Reaction of the silylene Si[(NCH2But)2C6H4-1,2] with the alkali metal silylamides M[N(SiMe3)R](M = Li, Na or K; R = SiMe3 or SiMe2Ph)

The thermally stable silylene Si[(NCH(2)Bu(t))(2)C(6)H(4)-1,2] 1 undergoes oxidative addition reactions with the alkali metal silylamides MN(SiMe(3))(2)(M = Li, Na or K) to afford the new alkali metal amides MN(SiMe(3))[(1)SiMe(3)][M = Li (2), Na (3) or K (4)]. Reaction of two equivalents of 1 with LiN(R)(SiMe(3)) leads in a two-step process to the compound LiN[(1)R][(1)SiMe(3)][R = SiMe(2)Ph (5) or SiMe(3) (6)]. Alternatively, 1 reacts with 3 to afford NaN[(1)SiMe(3)](2) (7). The structures of 2-5 and are presented and the formation of 2-7 is discussed.     

Reactions of M[CH(SiMe3)2] (M = Na or K) with PhCN, and related chemistry

A number of metal complexes containing one of the following ligands: the 1-azaallyl [N(R)C(Ph)C(H)R]- ([triple bond]L-), the 1,3-diazaallyl([triple bond]LL'-) and the isomeric beta-diketiminate [{N(R)C(Ph)]}2CH]- ( identical with LL-) have been prepared (R = SiMe(3)). These are the crystalline compounds H(LL) (2), Na(LL) (3), [Na(LL)(thf)2] (4), Na(L) (6), [Na(mu-LL')]8 (7), [K(mu-L)(eta6-C6H6)]2 (8), [K(mu-LL')(thf)]2 (9), [K(thf)2(mu-LL)](infinity) (10) and [Ni(LL')2] (11). A new synthesis of Na[C(H)R2] (1) involved Hg[C(H)R2]2 and Na/Hg as reagents. The beta-diketimine 2 was obtained from Li(LL) and cyclopentadiene. Under different conditions compounds 3, 6 and 7 were isolated from 1 and benzonitrile, and compounds 8, 9 and 10 from K[C(H)R2] and PhCN. Complex 11 was derived from [Li(LL')]2 and [NiBr(2)(dme)]. The solution obtained from 1 + 2 PhCN in Et2O at ambient temperature was a mixture (5) of 3 (predominantly) and 7. The 1-azaallyl complex 8 has the ligand bound to the metal as the enamide, and this is also probably (NMR) the case for 6. The molecular structures of the crystalline complexes 7, 8 and 11 are presented; that of 10 was published earlier. Compound 7, a cyclooctamer, is particularly interesting, in that each LL'- ligand is bridging via one of its N atoms to two neighbouring sodium ions and is not only N,N'- but also (eta2-C[=]C)-chelating to one of them.     

Magnesium(II) complexes of 2,4-N,N'-disubstituted 1,3,5-triazapentadienyl ligands: synthesis and characterization of [N(Dipp)C(NMe2)NC(NMe(2))N(SiMe3)MgBr]2 and [N(R)C(R')NC(R')N(SiMe3)]2Mg (Dipp = 2,6-iPr2-C6H3, R = Ph, SiMe(3); R' = NMe2, 1-piperidino)

Treatment of the appropriate lithium or sodium 2,4-N,N'-disubstituted 1,3,5-triazapentadienate [RNC(R')NC(R')N(SiMe(3))M](2) (R = Ph, 2,6-(i)Pr(2)-C(6)H(3)(Dipp) or SiMe(3); R' = NMe(2) or 1-piperidino; M = Li or Na) with one or half equivalent portion of MgBr(2)(THF)(2) in Et(2)O under mild conditions furnishes in good yield the first structurally characterized molecular magnesium 2,4-N,N'-disubstituted 1,3,5-triazapentadienates [DippNC(NMe(2))NC(NMe(2))N(SiMe(3))MgBr](2) (1), [{RNC(R')NC(R')N(SiMe(3))}(2)Mg] (R = Ph, R' = NMe(2) 2; R = Ph, R' = 1-piperidino 3; R = SiMe(3), R' = 1-piperidino 4). The solid-state structure of 1 is dimeric and those of 2, 3, and 4 are monomeric. The ligand backbone NCNCN in 1 adopts a W-shaped configuration, while in 2, 3 and 4 adopts a U-shaped configuration.     

