Stabilizing a Ni^II-aqua complex via intramolecular hydrogen bonds: synthesis, structure, and redox properties

Abstract

Hydrogen bonds within the secondary coordination sphere are effective in controlling the chemistry of synthetic metal complexes. Coupling the capacity of hydrogen bonds with those of redox-active ligands offers a promising approach to enhance the functional properties of transition metal complexes. These qualities were successfully illustrated with the [NNN]³⁻pincer ligand N,N′ -(azanediylbis(2,l-phenylene))bis(2,4,6-triisopropyl-benzene-sulfonamido ([ibaps]³⁻) through the preparation of the Ni^II-OH2 complex, [Ni^II(ibaps)(OH2)]⁻. The [ibaps]³⁻ ligand contains two appended sulfonamido groups that support the formation of intramolecular hydrogen bonds. The bulky 2,4,6-triisopropylphenyl rings are necessary to ensure that only one ligand binds to a single metal ion. The molecular structure of the complex shows a square planar N3O primary coordination sphere and two intramolecular hydrogen bonds involving the aqua ligand. Electrochemical measurements in acetonitrile revealed two oxidation events at potentials below that of the ferrocenium/ferrocene couple. Oxidation with 1 equiv of ferrocenium produced the one-electron oxidized species, [Ni(ibaps)(OH2)]. Experimental and computational studies support this assignment.

Introduction

The designs of multidentate ligands are important in stabilizing specific metal oxidation states, promoting redox reactions in solution, and in tuning the reactivity of transition metal ions by influencing the steric and the electronic properties. Additionally, interest in ligands with radical character has increased following their use in the discovery of new types of reactivities.[1] Understanding the interactions between transition metal ions and non-innocent ligands has been crucial in order to design new catalysts.[2][3] Radical ligands are known to be found in several metalloenzymes such as the Fe-containing cytochrome P450s [4][5] as well as Cu-containing galactose oxidase.[6–8] These two classes of biological metal-radical species are formed differently and have distinct properties and stabilities, but share the common feature that the redox-activity of a coordinated ligand is essential for function.[2][9] Many synthetic redox-active ligand frameworks have been developed to improve the functional capabilities of metal complexes: significant improvements are found when these types of ligands are coordinated to 3d metal ions to promote multi-electron transformations.[2,3,10–15] Several reports have shown that redox properties of complexes with phenolate and porphyrin ligands can be further influenced by various factors including solvent, temperature, and the nature of the metal ion and substituents on the ligands. For instance, Ni^II-containing square planar complexes with phenolate ligands [16–18] can undergo a reversible one-electron oxidative process that was assigned to the Ni^III/Ni^II redox couple in coordinating solvent; however, the redox process changes to a phenolate-phenoxyl couple when done in non-coordinating solvents such as CH₂Cl₂.

Another design aspect of redox-active ligands is the incorporation of functional groups to control the secondary coordination sphere around the metal ion(s).[1,19–21] Examples include the pincer ligands of Gilbertson that contain appended amine groups that form intramolecular hydrogen bonds (H-bonds) with external ligands. We have recently introduced a new redox-active ligand ([ibaps]³⁻) that contains a bis(aminophenyl)aminate ([NNN]³⁻) framework and shows rich redox properties when coordinated to Fe and Ga ions.[14,15,21] This ligand also has the potential to control the secondary coordination sphere around the metal ion through the formation of H-bonds involving the two appended sulfonamido groups. In this report, we describe the preparation of [Ni^II(ibaps)(OH2)]⁻ and its structural and redox properties. Our work demonstrates that [ibaps]³⁻ can indeed support intramolecular H-bonds and undergo multi-electron oxidations.

Experimental

General Methods.

The preparations of metal complexes were completed under a dinitrogen atmosphere in a VAC drybox. Solvents were sparged with argon and dried over columns containing Q-5 and molecular sieves. All reagents were purchased from commercial suppliers and used as received unless otherwise noted. Potassium hydride as a 30% suspension in mineral oil was filtered and washed five times each with Et2O and pentane and dried under vacuum. Ferrocenium tetrafluoroborate (FcBF₄) was recrystallized before use. The ligand precursor N,N′-(azanediylbis(2_,l-phenylene))bis(2_,4,6-triisopropyl-benzene-sulfonamide, H₃ibaps) was prepared according to literature procedures.[21][22]

Physical Methods.

