Proton magnetic resonance studies of compounds with bridgehead nitrogen atoms—XXIV: The stereochemistry of octahydroimidazo [1,5-a] pyridine and of some N-acetyloctahydroimidazo [1,5-a] pyridines

Octahydroimidazo[1,5-a]pyridine is shown to preferentially adopt the trans-fused ring conformation in solution at room temperature. The NMR spectra of the rotational isomers of some N-acetyloctahydroimidazo[1,5-a]pyridines and N-acetyloctahydro-1H-pyrido[1,2-c]pyrimidine are described, and the stereochemistry about the N-acetyl bonds and the cis or trans nature of the ring fusion defined.     

Formation of a chiral NPPN ligand via metallation of acyclic NPNCN systems

A new type of bidentate N,N'-chelating ligand containing a chiral phosphorus centre has been synthesized via the metallation of an acyclic NPNCN species. The zwitterionic ligand backbone contains a phosphenium centre stabilised by an imido phosphine fragment.     

New tellurium-containing ring systems

The recent discovery of a suitable synthesis of the monoanionic ditelluroimidodiphosphinate ligands [TePR₂NPR₂Te]⁻ (R = Ph, ⁱPr, ^tBu) has facilitated investigations of the fundamental chemistry of these chelating inorganic ligands. This article is focused on aspects of that chemistry in which the behaviour of this ditelluro PNP ligand differs from that of the well-studied dithio and diseleno congeners. The emphasis is on new tellurium-containing ring systems formed in: (a) redox transformations and (b) the synthesis of metal complexes.     

Stable spirocyclic neutral radicals: aluminium and gallium boraamidinates

Stable dark red (M = Al) or dark green (M = Ga) neutral radicals {[PhB(mu-NtBu)2]2M} are obtained by the oxidation of their corresponding anions with iodine, and EPR spectra supported by DFT calculations show that the spin density is equally delocalized over all four nitrogen atoms in these spiroconjugated systems.     

Electronic structures and molecular properties of chalcogen nitrides Se2N2 and SeSN2

The electronic structures and molecular properties of S2N2 as well as the currently unknown chalcogen nitrides Se2N2 and SeSN2 have been studied using various ab initio and density functional methods. All molecules share a qualitatively similar electronic structure and can be primarily described as 2pi-electron aromatics having minor singlet diradical character of 6-8% that can be attributed solely to the nitrogen atoms. This diradical character is manifested in the prediction of their molecular properties, in which coupled cluster and multiconfigurational approaches, as well as density functional methods, show the best performance. The conventional ab initio methods RHF and MP2 completely fail to describe these systems. Predictions for the vibrational frequencies, IR intensities, Raman activities, and 14N, 15N, and 77Se chemical shifts, as well as singlet excitation energies of Se2N2 and SeSN2, have been made. The computed high-level spectroscopic data will be of considerable value in future efforts aimed at the preparation of the conducting polymers (SeN)x and (SeNSN)x.     

Syntheses and X-ray structures of potassium derivatives and a paramagnetic nickel(II) complex of a cyclodiphosph(III/V)azane monoselenide

The cyclodiphosph(III/V)azane monoselenide [(t)Bu(H)N(Se)P(micro-N(t)Bu)(2)PN(H)(t)Bu] (6, H(2)cdppSe where cdpp = cyclodiphosphazane) is obtained in quantitative yield from the comproportionation reaction of equimolar amounts of H(2)cdpp (7) and H(2)cdppSe(2) (3) in toluene at 85 degrees C. The X-ray structures of 3 and 6 reveal that both the monoselenide and the diselenide adopt a cis,endo,exo arrangement of the (t)BuNH groups. Metalation of 6 with 1 equiv of KO(t)Bu or 2 equiv of KCH(2)Ph in THF produces [(THF)K[HcdppSe]](2) (8) or [[(THF)(2)K](2)[cdppSe]](2) (9), respectively. The initial deprotonation involves the (t)BuNH group attached to the P(V) center in 6. In the dimeric structure of 8, both of the monoanionic ligands are bis-N,Se-chelated to the two K(+) cations to give a distorted K(2)N(2)Se(2) octahedron. In the centrosymmetric dimer 9, the dianionic ligands adopt two different coordination modes to the bis-solvated K(+) ions, viz., tridentate N,N',N" coordination and N,Se-chelation involving both exo- and endocyclic nitrogen atoms. The dimer is linked through K-Se interactions. The reaction of 2 equiv of 8 with NiCl(2)(PMe(3))(2) in THF produces [Ni(HcdppSe)(2)], which has a distorted tetrahedral structure and exhibits anomalous (1)H, (13)C, and (31)P NMR chemical shifts owing to the proximity of the paramagnetic Ni(II) center.     

