Stereochemically non-rigid motions and structural characteristics of (η3-2-methallyl){tetrakis(3-t-butylpyrazolyl)borato}palladium(II)

Variable temperature H-1 NMR spectra of the t-butyl-substituted (polypyrazolylborato)palladium(II), [Pd(eta(3)-2-methallyl){B(3-t-Bupz)(4)}] (3-t-Bupz = 3-t-butylpyrazol-1-yl group) (1) were measured and stereochemically non-rigid motions of 1 were studied. Four t-butyl-substituted pyrazolyl groups in the ligating B(3-t-Bupz)(4) moiety showed their spectroscopic classifications of 1:1:1:1 and 2:1:1 at low and high temperatures, respectively, whereas syn and anti protons of the eta(3)-2-methallyl moiety resonated separately without their mutual interconversion. To clarify the classification change of the pyrazolyl groups in the B(3-t-Bupz)4, stereochemical characteristics of 1 were disclosed through its single crystal X-ray structural analysis, which confirmed eta(2)-coordination of the B(3-t-Bupz)4 and evaluated stereochemical bulkiness effects of the 3-t-butyl substituents on the non-rigid motions of 1. Temperature-dependent spectral change observed was attributed to the rotation of one uncoordinated pyrazolyl group near palladium, i.e. axial one, around the bond axis of its 1-N nitrogen with the boron atom. (C) 2002 Elsevier Science B.V. All rights reserved.