Titanium and zirconium complexes of the N , N ’ -bis(2,6-diisopropylphenyl)-1,4-diaza-butadiene ligand: syntheses, structures and uses in catalytic hydrosilylation reactions †

We report here a number of dianionic 1,4-diaza-1,3-butadiene complexes of titanium and zirconium synthesised by a salt metathesis reaction. The reaction of either CpTiCl 3 or Cp 2 TiCl 2 with the dilithium salt of N , N ’ -bis(2,6-diisopropylphenyl)-1,4-diaza-1,3-butadiene [ 1 ; abbreviated (Dipp) 2 DADLi 2 ] a ﬀ orded the mono-cyclopentadienyl titanium complex [ η 5 -CpTi((Dipp) 2 DAD)Cl] ( 2 ) bearing a dianionic ene-diamide ligand, while the analogous reaction of zirconocene dichloride (Cp 2 ZrCl 2 ) with the dilithium salt 1 gave the bis-cyclopen-tadienyl zirconium complex [Cp 2 Zr{(Dipp) 2 DAD}] ( 3 ). The metal dichloride complexes [Ti((Dipp) 2 DAD)Cl 2 ] ( 4 ) and [{(Dipp) 2 DADZrCl( μ -Cl)} 2 ( κ 3 -Cl)(Li)(OEt 2 ) 2 ] ( 5 ) were obtained by the reaction of 1 and anhydrous metal tetrachloride in a 1:1 molar ratio in diethyl ether at room temperature. Meanwhile, the homoleptic titanium complex [Ti{((Dipp) 2 DAD)} 2 ] ( 6 ) was isolated in good yield by the treatment of 1 with TiCl 4 in a 1:2 molar ratio in diethyl ether. The complexes 2 and 5 were further reacted with neosilyl lithium to a ﬀ ord mono-and bis-alkyl complexes of titanium [ η 5 -CpTi{(Dipp) 2 DAD}(CH 2 SiMe 3 )] ( 7 ) and zirconium [Zr{(Dipp) 2 DAD}(CH 2 SiMe 3 ) 2 ] ( 8 ) respectively. Molecular structures of the complexes 2 , 3 , and 5 – 8 in the solid states were con ﬁ rmed by single crystal X-ray di ﬀ raction analysis. The solid state structures of all the complexes reveal that the metal ions are chelated through the amido-nitrogen atoms and the ole ﬁ nic carbons of the [(Dipp) 2 DAD] 2 − moiety, satisfying the σ 2 , π coordination mode. Compound 8 was used as a catalyst for the intermolecular hydrosilyl-ation reaction of a number of ole ﬁ ns, and moderate activity of catalyst 8 was observed.


Introduction
Amido metal chemistry of the early transition metals has achieved significant momentum in the last 25 years with the design of novel amido ligands. 1 It was observed that, in the early stages of this field of study, most researchers focused on cyclopentadienyl-analogous amido ligands for comparison with, and for further investigation of, the well-known cyclopentadienyl moiety. 2 Amido-metal bonds are thermodynamically stable and less labile compared to metal-carbon bonds. However, nowadays the stable amido-metal bond is utilized in amido-metal chemistry to produce well-defined reaction centers in transition metal complexes. In this way, the reactivity of the resulting early transition metal compounds can be specifically tailored to allow applications in areas such as the activation of small, poorly reactive molecules, homogeneous catalysis, or organic synthesis. 3 Recently the use of diamide ligands has gained more importance in early transition metal chemistry for the stabilization of group 4 and 5 metal complexes, due to their ability to chelate metal centers with higher oxidation numbers through the formation of dianionic forms. 4 Metal complexes with these bis (amido) ligands exhibit a closer relationship to the metallocenes and particularly to the constrained-geometry half-sandwich amido-metal complexes, which have been studied as potential catalysts for homogeneous Ziegler-Natta polymerization. 5 The chelating diamide complexes of titanium and zirconium serve as precursors for the highly active and living polymerization of olefins. 6 The potential advantage of the bis (amido) metal system relative to the metallocene or the half-sandwich amido-metal complexes is their lower formal electron count which results in a more electrophilic and therefore potentially more active catalyst fragment. 7 Since the α-diimine ligand, 1,4-disubstituted diazabutadiene (DAD), was synthesized and utilized in the early 1960s, 8 various substituted DAD ligands have been synthesized by a number of research groups, even today, as it can be reduced to generate a diamido ligand. 