Synthesis, Structure, and Reactivity of Magnesium Pentalenides

Abstract

The first magnesium pentalenide complexes have been synthesized via deprotonative metalation of 1,3,4,6-tetraphenyldihydropentalene (Ph 4 PnH 2 ) with magnesium alkyls. Both the nature of the metalating agent and the reaction solvent influenced the structure of the resulting complexes, and an equilibrium between Mg[Ph 4 Pn] and [ n BuMg] 2 [Ph 4 Pn] was found to exist and investigated by NMR, XRD, and UV−vis spectroscopic techniques. Studies on the reactivity of Mg[Ph 4 Pn] with water, methyl iodide, and trimethylsilylchloride revealed that the [Ph 4 Pn] 2− unit undergoes electrophilic addition at 1,5-positions instead of 1,4-positions known for the unsubstituted pentalenide, Pn 2− , highlighting the electronic influence of the four aryl substituents on the pentalenide core. The ratio of syn/anti addition was found to be dependent on the size of the incoming electrophile, with methylation yielding a 60:40 mixture, while silylation yielded exclusively the anti-isomer.

Twinned versus linked organometallics - bimetallic “half-baguette” pentalenide complexes of Rh(i)

“…This rotation is less than what was found for Li 2 [Ph 4 Pn] 60 (29.3°–35.8°) but within the range seen for Mg[Ph 4 Pn] (17.1°–35.2)°. 61…”

“…With either Mg[Ph 4 Pn] or Li·K[Ph 4 Pn] the same transmetalation product was obtained, consistent with the notion that in ethereal solution s-block pentalenides exist as solvent-separated ion pairs. 60,61 However, Mg[Ph 4 Pn] provides practical advantages with cleaner formation of the transmetalation product and easy removal of MgCl 2 by induced precipitation of [MgCl 2 (dioxane)]. [Rh I (COD)] 2 [Ph 4 Pn] ( 1 ), [Ir I (COD)] 2 [Ph 4 Pn] ( 2 ), [Rh I (NBD)] 2 [Ph 4 Pn] ( 3 ) and [Rh I (C 2 H 4 ) 2 ] 2 [Ph 4 Pn] ( 4 ) all formed exclusively as linked anti -bimetallics, whilst the twinned syn -bimetallic [Rh I (CO) 2 ] 2 [Ph 4 Pn] ( 5 ) was obtained regardless whether the monometallic [Rh I (acac)(CO) 2 ] or bimetallic [Rh I (CO) 2 (μ-Cl)] 2 precursors were used.…”

“…1,3,4,6-Tetraphenyl-1,2-dihydropentalene (Ph 4 PnH 2 ), 60 Lithium potassium 1,3,4,6-tetraphenylpentalenide (Li·K[Ph 4 Pn]), 60 Magnesium 1,3,4,6-tetraphenylpentalenide (Mg[Ph 4 Pn]), 61 [Rh I (COD)(μ-Cl)] 2 , 116 [Ir I (COD)(μ-Cl)] 2 117 and [Rh I (NBD)(μ-Cl)] 2 118 were prepared according to literature procedures. [Rh I (CO) 2 (μ-Cl)] 2 and [Rh I (C 2 H 4 ) 2 (μ-Cl)] 2 were used as received.…”

“…We have recently described the facile and modular syntheses of tetra-substituted dihydropentalenes 59 and introduced the first tetra-arylated pentalenide, M 2 [Ph 4 Pn] (M = Li, Na, K) 60 and the first alkaline earth pentalenide, Mg[Ph 4 Pn] . 61,62 The increased solubility and ease of separation of MgX 2 transmetalation by-products (through induced precipitation by addition of 1,4-dioxane 63,64 ) are key advantages of Mg[Ph 4 Pn] over M 2 [Ph 4 Pn] which make it a prime candidate to explore the coordination chemistry of Ph 4 Pn 2− by way of salt metathesis. The majority of known pentalenide complexes contain pentalenide scaffolds containing electron-donating groups (such as alkyl and trialkylsilyl substituents), and the electron-withdrawing nature of the phenyl substituents should make Ph 4 Pn 2− significantly less reducing and thus more suited to bind to electron-rich, late d-block metals.…”