Elsevier

Chemical Physics Letters

Volume 554, 3 December 2012, Pages 96-101
Chemical Physics Letters

DFT study of metal-complex structural variation on tensile force profiles

https://doi.org/10.1016/j.cplett.2012.09.030Get rights and content

Abstract

We present calculations on metal–ligand complexes for the evaluation of mechanical properties as they pertain to the inclusion in polymer-linked supramolecular complexes. To this end, we investigate the energy profiles of stretching various complexes according to external forces exerted on each complex via the attached polymer strands. Zn2+ and Fe2+ complexated by 2,6-bisbenzimidazolyl-pyridine (BP) were considered in the presence of tetrafluoro borate. We find that the yield characteristics are subject to a complex interplay of steric and electronic effects of the ligands and metal center.

Highlights

► The rupture characteristics of organometallic complexes is tunable via the ligand. ► Electronic effects nontrivially impact rupture characteristics and materials design. ► The mechanism for bond rupture was found to be of closed-shell character. ► The initial response to tension is dominated by symmetric coordinative-bond-stretching. ► Rupture is concurrent with a sudden significant rearrangement of the ligands.

Introduction

Recently interest has increased in the integration of supramolecular complexes into polymeric materials. A supramolecular complex consists of reversible, highly directional non-covalent bonds formed spontaneously between well-designed host–guest molecules. In supramolecular polymers, the complexes are typically either integrated into the main-chain of the polymer or tethered along the polymer backbone. Polymers created with supramolecular chemistry often display thermal and mechanical properties comparable to their covalent counterparts but offer advantages in terms of processability, reversibility, and functionality. The construction of supramolecular materials can arise from a variety of non-covalent interactions including: ion–ion interactions, arene–arene interactions, hydrogen-bonding [1], [2] and metal–ligand complexation [3].

Incorporation of supramolecular complexes offers the possibility to prepare materials with a wide range of possible properties. Optoelectronic properties have been directed by employing Zn2+ and Fe2+ complexes [4]; other applications include microfluidics, microengineering of smart templates for bioseparation or data storage, sensors, as well as for the microfabrication of controlled release devices, via attenuation of surface properties [5]. Mechanical as well as kinetic properties can be tuned with the strength of the host–guest bond. The nature of the bond determines not only the structure of its immediate vicinity, but also influences the supramolecular interactions, which control the self-assembly of the aggregate [2], [6]. In micellar assemblies, their characteristic features are controlled by the stretching of its core-forming chains [7], which are dominated by the most labile interactions. In supramolecular complexes, the most labile interactions are the metal–ligand interactions. In conjunction with stoichiometry and kinetics of the supramolecular complex, the binding strength controls the dynamics and lifetime of the aggregate [6]. While the metal–ligand interactions are stable, they can also be kinetically labile. Hence, supramolecular complexes are attractive candidates for stimuli-responsive materials [6].

Of particular interest is the ability to tune the elastic response and energy dissipation of materials containing metallo-supramolecular modules. Materials fail under a variety of conditions and in some instances are even required to break or fail under prescribed stresses; e.g., a car’s crumple zone must retain its shape and stability during normal operation while irreversibly deforming under collision conditions. Another important characteristic of a material is its ability to absorb energy before breaking, its toughness, which plays a vital role in energy dissipation, but also in the thermal response of the polymer. The diversity of requirements produces a need for tunable materials with a wide range of (mechanical) properties. Various combinations of metals and counterions were shown to influence significantly thermo-, chemo- and mechanical as well as spectroscopic properties [4], [6], [8], [9], [10].

The advent of atomic force microscopy (AFM) has stimulated much interest in the simulation of such experiments, particularly with a focus on biomaterials [11], [12]. These simulations are typically molecular dynamics investigations of systems which have already been well characterized with force fields. Since common force fields do not incorporate bond-breaking, they are increasingly less suitable as the breaking point is approached [13], although the introduction of reactive force-fields to protein dynamics is a promising approach that has shown some initial success [14].

