Metal-alkane complexes are of importance because of their involvement in alkane C À H activation reactions [1] and hydrocarbon adsorption in alkali-metal-containing zeolites. [2] However, observable metal-alkane complexes are scarce as a consequence of the poor donor/acceptor character of alkanes and the low polarity of CÀH bonds. Examples detected by NMR spectroscopy include [(C 5 R 5 )Re(CO) 2 -(alkane)], [3,4] [(C 5 R 5 )M(CO)(PF 3 )(alkane)] (M = Re or Mn), [4] [TpRe(CO) 2 (alkane)], [5] [(PONOP)Rh(CH 4 )] + {PONOP = 2,6-(tBu 2 PO) 2 C 5 H 3 N}, [6] and [(C 6 Et 6 )W(CO) 2 -(n-pentane)], [7] but none of these complexes have proven sufficiently robust to allow isolation or crystallization. At the other end of the spectrum are the crystallographically characterized metal-alkane complexes [8] which have not been observed in solution. The only members of this group are the iron(II) double A-frame porphyrin-heptane complex reported by Reed and co-workers, [9] the uranium(III)-alkane complexes reported by Meyer and co-workers, [10] and a rhodium(I) norbornane complex reported by Weller and coworkers, [11] and in all cases the metal-alkane interaction is considered to possess some degree of covalency, perhaps with additional stabilization from interactions between the alkane and the ligand framework. Herein we describe potassium complexes of a new highly rigid and sterically encumbered NON-donor ligand, all of which feature remarkably short intermolecular potassium-alkane distances in the solid state.Palladium-catalyzed coupling of 4,5-dibromo-2,7-di-tertbutyl-9,9-dimethylxanthene with 2 equivalents of 2,6-dimesitylaniline afforded 4,5-bis(2,6-dimesitylanilino)-2,7-di-tertbutyl-9,9-dimethylxanthene [H 2 (XAT); Scheme 1], which is an extremely sterically hindered analogue of the known 4,5bis(2,6-diisopropylanilino)-2,7-di-tert-butyl-9,9-dimethylxanthene [12] and 4,5-bis (2,4,6-trimethylanilino)-2,7-di-tert-butyl-9,9-dimethylxanthene [13] pro-ligands. Reaction of H 2 (XAT) with excess KH in toluene yielded the dipotassium salt, and filtration and layering with hexanes at À30 8C deposited vibrant yellow X-ray quality crystals of [K 2 (XAT)(n-hexane)]·toluene (1; Scheme 1 and Figure 1). The potassium atoms in 1 are bound to bridging amido and ether donors, forming a square pyramidal K 2 N 2 O core with oxygen in the apical site.

Reaction of [(XA2)UCl3{K(dme)3}] (XA2 = 4,5-bis(2,6-diisopropylanilino)-2,7-di-tert-butyl-9,9-dimethylxanthene) with 2 equiv of ((trimethylsilyl)methyl)lithium or neopentyllithium afforded red-orange [(XA2)U(CH2SiMe3)2] (1) and dark red [(XA2)U(CH2CMe3)2] (2), respectively. Reaction of 1 with an additional 1 equiv of LiCH2SiMe3 in THF yielded yellow [Li(THF) x ][(XA2)U(CH2SiMe3)3] (3), and reaction of [(XA2)UCl3{K(dme)3}] with 3 equiv of methyllithium in dme afforded yellow [Li(dme)3][(XA2)UMe3] (4). Reaction of 1 with 2.1 equiv of LiCH2CMe3 in benzene resulted in rapid conversion to 2, with release of 2 equiv of LiCH2SiMe3. Similarly, reaction of 1 with 3.3 equiv of MeLi in THF provided 4 as the [Li(THF) x ]+ salt, accompanied by 2 equiv of LiCH2SiMe3. These unusual alkyl exchange reactions resemble salt metathesis reactions, but with elimination of an alkyllithium instead of a lithium halide. Addition of a large excess of LiCH2SiMe3 to 2 or 4 did not generate detectable amounts of 1 by NMR spectroscopy, suggesting that the equilibrium in these reactions lies far to the side of complexes 2 and 4. In contrast, the reaction of [(XA2)Th(CH2SiMe3)2] (1-Th) with 2.2 equiv of LiCH2CMe3 yielded an approximate 1:1:3:1 mixture of [(XA2)Th(CH2CMe3)2] (2-Th), [(XA2)Th(CH2SiMe3)(CH2CMe3)] (5-Th), LiCH2SiMe3, and LiCH2CMe3.

