Four different (β-diketonate)3V complexes [β-diketonate = 2,4-pentanedione (acac), 2-acetylcyclohexanone (Cy-acac), 2,2,6,6-tetramethyl-3,5-heptanedione (t-Bu-acac), and 1,1,1,5,5,5-hexafluoro-2,4-pentanedione (F-acac)] have been prepared and tested as catalysts for propylene−ethylene copolymerization with the aim of gaining insights into the structure of the active species. Data on polymer composition and catalyst activity indicate that one of the roles of the Al cocatalyst is to trigger a major ligand scrambling around the transition metal. Attempts to isolate the catalytically active species afforded a compound formulated as {[(β-diketonate)AlCl2][VCl2][(β-diketonate)2AlCl]}. This species, which displayed only a minor catalytic activity, arises from a parasite process (catalyst deactivation). The formulation was supported by chemical degradation experiments with THF, which afforded a mixture of [V2Cl3(THF)6][AlCl4] and [(acac)2Al(THF)2][Al Cl4] (5). Two unprecedented V(II) complexes (R-acac)2V(TMEDA) [R = H(7a), Cy(7b), t-Bu (7c)] have been prepared and reacted with halocarbons to model the reactivation process. The results indicated that the primary role of reactivating substances, commonly employed in the industrial processes, is to reoxidize V(II) to the trivalent state. The reaction formed the catalyst precursor (R-acac)2VCl(TMEDA), which was characterized on the basis of analytical and spectroscopic data. In agreement with this proposal, a trapping experiment carried out with ZnCl2 or oxidation with CuCl allowed the isolation and characterization of (t-Bu-acac)2V(TMEDA)][X] [X = ZnCl4 - (8), CuCl2 - (9)]. The structures of 5, 7a, 8, and 9 have been elucidated by X-ray diffraction. Crystal data are as follows. 5: triclinic space group P1̄, a = 8.916(3) Å, b = 9.802(3) Å, c = 15.390(5) Å, α = 88.156(4)°, β = 86.041(4)°, γ = 82.710(4)°, Z = 2. 7a: triclinic space group P1̄, a = 7.896(1) Å, b = 10.032(2) Å, c = 13.134(2) Å, α = 75.265(2)°, β = 88.520(2)°, γ = 71.445(2)°, Z = 2. 8: orthorhombic space group Pbcn, a = 18.723(7) Å, b = 19.675(7) Å, c = 18.836(7) Å, Z = 4. 9: monoclinic space group C2/c, a = 13.676(5) Å, b = 19.521(3) Å, c = 13.206(3) Å, β = 98.47(3)°, Z = 4.

] complex reported as a byproduct of the reaction between (acac) 3 V and AlCl 3 (THF) 3 (right-hand column, 14th line from the top), we wish to acknowledge that this complex was previously isolated upon reaction of Cl 2 Al(acac) with 2 equiv of THF in methylene chloride (Lewinski, J.; Pasynkiewicz, S. Inorg. Chim. Acta 1987, 130, 23-27) and correctly formulated on the basis of spectroscopic characterization. Several other relevant aluminum acetylacetonate complexes have been described and characterized by NMR spectroscopy and elemental analysis (Lewinski, J.; Pasynkiewicz, A.

Patients undergoing assisted reproduction techniques can be counseled for maternal serum Down syndrome screening with the same efficacy as patients with naturally conceived pregnancies.

Trisomy 21 maternal serum marker screening has led to screening for other anomalies, including trisomy 18. Trisomy 18 is generally prenatally diagnosed because of major morphological defects. However, in up to 30% of cases ultrasound signs are unclear, and in most cases diagnosis is performed late in pregnancy. Of the different maternal serum markers, PAPP-A is now considered as the best for trisomy 18 screening. However, pregnancy-associated plasma protein A (PAPP-A) is of value in first trimester screening for trisomy 21, but not in the second trimester. We therefore propose a two-step screening strategy. Based on 45 trisomy 18 cases, we confirm the values of alpha-fetoprotein (AFP) (median 0.61 MoM), free beta-human chorionic gonadotrophin (beta-hCG) (median 0.24 MoM) and of PAPP-A (median 0.08 MoM). In the first step, a 0.5 MoM cut-off for AFP or for free beta-hCG resulted in detection of 37/45 trisomy 18 cases (82%) with a 10% false-positive rate. The second step consisted of the measurement of PAPP-A for all these false-positive cases. Using a PAPP-A cut-off of 0.5 MoM, all the 37 trisomy 18 cases were detected, but now with a 0.1-0.2% false-positive rate. Amniocentesis was only offered to these few patients. This two-step second trimester screening will be of value for patients who have not been included in first trimester screening based on nuchal translucency (NT) measurement combined with the first trimester markers, PAPP-A and free beta-hCG.

Once technical errors have been excluded (repeat assay in a second run, calcium assayed to exclude the interference of EDTA for fluorimetric methods, dilution to exclude interfering antibodies, running on an alternative analyser, checking a second sample), very low second-trimester maternal serum AFP should prompt ultrasound examination in order to check fetal viability. Congenital AFP deficiency, an extremely rare disorder (1/100 000), should be suspected. It has no consequences for fetal and infant development, and parents should be reassured.