Heterolytic CH-bond activation in the synthesis of Ni{(2-aryl-κC2)pyridine-κN}2 and derivatives

Article abstract of DOI:10.1016/j.jorganchem.2007.06.047

Thermolysis of Ni(OTf)₂ in 2-phenyl-pyridine or 2-tolyl-pyridine afforded the cationic chelate derivatives, [bis(2-aryl-pyridine)Ni{(2-aryl-κC²)pyridine-κN}]OTf (aryl = phenyl, 1a; tolyl, 1b). Addition of KBr to 1a and LiBr to 1b provided the bromides, (2-aryl-pyridine)BrNi{(2-aryl-κC²)pyridine-κN} (aryl = phenyl, 2a; tolyl, 2b). When subjected to KO^tBu in Et₂O, the bromides generated the entitled bis-cyclometalated compounds, Ni{(2-aryl-κC²)pyridine-κN}₂ (aryl = phenyl, 3a; tolyl, 3b). These compounds insert diphenylacetylene into one cyclometalate arm to produce [(2-aryl-κC²)pyridine-κN]Ni[2-(2-(1,2-diphenylethenyl-κC²)aryl)pyridine-κN] (aryl = phenyl, 4a; p-tolyl, 4b). X-ray crystallographic studies were conducted on 1a, 2a, 3a and 4a, and a brief DFT study of 3a confirmed its low spin configuration and rippled geometry.

Full text of DOI:10.1016/j.jorganchem.2007.06.047

Journal of Organometallic Chemistry 692 (2007) 4774–4783
www.elsevier.com/locate/jorganchem
Heterolytic CH-bond activation in the synthesis of
Ni{(2-aryl-jC2)pyridine-jN}2 and derivatives
*
Emily C. Volpe, Andrew R. Chadeayne, Peter T. Wolczanski , Emil B. Lobkovsky
Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, NY 14853, United States
Received 3 May 2007; received in revised form 5 June 2007; accepted 7 June 2007
Available online 30 June 2007
Dedicated to Prof. Gerhard Erker on the occasion of his 60[th] birthday; Zirconium, it’s in our blood.
Abstract
Thermolysis of Ni(OTf)2 in 2-phenyl-pyridine or 2-tolyl-pyridine afforded the cationic chelate derivatives, [bis(2-aryl-pyridine)Ni{(2-
aryl-jC2)pyridine-jN}]OTf (aryl = phenyl, 1a; tolyl, 1b). Addition of KBr to 1a and LiBr to 1b provided the bromides, (2-aryl-pyri-
dine)BrNi{(2-aryl-jC2)pyridine-jN} (aryl = phenyl, 2a; tolyl, 2b). When subjected to KOtBu in Et2O, the bromides generated the entitled
bis-cyclometalated compounds, Ni{(2-aryl-jC2)pyridine-jN}2 (aryl = phenyl, 3a; tolyl, 3b). These compounds insert diphenylacetylene
into one cyclometalate arm to produce [(2-aryl-jC2)pyridine-jN]Ni[2-(2-(1,2-diphenylethenyl-jC2)aryl)pyridine-jN] (aryl = phenyl, 4a;
p-tolyl, 4b). X-ray crystallographic studies were conducted on 1a, 2a, 3a and 4a, and a brief DFT study of 3a confirmed its low spin
configuration and rippled geometry.
ꢀ 2007 Elsevier B.V. All rights reserved.
Keywords: Nickel; CH-bond activation; Alkyne; Insertion; Triflate; Cyclometalation
1. Introduction
tion (ArX/M0) of the aryl group (i.e., X = halogen;
M0 = Li, MgX), CH-bond activation eliminates these
steps. In principle, an electrophilic metal center MXn
can react with a chelate L-ArH to produce (L-Ar)MXnꢀ1
and HX, where X is likely to be halide, OTf, O2CCF3,
etc. Transition metal triflates are an attractive class of
starting materials for an exploratory study of this type,
and 2-phenyl-pyridine has considerable precedent in het-
erolytic CH-bond activations [8–18]. The use of Ni(OTf)2
[19] as a starting material capable of heterolytically acti-
vating 2-phenyl-pyridine, and subsequent reactivity stud-
ies are reported herein.
In order to impart stronger ligand fields on 1st-row tran-
sition metals, the use of metal–carbon bonds, especially
metal–aryl bonds, remains an intriguing possibility [1]. In
angular overlap parlance, the DEMAr for such bonds – the
energy match between metal and C-based orbitals – is small
when compared to more electronegative second row main
group elements (e.g., O, N), and the orbital overlap is also
significant in comparison [2]. The critical problem that must
be overcome is the intrinsic reactivity of the metal–aryl bond.
Metal–aryl bonds can be entrained within chelates to
add kinetic stability, and heterolytic CH-bond activation
was considered as an efficient means to synthesize com-
plexes containing such ligands [3–5]. There are various
approaches to the synthesis of metal–carbon bonds, the
most common being metathesis and oxidative addition
(OA) [6,7]. Since either of these requires a functionaliza-
2. Results
2.1. Heterolytic 2-phenyl-pyridine activation by Ni(OTf)2
2.1.1. Synthesis of [bis(2-aryl-pyridine)Ni{(2-aryl-
jC2)pyridine-jN}]OTf
*
As Scheme 1 illustrates, thermolysis of Ni(OTf)2 in neat
2-phenyl-pyridine or 2-tolyl-pyridine afforded the cationic
Corresponding author.
E-mail address: ptw2@cornell.edu (P.T. Wolczanski).
