Synthesis of binucleating macrocycles and their nickel(II) hydroxo- and cyano-bridged complexes with divalent ions: Anatomical variation of ligand features

Article abstract of DOI:10.1021/ic301506x

The planar NNN-pincer complexes [M^II(pyN₂ ^Me2)(OH)]^1- (M^II = Ni, Cu) fix CO₂ in η¹-OCO₂H complexes; results for the copper system are described. Mn^II, Fe^II, Co^II, and Zn ^II behave differently, forming [M^II(pyN₂ ^Me2)₂]^2- with N₄O₂ coordination. Incorporation of the Ni^II pincer into binucleating macrocycle 2 containing a triamino M^II locus connected by two 1,3-biphenylene groups affords proximal Ni^II and M^II sites for investigation of the synthesis, structure, and reactivity of Ni-X-M bridge units. This ligand structure is taken as a reference for variations in M ^II atoms and binding sites and bridges X = OH^- and CN ^- to produce additional members of the macrocyclic family with improved properties. Macrocycle 2 with a 22-membered ring is shown to bind M^II = Mn, Fe, and Cu with hydroxo bridges. Introduction of the 4-BuⁱO group (macrocycle 3) improves the solubility of neutral complexes such as those with Ni^II-OH-Cu^II and Ni ^II-CN-Fe^II bridges. Syntheses of macrocycle 5 with a 7-Me-[12]aneSN₃ and macrocycle 6 with a 1,8-Me₂-[14] aneN₄ M^II binding site are described together with hydoxo-bridged Ni/Cu and cyano-bridged Ni/Fe complexes. This work was motivated by the presence of a Ni...(HO)-Fe bridge grouping in a reactive state of carbon monoxide dehydrogenase. Attempted decrease in Ni-(OH)-M distances (3.70-3.87 A) to smaller values observed in the enzyme by use of macrocycle 4 having 1,2-biphenylene connectors led to a mononuclear octahedral Ni^II complex. Bridge structural units are summarized, and the structures of 14 macrocyclic complexes including 8 with bridges are described.

Full text of DOI:10.1021/ic301506x

Article
pubs.acs.org/IC
Synthesis of Binucleating Macrocycles and Their Nickel(II) Hydroxo-
and Cyano-Bridged Complexes with Divalent Ions: Anatomical
Variation of Ligand Features
Xiaofeng Zhang,† Deguang Huang,† Yu-Sheng Chen,‡ and R. H. Holm*,†
†Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138, United States
‡Center for Advanced Radiation Source, University of Chicago, c/o APS/ANL, Building 434 D, Argonne, Illinois 60439, United
States
S
* Supporting Information
ABSTRACT: The planar NNN-pincer complexes
[MII(pyN2Me2)(OH)]1− (MII = Ni, Cu) fix CO2 in η1-
OCO2H complexes; results for the copper system are
described. MnII, FeII, CoII, and ZnII behave differently, forming
[MII(pyN2Me2)2]2− with N4O2 coordination. Incorporation of
the NiII pincer into binucleating macrocycle 2 containing a
triamino MII locus connected by two 1,3-biphenylene groups
affords proximal NiII and MII sites for investigation of the
synthesis, structure, and reactivity of Ni−X−M bridge units.
This ligand structure is taken as a reference for variations in
MII atoms and binding sites and bridges X = OH− and CN− to produce additional members of the macrocyclic family with
improved properties. Macrocycle 2 with a 22-membered ring is shown to bind MII = Mn, Fe, and Cu with hydroxo bridges.
Introduction of the 4-BuiO group (macrocycle 3) improves the solubility of neutral complexes such as those with NiII−OH−CuII
and NiII−CN−FeII bridges. Syntheses of macrocycle 5 with a 7-Me-[12]aneSN3 and macrocycle 6 with a 1,8-Me2-[14]aneN4 MII
binding site are described together with hydoxo-bridged Ni/Cu and cyano-bridged Ni/Fe complexes. This work was motivated
by the presence of a Ni···(HO)−Fe bridge grouping in a reactive state of carbon monoxide dehydrogenase. Attempted decrease
in Ni−(OH)−M distances (3.70−3.87 Å) to smaller values observed in the enzyme by use of macrocycle 4 having 1,2-
biphenylene connectors led to a mononuclear octahedral NiII complex. Bridge structural units are summarized, and the structures
of 14 macrocyclic complexes including 8 with bridges are described.
replica of the binuclear NiII site, may be prepared and its
INTRODUCTION
■
reactivity examined separately.9,10 This complex reacts
The NiII···(HO)−FeII bridge grouping is a component of the
−
quantitatively and reversibly with CO2 to afford an η1-HCO3
catalytic site (C-cluster) in the Ni−Fe−S enzyme carbon
product in an exceptionally fast second-order reaction in DMF
monooxide dehydrogenase which catalyzes the reaction CO +
(k2 ≈ 106 M−1 s−1).
298
H2O ⇌ CO2 + 2H+ + 2e−.1−4 The site consists of a NiFe3S4
cubanoid cluster to which is bound via a μ3-S atom an exo FeII
atom that is part of the bridge. While the cluster portion has
been chemically synthesized,5,6 attainment of the complete
assembly NiFe3S4−Feexo has proven elusive. Consequently,
more recent research has concentrated on a different approach
which departs from a Ni−Fe−S support platform but allows an
investigation of the chemistry of NiII and FeII in combination.
Suitable binucleating macrocyclic ligands allow specific binding
of the two metals in bridging juxtaposition at a known distance
and variation of the atom or bridging group between them. The
dianion of 2 in Figure 1 is a first approach to the problem.7 The
upper NNN site, obviously related to the pincer ligand 1 with
two deprotonated amide groups, binds NiII, while the lower
triamine portion binds FeII. Various bridges including hydroxo,
formate, and cyano may be interposed between the two metal
sites. A further advantage of the system is that the terminal
hydroxo pincer complex [Ni(pyN2Me2)(OH)]1−,8 a virtual
We are engaged in producing a new set of macrocycles and
their complexes that sustain proximal Ni···M binding with
bridge formation and retain the pincer binding site but with
otherwise variable and/or improved properties. With attention
to the reference macrocyclic complex in Figure 2 which
includes hydroxo as a representative bridging group, we note
four regions of potential modification. Region I is the site of
atom M and associated ligands. Various atoms M and ligands,
bridging and terminal (if present), in this region are subject to
incorporation dependent on several factors. That of primary
interest in ligand design is the binding functionality present in
region IV. Introduction of a suitable substituent R in region II
would enhance the solubility of neutral complexes such as
[Ni(μ2-OH)FeCl(⊂22-pyN2dienMe3)] (see below), allowing
Received: July 11, 2012
Published: October 3, 2012
© 2012 American Chemical Society
11017
dx.doi.org/10.1021/ic301506x | Inorg. Chem. 2012, 51, 11017−11029

Inorganic Chemistry
Article
Figure 1. Depictions of NNN-pincer ligand 1 and binucleating macrocycles 2−6 with ligand abbreviations. Subscript to the symbol ⊂ refers to the
number of atoms in the macrocyclic ring.
Chart 1. Designation of Ligands and Complexes
Figure 2. Binuclear complex [Ni(OH)ML(⊂22-pyN2dienMe3)] with
regions I−IV designated for possible structural alteration.
solution studies. Adjustment of the spacer group between
binding sites in region III could control the Ni···M separation.
Variation of the ligand framework in region IV (M site) might
lead to tighter binding of atom M, a problem we encountered
on occasion in this work. Lastly, given the huge number of
known binucleating (or “compartmental”) ligands,11−13 it
needs be recognized that the binuclear complex in Figure 2
and any alterations of it constitute a small subset of possibilities
for examination of the chemistry of proximally bound metals. In
this report, we demonstrate certain modifications in regions I−
IV by synthesis and structure determinations. As will be seen,
all modifications are illustrative of further possibilities and all
but that in region III allow bridge formation.
min. The light green solution was treated slowly with Et4NOH (25%
EXPERIMENTAL SECTION
in methanol, 353 mg, 0.60 mmol), and the mixture was stirred for 12 h
to form a purple-blue solution and a small amount of sticky precipitate
which was removed by filtration. Ether (18 mL) was added to the
filtrate, resulting in a cloudy suspension from which crystals separated
over 3 h. Product was collected as a purple-blue crystalline solid (85
mg, 73%). Absorption spectrum (DMF): λmax (εM) 594 (br, 270) nm.
IR (KBr): vOH 3608 cm−1. Anal. Calcd for C31H42CuN4O3: C, 63.95;
H, 7.27; N, 9.62. Found: C, 63.85; H, 7.23; N, 9.52.
■
Preparation of Compounds. Unless otherwise stated, all
reactions and manipulations were performed under a dinitrogen
atmosphere. Volume reduction and drying steps were performed in
vacuo; filtrations were through Celite. Commercial-grade chemicals
were used without further purification. Acetonitrile, THF, and diethyl
ether were purified by an Innovative Technology or MBraun solvent
purification system. DMF was freshly distilled from CaH2 and dried
over molecular sieves for 24 h. Compounds were identified by
combinations of elemental analyses, spectroscopic measurements
(mainly 1H NMR where possible), and X-ray structure determination.
Representative compounds were analyzed (Midwest Microlab, Inc.).
The structures of the NNN-pincer ligand 1 and binucleating
macrocycles 2−6 are provided in Figure 1.
b. (Et4N)[Cu(pyN2Me2)(HCO3)]. A solution of (Et4N)[Cu(pyN2Me2)-
(OH)] (29 mg, 0.050 mmol) in DMF (3 mL) was bubbled with
carbon dioxide for 15 min. Diffusion of ether into the blue solution
afforded the product as blue crystals (25 mg, 80%). Absorption
spectrum (DMF): λmax (εM) 610 (br, 320) nm.
Me2
c. (Et4N)2[Fe(pyN2Me2)2]. A solution of H2pyN2
(112 mg, 0.30
mmol) in DMF (3 mL) was treated with NaH (15 mg, 0.60 mmol)
slowly to give a light yellow solution, to which was added Fe(OTf)2
(106 mg, 0.30 mmol). The dark purple reaction mixture was stirred for
2 h and filtered. Ether (20 mL) was added to the filtrate, causing
separation of a purple oil, which was treated with ether to form a
In the sections that follow, ligands and complexes are numerically
designated according to Chart 1 and Figures 1, 3−7, 9, 11, 12, and 14.
Me2
1. Complexes Derived from the Pincer Ligand H2pyN2
.
a. (Et4N)[Cu(pyN2Me2)(OH)]. H2pyN2
(75 mg, 0.20 mmol)7 and
Me2
Cu(OTf)2 (72 mg, 0.20 mmol) were stirred in DMF (3 mL) for 20
11018
dx.doi.org/10.1021/ic301506x | Inorg. Chem. 2012, 51, 11017−11029

