Modulation of Band Gaps toward Varying Conductivities in Heterometallic One-Dimensional Chains by Ligand Alteration and Third Metal Insertion.

A heterometallic one-dimensional (1-D) chain consisting of multiple kinds of metals, Rh, Pt, and Pd, with direct metal-metal bonds was successfully obtained by mixing a Rh dinuclear complex and Pt-Pd-Pt trinuclear complex. The Pt-Pd-Pt trinuclear complex can be reversibly one-electron-oxidized or -reduced, where the electron paramagnetic resonance spectrum of the one-electron-oxidized one shows an axially symmetric signal with hyperfine splitting by two Pt and Pd, indicating that an unpaired electron is delocalized to the d z 2 orbital of Pt-Pd-Pt. Utilized with the highest occupied molecular orbital and lowest unoccupied molecular orbital interaction at the d z 2 orbital, simple mixing of the Pt-Pd-Pt trinuclear complex and Rh dinuclear complex in adequate solvents afforded heterometallic 1-D chains, which are aligned as -Rh-Rh-Pt-Pd-Pt-. Several physical measurements revealed that the metal oxidation state is +2. Diffuse reflectance spectra and theoretical calculations show that heterometallic 1-D chains have σ-type conduction and valence bands where π*(Rh2) are lying between them, whose gaps become narrower than the prototype chains aligned as -Rh-Rh-Pt-Pt-Pt-Pt-. The narrower band gaps are induced by destabilization of the σ-type valence bands and accompanied by insertion of Pd ions because the d-orbital energy level of Pd is closer in value to Rh compared with Pt. Flash-photolysis time-resolved microwave conductivity measurements exhibited an increase in the product of charge carrier mobility and its generation efficiency (8.1 × 10-5 to 4.6 × 10-4 cm2 V-1 s-1) with narrowing the band gaps, suggesting that the better conductivity is attributed to shorter metal-metal distances in 1-D chains. These results imply the possibilities of controlling band gap with ligand modification and third metal insertion in heterometallic 1-D chains to show various conductivities.

Introduction

There is a growing interest in the crystal engineering of polymeric compounds such as metal–organic frameworks with diverse structures because of potential applications as well as fascinating architecture and topology.[1−5] One-dimensional (1-D) coordination polymers constructed using metals and bridging ligands are the simplest topological type of polymeric compounds.[6,7] Particularly, −M–M–X– chains,[8−15] wherein dimetallic units are axially linked through halide ligands, have provided simple but significant information with viewpoints from molecular-based electronic structures to understand their electronic behaviors in bulk solids. For example, A4[Pt2(pop)4I]·nH2O (A = cation, pop = P2O5H22–) is a semiconductor attributed to the charge density wave or alternate charge-polarized states based on −Pt(+2)–Pt(+2)–I–Pt(+3)–Pt(+3)–I– or −Pt(+2)–Pt(+3)–I–Pt(+3)–Pt(+2)–I–, respectively.[8,11,14] In contrast, [Pt2(S2CCH3)4I] exhibits metallic conduction attributed to the average valence oxidation state of −Pt(+2.5)–Pt(+2.5)–I–.[9,10,12] Despite having similar 1-D networks, the obvious different physical properties of bulk solids are attributed to modulated band structures caused by fine tuning of molecular environments such as the combination of metal species, ligands, and other contained molecules.[14] Because a slight difference around metals perturbs the bulk structure in 1-D networks, it is necessary to examine other direct metal–metal-bonded −M–M–M– chains to understand in-depth exploration of the discovery.

One-dimensional compounds have metal–metal bonds that are classified into four types: extended metal atom chains (EMACs);[16,17] heterometallic extended metal atom chain compounds (HEMACs), which are also known as heterometallic metal string compounds (HMSCs);[18−28] infinite 1-D chains;[29,30] and heterometallic 1-D chains.[28,31,32] Both EMACs and HEMACs are discrete chains of metal-bonded atoms, which is interesting not only from a theoretical point of view but also potential applications, particularly as nanoscale electronic devices, where single-molecule conductivities have been widely explored.[33−44] Compared with EMACs containing single metal species, HEMACs show negative differential resistance expecting a molecular rectifier[37,38] and an extraordinarily large ferromagnetic coupling through metal–metal bonds,[27] which is attributed to anomalous electronic structures induced by connecting multiple metals. Reproducible and well-defined synthesis of these compounds includes ligand-assisted reactions with conjugated polyenes and polydentate ligands; in these cases, the number of available coordination sites of a ligand determines the length of a metallic chain.[17,26,45,46]

In contrast to the ligand-assisted EMACs and HEMACs, infinite chains are obtained using metal–metal interaction: the oxidation of mononuclear or dinuclear complexes containing d8 square-planar metal centers or reduction of d7 metal compounds induces the metal–metal bonds via d orbitals to align infinitely, where obtained chains are expected for conducting materials with the partially oxidized z band (Scheme left). Classical compounds such as K2[Pt(CN)4]Br0.3·3H2O (KCP),[47] Magnus’ green salt,[48] and Ir chains[49] and recent Rh,[50−63] Pt,[64−72] and Pd wires[70,73−76] have been investigated and explored for characteristic physical properties. However, there are a few examples of infinite chains except for ones with d10 interactions,[77−80] and the metal species are very limited to Rh, Pd, Ir, and Pt because only these four metals favor to d8 configuration. On the basis of these backgrounds, we have proposed a simple concept for heterometallic infinite 1-D chains: the construction methodologies are the utilization of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) interaction at the d orbital (σ-type orbitals) between two kinds of complexes.[32] For example, simply mixing a Pt dinuclear complex of [Pt2(piam)2(NH3)4](PF6)2 (piam = pivalamidate) having σ* as the HOMO and [Rh2(O2CCH3)4] having σ* as the LUMO affords the crystals of the 1-D chain, [{Rh2(O2CCH3)4}{Pt2(piam)2(NH3)4}2](PF6)4·6nH2O (1) via interaction of σ-type orbitals.[81] In 1, [Rh2(O2CCH3)4] is sandwiched by two [Pt2(piam)2(NH3)4]2+ with unbridged Pt–Rh bonds to be a hexanuclear unit, where each unit is further linked with unbridged Pt–Pt bonds to be infinite chain aligned as −Pt–Pt–Rh–Rh–Pt–Pt– (Scheme , Figure S1). Interestingly, it is possible to synthesize a similar chain, [{Rh2(O2CCF3)4}{Pt2(piam)2(NH3)4}2](CF3CO2)4·2nEtOH·2nH2O (2), changing the bridging ligands at Rh parts from −O2CCH3 to −O2CCF3 (Figures S2),[81] showing universally applicable construction methodologies.

Scheme 1

Schematic Band Structures of 1-D Chains Consisting of Single-Metal Species Such as Pt (Left) and Two Kinds of Metal Species, Pt and Rh (Right)

Scheme 2

Structures of Heterometallic 1-D Chains for 1, 2, 5, and 6

The significant features found in these heterometallic 1-D chains are the tuning capabilities of their HOMOs. For example, in both 1 and 2 aligned as −Pt–Pt–Rh–Rh–Pt–Pt–, the HOMO–LUMO interaction at the σ* orbitals between Pt and Rh dinuclear complexes provides a σ-type conduction band (CB) and valence band (VB), where occupied π* orbitals of Rh–Rh lie between the bands as HOMOs (Scheme right).[81] It is possible to modulate their HOMOs by changing bridging ligands around Rh–Rh or the insertion of first transition metals to Pt–Pt parts; π*,[81,82] δ*,[83,84] and singly occupied molecular orbitals[85,86] can be either HOMOs with σ-type CB and VB. Although infinite 1-D chains composed of a single-metal species have σ-type orbitals as HOMOs (VB),[29,30,47−76] the HOMOs of heterometallic 1-D chains are various,[32,81−86] where the ligand alteration and third metal insertion afforded the frontier orbital engineering. Furthermore, in this study, we have demonstrated to modulate the σ-type band gaps in heterometallic 1-D chains by the insertion of the third metal as well as ligand modifications. Selecting Pd as the third metal, it was succeeded in obtaining heterometallic 1-D chains containing three kinds of metals, Rh, Pt, and Pd with metal–metal bonds, in which crystal and electronic structures based on the physical measurements and conductive properties will be shown. Similar to the modulation of band gaps as found in −M–M–X– chains,[13−15] these experimental results will prove the band gap modulation in heterometallic 1-D chains having direct metal–metal bonds.

