A crystallographically isolated dimeric hydrolyzed chlorophosphazene dianion

Single crystals of the title compound bis[bis(1-ethyl-3-methyl-imidazol-2-ylidene)silver(I)] 1,5,5,7,11,11-hexachloro-2,8-dioxa-4,6,10,12,13,14-hexaaza-1λ5,3,5λ5,7λ5,9,11λ5-hexaphosphatricyclo[7.3.1.13,7]tetradeca-1(13),4,7(14),10-tetraene-6,12-diide 3,9-dioxide, [Ag(C6H10N2)2](Cl6N6O4P6)0.5, were isolated from the reaction of the silver N-heteocyclic carbene complex [Ag(C6H10N2)2]Cl and hexachlorocyclotriphosphazene [NPCl2]3 in the presence of water. The asymmetric unit contains one silver carbene cation with the carbene ligands bound to the Ag(I) in an almost linear arrangement and one half of a hydrolyzed phosphazene dianion. The second cation and additional half of the anion are generated by an inversion center.

. It has long been contested that trace amounts of water not only accelerate the rate of polymerization but are necessray to generate active species needed to simply promote polymer formation (Allcock, 2003;Allcock et al., 1975). Isolation of hydrolyzed species gives insight into the still unclear role that water plays in this polymerization reaction (Gabler et al., 1990). As part of our broader studies on the irreproducibility in the synthesis of poly(dichlorophosphazene) we report the first crystal structure of a dimeric hydrolyzed chlorophosphazene dianion.
The asymmetric unit consists of one silver N-heteocyclic carbene cation [AgC 12 H 20 N 4 ] + and half of a hydrolyzed phosphazene dianion [Cl 6 N 6 O 4 P 6 ] 2-. The second cation and other half of the dianion are generated by a crystallographic inversion center at x,y,z (1/2,1/2,1/2) located between one of the bridging oxygen atoms of the dimeric dianion and its symmetry generated equivalent. Each of the Ag(I) atoms is bound to two identical N-heterocyclic carbene ligands in a linear fashion.
Two partially hydrolyzed phosphazene rings are joined by bridging oxygen atoms from a phosphorus atom of one ring to the other to form the dimeric dianion. The P-O-P bonds of the bridging oxygen atoms are inequivalent . The P-N bond distances of the individual rings deviate from the reported values of hexachlorocyclotriphosphazene, all six being virtually equivalent (Bartlett et al., 2006;Bullen, 1971). Two of the P-N bonds on each of the rings show significant double bond character while the remaining P-N bonds are lengthened to give more single bond character.

Refinement
Hydrogen atoms were calculated and palced in geometrically idealized positions with C-H distances of 0.95 Å (aromatic), 0.99 Å (methylene), and 0.98 Å (methyl). H atoms were constrained to ride on the parent carbon atom with U iso (H) = 1.2 U eq (C) for aromatic and methylene and U iso (H) = 1.5 U eq (C) for methyl protons.
supplementary materials sup-2 Figures Fig. 1. Structure of the title compound, first crystallographically characterized dimeric oxygen-bridged chlorophosphazene dianion. A numbering scheme of the non-H atoms is shown and thermal ellipsoids shown at 50% probability.