Stabilization of the double sandwich structure of mercury(II) porphyrins: Hg⋯Hg⋯Hg interactions and structure–function correlation

Abstract

A series of stable trinuclear “double sandwich” complexes of mercury(II) porphyrins with linear Hg₃ cores has been stabilized successfully utilizing both flexible and rigid porphyrin dimer frameworks. The gross structural patterns are similar: two terminal Hg(II) centers are above and below the porphyrin rings, whereas the middle Hg(II) center is sandwiched between the two rings. The mercury–nitrogen distances are quite different in the complexes. Mercurophilic interactions play a crucial role in stabilizing this unique structure, with a linear Hg⋯Hg⋯Hg unit overcoming the inherent instability arising from two coplanar aromatic (porphyrin) rings placed exactly on top of each other with eclipsed conformations, a hallmark of the double sandwich complexes reported here. Interestingly, the strongest mercurophilic interactions (with Hg⋯Hg distances of 3.1251(11) Å and 3.1333(16) Å) are observed with the highly flexible ethane-bridged porphyrin dimer. Extensive DFT calculations demonstrate that the mercurophilic interaction is evident when relativistic and dispersion effects are included and the distances are also in excellent agreement with the X-ray structures of the complexes. NBO and QTAIM analyses revealed distinct bond paths and bond critical points (BCPs) that are commonly recognized as key indicators of mercurophilic interactions. The absorption (with an MMLCT band at ∼350 nm) and photoluminescence properties of the complexes display direct correlation with the strength of the Hg⋯Hg interactions. Fluorescence decays at the blue end (related to the mercurophilic interactions) of the emission spectra are faster than those at the red end (associated with ligand emission) for all the complexes at both 298 K and 77 K.

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Introduction

Metallophilic interactions between closed shell atoms or ions (with d¹⁰ or s² configurations) in homo- and hetero-metallic species have become a prominent area for research recently.^1–4 Two or more closed-shell cations are expected to repel each other; however, their interactions become attractive for heavier elements due to dispersive forces magnified by relativistic effects.^1–4 Importantly, such interactions also enable interesting structural and physical properties such as polychromism, luminescence, electrical conductivity, etc., and therefore have attracted attention in the scientific community.^3–6

Hg(II) porphyrins have been known since 1951.⁷ However, their X-ray structures involving porphyrin and its derivatives are scarce.^8–10 Surprisingly, only two X-ray structures of mercury(II) porphyrin have been reported so far, in which all four porphyrinic nitrogens are available for possible coordination to the metal (Scheme 1).^9b,10b It is important to note here that the number of crystal structures displaying mercurophilic interactions is very small with higher coordination numbers (5 and 6).^4e Recently, we have briefly reported, as a Communication,¹¹ an unusual trinuclear double sandwich structure of mercury(II) porphyrin, for the first time, using an urea-bridged porphyrin dimer. The X-ray crystallographic analysis of the complex revealed a unique structural arrangement where one mercury atom is positioned above a porphyrin core, another intercalated between the two cores and a third one positioned below the second porphyrin core. Porphyrin rings are on top of each other in a fully eclipsed conformation in the molecule, which is assumed to result in a high level of repulsion in nature.¹² The urea-bridging might play a critical role in stabilizing such a particular arrangement of the macrocycles with a linear Hg₃ unit, which results in significant mercurophilic interactions. Carlo Floriani et al. have reported the X-ray structure of a double sandwich structure but with an alkaline earth metal Ca using a monomeric 5,10,15,20-tetrakis(4-tert-butylphenyl)porphyrin.¹³

Scheme 1 Structurally characterized mercury(II) porphyrins (A) reported previously and (B) in the present study.

To comprehensively understand what causes the stabilization of such an unusual and distinct double sandwich arrangement with a linear Hg₃ unit and for the investigation of the nature and unique spectroscopic and photophysical properties arising from such Hg⋯Hg⋯Hg interactions, a series of double sandwich complexes using completely different sets of ligand frameworks has been synthesized for an extensive structure–function correlation analysis in the present investigation. We have utilized here one highly flexible ethane-bridged and one rigid ethene-bridged porphyrin dimer that provide quite different molecular platforms for synthesizing such highly fluorescent molecules. The transition metal complexes of the ethane-bridged porphyrin dimer display large vertical and horizontal flexibilities and can easily be interconvertible between syn and anti-conformations (Scheme S1), with just slight environmental perturbation and geometrical constraints.¹⁴ On the other hand, a fairly rigid and relatively short ethene-bridged porphyrin dimer in the cis form is known to display only restricted vertical and horizontal movements between two cofacial porphyrin macrocycles.¹⁵ In this study, we synthesized and successfully determined the X-ray structures of two stable “double-sandwich” molecules (Scheme 1) using ethane- and ethene-bridged porphyrin dimers and compared these with urea-bridged complex 1·Hg (Scheme 1). The solid and solution state structures and properties of the molecules have been thoroughly scrutinized along with an extensive computational study to understand the origin of such stabilization of these unique “double-sandwich” arrangements with a linear Hg₃ core. Additionally, the mercurophilic interactions and their effects on the photophysical and photoluminescence properties were examined, both at 298 K and 77 K, providing valuable insights into the structure–property relationship.

