Pyrazine-imide complexes: Reversible redox and MOF building blocks

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ARTICLE TYPE

Pyrazine-imide complexes: reversible redox and MOF building blocks

Matthew G. Cowan, Reece G. Miller and Sally Brooker*

Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX

DOI: 10.1039/b000000x

5

The synthesis of the symmetric pyrazine imide ligand, N-(2-pyrazylcarbonyl)-2-pyrazinecarboxamide,

(Hdpzca) and five new first row transition metal complexes of it are reported: [M^II(dpzca)₂], M^II= Fe, Cu,

Zn; {[Cu^II(dpzca)(H₂O)_n]_{1 or 2}X}, X = BF₄^-or SiF₆^2-, n = 2 or 3. The crystal structures of Hdpzca,

[Co^II(dpzca)₂], [Cu^II(dpzca)₂], {[Co^III(dpzca)₂](BF₄)}₂·5CH₃CN and [Cu^II(dpzca)(H₂O)₃]₂SiF₆·2H₂O were

determined and reveal an orthogonal positioning of the ‘spare’ pyrazine nitrogen atoms and ‘spare’ pairs

¹⁰of imide oxygen atoms. The [M^II(dpzca)₂] complexes are therefore useful six-coordinate building blocks

for producing larger supramolecular assemblies. Two examples of secondary assembly of [M^II(dpzca)₂]

complexes, with M = Co and Ni, with silver nitrate gave single crystals: {[Co^III(dpzca)₂Ag](NO₃)₂

ꢀ2H₂O}_nand {([Ni^II(dpzca)₂Ag^I](½NO₃)(xH₂O}_nwere structurally characterised . The redox processes of

½

[M^II(dpzca)₂], with M^II= Fe, Ni, Cu and Zn, are reported and, as seen for M^II= Co, reversible metal- and

15 ligand-based redox processes are observed, with E_m(M^II/M^III) values 0.15-0.24 V higher than for the

analogous complexes of Hpypzca (non-symmetric pyridine/pyrazine imide ligand), and 0.35-0.36 V

higher than for the complexes of Hbpca (symmetric pyridine imide ligand).

Introduction

O

Imides are a relatively under-exploited ligand class, with only 12

²⁰imide ligands and 150 structurally characterised imide complexes

reported by mid-2011.¹The first structurally characterised imide

complex was reported in 1976, with the imide formed

accidentally, by hydrolysis of the initial triazine ligand.^{2, 3}By far

the most commonly employed imide is the symmetric pyridine

²⁵based ligand Hbpca (Fig. 1). Deprotonation of such imide ligands

results in a mono-anionic N₃-donor ligand and the formation of a

wide variety of complexes. The mononuclear [M(bpca)Y_n]^x+and

[M(bpca)₂]^x+complexes have a pair or two pairs, respectively, of

‘spare’ imide oxygen atoms which are available for secondary

30 coordination to additional metal ions (transition metal or

lanthanide ions). Hence they have been used as building blocks to

generate a large number of discrete and polymeric structural

motifs.¹The resulting 1-, 2- and 3-D assemblies are frequently

mixed-valent or mixed-metal complexes, and have displayed a

35 range of interesting properties including Single Molecule

N

H

N

Hpypzca

Hbpca

Hdpzca

Fig. 1 The most studied imide ligand to date, Hbpca, and two new imide

55 ligands, Hpypzca and Hdpzca, developed in this group.

5

7

Magnetism (SMM),^4,Single Chain Magnetism (SCM)^6,or

luminescence,⁸as well as generating anion-exchangeable⁹or CO₂

vs N₂selective¹⁰Metal Organic Frameworks (MOFs).

We have previously reported the synthesis, structures and

40 redox properties of a family of transition metal complexes of a

new, non-symmetric pyrazine/pyridine analogue, Hpypzca, of the

symmetric pyridine based imide ligand Hbpca (Fig. 1 & 2).¹¹

Consistent with replacing one pyridine ring by a pyrazine ring in

each ligand, the [M^II(pypzca)₂] complexes with M = Ni, Co, Fe

45 undergo reversible redox, M^II↔ M^III, at higher potentials than

observed for the analogous Hbpca complexes. We also noted that

the ‘spare’ pyrazine nitrogen atoms add another pair of potential

secondary coordination sites to these complexes, forming a

‘corner’ type building block, that the usual secondary

50 coordination through the pairs of ‘spare’ imide oxygen atoms can

extend along a third axis (Fig. 2, top right).

Fig.2 The structures of [Cu^II(pypzca)₂] (top left) and [Cu^II(dpzca)₂]

(bottom left). These complexes can be thought of as ‘building blocks’

(right). The ‘all pyrazine complex’ building block is reminiscent of an

60 octahedron, with the ‘spare’ pyrazine nitrogen atoms (blue arrows) the

four imide oxygen atoms (red arrows) positioned orthogonally.

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equivalents of the acid chloride were then added to ensure that

none of the amide starting material remained. Firstly, the excess

is necessary as the acid chloride decomposes during the reaction,

and secondly the amide proved to be difficult to separate from the

⁵⁵imide ligand. Once thin layer chromatography of the reaction

solution (using 1:1 dichloromethane/acetone) showed that no

amide reagent remained, methanol was added to the reaction

solution to convert any remaining acid chloride reagent to the

corresponding ester, thereby making it trivial to separate from the

⁶⁰imide ligand. The solution was evaporated to dryness under

reduced pressure, the resulting solid suspended in

dichloromethane and filtered through a celite plug. This removed

the purple by-product as it adheres to the celite (N.B. silica gel

does not work). The dichloromethane filtrate was taken to

⁶⁵dryness, acetone added and the resulting suspension sonicated

before filtration to give clean Hdpzca in good yield (77%). The

excess ester could then be recovered from the acetone solution.

Single crystals of Hdpzca suitable for structural characterisation

were obtained by vapour diffusion of diethylether into a

⁷⁰dichloromethane solution.

More recently, we have developed access to a new symmetric

pyrazine based ligand Hdpzca (Fig. 1), in which both of the

pyridine rings are replaced by pyrazine rings. The resulting

mononuclear [Co^II(dpzca)₂] complex was shown to be triply

5

switchable, undergoing temperature or pressure induced spin

13

crossover as well as reversible Co^II↔ Co^IIIredox.^12,

In

addition, the cobalt and nickel [M^II/III(dpzca)₂] building block

complexes (Figure 2 bottom; M = Co^II, Co^IIIand Ni^II) were used,

via secondary coordination to Ag^Iions (i.e. as secondary building

10 units, SBUs¹⁴), to generate two coordination polymers, or MOFs,

[Co^III(dpzca)₂Ag](BF₄)₂(1) and [Ni^II(dpzca)₂Ag]BF₄(2).¹⁰These

MOFs are robust and isostructural, but due to the different

oxidation states they differ in the number of anions in the

channels. Both selectively absorb CO₂over N₂, albeit with a low

¹⁵CO₂capacity. Interestingly 1 is more selective than 2, perhaps

due to the greater anion content of the channels.¹⁰

Here we detail the synthesis of the Hdpzca ligand, as well as

the preparation, structures, magnetic and electrochemical

properties of a series of mononuclear transition metal building

²⁰block complexes from it, and the synthesis and structures of two

coordination polymers by reaction of two of these SBUs with

silver(I) nitrate (Fig. 2 and 3).

Synthesis of [M^II(dpzca)₂] complexes

As for the synthesis of [Co^II(dpzca)₂] (red, 87%) and

12

[Ni^II(dpzca)₂] (tan, 85%),^10,

the complexes [Fe^II(dpzca)₂]

(purple, 94%), [Cu^II(dpzca)₂] (green, 84%) and [Zn^II(dpzca)₂]

75 (white, 44%) were prepared by combining a methanol or acetone

solution of the hydrated tetrafluoroborate metal salt and a

dichloromethane solution containing two equivalents of both

Hdpzca and triethylamine. The desired complexes immediately

precipitated and the resulting powders were isolated by filtration

⁸⁰before drying in vacuo.

Results and Discussion

Ligand Synthesis

25 The synthetic method used to produce Hpypzca and other imide

ligands^{8, 11, 15}was adapted for the preparation of the new imide

ligand Hdpzca. Accordingly, in the first step, 2-pyrazinecarbonyl

chloride was prepared from commercially available 2-

pyrazinecarboxylic acid using SOCl₂. In our hands, the

30 distillation methods reported in the literature¹⁶failed to

reproducibly purify the acid chloride from the dark purple by-

product (which the literature for the pyridine analogue¹⁷indicates

is the product of self-condensation of the acid chloride to form a

dihydropyrazine-pyrazinium salt). Hence we instead used the

³⁵crude, purple, acid chloride sample without further purification in

the next step, reaction with commercially available 2-

pyrazinecarboxamide to form the imide ligand (Fig. 3). More of

the dark purple contaminant was formed during the reaction to

form the imide, but it did not appear to interfere with the reaction

40 or to react with the amide to generate additional by-products.

Rather, the desired ligand, Hdpzca, was obtained.

At room temperature these [M^II(dpzca)₂] complexes are

insoluble in most common solvents and have very low solubility

in others (Tables S1-S5) which hindered attempts to generate

crystalline samples by recrystallisation. Instead, crystalline

⁸⁵samples of all of the complexes, except that of copper(II), were

obtained by carrying out the reactions by the slow diffusion of

triethylamine into

a well-mixed methanol/dichloromethane

solution of the metal salt and ligand. In contrast, mixing the

copper(II) salt and Hdpzca caused immediate precipitation of

⁹⁰[Cu^II(dpzca)₂] as a powder, without addition of base. That the

reaction with copper(II) does not require a base is unsurprising

given its well established ability to compete for nitrogen atom

donors with protons.¹⁸

Of the crystals grown using this slow diffusion of

95 triethylamine method, only those of [Co^II(dpzca)₂] were suitable

for structural characterisation.¹²Crystals of [Cu^II(dpzca)₂]

suitable for structural characterisation were grown from slow

evaporation of a small amount of dilute chloroform solution.

Synthesis of [M^III(dpzca)₂]X complexes

100 The

mild

oxidative

conditions

used

to

prepare

[Co^III(pypzca)₂]BF₄(addition of water to the reaction

suspension)¹¹did not work for the synthesis of

[Co^III(dpzca)₂]BF₄, probably due to the very low solubility of

[Co^II(dpzca)₂]. This also prevented facile chemical oxidation of

105 [Co^II(dpzca)₂] by the stronger oxidant hydrogen peroxide (Fig. 3).

Instead a synthetic route from the analogous amide ligand, (2-

pyridylmethyl)-2-pyridinecarboxamide, HL (Fig. 3), was

developed and optimised,¹⁰based on observations from the

investigations of related amide based ligands which show facile

110 oxidation of methylene links in amides to imides in the presence

of transition metal ions.^{1, 19-21}

Fig. 3 The preparation of the new symmetric pyrazine imide ligand

45

Hdpzca, and a summary of the two synthetic routes explored for the

preparation of [Co^III(dpzca)₂]BF₄.

The best yield of Hdpzca was obtained by refluxing a 40%

excess of the crude acid chloride with the amide in dry toluene

50 for three days (Fig. 3). For two reasons, an additional 0.85

The methylene-linked amide ligand, HL, was prepared, then

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combined with triethylamine and [Co^II(H₂O)₆](BF₄)₂in methanol

(N.B. this does not work in acetonitrile), and treated with 5.5

equivalents of hydrogen peroxide (Fig. 3). The crude

[Co^III(dpzca)₂]BF₄gradually precipitated and was isolated by

filtration, then washed with a large amount of methanol to

remove the excess and protonated triethylamine (traces readily

observed by mass spectrometry) and recrystallised from an

approximately 9:1 acetonitrile/water mixture. In practice this was

done by suspending approximately 100 mg of material in 15 mL

5

¹⁰of boiling acetonitrile and adding water until the suspension

became a solution. Crystals would not grow when the procedure

was attempted in methanol or in water. Furthermore, the scale of

the recrystallisation was unable to be successfully increased

beyond 150 mg. Crystals of {[Co^III(dpzca)₂]BF₄}₂·5CH₃CN were

¹⁵prepared on one occasion by vapour diffusion of diethylether into

an acetonitrile solution of [Co^II(dpzca)₂](BF₄) and silver

tetrafluoroborate. This method could not be used for purification

purposes as it does not separate {[Co^III(dpzca)₂](BF₄)}₂·5MeCN

from contaminants including unoxidised [Co^II(dpzca)₂] and Ag^o.

²⁰As reported for the series of complexes generated with

Hpypzca,¹¹the carbonyl stretch of [Co^III(dpzca)₂]BF₄(1719 cm^-1)

is considerably higher in energy than that of the [M^II(dpzca)₂]

complexes (1692-1701 cm^-1). The stretch is similar to those of the

[M^III(pypzca)₂]BF₄(M^III= Fe, Co) complexes (1719-1722 cm^-

25 ¹).¹¹

Fig.4 The structure of metal-free ligand Hdpzca. Note the planarity of the

entire ligand.

The structure of metal-free Hdpzca is planar (Fig. 4): the

60 largest deviation from the plane of all non-hydrogen atoms is the

imide nitrogen atom [-0.069(3) Å]. This feature is similar to the

symmetric quinoline based ligand Hbqca²³but not to the

pyridine/imide oligomers which adopt a helical structure.¹⁵The

remarkable planarity is the result of Hdpzca being fully

⁶⁵conjugated and the bifurcated intra-molecular hydrogen bonding

between the imide hydrogen atom H(3)X and the pyrazine

nitrogen atoms N(1) and N(5) [N(3)···N(1) 2.636(4),

N(3)···H(3)X···N(1)

111.9°;

N(3)···N(5)

2.631(4),

N(3)···H(3)X···N(5) 111.9°]. The ¹H-NMR signal for H(3)X is at

⁷⁰12.42 ppm in d₆-DMSO, comparable to those observed for

pyridine/imide oligomers (12.96 ppm CDCl₃, 12.66 ppm in d₆-

DMSO) which are also stabilised by intra-molecular hydrogen

bonds in the solid state.¹⁵Importantly the imide hydrogen signal

has been reported at 10.28 (CDCl₃) and 11.53 ppm (d₆-DMSO)

75 for imide molecules that cannot undergo similar intra-molecular

hydrogen bonding.²⁴Unfortunately these molecules have not been

Synthesis of [Cu^II(dpzca)(H₂O)_n]X complexes

The preparation of [Cu^II(dpzca)(H₂O)₂]BF₄·H₂O was attempted in

order to examine the expected differences in secondary

³⁰interactions between [Cu^II(dpzca)(H₂O)₂]BF₄·H₂O and its non-

symmetric ligand analogue [Cu^II(pypzca)(H₂O)₂]BF₄·H₂O.¹¹

Initially

[Cu^II(pypzca)(H₂O)₂]BF₄·H₂O was followed. A 1:1 mixture of

copper(II) and Hdpzca was combined in

the

same

reaction

protocol

used

for

structurally characterised, preventing

comparison.

a detailed structural

a

For the complexes of Hdpzca we observe that the ligand is

80 deprotonated and coordinated via the N₃donor set (Fig. 2, 5, 6,

S1), in an identical fashion to the analogues Hbpca and

35 methanol/dichloromethane solution, followed by the addition of

water and slow evaporation. The blue-green crystals obtained in

this manner were used for X-ray structure determination,

however they were found to be of very low quality and the

resulting structure of [Cu^II(dpzca)(H₂O)₂]BF₄·H₂O was

⁴⁰incomplete so is not presented here. During attempts to

recrystallise the sample from boiling water a slight colour change

to a darker green was observed. Structural characterisation of the

resulting crystals showed that the tetrafluoroborate anions had

presumably reacted with silicon dioxide from the glassware, as

45 they had formed hexafluorosilicate anions, producing crystals of

the complex [Cu^II(dpzca)(H₂O)₃]₂SiF₆·2H₂O in 29% yield. This

11

Hpypzca.^1,For Hdpzca this leaves two sets of imide oxygen

atoms and four ‘spare’ pyrazine nitrogen donors available for

secondary coordination.¹⁰The octahedral orientation of these

⁸⁵‘sets’ of assembly instructions is summarised in Table 1 and

illustrated in Figure 2.

