Journal of Coordination Chemistry ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: http://www.tandfonline.com/loi/gcoo20 Spin modulation and electrochemical behavior of a five-coordinate cobalt(III) salen complex James R. Buchwald, Subhadeep Kal, Marissa R. Civic, Ian M. deJoode, Alexander S. Filatov & Peter H. Dinolfo To cite this article: James R. Buchwald, Subhadeep Kal, Marissa R. Civic, Ian M. deJoode, Alexander S. Filatov & Peter H. Dinolfo (2016): Spin modulation and electrochemical behavior of a five-coordinate cobalt(III) salen complex, Journal of Coordination Chemistry, DOI: 10.1080/00958972.2016.1175001 To link to this article: http://dx.doi.org/10.1080/00958972.2016.1175001 View supplementary material Accepted author version posted online: 07 Apr 2016. Published online: 28 Apr 2016. Submit your article to this journal Article views: 10 View related articles View Crossmark data Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=gcoo20 Download by: [Rensselaer Polytechnic Institute] Date: 06 May 2016, At: 08:36 JOURNAL OF COORDINATION CHEMISTRY, 2016 http://dx.doi.org/10.1080/00958972.2016.1175001 Spin modulation and electrochemical behavior of a five- coordinate cobalt(III) salen complex James R. Buchwalda , Subhadeep Kala , Marissa R. Civica , Ian M. deJoodea, Alexander S. Filatovb,c and Peter H. Dinolfoa aDepartment of Chemistry and Chemical Biology, Rensselaer Polytechnic Institute, Troy, NY, USA; bDepartment of Chemistry, University at Albany, State University of New York, Albany, NY, USA; cDepartment of Chemistry, University of Chicago, Chicago, IL, USA ABSTRACT ARTICLE HISTORY We report the synthesis of a Co(III) complex with the five-coordinate salen- Received 22 January 2016 type ligand (N,N′-bis(3,5-di-tert-butyl-2-hydroxybenzyliden)-1,7-diamino-4- Accepted 23 March 2016 methyl-4-azaheptane). This complex is stable in air with a trigonal bipyramidal geometry and we show spectroscopically and computationally that a high- KEYWORDS Salen; spin modulation; spin triplet ground state is preferred. This spin state is readily modulated electron transfer; spin by introduction of an exogenous ligand (pyridine, acetonitrile) to yield a crossover; mixed-valence six-coordinate complex with low-spin ground state. The five-coordinate complex exhibits solvent- and ligand-dependent electrochemical behavior in solution for the CoII/III transition and undergoes a one-electron ligand oxidation to generate a phenoxyl radical species that is relatively stable in the absence of oxygen. We show that this phenoxyl radical species is a Class I mixed-valence compound that can undergo photoinduced inner-sphere charge transfer with the neighboring phenoxide. This process is mediated by the Co(III) center which acts as a bridge. Understanding this behavior will lead to a better understanding of a dicobalt bis-salen analog previously reported by our group as a proton reduction catalyst. Downloaded by [Rensselaer Polytechnic Institute] at 08:36 06 May 2016 1. Introduction Salen complexes of transition metals are classic examples of macrocyclic coordination complexes. Perhaps the most famous of this family of molecules is Jacobsen’s catalyst, a tetradentate salen ligand coordinated to a manganese(II), which is widely used for the asymmetric epoxidation of alkenes [1–3]. CONTACT Peter H. Dinolfo [email protected] Supplemental data for this article can be accessed at http://dx.doi.org/10.1080/00958972.2016.1175001. © 2016 Informa UK Limited, trading as Taylor & Francis Group 2 J. R. BuchwaLD ET AL. Salen complexes have found a wide range of applications in chemistry including catalysis [4], molecular magnetism [5, 6], and the preparation of nanostructured thin films [7]. Transition metal salen complexes have also been employed as biomimetic models to study the spectroscopic properties of enzyme active sites. A distinguishing feature of the salen ligand is the redox activity of the two phenolate moieties in the backbone. These groups are known to actively participate in oxidative chemistry alongside the metal center in analogous complexes containing Cu, Zn, and Ni metal centers [8]. Complexes of this type provide an excellent model for biochemical phenomena such as the CuII-tyrosyl radical found in galactose oxidase [9]. This “guilty ligand” behavior can also have significant consequences for any catalytic behavior of these complexes. Cobalt salens were first studied as early as 1938 when the “salcomine” complex, a square planar cobalt(II) complex, was reported to reversibly bind molecular oxygen [10]. This early example was based on a simple tetradentate ligand formed through condensation of salicylaldehyde with ethylenediamine. In 1946, Roberts and colleagues systematically varied the nitrogen-bearing “backbone” of the salen ligand with the goal of tuning the oxygen-binding properties of salcomine-like complexes [11]. This research eventually led Van Dort and Geursen in 1967 to discover that oxygen-bound cobalt salens could catalytically oxidize phenols to quinones [12]. Similarly, the cobalt analog of Jacobsen’s catalyst [1] was reported in 1999 to catalyze the enantioselective ring-opening of epoxides [13]. Our research group first became interested in the properties of first-row transition metal salens due to their similarity to the bis-salen architectures (M2BisSalen in scheme 1) we have employed as water oxidation and proton reduction catalysts [14–17]. Bis-salens feature a similar ligand architecture, again containing two redox-active phenolate moieties, but coordinate two transition metals through the introduction of an additional pair of Schiff bases ortho to the phenolic oxygen. The electrochemical and spectroscopic properties of these molecules can be complicated by strong electronic correlation between the two transition metals. Salen complexes are thus a natural choice of system in which to examine the interplay of ligand and metal electronic properties in the absence of strong correlation effects. In this paper, we report an unusual behavior of the ground-state electronic structure of a stable pentadentate cobalt(III) salen, CoL+. This five-coordinate trigonal bipyramidal complex, in contrast to the widely studied four-coordinate cobalt salens described above, leaves only one coordination site open and accessible on CoIII. We demonstrate that the coordination of one pyridine ligand to give the tetragonal complex CoLPy+ induces a drastic change in the spin state of the cobalt. This has a pro- nounced effect on the redox properties and electronic spectra of the complex. We characterize this Downloaded by [Rensselaer Polytechnic Institute] at 08:36 06 May 2016 + + Scheme 1. Schematic representation of M2BisSalen, CoL , and CoLPy . JOURNAL OF Coordination CHEMISTRY 3 unusual behavior using electronic absorbance spectra, electrochemical techniques, paramagnetic NMR studies, and density functional theory (DFT) calculations. 2. Results and discussion 2.1. Synthesis H2L (N,N′-bis(3,5-di-tert-butyl-2-hydroxybenzyliden)-1,7-diamino-4-methyl-4-azaheptane) was syn- thesized via a Schiff base condensation reaction between 3,3′-diamino-N-methyldipropylamine and 3,5-di-tert-butyl-2-hydroxybenzaldehyde in refluxing ethanol [8]. Addition of Co(ClO4)2 to a solution of H2L in refluxing ethanol under nitrogen yieldedCoL . This was subsequently oxidized with AgSbF6 in anhydrous dichloromethane with and without pyridine to obtain CoL+ and CoLPy+, respectively, as − the SbF6 salts. These compounds were confirmed by elemental analysis, mass spectrometry, Fourier 1 transform infrared spectroscopy (FTIR), H NMR, and UV–visible spectroscopy. The FTIR spectra of H2L, CoL, CoL+, and CoLPy+ are shown in figure S1. 2.2. X-ray crystal structure The X-ray crystal structures of CoL+ (CCDC No. 1447916) and CoLPy+ (CCDC No. 1447917) are shown in figure 1. Table 1 includes crystallographic data and structural refinement parameters, and table 2 contains selected bond lengths and angles. The structure of CoL+ is closely related to that of CoL’ reported by Boča et al., where L’ is N,N′-bis(3-tert-butyl-5-methyl-2-hydroxybenzyliden)-1,7-diamino- 4-methyl-4-azaheptane [18] (relevant data are also included in table 2) and ML type complexes for M = Zn(II), Ni(II), and Cu(II) [8]. The N-methyldipropylamine backbone enforces a butterfly-like structure of the salen ligand framework and trans-coordination of the Nimines to Co. There is some disorder in the N-methyldipropylamine backbone with respect to the position of the N-methyl group and one of the t-butyl groups. CoL+ assumes a slightly distorted trigonal bipyramidal geometry around Co with the two imine nitrogens (N1 and N3) occupying the axial positions and forming a 178.95° bond angle (N1– Co1–N3). The Ophenolates (O1 and O2) and the Namine (N2) occupy the equatorial positions with respect to Co1. The O1–Co1–O2 bond angle (129.39°) is slightly larger than the other two equatorial bond angles (i.e. O1–Co1–N3 and O2–Co1–N3) at 115.41 and 115.49°, respectively, leading to the distortion. Downloaded by [Rensselaer Polytechnic Institute] at 08:36 06 May 2016 Figure 1. Side and top view ORTEP diagrams of (a) [CoL](SbF6) and (b) [CoLPy](SbF6)·(CH2Cl2)0.5 showing 50% probability thermal ellipsoids. Non-coordinating anion, hydrogen, and solvent atoms are omitted for clarity. 4 J. R. BuchwaLD ET AL. Table 1. Crystallographic data and structural refinement parameters for [CoL](SbF6) and [CoLPy](SbF6)·(CH2Cl2)0.5. [CoL](SbF6) [CoLPy](SbF6)·(CH2Cl2)0.5 Formula C37H57F6CoN3O2Sb C42H62F6CoN4O2Sb fw 870.54 992.10 Crystal system Monoclinic Triclinic Space group C2/c P1̄ a (Å) 31.836(6) 11.8859(9) b (Å) 11.966(2) 13.6004(10) c (Å) 21.062(4) 14.3193(11) α (°) 90 92.6110(10) β (°) 99.122(2) 92.9470(10) γ (°) 90 103.0650(10) V (Å3) 7923(2) 2248.0(3) Z 8 2 T (K) 100(2) 100(2) λ (Å) 0.71073 0.71073 ρ (calcd, g cm−3) 1.46 1.466 μ (mm−1) 1.164 1.093 Reflections collected 9336 10,135 No.