Structural, Electronic, and Theoretical Description of a Series of Cobalt

ARTICLE pubs.acs.org/JPCA

Structural, Electronic, and Theoretical Description of a Series of Cobalt Clathrochelate Complexes in the Co(III), Co(II) and Co(I) Oxidation States Minh Thu Dinh Nguyen,† Marie-France Charlot,*,† and Ally Aukauloo*,†,‡ †

Laboratoire de Chimie Inorganique, Institut de Chimie Mol
eculaire et des Mat
eriaux d’Orsay UMR 8182, Universit
e Paris-Sud 11, F-91405 Orsay, France ‡ CEA, iBiTecS, Service de Bio
energ
etique Biologie Structurale et M
ecanismes (SB2SM), F-91191 Gif-sur-Yvette, France

bS Supporting Information ABSTRACT: Encaged hexacoordinated metal complexes have long been a fascinating family of complexes, as they confer the same ligand environment to metal ions in different oxidation states. We recently reported that cobalt clathrochelate complexes behave as hydrogen-producing catalysts at quite modest overpotential (Pantani et al. Angew. Chem., Int. Ed. 2008, 47, 9948). The electrochemical properties evidenced two quasireversible one-electron reduction waves starting from the cobalt(III) derivative, indicating that there are no dramatic changes in the coordination sphere. The intriguing question is the mechanistic pathways for this observed reactivity. In this work, we compare our observed electrochemical and spectroscopic data (UV-visible and EPR spectroscopies) with our theoretical findings based on DFT, TD-DFT, and CASSCF calculations. The properties of the Co(III) and Co(II) species can be explained as low-spin complexes. In contrast, the doubly reduced species, the “Co(I)” form, is a high-spin complex and its electronic description involves partial reduction of the ligand cage. This point is of major importance to understand the catalytic activity.

’ INTRODUCTION The intrinsic properties of metal complexes stem from the combination of a metal ion and a set of surrounding ligands. The chemical reactivity of a metal-containing molecular catalyst is inherent to the oxidation states of the metal ion and also the lability of the ligands in the coordinaton sphere of the metal ion. In certain cases, the ligands surrounding the metal ion can also participate in the description of the oxidation states; these ligands are termed “non-innocent”. The reactivity pattern of these metal complexes is attracting much interest in the quest for molecular catalytic systems.1,2 Metal cage compounds are a family of metal complexes in which the metal ion is wrapped by a macrobicyclic ligand, leading to a tight-fitting hexacoordinated sphere.3 The original synthetic challenge was to capture labile oxidation states of metal ions, thereby inhibiting any ligand exchange. The pioneering work of Sargeson4 on cobalt cage complexes evidenced the high chemical stability of their CoIII oxidation states, as well as the corresponding lability of the CoII oxidation states.5-7 The chemical reversibility of this redox process has allowed its tuning through variations in the nature of the surrounding ligands. In fact, these complexes have been used as electrontransfer agents in light-driven catalytic reactions, such as in the photoproduction of hydrogen using a molecular photoactive unit ([Ru(bpy)3]2þ) in the presence of a sacrificial electron donor and platinum as the catalyst.8 The structural arrangements of the r 2011 American Chemical Society

ligands around the metal ion after the redox process and the reorganization energy as a whole in these systems are elemental for these applications. A force field for molecular mechanics of cobalt(II) cage complexes has been used to analyze these effects in these systems.9 The encapsulation of metal ions has also received particular interest from a biological viewpoint because of their inherent high binding constants, therefore preventing the release of metal ions.10 It has also been argued that the organic scaffold around the metal ions prevents the access of small active molecules such as hydrogen peroxide, thereby inhibiting Fentonlike chemistry.4 Recently, we reported that the cobalt clathrochelate complexes shown in Scheme 1 behave as potential candidates as electrocatalysts for hydrogen production.11 Recent work by Sun et al.12 demonstrated that these complexes can be activated through a photodriven process in the presence of a sacrificial electron donor and protons. Meanwhile, Lee et al.13 demonstrated that cobalt chlathrochelate complexes can be used as robust structural motifs in the elaboration of conducting polymers through the π-conjugated organic fragments and the metal center. The electrochemical behavior of these complexes shows two quasireversible waves for the CoIII/CoII and CoII/CoI Received: October 29, 2010 Revised: December 20, 2010 Published: January 12, 2011 911

dx.doi.org/10.1021/jp1103643 | J. Phys. Chem. A 2011, 115, 911–922

The Journal of Physical Chemistry A

ARTICLE

Scheme 1

Table 1. Calculated Metric Data for Clathrochelates with X = F and R = Me

Co;N (Å)

ÆCo;Næ (Å)

processes. Moreover, spectroelectrochemical data indicate the lack of any chemical alteration of the identities of these complexes in solution. Crystallographic data for the cobalt complexes in the 2þ and 3þ oxidation states have been obtained for differently substituted chlathrochelate complexes.11,14 Voloshin and co-workers15 even structurally characterized a cobalt(I) derivative in this family. In all cases, the six imino nitrogen atoms of the macrobicyclic ligand have been found in the coordination sphere of the cobalt ion. We have been interested in describing the electronic properties of cobalt clathrochelate complexes based on density functional theory (DFT), timedependent DFT (TD-DFT), and complete active space selfconsistent field (CASSCF) analyses. We also report on the structural changes in this constrained architecture for the CoIII/ CoII and CoII/CoI redox processes. Knowledge of these electronic states should provide insight into the physical properties and chemical reactivities of these complexes. In this article, we first compare the calculated optimized geometries of the cobalt complexes in the three different oxidation states to the crystallographic structures. For clarity reasons, we denote the different redox states of the complexes as oxidized when the total charge (n) of the complex is þ1, neutral for n = 0, and reduced for n = -1 (see Scheme 1). We discuss the different structural rearrangements upon subsequent addition of an electron to the starting oxidized complex. Concerning the reduced species, we have investigated two different spin states (S = 0 and S = 1) that the complex can potentially adopt and draw conclusions at the end of this article on the appropriate state. We then describe the electronic structures for the three different oxidation states based on the molecular orbital (MO) diagram. For the three oxidation states, we interpret the electronic absorption spectra using comparisons between experimental data and computation results. Finally, we draw conclusions on the role of the ligand in the reduction processes and show how experimental electrochemical data can rule out the S = 0 spin state for the reduced complex.

a

oxidized

neutral

reduced

reduced

S=0

S = 1/2

S=0

S=1

1.92 1.93

1.90 1.97

1.86 1.87

2.02 2.05

1.93

1.95

1.89

2.03

1.93

1.91

1.86

2.04

1.91

2.16

2.16

2.01

1.92

2.18

(2.67)

2.05

1.92

2.01

1.93

2.03

Δ(Co;N)a (Å)

0.02

0.28

0.29

0.04

ÆN;Cæ (Å) Δ(N;C)a (Å)

1.31