Synthesis and reactivity of NiII(Phpy)2 (Phpy = 2-phenylpyridine)

Article abstract of DOI:10.1021/om100387z

This article describes the synthesis and reactivity of Ni ^II(Phpy)₂ (Phpy = 2-phenylpyridine) with a variety of oxidants, including O₂, Br₂, PhICl₂, N-fluoropyridinium salts, Cu^II salts, and N-halosuccinimides. High-oxidation-state Ni intermediates were not detected in any of these transformations. In all cases, the major organic product resulted from oxidatively induced C-C bond formation to generate the Phpy-Phpy dimer. Traces (2-16%) of organic products resulting from C-O, C-Br, C-Cl, and C-N bond-forming reductive elimination were also observed.

Full text of DOI:10.1021/om100387z

5446 Organometallics 2010, 29, 5446–5449
DOI: 10.1021/om100387z
Synthesis and Reactivity of NiII(Phpy)2 (Phpy = 2-Phenylpyridine)†
Andrew T. Higgs, Paul J. Zinn, and Melanie S. Sanford*
Department of Chemistry, University of Michigan, 930 N. University Avenue, Ann Arbor, Michigan 48109
Received April 30, 2010
This article describes the synthesis and reactivity of NiII(Phpy)2 (Phpy = 2-phenylpyridine) with a
variety of oxidants, including O2, Br2, PhICl2, N-fluoropyridinium salts, CuII salts, and N-halosucci-
nimides. High-oxidation-state Ni intermediates were not detected in any of these transformations. In all
cases, the major organic product resultedfrom oxidatively inducedC-C bond formation to generate the
Phpy-Phpy dimer. Traces (2-16%) of organic products resulting from C-O, C-Br, C-Cl, and C-N
bond-forming reductive elimination were also observed.
Introduction
current interest in organometallic chemistry.1 Such reac-
tions serve as the product release step of many catalytic
transformations, including C-H functionalization,2 allylic
substitution,3 carbonylation,4 cross-coupling,1a and alkene
functionalization.1a,5,6 Fundamental studies of C-X bond-
forming processes can provide insights for the design and
optimization of new catalytic transformations with im-
proved scope, efficiency, and selectivity.
Our group has been particularly interested in C-X
bond formation at high-oxidation-state group 10 metal
centers.7-10 As part of this effort, we recently reported that
the monocyclometalated NiII complex (Phpy)NiII(pic)(Br)
(1; Phpy = 2-phenylpyridine, pic = picoline) reacts with Br2,
CuBr2, and PhICl2 to afford halogenated organic products
(for an example, see eq 1).11 These transformations were
proposed to proceed via high-oxidation-state NiIII or NiIV
intermediates; however, such species could not be directly
detected in this system. As such, we sought to design ligand
architectures that could potentially stabilize high-oxidation-
state Ni organometallic complexes.12-14
Carbon-heteroatom (C-X) bond-forming reductive
elimination at late-transition-metal centers is a topic of great
† Part of the Dietmar Seyferth Festschrift.
*To whom correspondence should be addressed. E-mail: mssanfor@
umich.edu.
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r
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2010 American Chemical Society

