Improving the Solubility of Hexanuclear Heterometallic Extended Metal Atom Chain Compounds in Nonpolar Solvents by Introducing Alkyl Amine Moieties.

The highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) interaction at the d z 2 orbital between two kinds of metal complex is useful for obtaining heterometallic one-dimensional (1D) chains as well as heterometallic metal string compounds (HMSCs). Platinum dinuclear complexes, [Pt2(piam)2(NH2R)4]X2 (piam = pivalamidate, R = CH3, C2H5, C3H7, or C4H9, X = anion), comprising σ* as HOMO were mixed with [Rh2(O2CCH3)4] comprising σ* as LUMO in solvents to afford single crystals of [{Rh2(O2CCH3)4}{Pt2(piam)2(NH2R)4}2]X4 (2-5). Single-crystal X-ray analyses revealed that 2-5 are hexanuclear complexes that are one-dimensionally aligned as Pt-Pt-Rh-Rh-Pt-Pt with metal-metal bonds, where the alkyl moieties at end Pt atoms obstruct further 1D extension. Complexes 2-5 appear as if they are cut off from an infinite chain [{Rh2(O2CCH3)4}{Pt2(piam)2(NH3)4}2] n (PF6)4n ·6nH2O (1) aligned as -{Pt-Pt-Rh-Rh-Pt-Pt} n -. The diffuse reflectance spectrum of 1 depicts broad shoulder bands, which are not present in the spectra of 2-5, proving that the infinite chain 1 forms a band structure. Compounds 4 and 5 with propyl or butyl moieties at amine ligands, respectively, are soluble in nonpolar solvents, such as CH2Cl2, without the dissociation of their hexanuclear structures. Taking advantage of their solubility, measurement of cyclic voltammetry in CH2Cl2 become possible, which shows the quasi-reversible oxidation and reduction waves at 4: E ox = 0.86 V and E red = 0.69 V and 5: E ox = 0.87 V and E red = 0.53 V.

Introduction

The exploration of a useful synthetic method for manufacturing one-dimensional (1D) metal wires is still sustained by their advantageous physical properties of high anisotropy,[1−5] because 1D metal wires containing element block are used in various applications, such as light-emitting diodes, photovoltaic cells, and molecular sensors.[6−8] Extended metal atom chains (EMACs), where metals are one-dimensionally arranged by metal–metal bonds, are advantageous as various electronic structures can be obtained by tuning the metal species and their oxidation states.[9−24] The major synthetic approach for EMACs is the template method, where the metals are reacted with pre-programed ligands, such as oligopyridylamine,[9−17] oligophosphine,[18−21] and π-conjugated ligand,[22−24] to align metals with metal–metal bonds by controlling the number of metal atoms by the length of the organic ligand. Recently, heterometallic EMAC compounds (HEMACs), also known as heterometallic metal string compounds (HMSCs),[25,26] have been synthesized, where a few kinds of metal species are regularly aligned; for example, MA–MA–MB,[27−44] Ni–Ru–Ru–Ni–Ni,[45] and Ni–Ni–Ru–Ru–Ni–Ni–Ni,[46] with two kinds of metal species; MA–MB–MC, with three kinds of metal species;[47,48] and surprisingly Ni–Pt–Co–Co–Pd, with four kinds of metal species[49] were realized with a molecular rectifier[45] and large ferromagnetic coupling thorough metal–metal bonds.[42] To align metals by regulating number and species at will, rational and systematic synthesis methods are currently being explored.

In previous study, we synthesized heterometallic 1D chains using the highest occupied molecular orbital–lowest unoccupied molecular orbital (HOMO–LUMO) interaction in the σ* (d) orbital between two kinds of metal complexes.[50] For example, platinum dinuclear complex [Pt2(piam)2(NH3)4](PF6)2 (piam = pivalamidate) and rhodium dinuclear complex [Rh2(O2CCH3)4] with HOMO and LUMO in σ*, respectively, were mixed in an adequate solvent to yield the crystals for [{Rh2(O2CCH3)4}{Pt2(piam)2(NH3)4}2](PF6)4·6nH2O (1), which contains 1D chains aligned as −{Pt–Pt–Rh–Rh–Pt–Pt}– (Figure a).[51] Interestingly, this synthetic method is versatile: various 1D chains modulated in electronic structures can be attained by selecting the metal species and bridging ligands in dinuclear LUMO parts[52−54] or trinuclear complex Pt–M–Pt in HOMO parts;[54−56] thus, band gap modulation[54] and paramagnetism[53,55−57] are achieved through metal–metal bonds (Scheme ).

Figure 1

Crystal structures of (a) [{Rh2(O2CCH3)4}{Pt2(piam)2(NH3)4}2](PF6)4·6nH2O (1) and (b) [{Rh2(O2CCH3)4}{Pt2(piam)2(NH2CH3)4}2](PF6)4 (2).

Scheme 1

Heterometallic 1D Chains Constructed by HOMO–LUMO Interaction at d Orbitals with Variable Metal Alignments

In this study, we applied this rational metal extension method for infinite 1D chains, the HOMO–LUMO interaction in σ* (d) orbital, to synthesize HMSCs. As shown in Figure a, the unbridged Pt–Rh bonds in 1 are formed by not only HOMO–LUMO interactions in σ* but also quadruple hydrogen bonds between Pt-coordinated NH3 or NH (piam) and Rh-coordinated O atoms. The ligands coordinated to the end Pt atoms are also quadruple hydrogen-bonded, and thus, they support the formation of end Pt–Pt bonds as infinite −{Pt–Pt–Rh–Rh–Pt–Pt}– chains.[51] In contrast, when a dinuclear platinum complex has different co-ligands, such as NH2CH3 from NH3, a hexanuclear complex [{Rh2(O2CCH3)4}{Pt2(piam)2(NH2CH3)4}2](PF6)4 (2) aligned as Pt–Pt–Rh–Rh–Pt–Pt is afforded (Figure b), where end Pt–Pt bonds are not formed despite the formation of unbridged Pt–Rh bonds.[58] These results indicate that the unbridged Pt–Rh bond in this system is relatively strong, but the end Pt–Pt bond is not formed without the support of hydrogen bonds. Using these findings, novel hexanuclear HMSCs containing two unbridged Pt–Rh bonds were designed by introducing alkyl moieties in co-ligands in Pt atoms. In this study, the syntheses, structures, and physical properties of three hexanuclear HMSCs with NH2C2H5, NH2C3H7, or NH2C4H9 in co-ligands in Pt atoms are shown. Fortunately, the hexanuclear complexes comprising NH2C3H7 or NH2C4H9 are improved with solubility in nonpolar solvents without the collapse of their hexanuclear backbones. The electronic structures, solubilities, and redox behaviors are discussed by comparing several physical measurements and calculations.

