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Morpholine-N-carboxylate as a ligand in coordination chemistry – Syntheses and structures of three heteroleptic copper(ii ) and zinc complexes
Journal Pre-proof Morpholine-N-carboxylate as a ligand in coordination chemistry – Syntheses and structures of three heteroleptic copper(ii) and zinc complexes Nikola Bedeković, Vladimir Stilinović PII:
S0022-2860(19)31736-3
DOI:
https://doi.org/10.1016/j.molstruc.2019.127627
Reference:
MOLSTR 127627
To appear in:
Journal of Molecular Structure
Received Date: 14 November 2019 Revised Date:
18 December 2019
Accepted Date: 19 December 2019
Please cite this article as: N. Bedeković, V. Stilinović, Morpholine-N-carboxylate as a ligand in coordination chemistry – Syntheses and structures of three heteroleptic copper(ii) and zinc complexes, Journal of Molecular Structure (2020), doi: https://doi.org/10.1016/j.molstruc.2019.127627. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
CRediT author statement Nikola Bedeković: Validation, Formal analysis, Investigation, Data Curation, Writing - Original Draft, Writing - Review & Editing, Visualization
Vladimir Stilinović: Conceptualization, Validation, Investigation, Writing - Original Draft, Writing - Review & Editing, Supervision
Morpholine-N-carboxylate as a ligand in coordination chemistry – syntheses and structures of three heteroleptic copper(II) and zinc complexes
Nikola Bedeković & Vladimir Stilinović* University of Zagreb, Faculty of Science, Chemistry Department Horvatovac 102a, RH-10000 Zagreb, Croatia *E-mail: [email protected]
Abstract The first three morpholine-N-carboxylate complexes, with copper(II) and zinc(II) have been prepared and studied by single crystal X-ray diffraction. All three compounds comprise of discrete mono- or dinuclear heteroleptic complex molecules. Three different ways of bonding of morphCOO– anions have been observed – as a monodentate, chelating, and as a bridging ligand. In all cases only the carboxyl group of the morpholinecarboxylate anion was found to bind to the metal cation, while the oxygen atom of the morpholine ring participates only in weak C–H
O hydrogen
bonds. Changes in the geometry of the coordinated morphCOO– anion relative to the free ligand (in its morpholinium salt) were observed in all three modes of binding to the metal center. Introduction Morpholine (morphH), first prepared by Knorr in 1889,1 is generally recognized today as a convenient ligand for the synthesis of a wide range of organometallic2,3 and metal-organic compounds including discrete complexes4,5 and metal-organic polymers.6,7 Although a morphH molecule is potentially an ambidentate N- and Odonor ligand, binding of morphH to a metal center is most commonly accomplished through the nitrogen atom,8,9 except in cases where the nitrogen atom is protonated.10,11 This leaves the oxygen atom free to participate in supramolecular
1
interconnections, and can act as a halogen bond acceptor12 or participate in hydrogen bonding,13 which can result in many different supramolecular architectures. The hydrogen atom of the secondary amino group can be easily substituted by an electrophilic species, allowing for derivatization of morphH to corresponding hydrazines14, carbonyl compounds15,16 or Schiff bases17. A potentially interesting way of derivatizing morphH molecule is carboxylation of the nitrogen atom, which would result in morpholine-N-carboxylic acid (morphCOOH), or the respective morpholineN-carboxylate (morphCOO–) anion.18 This should act as an anionic ligand in coordinating metal ions through the carboxylate group.19 Furthermore, a coordinated morpholine-N-carboxylate should be an even better hydrogen or halogen bond acceptor than the coordinated morpholine molecule due to its negative charge (as has been noticed with other anionic ligands such as imines,20,21 β-diketones22 and other.23,24 While the morpholine-N-carboxylate was prepared already by Knorr by passing a stream of gaseous CO2 through liquid morpholine, obtaining solid morpholinium morpholine-N-carboxylate,1 to the best of our knowledge there have been no metalorganic compounds comprising morpholine-N-carboxylate reported to date. With this in mind, we have decided to explore the potential of morpholine-N-carboxylate as an anionic ligand, in particularly for coordinating Cu(II) and Zn(II) metal centres. Results and discussion: All attempts to prepare morpholine-N-carboxylato complexes from previously prepared morpholinium morpholine-N-carboxylate and Cu(II) and Zn(II) salts have failed. However, in a continuation of a previous study,8 we have accidentaly obtained a complex containing morpholine-N-carboxylate. This had occurred in an attempt to produce a heteroleptic copmplex bis(morpholine)(acetylacetonato)(chloro)copper(II) – the reaction mixture (a morpholine solution of copper(II) chloride and acetlyacetone) was left standing in an open vial for several weeks, yielding blue crystals which were subsequently identified as a copper(II) complex containing both morphCOO– and morphH ligands (II). The formation of the morpholine-N-carboxylate could only be accounted by absorption of atmospheric CO2 in liquid morpholine. As there have also been several reports that morpholinium morpholine-N-carboxylate could also be obtained by absorption of atmospheric CO2 in liquid morpholine over a prolonged 2
period,25,26,27 we have decided to attempt the synthesis of complexes by dissolving Cu(II) and Zn(II) salts in morpholine and exposing them to atmosphere for prolonged periods in order to achieve slow formation of morpholine-N-carboxylate in situ. As the starting salts we have opted for salts with non-coordinating anions (nitrate and sulphate) instead of the originally used chloride which could lead to formation of chloro complexes.8 Since both Cu(II) and Zn(II) salts are only sparingly soluble in morpholine, small amounts of diketones (acetlyacetone, Hacac, or benzoylacetone, Hbzac) were added in order to facilitate the dissolution through complex formation. This procedure was successful, and has led to formation and slow crystallization (over one month) of three heteroleptic coordination compounds (I–III) found to contain the morphCOO– ligand (Table 1).
Figure 1. a) A molecule of I; b) N–H···O hydrogen bonded chains of molecules of I along the b crystallographic axis (hydrogen atoms not participating in hydrogen bonding have been ommited for clarity).
Complex I, [Zn(morphCOO)2(morphH)2] was obtained from ZnSO4 in morpholine with addition of either Hacac or Hbzac. It crystallized in the C2/c space group with four molecules per unit cell. Zinc atom was tetracoordinated by nitrogen atoms of two morphH molecules and two carboxyl oxygen atoms of two morphCOO– anions placed in the vertices of a distorted tetrahedron (Figure 1a). The morphCOO– anions are bonded as monodentate ligands. The metal cation was positioned on the 2-fold axis and, consequently, both the two Zn–O and the two Zn–N bonds were of the 3
same length (1.931(1) Å and 2.055(2) Å respectively). The morphH ligands bind the zinc atom in the equatorial position of the morpholine ring, while the amine hydrogen occupies the axial position and serves as a hydrogen bond donor toward the noncoordinated carboxyl oxygen atom of a morphCOO– ligand (d = 2.862(5) Å, ∠(NHO) = 159.1(6)°) thus forming a hydrogen bonded chain a long the crystallographic b axis (Figure 1b). Table 1. An overview and crystallographic data of the prepared compounds. Compound Chemical formula M / g mol−1 Crystal system
I
II
III
ZnC18H34N4O8 Cu2C36H68N8O16 Cu2C38H52N4O12 499.9 monoclinic
498.0 triclinic
442.0 triclinic
Space group
C2/c
P−1
P−1
a/Å b/ Å c/ Å
19.9413(44) 5.97678) 18.2980(16) 90 97.731(15) 90 2160.99(45) 4 295 1.54
9.9281(7) 10.3730(7) 11.5405(6) 99.781(5) 92.998(5) 101.916(6) 1141.26(20) 2 295 1.45
9.4832(6) 9.9606(8) 12.3211(8) 97.259(6) 95.264(5) 117.502(7) 1009.03(48) 2 295 1.45
mm−1
1.190
1.007
1.120
θmin,max
4.4 ≤ θ ≤ 25.0 −23 ≤ h ≤ 23 −7 ≤ h ≤ 7 −21 ≤ h ≤ 21 1056.0
4.2 ≤ θ ≤ 25.0 −11 ≤ h ≤ −11 −12 ≤ h ≤ 12 −12 ≤ h ≤ 13 526.0
3.7 ≤ θ ≤ 27.0 −12 ≤ h ≤ 10 −12 ≤ k ≤ 11 −15 ≤ l ≤ 15 462.0
5915
8402
8762
1887
3993
4275
1542
3475
2653
0.078
0.041
0.028
191
333
258
α β γ V / Å3 Z T/K
ρcalc / g cm−3 µ (Mo-Kα) /
hmin, max kmin, max lmin, max F(000) Number of measured data Number of unique data Number of observed data Rint Number of refined parameters
4
R[F2 > 2σF2] wR(F2) S ∆ρmax / e Å−3 ∆ρmin / e Å−3
0.061 0.155 1.063 0.706 −0.946
0.032 0.097 0.877 0.286 −0.322
0.032 0.091 0.856 0.350 −0.328
Figure 2. a) A molecule of II (only the major component of the disordered morpoline ring is shown); b) Chains of molecules of II formed by C–H
O hydrogen bonding
along the crystallographic c axis (hydrogen atoms not participating in hydrogen bonding have been ommited for clarity). Complex compound [Cu2(morphCOO)4(morphH)4] (II) was obtained from CuSO4 disolved in morpholine with addition of Hacac. It was found to be a binuclear copper(II) complex which crystallized in the P−1 space group with one molecule per unit cell. The copper ions were pentacoordinated and their coordination polyhedron was a square pyramid. Nitrogen atoms of two morpholine ligands (N1 and N2) and morpholine carboxylate oxygen atoms of two morphCOO– anions (O6 and O7) defined the base of the pyramid, while the O8 atom of another morphCOO– anion occupyied
the
apical
position
(Figure
2a).
One
morphCOO–
anion
was
monodentately bonded to metal center, at the same time participating in an intramolecular N–H···O hydrogen bonding with a morphH ligand (d = 2.775(3) Å; ∠(NHO) = 156.2(3)°). In the monodentately morphCOO– ligand the morpholine ring was found to be disordered over two positions (Figure S4 in ESI). Another morphCOO– ligand was bridging between two metal centers with one oxygen 5
bonded in the apical and other in an equatorial position (Figure 2a). In crystal structure of II the molecules are connected into hydrogen bonded chains via weak C– H···Omorph hydrogen bonds (d = 2.507(2) Å) along the crystallographic c axis (Figure 2b). When Hbzac was used for increasing the solubility of CuSO4 in morpholine, a different heteroleptic complex, [Cu2(bzac)2(morphCOO)2(morphH)2] (III), was obtained. Here, along with morpholine and morpholine-N-carboxylate ligands, the benzoylacetonate anions (bzac) were also found to coordinate the Cu(II) centres. III was also a binuclear copper(II) complex which crystallized in the P−1 space group with one molecule per unit cell. Two metal centers are connected through a pair of bridging O3 atoms of two morphCOO– anions (Figure 3a). The donor atoms around the central metal ions (O1 and O2 of the bzac, O3 and O4 of the morphCOO– and N2 of morphH) were placed at vertices of distorted octahedra, the base of which (defined by the atoms O1, O2, O3 and N2) somewhat deviated from planarity (∠(O1– Cu–O3) = 165.27(8)° and ( ∠(O2–Cu–N2) = 169.90(1)°) (Figure 3a). The angle between two axial Cu-O bonds was ∠(O3–Cu–O4) = 135.8(7)° and bond lengths between axial oxygens and central copper atom were d(Cu–O3) = 2.512(2) Å and d(Cu–O4) = 2.727(2) Å. Crystal structure of III consisted of molecules bonded in chains through weak hydrogen bonding contacts C–H···O4 contacts (d = 3.417(2) Å) along the crystallographic a axis (Figure 3b).
