Article Novel Countercation in MMX-Type Mixed-Valence Chain Compound: Coexistence of Neutral and Protonated Amino Substituents

The first MMX-type quasi-one-dimensional (Q1D) Pt chain complex (MMX chain) that contains a mono-protonated diamine as countercation, {o-(H3NC6H4NH2)}4[Pt2(pop)4I]·H2O (pop = P2H2O52–), was synthesized. According to the crystal structural analysis, –NH2 group was hydrogen-bonded to either lattice H2O molecule or –NH3+ group in addition to typical hydrogen bond between –NH3+ group and pop ligand. To control the partial deprotonation of the countercation will be an important method for achieving the high-conductive MMX-chain polymer by the hole doping.


Introduction
Study of 1D electron system has been one of the most important subjects from the viewpoints of both fundamental and applied sciences.Conductive polymers found in the 1970s, such as polyacetylene [1,2],

OPEN ACCESS
have principally developed and support the current technological civilization.However, the study of conductive polymers to reveal their fundamental structure and properties has been limited because of the difficulty in making single crystals.On the other hand, conductive materials formed by the stacking of π-conjugated organic molecules were found earlier than conductive polymers [3].The discovery of metallic behavior of TTF-TCNQ [4] attracted many scientists and promoted the studies of various Q1D charge-transfer (CT) complexes.Because these CT complexes can be obtained as single crystal, these are suitable to study the fundamentals of 1D electron system.So far, various electronic states of Q1D materials and their relationships to attractive physical properties, such as non-linearity [5,6], superconductivity [7], ferroelectricity [8] etc., have been extensively studied.
Q1D halogen-bridged metal complexes have the characteristics of both conductive polymer and CT complex, that is, σ-bonded infinite chain structure and ease of making single crystal.These complexes are categorized into two groups by the number of metal ions per unit.One made from mononuclear complex (hereafter abbreviated as MX chains) has The other, made from lantern-type dinuclear complex (hereafter abbreviated as MMX chains) has MX chains has been attracting much attention because of their interesting physical properties, such as gigantic third-order nonlinear optical properties [9,10], midgap absorptions attributable to solitons and polarons [11], and charge-density-wave to Mott-Hubbard phase transition [12].Compared with MX chains, the electrical conductivity of MMX chains is higher.To date, the measurement of electrical conductivity was performed mainly in MMX chains with dithiocarboxylate ligand, [M 2 (RCS 2 ) 4 I] (M = Ni, Pt; R = alkyl chain group) [13][14][15][16][17].It was reported that Pt 2 (CH 3 CS 2 ) 4 I showed high conductivity, 13 S cm −1 , at room temperature, and metal-insulator transition at 300 K [14].On the other hand, MMX chains with pop ligand, A 4 [Pt 2 (pop) 4 X]•nH 2 O (A = Li, K, Cs, NH 4 etc.; X = Cl, Br, I) [18][19][20], have lower conductivity [14].However, pop-type MMX chains are more attractive because it is easier to synthesize various derivatives by changing countercations.Moreover, the new electronic state [21], photo-induced phase transition [22] and vapor-induced reversible switching of the electronic state and the electrical conductivity [23][24][25][26] have been reported, recently.
In order to achieve the higher electrical conductivity in pop-type MMX chains, the doping of hole or electron into 1D chain is a promising method, though no trials have succeeded.Herein, we report the first MMX chains containing mono-protonated diamine as countercation, {o-( (1), which can be the clue to the doping.

Materials
All commercially available chemicals are of reagent grade and used as received without further purification.

Synthesis of {o-(H 3 NC 6 H 4 NH 2 )} 4 [Pt 2 (pop) 4 I]•H 2 O (1)
K 4 [Pt 2 (pop) 4 ]•2H 2 O (29.9 mg, 0.026 mmol) and K 4 [Pt 2 (pop) 4 I 2 ] (30.5 mg, 0.022 mmol) were dissolved in water (4 mL).200 mM {o-(H 3 NC 6 H 4 NH 3 )}SO 4 aqueous solution was also prepared.Single crystals of 1 suitable for X-ray crystal structural analysis were grown by the standard diffusion method using glass cell. 1 mL of 200 mM {o-(H 3 NC 6 H 4 NH 3 )}SO 4 aqueous solution was loaded at the end of the cell, then 1 mL of the Pt complex solution was carefully loaded above and allowed to stand for a few days.Brown needle-like crystals of 1 were obtained in 44% yield with the red crystalline by-products, Pt(III) 2 dinuclear complex, in 4% yield.Further standing of the solution caused a redissolution of the crystals of 1 and an increase of the amount of by-products.

