Zinc complexes of the biomimetic N,N,O ligand family of substituted 3,3-bis(1-alkylimidazol-2-yl)propionates: the formation of oxalate from pyruvate

The coordination chemistry of the 2-His-1-carboxylate facial triad mimics 3,3-bis(1-methylimidazol-2-yl)propionate (MIm2Pr) and 3,3-bis(1-ethyl-4-isopropylimidazol-2-yl) propionate (iPrEtIm2Pr) towards ZnCl2 was studied both in solution and in the solid state. Different coordination modes were found depending both on the stoichiometry and on the ligand that was employed. In the 2:1 ligand-to-metal complex [Zn(MIm2Pr)2], the ligand coordinates in a tridentate, tripodal N,N,O fashion similar to the 2-His-1-carboxylate facial triad. However, the 1:1 ligand-to-metal complexes [Zn(MIm2Pr)Cl(H2O)] and [Zn(iPrEtIm2Pr)Cl] were crystallographically characterized and found to be polymeric in nature. A new, bridging coordination mode of the ligands was observed in both structures comprising N,N-bidentate coordination of the ligand to one zinc atom and O-monodentate coordination to a zinc second atom. A rather unique transformation of pyruvate into oxalate was found with [Zn(MIm2Pr)Cl], which resulted in the isolation of the new, oxalato bridged zinc coordination polymer [Zn2(MIm2Pr)2(ox)]·6H2O, the structure of which was established by X-ray crystal structure determination.
Introduction
The 2-His-1-carboxylate facial triad is a structural motif of increasing prominence among structurally characterized metalloenzymes. Next to the rapidly growing subgroup of non-heme iron enzymes [1, 2], several mononuclear zinc enzymes also feature this triad at their active site [3]. The proteases thermolysin, carboxypeptidase, and neutral protease, for example, catalyze the hydrolysis of peptide bonds. The zinc ion in these enzymes is coordinated by three endogenous residues, i.e., one glutamate and two histidine residues (Fig. 1). The fourth coordination site of the pseudo-tetrahedral zinc center is occupied by a catalytically important water or hydroxide ligand. This 2-His-1-carboxylate facial triad is a variation of the most commonly observed structural motif for the active sites of zinc-containing enzymes. Typically, the tetrahedral zinc centers in these enzymes are bound by a combination of histidine (N), glutamate or aspartate (O) and/or cysteine (S) residues. The resulting NxOySz donor set determines its biological function [3]. Accordingly, it is important to understand how a particular combination of donor groups modulates the reactivity of the Zn(II) metal center. Fig. 1Active site of a zinc-containing enzyme that features the 2-His-1-carboxylate facial triad: neutral protease from Bacillus cereus (Protein Data Bank accession code 1NPC.pdb) [4]
The study of small synthetic analogues has focused on establishing a basis for such a structure–function relationship for these mononuclear zinc-containing enzymes [3, 5]. Initially, pioneering studies by the groups of Vahrenkamp, Parkin, Kitajima, and others employed all-N donor ligands such as the trispyrazolylborates [6–8]. In order to obtain close structural models for the 2-His-1-carboxylate facial triad containing zinc enzymes, one would preferentially like to construct small analogues based on a tripodal N,N,O ligand framework that incorporates the biologically relevant donor groups. Of the complexes with a mixed N/O donor set reported to date [9–22], only a few make use of such tridentate, tripodal N,N,Ocarboxylato ligands. These tripodal ligands should be particularly well suited for the structural modeling of tetrahedral metal centers [23]. Dowling and Parkin [24] and Ghosh and Parkin [25] reported the first example of an N,N,O model complex with a carboxylato donor group, which resulted from the insertion of carbon dioxide into a borohydride bond. More recently, the groups of Burzlaff [9, 12] and Carrano [11, 18, 19] have reported studies on the zinc coordination chemistry of the bispyrazolylacetate ligand family. Beck et al. [9], for instance, reported that a 2:1 ligand–zinc complex was obtained with bis(3,5-dimethylpyrazol-2-yl)acetate (bdmpza), a ligand closely related to 3,3-bis(1-methylimidazol-2-yl)propionate (MIm2Pr). These 2:1 complexes were obtained regardless of the ratio of reactants, i.e., even with stoichiometric amounts of ligand and zinc(II). Very recently, Friese et al. [22] constructed mononuclear zinc models via a new approach, i.e., by the use of sterically hindered carboxylato and N-donor ligand building blocks.
We have been studying the copper and iron coordination chemistry of the new ligand family of the substituted 3,3-bis(1-alkylimidazol-2-yl)propionates as accurate structural mimics of the 2-His-1-carboxylate facial triad [26–28]. These ligands incorporate the biologically relevant donor groups, i.e., two imidazoles and a carboxylate group, into a monoanionic, tripodal framework. Their general biomimetic potential has been illustrated by the structurally characterized copper [27, 28] and iron [26] complexes of these ligands. Here, our studies are expanded to the structural modeling of the zinc-containing enzymes and describe the zinc coordination chemistry of the ligands MIm2Pr and 3,3-bis(1-ethyl-4-isopropylimidazol-2-yl)propionate (iPrEtIm2Pr) (Fig. 2). Furthermore, the attempted synthesis of zinc complexes with 3,3-bis(1-methylimidazol-2-yl)acetate (MIm2Ac), a direct analogue of the bispyrazolylacetates, is described. During the course of these studies, we found that zinc complexes of ligand MIm2Pr catalyze the unexpected and unprecedented conversion of pyruvate to oxalate in aqueous solution.Fig. 2Ligands 3,3-bis(1-methylimidazol-2-yl)propionate (MIm2Pr), 3,3-bis(1-ethyl-4-isopropylimidazol-2-yl)propionate (iPrEtIm2Pr), and 3,3-bis(1-methylimidazol-2-yl)acetate (MIm2Ac) and the synthesis of the zinc complexes [Zn(MIm2Pr)2], [Zn(MIm2Pr)Cl], and [Zn(iPrEtIm2Pr)Cl]
Materials and methods
Air-sensitive organic reactions were carried out under an atmosphere of dry, oxygen-free N2 using standard Schlenk techniques. Tetrahydrofuran (THF) and diethyl ether were dried over sodium benzophenone ketyl and distilled under N2 prior to use. Methanol was dried over magnesium methoxide and distilled under N2 prior to use. 1H and 13C NMR spectra were recorded using a Varian AS400, a Varian Inova 300, or a Varian Mercury 200 spectrometer, operating at 25 °C. IR spectra were recorded with a PerkinElmer Spectrum One Fourier transform IR instrument. Elemental microanalyses were carried out by the Microanalytisches Laboratorium Dornis & Kolbe, Mulheim a.d. Ruhr, Germany. ESI–MS spectra were recorded using a Micromass liquid chromatography time-of-flight mass spectrometer at the Biomolecular Mass Spectrometry Group, Utrecht University. Bis(1-methylimidazol-2-yl)methane (MIm2CH2) [29,] 3,3-bis(1-methylimidazol-2-yl)propionic acid (HMIm2Pr), potassium and tetrabutylammonium 3,3-bis(1-methylimidazol-2-yl)propionate (K[MIm2Pr] and [Bu4N][MIm2Pr]) [27], potassium 3,3-bis(1-ethyl-4-isopropylimidazol-2-yl)propionate (K[iPrEtIm2Pr]) [26], and zinc bis(trimethylsilyl)amide [30] were prepared according to published procedures. All other chemicals were obtained commercially and used as received.
Benzyl bis(1-methylimidazol-2-yl)acetate
To a cooled solution of MIm2CH2 (0.52 g, 3.0 mmol) in dry THF (50 mL) at −78 °C was added dropwise a solution of n-butyl lithium in hexane (2.0 mL, 3.2 mmol). The reaction mixture was stirred for 1 h at −78 °C, after which benzylchloroformate (1.19 mL, 3.5 mmol, 50 wt% solution in toluene) was added dropwise. During the addition, a white precipitate formed. The solution was allowed to warm to room temperature overnight. The reaction mixture was quenched with H2O (20 mL) and all volatiles were removed in vacuo. The aqueous layer was extracted with diethyl ether (4 × 20 mL) and the combined organic layers were dried over MgSO4, filtered and concentrated in vacuo. The crude oil was purified by column chromatography (SiO2, ethyl acetate–methanol 2:1). Benzyl bis(1-methylimidazol-2-yl)acetate (BnMIm2Ac) was isolated as a yellow oil in 62% yield (0.58 g, 1.9 mmol). 1H NMR (300 MHz, CD3CN, 25 °C): δ = 3.44 (s, 6H, NCH3), 5.23 (s, 2H, CH2), 5.62 (s, 1H, CH), 6.85 (s, 2H, Him), 6.96 (s, 2H, Him), 7.35 (s, 5H, HPh) ppm. Anal. for C17H18N4O2 (310.35): calc. C 65.79, H 5.85, N 18.05; found C 65.88, H 5.74, N 17.92. IR (solid): ν = 3,122.9, 2,911.5, 1,734.5, 1,547.4, 1,510.2, 1,454.4, 1,373.6, 1,263.4, 1,220.8, 1,150.4, 978.2, 958.3, 907.1, 847.8, 785.7, 776.5, 740.1, 696.7, 683.5, 665.3 cm−1. ESI–MS: m/z = 409.0 ([M–Cl]+, calc. 409.0).
[Zn(MIm2Pr)2]
To a hot, colorless solution of HMIm2Pr (282 mg, 1.20 mmol) in dry acetonitrile (100 mL) was added a solution of Zn[N(SiMe3)2]2 (232 mg, 0.60 mmol) in dry diethyl ether (20 mL) via a cannula. The resulting colorless, clear solution was stirred for 60 h at elevated temperature, during which gradually a white precipitate formed. The reaction mixture was concentrated in vacuo to give the crude product as a white powder. Recrystallization from an acetone–water mixture (4:1 v/v) gave [Zn(MIm2Pr)2] as a colorless crystalline solid (230 mg, 72%). 1H NMR (300 MHz, D2O, 25 °C): δ = 2.69 (d, 2H, J = 6.3 Hz, CHCH2), 3.74 (s, 6H, NCH3), 4.93 (t, 1H, J = 6.0 Hz, CHCH2), 6.69 (s, 2H, Him), 7.00 (s, 2H, Him) ppm. 13C{1H} NMR (50 MHz, D2O, 25 °C): δ = 30.2, 33.4, 43.6, 122.7, 125.7, 146.8, 177.2 ppm. Anal. for C22H26ZnN8O4 (531.90): calc. C 49.68, H 4.93, N 21.07; found C 49.76, H 5.06, N 20.97. IR (solid): ν = 3,120.2, 2,951.7, 2,907.1, 2,815.2, 1,580.5, 1,508.1, 1,427.1, 1,391.0, 1,306.4, 1,288.0, 1,229.3, 1,168.8, 1,143.3, 1,044.8, 956.9, 906.2, 769.3, 751.1 cm−1. ESI–MS: m/z = 531.18 ([M + H]+, calc. 531.14), 553.18 ([M+Na]+, calc. 553.13), 569.16 ([M + K]+, calc. 569.10).
[Zn(MIm2Pr)Cl]
To a hot, colorless solution of K[MIm2Pr] (255 mg, 0.93 mmol) in dry methanol (5 mL) was added a solution of ZnCl2 (127 mg, 0.93 mmol) in methanol (5 mL) via a cannula. Immediately upon addition a white precipitate formed and the suspension was stirred at elevated temperature for 1 h. The white precipitate was separated by centrifugation and washed with methanol (3 × 20 mL). [Zn(MIm2Pr)Cl] was obtained as a white powder in almost quantitative yield (305 mg, 98%). 1H NMR (300 MHz, D2O, 25 °C): δ = 2.80 (d, 3H, J = 3.3 Hz, CHCH2), 3.85 (s, 6H, NCH3), 4.99 (t, 1H, CH2CH), 6.99 (s, 2H, Him), 7.14 (s, 2H, Him) ppm. 13C{1H} NMR (50 MHz, D2O, 25 °C): δ = 29.8, 33.4, 42.6, 122.5, 125.5, 146.1, 177.3 ppm. Anal. for C11H13ClN4O2Zn (334.11): calc. C 39.55, H 3.92, N 16.77; found: C 39.46, H 4.05, N 16.85. IR (solid): ν = 3,138.0, 3,117.2, 1,614.8, 1,508.2, 1389.8, 1,299.8, 1,289.5, 1,151.7, 1,142.5, 1,089.7, 976.2, 949.7, 796.3, 765.3, 736.1 cm−1. ESI–MS: m/z = 296.94 ([M–Cl]+, calc. 297.03), 332.92 ([M + H]+, calc. 333.01), 413.97 ([3M–Zn–3Cl + H]2+, calc. 414.09), 531.02 ([2M–Zn–2Cl + H]+, calc. 531.15), 628.97 ([2M–Cl]+, calc. 629.03), 827.10 ([3M–Zn–3Cl]+, calc. 827.17).
[Zn(MIm2Pr)Cl(H2O)]
Colorless crystals of [Zn(MIm2Pr)Cl(H2O)] suitable for X-ray diffraction were obtained upon standing of a concentrated solution of [Zn(MIm2Pr)Cl] in H2O for several weeks. The coordination polymer obtained is insoluble in all common organic solvents and water of neutral pH. Anal. for C11H15ClN4O3Zn (352.12): calc. C 37.52, H 4.29, 15.91; found C 37.44, H 4.27, N 15.97. IR (solid): ν = 3,408.6, 3,113.5, 3,134.7, 1,594.5, 1,504.5, 1,403.1, 1,306.7, 1,255.5, 1,155.5, 1,136.3, 1,087.4, 983.7, 959.1, 934.9, 849.7, 764.6, 743.1, 717.6 cm−1.
[Zn(iPrEtIm2Pr)Cl]
To a colorless solution of ZnCl2 (30 mg, 0.22 mmol) in dry methanol (10 mL) was added a solution of K[iPrEtIm2Pr] (85 mg, 0.22 mmol) in dry methanol (15 mL) via a cannula. The clear solution was stirred overnight at room temperature and evaporated in vacuo. The product was redissolved in dichloromethane and insoluble KCl was separated off by centrifugation. The solution was filtered over Celite and concentrated in vacuo to give [Zn(iPrEtIm2Pr)Cl] as a white powder (97 mg, 99%). Colorless crystals suitable for X-ray diffraction were obtained from a methanolic solution of [Zn(iPrEtIm2Pr)Cl] upon standing. 1H NMR (300 MHz, D2O, 25 °C): δ = 1.22 (d, 6H, J = 7.2 Hz, CH3CHCH3), 1.25 (d, 6H, J = 6.8 Hz, CH3CHCH3), 1.44 (t, 6H, J = 7.2 Hz, CH3CH2), 2.88 (d, 2H, J = 6.0 Hz, CHCH2), 3.25 (m, 2H, CH3CHCH3), 4.21 (ABX3, 2H, J = 7.2, CHHCH3), 4.32 (ABX3, 2H, J = 7.2, CHHCH3), 5.02 (t, 1H, J = 5.7 Hz, CHCH2), 6.99 (s, 2H, Him) ppm. 13C{1H} NMR (100 MHz, CD3OD, 25 °C): δ = 16.6, 22.1, 24.1, 28.0, 31.1, 42.9, 116.0, 146.2, 149.9, 175.7 ppm. Anal. for C19H29ClN4O2Zn (446.32): calc. C 51.13, H 6.55, N 12.55; found: C 51.05, H 6.43, N 12.37. IR (solid): ν = 3,101.4, 2,967.9, 2,873.7, 1,626.6, 1,572.8, 1,494.8, 1,448.9, 1,464.1, 1,381.1, 1,324.8, 1,257.2, 1,175.8, 1,153.0, 1,027.6, 976.1, 801.2, 757.7, 660.5 cm−1. ESI–MS: m/z = 409.13 ([2M–2Cl]2+, calc. 409.16), 445.11 ([M + H]+, calc. 445.13), 483.04 ([M + K]+, calc. 483.09), 631.21 ([3M–2Cl]2+, calc. 631.22), 853.29 ([2M–Cl]+, calc. 853.29), 889.20 ([2M + H]+, calc. 889.26), 1297.02 ([3M + H]+, calc. 1297.42).
[Zn(BnMIm2Ac)Cl2]
To a hot solution of BnMIm2Ac (259 mg, 0.84 mmol) in dry methanol (5 mL) was added a solution of anhydrous ZnCl2 (115 mg, 0.84 mmol) in dry MeOH (5 mL) via a cannula. The reaction mixture was stirred at elevated temperature for 1 h and gradually the product formed as a white precipitate. The product [Zn(BnMIm2Ac)Cl2] was separated by centrifugation and was obtained as a white powder in quantitative yield. Single crystals suitable for X-ray analysis were obtained by slow evaporation of a solution of [Zn(BnMIm2Ac)Cl2] in acetone. 1H NMR (300 MHz, CD3CN, 25 °C): δ = 3.79 (2, 6H, CH3), 5.12 (s, 2H, CH2), 5.60 (s, 1H, CH), 7.13 (d, 2H, J = 0.75 Hz, Him), 7.22 (d, 2H, J = 0.75 Hz, Him), 7.25 (m, 2H, HPh), 7.33 (m, 3H, HPh) ppm. 13C{1H} NMR (100 MHz, CD3OD, 25 °C): δ = 34.9, 40.6, 69.7, 124.5, 126.5, 129.1, 129.5, 129.7, 136.1, 142.3, 166.2 ppm. Anal. for C17H18Cl2N4O2Zn (446.67): calc. C 45.71, H 4.06, N 12.54; found C 45.64, H 3.95, N 12.61. IR (solid): ν = 3,122.9, 2,911.5, 1,734.5, 1,547.4, 1,510.2, 1,454.4, 1,373.6, 1,263.4, 1,220.8, 1,150.4, 978.2, 958.3, 907.1, 847.8, 785.7, 776.5, 740.1, 696.7, 683.5, 665.3 cm−1. ESI–MS: m/z = 409.0 ([M–Cl]+, calc. 409.0).
[Zn2(MIm2Pr)2(ox)] (direct synthesis)
To a solution of [Zn(MIm2Pr)Cl] (50 mg, 0.15 mmol) in water (5 mL) was added a solution of potassium oxalate hydrate (14 mg, 0.075 mmol) in water (3 mL). The solution was stirred overnight at room temperature, during which gradually a white precipitate formed. The precipitate was separated by centrifugation and washed three times with water (3 × 20 mL). The product [Zn2(MIm2Pr)2(ox)] was dried in vacuo and obtained as a white powder in 82% yield (42 mg, 0.061 mmol). Anal. for C24H26N8O8Zn2·6H2O (685.33): calc. C 36.33, H 4.83, N 14.12; found: C 36.45, H 4.50, N 14.25. IR (solid): ν = 3,476.8, 3,135.4, 1,671.3, 1,651.6, 1,600.2, 1,544.9, 1,504.8, 1,423.6, 1,382.7, 1,320.1, 1,287.2, 1,158.1, 1,137.9, 985.1, 769.4, 745.1, 710.4 cm−1.
[Zn2(MIm2Pr)2(ox)]·6H2O, reaction of [Zn(MIm2Pr)Cl] with sodium pyruvate
For NMR investigation of reaction mixtures, stock solutions of [Zn(-MIm2Pr)Cl] (20 mg, 0.06 mmol) in H2O (1 mL) and sodium pyruvate (13.2 mg, 0.06 mmol) in H2O (2 mL) were prepared. NMR tubes were filled with 0.1 mL of each stock solution and kept either at room temperature or heated to 50 °C. The NMR samples were analyzed at different times after mixing the reagents. Before acquisition of the NMR spectrum, 0.4 mL D2O was added to the reaction mixture. Crystals usually deposited within 2 or 3 days in the NMR tube.
In the case of the catalytic runs, 0.1 mL of a solution containing 20 equiv of sodium pyruvate was added to 0.1 mL of the [Zn(MIm2Pr)Cl] stock solution and dimethyl sulfoxide (5 equiv, 11 μL) was added as an internal standard. The reaction mixture was kept at 50 °C and the NMR spectrum was recorded with addition of 0.4 mL D2O after 24 h.
On a preparative scale, to a solution of ZnCl2 (37 mg, 0.27 mmol) and K[MIm2Pr] (75 mg, 0.27 mmol) in H2O (10 mL) was added sodium pyruvate (30 mg, 0.27 mmol). The solution was heated to 50 °C and gradually an off-white precipitate was formed. After 1 week, the product was separated by centrifugation (29 mg, 31%). The IR spectrum of the product was identical to that of independently synthesized [Zn2(MIm2Pr)2(ox)].
