Synthesis, detailed geometric analysis and bond-valence method evaluation of the strength of π-arene bonding of two isotypic cationic prehnitene tin(II) complexes: [{1,2,3,4-(CH3)4C6H2}2Sn2Cl2][MCl4]2 (M = Al and Ga)

From solutions of prehnitene and the ternary halides (SnCl)[MCl4] (M = Al, Ga) in chlorobenzene, the new cationic SnII–π-arene complexes catena-poly[[chloridoaluminate(III)]-tri-μ-chlorido-4′:1κ2Cl,1:2κ4Cl-[(η6-1,2,3,4-tetramethylbenzene)tin(II)]-di-μ-chlorido-2:3κ4Cl-[(η6-1,2,3,4-tetramethylbenzene)tin(II)]-di-μ-chlorido-3:4κ4Cl-[chloridoaluminate(III)]-μ-chlorido-4:1′κ2Cl], [Al2Sn2Cl10(C10H14)2]n, (1) and catena-poly[[chloridogallate(III)]-tri-μ-chlorido-4′:1κ2Cl,1:2κ4Cl-[(η6-1,2,3,4-tetramethylbenzene)tin(II)]-di-μ-chlorido-2:3κ4Cl-[(η6-1,2,3,4-tetramethylbenzene)tin(II)]-di-μ-chlorido-3:4κ4Cl-[chloridogallate(III)]-μ-chlorido-4:1′κ2Cl], [Ga2Sn2Cl10(C10H14)2]n, (2), were isolated. In these first main-group metal–prehnitene complexes, the distorted η6 arene π-bonding to the tin atoms of the Sn2Cl22+ moieties in the centre of [{1,2,3,4-(CH3)4C6H2}2Sn2Cl2][MCl4]2 repeating units (site symmetry \overline{1}) is characterized by: (i) a significant ring slippage of ca 0.4 Å indicated by the dispersion of Sn—C distances [1: 2.881 (2)–3.216 (2) Å; 2: 2.891 (3)–3.214 (3) Å]; (ii) the non-methyl-substituted arene C atoms positioned closest to the SnII central atom; (iii) a pronounced tilt of the plane of the arene ligand against the plane of the central (Sn2Cl2)2+ four-membered ring species [1: 15.59 (11)°, 2: 15.69 (9)°]; (iv) metal–arene bonding of medium strength as illustrated by application of the bond-valence method in an indirect manner, defining the π-arene bonding interaction of the SnII central atoms as s(SnII—arene) = 2 − Σs(SnII—Cl), that gives s(SnII—arene) = 0.37 and 0.38 valence units for the aluminate and the gallate, respectively, indicating that comparatively strong main-group metal–arene bonding is present and in line with the expectation that [AlCl4]− is the slightly weaker coordinating anion as compared to [GaCl4]−.