Article pubs.acs.org/IC
Metal Ion Complexes of N,N′‑Bis(2-Pyridylmethyl)-trans-1,2Diaminocyclohexane-N,N′‑Diacetic Acid, H2bpcd: Cis/Trans Isomerization Equilibria Jan Florián, † Craig C. McLauchlan,‡ Daniel S. Kissel, † Chad C. Eichman, † and Albert W. Herlinger*, † † Department of Chemistry and Biochemistry, Loyola University Chicago, 1032 W Sheridan Road, Chicago, Illinois 60660 United States ‡ Department of Chemistry, Illinois State University, Campus Box 4160, Normal, Illinois 61790-4160 United States S Supporting Information *
ABSTRACT: The synthesis of N,N′-bis(2-pyridylmethyl)trans-1,2-diaminocyclohexane-N,N′-diacetic acid (H2bpcd) and its complexation of Ga(III) and Co(III) are reported. H2bpcd and the metal−bpcd2− complexes, isolated as hexafluorophosphate salts, were characterized by elemental analysis, X-ray crystallography, IR spectroscopy, and 1H and 13 C NMR spectroscopy. [Ga(bpcd)]PF6, [Ga(C22H26N4O4)]PF6, crystallized in the orthorhombic space group Ibca, with a = 13.8975(7) Å, b = 15.0872(7) Å, c = 22.2418(10) Å, and Z = 8. Ga is coordinated in a distorted octahedral geometry provided by a N4O2 donor atom set with trans-monodentate acetate groups and cis-2-pyridylmethyl N atoms, i.e., the trans-O,O isomer. The diamagnetic [Co(bpcd)]PF6, [Co(C22H26N4O4)]PF6, also crystallized from solution in the Ibca space group as the trans-O,O isomer. The 1H and 13C assignments for H2bpcd and metal−bpcd2− complexes were made on the basis of 2D COSY and HSQC experiments, which were used to differentiate among three possible isomers, i.e., one cis (C1 symmetry) and two trans (C2 symmetry). NMR results indicate that the [Ga(bpcd)]+, [Co(bpcd)]+, and cis-O,O, cis-Npy,Npy-[Ga(bppd)]+ cations, where bppd2− stands for bis(2pyridylmethyl)-1,3-diaminopropane diacetate, are present in solution as isomers with the same symmetry as observed in the solid state. The crystallographic data and the dramatic shift that occurs in the position of the cis/trans isomerization equilibria for the [Ga(bpad)]+ cations simply by increasing the number of bridging CH2 groups in the ligand’s diamine backbone represent a unique opportunity to assess the accuracy of modern computational methods. The performance of several local density functionals using a pseudopotential-based SDD basis set was compared with the more rigorous HF and MP2 ab initio calculations. The SVWN5 and SV5LYP functionals provide significantly better Ga−O and Ga−N distances than the HF method or the nonlocal BLYP functional. However, to provide proper isomerization energies the pseudopotential-DFT calculations must be augmented by MP2 single-point energies and calculations of solvation free energies.
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INTRODUCTION
Substituted diaminoacetic acids are of interest as metal ion complexation agents for a wide variety of applications.1−5 When coordinated to a radioactive group 13 nuclide like gallium-67 or gallium-68, for example, the complexes potentially could find use in nuclear medicine.2,6−9 N,N′-Bis(2-pyridylmethyl)-trans1,2-diaminocyclohexane-N,N′-diacetic acid, H2bpcd, is a symmetrically bis(2-pyridylmethyl)-substituted diaminodiacetic acid featuring a “rigid” preoriented chiral trans-diaminocyclohexane backbone. H2bpcd employs two acetic acid functionalities as well as two softer 2-pyridylmethyl donor groups for metal ion complexation, Figure 1 A. Incorporation of softer aromatic nitrogen atoms into a ligand’s structure is designed to increase the ligand’s selectivity for softer metal ions, while acetic acid groups provide enhanced stability. The restricted rotation about the C−C bonds in the cyclohexane ring constrains the amine nitrogen atoms into a © 2015 American Chemical Society
Figure 1. Two 2-pyridylmethyl-substituted diaminoacetic acids H2bpad: H2bpcd (A) and H2bppd (B), where bppd2− stands for N,N′-bis(2-pyridylmethyl)-1,3-diaminopropane-N,N′-diacetate.
conformation that favorably preorients the donor groups for metal ion complexation.10−12 The cyclohexane ring of the diamine backbone adopts a chair conformation and positions the amine N atoms of the ligand backbone in opposing spatial Received: August 2, 2015 Published: October 19, 2015 10361
DOI: 10.1021/acs.inorgchem.5b01586 Inorg. Chem. 2015, 54, 10361−10370
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Inorganic Chemistry orientations. Both N atoms are in equatorial positions on the ring; however, one nitrogen is oriented “up” above the mean plane of the cyclohexane ring while the other nitrogen is oriented “down” below the plane. This orientation positions the acetate functionalities in a trans position favorable for complexation to a metal ion in a pseudo-octahedral geometry. This preorganization of a ligand can reduce the entropic penalty for metal ion complexation and thereby provide improved metal−ligand complex stability.10−12 Further, the close proximity of the diamine nitrogen atoms in the transdiaminocyclohexane backbone of H2bpcd maximizes the number of five-membered chelate rings capable of forming upon metal ion complexation. The formation of five-membered chelate rings has been shown to be more favorable for larger metal ions than for smaller metal ions. In contrast, the longer 1,3-diaminopropane backbone in H2bppd (B in Figure 1) provides greater ligand flexibility and reach compared to those of H2bpcd as well as allows the formation of a six-membered chelate ring, which is more favorable for smaller metal ions.13 The synthesis of H2bpcd was previously reported as part of a study of alkene oxidation catalysis by iron(II) complexes of substituted N2Py2 ligands with acetic acid moieties.14 As part of the study, a structure was reported for a heptacoordinate [FeII(H2bpcd)(C3H6O)]2+ complex ion that was isolated as a perchlorate salt. In the structure, the FeII−H2bpcd complex ion has a distorted pentagonal bipyramidal geometry provided by a N4O3 donor atom set. The acetic acid moieties of the H2bpcd ligand, which are fully protonated, are coordinated through their carbonyl O atom as is the acetone molecule. The three O atoms along with the two amine N atoms (Nam) reside in the equatorial plane of the pentagonal bipyramid while the two pyridyl N atoms (Npy) occupy axial positions.14 This geometry most likely arises from the relatively weak binding of fully protonated carboxylate groups to a relatively large seven coordinate FeII and results in longer Fe−O and Fe−N bonds than typically observed. In the present case, H2bpcd was synthesized using simple starting reagents following a facile, two-step procedure previously described in detail for H2bppd.15 This procedure was employed in the synthesis of the H2bpcd sample used in the preparation of [Co(bpcd)]PF6, which is nearly isostructural with the [Ga(bpcd)]PF6 salt reported here. The Co atom in the CoIII-bpcd2− cation has a distorted octahedral geometry provided by a N4O2 donor atom set.16 The acetate groups in the cation are coordinated to Co in a trans configuration whereas the pyridyl N atoms are cis with respect to each other, i.e., the trans-O,O isomer, Figure 2.16 This investigation is devoted to exploring the structural and complexation properties of H2bpcd. The objectives of the investigation were (1) to determine the energetics of the isomerization equilibria for [M(bpad)]+ complexes in solution; (2) to understand the structural features of bpad2− ligands that give rise to the cis-O,O, cis-Npy,Npy isomer for [Ga(bppd)]+, but not for [Ga(bpcd)]+ or other smaller metal ions like CoIII; and (3) to assess computational methodologies capable of providing accurate structural and selectivity information for [M(bpcd)]+ complexes with larger metals (Z > 36).