Deprotonation reactions of zirconium and hafnium amide complexes H2N-M[N(SiMe3)2]3 and subsequent silyl migration from amide -N(SiMe3)2 to imide =NH ligands

Ammonolysis of previously reported Cl-M[N(SiMe3)2]3 (M = Zr, 1a; Hf, 1b) leads to the formation of peramides H2N-M[N(SiMe3)2]3 (M = Zr, 2a; Hf, 2b) which upon deprotonation by LiN(SiMe3)2 or Li(THF)3SiPh2But yields imides Li+(THF)n{HN(-)-M[N(SiMe3)2]3} (M = Zr, 3a; Hf, 3b). One -SiMe3 group in 3a-b undergoes silyl migration from a -N(SiMe3)2 ligand to the imide =NH ligand to give Li+(THF)2{Me3SiN(-)-M[NH(SiMe3)][N(SiMe3)2]2} (M = Zr, 4a; Hf, 4b) containing an imide =N(SiMe3) ligand. The kinetics of the 3a --> 4a conversion was investigated between 290 and 315 K and was first-order with respect to 3a. The activation parameters for this silyl migration are DeltaH++ = 13.3(1.3) kcal/mol and DeltaS++ = -34(3) eu in solutions of 3a (in toluene-d8 with 1.07 M THF) prepared in situ. THF in the mixed solvent promoted the 3a --> 4a reaction. The effect of THF on the rate constants of the conversion has been studied, and the kinetics of the reaction was 3.4(0.6)th order with respect to THF. Crystal and molecular structures of H2N-Zr[N(SiMe3)2]3 (2a) and 4a-b have been determined.     

Reactions of a gallium(II)-diazabutadiene dimer, [{{[(H)C(Bu(t))N]2}GaI}2], with [ME(SiMe3)2] (M = Li or Na; E = N, P, or As): structural, EPR, and ENDOR characterization of paramagnetic gallium(III) pnictide complexes

The reactions of the paramagnetic gallium(II) complex [{(Bu(t)-DAB)GaI}2] (Bu(t)-DAB = {(Bu(t))NC(H)}2) with the alkali metal pnictides [ME(SiMe3)2] (M = Li or Na; E = N, P, or As) have been carried out under a range of stoichiometries. The 1:2 reactions have led to a series of paramagnetic gallium(III)-pnictide complexes, [(Bu(t)-DAB)Ga{E(SiMe3)2}I] (E = N, P, or As), while two of the 1:4 reactions afforded [(Bu(t)-DAB)Ga{E(SiMe3)2}2] (E = P or As). In contrast, treatment of [{(Bu(t)-DAB)GaI}2] with 4 equiv of [NaN(SiMe3)2] resulted in a novel gallium heterocycle coupling reaction and the formation of the diradical species [(Bu(t)-DAB)Ga{N(SiMe3)2}{[CC(H)N2(Bu(t))2]Ga[N(SiMe3)2]CH3}]. The mechanism of this unusual reaction has been explored, and evidence suggests it involves an intramolecular transmethylation reaction. The X-ray crystal structures of all prepared complexes are reported, and all have been characterized by EPR and ENDOR spectroscopies. The observed spin Hamiltonian parameters provide a detailed picture of the distribution of the unpaired spin density over the molecular frameworks of the complexes.     

