Judith L. Eglin

Synthesis of multiply-bonded dichromium complexes with a variety of formamidinate ligands

Abstract A series of dichromium tetraformamidinate complexes with (ArNCHNAr)− where Ar is X2C6H3 or XC6H4 and X is the remote substituent 3,5-Cl2, 3,4-Cl2, p-Cl, p-CF3, m-CF3, p-OCH3 or m-OCH3 has been synthesized, and the structures of four of the compounds determined by X-ray crystallography. The Cr–Cr bond distances of the complexes structurally characterized are 1.9072(10) A [Cr2((p-ClC6H4N)2CH)4], 1.9162(10) A [Cr2((3,5-Cl2C6H3N)2CH)4], 1.9018((8) A [Cr2((m-CF3C6H4N)2CH)4 ], and 1.9178(11) A [Cr2((m-OCH3C6H4N)2CH)4]. 1H NMR spectroscopy and electrochemistry were performed on the series. The diamagnetic anisotropy of the Cr–Cr bond was calculated from X-ray crystallographic and 1H NMR data and correlates with the Hammett constant of the formamidine ligands.

Synthesis and structure of W2Cl4(μ-dppm)2(η2-μ-CH3CN)☆

Abstract A new ditungsten compound with a bridging ligand in a novel geometry, W2Cl4(μ-dppm)2(η2-μ-CH3CN) where dppm is bis(diphenylphosphino)methane, has been prepared from a quadruply bonded starting material. The structure of this compound is an example of a dinuclear tungsten compound with an η2-μ-bonded acetonitrile. The use of acetonitrile as a solvent during the preparation of the known compound W2Cl4(μ-dppm)2 results in the incorporation of an acetonitrile molecule in a bridging position with the C-N of this ligand perpendicular to the W-W axis. The bridging acetonitrile is no longer linear. The angle formed by the three-atom chain of acetonitrile, C-C-N, is 116.3(7)° with a C-N bond distance of 1.303(8) A. The W-W bond distance is 2.4981(10) A, significantly longer than the distance of 2.269(1) A observed in the parent compound W2Cl4(μ-dppm)2. Crystallographic data for this compound is as follows: P21/c with a=12.577(5), b=8.644(8), c=27.024(8) A, β=95.98(2)°, V=6302(4) A3 and Z=4.

Structural and spectroscopic characterization of the dirhenium acetamidate products resulting from the hydrolysis of acetonitrile

Abstract Unsaturated molecules such as nitriles are activated upon coordination to a dirhenium core. Acetonitrile is hydrolyzed in the presence of water to produce coordinated bridging acetamidate ligands as demonstrated by the reaction of [N(C 4 H 9 ) 4 ] 2 [Re 2 Cl 8 ] with CH 3 CN and water in ethanol to form the acetamidate complexes, [N(C 4 H 9 ) 4 ][Re 2 Cl 5 (μ-CH 3 C(O)NH)(μ-CH 3 C(OH)N)]·3CH 2 Cl 2 [ I ] and Re 2 Cl 4 (μ-CH 3 C(O)NH)(μ-dppm) 2 ·4 CH 2 Cl 2 /0.833 EtOH [ II ]. Compound II is formed when I is reduced upon coordination of bis(diphenylphosphino)methane (dppm). Compounds I and II were characterized using X-ray crystallography and a variety of other spectroscopic methods.

Nature of bonding in complexes containing "supershort" metal-metal bonds. raman and theoretical study of M2(dmp)4 [M = Cr (natural abundance Cr, 50Cr, and 54Cr) and Mo; dmp = 2,6-dimethoxyphenyl].

