This is an open access article published under a Creative Commons Attribution (CC-BY) License, which permits unrestricted use, distribution and reproduction in any medium, provided the author and source are cited. Article pubs.acs.org/Organometallics Reactions of Titanium Imides and Hydrazides with Boranes Simona Mellino,† Laura C. Stevenson,† Eric Clot,*,‡ and Philip Mountford*,† † Chemistry Research Laboratory, Department of Chemistry, University of Oxford, Mansfield Road, Oxford OX1 3TA, U.K. ‡ Institut Charles Gerhardt Montpellier, UMR 5253 CNRS-UM-ENSCM, Universitéde Montpellier, cc 1501, Place Eugené Bataillon, F-34095 Montpellier Cedex 5, France *S Supporting Information ABSTRACT: We report the first reactions of titanium imido, alkylidene hydrazido, and dimethylhydrazido compounds with the F F * i boranes H2BTex, 9-BBN, HBAr 2, and HBPin (Tex = tert-hexyl; Ar =C6F5). Reactions of Cp Ti{MeC(N Pr)2}(NTol) with F * i ′ − H2BTex, 9-BBN, or HBAr 2 resulted in the hydride-bridged adducts Cp Ti{MeC(N Pr2)2}{N(Tol)HBRR } without B H bond * i F β cleavage. Cp Ti{MeC(N Pr)2}(NNCPh2)(4) reacted with HBAr 2 via a sequence of steps involving adducts at the - and then α − -nitrogen of the NNCPh2 ligand, before slow 1,2-addition of B H across the N CPh2 double bond of 4, forming * i F Cp Ti{MeC(N Pr)2}{NN(BAr 2)CHPh2}(17). The other boranes reacted immediately with 4 to form homologues of 17. fi * i These products are the rst examples of borylhydrazido(2-) complexes. Reaction of Cp Ti{MeC(N Pr2)2}(NNMe2)(2) with F * i F − HBAr 2 gave the hydride-bridged adduct Cp Ti{MeC(N Pr2)2}{N(NMe2)HBAr 2}, whereas with HBPin B H bond cleavage * i occurred to form the borylhydrazide(1-)-hydride Cp Ti{MeC(N Pr)2}(H){N(BPin)NMe2}. Finally, reaction of 2 with 9-BBN * i dimer resulted in H2 elimination and formation of Me2NBC8H14 and Cp Ti{MeC(N Pr)2}(NBC8H14), a rare example of a borylimido compound. − * ■ INTRODUCTION addition to the Ti S bond of Cp 2Ti(S)(py) and to a strained 9c,d A series of reviews and recent papers have charted the tantalum imide was reported some time ago. Very recently a 1 scandium imide was shown to activate Si−H bonds by 1,2- remarkable developments in Group 4 imido (L)M(NR), 9e 2 addition to Sc−Nα, albeit in a reversible equilibrium. hydrazido (L)M(NNR2), and alkylidene hydrazido/diazoal- 3 The situation with hydrazides and alkylidene hydrazides is kane (L)M(NNCR2) compounds over the last ca. 25 years (L = supporting ligand set; R = H or hydrocarbyl). Other classes better established, at least for titanium. Andersen and Bergman * η2 of Group 4 imido-type compounds featuring heteroatom found that Cp 2Ti( -NNCHTol) reacted with PhSiH3 or − substituents have also been recently reported, namely the Ph2SiH2 by 1,2-addition to Ti Nα forming alkylidene titanium tert-butoxyimides (L)Ti(NOtBu)4 and borylimides hydrazide(1-)-hydride products, 1, as illustrated in Figure 1.3c 5 * (L)Ti(NBR2). In general, the dominant feature of these We subsequently reported that the dimethylhydrazide Cp Ti- compounds is their addition or insertion reactions with {MeC(NiPr) }(NNMe )(2) forms analogous 1,2-addition − 2 2 unsaturated compounds at the M Nα multiple bond itself products, 3 (Figure 1), with primary aryl silanes ArSiH3 or 2 4 6 (best described as a σ π triple bond). For certain hydrazido, BuSiH in a reversible equilibrium.10 No reaction occurred with 3c 4a,c 3 alkylidene hydrazido, and tert-butoxyimido compounds, the diphenylhydrazido or imido counterparts of 2. DFT − − reductive cleavage of the Nα Nβ or Nα Oβ bond can occur calculations found that, without additional coordination of with reducing substrates such as CO, isonitriles, and alkynes. the β-NMe group in 3,1,2-additiontoTi−Nα was Less commonly, dating from Bergman and Wolczanski’s first 2 7 thermodynamically unfavorable. The alkylidene hydrazide reports on these compounds, Group 4 imides can also activate * i − − analogue of 2,CpTi{MeC(N Pr)2}(NNCPh2)(4), also the C H bonds of organic substrates, and also the H H bond ′ 1a−c,e,g fi reacted with ArSiH3 or Ph2SiH2, forming N,N -disubstituted of H2. As documented in the reviews, many signi cant 3d,8 hydrazide(2-) products 5, featuring net 1,3 Si−H bond addition advances regarding these aspects have been achieved. − 3e − − across the N N CPh2 moiety of 4. Mechanistically, this In contrast, Si H and, especially, B H bond activation − − reactions of Group 4 imido and hydrazido-type compounds proceeds via 1,2-addition of Si HtoTiNα to give a with silanes and boranes are still poorly developed, although hydrazide(1-)-hydride intermediate, analogous to 1 and 3, they