Novel Triplet Ground State Silylenes: H–N = C = Si, CN–N = C = Si, and MeO–N = C = Si at DFT Levels

Summary.  DFT calculations predict the existence of three new triplet ground state silylenes: [(imino)-methylene]silylene, [(cyanoimino)methylene]silylene, and [(methoxyimino)methylene]silylene, with CNSiX formula (X = H, CN, and OMe, respectively). Discrepancies are found between DFT and some ab initio results.     

A quest for triplet silylenes XHSi3 at ab initio and DFT levels (X = H, F, Cl and Br)

Four ground state triplet silylenes are found among 30 possible silylenic XHSi₃ structures (X = H, F, Cl and Br), at seven ab initio and DFT levels including: B3LYP/6-311++G^∗∗, HF/6-311++G^∗∗, MP3/6-311G^∗, MP2/6-311+G^∗∗, MP4(SDTQ)/6-311++G^∗∗, QCISD(T)/6-311++G^∗∗ and CCSD(T)/6-311++G^∗∗. The latter six methods indicate that the triplet states of 3-flouro-1,2,3-trisilapropadienylidene, 1-chloro-1,2,3-trisilapropargylene and 3-chloro-1,2,3-trisilapropargylene are energy minima. These triplets appear more stable than their corresponding singlet states which cannot even exist for showing negative force constants. Also, triplet state of 1-flouro-1,2,3-trisilapropargylene is possibly accessible for being an energy minimum, since its corresponding singlet state is not a real isomer. Some discrepancies are observed between energetic and/or structural results of DFT vs. ab initio data.     

Magnetostructural studies on ferromagnetically coupled copper(II) cubanes of Schiff-base ligands

Three cubane copper(II) clusters, namely [Cu(4)(HL')4] (1), [Cu4L2(OH)2] (2), and [Cu4L2(OMe)2] (3), of two pentadentate Schiff-base ligands N,N'-(2-hydroxypropane-1,3-diyl)bis(acetylacetoneimine) (H3L') and N,N'-(2-hydroxypropane-1,3-diyl)bis(salicylaldimine) (H3L), are prepared, structurally characterized by X-ray crystallography, and their variable-temperature magnetic properties studied. Complex 1 has a metal-to-ligand stoichiometry of 1:1 and it crystallizes in the cubic space group P43n with a structure that consists of a tetranuclear core with metal centers linked by a mu(3)-alkoxo oxygen atom to form a cubic arrangement of the metal and oxygen atoms. Each ligand displays a tridentate binding mode which means that a total of eight pendant binding sites remain per cubane molecule. Complexes [Cu4L2(OH)2] (2) and [Cu4L2(OMe)2] (3) crystallize in the orthorhombic space group Pccn and have a cubane structure that is formed by the self-assembly of two {Cu2L}+ units. The variable-temperature magnetic susceptibility data in the range 300-18 K show ferromagnetic exchange interactions in the complexes. Along with the ferromagnetic exchange pathway, there is also a weak antiferromagnetic exchange between the copper centers. The theoretical fitting of the magnetic data gives the following parameters: J1 = 38.5 and J2 = -18 cm(-1) for 1 with a triplet (S = 1) ground state and quintet (S = 2) lowest excited state; J1 = 14.7 and J2 = -18.4 cm(-1) for 2 with a triplet ground state and singlet (S = 0) lowest excited state; and J1 = 33.3 and J2 = -15.6 cm(-1) for 3 with a triplet ground state and quintet lowest excited state, where J1 and J2 are two different exchange pathways in the cubane {Cu4O4} core. The crystal structures of 2 * 6 H2O and 3 * 2 H2O * THF show the presence of channels containing the lattice solvent molecules.     

