Stereochemical Geometries and Photoluminescence in Pseudo-Halido-Zinc(II) Complexes. Structural Comparison between the Corresponding Cadmium(II) Analogs

: Six pseudohalide zinc(II) containing a variety of N -donor auxiliary amines were structurally characterized. These include two mononuclear trigonal bipyramidal [Zn(NTB)(N 3 )]ClO 4 · 1 / 2 H 2 O ( 3 ) and [Zn(TPA)(NCS)]ClO 4 ( 4 ), two distorted octahedral [Zn(1,8-damnph) 2 (dca) 2 ] ( 5 ) and [Zn(8-amq) 2 (dca) 2 ] ( 6a ) as well as two 1D polymeric chains catena -[Zn(isq) 2 ( µ 1,5 -dca) 2 ] ( 7 ) and catena -[Zn( N , N -Me 2 en) 2 ( µ 1,5 -dca)]dca ( 8 ), where NTB = tris(2-benzimidazolylmethyl)amine, TPA = tris(2-pyridylmethyl)amine, 1,8-damnph = 1,8-diaminonaphthalene, 8-amq = 8-amino-quinoline, isq = isoquinoline (isq) and N , N -Me 2 en = N , N -dimethylethylenediamine. In general, with the exception of 6 and 8 , the complexes exhibited luminescence emission in MeOH associated with red shift of the emission maxima, and the strongest visible ﬂuorescence peak was detected at 421 nm ( λ ex = 330 nm) in the case of Complex 5 .

Caution! Salts of perchlorate and azide as well as their metal complexes are potentially explosive and should be handled with great care and in small quantities.

Preparation of the Compounds
To a solution containing 0.140 g of 2(ethyl-2-pyridyl)-2-methylquinolyl-methylamine, Meepmqa (0.50 mmol) dissolved in MeOH (20 mL) Zn(ClO 4 ) 2 ·6H 2 O (0.190 g, 0.50 mmol) was added and this was followed by the addition of NaN 3 (0.066 g, 0.5 mmol) dissolved in H 2 O (1-2 mL). The resulting mixture was heated to boiling for 10 min, filtered while hot and the resulting yellow solution was allowed to stand at room temperature. The yellow crystals, which were separated on the following day, were collected by filtration, washed with propan-2-ol and diethyl ether and air dried (yield: 0.132 g, 62% To a mixture containing Zn(ClO 4 ) 2 ·6H 2 O (0.192 g, 0.5 mmol) and Meepmqa (0.140 g, 0.5 mmol) dissolved in MeOH (15 mL), an aqueous solution (5 mL) of sodium dicyanamide (0.090 g, 1 mmol) was added and the resulting faint yellow solution was heated for 10 min on a steam-bath, filtered through celite and then allowed to crystallize at room temperature. After 2 h, the off-white crystalline compound that separated was collected by filtration, washed with propan-2-ol and Et 2 O and air dried (yield: 0.23 g, 89% The complex was prepared using a similar procedure as that described for complex 1 except tris(2-benzimidazolylmethyl)amine (NTB) was used instead of TPA (yield: 72%). Characterization: Anal. Calcd: C 24  The complex was prepared using a procedure similar to that described for 2, except two equivalents of 1,8-damnph were used instead of Meepmqa (yield: 0.203 g, 79%). The complex was prepared using a procedure similar to that described for 2, except two equivalents of isoquinoline (isq) were used instead of Meepmqa (yield: 77%). Characterization: Anal.

X-ray Crystal Structure Analysis
The X-ray single-crystal data of the six title compounds were collected on a Bruker-AXS APEX II CCD diffractometer (Bruker AXS, Karlsruhe, Germany) at 100(2) K. The crystallographic data, conditions retained for the intensity data collection and some features of the structure refinements are listed in Table 1. Data collections were performed with Mo-Kα radiation (λ = 0.71073 Å); data processing, Lorentz-polarization and absorption corrections were performed using the APEX and SADABS computer programs [57,58]. The structures were solved by direct methods and refined by full-matrix least-squares methods on F 2 , using the SHELX program library [59][60][61]. All non-hydrogen atoms were refined anisotropically. The hydrogen atoms were located from difference Fourier maps, assigned with isotropic displacement factors. Geometrical constraints (HFIX) were applied only for H atoms bonded to C atoms. Further programs used: Mercury and PLATON [62,63].

