An Aerobic Synthetic Approach toward Bis-Alkynyl Cobalt(III

Aug 9, 2017 - Lindsay E. Eddy , Parth U. Thakker , Colin D. McMillen , Jared A. Pienkos , John J. Cordoba , Carter E. Edmunds , Paul S. Wagenknecht...
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An Aerobic Synthetic Approach toward Bis-Alkynyl Cobalt(III) Compounds Sean N. Natoli, Matthias Zeller, and Tong Ren* Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States S Supporting Information *

ABSTRACT: Reported herein is an expanded investigation into a new method for the preparation of Co(III) cyclam bis-alkynyls (cyclam = 1,4,8,11-tetraazacyclotetradecane) under aerobic, weak base conditions. Treatment of trans-[Co(cyclam)(C2Ar)Cl]Cl-type complexes (Ar = C6F5 (1a), 4-C6H4NMe2 (1b)) with AgOTf in MeCN resulted in the doubly charged complexes [Co(cyclam)(C2Ar)(NCMe)](OTf)2 (Ar = C6F5 (2a), 4C6H4NMe2 (2b)). These solvento complexes 2a,b undergo rapid alkynylation under aerobic conditions in the presence of an organic base and HC2Ar′ to form the symmetrical or unsymmetrical bis-alkynyl complexes trans-[Co(cyclam)(C2Ar)(C2Ar′)](OTf) (Ar/Ar′ = C6F5 (3a), 4-C6H4NMe2 (3b); Ar = C6F5 and Ar′ = 4-C6H4NMe2 (3c), C2Ph (3d)) in good yields. Molecular structures of the new compounds were established using single-crystal X-ray diffraction. Structural studies revealed a notable trans influence for the Co−Cα bond lengths in the unsymmetrical complex 3c with a bond length of 1.929(7) Å for the electronwithdrawing −C2C6F5 ligand and 1.944(7) Å for −C2-4-C6H4NMe2. The optical HOMO−LUMO gaps for the bis-alkynyl complexes follow the trend 3a (2.83 eV) > 3d (2.77 eV) > 3c (2.70 eV) > 3b (2.64 eV). Although [Co(cyclam)(C2R)2]+ type complexes typically have irreversible electrochemical reductions, reversibility of the Co(+3/+2) couple improves in Co(III) cyclam complexes bearing more electron withdrawing substituents. Voltammetric analysis also revealed a modest NMe2/NMe2 coupling across the Co−alkynyl backbone in 3b, while DFT calculations identified the HOMO in 3b as the superexchange pathway for such coupling.

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INTRODUCTION Considerable attention has been given to organometallic compounds possessing σ-alkynyl ligands due to their structural diversity,1−4 high degree of conjugation,5−7 and increasing application in areas such as nonlinear optics,8 photovoltaics,9,10 luminescent sensors,11−13 molecular wires,14−19 and devices.20 As such, synthetic efforts have been undertaken leading to metal alkynyl5,7 and metal alkenyl21−23 complexes that utilize a range of transition metals. Noteworthy among recent pursuits is the work of Weinstein on a series of donor−bridge−acceptor (D-B-A) compounds with trans-bis-alkynyls with Pt(II) as the bridge, where the photoinduced electron transfer between D and A is significantly attenuated by the vibrational excitation of the Pt-bound CC bonds.24−27 Many of the aforementioned studies have been based on the combination of 3d/4d/5d metals and soft auxiliary ligands such as Cp/Cp* and phosphines. Recently, our laboratory started exploring both the preparative chemistry and physical properties of 3d metal alkynyl complexes supported by cyclam (1,4,8,11-tetraazacyclotetradecane) and its derivatives.28 While many examples of symmetrical trans-[M(cyclam)(C2R)2]+ have been reported with M as chromium,29−35 iron,36,37 and cobalt,38−40 unsymmetrical complexes are limited to those of trans-[Co(cyclam)(C2R)(C2R′)]+.41−43 With an interest to develop D-B-A molecular dyads, our laboratory explored the preparation of such complexes using lithio-alkynyl agents.43 However, progress was stymied by synthetic challenges posed © XXXX American Chemical Society

by this method, which led to a mixture of the desired unsymmetrical product and symmetrical byproducts.41,43 Recently we reported a new synthetic method for the selective preparation of unsymmetrical trans-[Co(cyclam)(C2R)(C2R′)]+ in moderate yields under aerobic conditions using milder organic bases (Et3N and Et2NH),44 where key to the success was the generation of trans-[Co(cyclam)(C2R)(MeCN)]2+. This protocol has now been expanded to the preparation of several new bis-alkynyl Co(cyclam) complexes including electron-withdrawing (−C2C6F5) and -donating (−C2-4-C6H4NMe2) ligands. In addition, these complexes have been characterized with structural, spectroscopic, and voltammetric techniques to gain insight into the impact of electron-donating/-withdrawing groups on both the molecular and electronic structures.

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RESULTS AND DISCUSSION As shown in Scheme 1, the reaction between [Co(cyclam)Cl2] Cl and 1.1 equiv of HC2C6F5 in the presence of Et3N afforded the complex trans-[Co(cyclam)(C2C6F5)Cl]Cl (1a), which was isolated in a yield of 77% after silica gel purification. Also by this weak base protocol, the electron-donor alkynyl compound trans-[Co(cyclam)(C2-4-C6H4NMe2)Cl]Cl (1b; 74%) was obtained from the reaction between [Co(cyclam)Cl2]Cl and Received: June 20, 2017

A

DOI: 10.1021/acs.inorgchem.7b01577 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 1. Synthesis of Complexes 1−3a

a

Conditions: (i) 1.1 equiv of HC2Ar, Et3N, MeOH, reflux, 24 h; (ii) 3 equiv of AgOTf, MeCN, reflux, 48 h; (iii) 4 equiv of HC2Ar′, Et3N, MeCN, reflux, 24 h.

MS, 1H NMR, elemental analysis, and single-crystal X-ray diffraction. The molecular structures of 1b, 2a,b, and 3a−d have been established by single-crystal X-ray diffraction studies, and the ORTEP plots of the cations are provided in Figures 1 and 2. Single crystals were grown by slow diffusion of diethyl ether into a concentrated solution of either MeOH (1b) or MeCN (2a,b) or by slow diffusion of hexanes into a concentrated solution of THF (3a−d). It is clear from the ORTEP plots that

1.1 equiv of HC2-4-C6H4NMe2. The orange-red compounds 1a,b appear to be indefinitely stable as solids under ambient conditions and are soluble in polar organic solvents such as MeCN, MeOH, and DMF. In accordance with earlier observations made in both the pioneering work by Shores39 and later studies by us,43,45,46 the use of alkynyl ligands in excess under the reaction conditions for 1a,b does not lead to bis-alkynyl complexes. Previous syntheses of both symmetric trans-M(cyclam)(C2R)236,37,40,41,47 and unsymmetric trans-[Co(cyclam)(C2R)(C2R′)]+ 43 were accomplished exclusively with lithiated alkynes. We surmised that bis-alkynyl Co(III) complexes could be synthesized with a more reactive complex under aerobic, weak base conditions. Thus, chloride abstraction from 1a was accomplished in a reaction with excess AgOTf, followed by recrystallization to afford the yellow complex 2a (71%), which was identified as trans-[Co(cyclam)(C2C6F5)(NCMe)](OTf)2 through crystallographic analysis. Treatment of 1b with AgOTf under similar conditions led to the red complex 2b (66%), which was also authenticated as trans-[Co(cyclam)(C24-C6H4NMe2)(NCMe)](OTf)2 through crystallographic analysis. Other silver salts, namely AgNO3, AgCF3CO2, AgBF4, and AgPF6, were found to facilitate chloride abstraction (see Table S1 in the Supporting Information) as well but resulted in cobalt complexes that had lower solubility, stability, or yields in comparison to 2a,b. The symmetrical bis-alkynyl complex trans-[Co(cyclam)(C2C6F5)2]OTf (3a; 72%) was readily obtained from a refluxing MeCN solution of 2a and HC2C6F5 in the presence of Et3N. Similarly, trans-[Co(cyclam)(C2-4-C6H4NMe2)2]OTf (3b; 44%) was obtained by refluxing a MeCN solution of 2b with HC2-4-C6H4NMe2. Interestingly, the reaction between 2a and HC2-4-C6H4NMe2 under conditions similar to those for 3a yielded the unsymmetrical complex trans-[Co(cyclam)(C2-4C6H4NMe2)(C2C6F5)]OTf (3c; 64%). Complex 3c was also synthesized from the reaction of 2b with HC2C6F5 under identical weak base conditions and isolated in a reduced yield of 47%. In contrast to the prior preparation using lithiated alkynes,41,43 the symmetrical byproducts due to the scrambling of alkynyl ligands were not detected in the reaction mixture (see Figure S26 in the Supporting Information for ESI-MS). To probe the scope of this synthetic methodology, the preparation of trans-[Co(cyclam)(C2Ph)(C2C6F5)]OTf (3d; 72%) was accomplished from the reaction of 2a and 4 equiv of HC2Ph, during which no alkynyl scrambling was observed. Bis-alkynyl complexes 3a−d have improved solubility in less polar solvents and are stable indefinitely as solids in dry environments. The reported complexes were unambiguously characterized by ESI-

