A dinuclear Ni(I) system having a diradical Ni2N2 diamond core resting state: Synthetic, structural, spectroscopic elucidation, and reductive bond splitting reactions

Article abstract of DOI:10.1021/ic801137p

One-electron reduction of the square-planar nickel precursor (PNP)NiCl (1) (PNP^- = N[2-P(CHMe₂)₂-4-methylphenyl] ₂) with KC₈ effects ligand reorganization of the pincer ligand to assemble a Ni(I) dimer, [Ni(μ₂-PNP)]₂ (2), containing a Ni₂N₂ core structure, as inferred by its solid-state X-ray structure. Solution magnetization measurements are consistent with a paramagnetic Ni(I) system likely undergoing a monomer ? dimer equilibrium. The room-temperature and 4 K solid-state X-band electron paramagnetic resonance (EPR) spectra display anisotropic signals. Low-temperature solid-state X-band EPR data at 4 K reveal rhombic values g _z = 1.980(4), g_x = 2. 380(4), and g_y = 2.225(4), as well as a forbidden signal at g = 4.24 for the ΔM_s = 2 half field transition, in accord with 2 having two weakly interacting metal centers. Utilizing an S = 1 model, full spin Hamiltonian simulation of the low-temperature EPR spectrum on the solid sample was achieved by applying a nonzero zero-field-splitting parameter (D = 0.001 cm^-1), which is consistent with an S = 0 ground state with a very closely lying S = 1 state. Solid-state magnetization data also corroborate well with our solid-state EPR data and reveal weak antiferromagnetic behavior (J = -1.52(5) cm^-1) over a 2-300 K temperature range at a field of 1 Tesla. Evidence for 2 being a masked "(PNP)Ni" scaffold originates from its reaction with N ₂CPh₂, which traps the Ni(I) monomer in the form of a T-shaped species, Ni(PNP=NNCPh₂), a system that has been structurally characterized. The radical nature of complex 2, or its monomer component, is well manifested through the plethora of cooperative H-X-type bond cleavage reactions, providing the nickel(II) hydride (PNP)NiH and the corresponding rare functionalities -OH, -OCH₃, -PHPh, and -B(catechol) integrated into the (PNP)Ni moiety in equal molar amounts. In addition to splitting H ₂, compound 2 can also engage in homolytic X-X bond cleavage reactions of PhXXPh to form (PNP)Ni(XPh) (X = S or Se).

Full text of DOI:10.1021/ic801137p

Inorg. Chem. 2008, 47, 10479-10490
A Dinuclear Ni(I) System Having a Diradical Ni2N2 Diamond Core
Resting State: Synthetic, Structural, Spectroscopic Elucidation, and
Reductive Bond Splitting Reactions
Debashis Adhikari,† Susanne Mossin,‡ Falguni Basuli,§ Benjamin R. Dible,| Mircea Chipara,⊥
Hongjun Fan,† John C. Huffman,† Karsten Meyer,*,‡ and Daniel J. Mindiola*,†
Department of Chemistry and the Molecular Structure Center, Indiana UniVersity, Bloomington,
Indiana 47405, UniVersity of Erlangen-Nu¨rnberg, Department of Chemistry and Pharmacy,
Inorganic Chemistry, Egerlandstraꢀe 1, Erlangen, Germany 91058, Nuclear Medicine, Brigham
and Women’s Hospital, HarVard Medical School, Boston, Massachusetts 02115, Department of
Chemistry, RC Box 270216, UniVersity of Rochester, Rochester, New York 14627, and Department
of Physics and Geology, UniVersity of Texas-Pan American, Edinburg, Texas 7854
Received June 19, 2008
One-electron reduction of the square-planar nickel precursor (PNP)NiCl (1) (PNP- ) N[2-P(CHMe2)2-4-methylphenyl]2)
with KC8 effects ligand reorganization of the pincer ligand to assemble a Ni(I) dimer, [Ni(µ2-PNP)]2 (2), containing
a Ni2N2 core structure, as inferred by its solid-state X-ray structure. Solution magnetization measurements are
consistent with a paramagnetic Ni(I) system likely undergoing a monomer T dimer equilibrium. The room-temperature
and 4 K solid-state X-band electron paramagnetic resonance (EPR) spectra display anisotropic signals. Low-
temperature solid-state X-band EPR data at 4 K reveal rhombic values gz ) 1.980(4), gx ) 2. 380(4), and gy )
2.225(4), as well as a forbidden signal at g ) 4.24 for the ∆MS ) 2 half field transition, in accord with 2 having
two weakly interacting metal centers. Utilizing an S ) 1 model, full spin Hamiltonian simulation of the low-temperature
EPR spectrum on the solid sample was achieved by applying a nonzero zero-field-splitting parameter (D ) 0.001
cm-1), which is consistent with an S ) 0 ground state with a very closely lying S ) 1 state. Solid-state magnetization
data also corroborate well with our solid-state EPR data and reveal weak antiferromagnetic behavior (J ) -1.52(5)
cm-1) over a 2-300 K temperature range at a field of 1 Tesla. Evidence for 2 being a masked “(PNP)Ni” scaffold
originates from its reaction with N2CPh2, which traps the Ni(I) monomer in the form of a T-shaped species,
Ni(PNPdNNCPh2), a system that has been structurally characterized. The radical nature of complex 2, or its
monomer component, is well manifested through the plethora of cooperative H-X-type bond cleavage reactions,
providing the nickel(II) hydride (PNP)NiH and the corresponding rare functionalities -OH, -OCH3, -PHPh, and
-B(catechol) integrated into the (PNP)Ni moiety in equal molar amounts. In addition to splitting H2, compound 2
can also engage in homolytic X-X bond cleavage reactions of PhXXPh to form (PNP)Ni(XPh) (X ) S or Se).
Introduction
of these species, some of the known Ni(I) systems play
significant roles in several chemically important processes such
as catalytic aliphatic C-C bond formation reactions, and the
cyclotrimerization and linear oligomerization of phenyl acety-
lene.2 In the catalytic aliphatic C–C coupling reaction, it was
noted in the solid state that the Ni(I) centers were not discrete
monomers, owing to Ni · · ·Ni interactions.2b By the same token,
dinuclear Ni(I) cores have also been demonstrated to be
important reagents relevant to important industrials processes.
For example, dinuclear Ni(I) cores have been used as models
for the hydrodesulfurization of sulfur-based heterocycles.3,4 The
Accessing well-defined Ni(I) complexes is a relatively rare
phenomenon given the difficulty in stabilizing this radical
and the low-valent oxidation state.1-9 Despite the paucity
* Authors to whom correspondence should be addressed. E-mail:
Karsten.Meyer@chemie.uni-erlangen.de (K.M.), mindiola@indiana.edu
(D.J.M.).
†
Indiana University.
University of Erlangen-Nu¨rnberg.
Harvard Medical School.
University of Rochester.
‡
§
|
⊥
University of Texas-Pan American.
10.1021/ic801137p CCC: $40.75  2008 American Chemical Society
Inorganic Chemistry, Vol. 47, No. 22, 2008 10479
Published on Web 10/15/2008

Adhikari et al.
responsible precursor in the process for desulfurization is a
bimetallic Ni(I) core bridged by two hydride ligands. Intuitively,
to understand the mechanism behind important transformations
such as C-C bond formation or desulfurization, it is necessary
to determine the electronic structure of the Ni(I) dimeric core,
especially since the Ni-Ni interaction appears weak enough
to promote redox events possibly via mononuclearity formation.
Bimetallic cores consisting of the most accessible oxidation state
of Ni, namely, Ni(II) centers, have been scrutinized in depth to
address the electronic interaction between the metal ions, more
specifically, the mechanism for the communication among the
two metal centers via superexchange through spin delocalization
or spin polarization.10 On the contrary, bimetallic centers
consisting of Ni(I) ions are a very scarce occurrence and hence
lack detailed scrutiny.3-8 As a result, investigating the electronic
and magnetic interactions between two Ni(I) centers should
provide clues to how these systems, if any, cooperatively
perform important chemical transformations relating to bond-
breaking and -forming reactions. In general, understanding the
interaction between two metal centers represents an important
paradigm in coordination chemistry since it can lead to
predictions on how dinuclear moieties operate, in concert and
along the way to performing unusual transformations.
In a bimetallic motif consisting of two d9 metal centers,
such as in a Ni(I)/Ni(I) dinuclear complex, one can categorize
three different types of electronic states that depend greatly
on the strength of metal-metal interactions.11 In one case,
the two metal centers in question can be virtually noninter-
active, essentially displaying properties amenable to an
isolated d9 unit and more consistent with a paramagnetic
monomer. In a second case scenario, whereby there is weak
interaction, the metal centers can be communicating very
weakly, thus leading to ferromagnetically and antiferromag-
netically coupled ground states, depending on whether the
total high-spin or low-spin state is attained. In such a case
of weak interaction, it is expected that the triplet excited state
should be close in energy to the singlet ground state and
that such a low-lying excited state should be populated
thermally. In the third case scenario, the two metal centers
could be interacting strongly, thereby forming a metal-metal
bond which eventually displays diamagnetic behavior. De-
ciphering these types of interactions can be challenging,
given the interplay between metal-metal bonds, ferro- and
antiferromagnetic coupling pathways through space or the
bridging ligands, and a possible caveat involving a monomer-
dimer equilibrium scenario in solution.
In this manuscript, we present a bimetallic system where
two Ni(I) centers are assembled in a diamond-like core form,
and where the metal ions in question are very weakly
interacting despite the connective amido bridges between
them. Intuitively, two 17-electron metal centers in proximity
and bridged by a strong field ligand such as amido are
inclined to interact strongly via the bridge or through space
along the way to metal-metal bond formation. Applying
X-ray crystallography and other spectroscopic techniques,
we elucidate our dinuclear Ni2(I,I) complex to be a rare and
special example of weakly coupled bimetallic framework.
Herein, we describe the synthesis of an electronically unusual
dinuclear Ni(I) complex, [Ni(µ2-PNP)]2, supported by the
pincer ligand PNP (PNP- ) N[2-P(CHMe2)2-4-methylphe-
nyl]2).12 Despite having a short Ni · · · Ni distance, this
complex exists at room temperature as a diradical having a
Ni2N2 diamond core resting state. Although the solution
magnetization data suggest this core to equilibrate to two
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10480 Inorganic Chemistry, Vol. 47, No. 22, 2008

