Synthesis, characterization and reactivity of a novel Di-nickel complex towards carbon dioxide

Inorganic Chemistry Communications 44 (2014) 177–179

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Synthesis, characterization and reactivity of a novel Di-nickel complex towards carbon dioxide William Seals, Hadi Arman, Stephan B.H. Bach, Ghezai T. Musie ⁎ Department of Chemistry, The University of Texas at San Antonio, San Antonio, TX 78249, USA

a r t i c l e

i n f o

Article history: Received 19 February 2014 Accepted 21 March 2014 Available online 29 March 2014

a b s t r a c t Newly synthesized dinickel complex 1, K2[Ni2(ccdp)(C5H7O2)(H2O)2], reacts with CO2 to produce a novel bis(μ-η2,η2)-carbonato bridged tetranickel complex 2, Cs6[Ni4(ccdp)2(μ-η2-η2-CO3)2], via an intermediate species that contains a bis(μ-OH) dinickel(II) moiety, complex 3. © 2014 Elsevier B.V. All rights reserved.

Keywords: Carbon dioxide Nickel complex Carbonato ligand Metal complex activation

Carbon dioxide is one of the major green-house gases. Although sequestration and/or fixation of carbon dioxide using metal complexes have been actively pursued, it still remains a long-standing and unresolved issue. Studies of metal complexes with terminal and/or bridging metal bound hydroxo groups that show reactivity towards CO2 are reported in the literature [1–9]. The strong nucleophilic hydroxo group of these complexes attacks and converts CO2 to carbonate or bicarbonate under different reaction conditions. Reactions of CO2 with bridging hydroxo nickel(II) complexes often resulted in the production of bridged carbonates with η2 or η3 coordination modes of bi- or trinickel complexes [1]. However, recently a series of terminal hydroxo containing nickel complexes prepared by Holm and coworkers have been shown to produce complexes with a rare η 1 -CO3 H ligation mode [7–9]. In this study, we report the reactivity of an in situ formed bis(μ-OH)-tetranickel complex with CO2 to generate a novel bis(μ-CO3-κ4O,O′:O,O″) bridged tetranickel complex. Complex 1 was synthesized at room temperature by the reaction of Ni(acac)2 and our previously reported ligand, H5ccdp, (N,N′-Bis[2carboxybenzomethyl]-N,N′-Bis[carboxymethyl]-1,3-diaminopropan-2ol) [11] in 2:1 molar ratio, in alkaline CH3OH/H2O medium. The crystal structure of 1, shown in Fig. 1, reveals that the binucleating ligand, ccdp5 −, is coordinated to two Ni(II) ions at Ni(1)`Ni(2) distance of 3.75 Å. The shared ccdp5− ligand, which acts as tetradentate ligand to each of the nickel centers, is coordinated in a fashion that the metal centers are missing two ligands in a cis- position to secure an octahedral

⁎ Corresponding author at: Department of Chemistry, The University of Texas at San Antonio, San Antonio, USA. E-mail address: [email protected] (G.T. Musie).

http://dx.doi.org/10.1016/j.inoche.2014.03.029 1387-7003/© 2014 Elsevier B.V. All rights reserved.