9 The diversity in coordination and redox properties of this ligand has resulted in a high level of interest in these compounds, which have already proved to have wide-ranging uses in the areas of both fundamental and applied research. 10 The neutral DAD molecule includes two lone electron pairs of nitrogen atoms and π-electrons of the multiple imine (N-C) bonds, and this molecule can act as both a σand π-donor, and coordinate to the metal atom as a neutral ligand. 11 Although the dianionic DAD ligands preferentially coordinate to early transition metals and alkaline metals in σ 2 -and σ 2 ,π-coordination modes, 12-14 in many cases the DAD ligands coordinate to group 3 metal atoms as a σ 2 -monoanion, 15 and, in addition, both monoanionic and dianionic coordination modes were observed for alkaline-earth metals, group 12, and group 13 metal complexes. 16,17 The possible flexible coordination modes depending on the types and redox properties of the central metal are shown in Chart 1.
The DAD ligand is widely utilized not only for early transition metals, 18,19 f-block metals, 20 and late transition metals, 21 but also for s-block and p-block main-group elements. [22][23][24] In the majority of these complexes, the DAD ligands are coordinated in their dianionic form as chelating enediamides to the metal (mode D) and therefore are reminiscent of diamide ligands.
To get more insight into the structure-reactivity relationships of early transition metal DAD complexes, and to explore their applications in organic transformations, we have studied this chemistry further. In this context, we present the synthesis of a number of dianionic 1,4-diaza-1,3-butadiene complexes with the molecular compositions [η 5 -CpTi((Dipp) 2 DAD)Cl] (2), [Cp 2 Zr- The solid state structures of complexes 2-3 and 5-8 are also reported. The catalytic hydrosilylation of various alkenes using complex 8 as a catalyst is also presented.

Cyclopentadienyl metal complexes
The cyclopentadienyl titanium complex [η 5 -CpTi((Dipp) 2 DAD)-Cl] (2) was isolated in good yield from the reaction of (Dipp) 2 -DADLi 2 ) (1) and CpTiCl 3 in a 1 : 1 molar ratio in diethyl ether at room temperature, followed by re-crystallisation from ether at −35°C (see Scheme 1). The titanium complex 2 could also be obtained by the reaction of 1 and titanocene dichloride (Cp 2 TiCl 2 ) under similar reaction conditions. Thus under the reaction conditions, one cyclopentadienyl moiety underwent elimination from Cp 2 TiCl 2 to LiCp along with one equivalent LiCl. Such phenomenon was recently observed by Sun et al. while treating triscyclopentadienyl yttrium with a lithium amidinate ligand. 25 In contrast, the reaction of zirconocene dichloride (Cp 2 ZrCl 2 ) with dianionic lithium salt 1 afforded the corresponding bis-cyclopentadienyl zirconium complex [Cp 2 Zr{(Dipp) 2 DAD}] (3) in good yield by elimination of two equivalents of lithium chloride (see Scheme 1). Both the titanium and zirconium complexes were characterized by spectroscopic techniques and the solid states of the complexes 2 and 3 were established by X-ray diffraction analysis. In 1 H NMR spectra measured in C 6 D 6 , the resonances of the Cp protons in 2 appear at 6.17 ppm as a sharp singlet. The signals for the analogous Cp protons in complex 3 are observed at 5.62 and 5.56 ppm, indicating two different chemical environments for the two cyclopentadienyl rings. The sharp singlets at 6.16 ppm for 2 and 5.35 ppm for 3 are assigned to the olefinic protons of the respective DAD ligand backbone. Therefore the resonances for the olefinic protons in 2 are significantly low field shifted compared to those of bis-cyclopentadienyl complex 3. Two septets for each complex (3.51 and 2.33 ppm for 2 and 3.71 and 2.92 ppm for 3) are observed for the isopropyl groups of the 2,6-diisopropylphenyl moiety present in the DAD ligand. The presence of