Carbonyl copper(I) complexes with hydrotris(1,2,4-triazolyl)borate, hydrotris(pyrazolyl)borate, and tris(pyrazolyl)methane ligands: a DFT study

Abstract

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.

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An impressive number of experimental contributions has been devoted to the study of the structural and electronic properties of scorpionate transition metal complexes.^1–6 N₃ tripodal ligands, such as hydrotris(pyrazolyl)borate (tp) and the neutral tris(pyrazolyl)methane (tpm), are both able to shield the metal center, and to act as reliable spectator ligands. They have also been proposed as modeling tools in the investigation of the catalytic activity of metal centers in biological systems. However, because of their low solubility, sterically bulky tp and tpm ligands are of limited use as models of enzyme active sites in an aqueous milieu.

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)] ( M_II), [Cu(tp)(CO)] ( II), and [Cu(tpm)(CO)]⁺ ( M_IIIIII) complexes (see Fig. 1), with M_II and M_IIIIII unsubstitued models of [Cu(ttz^tBu,Me)(CO)] ( I) and [Cu(tpm^iPr,iPr)(CO)]⁺ ( III) (ttz^tBu,Me and tpm^iPr,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)] ( M_II), [Cu(tp)(CO)] ( II), and [Cu(tpm)(CO)]⁺ ( M_IIIIII). Spheres evidenced by * represent N and C atoms in M_II and II/M_IIIIII, respectively, while the one evidenced by ‡ symbolizes B in M_II and II, and C in M_IIIIII. H atoms of triazolyl and pyrazolyl rings in M_II and II/M_IIIIII, respectively, are not displayed for the sake of clarity.

Optimized coordinates of M_II, II, and M_IIIIII are collected in Tables S1–S3 of the ESI†, while selected structural parameters are compared with X-ray data^4,6,15 in Table 1.

Table 1 Selected structural parameters for M_II, II, and M_IIIIII^a

	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 III^4 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

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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, M_II, II, and M_IIIIII are all characterized by the presence of a pseudo C₃ axis aligned with the H–X (X = B in M_II and II, X = C in M_IIIIII) 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 (ν_CO) 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, ν_CO 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 ν_CO are red shifted by 58, 74, and 23 cm⁻¹ in M_II, II, and M_IIIIII, 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 ΔE_es is the pure electrostatic interaction, ΔE_Pauli is the destabilizing two-orbital–four-electron interaction between the occupied orbitals of the interacting fragments ((ΔE_es + ΔE_Pauli) corresponds to the so called steric interaction (ΔE_st) contribution), ΔE_int derives from the stabilizing interaction between occupied and empty orbitals of the interacting fragments, and ΔE_prep provides information about the energy required to relax the structure of the free fragments to the geometry they assume in the final system.

Table 2 Q _Cu(L) , Q_CO and BE contributions (kcal mol⁻¹) to the Cu(L)–CO interaction (L = ttz, tp, and tpm in M_II, II, and M_IIIIII)

	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

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Data reported in Table 2 point out that: (i) CO always carries a negative charge, i.e. it is a net electron acceptor; (ii) Q_CO becomes more negative along the series M_IIIIII→M_II→II; (iii) the strength of the Cu(L)–CO interaction is substantially the same along the investigated series because the more negative ΔE_int term in the neutral adducts ( M_II and II) is compensated by the less positive ΔE_st contribution in the charged species.²¹ It is not surprising that the highest Q_CO value is computed for M_IIIIII; 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 Q_CO difference between M_II 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 ΔE_int of M_II, II, M_IIIIII. This hitch can be however overcome by considering that the M_II, II, and M_IIIIII 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 C_3v 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.

Table 3 a₁ (σ) and e (π) contributions (kcal mol⁻¹) to the Cu(L)–CO interaction (L = ttz, tp, and tpm in M_II, II, and M_IIIIII)

	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

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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.

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18. The experimental ν_CO of the free molecule is 2143 cm⁻¹; ν_CO values of I, II, and III lie at 2080,⁶ 2083,¹⁵ and 2107⁴ cm⁻¹, respectively. The [Cu(tp^tBu,Me)(CO)] ν_CO is 2059 cm⁻¹ (ref. 6).
19. The ν_CO evaluated for the free molecule within the harmonic approximation amounts to 2113 cm⁻¹.
20. F. L. Hirshfeld, Theor. Chim. Acta, 1977, 44, 129.
21. The Cu(L)–CO BEs of II and M_IIIIII are in excellent agreement with data reported by Fujisawa et al.⁴ for [Cu(tp^Me,Me)(CO)] and [Cu(tpm^Me,Me)(CO)]⁺.
22. The Mulliken charge density analysis¹⁴ points out that the topmost MOs of M_II, II, and M_IIIIII are all strongly localized on the Cu based 3d AOs.
23. Bonding energy differences between C_3v and C₁ calculations amount to 0.2 (M_II), 0.4 (II), and 0.1 (M_IIIIII) kcal mol⁻¹. C_3v geometrical parameters negligibly differ from those reported in Table 1.

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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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