Nickel(II) complexes containing a pyrrole-diphosphine pincer ligand

Article abstract of DOI:10.1021/ic301633q

A new pincer ligand, (P₂^PhPyr)^-, based on the anion of 2,5-bis[(diphenylphosphino)methyl]pyrrole has been prepared in four steps from pyrrole. The ligand undergoes oxidation to diphosphine oxide under ambient conditions and was therefore isolated as its borane adduct, H(P ₂^PhPyr)·2BH₃ (2). Delivery of the ligand to nickel(II) was accomplished by the direct reaction of NiCl₂ with 2 in the presence of Et₂NH to afford [NiCl(P₂ ^PhPyr)]. Salt metathesis reactions of the chloro complex afford new compounds including [Ni(CH₃)(P₂^PhPyr)] and [Ni(NCCH₃)(P₂^PhPyr)](OTf). In all cases, the ligand gives rise to diamagnetic square-planar complexes, which have been fully characterized in solution and the solid state. All complexes examined display an irreversible oxidation to nickel(III) according to cyclic voltammetry. Reduction of the chloro complex in dichloromethane results in an electrocatalytic process, whereas reduction in tetrahydrofuran leads to the irreversible formation of a nickel(I) species.

Full text of DOI:10.1021/ic301633q

Article
pubs.acs.org/IC
Nickel(II) Complexes Containing a Pyrrole−Diphosphine Pincer
Ligand
Gopaladasu T. Venkanna, Teresa Virginia M. Ramos, Hadi D. Arman, and Zachary J. Tonzetich*
Department of Chemistry, University of Texas at San Antonio (UTSA), San Antonio, Texas 78249, United States
S
* Supporting Information
ABSTRACT: A new pincer ligand, (P2PhPyr)−, based on the
anion of 2,5-bis[(diphenylphosphino)methyl]pyrrole has been
prepared in four steps from pyrrole. The ligand undergoes
oxidation to diphosphine oxide under ambient conditions and
was therefore isolated as its borane adduct, H(P2PhPyr)·2BH3
(2). Delivery of the ligand to nickel(II) was accomplished by
the direct reaction of NiCl2 with 2 in the presence of Et2NH to afford [NiCl(P2PhPyr)]. Salt metathesis reactions of the chloro
complex afford new compounds including [Ni(CH3)(P2PhPyr)] and [Ni(NCCH3)(P2PhPyr)](OTf). In all cases, the ligand gives
rise to diamagnetic square-planar complexes, which have been fully characterized in solution and the solid state. All complexes
examined display an irreversible oxidation to nickel(III) according to cyclic voltammetry. Reduction of the chloro complex in
dichloromethane results in an electrocatalytic process, whereas reduction in tetrahydrofuran leads to the irreversible formation of
a nickel(I) species.
linked by a methylene group, reasoning that the resulting
chelate angle would be ideal for transition-metal ions. We
report here our findings concerning a new class of PNP ligand,
(P2PhPyr)− (P2PhPyr = anion of 2,5-bis[(diphenylphosphino)-
methyl]pyrrole; Scheme 1), and its coordination chemistry with
nickel(II).
INTRODUCTION
■
Pincer ligands are used to great effect in modern coordination
chemistry to realize new forms of reactivity and to design better
catalysts.1−4 In particular, pincer ligands featuring a PNP
ligating motif have been demonstrated to support a variety of
interesting transition-metal complexes displaying unique geo-
metries and small-molecule reactivity.5−21 In many instances,
the enhanced reactivity of these metal complexes results from
the rigid meridional geometry imposed by the pincer ligand.22
Despite the large diversity of pincer ligands reported to date,
those based on a central pyrrole moiety have received less
attention than those containing a pyridine, aryl, or amine
unit.23−38 Pyrrole is a versatile ligand that finds widespread use
in coordination chemistry with both high- and low-oxidation-
state metals.39,40 The attenuated π basicity of the pyrrolic lone
pair is significant in this respect, allowing for binding to metal
ions with filled or partially filled dπ orbitals. In addition, the
potential for pyrrole-centered redox events41,42 makes pincer
ligands containing these units especially intriguing given the
beneficial ligand-centered redox and tautomerization demon-
strated by pincer ligands reported by the groups of Chirik and
Milstein.43,44
Nonmacrocyclic chelates based on pyrrole have been shown
to be effective ligands for a variety of transition-metal ions.