Results and Discussion

Synthesis

cis-[Pt(piam)2(NH3)2] could be considered as a good precursor of dinuclear or trinuclear complexes with a Pd2+ ion because it possesses pendant arms of amidate ligands (Scheme ), involving the electron donation from the full d orbital of Pt2+ to Pd2+.[87] By simply mixing cis-[Pt(piam)2(NH3)2]·2H2O, Na2[PdCl4] and NaPF6 in H2O, free oxygen atoms of the amidate-hanging Pt complex bind Pd ions to afford a heterometallic trinuclear complex [Pt2Pd(piam)4(NH3)4](PF6)2 (= [Pt–Pd–Pt], 3). As discussed later, compound 3 has a trinuclear Pt–Pd–Pt structure with overlapping of the d orbital between Pt and Pd atoms. Figure shows cyclic voltammograms of 3 recorded with THF and MeCN solutions containing 0.1 M Bu4NPF6 as a supporting electrolyte. In both solutions, reversible waves at E1/2 = 0.17 V (vs Fc/Fc+) in THF and E1/2 = 0.24 V in MeCN were observed, which are attributed to one-electron oxidation and reduction, [Pt–Pd–Pt]2+ ↔ [Pt–Pd–Pt]3+. The reversible profile indicates stability of the trinuclear structure of 3 during the redox reaction. Furthermore, an oxidation peak at E = 1.06 V in the THF solution and a weak quasi-reversible oxidation and reduction wave at E1/2 = 0.65 V in MeCN were also observed, which are attributed to [Pt–Pd–Pt]3+ → [Pt–Pd–Pt]4+.[19] The irreversible wave in THF solution indicates collapse of the trinuclear [Pt–Pd–Pt]4+ structure. In the case with treatment of 3 with AgPF6 as an oxidizing agent in THF, obtaining violet powder of [Pt2Pd(piam)4(NH3)4](PF6)3 (4) was successful.

Scheme 3

Synthetic Route for Heterometallic 1-D Chains of 5 and 6

Figure 1

Cyclic voltammograms of 1 mM 3 in (a) THF or (b) MeCN, with 0.1 M Bu4NPF6 as a supporting electrolyte using a glassy carbon disk working electrode, a Ag/Ag+ reference electrode, and a Pt wire auxiliary electrode (scan rate 100 mV/s). Electrode potentials were converted to those relative to Fc/Fc+.

Crystal Structures of [Pt2Pd(piam)4(NH3)4](PF6)2·4THF·2H2O (3·4THF·2H2O) and [Pt2Pd(piam)4(NH3)4](PF6)3·4THF (4·4THF)

Figure a shows the crystal structure of 3·4THF·2HO. A Pd atom is sandwiched by two Pt atoms with the four bridging piam ligands, affording a linear [Pt–Pd–Pt] alignment. The Pt–Pd distance is 2.8500(5) Å, which is similar to the [2.839(1) and 2.837(1) Å] values in other [Pt–Pd–Pt] complex [(NH3)2Pt(1-MeU)2Pd(1-MeU)2Pt(NH3)2](ClO4)2 (1-MeU = 1-methyluracilato).[19] Coordination environments of Pt and Pd are eclipsed (Figure a, right). The torsional angle between Pt(1) and Pd(1) coordination planes is 23.5°. Considering that the sum of the metal oxidation numbers of [Pt–Pd–Pt] is +6, the oxidation state for each metal in 3 is not changed from the original compounds, to be Pt(+2)–Pd(+2)–Pt(+2). Amine moieties are hydrogen-bonded to PF6– ions with distances of 3.1 Å, where PF6– ions bridge two trinuclear units. The amine moieties are also hydrogen-bonded to THF and H2O molecules, wherein H2O is further hydrogen-bonded to another THF molecule (Figure S3).

Figure 2

Molecular structure of the (a) trinuclear [Pt2Pd(piam)4(NH3)4]2+ cation of 3·4THF·2HO and (b) trinuclear [Pt2Pd(piam)4(NH3)4]3+ cation of 4·4THF in the crystal. The right sides show views along the metal–metal bonds.

Figure b shows the molecular structure of [Pt2Pd(piam)4(NH3)4](PF6)3·4THF (4·4THF). Similar to 3·4THF·2HO, the Pd atom is sandwiched by two Pt atoms, affording a linear [Pt–Pd–Pt] alignment. Contrary to 3·4THF·2HO containing two PF6– ions per [Pt–Pd–Pt], 4·4THF contains three PF6– ions, indicating that the sum of metal oxidation numbers in [Pt–Pd–Pt] is +7. The Pt–Pd distance is 2.6601(9) Å, which is about 0.2 Å shorter than that in 3·4THF·2HO. Such a shorter Pt–Pd distance was also found in the 1-MeU-bridged [Pt–Pd–Pt] complex, wherein the one-electron-oxidized compound of [(NH3)2Pt(1-MeU)2Pd(1-MeU)2Pt(NH3)2](NO3)3·11H2O has a shorter Pt–Pd distance of 2.641(1) Å compared with that of the original compound [2.839(1) and 2.837(1) Å].[18,19] As shown in Figure b, coordination environments of Pt and Pd are eclipsed, wherein the torsional angle between Pt(1) and Pd(1) coordination planes is 17.1°, which is also lower than that of 3·4THF·2HO. The amine and amidate moieties in the piam ligands are coordinated to Pt atoms hydrogen-bonded to THF or PF6– ions, obstructing the interaction between trinuclear complexes (Figure S4).

Physical Properties of [Pt2Pd(piam)4(NH3)4] (n = 2, 3)

In order to investigate whether trinuclear complexes of [Pt2Pd(piam)4(NH3)4](PF6)2 (3) and [Pt2Pd(piam)4(NH3)4](PF6)3 (4) are suitable for heterometallic 1-D chains, density functional theory (DFT) calculations were conducted to reveal electronic structures in detail. DFT calculations on closed-shell compound 3 with model [Pt2Pd(NHCOCH3)4(NH3)4]2+ reveal significant σ, π, and δ electron delocalization to [Pt–Pd–Pt]. The HOMO and LUMO of 3 are σ-type orbitals delocalized to three metal centers and Pd d– orbital, respectively (Figure a). Because the HOMO has Pd d and both ends of Pt d characters, oxidation should occur mostly on the σ-type orbital of Pt–Pd–Pt and is expected to affect the Pt–Pd distance less. As shown in Figure b, DFT calculations were also performed on 4 with complex [Pt2Pd(NHCOCH3)4(NH3)4]3+. The calculation result indicates the contraction of the Pt–Pd bond upon removal of an electron from 3. The obtained Pt–N and Pd–O distances showed good agreement to the experimental values, although the obtained Pt–Pd distance is somewhat longer than that in the crystal structure. Both α and β molecular orbitals display similar ordering, where the HOMO of α and the LUMO of β are σ-type orbitals with spin population for Pt, Pd, and Pt of 0.24, 0.42, and 0.24, respectively. Furthermore, [Pt2Pd(NHCOCH3)4(NH3)4]4+ was optimized (Figure S5), showing that the LUMO and HOMO are σ-type and δ-type orbital, respectively. The effect of one- or two-electron oxidation on metal–metal bonding along with the Pt–Pd–Pt bond can be analyzed by natural bond orbital analysis. The increase in the Pt–Pd bond ordering of [Pt2Pd(NHCOCH3)4(NH3)4] with 0 (n = 2), 0.25 (n = 3), and 0.47 (n = 4) is indicative of unpaired σ-type electron delocalization to Pt–Pd–Pt in 4.

Figure 3

Results of DFT calculation based on the model of (a) [Pt2Pd(NHCOCH3)4(NH3)4]2+ and (b) [Pt2Pd(NHCOCH3)4(NH3)4]3+.