Electronic absorption spectra were recorded in a 5 mm cuvette on a Cary 60 UV-vis spectrometer. Negative-mode electrospray ionization mass spectra were collected using a Micromass MS Technologies LCT Premier mass spectrometer. Fourier transform infrared spectra were collected a Thermo Scientific Nicolet iS5 spectrophotometer with an iD5 Attenuated Total Reflectance (ATR) attachment in a dinitrogen filled glovebox. ¹H NMR spectra were recorded as CDCl₃ solutions on a Bruker CRYO500 or GN500 spectrometer and referenced to the residual solvent peak. Elemental analyses were performed on a Perkin-Elmer 2400 CHNS analyzer. Cyclic voltammetric experiments were conducted using a CHI600C electrochemical analyzer under an N₂ or Ar atmosphere with 0.1 M tetrabutylammonium hexafluorophosphate as the supporting electrolyte. A 3.0-mm glassy carbon electrode was used as the working electrode with an AgCl reference electrode and a platinum wire counter electrode. Potentials are referenced to the FeCp₂⁺/FeCp₂ redox couple.

X-ray Crystallographic Methods.

A Bruker SMART or Bruker Kappa APEX II CCD diffractometer was used to collect data. The APEX25 program package was used to determine the unit-cell parameters and for data collection. The raw frame data were processed using SAINT[23] and SADABS[24] to yield the reflection data file. Subsequent calculations were carried out using the SHELXTL[25] program. The analytical scattering factors[26] for neutral atoms were used throughout the analysis. Hydrogen atoms were included using a riding model.

Density Functional Theory.

All calculations were performed using gaussian 09 revision C.01 program.[27] For the geometry optimizations, the B3LYP functional with a 6–31g* basis set was used on all atoms. A CPCM solvent continuum model was used with acetonitrile as the solvent. Geometry optimization on the [Ni(ibaps)(OH2)]⁻species was performed using the XRD coordinates as a starting point. The optimized geometry of [Ni(ibaps)(OH2)]⁻was used initially for the geometry optimization on the [Ni(ibaps)(OH2)] species. Frequency calculations showed no negative frequencies indicating ground state geometries were obtained. TD-DFT calculations on the B3LYP optimized structures were performed using the CAM-B3LYP functional, which is reported to more accurately predict charge transfer transitions.[28] Molecular orbitals, spin density plots, and natural transition orbitals (NTOs) were rendered using the Chemcraft program.[29]

Preparative Methods. (Et4N)[Ni^II(ibaps)OH₂].

A solution of H₃ibaps (0.20 g, 0.28 mmol) in 4 mL of dimethylacetamide (DMA) was treated with three equiv of solid KH (32 mg, 0.84 mmol). The reaction was stirred until gas evolution ceased and the solution was filtered. The NiCl₂.6H₂O (67 mg, 0.28 mmol) salt was then added. After 1 h, Et₄NBr (59 mg, 0.28 mmol) was added to the reaction, which was stirred for an additional two hours before filtering the solution and removing the solvent under vacuum. The crude product was washed with 20 mL of Et₂O to remove DMA and volatiles were removed under reduced pressure. The resulting oil was dissolved in dichloromethane (DCM) and the product was crystallized from vapor diffusion of Et2O into a CH₂Cl₂ solution of the salt. The dark green needle crystals were washed with ether (3 × 10 mL) and filtered to remove KBr to obtain 101 mg (40% yield) of (Et4N)[Ni^II(ibaps)OH2] as a green crystalline powder. ¹H NMR (500 MHz, CD₃CN): δ 1.11 (t, 12H), 1.22 (d, 36H), 2.86 (m, 8H), 3.16 (q, 6H), 6.4 (s, 4H), 6.88 (s, 4H), 7.16 (s, 4H). ESI-MS (5.10⁻⁵ M, CH₃CN, m/z,): Calcd. for (Et4N)[Ni^II(ibaps)OH₂]: 934.5; Found: 934.2, (positive ion mode). Elemental analysis calcd. For C₅₀H₇₆N₄NiO₅S₂.·0.5CH₂Cl₂ (978.45 g mol⁻¹): C, 61.99; H, 7.93; N, 5.73; Found for: C, 61.78; H, 8,36; N, 5.73. FTIR (ATAR, cm⁻¹, selected bands): 3230, 2956, 2927, 2867, 1600, 1570, 1556, 1491, 1480, 1451, 1438, 1418.

Results and Discussion.

Synthesis and Molecular Structure of (Et₄N)[Ni^II(ibaps)(OH₂)].

