New members of ternary rare-earth metal boride carbides containing finite boron-carbon chains: RE25B14C26 (RE=Pr, Nd) and Nd25B12C28

New ternary rare-earth metal boride carbides RE25B14C26 (RE=Pr, Nd) and Nd25B12C28 were synthesized by co-melting the elements. Nd25B12C28 is stable up to 1440 K. RE25B14C26 (RE=Pr, Nd) exist above 1270 K. The crystal structures were investigated by means of single-crystal X-ray diffraction. Nd25B12C28: space group P1-bar, a=8.3209(7) A, b=8.3231(6) A, c=29.888(2) A, α=83.730(9)o, β=83.294(9)o, γ=89.764(9)o. Pr25B14C26: space group P21/c, a=8.4243(5) A, b=8.4095(6) A, c=30.828(1) A, β=105.879(4)o, V=2100.6(2) A3, (R1=0.048 (wR2=0.088) from 2961 reflections with Io>2σ(Io)); for Nd25B14C26 space group P21/c, Z=2, a=8.3404(6) A, b=8.3096(6) A, c=30.599(2) A, β=106.065(1)o. Their structures consist of a three-dimensional framework of rare-earth metal atoms resulting from the stacking of slightly corrugated and distorted square nets, leading to cavities filled with cumulene-like molecules [B2C4]6- and [B3C3]7-, nearly linear [BC2]5- and bent [BC2]7- units and isolated carbon atoms. Structural and theoretical analysis suggests the ionic formulation for RE25B14C26: (RE3+)25[B2C4]6-([B3C3]7-)2([BC2]5-)4([BC2]7-)2(C4-)4.5e- and for Nd25B12C28: (Nd3+)25([B2C4]6-)3([BC2]5-)4([BC2]7-)2(C4-)4.7e-. Accordingly, extended Hueckel tight-binding calculations indicate that the compounds are metallic in character. - Graphical Abstract: New ternary rare-earth metal boride carbides RE25B14C26 (RE=Pr, Nd) and Nd25B12C28 were synthesized by co-melting the elements. Nd25B12C28 is stable up to 1440 K. RE25B14C26 (RE=Pr, Nd) exist above 1270 K. The crystal structures were investigated by means of single-crystal X-ray diffraction. Their structures consist of a three-dimensional framework of rare-earth metal atoms resulting from the stacking of slightly corrugated and distorted square nets, leading to cavities filled with cumulene-like molecules [B2C4]6- and [B3C3]7-, nearly linear [BC2]5- and bent [BC2]7- units and isolated carbon atoms, respectively. Structural and theoretical analysis suggests the ionic formulation for RE25B14C26: (RE3+)25[B2C4]6-([B3C3]7-)2([BC2]5-)4([BC2]7-)2(C4-)4.5e- and for Nd25B12C28: (Nd3+)25([B2C4]6-)3([BC2]5-)4([BC2]7-)2(C4-)4.7e-. Highlights: → New rare-earth metal boride carbides RE25B14C26 (RE=Pr, Nd) and Nd25B12C28. → Structures contain 1-D finite B3C3, B2C4, BC2 units as well as isolated C atoms. → B/C distribution in the crystal structures was approved by EH-TB calculations.