Results and discussion

Ethane- and ethene-bridged porphyrin dimers have been synthesized using procedures reported earlier.¹⁵ The highly flexible ethane-bridged porphyrin dimer can easily interconvert between syn and anti-conformations with slight environmental perturbation (Scheme S1).¹⁴ In the syn-form, two rings are in the face-to-face arrangement but slipped, which is the geometric requirement for strong π–π interactions.^12,14 To a CHCl₃ solution of the free base porphyrin dimers, three equivalents of mercuric chloride dissolved in methanol were added and refluxed for two hours under a strictly inert environment in the absence of light. Chromatographic purifications of the crude product yielded dark-green solids of 2·Hg and 3·Hg in good yields (Scheme 1).

The UV-vis spectra of 1·Hg,¹¹2·Hg, and 3·Hg have similar spectral features and are compared in Fig. 1. The spectrum of 1·Hg displays a band at 339 nm, a Soret band at 420 nm along with a shoulder at 430 nm and two Q bands at 538 and 572 nm in methanol at 298 K. The bands are red-shifted respectively to 343, 428, 560 and 594 nm for 2·Hg and 344, 432, 561 and 595 nm for 3·Hg. The band around 350 nm seems to be characteristic of these trinuclear double sandwich complexes and has not been observed in the case of Hg(II) porphyrin monomers reported previously. Interestingly, the band is red-shifted in the order 1·Hg < 2·Hg < 3·Hg, which is also the order of the strength of mercurophilic (Hg⋯Hg) interactions (vide infra), and this is tentatively assigned to metal–metal-to-ligand charge-transfer (MMLCT).¹⁶ The origin of the band is further supported by fluorescence, TD-DFT and DFT studies (vide infra). The ESI-MS spectrum revealed a peak at m/z 1764.5101 which is assigned to [3·Hg]⁺ (Fig. 1). The isotopic distribution of the experimental mass matches well with that of the theoretical one and thereby confirms the formation of the double sandwich complex (Fig. 1 and S1).

Fig. 1 (A) A comparison of the UV-visible spectra (in CH₃OH at 295 K) of 1·Hg (black line),¹¹2·Hg (blue line) and 3·Hg (red line), and (B) the isotopic distribution pattern (simulated (red) and experimental (black)) for the ESI-MS data of [3·Hg]⁺.

Crystallographic characterization

Dark-green needle-shaped crystals of 2·Hg and 3·Hg were grown via slow diffusion of hexane into the dichloromethane solutions of the respective complexes at room temperature in air in the absence of light, and the structures were unambiguously elucidated by single crystal X-ray structure determination. Fig. 2, S2, and S3 display the X-ray structures and molecular packings in the unit cells of the complexes reported here. 2·Hg and 3·Hg crystallizes in a monoclinic crystal system with the space groups C2/c and P2₁, respectively. The asymmetric unit of 2·Hg contains an independent half-molecule, while the other half is symmetry generated by the application of the crystallographic inversion center located at the middle Hg2 center. However, the asymmetric units of 1·Hg and 3·Hg contain full molecules. Table S1 displays the crystallographic parameters of the complexes, while Table 1 lists the structural and geometrical parameters of 1·Hg, 2·Hg and 3·Hg.

Fig. 2 Perspective views of 2·Hg [side (A) and top (B) views] and 3·Hg [side (C) and top (D) views] showing 50% thermal contours for all nonhydrogen atoms at 100 K (H atoms have been omitted for clarity).

