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], 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 ⋯M—X—M—X⋯ infinite 1D chain structure. The other, made from lantern-type dinuclear complex (hereafter abbreviated as MMX chains) has ⋯M-M—X—M-M—X⋯ infinite 1D chain structure. 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, [M₂(RCS₂)₄I] (M = Ni, Pt; R = alkyl chain group) [13-17]. It was reported that Pt₂(CH₃CS₂)₄I 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, A₄ [Pt₂(pop)₄X]·nH₂O (A = Li, K, Cs, NH₄ etc.; X = Cl, Br, I) [18-20], 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 [23-26] 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-(H₃NC₆H₄NH₂)}₄ [Pt₂(pop)₄I]·H₂O (1), which can be the clue to the doping.

ORTEP representation of the molecular structure of 1 at 150 K is shown in Figure 1. Except half of the [Pt₂(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.

Figure 2 shows the crystal structure of 1 with the representation of the unit cell. Two neighboring [Pt₂(pop)₄] units are bridged by an I⁻ ion, forming a ⋯Pt–Pt–I–Pt–Pt–I⋯ linear-chain structure along the α-axis. The bridging I⁻ ions are disordered with half occupancies at displaced position from the midpoints between the neighboring two [Pt₂(pop)₄] units. As shown in Table 2, Pt-Pt distance (d(Pt-Pt) = 2.8470(10) Å) is almost intermediate value of that of Pt(II)₂ dinuclear complex, K₄[Pt₂(pop)₄]·2H₂O, (d(Pt-Pt) = 2.925(1) Å) [18], and that of Pt(III)₂ dinuclear complex, K₄[Pt₂(pop)₄I₂], (d(Pt-Pt) = 2.754(1) Å) [30]. This indicates that 1 is Pt(II)-Pt(III) mixed-valence compound [18,20,31].

Symmetry transformations used to generate equivalent atoms: #1: −x, −y + 1, −z; #2: −x + 1, −y + 1, −z.

Interestingly, one [Pt₂(pop)₄] unit, one bridging I⁻ ion and four o-phenylenediamine molecules as countercations exist in a unit cell. Because the charge of the chain unit (⋯[Pt₂(pop)₄]I⋯) is −4, o-phenylenediamine molecules should be monocation, o-(H₃NC₆H₄NH₂)⁺, not dication, o-(H₃NC₆H₄NH₃)²⁺, 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 [32-36], 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-(H₃NC₆H₄NH₃)}SO₄, via the deprotonation. To our knowledge, only the organic-inorganic hybrid compounds, (NH₃-R-NH₃)(NH₃-R-NH₂)PbI₅ (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 −NH₂ and −NH₃⁺ 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 (RNH₃⁺) and oxygen atoms of pop ligands (–P=O⋯H-N⁺-H⋯O=P–). Consequently, the nitrogen atom which is close to oxygen atoms in both [Pt₂(pop)₄] units belongs to −NH₃⁺ group. As shown in Table 3, N1 and N4 are close (d(N…O) < 3.0 Å) to two or more oxygen atoms in pop ligands (O2 and O6 for N1, O3, O4 and O7 for N4). Therefore, N1 and N4 belong to −NH₃⁺ group, and N2 and N3 belong to −NH₂ group.

Symmetry transformations used to generate equivalent atoms: #1: −x, −y + 1, −z; #2: −x + 1, −y + 1, −z; #3: x, y − 1, z; #4: −x + 1, −y, −z; #5: −x + 1, −y + 1, −z + 1; #6: x, y, z + 1.

In addition, the short intermolecular N1–N2 distance (2.858(10) Å) suggests the existence of hydrogen bond between −NH₂ and −NH₃⁺ group. Focusing on the o-(H₃N(1)C₆H₄N(2)H₂)⁺ molecules, this N-H···N hydrogen bond induces the pairing of neighbor two molecules as shown in Figure 3(a). In case of o-(H₃N(4)C₆H₄N(3)H₂)⁺ molecules, relatively short N-O distances, d(N3-O11) = 3.031(12) Å and d(N4-O11) = 3.065(13) Å, suggest the existence of some interactions between these molecules and lattice water molecules. Although the position of the hydrogen atoms are unclear, N-H⋯O and N⋯;H-O hydrogen bonds are possibly the origin of these interactions. As shown in Figure 3(b), a o-(H₃N(4)C₆H₄N(3)H₂)⁺ molecule is paired with another one via a lattice water molecule.

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. Since fully protonated o-(H₃NC₆H₄NH₃)²⁺ ion, mono-protonated o-(H₃NC₆H₄NH₂)⁺ ion and neutral o-(H₂NC₆H₄NH₂) 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.

{o-(H₃NC₆H₄NH₂)}₄[Pt₂(pop)₄I]·H₂O (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 −NH₃⁺ 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.