Mononuclear and Binuclear Copper(I) Complexes Ligated by Bis(3,5-diisopropyl-1-pyrazolyl)methane:  Insight into the Fundamental Coordination Chemistry of Three-Coordinate Copper(I) Complexes with a Neutral Coligand

Abstract

By using the neutral bidentate nitrogen-containing ligand, bis(3,5-diisopropyl-1-pyrazolyl)methane (L1' '), the copper(I) complexes [Cu(L1' ')2](CuCl2) (1CuCl2), [Cu(L1' ')2](ClO4) (1ClO4), [Cu(L1' ')]2(ClO4)2 (2ClO4), [Cu(L1' ')]2(BF4)2 (2BF4), [Cu(L1' ')(NCMe)](PF6) (3PF6), [Cu(L1' ')(PPh3)](ClO4) (4ClO4), [Cu(L1' ')(PPh3)](PF6) (4PF6), [{Cu(L1' ')(CO)}2(mu-ClO4)](ClO4) (5ClO4), and the copper(II) complexes [{Cu(L1' ')}2(mu-OH)2(mu-ClO4)2] (6), and [Cu(L1' ')Cl2] (7) were systematically synthesized and fully characterized by X-ray crystallography and by IR and 1H NMR spectroscopy. In the case of copper(II), ESR spectroscopy was also applied. In comparison with the related neutral tridentate ligand L1', bis-chelated copper(I) complexes and binuclear linear-coordinated copper(I) complexes are easy to obtain with L1' ', like 1CuCl2, 1ClO4, 2ClO4, and 2BF4. Importantly, stronger and bulkier ligands such as acetonitrile (3PF6) and especially triphenylphosphine (4ClO4 and 4PF6) generate three-coordinate structures with a trigonal-planar geometry. Surprisingly, for the smaller ligand carbon monoxide, a mononuclear three-coordinate structure is very unstable, leading to the formation of a binuclear complex (5ClO4) with one bridging perchlorate anion, such that the copper(I) centers are four-coordinate. The same tendency is observed for the copper(II) bis(mu-hydroxo) compounds 6, which is additionally bridged by two perchlorate anions. Both copper(II) complexes 6 and 7 were obtained by molecular O2 oxidation of the corresponding copper(I) complexes. A comparison of the new copper(I) triphenylphosphine complexes 4ClO4 and 4PF6 with corresponding species obtained with the related tridentate ligands L1' and L1 (8ClO4 and 9, respectively) reveals surprisingly small differences in their spectroscopic properties. Density functional theory (DFT) calculations are used to shed light on the differences in bonding in these compounds and the spectral assignments. Finally, the reactivity of the different bis(pyrazolyl)methane complexes obtained here toward PPh3, CO, and O2 is discussed.

Copper Complexes with Neutral N4 Tripodal Ligands: Influence of the Number of Nitrogen Donors on Their Structures, Properties, and Reactivity

“…The axial position is occupied by the tertiary amine nitrogen (N41), which contrasts with the situation in other complexes. The coordination geometries of complexes 1 and 2 are similar to those of [Cu(L1ЈЈ)-Cl 2 ] [20,41] and [Cu(L1Ј)Cl 2 ], [42] which also have two bound chloride anions. The average Cu-Cl bond lengths in the three complexes [Cu(L1ЈЈ)Cl 2 ], [Cu(L1Ј)Cl 2 ], and 2, which have the same isopropyl substituents on the pyrazolyl rings, are 2.22, [20,41] 2.26, [42] and 2.29 Å, respectively.…”

“…This is in agreement with the distances between the N2P least-squares plane and the copper(I) ions [0.134(1) Å for Cu1 and 0.148(1) Å of Cu2], both of which show a higher planarity. The previously reported N2-type ligands L0ЈЈ and L1ЈЈ form a trigonal planar geometry in copper(I) triphenylphosphane complexes, [16,20] and the N3-type ligand L1Ј shows a tetrahedral geometry. [20] As shown in Table 4, the average Cu-N pz distance in complex 8 (2.023 Å) is almost the same as the average value for the N2-type complexes [Cu(L1ЈЈ)(PPh 3 )]-(ClO 4 ) (2.008 Å) [20] and [Cu(L0ЈЈ)(PPh 3 )](ClO 4 ) (2.019 Å) [16] but shorter than that for the N3-type complex [Cu(L1Ј)(PPh 3 )](ClO 4 ) (2.085 Å).…”

“…[ [20] [Cu(L1Ј)(PPh 3 )](ClO 4 ) 2.085 2.166(6) 126.0 N3P [20] [a] bqpa = bis(2-pyridylmethyl)(2-quinolylmethyl)amine.…”

“…Thus, whereas the Cu I CO complex with L1Ј is mononuclear, the analogous complex with L1ЈЈ is binuclear with one bridging ClO 4 -anion. [12,20] In addition, the 31 P NMR spectrum of the Cu I PPh 3 complex of L1ЈЈ shows a narrower signal than that of the corresponding complex with L1Ј. [20] These differences between tpzm (N3 donor set) and bpzm (N2 donor set) have led to renewed interest in their complexes.…”

“…[12,20] In addition, the 31 P NMR spectrum of the Cu I PPh 3 complex of L1ЈЈ shows a narrower signal than that of the corresponding complex with L1Ј. [20] These differences between tpzm (N3 donor set) and bpzm (N2 donor set) have led to renewed interest in their complexes. Herein we focus on neutral N4 tripodal tetradentate tris-(pyrazol-1-ylmethyl)amine ligands in order to gain more insights into the influence of the number of nitrogen donors on the structures, properties, and reactivity of copper complexes.…”