two distinct septets in each complex can be explained by the asymmetric attachment of the DAD ligand in each case. The isopropyl methyl protons show four doublet resonances with a coupling constant of 6.8 Hz in 2, due to the restricted rotation around the respective carbon nitrogen bond of the DAD ligand moiety; this indicates the presence of nonequivalent 2,6-diisopropylphenyl groups. However, in 3, we observed two doublets for one set of diastereotopic isopropyl CH 3 groups, indicating the presence of equivalent 2,6-diisopropylphenyl groups. In proton decoupled 13 C NMR spectra for 2, the resonances at 114.6 ppm and 108.3 ppm represent the C 5 of the Cp moiety and the olefinic carbons of the DAD ligand. For zirconium compound 3, the 13 C{ 1 H} NMR signals are 114.3 and 110.1 ppm for the two Cp rings and 106.7 ppm for the olefinic carbon atoms. All of the 1 H and 13 C-{ 1 H} NMR signals are in agreement with the values reported in the literature. 14 The molecular structure of the air-and moisture-sensitive complexes 2-3 were established by single crystal X-ray diffraction analysis. The complex 2 crystallizes in the monoclinic space group P2 1 /c and has four independent molecules in the unit cell (Fig. 1). The zirconium complex 3 crystallizes in the triclinic space group P1 and has two independent molecules along with one molecule of diethyl ether in the unit cell as a solvate (Fig. 2). The details of the structural and refinement parameters of the crystal structures of 2-3 are given in Table TS1 in the ESI. † Complex 2 is monomeric and the Chart 1 Different coordination modes of the DAD ligand. coordination polyhedron is formed by the chelation of two amido nitrogen atoms of the dianionic DAD ligand, η 5 -coordination of one cyclopentadienyl moiety, and one chloride atom.
The geometry around the titanium ion can be best described as pseudo tetrahedral, considering the η 5 -Cp ring as a pseudomonodentate ligand. The Ti-N distances [1.928(2) and 1.922(2) Å] Scheme 1 Syntheses of titanium and zirconium complexes 2-6 from 1.   (18)  are close to that of the Ti-N covalent bond. The Ti-C(Cp) distances, ranging from 2.331(3) to 2.358(3) Å, are within the agreement of reported Ti-C(Cp) values. The zirconium complex 3 is also monomeric, bearing two η 5 -Cp moiety and one DAD ligand. The geometry around the zirconium ion is pseudo tetrahedral, considering the η 5 -Cp ring as a pseudo-monodentate ligand. The Zr-N distances [2.105(1) and 2.141(1) Å] are slightly longer than the Ti-N distances, due to the larger ion radius of Zr(IV) ion, however they are in agreement with the Zr-N covalent bonds reported in the literature. The Zr-C(Cp) distances [2.52(2) to 2.590(2) Å] are also slightly longer than the Ti-C(Cp) distances, but are in the range of the Zr-C(Cp) distances reported for other zirconocene complexes. 26 Notably, the coordination of the dianionic DAD ligands in complex 2 and 3 are similar, and both complexes form a five-membered diazametallacyclopentene structure (Ti1-N1-C1-C2-N2 for 2 and Zr1-N1-C1-C2-N2 for 3). Both metallacycles are folded and the dihedral angles between the N1-M-N2 and N1-C1-C2-N2 planes are 50.62°(for 2) and 50.30°(for 3). In complex 2, the distances between the titanium ion and C1vC2 are short enough [2.427(3) and 2.433 (3) Å] for π bonding to display the σ 2 ,π-enediamide mode of the DAD ligand. However, no such π interactions between the zirconium ion and the olefinic carbon atoms are observed in the molecular structure. Thus for complex 3, the DAD ligand displayed only a σ 2 -diamide mode (C in Chart 1). Nevertheless, DAD ligation can be described as the elongation of the C-N bond [1.389(3) and 1.383(3) Å for 2; 1.392(2) and 1.389(2) Å for 3] and the shortening of the C-C bond [1.382(4) Å for 2; 1.377(2) Å for 3] i.e. a long-short-long sequence compared to the neutral DAD ligand. Similar coordination behavior has also been observed in recently reported DAD lanthanide complexes. 14

Metal dichloride complexes
