Ligand Dynamics in the Solid: Lithium‐fluorenide and Lithium‐benzo[b]fluorenide N,N,N′,N′‐Tetramethylethylenediamine (tmeda) Complexes

The dynamic behavior of the N,N,N′,N′‐tetramethylethylenediamine (tmeda) ligand has been studied in solid lithium‐fluorenide(tmeda) ( 3 ) and lithium‐benzo[b]fluorenide(tmeda) ( 4 ) using CP/MAS solid‐state ¹³C‐ and ¹⁵N‐NMR spectroscopy. It is shown that, in the ground state, the tmeda ligand is oriented parallel to the long molecular axis of the fluorenide and benzo[b]fluorenide systems. At low temperature (<250 K), the ¹³C‐NMR spectrum exhibits two MeN signals. A dynamic process, assigned to a 180° rotation of the five‐membered metallacycle (π‐flip), leads at elevated temperatures to coalescence of these signals. Line‐shape calculations yield ΔH^?=42.7 kJ mol^?1, ΔS^?=?5.3 J mol^?1 K^?1, and =44.3 kJ mol^?1 for 3 , and ΔH^?=36.8 kJ mol^?1, ΔS^?=?17.7 J mol^?1 K^?1, and =42.1 kJ mol^?1 for 4 , respectively. A second dynamic process, assigned to ring inversion of the tmeda ligand, was detected from the temperature dependence of T_1ρ, the ¹³C spin‐lattice relaxation time in the rotating frame, and led to ΔH^?=24.8 kJ mol^?1, ΔS^?=?49.2 J mol^?1 K^?1, and =39.5 kJ mol^?1 for 3 , and ΔH^?=18.2 kJ mol^?1, ΔS^?=?65.3 J mol^?1 K^?1, and =37.7 kJ mol^?1 for 4 , respectively. For (D₁₂)‐ 3 , the rotation of the CD₃ groups has also been studied, and a barrier E_a of 14.1 kJ mol^?1 was found.     

Ferromagnetic Interactions in a 1D Alternating Linear Chain of π‐Stacked 1,3‐Diphenyl‐7‐(thien‐2‐yl)‐1,4‐dihydro‐1,2,4‐benzotriazin‐4‐yl Radicals

X‐ray studies show that 1,3‐diphenyl‐7‐(thien‐2‐yl)‐1,4‐dihydro‐1,2,4‐benzotriazin‐4‐yl ( 6 ) adopts a distorted, slipped π‐stacked structure of centrosymmetric dimers with alternate short and long interplanar distances (3.48 and 3.52 Å). Cyclic voltammograms of 7‐(thien‐2‐yl)benzotriazin‐4‐yl 6 show two fully reversible waves that correspond to the ?1/0 and 0/+1 processes. EPR and DFT studies on radical 6 indicate that the spin density is mainly delocalized over the triazinyl fragment. Magnetic susceptibility measurements show that radical 6 obeys Curie–Weiss behavior in the 5–300 K region with C=0.378 emu K mol^?1 and θ=+4.72 K, which is consistent with ferromagnetic interactions between S=1/2 radicals. Fitting the magnetic susceptibility revealed the behavior is consistent with an alternating ferromagnetic chain (g=2.0071, J₁=+7.12 cm^?1, J₂=+1.28 cm^?1).     

Thermal Behavior of N,N＇Bis[N-（2,2,2-trinitroethyl）-N-nitro]ethylenediamine

The thermal behavior and kinetic parameters of the exothermic decomposition reaction of N‐N‐bis[N‐(2,2,2‐tri‐nitroethyl)‐N‐nitro]ethylenediamine in a temperature‐programmed mode have been investigated by means of differential scanning calorimetry (DSC). The results show that kinetic model function in differential form, apparent activation energy E_a and pre‐exponential factor A of this reaction are 3(1 ‐α)^2/3, 203.67 kJ·mol^?1 and 10^20.61s^?1, respectively. The critical temperature of thermal explosion of the compound is 182.2 °C. The values of ΔS^≠ ΔH^≠ and ΔG^≠ of this reaction are 143.3 J·mol^?1·K^?1, 199.5 kJ·mol^?1 and 135.5 kJ·mol^?1, respectively.     

