inorganics Article Adding to the Family of Copper Complexes Featuring Borohydride Ligands Based on 2-Mercaptopyridyl Units Joseph Goldsworthy 1, Simon D. Thomas 1 , Graham J. Tizzard 2 , Simon J. Coles 2 and Gareth R. Owen 1,* 1 School of Applied Sciences, University of South Wales, Pontypridd CF37 4AT, UK 2 UK National Crystallography Service, University of Southampton, Highfield, Southampton SO17 1BJ, UK * Correspondence: [email protected]; Tel.: +44-1443-65-4527 Received: 13 June 2019; Accepted: 19 July 2019; Published: 24 July 2019 Abstract: Borohydride ligands featuring multiple pendant donor functionalities have been prevalent in the chemical literature for many decades now. More recent times has seen their development into new families of so-called soft scorpionates, for example, those featuring sulfur based donors. Despite all of these developments, those ligands containing just one pendant group are rare. This article explores one ligand family based on the 2-mercaptopyridine heterocycle. The coordination chemistry of the monosubstituted ligand, [H3B(mp)]− (mp = 2-mercaptopyridyl), has been explored. (I) Reaction of Na[BH3(mp)] with one equivalent of Cu Cl in the presence of either triphenylphosphine or tricyclohexylphosphine co-ligands leads to the formation of [Cu{H3B(mp)}(PR3)] (R = Ph, 1; Cy, 2), respectively. Structural characterization confirms a κ3-S,H,H coordination mode for the borohydride-based ligand within 1 and 2, involving a dihydroborate bridging interaction (BH2Cu) with the copper centers. Keywords: scorpionate; copper; borohydride; ligand; sulfur 1. Introduction The coordination chemistry of borohydride and substituted borohydride units with transition metals has been a major focus of research over many decades now [1–9]. One particular focus of research has been on substituted borohydride units attached to other donor functional groups. This gave rise to a research area known as “scorpionate chemistry”, where the borohydride moiety is typically substituted by two or more pyrazolyl rings, thus forming multidentate ligand systems. This area of research has provided an expansive and fascinating array of compounds with wide ranging applications. These have been explored in homogeneous catalysis and bioinorganic chemistry, for example [10–16]. In many examples, the borohydride unit is positioned away from the metal center, playing a spectator role within the complex. The polyprazolylborates, for example, are known as “octahedral enforcers”, furnishing highly rigid stable complexes. More recent developments, have led to new generations of ligand systems, where the borohydride unit is positioned in direct contact with the metal center. In some cases these can undergo direct transformations at the boron center (Figure1)[ 17–22]. This occurs when an additional atom is incorporated between the boron and donor atom. The publication of this new generation of more “flexible scorpionates” opened up a new area of research with respect to the formation of Z-type ligands [17–22]. This revolutionized the field and altered the perspective on the coordination chemistry of such ligands. The first of the more flexible scorpionate ligands was [Tm]− [hydrotris(methylimidazolyl)borate] (Figure1; middle) [ 23]. This new ligand had two major Inorganics 2019, 7, 93; doi:10.3390/inorganics7080093 www.mdpi.com/journal/inorganics Inorganics 2019, 7, 93 2 of 11 Inorganics 2019, 7, x FOR PEER REVIEW 2 of 11 differences when compared to Trofimenko’s original scorpionates. The ligand was based on soft Inorganics 2019, 7, x FOR PEER REVIEW 2 of 11 sulfurdifferences donor atoms when [16compared], and perhaps to Trofimenko’s more significantly, original scorpionates. greater flexibilityThe ligand had was been based incorporated on soft sulfur donor atoms [16], and perhaps more significantly, greater flexibility had been incorporated into thedifferences ligand when by addition compared of theto Trofimenko’s extra atom origin betweenal scorpionates. the boron andThe theligand donor was atom.based on It wassoft this into the ligand by addition of the extra atom between the boron and the donor atom. It was this greatersulfur flexibility donor atoms within [16], the and ligand perhap structures more thatsignificantly, opened upgreater the potentialflexibility forhad activationbeen incorporated at the boron greater flexibility within the ligand structure that opened up the potential for activation at the boron into the ligand by addition of the extra atom between the boron and the donor atom. It was this bridgeheadbridgehead and and formation formation of metal-boraneof metal-borane (metallaboratrane) (metallaboratrane) complexescomplexes [17–20,24–27], [17–20,24–27 ],giving giving rise rise to greater flexibility within the