Mixed-valence dimolybdenum complexes containing hard oxo and soft carbonyl ligands: synthesis, structure, and electrochemistry of Mo₂(O)(CO)₂(μ-κ²-S(CH₂)_nS)₂(κ²-diphosphine)

Abstract

Mixed-valence dimolybdenum complexes Mo2(O)(CO)2{μ-κ2-S(CH2)nS}2(κ2-Ph2P(CH2)mPPh2) (n = 2, 3; m = 1, 2) (1-4) have been synthesized from one-pot reactions of fac-Mo(CO)3(NCMe)3 and dithiols, HS(CH2)nSH, in the presence of diphosphines. The dimolybdenum framework is supported by two thiolate bridges, with one molybdenum carrying a terminal oxo ligand and the second two carbonyls. The dppm (m = 1) products exist as a pair of diastereomers differing in the relative orientation of the two carbonyls (cis and trans) at the Mo(CO)2(dppm) center, while dppe (m = 2) complexes are found solely as the trans isomers. Small amounts of Mo(CO){κ3-S(CH2CH2S)2}(κ2-dppe) (5) also result from the reaction using HS(CH2)2SH and dppe. The bonding in isomers of 1-4 has been computationally explored by DFT calculations, trans diastereomers being computed to be more stable than the corresponding pair of cis diastereomers for all. The calculations confirm the existence of Mo[triple bond, length as m-dash]O and Mo-Mo bond orders and suggest that the new dimeric compounds are best viewed as Mo(v)-Mo(i) mixed-valence systems. The electrochemical properties of 1 have been investigated by CV and show a reversible one-electron reduction associated with the Mo(v) centre, while two closely spaced irreversible oxidation waves are tentatively assigned to oxidation of the Mo(i) centre of the two isomers as supported by DFT calculations.

Combining [MoVIO3] and [M0(CO)3] (M = Mo, Cr) Fragments within the Same Complex: Synthesis and Reactivity of the Single Oxo-Bridged Heterobimetallics Supported by Xanthene-Based Heterodinucleating Ligands

“…First, L 1 specifically favors molybdenum in the maximum/minimum oxidation states by utilizing hard (catecholate) and soft (iminopyridine) donors. Second, L 1 likely geometrically precludes the formation of the metal–metal bond usually found in the mixed-valent complexes. – Third, as stated above, efficient π back bonding in the fac -Mo­(CO) 3 leads to a particularly stable formally Mo(0) complex. To further demonstrate the fact that the stability of 4 is primarily due to the heterodinucleating ligand L 1 , we investigated the reactivity of Mo­(CO) 6 with (Et 4 N) 2 [MoO 4 ] in the absence of L 1 .…”

“…There are a significant number of stable mixed-valent dimolybdenum complexes in the literature. Some of these complexes combine molybdenum-oxo and molybdenum-carbonyl functionalities. – Most of these complexes demonstrate intermediate oxidation states of molybdenum, are supported by Mo–Mo bonds, and are obtained from the bimetallic Mo precursor [CpMo­(CO) 2 ] 2 . Complex 4 is a rare example in which two molybdenum centers are (1) in their (typical) minimum and maximum oxidation states and (2) the dimolybdenum complex is not supported by a Mo–Mo interaction.…”