1，1－二氨基2，2－二硝基乙烯（DADE）的热化学性质、热行为和热分解机理

The constant-volume combustion energy, △cU （DADE, s, 298.15 K）, the thermal behavior, and kinetics and mechanism of the exothermic decomposition reaction of 1,1-diamino-2,2-dinitroethylene （DADE） have been investigated by a precise rotating bomb calorimeter, TG-DTG, DSC, rapid-scan fourier transform infrared （RSFT-IR） spectroscopy and T-jump/FTIR, respectively. The value of △cHm （DADE, s, 298.15 K） was determined as （-8518.09±4.59） j·g^-1. Its standard enthalpy of combustion, △cU （DADE, s, 298.15 K）, and standard enthalpy of formation, △fHm （DADE, s, 298.15 K） were calculated to be （-1254.00±0.68） and （- 103.98±0.73） kJ·mol^-1, respectively The kinetic parameters （the apparent activation energy Ea and pre-exponential factor A） of the first exothermic decomposition reaction in a temperature-programmed mode obtained by Kissinger＇s method and Ozawa＇s method, were Ek=344.35 kJ·mol^-1, AR= 1034.50 S^-1 and Eo=335.32 kJ·mol^-1, respectively. The critical temperatures of thermal explosion of DADE were 206.98 and 207.08 ℃ by different methods. Information was obtained on its thermolysis detected by RSFT-IR and T-jump/FTIR.     

Breaking the Semi‐Quinoid Structure: Spin‐Switching from Strongly Coupled Singlet to Polarized Triplet State

2,7‐TMPNO (4,5,9,10‐tetramethoxypyrene‐2,7‐bis(tert‐butylnitroxide)) was found to exist in semi‐quinoid form with unprecedented strong intramolecular magnetic exchange interaction of 2 J/k_B=1185 K operating over a distance of 10 Å. Structural transformations with the activation energy of ΔE_eq=949 K were observed by varying the temperature, from more quinoid structure at low temperature to more biradicaloid structure at higher temperature. Moreover, this molecule undergoes a transient spin transition from singlet to polarized triplet state upon photoexcitation revealed by TREPR spectroscopy. The spin Hamiltonian parameters were determined to be S=1, g=2.0065, D=?0.0112 cm^?1, and E=?0.0014 cm^?1 by spectral simulation with the hybrid Eigenfield/exact diagonalization method.     

Encapsulation‐Induced Remarkable Stability of a Hydrogen‐Bonded Heterocapsule

Remarkably enhanced stability of the self‐assembled hydrogen‐bonded heterocapsule 1?2 by the encapsulation of 1,4‐bis(1‐propynyl)benzene 3 a was found with K_a=1.14×10⁹ M ^?1 in CDCl₃ and K_a2=1.59×10⁸ M ^?2 in CD₃OD/CDCl₃ (10 % v/v) at 298 K. The formation of 3 a @( 1?2 ) was enthalpically driven (ΔH°<0 and ΔS°<0) and there was a unique inflection point in the correlation between ΔH° versus ΔS° as a function of polar solvent content. The ab initio calculations revealed that favorable guest–capsule dispersion and electrostatic interactions between the acetylenic parts (triple bonds) of 3 a and the aromatic inner space of 1?2 , as well as less structural deformation of 1?2 upon encapsulation of 3 a , play important roles in the remarkable stability of 3 a @( 1?2 ).     

Mechanistic Aspects of Ligand Substitution on the Hydroxopentaaquarhodium(III) Ion in Aqueous Solution by Sulfur‐Containing Bioactive Ligands

The kinetics of the interactions between three sulfur‐containing ligands, thioglycolic acid, 2‐thiouracil, glutathione, and the title complex, have been studied spectrophotometrically in aqueous medium as a function of the concentrations of the ligands, temperature, and pH at constant ionic strength. The reactions follow a two‐step process in which the first step is ligand‐dependent and the second step is ligand‐independent chelation. Rate constants (k₁ ～10^?3 s^?1 and k₂ ～10^?5 s^?1) and activation parameters (for thioglycolic acid: ΔH₁^≠ = 22.4 ± 3.0 kJ mol^?1, ΔS₁^≠ = ?220 ± 11 J K^?1 mol^?1, ΔH₂^≠ = 38.5 ± 1.3 kJ mol^?1, ΔS₂^≠ = ?204 ± 4 J K^?1 mol^?1; for 2‐thiouracil: ΔH₁^≠ = 42.2 ± 2.0 kJ mol^?1, ΔS₁^≠ = ?169 ± 6 J K^?1 mol^?1, ΔH₂^≠ = 66.1 ± 0.5 kJ mol^?1, ΔS₂^≠ = ?124 ± 2 J K^?1 mol^?1; for glutathione: ΔH₁^≠ = 47.2 ± 1.7 kJ mol^?1, ΔS₁^≠ = ?155 ± 5 J K^?1mol^?1, ΔH₂^≠ = 73.5 ± 1.1 kJ mol^?1, ΔS₂^≠ = ?105 ± 3 J K^?1 mol^?1) were calculated. Based on the kinetic and activation parameters, an associative interchange mechanism is proposed for the interaction processes. The products of the reactions have been characterized from IR and ESI mass spectroscopic analysis. A rate law involving the outer sphere association complex formation has been established as      

