Low-temperature reaction of the tris(pyrazolyl)borate copper(II) hydroxide [(iPr2) TpCu]2 (μ-OH)2 with triphenylsilane under a dinitrogen atmosphere gives the bridging dinitrogen complex [(iPr2) TpCu]2 (μ-1,2-N2 ) (3). X-ray crystallography reveals an only slightly activated N2 ligand (N-N: 1.111(6) Å) that bridges between two monovalent (iPr2) TpCu fragments. While DFT studies of mono- and dinuclear copper dinitrogen complexes suggest weak π-backbonding between the d(10) Cu(I) centers and the N2 ligand, they reveal a degree of cooperativity in the dinuclear Cu-N2 -Cu interaction. Addition of MeCN, CNAr(2,6-Me) , or O2 to 3 releases N2 with formation of (iPr2) TpCu(L) (L=NCMe, CNAr(2,6-Me2) ) or [(iPr2) TpCu]2 (μ-η(2) :η(2) -O2 ) (1). Addition of triphenylsilane to [(iPr2) TpCu]2 (μ-OH)2 in pentane allows isolation of a key intermediate [(iPr2) TpCu]2 (μ-H) (5). Although 5 thermally decays under N2 to give 3, it reduces unsaturated substrates, such as CO and HC≡CPh to HC(O)H and H2 C=CHPh, respectively.

The life-sustaining reduction of N2 to NH3 is thermoneutral yet kinetically challenged by high-energy intermediates such as N2H2. Exploring intramolecular H-bonding as a potential strategy to stabilize diazene intermediates, we employ a series of [ xHetTpCu]2(μ-N2H2) complexes that exhibit H-bonding between pendant aromatic N-heterocycles (xHet) such as pyridine and a bridging trans-N2H2 ligand at copper­(I) centers. X-ray crystallography and IR spectroscopy clearly reveal H-bonding in [pyMeTpCu]2(μ-N2H2) while low-temperature 1H NMR studies coupled with DFT analysis reveals a dynamic equilibrium between two closely related, symmetric H-bonded structural motifs. Importantly, the xHet pendant negligibly influences the electronic structure of xHetTpCuI centers in xHetTpCu­(CNAr2,6‑Me2 ) complexes that lack H-bonding as judged by nearly indistinguishable ν­(CN) frequencies (2113–2117 cm–1). Nonetheless, H-bonding in the corresponding [ xHetTpCu]2(μ-N2H2) complexes results in marked changes in ν­(NN) (1398–1419 cm–1) revealed through resonance Raman studies. Due to the closely matched N–H BDEs of N2H2 and the pyH0 cation radical, the aromatic N-heterocyclic pendants may encourage partial H-atom transfer (HAT) from N2H2 to xHet through redox-non-innocent H-bonding in [ xHetTpCu]2(μ-N2H2). DFT studies reveal modest thermodynamic barriers for concerted transfer of both H-atoms of coordinated N2H2 to the xHet pendants to generate tautomeric [ xHetHTpCu]2(μ-N2) complexes, identifying metal-assisted concerted dual HAT as a thermodynamically favorable pathway for N2/N2H2 interconversion.

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 H-bonding 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 Tp-Cu−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 ν OH frequencies that tracks with the Hbond accepting ability of the pendant arm. Reversible protonation studies that interconvert [ XpyMe TpCu] 2 (μ-OH) 2 and [ XpyMe TpCu(OH 2 )]OTf species indicate that the acidity of the corresponding aquo ligand decreases with increasing H-bond accepting ability of the pendant arm.

A trio of dimeric copper(I) complexes of the formula [(Ph 3 P)Cu(μ-X) 2 Cu(PPh 3)] (X = OC 6 F 5 , 1, OC 4 F 9 , 2, OCPh(CF 3) 2 , 3) were prepared and characterized by X-ray crystallography, elemental analysis, and NMR spectroscopy (1 H, 13 C, 19 F, 31 P), as was the monomeric [(cy 3 P)Cu(OC 4 F 9)] (cy = cyclohexyl,4). Solution conductivity studies demonstrate that all three new dinuclearcompounds, as well as the known [(Ph 3 P)Cu(μ-OC 4 H 9) 2 Cu(PPh 3)], 2-H, are neutral species in solution and do not rearrange into ion pairs. The fold angle () varies among these four dimers and the Cu(I) … Cu(I) distance in the structure of 2, 2.8315(5) Å, is cuprophilic. The Cu(I) … Cu(I) distances for 1, 2-H, and 3 are 3.0533(5), 2.890(2), and 3.0169(6) Å respectively.Density functional theory (DFT) calculations were performed on 1, 2, 3, and 2-H, as well as the hypothetical 1-H, and several related models. FivePMe 3 models, 1(Me), 1-H(Me), 2(Me), 2-H(Me), and 3(Me) were also studied as well as five monomers [(Me 3 P)CuX]1-mon, 1-H(mon), 2-mon, 2-H(mon), and 3-mon to understand the electronic reasons for folding in this group of compounds. A Natural Energy Decomposition Analysis (NEDA) indicates that electrostatic stabilizations are the dominant factor governing the strength of interaction between monomeric fragments in 1(Me)-3(Me). NBO analysis reveals that 1(Me) and 1-H(Me) do not display any cuprophilic interactions. The folding angle observed in 2(Me), 2-H(Me), and 3(Me), which is correlated with an increased delocalization from the oxygen 2p z lone pairs, brings the metal centers into sufficient propinquity to have weak Cu … Cu orbital interactions. Weak luminescence behavior at room-temperature is also consistent with these assignments.

A modular synthesis provides access to a series of new tris(pyrazolyl)borate ligands ^XpyMeTpK that possess a single functionalized pendant pyridyl (py) or pyrimidyl (pyd) arm designed to engage in tunable intramolecular H-bonding to metal–bound functionalities. To illustrate such H-bonding interactions, a series of [^XpyMeTpCu]₂(𝜇–OH)₂(<b>6a</b><b>–6e</b>) complexes were synthesized from the corresponding ^XpyMeTpCu–OAc (<b>5a–5e</b>) complexes. Single crystal X-ray structures of three new dinuclear [^XpyMeTpCu]₂(𝜇–OH)₂complexes reveal H-bonding between the pendant heterocycle and bridging hydroxide ligands while the donor arm engages the copper center in an unusual monomeric ^DMAPMeTpCu–OH complex. Vibrational studies (IR) of each bridging hydroxide complex reveal reduced 𝜈_OHfrequencies that tracks with the H-bond accepting ability of the pendant arm.