In Search of the [PhB(μ-NtBu)2]2As• Radical: Experimental and Computational Investigations of the Redox Chemistry of Group 15 Bis-boraamidinates

Crystal structure of 1,3-di-tert-butyl-2-chloro-1,3,2-di-aza-phospho-rinane - a saturated six-membered phospho-rus nitro-gen heterocycle with a partially flattened chair conformation and a long PIII-Cl bond

Colourless blocks of 1,3-di-tert-butyl-2-chloro-1,3,2-di-aza-phospho-rinane, C11H24ClN2P (1), were obtained by sublimation in vacuo slightly above room temperature. The asymmetric unit of the monoclinic crystal structure of the six-membered N-heterocyclic compound is defined by one mol-ecule in a general position. The six-membered ring of the mol-ecule adopts a cyclo-hexane-like chair conformation; the chair at one side is to some extent flattened as a result of the approximately trigonal-planar coordination of both nitro-gen atoms. In detail, this modified chair conformation is characterized by an angle of 53.07 (15)° between the plane defined by the three carbon atoms and the best plane of the two nitro-gen atoms and the two carbon atoms bound to them, and an angle of 27.96 (7)° between the latter plane and the plane defined by the nitro-gen and phospho-rus atoms. The tert-butyl groups are oriented equatorially and the chloro substituent is oriented axially. The P-Cl bond length of 2.2869 (6) Å is substanti-ally longer than the P-Cl single-bond length in PCl3 [2.034 Å; Galy & Enjalbert (1982 ▸). J. Solid State Chem. 44, 1-23]. Inspection of the inter-molecular distances gives no evidence for inter-actions stronger than van der Waals forces. The closest contact is between the Cl atom and a methyl-ene group of a neighbouring mol-ecule with a Cl⋯C distance of 3.7134 (18) Å, excluding a significant influence on the P-Cl bonding.

Syntheses, X-ray structures, and redox behaviour of the group 14 bis-boraamidinates M[PhB(μ-N-t-Bu)2]2 (M = Ge, Sn) and Li2M[PhB(μ-N-t-Bu)2]2 (M = Sn, Pb)

The solid-state structures of the complexes M[PhB(μ-N-t-Bu)2]2 (1a, M= Ge; 1b, M = Sn) were determined to be spirocyclic with two orthogonal boraamidinate (bam) ligands N,N′-chelated to the group 14 centre. Oxidation of 1b with SO2Cl2 afforded the thermally unstable, blue radical cation {Sn[PhB(μ-N-t-Bu)2]2}•+, identified by electron paramagnetic resonance (EPR) spectroscopy supported by density functional theory (DFT) calculations, whereas the germanium analogue 1a was inert towards SO2Cl2. The reaction between Li2[PhB(μ-N-t-Bu)2]2 and SnCl2 or PbI2 in 2:1 molar ratio in diethyl ether produced the novel heterotrimetallic complexes Li2Sn[PhB(μ-N-t-Bu)2]2 (2b) and (Et2O·Li)LiPb[PhB(μ-N-t-Bu)2]2 (2c·OEt2), respectively. By contrast, treatment of Li2[PhB(μ-N-t-Bu)2]2 with C4H8O2·GeCl2 yielded the germanium(IV) complex 1a via a redox process. The X-ray structures of 2b and 2c·THF revealed polycyclic arrangements in which one bam ligand is N,N′-chelated to the Sn(II) or Pb(II) atom and one of the Li+ cations, while the second bam ligand exhibits a unique bonding mode, bridging all three metal centres. The fluctional behaviour of 2b was investigated by variable temperature, multinuclear NMR spectroscopy.