Results and Discussion

Synthetic Procedure

To rationally introduce alkyl moieties at amines coordinated to Pt atoms in the 1D heterometallic complexes, amidate-hanging platinum mononuclear complexes cis-[Pt(piam)2(NH2CH3)2]·H2O,[59]cis-[Pt(piam)2(NH2C2H5)2]·H2O,[60]cis-[Pt(piam)2(NH2C3H7)2]·H2O,[61] and cis-[Pt(piam)2(NH2C4H9)2][61] were used. These complexes were easily dimerized with aqua Pt complexes having corresponding alkyl amines, cis-[Pt(OH2)2(NH2R)2] (R = CH3, C2H5, C3H7 and C4H9), to provide dinuclear platinum complexes, [Pt2(piam)2(NH2R)4]X2 (X = anion). Similar to previous synthetic procedures[58] (Scheme ), novel heterometallic complexes were successfully obtained, [{Rh2(O2CCH3)4}{Pt2(piam)2(NH2C2H5)4}2](PF6)4 (3), [{Rh2(O2CCH3)4}{Pt2(piam)2(NH2C3H7)4}2](PF6)3(ClO4) (4), and [{Rh2(O2CCH3)4}{Pt2(piam)2(NH2C4H9)4}2](ClO4)4 (5), by selecting adequate counter anions and solvents.

Scheme 2

Synthetic Route for Heterometallic 1D Hexanuclear Complexes 1–5

Crystal Structures and Oxidation States of 3–5

Figure shows the crystal structure of 3. The dinuclear paddle wheel complexes of [Rh2(O2CCH3)4] are sandwiched by [Pt2(piam)2(NH2C2H5)4] at both ends with metal–metal bonds to form [Pt–Pt]–[Rh–Rh]–[Pt–Pt] units, where a crystallographic inversion center lies at the center of the rhodium complex (Figure a). These hexanuclear units in 3 are similar to those in 2. Three kinds of angles are defined here as that θ is the angle through metal–metal bonds; ϕ is the twist angles between the coordination plane; and τ is the dihedral angles of coordination planes (Scheme ). The platinum dinuclear complexes are bonded to a rhodium complex with a bond distance of Pt(2)–Rh(1) = 2.8321(5) Å and a torsion angle N–Pt–Rh–O (ϕ2) of about 16–23° (Figure b). Between [Pt–Pt] and [Rh–Rh], the ethyl groups of NH2C2H5 are tilted toward [Pt–Pt], which permits multiple hydrogen bonds between the nitrogen atoms of the amine/amidate ligands in [Pt–Pt] and the carboxylate oxygen atoms in [Rh–Rh] with N–O bond distances of 2.9–3.0 Å. The dihedral angle (τ2) between the RhO4 and PtN4 planes is small (2.6°), indicating that the Rh and Pt planes are arranged in a face-to-face fashion. The τ3 between the two Pt coordination planes is relatively large (37°), which is caused by the half-lantern fashion of the piam bridges. The average torsional twist angle (ϕ3) about the Pt–Pt axis within the [Pt–Pt] is 7.5°. In the entire crystal, each hexanuclear [Pt–Pt]–[Rh–Rh]–[Pt–Pt] unit is packed in parallel (Figure c), where the methyl groups of NH2C2H5 on terminal platinum atoms obstruct the close contact to the neighboring hexanuclear unit (Pt–Pt = 5.59 Å), resulting in no formation of infinite 1D chains.

Figure 2

(a) Crystal structure of [{Rh2(O2CCH3)4}{Pt2(piam)2(NH2C2H5)4}2](PF6)4 (3). (b) Stacking fashion between rhodium dinuclear complex and platinum dinuclear complex in 3. (c) Packing view of 3. The hydrogen atoms and anions are omitted for clarity.

Scheme 3

Angles (θ) through Metal–Metal Bonds, Torsional Twist Angles (ϕ), and Dihedral Angles (τ) in Heterometallic Hexanuclear Complexes

Figures and 4 show the crystal structures of [{Rh2(O2CCH3)4}{Pt2(piam)2(NH2C3H7)4}2](PF6)3(ClO4) (4) and [{Rh2(O2CCH3)4}{Pt2(piam)2(NH2C4H9)4}2](ClO4)4 (5), respectively. Each compound also forms hexanuclear complexes expressed as [Pt–Pt]–[Rh–Rh]–[Pt–Pt] (Figures a and 4a). The alkyl moieties at amine ligands coordinated to inner Pt atoms protrude along the coordination planes, where effective hydrogen bonds are formed between inner Pt and Rh coordination planes. The Pt–Rh distances are 2.7536(6) Å (4) and 2.8030(11) Å (5), where [Pt–Pt] and [Rh–Rh] are stacked in a staggered fashion with torsion angles N–Pt–Rh–O (ϕ2) of approximately 35–39° (4) and 31–34° (5). The end methyl groups at inner alkyl moieties are closer to –O2CCH3 ligands, as if surrounding the [Rh–Rh] parts. On the contrary, outer alkyl moieties at amine ligands coordinated to outer Pt atoms protrude along the metal–metal bonds, affording greater differences between hexanuclear complexes with longer alkyl moieties, where the distances between end platinum atoms of outer Pt and neighboring ones are 5.54 Å (4) and 10.66 Å (5). In 4, the determination of the position of ClO4– ions in the crystal is difficult due to the high disorder; however, the results of elemental analysis and electrospray ionization mass spectrometry (ESI-MS) spectra revealed that the formula of 4 is [{Rh2(O2CCH3)4}{Pt2(piam)2(NH2C3H7)4}2](PF6)3(ClO4).