We report an investigation of complexes of the type M(2)(dmp)(4) (M = Mo, Cr; dmp = 2,6-dimethoxyphenyl) using resonance Raman (RR) spectroscopy, Cr isotopic substitution, and density functional theory (DFT) calculations. Assignment of the Mo-Mo stretching vibration in the Mo(2) species is straightforward, as evidenced by a single resonance-enhanced band at 424 cm(-1), consistent with an essentially unmixed metal-metal stretch, and overtones of this vibration. On the other hand, the Cr(2) congener has no obvious metal-metal stretching mode near 650-700 cm(-1), where empirical predictions based on the Cr-Cr distance as well as DFT calculations suggest that this vibration should appear if unmixed. Instead, three bands are observed at 345, 363, and 387 cm(-1) that (a) have relative RR intensities that are sensitive to the Raman excitation frequency, (b) exhibit overtones and combinations in the RR spectra, and (c) shift in frequency upon isotopic substitution ((50)Cr and (54)Cr). DFT calculations are used to model the vibrational data for the Mo(2) and Cr(2) systems. Both the DFT results and empirical predictions are in good agreement with experimental observations in the Mo(2) complex, but both, while mutually consistent, differ radically from experiment in the Cr(2) complex. Our experimental and theoretical results, especially the Cr isotope shifts, clearly demonstrate that the potential energy of the Cr-Cr stretching coordinate is distributed among several normal modes having both Cr-Cr and Cr-ligand character. The general significance of these results in interpreting spectroscopic observations in terms of the nature of metal-metal multiple bonding is discussed.

Synthesis, characterization, and X-ray crystallography of a series of ditungsten complexes with bis(diphenylphosphino)amine

The W 2 (II, II) and W 2 (III, III) compounds, W 2 (μ-O 2 CC 6 H 5 ) 2 Cl 2 (μ-dppa) 2 ( 1 ), W 2 (μ-O 2 CC 6 H 5 ) 2 Br 2 (μ-dppa) 2 ( 2 ), W 2 (μ-O 2 CC 6 H 5 ) 2 I 2 (μ-dppa) 2 ( 3 ), W 2 Cl 4 (μ-dppa) 2 ( 4 ), W 2 (μ-Cl) 2 Cl 4 (μ-dppa) 2 ( 5 ), and W 2 (μ-H) 2 Cl 4 (μ-dppa) 2 ( 6 ), have been synthesized using the chelating phosphine ligand dppa (bis(diphenylphosphino)amine). The series of metal–metal multiply bonded complexes has been characterized by NMR and UV–vis spectroscopy, and the structures of 2 ·(THF) 2 , 5 ·(THF) 4 , and 6 ·(THF) 2 determined by X-ray crystallography.

Crystallographic and Spectroscopic Characterization of Tetrakis(μ‐N,N′‐diarylformamidinato)dichlorodirhenium(III,III) Compounds

Through a variation in the aryl substituents of the formamidinate ligand, a variety of dirhenium compounds has been synthesized with [XArNC(H)NArX]− where Ar is a substituted C6H5 or C6H4 aryl ring and × is p-MeO (1), H (3), m-MeO (4), p-Cl (5), m-Cl (6), m-CF3 (7), p-CF3 (8), 3,4-Cl2 (9), and 3,5-Cl2 (10a, 10b). UV/Vis and NMR spectroscopy and electrochemical data for 1and 3–10 have been obtained. X-ray crystallographic analysis of Re2Cl2(μ-form)4 with four different diarylformamidinate ligands and one analog with two different interstitial solvents are presented; Re2Cl2[(p-MeOC6H4)NCHN(p-MeOC6H4)]4 (1), Re2Cl2[(m-MeOC6H4)-NCHN(m-MeOC6H4)]4 · 2 CH2Cl2 (4), Re2Cl2[(3,4-Cl2C6H3)NCHN(3,4-Cl2C6H3)]4 · 2 CH2Cl2 (9), Re2Cl2[(3,5-Cl2C6H3)NCHN(3,5-Cl2C6H3)]4 · 4 CH2Cl2 (10a), and Re2Cl2[(3,5-Cl2C6H3)NCHN(3,5-Cl2C6H3)]4 · OC4H8 (10b).