Catena-[Zn(isq)2(μ1,5-dca)2] (7) and Catena-[Zn(N,N-Me2en)(μ1,5-dca)]dca (8)
The common feature of 7 and 8 is 1D polymeric chains (Figures 3 and 4 and bond parameters are shown in Table S1). In 7, neutral chains of catena-[Zn(isq)2(μ1,5-dca)2] are formed via bis μ1,5-dca bridging ligands, which connect the Zn(II) metal centers along the a-axis of the unit cell. Each ZnN6 octahedron is completed by two terminal isoquinoline molecules in trans disposition. The Zn-N bond distances vary from 2.1369(17) to 2.1735 (17) Å and N-Zn-N bond angles deviate less than 0.60° from ideal octahedral The common feature of 7 and 8 is 1D polymeric chains (Figures 3 and 4 and bond parameters are shown in Table S1). In 7, neutral chains of catena-[Zn(isq) 2 (µ 1,5 -dca) 2 ] are formed via bis µ 1,5 -dca bridging ligands, which connect the Zn(II) metal centers along the a-axis of the unit cell. Each ZnN 6 octahedron is completed by two terminal isoquinoline molecules in trans disposition. The Zn-N bond distances vary from 2.1369(17) to 2.1735 (17) Å and N-Zn-N bond angles deviate less than 0.60 • from ideal octahedral geometry. The parallel stacking of the isoquinoline molecules is stabilized by the formation of π···π ring···ring interactions (Table S3, Figure S5). The cationic chains in 8 are formed via single µ 1,5 -dca bridging ligands, which connect the metal centers in an alternate [Zn1···Zn1···Zn2] sequence along the b-axis of the unit cell. Each distorted ZnN 6 octahedron is completed by four N-donors of two chelating N,N-Me 2 en molecules in trans disposition. The Zn-N bond distances vary from 2.063(4) to 2.278(3) Å and N-Zn-N bond angles deviate up to 7.5 • from ideal octahedral geometry. Hydrogen bonds of type N-H···N are formed between N,N-Me 2 en and dca counter anions, which are arranged parallel to the µ 1,5 -dca bridging ligands, to generate a supramolecular 2D system oriented along the aand b-axis of the unit cell (Table S2, Figure S6). The dca anions in 7 and 8 have the following bond parameters: Zn-N-C: from 148.80 (15) to 168.49 (16)

Structural Comparison between Zinc(II) Complexes and Cadmium(II) Analogs
In this section, we are focusing on the Zn(II)-pseudohalide complexes and their corresponding structurally characterized Cd(II) analogs containing the same auxiliary ligands and pseudohalides. These compounds are collected in Table 2 for comparison. Inspection of the data in this table reveals that although the two metal ions can produce compounds with similar geometrical features and the same pseudohalide bonding modes (see entries 7-10, 12 and 13 and 20-25), still many other compounds show significant differences with respect to geometry, nuclearity and pseudohalide bonding modes (entries 1-5, 9 and 11 and 14-19). This behavior precludes any possibility of predicting the structure of any of the two complexes if one of them was determined. This could not only be

Structural Comparison between Zinc(II) Complexes and Cadmium(II) Analogs
In this section, we are focusing on the Zn(II)-pseudohalide complexes and their corresponding structurally characterized Cd(II) analogs containing the same auxiliary ligands and pseudohalides. These compounds are collected in Table 2 for comparison. Inspection of the data in this table reveals that although the two metal ions can produce compounds with similar geometrical features and the same pseudohalide bonding modes (see entries 7-10, 12 and 13 and 20-25), still many other compounds show significant differences with respect to geometry, nuclearity and pseudohalide bonding modes (entries 1-5, 9 and 11 and 14-19). This behavior precludes any possibility of predicting the structure of any of the two complexes if one of them was determined. This could not only be

Structural Comparison between Zinc(II) Complexes and Cadmium(II) Analogs
In this section, we are focusing on the Zn(II)-pseudohalide complexes and their corresponding structurally characterized Cd(II) analogs containing the same auxiliary ligands and pseudohalides. These compounds are collected in Table 2 for comparison. Inspection of the data in this table reveals that although the two metal ions can produce compounds with similar geometrical features and the same pseudohalide bonding modes (see entries 7-10, 12 and 13 and 20-25), still many other compounds show significant differences with respect to geometry, nuclearity and pseudohalide bonding modes (entries 1-5, 9 and 11 and 14-19). This behavior precludes any possibility of predicting the structure of any of the two complexes if one of them was determined. This could not only be attributed to the smaller ionic size of the Zn 2+ ion but also to its hard Lewis acid nature in regard to the soft Cd 2+ ion.