Inorganic Chemistry
Article
Figure 3. Synthetic scheme for macrocycle 5 via intermediates 5a−e.
precipitate, which was removed by filtration through Celite. Ether (15
mL) was added to the filtrate, causing formation of a red oil. Addition
of ether to the oil formed a red solid, which was washed with THF (5
mL) and dissolved in DMF (2 mL). Diffusion of ether into the
solution afforded the product as violet block-like crystals (53 mg,
50%). Anal. Calcd for C62H82CoN8O4: C, 70.10; H, 7.78; N, 10.55.
Found: C, 69.92; H, 7.92; N, 10.66.
e. (Et4N)2[Mn(pyN2Me2)2]. The preceding method was followed with
use of Mn(OTf)2 (71 mg, 0.20 mmol) and gave the product as a
yellow crystalline solid (70 mg, 66%). Anal. Calcd for C62H82MnN8O4:
C, 70.36; H, 7.81; N, 10.59. Found: C, 70.04; H, 7.87; N, 10.46.
f. (Et4N)2[Zn(pyN2Me2)2]. The preceding method was followed with
use of Zn(OTf)2 (73 mg, 0.20 mmol) and gave the product as a yellow
crystalline solid (77 mg, 72%). Anal. Calcd for C62H82N8O4Zn: C,
69.68; H, 7.73; N, 10.48. Found: C, 69.51; H, 7.91; N, 10.52
2. Binuclear Complexes Derived from Macrocycle ⊂22-
H2pyN2dienMe3. Macrocycle 2 and complexes 13 and 15 were
prepared as described.7
a. [Ni(μ2-OH)MnCl(⊂22-pyN2dienMe3)]. To a solution of (Et4N)-
[Ni(OH)(⊂22-pyN2dienMe3)] (28 mg, 0.040 mmol) in DMF (2 mL)
was added a solution of MnCl2 (5.0 mg, 0.040 mmol) in DMF (1 mL).
The reaction mixture was stirred for 1 h and filtered, and ether was
diffused into the deep red filtrate for 2 days. The solid was washed
with acetonitrile (3 mL) and dried to afford the product as red crystals
(11 mg, 42%). Anal. Calcd for C28H33ClMnN6NiO3: C, 51.68; H,
5.11; N, 12.92. Found: C, 51.62; H, 5.18; N, 13.01.
b. [Ni(μ2-OH)CuCl(⊂22-pyN2dienMe3)]. To a solution of (Et4N)[Ni-
(OH)(⊂22-pyN2dienMe3)] (21 mg, 0.030 mmol) in DMF (2 mL) was
added a solution of CuCl2 (4.1 mg, 0.030 mmol) in DMF (1 mL). The
reaction mixture was stirred for 30 min and filtered, and ether was
diffused into the brown-red filtrate to form a solid. This material was
washed with acetonitrile (3 mL) and dried in vacuo to give the product
as orange-yellow crystals (8.0 mg, 41%).
Figure 4. Synthesic scheme for macrocycle 6 via intermediates 6a−c.
3. Complexes Derived from Macrocycle ⊂22-4-BuiO-
H2pyN2dienMe3. The five-step synthesis of macrocycle 3, obtained
in 16% overall yield based on chelidamic acid, is given elsewhere.14
a. (Me4N)[Ni(OH)(⊂22-4-BuiO-pyN2dienMe3)]. Macrocycle 3 (56 mg,
0.10 mmol) and Ni(OTf)2 (37 mg, 0.10 mmol) were mixed and
stirred in DMF (2 mL) for 15 min to give a light yellow suspension.
Me4NOH (25% in methanol, 55 mg, 0.015 mmol) was added, and the
mixture was stirred for 1 h, forming a red suspension. A second equal
portion of Me4NOH was added, and the mixture was stirred for 4 h to
give a deep red solution with some sticky precipitate, which was
removed by filtration. Addition of ether (40 mL) to the filtrate
deposited an orange-red material, which was dissolved in DMF/THF
(6 mL, 1/5 v/v) to give a red solution. Ether (5 mL) was added, and a
white solid was filtered off. A second portion of ether (10 mL) was
purple solid. The solid was dissolved in THF (5 mL), and solution was
filtered through Celite to give a deep purple filtrate solution. A
solution of Et4NOH (25% in methanol, 176 mg, 0.30 mmol) was
slowly added, and the mixture was stirred for 2 h and filtered to
remove a sticky white precipitate. Ether (15 mL) was added to the
filtrate to form a blue solid, which was dissolved in DMF (2 mL).
Addition of ether by diffusion caused separation of the product as
blue-red crystals (71 mg, 45%). Anal. Calcd for C62H82FeN8O4: C,
70.30; H, 7.80; N, 10.58. Found: C, 70.19; H, 7.83; N, 10.66.
Me2
d. (Et4N)2[Co(pyN2Me2)2]. A mixture of H2pyN2
(75 mg, 0.20
mmol) and Co(OTf)2 (71 mg, 0.20 mmol) in DMF was stirred for 30
min. The pink-red solution was slowly treated with a solution of
Et4NOH (25% in methanol, 353 mg, 0.60 mmol), and the reaction
mixture was stirred for 6 h to form a red solution and a sticky
11019
dx.doi.org/10.1021/ic301506x | Inorg. Chem. 2012, 51, 11017−11029

Inorganic Chemistry
Article
Figure 6. Preparation of a set of O,N-bound complexes
[MII(pyN2Me2)2]2− (M = Mn, Fe, Co, Zn); ligand deprotonation
reagents are indicated. Structure of [CoII(pyN2Me2)2]2− is shown with
atom-labeling scheme. Crystallographically imposed C2 axis relates
atoms X(n) and X(nA). Selected bond distances (Å) and angles (deg):
Co−N(1) 2.134(3), Co−N(2) 2.028(3), Co−O(2) 2.226(2), N(1)−
Co−N(2) 77.1(1), N(2)−Co−O(2) 74.3(1), O(2)−Co−O(2A)
96.4(1), N(2)−Co−N(2A) 145.6(2), O(2)−Co−N(1) 149.4(1),
N(2)−Co−N(1A) 127.1(1). Other angles at the Co atom are in the
83−98° range.
Figure 5. UV−vis spectrum of the reaction of carbon dioxide with
[Cu(pyN2Me2)(OH)]1− after 15 min bubbling in DMF solution at
room temperature, and structures of the reactant and product
complexes. Selected bond distances (Å) and angles (deg): [Cu-
(pyN2Me2)(OH)]1− Cu−O(2) 1.845(3), Cu−N(1) 2.004(2), Cu−
N(2) 1.924, N(1)−Cu−N(2) 80.01(6), N(2)−Cu−O(2) 175.5(1);
[Cu(pyN2Me2)(HCO3)]1− Cu−O(3) 1.946(2), Cu−N(1) 2.005(3),
Cu−N(2) 1.912(3), Cu−N(3) 1.996(2), Cu···O(4) 2.542(3), N(2)−
Cu−O(3) 178.0(1). In this and succeeding X-ray structures, 50%
probability ellipsoids are depicted.
added, and the solution was stored overnight, resulting in separation of
a red crystalline solid (31 mg, 44%). Absorption spectrum (DMF):
λmax (εM) 400 (5460) nm. IR (KBr): vOH 3608 cm−1. 1H NMR (DMF-
d7): δ −4.71 (s, 1), 1.03 (d, 6), 2.05 (s, 3), 2.10 (m, 1), 2.18 (m, 4),
2.26 (m, 4), 2.28 (s, 6), 3.35 (s, 4), 3.99 (d, 2), 6.79 (d, 2), 7.04 (m,
4), 7.18 (s, 2), 7.25 (d, 2). Anal. Calcd for C36H53N7NiO4: C, 61.20;
H, 7.56; N, 13.88. Found: C, 60.29, H, 7.70; N, 14.02.
b. [Ni(μ2-OH)CuCl(⊂22-4-BuiO-pyN2dienMe3)]·DMF. CuCl2 (4.0 mg,
0.030 mmol) in DMF (0.5 mL) was added dropwise to a solution of
(Me4N)[Ni(OH)(⊂22-4-BuiO-pyN2-dienMe3)] (21 mg, 0.030 mmol)
in DMF (2 mL). The reaction mixture was stirred for 1 h and filtered.
Diffusion of ether into the filtrate deposited the product as tea-brown
block-like crystals (15 mg, 62%). IR (KBr): vOH 3557 cm−1. Anal.
Calcd for C35H48ClCuN7NiO5: C, 52.25; H, 6.01; N, 12.19. Found: C,
51.91; H, 5.90; N, 11.97.
Figure 7. Formation of binuclear hydroxo-bridged complexes 14−16
from NiII−OH complex 13 based on macrocycle 2.
c. [Ni(μ2-CN)FeCl(⊂22-4-BuiO-pyN2dienMe3)]·1/2Et2O. A mixture of
(Me4N)[Ni(OH)(⊂22-4-BuiO-pyN2dienMe3)] (35 mg, 0.050 mmol)
and Et4NCN (9.4 mg, 0.060 mmol) was stirred in DMF (3 mL) for 3
h and filtered. FeCl2 (6.3 mg, 0.050 mmol) in DMF (0.5 mL) was
added to the orange-red filtrate. The mixture was stirred for 1 h and
filtered, and ether (18 mL) was added slowly to the filtrate to deposit a
white precipitate, which was filtered off. Ether was diffused into the
filtrate, causing separation of a red crystalline solid. This material was
dissolved in 4 mL of DMF/propionitrile (1:1 v/v) and diffused with
11020
dx.doi.org/10.1021/ic301506x | Inorg. Chem. 2012, 51, 11017−11029

