Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
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Synthesis and Characterizations of Macrocyclic Cr(III) and Co(III) 1‑Ethynyl Naphthalene and 9‑Ethynyl Anthracene Complexes: An Investigation of Structural and Spectroscopic Properties Eileen C. Judkins, Matthias Zeller, and Tong Ren* Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States S Supporting Information *
ABSTRACT: Reported herein are the syntheses and structural and emission spectroscopic characterizations of new CrIII(HMC) and CoIII(cyclam) complexes bearing fluorophore alkynyl ligands, where HMC and cyclam are 5,5,7,12,12,14hexamethyl-1,4,8,11-tetraazacyclotetradecane and 1,4,8,11-tetraazacyclotetradecane, respectively. Two Cr(III) bis-1-ethynylnaphthalene (C2Np) complexes, trans-[Cr(HMC)(C2Np)2]Cl ([1]Cl) and cis-[Cr(HMC)(C2Np)2]Cl ([2]Cl), were prepared from the reactions between trans/cis-[Cr(HMC)Cl2]Cl and lithium 1-ethynylnaphthalene (LiC2Np) in yields of 73 and 66%, respectively. Also investigated are CoIII(cyclam) complexes bearing both C2Np and C2ANT (ANT = 9-anthryl), namely [Co(cyclam)(C2Ar)Cl]Cl (Ar = ANT ([3]Cl), Np ([4]Cl)), [Co(cyclam)(C2Np)(NCCH3)](OTf)2 ([5](OTf)2), and [Co(cyclam)(C2Np)2]OTf ([6]OTf). Complexes [3]Cl (72%) and [4]Cl (67%) were prepared from the reaction between [Co(cyclam)Cl2]Cl and Me3SiC2ANT or Me3SiC2Np, respectively, in the presence of triethylamine. The reaction of [4]Cl with excess silver triflate in CH3CN yielded complex [5](OTf)2 (78%), which was reacted with HC2Np in the presence of triethylamine to form complex [6]OTf in 39% yield. Single crystal X-ray diffraction studies of [1]+, [3]+, [4]+, and [6]+ revealed a pseudo-octahedral geometry around the Cr(III) or Co(III) center with the tetraaza-macrocyclic ligand occupying the equatorial plane and the alkynyl- and/or chloro-ligand occupying the apical positions. The absorption spectra of complexes [1]+ and [2]+ display structured d−d bands between 400 and 550 nm, a feature that is absent in the d−d absorption of the Co(III) complexes [3]+−[6]+. Contrasting emission behaviors were observed: the Cr(III) complexes display metal-centered phosphorescence, while the Co(III) species exhibit ligand-based fluorescence. Time-delayed phosphorescence measurements revealed lifetimes of 447 and 97 μs for [1]+ and [2]+ at 77 K, respectively, and a room temperature lifetime of 218 μs for [1]+.
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emission properties of CrIII(cyclam′) alkynyl complexes, where cyclam′ is the C-substituted cyclam HMC (5,5,7,12,12,14hexamethyl-1,4,8,11-tetraazacyclotetradecane)44 or DMC (5,12-dimethyl-1,4,8,11-tetraazacyclotetradecane). 45 While arylalkynyls with aryl as phenyl or its derivatives have been exploited frequently in these studies, alkynyls with expanded aromatic substituents were limited to the examples of RhIII/ CrIII(cyclam) with Ar as naphthalene and phenanthrene.32 Hence, it is of interest to us to investigate the [MIII(cyclam)(C2Ar)m]-type complexes with Ar as naphthalene or anthracene and m = 1 or 2. Though far less pervasive than phenylethynyls as ligands for metal−alkynyl chemistry, 1-ethynyl naphthalene and 9-ethynyl anthracene have been incorporated in a number of transitionmetal compounds. 1-Ethynyl naphthalene has been used as monodentate ligand for both mononuclear and polynuclear Pt compounds,46−48 exhibiting interesting properties such as twophoton absorption (TPA) abilities with a two-photon induced luminescence (TPIL)46 and ligand-based emission of triplet
