Characterization of the Terminal Iron(IV) Imides {[PhBP^tBu₂(pz‘)]Fe^IV⋮NAd}⁺

Abstract

New hybrid bis(phosphine)(pyrazole)borate tripodal ligands ([PhBP tBu 2 (pz')] − ) are reported that support pseudotetrahedral iron in the oxidation states +1, +2, +3, and +4. The higher oxidation states are stabilized by a terminal Fe≡NR linkage. Of particular interest is the generation and thorough characterization of an S = 1 Fe IV ≡NR + imide cation using this new ligand system. The latter species can be observed electrochemically and spectroscopically, and its solid-state crystal structure is reported.Although implicated as intermediates in group transfer reactions,1 mid-to-late first row transition metals (e.g., Mn, Fe, Co, Ni) featuring terminal imido/nitrene functionalities are rare. 5 To the best of our knowledge the single report of a complex that can be formulated as an Fe(IV) imide concerns Lee's tetranuclear cluster Fe 4 (µ 3 -N t Bu) 4 (N t Bu)Cl 3 , isolated in only 1-2% yield. 6 Mössbauer data for this species were consistent with a cluster featuring three Fe(III) centers and one Fe (IV) center, the latter most likely indicative of the Fe IV =N t Bu terminal linkage. Herein we describe a new hybrid pyrazolyl/phosphino-borate ligand that can support pseudotetrahedral iron in the +1, +2, +3, and +4 oxidation states. The +3 and +4 oxidation states are stabilized by the terminal Fe≡NR imide linkage.Access to the required bis(phosphino)pyrazolylborate 7 ligand is achieved by initial preparation of the bis(phosphino)borane precursor PhB(CH 2 P t Bu 2 ) 2 (1) via metathesis between PhBCl 2 and two equivalents of LiCH 2 P t Bu 2 (Scheme 1). Reaction of [pz]Li with 1 (pz = pyrazolyl), followed immediately by salt metathesis with TlPF 6 , leads to the clean formation of solid white [PhBP tBu 2 (pz)]Tl (2a) in 66% isolated yield. 8 The use of the bulky LiCH 2 P t Bu 2 carbanion is critically important in the preparation of this type of hybrid borate ligand because (i) effective di-rather than tri-substitution at boron can be achieved, which could not be realized using less hindered carbanions such as LiCH 2 P i Pr 2 and LiCH 2 PPh 2 ; (ii) the borane product, PhB (CH 2 P t Bu 2 ) 2 (1), does not appear to dimerize to an appreciable degree in solution. This fact allows the efficient introduction of a third donor arm. Metathesis of 2a with FeCl 2 leads to the clean formation of the high spin (S = 2) complex [PhBP tBu 2 (pz)]FeCl (3a), isolated in 69% yield as a yellow crystalline solid. Solution 1 H NMR data for 3a are suggestive of a monomeric structure in solution, in accord with the calculated solution Evans' method magnetic moment at 295 K (5.2 µ B ). 9 However, the 100 K X-ray crystal structure of 3a establishes a dimeric structure with bridging chlorides and a crystallographic center of symmetry in the solid-state (see SI for details).Following an overall methodology that proved effective for the synthesis of Fe(III) imides previously, one-electron reduction of 3a with sodium/mercury amalgam in the presence of excess PMe 3 generates the high spin d 7 precursor [PhBP tBu 2 (pz)]Fe(PMe 3 ) (4a) a...

Coordination Features of a Hybrid Scorpionate/Phosphane Ligand Exemplified with Iridium

“…[10] We have already anticipated that such a system could lead to new reactivity patterns and open up new possibilities for scorpionate ligands. [11] In this paper we report the results of a study of a series of iridium complexes produced with this ligand system, paying special attention to the possibilities of coordination through the four arms, and their reactions with protic acids, which reflect the versatility of this hybrid scorpionate when attached to an iridium center.…”

Copper Complexes of Mono‐ and Ditopic [(Methylthio)methyl]borates: Missing Links and Linked Systems En Route to Copper Enzyme Models

“…Treatment of the monotopic tripod [5] À with CuCl and CuBr 2 results in the formation of complexes K[Cu (5) ((N 2 P), [13,14] (NP 2 ) [15] ) or nitrogen and sulfur donors ((N 2 S), [16] (NS 2 ) [17][18][19][20][21][22] ). These derivatives are most interesting, because they allow a smooth modulation of the coordination environment between the extremes (N 3 ) and (P 3 )/(S 3 ).…”

Bio‐organometallic Approaches to Nitrogen Fixation Chemistry

“…This electronic arrangement gives rise to their corresponding singlet, doublet and triplet ground states (Fig. 3.24) [79,101,117,120]. As noted previously, it is possible that a single iron site initiates the N 2 reduction process, but that bi-or polymetallic N x H y intermediates are formed at later stages.…”

“…While efforts in our laboratories are ongoing with respect to the generation and characterization of the directly relevant parent imide derivatives, our group has achieved the preparation of well-defined [BP 3 ]Fe(NR) species in the Fe(II) and Fe(III) oxidation states [79,101,117]. We have most recently also realized the preparation of a related system, [PhB(CH 2 P t Bu 2 ) 2 (pyrazole)]Fe(NR) [120], that supports imide functionalities in the Fe(III) and Fe(IV) oxidation states. Remarkably the Fe-N bond distance changes only negligibly across the three iron oxidation states (Fe-N ranges between 1.63 and 1.65 Å) for this series of pseudotetrahedral imide complexes.…”

“…Remarkably the Fe-N bond distance changes only negligibly across the three iron oxidation states (Fe-N ranges between 1.63 and 1.65 Å) for this series of pseudotetrahedral imide complexes. This fact is consistent with the d orbital splitting diagram that we have proposed for low-spin, four-coordinate iron imides of approximate 3-fold symmetry, whereby two high-lying antibonding orbitals (3d xz , 3d yz ) with both r-and p-antibonding contributions are empty and three lower lying, largely nonbonding orbitals (3d xy , 3d x2-y2 , 3d z 2 ) house either six, five or four electrons for the Fe(II), Fe(III) and Fe(IV) imides, respectively [79,101,104,117,120]. This electronic arrangement gives rise to their corresponding singlet, doublet and triplet ground states (Fig.…”