Insights into the van der Waals Radius of Low-Spin Ni(ii) fromMolecular Mechanics Studies and the Crystal Structures of [Ni(cis-cyclohexane-1,3-diamine)2]Cl2, [Ni{(R)-5,5,7-trimethyl-1,4-diazacycloheptane}2]Cl2·H2O and [Ni(5,7-dimethyl-1,4-diazacycloheptane)2](ClO4)2. Synthesis of 5,7-Dimethyl-1,4-diazacycloheptane and an Improved Synthesis of cis-Cyclohexane-1,3-diamine

Abstract

The structures of three bis(diamine)nickel(II) complexes, chosen to shed light on the van der Waals radius of nickel(II), are described. [Ni(cis-1,3-chxn)₂]Cl₂ (cis-1,3-chxn = cis-cyclohexane-1,3-diamine) crystallizes in the monoclinic space group P2₁/n, with a 6.397(2), b 16.463(4), c 7.229(2) Å, b 90.70(2)°, and its structure has been refined to an R value of 0.031 on 1214F. [Ni{(R)-tmdz}₂]Cl₂·H₂O (tmdz = 5,5,7-trimethyl-1,4-diazacycloheptane) crystallizes in the orthorhombic space group P2₁2₁2₁, with a 10.678(1), b 11.073(5), c 17.968(6) Å, and its structure has been refined to an R value of 0.031 on 1586F. [Ni(dmdz)₂](ClO₄)₂ (dmdz = 5,7-dimethyl-1,4-diaza- cycloheptane) crystallizes in the monoclinic space group P2₁/n, with a 9.582(1), b 10.390(2), c 11.817(3) Å, β 96.19(2)°, and its structure has been refined to an R value of 0.059 on 817F. In all three structures, short Ni···H and Ni···C interactions, ranging from 2.37 to 2.61 Å and 2.99 to 3.03 Å, respectively, are observed. Using molecular mechanics modelling to reproduce these separations, we have arrived at a van der Waals radius of 1.35 Å for low-spin nickel(ii). Analysis of Ni···O contacts in the solid state leads to a van der Waals radius of about 1.26 Å, which is consistent with the molecular mechanics derived value since these are usually longer.