Figure 3

(a) Crystal structure of [{Rh2(O2CCH3)4}{Pt2(piam)2(NH2C3H7)4}2](PF6)3(ClO4) (4). (b) Stacking fashion between rhodium dinuclear complex and platinum dinuclear complex in 4. (c) Packing view of 4. The hydrogen atoms and anions are omitted for clarity.

Figure 4

(a) Crystal structure of [{Rh2(O2CCH3)4}{Pt2(piam)2(NH2C4H9)4}2](ClO4)4 (5). (b) Stacking fashion between rhodium dinuclear complex and platinum dinuclear complex in 5. (c) Packing view of 5. The hydrogen atoms and anions are omitted for clarity.

Table summarizes metal–metal distances and angles for 1–5. The values of θ, ϕ, and τ are almost similar. The ϕ2 values between [Pt–Pt] and [Rh–Rh] are in the range of 20–43°, showing twist fashion. No trend in Rh–Pt distances exists depending on the length of alkyl moieties. In 1–5, the Pt–Pt distances are 2.94–3.02 Å, which are typical for Pt(+2)–Pt(+2) oxidation states.[62]

Table 1

Comparison of Selected Bond Distances (Å) and Angles (deg) between 1, 2, [{Rh2(O2CCH3)4}{Pt2(piam)2(NH2C2H5)4}2](PF6)4 (3), [{Rh2(O2CCH3)4}{Pt2(piam)2(NH2C3H7)4}2](PF6)3(ClO4) (4), and [{Rh2(O2CCH3)4}{Pt2(piam)2(NH2C4H9)4}2](ClO4)4 (5)

compounds	Rh–Rh (Å)	Rh–Pt (Å)	Pt–Pt (Å)	θ1 (deg)	θ2 (deg)
1	2.3832(17)	2.7460(10)	2.9376(7)	177.75(6)	156.45(3)
2	2.3779(17)	2.7493(12)	2.9929(10)	177.04(5)	156.93(2)
3	2.3935(8)	2.8321(5)	3.0008(4)	176.68(3)	159.303(11)
4	2.3903(9)	2.7536(6)	2.9356(4)	177.04(3)	159.361(14)
5	2.3920(18)	2.8030(11)	3.0167(10)	174.96(6)	151.84(2)

Average of four angles.

Several hexanuclear EMACs bridged by polyphenylphosphine or polypyridyl amine ligands,[63−68] such as [Pt6(μ-dpmp)4(XylNC)2](PF6)4 (dpmp = bis(diphenylphosphanylmethyl)phenylphosphane, Xyl = 2,6-dimethylphenyl),[63] [Co6(μ6-bpyany)4(NCS)2]PF6 (H2bpyany = 2,7-bis(α-pyridylamino)-1,8-naphthyridine),[64] and [Ni6(μ6-bpmany)4(NCS)2]Cl (H2bpmany = 2,7-bis(α-pyrimidylamino)-1,8-naphthyridine),[66] have been reported. Compared to these EMACs, the characteristic features for 2–5 contain two unbridged metal–metal (Pt–Rh) bonds, which are similar to those for the previous hexanuclear compound HH,HT,HH-[Ir6(μ-OPy)6(I)2(CO)12] (Opy = 2-pyridonate), which comprises three dinuclear Ir complexes aligned as Ir–Ir–Ir–Ir–Ir–Ir.[69,70] In HH,HT,HH-[Ir6(μ-OPy)6(I)2(CO)12], six iridium atoms have a formal oxidation state of +1.33, where the sum of oxidation state is +8, indicating that 46 d-electrons are present over six iridium atoms. Moreover, in 2–5, the formulas based on the crystal structures and elemental analyses revealed that the sum of the metal oxidation state of hexanuclear Pt–Pt–Rh–Rh–Pt–Pt is +12, which indicates 46 d-electrons in four Pt and two Rh atoms. Considering that the Pt–Pt distances in the crystal structures revealed typical d8–d8 configurations, Rh–Rh comprised d7–d7 configurations and resulted in the formal oxidation state of Pt(+2)–Pt(+2)–Rh(+2)–Rh(+2)–Pt(+2)–Pt(+2), which were not changed from the original compounds. This oxidation state is similar to that in the prototype 1D chain 1. To further estimate the oxidation states, X-ray photoelectron spectroscopy (XPS) measurements were performed (Figure S5). The XPS spectra of 2–5 in the Rh 3d and Pt 4f regions at room temperature show that the Rh 3d5/2 binding energy values are 308.6 (2), 308.8 (3), 308.7 (4), and 308.8 (5) eV. In addition, the Pt 4f7/2 binding energy values are 73.1 (2), 73.2 (3), 72.9 (4), and 73.0 (5) eV. These values well coincide with those in 1, supporting the Pt(+2)–Pt(+2)–Rh(+2)–Rh(+2)–Pt(+2)–Pt(+2) oxidation state.

Compounds 2–5 are considered as if they are cut from the 1D chain 1 as hexanuclear units, because the metal repetition, metal–metal distances, and oxidation states are similar. In other previous metal complexes introducing alkyl moieties in coordinated ligands,[71−73] the hydrophobic interaction among side ligands induces the aggregation of metal cores, thus affording the infinite 1D chains. Furthermore, in 2–5, outer alkyl moieties obstruct the further extension to become HMSCs. The Pt–Rh bonds afforded by the HOMO–LUMO interaction along the z axis overcome the obstruction of inner alkyl moieties, indicating that these heterometallic bonds are relatively strong, but the unbridged Pt–Pt bonds found in 1 are relatively weak. This system was successful in cutting the hexanuclear units from 1 by introducing alkyl moieties at the co-ligands.

Electronic Structures and Absorption Spectra

Figures –7 and S6–S33 show the results of density functional theory (DFT) calculations of the model of [{Rh2(O2CCH3)4}{Pt2(NHCOR′)2(NH2R)4}2]4+ (R = CH3, C2H5, C3H7 or C4H9; R′ = CH3 or Bu) based on the crystal structures. Based on the crystal structures of 2–5, the model structures [{Rh2(O2CCH3)4}{Pt2(NHCOCH3)2(NH2R)4}2]4+ were calculated to obtain optimized structures na (n = 2, R = CH3; n = 3, R = C2H5; n = 4, R = C3H7; n = 5, R = C4H9), nb under the rigid condition of metal coordinates, and nc under the rigid condition of metal and coordinated atom coordinates. Furthermore, the model structures [{Rh2(O2CCH3)4}{Pt2(NHCOBu)2(NH2R)4}2]4+ were calculated to obtain optimized structures nd, structures ne under the rigid condition of metal coordinates, and structures nf under the rigid condition of metal and coordinated atom coordinates. Thus, for 2–5, six optimized structures na–nf were obtained, respectively. As shown in Figures S6–S33, comparing na–nc and nd–nf, cardinal electronic structures do not depend on whether CH3 or Bu is present at the bridging ligands of Pt dinuclear complexes; this indicates that the ligands of –NHCOR′ do not significantly influence the electronic structures of 2–5. Figure shows optimized structures and shapes of LUMO and HOMO for 2a–c. In the optimized structure of 2a, the metal–metal distances are longer than those in the crystal structure of 2 (Table S1), where the Pt–Pt distance is 3.55 Å and shows no significant metal–metal interactions. Moreover, the twist angle (ϕ2) between [Rh–Rh] and [Pt–Pt] is 5.6°, as it is close to face-to-face stacking. In contrast, in 2b and 2c, the values of twist angle ϕ2 calculated under rigid conditions are closer to the crystal structure. As shown in Figure , the LUMOs and HOMOs for 2a–c are σ-type orbitals over Pt–Rh–Rh–Pt (2a) or Pt–Pt–Rh–Rh–Pt–Pt (2b and 2c). The noteworthy feature is the shapes of HOMO at [Rh–Rh], which is not occupied by sole d orbitals but is occupied by dzx (or dyz), indicating that HOMOs are mixed with π-type orbitals. In other words, in the formation of unbridged Pt–Rh bonds with HOMO–LUMO interactions between Pt2 σ* and Rh2 σ* orbitals, the HOMO at Rh2 π* is also involved with the formation of unbridged bonds.

Figure 5

Optimized structures (left), LUMO (middle), and HOMO (right) obtained by DFT calculations based on the model of [{Rh2(O2CCH3)4}{Pt2(NHCOCH3)2(NH2CH3)4}2]4+, (a) 2a, (b) 2b under the rigid condition of Pt and Rh coordinates, and (c) 2c under the rigid condition of Pt, Rh, O, and N coordinates.

Figure 7

Results of DFT calculation based on the model of (a) [{Rh2(O2CCH3)4}{Pt2(NHCOCH3)2(NH2CH3)4}2]4+ (2c), (b) [{Rh2(O2CCH3)4}{Pt2(NHCOCH3)2(NH2C2H5)4}2]4+ (3c), (c) [{Rh2(O2CCH3)4}{Pt2(NHCOCH3)2(NH2C3H7)4}2]4+ (4c), and (d) [{Rh2(O2CCH3)4}{Pt2(NHCOCH3)2(NH2C4H9)4}2]4+ (5c).

Figure shows the result of the DFT calculation of 2c. As mentioned above, both HOMO and LUMO are σ-type orbitals over six metals, where HOMO are antibonding orbitals between Rh2 σ and two Pt2 σ* and LUMO are antibonding orbitals between Rh2 σ* and two Pt2 σ*. Other σ-type orbitals are found in HOMO – 4 (Figure S8), and more energetically stable ones are expected to attribute to the six aligned metals. The nodes of σ-type orbitals for LUMO, HOMO, and HOMO – 4 are five, four, and three, respectively. In addition, HOMO – 1 and HOMO – 2 are degenerate Rh2 π* and HOMO – 3 is Rh2 δ*. Although the order of energetic levels for each molecular orbital is similar, in na and nd with longer metal–metal distances, σ-type HOMOs are stabilized to be degenerate with three lower orbitals (Figures S30–S33). These results indicate that depending on metal–metal distances and twist angles between [Rh–Rh] and [Pt–Pt], HOMOs in hexanuclear Pt–Pt–Rh–Rh–Pt–Pt complexes are flexible.

Figure 6

Result of DFT calculation based on the model of [{Rh2(O2CCH3)4}{Pt2(NHCOCH3)2(NH2CH3)4}2]4+ (2c) under the rigid condition of Pt, Rh, O, and N coordinates.

Figure shows the energy levels of molecular orbitals for 2c, 3c, 4c, and 5c. Although the bridging ligand at the platinum parts did not affect the energy levels (Figures S30–S33), as alkyl groups in co-ligands at platinum atoms become longer, the overall orbital energy increases, but is only due to the change in composition. The HOMO–LUMO gaps in four compounds are similar: 3.12 (2c) eV, 3.02 (3c) eV, 3.02 (4c) eV, and 3.00 (5c) eV.

Figure shows diffuse reflection spectra for 1–5. In 1 and 2, three characteristic absorptions E1, E2, and E3 were observed,[51,58] which are also found in 3–5. Taking into account the fact that the metal oxidation number of the hexanuclear unit in 1–5 is [Pt(+2)2]–[Rh(+2)2]–[Pt(+2)2], six molecular orbitals are made from all possible combinations of the metal σ orbitals, σ(Pt2)−σ(Rh2)−σ(Pt2), σ(Pt2)−σ*(Rh2)−σ(Pt2), σ(Pt2)−σ(Rh2)−σ(Pt2), σ*(Pt2)−σ*(Rh2)−σ*(Pt2), σ*(Pt2)−σ(Rh2)−σ*(Pt2), and σ*(Pt2)−σ*(Rh2)−σ*(Pt2), where energy increases with the number of nodes along the chain direction.[70] As shown in the result of the DFT calculation (Figures and 7), the LUMO comprises σ*(Pt2)−σ*(Rh2)−σ*(Pt2) having all antibonding combinations of σ*(Pt2) and σ*(Rh2). Previous analyses[50,55] suggested that E1 is attributed to the transition from filled σ-type to vacant σ-type orbitals, whereas both E2 and E3 are attributed to transition from degenerate Rh2 π* to vacant σ-type orbitals. In addition, Figure shows the results of time-dependent density functional theory (TD-DFT) calculations for 5a–c as bars. The calculated results of TD-DFT for E1 well coincide the observed results, indicating that the strong absorption in the 2.88–3.05 eV regions is induced by the transition between σ-type orbitals. In 1, a broad shoulder was observed around 2.60 eV (Figure a). Since this broad shoulder is not present in 2–5, it is attributed to the formation of a band structure. In other words, the appearance of the shoulder band in 1 proved the significant interaction between end Pt and Pt atoms of hexanuclear Pt–Pt–Rh–Rh–Pt–Pt units. Among 2–5, the values of E1 are similar or slightly decreased with longer chain of alkyl groups at co-ligands.