Upon treating 1 with MCl 4 (M = Ti and Zr) either in toluene (in the case of Ti) at −78°C or in diethyl ether (in the case of Zr) at room temperature, a DAD titanium dichloride complex [Ti((Dipp) 2 DAD)Cl 2 ] (4) and an 'ate' complex for zirconium [{(Dipp) 2 DADZrCl(μ-Cl)} 2 (κ 3 -Cl)(Li)(OEt 2 ) 2 ] (5) were obtained respectively in good yields. Both the complexes 4 and 5 were characterized by spectroscopic and combustion analyses. The solid state structure of complex 5 was established by single crystal X-ray diffraction analysis. In the 1 H NMR spectrum of 5 in C 6 D 6 , a sharp singlet was observed at δ 5.81 ppm (6.18 ppm for 4), which was assigned to the olefinic protons of the DAD ligand backbone; a broad signal was observed at δ 3.31 ppm (2.98 ppm for 4); and two doublet resonances of a constant 5.6 Hz appeared at δ 1.20 and 1.01 ppm (1.14 ppm for 4), respectively, due to the CH hydrogen and isopropyl methyl hydrogen atoms of the ligand. The above values are quite similar to the corresponding values of compounds 2 and 3 (see above).

Homoleptic complex
The bis-DAD titanium complex [Ti-{(Dipp) 2 DAD} 2 ] (6) was isolated by the treatment of 1 with TiCl 4 in a 1 : 2 molar ratio, by the elimination of LiCl. The corresponding complex of zirconium was also recently synthesized by the reaction reduction of the neutral DAD ligand followed by a reaction with zirconium tetrachloride. 30 The complex 6 was characterized by 1 H, 13 C{ 1 H} NMR spectroscopy and combustion analysis, and its molecular structure was established by single crystal X-ray diffraction analysis. The 1 H NMR spectra of the complex 6 show two sets of signals for each DAD ligand. Four doublets at δ 1.24, 1.21, 1.17 and 1.15 ppm in a 12 : 12 : 12 : 12 ratio, and a coupling constant of 4.8 Hz in each case can be assigned to the resonances of 48 methyl protons distributed in four diisopropylphenyl moieties. The magnetically asymmetric protons indicate that the orientations of the two DAD ligands must be in different planes. The resonances for the olefinic protons of the two DAD ligands' backbones are observed at δ 6.18 and 6.05 ppm as doublets, indicating a clear distinction between the two ligands' magnetic environments. However two multiplets at δ 3.12 and 2.95 ppm are obtained for the eight isopropyl protons, due to the overlapping of two closely associated septets for each DAD ligand. In proton decoupled 13 C NMR spectra, we observed that the characteristic peaks for the two DAD ligands match with complexes 2 and 4 (see the Experimental section). 30 The X-ray quality crystal of titanium complex 6 was re-crystallized from diethyl ether at −35°C as a red crystal. Compound 6 crystallizes in the monoclinic space group P2 1 /c with four independent molecules in the unit cell. The solid state structure of complex 6 is given in Fig. 4 and details of the structural parameters are given in Table TS1 in the ESI. † All the hydrogen atoms were located in the Fourier difference map and were subsequently refined. The coordination polyhedron is formed by four amido nitrogen atoms from the two DAD ligands. The geometry around the titanium ion is best described as distorted tetrahedral. The Ti-N distances [Ti1-N1 1.968(14), Ti1-N2 1.920 (14) (17) Å] are sufficiently shorter, considering the σ bonds between the metal ion and the CvC backbone of the ligands. Two folded metallacycles Ti1-N1-C1-C2-N2 and Ti1-N3-C3-C4-N4 are formed by the ligation of two dianionic DAD ligands which satisfy the σ 2 ,π-enediamide mode of coordination to the zirconium ion, with a long-short-long sequence within the ligand fragments [N1-C1 1.390 (3), C1-C2 1.366(3) N2-C2 1.399(2); C3-N3 1.391(2), C3-C4 1.377(2), C4-N4 1.392(2) Å]. A dihedral angle of 59.2°was observed between the two planes containing N1, C1, C2, N2 atoms and N3, C3, C4, N4 atoms present in the two ligands. The center metal titanium ion is 1.104 and 1.101 Å, respectively, away from the above-mentioned two planes.