Figure 1. ORTEP plots of [1b]+ (top), [2a]2+ (middle), and [2b]2+ (bottom) at the 30% probability level. Hydrogen atoms, counterions, and solvent molecules are omitted for clarity. B

DOI: 10.1021/acs.inorgchem.7b01577 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

given in Tables 1 and 2, and relevant crystal data are provided in Tables 5 and 6. The molecular structure of complex 1b is typical of most trans-[Co(cyclam)(C2R)Cl]+ type compounds, where the Co− Cl bond (2.3177(4) Å) is slightly elongated from that in [Co(cyclam)Cl2]Cl (2.2533(4) Å)48 due to the trans influence of the acetylide ligand.46 The Co−C1 bond length (1.890(2) Å) in 1b is comparable to the Co−C1 bond length (1.898(2) Å) in the previously reported complex trans-[Co(cyclam)(C2Ph)Cl]+.39 The single-crystal X-ray structure of 2b highlights a feature key to its reactivity: the displacement of the axial chloro ligand by a labile acetonitrile molecule. Chloride displacement results in an increased electron deficiency at the cobalt center, which pulls the −C2-4-C6H4NMe2 ligand closer with a Co−C1 bond length (1.874(2) Å) shortened from that in 1b. This structural feature is also observed for 2a with a Co− C1 bond length of 1.873(2) Å. The average bond length for the ethynyl bonds (CC) (1.204 Å) in 1b and 2a,b is within the anticipated confines of a carbon−carbon triple bond.1,2 Due to the crystallographic inversion symmetry at the cobalt center in 3b, the two −C2-4-C6H4NMe2 ligands are coplanar. Complex 3b exhibits a significant elongation of the Co−C1 bond (1.942(3) Å) in comparison to those in complexes 1b and 2b, which stems from the trans influence of the second acetylide. The Co−C1 bond length in 3b measures between those of previously reported [Co(cyclam)(C 2 CF 3 ) 2 ] + (1.917(4) Å)40 and [Co(cyclam)(C2Ph)2]+ (2.001(3) Å).39 The molecular structure of 3a is similar to that of 3b, where an inversion center at the cobalt center results in a linear arrangement of the −C2C6F5 ligands. The more electron deficient nature of 3a results in a shorter Co−C1 bond (1.926(3) Å) in comparison to those measured for the more electron rich complex 3b. Structural features for the aryl substituents observed for 3a,b are similar to those of respective precursors 2a,b. A notable degree of push−pull effect across Co−Cα bonds was observed in the molecular structures of the unsymmetrical complexes 3c,d. While both Co−Cα bonds in 3c are longer than those in 1b and 2a,b, there is a clear distinction in bond lengths between the electron-withdrawing −C6F5 fragment (Co−C1 (1.929(7) Å)) and the electron-donating −4C6H4NMe2 fragment (Co−C9 (1.944(7) Å). This push−pull effect was not observed for the unsymmetrical complex

Figure 2. ORTEP plots of [3a]+ (top), [3b]+ (top middle), [3c]+ (bottom middle), and [3d]+ (bottom) at the 30% probability level. Hydrogen atoms, counterions, and solvent molecules are omitted for clarity.

all reported complexes adopt a pseudo-octahedral configuration, where the cyclam nitrogen atoms occupy the equatorial plane, leaving the chloro, acetylide, or solvento ligands to occupy the axial positions. Selected bond lengths and angles are

Table 1. Selected Bond Lengths (Å) and Angles (deg) for [1b]+, [2a]+, and [2b]+ [1b]+

[2a]+

Co1−N2 Co1−N3 Co1−N4 Co1−N5 Co1−Cl1 Co1−C1 C1−C2 C2−C3 C3−C4

1.982(2) 1.979(2) 1.972(2) 1.983(2) 2.3177(4) 1.890(2) 1.201(2) 1.444(2) 1.396 (2)

C11−Co1−C1 Co1−C1−C2 C1−C2−C3

178.42(4) 172.3(2) 174.3(2)

Co1−N2 Co1−N3 Co1−N4 Co1−N5 Co1−N1 Co1−C1 C1−C2 C2−C3 C3−C4 N1−C9 C9−C10 N1−Co1−C1 Co1−C1−C2 C1−C2−C3

[2b]+ 1.976(2) 1.976(2) 1.984(2) 1.982(2) 1.972(2) 1.873(2) 1.204(2) 1.429(2) 1.391(3) 1.139(2) 1.455(2) 177.31(7) 173.6(2) 174.1(2)

C

Co1−N1 Co1−N2 Co1−N3 Co1−N4 Co1−N2 Co1−C1 C1−C2 C2−C3 C3−C4 N2−C11 C10−C11 N1−Co1−C1 Co1−C1−C2 C1−C2−C3

1.975(2) 1.983(2) 1.987(2) 1.971(2) 1.976(2) 1.874(2) 1.208(2) 1.441(2) 1.383(2) 1.141(2) 1.455(2) 175.73(6) 170.5(2) 177.3(2) DOI: 10.1021/acs.inorgchem.7b01577 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 2. Selected Bond Lengths (Å) and Angles (deg) for [3a]+, [3b]+, [3c]+, and [3d]+ [3a]+

[3b]+

[3c]+

Co1−N1 Co1−N2 Co1−C1 C1−C2 C2−C3 C3−C4

1.985(3) 1.988(3) 1.926(3) 1.210(5) 1.428(5) 1.395(5)

Co1−N2 Co1−N3 Co1−C1 C1−C2 C2−C3 C3−C4

1.989(3) 1.983(3) 1.942(3) 1.204(4) 1.447(4) 1.391(4)

C1−Co1−C1′ Co1−C1−C2 C1−C2−C3

180.0 174.4(3) 172.6(3)

C1−Co1−C1′ Co1−C1−C2 C1−C2−C3

180.0 176.1(3) 176.3(3)

Co1−N1 Co1−N2 Co1−N3 Co1−N4 Co1−C1 C1−C2 C2−C3 C3−C4 Co1−C9 C9−C10 C1−Co1−C9 Co1−C1−C2 C1−C2−C3 Co1−C9−C10 C9−C10−C11

Table 3. IR Vibrational Data (cm−1) a b c d

1 (ν (Δνa))

2 (ν (Δνa))

2135 (HC2C6F5) 2100 (HC2-4C6H4NMe2) 2135 (HC2C6F5); 2100 (HC2-4C6H4NMe2) 2135 (HC2C6F5); 2110 (HC2Ph)

2133 (−2) 2113 (13)

2143 (8) 2128 (28)

1.977(6) 1.988(6) 1.987(6) 1.981(6) 1.929(7) 1.205(9) 1.428(9) 1.399(2) 1.944(7) 1.20(2) 177.1(3) 175.8(7) 177.8(8) 171.7(7) 177.7(8)

Co1−N1 Co1−N2 Co1−N3 Co1−N4 Co1−C1 C1−C2 C2−C3 C3−C4 Co1−C9 C9−C10 C1−Co1−C9 Co1−C1−C2 C1−C2−C3 Co1−C9−C10 C9−C10−C11

1.95(2) 1.99(2) 2.02(2) 1.986(9) 1.917(4) 1.211(6) 1.424(6) 1.373(7) 1.953(4) 1.187(6) 177.6(2) 171.7(4) 177.1(5) 172.6(4) 176.1(5)