X-ray crystal structure determinations of [Zn(MIm2Pr)Cl(H2O)], [Zn(iPrEtIm2Pr)Cl], ZnCl2(BnMIm2Ac)], [ZnCl2(MIm2CH2)], and [Zn2(MIm2Pr)2(ox)]·6H2O
Reflections were measured with a Nonius Kappa CCD diffractometer with a rotating anode (graphite monochromator, λ = 0.71073 Å) at −123 °C. The structures were refined with SHELXL-97 [31] against F2 of all reflections. Non-hydrogen atoms were refined with anisotropic displacement parameters. Geometry calculations and checking for higher symmetry were performed with the PLATON program [32]. Further details are given in Table 1. Table 1Crystallographic data for compounds [Zn(MIm2Pr)Cl(H2O)], [Zn(iPrEtIm2Pr)Cl], ZnCl2(BnMIm2Ac)], [ZnCl2(MIm2CH2)], and [Zn2(MIm2Pr)2(ox)]·6H2OCompound[Zn(MIm2Pr)Cl(H2O)][Zn(iPrEtIm2Pr)Cl] [ZnCl2(BnMIm2Ac)][ZnCl2(MIm2CH2)][Zn2(MIm2Pr)2(ox)]·6H2O FormulaC11H15ClN4O3ZnC19H29ClN4O2ZnC17H18Cl2N4O2ZnC9H12Cl2N4ZnC24H26N8O8Zn2·6H2OFW352.09446.28446.62312.50793.36Crystal size (mm3)0.30 × 0.12 × 0.090.18 × 0.03 × 0.030.60 × 0.03 × 0.030.30 × 0.15 × 0.060.24 × 0.18 × 0.09Crystal colorColorlessColorlessColorlessColorlessColorlessCrystal systemOrthorhombicMonoclinicMonoclinicMonoclinicTriclinicSpace groupP212121 (no. 19)Cc (no. 9)C2/c (no. 15)P21/c (no. 14)a (Å)8.7159(1)10.1766(2)35.5761(4)7.3547(1)9.9634(7)b (Å)10.1124(1)23.8561(6)7.4531(1)14.9538(2)12.0617(11)c (Å)15.1177(2)9.1995(2)14.6106(2)12.3489(2)14.8889(9)α (°)––––113.021(3)β (°)–107.0078(14)101.8858(5)117.5968(6)90.365(3)γ (°)––––102.513(3)V (Å)1332.45(3)2135.72(8)3790.97(8)1203.62(3)1599.4(2)Z44842Dcalc. (g cm−3)1.7551.3881.5651.7241.647μ (mm−1)2.0561.2961.5972.4611.580Absorption correctionMultiscanMultiscanMultiscanMultiscanMultiscanAbsorption correction range0.71–0.840.86–0.960.85–0.960.74–0.860.64–0.87sin(θ/λ)max (Å−1)0.650.650.600.650.65Reflections (measured/unique)21,618/3,06311,758/4,64428,566/3,42427,341/2,75935,709/7,342Parameters/restraints241/0251/2237/0193/0438/0R1/wR2 [I > 2σ(I)]0.0182/0.04470.0312/0.06940.0309/0.07040.0306/0.07930.0389/0.0867R1/wR2 (all reflections)0.0190/0.04520.0384/0.07230.0512/0.07860.0389/0.08520.0540/0.0947S1.0621.0411.0811.0891.083Flack parameter [37]−0.006(7)0.305(9)–––Residual density (e Å−3)−0.36/0.19−0.27/0.41−0.31/0.43−0.54/1.23−0.79/0.51MIm2Pr 3,3-bis(1-methylimidazol-2-yl)propionate, iPrEtIm2Pr 3,3-bis(1-ethyl-4-isopropylimidazol-2-yl)propionate, and BnMIm2Ac benzyl bis(1-methylimidazol-2-yl)acetate, 22, bis(1-methylimidazol-2-yl)methane
For Zn(MIm2Pr)Cl(H2O)] the structure was solved with SHELXS-97 [33] using direct methods. All hydrogen atoms were located in the difference Fourier map and refined freely with isotropic displacement parameters.
For [Zn(iPrEtIm2Pr)Cl] the structure was solved with DIRDIF-99 [34] using automated Patterson methods. Refinement was performed as an inversion twin. All hydrogen atoms were located in the difference Fourier map and refined with a riding model.
For ZnCl2(BnMIm2Ac)] the structure was solved with DIRDIF-99 [34] using automated Patterson methods. All hydrogen atoms were located in the difference Fourier map and refined with a riding model.
For [ZnCl2(MIm2CH2)] the structure was solved with SIR-97 [35] using Direct methods. All hydrogen atoms were located in the difference Fourier map and refined freely with isotropic displacement parameters.
For [Zn2(MIm2Pr)2(ox)]·6H2O the crystal was nonmerohedrally twinned with a twofold rotation about the crystallographic c-axis as a twin operation. This twin law was taken into account during the intensity integration using the program EvalCCD [36]. The structure was solved on the nonoverlapping reflections with DIRDIF-99 [34] using automated Patterson methods. All hydrogen atoms were located in the difference Fourier map and refined with a riding model. The twin fraction refined to 0.127(3).
Crystallographic data (without structure factors) for the structures reported in this paper have been deposited with the Cambridge Crystallographic Data Centre (CCDC) as supplementary publication nos. CCDC-637452 for Zn(MIm2Pr)Cl(H2O)], CCDC-637453 for [Zn(iPrEtIm2Pr)Cl], CCDC-637454 for ZnCl2(BnMIm2Ac)], CCDC-637455 for [ZnCl2(MIm2CH2)], and CCDC-637456 for [Zn2(MIm2Pr)2(ox)]·6H2O. Copies of the data can be obtained free of charge from the CCDC (12 Union Road, Cambridge CB2 1EZ, UK; Tel.: +44-1223-336408; Fax: +44-1223-336003; e-mail: deposit@ccdc.cam.ac.uk; Web site http://www.ccdc.cam.ac.uk).
Results
As part of our efforts to study the coordination chemistry of the substituted 3,3-bis(1-alkylimidazol-2-yl)propionate ligand family, the complexes of MIm2Pr and iPrEtIm2Pr with ZnCl2 were synthesized and structurally characterized (Fig. 2). MIm2Pr is the parent ligand of the family and iPrEtIm2Pr offers the most steric hindrance of the ligands studied so far. The coordination chemistry of MIm2Pr and iPrEtIm2Pr with zinc was found to be stoichiometry-dependent.
Zinc complexes of MIm2Pr
The formation of different species upon addition of either 0.5 or 1.0 equiv of ZnCl2 to a solution of the potassium salt of MIm2Pr in D2O is evidenced by the respective 1H NMR spectra in D2O (Fig. 3). Fig. 3400 MHz 1H NMR spectra of the titration of K[MIm2Pr] with ZnCl2 in D2O at room temperature: a K[MIm2Pr]; b K[MIm2Pr] and ZnCl2 2:1; c K[MIm2Pr] and ZnCl2 1:1. Asterisks denote the residual solvent peak
All four resonances associated with ligand MIm2Pr (spectrum a) shifted upon addition of 0.5 equiv of ZnCl2 (spectrum b). The observation of two sharp signals of equal intensity for the imidazole protons shows that all four imidazole groups are equivalent. This is indicative of symmetric binding of both ligands to the zinc ion. The resonances of the methyl groups and the methine proton at the bridging carbon shift downfield relative to the free ligand, whereas the propionate CH2 protons shift upfield. Note that the methine proton resonance of the free ligand is hidden under the residual solvent signal in spectrum a. The lack of any other resonances suggests the clean formation of a neutral, 2:1 [Zn(MIm2Pr)2] complex. Upon addition of another 0.5 equiv of ZnCl2 several resonances shift and signal broadening is observed (spectrum c). Especially, one of the two imidazole signals is severely broadened. Again, no free ligand is observed in the spectrum. These observations indicate a dynamic system with an overall 1:1 ligand–zinc stoichiometry.
The isolation and purification of the complex formed with 2 equiv of MIm2Pr with respect to ZnCl2 proved troublesome. The separation of the inorganic salts formed as a by-product in the synthesis by means of repetitive washing in all cases resulted in partial loss of coordinated ligand (as observed by NMR), regardless of the A[MIm2Pr] salt (A is K+, [Bu4N]+) originally employed. We therefore attempted the synthesis of this complex by the reaction of 2 equiv of the acid HMIm2Pr with Zn[N(SiMe3)2]2. Zinc bis(trimethylsilyl)amide can be synthesized in sizeable quantities [30] and was found to be a convenient reagent in this case. Indeed, the reaction of Zn[N(SiMe3)2]2 with 2 equiv of HMIm2Pr in acetonitrile resulted in the formation of a white precipitate, which was recrystallized from an acetonitrile–water solution and was identified as the 2:1 complex [Zn(MIm2Pr)2].
Unfortunately, all attempts at obtaining single crystals of [Zn(MIm2Pr)2] failed. On the basis of ESI–MS, IR, and the previously described NMR analysis, we propose that [Zn(MIm2Pr)2] (Fig. 4) is isostructural to the crystallographically characterized analogous complex [CuII(MIm2Pr)2] [27]. The position of the symmetric and asymmetric carbonyl stretching vibrations proved to be indicative of the binding mode of MIm2Pr (Bruijnincx et al., submitted) [27]. The νas(C=O) and νs(C=O) are found at 1,581 and 1,391 cm−1, respectively, indicative of a monodentate binding mode of the carboxylate [Δ(νas − νs) = 190 cm−1, identical to Δionic] [38]. Furthermore, the clean ESI–MS spectrum of [Zn(MIm2Pr)2] confirmed the structure of a mononuclear, neutral 2:1 species with [M + X]+ (X is H, Na, K) as the prominent ions. Fig. 4The 2:1 complex [Zn(MIm2Pr)2]
The 1:1 complex [Zn(MIm2Pr)Cl)] was synthesized by the reaction of equimolar amounts of ZnCl2 and K[MIm2Pr] in dry methanol. This led to the precipitation and isolation of [Zn(MIm2Pr)Cl] in almost quantitative yield. The broadened signals in the 1H NMR spectrum of [Zn(MIm2Pr)Cl)] in D2O prompted us to study this complex by variable-temperature NMR (Fig. 5). As complex [Zn(MIm2Pr)Cl)] is insoluble in organic solvents, the variable-temperature NMR studies were limited to aqueous solutions, which restricted the temperature range that could be studied. Spectra were recorded in the range from 2 to 70 °C. Fig. 51H NMR resonances of the imidazole rings of [Zn(MIm2Pr)Cl] at various temperatures in D2O
Figure 5 shows that at elevated temperatures two sharp imidazole resonances are observed. In the spectrum recorded at 70 °C, the (small) coupling between the two imidazole protons H1 and H2 can also be discerned. The imidazole signal at higher field broadens upon cooling and ultimately splits into two separate (broad) resonances. This implies that at higher temperature the imidazole H1 protons (Fig. 3) become magnetically equivalent, whereas at lower temperature exchange slows down with the result that decoalesence into two broad resonances occurs. This behavior can be tentatively explained by the possibility of two different spatial orientations of the imidazole rings with respect to the carboxylate group of a second ligand upon coordination to zinc (vide infra), which would result in nonequivalent imidazole H1 protons at lower temperature. At higher temperatures these orientations average on the NMR timescale and only one signal is observed.
The asymmetric and symmetric carbonyl stretch vibrations of isolated [Zn(MIm2Pr)Cl] were observed at 1,615 and 1,390 cm−1 [Δ(νas − νs) = 225 cm−1], respectively. The asymmetric stretch thus shifted to higher wavenumbers compared with that observed for [Zn(MIm2Pr)Cl] and a different, but still monodentate coordination mode of the carboxylate must therefore be present in [Zn(MIm2Pr)Cl]. The ESI–MS spectrum of [Zn(MIm2Pr)Cl] showed the presence of several different species, with ions corresponding to, e.g., [MIm2Pr + Zn]+, [2(MIm2Pr) + Zn + H]+, [2(MIm2Pr) + 2Zn + Cl]+, [3(MIm2Pr) +2Zn + H]2+ and [3(MIm2Pr) + 2Zn]+. On the basis of the NMR, IR, and ESI–MS data, the structure of [Zn(MIm2Pr)Cl] can be described as an oligomeric/polymeric species both in the solid state and in solution. This was confirmed by the single-crystal structure determination of [Zn(MIm2Pr)Cl(H2O)].
Crystal structure of [Zn(MIm2Pr)Cl(H2O)]
Colorless single crystals of complex [Zn(MIm2Pr)Cl(H2O)] suitable for X-ray diffraction were obtained from a solution of [Zn(MIm2Pr)Cl] in H2O upon standing. The molecular structure of [Zn(MIm2Pr)Cl(H2O)] is depicted in Fig. 6, with selected bond lengths and angles presented in Table 2. Fig. 6Molecular structure of [Zn(MIm2Pr)Cl(H2O)] in the crystal, showing three molecules of the carboxylato-bridged coordination polymer, forming a one-dimensional chain in the crystallographic c-direction. C–H hydrogen atoms have been omitted for clarity. Displacement ellipsoids are drawn at the 50% probability level. Symmetry operation a 3/2 − x, 1 − y, z − 1/2Table 2Selected bond lengths (angstroms) and bond angles (degrees) for [Zn(MIm2Pr)Cl(H2O)]BondBond lengthAngleBond angleAngleBond angleZn1–N22.1203 (15)N4–Zn1–N288.05 (6)N2–Zn1–O1a92.41 (5)Zn1–N42.0517 (14)N2–Zn1–O3173.58 (5)O1a–Zn1–O382.24 (5)Zn1–O1a1.9975 (11)O3–Zn1–N490.38 (5)N4–Zn1–Cl1119.50 (4)Zn1–Cl12.2844 (4)O3–Zn1–Cl190.23 (4)Cl1–Zn1–O1a130.40 (4)Zn1–O32.1860 (13)Cl1–Zn1–N295.97 (4)O1a–Zn1–N4109.55 (5)Symmetry operation a 3/2 − x, 1 − y, z − 1/2
The single-crystal structure determination reveals that the structure of the metal complex is that of a coordination polymer. The zinc atom is coordinated by two 1-methylimidazole groups of one MIm2Pr ligand, while the carboxylato group of this ligand coordinates to a neighboring zinc atom. In this way an infinite one-dimensional chain parallel to the crystallographic c-axis is formed. The chlorido anion and a coordinated water molecule complete the coordination sphere around each zinc center and render it five-coordinate. Thus, a new, bridging coordination mode of MIm2Pr is observed in the structure of [Zn(MIm2Pr)Cl(H2O)] comprising N,N-bidentate coordination to one metal center and O-monodentate coorrdination to a second one. This coordination mode complements the alternative N,N,O-tridentate facial capping mode of the ligand, which is commonly observed for divalent transition metals [26, 27, 39]. The coordination geometry around the five-coordinate Zn atom is distorted trigonal bipyrimidal (τ = 0.72) [40]. The equatorial positions are occupied by N4 and the monoanionic carboxylato O1a and chlorido Cl1 ligands with O1a–Zn1–N4, Cl1–Zn1–O1a, and N4–Zn1–Cl1 angles of 109.55(5), 130.40(4), and 109.50(4)°, respectively (Σangles = 360°). The axial positions are occupied by O3 of the water molecule and imidazole N2 [O3–Zn1–N2 173.58(5)°]. The Zn1–N2 and Zn1–N4 bond lengths are different [2.1203(15) and 2.0517(14) Å], consistent with their respective axial and equatorial positions. The Zn1–O1a distance of 1.9975(11) Å is slightly longer than the Zn–Ocarboxylato bond length observed in [Zn(bdatBu2,Me2)Cl]2 (1.942 Å) [12] [bpatBu2,Me2 is (3,5-di-tert-butylpyrazol-1-yl)(3′,5′-dimethylpyrazol-1-yl)acetate], in which a carboxylato group is similarly coordinated to the metal center. The zinc-aquo distance of 2.1860(13) Å in [Zn(MIm2Pr)Cl(H2O)] is rather long compared with most reported Zn–OH2 distances (mean Zn–OH2 distance for five-coordinate zinc complexes is 2.046 Å; Cambridge Structural Database version 5.27) [41]. In the related coordination polymer [Zn(bpatBu,Me)(H2O)](ClO4) [11] [bpatBu,Me is bis(5-tert-butyl-3-methylpyrazol-2-yl)acetate], where the water molecule occupies a position in the basal plane of the trigonal bipyramid, the zinc–water distance, for example, amounts to 1.961 Å. [Zn(MIm2Pr)Cl(H2O)] crystallizes in the noncentrosymmetric space group P212121 and the determination of the Flack parameter indicates that the crystal is enantiomerically pure.
The one-dimensional linear chains in Zn(MIm2Pr)Cl(H2O)] are interconnected via hydrogen-bonding interactions into a two-dimensional network (Fig. 7, Table 3). The water molecule binds via its two hydrogen atoms by hydrogen bonding to a carbonyl O and a chlorido anion as hydrogen acceptors of a [Zn(MIm2Pr)Cl(H2O)] molecule in a neighboring strand. Fig. 7Hydrogen-bonding network in [Zn(MIm2Pr)Cl(H2O)]. Hydrogen bonds connect the infinite one-dimensional chains into a two-dimensional network. C–H hydrogen atoms have been omitted for clarity. Symmetry operations i 1 − x, 1/2 + y, 1/2 − z; ii −1/2 + x, 3/2 − y, −zTable 3Selected hydrogen bond lengths (angstroms) and angles (degrees) for [Zn(MIm2Pr)Cl(H2O)]Donor–H···acceptorD–HH···AD···AD–H···AO3–H1O···O2i0.78 (3)1.95 (3)2.7307 (19)176 (3)O3–H2O···Cl1ii0.84 (3)2.31 (3)3.1472 (15)171 (3)Symmetry operations i 1 − x, 1/2 + y, 1/2 − z; ii −1/2 + x, 3/2 − y, −z
Zinc complexes of iPrEtIm2Pr
In order to assess the influence of the presence of steric bulk in the ligand framework, we also studied the zinc coordination chemistry of ligand iPrEtIm2Pr. The ethyl and isopropyl substituents of this ligand provide significant steric hindrance and, moreover, increase the solubility of its complexes in organic solvents. The 1:1 complex [Zn(iPrEtIm2Pr)Cl] was synthesized by the reaction of equimolar amounts of ZnCl2 and K[iPrEtIm2Pr] in methanol. The complex was isolated in almost quantitative yield. The IR spectrum of [Zn(iPrEtIm2Pr)Cl] showed the asymmetric and symmetric carbonyl stretch vibrations at 1,627 and 1,381 cm−1 [Δ(νas − νs) = 246 cm−1], respectively. The structure of [Zn(iPrEtIm2Pr)Cl] was determined by X-ray crystal structure determination.
Crystal structure of [Zn(iPrEtIm2Pr)Cl]
Colorless single crystals of [Zn(iPrEtIm2Pr)Cl] suitable for X-ray diffraction were obtained from a solution of [Zn(iPrEtIm2Pr)Cl] in methanol. The molecular structure of [Zn(iPrEtIm2Pr)Cl] is depicted in Fig. 8, with selected bond lengths and angles presented in Table 4. The crystal structure shows that whereas like [Zn(MIm2Pr)Cl(H2O)] [Zn(iPrEtIm2Pr)Cl] also crystallizes as a coordination polymer, the molecular structure of [Zn(iPrEtIm2Pr)Cl] is noticeably different. Unlike in the structure of [Zn(MIm2Pr)Cl(H2O)], no water molecule is found to coordinate to zinc in [Zn(iPrEtIm2Pr)Cl]. Instead of the five-coordinate structure in [Zn(MIm2Pr)Cl(H2O)], the zinc atom in [Zn(iPrEtIm2Pr)Cl] is four-coordinate. It has a pseudotetrahedral coordination geometry with two imidazole groups of one iPrEtIm2Pr ligand, a chlorido, and a carboxylato group of a neighboring ligand coordinated to it. The bridging mode of iPrEtIm2Pr observed in [Zn(iPrEtIm2Pr)Cl] is the same as found in [Zn(MIm2Pr)Cl(H2O)]. The deviation from ideal tetrahedral geometry is reflected in the observed bond angles [91.90(9)–119.66(6)°]. The smallest angle (N2–Zn1–N4) is caused by the inherent geometrical restrictions imposed by the ligand and the largest angle (Cl1–Zn1–O1a) probably reflects the steric influence of the isopropyl groups. The Zn–N, Zn–O, and Zn–Cl bond lengths in [Zn(iPrEtIm2Pr)Cl] are shorter than the corresponding distances in [Zn(MIm2Pr)Cl(H2O)], which reflects the stronger Lewis acidity of zinc(II) in a four-coordinate versus a five-coordinate geometry. The Zn1–O1a bond length of 1.9628(19) Å is comparable to values reported for other tetrahedral N,N,O zinc complexes with a monodentate carboxylate [9, 11, 12, 18]. The different coordination geometry at zinc results in a more folded zigzag polymer chain in [Zn(iPrEtIm2Pr)Cl], compared with the more stretched zigzag chain observed for [Zn(MIm2Pr)Cl(H2O)]. Fig. 8Molecular structure of [Zn(iPrEtIm2Pr)Cl] in the crystal, showing three molecules of the carboxylato-bridged coordination polymer. C–H hydrogen atoms have been omitted for clarity. Displacement ellipsoids are drawn at the 50% probability level. Symmetry operation a x, 1 − y, z − 1/2Table 4Selected bond lengths (angstroms) and bond angles (degrees) for [Zn(iPrEtIm2Pr)Cl]BondBond lengthAngleBond angleAngleBond angleZn1–N22.038 (2)Cl1–Zn1–N2116.85 (7)N2–Zn1–O1a108.85 (8)Zn1–N42.045 (2)Cl1–Zn1–N4112.12 (7)N4–Zn1–O1a103.15 (9)Zn1–O1a1.9628 (19)Cl1–Zn1–O1a119.66 (6)Zn1–Cl12.2146 (7)N2–Zn1–N491.90 (9)Symmetry operation a x, 1 − y, z − 1/2
The structure of [Zn(iPrEtIm2Pr)Cl] and more generally the coordination chemistry of iPrEtIm2Pr with Zn in methanol solution were studied by 1H NMR spectroscopy and ESI–MS. The 1H NMR spectra of a titration experiment comprising the addition of ZnCl2 to the potassium salt of iPrEtIm2Pr in methanol solution are shown in Fig. 9. Fig. 9400 MHz 1H NMR spectra of the titration of K[iPrEtIm2Pr] with ZnCl2 in CD3OD at room temperature: a K[iPrEtIm2Pr] and ZnCl2 1:0; b K[iPrEtIm2Pr] and ZnCl2 1:1; c K[iPrEtIm2Pr] and ZnCl2 2:1; d K[iPrEtIm2Pr] and ZnCl2 3:1. Asterisks denote the residual solvent peak and H2O
Comparison of the 1H NMR spectra of K[iPrEtIm2Pr] (Fig. 9, spectrum a) with that of an equimolar K[iPrEtIm2Pr]–ZnCl2 mixture shows significant shifts pointing to the formation of a [Zn(iPrEtIm2Pr)Cl] complex (spectrum b). In the 1H NMR spectrum of [Zn(iPrEtIm2Pr)Cl] one sharp resonance is observed for the imidazole protons at 6.99 ppm, which is shifted to lower field compared with that of the free ligand. The two imidazole rings are equivalent on the NMR timescale, suggesting that they are symmetrically bound to zinc. Importantly, the CH2 resonance of the ethyl groups is shifted and split into two signals of equal intensity (4.20 and 4.32 ppm). Similarly, the methyl groups of the isopropyl substituent now appear as two doublets of equal intensity. These splittings confirm that the CH2(Et) and CH3(iPr) groupings contain diastereotopic protons and methyl groups, respectively. No splitting or change in multiplicity of the resonances associated with the propionate backbone is observed, which is indicative of mirror symmetry of the complex in solution. Although at first sight the NMR data would allow an assignment of the solution structure for [Zn(iPrEtIm2Pr)Cl] as a mononuclear zinc complex with a facially capped ligand, a bridged oligomeric species (similar as the one discussed before for [Zn(MIm2Pr)Cl]) is preferred. ESI–MS data support the latter description since trimeric, dimeric, and monomeric species are observed. Prominent ions corresponding to [Zn(iPrEtIm2Pr)Cl + H]+, [Zn2(iPrEtIm2Pr)2Cl2 + H]+, and [Zn3(iPrEtIm2Pr)3Cl3 + H]+ are, for instance, observed. The structure of [Zn(iPrEtIm2Pr)Cl] in solution can therefore best be described as a coordination polymer similar to [Zn(MIm2Pr)Cl].
