Synthesis and crystal structures of two related Co and Mn complexes: a celebration of collaboration between the universities of Dakar and Southampton

The synthesis and structures are reported of two transition-metal complexes involving 2-(2-hydroxyphenyl)benzimidazole with cobalt and manganese, arising from a UK–Africa collaboration.


Chemical context
One of the conclusions from the Pan African Summit meeting (15-17 th October 2014, University of the Free State, Bloemfontein, South Africa; Roodt et al., 2014) held during the International Year of Crystallography, IYCr2014, was the lack of convenient crystallographic facilities across the continent.It was also recognised that crystallography is not well developed or studied in most of Africa and that opportunities for crystallographic researchers to interact with each other, as well as to those outside of the continent, are sparse.As a result, teaching activities are quite rare and the chance to develop new skills, competencies and capabilities, remained low.
To foster the progress of both practical and theoretical expertise within and around crystallography, programmes such as the IUCr Crystallography in Africa Initiative (Lecomte et al., 2014) aim to promote and develop crystallography (both teaching and research), throughout the continent.A primary goal is to create and support the operation of at least one national crystallography centre per country, ultimately to enable each of these to become independent hub of research in fields related to crystallography and the structural sciences.As part of the International Year of Crystallography activities, the IUCr-UNESCO OpenLab network was launched.This was a global activity to provide a network of functioning laboratories across developing nations, thereby enabling access to crystallographic instrumentation to a much broader range of researchers.In Africa, OpenLab was operated within the framework of the IUCr Crystallography in Africa initiative and acted as a starting point for a much wider range of researchers to fully engage with crystallographic experimentation.
Establishing the first Pan-African Crystallography Conference (PCCr1: Dschang, Cameroon, 2016) was a notable milestone towards the goal of coalescing crystallographybased research in Africa.In fact, its successes went much further than this, placing African crystallography on the world stage and attracting a notable number of delegates from further afield.Of particular success was the promotion of networking between delegates, both from within Africa and to those attending from all around the world.Subsequently PCCr2 and PCCr3 have been held in Accra, Ghana (2019) and Nairobi, Kenya (2023), respectively, have built on the platform established by PCCr1, resulting in far greater mobility, collaboration and empowerment for African crystallographers.
The Laboratoire de Chimie de Coordination Organique (LCCO) is a research unit linked to the Department of Chemistry at the University of Dakar, Senegal.It opened in 1995, under the guidance of Prof. Abdou Salam Sall (the former Rector of the University of Dakar) and Prof. Mohamed Lamine Gaye with a focus on structural chemistry, inorganic chemical synthesis, antioxidant activity, X-ray diffraction and spectroscopic characterization.Since its inception, over 20 students have successfully completed their PhD programs.It also continues to ramp up its research capacity, by expanding its complement of staff, both locally and with some based at other universities within Senegal.This expansion reflects the wider University strategy, which has seen the University of Dakar being declared the best university in Francophone Sub-Saharan Africa and the top-rated higher education institution in Senegal (by several independent online metric-based ranking sites, such as edurank.org,webmetrics.infoetc.).
Over its 40+ year history (Hursthouse et al., 2014), the National Crystallography Service (NCS) has provided access, primarily for UK researchers, to advanced crystallographic facilities and expertise to support those lacking facilities or to those with very challenging samples.With a strong history championing and investing into diffraction technology innovations, the NCS supports users from a wide variety of disciplines from over 30 different academic institutions in addition to several commercial clients.It also holds several teaching and outreach events each year, to promote science in general, and crystallography in particular.It is therefore well-placed to contribute (in part) to the development of African crystallography, by making some of these opportunities available.NCS has therefore been very fortunate to be involved in some of the significant development activities and has relished the opportunities that have since arisen, to collaborate closely with a range of new colleagues across Africa.
Although a connection had already been made, a strong collaboration between the LCCO and the NCS grew out of meeting face-to-face at PCCr2.Later in 2019, Dr Thiam undertook a two-week secondment to the NCS in Southampton, which cemented the basis of the collaboration.This visit was a valuable training opportunity covering crystallographic instrumentation and software, sample selection and handling, screening, collection strategy, data processing, structure solution and publication preparation.During the visit, over 20 full datasets were collected.This activity enabled considerable knowledge transfer to LCCO and subsequently, more than 70 further sample analyses have been successfully completed.Many novel and interesting structures have been characterized and have resulted in eleven joint publications in a range of journals, between 2018 and 2023 (Gaye et al., 2018(Gaye et al., , 2020;;Gaye, Fall et al., 2023;Gaye, Sarr et al., 2023;Gaye, Ndoye et al., 2021;Gaye, Kebe et al., 2021;Sokhna et al., 2023); Diallo et al., 2022;Diop et al., 2019;Faye et al., 2020;Sarr et al., 2018;Sylla-Gueye et al., 2020).Recent samples originated from more than a dozen current doctoral students from within the LCCO itself, the wider Department of Chemistry or from two additional joint projects with the University of Bambey, Senegal and Gaston Berger University of Saint-Louis, Senegal.
The volume of crystal structures being produced via this collaboration is significant, particularly for an African-based group.This means it is not possible, or necessary, to publish all these in the primary literature.Accordingly, we have made over 20 CSD Communications (see https://www.ccdc.cam.ac.uk/community/access-deposit-structures/deposit-a-structure/csd-communications/), as part of an ongoing exercise.
2-(2-Hydroxyphenyl)benzimidazoles are of interest for their photoluminescence properties and how the optical properties are effected by chelation to various metal centres has been previously reported (Zheng et al., 2003;Tong et al., 2005a,b).Exploration of these systems, particularly how the choice of metal centre and how modification of the coordination sphere may effect both crystal packing and the photoluminescence response, is of ongoing interest.Herein we report two recent structures resulting from this study.