Article
Organometallics, Vol. 29, No. 21, 2010 5447
byproducts proved challenging, and 6 could not be reliably
isolated in pure form from this reaction.
The bis-cyclometalated PdII complex PdII(Phpy)2 (2) is
known to react with strong oxidants (e.g., PhICl2, N-chloro-
succinimide (NCS), PhI(OAc)2, and Br2) to afford unusually
stable octahedral PdIV products such as 3 (eq 2).7a,b,d While
these compounds are typically isolable at room tempera-
ture, they react at 40-80 °C to afford mixtures of organic
products derived from carbon-heteroatom (4) and carbon-
carbon (5) bond-forming reductive elimination. We rea-
soned that this ligand set might also stabilize NiIV or NiIII
complexes and allow us to directly observe C-heteroatom
bond-forming reactions from such high-oxidation-state Ni
species. This paper describes the synthesis of NiII(Phpy)2 (6)
and its reactivity with diverse oxidants.
A recent report by Wolczanski demonstrated the forma-
tion of 6 via ortho deprotonation of coordinated 2-phenyl-
pyridine ligands in [(Phpy)NiII(Phpy-H)2]þ and (Phpy)NiII-
(Br)(Phpy-H) (7).17 On the basis of this precedent, we next
targeted a one-pot synthesis of 6 via the formation and in situ
deprotonation of (Phpy)NiII(Br)(Phpy-H) (7). Gratifyingly,
treatment of Ni(COD)2 with Phpy-Br in the presence of
1 equiv of 2-phenylpyridine followed by the addition of
potassium tert-amylate resulted in formation of the desired
product (eq 4). Recrystallization from benzene/pentanes
provided clean samples of 6 in modest yield (33% over two
steps), but in a highly reproducible fashion.
Complex 6 is a brick red solid that shows no signs of
decomposition after 1 month at -35 °C in a N2-filled drybox.
This bis-cyclometalated NiII adduct is also stable in benzene
solution for at least 24 h at room temperature under N2.
However, exposure of a benzene solution of 6 to dry O2 under
otherwise identical conditions resulted in a rapid (within
1 min) color change from red to yellow-brown accompanied
by the formation of Phpy dimer 5 (47% yield) and the phenol
product 8 (23% yield) (eq 5). This result demonstrates that 6 is
highly susceptible to oxidatively induced C-C and
C-heteroatom bond-forming reactions.
Results and Discussion
Our initial studies focused on developing an efficient
synthesis of NiII(Phpy)2 (6). The analogous PdII complex 1
is prepared by reaction of PdCl2(SEt2)2 with 2 equiv of ortho-
lithiated 2-phenylpyridine (Li-Phpy) (eq 2). On the basis of
this precedent, we first examined the reaction between NiII
dihalide precursors (including NiBr2(PPh3)2, NiCl2(DME)2,
and NiCl2) and 2 equiv of Li-Phpy at -78 °C in Et2O or
THF.15 Unfortunately, in all cases, intractable mixtures of
Ni0 and paramagnetic inorganic products were obtained,
suggesting that reduction of the NiII starting material by the
Li reagent predominates over the desired transmetalation in
these systems.16
We hypothesized that this competing reduction could be
limited by the use of a more electron rich Ni starting material.
As such, the reaction of Li-Phpy with the monocyclometa-
lated NiII complex (Phpy)NiII(Pic)(I) was next explored
(eq 3). Gratifyingly, 1H NMR spectroscopic analysis of the
crude reaction mixture showed a 23% yield of the desired
product 6. However, separation of this material from other
Treatment of a benzene solution of 6 with Br2 resulted in a
color change from red to dark brown within seconds at room
temperature. Even at -78 °C, this reaction was extremely
fast, and no high-oxidation-state intermediates were de-
tected by 1H NMR or EPR spectroscopy.18 Analysis of the
organic products showed the formation of Phpy dimer 5
(70%) along with significant amounts (16%) of the C-Br
(15) Wei, P.; Chan, K. T. K.; Stephan, D. W. Dalton Trans. 2003,
3804.
(16) For example, the use of NiIIBr2(PPh3)2 as the NiII starting
material produced NiIBr(PPh3)3 as the major inorganic product
(characterized by EPR spectroscopy and X-ray crystallography). For
a similar observation, see ref 15.
(17) Volpe, E. C.; Chadeayne, A. R.; Wolczanski, P. T.; Lobkovsky,
E. B. J. Organomet. Chem. 2007, 692, 4774.