Upon addition of 2 equiv of K[iPrEtIm2Pr] to ZnCl2 in CD3OD (Fig. 9, spectrum c), a second species is formed next to the 1:1 complex [Zn(iPrEtIm2Pr)Cl]. The fact that two species are observed, indicates that an equilibrium is obtained, i.e., the clean formation of a symmetrical 2:1 iPrEtIm2Pr to zinc complex, as previously observed for MIm2Pr, does not occur. Further inspection of the 1H NMR spectrum indicated that all resonances of the 1-ethyl-4-isopropylimidazole groups of the new species are split into two signals of equal intensity, e.g., the imidazole ring protons at 7.37 and 7.19 ppm. This suggests that the two imidazole groups are inequivalent and experience a drastically different magnetic environment. For instance, the two resonances of the CH protons of the two isopropyl groups are found at 2.96 and 1.03 ppm. Furthermore, the methine proton at the bridging carbon is still a single triplet, whereas the CH2 group of the propionate backbone has become diastereotopic. Variable-temperature NMR studies over a temperature range from −40 to 60 °C did not show significant spectral changes. The ESI–MS spectrum of a 2:1 K[iPrEtIm2Pr]–ZnCl2 solution showed prominent ions corresponding to the [Zn(iPrEtIm2Pr)2 + H]+ and [Zn(iPrEtIm2Pr)2 + 2H]2+ cations.
On the basis of these data, the structure of the new species is proposed to be a 2:1 ligand-to-zinc complex [Zn(iPrEtIm2Pr)2] in which each ligand is coordinated via its carboxylate group and one imidazole group, resulting in a four-coordinated zinc(II) ion (Fig. 10). The coordination of two of the three donor atoms in a 2:1 ligand-to-metal complex has also been observed with bulky N,N,S(thiolate) ligands in the complexes [Zn(L2S)2] [L2SH is (3-tert-butyl-5-methyl-2-thiophenyl)bis(3,5-dimethyl pyrazolyl) methane] and [Zn(L2)2] [L2 is 1-methoxy-2-methyl-1,1-bis(1-methyl-4,5-diphenyl-1H-imidazol-2-yl)propane-2-thiol] [42, 43]. The addition of a third equivalent of K[iPrEtIm2Pr] (spectrum d) results in the disappearance of the signals associated with [Zn(iPrEtIm2Pr)Cl] and both free ligand K[iPrEtIm2Pr] and [Zn(iPrEtIm2Pr)2] are detected. No attempts to isolate [Zn(iPrEtIm2Pr)2] were undertaken. Fig. 10Proposed structure for [Zn(iPrEtIm2Pr)2]. The structure was calculated by molecular mechanics geometry optimization (Spartan SGI [44] MMFF94)
Attempted synthesis of a zinc bis(1-methylimidazol-2-yl)acetate complex
Recently, the zinc coordination chemistry of bis(pyrazolyl)acetates (bpa) has been actively explored to mimic the 2-His-1-carboxylate facial triad found in several different zinc enzymes [9, 11, 12, 18, 19]. The backbone of bpa is more rigid than the backbone of the ligands MIm2Pr and iPrEtIm2Pr used in the present study. In particular, the bridging N,N-bidentate-O-monodentate coordination mode reported here is less favorable for bpa, for which a strong predisposition to an N,N,O facial capping mode is expected. The synthesis of zinc complexes of MIm2Ac would therefore be desirable, since it would allow a direct comparison of the respective pyrazole and imidazole donor groups (Fig. 11).Fig. 11Ligands benzyl bis(1-methylimidazol-2-yl)acetate (BnMIm2Ac), MIm2Ac, and bis(pyrazolyl)acetate (bpa) and the synthesis of complexes [ZnCl2(BnMIm2Ac)] and [ZnCl2(MIm2CH2)]
We attempted the synthesis of ligand MIm2Ac, but found that the common synthetic procedure, i.e., lithiation of MIm2CH2 followed by reaction with carbon dioxide [45], did not result in product formation in acceptable yields. Instead, it was found that the ligand or an ester precursor of the ligand decarboxylates rather easily, as was also observed by Peters et al. [39]. The benzyl ester of bis(1-methylimidazol-2-yl)acetic acid (BnMIm2Ac), however, was sufficiently stable and could be isolated and purified after reaction of lithio bis(1-methylimidazol-2-yl)methane with benzylchloroformate. Since hydrolysis and hydrogenolysis of the ester moiety resulted in decomposition of the ligand, the 1:1 zinc dichloride complex with BnMIm2Ac was synthesized in the hope that autocatalytic ester hydrolysis by the zinc complex would yield the desired zinc bis(1-methylimidazol-2-yl)acetate complex. A similar approach, involving the autocatalytic saponification of a zinc bis(picolyl)glycine ethyl ester complex, has been reported by Abufarag and Vahrenkamp [46].
The reaction of equimolar amounts of ZnCl2 and BnMIm2Ac gave the complex [ZnCl2(BnMIm2Ac)] as a white powder in quantitative yield. The structure of [ZnCl2(BnMIm2Ac)] was determined by X-ray diffraction studies and showed the BnMIm2Ac ligand bound to the zinc center in an N,N-bidentate way (Fig. 12, top). However, stirring of [ZnCl2(BnMIm2Ac)] in H2O at room temperature in an attempt to obtain [Zn(MIm2Ac)Cl] resulted in decomposition of the ligand and formation of [ZnCl2(MIm2CH2)], which was isolated as one of the decomposition products. [ZnCl2(MIm2CH2)] was also characterized crystallographically (Fig. 12, bottom). On the basis of these findings, it was concluded that the MIm2Ac monoanion is too unstable for practical studies and the pursuit of this ligand was abandoned. Fig. 12Molecular structures of [ZnCl2(BnMIm2Ac)] (top) and [ZnCl2(MIm2CH2)] (bottom) in the crystal. Displacement ellipsoids are drawn at the 50% probability level. All hydrogen atoms have been omitted for clarity. Selected bond lengths (angstroms) as follows: [ZnCl2(BnMIm2Ac)], Zn1–N2 2.011(2), Zn1–N4 1.994(2), Zn1–Cl1 2.2242(7), Zn1–Cl2 2.2554(7); [ZnCl2(MIm2CH2)], Zn1–N2 2.0247(17), Zn1–N4 2.0222(17), Zn1–Cl1 2.2110(6), Zn1–Cl2 2.2504(6)
Reaction of [Zn(MIm2Pr)Cl] with sodium pyruvate: oxalate formation
In an attempt to prevent the formation of coordination polymers of MIm2Pr and to obtain mononuclear complexes with the desired N,N,O facial capping mode of the ligand, several different monoanionic, bidendate coligands were used. In the course of this study, we also investigated the reaction of [Zn(MIm2Pr)Cl] with sodium pyruvate. The addition of 1 equiv of sodium pyruvate to a solution of [Zn(MIm2Pr)Cl] in D2O resulted in the appearance of a new singlet at 2.38 ppm in the 1H NMR spectrum (Fig. 13, top spectrum). The signal is not shifted with respect to the signal observed for free sodium pyruvate, which indicates that the interaction between [Zn(MIm2Pr)Cl] and the pyruvate anion is weak, at best. The ESI–MS spectrum of an aqueous solution of equimolar amounts of [Zn(MIm2Pr)Cl] and sodium pyruvate, however, did show some major ions corresponding to a mononuclear {[Zn(MIm2Pr)(pyruvate)] + Na}+ and a dinuclear {[Zn2(MIm2Pr)2(pyruvate)]}+ cation, in addition to the ions observed in the ESI–MS spectrum of a solution of [Zn(MIm2Pr)Cl] in water. Fig. 131H NMR spectra of a solution of [Zn(MIm2Pr)Cl] in D2O (asterisk) after addition of 1 equiv of sodium pyruvate and the same solution after 24 h
However, when the mixture of [Zn(MIm2Pr)Cl] and pyruvate in D2O was followed in time by NMR spectroscopy, the gradual decrease in intensity of the signal of the pyruvate methyl group was noticed until it ultimately disappeared (Fig. 13, bottom spectrum). After several days colorless crystals suitable for X-ray diffraction studies were formed in the NMR tube. To our surprise, the X-ray crystal structure showed the structure of the zinc complex [Zn2(MIm2Pr)2(ox)]·6H2O. [Zn2(MIm2Pr)2(ox)]·6H2O has the structure of a coordination polymer involving [Zn(MIm2Pr)]+ cations bridged by oxalato dianions.
To unequivocally prove the relevance of the structure obtained, zinc coordination polymer [Zn2(MIm2Pr)2(ox)] was also synthesized independently. Indeed, the addition of 0.5 equiv of K2C2O4·H2O to an aqueous solution of [Zn(MIm2Pr)Cl] yielded a white, insoluble powder. This powder was identified as [Zn2(MIm2Pr)2(ox)] by elemental analysis and IR spectroscopy. Typical features of the IR spectrum of [Zn2(MIm2Pr)2(ox)] include absorptions at 1,674, 1,638, 1,591, and 1,399 cm−1, which can be attributed to the oxalato and carboxylato groups in the polymer. Crystallization of [Zn2(MIm2Pr)2(ox)] proved difficult given its general insolubility. Single crystals could, however, be obtained via the gel crystallization method [47]. A gel formed from a solution of tetramethoxysilane and potassium oxalate hydrate in water was layered with an aqueous solution of K[MIm2Pr] and ZnCl2. Single crystals were obtained after several days and could be unequivocally identified as [Zn2(MIm2Pr)2(ox)] by X-ray crystal structure determination.