Figure 3. a) A molecule of III; b) C–H···O hydrogen bonded chain in the crystal structure of complex III along the crystallographic a axis (hydrogen atoms not participating in hydrogen bonding have been ommited for clarity). 6
In the three obtained compounds it has been shown that morphCOO– can serve as monodentate (mode 1), bidentate (mode 2), or bridging ligand (mode 3). All three binding modes are known in carboxylate complexes of transition metals (Scheme 1 and Table S1 in ESI).28,29 It is interesting to note, however, that the monodentate binding (mode 19) present in two structures (I and II) is extremely rare (occurs in less than 1% of crystal structures which contain both a carboxylate group and a transition metal ion). Its occurence here should not be ascribed to specific behaviour of the morphCOO–ligand, but rather to the fact that all three compounds contain neutral morphH molecules as ligands. These can act as intramolecular hydrogen bond donors, and such hydrogen bonds in I and II are formed with the non-coordinated carboxylate oxygen atom, thus stabilizing the structure. Table 1 shows geometrical parameters of the morphCOO– ligand in the prepared complexes compared to those of morphCOO– anion in crystal structure of morpholinium morpholine-N-carboxylate ((morphH2)(morphCOO); CSD Refcode MORPLN01).30 In all three binding modes the carboxyl group is less symmetrical than in in morphCOO– anion in MORPLN01. For mode 1 the C-O bonds with the coordinated oxygen atom, are 0.031 Å (in I) and 0.045 Å (in II) longer than the noncoordinated C-O bonds – a common feature of monoccordinated carboxylates (see Table S2 in ESI).31 The differences in C-O bond lengths for modes 2 (III) and 3 (II) are 0.027 Å and 0.051 Å respectively. In these compounds, the decrease of the oxygen-metal bond length is followed by an increase of the corresponding carbonoxygen bond length. The coordination of the morphCOO– anion also apparently affects the geometry of the morpholine ring – in monodentately bonded ligands both N-CH2 bonds are longer than the equivalent bonds in MORPLN01. In compound I the elongation is the largest (0.058 Å and 0.052 Å) while in II, where morphCOO– is disordered, it is still slightly less pronounced, but also not insignificant (0.025 Å and 0.023 Å). In modes 2 and 3, a similar elongation is present but much less than in the case of the monodentately bonded ligand. Finally, other geometrical parameters of the ligand bounded in any of observed three modes do not deviate notably from those in noncoordinated morphCOO–.
7
Table 2. Geometrical parameters of morphCOO– in prepared compounds and morpholinium-N-morpholincarboxylate. +
III
(morphH ) – (morphCOO ) (MORPLN01)
mode 3
mode 2
-
1.281(3)*
1.280(3)
1.289(4)
1.268
1.245(5)
1.236(3)
1.253(3)
1.238(4)
1.259
d(C-N) / Å
1.357(6)
1.375(3)
1.367(3)
1.375(4)
1.379
d1(N-CH2) / Å
1.509(1)
1.474(6) 1.476(4)
1.458(4)
1.455(5)
1.451
d2(N-CH2) / Å
1.503(1)
1.474(5) 1.474(5)
1.453(3)
1.451(6)
1.451
∠(O-C-O) / °
124.8(4)
125.9(2)
125.2(2)
123.6(3)
124.91
∠(O-C-N)1 / °
115.8(4)*
115.6(2)*
115.0(2)
116.0(2)
116.94
∠(O-C-N)2 / °
119.4(4)
118.6(2)
119.8(2)
120.4(3)
118.14
∠(C-N-CH2)1 / °
117.4(5)
122.3(3) 118.3(3)
121.2(2)
121.0(3)
120.46
∠(C-N-CH2)2 / °
120.4(5)
123.1(3) 121.7(3)
122.3(2)
121.4(3)
120.43
compound
I
II
type of bonding
mode 1
mode 1
d1(C-O) / Å
1.276(5)*
d2(C-O) / Å
* - values for oxygen atoms coordinated to the metal center −
**- morpholine ring in morphCOO ligand is disordered over two positions
Conclusion: As demonstrated by the described three complexes, the morpholine-N-carboxylate anion can act as a versatile ligand for coordination of transition metals in which it can act as the expected chelating or bridging ligand, but also rather unexpectedly, as a monodentate ligand binding only through one oxygen atom of the carboxyl group. In all three cases, morpholine-N-carboxylate invariably employs only the oxygen atoms 8
of the carboxyl group for coordinating the metal cation leaving the morpholine ring oxygen (and potentially even nitrogen) atoms free as potential hydrogen or halogen bond acceptors. This makes the morpholine-N-carboxylate complexes interesting and potentially
useful
building
blocks
for
crystal
engineering
of
metal-organic
multicomponent solids.
Experimental section: Synthesis General procedure for synthesis of studied compounds is as follows: Zinc(II) sulfate heptahydrate (80,0 mg) or Copper(II) sulfate pentahydrate (80,0 mg) was added in hot morpholine (1,0 mL) after which acetylacetone (50 µL; in case of I and II) or benzoylacetone (110 mg for III) was added to reaction mixture. Obtained clear solution was left to cool and evaporate. Single crystals of complexes have obtained after several weeks.