Inorganic Chemistry
Article
Figure 8. Structures of complexes containing the NiII−OH−MII bridge unit (MII = Mn (14), Fe (15), Cu (16)). Complex 157 is included for
comparison.
portion of Et4NOH was added, and the mixture was stirred for 10 h to
give a green solution and a white precipitate, which was removed by
filtration. Ether (20 mL) was added to the filtrate, forming a green oil.
The mixture was allowed to stand overnight, yielding the product as a
green crystalline solid (43 mg, 40%).
Me
5. Macrocylic Ligand ⊂-H2pyN2SN3
Synthesis of macrocycle 5 (11% overall yield based on 5a) is given
in Figure 3.
and Complexes.
a. Intermediates 5a−e. To a mixture of thioglycollic acid (21.0 g,
0.140 mol) in thionyl chloride (45 mL) was added 0.15 mL of dry
DMF as a catalyst. The mixture was stirred for 4 h at 45 °C to give a
clear orange solution. Excess thionyl chloride was partially removed to
leave a red-black solution from which pure 5a15 was obtained by
distillation (130°/3.0 mm Hg). Solutions of 5a (2.80 g, 15.0 mmol)
and 3-N-methylaminopentane-1,5-diamine (3.51 g, 30.0 mmol), each
in dry toluene (40 mL), were added simultaneously to chloroform
(500 mL) by syringe pump over 16 h. After removal of a white
precipitate and evaporation of the filtrate, the yellow-orange solid was
purified on a silica column eluted with CH2Cl2/MeOH/NH4OH
1
(300/80/1 v/v) to yield 5b as a white solid (1.80 g, 52%). H NMR
(CDCl3): δ 2.30 (s, 3), 2.48 (t, 4), 3.24 (s, 4), 3.30 (q, 4), 7.26 (s, 2).
ESMS: m/z 232 (M + H)+.
To a suspension of 5b (1.80 g, 7.8 mmol) in dry THF (20 mL) was
added a solution of Me2SBH3 (16 mL, 32 mmol). The mixture was
refluxed at 75 °C for 5 h. Methanol was added to the white slurry, the
mixture was stirred for 1 h, and the solvent was removed to leave a
foam-like residue. This material was refluxed in 6 M aqueous HCl (35
mL) for 3 h, and solvent was removed to give a white solid, which was
treated with 2 M aqueous NaOH until pH ≈ 14 was reached. The
mixture was extracted with chloroform (5 × 70 mL), the combined
organic layers were dried (MgSO4), and solvent was removed to leave
1
5c as a colorless crystalline solid (0.95 g, 60%). H NMR (CDCl3): δ
2.17 (s, 3), 2.37 (t, 4), 2.55 (q, 8), 2.63 (t, 4), 2.68 (br, s, 2).
A mixture of N-(tert-butoxycarbonyl)-3-bromomethylaniline16 (1.35
g, 4.72 mmol), 5c (0.48 g, 2.36 mmol), K2CO3 (0.65 g, 4.72 mmol),
and KI (0.39 g, 2.36 mmol) was refluxed in acetonitrile (200 mL) for
36 h, the mixture was filtered, and the solvent was evaporated to leave
a light yellow solid residue. This material was dissolved in
dichloromethane (100 mL), the solution was filtered, and the solvent
was removed. The residue was purified on a silica column eluted with
CH2Cl2/MeOH/NH4OH (70/8/5 v/v) to yield 5d as a white foam-
like solid (1.04 g, 72%). 1H NMR (CDCl3): δ 1.53 (s, 18), 2.11 (s, 3),
2.45 (s, 4), 2.54 (s, 4), 2.76 (t, 4), 2.98 (t, 4), 3.59 (s, 4), 6.67 (s, 2),
7.05 (d, 2), 7.23 (t, 2), 7.30 (d, 2), 7.35 (s, 2). ESMS: m/z 614 (M +
H)+.
Figure 9. Formation of solubilized binuclear-bridged complexes 18
and 19 from NiII−OH complex 17 based on macrocycle 3.
ether to deposit the product as orange-red crystals (17 mg, 44%). IR:
vCN 2117 cm−1. Anal. Calcd for C35H45ClFeN7NiO3.5: C, 54.61; H,
5.89; N, 12.74. Found: C, 54.59; H, 5.85; N, 12.80.
4. NiII Complex Derived from Macrocycle
⊂ -
H2pyN2dienMe3. Preparation of macrocycle 4 (25% overall yi2e0ld
based on o-nitrobenzaldehyde) is described elswhere.14
a. [Ni(⊂20-pyN2dienMe3)]. Macrocycle 4 (97 mg, 0.20 mmol) and
Ni(OTf)2 (71 mg, 0.20 mmol) were reacted in DMF/THF (5 mL, 3:2
v/v) for 1 h to give a light yellow suspension. Et4NOH (25% in
methanol, 118 mg, 0.20 mmol) was added, and the reaction mixture
was stirred for 1 h, forming a light green suspension. A second equal
To a solution of 5d (1.04 g, 1.70 mmol) in dichloromethane (10
mL) was added trifluoroacetic acid (3 mL); the mixture was stirred for
11021
dx.doi.org/10.1021/ic301506x | Inorg. Chem. 2012, 51, 11017−11029