geometry. Whereas two water molecules complete the octahedral coordination around Ni(1), the bidentate acac − ligand takes the missing cis-positions around the Ni(2) center. The extent of distortion around the Ni(1) center is pronounced by the large bite angle of O(13)-Ni(1)-N(1) = 107.5° as well as in the small bite angles for the O(5)-Ni(1)-N(1) = 82.0°, O(13)-Ni(1)-O(12) = 82.1° and O(3)Ni(1)-N(1) = 82.3°. Another interesting structural feature of the complex is the 133.4° for the Ni(1)\O(5)\Ni(2) bond angle. The angle is wider than expected for sp3 hybridized oxygen but is slightly smaller than values observed for similar complexes reported in the literature [10]. The average Ni\Owater, Ni\Obenzoate, Ni\Oacetate, Ni\Oalkoxo, Ni\Oacac and Ni\Namine bond distances are 2.110, 2.076, 2.081, 2.042, 2.029 and 2.075 Å, respectively. The observed metric values are comparable to values reported for similar complexes in the literature [11–15]. Electronic spectrum of the aqueous solution of 1 shows absorption peaks at 634 (ε = 36 mol·L − 1 ·cm − 1 ) and 1020 (ε = 23 mol·L− 1·cm− 1) nm. While complex 1 does not show any reactivity towards CO2 under neutral reaction conditions, in slightly basic medium it produces a novel bis(μ-carbonato-κ4O,O′:O,O″) bridged tetranickel complex, 2, vide infra. In this context, the reaction can be recognized as an activation of CO2, in the subsequent conversion to carbonate, by the complex. Under the same experimental conditions but in the absence of 1, no carbonate formation was detected when the resulted solution was subjected to either CaCl2 or examination of the reaction mixture using Kettrup's method of carbonate analysis [16,17]. In an effort to characterize the active nickel species involved in the activation of CO2, mass and electronic spectroscopic studies were conducted. In the mass spectral study, aqueous solutions were negative-ion-electrosprayed into a quadrupole ion-trap mass spectrometer (ESMS) and subjected to collision-

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W. Seals et al. / Inorganic Chemistry Communications 44 (2014) 177–179

Fig. 1. Perspective view of 1 with atomic numbering scheme, all H atoms, solvent molecules and counter ions are omitted for clarity.

induced dissociation (CID) to gain structural information [18]. The mass spectrum of 1 at pH 9.5 contains not only the parent signal that corresponds to [(NH4 ) 6 (OH)2 Ni2(ccdp)(μ-OH)]− 2 at m/z = 1349 (20%) but also signals that correspond to its fragmentation products at m/ z = 1211, 843, 723, 684 and 585, Figure S1. In contrast, the mass spectrum of 1 dissolved in just pure water features a parent peak with m/z = 843 at relatively low intensity (35%) with the typical isotope pattern. In the stepwise fragmentation of the parent ion, loss of the potassium ions (m/z = 723, 45%) was followed by the losses of 2H2O (m/z = 684, 25%) and acac− (m/z = 585, 65%) Figure S2. These are the same signals observed in the spectrum obtained for complex 1. The differences noted between the two spectra are the featured peak values at m/z 1349 and 1211, values that correspond to anionic cluster of [(NH4)6(OH)2Ni2(ccdp)(μ-OH)]2, and its fragmentation product [Ni2(ccdp)(μ-OH)]2, respectively. Additionally, using the Isotope Viewer in Xcalibur (Thermo Finnigan), simulations of the stable isotope patterns of the 1349 and 1211 peaks were calculated and compared to the experimental results, Figs. S3 & S4, respectively. The excellent agreement of isotope patterns between the experimental data and the simulations confirms the formation

of the μ-OH species when 1 is subjected to the alkaline reaction conditions. The electronic spectrum of a fresh aqueous solution produced by the reaction of 1 and a base, such as NH4OH or (CH3)4NOH, has absorption peaks at 379, 625 and 1028 nm, Fig. S5. The values of these peaks are different from the ones observed in the electronic spectra for complexes 1 (which has absorption peaks at 634 and 1020 nm) and 2 (peaks at 388, 644, 659 and 1022 nm) in aqueous solutions, Figure S5. The observed wavelength values for 3 are comparable to the values reported in the literature for μ-OH nickel complexes [1–9]. So far, our efforts to grow single crystals of 3 for X-ray diffraction have not been successful, nevertheless, our efforts continue. The electronic spectrum of the above solution after reacted with CO2 (added as a dry ice) has the same features as the spectrum of 2. Nickel bridged hydroxide groups are strong nucleophiles that attack and hydrolyze electrophilic centers of very stable substrates, such as CO2 and phosphoesters [19–23]. In this investigation, the possible reaction pathway involved in the synthesis of 2 is depicted in Fig. 2. The nucleophlic attack of CO2 by one of the μ-OH group of 3 results in the formation of the nickel bound bicarbonate intermediate, 4. Subsequent deprotonation of the nickel bound bicarbonate moiety, thus, produces the (μ-CO3-κ4O,O′:O,O″-μ-OH) tetranickel intermediate, 5. The same steps undergo at the second bridging hydroxide to yield the stable bis(μ-CO3-κ4O,O′:O,O″) tetranickel complex, 2. Although similar reaction pathways with μ-OH-dinickel complexes are reported in the literature, the formation of the bis(μ-CO3) complex to form a tetranickel complex and the structure of 2 are not common [1–9,24]. The unit cell of the crystal structure contains [Ni2(ccdp)(μ-CO3-κ4O, O′:O,O″)]62 −, and six cesium counter ions, and 23 water molecules of crystallization. The depiction of the molecular structure of the anion of 2 is shown in Fig. 3. The X-ray characterization of 2 reveals that the complex is tetranickel cluster produced as a result of self-assembly of two [Ni2(ccdp)]− units and two bridging CO23 − ions coordinated to the nickel centers in the rare anti-anti, pseudo-oxo, μ-CO3-κ4O,O′:O,O″ fashion. The coordination geometries around each Ni(II) center are best described as a distorted octahedral geometry defined by NO5 coordination environments. All the nickel centers are coordinated to the μ-alkoxo, the acetate oxygen, the benzoate oxygen, the tertiary amine nitrogen of the ccdp5 − ligand and two oxygens from the bridging carbonato ligand. The octahedral geometry around Ni(1) is defined by the O(10),