In addition to the calculations, spectroscopic analyses were conducted to reveal further electronic nature in trinuclear complexes of 3 and 4. Figure shows ultraviolet–visible (UV–vis) spectra of 3 (0.5 mM) in MeOH and 4 (0.1 mM) in H2O. Compound 3 only absorbs in the UV region attributed to metal-to-ligand charge transfer around Pd ions. In contrast, compound 4 shows peaks around 357 and 571 nm, which are similar to those in previous reports on compound [(NH3)2Pt(1-MeU)2Pd(1-MeU)2Pt(NH3)2](NO3)3·11H2O.[18,19] Time-dependent (TD) DFT calculations performed on 4 with the complex [Pt2Pd(NHCOCH3)4(NH3)4]3+ predict that the lower energy absorption around 571 nm is attributed to the transition from HOMO – 3 to LUMO, both of which are the σ-type orbital. As the concentration of 4 solution was diluted, the absorption intensities become lower, indicating that 4 is easily reduced to 3 under dilution conditions. As found in UV–vis spectra of solutions containing 3 (Figure S6), the molar extinction coefficient in the near-infrared region depends on the concentration of 3. Considering that there is no prediction performed using TD-DFT calculations for the near-infrared region, absorption of both 3 and 4 could be attributed to the aggregation of [Pt–Pd–Pt], probably with metal–metal interaction.

Figure 4

UV–vis spectra of (a) 0.5 mM MeOH solution containing [Pt2Pd(piam)4(NH3)4](PF6)2 (3) and (b) aqueous solution containing [Pt2Pd(piam)4(NH3)4](PF6)3 (4) with the concentration of 100, 50, 25, and 12.5 μM. Blue or red bars show the results of TD-DFT calculation based on [Pt2Pd(NHCOCH3)4(NH3)4]2+ or [Pt2Pd(NHCOCH3)4(NH3)4]3+, respectively.

Figure shows the electron paramagnetic resonance (EPR) spectrum of powder sample 4 measured at 77 K. The spectrum exhibits broad perpendicular and parallel peaks, each splits into at least five lines because of 195Pt nuclei (I = 1/2 with abundance of 33.7%) and 105Pd nuclei (I = 5/2 with abundance of 22%). EPR parameters have been determined by computer simulation as g⊥ = 2.32, g// = 1.98, and gav = 2.21 and A⊥ = 350 × 10–4 cm–1 and A// = 330 × 10–4 cm–1. The value of g is slightly higher than the previously reported compound [(NH3)2Pt(1-MeU)2Pd(1-MeU)2Pt(NH3)2](NO3)3·11H2O (g⊥ = 2.167, g// = 1.986, and gav = 2.107),[18] as well as a dinuclear Pd(+3)–Pd(+2) complex (g⊥ = 2.17, g// = 1.98, and gav = 2.11)[88] and mononuclear Pd(+3) complex (g⊥ = 2.049, g//= 2.009, and gav = 2.036)[89] but similar to [Pt4(piam)4(NH3)8](PF6)4(ClO4)·2H2O (g⊥ = 2.393, g// = 1.979, and gav = 2.255)[90] with a Pt(+2.25)4 oxidation state, which is named platinum blue.[91−93] The obtained g values of 4 are well-interpreted in terms of a d hole state with an admixture of lower-lying d and d states because of spin–orbit coupling.[92,93] Typical examples for d-type Pt complexes are Pt(IV)-doped [Pt(NH3)4][PtCl4] (g⊥ = 2.504, g// = 1.939, and gav = 2.316)[94] and KCP (g⊥ = 2.336, g// = 1.946, and gav = 2.206),[95] whose values are attributed to the d hole state of Pt(+3).

Figure 5

Continuous wave EPR spectra for (a) powder samples of 4 at 77 K and (b) simulation. Experimental settings: microwave frequency, 9.0709 GHz; microwave power, 6 mW; and field modulation, 0.2 mT.

Although the spectrum of [(NH3)2Pt(1-MeU)2Pd(1-MeU)2Pt(NH3)2](NO3)3·11H2O shows no hyperfine coupling and that the signal was attributed to localized Pd(+3) spin,[18] the observed five splitting of 4 was caused by hyperfine coupling of two-end Pt atoms with an intensity ratio of 1:8:18:8:1, whose simulation is well-matched with the observed one as shown in Figure b. The obtained A values of 4 are similar to those of [Pt4(piam)4(NH3)8](PF6)4(ClO4)·2H2O (A1⊥ = 143 × 10–4 cm–1, A1// = 166 × 10–4 cm–1, A2⊥ = 287 × 10–4 cm–1, and A2// = 227 × 10–4 cm–1).[90] Considering that Pt(+3) complexes favor being axially coordinated to anions[96,97] and not axially coordinated to the Pt atom in 4·4THF, the Pt(+3) spin in [Pt–Pd–Pt] does not localize on one side. Generally, the A values of known Pd(+3) compounds are lower than 35 × 10–4 cm–1,[88,89,98−101] which results in the separation of linewidths that are typically between 10 and 20 mT. Therefore, because of the broadness of such signals, splitting due to 105Pd hyperfine interaction is not observed and generally hidden underneath the main signal.[98] Consequently, an unpaired electron in 4 delocalized to the σ-type orbital of [Pt–Pd–Pt], which coincides with the result of DFT calculations. Furthermore, the EPR spectra of Me2CO and THF glasses containing 4 at 77 K also showed the axial symmetry signals with complicated splitting (Figure S7), which is probably because of aggregation of [Pt–Pd–Pt].

Synthesis and Crystal Structure of Heterometallic 1-D Chains

Both experiment and calculation results showed that trinuclear complexes 3 and 4 have a σ-type orbital of [Pt–Pd–Pt] as the HOMO and singly occupied molecular orbital, respectively. As found in both 1 and 2 aligned as −Pt–Pt–Rh–Rh–Pt–Pt–, wherein [Rh2(O2CR)4] is bridged by two [Pt2(piam)2(NH3)4]2+, the components of [Pt2(piam)2(NH3)4]2+ also have a σ-type orbital of [Pt–Pt] as the HOMO. Thus, trinuclear complexes 3 and 4 would be good components for heterometallic 1-D chains with [Rh2(O2CR)4]. Because compound 3 has a full d orbital, HOMO–LUMO interaction at the σ-type orbital with [Rh2(O2CCH3)4] or [Rh2(O2CCF3)4], which has vacant σ* (d) orbitals, is expected.[32] Gently mixing of 3 with [Rh2(O2CCH3)4] at a ratio of 1:1 in MeOH will afford single crystals of [{Rh2(O2CCH3)4}{Pt2Pd(piam)4(NH3)4}](PF6)2 (5). In contrast, the mixture with [Rh2(O2CCF3)4] did not afford single crystals but only blue powder of [{Rh2(O2CCF3)4}{Pt2Pd(piam)4(NH3)4}](PF6)2 (6) after stirring 3 and [Rh2(O2CCF3)4] in an aqueous solution. The blue powder of 6 was soluble in Me2CO, wherein the 1-D structure was decomposed to each module and assembled to an octanuclear compound [{Rh2(O2CCF3)4}{Pt2Pd(piam)4(NH3)4}2](PF6)3(CF3CO2) (6′) as single crystals after the addition of a poor solvent to the solution.

