Synthesis, characterization, and catalytic activity of nickel(II) alkyl complexes supported by pyrrole-diphosphine ligands

Article abstract of DOI:10.1021/om400630q

The organometallic Ni(II) chemistry of the pyrrole-based pincer ligands (P₂^RPyr)^- (P₂^RPyr = 2,5-(R₂PCH₂)₂C₄H₂N, R = Ph, Cy) is reported. Reactions of Grignard reagents with [NiCl(P ₂^RPyr)] afford a variety of alkyl and aryl complexes (methyl, ethyl, benzyl, phenyl, and allyl) that all display square-planar geometries about nickel. The hydride complex [NiH(P₂ ^CyPyr)] can also prepared either through treatment of [NiCl(P ₂^CyPyr)] with LiHBEt₃ or by reaction of H(P₂^RPyr) with [Ni(COD)₂] (COD = 1,4-cyclooctadiene). Reactions of the methyl and hydride complexes with CO and CO₂, respectively, evince clean migratory insertion chemistry of the Ni-C and Ni-H bonds. Both the alkyl and chloride complexes are active catalysts for the Kumada coupling of aryl chlorides and aryl or alkyl Grignard reagents at room temperature. The solid-state structures of several of the complexes are reported.

Full text of DOI:10.1021/om400630q

Article
pubs.acs.org/Organometallics
Synthesis, Characterization, and Catalytic Activity of Nickel(II) Alkyl
Complexes Supported by Pyrrole−Diphosphine Ligands
Gopaladasu T. Venkanna, Swetha Tammineni, Hadi D. Arman, and Zachary J. Tonzetich*
Department of Chemistry, University of Texas at San Antonio, San Antonio, Texas 78249, United States
S
* Supporting Information
ABSTRACT: The organometallic Ni(II) chemistry of the
R
R
pyrrole-based pincer ligands (P2 Pyr)− (P2 Pyr = 2,5-
(R2PCH2)2C4H2N, R = Ph, Cy) is reported. Reactions of
R
Grignard reagents with [NiCl(P2 Pyr)] afford a variety of alkyl
and aryl complexes (methyl, ethyl, benzyl, phenyl, and allyl)
that all display square-planar geometries about nickel. The
hydride complex [NiH(P2CyPyr)] can also prepared either
through treatment of [NiCl(P2CyPyr)] with LiHBEt3 or by
R
reaction of H(P2 Pyr) with [Ni(COD)2] (COD = 1,4-cyclooctadiene). Reactions of the methyl and hydride complexes with CO
and CO2, respectively, evince clean migratory insertion chemistry of the Ni−C and Ni−H bonds. Both the alkyl and chloride
complexes are active catalysts for the Kumada coupling of aryl chlorides and aryl or alkyl Grignard reagents at room temperature.
The solid-state structures of several of the complexes are reported.
In this contribution, we extend the chemistry of the pyrrole-
based PNP ligand P2PhPyr− to a series of organometallic
complexes of Ni(II). In addition, we report the synthesis and
coordination chemistry of the new dicyclohexylphosphine-
derived ligand P2CyPyr−. This aliphatic pyrrole−diphosphine
ligand is prepared in a straightforward fashion and allows access
to a variety of complexes not obtainable with the diphenylphos-
phine analogue P2PhPyr−. Nickel(II) complexes of both PNP
ligands appear to be active catalysts for a variety of Kumada-type
C−C cross-coupling reactions.
INTRODUCTION
■
The efficacy of pincer-type ligands in transition-metal catalysis is
well documented and continues to garner attention as new forms
of reactivity are discovered for complexes containing this class of
supporting ligands.1−8 The variety of pincers reported to date
attests to the usefulness of these ligands and the high degree to
which they can be modified to suit catalytic applications.9−15 In
the area of nickel chemistry, complexes of pincer ligands have
been demonstrated to catalyze a variety of carbon−carbon and
carbon−heteroatom couplings.16 In certain instances, the
stability of the pincer framework allows for detailed mechanistic
information to be surmised, as reactive intermediates can be
observed directly.17,18
We have been interested in exploring the chemistry of the new
pyrrole-based pincer ligand 2,5-bis[(diphenylphosphino)-
methyl]pyrrolide (abbreviated P2PhPyr−), which was recently
demonstrated to bind transition metals in a meridional fashion,
giving rise to square-planar complexes of the type [MX-
(P2PhPyr)] for Ni and Pd.19−21 These complexes were prepared
in a straightforward fashion, and several examples containing
different X-type ligands were accessed readily through salt
metatheses. We were thus intrigued to learn whether such
compounds display an interesting organometallic chemistry,
especially with hydrocarbyl-derived ligands. Compounds of the
type [NiX(P2PhPyr)] (X = Cl, alkyl, aryl) were envisioned to
serve as active catalysts for select organic transformations based
on the precedent set with other pincer systems.16 Furthermore,
chelating ligands based on pyrrole motifs have been shown to
produce unique catalysts when paired with certain transition
metals.22 In the context of PNP ligands, the role of the pyrrolide
group in modulating the catalytic activity of pincer-supported
transition-metal complexes remains a question.23−32
RESULTS AND DISCUSSION
■
Our initial synthesis of the pyrrole−diphosphine H(P2PhPyr) (1)
involved reduction of the corresponding phosphine oxide to
afford a BH3-protected precursor that was used directly in the
synthesis of Ni(II) complexes.21 Concurrent reports by the
groups of Gade19 and Mani20 demonstrated an alternative
method for the preparation of 1 using the secondary phosphines
(or potassium salts thereof) directly in displacement reactions
with bis(aminomethyl)pyrrole precursors. We have adopted this
synthetic methodology for the preparation of 1 and have
extended it to include the formation of the dicyclohexylphos-
phine derivative 2, as shown in Scheme 1. The formation of 2
from bis(dimethylaminomethyl)pyrrole occurs in higher yield
than that for 1, presumably due to the increased nucleophilicity
of the secondary alkylphosphine. 31P NMR spectra of 2 display a
single peak at −7.67 ppm consistent with trivalent phosphorus.33
Similar to the case for compound 1, no JHP coupling is observed
between the methylene protons of the pincer “arms” and the
phosphorus nuclei by 1H NMR spectroscopy (see the
Received: June 28, 2013
© XXXX American Chemical Society
A
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Scheme 1. Synthesis of 1 and 2
Figure 1. Thermal ellipsoid drawing (50%) of 4. Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (deg): Ni(1)−Cl(1) =
2.1771(8), Ni(1)−N(1) = 1.848(2), Ni(1)−Pav = 2.200(1); P(1)−Ni(1)−P(2) = 166.01(3), N(1)−Ni(1)−Cl(1) = 177.77(7).
Scheme 2. Electrocatalytic Behavior of Complex 3 in Dichloromethane
Supporting Information). Thus far, compound 2 has resisted
efforts at crystallization and its sensitivity to air has hampered
chromatographic purification. Fortunately, however, the prep-
aration of compound 2 directly affords material of sufficient
purity for attachment to nickel(II).
Å. No deformation of the pyrrolic unit is apparent, nor is there
any unexpected bond length alternation within the heterocycle.
The remaining bond metrics are unremarkable and are consistent
with those of other Ni(II) complexes bearing the P2PhPyr−
ligand.21
The reaction of 2 with anhydrous NiCl2 in the presence of
Et3N affords [NiCl(P2CyPyr)] (4) in a fashion identical with that
for the phenylphosphine analogue 3. Complex 4 and all others
bearing the P2CyPyr− ligand display greater solubility in organic
solvents than similar compounds containing the P2PhPyr− ligand.
Much like 3, complex 4 exists as an orange-red solid (λmax 438
Previous electrochemical experiments with complex 3
demonstrated that an electrocatalytic process occurs in
methylene chloride at low potentials.21 This process most likely
corresponds to the abstraction of a chlorine atom by the dimeric
Ni(I) species [NiI(μ2-P2PhPyr)]2 (13),19 which is formed by
reduction of 3 around −2.1 V versus the ferrocene/ferrocenium
couple (Scheme 2). To examine whether a similar process occurs
for 4, its cyclic voltammogram was recorded in CH2Cl2. In
contrast to the case for 3, the CV of 4 in CH2Cl2 displays no
cathodic events within the solvent window. Such a result is
consistent with the more reduced nature of 4 with respect to 3
1
nm) displaying a diamagnetic H NMR spectrum with the
methylene protons of the ligand appearing as a pseudotriplet,
consistent with κ3-PNP coordination. The solid-state structure of
4 is shown in Figure 1. The geometry of complex 4 is square
planar about nickel with a short Ni−Npyrrole distance of 1.848(2)
B
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Scheme 3. Synthesis of Ni(II) Alkyl Complexes
Figure 2. Thermal ellipsoid drawings (50%) of compounds 7 (left) and 11 (right). Hydrogen atoms are omitted for clarity. For compound 7, one of the
two crystallographically independent molecules of the asymmetric unit is displayed. Selected bond distances and angles can be found in Table 1.
Figure 3. Thermal ellipsoid drawings of compounds 6 and 14. Hydrogen atoms and minor components of the disorder (for 14) are omitted for clarity.
Selected bond distances and angles can be found in Table 1.
due to the presence of the more electron-donating dicyclohex-
ylphosphine groups of the ligand. In THF, the CV of 4 does
display an irreversible cathodic event at low potentials (see the
Supporting Information). Although they are not as well resolved
as those observed for complex 3, the cathodic events are
somewhat comparable. Whether this process corresponds to
formation of an analogous Ni(I) dimeric species is not known at
this time. However, treatment of 4 with hydride sources does not
result in formation of an analogous NiI dimer as observed with 3
(vide infra).19
Much like 3, complex 4 also displays an irreversible anodic
process corresponding to oxidation of the Ni(II) complex. For
C
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Table 1. Selected Bond Distances (Å) and Angles (deg) for Nickel(II) Hydrocarbyl Complexes
compd
Ni(1)−C(31)
Ni(1)−N(1)
Ni(1)−Pav
P(1)−Ni(1)−P(2)
Ni(1)−C(31)−C(32)
6
1.967(2)
1.997(2)
1.980(1)
1.903(1)
1.993(2)
1.886(2)
1.891(2)
1.890(1)
1.884(1)
1.893(1)
2.178(2)
2.182(1)
2.198(1)
2.179(1)
2.199(1)
165.11(2)
166.60(3)
158.83(2)
167.57(1)
166.04(2)
N/A
a
7
111.4(1)
110.9(1)
122.3(1)
114.1(2)
10
11
14
a
Data shown for one of two chemically identical but crystallographically independent molecules of 7 in the asymmetric unit. See the Supporting
Information for more information.