Dramatic Influence of an Anionic Donor on the Oxygen‐Atom Transfer Reactivity of a MnV–Oxo Complex

Addition of an anionic donor to an Mn^V(O) porphyrinoid complex causes a dramatic increase in 2‐electron oxygen‐atom‐transfer (OAT) chemistry. The 6‐coordinate [Mn^V(O)(TBP₈Cz)(CN)]^? was generated from addition of Bu₄N⁺CN^? to the 5‐coordinate Mn^V(O) precursor. The cyanide‐ligated complex was characterized for the first time by Mn K‐edge X‐ray absorption spectroscopy (XAS) and gives Mn?O=1.53 Å, Mn?CN=2.21 Å. In combination with computational studies these distances were shown to correlate with a singlet ground state. Reaction of the CN^? complex with thioethers results in OAT to give the corresponding sulfoxide and a 2e^?‐reduced Mn^III(CN)^? complex. Kinetic measurements reveal a dramatic rate enhancement for OAT of approximately 24 000‐fold versus the same reaction for the parent 5‐coordinate complex. An Eyring analysis gives ΔH^≠=14 kcal mol^?1, ΔS^≠=?10 cal mol^?1 K^?1. Computational studies fully support the structures, spin states, and relative reactivity of the 5‐ and 6‐coordinate Mn^V(O) complexes.     

EPR Studies on Branched High‐Spin Arylnitrenes

The UV (λ>305 nm) photolysis of triazide 3 in 2‐methyl‐tetrahydrofuran glass at 7 K selectively produces triplet mononitrene 4 (g=2.003, D_T=0.92 cm^?1, E_T=0 cm^?1), quintet dinitrene 6 (g=2.003, D_Q=0.204 cm^?1, E_Q=0.035 cm^?1), and septet trinitrene 8 (g=2.003, D_S=?0.0904 cm^?1, E_S=?0.0102 cm^?1). After 45 min of irradiation, the major products are dinitrene 6 and trinitrene 8 in a ratio of ～1:2, respectively. These nitrenes are formed as mixtures of rotational isomers each of which has slightly different magnetic parameters D and E. The best agreement between the line‐shape spectral simulations and the experimental electron paramagnetic resonance (EPR) spectrum is obtained with the line‐broadening parameters Γ(E_Q)=180 MHz for dinitrene 6 and Γ(E_S)=330 MHz for trinitrene 8 . According to these line‐broadening parameters, the variations of the angles Θ in rotational isomers of 6 and 8 are expected to be about ±1 and ±3°, respectively. Theoretical estimations of the magnetic parameters obtained from PBE/DZ(COSMO)//UB3LYP/6‐311+G(d,p) calculations overestimate the E and D values by 1 and 8 %, respectively. Despite the large distances between the nitrene units and the extended π systems, the zero field splitting (zfs) parameters D are found to be close to those in quintet dinitrenes and septet trinitrenes, where the nitrene centers are attached to the same aryl ring. The large D values of branched septet nitrenes are due to strong negative one‐center spin–spin interactions in combination with weak positive two‐center spin–spin interactions, as predicted by theoretical considerations.     

