Gold dipyrrin-bisphenolates: a combined experimental and DFT study of metal–ligand interactions

Abstract

Given that noninnocent and metalloradical-type electronic structures are ubiquitous among dipyrrin-bisphenolate (DPP) complexes, we synthesized the gold(iii) derivatives as potentially innocent paradigms against which the properties of other metallo-DPP derivatives can be evaluated. Electronic absorption spectra, electrochemical studies, a single-crystal X-ray structure, and DFT calculations all suggest that the ground states of the new complexes indeed correspond to an innocent Au^III–DPP³⁻, paralleling a similar description noted for Au corroles. Interestingly, while DFT calculations indicate purely ligand-centered oxidations, reduction of AuDPP is predicted to occur across both the metal and the ligand.

The first gold dipyrrin-bisphenolates have been synthesized. Like their corrole analogues, they exhibit Au^III–L³⁻ ground states, providing rare innocent paradigms for a class of complexes that commonly occur as metalloradicals.

1. Introduction

The dipyrrin-bisphenols (H₃DPP) are an emerging class of ligands that share a number of similarities with corroles (Fig. 1).¹ Thus, they are triprotic, afford a square-planar environment for coordinated metals, and even exhibit redox-active behavior remarkably similar to that of corroles.^2,3 Thus, like their corrole analogues,^4–6 a number of M[DPP] derivatives (M = Co,^7,8 Ni,⁷ Cu⁹) are best regarded as M^II–L˙²⁻, as opposed to M^III–L³⁻. These similarities promise applications of M[DPP] derivatives in catalytic transformations, in which metallocorroles have already proved useful.^10,11 To better understand the role of metalloradical or noninnocent states in DPP chemistry, we have for some time sought innocent M[DPP] complexes, whose properties can serve as standards against which other DPP derivatives can be evaluated. Given that gold(iii) corroles have emerged as archetypes of innocent metallocorroles,^12–15 we synthesized a series of gold(iii) dipyrrin-bisphenolate derivatives, which, as described below, also appear to exhibit innocent Au^III–DPP³⁻ ground states.

Fig. 1. Free-base dipyrrin-bisphenol and meso-triarylcorrole ligands.

2. Results and discussion

2.1. Physical measurements

Three Au(iii) meso-para-X-phenyl dipyrrin-bisphenolate complexes Au[XDPP] with X = CF₃, H, and Me were obtained as blue solids in 50–77% yields via the interaction of the corresponding free ligands and a threefold molar excess of Au(iii) acetate in pyridine.¹² A single-crystal X-ray structure (Table 1 and Fig. 2) could be obtained for one of the complexes (X = CF₃). The X-ray structure reveals Au–N/O distances of around 1.97 Å, which are approximately 0.02–0.03 Å longer than Au–N distances typically observed for Au(iii) corroles.^12–15 An examination of the skeletal bond distances of Au[CF₃DPP] led to the interesting observation that the C–C bonds in the phenolate moieties span a narrower range (∼0.04 Å) relative to M(DPP) derivatives (∼0.08 Å) that are unambiguously describable as metalloradicals, as for M = Cu (CCSD: FICCEC, FICCUS)¹⁶ and Pt (LACCUQ, LACDAX).¹⁷ Similarly narrow C–C bond distance ranges are also observed for nonradicaloid Ge (VIVNAR, PONGOQ, SIRFOQ),¹⁸ Mn (UTOVEF¹⁹ and EXOBAV²⁰), Al (NABFII),^21–23 Ga (WOMPAS),²⁴ and In (WOMPIA)²⁴ DPP complexes.

Crystal data and structure refinement for Au[CF₃DPP].

Sample	Name
Chemical formula	C28H16AuF3N2O2
Formula mass	666.39
Crystal system	Monoclinic
Crystal size (mm3)	0.090 × 0.040 × 0.020
Space group	P21/c
λ (Å)	0.7288
a (Å)	6.8844(4)
b (Å)	16.4488(8)
c (Å)	19.8240(10)
α (°)	90
β (°)	98.802(2)
γ (°)	90
Z	4
V (Å3)	2218.4(2)
Temperature (K)	100(2)
Density (calculated)	1.995 Mg m−3
Measured reflections	89 167
Unique reflections	8107
Parameters	0
Restraints	325
R int	0.0502
θ range (°)	2.482 to 33.562
R1, wR2 all data	0.0462, 0.1131
S (GooF) all data	1.332
Max/min res. dens. (e Å−3)	3.137/−2.242

Fig. 2. Molecular structure of Au[CF₃DPP]: top view (above) with selected distances (Å) and side view (below).

