Isotypical N,N-dialkylcarbamato lanthanide complexes covering a range of 11 atomic numbers: direct experimental assessment of the lanthanide contraction in trivalent molecular compounds Ulrich Abram, a Daniela Belli Dell’Amico, b Fausto Calderazzo,* b Cinzia Della Porta, b Ulli Englert, c Fabio Marchetti d and Alessandra Merigo b a Forschungszentrum Rossendorf, Institute of Radiochemistry, c/o Dresden University of Technology, Institute of Analytical Chemistry, 01062 Dresden, Germany b Dipartimento di Chimica e Chimica Industriale, Università di Pisa, via Risorgimento 35, 56126 Pisa, Italy. E-mail: facal@dcci.unipi.it c Institut für Anorganische Chemie der RWTH, Professor-Pirlet-Strasse 1, D-52074 Aachen, Germany d Dipartimento di Ingegneria Chimica, dei Materiali, delle Materie Prime e Metallurgia, Università «La Sapienza» di Roma, via del Castro Laurenziano, 7, Box 15 Roma 62, I-00185 Roma, Italy Received (in Basel, Switzerland) 15th June 1999, Accepted 7th September 1999 The N,N-diisopropylcarbamato complexes of lanthanide- s(iii) from neodymium (Z = 60) to ytterbium (Z = 70) were found to be isotypical, with Ln–O distances decreasing steadily with increasing Z, the lanthanide contraction in this series of molecular complexes being provisionally assigned to an increasing bond strength with increasing atomic number of the central metal atom. The well known lanthanide contraction was originally recog- nized 1a on the basis of X-ray diffractometric data on the ionic oxides and fluorides of the trivalent cations and estimated to correspond to about a 15% decrease of the radii on going from cerium to lutetium, 1 a correction 1b,c being required for the decreasing coordination numbers (CN) along the series. In fact, no isotypical coordination compounds of lanthanides are known, for halide or oxide ligands, to cover the whole series; normally, changes of both space group and CN are encountered, on going from the first part to the second one 1d,e of the series. Coordination chemistry of lanthanides (Ln) is a subject of increasing interest, 2 the focus of the current studies being on uncharged complexes of Ln(iii), 3 in view of their role in biochemistry 3a and in carbon–carbon and carbon–nitrogen bond forming reactions, 3b–r mainly catalyzed by organometallic derivatives 4 of these elements. A better understanding of these systems should originate from the synthesis of isotypical (i.e. with constant CN and geometry) and neutral compounds and by the determination of their interatomic parameters through diffraction methods. We now report the preparation of some isotypical N,N- dialkylcarbamato complexes of the lanthanides. Although N,N- dialkylcarbamato complexes of transition d elements are well established, 5 such lanthanide derivatives are still in their infancy, mainly due to synthetic difficulties. 6 The synthesis of the new N,N-diisopropylcarbamato deriva- tives of La, Nd, Eu, Gd and Ho, of the same general formula, has now been achieved by using the ether complexes LnCl 3 (ether) x (ether = THF or Et 2 O), 7 as starting materials [eqn. (1)]. Yields of recrystallized product are generally moderate (20%) to satisfactory (50%). The new isopropyl derivatives of Nd(iii), Eu(iii), Gd(iii) and Ho(iii) were found to be moderately soluble in hydrocarbons; these compounds are isotypical with one another and with the already known Yb(iii) analogue. 6 Complete crystal data were collected for the Nd (Z = 60), Gd (Z = 64), and Ho (Z = 67) derivatives,† to be compared with the already available information for the isostructural Yb species (Z = 70). 6 In these tetranuclear compounds, the heptacoordinate lanthanide is surrounded by terminal bidentate, bridging bidentate and bridging tridentate diisopropylcarba- mato groups. The molecular core of the tetranuclear derivatives is schematically shown in Fig. 1. The Ln–O average bond distances of the new N,N-diisopro- pylcarbamato complexes can be factorized as a function of the type of bonding (Table 1, Fig. 2). The Ln–O bond distance contracts along our series with increasing Z; more precisely, by assuming that the lanthanide-coordinated oxygen has the same radius of 1.21 Å as that of bicoordinated oxygen, 1c the lanthanide contraction, averaged over the four different types of ligands (Table 1) is ca. 8% over the eight Z values from Nd to Fig. 1 Molecular structure of the lanthanide derivatives M 4 (O 2 CNPr i 2 ) 12 ; M = Nd (Z = 60), Gd (Z = 64), Ho (Z = 67), Yb (Z = 70). The heptacoordinated metal atoms are shown with their oxygen donor atoms. Table 1 Average Ln–O bond distances (Å) in tetranuclear N,N-diisopro- pylcarbamato complexes of lanthanides, for different coordination modes. The quadratic dispersion from the average values is given in parentheses. This journal is © The Royal Society of Chemistry 1999 Chem. Commun., 1999, 2053–2054 2053