Thermodynamic, Kinetic, Structural, and Computational Studies of the Ph3Sn-H, Ph3Sn-SnPh3, and Ph3Sn-Cr(CO)3C5Me5 Bond Dissociation Enthalpies

Article

pubs.acs.org/IC

Thermodynamic, Kinetic, Structural, and Computational Studies of

the Ph₃Sn−H, Ph₃Sn−SnPh₃, and Ph₃Sn−Cr(CO)₃C₅Me₅Bond

Dissociation Enthalpies

Xiaochen Cai,^†Subhojit Majumdar,^†George C. Fortman,^†Anjaneyulu Koppaka,^†Leonardo Seraﬁm,^†

Burjor Captain,*^,†Manuel Temprado,*^,‡and Carl D. Hoﬀ*^,†

^†Department of Chemistry, University of Miami, 1301 Memorial Drive, Coral Gables, Florida 33124, United States

^‡Department of Analytical Chemistry, Physical Chemistry and Chemical Engineering, Universidad de Alcala,

́

Ctra. Madrid-Barcelona

Km. 33,600, Madrid 28871, Spain

S

* Supporting Information

ABSTRACT: The kinetics of the reaction of Ph₃SnH with excess •Cr(CO)₃C₅Me₅

= •Cr, producing HCr and Ph₃Sn−Cr, was studied in toluene solution under 2−3

atm CO pressure in the temperature range of 17−43.5 °C. It was found to obey the

rate equation d[Ph₃Sn−Cr]/dt = k[Ph₃SnH][•Cr] and exhibit a normal kinetic

isotope eﬀect (k_H/k_D= 1.12 0.04). Variable-temperature studies yielded ΔH^‡=

15.7 1.5 kcal/mol and ΔS^‡= −11 5 cal/(mol·K) for the reaction. These data are

interpreted in terms of a two-step mechanism involving a thermodynamically uphill

hydrogen atom transfer (HAT) producing Ph₃Sn• and HCr, followed by rapid

trapping of Ph₃Sn• by excess •Cr to produce Ph₃Sn−Cr. Assuming an overbarrier of

2

1 kcal/mol in the HAT step leads to a derived value of 76.0 3.0 kcal/mol for

the Ph₃Sn−H bond dissociation enthalpy (BDE) in toluene solution. The reaction

enthalpy of Ph₃SnH with excess •Cr was measured by reaction calorimetry in toluene

solution, and a value of the Sn−Cr BDE in Ph₃Sn-Cr of 50.4 3.5 kcal/mol was

derived. Qualitative studies of the reactions of other R₃SnH compounds with •Cr are

described for R = ⁿBu, ^tBu, and Cy. The dehydrogenation reaction of 2Ph₃SnH → H₂+ Ph₃SnSnPh₃was found to be rapid and

quantitative in the presence of catalytic amounts of the complex Pd(IPr)(P(p-tolyl)₃). The thermochemistry of this process was

also studied in toluene solution using varying amounts of the Pd(0) catalyst. The value of ΔH = −15.8 2.2 kcal/mol yields a

value of the Sn−Sn BDE in Ph₃SnSnPh₃of 63.8 3.7 kcal/mol. Computational studies of the Sn−H, Sn−Sn, and Sn−Cr BDEs

are in good agreement with experimental data and provide additional insight into factors controlling reactivity in these systems.

The structures of Ph₃Sn−Cr and Cy₃Sn−Cr were determined by X-ray crystallography and are reported. Mechanistic aspects of

oxidative addition reactions in this system are discussed.

appear to have been reported for tin, it was shown for lead

compounds¹⁰that the rotating-bomb method was critical to

obtaining reliable results. However, Davies, Pope, and Skinner¹¹

found that combustions performed at higher than usual

pressures gave satisfactory results in static-bomb calorimetry

for organic tin compounds.¹²There are a limited number of tin

compounds for which independent measurements of enthalpies

of formation have been reported to check the consistency of the

values. In most of the cases, reasonable agreement between the

diﬀerent values obtained by independent means is observed.

However, in several cases such as the enthalpy of formation of

Ph₃SnSnPh₃, a marked lack of agreement in the results of

almost 25 kcal/mol was found.^13,14In addition to the

combustion experiments, photoacoustic calorimetric,^15,16

threshold photoelectron−photoion coincidence,¹⁷appearance

potential,^{11,16,18−20}gas-phase acidity,²¹kinetics,²²and variable-

INTRODUCTION

■

Recently in individual¹⁻⁴and collaborative⁵work, we reported

investigations of use of the sterically demanding triorganotin

t

hydride Bu₃SnH with transition metal complexes capable of

small molecule activation. There is a rich history of synthesis of

bulky tin ligands in the pioneering work of Neumann,⁶

Lappert,⁷Davies,⁸and others. In comparison, there is a dearth

of thermochemical data on organotin complexes due to the

experimental challenges innate in their study. Experimental

enthalpies of formation of organotin compounds are scarce, and

the existing values in the literature were mainly determined

before 1970. There are several compilations and revisions of

these data, since newer and more reliable data used to derive

the reported values are now available.⁹However, some of the

articles do not give enough experimental information to allow

recalculating the values, and some data were never published in

the primary literature. The enthalpies of formation of tin

compounds have been determined mainly by static-bomb

calorimetry. While no rotating-bomb calorimetric studies

Received: August 15, 2016

A

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Article

temperature electron spin resonance (ESR)²³studies were

performed to derive Sn−H, Sn−Sn, and Sn−M (M = transition

metal) bond dissociation enthalpies (BDE).

(CO)₃C₅Me₅indicated that the best conditions for kinetic

study would involve reaction under CO pressures of 2−3 atm

of Ph₃SnH in the presence of a large pseudo-ﬁrst-order excess

of •Cr(CO)₃C₅Me₅.

In spite of the wide use of stannanes as reducing agents in

organic reactions, there are only sparse reports of experimental

determination²⁴of the Sn−H BDE in R₃SnH compounds.

