax: +44-
1223-336-033. E-mail: deposit@ccdc.cam.ac.uk.
(28) Mueller, K. T.; Kunin, A. J.; Greiner, S.; Henderson, T.; Kreilick, R.
W.; Eisenberg, R. J. Am. Chem. Soc. 1987, 109, 6313.
IC061185Z
10328 Inorganic Chemistry, Vol. 45, No. 25, 2006