1,1 '-diphosphaferrocene derivatives for labeling of molecules of biological interest. The X-ray molecular structures of 4-oxo-4-(3,3 ',4,4 '-tetramethyl-1,1 '-diphosphaferrocen-2-yl)butanoic acid and its bis-W(CO)(5) complex

3,3',4,4'-Tetramethy1-1,1'-diphosphaferrocene (1) reacts with succinic anhy dride in the presence of AlCl3 to give 4-oxo- 4-(3,3',4,4'-tetramethyl-1,1- diphosphaferrocen-2-yl)butanoic acid (2) in 80% yield. This complex treated with 1 equiv of W(CO)(5)(THF) gave a mixture of the products in which the W(CO)(5) moiety is coordinated to P(1') (3a) and to P(1) (3b) in the ratio similar to 7:1. With two equiv of W(CO)(5)(THF) the bis-W(CO)(5) adduct (4) was obtained and transformed into the corresponding N-succinimidyl ester ( 5) by treatment with N-hydroxysuccinimide in dichloromethane in the presenc e of DCC (dicyclohexylcarbodiimide). This activated ester reacts with benzy lamine and glycine methyl ester to give the corresponding amides in good yi elds. The crystal structures of 2 and 4 have been determined by X-ray diffr action. Both structures show unexpected conformations with short intramolec ular P- - -P distances, indicating secondary bonding between phosphorus ato ms. This contrasts with the X-ray structure of 1 displaying longer P- - -P distance (no secondary bonding) and theoretical calculations showing destab ilizing, repulsive P---P interactions in the 1,1'-diphosphaferrocene system (Ashe, A. J., III; Al-Ahmad, S. Adv. Organomet. Chem 1996, 39, 325, and re ferences therein). Apparently, conformational preferences of 1,1'-diphospha ferrocenes are small, since they are so easily changed by small changes in the substitution. Furthermore, the structure of 2 shows intermolecular P- - -P secondary bonding, giving rise to the formation of stacking columns alo ng the z axis. The structural differences between 2 and 4 and between the W (CO)(5) moiety in 4 and this moiety coordinated to nitrogen in W(CO)(5)-(az aferrocene) complexes have also been analyzed.