Hydrogen bridging in the compounds X2H (X=Al,Si,P,S)

X2H hydrides (X=Al, Si, P, and S) have been investigated using coupled cluster theory with single, double, and triple excitations, the latter incorporated as a perturbative correction [CCSD(T)]. These were performed utilizing a series of correlation-consistent basis sets augmented with diffuse functions (aug-cc-pVXZ, X=D, T, and Q). Al2H and Si2H are determined to have H-bridged C2v structures in their ground states: the Al2H ground state is of 2B1 symmetry with an Al-H-Al angle of 87.6 degrees, and the Si2H ground state is of 2A1 symmetry with a Si-H-Si angle of 79.8 degrees. However, P2H and S2H have nonbridged, bent Cs structures: the P2H ground state is of 2A' symmetry with a P-P-H angle of 97.0 degrees, and the S2H ground state is of 2A' symmetry with a S-S-H angle of 93.2 degrees. Ground state geometries, vibrational frequencies, and electron affinities have been computed at all levels of theory. Our CCSD(T)/aug-cc-pVQZ adiabatic electron affinity of 2.34 eV for the Si2H radical is in excellent agreement with the photoelectron spectroscopy experiments of Xu et al. [J. Chem. Phys. 108, 7645 (1998)], where the electron affinity was determined to be 2.31+/-0.01 eV.     

Triplet state dipole moments of aminobenzonitriles

The triplet state dipole moments mu(T) of a series of 4-amino- and 3-aminobenzonitriles in cyclohexane, benzene, and 1,4-dioxane are recalculated from previously published [J. Phys. Chem. 1992, 96, 10809] time-resolved microwave conductivity data, on the basis of newly measured intersystem crossing yields. For 4-(dimethylamino)benzonitrile (DMABN), the following values are now determined for mu(T): 8.3 D (cyclohexane), 8.9 D (benzene), and 9.7 D (1,4-dioxane), as compared with the previously reported dipole moment of 12 D for the first and the last solvent. With the other aminobenzonitriles, similar mu(T) data are obtained, between 6.9 D for 4-aminobenzonitrile (ABN) in n-hexane and 10.0 D for 4-(di-n-decylamino)benzonitrile (DDABN) in 1,4-dioxane. The increase of mu(T) observed for all aminobenzonitriles when going from cyclohexane via benzene to 1,4-dioxane may indicate that their triplet dipole moments become larger with increasing solvent polarity. The present mu(T) of DMABN, between 8.3 and 9.7 D, although larger than the ground state dipole moment mu(0) of 6.6 D, is somewhat smaller than that of the locally excited (LE) state (9.9 D) but considerably smaller than the dipole moment of the intramolecular charge transfer (ICT) state (17 D). By comparing these mu(X) data with the frequency (CN) of the cyano vibration in each state, it appears that at least for DMABN in the triplet state (CN) is not a reliable indication of the extent of charge transfer as compared with the other states S0, LE, and ICT.     

Reactions of Silicon Atoms with Methanol: A Combined Matrix‐Spectroscopic and Density Functional Theory Study

The reaction of silicon atoms with methanol ( 4 ) has been studied in an argon matrix at 10 K. In the initial step a triplet n‐adduct T‐5 between a silicon atom and 4 is formed. It cannot be detected directly as long as a low concentration of 4 is used. But T‐5 must be generated since upon simultaneous irradiation during cocondensation methylsilanone ( 15 ) is found. Without irradiation T‐5 undergoes immediately O,H insertion and methoxysilylene ( S‐7‐c ) is isolated, which establishes a photoequilibrium between the s,trans ( S‐7‐t ) and s,cis form ( S‐7‐c ). If a high concentration of 4 is applied the silylenes exist as complexes S‐17 . The next step needs photochemical activation. Products are dimethoxysilane ( 3 ) and (hydroxy)(methoxy)methylsilane ( 19 ). The picture becomes even more complicated when deuteromethanol ( [D]4 ) is treated with silicon atoms. In this case the primarily formed n‐adduct ( [D]T‐5 ) is stable under matrix conditions. As long as a low concentration is applied, the subsequent step has to be induced by long wavelength irradiation and leads – different from the undeuterated case – to O,CH₃ insertion, giving (hydroxy)methylsilylenes ( [D]S‐11‐c ) and ( [D]S‐11‐t ). If a high concentration is used O,D insertion is preferred instead and even without irradiation the first observable products are methanol‐solvated methoxysilylenes ( [D₂]S‐17‐c ) and ( [D₂]S‐17‐t ). Subsequent irradiation of complexes [D₂]S‐17 gives in accordance with the protonated series a mixture of [D₂]3 and [D₂]19 . Our goal, to find a way to dimethylsilanediol ( [D₂]2 ), was finally reached by preparing silylenes [D]S‐11 in a diluted matrix, its specific solvation with [D]4 and final irradiation of complexes [D₂]S‐18 . The structural elucidation of all new species is based on the comparison of the experimental observations with density functional theory calculations. Upon cocondensation of silicon atoms with pure methanol at 77 K dimethoxysilane ( 3 ) and 1,1,2‐trimethoxydisilane ( 25 ) are produced. The also present methoxysilanes 22‐24 have to be regarded as secondary products of 3 .     

