Haptotropic Shift of [5]Cumulenes in Zirconocene Complexes and Effects of Steric Factors

Abstract

Zirconium complexes of some [5]cumulene derivatives were studied for their variable coordination modes and haptotropic shifts. Some [5]cumulene compounds reacted with zirconocene(II) species to afford 1-zirconacyclopent-3-yne complexes that have five-membered cycloalkyne structures. Only a few [5]cumulene compounds afforded η 2 -coordinated complexes in the presence of neutral ligands such as trimethylphosphine and tert-butyl isocyanide. Interconversion between the five-membered structure and the η 2 -complex was observed. Investigation of [5]cumulene derivatives of various cycloalkylidene moieties indicated that the η 2 -complex was preferred when the [5]cumulene has bulkier substituents. A [5]cumulene with 2,2,6,6-tetramethylcyclohexylidene groups much preferred the 1-zirconacyclopent-3-yne structure to η 2 -coordination. In sharp contrast, the η 2 -coordinated complex was favored for a [5]cumulene with 2,2,7,7-tetramethylcycloheptylidene groups in the presence of PMe 3 . Small differences in steric environments caused totally different reactivity in [5]cumulene complexes. DFT calculations on the formation enthalpy were consistent with the experimental results, although that cannot fully rationalize the difference.

The Role of Bulkiness in Haptotropic Shifts of Metal–Cumulene Complexes

“…Contrary to 2a , the tautomer that crystallized was the symmetric one (Figure ); this was the first late‐transition‐metal complex that crystallized in configuration A. The elongation of the carbon double bond with respect to that of a similar free ligand is comparable to the elongation observed for 2a , also around 0.13 Å. The elongation is less pronounced in the case of an equivalent early‐transition‐metal complex, t Bu 4 [5]cumul~eneulene–[Cp 2 ZrPMe 3 ] ( 5b ) .…”

“…Interestingly, some of these few examples present haptotropic shifts . The metal moves between the central double bond (tautomer A) and the contiguous bond (tautomer B), as shown in Scheme . In no case is the metal coordinated to the terminal double bond of the cumulene (tautomer C).…”

Formation of Binuclear Zigzag Hexapentaene Titanium Complexes via a Titanacumulene [Ti=C=C=CH2] Intermediate

“…Theb ond lengths of the terminal methylene groups C61ÀC62 (1.3430-(14) )a nd C65ÀC66 (1.3425 (14) )a re in typical range of C sp 2 À C sp 2 double bonds (1.32 [19] )a sw ell as the newly formed C63 À C64 bond (1.3592 (14) ). These elongated bond lengths are due to the coordination of the hexapentaene to the two titanocene fragments repealing the linear cumulene motif.T his feature compares to those found for olefine [21] or cumulene [20,22] complexes as well as the Ti À Cd istances from 2.0930 (11) to 2.3028 (11) . These elongated bond lengths are due to the coordination of the hexapentaene to the two titanocene fragments repealing the linear cumulene motif.T his feature compares to those found for olefine [21] or cumulene [20,22] complexes as well as the Ti À Cd istances from 2.0930 (11) to 2.3028 (11) .…”

Formation of Binuclear Zigzag Hexapentaene Titanium Complexes via a Titanacumulene [Ti=C=C=CH2] Intermediate

“…Zuschriften the coordination of the hexapentaene to the two titanocene fragments repealing the linear cumulene motif.T his feature compares to those found for olefine [21] or cumulene [20,22] complexes as well as the Ti À Cd istances from 2.0930 (11) to 2.3028 (11) . [22b,23] Ti2b crystallizes as green plates in the triclinic space group P " 1, the molecular structure is shown in Figure 2.…”