Mono- vs. bi-metallic assembly on a bulky bis(imino)terpyridine framework: a combined experimental and theoretical study

The bis(imino)terpyridine ligands, 6,6″-{(2,6-i-Pr[subscript 2]C[subscript 6]H[subscript 3])N=CR}[subscript 2]-2,2′:6′,2″-C[subscript 15]H[subscript 9]N[subscript 3] (R = H L1, Me L2), have been prepared in high yield from the condensation reaction of the corresponding carbonyl compound with two equivalents of 2,6-diisopropylaniline. The molecular structure of L2 reveals a transoid relationship between the imino and pyridyl nitrogen groups throughout the ligand framework. Treatment of aldimine-containing L1 with one equivalent or an excess of MX[subscript 2] in n-BuOH at 110 °C gives the mononuclear five-coordinate complexes, [(L1)MX[subscript 2]] (M = Fe, X = Cl 1a; M = Ni, X = Br 1b; M = Zn, X = Cl 1c), in which the metal centre occupies the terpyridine cavity and the imino groups pendant. Conversely, reaction of ketimine-containing L2 with excess MX[subscript 2] in n-BuOH at 110 °C affords the binuclear complexes, [(L2)M[subscript 2]X[subscript 4]] (M = Fe, X = Cl 3a; M = Ni, X = Br 3b; M = Zn, X = Cl 3c), in which one metal centre occupies a bidentate pyridylimine cavity while the other a tridentate bipyridylimine cavity. [superscript 1]H NMR studies on diamagnetic 3c suggests a fluxional process is operational at ambient temperature in which the central pyridine ring undergoes an exchange between metal coordination. Under less forcing conditions (room temperature in dichloromethane), the monometallic counterpart of 1b [(L2)NiBr[subscript 2]] (2b) has been isolated which can be converted to 3b by addition of one equivalent of (DME)NiBr[subscript 2] (DME = 1,2-dimethoxyethane) in n-BuOH at 110 °C. Quantum mechanical calculations (DFT) have been performed on [(L1)ZnCl[subscript 2]] and [(L2)ZnCl[subscript 2]] for different monometallic conformations and show that 1a is the energetically preferred structure for L1 while there is evidence for dynamic behaviour in L2-containing species leading to bimetallic formation. Single-crystal X-ray diffraction studies have been performed on 1a, 1b, 1c, 2b, 3a, 3b(H[subscript 2]O) and 3c.