Comprehensive theoretical studies of organostannane thermo-

chemistry were recently reported by Boyd and co-workers²⁵

who tested 14 diﬀerent levels of theory including high-level

CCSD(T) calculations²⁶in computation of the Sn−H BDE in

Me₃SnH. Eight of the 14 results agreed to within 1 kcal/mol or

better with the experimental value of 76 kcal/mol reported by

Drozdova and Denisov.²²

In this paper, we report our initial eﬀorts at mapping the

solution thermochemistry of organotin compounds. This stems

from our interest in oxidative addition reactions of bulky

stannane ligands and the strength of the Pt−Sn bond in L_nPt−

SnR₃complexes. Prior to initiating those investigations, it was

deemed necessary to generate data in toluene solution to allow

comparison of these BDE values to existing gas-phase and

computational data.

The second method utilized for entry into the organotin

solution thermochemistry is based on attempts to develop a

direct method to measure the enthalpy of the well-known

spontaneous dehydrogenation of tin hydrides as shown in eq 3.

2R₃SnH → H₂+ R₃Sn−SnR₃

(3)

Dehydrogenation of tin hydrides such as shown in reaction 3

is normally something to be avoided. It is well-known that

solutions of tin hydrides lose their activity due to slow

formation of distannanes. As cited in the classic book of

Neumann,⁶reaction 3 can be catalyzed by several agents

including PtCl₂. In this work, we deliberately sought active

catalysts capable of accelerating reaction 3 for the purpose of

calorimetric measurement of it under diﬀerent loads of catalyst

to give an accurate and reliable direct value for its reaction

enthalpy. This presents a novel approach where a transition

metal catalyst is used to obtain valid thermochemical data for a

main-group organometallic reaction.

A complementary approach is utilization of high-level

computational techniques. The value of calculations to

thermochemistry in the hitherto primarily experimental

NIPER group was realized as early as in the 1990s.³⁰The

blending of experiment and theory is now standard in physical

organometallic chemistry. In the current work, agreement

between computed and experimental data gives needed

conﬁdence to both approaches.

Two experimental approaches are taken in this work. The

ﬁrst is based on studies of hydrogen atom transfer (HAT) from

R₃SnH to the 17 e⁻radical complex •Cr(CO)₃C₅Me₅. Baird

and co-workers²⁷had reported earlier that the dimeric complex

[Cr(CO)₃C₅H₅]₂reacts relatively rapidly with ⁿBu₃SnH to

yield 1 mol of HCr(CO)₃C₅H₅and 1 mol of ⁿBu₃Sn−

Cr(CO)₃C₅H₅, but that HCr(CO)₃C₅H₅may also undergo

further reaction with excess stannane. This reaction was

believed to occur by attack of the •Cr(CO)₃C₅H₅radical

formed by dissociation of [Cr(CO)₃C₅H₅]₂. Subsequent

synthetic and reactivity studies of a host of reagents with the

•Cr(CO)₃C₅Me₅radical also included spectroscopic data in

EXPERIMENTAL SECTION

■

Unless stated otherwise, all operations were performed in a Vacuum

Atmospheres or MBraun drybox under an atmosphere of puriﬁed

nitrogen or argon. Toluene was dried and deoxygenated by distillation

from sodium benzophenone ketyl under argon atmosphere.

Spectroscopic-grade anhydrous heptane was degassed prior to use.

Anhydrous benzene-d₆and toluene-d₈were purchased from Aldrich

and degassed by evacuation and reﬁlling with argon and stored over 4

Å molecular sieves in the glovebox prior to use. Triphenyl tin hydride

was obtained from Aldrich and stored in the glovebox at an ambient

temperature of ∼20 °C. A small amount of supernatant liquid,

presumably an impure eutectic mixture, was found above the pure

solid (as determined by NMR), and this was decanted. Triphenyl tin

n

keeping with oxidative addition of R₃SnH (R = Bu) by 2 mol

of •Cr(CO)₃C₅Me₅as shown in reaction 1.

n

2•Cr(CO)₃C₅Me₅+ Bu₃SnH

n

→ Bu₃Sn−Cr(CO)₃C₅Me₅+ HCr(CO)₃C₅Me₅

(1)

n

The formed complex Bu₃Sn−Cr(CO)₃C₅Me₅was report-

edly too labile to isolate, but spectroscopic and other relevant

synthetic studies performed by Baird and co-workers conﬁrmed

the product assignments. Provided appropriate conditions

could be found, study of reaction 1 in principle provides a

convenient entry into thermochemical study of stannanes, since

the value of the Cr−H BDE in HCr(CO)₃C₅Me₅is known²⁸

(62.3 1.0 kcal/mol). In addition, if conditions could be found

where the rate-determining step in reaction 1 corresponded to

the thermodynamically uphill HAT reaction shown in eq 2,

then information regarding the Sn−H BDE could be derived

from kinetic data.

deuteride was prepared from Ph₃SnCl by use of LiAlD₄in place of

31

LiAlH₄and recrystallized from a concentrated heptane solution at

̀

3

t

−20 °C. Preparation of Bu₃SnH was done as described earlier.

ⁿBu₃SnH and Cy₃SnH were obtained from Aldrich and Strem,

respectively, and used without further puriﬁcation. FTIR spectra were

obtained using a PerkinElmer Spectrum 400 FTIR Spectrometer.

Calorimetric measurements were made using a Setaram C-80 Calvet

microcalorimeter.

Qualitative Study of Reactions of •Cr(CO)₃C₅Me₅and

Stannanes. Qualitative studies were performed using techniques

similar to those described in a later section. The presence of

•Cr(CO)₃C₅Me₅with known FTIR bands at 1994 and 1890 cm⁻¹in

toluene solution as well as the formation of HCr(CO)₃C₅Me₅with

bands at 1995 and 1912 cm⁻¹were readily identiﬁed, and iterative

computer subtraction was used to identify bands due to the R₃Sn−

Cr(CO)₃C₅Me₅products formed. Doubt regarding product identity

based on the in situ FTIR data is small due to structural determination

by single-crystal X-ray diﬀraction analysis of two of the Cr−Sn

products. In toluene solution, the complexes R₃Sn−Cr(CO)₃C₅Me₅

•Cr(CO)₃C₅Me₅+ R₃SnH → HCr(CO)₃C₅Me₅+ R₃Sn•

(2)

The overbarrier for related radical reactions is very low and

on the order of 2 1 kcal/mol as shown by Franz and co-

workers.²⁹

The earlier observation of Baird and co-workers regarding

the presence of side reactions and the lability of the products

formed indicated that a search for suitable reaction conditions

to allow kinetic study would have to be made, since the critical

part of any kinetic study is establishment of the mechanism.