are better known for transition metal−heteroatom multiple bonds in general.9 There are no reported Si−H Received: June 23, 2017 additions to Group 4 imides. In related chemistry, Si−H 1,2- © XXXX American Chemical Society A DOI: 10.1021/acs.organomet.7b00477 Organometallics XXXX, XXX, XXX−XXX Organometallics Article − − ′ i 3c,e,5b,9f,10 Figure 1. Examples of products of Si HorB H bond activation with imido and hydrazido compounds (Ar = 2,6-C6H3 Pr2). followed by hydride migration to the electrophilic NβCPh2 carbon. Very recently we also reported the reductive Nα−Nβ bond Me cleavage reactions of Ti(N2N )(NNPh2)(py) with PhSiH3 5b and Ph2SiH2. DFT and kinetic studies found that this reaction proceeds via concerted Si−H bond addition across the Ti−Nα bond to again form a hydrazide(1-)-hydride intermediate (cf. 1 ff and 3). Subsequent hydride migration to the Nα atom of the so- with di erent types of primary and secondary boranes. A part of η2 − 5b formed -N(SiH2R)NPh2 ligand is coupled with Nα Nβ bond this work has been reported in a preliminary communication. cleavage, forming 6 (Figure 1). Furthermore, reaction of Me μ ■ RESULTS AND DISCUSSION Ti(N2N )(NNPh2)(py) with 9-BBN ([( -H)BC8H14]2) formed the borylamide counterpart 7, and DFT and kinetic As described in the Introduction, we found previously that studies established an analogous mechanism to that for 6.5b,11 A titanium half-sandwich amidinate compounds of the type * i ′ homologous compound to 6 was formed using HBMes2 (Mes Cp Ti{MeC(N Pr)2}(NR) (R = hydrocarbyl, NCPh2 or NR 2) ’ − = 2,4,6-C6H2Me3), but with pinacol borane (HBPin) or Piers provided good starting points for developing Si H bond 12 F F 3e,10 borane (HAr 2,Ar =C6F5) mixtures or undesired products activation chemistry of imido and hydrazido compounds. of ligand transmetalation were obtained.11 We therefore used this class of compound for our current study The Nα−Nβ bond cleavage processes leading to 6 and 7 were with boranes. Preliminary screening with several borane types the first of their type with silanes or boranes. More generally, (vide infra) established that the tolylimide Cp*Ti{MeC- Me i * the reactions of Ti(N2N )(NNPh2)(py) with HBR2 were the (N Pr)2}(NTol) (10), alkylidene hydrazide Cp Ti{MeC- i * first B−H activation reactions reported for any metal hydrazide. (N Pr)2}(NNCPh2)(4), and dimethylhydrazide Cp Ti{MeC- i Furthermore, the first B−H activation reaction of an imide was (N Pr)2}(NNMe2)(2)(Figure 2) gave the best outcomes in described only recently by Chen for a scandium compound terms of reactivity and product isolation. For example, the tert- 9f * i t reacting with 9-BBN, forming 8 (Figure 1). Subsequently butylimide Cp Ti{MeC(N Pr)2}(N Bu) did not react with * i Walter et al. reported the first B−H 1,2-addition reaction of an boranes, and the diphenylhydrazide Cp Ti{MeC(N Pr)2}- η t actinide imide for ( -C5H2 Bu3)2Th(NTol) (Tol = 4-C6H4Me) (NNPh2) reacted only with 9-BBN, under forcing conditions with 9-BBN.13 No well-defined reactions of Group 4 imides as discussed later on. In a similar way, we screened a number of with the B−H bonds of boranes have so far been reported. However, Lancaster recently showed that reaction of Cp2HfCl2 F with 2 equiv of the lithiated borylamide LiNH2BHAr 2 formed F Cp2Hf{N(H)HBAr 2}(9) as one component of a mixture, with F 14 the overall elimination of H3NBHAr 2 and LiCl. Although 9 appears formally to be the product of the 1,2-addition of F − HBAr 2 to the Hf Nα bond of Cp2Hf(NH) (not known as an isolated species), the mechanism of its formation is unclear and probably proceeds via an amidohydroborate intermediate that is subsequently dehydrohalogenated by additional F LiNH2BHAr 2. Overall, the B−H bond activation reactions of Group 4 imides and hydrazides/alkylidene hydrazides have not been developed in any detail, although there are encouraging and interesting precedents as outlined above. In this contribution Figure 2. Principal titanium compounds and hydroboranes (shown in we report the reactions of titanium compounds of this type their monomeric form) used in this study. B DOI: 10.1021/acs.organomet.7b00477 Organometallics XXXX, XXX, XXX−XXX Organometallics Article hydroboranes possessing different substitution patterns at associated angles for the supporting ligand sets are comparable boron. The successful candidate boranes carried forward from and within the expected ranges.21 The Ti(1)−N(1) bond these trials are listed in Figure 2 and represent primary distances are very similar, and their average value of 1.877(1) Å 15 fi (H2BTex, Tex = tert-hexyl) and secondary boranes.