The three Au complexes exhibit similar optical spectra, with the strongest absorption occurring in the red at 639 ± 5 nm (Fig. 3). The molar absorptivities turned out to be around 3.0 × 10⁴ M⁻¹ cm⁻¹, over three times the value observed for Cu[CF₃DPP], consistent with an innocent electronic-structural description for the Au complexes and a radical description for the Cu complex (Fig. 4).^25,26

Fig. 3. UV-vis spectra of Au[XDPP] (X = CF₃, H, and CH₃) in dichloromethane.

Fig. 4. UV-vis spectra of M[CF₃DPP] (M = Cu and Au) in dichloromethane.

Cyclic voltammetric measurements, indicating relatively high oxidation potentials of around +0.95 V against the saturated calomel electrode (SCE),²⁷ relatively low reduction potentials of around −0.90 V, and substantial electrochemical HOMO–LUMO gaps of around 1.85 V, are also suggestive of an innocent description for the Au[XDPP] complexes (Fig. 5). In contrast, Cu[CF₃DPP] (Fig. 6) was found to exhibit an electrochemical HOMO–LUMO gap of just under 1.0 V (qualitatively consistent with similar observations elsewhere⁹). These electrochemical trends closely mirror those observed for analogous coinage metal corroles.^{6a,d,g,h,12,15b}

Fig. 5. Cyclic voltammograms of Au[XDPP] (X = CF₃, H and CH₃) in benzonitrile. Scan rate: 0.1 V s⁻¹.

Fig. 6. Cyclic voltammograms of M[CF₃DPP] (M = Cu and Au) in benzonitrile. Scan rate: for Cu is 0.05 V s⁻¹ and 0.1 V s⁻¹ for Au.

2.2. DFT calculations

To obtain a unified interpretation of the above findings, we carried out scalar-relativistic DFT (OLYP^28,29/STO-TZ2P) calculations on the para-unsubstituted complexes M(DPP) for M = Cu and Au. For Cu, the ground state turned out to be a triplet, consistent with a Cu^II–L˙²⁻ description.⁹ For Au, the ground state was unambiguously a singlet, with the triplet approximately 1.1 eV higher in energy. Fig. 7, which juxtaposes the optimized skeletal bond distances in the Cu- and Au-DPP complexes, confirms that the phenolate C–C bonds in the Au complex span a significantly smaller range than those in Cu[DPP].

Fig. 7. Juxtaposition of the OLYP/STO-TZ2P optimized geometries (Å) Cu (left) and Au (right) DPP complexes.

An examination of the Kohn–Sham frontier orbitals of Au[DPP] and their eigenvalues showed that while the ligand-based HOMO is energetically well-separated from the other MOs, the two LUMOs – a DPP-based π-symmetry MO and an Au 5d_x²−y²-based σ-symmetry MO – are near-degenerate under C_2v symmetry (Fig. 8). Relaxing the point group symmetry to C₂ allows the two LUMOs to mix, as is clear from both the HOMO and spin density profiles of the Au[DPP]⁻ anion. Thus, while confirming pure ligand-centered oxidation,³⁰ the calculations provide a more nuanced picture of the reduction process, which occurs over both the Au and the DPP ligand (Fig. 9). In this respect, the Au–DPP complexes differ from simple Au corroles, which exhibit purely ligand-centered reduction.³¹ Presumably, the open-chain DPP ligands afford a more flexible coordination cavity for a larger, reduced Au center relative to the much more sterically constrained corroles.

Fig. 8. OLYP/STO-TZ2P frontier MOs of Au[DPP] under a C_2v symmetry constraint.

Fig. 9. OLYP/STO-TZ2P spin density and HOMO profiles of the cationic and anionic states of Au[DPP] optimized with a C₂ symmetry constraint.

3. Conclusion

The first Au(iii) dipyrrin-bisphenolate complexes, Au[XDPP] with X = CF₃, H, and Me, have been synthesized in fair to good yields (50–77%) yields via the interaction of the corresponding free ligands and a threefold molar excess of Au(iii) acetate in pyridine. X-ray structure analysis, optical spectroscopy, electrochemistry and DFT calculations all suggest an innocent Au^III–DPP³⁻ description for the complexes. The calculations, however, also suggest that while the compounds undergo ligand-centered oxidation, reduction occurs across both the Au(5d_x²−y²) orbital and the DPP π-LUMO. In the latter respect, the Au–DPP complexes differ from simple Au corroles, which undergo exclusively corrole-centered reductions.

4. Experimental section

4.1. Materials and instruments

All reagents and solvents were used as purchased unless noted otherwise. Benzonitrile was distilled from P₄O₁₀ and stored over activated 4 Å molecular sieves. Ultraviolet-visible (UV-vis) spectra were recorded in CH₂Cl₂ on an HP 8454 or a Varian Cary 50 spectrophotometer. Unless otherwise mentioned, ¹H (400 MHz) and ¹⁹F (376 MHz) NMR spectra were recorded in CDCl₃ on a 400 MHz Bruker Avance III HD spectrometer equipped with a 5 mm BB/¹H (BB = ¹⁹F, ³¹P, and ¹⁵N) SmartProbe and referenced to residual CHCl₃ (δ = 7.26 ppm) and 2,2,2-trifluoroethanol-d₃ (δ = −77.8 ppm), respectively. In the case of the free ligands, ¹H NMR spectra were recorded on a Bruker Avance III 500 spectrometer operating at 500 MHz and ¹⁹F NMR spectra were recorded on a Bruker Avance III 600 spectrometer operating at 564 MHz and available at the PACSMUB-WPCM technological platform, which relies on the “Institut de Chimie Moléculaire de l’Université de Bourgogne” and Satt Sayens “TM”, a Burgundy University private subsidiary. All NMR shift values are expressed as ppm. ¹H and ¹⁹F spectra were calibrated using the residual peak of chloroform at 7.26 ppm or acetone-d₆ at 2.05 ppm. High-resolution electrospray ionization mass spectra were recorded on an LTQ Orbitrap XL spectrometer. MALDI-TOF mass spectra were recorded on a Bruker Ultraflex Extreme MALDI Tandem TOF Mass Spectrometer using dithranol as the matrix. Cyclic voltammetry was performed with an EG&G Model 263A potentiostat having a three-electrode system, including a glassy carbon working electrode, a platinum wire counter electrode, and a saturated calomel reference electrode (SCE). Tetra(n-butyl)ammonium perchlorate (TBAP) was recrystallized three times from absolute ethanol and dried in vacuo for at least one week prior to use as supporting electrolyte. The SCE was separated from the bulk solution by a fritted-glass bridge filled with the solvent/supporting-electrolyte mixture. Sample solutions in dry benzonitrile were purged with argon for at least 5 min prior to electrochemical measurements, which were also carried out under an argon blanket. All potentials are referenced to the SCE. The dipyrrin-bisanisole H₃[HDPPOMe] and the corresponding dipyrrin-bisphenol H₃[HDPP] were synthesized as described in the literature.³²

4.2. General synthetic procedure for dipyrrin-bisanisoles

To a stirred solution of the appropriate benzaldehyde (0.86 mmol, 1.0 eq.) and 2-(2-methoxyphenyl)pyrrole (synthesized according to a literature procedure,³³ 299 mg, 1.73 mmol, 2.0 eq.) in CH₂Cl₂ (15 mL) under argon, trifluoroacetic acid (22 μL) was added and the mixture was stirred for 2 h at room temperature. 2,3-Dichloro-5,6-dicyano-1,4-benzoquinone (DDQ, 200 mg, 0.88 mmol) was then added and the resulting solution stirred overnight at room temperature. The reaction mixture was washed with saturated NaHCO₃ aqueous solution and extracted with CH₂Cl₂. The organic phase was dried over MgSO₄, evaporated to dryness, and subjected to column chromatography as described below.

