An Investigation of the Formation Mechanism of Copper(II) Carbodiimide Richard Dronskowski, Xiaohui Liu To cite this version: Richard Dronskowski, Xiaohui Liu. An Investigation of the Formation Mechanism of Copper(II) Car- bodiimide. Journal of Inorganic and General Chemistry / Zeitschrift für anorganische und allgemeine Chemie, Wiley-VCH Verlag, 2009, 636 (1), pp.121. 10.1002/zaac.200900320. hal-00528952 HAL Id: hal-00528952 https://hal.archives-ouvertes.fr/hal-00528952 Submitted on 23 Oct 2010 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. 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ZAAC An Investigation of the Formation Mechanism of Copper(II) Carbodiimide Journal: Zeitschrift für Anorganische und Allgemeine Chemie Manuscript ID: zaac.200900320.R1 Wiley - Manuscript type: Article Date Submitted by the 01-Sep-2009 Author: Complete List of Authors: Dronskowski, Richard; Institute of Inorganic Chemistry, RWTH Aachen Liu, Xiaohui; Institute of Inorganic Chemistry, RWTH Aachen Keywords: carbodiimide, copper(II), synthesis, crystal structure, copper(I) Wiley-VCH Page 1 of 25 ZAAC 1 2 3 4 An Investigation of the Formation 5 6 Mechanism of Copper(II) Carbodiimide 7 8 9 Xiaohui Liu, Hari Puspitosari, and Richard Dronskowski* 10 11 Institute of Inorganic Chemistry, RWTH Aachen University 12 13 D-52056 Aachen, Germany 14 15 16 Dedicated to Professor Arndt Simon on the Occasion of his 70 th Birthday 17 18 19 20 21 22 Abstract 23 24 25 The formation mechanism of copper(II) carbodiimide in aqueous ammonia solution has been 26 27 studied by synthetic and crystallographic means. The reduction of the starting Cu(II) solution 28 29 leads to the synthesis of solid Cu(I) intermediates, and the crystal structures of the two new 30 31 compounds Cu(NCNH 2)Cl (a = 8.2925(4), b = 3.7275(1), c = 12.4534(4) Å, Pnma ) and the 32 iso-structural Cu(NCNH2)Br ( a = 8.406(3), b = 3.9229(9), c = 12.656(3) Å, Pnma ) have been 33 34 elucidated. A further increase in pH value by adding more ammonia leads to the crystalliza- 35 36 tion of two additional Cu(I) intermediates, Cu 2NCN and the ammine adduct Cu 4(NCN) 2NH 3. 37 38 The former can be also made by the thermal decomposition of the latter whereas mild oxida- 39 tion of Cu (NCN) NH leads to the formation of perfectly crystalline CuNCN. 40 4 2 3 41 42 43 Keywords: carbodiimide / cyanamide / copper / crystal structure / reaction mechanism 44 45 46 47 48 Introduction 49 50 51 52 The motivation for carrying out exploratory syntheses in the fields of carbodiimides and cy- 53 5 9 54 anamides of the divalent magnetic transition metals (d – d ) is naturally linked with the struc- 55 tural and physical properties of these novel types of inorganic materials.[1–7] Among them, 56 57 black powderous copper carbodiimide, CuNCN, stands for the nitrogen-based analogue of 58 [2,5] 59 CuO, and CuNCN was first made in large, phase-pure and well-crystallized quantities only 60 five years ago.[2] The synthetic difficulty lies in the fact that aforementioned M 2+ transition- metal ions prefer to form cyanamide complexes in aqueous solution [8, 9] such that, as a charac- 1 Wiley-VCH ZAAC Page 2 of 25 1 2 3 2+ 2+ teristic example, Cu cations merely form coordination complexes like [Cu(NCNH 2)4] from 4 5 which the protons can not be fully removed. The brute-force alternative of a (strong) basic 6 7 aqueous Cu(II) solution reacting with anionic cyanamide units, however, yields a black mate- 8 [10,11] 9 rial whose amorphous nature renders its structure determination impossible. As said be- 10 fore, we were fortunate to discover a new synthetic approach for perfectly crystalline CuNCN 11 12 a while ago.[2] The two-step method is essentially based on an ammine Cu(I) cyanamide com- 13 + 14 pound, Cu 4(NCN) 2NH 3, which can be obtained from the mild reaction of [Cu(NH 3)2] and 15 [12] 16 H2NCN under aqueous ammonia conditions. The likewise mild oxidation of 17 18 Cu 4(NCN) 2NH 3 then yields the target phase CuNCN. Schematically, the entire procedure may 19 be summarized as follows: 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 In order to come up with a less formal description, we have now examined the reaction 37 mechanism of CuNCN formation in more detail. By doing so, the syntheses and crystal struc- 38 39 tures of two new solid-state copper(I) halide complexes have been accomplished, together 40 41 with yet another new Cu(I) cyanamide solid-state material. 42 43 44 45 46 Experimental Section 47 48 49 50 In order to elucidate possible reaction pathways, alternative syntheses were tested, namely a 51 52 total of six different preparations. 53 Experiment 1: A mixture of CuCl 2·2H 2O (1.71 g, 10 mmol) and cyanamide H 2NCN (0.84 g, 54 55 20 mmol) was stirred at room temperature in water (10 mL) under argon gas. A solution of 56 57 Na 2SO 3 (1.26 g, 10 mmol) in 10 mL water was added dropwise to the above mixture resulting 58 59 in colorless needle-like crystals of the new phase Cu(NCNH 2)Cl ( 1a ) which were then filtered 60 and dried under vacuum. Analytical data (%) for Cu(NCNH 2)Cl (141.04 g/mol): Cu 44.76 (calc. 45.05); C 8.32 (8.52); H 2.01 (1.42); N 19.34 (19.86). 2 Wiley-VCH Page 3 of 25 ZAAC 1 2 3 Experiment 2: It was analogous to experiment 1 but using CuBr 2 (2.23 g, 10 mmol). The 4 5 product crystallized as colorless needle-like crystals of the new phase Cu(NCNH 2)Br ( 1b) but 6 7 with a small amount of a CuBr impurity detectable from X-ray powder diffraction data 8 9 (XRPD). Fortunately, a single crystal of 1b was isolated which turned out as suitable for sin- 10 gle-crystal measurement as well as solution and refinement. 11 12 Experiment 3: When 1a had been formed in experiment 1, adding 10 mL of a 25 wt.% am- 13 14 monia solution (NH 3·H 2O) resulted in the precipitation of pure white Cu 4(NCN) 2NH 3 ( 2) 15 [12] 16 which was filtered and dried under vacuum. 17 18 Experiment 4: When 1a had been formed in experiment 1, 10 mL of a one-molar NH 3·NH 4Cl 19 buffer solution were added. A pale-yellow precipitation was obtained, then filtered and dried 20 21 under vacuum. XRPD analysis showed it to be a mixture of 2 and the new phase Cu 2NCN ( 3). 22 23 Experiment 5: 0.1 g of 2 was heated at 150 °C under vacuum for two days in one side of a 24 25 long Schlenk glass. Yellow powderous 3 was obtained, and the presence of ammonia was 26 verified in the attached cooling trap. 3 appeared as very well crystallized. XRPD analysis 27 28 showed that 3 was a pure phase which could be indexed using an orthorhombic cell. Analyti- 29 30 cal data (%) for Cu 2NCN (167.09 g/mol): Cu 75.30 (calc. 76.06); C 7.20 (7.18); H 0.14 (0); N 31 –1 32 17.0 (16.76). IR (KBr, cm ): νas (NCN) = 2138 and 2193(s), νs(NCN) = 1183 (s) and δ(NCN) 33 34 = 655 (m). 35 Experiment 6: Upon stirring the suspension from experiment 3 over night, 2 was slowly oxi- 36 37 dized at room temperature due to the presence of small amounts of atmospheric oxygen 38 [2] 39 within the reaction flask, thereby yielding black crystalline CuNCN. 40 41 42 43 44 X-ray Crystallography 45 46 47 48 Structure data of 1a. The X-ray diffraction data were recorded at room temperature using a 49 50 G670 Imageplate Guinier diffractometer (Huber, Rimsting) equipped with a Johansson Ge 51 monochromator for Cu Kα1 radiation and a flat sample holder in the 2θ range between 6 and 52 53 100°. The XRPD pattern of 1a indicated a pure phase such that its structure was determined 54 55 using Rietveld analysis based upon the iso-structural model of 1b . The FULLPROF program 56 [21] 57 package was used with a pseudo-Voigt profile function, and the background data were 58 manually subtracted by linear interpolation. Because of strongly preferred orientation and 59 60 likewise asymmetric Bragg peaks, the atomic distances of Cu–N and C–N and the N–C–N angle had to be soft-restrained to yield reasonable bond lengths and a more reliable refine- 3 Wiley-VCH ZAAC Page 4 of 25 1 2 3 ment. Likewise, all H positional parameters were fixed to N–H distances of 0.89 Å. Empirical 4 5 formula CH 2ClCuN 2, Mr = 141.04, orthorhombic, Pnma , a = 8.2925(4), b = 3.7275(1), c = 6 3 7 12.4534(4) Å, V = 384.48(2) Å , Z = 4. The residual values arrived at 0.072 ( Rp), 0.102(Rwp ), 8 9 and 0.18 ( RBragg ) for a total of 22 variables, 4 restraints and 248 Bragg reflections.