Steric Effects of Alkyl Substituents at N-Donor Bidentate Amines Direct the Nuclearity , Bonding and Bridging Modes in Thiocyanato-Copper ( II ) Complexes

1 Institut für Physikalische and Theoretische Chemie, Technische Universität Graz, A-8010 Graz, Austria; mautner@tugraz.at 2 Institut für Anorganische Chemie, Technische Universität Graz, Stremayrgasse 9/V, A-8010 Graz, Austria; roland.fischer@tugraz.at, ana.torviscogomez@tugraz.at 3 Department of Chemistry and Biochemistry, University of California, Los Angeles, CA 900951569, U.S.A. henary@chem.ucla.edu 4 Department of Chemistry, University of Louisiana at Lafayette, P.O. Box 43700 Lafayette, LA 70504, U.S.A. tolga.karsili@louisiana.edu (T.K.); frl6631@louisiana.edu (F.R.L.); ssmassoud@louisiana.edu (S.S.M.); C00250551@louisiana.edu (A.M.); C00063128@louisiana.edu (H.De.V)


IR and UV-VIS Spectra of the Complexes
The IR asymmetric stretching frequencies of the thiocyanato-Cu(II) complexes under investigation and their corresponding UV-Vis spectra in acetonitrile solution are collected in Table 1.It has been stated that the νas(C≡N) frequencies could be used as a criterion to differentiate between the terminally N-bonded thiocyanato anions which in most cases display a value below 2100 cm -1 , whereas in the corresponding S-bonded and/or µ-1,3-bridging thiocyanato anions, the stretching vibrations are observed above 2100 cm -1 [12,[28][29][30][31]42,43,45].The IR spectra of complexes 1-4 display very strong band over the 2095-2131 cm -1 region due to the asymmetric stretching vibration,νas(C≡N) of the bridged-thiocyanato ligands.In addition, the same series of complexes as well as 5 showed a strong to medium intensity band of the region 2059-3086 cm -1   which can be attributed to the N-bonded terminal thiocyanate ligand(s).This data are fully consistent with N-bonded thiocyanate in all complexes and µ-1,3-bridging thiocyanate in complexes 1-4 which was also confirmed by X-ray (section 2.3).
Inspection of the acetonitrile solutions of the UV-Vis spectral data of the complexes 1-4 shown in Table 1 reveals the common spectral feature as they display a broad absorption band in the 615-656 nm region.This feature is consistent with five-coordinate Cu(II) complexes with square pyramidal geometry (SP) which may be associated with a low-energy shoulder at λ > 800 nm [28][29][30][31]42,43,[45][46][47][48][49][50].The presence of a small intense band at 974 nm in complex 1 may reflect the increased distortion towards the trigonal bipyramidal geometry (TBP) [45].Obviously, complex 4 does not belong to the observed geometry but instead it has one visible band at ~766 nm, resulting from d-d transition; 2 E ← 2 T in tetrahedral environment [51].The very strong intense band located at 469 nm can be assigned to ligand-metal charge transfer transition (L→M CT), whereas the 299 nm band is most likely attributed to electronic transition within the NCS -ligand (π→π*).The geometrical finding around the Cu(II) ion in acetonitrile solution was retained in the solid state as it supported with the X-ray structural data.

Description of the Structures
Perspective views together with the atom numbering scheme for the polymeric complexes 1 -3 are presented in Figures 1 -3, respectively, and selected bond parameters are given in Tables S1 -

S3
. The copper centers are linked by single μ1,3-thiocyanato bridges to form polymeric chains of polyhedra (1D).Each metal center is penta-coordinated by two N-donor atoms of the amine ligands, two N-atoms of terminal and bridging NCS -anions and S-atom of bridging NCS -anion.Their CuN4S chromophores may be described as distorted as SP geometry with τ values of 0.01, 0.42 and 0.21, for 1 -3, respectively (τ values of 0 and 1 refer to ideal SP and TBP geometries, respectively) [52].The ] to form a supramolecular 2D system in case of 3.    We think it would be interesting to compare our structural results of compounds 1-5 with the literature data of Cu(II)-thiocyanato complexes which derived from related bidentate amines and these compounds together with ours are collected in Table 2. Inspection of this data may show some general trends about the coordination behavior of these complexes.The thiocyanato complexes which are constructed from ethylenediamine (en) [53][54][55][56][57], mono-N-alkyl ethylenediamine (Me-en, propen) [61,62], less hindered di(N,N`-alkyl)-ethylenediamine (N,N`-Me2en) [60] and DACO [63]