Investigation of reactions of several stannanes with •Cr-

n

had bands in the FTIR spectrum at 1959 and 1872 cm⁻¹(R = Bu);

1955 and 1872 cm⁻¹(R = Cy); 1957 and 1887 cm⁻¹(R = ^tBu); 1970

and 1889 cm⁻¹(R = Ph). The presence of the triply bonded CrCr

B

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Article

complex [Cr(CO)₂C₅Me₅]₂was detected by its characteristic bands at

1850 and 1870 cm⁻¹. This complex was at times present in stock

solutions of •Cr(CO)₃C₅Me₅that had been stored overnight and did

not interfere with kinetics studies. In some cases, as discussed in the

results section, small amounts of [Cr(CO)₂C₅Me₅]₂were formed in

reactions performed under Ar atmosphere but not under CO. All

qualitative studies of stannanes and distannanes were performed using

standard Schlenk tube techniques and are discussed speciﬁcally in the

results section.

[Cr(CO)₃C₅Me₅]₂present in solutions of •Cr(CO)₃C₅Me₅and

which occurs due to dilution eﬀects during the reaction as

•Cr(CO)₃C₅Me₅is consumed and some [Cr(CO)₃C₅Me₅]₂disso-

ciates in keeping with previously reported equilibrium data.³³The ﬁnal

accepted value is −36.7

1.8 kcal/mol and corresponds to the

reaction enthalpy if [Cr(CO)₃C₅Me₅]₂was completely dissociated to

•Cr(CO)₃C₅Me₅under calorimetric condition measurements.

Calorimetric Measurement of Catalytic Dehydrogenation of

Ph₃SnH by Pd(IPr)(P(p-tolyl)₃). In the glovebox, a solution of

crystalline Ph₃SnH (0.1975 g) was prepared in 2 mL of C₆D₆, and this

solution was loaded into one chamber of a slightly modiﬁed Setaram

Calvet calorimeter mixing cell. A second solution containing between

1 and 5 mol % of Pd(IPr)(P(p-tolyl)₃) was dissolved in 1 mL of C₆D₆

and added to the second mixing chamber. The calorimeter cell was

sealed, taken from the glovebox, and loaded into the calorimeter.

Following temperature equilibration, the reaction was initiated, and

the calorimeter was rotated to achieve mixing. Following return to

baseline the calorimeter cell was taken into the glovebox. An aliquot

was taken of the calorimetric solution and analyzed by NMR

spectroscopy. Conversion of Ph₃SnH to Ph₃SnSnPh₃was found to

be quantitative within 1%. Use of one-half or twice the amount of

catalytic Pd(IPr)(P(p-tolyl)₃) showed no apparent change in enthalpy

of reaction leading to the conclusion that the dominant heat eﬀect

measured corresponded to dehydrogenation of Ph₃SnH with negligible

contributions from changes in Pd(IPr)(P(p-tolyl)₃). The accepted

Qualitative Study of Catalytic Dehydrogenation of Ph₃SnH

and Cy₃SnH. Elimination of H₂from R₃SnH was investigated by

reaction of a mixture of ∼0.2 g of R₃SnH in 5 mL of C₆D₆to which

∼0.006 g of catalyst was added. The complex Pd(IPr)(P(p-tolyl)₃)³²

was found to be an eﬃcient catalyst for dehydrogenation of both

5

Ph₃SnH and Cy₃SnH. The complex [(μ-H)Pt(I^tBu)(Sn^tBu₃)]₂was

found to show only limited activity.

Quantitative Kinetic Study of Reaction of •Cr(CO)₃C₅Me₅

and R₃SnH. Preliminary qualitative studies had shown that reactions

run under argon were faster than those under CO. Reactions run at an

overpressure of 10, 20, and 30 psi of CO showed that at temperatures

lower than 45 °C there was a decrease in rate between 10 and 20 psi

but no signiﬁcant decrease in rate between 20 and 30 psi overpressure

of CO. Therefore, data were typically collected at 20−25 psi CO

pressure in a thermostated glass reactor ﬁtted with thick-walled Teﬂon

tubing lines used to transfer 1−3 mL of solution to a high-pressure

Fourier transform infrared (FTIR) cell allowing a fresh sample to be

studied for each data point. The procedure described below for

studying the rate of reaction of Ph₃SnH and Ph₃SnD is representative

of all studies.

In the glovebox, a stock solution of •Cr(CO)₃C₅Me₅was prepared

by dissolving 4.6773 g of [Cr(CO)₃C₅Me₅]₂in 560 mL of toluene and

was ﬁltered in the glovebox into a separate Schlenk ﬂask. In separate

vials, stock solutions of Ph₃SnH (0.0425 g) and Ph₃SnD (0.0474 g)

were each prepared in 2 mL of toluene. A 33 mL sample of the

•Cr(CO)₃C₅Me₅solution was loaded into a syringe, taken from the

glovebox, and loaded under an atmosphere of 22 psi CO into the glass

reactor thermostated at 38.5 °C. Approximately 20 min were allowed

for temperature equilibration. A 3 mL aliquot of the stock solution of

the Cr radical was taken to run an FTIR spectrum prior to initiation of

the reaction. A 0.500 mL sample of the Ph₃SnH solution was loaded

into a Hamilton gas-tight syringe ﬁtted with a valve. Reaction was

initiated by injection of the Ph₃SnH solution followed by periodic

sampling of the reaction progress by removing a 2−3 mL aliquot to

the FTIR cell and recording the spectrum. A total of six runs were

studied using three samples of Ph₃SnH and three samples of Ph₃SnD

and alternating the order in which the samples were run. The reactions

were run under pseudo-ﬁrst-order conditions of excess •Cr-

(CO)₃C₅Me₅and were found to be ﬁrst order in Ph₃SnH(D) through

at least three half-lives. Reactions run at diﬀering absolute •Cr-

(CO)₃C₅Me₅concentration showed that the reaction was ﬁrst order in

this reagent as well. Reported experimental data are typically the

average of at least three separate runs at each temperature.

Representative spectroscopic data and kinetic plots and an Eyring

plot are found in the results section.

value of −15.8

measurements.

