Strategies to Improve Electrical Conductivity in Metal-Organic Frameworks: A Comparative Study

Metal-organic frameworks (MOFs), formed by the combination of both inorganic and organic components, have attracted special attention for their tunable porous structures, chemical and functional diversities, and enormous applications in gas storage, catalysis, sensing, etc. Recently, electronic applications of MOFs like electrocatalysis, supercapacitors, batteries, electrochemical sensing, etc., have become a major research topic in MOF chemistry. However, the low electrical conductivity of most MOFs represents a major handicap in the development of these emerging applications. To overcome these limitations, different strategies have been developed to enhance electrical conductivity of MOFs for their implementation in electronic devices. In this review, we outline all these strategies employed to increase the electronic conduction in both intrinsically (framework-modulated) and extrinsically (guests-modulated) conducting MOFs.

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.

One-dimensional complexes extended by unbridged metal-metal bonds based on a HOMO-LUMO interaction at the dz2 orbital between platinum and heterometal atoms

In this review, the crystal structures of seventeen heterometallic one-dimensional compounds 1-17 are shown, and their electronic structures are discussed based on the diffuse reflectance spectra and other physical measurements. Compounds 1-17 comprise two kinds of complexes with HOMO-LUMO interactions at σ-type (d_z²) orbitals between platinum and heterometal atoms, where two or three kinds of metal are regularly aligned through metal-metal bonds. In [{PtRh(piam)₂(NH₃)₂Cl_2.5}₂{Pt₂(piam)₂(NH₃)₄}₂]_n(PF₆)_6n·2nMeOH·2nH₂O (3, piam = pivalamidate), the platinum and rhodium atoms are aligned as -Pt-Rh-Pt-Pt-Pt-Pt-Rh-Pt-Cl- with a mixed valence, where an unpaired electron hops from one Rh atom to another. [{Rh₂(O₂CCF₃)₄}{Pt₂(piam)₂(NH₃)₄}₂]_n(CF₃CO₂)_4n·2nEtOH·2nH₂O (6) with -Pt-Pt-Rh-Rh-Pt-Pt- alignment has a conduction band (CB) and a valence band (VB) attributed to σ-type orbitals, exhibiting narrower gaps between CB and VB than other analogues. [{Ru₂(O₂CCH₃)₄}{Pt₂(piam)₂(NH₃)₄}₂]_n(PF₆)_4n·4nH₂O (14) with -Pt-Pt-Ru-Ru-Pt-Pt- and [{Rh₂(O₂CCH₃)₄}{Pt₂Cu(piam)₄(NH₃)₄}]_n(PF₆)_2n (15) with -Rh-Rh-Pt-Cu-Pt- are paramagnetic chains, where one or two unpaired electrons reside at each metal per repeating unit. Thus, this system is diverse in modulating the electronic structures, band structures and the expected physical properties based on the unique oxidation states and redox properties attributed to the metal-metal interaction.

Heterometallic one-dimensional chain with tetradeca metal repetition constructed by amidate bridged dirhodium and pivalate bridged diplatinum complexes influenced by hydrogen bonding

A novel platinum and rhodium based heterometallic one-dimensional chain complex with tetradeca metal repetition was obtained.

Two types of heterometallic one-dimensional alignment composed of acetamidate-bridged dirhodium and pivalamidate-bridged diplatinum complexes

Two types of heterometallic one-dimensional chains, [{Rh2(acam)4}{Pt2(piam)2(NH3)4}2]n(CF3SO3)4n·2nMeOH (2, where acam = acetamidate, piam = pivalamidate) and [{Rh2(acam)4}{Pt2(piam)2(NH3)4}]n(CF3CO2)2n·2nEtOH (3), have been synthesized and characterized by single-crystal X-ray analyses. The chain structures in 2 and 3 are composed of two kinds of dinuclear complexes, [Rh2(acam)4] (i.e., [Rh2]) and [Pt2(piam)2(NH3)4] (i.e., [Pt2]), where Rh and Pt atoms are axially linked by metal-metal bonds. In 2 and 3, each complex is one-dimensionally aligned as -{[Rh2]-[Pt2]-[Pt2]}n- or -{[Rh2]-[Pt2]}n-, respectively, in which different alignments are caused by different isomers of [Pt2] that are HH (head-head) and HT (head-tail) orientation of piam ligands and their hydrogen bonding modes. Considering the crystal structures and X-ray photoelectron spectra (XPS) measurements in 2 and 3, the oxidation states of the metal atoms are -{[Rh2(II,II)]-[Pt2(II,II)]-[Pt2(II,II)]}n- and -{[Rh2(II,II)]-[Pt2(II,II)]}n-, which are unchanged from those in the starting compounds. The diffuse reflectance spectra show that LUMOs are M-M σ-type orbitals. The gap between filled and vacant σ-type orbitals in 3 is narrower than that in 2, and is attributed to the relative higher destabilized filled σ-type orbitals caused by lower numbers of linking platinum atoms.

Paramagnetic one-dimensional chains comprised of trinuclear Pt-Cu-Pt and paddlewheel dirhodium complexes with metal-metal bonds

One-dimensional (1-D) chain complexes constructed by metal-metal bonds containing three types of metal species-platinum, rhodium, and copper-have been rationally synthesized and characterized by single-crystal X-ray analyses and physical measurements. The paddlewheel or lantern type complex, [Rh2(O2CCH3)4] (i.e., [Rh2]), has a vacant σ* orbital which accepts the electrons from the filled dz(2) orbital of cis-[Pt(piam)2(NH3)2]·2H2O (1, i.e. [Pt], where piam = pivalamidate) to afford a tetranuclear complex, [{Rh2(O2CCH3)4}{Pt(piam)2(NH3)2}2]·2H2O (2). Compound 2 forms a linear alignment as [Pt]-[Rh2]-[Pt] with unbridged Rh-Pt bonds, where the oxygen atoms of the piam ligands in the [Pt] are noncoordinated, showing the capability of binding another metal ion. Simply mixing [Rh2] and the heterometallic trinuclear complex [Pt2Cu(piam)4(NH3)4](PF6)2 (3, i.e. [Pt-Cu-Pt]) in a ratio of 1:1 in MeOH, EtOH, or Me2CO affords [{Rh2(O2CCH3)4}{Pt2Cu(piam)4(NH3)4}]n(PF6)2n (4), [{Rh2(O2CCH3)4}{Pt2Cu(piam)4(NH3)4}]n(PF6)2n (5), or [{Rh2(O2CCH3)4}{Pt2Cu(piam)4(NH3)4}]n(PF6)2n·6nMe2CO (6), respectively. Compounds 4-6 form infinite chains with the repetition of -{[Rh2]-[Pt-Cu-Pt]}n-, which to our knowledge, are the first examples of heterometallic 1-D chains comprised of three types of metal species with direct metal-metal bonds. The CF3CO2(-), ClO4(-), and water molecules influence the crystal packing to form an octanuclear complex of [{Rh2(O2CCH3)4}{Pt2Cu(piam)4(NH3)4}2](CF3CO2)2(ClO4)2·2H2O (7) with [Pt-Cu-Pt]-[Rh2]-[Pt-Cu-Pt] alignment. Considering the crystal structures and X-ray photoelectron spectra (XPS) measurements in 4-7, the oxidation states of the metal atoms are -{[Rh2(II,II)]-[Pt(II)-Cu(II)-Pt(II)]}n- or [Pt(II)-Cu(II)-Pt(II)]-[Rh2(II,II)]-[Pt(II)-Cu(II)-Pt(II)], which are unchanged from those in the starting compounds. Electron paramagnetic resonance spectra of 4-7 show axially symmetric spectra with g∥ > g⊥, indicating that the HOMO (SOMO) is a Cu d(x(2)-y(2)) orbital. In 7, the hyperfine coupling in the spectrum indicates that the unpaired spin on Cu is perturbed by the Pt atoms.