X-ray diffraction experiments: Single crystal diffraction experiments were made on an Oxford Diffraction Xcalibur Kappa CCD X-ray diffractometer with graphite-monochromated MoKα (λ = 0.71073 Å) radiation32. The data sets were collected using the ω scan mode over the 2θ range up to 54°. The structures were solved by dire ct methods and refined using the SHELXS and SHELXL programs, respectively33. The structural refinement was performed on F2 using all data. The hydrogen atoms were placed in calculated positions and treated as riding on their parent atoms. All calculations were performed using the WinGX crystallographic suite of programs34. The hydrogen- and halogen bond geometries were calculated using Parst35 and Platon36. Further details are available from the Cambridge Crystallographic Centre. Molecular structures of compounds are presented by ORTEP37 and their packing diagrams were prepared using Mercury38. Crystallographic information files are also available from the Cambridge
Crystallographic
Data
Center
(CCDC)
upon
(http://www.ccdc.cam.ac.uk, CCDC deposition numbers 1965833-1965835). 9
request
Acknowledgements This research was supported by the Croatian Science Foundation through the grant IP-2014-09-736.
References: 1. L. Knorr, Ber. 22, (1889), 2081. 2. M. Beller, H. Trauthwein, M. Eichberger, C. Breindl, T. E. Muller, Eur. J. Inorg.Chem., 7, (1999), 1121–1132. 3. M. Beller, H. Trauthwein, M. Eichberger, C. Breindl, J. Herwig, T. E. Muller, O. R. Thiel, Chem.-Eur. J., 5, (1999), 1306–1319. 4. V. Stilinović, K. Užarević, I. Cvrtila, B. Kaitner, CrystEngComm, 14, (2012), 7493– 7501. 5. C. Doring, P. G. Jones, Z. Naturforsch.,B:Chem. Sci., 68, (2013), 474–492. 6. M. Strey, C. Doring, Z. Naturforsch., B:Chem. Sci., 73, (2018), 231–241. 7. H.-Sil Moon, C.-H. Kim, S.-G. Lee, Bull. Korean Chem.Soc., 21, (2000), 339. 8. I. Cvrtila, V.Stilinović, B. Kaitner, Struct.Chem., 23, (2012), 587–594. 9. M. Cindrić, G.Pavlović, T.Hrenar, M.Uzelac, M.Ćurić, Eur. J. Inorg. Chem., 4, (2013), 563–571. 10. H.-H. Li, Z. R. Chen, L.-C. Cheng, Y.-J. Wang, M. Feng, M. Wang, Dalton Trans., 39, (2010), 11000–11007. 11. R. D. Willett, R. Butcher, C. P. Landee, B. Twamley, Polyhedron, 24, (2005), 2222–2231. 12. G. Lapadula, N. Judaš, T. Friščić, W. Jones, Chem. Eur. J., 16, (2010), 7400– 7403. 13. P. Weinberger, R. Schamschule, K. Mereiter, L. Dlhán, R. Boca, W. Linert, J. Mol. Struct, 446, (1998), 115–126. 10
14. A. L. Johnson, N. Hollingsworth, A. Kingsley, G. Kociok-Kohn, K. C. Molloy, Organometallics, 28, (2009), 2650–2653. 15. C. Cheadle, J. Ratcliff, M. Berezin, V. Pal'shin, V. N. Nemykin, N. N. Gerasimchuk, Dalton Trans., 46, (2017), 13562–13581. 16. M. Tazi, W. Erb, Y. S. Halauko, O. A. Ivashkevich, V. E. Matulis, T. Roisnel, V. Dorcet, F. Mongin, Organometallics, 36, (2017), 4770–4778. 17. J. Hellmann, I. Rhotert, H. Westenberg, R. Frohlich, B. Wibbeling, W. Uhl, E.-U. Wurthwein, Eur. J. Org. Chem., 16, 3356–3368. 18. C. J. Brown, L. R. Gray, Acta Cryst. Sect. A., (1981), 37, C202. 19. C. N. R. Rao, S. Natarajan, R. Vaidhyanathan, Angew. Chem. Int. Ed., 43, (2004), 1466–1496. 20. V. Nemec, L. Fotović, T. Vitasović, D. Cinčić, CrystEngComm, 21, (2019), 3251– 3255. 21. V. Nemec, L. Fotović, T. Friščić, D. Cinčić, Cryst. Growth Des., 17, (2017), 6169– 6173. 22. V. Stilinović, T. Grgurić, T. Piteša, V. Nemec, D. Cinčić, Cryst. Growth Des., 19, (2019), 1245–1256. 23. M. Borovina, I. Kodrin, M. Đaković, CrystEngComm, 20, (2018), 539−549. 24. C. Merkens, F. Pan, U. Englert, CrystEngComm, 15, (2013), 8153−8158. 25. Knorr, Annelen, 301, (1898), 1–18. 26. M. Czugler, P. Bombicz, I. Csoregh, O. Gallardo, Z.Krist.Cryst.Mater., 1995, 368. 27. H. Jiang, S. Zhang, Y. Xu, J. Mol. Struct., 919, (2009), 21–25. 28. D. Martínez, M. Motevalli, M. Watkinson, Dalton Trans., 39, (2010), 446-455. 29. M. A. Nadeem, D. J. Craig, R. Bircher, J. A. Stride, Dalton Trans., 39, (2010), 4358-4362. 30. R. B. Von Dreele, R. L. Bradshaw, W. J. Burke, Acta. Cryst. C, 39, (1983), 253– 255.
11
31. C. R. Groom, I. J. Bruno, M. P. Lightfoot, S. C. Ward, Acta Cryst. B72, (2016), 171–179. 32. Oxford Diffraction (2003), CrysAlis CCD and CrysAlis RED. Version 1.170., Oxford Diffraction Ltd, Wroclaw, Poland. 33. Sheldrick, G. M. Acta Cryst., A64, (2008), 112. 34. L. J. Farrugia, J. Appl. Cryst., (1999), 32, 837. 35. M. Nardelli, J. Appl. Cryst., (1995), 28, 659. 36. A. L. Spek, J. Appl. Cryst., (2003), 36, 7. 37. L. J. Farrugia, J. Appl. Cryst., (1997), 30, 565. 38. C. F. Macrae, I. J. Bruno, J. A. Chisholm, P. R. Edgington, P. McCabe, E. Pidcock, L. Rodriguez-Monge, R. Taylor, J. van de Streek, P. A. Wood, J. Appl. Cryst. (2008), 41, 466.
12
Highlights
-
Morpholinecartboxylate has been successfully employed as a ligand Its complexes with Cu(II) and Zn(II) have been synthesized and studied It always coordinates metal atoms solely with the carboxylate oxygen atoms It can coordinate as a monodentate, chelating or a bridging ligand
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0022-2860(19)31736-3
DOI:
https://doi.org/10.1016/j.molstruc.2019.127627
Reference:
MOLSTR 127627
To appear in:
Journal of Molecular Structure
Received Date: 14 November 2019 Revised Date:
18 December 2019
Accepted Date: 19 December 2019
Please cite this article as: N. Bedeković, V. Stilinović, Morpholine-N-carboxylate as a ligand in coordination chemistry – Syntheses and structures of three heteroleptic copper(ii) and zinc complexes, Journal of Molecular Structure (2020), doi: https://doi.org/10.1016/j.molstruc.2019.127627. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
CRediT author statement Nikola Bedeković: Validation, Formal analysis, Investigation, Data Curation, Writing - Original Draft, Writing - Review & Editing, Visualization
Vladimir Stilinović: Conceptualization, Validation, Investigation, Writing - Original Draft, Writing - Review & Editing, Supervision
Morpholine-N-carboxylate as a ligand in coordination chemistry – syntheses and structures of three heteroleptic copper(II) and zinc complexes
Nikola Bedeković & Vladimir Stilinović* University of Zagreb, Faculty of Science, Chemistry Department Horvatovac 102a, RH-10000 Zagreb, Croatia *E-mail: [email protected]
Abstract The first three morpholine-N-carboxylate complexes, with copper(II) and zinc(II) have been prepared and studied by single crystal X-ray diffraction. All three compounds comprise of discrete mono- or dinuclear heteroleptic complex molecules. Three different ways of bonding of morphCOO– anions have been observed – as a monodentate, chelating, and as a bridging ligand. In all cases only the carboxyl group of the morpholinecarboxylate anion was found to bind to the metal cation, while the oxygen atom of the morpholine ring participates only in weak C–H
O hydrogen
bonds. Changes in the geometry of the coordinated morphCOO– anion relative to the free ligand (in its morpholinium salt) were observed in all three modes of binding to the metal center. Introduction Morpholine (morphH), first prepared by Knorr in 1889,1 is generally recognized today as a convenient ligand for the synthesis of a wide range of organometallic2,3 and metal-organic compounds including discrete complexes4,5 and metal-organic polymers.6,7 Although a morphH molecule is potentially an ambidentate N- and Odonor ligand, binding of morphH to a metal center is most commonly accomplished through the nitrogen atom,8,9 except in cases where the nitrogen atom is protonated.10,11 This leaves the oxygen atom free to participate in supramolecular
1
interconnections, and can act as a halogen bond acceptor12 or participate in hydrogen bonding,13 which can result in many different supramolecular architectures. The hydrogen atom of the secondary amino group can be easily substituted by an electrophilic species, allowing for derivatization of morphH to corresponding hydrazines14, carbonyl compounds15,16 or Schiff bases17. A potentially interesting way of derivatizing morphH molecule is carboxylation of the nitrogen atom, which would result in morpholine-N-carboxylic acid (morphCOOH), or the respective morpholineN-carboxylate (morphCOO–) anion.18 This should act as an anionic ligand in coordinating metal ions through the carboxylate group.19 Furthermore, a coordinated morpholine-N-carboxylate should be an even better hydrogen or halogen bond acceptor than the coordinated morpholine molecule due to its negative charge (as has been noticed with other anionic ligands such as imines,20,21 β-diketones22 and other.23,24 While the morpholine-N-carboxylate was prepared already by Knorr by passing a stream of gaseous CO2 through liquid morpholine, obtaining solid morpholinium morpholine-N-carboxylate,1 to the best of our knowledge there have been no metalorganic compounds comprising morpholine-N-carboxylate reported to date. With this in mind, we have decided to explore the potential of morpholine-N-carboxylate as an anionic ligand, in particularly for coordinating Cu(II) and Zn(II) metal centres. Results and discussion: All attempts to prepare morpholine-N-carboxylato complexes from previously prepared morpholinium morpholine-N-carboxylate and Cu(II) and Zn(II) salts have failed. However, in a continuation of a previous study,8 we have accidentaly obtained a complex containing morpholine-N-carboxylate. This had occurred in an attempt to produce a heteroleptic copmplex bis(morpholine)(acetylacetonato)(chloro)copper(II) – the reaction mixture (a morpholine solution of copper(II) chloride and acetlyacetone) was left standing in an open vial for several weeks, yielding blue crystals which were subsequently identified as a copper(II) complex containing both morphCOO– and morphH ligands (II). The formation of the morpholine-N-carboxylate could only be accounted by absorption of atmospheric CO2 in liquid morpholine. As there have also been several reports that morpholinium morpholine-N-carboxylate could also be obtained by absorption of atmospheric CO2 in liquid morpholine over a prolonged 2
period,25,26,27 we have decided to attempt the synthesis of complexes by dissolving Cu(II) and Zn(II) salts in morpholine and exposing them to atmosphere for prolonged periods in order to achieve slow formation of morpholine-N-carboxylate in situ. As the starting salts we have opted for salts with non-coordinating anions (nitrate and sulphate) instead of the originally used chloride which could lead to formation of chloro complexes.8 Since both Cu(II) and Zn(II) salts are only sparingly soluble in morpholine, small amounts of diketones (acetlyacetone, Hacac, or benzoylacetone, Hbzac) were added in order to facilitate the dissolution through complex formation. This procedure was successful, and has led to formation and slow crystallization (over one month) of three heteroleptic coordination compounds (I–III) found to contain the morphCOO– ligand (Table 1).