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Figure 2. Isomerization equilibria for the three possible isomers for a [Ga(bpcd)]+ cation. The trans-O,O isomer lies at the lowest free energy for [Ga(bpcd)]+, but the relative free energies of the cis-O,O, cis-NpyNpy, and trans-O,O isomers are reversed for [Ga(bppd)]+. The double-headed arrows in the figure indicate the open X−M−Y angle in Ga’s coordination sphere formed by the donor atoms trans to the N atoms of the diamine. This angle is central to understanding the results of the calculations discussed below. anol, 2-pyridinecarboxaldyde, bromoacetic acid, 30% by weight hydrogen peroxide, chloroform-d (CDCl3), acetonitrile-d3 (CD3CN), dimethyl sulfoxide (DMSO-d6), sodium borohydride, and deuterium oxide (D2O), obtained from Sigma-Aldrich Chemical Co., were used without further purification. Dowex 50W-X8 (100−200 mesh) cation exchange resin was obtained from Fisher Scientific and prepared as previously described.15 Methods. Combustion analyses were done by Galbraith Laboratories, Inc., Knoxville, TN, using GLI Procedure ME-14. Equivalent weight titrations with 0.02 M NaOH, standardized against KHP, were conducted in a 50 mL glass flow through cell using phenolphthalein as the indicator. Magnetic moments of the complexes were determined using a Johnson Matthey magnetic susceptibility balance. The 1H NMR spectra for N,N′-bis(2-pyridylmethyl)-1,2-diaminocyclohexane, bpmdac, were recorded in CDCl3 on a Varian Unity INOVA 300 MHz spectrometer. The other 1H and 13C NMR spectra as well as the 2D NMR experiments were recorded at 25 °C on a Varian Unity INOVA 500 MHz spectrometer. The 1H NMR spectra for H2bpcd·2HCl were recorded at 50 °C. 1H and 13C NMR spectra recorded in CDCl3, acetonitrile-d3, and DMSO-d6 were referenced internally to the residual solvent proton resonances.17 1H and 13C NMR spectra recorded for samples in D2O were referenced to internal and/or external DSS. Infrared spectra were obtained on a Thermo Nicolet Nexus 470 FTIR spectrometer calibrated in the 4000−400 cm−1 spectral range using polystyrene. Samples were prepared as fluorolube mulls and run with air as the background. The spectra were checked as Nujol mulls and KBr pellets. The symmetric carboxylate stretching mode, νs(COO−), is most easily identified in a spectrum run as a KBr pellet. Routine spectra were recorded collecting 32 scans at 4 cm−1 resolution. Synthesis of Ligands. The syntheses of the 2-pyridylmethylsubstituted parent diamine bpmdac and its subsequent elaboration to the diacetic acid were achieved using the procedures reported previously for the 1,3-diaminopropane analogues, bpmdap and H2bppd.15 N,N′-Bis(2-pyridylmethyl)-1,2-diaminocyclohexane, bpmdac. Yield: 4.1 g (14 mmol, 91%). 1H NMR (ppm, CDCl3): 1.05 (p, b, 2H, NHCH(C4H8)CHNH, J = 8.4 Hz), 1.21 (t, 2H, NHCH(C4H8)CHNH, J = 10.1 Hz), 1.70 (d, b, 2H, NHCH(C4H8)CHNH, J = 8.1 Hz), 2.12 (d, 2H, NHCH(C4H8)CHNH, J = 13.2 Hz), 2.30 (t, 2H, NHCH(C4H8)CHNH, J = 4.5 Hz), 2.53 (s, b, 2H, NH(C6H10)NH), 3.82, 4.01 (AB q, 4H, NHCH2py, J = 14.1 Hz), 7.11 (td, 2H, NCCHCHCHCH, J = 6.15 Hz, 1.2 Hz), 7.37 (d, 2H, NCCHCHCHCH, J = 7.8 Hz), 7.60 (td, 2H, NCCHCHCHCH, J = 7.7 Hz, 1.8 Hz), and 8.51 (d, 2H, NCCHCHCHCH, J = 6.6 Hz). N,N′-Bis(2-pyridylmethyl)-1,2-diaminocyclohexane-N,N′-diacetic Acid Dihydrochloride, H2bpcd·2HCl. H2bpcd was synthesized from a solution of bpmdac (4.1 g, 14 mmol) as previously described15 using excess bromoacetic acid (4.38 g, 31.5 mmol). The
EXPERIMENTAL SECTION
Reagents. Reagent grade cobalt chloride hexahydrate and gallium nitrate hydrate, obtained from Fisher Scientific, were used as received. Reagent grade sodium hexafluorophosphate, potassium hydrogen phthalate (KHP), trans-1,2-diaminocyclohexane, anyhydrous meth10362
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Inorganic Chemistry
for absorption using the SAINT+ Software Suite.19 Structure solutions were obtained by direct methods and were refined on F2 with the use of full-matrix least-squares techniques.20 All non-hydrogen atoms were refined anisotropically, and hydrogen atoms were refined with a riding model. Selected crystallographic parameters are shown in Table 1. More extensive crystallographic details are included in Supporting Information.
