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. 2022 Oct 21;13(45):13387-13392.
doi: 10.1039/d2sc05024b. eCollection 2022 Nov 23.

Towards new coordination modes of 1,2,3-triazolylidene: controlled by the nature of the 1st metalation in a heteroditopic bis-NHC ligand

Praseetha Mathoor Illam  1 , Chandra Shekhar Tiwari  1 , Arnab Rit  1
Affiliations

Towards new coordination modes of 1,2,3-triazolylidene: controlled by the nature of the 1st metalation in a heteroditopic bis-NHC ligand

Praseetha Mathoor Illam et al. Chem Sci. .

Abstract

An unusual effect of the nature of the first metal coordination of a heteroditopic N-heterocyclic carbene ligand (L2) towards the coordination behavior of 1,2,3-tzNHC is explored. The first metal coordination at the ImNHC site (complexes 3 and 4) was noted to substantially influence the electronics of the 1,2,3-triazolium moiety leading to an unprecedented chemistry of this MIC donor. Along this line, the RhIII/IrIII-orthometalation in complexes 4 makes the triazolium C4-H more downfield shifted than C5-H, whereas a reverse trend, although to a lesser extent, is observed in the case of the non-chelated PdII-coordination. This difference in behavior assisted us to achieve the selective activation of triazole C4/C5 positions, not observed before, as supported by the isolation of the homo- and hetero-bimetallic complexes, 5, 6 and 7-9via C5- and C4-metalation, respectively. Furthermore, the %V bur calculations eliminate any considerable steric influence and the DFT studies strongly support the selectivity observed during bimetalation.

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Conflict of interest statement

There are no conflicts to declare.

Figures

Fig. 1
Fig. 1. Comparison of previous reports on 1,2,3-triazolylidene complexes with the present work.
Scheme 1
Scheme 1. (a) Synthesis of [L1-H]Br and its palladium (1) and gold (2) complexes: (i) NaNO2, HCl/H2O (10% solution), 0 °C, 1 h; (ii) NaN3, 0 °C-RT, 12 h; (iii) vinyl acetate, reflux, 24 h; (iv) EtBr, CH3CN, reflux, 24 h; (v) [Pd(CH3CN)2Cl2], Cs2CO3, KBr, CH3CN/pyridine, 70 °C, 24 h; (vi) [Au(SMe2)Cl], Cs2CO3, CH3CN, 70 °C, 24 h; (b) molecular structures of 1 and 2 with ellipsoids at a 50% probability level. Hydrogen atoms except H8 and Me moieties of the N–Et groups are omitted for clarity. Pyridine is shown in capped stick.
Scheme 2
Scheme 2. (a) Synthesis of bisazolium salt, [L2-H2]Br2: (i) NaNO2, HCl/H2O (10% solution), 0 °C, 1 h; (ii) NaN3, 0 °C–RT, 12 h; (iii) vinyl acetate, reflux, 24 h; (iv) imidazole, K2CO3, CuO, DMSO, 150 °C, 48 h; (v) EtBr, DMF, reflux, 24 h. (b) Molecular structure of [L2-H2]Br2 with ellipsoids at a 50% probability level. Hydrogen atoms except H5, H12, and H13, counterions, solvent of crystallization and Me moieties of the N–Et groups are omitted for clarity.
Scheme 3
Scheme 3. (a) Synthesis of palladium (3), iridium (4a and a′), and rhodium (4b) complexes: (i) [Pd(CH3CN)2Cl2], Cs2CO3, KBr, CH3CN/pyridine, RT, 12 h. (ii) [M(Cp*)Cl2]2 (M = Ir/Rh), NaOAc, K2CO3/Cs2CO3, KBr (for 4a and b) or KI (for 4a′), CH3CN, 75 °C, 24 h. (b) Molecular structures of 3 and 4a′ with ellipsoids at 50% probability level. Hydrogen atoms except H15/H16 in 3 and H20/H21 in 4a′, counterions, and the Me groups of N–Et moieties are omitted for clarity. Pyridine and Cp* moieties are shown in capped stick.
Fig. 2
Fig. 2. 2D-NOESY NMR spectrum of 4a showing the interaction of triazolium C4–H with the N–CH2 protons of the ethyl group.
Scheme 4
Scheme 4. (a) Synthesis of the bimetallic complexes 5 and 6 from 3: (i) [Pd(CH3CN)2Cl2], Cs2CO3, KBr, CH3CN/pyridine, 60 °C, 24 h; (ii) Ag2O, DCM, RT, 12 h; (iii) [Au(SMe2)Cl], RT, 12 h. (b) Molecular structures of 5 and 6 with ellipsoids at a 50% probability level. Hydrogen atoms except H3 in 5, H15 in 6, and the Me groups of N–Et moieties are omitted for clarity. Pyridine groups are shown in capped stick.
Scheme 5
Scheme 5. (a) Synthesis of the heterobimetallic complexes 7–9: (i) [Pd(CH3CN)2Cl2], Cs2CO3, KBr, CH3CN/pyridine, 70 °C, 24 h. (ii) Ag2O, DCM, RT, 12 h and then [Au(SMe2)Cl], RT, 12 h. (b) Molecular structures of 7 and 8 with ellipsoids at a 50% probability level. Hydrogen atoms except H20 in 7 and H21 in 8, and the Me moieties of N–Et groups are omitted for clarity. Pyridine and Cp* moieties are shown in capped stick.
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