Complex 3c possesses two ν(CC) stretches at 2115 and 2094 cm−1, which are tentatively assigned to the alkynyl −C6F5 and −4-C6H4NMe2 fragments, respectively. Matching of the stretching frequency in this order corroborates the structural analysis of ethynyl bond length. For the unsymmetrical complex 3d the change in frequency (Δν = −18 cm−1 (C6F5), Δν = −4 cm−1 (C2Ph)) is reduced in comparison to that of 3c. Consistent with a prior discussion by Wagenknecht and co-workers,40 the trend observed points to the role of the filled−filled type dπ−π(CC) interaction49 in dictating the change in ν(CC). Replacement of the chloro ligand (a weak π donor) in 1 by acetonitrile (pure σ donor) in 2 reduces the destabilization of the filled dπ, which in turn alleviates the dπ−π(CC) antibonding interaction and results in a stronger CC bond. Addition of the second alkynyl to replace acetonitrile in 3 restores the dπ−π(CC) antibonding interaction in both directions and yields weaker CC bonds. The Co(III) complexes reported herein were isolated as red or yellow crystalline materials, with colors originating from a d−d transition (1A1g → 1T1g) between 438 and 510 nm. UV− vis spectra of compounds 1a−3a recorded in MeCN are shown in Figure 3 (see Figure S9 in the Supporting Information for spectra of 1b, 2b, and 3b−d). The removal of the chloro ligand in 1a (λmax 476 nm) results in a blue shift of the d−d absorption maximum observed for complex 2a (λmax 450 nm), while the addition of a second acetylide as for 3a (λmax = 438 nm) leads to a further blue shift. Optical gaps were calculated from the λmax value of the d−d band for the bis-alkynyl complexes, revealing a slight decrease in energy from the most electron poor to the most electron rich: 3a (2.83 eV) > 3d (2.77 eV) > 3c (2.70 eV) > 3b (2.64 eV). Higher energy transitions are observed between 270 and 380 nm. This is most evident for complex 1a, where the higher energy 1A1g → 1T2g transition is observed. Organometallic complexes often display rich redox chemistry, and complexes 1−3 are no exception. As established in previous studies, the [Co(cyclam)(C2R)Cl]+-type complexes exhibit as many as three one-electron metal-centered redox processes: an oxidation (A) and a pair of reductions (C and D) (Scheme 2).42,52 The appearance of these peaks within the potential window permitted by solvent is strongly dependent on the electronic properties and solubility of the Co(III) complexes. For example, bis-alkynyl complexes typically only

[Co(cyclam)(C2Ph)(C2SiMe3)]Cl, which displays Co−Cα bond lengths identical within experimental error.41 Complex 3d exhibits a similar push−pull effect, where the −C6F5 fragment has a Co−C1 length of 1.917(4) Å and the −Ph fragment has a Co−C9 bond length of 1.953(4) Å. Complexes 3c,d both crystallized with the aryl rings in a nearly coplanar arrangement, as shown by the small dihedral angles between the aryl groups (9.6(4)° for 3c and 16.3(3)° for 3d). The C1− Co−C9 angle in 3c,d is slightly bent from linearity at 177.1(3) and 177.6(2)°, respectively. The nature of metal−alkynyl bonding and strength of CC bonds are often evaluated from their vibrational stretching frequencies.49 Insight into the electronic state of a coordination complex can be gleaned upon examination of Δν (metal complex versus parent alkyne).30 All of the reported Co(III) complexes display a νa(CC) stretching frequency around 2100 cm−1, and a list is provided in Table 3. A slight increase in

free ligand ν

[3d]+

3 (ν (Δνa)) 2116 (−19) 2100 (0) 2115 (−20), -C6F5; 2094 (−6), −4C6H4NMe2 2117 (−18), _-C6F5; 2106 (−4), −Ph

Difference in stretching frequencies = (metal complex) − (parent alkyne).

a

the CC stretching frequency is measured upon metalation of the parent ligand HC2-4-C6H4NMe2 (2100 cm−1) to form complex 1b (2113 cm−1).50 Removal of the axial chloride yielding the electron-poor complex 2b led to a more pronounced increase in ν(CC) (2128 cm−1). For the electron-withdrawing ligand HC2C6F5 (2135 cm−1), complex 1a (Δν = −2 cm−1) has a negligible change in ν(CC) frequency upon metalation,51 while the electron-deficient complex 2a (2143 cm−1) shows a stark shift in frequency similar to that of 2b. A decrease in frequency is measured for both complexes 3a,b in comparison to complexes 2a,b. D

DOI: 10.1021/acs.inorgchem.7b01577 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 3. UV−vis spectra of compounds 1a−3a in MeCN. Figure 5. CVs of 3a (top), 3b (top middle), 3c (bottom middle), and 3d (bottom) recorded in a 0.1 M DCM solution of Bu4NPF6 at a scan rate of 0.10 V/s. Forward scan directions are indicated in the CV of 3a, and blue traces in 3b,c represent anodic scans up to 0.6 V.

Scheme 2. Metal-Centered Redox Couple Assignments for Co(III) Cyclam Complexes

observed for complex 1a within the potential window allowed by MeCN. The Co(III) center in 1b has reduction potentials at −1.62 (C) and −2.02 V (D), which are cathodically shifted from those of 1a as a result of electron donation from the −NMe2 group. For 1b, the anodic region consists of an irreversible oxidation at 0.64 V (A, Co4+/3+) and a ligand-based oxidation at 0.26 V (B, −NMe2 oxidation). The latter becomes more reversible when the forward sweep is limited to 0.6 V (blue trace), suggesting that the irreversible appearance (dark trace) was due to the degradation of 1b at more positive potentials. Electrochemical characterization confirms that the synthesis of compounds 2a,b leads to a more electron poor Co(III) center, where the first reduction event (C) for compounds 2a,b is anodically shifted by nearly 250 mV in comparison to 1a,b. For compound 2b there is a strong anodic shift in the metal-centered oxidation potential at 1.22 V (A) but little effect on the reversibility or potential (0.31 V) for the ligand-based oxidation (B). Complex 3a possesses a reversible reduction potential (C) that is cathodically shifted by ca. 450 mV from that of 2a, indicating a more electron rich Co(III) metal center stemming from the inclusion of the second alkynyl ligand. Consequently, the Co2+/+ couple is no longer observed in the potential window permitted by DCM or MeCN. For 3b, there is an irreversible oxidation at 0.92 V (A, Co4+/3+), an NMe2-based two-electron oxidation at 0.23 V (B′), and an irreversible reduction at −2.08 V (C, Co3+/2+). The unsymmetrical complex 3c displays a cobalt-based oxidation at 1.05 V (A), an NMe2based oxidation at 0.23 V (B), and a quasi-reversible reduction at −1.86 V (C, Co3+/2+). The reduction potential measured for 3c is nearly equal to the average of the potentials between 3a and 3b. Complex 3d possesses a reduction potential at −1.84 V and a single oxidation peak at 1.20 V which are anodically shifted in comparison to those of 3b,c. A clear trend for bisalkynyl complexes is obtained from voltammetric analysis, where the order of electron richness of the Co(III) center is 3b

exhibit a single oxidation (A) and reduction (C) process.40,44 Cyclic voltammograms (CVs) of complexes 1a,b and 2a,b are shown in Figure 4, and CVs of 3a−d are shown in Figure 5. Electrochemical parameters are also reported in Table 4. As shown in Figure 4, compound 1a possesses two oneelectron reductions at −1.50 V (C) and −1.82 V (D), which are anodically shifted in comparison to those reported for [Co(cyclam)(C2TIPS)Cl]Cl.52 No oxidation peaks were

Figure 4. CVs of 1a (top), 1b (top middle), 2a (bottom middle), and 2b (bottom) recorded in a 0.1 M MeCN solution of Bu4NPF6 at a scan rate of 0.10 V/s. Forward scan directions are indicated in the CV of 1a, and blue traces in 1b and 2b represent anodic scans up to 0.6 V. E

DOI: 10.1021/acs.inorgchem.7b01577 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 4. Electrode Potentials (V, versus Fc/Fc+) for 1a,b, 2a,b, and 3a−d E(Co4+/Co3+) (A) 1a (MeCN) 1b (MeCN) 2a (MeCN) 2b (MeCN) 3a (DCM) 3b (DCM) 3c (DCM) 3d (DCM) a

E(NMe2+/NMe2) (B) (ΔE1/2, iback/iforward)

E(Co3+/Co2+) (C) (ΔE1/2, iback/iforward)

E(Co2+/Co1+) (D) (ΔE1/2, iback/iforward)

−1.50 −1.62b −1.25b −1.39b −1.72 (0.106, 0.84) −2.08b −1.86c (0.134, 0.59) −1.84c (0.183, 0.55)

−1.82 (0.118, 0.91) −2.02b −1.80 (0.072, 0.80) −2.0b

b

0.64a

0.26 (0.072, 0.56)

1.22a

0.31 (0.074, 0.87)

0.92a 1.05a 1.20a

0.23 (0.186, 0.90) 0.23 (0.126, 0.93)

Irreversible couple; Epa is reported. bIrreversible couple; Epc is reported. cQuasi-reversible couple.

Figure 6. Molecular orbital diagram of complexes [3a′]+, [3b′]+, and [3c′]+ obtained from DFT calculations.

electron withdrawing groups are better suited to stabilize the electron-rich Co(II) state.40 Electronic coupling between two Co(III) cyclam units across a carbon-rich bridge were investigated previously, but with limited success due to the irreversible nature of the Co3+/2+ couples.40,42,52 The reversibility of the −NMe2 oxidation couple in 3b allows for a reliable assessment of the coupling between two NMe2 groups across the entire Co(III) cyclam bis-alkynyl fragment. Though it appears to be a single 2e− oxidation step, the ΔEp value for wave B′ in the CV (186 mV) is significantly broader than the theoretical value expected for a true 2e− wave (29 mV), indicating that B′ might be the coalescence of two closely spaced 1e− waves.53 To estimate the separation of these two waves (ΔE1/2), a differential pulse voltammogram (DPV) of B′ was measured under the conditions prescribed by Taube and Richardson53 to yield a width at half height of 143 mV, which corresponds to a ΔE1/2 value of 73 mV (Figure S29 in the Supporting Information). A comparable ΔE1/2 value (65 mV) was previously reported for the oxidation of the triarylamine groups in trans-[Pt(PEt3)2{CC-1,4-C6H4-N(C6H4OMe-4)2}2], for which a Robin−Day class II designation was inferred.54 It is likely that the 1e− oxidation derivative of 3b, namely (NMe2-NMe2)+, is also a class II mixed-valence species on the basis of similarity to the Pt species in both ΔE1/2 and the N−N distance (17.6 Å in 3b and 17.8 Å in the case of Pt).

> 3c > 3d > 3a. This order is consistent with results from both vibrational and absorption spectroscopy. The electrochemical reduction of Co(III) cyclam alkynyl complexes are typically irreversible,39−41,44,52 likely a result of the formation of a labile Co(II) center. Irreversibility for the first electrochemical reduction (C) for [Co(cyclam)(C2R)(Cl)]+-type compounds is due to the dissociation of the more weakly σ donating chloride ion, followed by association of either a chloride or solvent upon oxidative return.39,42,52 Careful analysis of the CVs of the compounds bearing the electron-withdrawing −C2C6F5 group, namely 1a, 2a, and 3c,d, in the coordinating solvent MeCN revealed that the daughter return peak after reduction had identical potentials for all of the complexes at −1.80 V (Figure S27 in the Supporting Information), suggesting that acetonitrile may facilitate irreversible behavior. Therefore, voltammetry studies were carried out in the noncoordinating solvent DCM. As shown in Figure 5, a quasi-reversible reduction peak is now observed in DCM solutions for 3c,d, suggesting that the reduced Co(II) state is stabilized by the −C2C6F5 ligand and maintains both alkynyl ligands in noncoordinating solvents. The stability gained with the −C2C6F5 ligand is further evidenced by the reversible reduction of 3a. These results support the conclusions initially reported by Wagenknecht and co-workers, where the nature of the ligand confers reversibility and strongly F

DOI: 10.1021/acs.inorgchem.7b01577 Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

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The electronic structures of complexes 3a−c were examined by density functional theory (DFT) at the B3LYP55 level of theory with additional computational details provided at the end of the Experimental Section. Complexes 3a−c were optimized from their coordinates generated from X-ray crystallographic analysis. The average calculated bond lengths and angles are in agreement with those determined experimentally (see the Supporting Information). The frontier molecular orbitals (FMOs), provided in Figure 6, are in agreement with ligand-field theory predictions of a d6 metal center in a strong field, where the LUMO and LUMO+1 are dominated by dx2−y2 and dz2, respectively. Contribution from π(CC) orbitals is missing from the LUMO orbital in each of the model compounds due to their orthogonality to the dx2−y2 orbital. For both of the symmetrical compounds [3a′]+ and [3b′]+, the LUMO+1 involves the Co dz2 minimally interacting with the σ* orbitals of alkynyl ligands. It is clear from Figure 6 that the HOMO energy level is significantly stabilized by the presence of the C2C6F5 ligand: the more C2C6F5 that is present, the more electron deficient the Co center is and the lower in energy the HOMO becomes. Furthermore, the energy levels of the corresponding MOs also follow the trend [3b′]+ > [3c′]+ > [3a′]+ (in general). The fact that the HOMO of [3a′]+ is deeply buried is consistent with the absence of anodic peaks within the window permitted by the solvent. The compositions of HOMO and HOMO-2 are similar among [3a′]+ and [3b′]+ and are primarily the interactions between dπ (dyz and dxz) orbitals and π orbitals localized on alkynyl ligands. The orbital mixings are of the filled−filled (antibonding) type, which has been noted for a number of transition-metal acetylide complexes.49,56,57 In both the HOMO and HOMO-2 of [3b′] a significant degree of mixing of π-type orbitals is observed across the entire ArC2−Co−C2Ar linkage, providing a π-orbital route for electronic coupling, corroborating the communication observed via voltammetric methods. Another noteworthy feature is the spatial separation of the FMO contours of [3c′]+ (Figure 6): the LUMO+1 is predominantly the combination of the Co dz2 orbital and the π orbital of the −C2C6F5 ligand, while the HOMO is dominated by contributions from the dπ (dyz) orbitals and π orbitals localized on the donating −C2-4-C6H4NMe2 ligand. This gives credence to the desire and viability of building Co cyclam based donor−bridge−acceptor dyads bearing elaborately designed chromophore donors and acceptors.

Article

EXPERIMENTAL SECTION

General Considerations. (Pentafluorophenyl)acetylene58 and [Co(cyclam)Cl2]Cl59 were prepared using literature procedures. All other reagents were purchased commercially and used as received without purification. UV−vis spectra were obtained with a JASCO V670 spectrophotometer. FT-IR spectra were measured on a JASCO FT/IR-6300 instrument as neat samples. 1H NMR spectra were obtained using a Varian Mercury 300 NMR, with chemical shifts (δ) referenced to the residual solvent signal (MeCN at 1.93 ppm). Electrospray ionization mass spectrometry (ESI-MS) spectra were recorded on a Waters 600 LC/MS apparatus. Elemental analysis (EA) was performed by Atlantic Microlab, Norcross, GA. Voltammograms were recorded on a CHI620A voltammetric analyzer with a glassycarbon working electrode (diameter 2 mm), a Pt-wire auxiliary electrode, and a Ag/AgNO3 reference electrode filled with 10 mM AgNO3 and 0.1 M Bu4NPF6 in dry MeCN or DCM. The concentration of analyte was always 1.0 mM in 4 mL of dry MeCN or DCM (thoroughly degassed by Ar purging). Potentials were corrected using a ferrocene standard at the end of the runs. [Co(cyclam)(C2C6F5)Cl]Cl (1a). In a round-bottom flask, 400 mg (1.10 mmol) of [Co(cyclam)Cl2]Cl was dissolved in 70 mL of MeOH. To the solution were added 1 mL (7 mmol) of Et3N and 231 mg (1.20 mmol) of HC2C6F5. The solution was refluxed for 24 h, and a gradual shift in solution color from green to red was observed. Solvent was removed, and purification was performed on a silica gel pad by rinsing with EtOAc and then eluting 1a with an EtOAc/MeOH (5/1) mixture. Complex 1a was then recrystallized by the addition of Et2O to a concentrated solution in MeOH, giving a red crystalline material. Yield: 439 mg (0.84 mmol) (77% based on [Co(cyclam)Cl2]Cl). Data for 1a are as follows. ESI-MS (MeOH): m/z 485, corresponding to [Co(cyclam)(C2C6F5)Cl]+. Anal. Found (calcd) for C18H24N4F5CoCl2 (1a): C, 42.00 (41.48); H, 4.82 (4.64); N, 10.75 (10.75). FT-IR (cm−1): Co−CC− 2133. 1H NMR (CD3CN, δ): 4.64 (br s, 4H, NH), 2.89−2.42 (m, 16H, CH2), 1.96 (t, 2H, CH2), 1.53 (q, 2H, CH2). UV−vis (MeCN): λmax (εmax, L mol−1 cm−1): 476 (151), 379 (267), 354 (544), 327 (966). [Co(cyclam)(C2-4-C6H4NMe2)Cl]Cl (1b). In a round-bottom flask, 350 mg (0.96 mmol) of [Co(cyclam)Cl2]Cl was dissolved in 70 mL of MeOH. To the solution were added 1 mL (7 mmol) of Et3N and 153 mg (1.05 mmol) of HC2-4-C6H4NMe2. The solution was refluxed for 24 h, and a gradual shift in color from green to red was observed. Solvent was removed, and purification was performed on a silica gel pad, rinsing first with EtOAc and then eluting 1b with an EtOAc/ MeOH (5/1) mixture. Complex 1b was then recrystallized by the addition of Et2O to a concentrated solution in MeOH, giving a red crystalline material. Yield: 335 mg (0.71 mmol) (74% based on [Co(cyclam)Cl2]Cl). Data for 1b are as follows. ESI-MS (MeOH): m/ z 438, corresponding to [Co(cyclam)(C2-4-C6H4NMe2)Cl]+. Anal. Found (calcd) for C20H34N5CoCl2 (1b·H2O): C, 48.31 (48.79); H, 7.67 (7.31); N, 13.07 (14.22). FT-IR (cm−1): Co−CC− 2113 (s). 1 H NMR (CD3CN, δ): 7.29 (d, J = 9.0 Hz, 2H, Ar-H), 6.66 (d, J = 9.3 Hz, 2H, Ar-H), 4.56 (br s, 2H, N-H), 4.48 (br s, 2H, N-H), 2.92 (s, 6H, N-CH3), 2.90−2.39 (m, 16H, CH2), 1.90 (t, 2H, CH2), 1.49 (q, 2H, CH2). UV−vis (MeCN): λmax (εmax, L mol−1 cm−1): 510 (403), 395 (892). [Co(cyclam)(C2C6F5)NCMe](OTf)2 (2a). In a round-bottom flask, 300 mg (0.58 mmol) of 1a was dissolved in 50 mL of MeCN. To the solution was added 444 mg (1.73 mmol) of AgOTf. The mixture was refluxed for 48 h, and a gradual shift in solution color from red to yellow was observed. Solvent was removed, and purification was performed on a silica gel pad by rinsing with EtOAc and then eluting 2a with MeCN. Complex 2a was then recrystallized by the addition of Et2O/EtOAc (1/1 v/v) to a concentrated solution in MeCN, giving a yellow crystalline material. Yield: 322 mg (0.41 mmol) (71% based on Co of 1a). Data for 2a are as follows. ESI-MS (MeCN): m/z 599, corresponding to [Co(cyclam)(OTf)(C2C6F5)]+. Anal. Found (calcd) for C22H27CoF11N5O6S2 (2): C, 33.75 (33.47); H, 3.42 (3.45); N, 8.85 (8.87). FT-IR (cm−1): Co−CC− 2143. 1H NMR (CD3CN, δ): 4.88 (br s, 2H, N-H), 4.62 (br s, 2H, N-H), 2.80−2.39 (m, 16H, CH2), 2.00