A rigid NSN-donor proligand, 4,5-bis(2,6-diisopropylanilino)-2,7-di-tert-butyl-9,9-dimethylthioxanthene (H(2)[TXA(2)], 1) was prepared by palladium-catalyzed coupling of 2,6-diisopropylaniline with 4,5-dibromo-2,7-di-tert-butyl-9,9-dimethylthioxanthene. Deprotonation of 1 using (n)BuLi provided Li(2)(DME)(2)[TXA(2)] (2), and subsequent reaction with UCl(4) afforded [Li(DME)(3)][(TXA(2))UCl(3)] (4). The analogous NON-donor ligated complex [(XA(2))UCl(3)K(DME)(3)] [3; XA(2) = 4,5-bis(2,6-diisopropylanilino)-2,7-di-tert-butyl-9,9-dimethylxanthene] was prepared by the reaction of K(2)(DME)(x)[XA(2)] with UCl(4). A cyclic voltammogram (CV) of 3 in THF/[NBu(4)][B(C(6)F(5))(4)] at 200 mV s(-1) showed an irreversible reduction to uranium(III) at E(pc) = -2.46 V versus FeCp(2)(0/+1), followed by a product wave at E(1/2) = -1.83 V. Complex 4 also underwent irreversible reduction to uranium(iii) [E(pc) = -2.56 V], resulting in an irreversible product peak at E(pa) = -1.83 V. One-electron reduction of complexes 3 and 4 using K(naphthalenide) under an argon atmosphere in DME yielded 6-coordinate [(XA(2))UCl(DME)] (5) and the thermally unstable 7-coordinate [(TXA(2))U(DME)Cl(2)Li(DME)(2)] (6), respectively. The U-S distances in 4 and 6 are uncommonly short, the C-S-U angles are unusually acute, and the thioxanthene backbone of the TXA(2) ligand is significantly bent. By contrast, the xanthene backbone in XA(2) complexes 3 and 5 is planar. However, κ(3)-coordination and an approximately meridional arrangement of the ancillary ligand donor atoms is maintained in all complexes. DFT and Atoms in Molecules (AIM) calculations were carried out on 3, 4, 5, 6, [(XA(2))UCl(3)](-) (3B), [(TXA(2))UCl(2)(DME)](-) (6B) and [(TXA(2))UCl(DME)] (6C) to probe the extent of covalency in U-SAr(2) bonding relative to U-OAr(2) bonding.

Palladium-catalyzed coupling of 1-adamantylamine (2 equiv.) with 4,5-dibromo-2,7-di-tert-butyl-9,9-dimethylxanthene afforded the proligand 4,5-bis(1-adamantylamino)-2,7-di-tert-butyl-9,9-dimethylxanthene, H2[XAd] (1), which upon deprotonation with excess KH or KCH2Ph in THF or dme generated [{K(THF)3}2(XAd)] (2a) and [K2(XAd)(dme)] (2b). Subsequent reaction of in situ generated 2a or 2b with [ThCl4(dme)2] yielded [(XAd)ThCl4K2]·x(dme) (3; x = 0.5-2), which reacted with 2 equiv. of LiCH2SiMe3 or K[allylTMS] to afford the bis(hydrocarbyl) complexes [(XAd)Th(CH2SiMe3)2(THF)] (4) and [(XAd)Th(η3-allylTMS)2] {5; allylTMS = 1-(SiMe3)C3H4}. Dialkyl complex 4 was stable at 80 °C for at least 2.5 hours, but decomposed over 4.5 hours at 110 °C. Diallyl complex 5 was stable for hours at 85 °C, and suffered less than 5% decomposition after 10 minutes at 155 °C. At room temperature, averaging of the syn and anti protons of the allyl CH2 groups of 5 occurred on the NMR timescale, as a consequence of rapid π-σ-π hapticity changes. Additionally, low temperature 1H and 13C NMR spectroscopy indicates that 5 exists as a rapidly exchanging mixture of two isomers with C1 and C2 symmetry. Compounds 2a, 4 and 5 were crystallographically characterized.

Uranium(iv) alkyl cations have been prepared, and arene solvent coordination was shown to have a major impact on ethylene polymerization activity. Actinide-arene binding was studied experimentally in the solid state and solution, and computationally.