Figure 1. a) A molecule of I; b) N–H···O hydrogen bonded chains of molecules of I along the b crystallographic axis (hydrogen atoms not participating in hydrogen bonding have been ommited for clarity).
Complex I, [Zn(morphCOO)2(morphH)2] was obtained from ZnSO4 in morpholine with addition of either Hacac or Hbzac. It crystallized in the C2/c space group with four molecules per unit cell. Zinc atom was tetracoordinated by nitrogen atoms of two morphH molecules and two carboxyl oxygen atoms of two morphCOO– anions placed in the vertices of a distorted tetrahedron (Figure 1a). The morphCOO– anions are bonded as monodentate ligands. The metal cation was positioned on the 2-fold axis and, consequently, both the two Zn–O and the two Zn–N bonds were of the 3
same length (1.931(1) Å and 2.055(2) Å respectively). The morphH ligands bind the zinc atom in the equatorial position of the morpholine ring, while the amine hydrogen occupies the axial position and serves as a hydrogen bond donor toward the noncoordinated carboxyl oxygen atom of a morphCOO– ligand (d = 2.862(5) Å, ∠(NHO) = 159.1(6)°) thus forming a hydrogen bonded chain a long the crystallographic b axis (Figure 1b). Table 1. An overview and crystallographic data of the prepared compounds. Compound Chemical formula M / g mol−1 Crystal system
I
II
III
ZnC18H34N4O8 Cu2C36H68N8O16 Cu2C38H52N4O12 499.9 monoclinic
498.0 triclinic
442.0 triclinic
Space group
C2/c
P−1
P−1
a/Å b/ Å c/ Å
19.9413(44) 5.97678) 18.2980(16) 90 97.731(15) 90 2160.99(45) 4 295 1.54
9.9281(7) 10.3730(7) 11.5405(6) 99.781(5) 92.998(5) 101.916(6) 1141.26(20) 2 295 1.45
9.4832(6) 9.9606(8) 12.3211(8) 97.259(6) 95.264(5) 117.502(7) 1009.03(48) 2 295 1.45
mm−1
1.190
1.007
1.120
θmin,max
4.4 ≤ θ ≤ 25.0 −23 ≤ h ≤ 23 −7 ≤ h ≤ 7 −21 ≤ h ≤ 21 1056.0
4.2 ≤ θ ≤ 25.0 −11 ≤ h ≤ −11 −12 ≤ h ≤ 12 −12 ≤ h ≤ 13 526.0
3.7 ≤ θ ≤ 27.0 −12 ≤ h ≤ 10 −12 ≤ k ≤ 11 −15 ≤ l ≤ 15 462.0
5915
8402
8762
1887
3993
4275
1542
3475
2653
0.078
0.041
0.028
191
333
258
α β γ V / Å3 Z T/K
ρcalc / g cm−3 µ (Mo-Kα) /
hmin, max kmin, max lmin, max F(000) Number of measured data Number of unique data Number of observed data Rint Number of refined parameters
4
R[F2 > 2σF2] wR(F2) S ∆ρmax / e Å−3 ∆ρmin / e Å−3
0.061 0.155 1.063 0.706 −0.946
0.032 0.097 0.877 0.286 −0.322
0.032 0.091 0.856 0.350 −0.328
Figure 2. a) A molecule of II (only the major component of the disordered morpoline ring is shown); b) Chains of molecules of II formed by C–H
O hydrogen bonding
along the crystallographic c axis (hydrogen atoms not participating in hydrogen bonding have been ommited for clarity). Complex compound [Cu2(morphCOO)4(morphH)4] (II) was obtained from CuSO4 disolved in morpholine with addition of Hacac. It was found to be a binuclear copper(II) complex which crystallized in the P−1 space group with one molecule per unit cell. The copper ions were pentacoordinated and their coordination polyhedron was a square pyramid. Nitrogen atoms of two morpholine ligands (N1 and N2) and morpholine carboxylate oxygen atoms of two morphCOO– anions (O6 and O7) defined the base of the pyramid, while the O8 atom of another morphCOO– anion occupyied
the
apical
position
(Figure
2a).