ligand structure that opened up the potential for activation at the boron reactivityto reactivity not observed not observed in the in analogous the analogous polypyrazolylborate polypyrazolylborate ligands ligands [[10–16].10–16]. bridgehead and formation of metal-borane (metallaboratrane) complexes [17–20,24–27], giving rise to reactivity not observed in the analogous polypyrazolylborate ligands [10–16]. FigureFigure 1. Selected 1. Selected examples examples of of ligands ligands inin whichwhich the borohydride borohydride unit unit is positioned is positioned away away from fromthe the metalmetal center center (left ),(left), directed directed towards towards the metalthe metal center center (middle (middle),), and and a system a system in which in which a B-H a activationB‒H Figure 1. Selected examples of ligands in which the borohydride unit is positioned away from the activation has occurred (right). The additional atom between boron and the donor atoms is typically has occurredmetal center (right (left),). The directed additional towards atom the between metal center boron (middle), and the donorand a atomssystem isin typically which a necessaryB‒H necessary for metal‒boron bond formation. for metal-boronactivation has bond occurred formation. (right). The additional atom between boron and the donor atoms is typically necessary for metal‒boron bond formation. OverOver the the following following twenty twenty years years sincesince thethe first first report report of of hydride hydride migration migration from from the boron the boron center of a scorpionate ligand, a number of research groups have focused on new, more flexible center ofOver a scorpionate the following ligand, twenty a numberyears since of the research first report groups of hydride have focused migration on from new, the more boron flexible borohydride ligands containing a range of supporting units based on nitrogen [28–31] and other borohydridecenter of ligandsa scorpionate containing ligand, aa rangenumber of of supporting research groups units have based focused on nitrogen on new, [28more–31 flexible] and other sulfur heterocycles [32–46]. As part of our research, we have focused on providing new derivative borohydride ligands containing a range of supporting units based on nitrogen [28–31] and other sulfurligand heterocycles systems. In [32 2009,–46]. we As introd partuced of our a new research, family of we flexible have sc focusedorpionate on ligands providing derived new from derivative the sulfur heterocycles [32–46]. As part of our research, we have focused on providing new derivative ligand2-mercaptopyridine systems. In 2009, heterocycle we introduced [36]. This a new origin familyal report of flexible provided scorpionate a borohydride-based ligands derived ligand from ligand systems. In 2009, we introduced a new family of flexible scorpionate ligands derived from the the 2-mercaptopyridinesubstituted by two and heterocycle three of these [36 heterocycles.]. This original Last reportyear, we provided extended athis borohydride-based family to include the ligand 2-mercaptopyridine heterocycle [36]. This original report provided a borohydride-based ligand substitutedmonosubstitued by two andligand, three [H3 ofB(mp)] these− (where heterocycles. mp = 2-mercaptopyridyl; Last year, we extended Figure 2) this [37]. family Herein, to we include substituted by two and three of these heterocycles. Last year, we extended this family to include the the monosubstituedreport the synthesis ligand, and characterization [H3B(mp)]− (where of the fi mprst copper= 2-mercaptopyridyl; complexes containing Figure this2 new)[ 37 ligand.]. Herein, monosubstitued ligand, [H3B(mp)]− (where mp = 2-mercaptopyridyl; Figure 2) [37]. Herein, we we reportThe complexes the synthesis have andbeen characterizationstructurally characterized of the first and copper compared complexes to related containing complexes. this new ligand. report the synthesis and characterization of the first copper complexes containing this new ligand. The complexesThe complexes have have been been structurally structurally characterized characterized and compared compared to to related related complexes. complexes. Figure 2. The monosubstituted borohydride salt, Na[H3B(mp)]. Figure 2. The monosubstituted borohydride salt, Na[H3B(mp)]. 2. Results and DiscussionFigure 2. The monosubstituted borohydride salt, Na[H3B(mp)]. 2. Results and Discussion 2.2.1. Results Synthesis and and Discussion Characterization of Copper Complexes 2.1. Synthesis2.1. SynthesisThe andcoordination and Characterization Charact chemistryerization of of Copperof Copper [H3B(mp)] ComplexesComplexes− is limited to one example to date. The complex [Rh{κ3-B,H,H-H3B(mp)}(NBD)] (where NBD = 2,5-norbornadiene), was reported