Luminescence Emission
The optical properties of the synthesized pseudo-halido-zinc(II) complexes were investigated in methanol by UV-Vis and luminescence spectroscopy. To examine the effect of the pseudohalide on the luminescence emission, the TPA series of [Zn(TPA)(X)]ClO 4 (X = NCS − , 4; X = N 3 − , 4a [68]; X = dca, 4b [72]) were studied. These complexes, together with containing a TPA ligand, show two absorption bands centered around 205 and 260 nm, which can be assigned to ligand-centered π-π* and n-π* transitions of the pyridyl rings and those containing N atoms. As seen in Figure 5, upon photoexcitation at 260 nm, complexes 4, 4a and 4b showed pronounced emission maxima at~450 nm and a large red shift up to ∆λ = 190 nm was observed, which can be ascribed to ligand to sensitized charge transfer transition (LCT) from the lowest excited energy states within the whole complex.
Interestingly, parent ligand-centered charge transfer (LCT) emission in the azido complex 4a was quenched in 4 and 4b. In addition, it is noted from normalized emission intensities that complex 4a with N 3 − exhibited the strongest luminescence at~450 nm among the TPA series ( Figure 5, left). The luminescence of 4a is found to be nearly 1.6 times greater than the visible luminescence of complex 4b with dca.
which can be assigned to ligand-centered π-π* and n-π* transitions of the pyridyl rings and those containing N atoms. As seen in Figure 5, upon photoexcitation at 260 nm, complexes 4, 4a and 4b showed pronounced emission maxima at ~450 nm and a large red shift up to ∆ = 190 nm was observed, which can be ascribed to ligand to sensitized charge transfer transition (LCT) from the lowest excited energy states within the whole complex. Interestingly, parent ligand-centered charge transfer (LCT) emission in the azido complex 4a was quenched in 4 and 4b. In addition, it is noted from normalized emission intensities that complex 4a with N3 − exhibited the strongest luminescence at ~450 nm among the TPA series ( Figure 5, left). The luminescence of 4a is found to be nearly 1.6 times greater than the visible luminescence of complex 4b with dca. The influence of the auxiliary ligands, L, was also investigated for the azido compounds, 1, 3 and 4a. As illustrated in Figure 6, all the azido complexes when excited at their second or higher wavelength absorption band displayed luminescence emission peaks associated with LCT transitions (Figure 6, UV spectra). These complexes show bathochromic shifts consistent with their corresponding electronic absorption spectra. Therein, Complex 1 showed the largest bathochromic shift (∆ 115 nm with the emission peak maximum centered at 375 nm (λex = 260 nm). Comparison of the absolute emission intensities' wavelengths and UV-Vis. spectra of the dicyanamido Zn(II) complexes 2, 5, 6, 7 and 8 containing different coordinating ligands in methanol was performed ( Figure  S7, SM). Therein, complex 5 shows the strongest visible fluorescence peak at 421 nm (λex = 330 nm). The UV-Vis. spectrum 5 ( Figure S7-Right, SM) exhibits a broad absorption peak centered around 330 nm, which is assigned to the n-π* transition of the amino group and another absorption peak in the UV region at 230 nm ascribed to the π-π* transition of the naphthalene system. The fluorescence quantum yield (FL QY) of the complex is estimated to be around 9% and close to the quantum yield (QY) of 1,8-diaminonaphthalene ligand in methanol [73]. The absolute emission intensities of 2 and 7, seen in the UV region, roughly determined to be ~6.8-and ~9-fold, respectively, are lower than the reference complex 5 ( Figure S7-FL, SM). The complexes [Zn(8-amq)2(dca)]ClO4 (6) and catena-[Zn(N,N-Me2en)(μ1,5-dca)]dca (8) did not show any detectable luminescence emission spectra ( Figure S7) because of the fluorescence quenching of the 8-amq in polar protic solvents due to hydrogen bonding with the MeOH [74] in the former complex and the lack of π-π* transition in the N,N-Me2en of Complex 8. The influence of the auxiliary ligands, L, was also investigated for the azido compounds, 1, 3 and 4a. As illustrated in Figure 6, all the azido complexes when excited at their second or higher wavelength absorption band displayed luminescence emission peaks associated with LCT transitions (Figure 6, UV spectra). These complexes show bathochromic shifts consistent with their corresponding electronic absorption spectra. Therein, Complex 1 showed the largest bathochromic shift (∆λ = 115 nm) with the emission peak maximum centered at 375 nm (λ ex = 260 nm). Comparison of the absolute emission intensities' wavelengths