Über den Phosphandiyl‐Transfer von invers‐polarisierten Phosphaalkenen R1P=C(NMe2)2 (R1 = tBu,Cy, Ph,H) auf Phospheniumkomplexe [(η5‐C5H5)(CO)2M=P(R2)R3] (R2 = R3 = Ph; R2 = tBu,R3 = H; R2 = Ph,R3 = N(SiMe3)2)

Phosphanediyl Transfer from Inversely Polarized Phosphaalkenes R¹P=C(NMe₂)₂ (R¹ = tBu, Cy, Ph, H) onto Phosphenium Complexes [(η⁵‐C₅H₅)(CO)₂M=P(R²)R³] (R² = R³ = Ph; R² = tBu, R³ = H; R² = Ph, R³ = N(SiMe₃)₂) Reaction of the freshly prepared phosphenium tungsten complex [(η⁵‐C₅H₅)(CO)₂W=PPh₂] ( 3 ) with the inversely polarized phosphaalkenes RP=C(NMe₂)₂ ( 1 ) ( a : R = tBu; b : Cy; c : Ph) led to the η²‐diphosphanyl complexes ( 9a‐c ) which were isolated by column chromatography as yellow crystals in 24‐30 % yield. Similarly, phosphenium complexes [(η⁵‐C₅H₅)(CO)₂M=P(H)tBu] (M = W ( 6 ); Mo ( 8 )) were converted into (M = W ( 11 ); Mo ( 12 )) by the formal abstraction of the phosphanediyl [PtBu] from 1a . Treatment of [(η⁵‐C₅H₅)(CO)₂W=P(Ph)N(SiMe₃)₂] ( 4 ) with HP=C(NMe₂)₂ ( 1d ) gave rise to the formation of yellow crystalline ( 10 ). The products were characterized by elemental analyses and spectra (IR, ¹H, ¹³C‐, ³¹P‐NMR, MS). The molecular structure of compound 10 was elucidated by an X‐ray diffraction analysis.     

Synthesis and structures of beta-diketiminatotin(II) halides, an amide and of Sn(=E)[{N(R)C(Ph)}2CH](NR2) (E = S or Se, R = SiMe3)

Treatment of [Li(L1)]2 (1) or K(L2) (2) with SnX2 in Et2O yielded the heteroleptic beta-diketiminatotin(II) halides Sn(L1)Cl (3a), Sn(L1)Br (3b) or Sn(L2)Cl (4), even when an excess of the alkali metal beta-diketiminate was used [L1={N(R)C(Ph)}2CH, L2={N(R)C(Ph)CHC(But)N(R)}, R = SiMe3]. From and half an equivalent each of SnCl2.2H2O and SnCl2, or one equivalent of SnCl2.2H2O, the product was Sn(L3)Cl (5) or Sn(L4)Cl (6), in which one or both of the N-R bonds of L1 had been hydrolytically cleaved; the compound Sn(L5)Cl (7) was similarly obtained from and an equivalent portion of SnCl2.2H2O [L3={N(R)C(Ph)CHC(But)N(H)}, L4={N(H)C(Ph)CHC(But)N(H)} and L5={N(H)C(Ph)}2CH]. The halide exchange between 3a and 3b, studied by two-dimensional (119)Sn{1H}-NMR spectroscopy, is attributed to implicate a (mu-Cl)(mu-Br)-dimeric intermediate or transition state. The 13C{1H}-NMR spectra of or showed two distinct resonances for each group, which coalesced on heating, corresponding to DeltaG(338 K)= 69.4 (3a) or 72.8 (3b) kJ mol(-1). The chloride ligand of was readily displaced by treatment with NaNR2, CF3SO3H or CH2(COPh)2, yielding Sn(L1)X [X = NR2 (8), O3SCF3 (9) or {OC(Ph)}2CH (10)]. Oxidative addition of sulfur or selenium to gave the tin(IV) terminal chalcogenides Sn(E)(L1)(NR2)[E = S (11) or Se (12)]. The X-ray structures of the cocrystal of 3a/3b and of the crystalline compounds 5, 6, 8, 11 and are presented, as well as multinuclear NMR spectra of each of the new compounds.     

