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)[ M Cl 4 ] ( M = Al, Ga) in chlorobenzene, the new cationic Sn II –π-arene complexes catena -poly[[chloridoaluminate(III)]-tri-μ-chlorido-4′:1κ 2 Cl ,1:2κ 4 Cl -[(η 6 -1,2,3,4-tetramethylbenzene)tin(II)]-di-μ-chlorido-2:3κ 4 Cl -[(η 6 -1,2,3,4-tetramethylbenzene)tin(II)]-di-μ-chlorido-3:4κ 4 Cl -[chloridoaluminate(III)]-μ-chlorido-4:1′κ 2 Cl ], [Al 2 Sn 2 Cl 10 (C 10 H 14 ) 2 ] n , ( 1 ) and catena -poly[[chloridogallate(III)]-tri-μ-chlorido-4′:1κ 2 Cl ,1:2κ 4 Cl -[(η 6 -1,2,3,4-tetramethylbenzene)tin(II)]-di-μ-chlorido-2:3κ 4 Cl -[(η 6 -1,2,3,4-tetramethylbenzene)tin(II)]-di-μ-chlorido-3:4κ 4 Cl -[chloridogallate(III)]-μ-chlorido-4:1′κ 2 Cl ], [Ga 2 Sn 2 Cl 10 (C 10 H 14 ) 2 ] n , ( 2 ), were isolated. In these first main-group metal–prehnitene complexes, the distorted η 6 arene π-bonding to the tin atoms of the Sn 2 Cl 2 2+ moieties in the centre of [{1,2,3,4-(CH 3 ) 4 C 6 H 2 } 2 Sn 2 Cl 2 ][ M Cl 4 ] 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 Sn II central atom; (iii) a pronounced tilt of the plane of the arene ligand against the plane of the central (Sn 2 Cl 2 ) 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 Sn II central atoms as s (Sn II —arene) = 2 − Σ s (Sn II —Cl), that gives s (Sn II —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 [AlCl 4 ] − is the slightly weaker coordinating anion as compared to [GaCl 4 ] − .

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