Dinitrogen-derived (diarylboryl)diazenido complexes with differing coordination to the thallium cation

All-Inorganic sp-Chain Ligands: Isoelectronic B/N Analogues of E. O. Fischer's Alkynylcarbynes

Borylation of a tungsten-bound N2 ligand and halide abstraction provides access to a cationic complex with an unprecedented linear NNBR ligand. This complex undergoes [3+2] cycloaddition with azides, and an unexpected chain-extension reaction with an iminoborane, leading to a complex with a five-atom B/N chain. These two [NNBR]-containing complexes, inorganic analogues of E. O. Fischer's alkynylcarbynes, are very rare examples of molecules containing all-inorganic chains of sp-hybridized atoms.

Seven-Coordinate Group 6 Metal Hydrides Obtained by H2 Activation at B(C6F5)3 Adducts of N2 Complexes: Frustrated Lewis Pair-Type Reactivity of The B-N Linkage

The adducts 2M,R of general formula trans-[(L)M{R2P(CH2)2PR2}2{N2B(C6F5)3}] (L=ø or N2, M=Mo or W, R=Et or Ph), formed from Lewis acid-base pairing of B(C6F5)3 to a dinitrogen ligand of zero-valent group 6 bis(phosphine) complexes trans-[M{R2P(CH2)2PR2}2(N2)2] are shown to react with dihydrogen to afford hepta-coordinated bis(hydride) complexes [M(H)2{R2P(CH2)2PR2}{N2B(C6F5)3}] 3M,R which feature the rare ability to activate both dinitrogen and dihydrogen at a single metal center, except in the case where M=Mo and R=Ph for which fast precipitation of insoluble [Mo(H)4(dppe)2] (dppe=1,2-bis(diphenylphosphino)ethane) occurs. The frustrated Lewis pair (FLP)-related reactivity of the B-N linkage in compounds 3W,R was explored and led to distal N functionalization without involvement of the hydride ligands. It is shown in one example that the resulting bis(hydride) diazenido compounds may also be obtained through a sequence involving first FLP-type N-functionalization followed by oxidative addition of H2. Those oily compounds were found to have limited stability in solution or in their isolated states. Finally, treatment of 3W,Et with the Lewis base N,N-dimethylaminopyridine (DMAP) affords the simple but unknown bis(hydride)-dinitrogen species [W(H)2(depe)2(N2)] 11Et (depe=1,2-bis(diethylphosphino)ethane) which direct, selective formation from trans-[W(N2)2(depe)2] is not possible.

Iron(I) Complex Bearing an Open-Shell Diazenido Ligand

Low-valent transition-metal diazenido species are important intermediates in transition-metal-mediated dinitrogen reduction reactions. Isolable complexes of the type unanimously feature closed-shell diazenido ligands. Those bearing open-shell diazenido ligands have remained elusive. Herein, we report the synthesis, characterization, and reactivity of a d7 iron(I) complex featuring an open-shell silyldiazenido ligand, [(ICy)Fe(NNSiiPr3)(η2:η2-dvtms)] (1, ICy = 1,3-dicyclohexylimidazole-2-ylidene, dvtms = divinyltetramethyldisiloxane). Complex 1 is prepared in good yield by silylation of the iron(-I)-N2 complex [K(18-crown-6)][(ICy)Fe(N2)(η2:η2-dvtms)] with iPr3SiOTf and has been fully characterized by various spectroscopic methods. Theoretical studies, in combination with characterization data, established an S = 1/2 ground spin-state for 1 that can best be described as a quartet iron(I) center featuring an antiferromagnetically coupled triplet silyldiazenido ligand. The diazenido and alkene ligands in 1 are labile, as indicated by the facile disproportionation reaction of 1 at ambient temperature to transform into the iron(II) bis(diazenido) species [(ICy)(NNSiiPr3)2Fe(dvtms)Fe(NNSiiPr3)2(ICy)] (2) and the iron(0) species [(ICy)Fe(η2:η2-dvtms)] and also the alkene-exchange reaction of 1 with PhCH═CHBC8H14 to form [(ICy)Fe(NNSiiPr3)(η2-trans-PhCH═CHBC8H14)] (3). Complex 1 is light-sensitive. Upon photolysis, it undergoes a SiiPr3 radical-transfer reaction to yield [(ICy)Fe(σ:η2-MeCHSiMe2OSiMe2CH═CHSiiPr3)] (4) and N2. The reactions of 1 with the trityl radical and organic bromides yield iron(II) complexes, which indicates its reducing nature. Moreover, 1 is a weak hydrogen-atom abstractor, as indicated by its inertness toward HSi(SiMe3)3 and cyclohexa-1,4-diene and the low calculated N-H bond dissociation energy (48 kcal/mol) of its corresponding iron(II) iso-hydrazenido species.

