Mono‑, Di‑, and Triborylphosphine Analogues of Triarylphosphines

Diazaborinylphosphines based on the 1,8-diaminonaphthylboronamide heterocycle are prepared by a chlorosilane-elimination reaction, and their structural and bonding properties are compared to those of PPh₃. The precursor chloroborane ClB­{1,8-(NH)₂C₁₀H₆} (<b>I</b>) is fully characterized including its crystal structure, which features intermolecular π–π stacking, B···N interactions, and N–H···Cl hydrogen bonding. Treatment of <b>I</b> with Ph_3–<i>n</i>P­(SiMe₃)_<i>n</i> gave the corresponding Ph_3–<i>n</i>P­(B­{1,8-(NH)₂C₁₀H₆})_<i>n</i>, {<b>L</b>_<b>1</b> (<i>n</i> = 1), <b>L</b>_<b>2</b> (<i>n</i> = 2), and <b>L</b>_<b>3</b> (<i>n</i> = 3)}. The crystal structures of <b>L</b>_{<b>1</b>–<b>3</b>} reveal an increase in the planarity at P as a function of <i>n</i>, and the steric bulk of the diazaborinyl substituent B­{1,8-(NH)₂C₁₀H₆} is similar to that of a phenyl. Nucleus-independent chemical shift calculations were carried out that suggest that the 14 π-electron diazaborinyl substituent can be described as aromatic overall, though the BN₂-containing ring is slightly antiaromatic. The complexes <i>cis-</i>[Mo­(<b>L</b>_{<b>1</b>–<b>3</b>})₂(CO)₄] (<b>1</b>–<b>3</b>) are prepared from [Mo­(nbd)­(CO)₄] (nbd = norbornadiene) and <b>L</b>_{<b>1</b>–<b>3</b>}. From the position of the ν­(CO) (<i>A</i>₁) band in the IR spectra of <b>1</b>–<b>3</b>, it is deduced that the diazaborinyl substituent has a donating capacity similar to an alkyl group

Cobalt PCP Pincer Complexes via an Unexpected Sequence of Ortho Metalations

The cobalt PCP pincer complexes [Co­{2,6-(CH₂PPh₂-κ<i>P</i>)₂C₆H₃-κ<i>C</i>¹}­(L)₂], where L = PMe₃ (<b>1</b>), CO (<b>2</b>), have been prepared. Complex <b>1</b> is obtained by a transmetalation reaction between 1-lithio-2,6-bis­((diphenylphosphino)­methyl)­benzene and [CoCl­(PMe₃)₃]. Subsequent exposure of <b>1</b> to CO gave complex <b>2</b>. Complexes <b>1</b> and <b>2</b> can also be obtained from 1,3-bis­((diphenylphosphino)­methyl)­benzene and [CoMe­(PMe₃)₄]. Instead of ortho metalation occurring directly at the C2 (pincer) position of the diphosphine, ortho metalation first occurs at the C4 position to form [Co­{2-(CH₂PPh₂-κ<i>P</i>)-4-(CH₂PPh₂)-C₆H₃-κ<i>C</i>¹}­(PMe₃)₃] (<b>4</b>). After reflux of the reaction mixture for 24 h, a rearrangement of <b>4</b> occurs to give pincer complex <b>1</b> with loss of PMe₃ in ca. 50% yield; this rearrangement was accompanied by some decomposition. The mechanism for the conversion of <b>4</b> to <b>1</b> has been probed using 1-deuterio-2,6-bis­((diphenylphosphino)­methyl)­benzene. Unexpectedly, the labeled ligand led to 15% deuterium enrichment of an ortho CH of the terminal PPh₂ group in the product complex <b>1</b>, and the proposed mechanism for this rearrangement involves a four-membered cobaltacyclic intermediate

Cobalt PCP Pincer Complexes via an Unexpected Sequence of Ortho Metalations

The cobalt PCP pincer complexes [Co­{2,6-(CH₂PPh₂-κ<i>P</i>)₂C₆H₃-κ<i>C</i>¹}­(L)₂], where L = PMe₃ (<b>1</b>), CO (<b>2</b>), have been prepared. Complex <b>1</b> is obtained by a transmetalation reaction between 1-lithio-2,6-bis­((diphenylphosphino)­methyl)­benzene and [CoCl­(PMe₃)₃]. Subsequent exposure of <b>1</b> to CO gave complex <b>2</b>. Complexes <b>1</b> and <b>2</b> can also be obtained from 1,3-bis­((diphenylphosphino)­methyl)­benzene and [CoMe­(PMe₃)₄]. Instead of ortho metalation occurring directly at the C2 (pincer) position of the diphosphine, ortho metalation first occurs at the C4 position to form [Co­{2-(CH₂PPh₂-κ<i>P</i>)-4-(CH₂PPh₂)-C₆H₃-κ<i>C</i>¹}­(PMe₃)₃] (<b>4</b>). After reflux of the reaction mixture for 24 h, a rearrangement of <b>4</b> occurs to give pincer complex <b>1</b> with loss of PMe₃ in ca. 50% yield; this rearrangement was accompanied by some decomposition. The mechanism for the conversion of <b>4</b> to <b>1</b> has been probed using 1-deuterio-2,6-bis­((diphenylphosphino)­methyl)­benzene. Unexpectedly, the labeled ligand led to 15% deuterium enrichment of an ortho CH of the terminal PPh₂ group in the product complex <b>1</b>, and the proposed mechanism for this rearrangement involves a four-membered cobaltacyclic intermediate