Inorganic Chemistry
Article
Figure 10. Structures of NiII−OH complex 17 and binuclear complexes containing the bridge units NiII−OH−CuII (18) and NiII−CN−FeII (19).
Ni−N3 site of 17: Ni−N(1) 1.912(2) Å, Ni−N(3) 1.916(2) Å, Ni−N(2) 1.828(2) Å, N(1)−Ni−N(2) 82.5(1)°, N(2)−Ni−N(3) 82.2(1)°, N(1)−
Ni−N(3) 163.8(1)°. Cu site of 18: Cu−N(5) 2.033(3) Å, Cu−N(6) 2.130(3) Å, Cu−Cl(1) 2.239(1) Å, Cu···N(4) 2.702(3) Å.
2 h. The orange-red mixture was added slowly to a 2 M aqueous
NaOH solution (50 mL), stirred for 20 min, and diluted with water
(20 mL). The organic layer was collected, and the aqueous layer was
extracted with dichloromethane. The combined organic layers were
washed with brine (2 × 70 mL) and dried (MgSO4). Solvent was
7.27 (d, 2), 7.52 (d, 2), 7.67 (s, 2), 8.05 (t, 1). Anal. Calcd for
C34H47N7NiO3S: C, 58.97; H, 6.84; N, 14.16. Found: C, 59.02; H,
6.79; N, 13.97.
[Ni(OH2)(⊂-pyN2SN3Me)]. A solution of (Me4N)[Ni(OH)(⊂-
pyN2SN3Me)] (21 mg, 0.03 mmol) was added to solid NH4HCO3
(4.7 mg, 0.06 mmol). The mixture was stirred for 3 h and filtered. The
filtrate was diffused with ether to precipitate some colorless crystals,
which were removed by filtration. The red filtrate was reduced to a red
oily solid, which was dissolved in THF (0.5 mL). The solution was
diffused with ether to deposit a red microcystalline product (8.0 mg,
43%), which was identified by X-ray structure determination.
[NiClFeCl(⊂-pyN2SN3Me)]. FeCl2 (3.8 mg, 0.030 mmol) in DMF
(0.5 mL) was added dropwise to a red solution of (Me4N)[Ni(OH)-
(⊂-pyN2SN3Me)] (21 mg, 0.030 mmol) in DMF (3 mL). The reaction
mixture was stirred for 2 h and filtered to remove some black
precipitate. A second equal portion of FeCl2 in DMF (0.5 mL) was
added slowly to the red-brown filtrate. The mixture was stirred for 2 h
and filtered. Diffusion of ether into the filtrate afforded the red
crystalline product (10 mg, 41%).
1
removed to afford 5e as a pale yellow oil (0.674 g, 96%). H NMR
(CDCl3): δ 2.12 (s, 3), 2.48 (s, 4), 2.55 (s, 4), 2.76 (t, 4), 2.96 (t, 4),
3.53 (s, 4), 3.66 (s, 4), 6.56 (d, 2), 6.71−6.74 (m, 4), 7.08 (t, 2).
b. Macrocycle ⊂-H2pyN2SN3Me. To a solution of Et3N (6.0 mL) in
dry THF (350 mL) was added simultaneously solutions of pyridine-
2,6-dicarbonylchloride (0.365 g, 1.79 mmol) in THF (50 mL) and 5e
(0.674 g, 1.63 mmol) in THF/acetonitrile (50 mL, 4/1 v/v) by syringe
pump over 20 h. The mixture was filtered, and solvent was evaporated
to give a yellow-brown solid, which was dissolved in dichloromethane
(7 mL). Upon standing for several hours, the solution deposited a
colorless crystalline solid which was collected and washed with
dichloromethane (2 × 3 mL). The solid was dissolved in dichloro-
methane/methanol (8/2 mL) and dried to give the macrocyclic 5 as a
1
white powder (0.47 g, 53%). H NMR (Me2SO-d6): δ 2.53 (m, 2),
2.65−2.73 (m, 7), 2.75−2.87 (m, 6), 3.21 (m, 2), 3.64 (s, 4), 3.70 (m,
2), 7.05 (d, 2), 7.37 (t, 2), 7.53 (s, 2), 8.32 (t, 1), 8.39 (m, 4), 11.14 (s,
2). ESMS: m/z 545 (M + H)+.
[Ni(μ2-CN)FeCl(⊂-pyN2SN3Me)]·Me2SO. A mixture of (Me4N)[Ni-
(OH)(⊂-pyN2SN3Me)] (21 mg, 0.030 mmol) and Et4NCN (5.6 mg,
0.036 mmol) was stirred in DMF (2 mL) for 5 h and filtered. Ether
was added to deposit a red solid, which was washed with THF/ether
(3 mL, 2:1 v/v) and dissolved in DMF (1 mL). FeCl2 (3.8 mg, 0.030
mmol) in DMF (0.5 mL) was added slowly to the red solution, which
was stirred for 1 h and filtered. Ether (20 mL) was added to the filtrate
to obtain a red solid, which was washed with ether (1 mL) and
acetonitrile (1 mL). The red solid was dissolved in Me2SO (1 mL),
and the solution was layered with ethyl acetate to cause separation of a
red crystalline solid and a white precipitate. The red solid was
manually separated and dried to yield the product (11 mg, 46%). IR:
vCN 2137 cm−1.
c. Complexes. (Me4N)[Ni(OH)(⊂-pyN2SN3Me)]. Macrocycle 5 (82
mg, 0.15 mmol) and Ni(OTf)2 (54 mg, 0.15 mmol) were stirred in
DMF (3 mL) for 12 h to give a light yellow suspension. Me4NOH
(25% in methanol, 82 mg, 0.225 mmol) was added dropwise, and the
mixture was stirred for 4 h. An equal portion of Me4NOH was added,
and the mixture was stirred for 4 h and filtered to remove a sticky
precipitate. Ether was added to the filtrate to deposit an orange-red
solid that was collected and washed with ether (3 mL). The solid was
dissolved in DMF (7 mL) and filtered. The red filtrate was diffused
with ether to cause separation of a red solid together with some white
precipitate. The red product was separated manually and dried (52 mg,
50%). Absorption spectrum (DMF): λmax (εM) 412 (5500), 488 (sh,
1800) nm. IR (KBr): vOH 3601 cm−1. 1H NMR (DMF-d7): δ −4.20 (s,
1), 1.94 (m, 2) 2.12 (s, 3), 2.41 (m, 2), 2.56 (m, 4), 2.80 (m, 2), 2.87
(m, 2), 2.98 (m, 4), 3.48 (d, 2), 3.59 (d, 2), 6.73 (d, 2), 6.98 (t, 2),
[Ni(μ2-OH)Cu(⊂-pyN2SN3Me)](ClO4). A solution of (Me4N)[Ni-
(OH)(⊂-pyN2SN3Me)] (21 mg, 0.030 mmol) in DMF (2.5 mL) was
added quickly to a solution of Cu(ClO4)2·6H2O (11 mg, 0.030 mmol)
in DMF (0.5 mL). The mixture was stirred for 10 min and filtered.
The filtrate was diffused with ether, causing the product to separate as
11022
dx.doi.org/10.1021/ic301506x | Inorg. Chem. 2012, 51, 11017−11029

Inorganic Chemistry
Article
Figure 12. Formation of mononuclear NiII−OH complex 21, aquo
complex 22, unbridged binuclear Ni/Fe complex 23, and bridged
binuclear complexes 24 and 25 based on macrocycle 5.
Figure 11. Formation of the NiII complex 20 derived from macrocycle
4, and structures of the macrocycle and complex.
give 6b as a pale yellow solid (0.96 g, 32%). 1H NMR (CDCl3): δ 1.64
(m, 4), 2.11 (s, 6), 2.46 (m, 8), 2.58 (m, 8), 3.61 (s, 4), 7.27 (d, 2),
7.41 (m, 6), 7.78 (m, 4), 7.94 (m, 4). ESMS: m/z 699 (M + H)+, 350
(M + 2H)2+/2.
a black-green crystalline solid (14 mg, 60%). IR (KBr): vOH 3516 cm−1.
Anal. Calcd for C30H35ClCuN6NiO7S: C, 46.11; H, 4.51; N, 10.76.
To a hot solution of 6b (0.96 g, 1.38 mmol) in 30 mL of THF and
10 mL of ethanol was added N2H4·H2O (1.0 g, 20 mmol). The
mixture was refluxed at 80 °C for 15 h to give a white suspension.
Ethanol (50 mL) was added, and the mixture was filtered. Solvent
removal gave a pale yellow oil. The product was purified on a silica gel
column eluted with dichloromethane/methanol/NH4OH (15/3/2 v/
v) to give 6c as a colorless oily solid (453 mg, 75%). 1H NMR
(CDCl3): δ 1.67 (m, 4), 2.14 (s, 6), 2.49 (m, 8), 2.57 (m, 8), 3.46 (s,
4), 3.67 (br s, 4), 6.55 (d, 2), 6.70 (m, 4), 7.06 (t, 2). ESMS: m/z 439
(M + H)+.
Found: C, 45.93; H, 4.47; N, 10.54.
Me2
6. Macrocyclic Ligand ⊂-H2pyN2N4
synthetic route to macrocycle 6 (10% overall yield based on 6a) is
provided in Figure 4.
and Complexes. The
a. Intermediates 6a−c. Compound 6a has been previously used,
but no preparation was reported.17 PPh3 (4.72 g, 18.0 mmol) was
added to a solution of 2-[(3-hydroxymethyl)phenyl]isoindoline-1,3-
dione18 (3.04 g, 12.0 mmol) in dichoromethane (30 mL). The mixture
was cooled to −5 °C, and CBr4 (5.97 g, 18.0 mmol) in
dichloromethane (30 mL) was added dropwise. The mixture was
stirred at −5 °C for 2 h and allowed to warm to room temperature.
Ethyl acetate (150 mL) was added, and the solution was washed with
brine (4 × 70 mL). The organic layers were combined and dried over
MgSO4, and solvent was removed to leave a light yellow-brown
residue. The crude product was purified on a silica column eluted with
dichloromethane/hexanes (1:1 v/v) to give 6a as a white solid (3.21 g,
85%). 1H NMR (CDCl3): δ 4.54 (s, 2), 7.42 (m, 2), 7.50 (m, 2), 7.81
(m, 2), 7.96 (m, 2). ESMS: m/z 338 (M + Na)+.
Compound 6a (2.71 g, 8.60 mmol), 1,8-dimethyl-1,4,8,11-
tetraazacyclotetradecane (1,8-Me2-[14]aneN4) (0.98 g, 4.3 mmol),
K2CO3 (1.19 g, 8.60 mmol), and KI (0.71 g, 4.3 mmol) were refluxed
in acetonitrile (300 mL) for 60 h. The mixture was filtered, and solvent
was removed to leave a light brown solid. This material was purified on
a silica column eluted with dichloromethane/methanol/NH4OH to
b. Macrocycle ⊂-H2pyN2N4Me2. To a solution of Et3N (5.5 mL) in
dry THF (300 mL) was added simultaneously solutions of pyridine-
2,6-dicarbonylchloride (0.245 g, 1.20 mmol) in THF (40 mL) and 6c
(0.44 g, 1.00 mmol) in THF/acetonitrile (40 mL, 3:1 v/v) by syringe
pump over 20 h. The mixture was filtered and solvent removed to
leave a light yellow solid. The crude product was purified on a silica
column eluted with dichloromethane/methanol/NH4OH (60/6/1 v/
1
v) to afford the macrocycle 6 as a white powder (285 mg, 50%). H
NMR (CDCl3): δ 1.70 (m, 2), 1.77 (m, 2), 2.13 (s, 6), 2.21 (m, 6),
2.45 (m, 4), 2.62 (m, 2), 2.92 (m, 2), 3.16 (m, 2), 3.47 (m, 2), 3.74
(m, 2), 6.97 (d, 2), 7.35 (t, 2), 7.44 (s, 2), 8.15 (t, 1) 8.47 (d, 2), 8.56
(d, 2), 9.61 (s, 2). ESMS: m/z 570 (M + H)+, 285 (M + 2H)2+/2.
c. Complexes. (Me4N)[Ni(OH)(⊂-pyN2N4Me2)]·1/2Et2O. Ni(OTf)2
(36 mg, 0.10 mmol) in DMF/THF (4 mL, 3:1 v/v) was stirred for
12 h to give a light yellow suspension. Macrocycle 6 (57 mg, 0.10
11023
dx.doi.org/10.1021/ic301506x | Inorg. Chem. 2012, 51, 11017−11029