Table 1 Selected structural and geometric parameters

	1·Hg	2·Hg	3·Hg	Hg(p-CN)4 tpp
a Average displacement of atoms from the least-squares plane of the C20N4 pophyrinato core. b Displacement of Hg1/Hg3 from the least-squares plane of the C20N4 porphyrinato core. c Displacement of Hg2 from the least-squares planes of the C20N4 pophyrinato core. d Average distance between the two intramolecular least-squares planes of the C20N4 pophyrinato core. e Non-bonding distance.
Hg–Cl (Å)	2.321(2), 2.315(2)	2.310(3), 2.310(3)	2.331(5), 2.336(6)	—
Hg1–Npor (Å)	2.154(7), 2.457(7), 2.472(7)	2.146(8), 2.445(8), 2.460(8)	2.228(18), 2.432(17), 2.460(15)	2.177(6), 2.255(6), 2.169(6), 2.212(6)
Hg2–Npor (Å)	2.097(7), 2.108(7), 2.576(7), 2.758(7)	2.117(7), 2.742(8), 2.748(8)	2.250(2), 2.327(18), 2.410(2), 2.503(17)	—
Hg3–Npor (Å)	2.146(7), 2.486(7), 2.498(7)	2.146(8), 2.445(8), 2.460(8)	2.124(17), 2.422(17), 2.456(15)	—
Δ24 ^a (Å)	0.12, 0.14	0.10, 0.10	0.09, 0.09
Δ^Hg24 ^b (Å)	1.30, 1.44	1.31, 1.36	1.31, 1.35	0.64
Δ^Hg224 ^c (Å)	1.78, 1.63	1.74, 1.67	1.73, 1.68
MPS^d	3.40	3.42	3.43
Hg⋯Hg^e (Å)	3.1911(5), 3.2056(5)	3.1776(4), 3.1776(4)	3.1251(11), 3.1333(16)
Ref.	11	tw	tw	10b

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The gross structural patterns of 2·Hg and 3·Hg are similar to that of 1·Hg, having two types of Hg atoms, where the terminal Hg(II) centers are above and below the porphyrin rings, whereas the middle Hg(II) center is sandwiched between the two rings. The mercury–nitrogen distances are quite different in the complexes (Table 1) compared to the reported^10b lone monomeric structure of Hg(p-CN)₄tpp. Each terminal Hg(II) center binds strongly with three porphyrinic nitrogen atoms. The distance is longest for the central Hg–N_por bond (Hg–N, 2.460(8) Å) and shorter for the other two flanking Hg–N_por distances for 2·Hg. However, in the case of 3·Hg, this distance is shortest for the central Hg–N_por bond (Hg–N, 2.228(18) Å and 2.124(17) Å) and longer for the other flanking Hg–N_por distances, which are also similar in the case of 1·Hg as reported earlier. The Hg–Cl distances are 2.310(3) Å for 2·Hg and 2.331(5) and 2.336(6) Å for 3·Hg, directly opposite to the longest and shortest Hg–N_por bonds, respectively. In the case of 1·Hg, the Hg–Cl bond was also directed to the opposite of the shortest Hg–N_por bond, producing N–Hg–Cl bond angles of 159.5(2) and 156.7(2)° for Hg1 and Hg3, respectively.

It is known that mercury(II), having a larger size,^9,10 can't fit well into the porphyrinato core and, therefore, sits 0.64 Å above the mean plane of the porphyrin macrocycle in the monomeric complex, Hg(p-CN)₄tpp.^10b Interestingly, the metal displacements are much larger in the double sandwich complexes reported here. For example, Hg1 and Hg3 are displaced by 1.31 and 1.36 Å for 2·Hg and 1.31 and 1.35 Å for 3·Hg. Hg2 is also displaced by 1.74 and 1.67 Å for 2·Hg and 1.73 and 1.68 Å for 3·Hg. The values are also similar to those for 1·Hg, where Hg1 and Hg3 were displaced by 1.44 and 1.30 Å and Hg2 by 1.78 and 1.63 Å.

For strong π–π interaction, two porphyrin macrocycles should be in a cofacial arrangement but slipped.¹² The average mean plane separations between the porphyrin macrocycles are 3.40, 3.42, and 3.43 Å for 1·Hg, 2·Hg, and 3·Hg, respectively. It is interesting to note here that the two porphyrin rings are coplanar and placed on the top of each other in the double sandwich structure of the complexes reported. Two rings are in a fully eclipsed conformation in 1·Hg and 2·Hg, with twist angles of 3.37° and 2.9°, respectively. These are the cases where two rings of the porphyrin dimer are connected either with a urea-bridge or with an ethene-bridge having only a limited horizontal flexibility. Interestingly, with the highly flexible ethane-bridged porphyrin dimer in 3·Hg, the rings are slightly twisted with an angle of 19.7°.

The van der Waals radius for the mercury(II) ion is around 1.75 Å and, thus, a Hg⋯Hg distance lower than 3.50 Å is associated with mercurophilic interaction.^4,10 It has also been reported^4f that the Hg⋯Hg distance further moves to higher values when the coordination number of mercury increases from two to a larger value.^4f In the present work, Hg1⋯Hg2 and Hg2⋯Hg3 distances are 3.1251(11) Å and 3.1333(16) Å for 3·Hg, 3.1776(4) Å and 3.1776(4) Å for 2·Hg, and 3.2056(5) Å and 3.1911(5) Å for 1·Hg, which, therefore, reflect very strong mercurophilic interactions in the complexes with a higher coordination number of Hg. The Hg⋯Hg distance progressively decreases in the order 1·Hg < 2·Hg < 3·Hg, which demonstrates that the more flexible ethane-bridged porphyrin dimer produces the strongest mercurophilic interaction, followed by the ethene-bridged porphyrin dimer, while it is weakest for the urea-bridged porphyrin dimer. The Hg1–Hg2–Hg3 angles are nearly linear at 176.41(2)°, 180.0° and 175.07(4) Å for 1·Hg, 2·Hg, and 3·Hg, respectively. Interestingly, the absorption and photoluminescence properties of the complexes also display direct correlation with the strength of the Hg⋯Hg interactions (vide infra).