The crystal structure of 5 is shown in Figure . The most remarkable structural feature of 5 is that [Pt2Pd(piam)4(NH3)4] is linked by the paddlewheel complex [Rh2(O2CCH3)4] on both ends with metal–metal bonds to generate 1-D chains expressed as −Rh–Rh–Pt–Pd–Pt–. The Pt atoms are bonded to a Rh complex with a distance of Pt(1)–Rh(1) = 2.7567(10) Å (Figures a and S8), wherein multiple hydrogen bonds between N atoms at amine/amidate ligands coordinated to Pt atoms and carbonyl O atoms in a Rh complex with a distance range of 3.02–3.47 Å supporting these unbridged metal–metal bonds. The distance between the Pt and Pd ions is 2.7683(7) Å, which is shorter than that in 3·4THF·2HO (2.8500(5) Å) but longer than that in 4·4THF (2.6601(9) Å). Although the torsional angle between the RhO4 and PtN4 planes is low (1.9°), the angle between the PtN4 and PdO4 planes is relatively high (20.6°), which is caused by the half lantern fashion of piam bridges in the [Pt2Pd(piam)4(NH3)4] unit. As shown in Figure b, the Rh and Pt planes are arranged in a staggered manner, which induces bending of the angles of Rh–Pt–Pd that results in zigzagged backbones. The PF6– ions are hydrogen-bonded to amine ligands coordinated to Pt atoms with a distance of 3.1 Å (Figure c). Each chain is aligned in a parallel fashion within the whole crystal (Figure d).

Figure 6

(a) Crystal structure of [{Rh2(O2CCH3)4}{Pt2Pd(piam)4(NH3)4}](PF6)2 (5). (b) View along the metal–metal bond in 5. (c) Hydrogen bonds between the chain and PF6– ions shown as dotted lines. (d) Crystal packing of the 1-D chains in 5. Hydrogen atoms and PF6– ions are omitted for clarity.

Figure shows the crystal structure of 6′. In 6′, [Pt2Pd(piam)4(NH3)4] is axially connected to the paddlewheel dinuclear complex [Rh2(O2CCF3)4] on the both ends with metal–metal bonds to generate one-dimensionally aligned octanuclear complex Pt–Pd–Pt–Rh–Rh–Pt–Pd–Pt, wherein a crystallographic inversion center is positioned at the center of the Rh complex (Figure a). In the crystal, there are two crystallographically independent octanuclear complexes (Figure S9), [Pt(2)–Pd(1)–Pt(1)]–[Rh(1)–Rh(1′)]–[Pt(1′)–Pd(1′)–Pt(2′)] and [Pt(4)–Pd(2)–Pt(3)]–[Rh(2)–Rh(2′)]–[Pt(3′)–Pd(2′)–Pt(4′)], which both have a similar bond distance and angle. As shown in Figure b, [Rh–Rh] and [Pt–Pd–Pt] are stacked with a torsion angle range of 17–30°, which is smaller than that of 5. The distances between the inner Pt and Pd ions are 2.7367(17) and 2.729(2) Å, which are shorter than that in 5 (2.7683(7) Å), and those between the outer Pt and Pd ions are 2.8027(17) and 2.781(2) Å. Compared with 5, the significant feature of 6′ is octanuclear fashion, which is attributed to the obstruction of further extension by the hydrogen bonding between NH3 coordinated to end Pt(2) or Pt(4) and counter anions of PF6– or CF3CO2–. The CF3CO2– ions were probably generated from the decomposition of [Rh2(O2CCF3)4]. Despite the obstruction by counter anions, the distances between end Pt and neighboring end Pt are relatively close (Figure c), Pt(2)···Pt(2″) = 4.3 Å and Pt(4)···Pt(4″) = 3.7 Å, forming a quasi 1-D structure with the repetition of octanuclear complexes. As shown in Figure d, each octanuclear complex is parallel to the ab plane, where quasi 1-D chains in the neighboring ab plane are twisted and packed at about 80°.

Figure 7

(a) Crystal structure of [{Rh2(O2CCF3)4}{Pt2Pd(piam)4(NH3)4}2](PF6)3(CF3CO2) (6′). (b) View along the metal–metal bond in 6′. (c) Close contact between the end Pt atom and neighboring one as a dotted line. (d) Crystal packing of the octanuclear complexes in 6′. Hydrogen atoms, CF3CO2– ions, and PF6– ions are omitted for clarity.

Table summarizes the Rh–Pt distance of 5, 6′, and related compounds.[81−85,102,103] In all compounds, unbridged Rh–Pt distances are shorter than the sum of the van der Waals radii (4.1 Å) of the Rh and Pt atoms,[104] which indicates overlapping of d orbitals. Compounds 5 and 6′ have similar metal repetition to copper-containing 1-D chains and octanuclear complex, [{Rh2(O2CCH3)4}{Pt2Cu(piam)4(NH3)4}](PF6)2 (A) and [{Rh2(O2CCH3)4}{Pt2Cu(piam)4(NH3)4}2](CF3CO2)2(ClO4)2·2H2O aligned as −Rh–Rh–Pt–Cu–Pt– and Pt–Cu–Pt–Rh–Rh–Pt–Cu–Pt, respectively.[85] Compared with those in copper-containing 1-D chains and octanuclear complex, the Rh–Pt distances in 5 and 6′ are relatively shorter, indicating that the Pd ion influences the energy level of the Pt d orbital more than the Cu ion.

Table 1

Comparison of Rh–Pt Distances (Å) in 1, 2, 5, 6′, and Reported Compounds

compoundsa	metal repetition	unbridged Rh–Pt (Å)	ref
1	–Rh–Rh–Pt–Pt–Pt–Pt–	2.7460(10)	(81)
2	–Rh–Rh–Pt–Pt–Pt–Pt–	2.7473(15)	(81)
[{Rh2(O2CCH3)4}{Pt2(piam)2(NH2CH3)4}2](PF6)4	Pt–Pt–Rh–Rh–Pt–Pt	2.7493(12)	(102)
[{Rh2(O2CCH3)4}{Pt2(piam)2(bpy)2}2](PF6)4	Pt–Pt–Rh–Rh–Pt–Pt	2.7310(5)	(102)
[{Rh2(O2CCH3)4}{Pt(piam)2(NH3)2}2]·2H2O	···Pt–Rh–Rh–Pt···	2.8208(8)b	(85)
[{Rh2(O2CCH3)4}{Pt2Cu(piam)4(NH3)4}]n(PF6)2n (A)	–Rh–Rh–Pt–Cu–Pt–	2.7749(11)	(85)
[{Rh2(O2CCH3)4}{Pt2Cu(piam)4(NH3)4}]n(PF6)2n (B)	–Rh–Rh–Pt–Cu–Pt–	2.7702(4)	(85)
[{Rh2(O2CCH3)4}{Pt2Cu(piam)4(NH3)4}]n(PF6)2n·6nMe2CO	–Rh–Rh–Pt–Cu–Pt–	2.7954(6)	(85)
[{Rh2(O2CCH3)4}{Pt2Cu(piam)4(NH3)4}2](CF3CO2)2(ClO4)2·2H2O	Pt–Cu–Pt–Rh–Rh–Pt–Cu–Pt	2.8155(5)	(85)
[{Rh2(acam)4}{Pt2(piam)2(NH3)4}2]n(CF3SO3)4n·2nMeOH (C)	–Rh–Rh–Pt–Pt–Pt–Pt–	2.7624(11)	(83)
[{Rh2(acam)4}{Pt2(piam)2(NH3)4}]n(CF3CO2)2n·2nEtOH (D)	–Rh–Rh–Pt–Pt–	2.7641(14), 2.7894(14)	(83)
[{Rh2(acam)4}{Pt(piam)2(NH3)2}2]·4MeOH·3H2O	···Pt–Rh–Rh–Pt···	2.7660(6)	(103)
[{Ru2(O2CCH3)4}{Pt2(piam)2(NH3)4}2]n(PF6)4n·4nH2O (E)	–Ru–Ru–Pt–Pt–Pt–Pt–	2.7889(18)c	(82)
[{Rh2(acam)4}3{Pt2(OPiv)2(NH3)4}4](ClO4)8 (F)	–{[Pt2]–[Rh2]}3–[Pt2]–	2.8520(17), 2.864(2), 2.8002(18)b	(84)
[{Rh2(O2CCH3)4}{Pt2Co(piam)4(NH3)4}]n(PF6)2n	–Rh–Rh–Pt–Co–Pt–	2.8415(15), 2.8388(15)b	(86)
5	–Rh–Rh–Pt–Pd–Pt–	2.7567(10)b	this work
6′	Pt–Pd–Pt–Rh–Rh–Pt–Pd–Pt	2.7733(17), 2.721(2)b	this work

Abbreviation: bpy = 2,2′-bipyridine, acam = acetamidate, and OPiv = pivalate.