A Dinuclear Ni(I) System
Scheme 1. Synthesis of Complex 2 via Reduction of 1 with KC8
electron reductant should be used,14 since chemical treatment
of 1 with 1% Na/Hg in THF resulted in no detectable reaction
1
when the mixture was assayed by H NMR spectroscopy.
However, assembling a reactive Ni(I) species was possible
when 1 was treated with 1 equiv of KC8 in THF, as indicated
by an immediate color change from green to dark brown.
Upon workup of the mixture, dark crystals of the Ni(I) dimer,
[Ni(µ2-PNP)]2 (2), were isolated in 62% yield (Scheme 1).
The moderate isolation yield of 2 is due to its high solubility
in most common organic solvents as well as the presence of
a byproduct, namely, the Ni(II) hydride (PNP)NiH (3)
(5-15% yield based on 1H NMR spectra having an internal
standard). Complex 3 has been previously reported by
Ozerov and co-workers and displays a diagnostic hydride
resonance at -18.4 ppm (JH-P ) 60 Hz).13 Fortunately, we
found that the complex can be obtained in very pure form,
and in reproducible yields, when 1 is subjected to freshly
prepared KC8. In addition, the separation of 2 from traces
of 3 can be achieved by fractional crystallization of the
mixture at -35 °C using hexane as solvent. Independently,
we have found that the treatment of 2 with excess KC8 in
THF does not produce 3 (no reaction), suggesting that the
over-reduction (if occurring) of Ni(I) is not a competing
process. Notably, the origin of the hydride ligand in 3 was
not from the solvent medium, THF, since the treatment of 2
with 1 equiv of KC8 in THF-d8 did not yield the expected
isotopologue (PNP)NiD (3)-d1 (likewise, 3 + KC8 does not
yield 2 and KH). Therefore, these results advocate that 2 is
not a precursor to 3 and that the hydride must originates
from ligand degradation of the pincer ancillary in 1 or from
a byproduct or impurity originated from KC8.
The formation of 2 is likely occurring via a transient three-
coordinate Ni(I) putative species,9 “(PNP)Ni” (Scheme 1),
which subsequently dimerizes by substantial reorganization
of the PNP pincer framework. The degree of aggregation
and rearrangement of the former pincer-type ligand is inferred
by single-crystal X-ray diffraction analysis (XRD; Figure 2,
top) and clearly portrays a Ni2N2 diamond core framework
whereby the PNP ancillary is severely reorganized via
bridging of the pincer amide nitrogen. The bottom of Figure
2 emphasizes the first coordination sphere comprising the
diamond core in the solid state structure of 2. Congruently,
the phosphorus atoms composing a PNP ligand extend across
each metal center. This expansion of the phosphine pendant
arm adds some strain to the aryl framework, which is reflected
by the dihedral angles N-C-C-P (1.4(4), 1.8(4), 6.2(4), and
6.8(4)°). Therefore, this discrepancy in dihedral angles implies
Figure 1. Cyclic voltammagram of 1 in THF using 0.3 M TBAH as an
electrolyte and referenced to Fc+/0 at 0.0 V. The scan rate is 100 mV/s.
equivalents of the corresponding monomer “(PNP)Ni or
solvated form”, the dinuclear core along with its paramagnet-
ism is retained in the solid state. Structural analysis of this
species confirms that each PNP unit assembling the Ni2N2
diamond core slightly differs in geometry. The combination
of solid-state magnetization and solid-state electron para-
magnetic resonance (EPR) spectroscopic data are in accord
with 2 having an accessible S ) 1 electronic configuration
where each Ni radical center weakly antiferromagnetically
couples with an overall S ) 0 ground state and has a J )
-1.52(5) cm-1 over a 2-300 K range. We also demonstrate
that the Ni2N2 core is a masked (PNP)Ni(I) surrogate and
can undergo binuclear oxidative additions by splitting H-X
bonds of various substrates (X ) H, OH, OCH3, PHPh, and
B(catechol)), to form equal mixtures of (PNP)Ni-H and
(PNP)Ni-X moieties. Some of the latter complexes have
been prepared independently and free of hydride. Finally,
the solid-state structures for a family of rare, terminal
moieties such as (PNP)Ni(OH) and (PNP)Ni(NH2) are
presented and discussed.
Results and Discussion
Synthesis and Solid-State Structure of Complex
[Ni(µ2-PNP)]2 (2). The square-planar complex (PNP)NiCl
(1) was originally reported by Ozerov and co-workers and
can be readily prepared by the addition of (PNP)H to
anhydrous NiCl2 in the presence of NEt3 or by salt metathesis
involving (PNP)Li and NiCl2(THF)1.5 (see the Experimental
Section).13 To assess whether compound 1 could be reduced
by one electron, we examined its electrochemical behavior
in tetrahydrofuran (THF) using a 0.3 M solution of tetrabu-
tylammonium hexafluorophosphate (TBAH) electrolyte in
THF. As shown in Figure 1, complex 1 displays a reversible
0/+
anodic wave close to the FeCp2 redox couple and an
irreversible endothermic wave at a potential of -2.48 V. The
high reduction potential of 1 suggested that a powerful one-
(12) (a) Fan, L.; Foxman, B. M.; Ozerov, O. V. Organometallics 2004,
23, 326. (b) Fan, L.; Yang, L.; Guo, C.; Foxman, B. M.; Ozerov, O. V.
Organometallics 2004, 23, 4778. (c) Ozerov, O. V.; Guo, C.; Papkov,
V. A.; Foxman, B. M. J. Am. Chem. Soc. 2004, 126, 4792.
(13) Ozerov, O. V.; Guo, C.; Fan, L.; Foxman, B. M. Organometallics
2004, 23, 5573.
(14) Connelly, N. G.; Geiger, W. E. Chem. ReV. 1996, 96, 877.
Inorganic Chemistry, Vol. 47, No. 22, 2008 10481

Adhikari et al.
2.084 (3) vs Ni(2)-N(11), 2.062 (3); Ni(2)-N(40), 2.070(3)
Å), thus hinting that each metal center is experiencing a
slightly different chemical environment. Given the acute
Ni-N-Ni angles, each metal center is proximal, having a
distance of 2.3288(7) Å between them (Figure 2). Other
notable metrical parameters for 2 are listed with Figure 2.
An analogous geometry to 2 has been recently reported for
Co(I)15 and Cu(I)16 systems bearing a similar PNP ligand.
The diamond core structure presented here does differ
significantly from a Ag(I) dimeric structure formed with this
same PNP ligand.17
Magnetism of Complex 2. Logically, the radical nature
of each Ni(I) (formally 17 valence electrons per Ni center)
should be quenched, per se, via the formation of a Ni · · ·Ni
bond, a metrical parameter which is extremely short in our
case, 2.3288(7) Å (vide supra). Considering the short Ni · · ·Ni
distance found in 2, a strong antiferromagnetic interaction
between the centers was anticipated, which would therefore
result in an overall diamagnetic ground state for 2. A similar
coupling phenomenon has been observed in the case of the
hydride dimers [(tBu2P)2CH2CH2)Ni]2(µ2-H)2 and [((iPr2P)2-
CH2CH2CH2)Ni]2(µ2-H)2 where the Ni-Ni distances are in
fact slightly longer at 2.433(1) (structure severely disordered)
and 2.438(1) Å, respectively.6,18 Despite having a very short
Ni · · · Ni distance, a value comparable with the calculated
covalent radii (2.33 Å),19 compound 2 was found to be
paramagnetic. The magnetic moment, µeff, of a solution of 2
(calculated for the monomer), measured by the Evans
method, was determined to be 1.78(1)µΒ at 25 °C in toluene.
The magnetic moment is slightly temperature-dependent,
varying from 1.45 µΒ at 218 K to 1.74 µΒ at 348 K,
suggesting that 2 might exist as a mixture of monomers
“(PNP)Ni, (PNP)Ni(solvent)”, and a dinuclear compound.20
The same magnetic behavior has been observed in pyridine
but not in cyclohexane, thus implying that 2 might be
undergoing a ligand rearrangement in coordinating solvents.20
A similar equilibrium scenario has been proposed for the
Co(I) analogue, [Co(µ2-PNP)]2, recently reported by our
group.15 However, given our inability to identify the species
present in solution, we studied the magnetism of microc-
rystalline solid samples of 2. This feature would also allow
for an analysis and direct comparison with the solid-state
structure, as determined by XRD (vide supra).
Figure 2. Molecular structure of 2 (top) depicting thermal ellipsoids at
the 50% probability level with H atoms, the Et2O molecule, and isopropyl
methyls omitted for clarity. The bottom figure emphasizes the Ni2N2 core
along with the bridging phosphines. Selected metrical parameters are
reported in angstroms and degrees. Ni(1)-Ni(2), 2.3288(7); Ni(1)-N(11),
2.050(3); Ni(1)-N(40), 2.084(3); Ni(1)-P(48), 2.2138(10); Ni(1)-P(19),
2.2195(10); Ni(2)-N(11), 2.062(3); Ni(2)-N(40), 2.070(3); Ni(2)-P(32),
2.2287(10); Ni(2)-P(3), 2.2344(10); P(19)-Ni(1)-P(48), 122.17(4);
P(32)-Ni(2)-P(3), 119.30(4); Ni(1)-N(11)-Ni(2), 68.99(9); Ni(1)-
N(40)-Ni(2), 68.19(9); N(11)-Ni(1)-N(40), 111.33(11); N(11)-Ni(2)-
N(40), 111.43(11); N(11)-Ni(1)-P(19), 89.64(8); N(11)-Ni(1)-P(48),
126.00(8); N(11)-Ni(2)-P(3), 89.09(8); N(11)-Ni(2)-P(32), 124.66(8);
N(40)-Ni(1)-P(19), 120.72(8); N(40)-Ni(1)-P(48), 89.72(8); N(40)-
Ni(2)-P(3), 127.51(8); N(40)-Ni(2)-P(32), 88.63(8).
that one PNP ligand in 2 is undergoing more tension than the
other, and that each half of the molecule is not strictly
equivalent. Likewise, the C-N-C-C angles (42.5 (4), 42.6
(4), 44.5 (4), and 47.7(4)°) also depict a considerable amount
of torsion involved in the ligand framework. The angles
between two aryl rings connected to nitrogen are 74.07(11)
and 72.45(11)°, respectively, which might be the result of
strain. As a result of these torsions, the molecule cannot
possess an inversion center, despite each Ni environment and
its first coordination sphere being grossly identical. Although
the Ni2N2 diamond core appears symmetrical at first sight
(Figure 2), closer inspection of other salient metrical
parameters exposes subtle differences in the Ni-P distances
(Ni(1)-P(19), 2.2195(10); Ni(1)-P(48), 2.2138(10) vs
Ni(2)-P(3), 2.2344(10); Ni(2)-P(32), 2.2287(10) Å) and
Ni-N distances (Ni(1)-N(11), 2.050 (3); Ni(1)-N(40),
The magnetizations of crystalline samples of 2 were
measured by SQuID magnetometry in the temperature range
between 2 and 300 K at a field of 1 Tesla. The data were
(15) Fout, A. R.; Basuli, F.; Fan, H.; Tomaszewski, J.; Huffman, J. C.;
Baik, M.-H.; Mindiola, D. J. Angew. Chem., Int. Ed. 2006, 45, 3291.
(16) (a) Harkins, S. B.; Peters, J. C. J. Am. Chem. Soc. 2005, 127, 2030.
(b) Mankad, N. P.; Rivard, E.; Harkins, S. B.; Peters, J. C. J. Am.
Chem. Soc. 2005, 127, 16032. (c) Harkins, S. B.; Mankad, N. P.;
Miller, A. J. M.; Szilagyi, R. K.; Peters, J. C. J. Am. Chem. Soc. 2008,
130, 3478. (d) Harkins, S. B.; Peters, J. C. J. Am. Chem. Soc. 2004,
126, 2885.
(17) DeMott, J. C.; Basuli, F.; Kilgore, U. J.; Foxman, B. M.; Huffman,
J. C.; Ozerov, O. V.; Mindiola, D. J. Inorg. Chem. 2007, 46, 6271.
(18) Fryzuk, M. D.; Clentsmith, G. K. B.; Leznoff, D. B.; Rettig, S. J.;
Geib, S. J. Inorg. Chim. Acta 1997, 265, 169.
(19) Pauling, L.; The Nature of Chemical Bond; Cornell University Press:
Ithaca, NY, 1939.
(20) See the Supporting Information.
10482 Inorganic Chemistry, Vol. 47, No. 22, 2008