Two strong mercurophilic interactions bring two porphyrin macrocycles on the exact top of each other with eclipsed geometries in the complexes, which are, otherwise, unstable. The closeness between the two porphyrin rings in the complexes is promoted further by the “outward” direction of the peripheral ethyl moieties as observed in their X-ray structures. Interestingly, out of all the three complexes, the strongest mercurophilic interactions (shortest Hg⋯Hg distance) are observed in 3·Hg, for which the highly flexible ethane-bridged porphyrin dimer is used.

¹H NMR spectroscopy

¹H NMR spectra of the complexes provide deeper insights into their double sandwich structure in solution. Sharp signals, characteristic of diamagnetic complexes, were observed as demonstrated in Fig. 3.

Fig. 3 ¹H NMR spectra (in CDCl₃ at 298 K) of (A) 3·Hg, (B) 2·Hg and (C) 1·Hg. The insets display the expanded regions of the –CH₂ protons.

Only one set of signals is observed in the ¹H NMR spectrum of the complex, which confirms that the two terminal porphyrin units are indeed equivalent, as observed in the X-ray structure. 1·Hg displays a bridging –NH signal at 9.27 ppm, while 2·Hg and 3·Hg show bridging –CH and –CH₂ signals at 4.2 ppm and 5.6 ppm, respectively (Fig. 3). All three complexes display four –CH₂ resonances between 3.5 and 4.2 ppm, four –CH₃ proton signals between 1.5 and 2.0 ppm and two meso signals in a 2 : 1 intensity ratio between 8 and 9 ppm. For example, the ¹H NMR spectrum of 3·Hg showed –CH₃ peaks ranging from 1.49 ppm to 1.84 ppm appearing as four sharp triplet signals due to spin–spin coupling with adjacent –CH₂ protons (J = 7.5 Hz). Interestingly, the –CH₂ protons ranging from 3.55 to 4.17 ppm exhibited four sets of multiplets with coupling constants (J) ranging from 2 to 8 Hz; the presence of more than one type of spin–spin coupling is clearly evident in Fig. 4. Notably, alongside the expected vicinal coupling (J = 7.5 Hz) with the –CH₃ group and geminal coupling arising from inequivalent diastereotopic –CH₂ protons, additional splitting (J = 1.5–3 Hz) was also observed. This fine splitting likely arises from the long-range coupling¹⁸ with the bridging –CH₂ protons, enabled by the close spatial arrangement of porphyrin units within the trinuclear double-sandwich framework. Such a splitting pattern has not been reported previously for other metal complexes of ethane-bridged porphyrin dimers, underscoring the unique structural arrangement of the complexes.

Fig. 4 Selected ¹H NMR spectra of 3·Hg in CDCl₃ at 298 K: (A) –CH₃ and (B) –CH₂ proton resonances. Various coupling constant values are shown in the figure. (C) A selected ¹H–¹H NOESY NMR spectrum of 3·Hg in CDCl₃ at 298 K.

The origin of these complex splitting patterns observed for the –CH₂ protons in 3·Hg has been confirmed by detailed ²D NMR (¹H–¹H NOESY and COSY) spectroscopic investigations as well as homo-decoupled experiments (Fig. 4 and S4–S6). The ¹H–¹H NOESY of 3·Hg clearly demonstrated through-space interactions between the –CH₂ and bridging –CH₂ protons (Fig. 4 and S4).

Consistent cross-peaks were also observed in the ¹H–¹H NOESY spectrum of 2·Hg, corroborating the presence of similar spin–spin splitting observed in its ¹H NMR spectrum (Fig. S7 and S8). ¹³C NMR spectra of 3·Hg (Fig. S9) and 2·Hg (Fig. S10) are also found to be similar to the DFT calculated spectra of the respective complexes. These observations strongly suggest that the solid-state structures are preserved in solution.

¹⁹⁹Hg NMR spectroscopy

¹⁹⁹Hg NMR is a sensitive indicator for probing the Hg(II) coordination environment in solution.¹⁹ The comparative ¹⁹⁹Hg NMR spectra of all three complexes are shown in Fig. 5, while Table 2 compares the NMR peak positions of the complexes. There are two types of Hg(II) centers in each of the complexes. 1·Hg displays two sharp signals at −1911.2 (for Hg2) and −1925.5 ppm (for Hg1 and Hg3) in 1 : 2 intensity ratios. Similarly, signals at −1911.1 (Hg2) and −1925.3 ppm (Hg1 and Hg3) are observed for 2·Hg and −1923.3 (Hg2) and −1937.7 ppm (Hg1 and Hg3) are observed in 3·Hg.

