Reactions of the terminal NiII-OH group in substitution and electrophilic reactions with carbon dioxide and other substrates: Structural definition of binding modes in an intramolecular NiIIFeII bridged site

Article abstract of DOI:10.1021/ja1003125

A singular feature of the catalytic C-cluster of carbon monoxide dehydrogenase is a sulfide-bridged NiFe locus where substrate is bound and transformed in the reversible reaction CO + H₂O → CO₂ + 2H⁺ + 2e^-. A similar structure has been sought in this work. Mononuclear planar Ni^II complexes [Ni(pyN₂ ^Me2)L]^1- (pyN₂^Me2 = bis(2,6-dimethylphenyl)-2,6-pyridinedicarboxamidate(2-)) derived from a NNN pincer ligand have been prepared including L = OH^- (1) and CN ^- (7). Complex 1 reacts with ethyl formate and CO₂ to form unidentate L = HCO₂^- (5) and HCO₃^- (6) products. A binucleating macrocycle was prepared which specifically binds Ni^II at a NNN pincer site and five-coordinate Fe^II at a triamine site. The Ni^II macrocyle forms hydroxo (14) and cyanide complexes (15) analogous to 1 and 7. Reaction of 14 with FeCl₂ alone and with ethyl formate and 15 with FeCl₂ affords molecules with the Ni^II-L-Fe^II bridge unit in which L = μ₂: η¹-OH^- (17) and μ₂:η²- HCO₂^- (18) and -CN^- (19). All bridges are nonlinear (17, 140.0°; 18, M-O-C 135.9° (Ni), 120.2° (Fe); 19, Ni-C-N 170.3°, Fe-N-C 141.8°) with NiFe separations of 3.7-4.8 A. The Ni^IIFe^II complexes, lacking appropriate Ni-Fe-S cluster structures, are not site analogues, but their synthesis and reactivity provide the first demonstration that molecular Ni^IIFe^II sites and bridges can be attained, a necessity in the biomimetic chemistry of C-clusters.

Full text of DOI:10.1021/ja1003125

Published on Web 03/10/2010
Reactions of the Terminal NiII-OH Group in Substitution and
Electrophilic Reactions with Carbon Dioxide and Other
Substrates: Structural Definition of Binding Modes in an
Intramolecular NiII · · ·FeII Bridged Site
Deguang Huang and R. H. Holm*
Department of Chemistry and Chemical Biology, HarVard UniVersity,
Cambridge, Massachusetts 02138
Received January 13, 2010; E-mail: holm@chemistry.harvard.edu
Abstract: A singular feature of the catalytic C-cluster of carbon monoxide dehydrogenase is a sulfide-
bridged Ni · · ·Fe locus where substrate is bound and transformed in the reversible reaction CO + H2O h
CO2 + 2H+ + 2e-. A similar structure has been sought in this work. Mononuclear planar NiII complexes
[Ni(pyN2Me2)L]1- (pyN2Me2 ) bis(2,6-dimethylphenyl)-2,6-pyridinedicarboxamidate(2-)) derived from a NNN
pincer ligand have been prepared including L ) OH- (1) and CN- (7). Complex 1 reacts with ethyl formate
and CO2 to form unidentate L ) HCO2- (5) and HCO3- (6) products. A binucleating macrocycle was prepared
which specifically binds NiII at a NNN pincer site and five-coordinate FeII at a triamine site. The NiII macrocyle
forms hydroxo (14) and cyanide complexes (15) analogous to 1 and 7. Reaction of 14 with FeCl2 alone
and with ethyl formate and 15 with FeCl2 affords molecules with the NiII-L-FeII bridge unit in which L )
µ2:η1-OH- (17) and µ2:η2-HCO2 (18) and -CN- (19). All bridges are nonlinear (17, 140.0°; 18, M-O-C
-
135.9° (Ni), 120.2° (Fe); 19, Ni-C-N 170.3°, Fe-N-C 141.8°) with Ni· · ·Fe separations of 3.7-4.8 Å.
The NiIIFeII complexes, lacking appropriate Ni-Fe-S cluster structures, are not site analogues, but their
synthesis and reactivity provide the first demonstration that molecular NiII · · ·FeII sites and bridges can be
attained, a necessity in the biomimetic chemistry of C-clusters.
MtCODH/ACS12) point toward a consistent structure based on
Introduction
a NiFe3S4 cluster to which is appended an external (exo) iron
Among the many challenges in the biomimetic chemistry of
metal clusters1 is the construction of molecules that approach
or achieve the structures of the catalytic sites of carbon
monoxide dehydrogenases (CODHs2).3,4 These enzymes contain
either a Mo-Cu-S or a Ni-Fe-S active site and catalyze the
reversible reaction CO + H2O h CO2 + 2H+ + 2e-, a process
of prime significance in the global carbon cycle. Nickel-
containing CODHs5,6 utilize as their catalytic site for CO/CO2
interconversion the structurally complex C-cluster, which has
been crystallographically investigated in enzymes from several
different anaerobic bacteria.7-9 The most recent results (2007-09)
for enzymes from three sources (MtCODH,10 MbCODH/ACS,11
atom. As shown in Figure 1 (a), the structure is a bridged
assembly of core composition NiFe4S4 with an aquo or hydroxo
ligand bound to the exo iron with approximate tetrahedral
geometry. The consensus structure-based mechanistic proposal
is that CO binds at the nickel site, undergoes nucleophilic attack
by coordinated hydroxide to form a carbon-bound hydroxycar-
bonyl bridged intermediate such as (b). This species subse-
quently liberates CO2 and two electrons upon reaction with water
to regenerate (a).
Previous synthetic work directed toward the C-cluster, some
of it prior to the availability of structural information, has
resulted in clusters with certain relevant features. Phosphine
ligation in the cubanes [(Ph3P)NiFe3S4(SR)3]2- simulates the
binding of CO and other π-acid ligands at a tetrahedral nickel
site.13-15 Further, the approximately planar Ni sites in inter-
mediate (b) and the cyanide-ligated C-clusters (c) and (d) (Figure
1) are approached in clusters such as [(tdt)NiFe3S4(LS3)]3-. This
and related cubanoid clusters are produced by phosphine
substitution with a strong in-plane chelating ligand and attendant
(1) Groysman, S.; Holm, R. H. Biochemistry 2009, 48, 2310–2320.
(2) Abbreviations are given in Chart 1.
(3) Ragsdale, S. W.; Kumar, M. Chem. ReV. 1996, 96, 2515–2539.
(4) Ragsdale, S. W. Crit. ReV. Biochem. Mol. Biol. 2004, 39, 165–195.
(5) Lindahl, P. A. Biochemistry 2002, 41, 2097–2105.
(6) Lindahl, P. A.; Graham, D. E. Metal Ions Life Sci. 2007, 2, 357–415.
(7) Drennan, C. L.; Doukov, T. I.; Ragsdale, S. W. J. Biol. Inorg. Chem.
2004, 9, 511–515.
(8) Dobbek, H.; Svetlitchnyi, V.; Liss, J.; Meyer, O. J. Am. Chem. Soc.
2004, 126, 5382–5387.