Kristallstruktur des Zinkamids Zn[N(SiMe3)2]2

Crystal Structure of the Zinc Amide Zn[N(SiMe₃)₂]₂ X‐ray quality crystals of Zn[N(SiMe₃)₂]₂ (monoclinic, P2₁/c) are obtained by sublimation of the zinc amide Zn[N(SiMe₃)₂]₂ at —30 °C in vacuo (300 torr). According to the result of the X‐ray structural analysis, Zn[N(SiMe₃)₂]₂ contains an almost linear N‐Zn‐N unit with two short N‐Zn bonds.     

Das iso-Tetraphosphan P[P(SiMe3)Me]2[P(SiMe3)2]

H ? C Bond Cleavage in Ferrocene by Organylruthenium Complexes Cp*(Me₃P)₂RuCH₂CMe₃ ( 1 ) reacts at 85°C with ferrocene ( 2 ) by cleavage of one H? C bond in 2 to give CpFe[η⁵-C₅H₄Ru(PMe₃)₂Cp*] ( 3 ) (Cp = η⁵-C₅H₅; Cp* = η⁵-C₅Me₅) and neopentane. The ruthenium atom in 3 has a distorted tetrahedral geometry, the planar Cp ligands in the ferrocenyl fragment are eclipsed. Solutions of 3 in [D₆]benzene or [D₈]THF exhibit H? D exchange of the ferrocenyl protons. In the [D₈]THF molecule only the α-deuterium atoms are exchanged. Reaction pathways for this exchange are discussed.     

Platinum,iridium and rhodium complexes with substituted bis-(diphenylphosphino)methane ligands Ph2PCH(R)PPh2 (R = Me,Ph or SiMe3)

Substituted phosphines of the type Ph₂PCH(R)PPh₂ and their Pt^II complexes [PtX₂{Ph₂PCH(R)PPh₂}] (R = Me, Ph or SiMe₃; X = halide) were prepared. Treatment of [PtCl₂(NCBu^t)₂] with Ph₂PCH(SiMe₃)-PPh₂ gave [PtCl₂(Ph₂PCH₂PPh₂)], while treatment with Ph₂PCH(Ph)PPh₂ gave [Pt{Ph₂PCH(Ph)PPh₂}₂]Cl₂. Reaction of p-MeC₆H₄C≡CLi or PhC≡CLi with [PtX₂{Ph₂PCH(Me)PPh₂}] gave [Pt(C≡CC₆H₄Me-p)₂-{Ph₂PCH(Me)PPh₂}] (X = I) and [Pt{Ph₂PC(Me)PPh₂}₂](X = Cl),while reaction of p-MeC₆H₄C≡CLi with [Pt{Ph₂PCH(Ph)PPh₂}₂]Cl₂ gave [Pt{Ph₂PC(Ph)PPh₂}₂]. The platinum complexes [PtMe₂(dpmMe)] or [Pt(CH₂)₄(dpmMe)] fail to undergo ring-opening on treatment with one equivalent of dpmMe [dpmMe = Ph₂PCH(Me)PPh₂]. Treatment of [Ir(CO)Cl(PPh₃)₂] with two equivalents of dpmMe gave [Ir(CO)(dpmMe)₂]Cl. The PF₆ salt was also prepared. Treatment of [Ir(CO)(dpmMe)₂]Cl with [Cu(C≡CPh)₂], [AgCl(PPh₃)] or [AuCl(PPh₃)] failed to give heterobimetallic complexes. Attempts to prepare the dinuclear rhodium complex [Rh₂(CO)₃(μ-Cl)(dpmMe)₂]BPh₄ using a procedure similar to that employed for an analogous dpm (dpm = Ph₂PCH₂PPh₂) complex were unsuccessful. Instead, the mononuclear complex [Rh(CO)(dpmMe)₂]BPh₄ was obtained. The corresponding chloride and PF₆ salts were also prepared. Attempts to prepare [Rh(CO)(dpmMe)₂]Cl in CHCl₃ gave [RhHCl(dpmMe)₂]Cl. Recrystallization of [Rh(CO)(dpmMe)₂]BPh₄ from CHCl₃/EtOH gave [RhO₂(dpmMe)₂]BPh₄. Treatment of [Rh(CO)₂Cl₂]₂ with one equivalent of dpmMe per Rh atom gave two compounds, [Rh(CO)(dpmMe)₂]Cl and a dinuclear complex that undergoes exchange at room temperature between two formulae: [Rh₂(CO)₂(μ-Cl)(μ-CO)(dpmMe)₂]Cl and [Rh₂(CO)₂-(μ-Cl)(dpmMe)₂]Cl. This revised version was published online in June 2006 with corrections to the Cover Date.     