Fig. 5 ¹⁹⁹Hg NMR spectra (in CDCl₃ at 298 K) of (A) 3·Hg, (B) 1·Hg and (C) 2·Hg.

Table 2 Selected structural, absorption, and ¹⁹⁹Hg NMR spectral data

Complex	X-ray structural parameters		UV-vis spectral data^a			^199Hg NMR^b
	Hg⋯Hg (Å)	MPS (Å)	MMLCT band (nm)	Soret band (nm)	Q bands (nm)	δ (ppm)
a In dichloromethane at 298 K. b In CDCl3 at 298 K.
1·Hg	3.1911(5), 3.2056(5)	3.40	339	420	538, 572	−1911.2, −1925.5
2·Hg	3.1776(4), 3.1776(4)	3.42	343	428	560, 594	−1911.1, −1925.3
3·Hg	3.1251(11), 3.1333(16)	3.43	344	432	561, 595	−1923.3, −1937.7

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There is no change in the NMR shift positions upon going from 1·Hg to 2·Hg. However, as the Hg⋯Hg distance decreases further for 3·Hg, the peaks are upfield shifted to −1923.3 (for Hg2) and −1937.7 ppm (for Hg1 and Hg3). The NMR shifts have also been found to be dependent on the Hg–Cl_ax bond lengths. In 3·Hg, Hg–Cl distances are longer as compared to 1·Hg and 2·Hg. Hence, there is a net accumulation of electron density and subsequent deshielding on the mercury atoms, which might be responsible for the upfield shift.

Fluorescence spectroscopy

All the complexes are highly fluorescent in their methanolic solutions (Fig. 6, S11, S12 and Table S2), but not in the solid state. 1·Hg and 2·Hg exhibit bright red emission under UV light while 3·Hg exhibits an intense blue emission (Fig. 6 and S13–S15). Fluorescence spectra with excitation wavelength λ_ex = 350 nm exhibit two types of feature at 298 K: a structured emission with three sharp bands (with maxima at 580, 630 and 690 nm for 1·Hg and 2·Hg and 576, 628 and 675 nm for 3·Hg) and a minor, broad band at 455 nm for 1·Hg and 2·Hg.

Fig. 6 Normalized spectra at (A–C) 298 K and (D–F) 77 K of 1·Hg (A and D), 2·Hg (B and E), and 3·Hg (C and F) in methanol. Absorption and emission (λ_ex = 350 nm) are in black and red, respectively. Excitation spectra in blue are for λ_em = 690 nm (A, B, D, E), 675 nm (C), and 550 nm (F). Those in green are for λ_em = 455 nm (A–C), 525 nm (D) and 538 nm (E).

The excitation spectra recorded with emission wavelength λ_em = 455 nm are superimposable with the absorption band with a maximum (λ^max_abs) at 340 nm for 1·Hg and at 347 nm for 2·Hg. For 3·Hg, the broad, blue shifted band is the major one and the excitation spectrum for λ_em = 455 is superimposable with the 350 nm absorption band (Fig. 6). Hence, it is inferred that the emission bands with λ^max_em = 455 nm arise due to the species with λ^max_abs = 350 nm. This is an extra feature that arises in the Hg complexes only and becomes more prominent as the Hg⋯Hg distance decreases. The structured emission, on the other hand, is similar to the emission spectra in the corresponding free base ligands (Fig. S15). At 77 K, the 455 nm feature becomes predominant in 2·Hg and more so in 3·Hg, possibly because of enhanced mercurophilic interactions at low temperature. The absolute fluorescence quantum yields (Φ_F) (Fig. S11 and S12) were recorded in the solution state. The Φ_F of 2·Hg and 3·Hg in methanol were determined to be 10.94% and 15.76%, respectively.

Fluorescence decays at the blue end of the emission spectra are faster than those at the red end for all the complexes at both 298 K and 77 K (Fig. 7). Since the emission due to mercurophilic interactions occurs at the blue end, this observation implies that the emission associated with such interactions is faster than that for the ligand emission. Considering no other emissive species to be involved, such a situation is best described by a two-component global analysis²⁰ for the fluorescence decays of each complex, in which the shorter component is due to the mercurophilic interaction and the longer one is due to the ligand (porphyrin ring) emission. Interestingly, such a global analysis yields the same values (ca. 1.5 ns at room temperature and ca. 2.5 ns at 77 K, Table 3) for the shorter component, for all three complexes, even though the analysis is performed independently for each. Hence, this omnipresent component is assigned to the mercurophilic interactions. The long components are different for the different complexes and are attributed to the ligands bound to the metal ions.

