Balance between Metal and Ligand Reduction in Diiminepyridine Complexes of Ti

Abstract

Reaction of the diiminepyridine ligand Et DIP (2,6-Et 2 -C 6 H 3 NCMe) 2 C 5 H 3 N) with TiCl 3 (THF) 3 gave the corresponding Ti(III) complex ( Et DIP)TiCl 3 (1). Reduction of 1 with 1 equiv of KC 8 produced the formally Ti(II) complex ( Et DIP)TiCl 2 (2). From this, ( Et DIP)TiClR complexes (R = Me (3a), Me 3 SiCH 2 (3b), Ph (3c)) were obtained by addition of 1 equiv of RLi. Similarly, dialkyl complexes ( Et DIP)TiR 2 (R = Me (4a), Me 3 SiCH 2 (4b)) were obtained with 2 equiv of RLi. All new complexes except 3b were characterized by single-crystal X-ray diffraction. EPR studies indicate that complex 1 is best regarded as a true Ti(III) complex with an "innocent" DIP ligand. Complexes 2−4 are all diamagnetic. In contrast to DIP complexes of the late transition metals Fe and Co, the new complexes 2−4 show strong upfield 1 H NMR shifts for the pyridine β and γ protons caused by transfer of negative charge to the DIP ligand. On the basis of this and the CN and C imine −C Py bond lengths, a description involving Ti(IV) and a dianionic ligand seems most appropriate, and DFT calculations support this interpretation. This means that reduction of Ti(III) complex 1 results in oxidation of the metal center to Ti(IV). VT-NMR studies of 4a suggest a small and temperature-dependent thermal population of a triplet state, and indeed calculations indicate that 4a has the lowest singlet− triplet energy difference of the systems studied.

Formation and Rearrangement of Reduced Diiminepyridine Complexes of Zr and Hf

“…The spectrum of 2b sharpens on cooling to -5°C and does not undergo changes on further cooling. This behaviour is consistent with the rapidly inverting square pyramidal type structure reported previously for [( Et DIP)TiCl 2 ], [36] and a single-crystal Xray structure confirms that proposal ( Figure 3). The elongated imine C=N bonds and shortened C im -C py bonds indicate that reduction has happened at the ligand, just as in the Ti series.…”

“…Analyses were problematic in several cases. Hf complexes ( 2b , 3b and 4b ) appear to give too‐low C values, which we tentatively attribute to partial formation of Zr carbides during combustion (like related Ti complexes described in our previous paper). In addition, several complexes crystallized with solvent of crystallization that was likely only loosely bound ( 2a , 2b , 4b ) and may have been partly lost on drying.…”

“…Both showed sharp NMR spectra indicative of effective C 2 v symmetry. In this respect they differ from the corresponding Ti complexes . [( Et DIP)TiMe 2 ] showed temperature‐dependent shifting and broadening of the signals, which was attributed to thermal population of a triplet state; for the corresponding Hf complex the singlet–triplet gap appears to be larger.…”

“…Treatment of 1a / b with two equivalents of KC 8 in toluene resulted in formation of purple, formally divalent complexes [( Et DIP)MCl 2 ] ( 2a / b , Scheme ). Use of less than two equivalents of KC 8 led to mixtures of 2 and starting material; formation of [( Et DIP)MCl 3 ] complexes analogous to known [( Et DIP)TiCl 3 ] was not observed (disproportionation of Zr III and Hf III complexes is common but not universal).…”

“…Treatment of 1a/b with two equivalents of KC 8 [36] was not observed (disproportionation of Zr III and Hf III complexes is common but not universal [38] ).…”