0022-328X/$ - see front matter ꢀ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2007.06.047

E.C. Volpe et al. / Journal of Organometallic Chemistry 692 (2007) 4774–4783
4775
+
-
Ni(OTf)
+
OTf
2
180˚C
18 h
KBr
Br
L
N
L
L
N
N
(or LiBr)
Ni
Ni
6 h,THF
R
(excess)
R = H,
R = H,
R = H, 2-Ph-py
Me, 2-tol-py
2a
2b
1a
1b
60%
62%
43%
L = 2-Ph-py
L = 2-Ph-py
R
R
R = Me,
L = 2-tol-py
R = Me,
L = 2-tol-py
45%
t
KO Bu
4a' (insertion isomer)
3 h
23˚C
- KBr
Et O
2
t
KO Bu
- KOTf
Et O
- L
2
1a
23˚C
K
= 0.43
R
4a->4a'
3a
3b
R = H
32%
34%
Ph
N
N
Ph
52% 4a
R = H
Ni
R
PhC CPh
R = Me
Ni
benzene
100˚C, 8 h
50%
4b
R = Me
N
N
R
R
Scheme 1.
chelate derivatives, [bis(2-aryl-pyridine)Ni{(2-aryl-jC2)pyr-
idine-jN}]OTf (aryl = phenyl, 1a; tolyl, 1b), in modest yield
(ꢁ40–45%). 1H NMR spectroscopy on these sparingly solu-
ble yellow compounds revealed two g1-N-bound 2-aryl-
pyridines, and a related group lacking an ortho-H on an
asymmetric phenyl residue, suggesting that heterolytic
CH-bond activation had indeed occurred. 1H NMR spectra
for all new compounds can be found in Table 1, and
13C{1H} NMR spectra for those compounds soluble
enough for ready characterization are found in Table 2.
Table 1
1H NMR data (d, J (Hz); benzene-d6) for [bis(2-aryl-pyridine)Ni{(2-aryl-jC2)pyridine-jN}]OTf (aryl = phenyl, 1a; tolyl, 1b), [(2-aryl-pyridine)BrNi{(2-
aryl-jC2)pyridine-jN}] (aryl = phenyl, 2a; tolyl, 2b), Ni{(2-aryl-jC2)pyridine-jN}2 (aryl = phenyl, 3a; tolyl, 3b), and [(2-aryl-jC2)pyridine-jN]Ni[2-(2-
(1,2-diphenylethenyl-jC2)aryl)pyridine-jN] (aryl = phenyl, 4a; tolyl, 4b)a
b
j
s
j
a
c
d
e
f
i
k
l
t
i
r
k
l
N
g
u
N
o
N
o
v
Ni
w
m
n
q
p
m
n
x
y
h
p
aa
z
R
R
R
ab
(i)
c
d
e
f
g
h
q
(r–aa)
(j)
(k)
(l)
(m)
(n)
(o)
(p)
1a
1b
2a
2b
3a
4a
4b
a
9.29(5.5)
8.56(8.5)
9.36(5.5)
8.60(8.5)
9.94(7.0)
9.45(6.5)
9.54(5.5)
9.98(6.0)
7.72(5.0)
7.73(5.6)
7.79(5.5)
7.06(7.5)
7.84(4.0)
7.11(7.5)
6.08
6.87–6.76
6.06
6.46(8.5)
6.64–6.58
6.47
6.52–6.58
6.56–6.63
6.97(9.5)
6.60
6.72–6.81
6.92(7, 8)
6.94(10.0)
6.77(7.5)
6.85
6.52
8.40(6.0)
7.99(7.5)
8.43(6.0)
8.00
6.68–6.77
8.90(9.0)
6.72–6.81
8.93(8.0)
7.31(7.5)
7.33(8.0)
7.43(7.5)
7.30
7.12
7.12
7.03
2.09
1.89,1.91
6.83
7.19
1.84
1.98
7.15
2.35
7.30
7.30
2.49
1.78
5.49(8.0)
7.21–7.20
5.41
7.99(7.5)
8.00
6.76–6.87
6.66(8.0)
6.87
6.83(5.0)
7.19
6.67(8.0)
6.86(8.0)
7.12(8.5)
7.01(8.4)
7.22
7.21–7.20
6.73(8.0)
7.00
6.56–6.63
6.68–6.77
6.60
7.03(7.5)
7.35(7.0)
7.36(8.0)
7.30
7.11(7.5)
7.34(7.5)
7.34(7.5)
7.02
7.08
6.12(8.0)
6.30(8.0)
6.12(6.5)
6.33(6.5)
6.27(6.5)
6.26(8.0)
6.28(7.0)
6.04(6.0)
6.28(5.0)
6.04(5.0)
5.36(9.5)
6.68–6.77
5.23
7.03
8.39(7.5)
8.22
7.49(8.0)
8.47(7.5)
7.44
8.90(9.0)
8.93(8.1)
6.86–7.01
6.87–7.06
7.22
7.18
7.18
6.78
6.78
7.40(7.5)
7.40(7.5)
8.31
Shift assignments are based on correlations with free ligands, literature precedent and the accompanying compounds; the assignments are tentative.
Coupling constants, where quoted, are phenomenological. First row corresponds to {(2-aryl-jC2)pyridine-jN} ligand as shown in illustration; second row
corresponds to remaining 2-aryl-pyridine or pyridine-derived fragment.