Measured at room temperature. Others were measured at 123 K.

Unbridged Ru–Pt (Å).

Although [Rh2(O2CCH3)4] and [Pt2Pd(piam)4(NH3)4](PF6)2 are easily linked in MeOH to afford single crystals for the infinite 1-D chain of 5, [Rh2(O2CCF3)4] needs H2O to infinitely link [Pt2Pd(piam)4(NH3)4](PF6)2. The plausible reason is less stability of [Rh2(O2CCF3)4] in the presence of [Pt2Pd(piam)4(NH3)4](PF6)2 in organic solvents, wherein the partial decomposition of the dinuclear Rh structure releases CF3CO2– ions, being components of single crystals for octanuclear complex 6′, whose ions obstruct further 1-D extension by hydrogen bonding. As described in the Experimental Section, elemental analyses evidently indicate whether infinite chain or octanuclear compound: the weight ratio of C, H, and N in the octanuclear compound becomes higher than those in the infinite chain because the amount of piam and NH3 in the octanuclear compound becomes larger. The result of the elemental analysis of 6, which is also supported by spectroscopic analyses (vide infra), shows a lower weight ratio than that of 6′, indicating that 6 has an infinite 1-D structure similar to 5. Compound 6 was obtained in H2O, wherein the hydrophobic CF3 moieties in ligands prevent the decomposition of its dinuclear structure [Rh2(O2CCF3)4], resulting in the CF3CO2– ions not being released to H2O.

Electronic Structures of Heterometallic 1-D Chains

Considering the chemical formula of 5 and 6′, the sum of the oxidation state of Rh–Rh–Pt–Pd–Pt in 5 and Rh–Pt–Pd–Pt in 6′ is +10 and +8, respectively. The X-ray photoelectron spectroscopy (XPS) spectra of 3, 4, 5, 6, 6′, and related compounds in the Rh 3d, Pd 3d, and Pt 4f regions at room temperature are shown in Figure S11. The Rh 3d5/2 binding energy values of 5, 6, and 6′ are 308.6, 309.4, and 309.3 eV, respectively, which are similar to those of original complexes [Rh2II,II(O2CCH3)4] (308.9 eV) and [Rh2II,II(O2CCF3)4] (310.2 eV). The higher energy value of 6 and 6′ compared with that of 5 is attributed to stabilization of the Rh core because of the electron-withdrawing ligands of −O2CCF3. The Pd 3d5/2 binding energy values of 3, 4, 5, 6, and 6′ are 337.7, 337.8, 337.8, 338.0, and 338.1 eV and are closer to that of PdIICl2 (338.0 eV)[105] and K2PdIICl4 (338.4 eV)[106] than that of K2PdIVCl6 (340.3 eV).[106] As mentioned above, the sum of the oxidation state of Pt–Pd–Pt in 3 and 4 is +6 and +7, respectively, wherein the Pd ion in 4 has a higher oxidation state than 3. However, the observed Pd 3d5/2 binding energy values of 3 and 4 do not reflect the slight difference in the oxidation state and also in Pt binding energies. The Pt 4f7/2 binding energy values of 3, 4, 5, 6, and 6′ are determined to be 73.1, 73.4, 73.2, 73.5, and 73.5 eV, respectively, which are closer to that of [Pt2II,II(en)2(α-pyridonato)2](NO3)2 (73.1 eV; en = ethylenediamine) than that of [Pt2III,III(NH3)4(α-pyrrolidonato)2(NO3)2](NO3)2 (74.6 eV).[107] Considering that the value of cis-[Pt(piam)2(NH3)2], which is the original mononuclear Pt compound, is 72.7 eV (Figure S11k), the higher energy shift might be caused by charge fluctuation in the Pt atoms originating from the close Pt–Pd contact. Consequently, the formal oxidation states in 5 and 6′ are −Rh(+2)–Rh(+2)–Pt(+2)–Pd(+2)–Pt(+2)– and Pt(+2)–Pd(+2)–Pt(+2)–Rh(+2)–Rh(+2)–Pt(+2)–Pd(+2)–Pt(+2), wherein 6 also has a similar oxidation state to 5, considering the results of physical measurements.

Figures and S12 show the result of DFT calculations on closed-shell compound 6′ with model [{Rh2(O2CCF3)4}{Pt2Pd(NHCOCH3)4(NH3)4}2]4+. The obtained distances and angles showed good agreement to the experimental values (Tables S5 and S6). The LUMO, HOMO, and HOMO – 1 are σ-type orbitals delocalized to eight metal centers, where the number of nodes are 7, 6, and 5, respectively. The other σ-type orbitals with less numbers of nodes are expected in the energetically stable region, where HOMO – 9 with 4 nodes is found (Figure S12). The HOMO – 2 and HOMO – 3 occupy degenerated Rh2 π* orbitals, which are HOMOs in [Rh2(O2CCF3)4(Lax)2] (Lax = axial ligands such as solvents),[96] indicating effective electron donation of axially bonded Pt atoms, because resulted σ-type orbitals are energetically higher than Rh2 π* orbitals. Furthermore, the natural bond orbital analysis showed that unbridged Rh–Pt bond ordering is 0.27, showing an effective metal–metal bond between [Rh2(O2CCF3)4] and [Pt–Pd–Pt] in 6′.

Figure 8

Result of DFT calculation based on the model of [{Rh2(O2CCF3)4}{Pt2Pd(NHCOCH3)4(NH3)4}2]4+.

As reported previously,[81] compounds 1 and 2 have a formal oxidation state of −Rh(+2)–Rh(+2)–Pt(+2)–Pt(+2)–Pt(+2)–Pt(+2)–. Figure a shows a schematic molecular orbital diagram on the basis of the hexanuclear complex aligned as Pt(+2)2–Rh(+2)2–Pt(+2)2. Although the energy levels of Rh 4d and Pt 5d atomic orbitals are similar, the energy splitting caused by ligand coordination with D4 symmetry affords a more stabilized Pt 5d orbital than a Rh 4d one, resulted in that the σ*(Rh2) is energetically higher than σ*(Pt2). Six molecular orbitals are made from all possible combinations of metal orbitals, wherein energy increases with increase in the number of nodes along the chain direction.[32,85] The oxidation states of 1 and 2 resulted in vacant σ*(Pt2)−σ*(Rh2)−σ*(Pt2) with all antibonding combinations of σ*(Pt2) and σ*(Rh2) as the LUMO and other stable and full σ-type orbitals. Furthermore, the infinite overlapping of σ-type orbitals along the 1-D chain leads to the emergence of a band structure, wherein LUMO and full σ-type orbitals afford CB and VB, respectively (Figures a, right). The band gaps between σ-type CB and VB are reflected as strong absorption at E1 in diffuse reflectance spectra (Figure ).[32]E1 absorption is observed at 3.01 and 2.60 eV in 1 and 2.67 and 2.23 eV in 2, indicating that the gap in 2 is about 0.4 eV narrower than that in 1, whereas other two absorptions E2 and E3 are attributed to the transition from π*(Rh2) to CB,[32] where π*(Rh2) lies between σ-type CB and VB as the HOMO in 1 and 2.

Figure 9

Schematic molecular orbital diagrams of (a) [{Rh2(O2CR)4}{Pt2(piam)2(NH3)4}2]4+ (R = CH3 or CF3) and (b) [{Rh2(O2CCF3)4}{Pt2Pd(piam)4(NH3)4}2]4+.

Figure 10

Diffuse reflectance spectra of (a) 1, (b) 2, (c) 5, (d) 6, and (e) 6′ with MgO at room temperature. Purple bars show the results of TD-DFT calculation based on [{Rh2(O2CCF3)4}{Pt2Pd(NHCOCH3)4(NH3)4}2]4+.