Formation of N-I charge-transfer bonds and ion pairs in polyiodides with imidotellurium cations

[((t)BuNH)Te(mu-N(t)Bu)(2)Te(N(t))Bu)][OSO(2)CF(3)] (4a) is obtained in quantitative yields by the treatment of [((t)BuN)Te(mu-N(t)Bu)(2)Te(N(t)Bu)] (1) with HCF(3)SO(3). The reaction of 4a with LiI and iodine in the molar ratio 1:1:4.5 affords a product that, upon recrystallization from acetonitrile, was found to be a solid solution of [((t)BuNH)Te(mu-N(t)Bu)(2)Te(N(t)Bu)](2)I(20) (5a) and [((t)BuNH)Te(mu-N(t)Bu)(2)Te(NH(t)Bu)](2)I(18) (5b). Consequently, the crystal structure is disordered, containing 88.3(1)% of 5a.2MeCN and 11.7(1)% of 5b.2MeCN. The I(20) framework is involved in two symmetry-equivalent N-I-I-I-I fragments, two I(3)(-) ions, and three I(2) molecules that are linked together by I...I secondary bonding interactions. The bonding in the N-I-I-I-I fragment can be considered in terms of the lp(N) --> sigma*(I(2)) and pi(I(2)) --> sigma*(I(2)) charge-transfer interactions involving one [((t)BuNH)Te(mu-N(t)Bu)(2)Te(N(t)Bu)](+) cation and two I(2) units. The N-I bond length of 2.131(7) A, the I-I distances of 3.118(1), 3.095(2), and 2.788(2) A, and the angle I(2)-I(2) angle of 84.75(4) degrees are consistent with this bonding scheme. The I-I bond distances in the two symmetry-equivalent I(3)(-) ions are 3.113(1) and 2.792(2) A, and those in two crystallographically independent I(2) molecules are 2.736(2) and 2.743(1) A. The formal I(18)(4)(-) anion in 5b.2MeCN consists of four I(3)(-) anions and three I(2) molecules linked by I...I secondary bonds. One crystallographically independent I(3)(-) anion is connected to the [((t)BuNH)Te(mu-N(t)Bu)(2)Te(HN(t)Bu)](2+) cation by two hydrogen bonds [H...I = 2.823(5) and 2.983(5) A; N...I = 3.697(8) and 3.857(9) A]. The I(3)(-) anions and I(2) molecules in 5b show virtually identical bond parameters to those in 5a. The treatment of 1 with iodine and the reactions of its methylated derivatives, [((t)BuNMe)Te(mu-N(t)Bu)(2)Te(N(t)()Bu)][OSO(2)CF(3)] and [((t)BuNMe)Te(mu-N(t)Bu)(2)Te(MeN(t)Bu)][OSO(2)CF(3)](2), with LiI and iodine also afford highly moisture-sensitive polyiodides, either by the formation of N-I charge-transfer complexes or by ionic interactions. The crystal structures of the partially hydrolyzed products, [((t)BuIN)Te(mu-N(t))Bu)(2)Te(mu-O)](2)(I(3))(2) (3), [((t)BuMeN)Te(mu-N(t)Bu)(2)Te(mu-O)](2)(I(3))(2) (6), and 6.2MeCN, are also reported.     

Synthetic and structural investigations of monomeric dilithium boraamidinates and bidentate NBNCN ligands with bulky N-bonded groups