INTRODUCTION Transition-metal alkynyl complexes have garnered much interest in the field of molecular electronic materials such as molecular wires,1−6 molecular devices,7−11 photovoltaics,12−14 sensors,15,16 and nonlinear optics17,18 due in part to their structural diversity as well as promising spectral properties for applications in optoelectronics. Lately, photoinduced electron/ energy transfer (PET) processes in transition-metal alkynyl complexes have drawn intense interest. Especially noteworthy is the vibronic attenuation of PET processes in donor−bridge− acceptor dyads based on trans-bis-alkynyl Pt(II) species.19,20 Also of interest is the recent demonstration of a Ru(II) bisalkynyl bridge as redox switch of molecular magneto and optical responses by the laboratory of Rigaut.21,22 Much of the aforementioned efforts are based on 4d and 5d metals, while similar exploration based on 3d metal alkynyls is limited largely to Fe species pioneered by Lapinte.23,24 During recent years, our laboratory,25 along with those of Berben,26 Shores,27 Nishijo,28−31 and Wagenknecht,32 have developed alkynylation chemistry of MIII(cyclam) (cyclam = 1,4,8,11-tetraazacyclotetradecane) with M as Cr,33 Fe,34−36 and Co.37−43 In addition, we also investigated the synthesis and © XXXX American Chemical Society
Received: December 12, 2017
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DOI: 10.1021/acs.inorgchem.7b03128 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry parentage.46,48 1,8-Diethynylanthracene was used as a bidentate bridging ligand for dipalladium complexes in an effort to synthesize molecular rectangles for sensing applications.49 Similar bidentate ligand bridged complexes have been made with Au, exploiting the capability of alkynyl gold complexes to form supramolecular arrays.50−52 Other examples of binuclear gold alkynyl complexes feature 9-ethynylanthracene as terminal ligand52 as well as complexes containing 1,5-ethynylnaphthalene as bridging moiety,51 each displaying rich photoluminescent behavior of predominantly ligand parentage and of both fluorescent and phosphorescent nature. However, few examples exist for 3d transition-metal-based systems. Lapinte et al. demonstrated the feasibility of 9,10diethynylanthracene as bridging moiety between two iron centers, displaying excellent π-electron delocalization properties.53 Subsequent photophysical studies of compounds with both bridging 9,10-diethynylanthracene and monodentate 9ethynyl anthracene revealed anthracene-based fluorescences with significantly reduced quantum yields compared to the free ligand.54,55 The trinickel extended metal atom chain compound bearing axial 9-ethynylanthracene was reported by Cotton and displays an anthracene-based fluorescence red-shifted from that of the free ligand.56 1-Ethynyl naphthalene was used as the capping ligand of a planar tricopper(I) cluster, which displays unusually long phosphorescence lifetimes from predominantly ligand-based emissive states.51 Explored in this work are the preparation of new CrIII(HMC) and CoIII(cyclam) complexes (Schemes 1 and 2) bearing 1-ethynyl naphthalene or 9-ethynyl anthracene as ligands and their structural, spectral, and photoluminescent properties.
Scheme 1. General Synthesis of trans/cis-CrIII(HMC) Bisalkynyl Complexes
RESULTS AND DISCUSSION Synthesis. As shown in Scheme 1, trans/cis-[Cr(HMC)Cl2] Cl reacts with excess LiC2Np to yield trans/cis-[Cr(HMC)(C2Np)2]Cl ([1]Cl/[2]Cl). The stereochemistry of the product is determined by the metal macrocycle starting material. Consequently, reacting ∼3 equiv of LiC2Np with trans-[Cr(HMC)Cl2]Cl yields [1]Cl as a salmon colored solid in 73% yield after purification over silica. Complex [2]Cl was synthesized in a similar manner, reacting ∼3 equiv of LiC2Np with cis-[Cr(HMC)Cl2]Cl to form a crystalline, bright orange solid in 66% yield. Attempts to synthesize analogous 9ethynylanthracene species under similar conditions were unsuccessful, likely due to the instability of the lithiated 9ethynylanthracene. The CoIII(cyclam) complexes [3]+−[6]+ were synthesized using a weak base method (Scheme 2).41 Refluxing [Co(cyclam)Cl2]Cl with excess Me3SiC2ANT in methanol and in the presence of triethylamine (Et3N) for 24 h yielded [Co(cyclam)(C2ANT)Cl]Cl ([3]Cl) as a red-orange crystalline material in 72% yield after purification over silica. Complex [Co(cyclam)(C2Np)Cl]Cl ([4]Cl) was similarly prepared from [Co(cyclam)Cl2]Cl and excess Me3SiC2Np in the presence of Et3N and isolated as a coral colored solid in 67% yield after purification. Synthesis of a bis-alkynyl species trans-[Co(cyclam)(C2Np)2]OTf ([6]OTf) was achieved using a recently developed procedure by our group.43 The abstraction of chloride was realized by refluxing [4]Cl with an excess of AgOTf in acetonitrile over 2 d to yield [Co(cyclam)(C2Np)(NCMe)](OTf)2 ([5](OTf)2) as a feathery, orange solid in 78% yield. The bis-alkynyl species trans-[Co(cyclam)(C2Np)2]OTf ([6]OTf) was readily synthesized by reacting [5](OTf)2