Synthesis and structural characterization of [kappa3-B,S,S-B(mimR)3]Ir(CO)(PPh3)H (R = Bu(t), Ph) and [kappa4-B(mim(Bu(t))3]M(PPh3)Cl (M = Rh, Ir): analysis of the bonding in metal borane compounds

A series of iridium and rhodium complexes that feature M-->B dative bonds, namely [kappa(3)-B,S,S-B(mim(R))3]Ir(CO)(PPh3)H (R = But, Ph) and [kappa4-B(mim(Bu)t)3]M(PPh3)Cl (M = Rh, Ir), has been synthesized via (i) the reactions of Ir(PPh3)2(CO)Cl with [Tm(Bu)t]Tl and [Tm(Ph)]Li and (ii) the reactions of (COD)M(PPh3)Cl with [Tm(Bu)t]K. The complexes have been structurally characterized by X-ray diffraction, thereby demonstrating the presence of a M-->B dative bond in each complex. The nature of the M-->B interaction in these complexes has been addressed by computational methods which indicate that the metal centers possess a d(6) configuration. The d(6) configuration is in accord with the value predicted by using a method that employs the valence to determine d(n)(), but is not in accord with the d8 configuration that is predicted using the oxidation number. Thus, even though B(mim(R))3 may be regarded as a neutral closed-shell ligand, coordination to a d(n) transition metal via the boron results in the formation of a complex in which the metal center possesses a d(n-2) configuration.     

1-D and 2-D homoleptic dicyanamide structures, [Ph4P]2[CoII[N(CN)2]4] and [Ph4P][M[N(CN)2]3] (M = Mn, Co)

The homoleptic complexes [Ph(4)P](2)[Co[N(CN)(2)](4)] and [Ph(4)P][M[N(CN)(2)](3)] [M = Co, Mn] have been structurally as well as magnetically characterized. The complexes containing [M[N(CN)(2)](4)](2-) form 1-D chains, which are bridged via a common dicyanamide ligand in [M[N(CN)(2)](3)](-) to form a 2-D structure. The five-atom [NCNCN](-) bridging ligands lead to weak magnetic coupling along a chain. The six [NCNCN](-) ligands lead to a (4)T(1g) ground state for Co(II) which has an unquenched spin-orbit coupling that is reflected in the magnetic properties. Long-range magnetic ordering was not observed in any of these materials.     

The open-chain triphosphanes RMe2SiCH2P(PR'2)2 (R = Me, Ph; R' = SiMe3, Cy, Ph)

The triphosphanes RMe(2)SiCH(2)P(PR'(2))(2) (R = Me, Ph; R' = SiMe(3), Cy) are synthesised in good yield via metathesis of organodichlorophosphanes and LiPR'(2), while for R' = Ph a propensity to form (Ph(2)P)(2) precludes isolation of the in situ characterised triphosphanes. Where R = Me and R' = SiMe(3) the triphosphane has also been characterised by single crystal X-ray diffraction and exhibits a single geometric conformer in the solid state, though solution-phase NMR spectra are indicative of facile conformational exchange across a wide temperature range. All of the described triphosphanes exhibit comparable behaviour, with their respective (31)P{(1)H} NMR spectra manifesting anomalous 'second-order' characteristics, which are considered using full spin-Hamiltonian simulation. Preliminary studies of coordination chemistry and ancillary reactivity of the triphosphanes are described.     