complex 4 this event occurs at +0.23 V (vs Fc/Fc+), lower than
that observed for 3 (+0.45 V) as expected. A quasi-reversible
event is also observed for 4 at potentials higher than +0.23 V (see
the Supporting Information). The current response for this
process is less than that expected for a one-electron process and
therefore may represent a subsequent redox event of the species
generated from the irreversible oxidation at +0.23 V, although
further experiments are required to test this possibility. Although
uncommon, Ni(III) species have been invoked as intermediates
in C−C coupling reactions.11,17,34−38
at 50 °C in benzene-d6 afforded the corresponding acyl insertion
1
product after 24 h (eq 1), as judged by H NMR and IR
spectroscopy (see the Supporting Information).40 Reaction of
the phenyl complex 11 with CO(g), however, did not afford the
benzoyl species under similar conditions.
In addition to hydrocarbyl species, we also examined the
synthesis of a Ni(II) hydride complex. Previous attempts to
prepare a nickel hydride complex by treatment of 1 with
[Ni(COD)2] were reported by Gade to yield the dimeric NiI
specie 13.19 We also confirmed this result with 3. Upon switching
to the P2CyPyr− ligand, however, the hydride species 15 was
obtained in good yield either through reaction of 2 with
[Ni(COD)2] or by treatment of 4 with LiHBEt (eqs 2 and 3,
Compounds 3 and 4 both react cleanly with several Grignard
reagents to afford the corresponding hydrocarbyl complexes
(Scheme 3). As with 3 and 4, each of the alkyl complexes is
diamagnetic, displaying a virtual triplet resonance for the
methylene protons of the PNP ligand. The solid-state structures
of compounds 7 and 11 and compounds 10 and 14 are shown in
Figures 2 and 3, respectively. Additionally, the solid-state
structure of 6 can be found in the Supporting Information.
Relevant metric parameters for each complex appear in Table 1.
Each of the structures is similar, displaying square-planar
geometries about nickel. The only structure that appears slightly
distorted is that of the benzyl complex 10. This species displays
an increased contraction of the P(1)−Ni(1)−P(2) bond angle
(158.8°) relative to the other structurally characterized
compounds. The allyl complex 14 displays an η1 coordination
mode as expected for a 16-electron square-planar compound,
displaying a C(31)−C(32) bond distance of 1.501(5) Å and a
C(32)−C(33) bond distance of 1.373(5) Å consistent with
1
single and double bonds, respectively. The H NMR spectrum
for 14 is also indicative of η1 coordination in solution at room
temperature (see the Supporting Information).
Unexpectedly, complex 14 could only be prepared with the
P2CyPyr− ligand. Attempted reactions of 3 with allyl Grignard
afforded a mixture of starting material 3 and the NiI dimer 13
(Scheme 2). The nature of the reductant in this reaction is not
known at present. β-Hydride elimination of [Ni(η1-C3H5)-
(P2PhPyr)] to afford allene and a putative nickel hydride is an
attractive proposal, given the reported instability of “[NiH-
(P2PhPyr)]” toward formation of 13.19 The stability of complex 7
toward elimination, however, argues against such a possibility
(vide infra). Attempted observation of the alkylation reaction
respectively). The former procedure was found to be preferable
as a means of preparing 15. The dichotomy in reactivity between
1 and 2 with hydride sources is consistent with PCP systems,41
but notable given the recent finding that a related carbazole-
based pincer ligand containing diphenylphosphine groups is
capable of stabilizing a nickel(II) hydride.42 Thus, the observed
propensity for formation of a Ni(I) dimer with compounds
containing the P2PhPyr− ligand in the presence of certain
nucleophiles (hydride and allyl sources) may be both a function
of the more electron deficient nature of the ligand and its smaller
steric profile with respect to P2CyPyr− (cone angles of
diphenylphosphinoethane and dicyclohexylphosphinoethane
are 125 and 142°, respectively).43
1
between 3 and allylmagnesium chloride by H NMR spectros-
copy in benzene-d6 provided no further insight into the reaction,
indicating peaks solely due to the starting materials. In addition
to allyl Grignard, divergent reactivity between compounds 3 and
4 was also observed with hydride sources (vide infra).
All alkyl complexes described in Scheme 3 are stable
indefinitely under an inert atmosphere at room temperature.
Furthermore, the ethyl species 7 proved resistant to β-H
elimination even upon heating to 80 °C overnight.39 Despite
their thermal stability, select alkyl complexes did react cleanly
with carbon monoxide. Reaction of complex 5 with 1 atm of CO
We have not yet been able to grow crystals of 15 suitable for X-
ray diffraction, but NMR data are consistent with a monomeric,
square-planar Ni(II) species. Compound 15 displays a triplet
resonance for the hydride ligand at −17.61 ppm in benzene-d6
with a coupling constant of 59.3 Hz to 31P (see the Supporting
Information). Unlike the alkyl complexes described above, 15
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does not react cleanly with CO(g), as judged by NMR
spectroscopy. The compound does insert CO2 readily, however,
to give the putative metal formate species [Ni(O2CH)(P2CyPyr)]
(eq 4; see also the Supporting Information).44,45
CONCLUSIONS
■
In conclusion, we have prepared and characterized a variety of
alkyl species containing the new PNP ligands P2PhPyr− and
P2CyPyr−. Each of the complexes demonstrates good thermal
stability, including those containing alkyl ligands with β-
hydrogen atoms. In addition to alkyls, a nickel hydride complex
was isolated with the P2CyPyr− ligand. This species is inaccessible
with the P2PhPyr− ligand, demonstrating the significant differ-
ences between pyrrole-based pincer ligands containing alkyl- and
arylphosphine groups. Both alkyl and hydride complexes
demonstrate insertion chemistry of unsaturated C−O bonds.
The methyl complex [Ni(CH3)(P2PhPyr)] was found to insert
CO cleanly at elevated temperatures to generate an acyl complex.
In contrast, the hydride complex did not react cleanly with CO
but was found to insert CO2 readily at ambient temperature,
affording a Ni(II) formate. Finally, nickel(II) complexes of both
PNP ligands are efficient catalysts for the Kumada coupling of
aryl chlorides and Grignard reagents at room temperature.
Similar results with a variety of different precatalysts suggest that
a common active species is responsible for catalytic activity.
Future work will further probe the catalytic potential of these
nickel(II) complexes and investigate in detail their catalytic
mechanisms.
Catalytic Reactions. The nickel(II) complexes described
above were examined as catalysts for the Kumada coupling of aryl
chlorides with a variety of Grignard reagents. Each complex
examined gave comparable results for the representative
coupling of 4-chloroanisole with o-tolylmagnesium chloride
(Table 2). The couplings were found to proceed in high yield at
Table 2. Kumada Coupling of 4-Chloroanisole and o-
Tolylmagnesium Chloride Employing Various Nickel(II)
a
Precatalysts
EXPERIMENTAL SECTION
■
entry
catalyst
yield, %
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, and toluene were purified by
sparging with argon and passing through two columns packed with 4 Å
molecular sieves or activated alumina (THF). Benzene and benzene-d6
were dried over sodium ketyl and vacuum-distilled prior to use. NMR
spectra were recorded in benzene-d6 on a Varian INOVA spectrometer
1
2
3
4
5
6
7
8
3
92
89
90
87
82
82
84
91
4
5
6
11
12
13
14
operating at 500 MHz (1H) and referenced to the residual H (7.16
1
ppm) or 13C (128.39 ppm) resonance of the solvent. For 31P NMR
spectra an external standard of neat 85% H3PO4(aq) was used (0.00
ppm). All reported coupling constants are in units of Hz. UV−vis
spectra were recorded on a Cary-60 spectrophotometer in airtight
Teflon-capped quartz cells. Infrared spectra were recorded as thin films
on a KBr plate using a Nicolet iS10 FTIR spectrometer. Cyclic
voltammetry was performed at 23 °C on a CH Instruments 620D
electrochemical workstation. A three-electrode setup was employed,
comprising a glassy-carbon working electrode, platinum-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 Atlantic Microlab, Inc. of
Norcross, GA, or Midwest Microlab, LLC of Indianapolis, IN.
a
Conditions: 1.0 mmol of 4-chloroanisole; 1.3 mmol of o-tolyl
Grignard; 3 mL of toluene; 12 h. Yields are reported for isolated
product.
room temperature with low catalyst loadings (<1 mol %). Even
the dimeric Ni(I) complex 13 was found to give productive
coupling (Table 2, entry 7). No significant differences were
found for the P2PhPyr− ligand versus P2CyPyr−, and therefore
compound 3 was employed in a screen of different coupling
partners. In addition, complex 3 is stable under ambient
conditions in the solid state, facilitating its use as a precatalyst.
Table 3 displays the results of several Kumada couplings
catalyzed by complex 3. As is evident from the table, the catalyst
system exhibited good substrate scope for aryl chlorides
containing both electron-releasing and electron-withdrawing
substituents. Added steric bulk to the electrophilic coupling
partner resulted in slightly lower yields, as demonstrated by
couplings using 2-chlorotoluene (entry 3). In contrast, added
bulk to the nucleophilic coupling partner actually lead to a slight
increase in reaction yield (entries 7−12). This result is most
likely due to suppression of the small amount of homocoupling
product observed with phenylmagnesium chloride (<5%).
Couplings employing alkyl Grignard reagents were also
successful with catalyst 3 (entries 14 and 15). In sum, complex
3 appears to demonstrate good substrate scope and catalytic
efficacy at room temperature for a variety of Kumada couplings
involving aryl chlorides.
Materials. H(P2PhPyr) (1),19−21 [Ni(CH3)(P2PhPyr)] (5),21 and
bis(dimethylaminomethyl)pyrrole46 were prepared according to
literature procedures. Anhydrous nickel(II) chloride, bis-
(cyclooctadiene)nickel(0), dicyclohexylphosphine, Grignard reagents
(as solutions in THF or Et2O), lithium triethylborohydride, and catalytic
substrates were purchased from Strem Chemicals or Sigma-Aldrich and
used as received. Carbon monoxide and carbon dioxide were purchased
from Praxair and used as received.
