Strontium nitride carbodiimide, Sr 4 N 2 (CN 2 )

Strontium nitride carbodiimide, Sr4N2(CN2), is isostructural with the calcium analogue and consists of a framework of edge- and vertex-sharing Sr6N octa­hedra forming channels within which almost linear and almost symmetrical carbo­diimide anions reside, surrounded by eight strontium ions.

Strontium nitride carbodiimide, Sr 4 N 2 (CN 2 ), is isostructural with the calcium analogue and consists of a framework of edge-and vertex-sharing Sr 6 N octahedra forming channels within which almost linear and almost symmetrical carbodiimide anions reside, surrounded by eight strontium ions.

Comment
There is increasing interest in the chemistry of the nitrides of the elements and one way to grow crystals of alkaline earth main group nitrides is to make use of a molten sodium flux (Yamane & DiSalvo, 1996;Reckeweg & DiSalvo, 2000). In attempting to grow crystals of strontium aluminium nitrides we grew crystals of the title phase. Strontium nitride carbodiimide is isostructural with the calcium analogue Ca 4 N 2 (CN 2 ) (Reckeweg & DiSalvo, 2000) and with Ca 3.2 Sr 0.8 N 2 (CN 2 ) (Hö hn et al., 2000). The structure consists of a three-dimensional framework of Sr 6 N octahedra, centred by atoms N3 and N4, linked by their edges and vertices. Channels are formed which accommodate the carbodiimide anions. Each N atom of the carbodiimide anion is within 3.0 Å of four strontium ions and the [CN 2 ] 2À anions should be considered eight-coordinate by strontium cations. Atoms Sr1 and Sr3 are coordinated by five N atoms within 3 Å , Sr2 is in approximately octahedral coordination by six N atoms, and Sr4 is in distorted tetrahedral coordination by four N atoms within 2.7 Å , with a fifth N atom 3.228 (4) Å distant. The carbodiimide anions are almost linear, with an N-C-N bond angle of 178.0 (5) , and the anion is in the symmetrical carbodiimide form, with C-N bond lengths of 1.240 (6) and 1.235 (6) Å , which are equal within experi-mental uncertainty. The geometry of the carbodiimide anions in Ca 4 N 2 (CN 2 ) is similar: C-N bond lengths of 1.22 (1) and 1.24 (1) Å , and an N-C-N angle of 179.7 (10) (Reckeweg & DiSalvo, 2000). The structure of Sr 4 N 2 (CN 2 ) is shown in Fig. 1.

S1. Comment
There is increasing interest in the chemistry of the nitrides of the elements and one way to grow crystals of alkaline earth main group nitrides is to make use of a molten sodium flux (Yamane & DiSalvo, 1996;Reckeweg & DiSalvo, 2000). In attempting to grow crystals of strontium aluminium nitrides we grew crystals of the title phase. Strontium nitride carbodiimide is isostructural with the calcium analogue Ca 4 N 2 (CN 2 ) (Reckeweg & DiSalvo, 2000) and with Ca 3.2 Sr 0.8 N 2 (CN 2 ) (Höhn et al., 2000). The structure consists of a three-dimensional framework of Sr 6 N octahedra, centred by atoms N3 and N4, linked by their edges and vertices. Channels are formed which accommodate the carbodiimide anions. Each N atom of the carbodiimide anion is within 3.0 Å of four strontium ions and the [CN 2 ] 2− anions should be considered eightcoordinate by strontium cations. Atoms Sr1 and Sr3 are coordinated by five N atoms within 3 Å, Sr2 is in approximately octahedral coordination by six N atoms, and Sr4 is in distorted tetrahedral coordination by four N atoms within 2.7 Å, with a fifth N atom 3.228 (4) Å distant. The carbodiimide anions are almost linear, with an N-C-N bond angle of 178.0 (5)°, and the anion is in the symmetrical carbodiimide form, with C-N bond lengths of 1.240 (6) and 1.235 (6) Å, which are equal within experimental uncertainty. The geometry of the carbodiimide anions in Ca 4 N 2 (CN 2 ) is similar: C-N bond lengths of 1.22 (1) and 1.24 (1) Å, and an N-C-N angle of 179.7 (10)° (Reckeweg & DiSalvo, 2000). The structure of Sr 4 N 2 (CN 2 ) is shown in Fig. 1.

S2. Experimental
Strontium nitride carbodiimide was synthesized by reacting together Sr (99%, Aldrich, 100 mg), NaN 3 (99%, Aldrich, 85 mg), Al (99.99%, Aldrich, 31 mg) and Na (99+ %, BDH, 200 mg) in a sealed nickel tube at 1073 K for 4 d, with slow cooling to 673 K prior to removal of the tube from the furnace. A small numbers of colourless crystals of the product were obtained after sublimation of excess sodium from the reactants. No other crystalline products were identified in the reaction. The carbon forming the carbodiimide units presumably arises adventitiously from the nickel tube or from one or more of the reactants, as noted by Reckeweg & DiSalvo (2000). supporting information sup-2 . E61, i221-i222

Figure 1
The structure of Sr 4 N 2 (CN 2 ), showing the framework of Sr 6 N octahedra and the channels containing the carbodiimide anions. The detail shows the asymmetric unit depicting 99% displacement ellipsoids. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq