L.S. Morris et al. / Inorganica Chimica Acta 413 (2014) 149–159
151
Packard 8425A Diode Array spectrophotometer using 1 cm quartz
2.3.2. N,N0-dimethyl-N,N0-bis(quinolin-2-ylmethyl)-1,2-
diaminoethane (L4)
cells. Elemental analyses for C, H and N were obtained from the
Microanalysis Lab at the University of Illinois at Urbana-
Champaign.
Quinoline-2-carboxaldehyde (1.100 g, 7.0 mmol) and N,N0-
dimethylethane-1,2-diamine (0.295 g, 3.3 mmol) were added to a
suspension of sodium triacetoxyborohydride (2.700 g, 12.8 mmol)
in anhydrous 1,2-dichloroethane (80 mL). After 24 h of stirring un-
der a dinitrogen atmosphere, the deep red solution was quenched
with saturated NaHCO3 (50 mL). The organic layer was isolated,
and the aqueous fraction extracted with ethyl acetate
(3 ꢀ 50 mL). The organic extracts were dried over anhydrous mag-
nesium sulfate, filtered and solvent was removed under reduced
pressure to yield an orange oil. The oil was dissolved in anhydrous
THF (50 mL), and the solution transferred to a flask containing NaH
(0.250 g, 10.4 mmol). The mixture was stirred for 24 h. Solvent was
removed under reduced pressure, and the product extracted with
pentane (4 ꢀ 50 mL) and dichloromethane (4 ꢀ 50 mL). Removal
of solvent from the extract under reduced pressure yielded pure
product as a yellow-brown oil (0.89 g, 71% yield). 1H NMR (CDCl3,
298 K): d 8.07 (t, J = 8.5 Hz, 4H, Ar), 8.06 (d, J = 8.1 Hz, 2H, Ar), 7.67
(t, J = 8.0 Hz, 2H, Ar), 7.59 (d, J = 7.7 Hz, 2H, Ar) 7.50 (t, J = 7.6 Hz,
2H, Ar) 3.87 (s, 4H, N–CH2–Ar), 2.70 (s, 4H, N–CH2), 2.31 (s, 6H,
N–CH3) ppm. 13C NMR (CDCl3, 298 K): d 160.6 (Ar), 144.8 (Ar),
136.5 (Ar), 129.5 (Ar), 129.3 (Ar), 127.7 (Ar), 127.5 (Ar), 126.2
(Ar), 121.3 (Ar), 65.2 (N–CH2–Ar), 55.8 (N–CH2), 43.3 (N-CH3)
ppm. MS: m/z 370.1 (M+), m/z 185.0 (Ar–CH2–NCH3–CH2+).
Cyclic voltammetry measurements were carried out using a
BASi Epsilon Electrochemical Workstation and the associated Epsi-
lon-EC software. These electrochemical experiments were per-
formed at room temperature in a glovebox under a dinitrogen
atmosphere, using a BASi VC-2 voltammetry cell with a platinum
working electrode (1.6 mm dia.), a platinum wire auxiliary elec-
trode, and a non-aqueous silver/silver ion reference electrode.
The reference electrode contained a silver wire immersed in a solu-
tion of 0.01 M silver nitrate dissolved in a 0.1 M solution of (Bu4-
N)(PF6) in acetonitrile. Scan rates were varied from 200 to
20 mV sꢁ1, with values reported at 50 mV sꢁ1 unless stated other-
wise. The complexes were dissolved in 3 mL of acetonitrile or
dichloromethane containing 0.1 M (Bu4N)(PF6) as supporting elec-
trolyte. To facilitate comparisons to electrochemical measure-
ments made by other research groups, electrochemical
measurements were also run in presence of ferrocene as an inter-
nal standard, and potentials are reported relative to the position of
the ferrocenium/ferrocene couple [32]. In our experimental set-up,
+
values for E°Fc
ranged from 0.104 to 0.109 V (DE1/2 ranges 94–
/Fc
120 mV; ipf/ipr ranges 0.93–0.99) in acetonitrile and from 0.230
to 0.246 V (DE1/2 ranges 160–270 mV; ipf/ipr ranges 0.88–0.99) in
dichloromethane versus the Ag+/Ag reference electrode employed.
2.3.3. N,N0-dimethyl-N,N0-bis(5-chloropyridin-2-ylmethyl)-1,2-
diaminoethane (L5)
2.3. Ligand syntheses
5-Chloro-2-formylpyridine (0.428 g, 3.0 mmol) and N,N0-
dimethylethane-1,2-diamine (0.121 g, 1.4 mmol) were added to a
suspension of sodium triacetoxyborohydride (1.019 g, 4.8 mmol)
in anhydrous 1,2-dichloroethane (20 mL). After 13 hours of stirring
under an inert atmosphere, the reaction was quenched with 5%
aqueous NaHCO3 (20 mL). The organic layer was isolated, and the
aqueous fraction extracted with dichloromethane (3 ꢀ 20 mL).
