Ditungsten Tetracarboxylates (Quadruple Bond, Ditungsten).

Abstract

W(,2)(O(,2)CCF(,3))(,4), 1, and W(,2)(O(,2)C('t)Bu)(,4), 2, are obtained in ca. 50% yields by sodium amalgam reduction in THF of polymeric tungsten tetrachloride, (WCl(,4))(,x), in the presence of NaO(,2)CCF(,3) and NaO(,2)C('t)Bu, respectively. Metathesis of W(,2)(O(,2)CCF(,3))(,4) in toluene with 4 equivalents of {('n)Bu(,4)N}{O(,2)CCH(,3)} affords W(,2)(O(,2)CCH(,3))(,4), 3, in quantitative yield. These oxygen-sensitive, yellow micro-crystalline complexes were characterized by cyclic voltammetry, electronic absorption, infrared, Raman, NMR (('1)H, ('19)F, ('31)P, and ('183)W), and mass spectroscopies. The Raman active totally symmetric W-W stretching vibrations were observed at 310, 313, and 304 (+OR-) 2 cm('-1) for 1, 2, and 3, respectively. In the solid state, the diglyme adduct of 1 consists of two independent W(,2)(O(,2)CCF(,3))(,4) units with W(,)('4)W bond lengths of 2.207(2) and 2.212(2) (ANGSTROM). The reaction of 1 and 2 with two equivalents of PPh(,3) in toluene provides the axial adducts W(,2)(O(,2)CCF(,3))(,4)(.)2PPh(,3), 4, and W(,2)(O(,2)C('t)Bu)(,4)(.)2PPh(,3), 5. The symmetric W-W stretching frequencies for these latter two adducts were found at 280 and 287 (+OR-) 2 cm('-1), respectively. Characterization by low temperature X-ray crystallography of the latter two axial adducts, revealed W(,)('4)W bond lengths of 2.242{2} ave and 2.218(1) (ANGSTROM), respectively. The bis-equatorial adducts, W(,2)(O(,2)CCF(,3))(,4)(.)2PR(,3) (R = Me (6), Et (7), and ('n)Bu (8) are formed when 1 reacts with two equivalents of the individual trialkylphosphines in toluene. Investigations by ('31)P{('1)H} and ('19)F NMR indicate only one of the six possible geometric, bis-equatorial isomers is present in solution. In view of this result, the molybdenum analogues of 6, 7, and 8 were reinvestigated by variable temperature ('31)P{('1)H} and ('19)F NMR. In toluene at -75(DEGREES)C, the molybdenum equatorial adducts demonstrate behavior analogous to the tungsten compounds. The P(('n)Bu)(,3) adducts of M(,2)(O(,2)CCF(,3))(,4)(.)2PR(,3) (M = W (8), Mo (9)) were studied by low temperature X-ray crystallography. In the solid state, 8 and 9 are isostructural with M(,)('4)M and M-P bond lengths of 2.224(1) and 2.489(3) (ANGSTROM) in 8 and 2.105(1) and 2.542(2) (ANGSTROM) in 9. The electronic absorption spectra of 1-3 were recorded in THF; however, the (delta) (--->) (delta)* (('1)A(,1g) (--->) ('1)A(,2u)) transition was not observed. Further studies indicate this b and is masked by the more intense (delta) (--->) (pi)*(OCO) transition in the spectra. The (delta) (--->) (delta)* transition was observed in the spectrum of 6 at 562 nm.