Electronic Challenges of Retrofitting 2D Electrically Conductive MOFs to Form 3D Conductive Lattices

Summarises the author interpretation: 3D conductive MOFs are plausible when neutral N-donor pillars connect 2D sheets containing square-planar metals with reduced dz2 occupancy.

Relevance: Core · 5 · Conclusions

Develops the conclusion that earlier transition metals can form axial bonds and maintain both covalent-plane and out-of-plane band dispersion after retrofitting.

Relevance: Core · 5 · Retrofitting Cr- and Fe-HIB Derivatives · Figure 5

Frames MOFs as high-porosity materials whose electronic applications are limited by low conductivity, then reviews carrier, mobility, bandgap, oxidation, and geometry constraints in conductive MOFs.

Relevance: Core · 1 · Introduction

Assesses DABCO, pyrazine, and bipyridine as axial pillars for Ni3(HIB)2 and interprets why axial ligation of square-planar Ni2+ destabilises rather than produces robust 3D conduction.

Relevance: Core · 3 · Retrofitting Square-Planar Ni2+ · Figure 3; Table 1

Introduces retrofitting as a postsynthetic ligand-modification route intended to strengthen intersheet interactions and preserve porosity in conductive 2D MOFs.

Relevance: Core · 2 · Introduction

Compares stacking conformations and interlayer spacing in Ni3(HIB)2, separating covalent-plane dispersion from out-of-plane interactions.

Relevance: Core · 2 · Results and Discussion · Figure 1

Motivates metal substitution as a route to reduce dz2 repulsion and enable axial bonding in 3D-retrofitted conductive lattices.

Relevance: Core · 4 · Transmetalation · Figure 4

Routes to increased MOF conductivity

The introduction groups prior conductivity enhancement efforts into ligand design, valency control, defects, and carrier generation by oxidation.

Categories: ligand selection for reduced or zero bandgaps · mixed-valency doping · defect engineering · chemical oxidation

1 · Introduction

Conductivity determinants

The article frames MOF conductivity through the standard decomposition into carrier concentration, charge, and mobility, with mobility constrained by band flatness and scaffold geometry.

Categories: charge-carrier number · carrier charge · carrier mobility

1 · Introduction

Radical-stabilisation and delocalisation axes

The review framing distinguishes delocalisation in the covalent plane, through interlayer pi overlap, and through axial metal-ligand bonding after retrofitting.

Categories: in-plane covalent-sheet delocalisation · out-of-plane van der Waals or pi-stacked delocalisation · out-of-plane through-bond delocalisation after pillaring

2 · Introduction

Metal dz2 occupancy classes

The article organises retrofitting feasibility around whether the square-planar metal centre can accept axial donor interactions without destructive dz2 repulsion.

Categories: filled dz2 late transition metals · partially filled dz2 earlier transition metals · unoccupied or reduced dz2 density targets

4 · Transmetalation

Axial pillar electronic character

DABCO is used as a pure sigma-donor example, while pyrazine and bipyridine are treated as sigma donors with stronger pi-acceptor character; the outcome depends on whether real axial bonding forms.

Categories: sigma-donor only · sigma-donor and pi-acceptor · intercalated nonbonding pillar

3 · Retrofitting Square-Planar Ni2+ · Figure 3

2D sheet stacking orientations

Ni3(HIB)2 and Ni3(HITP)2 are interpreted by whether layers retain eclipsed orbital overlap, move into staggered arrangements, or slip enough to occlude pores.

Categories: eclipsed · staggered · parallel-displaced or slipped

2 · Results and Discussion · Figure 1

M3(HIB)2 analogues in which earlier transition metals are substituted for Ni to reduce dz2 occupancy and support axial bonding during retrofitting.

Conduction: Predicted to retain covalent-plane dispersion and regain out-of-plane dispersion through metal-axial ligand through-bond pathways.

Representative materials: Cr3(HIB)2 · Fe3(HIB)2 · Cr3(HIB)2(1,4-pyrazine)3 · Fe3(HIB)2(DABCO)3

Nodes / linkers: Cr · Fe · hexaiminobenzene · DABCO · pyrazine · 4,4'-bipyridine

5 · Retrofitting Cr- and Fe-HIB Derivatives · Figure 5

Porous metal-ligand scaffolds with ultrahigh surface area and adsorption or catalytic utility, but often poor electrical conductivity because of ionic metal-ligand interfaces.

Conduction: Often insulating or low-conductivity; charge mobility is limited by flat bands associated with ionic metal-ligand bonding.

Representative materials: UiO-66 · Zn-BTC · Fe(1,2,3-triazolate)2(BF4)x

Nodes / linkers: various · carboxylates · triazolates · functionalised ligands

1 · Introduction

A metallic 2D MOF based on Ni nodes and hexaiminobenzene linkers, where a half-populated HIB band gives monolayer metallicity.

Conduction: Unlike Ni3(HITP)2, both monolayer and bulk forms are described as metallic, making transport less dependent on stacking orientation.

Representative materials: Ni3(HIB)2

Nodes / linkers: Ni · hexaiminobenzene

2 · Introduction

A 2D conductive MOF family built from Ni nodes and hexaiminotriphenylene-type linkers; bulk metallicity is associated with stacked linker interactions.

Conduction: The monolayer is described as having a narrow discrete bandgap, while the bulk form becomes metallic through weak out-of-plane pi orbital overlap.

Representative materials: Ni3(HITP)2 · Cu3(HITP)2

Nodes / linkers: Ni · Cu · 2,3,6,7,10,11-hexaiminotriphenylene

1 · Introduction

Hypothetical 3D-connected variants formed by adding DABCO, pyrazine, or 4,4'-bipyridine as axial linkers between Ni3(HIB)2 sheets.

Conduction: Retrofitting the Ni analogue can retain or alter metallic bands but is interpreted as unstable because axial Ni-ligand bonding is weak or energetically costly.

Representative materials: Ni3(HIB)2(DABCO)3 · Ni3(HIB)2(1,4-pyrazine)3 · Ni3(HIB)2(4,4'-bipyridine)3

Nodes / linkers: Ni · DABCO · pyrazine · 4,4'-bipyridine · hexaiminobenzene

3 · Retrofitting Square-Planar Ni2+ · Figure 3; Table 1

Ligand selection to reduce or eliminate bandgaps

Design linkers and metal-ligand connectivity to create narrower bandgaps or metallic states in MOFs.

Claimed effects: Can increase conductivity by changing band structure, but transport may remain limited by geometry and anisotropy.

Controlling variables: linker conjugation · metal-ligand covalency · band dispersion · formal oxidation state

Representative materials: Zn-BTC derivatives · Ni3(HITP)2 · Ni3(HIB)2

Caveat: The review framing cautions that conductivity is not only a bandgap problem; carrier mobility and directionality are also scaffold dependent.

1 · Introduction

Mixed valency and chemical oxidation

Introduce or stabilise charge carriers by mixed valency or oxidation, often interpreted as hole formation in conductive MOFs.

Claimed effects: Increasing carrier number can increase conductivity, but scaffold geometry still constrains mobility and transport direction.

Controlling variables: oxidation state · charge delocalisation · carrier density · redox chemistry

Representative materials: Fe(1,2,3-triazolate)2(BF4)x · oxidised MOF conductors

Caveat: Carrier formation alone does not guarantee high mobility because ionic interfaces and flat bands can still dominate.

1 · Introduction

Retrofitting by postsynthetic ligand modification

Install bridging ligands after framework formation to strengthen intersheet interactions, hold sheets apart, and potentially create 3D-connected conductive lattices.

Claimed effects: May preserve porosity and introduce 3D connectivity, but can destroy out-of-plane covalency or create unstable structures if metal orbitals are incompatible.

Controlling variables: pillar length · axial ligand donor and acceptor character · metal coordination preference · interlayer spacing · dynamic stability

Representative materials: Ni3(HITP)2 pillared analogue · Ni3(HIB)2(DABCO)3 · Ni3(HIB)2(1,4-pyrazine)3

Caveat: The article interprets retrofitting as highly sensitive to whether axial bonding is electronically favourable.

2 · Introduction

Transmetalation to tune dz2 occupancy

Alter the metal composition of a 2D conductive MOF so that axial ligands can form stronger bonds and support through-bond out-of-plane transport.

Claimed effects: Earlier transition metals such as Cr2+ and Fe2+ are proposed to favour axial bonding relative to Ni2+ or Cu2+ analogues.

Controlling variables: metal identity · dz2 occupancy · spin state · Jahn-Teller distortion · axial bond order

Representative materials: Cr3(HIB)2 · Fe3(HIB)2 · Cr3(HIB)2(1,4-pyrazine)3

Caveat: Direct synthesis of the Cr and Fe analogues is acknowledged as potentially intractable, so ligand pillaring and metal exchange are proposed rather than demonstrated.

5 · Conclusions

Conductive 2D MOFs are known, but turning them into porous 3D-connected conductors without destroying electronic delocalisation remains unresolved.

Proposed direction: Use retrofitting only where metal orbitals can support axial bonding and through-bond transport.

2 · Introduction

Favourable computed formation energies do not guarantee usable retrofitted frameworks because phonon calculations can indicate imaginary modes.

Proposed direction: Pair energetic screening with phonon, disorder, and finite-temperature stability checks before claiming practical 3D conductors.

5 · Retrofitting Cr- and Fe-HIB Derivatives

The article identifies dz2 occupancy as a critical but underdeveloped design rule for whether axial ligation creates conduction or destabilisation.

Proposed direction: Prioritise metals with reduced dz2 density and evaluate axial bond order, band dispersion, and dynamic stability together.

2 · Introduction

The most promising Cr and Fe retrofitted structures may be difficult to access directly, so the synthetic route is not solved by the electronic-structure argument.

Proposed direction: Explore ligand pillaring and metal exchange routes as synthetic access strategies for predicted 3D conductors.

5 · Conclusions

Carrier generation and metallicity alone are insufficient; the directionality of band dispersion and scaffold geometry still limit useful conductivity.

Proposed direction: Discuss conductive MOFs by separating in-plane covalent transport, out-of-plane through-space transport, and out-of-plane through-bond transport.

1 · Introduction