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CONCLUSION In summary, the expanded investigation of a new synthetic method has been described that allows for the facile preparation of Co cyclam bis-alkynyls under aerobic conditions in the presence of a weak base. The synthesis of the solvento compounds 2a,b enables the introduction of a second alkynyl ligand without the use of lithiated alkynyls, hence avoiding the scrambling of alkynyl ligands in preparing unsymmetric bisalkynyl complexes such as 3c,d and circumventing the need for a protective inert gas atmosphere. A significant advantage of the weak base approach over the lithiated alkynyl method is the functional group tolerance, which should facilitate selective synthesis of donor−bridge−acceptor dyads with elaborately designed donor and acceptor chromophores. In addition, electronic coupling between two distal −NMe2 groups in 3b was demonstrated via voltammetric studies, and DFT calculations identified the HOMO of 3b as a proper superexchange pathway. G

DOI: 10.1021/acs.inorgchem.7b01577 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 5. Crystal Data for Compounds 1b and 2a,b mol formula formula wt space group a, Å b, Å c, Å β, deg V, Å3 Z ρcalcd, g cm−3 T, K final R indices (I > 2σ(I)) R1, wR2 GOF on F2

1b·2CH3OH

2a

2b

C22H42Cl2CoN5O2 538.43 P21/n 10.3649(2) 19.2578(5) 12.8446(3) 97.7118(19) 2540.66(10) 4 1.408 100 0.0313, 0.0764 1.060

C22H27CoF11O6S2N5 789.54 C2/c 30.9689(8) 11.9586(2) 16.7011(4) 103.1274(14) 6023.5(2) 8 1.741 100 0.0368, 0.0865 1.042

C24H37CoF6O6N6S2 742.63 P21/n 9.2529(1) 23.1380(3) 15.0659(2) 95.7595(5) 3209.23(7) 4 1.537 100 0.0331, 0.0866 1.051

(t, 2H, CH2), 1.62 (q, 2H, CH2). UV−vis (MeCN): λmax (εmax, L mol−1 cm−1) 450 (172), 360 (188). [Co(cyclam)(C2-4-C6H4NMe2)(NCMe)](OTf)2 (2b). In a roundbottom flask, 300 mg (0.63 mmol) of 1b was dissolved in 50 mL of MeCN. To the solution was added 490 mg (1.91 mmol) of AgOTf. The solution was refluxed for 48 h. Solvent was removed, and purification was performed on a silica gel pad by first rinsing with EtOAc and then eluting 2b with MeCN. Complex 2b was then recrystallized by the addition of Et2O/EtOAc (1/1 v/v) to a concentrated solution in MeCN, giving a red-orange crystalline material. Yield: 310 mg (0.42 mmol) (66% based on Co of 1b). Data for 2b are as follows. ESI-MS (MeCN): m/z 552, corresponding to [Co(cyclam)(OTf)(C2-4-C6H4NMe2)]+. Anal. Found (calcd) for C24H39N6O7CoF6S2 (2b·H2O): C, 37.72 (37.90); H, 4.88 (5.17); N, 10.84 (11.05). FT-IR (cm−1): Co−CC− 2128. 1H NMR (CD3CN, δ): 7.05 (d, J = 8.4 Hz, 2H, Ar-H), 6.42 (d, J = 7.8 Hz, 2H, Ar-H), 4.48 (br s, 2H, N-H), 4.36 (br s, 2H, N-H), 2.66 (s, 6H, N-CH3), 2.58− 2.12 (m, 16H, CH2), 1.71 (t, 2H, CH2), 1.35 (q, 2H, CH2). UV−vis (MeCN): λmax (εmax, L mol−1 cm−1) 489 (252), 379 (338). [Co(cyclam)(C2C6F5)2]OTf (3a). In a round-bottom flask, 100 mg (0.13 mmol) of 2a was dissolved in 50 mL of MeCN. To the solution were added 1 mL (7 mmol) of Et3N and 100 mg (0.52 mmol) of HC2C6F5. The solution was refluxed for 24 h. Solvent was removed, and purification was performed on a silica gel pad by rinsing with EtOAc and then eluting 3a with an EtOAc/MeOH (9/1) mixture. Complex 3a was then recrystallized by the addition of pentane to a concentrated solution in methylene chloride, giving a yellow crystalline material. Yield: 72 mg (0.09 mmol) (72% based on 2a). Data for 3a are as follows. ESI-MS (MeCN): m/z 641, corresponding to [Co(cyclam)(C2C6F5)2]+. Anal. Found (calcd) for C27H24CoF13N4O3S (3a): C, 40.68 (41.02); H, 3.05 (3.06); N, 7.01 (7.09). FT-IR (cm−1): Co−CC− 2116. 1H NMR (CD3CN, δ): 4.14 (br s, 4H, N-H), 2.82−2.35 (m, 16H, CH2), 1.89 (t, 2H, CH2), 1.34 (q, 2H, CH2). UV−vis (MeCN): λmax (εmax, L mol−1 cm−1) 438 (152), 352 (566), 331 (731). [Co(cyclam)(C2-4-C6H4NMe2)2]OTf (3b). In a round-bottom flask, 100 mg (0.13 mmol) of 2b was dissolved in 50 mL of MeCN. To the solution were added 1 mL (7 mmol) of Et3N and 80 mg (0.55 mmol) of HC2-4-C6H4NMe2. The solution was refluxed for 24 h, and a gradual shift in color from red to yellow was observed. Solvent was removed, and purification was performed on a silica gel pad by first rinsing with EtOAc and then eluting 3b with an EtOAc/MeOH (9/1) mixture. Complex 3b was then recrystallized by the addition of pentane to a concentrated solution in methylene chloride, giving a yellow crystalline material. Yield: 41 mg (0.06 mmol) (44% based on 2b). Data for 3b are as follows. ESI-MS (MeCN): m/z 547, corresponding to [Co(cyclam)(C2-4-C6H4NMe2)2]+. Anal. Found (calcd) for C31H44N6CoF3O3S (3b. H2O): C, 52.61 (52.09); H, 6.49 (6.49); N, 11.99 (11.76). FT-IR (cm−1): Co−CC− 2100. 1H NMR (CD3CN, δ): 7.06 (d, J = 8.7 Hz, 4H, Ar-H), 6.43 (d, J = 9.0 Hz, 4H, Ar-H), 3.87 (br s, 4H, N-H), 2.67 (s, 12H, N-CH3), 2.62−2.12 (m,

