Maurizio
Casarin
*ab,
Luciano
Pandolfo
a and
Andrea
Vittadini
b
aDipartimento di Scienze Chimiche, via Francesco Marzolo 1, 35131, Padova, Italy. E-mail: maurizio.casarin@unipd.it; Fax: +39 049 8275161; Tel: +39 049 8275164
bCR-INSTM “Village”, and, Istituto di Scienze e Tecnologie Molecolari, via Francesco Marzolo 1, 35131, Padova, Italy
First published on 18th November 2008
Theoretical evidence supporting the use of hydrotris(1,2,4-triazolyl)borate (ttz) ligands as a proper alternative to the hydrotris(pyrazolyl)borate (tp) ones is provided by density functional calculations.
In the recent past, Papish and coworkers6 have reported the synthesis as well as the structural and spectroscopic characterization of a series of tris(triazolyl)borate (ttz) metal complexes demonstrating: (i) the higher water solubility of these compounds compared to that of their tp analogues; (ii) the negligible effect of the third nitrogen atom of the triazole ring on the complex coordination geometry; (iii) the weaker electron donor capability of ttz with respect to tp.
As part of a systematic theoretical investigation of scorpionate binding capabilities as a function of the ligand hapticity and charge (−1 for ttz and tp, 0 for tpm),5 we report here the results of a series of numerical experiments7 on [Cu(ttz)(CO)] ( MII), [Cu(tp)(CO)] ( II), and [Cu(tpm)(CO)]+ ( MIIIIII) complexes (see Fig. 1), with MII and MIIIIII unsubstitued models of [Cu(ttztBu,Me)(CO)] ( I) and [Cu(tpmiPr,iPr)(CO)]+ ( III) (ttztBu,Me and tpmiPr,iPr stand for tris(3-tert-butyl-5-methyl-1,2,4-triazolyl)borate6 and tris(3,5-diisopropylpyrazolyl)borate ligands,4 respectively).
Fig. 1 Schematic representation of [Cu(ttz)(CO)] ( MII), [Cu(tp)(CO)] ( II), and [Cu(tpm)(CO)]+ ( MIIIIII). Spheres evidenced by * represent N and C atoms in MII and II/MIIIIII, respectively, while the one evidenced by ‡ symbolizes B in MII and II, and C in MIIIIII. H atoms of triazolyl and pyrazolyl rings in MII and II/MIIIIII, respectively, are not displayed for the sake of clarity. |
Optimized coordinates of MII, II, and MIIIIII are collected in Tables S1–S3 of the ESI†, while selected structural parameters are compared with X-ray data4,6,15 in Table 1.
MII Theory/exp. | II Theory/exp.b | MIIIIII Theory/exp. | |
---|---|---|---|
a Bond lengths and bond angles in Å and °, respectively. Experimental values for I,6II,15 and III4 are reported for comparison. The numbering scheme of the atoms is the same as that adopted in ref. 6. b Complex II has precise (i.e., crystallographically dictated) C3 symmetry. | |||
O1–C1 | 1.151/1.132(2) | 1.153/1.120(13) | 1.146/1.127(6) |
Cu1–C1 | 1.815/1.8064(19) | 1.810/1.755(11) | 1.825/1.777(5) |
Cu1–N7 | 2.121/2.0565(15) | 2.112/2.048(4) | 2.142/2.030(4) |
Cu1–N1 | 2.121/2.0778(14) | 2.113/2.048(4) | 2.143/2.050(4) |
Cu1–N4 | 2.123/2.0846(14) | 2.114/2.048(4) | 2.143/2.043(3) |
Average Cu–N | 2.122/2.0730 | 2.113/2.048 | 2.143/2.041 |
O1–C1–Cu1 | 180.0/179.41(19) | 180.0/180.0(-) | 180.0/176.7(6) |
C1–Cu1–N7 | 125.9/125.27(8) | 125.1/124.4(1) | 127.4/126.5(2) |
C1–Cu1–N1 | 126.0/124.41(7) | 125.4/124.4(1) | 127.9/130.1(2) |
C1–Cu1–N4 | 125.1/122.09(7) | 125.4/124.4(1) | 127.9/123.6(2) |
Average C1–Cu–N | 125.7/123.92 | 125.3/124.4 | 127.7/126.7 |
N7–Cu1–N1 | 89.4/90.60(6) | 90.0/91.3(2) | 86.5/86.3(2) |
N7–Cu1–N4 | 89.4/92.85(5) | 89.9/91.3(2) | 86.5/89.0(1) |
N1–Cu1–N4 | 89.4/92.20(5) | 89.9/91.3(2) | 86.6/88.3(1) |
Average N–Cu–N | 89.4/91.88 | 89.9/91.3 | 86.5/87.9 |
The good agreement between experiment and theory indicates that, for Cu(I) complexes, bulky substituents in positions 3 and 5 of the triazolyl and pyrazolyl rings negligibly affect the structure of the complex cores. Moreover, though no symmetry has been assumed in our calculations, MII, II, and MIIIIII are all characterized by the presence of a pseudo C3 axis aligned with the H–X (X = B in MII and II, X = C in MIIIIII) and Cu–C–O fragments.
It is well known that IR spectroscopy is a very useful tool to get information about the M–CO (M = metal) interaction,16 the C–O stretching frequency (νCO) being very sensitive to the M→CO back-donation strength, to the M oxidation state as well as to its coordination geometry.17
On passing from the free CO to the coordinated one, νCO is red shifted by 63, 60, and 36 cm−1 in I, II, and III,18 respectively. In very good agreement with experimental evidence, theoretical νCO are red shifted by 58, 74, and 23 cm−1 in MII, II, and MIIIIII, respectively.19 These results indicate that an extensive Cu(I)→CO back-donation from the completely occupied Cu(I) 3d atomic orbitals (AOs) into the CO 2π lowest unoccupied molecular orbital (LUMO) is present,17 but they are of little use in assessing the strength of the concomitant CO→Cu(I) donation from the CO 5σ highest occupied MO (HOMO) into the Cu(I) 4s and 4p empty AOs.
In order to get further insight into the bonding scheme of the title molecules we applied the Ziegler extended transition state (ETS) method12 to the Cu(L) (L = ttz, tp, and tpm) and CO fragments. Cu(L) and CO Hirshfeld charges20 (Q) and Cu(L)–CO (binding energy) BE contributions are reported in Table 2, where ΔEes is the pure electrostatic interaction, ΔEPauli is the destabilizing two-orbital–four-electron interaction between the occupied orbitals of the interacting fragments ((ΔEes + ΔEPauli) corresponds to the so called steric interaction (ΔEst) contribution), ΔEint derives from the stabilizing interaction between occupied and empty orbitals of the interacting fragments, and ΔEprep provides information about the energy required to relax the structure of the free fragments to the geometry they assume in the final system.
MII | II | MIIIIII | |
---|---|---|---|
Q CO | −0.13 | −0.15 | −0.10 |
Q Cu(L) | 0.13 | 0.15 | 1.10 |
ΔEPauli | 122.00 | 127.44 | 109.34 |
ΔEes | −98.80 | −101.97 | −92.23 |
ΔEst | 23.08 | 25.46 | 17.11 |
ΔEint | −61.47 | −64.49 | −56.14 |
ΔEprep | 2.63 | 3.02 | 2.64 |
BSSE (basis set superposition errors) | 0.84 | 0.81 | 0.96 |
BE | −34.92 | −35.20 | −35.43 |
Data reported in Table 2 point out that: (i) CO always carries a negative charge, i.e. it is a net electron acceptor; (ii) QCO becomes more negative along the series MIIIIII→MII→II; (iii) the strength of the Cu(L)–CO interaction is substantially the same along the investigated series because the more negative ΔEint term in the neutral adducts ( MII and II) is compensated by the less positive ΔEst contribution in the charged species.21 It is not surprising that the highest QCO value is computed for MIIIIII; in fact, CO interacts with the positively charged [Cu(tpm)]+ fragment and is unwilling to back-donate. Similar concepts can be applied to explain the QCO difference between MII and II. Actually, despite ttz and tp both carrying a negative charge, the three N atoms of each triazolyl ring makes ttz more electronegative than tp and, consequently, [Cu(ttz)] is a weaker electron donor than [Cu(tp)].6 This can be better realized from Fig. 2, where the Cu(I) 3d partial density of states (PDOS) of [Cu(ttz)], [Cu(tp)], and [Cu(tpm)]+ fragments are compared. The inspection of the PDOS curves clearly testifies that: (i) topmost occupied states are strongly localzed on Cu(I) 3d AOs;22 (ii) the Cu(I) 3d PDOS of the positively charged [Cu(tpm)]+ lie at significantly lower energies than those of the neutral [Cu(ttz)] and [Cu(tp)] fragments; (iii) the Cu(I) 3d PDOS of [Cu(ttz)] lie at lower energies than that of [Cu(tp)].
Fig. 2 Cu PDOS of [Cu(ttz)], [Cu(tp)], and [Cu(tpm)]+ fragments. Vertical lines at −8.29, −5.08, and −4.33 eV correspond to the HOMO energy of [Cu(tpm)]+, [Cu(ttz)], and [Cu(tp)], respectively. |
Because of the absence of symmetry, no partitioning in the σ and π contributions is possible for ΔEint of MII, II, MIIIIII. This hitch can be however overcome by considering that the MII, II, and MIIIIII optimized structures are all characterized by the presence of a pseudo C3 axis. On this basis we decided to carry out a further series of numerical experiments on the same model systems, where a C3v symmetry was assumed.23
The a1 (σ) and e (π) contributions to the Cu(L)–CO interaction (see Table 3) indicates that, despite the leading role played by the Cu(L)→CO back-donation, the CO→Cu(L) σ donation significantly contributes to the Cu(L)–CO bond.
MII | II | MIIIIII | |
---|---|---|---|
a1 (σ) | −24.92 | −25.14 | −24.53 |
e (π) | −36.95 | −39.57 | −32.65 |
ΔEint | −61.87 | −64.71 | −57.18 |
In conclusion, DFT results reported herein provide a theoretical support to the use of ttz as a proper alternative to tp, thus corroborating Papish’s suggestion6 that ttz ligands could be useful in model studies of the catalytic activity of metal centers in biological systems.
The “Laboratorio Interdipartimentale di Chimica Computazionale” (LICC) at the Department of Chemical Sciences of the University of Padova is acknowledged for support of the computer facilities. L. P. thanks MIUR for the PRIN 2006038447 grant.
Footnote |
† Electronic supplementary information (ESI) available: Optimized coordinates of [Cu(ttz)(CO)], [Cu(tp)(CO)], and [Cu(tpm)(CO)]+ (Tables S1–S3). See DOI: 10.1039/b815095h |
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