New Palladium-Catalyzed Cross-Coupling Routes to Carbon Functionalized Metallatricarbadecaboranes

A general method for the synthesis of cage-carbon-functionalized
cyclopentadienyl iron and cyclopentadienyl ruthenium tricarbadecaboranyl
complexes has been developed that employs palladium-catalyzed Sonogashira,
Heck, and Stille cross-coupling reactions directed at a cage-carbon
haloaryl substituent. The key Li+[6-(p-XC6H4)-nido-5,6,9-C3B7H9–] (X = I (1), Br (2), Cl (3)) haloaryl–tricarbadecaboranyl
anionic ligands were synthesized in high yields via the reaction of
the arachno-4,6-C2B7H12– anion with the corresponding p-halobenzonitriles (p-XC6H4-CN). The reactions of the salts 1–3 with (η5-C5H5)­Fe­(CO)2I and (η5-C5H5)­Ru­(CH3CN)3PF6 were then used to produce the
haloaryl complexes 1-(η5-C5H5)-2-(p-XC6H4)-closo-1,2,3,4-MC3B7H9 (M = Fe, X = I
(4), Br (5), Cl (6) and M =
Ru, X = I (7), Br (8), Cl (9)). The sonication-promoted Sonogashira coupling reactions of 4 with terminal alkynes catalyzed by Pd­(dppf)2Cl2/CuI yielded the alkynyl-linked derivatives 1-(η5-C5H5)-2-p-RC6H4-closo-1,2,3,4-FeC3B7H9 (R = (PhCC)- (10), (CH3CH2C­(O)­OCH2CC)- (11), ((η5-C5H5)­Fe­(η5-C5H4CC))- (12)).
Heck reactions of 4 with terminal alkenes catalyzed by
Pd­(OAc)2 yielded the alkene-functionalized products 1-(η5-C5H5)-2-p-RC6H4-closo-1,2,3,4-FeC3B7H9 (R = (PhCH2CHCH)- (13), (CH3(CH2)2CHCH)-
(14)), while the Stille cross-coupling reactions of 4 with organotin compounds catalyzed by Pd­(PPh3)2Cl2 afforded the complexes 1-(η5-C5H5)-2-p-RC6H4-closo-1,2,3,4-FeC3B7H9 (R = Ph- (15), (CH2CH)-
(16), (CH2CHCH2)- (17)). These reactions thus provide facile and systematic access
to a wide variety of new types of functionalized metallatricarbadecaboranyl
complexes with substituents needed for potential metallocene-like
biomedical and/or optoelectronic applications.