Single electron transfer-promoted photocyclization reactions of linked acceptor-polydonor systems: effects of chain length and type on the efficiencies of macrocyclic ring-forming photoreactions of tethered alpha-silyl ether phthalimide substrates

Results of an investigation, aimed at gaining information about the factors governing the efficiencies of single electron transfer (SET)-promoted photocyclization reactions of linked acceptor-polydonor systems, are described. One set of substrates used in this effort includes alpha-trimethylsilyl ether terminated, polymethylene- and polyethylenoxy-tethered phthalimides and 2,3-naphthalimides. Photocyclization reactions of the polyethylenoxy-linked phthalimides and naphthalimides were observed to take place in higher chemical yields and with larger quantum efficiencies than those of analogs containing polymethylene tethers of near equal length. These findings show that the rates of formation of 1,omega-zwitterionic biradicals that serve as key intermediates in the photocyclization processes are enhanced in substances that contain oxygen donor sites in the chain. The findings suggest that these donor sites facilitate both initial SET to acceptor excited states and ensuing intrachain SET, resulting in migration of the cation radical center to the terminal alpha-trimethylsilyl ether position. In addition, an inverse relationship was observed between the quantum yields of photocyclization reactions of the tethered phthalimides and naphthalimides and the length of the polyethylenoxy chain. Finally, the roles played by chain type and length in governing photoreaction efficiencies were investigated by using intramolecular competition in photoreactions of polyethylenoxy and polymethylene bis-tethered phthalimides. Mechanistic interpretations and synthetic consequences of the observations made in this study are discussed.     

A strategy for the preparation of cyclic polyarenes based on single electron transfer-promoted photocyclization reactions

Single electron transfer (SET)-promoted photocyclization reactions of substrates comprised of benzylsilane tethered to phthalimides were subjected to an exploratory study in order to probe a new approach for the preparation of cyclic polyarenes. The results show that UV irradiation of the substrates leads to efficient photochemical reactions that are initiated by SET from benzylsilane moieties to the excited phthalimide acceptor. Ensuing desilylation reactions of the benzylsilane cation radical moieties in the intermediate zwitterionic biradicals and proton transfer gives biradical precursors of the cyclic polyarene products. The observations made in this effort suggests that SET photochemical methods, which have been employed earlier to generate cyclic poly-ethers, -thioethers and -amides, serve as a useful method to access potentially interesting macrocyclic targets.     

Applications of phthalimide photochemistry to macrocyclic polyether, polythioether, and polyamide synthesis.

Irradiation of phthalimides which contain N-linked omega-trimethylsilylmethyl-substituted polyether, polythioether, and polysulfonamide chains results in efficient production of the corresponding macrocyclic polyether, polythioether, and polysulfonamide products. These photocyclization reactions follow sequential single electron transfer (SET)-desilylation pathways. Only in the cases of phthalimides, bearing mixed ether-thioether N-substituents, do these excited-state cyclization reactions proceed with lower degrees of regioselectivity. This is a result of competitive desilylation and alpha-to-sulfur deprotonation reactions of the zwitterionic diradical intermediates formed by initial SET.     

Studies aimed at elucidating factors involved in the control of chemoselectivity in single electron transfer promoted photoreactions of branched-polydonor substituted phthalimides

Factors that govern the chemical selectivities and efficiencies of SET-promoted photocyclization reactions of acceptor-polydonor substrates were explored by using systems comprised of phthalimide acceptors linked via polymethylene or polyethylenoxy chains to α-silylether and thioether donors. A number of linear and branched substrates of this type were prepared and their photochemical behavior was explored. The results of this effort have led to the identification of several key factors that govern the chemoselectivities and efficiencies of the competitive reaction pathways followed. The observations suggest that the length and nature of the chain linking the phthalimide acceptor and α-silyl donor sites are important factors in controlling the rates of formation of zwitterionic biradicals that serve as penultimate intermediates in routes for product formation. In addition, the rates of methanol promoted desilylation at cation radical centers in intermediate zwitterionic biradicals also play important roles especially in cases where chain length/type is not a factor. The results are discussed in terms of both their mechanistic and synthetic significance.     

A synthetic strategy for the preparation of cyclic peptide mimetics based on SET-promoted photocyclization processes

A novel method for the synthesis of cyclic peptide analogues has been developed. The general approach relies on the use of SET-promoted photocyclization reactions of peptides that contain N-terminal phthalimides as light absorbing electron acceptor moieties and C-terminal alpha-amidosilane or alpha-amidocarboxylate centers. Prototypical substrates are prepared by coupling preformed peptides with the acid chloride of N-phthalimidoglycine. Irradiation of these substrates results in the generation of cyclic peptide analogues in modest to good yields. The chemical efficiencies of these processes are not significantly affected by (1) the lengths of the peptide chains separating the phthalimide and alpha-amidosilane or alpha-amidocarboxylate centers and (2) the nature of the penultimate cation radical alpha-heterolytic fragmentation process (i.e., desilylation vs decarboxylation). An evaluation of the effects of N-alkyl substitution on the amide residues in the peptide chain showed that N-alkyl substitution does not have a major impact on the efficiencies of the photocyclization reactions but that it profoundly increases the stability of the cyclic peptide.     

Synthesis and structures of heteroleptic silylenes

The reaction of benzamidinato silicon trichloride [{PhC(NR)2}SiCl3] [R = Bu(t) (1), SiMe3 (2)] with 2 equiv of potassium in THF afforded mononuclear chlorosilylene [{PhC(NBu(t))2}SiCl] (3) and [{PhC(NSiMe3)2}2SiCl2] (4), respectively. Compound 4 was formed by the disproportionation of unstable [{PhC(NSiMe3)2}SiCl]. The reaction of [{PhC(NBu(t))2}SiCl3] (1) with 1 equiv of LiR (R = NMe2, OBu(t), OPr(i), PPr(i)2) in THF yielded [{PhC(NBu(t))2}SiCl2R] [R = NMe2 (5), OBu(t) (6), OPr(i) (7), PPr(i)2 (8)]. Treatment of 5-8 with 2 equiv of potassium in THF resulted in the novel heteroleptic silylene [{PhC(NBu(t))2}SiR] [R = NMe2 (9), OBu(t) (10), OPr(i) (11), PPr(i)2 (12)]. Compounds 4, 9, and 12 have been analyzed by X-ray crystallography.     

Unusual equilibria involving group 4 amides, silyl complexes, and silyl anions via ligand exchange reactions

Silyl anion SiButPh2- (2) was found to substitute an amide ligand in Zr(NMe2)4 (3) to give the disilyl complex Zr(NMe2)3(SiButPh2)2- (1a) and Zr(NMe2)5- (1b) in THF. The reaction is reversible, and nucleophilic amide NMe2- attacks the Zr-SiButPh2 bonds in 1a or Zr(NMe2)3(SiButPh2) in the reverse reaction, leading to an unusual ligand exchange equilibrium 2 3 + 2 2 right harpoon over left harpoon 1a + 1b (eq 1). The silyl anion 2 selectively attacks the -N(SiMe3)2 ligand in Zr(NMe2)3[N(SiMe3)2] (6) to give 1a and N(SiMe3)2- (7). Reversible reaction occurs as well, where 7 selectively substitutes the silyl ligand in Zr(NMe2)3(SiButPh2)2- (1a) or Zr(NMe2)3(SiButPh2), giving the equilibrium 6 + 2 2 right harpoon over left harpoon 1a + 7 (eq 3). The thermodynamics of these equilibria has been studied: For eq 1, DeltaH degrees = -8.3(0.2) kcal/mol, DeltaS degrees = -23.3(0.9) eu, and DeltaG degrees 298K = -1.4(0.5) kcal/mol at 298 K; for eq 3, DeltaH degrees = -1.61(0.12) kcal/mol, DeltaS degrees = -2.6(0.5) eu, and DeltaG degrees 298K = -0.8(0.3) kcal/mol. In both equilibria, the enthalpy changes for the forward reactions outweigh the entropy changes, and therefore the substitutions of amide ligands in Zr(NMe2)4 (3) and Zr(NMe2)3[N(SiMe3)2] (6) to afford the disilyl complex 1a are thermodynamically favored. The following equilibria were also observed and studied: Zr(NMe2)3[N(SiMe3)2] (6) + Si(SiMe3)3- (9) right harpoon over left harpoon Zr(NMe2)3[Si(SiMe3)3] (10) + N(SiMe3)2- (7) and Zr(NMe2)4 (3) + 9 right harpoon over left harpoon 10 + Zr(NMe2)5- (1b).     

Reactions of tetrakis(dimethylamide)-titanium, -zirconium and -hafnium with silanes: synthesis of unusual amide hydride complexes and mechanistic studies of titanium-silicon-nitride (Ti-Si-N) formation

M(NMe(2))(4) (M = Ti, Zr, Hf) were found to react with H(2)SiR'Ph (R' = H, Me, Ph) to yield H(2), aminosilanes, and black solids. Unusual amide hydride complexes [(Me(2)N)(3)M(mu-H)(mu-NMe(2))(2)](2)M (M = Zr, 1; Hf, 2) were observed to be intermediates and characterized by single-crystal X-ray diffraction. [(Me(2)N)(3)M(mu-D)(mu-NMe(2))(2)](2)M (1-d(2), 2-d(2)) were prepared through reactions of M(NMe(2))(4) with D(2)SiPh(2). Reactions of (Me(2)N)(3)ZrSi(SiMe(3))(3) (5) with H(2)SiR'Ph were found to give aminosilanes and (Me(2)N)(2)Zr(H)Si(SiMe(3))(3) (6). These reactions are reversible through unusual equilibria such as (Me(2)N)(3)ZrSi(SiMe(3))(3) (5) + H(2)SiPh(2) right arrow over left arrow (Me(2)N)(2)Zr(H)Si(SiMe(3))(3) (6) + HSi(NMe(2))Ph(2). The deuteride ligand in (Me(2)N)(2)Zr(D)Si(SiMe(3))(3) (6-d(1)) undergoes H-D exchange with H(2)SiR'Ph (R' = Me, H) to give 6 and HDSiR'Ph. The reaction of Ti(NMe(2))(4) with SiH(4) in chemical vapor deposition at 450 degrees C yielded thin Ti-Si-N ternary films containing TiN and Si(3)N(4). Ti(NMe(2))(4) reacts with SiH(4) at 23 degrees C to give H(2), HSi(NMe(2))(3), and a black solid. HNMe(2) was not detected in this reaction. The reaction mixture, upon heating, gave TiN and Si(3)N(4) powders. Analyses and reactivities of the black solid revealed that it contained -H and unreacted -NMe(2) ligands but no silicon-containing ligand. Ab initio quantum chemical calculations of the reactions of Ti(NR(2))(4) (R = Me, H) with SiH(4) indicated that the formation of aminosilanes and HTi(NR(2))(3) was favored. These calculations also showed that HTi(NH(2))(3) (3b) reacted with SiH(4) or H(3)Si-NH(2) in the following step to give H(2)Ti(NH(2))(2) (4b) and aminosilanes. The results in the current studies indicated that the role of SiH(4) in its reaction with Ti(NMe(2))(4) was mainly to remove amide ligands as HSi(NMe(2))(3). The removal of amide ligands is incomplete, and the reaction thus yielded "=Ti(H)(NMe(2))" as the black solid. Subsequent heating of the black solid and HSi(NMe(2))(3) may then yield TiN and Si(3)N(4), respectively, as the Ti-Si-N materials.     

From lithium bis(trimethylsilyl)amide with cyanoamine into triazine compounds: synthesis and structures of lithium 6-((trimethylsilyl)amido)-2,4-bis(dimethylamino)[1,3,5]triazines and their manganese and cobalt complexes

Addition reactions of lithium bis(trimethylsilyl)amide with dimethylcyanamide lead to novel lithium salts of 6-((trimethylsilyl)amido)-2,4-bis(dimethylamino)[1,3,5]triazines [LLi(D)](2) (L = NC(NMe(2))NC(NMe(2))NC(NSiMe(3)); D = Me(2)NCN (1), Et(2)O (2)) and to the Mn and Co complexes [LL'M] (L' = N{N(SiMe(3))C(NMe(2))}(2); M = Mn (3), Co (4)); the structures of crystalline 1, 3, and 4 are reported. Their formation involves trimethylsilyl shifts, ring formation, and unusual Me(2)NSiMe(3) elimination.     

Amido/amine triazacyclononane-based zirconium complexes: syntheses, reactivity, and structures

The reactions of [Zr(NMe2)4]2 with triamido-triazacyclonane ligand precursors, {NH(Ph)SiMe2}3tacn (H3N3[9]N3) and {NH(C6H4F)SiMe2}3tacn (H3N3-F[9]N3), led to the formation of complexes [Zr(NMe2)2{N(Ph)SiMe2}2{NH(Ph) SiMe2}tacn], 1, and [Zr(NMe2)2{N(o-C6H4F)SiMe2}2{NH(o-C6H4F)SiMe2} tacn], 2, where the zirconium is coordinated to two remaining dimethylamido ligands and to a dianionic tacn-based ligand, [{N(Ph')SiMe2}2{NH(Ph')SiMe2}tacn]2-, that formed from deprotonation of two amine pendent arms of the ligands' precursors. The third pendent arm of H3N3[9]N3 and H3N3-F[9]N3 remains neutral and not bonded to the zirconium. Treatment of 1 with NaH led to the synthesis of [Zr(NMe2){N(Ph)SiMe2}2tacn], 3, that results from the cleavage of the N-Si bond of the original neutral pendent arm. Complexes [ZrCl{N(Ph')SiMe2}2tacn] (Ph' = C6H5, 4, and C6H4F, 5) have been obtained by reactions of ZrCl4 with {MN(Ph')SiMe2}3tacn.2THF (M = Li, Na). Reactions of 4 and 5 with LiC triple bond CPh led to the syntheses of [Zr(CCPh){N(Ph')SiMe2}2tacn] (Ph' = C6H5, 6, and C6H4F, 7). The solid-state structure of 3 shows a chiral metal center.     

Unexpected formation of a trinuclear complex containing a Ta(IV)-Ta(IV) bond in the reactions of Bu(t)N=Ta(NMe2)3 with silanes

A new trinuclear species containing a Ta(IV)-Ta(IV) bond, Ta(3)(μ-H)(μ-NMe(2))(μ=NBu(t))(2)(=NBu(t))(NMe(2))(5), has been formed by reductive elimination of H(2). Ta(2)H(2)(μ-NMe(2))(2)(NMe(2))(2)(=NBu(t))(2) has also been isolated. O(2) oxidizes the Ta(IV)-Ta(IV) bond to yield Ta(3)(μ(3)-O)(H)(μ=NBu(t))(μ-NMe(2))(2)(NMe(2))(4)(=NBu(t))(2) under ligand exchange. Delocalization of d electrons is discussed.     

Reactivity of the inversely polarized phosphaalkenes RP=C(NMe2)2 (R = tBu, Me3Si, H) towards arylcarbene complexes [(CO)5M=C(oEt)Ar] (Ar= Ph, M = Cr, W; Ar = 2-MeC6H4, 2-MeOC6H4, M = W)

The reaction of the arylated Fischer carbene complexes [(CO)5M=C(OEt)Ar] (Ar=Ph; M = Cr, W; 2-MeC6H4; 2-MeOC6H; M = W) with the phosphaalkenes RP=C(NMe2), (R=tBu, SiMe3) afforded the novel phosphaalkene complexes [[RP=C(OEt)Ar]M(CO)5] in addition to the compounds [(RP=C(NMe2)2]M(CO)5]. Only in the case of the R = SiMe3 (E/Z) mixtures of the metathesis products were obtained. The bis(dimethylamino)methylene unit of the phosphaalkene precursor was incorporated in olefins of the type (Me2N)2C=C(OEt)(Ar). Treatment of [(CO)5W=C(OEt)(2-MeOC6H4)] with HP=C(NMe2)2 gave rise to the formation of an E/Z mixture of [[(Me2N)2CH-P=C(OEt)(2-MeOC6H4)]W(CO)5] the organophosphorus ligand of which formally results from a combination of the carbene ligand and the phosphanediyl [P-CH(NMe2)2]. The reactions reported here strongly depend on an inverse distribution of alpha-electron density in the phosphaalkene precursors (Pdelta Cdelta+), which renders these molecules powerfu] nucleophiles.     

Amino-functionalised diarylphosphide complexes of the alkali metals and lanthanum

The secondary phosphines Ar(C6H4-2-CH2NMe2)PH [Ar = mes (3), Tripp (4)] may be isolated in good yields from reactions between Li(C6H4-2-CH2NMe2) and the respective dichlorophosphine, followed by reduction with LiAlH4 [mes = 2,4,6-Me3C6H2, Tripp = 2,4,6-Pri3C6H2]. Metalation of either 3 or 4 with BunLi gives the corresponding lithium compound; the lithium derivative of 3 was isolated as the separated ion pair complex [Li(12-crown-4)2][(mes)(C6H4-2-CH2NMe2)P].THF (5). The lithium complexes Ar(C6H4-2-CH2NMe2)PLi undergo metathesis reactions with either NaOBut or KOBut to give the heavier alkali metal phosphides {Ar(C6H4-2-CH2NMe2)P}M.1/2OEt2 [Ar = mes, M = Na (8), K (9); Ar = Tripp, M = K (10)]. Metathesis reactions between 9 and LaI3(THF)4 give only intractable products; in contrast, a metathesis reaction between 10 and LaI3(THF)4 yields the heteroleptic complex {(Tripp)(C6H4-2-CH2NMe2)P}2LaI (11). Compound 11 reacts cleanly with K{N(SiMe3)2} to give {(Tripp)(C6H4-2-CH2NMe2)P}2La{N(SiMe3)2} (14). Compounds 3-5, 8-11 and 14 have been characterised by multi-element NMR spectroscopy; in addition, compounds 5, 11 and 14 have been studied by X-ray crystallography.     

On the presence or absence of geminal Si...N interactions (alpha-effect) in pentafluorophenylsilyl compounds with SiCN, SiNN and SiON backbones

The silanes C6F5SiF2CH2NMe2 (1), C6F5SiF2N(SiMe3)NMe2 (2) and C6F5SiF2ONMe2 (3) with pentafluorophenyl substituents and geminal N atoms have been prepared by the reaction of C6F5SiF3 with LiCH2NMe2, LiN(SiMe3)NMe2 and LiONMe2, respectively. The compounds have been characterised by spectroscopic methods and crystal structure determination. Comparison of measured and calculated IR spectra has provided insight into the conformational composition of the vapour of . Whereas and show interactions between the geminal Si and N atoms, does not. Further analysis of the bonding situation has been undertaken by quantum chemical calculations of the rotation and bending potentials of the C6F5SiF2-X-NMe2 units.     

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.     

Solution dynamics and molecular structure elucidation of novel aluminium derivatives containing diaminodimethylsilane ligands

The interaction of dimethyldiaminosilane ligands of general formula SiMe2(NR2)(NR'2)(NR2, NR'2 = NiHPr, NHtBu, NC4H8, NHCH2CH2NMe2) with AlX3 (X = Cl, Me) has been investigated and the synthesis of novel aluminium derivatives is reported, namely AlMe3[SiMe2(NR2)(NR'2)], AlX2[SiMe2(NR)(NR'2)] and AlMe[SiMe2(NR)2], containing the silane ligand as neutral, monoanionic and dianionic species, respectively. Moreover, the solution molecular structures and dynamics have been elucidated via 1D/2D variable temperature NMR spectroscopy showing the influence of the N-substituents of the silane ligand and of the aluminium ancillary ligands.     

Synthesis and structures of selected triazapentadienate of Li, Mn, Fe, Co, Ni, Cu(I), and Cu(II) using 2,4-N,N'-disubstituted 1,3,5-triazapentadienate anions as ancillary ligands: [N(Ar)C(NMe2)NC(NMe2)N(R)](-) (Ar = Ph, 2,6-(i)Pr2-C6H3; R = H, SiMe3)