Figure 8

Diffuse reflectance spectra of (a) 1, (b) 2, (c) 3, (d) 4, and (e) 5 with MgO at room temperature.

Figure 9

Diffuse reflectance spectra of 5 with schematic electronic structure. Green, blue, and red bars show the results of TD-DFT calculation for 5a, 5b, and 5c, respectively.

Although E1 is exactly assigned by the TD-DFT calculations, the estimation of both E2 and E3 is difficult. Results of TD-DFT calculations for 5a–c showed weak adsorption around <2.0 eV, which are caused by the transition from HOMO – 1 and HOMO – 2 of degenerate Rh2 π* to LUMO admixed with transitions from HOMO to LUMO. The absorption of E2 in lower energetic regions attributed to wider transition from HOMO – 1 to LUMO than HOMO to LUMO is conflicting; a probable reason for this is that calculated energy levels are based on static structures, and they do not consider local differences of structures caused by vibrations. In reality, in [{Rh2(O2CCH3)4}{Pt2Cu(piam)4(NH3)4}](PF6)2[55] and [{Rh2(O2CCH3)4}{Pt2Co(piam)4(NH3)4}](PF6)2[56] with staggered and eclipsed stacking between Pt and Rh coordination planes, respectively, the peak strengths of both E2 and E3 in the latter are weaker than those in the former,[56] depending on twist angles between the two complexes. As shown in Figure d,e, absorption peaks of E3 in 4 and 5 were not observed. Since the energetic differences between E2 and E3 observed in 1–3 are small and similar, the observation of E3 is related to whether Rh2 π* is degenerate or not. This trend evidently revealed that both E2 and E3 are involved with degenerate Rh2 π* orbitals.

Stabilities of Hexanuclear Structures in Nonpolar Solvents

In general, the molecule with longer alkyl chains has more hydrophobicity. The previously reported trinuclear Pt–Cu–Pt complexes comprising propyl or butylamine coordinated to end Pt atoms are soluble in nonpolar solvents, such as CH2Cl2, although they are cationic complexes.[61] In these trinuclear complexes aligned as Pt–Cu–Pt, each metal is bridged by piam ligands. In contrast, hexanuclear 2–5 have two unbridged Pt–Rh bonds. When compounds 2–5 are immersed in polar solvents, such as MeOH, most of the hexanuclear molecules are dissociated to afford dinuclear complexes, due to the coordination of solvent molecules. Figure shows ESI-MS spectra of the MeOH solution containing 4 or 5. In 4, weak signals at m/z = 1897, 1943, 2339, and 2385 corresponding to {[Pt2(piam)2(NH2C3H7)4]2(PF6)(ClO4)–H}+, {[Pt2(piam)2(NH2C3H7)4]2(PF6)2–H}+, {[{Rh2(O2CCH3)4}{Pt2(piam)2(NH2C3H7)4}2](PF6)(ClO4)–H}+, and {[{Rh2(O2CCH3)4}{Pt2(piam)2(NH2C3H7)4}2](PF6)2–H}+ were observed. Furthermore, in 5, weak signals at m/z = 2110, 2155, 2201, 2260, 2305, and 2351 corresponding to {[Pt2(piam)2(NH2C4H9)4]2(PF6)(ClO4)2}+, {[Pt2(piam)2(NH2C4H9)4]2(PF6)2(ClO4)}+, {[Pt2(piam)2(NH2C4H9)4]2(PF6)3}+, {[{Rh2(O2CCH3)4}{Pt2(piam)2(NH2C4H9)3}2](ClO4)2–H}+, {[{Rh2(O2CCH3)4}{Pt2(piam)2(NH2C4H9)3}2](PF6)(ClO4)–H}+, and {[{Rh2(O2CCH3)4}{Pt2(piam)2(NH2C4H9)3}2](PF6)2–H}+ were observed. Both 4 and 5 contained PF6– and ClO4– as counter anions, which are mixed during synthetic and crystallized processes; both anions have similar molecular sizes and spherical shapes.[74,75] Although hexanuclear units of 4 and 5 are observed in MeOH, indicating the partial maintenance of their hexanuclear backbones, the unbridged Pt–Rh bonds are dissociated by the MeOH molecules.

Figure 10

ESI-MS (positive) spectra for (a) 4 and (b) 5 measured by dilution in MeOH. Selected peaks are simulated with isotope patterns as bars.

Furthermore, both 4 and 5 have enough solubilities in CH2Cl2. Figure shows the ultraviolet–visible (UV–vis) spectra of the CH2Cl2 solution containing 4 or 5 with their photograph, in comparison to the results of diffuse reflection spectra (Figure d,e). In both 4 and 5, three absorption peaks were observed at 356, 427 (452), and 692 nm for 4 and at 363, 429, and 690 nm for 5. The energetically highest peaks at 356 (4) and 363 (5) were also found in their solid state as sub-maxima peaks, although the measured range was out of resolution for our instrument system. Both 427 (4) and 429 (5) well coincide with 420 (=2.95 eV, 4) and 430 (=2.88 eV, 5), which correspond to E1. Moreover, E2 was found at 692 (4) and 690 (5). The spectra of the solution and solid states well coincide, revealing that both 4 and 5 maintain their hexanuclear backbones in CH2Cl2.

