was quenched by dropwise addition of concentrated H2SO4.
White precipitate of CaSO4 was filtered off, followed by solvent
evaporation. Extraction of the resulting mixture with CHCl3 and
H2O, followed by solvent evaporation gave pale yellow solid of
5-ethoxycarbonyl-2-hydroxymethylpyridine (0.94 g, 69%). 1H
NMR (CDCl3): d 8.85 (s, 1H), 8.07 (d, J ¼ 8 Hz, 1H), 7.34 (d, J ¼
8 Hz, 1H), 5.26 (s, 1H), 4.65 (s, 2H), 4.20 (q, J ¼ 7 Hz, 2H), 1.21
(t, J ¼ 7 Hz, 3H). MS (ESI) (m/z) (CHCl3): 182.3 (M + H)+, 204.1
(M + Na). Anal. Calc. for C9H11NO3: C, 59.66; H, 6.12; N,
7.73%. Found: C, 60.01; H, 6.35; N, 7.92%.
8.28 (s, 1H), 7.63 (s, 1H), 4.89 (s, 1H), 4.63 (s, 2H), 4.32 (s, 2H),
1.32 (s, 3H). Anal. Calc. for C27.33H34.32Br2N3NiO9.33 (4): C,
42.48; H, 4.48; N, 5.44%. Found: C, 41.82; H, 4.50; N, 5.98%. IR
(KBr) for 4: 1715 (C]O), 1613 (C]Npyr), 1456 (C]Cpyr), 1038
(C–O) cmꢁ1
.
[Ni4(5-ehmp)4(m-OAc)2(OAc)2(H2O)2] (6)
Ni(OAc)2$4H2O (0.25 g, 1 mmol) in MeOH (5 mL) was mixed
with 5-ethoxycarbonyl-2-hydroxymethylpyridine (0.20 g, 1.1
mmol) in MeCN (5 mL). The resulting blue solution was stirred
for 4 h at rt. Blue crystals of the title compound were obtained by
slow diffusion of ether vapour into the resulting solution over 3
weeks (0.85 g, 69%). 1H NMR analysis could not be done for this
complex due to its strong paramagnetic nature. Anal. Calc. for
C44H56N4Ni4O22: C, 43.05; H, 4.60; N, 4.56%. Found: C, 42.36;
H, 4.41; N, 4.40%. IR (KBr): 1715 (C]O), 1610 (C]Npyr), 1421
[Ni(hmpH)2(H2O)2]Br2 (1)
NiBr2 (0.22 g, 1 mmol) in MeOH (5 mL) was mixed with hmpH
(0.24 g, 2.2 mmol) in MeCN (5 mL). The resulting royal-green
solution was stirred for 4 h at rt. Royal-green crystals of the title
compound were obtained by slow diffusion of Et2O vapour into
the resulting solution over 2 weeks (0.35 g, 75%). 1H NMR
(DMSO-d6): d 8.77 (d, J ¼ 5 Hz, 1H), 8.57 (t, J ¼ 8 Hz, 1H), 8.01
(d, J ¼ 8 Hz, 1H), 7.92 (t, J ¼ 6 Hz, 1H), 4.87 (s, 2H), 4.32 (s, 2H).
Anal. Calc. for C12H18Br2N2NiO4: C, 30.49; H, 3.84; N, 5.93%.
Found: C, 30.01; H, 3.59; N, 5.80%. IR (KBr): 1612 (C]Npyr),
(C]Cpyr), 1039 (C–O) cmꢁ1
.
Results and discussion
Synthesis
1442 (C]Cpyr), 1046 (C–O) cmꢁ1
.
The synthesis of 5-ehmpH ligand follows the synthetic proce-
dures reported by Kojima et al. in their preparation of 4-
ethoxycarbonyl-2-hydroxymethylpyridine (4-ehmpH).19 As
shown in Scheme 1, the first step involves the acid catalyzed
esterification of 2,5-pyridinedicarboxylic acid with ethanol to
form 2,5-diethoxycarbonylpyridine. The ester group at 2-posi-
tion is then reduced by ethanolic NaBH4 to give the targeted 5-
ehmpH in 52% overall yield.
The 1H NMR spectra of 5-ehmpH and hmpH in CDCl3 show
that the OH group in 5-ehmpH is more acidic (5.26 ppm for 5-
ehmpH vs. 4.96 ppm for hmpH). For a direct comparison of the
reactivity between the two ligands, we carried out reactions of
hmpH and 5-ehmpH with NiX2 (X ¼ Br and OAc) under similar
reaction conditions. As shown in Scheme 2, reactions of hmpH
and 0.5 eq. NiBr2 in a solvent mixture of MeOH/MeCN gave rise
to an ionic aqua complex [Ni(hmpH)2(H2O)2]Br2 (1) in 75%
yield. Adjusting the metal-to-ligand ratio to 1 : 1 also yielded
complex 1, but in a lower yield (38%). Similar reaction using
hmpH and Ni(OAc)2$4H2O in 2 : 1 and 1 : 1 molar ratio
afforded two stoichiometrically directed neutral complexes
[Ni(hmpH)2(OAc)2] (71% yield)20 and [Ni4(hmp)4(m-OAc)4] (2)
(40% yield) respectively. Further reaction of [Ni(hmpH)2(OAc)2]
with 1 equivalent of Ni(OAc)2$4H2O in a dilute MeCN/MeOH
(1 : 1, v/v) solution yielded [Ni4(hmp)4(m-OAc)2(OAc)2-
(MeOH)2]$1.4(H2O) (ESI) whose structure is closely related
to the reported complex [Ni4(hmp)4(m-OAc)2(OAc)2-
(H2O)2]$2.25(H2O)$0.5(1,4-dioxane).20
[Ni4(hmp)4(m-OAc)4] (2)
Ni(OAc)2$4H2O (0.25 g, 1 mmol) in MeOH (5mL) was mixed
with hmpH (0.12 g, 1.1 mmol) in MeCN (5 mL). The resulting
dark green solution was stirred for 4 h at rt. Dark green crystals
of the title compound were obtained by slow diffusion of Et2O
vapour into the resulting solution over 1 week (0.12 g, 40%). 1H
NMR for this complex cannot be done due to its strong para-
magnetic nature. Anal. Calc. for C16H20N2NiO6: C, 42.54; H,
4.02; N, 6.20%. Found: C, 41.82; H, 4.50; N, 5.98%. IR (KBr):
1570 (very broad peak) (C]Npyr and conjugated C–O in
CH3COO), 1416 (C]Cpyr), 1049 (C–O) cmꢁ1
.
[Ni2(m-Br)2(5-ehmpH)4]Br2$0.33MeCN$0.67H2O (3)
NiBr2 (0.22 g, 1 mmol) in MeOH (5 mL) was mixed with
a mixture of 5-ethoxycarbonyl-2-hydroxymethylpyridine (0.40 g,
2.2 mmol) in MeCN (5 mL). The resulting dark-blue solution
was stirred for 4 h at rt. Dark-blue crystals of the title compound
were obtained by slow diffusion of Et2O vapour into the resulting
solution over 2 weeks (0.42 g, 69%). 1H NMR (DMSO-d6): d 8.98
(s, 1H), 8.34 (s, 1H), 7.66 (s, 1H), 4.89 (s, 1H), 4.64 (s, 2H), 4.34(s,
2H), 1.32 (s, 3H). Anal. Calc. for C36.66H46.33Br4N4.33Ni2O12.66
:
C, 37.09; H, 3.93; N, 5.11%. Found: C, 38.91; H, 4.02; N, 4.76%.
IR (KBr): 1729 (C]O), 1611 (C]Npyr), 1427 (C]Cpyr), 1038
(C–O) cmꢁ1
.
[Ni(5-ehmpH)3]Br2$0.33MeOH (4) and [Ni4(5-
ehmp)4(H2O)4Br4] (5)
NiBr2 (0.22 g, 1 mmol) in MeOH (5mL) was mixed with
a mixture of 5-ehmpH (0.20 g, 1.1 mmol) in MeCN (5 mL). The
resulting dark-blue solution was stirred for 4 h at rt. Blue crystals
of 4 (0.26 g, 26%) and dark-green crystals of 5 (trace amount)
were obtained by slow diffusion of Et2O vapour into the resulting
solution over 2 weeks. 1H NMR (DMSO-d6) for 4: d 8.98 (s, 1H),
Scheme 1 Two-step synthesis of 5-ehmpH.
CrystEngComm, 2011, 13, 2915–2922 | 2917
This journal is ª The Royal Society of Chemistry 2011