F. Luzardo et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 183 (2017) 45–52
49
when some conformational changes were detected (Cu′\\O2′ and Cu
′\\O3′ bond lengths, O1′\\Cu′\\O2′ and O1′\\Cu′\\O3′ angles).
Calculated NPA charges on selected atoms are collected in Table 4.
The NPA result for copper is smaller than the formal value of +2 as a re-
sult of the electron-density donation process. The metal ion ends with a
charge of about +1.09, while the non-metal atoms support a charge
ranging from −0.528 to −0.942. In all cases, the oxygen apically coor-
dinated delivers a charge density in minor extension than the one of the
dipic anion. Nitrogen atoms of the pyridyl ligand and the dipic fragment
exhibit similar values of the charge density. The NPA results for the
{Cu(dipic)(OH2)} units in 6 and 7 seems to be not affected by the pres-
ence of the N,N-bipyridyl connector.
3.3. Electronic Spectra
Complexes 2–6 were characterized by UV–VIS measurements in so-
lution (MeOH) and in the solid state (reflectance measurements), com-
plex 7 being included only in the last studies due to solubility problems
in common solvents. TD-DFT calculations led to results in the presence
of the solvent with no significant shifts in comparison to those in the
gas phase.
In solution, a well-defined band was experimentally observed in the
visible region at about 730 nm, which seems to show no dependence on
the nature of the substituent on the pyridyl-ring. The UV-portion of the
spectrum exhibited an intense and complex band at about 280 nm. In
going from the solution to the solid state, remarkable changes were de-
tected. For mononuclear complexes, the nature of the substituent affects
the position of the band at the low-energy region. The change in the
pyridyl-based ligand from py to pyph led to a systematic hypsochromic
shift (of up to −45 nm) (for further details, see Fig. S1). At the same
time, the band at the visible emerged as very intense, which is indeed
comparable to the one detected in the UV-region. In case of dinuclear
complexes, the position of the visible band is not affected by an increase
in the length of the connecting ligand (pyz and pypy). It is worth men-
tioning that the intensity of this band for dinuclear species resulted
comparable to the one at the high-part of the spectrum, this observation
being in agreement with the results for mononuclear species.
TD-DFT calculations were conducted for species 6 and 7, the last one
being studied in the gas phase in a cis and a trans orientation of the api-
cally coordinate water molecules (Fig. 4). The TD-B3LYP/6-31G(d) data
for 6 and for trans-7 reasonably reproduced the experimental evidence
in the solid state (Figs. 5 and 6). In the range 400–800 nm, one well-de-
fined and asymmetric absorption band was detected, for which a blue-
shift of up to −80 nm with respect to the experimental information
(complex 6) was calculated. Unlike the one calculated for the trans iso-
mer, the simulation for cis-7 accounted for the presence of two bands
(Fig. S2). It is worth mentioning that the calculations for all dinuclear
complexes were not able to reproduce the relative intensity of the
band in the visible portion of the spectrum experimentally observed
in the solid state.
By examining the high-energy part of the spectrum, interesting fea-
tures emerged. For complex 6, three simulated bands (comparable in in-
tensity) originate the experimental one observed in the UV-region. The
results for complex 7, for which no influence on the disposition of the
coordinated water molecules was detected, show a very intense and
asymmetric band peaked at about 330 nm, and a second one — signifi-
cantly less intense — located at about 240 nm. The nature of the bridging
ligand promotes differences in charge delocalization of the MOs in-
volved in the UV-region excitations. This finding promotes the afore-
mentioned differences in intensity ratio in the high-energy portion of
the spectrum for complexes 6 and 7 (further details are discussed in
text below).
Fig. 6. Experimental UV–VIS spectrum of 7 measured in the solid state (solid line), and
calculated for trans-7 in the gas phase with B3LYP/6-31G(d) (dashed line; inset:
calculated spectrum in the VIS region).
Geometry optimization led to a minimum as stationary point in all
cases (Tables S1–S7). All optimized structures display the metal residing
in a slight distorted square-pyramidal environment. Selected optimized
parametric are collected in Table 3. The calculated geometries in the gas
phase reasonable match the experimental evidence [40,46,47] consider-
ing the absence of packing-forces and H-bond effects in the calculations.
It is worth mentioning that no systematic improvement of the geomet-
ric data has been obtained by employing more extended basis sets (cc-
pVDZ and cc-pVTZ; for further details, see Supplementary Information
in Table S8).
For dinuclear species, the presence of pyz as bridging-ligand pro-
motes slight modification in the coordination sphere with respect to
mononuclear complexes. In case of pypy as bipyridyl-bridge, the select-
ed geometric parameters resemble those calculated for mononuclear
compounds. It should be noted that these results are practically not af-
fected by the disposition of the coordinated water molecules, even
Table 5
Selected orbital excitations calculated for 6 in the gas phase employing TD-B3LYP in com-
bination with 6-31G(d).
λcalc
(nm)
Most important excitationsa,b
f
λ
exp (nm)c
Origind
H − 25(β) → L + 1(β),
H − 9(β) → L + 1(β)
H − 23(β) → L + 2(β),
H − 7(β) → L + 2(β)
0.0023 594.9
0.0024 591.2
672 (720)
MLMLCT
H − 12(β) → L(β), H − 10(β) 0.0434 358.5
→ L(β)
365 (313)
MLLCT
H − 16(α) → L(α)
0.1202 307.1
0.0196 275.3
0.0167 273.2
0.0106 271.1
0.0320 242.7
0.0320 239.5
0.2048 211.7
0.1634 204.4
0.1829 203.6
260 (253, 260, 267, LLCT
277)
H − 9(β) → L + 3(β)
H − 7(β) → L + 5(β)
H − 17(β) → L(β)
H − 15(β) → L + 6(β)
H − 12(β) → L + 6(β)
H − 27(α) → L(α)
H − 15(α) → L + 3(α)
H − 15(α) → L + 3(α),
H − 1(α) → L + 8(α)
220 (224)
LLCT
Not detected
The analysis of MOs is very important to get deeper insight into the
origin of absorption bands. In Tables 5 and 6, the most important orbital
excitations associated with the band experimentally located at 672 and
675 nm for 6 and trans-7, respectively, are summed up (data for cis-7
a
Only those excitations with contribution larger than 15% were considered.
H = HOMO; L = LUMO.
b
c
λexp (nm) measured in the solid state; the value obtained in solution in parentheses.
d
See in text for acronyms employed in assigning origin of absorption bands.