One
morphCOO–
anion
was
monodentately bonded to metal center, at the same time participating in an intramolecular N–H···O hydrogen bonding with a morphH ligand (d = 2.775(3) Å; ∠(NHO) = 156.2(3)°). In the monodentately morphCOO– ligand the morpholine ring was found to be disordered over two positions (Figure S4 in ESI). Another morphCOO– ligand was bridging between two metal centers with one oxygen 5
bonded in the apical and other in an equatorial position (Figure 2a). In crystal structure of II the molecules are connected into hydrogen bonded chains via weak C– H···Omorph hydrogen bonds (d = 2.507(2) Å) along the crystallographic c axis (Figure 2b). When Hbzac was used for increasing the solubility of CuSO4 in morpholine, a different heteroleptic complex, [Cu2(bzac)2(morphCOO)2(morphH)2] (III), was obtained. Here, along with morpholine and morpholine-N-carboxylate ligands, the benzoylacetonate anions (bzac) were also found to coordinate the Cu(II) centres. III was also a binuclear copper(II) complex which crystallized in the P−1 space group with one molecule per unit cell. Two metal centers are connected through a pair of bridging O3 atoms of two morphCOO– anions (Figure 3a). The donor atoms around the central metal ions (O1 and O2 of the bzac, O3 and O4 of the morphCOO– and N2 of morphH) were placed at vertices of distorted octahedra, the base of which (defined by the atoms O1, O2, O3 and N2) somewhat deviated from planarity (∠(O1– Cu–O3) = 165.27(8)° and ( ∠(O2–Cu–N2) = 169.90(1)°) (Figure 3a). The angle between two axial Cu-O bonds was ∠(O3–Cu–O4) = 135.8(7)° and bond lengths between axial oxygens and central copper atom were d(Cu–O3) = 2.512(2) Å and d(Cu–O4) = 2.727(2) Å. Crystal structure of III consisted of molecules bonded in chains through weak hydrogen bonding contacts C–H···O4 contacts (d = 3.417(2) Å) along the crystallographic a axis (Figure 3b).
Figure 3. a) A molecule of III; b) C–H···O hydrogen bonded chain in the crystal structure of complex III along the crystallographic a axis (hydrogen atoms not participating in hydrogen bonding have been ommited for clarity). 6
In the three obtained compounds it has been shown that morphCOO– can serve as monodentate (mode 1), bidentate (mode 2), or bridging ligand (mode 3). All three binding modes are known in carboxylate complexes of transition metals (Scheme 1 and Table S1 in ESI).28,29 It is interesting to note, however, that the monodentate binding (mode 19) present in two structures (I and II) is extremely rare (occurs in less than 1% of crystal structures which contain both a carboxylate group and a transition metal ion). Its occurence here should not be ascribed to specific behaviour of the morphCOO–ligand, but rather to the fact that all three compounds contain neutral morphH molecules as ligands. These can act as intramolecular hydrogen bond donors, and such hydrogen bonds in I and II are formed with the non-coordinated carboxylate oxygen atom, thus stabilizing the structure. Table 1 shows geometrical parameters of the morphCOO– ligand in the prepared complexes compared to those of morphCOO– anion in crystal structure of morpholinium morpholine-N-carboxylate ((morphH2)(morphCOO); CSD Refcode MORPLN01).30 In all three binding modes the carboxyl group is less symmetrical than in in morphCOO– anion in MORPLN01. For mode 1 the C-O bonds with the coordinated oxygen atom, are 0.031 Å (in I) and 0.045 Å (in II) longer than the noncoordinated C-O bonds – a common feature of monoccordinated carboxylates (see Table S2 in ESI).31 The differences in C-O bond lengths for modes 2 (III) and 3 (II) are 0.027 Å and 0.051 Å respectively. In these compounds, the decrease of the oxygen-metal bond length is followed by an increase of the corresponding carbonoxygen bond length. The coordination of the morphCOO– anion also apparently affects the geometry of the morpholine ring – in monodentately bonded ligands both N-CH2 bonds are longer than the equivalent bonds in MORPLN01. In compound I the elongation is the largest (0.058 Å and 0.052 Å) while in II, where morphCOO– is disordered, it is still slightly less pronounced, but also not insignificant (0.025 Å and 0.023 Å). In modes 2 and 3, a similar elongation is present but much less than in the case of the monodentately bonded ligand. Finally, other geometrical parameters of the ligand bounded in any of observed three modes do not deviate notably from those in noncoordinated morphCOO–.
7
Table 2. Geometrical parameters of morphCOO– in prepared compounds and morpholinium-N-morpholincarboxylate. +
III
(morphH ) – (morphCOO ) (MORPLN01)
mode 3
mode 2
-
1.281(3)*
1.280(3)
1.289(4)
1.268
1.245(5)
1.236(3)
1.253(3)
1.238(4)
1.259
d(C-N) / Å
1.357(6)
1.375(3)
1.367(3)
1.375(4)
1.379
d1(N-CH2) / Å
1.509(1)
1.474(6) 1.476(4)
1.458(4)
1.455(5)
1.451
d2(N-CH2) / Å
1.503(1)
1.474(5) 1.474(5)
1.453(3)
1.451(6)
1.451
∠(O-C-O) / °
124.8(4)
125.9(2)
125.2(2)
123.6(3)
124.91
∠(O-C-N)1 / °
115.8(4)*
115.6(2)*
115.0(2)
116.0(2)
116.94
∠(O-C-N)2 / °
119.4(4)
118.6(2)
119.8(2)
120.4(3)
118.14
∠(C-N-CH2)1 / °
117.4(5)
122.3(3) 118.3(3)
121.2(2)
121.0(3)
120.46
∠(C-N-CH2)2 / °
120.4(5)
123.1(3) 121.7(3)
122.3(2)
121.4(3)
120.43
compound
I
II
type of bonding
mode 1
mode 1
d1(C-O) / Å
1.276(5)*
d2(C-O) / Å
* - values for oxygen atoms coordinated to the metal center −
**- morpholine ring in morphCOO ligand is disordered over two positions
Conclusion: As demonstrated by the described three complexes, the morpholine-N-carboxylate anion can act as a versatile ligand for coordination of transition metals in which it can act as the expected chelating or bridging ligand, but also rather unexpectedly, as a monodentate ligand binding only through one oxygen atom of the carboxyl group. In all three cases, morpholine-N-carboxylate invariably employs only the oxygen atoms 8
of the carboxyl group for coordinating the metal cation leaving the morpholine ring oxygen (and potentially even nitrogen) atoms free as potential hydrogen or halogen bond acceptors. This makes the morpholine-N-carboxylate complexes interesting and potentially
useful
building
blocks
for
crystal
engineering
of
metal-organic
multicomponent solids.