Inorganic Chemistry
Article
Figure 13. Structures of macrocycle 5, mononuclear NiII complexes 21 (Ni−O 1.824(4) Å) and 22 (Ni−O 1.917(2) Å) derived from 5, unbridged
23, and bridged reaction products 24 and 25 from 21. Fe site in 24 (Å): Fe−Cl(1) 2.342(1), Fe−N 2.084(3)−2.339(2), Fe−S 2.528(1). Cu site in
25 (Å): Cu−N(5) 2.018(4), Cu−N(6) 2.037(3), Cu−S(1) 2.358(1), Cu···N(4) 3.011(4).
mmol) was added, followed by dropwise addition of Me4NOH (25%
in methanol, 55 mg, 0.15 mmol). The mixture was stirred for 2 h, a
second equal portion of Me4NOH was introduced, and stirring was
continued for 4 h. The mixture was filtered to remove a sticky
precipitate, and ether (20 mL) was added to the filtrate. The orange
solid that separated was washed with ether (3 mL) and dissolved in
DMF (2 mL), and THF (2 mL) was added. The solution was filtered,
and the filtrate was diffused with ether to give a mixture of red and
white solids. This was washed with THF (1 mL) and DMF (0.5 mL)
to afford the product as red plate-like crystals (35 mg, 46%).
Absorption spectrum (DMF): λmax (εM) 412 (5350), 488 (sh, 1800)
[Ni(μ2-OH)(DMF)Cu(⊂-pyN2N4Me2)](ClO4). The preceding com-
pound was recystallized from DMF/THF/C6H5CF3 (1:10:25 v/v)
to give the product in essentially quantitative yield as a green-black
crystalline solid. The compound was identified by X-ray structure
determination.
7. X-ray Structure Determinations. Twenty of the compounds
in Chart 1 were structurally characterized; for brevity, compounds that
are salts are referred to by their cation or anion number. Diffraction-
quality crystals were obtained from the following solvents: dichloro-
methane/ether/hexane, 4; DMF/acetonitrile/ether, 5; methanol/
chloroform/ethyl acetate, 6; DMF/ether, 7, 8, 11, 14, 16, 18, 20,
21, 23, 25−27; DMF/toluene/ether, 17, 22; DMF/propionitrile/
ether, 19; Me2SO/ethyl acetate, 24; DMF/THF/PhCF3, 28.
Diffraction data for 16 (15 K) and 22 (95 K) were obtained using
synchrotron radiation at the ChemMatCARS, Advanced Photon
Source (APS), Argonne National Laboratory. Intensities of reflections
were collected by means of a Bruker APEX II CCD together with the
D8 Diffractometer equipped with an Oxford Instrument Helijet/
Cryojet open flow apparatus. The collection method involved 0.5°
scans in φ at −5° in 2θ. Other diffraction data were collected with a
Bruker CCD area detector diffractometer equipped with an Oxford
700 low-temperature nitrogen apparatus (100 K). Data integration was
carried out using SAINT V7.46A with reflection size optimization and
absorption corrections made with SADABS (Bruker AXS APEX II,
Bruker AXS Inc., Madison, WI). Structures were solved by direct
1
nm. IR (KBr): vOH 3603 cm−1. H NMR (DMF-d7): δ −4.15 (s, 2),
1.45 (m, 2), 1.61 (m, 2), 2.02 (s, 6), 2.07 (m, 2), 2.18 (m, 2), 2.31 (m,
2), 2.42 (t, 4), 2.69 (m, 2), 2.82 (m, 2), 2.99 (m, 2), 3.22 (m, 2), 3.50
(m, 2), 6.72 (d, 2), 6.97 (t, 2), 7.26 (d, 2), 7.52 (m, 4), 8.04 (t, 1).
Anal. Calcd for C39H59N8NiO3.5: C, 62.07; H, 7.88; N, 14.85. Found:
C, 62.16; H, 7.91; N, 14.45.
[Ni(μ2-OH)(DMF)2Cu(⊂-pyN2N4Me2)](ClO4). Cu(ClO4)2·6H2O (9.3
mg, 0.025 mmol) in DMF (0.5 mL) was added dropwise to a solution
of (Me4N)[Ni(OH)(⊂-pyN2N4Me2)] (18 mg, 0.025 mmol) in DMF
(1.5 mL). The mixture was stirred for 1 h and filtered. The filtrate was
diffused with ether to deposit the product as green-black microcrystals
(19 mg, 80%). IR (KBr): vOH 3565 cm−1. Anal. Calcd for
C39H56ClCuN9NiO9: C, 49.17; H, 5.93; N, 13.23. Found: C, 49.21;
H, 5.79; N, 13.00
11024
dx.doi.org/10.1021/ic301506x | Inorg. Chem. 2012, 51, 11017−11029

Inorganic Chemistry
Article
orthorhombic, a = 12.723(5) Å, b = 28.53(1) Å, c = 15,467(6) Å, V =
5614(9), Z = 4, Pbcn. Sufficient data were collected to show that 9 and
10 have the same structure as 11 in its Et4N+ salt. Complex 12 is
assigned the structure of 11 based on crystal isomorphism.
The metal contents in selected binuclear complexes were
determined at the APS by multiple wavelength anomalous dispersion
techniques.14 The following electron densities normalized to one metal
atom per molecule were determined and Ni:M atom ratios calculated.
19: Ni 0.967(3), Fe 0.961(3), 1.01:1. 24: Ni 0.938(5), Fe 0.851(5),
1.10:1. [25](ClO4): Ni 1.000(5), Cu 0.964(5), 1.04:l. [28](ClO4): Ni
0.968(5), Cu 0.953(3), 1.02:1.
RESULTS AND DISCUSSION
■
1. Pincer Complexes with Other Metals. a. CuII. Prior to
pursuing ligand alterations in regions I−IV of the complex in
Figure 2, we wished to know what other divalent metals might
bind in the NNN pincer site. Such metals would be candidates
for incorporation into binucleating macrocycles containing that
site and also may engage in CO2 fixation. To do so, we utilized
for simplicity ligand 1. Planar complexes [M(pyN2Me2)(OH)]1−
(M = NiII, PdII) have been previously described. Both react
cleanly with CO2 to afford [M(pyN2Me2)(HCO3)]1− in good
yield.7 The corresponding CuII−OH complex 7 is easily
prepared by reaction of Cu(OTf)2 and Et4NOH in DMF. It
reacts immediately with CO2 in DMF to form the η1-HCO3
−
Figure 14. Formation of mononuclear NiII−hydroxo complex 26 and
complex 8, which is isolable in 80% yield. Spectrophotometric
conversion 7 → 8 is illustrated with Figure 5 together with the
planar structures and selected dimensions of the two complexes
which are isostructural with their NiII and PdII counterparts and
nearly isometric with the NiII complex. Mononuclear CuII−OH
complexes in different coordination environments have been
described.19−21 However, nearly all CuII-mediated CO2 fixation
reactions have been carried out with polynuclear bridged oxo
and hydroxo reactants and afford carbonate-bridged prod-
ucts.22−25 Only one four-coordinate η1-bicarbonate complex,
also a pincer species, has been previously characterized.26,27 It
and 8 crystallize as centrosymmetric hydrogen-bonded dimers.
The long Cu−O4 distance of 2.542 Å is consistent with a four-
coordinate description of 8. Evidently, the system in Figure 5 is
the simplest yet devised for CO2 fixation by a CuII reactant.
b. MnII, FeII, CoII, and ZnII. To seek a broader set of reactants,
we examined reactions of other divalent metals with equimolar
H2pyN2Me2. The results with four divalent ions are summarized
in Figure 6, from which it is evident that despite the 1:1
reaction stoichiometry six-coordinate bis-ligand complexes 9−
binuclear hydroxo-bridged complexes 27 and 28 based on macrocycle
6.
methods using the SHELX program package. All non-hydrogen atoms
were refined anisotropically, and temperature factors of those
disordered atoms were restrained by DELU/SIMU to achieve an
anisotropic average. Hydrogen atoms were not added to the cations (7,
11) or solvate molecules (6, 23) owing to highly disordered carbon
atoms. Hydrogen atom(s) of the −OH groups (7, 17, 18, 28) and
−HCO3 group (8) were located geometrically. Hydrogen atoms of
other −OH groups (14, 16, 18, 22, 25−27) were located from
difference Fourier maps and refined isotropically. Crystal data and
refinement details area given in Tables S1−S4, Supporting
Information.14 Other refinement details and explanations (wherever
necessary) are included in individual CIF files.
The compounds (Et4N)2[M(pyN2Me2)2] with M = Fe, Mn, and Zn
yielded the following crystal data. M = Fe (10): triclinic, a = 15.529(3)
Å, b = 25.509(5) Å, c = 31.076(6) Å, α = 65.966(4)°, β = 89.778(5)°,
γ = 89.945(5)°, V = 11243(4) Å3, Z = 8, P1. M = Mn (9): triclinic, a =
̅
10.4628(4) Å, b = 16.2333(6) Å, c = 18.9633(7) Å, α = 78.432(1)°, β
= 80.522(1)°, γ = 83.378(1)°, V = 3100.9(2), Z = 2, P1. M = Zn (12):
̅
Table 1. Ni−X−M Bridge Matric Parameters
a
compounds, Ni−X−M
14, Ni−OH−Mn
15, Ni−OH−Fe
16, Ni−OH−Cu
18, Ni−OH−Cu
19, Ni−CN−Fe
Ni−O/C/Cl (Å)
M−O/N/Cl (Å)
Ni−O−M (deg)
Ni−C−N (deg)
Fe−N−C (deg)
Ni···M (Å)
1.861(3)
2.089(4)
142.5(2)
1.927(4)
2.059(3)
140.0(2)
1.899(3)
2.134(4)
147.6(2)
1.895(2)
2.026(2)
141.1(1)
1.878(3)
2.099(2)
170.2(2)
138.3(2)
3.741(2)
3.747(2)
3.873(2)
3.697 (1)
4.709(1)
23, Ni−Cl Cl−Fe
24, Ni−CN−Fe
25, Ni−OH−Cu
27, Ni−OH−Cu
28, Ni−OH−Cu
Ni−O/C/Cl (Å)
M−O/N/Cl (Å)
Ni−O−M (deg)
Ni−C−N (deg)
Fe−N−C (deg)
Ni···M (Å)
2.178(2)
2.236(2)
1.853(3)
2.084(3)
1.890(3)
1.936(3)
150.4(2)
2.056(4)
1.924(4)
153.1(2)
2.008(3)
1.936(3)
152.7(2)
170.5(3)
157.2(2)
4.927(1)
5.315(1)
3.699(1)
3.871(1)
3.833(2)
a
Reference 7.
11025
dx.doi.org/10.1021/ic301506x | Inorg. Chem. 2012, 51, 11017−11029

Inorganic Chemistry
Article
Figure 15. Structures of macrocycle 6, mononuclear NiII−OH complex 26, and binuclear complex 27 containing a NiII−OH−CuII bridge. Ni site in
26 (Å): Ni−N(1) 1.917(9), Ni−N(2) 1.828(3), Ni−N(3) 1.902(8), Ni−O(3) 1.821(2). Ni site in 27 (Å): Ni−N(1) 2.110(4), Ni−N(2) 1.991(4),
Ni−N(3) 2.149(4). Cu site in 27 (Å): Cu−N(5) 2.073(4), Cu−N(6) 2.084(4), Cu−N(7) 2.067(5), Cu···N(4) 3.070(4).
12 are formed. Because of ligand steric factors, these
isostructural complexes adopt a MN4O2 coordination unit in
which one N substituent of each ligand is rotated in order to
place two oxygen atoms mutually cis, thus reducing
destabilizing steric interactions between N substituents. The
structure of 11 (Figure 6) reveals a C2-symmetric configuration
far removed from octahedral with approximately cis oxygen
atoms and trans O/N−Co−N angles much smaller than 180°.
This situation arises because, unlike low-spin d8 and d9, the
other four metal ions do not have an intrinsic planar
respectively. Stereochemistry at five-coordinate M sites is
approximated in terms of the shape parameter τ = (α − β)/60,
where α and β are largest and next-largest bond angles,
respectively.33 For idealized square pyramidal (SP) τ = 0, and
for trigonal bipyramidal (TBP) τ = 1. Note that for four
binuclear complexes, mixed metal contents were determined
directly by anomalous dispersion methods.
a. Region I. Different metals are incorporated by binding in
the vacant site of mononuclear NiII−OH complexes 13, 17, 21,
and 26. These precursors are readily prepared and recognized
by upfield −OH resonances from −4.1 to −4.7 ppm (DMF).
As shown in Figure 7, different metals M = MnII (14) and CuII
(16) can be readily inserted into the vacant triamine site of
terminal hydroxo complex 13 in ca. 40% yield by reaction with
metal chlorides in DMF. The FeII complex 15, the first
binuclear complex of macrocyle 2,7 is included for comparison.
The M sites are five-coordinate with intermediate shapes (τ =
0.52 for 14 and 0.50 for 15) and approach TBP with 16 (τ =
0.78). Complexes 19 (44%, Figure 9) and 23 (46%, Figure 12)
containing the NiII−CN−MII bridge were obtained by cyanide
displacement of hydroxo.
b. Region II. Neutral complexes 14−16 are insoluble or
sparingly soluble in organic solvents such as DMF and
acetonitrile; as such they are not amenable to studies of
solution properties and reactivity. We find that substitution of
an isobutoxy group in the 4-position of the pyridine ring affords
complexes that are freely soluble in DMF and soluble (but less
so) in acetonitrile. These complexes are derived from
macrocycle 3, which is accessible by five steps in 16% overall
yield from chelidamic acid.14 Two complexes, hydroxo-bridged
18 (M = CuII, 62%) and cyano-bridged 19 (M = FeII, 44%),
were prepared from 17 as indicated in Figure 9. Structures are
set out in Figure 10. The bridge structural parameters of 18 are
similar to those of the Ni/Cu complex 16 except for the
Ni···Cu separation, which is 0.17 Å shorter. The CuII site does
not approach the TBP shape of 16. Instead, the site reveals a
highly irregular four-coordinate CuN2OCl unit with a NNCu/
CuOCl dihedral angle of 38.6° and a nonbonded Cu−N4
distance of 2.702(3) Å. This arrangement apparently influences
stereochemical preference based on d-orbital energetics.
Me2
Further, the pyN2
ligand dianion does not engender bond
angles in the usual tetrahedral range. This type of coordination
isomerism has several precedents among 2,6-pyridinedicarbox-
amido complexes.28−32 These four metal ions are unsuitable for
coordination in the pincer site of a macrocycle such as that in
Figure 2 but are candidates for binding in the other site.
2. Modifications in Complexes of Binucleating
Ligands. This phase of our work on complexes of binucleating
ligands is focused on synthesis and structure proofs by an X-ray
methods. Physical properties will be described subsequently as
needed in reactivity studies. Displayed in Figure 2 is the
reference molecule subject to modification. The changes that
follow are illustrative of a much broader set of potential
alterations. Yields of isolated compounds are indicated. For
example, variation in divalent M is confined to MnII, FeII, and
CuII and bridging ligands to hydroxo and cyano, although other
M atoms and bridges are surely possible. Cyanide was
employed to determine if macrocycle flexibility would
accommodate a two-atom bridge NiII−CN−MII approaching
linearity. Further, cyanide is an inhibitor of CODH. X-ray
structures of macrocycles 4−6 and complexes 14 and 16−28
were determined; those of macrocycle 2 and complexes 13 and
15 were described earlier.7 Metric features of bridge units are
collected in Table 1; other structural information is available in
figure legends and elsewhere.14 All complexes except 27 and 28
contain planar NiII coordinated by the NNN pincer ligand.
Parameters of the NiN3 unit are nearly identical across the set
13−26; data for 17 and 26 are included in Figures 10 and 15,
11026
dx.doi.org/10.1021/ic301506x | Inorg. Chem. 2012, 51, 11017−11029

Inorganic Chemistry
Article
the metal−metal distances, but why substitution at the remote
4-position of the pyridyl ring affects structure is unclear. The
DMF solvate molecule14 does not interact with this site.
Cyanide can be readily interposed in the macrocyclic structure
19, affording nearly linear Ni−C−N (170°) and bent C−N−Fe
(138°) bridge segments and an increase of nearly 1.0 Å in
metal−metal distance. The FeII site approaches SP coordina-
tion (τ = 0.16) with chloride in the apical position. Thus, the
reference macrocycle can stabilize two-atom bridges.
macrocycle and its complexes are presented in Figure 15.
Reaction of 6 with Ni(OTf)2 leads to mononuclear 26 (46%)
whose coordination unit is isostructural with 13, 17, and 21.
Reaction with CuII affords the bis(DMF) hydroxo-bridged
complex 27 (80%), which upon recrystallization from a solvent
mixture containing a small amount of DMF converts to the
mono-DMF adduct 28.14 Bridge dimensions of the two
complexes are quite similar (Table 1).
Complex 27 contains two unexpected features. The NiII site
is six coordinate with trans DMF ligands (O−Ni−O =
177.7(2)°) and bond lengths (2.14−2.15 Å) ca. 0.10−0.15 Å
longer than in (paramagnetic) [Ni(DMF)6]2+ (Ni−O = 2.04−
2.06 Å)39,40 and various cationic pincer complexes (unreported
magnetism).41−43 In 28 this distance decreases to 2.052(4) Å in
the absence of a trans ligand.43 All NiII−OH complexes in DMF
solution show 1H NMR spectra indicative of diamagnetism and
weak or nil out-of-plane coordination. We have not
encountered additional ligation in any other NiII NNN pincer
coordination unit where the strong in-plane ligand field is
expected to favor a planar structure. However, compared to
diamagnetic 26, the Ni−N bond lengths in 27 and 28 are 0.16−
0.25 Å and Ni−OH bond lengths are 0.19−0.24 Å longer
(Figure 15, Table 1). These results imply a spin state change at
NiII to S = 1 (as in [Ni(DMF)6]2+) upon DMF binding.
The CuII atom in 27 and 28 does not assume CuN4 binding
in the cyclam ring but instead assumes an irregular four-
coordinate CuN3O structure with N4 uncoordinated at 3.07 Å;
bonded Cu−N distances are 2.07−2.08 Å. Similar distorted
stereochemistry occurs at the CuII sites in 18, 25, and 28. Given
the binding affinity of cyclam-type ligands toward CuII and the
occurrence of the planar CuN4 unit in such complexes,44−46 we
conclude that the macrocycle conformation does not favor this
conventional coordination mode. A similar factor apparently is
operative in 25 as well. Nonetheless, CuII is bound tightly in
these structures.
3. Bridge Structures. Macrocyclic binucleating ligands
facilitate entry to and sustain the viability of supported [NiII−
X−MII] bridges with X = OH− and CN− and variable M.
Whereas homometallic hydroxo and cyano bridges between
divalent ions are commonplace, supported or unsupported
heterometallic bridgesespecially one such bridge per
moleculeare not. Such bridges nearly always require one
trivalent ion, as exemplified by, e.g., the assemblies [FeIII−
(OH)−CuII]47 and [FeIII−(CN)−CuII,I]46,48,49 prepared in this
laboratory. We examine briefly the metric features collected in
Table 1.
c. Region III. Inasmuch as the Ni···Fe separation in active
and cyanide-inhibited CODH is less than 3.0 Å,1,4,34 we wished
to decrease the Ni···M distance in the reference macrocycle.
This is most readily accomplished synthetically by changing the
two connecting phenylene rings from 1,3- to 1,2-substitution.
Macrocycle 4 differs from 2 and 3 by having 20 rather than 22
atoms in its inner ring. Relevant information is provided in
Figure 11, which includes a structure proof of the macrocycle.
Reaction of 4 with Ni(OTf)2 and base leads to the green
distorted octahedral complex 20 (40%) instead of a complex
with an NiII−OH group and a vacant triamine locus. While the
pincer site is retained, the smaller ring and the proclivity of NiII
for octahedral coordination favor the observed product. In this
context, we note that another NNN pincer macrocycle with a
20-membered ring and a potential pyridyldiamino binding site
forms a distorted octahedral complex with FeII.35 A shorter
metal−metal distance requires a different macrocycle design.
d. Region IV. Two changes were made in the M binding site
of the reference molecule by replacement of the triamine chain
with macrocyclic components. The 1,3-phenylene connectors
were retained in both cases. The 7-Me-[12]aneSN3 ring was
appended by the procedure of Figure 3 to give macrocycle 5
with a 22-membered internal ring. The unsubstituted [12]-
aneSN3 ring has been prepared and binds divalent ions
including CuII and ZnII with three M−N bonds and a longer
M−S interaction.36 The macrocycle readily forms pincer
hydroxo complex 21 (50%) from which 22−25 (40−60%)
are prepared as indicated in Figure 12. Structures of the
macrocycle and these complexes are collected in Figure 13.
NH4HCO3 acts as a proton donor to 21, yielding aquo complex
22, whose Ni−O bond is 0.10 Å longer than 21; 22 should be
reactive to substitution at the NiII site. Two equivalents of
FeCl2 results in unexpected removal of the hydroxo group for
chloride and insertion of FeII in the SN3 site to afford the
unbridged complex 23 with a TBP iron site (τ = 0.91). Cyano-
bridged Ni/Fe complex 24 results from hydroxo substitution of
21. It features a bridge more closely approaching linearity than
19 and a six-coordinate iron site with a weak Fe−S interaction
(2.528(1) Å). Hydroxo-bridged Ni/Cu complex 25 is readily
formed by reaction of CuII with 21 and contains a nonplanar
CuN2OS site with one nitrogen atom uncoordinated. These
results reveal that macrocycle 5 can stabilize binuclear
unbridged and hydroxo- and cyano-bridged complexes with
variable coordination at the M site. This structural design
should be capable of elaboration with other macrocycles as the
In the mononuclear diamagnetic NiII−OH complexes 13, 17,
21, and 26, Ni−O bond lengths are nearly constant at
1.802(4)−1.825(2) Å. Hydroxo-bridged complexes 14−16, 18,
and 25 when compared against the appropriate member of this
set show Ni−O bond length increases of 0.06−0.10 Å
indicative of diamagnetic NiII. With 27 and 28, increases are
larger (0.19, 0.24 Å) because of apparent high-spin NiII in these
complexes. Increases in Ni−O bond lengths are to be expected
upon bridge formation and relatively small with diamagnetic
NiII. Hydroxo bridges of 14−18 exhibit small variations in Ni−
O−M angles (140−147°) and Ni···M distances (3.70−3.87 Å).
The shortest distance is associated with the near-TBP CuII
stereochemistry in 16 and the longest with the unexpected
four-coordinate CuII site in 18. The structures in Figures 8 and
10 and the data of Table 1 delineate the NiII−(OH)−MII
bridge geometry supported by macrocycles 2 and 3 for metal
ionic radii of 0.7−0.9 Å. Figures 13 and 15 and Table 1 convey
37
M site. One closely related example is 1,7-Me2-[12]aneN4
(1,7-Me2-cyclen), which binds divalent metal ions with high
affinity.38 Another example is considered next.
The procedure in Figure 4, which involves reaction of the
benzylic bromide 6a with the substituted cyclam 1,8-Me2-
[14]aneN4, deprotection of 6b with hydrazine, and coupling of
6c with pyridine-2,6-dicarbonylchloride affords macrocycle 6
with the substituted cyclam ring as the M site. Reactions of 6
are summarized in Figure 14, and the structure of the
11027
dx.doi.org/10.1021/ic301506x | Inorg. Chem. 2012, 51, 11017−11029

