Crystal structure of [Cu(tmpen)](BF4)2 {tmpen is N,N,N′,N′-tetrakis[(6-methylpyridin-2-yl)methyl]ethane-1,2-diamine}

The mononuclear copper complex {N,N,N′,N′-tetrakis[(6-methylpyridin-2-yl)methyl]ethane-1,2-diamine-κ6 N}copper(II) bis(tetrafluoridoborate) shows a distorted octahedral environment around the CuII cation. The presence of the 6-methyl substituent hinders the approach of the pyridine group to the CuII core. The bond lengths about CuII are significantly longer than those of analogues without the 6-methyl substituents.


Chemical context
Copper complexes with polypyridine ligands are of great interest in catalytic reactions. For example, the copper-based complex CuBr[N,N,N 0 ,N 0 -tetrakis(2-pyridylmethyl)ethylenediamine] (TPEN) is reported as a versatile and highly active catalyst for acrylic, methacrylic and styrenic monomers (Tang et al., 2006). Copper(II) N-benzyl-N,N 0 ,N 0 -tris(pyridin-2-ylmethyl)ethylenediamine (bztpen) displays high catalytic activity for electrochemical proton reduction in acidic aqueous solutions, with a calculated hydrogen-generation rate constant (k obs ) of over 10000 s À1 (Zhang et al., 2014). [Cu 2 (m-xpt) 2 -(NO 3 ) 2 ](PF 6 ) 2 [m-xpt = m-xylylenebis(pyridyltriazole)] can selectively capture CO 2 from air and reduce it to oxalate, in the form of an oxalate-bridged complex (Pokharel et al., 2014). Generally, the reduction of a metal complex is accompanied by ligand dissociation (reductive dissociation), which is able to give the appearance of an open site for catalytic reaction. Herein, we describe the structure of the title complex, 1. ISSN 2056-9890

Structural commentary
In the title complex ( Fig. 1), the coordination sphere of the copper(II) atom is distorted octahedral, presumably as a result of the introduction of the 6-methyl substituent. Two pyridine nitrogen atoms (N1, N1 0 ) and two amino nitrogen atoms (N2, N2 0 ) form the equatorial planar coordination, while the apical positions are occupied by the other two pyridine nitrogen atoms (N3, N3 0 ). The Cu II ion lies almost in the equatorial plane. The Cu-N bond lengths for the two axial pyridinenitrogen atoms [Cu-N3 = 2.5742 (13) Å ] are significantly longer than those for the other four nitrogen atoms [Cu-N1 = 2.0571 (13), Cu-N2 = 2.0311 (13) Å ]. The long Cu-N3 distance indicates a weak connection between copper and pyridine, which is apt to dissociate under reductive conditions (Tang et al., 2006). As a result of steric hindrance from the methyl group, the N3-Cu1-N3 0 bond angle is not linear but rather 164.94 (5) . The pyridine rings in the equatorial plane (N1/C2-C6 and N1 0 /C2 0 -C6 0 ) subtend a dihedral angle of 35.03 (9) .
The distortion about the Cu II atom is in favour of the reductive dissociation of one pyridine group. On a cathodic scan under Ar, complex 1 features one reversible couple based on copper at 0.26 V (vs Fc +/0 ), assigned to Cu II/I (Fig. 2). The free ligand tmpen is electrochemically silent in the potential range (Fig. 3). The good reversibility of the couple indicates negligible change in the configuation of 1 under reductive conditions.

Supramolecular features
While there are no classical hydrogen bonds in the crystal structure, C-HÁ Á ÁN and C-HÁ Á ÁF interactions are observed (Fig. 4, Table 1).

Figure 1
The molecular entities in the structure of complex 1. Atoms N1A, N2A and N3A are generated by the symmetry operation Àx, y, 1 2 À z.

Figure 2
Cyclic voltammograms of complex 1 (1 mM) under Ar in CH 3 CN with 0.1 M n Bu 4 NBF 4 as the supporting electrolyte.

Database survey
There are four published reports of polypyridine copper complexes (Kaur et al., 2015;Meyer et al., 2015;Bania & Deka, 2012;Yoon et al., 2005) , but to the best of our knowledge, the title compound has not been reported previously. Among the earliest reports, the copper complex with an N,N,N 0 ,N 0 -tetrakis(2-pyridylmethyl)ethylenediamine (TPEN) ligand is most similar to title complex in configuration. In the presence of ascorbic acid as a reducing agent, Cu 2+ (TPEN) displays high activity in atom-transfer radical addition (ATRA) reactions (Kaur et al., 2015). In contrast to Cu 2+ (TPEN), the title complex exhibits greater steric hindrance, which results in an evident Jahn-Teller effect on the configuration. In the title complex, the axial Cu-N bonds to pyridyl nitrogen atoms [2.5742 (13) Å )] are significantly longer than in Cu 2+ (TPEN) [2.377 (3) and 2.308 (2) Å ] while the differences in the equatorial Cu-N distances are negligible (Yoon et al., 2005). The other two reported polypyridine copper complexes show similar distorted octahedral coordination spheres around the Cu 2+ cation, but the ligands are evidently different from the title complex.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All F atoms of the BF 4 group were split into two groups and their ccupancies determined via a free variable refinement. All hydrogen atoms were refined in riding mode with C-H= 0.93-0.97 and U iso (H) = 1.2U eq (C) or 1.5U eq (C) for methyl H atoms.  Computer programs: SMART and SAINT (Bruker, 2013), SHELXTL (Sheldrick, 2008) and SHELXL2014 (Sheldrick, 2015).

{N,N,N′,N′-Tetrakis[(6-methylpyridin-2-yl)methyl]ethane-1,2-diamine-κ 6 N}copper(II) bis(tetrafluoridoborate)
Crystal data 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.