A Dinuclear Ni(I) System
agreement with the g values obtained from solution and solid-
state EPR data (2.205 and 2.214, vide infra).
On the grounds of our SQuID data, case 1 is discarded
since the g value obtained in this fit (1.95) deviates
significantly from the one found by EPR spectroscopy (vide
infra). We also find it unlikely to observe such an extremely
strong ferromagnetic coupling regardless of the Ni-Ni
distance. In the case of isoelectronic Cu(II) dimers, no
documented cases having a ferromagnetic coupling to even
a third of this magnitude have been experimentally ob-
served.16 Case 2 is less easily discarded, and we disfavor
the uncoupled model on the basis of the fit of the magnetiza-
tion data in conjunction with the solid-state X-band EPR
spectroscopic data (vide infra).
X-Band EPR Spectroscopy of Complex 2. EPR spectra
of compound 2 were simulated using the spin Hamiltonian
H ) µBgxSxBx + µBgySyBy + µBgzSzBz + D(Sz2 - 1/3S(S +
1)). The X-band EPR spectrum of 2 recorded in toluene
solution at room temperature (298 K) shows a virtually
featureless single resonance line that can be simulated with
an isotropic g value of 2.205(5) (Figure S5, Supporting
Information). However, the X-band EPR performed on 2 in
the solid state displays an anisotropic rhombic signal at 298
K (Figure 4, top). The spectrum was simulated using three
different g values of 2.393(4), 2.238(4), and 1.992(4). The
average of the anisotropic g values is 2.214(4) and within
the uncertainty; this is the same as the isotropic value
measured in solution, indicating that both the solution and
solid-state EPR signals originate from the same species. We
cannot refute, however, whether the solution spectrum is a
superposition of dimer and monomer spectra, especially if
they have similar g values. The g values observed for
complex 2 are analogous to that found by others.22b The
nonaxiality of the coordination environment for the nickel
centers (Figure 2) is clearly reflected in the rhombic
anisotropy of the experimental g values.
As discussed earlier, magnetization data have shown that
the Ni(I) centers are weakly antiferromagnetically coupled,
and this observation prompted us to corroborate these
findings by performing low-temperature EPR spectroscopy.
Accordingly, the solid-state spectrum of 2 at 4 K is shown
in Figure 4 (middle). The spectroscopic features are very
similar to the room-temperature spectra but with slightly
lower g values of 2.380(4), 2.225(4), and 1.980(4).
On the basis of magnetization and EPR data, we wish to
distinguish between case 2 and case 3 (vide supra). The main
lines in the solid-state EPR spectrum of complex 2 were
simulated using both models (Figure 4, top and middle). A
low-intensity feature at approximately half field (positioned
at g ) 4.24) was observed at 4 K, and this feature should
only be present in case 3: a weak interaction in a dinuclear
complex (Figure 4, bottom).22 The red lines in Figure 4
display the simulations using the dimer model: a spin
Hamiltonian with S ) 1. At 4 K: S ) 1, gx ) 2.380(4), gy
) 2.225(4,), gz ) 1.980(4), D ) 0.001 cm-1, line width )
Figure 3. Plot of µeff as a function of temperature shown for 2 in the
temperature interval 2-300 K. Open spheres and blue spheres represent
runs performed on two independently prepared samples. The line shows
the fit using the dimer model and the parameters described in the text.
corrected for underlying diamagnetism using tabulated Pas-
cals constants21 and plotted as the effective magnetic moment
versus temperature (Figure 3). The effective magnetic
moment varies from 1.6 µB at 2 K to 2.9 µB at 300 K.
Disregarding a temperature-independent paramagnetism (TIP)
of 450 × 10-6 cm3 · mol-1 per dimer molecule, the suscep-
tibility is fairly constant over the temperature interval
50-300 K. This data was reproducible with an independently
prepared sample of microcrystalline 2 (referred to, in Figure
3, as Run 2).
Qualitatively, the magnetization data of 2 could correspond
to three different situations. Case 1 is a very strong
ferromagnetic coupling between the S ) 1/2 nickel centers
with a triplet ground state (Stotal ) 1) and an exited singlet
state (Stotal ) 0), which is not partially populated even at
room temperature (>500 cm-1 higher in energy). Case 2
corresponds to magnetically isolated S ) 1/2 centers, while
case 3 is a weakly antiferromagnetically coupled dimer with
a singlet ground state and an excited triplet state only several
cm-1 higher in energy.
In order to address the most logical case for 2, the
experimental data were fitted to all three cases using the
appropriate spin Hamiltonian. In each case, there is a gradual
decrease of the effective magnetic moment at very low
temperatures. In cases 1 and 2, the latter feature will be due
to intermolecular interactions with neighboring nickel centers
and is expressed as a Weiss temperature, while in case 3,
this lowering in effective magnetic moment is mainly due
to the intramolecular antiferromagnetic interaction in the
dimer.
Our best fit is case 3 and is displayed in Figure 3 as a
solid black line. The model used is a dimer model using the
usual spin Hamiltonian H ) -2J(S1 × S2) + gµBB(S1 + S2)
with S1 ) S2 ) 1/2. The parameters found are a small
coupling component J ) -1.52(5) cm-1, an isotropic g value
of 2.21(1), a TIP of 450 × 10-6 cm3 · mol-1, and a Weiss
temperature of θ ) -0.4 K. The g value is in excellent
(21) (a) O’Connor, C. J. Prog. Inorg. Chem. 1982, 29, 203. (b) David,
R. L. CRC Handbook of Chemistry and Physics, 60th ed.; CRC Press:
Boca Raton, FL, 1979.
(22) (a) Meyer, K.; Bendix, J.; Bill, E.; Weyhermuller, T.; Wieghardt, K.
Inorg. Chem. 1998, 37, 5180. (b) Muresan, N.; Weyhermu¨ller, T.;
Wieghardt, K. Dalton Trans. 2007, 4390.
Inorganic Chemistry, Vol. 47, No. 22, 2008 10483

