Chromous siloxides of variable nuclearity and magnetism.

Treatment of Cr[N(SiMe 3 ) 2 ] 2 (thf) 2 with HOSiR 3 (R = Et, i Pr) in THF afforded the bridged Cr II siloxide complexes Cr 3 (OSiEt 3 ) 2 (μ-OSiEt 3 ) 4 (thf) 2 and Cr 2 (OSi i Pr 3 ) 2 (μ-OSi i Pr 3 ) 2 (thf) 2 in high yield. Exposure of these compounds to vacuum in aliphatic solvents led to the loss of coordinated THF and to the formation of the homoleptic chromous siloxides Cr 4 (μ-OSiEt 3 ) 8 and Cr 3 (OSiiPr 3 ) 2 (μ-OSi i Pr 3 ) 4 , respectively, in moderate to high yield. Use of TMEDA as a potentially bidentate donor molecule gave the monomeric cis -coordinated siloxide Cr(OSiiPr 3 ) 2 (tmeda) (tmeda = N , N , N ', N '-tetramethylethane-1,2-diamine). Oxidation of Cr 2 (OSi i Pr 3 ) 2 (μ-OSi i Pr 3 ) 2 (thf) 2 with CHI 3 and C 2 Cl 6 produced the trigonal bipyramidal chromic compound Cr III (OSi i Pr) 3 (thf) 2 and asymmetrically coordinated Cr 2 Cl 3 (OSi i Pr 3 ) 3 (thf) 3 , respectively. Magnetic measurements (Evans and SQUID) hinted at (a) antiferromagnetic interactions between the Cr II centres, (b) revealed higher effective magnetic moments ( μ eff ) for cis -coordinated monomeric heteroleptic complexes compared to trans -coordinated ones, and (c) pointed out the highest ( μ eff ) for the tetranuclear complex Cr 4 (μ-OSiEt 3 ) 8 (6.26 μ B , SQUID, 300 K; Cr⋯Cr adjacent avg. 2.535 A).