Pyrazole as a Donor Function in Neopentane-Based Tripod Ligands RCH2C(CH2pyrazol-1-yl)3–n(CH2PR2)n – Synthesis and Coordination Chemistry

Abstract

The chlorine functions of CH3C(CH2Cl)3, 1, may be replaced by pyrazolyl (pz) as well as imidazolyl (im) residues under the conditions of nucleophilic substitution leading to tripodal ligands CH3C(CH2X)3, X = pz, 2; X = im, 3. As a means of introducing two nitrogen donors and one phosphorus donor into a tripod ligand, substitution of the Br and OMs functions in O(CH2)2C(CH2Br)(CH2OMs), 8, by nitrogen nucleophiles and subsequent cleavage of the oxetane ring by a phosphide nucleophile to give HOCH2C(CH2PPh2)(CH2X)2 has been developed, furnishing 10a (X = pz) and 10d (X = NEt2), respectively. For the synthesis of 10a, K-pz was used as the nucleophile, while 10d was prepared using azide in the initial step, which then had to be transformed into NEt2 in two subsequent steps. The nucleophugic functions of the oxetane 8 undergo selective substitution by K-pz and KPPh2 in THF to produce O(CH2)2C(CH2PPh2)(CH2pz), 9b. Phosphide cleavage of the oxetane function leads to HOCH2C(CH2PPh2)(CH2PR2)(CH2pz), R = Ph, 10b; R = 3,5-Me2(C6H3), 10c. – The tris(pyrazolyl) tripod ligand 2 reacts with (MeCN)3Mo(CO)3to give 2 · Mo(CO)3(MeCN), 12a, in which only two of the three donor functions are coordinated. Upon reaction with 10a, the same reagent gives 10a · Mo(CO)4, 12b, with one pyrazolyl coordinated and the other involved in intramolecular hydrogen bonding to the CH2OH function (N···H–O distance 280 pm). Blocking of the OH function of 10a by etherification, i.e. to form EtOCH2C(CH2PPh2)(CH2pz)2, 11, does not dramatically affect the coordination capabilities with 11 · Mo(CO)3(MeCN), 12d, being formed upon treatment with (MeCN)3Mo(CO)3. Again only one pz function is coordinated to the metal. Bidentate coordination by two phosphorus donors of 10c is observed in 10c · Mo(CO)3(MeCN), 12d. The dangling arm pz donor function and the CH2OH group are intermolecularly hydrogen-bonded in this case. When the bulky P[3,5-Me2(C6H3)]2 substituent of 10c is replaced by the less sterically demanding PPh2 donor in 10b, η3-coordination is finally observed with the formation of 10b · Mo(CO)3, 13. The coordination capabilities of the new ligands are rationalized in terms of the size (six-, seven-, and eight-membered rings) and interference of the chelate cycles. All compounds have been characterized by the usual analytical and spectroscopic methods, with a complete assignment of the NMR data achieved by a combination of 2D-NMR techniques in some cases. The structures of the coordination compounds have additionally been deduced by X-ray methods.