New Bonding Modes for Boraamidinate Ligands in Heavy Group 15 Complexes:  Fluxional Behavior of the 1:2 Complexes, LiM[PhB(N t Bu)2]2 (M = As, Sb, Bi)

The reactions of MCl3 with Li2[PhB(N t Bu)2] in 1:1, 1:1.5, and 1:2 molar ratios in diethyl ether produced the monoboraamidinates ClM[PhB(N t Bu)2] (1a, M = As; 1b, M = Sb; 1c, M = Bi), the novel 2:3 boraamidinate complexes [PhB(N t Bu)2]M-μ-N( t Bu)B(Ph)N( t Bu)M[PhB(N t Bu)2] (2b, M = Sb; 2c, M = Bi), and the bisboraamidinates LiM[PhB(N t Bu)2]2 (3a, 3a·OEt2, M = As; 3b, M = Sb; 3c·OEt2, M = Bi), respectively. The 2:3 complexes 2b and 2c were also observed in the reactions carried out in a 1:2 molar ratio at room temperature. All complexes have been characterized by multinuclear NMR spectroscopy (1H, 7Li, 11B, and 13C) and by single-crystal X-ray structural determinations. The molecular units of the mono-boraamidinates 1a−c are isostructural, but their crystal packing is distinct as a result of stronger intermolecular close contacts going from 1a to 1c. In the novel 2:3 bam complexes 2b and 2c, each metal center is N,N‘-chelated by a bam ligand and these two [M(bam)]+ units are bridged by the third [bam]2- ligand. The structures of the unsolvated bis-boraaminidate complexes 3a and 3b consist of [Li(bam)]- and [M(bam)]+ monomeric units linked by Li−N and M−N bonds to give a tricyclic structure. Solvation of the Li+ ion by diethyl ether results in a bicyclic structure composed of four-membered BN2As and six-membered BN3AsLi rings in 3a·OEt2. In contrast, the analogous bismuth complex 3c·OEt2 exhibits a tetracyclic structure. Variable-temperature NMR studies reveal that the nature of the fluxional behavior of 3a−c in solution is dependent on the group 15 center.