pure H2bpcd product was eluted as the dihydrochloride salt using 6 column volumes of 4.0 M HCl on a Dowex 50W-X8 cation exchange column (12 g, 20 mm × 38 cm, resin height 13 cm). H2bpcd elutes more rapidly at higher HCl concentrations, but a lower purity product may be obtained. The diacetic acid was obtained as a white, hygroscopic solid by evaporation of the 4 M HCl eluent under reduced pressure at 60 °C and dried in vacuo overnight at 60 °C. Yield: 4.4 g (9.0 mmol, 64%). Anal. obsd (calcd) for C22H28N4O4· 2HCl: C, 54.33 (54.44); H, 6.38 (6.23); N, 11.70 (11.54). 1H NMR (ppm, D2O, 50 °C): 1.55 (t, 2H, NCH(C4H8)CHN, J = 9.5 Hz), 1.75 (d, 2H, NCH(C4H8)CHN, J = 8.5 Hz), 2.09 (d, 2H, NCH(C4H8)CHN, J = 9.0 Hz), 2.46 (d, 2H, NCH(C4H8)CHN, J = 13.0 Hz), 3.63 (m, 2H, NCH(C4H8)CHN, 3.2 Hz), 3.91, 4.10 (AB q, 4H, NCH2py, JAB = 18.0 Hz), 4.67 (s, 4H, NCH2COOH), 8.11 (t, 2H, NCCHCHCHCH, J = 6.5 Hz), 8.23 (d, 2H, NCCHCHCHCH, J = 8.0 Hz), 8.62 (t, 2H, NCCHCHCHCH, J = 7.8 Hz), and 8.87 (d, 2H, NCCHCHCHCH, J = 4.5 Hz). 13C NMR (ppm, D2O): 26.27, 26.66, 53.51, 57.16, 66.45, 129.54, 130.49, 146.35, 148.46, 151.27, and 175.58. IR (ν(cm−1), fluorolube): 3400 (m, b, OH and NH+ str), 3251 (m, CH str), 2935 (s, CH2 str), 2862 (s, CH2 str), 1712 (s, CO str), 1617 (s, NH+ def), 1542 (m, py str), 1449 (m, CH2 def), 1402 (m, CO str). Equiv wt: obsd 123 g/eqH+; calcd 121 g/ eqH+. Synthesis of Metal Complexes. The [Ga(bpcd)]PF6 and [Co(bpcd)]PF6 salts were synthesized following procedures reported previously for their 1,3-diaminopropane analogues.15,18 Na2bpcd. H2bpcd·2HCl (43.7 mg, 0.09 mmol) was dissolved in 5 mL of deionized water and neutralized with NaOH (14.4 mg in 225 μL of H2O, 0.36 mmol). The solution was allowed to stir for 30 min, and a white solid was collected by evaporation at 60 °C under reduced pressure. Yield: 37.1 mg, (0.086 mmol, 95%). IR (ν(cm−1), fluorolube): 3356 (s, b, OH str), 2923 (m, CH2 str), 2851 (m, CH2 str), 1590 (s, COO− str), 1433 (m, CH2 def), 1408 (m, COO− str). [Co(bpcd)]PF6. Yield: 119 mg (0.2 mmol), 40%. Anal. obsd (calcd) for CoC22H26N4O4PF6: C, 42.56 (43.00); H, 3.85 (4.26); N 8.94 (9.11). Magn susc μeff = 0.0 μB. 1H NMR (ppm, acetonitrile-d3): 1.07 (s, 2H, NCH(C4H8)CHN), 1.18 (s, 2H, NCH(C4H8)CHN), 1.62 (s, 2H, NCH(C4H8)CHN), 1.79 (s, 2H, NCH(C4H8)CHN), 2.85, 3.90 (AX q, 4H, NCH2py, J = 18.3 Hz), 3.61 (s, 2H, NCH(C4H8)CHN), 4.07, 4.57 (AX q, 4H, NCH2COO, J = 15.0 Hz), 7.48 (d, 2H, NCCHCHCHCH, J = 8.0 Hz), 7.60 (t, 2H, NCCHCHCHCH, J = 7.6 Hz), 8.01 (t, 2H, NCCHCHCHCH, J = 7.0 Hz), and 8.65 (d, 2H, NCCHCHCHCH, J = 5.5 Hz). 1H NMR (ppm, D2O): 1.42 (d, 2H, NCH(C4H8)CHN, J = 9.5), 1.97 (d, 2H, NCH(C4H8)CHN, J = 8.3), 2.11 (d, 2H, NCH(C4H8)CHN, J = 10.0), 2.41 (d, 2H, NCH(C4H8)CHN, J = 12.5), 4.08 (m, 2H, NCH(C4H8)CHN, J = 9.0), 4.75−4.93 (m, 8H, NCH2py and NCH2COO), 7.83 (d, 2H, NCCHCHCHCH, J = 8.0 Hz), 7.93 (t, 2H, NCCHCHCHCH, J = 6.5 Hz), 8.32 (t, 2H, NCCHCHCHCH, J = 7.8 Hz), and 9.03 (d, 2H, NCCHCHCHCH, J = 5.5 Hz). IR (ν(cm‑1), fluorolube): 3065 (m, CH aryl str), 2936 (m, CH2 str), 2870 (m, CH2 str), 1683 (vs, COO− str), 1609 (m, py str), 1478 (w, py str), 1446 (m, CH2 def), 1346 (s, COO− str). [Ga(bpcd)]PF6. Yield: 161 mg (0.38 mmol, 84%). Anal. obsd (calcd) for GaC22H26N4O4PF6: C, 42.65 (42.27); H, 3.98 (4.19); N, 8.69 (8.96). Magn susc μeff = 0.0 μB. 1H NMR (ppm, DMSO-d6): 0.53 (s, b, 2H, NCH(C4H8)CHN), 0.89 (s, b, 4H, NCH(C4H8)CHN), 1.13 (s, b, 2H, NCH(C4H8)CHN), 2.41 (s, 2H, NCH(C4H8)CHN), 2.30, 3.49 (AX q, 4H, NCH2py, J = 18.0 Hz), 3.17, 3.92 (AX q, 4H, NCH2COO, J = 15.3 Hz), 7.02 (t, 4H, NCCHCHCHCH, J = 9.0 Hz), 7.55 (s, b, 2H, NCCHCHCHCH), and 8.26 (s, 2H, NCCHCHCHCH). 13C NMR (ppm, DMSO-d6): 22.67, 23.92, 52.30, 59.412, 63.42, 126.19, 126.38, 142.85, 146.70, 152.15, and 170.39. IR (ν(cm−1), fluorolube): 2939 (m, CH2 str), 2868 (m, CH2 str), 1670 (s, COO− str), 1612 (m, py str), 1571 (w, py str), 1488 (m, py str), 1445 (m, CH2 def), 1342 (m, COO−). X-ray Crystallography. Intensity data were collected for single crystals of [Ga(bpcd)]PF6 at −173 °C on a Bruker SMART Apex 2 diffractometer equipped with a CCD area detector using graphite monochromated Mo Kα radiation. Data were reduced and corrected
Table 1. Crystallographic Parameters for [Ga(bpcd)]PF6 empirical formula moiety formula fw T, K λ (Å) cryst syst space group a (Å) b (Å) c (Å) V (Å3) Z Dcalcd (Mg m−3) μ(mm−1) F(000) R1 [I > 2σ(I)]a wR2 [I > 2σ(I)]b GOF on F2
C22H26F6GaN4O4P C22H26GaN4O4, F6P 625.16 100(2) 0.710 73 orthorhombic Ibca 13.8975 (7) 15.0872 (7) 22.2418 (10) 4663.5(4) 8 1.781 1.34 2544 0.022 0.060 1.03
a R1 = ∑||Fo| − |Fc||/∑|Fo|. bwR2 = {∑[w(Fo2 − Fc2)2/∑w(Fo2)2]}1/2; w = 1/[σ2(Fo2) + (0.0325P)2 + 5.066P], where P = (Fo2 + 2Fc2)/3.
