Phosphorus–nitrogen compounds. Part VI. Aminolysis of octachlorocyclotetraphosphazatetraene and the crystal structure of 2-trans-6-bis(n-propylamino)-2,4,4,6,8,8-hexakis-tert-butylaminocyclo-2λ5, 4λ5, 6λ5, 8λ5-tetraphosphazatetraene