Nitrido Complexes of Transition Metals

Molecular and electronic structure of chromium(v) nitrido complexes with azide and isothiocyanate ligands

The first use of [Cr(N)Cl4]2- as a starting material in chromium(v) nitrido chemistry is demonstrated in simple, high yield, metathesis reactions with the pseudohalogens SCN- and N3- yielding five-coordinate, labile complexes: [Cr(N)(NCS)4]2- and [Cr(N)(N3)4]2-, which have been crystallized and characterized by single-crystal X-ray diffraction. Reaction of [Cr(N)(NCS)4]2- with 1,10-phenanthroline furnishes six-coordinate [Cr(N)(NCS)3(phen)]- wherein phenanthroline coordinates to the position trans to the nitrido ligand. The trans influence of the nitrido ligand leads to a bond length difference of 0.223 A between the axial and equatorial ligators from the phenanthroline ligand. The absorption band with lowest energy in these pseudo-linear complexes is assigned as the electric dipole forbidden transition d(xy) --> d(x-y) based on intensities and its variation with the nature of the equatorial ligators. This absorption provides the spectrochemical series for the equatorial ligands, which is found to be numerically almost identical to that determined for chromium(III). DFT calculations reproduce the observed structures and corroborate the ligand field picture of the electronic structure of these complexes.

Organic Nitriles from Acid Chlorides: An Isovalent N for (O)Cl Exchange Reaction Mediated by a Tungsten Nitride Complex

Nitride NW(N[i-Pr]Ar)3 (1, Ar = 3,5-C6H3Me2) was synthesized in two steps from known NW(O-t-Bu)3 (41% overall yield). Complex 1 is the tungsten congener of NMo(N[i-Pr]Ar)3, a known molecule that has been synthesized using N2 as the nitrido nitrogen source, but which undergoes no reaction with pivaloyl chloride. Compound 1 undergoes metathesis with pivaloyl chloride at 25 degrees C to form the corresponding nitrile in 97% yield. Another substrate examined in this work was the labeled acid chloride 1-Ad13C(O)Cl (Ad = adamantyl). The "(O)Cl" moiety is transferred to tungsten forming an oxo-chloride, (Ar[i-Pr]N)3W(O)Cl (3), as the final tungsten product; both 1 and 3 were characterized structurally by X-ray diffraction. An intermediate observed in the nitrile-forming reaction was characterized spectroscopically to be a tungsten acylimido complex. The latter assignment was substantiated by the synthesis and structural characterization of the compound (Ar[i-Pr]N)3W(NC(O)CF3)(O2CCF3) (2m). In addition, density functional theory calculations performed using ADF lent insight into the thermochemistry of the overall process.

[Cr(N)Cl4]2-: a simple nitrido complex synthesized by nitrogen-atom transfer

A new general route to nitrido complexes of Cr(V) based on nitrogen-atom transfer from Mn(N)(salen) to labile CrCl3(THF)3 is presented. By this approach, the simplest nitrido complex of a first row transition metal, [Cr(N)Cl4]2-, has been synthesized and isolated. [[N(CH3)4]2[Cr(N)Cl4].H2O crystallizes in the cubic space group Fm-3m with disordered anions. Cr-N is 1.555(19) A, Cr-Cl is 2.2912(16) A, and N-Cr-Cl is 101.24(4) degrees . The orbital splitting scheme of [Cr(N)Cl4]2- is extreme with the dx2-y2 orbital 10 000 cm-1 lower in energy than the degenerate {dzx, dyz} set of orbitals destabilized by pi-bonding with the nitrido ligand. Hydrolysis of [Cr(N)Cl4]2 preserves the {CrN}2+ moiety.

Tristhiolatomolybdenum nitrides, (RS)(3)Mo[triple bond]N where R = (i)Pr and (t)Bu, preparation, characterization and comparisons with related trialkoxymolybdenumnitrides

The addition of thiols to ((t)BuO)(3)Mo[triple bond]N in toluene leads to the formation of (RS)(3)Mo[triple bond]N compounds as yellow, air-sensitive compounds, where R = (i)Pr and (t)Bu. The single-crystal structure of ((t)BuS)(3)Mo[triple bond]N reveals a weakly associated dimeric structure where two ((t)BuS)(3)Mo[triple bond]N units (Mo-N = 1.61 A, Mo-S = 2.31 A (av)) are linked via thiolate sulfur bridges with long 3.03 A (av) Mo-S interactions. Density functional theory calculations employing Gaussian 98 B3LYP (LANL2DZ for Mo and 6-31G* for N, O, S, and H) have been carried out for model compounds (HE)(3)Mo[triple bond]N and (HE)(3)MoNO, where E = O and S. A comparison of the structure and bonding within the related series ((t)BuE)(3)Mo[triple bond]N and ((t)BuE)(3)MoNO is made for E = O and S. In the thiolate compounds, the highest energy orbitals are sulfur lone-pair combinations. In the alkoxides, the HOMO is the N 2p lone-pair which has M-N sigma and M-O pi* character for the nitride. As a result of greater O p pi to Mo pi interactions, the M-N pi orbitals of the Mo-N triple bond are destabilized with respect to their thiolate counterpart. For the nitrosyl compounds, the greater O p pi to Mo d pi interaction favors greater back-bonding to the nitrosyl pi* orbitals for the alkoxides relative to the thiolates. The results of the calculations are correlated with the observed structural features and spectroscopic properties of the related alkoxide and thiolate compounds.

DFT analysis of the activation and breaking of the Mo-N bond in a (mu-nitrido)dimolybdenum complex: is molybdenum tris(thiolate) an elusive intermediate?

The electronic structure, conformation, synthesis, and thermal decomposition pathways of the recently characterized dimolybdenum mu-nitrido complex (AdS)(3)Mo(mu-N)Mo(N[(t)Bu]Ph)(3) (1exp, Ad = adamantyl) are investigated by means of DFT calculations carried out on the model system (HS)(3)Mo(mu-N)Mo(NH(2))(3) (1). The observed asymmetry of the Mo(mu-N)Mo core is reproduced in the optimal conformation of 1 and assigned to the tendency for the electron density of the metal atoms to be preferably accommodated in the pi orbitals of Mo(thiolate). The balance in the metal-ligand and ligand-ligand interactions conditioning the flow of the electron density along the Mo-(mu-N)-Mo framework, and eventually the relative activation of the molybdenum-nitrido bonds, appears very sensitive to the nature of the ancillary substituents on both the thiolate and the amido sides. On one hand, replacing HS by AdS in 1 increases the calculated value of Delta(mu-N-Mo) from 0.053 to 0.094 A, close to the experimental value of 0.111 A. The mu-nitrido complex with bulky thiolates 1a is also less stable than 1 by 7.3 kcal x mol(-1) with respect to its monometallic constituents. On the other hand, substituting the bulky N[(t)Bu]Ph for NH(2) in the model complex 1b induces an important charge transfer toward the thiolate moiety resulting in structural and energetic consequences of similar magnitude. Even though these substitutional effects are not likely to be fully additive in the real complex, both should contribute to an increase of the mu-N-Mo(thiolate) bond activation in 1exp. The importance of this activation conditions the feasibility of the thermal decomposition of 1exp promoted by benzonitrile which eventually yields the molybdenum thiolate dimer (RS)(3)Mo [triple bond] Mo(SR)(3). The energy profile calculated for this reaction with model complex1 in the presence of one or two molecules of acetonitrile shows that the axial fixation of the promoter on one or both molecular ends forms intermediates in which the mu-N-Mo(thiolate) bond is further activated with respect to the original complex. The consequence is an important, but still insufficient, decrease of the barrier to Mo-N bond breaking, from 53 to 37 kcal x mol(-1). Furthermore, the thermodynamic balance of the reaction leading from the acetonitrile adducts of 1 to (HS)(3)Mo [triple bond] Mo(SH)(3) remains endothermic by 6.5 kcal x mol(-1) for the monoadduct, and more for the diadduct. It therefore appears that bulky substituents on both ends of the dinuclear complex are essential to the completion of the reaction, from both the thermodynamic and the kinetic viewpoints.

Reversible osmium(VI) nitrido to osmium(II) ammine interconversion in complexes containing polypyrazolyl ligands

This paper describes the 4e-/3H+ interconversion between NH3 and N3-, which is reversible in the coordination spheres of Os complexes containing either tpm (tpm = tris(1-pyrazolyl)methane) or Tp (Tp = hydrotris(1-pyrazolyl)borate anion) ligands. Electrochemical or chemical reduction of the nitrido complexes [Os(VI)(tpm)(Cl)2(N)]+ (1) and Os(VI)(Tp)(Cl)2(N) (2) in acidic aqueous solution gives the corresponding Os(II)-ammine complexes, which, after air oxidation and workup, are isolated and structurally characterized as [Os(III)(tpm)(Cl)2(NH3)](PF6) (3) and Os(III)(Tp)(Cl)2(NH3) (4). The Os(III)-ammine complexes are reoxidized electrochemically to the nitrido complexes by stepwise mechanisms involving the loss of both electrons and protons and sequential Os(III-->IV) and Os(IV-->VI) oxidations.

Ammonolysis of mono(pentamethylcyclopentadienyl) titanium(IV) derivatives

Ammonolyses of mono(pentamethylcyclopentadienyl) titanium(IV) derivatives [Ti(eta5-C5Me5)X3] (X = NMe2, Me, Cl) have been carried out in solution to give polynuclear nitrido complexes. Reaction of the tris(dimethylamido) derivative [Ti(eta5-C5Me5)(NMe2)3] with excess of ammonia at 80-100 degrees C gives the cubane complex [[Ti(eta5-C5Me5)]4(mu3-N)4] (1). Treatment of the trimethyl derivative [Ti(eta5-C5Me5)Me3] with NH3 at room temperature leads to the trinuclear imido-nitrido complex [[Ti(eta/5-CsMes)(mu-NH)]3(mu3-N)] (2) via the intermediate [[Ti(eta5-C5Me5)Me]2(mu-NH)2] (3). The analogous reaction of [Ti(eta5-C5Me5)Me3] with 2,4,6-trimethylaniline (ArNH2) gives the dinuclear imido complex [[Ti(eta5-C5Me5)Me])2(mu-NAr)2] (4) which reacts with ammonia to afford [[Ti(eta5-C5Me5)(NH2)]2(mu-NAr)2] (5). Complex 2 has been used, by treatments with the tris(dimethylamido) derivatives [Ti(eta5-C5H5-nRn)(NMe2)3], as precursor of the cubane nitrido systems [[Ti4(eta5-C5Me5)3(eta5-C5H5-nRn)](mu3-N)4] [R = Me n = 5 (1), R = H n = 0 (6), R = SiMe3 n = 1 (7), R = Me n = 1 (8)] via dimethylamine elimination. Reaction of [Ti(eta5-C5Me5)Cl3] or [Ti(eta5-C5Me5)(NMe2)Cl2] with excess of ammonia at room temperature gives the dinuclear complex [[Ti2(eta5-C5Me5)2Cl3(NH3)](mu-N)] (9) where an intramolecular hydrogen bonding and a nonlineal nitrido ligand bridge the "Ti(eta5-C5Me5)Cl(NH3)" and "Ti(eta5-C5Me5)Cl2" moieties. The molecular structures of [[Ti(eta5-C5Me5)Me]2 (mu-NAr)2] (4) and [[Ti2(eta5-C5Me5)2Cl3(NH3)](mu-N)] (9) have been determined by X-ray crystallographic studies. Density functional theory calculations also have been conducted on complex 9 to confirm the existence of an intramolecular N-H...Cl hydrogen bond and to evaluate different aspects of its molecular disposition.

Reactions of Nitridorhenium(V) and -Osmium(VI) Complexes with Acylating Agents

Interaction of Re(N)L(2) [L = N(PSPh(2))(2)] 1 with (CF(3)CO)(2)O or RCOCl afforded air-sensitive acylimido-Re(V) complexes trans-Re[NC(O)CF(3)](OCOCF(3))L(2) 2 or trans-Re[NC(O)R]ClL(2) (R = CCl(2)H 3, CClH(2) 4, CH(3) 5), respectively. Treatment of 1 with (CX(3)CO)(2)O followed by recrystallization from CH(2)Cl(2)/hexane in air led to the formation of the corresponding parent imido complexes trans-Re(NH)(OCOCX(3))L(2) (X = F 6, Cl 7). The structure of 7 has been characterized by X-ray crystallography. The Re-N, average Re-S, and Re-O distances are 1.664(3), 2.441, and 2.116(3) Å, respectively. Deprotonation of 6 or 7 with Et(3)N gave 1. Recrystallization of 3 from CH(2)Cl(2)/hexane in air resulted in oxo-imido exchange and the isolation of the oxo-Re(V) species trans-Re(O)ClL(2). Treatment of 1 with tosyl anhydride gave trans-Re(NH)(OTs)L(2) (OTs = tosyl) 8. Reaction of [n-Bu(4)N][OsNCl(4)] with KL afforded trans-Os(N)ClL(2) 9, which has been characterized by X-ray crystallography. The Os-N, Os-Cl, and average Os-S bond distances in 9 are 1.64(1), 2.577(4), and 2.429 Å, respectively. Treatment of 10 with (CF(3)CO)(2)O, Ag(CF(3)CO(2)), or CF(3)CO(2)H resulted in chloride substitution and the formation of trans-Os(N)(OCOCF(3))L(2) 10. The Os-N, Os-O, and average Os-S distances in 10 are 1.643(5), 2.271(4), and 2.419 Å, respectively. Treatment of 1 with [Ph(3)C]BF(4) resulted in the isolation of trans-Re(NCPh(3))(F)L(2) 11, presumably via the cationic tritylimido intermediate [Re(NCPh(3))L(2)](+). Reaction of 9 with [Ph(3)C]BF(4) led to chloride abstraction and the formation of five-coordinate [Os(N)L(2)]BF(4) 12. The Os-N and average Os-S distances in 12 are 1.646(5) and 2.364 Å, respectively.

Reactivity of Osmium(VI) Nitrides with the Azide Ion. A New Synthetic Route to Osmium(II) Polypyridyl Complexes

There is an extensive reactivity chemistry between trans-[Os(VI)(tpy)(Cl)(2)(N)](+) (1) (tpy = 2,2':6',2"-terpyridine) and N(3)(-). Reaction of 1 with N(3)(-) in CH(2)Cl(2) or acetone occurs by electron transfer to give trans,trans-(tpy)(Cl)(2)Os(II)(N(2))Os(II)(Cl)(2)(tpy). In CH(3)CN, trans-Os(II)(tpy)(Cl)(2)(N(2)) forms but undergoes solvolysis to give trans-Os(II)(tpy)(Cl)(2)(CH(3)CN). 1 reacts with excess N(3)(-) in CH(3)CN to give Os(III)(tpy)(Cl)(2)(5-CH(3)-tetrazolate), which has been characterized by X-ray crystallography. This is the first known Os-tetrazolato complex. 1 reacts with N(3)(-) in the presence of CS(2) to give trans-[Os(II)(tpy)(Cl)(2)(NS)](+), SCN(-), and N(2).

Titanium Imido Complexes Supported by Amidinate Ligands: Synthesis, Solution Dynamics, and Solid State Structures

Reaction of Li[PhC(NSiMe(3))(2)] with the complexes [Ti(NR)Cl(2)(py)(3)] affords the corresponding (N,N'-bis(trimethylsilyl)benzamidinato)titanium imido derivatives [Ti(NR){PhC(NSiMe(3))(2)}Cl(py)(2)] [R = Bu(t) (1), 2,6-C(6)H(3)Me(2) (2), 2,6-C(6)H(3)Pr(i)(2) (3)], which, in solution, exist in temperature-dependent, dynamic equilibrium with their mono(pyridine) homologues [Ti(NR){PhC(NSiMe(3))(2)}Cl(py)] and free pyridine. Kinetic and thermodynamic data for these processes are reported, and the relative contributions of the DeltaH and DeltaS terms associated with all three equilibria are identified. The arylimido complexes 2 and 3 may also be prepared by treating 1 with the appropriate arylamine. Reaction of Li[MeC(NC(6)H(11))(2)] with [Ti(NBu(t))Cl(2)(py)(3)] gives the binuclear N,N'-bis(cyclohexyl)acetamidinato derivative [Ti(2)(&mgr;-NBu(t))(2){MeC(NC(6)H(11))(2)}(2)Cl(2)] (4). The X-ray structures of 2 and 4 have been determined. Crystal data for 2: triclinic, P&onemacr;, a = 11.219(5) Å, b = 12.131(6) Å, c = 13.208(7) Å, alpha = 80.34(5) degrees, beta = 87.41(4) degrees, gamma = 75.13(3) degrees, V = 1722.1(15) Å(3), Z = 2, R = 0.054, R(w) = 0.056. Crystal data for 4: triclinic, P&onemacr;, a = 10.455(3) Å, b = 10.637(5) Å, c = 11.024(3) Å, alpha = 90.52(4) degrees, beta = 112.62(3) degrees, gamma = 114.10(3) degrees, V = 1012.8(10) Å(3), Z = 1, R = 0.0453, R(w) = 0.0495.

Oxo Ligand Reactivity in the [ReO(2)L(4)](+) Complex of 1-Methylimidazole. Preparation and Crystal Structures of Salts Containing the ReOL(4)(3+) Core and Apical CH(3)O(-), BF(3)O(2)(-), and (CH(3)O)(2)PO(2)(-) Groups

The oxo group of [ReO(2)(1-MeIm)(4)](+) can be methylated with excess methyl trifluoromethanesulfonate in CH(2)Cl(2) under mild conditions, leading to [ReO(OCH(3))(1-MeIm)(4)](CF(3)SO(3))(2), which is converted to the B(C(6)H(5))(4)(-) and PF(6)(-) salts. When the reaction is carried out in methanol in the presence of excess PF(6)(-), the phosphate ester complex [ReO{OP(O)(OCH(3))(2)}(1-MeIm)(4)](PF(6))(2) is isolated. [ReO(2)(1-MeIm)(4)](+) in the presence of acid, 2,2-dimethoxypropane, and excess BF(4)(-) transforms to the oxo-boron adduct [ReO(OBF(3))(1-MeIm)(4)](+). The presence of the trans-O=Re-OR core was demonstrated by crystallographic studies on one compound of each type: [ReO(OCH(3))(1-MeIm)(4)](PF(6))(2), tetragonal, P4/ncc, Z = 4, a = 13.022(3) Å, c = 17.218(5) Å, R = 0.0271; [ReO{OP(O)(OCH(3))(2)}(1-MeIm)(4)](PF(6))(2).toluene, monoclinic, P2(1)/c, Z = 4, a = 14.165(4) Å, b = 13.052(6) Å, c = 20.931(6) Å, beta = 95.83(2) degrees, R = 0.0373; [ReO(OBF(3))(1-MeIm)(4)](I(3)), monoclinic, P2(1)/c, Z = 4, a = 13.373(4) Å, b = 13.237(3) Å, c = 16.689(6) Å, beta = 103.67(3) degrees, R = 0.0445. The IR spectra show nu(Re=O) vibrations near 960 cm(-)(1) in all cases. The solution UV-vis spectra of the CH(3)O(-) and BF(3)O(2)(-) compounds are similar to that of the parent oxo-hydroxo [ReO(OH)(1-MeIm)(4)](2+) species, whereas the visible spectrum of the blue phosphate ester compound resembles that of the oxo-aquo [ReO(OH(2))(1-MeIm)(4)](3+) ion. The compounds were also characterized by solution (1)H, (13)C, and (31)P NMR spectroscopy. This work confirms earlier conclusions that ReO(2)L(4)(+) units with good sigma-donor imidazole ligands are more resistant to ligand loss than the pyridine analogues.