Article pubs.acs.org/Organometallics Tetranuclear Zirconium and Hafnium Polyhydride Complexes “ ” Composed of the CpMH2 Units Shaowei Hu,† Takanori Shima,*,† Yi Luo,*,‡ and Zhaomin Hou*,† † Organometallic Chemistry Laboratory, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan ‡ State Key Laboratory of Fine Chemicals and School of Pharmaceutical Science and Technology, Dalian University of Technology, Dalian 116024, People’s Republic of China *S Supporting Information ABSTRACT: A series of tetranuclear group 4 transition metal μ octahydride complexes [(C5Me4R)4M4( -H)8](2-Zr, M = Zr, R = SiMe3; 2-Hf, M = Hf, R = SiMe3; 3, M = Zr, R = Me) were synthesized by the hydrogenolysis of the half-sandwich tris(trimethylsilylmethyl) complexes [(C5Me4R)M- (CH2SiMe3)3](1-Zr, M = Zr, R = SiMe3; 1-Hf, M = Hf, R ′ ff = SiMe3; 1-Zr , M = Zr, R = Me). X-ray di raction studies μ μ revealed that these hydride clusters possess a tetrahedral M4 framework which is connected by two 3-H and six 2-H ligands. fi Such bonding modes have been further clari ed by DFT studies. The reaction of 2-Zr with SePPh3 resulted in oxidation of two 2− of the four Zr(III) ions in 2-Zr to Zr(IV) and reduction of SePPh3 to Se , yielding the selenium-capped hydride cluster μ μ [(C5Me4SiMe3)4Zr4( 3-Se)( -H)8](4) with release of PPh3. 13 ■ INTRODUCTION (CH2C6H4NMe2-o)2] (R = SiMe3, Me, Et, H) could be Group 4 transition metal hydride complexes have received easily transformed into the corresponding dihydride species “(Cp)LnH ” by hydrogenolysis with H . The resulting much interest because of their importance in various chemical 2 2 1−5 dihydride species showed unique polynuclear structures and transformations. So far, a large number of group 4 transition reactivities which are different from those of the conventional metal hydride compounds of the general types [(Cp) MH ] 2 2 metallocene hydride complexes bearing two Cp ligands per and [(Cp) MHX] (Cp = cyclopentadienyl derivatives) bearing − 2 metal.14 27 During these studies, we became interested in the two cyclopentadienyl ligands per metal (for example, analogous group 4 metal hydride clusters. In this paper, we [Cp* ZrH ]; Cp* =CMe )6 have been reported and 2 2 5 5 report that half-sandwich group 4 metal tris- extensively studied. In contrast, group 4 metal hydride (trimethylsilylmethyl) complexes such as [(C Me R)M- complexes of the half-sandwich type “[(Cp)MH ]”, which 5 4 n (CH SiMe ) ] (M = Zr, Hf; R = SiMe , Me) can serve as bear one cyclopentadienyl ligand per metal, have hardly been 2 3 3 3 excellent precursors for the synthesis of the tetranuclear studied, although such complexes are of much interest both 7 zirconium and hafnium polyhydride complexes [(C5Me4R)M- structurally and chemically. μ “ ( -H)2]4, which are formally composed of four (C5Me4R)- In 1982, Wolczanski and Bercaw reported the first mono-Cp- ” * μ MH2 units. DFT studies on a zirconium complex to elucidate coordinated zirconium hydride complex, [{Cp Zr(BH4)H( - 8 the Zr4H8 core structure, as well as a preliminary reactivity H)}2], in combination with a tetrahydroborate unit. Since study on the Zr hydride cluster, are also reported. then, several analogous mono-Cp-coordinated group 4 metal * μ η2 μ hydride complexes, such as [{Cp Zr(BH4)}2( - -BH4)( - * μ μ 8,9 * μ ■ RESULTS AND DISCUSSION H)3]2,[{CpZr(BH4)}2( -H)( -H)3]2, [(Cp MCl)( - μ 9,10 * μ μ Synthesis and Structure of C Me SiMe -Ligated H)( 3-H)]4 (M = Zr, Hf), and [(Cp Hf)4( -C6H8)( - 5 4 3 11 Tetranuclear Zr and Hf Octahydride Complexes. Hydro- H)6], have been reported. However, group 4 metal hydride “ ” genolysis of the half-sandwich Zr tris(trimethylsilylmethyl) complexes composed of only the CpMHn unit without a third component have not yet been reported. Previous attempts to complex [(C5Me4SiMe3)Zr(CH2SiMe3)3](1-Zr) with H2 (10 ° ff prepare such half-sandwich hydride complexes by hydro- atm) in hexane at 80 C for 1 day a orded the tetranuclear μ genolysis of the corresponding half-sandwich alkyl precursors zirconium octahydride complex [(C5Me4SiMe3)4Zr4( -H)8] * * * 8 (2-Zr) in 87% yield (Scheme 1). The formation of 2-Zr can such as [Cp ZrMe3], [Cp Zr(CH2Ph)3], [Cp ZrPh3], and * 10 be viewed as a result of the hydrogenation of the three alkyl [Cp HfMe3] with