(12) Kung, Y.; Doukov, T. I.; Seravalli, J.; Ragsdale, S. W.; Drennan, C. L.
Biochemistry 2009, 48, 7432–7440.
(9) Volbeda, A.; Fontecilla-Camps, J. C. J. Chem. Soc., Dalton Trans.
2005, 3443–3450.
(13) Ciurli, S.; Ross, P. K.; Scott, M. J.; Yu, S.-B.; Holm, R. H. J. Am.
Chem. Soc. 1992, 114, 5415–5423.
(10) Jeoung, J.-H.; Dobbek, H. Science 2007, 318, 1461–1464.
(11) 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.
(14) Zhou, J.; Scott, M. J.; Hu, Z.; Peng, G.; Mu¨nck, E.; Holm, R. H. J. Am.
Chem. Soc. 1992, 114, 10843–10854.
(15) Zhou, J.; Raebiger, J. W.; Crawford, C. A.; Holm, R. H. J. Am. Chem.
Soc. 1997, 119, 6242–6250.
9
10.1021/ja1003125  2010 American Chemical Society
J. AM. CHEM. SOC. 2010, 132, 4693–4701 4693

A R T I C L E S
Huang and Holm
substrate and inhibitor binding at one or both metal centers in
terminal or bridge modes. Other than with the enzymes
themselves, there is no information on reactivity and structures
of binuclear NiIIFeII molecules with any species.
Experimental Section
Preparation of Compounds. All reactions and manipulations
were performed under a pure dinitrogen atmosphere using Schlenk
techniques or an inert atmosphere box. Volume reduction and drying
steps were performed in vacuo. DMF was freshly distilled and dried
over 3-Å molecular sieves for 3 d. Other solvents were passed
through an Innovative Technology or MBraun solvent purification
system. All solvents were degassed before use. Compounds were
identified by spectroscopic and/or crystallographic characterization;
representative compounds were analyzed (Kolbe Microanalytical
Laboratory, Mu¨lheim, Germany). Cation resonances are omitted
from NMR data. Compounds with no NMR data were insufficiently
soluble for measurement and were identified crystallographically.
In the sections that follow, complexes and ligands are numerically
designated according to Chart 1.
Figure 1. Structures of C-clusters in different reaction states with selected
interatomic distances (Å) and angles. (a) ChCODH in the Cred1 or -320
mV (resting) state: Ni-O, 2.72; Fe-O, 1.95; Ni-Fe, 2.85.10 (b) ChCODH
in the -600 + CO2 (intermediate) state: Ni-C, 1.96; Fe-O, 2.05; C-OFe,
1.25; Ni-Fe, 2.76; Ni-C-O, 119°; Fe-O-C, 105°.10 (c) ChCODH in
the cyanide-bound state: Ni-C, 1.79; Ni-Fe, 2.56; Ni-C-N, 170°.10 (d)
MtCODH/ACS in the cyanide-bound state: Ni-C, 1.99; Fe-O, 2.05;
Ni-C-N, 114°.12 In (a) and (d), OH-/OH2 is bound to Fe. Terminal
cysteinate ligation at three Fe atoms in each structure is omitted.
Chart 1. Abbreviations and Designation of Ligands and
Complexesa
rupture of the axial Ni-(µ3-S) bond.16,17 However, key attributes
of the C-cluster have not been experimentally attained. These
include an exo iron site linked to the NiFe3S4 portion through
a µ3-S bridge (or any bridge), coordinative unsaturation at the
nickel site as manifested in the apparent three-coordinate “T-
shaped”10 Ni(µ3-S)2SCys fragment (Figure 1 (a)) and the juxta-
position of a terminal OH-/OH2 ligand at a distance too long
to represent a bridging interaction to the nickel site (Ni · · ·OH
2.7 Å). Multiple attempts to bridge FeII to an NiFe3S4 unit have
not succeeded.
Given the current synthetic inaccessibility of a complete
C-cluster analogue, we have utilized a different ligand construct
with which to pursue reactivity with CO and CO2 by NiII alone
and in the presence of proximal FeII. The few terminal FeII-OH
complexes known are stabilized by steric factors18 or hydrogen-
bonding interactions;19,20 reactivity toward CO or CO2 has not
been reported. Pincer NiII, PdII, and PtII terminal hydroxo
complexes with PCP donor atom sets react with these substrates,
in several cases affording C-bound hydroxycarbonyl products21-23
similar to intermediate (b) but bound in the η1 mode. Conse-
quently, we have utilized pincers with a less-abiological NNN
donor set as a ligand for NiII in mononuclear complexes and in
a binucleating macrocycle incorporating a triamine (dien)
binding site for FeII. This investigation is an initial step toward
a binuclear NiII · · ·FeII system allowing examination of biological
a ACS ) acetyl coenzyme A synthase; Ch ) Carboxydothermus
hydrogenoformans; CODH ) carbon monoxide dehydrogenase; dien )
diethylenetriamine; dme ) 1,2-dimethoxyethane; Mb ) Methanosarcina
barkeri; Me6tren ) tris(N,N′-dimethyl-2-aminoethyl)amine; Mt ) Moorella
thermoacetica; pyN2 ) N,N′-bis(phenyl)-2,6-pyridinedicarboxamidate(2-); pdtc
) pyridine-2,6-dithiocarboxylate(2-); pyN2iPr2 ) N,N′-bis(2,6-diisopropy-
lphenyl)-2,6-pyridinedicarboxamidate(2-); pyN2Me2 ) bis(2,6-dimethylphe-
nyl)-2,6-pyridinedicarboxamidate(2-); pyN2dienMe3 ) bis(phenyl-3,3′-
(2,5,8-tri-N-methylamino-1,9-nonyl))-2,6-pyridinedicarboxamidate(2-); tdt
) toluene-3,4-dithiolate(2-); TfO- ) triflate.
(a) Pincer Ligand and Mononuclear Complexes. N,N′-Bis(2,6-
dimethylphenyl)-2,6-pyridinedicarboxamide (H2pyN2Me2). A solu-
tion of pyridine-2,6-dicarbonyl chloride (1.02 g, 5.00 mmol) at
0 °C in THF (150 mL) was added slowly to a mixture of 2,6-
dimethylaniline (1.21 g, 10 mmol) and triethylamine (1.41 mL,
10 mmol). The reaction mixture was warmed to room temper-
ature, stirred for 5 h, and filtered. The light yellow residue
obtained by removal of solvent was treated with hexane (10 mL)
and washed with a small volume of dichloromethane/ether (1:5
v/v) to give the pure product as a white powder (1.58 g, 85%).
1H NMR (CDCl3): δ 2.27 (s, 12), 7.13 (m, 6), 8.14 (t, 1), 8.51
(d, 2) 9.17 (s, 2).
Synthetic reactions for complexes are summarized in Figure 2.