5448 Organometallics, Vol. 29, No. 21, 2010
Higgs et al.
Table 1. Reaction of 6 with Electrophilic Halogenating Reagents
Table 2. Comparison of Reaction of 6/NBS to Reductive Elim-
ination from 12
entry
oxidant
yield C-X (4),a %
yield C-C (5),a %
1
2
3
4
5
6
7
Br2
16
10
2
70
53
59
69
68
79
58
NBSb
CuBr2
PhICl2
NCS
3
ndc
ndc
ndc
CuCl2
NFTPTd
a Yields determined by 1H NMR spectroscopy based on an average of
two runs. b The C-N coupled product 11 was also formed in 7% yield
(see Table 2). c nd = not detected. d N-Fluoro-2,4,6-trimethylpyridinium
triflate.
entry reacn yield C-X (4), % yield C-N (11), % yield C-C (5), %
1a 6/NBS
2b 12
10
13
7
6
53
23
coupled product Phpy-Br (4-Br) (eq 6). Attempts to directly
characterize the inorganic byproducts were complicated by
the low crystallinity and paramagnetic nature of these Ni
species. However, the addition of 4 equiv of dppe to the crude
reaction mixture resulted in formation of the diamagnetic
NiII species [(dppe)2NiIIBr]Br (9) in 78% yield, as determined
by 31P NMR spectroscopy. The paramagnetic NiI species
[NiI(dppe)2]Br (10) was also formed as a minor side product
of this reaction (7% isolated yield of yellow crystals) and was
characterized by X-ray crystallography (see the Supporting
Information for details). On the basis of these results, we
propose that the initial inorganic products of this transfor-
mation are a mixture of NiII and NiI complexes containing
arylpyridines 4-Br and 5 as L-type ligands.
a Yields determined by 1H NMR spectroscopy based on an average of
2 runs. b Data from ref 7b.
of C-halogen coupled Phpy-Br (4-Br) (2-16%), along with
5 as the major organic product (entries 1-3).19
The reaction of 6 with NBS was particularly notable,
because it afforded C-N coupled product 11 (7%) along
with 4-Br (10%) and 5 (53%) (Table 2, entry 1). Interestingly,
thermolysis of the palladium(IV) complex PdIV(Phpy)2(Cl)-
(succinimide) (12) in benzene was previously reported to
afford a similar distribution of C-N, C-X, and C-C
products (Table 2, entry 2).7b This result suggests the possi-
bility that the Ni reaction may proceed via a similar high-
oxidation-state MIV intermediate. However, other pathways
for oxidatively induced C-Br/C-N/C-C bond formation
at Ni, including the formation of a NiIII species20 and/or
Ni-C bond homolysis/free radical based coupling processes,
are also potential routes to these products.
Conclusions
In conclusion, this paper describes the reactivity of NiII-
(Phpy)2 (6) with a number of different oxidants. In contrast
to analogous Pd systems, no high-oxidation-state Ni inter-
mediates were detected. Additionally, the Ni-bound Phpy
ligands appear to be more susceptible to oxidatively induced
C-C coupling in comparison to their Pd analogues. These
features make it difficult to draw definitive conclusions
about the mechanism of C-X bond formation at Ni in these
systems. This study clearly indicates that the stabilization/
isolation of high-oxidation-state Ni intermediates in carbon-
heteroatom bond-forming processes will require support-
ing ligands that are not susceptible to competing C-C
coupling.13,14 Efforts in this area are underway in our labora-
tory and will be reported in due course.
As summarized in Table 1, a variety of other electrophilic
brominating, chlorinating, and fluorinating reagents reacted
with 6 in a fashion similar to that for Br2. In all of these cases,
C-C bond formation to generate 5 predominated over
C-halogen coupling. The brominating reagents Br2, NBS,
and CuBr2 provided the highest (although still modest) yields
(18) There were no clearly identifiable peaks in the low-temperature
NMR spectra; however, there were several very broad resonances that
may be indicative of paramagnetic intermediates. Unfortunately, these
proved to be too transient and to be present in concentrations too low to
observe by EPR.
(19) Numerous spectroscopic methods (NMR, GC, GCMS) and
workup procedures (acidic, basic) have been attempted in an effort to
account for the mass balance of the organic ligands in these transforma-
tions. The best results are shown in Table 1. We believe that the
remaining organic products likely undergo decomposition in the pre-
sence of transient coodinatively unsaturated NiI or NiII byproducts of
these reactions. Similar issues have been observed previously in oxida-
tively induced reductive elimination reactions at PdIV and PdIII. For
examples, see: (a) Reference 7. (b) Reference 11. (c) Powers, D. C.;
Geibel, M. A. L.; Klein, J. E. M. N.; Ritter, T. J. Am. Chem. Soc. 2009,
131, 17050. (d) Kalyani, D.; Deprez, N. R.; Desai, L. V.; Sanford, M. S.
J. Am. Chem. Soc. 2005, 127, 7330.
Experimental Section
General Procedures. NMR spectra were obtained on Varian
Inova 500 (499.90 MHz for 1H; 125.70 MHz for 13C) and Varian
Inova 400 instruments (399.96 MHz for H; 100.57 MHz for
1
(20) For formation of NiIII via reaction of (PPhMe2)2NiBr(C6Cl5)
with NBS, see: Oguro, K.; Wada, M.; Sonoda, N. J. Organomet. Chem.
1979, 165, C10.