4776
E.C. Volpe et al. / Journal of Organometallic Chemistry 692 (2007) 4774–4783
2.1.2. Structure of [bis(2-phenyl-pyridine)Ni{(2-phenyl-jC2)
pyridine-jN}]OTf
ment, and the orientation of the bound 2-phenyl-pyridines
hampers associative substitution processes expected for
square planar Ni(II) by blocking sites above and below the
plane. Persistent difficulties in substitution with neutral L0
prompted a switch to anionic nucleophiles in an effort to gen-
erate neutral Ni(II) species. Various Rꢀ equivalents (MeLi,
tBuCH2Li, MeMgBr, etc.) generated red solutions from
which no tractable material could be isolated. Success was
achieved through the addition of KBr to 1a and LiBr to 1b
to afford the bromides, (2-aryl-pyridine)BrNi{(2-aryl-jC2)-
pyridine-jN} (aryl = phenyl, 2a; tolyl, 2b). The compounds
were again characterized by one complete 2-aryl-pyridine
fragment and one lacking an ortho-H on the aryl, as Tables
1 and 2 reveal. The conformation of 2a, as confirmed by a
single crystal X-ray structure determination, is square planar,
with the Brꢀ opposite the Ni–aryl bond. This geometry con-
forms to expectations based on the stronger trans-effect of
the Ni–aryl bond in comparison to the pyridine–Ni bond of
the chelate. In addition, no noticeable isomerization was
observed under a variety of solvents (THF, toluene, benzene,
CH3CN) and conditions. In the trans-influence series, the
Table 3 lists crystallographic information pertaining to
the structure of [bis(2-phenyl-pyridine)Ni{(2-phenyl-
jC2)pyridine-jN}]OTf (1a), and Fig. 1 provides a molecu-
lar view of the square planar cation. Unfortunately, while
the structure confirms the cyclometalated (2-phenyl-
jC2)pyridine-jN unit, there is a 1:1 disorder in the pyridine
and activated phenyl groups, thereby rendering virtually all
metric parameters an average of the two orientations. As
expected, the two 2-phenyl-pyridine ligands are oriented
in the more sterically favorable ‘‘up and down’’ orientation
with respect to the (Npy)2Ni(jC,jN) plane, giving the mol-
ecule a crystallographic pseudo-2-fold axis bisecting the
Npy–Ni–Npy angle.
2.1.3. Substitution by bromide
Displacement of the 2-aryl-pyridine ligands from [bis(2-
aryl-pyridine)Ni{(2-aryl-jC2)pyridine-jN}]OTf (aryl = phe-
nyl, 1a; tolyl, 1b) proved to be difficult. The low volatility of
2-phenyl-pyridine prevented its ready removal after displace-
Table 2
13C{1H} NMR data (benzene-d6) for [bis(2-phenyl-pyridine)Ni{(2-phenyl-jC2)pyridine-jN}]OTf (1a), [(2-aryl-pyridine)BrNi{(2-aryl-jC2)pyridine-jN}]
(aryl = phenyl, 2a; tolyl, 2b), Ni{(2-aryl-jC2)pyridine-jN}2 (aryl = phenyl, 3a; tolyl, 3b), and [(2-aryl-jC2)pyridine-jN]Ni[2-(2-(1,2-diphenylethenyl-
jC2)aryl)pyridine-jN] (aryl = phenyl, 4a; tolyl, 4b)a,b
cc
b
n
n
bb
y
a
c
d
dd
o
p
m
m
o
p
N
N
N
aa
e
i
q
u
q
u
f
Ni
r
r
ff
z
g
h
s
t
w
v
gg
hh
s
t
k
j
w ee
v
l
x
R
x
R
R
a
b
n
z
c
d
p
e
f
g
h
i
j
k
w
ii
l
m
y
o
aa
q
r
dd
s
t
u
gg
v
hh
x
bb
cc
ee
ff
1a
153.4
139.0
150.7
117.5
122.3
121.1
125.6
122.8
125.8
138.4
130.4
136.0
163.3
143.6
148.9
147.2
138.3
140.6
130.2
129.4
129.8
123.2
128.7
129.3
127.7
122.5
128.7
129.3
162.8
129.4
129.8
c
129.2
c
2a
2b
153.8
156.2
117.2
121.7
125.6
138.0
130.0
164.6
151.1
147.1
141.3
134.5
136.9
124.9
122.4
122.2
128.9
162.1
136.9
c
128.9
c
c
153.7
156.1
117.0
121.3
125.5
125.9
137.9
136.9
164.7
150.9
144.6
140.1
135.4
129.8
138.5
138.6
122.1
c
162.2
129.8
21.5
22.3
3a
3b
4a
148.9
148.8
118.4
118.1
121.7
121.3
142.7
143.6
165.4
165.5
147.7
145.3
147.4
124.2
125.2
137.1
137.0
130.4
139.8
122.3
122.1
165.1
165.2
22.5
148.8
146.6
184.4
118.0
120.9
148.9
125.2
125.1
143.7
137.1
136.3
130.0
164.1
148.5
129.9
131.6
123.9
132.0
122.7
125.7
131.2
124.5
c
122.5
141.6
128.9
162.6
166.0
139.4
c
129.8
4b
148.7
146.6
117.8
120.5
125.2
125.1
137.1
136.2
164.2
145.0
144.2
139.8
137.0
124.3
122.7
124.9
125.1
c
122.4
142.6
162.7
166.1
21.5
23.0
c
c
c
c
c
129.9
148.8
131.2
132.2
a
Shift assignments are based on correlations with free ligands, literature precedent and the accompanying compounds; the assignments are tentative.
First row corresponds to {(2-aryl-jC2)pyridine-jN} ligand as shown in illustration; second row corresponds to remaining 2-aryl-pyridine (1a, 2a, b) or
pyridyl fragment (4a, b); third row corresponds to other 2-phenyl-pyridine (1a) or diphenylacetylene fragment (4a, b).
b
Limited solubility of 1b prevented data acquisition.