16H, CH2), 1.66 (t, 2H, CH2), 1.05 (q, 2H, CH2). UV−vis (MeCN): λmax (εmax, L mol−1 cm−1) 470 (180), 328 (10155). [Co(cyclam)(C2-4-C6H4NMe2)(C2C6F5)]OTf (3c). Method A. In a round-bottom flask, 100 mg (0.13 mmol) of 2a was dissolved in 50 mL of MeCN. To the solution were added 1 mL (7 mmol) of Et3N and 75 mg (0.52 mmol) of HC2-4-C6H4NMe2. The solution was refluxed for 24 h. Solvent was removed, and purification was performed on a silica gel pad by first rinsing with EtOAc and then eluting 3c with an EtOAc/MeOH (6/1) mixture. Solvent was removed, and complex 3c was then recrystallized by the addition of pentane to a concentrated solution in methylene chloride, giving a yellow crystalline material. Yield: 60 mg (0.08 mmol) (64% based on 2a). Data for 3c are as follows. ESI-MS (MeOH): m/z 594, corresponding to [Co(cyclam)(C2C6F5)(C2-4-C6H4NMe2)]+. Anal. Found (calcd) for C29H34N5CoF8O3S (3c·2H2O): C, 44.50 (44.68); H, 4.48 (4.91); N, 8.78 (8.98). FT-IR (cm−1): Co−CC−C6F5 2115, Co−CC−C6H4N(CH3)2 2094. 1H NMR (CD3CN, δ): 7.11 (d, J = 8.7 Hz, 2H, Ar-H), 6.47 (d, J = 8.7 2H, Ar-H), 4.02 (br s, 2H, N-H), 3.83 (br s, 2H, N-H), 2.69 (s, 6H, N-CH3), 2.63−2.13 (m, 16H, CH2), 1.65 (t, 2H, CH2), 1.12 (q, 2H, CH2). UV−vis (MeCN): λmax (εmax, L mol−1 cm−1) 460 (183), 369 (703). Method B. In a round-bottom flask, 100 mg (0.13 mmol) of 2b was dissolved in 50 mL of MeCN. To the solution were added 1 mL (7 mmol) of Et3N and 110 mg (0.58 mmol) of HC2C6F5. The solution was refluxed for 24 h, and a gradual shift in color from red to yellow was observed. Solvent was removed, and purification was performed on a silica gel pad by first rinsing with EtOAc and then eluting 3c with an EtOAc/MeOH (7/1) mixture. Complex 3c was then recrystallized by the addition of pentane to a concentrated solution in methylene chloride, giving a yellow crystalline material. Yield: 47 mg (0.06 mmol) (47% based on 2b). [Co(cyclam)(C2C6F5)(C2Ph)]OTf (3d). In a round-bottom flask, 100 mg (0.13 mmol) of 2a was dissolved in 50 mL of MeCN. To the solution were added 1 mL (7 mmol) of Et3N and 0.1 mL (0.91 mmol) of HC2Ph. The solution was refluxed for 24 h. Solvent was removed, and purification was performed on a silica gel pad by rinsing with EtOAc and then eluting 3d with an EtOAc/MeOH (7/1) mixture. Complex 3d was then recrystallized by the addition of pentane to a concentrated solution in methylene chloride, giving a yellow crystalline material. Yield: 64 mg (0.09 mmol) (72% based on 2a). Data for 3d are as follows. ESI-MS (MeOH): m/z 551, corresponding to [Co(cyclam)(C 2 C 6 F 5 )(C 2 Ph)] + . Anal. Found (calcd) for C27H31CoF8N4O4S (3d. H2O): C, 44.58 (45.13); H, 4.14 (4.35); N, 7.55 (7.80). FT-IR (cm−1): Co−CC−C6F5 2117, Co−CC−Ph 2106. 1H NMR (CD3CN, δ): 7.50 (d, 2H, Ar-H), 7.33 (t, 2H, Ar-H), 7.23 (t, 1H, Ar-H), 4.24 (br s, 2H, N-H), 4.05 (br s, 2H, N-H), 2.84− 2.38 (m, 16H, CH2), 1.91 (t, 2H, CH2), 1.34 (q, 2H, CH2). UV−vis (MeCN): λmax (εmax, L mol−1 cm−1) 447 (147), 355 (490). X-ray Structural Analysis of 1b, 2a,b, and 3a−d. X-ray diffraction data were collected on either a Rigaku RAPID-II image plate diffractometer using Cu Kα (λ = 1.54184 Å) radiation (3a,c) or a H

DOI: 10.1021/acs.inorgchem.7b01577 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 6. Crystal Data for Compounds 3a−d mol formula formula wt space group a, Å b, Å c, Å α, deg β, deg γ, deg V, Å3 Z ρcalcd, g cm−3 T, K final R indices (I > 2σ(I)) R1, wR2 GOF on F2

3a·2THF

3b·0.346THF·0.516MeCN

3c

3d

C35H40CoO5F13N4S 934.70 P1̅ 9.3814(3) 12.7849(4) 16.1671(7) 89.918(3) 84.075(3) 79.915(2) 1898.65(12) 2 1.635 100 0.0565, 0.1549 1.080

C33.42H49F3O3.34SCoN6.52 743.49 P1̅ 13.6550(3) 16.9720(3) 17.1983(5) 93.6056(8) 99.2916(8) 110.8429(9) 3644.63(15) 4 1.355 100 0.0599, 0.1751 1.015

C29H34F8O3SCoN5 743.60 Pbca 22.9383(13) 12.1841(8) 23.0305(18)

C26.45H29CoF9.64N4O1.36P0.55S0.45 698.22 P21/c 21.111(2) 11.8664(8) 11.7589(8)

2864.6(4) 4 1.619 100 0.0729, 0.1655 1.088

Tong Ren: 0000-0002-1148-0746 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

(1) Szafert, S.; Gladysz, J. A. Carbon in One Dimension: Structural Analysis of the Higher Conjugated Polyynes. Chem. Rev. 2003, 103, 4175−4206. (2) Szafert, S.; Gladysz, J. A. Update 1 of: Carbon in one dimension: Structural analysis of the higher conjugated polyynes. Chem. Rev. 2006, 106, PR1−PR33. (3) Low, P. J.; Bruce, M. I. Transition Metal Chemistry of 1,3-Diynes, Poly-ynes, and Related compounds. Adv. Organomet. Chem. 2001, 48, 71−288. (4) Bruce, M. I.; Low, P. J. Transition Metal Complexes Containing All-Carbon Ligands. Adv. Organomet. Chem. 2004, 50, 179−444. (5) Paul, F.; Lapinte, C. Organometallic molecular wires and other nanoscale-sized devices: An approach using the organoiron (dppe)Cp*Fe building block. Coord. Chem. Rev. 1998, 178−180, 431−509. (6) Ren, T. Diruthenium sigma-alkynyl compounds: A new class of conjugated organometallics. Organometallics 2005, 24, 4854−4870. (7) Costuas, K.; Rigaut, S. Polynuclear carbon-rich organometallic complexes: clarification of the role of the bridging ligand in the redox properties. Dalton Trans. 2011, 40, 5643−5658. (8) Zhou, G. J.; Wong, W. Y. Organometallic acetylides of Pt-II, Au-I and Hg-II as new generation optical power limiting materials. Chem. Soc. Rev. 2011, 40, 2541−2566. (9) Wong, W.-Y.; Wang, X.-Z.; He, Z.; Djurisic, A. B.; Yip, C.-T.; Cheung, K.-Y.; Wang, H.; Mak, C. S. K.; Chan, W.-K. Metallated conjugated polymers as a new avenue towards high-efficiency polymer solar cells. Nat. Mater. 2007, 6, 521−527. (10) Wong, W. Y.; Ho, C. L. Organometallic Photovoltaics: A New and Versatile Approach for Harvesting Solar Energy Using Conjugated Polymetallaynes. Acc. Chem. Res. 2010, 43, 1246−1256. (11) Yam, V. W.-W. Molecular Design of Transition Metal Alkynyl Complexes as Building Blocks for Luminescent Metal-Based Materials: Structural and Photophysical Aspects. Acc. Chem. Res. 2002, 35, 555− 563. (12) Wong, K. M. C.; Yam, V. W. W. Self-Assembly of Luminescent Alkynylplatinum(II) Terpyridyl Complexes: Modulation of Photophysical Properties through Aggregation Behavior. Acc. Chem. Res. 2011, 44, 424−434.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01577. IR, ESI-MS, and 1H NMR spectra for all complexes presented, differential pulse voltammogram for 3b, computational details and relevant geometric parameters for the optimized structures of [3a′]+, [3b′]+, [3c′]+, and details of the single crystal structure determinations (general procedures, handling of disorder, and twinning) (PDF) Computed Cartesian coordinates of all of the molecules reported in this study (XYZ) Accession Codes

CCDC 1557173, 1557175, and 1557178 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/ data_request/cif, or by emailing [email protected]. uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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6436.6(8) 8 1.535 100 0.0969, 0.2894 1.038

ORCID

Nonius Kappa CCD using Mo Kα (λ = 0.71073 Å) radiation (1b, 2a,b, 3b,d). The structures were solved by direct methods and refinded using Shelxl.60 Details on data collection, refinement, and disorder (2b, 3b,d) or twinning (3b) are given in the Supporting Information. Complete crystallographic data, in CIF format, have been deposited with the Cambridge Crystallographic Data Center: CCDC 1498233 (1b), 1557173 (2a), 1498234 (2b), 1557175 (3a), 1498235 (3b), 1498236 (3c), and 1557178 (3d). Crystal data for compounds 1b, 2a,b, and 3a−d are given in Tables 5 and 6. Computational Details. The electronic structures for complexes 3a−c were examined using the Gaussian03 suite of software61 at the B3LYP55 (Beck’s three-parameter hybrid functional using the Lee− Yang−Parr62 correlation function) level. Full geometry optimizations were performed from the crystal structures reported in this work, and stationary points were determined to be at global minima using analytical frequency calculations with the LanL2DZ basis set. For cobalt, the LanL2DZ basis set was used. The Pople double-ζ quality basis set 6-31G(d,p)63,64 was used for all remaining atoms.

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103.481(5)

AUTHOR INFORMATION

Corresponding Author

*E-mail for T.R.: [email protected]. I

DOI: 10.1021/acs.inorgchem.7b01577 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry (13) Yam, V. W. W.; Au, V. K. M.; Leung, S. Y. L. Light-Emitting Self-Assembled Materials Based on d(8) and d(10) Transition Metal Complexes. Chem. Rev. 2015, 115, 7589−7728. (14) Schull, T. L.; Kushmerick, J. G.; Patterson, C. H.; George, C.; Moore, M. H.; Pollack, S. K.; Shashidhar, R. Ligand effects on charge transport in platinum(II) acetylides. J. Am. Chem. Soc. 2003, 125, 3202−3203. (15) Blum, A. S.; Ren, T.; Parish, D. A.; Trammell, S. A.; Moore, M. H.; Kushmerick, J. G.; Xu, G. L.; Deschamps, J. R.; Pollack, S. K.; Shashidhar, R. Ru-2(ap)(4)(sigma-oligo(phenyleneethynyl)) molecular wires: Synthesis and electronic characterization. J. Am. Chem. Soc. 2005, 127, 10010−10011. (16) Mahapatro, A. K.; Ying, J. W.; Ren, T.; Janes, D. B. Electronic transport through ruthenium-based redox-active molecules in metalmolecule-metal nanogap junctions. Nano Lett. 2008, 8, 2131−2136. (17) Ballmann, S.; Hieringer, W.; Secker, D.; Zheng, Q.; Gladysz, J. A.; Görling, A.; Weber, H. B. Molecular Wires in Single-Molecule Junctions: Charge Transport and Vibrational Excitations. ChemPhysChem 2010, 11, 2256−2260. (18) Schwarz, F.; Kastlunger, G.; Lissel, F.; Riel, H.; Venkatesan, K.; Berke, H.; Stadler, R.; Lortscher, E. High-Conductive Organometallic Molecular Wires with De localized Electron Systems Strongly Coupled to Metal Electrodes. Nano Lett. 2014, 14, 5932−5940. (19) Zhang, X. Y.; Zheng, Q.; Qian, C. X.; Zuo, J. L. Some New Progress on Molecular Wires. Chin. J. Inorg. Chem. 2011, 27, 1451− 1464. (20) Zhu, H.; Pookpanratana, S. J.; Bonevich, J. Y.; Natoli, S. N.; Hacker, C. A.; Ren, T.; Suehle, J. S.; Richter, C. A.; Li, Q. L. RedoxActive Molecular Nanowire Flash Memory for High-Endurance and High-Density Nonvolatile Memory Applications. ACS Appl. Mater. Interfaces 2015, 7, 27306−27313. (21) Liu, S. H.; Xia, H. P.; Wen, T. B.; Zhou, Z. Y.; Jia, G. C. Synthesis and characterization of bimetallic ruthenium complexes with (CH)(6) and related bridges. Organometallics 2003, 22, 737−743. (22) Liu, S. H.; Hu, Q. Y.; Xue, P.; Wen, T. B.; Williams, I. D.; Jia, G. C. Synthesis and characterization of C10H10-bridged bimetallic ruthenium complexes. Organometallics 2005, 24, 769−772. (23) Kong, D. D.; Xue, L. S.; Jang, R.; Liu, B.; Meng, X. G.; Jin, S.; Ou, Y. P.; Hao, X.; Liu, S. H. Conformational Tuning of the Intramolecular Electronic Coupling in Molecular-Wire Biruthenium Complexes Bridged by Biphenyl Derivatives. Chem. - Eur. J. 2015, 21, 9895−9904. (24) Delor, M.; Keane, T.; Scattergood, P. A.; Sazanovich, I. V.; Greetham, G. M.; Towrie, M.; Meijer, A.; Weinstein, J. A. On the mechanism of vibrational control of light-induced charge transfer in donor-bridge-acceptor assemblies. Nat. Chem. 2015, 7, 689−695. (25) Delor, M.; Scattergood, P. A.; Sazanovich, I. V.; Parker, A. W.; Greetham, G. M.; Meijer, A.; Towrie, M.; Weinstein, J. A. Toward control of electron transfer in donor-acceptor molecules by bondspecific infrared excitation. Science 2014, 346, 1492−1495. (26) Scattergood, P. A.; Delor, M.; Sazanovich, I. V.; Bouganov, O. V.; Tikhomirov, S. A.; Stasheuski, A. S.; Parker, A. W.; Greetham, G. M.; Towrie, M.; Davies, E. S.; Meijer, A. J. H. M.; Weinstein, J. A. Electron transfer dynamics and excited state branching in a chargetransfer platinum(II) donor-bridge-acceptor assembly. Dalton Trans. 2014, 43, 17677−17693. (27) Rubtsov, I. V. STATE-SPECIFIC ELECTRON TRANSFER Shake it off. Nat. Chem. 2015, 7, 683−684. (28) Ren, T. Sustainable metal alkynyl chemistry: 3d metals and polyaza macrocyclic ligands. Chem. Commun. 2016, 52, 3271−3279. (29) Berben, L. A. Toward acetylide- and N-hetercycle-bridged materials with strong electronic and magnetic coupling. Ph.D. Dissertation, University of California, 2005. (30) Sun, C.; Turlington, C. R.; Thomas, W. W.; Wade, J. H.; Stout, W. M.; Grisenti, D. L.; Forrest, W. P.; VanDerveer, D. G.; Wagenknecht, P. S. Synthesis of cis and trans Bis-alkynyl Complexes of Cr(III) and Rh(III) Supported by a Tetradentate Macrocyclic Amine: A Spectroscopic Investigation of the M(III)−Alkynyl Interaction. Inorg. Chem. 2011, 50, 9354−9364.