4.3. H₃[CF₃DPPOMe]

This compound was purified by silica gel column using CH₂Cl₂ and a 9 : 1 mixture of CH₂Cl₂/MeOH as eluent and by alumina using a 2 : 3 mixture of CH₂Cl₂/heptane. Yield 281 mg (0.56 mmol, 65%). UV-vis λ_max [nm, ε × 10⁴ (M⁻¹ cm⁻¹)]: 526 (2.62), 610 (0.27). ¹H NMR δ (CDCl₃, δ = 7.26 ppm): 13.66 (s, 1H, NH); 8.04 (d, 2H, J = 7.5 Hz, phenylmethoxy); 7.72 (d, 2H, J = 8.0 Hz, m or o 10-pCF₃C₆H₄); 7.67 (d, 2H, J = 8.0 Hz, m or o 10-pCF₃C₆H₄); 7.35 (t, 2H, J = 7.5 Hz, phenylmethoxy); 7.05 (t, 2H, J = 7.5 Hz, phenylmethoxy); 7.00 (d, 2H, J = 7.5 Hz, phenylmethoxy); 6.94 (d, 2H, J = 4.5 Hz, β-H); 6.54 (d, 2H, J = 4.5 Hz, β-H); 3.87 (s, 6H, OMe). ¹⁹F NMR δ: −62.96 (s, 3F, 10-pCF₃C₆H₄). LRMS (MALDI/TOF) [M]⁺˙: 500.65 (expt), 500.17 (calcd). HRMS (ESI) [M + H]⁺: 501.1781 (expt), 501.1784 (calcd).

4.4. H₃[MeDPPOMe]

This compound was purified by silica gel column using a 4 : 1 mixture of heptane/AcOEt as eluent. Yield 212 mg (0.47 mmol, 55%). UV-vis λ_max [nm, ε × 10⁴ (M⁻¹ cm⁻¹)]: 316 (2.08), 521 (2.96), 601 (0.60). ¹H NMR δ (CDCl₃, δ = 7.26 ppm): 13.75 (s, 1H, NH); 8.06 (d, 2H, J = 7.5 Hz, phenylmethoxy); 7.45 (d, 2H, J = 8.0 Hz, m or o 10-pMeC₆H₄); 7.35 (t, 2H, J = 7.5 Hz, phenylmethoxy); 7.27 (d, 2H, J = 8.0 Hz, m or o 10-pMeC₆H₄); 7.05 (t, 2H, J = 7.5 Hz, phenylmethoxy); 7.01 (d, 2H, J = 7.5 Hz, phenylmethoxy); 6.95 (d, 2H, J = 4.5 Hz, β-H); 6.67 (d, 2H, J = 4.5 Hz, β-H); 3.87 (s, 6H, OMe); 2.47 (s, 3H, CH₃). LRMS (MALDI/TOF) [M]⁺˙: 446.58 (expt), 446.20 (calcd). HRMS (ESI) [M + H]⁺: 447.2059 (expt), 447.2067 (calcd).

4.5. General synthetic procedure for dipyrrin-bisphenols

The experimental procedure was adapted from methodology described in the literature for the preparation of the unsubstituted phenyl ligand H₃[HDPP].³² To a stirred solution of the corresponding dipyrrin-bisanisole (0.282 mmol) in CH₂Cl₂ (14 mL) under an argon atmosphere, BBr₃ (1.0 M in heptane, 5.63 mL, 5.63 mmol) was added at 0 °C. The reaction mixture was stirred and allowed to warm up to room temperature and left for 3 days before quenching with methanol (14 mL). Concentrated HCl (37%, 1.35 mL) was then added and the resulting mixture was refluxed for 3 h. After cooling, the mixture was neutralized with saturated aqueous NaHCO₃ and extracted with ethyl acetate. The organic layer was dried over MgSO₄ and evaporated to dryness; the residue was then subjected to column chromatography to yield the desired free dipyrrin-bisphenol ligands.