Adhikari et al.
Scheme 2. Synthesis of Complex 4 from 2 and 2 equiv of N2CPh2
enough evidence, however, to unambiguously determine the
magnitude or main axis of the zero-field-splitting and can
only therefore confirm this parameter to indeed be nonzero
at liquid helium temperatures, and that it is maximally at
0.0015 cm-1. Otherwise, splitting in the main lines would
be present, which we do not observe in our case. Due to the
anisotropy in the g values, the position of the half-field line
is dependent on which g value is assigned to the z axis. Our
findings point toward the lower g value being assigned to
the z axis (axis of the zero-field splitting parameter D).
It is noted that we did not observe the weak half-field line
in background measurements. It is unlikely that a decom-
position/impurity byproduct of 2 would be responsible for
this signal given our consistent magnetization data collected
for two independently crystallized samples. Although ex-
tremely small amounts of iron(III) impurities can generate a
weak signal at 4.3 K, we are confident that compound 2 is
analytically pure material which is prepared under highly
reducing conditions. Hence, the plausibility of an iron(III)
impurity present in the final product is unlikely, especially
if the data are reproducible with independently prepared
samples of crystalline 2.
Trapping of the Ni(I) Monomer. Despite our definitive
evidence for 2 being a diradical species in the solid state both
at room temperature and near absolute zero, the possibility of
such species dissociating into the corresponding monomer, in
solution, is still questionable. The results obtained by Evans,
although inconclusive, do indeed suggest that a monomer T
dimer equilibrium could be operative in 2, since its magnetiza-
tion measurements slightly varied over a temperature range and
depend on the nature of the solvent medium (vide supra). Gas
chromatography-mass spectrometry measurements also sug-
gested that the dimeric core in 2 was fragile enough to break
into its corresponding monomers (calcd, 486.1989; found,
486.1977). However, the most conclusive proof for the forma-
tion of a monomer in solution stems from molecular weight
measurements obtained by the freezing point depression method
of a 1.0% w/w solution of 2 in naphthalene at its melting point.24
The average observed value collected on four different samples,
379(107) g/mol, is lower than the expected value for the
monomer but within the limitation of the method (see the
Experimental Section). Therefore, these results imply that 2 is
mostly a monomer near the melting point of naphthalene (∼80
°C). Despite unsuccessful attempts to trap the elusive three-
coordinate Ni(I) species with donors such as THF, P(CH3)3,
and pyridine, the treatment of 2 with 2 equiv of N2CPh2 resulted
Figure 4. X-band EPR spectra of solid state 2 at room temperature (top)
and at 4 K (middle). At the bottom, the 4 K spectra has been expanded by
a factor of 150 to show the weak line found at approximately half field (g
value of the position of this line is 4.24). Experimental conditions at 300
K: microwave frequency, 9.390083 GHz; microwave power, 0.2012 mW;
modulation frequency, 100 kHz; modulation amplitude, 0.8 mT. At 4 K,
middle spectrum: microwave frequency, 9.016023 GHz; microwave power,
0.998 mW; modulation frequency, 100 kHz; modulation amplitude, 1.0 mT.
At 4 K, bottom spectrum: microwave frequency, 9.014827 GHz; microwave
power, 0.998 mW; modulation frequency, 100 kHz; modulation amplitude,
0.5 mT, collected over three runs. The simulations shown as red lines were
performed with the spin Hamiltonian model and the parameters described
in the text. The field scales of the middle and bottom spectra have been
corrected for the differing frequencies compared to the top spectrum to
facilitate comparisons.
25 G, and Lorentzian line shapes. At 25 °C: S ) 1, gx )
2.393(4), gy ) 2.238(4), gz ) 1.992(4), D ) 0.000 cm-1,
line width ) 17 G, and Lorentzian line shapes. We have
also recorded a series of solution EPR spectra in the
temperature range 280-175 K and found that the intensity
of the g ) 2.205 signal decreases gradually upon lowering
the temperature, thus consistent with a diamagnetic S ) 0
ground state being populated over the S ) 1 low-lying
excited state.22a The presence of a small nonzero zero-field-
splitting parameter, D, in the spin Hamiltonian will induce
a slight mixing of the three energy levels in the triplet state
(MS ) -1, 0, and +1), and this will relax the selection rules,
therefore inducing intensity in the forbidden ∆MS ) 2 line
found at the half-field signal (g ) 4.24) in our simulation.
The observed experimental line is more than a magnitude
stronger than the simulated line, and we tentatively assign
this to the fact that the nickel centers are not truly equal.
This is in fact the case when examining the solid-state
structure of 2 (vide supra). It has been demonstrated that
lowering the symmetry of the system is very effective in
increasing intensity in the forbidden line.23 We do not have
(23) Abragam, A.; Bleaney, B. Electron Paramagnetic Resonance of
Transition Ions; Clarendon Press: Oxford, U.K., 1970.
(24) Smith, J. M.; Lachicotte, R. J.; Holland, P. L. J. Am. Chem. Soc. 2003,
125, 15752.
10484 Inorganic Chemistry, Vol. 47, No. 22, 2008

A Dinuclear Ni(I) System
pincer amide group. Coincidently, one of the phenyl groups
from the former diazo reagent blocks the open coordination
site of the Ni(I) ion, but the closest Ni · · · C distance is too
far to be considered a significant interaction (>2.92 Å).
Although complex 4 represents a “trapped” version of the
Ni(I) monomer in 2, the formation of this complex still casts
some doubt as to whether this species was generated from
the monomer or dimer. Given the sterically congested
environment in 2, however, it thus seems logical to propose
that 4 originates from the reaction of N2CPh2 with a putative
“(PNP)Ni” monomer in solution.
Reductive Bond Splitting Reactions Promoted by
Complex 2. The radical nature of 2 allows this species to
behave as a two-electron reductant with each nickel center
acting as a masked three-coordinate “(PNP)Ni” source.
As a result, the treatment of 2 with substrates such as
H-X (X ) H, OH, OCH3, PHPh, B(catechol)) im-
mediately leads to splitting of the H-X bond concurrent
with formation of an equal mixture of Ni(II) products
(PNP)NiH13 and (PNP)NiX (X ) H13 (3), OH (5), OCH3
(6), PHPh (7), and the boryl B(catechol) (8)) according
to Scheme 3. The identities of 3 and 5-7 have been
confirmed independently by the salt metathesis of 1 with
the appropriate MX reagent (M ) Na, X ) BH4;13 M )
K, X ) OH; M ) Na, X ) OCH3; M ) Li, X ) PHPh;
Scheme 3). With the exception of 5, the transmetalation
of 1 with X- is very clean, if not quantitative. The identity
of 3 was compared to independently prepared samples
originally reported by Ozerov and co-workers.13 The
formation of hydride-free 8 on the other hand was
achieved by the addition of biscatecholdiborane
((catechol)2B2) to 2 (Scheme 3), a process that involves
an unprecedented B-B bond cleavage promoted by two
Ni(I) centers. Analogous to 3, the 31P and 13C NMR spectra
for 5-8 display resonances consistent with square-planar
Figure 5. Molecular structure of 4 depicting thermal ellipsoids at the 50%
probability level with H atoms, the Et2O molecule, and isopropyl methyls
omitted for clarity. Selected metrical parameters are reported in angstroms
and degrees. Ni(1)-N(31), 1.913(8); Ni(1)-N(10), 1.979(9); Ni(1)-P(2),
2.158(3); P(18)-N(31), 1.678(8); N(31)-N(32), 1.408(10); N(31)-Ni(1)-
N(10), 98.5(3); N(31)-Ni(1)-P(2), 165.7(3); N(10)-Ni(1)-P(2), 88.4(3);
P(18)-N(31)-Ni(1), 115.2(4); N(32)-N(31)-P(18), 108.5(7); N(32)-
N(31)-Ni(1), 125.8(7); C(33)-N(32)-N(31), 114.4(9).
in a gradual color change from dark to brown concurrent with
precipitation of the three-coordinate Ni(I) complex
Ni(PNPdNNCPh2) (4) as a brown solid (Scheme 2). Complex
4 is a paramagnetic Ni(I) species (µeff ) 2.22(3) µΒ) and,
therefore, shows a broad isotropic X-band EPR signal at giso
)
2.19 (Figure S6, Supporting Information).20 The line is probably
broadened (line width, W ) 55 G) due to unresolved hyperfine
coupling with proximal nuclear spins such as 31P (I ) 1/2) and
14N (I ) 1).
To determine the geometry at the metal center and confirm
the degree of aggregation in 4, we resorted to XRD studies.
Accordingly, the molecular structure of 4 reveals a T-shaped
Ni(I) center9 in which one of the pendant phosphorus arms
composing the PNP framework has been oxidized by the
diazo unit to furnish a mono-substituted (phosphoranylidene)-
hydrazone group (PNPdNNCPh2). The R-N of the (phos-
phoranylidene)hydrazone moiety is bound to the nickel center
(Figure 5). Ylide bond formation (NdP distance is 1.678(8)
Å) results via the coordination of this nucleophilic site to
the Ni(I) center (Ni-N, 1.913(8) Å), a parameter comparable
to the Ni-Namide distance of 1.979(9) Å obtained for the
1
Ni(II) environments, while the H NMR spectra reveal
diagnostic resonances for the terminal hydroxyl (-5.06
3
ppm, JH-P ) 6 Hz), methoxide (3.30 ppm), and phe-
1
nylphosphide (doublet of triplets, 3.64 ppm, JH-P ) 179
3
Hz and JH-P ) 20 Hz). In the case of compound 8, the
Scheme 3. Bond Splitting Reactions Involving 2 and Independent Routes to Forming Complexes 3, 5-8
Inorganic Chemistry, Vol. 47, No. 22, 2008 10485

Adhikari et al.
Table 1. Selected Metrical Parameters for the Molecular Structures of 5
and 9a
5
9
Ni-X
1.8634(9)
2.1883(4), 2.2025(4)
1.9125(8)
1.884(9)
2.192(4), 2.195(4)
1.931(7)
167.79(10)
84.0(3), 84.0(3)
96.1(4), 95.9(4)
179.3(6)
Ni-P
Ni-N
P-Ni-P
N-Ni-P
P-Ni-X
N-Ni-X
Nideviation
167.986(11)
84.13(3), 83.98(2)
92.39(3), 99.50(3)
176.52(4)
0.016
0.014
a Metrical parameters reported in angstroms and degrees, where X-
represents OH for complex 5 and NH2 for complex 9.
In addition to H2, complex 2 can engage in other homolytic
bond-splitting reactions since the addition of 1 equiv of
PhXXPh smoothly oxidizes the Ni2(I,I) core to the corre-
sponding square-planar complexes (PNP)Ni(XPh) (X ) S,
10; X ) Se, 11; Scheme 3) quantitatively. The most notable
spectroscopic feature for compounds 10 and 11 includes the
observation of a C2 symmetric system (31P NMR: 33.6 and
36.4 ppm, respectively). Riordan et al.29 have reported S-S
reductive bond cleavage reactions of elemental sulfur by Ni(I)
centers, while Hillhouse et al.1a have reported a similar
reaction to ours: reductive cleavage of the S-S bond in
PhSSPh by a dimeric Ni(I) complex.
Figure 6. Molecular structures of 5 (left) and 9 depicting thermal ellipsoids
at the 50% probability level with H atoms, solvent molecules, and isopropyl
methyls omitted for clarity. Selected metrical parameters are listed in Table 1.
11B NMR spectrum clearly reveals the formation of a rare
example of a nickel-boryl (∼47 ppm). Complex 8 can be
also independently prepared via treatment of 3 with
catecholborane,25 and its solid-state structure has been also
recently reported by us.25
As noted before, the formation of 5 via salt metathesis
was not quantitative given the presence of water and soluble
Cl- to regenerate 1 (Scheme 3). The presence of a large
excess of KOH modestly improved the formation of 5 and
suggested that an equilibrium scenario was hampering the
generation of pure 5. However, in the course of finding a
better leaving group, we discovered that H2O cleanly reacts
with (PNP)Ni(NH2) (9), prepared readily from 1 and NaNH2
in THF, to extrude NH3 concomitant with the formation of
5 (Scheme 3). Attempts to generate 9 from a binuclear
oxidative addition of NH3 to 226 resulted in only the
formation of an equal molar amount of 3 and 5. Despite
extensive attempts to generate an anhydrous NH3 reagent,
the formation of 3 and 5 was unavoidable, possibly due to
adventitious H2O.
Given the rarity of terminal Ni(II) hydroxyls, and parent
amides,27,28 we proceeded to collect solid-state structural data
for 5 and 9. As indicated in Figure 6, the crystal structure
for each displays square-planar Ni(II) systems bearing
terminal -OH and -NH2 functionalities, respectively. The
Ni-O (1.8634(9) Å) and Ni-N (1.884(9) Å) distances in 5
and 9 are comparable to the few examples of crystallogra-
hically characterized terminal nickel amides, 1.872(2) Å,27a
and hydroxides, 1.865(17),27b 1.877(4),27c and 1.955(2)28 Å.
A comparison of the metrical parameters between 5 and 9
is reported in Table 1.
Conclusions
X-ray, magnetization, and EPR data corroborate well with
the Ni2N2 unit in 2, having two unpaired electrons in an
overall S ) 0 ground state with reachable S ) 1, where the
nickel centers are not isolated. The unpaired electrons do
couple, but this interaction is weak and occurs at lower
temperatures (< 50 K). The possibility of 2 undergoing a
monomer T dimer equilibrium in solution should not be
refuted, given the findings of molecular weight determination
in solution along with our ability to trap a Ni(I) monomer
with substrates such as N2CPh2. Our results present compel-
ling evidence to suggest that a dinuclear Ni(I) radical-based
system possessing a Ni2N2 diamond core resting state can
engage in H-X splitting reactions.30 The two Ni(I) centers
in 2 appear to be reducing these substrates in concert and in
a manner in accord with a binuclear oxidative addition,30
since no H2 has been detected for the aforementioned
reactions. However, we cannot rule out the possibility of the
PNP framework in 2 rearranging, in the presence of
substrates, to a metal-metal bonded species, (PNP)Ni-
Ni(PNP). Regardless of the mechanism, complex 2 reacts
analogously to a Pd analogue recently reported by Ozerov
and co-workers.26a The dissociation of 2 into its correspond-
ing monomers, “(PNP)Ni”, and promotion of a radical chain
reaction could also be operative, analogous to well docu-
(25) Adhikari, D.; Huffman, J. C.; Mindiola, D. J. Chem. Commun. 2007,
4489.
(26) (a) Fafard, C. M.; Adhikari, D.; Foxman, B. M.; Mindiola, D. J.;
Ozerov, O. V. J. Am. Chem. Soc. 2007, 129, 10318. Oxidative addition
of NH3 at a single metal center has been also reported. (b) Zhao, J.;
Goldman, A. S.; Hartwig, J. F. Science 2005, 307, 1080.
(27) (a) Campora, J.; Palma, P.; del Rio, D.; Conejo, M. M.; Alvarez, E.
Organometallics 2004, 23, 5653. (b) Campora, J.; Palma, P.; del Rio,
D.; Alvarez, E. Organometallics 2004, 23, 1652. (c) Campora, J.;
Matas, I.; Palma, P.; Graiff, C.; Tiripicchio, A. Organometallics 2005,
24, 2827. (d) Siegfried, L.; Kaden, T. A.; Meyer, F.; Kircher, P.;
Pritzkow, H. Dalton Trans. 2001, 15, 2310. (e) Meyer, F.; Kaifer, E.;
Kircher, P.; Heinze, K.; Pritzkow, H. Chem.sEur. J. 1999, 5, 1617.
(28) Kieber-Emmons, M. T.; Schenker, R.; Yap, G. P. A.; Brunold, T. C.;
Riordan, C. G. Angew, Chem., Int. Ed. 2004, 43, 6716.
(29) Cho, J.; Van Heuvelen, K. M.; Yap, G. P. A.; Brunold, T. C.; Riordan,
C. G. Inorg. Chem. 2008, 47, 3931.
(30) (a) Crabtree, R. H. The Organometallic Chemistry of the Transition
Metals; Wiley Interscience: New York, 2001. (b) Jamali, S.; Na-
bavizadeh, S. M.; Rashidi, M. Inorg. Chem. 2005, 44, 8594. (c) Cui,
W.; Zhang, P. X.; Wayland, B. B. J. Am. Chem. Soc. 2003, 125, 4994.
(d) Jimenez, M. V.; Sola, E.; Lopez, J. A.; Lahoz, F. J.; Oro, L. A.
Chem.sEur. J. 1998, 4, 1398.
10486 Inorganic Chemistry, Vol. 47, No. 22, 2008

