Mixed Chloride/Amine Complexes of Dimolybdenum(II). 1. Preparation, Characterization, and Crystal Structure of Mo2Cl4(NHEt2)4: A Quadruply-Bonded Dimolybdenum Compound with Diethylamine Ligands

Diverse Post-Metathesis Reactivities of Mixed Amido-Isothiocyanato Molybdenum Complexes

Attempts to prepare mixed isothiocyanato-bis(imido) MoVI complexes led to the discovery of post-metathesis rearrangements toward three distinct products (1-3), which feature the NCS-derived chelators [N(NMe2)CS]2- (L1 in dinuclear 1 and 2) and [N(SiMe3)(NMe2)CS]- (L2 in mononuclear 3). Notably, the preparation of bidentate ligand L1 and its coordination chemistry are unprecedented. Together with computational studies, it is proposed that the putative "mono-substituted" intermediate [Mo(NtBu)2(NMe2)(NCS)] serves as the common starting point for the observed molecular transformations. Construction of the [Mo(NtBu)2(NCS)2] core was ultimately possible in the presence of additional stabilizing donors (THF or PMe3), which yielded the complexes [Mo(NtBu)2(NCS)2(THF)2] (4) and [Mo(NtBu)2(NCS)2(PMe3)2] (5).

Mixed chloride/amine complexes of dimolybdenum(II,II). 6. Stepwise substitution of amines by tertiary phosphines and vice versa: stereochemical hysteresis

The substitution reactions of primary amines by tertiary phosphines in quadruply bonded dimolybdenum(II,II) complexes Mo2Cl4(NH2R)4 have been studied. The exchange reaction has been shown to result at room temperature in disubstituted species Mo2Cl4(NH2R)2(PR3)2 (PR3 = PMe3, NH2R = NH2Prn (1a), NH2But (2a), NH2Cy (3a); PR3 = PMe2Ph, NH2R = NH2Cy (4a)), while heating is needed to obtain fully substituted complexes Mo2Cl4(PR3)4. The crystal structure of disubstituted products has been investigated by X-ray crystallography and revealed that they all belong to the alpha-isomer, having both phosphine groups at the same Mo atom. Crystal data are as follows: for 1a, tetragonal space group I4(1/a) with a = 17.737(2) A, c = 15.6915(6) A, and Z = 8; for 3a, monoclinic space group P2(1) with a = 10.963(3) A, b = 10.117(2) A, c = 13.323(4) A, beta = 90.05(2) degrees, and Z = 2; for 4a, triclinic space group P1 with a = 9.329(3) A, b = 10.206(2) A, c = 18.975(3) A, alpha = 85.45(2) degrees, beta = 87.10(1) degrees, gamma = 80.88(1) degrees, and Z = 2. The substitution processes for the direct and reverse reactions have been monitored by 31P NMR. They both proceed in a stepwise manner; however, a stereochemical hysteresis is taking place, i.e., the back reaction, the substitution of phosphines by amines, goes through another isomer of Mo2Cl4(NH2R)2(PR3)2, having phosphine ligands on different Mo atoms. This beta-isomer is more thermodynamically stable and can be obtained by thermal conversion of the alpha-form. All chemical equilibria studied in the paper have been explained as governed by a higher trans effect of PR3 groups compared to NH2R groups.

Mixed Chloride/Amine Complexes of Dimolybdenum(II,II). 4. Rotational Isomers of Mo(2)Cl(4)(R-py)(4) (R-py = 4-Picoline, 3,5-Lutidine, and 4-tert-Butylpyridine)

The quadruply bonded dimolybdenum complexes with alkyl-substituted pyridines of the formula Mo(2)Cl(4)(R-py)(4) (R-py = 4-pic (4-methylpyridine) (1), 3,5-lut (3,5-dimethylpyridine) (2), and 4-Bu(t)-py (4-tert-butylpyridine) (3)) have been prepared. Nine different compounds, in which there are 11 independent molecules, have been obtained in crystalline form, and their crystal structures have been investigated by X-ray diffraction. Three types of geometric isomers which differ by the angle of internal rotation about the Mo-Mo axis have been recognized in these structures. The eclipsed structures 1-3a display pyridine ligands opposite to each other across the metal-metal bond and have a virtual symmetry D(2)(h)(). The noncentrosymmetric D(2)(d)() structures 1-3b have each pyridine ligand opposite to a Cl atom. The molecules 3c-f have a partially staggered D(2) geometry with N-Mo-Mo-N torsion angles ranging from 10.4 degrees to 25.2 degrees. As a result of this work it is now clear that Mo(2)Cl(4)(R-py)(4) compounds do not show a preference for D(2)(h)() conformation rather than D(2)(d)() conformation. In fact, they seem unusually unrestricted in their rotational conformation, and packing forces appear to have a major influence on the conformation adopted. Further clarification of this question will require spectroscopic study of solutions.

Mixed Chloride/Amine Complexes of Dimolybdenum(II,II). 2. Reactions of Mo(2)Cl(4)(NHEt(2))(4) with Monodentate and Bidentate Phosphines. New Type of Compounds Mo(2)Cl(4)(diphosphine)(4)

The complex Mo(2)Cl(4)(NHEt(2))(4) (1) undergoes facile substitution reactions of the amine ligands by phosphines to give compounds with the same core structure, Mo(2)Cl(4)(phosphine)(4), where the phosphine is PMe(3) (2), PMe(2)Ph (3), PHEt(2) (4), dmpm (bis(dimethylphosphino)methane) (5), or dmpe (1,2-bis(dimethylphosphino)ethane) (6). Complexes of the type M(2)X(4)L(4) are well-known with monodentate ligands, but there is no previous crystallographically verified example of the M(2)X(4)L(4) type of compound in which L is a potentially bidentate ligand acting as a unidentate ligand. Mo(2)Cl(4)(eta(1)-dmpm)(4) (5) can be isolated in good yield as the kinetic product of the substitution reaction at room temperature. When a solution of 5 in THF is heated, transformation into the more stable compound Mo(2)Cl(4)(&mgr;-dmpm)(2) (7) takes place. Mo(2)Cl(4)(eta(1)-dmpe)(4) (6) is stable in solution in the presence of free phosphine and can be detected by NMR. Over time it converts to a polymeric material which precipitates from the solution. For dppm (bis(diphenylphosphino)methane) (8) and dppa (bis(diphenylphosphino)amine) (9), only products of the stoichiometry Mo(2)Cl(4)(diphosphine)(2) were obtained. The crystal structures of the complexes 4 and 5 have been investigated by X-ray diffraction. The crystallographic parameters for them are as follows: for 4, orthorhombic space group Pbcn with a = 7. 6015(8) Å, b = 20.120(8) Å, c = 19.070(6) Å, and Z = 4; for 5, hexagonal space group P6(4)22 with a = 12.396(1) Å, c = 21.960(2) Å, and Z = 3. Both structures have a Mo(2)Cl(4)P(4) core where the phosphorus atoms are trans one to another on each metal center. The Mo-Mo distances of 2.137(1) Å (4) and 2.137(1) Å (5) are consistent with quadruple bonding.