Synthesis and Photoisomerization of Divinyltetramethyldisiloxane and Divinyltetramethyldisilazane Complexes of (η5-C5R5)Rh (R = H, Me). Crystal and Molecular Structure of (η5-C5Me5)Rh[η4-(CH2CH)Me2SiOSiMe2(CHCH2)]

The synthesis and photochemistry of a series of new rhodium complexes of 1,3-divinyl-1,1,3,3-tetramethyldisiloxane, (CH2CHSiMe2)2O, and its disilazane analogue (CH2CHSiMe2)2NH are reported. The divinyldisiloxane or divinyldisilazane ligands in (η5-C5R5)Rh[η4-(CH2CHSiMe2)2X] (R = H, Me; X = O, NH; type 1 complexes) are coordinated through the vinyl groups as for a diene. The structure of (η5-C5Me5)Rh[η4-(CH2CHSiMe2)2O] was established crystallographically. The two coordinated double bonds are arranged with their Si substitutents in a cis orientation with respect to the metal. The strain in the structure is suggested by the scissor geometry of the vinyl CC bonds relative to the C5 plane of the ring, combined with unequal Rh-C and CC bond lengths. Photolysis of (η5-C5R5)Rh(C2H4)2 (R = H, Me) in the presence of (CH2CHSiMe2)2X (X = O, NH) results in the formation of the complexes of type 1, together with an isomer in which the substituents on the coordinated CC bonds are oriented trans at rhodium (2) and species containing two dangling divinyldisiloxane ligands, (η5-C5R5)Rh[η2-(CH2CHSiMe2)2X]2, in both cis and trans configurations. Direct irradiation of species of type 1 converts them to the photoisomer 2. The 16-electron intermediate with one η2-divinyldisiloxane ligand may be trapped by addition of CH2CHSiMe3 or excess (CH2CHSiMe2)2X (X = O, NH). The isomerization of type 2 species to type 1 species proceeds by first-order kinetics. For trans-(η5-C5H5)Rh[η4-(CH2CHSiMe2)2O] (2a) the values of ΔH ⧧ and ΔS ⧧ are 114 ± 1 kJ mol-1 and 41 ± 4 J mol-1 K-1, consistent with rate-determining dissociation of one double bond. Replacement of C5H5 by C5Me5 increases H ⧧ by 9 ± 2 kJ mol-1. The assignment of the photoisomers 2 as the trans species is supported by further observations that (i) a mixture of 1 and 2 can be synthesized by hydrolysis of chlorovinylsilane precursors (η5-C5H5)Rh(η2-CH2CHSiMe2Cl)2, a method which generates the Si−O−Si linkage in situ, (ii) the conformationally locked species with a cyclic trivinyltrimethyltrisiloxane ligand, (η5-C5H5)Rh[η4-(CH2CHSi(Me)O)3], does not undergo photoisomerization, and (iii) the species with the Si−O−Si linkage replaced by a C−O−Si linkage (η5-C5H5)Rh[η4-(CH2CMe)CH2OSiMe2(CHCH2)] also fails to undergo photoisomerization. These complexes represent the first examples of adoption of a trans configuration at a metal by η4-divinyldisiloxane ligands and the first examples of this type of isomerization.