The salt was prepared via 3-step synthesis (Scheme 1): H₃ibaps was allowed to react with three equiv of KH and the resultant mixture was treated with solid NiCl₂·6H₂O in one portion and stirred for 1 hour. Without isolation, the potassium salt was metathesized with tetraethylammonium bromide to afford (Et₄N)[Ni^II(ibaps)(OH₂)], which was isolated in 40% yield as green crystals after recrystallization via slow diffusion of Et₂O into a solution of the tetraethylammonium salt. FTIR studies of the Ni^II complex, [Ni^II(ibaps)(OH2)]⁻ contained a peak at 3230 cm⁻¹ which is consistent with the υ(OH) of an aqua ligand. For example, the Ni^II-OH₂ complex with the tripodal sulfonamido ligand, N,N′, N″-[2,2′,2″-nitrilotris(ethane-2,l-diyl)]tris(2,4,6-trimethylbenzenesulfonamido, [MST]³⁻) has a υ(OH) = 3259 cm⁻¹. [30] ¹Ή NMR studies indicate that the complex is stable in solution with observable resonances associated with the [ibaps]³⁻ ligand that are at different chemical shifts than that of the free ligand (Figure S1). Moreover, the chemical shifts of these signals suggest that [Ni^II(ibaps)(OH₂)]⁻ is diamagnetic as expected for a Ni^II complex with a square planar ligand field.

Scheme 1.

The molecular structure of (Et₄N)[Niⁿ(ibaps)(OH₂)] was determined by X-ray diffraction methods and found that [Ni^II(ibaps)(OH₂)]⁻ is indeed a mononuclear Ni^II-OH₂ complex in the crystalline phase (Figure 2, Tables 1 and S1). The Ni^II center resides in a slightly distorted square planar coordination geometry based on a τ₄ = 0.01 in which an ideal square planar geometry has a τ₄ = 0.[31] The primary coordination sphere contains a N₃O donor set composed of three deprotonated N-atoms from the [ibaps]³⁻ ligand and a coordinated O-atom from an external water molecule. The Ni1-N1 bond length is 1.833(1) Å which is significantly shorter than the bond distances found for Ni1-N2 and Ni1-N3 which are 1.930(1) and 1.910(1) Å. A relatively short Ni1-N1 bond length of 1.835(2) Å was also found in the related Ni complex that contains an amido-bis(amine) pincer ligand. [32] The Ni1-O1 bond length is found to be 1.908(1) Å which is statistically shorter than the 2.074(2) Å bond length observed for the same bond in [Ni^IIMST(OH₂)]⁻−.[30] Note that the N1-Ni1-O1 and N2-N1-N3 bond angles are 176.02(6) and 169.09(6)°.

Figure 2.

Thermal ellipsoid diagram of [Ni^II(ibaps)(OH₂)]⁻. The ellipsoids are drawn at 50% probability level and only the aqua hydrogen atoms are presented for clarity.

Table 1.

Selected structural parameters for of [Ni(ibaps)(H₂O)]⁻ and a comparison to those calculated from DFT.

Metric	Exp (Å or °)	Calc (Å or °)	|Δ|
Ni1-N1	1.833(1)	1.833	0.001
Ni1-N2	1.929(1)	1.936	0.007
Ni1-N3	1.910(1)	1.921	0.011
Ni1-O1	1.908(1)	1.907	0.002
O1-H1	0.84(3)	0.99	0.15
O1-H2	0.83(3)	0.99	0.16
O1⋯O2	2.532(2)	2.566	0.033
O1⋯O4	2.592(2)	2.611	0.019
H1⋯O2	1.75(3)	1.62	0.13
H2⋯O4	1.86(3)	1.69	0.17
N1-Ni1-O1	176.02(6)	170.51	5.51
N1-Ni1-N3	84.11(6)	84.33	0.22
O1-Ni1-N3	93.05(6)	94.82	1.77
N1-Ni1-N2	85.06(6)	84.78	0.28
O1-Ni1-N2	97.70(6)	96.92	0.78
N3-Ni1-N2	169.09(6)	167.56	1.53
O1-H1-O2	155(3)	158	3
O1-H2-O4	147(3)	152	5

A distinguishing feature of [Ni^II(ibaps)(OH₂)]⁻ is the presence of intramolecular H-bonds involving the coordinated aqua ligand. Both O-H bonds of the aqua ligand form H-bonds to the sulfonamido groups from [ibaps]³⁻ as indicated by the O1⋯O2 and O1⋯O4 distances of 2.532(2) and 2.592(2) Å. In addition, HI and H2 were located from a difference-Fourier map and refined (x,y,z and U_ISO) to give O1-H1 and O1-H2 bond lengths of 0.84(3) and 0.83(3) Å and H1⋯O2 and H2⋯O4 distances of 1.75(3) and 1.86(3) Å. The presence of H-bonds causes the phenyl rings of the sulfonamido groups to be arranged in a syn-orientation on opposite faces of the [ibaps]³⁻ ligand.