Contrary to 1 and 2, compounds 5 and 6 have a formal oxidation state of −Rh(+2)–Rh(+2)–Pt(+2)–Pd(+2)–Pt(+2)–. Considering the trend in diffuse reflectance spectra (Figure c,d), the electronic band structures of both 5 and 6 are similar to those of 1 and 2, wherein three characteristic bands, E1, E2, and E3, are also observed; for 5, E1 = 2.48, E2 = 1.76, and E3 = 1.49 eV; and for 6, E1 = 2.10, E2 = 1.76, and E3 = 1.50 eV. The significant feature of the four 1-D compounds is the shift in E1 values of 1 > 2 ≈ 5 > 6, although both E2 and E3 values are similar. These results indicate that the band gaps between σ-type CB and VB become narrower in the order of 1 > 2 ≈ 5 > 6 without a change in the gap between σ-type CB and π*(Rh2). Indeed, the calculations of partial densities of states (PDOSs) in Figure S13 also suggested the narrower band gap of 5 than that of 1, where the bands are composed of the d orbital of metals.

Figure b shows a schematic molecular orbital diagram of 6′ based on the result of DFT calculation. Although the metal repetitions of both 5 and 6 are different from 6′, it is possible to understand the electronic structure of 6 qualitatively in this case. Considering that the energy levels of Rh 4d orbitals are higher than those of Pt 5d orbitals, the electron-withdrawing group of CF3 in Rh2 stabilizes Rh2 molecular orbitals, resulting in being close to those of Pt. In contrast, because the energy levels of Pd 4d orbitals are higher than those of Pt 5d orbitals, it is valid to say that the energy levels of the molecular orbitals of Pt–Pd–Pt are higher than that of Pt–Pt–Pt–Pt and close to that of Rh2. Therefore, a trend that band gaps between σ-type orbitals become narrower when the difference in the energy level of two components is lower was observed. Furthermore, the absence of change in the gap between σ-type CB and π*(Rh2) in 1, 2, 5, and 6 indicates that the narrower band gaps are accompanied by destabilization of the VB, whose energy levels and π*(Rh2) are closer in 6. As shown in Figure b, the destabilization of the filled σ-type orbital is attributed to the instability of the antibonding orbital between the σ(Rh2) and σ-type orbital of the Pt complex induced by the shorter Rh–Pt distance at a lower difference in the energy level of two components. As shown in Figure e, the results of TD-DFT calculations show three intense absorptions, 2.21, 2.15, and 1.87 eV, which are well-coincided with the observed ones. Both absorption at 2.21 and 2.15 eV are attributed to the admixture of transition from HOMO to LUMO or σ*(Rh–O), corresponding to the E1. In contrast, the absorption at 1.87 eV is attributed to the transition from HOMO – 3 to LUMO admixed from HOMO to LUMO, corresponding to the E2 (E3). Compared with 6′, the E1 value of 6 is slightly lower, indicating that the gap between HOMO and LUMO becomes narrower which come from infinite backbone.

Photoconductivities of Heterometallic 1-D Chains

Table summarizes the values of E1, E2, and E3 in heterometallic 1-D chains.[81−85,102,103] The E1 varies within the range of 2.10–3.51 eV and depends on metal species, repetition, and ligands, although there is a little difference in E2 and E3 values. Figure shows the plots of E1, E2, and E3 shown in Table against unbridged Rh–Pt distances (Å) shown in Table . A linear trend that E1 values lower when the distance of unbridged Rh–Pt bonds is shorter was observed, indicating that the stronger Rh–Pt bonds induce the narrower σ-type band gaps. As was found with 5 and 6, the stronger Rh–Pt bonds are realized with energy level conformity between two components. In contrast, both E2 and E3 are almost constant or slightly increased with shorter distances. The systematic heterometallic 1-D chains would be promising compounds for electrical conductivity measurements. However, the electrical conductivity values (two probes, d.c.) for the pellet samples of 1, 2, 5, and 6 exhibited high resistivity (>10 GΩ) at room temperature, which is attributed to the full VB orbital wherein all metal oxidation states in the chain are +2 in the four compounds. Because it is possible to oxidize compound 3 at d orbitals to be 4, an effort was made to obtain 1-D chains with 4 and dinuclear Rh complexes; however, deposited samples from the solution containing both compounds become 5 or 6 accompanied by reduction during synthetic processes.

Table 2

Comparison of E1, E2, and E3 (eV) Found in 1, 2, 5, 6, 6′, and Reported Compounds

compoundsa	E1 (eV)	E2 (eV)	E3 (eV)	ref
1	3.01, 2.60	1.78	1.50	(81)
2	2.67, 2.23	1.85	1.55	(81)
[{Rh2(O2CCH3)4}{Pt2(piam)2(NH2CH3)4}2](PF6)4	3.05	1.80	1.53	(102)
[{Rh2(O2CCH3)4}{Pt2(piam)2(bpy)2}2](PF6)4	3.21	1.70	1.44	(102)
[{Rh2(O2CCH3)4}{Pt(piam)2(NH3)2}2]·2H2O	3.45, 2.84	1.74	1.45	(85)
[{Rh2(O2CCH3)4}{Pt2Cu(piam)4(NH3)4}]n(PF6)2n (A)	3.41, 2.85	1.65	1.36	(85)
[{Rh2(O2CCH3)4}{Pt2Cu(piam)4(NH3)4}]n(PF6)2n (B)	3.26, 2.85	1.65	1.35	(85)
[{Rh2(O2CCH3)4}{Pt2Cu(piam)4(NH3)4}2](CF3CO2)2(ClO4)2·2H2O	3.43, 2.88	1.62	1.32	(85)
[{Rh2(acam)4}{Pt2(piam)2(NH3)4}2]n(CF3SO3)4n·2nMeOH (C)	2.90	1.80	1.52	(83)
[{Rh2(acam)4}{Pt2(piam)2(NH3)4}]n(CF3CO2)2n·2nEtOH (D)	2.47	1.70	1.43	(83)
[{Rh2(acam)4}{Pt(piam)2(NH3)2}2]·4MeOH·3H2O	3.50, 3.02	1.77	1.47	(103)
[{Ru2(O2CCH3)4}{Pt2(piam)2(NH3)4}2]n(PF6)4n·4nH2O (E)	3.43	1.99	1.68	(82)
[{Rh2(acam)4}3{Pt2(OPiv)2(NH3)4}4](ClO4)8 (F)	3.41, 2.93	1.67	1.38	(84)
[{Rh2(O2CCH3)4}{Pt2Co(piam)4(NH3)4}]n(PF6)2n	3.51, 2.90	1.76	1.55	(86)
5	2.48	1.76	1.49	this work
6	2.10	1.76	1.50	this work
6′	2.23	1.88	1.59	this work

Abbreviation: bpy = 2,2′-bipyridine, acam = acetamidate, and OPiv = pivalate.

Figure 11

Plots of E1 (eV, open circle), E2 (eV, triangle), and E3 (eV, square) against unbridged Rh–Pt distances (Å) in known heterometallic 1-D chains shown in Tables and 2.

Then, to evaluate a conductive nature of the metal complex wire, measurements with flash-photolysis time-resolved microwave conductivity (TRMC) were conducted, which provides the nanometer-scale photoconductivity of charge carriers generated by laser pulse irradiation under a low-power microwave electric field.[108−112] Micrometer-scale crystals were spread on an adhesive tape at a quartz plate placed in a microwave resonance cavity and photoexcited at 355 nm. Figure a shows the pseudo-photoconductivity (ϕΣμ) transients of 1, 2, 5, and 6. Notably, 6 exhibits considerably higher photoconductivity than the others, where the ϕΣμ maximum (ϕΣμmax) is 1: 8.1 × 10–5 cm2 V–1 s–1, 2: 1.6 × 10–4 cm2 V–1 s–1, 5: 1.2 × 10–4 cm2 V–1 s–1, and 6: 4.6 × 10–4 cm2 V–1 s–1. Although the value of 6 is lower than but in a similar order to that of rubrene for high-performance organic field effect transistors,[109]6 has higher charge carrier mobility compared with other related heterometallic 1-D chains (Figures b and S14). Although 6′ has an octanuclear structure not infinite chain, the conductivity of 6′ is better than other related compounds, which is probably because of the band structure caused by the closer distances of the end and end Pt atoms (Figure d). Figure c shows the plots of ϕΣμmax against E1 in Table . The trend that the values of ϕΣμmax are higher as E1 are lower is observed, indicating the higher photoconductivity in the heterometallic 1-D chain with a narrower σ-type band gap. Considering that Σμ represents the sum of hole and electron mobilities, and all measured compounds were certainly excited by the 355 nm (3.5 eV) light, it is impossible to determine which is the main carrier in this case, however, the conductive path of either hole or electron is undoubtedly the σ-type band. As shown in Figure , lower E1 values are regarded as shorter unbridged Rh–Pt bonds, resulting in closer distances among two components of [Rh2(O2CR)4] and Pt complexes. Consequently, the better conductivity is attributed to the shorter metal–metal distances over the entire 1-D backbones along the z axis, which can be realized in a combination taking into account the energy levels of the two components.