A Dinuclear Ni(I) System
mented cases such as [Co(CN)5]3-, and other metal radicals
such as Rh(II), and mixed valent Ir2 cores.30,31 Given the
ability of 2 to generate unusual terminal groups on nickel(II)
presumably by homolytic and heterolytic bond cleavage
processes, we are currently exploring other substrates prone
to two-electron reduction reactions, since this will allow us
to assemble other rare functional groups onto nickel for
subsequent group-transfer chemistry. Since 2 presents a
special and rare example of a very weakly coupled bimetallic
Ni(I) system, the underlying reason behind this uncoupled
behavior as well as the mechanism for antiferromagnetic
coupling will be interesting aspects to investigate. The
governing factors behind such a weakly coupled behavior
in 2, as well as the mechanistic detail of the bond cleavage
reactions promoted by the Ni2(I,I) core, are currently being
studied in our laboratories.
Table 2. Molecular Weight (MW) Determinations from Melting Point
Determination of Four Samples of 2 in Naphthalene (napth)a
T
∆T
MW
napth (average)
sample 1
sample 2
sample 3
sample 4
80.25
80.08
80.12
80.13
80.17
0.17
0.13
0.12
0.08
259
339
367
551
a Temperatures are reported in °C. The rate of heating was 0.005 °C/
sec.
Planck-Institut fu¨r Bioanorganische Chemie, Mu¨hlheim an der Ruhr,
Germany. X-ray diffraction data were collected on a SMART6000
(Bruker) system under a stream of N2 (g) at low temperatures. The
EPR measurements were performed in quartz tubes equipped with
J-young valves. EPR data for 4 were recorded with a Bruker EMX
EPR instrument. Other EPR spectra were recorded on a JEOL
continuous wave spectrometer JES-FA200 equipped with an X-band
Gunn diode oscillator, a cylindrical mode cavity, and a helium
cryostat. EPR data were simulated using the full matrix diagonal-
ization program ESRSIM written by Høgni Weihe, University of
Copenhagen, Denmark.38
Molecular Weight Determination. This procedure follows an
earlier protocol reported by Holland and co-workers.24 A mixture
of 2 (1.8 mg) and naphthalene (sublimed, 185 mg) was heated until
an brown solution formed. After cooling, an aliquot of the brown
solid was placed in a capillary, which was then plugged with
silicone grease and flame-sealed. Melting point depressions,
compared to that of authentic naphthalene (average of two sample
runs which were sublimed prior to usage), were measured on a
Thomas-Hoover melting point apparatus, using a VWR Precision
0.01 Thermometer equipped with a Pt-100Ω sensor. Molecular
weights were determined using the formula molecular weight )
K·w·1000/∆T·W, where K ) 4.9 for naphthalene, w ) mass of
complex 2, W ) mass of naphthalene, and ∆T ) temperature
depression. The results of these measurements (four samples) are
listed in Table 2.
Cyclic Voltammetry (CV) Details. In a typical CV experiment,
0.3 M [N(nBu)4]PF6 in THF was used as a supporting electrolyte
where 15-20 mg of crystalline 1 was dissolved in ∼5 mL of TBAH
solution in THF at 25 °C. A platinum disk (2.0 mm diameter,
Bioanalytical Systems), a platinum wire, and a silver wire were
employed as the working electrode, the auxiliary, and the reference
electrode, respectively. A one-compartment cell was used in the
CV measurement. The electrochemical response was collected with
the assistance of an E2 Epsilon (BAS) autolab potentiostat/
galvanostat with the BAS software. All of the potentials were
reported against the ferrocenium/ferrocene (Fc+/0) couple (0.0 V)
measured as an internal standard, and spectra were recorded under
a N2 atmosphere.
Synthesis of Complex (PNP)NiCl (1). Complex 1 was prepared
by a procedure modified from the literature, and the identity of the
product was confirmed by 1H and 31P NMR spectra from indepen-
dently prepared samples.4 In a 500 mL round-bottom flask,
NiCl2(THF)1.5 (500 mg, 2.10 mmol) was suspended in toluene (250
mL), and Li(PNP) (915.50 mg, 2.10 mmol) in 10 mL of toluene
was added dropwise to the suspension. The mixture was stirred
for 12 h at room temperature, after which the volatiles were
evaporated completely in vacuo. The residue was extracted with
toluene and filtered and the filtrate concentrated and cooled to -35
°C to afford green crystals of (PNP)NiCl (1) (1.05 g, 2.02 mmol,
Experimental Section
General Procedures. Unless otherwise stated, all operations
were performed in an M. Braun Laboratory Master double-dry box
under an atmosphere of purified nitrogen or using high-vacuum
standard Schlenk techniques under an argon or dinitrogen atmo-
sphere. Anhydrous n-hexane, pentane, toluene, and benzene were
purchased from Aldrich in sure-sealed reservoirs (18 L) and dried
by passage through two columns of activated alumina and a Q-5
column.32 Diethylether was dried by passage through a column of
activated alumina.32 THF was distilled, under nitrogen, from purple
sodium benzophenone ketyl and stored under sodium metal.
Distilled THF was transferred under vacuum into a thick-walled
reaction vessels before being carried into a dry box. Deuterobenzene
was purchased from Cambridge Isotope Laboratory, degassed, and
vacuum-transferred to 4 Å molecular sieves. All other deuteron
solvents were stored under sodium metal. Celite, alumina, and 4 Å
molecular sieves were activated under vacuum for overnight at 200
°C. NiCl2(THF)1.5,
33 (PNP)H,12 Li(PNP),12 KC8,34 Ph2CN2,35 and
LiPHPh36 were prepared according to the literature methods. All
other chemicals were purchased from Strem Chemicals or Aldrich
and used as received. CHN analyses were performed by Desert
1
Analytics, Tucson, Arizona. H, 13C, 11B, and 31P NMR spectra
were recorded on Varian 500, 400, or 300 MHz NMR spectrom-
1
eters. H and 13C NMR are reported with reference to residual
solvent resonances. 31P NMR chemical shifts are reported with
respect to external H3PO4 (0.0 ppm). 11B NMR chemical shifts are
reported with respect to external BF3 ·OEt2 (δ 0.0 ppm). Magnetic
moments were obtained by the method of Evans.37 UV-visible
spectra were recorded on a Cary 5000 UV-vis-NIR spectropho-
tometer. For the variable-temperature UV-vis experiment, the
sample in the cuvette was cooled and heated by an external cooler
regulated by the Peltier effect. Magnetization measurements were
performed on a Quantum Design SQuID susceptometer at 0-5
Tesla and 2-300 K. Susceptibility data were fitted using the
program JulX, version 1.3, written by Dr. Eckhard Bill, Max-
(31) Halpern, J. Pure Appl. Chem. 1979, 51, 2171.
(32) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J. Organometallics 1996, 15, 1518.
(33) Eckert, N. A.; Bones, E. M.; Lachicotte, R. J.; Holland, P. L. Inorg.
Chem. 2003, 42, 1720.
(34) Schwindt, M. A.; Lejon, T.; Hegedus, L. S. Organometallics 1990, 9,
2814.
(35) Miller, J. B. J. Org. Chem. 1959, 24, 560.
(36) Dias, H. V. R.; Power, P. P. J. Am. Chem. Soc. 1989, 111, 144.
(37) (a) Sur, S. K. J. Magn. Reson. 1989, 82, 169. (b) Evans, D. F. J. Chem.
Soc. 1959, 2003.
(38) Jacobsen, C. J. H.; Pedersen, E.; Villadsen, J.; Weihe, H. Inorg. Chem.
1993, 32, 1216.
Inorganic Chemistry, Vol. 47, No. 22, 2008 10487

Adhikari et al.
1
96% yield). Anal. calcd for C26H40NP2NiCl: C, 59.74; H, 7.71; N,
2.68. Found: C, 60.92; H, 7.69; N, 2.46. Mp: 178(2) °C.
mixture was dried in vacuo. H and 31P NMR spectra confirmed
the formation of 3 and 6 in an equimolar amount.
Synthesis of Complex [Ni(µ2-PNP)]2 (2). Compound 1 (200
mg, 0.38 mmol) was taken in a vial with ∼5 mL of THF and the
solution cooled to -35 °C. An analogously cold suspension of KC8
in THF (56 mg, 0.42 mmol) was added to the nickel solution. The
reaction mixture was stirred for 1 h, and the dark suspension was
then dried under reduced pressure. The dried mass was extracted
with hexane and filtered and the filtrate kept cold (-35 °C) for
two days to isolate brown crystals of [Ni(µ2-PNP)]2 (2) (115 mg,
Bond-Splitting Reaction of 2 with H2PPh. Synthesis of
Complex (PNP)Ni(PHPh) (7) and 3. In a vial was dissolved 2
(40 mg, 0.041 mmol) in 5 mL of THF and the solution cooled to
-35 °C. An analogously cooled solution of phenylphosphine (4.52
mg, 0.041 mmol) in 2 mL of hexane was added dropwise to the
cooled solution of 2. The reaction mixture was stirred for 2 h, during
which the color of the solution changed immediately from brown
to red. The solution was then dried under reduced pressure, and
1H and 31P NMR spectra confirmed an equimolar formation of
compounds 3 and 7.
1
0.12 mmol, 62% yield). H NMR (25 °C, 399.8 MHz, C6D6): δ
16.68 (br, ν1/2 ) 279 Hz), 8.41 (br, ν1/2 ) 96 Hz), 4.53 (br, ν1/2
)
53 Hz), 2.85 (br, ν1/2 ) 347 Hz), 1.25 (br, ν1/2 ) 22 Hz), 0.89 (br,
ν1/2 ) 6 Hz). µeff: 2.84 (1) µB (C6D6, 25 °C, Evans method). Anal.
calcd for C52H80N2P4Ni: C, 64.08; H, 8.23; N, 2.87. Found: C,
64.36; H, 8.17; N, 2.86. UV-vis (hexane, λ in nm, ε in mol-1
cm-1): 616 (826), shoulder at 475 (2521), shoulder at 438 (4526),
shoulder at 390 (8383). Mp: 148(2) °C.
Bond-Splitting Reaction of 2 with HB(catechol). Synthesis
of Complex (PNP)Ni[B(catechol)]) (8) and 3. In a vial, compound
2 (50 mg, 0.053 mmol) was dissolved in 5 mL of hexane and the
solution cooled to -35 °C. In a separate vial, an analogously cooled
solution of catecholborane (6.28 mg, 0.053 mmol) in hexane was
added dropwise to the Ni(I) solution. The solution was stirred for
3 h, during which the color gradually changed from brown to
Bond Splitting Reaction of
2 with H2. Synthesis of
yellowish. The solution was evaporated to dryness, and 1H and 31
P
Complex (PNP)NiH (3). In a J-Young tube, complex 2 (40 mg,
0.041 mmol) was dissolved in C6D6 and degassed by a standard
freeze-pump-thaw technique. An excess amount of dihydrogen
gas was passed through the solution, during which the brown color
changed to yellowish after 10 min. Examination by 1H and 31P NMR
spectra confirmed the quantitative formation of compound 3.
Synthesis of Complex Ni(PNPdNNCPh2) (4). In a vial, 2 (75
mg, 0.08 mmol) was dissolved in ∼5 mL of hexane and cooled to
-35 °C. In a separate vial, Ph2CN2 (29.9 mg, 0.15 mmol) was taken
in hexane and cooled to -35 °C. Ph2CN2 solution was added
dropwise to 2 and stirred for 2 h. During the course of the reaction
(∼20 min), a brown-colored precipitate appeared. After completion
of the reaction, the reaction mixture was filtered, and the brown-
colored residue was collected and washed with 10 mL of hexane.
The product was dried and redissolved in a minimum of toluene
(∼2 mL), and then two drops of hexane in a toluene solution were
added, from which brown-colored crystals were obtained at -35
°C (63.8 mg, 0.09 mmol, 61% yield). 1H NMR (25 °C, 399.8 MHz,
C6D6): δ 8.21 (br, ν1/2 ) 36 Hz), 7.69 (m, aryl), 7.35 (m, aryl),
7.02 (m, aryl), 2.11-1.99 (br, MeAr and CHMe2 resonances
overlapped), 1.23 (br, CHMe2), 0.89 (br, CHMe2). µeff: 2.22(3) µB
(C6D6, 25 °C, Evans method). Mp: 118(2) °C. Multiple attempts
to obtain satisfactory combustion analysis data failed given the
thermal instability of the complex.
NMR spectra of the mixture confirmed an equimolar formation of
8 and 3 as the only two products.
Attempted Bond Splitting Reaction of 2 with NH3. Complex
2 (60 mg, 0.062 mol) was taken in a Schlenk flask in ∼5 mL of THF
and in situ generated NH3 [generated from LiNH2 (7.02 mg, 0.31
mmol) and 2,6-diisopropylphenol (22.05 mg, 0.31 mmol) in ∼ 10 mL
THF] was passed through a vacuum adapter. The solution was stirred
for 1 h, during the course of which the color of the nickel solution
changed from brown to green. 1H and 31P NMR spectra of the reaction
revealed the formation of equimolar amounts of 3 and 5. Attempts to
generate dry NH3 from Na/NH3 electride solution also resulted in the
quantitative formation of 3 and 5 from 2.
Synthesis of Complex (PNP)Ni(NH2) (9). Complex 1 (150 mg,
0.29 mmol) was dissolved in diethyl ether (∼5 mL) and cooled to
-35 °C, and a suspended solution of NaNH2 (11.5 mg, 0.29 mmol)
in diethyl ether was added dropwise. After stirring the reaction
mixture for 8 h, the mixture was filtered and concentrated to ∼2
mL. The concentrated solution was cooled to -35 °C for 48 h to
yield green crystals of 9 (99 mg, 0.20 mmol, 69% yield). 1H NMR
(25 °C, 399.8 MHz, C6D6): δ 7.58 (d, 2H, C6H3), 6.97 (s, 2H, C6H3),
6.77 (d, 2H, C6H3), 2.16-2.02 (br, 4H, CHMe2), 2.12 (s, 6H,
MeAr), 1.40 (dd, 12H, CHMe2), 1.26 (dd, 12H, CHMe2), -2.30 (t,
2H, NH2). 13C NMR (25 °C, 100.6 MHz, C6D6): δ 162.05 (aryl),
132.06 (aryl), 131.86 (aryl), 123.87 (aryl), 120.64 (aryl), 116.07
(aryl), 23.86 (CHMe2), 20.56 (MeAr), 18.74 (CHMe2), 17.99
(CHMe2). 31P NMR (25 °C, 121.5 MHz, C6D6): δ 30.77. MS-CI,
[M + H]+: calcd, 502.217; found, 502. 189. Mp: 166(3) °C.
Independent Synthesis of Complex 5. Method A. Complex 1
(120 mg, 0.24 mmol) was dissolved in 50 mL of THF in a Schlenk
flask. In a separate Schlenk flask, KOH (752 mmol solution, large
excess) in water (1.92 mL, 1.44 mmol) was deareated by a standard
freeze-pump-thaw technique. To the cold (-78 °C) solution of
nickel, the KOH solution in water was transferred via cannula, and
the reaction mixture was slowly allowed to warm up to room
temperature. After 4 h of stirring, the reaction mixture was dried
under reduced pressure and extracted with pentane. The green-
colored extract was filtered and cooled to isolate the desired product,
5 (53.2 mg, 0.106 mmol, 46% yield). The low yield was due to the
unachieved completion of the reaction, supposedly stemming from
the solubility of byproduct KCl, which can drive the reaction in
the backward direction. Crude product was always isolated along
with the contamination of 1. Method B. In a Schlenk flask, complex
9 (100 mg, 0.20 mmol) was dissolved in 10 mL of hexane. An
Bond-Splitting Reaction of 2 with H2O. Synthesis of Complex
(PNP)Ni(OH) (5) and 3. In a Schlenk flask, complex 2 was
dissolved (60 mg, 0.062 mmol) in 20 mL of THF. To the solution
was added degassed water (purged with argon over 10 min, 0.04
mL, 1.24 mmol) by means of cannula transfer. The solution was
stirred for 2 h, during which the color of the reaction mixture rapidly
changed from brown to green. After the completion of the reaction,
1
the solution was dried under reduced pressure. H and 31P NMR
spectra of the reaction mixture revealed an equimolar formation of
3 and 5.
Bond-Splitting Reaction of 2 with CH3OH. Synthesis of
Complex (PNP)Ni(OCH3) (6) and 3. CH3OH (10 mL) was stirred
over a few pieces of thin sodium film (∼500 mg) for 16 h and the
solution filtered through alumina to obtain the anhydrous reagent.
Complex 2 (60 mg, 0.062 mmol) was taken in a vial and dissolved
in THF (5 mL). To the brown solution was added via syringe
anhydrous CH3OH (1.98 mg, 0.062 mol) and the mixture stirred.
Over the course of 2 h, the reaction mixture gradually changed
from brown to reddish brown. After 2 h of stirring, the reaction
10488 Inorganic Chemistry, Vol. 47, No. 22, 2008

A Dinuclear Ni(I) System
1
excess amount of deoxygenated water was added dropwise under
a positive pressure of N2 and the solution stirred for 10 min. After
this time, the solution was dried in vacuo, and a green-colored
obtained (55 mg, 0.09 mmol, 45% yield). H, 31P, and 11B NMR
spectroscopic data confirmed the formation of 8 when compared
to those of an independently prepared sample.25
1
Synthesis of Complex (PNP)Ni(SPh) (10). Complex 2 (20 mg,
0.021 mmol) and diphenyldisulfide (4.6 mg, 0.021 mmol) were
loaded into a J-Young tube in hexane. No immediate color change
was observed after the loading. The reaction mixture was stirred
for an hour, and the brown color of the solution was intensified
during the course of the reaction. The mixture was monitored by
31P NMR spectroscopy until complete (1 h). After this time, the
product was collected (96 mg, 0.19 mmol, 96% yield). H NMR
(25 °C, 399.8 MHz, C6D6): δ 7.52 (d, 2H, C6H3), 6.92 (s, 2H, C6H3),
6.70 (d, 2H, C6H3), 2.28-2.10 (br, CHMe2, 4H), 2.16 (s, 6H, MeAr),
1.51 (dd, 12H, CHMe2), 1.29 (dd, 12H, CHMe2), -5.06 (t, 1H,
OH). 13C NMR (25 °C, 100.6 MHz, C6D6): δ 162.26 (aryl), 132.10
(aryl), 131.94 (aryl), 124.16 (aryl), 120.78 (aryl), 116.94 (aryl),
23.46 (CHMe2), 20.49 (MeAr), 18.62 (CHMe2), 17.87 (CHMe2).
31P NMR (25 °C, 121.5 MHz, C6D6): δ 26.89. MS-CI [M + H]+:
calcd, 502.217; found, 502. 189 along with 486.2 corresponding
to (PNP)Ni+. Mp: 82(2) °C.
1
reaction mixture was dried in vacuo, and the H NMR spectrum
showed quantitative formation of the product, 10. H NMR (25
1
°C, 399.8 MHz, C6D6): δ 8.04 (d, SC6H5, 2H), 7.54 (d, C6H3, 2H),
7.02 (t, SC6H5, 1H), 6.89 (m, overlapped resonances of aryl protons,
4H), 6.77 (d, C6H3, 2H), 2.22-2.08 (br, CHMe2, 4H), 2.14 (s, MeAr,
6H), 1.40 (dd, CHMe2, 12H), 1.17 (dd, CHMe2, 12H). 13C NMR
(25 °C, 100.6 MHz, C6D6): δ 162.04 (aryl), 133.33 (aryl), 132.47
(aryl), 131.81 (aryl), 129.33 (aryl), 127.66 (aryl), 125.02 (aryl),
122.02 (aryl), 121.41 (aryl), 116.15 (aryl), 24.54 (CHMe2), 20.53
(MeAr), 18.53 (CHMe2), 17.65 (CHMe2). 31P NMR (25 °C, 121.5
MHz, C6D6): 33.65 ppm. NMR spectra (1H, 31P, and 13C) are
included in the Supporting Information as proof of bulk purity.
Synthesis of Complex (PNP)Ni(SePh) (11). Complex 2 (20 mg,
0.021 mmol) and diphenyldiselenide (6.6 mg, 0.021 mmol) were
loaded into a J-Young tube along with hexane. The reaction mixture
was stirred for an hour and monitored by 31P NMR spectroscopy,
and during the course of the reaction, the brown color of the solution
was intensified. After 1 h, the reaction mixture was dried in vacuo,
Independent Synthesis of Complex 6. In a Schlenk flask,
complex 1 (300 mg, 0.60 mmol) was dissolved in 50 mL of THF.
In a separate Schlenk flask, 1 was charged with KOH in methanol
(0.58mL,0.60mmol)anddeareatedbyastandardfreeze-pump-thaw
technique. To the cold solution (-78 °C) of nickel, the KOH in
methanol was transferred via cannula, and the reaction mixture was
allowed to warm up to room temperature. After 3 h of stirring, the
reaction mixture was dried in vacuo, extracted with pentane, and
filtered. From the filtrate, brown-colored crystals of (PNP)Ni(OCH3)
(6) were isolated after cooling the solution to -35 °C for 48 h
1
(234 mg, 0.45 mmol, 79% yield). H NMR (25 °C, 399.8 MHz,
C6D6): δ 7.43 (d, 2H, C6H3), 6.92 (s, 2H, C6H3), 6.71(d, 2H, C6H3),
3.30 (s, 3H, OCH3), 2.28-2.12 (br, 4H, CHMe2), 2.15 (s, 6H,
MeAr), 1.54 (dd, 12H, CHMe2), 1.32 (dd, 12H, CHMe2). 13C NMR
(25 °C, 100.6 MHz, C6D6): δ 162.26 (aryl), 132.12 (aryl), 131.95
(aryl), 124.52 (aryl), 120.03 (aryl), 116.61 (aryl), 57.37 (OCH3),
23.63 (CHMe2), 20.50 (MeAr), 18.52 (CHMe2), 17.78 (CHMe2).
31P NMR (25 °C, 121.5 MHz, C6D6): δ 24.61. MS-CI [M + H]+:
calcd, 517.217; found, 517.215. Mp: 128(2) °C.
1
and the H NMR spectrum showed quantitative formation of the
product, 11. 1H NMR (25 °C, 399.8 MHz, C6D6): δ 8.07 (d, SC6H5,
1H), 7.54 (m, overlapped resonances of aryl protons, 2H), 6.95
(m, overlapped resonances of aryl protons, 6H), 6.79 (d, C6H3, 2H),
2.21-2.06 (br, CHMe2, 4H), 2.15 (s, MeAr, 6H), 1.41 (dd, CHMe2,
12H), 1.18 (dd, CHMe2, 12H). 13C NMR (25 °C, 100.6 MHz, C6D6):
δ 161.70 (aryl), 136.28 (aryl), 132.39 (aryl), 131.90 (aryl), 131.73
(aryl), 129.40 (aryl), 124.94 (aryl), 124.00 (aryl), 121.86 (aryl),
116.13 (aryl), 24.83 (CHMe2), 20.54 (MeAr), 18.94 (CHMe2), 17.84
(CHMe2). 31P NMR (25 °C, 121.5 MHz, C6D6): 36.44 ppm. NMR
spectra (1H, 31P, and 13C) are included in the Supporting Information
as proof of bulk purity.
Independent Synthesis of Complex 7. Complex 1 (100 mg,
0.19 mmol) was dissolved in 10 mL of diethyl ether and cooled to
-35 °C. In a separate vial, LiPHPh (22.2 mg, 0.19 mmol) was
dissolved and the solution cooled to -35 °C. A LiPHPh solution
was added dropwise to the nickel solution, and the color changed
to reddish brown after 5 min. After allowing the mixture to stir
for 2 h, the volatiles were removed under reduced pressure, the
solid extracted with pentane, and the solution filtered. The filtrate
was then concentrated and cooled to -35 °C over 4 days to
afford red crystals of (PNP)Ni(PHPh) (7) (81 mg, 0.14 mmol,
Crystallographic Details. Single crystals of 2·Et2O (Et2O),
4·Et2O (Et2O), 5 (pentane), and 9 (Et2O) were grown at -35 °C
from concentrated solutions. Inert-atmosphere techniques were used
to place the crystal onto the tip of a diameter glass capillary (0.05
mm) mounted on a SMART6000 (Bruker) at 113-137(2) K. A
preliminary set of cell constants was calculated from reflections
obtained from three nearly orthogonal sets of 10-30 frames. The
data collection was carried out using graphite-monochromated Mo
KR radiation with a frame time of 10 s and a detector distance of
5.0 cm. A randomly oriented region of a sphere in reciprocal space
was surveyed. In general, three sections of 606 frames were
collected with 0.30° steps in ω at different φ settings with the
detector set at -43° in 2θ. Final cell constants were calculated
from the xyz centroids of strong reflections from the actual data
collection after integration (SAINT).39 The structure was solved
using SHELXS-97 and refined with SHELXL-97.40
1
71% yield). H NMR (25 °C, 399.8 MHz, C6D6): δ 8.01 (t, 1H,
PHAr), 7.68 (d, C6H3, 2H), 7.09-6.98 (br, 4H, C6H3), 6.94 (d,
2H, PHAr), 6.84 (d, 2H, PHAr), 3.64 (doublet of triplets, 1H,
NiPHPh), 2.23-2.11 (br, 4H, CHMe2), 2.18 (s, 6H, MeAr), 1.26
(dd, 12H, CHMe2), 1.11 (dd, 12H, CHMe2). 13C NMR (25 °C,
100.6 MHz, C6D6): δ161.24 (aryl), 136.67 (aryl), 132.44 (aryl),
132.04 (aryl), 129.33 (aryl), 127.50 (aryl), 126.00 (aryl), 124.33
(aryl), 121.02 (aryl), 115.53 (aryl), 24.70 (CHMe2), 20.64
(MeAr), 19.02 (CHMe2), 17.83 (CHMe2). 31P NMR (25 °C, 121.5
MHz, C6D6): δ -59.33 (ArPiPr2, 3JP-H ) 20 Hz), 38.36 (PHAr,
1JP-H ) 179 Hz). Anal. calcd for C32H46NP3Ni: C, 64.45; H,
7.77; N, 2.35. Found: C, 64.59; H, 7.92; N, 2.39. Mp: 72(2) °C.
Independent Synthesis of Complex 8. In a vial, complex 2 (100
mg, 0.10 mmol) was taken in hexane and the solution cooled to
-35 °C. An analogously cooled solution of dicatechol diborane,
(catechol)2B2 (16.9 mg, 0.10 mmol), was added dropwise to the
Ni(I) solution. During the course of the reaction, the solution color
slowly changed from brown to yellow. After 3 h of stirring, the
solution was dried in vacuo, the solid extracted with ether and
filtered, and the filtrate concentrated. The ether solution was kept
at -35 °C for 24 h, from which yellow-colored crystals of 8 were
Crystallographic Details for 2 ·Et2O. The crystals occurred as
very small dark needles that appeared to have a red or green tinge.
Several crystals were examined and were found to be twinned, and
(39) SAINT, 6.1 ed.; Bruker Analytical X-ray Systems: Madison, WI,
1991-2003.
(40) SHELXL-Plus, 5.10 ed.; Bruker Analytical X-ray Systems: Madison,
WI, 1999.
Inorganic Chemistry, Vol. 47, No. 22, 2008 10489