Synthesis and structures of the [benzamidinato]3- complexes Li3(tmeda)(L1)]2 and [Li(thf)4][Li5(L2)(OEt2)2] [L1 = N(SiMe3)C(Ph)N(SiMe3) and L2 = N(SiMe3)C(C6H4-4)NPh

Reduction at ambient temperature of each of the lithium benzamidinates [Li(L(1))(tmeda)] or [{Li(L(2))(OEt(2))(2)}(2)] with four equivalents of lithium metal in diethyl ether or thf furnished the brown crystalline [Li(3)(L(1))(tmeda)] (1) or [Li(thf)(4)][Li(5)(L(2))(2)(OEt(2))(2)] (2), respectively. Their structures show that in each the [N(R(1))C(R(3))NR(2)](3-) moiety has the three negative charges largely localised on each of N, N' and R = Aryl); a consequence is that the "aromatic" 2,3- and 5,6-CC bonds of R(3) approximate to being double bonds. Multinuclear NMR spectra in C(6)D(6) and C(7)D(8) show that 1 and 2 exhibit dynamic behaviour. [The following abbreviations are used: L(1) = N(SiMe(3))C(Ph)N(SiMe(3)); L(2) = N(SiMe(3))C(C(6)H(4)Me-4)N(Ph); tmeda = (Me(2)NCH(2)-)(2); thf = tetrahydrofuran.] This reduction is further supported by a DFT analysis.     

Disilynes. III [1] A Relatively Stable Disilyne RSi≡SiR (R = SiMe(SitBu3)2)

The disilyne R**Si≡SiR** (R** = SiMe(SitBu₃)₂), prepared as the first isolable and realtively stable silicon compound with a SiSi triple bond two years ago by dehalogenation of trans‐R**ClSi=SiClR** with LiC₁₀H₈ in thf at ‐78 °C (calc.: Si≡Si distance 2.072Å, Si‐Si≡Si bond angle 148°), forms with CH₂=CH₂ a [2+2] and with CH₂=CH‐CH=CH₂ a [2+4] cycloadduct. The ethene adduct takes up oxygen very easily with change of the Si=Si group into a SiOSiO ring with formation of R**Si(μ‐O)(μ‐O)(μ‐C₂H₄)SiR**. By heating the disilyne in heptane to ca. 50 °C in the presence of traces of thf it transforms into a monoxide of the ethene adduct with formation of R**Si(μ‐O)(μ‐C₂H₄)SiR**. In thf, the disilyne rearranges at r.t. and below by migration of a SitBu₃ group with formation of a silyl substituted cyclotrisilene. X‐ray structure determinations of the ethene adduct and its mono‐ and dioxide are presented.     

Solution structures and dynamic properties of chelated d(0) metal olefin complexes [eta(5): eta(1)-C(5)R(4)SiMe(2)N(t)Bu]Ti(OCMe(2)CH(2)CH(2)CH=CH(2))(+) (R = H, Me): Models for the [eta(5): eta(1)-C(5)R(4)SiMe(2)N(t)Bu]Ti(R')(olefin)(+) intermediates in "constrained geometry" catalysts.