2.2 kcal/mol is the average of six separate

Growth of Crystals of Ph₃Sn−Cr(CO)₃C₅Me₅. In the glovebox, a

solution was made in ∼15 mL of heptane of 0.0284 g of

[Cr(CO)₃C₅Me₅]₂and 0.0368 g of Ph₃SnH. The solution was kept

in a sealed vial for over a week in the glovebox and in the dark. During

that time yellow crystals grew, and these were separated, covered with

mineral oil, and taken for crystallographic study.

Growth of Crystals of Cy₃Sn−Cr(CO)₃C₅Me₅. In the glovebox, a

solution was made in 5 mL of toluene of 0.0453 g of [Cr-

(CO)₃C₅Me₅]₂and 0.0587 g of Cy₃SnH. The solution was kept in a

sealed vial for over a week in the glovebox and in the dark. During that

time yellow crystals grew, and these were separated, covered with

mineral oil, and taken for crystallographic study.

Crystallographic Analyses. Each data crystal was glued onto the

end of a thin glass ﬁber. X-ray intensity data were measured by using a

Bruker SMART APEX2 CCD-based diﬀractometer using Mo Kα

radiation (λ = 0.710 73 Å).³⁴The raw data frames were integrated with

the SAINT+ program by using a narrow-frame integration algorithm.³⁴

Corrections for Lorentz and polarization eﬀects were also applied with

SAINT+. An empirical absorption correction based on the multiple

measurement of equivalent reﬂections was applied using the program

SADABS. All structures were solved by a combination of direct

methods and diﬀerence Fourier syntheses and were reﬁned by full-

matrix least-squares on F²by using the SHELXTL software package.³⁵

All non-hydrogen atoms were reﬁned with anisotropic displacement

parameters. Hydrogen atoms were placed in geometrically idealized

positions and included as standard riding atoms during the least-

squares reﬁnements. Crystal data, data collection parameters, and

results of the analyses are listed in Table 1.

Yellow single crystals of Ph₃Sn−Cr(CO)₃C₅Me₅suitable for X-ray

diﬀraction analyses were obtained by reaction in heptane solvent as

described previously and crystallized in the monoclinic crystal system.

The systematic absences in the intensity data were consistent with

either of the space groups Cc or C2/c, the latter of which was

conﬁrmed by the successful solution and reﬁnement of the structure.

With Z = 16, there are two formula equivalents of the molecule

present in the asymmetric crystal unit. Yellow single crystals of

Cy₃Sn−Cr(CO)₃C₅Me₅suitable for X-ray diﬀraction analyses were

obtained by reaction in heptane solvent as described previously and

crystallized in the monoclinic crystal system. The systematic absences

in the intensity data were consistent with the unique space group

P2₁/c.

Calorimetric Measurement of Reaction of •Cr(CO)₃C₅Me₅

and Ph₃SnH. In the glovebox, a solution of crystalline Ph₃SnH

(0.0866 g) was prepared in 2 mL of C₆D₆, and and aliquot of 0.250

mL of this solution was loaded into one chamber of a slightly modiﬁed

Setaram Calvet calorimeter mixing cell. A second solution containing

0.0449 g of [Cr(CO)₃C₅Me₅]₂was dissolved in 6 mL of C₆D₆.

Approximately 5.0 mL of this solution was ﬁltered through a syringe

ﬁlter into a second mixing chamber. The calorimeter cell was sealed,

taken from the glovebox, and loaded into the Setaram C-80

calorimeter. Following temperature equilibration, the reaction was

initiated, and the calorimeter was rotated to achieve mixing. Following

return to baseline the calorimeter cell was taken into the glovebox.

Aliquots were taken of the calorimetric solution and analyzed by FTIR

and NMR spectroscopy. The measured value of −34.3 1.1 kcal/mol

is adjusted to be more exothermic by −2.3

0.7 kcal/mol to

Computational Details. Electronic structure calculations were

compensate for endothermic dissociation of small amounts of

performed using the B3LYP density functional³⁶with the D3(BJ)

C

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Inorganic Chemistry

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Table 1. Crystallographic Data for R₃Sn−Cr(CO)₃C₅Me₅(R

a

= Ph, Cy) Compounds

Ph₃Sn−

Cy₃Sn−

Cr(CO)₃C₅Me₅

empirical formula

formula weight

crystal system

lattice parameters

a (Å)

CrSnO₃C₃₁H₃₀

621.24

CrSnO₃C₃₁H₄₈

639.38

monoclinic

36.9426(19)

8.7211(5)

37.1627(19)

90

18.1471(8)

17.7577(8)

9.7232(4)

90

b (Å)

c (Å)

α (deg)

β (deg)

109.098(1)

90

95.814(1)

90

γ (deg)

V (Å³)

11 314.1(10)

C2/c (No. 15)

16

3117.2(2)

P2₁/c (No. 14)

4

space group

Z value

Figure 1. An ORTEP of the molecular structure of Ph₃Sn−

Cr(CO)₃C₅Me₅showing 40% probability thermal ellipsoids. Cr1−

Sn1 = 2.7393(4) Å; Cr2−Sn2 = 2.7294(4) Å (molecule 2).

ρ_calc(g/cm³)

μ (Mo Kα) (mm⁻¹

temperature (K)

2Θ_max(deg)

1.459

1.362

)

1.296

1.177

296(2)

59.00

No. obs. (I > 2σ(I))

12268

6250

No. parameters

659

330

goodness of ﬁt

1.035

1.062

max. shift in cycle

0.004

0.003

a

residuals : R1; wR2

0.0332; 0.0693

0.0454; 0.1177

absorption correction, max/min multiscan

0.8813/0.5635

multiscan

0.9768/0.6377

largest peak in ﬁnal diﬀ. map

0.516

1.621

(e⁻/ Å³)

a

R1 = ∑_hkl(||F_obs| − |F_calc||)/∑_hkl|F_obs|; wR2 = [∑_hklw(|F_obs| − |F_calc|)²/

2

∑_hklwF_obs

] −

^1/2, w = 1/σ²(F_obs); GOF = [∑_hklw(|F_obs| − |F_calc|)²/(n_data

n_vari)]^1/2

.

