Chemistry of o-xylidene-metal complexes. Part 1. o-Xylidene-magnesium reagents as metallocyclic precursors and synthesis of [Pt(CH₂C₆H₄CH₂-o)(cod)] (cod = cyclo-octa-1,5-diene); the X-ray crystal structure of the macrometallocycle [{Mg(CH₂C₆H₄CH₂-o)(C ₄H₈O)₂}₃]

A high-yield synthesis of a tetrahydrofuran (thf) solution of the di-Grignard reagent (A) derived from o-bis(chloromethyl)benzene is described, as well as that of an analogue obtained from 1,2-bis(chloromethyl)-4,5-dimethylbenzene. Cooling (A) to ca. -40 °C yields the colourless chloride-free Mg(CH₂C₆H₄CH₂-o)(thf) of unknown structure, whereas at ambient temperature a concentrated solution (> ca. 0.1 mol dm^-3) slowly (days) deposits colourless needles of [{Mg(CH₂C₆H₄CH₂-o)(thf) ₂}₃]. A single-crystal X-ray structure determination of the latter (R = 0.054 for 1 117 'observed' reflections at 295 K) shows it to be a trimer. Crystals are orthorhombic, space group F2dd. with a = 24.706(8), b = 8.948(3), c = 44.315(9) Å, and Z = 8; the trimeric unit lies on a crystallographic two-fold axis. Each of the three magnesiums is bridged to the other two by a ^-CH₂C₆H₄CH₂^--o ligand, the pseudo-tetrahedral co-ordination about each magnesium atom being completed by a pair of thf molecules. The utility of the di-Grignard reagent as a general metallocycle precursor is illustrated by the synthesis of [Pt(CH₂C₆H₄CH₂-o)(cod)] (cod = cycle-octa-1,5-diene) from [Ptl₂(cod)]. In contrast [PtCl₂(dppe)] with [{o-C₆H₄(CHSiMe₃)₂}{Li(tmen)} ₂] (dppe = Ph₂PCH₂CH₂PPh₂, tmen = Me₂NCH₂CH₂NMe₂) affords [Pt(dppe)₂].