(31) Forrest, W. P.; Cao, Z.; Hambrick, H. R.; Prentice, B. M.; Fanwick, P. E.; Wagenknecht, P. S.; Ren, T. Photoactive Chromium(III)-Cyclam Complexes with Axially Bound geminal-Diethynylethenes. Eur. J. Inorg. Chem. 2012, 2012, 5616−5620. (32) Nishijo, J.; Judai, K.; Numao, S.; Nishi, N. Chromium Acetylide Complex Based Ferrimagnet and Weak Ferromagnet. Inorg. Chem. 2009, 48, 9402−9408. (33) Nishijo, J.; Judai, K.; Nishi, N. Weak Ferromagnetism and Strong Spin-Spin Interaction Mediated by the Mixed-Valence Ethynyltetrathiafulvalene-Type Ligand. Inorg. Chem. 2011, 50, 3464− 3470. (34) Nishijo, J.; Enomoto, M. A Series of Weak Ferromagnets Based on a Chromium-Acetylide-TTF Type Complex: Correlation of the Structures and Magnetic Properties and Origin of the Weak Ferromagnetism. Inorg. Chem. 2013, 52, 13263−13268. (35) Tyler, S. F.; Judkins, E.; Song, Y.; Cao, F.; McMillin, D. R.; Fanwick, P. E.; Ren, T. Cr(III)-HMC (HMC = 5,5,7,12,12,14hexamethyl-1,4,8,11-tetraazacyclotetradecane) Alkynyl Complexes Preparation and Emission Properties. Inorg. Chem. 2016, 55, 8736− 8743. (36) Cao, Z.; Forrest, W. P.; Gao, Y.; Fanwick, P. E.; Zhang, Y.; Ren, T. New Iron(III) Bis(acetylide) Compounds Based on the Iron Cyclam Motif. Inorg. Chem. 2011, 50, 7364−7366. (37) Cao, Z.; Forrest, W. P.; Gao, Y.; Fanwick, P. E.; Ren, T. transFe(cyclam)(C2R)(2) (+): A New Family of Iron(III) Bis-Alkynyl Compounds. Organometallics 2012, 31, 6199−6206. (38) Hoffert, W. A.; Shores, M. P. Crystallographic coincidence of two bridging species in a dinuclear CoIII ethynylbenzene complex. Acta Crystallogr., Sect. E: Struct. Rep. Online 2011, 67, m853−m854. (39) Hoffert, W. A.; Kabir, M. K.; Hill, E. A.; Mueller, S. M.; Shores, M. P. Stepwise acetylide ligand substitution for the assembly of ethynylbenzene-linked Co(III) complexes. Inorg. Chim. Acta 2012, 380, 174−180. (40) Thakker, P. U.; Aru, R. G.; Sun, C.; Pennington, W. T.; Siegfried, A. M.; Marder, E. C.; Wagenknecht, P. S. Synthesis of trans bis-alkynyl complexes of Co(III) supported by a tetradentate macrocyclic amine: A spectroscopic, structural, and electrochemical analysis of pi-interactions and electronic communication in the C equivalent to C-M-C equivalent to C structural unit. Inorg. Chim. Acta 2014, 411, 158−164. (41) Cook, T. D.; Fanwick, P. E.; Ren, T. Unsymmetric Mononuclear and Bridged Dinuclear Co-III(cyclam) Acetylides. Organometallics 2014, 33, 4621−4624. (42) Natoli, S. N.; Azbell, T. J.; Fanwick, P. E.; Zeller, M.; Ren, T. A Synthetic Approach to Cross-Conjugated Organometallic Complexes Based on geminal-Diethynylethene and CoIII(cyclam). Organometallics 2016, 35, 3594−3603. (43) Banziger, S. D.; Cook, T. D.; Natoli, S. N.; Fanwick, P. E.; Ren, T. Synthetic and structural studies of mono-acetylide and unsymmetric bis-acetylide complexes based on Co-III-cyclam. J. Organomet. Chem. 2015, 799−800, 1−6. (44) Natoli, S. N.; Zeller, M.; Ren, T. Stepwise Synthesis of BisAlkynyl CoIII(cyclam) Complexes under Ambient Conditions. Inorg. Chem. 2016, 55, 5756−5758. (45) Cook, T. D.; Natoli, S. N.; Fanwick, P. E.; Ren, T. Dimeric Complexes of Co-III(cyclam) with a Polyynediyl Bridge. Organometallics 2015, 34, 686−689. (46) Natoli, S. N.; Cook, T. D.; Abraham, T. R.; Kiernicki, J. J.; Fanwick, P. E.; Ren, T. Cobalt(III) Bridged by gem-DEE: Facile Access to a New Type of Cross-Conjugated Organometallics. Organometallics 2015, 34, 5207−5209. (47) Sun, C.; Thakker, P. U.; Khulordava, L.; Tobben, D. J.; Greenstein, S. M.; Grisenti, D. L.; Kantor, A. G.; Wagenknecht, P. S. Trifluoropropynyl as a Surrogate for the Cyano Ligand and Intense, Room-Temperature, Metal-Centered Emission from Its Rh(III) Complex. Inorg. Chem. 2012, 51, 10477−10479. (48) Ivanikova, R.; Svoboda, I.; Fuess, H.; Maslejova, A. transdichloro(1,4,8,11-tetraazacyclotetradecane)-cobalt(III) chloride. Acta Crystallogr., Sect. E: Struct. Rep. Online 2006, 62, M1553−M1554. J

DOI: 10.1021/acs.inorgchem.7b01577 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry (49) Manna, J.; John, K. D.; Hopkins, M. D. The Bonding of MetalAlkynyl Complexes. Adv. Organomet. Chem. 1995, 38, 79−154. (50) Rodriguez, J. G.; Tejedor, J. L.; Rumbero, A.; Canoira, L. Stereospecific synthesis of conjugated (1E,3E)- and (1Z,3Z)-1,4-di(nN,N-dimethylaminophenyl)-1,3-butadienes from 2-chloro-1-(n-N,Ndimethylaminophenyl)ethenes: fluorescence properties. Tetrahedron 2006, 62, 3075−3080. (51) Neenan, T. X.; Whitesides, G. M. Synthesis of High-Carbon Materials from Acetylenic Precursors - Preparation of Aromatic Monomers Bearing Multiple Ethynyl Groups. J. Org. Chem. 1988, 53, 2489−2496. (52) Cook, T. D.; Natoli, S. N.; Fanwick, P. E.; Ren, T. CoIII(cyclam) Oligoynyls: Monomeric Oligoynyl Complexes and Dimeric Complexes with an Oligoyn-diyl Bridge. Organometallics 2016, 35, 1329−1338. (53) Richardson, D. E.; Taube, H. Determination of E20-E10 in multistep charge transfer by stationary-electrode pulse and cyclic voltammetry: application to binuclear ruthenium ammines. Inorg. Chem. 1981, 20, 1278−1285. (54) Jones, S. C.; Coropceanu, V.; Barlow, S.; Kinnibrugh, T.; Timofeeva, T.; Brédas, J.-L.; Marder, S. R. Delocalization in PlatinumAlkynyl Systems: A Metal-Bridged Organic Mixed-Valence Compound. J. Am. Chem. Soc. 2004, 126, 11782−11783. (55) Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J. Ab Initio Calculation of Vibrational Absorption and Circular Dichroism Spectra Using Density Functional Force Fields. J. Phys. Chem. 1994, 98, 11623−11627. (56) Lichtenberger, D. L.; Renshaw, S. K.; Bullock, R. M. Metal Acetylide Bonding in (Eta-5-C5h5)Fe(Co)2ccr Compounds Measures of Metal-D-Pi-Acetylide-Pi Interactions from Photoelectron-Spectroscopy. J. Am. Chem. Soc. 1993, 115, 3276−3285. (57) Lichtenberger, D. L.; Renshaw, S. K.; Wong, A.; Tagge, C. D. Investigation of Metal-D-Pi-Butadiynyl-Pi Interactions in (Eta(5)C5h5)(Co)2fec-Equivalent-to-Cc-Equivalent-to-Ch Using Photoelectron-Spectroscopy. Organometallics 1993, 12, 3522−3526. (58) Zhang, Y.; Wen, J. A Convenient Synthesis of Bis(polyfluorophenyl)butadiyne Monomers. Synthesis 1990, 1990, 727− 728. (59) Bosnich, B.; Poon, C. K.; Tobe, M. L. Complexes of Cobalt(III) with a Cyclic Tetradentate Secondary Amine. Inorg. Chem. 1965, 4, 1102−1108. (60) Sheldrick, G. A short history of SHELX. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, 64, 112−122. (61) Frisch, M. J., et al. Gaussian 03; Gaussian, Inc., Wallingford, CT, 2003. (62) Lee, C.; Yang, W.; Parr, R. G. Development of the Colle-Salvetti correlation-energy formula into a functional of the electron density. Phys. Rev. B: Condens. Matter Mater. Phys. 1988, 37, 785−789. (63) Hariharan, P. C.; Pople, J. A. The influence of polarization functions on molecular orbital hydrogenation energies. Theor. Chim. Acta 1973, 28, 213−222. (64) Francl, M. M.; Pietro, W. J.; Hehre, W. J.; Binkley, J. S.; Gordon, M. S.; DeFrees, D. J.; Pople, J. A. Self-consistent molecular orbital methods. XXIII. A polarization-type basis set for second-row elements. J. Chem. Phys. 1982, 77, 3654−3665.

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DOI: 10.1021/acs.inorgchem.7b01577 Inorg. Chem. XXXX, XXX, XXX−XXX