(Et4N)[Ni(pyN2Me2)(OH)]. H2pyN2Me2 (149 mg, 0.40 mmol) and
Ni(OTf)2 (143 mg, 0.40 mmol) were stirred in THF (5 mL) for
2 h. The light yellow suspension was treated with Et4NOH (25%
in methanol, 353 mg, 0.60 mmol) and stirred for 2 h to give a red
suspension. A second equal portion of Et4NOH was added, and
the mixture was stirred for 10 h to afford a deep red solution and
some sticky precipitate, which was removed by filtration through
Celite. Hexane (10 mL) was added to the filtrate resulting in
separation of a red oil. Addition of ether (5 mL) to the oil resulted
in a solid, which was washed with THF and ether and dried to
give the product as a red crystalline solid (138 mg, 60%). IR (KBr):
(16) Panda, R.; Berlinguette, C. P.; Zhang, Y.; Holm, R. H. J. Am. Chem.
Soc. 2005, 127, 11092–11101.
(17) Sun, J.; Tessier, C.; Holm, R. H. Inorg. Chem. 2007, 46, 2691–2699.
(18) Hikichi, S.; Ogihara, T.; Fujisawa, K.; Kitajima, N.; Akita, M.; Moro-
oka, Y. Inorg. Chem. 1997, 36, 4539–4547.
(19) MacBeth, C. E.; Hammes, B. S.; Young, V. G., Jr.; Borovik, A. S.
Inorg. Chem. 2001, 40, 4733–4741.
(20) Be´nisvy, L.; Halut, S.; Donnadieu, B.; Tuchagues, J.-P.; Chottard, J.-
C.; Li, Y. Inorg. Chem. 2006, 45, 2403–2405.
(21) Bennett, M. A.; Jin, H.; Willis, A. C. J. Organomet. Chem. 1993,
451, 249–256.
´
(22) Ca´mpora, J.; Palma, P.; del Rio, D.; Alvarez, E. Organometallics 2004,
ν
OH 3594 cm-1. Absorption spectrum (DMF): λmax (εM) 411 (5500),
23, 1652–1655.
1
(23) Johansson, R.; Wendt, O. F. Organometallics 2007, 26, 2426–2430.
483 (sh, 2600) nm. H NMR (CD2Cl2): δ -4.95 (s, 1), 2.48 (s,
9
4694 J. AM. CHEM. SOC. VOL. 132, NO. 13, 2010

Reactions of the Terminal NiII-OH Group
A R T I C L E S
Me2
(Et4N)[Ni(pyN2Me2)(CN)]. To a solution of (Et4N)[Ni(pyN2
)
(OH)] (23 mg, 0.040 mmol) in DMF (1 mL) was added a solution
of Et4NCN (9.4 mg, 0.060 mmol) in DMF (0.5 mL). The reaction
mixture was stirred for 1 h and filtered. Ether diffusion into the
filtrate caused separation of the product as orange-red crystals (19
mg, 81%). IR (KBr): νCN 2127 cm-1. H NMR (CD2Cl2): δ 2.40
1
(s, 12), 6.90 (m, 6), 7.69 (d, 2), 8.07 (t, 1).
[Ni(pyN2Me2)(CN)Fe(Me6tren)](OTf). To
a
solution of
(Et4N)[Ni(pyN2Me2)(CN)] (19 mg, 0.032 mmol) in DMF (1 mL)
was added a solution of [Fe(Me6tren)(OTf)](OTf)24 (19 mg, 0.032
mmol) in DMF (1 mL). The reaction mixture was stirred for 40
min and filtered. Diffusion of ether into the filtrate affords the
product as thin platelike light red crystals (12 mg, 42%). IR (KBr):
νCN 2135 cm-1
.
(Et4N)[Pd(pyN2Me2)(OH)]. H2pyN3Me2 (75 mg, 0.20 mmol) and
[Pd(MeCN)4](OTf)2 (114 mg, 0.2 mmol) were stirred in aceto-
25
nitrile for 90 h. Solvent was reduced to ca. 3 mL and ether (13
mL) added to deposit a yellow-brown solid, which was washed
with acetonitrile/THF (2 mL, 1:3 v/v) and dried. The solid was
dissolved in DMF/THF (4 mL, 1:3 v/v), and the yellow solution
was treated with Et4NOH (25% in methanol, 118 mg, 0.20 mmol)
and stirred for 4 h. The mixture was filtered and ether diffused
into the filtrate to afford the product as light yellow-brown
crystalline solid, which was washed with THF and dried in vacuo
(88 mg, 70%). Absorption spectrum (DMF): λmax (εM) 335 (8600)
1
nm. H NMR (DMF-d7): δ -3.14 (s, 1), 2.35 (s, 12), 6.83 (t, 2),
6.92 (d, 4), 7.74 (d, 2), 8.20 (t, 1).
Figure 2. Scheme showing the preparation of pincer complexes
[Ni(pyN2Me2)L]1-, including hydroxo complex 1 and complexes 2-7 derived
from it. Binuclear 8 is obtained from 7.
Me2
(Et4N)[Pd(pyN2Me2)(HCO3)]. A solution of (Et4N)[Pd(pyN2
)
(OH)] (25 mg, 0.040 mmol) in DMF was bubbled with carbon
dioxide for 30 min. Diffusion of ether into the solution resulted in
separation of the product as block-like yellow crystals (24 mg,
90%). IR (KBr): 1615 (vs), 1584 (w), 1469 (m), 1445 (s), 1366
(s), 1304 (w) cm-1. Absorption spectrum (DMF): λmax (εM) 276
(sh, 8800), 335 (sh, 5000) nm. 1H NMR (DMF-d7): δ 2.36 (s, 12),
6.78 (t, 2), 6.85 (d, 4), 7.74 (d, 2), 8.28 (t, 1).
12), 6.88 (m, 6), 7.51 (d, 2), 7.87 (t, 1). Anal. Calcd for
C31H42N4NiO3: C, 64.49; H, 7.33; N, 9.70. Found: C, 64.36; H,
7.40; N, 9.69.
Me2
(Et4N)[Ni(pyN2Me2)Cl].
A
solution of (Et4N)[Ni(pyN2
)
(b) Macrocyclic Ligand and Complexes. Reactions leading to
the ligand and complexes are outlined in Figures 3 and 4.
Intermediates 9-12. 3-Nitrobenzaldehyde (7.56 g, 50 mmol)
and 3-N-methylaminopentane-1,5-diamine (2.93 g, 25 mmol) were
reacted in ethanol (150 mL) for 12 h. The solid collected by
filtration and a second batch obtained by volume reduction of the
filtrate were combined and washed with ethanol to afford light
yellow 9 (8.63 g, 90%; 1H NMR (CDCl3) δ 2.41 (s, 3), 2.83 (t, 4),
3.78 (t, 4), 7.51 (t, 2), 8.00 (d, 2), 8.21 (d, 2), 8.32 (s, 2), 8,48 (s,
2)). NaBH4 (3.40 g, 90 mmol) was added to a solution of 9 (8.63
g, 22.5 mmol) in methanol (150 mL) at 50 °C. The solution was
cooled to room temperature, solvent was removed, and the orange
solid was extracted with dichloromethane/water (200 mL/100 mL).
The organic layer was washed with water (2 × 100 mL), dried
over MgSO4, and filtered. Removal of solvent gave 10 as a pale
(OH)] (29 mg, 0.050 mmol) in dichloromethane (1 mL) was allowed
to stand for 4 d. Ether was diffused into the solution causing
deposition of the product as block-like brown-red crystals (24 mg,
1
81%). H NMR (CD2Cl2): δ 2.42 (s, 12), 6.86 (m, 6), 7.57 (d, 2),
7.97 (t, 1).