Bidentate ligands featuring phosphinopyrroles find precedent in
nickel chemistry, suggesting that an analogous pincer-type
ligand would be successful in supporting various metal
complexes.45 Furthermore, related diphosphinocarbazole li-
gands have found use in lanthanide chemistry (scandium,
yttrium, and erbium), although the larger bite angle of these
systems is likely incompatible with coordination to smaller 3d
metals.46 We therefore sought to prepare a new pincer ligand
containing ligating phosphine atoms and a central pyrrole
RESULTS AND DISCUSSION
■
The preparation of H(P2PhPyr) (3) proceeded by the addition
of 2 equiv of diphenylphosphine to the previously reported
pyrrole-2,5-dicarboxaldehyde47 (Scheme 1) following a proce-
dure adapted from the synthesis of phosphinopyrrole reported
by Bochmann et al.45 The resulting phosphine oxide (1) could
be prepared on gram scales in satisfactory yield without
chromatographic purification. The 31P NMR spectrum of 1
displays a single resonance at 27.5 ppm in chloroform-d with a
JPH = 12 Hz coupling to the protons of the methylene linker.
The reduction of 1 in tetrahydrofuran (THF) with LiAlH4/
CeCl3 produced the desired diphosphine without requiring
protection of the pyrrolic nitrogen atom. Reactions carried out
in the absence of CeCl3 resulted in incomplete reduction, likely
because of inactivation of LiAlH4 by coordination of 1.48,49
The colorless diphosphinopyrrole, 3, is not air-stable as
1
judged by the reappearance of resonances for 1 in the H and
31P NMR spectra of solutions exposed to ambient conditions.
Furthermore, attempted chromatographic purification of 3 on
silica resulted in the isolation of diphosphine oxide. As a result,
purification and complete characterization of this species was
not pursued. As an alternative, the borane adduct, H-
(P2PhPyr)·2BH3 (2), was prepared by the addition of NaBH4
Received: July 27, 2012
Published: November 16, 2012
© 2012 American Chemical Society
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Scheme 1. Synthesis of 3
to the reduction conditions (Scheme 1). Compound 2 proved
to be indefinitely stable and could be purified by alumina
chromatography. The spectroscopic features of 2 are in line
with the structure displayed in Scheme 1 and consistent with
other phosphinoboranes50,51 including those that have been
subjected to structural characterization.52−54 The 31P NMR
spectrum of 2 displays a broad resonance at 16.1 ppm in
benzene-d6 consistent with reduction of the two phosphorus
atoms and coordination of the BH3 group. The free
diphosphine 3 could be easily accessed from 2 by refluxing
the compound in methanol under nitrogen. The spectroscopic
features of 2 are in line with that expected for a diphosphine,
displaying a 31P chemical shift of −16.2 ppm for the
phosphorus atoms.
displaying a virtual triplet for the methylene protons of the
P2PhPyr ligand, as has been observed in other NiIIPNP
systems.55 The 31P NMR resonance for complex 4 appears at
30.3 ppm, substantially downfield of that observed for 3.
Crystallization of 4 from benzene/pentane afforded material
suitable for X-ray diffraction. The solid-state structure of the
complex is depicted in Figure 1 (crystallographic and metric
In order to deliver (P2PhPyr)− to nickel(II), we attempted the
reaction of 2 with anhydrous NiCl2 in the presence of several
equivalents of HNEt2. It was our hope that the diethylamine
would serve to both deprotect the phosphinoborane groups
and remove the pyrrolic NH proton. Gratifyingly, heating a
mixture of anhydrous NiCl2 in the presence of 1 equiv of 2 and
an excess of HNEt2 in THF afforded the desired nickel(II)
complex, [NiCl(P2PhPyr)] (4), as an orange solid in 62% yield
(eq 1). The reaction of compound 2 with excess Et2NH in the
Figure 1. Thermal ellipsoid drawing (50%) of 4. Hydrogen atoms are
omitted for clarity. See Table 2 for selected bond lengths and angles.