Crystal structure of [Zn2(MIm2Pr)2(ox)]·6H2O
The molecular structure of [Zn2(MIm2Pr)2(ox)]·6H2O is shown in Fig. 14, with selected bond lengths and angles presented in Table 5.1 The polymeric structure of [Zn2(MIm2Pr)2(ox)]·6H2O comprises two crystallographically inequivalent zinc atoms. Both zinc centers are five-coordinate and differ only in the slightly different bond lengths and angles. Each zinc atom is coordinated in an N,N-bidentate fashion by the 1-methylimidazole groups of one MIm2Pr ligand, two oxygen atoms of a bis O,O-bidentate bridging oxalato group, and a carboxylato O donor atom of a neighboring MIm2Pr ligand. The combination of the bridging oxalato and MIm2Pr ligands constructs a polymeric structure leading to the formation of two-dimensional sheets. Fig. 14Molecular structure of [Zn2(MIm2Pr)2(ox)]·6H2O in the crystal. C–H hydrogen atoms and all solvent water molecules have been omitted for clarity. Displacement ellipsoids are drawn at the 50% probability levelTable 5Selected bond lengths (angstroms) and angles (degrees) for [Zn2(MIm2Pr)2(ox)]·6H2OBondBond lengthAngleBond angleAngleBond angleZn1–O312.258 (2)O11–Zn1–O3189.48 (8)O21–Zn2–O4190.33 (8)Zn1–O322.018 (2)O11–Zn1–O32124.09 (9)O21–Zn2–O42125.19 (8)Zn1–O111.9694 (19)O11–Zn1–N22b108.91 (9)O21–Zn2–N12109.08 (9)Zn1–N24b2.046 (2)O11–Zn1–N24b107.61 (9)O21–Zn2–N14107.85 (9)Zn1–N22b2.088 (2)O31–Zn1–O3277.16 (9)O41–Zn2–O4277.84 (8)Zn2–O412.233 (2)O31–Zn1–N22b161.43 (9)O41–Zn2–N12160.51 (8)Zn2–O422.0179 (19)O31–Zn1–N24b87.69 (9)O41–Zn2–N1487.24 (9)Zn2–O211.9630 (19)O32–Zn1–N22b89.94 (8)O42–Zn2–N1289.12 (8)Zn2–N122.089 (2)O32–Zn1–N24b125.38 (9)O42–Zn2–N14124.42 (9)Zn2–N142.052 (2)N22b–Zn1–N24b89.06 (9)N12–Zn2–N1488.34 (9)C31–O311.248 (4)C41–O411.250 (3)C31d–O321.252 (4)C41c–O421.254 (3)C31–C31d1.544 (6)C41–C41c1.537 (6)Symmetry operations b 1 + x, y, z; c −x, 1 − y, −z; d 1 − x, 1 − y, 1 − z
The coordination geometry around each zinc center is best described as a severely distorted trigonal bipyramid, with τ values of 0.60 and 0.59 for Zn1 and Zn2, respectively [40]. The equatorial positions are occupied by N24b/N14, and the anionic carboxylato O11/O21 and oxalato O32/O42 ligands for Zn1/Zn2, respectively. The axial positions are occupied by O31/O41 of the oxalato group and imidazole N22b/N12 for Zn1/Zn2 [O–Zn–N angles of 161.43(9)°/160.51(8)°]. The planar oxalato moieties are rather asymmetrically bound to the metal centers, with the Zn1–O31 and Zn1–O32 distances amounting to 2.258(2) and 2.018(2) Å, for instance. This asymmetry has also been observed in other bisbidentate bridged zinc–oxalato complexes with five-coordinated metal centers and is consistent with the occupation of one equatorial and one axial site of the trigonal bipyramid [10, 48]. The shortest Zn···Zn distances via the bridging oxalato group are 5.5667(7) Å for Zn1···Zn1d and 5.5332(7) Å for Zn2···Zn2c. The distances of the metal centers to the donor atoms of MIm2Pr follow the same trend as observed in [Zn(MIm2Pr)Cl(H2O)]. The crystal structure of [Zn2(MIm2Pr)2(ox)]·6H2O is further stabilized by 14 different hydrogen-bonding interactions, which involve the cocrystallized water molecules, the oxalato bridges, and the carboxylato groups (Fig. 15). The oxalato groups are, for instance, involved in two chelated, three-centered hydrogen bonds with two water molecules via four O–H···O hydrogen bonds. Fig. 15Hydrogen bonding network and crystal packing of [Zn2(MIm2Pr)2(ox)]·6H2O
Discussion
The aim of this investigation was to explore the biomimetic modeling potential of the MIm2Pr ligand family with respect to mononuclear zinc enzymes featuring the 2-His-1-carboxylate facial triad. Complex [Zn(MImPr)2] shows that the facial capping mode of MIm2Pr, as earlier observed with copper [27, 28] and iron (Bruijnincx et al., submitted) [26], is also accessible for zinc. The crystal structure of the 1:1 complex [Zn(MIm2Pr)Cl(H2O)], however, shows that MIm2Pr can also adopt other coordination modes. The formation of the oligomeric/polymeric structure of [Zn(MIm2Pr)Cl], both in solution and in the solid state, can be attributed to the flexibility of the ligand due to the CH2 spacer of the propionate backbone. The formation of polymeric [Zn(MIm2Pr)Cl(H2O)] is further aided by the insolubility of the resulting coordination polymer. Two strategies were explored to circumvent polymer formation, i.e., increasing the steric bulk of the ligand and elimination of the CH2 spacer. The latter approach demanded the synthesis of the bis(1-methylimidazol-2-yl)acetate analogue of MIm2Pr, which was found to be rather unstable and its zinc complexes could not be obtained. The effect of more steric bulk was studied with iPrEtIm2Pr, which contains the sterically more demanding isopropyl and ethyl substituents. Zinc complexes of iPrEtIm2Pr are polymeric in nature as well, albeit with a different coordination geometry at the zinc atom as observed in [Zn(MIm2Pr)Cl(H2O)]. The first coordination spheres of [Zn(MIm2Pr)Cl(H2O)] and [Zn(iPrEtIm2Pr)Cl] are compared in Fig. 16. The added steric bulk in iPrEtIm2Pr causes the zinc ion to adopt a pseudotetrahedral geometry in [Zn(iPrEtIm2Pr)Cl], instead of the distorted trigonal bipyramid observed for [Zn(MIm2Pr)Cl(H2O)]. The coordination around zinc in [Zn(iPrEtIm2Pr)Cl] resembles the first coordination sphere of the active sites of the zinc enzymes thermolysin, carboxypeptidase A, and neutral protease. The polymeric rather than monomeric nature of the complex, however, clearly limits its potential as a biological mimic. Fig. 16Comparison of the first coordination spheres of the zinc atoms in the coordination polymers [Zn(MIm2Pr)Cl(H2O)] (top) and [Zn(iPrEtIm2Pr)Cl] (bottom)
The isolation of [Zn2(MIm2Pr)2(ox)] from the reaction of [Zn(MIm2Pr)Cl] with sodium pyruvate was rather unexpected and is, to the best of our knowledge, the first report of the nonoxidative formation of oxalate from pyruvate (Scheme 1).2 Interestingly, Takeste and Vahrenkamp [51] very recently reported a trispyrazolylborato–zinc complex with pyruvate, representing the first structurally characterized zinc complex of an α-keto acid. Although in their case the zinc–pyruvate complex obtained was stable, unexpected cleavage reactions with other α-keto acids, such as benzoylformic acid, and α-diketones were observed.Scheme 1The observed transformation of pyruvate to oxalate mediated by [Zn(MIm2Pr)Cl]
A control experiment showed that an aqueous sodium pyruvate solution is stable under the reaction conditions. Apparently, [Zn(MIm2Pr)Cl] mediates the conversion of pyruvate to oxalate. The product [Zn2(MIm2Pr)2(ox)] invariably deposited as crystals in experiments done on the NMR scale. On a preparative scale (0.3 mmol), the reaction also takes place, yet the product was isolated as an insoluble white powder. The IR spectrum of this powder was identical to that obtained for independently synthesized [Zn2(MIm2Pr)2(ox)]. Most importantly, further experiments revealed that the reaction is catalytic in zinc. For example, addition of 20 equiv of sodium pyruvate to a solution of [Zn(MIm2Pr)Cl] in water under ambient conditions resulted in ten turnovers after 24 h. The disappearance of the pyruvate methyl signal was also observed in a solution containing ZnCl2 and sodium pyruvate without any further additives. Concomitantly, a precipitate was observed in the NMR tube. However, the composition of this precipitate, i.e., the formation of oxalate, could not be unambiguously established.
Since it is known that pyruvate keto–enol tautomerization is catalyzed by divalent metal ions such as Zn(II) [52, 53], the disappearance of the methyl signal could in principle also be attributed to consecutive H/D-exchange steps. This would yield the deuterated pyruvate-d3 anion, which is also invisible in 1H NMR spectroscopy. To be able to exclude this possibility, experiments were performed in H2O and samples were diluted with 2 equiv of D2O just before the measurement. The same observations were made as previously, i.e., gradual disappearance of the methyl signal and slow crystallization of the product in the NMR tube. The presence of appreciable amounts of enolate should also lead to the detection of the dimerization product of pyruvate [52, 53]. This pyruvate dimer, which results from an aldol condensation reaction and has characteristic signals at 1.38 (singlet) and 3.35 (AB pattern) ppm [52–54], was not observed under these conditions.
Phenyl pyruvic acid (and other α-ketoacids) is oxidized by molecular oxygen to give benzaldehyde and oxalate or the carbon oxides (CO and/or CO2) [55, 56]. This reaction is catalyzed by several metal ions, such as Mn(II), Fe(II), and Cu(II), but not by Zn(II) [57, 58], and has been attributed to the presence of the enol tautomer, which is capable of reacting with dioxygen [61]. In the absence of dioxygen, i.e., under an Ar atmosphere, pyruvate does react with [Zn(MIm2Pr)Cl] to form oxalate. The anaerobic reactivity cannot be attributed to any residual dioxygen traces, since phenyl pyruvate (a substrate more prone to the reaction with dioxygen) did not show any reactivity under these conditions.
The observed conversion of pyruvate to oxalate under anaerobic conditions clearly places it outside previously observed formation of oxalate from oxidative transformations of α-keto acids [55, 56, 59–63]. The observation is important, since pyruvate is a major metabolic junction and it is therefore of prime importance to understand its reactivity. At present no biological counterpart has been reported for the pyruvate–oxalate conversion catalyzed by [Zn(MIm2Pr)Cl]. The chemical transformation as reported here, however, takes place at room temperature and physiological pH and should therefore be considered as a possible alternative pathway in the reactivity of pyruvate. The mechanism of this conversion is currently not clear. No other products could be detected by NMR during the reaction, which precludes the formulation of a mass balance for the reaction and clearly hampers a mechanistic interpretation. In any case, the pyruvate to oxalate conversion requires the scission of a C–C bond. On the other hand, the reactivity of zinc enzymes and their model complexes is often of hydrolytic nature. This can be attributed to the generation of a nucleophilic hydroxide species and/or the activation of a coordinated substrate towards attack of such a nucleophile [3]. The formation of oxalate from pyruvate via a hydrolytic mechanism is, however, difficult to envision. Further investigations have to be performed to address the mechanistic questions concerning this transformation.