and UV-Vis. spectra of the dicyanamido Zn(II) complexes 2, 5, 6, 7 and 8 containing different coordinating ligands in methanol was performed ( Figure S7, SM). Therein, complex 5 shows the strongest visible fluorescence peak at 421 nm (λ ex = 330 nm). The UV-Vis. spectrum 5 ( Figure S7-Right, SM) exhibits a broad absorption peak center around 330 nm, which is assigned to the n-π* transition of the amino group and another absorption peak in the UV region at 230 nm ascribed to the π-π* transition of the naphthalene system. The fluorescence quantum yield (FL QY) of the complex is estimated to be around 9% and close to the quantum yield (QY) of 1,8-diaminonaphthalene ligand in methanol [73]. The absolute emission intensities of 2 and 7, seen in the UV region, roughly determined to be~6.8-and~9-fold, respectively, are lower than the reference complex 5 ( Figure S7-FL, SM). The complexes [Zn(8-amq) 2 (dca)]ClO 4 (6) and catena-[Zn(N,N-Me 2 en)(µ 1,5 -dca)]dca (8) did not show any detectable luminescence emission spectra ( Figure S7) because of the fluorescence quenching of the 8-amq in polar protic solvents due to hydrogen bonding with the MeOH [74] in the former complex and the lack of π-π* transition in the N,N-Me 2 en of Complex 8. The fluorescence quantum yield (FL QY) of complex 5 is estimated to be around 9% and close to the QY of 1,8-diaminonaphthalene ligand in methanol [73]. The absolute emission intensities of 2 and 7, seen in the UV region, roughly determined to be ~6.8-and ~9-fold, respectively, are lower than the reference complex 5 ( Figure S7-FL, SM). The complexes [Zn(8-amq)2(dca)]ClO4 (6) and catena-[Zn(N,N-Me2en)(μ1,5-dca)]dca (8) did not show any detectable luminescence emission spectra ( Figure S7) because of the fluorescence quenching of the 8-amq in polar protic solvents due to hydrogen bonding with the MeOH [74] in the former complex and the lack of π-π* transition in the N,N-Me2en of complex 8. The FL QY of the pseudohalido-Zn(II) complexes under investigation in MeOH together with λmax absorption and emission is tabulated in Table 3. The FL QY of all complexes was measured in MeOH by an optically dilute relative method using 1,8 diaminonaphthalene (1,8-damnaph) as a fluorescent standard of known QY in methanol [73]. The luminescence emission observed in the complexes 3-6a may be attributed to the CHE effect, which tends to reduce energy loss via radiationless thermal vibrations, and the intraligand π*-π emission band may shift due to perturbations in the electronic states of the ligands upon coordination with Zn 2+ ions [52,53]. Attempts made to correlate the fluorescence enhancements to the rigidity of chelation or to strong Zn-Nav(amine) bond lengths were unsuccessful. For example, in the case of Zn-TPA series, all Zn-Nav (TPA): The fluorescence quantum yield (FL QY) of complex 5 is estimated to be around 9% and close to the QY of 1,8-diaminonaphthalene ligand in methanol [73]. The absolute emission intensities of 2 and 7, seen in the UV region, roughly determined to be~6.8-and~9-fold, respectively, are lower than the reference complex 5 ( Figure S7-FL, SM). The complexes [Zn(8-amq) 2 (dca)]ClO 4 (6) and catena-[Zn(N,N-Me 2 en)(µ 1,5 -dca)]dca (8) did not show any detectable luminescence emission spectra ( Figure S7) because of the fluorescence quenching of the 8-amq in polar protic solvents due to hydrogen bonding with the MeOH [74] in the former complex and the lack of π-π* transition in the N,N-Me 2 en of complex 8. The FL QY of the pseudohalido-Zn(II) complexes under investigation in MeOH together with λ max absorption and emission is tabulated in Table 3. The FL QY of all complexes was measured in MeOH by an optically dilute relative method using 1,8 diaminonaphthalene (1,8-damnaph) as a fluorescent standard of known QY in methanol [73]. The luminescence emission observed in the complexes 3-6a may be attributed to the CHE effect, which tends to reduce energy loss via radiationless thermal vibrations, and the intraligand π*-π emission band may shift due to perturbations in the electronic states of the ligands upon coordination with Zn 2+ ions [52,53]. Attempts made to correlate the fluorescence enhancements to the rigidity of chelation or to strong Zn-N av (amine) bond lengths were unsuccessful. For example, in the case of Zn-TPA series, all Zn-N av (TPA): 2.111 (4), 2.111 (4a) [68] and 2.094 Å (4b) [72] were very close to providing a satisfactory explanation for the fluorescence enhancement differences between the three TPA complexes, and in some cases the lack of crystal structures as compounds 1 and 2.