Fig. 2. Proposed pathway for the synthesis of 2 from 1 via metal bound bridging hydroxide and bicarbonate intermediates.

W. Seals et al. / Inorganic Chemistry Communications 44 (2014) 177–179

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Acknowledgments Financial support from the Welch Foundation in the form of Grant AX-1540 is greatly appreciated. The authors also would like to thank the Chemistry Department, University of Texas at San Antonio (1980170350), for funds to upgrade the X-ray instrument and computers.

Appendix A. Supplementary material Electronic Supplementary Information (ESI) available: X-ray crystallographic data for complexes 1 and 2 in CIF format, detailed experimental procedures and spectroscopic data of mass, Uv–vis and IR. See http:// dx.doi.org/10.1016/j.inoche.2014.03.029.

References Fig. 3. Perspective view of 2 with atomic numbering scheme, all H atoms, solvent molecules and counter ions are omitted for clarity.

O(11), O(5), and N(1) atoms at the equatorial positions and the O(1) and O(3) at the axial positions. While the benzoate and the acetate arms of the ccdp 5 − ligand take the distorted axial positions, the amine, the alkoxo and the carbonate groups occupy the equatorial positions of the octahedral. The bond angles for O(1)\Ni(1)\O(3), O(5)\Ni(1)\O(10), O(10)\Ni(1)\O(11), O(10)\Ni(1)\N(1) and O(5)\Ni(1)\N(1) are 169°, 105°, 63°, 105°, and 87°, respectively. Similar coordination behaviors were observed around other three Ni(II) centers of the complex. These metric values indicate the severity of distortion of the octahedral geometry around the nickel centers. Furthermore, the crystal structure indicates displacement of the nickel centers from their respective equatorial planes by 0.08 Å and 0.11 Å for Ni(1) and Ni(2), respectively. The electronic spectrum of 2 in water displays absorption peaks at 388, 644, 659 and 1022 nm. IR active vibrational bands of metal bound carbonato group that correspond to the E′ mode occur in the 1600–1200 cm−1 region [25]. The ATR-FTIR spectrum of 2 measured in a powder form presents absorption bands at 1578(m), 1527(s), 1444(m), 1392(s), and 1237(m) cm−1, Fig. S6. The strong absorption bands at 1527 and 1392 cm− 1 are assigned to the υasym (CO2) and υsym (CO2) vibrational modes of the carbonate ligands, respectively. The values of these absorption bands are comparable to the values obtained for similar complexes reported in the literature [26–29]. In conclusion, we here described the reaction of the nickel complex with CO2 and its subsequent formation of a unique anti-anti, pseudo-oxo (μ-CO3-κ4O,O′:O,O″) tetra-nickel complex. Our investigations revealed that the in situ synthesized hydroxo-bridged tetranickel complex is the active species involved in the activation of CO2. The crystallization of 3 and research on the activation of CO2 using copper(II) and zinc(II) complexes of H5ccdp ligand are under development.

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