Quantum Mechanical Calculations. The geometries for each of the three possible geometric isomers for [Ga(bpcd)]+ and [Ga(bppd)]+ were optimized in the gas-phase using Hartree−Fock (HF), density functional B-LYP, and S-V5LYP methods.21−24 The HF geometries served as a starting point for DFT geometry optimizations. The 6-31G*, SDD, and SDD(d) basis sets were used. The SDD basis set combines Stuttgart effective core potential for core electrons with Dunning’s D95 basis set for valence electrons,25 which was augmented in the SDD(d) basis set by additional d-polarization functions on C, N, and O atoms with exponents 0.75, 0.80, and 0.85, respectively. Additional single-point calculations were carried out at the MP2 level.21 The solvation free energies were calculated using the polarized continuum model (PCM) and the dielectric constant of water (ε = 78.5).26 All quantum mechanical calculations were carried out using the Gaussian 09 program.27
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RESULTS Synthesis. H2bpcd was previously prepared in a study of the efficiency of alkene oxidation catalysis by FeII complexes of substituted N2Py2 ligands with acetic acid moieties.14 In this study, H2bpcd was synthesized by a two-step route involving alkylation of N,N′-bis(2-pyridylmethyl)-1,2-diaminocyclohexane (bpmdac) by tert-butyl 2-bromoacetate followed by quantitative hydrolysis by trifluoroacetic acid and characterized by 13C NMR spectroscopy. The total reported overall yield for the two steps was 41%. Complexes of the fully protonated H2bpcd ligand were synthesized by a 1:1 reaction with Fe(ClO4)2. In the present case, H2bpcd was synthesized by a facile, twostep procedure developed for the synthesis of H2bppd using simple starting reagents.15 Step 1 is a sequential one-pot preparation of the 2-pyridylmethyl-substituted diamine. In this step, the parent diamine, trans-1,2-diaminocyclohexane, reacts with 2-pyridinecarboxaldehyde to form a diimine, which 10363
DOI: 10.1021/acs.inorgchem.5b01586 Inorg. Chem. 2015, 54, 10361−10370
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Inorganic Chemistry reduces nearly quantitatively with NaBH4 to bpmdap. In step 2, the substituted diamine is elaborated directly to the diacetic acid H2bpcd by reaction with 2 equiv of bromoacetic acid, a strong alkylating agent. Ion exchange chromatography is used to remove the sodium halide byproducts of the alkylation reaction that results in H2bpcd being isolated as a hydrochloride, which is hygroscopic. H2bpcd was characterized by elemental analysis, equivalent weight titration, and 1H and 13C NMR spectroscopy as well as infrared spectroscopy. Typical yields for this preparation are ∼60%. Formation of a [M(bpcd)]+ cation typically involves reaction of a MIII nitrate in anhydrous methanol with the fully deprotonated bpcd2− ligand. The complex ion is isolated from solution as a salt by precipitation with a large anion such as PF6−. The isolated [Ga(bpcd)]PF6 and [Co(bpcd)]PF6 compounds are diamagnetic. The yield of a given [M(bpad)]PF6 salt can vary widely depending upon the metal (30−80%). X-ray Structure. X-ray quality crystals of [Ga(bpcd)]PF6 were grown by slow evaporation of a supersaturated D2O solution overnight at room temperature. The compound crystallized in the orthorhombic space group Ibca with onehalf molecule in the asymmetric unit and eight asymmetric units (four formula units) per unit cell. The two halves of the cation are related via 2-fold rotation. The cation and anion are well-resolved in the structure. Relevant crystallographic information is shown in Table 1 and in the Supporting Information. Displacement ellipsoid plots of the [Ga(bpcd)]+ and [Ga(bppd)]+ cations are shown in Figure 3. The Ga atom in [Ga(bpcd)]+ is surrounded by the bpcd2− ligand in an approximately octahedral environment of general formula GaN4O2. The coordination environment can be described as the trans-O,O configuration with the acetate groups coordinated in a monodentate fashion. Selected bond distances and angles are shown in Table 2. Infrared Spectra. The infrared spectra of the isolated GaIII and Co III bpcd2− compounds are very similar. These compounds exhibit absorption bands in regions characteristic of aromatic and aliphatic stretching, bending, and deformation modes; carboxylate stretching modes; and frequencies associated with the PF6− anion at ∼915, 840, and 555 cm−1.28 The frequencies and tentative assignments of the absorption bands in the 3500−1300 cm−1 region of the IR spectrum are given in the Experimental Section with the characterization data for each compound. The antisymmetric COO−, ν a(COO−), and symmetric COO−, νs(COO−), stretching bands have been assigned empirically by comparison with analogous metal-bppd2− cations15 as well as a variety of metal-acetato and metal-aminoacetato complexes.29,30 The carboxylate stretching frequencies along with the difference between the antisymmetric and symmetric stretching bands, Δν = νa(COO−) − νs(COO−), for H2bpcd and the isolated bppd2−compounds are given in Table 3. NMR Spectroscopy. The 1H and 13C assignments for bpmdac, H2bpcd, and the trivalent metal-bppd2− complexes were made on the basis of 1D and 2D COSY and HSQC experiments. The COSY and HSQC experiments established the detected 1H−1H and 1H−13C correlations of the resonances observed in the 1H and 13C spectra.31 The NMR data obtained for bpmdac, H2bpcd, and the GaIII and CoIII complexes are presented in the Experimental Section. The spectra for H2bpcd, [Ga(bpcd)]+, and [Co(bpcd)]+ show the expected number of resonances with splitting patterns, intensities, and chemical shifts characteristic of a diaminediacetato ligand bearing 2-
Figure 3. Displacement ellipsoid plots for [Ga(bpcd)]+ (a) and [Ga(bppd)]+ (b) Kissel et al.15 Hydrogen atoms are shown as spheres of arbitrary size. i. Symmetry code: 1 − x, 1/2 − y, z.
Table 2. Selected Bond Distances (Å) and Angles (deg) for [Ga(bpcd)][PF6]a Ga1O1 Ga1N1 Ga1N2 O1Ga1O1i O1Ga1N1 O1Ga1N2 O1Ga1N1i O1Ga1N2i a
1.9343 (9) 2.0575(10) 2.0866 (10) 176.556 (5) 91.19 (4) 85.36 (4) 86.96 (4) 97.19 (4)
C8−O1 C8O2
1.3025 (15) 1.2151 (15)
N1Ga1N1i N2Ga1N2i N1Ga1N2 O2C8O1 C8O1Ga1
114.95 (6) 85.30 (5) 80.51 (4) 125.09 (12) 115.26 (8)
Symmetry code i: 1 − x, 1/2 − y, z.
Table 3. Carboxylate Stretching Frequencies and Δν Values for H2bpcd, Na2bpcd, [Co(bpcd)]PF6, and [Ga(bpcd)]PF6 compd
νa(COO−)
νs(COO−)
Δν (cm−1)
H2bpcd Na2bpcd [Co(bpcd)]PF6 [Ga(bpcd)]PF6
1712 1590 1683 1670
1402 1408 1346 1342
310 182 337 328
methylpyridyl functionalities and a trans-1,2-diaminocyclohexane backbone. There are 11 13C and 11 1H NMR resonances expected for a species with C2 symmetry; i.e., 5 resonances at high field from protons of the disubstituted cyclohexane ring since the axial and equatorial methylene ring protons are nonequivalent, 2 resonances at intermediate field strength from 10364
DOI: 10.1021/acs.inorgchem.5b01586 Inorg. Chem. 2015, 54, 10361−10370
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Inorganic Chemistry
Hz) ratio ΔνAB/J of 5.4. The 1H NMR spectrum for the parent 2-pyridylmethyl-substituted diamine bpmdac is shown in Supporting Information, Figure S4. The 1H NMR spectra for the [Co(bpcd)]+ cation is similar to that of the pro-ligand. The 1H spectrum for [Co(bpcd)]PF6 in CD3CN shows 11 resonances consistent with a solution species of C2 symmetry (Supporting Information Figure S5, top). The signals centered at 2.85 and 3.90 ppm (ΔνAB/J = 28.7), and 4.07 and 4.57 ppm (ΔνAB/J = 16.8), are AX quartets. The spectrum of [Co(bpcd)]PF6 in D2O clearly shows 9 resonances with two additional signals, the AX quartets, that are partially obscured by the solvent (Figure S5, bottom). There are 11 13C and 9 1H resonances observed in the NMR spectra for [Ga(bpcd)]PF6 in DMSO (Supporting Information Figure S6). The decrease from 11 to 9 observed 1H signals for [Ga(bpd)]+ results from overlapping signals in the aromatic region and in the high field methylene region. Resonances for the H(2) and H(4) protons of the pyridine ring overlap as do resonances for two CH2 groups in the cyclohexane ring (for labeling, see Figure 3a). These signals, which have relative intensity 4, appear at 7.02 and 0.89 ppm, respectively. The signals for the methyne protons in the cyclohexane ring and a portion of the 2-pyridylmethyl AX quartet at 2.41 and 2.30 ppm, respectively, are obscured because of their close proximity to the DMSO-d6 solvent signal. Quantum Mechanical Calculations. The relative stability of the trans-O,O and cis-O,O, cis-Npy,Npy geometric isomers of [Ga(bpcd)]+ and [Ga(bppd)]+ were correctly predicted by a composite quantum mechanical approach. In this approach, the free energy differences in aqueous solution were approximated by the sum of the relative gas-phase MP2/6-31G* energies (ΔEgas,MP2 in Tables 4 and 5) and solvation free energies (ΔΔGsolv in Tables 4 and 5),32 evaluated at the gas-phase HF/ 6-31G* geometries of the cations. The calculations revealed that the cis-O,O, cis-Npy,Npy isomer is unfavorable for [Ga(bpcd)]+ by ΔGMP2 in solution by 3.9 kcal/mol, Table 4, whereas this isomer becomes more stable by 0.4 kcal/mol with the bppd2− ligand, Table 5. The trans-Npy,Npy isomers of [Ga(bpcd)]+ and [Ga(bppd)]+ are both significantly destabilized in aqueous solution relative to the lowest free energy isomer; i.e., ΔGMP2 in solution for trans-Npy,Npy-[Ga(bpcd)]+ and trans-Npy,Npy-[Ga(bppd)]+ are 5.2 and 3.2 kcal/mol,
pendant arm methylene groups, and 4 resonances at low field from the aromatic protons of the pyridine rings, see Figure 4
Figure 4. 13C NMR spectrum of H2bpcd in D2O at 25 °C (top), with the * indicating internal DSS. 1H NMR spectrum of H2bpcd in D2O at 50 °C (bottom), with the * indicating HOD or impurity. The signals at 3.91 and 4.10 ppm are the AB quartet (ΔνAB/J = 5.4) with relative intensity 4, which arises from the diastereotopic NCH2py protons.