Experimental section: Synthesis General procedure for synthesis of studied compounds is as follows: Zinc(II) sulfate heptahydrate (80,0 mg) or Copper(II) sulfate pentahydrate (80,0 mg) was added in hot morpholine (1,0 mL) after which acetylacetone (50 µL; in case of I and II) or benzoylacetone (110 mg for III) was added to reaction mixture. Obtained clear solution was left to cool and evaporate. Single crystals of complexes have obtained after several weeks.
X-ray diffraction experiments: Single crystal diffraction experiments were made on an Oxford Diffraction Xcalibur Kappa CCD X-ray diffractometer with graphite-monochromated MoKα (λ = 0.71073 Å) radiation32. The data sets were collected using the ω scan mode over the 2θ range up to 54°. The structures were solved by dire ct methods and refined using the SHELXS and SHELXL programs, respectively33. The structural refinement was performed on F2 using all data. The hydrogen atoms were placed in calculated positions and treated as riding on their parent atoms. All calculations were performed using the WinGX crystallographic suite of programs34. The hydrogen- and halogen bond geometries were calculated using Parst35 and Platon36. Further details are available from the Cambridge Crystallographic Centre. Molecular structures of compounds are presented by ORTEP37 and their packing diagrams were prepared using Mercury38. Crystallographic information files are also available from the Cambridge
Crystallographic
Data
Center
(CCDC)
upon
(http://www.ccdc.cam.ac.uk, CCDC deposition numbers 1965833-1965835). 9
request
Acknowledgements This research was supported by the Croatian Science Foundation through the grant IP-2014-09-736.
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14. A. L. Johnson, N. Hollingsworth, A. Kingsley, G. Kociok-Kohn, K. C. Molloy, Organometallics, 28, (2009), 2650–2653. 15. C. Cheadle, J. Ratcliff, M. Berezin, V. Pal'shin, V. N. Nemykin, N. N. Gerasimchuk, Dalton Trans., 46, (2017), 13562–13581. 16. M. Tazi, W. Erb, Y. S. Halauko, O. A. Ivashkevich, V. E. Matulis, T. Roisnel, V. Dorcet, F. Mongin, Organometallics, 36, (2017), 4770–4778. 17. J. Hellmann, I. Rhotert, H. Westenberg, R. Frohlich, B. Wibbeling, W. Uhl, E.-U. Wurthwein, Eur. J. Org. Chem., 16, 3356–3368. 18. C. J. Brown, L. R. Gray, Acta Cryst. Sect. A., (1981), 37, C202. 19. C. N. R. Rao, S. Natarajan, R. Vaidhyanathan, Angew. Chem. Int. Ed., 43, (2004), 1466–1496. 20. V. Nemec, L. Fotović, T. Vitasović, D. Cinčić, CrystEngComm, 21, (2019), 3251– 3255. 21. V. Nemec, L. Fotović, T. Friščić, D. Cinčić, Cryst. Growth Des., 17, (2017), 6169– 6173. 22. V. Stilinović, T. Grgurić, T. Piteša, V. Nemec, D. Cinčić, Cryst. Growth Des., 19, (2019), 1245–1256. 23. M. Borovina, I. Kodrin, M. Đaković, CrystEngComm, 20, (2018), 539−549. 24. C. Merkens, F. Pan, U. Englert, CrystEngComm, 15, (2013), 8153−8158. 25. Knorr, Annelen, 301, (1898), 1–18. 26. M. Czugler, P. Bombicz, I. Csoregh, O. Gallardo, Z.Krist.Cryst.Mater., 1995, 368. 27. H. Jiang, S. Zhang, Y. Xu, J. Mol. Struct., 919, (2009), 21–25. 28. D. Martínez, M. Motevalli, M. Watkinson, Dalton Trans., 39, (2010), 446-455. 29. M. A. Nadeem, D. J. Craig, R. Bircher, J. A. Stride, Dalton Trans., 39, (2010), 4358-4362. 30. R. B. Von Dreele, R. L. Bradshaw, W. J. Burke, Acta. Cryst. C, 39, (1983), 253– 255.
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31. C. R. Groom, I. J. Bruno, M. P. Lightfoot, S. C. Ward, Acta Cryst. B72, (2016), 171–179. 32. Oxford Diffraction (2003), CrysAlis CCD and CrysAlis RED. Version 1.170., Oxford Diffraction Ltd, Wroclaw, Poland. 33. Sheldrick, G. M. Acta Cryst., A64, (2008), 112. 34. L. J. Farrugia, J. Appl. Cryst., (1999), 32, 837. 35. M. Nardelli, J. Appl. Cryst., (1995), 28, 659. 36. A. L. Spek, J. Appl. Cryst., (2003), 36, 7. 37. L. J. Farrugia, J. Appl. Cryst., (1997), 30, 565. 38. C. F. Macrae, I. J. Bruno, J. A. Chisholm, P. R. Edgington, P. McCabe, E. Pidcock, L. Rodriguez-Monge, R. Taylor, J. van de Streek, P. A. Wood, J. Appl. Cryst. (2008), 41, 466.
12
Highlights
-
Morpholinecartboxylate has been successfully employed as a ligand Its complexes with Cu(II) and Zn(II) have been synthesized and studied It always coordinates metal atoms solely with the carboxylate oxygen atoms It can coordinate as a monodentate, chelating or a bridging ligand
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: