Multitopic third generation tris(pyrazolyl)methane ligands built on alkyne structural scaffolding: first preparation of mixed tris(pyrazolyl)methane/tris(pyrazolyl)borate ligands

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Abstract
A series of new multitopic ligands with rigid linear geometry are formed by joining tris(pyrazolyl)methane and tris(pyrazolyl)borate units with arenyl and alkynyl linkers using Sonogashira and related alkynyl coupling reactions. These ligands are new examples of “third generation” poly(pyrazolyl)borate and poly(pyrazolyl)methane ligands, ligands functionalized at the non-coordinating “back” positions of either the boron or central carbon atoms. The reaction of Na[OCH2C(pz)3] with propargyl bromide yields HC2CH2OCH2C(pz)3 (2) and homocoupling of this alkyne yields [-C2CH2OCH2C(pz)3]2. The reaction of Na[OCH2C(pz)3] with 3,5-(BrCH2)2C6H3I yields 3,5-[(pz)3CCH2OCH2]2C6H3I (4), which can be converted to 3,5-[(pz)3CCH2OCH2]2C6H3(C2H) (6) by reaction with HC2SiMe3 followed by removal of the SiMe3 group. Compounds 4 and 6 can be combined to form {3,3′,5,5′-[(pz)3CCH2OCH2]4(1,1′-C6H3C2C6H3) (7) and 6 homocoupled to form {3,5-[(pz)3CCH2OCH2]2C6H3C2-}2. Compound 6 reacts with p-I2C6H4 to produce 3,3′,5,5′-[(pz)3CCH2OCH2]4[p-(1,1′-C6H3C2)2C6H4], which can also be formed by the reaction of 4 with bis(ethynyl)benzene. The reaction of 2 with Fe[(p-IC6H4)B(pz)3]2 yields the bitopic, metalloligand Fe[(pz)3CCH2OCH2-C2-C6H4B(κ3-N,N′,N″-pz)3]2 (10) and a similar reaction with 6 yields the tetratopic metalloligand Fe[{3,5-[(pz)3CCH2OCH2]2C6H3C2}C6H4B(κ3-N,N′,N″-pz)3]2. The molecular structures of 2, 4, 7, and 10·4CH2Cl2 are reported and their supramolecular structures, organized by a series of CH⋯I and CH–π interactions, are detailed.