2,5-Bis((dicyclohexylphosphino)methyl)pyrrole (H(P2CyPyr),
2). This compound was prepared in a fashion similar to that for the
phenyl derivative.20 A flask was charged with 1.08 g (5.96 mmol) of 2,5-
bis(dimethylaminomethyl)pyrrole and 2.4 mL of dicyclohexylphos-
phine (12 mmol). The reaction mixture was heated to 140 °C and stirred
for 20 h. After cooling, all volatiles were removed in vacuo to afford 2.77
g (95%) of a white residue. The material was used directly without
further purification. 1H NMR: δ 8.24 (s, 1 NH), 6.05 (d, 2 pyr-CH), 2.67
(s, 4 CH2), 1.79 (m, 4 Cy-CH), 1.69 (m, 12 Cy-CH2), 1.61 (m, 4 Cy-
CH), 1.51 (m, 4 Cy-CH), 1.17 (m, 20 Cy-CH2). 13C{1H} NMR: δ
128.28 (d, obscured by solvent), 107.36 (d, JCP = 4.8), 34.25 (d, JCP
=
E
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a
Table 3. Kumada Coupling of Various Aryl Chlorides Using 3 as Precatalyst
a
Conditions: 1.0 mmol of aryl chloride; 1.3 mmol of Grignard; 0.4 mol % of 3; 3 mL of toluene; 23 °C. Yields are reported for isolated product.
15.3), 30.64 (d, JCP = 13.3), 29.71 (d, JCP = 8.7), 28.09 (d, JCP = 14.4),
28.02 (d, JCP = 11.4), 27.27, 21.71 (d, JCP = 19.6). 31P{1H} NMR: δ
−7.67. ESI-MS (positive mode): calcd for [M + H]+ m/z 488.4, [M(O)
+ H]+ m/z 504.3; found for [M + H]+ m/z 488.1, [M(O) + H]+ m/z
504.1.
benzene extract was filtered through a pad of Celite and evaporated to
dryness to afford 1.48 g (85%) of a red solid. Crystals suitable for X-ray
diffraction were grown by slow cooling of a concentrated diethyl ether
solution at −30 °C. Mp: 210 °C dec. 1H NMR: δ 6.37 (s, 2 pyr-CH),
2.64 (app t, 4 CH2, JHP = 4.5), 2.45 (m, 4 Cy-CH), 1.95 (m, 4 Cy-CH),
1.79 (m, 4 Cy-CH), 1.69 (m, 8 Cy-CH2), 1.59 (m, 12 Cy-CHn), 1.15 (m,
8 Cy-CH), 1.06 (m, 4 Cy-CH). 13C{1H} NMR: δ 138.50 (t, JCP = 7.6),
[NiCl(P2CyPyr)] (4). A flask was charged with 1.46 g of H(P2CyPyr)
(2.99 mmol) and 20 mL of THF. To the resulting solution was added
0.388 g of anhydrous NiCl2 (2.99 mmol) and 0.83 mL of Et3N (6.0
mmol). The mixture was stirred for 12 h at room temperature, during
which time it became dark red. All volatiles were removed in vacuo,
leaving a dark red residue that was extracted into 50 mL of benzene. The
106.06 (t, JCP = 5.2), 33.11 (t, JCP = 10.5), 28.84, 28.56, 27.51 (t, JCP
=
6.3), 27.34 (t, JCP = 5.2), 26.82, 23.25 (t, JCP = 10.9). 31P{1H} NMR: δ
51.10. Anal. Calcd for C30H50ClNNiP2: C, 62.04; H, 8.68; N, 2.41.
Found: C, 62.55; H, 8.56; N, 2.34.
F
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General Procedure for Preparation of Alkyl Complexes. A flask
was charged with 0.200 mmol of [NiCl(P2 Pyr)] (R = Ph, Cy) and 10
(m, 8 Ar-H), 6.96−6.89 (m, 15 m,p-C6H5 + Ar-H), 6.64 (s, 2 pyr-CH),
3.65 (app t, 4 CH2, JHP = 4.7). 13C{1H} NMR: δ 149.97 (t, JCP = 26.2),
139.52 (t, JCP = 3.6), 135.35 (t, JCP = 7.1), 133.44 (t, JCP = 5.6), 133.07 (t,
JCP = 20.0), 130.41, 128.95 (t, JCP = 4.7), 126.94, 106.76 (t, JCP = 5.2),
35.27 (t, JCP = 13.1). 31P{1H} NMR: δ 34.91. Anal. Calcd for
C36H31NNiP2: C, 72.27; H, 5.22; N, 2.34. Found: C, 70.04; H, 5.32; N,
2.45.
R
mL of THF. The resulting red-orange solution was chilled to −30 °C, at
which point 1 equiv (0.20 mmol) of the desired Grignard reagent as a
solution in THF or Et2O was added dropwise. The reaction mixture was
warmed to room temperature and stirred for 3 h, during which time the
color lightened to a dark yellow-brown. All volatiles were removed in
vacuo, and the remaining residue was extracted into 10 mL of benzene.