The general structural features of these four complexes are

similar to those summarised for the analogous complexes of

Hpypzca.¹¹

In

{[Co^III(dpzca)₂]BF₄}₂·5CH₃CN,

and

90 [Cu^II(dpzca)₂], two meridionally coordinated dpzca anions are

bound to the metal ion at almost right angles to one another,

providing an approximately octahedral coordination geometry

(Fig. 2, 6 and S1, Table 1). As anticipated from previous studies,

all of the M-N bond lengths are shorter for cobalt(III) than for the

95 divalent transition metal ions. The copper(II) ion of

[Cu^II(dpzca)₂] is slightly more Jahn-Teller distorted than the

2-

synthesis was reproducible. The efficient production of SiF₆

anions has been reported previously, including for copper(II)

complexes of the related amide ligand H₂L^S1.²²

50 Crystal structures

Hdpzca,

[Co^II(dpzca)₂],¹²

{[Co^III(dpzca)₂]BF₄}₂·5CH₃CN,

copper(II) ion of [Cu^II(pypzca)₂] (T

respectively).¹¹

= 0.87 and 0.90

[Cu^II(dpzca)₂], and [Cu^II(dpzca)(H₂O)₃]₂SiF₆·2H₂O have been

structurally characterised (Fig. 2, 4-6, S1 and Tables 1 and S6-

S8). The structural characteristics of the high and low spin forms

55 of [Co^II(dpzca)₂] are discussed in detail in our previous paper.¹²

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Table 1 Selected bond lengths (Å), angles (º) and other parameters for the structurally characterised monometallic [M(dpzca)₂]^0/+complexes.

12

Parameter

[Co^II(dpzca)₂]^d,

[Co^III(dpzca)₂]BF₄·5CH₃CN

[Cu^II(dpzca)₂]

Temperature (K)

M-N_pz

298

93

90

1.929(4), 1.925(4)

1.931(4), 1.930(4)

2.306(2), 2.320(2)

2.063(2), 2.059(2)

2.145(3)

2.049(4)

77.51(7)

102.49(7)

M-N_imide

1.882(4), 1.894(4)

1.993(2), 1.955(2)

83.8(2), 83.6(2)

84.3(2), 83.0(2)

78.18(8), 77.25(8)

81.24(8), 80.86(2)

N_pz-M-N_imide

N_pz-M-N_pz

167.4(2), 167.4(2)

155.43(7), 161.91(8)

O(1) 0.167(7)

O(2) -0.169(7)

O(21) 0.307(7)

O(22) -0.387(7)

O(1) -0.376

O(1A) 0.376

-0.15(1); 0.25(1)

0.105(9); -0.100(8)

O_imideout of N₃plane^a

78.7, 79.2

79.6, 79.3

70.7, 70.9

76.0, 76.3

c

Intra-ligand centroid_OO^b···M···N_pz’

73.3

106.7

100, 100

101, 100

102, 106

105, 113

c

Inter-ligand centroid_OO^b···M···N_pz’

158.79(7)

157.92(9)

141.60(4)

152.07(4)

Intra-ligand N_pz’···M···N_pz’

155.0(1)

95.69(8), 88.53(8)

90.92(8), 92.88(8)

90.22(4), 97.34(4)

93.50(4), 97.12(4)

Inter-ligand N_pz’···M···N_pz’

94.73(3)

Distortion parameter ^e

Σ = 110.6

Σ = 54.6

Σ = 83.7, T = 0.87

^aThe plane is defined by the coordinated N₃donor set of the same ligand; ^bcentroid of the oxygen imide atoms; ^cpz and pz’ refer to the coordinated and

‘spare’ pyrazine donors respectively; ^dNote all pyrazine rings are equivalent due to crystallographic symmetry; ^eDistortion parameters were calculated as

described in the following references T,²⁵Σ.²⁶

5

Fig. 5 The packing of [Cu^II(dpzca)(H₂O)₃]₂SiF₆·2H₂O. The complex forms dimers (top left, A = 1-x, 1-y, 2-z). These dimers are assembled through

hydrogen bonding interactions into double stranded chains (top right (B = x+1, y, z) and bottom left (B = x, y-1, z)). These stacks are separated by layers

of SiF₆^2-counterions and water molecules (bottom right A = -x, 2-y, 1-z)). Note that non-acidic hydrogen atoms, solvent molecules and anions have been

excluded for clarity where appropriate.

10

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between two of the three coordinated water molecules and the

uncoordinated water molecules of solvation [O(51)-H···O(11)

45 2.627 Å, 158°; O(12)-H···O(41) 2.782 Å, 166°].

Electrochemical studies

Despite the low solubility of the [M^II(dpzca)₂] complexes in

acetonitrile, the electrochemical processes could be observed

(Table 1), either by using solutions of < 1 mmolL^-1complex

50 and/or by studying a suspension. As for the pypzca complexes,¹¹

the magnitude of the current signal was increased by using bulk

electrolysis of the suspension to provide the more soluble

[M^III(dpzca)₂]⁺form in situ (Fig. 7 & 8). The quasi-reversible

oxidation of [Ni^II(dpzca)₂] ↔ [Ni^III(dpzca)₂]⁺was accompanied

⁵⁵by a significant colour change from an almost colourless

suspension to a dark green solution (Fig. S11). Likewise the

reversible oxidation of [Fe^II(dpzca)₂] ↔ [Fe^III(dpzca)₂]⁺was

accompanied by a significant colour change from a dark purple

suspension to an orange solution (Fig. 7). The bulk electrolysis

⁶⁰carried out on this compound (Fig. S5 and S6) required

significantly longer (15000 vs. 4000 seconds) than that on

[Ni^II(dpzca)₂], as [Fe^II(dpzca)₂] is much less soluble, indeed is

almost completely insoluble, in acetonitrile.

Fig. 6 The structure of {[Co^III(dpzca)₂](BF₄)}₂·5CH₃CN (H atoms,

solvent molecules and a second cation/anion pair in the asymmetric unit

have been excluded for clarity.

5

The secondary coordination sites in the [M^II/III(dpzca)₂]^0/+units

are arranged in an approximately octahedral fashion (Fig. 2,

Table 1). This makes the complexes useful as SBUs, for example

in the construction of MOFs,^{27, 28}as we recently demonstrated for

the M = Co^IIIand Ni^IIcomplexes using a silver(I) connector, with

10 the resulting pair of robust isomorphous MOFs, 1 and 2, capable

of selective gas uptake.¹⁰Similar to the complexes of Hpypzca

the angles between the ‘spare’ pyrazine nitrogen atoms on

separate ligands are approximately orthogonal (88.53-97.34°).

However the angle between ‘spare’ pyrazine donors within the

¹⁵same ligand varies between 141.60 and 158.79°, quite distorted

from the ‘ideal’ 180°. Similarly the angles between the centroid

of the imide oxygen atoms and the spare pyrazine nitrogen atoms

is distorted away from the orthogonal ideal for both for intra-

(70.7-79.6°) and inter-ligand (100.4-113.2°) cases.

The reversible ligand-based electrochemical processes of the

⁶⁵complexes [Co^II(dpzca)₂],¹²[Fe^II(dpzca)₂], [Ni^II(dpzca)₂] and

[Zn^II(dpzca)₂] (Fig. 8, S4-S16, Table 2) are similar to those

reported for [M^II/III(bpca)₂]^0/+and [M^II/III(pypzca)₂]^0/+(Tables 3,

4). However, the M^II↔ M^IIIprocesses occur 0.15-0.24 V higher

than those of the pyrazine-pyridine imide complexes,

70 [M^II/III(pypzca)₂]^0/+, and in turn these occur at 0.11-0.20 V higher

than for the pyridine-pyridine imide complexes, [M^II/III(bpca)₂]^0/+

,

indicating that the dpzca anion is the best of these three imide

ligands at stabilising the lower M^IIoxidation state. This is also

consistent with the expectation that of these three imide ligands

⁷⁵the pyrazine-pyrazine dpzca anion is the poorest σ-donor and best

π-acceptor, whereas the pyridine-pyridine bpca anion is the best

σ-donor and poorest π-acceptor. Replacing the first two

coordinated pyridine rings with pyrazine rings increases

E_m(M^II/III) by about +0.1 to +0.2 V, and the second two by

⁸⁰another +0.15 to +0.24 V, which may be useful for the informed

design and tuning of electrochemical properties in future systems.

20

Similar to the copper(II) complex of the non-symmetric

(pyridine/pyrazine imide) ligand, [Cu^II(pypzca)(H₂O)₂]BF₄·H₂O,

that of the symmetric (all pyrazine imide) ligand,

[Cu^II(dpzca)(H₂O)₃]₂SiF₆·2H₂O, features

a large number of

strong hydrogen bonding interactions (Fig. 5). The

25 [Cu^II(dpzca)(H₂O)₃]⁺units are linked together into dimers

through uneven ‘two acceptor’ bifurcated hydrogen bonds to the

axially

coordinated

water

molecule

[O(31)]

[O(31)-H···O(1A)/O(2A) 3.387, 2.782 Å; 125.60, 160.25°]. The

dimers are stacked into double stranded chains by O(31)

30 accepting a hydrogen bond from the other axially coordinated

water molecule [O(12)] on an adjacent water molecule [O(12)-

H···O(31B) 2.857 Å; 176°]. These chains are connected in the

second dimension by O(31) hydrogen bonding to a ‘spare’

pyrazine nitrogen atom on an neighbouring complex [H-

³⁵O31···N2 2.937 Å, 170°]. These chains are separated by layers of

hexafluorosilicate anions and solvent water molecules.

A

hydrogen bond network is formed between the solvent water

molecules and anions [O(51)-H···O(41) 2.935 Å, 163°;

[O(41A)-H···F(11B) 3.151 Å, 119°; O(41A)-H···F(12C)

40 2.708 Å, 143°; O(51)-H···F(11) 2.877 Å, 109°; O(51)-H···F(13)

2.901 Å, 123°]. A connection between the chains of complexes

and the anion/solvent layers occurs through hydrogen bonds

Fig. 7. The reversible colour change observed during the oxidation of

[Fe^II(dpzca)₂] (left) to [Fe^III(dpzca)₂]⁺(right). Controlled potential

85 electrolysis was performed at 0.65 V vs 0.01 M AgNO₃/Ag, on 10 mL of

a 0.518 mmol L^-1suspension of [Fe^II(dpzca)₂] in acetonitrile, resulting in

the transfer of 0.94 electron equivalents per mole.

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Fig. 8 A scan rate study on a 0.518 mmol L^-1solution of [Fe^III(dpzca)₂]⁺in acetonitrile (after bulk electrolysis of a suspension of [Fe^II(dpzca)₂] at 0.65 V

passed 0.94 electron equivalents per mole) of (left) the ligand-based processes (-1→-1.9→-1 V) and (right) the reversible metal based processes

(0→1.0→0 V). Reference 0.01 M AgNO₃/Ag.

5

Table 2 E_m(V) and [ꢁE] (mV) values in acetonitrile for the [M(dpzca)₂] complexes reported in this work (referenced to 0.01 M Ag⁺/AgNO₃).

M^II/III

E_m[∆E] for [M(dpzca)₂]

1^stLigand Reduction

-1.49 [70]

Metal Based Process

2^ndLigand Reduction

Fe^a

Co^{b, ref 12}

Ni^c

0.40 [70]

-1.67 [80]

-0.22 [61]

0.96 [43]

-0.79^d

-1.46 [71]

-1.60 [22]

-1.84 [77]

-1.78 [63]

Cu

Zn

-1.62 [30]

-1.77 [18]

^aAfter bulk electrolysis at 0.65 V transferred 0.94 electron equivalents. ^bAfter bulk electrolysis at 0.0 V transferred 1.04 electron equivalents. ^cAfter bulk

electrolysis to 1.1 V transferred 0.95 electron equivalents. ^dThis process is very weak, due to the low solubility of this complex (see ESI).

10 Table 3 The differences in E_m(V) for the metal based processes of the mononuclear complexes [M(bpca)₂],²⁹[M(pypzca)₂],¹¹and [M(dpzca)₂].

E_mfor

[M(bpca)₂]²⁹

(-0.74) -1.072^a

(+0.35) +0.048^a

(-0.28) -0.582^a

∆

E_m[∆E] for

∆

E_m[∆E] for [M(dpzca)₂]

(present work)

M^II/III

[M(pypzca)₂]¹¹

Cr

Fe

Co

Ni

0.11

0.21

+0.16 [73]

-0.37 [79]

+0.78 [68]

0.24

0.15

0.18

0.40 [70]

-0.22 [61]

0.96 [43]

^aThe values in brackets were recorded as 1 mM solutions in a 0.1 M nBu₄NPF₆acetonitrile solution using a glassy carbon electrode at a scan rate of 50

mVs^-1vs. SSCE. In order to compare these SSCE referenced literature values with those recorded for the [M(pypzca)₂] and [M(dpzca)₂] complexes which

used a 0.01 M Ag⁺/AgNO₃reference, 0.302 V has been subtracted from the literature values.³⁰

15 Table 4 The differences in E_m(V) for the first ligand-based processes of the mononuclear complexes [M(bpca)₂],²⁹[M(pypzca)₂],¹¹and [M(dpzca)₂].

E_mfor

[M(bpca)₂]²⁹

(-1.47) -1.77^a

∆

E_m[∆E] for

∆

E_m[∆E] for [M(dpzca)₂]

M^II/III

[M(pypzca)₂]¹¹

Cr

Fe

Co

Ni

Zn

-1.64 [70]

-1.56 [70]

-1.72 [78]

-1.72 [81]

0.15

0.1

-1.49 [70]

-1.46 [71] ^{ref 12}

-1.60 [22]

(-1.66) -1.96^a

0.4

0.12

0.1

-1.62 [30]

^aThe values in brackets were recorded as 1 mM solutions in a 0.1 M nBu₄NPF₆acetonitrile solution using a glassy carbon electrode at a scan rate of 50

mVs^-1vs. SSCE. In order to compare these SSCE referenced literature values with those recorded for the [M(pypzca)₂] and [M(dpzca)₂] complexes which

used a 0.01 M Ag⁺/AgNO₃reference, 0.302 V has been subtracted from the literature values.³⁰

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through coordination to the ‘spare’ pyrazine nitrogen atoms on

40 four of the adjacent SBUs and to a pair of ‘spare’ bidentate imide

oxygen atoms on a fifth adjacent SBU (Fig. 12). The remaining

‘spare’ imide oxygen pair on each SBU forms one wall of the

anion containing channels through non-classical C-H···O

hydrogen bonds with the hydrogen atoms on adjacent pyrazine

⁴⁵rings [C(H)···O 3.009(11) Å, 179.6°; C(H)···O 3.271(11) Å,

154.2°].

Magnetic Properties

To our knowledge, [Co^II(dpzca)₂] is only the second cobalt(II)

imide complex which has been reported in the literature,¹the first

being the analogous complex, [Co^II(pypzca)₂], of the non-

symmetric pyridine/pyrazine imide ligand.¹¹As we reported

earlier, whilst the latter is high spin throughout the temperature

range studied,¹¹the former exhibits remarkable magnetic

properties (pressure and temperature induced spin crossover with

hysteresis; and reversible redox properties).^{12, 13}

5

10

Both [Fe^II(dpzca)₂] and [Co^III(dpzca)₂]BF₄are low spin and

hence diamagnetic at room temperature. The room temperature

ꢂ_effvalues for [Ni^II(dpzca)₂] (3.3 BM) and [Cu^II(dpzca)₂] (2.2

BM) are within the range expected for two and one unpaired

electron, respectively.

¹⁵Supramolecular architectures: secondary assembly of SBUs

The [M^II/III(dpzca)₂]X_nbuilding block complexes with M = Co^II/III

or Ni^IIwere successfully assembled with silver(I)

tetrafluoroborate to form CO₂-selective MOFs.¹⁰Hence assembly

of these two SBUs using a different silver(I) salt, specifically

20 silver(I) nitrate, were carried out in order to test whether or not

analogous MOFs with nitrate instead of tetrafluoroborate anions

in the channels, could be accessed. However, unlike the

previously

reported

assembly

reactions

with

silver

tetrafluoroborate, the reactions with silver nitrate did not produce

²⁵a pair of isostructural Co^IIIand Ni^IIMOFs. Rather, the reaction of

[Co^II(dpzca)₂]

resulted

in

the

3D

framework

{[Co^III(dpzca)₂Ag^I](NO₃)₂}_nꢀ2H₂O (3), that is structurally distinct

from those previously reported (Fig. 9-10). In contrast, the

reaction of [Ni^II(dpzca)₂] with silver nitrate in 1:1 water/acetone

30 resulted in some green single crystals of

a 1D chain,

{([Ni^II(dpzca)₂Ag^I](½NO₃)(xH₂O}_n(4) (Fig. 11 and S1).

½

The new nitrate-containing MOF, 3, crystallised in the

monoclinic space group P2₁/m rather than the orthorhombic

Cmcm observed for the pair of previously reported

³⁵tetrafluoroborate-containing MOFs, {[Co^III(dpzca)₂Ag](BF₄)₂ꢀ

Fig. 9. Perspective views of the 3D framework

{[Co^III(dpzca)₂Ag^I](NO₃)₂}_nꢀ2H₂O (3), showing only one component of

50 the solvent and anion disorder (in the channels) for clarity. Top: looking

down the channels at an angle of 45° to the crystallographic a-axis.

Bottom: a channel running left to right, viewed down the b-axis.

(H₂O)₂}_n(1) and {[Ni^II(dpzca)₂Ag](BF₄) (acetone)_0.5

} (2), but

n

overall is otherwise quite similar (Fig. 9, top). As was observed

for 1 and 2, the silver(I) linker in 3 adopts a rare N₄O₂donor set

Fig. 10. Views showing (left): the stepping of the SBUs by 1.3 Å down the channels in nitrate MOF 3 (viewed with the plane of the page bisecting the a-

55 and c-axes in the vertical, and the b-axis horizontal), as compared to (right) the corresponding view of tetrafluoroborate MOF 1 (as viewed down the b-

axis). The mean planes indicated in magenta are defined by the pyrazine rings in each case. The anions are omitted for clarity.

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Table 5. Selected structural comparisons (distances in Å; angles in °) between the two new architectures and those previously reported.¹⁰

{[Co^III(dpzca)₂Ag](BF₄)₂

{[Ni^II(dpzca)₂Ag](BF₄)

{[Co^III(dpzca)₂Ag^I](NO₃)₂

{([Ni^II(dpzca)₂Ag^I](½NO₃)

½

ꢀ(H₂O)₂}_n(1)

ꢀ(acetone)_0.5}_n(2)

·2(H₂O)}_n(3)

(xH₂O}_n(4)

Type

Crystal system

Space Group

3D Framework

Orthorhombic

Cmcm

3D Framework

Orthorhombic

Cmcm

3D Framework

Monoclinic

1D Chain

Monoclinic

C2/m

P2₁/m

a,b,c (Å)

13.708(3), 14.079(2),

13.798(3)

14.0140(14), 14.4385(14),

13.6760(14)

90

9.3678(4), 13.6094(6),

10.1421(4)

14.619(3), 24.806(4),

9.861(4)

β (°)

V (Å³)

90

90.273(2)

1293.00(9)

122.003(7)

3032.2(14)

2663.1(10)

2767.2(5)

Ag coordination

N₄O₂octahedral

N₄square planar

Average Ag–N length

Average Ag-O length

Ag distortion

2.472

2.449

∑ = 97.94

2.483

2.552

∑ = 72.05

2.441

2.568

∑ = 131.8

2.367

N/A

τ₄= 0

parameter^a

a

Either the octahedral distortion parameter, ∑ (the sum of the absolute values of the difference of each of the 12 cis angles from 90°; 0 for a perfect

octahedron), or the square planar/tetrahedral distortion parameter, τ₄(1 for tetrahedral, 0 for square planar),³³as appropriate.

5

The key difference seen for the nitrate-containing MOF 3 is that

The asymmetric unit of the 1D chain, 4, comprises a half

the Ag(I) linker has a significantly greater octahedral distortion 35 occupancy nickel centre (2 fold axis), one full imide ligand, a

parameter than it does in the previously reported tetrafluoroborate

quarter occupancy silver centre (2/m site) and a quarter

MOFs 1 and 2 (Fig. 12, Table 5). This is due to sequential ligand

10 planes of the SBU perpendicular to the a,c diagonal stepping by

1.3 Å per plane/cobalt centre along that diagonal. This is seen

occupancy nitrate ion (N31 atom on a mirror plane; O32 on a

two-fold axis at right angles to the mirror plane; symmetry

therefore generates four quarter occupancy nitrates in the same

most clearly when viewed either down the resulting sloping ⁴⁰region of space, only one of which can be present at any one

channels (Fig. 10) or viewed side-on noting the angled channel

roof and floor components (Fig. 9, bottom), and comparing these

¹⁵with those in MOF 1. There is no guest accessible void volume in

the as-synthesised structure. The asymmetric unit of MOF 3

time). Applying the symmetry elements generates a 1D chain

(Fig. 11) with almost-square pockets [N_pz1– Ni1 – N_pz1A89.58°;

N_pz2– Ag1 – N_pz2, 94.74°; Ni1···Ag1 = Ni1A···Ag1 7.203(10)

Å], generated from linking a pair of nickel(II) building block

contains one nitrate anion disordered over two half occupancy 45 complexes via

a silver(I) linker to the next pair, that

sites and a single water molecule is disordered over the same two

sites. The closest O···O distances between adjacent partial

20 occupancy nitrate sites is very short [1.89(2) and 2.03(2) Å] so no

two adjacent sites would be occupied at the same time within a

single channel. Rather, the half occupancy nitrate anions alternate

with the half occupancy water molecules as shown (Fig. 9

bottom), with the other component of the disorder similar, but

²⁵with their relative positions reversed. This facilitates H-bonds

between them [O₃₁···(H)O₆₁3.16(2) Å, 123°; O₄₁···(H)O₆₁

2.77(2) Å, 134°; O₃₃···(H)O₅₁2.89(2) Å, 170°; O₄₂···(H)O₅₁

3.05(2) Å, 178°].

accommodate the disordered nitrate anions. The disordered water

molecules of solvation could not be resolved so SQUEEZE was

applied, resulting in an electron count (50) consistent with the

presence of approximately 4-5 water molecules per Ni^IIion.

50

Fig. 12. Perspective views highlighting the differing coordination spheres

of the silver(I) connectors, which are comprised of some of the ‘spare’

donors of the [M^II/III(dpzca)₂]^0/+building blocks, present in the

55 coordination polymers: 3D MOF 3 (M = Co^III, left, N₄O₂) and 1D linear

chain polymer 4 (M = Ni^II, right, N₄).

In contrast to MOFs 1-3, the silver(I) linker in the 1D chain

polymer 4 binds only to the ‘spare’ pyrazine nitrogen atom (N2)

60 (Fig. 12) on each of four symmetry generated adjacent building

block complexes (2 on each side), giving an N₄-donor square

planar coordination. The result is a straight, square ‘concertina’

30

Fig. 11. Perspective view of the 1D chain

{([Ni^II(dpzca)₂Ag^I](½NO₃)(xH₂O}_n(4) in the bc plane. Showing only

½

one component of the four-fold anion disorder; waters of crystallisation

absent as Platon SQUEEZE was applied.^{31, 32}

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Experimental

type structure with the silver and nickel ions as the ‘hinges’, and

each silver ion bridging four building block complexes. Within

the SBUs, the nickel(II) centres coordinate to four pyrazine

nitrogens (N1, N1A, N5, N5A) and two imide nitrogen atoms

(N3, N3A), retaining an octahedral N₆-donor coordination

environment.

General procedures

⁶⁰Unless otherwise stated, all reagents (including 2-pyrazine

carboxylic acid and 2-pyrazine carboxamide) and solvents were

commercially available and used as received. If desired, 2-

pyrazine carboxamide can be prepared from 2-pyrazine

carboxylic acid as described below. Dry toluene was obtained

⁶⁵from a MD-6 solvent purification system. Acetonitrile and

triethylamine were both distilled over calcium hydride before use.

Powder samples of [Ni^II(dpzca)₂] and [Co^III(dpzca)₂](BF₄) were

prepared as described elsewhere.¹⁰

5

Also in contrast to 1-3 in which the M:Ag molar ratio is 1:1,

this ratio is 2:1 in 4, which leaves two ‘spare’ nitrogen atoms per

¹⁰SBU (N4, N4A) that remain uncoordinated. These are instead

involved in weak, non-classical H-bonds with C1 and C2 of a

pyrazine ring on the adjacent chain [C₁(H)···N₄3.127(7) Å,

113.4°; C₂(H)···N₄3.391(9) Å, 98.4°]. The ‘spare’ imide oxygen

atoms also remain uncoordinated, and instead form bifurcated,

¹⁵non-classical, O···H-C hydrogen bonds with pyrazine hydrogens

on adjacent 1-D chains [C₁₀(H)···O₁3.249(7) Å, 151.4°;

C₁₀(H)···O₂3.089(10) Å, 134.4°] (Fig. S2).

Instrumentation and measurements

70 Infrared spectra were recorded on a Bruker Alpha-P diamond

anvil system. NMR spectra were recorded on a Varian 500 MHz

Ar spectrometer. Mass spectra were recorded on a Bruker

MicrOTOF-Q spectrometer. Microanalysis was performed by the

Campbell Microanalytical Laboratory at the University of Otago.

Conclusion

⁷⁵UV-Vis spectra were recorded on

a

JASCO V550

20 The synthesis and structures of four new building block

complexes of the symmetric pyrazine-based imide ligand Hdpzca

are detailed herein. Structural analysis of these complexes has

shown that the secondary assembly instructions, in the form of

four ‘spare’ pyrazine nitrogen atoms and two pairs of ‘spare’

²⁵imide oxygen atoms, are positioned in an approximately

octahedral arrangement. This facilitates the use of these

complexes as building blocks for the production of larger discrete

or polymeric assemblies, as demonstrated herein by the reaction

of [Co^II(dpzca)₂] and [Ni^II(dpzca)₂] with silver nitrate to give the

³⁰3D MOF {[Co^III(dpzca)₂Ag^I](NO₃)₂·2(H₂O)}_n(3) and the 1D

Spectrophotometer. Room temperature magnetic moments were

recorded on an Alfa Products MK I Magnetic Susceptibility

Balance or a Quantum Design Physical Property Measurement

System equipped with a vibrating sample mount, using an applied

⁸⁰field of 0.1 Tesla at Industrial Research Limited (IRL; now RRI

Robinson Research Institute) Lower-Hutt, New Zealand.

X-ray data were collected on a Bruker APEX II area detector

diffractometer at the University of Otago using graphite-

monochromated Mo_Kαradiation (λ = 0.71073 Å). The data were

85 corrected for Lorentz and polarisation effects, and semi-empirical

absorption corrections (SCALE) were applied. The structures

were solved by direct methods (SHELXS-97)³⁴and refined

against all F²data (SHELXL-97).^{35, 36}Other than the exceptions

noted in the CIF files, the hydrogen atoms were inserted at

⁹⁰calculated positions and rode on the atoms to which they were

attached and all non-hydrogen atoms were made anisotropic.

Crystal structure determination details are summarized in Tables

S6-S7. The Cambridge CCDC 1030793-1030799 contains the

supplementary crystallographic data for this paper and can be

⁹⁵obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif.

Electrochemistry samples were 10 mL of a 1 mmol L^-1

solution or suspension (see Figure captions) in dried acetonitrile

(freshly distilled from CaH₂) with 0.1 mol L^-1

tetrabutylammonium hexafluorophosphate as the supporting

100 electrolyte. Data were collected under an argon atmosphere on an

EG & G Princeton Applied Research Potentiostat/Galvanostat

Model 273A, using PowerSuite V2.58 software. The working

electrode was a 1 mm diameter Pt disk electrode and the

reference electrode was Ag/Ag⁺(0.01 mol L^-1AgNO₃in 0.1 mol

105 L^-1tetrabutylammonium hexafluorophosphate with acetonitrile as

the solvent). The Fc/Fc⁺couple was consistently observed at E_m=

0.100 0.005 V with a ꢁE_Pof 0.081 V. E_mis the midpoint

potential (approximately the reversible formal potential),

calculated from the average of E_PAand E_PC. ꢁE_Pis the absolute

chain coordination polymer {([Ni^II(dpzca)₂Ag^I](½NO₃)(xH₂O}_n

½

(4), respectively. In both cases each [M^II(pypzca)₂]^+/0SBU has

N₄O₄secondary donors available: the different structural types

come about due to the silver(I) connector using all but two imide

³⁵oxygen atoms in forming 3, giving it an N₄O₂environment, but

using only two of the pyrazine nitrogen donors, giving it an N₄

environment, in forming 4. The difference in donor type between

the oxygen and nitrogen secondary donor sets also offers the

future possibility of an elegant, stepwise assembly of larger

40 structures by judicious choice of linking metal ions.

As for the previously reported pyridine-pyrazine imide

complexes [M^II(pypzca)₂],¹¹the very low solubility of the all-

pyrazine analogues, [M^II(dpzca)₂], was overcome by using bulk

electrolysis to first prepare the soluble [M^III(dpzca)₂]⁺complexes

45 in situ. For the all pyrazine [M^II(dpzca)₂] complexes the quasi-

reversible metal based processes were observed at potentials

between 0.15-0.24 V higher than those reported for complexes of

Hpypzca (pyridine/pyrazine) and 0.35-0.36 V higher than those

reported for complexes of Hbpca (all pyridine). This indicates

⁵⁰that the electronic differences imposed on the metal centre by a

coordinated pyrazine rather than a pyridine ring is equivalent to

lifting the E_m(M^II/III) by about 0.1 V per ring, significantly

increasing the stability of the +2 oxidation state over the +3. This

information may be useful for the fine tuning of electron density

55 dependent properties of such complexes for future applications as

catalysts or spin crossover-based nanocomponents.

110 value of the difference between E_PAand E_PC

.

2-Pyrazinecarboxamide·H₂O

2-Pyrazine carboxylic acid (100 mg, 0.806 mmol) was suspended

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(2-Pyrazylmethyl)-2-pyrazinecarboxamide (HL)

in ammonium hydroxide (5 mL) and stirred at room temperature

for 12 hours. To the resulting clear colourless solution was added

acetone (25 mL) causing immediate precipitation of 2-

pyrazinecarboxamide as a white microcrystalline solid which was

filtered and washed with acetone (3 x 25 mL), air dried and then

further dried in vacuo (94.3 mg ,0.766 mmol, 95%) Elemental

analysis calcd (%) for C₆H₇N₂O₃(139.113 g mol^-1): Calc. C

42.55, H 5.00, N 29.77; found: C 42.58, H 5.06, N 29.77. H

NMR (400 MHz, CDCl₃): δ (ppm) 9.42 (d, J_3-2= 1.5 Hz 1H, H₃),

10 8.78 (d, J_1-2= 2.5 Hz, 1H, H₁), 8.56 (dd, J_2-1= 2.5 Hz, J_2-

₃= 2.5 Hz 1H, H₂), 7.63 (br s, 1H, H_4a), 5.70 (br s, 1H, H_4b).

A solution of pyrazine-2-carboxylate (2.0 g, 14.5 mmol) and 2-

⁶⁰aminomethylpyrazine (1.5 mL, 13.2 mmol) in methanol (80 mL)

was refluxed for one week. The solution was evaporated to

dryness and the solid purified by column chromatography using

5% methanol/dichloromethane on silica gel (R_f= 0.66). The

product was collected as a white solid (2.37 g, 83%). MS (+ESI)

65 (Methanol): m/z 238.0712 [C₁₀H₉N₅ONa]⁺calc. 238.0699.

Elemental analysis calcd (%) for C₁₀H₉N₅O) (215.21 g mol^-1):

Calc. C 55.81 H 4.22 N 35.24; found: C 55.92 H 4.23 N 32.76.