Article
Organometallics, Vol. 29, No. 21, 2010 5449
13C; 376.34 MHz for 19F). 1H and 13C NMR chemical shifts are
reported in parts per million (ppm) relative to TMS, with the
residual solvent peak used as an internal reference. Multiplicities
are reported as follows: singlet (s), doublet (d), doublet of
doublets (dd), doublet of doublets of doublets (ddd), doublet
of triplets (dt), triplet (t), quartet (q), quintet (quin), multiplet
(m), and broad resonance (br).
HCl were added, and then the organic layer was neutralized with
saturated sodium bicarbonate. The EtOAc layer was then
extracted with 1.0 M aqueous sodium citrate (2 ꢀ 20 mL) and
saturated aqueous sodium thiosulfate (1 ꢀ 20 mL). The organic
layer was collected and concentrated under vacuum, an internal
standard (1,3,5-trimethoxybenzene) was added, and the reac-
tion mixture was analyzed by 1H NMR spectroscopy.
Synthesis of 6. In a glovebox, Ni(COD)2 (0.250 g, 0.91 mmol,
1.00 equiv) was dissolved in THF (5 mL) in a dry 20 mL
scintillation vial. In a second 4 mL vial, 2-phenylpyridine
(0.143 g, 0.92 mmol, 1.01 equiv) and 2-(2-bromophenyl)pyridine
(0.215 g, 0.92 mmol, 1.01 equiv) were also dissolved in THF
(2 mL). Both vials were cooled to -35 °C, and then the ligand
solution was added to the Ni(COD)2 solution. The resulting
yellow-brown mixture was stirred for 10 min at room tempera-
ture. A solution of potassium tert-amylate (0.117 g, 0.92 mmol,
1.02 equiv) in THF (4 mL) was then added, which resulted in an
immediate color change to blood red. The mixture was stirred
for an additional 10 min, and then the solvent was removed
under vacuum. The resulting red solids were dissolved in dry
benzene (15 mL) and filtered through a pad of Celite. Pentanes
(100 mL) was added to the red filtrate, and this solution was
cooled at -35 °C overnight. The resulting precipitate was
collected and dried under vacuum to afford 6 as a brick red
Reaction of 6 with Br2. In a glovebox, complex 6 (9.5 mg, 0.026
mmol, 1.0 equiv) was dissolved in benzene (2 mL) in a 4 mL vial.
The vial was equipped with a magnetic stir bar and sealed with
a Teflon-lined cap fitted with a rubber septum. The vial was
removed from the glovebox, and the oxidant (0.104 mmol, 4.0
equiv) was added via syringe. The reaction mixture was stirred at
room temperature overnight. The solvent was then removed
under vacuum, the resulting material was taken up in EtOAc
(10 mL), and the EtOAc solution was extracted with 1.0 M
aqueous sodium citrate (2 ꢀ 20 mL) and saturated aqueous
sodium thiosulfate (1 ꢀ 20 mL). The organic layer was collected
and concentrated under vacuum, an internal standard (1,3,5-
trimethoxybenzene) was added, and the reaction mixture was
1
analyzed by H NMR spectroscopy. The isolation and char-
acterization of inorganic byproducts from this transformation is
described in the Supporting Information.
1
solid (112 mg, 33% yield). The H and 13C NMR data for 6
Acknowledgment. We thank the National Science
Foundation (No. CHE-0545909) for support of this work.
We thank Jeff Kampf for X-ray crystallographic charac-
terization of complexes 9 and 10.
matched those reported in the literature.17
Reaction of 6 with O2. In a glovebox, complex 6 (4.8 mg, 0.013
mmol, 1.0 equiv) was dissolved in benzene (1 mL) in a J. Young
NMR tube. The sample was removed from the glovebox,
degassed via three freeze-pump-thaw cycles, and then filled
with 1 atm of dry O2. The sample was shaken vigorously, which
resulted in a color change from deep red to yellow-brown. The
resulting mixture was then allowed to stand at room tempera-
ture overnight. The solvent was removed under vacuum, and the
solid residue was taken up in EtOAc (10 mL). Several drops of
Supporting Information Available: Text and figures giving
experimental details and spectroscopic data for new compounds
and a CIF file giving crystallographic data for complex 10. This
material is available free of charge via the Internet at http://
pubs.acs.org.