Regioselective B-Cyclometalation of a Bulky <i>o-</i>Carboranyl Phosphine and the Unexpected Formation of a Dirhodium(II) Complex

The bulky carboranyl monophosphine <i>closo-</i>1,2-B₁₀H₁₀C­(H)­C­(P^tBu₂) (<b>L</b>) has been prepared in a one-pot procedure from <i>o</i>-carborane. The reaction of [PdCl₂(NCPh)₂] with <b>L</b> rapidly gave the binuclear B-cyclopalladate [Pd₂(μ-Cl)₂(κ²-<b>L</b>′)₂] (<b>L</b>′ = <b>L</b> deprotonated at B3) as a mixture of two diastereoisomers, assigned structures <b>1a</b> and <b>1a</b>′. The Cl bridges of <b>1a</b>/<b>1a</b>′ are cleaved by the addition of PEt₃ to give the mononuclear [PdCl­(κ²-<b>L</b>′)­(PEt₃)] (<b>2</b>) as a single isomer, with the P atoms mutually trans. The metalation occurs at boron positions 3 and 6 in the carborane cluster, and DFT calculations show that the 3,6-borometalate is lower in energy than the isomeric 4,5-borometalate and 2-carbometalate. Treatment of [Rh₂Cl₂(CO)₄] with <b>L</b> led to the slow precipitation of the dirhodium­(II) species [Rh₂(μ-Cl)₂(CO)₂(κ²-<b>L</b>′)₂] (<b>3</b>). The crystal structures of ligand <b>L</b> and complexes <b>1a</b>, <b>2</b>, and <b>3</b> have been determined

Unexpectedly High Barriers to M–P Rotation in Tertiary Phobane Complexes: PhobPR Behavior That Is Commensurate with ^tBu₂PR

The four isomers of 9-butylphosphabicyclo[3.3.1]­nonane, <i>s-</i>PhobPBu, where Bu = <i>n</i>-butyl, <i>sec</i>-butyl, isobutyl, <i>tert</i>-butyl, have been prepared. Seven isomers of 9-butylphosphabicyclo[4.2.1]­nonane (<i>a</i>₅<i>-</i>PhobPBu, where Bu = <i>n</i>-butyl, <i>sec</i>-butyl, isobutyl, <i>tert</i>-butyl; <i>a</i>₇<i>-</i>PhobPBu, where Bu = <i>n-</i>butyl, isobutyl, <i>tert</i>-butyl) have been identified in solution; isomerically pure <i>a</i>₅<i>-</i>PhobPBu and <i>a</i>₇<i>-</i>PhobPBu, where Bu = <i>n</i>-butyl, isobutyl, have been isolated. The σ-donor properties of the PhobPBu ligands have been compared using the <i>J</i>_PSe values for the PhobP­(Se)­Bu derivatives. The following complexes have been prepared: <i>trans-</i>[PtCl₂(<i>s-</i>PhobPR)₂] (R = ⁿBu (<b>1a</b>), ⁱBu (<b>1b</b>), ^sBu (<b>1c</b>), ^tBu (<b>1d</b>)); <i>trans-</i>[PtCl₂(<i>a</i>₅<i>-</i>PhobPR)₂] (R = ⁿBu (<b>2a</b>), ⁱBu (<b>2b</b>)); <i>trans-</i>[PtCl₂(<i>a</i>₇<i>-</i>PhobPR)₂] (R = ⁿBu (<b>3a</b>), ⁱBu (<b>3b</b>)); <i>trans-</i>[PdCl₂(<i>s-</i>PhobPR)₂] (R = ⁿBu (<b>4a</b>), ⁱBu (<b>4b</b>)); <i>trans-</i>[PdCl₂(<i>a</i>₅<i>-</i>PhobPR)₂] (R = ⁿBu (<b>5a</b>), ⁱBu (<b>5b</b>)); <i>trans-</i>[PdCl₂(<i>a</i>₇<i>-</i>PhobPR)₂] (R = ⁿBu (<b>6a</b>), ⁱBu (<b>6b</b>)). The crystal structures of <b>1a</b>–<b>4a</b> and <b>1b</b>–<b>6b</b> have been determined, and of the ten structures, eight show an anti conformation with respect to the position of the ligand R groups and two show a syn conformation. Solution variable-temperature ³¹P NMR studies reveal that all of the Pt and Pd complexes are fluxional on the NMR time scale. In each case, two species are present (assigned to be the syn and anti conformers) which interconvert with kinetic barriers in the range 9 to >19 kcal mol^–1. The observed trend is that, the greater the bulk, the higher the barrier. The magnitudes of the barriers to M–P bond rotation for the PhobPR complexes are of the same order as those previously reported for ^tBu₂PR complexes. Rotational profiles have been calculated for the model anionic complexes [PhobPR-PdCl₃]⁻ using DFT, and these faithfully reproduce the trends seen in the NMR studies of <i>trans-</i>[MCl₂(PhobPR)₂]. Rotational profiles have also been calculated for [^tBu₂PR-PdCl₃]⁻, and these show that the greater the bulk of the R group, the lower the rotational barrier: i.e., the opposite of the trend for [PhobPR-PdCl₃]⁻. Calculated structures for the species at the maxima and minima in the M–P rotation energy curves indicate the origin of the restricted rotation. In the case of the PhobPR complexes, it is the rigidity of the bicycle that enforces unfavorable H···Cl clashes involving the Pd–Cl groups with H atoms on the α- or β-carbon in the R substituent and H atoms in 1,3-axial sites within the phosphabicycle