Figure 12

(a) FP-TRMC transients of 1 (yellow), 2 (blue), 5 (green), and 6 (red). (b) Comparison of the maximum of ϕΣμ (10–4 cm2 V–1 s–1). (c) Plots of the maximum of ϕΣμ (10–4 cm2 V–1 s–1) against the value of E1 (eV) between 1 (yellow), 2 (blue), 5 (green), 6 (red), 6′ (red), and A–F (black).

Conclusions

In this study, 1-D chains composed of three kinds of metals including Pd ions were successfully obtained by mixing a trinuclear complex [Pt2Pd(piam)4(NH3)4](PF6)2 that has a σ-type orbital as the HOMO and [Rh2(O2CR)4] (R = CH3 or CF3) that has a σ-type orbital as the LUMO, whose chains afford the σ-type CB and VB. Diffuse reflectance spectra revealed that the σ-type band gaps of the chains aligned as −Rh–Rh–Pt–Pd–Pt– become narrower than those of the prototype of heterometallic 1-D chains aligned as −Rh–Rh–Pt–Pt–Pt–Pt–. Narrower band gaps were also realized by ligand alteration of the Rh–Rh parts from −O2CCH3 to −O2CCF3, where electron-withdrawing ligands stabilize the d orbital of Rh–Rh that resulted in being close to the orbital levels of Pt-based complexes. Those band gap modulations are attributed to the stability or destabilization of the σ-type VB in heterometallic 1-D chains when the orbital levels of two kinds of complexes are energetically close; the VB becomes unstable, resulting in a narrower band gap and inducing better charge mobility as was found with 6. These results revealed the possibility of band gap modulations in heterometallic 1-D chains by third metal insertion and ligand alteration with expectations for wide varieties of the electronic structure with direct metal–metal bonds. Because molecular design is rational, it would be possible to obtain various chains composed of other metal species by controlling their metal oxidation states, opening up a new aspect in band structure engineering of heterometallic 1-D chains.

Experimental Section

Materials

PdCl2, RhCl3·3H2O, and K2PtCl4 were obtained from Tanaka Kikinzoku Co. Na2[PdCl4],[113]cis-[Pt(piam)2(NH3)2]·2H2O,[114] [Rh2(O2CCH3)4],[115] and [Rh2(O2CCF3)4][116] were synthesized according to previous procedures.

Synthesis of [Pt2Pd(piam)4(NH3)4](PF6)2 (3)

An aqueous solution (1 mL) of Na2[PdCl4] (8.8 mg, 30 μmol) and NaPF6 (25.5 mg, 152 μmol) was stirred for several minutes and mixed with cis-[Pt(piam)2(NH3)2]·2H2O (26.3 mg, 57 μmol) at room temperature overnight to afford green powder [Pt2Pd(piam)4(NH3)4](PF6)2 (14.3 mg). Yield was 40%. For elemental analysis, the sample was dried in vacuo for several hours. Elemental analysis calcd for C20H52F12N8O4P2PdPt2: C, 19.14%; H, 4.18%; and N, 8.93%. Found: C, 19.15%; H, 4.24%; and N, 8.92%. IR (KBr): 1629, 1566, 1487, 1362, 1297, 1255, 1244, 1192, 869, and 561 cm–1. To obtain single crystals suitable for X-ray analysis, the green powder was dissolved in THF at −30 °C. After several days, green block single crystals of [Pt2Pd(piam)4(NH3)4](PF6)2·4THF·2H2O (3·4THF·2HO) were successfully obtained.

Synthesis of [Pt2Pd(piam)4(NH3)4](PF6)3 (4)

A THF solution (8 mL) of AgPF6 (31.8 mg, 126 μmol) was poured into a THF solution (8 mL) of 3 (75.5 mg, 60 μmol) and stirred for 2 h at room temperature. After removal of deposited powder, violet solution was evaporated and washed with water to obtain violet powder of [Pt2Pd(piam)4(NH3)4](PF6)3 (37 mg). Yield was 44%. Elemental analysis calcd for C20H52F18N8O4P3PdPt2: C, 17.16%; H, 3.74%; and N, 8.00%. Found: C, 16.62%; H, 3.81%; and N, 7.68%. IR (KBr): 1643, 1567, 1487, 1429, 1366, 1303, 1254, 1224, 1188, 1084, 853, 741, 649, 560, and 482 cm–1. UV–vis (H2O) λmax: 357 and 571 nm. To obtain single crystals suitable for X-ray analysis, the violet THF solution was kept at −30 °C to obtain blue block single crystals of [Pt2Pd(piam)4(NH3)4](PF6)3·4THF (4·4THF).

Synthesis of [{Rh2(O2CCH3)4}{Pt2Pd(piam)4(NH3)4}](PF6)2 (5)

[Rh2(O2CCH3)4] (2.4 mg, 5.4 μmol) was added to a MeOH solution (2 mL) of 3 (6.3 mg, 5.0 μmol) and stirred at room temperature. After 1 day, dark red powder of [{Rh2(O2CCH3)4}{Pt2Pd(piam)4(NH3)4}](PF6)2 was collected by filtration (5.8 mg). Yield was 68%. Elemental analysis calcd for C28H64F12N8O12P2PdPt2Rh2: C, 19.82%; H, 3.80%; and N, 6.60%. Found: C, 19.80%; H, 3.82%; and N, 6.57%. IR (KBr): 1591, 1563, 1487, 1437, 1363, 1299, 1254, 1226, 1193, 869, 702, and 561 cm–1. UV–vis (solid) λmax: 500, 704, and 832 nm. To obtain single crystals suitable for X-ray analysis, a MeOH solution containing [Rh2(O2CCH3)4] was gently layered on a MeOH solution of 3. After 2 weeks, green single-crystal blocks were successfully obtained.

Synthesis of [{Rh2(O2CCF3)4}{Pt2Pd(piam)4(NH3)4}](PF6)2 (6)

An aqueous solution (3 mL) of [Rh2(O2CCF3)4] (10.6 mg, 16.1 μmol) was added to an aqueous solution (3 mL) of 3 (18.6 mg, 14.8 μmol) and stirred at room temperature. After 1 day, blue powder of [{Rh2(O2CCF3)4}{Pt2Pd(piam)4(NH3)4}](PF6)2 was collected by filtration (12 mg). Yield was 42%. Elemental analysis calcd for C28H52F24N8O12P2PdPt2Rh2: C, 17.58%; H, 2.74%; and N, 5.86%. Found: C, 17.05%; H, 2.93%; and N, 5.58%. IR (KBr): 1667, 1563, 1488, 1460, 1431, 1366, 1305, 1256, 1194, 869, 785, 739, and 563 cm–1. UV–vis (solid) λmax: 590, 704, and 827 nm.

Synthesis of [{Rh2(O2CCF3)4}{Pt2Pd(piam)4(NH3)4}2](PF6)3(CF3CO2) (6′)

In a Me2CO solution (1.0 mL) containing 6 (13 mg), n-hexane (3 mL) was gently layered to obtain microcrystalline [{Rh2(O2CCF3)4}{Pt2Pd(piam)4(NH3)4}2](PF6)3(CF3CO2) (6′) with yellow metallic luster (6.7 mg). Yield was 62%. Elemental analysis calcd for C50H104F33N16O18P3Pd2Pt4Rh2: C, 19.15%; H, 3.34%; and N, 7.15%. Found: C, 18.49%; H, 3.13%; and N, 6.37%. A MeOH solution (2 mL) containing [Rh2(O2CCF3)4] (3.3 mg, 5.0 μmol) and 3 (6.1 mg, 4.9 μmol) was slowly evaporated to obtain several dark green single crystals of 6′. IR (KBr): 1670, 1564, 1489, 1460, 1430, 1364, 1311, 1256, 1194, 869, 784, 739, and 562 cm–1. UV–vis (solid) λmax: 556, 659, and 780 nm.