Electron binding capabilities of some silylenes having small singlet-triplet splittings or triplet ground states

Several silyl and alkaline metal substituted silylenes have been investigated using the CAS-ACPF method in conjunction with the aug-cc-pVTZ basis sets. Silylsilylene and disilylsilylene are found to have singlet ground states with DeltaEST(-) values of 0.676 and 0.319 eV, respectively. The adiabatic ground state electron affinities are found to be 1.572 and 2.361 eV for HSiSiH(3) and Si(SiH(3))(2). respectively. Both silylenes possesses a stable 2A1 excited negative ion state, with respective adiabatic EA values of 0.037 and 1.000 eV. In contrast, all silylenes with at least one akaline metal substituent exhibit triplet neutral ground states. The metalated silylenes HSiLi, HSiNa, LiSiSiH(3), NaSiLi, SiLi(2), and SiNa(2) have adiabatic ground state EAs somewhat below 1 eV, but each of these negatively charged system possesses up to three bound excited negative ion states, some of which are dipole-bound states.     

New routes to polymetallic clusters: fluoride-based tri-, deca-, and hexaicosametallic MnIII clusters and their magnetic properties

The syntheses, structures and magnetic properties of three new MnIII clusters, [Mn26O17(OH)8(OMe)4F10(bta)22(MeOH)14(H2O)2] (1), [Mn(0O6(OH)2(bta)8(py)8F8] (2) and [NHEt3]2[Mn3O(bta)6F3] (3), are reported (bta=anion of benzotriazole), thereby demonstrating the utility of MnF3 as a new synthon in Mn cluster chemistry. The "melt" reaction (100 degrees C) between MnF(3) and benzotriazole (btaH, C6H5N3) under an inert atmosphere, followed by dissolution in MeOH produces the cluster [Mn26O17(OH)8(OMe)4F10(bta)22(MeOH)14(H2O)2] (1) after two weeks. Complex 1 crystallizes in the triclinic space group P1, and consists of a complicated array of metal tetrahedra linked by mu3-O2- ions, mu3- and mu2-OH- ions, mu2-MeO- ions and mu2-bta- ligands. The "simpler" reaction between MnF3 and btaH in boiling MeOH (50 degrees C) also produces complex 1. If this reaction is repeated in the presence of pyridine, the decametallic complex [Mn10O6(OH)2(bta)8(py)8F8] (2) is produced. Complex 2 crystallizes in the triclinic space group P1 and consists of a "supertetrahedral" [Mn(III)10] core bridged by six mu3-O2- ions, two mu3-OH- ions, four mu2-F- ions and eight mu2-bta- ions. The replacement of pyridine by triethylamine in the same reaction scheme produces the trimetallic species [NHEt3]2[Mn3O(bta)6F3] (3). Complex 3 crystallises in the monoclinic space group P2(1)/c and has a structure analogous to that of the basic metal carboxylates of general formula [M3O(RCO2)6L3]0/+, which consists of an oxo-centred metal triangle with mu2-bta- ligands bridging each edge of the triangle and the fluoride ions acting as the terminal ligands. DC magnetic susceptibility measurements in the 300-1.8 K and 0.1-7 T ranges were investigated for all three complexes. For each, the value of chi(M)T decreases with decreasing temperatures; this indicates the presence of dominant antiferromagnetic exchange interactions in 1-3. For complex 1, the low-temperature value of chi(M)T is 10 cm(3) K mol(-1) and fitting of the magnetisation data gives S=4, g=2.0 and D=-0.90 cm(-1). For complex 2, the value of chi(M)T falls to a value of approximately 5.0 cm(3) K mol(-1) at 1.8 K, which is consistent with a small spin ground state. For the triangular complex 3, the best fit to the experimental chi(M)T versus T data was obtained for the following parameters: Ja = -5.01 cm(-1), Jb = +9.16 cm(-1) and g=2.00, resulting in an S=2 spin ground state. DFT calculations on 3, however, suggest an S=1 or S=0 ground state with J(a)=-2.95 cm(-1) and J(b)=-2.12 cm(-1). AC susceptibility measurements performed on 1 in the 1.8-4.00 K range show the presence of out-of-phase AC susceptibility signals, but no peaks. Low-temperature single-crystal studies performed on 1 on an array of micro-SQUIDS show the time- and temperature-dependent hysteresis loops indicative of single-molecule magnetism behaviour.     

Asymmetry in platinum acetylide complexes: confinement of the triplet exciton to the lowest energy ligand

To determine structure-optical property relationships in asymmetric platinum acetylide complexes, we synthesized the compounds trans-Pt(PBu3)2(C[triple bond]CC6H5)(C[triple bond]C-C6H4-C[triple bond]CC6H5) (PE1-2), trans-Pt(PBu3)2(C[triple bond]CC6H5)(C[triple bond]C-C6H4-C[triple bond]C-C6H4-C[triple bond]CC6H5) (PE1-3) and trans-Pt(PBu3)2(C[triple bond]C-C6H4-C[triple bond]CC6H5)(C[triple bond]C-C6H4-C[triple bond]C-C6H4-C[triple bond]CC6H5) (PE2-3) that have different ligands on either side of the platinum and compared their spectroscopic properties to the symmetrical compounds PE1, PE2 and PE3. We measured ground state absorption, fluorescence, phosphorescence and triplet state absorption spectra and performed density functional theory (DFT) calculations of frontier orbitals, lowest lying singlet states, triplet state geometries and energies. The absorption and emission spectra give evidence the singlet exciton is delocalized across the central platinum atom. The phosphorescence from the asymmetric complexes comes from the largest ligand. Time-dependent (TD) DFT calculations show the S1 state has mostly highest occupied molecular orbital (HOMO) --> lowest unoccupied molecular orbital (LUMO) character, with the LUMO delocalized over the chromophore. In the asymmetric chromophores, the LUMO resides on the larger ligand, suggesting the S1 state has interligand charge transfer character. The triplet state geometries obtained from the DFT calculations show distortion on the lowest energy ligand, whereas the other ligand has the ground state geometry. The calculated trend in the triplet state energies agrees very well with the experimental trend. Calculations of triplet state spin density also show the triplet exciton is confined to one ligand. In the asymmetric complexes the spin density is confined to the largest ligand. The results show Kasha's rule applies to these complexes, where the triplet exciton moves to the lowest energy ligand.     

Distinct pi-bonding capability between phosphinidene and phosphonium ion: a computational study

The heavy dipnictenes (RE=ER, where E=P, As, Sb, and Bi with the substituent R) have essentially planar geometry and appreciable strength in pi-bonding, unlike related heavier main group 14 analogues of alkenes as concluded recently by Power. This work demonstrated that the protonated pnictenes behave more like the heavy carbene for their weak pi-bonding character from the computational study with the B3LYP/6-311++G** method. For example, although both phosphinidene (HP) and the phosphonium ion (H2P+) are isoelectronic to silylenes, the pi-bonding tendency of the former is rather strong and it forms a planar adduct with both the stable carbene and stable silylene ((HCNH)2E, where E=C and Si). In contrast, the latter forms trans-bent adducts with the two species. These results can be interpreted in terms of the Carter-Goddard-Malrieu-Trinquier (CGMT) model, and the fact that the value of DeltaEST [E(triplet)-E(singlet)] of the HP fragment increases significantly after protonation. All other heavy pnictenes resemble the phosphinidene. In contrast, nitrene (HN) and nitrenium (H2N+) have a ground triplet state, thus both have strong pi-bonding character similar to that of carbene.     

Carboxylate-free Mn(III)2Ln(III)2 (Ln = lanthanide) and Mn(III)2Y(III)2 complexes from the use of (2-hydroxymethyl)pyridine: analysis of spin frustration effects

The initial employment of 2-(hydroxymethyl)pyridine for the synthesis of Mn/Ln (Ln = lanthanide) and Mn/Y clusters, in the absence of an ancillary organic ligand, has afforded a family of tetranuclear [Mn(III)(2)M(III)(2)(OH)(2)(NO(3))(4)(hmp)(4)(H(2)O)(4)](NO(3))(2) (M = Dy, 1; Tb, 2; Gd, 3; Y; 4) anionic compounds. 1-4 possess a planar butterfly (or rhombus) core and are rare examples of carboxylate-free Mn/Ln and Mn/Y clusters. Variable-temperature dc and ac studies established that 1 and 2, which contain highly anisotropic Ln(III) atoms, exhibit slow relaxation of their magnetization vector. Fitting of the obtained magnetization (M) versus field (H) and temperature (T) data for 3 by matrix diagonalization and including only axial anisotropy (zero-field splitting, ZFS) showed the ground state to be S = 3. Complex 4 has an S = 0 ground state. Fitting of the magnetic susceptibility data collected in the 5-300 K range for 3 and 4 to the appropriate van Vleck equations revealed, as expected, extremely weak antiferromagnetic interactions between the paramagnetic ions; for 3, J(1) = -0.16(2) cm(-1) and J(2) = -0.12(1) cm(-1) for the Mn(III)···Mn(III) and Mn(III)···Gd(III) interactions, respectively. The S = 3 ground state of 3 has been rationalized on the basis of the spin frustration pattern in the molecule. For 4, J = -0.75(3) cm(-1) for the Mn(III)···Mn(III) interaction. Spin frustration effects in 3 have been quantitatively analyzed for all possible combinations of sign of J(1) and J(2).     

New linear high-valent tetranuclear manganese-oxo cluster relevant to the oxygen-evolving complex of photosystem II with oxo, hydroxo, and aqua coordinated to a single Mn(IV)

An unprecedented atom connectivity, MnIV(mu-O)MnIV(mu-O)2MnIV(mu-O)MnIV, is found in the complex [MnIV4O4(EtO-terpy)4(OH)2(OH2)2](ClO4)(6).8H2O (EtO-terpy=4'-ethoxyl-2,2':6',2' '-terpyridine), which has been characterized by X-ray crystallography, X-ray powder diffraction, EPR spectroscopy, and magnetic studies. This complex is the first example of a compound where a MnIV ion is coordinated by all three types of water-derived ligands: oxo, hydroxo, and aqua. Bond distances and angles for this complex are consistent with a MnIV4 oxidation state assignment. The di-mu-oxo- and mono-mu-oxo-bridged Mn-Mn distances are 2.80 and 3.51 A, respectively. The variable-temperature magnetic susceptibility data for this complex, in the range of 10-300 K, are consistent with an S=0 ground state and were fit using the spin Hamiltonian HHDvV=-J1S2S1-J2S1S1A-J1S1AS2A (S1=S1A=S2=S2A=3/2) with J1=-432 cm-1 and J2=-164 cm-1 (where J1 and J2 are exchange constants through the mono-mu-oxo and the di-mu-oxo bridges, respectively). The first excited spin state of this tetramer is a spin triplet state at 279 cm-1 above the diamagnetic ground state. The next spin states are the S=1 and S=2 levels at about 700 and 820 cm-1 above the S=0 ground state, respectively. These large energy gaps are consistent with the absence of an EPR signal for this complex, even at high temperature.     

Time-dependent quantum study of the kinetics of the H(2S) + FO(2II) OH(2II) + F(2P) reaction

Quantum mechanical wave packet calculations are carried out for the H((2)S) + FO((2)II) --> OH((2)II) + F((2)P) reaction on the adiabatic potential energy surface of the ground (3)A' triplet state. The state-to-state and state-to-all reaction probabilities for total angular momentum J = 0 have been calculated. The probabilities for J > 0 have been estimated from the J = 0 results by using J-shifting approximation based on a capture model. Then, the integral cross sections and initial state-selected rate constants have been calculated. The calculations show that the initial state-selected reaction probabilities are dominated by many sharp peaks. The reaction cross section does not manifest any sharp oscillations and the initial state-selected rate constants are sensitive to the temperature.     

Reactive carbon-chain molecules: synthesis of 1-diazo-2,4-pentadiyne and spectroscopic characterization of triplet pentadiynylidene (H-C[triple bond]C-:C-C[triple bond]C-H)

1-Diazo-2,4-pentadiyne (6a), along with both monodeuterio isotopomers 6b and 6c, has been synthesized via a route that proceeds through diacetylene, 2,4-pentadiynal, and 2,4-pentadiynal tosylhydrazone. Photolysis of diazo compounds 6a-c (lambda > 444 nm; Ar or N2, 10 K) generates triplet carbenes HC5H (1) and HC5D (1-d), which have been characterized by IR, EPR, and UV/vis spectroscopy. Although many resonance structures contribute to the resonance hybrid for this highly unsaturated carbon-chain molecule, experiment and theory reveal that the structure is best depicted in terms of the dominant resonance contributor of penta-1,4-diyn-3-ylidene (diethynylcarbene, H-C[triple bond]C-:C-C[triple bond]C-H). Theory predicts an axially symmetric (D(infinity h)) structure and a triplet electronic ground state for 1 (CCSD(T)/ANO). Experimental IR frequencies and isotope shifts are in good agreement with computed values. The triplet EPR spectrum of 1 (absolute value(D/hc) = 0.6157 cm(-1), absolute value(E/hc) = 0.0006 cm(-1)) is consistent with an axially symmetric structure, and the Curie law behavior confirms that the triplet state is the ground state. The electronic absorption spectrum of 1 exhibits a weak transition near 400 nm with extensive vibronic coupling. Chemical trapping of triplet HC5H (1) in an O2-doped matrix affords the carbonyl oxide 16 derived exclusively from attack at the central carbon.     

Model studies of hydrogen atom addition and abstraction processes involving ortho-, meta-, and para-benzynes.

H-atom addition and abstraction processes involving ortho-, meta-, and para-benzyne have been investigated by multiconfigurational self-consistent field methods. The H(A) + H(B)...H(C) reaction (where r(BC) is adjusted to mimic the appropriate singlet-triplet energy gap) is shown to effectively model H-atom addition to benzyne. The doublet multiconfiguration wave functions are shown to mix the "singlet" and "triplet" valence bond structures of H(B)...H(C) along the reaction coordinate; however, the extent of mixing is dependent on the singlet-triplet energy gap (DeltaE(ST)) of the H(B)...H(C) diradical. Early in the reaction, the ground-state wave function is essentially the "singlet" VB function, yet it gains significant "triplet" VB character along the reaction coordinate that allows H(A)-H(B) bond formation. Conversely, the wave function of the first excited state is predominantly the "triplet" VB configuration early in the reaction coordinate, but gains "singlet" VB character when the H-atom is close to a radical center. As a result, the potential energy surface (PES) for H-atom addition to triplet H(B)...H(C) diradical is repulsive! The H3 model predicts, in agreement with the actual calculations on benzyne, that the singlet diradical electrons are not coupled strongly enough to give rise to an activation barrier associated with C-H bond formation. Moreover, this model predicts that the PES for H-atom addition to triplet benzyne will be characterized by a repulsive curve early in the reaction coordinate, followed by a potential avoided crossing with the (pi)1(sigma*)1 state of the phenyl radical. In contrast to H-atom addition, large activation barriers characterize the abstraction process in both the singlet ground state and first triplet state. In the ground state, this barrier results from the weakly avoided crossing of the dominant VB configurations in the ground-state singlet (S0) and first excited singlet (S1) because of the large energy gap between S0 and S1 early in the reaction coordinate. Because the S1 state is best described as the combination of the triplet X-H bond and the triplet H(B)...H(C) spin couplings, the activation barrier along the S0 abstraction PES will have much less dependence on the DeltaE(ST) of H(B)...H(C) than previously speculated. For similar reasons, the T1 potential surface is quite comparable to the S0 PES.     

Cu3(mu-S)2]3+ clusters supported by N-donor ligands: progress toward a synthetic model of the catalytic site of nitrous oxide reductase

By treating Cu(I) complexes of neutral, bidentate N-donor ligands with S8, clusters with novel delocalized mixed-valence [Cu3(mu-S)2]3+ cores have been isolated. X-ray crystal structures and UV-vis and resonance Raman spectral features of these clusters reveal similarities to the tetracopper-sulfide "CuZ" site in nitrous oxide reductase. A delocalized S = 1 ground state for the mixed-valent CuIIICu2II cores is supported by the observation of high symmetry in the X-ray structures and 10-line hyperfine features arising from coupling to three equivalent Cu ions in EPR spectra obtained at room temperature (shown) and 10 K. The delocalization we observe contrasts with the localization reported previously for a [Cu3(mu-O)2]3+ analogue (Root, D. E.; Henson, M. J.; Machonkin, T.; Mukherjee, P.; Stack, T. D. P.; Solomon, E. I. J. Am. Chem. Soc. 1998, 120, 4982), which we rationalized through DFT calculations.     

Photolysis of (3-methyl-2H-azirin-2-yl)-phenylmethanone: direct detection of a triplet vinylnitrene intermediate

The photoreactivity of (3-methyl-2H-azirin-2-yl)-phenylmethanone, 1, is wavelength-dependent (Singh et al. J. Am. Chem. Soc. 1972, 94, 1199-1206). Irradiation at short wavelengths yields 2P, whereas longer wavelengths produce 3P. Laser flash photolysis of 1 in acetonitrile using a 355 nm laser forms its triplet ketone (T(1K), broad absorption with λ(max) ~ 390-410 nm, τ ~ 90 ns), which cleaves and yields triplet vinylnitrene 3 (broad absorption with λ(max) ~ 380-400 nm, τ = 2 μs). Calculations (B3LYP/6-31+G(d)) reveal that T(1K) of 1 is located 67 kcal/mol above its ground state (S(0)) and has a long C-N bond (1.58 ?), and the calculated transition state to form 3 is only 1 kcal/mol higher in energy than T(1K) of 1. The calculations show that 3 has significant 1,3-carbon iminyl biradical character, which explains why 3 reacts efficiently with oxygen and decays by intersystem crossing to the singlet surface. Photolysis of 1 in argon matrixes at 14 K produced ketene imine 7, which presumably is formed from 3 intersystem crossing to 7. In comparison, photolysis of 1 in methanol with a 266 nm laser produces mainly ylide 2 (λ(max) ~ 380 nm, τ ~ 6 μs, acetonitrile), which decays to form 2P. Ylide 2 is formed via singlet reactivity of 1, and calculations show that the first singlet excited state of the azirine chromophore (S(1A)) is located 113 kcal/mol above its S(0) and that the singlet excited state of the ketone (S(1K)) is 85 kcal/mol. Furthermore, the transition state for cleaving the C-C bond in 1 to form 2 is located 49 kcal/mol above the S(0) of 1. Thus, we theorize that internal conversion of S(1A) to a vibrationally hot S(0) of 1 forms 2, whereas intersystem crossing from S(1K) to T(1K) results in 3.     

An "S"-shaped pentanuclear CuII cluster derived from the metal-assisted hydrolysis of pyCOpyCOpy: structural, magnetic and spectroscopic studies

Reaction of [Cu2(O2CMe)4(H2O)2] with 2,6-di-(2-pyridylcarbonyl)-pyridine (pyCOpyCOpy or dpcp) in MeCN-H2O 10:1, led to the pentanuclear copper(II) complex [Cu5(O2CMe)6{pyC(O)(OH)pyC(O)(OH)py}2] () which crystallizes in the triclinic P1 space group. The copper(II) atoms are arranged in an "S"-shaped configuration, and are bridged by the doubly deprotonated bis(gem-diol) form of the ligand, pyC(O)(OH)pyC(O)(OH)py2-. Magnetic susceptibility data indicate the interplay of both ferro- and antiferromagnetic intramolecular interactions stabilizing an S=3/2 ground state. Fitting of the data according to a next-nearest-neighbour model {H=-[J1(S1S2+S1'S2')+J2(S2S3+S3'S2')+J3(S1S3+S3'S1')+J4(S2S2')]} yields exchange coupling constants J1=+39.7 cm(-1), J2=-15.9 cm(-1), J3=-8.3 cm(-1) and J4=+4.3 cm(-1), leading to an S=3/2 ground state. X-Band EPR spectroscopy indicates a zero-field splitting of the ground state with |D3/2|=0.38 cm(-1).     