Metal mono-and bis-alkyl complexes
To learn more about the reactivity of metal halide complexes 2 and 5, we were interested in synthesizing their alkyl derivatives. Metal alkyl complexes are important precursors to catalysts for a number of organic transformations. 31 To explore the reactivity of titanium and zirconium halide complexes 2 and 5, we decided to isolate the corresponding metal alkyl complexes. Both the complexes 2 and 5 were reacted with trimethylsilylmethyl lithium in diethyl ether as a solvent to afford the corresponding mono-alkyl [η 5 -CpTi((Dipp) 2 DAD)(CH 2 SiMe 3 )] (7) and bis-alkyl [Zr-{(Dipp) 2 DAD}(CH 2 SiMe 3 ) 2 ] (8) complexes, respectively, in good yields after re-crystallisation from hexane at −35°C (Scheme 2). Compounds 7 and 8 are soluble in THF, toluene and hydrocarbon solvents like pentane and hexane. Both air-and moisture-sensitive complexes were characterized by spectroscopic analysis and the solid state structures of the complexes 7-8 were established by single crystal X-ray diffraction analysis.
The 1 H NMR spectrum of 7 in C 6 D 6 is very similar to the spectrum recorded for complex 2, exhibiting four characteristic doublet resonances in a 6 : 6 : 6 : 6 ratio for the four different types of isopropyl methyl groups present in the DAD ligand, along with two high field septet resonances at 3.25 and 2.42 ppm for the isopropyl -CH hydrogen atoms. Thus it is evident that the chemical and magnetic environments of isopropyl methyl and -CH protons are different due to the presence of the alkyl group attached to the titanium ion. Between two sharp singlets, the signal at δ 6.19 ppm can be assigned to the five protons present in the cyclopentadienyl ring, whereas the signal at δ 5.95 ppm was confirmed for the olefinic protons (CvC) of the DAD ligand. For the neosilyl (CH 2 SiMe 3 ) group in 7, one singlet at δ 0.18 ppm (SiMe 3 ) and one singlet at δ −0.46 ppm is observed at high field, which can be assigned to methylene (CH 2 ) hydrogen atoms. In the 1 H NMR spectrum, zirconium bis-alkyl complex 8 exhibits two doublets at δ 5.97 and 5.91 ppm, assignable to the olefinic protons of the DAD ligand backbone and two septets at δ 3.54 and 3.17 ppm for two chemically different isopropyl -CH protons, while four doublet resonances with a coupling constant of 6.8 Hz appeared at δ 1.33, 1.22, 1.10 and 0.92 ppm in a 6 : 6 : 6 : 6 ratio for the methyl protons of the ligand. In addition, two singlets at δ 0.10 and 0.03 ppm were observed for the two neosilyl (CH 2 SiMe 3 ) groups present in 8. Similar chemical shift values for the neosilyl groups (δ 0.13 and 0.04 ppm) were reported for Cp″ 2 Zr(CH 2 SiMe 3 ) 2 (Cp″ = CH 2 vCHCH 2 C 5 H 4 ) by Piers et al. 32 Although there has been ongoing interest in the alkyl complexes of group 4 organometallics, and particularly in the cyclopentadienyl chemistry of these elements, to the best of our knowledge complexes 7-8 represent the first titanium and zirconium alkyl complexes containing a dianionic 1,4-diaza-1,3-butadiene ligand and a neosilyl group attached to it. 18c Therefore, their molecular structures in the solid state were determined by X-ray diffraction analysis. Both the titanium and zirconium complexes 7 and 8 crystallize in the monoclinic space group P2 1 /c and have