for H2bpcd. The resonances in the 1H NMR spectrum of H2bpcd in D2O at 50 °C appear at decreasing field strength as follows: H(11) a triplet, H(10) a doublet, H(11) a doublet, H(10) a doublet, H(9) a multiplet, H(6) an AB-quartet, H(7) a singlet, H(2) a triplet, H(4) a doublet, H(3) a triplet, H(1) a doublet (for labeling, see Figure 3a). The AB-quartet for H2bpcd shown at 3.9, 4.10 ppm in Figure 4 with relative intensity 2:3:3:2 is consistent for an AB quartet with a chemical shift difference (ΔνAB = 98 Hz) to coupling constant (J = 18
Table 4. Calculated and Observed Bond Distances (Å) and Angles (deg) for the trans-O,O-[Ga(bpcd)]+ Cationa method
HF
HF
BLYP
SVW5
SV5LYP
SV5LYP
basis set Ga1−O1 Ga1−N1 Ga1−N2 N1−Ga−N1c N2−Ga−N2 % errord ΔEgas ΔEgas,MP2e ΔΔGsolvf ΔG in solutiong ΔGMP2 in solutionh
6-31G* 1.865 2.055 2.118 116.0 84.6 1.8 13.1 10.5 −6.6 6.5 3.9
SDD 1.889 2.075 2.158 117.1 84.7 2.1 12.0 9.0 −7.3 4.7 1.7
SDD 1.962 2.120 2.170 115.7 85.4 2.4 10.4 9.5 −6.1 4.3 3.4
SDD 1.957 2.062 2.128 114.8 85.6 1.1 10.2 8.6 −6.8 3.4 1.8
SDD 1.967 2.049 2.118 114.4 85.9 1.1 9.9 8.1 −7.4 2.5 0.7
SDD(d) 1.957 2.074 2.116 116.4 85.1 1.1 7.7 7.0 −6.0 1.7 1.0
obsdb 1.934 2.057 2.087 115.0 85.3
a For atom numbering see Figure 3a. Stability (kcal/mol) of the cis-O,O, cis-Npy,Npy isomer relative to the trans-O,O isomer. bThis work. cX−M−Y describes the open angle in Ga’s coordination sphere formed by donor atoms trans to the N atoms of the diamine, where X = Y = Npy, see Figure 2. d 100((∑((x − x0i)/x0i)2)/N)1/2, where x0 and x correspond, respectively, to a subset of X-ray and calculated geometric parameters shown in this table. eSingle-point MP2 calculations. fSolvation free energy, i.e., free energy for the transfer of 1 M solute from the gas-phase to aqueous solution. g ΔG = ΔEgas + ΔΔGsolv. hΔGMP2 = ΔEgas, MP2 + ΔΔGsolv.
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Table 5. Calculated and Observed Bond Distances (Å) and Angles (deg) for the cis-O,O, cis-Npy,Npy-[Ga(bppd)]+ Cationa method
HF
HF
BLYP
SVWN5
SV5LYP
SV5LYP
basis set Ga−Oac1 Ga−Oac2 Ga−Npy1 Ga−Npy2 Ga−Nam1 Ga−Nam2 Npy−Ga−Oacc Nam−Ga−Nam % errord ΔEgas ΔEgas,MP2e ΔGsolvf ΔG in solutiong ΔGMP2 in solutionh
6-31G* 1.879 1.870 2.025 2.096 2.118 2.148 97.9 96.1 2.0 6.8 4.5 −4.9 1.9 −0.4
SDD 1.895 1.860 2.027 2.188 2.155 2.193 100.0 95.4 2.6 6.0 3.1 −4.0 2.0 −0.9
SDD 1.960 1.924 2.075 2.230 2.170 2.208 97.8 95.8 3.2 5.6 3.7 −4.6 1.0 −0.9
SDD 1.950 1.908 2.032 2.158 2.138 2.152 96.8 96.3 1.8 5.3 2.9 −3.6 1.7 −0.7
SDD 1.952 1.908 2.020 2.144 2.128 2.138 96.1 96.4 1.7 4.6 2.8 −3.6 1.0 −0.8
SDD(d) 1.942 1.898 2.046 2.183 2.130 2.137 97.0 95.2 1.8 3.3 1.7 −3.0 0.3 −1.3
obsdb 1.946 1.896 2.016 2.138 2.067 2.106 99.4 96.5
a For atom numbering see Figure 3b. Stability (kcal/mol) of the cis-O,O, cis-Npy,Npy isomer relative to the trans-O,O isomer. bKissel et al.15 cX−M− Y describes the open angle in Ga’s coordination sphere formed by donor atoms trans to the N atoms of the diamine, where X = Npy and Y = Oac, see Figure 2. d100((∑((x − x0i)/x0i)2)/N)1/2, where x0 and x correspond to a subset of X-ray and calculated geometric parameters shown in this table. e Single-point MP2 calculations. fSolvation free energy, i.e., free energy for the transfer of 1 M solute from the gas-phase to aqueous solution. gΔG = ΔEgas + ΔΔGsolv. hΔGMP2 = ΔEgas, MP2 + ΔΔGsolv.