Figure 11

UV–vis spectra of 0.05 mM CH2Cl2 solution containing (a) 4 and (c) 5. Diffuse reflectance spectra of (b) 4 and (d) 5 with MgO at room temperature. Green, blue, and red bars show the results of TD-DFT calculation for na, nb, and nc, respectively. Photograph courtesy of “Kazuhiro Uemura”. Copyright 2021.

The UV–vis spectra of MeOH solution containing [Rh2(O2CCH3)4] and [Pt2(piam)2(NH2R)4](PF6)2 (R = C3H7 or C4H9), which are the starting materials for 4 and 5, in different ratios show no absorption peaks (Figure S34a,c), indicating that two complexes do not associate in MeOH. However, in CH2Cl2, as [Pt2(piam)2(NH2R)4](PF6)2 are added, absorption peaks around 300–450 nm become intense, indicating that the two complexes do associate. In both cases, however, the absorbances of the E1 absorption at 420 (4) and 430 nm (5), which are characteristic for the formation of hexanuclear backbones, are low. Considering that the growing peaks are observed at lower wavelengths, mixing two complexes in CH2Cl2 dominantly affords tetranuclear complexes, [Rh2]–[Pt2].

Figure shows the 1H NMR spectrum of the hexanuclear complex 5 dissolved in CD2Cl2. In addition to the CH3 group of [Rh2(O2CCH3)4] and the Bu group of [Pt2(piam)2(NH2C4H9)4]2+, the C4H9 groups were observed as broad peaks. For all four kinds of proton, sharper (a–d) and broader (a′–d′) peaks were observed at the lower and higher fields, respectively. The broader peaks (a′–d′) are attributed to the inner butylamine in the hexanuclear complex, due to the inhibition of free rotation in the molecule, whereas the sharper peaks (a–d) are attributed to the outer butylamine, where the integral values well coincided with the chemical formula.

Figure 12

1H NMR spectrum (600 MHz in CD2Cl2, 0.5 mM) of 5.

As shown in Figure , 1H NMR and UV–vis spectra of 5 with 3 equiv of [Pt2(piam)2(NH2C4H9)4](PF6)2 showed no significant change from those of 5. Therefore, the dissociation of 5 is suppressed in CH2Cl2, maintaining the hexanuclear backbone. Despite the cationic hexanuclear complexes, the reason of solubility in CH2Cl2 is the hydrophobic interaction of propyl or butyl moieties at amine ligands coordinated Pt atoms. Considering the crystal structures (Figure ), the eight alkyl chains avoid the dissociation of the unbridged Pt–Rh bonds by surrounding their molecules (Scheme ). Thus, soluble hexanuclear Pt–Pt–Rh–Rh–Pt–Pt units were successfully appeared as if HMSCs from infinite heterometallic 1D chains 1 aligned as −{Pt–Pt–Rh–Rh–Pt–Pt}–. Similar measurements were also carried out for hexanuclear complex 4 (Figures S35 and S36). When 3 equiv [Pt2(piam)2(NH2C3H7)4](PF6)2 were added to 4, although a similar trend was observed, indistinguishable peaks appeared in 1H NMR spectra and absorption peaks are slightly shifted to lower wavelengths in the UV–vis spectra. These results suggest that the hexanuclear complex 4 with propylamine is in an equilibrium reaction with some dissociation even in CH2Cl2.

Figure 13

(a) 1H NMR spectra (600 MHz in CD2Cl2, 0.5 mM) of 5 with 0, 1, 2, and 3 equiv [Pt2(piam)2(NH2C4H9)4](PF6)2. (b) UV–vis spectra of 0.05 mM CH2Cl2 solution containing 5 with 0, 0.25, 0.5, ..., and 3.0 equiv [Pt2(piam)2(NH2C4H9)4](PF6)2.

Scheme 4

Alkyl Moieties Support the Maintaining of Hexanuclear Backbone in Hydrophobic Solvents

The direct benefit of the solubility of compounds without changes to their molecular structures is the extension of the experimental approach to solution states. For example, Figure shows cyclic voltammograms of 4 and 5 recorded with CH2Cl2 solution containing 0.1 M Bu4NPF6 as a supporting electrolyte, in comparison of the cyclic voltammograms of [Rh2(O2CCH3)4(CH3CN)2]. In [Rh2(O2CCH3)4(CH3CN)2], a reversible wave at E1/2 = 0.62 V (vs Fc/Fc+) was observed, which are attributed to one-electron oxidation and reduction at the Rh2 π* orbital.[76−78] In contrast, both 4 and 5 afford the quasi-reversible oxidation and reduction waves at 4: Eox = 0.86 V and Ered = 0.69 V and 5: Eox = 0.87 V and Ered = 0.53 V. As shown in Figure S37, there are no significant oxidation waves in the cyclic voltammograms of the mononuclear or dinuclear Pt complexes, cis-[Pt(piam)2(NH2C3H7)2]·H2O, [Pt2(piam)2(NH2C3H7)4](PF6)2, cis-[Pt(piam)2(NH2C4H9)2], and [Pt2(piam)2(NH2C4H9)4](PF6)2, which are the modules for 4 and 5. The difficulty is probably due to that the oxidation of Pt(+2) atom is involved with the axial coordination.[79] Considering the results of DFT calculation, the oxidation and reduction of 4 and 5 occurred at HOMOs, which consist of σ* over the hexanuclear metals admixed with Rh2 π*. Using the obtained Eox values, the absolute values of EHOMO were successfully estimated as −5.66 (4) eV and −5.67 (5) eV, according to the equation of EHOMO (eV) = −(4.8 + Eox (V vs Fc/Fc+)).[80−82] Furthermore, for 5, electron paramagnetic resonance (EPR) measurement was conducted after the electrolytic oxidation. The CH2Cl2 solution containing 1 mM 5 with 0.1 M Bu4NPF6 as supporting electrolyte was electrolytically oxidized and immediately frozen under 77 K. The EPR spectrum for the CH2Cl2 glass shows a broad isotropic signal with g = 2.52 (Figure S38). Considering that the value of g is higher than the spin-only value, the unpaired electron was affected by the significant spin–orbit coupling from the metals, resulting in that HOMO of 5 is on the metal, which coincided with the calculations. We also tried to chemically oxidize 5 with I2 or H2O2 but did not succeeded in isolating. Hence, we performed an one-electron oxidized DFT calculation based on the model 2c with a different charge, [{Rh2(O2CCH3)4}{Pt2(NHCOCH3)2(NH2CH3)4}2]5+ (2c), under the rigid condition of Pt, Rh, O, and N coordinates (Figure S39). As shown in Figure , the molecular orbital corresponding to the SOMO was not different from the HOMO of 2c, but was a mixture of σ* delocalized in the hexanuclear metal and Rh2 π* orbitals.