Obscured by solvent or unobserved.
c

E.C. Volpe et al. / Journal of Organometallic Chemistry 692 (2007) 4774–4783
4777
Table 3
Crystallographic data for [bis(2-phenyl-pyridine)Ni{(2-phenyl-jC2)pyridine-jN}]OTf (1a), [(2-phenyl-pyridine)BrNi{(2-phenyl-jC2)pyridine-jN}] (2a),
Ni{(2-phenyl-jC2)pyridine-jN}2 (3a), and [(2-phenyl-jC2)pyridine-jN]Ni[2-(2-(1,2-diphenylethenyl-jC2)phenyl)pyridine-jN] (4a)
1a
2a
3a
4a
Formula
C34H26N3O3F3SNia
1499.92a
C22H17BrN2Ni
520.11e
C22H16N2Ni
444.69f
C2/c
C36H26N2Ni
545.32
P21/c
Formula weight
Space group
Z
ꢀ
ꢀ
P1
P1
2a
2
8
4
˚
a (A)
15.0530(9)
15.1416(10)
15.6322(10)
95.366(2)
102.954(2)
93.317(3)
3445.7(4)
1.446
10.001(3)
10.593(4)
11.718(4)
81.212(18)
71.227(18)
84.383(18)
1160.0(7)
1.489
24.2023(8)
11.7116(4)
14.5953(4)
90
92.563(1)
90
4132.9(2)
1.431
0.959
10.6170(5)
16.0815(6)
15.6065(6)
90
96.212(2)
90
2648.97(19)
1.367
0.761
˚
b (A)
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
3
˚
V (A )
rcalc (g cmꢀ3
)
l (mmꢀ1
)
0.684
2.579
Temperature (K)
173(2)
173(2)
173(2)
173(2)
˚
k (A)
0.71073
0.71073
0.71073
R1 = 0.0326
wR2 = 0.0842
R1 = 0.0429
wR2 = 0.0896
1.041
0.71073
R1 = 0.0333
wR2 = 0.0877
R1 = 0.0516
wR2 = 0.0952
1.061
R indices [I > 2r(I)]b,c
R1 = 0.0431
wR2 = 0.1196
R1 = 0.0703
wR2 = 0.1307
1.054
R1 = 0.0266
wR2 = 0.0679
R1 = 0.0384
wR2 = 0.0708
1.066
R indices (all data)b,c
Goodness-of-fitd
a
The asymmetric unit is twice the formula unit plus one free 2-phenyl-pyridine molecule; the formula weight refers to the asymmetric unit, as does Z.
P
P
b
c
R1 = jjFoj ꢀ jFcjj/ jFoj.
h
i
1=2
P
P
2
wR2 ¼
wðjF oj ꢀ jF cjÞ = wF o2
.
P
d
e
f
GOF (all data) = [ w(jFoj ꢀ jFcj)2/(n ꢀ p)]1/2, n = number of independent reflections, p = number of parameters.
The asymmetric unit contains one formula unit and a molecule of THF, as does Z.
The asymmetric unit contains one formula unit and 1/2 of a 2-phenyl-pyridine molecule, as does Z.
Fig. 2. Molecular view of (2-phenyl-pyridine)BrNi{(2-phenyl-jC2)pyri-
dine-jN} (2a).
lographic details and core metric parameters are given in
Tables 3 and 4, respectively. The bite angle of the cyclomet-
alate is 84.62(7)ꢁ, and the accompanying Ni–jC and Ni–
Fig. 1. Molecular view of one disorder model of the [bis(2-phenyl-
pyridine)Ni{(2-phenyl-jC2)pyridine-jN}] cation of the triflate salt 1a.
positionofBrꢀ versuspyridineissomewhatnebulous, henceit
is unknown whether 2a and 2b are the thermodynamic
products.
jN distances are 1.9044(18) and 1.9169(16) A, respectively.
˚
The Ni–jN bond is shorter than in the other examples
below, in part because it is opposite the relatively weak
2-phenyl-pyridine donor. There is a strain affiliated with
the cyclometalate evidenced in internal versus external
Ni–jC–C angles of 112.60(13)ꢁ and 130.09(14)ꢁ, and
related Ni–jN–C angles of 115.25(12)ꢁ and 126.76(14)ꢁ,
respectively. The (2-phenyl-jC2)pyridine-jN ligand is
2.1.4. Structure of (2-phenyl-pyridine)BrNi{(2-phenyl-
jC2)pyridine-jN}
A molecular view of (2-phenyl-pyridine)BrNi{(2-phenyl-
jC2)pyridine-jN} (2a) is given in Fig. 2, while the crystal-

4778
E.C. Volpe et al. / Journal of Organometallic Chemistry 692 (2007) 4774–4783
1
Table 4
<10% yield by H NMR spectroscopy. Along with black
material presumed to be Ni0, GC–MS confirmed the pres-
ence of 2-(2,4-xylyl)-pyridine and 2-tolyl-pyridine in a
rough 1:2 ratio. The products implicate reductive elimina-
tion from (2-tolyl-pyridine)MeNi{(2-tolyl-jC2)pyridine-
jN} as potential degradation path [21].