Thermodynamics of Hydrogen Adsorption on Metal‐Organic Frameworks

Interaction between adsorbed hydrogen and the coordinatively unsaturated Mg²⁺ and Co²⁺ cationic centres in Mg‐MOF‐74 and Co‐MOF‐74, respectively, was studied by means of variable‐temperature infrared (VTIR) spectroscopy. Perturbation of the H₂ molecule by the cationic adsorbing centre renders the H? H stretching mode IR‐active at 4088 and 4043 cm^?1 for Mg‐MOF‐74 and Co‐MOF‐74, respectively. Simultaneous measurement of integrated IR absorbance and hydrogen equilibrium pressure for spectra taken over the temperature range of 79–95 K allowed standard adsorption enthalpy and entropy to be determined. Mg‐MOF‐74 showed ΔH⁰=?9.4 kJ mol^?1 and ΔS⁰=?120 J mol^?1 K^?1, whereas for Co‐MOF‐74 the corresponding values of ΔH⁰=?11.2 kJ mol^?1 and ΔS⁰=?130 J mol^?1 K^?1 were obtained. The observed positive correlation between standard adsorption enthalpy and entropy is discussed in the broader context of corresponding data for hydrogen adsorption on cation‐exchanged zeolites, with a focus on the resulting implications for hydrogen storage and delivering.     

Wolves in Sheep's Clothing: μ‐Oxo‐Diiron Corroles Revisited

For well over 20 years, μ‐oxo‐diiron corroles, first reported by Vogel and co‐workers in the form of μ‐oxo‐bis[(octaethylcorrolato)iron] (Mössbauer δ 0.02 mm s^?1, ΔE_Q 2.35 mm s^?1), have been thought of as comprising a pair antiferromagnetically coupled low‐spin Fe^IV centers. The remarkable stability of these complexes, which can be handled at room temperature and crystallographically analyzed, present a sharp contrast to the fleeting nature of enzymatic, iron(IV)‐oxo intermediates. An array of experimental and theoretical methods have now shown that the iron centers in these complexes are not Fe^IV but intermediate‐spin Fe^III coupled to a corrole^.2?. The intramolecular spin couplings in {Fe[TPC]}₂(μ‐O) were analyzed via DFT(B3LYP) calculations in terms of the Heisenberg–Dirac–van Vleck spin Hamiltonian H=J_Fe–corrole(S_Fe?S_corrole)+J_Fe–Fe′(S_Fe?S_Fe′)+J_{Fe′–corrole}(S_Fe′?S_corrole′), which yielded J_Fe–corrole=J_{Fe′–corrole′}=0.355 eV (2860 cm^?1) and J_Fe–Fe′=0.068 eV (548 cm^?1). The unexpected stability of μ‐oxo‐diiron corroles thus appears to be attributable to charge delocalization via ligand noninnocence.     

Thermodynamics of Carbon Dioxide Adsorption on the Protonic Zeolite H‐ZSM‐5

Adsorption of carbon dioxide on H‐ZSM‐5 zeolite (Si:Al=11.5:1) was studied by means of variable‐temperature FT‐IR spectroscopy, in the temperature range of 310–365 K. The adsorbed CO₂ molecules interact with the zeolite Brønsted‐acid OH groups bringing about a characteristic red‐shift of the O? H stretching band from 3610 cm^?1 to 3480 cm^?1. Simultaneously, the ν₃ mode of adsorbed CO₂ is observed at 2345 cm^?1. From the variation of integrated intensity of the IR absorption bands at both 3610 and 2345 cm^?1, upon changing temperature (and CO₂ equilibrium pressure), the standard adsorption enthalpy of CO₂ on H‐ZSM‐5 is ΔH⁰=?31.2(±1) kJ mol^?1 and the corresponding entropy change is ΔS⁰=?140(±10) J mol^?1 K^?1. These results are discussed in the context of available data for carbon dioxide adsorption on other protonic, and also alkali‐metal exchanged, zeolites.     

Kinetic Studies on the Reaction of Sodium Hexacyanoferrate（ll） Complex with Peroxydisulfate Anion

The oxidation of Na₄Fe(CN)₆ complex by S₂O anion was found to follow an outer‐sphere electron transfer mechanism. We firstly carried out the reaction at pH=1. The specific rate constants of the reaction, k_ox, are (8.1±0.07)×10^?2 and (4.3±0.1)×10^?2 mol^?1·L·s^?1 at μ=1.0 mol·L^?1 NaClO₄, T=298 K for pH=1 (0.1 mol·L^?1 HCl0₄) and 8, respectively. The activation parameters, obtained by measuring the rate constants of oxidation 283–303 K, were ΔH^≠=(69.0±5.6) kJ·mol^?1, ΔS^≠=(?0.34±0.041)×10² J·mol^?1·K^?1 at pH=l and ΔH^≠=(41.3±5.5) kJ·mol^?1, ΔS^≠=(?1.27±0.33)×10² J·mol^?1·K^?1 at pH=8, respectively. The cyclic voltammetry of Fe(CN) shows that the oxidation is a one‐electron reversible redox process with E_1/2 values of 0.55 and 0.46 V vs. normal hydrogen electrode at μ=1.0 mol·L^?1 LiClO₄, for pH=1 and pH=8 (Tris). respectively. The kinetic results were discussed on the basis of Marcus theory.     

