Complexes of carbene-functionalized diamondoids and metal atoms: Electronic properties

Highlights

• Reduction of electronic band-gaps in carbene-functionalized diamondlike cages.

• Carbene-functionalized diamondoids are thermally stable.

• Strong binding of carbene-functionalized diamondoids to metal atoms.

Abstract

Tiny carbon cages known as diamondoids have recently attracted attention and can be selectively chemically modified. In this work, we focus on lower diamondoids, from adamantane (C₁₀H₁₆) up to (121)-tetramantane ((121)C₂₂H₂₈). Specifically, we investigate a chemical modification based on a member of the carbene family, the imidazolylidene molecule (C₃N₂H₆). One carbon site of the lower diamondoids has been replaced with imidazolylidene. The electronic properties and the thermal stability of these modified diamondoids are analyzed. In view of practical applications involving self-assembled diamondoid metallic surfaces, the interaction of the modified diamondoids with a metal atom (Au, Ag, Cu or Pt) is evaluated. Our results are based on quantum-mechanical calculations within the density-functional-theory approach. The structural characteristics, the energetics, and the electronic properties of the carbene-functionalized diamondoids and their complexes with metal atoms are investigated. We find that the carbene-functionalized diamondoids form thermally stable structures, show a considerable reduction in their electronic band-gap with respect to the unmodified diamondoids, and retain the metal-bonding characteristics of carbene. For their metal complexes, a higher affinity and a stronger bond, for binding to platinum was evident. The platinum complex is also the only carbene-functionalized metal complex, which retains a non-metallic character. the high stability of these complexes and the strong bonding therein underlines the strong potential of carbene-functionalized diamondoids as building blocks in novel applications.

Introduction

Nano-sized hydrogen terminated diamondoids can assume various sizes and chemical modifications and can be found in petroleum [1], [2]. The diamondoids can also be selectively synthesized in the laboratory [3] or nucleated from energetic species [4]. These nanostructures have shown a high potential to be used in nanotechnology, from drugs to field emitting devices [5], [6], [7], [8]. The first and smallest member of the diamondoid family is known as adamantane (C₁₀H₁₆). These nano-diamond cages can attach on metallic surfaces through a thiol group [9], [10] and form self-assembled monolayers (SAMs) with a negative electron affinity [5], [11] and a strong monochromatic emission [5], [12]. Such properties make diamondoids very promising candidates for electronics applications especially electron emitting devices. In the past, thiol based SAMs on metal surfaces have led to significant applications in the field of surface emission, sensing, electrochemistry, drug delivery, and microelectronics [13], [14], [15], [16], [17]. Nevertheless, the thermal instability of these thiol-based diamondoid SAMs on gold is questionable. There have been indications, that changes in the environmental conditions have a strong effect on these materials and their use for industrial purposes could be problematic [18], [19], [20], [21], [22]. Thiol-based SAMs are found to be stable only when stored in an ultra high vacuum in the absence of light, but tend to degrade after few weeks at room temperature [23], [24], [25].

As an alternative to thiol, carbene molecules have recently been used to promote binding of diamondoids on metal surfaces [26]. These molecules belong to the family of N-heterocyclic carbenes to which we refer to in the following. Carbene molecules (R-(C:)-R′ or R = C:) are usually defined as neutral compounds having a divalent carbon atom with a six-electron valence shell and are powerful tools in organic chemistry having numerous applications in chemical processes [27]. Various classes of carbene molecules can be synthesized using different methods [28] and most commonly contain at least one nitrogen atom within the ring structure [29]. N-heterocyclic carbenes (e.g. (R2N)2C:, where the ‘R’s are typically alkyl and aryl groups) are typically used as ligands for transition metals, upon coordination to p-block elements and as organocatalysts and are used in relevant applications in important catalytic transformations in the chemical industry and as organocatalysts. The reactivity of carbene molecules upon coordination to main group elements is an additional important property in view of applications. Imidazolylidene (C₃N₂H₆), a simple ring structure with two nitrogen atoms is one member of the carbene family. This molecule can form metal and transition metal complexes [30], [31] to be used in catalysis and other chemical reactions.

In this work, we study the structural and electronic properties of imidazolylidene-functionalized diamondoids and their bonding and interaction to metal atoms. In the following sections, we present the methodology of our investigation and move on to the analysis of the results on the structural and electronic properties of carbene-functionalized diamondoids, their thermal stability, and the characteristics on their interactions with metals.