Fig. 7 Fluorescence decays for mercurophilic emission maxima (green) and for ligand emission maxima (pink) of (A) 1·Hg, (B) 2·Hg and (C) 3·Hg in methanol at 298 K and (D) 1·Hg, (E) 2·Hg, and (F) 3·Hg in methanol at 77 K.

Table 3 Fluorescence decay parameters^a of mercury complexes obtained from the global analysis of TCSPC data. λ_ex = 365 nm, at 298 K and 77 K

Complex	298 K				77 K
	λ em	a 1	τ 1	τ 2	λ em	a 1	τ 1	τ 2
a λ em (nm), τ (ns).
1·Hg	455	0.98	1.52	11.10	525	0.97	2.46	21.34
	630	0.05	1.52	11.10	625	0.23	2.46	21.34
2·Hg	455	0.93	1.49	8.27	538	0.93	2.89	13.59
	630	0.39	1.49	8.27	625	0.78	2.89	13.59
3·Hg	455	0.96	1.49	10.40	550	0.92	2.46	12.26
	630	0.23	1.49	10.40	625	0.69	2.46	12.26

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TD-DFT and DFT calculations

A series of density functional theory (DFT) calculations was performed to gain deeper insights into the nature of the mercurophilic interactions and the intriguing stabilization observed in the double sandwich complexes. The geometries of 1·Hg, 2·Hg, and 3·Hg were optimized (Fig. 8), and the results closely matched with the geometric parameters observed in their X-ray structures. This confirmation underscores the reliability of the computational method in accurately representing the intricate structures of these complexes. Building upon the structural insights provided by the DFT calculations, time-dependent DFT (TD-DFT) calculations were performed to simulate and analyze the UV-vis spectra of the complexes, and the results are tabulated in Tables S3 and S4. The computational results successfully reproduced the experimental spectra (Fig. 9), further validating the accuracy and reliability of the theoretical approach. Specifically, the origin of the MMLCT band¹⁷ from the calculated absorption bands at 343 and 344 nm correspond to transitions from HOMO−5 to LUMO (f = 0.4061) (Fig. 9), and from HOMO−4 to LUMO+1 (f = 0.4556) (Fig. S16) for 2·Hg and 3·Hg, respectively. In the case of 1·Hg, the calculated absorption band at 339 nm corresponds to transitions from HOMO−11 to LUMO (oscillator strength f = 0.1284). These computational findings provide a comprehensive understanding of the optical properties and their correlation with the unique electronic features of these double sandwich complexes.

Fig. 8 B3LYP/D3BJ optimized geometry of (A) 1·Hg, (B) 2·Hg and (C) 3·Hg with bond lengths in Å as calculated using the SARC-ZORA-TZVP basis set for Hg atoms, “ZORA-def2-TZVP(-f)” for nitrogen and chlorine atoms and “def2-SVP” for all other atoms. Parentheses contain the experimental values obtained from the X-ray structure.

Fig. 9 The absorption spectrum (curved line, left axis) of 2·Hg in methanol and oscillator strengths (vertical lines, right axis) obtained from TD-DFT calculations at the cam-B3LYP/6-31G**/SDD level of theory.

To obtain more information related to the existence of mercurophilic interactions in these double sandwich structures, 3·Hg has been optimized with and without dispersion and also without the ethane-bridge in the molecule (Fig. 10 and Table S5). Although the gross structure remains the same, large changes in the structure and geometrical parameters are found. This is clearly reflected in the Hg⋯Hg distances obtained in the DFT optimizations: 3.14 Å and 3.18 Å (with dispersion), 3.46 Å and 3.37 Å (without dispersion) and 3.48 Å and 3.50 Å (without dispersion and also without the ethane-bridge). It can be seen that the Hg⋯Hg distances increase significantly in the absence of dispersion. In addition, there is also a large increase in the distance between two terminal Hg atoms with and without the dispersion effect (from 6.32 Å to 6.80 Å to 6.99 Å, respectively, Fig. 10). Our DFT calculations thus demonstrate that the mercurophilic interaction is evident when relativistic and dispersion effects are included and the distances are also in excellent agreement with the X-ray structure of the complex. Additionally, the absence of the ethane bridge leads to increased Hg⋯Hg distances, further confirming the role of the bridge in facilitating metallophilic interactions.

Fig. 10 B3LYP-optimized geometries of 3·Hg under different computational conditions: (A) including both dispersion (D3BJ) and relativistic (ZORA) effects; (B) excluding both dispersion and relativistic effects; and (C) excluding dispersion and relativistic effects as well as the ethane bridge. Bond lengths are given in Å, with experimental values from the X-ray structure shown in parentheses. The twist angle (φ) is an average of the four N–Hg1–Hg3–N′ dihedral angles.