Figure 14

Cyclic voltammograms of (a) [Rh2(O2CCH3)4(CH3CN)2] in CH3CN, (b) 4, and (c) 5 in CH2Cl2 in the presence of 0.1 M Bu4NPF6 as the supporting electrolyte (scan rate: 100 mV s–1). The values are given with regard to Fc/Fc+, which has been used as internal standard for calibration of the Ag/AgCl reference electrode.

Figure 15

Result of DFT calculation based on the model of [{Rh2(O2CCH3)4}{Pt2(NHCOCH3)2(NH2CH3)4}2]5+ (2c) under the rigid condition of Pt, Rh, O, and N coordinates.

Conclusions

This study demonstrated the syntheses and characterization of hexanculear complexes as novel HMSCs containing two unbridged metal–metal bonds constructed by the HOMO–LUMO interaction at d orbitals. By introducing alkyl moieties at co-ligand-coordinated Pt atoms, further extension was obstructed, and the obtained hexanuclear complexes appeared as if they were excised from the prototype heterometallic 1D chain 1. Comparing the infinite chain 1 with hexanuclear 2–5, unclear problems for 1 were revealed, such as shoulder absorption around E1 attributed to band structure or exact molecular orbitals. Interestingly, when the alkyl moieties were propyl or butyl, hydrophobic interaction provided enough solubility in nonpolar solvents. In addition to the infinite chains in solid state, the HOMO–LUMO interactions at d orbitals are effective for HMSCs, comprising unique compounds with various numbers of metal and metal species.

Experimental Section

Materials

Rhodium(III) chloride trihydrate and potassium tetrachloroplatinate(II) were obtained from Tanaka Kikinzoku Co. 40% NH2CH3 aqua solution, 70% NH2C2H5 aqua solution, NH2C4H9, NaClO4, and KI were obtained from Nacalai Tesque Co. NaPF6, pivalonitrile, and NH2C3H7 were obtained from Tokyo Chemical Industry Co. AgNO3 and AgPF6 were obtained from Wako Co. cis-[PtI2(NH2CH3)2], cis-[PtI2(NH2C2H5)2], cis-[PtI2(NH2C3H7)2], and cis-[PtI2(NH2C4H9)2] were synthesized by a modified version of Dhara’s method.[83,84]cis-[Pt(piam)2(NH2CH3)2]·H2O,[59]cis-[Pt(piam)2(NH2C2H5)2]·H2O,[60]cis-[Pt(piam)2(NH2C3H7)2]·H2O,[61]cis-[Pt(piam)2(NH2C4H9)2],[61] [Rh2(O2CCH3)4],[85] [{Rh2(O2CCH3)4}{Pt2(piam)2(NH3)4}2](PF6)4·6nH2O (1),[51] and [{Rh2(O2CCH3)4}{Pt2(piam)2(NH2CH3)4}2](PF6)4 (2)[58] were synthesized according to the previous procedures.

Synthesis of [{Rh2(O2CCH3)4}{Pt2(piam)2(NH2C2H5)4}2](PF6)4 (3)

An aqueous solution (5 mL) of [PtI2(NH2C2H5)2] (0.29 g, 0.54 mmol) was stirred with AgPF6 (0.27 g, 1.1 mmol) overnight in the dark, and AgI was then removed by filtration. The colorless filtrate was evaporated and stirred with cis-[Pt(piam)2(NH2C2H5)2]·H2O (0.26 g, 0.52 mmol) in MeOH (5 mL) for 2.5 h. The resulting light yellow solution was evaporated to obtain crude [Pt2(piam)2(NH2C2H5)4](PF6)2 (0.38 g). A EtOH solution (0.75 mL) of crude powder (9.6 mg) was layered on a EtOH solution (0.75 mL) of [Rh2(O2CCH3)4] (2.2 mg, 5.0 μmol). After several days, yellow-green crystals with metallic luster were obtained (5.2 mg). Yield 41%. Elemental analysis calcd for C44H108F24N12O12P4Pt4Rh2: C, 20.62; H, 4.25; N, 6.56%, found: C, 21.10; H, 4.38; N, 6.74%.

Synthesis of [{Rh2(O2CCH3)4}{Pt2(piam)2(NH2C3H7)4}2](PF6)3(ClO4) (4)

An aqueous solution (5 mL) of [PtI2(NH2C3H7)2] (0.32 g, 0.56 mmol) was stirred with AgPF6 (0.28 g, 1.1 mmol) overnight in the dark, and AgI was then removed by filtration. The colorless filtrate was evaporated and stirred with cis-[Pt(piam)2(NH2C3H7)2]·H2O (0.29 g, 0.55 mmol) in MeOH (7 mL) for 2.5 h. The resulting light blue solution was evaporated to obtain crude [Pt2(piam)2(NH2C3H7)4](PF6)2 (0.51 g). An EtOH solution (1.0 mL) of crude powder (7 mg), [Rh2(O2CCH3)4] (1 mg, 2.3 μmol), and NaClO4 (0.4 mg, 3.3 μmol) layered on H2O (3.0 mL) was slowly evaporated. After several days, green crystals with metallic luster were obtained (2 mg). Yield 33%. Elemental analysis calcd for C52H124ClF18N12O16P3Pt4Rh2: C, 23.75; H, 4.75; N, 6.39%, found: C, 23.86; H, 4.79; N, 6.43%.

