Crystal structure of (4,4′-bipyridyl-κN)bis[N-(2-hydroxyethyl)-N-isopropyldithiocarbamato-κ2 S,S′]zinc(II)–4,4′-bipyridyl (2/1) and its isostructural cadmium(II) analogue

The NS4 donor set in Zn[S2CN(i-Pr)CH2CH2OH]2(4,4′-bipyridyl).0.5(4,4′-bipyridyl), which features monodentate and non-coordinating 4,4′-bipyridyl molecules, is based on a trigonal bipyramid; the cadmium(II) analogue is isostructural.


Chemical context
The ditopic ligand 4,4 0 -bipyridyl is ubiquitous in coordination chemistry, usually providing bridges between metal centres to generate coordination polymers. While bidentate bridging is normally observed in the structural chemistry of zinc(II) bis(N,N 0 -dialkyldithiocarbamate)s, these more often than not lead to binuclear species of the general formula [Zn(S 2 CNRR 0 ) 2 ] 2 (4,4 0 -bipyridyl) as first observed in the archetypal compound [Zn(S 2 CNEt 2 ) 2 ] 2 (4,4 0 -bipyridyl) (Zemskova et al., 1994) and in other compounds relevant to the present study, such as {Zn[S 2 CN(R)CH 2 CH 2 OH] 2 } 2 (4,4 0bipyridyl) for R = Me, Et and CH 2 CH 2 OH (Benson et al., 2007). The exceptional structure is that of Zn[S 2 CN(n-Pr) 2 ] 2 -(4,4 0 -bipyridyl), which features a relatively rare monodentate coordination mode for the 4,4 0 -bipyridyl molecule (Klevtsova et al., 2001). The analogous chemistry for cadmium(II) bis-(N,N 0 -dialkyldithiocarbamate)s is considerably less explored with the only example in the Cambridge Structural Database (Groom et al., 2016) being a linear coordination polymer in the crystal of {Cd[S 2 CN(CH 2 Ph) 2 ] 2 (4,4 0 -bipyridyl)} n (Fan et al., 2007). The difference in chemistry between zinc and cadmium dithiocarbamates can be rationalized in terms of the larger size of cadmium versus zinc but, also in terms of the reduced Lewis acidity of the zinc atom owing to the strong chelation mode of the dithiocarbamate ligand. This is also true for cadmium whereby unusual coordination modes are found for ISSN 2056-9890 related pyridyl-containing molecules that might otherwise be expected to be bridging. This is discussed further below in Database survey. In the present report, the crystal and molecular structures of two compounds, formulated as Zn[S 2 CN(i-Pr)CH 2 CH 2 OH] 2 (4,4 0 -bipyridyl)Á0.5(4,4 0 -bipyridyl) (I) and the cadmium analogue (II), are described, i.e. featuring monodentate and non-coordinating 4,4 0 -bipyridine molecules.

Structural commentary
The molecular structure of the constituents of (I) are shown in Fig. 1a and selected geometric parameters are collected in Table 1. The asymmetric unit comprises an entire molecule of Zn[S 2 CN(i-Pr)CH 2 CH 2 OH] 2 (4,4 0 -bipyridyl) and half a molecule of 4,4 0 -bipyridine, the latter being disposed about a centre of inversion. The zinc atom is coordinated by two dithiocarbamate ligands that form disparate Zn-S bond lengths. This is seen in the values of Á(Zn-S) = Zn-S long À Zn-S short , which compute to 0.19 and 0.23 Å for the S1-and S3-dithiocarbamate ligands, respectively. The fifth position in the coordination geometry is occupied by a pyridyl-N atom. Based on the value of (Addison et al., 1984), which equals to 0.0 and 1.0 for ideal square-pyramidal and trigonal-bipyramidal geometries, respectively, it is possible to assign a coordination geometry based on the NS 4 donor set. In (I), = 0.64 indicating a highly distorted coordination geometry but, one approximating a trigonal bipyramid. In this description, the less tightly bound S2 and S4 atoms define the axial positions, Table 1. The coordinated 4,4 0 -bipyridyl molecule is non-planar with the dihedral angle between the two residues being 28.12 (14) .
Crystals of (II) are isostructural to those of (I), Fig. 1b and Table 2. Some differences in molecular geometry are apparent, most notably in the degree of symmetry in the Cd- Table 1 Selected geometric parameters (Å , ) for (I).  2.077 (2) C1-S1

Figure 1
The molecular structures of the constituents of (a) (I) and (b) (II) showing the atom-labelling scheme and displacement ellipsoids at the 70% probability level. For each of (I) and (II), the 4,4 0 -bipyridine molecule has been expanded to show the entire molecule; unlabelled atoms are related by the symmetry operation Àx, 2 À y, Àz.
S bond lengths, i.e. Á(Cd-S) = 0.09 and 0.11 Å for the S1-and S3-dithiocarbamate ligands, respectively. This is reflected in the narrower ranges in the C-S bond lengths in (II) cf. (I), Tables 1 and 2. The value of = 0.67 suggests a coordination geometry marginally closer to trigonal bipyramidal in (II) than for (I). The dihedral angle between the two rings comprising the coordinated 4,4 0 -bipyridyl molecule is 28.86 (7) .
Cg1 is the centroid of the Zn/S3/S4/C7 chelate ring. the ability of the dithiocarbamate ligand to form strong chelating interactions (see above). The molecular packing for isostructural (II) follows that just described for (I), Table 4. However, in this case, the putative pyridyl-C-HÁ Á Á(Cd/S3/S4/C7) interaction is just beyond the sum of the van der Waals radii for this type of contact (Spek, 2009).

Synthesis and crystallization
All chemicals and solvents were used as purchased without purificationÁThe melting point was determined using an Krü ss KSP1N melting point meter. The IR spectra were obtained by the attenuated total reflectance (ATR) technique on a Perkin Elmer RX1 FTIR spectrophotometer from 4000 to 400 cm À1. Tan and Tiekink [Zn(C 6 H 12 NOS 2 ) 2 (C 10 H 8 N 2 )]Á0.5C 10 H 8 N 2 and its Cd analogue 1645

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 5. For each of (I) and (II), carbonbound H atoms were placed in calculated positions (C-H = 0.95-1.00 Å ) and were included in the refinement in the riding-model approximation, with U iso (H) set to 1.2-1.5U eq (C). The O-bound H atoms were located in difference-Fourier maps but were refined with a distance restraint of O-H = 0.84AE0.01 Å , and with U iso (H) set to 1.5U eq (O). For (I), owing to poor agreement, two reflections, i.e. (0 0 6)

(4,4′-Bipyridyl-κN)bis[N-(2-hydroxyethyl)-N-isopropyldithiocarbamato-κ 2 S,S′]zinc(II)-4,4′-bipyridyl (2/1) (I)
Crystal data [Zn(C 6  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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq