Dual Role of Acetate as a Nucleophile and as an Internal Base in Cycloplatination Reaction of sym-N,N′,N″-Triarylguanidines

Abstract

Reaction of cis-[Cl(2)Pt(S(O)Me(2))(2)] with 1 equiv of sym-N,N',N″-triarylguanidines, ArN═C(NHAr)(2) (sym = symmetrical; Ar = 2-MeC(6)H(4) (LH(2)(2-tolyl)), 2-(MeO)C(6)H(4) (LH(2)(2-anisyl)), 4-MeC(6)H(4) (LH(2)(4-tolyl)), 2,5-Me(2)C(6)H(3) (LH(2)(2,5-xylyl)), and 2,6-Me(2)C(6)H(3) (LH(2)(2,6-xylyl))) in toluene under reflux condition for 3 h afforded cis- or trans-[Cl(2)Pt(S(O)Me(2))(ArN═C(NHAr)(2))] (Ar = 2-MeC(6)H(4) (1), 2-(MeO)C(6)H(4) (2), 4-MeC(6)H(4) (3), 2,5-Me(2)C(6)H(3) (4), and 2,6-Me(2)C(6)H(3) (5), respectively) in 83-96% yield. Reaction of cis-[Cl(2)Pt(S(O)Me(2))(2)] with 1 equiv of LH(2)(2-tolyl) and LH(2)(4-tolyl) in the presence of 1 equiv of NaOAc in methanol under reflux condition for 3 h afforded acetate-substituted products, cis-[(AcO)ClPt(S(O)Me(2))(ArN═C(NHAr)(2))] (Ar = 2-MeC(6)H(4) (6) and 4-MeC(6)H(4) (7)) in 83% and 84% yields, respectively. Reaction of cis-[Cl(2)Pt(S(O)Me(2))(2)] with 1 equiv of LH(2)(2-anisyl) and LH(2)(2-tolyl) in the presence of 1 equiv of NaOAc in methanol under reflux condition for 3 and 12 h afforded six-membered [C,N] platinacycles, [Pt{κ(2)(C,N)-C(6)H(3)R-3(NHC(NHAr)(═NAr))-2}Cl(S(O)Me(2))] (Ar = 2-RC(6)H(4); R = OMe (8) and Me (9)), in 92% and 79% yields, respectively. The new complexes have been characterized by analytical and spectroscopic techniques, and further the molecular structures of 1, 2, 4, 5, 6, and 8 have been determined by single-crystal X-ray diffraction. The platinum atom in 1, 4, and 5 exhibited the trans configuration, while that in 2, 6, and 8 exhibited the cis configuration. Complex 6 is shown to be the precursor for 9, and the former is suggested to transform to the latter possibly via an intramolecular C-H activation followed by elimination of AcOH. The solution behavior of new complexes has been studied by multinuclear NMR ((1)H, (195)Pt, and (13)C) spectroscopy. The new complexes exist exclusively as a single isomer (trans (1 and 5) and cis (6 and 7)), a mixture of cis and trans isomers with the former isomer being predominant in the case of 2 and the latter isomer being predominant in the case of 3. Complex 5 in the trans form revealed the presence of one isomer at 0.007 mM concentration and two isomers in about 1.00:0.12 ratio at 0.154 mM concentration as revealed by (1)H NMR spectroscopy, and this has been ascribed to the restricted Pt-S bond rotation at higher concentration. Platinacycle 8 exists as one isomer, while 9 exists as a mixture of seven isomers in solution. The influence of steric factor, π-acceptor property of the guanidine, subtle solid-state packing forces upon the configuration of the platinum atom, and the number of isomers in solution have been outlined. Factors that accelerate or slow down the cycloplatination reaction, the role of NaOAc, and a plausible mechanism of this reaction have been discussed.

Critical Role of Anions in Platinum(II) Precursors upon the Structural Motifs of Six-Membered Cycloplatinated N,N′,N″-Triarylguanidines

“…The δ( 195 Pt) values reported for 4–7 are also upfield-shifted than that reported for five-membered cycloplatinated ferrocenylamine, [Pt{κ 2 ( C , N )}(OAc)(DMSO)] (δ( 195 Pt) −2078 ppm 43a ) not only due to the difference in the [Pt{κ 2 ( C , N )}] ring size but also due to the difference in the nature of ligands. The presence of one solution species for 4 contrasts with the presence of seven solution species reported for its chloro analogue, [Pt{κ 2 ( C , N )}Cl(DMSO)] (δ( 195 Pt) −2645 (minor), −2710 (major), −2760 (minor), −2839 (minor), −2851 (minor), −2958 (major), and −2995 (minor) ppm 24 ). This spectral difference in the number of solution species arises because of larger size of the TFA in platinacycle 4 .…”

“…The configuration of Pd(II)/Pt(II) atoms in cyclometallated N-donor ligands was explained by invoking steric factor and antisymbiosis / transphobia of two mutually trans-oriented ligands and the influence of the latter factor was independently verified in one study by theoretical calculations. 4,24,38−40 The cisoid configuration of the Pt(II) atom in 1 ·1/2CH 2 Cl 2 and 3 is unfavorable from the point of view of degree of transphobia , as hard imine N atom of both the guanidines lies trans to each other. In a hypothetical transoid configuration such as that illustrated in M , there would be greater steric repulsion between 2-tolyl substituents of the imine N atom of both platinated and nonplatinated guanidines (see Figure 9).…”

“…This spectral difference in the number of solution species arises because of larger size of the TFA in platinacycle 4 . As a result, only one conformer, α3, out of six possible conformers (α1, α2, α3, β1, β2, and β3 24 ) is possible for 4 even in solution. The α and β nomenclature indicates conformers that arise due to three distinct orientations of DMSO with respect to the plane of the Pt(II) atom and two distinct orientations of the o -substituted aryl ring in the =NAr unit of the C,N chelate with respect to the basal plane of the boat, respectively.…”

Photophysical and DFT studies on cycloplatinated complexes: modification in luminescence properties by expanding of π-conjugated systems

“…A long-standing interest and signicant research effort are currently focused on the study of cyclometalated square-planar platinum(II) complexes due to the wide range of their potential applications in many elds, such as catalysis, [1][2][3][4][5][6] organic and organometallic synthesis, [7][8][9][10][11] photochemistry, 12-14 photonic [15][16][17][18][19] and chemical sensors, [20][21][22] with the complexes bearing a single cyclometalating ligand receiving special attention owing to their ability to generate unique emitter devices. [23][24][25][26][27] The best known types of luminescent complexes carrying a mono cycloplatinated(II) fragment have been reported in the literature based on ĈN aromatic chelate derivatives such as 2-arylpyridine, in which the ligand includes donor, C-bonded aryl group, and acceptor, N-bonded pyridine ring, abilities.…”

Unusual [Pt{κ2(C,N)}]+ → [Pt{κ2(N,N)}]+ Coordination Mode Flip of the Guanidinate(1−) Ligand in Cationic N,N′,N″-Tris(3,5-xylyl)guanidinatoplatinum(II) Bis(phosphine) Complexes. Syntheses, Structural and Theoretical Studies

“…Cycloplatinated imine, oxime, ketimine, and related N -donor complexes have been extensively studied in the literature due to their relevance in Pt­(II)-mediated C–H activation and their utility as luminescent materials and metallomesogens. − The mechanism of C–H activation involving Pt­(II) salts/precursors and an appropriate N -donor ligand was shown to critically depend upon various factors such as the nature of the Pt­(II) precursors, the steric, electronic, and conformational properties of the N -donor ligands, and the nature of the external base and solvent. , One of the simplest methods to cycloplatinate the N -donor ligands is to use cis -[PtCl 2 (S­(O)­Me 2 ) 2 ] as the Pt­(II) precursor in the presence of NaOAc as an external base in methanol or a methanol/toluene mixture under heating conditions. In this method, the role of NaOAc was thoroughly scrutinized and the OAc – group was shown to enter into the coordination sphere of the Pt­(II) atom during the course of the cycloplatination reaction. ,,, …”