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

Abstract

Separate reactions of [Pt­{κ2(C,N)}­X­(S­(O)­Me2)] (κ2(C,N) = N,N′,N″-triarylguanidinate­(1−); Ar = 2-(MeO)­C6H4 (3), 2-MeC6H4 (4), 4-MeC6H4 (5), 2,4-Me2C6H3 (6), 2,5-Me2C6H3 (7), and 3,4-Me2C6H3 (8); X = Cl (3), OC­(O)­CF3 (4–8)) with 1,1-bis­(diphenylphosphino)­methane (dppm) afforded the respective cationic complexes [Pt­{κ2(C,N)}­{κ2(dppm)}]­[X] (10–15) in high yields. In contrast, the separate reactions of [Pt­{κ2(C,N)}­(OC­(O)­CF3)­(S­(O)­Me2)] (κ2(C,N) = N,N′,N″-tris­(3,5-xylyl)­guanidinate­(1−); 9) with dppm and 1,2-bis­(diphenylphosphino)­ethane (dppe) afforded the cationic guanidinatoplatinum­(II) complexes [Pt­{κ2(N,N)}­{κ2(P,P)}]­[OC­(O)­CF3] (P,P = dppm (16), dppe (17), respectively) in high yields. Further, the separate reactions of 9 with dppe and 1,3-bis­(diphenylphosphino)­propane (dppp) afforded the cationic complexes [Pt­{κ2(C,N)}­{κ2(P,P)}]­[OC­(O)­CF3] (P,P = dppe (18), dppp (19), respectively) in high yields. The salt metathesis reaction of 19 with an excess of NH4PF6 afforded the cationic complex [Pt­{κ2(C,N)}­{κ2(dppp)}]­[PF6] (20) in good yield. The new Pt­(II) complexes were characterized by elemental analyses, mass spectrometry, IR, multinuclear (195Pt, 31P, 1H, 13C, and 19F) NMR spectroscopy, and conductivity measurements. Further, the molecular structures of 10·CHCl3, 11·CHCl3, 12·CH2Cl2, 14·1.5C7H8, 15·CH2Cl2, 16, 17, 18·0.5C7H8, 19·C4H10O, and 20·C7H8 were determined by single-crystal X-ray diffraction. To the best of our knowledge, 17 and 18·0.5C7H8 represent the first pair of crystallographically characterized metallacyclic structural isomers to be reported. The reactions of 9 with dppm and dppe carried out separately in CDCl3 at RT were monitored by variable-time 31P NMR spectroscopy, which revealed the formation of a transient species, [Pt­{κ2(C,N)}­{κ2(dppm)}]­[OC­(O)­CF3] (22), and 18 before finally forming the end products 16 and 17, respectively. The contributions of various factors in dictating the coordination flip of the guanidinate(1−) ligand from κ2(C,N) in 22 and 18 to κ2(N,N) in 16 and 17, respectively, were unraveled. The mechanism associated with the coordination flip of the guanidinate(1−) ligand was mapped with the aid of DFT calculations on a model complex, [Pt­{κ2(C,N)}­{κ2(P,P)}]­[OC­(O)­CF3] (22′), which revealed a pathway involving a Wheland intermediate, F, and further point out that the product, [Pt­{κ2(N,N)}­{κ2(P,P)}]­[OC­(O)­CF3] (16′), is a kinetically controlled product.

Influence of the Steric/Electronic Properties of N-Aryl Substituents in Cycloplatinated Guanidinate(1−) Complexes on the Formation of Discrete Pt → Ag Complexes and One-Dimensional Coordination Polymer

“…195 Pt NMR spectra of 5 and 6 revealed a singlet at −3717 and −3690 ppm, respectively. These δ Pt values are slightly more upfield shifted than the shifts reported for 1 – 4 [−3650 ppm ( 1 ), −3674 ppm ( 2 ), −3655 ppm ( 3 ), and −3657 ppm ( 4 )] and the cycloplatinated guanidinate(1−) complex, [Pt­(TAG) (TFA) (S­(O)­Me 2 )] (TAG = 3,4-XylG; −3663 ppm) …”

“…When the aryl rings in the TAG ligand of C is 3,5-xylyl, this intermediate further rearranges to E via the formation of two Pt–Ag bonds; one Pt–Ag bond is also bridged by the platinated carbon of the six-membered [Pt­(TAG)] ring. It is to be noted that Pt­(II) bound carbon atom of the 3,5-xylyl ring in model complex F was nucleophilic to be protonated by CF 3 C­(O)­OH to form a Wheland intermediate G . The protonation of G was shown to be slightly endergonic in nature by 3.0 kcal/mol (see Figure ).…”

“…Structures of F and the Wheland intermediate, G . Reproduced from ref . Copyright 2020 American Chemical Society.…”

“…These δ Pt values are slightly more upfield shifted than the shifts reported for 1−4 [−3650 ppm (1), −3674 ppm (2), −3655 ppm (3), and −3657 ppm (4)] 32 and the cycloplatinated guanidinate(1−) complex, [Pt(TAG) (TFA) (S(O)Me 2 )] (TAG = 3,4-XylG; −3663 ppm). 38 H and 19 F NMR spectra of 9 revealed the presence of three species, possibly 9, 9′ (or 9‴), and 9″ in about 1.00:0.12:0.39 ratio, as estimated from the integrals of CH 3 protons and CF 3 fluorines, respectively. We were able to detect the third species, 9′ (or 9‴) by 1 H and 19 F NMR spectroscopy as the abundance of these nuclei is greater than that of 195 Pt (100% vs ∼34%; see Interestingly, complex 19 also revealed the presence of two solution species in about 1.00:0.83 ratio as identified by 1 H NMR spectroscopy.…”

“…The intermediate B is isolable in the form of 11 due to the presence of Me groups in third and sixth positions of the platinated xylyl ring, which provides stability to the complex. 38 The protonation of G was shown to be slightly endergonic in nature by 3.0 kcal/mol (see Figure 2). Thus, the relatively electron-rich platinated carbon atom of the six-membered [Pt(TAG)] ring in C with aryl ring being 3,5-xylyl could have facilitated the formation of E. We chose to study the reactivity of 4 with 1.5 equiv.…”