"Embroidered" square pyramidal coordination caps for nickel(II): Mono-, Di- and tetrafunctionalisation of a tetrapodal pentadentate ligand with an NN4 donor set

The tetrapodal pentaamine ligand 2,6-bis(1',3'-diamino-2'-methylprop-2'-yl) pyridine ill, which contains four equivalent primary amino groups, can be d erivatised partly or completely by Schiff base condensation with suitable c arbonyl compounds. The new ligands thus obtained are mononucleating, as sho wn by the X-ray crystal structures of their respective nickel(II) complexes . Reaction of 1 with 1 equiv. of salicylaldehyde and subsequent reduction a llows the selective modification of one of the four sidearms. The resulting hgand 2 is hexadentate and uninegative in its nickel(II) complex {[(2)Ni]P F6} (3) with both the secondary amine and the phenoxide functionalities coo rdinated to the metal centre. The unreduced Schiff base form of the ligand, 4, does not form a complex with nickel(II) as readily, and only small quan tity of the mixed salt {[(4)Ni][(1)Ni(H2O)](Br)(2)(PF6)} (5) has been obtai ned. While the overall coordination of 4 resembles that of 2, there is cons iderably more strain in the appended chelate ring, due to the presence of t he C=N double bond. Modification of one arm in 1 can also be achieved by co ndensation with 1 equiv, of acetylacetone, to give the new Ligand 6 which, likewise, is hexadentate in its Ni-II complex {[(6)Ni](PF6)(2)} (7). In thi s case, however, the N/O-functional sidearm is not deprotonated. Rather, it is coordinated as the keto-imine tautomer, making 7 a rare example of a me tal complex containing this structural fragment. Two-fold functionalisation of 1 is observed upon reaction with acetone, regardless of whether the ket one is present in stoichiometric amounts or in excess, to give the pentaden tate ligand 8 with two diagonally juxtaposed isopropylidene-imine units. Th e complex isolated with this ligand {[(8)Ni](PF6)(2)} (9) contains pentacoo rdinate Ni-II, the sixth coordination site being blocked by the rigidly pos itioned isopropylidene groups. When reacted with 4 equiv, of trans-cinnamal dehyde, all the primary amino groups in 1 condense to give the four-fold Sc hiff base 10, which acts as a pentadentate podand towards nickel(II). In th is complex, {[(10)Ni(OH2)]Br-2} (11), an aqua Ligand completes the coordina tion octahedron. All ligands are stable towards hydrolysis when coordinated to the metal, despite the presence of alkyl-imine groups in some cases.