Isolation of mixed valence charge-neutral Ag₁₂, and dicationic Ag₁₀ nano-clusters stabilized by carbene-phosphaalkenides

Abstract

Cyclic alkyl(amino) carbene (cAAC)-supported phosphaalkenides (cAAC=P)⁻ have been employed as ligands for the isolation of two atomically precise mixed valence paramagnetic Ag^I/0₁₂Cl₃, and Ag^I/0₁₀, nano-clusters [(Me₂-cAAC=P)₆Ag₁₂Cl₃] (2), and [(Me₂-cAAC=P)₆Ag₁₀](NTf₂)₂ (4). 2 and 4 have been structurally characterized by single-crystal X-ray diffraction revealing the presence of three Ag⁰ atoms, nine Ag^I ions (2); and two Ag⁰ atoms, eight Ag^I ions (4), respectively. The clustering inorganic unit Ag₁₂Cl₃ in 2 has been found to be surrounded by six mono-anionic μ₃-cAAC=P moieties having 3-bar symmetry. 2 and 4 have been studied by cyclic voltammetry, UV-vis, ESI-MS, XPS, EPR spectroscopy, and DFT calculations.

---

Silver nanoclusters (NCs) are known to exhibit unique electronic, and optical properties, such as strong photoluminescence, and various catalytic activities.¹ They are utilised as attractive luminescent probes for sensing and bioimaging.² Structurally well-defined NCs possessing Ag⁰ atoms are highly reactive, and easier to oxidize compared to their heavier Au analogues, which makes their synthesis challenging. Metallic silver is non-magnetic in its bulk form. However, Pereiro et al. theoretically predicted that the small nuclearity silver NCs, Ag_n, could be magnetic with variation of the magnetic moment depending on the value of n [n = 2–22].³ Liu et al. reported the isolation of the chiral open-shell Ag₂₃ NC with five Ag⁰ atoms, which was structurally characterized by single-crystal X-ray diffraction.⁴ The open-shell behaviour of this Ag₂₃ NC was confirmed by a weak electron paramagnetic resonance (EPR) signal, characteristic of s  =  1/2, with g  =  1.959 and 1.955.⁴ The first atomically precise mixed valence Ag^0/I₂₂ NC displaying thermally activated delayed fluorescence (TADF) was reported in 2022 by Sun and co-workers.⁵ In the same year, two other mixed valence NCs Ag^0/I₈ and Ag^0/I₂₉ exhibiting strong EPR signals were reported.⁶ The heavily Ag⁰-doped KCl:AgCl crystals in different chemical defect positions [cationic and anionic holes] were studied by EPR spectroscopy.^7a The generation of Ag₄³⁺, and Ag₆⁺ clusters was also reported under different chemical conditions.^7b,c However, the mixed valence silver NCs are rarely reported.⁸ The first closed-shell mixed valence Ag^I/III₂ [Ag⋯Ag 7.4 Å] containing N₈-donor macrocyclic ligand was isolated by Qin-Hui et al. in 1994.^9a The 1D zig-zag chain of Ag^I/II₂ (ref. 9b) stabilized by the cyclic alkyl(amino) carbene (cAAC)-supported mono-anion of the inversely polarized phosphaalkene, viz., the phosphaalkenide¹⁰ was reported by Roy and co-workers. N-heterocyclic carbene (NHC), and P-SiMe₃ ligated diamagnetic Ag^I₁₂, and Ag^I₂₆ clusters were reported by Corrigan et al.¹¹ A plethora of other Ag^I clusters have been synthesized, and characterized employing alkyne, thiolate, sulfide, selenide, phosphine, perchlorate, etc. as ligands.¹² Moreover, clusters with Ag–H moieties have also been isolated, and further studied by mass spectrometry.¹³ However, isolation of stable silver NCs introducing carbene-phosphaalkenides as ligands is still scarce. Herein, we report on the solid-state isolation, and structural characterization of two novel structurally well-defined mixed valence paramagnetic silver NCs with Ag^I/0₁₂Cl₃, and Ag^I/0₁₀ metallic cores.

The dark red crystals of Me₂-cAAC=P–K (1)¹⁴ (Me₂-cAAC=C(N-2,6-ⁱPr₂C₆H₃)(CMe₂)₂(CH₂)) were reacted with AgNTf₂ in a 3 : 2 molar ratio in toluene at 0 °C to room temperature (rt) for 12 h to obtain a dark brown-red reaction mixture. Upon drying, the resultant crystalline solid obtained was dissolved in freshly distilled DCM, and stored for crystallization upon concentration to ∼1 mL in a freezer at −40 °C. After one-week, dark red blocks of [(Me₂-cAAC=P)₆(Ag)₁₂(Cl)₃] (2) were isolated in 40% yield (Scheme 1). The presence of the three chloride ions in 2 can be attributed to the decomposition of DCM, the usage of which is found to be crucial for the isolation of 2. A different mixed valence dicationic cluster [(Me₂-cAAC=P)₆(Ag)₁₀](NTf₂)₂ (4) was isolated in 60% yield as yellow blocks when [Me₂-cAAC=P–B(N(ⁱPr)₂)₂] (3)^9b was reacted with AgNTf₂ in 2 : 1 molar ratio in toluene at rt for 12 h, followed by crystallization of the resultant precipitate from concentrated DCM solution, stored at 0 °C in a refrigerator (Scheme 2). The (cAAC=P)⁻ ligands are found to be redox non-innocent, and undergo oxidation to produce the corresponding radical followed by dimerization, resulting in the formation of (Me₂-cAAC)₂P₂ with a subsequent reduction of Ag^I to Ag⁰ in solution affording the mixed valence Ag-NCs 2 and 4.⁶ The presence of Ag⁰ and Ag^I centres in 2 and 4 is supported by XPS spectroscopy (see ESI†). The formation of (Me₂-cAAC)₂P₂ in the reaction mixture is confirmed by ³¹P NMR spectroscopy (δ = 55.1 ppm).

Scheme 1 Synthesis of mixed-valence neutral NC [(Me₂-cAAC=P)₆Ag₁₂(Cl₃)] (2).

Scheme 2 Synthesis of mixed-valence dicationic NC [(Me₂-cAAC=P)₆Ag₁₀](NTf₂)₂ (4).

The red/yellow blocks of 2/4 are found to be stable under an argon atmosphere for six months inside a glove box at rt. The powders of 2 and 4 decompose above 205 and 165 °C, respectively. 2 and 4 are found to be NMR silent, and EPR active.

2 and 4 have been structurally characterized by single-crystal X-ray diffraction. The dodeca-nuclear Ag-NC [(Me₂-cAAC=P)₆(Ag)₁₂(Cl)₃] (2) crystallizes in the trigonal R3̄ space group (Fig. 1). 2 comprises six (Me₂-cAAC=P)⁻ ligands, three chloride ions, and twelve Ag-atoms/ions. Fig. 2 shows that the asymmetric unit of 2 starting at position-1, generates six different symmetry equiv. points (1–6). Positions 1, 3 and 5 are non-inverted, while positions 2, 4 and 6 are three inverted-symmetry equiv. positions. There are three Ag⁰ atoms in 2, which is suggested from charge balance consideration, since there are a total of nine mono-anionic ligands present.

Fig. 1 Molecular structure of NC [(Me₂-cAAC=P)₆(Ag)₁₂(Cl₃)] (2). Hydrogen atoms are omitted for clarity.

Fig. 2 Structural topology and symmetry [3-bar] in NC 2.

The space filling model of 2 has been shown in Fig. 3 (left), which represents how the central Ag₁₂ metallic core is well protected by the surrounding cAAC-supported phosphaalkenide ligands. The natural bond orbital (NBO) analyses of NCs 2 and 4 at neutral doublet, and dicationic triplet states, respectively, were performed at the UBP86-D3(BJ)/Def2-SVP level of theory (see ESI† for details). The NBO analysis of 2 revealed that the α-LUMO+1, α-LUMO, and α-SOMO are very close in energy (ΔE = 0.01–0.05 eV) suggesting that the unpaired electron in 2 can span all these three orbitals involving the C=N–P unit, Cl and Ag atoms (see ESI†). This is also reflected in the Mulliken α-spin densities of 2 showing delocalization of spin densities due to the unpaired electron (Fig. 3 (right), see ESI†).

Fig. 3 Space filling model (left), and Mulliken α-spin densities (right) of [(Me₂-cAAC=P)₆Ag₁₂Cl₃] (2).

The Ag–Ag distances of charge-neutral NC 2 are found to be 2.883(1), and 3.024(2) Å, which are significantly shorter than those of the previously isolated tri-cationic closed-shell Ag NC 2³⁺(OTf⁻)₃ (2.9593(14), 2.9174 (12), 3.2026(15), 3.1541(13) Å).⁶ The Ag–P distances of 2 are 2.379(4), 2.401(3), and 2.455(2) Å, which are close to those of 2³⁺(OTf⁻)₃ (Ag–P 2.393(4)–2.431(4) Å).⁶ The C–P bond in 2 is 1.75(1) Å, which is slightly shorter than those of 2³⁺(OTf⁻)₃ (1.780(11)–1.801(11) Å).⁶ The Ag–Cl distances in 2 are found to be 2.4530(14), and 2.572(7) Å, which are either very similar in value or significantly smaller than those of 2³⁺ (2.475(9), 2.705(5), 2.638 (4), 2.7143(11), 2.732(4), 2.8254(10) Å).⁶ The diameter of the outer/peripheral Ag₆ ring of 2 having a three-fold symmetry is 8.37 Å, which is very close to those of 2³⁺ with two-fold symmetry (8.26, 8.36, 8.48 Å).⁶ The Cl–Cl distance between the two extreme Cl-ions in 2 is 5.8 Å, which is shorter than that of 2³⁺ (6.5 Å).⁶ The P–P distance in 2 is 9.02 Å, which is slightly greater than the average value of 2³⁺ (8.46 Å; 8.26, 8.36, 4.48, 8.88, 9.22 Å).⁶2 represents the first example of an Ag^I/0 based mixed valence NC, which has been structurally characterized in two different charged (0 (2) vs. +3 (2³⁺)], and spin states [doublet (2) vs. singlet (2³⁺)).⁶

The dicationic deca-nuclear Ag NC [(Me₂-cAAC=P)₆Ag₁₀](NTf₂)₂ (4) crystallizes in triclinic space group P1̄ (Fig. 4). The entire molecule of 4 appears in the crystallographic asymmetric unit, which possesses six mono-anionic ligands (Me₂-cAAC=P)⁻, and ten Ag-atoms/ions. There are two non-coordinating/free mono-anionic NTf₂⁻ anions present for the electrical charge balance. The charge balance consideration suggests that there are two Ag⁰ atoms in 4. The four arms of the central Ag₄ unit of 4 have been bridged by four P-atoms of mono-anionic Me₂-cAAC=P⁻ ligands producing a (Me₂-cAAC=P)₄Ag₄ unit. Two Ag₂ units have been placed in anti-fashion above and below the irregular square-like Ag₄ unit (Fig. 4 and 5).

Fig. 4 Molecular structure of NC [(Me₂-cAAC=P)₆Ag₁₀](NTf₂)₂ (4). Hydrogen atoms are omitted for clarity. Two triflimide anions [N(SO₂CF₃)₂⁻] are omitted for clarity.

Fig. 5 Core topology of [(Me₂-cAAC=P)₆Ag₁₀](NTf₂)₂ (4). Two Ag₆ square prisms sharing the blue colored pseudo-Ag₄-square with a 4-bar symmetry operation.

An additional Ag-atom is present above the Ag₂ unit (Fig. 4 and 5), while another Ag atom is bridged by a μ₃-P atom (right, Fig. 5). The Ag–Ag distances of the central four-membered Ag₄ ring in 4 are 2.90, 3.0, and 3.20 Å, which are longer than those (∼2.96 Å) of the Ag^0/I₈ complex.⁶

The Ag₂ unit, which is situated on the top of the central Ag₄ unit, possesses the Ag–Ag distance of 2.91 Å, which is significantly shorter than that of the Ag₃ triangle of 4 (3.04 Å). The corresponding Ag–Ag distance in previously reported Ag^0/I₈ complexes is 3.07 Å.⁶ Notably, the distal Ag atom (Ag9) (Ag7–Ag9, Ag8–Ag9; 2.472(3), 2.458(3) Å) is significantly closer to the Ag₂ arm (Ag8–Ag9), which is situated above the central Ag₄ unit of 4. The Ag–P bond lengths of NC Ag^0/I₁₀ (4) range from 2.23 Å to 2.48 Å, and are significantly different than those of the neutral Ag₈ cluster (2.404(5)–2.417(4) Å).⁶ The space filling diagram of a ten Ag atoms/ions unit of 4 has been shown in Fig. 6 (left). The Ag atom placed above the Ag₂ unit is situated in between the two aromatic rings of the Dipp ligands. The NBO, and Mulliken spin density analyses of 4 (Fig. 6 (right)) showed the major spin densities on the C=N–P unit with very small values on the Ag-atoms except for one Ag-atom (5.6%) and thus typical EPR features of Ag–P systems were not observed⁶ in the experimental EPR spectrum of 4.

Fig. 6 Space filling model of total molecule (left), and Mulliken α-spin densities (right) of [(Me₂-cAAC=P)₆Ag₁₀](NTf₂)₂ (4).