An orbitally adapted push-pull template for N2 activation and reduction to diazene-diide

A Lewis superacidic bis(borane) C6F4{B(C6F5)2}2 was reacted with tungsten N2-complexes [W(N2)2(R2PCH2CH2PR2)2] (R = Ph or Et), affording zwitterionic boryldiazenido W(ii) complexes trans-[W(L)(R2PCH2CH2PR2)2(N2{B(C6F5)2(C6F4B(C6F5)3})] (L = ø, N2 or THF). These compounds feature only one N-B linkage of the covalent type, as a result of intramolecular boron-to-boron C6F5 transfer. Complex trans-[W(THF)(Et2PCH2CH2PEt2)2(N2{B(C6F5)2C6F4B(C6F5)3})] (5) was shown to split H2, leading to a seven-coordinate complex [W(H)2(Et2PCH2CH2PEt2)2(N2{B(C6F5)2}2C6F4)] (7). Interestingly, hydride storage at the metal triggers backward C6F5 transfer. This reverts the bis(boron) moiety to its bis(borane) state, now doubly binding the distal N, with structural parameters and DFT computations pointing to dative N→B bonding. By comparison with an N2 complex [W(H)2(Et2PCH2CH2PEt2)2(N2{B(C6F5)3}] (10) differing only in the Lewis acid (LA), namely B(C6F5)3, coordinated to the distal N, we demonstrate that two-fold LA coordination imparts strong N2 activation up to the diazene-diide (N22-) state. To the best of our knowledge, this is the first example of a neutral LA coordination that induces reduction of N2.

Boron Insertion into the N≡N Bond of a Tungsten Dinitrogen Complex

The 1,3-addition of 1,2-diaryl-1,2-dibromodiboranes (B2Br2Ar2) to trans-[W(N2)2(dppe)2] (dppe = κ2-(Ph2PCH2)2), which is accompanied by a Br-Ar substituent exchange between the two boron atoms, is followed by a spontaneous rearrangement of the resulting tungsten diboranyldiazenido complex to a 2-aza-1,3-diboraallenylimido complex displaying a linear, cumulenic B=N=B moiety. This rearrangement involves the splitting of both the B-B and N=N bonds of the N2B2 ligand, formal insertion of a BAr boranediyl moiety into the N=N bond, and coordination of the remaining BArBr boryl moiety to the terminal nitrogen atom. Density functional theory calculations show that the reaction proceeds via a cyclic NB2 intermediate, followed by dissociation into a tungsten nitrido complex and a linear boryliminoborane, which recombine by adduct formation between the nitrido ligand and the electron-deficient iminoborane boron atom. The linear B=N=B moiety also undergoes facile 1,2-addition of Brønsted acids (HY = HOPh, HSPh, and H2NPh) with concomitant Y-Br substituent exchange at the terminal boron atom, yielding cationic (borylamino)borylimido tungsten complexes.

Spontaneous N2-diboranylation of [W(N2)2(dppe)2] with B2Br4(SMe2)2

The 1,3-bromoboration of [W(N₂)₂(dppe)₂] (dppe = 1,2-bis(diphenylphosphino)ethane) with B₂Br₄(SMe₂)₂ in the presence of various Lewis bases L yields diboranyldiazenido complexes, with L coordinating either at the terminal or internal boron atom. The 2 : 1 reaction of [W(N₂)₂(dppe)₂] and B₂Br₄(SMe₂)₂ yields a 1,2-bis(diazenido)diborane-bridged ditungsten complex with a fully planar π-conjugated BrWN₂B₂Br₂N₂WBr core.