Addition reaction of ArN(SiMe 3)M (Ar = Ph or 2,6 - (i) Pr 2-C 6H 3 (Dipp); M = Li or Na) to 2 equivalents of alpha-hydrogen-free nitrile RCN (R = dimethylamido) gave the dimeric [M{N(Ar)C(NMe 2)NC(NMe 2)N(SiMe 3)}] 2 ( 1a, Ar = Ph, M = Li; 1b, Ar = Ph, M = Na; 1c, Ar = Dipp, M = Li). 1d was obtained by hydrolysis of 1c at ambient temperature. Treatment of a double ratio of 1a or 1b with anhydrous MCl 2 (M = Mn, Fe, Co) yielded the 1,3,5-triazapentadienato complexes [M{N(Ph)C(NMe 2)NC(NMe 2)N(SiMe 3)} 2] (M = Mn, 2; Fe, 3; Co, 4) and with NiCl 2.6H 2O gave [M{N(Ph)C(NMe 2)NC(NMe 2)N(H)} 2] (M = Ni, 5). Treatment of an equiv of 1c with anhydrous CuCl in situ and in air led to complexes [{N(Dipp)C(NMe 2)NC(NMe 2)N(SiMe 3)}CuPPh 3] 6 and [Cu{N(Dipp)C(NMe 2)NC(NMe 2)N(H)} 2] 7, respectively. 1c, 1d, and 2- 7 were characterized by X-ray crystallography and microanalysis. 1c, 1d, 5, and 6 were well characterized by (1)H, (13)C NMR, 1c by (7)Li, and 6 by (31)P NMR as well. The structural features of these complexes were described in detail.     

Activation and desoxygenation of CO, CO2, and SO2 with sulfur-rich azide and amide

The azide and amide complexes (NBu4)[Ni(N3)('S3')] (2) and (NBu4)[Ni[N(SiMe3)2]('S3')] (4) were found to react with CO, CO2, and SO2 under very mild conditions at temperatures down to -50 degrees C. Depending on the N oxidation state of the nitrogen ligands, addition or partial to complete desoxygenation of the oxides takes place. The reaction between 2 and CO gives (NBU4)[Ni(NCO)('S3')] (3). The reactions between 4 and CO, CO2, and SO2 afford selectively the cyano, isocyanato, and sulfinylimido complexes (NBu4)[Ni(X)('S3')] with X = CN- (5), NCO- (3), and NSO- (6). The silyl groups act as oxygen acceptors. Mechanisms are suggested which have in common the formation of reactive five-coordinate (NBu4)[Ni(L)(L')('S3')] intermediates. In these reactions, highly activated L and L' react with each other. The complexes were characterized by standard methods, and (NBu4)[Ni(CN)('S3')] (5) was also analyzed by X-ray crystallography.     

Amide-silyl ligand exchanges and equilibria among group 4 amide and silyl complexes

M(NMe2)4 (M = Zr, 1a; Hf, 1b) and the silyl anion (SiButPh2)- (2) in Li(THF)2SiButPh2 (2-Li) were found to undergo a ligand exchange to give [M(NMe2)3(SiButPh2)2]- (M = Zr, 3a; Hf, 3b) and [M(NMe2)5]- (M = Zr, 4a; Hf, 4b) in THF. The reaction is reversible, leading to equilibria: 2 1a (or 1b) + 2 2 <--> 3a (or 3b) + 4a (or 4b). In toluene, the reaction of 1a with 2 yields [(Me2N)3Zr(SiButPh2)2]-[Zr(NMe2)5Li2(THF)4]+ (5) as an ionic pair. The silyl anion 2 selectively attacks the -N(SiMe3)2 ligand in (Me2N)3Zr-N(SiMe3)2 (6a) to give 3a and [N(SiMe3)2]- (7) in reversible reaction: 6a + 2 2 <--> 3a + 7. The following equilibria have also been observed and studied: 2M(NMe2)4 (1a; 1b) + [Si(SiMe3)3]- (8) <--> (Me2N)3M-Si(SiMe3)3 (M = Zr, 9a; Hf, 9b) + [M(NMe2)5]- (M = Zr, 4a; Hf, 4b); 6a (or 6b) + 8 <--> 9a (or 9b) + [N(SiMe3)2]- (7). The current study represents rare, direct observations of reversible amide-silyl exchanges and their equilibria. Crystal structures of 5, (Me2N)3Hf-Si(SiMe3)3 (9b), and [Hf(NMe2)4]2 (dimer of 1b), as well as the preparation of (Me2N)3M-N(SiMe3)2 (6a; 6b) are also reported.