Structural commentary
Complex 1 is a Z = 0.5 structure in P2 1 /c, comprised of a discrete � 2 oxo-bridged bi-nuclear complex of Co II ions [Co� � �Co = 3.1326 (6) A ˚], with each metal centre being fivecoordinate (trigonal bipyramidal), with a trigonality (or Addison) index of � = 0.82 (Addison et al., 1984) denoting a slight deformation away from an idealized geometry (further details given in the supporting information).Their coordination spheres (related through an inversion centre), include one SCN À , one EtOH and a 2hpbi ligand, through its N1 and O1 (bridging) atoms, as shown in Fig. 1.
Similarly, 2 is also a Z = 0.5 structure, although in this case in the space group C2/c.The complex also comprises a discrete � 2 oxo-bridged bi-nuclear complex; however, the metal centres have undergone oxidation (during formation, in air), from Mn II to Mn III [Mn� � �Mn = 3.3769 (7) A ˚].Both Mn ions are six-coordinate (octahedral) and their coordination spheres are related by rotational symmetry.Each Mn III coordinates to one SCN À and two 2hpbi ligands, one via atoms N1 and O1 (bridging) and the other through atoms N3 and O2 (Fig. 2).

Supramolecular features
Packing in 1 is directed by a 2-D network of N-H� � �S and O-H� � �S hydrogen bonds (Table 1), depicted in Fig. 3. Comparison with similar examples (CSD-Materials -CSD v5.44, Apr 2023; Macrae et al., 2020) confirmed that these interactions were typical of their type (summarized in S1; for further details, see supporting information section SI-1).
Analysis of the Hirshfeld surface and associated fingerprint plots (Spackman et al., 2021) of complex 1 provide further evidence for these contacts.Fig. 4 shows the Hirshfeld surface mapped over d norm (normalized contact distance) in the range À 0.4413 A ˚(red) to 1.4142 A ˚(blue), where donors and acceptors of these contacts are the bright-red areas.Twodimensional fingerprint plots are shown in Fig. 5, with characteristic sharp features arising from hydrogen bonding being present.
The contacts directing the crystal packing in 2 identified a more complex mixture of hydrogen bonds: O-H� � �O and O-H� � �N involving the included solvent water and an N-H� � �S contact, of type similar to that seen in 1.These  Co 2 (C 13 H 9 N 2 O) 2 (NCS) 2 (C 2 H 6 O) 2 ] and [Mn 2 (C 13 H 9 N 2 O) 4 (NCS) 2 ]•2H 2 O 1111   Figure 1 Bi-nuclear Co II complex of 1, ellipsoids shown at 50% probability.

Figure 2
Bi-nuclear Mn III complex of 2, ellipsoids shown at 50% probability and with symmetry-equivalent atoms ghosted.

Figure 4
Hirshfeld surfaces for 1 mapped over d norm in the range À 0.44 A ˚(red) to 1.41 A ˚(blue).
collectively form a 1D chain-like array, Fig. 6, summarized in Table 2 and with further details in supporting information section SI-2.Fig. 7 shows the Hirshfeld surface of complex 2, mapped over d norm range À 0.4884 A ˚(red) to 1.5602 A ˚(blue), again highlighting the hydrogen bonding present.Two-dimensional fingerprint plots are shown in Fig. 8, which, when compared to similar contacts in the CSD, provide further insight into the crystal packing.The strongest and sharpest features correspond to the O-H� � �O(water) contacts, which are now both the strongest hydrogen-bond donor and acceptor in the structure.The N-H� � �S features are noticeably weaker than in structure 1, reflected in both its relative lengthening (0.3 A ˚) and being further perturbed from a linear geometry (163 to 152 � ).
Also of minor note, the second hydrogen (H3B) of the solvent water (containing O3) is orientated towards atom S1 of an adjacent complex as though to form a hydrogen bond.The distance, however (3.8A ˚), is much longer than the bulk of comparable contacts in the CSD (3.2-3.4A ˚) and its position is simply optimized for steric packing.