To model the Ti-olefin interaction in the putative [eta(5): eta(1)-C(5)R(4)SiMe(2)N(t)Bu]Ti(R')(olefin)(+) intermediates in "constrained geometry" Ti-catalyzed olefin polymerization, chelated alkoxide olefin complexes [eta(5): eta(1)-C(5)R(4)SiMe(2)N(t)Bu]Ti(OCMe(2)CH(2)CH(2)CH=CH(2))(+) have been investigated. The reaction of [eta(5): eta(1)-C(5)R(4)SiMe(2)N(t)Bu]TiMe(2) (1a,b; R = H, Me) with HOCMe(2)CH(2)CH(2)CH=CH(2) yields mixtures of [eta(5)-C(5)R(4)SiMe(2)NH(t)Bu]TiMe(2)(OCMe(2)CH(2)CH(2)CH=CH(2)) (2a,b) and [eta(5): eta(1)-C(5)R(4)SiMe(2)N(t)Bu]TiMe(OCMe(2)CH(2)CH(2)CH=CH(2)) (3a,b). The reaction of 2a/3a and 2b/3b mixtures with B(C(6)F(5))(3) yields the chelated olefin complexes [[eta(5): eta(1)-C(5)R(4)SiMe(2)N(t)Bu]Ti(OCMe(2)CH(2)CH(2)CH=CH(2))][MeB(C(6)F(5))(3)] (4a,b; 71 and 89% NMR yield). The reaction of 2b/3b with [Ph(3)C][B(C(6)F(5))(4)] yields [[eta(5): eta(1)-C(5)Me(4)SiMe(2)N(t)Bu]Ti(OCMe(2)CH(2)CH(2)CH=CH(2))][B(C(6)F(5))(4)] (5b, 88% NMR yield). NMR studies establish that 4a,b and 5b exist as mixtures of diastereomers (isomer ratios: 4a/4a', 62/38; 4b/4b', 75/25; 5b/5b', 75/25), which differ in the enantioface of the olefin that is coordinated. NMR data for these d(0) metal olefin complexes show that the olefin coordinates to Ti in an unsymmetrical fashion primarily through C(term) such that the C=C pi bond is polarized with positive charge buildup on C(int). Dynamic NMR studies show that 4b/4b' undergoes olefin face exchange by a dissociative mechanism which is accompanied by fast inversion of configuration at Ti ("O-shift") in the olefin-dissociated intermediate. The activation parameters for the conversion of 4b to 4b' (i.e., 4b/4b' face exchange) are: DeltaH = 17.2(8) kcal/mol; DeltaS = 8(1) eu. 4a/4a' also undergoes olefin face exchange but with a lower barrier (DeltaH = 12.2(9) kcal/mol; DeltaS = -2(3) eu), for the conversion of 4a to 4a'.     

Ligand rotation in [Ar(R)N]3M-N2-M'[N(R)Ar]3(M, M' = MoIII, NbIII; R = iPr and tBu) dimers

Earlier calculations on the model N2-bridged dimer (micro-N2)-{Mo[NH2]3}2 revealed that ligand rotation away from a trigonal arrangement around the metal centres was energetically favourable resulting in a reversal of the singlet and triplet energies such that the singlet state was stabilized 13 kJ mol(-1) below the D(3d) triplet structure. These calculations, however, ignored the steric bulk of the amide ligands N(R)Ar (R =iPr and tBu, Ar = 3,5-C6H3Me2) which may prevent or limit the extent of ligand rotation. In order to investigate the consequences of steric crowding, density functional calculations using QM/MM techniques have been performed on the Mo(III)Mo(III) and Mo(III)Nb(III) intermediate dimer complexes (mu-N(2))-{Mo[N(R)Ar]3}2 and [Ar(R)N]3Mo-(mu-N2)-Nb[N(R)Ar]3 formed when three-coordinate Mo[N(R)Ar]3 and Nb[N(R)Ar]3 react with dinitrogen. The calculations indicate that ligand rotation away from a trigonal arrangement is energetically favourable for all of the ligands investigated and that the distortion is largely electronic in origin. However, the steric constraints of the bulky amide groups do play a role in determining the final orientation of the ligands, in particular, whether the ligands are rotated at one or both metal centres of the dimer. Analogous to the model system, QM/MM calculations predict a singlet ground state for the (mu-N2)-{Mo[N(R)Ar]3}2 dimers, a result which is seemingly at odds with the experimental triplet ground state found for the related (mu-N2)-{Mo[N(tBu)Ph]3}2 system. However, QM/MM calculations on the (mu-N2)-{Mo[N(tBu)Ph]3}2 dimer reveal that the singlet-triplet gap is nearly 20 kJ mol(-1) smaller and therefore this complex is expected to exhibit very different magnetic behaviour to the (mu-N2)-{Mo[N(R)Ar]3}2 system.