IR: ν / cm^-1= 3087, 1719, 1654, 1605, 1585, 1411, 1325, 1028,

632. ¹H NMR (400 MHz, CDCl₃): δ (ppm) 9.43 (dd, J_3-2= 1.2 Hz,

70 J_3-1= 0.4 Hz, 1H, H₃), 8.77 (dd, J_1-2= 2.4 Hz, J_1-3= 0.4 Hz, 1H,

H₁), 8.68 (d, J_6-7= 0.8 Hz, ½ 2H, H₆), 8.66 (s(br), ½ 2H, H₄),

5

1

N-(2-Pyrazylcarbonyl)-2-pyrazinecarboxamide (Hdpzca)

Method A. 2-Pyrazine carboxylic acid (0.978 g, 7.88 mmol) was

dissolved in thionyl chloride (10 mL) and refluxed at 120 °C for

¹⁵two hours. The dark purple solution was then evaporated under

reduced pressure and the resulting solid was dissolved in dry

toluene (20 mL). 2-Pyrazine carboxamide (0.530 g, 4.27 mmol)

was then added and the resulting suspension was refluxed at 120

^°C for 3 days. The resulting solution was evaporated and the

²⁰collected solids were redissolved in dichloromethane and filtered

through celite to remove the purple contaminants. The filtrate was

evaporated to dryness and the solids were purified by column

chromatography with 1:1 dichloromethane/acetone on silica gel

(R_f= 0.72). Hdpzca was collected as an off white solid (0.663 g,

25 67%). MS (+ESI) (Acetone): m/z 252.0483 [C₁₁H₈N₄O₂Na]⁺calc.

252.0492. Elemental analysis calcd (%) for C₁₀H₆N₅O₂(229.06):

Calc. C 52.40 H 3.08 N 30.56; found: C 52.22 H 3.06 N 30.31.

IR: ν / cm^-1= 3290, 1757, 1579, 1527, 1462, 1411, 1396, 1288,

1240, 1173, 1127, 1069, 1017, 869, 819, 762, 719, 650, 614, 437.

30 ¹H NMR (500 MHz, d₆-DMSO): δ (ppm) 12.42 (s, 1H, H₄), 9.38

(d, J_3-2= 1.2 Hz, 1H, H₃), 9.05 (d, J_1-2= 2.4 Hz, 1H, H₁), 8.91

(dd, J_2-1= 2.4 Hz, J_2-3= 1.5 Hz, 1H, H₂). ¹³C NMR (500 MHz,

d₆-DMSO): δ (ppm) 160.9 (C_E), 149.2 (C_A), 144.4 (C_C), 143.6

(C_B), 142.9 (C_D).

8.56 (m, 2H, H_2/8), 8.52 (d, J _7-8= 2.8 Hz, 1H, H₆), 4.85 (d, J_5-4

=

4.2 Hz, 2H, H₅). ¹³C NMR (400 MHz, CDCl₃): δ (ppm) 169.9

(C_E), 152.2 (C_D), 147.5 (C_A), 144.5 (C_C), 144.2(C_G), 144.0 (C_B),

⁷⁵143.9 (C_H), 143.7 (C_J), 142.9 (C_I), 42.4 (C_F).

[Fe^II(dpzca)₂]

Solutions of Hdpzca (96.6 mg, 0.422 mmol) in dichloromethane

(5 mL) and iron(II) tetrafluoroborate hexahydrate (74 mg, 0.219

mmol) in methanol (5 mL) were mixed thoroughly and put in one

⁸⁰arm of an H-tube. Triethylamine (3-5 mL) was then placed in the

other arm and the tube was sealed. The triethylamine vapour then

diffused into the mixture and the resulting dark purple crystals

were isolated by filtration and washed with water and

dichloromethane, then dried under vacuum to yield [Fe^II(dpzca)₂]

85 (106 mg, 94%). MS (+ESI) (CH₂Cl₂): m/z 535.0237

[Fe(C₁₀H₆N₅O₂)₂Na]⁺

calc.

535.0285,

513.0386

[Fe(C₁₀H₆N₅O₂)₂H]⁺513.0465. Elemental analysis calcd (%) for

[Fe(C₁₀H₆N₅O₂)₂] (512.22 g mol^-1): Calc. C 46.90 H 2.36 N

27.34; found: C 46.90 H 2.46 N 27.51. IR: ν / cm^-1= 3086 (w),

90 3053 (w), 1692 (s), 1609 (s), 1577 (m), 1518 (w), 1407 (s), 1312

(m), 1255 (s), 1198 (m), 1170 (m), 749 (s), 656 (s). (SQUID, 298

35

Method B. Pyrazine carboxylic acid (1.606 g, 13.1 mmol) was

dissolved in thionyl chloride (10 mL) and refluxed at 120 °C for

two hours. The dark purple solution was then evaporated under

reduced pressure and the resulting solid dissolved in dry toluene

K) 0 BM (diamagnetic). UV-Vis (CH₃NO₂): ε₅₁₄= 9636, ε₅₆₇

6568, ε₆₉₀= 1920 L cm^-1mol^-1.

=

{[Co^III(dpzca)₂](BF₄)}₂·5MeCN

(20 mL). 2-Pyrazinecarboxamide (1.13 g, 9.11 mmol) was then

95 Solid silver tetrafluoroborate (13.4 mg, 0.0688 mmol) was added

to an orange solution of [Co^II(dpzca)₂] (17.4 mg, 0.0289 mmol) in

acetonitrile. No immediate colour change was observed and the

solution was stirred at ambient temperature for 12 hours before

being subjected to vapour diffusion of diethyl ether. After 2

¹⁰⁰weeks a few orange plate shaped crystals suitable for X-ray

crystallography were obtained. Due to the low yield (<1 mg) and

obvious physical contaminants these were only characterised by

X-ray crystallography.

°

⁴⁰added and the resulting suspension was refluxed at 120 C for 3

days. In a separate flask, pyrazine carboxylic acid (0.960 g, 7.8

mmol) was dissolved in thionyl chloride (5 mL) and refluxed at

120°C for two hours, evaporated to dryness under reduced

pressure and the resulting purple solids were added to the first

45 reaction solution. Refluxing at 120°C was continued for three

more days. Methanol (20 mL) was added to the reaction solution

and it was stirred for two hours. The solution was evaporated to

dryness then dissolved in dichloromethane and filtered through

celite to remove the purple contaminants. The filtrate was

50 evaporated to dryness again, and then acetone (100 mL) was

added. The resulting suspension was shaken, sonicated, then

filtered to produce Hdpzca (1.632 g, 77%). Elemental analysis

calcd (%) for C₁₀H₆N₅O₂·0.2H₂O (232.66): Calc. C 51.59 H 3.20

N 30.08; found: C 51.59 H 3.20 N 30.08. ¹H NMR (300 MHz, d₆-

55 DMSO): δ (ppm) 9.39 (d, J_3-2= 0.9 Hz, 1H, H₃), 9.06 (d,

J_1-2= 1.8 Hz, 1H, H₁), 8.91 (dd, J_2-1= 1.8 Hz, J_2-3= 0.9 Hz, 1H,

H₂).

[Cu^II(dpzca)₂]

105 A solution copper(II) tetrafluoroborate hexahydrate (81.7 mg,

0.244 mmol) in acetone (10 mL) was added to a suspension of

Hdpzca (110.4 mg, 0.482 mmol) (10 mL) and dichloromethane (2

mL) with triethylamine (67 ꢂL, 0.481 mmol). After two hours the

resulting green solid was filtered dried under vacuum to yield

110 [Cu^II(dpzca)₂] (106.8 mg, 84%). MS (+ESI) (CHCl₃/CH₃OH):

m/z 542.0264 [Cu(C₁₀H₆N₅O₂)₂Na]⁺calc. 542.0231, 290.9829

[Cu(C₁₀H₆N₅O₂)]⁺290.9812, 252.0507 [(C₁₀H₇N₅O₂)Na]⁺

Dalton Trans., [year], [vol], 00–00 | 10

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252.0492. Elemental analysis calcd (%) for [Cu^II(C₁₀H₆N₅O₂)₂]