X-ray Structure Determination

X-ray diffraction measurements were performed using a Rigaku AFC7R diffractometer equipped with a normal focus Mo-target X-ray tube (λ = 0.71070 Å) operated at 15 kW power (50 kV, 300 mA) and a Rigaku Mercury CCD two-dimensional detector. A total of 744 frames were collected with a scan width of 0.5° and an exposure time of 5 s/frame (3·4THF·2HO), 10 s/frame (4·4THF), 5 s/frame (5), and 10 s/frame (6′). Empirical absorption correction[117] was performed on all data. The structure was solved by the direct method[118] with subsequent difference Fourier synthesis and refinement using SHELX-2017[119] controlled using a Yadokari-XG software package.[120] Nonhydrogen atoms were anisotropically refined, and all hydrogen atoms were treated as riding atoms. In 3·4THF·2HO, a THF molecule containing O4 and C15–18 atoms was refined under rigid conditions, and the initial position of hydrogen atoms in a water molecule containing an O5 atom was found and refined under rigid conditions. In 6′, a piam ligand containing C30–34 atoms and −O2CCF3 containing C47–48 and F10–12 were refined under rigid conditions, and the C5 atom was isotropically refined. The data were applied to a solvent mask using PLATON/SQUEEZE.[121] Crystal data and structure refinement results are summarized in Table .

Table 3

Crystallographic Data and Structure Refinements for [Pt2Pd(piam)4(NH3)4](PF6)2·4THF·2H2O (3·4THF·2HO), [Pt2Pd(piam)4(NH3)4](PF6)3·4THF (4·4THF), [{Rh2(O2CCH3)4}{Pt2Pd(piam)4(NH3)4}](PF6)2 (5), and [{Rh2(O2CCF3)4}{Pt2Pd(piam)4(NH3)4}2](PF6)3(CF3CO2) (6′)

	3·4THF·2H2O	4·4THF	5	6′
empirical formula	C36H88F12N8O10P2PdPt2	C36H84F18N8O8P3PdPt2	C28H64F12N8O12P2PdPt2Rh2	C50H104F33N16O18P3Pd2Pt4Rh2
formula weight	1579.66	1688.60	1697.21	3136.38
crystal system	monoclinic	monoclinic	monoclinic	triclinic
space group	P21/n	C2/c	P21/n	P1̅
a (Å)	11.331(2)	30.731(13)	13.336(3)	13.872(6)
b (Å)	22.019(5)	11.615(4)	14.243(3)	18.090(8)
c (Å)	12.253(3)	22.369(10)	14.118(3)	20.222(8)
α (deg)	90	90	90	88.162(12)
β (deg)	95.145(3)	128.996(5)	97.164(3)	85.8430(10)
γ (deg)	90	90	90	88.603(8)
V (Å3)	3044.8(11)	6205(4)	2660.8(10)	5057(4)
Z	2	4	2	2
temperature (K)	293	293	293	293
Dc (Mg m–3)	1.723	1.807	2.118	2.060
absorption coefficient (mm–1)	5.015	4.963	6.335	6.342
F(000)	1560	3316	1628	2992
crystal size (mm3)	0.49 × 0.40 × 0.36	0.37 × 0.32 × 0.25	0.13 × 0.10 × 0.09	0.47 × 0.39 × 0.36
measured reflections	24,184	24,270	21,653	22,774
independent reflections	6969	7069	6091	22,774
Goodness-of fit on F2	1.123	1.133	1.086	1.094
R [I > 2σ(I)]	0.0364	0.0699	0.0544	0.0925
R (all data)	0.0440	0.1027	0.0830	0.1735

Physical Measurements

UV–vis spectra were recorded using a Shimadzu UV3100PC (range 200–1400 nm) at room temperature. Infrared spectra were recorded using a Perkin Elmer Spectrum 400 (range 400–2000 cm–1) at room temperature. XPS measurements were performed using a Quantera-SXM spectrometer at room temperature. Binding energies were measured relative to the C 1s peak (284.8 eV) of internal hydrocarbons. EPR spectra were measured on a JEOL TE-200 spectrometer. Diffuse reflectance spectra were recorded using a Hitachi U-4000 spectrophotometer (range 200–2500 nm) at room temperature. Obtained reflectance spectra were converted to absorption spectra using the Kubelka–Munk function F(R∞). Cyclic voltammetric measurements were conducted at room temperature using a BAS 617E electrochemical analyzer. Cyclic voltammograms were recorded with THF or MeCN solutions containing 0.1 M Bu4NPF6 as a supporting electrolyte. Conventional three-electrode arrangement consisting of a glassy carbon working electrode, Ag/Ag+ reference electrode, and Pt wire counter electrode was used.

DFT Calculation

Electronic structures of model compound [Pt2Pd(NHCOCH3)4(NH3)4] (n = 2, 3, and 4) and [{Rh2(O2CCF3)4}{Pt2Pd(NHCOCH3)4(NH3)4}2]4+ were determined using the DFT method with the B3LYP function[122−124] and Gaussian 09 program package.[125] For Pt, Rh, and Pd, the LANL2DZ basis set was used together with the effective core potential of Hay and Wadt.[126] For the other elements, 6-31G* basis sets[127] were selected. The initial models of [Pt2Pd(NHCOCH3)4(NH3)4] and [{Rh2(O2CCF3)4}{Pt2Pd(NHCOCH3)4(NH3)4}2]4+ for optimization were prepared using the geometrical parameters obtained from the crystal structure data of 3·4THF·2HO and 6′, respectively. For the models, full geometry optimization was conducted; based on the structure, 40 excited states were obtained to determine vertical excitation energy using TD-DFT calculation.[128,129] For comparison of PDOS, DFT calculations of 1 and 5 were performed under periodic boundary condition using Quantum ESPRESSO.[130] All calculations were carried out using the plane-wave basis set with 800 eV cutoff energy and the ultrasoft-type pseudopotential.[131] The Perdew, Burke, and Ernzerhof-parameterized generalized gradient approximation functional[132] was used for describing exchange and correlation. Brillouin-zone sampling was performed using the 1 × 1 × 1 Monkhorst–Pack mesh scheme for the atomic position and cell parameter optimizations using the crystal structures as the initial structures, while 4 × 4 × 4 k-points were employed to plot PDOS. The thresholds of convergence for energy and force were set to 10–8 eV and 10–6 eV/Å, respectively. To consider dispersion interaction, Grimme-type dispersion correction was employed.[133]

Flash-Photolysis Time-Resolved Microwave Conductivity Measurement

Nanosecond laser pulses from a Nd:YAG laser (third harmonic generation, THG (355 nm) from a Continuum Inc., Surelite II, FWHM 5–8 ns) were used as an excitation source. Incident photon density of the laser was set at 4.6 × 1015 photons cm–2 pulse–1. The microwave frequency and power were set at ca. 9.1 GHz and 3 mW, respectively, so that the motion of the charge carriers was not disturbed by the low electric field of microwave. The TRMC signal picked using a diode (rise time <1 ns) is monitored with a digital oscilloscope. All experiments mentioned above were conducted at room temperature. Pseudo-photoconductivity is given using the equation ϕΣμ = (1/eAI0Flight) (ΔPr/Pr), where ϕ, Σμ, e, A, I0, Flight, ΔPr, and ΔPr denote the photocarrier generation yield, the sum of charge carrier mobility, the unit charge of single electron, the sensitivity factor (S–1 cm), the incident photon density of excitation laser (photon cm–2 pulse–1), the filling factor (cm–1), and the reflected microwave power and change, respectively.