The dilithiated boraamidinate complexes [Li(2)[PhB(NDipp)(2)](THF)(3)] (7a) (Dipp = 2,6-diisopropylphenyl) and [Li(2)[PhB(NDipp)(N(t)Bu)](OEt(2))(2)] (7b), prepared by reaction of PhB[N(H)Dipp][N(H)R'] (6a, R' = Dipp; 6b, R' = (t)Bu) with 2 equiv of (n)BuLi, are shown by X-ray crystallography to have monomeric structures with two terminal and one bridging THF ligands (7a) or two terminal OEt(2) ligands (7b). The derivative 7a is used to prepare the spirocyclic group 13 derivative [Li(OEt(2))(4)][In[PhB(NDipp)(2)](2)] (8a) that is shown by an X-ray structural analysis to be a solvent-separated ion pair. The monoamino derivative PhBCl[N(H)Dipp] (9a), obtained by the reaction of PhBCl(2) with 2 equiv of DippNH(2), serves as a precursor for the synthesis of the four-membered BNCN ring [[R'N(H)](Ph)B(mu-N(t)Bu)(2)C(n)Bu] (10a, R' = Dipp). The X-ray structures of 6a, 9a, and 10a have been determined. The related derivative 10b (R' = (t)Bu) was synthesized by the reaction of [Cl(Ph)B(mu-N(t)Bu)(2)C(n)Bu] with Li[N(H)(t)Bu] and characterized by (1)H, (11)B, and (13)C NMR spectra. In contrast to 10a and 10b, NMR spectroscopic data indicate that the derivatives [[DippN(H)](Ph)B(NR')(2)CR(NR')] (11a: R =( t)Bu, R' = Cy; 11b: R = (n)Bu, R' = Dipp) adopt acyclic structures with three-coordinate boron atoms. Monolithiation of 10a produces the novel hybrid boraamidinate/amidinate (bamam) ligand [Li[DippN]PhB(N(t)Bu)C(n)Bu(N(t)Bu)] (12a).     

Experimental and Theoretical Investigations of the Formation of the Diazene PhSN=C(H)N=NC(H)=NSPh from HCN(2)(SPh)(3) by a Thiyl-Radical-Catalyzed Mechanism: Identification of the HC(NSPh)(2)(*) Radical and X-ray Structures of HCN(2)(SPh)(3) and PhSN=C(H)N=NC(H)=NSPh

The reaction of HCN(2)(SiMe(3))(3) with benzenesulfenyl chloride in a 1:3 molar ratio produces HCN(2)(SPh)(3) (4) as thermally unstable, colorless crystals. The decomposition of (4) in toluene at 95 degrees C was monitored by UV-visible, (1)H NMR and ESR spectroscopy. The major final products of the decomposition were identified as PhSN=C(H)N=NC(H)=NSPh (5) and PhSSPh. The structures of 4 and 5 were determined by X-ray crystallography. The crystals of 4 are monoclinic, space group P2(1)/a, with a = 9.874(2) ?, b = 19.133(2) ?, c = 10.280(2) ?, beta = 113.37(1) degrees, V = 1782.8(5) ?(3), and Z = 4. The final R and R(w) values were 0.042 and 0.049, respectively. The crystals of 5 are monoclinic, space group P2(1)/n, with a = 5.897(6) ?, b = 18.458(10) ?, c = 7.050(8) ?, beta = 110.97(5) degrees, V = 716(1) ?(3), and Z = 2. The final R and R(w) values were 0.075 and 0.085, respectively. The diazene 5 adopts a Z,E,Z structure with weak intramolecular S.N contacts of 2.83 ?, giving rise to four-membered NCNS rings. During the thermolysis of 4 at 95 degrees C in toluene a transient species (lambda(max) 820 nm) was detected. It decomposes with second-order kinetics to give 5 (lambda(max) 450 nm). The ESR spectrum of the reaction mixture consisted of the superposition of a three-line 1:1:1 spectrum (g = 2.0074, A(N) = 11.45 G), attributed to (PhS)(2)N(*), upon a doublet of quintets (1:2:3:2:1) with g = 2.0070, A(N) = 6.14 G, A(H) = 2.1 G assigned to the radical HCN(2)(SPh)(2)(*). Density functional theory (DFT) calculations for the models of the radical showed the E,Z isomer to have the lowest energy. Thermochemical calculations indicate that the decomposition of HCN(2)(SH)(3) into the diazene (Z,E,Z)-HSN=C(H)N=NC(H)=NSH (and 2 HSSH) is substantially more exothermic (DeltaH = -176.1 kJ mol(-)(1)) than the corresponding formation of the isomeric eight-membered ring (HC)(2)N(4)(SH)(2) (DeltaH = -40.6 kJ mol(-)(1)). These calculations also indicate that the diazene is formed by a mechanism in which the RS(*) radical acts as a catalyst.