Communication pubs.acs.org/IC
Stepwise Synthesis of Bis-Alkynyl CoIII(cyclam) Complexes under Ambient Conditions Sean N. Natoli, Matthias Zeller, and Tong Ren* Department of Chemistry, Purdue University, West Lafayette, Indiana 47906, United States S Supporting Information *
Scheme 1. Synthesis of Complex 1−4a
ABSTRACT: Reported herein is a new synthetic method for the synthesis of CoIII(cyclam) bis-alkynyls (cyclam = 1,4,8,11-tetraazacyclotetradecane) under aerobic conditions. Upon the treatment of AgOTf in acetonitrile, complex trans-[Co(cyclam)(C2C6H4NMe2)Cl]Cl (1) was converted to trans-[Co(cyclam)(C 2 C 6 H 4 NMe 2 ) (NCMe)](OTf)2 (2), and 2 was in turn reacted with HC2Ar under weakly basic conditions to afford the novel bis-alkynyls trans-[Co(cyclam)(C2C6H4NMe2)(C2Ar)](OTf) (Ar = C6H4NMe2 (3) and C6F5 (4)) in reasonable yields. Voltammetric analysis revealed a modest NMe2/ NMe2 coupling across the Co-alkynyl backbone in 3, while DFT calculations identified the HOMO in 3 as the superexchange pathway for such coupling.
a
Conditions: (i) 3 equiv AgOTf, MeCN, reflux 48 h; (ii) R = 4 equiv HC2Ar, Et3N, MeCN, reflux 24 h.
crystallographic analysis. With a labile acetonitrile as the sixth ligand, compound 2 is a versatile synthon. Previous syntheses of both symmetric trans-M(cyclam)(C2R)2 and unsymmetric trans-Co(cyclam)(C2R)(C2R′) rely exclusively on the use of lithium acetylides. 6 In contrast, trans- [Co(cyclam)(C2C6H4NMe2)2]OTf (3, 44%) was obtained by heating an MeCN solution of 2 with 4 equiv of HC2C6H4NMe2 and Et3N to reflux. More interestingly, the reaction between 2 and 4 equiv of HC2C6F5 under conditions similar to that of 3 resulted in trans-[Co(cyclam)(C2C6H4NMe2)(C2C6F5)]OTf (4, 47%). Unlike 2, the reaction of 1 under similar conditions did not yield bis-alkynyls, even with a larger excess of alkynyl ligand. Although the isolated yields for 3 and 4 are modest, ESI-MS did not reveal either of the symmetric byproducts in the preparation of 4 (see Supporting Information for ESI-MS), indicating the absence of alkynyl ligand scrambling, which is a limitation when using lithium acetylides in the preparation of unsymmetrical Co(cyclam) complexes.8 Single crystal X-ray diffraction analysis of 1 (see Supporting Information for the structure plot) revealed a CoIII center in an octahedral environment similar to other [Co(cyclam)(C2R)Cl]+ species,8−11 where the cyclam nitrogens occupy the equatorial plane, leaving the chloro and acetylide ligands to occupy the axial positions. The single crystal structure analysis of 2 (Figure 1) highlights a key structural feature responsible for its reactivity toward bis-alkynyl formation: the displacement of the axial chloro in 1 by a labile acetonitrile molecule. Such a displacement results in an electron deficient CoIII center, which pulls the -C2C6H4NMe2 ligand closer with a Co−C bond (1.874(2) Å) shortened from that in 1 (1.890(2) Å). Complex 3 crystallized with an inversion center at the Co center (Figure 2). The Co−C1 bond length is 1.942(3) Å with a C1′−Co1−C1 angle of 180°. Significant elongation of the Co−C1 bond compared to those in complexes 1 and 2 stems from the trans-influence of the second acetylide. The Co−C
ransition metal σ-alkynyl compounds have been studied as potential molecular wires1,2 and nonlinear optical3 and photovoltaic materials4 because of their rigid and highly conjugated structures. Noteworthy among recent development is the work of Weinstein on a series of donor−bridge−acceptor (D-B-A) compounds with trans-Pt(II)-bis-alkynyl as the bridge, where the photoinduced electron transfer between D and A is significantly attenuated by the excitation of the Pt-bound CC bonds.5 Since many of the aforementioned studies have been based on the combination of 4d/5d metals and soft auxiliary ligands, we have endeavored to explore both the preparative chemistry and materials properties of 3d metal alkynyl complexes supported by cyclam (1,4,8,11-tetraazacyclotetradecane) and its derivatives.6 While many examples of symmetric trans-M(cyclam)(C2R)2 have been reported with M as Cr, Fe, and Co, few unsymmetric trans-Co(cyclam)(C2R)(C2R′) complexes are known, and all were prepared using lithioalkynyls in low to modest yields.7−9 Reported in this Communication is a