Pursuit of an Electron Deficient Titanium Nitride

Abstract

The nitride salt [(PN) 2 Ti≡N{μ 2 -K(OEt 2 )}] 2 (1) (PN − = (N-(2-P i Pr 2 -4-methylphenyl)-2,4,6-Me 3 C 6 H 2 ) can be oxidized with two equiv of I 2 or four equiv of ClCPh 3 to produce the phosphinimide-halide complexes (NPN'), respectively. In the case of 2, H 2 was found to be one of the other products; whereas, HCPh 3 and Gomberg's dimer were observed upon the formation of 3. Independent studies suggest that the oxidation of 1 could imply the formation of the transient nitridyl species [(PN) 2 Ti(≡N•)] (A), which can either oxidize the proximal phosphine atom to produce the Ti(III) intermediate [(NPN')(PN)Ti] (B) or, alternatively, engage in H atom abstraction to form the parent imido (PN) 2 Ti≡NH (4). The latter was independently prepared and was found to photochemically convert to the titanium-hydride, (NPN')(PN)Ti(H) (5). Isotopic labeling studies using (PN) 2 Ti≡ND (4-d 1 ) as well as reactivity studies of 5 with a hydride abstractor demonstrate the presence of the hydride ligand in 5. An alternative route to putative A was observed via a photochemically promoted incomplete reduction of the azide ligand in (PN) 2 Ti(N 3 ) (6) to 4. This process was accompanied by some formation of 5. Frozen matrix X-band EPR studies of 6, performed under photolytic conditions, were consistent with species B being formed under these reaction conditions, originating from a low barrier N-insertion into the phosphine group in the putative nitridyl species A. Computational studies were also undertaken to discover the mechanism and plausibility of the divergent pathways (via intermediates A and B) in the formation of 2 and 3, and to characterize the bonding and electronic structure of the elusive nitrogen-centered radical in A.

Divalent Titanium via Reductive N−C Coupling of a TiIV Nitrido with π‐Acids

“…Although treatment of 1 with CO and CNAd in the presence of Kryptofix222 and NaN 3 in thf resulted in immediate formation of parent cumulene salts such as NaNCO and NaNCNAd, respectively (confirmed by IR spectroscopy), [17] catalysis was shunned due to the deleterious formation of the parent imide [(PN) 2 Ti(NH)] via an unstable and putative Ti II azide intermediate [K(Kryptofix222)][(PN) 2 Ti(N 3 )]. Previous studies have invoked the extrusion of N 2 from [(PN) 2 Ti(N 3 )] to accompany the formation of a transient nitridyl radical [(PN) 2 Ti(N⋅)], which engages in H‐atom abstraction, most likely from the PN ligand [28] . As a result, a catalytic cycle involving N‐atom transfer from 1 to CO and CNAd is not yet possible due to the inaccesibility of direct formation of titanium nitride anion complex, 1 , rather the detrimental parent imide, 12 is formed (Scheme 3).…”

“…Previous studies have invoked the extrusion of N 2 from [(PN) 2 Ti(N 3 )] to accompany the formation of a transient nitridyl radical [(PN) 2 Ti(N * )], which engages in H-atom abstraction, most likely from the PN ligand. [28] As a result, a catalytic cycle involving N-atom transfer from 1 to CO and CNAd is not yet possible due to the inaccesibility of direct formation of titanium nitride anion complex, 1, rather the detrimental parent imide, 12 is formed (Scheme 3). Despite the parent imido forming from this reaction, we could close the synthetic cycle for N-atom transfer by treating [(PN) 2 Ti-(NH)] with KCH 2 Ph to reform the nitride moiety as shown in Scheme 3.…”

Untangling ancillary ligand donation versus locus of oxidation effects on metal nitride reactivity

“…[33][34][35][36][37][38][39][40][41][42][43][44][45] Conversely, early transition-metal nitrides are generally more stable [46][47][48] and in some cases demonstrate nucleophilic reactivity via lled M^N p orbitals or the nitride lone pair. [49][50][51][52][53] Certain metal nitride complexes have been reported to exhibit ambiphilic reactivity, highlighting that subtle modications to the coordination environment and/or redox changes can enable diverse reactivity. 35,[54][55][56] Previous studies by Mayer et al 57,58 and Lau et al 59 have elucidated how changes in ancillary ligands can impact nitride reactivity, work by Holland et al demonstrated that oxidation of both the metal and ancillary ligand (from amide to nitroxide) led to a change from nucleophilic to electrophilic nitride reactivity, 60 and a recent study by Burger and co-workers showed that oxidation of an Ir nitride complex resulted in insertion of the nitride into an aromatic C-C bond of ferrocene.…”