The benzene solution was filtered through a pad of Celite and
evaporated to dryness to afford the alkyl complex as a yellow or orange
microcrystalline solid that was washed with pentane. In the case of
complexes containing the P2PhPyr− ligand, the products were further
washed with Et2O. Characterization data and yields for each complex
appear below. For compounds 9 and 11, combustion analyses with
multiple samples consistently returned low results for carbon despite
satisfactory H and N values. For compound 14, analyses were repeatedly
low for all elements, suggesting poor or incomplete combustion. The 1H
NMR spectrum of 14 is provided in the Supporting Information.
[Ni(CH3)(P2CyPyr)] (6). Yield: 96%. Crystals suitable for X-ray
diffraction were grown by slow cooling of a saturated Et2O solution at
−30 °C. 1H NMR: δ 6.53 (s, 2 pyr-CH), 2.91 (app t, 4 CH2, JHP = 4.3),
2.13 (m, 4 Cy-CH), 1.83 (m, 4 Cy-CH), 1.69 (m, 8 Cy-CH2), 1.58 (m, 8
Cy-CH2), 1.44 (m, 8 Cy-CH), 1.13 (m, 4 Cy-CH), 1.08 (m 8 Cy-CH),
−0.36 (t, 3 CH3, JHP = 8.5). 13C{1H} NMR: δ 135.92 (t, JCP = 7.4),
[Ni(C6H5)(P2CyPyr)] (12). Yield: 93%. 1H NMR: δ 7.80 (d, 2 o-C6H5),
7.19 (t, 2 m-C6H5), 6.95 (t, 1 p-C6H5), 2.94 (app t, 4 CH2, JHP = 4.5),
1.99 (m, 4 Cy-CH), 1.78 (m, 4 Cy-CH), 1.56 (m, 20 Cy-CH + CH2),
1.10 (m, 8 Cy-CH), 0.98 (m, 8 Cy-CH). 13C{1H} NMR: δ 153.84,
140.38 (t, JCP = 2.9), 136.30 (t, JCP = 7.1), 126.63 (t, JCP = 1.7), 122.04,
105.34 (t, JCP = 5.1), 32.99 (t, JCP = 10.6), 28.37, 28.05, 27.66 (t, JCP
=
6.1), 27.35 (t, JCP = 5.0), 26.78, 25.18 (t, JCP = 10.6). 31P{1H} NMR: δ
48.56. Anal. Calcd for C36H55NNiP2: C, 69.46; H, 8.91; N, 2.25. Found:
C, 69.09; H, 8.69; N, 2.31.
[Ni(η1-C3H5)(P2CyPyr)] (14). Yield: 91%. Crystals suitable for X-ray
diffraction were grown by slow cooling of a saturated Et2O solution at
−30 °C. 1H NMR: δ 6.49 (s, 2 pyr-CH), 6.41 (m, 1 allyl-CH), 5.14 (m, 1
allyl-CH), 4.88 (m, 1 allyl-CH), 2.86 (app t, 4 CH2, JHP = 4.3), 2.17 (m, 4
Cy-CH), 1.89 (m, 4 Cy-CH), 1.71 (m, 2 allyl-CH2), 1.68 (m, 8 Cy-
CH2), 1.57 (m, 8 Cy-CH2), 1.17 (m, 4 Cy-CH), 1.07 (m, 8 Cy-CH2).
13C{1H} NMR: δ 148.62 (t, JCP = 3.5), 135.75 (t, JCP = 6.7), 104.99 (t,
J
CP = 4.7), 103.89, 34.09 (t, JCP = 9.6), 29.31, 28.88, 27.73 (t, JCP = 6.1),
104.79 (t, JCP = 4.8), 33.43 (t, JCP = 9.9), 29.33, 28.84, 27.74 (t, JCP
=
27.50 (t, JCP = 4.6), 26.95, 25.75 (t, JCP = 11.3), 2.61 (t, JCP = 17.9).
31P{1H} NMR: δ 47.43. HRMS (ESI, positive mode): calcd for [M]+ m/
z 585.3163; found for [M]+ m/z 585.3149.
6.1), 27.47 (t, JCP = 4.7), 27.01, 25.89 (t, JCP = 10.5). 31P{1H} NMR: δ
52.11. Anal. Calcd for C31H53NNiP2: C, 66.44; H, 9.53; N, 2.50. Found:
C, 66.02; H, 9.16; N, 2.29.