Ground- and excited-state aromaticity and antiaromaticity in benzene and cyclobutadiene

The aromaticity and antiaromaticity of the ground state (S 0), lowest triplet state (T 1), and first singlet excited state (S 1) of benzene, and the ground states (S 0), lowest triplet states (T 1), and the first and second singlet excited states (S 1 and S 2) of square and rectangular cyclobutadiene are assessed using various magnetic criteria including nucleus-independent chemical shifts (NICS), proton shieldings, and magnetic susceptibilities calculated using complete-active-space self-consistent field (CASSCF) wave functions constructed from gauge-including atomic orbitals (GIAOs). These magnetic criteria strongly suggest that, in contrast to the well-known aromaticity of the S 0 state of benzene, the T 1 and S 1 states of this molecule are antiaromatic. In square cyclobutadiene, which is shown to be considerably more antiaromatic than rectangular cyclobutadiene, the magnetic properties of the T 1 and S 1 states allow these to be classified as aromatic. According to the computed magnetic criteria, the T 1 state of rectangular cyclobutadiene is still aromatic, but the S 1 state is antiaromatic, just as the S 2 state of square cyclobutadiene; the S 2 state of rectangular cyclobutadiene is nonaromatic. The results demonstrate that the well-known "triplet aromaticity" of cyclic conjugated hydrocarbons represents a particular case of a broader concept of excited-state aromaticity and antiaromaticity. It is shown that while electronic excitation may lead to increased nuclear shieldings in certain low-lying electronic states, in general its main effect can be expected to be nuclear deshielding, which can be substantial for heavier nuclei.     

Single-molecule magnets: a new class of tetranuclear manganese magnets

The preparation, X-ray structure, and detailed physical characterization are presented for a new type of single-molecule magnet [Mn4(O2CMe)2(pdmH)6](ClO4)2 (1). Complex 1.2MeCN.Et2O crystallizes in the triclinic space group P1, with cell dimensions at 130 K of a = 11.914(3) A, b = 15.347(4) A, c = 9.660(3) A, alpha = 104.58(1) degree, beta = 93.42(1) degree, gamma = 106.06(1) degree, and Z = 1. The cation lies on an inversion center and consists of a planar Mn4 rhombus that is mixed-valent, MnIII2MnII2. The pdmH- ligands (pdmH2 is pyridine-2,6-dimethanol) function as either bidentate or tridentate ligands. The bridging between Mn atoms is established by either a deprotonated oxygen atom of a pdmH- ligand or an acetate ligand. The solvated complex readily loses all acetonitrile and ether solvate molecules to give complex 1, which with time becomes hydrated to give 1.2.5H2O. Direct current and alternating current magnetic susceptibility data are given for 1 and 1.2.5H2O and indicate that the desolvated complex has a S = 8 ground state, whereas the hydrated 1.2.5H2O has a S = 9 ground state. Ferromagnetic interactions between MnIII-MnII and MnIII-MnIII pairs result in parallel spin alignments of the S = 5/2 MnII and S = 2 MnIII ions. High-frequency EPR spectra were run for complex 1.2.5H2O at frequencies of 218, 328, and 436 GHz in the 4.5-30 K range. A magnetic-field-oriented polycrystallite sample was employed. Fine structure is clearly seen in this parallel-field EPR spectrum. The transition fields were least-squares-fit to give g = 1.99, D = -0.451 K, and B4 degrees = 2.94 x 10(-5) K for the S = 9 ground state of 1.2.5H2O. A molecule with a large-spin ground state with D < 0 can function as a single-molecule magnet, as detected by techniques such as ac magnetic susceptibility. Out-of-phase ac signals (chi' M) were seen for complexes 1 and 1.2.5H2O to show that these complexes are single-molecule magnets. A sample of 1 was studied by ac susceptibility in the 0.4-6.4 K range with the ac field oscillating at frequencies in the 1.1-1000 Hz range. A single peak in chi' M vs temperature plots was seen for each frequency; the temperature of the chi' M peak varies from 2.03 K at 995 Hz to 1.16 K at 1.1 Hz. Magnetization relaxation rates were evaluated in this way. An Arrhenius plot gave an activation energy of 17.3 K, which, as expected, is less than the 22.4 K value calculated for the thermodynamic barrier for magnetization direction reversal for an S = 8 complex with D = -0.35 K. The 1.2.5H2O complex with an S = 9 ground state has its chi' M peaks at higher temperatures.