Low temperature redetermination of the crystal structure of catena-poly[[tri-4-fluorobenzyltin(IV)]μ2-pyridine-4-carboxylato-κ2N:O], {C27H22F3NO2Sn}n

Abstract C27H22F3NO2Sn, monoclinic, P21/n (no. 14), a = 8.7913(2) Å, b = 19.0856(5) Å, c = 13.9417(4) Å, β = 91.017(2)°, V = 2338.87(11) Å3, Z = 4, Rgt(F) = 0.0324, wRref(F2) = 0.0873, T = 100(2) K.


Experimental details
The C-bound H atoms were geometrically placed (C-H = 0.95-0.99 Å) and refined as riding with U iso (H) = 1.2Ueq(C). The maximum and minimum electron density peaks of 1.45 and 0.61 e Å −3 , respectively, were located 1.01 and 0.81 Å, respectively, from the Sn atom.
Owing to poor agreement, the (4 15 0) reflection was omitted from the final cycles of refinement.

Comment
Steric bulk associated with heavy element-and/or ligandbound organic substituents are well-documented to moderate the formation of secondary M· · · O/S bonding interactions [6,7] in crystals of the zinc-triad [8], tin [9], arsenictriad [10] and tellurium [11] carboxylates/1,1-dithiolates. This effect is nicely illustrated in the series of solvent-free crystal structures of triorganotin(pyridine-4-carboxylato) molecules, i.e. molecules of the general formula R 3 Sn(O 2 CC5H4N-4). The common feature of these structures is the presence of five-coordinate tin-atom geometries defined by trans-C 3 NO donor sets owing to the presence of µ 2 -pyridine-4carboxylato bridges. Thus, derivatives with R = methyl [12,13], n-butyl [14], cyclohexyl [15], benzyl [16], 2-chlorobenzyl [17], 2-fluorobenzyl [18], 4-fluorobenzyl [19], phenyl [20] and the mixed species R 3 = phenyl 2 (ethyl) [21] are known. The title compound, i.e. the species with R = 4-fluorobenzyl, (I), was isolated as part of continuing interest in this area [9,16]; over and above structural considerations, recent work has indicated both nicotinic and isonicotinic acid derivatives of organotins [12], including benzyltin compounds, display potential as anti-cancer agents [22]. During characterisation of the previosuly described R = 4-fluorobenzyl derivative [19] by X-ray crystallography at 100(2) K, significant differences in some tin-bound parameters were noted between the new data and the literature analysis, conducted at 298(2) K [19]. Hence, herein the crystal and molecular structures of the R = 4-fluorobenzyl compound, redetermined at 100(2) K. are described and compared with literature structures and more specifically with the room temperature structure [19]. A portion of the polymeric structure of (I) is shown in the figure with the crystallographic asymmetric unit, corresponding to a repeat unit, labelled (70% displacement ellipsoids; symmetry operations i: 1/2 − x, 1/2 + y, 1/2 − z and ii: 1/2 − x, −1/2 + y, 1/2 − z). The structure is a one-dimensional coordination polymer and being propagated by a 21 screw symmetry along the b-axis direction, has a helical topology. A trans-C 3 NO donor set coordinates the tin atom: the donor atoms are derived from the carbon atoms of the three benzyl substituents, an oxygen atom from the carboxylato group and a pyridyl-nitrogen atom from a symmetry-related pyridine-4carboxylato ligand. The carboxylato ligand coordinates in a monodentate mode with Sn-O1 of 2.149(2) Å being significantly shorter than Sn· · · O2 of 3.090(2) Å. The disparity in the Sn-O separations is also reflected in the large difference in the C1-O1, O2 bond lengths of 1.287(4) and 1.223(4) Å, respectively, i.e. following the anticipated trends. The Sn-N1 i bond length is 2.598(3) Å, and the Sn-C bonds are experimentally equivalent, ranging from 2.149(4) Å, for Sn-C21, to 2.166(4) Å, for Sn-C11. The trans O1-Sn-N1 i angle is 175.62(10)°, and the C-Sn-C angles lie in a range from narrow 114.89(14)°, for C21-Sn-C31, to a wide 128.15 (14)°, for C11-Sn-C21. This disparity in the C-Sn-C angles is traced to the close approach of the loosely associated O2 atom which serves to widen the C11-Sn-C21 angle. Overall, the coordination geometry is based on a trigonal-bipyramid.
A comment on the topologies of the formed onedimensional coordination polymers of R 3 Sn(O 2 CC5H4N-4) is appropriate. Only two other of the literature precedents form a helical chain in its crystals, namely in the R = 2-chlorobenzyl [17] and 2-fluorobenzyl [18] structures. The chain in the R = phenyl derivative adopts a zig-zag topology (glide symmetry) [20] whereas the remaining coordination polymers have linear topologies.
Finally, a comparison of the new data with previously described room temperature determination of the R = 4fluorobenzyl [19] compound is made. While the majority of the interatomic parameters about the tin atoms in both structures are equal within experimental error, a considerable shortening of the Sn· · · N interaction is evident. Thus, Sn· · · N of 2.639(4) Å in the 298(2) K structure contracts to 2.598(3) K at 100(2) K which is also reflected in the intra-chain Sn· · · Sn separations of 2.671(8) and 2.63261(6) Å, respectively. The contraction of the Sn· · · N interaction with reducing temperature is entirely consistent with this contact being primarily a secondary bonding interaction [6,7].