isomerization free energies for the [Ga(bpcd)]+ and [Ga(bppd)]+ cations were obtained. The HF/SSD method, however, significantly overestimates the Ga−Npy and Ga−Nam bond lengths. Consequently, the performances of several density functional methods vis-à-vis the geometry and the isomerization stability of [Ga(bpcd)]+ and [Ga(bppd)]+ were also examined, Tables 4 and 5. Use of BLYP, a popular nonlocal pure DFT functional,22,24 further worsened the agreement with experimental distances and angles involving Ga, whereas the more economical SVWN5 local functional resulted in a significant improvement of the predicted Ga−O and Ga−N distances. The use of the more sophisticated SV5LYP functional, a pure DFT method that employs Slater’s local spin density exchange functional and the V5LYP correlation functional that combines VWN5 local and LYP nonlocal functionals,23,24 did not lead to a notable improvement of the calculated geometry around the metal. Similarly, augmenting the SDD basis function for the SV5LYP calculations by an additional set of d-polarization functions on C, N, and O atoms did not provide notable improvement over lower level SVWN5/SDD or SV5LYP/SDD geometries. The computational methods examined in this investigation all provided overall geometries for trans-O,O-[Ga(bpcd)]+ and cis-O,O, cis-Npy,Npy-[Ga(bppd)]+ cations that are remarkably similar to those observed in the solid state. For example, the edge to face twisted orientation of the pyridine rings in the [Ga(bppd)]+ structure results in the planes of the pyridine rings being nearly perpendicular to each other, Figure 3b. Since these methods also yield similar bond angles around Ga, only those angles calculated using the SV5LYP/SDD(d) functional are considered. At this computational level, the Nam−Ga−Nam angle of 95.2° in [Ga(bppd)]+, Table 5, contracts to 85.1° in [Ga(bpcd)]+, Table 4. A 10° contraction of the Nam−Ga−Nam angle in [Ga(bpcd)]+ might be anticipated since the alkyl bridge in the diamine backbone of H2bpcd contains one less CH2 group than that in H2bppd’s backbone. The calculated angles are also in good agreement with the bond angles observed in the structures of [Ga(bppd)]PF6 and [Ga(bpcd)]PF6, i.e., 96.5° and 85.3°, respectively (Table 3 in Kissel et al.15
respectively. The results for trans-Npy,Npy isomers are not included in Table 4 or 5. Regardless of the identity of the chelating bpad2− ligand or the trivalent metal ion, the trans-O,O isomer is always stabilized relative to the two other isomers by its gas-phase free energy (Tables 4 and 5 and Table 4 in Kissel et al.15). The relative gasphase free energy is dominated by variations in the electrostatic interactions between the metal ion and the bpad2− ligand. Steric interactions between the ligand’s pendant arms can also be important with smaller metal ions. Opposite to this effect, the cis-O,O, cis-Npy,Npy isomer has the most favorable solvation free energy. For example, ΔGsolv for the trans-O,O and cis-O,O, cisNpy,Npy isomers of [Ga(bpcd)]+, at the HF/6-31G* level, are −56.7 and −63.3 kcal/mol, respectively. Overall, using the highest-level computational method, the calculated energetic preference for the trans-O,O-[Ga(bpcd)]+ cation in solution over the other isomers is 1.0 kcal/mol, Table 4. This means roughly 85% of the cationic species present in solution at room temperature should be present as the trans-O,O isomer. At the same PCM/MP2/SDD(D)//SV5LYP/SDD(d) level, the calculated free energy preference for the cis-O,O, cis-Npy,Npy[Ga(bppd)]+ cation in solution over the other two possible isomers is 1.3 kcal/mol, Table 5. This energy preference translates to approximately 90% of the cationic species being present in solution as the all-cis isomer. In these cases, the lowest energy (most stable) isomer would be the predominant or only cationic species in solution. Consequently, cis/trans isomerization equilibria for [M(bpad)]+ complex ions in solution generally have not been observed by NMR spectroscopy. In contrast, the cis and trans isomers formed VV and ethylenediamine-N,N′-diacetic acid (EDDA), i.e., cis- and trans[VO2(EDDA)]+, are sufficiently different in energy that both are observed in solution.33 Because the 6-31G* basis set is limited to atoms with Z ≤ 36, the performance of the SSD pseudopotential-based basis set, which is available for heavy atoms, e.g., actinides, was explored. Using an analogous composite QM approach, in this case a combination of the PCM and MP2/SSD energetics calculated at HF/SSD geometries, the correct relative cis-to-trans 10366
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binding, the antisymmetric stretching frequency increases compared to that for ionic binding while the frequency of the symmetric stretching mode decreases. This leads to larger separations between νa(COO−) and νs(COO−), i.e., Δν = 190−350 cm−1. The IR results in Table 3 for [Co(bpcd)]PF6 and[Ga(bpcd)]PF6 show a significant increase in the antisymmetric stretching frequency and large Δν values, 337 and 328 cm−1, respectively, that demonstrate this behavior. Chelation or symmetric bridging shifts both stretching bands in the same direction and leads to smaller Δν values. Bridging acetate groups generally have moderate Δν values, 150−190 cm−1, while chelation leads to smaller Δν values, 90−150 cm−1. Thus, the IR spectra of [Co(bpcd)]PF6 and[Ga(bpcd)]PF6 are consistent with monodentate coordination as observed in the solid state for the acetate groups in bpcd2−. X-ray Structure. Crystals of [Ga(bpcd)]PF6 were isolated and examined via X-ray diffraction. The structure of the [Ga(bpcd)]+ cation is shown in Figure 3, and selected geometric parameters are listed in Table 2. The Ga atom is surrounded by the bpcd2− ligand in a distorted octahedral geometry provided by a N4O2 donor set. The bpcd2− ligand is bound to Ga in a trans-O,O configuration to afford a cation with idealized C2 symmetry. Selected bond distances and angles are shown in Tables 6 and 7, respectively. The Ga−O distance
and Table 2, above). Since the lengths of the acetate and 2pyridylmethyl pendant arms in H2bpcd and H2bppd are identical, the smaller Nam−Ga−Nam angle in H2bpcd limits the reach of these arms to “wrap” around a given metal ion. Consequently, the Npy−Ga−Npy open angle in trans-O,O[Ga(bpcd)]+ (116.4° calculated vs 115.0° observed, Table 4) is significantly wider than the corresponding calculated open angle in trans-O,O [Ga(bppd)]+, 104.3°. (Note: X−M−Y describes the open angle in a metal’s coordination sphere formed by donor atoms trans to the N atoms of the diamine, where here X = Npy and Y = Npy or Oac; also, see Figure 2.) H2bpcd with the larger open angle is expected to favor binding to larger metals than H2bppd, especially for metal ions that prefer coordination numbers greater than 6. The calculated open angles for [Ga(bpcd)]+ and [Ga(bppd)]+ also provide insight into the structural origin of the different relative stabilities of the trans-O,O and cis-O,O, cisNpy,Npy isomers for the H2bpcd and H2bppd complexes with Ga. The change in isomer geometry changes the identity of the open angle from Npy−Ga−Npy for the trans isomer to Npy− Ga−Oac for the all-cis isomer that accompanies the decreased magnitude of the open angle. This contraction is significantly more pronounced in the calculated open angles for [Ga(bppd)]+ (104.3° vs 97.0°) than for [Ga(bpcd)]+ (116.4° vs 113.5°), which is undoubtedly due to greater flexibility of H2bppd. In the absence of steric repulsion between the pendant arms of the ligand, which is the case for the acetate (Oac) and 2pyridylmethyl (Npy) groups that form the open angle of the cisO,O, cis-Npy,Npy isomer, the open angle is close to ideal for an octahedral complex (∼90°). Although the geometry of the lone-pair orbital on the donor atoms forming an open bond angle is not the only factor determining the relative energy of the isomer, this contribution to a first approximation is expected to be proportional to the magnitude of the change in this angle. Consequently, the stabilizing energy provided from changes in ion-pair orbital geometry is expected to be larger for bppd2− than for bpcd2− when coordinated to Ga. For smaller metals, for example, Co, the stabilizing effect of a more ideal open angle is offset by the intramolecular steric repulsions between pendant arms that are greater for the all-cis isomer than the trans-O,O isomer. Further, this effect increases with decreasing metal ion size. Larger metals favor larger open angles afforded by the transO,O isomer. Ga’s size then is optimal for allowing the energetic benefits of an ideal open angle arrangement to prevail over the overall intracomplex steric and electrostatic interactions that favor the trans-O,O geometry, which is consistent with the calculated gas-phase energy differences.
Table 6. Selected Experimental Bond Distances (Å) for Co(bpcd)+, Ga(bpcd)+, and Ga(bppd)+ bond (Å) MOac1 MOac2 MNam1 MNam2 MNpy1 MNpy2 COac1 COac1 COac2 COac2 M above N/N/N/N plane T, K
Co(bpcd)+a
Ga(bpcd)+b
Ga(bppd)+c
1.8868 d 1.9548 d 1.9448 d 1.3029 1.2212 d d 0f 100
1.9343 1.9344 2.0866 d 2.0574 2.0575 1.3025 1.2151 d d 0f 100
1.9459 (15) 1.8956 (14) 2.0673 (16) 2.1061 (17) 2.0156 (17) 2.1377 (17) 1.286 (3) 1.211 (3) 1.287 (3) 1.208 (3) 0.0387 (9) 296
(8) (9) (9) (13) (14)
(9) (9)e (10) (10) (10)e (15) (15)
a
McLauchlan et al.16 bThis work. cKissel et al.15 d N/A. Symmetry equivalent. eSymmetry equivalent, but a difference in the distances is noted in the refinement output file. f Special position.