with 8 equiv of HC2Np in acetonitrile and in the presence of Et3N overnight. After purification, complex [6]OTf was isolated as a bright yellow solid in 39% yield. Molecular Structures. Single crystals of X-ray diffraction quality were grown for complexes [1]Cl, [3]Cl, [4]OTf, and [6]OTf via slow solvent−solvent diffusion. Crystals grown via slow evaporation for [5](OTf)2 displayed high levels of disorder, and the structural features of [5]2+ will not be discussed. An ORTEP plot57 of [5]2+ is included in the Supporting Information (Figure S1). Crystallization attempts for complex [2]Cl failed due to instability of [2]Cl in solution. Molecular structures of [1]Cl, [3]Cl, [4]OTf, and [6]OTf were determined via single crystal X-ray diffraction, and ORTEP plots57 of their cations are shown in Figures 1−4, respectively. In all four complexes, the nitrogen atoms occupy the equatorial plane positions with the metal center adopting a pseudooctahedral geometry and the chloro and/or alkynyl ligands residing in the axial positions. In the case of complex [1]+, the HMC macrocycle adopts a C-meso configuration, while the stereochemistry around the nitrogen atoms of the ring presents the expected trans-III configuration of RRSS (see Figure S2). Selected bond lengths and angles are compiled in Table 1. Similarities in the first coordination sphere can be found between [1]+ and trans-[Cr(HMC)(C2Ph)2]Cl. The averaged Cr1−C bond (2.077[5] Å) in complex [1]Cl is slightly shorter than but within the experimental errors of that observed for the related trans-[Cr(HMC)(C2Ph)2]Cl complex (2.085[2] Å).44 Both CC bonds in [1]+ display a slight difference in length (1.206(7) Å and 1.22(2) Å for C1−C2 and C3−C4, respectively), and they are within experimental error of triple bond lengths observed for trans-[Cr(HMC)(C2 Ph) 2]Cl
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DOI: 10.1021/acs.inorgchem.7b03128 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry Scheme 2. General Synthesis of Mono- and Bis-CoIII(cyclam) Alkynyl Complexes Using a Weak Base Method
Figure 3. ORTEP plot of [4]+ at 30% probability level. Hydrogen atoms, (OTf)− counterion, and solvent molecules were omitted for clarity.
Figure 1. ORTEP plot of [1]+ at 30% probability level. Hydrogen atoms and Cl− counterion were omitted for clarity.
Figure 4. ORTEP plot of [6]+ at 30% probability level. Hydrogen atoms, disorder, and (OTf)− counterion were omitted for clarity.
Structures of Co(III) complexes [3]+, [4]+, and [6]+, which are based on cyclam, show the ring adopting the same trans-III configuration around the nitrogen centers (see Figures S3−S5), as noted for complex [1]Cl as well. The arylacetylide ligands in complexes [3]+ and [4]+ exert a pronounced trans-influence on the opposite chloro ligand, manifesting in the significant elongation of the respective Co1−Cl1 bonds at 2.3323(3) and 2.3217(3) Å, compared to the Co−Cl bond length for [Co(cyclam)Cl2]Cl (2.2533(4) Å).58 The Co1−C1 bond lengths for [3]+ and [4]+ are 1.875(1) and 1.873(1) Å, respectively, and slightly less than the Co−C1 bond length in [Co(cyclam)(C2Ph)Cl]Cl (1.898(2) Å).27 The shortening of the Co−Cl bond is attributed to a decrease in the π-donation from the CC bond due to the extended conjugation of the Np/ANT ring. The tilt of the Np ring noted earlier in [1]+ is also observed in [4]+.
Figure 2. ORTEP plot of [3]+ at 30% probability level. Hydrogen atoms, Cl− counterion, and solvent molecules were omitted for clarity.
(1.211[2] Å)44 and trans-[Cr(DMC)(C2Ph)2]Cl (1.213(3) Å).45 The C1−Cr1−C3 angle (176.5(6)°) shows only a slight deviation from linearity, though the appraisal of the Cr1−C1− C2 (165.3(9)°), Cr1−C3−C4 (164(1)°), and N−Cr−C (