The strong Hg(II)⋯Hg(II) interaction, a key characteristic of these systems, is primarily attributed to relativistic effects and dispersive forces. Besides these, the ligand-induced coordination/electronic effects also promote a linear arrangement of the three Hg atoms, a structural motif corroborated by the X-ray crystallographic data (Fig. 2). These interactions play a crucial role in stabilizing the double sandwich structure with a linear Hg⋯Hg⋯Hg unit, overcoming the inherent instability arising from two cofacial aromatic porphyrin rings on top of each other but not slipped.¹²

To gain deeper insights into the underpinning mercurophilic interactions in these complexes, a detailed analysis was performed using natural bond orbital (NBO) perturbation theory.²¹ This approach enabled the evaluation of interactions between the filled (donor) Lewis-type NBOs and the empty (acceptor) non-Lewis NBOs corresponding to Hg atoms. Consistent with previous findings for 1·Hg, the NBO analysis revealed that Hg1 and Hg3 serve as electron donors, while Hg2 functions as the central electron acceptor in both 2·Hg (Fig. 11) and 3·Hg (Fig. S17). The stabilization energy ΔE for Hg1⋯Hg2 and Hg2⋯Hg3 interactions, respectively, increases from 4.20 and 5.16 kcal mol⁻¹ for 1·Hg to 4.47 and 5.39 kcal mol⁻¹ for 2·Hg and 4.63 and 5.67 kcal⁻¹ mol⁻¹ for 3·Hg (Table 4). Similarly, the Wiberg bond order values, which quantify the bond strength, follow the same trend. For 1·Hg, the bond orders were calculated to be 0.11 (Hg1⋯Hg2) and 0.13 (Hg2⋯Hg3), while for 2·Hg and 3·Hg, the respective values were found to be 0.12 and 0.14. This increase in both stabilization energies and bond orders is directly correlated with the decreasing Hg⋯Hg distances observed in the respective X-ray structures, which follow the trend 1·Hg > 2·Hg > 3·Hg.

Fig. 11 LP(Hg) → LP*(Hg) donor–acceptor orbital interactions derived from the NBO analysis of 2·Hg. Hydrogen atoms are omitted for clarity (isovalue = 0.03).

Table 4 Hg⋯Hg distances and stabilization energies (ΔE) obtained from NBO analysis

Complex	Hg⋯Hg (Å)	ΔE (kcal mol^−1) [Lp(5dz^2)Hg(1) → σ*Hg(2)]	ΔE (kcal mol^−1) [Lp(5dz^2)Hg(3) → σ*Hg(2)]	Ref.
1·Hg	3.1911(5), 3.2056(5)	4.20	5.16	11
2·Hg	3.1776(4), 3.1776(4)	4.47	5.39	tw
3·Hg	3.1251(11), 3.1333(16)	4.63	5.67	tw

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Complex 1·Hg exhibits the highest dipole moment (6.615 D), complex 2·Hg shows an intermediate value (1.966 D), and complex 3·Hg has the lowest dipole moment (0.662 D), consistent with a more symmetric charge distribution and enhanced intrinsic stability. Additional computational tests support this interpretation: omission of dispersion and relativistic corrections increases the dipole moment of 3·Hg slightly (0.752 D), while removal of the ethane bridge decreases it (0.367 D), indicating that these factors influence the degree of electronic asymmetry. Overall, the results demonstrate that lower dipole moments correlate with greater intrinsic stability in this system, establishing the relative order of stability as 3·Hg > 2·Hg > 1·Hg.

To complement the NBO analysis, a topological examination of the electron density was conducted using Bader's quantum theory of atoms in molecules (QTAIM) method for 2·Hg and 3·Hg.²² This analysis revealed distinct bond paths and bond critical points (BCPs), that are commonly recognized as key indicators of mercurophilic interactions. The electron densities at the bond critical points were found to be 0.01724 and 0.01723 a.u. for 2·Hg, which was similar to those calculated for 1·Hg, but the values increased to 0.01916 and 0.02114 for 3·Hg (Fig. 12). Moreover, some other topological parameters were found to increase in the order 1·Hg < 2·Hg < 3·Hg, which is again consistent with decreasing Hg⋯Hg distances. The small value of ρ(r) and positive sign of ∇²ρ(r) at the BCPs along the Hg⋯Hg bond path (Table 5) visibly indicate the presence of significant mercurophilic interactions which increase in the order: 1·Hg < 2·Hg < 3·Hg (Fig. S19).

Fig. 12 Views of the computed electron density maps of (A) 1·Hg, (B) 2·Hg, and (C) 3·Hg. The paths (green lines) and bond critical points (blue circles) are also shown.