[NiH(P2CyPyr)] (15). A flask was charged with 0.488 g of H(P2CyPyr)
(1.00 mmol), 0.275 g of [Ni(COD)2] (1.00 mmol), and 20 mL of
toluene. The yellow-brown solution was stirred at room temperature for
8 h. All volatiles were removed in vacuo, and the resulting solid was
washed with pentane to afford 0.496 g (91%) of a yellow microcrystal-
line solid. The material was further purified by recrystallization in Et2O
at −30 °C. 1H NMR: δ 6.60 (s, 2 pyr-CH), 3.01 (app t, 4 CH2, JHP = 4.3),
2.05 (m, 4 Cy-CH), 1.76 (m, 4 Cy-CH), 1.72 (m, 4 Cy-CH), 1.66 (m, 4
Cy-CH), 1.62 (m, 4 Cy-CH), 1.54 (m, 4 Cy-CH), 1.46 (m, 4 Cy-CH),
[Ni(CH2CH3)(P2PhPyr)] (7). Yield: 90%. Crystals suitable for X-ray
diffraction were grown by vapor diffusion of pentane into a concentrated
benzene solution at room temperature. 1H NMR: δ 7.62 (m, 8 Ar-H),
7.03 (m, 4 Ar-H), 6.99 (m, 8 Ar-H), 6.55 (s, 2 pyr-CH), 3.67 (app t, 4
CH2, JHP = 4.5), 0.98 (m, 2 CH2CH3), 0.83 (t, 3 CH2CH3). 13C{1H}
NMR: δ 134.58 (t, JCP = 7.2), 133.89 (t, JCP = 18.4), 133.55 (t, JCP = 5.9),
130.39, 129.16 (t, JCP = 4.4), 106.28 (t, JCP = 5.2), 36.23 (t, JCP = 12.9),
15.62 (t, JCP = 3.1), −6.40 (t, JCP = 18.5). 31P{1H} NMR: δ 39.39. Anal.
Calcd for C32H31NNiP2·Et2O: C, 69.25; H, 6.62; N, 2.24. Found: C,
68.66; H, 6.20; N, 2.55.
1.35 (m, 4 Cy-CH), 1.07 (m, 12 Cy-CH + CH2), −17.61 (t, NiH, JHP
=
59.3). 13C{1H} NMR: δ 136.38 (t, JCP = 7.5), 104.98 (t, JCP = 4.9), 34.52
(t, JCP = 11.6), 30.11, 29.27, 27.40 (app t, JCP = 5.4, two overlapping
triplets), 26.90, 26.23 (t, t, JCP = 10.1). 31P{1H} NMR: δ 67.21. Anal.
Calcd for C30H51NNiP2: C, 65.95; H, 9.41; N, 2.56. Found: C, 65.60; H,
9.22; N, 2.49.
[Ni(CH2CH3)(P2CyPyr)] (8). Yield: 95%. H NMR: δ 6.53 (s, 2 pyr-
1
CH), 2.92 (app t, 4 CH2, JHP = 4.3), 2.15 (m, 4 Cy-CH), 1.89 (m, 4 Cy-
CH), 1.70 (m, 8 Cy-CH2), 1.59 (m, 8 Cy-CH2), 1.45 (m, 8 Cy-CH2),
1.38 (t, 3 CH2CH3), 1.15 (m, 4 Cy-CH), 1.08 (m, 8 Cy-CH2), 0.77 (m-
qd, 2 CH2CH3). 13C{1H} NMR: δ 135.53 (t, JCP = 7.2), 104.80 (t, JCP
4.8), 34.19 (t, JCP = 9.6), 29.40, 28.87, 27.83 (t, JCP = 5.9), 27.56 (t, JCP
4.6), 26.99, 26.10 (t, JCP = 10.9), 17.82 (t, JCP = 3.3), −12.73 (t, JCP
=
=
=
[Ni(C{O}CH3)(P2PhPyr)]. This species was observed in solution by
adding ∼1 atm of CO(g) to a 50 mM solution of [Ni(CH3)(P2PhPyr)] in
benzene-d6. After it was heated to 50 °C for 15 h, the acyl complex was
observed to form, as judged by NMR and IR spectroscopy. 1H NMR: δ
7.64 (m, 8 Ar-H), 6.97 (m, 12 Ar-H), 6.57 (s, 2 pyr-CH), 3.61 (app t, 4
CH2, JHP = 4.7), 1.91 (s, 3 C(O)CH3). 13C{1H} NMR: acyl carbon not
21.3). 31P{1H} NMR: δ 47.32. Anal. Calcd for C32H55NNiP2: C, 66.91;
H, 9.65; N, 2.44. Found: C, 66.59; H, 9.68; N, 2.37.
[Ni(CH2C6H5)(P2PhPyr)] (9). Yield: 87%. 1H NMR: δ 7.49 (m, 8 Ar-H),
7.04 (m, 4 Ar-H), 6.98 (m, 8 Ar-H), 6.79 (t, 1 p-CH2C6H5), 6.74 (t, 2 m-
CH2C6H5), 6.54 (d, 2 o-CH2C6H5), 6.48 (s, 2 pyr-CH), 3.57 (app t, 4
CH2, JHP = 4.5), 2.27 (t, 2 CH2C6H5, JHP = 9.3). 13C{1H} NMR: δ 151.30
observed, δ 134.00 (t, JCP = 6.7), 133.70 (t, JCP = 19.8), 133.45 (t, JCP
=
=
5.8), 130.65, 129.29 (t, JCP = 4.8), 106.72 (t, JCP = 4.9), 39.40 (t, JCP
8.2, C(O)CH3), 35.20 (t, JCP = 14.4). 31P{1H} NMR: δ 32.69. IR (film,
KBr, cm−1): 1614 (νCO).
(t, JCP = 2.3), 134.74 (t, JCP = 6.7), 133.68 (t, JCP = 5.7), 132.96 (t, JCP
=
[Ni(O2CH)(P2CyPyr)]. This species was observed in solution by
adding ∼1 atm of CO2(g) to a 30 mM solution of [NiH(P2CyPyr)] in
benzene-d6. After standing at room temperature for 24 h, the formate
complex was observed to form, as judged by NMR and IR spectroscopy.