4-硝基苯甲醇的低温热容和标准摩尔生成焓

用精密自动绝热量热计测定了4-硝基苯甲醇（4-NBA）在78 ~ 396 K温区的摩尔热容。其熔化温度、摩尔熔化焓及摩尔熔化熵分别为:(336.426 ± 0.088) K, (20.97 ± 0.13) kJ×mol-1 和 (57.24 ± 0.36) J×K-1×mol-1.根据热力学函数关系式,从热容值计算出了该物质在80 ~ 400 K温区的热力学函数值 [HT - H298.15 K] 和[ST - S298.15 K]. 用精密氧弹燃烧量热计测定了该物质在T＝298.15 K的恒容燃烧能和标准摩尔燃烧焓分别为 (C7H7NO3, s)＝- ( 3549.11 ± 1.47 ) kJ×mol-1 和 (C7H7NO3, s)＝- ( 3548.49 ± 1.47 ) kJ×mol-1. 利用标准摩尔燃烧焓和其他辅助热力学数据通过盖斯热化学循环, 计算出了该物质标准摩尔生成焓 (C7H7NO3, s)＝- (206.49 ± 2.52) kJ×mol-1 .     

Evidence for a Rapid Degenerate Hetero‐Cope‐Type Rearrangement in [Cp*W(S)2S‐CH2‐CHCH2]

Treatment of the salt [PPh₄]⁺[Cp*W(S)₃]^? ( 6 ) with allyl bromide gave the neutral complex [Cp*W(S)₂S‐CH₂‐CH?CH₂] ( 7 ). The product 7 was characterized by an X‐ray crystal structure analysis. Complex 7 features dynamic NMR spectra that indicate a rapid allyl automerization process. From the analysis of the temperature‐dependent NMR spectra a Gibbs activation energy of ΔG^≠ (278 K)≈13.7±0.1 kcal mol^?1 was obtained [ΔH^≠≈10.4±0.1 kcal mol^?1; ΔS^≠≈?11.4 cal mol^?1 K^?1]. The DFT calculation identified an energetically unfavorable four‐membered transition state of the “forbidden” reaction and a favorable six‐membered transition state of the “Cope‐type” allyl rearrangement process at this transition‐metal complex core.     

The Unexpected Mechanism Underlying the High‐Valent Mono‐Oxo‐Rhenium(V) Hydride Catalyzed Hydrosilylation of CN Functionalities: Insights from a DFT Study

In this study, we theoretically investigated the mechanism underlying the high‐valent mono‐oxo‐rhenium(V) hydride Re(O)HCl₂(PPh₃)₂ ( 1 ) catalyzed hydrosilylation of C?N functionalities. Our results suggest that an ionic S_N2‐Si outer‐sphere pathway involving the heterolytic cleavage of the Si?H bond competes with the hydride pathway involving the C?N bond inserted into the Re?H bond for the rhenium hydride ( 1 ) catalyzed hydrosilylation of the less steric C?N functionalities (phenylmethanimine, PhCH=NH, and N‐phenylbenzylideneimine, PhCH=NPh). The rate‐determining free‐energy barriers for the ionic outer‐sphere pathway are calculated to be ～28.1 and 27.6 kcal mol^?1, respectively. These values are slightly more favorable than those obtained for the hydride pathway (by ～1–3 kcal mol^?1), whereas for the large steric C?N functionality of N,1,1‐tri(phenyl)methanimine (PhCPh=NPh), the ionic outer‐sphere pathway (33.1 kcal mol^?1) is more favorable than the hydride pathway by as much as 11.5 kcal mol^?1. Along the ionic outer‐sphere pathway, neither the multiply bonded oxo ligand nor the inherent hydride moiety participate in the activation of the Si?H bond.     

Diagnostic cyclisation reactions which follow phosphate transfer to carboxylate anion centres for energised [M-H]- anions of pTyr-containing peptides

The low‐energy negative ion phosphoTyr to C‐terminal ‐CO₂PO₃H₂ rearrangement occurs for energised peptide [M–H]^– anions even when there are seven amino acid residues between the pTyr and C‐terminal amino acid residues. The rearranged C‐terminal ‐CO₂PO₂H(O^–) group effects characteristic S_Ni cyclisation/cleavage reactions. The most pronounced of these involves the electrophilic central backbone carbon of the penultimate amino acid residue. This reaction is aided by the intermediacy of an H‐bonded intermediate in which the nucleophilic and electrophilic reaction centres are held in proximity in order for the S_Ni cyclisation/cleavage to proceed. The ΔG_reaction is +184 kJ mol^?1 with the barrier to the S_Ni transition state being +240 kJ mol^?1 at the HF/6‐31 + G(d)//AM1 level of theory. A similar phosphate rearrangement from pTyr to side chain CO₂^– (of Asp or Glu) may also occur for energised peptide [M–H]^– anions. The reaction is favourable: ΔG_reaction is ?44 kJ mol^?1 with a maximum barrier of +21 kJ mol^?1 (to the initial transition state) when Asp and Tyr are adjacent. The rearranged species R¹‐Tyr‐NHCH(CH₂CO₂PO₃H^–)COR² (R¹ = CHO; R² = OCH₃) may undergo an S_Ni six‐centred cyclisation/cleavage reaction to form the product anion R¹‐Tyr(NH^–). This process has a high energy requirement [ΔG_reaction = +224 kJ mol^?1, with the barrier to the S_Ni transition state being +299 kJ mol^?1]. Copyright © 2011 John Wiley & Sons, Ltd.     

A Molecular Copper Catalyst for Electrochemical Water Reduction with a Large Hydrogen‐Generation Rate Constant in Aqueous Solution

The copper complex [(bztpen)Cu](BF₄)₂ (bztpen=N‐benzyl‐N,N′,N′‐tris(pyridin‐2‐ylmethyl)ethylenediamine) displays high catalytic activity for electrochemical proton reduction in acidic aqueous solutions, with a calculated hydrogen‐generation rate constant (k_obs) of over 10000 s^?1. A turnover frequency (TOF) of 7000 h^?1 cm^?2 and a Faradaic efficiency of 96 % were obtained from a controlled potential electrolysis (CPE) experiment with [(bztpen)Cu]²⁺ in pH 2.5 buffer solution at ?0.90 V versus the standard hydrogen electrode (SHE) over two hours using a glassy carbon electrode. A mechanism involving two proton‐coupled reduction steps was proposed for the dihydrogen generation reaction catalyzed by [(bztpen)Cu]²⁺.     

Ambident PCN Heterocycles: N‐ and P‐Phosphanylation of Lithium 1,3‐Benzazaphospholides

Synthetic and structural aspects of the phosphanylation of 1,3‐benzazaphospholides 1_Li , ambident benzofused azaphosphacyclopentadienides, are presented. The unusual properties of phospholyl‐1,3,2‐diazaphospholes inspired us to study the coupling of 1_Li with chlorodiazaphospholene 2 , which led to the N‐substituted product 3 . Reaction of 1_Li with chlorodiphenyl‐ and chlorodicyclohexylphosphane likewise gave N‐phosphanylbenzazaphospholes 4 and 5 , whereas with the more bulky di‐tert‐butyl‐ and di‐1‐adamantylchlorophosphanes, the diphosphanes 6 and 7 are obtained; in the case of 7 they are isolated as a dimeric LiCl(THF) adduct. Structural information was provided by single‐crystal X‐ray diffraction and solution NMR spectroscopy experiments. 2D exchange spectroscopy confirmed the existence of two rotamers of the aminophosphane 5 at room temperature; variable‐temperature NMR spectroscopy studies of 6 revealed two dynamic processes, low‐temperature inversion at ring phosphorus (ΔH^≠=22 kJ mol^?1, ΔS^≠=2 J K^?1 mol^?1) and very low‐temperature rotation of the tBu₂P group. Quantum chemical studies give evidence that 2‐unsubstituted benzazaphospholides prefer N‐phosphanylation, even with bulky chlorophosphanes, and that substituents at the 2‐position of the heterocycle are crucial for the occurrence of P–N rotamers and for switching to alternative P‐substitution, beyond a threshold steric bulk, by both P‐ and 2‐position substituents.