Tris(η4-naphthalene)- and Tris(1-4-η4-anthracene)tantalate(1−):  First Homoleptic Arene Complexes of Anionic Tantalum

Abstract

Reduction of tantalum pentachloride by 6 equiv of sodium naphthalene in 1,2-dimethoxyethane provided, after recrystallization from tetrahydrofuran, 50-55% yields of yellow, pyrophoric [Na(THF)][Ta(C10H8)3]. The product was shown by 1H and 13C NMR spectra and an X-ray study (on the corresponding [Na(crypt 2.2.2)]salt) to be tris(eta4-naphthalene)tantalate(1-), 1, the first homoleptic naphthalene complex of a third row (5d) transition metal. Salts of 1 react under mild conditions with excess CO (1 atm pressure, -60 degrees to +20 degrees C) and 3 equiv of anthracene, C14H10 (20 degrees C), to give 99 and 52% yields of yellow [Ta(CO)6]- and orange [Ta(C14H10)3]-, (2), respectively. The latter is the first homoleptic anthracene complex of a group 5 element and only the third one known, the others being Cr(eta6-C14H10)2 and [Co(eta4-C14H10)2]-. NMR spectra and X-ray structural characterization, as the [Na(crypt 2.2.2)] salt, established 2 to be [Ta(1-4-eta4-C14H10)3]- and is very similar to 1 in solution and in the solid state. Salts of 2 also undergo facile ligand substitution reactions. For example, it reacts with 1,3,5,7-cyclooctatetraene, COT, at 20 degrees in THF to give high yields of the previously known [Ta(COT)3]-, which was structurally characterized as the Na(crypt 2.2.2)salt. One particularly important feature concerning 1 and 2 is that they are the first available synthons for "naked" atomic Ta- and promise to be useful reagents for the general exploration of low-valent tantalum chemistry. Also, 1 and 2 represent the first homoleptic arene tantalum complexes to have been prepared by conventional syntheses. The only previously known substance of this class is the neutral bis(benzene)tantalum(0), which was accessed by the co-condensation of atomic tantalum and benzene vapor in a sophisticated (electron-gun furnace equipped) metal atom reactor.

Structurally Distinct Homoleptic Anthracene Complexes, [M(C14H10)3]2−, M=Titanium, Zirconium, Hafnium: Tris(arene) Complexes for a Triad of Transition Metals

“…[3] The first well-characterized homoleptic polyarenemetalate, tris(naphthalene)zirconate(2À) (1), [4] was reported nearly 15 years ago, and related complexes are now documented for Nb, [5] Ta, [6] Fe, [3] and Co. [7] However, little or nothing is known about the analogous metalates of other d-block elements. [2,8] We report herein the synthesis, isolation, and structural characterization of the first set of homoleptic polyarenemetalates for a triad of transition metals: the tris(anthracene)metalates(2À), [M-(C 14 H 10 ) 3 ] 2À , M = Ti (2), Zr (3), and Hf (4).…”

“…[2,8] We report herein the synthesis, isolation, and structural characterization of the first set of homoleptic polyarenemetalates for a triad of transition metals: the tris(anthracene)metalates(2À), [M-(C 14 H 10 ) 3 ] 2À , M = Ti (2), Zr (3), and Hf (4). In addition, the triad of Group 4 elements has been "completed" for homoleptic naphthalene complexes with access to the tris(naphthalene)metalates(2À), [M(C 10 H 8 ) 3 ] 2À , for M = Ti (5), Hf (6). Hafnates 4 and 6 are of particular interest as they are the first polyarene complexes of Hf and are unprecedented examples of hydrocarbon-stabilized negative-valent hafnium, Hf 2À , previously established for only one other compound, [Hf(CO) 6 ] 2À .…”

“…In addition, the triad of Group 4 elements has been "completed" for homoleptic naphthalene complexes with access to the tris(naphthalene)metalates(2À), [M(C 10 H 8 ) 3 ] 2À , for M = Ti (5), Hf (6). Hafnates 4 and 6 are of particular interest as they are the first polyarene complexes of Hf and are unprecedented examples of hydrocarbon-stabilized negative-valent hafnium, Hf 2À , previously established for only one other compound, [Hf(CO) 6 ] 2À . [9,10] Reactions of [MCl 4 (thf) 2 ] (M = Ti, Zr) with monopotassium anthracene (K[C 14 H 10 ]) in THF were carried out under nearly the same conditions as recently described for the synthesis of bis(anthracene)cobaltate(1À), [7] and resulted in the formation of anions 2 or 3, isolated in 30-40 % yields as dark purple [K( [18] [6].…”

“…Hafnates 4 and 6 are of particular interest as they are the first polyarene complexes of Hf and are unprecedented examples of hydrocarbon-stabilized negative-valent hafnium, Hf 2À , previously established for only one other compound, [Hf(CO) 6 ] 2À . [9,10] Reactions of [MCl 4 (thf) 2 ] (M = Ti, Zr) with monopotassium anthracene (K[C 14 H 10 ]) in THF were carried out under nearly the same conditions as recently described for the synthesis of bis(anthracene)cobaltate(1À), [7] and resulted in the formation of anions 2 or 3, isolated in 30-40 % yields as dark purple [K( [18] [6]. [11] Efforts are underway to identify intermediates in these intriguing formal six-electron phosphane-mediated reductions, in order to help understand why HfCl 4 behaves differently from ZrCl 4 and TiCl 4 , which do not require the presence of PMe 3 to directly access negativevalent complexes.…”

“…These structures are unusual compared to other known tris(polyarene)metal complexes. Thus, although titanium has approximately the same atomic and covalent radii as Nb and Ta, [21] which provide unexceptional 18-electron tris(1,2,3,4-h 4 -anthracene)metalates [5,6] and tris(h 4 -naphthalene)tantalate, [6] the Ti À C bonds in 2 and 5 are all significantly shorter (on average about 0.05 ) than the corresponding MÀC bonds in the Group 5 complexes. [22] Undoubtedly this difference arises from a greater degree of d-or back-bonding from Ti to the polyarene, which draws the polyarene groups closer to the Ti centers and results in sufficiently strong intramolecular polyarene repulsions to cause reduced hapticity (h 2 ) in one ring in both 2 and 5.…”