Preparation and structures of 1-dimethylamino-2-bis(dimethylamino)- and 1-chloro-2-bis(diethylamino)-1-phospha-2-phosphonium acenaphthene: the first examples of the 1,2-dihydro-1,2-diphospha-acenaphthene ring system

This paper describes the preparation of 1,8-bis[bis(dimethylamino)phosphino]naphthalene (2a) and the attempted preparation of its isopropyl analogue 2c, which led to the formation of 1-naphthyl-bis(diisopropylamino)phosphine (4), and to other unidentified products. The X-ray structures of 2a and 4 are discussed in comparison to those of 1,8-bis[bis(diethylamino)phosphino]naphthalene (2b) and 1-naphthyl-di-tert-butylphosphine (5), respectively. In the structures of 2a, 2b and 4 the (R2N)2P groups (R=alkyl) are eclipsed with respect to the naphthalene plane, whereas the R2P groups (R=alkyl or aryl) in 5, as in 1,8-bis(diorganophosphino)naphthalenes in general, adopt a conformation between bisecting and eclipsed. Furthermore, the reactions of 2a and 2b with BX3 ether adducts (X=F, Cl) are described, which furnished the heterocyclic [σ3P–σ4P+]-diphosphorus compounds, 1-dimethylamino-2-bis(dimethylamino)-, 1-diethylamino-2-bis(diethylamino)- and 1-chloro-2-bis(diethylamino)-1-phospha-2-phosphonium-acenaphthene (6a, 6b and 7b); the first examples of the 1,2-dihydro-1,2-diphospha-acenaphthene ring system. The X-ray structures of 6a and 7b display a relief of strain compared to the parent bis-aminophosphines 2a and 2b, quantified by negative splay angles [−7.89° (6a) and −9.40° (7b); cf. +12.16° (2a) and +12.0° (2b)] and the bonded [225.38 (6a) and 223.16 (7b) pm] compared to the non-bonded phosphorus–phosphorus distances [311.9 (2a) and 311.7 (2b) pm]. A mechanism is discussed for the formation of 6b and 7b from 2b and gaseous hydrogen chloride.