4.6. H₃[CF₃DPP]

The compound was purified by silica gel column using 3 : 1 heptane/ethyl acetate as eluent. Yield 58.6 mg (0.12 mmol, 44%). UV-vis λ_max [nm, ε × 10⁴ (M⁻¹ cm⁻¹)]: 301 (1.17), 545 (1.53), 612 (0.47). ¹H NMR δ (acetone-d₆, δ = 2.05 ppm): 7.90 (d, 2H, J = 8.0 Hz, m or o 10-pCF₃C₆H₄); 7.87 (d, 2H, J = 7.5 Hz, phenoxy); 7.80 (d, 2H, J = 8.0 Hz, m or o 10-pCF₃C₆H₄); 7.30 (t, 2H, J = 7.5 Hz, phenoxy); 7.16 (m, 2H, β-H); 7.06 (d, 2H, J = 7.5 Hz, phenoxy); 6.98 (t, 2H, J = 7.5 Hz, phenoxy); 6.67 (m, 2H, β-H). ¹⁹F NMR δ: −63.01 (s, 3F, 10-pCF₃C₆H₄). LRMS (MALDI/TOF) [M]⁺˙: 472.64 (expt), 472.14 (calcd). HRMS (ESI) [M + H]⁺: 473.1459 (expt), 473.1471 (calcd).

4.7. H₃[MeDPP]

The compound was purified by silica gel column using a 4 : 1 heptane/ethyl acetate as eluent. Yield 89.2 mg (0.213 mmol, 75%). UV-vis λ_max [nm, ε × 10⁴ (M⁻¹ cm⁻¹)]: 323 (1.27), 373 (0.53), 545 (2.04), 601 (0.52). ¹H NMR δ (acetone-d₆, δ = 2.05 ppm): 7.86 (d, 2H, J = 7.5 Hz, phenoxy); 7.46 (d, 2H, J = 8.0 Hz, m or o 10-pMeC₆H₄); 7.37 (t, 2H, J = 8.0 Hz, m or o 10-pMeC₆H₄); 7.27 (t, 2H, J = 7.5 Hz, phenoxy); 7.13 (d, 2H, J = 4.5 Hz, β-H); 7.03 (d, 2H, J = 7.5 Hz, phenoxy); 6.95 (t, 2H, J = 7.5 Hz, phenoxy); 6.75 (m, 2H, β-H); 2.48 (s, 3H, CH₃). LRMS (MALDI/TOF) [M]⁺˙: 418.55 (expt), 418.17 (calcd). HRMS (ESI) [M + H]⁺: 419.1748 (expt), 419.1754 (calcd).

4.8. General synthetic procedure for gold dipyrrin-bisphenolates

Gold acetate (3 equiv.) was added to a pink solution of the free dipyrrin-bisphenol ligand (30 mg) in pyridine (6 mL). The resulting suspension was stirred for 24 h and monitored with TLC and mass spectrometry. The blue suspension that was finally obtained was passed through Celite, the resulting solution was filtered, and the filtrate was rotary-evaporated to dryness. The brown residue obtained was dissolved in THF or CHCl₃ and filtered twice through a double-layer of filter paper. The resulting blue filtrate was rotary-evaporated to yield a blue solid, which was thoroughly washed with n-hexane and dried under vacuum. Unfortunately, the compounds proved quite light-sensitive, especially in the presence of air, preventing us from obtaining satisfactory elemental analyses. Fortunately, X-ray quality crystals could be obtained for Au[CF₃DPP] via slow diffusion of methanol into a concentrated chloroform solution in about 2 weeks.

4.9. Au[CF₃DPP]

Yield 21 mg (0.031 mmol, 50%). UV-vis λ_max [nm, ε × 10⁴ (M⁻¹ cm⁻¹)]: 322 (2.26), 435 (0.65), 643 (2.25). ¹H NMR δ (1,1,2,2-tetrachloroethane-d₂, δ = 6.00 ppm): 7.82 (d, 2H, J = 8.0 Hz, m or o-10-pCF₃C₆H₄); 7.78 (d, 2H, J = 7.8 Hz, phenoxy); 7.71 (d, 2H, J = 8.0 Hz, o or m-10-pCF₃Ph); 7.39 (t, 2H, J = 7.8 Hz, phenoxy); 7.32 (d, 2H, J = 8.2 Hz, phenoxy); 7.25 (d, 2H, J = 4.8 Hz, β-H); 6.96 (t, 2H, J = 7.4 Hz, phenoxy); 6.83 (d, 2H, J = 4.8 Hz, β-H). ¹⁹F NMR δ: −63.16 (s, 2F, 10-pCF₃C₆H₄); −63.18 (s, 1F, 10-pCF₃C₆H₄). HRMS (ESI, major isotopomer) [M]⁺: 666.0789 (expt), 666.0824 (calcd).