Quantum–mechanical (QM) and density functional theoretical (DFT) methods inherently incorporate bond-breaking or building processes, but are computationally too expensive to treat realistic polymer environments. While the stress–strain curve is dominated by polymer unfolding in the beginning [15], the rupture point is dominated by bond-breaking processes [1], [2], which break the coordination bonds in the case of incorporated metal complexes. It is therefore necessary to investigate the bond-breaking process quantum–mechanically or via DFT by considering the metal complex alone in order to inform or validate reactive force fields such as ReaxFF [16], [17]. Although QM and DFT potential energy surfaces (PES) of organo-metallic complexes are often investigated to determine reactivity [18], [19] and catalytic activities [20], [21], little attention has been given to mechanical implications until recently [22], [23], [24]. The former rely on transition states or minima of the PES, while the mechanics depend on characteristics of entire regions of the PES.

The Letter is organized as follows: Since the complexes undergo bond-breaking, the appropriate level of DFT and the need for unrestricted versus closed-shell treatments of the complexes are assessed first. Then, the effects on iron and zinc as the central metal atoms of the complex are compared. We proceed to investigate the sensitivity of the results with respect to the chosen numerical parameters. Finally, we study the effect of substituents with varying degrees of inductive and steric effects.

Section snippets

Methods

Stress–strain curves visualize the mechanical properties of a material. The engineering and true strain, ε and εtrue, respectively,ε=Δll0×100,εtrue=ln(1+ε/100),where l0 is the reference length and Δl is the elongation, exerted on a material expresses the deformation from equilibrium of the material. We consider an organometallic complex a single body and compute the molar tensile stress σ, i.e., the stress per complex, viaAmσ(ε)=dEdl(l0(1+ε/100)),where E is the energy of the complex, Am is the

Results

Figure 1 shows the equilibrium as well as the ruptured structures of Zn-HH, as computed with M05, which has been calibrated for zinc complexes and has shown superior performance in predicting geometries, dipole moments and energetics [37], [38]. The structures are representative of all complexes with respect to the general features. The equilibrium structure is in all cases C2-symmetric, in which the ligands are slightly curved along the π-system, while the ruptured structure is comprised of Zn

Conclusions

We have investigated the stress–strain relationships of 10 metal–ligand complexes. The results show that both closed-shell and unrestricted M05 as well as B3LYP at the 6-31G(d) level of theory render similar relaxed potential energy surfaces for the stretching of these complexes at first. While the unrestricted and closed-shell treatment of M05 does not change the results appreciably, considerable disagreement is found between M05 and B3LYP after the rupture point. Since the B3LYP curves imply

References (49)

  • D.L. Guzman et al.

    Polymer

    (2008)
  • M.J. Buehler et al.

    Prog. Mater. Sci.

    (2008)
  • M.J. Buehler et al.

    Biophys. J.

    (2007)
  • S. Keten et al.

    J. Mech. Behav. Biomed. Mater.

    (2012)
  • D. Troya et al.

    Chem. Phys. Lett.

    (2003)
  • T. Zhu et al.

    J. Mech. Phys. Solids

    (2005)
  • J. Baker et al.

    Chem. Phys. Lett.

    (1993)
  • A.M. Kushner et al.

    J. Am. Chem. Soc.

    (2007)
  • O.C. Compton et al.

    ACS Nano

    (2011)
  • J.B. Beck et al.

    J. Am. Chem. Soc.

    (2003)
  • M. Burnworth et al.

    Macromolecules

    (2008)
  • C. Haensch et al.

    Langmuir

    (2008)
  • W.G. Weng et al.

    J. Am. Chem. Soc.

    (2006)
  • P. Guillet et al.

    Macromolecules

    (2006)
  • W.W. Gerhardt et al.

    Chem.: A Eur. J.

    (2007)
  • A.J. Goshe et al.

    Proc. Natl. Acad. Sci. USA

    (2002)
  • W.G. Weng et al.

    Macromolecules

    (2009)
  • B.L. Smith

    Nature

    (1999)
  • K. Chenoweth et al.

    J. Phys. Chem. A

    (2008)
  • K.D. Nielson et al.

    J. Phys. Chem. A

    (2005)
  • M. Armelin et al.

    Chem. Eur. J.

    (2008)
  • V. Georgiev et al.

    J. Biol. Inorg. Chem.

    (2008)
  • M. Bittner et al.

    J. Phys. Chem. A

    (2007)
  • D.C. Graham et al.

    Organometallics

    (2007)
  • Cited by (0)

    View full text