Extremely long axial Cu-N bonds in chiral one-dimensional zigzag cyanide-bridged Cu^II-Ni^II and Cu^II-Pt ^II bimetallic assemblies

Preparations, crystal structures, and spectral and magnetic properties of two new chiral one-dimensional cyano-bridged coordination polymers, [Cu ^IIL₂][M^II(CN)₄]·2H ₂O (M^II = Ni^II (1) and Pt^II (2), L = trans-cyclohexane-(1 R,2R)-diamine) have been presented. Complex 1 crystallizes in the monoclinic P2₁ space group with a = 9.864(4) Å, b = 15.393(8) Å, c = 7.995(4) Å, β = 110.32(3)°, V = 1138.4(10) ų, and Z = 2, while 2 crystallizes in the monoclinic P2 ₁ space group with a = 9.899(3) Å, b = 15.541(4) Å, c = 8.102(2) Å, β = 111.02(2)°, V = 1163.6(5) ų, and Z = 2. The unique zigzag cyano-bridged chains along the crystallographic b axis consist of alternate chiral [CuL₂]²⁺ cations and square-planar [M(CN)₄]^2- anions. One side of the axial Cu-N(≡C) bond distances are 2.324(6) and 2.34(1) Å with Cu-N≡C angles of 137.8(6)° and 138.2(9)° for 1 and 2, respectively. On the other hand, the opposite side of the axial Cu-N(≡C) bond distances are 3.120(8) and 3.09(1) Å with significantly large bent Cu-N≡C angles of 97.9(5)° and 96.8(7)° for 1 and 2, respectively. The novel axial bonding features of extremely long semi-coordination Cu-N bonds are attributed to coexistence of pseudo-Jahn-Teller elongation and electrostatic interaction in the unique zigzag cyano-bridged chains. The characteristic bonding features with overlap between small 3d (Ni^II) or large 5d (Pt^II) and 3d (Cu^II) orbitals results in larger shifts in XPS peaks of not only Cu2p_1/2 and Cu2p_3/2 but also Ni2p_1/2 and Ni2p_3/2 for 1 than those of 2, which is also consistent with weak antiferromagnetic interactions with Weiss constants of -5.31 and -5.94 K for 1 and 2, respectively. The d-d, π-π*, and CT bands in the electronic, CD, and MCD spectra for 1 and 2 in the solid state at room temperature are discussed from the viewpoint of magneto-optical properties.