The EPR spectra of 2 and 4 (in DCM) are shown in Fig. 7 and 8. The EPR simulation/fitting of 2 considering coupling of the unpaired electron with the nuclei of ^107,109Ag, ³¹P and ^35,37Cl atoms is quite satisfactory (Fig. 7). Three board lines near g = 2.0033 were observed for the previously reported Ag^0/I₈ cluster containing four Ag⁰ atoms.^6,9b However, the EPR spectrum of 2 shows three sets of multiple hyperfine lines, which have been simulated with EasySpin.

Fig. 7 Experimental (black), and simulated (red) X-band EPR spectra of [(Me₂-cAAC=P)₆(Ag)₁₂(Cl₃)] (2) in DCM at 293 K. EasySpin, simulated parameters: one ^107,109Ag(0) [A = 9.43 MHz], two ³¹P [A = 111.79, 111.75 MHz], one ^35,37Cl [A = 9.81 MHz], g_x = 2.00766, g_y = 2.00885, g_z = 1.99959, LW1 = 0.016 mT, LW2 = 0.166 mT. Experimental frequency = 9.436369 GHz.

Fig. 8 X-band EPR spectrum of the NC [(Me₂-cAAC=P)₆(Ag)₁₀](NTf₂) (4) in DCM at 293 K. Black line represents the experimental spectrum, and the red line represents the simulated spectrum using the EasySpin program [g_x = 2.00165, g_y = 2.00006, g_z = 1.99703, LWPP₁ = 0.0678384 mT, LWPP₂ = 0.0817786 mT, A_Ag = 2.51891 MHz].

The coupling constants of one ^107,109Ag⁰, two ³¹P and one ^35,37Cl nuclei are 9.43, 111.79/111.75, and 9.81 MHz, respectively, with a slight rhombic nature of g [g_x = 2.00766, g_y = 2.00885, g_z = 1.99959]. The EPR spectrum of 4 appears as unsymmetrical showing two resonances around g ≈ 2 [g_x = 2.00165, g_y = 2.00006, g_z = 1.99703] (Fig. 8). The coupling constant of Ag⁰ is 2.51 MHz, which is significantly smaller than that of 2 (9.43 MHz). The ESI-MS studies show that 2 (see ESI†) and 4 can fly as dications. The ESI-MS spectrum of 4²⁺ corroborates the loss of two isopropyl groups with the intake of a sodium ion as [(Me₂cAACP)₆Ag₁₀ + Na–2C₃H₇–H]²⁺. The cyclic voltammetry studies of 2 and 4 suggest the possible oxidation (see ESI†). The UV-vis spectra (in THF at 298 K) of 2 and 4 exhibited the absorption maxima (λ_max) at 365 and 367 nm, respectively (see ESI†). 2 and 4 were observed to be non-emissive.

In conclusion, two novel mixed valence Ag-NCs (Ag₁₀, 2; Ag₁₂, 4) were isolated as red/yellow blocks. 2 and 4 have been structurally characterized by X-ray single-crystal diffraction. The Ag₁₀, and Ag₁₂ NCs possess three and two Ag⁰ atoms, respectively. The NMR silent NCs 2 and 4 were found to be EPR active. The unpaired electrons couple with the ^35,37Cl, ³¹P and ^107,109Ag nuclei. The strongest coupling constant was obtained for ³¹P-nuclei (111.75 MHz) in 2. The coupling constant of ^107,109Ag of 2 (9.43 MHz) is four times stronger than that of 4 (2.51 MHz). The distributions of electron densities in 2 and 4 were estimated by computation of the Mulliken spin densities, and further correlated with ERP simulation.

SR gratefully acknowledges STARS-IISC, MoE (MoE-STARS/STARS-2/2023-0666), IISERT, CSIR for the generous financial support. We thank Dr S. S. Sen (NCL-Pune), and KB for the XPS measurements.

Data availability

The data supporting this article (experimental details, UV-vis, HRMS, EPR, XPS, single-crystal X-ray data, and computational details) have been included as part of the ESI.† Crystallographic data for 2 and 4 have been deposited at the CCDC (2242239, 2194453†), and can be obtained from https://www.ccdc.cam.ac.uk/.

Conflicts of interest

There are no conflicts to declare.

Notes and references

1. (a) C. M. Aikens, J. Phys. Chem. Lett., 2011, 2, 99; (b) Y. Du, H. Sheng, D. Astruc and M. Zhu, Chem. Rev., 2020, 120, 526.
2. (a) T. Udayabhaskararao and T. Pradeep, J. Phys. Chem. Lett., 2013, 4, 1553; (b) K. Zheng, X. Yuan, N. Goswami, Q. Zhang and J. Xie, RSC Adv., 2014, 4, 60581.
3. M. Pereiro, D. Baldomir and J. E. Arias, Phys. Rev. A, 2007, 75, 063204.
4. C. Liu, T. Li, H. Abroshan, Z. Li, C. Zhang, H. J. Kim, G. Li and R. Jin, Nat. Commun., 2018, 9, 744.
5. Z.-R. Yuan, Z. Wang, B.-L. Han, C.-K. Zhang, S.-S. Zhang, Z.-Y. Zhu, J.-H. Yu, T.-D. Li, Y.-Z. Li, C.-H. Tung and D. Sun, Angew. Chem., Int. Ed., 2022, 61, e202211628.
6. E. Nag, S. Battuluri, K. C. Mondal and S. Roy, Chem. – Eur. J., 2022, 28, e202202324.
7. (a) P. G. Baranov, N. G. Romanov, V. A. Khramtsov and V. S. Vikhnin, J. Phys.: Condens. Matter, 2001, 13, 2651; (b) J. Sadło, J. Michalik and L. Kevan, EPR and ESEEM study of silver clusters in ZK-4 molecular sieves, Nukleonika, 2006, 51, 49; (c) A. Baldansuren, R. Eichel and E. Roduner, Phys. Chem. Chem. Phys., 2009, 11, 6664.
8. H. Hirai, S. Ito, S. Takano, K. Koyasu and T. Tsukuda, Chem. Sci., 2020, 11, 12233.
9. (a) Y. Shu-Yan, L. Qin-Hui and S. Meng-Chang, Inorg. Chem., 1994, 33, 1251; (b) E. Nag, S. Battuluri, B. B. Sinu and S. Roy, Inorg. Chem., 2022, 61, 3007.
10. L. Weber, U. Lassahn, H.-G. Stammler and B. Neumann, Eur. J. Inorg. Chem., 2005, 4590.
11. B. K. Najafabadi and J. F. Corrigan, Chem. Commun., 2014, 51, 665.
12. (a) D. Fenske and F. Simon, Angew. Chem., Int. Ed. Engl., 1997, 36, 230; (b) X. He, H.-X. Liu and L. Zhao, Chem. Commun., 2016, 52, 5682; (c) X.-J. Wang, T. Langetepe, C. Persau, B.-S. Kang, G. M. Sheldrick and D. Fenske, Angew. Chem., Int. Ed., 2002, 41, 3818; (d) D. Fenske, C. Persau, S. Dehnen and C. E. Anson, Angew. Chem., Int. Ed., 2004, 43, 305; (e) M. Ueda, Z. L. Goo, K. Minami, N. Yoshinari and T. Konno, Angew. Chem., Int. Ed., 2019, 58, 14673; (f) Z.-A. Nan, Y. Wang, Z.-X. Chen, S.-F. Yuan, Z.-Q. Tian and Q.-M. Wang, Chem. Commun., 2018, 1, 1; (g) J.-W. Liu, H.-F. Su, Z. Wang, Y.-A. Li, Q.-Q. Zhao, X.-P. Wang, C.-H. Tung, D. Sun and L.-S. Zheng, Chem. Commun., 2018, 54, 4461; (h) L. G. AbdulHalim, M. S. Bootharaju, Q. Tang, S. Del Gobbo, R. G. AbdulHalim, M. Eddaoudi, D.-E. Jiang and O. M. Bakr, J. Am. Chem. Soc., 2015, 137, 11970; (i) S.-F. Yuan, Z.-J. Guan, W.-D. Liu and Q.-M. Wang, Nat. Commun., 2019, 10, 1; (j) C. Sun, B. K. Teo, C. Deng, J. Lin, G.-G. Luo, C.-H. Tung and D. Sun, Coord. Chem. Rev., 2021, 427, 213576; (k) M. S. Bootharaju, R. Dey, L. E. Gevers, M. N. Hedhili, J.-M. Basset and O. M. Bakr, J. Am. Chem. Soc., 2016, 138, 13770.
13. M. Jash, A. C. Reber, A. Ghosh, D. Sarkar, M. Bodiuzzaman, P. Basuri, A. Baksi, S. N. Khanna and T. Pradeep, Nanoscale, 2018, 10, 15714.
14. A. Kulkarni, S. Arumugam, M. Francis, P. G. Reddy, E. Nag, S. M. N. V. T. Gorantla, K. C. Mondal and S. Roy, Chem. – Eur. J., 2021, 27, 200.

---

Footnotes

† Electronic supplementary information (ESI) available. CCDC 2242239 (2), 2194453 (4). For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4cc05628k

‡ Both authors contributed equally.

---