Nickel complexes in electroreduction
Russ.Chem.Bull., Int.Ed., Vol. 51, No. 5, May, 2002
797
tion mixtures and prepared compounds were carried out
respectively. Electrolysis was carried out in the potentiostatic
mode at a working electrode potential of –1.52 V vs. Ag/0.01 М
AgNO3 in MeCN. Electricity (27 mA h–1) was passed through
the electrolyte. The amounts of electricity passed for the prepaꢀ
ration of Tol2NiBrbpy, MesNiBrbpy, and Mes2Nibpy were 40.5,
27, and 54 mA h–1, respectively. After completion of electrolyꢀ
sis, the solvent was distilled off, and the products were exꢀ
tracted from the residue into ether. Then the ether was evapoꢀ
rated and the products were dried in a vacuum chamber at 30 °C.
(2,2´ꢀBipyridine)bromobis(2ꢀtolyl)nickel, m.p. 145 °C
(decomp.). Found (%): C, 60.34; H, 5.23; Br, 16.58; N, 6.03;
Ni, 12.37. C24H22NiN2Br. Calculated (%): C, 60.38; H, 4.61;
Br, 16.77; N, 5.87; Ni, 12.37. 1H NMR, δ: 8.56—8.57 (m, 2 H);
8.33—8.37 (d, 2 H); 7.79—7.82, 7.28—7.31 (both m, each 2 H);
7.10—7.14 (m, 6 H, C6H4); 6.92 (m, 2 H, C6H4); 1.91
(s, 6 H, Me).
(2,2´ꢀBipyridine)bromo(mesityl)nickel(II), m.p. 149 °C
(decomp.). Found (%): C, 55.03; H, 4.60; Br, 18.90; N, 6.03;
Ni, 14.16. C19H19NiN2Br. Calculated (%): C, 55.14; H, 4.59;
Br, 19.31; N, 6.77; Ni, 14.18. 1H NMR, δ: 8.56—8.57 (m, 2 H);
8.33—8.37 (d, 2 H); 7.79—7.82, 7.28—7.31 (both m, each 2 H);
6.45 (m, 2 H, C6H2); 2.59 (s, 6 H, Me); 2.17 (s, 3 H, Me).
(2,2´ꢀBipyridine)bis(mesityl)nickel, m.p. 215 °C (decomp.).
Found (%): C, 74.08; H, 6.83; N, 6.28; Ni, 12.37. C28H30NiN2.
Calculated (%): C, 74.19; H, 6.62; N, 6.18; Ni, 12.96. 1H NMR,
δ: 8.56—8.57 (m, 2 H); 8.33—8.37 (d, 2 H); 7.79—7.82,
7.28—7.31 (both m, each 2 H); 6.45 (m, 4 H, C6H2); 2.59 (s,
12 H, Me); 2.17 (s, 6 H, Me).
on
a Chromꢀ5 chromatograph (Czechoslovakia), using
helium as the carrier gas, a heat conductivity detector, a
120×0.3 cm glass column, and 5% SEꢀ30 on Chromaton NꢀAW
(0.125—0.160 mm).
The MeCN solvent was purified by three fractional distillaꢀ
tions over P2O5 with the addition of potassium permanganate.
The concentration of the residual water was 1.8•10–3%. DMF
was purified by threefold vacuum distillation with intermediate
drying over calcined potassium carbonate and molecular sieves.
The salts used as supporting electrolytes were twice recrystalꢀ
lized (Et4NBr, from MeCN and Et4NBF4, from EtOH) and
dried in a vacuum drying chamber for 2 days at 100 °C. The
alcohols were refluxed for 5 h over freshly prepared barium
oxide and then distilled. Benzene and ether were dried by disꢀ
tillation over Na; chloroform was dried by refluxing over P2О5
with subsequent distillation.
The NiBr2bpy and Ni(BF4)2bpy3 complexes were prepared
from the corresponding nickel salt and the required amount of
bpy in EtOH with stirring for 5 h. The precipitates formed were
filtered off and dried in a vacuum chamber at 30 °C for 24 h.
The organic halides (RX) used were chemically pure grade
commercial reagents, which were purified by distillation until
constant physical parameters were attained. The (BuO)3РО
used as the label was purified by heating with metallic Na for
4 h at 100 °C followed by three vacuum distillations.
Processing of the results of voltammetric studies. The error
of the peak potential measurements was ≤10 mV. The number
of electrons transferred from an electrode to the nickel comꢀ
plex was determined by comparing the peak currents of the
compound under study with the current of the first diffusion
peak of benzophenone reduction (1e) under similar conditions.
The apparent rate constants (kapp) for catalyst regeneration
were calculated by the Saveant method2 using the approximatꢀ
ing equation that relates the Ipk/Ipd ratio to kapp (Ipk and Ipd are
the catalytic (kinetic) and diffusion currents, respectively).
The rate constants (k1) for the oxidative addition of organic
halides to σꢀorganonickel complex (mediator) were calculated
by a previously described procedure3 using the calibrating curves
Results and Discussion
Reduction of aliphatic halides by nickel(0) bpy comꢀ
plexes. When RI (R = Pri, Ami) is added to a solution of
NiIIbpyn (n = 1 or 3), the reduction current at the firstꢀ
peak potential (NiII/Ni0) substantially increases, while
the anodic component disappears (Fig. 1).
When the NiBr2bpy is used as the initial complex, a
new peak (Red4) appears in the presence of RI. The peak
occurs at more cathodic potentials (Table 1) and corꢀ
responds apparently to the reduction of RNiIIIbpyn
0
0
for different excess factors CS0/CA (CS is the substrate bulk
0
concentration, CA is the mediator concentration) and the plot
for the Ipk/Ipd ratio vs. logχk, where χk is the kinetic parameter
equal to k1CA0RT/(FV ).
–i/µA
Electrochemical reduction of NiBr2bpy in the presence of
alkyl iodides. The working solution (30 mL) was prepared
by dissolving NiBr2bpy (18.75•10–2 g, 5•10–4 mol), RI
(5•10–3 mol; 0.5 mL of PriI or 0.65 mL of AmiI), and Et4NBF4
(1.085 g, 5•10–3 mol) in DMF. Electrolyses were carried out in
both divided and undivided electrochemical cells. During elecꢀ
trolysis, 0.27 А h–1 of electricity was passed at a controlled
potential (–1.55 V). The reaction mixture was stirred under a
continuous argon stream and analyzed by GLC when the elecꢀ
trolysis had been completed.
Electrochemical reduction of NiBr2bpy in the presence of
aryl bromides. In the case of preparative reduction, a 30ꢀmL
working solutions was prepared by dissolving NiBr2bpy
(0.1875 g, 5•10–4 mol), Et4NBr (1.05 g, 5•10–3 mol), and RBr
(1•10–3 mol; 0.1 mL of PhBr or 0.12 mL of 2ꢀBrTol) in DMF.
The complexes Tol2NiBrbpy, MesNiBrbpy, and Mes2Nibpy
were prepared using 2ꢀBrTol (0.12 mL, 1•10–3 mol), MesBr
(0.075 mL, 5•10–4 mol), or MesBr (0.15 mL, 1•10–3 mol),
Red4
125
3
Red1
100
75
50
25
0
2
Red3
Red2
1
Ox2
Ox3 Ox1
1
2
–E/V
Fig. 1. CV curves for NiIIbpy (0.01 mol L–1) in the absence (1)
and in the presence (2, 3) of PriI in a concentration of 0.01 (2)
and 0.1 mol L–1 (3).