Redox-enhanced hemilability of a tris(tert-butoxy)siloxy ligand at cerium

Abstract

The stabilizing effect of a tris(tert-butoxy)siloxy ligand on cerium(iv) is revealed by electrochemical and computation methods as well as by targeted redox chemistry. Ceric homoleptic complex Ce[OSi(OtBu)3]4 was obtained by the reaction of [Et4N]2[CeCl6] with NaOSi(OtBu)3 at ambient temperature in acetonitrile, while cerous ion-separated complex [Ce{OSi(OtBu)3}4][K(2.2.2-crypt)] was readily synthesized from [Ce{OSi(OtBu)3}4K] and cryptand. The solid-state structures of monocerium complexes Ce[OSi(OtBu)3]4 and Ce[OSi(OtBu)3]4(THF) show 5- and 6-coordinate CeIV centers (one κ2-bonded siloxy ligand), while complex [Ce{OSi(OtBu)3}4][K(2.2.2-crypt)] exhibits a 4-coordinate CeIII center (all-terminal siloxy coordination). A comparative electrochemical study of Ce[OSi(OtBu)3]4 and [Ce{OSi(OtBu)3}4][K(2.2.2-crypt)] suggests a redox-modulated molecular rearrangement process, featuring oxidation-state dependent formation and release of a CeOtBu coordination. While the overall stabilization of CeIV by the siloxy ligand is evident, significant extra stabilization is gained if the siloxy ligand coordinates in a chelating fashion, which is further supported by DFT calculations. Natural bond orbital (NBO) analysis indicates an enhanced donation of the siloxy ligand electron density into the unfilled CeIV 6s, 4f, and 5d orbitals. CeIV to CeIII reduction readily occurs when homoleptic complex Ce[OSi(OtBu)3]4 is treated with cobaltocene, affording the separated ion pair [Ce{OSi(OtBu)3}4][CoCp2], featuring exclusive terminal siloxy bonding in the solid-state, similar to that detected for [Ce{OSi(OtBu)3}4][K(2.2.2-crypt)].

Ceric Ammonium Nitrate and Ceric Ammonium Chloride as Precursors for Ceric Siloxides: Ammonia and Ammonium Inclusion

“…The solid‐state structure of [Ce{OSi(O t Bu) 3 } 4 (NH 3 ) 2 ] ( 2 ) revealed a distorted octahedral coordination geometry of the cerium atom with the two ammonia molecules positioned cis to each other, as observed for 1 (Figure ) or, e.g., in dimeric [Pr[OCMe(CF 3 ) 2 ] 2 (NH 3 ) 2 ] 2 . [37a] The Ce–O bond lengths in 2 [2.120(2) – 2.170(2) Å] match those in complex 1 [2.142(3) – 2.145(2) Å], but are slightly longer than in homoleptic five‐coordinate complex [Ce{OSi(O t Bu) 3 } 4 ] [Ce–O(siloxy) 2.089(2) – 2.102(2) Å] . The Ce–N distances [Ce–N1 2.602(3), Ce–N2 2.579(3) Å] are slightly shorter than found in 1 but match those of eight‐coordinate cerium complex [CeF 4 (NH 3 ) 4 ] 2.586(2) Å.…”

“…Apparently (and surprisingly), the solid‐state structural arrangement is retained in solution at ambient temperature ( t Bu proton ratio of 6:2:1; Figure S2), suggesting hindered rotation about the Ce–O5–Si1 axis. The Ce–O(siloxy) bond lengths in 1 [2.142(3) – 2.145(2) Å] are slightly elongated in comparison with those in six‐coordinate complex [Ce{OSi(O t Bu) 3 } 3 Cl(THF) 2 ] [Ce–O Si : 2.097(4) – 2.104(3) Å] . The Ce ··· NH 3 distances of 2.613(4) and 2.631(4) Å are slightly shorter than those reported for six‐coordinate trivalent alkoxide [Pr{OCMe(CF 3 ) 2 } 3 (NH 3 ) 2 ] 2 [2.687(7) Å, 2.692(7) Å][37a] but significantly longer than those in [CeF 4 (NH 3 ) 4 ] [2.586(2) Å].…”

“…This distinct arrangement of the tert ‐butoxy groups in the solid‐state seems to be also observable in the ambient temperature 1 H NMR spectrum of 3 featuring two distinct environments in a 2:1 ratio (Figure S4). The Ce–O bond lengths in seven‐coordinate 3 [2.104(2) – 2.1172(19) Å] appear marginally shorter than found in 1 and 2 , but slightly elongated compared to [Ce{OSi(O t Bu) 3 } 3 Cl(THF) 2 ] …”

“…We and others have recently introduced the tris( tert‐ butoxy)siloxy [OSi(O t Bu) 3 ] – ligand into cerium(IV) chemistry . This ligand system has been routinely employed in metal silicate precursor chemistry, as a model for silica surface reactions, and for stabilizing metals in high oxidation states , , .…”

“…In the present study on ceric siloxide complexes “Ce IV ‐OSiR 3 ”, we draw a comparison between alkoxysiloxy (R = O t Bu) and siloxy (R = Et, i Pr) derivatives applying both CAN {= (NH 4 ) 2 Ce(NO 3 ) 6 } and Ceric organoAmmonium Chloride “CAC R” {= (R 4 N) 2 [CeCl 6 ]; R = Me, Et} – based metathesis protocols. [32e] The stabilizing influence of the siloxy ligands on the tetravalent cerium ion is supported by electrochemical investigations.…”