four molecules of either 7 or 8 in the respective unit cells. The details of the structural parameters are given in Table TS1 in the ESI. † The solid state structures of complexes 7 and 8 are shown in Fig. 5 and 6, respectively. The coordination polyhedron of half sandwich titanium alkyl complex 7 is formed by η 5 coordination of the cyclopentadienyl ring with an average Ti-C(Cp) distance of 2.368 Å, which is similar to the corresponding value in compound 2 (2.344 Å) and other titanocene complexes in the literature. 33 Beside the Cp ring, the DAD ligand is chelated in a dianionic ene-diamide canonical form to the titanium ion through two amido-nitrogen atoms, and one neosilyl (Me 3 SiCH 2 ) group is ligated to the center metal through a carbon atom. The DAD ligand is folded to have a titanium olefin interaction, which is observed in all of the DAD metal complexes reported in this work. In contrast, the zirconium coordination sphere in 8 is constructed by a folded DAD ligand moiety similar to compound 7, and two neosilyl groups.  ligand. Thus in both complexes 7 and 8, the DAD ligand maintained its σ 2 ,π-enediamide mode of coordination to the metal ion with a long-short-long sequence within the ligand fragments [N1-C1 1.386 (3) One four-membered metallacycle in each complex (Ti1-N1-C1-C2 for 7 and Zr1-N1-C1-C2 for 8) is formed by the coordination of the DAD ligand to the metal ion. The titanium ion is 1.108 Å away from the plane containing the N1-C1-C2 and N2 atoms, and this plane is orthogonal to the cyclopentadienyl plane. The center metal titanium possesses distorted pseudotetrahedral geometry if we consider Cp − as a pseudo-monodentate ligand. The fourth coordination site of the titanium atom in 7 (third and fourth for zirconium complex 8) is occupied by a neosilyl group and the Ti-C bond distance of 2.174(3) Å is within the range of Ti-C distances reported for titanium alkyl complexes. 34 The Zr-C distances of 2.236(3) and 2.240(3) Å are also in the accepted range for reported organozirconium complexes. 35

Catalytic hydrosilylation of alkenes
The catalytic addition of an organic silane Si-H bond to alkenes or alkynes (hydrosilylation) to give silicon-containing molecules is of great interest. 36 Currently, most organosilanes are made using multistep syntheses that produce significant amounts of waste. Therefore, hydrosilylation offers an attractive alternative route to obtain silicon-containing molecules that are important for the preparation of fine chemicals and pharmaceuticals. It has been demonstrated that group 3 metal complexes with Cp 37,38 and non-Cp 39,40 ligands are efficient catalysts or precatalysts for the hydrosilylation of olefins, and the mechanism is generally believed to involve the insertion of the olefin into the M-Si or M-H bond of a metal-silyl or metalhydride species, followed by σ-bond metathesis. 40,41 In our study, the mono and bis(neosilyl) complexes 7 and 8 proved to be highly efficient pre-catalysts for the intermolecular hydrosilylation of hexene and octene, using a small excess (10%) of phenylsilane (PhSiH 3 ) and 5 mol% catalyst loadings. However it was observed that complex 7 is poorly active for intermolecular hydrosilylation and thus the screening was tested using only zirconium complex 8.