H2 did not give a structurally character- izable product. groups in 1-Zr followed by tetramerization of the resulting “ ′ ” We recently found that half-sandwich rare-earth bis(alkyl) trihydride species [Cp ZrH3] and liberation of two molecules complexes such as [(C5Me4SiMe3)Ln(CH2SiMe3)2(THF)] (Ln − = Y, Lu, Sc, Gd Yb), [(C5Me4SiMe3)Ln(CH2C6H4NMe2-o)2] Received: January 8, 2013 12 (Ln=La,Ce,Pr,Nd,Sm), and [(C5Me4R)Y- Published: March 15, 2013 © 2013 American Chemical Society 2145 dx.doi.org/10.1021/om400012a | Organometallics 2013, 32, 2145−2151 Organometallics Article Scheme 1. Synthesis of C5Me4SiMe3-Ligated Tetranuclear Zirconium and Hafnium Octahydride Complexes of H2. The oxidation state of the four metal centers was reduced from Zr(IV) in 1-Zr to Zr(III) in 2-Zr. Similarly, the hydrogenolysis of [(C5Me4SiMe3)Hf(CH2SiMe3)3](1-Hf) ° with H2 (10 atm) in benzene at 90 C for 4 days gave μ [(C5Me4SiMe3)4Hf4( -H)8](2-Hf) in 58% isolated yield (Scheme 1). Compounds 2-Zr and 2-Hf are soluble in common organic solvents such as benzene, hexane, THF, and Et2O. Single crystals of 2-Zr and 2-Hf suitable for X-ray structure determinations were obtained by recrystallization from hexane. Both complexes adopted a similar solid structure, and the X-ray structure of 2-Zr is shown in Figure 1. Each Zr atom is bonded η5 to one C5Me4SiMe3 ligand in an bonding mode. There are eight hydride ligands in the tetranuclear Zr framework, two of μ which are face-capped in a 3-H fashion and six are edge- μ bridged in a 2-H form. Unlike the analogous tetranuclear μ μ yttrium octahydride complex [(C5Me4SiMe3)4Y4( 4-H)( 3- μ 23 μ Figure 1. (a) X-ray full structure of Zr4H8 in 2-Zr recrystallized from H)( 2-H)6], no body-centered interstitial 4-H ligand was found in the Zr tetrahedron cavity in 2-Zr. The short Zr−Zr hexane with 30% thermal ellipsoids. (b) Core structure of 2-Zr. 4 Selected bond lengths (Å): Zr1−Zr4, 3.0679(4); Zr1−Zr2, 3.0806(4); distances in 2-Zr (3.0670(4)−3.0949(5) Å (average 3.0787 Å)) − − − − 28 Zr1 Zr3, 3.0885(4); Zr2 Zr4, 3.0735(4); Zr2 Zr3, 3.0949(5); Zr3 in comparison with the sum of the atomic radii of Zr (3.10 Å) Zr4, 3.0670(4); Zr1−H1, 1.97(3); Zr1−H4, 1.79(5); Zr1−H6, and the diamagnetism of 2-Zr (vide infra) might indicate the 1.88(5); Zr1−H7, 2.07; Zr1−H8, 2.22(7); Zr2−H2, 1.92(3); Zr2− existence of Zr−Zr σ interactions. The Zr−Zr distances in 2-Zr H3, 2.00(3); Zr2−H6, 1.87(4); Zr2−H7, 2.16; Zr3−H1, 1.91(3); fall into a narrow range, in contrast with the case for the Zr3−H3, 1.94(3); Zr3−H5, 1.87(6); Zr3−H8, 1.92(7); Zr4−H2, * μ μ − − − − analogous mixed chloride/hydride cluster [{Cp ZrCl( -H)( 3- 2.03(3); Zr4 H4, 1.93(5); Zr4 H5, 1.85(5); Zr4 H7, 2.04; Zr4 9 fl H8, 2.11(7). The H7 atom was not refined. H)}4], which adopted a butter y-like core structure (Zr- - -Zr, − −μ − 3.2808(4) 5.635 Å). The Zr 2-H bond distances (1.79(5) −μ 2.03(3) Å (average 1.91 Å)) and the Zr 3-H bond distances (1.92(7)−2.22(7) Å (average 2.09 Å)) in 2-Zr are comparable * μ μ −μ − to those in [{Cp ZrCl( -H)( 3-H)}4] (Zr 2-H, 1.79(5) −μ − 2.09(6) Å; Zr 3-H, 1.96(8) 2.33(5) Å). 1 The HNMRspectrumof2-Zr in C6D6 at room temperature showed a singlet at δ 0.61 (8H) for the hydride ligands, which are shifted significantly to high field in * μ μ δ comparison with those in [{Cp ZrCl( -H)( 3-H)}4]( 3.88, 9 * μ δ 8 3.50) and [{Cp Zr(BH4)H( -H)}2]( 1.31). The line shape and intensity of the hydride signal in 2-Zr remained unchanged − ° in the temperature range of +80 to 80 C in toluene-d8 or in THF-d , suggesting that these hydride ligands are highly 8 · fluxional in solution. Complex 2-Hf showed similar behavior in Figure 2. X-ray structure of the Zr4H8 core in 2-Zr THF recrystallized 1 from THF with 30% thermal ellipsoids. The hydride ligands H4 and its H NMR spectrum. * fi In an attempt to recrystallize 2-Zr from THF, single crystals H4 are re ned at 25% occupancy due to a disorder problem. Selected bond lengths (Å): Zr−Zr, 3.0748(4)−3.0886(5) (average 3.0794); having a lattice THF molecule (2-Zr·THF) were obtained. An −μ − −μ − Zr 2-H(2,3), 1.87(3) 1.91(2) (average 1.90); Zr 3-H4, 2.13(8) X-ray diffraction study revealed that 2-Zr·THF adopts a −μ 2.14(7) (average 2.14), Zr 4-H1, 1.8857(3). tetranuclear structure similar to that of 2-Zr.