The electronic structures of early actinide (Th-Pu) compounds present unique challenges to study because of their very large spin-orbit coupling and appreciable 5f/6d metalligand orbital overlap. In fact, it is the interplay of (non)participating 5f/6d orbitals in bonding that is the primary factor responsible for the unusual properties of Pu metal, which lies at the interface of localized and delocalized (electronically itinerant) behavior in the actinides, [1] and for the emergence of diverse magnetic and conducting phases in early actinide intermetallics. [2] Recent studies have shown that multimetallic molecular systems may serve as models for understanding electronic structures in complex actinide materials, [3] wherein 5f/6d electrons can span the continuum from corelike and nonbonding to delocalized with appreciable covalent character. Although mixed valency and the corresponding electrochemical, spectroscopic, and magnetic signatures for classical Robin-Day delocalization are well established for multimetallic transition-metal complexes, [4] they are essentially unknown for the actinides. [5] We previously described the synthesis of [Cp* 2 An-{N=C(Bz)(tpy)} 2 ] (An = Th (1 a), U (1 b); Bz = CH 2 Ph; tpy = 2,2':6',2''-terpyridine; Cp* = C 5 Me 5 ). [3b] Reaction of 1 a,b with the Yb II precursor [Cp* 2 Yb(OEt 2 )] leads to the formation of the mixed-metal complexes [Cp* 2 An-{N=C(Bz)(tpy-YbCp* 2 )} 2 ] (An = Th (2 a), U (2 b)).
As these complexes showed the first distinctive signs of 5f-4f metalmetal magnetic and electrochemical communication, we were compelled to prepare the all-actinide analogues in this structural motif. Herein, we report mixed-valent homo-and hetero-trimetallic actinide complexes that show intriguing signs of Robin-Day delocalization.
In the absence of a readily available divalent uranium synthon, we discovered that addition of a slight excess of potassium and two equivalents [Cp °2UI(thf)] (Cp °= C 5 Me 4 Et) [6] to 1 a,b produced the actinide multimetallic complexes 3 a,b [Eq. ( 1)] in good yields.
Experimental Section
All manipulations were performed in either a recirculating Vacuum Atmospheres (Model HE-553-2 with a MO-40-2 Dri-Train or NEXUS with a 40CFM Dual Purifier NI-Train) drybox (N 2 ), or by using standard Schlenk line techniques.
3 a: Potassium mirror (0.034 g, 0.87 mmol, 2.1 equiv) was prepared on the bottom of a 125-mL side-arm flask, and toluene (30 mL) was added. With stirring, 1 a (0.50 g, 0.42 mmol) and [Cp °2U(I)(thf)] (0.64 g, 0.87 mmol, 2.1 equiv) were added simultaneously, producing a dark green solution that was stirred at room temperature for 48 h. Over this time the reaction mixture became dark blue in color and a solid precipitated from the solution. The solution was filtered through a celite-padded coarse frit, and the filtrate was collected and concentrated to 5 mL under reduced pressure. Solid 3 a was precipitated from the toluene solution with addition of (Me 3 Si) 2 O (50 mL). Crystalline, analytically pure 3 a was obtained from saturated solutions of (Me 3 Si) 2 O/toluene (10:1 v/v) set at room temperature for three days. Yield: 0.59 g, 0.24 mmol (62 %). 1 H NMR ([D 8 ]toluene, 100 8C): d = 17.88 (br s, 12 H, C 5 Me 4 (CH 2 CH 3 )), 12.22 (br s, 24 H, C 5 (CH 3 ) 2 (CH 3 ) 2 Et), 11.47 (br s, 24 H, C 5 (CH 3 ) 2 (CH 3 ) 2 Et), 4.54 (br s, 2 H, p-H in Bz), 3.84 (br s, 4 H, ArH), 2.93 (br s, 8 H, C 5 Me 4 (CH 2 CH 3 )), 0.64 (br s, 4 H, CH 2 Ph), À0.23 (s, 30 H, C 5 (CH 3 ) 5 ), À2.15 (br s, 4 H, ArH), À3.04 (br s, 4 H, ArH), À16.84 (br s, 4 H, ArH), À19.38 (br s, 8 H, ArH), À24.05 ppm (br s, 4 H, ArH). Elemental analysis (%) calcd for C 110 H 131 N 8 U 2 Th•2 (C 6 H 18 OSi 2 ) (2596.37 g mol À1 ): C 56.40, H 6.48, N 4.31; found: C 56.67, H 6.55, N 4.36. 3 b: The synthesis of 3 b is analogous to that of 3 a. Yield: 0.638 g, 0.258 mmol (60 %). 1 H NMR ([D 8 ]toluene, 90 8C): d = 18.02 (br s, 12 H, C 5 Me 4 (CH 2 CH 3 )), 12.75 (br s, 24 H, C 5 (CH 3 ) 2 (CH 3 ) 2 Et), 11.97 (br s, 24 H, C 5 (CH 3 ) 2 (CH 3 ) 2 Et), 4.93 (br s, 2 H, p-H in Bz), 4.13 (br s, 4 H, ArH), 3.60 (br s, 8 H, C 5 Me 4 (CH 2 CH 3 )), 1.20 (br s, 4 H, CH 2 Ph), À4.25 (s, 30 H, C 5 (CH 3 ) 5 ), À23.75 (br s, 4 H, ArH), À87.09 ppm (br s, 4 H, ArH), additional tpy-H resonances were not observed. Elemental analysis (%) calcd for C 110 H 131 N 8 U 3 (2279.36 g mol À1 ): C 57.96, H 5.79, N 4.92; found: C 57.95, H 5.69, N 5.14.