The DFT Computational Results
DFT was used in order to garner insights into the aggregation properties of the various Cu complexes.In particular, we are interested in the intrinsic reasons for the ligand specific variations in adapting certain mono-or polynuclear chain size.Figure 6 shows the geometries and energetics associated with various oligomeric arrangements of 5 and 4. Figure 6(a) shows the ground state minimum energy geometry of Cu(I), Cu(II) and Cu(III) oxidation states of monomeric 5; The N,N't-Bu2en ligand tends to form a mononuclear tetrahedral arrangement around the metal center which is in good agreement with the X-ray crystal structure results.The Cu(I) is the lowest energy charge of 5 is -1, which can be understood by considering that the entire complex is closed-shell when Cu is in the +1 oxidation state.Attempts were made to dimerizing 5 with overall charges of neutral, +1 and -1.Upon optimization, the neutral complex associates and forms a stable dimer, the +1 and -1 charged 5 dimers dissociate and form a monomer pair at an asymptotic separation.The neutral complex forms a stable dimer but it is at a higher energy state when compared the -1 complex of 5.
Thus, the Cu(I) complex 5 is the lowest energy oxidation state of the metal-center and yields a complex with an overall charge of -1; The -1 charge of 5 leads to prompt dissociation of a dimer upon geometry optimization -forming two monomers.dimeric crystal structure but unable to maintain a larger oligomeric arrangement since (en)2 remains a bulky group that introduced steric hindrances.With the available resources and large molecular sizes, complexes 1, 2 and 3 produced computationally challenging.However, using the available computational data for 5 and 4 we are able to postulate that similar steric arguments would hold in the cases of 1 -3 since the bulky N-alkyl substituents at the ethylenediamine coligand becomes progressively less favorable across this series.

X-ray crystal structure analysis and refinement
The X-ray single-crystal data of compounds 1-5 were collected on a Bruker-AXS APEX II CCD diffractometer at 100(2) K.The crystallographic data, conditions retained for the intensity data collection and some features of the structure refinements are listed in Tables 3 and 4. The intensities were collected with Mo-Kα radiation (λ= 0.71073 Å).Data processing, Lorentz-polarization and absorption corrections were performed using SAINT, APEX and the SADABS computer programs [68].The structures were solved by direct methods and refined by full-matrix least-squares methods on F 2 , using the SHELXTL program package [69].All non-hydrogen atoms were refined anisotropically.The hydrogen atoms were located from difference Fourier maps, assigned with isotropic displacement factors and included in the final refinement cycles by use of geometrical constraints.Molecular plots were performed with the MERCURY program [70].
CCDC 1878247-1878251 contain the supplementary crystallographic data for complexes 1 -5, respectively.These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

The Computational Methodology
The ground state minimum energy geometry of the various structures was optimized with density functional theory (DFT), using the Becke-3 parameter-Lee-Yang-Parr (B3LYP) functional [71] and the 6-31G(d) Pople basis set [72].The increasing size and molecular complexity of the oligomeric metal complexes precluded the use of higher levels of theory.The B3LYP/6-31G(d) level is therefore an adequate compromise between accuracy and computational expense and allows for a qualitative comparison between polymers of varying size.The individual Cu-complexes may associate to form oligomers of varying chain length.Association energies were calculated by optimizing a given polymer and their associated monomers and smaller oligomers that make up the polymers.Computations were performed on both Cu(I) and Cu(III) oxidation states in order to determining the dominant oxidation state in a given oligomer.The Gaussian 09 computational package was used to perform all computations [73].

Scheme 1 .
Scheme 1. Structures of the ligands used in this study together with other related compounds ) and [Cu(N,N`-t-Bu2en)(NCS)2] (5) were synthesized in reasonably good yield (71-92%) and by the reaction of a methanolic solution containing equimolar amounts of Cu(NO3)•3H2O or Cu(ClO4)2•6H2O and the corresponding diamine ligand with four equivalents of NH4NCS solution dissolved in H2O or MeOH.Crystals suitable for X-ray analysis were obtained from dilute solutions or by recrystallization from CH3CN or anhydrous MeOH.The purity of the complexes was confirmed by elemental microanalyses (see experimental section).Molar

Analogous computations on 4
also returned Cu(I) as the lowest energy oxidation state of the metal-center.Attempts to dimerizing 4 were successful for the lowest energy triplet state of Cu(II) (henceforth 3 Cu(II)) oxidation state and, unlike 5, did not dissociated to form two monomers upon optimization.The dimer form of triplet-Cu(II) of 4 is lower that the monomer forms by ~16 kcal/ mol.All other oxidation states of Cu of 4 failed to converge to a dimer and yielded two monomers.However, further attempts to trimerize 4 were unsuccessful and yielded a dimer + monomer at an infinite separation upon optimization.This observation is again in line with the observed dimeric crystal structure of 4.

Figure 6 .In comparing 5 with 4 ,
Figure 6.Geometries associated with the ground state minima of (a) 5 and (b) 4. The Dimer geometries are relative to the energy of the Cu(II) + Cu(II) monomer asymptote at infinite separation.

Figure 6 .Figure S1 .
Figure 6.Geometries associated with the ground state minima of (a) 5 and (b) 4. The Dimer geometries are relative to the energy of the Cu(II) + Cu(II) monomer asymptote at infinite separation.

Table 1 .
The IR asymmetric stretching frequency of the coordinated thiocyanato groups and UV-Vis spectra of the complexes 1-5 in CH3CN solution.

Table 2 .
Some structural details of Cu(II)-thiocyanate complexes derived from some N-alkyl bidentate amine ligands.a)

Table 4 .
Crystallographic data and processing parameters for 4 and 5.

Table 1 .
The IR asymmetric stretching frequency of the coordinated thiocyanato groups and UV-Vis spectra of the complexes 1-5 in CH3CN solution.

Table 2 .
Some structural details of Cu(II)-thiocyanate complexes derived from some N-alkyl bidentate amine ligands.a)

Table 4 .
Crystallographic data and processing parameters for 4 and 5.