parameters can be found in Tables 1 and 2, respectively). The
geometry about nickel(II) is planar [sum of the angels about Ni
= 360.3(1)°] with a slight contraction of the P−Ni−N bond
angles [avg = 83.4(1)°] away from 90°. The pyrrole group is
canted at an angle of 13.3° with respect to the coordination
plane but remains planar. The Ni−Cl distance of 2.1768(9) Å is
in the range encountered for similar square-planar nickel(II)
complexes, as is the average Ni−P distance of 2.20 Å.45 By
contrast, the Ni−N distance of 1.856(2) Å is somewhat shorter
than other reported nickel pyrrolide compounds.45,56 In
summary, the bond metrics indicate that (P2PhPyr)− is capable
of supporting square-planar complexes of nickel(II) with
minimal geometric distortion.
absence of NiCl2 produced free diphosphine 3, as judged by 1H
NMR spectroscopy, confirming initial deprotection prior to
metalation. As an alternative delivery route, the reaction of
NiCl2 with Li(P2PhPyr), generated in situ from the reduction of
1 with LiAlH4/CeCl3, was examined. This route led to the same
nickel(II) chloride complex, albeit in lower yields. Additionally,
complex 4 could also be prepared by the direct reaction of 3
with NiCl2 in the presence of Et3N.
Complex 4 is an orange, air-stable crystalline solid that is
The chloride ligand in 4 can be replaced in straightforward
fashion via salt metathesis reactions (Scheme 2). Alkylation of
complex 4 with MeMgCl in THF produced the methyl
1
soluble in THF, CH2Cl2, and aromatic solvents. H NMR
spectra in benzene-d6 indicate a diamagnetic compound,
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a
Table 1. Crystallographic Data and Refinement Parameters
4
5
6
empirical formula
fw (g/mol)
C30H26ClNNiP2
556.62
C31H29NNiP2
536.20
C33H29F3N2NiO3P2S
694.25
temperature (K)
cryst syst, space group
unit cell dimens (Å, deg)
a (Å)
98(2)
98(2)
98(2)
monoclinic, P21/c
monoclinic, P21/c
monoclinic, P21/n
14.391(6)
16.192(6)
11.431(5)
107.556(7)
2539.6(2)
4
14.261(2)
16.269(2)
11.528(2)
106.917(8)
2558.8(6)
4
9.3166(9)
23.499(2)
15.009(1)
103.738(7)
3191.8(5)
4
b (Å)
c (Å)
β (deg)
volume (Å3)
Z
calcd density (g/cm3)
1.456
1.392
1.445
abs coeff (mm−1
)
1.02
0.904
0.826
F(000)
1152
1120
1430
cryst size (mm3)
θ range (deg)
limiting indices
0.35 × 0.24 × 0.03
2.25−26.00
0.45 × 0.16 × 0.15
3.11−25.50
0.20 × 0.19 × 0.09
3.04−25.50
−17 ≤ h ≤ 15, −19 ≤ k ≤ 19, −13 ≤ l ≤ −17 ≤ h ≤ 17, −19 ≤ k ≤ 16, −10 ≤ l ≤ −10 ≤ h ≤ 11, −28 ≤ k ≤ 28, −18 ≤ l ≤
14
13
18
reflns collected/unique
completeness to θ (%)
abs corrn
15750/4978 (Rint = 0.039)
99.7
13313/4752 (Rint = 0.027)
99.8
17146/5873 (Rint = 0.054)
98.7
multiscan ABSCOR
0.813 and 1.000
4978/0/316
multiscan ABSCOR
0.811 and 1.000
4752/0/316
multiscan ABSCOR
0.229 and 1.000
5873/0/454
min and max transmn
data/restraints/param
GOF on F2
1.002
1.011
1.036
final R indices [I > 2σ(I)]
R indices (all data)
R1 = 0.0397, wR2 = 0.0993
R1 = 0.0455, wR2 = 0.1053
0.634 and −0.517
R1 = 0.0294, wR2 = 0.0809
R1 = 0.0321, wR2 = 0.0830
0.543 and −0.641
R1 = 0.0613, wR2 = 0.1320
R1 = 0.0789, wR2 = 0.1415
0.916 and −0.608
largest diff peak and hole
(e/Å3)
a
2
The refinement method was full-matrix least squares on F2; wavelength = 0.71073 Å. R1 = ∑||Fo| − |Fc||/∑|Fo|; wR2 = {∑[w(Fo − Fc2)2]/
2
∑[w(Fo )2]}1/2
.
Table 2. Selected Crystallographic Bond Distances (Å) and
Scheme 2. Nickel(II) Complexes Prepared from 4
Angles (deg) for Complexes 4−6
4
5
6
Ni(1)−N(1)
Ni(1)−P(1)
Ni(1)−P(2)
Ni(1)−X
1.856(2)
2.198(1)
2.208(1)
2.1768(9)
1.898(1)
2.1910(6)
2.1824(6)
1.999(2)
1.820(3)
2.211(1)
2.203(1)
1.838(4)
P(1)−Ni(1)−P(2)
N(1)−Ni(1)−X
Ni(1)−N(2)−C(31)
166.19(3)
175.83(7)
n/a
165.59(2)
177.01(7)
n/a
163.96(5)
172.0(2)
167.1(4)
complex [Ni(CH3)(P2PhPyr)] (5). This species is yellow and
displays a 1H NMR resonance for the methyl protons at −0.05
ppm in benzene-d6. The solid-state structure of complex 5 is
nearly isomorphous to that of 4 (Figure 2 and Tables 1 and 2),
displaying a Ni−C bond length of 1.999(2) Å comparable to
that of other nickel(II) alkyl complexes.57 The Ni−PNP bond
metrics are similar to those of 4 except for a slightly elongated
Ni−N distance of 1.898(1) Å.
Removal of the chloride ligand in 4 with AgOTf, TlOTf, or
AgBF4 in the presence of CH3CN afforded the cationic solvent
adducts [Ni(CH3CN)(P2PhPyr)](X) [X = OTf (6) or BF4
(6′)]. These red-orange species are insoluble in benzene but
dissolve readily in more polar solvents such as THF, CH2Cl2,
and CH3CN. IR spectroscopy of the triflate salt (6) revealed a
CN stretch for the coordinated acetonitrile ligand at 2293 cm−1
in KBr. The solid-state structure of complex 6 is depicted in
Figure 3 (see also Tables 1 and 2). As expected, the cationic
nature of [Ni(CH3CN)(P2PhPyr)]+ results in a slightly
contracted Ni−Npyrrole distance of 1.820(3) Å [cf. 1.856(2)
and 1.898(1) Å for the chloro and methyl complexes,
respectively]. The complex also displays a short Ni−Nnitrile
distance of 1.838(4) Å comparable to other reported CH3CN
adducts.58 The remaining metric parameters of the PNP ligand
are unremarkable with respect to those of the chloro and
methyl complexes.
To explore the possible redox chemistry of the nickel(II)
complexes, we next examined their electrochemical behavior.
The cyclic voltammogram of complex 4 in CH2Cl2 displays an
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Figure 2. Thermal ellipsoid drawing (50%) of 5. Hydrogen atoms are
omitted for clarity. See Table 2 for selected bond lengths and angles.
Figure 4. Cyclic voltammogram of 4 in THF (1.8 mM) at a glassy
carbon electrode demonstrating the catalytic current wave observed
upon an increase in the concentration of CH2Cl2. The scan rate is 50
mV/s, and the supporting electrolyte is 0.15 M Bu4NPF6.
Figure 3. Thermal ellipsoid drawing (30%) of the cation of 6.
Hydrogen atoms and minor components of the disorder are omitted
for clarity. See Table 2 for selected bond lengths and angles.
irreversible anode process at +0.45 V (all potentials reported vs
ferrocene/ferrocenium), which we assign as oxidation to
nickel(III) based on observations with other NiIIPXP systems
(X = N or C).59−61 Upon scanning to negative potentials,
complex 4 exhibits a catalytic current wave near −2.1 V,
suggesting that reduction of the complex to nickel(I) results in
reactivity with the methylene chloride solvent. To examine this
process in more detail, increasing amounts of CH2Cl2 were
added to a THF solution of 4 and cyclic voltammetry (CV) was
repeated (Figure 4). Consistent with a catalytic process, the
current response was found to increase with larger concen-
trations of CH2Cl2.
Conducting CV in pure THF results in a similar irreversible
anode process at +0.38 V, but no catalytic event is observed
within the solvent window. Instead, an irreversible cathodic
event takes place at −2.06 V, which gives rise to a new
irreversible anodic event at −0.37 V (Figure 5). Furthermore, a
quasi-reversible cathodic process centered at −2.53 V is also
observed in THF upon scanning to lower potentials (Figure 5,
inset).
Figure 5. Cyclic voltammogram of 4 in THF (2.0 mM) at a glassy
carbon electrode displaying the irreversible reduction at −2.06 V and
its corresponding return wave at −0.37 V. The inset displays the quasi-
reversible couple at −2.53 V. The scan rate is 100 mV/s, and the
supporting electrolyte is 0.15 M Bu4NPF6.
(Scheme 3). In THF, a nickel(I) species is formed irreversibly,
which demonstrates a further quasi-reversible process at −2.53
V. The new nickel(I) species is observed to undergo oxidation
at a potential of −0.37 V, perhaps resulting in regeneration of 4.
The electrochemical properties of 6 show notable differences
with those of complex 4. Similar to 4, complex 6 displays an
irreversible oxidation to nickel(III) in CH2Cl2 albeit at a slightly
higher potential of +0.52 V, consistent with more difficult
oxidation in the cationic complex. Interestingly, however,
complex 6 shows no electrocatalytic activity when scanned to
lower potentials in CH2Cl2. Instead, an irreversible cathodic
reduction at −1.40 V takes place for 6 similar to that observed
for 4 in THF (see Supporting Information). Thus, formation of
the nickel(I) complex from 6 produces a species that is stable
to CH2Cl2 on the electrochemical timescale. Scanning to more
negative potentials (in CH3CN) reveals a quasi-reversible
process centered at −2.40 V also similar to that observed for 4
in THF. It therefore appears that analogous reduced nickel(I)
species may result from the reduction of complex 4 or 6.
Taken together, these observations are consistent with a
process involving one-electron reduction of 4 followed by rapid
loss of the Cl− ligand. In CH2Cl2, the resulting [NiI(P2PhPyr)]
fragment appears to be capable of chlorine atom abstraction
from solvent, which regenerates the nickel(II) chloride
complex, leading to an electrocatalytic dechlorination sequence
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wire auxiliary electrode, and a Ag/AgCl quasi-reference electrode.
Triply recrystallized Bu4NPF6 was used as the supporting electrolyte.
All electrochemical data were referenced internally to the ferrocene/
ferrocenium couple at 0.00 V. Elemental analyses were performed by
Midwest Microlab, LLC, in Indianapolis, IN.
Scheme 3. Possible Catalytic Sequence for the
Electrochemical Reactivity of 4 in CH2Cl2
Materials. Pyrrole was purchased from commercial suppliers
(Aldrich or Acros) and distilled prior to use. Anhydrous nickel(II)
chloride, diphenylphosphine, methylmagnesium chloride solution
(THF), lithium aluminum hydride, sodium borohydride, cerium(III)
chloride, silver tetrafluoroborate, and silver triflate were purchased
from Strem Chemicals Inc. and used as received.
Pyrrole-2,5-dicarbaldehyde. The dialdehyde was prepared by the
two-step method of Knizhnikov et al.47 In our hands, Soxhlet
extraction of the crude product with hexanes was found to give
material of insufficient purity, so an alternate purification method was
developed. The crude material was purified by flash chromatography
on alumina using hexanes/ethyl acetate (75:25) as the eluent. The
resulting pure dialdehyde was found to be a white solid. Spectroscopic
features matched those reported previously. 1H NMR (500 MHz,
CDCl3): δ 10.67 (br s, NH), 9.79 (s, 2CHO), 7.04−6.99 (m, 2pyr-
CH). 13C{1H} NMR (125 MHz, CDCl3): δ 181.83, 136.04, 119.82.
[(Pyrrole-2,5-diyl)dimethylene]bis(diphenylphosphine oxide) (1).
A round-bottomed flask was charged with 2.87 mL (16.5 mmol) of
diphenylphosphine and 1.0 mL of concentrated HCl under an
atmosphere of dinitrogen. In a separate flask, 1.0 g (8.1 mmol) of
pyrrole-2,5-dicarbaldehyde was dissolved in 8 mL of methanol under
an atmosphere of dinitrogen. The methanolic solution of dialdehyde
was added dropwise to the mixture containing diphenylphosphine.
After the addition was complete, the reaction mixture was heated at
reflux overnight. After cooling, methanol was removed by rotary
evaporation, and the remaining residue was extracted into dichloro-
methane (3 × 75 mL). The combined extracts were dried over Na2SO4
and evaporated to dryness. The remaining residue was washed
thoroughly with diethyl ether (4 × 15 mL) to afford 2.70 g (67%) of
However, reactivity with CH2Cl2 is only observed in the case of
4, suggesting that the initial product of reduction is not the
same in both cases.
CONCLUSIONS
■
In conclusion, we have prepared a new PNP pincer ligand
featuring a central pyrrolide unit. The synthesis of the ligand is
accomplished from a general precursor without the need of
protecting group chemistry at pyrrole and is therefore well
suited to the preparation of other alkylphosphine derivatives (t-
Bu, cyclohexyl, etc.). The borane-protected species, 2, is
especially promising in this regard because alkylphosphine
derivatives are expected to demonstrate even greater air
sensitivity than the phenyl derivative reported here. The pincer
ligand is delivered to nickel(II) via its borane adduct, giving rise
to diamagnetic square-planar complexes. The electrochemistry
of these complexes reveals a rich reduction chemistry. Further
work will examine the small-molecule reactivity of these
reduced nickel complexes and the coordination chemistry of
the ligand with other metals.
1
diphosphine oxide as a white solid. H NMR (500 MHz, CDCl3): δ
9.55 (br s, NH), 7.63 (m, 8o-H), 7.50 (t, 4p-H), 7.41 (t, 8m-H), 5.68
(s, 2pyr-CH), 3.59 (d, 4CH2, JHP = 12 Hz). 13C{1H} NMR (125 MHz,
CDCl3): δ 132.30 (d, JCP = 98.9 Hz), 132.01, 131.22 (app m), 128.71
(app m), 121.25 (m), 108.97 (m), 30.57 (d, CH2, JCP = 69.6 Hz).
31P{1H} NMR (202 MHz): δ 27.5. ESI-MS (negative mode). Calcd
for [M − H]−: m/z 494.1. Found: m/z 494.1.
2,5-Bis[(diphenylphosphino)methyl]pyrrole with Borane Adduct,
H(P2PhPyr)·2BH3 (2). A round-bottomed flask was charged with 1.45 g
(5.88 mmol) of anhydrous CeCl3, 0.227 g (6.00 mmol) of NaBH4, and
15 mL of THF. The mixture was was cooled to −30 °C, at which point
0.495 g (1.00 mmol) of diphosphine oxide was added followed by
0.152 g (4.00 mmol) of LiAlH4 in portions. The reaction mixture was
allowed to warm to room temperature and stir for 15 h under an
atmosphere of dinitrogen. The mixture was diluted with 25 mL of
benzene and poured slowly onto 150 mL of ice−water containing 4
mL of concentrated HCl. The aqueous layer was extracted twice with
25 mL of benzene. The combined extracts were dried over Na2SO4
and evaporated to dryness. The resulting residue was purified by
chromatography on alumina, eluting with 90:10 hexane/ethyl acetate
EXPERIMENTAL SECTION
■
General Comments. Manipulations of air- and moisture-sensitive
materials were performed under an atmosphere of nitrogen gas using
standard Schlenk techniques or in a Vacuum Atmospheres glovebox.
Tetrahydrofuran, diethyl ether, pentane, dichloromethane, toluene,
and acetonitrile were purified by sparging with argon and passage
through two columns packed with 4 Å molecular sieves or activated
alumina (CH3CN). Benzene and benzene-d6 were dried over sodium
ketyl and vacuum-distilled prior to use. Dichloromethane-d2 and
acetonitrile-d3 were sparged with nitrogen and stored over 4 Å
molecular sieves prior to use. Chloroform-d was used as received.
NMR spectra were recorded on Varian spectrometers operating at 300
1
to afford 0.295 g (60%) of diphosphinoborane as a colorless oil. H
NMR (500 MHz, C6D6): δ 8.48 (br s, NH), 7.52 (t, 8Ar-H), 7.00 (m,
12Ar-H), 5.68 (s, 2H, pyr-CH), 3.16 (d, 4CH2, JHP = 10 Hz), 2.1−1.3
(v br m, 6BH3). 13C{1H} NMR (75 MHz, C6D6): δ 133.12 (d, JCP
=
8.9 Hz), 131.55 (d, JCP = 2.0 Hz), 129.90 (d, JCP = 52.9 Hz), 129.21
(d, JCP = 9.7 Hz), 122.47 (app m), 110.15 (d, JCP = 4.3 Hz), 27.54 (d,
JCP = 34.3 Hz). 31P{1H} NMR (202 MHz): δ 16.0 (br m). 11B NMR
(150 MHz): δ −40.9. ESI-MS (negative mode). Calcd for [M − H]−:
m/z 490.2. Found: m/z 490.1.
or 500 MHz (1H) and referenced to the residual H or 13C resonance
1
of the solvent. For 31P, 19F, and 11B NMR spectra, external standards
of neat 85% H3PO4(aq), neat CFCl3, and 10% BF3·Et2O in benzene-d6
were used, respectively (δ = 0.00 ppm). UV−vis spectra were recorded
on a Cary 60 spectrophotometer in airtight Teflon-capped quartz cells.
IR spectra were recorded as pressed KBr windows on a Nicolet iS10
FTIR spectrometer. CV was performed at 23 °C on a CH Instruments
620D electrochemical workstation. A three-electrode setup was
employed comprising a glassy carbon working electrode, a platinum
2,5-Bis[(diphenylphosphino)methyl]pyrrole, H(P2PhPyr) (3). The
free diphosphine was prepared by refluxing phosphinoborane in
methanol. The volatiles were then removed in vacuo to afford
diphosphine as a colorless oil in quantitative yield. Because of the air
sensitivity of the material, it was used immediately without further
purification. 1H NMR (300 MHz, C6D6): δ 7.45 (br s, NH), 7.35 (m,
8Ar-H), 7.05 (m, 12Ar-H), 5.85 (d, 2pyr-CH, J = 2.0 Hz), 3.11 (s, 4
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CH2). 13C{1H} (75 MHz, C6D6): δ 139.56 (d, JCP = 15.9 Hz), 133.50
(d, JCP = 18.5 Hz), 129.14, 129.02 (app m, JCP = 6.5 Hz), 126.49 (d,
JCP = 14.1 Hz), 108.52 (d, JCP = 5.3 Hz), 28.90 (d, JCP = 15.7 Hz).
31P{1H} NMR (202 MHz): δ −16.2. Attempted observation by liquid
chromatography−mass spectrometry yielded the monooxidized
product. ESI-MS (positive mode). Calcd for [M(O) + H]+: m/z
480.2. Found: m/z 479.9.
transmission factors, were accomplished with CrystalClear and
ABSCOR,64 respectively. All structures were solved by direct methods
and refined on F2 using full-matrix least-squares techniques with
SHELXL-97.65,66 Non-hydrogen atoms were refined with anisotropic
displacement parameters. All carbon-bound hydrogen-atom positions
were determined by geometry and refined by a riding model. The
structure of 6 was found to contain an unbound chloride counterion in
partial occupancy with the triflate. Refinement of the cooccupancy to
85% OTf/15% Cl resulted in a satisfactory structure. In addition, the
structure of 6 was also found to contain substantial disorder about the
phenyl rings. Diffraction experiments utilizing the BF4 salt (6′) also
resulted in disordered structures.
[NiCl(P2PhPyr)] (4). A flask was charged with 0.122 g of 2 (0.248
mmol) and 0.064 g of anhydrous NiCl2 (0.49 mmol). The solids were
suspended in 20 mL of THF, and 0.52 mL of Et2NH was added. The
mixture was stirred for 18 h at room temperature, during which time it
became dark brown. All volatiles were removed in vacuo, leaving a
dark-colored residue that was extracted into 25 mL of benzene. The
benzene extract was filtered through a pad of Celite to give an orange-
red solution. The solution was evaporated to dryness to afford 0.085 g
(62%) of the desired nickel complex. Crystals suitable for X-ray
diffraction were grown by vapor diffusion of pentane into a saturated
ASSOCIATED CONTENT
■
S
* Supporting Information
Additional spectra, cyclic voltammograms, and crystallographic
information in CIF format. This material is available free of
charge via the Internet at http://pubs.acs.org.
