Synthesen und Strukturen von neuen 1,3,5‐Triphospha‐2,4,6‐trisilacyclohexanen und Untersuchung ihrer Ligandeneigenschaften gegenüber [M(CO)3]‐Komplexfragmenten (M = Cr, Mo)

Abstract

Syntheses and Structures of New 1,3,5‐Triphospha‐2,4,6‐trisilacyclohexanes and Investigation of Their Ligand Properties Toward [M(CO)3] Complex Fragments (M = Cr, Mo)*The synthesis of the new P3Si3‐heterocyclohexanes 1b‐1e are reported. They are easily accessible by salt elimination reaction of the corresponding dichloro organosilanes with lithium cyclohexylphosphanide. Their NMR‐spectroscopic properties, structural features and reactivity toward [M(CO)3(NCMe)3] are compared to those of the previous described derivative 1a. The structure of 1b has been established by X‐ray crystallography, showing that the Si3P3six‐membered ring has a slightly distorted chair‐like conformation and the organo substituents at silicon and phosphorus are located in the equatorial positions. The phosphorus atoms are pyramidal surrounded [sum of bond angles 302.1 (P1), 304.8 (P2) and 303.1° (P3)] and the Si‐P distances are identical with those in 1a and in other silacyclophosphanes. The complexation of the triphosphane‐coronands 1b‐e with [M(CO)3(NCMe)3] (M = Cr, Mo) gives the desired triphos‐phane crown‐like complexes 2b‐e and 3e in 38–53% yield. The X‐ray crystal structures of the Cr complexes 2b and 2e exhibit that the organo groups at silicon, surprisingly, being axial oriented and the cyclohexyl groups being in equatorial positions, whereas in case of the Mo analogue of 2e, that is 3e, all organo substituents prefer equatorial sites. The different configuration of the free ligand 1b compared with that in the complex 2b and the different configuration of the ligand 1e in the complexes 2e and 3e is due to the steric congestion (bulky aryl and [M(CO)3] groups) in these molecules. The change of configurations can be achieved by phosphorus inversion during the complexation process; the ease of inversion is rationalized by a intrinsic low inversion barriere at phosphorus (10–20 kcal mol−1) in silacyclophosphanes. Another complex type has been observed for the previous reported unusual Cr complex 2a, in which one Si‐H bond ousts a phosphorus atom in the coordination to the Cr center. The different structures of the Cr complexes 2b‐d compared to the structure of 2a clearly proves that the formation of a Si, H, Cr three‐center two‐electron bond in 2a is determined by the second ortho‐methyl group at the aryl substituent (mesityl) and the tiny metal center (Cr).