Adhikari et al.
the structure was initially solved using a twinned crystal. Further
examination revealed a single crystal of approximate dimensions
0.20 × 0.05 × 0.04 mm, which was used as the sample. Intensity
statistics and systematic absences suggested the centrosymmetric
space group P21/c, and subsequent solution and refinement con-
firmed this choice. A direct-methods solution was calculated which
provided most non-hydrogen atoms from the E-map. Full-matrix
least-squares/difference Fourier cycles were performed, which
located the remaining non-hydrogen atoms. A slight disorder occurs
in one isopropyl group, and a diethyl ether solvent is present. All
non-hydrogen atoms were refined with anisotropic displacement
parameters. With the exception of those associated with the
disordered atoms, all hydrogen atoms were located in subsequent
Fourier maps and included as isotropic contributors in the final
cycles of refinement. All hydrogen atoms associated with the
disorder were placed in ideal positions and refined as riding atoms
with relative isotropic displacement parameters.
Crystallographic Details for 4 ·Et2O. The sample consisted of
extremely small red needles of maximum diameter of only a few
microns. Inert atmosphere techniques were used to place a typical
crystal of dimensions 0.20 × 0.03 × 0.03 mm onto the tip of a
0.02-mm-diameter glass fiber. Intensity statistics and systematic
absences suggested the centrosymmetric space group P21/c, and
subsequent solution and refinement confirmed this choice. A direct-
methods solution was calculated, which provided most non-
hydrogen atoms from the E-map. Full-matrix least-squares/
difference Fourier cycles were performed, which located the
remaining non-hydrogen atoms. Non-hydrogen atoms were refined
with anisotropic displacement parameters except for two carbon
atoms that converged to nonpositive values. The latter were refined
isotropically. All hydrogen atoms were placed in ideal positions
and refined as riding atoms with relative isotropic displacement
parameters.
Crystallographic Details for 5. Crystals were typically elongated
parallelogram-shaped prisms of varying sizes. Inert atmosphere
techniques were used to place a typical crystal of approximate
dimensions 0.30 × 0.13 × 0.05 mm. Intensity statistics and systematic
absences suggested the centrosymmetric space group P21/n, and
subsequent solution and refinement confirmed this choice. A direct-
methods solution was calculated, which provided most non-hydrogen
atoms from the E-map. Full-matrix least-squares/difference Fourier
cycles were performed, which located the remaining non-hydrogen
atoms. All non-hydrogen atoms were refined with anisotropic displace-
ment parameters. After the conclusion of the initial refinement, it was
recognized that there was a “ghost” image of the molecule translated
by 0.5 along the a axis. Using the SAME command in SHELX, it
was possible to refine to a suitable model with a relative occupancy
of 0.958:0.042. Examination of the packing reveals that the molecules
are stacked in channels parallel to the a axis. Two additional crystals
were examined, and the same translational disorder was located,
differing only in the occupancy. All hydrogen atoms were placed in
ideal positions and refined as riding atoms with relative isotropic
displacement parameters.
Crystallographic Details for 9. Inert atmosphere techniques
were used to place a green crystal of approximate dimensions 0.15
× 0.10 × 0.03 mm. Several other crystals were examined, and all
were badly split or twinned. During data collection, two major
components were identified using George Sheldrick’s CELL_NOW
program. The Bruker-AXS SAINT program was used to generate
an HKL5 file that was used in the refinement. The intensity data
were corrected for absorption (TWINABS).41 Intensity statistics
and systematic absences suggested the centrosymmetric space group
j
P1, and subsequent solution and refinement confirmed this choice.
A direct-methods solution was calculated, which provided most non-
hydrogen atoms from the E-map. Full-matrix least-squares/differ-
ence Fourier cycles were performed, which located the remaining
non-hydrogen atoms. It was necessary to use isotropic thermal
parameters for several of the atoms, and examination of the ORTEP
drawings shows that the model is not 100% agreeable. All hydrogen
atoms were placed in ideal positions and refined as riding atoms
with relative isotropic displacement parameters. It was not possible
to locate the two hydrogens on the N atom.
Acknowledgment. We thank Indiana UniversitysBloom-
ington, the Dreyfus Foundation, the Sloan Foundation, the
NSF (CHE-0348941, PECASE award to D.J.M.), and the
Chemical Sciences, Geosciences and Biosciences Division,
Office of Basic Energy Science, Office of Science, U.S.
Department of Energy (DE-FG02-07ER15893) for financial
support of this research. Mr. Patrick Feng is also acknowl-
edged for collecting preliminary magnetization data for 2.
Professor Patrick L. Holland is acknowledged for his
assistance in molecular weight determination by the freezing
point depression method. K.M. acknowledges the DFG
(SFB583) for financial support. Dr. Maren Pink is also
thanked for insightful comments to the crystallographic
studies presented in this work.
Supporting Information Available: Complete crystallographic
data (2, 4, 5, and 9), figures, and additional discussion. This material
is available free of charge via the Internet at http://pubs.acs.org.
IC801137P
(41) An empirical correction for absorption anisotropy was used: Blessing,
R. Acta Crystallogr. 1995, A51, 33.
10490 Inorganic Chemistry, Vol. 47, No. 22, 2008