Polymorphism in the Crystallization Behavior of Trinitrocobalt(III) Complexes with Tridentate Amine Ligands: Hydrogen-Bonding Analysis and Syntheses of Racemic and Conglomerate mer-Co(dpt)(NO2)3
CRYSTAL GROWTH & DESIGN 2001 VOL. 1, NO. 1 67-72
Hyungphil Chun and Ivan Bernal* Department of Chemistry, University of Houston, Houston, Texas 77204-5641 Received July 13, 2000
W This paper contains enhanced objects available on the Internet at http://pubs.acs.org/crystal. ABSTRACT: Three trinitrocobalt(III) complexes with three different chelating amine ligands have been prepared and structurally characterized in order to record their crystallization modes. The meridional isomers of Co(Meaepn)(NO2)3‚H2O and Co(Me-dpt)(NO2)3 (Me-aepn ) N,N-bis((2-aminoethyl)methyl)-1,3-propanediamine; Me-dpt ) 3,3′-diamino-N-methylpropanediamine) crystallize as simple racemates, but intermolecular hydrogen bonds between homochiral molecules are observed in their packing structures. Both racemic (Pcab) and conglomerate (P212121) forms of mer-Co(dpt)(NO2)3 (dpt ) N-(3-aminoethyl)-1,3-propanediamine) have also been prepared by two different synthetic approaches, and their molecular structures are compared. The two are different in the torsional angles associated with the three NO2 groups and in the conformation of one of the two six-membered chelate rings. Introduction We are investigating crystal structures of neutral Co(III) complexes in order to understand the relationship between molecular packing and various crystallization modes, such as racemic or conglomerate crystallization. Co(III) complexes are nicely suited for that purpose, since they almost always adopt octahedral geometry, eliminating the potential effect of molecular shape on crystallization behavior, which has recently been studied in detail.1 Studying neutral compounds further simplifies the analysis of molecular packing structures because hydrogen bonding becomes the most important secondary interaction in such cases. In particular, trinitrocobalt(III) complexes caught our attention since both facial2 and meridional3 isomers of Co(NH3)3(NO2)3 have been known to crystallize as conglomerates in space groups P21 and P212121, respectively. When the NH3 groups of those compounds are replaced with chelating amine ligands, such as dien (diethylenetriamine),4 aepn (N-(2-aminoethyl)-1,3-propanediamine),5 en (ethylenediamine),6 or ptn (1,2,3-propanetriamine),7 all of the resulting trinitrocobalt(III) compounds crystallize as simple racemates. This observation gave us the impression that the peripheral CH2 groups of the chelating ligands make a racemic molecular packing much more favorable to an enantiomorphic packing. Such an oversimplification, however, was proved not to be true when Bernal et al. reported8 the crystal structure of mer-Co(dpt)(NO2)3 (dpt ) N-(3-aminopropyl)-1,3propanediamine), which crystallized as a conglomerate in space group P212121. Therefore, we decided to study more trinitrocobalt(III) complexes in order to obtain a more generalized idea of the crucial factor that plays a key role in the crystallization process of those compounds. * To whom correspondence should be addressed. Phone: (713) 7432718. Fax: (713) 743-2709. E-mail:
[email protected].
In this study, we report the crystal structures of neutral trinitrocobalt(III) complexes with Me-aepn, dpt, and Me-dpt ligands, where Me-aepn and Me-dpt are N-methylated analogues of aepn and dpt, respectively. Also, it is reported that both racemic and conglomerate forms of mer-Co(dpt)(NO2)3 can be prepared separately and recrystallized without an interconversion. The structure of the unknown racemic form is compared with that of the previously known conglomerate form. Experimental Section Syntheses. General Considerations. N-Methylethylenediamine, 3-bromopropylamine hydrobromide, KNO2, CoCl2‚ 6H2O, KOH, dpt, NaNO2, and Me-dpt were obtained from commercial sources and used as received. Na3[Co(CO3)3]‚3H2O was prepared according to a known method.9 Elemental analyses were carried out by Galbraith Laboratories.10 mer-Co(Me-aepn)(NO2)3‚H2O (1). A mixture of N-methylethylenediamine (6.85 g, 92.4 mmol) and 3-bromopropylamine hydrobromide (13.77 g, 62.9 mmol) in 100 mL of absolute ethanol was heated at 70 °C for 2 h with a loose cover. After the solution was cooled to room temperature, 10 g (178.2 mmol) of KOH was added, and the resulting solution was warmed for 10 min and then cooled to room temperature. After KBr was filtered off, the solution was fractionally distilled in order to remove the solvent and unreacted N-methylethylenediamine. The dark brown oily liquid (4.82 g) thus obtained and CoCl2‚6H2O (8.02 g, 33.7 mmol) were mixed with 40 mL of water, and the solution was aerated for 3 h in an ice bath. After KNO2 (8.61 g, 101.2 mmol) dissolved in 5 mL of water was added, aeration was continued for 3 h more, upon which the mustard yellow precipitate that had formed was collected. More precipitate was obtained by heating the filtrate (total 5.00 g, 23% based on 3-bromopropylamine hydrobromide). Single-crystals for X-ray study were obtained by recrystallization from hot water. Anal. Calcd for C6H19N6O7Co (FW ) 346.20): C, 20.81; H, 5.54; N, 24.28. Found: C, 20.77; H, 5.54; N, 24.34. rac,mer-Co(dpt)(NO2)3 (2). CoCl2‚6H2O (12.54 g, 52.7 mmol) dissolved in 50 mL of water and dpt (6.92 g, 52.7 mmol) dissolved in 50 mL of water were mixed together, and a stream of air was introduced into the solution overnight. While the
10.1021/cg005505f CCC: $20.00 © 2001 American Chemical Society Published on Web 11/15/2000
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Crystal Growth & Design, Vol. 1, No. 1, 2001
Chun and Bernal
Table 1. Crystal Data and Structure Refinement for mer-Co(Me-aepn)(NO2)3‚H2O (1), rac-mer-Co(dpt)(NO2)3 (2), and mer-Co(Me-dpt)(NO2)3 (3)a formula fw cryst syst space group a (Å) b (Å) c (Å) β (deg) V (Å3) cryst size (mm) 2θ range (deg) index ranges (h, k, l) no. of obsd rflns (I > 2σ(I)) no. of data/restraints/params GOFb on F2 final R indicesc (I > 2σ(I)) R indices (all data) Flack x param largest diff peak/hole (e/Å3)
1
2
3
C6H19CoN6O7 346.20 monoclinic P21/n 7.530(2) 22.782(4) 7.722(1) 96.00(1) 1317.4(4) 0.50 × 0.21 × 0.19 4-50 (8, +27, +9 1632 2308/0/257 1.026 R1 ) 0.0357, wR2 ) 0.0942 R1 ) 0.0650, wR2 ) 0.1047
C6H17CoN6O6 328.19 orthorhombic Pcab 11.009(5) 13.547(4) 16.542(7) 90 2467(2) 0.52 × 0.51 × 0.33 4-50 +13, +16, +19 1545 2157/0/172 1.079 R1 ) 0.0628, wR2 ) 0.1684 R1 ) 0.0876, wR2 ) 0.1785
0.556/-0.674
1.402/-0.826
C7H19CoN6O6 342.21 orthorhombic Pbc21 10.799(7) 12.849(3) 9.821(4) 90 1362.7(11) 0.56 × 0.53 × 0.41 4-55 +14, +16, +12 860 1644/1/181 1.074 R1 ) 0.0549, wR2 ) 0.1415 R1 ) 0.1187, wR2 ) 0.1594 0.10(6) 0.550/-0.505
Common for all structures: room temperature, λ ) 0.710 73 Å (Mo KR), absorption correction by ψ-scans, refinements by full-matrix least squares on F2. b GOF ) {∑[w(Fo2 - Fc2)2]/((no. of reflections) - (no. of parameters refined))}1/2. c R1 ) ∑||Fo| - |Fc||/∑|Fo|, wR2 ) [∑w(Fo2 - Fc2)2/∑wFo4]1/2. w ) 1/[σ2Fo2 + (aP)2], where P ) (Fo2 + 2Fc2)/3. a
Table 2. Selected Bond Lengths (Å) and Angles (deg) for 1-3 1
2
3
Co-N(1) Co-N(2) Co-N(3) Co-N(4) Co-N(5) Co-N(6) N(4)-O(1) N(4)-O(2) N(5)-O(3) N(5)-O(4) N(6)-O(5) N(6)-O(6)
1.947(3) 2.031(3) 1.967(3) 1.966(3) 1.938(3) 1.958(3) 1.238(4) 1.225(4) 1.221(4) 1.230(4) 1.223(4) 1.228(4)
1.968(5) 1.998(5) 1.967(5) 1.953(5) 1.944(5) 1.916(5) 1.236(7) 1.236(7) 1.233(7) 1.231(7) 1.250(7) 1.232(7)
1.993(9) 2.106(8) 1.941(9) 1.995(9) 1.959(9) 1.935(9) 1.209(11) 1.222(12) 1.197(12) 1.237(12) 1.287(12) 1.199(12)
N(1)-Co-N(3) N(2)-Co-N(5) N(4)-Co-N(6) N(5)-Co-N(4) N(5)-Co-N(6) O(1)-N(4)-O(2) O(3)-N(5)-O(4) O(5)-N(6)-O(6)
177.03(14) 176.46(12) 169.53(12) 83.95(12) 85.69(12) 118.6(3) 119.1(3) 118.8(3)
177.4(2) 177.7(2) 176.7(2) 88.2(2) 88.6(2) 119.0(5) 118.6(5) 119.0(5)
174.3(4) 177.4(6) 171.1(4) 84.3(4) 87.0(4) 120.6(10) 120.4(10) 117.6(10)
solution was heated, a 20 mL aqueous solution containing 10.91 g (158.1 mmol) of NaNO2 was added. The mixture of yellow and green precipitates thus obtained (6.5 g) was stirred in water-methanol (1:1) solution, upon which the greenish residue was dissolved, leaving a mustard yellow precipitate. Recrystallization of the yellowish powder from hot water with charcoal treatment gave brown crystals. Anal. Calcd for C6H17N6O6Co (FW ) 328.19): C, 21.96; H, 5.22; N, 25.61. Found: C, 22.51; H, 5.17; N, 25.52. In a separate experiment, a 10 mL aqueous solution containing NaNO2 (2.76 g, 40 mmol) and dpt (1.32 g, 10 mmol) was added to a slurry of Na3[Co(CO3)3]‚3H2O (3.62 g, 10 mmol) in 10 mL of water. The mixture was heated, and 7 mL of 6 M acetic acid was added dropwise. After 1 h the solution was cooled to room temperature, and an orange precipitate was collected (2.20 g, 67%). A brown crystalline precipitate was obtained from the filtrate by standing at room temperature. When examined on a diffractometer, these crystals gave the same unit cell parameters and Laue symmetry as those of the conglomerate mer-Co(dpt)(NO2)3;8 therefore, X-ray data were not collected. mer-Co(Me-dpt)(NO2)3 (3). A mixture of CoCl2‚6H2O (8.74 g, 36.7 mmol) in 30 mL of water and Me-dpt (5.50 g, 37.9 mmol) in 10 mL of water was aerated overnight, after which
Figure 1. Molecular structure of 1 drawn at the 40% probability level. The water molecule and hydrogen atoms of the chelating amine ligand have been omitted for clarity. the solution had dried out. The dark brown residue was redissolved in 40 mL of water, and KNO2 (9.38 g, 110.2 mmol) dissolved in 7 mL of water was added. Aeration was continued for 3 h more, and then the solution was heated for 30 min. A brown powdery precipitate was collected after cooling to room temperature (5.07 g, 40%). Single crystals for X-ray study were obtained by recrystallization of the product from hot water with charcoal treatment. Anal. Calcd for C7H19N6O6Co (FW ) 342.21): C, 24.57; H, 5.61; N, 24.56. Found: C, 24.61; H, 5.70; N, 24.47. Crystallography. The intensity data were collected with an Enraf-Nonius CAD-4 diffractometer at room temperature, and SHELXS-8611 (direct methods) and SHELXL-9312 were used to solve and refine the structures. All the hydrogen atoms of 1 were located from difference maps and refined isotropically; those of 2 and 3 were placed in their geometrically ideal positions with isotropic temperature factors 1.2 times of those of the attached non-hydrogen atoms. A relatively large residual electron density (1.40 e/Å3) found in the final refinement of 2 was ignored, since it was too close to cobalt (0.905 Å). Many crystals of 3 with excellent appearance showed poor reflection profiles when mounted on a diffractometer, and therefore, several different crystals were examined before collecting a full data set. Despite the effort, refinements of the structure of 3 suffered from a large number of weak reflections, resulting
Crystallization Behavior of CrIII(NO2)3 Complexes
Crystal Growth & Design, Vol. 1, No. 1, 2001 69
Figure 2. ORTEP views of 2 (left) and 3 (right). Hydrogen atoms of the amine ligands have been omitted for clarity. in relatively large esds in bond lengths and angles. Table 1 summarizes the parameters for data collection and structure refinements, and Table 2 gives selected bond lengths and angles.
Results and Discussion Compound 1 was prepared in order to see the effect of a methyl substitution of the -NH- hydrogen of the aepn ligand of mer-Co(aepn)(NO2)35 on the hydrogen bonding scheme. Therefore, a nucleophilic substitution reaction of NH2CH2CH2CH2Br with NH2CH2CH2NH(CH3) was carried out for the preparation of the aepn ligand with a methyl substituent on the secondary nitrogen atom. Although the reaction is not regiospecific by itself for the ligand intended, the desired metal complex was obtained in 43% yield based on CoCl2‚ 6H2O. In the structure of 1 (Figure 1), the cobalt atom is placed at the center of a distorted octahedron where the two nitro groups, trans to each other, are bent away from the chelating amine ligand with a methyl substituent (∠N(4)-Co-N(6) ) 169.5(1)°). The two bonds that are trans to each other have the longest (Co-N(2) ) 2.031(3) Å) and shortest (Co-N(5) ) 1.938(3) Å) distances among the six Co-N bonds. All six N-O distances and three O-N-O angles of the three nitro groups are statistically identical, with the averages 1.228(4) Å and 118.8(3)°, respectively. A routine air oxidation procedure was used for the synthesis of 2 and 3, and no particular difficulty was met except for obtaining single crystals of good quality with respect to X-rays. As shown in Figure 2 and Table 2, the geometry around the cobalt atom of the two compounds shows little difference from that of 1. Apparently, it seems that there is no significant difference in the packing structures of mer-Co(aepn)(NO2)35 and of mer-Co(Me-aepn)(NO2)3‚H2O (1), because they both crystallize as racemates in the same space group (P21/c). In the packing structure of 1, however, intermolecular hydrogen bonds between homochiral molecules exist, which is clear from Figure 3. In the figure, water molecules of crystallization play a bridging role by hydrogen-bonding neutral cobalt molecules of the same chirality which are related only by translation in the c direction of the unit cell. Such a string of homochiral molecules makes contacts with a neighboring string formed by molecules of the opposite chirality which resulted from n-glide plane symmetry.
Figure 3. Packing diagram of 1 showing hydrogen bonds as dotted lines (