Redox Properties of [Ni^IIibaps(OH₂)]⁻.

The electrochemical properties of [Ni^II(ibaps)(OH₂)]⁻ were investigated using cyclic voltammetry in MeCN and showed two reversible one-electron oxidation events at E_1/2 (1) = −0.41 and E_1/2 (2) = −0.035 V (Figure 3). The redox performance changed when the measurement was done in CH₂CL₂: the first oxidation process is reversible with an E′_1/2 = −0.59 V. The other redox event has an E_pa′ = −0.029 V but is not reversible. The loss of reversibility could be caused by interactions with Cl⁻ ions from the solvent (Figure 3). To check this possibility, various amounts of iPr₄NCl were added to the electrochemical cell containing [Ni^II(ibaps)(OH₂)]⁻ in CH₃CN solution with 0.1 M TBABF₆ (Figure 4). The low-potential redox couple showed little change upon addition of the chloride ions and remained reversible. However, the second redox process becomes less reversible with increasing concentration of chloride ions, which supports the premise that this ion influences the second oxidation. Whether the chloride ion binds to the Ni center or displaces the aqua ligand are not known and requires further studies.

Figure 3.

Cyclic voltammograms of [Ni^II(ibaps)(OH₂)]⁻ in MeCN (solid line) and in CH₂Cl₂ (dashed line) referenced to the [Fe^III/^IICp2]^+/0 couple. Conditions: 0.1 M of TBAPF₆, with a glassy carbon working electrode at 100 mV·s⁻¹.

Figure 4.

Cyclic voltammograms of [Ni^II(ibaps)(OH₂)]⁻ in MeCN after addition of 0 (black) 0.2 (red), 0.4 (blue), 0.6 (deep green), 0.8 (pink), 1 equiv. (green) of iPr₄NCl. 0.1M of TBAPF₆, on a glassy carbon electrode at 100 mV.s⁻¹. The CVs are referenced to the [Fe^III/IICp₂]^+/0 couple.

Oxidation and Related Spectral Changes.

The relatively low-potential associated with the first redox couple of [Ni^II(ibaps)(OH₂)]⁻ and its reversibility under various conditions prompted us to investigate the product of this oxidation. The reaction was monitored spectrophotometrically in CH₃CN at room temperature (Figure 5). The absorbance spectrum of [Ni^II(ibaps)(OH₂)]⁻ contained bands at λ_max = 770 nm (ε_M= 280) and λ_max = 470 nm (ε_M = 340) that are similar to features found for other N^II-OH₂ complexes[30] and are assigned to ligand-field transitions. These bands disappear when the complex is allowed to react with 1 equiv of [Fe^IIICp₂]⁺ to produce [Ni(ibaps)(OH₂)] (Scheme 1, Figure 5) that had more intense peaks at λ_max = 920 nm (ε_M = 4160), 790 (sh), 620 (ε_M = 1440), 458 nm (ε_M = 4320), and 360 nm (ε_M = 11600). These optical features are similar to those found in [Fe(ibaps)(bpy)] which has an electronic structure that did not allow assignment of formal oxidation state to the Fe center. The reaction was also probed using electron paramagnetic resonance (EPR) spectroscopy. The diamagnetic [Ni^II(ibaps)(OH₂)]⁻ complex was EPR silent but after oxidation to [Ni(ibaps)(OH₂)] a perpendicular-mode spectrum was observed with g-values at 2.28, 2.23 and 2.14. These signals are consistent with a rhombic species having an S = 1/2 spin ground state [33] but again, from these data we were unable to assign the unpaired spin to either a discrete ligand radical or a metal-centered oxidation.

Figure 5.