Conclusions
The difference in applied steric bulk by the ligands MIm2Pr and iPrEtIm2Pr is reflected in the coordination chemistry of the two ligands towards ZnCl2. Octahedral (MIm2Pr) versus tetrahedral (iPrEtIm2Pr) and trigonal pyramidal (MIm2Pr) versus tetrahedral (iPrEtIm2Pr) coordination geometries were obtained in the 1:1 and 2:1 ligand-to-metal complexes, respectively. The bridging binding mode of the two crystallographically characterized coordination polymers illustrates the intrinsic flexibility of the 3,3-bis(imidazol-2-yl)propionate ligand framework. The polymeric nature of the complexes could limit their biological relevance with respect to the 2-His-1-carboxylate facial triad in the case of zinc. On the other hand, attempts aimed at obtaining structurally more faithful mononuclear complexes led to the observation of the unprecedented Zn-mediated pyruvate to oxalate conversion. This new, nonoxidative transformation of an important metabolic junction is intriguing and warrants further investigation.

zinc
n,n,o ligands
pyruvic acid
oxalic acid


The Business Case for Preconception Care: Methods and Issues

Only a limited number of economic evaluations have addressed the costs and benefits of preconception care. In order to persuade health care providers, payers, or purchasers to become actively involved in promoting preconception care, it is important to demonstrate the value of doing so through development of a “business case”. Perceived benefits in terms of organizational reputation and market share can be influential in forming a business case. In addition, it is standard to include an economic analysis of financial costs and benefits from the perspective of the provider practice, payer, or purchaser in a business case. The methods, data needs, and other issues involved with preparing an economic analysis of the likely financial return on investment in preconception care are presented here. This is accompanied by a review or case study of economic evaluations of preconception care for women with recognized diabetes. Although the data are not sufficient to draw firm conclusions, there are indications that such care may yield positive financial benefits to health care organizations through reduction in maternal and infant hospitalizations. More work is needed to establish how costs and economic benefits are distributed among different types of organizations. Also, the optimum methods of delivering preconception care for women with diabetes need to be evaluated. Similar assessments should also be conducted for other forms of preconception care, including comprehensive care.
Introduction
The opportunity to assist a woman before she becomes pregnant to improve her chances of a healthy pregnancy is compelling. Adverse pregnancy outcomes, including birth defects, preterm birth, and perinatal complications, are the leading causes of infant mortality in the United States [1]. Further, developmental disabilities such as cerebral palsy result in direct and indirect economic costs that can exceed $1 million over a given child’s lifetime [2], in addition to the social and emotional costs incurred by families. A number of additional conditions are associated with serious complications of pregnancy, such as uncontrolled diabetes, hypothyroidism, hypertension, use of alcohol or tobacco, and obesity. The economic burden of most of these conditions in conjunction with pregnancy has been poorly studied but is likely to be enormous.
This paper discusses the potential economic benefits of preconception care due to the prevention of adverse pregnancy outcomes with a focus on the data that would be needed to assess costs and benefits. Currently, the needed data and analyses are largely lacking. In the absence of information about the costs and benefits of comprehensive preconception care, which should ideally include nutritional counseling, evaluation of medications, and family history evaluation and genetic counseling, this paper focuses on principles and methods. Three published studies have analyzed the expected financial benefits from counseling women with diabetes prior to pregnancy. These studies are compared and lessons drawn, with discussion of implications of preconception care for women with other conditions.
In order to argue that preconception care is a “good buy,” its costs and benefits must be assessed from the perspectives of a variety of stakeholders involved in health care services. This paper will first define a “business case” analysis and outline the methods for determining the “return on investment” from the perspective of various health care organizations. Similarities and differences between such an analysis and a standard cost-effectiveness analyses that calculate returns to the health care system as a whole or to society will be explained. Second, the paper reviews the literature on business case analyses from areas other than preconception care. Third, the paper discusses what data sources are available and needed to apply business case methods in preconception care with particular focus on women with diabetes. Finally, the paper draws conclusions about future steps needed to assess the business case for preconception care.
What is a business case analysis?
Concepts and definitions
A business case is an argument for a financial investment couched in terms of potential economic or market advantages to an organization, most commonly a private corporation [3]. The core of a business case is usually a budget impact analysis, a calculation of the financial return on investment (ROI) made by the firm(s) or organization(s) [4]. In addition to financial returns, such as cost savings or cost avoidance, a business case can be based on perceived strategic advantages such as enhanced organizational reputation, market positioning, or increased employee and customer satisfaction. This paper focuses just on financial business case analyses.
According to the Institute for Healthcare Improvement (IHI) and the Commonwealth Fund [3]:A business case for a health care improvement intervention exists if the entity that invests in the intervention realizes a financial return on its investment in a reasonable time frame, using a reasonable rate of discounting. This may be realized as “bankable dollars” (profit), a reduction in losses for a given program or population, or avoided costs. In addition, a business case may exist if the investing entity believes that a positive indirect effect on organizational function and sustainability will accrue within a reasonable time frame.
A business case related to health care is based on one or several of the perspectives of specific payers, purchasers, or providers. Because health care providers, payers, and purchasers often have different, and sometimes conflicting, economic incentives, the business case for the same intervention will probably differ among these groups. For example, improved ambulatory care may lead to reduced inpatient admissions, which could result in financial savings for payers, lost revenue for hospitals, and higher costs for ambulatory care providers. The resulting misaligment of incentives in the health care system may, and often does, lead to inappropriate provision or use of certain services and underutilization of others [3].
Although business case analyses by definition focus on businesses, they are relevant to other stakeholders as well. In particular, the consumer perspective is also important to keep in mind. If consumers do not consider a service such as preconception care to provide good value for their investment of time and money, low demand and utilization could render the business case unfavorable. Further, government agencies must meet budgets, and the short term financial impact of policy changes may influence funding decisions. In the long run, though, public health programs are expected to optimize population health subject to resource availability, which requires analysis of both short-term and long-term health outcomes and economic consequences.
Analytical techniques and outcome measures
Financial analyses that underlie a business case are variably referred to as budget impact and ROI analyses. It is important to situate such analyses in the context of two commonly used methods of economic evaluation, cost-effectiveness analysis (CEA) and cost-benefit analysis (CBA) [5]. A CEA calculates the net monetary cost per unit of health outcome achieved. Certain preventive services are both more effective than alternatives and less costly [6]. A CEA that yields evidence of net cost savings from the payer perspective could be used to frame a business case, but CEAs rarely report payer-specific costs. In a CBA, all outcomes are expressed in dollar values, and the summary measure is expressed as either a net economic benefit or benefit-cost ratio. For example, a CBA of prenatal care for undocumented immigrants in California estimated that public funding cost the state $58 million but saved $211 million in long-term costs of care, for a projected net benefit of $153 million and a benefit-cost ratio of 4.6:1 [7].
A financial analysis of a business case can be viewed as a CBA conducted from the perspective of an individual firm or organization. One important distinction between financial analyses and standard economic evaluations is that the latter are supposed to reflect the opportunity cost or value of resources from a societal perspective whereas a financial analysis considers the direct monetary or budgetary impact to a specific organization.
The ROI metric is typically calculated as the ratio of dollars in increased revenue or decreased cost divided by investment cost, although this can also be expressed as net cost difference [8]. Various other metrics can be used in a financial analysis, such as payback period or internal rate of return. For example, a recent study estimated the number of years that could be required for employers to realize a positive net benefit from bariatric surgery for obese employees as a result of reduced medical costs (all of which were assumed to be paid by the employer) and loss of work time [9]. ROI analyses require payer-specific costs, which are rarely reported in standard CBA studies.
Time horizon and discount rate
Business case analyses generally employ a short time horizon; for example, only changes in use and costs over a period of 2 to 3 years following an intervention are typically considered. This reduces the problem of enrollee or employee turnover. When individuals switch plans or employers, the return on investments in the long-term health of that individual made by the former health plan or employer will be realized by someone else. This can make financial returns to employers or health plans substantially lower than the economic benefits to society [3, 10, 11]. A payback period ranging from 5 years to 10 years [9] is not likely to be attractive to employers.
In economic evaluations from the societal perspective, it is conventional to discount costs occurring in future years using a discount rate of 3%, which reflects the social return on capital [5]. For a business case analysis employing a short time horizon, long-term future costs or benefits may not need to be counted or discounted. In the short run, when ROI analysis does employ discounting, the discount rate should reflect the opportunity cost of capital in the private sector or individual firm.
Prospective and retrospective evaluations
Financial and health economic analyses of an intervention can be performed before or after its implementation. A prospective analysis predicts the financial return for a new initiative or program before it begins. This presumes that there is reliable information about the effectiveness of the intervention and accurate historical cost and revenue data. Analysts construct a simulation model that projects likely costs, health outcomes, and benefits based on a set of assumptions. The benefits include anticipated savings in health care costs and, for analyses from the employer perspective, worker productivity [12, 13].
A limitation of prospective analyses is that key assumptions may turn out to be unrealistic. For example, analyses in the 1980s and early 1990s projected cost savings to Medicaid programs from reductions in the number of low birth weight (LBW) births if prenatal care coverage were expanded (e.g., [14–16]). In conjunction with effective political advocacy and policy changes, these arguments succeeded in more than doubling the number of pregnancies covered by Medicaid prenatal care between 1986 and 1991. Despite this expansion, little or no reduction in LBW rates could be confirmed to have occurred among eligible population groups [17]. The lack of an observed effect may reflect deficiencies in the effectiveness of the routine prenatal care that was offered, as well as barriers to access, and does not rule out a protective effect of an optimal prenatal care protocol. Although prenatal care expansion had other health benefits and can easily be defended, the cost-saving argument appears in retrospect to have been overstated [18].
In the absence of information about intervention effectiveness, a hypothetical calculation can be made regarding the potential magnitude of benefits if an effective intervention were to exist. One way to make such a calculation is to take cost-of-illness estimates of the economic burden of diseases and project the potential cost reduction that might occur as a result of prevention. Only short-term costs, which can be much smaller in magnitude than the long-term costs included in cost-of-illness studies, are likely to be relevant to employers or payers [19]. For example, one group of investigators calculated that poor birth outcomes such as preterm birth cost employer-sponsored health plans $5.6 billion in 1990 and projected that prevention of 10% of preterm births would lead to a cost savings of $560 million [20].
A retrospective or ex post analysis is based on observed costs and benefits. For example, a recent ex post CBA of folic acid fortification calculated that the economic returns were substantially greater, and the costs somewhat lower, than had been projected in previously published analyses conducted prior to fortification [21]. Although actual data can yield more reliable results, it is challenging to collect data on multiple outcomes. Furthermore, interactions among the outcomes or confounding of results from other sources can make it difficult to precisely estimate the impact of the initiative or program alone.
Lessons from business case literature
A number of business case studies of disease management and health promotion initiatives have been published [8, 22, 23]. However, there is a paucity of well-conducted analyses and studies that contain detailed data on intervention and outcome costs [24]. Readers of business case analyses should beware of publication bias [22] and “wish bias” leading to optimistic estimates of effectiveness and compliance with behavioral changes [10].
