Reactions of heteroallenes with cyclam-based Zr(iv) complexes

Abstract

This work describes reactions of heteroallenes with diamido–diamine cyclam-based Zr(IV) complexes of the general formula (Bn₂Cyclam)ZrX₂ (X = O^tBu, 1, OⁱPr, 2, SPh, 3, NH^tBu, 14) as well as the di-orthometallated species ((C₆H₄CH₂)₂Cyclam)Zr, 17. The reactions of isocyanates or isothiocyanates with 1, 2 or 3 resulted in the formation of N-bonded ureate or thioureate cyclam complexes upon [2 + 2] cycloaddition of the Zr–N_amido bonds of the cyclam to the heteroallene (4–9). DFT calculations showed that κ²-N,N′-ureate bonding is favoured over κ²-N,O-ureates, which in turn may be formed in reactions with bulky isocyanates as 1-naphthyl isocyanate (NpNCO). The reactions of 2 with N,N′-cyclohexylcarbodiimide (CyNCNCy) and carbon disulfide afforded guanidinate and dithiocarbamate fragments, respectively, appended to one of the nitrogen atoms of the cyclam ligand. These reactions represent a reliable method for the synthesis of asymmetrically N-functionalized cyclams giving rise to C₁ symmetry Zr(IV) species by addition of one equivalent of heteroallenes. The reaction of (Bn₂Cyclam)Zr(NH^tBu)₂, 14, with one equivalent of mesityl isocyanate (MesNCO) also proceeds through insertion, involving one Zr–NH^tBu bond. However, it was observed that the reaction of (Bn₂Cyclam)Zr(NH^tBu)₂, 14, with MesNCO follows a different path if the reaction is carried out at 60 °C. In this case the reaction leads to [2 + 2] addition of the Zr–N_cyclam bond to the isocyanate, with a concomitant occurrence of orthometallation of the one benzyl pending group of the cyclam ring. The reaction of ^tBuNCO with the di-orthometallated complex ((C₆H₄CH₂)₂Cyclam)Zr, 17, also gave a κ²-N,N′-ureate fragment, by isocyanate addition to the macrocycle. DFT calculations on these systems were conducted in an attempt to rationalise the reactivity patterns observed.