Bis(acetylacetonato)tricarbonyl Tungsten(II):  A Convenient Precursor to Chiral Bis(acac) Tungsten(II) Complexes

Abstract

Two equivalents of acetylacetonate (acac) have been successfully introduced into a monomeric tungsten(II) coordination sphere. With the tetracarbonyltriiodotungsten(II) anion as a precursor, the formation of a tungsten(II) bis(acac) tricarbonyl complex, W(CO)3(acac)2, 1, has been accomplished. The addition of PMe3 or PMe2Ph to tricarbonyl complex 1 formed tungsten(II)bis(acac)dicarbonylphosphine complexes 2a and 2b, respectively. Single-crystal X-ray diffraction studies of the parent tricarbonyl complex, 1, and dicarbonyl trimethylphosphine complex 2a confirmed seven-coordinate geometries for both complexes. Variable-temperature 1H and 13C{1H} NMR spectroscopy revealed fluxional behavior for these seven-coordinate molecules: rapid exchange of the three carbon monoxide ligands in 1 was observed, and movement of the phosphine ligand through a mirror plane in a C(S) intermediate species was observed for both 2a and 2b. Tricarbonyl complex 1 reacted readily with alkyne reagents to form bis(acac)monocarbonylmonoalkynetungsten(II) complexes 3a (PhC(triple bond)CH) and 3b (MeC(triple bond)CMe). Variable-temperature 1H NMR spectroscopy was used to probe rotation of the alkyne ligand in 3a and 3b. The introduction of two alkyne ligands was accomplished thermally using excess PhC(triple bond)CPh to form bis(alkyne) complex 4 which was characterized crystallographically, as well as by 1H and 13C NMR spectroscopy. The availability of W(CO)3(acac)2 as a source of the W(acac)2 d4 moiety lies at the heart of the chemistry reported here.

Synthesis and reactivity of oxygen ligated molybdenum(II) carbonyl complexes

“…By using both trop and acac ligands the effect of chelate bite angle can be probed in addition to the difference in oxygen versus sulfur donor atoms. The tropolonate ligand has a small bite angle of 73-75°which is closer to the dtc ligand values of 69-70°than the larger acac ligand 81-85° [30]. Many group VI d 4 would tend to favor the smaller bite angle ligands.…”

“…These resonances occur up to $0.40 ppm upfield from the similar W analogs (acac) 2 W(CO)(PhC"CH) and (acac) 2 W(CO)(MeC"CMe) [30]. The spectrum also shows two unique acac methine protons with resonances at 5.53 and 5.40 ppm.…”

“…NMR solvents CD 2 Cl 2 , CDCl 3 , C 6 D 6 , and acetone-d 6 were dried by passage through a pipet containing activated alumina in the glove box. [NEt 4 ][Mo(CO) 5 I] was synthesized according to published procedures [30,43]. All other reagents were used as received.…”

“…The bis(ketoenolate) ligated Mo II complexes, (LX) 2 Mo(CO) (RC"CR 0 ) where (LX = trop or acac) were synthesized using an analogous method to what has been employed for (LX) 2 W(CO) (RC"CR 0 ) [30,45]. These compounds are similar in structure to bis(dithiocarbamate) compounds of Mo II [5], but are the first reported oxygen ligated low-valent Mo species of this type.…”

“…Moving to an oxygen based chelate will eliminate these undesirable side reactions and stabilize the metal with stronger field oxygen donor atoms. Despite the widespread use of chelating oxygen ligands, particularly acetylacetonate, across the periodic table [23][24][25][26][27][28], their substitution for dithiocarbamates on low valent group VI metals has only recently been reported for tungsten and is mostly unexplored for molybdenum [29][30][31][32][33][34], though a few mono ligand species are known [35]. The unusual ability of (acac) 2 W complexes to coordinate nitriles in a p-bound fashion and subsequently derivatize them in a stoichiometric fashion is a substantial difference from dtc chemistry and provides further motivation to explore the chemistry of low valent Mo complexes of acetylacetonate (acac) and tropolonate (trop) [30][31][32][33][34]36].…”