The organic extracts were dried over anhydrous magnesium
sulfate, filtered and solvent was removed to yield a pale yellow-
white solid. The solid was dissolved in anhydrous THF (20 mL),
and the solution transferred to a flask containing NaH (103 mg,
4.29 mmol). The mixture was stirred for 24 h. Solvent was removed
under reduced pressure, and the product extracted with diethyl
ether (3 ꢀ 10 mL). Solvent removal under reduced pressure yielded
product as a light-yellow crystalline solid (0.402 g, 86% yield). 1H
NMR (CDCl3, 298 K): d 8.45 (d, J = 2.5 Hz, 2H, py), 7.61 (d of d,
J = 2.5 Hz, J = 8.0 Hz, 2H, py), 7.38 (d, J = 8.0 Hz, 2H, py), 3.65 (s,
4H, N–CH2–pyr), 2.61 (s, 4H, N–CH2), 2.26 (s, 6H, N–CH3) ppm.
13C NMR (CDCl3, 298 K): d 158.1 (Cl–C), 148.1 (py), 136.4 (py),
130.5 (py), 124.0 (py), 63.7 (N–CH2–pry), 55.7 (N–CH2), 43.1 (N–
CH3) ppm. MS: m/z 338.1 (M+), m/z 169.0 (Cl–pyr–CH2–N–CH3–
CH2+). m.p. = 54–58 °C.
The ligands N,N0-dimethyl-N,N0-bis(2-pyridylmethyl-1,2-diami-
noethane (bpmen, L1) and N,N0-dimethyl-N,N0-bis((6-methylpyri-
din-2-yl)methyl)-1,2-diaminoethane
(L2)
were
prepared
following published procedures [10,33,35]. L3, L4 and L5 were syn-
thesized by modifications of a previously published protocol for
the synthesis of L1 [33]. The synthesis of (2R,5S)-2,5-dimethyl-
1,4-bis(pyridin-2-ylmethyl)piperazine (L6) was reported else-
where [34].
2.3.1. N,N0-dimethyl-N,N0-bis(6-methoxypyridin-2-ylmethyl)-1,2-
diaminoethane (L3)
6-Methoxy-pyridinecarboxaldehyde (1.382 g, 10.1 mmol) and
N,N0-dimethylethane-1,2-diamine (0.296 g, 3.4 mmol) were added
to
a suspension of sodium triacetoxyborohydride (2.772 g,
13.1 mmol) in anhydrous 1,2-dichloroethane (80 mL). After 18
hours of stirring under a dinitrogen atmosphere, the reaction was
quenched with saturated NaHCO3 (50 mL). The organic layer was
isolated, and the aqueous fraction extracted with ethyl acetate
(3 ꢀ 50 mL). The organic extracts were dried over anhydrous mag-
nesium sulfate, filtered and solvent was removed under reduced
pressure to yield a yellow-brown oil. The oil was dissolved in anhy-
drous THF (50 mL), and the solution transferred to a flask contain-
ing NaH (0.161 g, 6.72 mmol). The mixture was stirred for 24 h.
Solvent was removed under reduced pressure, and the product ex-
tracted with pentane (4 ꢀ 50 mL). Removal of solvent from the ex-
tract under reduced pressure yielded an orange oil. Further
purification by column chromatography using eluent gradients of
ethyl acetate, ethyl acetate/methanol (95/5) and ethyl acetate/
methanol/triethylamine (90/5/5; Rf = 0.60) followed by solvent re-
moval yielded pure product L3 as a yellow oil (0.505 g, 46% yield).
1H NMR (CDCl3, 298 K): d 7.50 (t, J = 8.0 Hz, 2H, py), 6.96 (d,
J = 7.0 Hz, 2H, py), 6.58 (d, J = 8.0 Hz, 2H, py), 3.90 (s, 6H, O–CH3),
3.62 (s, 4H, N–CH2–pyr), 2.66 (s, 4H, N–CH2), 2.32 (s, 6H, N–CH3)
ppm. 13C NMR (CDCl3, 298 K): d 163.7 (O–py), 157.2 (py), 138.8
(py), 115.7 (py), 108.5 (py), 63.8 (py-CH2–N), 55.5 (N–CH2), 53.4
(O–CH3), 41.2 (N–CH3) ppm. MS: m/z 330.1 (M+), m/z 165.0 (Me–
O–pyr–CH2–N–CH3–CH2+).
2.4. Complex syntheses
The complexes [(bpmen)Fe(OTf)2] (C1) and [(L2)Fe(OTf)2] (C2)
were prepared following published procedures [11], and product
identities confirmed by ESI-MS: m/z 470 ([(bpmen)Fe(OTf)]+) and
m/z 503 ([(L2)Fe(OTf)]+).
2.4.1. N,N0-dimethyl-N,N0-bis(6-methoxypyridin-2-ylmethyl)-1,2-
diaminoethane iron(II) bis(triflate) [(L3)Fe(OTf)(CH3CN)]OTf (C3)
L3 (0.355 g, 1.08 mmol) was added to a stirred suspension of
Fe(OTf)2 (0.360 g, 1.02 mmol) in THF (30 mL). The tan-yellow sus-
pension was stirred for 14 hours, after which the tan precipitate
was allowed to settle and the translucent yellow supernatant
was removed by pipet. The remaining solid was washed with
diethyl ether and dried to yield [(L3)Fe(OTf)2] (0.511 g, 69% yield)