(Et4N)[Ni(pyN2Me2)(OMe)]. The preceding method with use of
methanol (1 mL) afforded a red crystalline product (21 mg, 71%).
1H NMR (CD2Cl2): δ 2.50 (s, 12), 3.26 (s, 3), 6.89 (m, 6), 7.54 (d,
2), 7.89 (t, 1).
Me2
(Et4N)[Ni(pyN2Me2)(SH)]. To a solution of (Et4N)[Ni(pyN2
)
(OH)] (23 mg, 0.040 mmol) in DMF (1 mL)was added a solution
of Et4NSH (9.8 mg, 0.040 mmol) in an equal volume of DMF.
The reaction mixture was stirred for 50 min and filtered. Diffusion
of ether into the deep red filtrate gave the product as red crystals
1
(22 mg, 93%). H NMR (DMF-d7): δ -3.51 (s, 1), 2.36 (s, 12),
1
yellow oil (7.67 g, 88%; H NMR (CDCl3) δ 1.85 (s, br, 2), 2.20
6.81 (m, 6), 7.66 (d, 2), 8.19 (t, 1). Anal. Calcd for C31H42N4NiO2S:
(s, 3), 2.52 (t, 4), 2.70 (t, 4), 3.90 (s. 4), 7.45 (t, 2), 7.64 (d, 2),
8.07 (d, 2), 8.20 (s, 2)). To a solution of 10 (7.67 g, 20 mmol) in
acetonitrile (540 mL) and acetic acid (60 mL) was added an aqueous
solution of formaldehyde (28 mL, 37%). The mixture was stirred
for 1 h and cooled to 0 °C, and NaBH4 (3.74 g, 99 mmol) was
slowly added. Stirring was continued for 2 h and for 36 h at room
temperature. After filtration, solvent was removed from the filtrate
to give a light yellow sticky solid, which was stirred in dichlo-
romethane (200 mL). A solution of 2 M NaOH was added until
the pH was ca. 14. The aqueous layer was extracted with
dichloromethane (2 × 50 mL), and the combined organic layers
were washed with water (100 mL), dried over MgSO4, and filtered.
Removal of solvent from the filtrate gave 11 as a light yellow oil
(7.07 g, 86%; 1H NMR (CDCl3) δ 2.21 (s, 6), 2.22 (d, 3), 2.53 (m,
C, 62.74; H, 7.13; N, 9.44. Found: C, 62.64; H, 7.17; N, 9.44.
Me2
(Et4N)[Ni(pyN2Me2)(HCO2)]. A solution of (Et4N)[Ni(pyN2
)
(OH)] (12 mg, 0.020 mmol) in DMF (0.5 mL) was layered with
ethyl formate (4 mL). After 2 d, the product was collected as orange-
red crystals (7.0 mg, 58%). IR (KBr): 1615 (s), 1587 (w), 1470
1
(m), 1437 (vw), 1368 (m), 1300 (m) cm-1. H NMR (CD2Cl2): δ
2.49 (s, 12), 6.85 (m, 6), 7.54 (d, 2), 7.94 (t, 1). Anal. Calcd for
C32H42N4NiO4: C, 63.49; H, 6.99; N, 9.26. Found: C, 63.85; H,
7.10; N, 9.11.
Me2
(Et4N)[Ni(pyN2Me2)(HCO3)]. A solution of (Et4N)[Ni(pyN2
)
(OH)] (23 mg, 0.040 mmol) in DMF (2 mL) was bubbled with
carbon dioxide for 15 min. Diffusion of ether into the red solution
gave the product as orange-red crystals (21 mg, 85%). IR (KBr):
1618 (vs), 1585 (w), 1470 (m), 1439 (w), 1360 (s), 1310 (w) cm-1
.
Absorption spectrum (DMF): λmax (εM) 300 (sh, 7800), 381 (5500),
476 (sh, 1500) nm. 1H NMR (CD3CN): δ 2.40 (s, 12), 6.90 (s, 6),
7.63 (s, 2), 8.15 (t, 1). Anal. Calcd for C32H42N4NiO5: C, 61.85; H,
6.81; N, 9.02. Found: C, 61.44; H, 7.07; N, 8.99.
(24) Britovsek, G. J. P.; England, J.; White, A. J. P. Inorg. Chem. 2005,
44, 8125–8134.
(25) Wendt, O. F.; Kaiser, N.-F. K.; Elding, L. I. J. Chem. Soc., Dalton
Trans. 1997, 4733–4737.
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A R T I C L E S
Huang and Holm
and pyridine-2,6-dicarbonyl chloride (0.79 g, 3.90 mmol), each in
THF (50 mL), by syringe pump over 24 h. The reaction mixture
was stirred for 3 h and filtered, and the filtrate was evaporated to
leave a yellow-orange solid. This residue was stirred with methanol
(120 mL, 1 h), and the solution was filtered. The yellow filtrate
was reduced to ca. 6 mL and heated to dissolve any solid. Cooling
the solution caused separation of colorless crystals. The steps of
filtrate volume reduction and cooling were repeated several more
times to obtain additional amounts of white crystals. The crops were
combined and dried to give the product as a white solid (0.72 g,
1
38%). H NMR (DMSO-d6): δ 2.13 (s, 6), 2.25 (s, 3), 2.53 (t, 4),
2.60 (t, 4), 3.51 (s, 4), 7.01 (d, 2), 7.35 (t, 2), 7.66 (s, 2), 8.30 (t,
1), 8.39 (m, 4), 11.22 (s, 2). Anal. Calcd for C28H34N6O2: C, 69.11;
H, 7.04; N, 17.27. Found: C, 68.01; H, 7.03; N, 17.01.
(Et4N)[Ni(OH)(⊂-pyN2dienMe3)]. The macrocycle (73 mg, 0.15
mmol) and Ni(OTf)2 (53 mg, 0.15 mmol) were mixed and stirred
in DMF (2 mL) for 1 h to give a light yellow suspension. Et4NOH
(25% in methanol, 118 mg, 0.20 mmol) was added, and the mixture
was stirred for 2 h forming a red suspension. A second portion of
Et4NOH (147 mg, 0.25 mmol) was added, and the mixture was
stirred to 10 h to give a deep red solution with some sticky
precipitate. The latter was removed by filtration through Celite.
Addition of ether (25 mL) to the filtrate and storage at -20 °C for
2 d yielded the product as red crystals (53 mg, 51%). IR (KBr):
ν
OH 3606 cm-1. Absorption spectrum (DMF): λmax (εM) 411 (5400),
483 (sh, 1900) nm. 1H NMR (DMF-d7): δ -4.40 (s, 1), 2.01 (s, 3),
2.20 (m, 4), 2.28 (m, 10), 3.35 (s, 4), 6.80 (d, 2), 7.05 (t, 2), 7.20
(s, 2), 7.28 (d, 2), 7.54 (d, 2), 8.06 (t, 1).
Figure 3. Scheme for the synthesis of the binucleating pincer macrocycle
⊂-H2pyN2dienMe3 (13).