Table 7. Selected Bond Angles (deg) for Co(bpcd)+, Ga(bpcd)+, and Ga(bppd)
■
DISCUSSION Infrared Spectra. Idealized metal−carboxylate binding modes for ionic and coordinated acetate groups, and the magnitude of the frequency difference between the antisymmetric and symmetric bands, Δν = νa(COO−) − νs(COO−), for these types of binding have been discussed in detail.15,29,34 The C−O bonds in an ionically bound carboxylate group are equivalent, and moderate Δν values, 150−190 cm−1, are associated with ionic acetate groups. The Δν value reported in Table 3 for Na2bpcd is 182 cm−1, which is consistent with ionic bonding. Monodentate coordination removes the equivalency of the C−O bonds in the carboxylate group and increases in the magnitude of Δν compared to that for ionic salts and other types of acetate complexation. Typically, for monodentate
angle (deg)
Co(bpcd)+a
Ga(bpcd)+b
Ga(bppd)+c
Oac1MOac2 Nam1MNam2 Npy1MNpy2 Nam1MNpy1 Nam2MNpr2 Nam1MOac1 Npyr1MOac1 OCOac
176.08 (5) 89.33 (5) 106.74 (5) 82.17 94) d 87.84 (4) 89.92 (4) 124.95 (10) d 114.57 (7) d
176.56 (5) 85.30 (5) 114.95 (6) 80.51 (4) d 85.36(4) 91.19 (4) 125.09 (12) d 115.26 (8) d
91.83 (7) 96.47 (7) 90.80 (6) 81.46 (7) 80.39 (7) 83.59 (7) 99.41 (7) 125.0 (3) 123.5 (2) 117.81 (15) 114.77 (14)
C(O)OacM
a
McLauchlan et al.16 bThis work. cKissel et al.15 dN/A. Symmetry equivalent. 10367
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angle, is even larger (115°). Finally, the GaIII in the [Ga(bpcd)]+ structure is situated directly in the N4 plane of the equatorial nitrogen atoms as is the CoIII in the [Co(bpcd)]+ cation, Table 6. NMR Spectroscopy. The 1H NMR spectra for H2bpcd· 2HCl were recorded at 50 °C to obviate the line broadening that occurs at lower temperatures. The line broadening could be the result of strong intramolecular hydrogen bonding augmented by the presence of HCl in solution or slower inversion of the cyclohexyl ring at lower temperatures.8,9 The splitting patterns and chemical shifts of the 1H and 13C NMR spectra for H2bpcd are consistent with a species displaying C2 symmetry. The methylene protons of the 2-pyridylmethyl groups in [Ga(bpcd)]+, H(6) in Figure 3a, reside in different magnetic environments that give rise to a splitting pattern typical of an AX-quartet resulting from geminal coupling between methylene protons of the same carbon atom, a characteristic of diastereotopic nuclei.37 This splitting pattern in the pro-ligand is the result of the chiral cyclohexyl ring. The same splitting pattern is observed in the 1H NMR spectrum of the parent diamine bpmdac, Figure S4. The methylene protons of the acetic acid moieties in H2bpcd are also diastereotopic but appear as a single peak in the pro-ligand. Classic 1H and 13C NMR spectroscopies were used to characterize the [Co(bpcd)]+ and [Ga(bpcd)]+ complex ions in solution. The nonequivalency of the methylene protons in both pendant arms, H(6) and H(7) for the 2-pyridylmethyl and acetate groups, respectively, for these complexes indicates metal ion complexation. This change in equivalency is typical for inorganic compounds with coordination geometries that feature the formation of chelate rings.38−40 In the present case, bpcd2− forms five chelate rings upon coordination that give rise to three sets of nonadjacent and non-coplanar rings. This creates nonequivalency in the methylene protons of the 2-pyridylmethyl and acetate functionalities and allows for the possibility of resonances arising from both pro-R and pro-S hydrogen atoms.37 Indeed, the presence of diastereotopic nuclei is evident in the splitting patterns observed for H(6) and H(7) in the 1H NMR spectra of [Co(bpcd)] + and [Ga(bpcd)]+ . The distinguishable proton signals in the GaIII and CoIII complexes show geminal couplings (2JHH) of approximately 15−18 Hz upon loss of equivalency. Resonances arising from these methylene protons split as AX-quartets with large chemical shift differences ΔνAB. The resonances are field dependent with the ΔνAB value largest for [Co(bpcd)]+, which at 500 MHz is 525 Hz. The 1H and 13C NMR spectra for H2bpcd (11 1H and 11 13C resonances) as well as the spectra for the GaIII and CoIII complexes show the number of resonances with splitting patterns, intensities, and chemical shifts expected for species with C2 symmetry, e.g., a trans-O,O isomer. The number of resonances for a cis-O,O, cis-Npy,Npy [M(bpcd)]+ isomer with C1 symmetry is expected to double, i.e., 22 1H and 22 13C resonances, similar to that observed for the all-cis [Ga(bppd)]+ isomer.15 The NMR findings then are consistent with the solid state structures for the [M(bpcd)]PF6 salts and the [M(bpcd)]+ isomers calculated to be most stable in solution. Quantum Mechanical Calculations. A systematic study of [Ga(bpcd)]+ and [Ga(bppd)]+, which corroborated crystallographic and solution NMR experiments, generated a combination of structural data that are well-suited for benchmarking the performance of various computational approaches. In general, such a comparison of the results
of 1.934 Å, and Ga−N distances of 2.058 and 2.086 Å to the pyridyl (Npy) and diamine (Nam) nitrogen atoms, respectively, are typical of those reported for GaN4O2 structures.15,35 Many of the angles defined by the N4O2 coordination sphere around Ga deviate significantly from ideal octahedral values, with the fairly constrained nature of the polydentate chelating bpcd2− ligand dictating those distortions. The diaminocyclohexane backbone in [Ga(bpcd)]+ is chiral, and both enantiomers are crystallized in this centrosymmetric structure in the orthorhombic space group Ibca. The trans-(1S,2S) enantiomer of the [Ga(bpcd)]+ cation is shown in Figure 3a. Little structural information has been reported for metal− bpcd2− complexes. A structure has been described for a heptacoordinate [FeII(H2bpcd)(C3H6O)]2+ complex ion that was isolated as a perchlorate salt.14 We previously reported a structure for a hexacoordinate [Co(bpcd)]+ complex ion isolated as a PF6− salt that is nearly isostructural to [Ga(bpcd)]PF6.16 [Ga(bpcd)]PF6 has a slightly larger unit cell volume, but the geometry and coordination are similarly described in both. The [Ga(bpcd)]+ cation in this structure occurs as the trans-O,O isomer, Figures 2 and 3a. A structure of GaIII with a derivative of bpcd2− with additional carboxylic acid units on each of the pyridine rings (“H4CHXoctapa”) has recently been reported, but this ligand adopts a N3O3 donor set when coordinated to Ga8 Although only two structures of metal−bpcd2− complexes have been reported, earlier we reported a structure for a pseudo-octahedral GaIII complex with the related bis-2pyridylmethyl-substituted diaminoacetic acid H2bppd, B in Figure 1.15 In [Ga(bppd)]PF6, the [Ga(bppd)]+ cation forms a hexadentate structure provided by a N4O2 donor atom set, but the acetate and 2-pyridylmethyl groups adopt cis orientations leading to an all-cis geometry. In the present case, the fully deprotonated bpcd2− ligand binds to GaIII in a pseudooctahedral fashion to form a hexacoordinate complex ion with trans monodentate acetate groups. The different orientations of the acetate groups represent the most dramatic difference between the two structures. Bond distances and angles for the [Co(bpcd)]+, [Ga(bpcd)]+, and [Ga(bppd)]+ cations are compared in Tables 6 and 7. The metal−donor atom distances (Ga−O and Ga−N distances) in the [Ga(bpcd)]+ cation are very similar to the averages of the corresponding distances in the [Ga(bppd)]+ cation. The CoO and CoN bond distances in the [Co(bpcd)]+ cation, however, are systematically ∼0.1 Å smaller than the bond distances in the GaIII complex ions, consistent with the smaller effective ionic radii for CoIII (0.545 Å) compared to GaIII (0.620 Å).36 The CO and CO bond distances in all three complex ions are also very similar. A comparison of the bond distances and angles for Ga(HCHXoctapa) and [FeII(H2bpcd)(C3H6O)]2+ with those reported here is included in the Supporting Information (Tables S1 and S2). Many of the angles given in Table 7 deviate from ideal octahedral values, with the largest deviations usually appearing in the [Ga(bpcd)]+ structure. The Nam1CoNam2 angle in the [Co(bpcd)]+ cation is close to ideal (89.3°), while the Nam1GaNam2 bond angle in the [Ga(bpcd)]+ cation deviates considerably from ideal (85.3°). The open angle formed by the donor atoms trans to the N atoms of the diamine, the Npy1CoNpy2 angle in the [Co(bpcd)]+ structure, is considerably larger than ideal (107°), while the open angle in the [Ga(bpcd)]+ structure, the Npy1GaNpy2 10368