Database survey
A CSD search (v5.44,Apr 2023; Groom et al., 2016) for complexes involving the (unfunctionalized) 2hpbi ligand returned 69 hits, nine of which are coordinated by Co or Mn and none containing thiocyanates.Eight of these Co or Mn complex species consist of a single metal centre, with the remaining structure, PULGUZ (Duan et al., 2010), forming a bi-nuclear complex, though quite dissimilar in structure to 2. PULGUZ contains two six-coordinate Mn IV centres, each chelated by two 2hpbi ligands then bridged by two O 2À ions [Mn� � �Mn = 2.7772 (9) A ˚].

Figure 5
Two-dimensional fingerprint plots of 1.

Figure 7
Hirshfeld surfaces for 2 mapped over d norm in the range À 0.49 A ˚(red) to 1.56 A ˚(blue).

Figure 8
Two-dimensional fingerprint plots of 2.
ethanol) into a flask containing orthophenyldiamine (30mmol, in ethanol), in the presence of few drops of glacial acetic acid.On cooling, no precipitate was observed and the reaction mixture was evaporated to dryness, to give a yellow oil.A yellow precipitate was obtained on the addition of diethyl ether, which was separated and thoroughly washed with further ether, then dried over P 4 O 10 .FT-IR and NMR spectra for the 2hpbi ligand are consistent to those reported (values below, further details in supporting information section SI-3), yield 93%. Crystallization from ethanol and slow evaporation led to the formation of crystals suitable for single crystal X-ray diffraction, after �1 week.Transition-metal complexes 1 and 2 were synthesized by first suspending the respective metal salt: CoCl 2 •2H 2 O (0.042g, 0.25mmol) or MnCl 2 •2H 2 O (0.040g, 0.25mmol) with NH 4 SCN (0.038g, 0.5mmol) in ethanol (and a few drops of water).These mixtures were then filtered and added to ethanol solutions of ligand 2-(1H-benzimidazol-2-yl) phenol (0.053g, 0.25mmol), subsequently stirred for 1 h, then filtered and allowed to slowly evaporate (1 week).In both cases, crystals suitable for scXRD were produced, dark-red crystals of the cobalt complex 1 (m.p. = 495 K) and pale-brown crystals of the manganese complex 2 (m.p. > 533 K).Crystallographic analysis identified 1 as the complex [Co 2 (C 13 H 9 N 2 O) 2 (NSC) 2 (CH 3 CH 2 OH) 2 ] and 2 as the complex [Mn 2 (C 13 H 9 N 2 O) 4 (NSC) 2 ]•2H 2 O.

Refinement
Details of the crystal data, data collection and structure refinement are summarized in Table 3.

Reflections and Future Work
This collaboration between the universities of Dakar and Southampton is just one of the many ways in which the initiatives promoting crystallography and related fields across African nations are helping to raise the profile of African research.The number of new compounds and crystals produced by LCCO is impressive and the value of these outputs are greatly increased via access to crystallographic facilities.The benefits for both institutions are clear when considering the number of new crystal structures generated and that the publication rate averages over two articles per year.The NCS benefits from a number of such collaborations, most of which have stemmed from networking at Pan-African conferences and related events and so we encourage researchers from outside the continent to attend these and engage with the wealth of science being conducted.The outputs from projects supported by the collaboration and the secondment to Southampton both contributed strongly to the recent promotion of Dr Thiam to the rank of Professor.
Finally, the NCS is launching a service for single-crystal structure analysis by Electron Diffraction to complement its established state-of-the-art X-ray facilities.This technique is on the verge of becoming more routine and provides many opportunities for new structural chemistry research and the ability to examine nanocrystals not only extends the capabilities of X-rays, but also opens up our technique to areas of chemistry that could not previously benefit.The NCS is currently exploring routes by which it can make these capabilities available to colleagues in Africa.

Special details
Geometry.All esds (except the esd in the dihedral angle between two l.s.planes) are estimated using the full covariance matrix.The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry.An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s.planes.

Special details
Geometry.All esds (except the esd in the dihedral angle between two l.s.planes) are estimated using the full covariance matrix.The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry.An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s.planes.Refinement.The N-H bond lengths (N2-H2 and N4-H4a) have been restrained, otherwise they refined to unrealistic values.A peak of residual electron density (c.a.1.2e-) at position (0.5, 0.264, 0.5 -close to atoms S1 and O3) remains unmodelled in the final refinement.During Structure development, a low occupancy water was modelled at this site (which also necessitated some disorder of the adjacent SCN ion).This arrangement gave rise to a chemically sensible hydrogen bond network and improved the refinement statistics; however, this arrangement freely refined to <8% occupancy and it has been omitted as this was deemed too low for reasonable certainty of the modelled disorder.

Table 3
Experimental details.