˚
Core distances (A) and angles (ꢁ) for [(2-phenyl-pyridine)BrNi{(2-phenyl-
jC2)pyridine-jN}] (2a), Ni{(2-phenyl-jC2)pyridine-jN}2 (3a), and [(2-
phenyl-jC2)pyridine-jN]Ni[2-(2-(1,2-diphenylethenyl-jC2)phenyl)pyri-
dine-jN] (4a)
2a
3a
4a
Ni–jC
Ni–jN
1.9044(18)
1.9169(16)
1.887(2)
1.892(2)
1.9794(16)
1.9610(18)
1.8911(10)
1.9556(9)
2.2. Double cyclometalation
Ni–N(jN0)
Ni–Br
Ni–jC0
C0@C
1.9018(15)
2.4410(7)
1.9336(10)a
2.2.1. Synthesis of Ni{(2-aryl-jC2)pyridine-jN}2
While attempting to synthesize a neutral cyclometala-
tion species through the addition of Rꢀ and Xꢀ, KOtBu
was utilized in Et2O, and the aforementioned red color
again appeared. With this reagent, the red, crystalline dou-
ble cyclometalation product, Ni{(2-phenyl-jC2)pyridine-
jN}2 (3a), was produced from its corresponding cation,
[bis(2-phenyl-pyridine)Ni{(2-phenyl-jC2)pyridine-jN}]OTf
(1a) [16,22]. Difficulties in separating 2-phenyl-pyridine
from the product limited isolation procedures to slow crys-
tallizations, and yields exceeding 40% were sometimes pro-
duced; most were substantially lower. In addition to the
aforementioned alkyl anion equivalents, amide bases were
also tried with limited success. These problems prompted
a change in synthetic procedures, and the bromides (2-phe-
nyl-pyridine)BrNi{(2-phenyl-jC2)pyridine-jN} (2a) and
(2-tolyl-pyridine)BrNi{(2-tolyl-jC2)pyridine-jN} (2b) were
subjected to KOtBu in Et2O to afford Ni{(2-aryl-jC2)pyri-
dine-jN}2 (aryl = phenyl, 3a; tolyl, 3b) in modest, but
reproducible yields (>30%). The two cyclometalated
1.8844(10)a
1.3462(15)a
jN–Ni–jC
84.62(7)
84.66(8)
84.78(4)
84.73(8)
Ni–jC–C
Ni–jN–C
112.60(13)b
130.09(14)c
113.26(15)b
130.23(16)c
112.73(16)b
130.48(17)c
112.46(13)b
126.99(15)c
113.95(14)b
126.32(15)c
113.07(8)b
130.94(8)c
115.25(12)b
126.76(14)c
113.69(7)b
127.87(9)c
jN–Ni–N
jC–Ni–N
jN–Ni–Br
jC–Ni–Br
N–Ni–Br
jN0–Ni–jC0
jN–Ni–jC0
173.29(6)
90.01(7)
96.24(5)
167.32(5)
89.89(5)
84.53(4)e
175.41(4)e
171.61(4)e
97.42(4)e
93.87(5)e
115.81(8)e
119.79(8)e
156.24(8)d
161.08(8)d
101.27(7)d
97.07(9)d
jN–Ni–jN0
jC–Ni–jC0
Ni–jC0–C(Ph)
Ni–jC0@C
a
1
ligands appear equivalent in H and 13C{1H} NMR spec-
tra, consistent with a 2-fold symmetry axis, but two struc-
tural isomers were deemed plausible, prompting an X-ray
structural study.
Prime refers to carbon of cyclometalate insertion ring.
Internal cyclometalate angle.
External cyclometalate angle.
Angles defined with primes are between two different cyclometalates.
Angles defined with primes are between the cyclometalate and the
b
c
d
e
2.2.2. Structure of Ni{(2-phenyl-jC2)pyridine-jN}2
Structural details are listed in Table 3 and core features
of Ni{(2-phenyl-jC2)pyridine-jN}2 (3a) are listed in Table
4. Fig. 3 illustrates the rippled geometry of 3a, whose two
cyclometalates possess bite angles of 84.66(8)ꢁ and
84.73(8)ꢁ. The pinched geometry of this unit is also
observed in the aforementioned internal versus external
Ni–jC–C and Ni–jN–C angles to the same degree as in
2a. In this derivative, the d(Ni–jN) of 1.9794(16) and
cyclometalate insertion ring (primed).
clearly ‘‘pinched’’ in binding to the first row metal. There is
a slight rippling of the pseudo square planar molecule, as the
jN–Ni–N angle is 173.29(6)ꢁ and the jC–Ni–Br angle is
167.32(5)ꢁ. Two of the remaining core angles (jC–Ni–
N = 90.01(7)ꢁ and N–Ni–Br = 89.89(5)ꢁ) are regular such
that the Br–Ni–jC–N plane is flat. The Ni–Br distance is
˚
1.9610(18) A are substantially longer than in 2a, perhaps
˚
˚
normal (92.4410(7) A) as is the remaining Ni–N(py) dis-
reflecting the strong trans-influence of the opposing
nickel–carbon bonds, while the d(Ni–jC) are essentially
tance of 1.9018(15) A.
˚
the same at 1.887(2) and 1.892(2) A. Accompanying the
2.1.5. Alkylation attempts
longer Ni–jN is a wider jN–Ni–jN angle of 101.27(7)ꢁ
in comparison to the jC–Ni–jC angle of 97.07(9)ꢁ. The
distortion from square planar results in intermetalate jC–
Ni–jN0 angles of 161.08(8)ꢁ and 156.24(8)ꢁ, and a dihedral
angle between cyclometalate jC–Ni–jN planes of 31.9ꢁ.
The structurally characterized Pt analogue has very similar
metric parameters aside from chelate bite angles of 79.3ꢁ,
The generation of alkyl derivatives was sought as a
means to investigate the potential of this ligand framework
with regard to olefin oligomerization [20]. Unfortunately,
repeated attempts to alkylate the slightly more soluble tolyl
bromide
derivative,
(2-tolyl-pyridine)BrNi{(2-tolyl-
jC2)pyridine-jN} (2b), were unsuccessful. In certain runs
utilizing MeLi or Me2Zn, a red material, possessing a
new aromatic signature and a singlet in a region (d 0.52)
potentially indicative of metalation, was observed in
˚
which are a consequence of longer Pt–C (1.984, 2.002 A)
˚
and Pt–N (2.125, 2.128 A) bonds, and accompanying jC–
Ni-jN0 angles of 176.4ꢁ and 176.8ꢁ. The latter renders

E.C. Volpe et al. / Journal of Organometallic Chemistry 692 (2007) 4774–4783
4779
insertion product 4a may have more than one conformer;
over time, solutions of 4a form an equilibrium mixture with
another minor species (4a0) possessing related spectral fea-
tures. No further evaluation of this process was conducted.
2.2.4. Structure of [(2-phenyl-jC2)pyridine-jN]Ni[2-(2-
(1,2-diphenylethenyl-jC2)phenyl)pyridine-jN]
Fig. 4 illustrates a molecular view of [(2-phenyl-jC2)pyr-
idine-jN]Ni[2-(2-(1,2-diphenylethenyl-jC2)phenyl)pyri-
dine-jN] (4a), and crystallographic details can be found in
Table 3, while selected metric parameters are given in Table
4. The geometry of the (2-phenyl-jC2)pyridine-jN ligand is
similar to 3a, with d(Ni–jC) and d(Ni–jN) of 1.8911(10)
˚
and 1.9556(9) A, respectively, and
a bite angle of
84.78(8)ꢁ along with the usual distortions. The pyridine
and its accompanying phenyl ring are greatly twisted rela-
tive to each other, as are the phenyls of the Ph2C2 frag-
ment, and the buckling of the seven-membered ring
(primed) enables the pseudo square planar molecule to be
only modestly distorted: \jN0–Ni–jC0 = 84.53(4)ꢁ, \jN–
Ni–jN0 = 97.42(4)ꢁ, \jC–Ni–jC0 = 93.87(4)ꢁ, and \jN–
Ni–jC0 = 175.41(4)ꢁ, 171.61(4)ꢁ. The remaining angles
and distances of the ring-expanded fragment are fairly nor-
mal, with d(Ni–jC0) = 1.8844(10)ꢁ and d(Ni–jN0) =
1.9336(10)ꢁ.
Fig. 3. Molecular view of Ni{(2-phenyl-jC2)pyridine-jN}2 (3a).
3. Discussion
3.1. Heterolytic CH-bond activation
As according to precedent, the 2-aryl-pyridine ligands
were found to undergo heterolytic CH-bond activation,
but the exact details of the process remain uncertain.
Scheme 2 illustrates some mechanistic possibilities [26].
Alkylation of (2-tolyl-pyridine)BrNi{(2-tolyl-jC2)pyri-
dine-jN} (2b) provided hints of an intermediate Ni–Me
complex, but this species did not generate Ni{(2-tolyl-
jC2)pyridine-jN}2 (3b) via the loss of MeH; instead, evi-
dence of reductive elimination and degradation was found.
The addition of tert-butoxide to (2-phenyl-pyri-
dine)BrNi{(2-phenyl-jC2)pyridine-jN} (2a) and (2-tolyl-
pyridine)BrNi{(2-tolyl-jC2)pyridine-jN} (2b) did generate
Fig. 4. Molecular view of [(2-phenyl-jC2)pyridine-jN]Ni[2-(2-(1,2- diphe-
nylethenyl-jC2)phenyl)pyridine-jN] (4a).
the aforementioned dihedral angle to be only 4.0ꢁ, hence
the Pt derivative is relatively flat [23].
the
bis-cyclometalates,
Ni{(2-aryl-jC2)pyridine-jN}2
(aryl = phenyl, 3a; tolyl, 3b), but the path to product is
unclear. It is conceivable that a Ni–OtBu species forms
and it is electrophilic enough to enable an abstraction, or
perhaps dissociation of tBuOꢀ is necessary. In this
2.2.3. Diphenylacetylene insertion
The stronger field imparted by the Ni–carbon bonds in
this system may be offset by the less than perfect bite angle
of the cyclometalated ligand. In an attempt to generate
five-coordinate species more accommodating to the
bite angle of the (2-phenyl-jC2)pyridine-jN ligand,
Ni{(2-aryl-jC2)pyridine-jN}2 (aryl = phenyl, 3a; tolyl,
3b) were treated with diphenylacetylene in the hopes of
making an adduct. Instead, the PhCCPh group inserted
into the carbon arm of one of the cyclometalates [24,25],
generating [(2-aryl-jC2)pyridine-jN]Ni[2-(2-(1,2-diphenyl-
ethenyl-jC2)aryl)pyridine-jN] (aryl = phenyl, 4a; p-tolyl,
4b) according to Scheme 1. The seven-membered ring of
t
instance, an exogenous BuOꢀ would deprotonate a cat-
ionic intermediate coordinated by an ortho-CH-bond of
the aryl group. In an attempt to isolate this intermediate
as a B(C6H3-3,5-(CF3)2)4 salt, H[BArF4 ] was added to
Ni{(2-tolyl-jC2)pyridine-jN}2 (3b) [27], but the reagent
merely induced a disproportion to produce [bis(2-tolyl-pyr-
idine)Ni{(2-tolyl-jC2)pyridine-jN}][BArF4 ] (1b0). This reac-
tion
was confirmed
by
stoichiometry
studies:
3=2 3b þ 2H½BArF4 ꢂ ! 1b0 þ 1=2½NiðIIÞꢂ½BAr4Fꢂ2. Loss of
ꢀ
tBuOꢀ may occur to a greater extent than Rꢀ or NR2
,

4780
E.C. Volpe et al. / Journal of Organometallic Chemistry 692 (2007) 4774–4783
Br
N
R
Me
N
N
R
N
N
N
Ni
MeLi
Ni
+
Me
- LiBr
+
0
"Ni "
R
R
R
R
H
R = H 2a
2b
R = Me
t
KO Bu - KBr
t
t
Bu
Bu
O
-
t
O
H
H
O Bu
or
R
+
R
R
N
N
N
Ni
Ni
Ni
-
X
N
N
N
R
R
R
F
H[BAr
]
4
-
F
[BAr
]
4
R = H 3a
F
+
H[BAr
]
L
L
N
N
4
N
R = Me,
L = 2-tol-py
Ni
Ni
R = Me 3b
1b'
R
R
R
Scheme 2.
perhaps favoring the dissociative mechanism. It may sim-
ply be the competition between a non-productive reductive
elimination path, and an internal abstraction/deprotona-
tion that favors the tert-butoxide as the effective base for
the second cyclometalation.
10 Dq of >40000 cmꢀ1, which is well above fields sup-
ported solely by more electronegative donors (i.e., halides,
O- and N-based ligands) [28]. Despite the rippled geometry
that would tend to attenuate the antibonding character of
dz2 , this orbital is still the HOMO in the system. In order
to determine the preference for the trans-jC–jN configura-
tion of the cyclometalates in 3a, one of these ligands was
‘‘flipped’’ to give an trans-jN–jN, trans-jC–jC geometry,
and geometry optimization gave a structure with very
similar metric parameters. This arrangement was found
to be ꢁ3 kcal/mol above the true geometry, lending cre-
dence to the strong trans-influence of the nickel–aryl
bonds, even though the distortion renders these interac-
tions well away from 180ꢁ (156ꢁ and 161ꢁ).
3.2. Nature of the bonding in Ni{(2-aryl-jC2)pyridine-jN}2
This study purported to take advantage of the strong
fields imparted by metal–aryl bonds, and the low spin, dia-
magnetic character of the compounds containing the
(2-aryl-jC2)pyridine-jN certainly provides supporting
evidence. In addition, a standard DFT calculation was con-
ducted on Ni{(2-phenyl-jC2)pyridine-jN}2 (3a) and
according to the d-orbital splitting diagram in Fig. 5, a
rather strong field is indeed present. While the actual ener-
gies provided by the calculation appear overestimated, they
represent a large splitting between the sigma-antibonding
4. Experimental
4.1. General considerations
dx2ꢀy2 orbital and the essentially non-bonding set of dz2
,
dyz, dxy and dxz orbitals. A rough assumption of square
All manipulations were performed using either glovebox
or high-vacuum techniques. Hydrocarbon and ethereal sol-
planar geometry (10 Dq ꢁ Eðdx2ꢀy2 Þ ꢀ EðdxyÞ) affords a

E.C. Volpe et al. / Journal of Organometallic Chemistry 692 (2007) 4774–4783
4781
solid (43%). Recrystallization of 1a from hot toluene
resulted in formation of bright yellow crystals suitable for
X-ray diffraction. The same procedure was followed for
1b, yielding a yellow-green solid (45%).
4.2.2. (2-Aryl-pyridine)BrNi{(2-aryl-jC2)pyridine-jN}
(aryl = phenyl, 2a; tolyl, 2b)
To a mixture of 1a (1.49 mmol) and KBr (2.00 g,
16.8 mmol) was distilled 20 ml THF at ꢀ78 ꢁC. A dark
orange color developed as the solution was warmed to
23 ꢁC and stirred for 10 min. After stirring for 6 h, the
amber solution was filtered, the salt cake washed several
times with THF, and the filtrate reduced in volume to
5 ml. Ether (15 ml) was added and the mixture was stirred
for 10 min at 23 ꢁC, yielding 400 mg (60%) of bright orange
solid (2a) which was collected by filtration. The same pro-
cedure was followed for 2b except that LiBr was used in
place of KBr. A red-orange solid was isolated in 62% yield.
Anal. Calc. for H21C24BrN2Ni (4b): C, 60.55; H, 4.45; N,
5.88. Found: C, 60.33; H, 4.73; N, 5.59%.
4.2.3. Ni{(2-aryl-jC2)pyridine-jN}2 (aryl = phenyl, 3a;
tolyl, 3b)
To a 250 ml flask charged with placed 2a (2.00 g,
5.45 mmol) and KOtBu (612 mg, 5.45 mmol) and attached
to a swivel frit was distilled 100 ml ether at ꢀ78 ꢁC. The
solution was allowed to warm to 23 ꢁC and stirred for
3 h. Upon warming, the solution attained a dark red
color. The resulting solution was filtered through the frit,
the solid washed five times with ether, and the red solu-
tion concentrated to 10 ml and cooled to ꢀ78 ꢁC. This
was filtered cold to leave a red solid (536 mg, 32%). X-
ray diffraction quality crystals were obtained by cooling
a concentrated solution of the solid in ether to ꢀ35 ꢁC.
The same procedure was followed for 3b with 2b as the
starting material (34%). Anal. Calc. for H20C24N2Ni
(2b): C, 72.95; H, 5.10; N, 7.09. Found: C, 70.55; H,
4.84; N, 6.80%.
Fig. 5. d-Orbital splitting diagram for Ni{(2-phenyl-jC2)pyridine-jN}2
(3a).
vents were dried over and vacuum transferred from sodium
benzophenone ketyl (3-4 ml tetraglyme/L was added to
hydrocarbons). Benzene-d6 was sequentially dried over
˚
sodium and stored over sodium or 4 A molecular sieves.
All glassware was base-washed and oven dried. Nickel(II)tri-
flate was prepared according to literature procedures [19].
Diphenylacetylene (Aldrich) was used as received; 2-phe-
nyl-pyridine and 2-p-tolyl-pyridine (Aldrich) were degassed
and stored under nitrogen; KBr (Mallinckrodt) and LiBr
(Aldrich) were dried for 4 days in a 150 ꢁC oven and stored
under nitrogen. KOtBu (Alfa Aesar) was sublimed and
stored under nitrogen.
4.2.4. [(2-Aryl-jC2)pyridine-jN]Ni[2-(2-(1,2-
diphenylethenyl- jC2)aryl)pyridine-jN] (aryl = phenyl, 4a;
tolyl, 4b)
1H- and 13C{1H}-NMR spectra were obtained using
Mercury-300, Inova-400, and Inova-500 spectrometers.
Combustion analyses were performed by Robertson
Microlit Laboratories, Madison, NJ.
To a bomb reaction vessel of 3a (70.0 mg, 0.191 mmol)
and diphenylacetylene (34.0 mg, 0.191 mmol) was distilled
10 ml benzene at ꢀ78 ꢁC. The vessel was allowed to warm
to 23 ꢁC and then placed in a 35 ꢁC oil bath. This was
slowly heated to 100 ꢁC and remained at this temperature
for 8 h. The resulting dark solution was stripped of solvent,
triturated with ether, and filtered to give 54 mg of orange-
brown solid (52% yield). Covering a solution of 4a in tolu-
ene with hexanes yielded orange-red, X-ray diffraction
quality crystals. Leaving 4a in benzene solution led to the
4.2. Procedures
4.2.1. [Bis(2-aryl-pyridine)Ni{(2-aryl-jC2)pyridine-
jN}]OTf (aryl = phenyl, 1a; tolyl, 1b;
OTf = trifluoromethansulfonate)
A neat mixture of nickel(II)triflate (10.0 g, 28.0 mmol)
and 2-phenyl-pyridine (20.0 g, 118 mmol) was heated at
180 ꢁC in a bomb reaction vessel for 18 h. The resulting
dark brown oil was triturated with THF (60 ml), giving a
brown solution with yellow solid. This mixture was filtered
and washed with THF (3 · 20 ml) to yield 8.5 g of yellow
1
appearance of a new product by H NMR, proposed to
be a conformational isomer (4a0). Keq(4a ! 4a0) = 0.43.
1H NMR (benzene-d6): d 9.05 (d, 1H, J = 6.0), 8.51 (d,
1H, J = 5.0), 8.06 (d, 1H, J = 5.5), 7.58 (d, 1H, J = 7.5),
7.51 (m, 1H), 7.28–6.75 (m, 17H), 6.52 (d, 2H, J = 5.5),

4782
E.C. Volpe et al. / Journal of Organometallic Chemistry 692 (2007) 4774–4783
[12] V. Grushin, N. Herron, D.D. LeCloux, W.J. Marshall, V.A. Petrov,
Y. Wang, J. Chem. Soc., Chem. Commun. (2001) 1494.
[13] (a) S. Lamansky, P. Djurovich, D. Murphy, F. Razzaq-Abdel, H.-E.
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Am. Chem. Soc. 123 (2001) 4304;
6.34 (‘‘t,’’ 1H, J = 6.0), 6.19 (‘‘t,’’ 1H, J = 6.0). The same
procedure was followed for 4b, which was isolated as an
orange-brown solid (50%).
4.3. Calculation details
(b) A. Tamayo, B. Alleyne, P.I. Djurovich, I.T. Lamansky, N.
Ho, R. Bau, M.E. Thompson, J. Am. Chem. Soc. 125 (2003)
7377.
All calculations were done using the GAUSSIAN 03 pro-
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level of theory, employing Becke’s three-parameter hybrid
DFT/HF exchange functional [30] and Perdew and Wang’s
non-local exchange parameter [31]. The CEP-31G effective
core potential basis set was used [32–34]. Atomic coordi-
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cases geometry optimizations were carried out without
symmetry constraints.
[14] R.L. Lagadec, L. Alexandrova, H. Estevez, M. Pfeffer, V. Lauri-
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[16] A.R. Dick, J.W. Kampf, M.S. Sanford, J. Am. Chem. Soc. 127 (2005)
12790.
[17] K.L. Hull, W.Q. Anani, M.S. Sanford, J. Am. Chem. Soc. 128 (2006)
7134, C–H activation of quinoline and 2-phenylpyridine derivatives
by Pd(II), followed by fluorination resulted in a variety of ortho-
fluorinated compounds.
Acknowledgements
[18] K.L. Hull, E.L. Lanni, M.S. Sanford, J. Am. Chem. Soc. 128 (2006)
14047, The Pd(II)/(IV) cycle is employed in the oxidative coupling of
several 2-phenylpyridine derivatives, resulting in a variety of func-
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[19] Y. Inada, Y. Nakano, M. Inamo, M. Nomura, S. Funahashi, Inorg.
Chem. 39 (2000) 4793.
P.T.W. thank the National Science Foundation (CHE-
0415506) and Cornell University for financial support.
The DFT study was done as an assignment for Chem 665
under the direction of Prof. Barry K. Carpenter.
[20] (a) T.R. Younkin, E.F. Connor, J.I. Henderson, S.K. Friedrich,
R.H. Grubbs, D.A. Bansleben, Science 287 (2000) 460;
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[21] K. Tamao, J. Organometal. Chem. 653 (2002) 23.
[22] M. Sanford has produced the compound from 2 equiv of 2-(2-
lithiophenyl)-pyridine and a Ni(II) source; personal communication.
CCDC 645421, 645423, 645422 and 645424 contain the
supplementary crystallographic data for 1a, 2a, 3a and 4a.
These data can be obtained free of charge via http://
www.ccdc.cam.ac.uk/conts/retrieving.html, or from the
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK, fax: (+44) 1223-336-033, or
e-mail: deposit@ccdc.cam.ac.uk. Supplementary data asso-
ciated with this article can be found, in the online version,
at doi:10.1016/j.jorganchem.2007.06.047.
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