W2(μ-H)2Cl4(μ-dppm)2: the bulk synthesis of a bridging hydride

Abstract The synthesis of the ditungsten species W2(μ-H)2Cl4(μ-dppm)2 marks the first bulk preparation of a bridging hydride W(III–III) species. The compound retains the characteristic edge-sharing bioctahedral (ESBO) geometry with a W W bond distance of 2.3918(7)A. Although several Group VI ESBO complesxes have been prepared, only two with bridging hydride groups, W2(μ-H)2Cl4(μ-dppm)2 and W2(μ-H)2(μ-O2CC6H5)2 Cl2(P(C6H5)3)2, have been reported to date. The complex W2(μ-H)2Cl4(μ-dppm)2 has been characterized with31P1H,1H NMR, UV-vis, and IR spectroscopic techniques. A singlet is observed in the31P1H NMR spectra of W2(μ-H)2Cl4(μ-dppm)2 at δ = 20.464 ppm with JP−W = 126Hz and the IR spectra includes the tungsten-hydride symmetric stretch at 1602 cm−1.

The synthesis and characterization of α-W2Cl4(dppp)2

To date, few quadruply bonded ditungsten compounds have been synthesized. Only four bidentate phosphine ligands (LL) have been utilized in the synthesis of compounds of the general type W2Cl4(LL)2. These phosphine ligands include: bis(diphenylphosphino)methane (dppm) [1], 1,2-bis(diphenylphosphino)ethane (dppe) [2], 1,2-bis(dimethylphosphino)ethane (dmpe) [3] and 1,3-bis(diisopropylphosphino)propane (dippp) [4]. Of particular interest is the ability of these bidentate ligands to chelate to one metal center (α form) or bridge both metal centers (β form). Results from the synthesis and characterization of α-W2Cl4(dppp)2 (dppp = 1,3bis(diphenylphosphino)propane) are discussed. Crystallographic data for this compound are as follows: P1 (No. 2) with a = 11.934(9), b = 12.315(7), c = 12.513(4) A, α = 83.82(4), β = 75.03(5), γ = 62.01(5)°, V = 1569(2) A3 and Z = 1.

Tungsten to tungsten quadruple bonds: over 30 years of research, 50 structurally characterized compounds and 100 known compounds

Abstract Since the first reported structure of a quadruply bonded ditungsten complex in 1977, the field of W 2 (II, II) chemistry has expanded as new synthetic methodologies have been developed. Based on a recently developed synthetic route in our laboratory, the synthesis and structures of two new W 2 (II, II) complexes with substituted formamidinate ligands are reported. With the addition of these two new structures, the number of structurally characterized quadruply bonded W 2 (II, II) complexes exceeds 50 and over 100 W 2 (II, II) complexes have been synthesized. A compilation of the quadruply bonded W 2 (II, II) compounds, W–W bond distances, and synthetic methodologies is presented.

The evaluation of singlet-triplet separations for [W2(μ-H)(μ-Cl)Cl4(μ-dppm)2] and [W2(μ-H)2 (μ-O2CC6H5)2(P(C6H5)3)2] based on 31P NMR and crystallographic data

Abstract The singlet-triplet separations for the edge-sharing bioctahedral (ESBO) complex W2(μ-H)(μ-Cl)(Cl4(μ-dppm)2 · (THF)3 (II) has been studied by 31P NMR spectroscopy. The structural characterization of [W2(μ-H)2(μ-O2CC6H5)2Cl2(P(C6H5)3)2] (I) by single-crystal X-ray crystallography has allowed the comparison of the energy of the HOMOLUMO separation determined using the Fenske-Hall method for a series of ESBO complexes with two hydride bridging atoms, two chloride bridging atoms and the mixed case with a chloride and hydride bridging atom. The complex representing the mixed case, [W2(μ-H)(μ-Cl)Cl4(μ-dppm)2 · (THF)3] (II), has been synthesized and the value of −2J determined from variable-temperature 31P NMR spectroscopy.