(Et4N)[Ni(HCO3)(⊂-pyN2dienMe3)].Asolutionof(Et4N)[Ni(OH)(⊂-
pyN2dienMe3)] (21 mg, 0.030 mmol) in DMF (1.5 mL) was bubbled
with carbon dioxide for 15 min. Diffusion of ether in the red
solution caused separation of the product as orange-red crystals
(17 mg, 77%). IR (KBr): 1656 (s), 1619 (vs), 1580 (m), 1481 (w),
1456 (m), 1369 (m), 1328 (s) cm-1. Absorption spectrum (DMF):
8), 3.59 (s, 4), 7.45 (t, 2), 7.64 (d, 2), 8.08 (d, 2), 8.19 (s, 2)). To
a mixture of 11 (2.49 g, 6.0 mmol) and Pd/C (160 mg) in anhydrous
ethanol was added a solution of N2H4 ·H2O (20 mL). The mixture
was stirred for 30 min and for 12 h at 60 °C and filtered. Volume
reduction of the filtrate gave a light orange oil. The crude product
was purified in the air on a silica column eluted with dichlo-
romethane/methanol/NH4OH to give 12 as a light yellow oil (1.38
λ
max (εM) 381 (5500), 476 (sh, 880) nm. 1H NMR (CD3CN): δ 1.88
(s, 3), 2.21 (br, s, 4), 2.32 (s, 6), 2.42 (br, s, 4), 3.43 (br, s, 4), 6.78
(d, 2), 6.90 (d, 2), 7.02 (s, 2), 7.09 (t, 2), 7.75 (d, 2), 8.01 (t, 1).
(Et4N)[Ni(CN)(⊂-pyN2dienMe3)]. To a solution of (Et4N)
[Ni(OH)(⊂-pyN2dienMe3)] (34.5 mg, 0.050 mmol) in DMF (2 mL)
was added a solution of Et4NCN (7.8 mg, 0.050 mmol) in DMF (1
mL). The reaction mixture was stirred for 1 h and filtered. Addition
of ether (15 mL) to the red filtrate deposited the product as an
1
g, 65%; H NMR (CDCl3) δ 2.18 (s, 3), 2.19 (s, 6), 2.49 (m, 8),
3.39 (s, 2), 3.43 (s, 2), 3.68 (s, br, 4), 6.55 (d, 2), 6.66 (m, 4), 7.06
(t, 2)).
⊂-H2pyN2dienMe3. To a solution of Et3N (13 mL) in THF (350
mL) was added simultaneously solutions of 12 (1.38 g, 3.90 mmol)
orange crystalline solid (26 mg, 75%). IR (KBr): νCN 2129 cm-1
.
Figure 4. Preparation of macrocyclic complexes [Ni(L)(⊂-pyN2dienMe3)]1- with L ) OH- (14) and CN- (15) and bridged complexes [Ni(L)FeCl(⊂-
-
pyN2dienMe3)] with L ) OH- (17), HCO2 (18), and CN- (19).
9
4696 J. AM. CHEM. SOC. VOL. 132, NO. 13, 2010

Reactions of the Terminal NiII-OH Group
A R T I C L E S
1H NMR (CD2Cl2): δ 2.24 (s, 3), 2.32 (s, 6), 2.49 (m, 4), 2.60 (m,
4), 3.51 (s, 4), 6.86 (d, 2), 7.03 (d, 2), 7.08 (t, 2), 7.41 (s, 2), 7.66
(d, 2), 8.00 (t, 1).
were recorded on a Varian AM-400 instrument. Infrared spectra
were obtained with a Nicolet 5PCFT-IR spectrometer.
Results and Discussion
[Ni(OH)FeCl(⊂-pyN2dienMe3)]. To
a solution of (Et4N)
[Ni(OH)(⊂-pyN3dienMe3)] (28 mg, 0.040 mmol) in DMF (2 mL)
was added a solution of FeCl2 (5.1 mg, 0.040 mol) in DMF (1
mL). The reaction mixture was stirred for 1 h, filtered through
Celite, and ether was slowly diffused into the deep red filtrate over
3 d. The crystalline solid was washed with acetonitrile (5 mL) and
dried to afford the product as red crystals (10 mg, 38%). IR (KBr):
νOH 3597 cm-1. Anal. Calcd for C28H33ClFeN6NiO3: C, 51.61; H,
5.11; N, 12.90; Fe, 8.57; Ni, 9.01. Found: C, 51.44; H, 5.39; Fe,
8.47; N, 12.79; Ni, 8.88.
Because the CO2 intermediate (b) and cyanide-inhibited
C-clusters (Figure 1, (c, d)) involve binding to nickel, we first
focused attention on reactions at NiII in an NNN pincer ligand
environment. Reaction of deprotonated N,N′-bis(phenyl)-2,6-
pyridinedicarboxamide and NiCl2(dme) in a 2:1 molar ratio
28
affords octahedral [Ni(pyN2)2]2-
.
When the reaction is con-
ducted with a 1:1 ratio, we find the same complex formed (in
reduced yield) owing to the favorable d-electron stabilization
of octahedral d8. However, with 1:1 deprotonated N,N′-bis(2,6-
diisopropylphenyl)-2,6-pyridinedicarboxamide and NiII, the
planar monopincer complexes [Ni(pyN2iPr2)L] (L ) OH2, PMe3)
result.29 Lesser steric hindrance that destabilizes a bis-pincer
complex occurs with 2,6-dimethylphenyl N-substituents, as
shown by formation of the [Ni(pyN2Me2)L]1- series of complexes
1-7 (Figure 2).
[Ni(HCO2)FeCl(⊂-pyN2dienMe3)]. To a solution of (Et4N)
[Ni(OH)(⊂-pyN2dienMe3)] (28 mg, 0.040 mmol) in DMF (1 mL)
was added a solution of FeCl2 (5.1 mg, 0.040 mmol) in DMF (1
mL). The reaction mixture was stirred for 1 h and filtered through
Celite. The deep red filtrate was layered with ethyl formate (4 mL)
for 4 d, filtered, and ether diffused into the filtrate to deposit the
product as a red crystalline solid (8.0 mg, 30%). IR (KBr): 1631
(vs), 1599 (vs), 1485 (w), 1462 (m), 1441 (w), 1366 (s), 1311 (w)
Mononuclear NiII Binding and Structures. The key species
in the foregoing series is hydroxo complex 1, readily formed
by deprotonation of the pincer ligand with Et4NOH in the
cm-1
.
[Ni(CN)FeCl(⊂-pyN2dienMe3)]. To
a solution of (Et4N)
[Ni(CN)(⊂-pyN2dienMe3)] (26 mg, 0.037 mmol) in DMF (1 mL)
was added a solution of FeCl2 (4.7 mg, 0.050 mmol) in DMF (0.5
mL). The reaction mixture was stirred for 1 h, filtered, and ether
was added to the orange-red filtrate. The deposited solid was washed
with acetonitrile (3 mL), stirred with methanol (1 mL) to form a
suspension, which was filtered. The solid was dissolved in Me2SO/
DMF/methanol (1:1:1 v/v/v, 1.5 mL) and ether added by diffusion
to give the product as orange crystals (8.0 mg, 33%). IR (KBr):
presence of Ni(OTf)2 in THF and obtained in 60% yield.
30,31
Compared to the double bridge NiII(OH)2NiII
and the
supported bridge Ni-(OH)-Ni,32,33 the terminal NiII-OH group
is uncommon23,34-37 and is usually stabilized by hydrogen
bonding or, as is the case here, by steric protection. The upfield
-OH resonance (δ -4.95), similar in position to other Ni-OH
groups,30,38 is indicative of nucleophilic reactivity, as displayed
in reaction with dichloromethane (2, 81%) and reactions 1 with
ethyl formate (5, 58%) and CO2 (6, 85%)). The complex
deprotonates methanol (3, 71%) and undergoes substitution by
L ) HS- (4, 93%) or CN- (7, 81%) in reaction 2. Products
were isolated as Et4N+ salts in the indicated yields. It was also
demonstrated that 7 reacts with [Fe(Me6tren)(OTf)]1+ to form
binuclear complex 8, obtained as the triflate salt (42%).
νCN 2154 cm-1
.
Attempted Reactions with Carbon Monoxide. No reaction was
detected when CO was bubbled through DMF solutions of NiII
hydroxo complexes 1 and 14 for 15-20 min. The same treatment
of 17 led to an off-white ill-defined solid after ether diffusion into
the reaction mixture, and with PdII hydroxo complex 20 a black
precipitate formed within ca. 2 min, and no further reaction occurred
after 20 min. Under similar conditions, all complexes but 17 readily
formed soluble bicarbonate complexes with CO2.
[Ni(pyNMe2)2(OH)]1- + HCO2Et/CO2 f
[Ni(pyNMe2)2(HCO2)/(HCO3)]1- + EtOH/- -
(1)
[Ni(pyNMe2)2(OH)]1- + L- f [Ni(pyNMe2)2L]1- + OH-
(2)
X-ray Structure Determinations. The structures of the 15
compounds in Tables S1-S3 (Supporting Information) were
determined; for brevity they are referred to by their anion or cation
number. Diffraction-quality crystals were obtained from the fol-
lowing solvents: DMF/ether (1, 4, 6-8, 14, 17, 20, 21), dichlo-
romethane/ether (2), methanol/ether (3, 13), DMF/ethyl formate (5,
18), and Me2SO/DMF/methanol/ether (19). Diffraction data were
collected on a Bruker CCD area detector diffractometer equipped
with an Oxford 700 low temperature apparatus. Single crystals were
coated with Paratone-N oil and mounted on a Nylon loop. Structures
were solved by direct methods using the SHELX program pack-
age.26 Hydrogen atoms were not added to the cations or solvate
molecules in 1-3, 6, 7, 14, 18, and 20 owing to disordered carbon
atoms but are included in the compound formulas. In 13, hydrogen
atoms of two HCl molecules were not located from Fourier maps
but are included for charge balance. The hydrogen atoms of the
-OH groups in 1, 14, 17, the -SH group in 4, and the -HCO3
group in 6 and 21 were located from difference Fourier maps and
refined isotropically. Crystal data and refinement details are given
in Tables S1-S3 (Supporting Information).27
The complexes 1-7 are planar; structures of hydroxo,
formate, bicarbonate, and cyanide complexes are set out in
Figure 5. Because dimensions of the NiN3 portions of the
coordination units are practically invariant, only the values for
(28) Patra, A. K.; Mukherjee, R. Inorg. Chem. 1999, 38, 1388–1393.
(29) Wasilke, J.-C.; Wu, G.; Bu, X.; Kehr, G.; Erker, G. Organometallics
2005, 24, 4289–4297.
(30) Carmona, E.; Mar´ın, J. M.; Palma, P.; Paneque, M.; Poveda, M. L.
Inorg. Chem. 1989, 28, 1895–1900.
(31) Kitajima, N.; Hikuchi, S.; Tanaka, M.; Moro-oka, Y. J. Am. Chem.
Soc. 1993, 115, 5496–5508.
(32) Barrios, A. M.; Lippard, S. J. J. Am. Chem. Soc. 2000, 122, 9172–
9177.
(33) Kersting, B. Angew. Chem., Int. Ed. 2001, 40, 3988–3990.
(34) Orlandini, A.; Sacconi, L. Inorg. Chem. 1976, 15, 78–85.
(35) Meyer, F.; Kaifer, E.; Kircher, P.; Heinze, K.; Pritzkow, H.
Chem.sEur. J. 1999, 5, 1617–1630.
Other Physical Measurements. Absorption spectra were de-
termined with a Cary 50 Bio spectrophotometer. 1H NMR spectra
(36) Ca´mpora, J.; Matas, I.; Palma, P.; Graiff, C.; Tiripicchio, A. Orga-
nometallics 2005, 24, 2827–2830.
(37) Kieber-Emmons, M. T.; Schenker, R.; Yap, G. P. A.; Brunold, T. C.;
Riordan, C. G. Angew. Chem., Int. Ed. 2004, 43, 6716–6718.
(38) Lo´pez, G.; Garcia, G.; Sa´nchez, G.; Garcia, J.; Vicente, C. J. Chem.
Soc., Chem. Commun. 1989, 1045–1046.
(26) Sheldrick, G. M. Acta Crystallogr. 2009, A64, 112–122.
(27) See the paragraph at the end of this article for Supporting Information
available.
9
J. AM. CHEM. SOC. VOL. 132, NO. 13, 2010 4697

A R T I C L E S
Huang and Holm
-
-
Figure 5. Structures of five pincer complexes [Ni(pyN2Me2)L]1- with L ) OH- (1), HCO2 (5), HCO3 (6), CN- (7), and (CN)Fe(tren)1+ (8). Atom
numbering schemes and 50% probability ellipsoids are shown. Selected bond distances (Å) and angles (deg). 1: Ni-N1, 1.911(2); Ni-N2, 1.826(3); Ni-O2,
1.825(3); N1-Ni-N2, 82.73(7); N1-Ni-O2, 97.29(6); N1-Ni-N1A, 165.4(1); N2-Ni-O2, 176.9(1). 5: Ni-O2, 1.857(5); C13-O2, 1.27(1); C13-O3,
1.21(1); Ni-O2-C13, 114.2(5); O2-C13-O3, 127.4(8). 6: Ni-O2, 1.817(4); C13-O2, 1.253(7); C13-O3, 1.286(7); C13-O4, 1.293(6); Ni-O2-C13,
113.4(3); O2-C13-O3, 121.5(5); O2-C13-O4, 118.5(5); O3-C13-O4, 120.0(5). 7: Ni-C10, 1.849(7); C10-N3, 1.15(1); Ni-C10-N3, 180.0. 8: Ni-C13,
1.853(7); C13-N3, 1.129(8); Fe-N3, 2.085(5); av Fe-N(4-6), 2.146[8]; N3-Fe-N5, 179.9(2). Atoms N(1) and N(1A) and O(1) and O(1A) are related
by a mirror plane. Disorder of the carbon atoms of a phenyl ring (5) and of methylene carbon atoms (8) over two equally populated positions is omitted; one
position in shown for each molecule.
Figure 6. Absorption spectra for the conversion of NiII and PdII hydroxo to bicarbonate complexes in DMF upon reaction with CO2. Left: 1 f 5 and 14
f 16. Right: 20 f 21.
1 are given. Cyanide, an inhibitor of CODH which binds at
nickel (Figure 1), forms a linear Ni-CtN unit in 7. Reaction
with ethyl formate gives 5 with bent formate binding (Ni-O2-
C13 ) 114.2(5)°) and a dihedral angle of 90° between the
formate and NiN3 planes. Bicarbonate complex 6 was prepared
by passing CO2 through a DMF solution of 1 and can also be
obtained by exposure of 1 to the air in an immediate reaction.
As observed in Figure 6, UV-vis spectral changes associated
with the reaction are very clear, resulting in a shift of the most
intense band from 411 to 381 nm. Bicarbonate binding is very
similar to formate, coordinating in the η1 mode with Ni-O2-C13
) 113.4(3)° and a CO3/NiN3 dihedral angle of 90°. These are
the initial examples of the reaction of a terminal NiII-OH group
with a formate ester or CO2 itself, leading to the first structure
9
4698 J. AM. CHEM. SOC. VOL. 132, NO. 13, 2010

Reactions of the Terminal NiII-OH Group
A R T I C L E S
Figure 7. Structures of free macrocycle ⊂-H2pyN2dienMe3 (13), [Ni(OH)(⊂-pyN2dienMe3)]1- (14), and [Ni(L)FeCl(⊂-pyN2dienMe3)] with L ) OH- (17),
-
HCO2 (18), CN- (19). Atom numbering schemes and 50% probability ellipsoids are shown. Selected bond distances (Å) and angles (deg). 14: Ni-N1,
1.927(4); Ni-N2, 1.821(6); Ni-O2, 1.802(4); N1-Ni-N2, 82.7(1); N1-Ni-O2, 97.3(1). 17: Ni-O3, 1.927(4); Fe-O3, 2.059(3); Fe-Cl, 2.334(2);
Ni-O3-Fe, 140.0(2); Ni· · ·Fe, 3.747(2). 18: Ni-O3, 1.842(3); Fe-O4, 2.032(3); Fe-Cl, 2.252(2); C29-O3, 1.216(6); C29-O4, 1.236(5); Ni-O3-C29,
135.9(3); O3-C29-O4, 125.2(4); Fe-O4-C29, 120.2(3); Ni · · ·Fe, 4.792(1). 19: Ni-C29, 1.827(3); C29-N7, 1.145(4); Fe-N7, 2.022(2); Ni-C29-N7,
170.3(3); Fe-N7-C29, 141.8(2); Ni · · ·Fe, 4.631(1). Complex 18 occurs as two independent molecules of very similar structure; the data refer to one
structure. Disorder in the positions of certain nitrogen and carbon atoms in a 1:1 population in 14 is omitted; only one position is shown.
proofs of the binding modes NiII-(η1-OCHO) and NiII-(η1-
OC(O)OH) (vide infra). In comparison to these reactions, 1 does
not react with CO.
Macrocycle formation was verified by the X-ray structure in
Figure 7. The molecule is approximately planar except for the
dien-type fragment whose N2C4 least-squares plane forms a
dihedral angle of 82.3° with the least-squares plane of the
remainder of the molecule (atom deviations of e0.20 Å).
Preparations of mononuclear and binuclear macrocyclic
complexes are summarized in Figure 4; structures are reported
in Figure 7. Using the same procedure as for 1, NiII was taken
up in the pincer site to yield hydroxo complex 14 (51%), which
was readily converted to cyanide complex 15 by ligand
substitution. Binding of NiII in the macrocycle results in a
conformational change in the dien portion and reorientation of
the phenyl rings from a dihedral angle of 77.6° with the NiN3
plane in 1 to 60.4° in 14. Both complexes have mirror symmetry,
and metric features of their NiN3O coordination units are
insignificantly different. Bicarbonate complex 16 was prepared
from 14 and CO2. An X-ray structure was not determined, but
the spectral comparison with 5 (Figure 6) demonstrates forma-
tion of the complex.
Macrocyclic Mononuclear NiII and Binuclear NiIIFeII
Systems. Having examined the reactivity of the NNN pincer
NiII site in mononuclear complexes, a binuclear system contain-
ing this site and a binding site for FeII was sought. We utilized
the binucleating macrocycle concept39,40 with a ligand design
related to NCN pincer-crown ethers41,42 and binding sites
differentiated for selective metal incorporation. The target
macrocycle ⊂-H2pyN2dienMe3 (13) was synthesized via inter-
mediates 9-12 by the reaction sequence in Figure 3, which
results in an overall 17% yield from 3-nitrobenzaldehyde.
(39) Gavrilova, A. L.; Bosnich, B. Chem. ReV. 2004, 104, 349–383.
(40) Vigato, P. A.; Tamburini, S.; Bertolo, L. Coord. Chem. ReV. 2007,
251, 1311–1492.
(41) Mahoney, J. M.; Nawaratna, G. U.; Beatty, A. M.; Duggan, P. J.;
Smith, B. D. Inorg. Chem. 2004, 43, 5902–5907.
(42) Mahoney, J. M.; Stucker, K. A.; Jiang, H.; Carmichael, I.; Brinkmann,
N. R.; Beatty, A. M.; Noll, B. C.; Smith, B. D. J. Am. Chem. Soc.
2005, 127, 2922–2928.
Three NiIIFeII complexes, with hydroxo (17), formate (18),
and cyanide bridges (19), were prepared. Complexes 17 and
9
J. AM. CHEM. SOC. VOL. 132, NO. 13, 2010 4699

A R T I C L E S
Huang and Holm
Table 1. Bridge Dimensions (Å, deg) in Macrocyclic Complexes
to 7. The complete Ni-CtN-Fe bridge unit in 8 is linear,
whereas in 19 the Ni-CtN portion is linear but the Fe-NtC
17
18
19
sequence is bent (142°) to
a larger extent than in
Ni-O
1.927(4)
2.059(3)
1.842(3)
2.032(3)
[Ni(pdtc)CNFe(Me6tren)]1+ (166°),44 the only other molecular
example of a single NiII-CN-FeII bridge. The bent configu-
ration is probably due to macrocycle constraints which with a
cyanide bridge impose a 4.63 Å Ni · · ·Fe separation.
NiII and PdII Reactivity with CO and CO2. Prior reactions of
NiII with CO2 have involved only binuclear hydroxo-bridged
complexes, which lead to binuclear products containing µ2:η3-
CO3 bridges with one or two chelate rings.30,31,49 In another
case, reaction in methanol affords a µ2:η2 methylcarbonate
bridge.33 These result underscore the unique NiII η1-bicarbonate
binding mode in 6, whose ligand structure prevents bridge
formation.
Carbonylation reactions of pincer complexes have recently
been summarized.50 We briefly note several observations
pertinent to this work with PCP pincer species. Reaction 5 (M
) PdII) affords the η1 C-bound hydroxycarbonyl complex; with
CO2 the product is the η1-bicarbonate complex.23 Reaction 6
(M ) NiII, PdII) with a less hindered ligand forms a binuclear
η2(C,O):µ2-CO2 product which with additional CO can be
cleaved to the mononuclear PdII hydroxycarbonyl.22 These
results suggested examination of reactions with the PdII analogue
of the NiII-hydroxo complex 1.
Fe-O
Ni-C
Fe-N
1.827(3)
2.022(2)
Ni-O-Fe
Ni-O-C
Ni-C-N
Fe-O-C
Fe-N-C
O-C-O
Ni· · ·Fe
140.0(2)
135.9(3)
170.3(3)
141.8(2)
4.631(1)
120.2(3)
125.2(4)
3.747(2)
4.792(1)
19 were obtained by insertion of FeII in reactions 3 with 14 (L
) OH-, 38%) and 15 (L ) CN-, 75%), whereas 18 was
prepared from 17 generated in situ and ethyl formate in reaction
4. In these species, NiII is planar while the FeIIN3OCl units are
intermediate between trigonal pyramidal and square pyramidal
(τ ) 0.51 (17), 0.43 (19)) or approach trigonal bipyramidal (τ
) 0.17, 0.19 (18)). The shape parameter τ ) 0 for square
pyramidal and 1 for trigonal bipyramidal structures.43 Ring
conformations in the dien region are similar and the NiN3/phenyl
dihedral angles (53.5-56.4°) indicate that inclusion of the bridge
units has a minor effect on phenyl group orientations in the
three complexes. Attempts to prepare complexes with hydroxy-
carbonyl as bridges were unsuccessful. In particular, 14, as 1,
is unreactive toward CO and 17 did not yield a tractable product.
[M(OH)(C6H3-2,6-(CH2PBut2)2)] + CO f
[M(CO2H)(C6H3-2,6-(CH2PBut2)2)]
(5)
2[M(OH)(C6H3-2,6-(CH2PPri2)2)] + CO f
[Ni(L)(⊂-pyN2dienMe3)]1- + FeCl2 f
{[M(C6H3-2,6-(CH2PPri2)2)]2(CO2)} + H2O
(6)
[Ni(L)FeCl(⊂-pyN2dienMe3)] + Cl-
(3)
Reaction of [Pd(MeCN)4]2+ and H2pyN2
followed by
Me2
[Ni(OH)FeCl(⊂-pyN2dienMe3)] +
HCO2Et f [Ni(HCO2)FeCl(⊂-pyN2dienMe3)] + EtOH
(4)
addition of Et4NOH gave (Et4N)[20] (70%). The complex is
isostructural with 1, having Pd-N1 ) 2.023(4) Å, Pd-N2 )
1.906(7) Å, and Pd-O ) 1.959(6) Å. Reaction with CO2 affords
(Et4N)[21] (90%) with η1-bicarbonate ligation and Pd-O )
2.041(3) Å.27 Spectral changes parallel those with NiII but at
higher energies (Figure 6). However, exposure to CO resulted
in the formation of an insoluble black precipitate; no tractable
product was obtained. It is now apparent that the NNN pincer
environment, unlike PCP ligands, with NiII-OH or PdII-OH
functional groups does not support either CO binding or
formation of a hydroxycarbonyl complex.51
There are no prior reports of NiII-L-FeII bridges with L )
-
OH- and HCO2 and only one other case of a single intramo-
lecular NiII-CN-FeII bridge.44 The most closely related bridges
with these three linking groups are of the unsupported
CuII-L-FeIII type45-48 and require one trivalent metal center
for stability. Complexes 17-19 provide the first insights into
structure modes of the three bridge groups between these
elements. The bridges are supported but the macrocycle is
sufficiently flexible so as to sustain bond sequences of different
lengths, varying from 3.7 to 4.8 Å. Metric data for bridge units
are summarized in Table 1. The hydroxo bridge in 17 features
an angle of 140° and bond lengths that separate the metal centers
by 3.75 Å. Relative to 14, the Ni-O interaction has increased
by ca. 0.12 Å upon bridge formation. In 18, formate functions
as an µ2:η2 bridge with the metal centers in a syn orientation,
M-O-C bond angles of 136° (M ) Ni) and 120° (M ) Fe),
and a Ni· · ·Fe distance of 4.79 Å. In 19, the Ni-CN and CtN
bonds are not significantly altered by bridge formation compared
Summary
With use of a binucleating macrocycle whose NNN pincer
and triamine portions provide selective metal binding sites, the
first examples of NiII-L-FeII-bridged molecules have been
synthesized and structural modes of three bridging groups
(µ2-OH-, µ2:η2-HCO2- and -CN-) established. Results include
necessary supporting information on reactivity and structure at
the NiII site in mononuclear NNN pincer complexes. This
investigation is motivated by the biomimetic Ni-CODH C-
cluster problem whose difficulties have been noted at the outset.
The NiIIFeII molecules are not site analogues because of
substantial structural departures from the native site (Figure 1),
but their accessibility and demonstrated bridging capability offer
(43) Addison, A. W.; Nageswara Rao, T.; Reedijk, J.; van Rijn, J.;
Verschoor, G. C. J. Chem. Soc., Dalton Trans. 1984, 1349–1356.
(44) Huang, D.; Deng, L.; Sun, J.; Holm, R. H. Inorg. Chem. 2009, 48,
6159–6166.
(45) 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.
(46) Scott, M. J.; Goddard, C. A.; Holm, R. H. Inorg. Chem. 1996, 35,
2558–2567.
(49) Lozano, A. A.; Sa´ez, M.; Pe´rez, J.; Garc´ıa, L.; Lezama, L.; Rojo, T.;
Lo´pez, G.; Garc´ıa, G.; Santana, M. D. Dalton Trans. 2006, 3906–
3911.
(47) Scott, M. J.; Holm, R. H. J. Am. Chem. Soc. 1994, 116, 11357–11367.
(48) Lim, B. S.; Holm, R. H. Inorg. Chem. 1998, 37, 4898–4908.
(50) Morales-Morales, D. In Modern Carbonylation Methods; Kolla´r, L.,
Ed.; Wiley-VCH: Weinheim, 2008; pp 27-64.
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Reactions of the Terminal NiII-OH Group
A R T I C L E S
the first indication that molecular NiII · · ·FeII sites and bridges
can be constructed. Any further elaboration of these binuclear
systems should include reduced intermetal separation to <∼3.0
Å as in the C-cluster (Figure 1), which might prove advanta-
geous for bridged hydroxycarbonyl formation from a CO
reaction and unidentate linear or bent cyanide ligation. Ulti-
mately, the bridging unit should be integrated into a
Fe· · ·NiFe3S4 bridged assembly, a future goal.
Acknowledgment. This research was supported by NIH Grant
GM No. 28856. We thank Dr. S.-L. Zheng for assistance with
crystallography.
Supporting Information Available: Crystallographic data for
the 15 compounds in Tables S1-S3 and structures of com-
pounds 2-4, 20, and 21 in Figure S1. This material is available
free of charge via the Internet at http://pubs.acs.org.
(51) While reaction of CO with reduced Ni complexes remains a possibility,
preliminary investigation of 1 (DMF) and 7 (acetonitrile) by cyclic
voltammetry reveals only quasi-reversible reductions at-1.40 and-
1.31 V, respectively, vs SCE. Both complexes showed reversible
oxidations (1, +0.67 V; 7, +1.04 V).
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