Electronic absorbance spectra of [Ni^II(ibaps)(OH₂)]⁻ (—) and after addition of 1 eq of [Fe^IIICp₂]BF₄ (—). Spectra were recorded at room temperature in CH₃CN. Concentrations for [Ni^II(ibaps)(OH₂)]⁻: 1.0 mM and for generation of [Ni(ibaps)(OH₂)]: 0.25mM.

Density Functional Theory.

We have used density functional theory to further examine the electronic structures of [Ni^II(ibaps)(OH₂)]⁻ and [Ni(ibaps)(H₂O)]. In particularly, we wanted to probe if we could locate where the unpaired spin resided in [Ni(ibaps)(H₂O)] that produced the EPR shown in Figure 6. Broken symmetry density functional theory (DFT) calculations were performed on [Ni(ibaps)(OH₂)]⁻ (S = 0) and the one-electron oxidized [Ni(ibaps)(H₂O)] (S = 1/2)species. A comparison between the calculated metal-ligand bond and H-bond distances (that is, O⋯O distances) for [Ni^II(ibaps)(OH₂)]⁻ to those experimentally determined show a mean absolute deviation of 0.012 Å, which indicates excellent agreement between the calculations and experiment (Figure S2, Tables 1 and S1). It is noteworthy that the intramolecular H-bonds between the aqua ligand and the sulfonamido oxygen atoms were maintained in the ground-state optimized structure. The optimized structure for [Ni(ibaps)(H₂O)] showed M-L bond distances of 1.852 Å, 1.949 Å, 1.939 Å, and 1.903 Å for Ni1-N1, Ni1-N2, Ni1-N3, and Ni1-O1 bonds, respectively (Figure 7A, Table S2). These bond distances indicated just a slight increase in M-N bond lengths of on average 0.017 Å and a small decrease in the M-O bond length of 0.004 Å compared to the reduced species. Intramolecular H-bonds were also maintained with O1⋯O2 and O1⋯O4 distances of 2.57 Å and 2.62 Å, respectively, an average increase of 0.006 Å relative to the reduced species.

Figure 6.

EPR spectrum of [Ni^II(ibaps)(OH₂)]-after addition of 1 eq of [Fe^IIICp₂]BF₄. Spectrum was recorded at X-band as a frozen CH₃CN solution at 10 K.

Figure 7.

DFT computed structure (A) and rendered spin density plot (B) for [Ni(ibaps)(H₂O)].

A relative Mulliken spin density analysis for [Ni(ibaps)(H₂O)] was used to qualitatively understand the degree of metal or ligand character for the first oxidation. This determination is indicated on the rendered spin density plots for [Ni(ibaps)(H₂O)] (Figure 7B). The four-coordinate [Ni(ibaps)(H₂O)] has significant spin density delocalized over the [ibaps]³⁻ ligand and minimal spin density of 0.06 on the nickel which suggests the first oxidation is primarily ligand centered. Excited state TD-DFT calculations were also performed on [Ni^II(ibaps)(OH₂)] to help assign the transitions in the electronic absorbance spectra. The calculated wavelengths of the vertical excitations are compared to the λ_max values of the experimentally observed transitions and show good agreement (Figure S3, Table S3). The nature of each transition as determined by the calculations is also indicated (Figures S4 and S5) and shows that those observed in the reduced [Ni(ibaps)(H₂O)]⁻ complex were a result of d-d transitions while those in the oxidized [Ni(ibaps)(H₂O)] species were a result of MLCT transitions at lower energies and intraligand π*-π* transitions at higher energies.

Summary

In this report we demonstrated that the trianionic pincer ligand [ibaps]³⁻ can serve as a redox-active ligand and support H-bonds to an exogenous ligand. The molecular structure of [Ni^II(ibaps)(OH₂)]⁻ revealed two intramolecular H-bonds are formed between a coordinated water molecule and the sulfonamido O-atoms of the [ibaps]³⁻ ligand. Electrochemical measurements indicate that the complex has two oxidative redox events with both processes being reversible in acetonitrile. Oxidation of [Ni^II(ibaps)(OH₂)]⁻ with 1 equiv of ferrocenium produced a species that is tentatively assigned as the one-electron oxidized complex, [Ni(ibaps)(OH₂)] which is paramagnetic and has an electronic absorbance spectrum with strong features throughout the visible region. Computational studies using DFT methods are in agreement with our experiments and suggest that the first oxidation predominately involves the [ibaps]³⁻ ligand and that the intramolecular H-bonds still remain intact upon oxidation.

Figure 1.

The redox-active, H-bonding ligand H₃ibaps.