X-Ray Single Crystal Structural Analysis
The X-ray diffraction experiments for 1 were carried out on a Bruker SMART CCD diffractometer with graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å).The crystal structures were solved by the directed method followed by Fourier syntheses.Structure refinement was performed by full matrix least-squares procedures using SHELXL-97 on F 2 [29].Hydrogen atoms binding to aromatic ring were treated with a riding model, though those binding to nitrogen or oxygen atoms were not refined because of low reliability especially for 1, which contains heavy atoms.Crystallographic data and structural refinement parameters for 1 are summarized in Table 1.

Results and Discussion
ORTEP representation of the molecular structure of 1 at 150 K is shown in Figure 1.Except half of the [Pt 2 (pop) 4 ]I chain unit generated by symmetry operation, labeled atoms belong to the asymmetric unit.Since bridging I − ion (I1) is disordered and lattice water molecule (O11) was at the inversion center, the occupancy of I1 and O11 is 50%.Although relatively large thermal ellipsoids observed in N3, N4 and C9 to C12 may suggest the disorder of the molecule, neither the trial to separate the two components nor that to select lower symmetric space group (P1) provided appropriate thermal factors., that we have expected.This partial deprotonation of diammonium ion has not been observed in any MMX chains synthesized so far.Although several kinds of compounds containing mono-protonated diamines as countercation have been reported [32][33][34][35][36], these compounds were synthesized from neutral diamines with acid.It should be noted that 1 differs from them in that it was synthesized from fully protonated diammonium, {o-(H 3 NC 6 H 4 NH 3 )}SO 4 , via the deprotonation.To our knowledge, only the organic-inorganic hybrid compounds, (NH 3 -R-NH 3 )(NH 3 -R-NH 2 )PbI 5 (R = 5,5'-bis(ethylsulfanyl)-2,2'-bithiophene) [37], is the other example which was synthesized through similar mechanism and whose structure was well-defined by the X-ray single crystal structural analysis.
In order to reveal the detail of the partial deprotonation, we focused on the assignment of -NH 2 and -NH 3 + group.It has been known that the chain structure of MMX chains with pop ligands is stabilized by the hydrogen bonds between alkyl ammonium ions (RNH 3

+
) and oxygen atoms of pop ligands   The most promising property expected from partial deprotonation is the hole doping of the semiconducting chain.Very recently, the hole doping by the deprotonation of counteranion was achieved in a conducting CT complex [38], thus this method can be effective for MMX chains.However, 1 is still in closed-shell system because total positive charge of countercation is +4 per formula.In order to achieve the hole doping and high conductivity, it is necessary to decrease the total positive charge to non-integral value with keeping four o-phenylenediamine molecules in a formula.1) was synthesized as the first MMX chain containing partially deprotonated countercation.The crystal structure of 1 was solved by the X-ray single crystal structural analysis.Since four o-phenylenediamine molecules exist in a formula, half of nitrogen atoms were expected to be deprotonated.A close examination of the structure confirmed this expectation.N1 and N4 were protonated nitrogen supporting 1D chain structure by hydrogen bond to pop ligands.On the other hand, N2 and N3 were neutral nitrogen forming hydrogen bond with -NH 3 + group and lattice water molecules, respectively.Further tuning of the synthetic condition of 1 will develop an effective method for partial deprotonation, which is one of the keys to achieve hole doping, inducing high conductivity of MMX chains.

Figure 1 .
Figure 1.ORTEP representation of the molecular structure of 1. Thermal ellipsoids are displayed at the 50% probability level.Hydrogen atoms are omitted for clarity.The occupancy of I1 and O11 are 50%.Symmetry transformations used to generate equivalent atoms: #1: −x, −y + 1, −z.

Figure 2 .
Figure 2. Crystal structure of 1 viewed along (a) a-axis and (b) b-axis.Hydrogen atoms are omitted for clarity.Unit cell is described as blue dashed line.(black, C; blue, N; red, O; orange, P; purple, I; yellow, Pt).

Table 1 .
Crystallographic data and structural refinement parameters for 1.Crystallographic data for 1 have been deposited with the Cambridge Crystallographic Data Center as supplementary publication No. CCDC-836700.These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Since fully protonated o-(H 3 NC 6 H 4 NH 3 ) 2+ ion, mono-protonated o-(H 3 NC 6 H 4 NH 2 ) + ion and neutral o-(H 2 NC 6 H 4 NH 2 ) molecule are held in equilibrium in aqueous solution, the tuning of synthetic condition such as pH and concentration may provide the desired material.These further works are currently in progress.