One group that developed seven business case studies for health care quality improvement initiatives, such as disease management, concluded that health care providers, payers, and purchasers cannot predictably expect to reap positive financial returns from initiatives to improve health care quality within short time horizons [3]. In the majority of case studies, short-term financial returns to health plan entities were negative. Furthermore, returns to health care providers were almost uniformly negative, as any cost savings were generally recouped by purchasers, and not by providers. In several cases, managers felt that there was a business case for investing in improved quality based on organizational benefits such as “retained market share, increased staff loyalty, and reputation among employers” rather than cost savings [3].
Similarly, the Child Health Business Case Working Group developed a set of business case studies [25]. For example, a case study of postpartum screening for risk factors in new mothers (e.g., depression, smoking, domestic violence) followed by referral to community resources found that such a program would lead to higher costs for the practice without additional reimbursements. A positive business case for providers may require a financial mechanism for tying reimbursements by payers to improved outcomes [3]. Integrated health service providers under capitation payments may have more opportunity to internalize the financial gains resulting from improved care [3, 25].
Several studies reported positive financial returns to managed care organizations (MCOs) from prenatal or perinatal interventions targeted to high-risk pregnant women [26–29], although these reports require validation and replication in well-controlled studies. For example, a high-risk pregnancy management program in upstate New York implemented by a managed care program serving Medicaid and other lower-income beneficiaries reported that cost savings from reduced NICU (neonatal intensive care unit) admissions were more than twice as great as the costs of the intervention [28]. A similar program implemented in another upstate New York MCO during the same time period reported investment costs (including provider reimbursements) of $573,355, cost savings of $789,621 resulting from the estimated prevention of 48 LBW births, and a net benefit of $216,266 [27]. The authors reported an internal rate of return of 37% on the investment, which is equivalent to an ROI ratio of 1.37.
Economic analyses of preconception care
Finally, we turn to the application of business case methods to preconception care. Preconception care refers to three types of services and target populations:Preventive services and screenings offered to women who expect to become pregnant in the near future;Interconception care for women who have given birth and intend to bear another child at some point in the future;Counseling about the impact of pre-existing health risks or conditions for affected women of childbearing age and their impact on pregnancy outcomes.
Few economic evaluations of individual screenings or services in the pre-pregnancy period have been published. One CEA estimated that promoting folic acid supplementation to all women of reproductive age would be cost effective, although not cost saving [30]. If multivitamins containing folic acid were targeted instead to women intending pregnancy, supplementation might be cost saving. On the other hand, because most of the averted costs of caring for children with spina bifida occur years in the future and are shared by multiple payers, this type of analysis cannot be used to calculate the business case for folic acid supplementation strategies.
One preconception care service with published analyses of short-term financial costs and benefits is preconception counseling for women with diabetes. Women with poorly controlled diabetes before pregnancy have an elevated risk of having an infant with a birth defect [31], fetal death, or macrosomia. Women with poorly controlled diabetes prior to pregnancy can also experience medical risks during pregnancy, including diabetic retinopathy, nephropathy, and ketoacidosis, as well as pregnancy-induced hypertension and uteroplacental insufficiency [32].
A substantial body of evidence on infant health outcomes associated with preconception counseling for women with diabetes yields consistent findings. According to a meta-analysis of 14 epidemiologic studies, pregnancies preceded by preconception care result in one third as many major congenital malformations as pregnancies not preceded by preconception care [31]. The Maine Diabetes in Pregnancy Program found 4-fold lower rates of both congenital anomalies and fetal and neonatal deaths among pregnancies with preconception care [33]. More recently, the Diabetes Complications Control Trial (DCCT) of 1441 adults with Type 1 diabetes randomly assigned to conventional or intensive therapy, including 180 women who became pregnant, reported 8 congenital malformations in the conventional therapy group and only 1 in the intensive therapy group [34]. Thus, in the case of maternal diabetes, randomized trial data confirm the findings of numerous observational studies that control of diabetes prior to pregnancy results in dramatically fewer birth defects.
Three economic evaluations of preconception and pregnancy care programs for women with non-gestational diabetes were identified in the published literature [35–37]. All reported calculations of net short-term economic benefit based on reduced costs of maternal and neonatal hospitalizations and did not take into account long-term averted costs of care associated with the prevention of birth defects. Although the analyses were not developed as business case analyses, the focus on short-term costs and benefits is consistent with that approach. Regrettably, the lack of common metrics of cost and health outcomes makes it difficult to make direct comparisons of assumptions and findings of the three studies.
The first study is a prospective analysis of a hypothetical program of comprehensive preconception diabetes care consisting of 20 visits prior to prenatal care [35]. The preconception visits were projected to cost $2,638 per enrollee in 1989 dollars. The investigators calculated that reduced maternal and infant hospitalizations would result in net benefits of $1,720 per enrollee. The majority of the hospitalization cost savings were projected to result from an 8-fold reduction in major congenital malformations, from 3.2 to 0.4%. Using the cost of the intervention as the denominator, the benefit-cost ratio from the health care system perspective can be calculated to be 1.6. This means that, on average, for every $1 spent on the program, reduced hospitalization costs of $1.60 would be expected. Although the projected percentage reduction in major malformations appears high relative to the literature, the projected intervention cost was also rather high, since it is uncertain that 20 visits would be needed for preconception care for women with diabetes.
The second study, a retrospective analysis of data for the years 1986–1988 from the California Diabetes and Pregnancy Program (CDAPP), found that matched perinatal hospital charges for mother and child were 41% higher for controls than for program participants [36]. The benefit-cost ratio was 5.19, with reduced maternal and infant hospitalization costs of $5.19 observed for each $1 spent on the program. This finding likely contributed to the ongoing funding of the CDAPP. It is difficult to apply these findings to preconception care, though. Only 24 of 102 participants were enrolled in the program prior to 8 weeks of gestation, the time during which major congenital malformations are likely to occur. Although those participants had average charges that were 44% lower than those who enrolled after 8 weeks, women who enrolled in the program prior to conception incurred higher costs per delivery because of the longer time during which services were received and the fact that not all women who enrolled became pregnant. Therefore, positive net financial returns to the component of preconception care were not demonstrated in that study.
The third study is a retrospective evaluation of prospectively gathered data on pregnancy-related costs among women with type 1 diabetes who received both preconception care and prenatal care or prenatal care only at selected Michigan hospitals [37]. Women in the preconception care group received an additional two outpatient visits on average, at an added cost of $132. The rate of congenital malformation was 4.2% in the preconception care group compared with 13.5% in the prenatal care-only group, a reduction of 69%. The rate of maternal and neonatal intensive care unit (NICU) hospitalizations was 50% lower in the preconception care group. Combining maternal and child hospitalizations, cost savings averaged $34,000 per woman enrolled in both preconception and prenatal care compared to those who received standard prenatal care. Although no information was presented on the distribution of costs in this study, an implication of the findings is that an integrated health plan could likely achieve cost savings by paying for a modest number of preconception care visits for women with type 1 diabetes.
Discussion
The economic and business case literature on preconception care is quite limited. There is some evidence of positive financial returns for preconception counseling for women with diabetes, based on savings in hospitalization costs [35–37]. Although none of the studies involved randomized assignment, the relative differences in frequencies of birth defects are consistent with findings from other studies and increase the confidence that can be placed in the estimates of cost reductions. None of the studies indicate who was expected to bear the costs of counseling or the magnitude of cost savings to specific stakeholders. The studies are all based on relatively old data; newer studies with more complete accounting of costs would be more informative. Furthermore, determination of the optimal number and cost of preconception visits is essential.
The American Diabetes Association [38] and the American College of Obstetricians and Gynecologists [39] recommend that all women of childbearing age with diabetes receive counseling about the importance of glucose control before becoming pregnant. Despite these guidelines and positive economic analyses, many women with diabetes still do not receive such advice. A recent survey of women with diabetes ages 18 to 45 years enrolled in managed care plans found that only about half recalled receiving advice about the need for glucose control prior to pregnancy [40]. Research is needed to determine why some women with diabetes seek out preconception care and adopt intensive glucose control measures and others do not [41]. Potential barriers, such as opportunity cost of time, financial cost (e.g., co-pays), invasive testing, and medical risks [32] need to be analyzed from the consumer perspective. For example, it has been estimated that for patients with type 2 diabetes controlled by oral agents, following all self-care practices recommended by the American Diabetes Association could require more than 2 hours daily [42].
Better data are needed to establish the business case for preconception care. Although benefits of preconception care can be projected, data derived from the evaluation of actual preconception care initiatives are needed to make a compelling business case. Further, observational data comparing pregnancy outcomes among women who choose to use preconception care and those who do not are subject to biases due to non-randomized study design. A business case analysis of interventions with both demonstrated efficacy (works if followed) and effectiveness (behaviors are actually changed and lead to improved outcomes) is likely to be more persuasive. In order to develop a compelling business case for preconception care, studies of preconception care interventions accompanied by rigorous evaluations with randomized or matched groups of controls are required.
Preparing a business case analysis based on intervention data poses challenges. It is logistically difficult but essential to prospectively collect data on costs and outcomes: “To understand which quality-enhancing interventions are likely to produce positive returns on investments, data collection and analysis must include tracking the investment and operating costs of implementing the intervention as well as the changes in revenues and costs that result from the intervention” [24]. Easily accessible or extant data sets typically do not allow one to calculate costs from the perspective of an individual organization. This requires quantifying actual reimbursements, co-pays, deductibles, etc., rather than using standard charges for a clinic visit. It also requires tracking which patients are covered by different health plans and patterns of enrollment and disenrollment.
Conclusion and the way forward
Research demonstrating the effectiveness and financial returns to investments in preconception care is necessary to make a financial business case for preconception care. In particular, the only economic evaluations that have been published to date have been of specific components of preconception care, such as counseling women with diabetes or promoting preconception use of folic acid supplements. Evaluations of the costs and benefits of different “bundles” of preconception care services or comprehensive preconception care are needed. Further, such evaluations must include payer-specific cost data to be relevant to framing the business case for preconception care.
Although more data are needed, such analyses may not be sufficient to ensure access to or provision of preconception care, as the diabetes preconception care experience suggests. Promotion of preconception care through carefully researched health marketing campaigns is also critical [41]. If, as a result of effective health marketing, consumers were to demand preconception care, providers and payers would be more likely to perceive a business case for providing it. Similarly, a business case might also be supported if health care professionals working with women of childbearing age were to advocate that their own organizations provide such services. Health care providers often lack the incentive to adopt a new service that improves outcomes; therefore, particular attention must be paid to the structure of incentives by both public and private payers and purchasers [3].
Public health programs should fund studies evaluating the effectiveness of preconception care and develop marketing strategies to promote awareness of strategies that are proven effective. Studies that test and evaluate preconception care strategies should collect the financial data needed to conduct business case analyses [24]. Controlled evaluations of preconception care need to be accompanied by financial analyses to help make the business case for providing and paying for such services. Better quality evidence of both the financial and health costs of poor pregnancy outcomes and the benefits from preconception and early prenatal care is needed to provide a compelling business case.

economic evaluation
birth defects
cost-benefit analysis
prenatal care
diabetes in pregnancy