1H NMR: δ 7.91 (t, 1 CO2-H, JHP = 3.3), 6.32 (s, 2 pyr-CH), 2.61 (app t,
4 CH2, JHP = 4.5), 2.56 (m, 4 Cy-CH), 1.85 (m, 4 Cy-CH), 1.74 (m, 4
Cy-CH), 1.69 (m, 8 Cy-CH2), 1.63 (m, 4 Cy-CH), 1.56 (m, 8 Cy-CH),
1.25 (m, 4 Cy-CH), 1.16 (m, 4 Cy-CH), 1.08 (m, 4 Cy-CH). 13C{1H}
NMR: δ 167.80 (O2CH), 138.39 (t, JCP = 8.5), 106.31 (t, JCP = 5.2),
33.72 (t, JCP = 9.7), 28.67, 28.52, 27.62 (t, JCP = 6.4), 27.40 (t, JCP = 4.9),
26.77, 22.17 (t, JCP = 11.6). 31P{1H} NMR: δ 47.86. IR (film, KBr,
cm−1): 1631 (νCO), 1298 (νCO).
General Procedure for Cross-Coupling Reactions. A Schlenk
tube was charged with 2.0 mg (4 μmol) of the desired nickel complex
and 3 mL of toluene or THF. The electrophilic coupling partner (1.0
mmol) was then added, followed by the Grignard reagent (1.3 mmol) as
a solution in THF. The reaction mixture was stirred at room
temperature for 6−12 h before being quenched with 5 mL of deionized
18.5), 130.41, 129.17 (t, JCP = 4.7), 128.52, 128.33, 122.42, 106.41 (t, JCP
= 4.8), 36.31 (t, JCP = 13.2), 7.22 (t, JCP = 16.2). 31P{1H} NMR: δ 37.52.
Anal. Calcd for C37H33NNiP2: C, 72.58; H, 5.43; N, 2.29. Found: C,
67.44; H, 5.64; N, 2.04.
1
[Ni(CH2C6H5)(P2CyPyr)] (10). Yield: 95%. H NMR: δ 7.45 (d, 2 o-
CH2C6H5), 7.18 (t, 2 m-CH2C6H5), 7.00 (t, 1 p-CH2C6H5), 6.49 (s, 2
pyr-CH), 2.86 (app t, 4 CH2, JHP = 4.3), 2.10 (m, 4 Cy-CH), 2.07 (t, 2
CH2C6H5, JHP = 8.0), 1.64 (m, 12 Cy-CH + CH2), 1.58 (m, 8 Cy-CH2),
1.39 (m, 8 Cy-CH2), 1.08 (m, 12 Cy-CH + CH2). 13C{1H} NMR: one
aromatic 13C resonance obscured by C6D6, δ 155.07 (t, JCP = 4.3), 135.86
(t, JCP = 7.0), 129.21, 122.47, 105.05 (t, JCP = 4.7), 33.80 (t, JCP = 9.4),
29.61, 28.97, 27.69 (t, JCP = 6.1), 27.62 (t, JCP = 4.2), 26.96, 25.19 (t, JCP
= 11.2), 2.73 (t, JCP = 19.1). 31P{1H} NMR: δ 47.19. Anal. Calcd for
C37H57NNiP2: C, 69.82; H, 9.03; N, 2.20. Found: C, 69.25; H, 8.94; N,
2.14.
[Ni(C6H5)(P2PhPyr)] (11). Yield: 84%. Crystals suitable for X-ray
diffraction were grown by vapor diffusion of pentane into a saturated
benzene solution of the complex. 1H NMR: δ 7.33 (d, 2 o-C6H5), 7.29
G
dx.doi.org/10.1021/om400630q | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
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solvents the crude product was purified by column chromatography on
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ASSOCIATED CONTENT
■
S
* Supporting Information
Figures giving additional NMR spectra and cyclic voltammo-
grams and tables and CIF files giving crystallographic data. This
material is available free of charge via the Internet at http://pubs.
acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail for Z.J.T.: zachary.tonzetich@utsa.edu.
Notes
(34) Jones, G. D.; Martin, J. L.; McFarland, C.; Allen, O. R.; Hall, R. E.;
Haley, A. D.; Brandon, R. J.; Konovalova, T.; Desrochers, P. J.; Pulay, P.;
Vicic, D. A. J. Am. Chem. Soc. 2006, 128, 13175−13183.
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Meyer, K.; Mindiola, D. J. J. Am. Chem. Soc. 2008, 130, 3676−3682.
(37) Kozhanov, K. A.; Bubnov, M. P.; Cherkasov, V. K.; Fukin, G. K.;
Vavilina, N. N.; Efremova, L. Y.; Abakumov, G. A. J. Magn. Reson. 2009,
197, 36−39.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by a TRAC award from UTSA and by a
grant from the Welch Foundation (AX-1772 to Z.J.T.). The
authors also thank Dr. David Black for assistance with HRMS
measurements. Mass spectrometer facilities are supported by a
grant from the NIMHD (G12MD007591).
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Chem. 2011, 50, 2661−2674.
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(41) Boro, B. J.; Duesler, E. N.; Goldberg, K. I.; Kemp, R. A. Inorg.
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