Tris(pyrazolyl)borate Copper Hydroxide Complexes Featuring Tunable Intramolecular H-bonding
A modular synthesis provides access to a series of new tris(pyrazolyl)borate ligands XpyMe TpK that possess a single functionalized pendant pyridyl (py) or pyrimidyl (pyd) arm designed to engage in tunable intramolecular Hbonding to metal-bound functionalities. To illustrate such H-bonding interactions, a series of [ XpyMe TpCu]2(𝜇-OH)2 (6a-6e) complexes were synthesized from the corresponding XpyMe TpCu-OAc (5a-5e) complexes. Single crystal X-ray structures of three new dinuclear [ XpyMe TpCu]2(𝜇-OH)2 complexes reveal H-bonding between the pendant heterocycle and bridging hydroxide ligands while the donor arm engages the copper center in an unusual monomeric DMAPMe TpCu-OH complex. Vibrational studies (IR) of each bridging hydroxide complex reveal reduced 𝜈OH frequencies that tracks with the H-bond accepting ability of the pendant arm.
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<p>In metalloenzymes, hydrogen bonding (H-bonding) interactions prevalent in the tightly regulated microenvironment of metal ions are closely tied to functionality. [1][2][3] When applied in synthetic systems, arrays of rigid Hbonding frameworks have had notable effects on chemical reactivity. [4][5][6][7][8] The number and diversity of new synthetic systems featuring secondary coordination sphere H-bonding is expanding as efforts to better understand these phenomena continue.</p><p>Secondary sphere H-bond donor and/or acceptor groups are featured especially often in tripodal systems (Figure 1). [9][10][11][12][13][14][15] For instance, several synthetic systems showcase the utility of H-bonding interactions with oxygen-containing small molecules. The stabilization of the peroxide anion (O2 2-) via interactions with H-bond donors, first observed in Cummins' 16 metal-free cryptand, is underscored by Szymczak's 17 tripodal Zn(II) complex, where secondary coordination sphere H-bonding governs O2 2binding at Zn (Figure 1a). Fewer systems possessing H-bond acceptors have been reported 14,18,19 but include Fout's 10 tripodal scaffold with pyrrole-imine Hbond acceptors that H-bond to coordinated water (Figure 1b). In addition, this ligand can undergo redox tautomerization to form a "hybrid" H-bond donor/acceptor system as well as an all H-bond donor variant. Borovik's 20 recent example of this hybrid class is a tripodal scaffold containing a mix of two amine H-bond donors and one phosphinic amide H-bond acceptor (Figure 1c).</p><p>Since it can be difficult to directly examine the nature of the hydrogen bonding with highly reactive intermediates, more stable hydroxide complexes possessing secondary sphere H-bonding serve useful roles to survey H-bond contributions. [21][22][23] Such hydroxide species have been important for revealing the influence of the number of H-bonding groups as well as the type of Figure 1. Examples of systems incorporating second sphere H-bond donor and /or acceptor groups. H-bonding groups on the properties of the complex. 11,14,21,24 In a landmark report, Kitajima and Fujisawa established TpCu scaffolds (Tp = tris(pyrazolyl)borate) as important biorelevant models for O 2 reactivity at copperbased hemocyanin with [ iPr2 TpCu] 2 ( -O 2 ) and the related [ iPr2 TpCu] 2 ( -OH) 2 . 25 Molecular copper-oxygen complexes attract interest as intermediates in chemical transformations mediated by hemocyanin, [25][26][27] copper oxidases, [28][29][30] and other copper-based enzymes. 31,32 We set out to construct a new family of Tp ligands with tunable secondary sphere H-bonding through only a single heterocyclic pyridyl (py) or pyrimidyl group (pyd). While examples of Tp scaffolds bearing pendant H-bonding groups [33][34][35][36][37] or ancillary ligands capable of Hbonding have been reported, [38][39][40] particularly inspiring is a py3 TpK ligand 35,37,41 that bears three pendant pyridyl groups that engages in secondary sphere H-bonding with an aqua ligand at Cu II (Figure 2a). 35,37 The targeted Tp ligand, however, bears only a single pendant base, since Tp systems with three pyridyl groups-one pyridyl arm per pyrazole-lead to an array of Cu coordination motifs involving Cu-py interactions (Figure 2). 35,37,41 We report herein a new, modular synthesis for singlyfunctionalized heteroscorpionates XpyMe TpK along with crystallographic and spectroscopic characterization of corresponding [ XpyMe TpCu] 2 ( -OH) 2 complexes that demonstrate tunable secondary coordination sphere Hbonding.</p><!><p>A series of substituted pyrazoles were prepared starting with the Claisen condensation of a series of commercially available py or pyd ester derivatives with acetone to give the corresponding diketone (Scheme 1). This reaction most reliably proceeds with a nonnucleophilic base such as KH in Et 2 O. Condensation of respective diketones 3a-3d with anhydrous hydrazine in methanol affords the py and pyd functionalized pyrazoles ( XpyMe PzH) 4a-4d in high yields (Scheme 2). This approach was not successful, however, in the synthesis of 3e that possesses a dimethylamino-pyridine (DMAP) functionality. Instead, nucleophilic aromatic substitution of chloropyridine-pyrazole 3d with neat 40% aqueous dimethyl amine under reflux gives 3e in 73% yield (Scheme S4). Single crystal X-ray diffraction structures obtained for 3c and 3e illustrate intermolecular X py⋯H-Pz H-bonding interactions (Figures S10 and  S11).</p><p>A pyrazole exchange reaction starting from the symmetric iPr2 TpK ligand 42 conveniently leads to singlysubstituted XpyMe TpK heteroscorpionates (4a-4e) (Scheme 3). Unlike other heteroscorpionates that bear non-symmetric alkyl groups, 43,44 parallel efforts to generate the monosubstituted XpyMe TpK from a molten reaction of KBH 4 2:1 ratio of the corresponding pyrazoles were unsuccessful. Instead, XpyMe TpK can be synthesized by combining equimolar amounts of iPr2 TpK and XpyMe PzH (3a -3e) at 160 °C under a dynamic vacuum. These conditions facilitate both pyrazole exchange and removal of iPr2 PzH via sublimation onto a cold finger, recycled for future use (Figure S1). Crystallization from minimal dichloromethane (DCM) with several drops of acetonitrile provide XpyMe TpK ligands 4a-4e as acetonitrile adducts, which can be dried in vacuo to provide solvent-free species in 46-60% yields. X-ray crystallography of acetonitrile adducts reveals either mono-or dinuclear structures (Figure 3). For instance, 3a crystallizes as monomeric CF3pyMe TpK(NCMe) 2 whereas 3c is     2 ( -OH) 2 com uOAc (5) in a ure, similar t OH) 2 (7). 25 red with MeC -OH) 2 (6a-6e tructures of [ X intramolecula dant py and p ps (Figure 5).</p><!><p>The modula single pyridine access to a new</p>