Bisphosphino ureas: flexible ligands with rigid backbone q Olaf Ku ¨hl * , Walter Langel Institut f€ ur Chemie und Biochemie, Ernst-Moritz-Arndt-Universit€ at Greifswald, Soldmannstr. 16, D-17487 Greifswald, Germany Received 26 July 2002; accepted 14 October 2002 Dedicated to Professor Frederick Jardine on the occasion of his 65th birthday. Abstract The nickel(0) complex ½Nifg 2 -ðPPh 2 NMeÞ 2 COgfg 1 -ðPPh 2 NMeÞ 2 COg 1 was obtained by the reaction of NiCl 2  6H 2 O with two equivalents of the ligand ðPPh 2 NMeÞ 2 CO in the presence of zinc as reducing agent. The structure of 1 was unambiguously de- termined by 1 H and 31 P–{ 1 H} NMR spectroscopy and compared to the known structures of the extremely strained Pd(II) and Mo(0) complexes of ðPPh 2 NMeÞ 2 CO. Ó 2002 Elsevier Science B.V. All rights reserved. 1. Introduction Bisphosphino urea derivatives have recently attracted renewed attention [1,2]. Only comparatively few transi- tion metal complexes of this ligand class are known to date. The structurally charaterised examples are even fewer [1]. As catalytic properties of diphosphine ligands are determined by electronic properties and by the value of the natural bite angle [3] it is of great interest to know this angle. There are several methods to determine the natural bite angle of a ligand [3]. Amongst the most prominent ones are the ‘‘ligand preferred’’ bite angle [4] in which a ‘‘dummy metal’’ is employed and a statistical approach. In the dummy metal method the potential minimum of the ligand conformation is determined ir- respective of the preferences of the metal. The statistical method uses average P–P distances found in the Cam- bridge Crystallographic Database (CSD) [3]. However, the few structurally characterised transi- tion metal complexes of bisphosphino urea ligands give reason to believe that both approaches might not give justice to this ligand class. To address this problem the ligand ðPPh 2 NMeÞ 2 CO was reacted with a Ni(II) salt in the presence of zinc as reducing agent. The aim was to supply the closest ap- proach to a dummy metal possible in a real system. In the present case this was the choice between a tetrahe- dral NiL 4 complex and metal preferred bite angle of 109.5° and a trigonal planar arrangement NiL 3 with a metal preferred bite angle of 120°. 2. Experimental 2.1. Synthesis of ½Nifg 2 -ðPPh 2 NMeÞ 2 COgfg 1 -ðPPh 2 NMeÞ 2 COg (1) Solid NiCl 2  6H 2 O (324 mg, 1.36 mmol) was added to a solution of 1.243 g (2.73 mmol) ðPPh 2 NMeÞ 2 CO in 25 ml thf. After stirring for 10 min 178 mg zinc dust was given to the slurry. Immediately the solution turned dark yellow and all the solids with the exception of ex- cess zinc dissolved. After stirring overnight, the reaction mixture was filtered and the filtrate concentrated under reduced pressure. The yellow product was precipitated with hexane. After filtration the product was dried un- der vacuum to afford 1.35 g (98% yield) of complex 1  0:5 thf, melting point 126–128 °C. Anal. Calcd. for C 54 H 52 N 4 NiO 2 P 4 : C, 66.75%; H, 5.39%; N, 5.77%. Found: C, 66.48%; H, 5.57%; N, 5.63%. No crystals Inorganic Chemistry Communications 6 (2003) 74–77 www.elsevier.com/locate/inoche q Work presented at the 35th International Conference on Coordi- nation Chemistry held in Heidelberg, Germany from 21 to 26 July 2002. * Corresponding author. Tel.: +49-3834-864319; fax: +49-3834- 864329. E-mail address: kuhl@uni-greifswald.de (O. Ku ¨hl). 1387-7003/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII:S1387-7003(02)00689-5