4.10. Au[HDPP]

Yield 23 mg (0.038 mmol, 52%). UV-vis λ_max [nm, ε × 10⁴ (M⁻¹ cm⁻¹)]: 325 (2.38), 429 (0.85), 636 (2.79). ¹H NMR δ: 7.71 (d, 2H, J = 8.0 Hz, phenoxy); 7.56 to 7.50 (d, 5H, Ph), 7.33 to 7.28 (m, 4H, phenoxy); 7.16 (d, 2H, J = 4.8 Hz, β-H), 6.91 to 6.85 (m, 2H, phenoxy), 6.83 (d, 2H, J = 4.8 Hz, β-H). HRMS (ESI, major isotopomer) [M]⁺: 598.0926 (expt), 598.0950 (calcd).

4.11. Au[MeDPP]

Yield 34 mg (0.055 mmol, 77%). UV-vis λ_max [nm, ε × 10⁴ (M⁻¹ cm⁻¹)]: 310 (1.85), 336 (1.89), 427 (0.97), 634 (2.93). ¹H NMR δ (1,1,2,2-tetrachloroethane-d₂, δ = 6.00 ppm): δ 7.77 (d, 2H, J = 7.8 Hz, phenoxy); 7.44 (d, 2H, J = 7.9 Hz, m or o-10-pCH₃C₆H₄), 7.39 to 7.30 (m, 4H, phenoxy; 2H, o or m-10-pCH₃C₆H₄); 7.24 (d, 2H, J = 4.7 Hz, β-H), 6.97 to 6.92 (overlapping d, 2H, J = 4.7 Hz, β-H and t, 2H, J = 7.8 Hz, phenoxy), 2.49 (3H, CH₃, 10-pCH₃C₆H₄). HRMS (ESI, major isotopomer) [M]⁺: 612.1096 (expt), 612.1107 (calcd).

4.12. Synthesis of Cu[CF₃DPP]

Copper acetate (11 mg, 0.055 mmol, 5 equiv.) was added to a pink solution of the H₃[CF₃DPP] ligand (5 mg, 0.011 mmol) in pyridine (2 mL). The suspension was stirred for 1 h, at the end of which the reaction was complete, as indicated by TLC (CHCl₃-2% CH₃OH) and mass spectrometry. The blue suspension obtained was filtered through Celite and the resulting solution was filtered twice before evaporation under vacuum. Yield 5.5 mg (0.010 mmol, 91%). UV-vis λ_max [nm, ε × 10⁴ (M⁻¹ cm⁻¹)]: 312 (0.74), 572 (0.35), 614 (0.92). HRMS (ESI, major isotopomer) [M + H]⁺ = 533.0527 (expt), 533.0533 (calcd).

4.13. X-ray structure determination

X-ray diffraction data were collected on beamline 12.2.1 at the Advanced Light Source of Lawrence Berkeley National Laboratory, Berkeley, California. The samples were mounted on MiTeGen® kapton loops and placed in a 100(2) K nitrogen cold stream provided by an Oxford Cryostream 700 Plus low temperature apparatus on the goniometer head of a Bruker D8 diffractometer equipped with PHOTONII CPAD detector. Diffraction data were collected using synchrotron radiation monochromated with silicon(111) to a wavelength of 0.7288(1) Å. In each case, an approximate full-sphere of data was collected using 1° ω scans. Absorption corrections were applied using SADABS.³⁴ The structure was solved by intrinsic phasing (SHELXT)³⁵ and refined by full-matrix least squares on F² (SHELXL-2014)³⁶ using the ShelXle GUI.³⁷ Appropriate scattering factors were applied using the XDISP³⁸ program within the WinGX suite.³⁹ All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were geometrically calculated and refined as riding atoms.

4.14. Computational methods

DFT calculations were carried out at the scalar-relativistic level with the ZORA (Zeroth Order Regular Approximation to the Dirac equation)^40–42 Hamiltonian, the OLYP^28,29 exchange-correlation functional, and all-electron ZORA STO-TZ2P relativistic basis sets, all as implemented in the ADF program system.^43,44

Conflicts of interest

There are no conflicts of interest to declare.