Inorganic Chemistry
Article
(13) Vigato, P. A.; Peruzzo, V.; Tamburini, S. Coord. Chem. Rev.
2012, 256, 953−1114.
this information for complexes derived from macrocycles 5 and
6.
(14) See paragraph at the end of this article for what is included in
the Supporting Information.
Cyano-bridged complexes 19 and 24, based on different
macrocycles 3 and 5, respectively, reveal Ni−C bonds that
differ by 0.04 Å, expected nearly linear Ni−C−N fragments
(170°) and bent Fe−N−C segments whose angles (138°,
157°) are presumably dependent on macrocyclic constraints.
These results together with the structure of [Ni(CN)FeCl(⊂22-
pyN2dienMe3)]7 show that 2, 3, and 5 are capable of supporting
two-atom bridges, here at Ni···Fe distances of 4.71 and 4.93 Å.
Lastly, unbridged 23 with two oppositely placed chloride atoms
in the bridging void, exhibits a Ni···Fe distance of 5.315(1) Å,
the best current measure of the maximum distance available for
insertion of a bridging group in macrocycle 5. No such complex
has been isolated with the other macrocycles.
(15) Coulter, K. R.; McAuley, A.; Rettig, S. Can. J. Chem. 2001, 79,
930−937.
(16) Brown, F. J.; Bernstein, P. R.; Cronk, L. A.; Dosset, D. L.;
Hebbel, K. C.; Maduskuie, T. P., Jr.; Shapiro, H. S.; Vacek, E. P.; Yee,
Y. K.; Willard, A. K.; Krell, R. D.; Snyder, D. W. J. Med. Chem. 1989,
32, 807−826.
(17) Rensing, S.; Schrader, T. Org. Lett. 2002, 2, 2161−2164.
(18) dos Santos, J. L.; Lanaro, C.; Lima, L. M.; Gambero, S.; Franco-
Penteado, C. F.; Alexandre-Moreira, S.; Wade, M.; Yerigenhally, S.;
Kutlar, A.; Meiler, S. E.; Costa, F. F.; Chung, M. J. Med. Chem. 2011,
54, 5811−5819.
(19) (a) Harata, M.; Hasegawa, K.; Jitsukawa, K.; Masuda, H.; Einaga,
H. Bull. Chem. Soc. Jpn. 1998, 71, 1031−1038. (b) Fujisawa, K.;
Kobayashi, T.; Fujita, K.; Kitajima, N.; Moro-oka, Y.; Miyshita, Y.;
Yamada, Y.; Okamoto, K. Bull. Chem. Soc. Jpn. 2000, 73, 1797−1804.
(c) Berreau, L. M.; Mahapatra, S.; Halfen, J. A.; Young, V. G., Jr.;
Tolman, W. B. Inorg. Chem. 1996, 35, 6399−6342. (d) Jitsukawa, K.;
Harata, M.; Arii, H.; Sakurai, H.; Masuda, H. Inorg. Chim. Acta 2001,
324, 108−116. (e) Tubbs, K. J.; Fuller, A. L.; Bennett, B.; Arif, H.;
Berreau, L. M. Inorg. Chem. 2003, 42, 4790−4791. (f) Price, J. R.;
Fainerman-Melnikova, M.; Fenton, R. R.; Gloe, K.; Lindoy, L. F.;
Rambusch, T.; Skelton, B. W.; Turner, P.; White, A. H.; Wichmann, K.
Dalton Trans. 2004, 3715−3726. (g) Cheruzel, L. E.; Cecil, M. R.;
Edison, S. E.; Mashuta, M. S.; Baldwin, M. J.; Buchanan, R. M. Inorg.
Chem. 2006, 45, 3191−3202. (h) Yao, L.; Li, W.-H. Acta Crystallogr.
2009, E65, m1532. (i) Huang, M.-L. Acta Crystallogr. 2010, E66,
m1312. (j) Yoshida, Y.; Miyamoto, R.; Nakato, A.; Santo, R.;
Kuwamura, N.; Gobo, K.; Nishioka, T.; Hirotsu., M.; Ichimura, A.;
Hashimoto, H.; Kinoshita, I. Bull. Chem. Soc. Jpn. 2011, 64, 600−611.
(20) Lee, S. C.; Holm, R. H. J. Am. Chem. Soc. 1993, 115, 11789−
11798.
ASSOCIATED CONTENT
■
S
* Supporting Information
Crystallographic data for 20 compounds (Tables S1−S4),
structure of complex 28, synthesis of macrocycles 3 and 4,
UV−vis spectra of NiII−OH compounds 17, 21, and 26. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: holm@chemistry.harvard.edu.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
(21) Donoghue, P. J.; Tehranchi, J.; Cramer, C. J.; Sarangi, R.;
Solomon, E. I.; Tolman, W. B. J. Am. Chem. Soc. 2011, 133, 17602−
17605.
(22) Kitajima, N.; Hikuchi, S.; Tanaka, M.; Moro-oka, Y. J. Am.
Chem. Soc. 1993, 115, 5496−5508.
(23) Bazzicalupi, C.; Bencini, A.; Bencini, A.; Bianchi, A.; Corana, F.;
Fusi, V.; Giorgi, C.; Paoli, P.; Paoletti, P.; Valtancoli, B.; Zanchini, C.
Inorg. Chem. 1996, 35, 5540−5548.
(24) Nishida, Y.; Yatani, A.; Nakao, Y.; Taka, J.; Kashino, S.; Mori,
W.; Suzuki, S. Chem. Lett. 1999, 135−136.
This research was supported by NIH Grant GM 28856. We
thank Dr. Shao-Liang Zheng for crystallographic assistance and
useful discussions. ChemMetCARS Sector 15 at the APS is
principally supported by the National Science Foundation/
Department of Energy under grant number NSF/CHE-
0822838. Use of the APS was supported by the U.S.
Department of Energy, Office of Science, Office of Basic
Energy Sciences, under Contract No. DE-AC02-06CH11357.
(25) Fenandes, C.; Neves, A.; Bortoluzzi, A. J.; Szpoganicz, B.;
Schwingel, E. Inorg. Chem. Commun. 2001, 4, 354−357.
(26) Jones, P. G.; Hrib, C. G.; Petrovic, D.; Tamm, M. Acta
Crystallogr. 2007, E63, m2768.
REFERENCES
■
(1) Jeoung, J.-H.; Dobbek, H. Science 2007, 318, 1461−1464.
(2) Gong, W.; Hao, B.; Wei, Z.; Ferguson, D. J.; Tallant, T.; Krzycki,
J. A.; Chan, M. K. Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 9558−9563.
(3) Kung, Y.; Doukov, T. I.; Seravalli, J.; Ragsdale, S. W.; Drennan, C.
L. Biochemistry 2009, 48, 7432−7440.
(27) Petrovic, D.; Bannenberg, T.; Randoll, S.; Jones, P. G.; Tamm,
M. Dalton Trans. 2007, 2812−2822.
(28) Wasilke, J.-C.; Wu, G.; Bu, X.; Kehr, G.; Erker, G.
Organometallics 2005, 24, 4289−4297.
(4) Amara, P.; Mouesca, J.-M.; Volbeda, A.; Fontecilla-Camps, J. C.
Inorg. Chem. 2011, 50, 1868−1878.
(29) Dwyer, A. N.; Grossel, M. C.; Horton, P. N. Supramol. Chem.
2004, 16, 405−410.
(5) Panda, R.; Berlinguette, C. P.; Zhang, Y.; Holm, R. H. J. Am.
Chem. Soc. 2005, 127, 11092−11101.
(30) Wallenhorst, C.; Axenov, K. V.; Kehr, G.; Samec, J. S. M.;
Frohlich, R.; Erker, G. Z. Naturforsch. 2007, 62b, 783−790.
̈
(6) Sun, J.; Tessier, C.; Holm, R. H. Inorg. Chem. 2007, 46, 2691−
2699.
(31) Tyler, L. A.; Olmstead, M. M.; Mascharak, P. K. Inorg. Chim.
Acta 2001, 321, 135−141.
(7) Huang, D.; Holm, R. H. J. Am. Chem. Soc. 2010, 132, 4693−4701.
(8) Abbreviations are given in Chart 1.
(9) Huang, D.; Makhlynets, O. V.; Tan, L. L.; Lee, S. C.; Rybak-
Akimova, E. V.; Holm, R. H. Proc. Natl. Acad. Sci. U.S.A. 2011, 108,
1222−1227.
(10) Huang, D.; Makhlynets, O. V.; Tan, L. L.; Lee, S. C.; Rybak-
Akimova, E. V.; Holm, R. H. Inorg. Chem. 2011, 50, 10070−10081.
(11) Vigato, P. A.; Tamburini, S.; Bertolo, L. Coord. Chem. Rev. 2007,
251, 1311−1492.
(12) Vigato, P. A.; Tamburini, S. Coord. Chem. Rev. 2008, 252, 1871−
1995.
(32) Harrop, T. C.; Tyler, L. A.; Olmstead, M. M.; Mascharak, P. K.
Eur. J. Inorg. Chem. 2003, 475−481.
(33) Addison, A. W.; Nageswara Rao, T.; Reedijk, J.; van Rijn, J.;
Verschoor, G. C. J. Chem. Soc., Dalton Trans. 1984, 1349−1356.
(34) Jeoung, J.-H.; Dobbek, H. J. Am. Chem. Soc. 2009, 131, 9922−
9923.
(35) Brooker, S.; Simpson, T. J. J. Chem. Soc., Dalton Trans. 1998,
1151−1154.
(36) Marcus, S. T.; Bernhardt, P. V.; Grondahl, L.; Gahan, L. R.
Polyhedron 1999, 18, 3451−3460.
11028
dx.doi.org/10.1021/ic301506x | Inorg. Chem. 2012, 51, 11017−11029

Inorganic Chemistry
Article
(37) Piersanti, G.; Varrese, M. A.; Fusi, V.; Giorgi, L.; Zappia, G.
Tetrahedron Lett. 2010, 51, 3436−3438.
(38) Ciampolini, M.; Micheloni, M.; Nardi, N.; Paoletti, P.;
Dapporto, P.; Zanobini, F. J. Chem. Soc., Dalton Trans. 1984, 1357−
1362.
(39) Li, W.-S.; Blake, A. J.; Champness, N. R.; Schroder, M.; Bruce,
̈
D. W. Acta Crystallogr. 2012, C54, 349−351.
(40) McKee, V.; Metcalfe, T.; Wikaira, J. Acta Crystallogr. 1996, C52,
1139−1141.
(41) Behrens, H.; Frohlich, R.; Wurthwein, E.-U. Eur. J. Org. Chem.
̈
̈
2005, 3891−3899.
(42) Chen, X.; Zhang, L.; Yu, J.; Hao, X.; Liu, H.; Sun, W.-H. Inorg.
Chim. Acta 2011, 370, 156−163.
(43) Huang, D.; Deng, L.; Sun, J.; Holm, R. H. Inorg. Chem. 2009, 48,
6159−6166.
(44) Tasker, P. A.; Sklar, L. J. Cryst. Mol. Struct. 1975, 5, 329−344.
(45) Elias, H. Coord. Chem. Rev. 1999, 187, 37−73.
(46) Scott, M. J.; Holm, R. H. J. Am. Chem. Soc. 1994, 116, 11357−
11367.
(47) Scott, M. J.; Zhang, H. H.; Lee, S. C.; Hedman, B.; Hodgson, K.
O.; Holm, R. H. J. Am. Chem. Soc. 1995, 117, 568−569.
(48) Scott, M. J.; Lee, S. C.; Holm, R. H. Inorg. Chem. 1994, 33,
4651−4662.
(49) Lim, B. S.; Holm, R. H. Inorg. Chem. 1998, 37, 4898−4908.
11029
dx.doi.org/10.1021/ic301506x | Inorg. Chem. 2012, 51, 11017−11029