Table 5 Topological parameters of electron density: electron density ρ(r), its Laplacian distribution ∇²ρ(r), potential energy density V(r), kinetic energy density G(r), and electronic energy density H(r) at the bond critical points (3, −1), corresponding to Hg⋯Hg interactions and energies for these contacts E_int (kcal mol⁻¹), defined by two approaches. The parameters are all in atomic units (a.u)

Complex	Atom pair	ρ(r)	∇^2ρ(r)	V(r)	G(r)	H(r)	E int ^a	E int ^b	Ref.
a E int = −V(r)/2. b E int = 0.429G(r).
1·Hg	Hg1⋯Hg2	0.01728	0.05856	−0.01259	0.01361	0.00102	3.95	3.66	11
	Hg2⋯Hg3	0.01757	0.06041	−0.01300	0.01405	0.00105	4.17	3.78
2·Hg	Hg1⋯Hg2	0.01724	0.06207	−0.01288	0.01415	0.00136	4.72	3.92	tw
	Hg2⋯Hg3	0.01723	0.06223	−0.01281	0.01418	0.00137	4.75	4.05
3·Hg	Hg1⋯Hg2	0.01916	0.06207	−0.01657	0.01596	0.00796	5.19	4.29	tw
	Hg2⋯Hg3	0.02114	0.06223	−0.01703	0.01689	0.00801	5.34	4.54

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These findings reinforce the conclusion that the strength and nature of these interactions evolve predictably across the series, with 3·Hg displaying the strongest interactions among the three complexes (Fig. S18). Together, these results from NBO and QTAIM analyses provide a coherent explanation of the electronic and topological factors that stabilize these remarkable complexes, offering a nuanced understanding of the mercurophilic interactions driving their structural and energetic trends.

Conclusions

We present here the structures and properties of a series of stable trinuclear “double sandwich” complexes having linear Hg₃ cores. In the structure, two porphyrin rings are coplanar and placed on top of each other. Two strong mercurophilic interactions bring two porphyrin macrocycles exactly on top of each other with eclipsed geometries in the complexes, which would otherwise produce highly unstable arrangements. Interestingly, out of all three complexes, the strongest mercurophilic interactions (shortest Hg⋯Hg distance) are observed in 3·Hg, for which a highly flexible ethane-bridged porphyrin dimer is used. Extensive DFT calculations along with NBO and QTAIM analyses provide a coherent explanation of the electronic and topological factors that stabilize these remarkable complexes, offering insights into the mercurophilic interactions that drive their structural and energetic trends. All the complexes are highly fluorescent in their methanolic solutions. 1·Hg and 2·Hg exhibit bright red emission under UV light while 3·Hg exhibits intense blue emission. The mercurophilic interactions increase (with decreasing Hg⋯Hg distances) in the order 1·Hg < 2·Hg < 3·Hg. Our investigation clearly demonstrates that the absorption (MMLCT band: ∼350 nm) and photoluminescence properties of the trinuclear sandwich complexes are directly correlated with the strength of the Hg⋯Hg interactions.

The fluorescence spectra exhibited two types of feature upon excitation at 350 nm at 298 K: a structured emission with three sharp bands (with maxima at 580, 630 and 690 nm for 1·Hg and 2·Hg and 576, 628 and 675 nm for 3·Hg) and a minor, broad band at 455 nm and 463 nm for 1·Hg and 2·Hg, respectively. Fluorescence decays at the blue end of the emission spectra are faster than those at the red end for all the complexes at both 298 K and 77 K. Since the emission due to mercurophilic interactions occurs at the blue end, this observation implies that the emission decay associated with such interactions is faster than that for ligand emission. A global analysis yields similar values (ca. 1.5 ns at room temperature and ca. 2.5 ns at 77 K) for the shorter component for all three complexes, even though the analysis is performed independently for each. Hence, this omnipresent component is assigned to the mercurophilic interactions.

Author contributions

SPR conceptualized and supervised the work. AS, DC, MNB, AKP, ND and CP performed the experiments. AS, AKP and MU carried out the DFT calculations. AS, MNB and AD analysed the photophysical properties. All the authors analysed the results together and wrote the manuscript.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information: text, figures, and tables depicting detailed experimental procedures and characterisation data including UV-vis, X-ray crystallography and DFT. See DOI: https://doi.org/10.1039/d5qi01593f.

CCDC 2418712 (2·Hg) and 1992243 (3·Hg) contain the supplementary crystallographic data for this paper.^23a,b

Acknowledgements

We thank the Science and Engineering Research Board (SERB), India and J. C. Bose Grant from Anusandhan National Research Foundation (ANRF), New Delhi for financial support.

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Footnote

† Dedicated to Prof. Dr. Franc Meyer on the occasion of his 60th birthday.

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