new method for the preparation of trans-Co(cyclam)(C2R)(C2R′) derivatives in respectable yields under aerobic conditions. Our effort is built on the high yield preparation of trans[Co(cyclam)(C2Ph)Cl]+ from the reaction between [Co(cyclam)Cl2]Cl and HC2Ph in the presence of Et3N by Shores and co-workers.10 Using a similar approach, the reaction between [Co(cyclam)Cl2]Cl and 1.1 equiv of HC2C6H4NMe2 afforded the red complex trans-[Co(cyclam)(C2C6H4NMe2)Cl]Cl (1, 74%). Subsequently, 1 was treated with AgOTf in acetonitrile (Scheme 1), and subsequent recrystallization afforded red-orange compound 2 (66%), which was identified as trans-[Co(cyclam)(C2C6H4NMe2)(NCMe)](OTf)2 through
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© XXXX American Chemical Society
Received: May 1, 2016
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DOI: 10.1021/acs.inorgchem.6b01076 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry
Figure 1. ORTEP of 22+ at the 30% probability level. Hydrogen atoms are omitted for clarity. Co1−Nav, 1.98[1]; Co1−C1, 1.874(2); Co1− N6, 1.976(1); C1−C2, 1.208(2); C1−Co1−N6, 175.73(3).
Figure 2. ORTEP plot of 3+ at the 30% probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Co1−Nav, 1.986[3]; Co1−C1, 1.942(3); C1−C2, 1.204(4); C2−C1−Co1, 176.1(3).
Figure 4. CVs recorded in 0.1 M solution of Bu4NPF6 for 2 (MeCN), 3, and 4 (DCM) at a scan rate of 0.10 V/s.
bond length in 3 measures between that of previously reported X-ray structures of [Co(cyclam)(C2CF3)2]+ (1.917(4) Å)12 and [Co(cyclam)(C2Ph)2]+ (2.001(3) Å).10 The intramolecular N(Me2)−N(Me2) distance across 3 was measured at 17.6 Å, while the two aryls are coplanar due to the crystallographic inversion symmetry. The structure of complex 4 was also obtained as shown in Figure 3, where the coplanarity of C6F5
Co2+/1+) for 2. CVs of bis-alkynyls 3 and 4 are significantly different from that of 2. The potentials are cathodically shifted from those of 2 with the inclusion of the second alkynyl ligand, which significantly increases electron density at the Co center. Consequently, the Co2+/1+ couple is no longer observed in the potential window permitted in DCM or MeCN. For 3, there is an irreversible oxidation at 0.92 V (A, Co4+/3+), a 2e− NMe2based oxidation at 0.23 V (B′), and an irreversible reduction at −2.08 V (C, Co3+/2+). Complex 4 displays Co-based oxidation at 1.05 V (A), NMe2-based oxidation at 0.23 V (B), and a quasi-irreversible reduction at −1.86 V (C, Co3+/2+). The replacement of C6H4NMe2 by C6F5 has improved the reversibility of couple C and resulted in a remarkable potential shift of +200 mV. Electronic coupling between two CoIII(cyclam) units across a carbon-rich bridge were investigated previously with limited success due to the irreversible nature of the Co 3+/2+ couples.7,9,11 The reversibility of the -NMe2 oxidation couple in 3 allows for a reliable assessment of the coupling between two NMe2 groups across the CoIII(cyclam) center. Though appearing to be a single 2e− oxidation step, the ΔEp for wave B′ in CV (186 mV) is significantly broader than the theoretical value expected for a true 2e− wave (29 mV), indicating that B′ is the coalescence of two closely spaced 1e− waves. To resolve the separation (ΔE1/2), a differential pulse voltammogram (DPV) of B′ was measured under the conditions prescribed by Taube and Richardson14 to yield a width at half height of 143 mV, which corresponds to a ΔE1/2 value of 73 mV. A comparable ΔE1/2 value (65 mV) was previously reported for the oxidation of the triarylamine groups in trans-[Pt(PEt3)2{CC-1,4-C6H4−N(C6H4OMe-4)2}2], for which a Robin-Day class II designation was inferred.15 It is likely that the 1e− oxidation derivative of 3, namely (NMe2-NMe2)+, is also a class II mixed valent species on the basis of similarity to the Pt species in both ΔE1/2 and the N−N distance (17.6 Å in 3 and 17.8 Å in the case of Pt). Further insight into molecular orbital interactions in these new Co-alkynyl species can be gleaned from density functional
Figure 3. ORTEP plot of 4+ at the 30% probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Co1−Nav, 1.983[6]; Co1−C1, 1.929(7); Co1−C9, 1.944(7); C1−C2, 1.205(9); C1−Co1−C9, 177.1(3); C2−C1−Co1, 175.8(7); C10−C9−Co1, 171.7(7); C9−C10−C11, 177.7(8).
and C6H4NMe2 rings is clear (dihedral angle between the two aryls is 9.6(4)°). The C1−Co−C9 angle is 177.1°. Reflecting the electron withdrawing nature of C6F5, the Co−C1 bond (1.929(7) Å) is slightly shorter than that of Co−C9 (1.944 (7) Å), while both are longer than the Co−C bond lengths in 1 and 2. Transition metal alkynyls often display rich redox chemistry,2,13 and complexes 1−4 are no exception. Cyclic voltammograms (CVs) of complexes 2−4 are shown in Figure 4. For 2, the anodic region consists of an irreversible oxidation at 1.22 V (A, Co4+/3+) and a ligand based oxidation at 0.31 V (B, -NMe2 oxidation). The latter becomes reversible when the forward sweep is limited to 0.6 V (blue trace), suggesting the irreversible appearance (dark trace) was due to the degradation of 2 at more positive potentials. Two irreversible reductions were observed at −1.39 V (C, Co3+/2+) and −2.02 V (D, B
DOI: 10.1021/acs.inorgchem.6b01076 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry
calculations identified the HOMO in 3 as a proper superexchange pathway.
calculations performed at the B3LYP/LanL2DZ level using the Gaussian 03 suite on the optimized model cations [3′]+ and [4′]+.16 The optimized geometries are in good agreement with crystallographic data (see the Supporting Information). The frontier molecular orbital contours [3′]+ and [4′]+ are provided in Figure 5. Frontier orbitals are in agreement with ligand-field
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01076. Experimental details, relevant characterizations, computational details, and geometric parameters for [3′]+ and [4′]+(PDF) Crystallographic data (CIF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully acknowledge financial support from the National Science Foundation (CHE 1362214). REFERENCES
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Figure 5. Molecular orbital diagram of compounds [3′]+ and [4′]+ obtained from DFT calculation.
theory of a d6 metal center in a strong field, where the LUMO and LUMO+1 correspond to the eg set (dx2−y2 or dz2). Contribution from π(CC) orbitals is missing from the LUMOs due to their orthogonality to the dx2−y2 orbital. The LUMO+1 involves the Co dz2 minimally interacting with the σ* orbitals on the -C2C6H4NMe2 substituent, with significant orbital contribution from the electron withdrawing -C2C6F5 ligand observed in [4′]+. The compositions of HOMO and HOMO−2 are similar among [3′]+ and [4′]+ and are dominated by the interactions between dπ (dyz) orbitals and π-orbitals localized on the -C2C6H4NMe2 ligand. The orbital mixings are of the filled− filled (antibonding) type, which has been noted for a number of transition metal acetylide complexes.17 A significant degree of mixing of π-type orbitals is observed across the entire ArC2− Co−C2Ar linkage in the HOMO of [3′]+, providing a π-orbital route for electronic coupling, corroborating the modest communication observed via voltammetry methods. In summary, we have described a new synthetic approach that allows for the facile synthesis of Co(cyclam) bis-alkynyls under weakly basic conditions. The presence of a coordinated solvent in 2 allows for the introduction of a second alkynyl ligand without the use of lithio-alkynyls and hence avoids scrambling of the alkynyl ligand in preparing unsymmetric bisalkynyl complexes such as 4 and circumvents the need for a protective inert gas atmosphere. A great advantage of the weak base approach over the lithium acetylide method is the functional group tolerance, which should facilitate selective synthesis of donor−bridge−acceptor dyads with elaborately designed chromophore donors and acceptors. In addition, modest but significant electronic coupling between two distal NMe2 groups was demonstrated via voltammetric study. DFT C
DOI: 10.1021/acs.inorgchem.6b01076 Inorg. Chem. XXXX, XXX, XXX−XXX