(515.07 g mol^-1): Calc. C 46.20 H 2.35 N 27.18; found: C 46.09

H 2.27 N 26.94. IR: ν / cm^-1= 3096 (w), 3061 (w), 3024 (w),

The complex [Co^II(dpzca)₂] (71.2 mg, 0.138 mmol) was

dissolved in 1:1 water/acetone (100 mL) and heated to 100 °C. A

solution of silver(I) nitrate (193 mg, 1.14 mmol) in water (100

1701 (s), 1696 (m), 1617 (m), 1581 (w), 1528 (m), 1320 (s), 1278 ⁶⁰mL) was then added, resulting in an immediate precipitate. After

5

(m), 1151 (s), 1030 (s), 818 (m), 649 (m). µ_eff(Johnson-Matthey,

approximately 30 minutes the suspension cleared and the solution

was refluxed for a further 6 hours. The solution was then filtered

through cotton wool into a conical flask wrapped in tinfoil and

allowed to slowly evaporate. After 2 weeks red single crystals of

⁶⁵{[Co^III(dpzca)₂Ag^I](NO₃)₂·2(H₂O)}_n(3) suitable for X-ray

crystallography were obtained. These were subsequently

collected and air dried (87 mg, 78%). Elemental analysis calcd

(%) for [Co^III(C₁₀H₆N₅O₂)₂Ag^I](NO₃)₂(H₂O)₃(C₃H₆O)_0.15(809.95

g mol^-1): Calc. C 30.33 H 2.35 N 20.75; found: C 30.54 H 1.99 N

70 20.43. IR: ν / cm^-1= 3193 (m), 3089 (w), 1712 (m), 1697 (m),

1624 (m), 1289 (s, br), 1162 (m), 1083 (m), 1038 (m), 781 (m),

640 (m).

298 K) = 2.2 BM. UV-Vis (CH₃NO₂): ε₆₂₆= 317 L cm^-1mol^-1.

[Zn^II(dpzca)₂]

A solution of zinc(II) tetrafluoroborate hydrate (58.3 mg, 0.244

mmol) in methanol (10 mL) was added to a solution of Hdpzca

10 (105.4 mg, 0.460 mmol) in dichloromethane (20 mL) with

triethylamine (64 ꢂL, 0.460 mmol). After two hours the resulting

white solid was filtered and dried under vacuum to yield

[Zn^II(dpzca)₂] (56.1 mg, 44%). MS (+ESI) (CH₃OH): m/z

543.0131 [Zn(C₁₀H₆N₅O₂)₂Na]⁺calc. 543.0227, 252.0455

¹⁵[(C₁₀H₇N₅O₂)Na]⁺252.0492. Elemental analysis calcd (%) for

[Zn^II(C₁₀H₆N₅O₂)₂] (521.76 g mol^-1): Calc. C 46.04 H 2.32 N

26.85; found: C 46.12 H 2.15 N 26.70. IR: ν / cm^-1= 1699 (s),

1617 (m), 1582 (w), 1528 (w), 1468 (m), 1402 (m), 1324 (s),

{([Ni^II(dpzca)₂Ag^I](½NO₃)(xH₂O}_n

½

1279 (m), 1201 (m), 1152 (m), 1085 (w), 1038 (s), 983 (w), 789 75 The complex [Ni^II(dpzca)₂] (67.8 mg, 0.132 mmol) was dissolved

²⁰(m), 649 (m), 631 (s), 454 (s).

in 100 mL of 1:1 acetone/water and heated to 120 °C. A solution

of silver(I) nitrate (176 mg, 1.04 mmol) in water (50 mL) was

then added and the resulting solution refluxed at 120 °C for 6

hours. The solution was then transferred to a conical flask

⁸⁰wrapped in tinfoil and allowed to slowly evaporate. After 3

[Cu^II(dpzca)(H₂O)₂]BF₄·H₂O

Hdpzca (13.9 mg, 0.061 mmol) was dissolved in dichloromethane

(5 mL) and mixed with a solution of copper(II) tetrafluoroborate

hydrate (58.0 mg, 0.253 mmol) in methanol (5 mL). The resulting

25 suspension of blue-green solid was filtered after five hours,

subsequent recrystallisation by slow evaporation from water

yielded [Cu^II(dpzca)(H₂O)₂]BF₄·H₂O (24.1 mg, 91%). MS

(+ESI) (CH₃OH): m/z 313.9724 [Cu(C₁₀H₆N₅O₂)Na]⁺303.9710,

weeks some green single crystals of {([Ni^II(dpzca)₂Ag^I](½NO₃)

½

(xH₂O}_n(4) suitable for X-ray crystallography were obtained.

Acknowledgements

290.9803

[Cu(C₁₀H₆N₅O₂)]⁺

290.9812,

229.9605 85 We thank the University of Otago for funding this research,

30 [(C₁₀H₇N₅O₂)H]⁺230.0673. Elemental analysis calcd (%) for

[Cu^II(C₁₀H₆N₅O₂)(H₂O)₂]BF₄·H₂O (432.58 g mol^-1): Calc. C

29.62 H 2.24 N 17.27; found: C 29.98 H 2.07 N 16.86. IR: ν /

cm^-13399 (w), 3230 (w), 3112 (w), 1715 (m), 1568 (m), 1473

including postgraduate scholarships to MGC and RGM. We are

grateful to Scott A. Cameron for assisting with ‘de-twinning’ the

structure of Hdpzca, Assoc. Prof. David S. Larsen for helpful

discussions, and Dr Juan Olguín and Dr Humphrey L. C. Feltham

(m), 1450 (m), 1076 (s), 1049 (s), 1003 (s), 762 (m), 638 (m). ⁹⁰for collecting the magnetic data. We thank the MacDiarmid

³⁵UV-Vis (H₂O): ε₆₅₄= 133; ε₂₇₈= 15,472; ε₂₀₈= 16,554

Institute for funding the magnetometers and Dr Jeffrey L. Tallon

for facilitating access to them, as well as the Campbell

Microanalytical Laboratory (University of Otago) for performing

the microanalyses.

dm³cm^-1mol^-1.

[Cu^II(dpzca)(H₂O)₃]₂SiF₆

Hdpzca (111.7 mg, 0.488 mmol) was dissolved in

dichloromethane (10 mL) and mixed with a solution of copper(II)

40 tetrafluoroborate hydrate (163.8 mg, 0.490 mmol) in methanol

(10 mL). The resulting suspension of blue-green solid was stirred

for five hours then evaporated to dryness. The solids were

recrystallised from boiling water to yield dark green block shaped

crystals a few of which were kept in the mother liquor for X-ray

95 Notes and references

a

Department of Chemistry and the MacDiarmid Institute for Advanced

Materials and Nanotechnology, University of Otago, PO Box 56, Dunedin

9054, New Zealand. Fax: +64-3-479 7906; Tel: +64-3-479 7919; E-mail:

sbrooker@chemistry.otago.ac.nz

100 † Electronic Supplementary Information (ESI) available: Solubility

information for the [M^II(dpzca)₂] complexes, summary of unsuccessful

syntheses, additional figures and details of X-ray crystal structure

determinations, additional cyclic voltammetry and controlled potential

coulometry data, UV-visble spectra and a figure showing the NMR

45 crystallography ([Cu^II(dpzca)₂(H₂O)₃]SiF₆∙2H₂O). The rest were

2

filtered, washed with cold H₂O and dried under vacuum for 5

days to yield [Cu^II(dpzca)₂(H₂O)₃]SiF₆(62.4 mg, 29%). MS

2

(+ESI) (CH₃OH): m/z 626.9616 [Cu₂(C₁₀H₆N₅O₂)₂SiF]⁺105 numbering, are available. See DOI: 10.1039/b000000x/

626.9226, 308.9872 [Cu(C₁₀H₆N₅O₂)]⁺308.9918, 290.9842

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TOC Entry

Pyrazine-imide complexes: reversible redox

and MOF building blocks

Matthew G. Cowan, Reece G. Miller and Sally Brooker

5

Stepwise exchange of pyrazine for pyridine, in three bis-

heterocycle-based imide ligands, tunes the potential of the

[M^II/III(ligand)₂]^0/+redox couple, increasing the stability of the M^II

¹⁰oxidation state by about 0.1 V per exchange.

Dalton Trans., [year], [vol], 00–00 | 13

Article Doi

Pyrazine-imide complexes: Reversible redox and MOF building blocks

DOI: 10.1039/c4dt03407d

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Article abstract of DOI:10.1039/c4dt03407d

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