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Inorganic Chemistry
explored experimentally and computationally. The structure of [Ga(bpcd)]PF6 was determined by X-ray crystallography and compared to nearly isostructural [Co(bpcd)]PF6 as well as related [FeII(H2bpcd)(C3H6O)](ClO4)2, [Ga(bppd)]PF6, and [Co(bppd)]PF6 salts. 1 H and 13C NMR spectroscopy established that the complex ion present in solutions of a [M(bpad)]+ salt has the same symmetry as observed in the solid state structure, i.e., trans-O,O-[Ga(bpcd)]+, trans-O,O[Co(bpcd)]+, cis-O,O, cis-Npy,Npy-[Ga(bppd)]+, and trans-O,O[Co(bppd)]+. The calculated free energies of [M(bpad)]+ cations in aqueous solution provide valuable insight into the effect that structural features of bpad2− ligands have on the relative stability of their cis and trans geometric isomers. The trans-Npy,Npy isomer is always the least stable of the three isomers possible for the [M(bpad)]+ complexes investigated, and the trans-O,O-isomer generally is the most stable isomer.2 The single exception is the cis-O,O, cis-Npy,Npy-[Ga(bppd)]+ cation.15 In this case, the greater chain length and flexibility of the propyl linker in the bppd2− backbone provide the extra stability necessary with “right” sized GaIII to make the all-cis isomer the most stable isomer. It was established that the acetate groups in metal [M(bpad)]+ complexes of fully deprotonated H2bpad ligands exhibit monodentate coordination. Finally, reliable and efficient computational methodologies capable of providing accurate structural and selectivity information for [M(bpad)]+ complex ions with larger metals such as lanthanides and actinides have been identified.
provided by several theoretical methods provides useful insights into structure−function relationships. For example, comparison of HF and MP2 results provides information about electron correlation effects. When a sufficiently reliable, computational approach is available to prescreen ligands for performing a specific task, only the candidates most likely to provide the best performances could be selected for testing, thereby minimizing the time and effort needed to find the best candidates as well as mitigating the environmental impact of testing. H2bpcd, which forms a five-membered chelate ring via its trans-1,2-diaminocyclohexane backbone, favors chelation of larger metal ions. The restricted rotation about the C−C bonds in the cyclohexane ring of the diamine backbone fixes the position of the amine nitrogen atoms and favorably preorganizes the ligand for metal complexation. Consequently, H2bpcd is preorientated for favorable metal binding.10−12 The presence of aromatic 2-pyridylmethyl functionalities increases the soft character of a ligand5 and makes H2bpcd a good candidate for applications that require selectivity for softer metal ions. For example, the separation of same sized and charge density metal ions with only slightly different hard−soft acid−base characteristics, e.g., trivalent actinides (AnIII) and lanthanides (LnIII).41 Thus, AnIII/LnIII separations using H2bpad’s could greatly benefit from establishing reliable and efficient computational methodology for predicting the structure and relative stability of AnIII and LnIII complex ions. The calculations also provide an opportunity to examine the geometries of isomers that are not accessible experimentally due to their low abundance in solution or the solid state. This analysis pointed to the greater length and flexibility of the propyl linker in the diamine backbone of H2bppd, which allows for greater variation in the open bond angle than possible for H2bpcd, as the main cause for the dramatically different positions of the cis/trans isomerization equilibrium for [Ga(bpcd)]+ compared to that of [Ga(bppd)]+. The tighter open angle due to greater arm reach in H2bppd provides extra stability to the cis-O,O, cis-Npy,Npy isomer for [Ga(bppd)]+ with “right” sized GaIII. In spite of the preference of six-member chelate rings for “smaller” metal ions, this extra stability is insufficient to overcome the greater intraligand steric repulsion between the pendant arms in bppd2− complexes with smaller metal ions such as Co.18 The extra stability is also insufficient to overcome the overall electrostatics favoring the trans-O,O isomer for larger metal ions such as LaIII. The electrostatics with larger metal ions can be enhanced further via a wider “open angle” that allows more preferred coordination numbers (8 or 9) to be adopted by acquiring additional ligands, e.g., H2O.3,15 Surprisingly, when combined with a pseudopotential-based basis set, the local SVWN5 functional performed better than the more rigorous BLYP method, which overestimated the Ga− N bond length by as much as 0.1 Å. While none of the HF or pseudopotential-DFT methods on their own or in combination with PCM solvation energetics correctly predicted the all-cis isomer for [Ga(bppd)]+ to be favored in aqueous solution, subsequent single-point MP2 calculations were sufficient in identifying the correct isomer. These benchmark results open the way for reliable structural predictions and stability studies of substituted diaminodiacetic acid ligands coordinated to fourth row and larger metal ions.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b01586. Structural details, 1H NMR spectrum for N,N′-bis(2pyridylmethyl)-1,2-diaminocyclohexane, bpmdac, in CDCl3 at 25 °C, 1H NMR spectra of [Co(bpcd)]+ in CD3CN and D2O at 25 °C, and 13C and 1H NMR spectra of [Ga(bpcd)]+ in DMSO-d6 at 25 °C (PDF) Crystallographic data (CIF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by Loyola University Chicago, Illinois State University, and the National Science Foundation (US, CHE-0645081). C.C.M. acknowledges the National Science Foundation for the purchase of the Bruker APEXII (CHE-1039689). D.S.K. wishes to thank Loyola University Chicago and the Schmitt Foundation for fifth year fellowship support. A.W.H. thanks Dr. Peidong Zhao of Loyola University Chicago for helpful discussions of NMR data acquisition and analysis.
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REFERENCES
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CONCLUSIONS The structural and complexation chemistry of the 2pyridylmethyl-substituted diaminodiacetic acid H2bpcd was 10369
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