1
benzene solution of the complex. H NMR (300 MHz, CD2Cl2): δ
7.91 (m, 8Ar-H), 7.46 (m, 12Ar-H), 5.93 (s, 2pyr-CH), 3.58 (app t,
4CH2, JHP = 5.1 Hz). 13C{1H} NMR (75 MHz, CD2Cl2): δ 136.69
(app t, JCP = 7.4 Hz), 133.20 (app t, JCP = 5.6 Hz), 131.25, 130.84 (app
AUTHOR INFORMATION
■
t, JCP = 21.0 Hz), 129.18 (app t, JCP = 10.5 Hz), 106.51 (app t, JCP
=
5.6 Hz), 31.98 (app t, CH2, JCP = 13.5 Hz). 31P{1H} NMR (202
MHz): δ 30.3. Anal. Calcd for C30H26ClNNiP2: C, 64.73; H, 4.71; N,
2.52. Found: C, 64.44; H, 4.77; N, 2.60.
Corresponding Author
*E-mail: zachary.tonzetich@utsa.edu.
Notes
[Ni(CH3)(P2PhPyr)] (5). A flask was charged with 0.111 g (0.199
mmol) of 4 and 20 mL of THF, and the resulting orange solution was
cooled to −30 °C. To the cooled solution was added 66 μL (0.20
mmol) of CH3MgCl (3.0 M in THF). The mixture was allowed to stir
at room temperature for 2 h, during which time the mixture lightened.
All volatiles were removed in vacuo, leaving a yellow residue, which
was extracted into 10 mL of benzene and filtered through a pad of
Celite. The benzene was removed in vacuo and the resulting solid
washed with Et2O to afford 0.095 g (85%) of the desired product as a
yellow microcrystalline solid. Crystals suitable for X-ray diffraction
were grown by pentane diffusion into a saturated benzene solution of
the complex. 1H NMR (500 MHz, C6D6): δ 7.59 (m, 8Ar-H), 6.97 (m,
12Ar-H), 6.58 (s, 2pyr-CH), 3.65 (app t, 4CH2, JHP = 4.7 Hz), −0.05
(t, 3CH3, JHP = 8.0 Hz). 13C{1H} NMR (125 MHz, C6D6): δ 135.08
(app t, JCP = 7.3 Hz), 133.79 (app t, JCP = 18.9 Hz), 133.45 (app t, JCP
The authors declare no competing financial interest.
After this paper was submitted, alternative syntheses of ligand 3
and complex 4 were reported by Gade et al.62 In that work, the
chemical reduction of 4 was demonstrated to result in a dimeric
nickel(I) complex featuring a Ni−Ni bond. The formation of
such a species in THF and subsequent quasi-reversible one-
electron reduction are fully consistent with the data presented
above. However, the reactivity of the nickel(I) dimer with
CH2Cl2 was not reported.
ACKNOWLEDGMENTS
■
This work was supported by startup funding from UTSA and
by a grant from the Welch Foundation (Grant AX-1776).
= 6.0 Hz), 130.39, 129.18 (app t, JCP = 4.7 Hz), 106.14 (app t, JCP
=
5.4 Hz), 35.93 (app t, CH2, JCP = 13.0 Hz), −18.76 (t, CH3, JCP = 21.3
Hz). 31P{1H} NMR (202 MHz): δ 31.1. Anal. Calcd for C31H29NNiP2:
C, 69.44; H, 5.45; N, 2.61. Found: C, 69.89; H, 5.74; N, 2.66.
[Ni(CH3CN)(P2PhPyr)](OTf) (6). A flask was charged with 0.100 g
(0.181 mmol) of 4 and 0.046 g (0.18 mmol) of AgOTf. The solids
were dissolved in 15 mL of CH2Cl2 containing 0.5 mL of CH3CN.
The resulting red-orange mixture was stirred for 5 h at room
temperature. The mixture was filtered through a pad of Celite and
evaporated to dryness. The resulting residue was washed thoroughly
with benzene to afford 0.090 mg (70%) of a red powder. Crystals
suitable for X-ray diffraction were grown by vapor diffusion of Et2O
into a saturated CH2Cl2 solution of the complex. 1H NMR (500 MHz,
CD3CN): δ 7.79 (m, 8Ar-H), 7.65 (t, 4Ar-H), 7.57 (t, 8Ar-H), 5.91 (s,
2pyr-CH), 3.73 (app t, 4CH2, JHP = 5.5 Hz), 1.96 (s, 3 CH3CN).
31P{1H} NMR (202 MHz): δ 43.1. 19F NMR (470 MHz): −79.1. IR
(KBr, cm−1): 2293 (CN). Anal. Calcd for C33H29F3N2NiO3P2S: C,
55.72; H, 4.11; N, 3.94. Found: C, 55.60; H, 3.96; N, 3.29.
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