Selected data obtained from the catalytic hydrosilylation reaction of various alkenes with respect to complex 8 are given in Table 1. In entries 1-3 and 6-7, the substrates (1-hexene,  The reaction was done in C 6 D 6 at r.t. The conversion and product selectivity was calculated from 1 H NMR. a 60°C. 1-octene, vinyl cyclohexane, 1,5-hexadiene and 1-bromopentene) essentially show complete conversion to the corresponding organosilanes in 2 hours at ambient temperature, as judged by 1 H NMR spectroscopy. Full selectivity for the n-products and no side reactions were observed (for example isoproducts, hydrogenation, alkene dimerization, and/or dehydrogenative coupling of organosilanes). In the case of using dodecene as the substrate, only 26% conversion was observed after 24 hours even at an elevated temperature (entry 4). The lower activity of dodecene in contrast to those of 1-hexene and 1-octene is not surprising. It seems that the presence of a longer alkyl chain in dodecene causes its sluggish reactivity in catalytic hydrosilylation. When we tried the hydrosilylation reaction using 8 in combination with B(C 6 F 5 ) 3 , the reactivity slightly improved in entry 4; however, it still remains lower than those of 1-hexene and 1-octene. Styrene gave a complete conversion to a mixture of products (27% n-product and 73% iso-product) after 24 hours at room temperature (entry 5). We also screened the alkenes with a halo functionality as substrates, and observed that even 1-bromopentene can be completely converted to the corresponding n-product in 2 hours at room temperature (entry 7), while 1-bromohexene shows 86% conversion even after 24 hours at the same temperature (entry 8). The lower reactivity for 1-bromohexene can be explained by the deactivation of the catalyst due to the presence of the bromine atom, followed by a sluggish reactivity towards hydrosilylation. Thus a sluggish reactivity in the hydrosilylation of the olefins is observed in the zirconium bis-alkyl complex 8 compared to catalysts known in the literature. 40
X-Ray crystallographic studies of 2, 3 and 5-8 Single crystals of compounds 2, 3, 5 and 6 were grown from diethyl ether at −35°C under an inert atmosphere. Compounds 7 and 8 were grown from either hexane (for 7) or pentane (for 8) at −35°C under an inert atmosphere. For compounds 2, 3 and 5-8, a crystal of suitable dimensions was mounted on a CryoLoop (Hampton Research Corp.) with a layer of light mineral oil, and placed in a nitrogen stream at 150(2) K. All measurements were made on an Agilent Supernova X-calibur Eos CCD detector with graphite-monochromatic Cu-Kα (1.54184 Å) radiation. Absorption corrections were performed on the basis of multi-scans. Crystal data and structure refinement parameters are summarised in Table TS1 in the ESI. † The structures were solved by direct methods (SIR92) 44 and refined on F 2 by the full-matrix least-squares method, using SHELXL-97. 45 Non-hydrogen atoms were anisotropically refined. H atoms were included in the refinement in calculated positions riding on their carrier atoms. No restraint has been made for any of the compounds. The function minimised was [∑w(

Conclusion
In this contribution, we have presented homoleptic and heteroleptic titanium and zirconium complexes with dianionic 1,4-diaza-1,3-butadiene in the backbone, to explore their coordination modes in straightforward synthesis. The titanium and zirconium alkyl complexes were also synthesized from the respective chloride complexes 2 and 5 and trimethylsilylmethyl lithium. In the solid state structures of all the DAD complexes, it was observed that the dianionic 1,4-diaza-1,3-butadiene ligand displayed a σ 2 ,π-enediamide mode towards the titanium and zirconium centers with a long-short-long sequence within the ligand fragments. The metal alkyl complexes were tested as catalysts for the intermolecular hydrosilylation of alkenes, and moderate activity was observed for the zirconium complex 8.