Synthesis of [{Rh2(O2CCH3)4}{Pt2(piam)2(NH2C4H9)4}2](ClO4)4 (5)

An aqueous solution (5 mL) of [PtI2(NH2C4H9)2] (0.29 g, 0.49 mmol) was stirred with AgPF6 (0.24 g, 0.95 mmol) for overnight in the dark, and AgI was then removed by filtration. The colorless filtrate was evaporated and stirred with cis-[Pt(piam)2(NH2C4H9)2] (0.26 g, 0.48 mmol) in MeOH (7 mL) for a day. The resulting brown solution was evaporated to obtain crude [Pt2(piam)2(NH2C4H9)4](PF6)2 (0.38 g). An EtOH solution (1.0 mL) of crude powder (7 mg), [Rh2(O2CCH3)4] (1 mg, 2.3 μmol), and NaClO4 (0.4 mg, 3.3 μmol) layered onH2O (3.0 mL) was slowly evaporated. After several days, green crystals with metallic luster were obtained (3.0 mg). Yield 50%. Elemental analysis calcd for C60H140Cl4N12O28Pt4Rh2: C, 27.66; H, 5.42; N, 6.45%, found: C, 28.03; H, 5.61; N, 6.57%. As mentioned in the text, bulk synthesized product contains several amounts of PF6– instead of ClO4–.

Physical Measurements

UV–vis spectra were recorded using a Shimadzu UV3100PC at room temperature. The XPS measurements were carried out on a Quantera-SXM spectrometer at room temperature. Binding energies were measured relative to the C 1s peak (284.8 eV) of an internal hydrocarbon. The diffuse reflectance spectra were recorded on a Hitachi U-4000 spectrophotometer over the range from 200 to 2500 nm at room temperature. The infrared spectra were recorded on a PerkinElmer Spectrum 400 over the range from 400 to 4000 cm–1 at room temperature. Cyclic voltammetric measurements were conducted at room temperature using a BAS CV-50W or BAS 617E electrochemical analyzer. Cyclic voltammograms were recorded with CH3CN or CH2Cl2 solutions containing 0.1 M Bu4NPF6 as the supporting electrolyte. Conventional three-electrode arrangement consisting of glassy carbon or Pt working electrode, Ag/Ag+ reference electrode, and Pt wire counter electrode was used. EPR spectra were measured on a JEOL TE-200 spectrometer. 1H NMR were conducted on a JEOL ECA-600.

Density Functional Theory (DFT) Calculation

The electronic structures of model compound [{Rh2(O2CCH3)4}{Pt2(NHCOR′)2(NH2R)4}2]4+ (R = CH3, C2H5, C3H7 or C4H9, R′ = CH3 or Bu) were determined using the DFT method with the B3LYP function[86−88] and Gaussian 16 program package.[89] For Pt and Rh, LANL2DZ basis set was used together with the effective core potential of Hay and Wadt.[90] For the other elements, 6-31G* basis sets[91] were selected. The initial models of [{Rh2(O2CCH3)4}{Pt2(NHCOR′)2(NH2R)4}2]4+ for optimization were prepared using the geometrical parameters obtained from the crystal structure data. For the models, full geometry optimization was conducted without rigid condition, under the rigid condition of coordinates for the metal, or under the rigid condition of coordinates for the metal and coordination atoms. Based on structure, 40 singlets in excited state were obtained to determine vertical excitation energy using time-dependent (TD) DFT calculation.[92,93]

X-ray Structure Determination

Measurements were carried out on a Rigaku AFC7R Mercury CCD diffractometer equipped with a normal-focus Mo-target X-ray tube (λ = 0.7107 Å) operated at 15 kW power (50 kV, 300 mA) and a CCD two-dimensional detector. A total of 744 frames were collected with a scan width of 0.5° with an exposure time of 5 s/frame. Empirical absorption correction[94] were performed for all data. The structure was solved by direct method[95] with subsequent difference Fourier synthesis and refinement using SHELX-2017[96] controlled by a Yadokari-XG software package.[97] Nonhydrogen atoms were refined anisotropically and all hydrogen atoms were treated as riding atoms. In 4, the oxygen atom O7 of water molecule was refined isotropically without hydrogen atoms. The crystal data and structure refinement results are summarized in Table .

Table 2

Crystallographic Data and Structure Refinements for [{Rh2(O2CCH3)4}{Pt2(piam)2(NH2C2H5)4}2](PF6)4 (3), [{Rh2(O2CCH3)4}{Pt2(piam)2(NH2C3H7)4}2](PF6)3(ClO4) (4), and [{Rh2(O2CCH3)4}{Pt2(piam)2(NH2C4H9)4}2](ClO4)4 (5)

	3	4	5
empirical formula	C44H108F24N12O12P4Pt4Rh2	C52H124F18N12O14P3Pt4Rh2	C60H140Cl4N12O28Pt4Rh2
formula weight	2563.48	2562.71	2605.81
crystal system	monoclinic	triclinic	triclinic
space group	C2/c	PI̅	P1̅
a (Å)	35.658(4)	12.6800(16)	10.799(3)
b (Å)	12.1247(13)	12.8383(15)	14.482(4)
c (Å)	20.754(3)	15.356(2)	15.787(5)
α (deg)	90	80.918(5)	77.164(9)
β (deg)	119.4765(14)	89.778(6)	89.966(11)
γ (deg)	90	64.518(3)	77.378(8)
V (Å3)	7811.6(16)	2222.5(5)	2346.1(12)
Z	4	1	1
temperature (K)	123	123	123
Dc (Mg m–3)	2.180	1.915	1.844
absorption coefficient (mm–1)	7.745	6.781	6.471
F(000)	4920	1241	1278
crystal size (mm3)	0.30 × 0.20 × 0.10	0.30 × 0.25 × 0.10	0.20 ×0.10 × 0.10
measured reflections	30 945	17 778	19 015
independent reflections	8941 [Rint = 0.0314]	10 024 [Rint = 0.0287]	10 576 [Rint = 0.0527]
data/restraints/parameters	8941/0/474	10024/0/482	10576/0/508
goodness of fit on F2	1.066	1.105	1.199
R [I > 2σ(I)]	0.0347	0.0450	0.0620
R (all data)	0.0387	0.0521	0.0935