empirical dispersion correction³⁷along with the Def2-SV(P)³⁸basis

set. Geometry optimizations were performed without any symmetry

restrictions, and all stationary points were optimized by computing

analytical energy gradients. The obtained minima were characterized

by performing energy second derivatives, conﬁrming them as minima

by the absence of negative eigenvalues of the Hessian matrix of the

energy. To further reﬁne the energies obtained from the B3LYP-

D3(BJ)/Def2-SV(P) calculations, single-point calculations were

performed using the larger Def2-TZVP³⁸basis set. In addition,

analogous single-point calculations were also performed with the PBE0

density functional.³⁹To test the inﬂuence of the solvation eﬀects in the

BDEs calculated, single-point calculations in toluene solution using the

IEF-PCM model⁴⁰on the previously optimized gas-phase structures

were ﬁnally performed for selected species. Computed electronic

energies were corrected for zero-point energy and thermal energy to

determine ΔH⁰(298 K) values. The X-H BDE were calculated

computing the enthalpy of reaction with molecular H₂for the

corresponding radicals using the H−H BDE in H₂of 104.2 kcal/mol⁴¹

analogously to the procedure described by Lee and Holm⁴²to derive

X-O BDE values. All calculations were performed with the Gaussian 09

suite of programs.⁴³

Figure 2. An ORTEP of the molecular structure of Cy₃Sn−

Cr(CO)₃C₅Me₅showing 40% probability thermal ellipsoids. Cr1−

Sn1 = 2.8158(6) Å.

would appear to imply a stronger bond for the aryl stannane

derivative. The most closely related literature structure the

authors could ﬁnd was that of Cl₂Sn[Cr(CO)₃C₅H₅]₂, which

has two Sn−Cr bond lengths (2.695 and 2.701 Å)⁴⁴that are

similar to the value measured here for Ph₃Sn−Cr(CO)₃C₅Me₅.

The ν_CObands at 1955 and 1872 cm⁻¹for Cy₃Sn−

Cr(CO)₃C₅Me₅and at 1970 and 1889 cm⁻¹for Ph₃Sn−

Cr(CO)₃C₅Me₅are in keeping with the more electron-donating

power of the Cy group compared to Ph.

Reaction of R₃SnH and •Cr(CO)₃C₅Me₅. Production of

the Sn−Cr products can occur from the reaction of the

stannane with either •Cr(CO)₃C₅Me₅as shown in reaction 4

or with HCr(CO)₃C₅Me₅as shown in reaction 5. It can also

exist as an admixture of reaction 4 + reaction 5 as shown in eqs

6 (α = 1) and 7 (0 ≤ α ≥ 1).

RESULTS

■

X-ray Structures of R₃Sn−Cr(CO)₃C₅Me₅(R = Ph, Cy).

Pure R₃Sn−Cr(CO)₃C₅Me₅complexes were prepared by slow

reaction of •Cr(CO)₃C₅Me₅in the presence of excess R₃SnH.

X-ray quality crystals were produced in this way for Ph₃Sn−

Cr(CO)₃C₅Me₅and Cy₃Sn−Cr(CO)₃C₅Me₅, and their struc-

tures are shown in Figures 1 and 2.

2•Cr(CO)₃C₅Me₅+ R₃SnH

The Sn−Cr bond distance is longer for the Cy₃Sn (2.816 Å)

derivative compared to the Ph₃Sn complex (2.734 Å), which

→ R₃Sn−Cr(CO)₃C₅Me₅+ HCr(CO)₃C₅Me₅

(4)

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due to Cy₃Sn−Cr(CO)₃C₅Me₅continue to grow, and the

bands due to HCr(CO)₃C₅Me₅(1995 and 1912 cm⁻¹) shrink,

as does the band near 1784 cm⁻¹due to Cy₃SnH. Note that the

extinction coeﬃcient of ν_Sn−His much lower than the carbonyl

bands, and the stannane was present in 10-fold molar excess.

Note also that the inset to Figure 3 shows that near 1906 cm⁻¹

there is an unstable isosbestic point, which is held through

∼30% reaction. Following that it shifts away. This is because at

ﬁrst it is predominantly reaction 4, and then it is a changing

mixture of reactions 4 and 5 that occurs (eq 7).

HCr(CO)₃C₅Me₅+ R₃SnH

→ R₃Sn−Cr(CO)₃C₅Me₅+ H₂

(5)

(6)

(7)

2•Cr(CO)₃C₅Me₅+ 2R₃SnH

→ H₂+ 2R₃Sn−Cr(CO)₃C₅Me₅

2•Cr + (1 + α)R₃SnH

→ (1 + α)R₃Sn−Cr + (α)H₂+ (1 − α)HCr

The data in Figure 3 are plotted as a function of time in

Figure 4 for the absorption at 1955 (Sn−Cr complex) and 1915

Reactions 4 and 5 refer only to the source of the Sn−Cr

product being either the chromium radical (eq 4) or the

corresponding hydride (eq 5) and not the mechanism, which

may be more complex and can also vary as a function of

reaction conditions (see below). In the absence of added

additional chromium hydride, at the start of the reaction, the

initial [HCr(CO)₃C₅Me₅] = 0, and reaction 5 depends upon its

generation by reaction 4. At the start of the reaction, reaction 4

predominates, and the concentration of HCr(CO)₃C₅Me₅will

increase in parallel to that of the Sn−Cr product. Under some

reaction conditions, once suﬃcient chromium hydride has been

generated from reaction 4, reaction 5 may gain in signiﬁcance.

A mixture of the two reactions will result in little change in

HCr(CO)₃C₅Me₅concentration even as the Sn−Cr complex

increases. Because of the highly air-sensitive nature of the

reactants and the unusual pseudo-ﬁrst-order reaction conditions

studied, such overall views are useful in delineating the

mechanism.

Kinetic Studies of the Reaction of •Cr(CO)₃C₅Me₅with

Pseudo-First-Order Conditions of Excess Cy₃SnH. The

initial reaction studied was between Cy₃SnH present in pseudo-

ﬁrst-order excess and •Cr(CO)₃C₅Me₅in toluene under either

Ar or CO atmosphere. This reaction proved unsuitable for

kinetic study, because reactions 4 and 5 could not be separated,

which made accurate kinetic analysis diﬃcult. Typical

spectroscopic data under Ar atmosphere are shown in Figure 3.

The major FTIR band of the radical at 1994 cm⁻¹decreases

to about half its intensity and converts to the 1995 cm⁻¹

(HCr(CO)₃C₅Me₅) as the bands at 1955 and 1872 cm⁻¹due to

Cy₃Sn−Cr(CO)₃C₅Me₅grow. This process is relatively rapid

and occurs during the ﬁrst 20 min. Over the next 2 h, the bands

Figure 4. Plot of decrease in A₁₈₉₆(•Cr) and increase in A₁₉₅₅(Sn−

Cr) (magenta) and A₁₉₁₅(HCr) (green) vs time for the data in Figure

3. From ∼1000 s the concentration of the chromium hydride stays

nearly constant and then begins to decrease after 3500 s.

cm⁻¹(Cr hydride). It can be seen that for the ﬁrst six or seven

points the two curves show similar rates of increases. Following

that, the rate of increase of the Sn−Cr product continues, but

between 1000 and 3000 s the concentration of the hydride is

roughly constant and then begins to decrease. This is attributed

to the predominant channel being reaction 4 for the ﬁrst 1000

s, followed by a mixture of eqs 4 and 5 between 1000 and 3000

s, and a dominance of reaction 5 following that. The reaction

was not followed to completion, but in synthetic studies a

nearly quantitative yield of Cy₃Sn−Cr(CO)₃C₅Me₅is obtained

with essentially no HCr(CO)₃C₅Me₅production indicating that

the slower reaction 5 does continue on. Note that almost all the

Cr radical is consumed in the ﬁrst step, and we believe that it

serves as a catalyst in the second step as discussed later.

Finally, it is informative in terms of interpretation of other

data to look at the spectroscopic diﬀerences at the beginning

and end of the reaction. Shown in Figures 5 and 6 are the

spectroscopic changes that occur at the beginning and end of

the reaction.

No attempt to resolve these two steps quantitatively into

their separate rate laws was made, since extracting one reaction

from the other was deemed as being too diﬃcult. Instead,

better reaction conditions were sought. The data from these

studies, however, are in qualitative agreement with the kinetic

behavior studied under diﬀerent conditions, when allowances

are made for the diﬀering concentrations. A probable

mechanism for both reactions 4 and 5 is discussed in a later

section.

Figure 3. FTIR spectra of the reaction of •Cr(CO)₃C₅Me₅(∼0.015

M) with Cy₃SnH (∼0.075 M, a 10-fold molar excess according to the

stoichiometry of reaction 4) in toluene at 25 °C under argon

atmosphere.

Kinetic Studies of the Reaction of Cy₃SnH with

Pseudo-First-Order Conditions of Excess •Cr(CO)₃C₅Me₅

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Figure 5. Spectroscopic changes at the beginning of the reaction

showing 2−1, 3−1, 4−1, 5−1 in which the ﬁrst spectrum is subtracted

from each of the following four spectra. These data show a clear

increase in the chromium hydride and the Sn−Cr product as well as a

decrease in Cr radical. Because of overlap of the peaks some reaction

changes nearly cancel. For example, the Cr radical has an absorption at

1994 cm⁻¹and the hydride at 1995 cm⁻¹, and so only a small growth

of the band at 1995 cm⁻¹is observed. The growth of the hydride at

1912 cm⁻¹however is clear in spite of the fact that there is some

overlap with the strongly decreasing band of the Cr radical at 1896

cm⁻¹.

Figure 7. FTIR spectra of the reaction of •Cr(CO)₃C₅Me₅with

Cy₃SnH in toluene at 36 °C under CO atmosphere. The chromium

radical is present in ∼15-fold molar excess as shown by the dominant

peaks near 1994 and 1890 cm⁻¹.

was generated as the reaction proceeded throughout the

reaction period.

The small band near 1955 cm⁻¹assigned to the Sn−Cr

product and shown in the inset allows accurate quantitative

analysis. Note that in the region between 1920 and 1900 cm⁻¹,

due to a higher initial Cr radical concentration, the isosbestic

point near 1905 does not result in line crossing, but the

behavior is clean and in keeping with only reaction 4 occurring

over this time period under CO atmosphere. The limited

Cy₃SnH reacts preferentially with the Cr radical, which is

present in ∼20-fold excess relative to the chromium hydride.

The presence of CO rather than Ar serves to reduce to

negligible reaction 5, and the kinetics studied correspond

predominantly to reaction 4. A ﬁrst-order plot of ln(A_∞− A)

versus time is linear through four half-lives as shown in Figure

8.

Figure 6. Spectroscopic changes at the end of the reaction showing

13−12, 14−12, 15−12, 16−12 in which the ﬁrst spectrum is

subtracted from each of the following four spectra. These data show

that, whereas there is an increase in the Sn−Cr product, the

concentration of the chromium hydride remains constant and then

decreases as the path changes from reaction 4, to reaction 7 (4 + α 5),

and then to reaction 5.

under Ar and CO Atmosphere. The diﬃculty in kinetically

resolving these two processes led us to seek other reaction

conditions for study. Reaction of a limited amount of Cy₃SnH

in the presence of a large excess of •Cr(CO)₃C₅Me₅was

studied under Ar atmospheres. In spite of changing the reagent

present in excess from Cy₃SnH to •Cr(CO)₃C₅Me₅, toward

the end of the reaction, reaction 5 became signiﬁcant. The only

situation where reliable monophasic kinetic data could be

obtained was under CO pressure and excess •Cr(CO)₃C₅Me₅.

Representative spectroscopic data are shown in Figure 7.

Analysis of the band shapes for the large radical peaks indicated

that reaction 4 was followed and that the chromium hydride

Figure 8. Plot of ln(A_∞− A) vs time for the data in Figure 7.

Reaction of ⁿBu₃SnH and ^tBu₃SnH with •Cr-

n

(CO)₃C₅Me₅. The reaction pattern for Bu₃SnH was found to

be similar to that of Cy₃SnH in all respects. Because of the

n

greater tendency of solutions of Bu₃SnH to lose H₂and form

ⁿBu₃SnSnⁿBu₃, only one series of kinetic data was generated for

ⁿBu₃SnH as listed later in Table 2. In contrast, the reactivity

t

observed for Bu₃SnH was diﬀerent in behavior to the other

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Table 2. Derived Rate Constants for Reactions of R₃SnH in

Toluene under Pseudo-First-Order Conditions of Excess

•Cr(CO)₃C₅Me₅

T (°C)

stannane

atmosphere

k (M⁻¹s⁻¹

)

17

Ph₃SnH

Cy₃SnH

ⁿBu₃SnH

Ph₃SnH

Ph₃SnD

Ph₃SnH

1.7 CO

2.7 CO

3.2 CO

1.7 Ar

0.028

0.029

0.031

0.165

0.097

0.188

0.07

17

27

1.7 CO

2.7 CO

2−3 CO

1.7 CO

1.7 Ar

27

35.5

36

0.17

36

2−3 CO

0.05

38.5

43.5

0.236

0.210

0.395

Figure 9. Plot of ln(A_∞− A) vs time through the ﬁrst three to four

half-lives showing that the rate of reaction under Ar is faster than

under CO but that studies at 10 psi (1.7 atm CO absolute pressure)

and 27 psi (2.7 atm CO absolute pressure) are equivalent within

experimental error. At this temperature, 27 °C, reaction 5 did not

become signiﬁcant until after the third half-life for Ph₃SnH.

stannanes studied. Oxidative addition producing HCr-

(CO)₃C₅Me₅and ^tBu₃Sn−Cr(CO)₃C₅Me₅was observed

spectroscopically; however, further reaction with excess free

stannane occurred before the reaction was completed. In

contrast to the other stannanes studied, where the further

reaction increased the yield of the R₃Sn−Cr(CO)₃C₅Me₅

product, the Sn−Cr complex was found to convert

quantitatively to the chromium hydride as shown in reaction 8.

^tBu₃Sn−Cr(CO)₃C₅Me₅+ ^tBu₃SnH

→ HCr(CO)₃C₅Me₅+ unknown Sn^tBu₃product

(8)

t

The presumed product of reaction is Sn₂Bu₆, but no eﬀort

was made to study further this reaction, which did not yield a

stable R₃Sn−Cr(CO)₃C₅Me₅complex.

Kinetic Studies of the Reaction of Ph₃SnH Under CO

with Pseudo-First-Order Excess of •Cr(CO)₃C₅Me₅. As

determined from studies of Cy₃SnH, the best conditions to

obtain kinetic data proved to be under a CO atmosphere using

a pseudo-ﬁrst-order of the Cr radical. Kinetic data under

pseudo-ﬁrst-order conditions are linear through four half-lives.

As was the case with Cy₃SnH, CO pressure was found to slow

the reaction, and this eﬀect leveled oﬀ at 1−2 atm at lower

temperatures and 2−3 atm at higher temperatures. Spectro-

scopic data correspond closely to those shown above for

Cy₃SnH. A plot of rate data at diﬀerent conditions is shown in

Figure 9.

As described later, a qualitative investigation at 36 atm CO

showed that reaction 1 occurred at a rate in keeping with data

at 2−3 atm pressure. In all cases, the reaction was found to be

ﬁrst order in Ph₃SnH. Doubling or cutting in half the

concentration of excess chromium radical resulted in doubling

or halving the observed rate of reaction indicating the reaction

was also ﬁrst order and not second order in metal radical. A

normal kinetic isotope ratio of 1.12 was measured at 38.5 °C.

The reactions of both Ph₃SnH and Cy₃SnH are faster under Ar

by a factor of 2−3, and the reactions of Ph₃SnH under CO are

2−3 times faster than Cy₃SnH and 3−4 times faster than

ⁿBu₃SnH. Collected data are shown in Table 2.

Figure 10. Eyring plot for reaction of Ph₃SnH and •Cr(CO)₃C₅Me₅

in the temperature range of 15−43.5 °C from which derived activation

parameters are ΔH^‡= 15.7 1.5 kcal/mol and ΔS^‡= −11 5 cal/

(mol·K).

computational data in this work (see below) yields an estimate

of the thermochemical diﬀerence between the Cr−H and Sn−

H bonds of 13.7 1.8 kcal/mol. Adopting a value of 62.3 1.0

kcal/mol for the Cr−H BDE²⁸gives an estimate of 76.0 3.0

kcal/mol for the Ph₃Sn−H BDE in toluene solution.

Reaction of Ph₃SnH under CO at 36 atm with Pseudo-

First-Order Excess of •Cr(CO)₃C₅Me₅. The greater rate of

reaction under Ar versus CO atmosphere led us to test the

reaction of 2 equiv of •Cr(CO)₃C₅Me₅and 1 equiv of Ph₃SnH

in toluene at 36 atm of CO. The stannane solution was isolated

from the Cr radical solution in a small glass tube in a Hoke

bomb. After the system was pressurized with 36 atm of CO at

room temperature in toluene solution, the reaction was

initiated by inverting the bomb and mixing the two solutions.

After ∼1 h the CO was vented, and a sample was loaded into an

IR cell. As shown in the spectra in Figure 11, the reaction had

gone to completion, and the HCr and Sn−Cr products were

present in the correct stoichiometric ratio of reaction 4.

Solution Calorimetric Studies of Reaction of •Cr-

(CO)₃C₅Me₅and Ph₃SnH. The reaction enthalpy of Ph₃SnH

Variable-temperature studies for reaction of Ph₃SnH from

data in Table 2 were used to contruct an Eyring plot as shown

in Figure 10.

Using the experimental value of ΔH^‡= 15.7 1.5 kcal/mol

and assuming an overbarrier of ∼2

1 kcal/mol and the

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conﬁdence to the assigned values in spite of rather large

assigned error limits.

Computational Results. Density functional theory (DFT)

calculations were performed to further support the exper-

imentally determined BDE values in this work. Moreover, other

Sn−H, Sn−Sn, and Sn−Cr BDE values were computed for

several R₃SnH, R₃SnSnR₃, and R₃Sn−Cr(CO)₃C₅Me₅species,

for which it was not possible to determine these data

experimentally. Table 3 collects a summary of some selected

structural parameters computed for the compounds studied.

Table 3. Selected Bond Lengths (Å) Computed for the

Species Studied at the B3LYP-D3(BJ)/Def2-SV(P) Level

a

R

Sn−H in R₃Sn−H Sn−Sn in R₃Sn−SnR₃

Sn−Cr in R₃Sn−Cr

b

Me

Cy

^tBu

Ph

1.741

1.747

1.751

1.735

2.805 [2.764]

2.795 [2.805]

2.907 [2.894]

2.766 [2.763]

2.771

c

e

f

d

2.810 [2.816]

Figure 11. FTIR spectra of the stock solution of Cr radical (black) and

product of reaction with Ph₃SnH (red) in toluene solution at 22 °C

under 36 atm of CO pressure. Quantitative production of an

equimolar mixture of the HCr and SnCr products occurs in ∼1 h in

keeping with kinetic studies and indicating that the radical reaction is

not inhibited by higher pressures of CO. The intensity of the product

bands relative to starting material bands is somewhat higher than

occurs during kinetic studies where ﬁltered solutions of the Cr radical

are used. In this study the reaction was not ﬁltered into the bomb, and

some microcrystalline Cr−Cr dimer may have dissolved and reacted

during the reaction.

2.934

d

2.739 [2.739; 2.729]

a

Cr = Cr(CO)₃C₅Me₅. Available experimental values determined by

b

c

X-ray crystallography between brackets. Taken from ref 46. Taken

from ref 47. This work. Taken from ref 48. Taken from ref 49.

d

e

f

There is a perfect agreement between the gas-phase

computed structures and those obtained by X-ray crystallog-

raphy. Dispersion eﬀects were found to be important especially

for the description of the structures in crowded R₃Sn−SnR₃

and R₃Sn−Cr(CO)₃C₅Me₅species. For instance, when the

t

corresponding R = Bu₃species were optimized without the

with 2 mol of •Cr(CO)₃C₅Me₅(reaction 4) was measured in

toluene solution at 30 °C using a Setaram Calvet calorimeter in

C₆D₆solution, and the reaction stoichiometry was conﬁrmed

by both NMR and FTIR analysis. The value obtained of ΔH =

−36.7 1.8 kcal/mol can be used to estimate a value of the

Sn−Cr BDE in Ph₃Sn−Cr(CO)₃C₅Me₅. Adopting a value of

62.3 1.0 kcal/mol for the Cr−H BDE,²⁸as well as a value of

76.0 3.0 kcal/mol for the Ph₃Sn−H BDE (see above), yields

inclusion of dispersion eﬀects, Sn−Sn and Sn−Cr bond lengths

of 3.005 and 3.061 Å, respectively, were obtained, ∼0.1 Å

longer than the experimentally determined distances.

As it can be seen in Table 3, the Sn−H bond length in the

optimized structure of the stannanes is almost constant at 1.745

0.010 Å being not greatly inﬂuenced by the R substituents

bound to the Sn atom. However, the Sn−Sn and Sn−Cr bond

lengths are more dramatically aﬀected by the steric hindrance

imposed by the R substituents. The Sn−Sn bond length

calculated is similar in Me₃SnSnMe₃and Cy₃SnSnCy₃

a derived value of 50.4

3.5 kcal/mol for the Ph₃Sn−

Cr(CO)₃C₅Me₅BDE in toluene solution.

t

Catalytic Dehydrogenation of Ph₃SnH by Late

Transition Metal Complexes. The recently reported⁵

complex [(μ-H)Pt(I^tBu)(Sn^tBu₃)]₂was tested as a catalyst

for dehydrogenation of Ph₃SnH as shown in eq 9 but was found

to be of low activity.

compounds, but it increases by ∼0.1 Å when bulkier Bu

substituents are bound to the tin. Likewise, the Cr−Sn bond

distance is more than 0.1 Å longer in the ^tBu₃Sn−

Cr(CO)₃C₅Me₅complex as compared to the analogous Cy

complex. The sum of the three R−Sn−R angles in the

optimized structure of all R₃Sn• radicals is in the range of 318−

330° indicating the pyramidal geometry of the Sn atom in these

species as it has been previously reported.⁵⁰However, the

∑R−Sn−R value increases for bulkier substituents in alkyl

radicals (318.6° (R = Cy) vs 329.5° (R = ^tBu)). The computed

Mulliken spin density located on the Sn atom in alkyl radicals is

2Ph₃SnH → H₂+ Ph₃SnSnPh₃

(9)

Nolan and co-workers have reported⁴⁵synthesis and

structure of Pd(IPr)(PPh₃), and we prepared in strict analogy

to the published procedure the closely related but more

toluene-soluble complex Pd(IPr)(P(p-tolyl)₃). This Pd(0)

complex proved to be highly eﬀective in catalysis of reaction

9. In typical reaction conditions samples of ∼0.200 g of Ph₃SnH

were quantitatively converted to Ph₃SnSnPh₃in ∼5 mL of

C₆D₆using 0.002 to 0.010 g of Pd(IPr)(P(p-tolyl)₃). The

enthalpy of reaction was found to be independent of the

amount of catalyst used provided it was present in less than 5

mol percent. The enthalpy of reaction 9, ΔH = −15.8 2.2

kcal/mol, is the average of six separate measurements. Adopting

the values of 76.0 3.0 and 104.2 kcal/mol for the Ph₃Sn−H

(see above) and H₂⁴¹BDEs, respectively, a value of 63.8 3.7

kcal/mol can be derived for the Ph₃Sn−SnPh₃BDE in toluene

solution. There is good agreement between theory and

experiment, as discussed later, and this gives some degree of

t

nearly constant (0.79 (R = Me), 0.76 (R = Cy, Bu)). In the

case of Ph₃Sn•, the spin density slightly decreases (0.74)

indicating little delocalization of the unpaired electron in the

phenyl rings as previously reported.⁵¹

BDEs calculated at the PBE0-D3(BJ)/Def2-TZVP//B3LYP-

D3(BJ)/Def2-SV(P) level are collected in Table 4. The BDEs

computed in gas phase and in toluene solution are nearly

identical with the larger diﬀerences lower than 1 kcal/mol. As

previously reported by Boyd and co-workers,²⁵the serious lack

of well-established accurate experimental BDE values for

organotin compounds make it diﬃcult to assess the perform-

ance of the DFT methods employed. In this regard, they used

high-level CCSD(T) calculations to test the reliability of their

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kcal/mol has been previously proposed by Steele and co-

workers¹³based on their enthalpy of formation measurements

of Ph₃SnSnPh₃by combustion calorimetry in combination with

appearance potential studies reported by Chambers and

Glocking.¹⁹

Table 4. Experimental (in bold) and Computational (in

italics) Sn−H, Sn−Sn, and Sn−Cr Bond Dissociation

Enthalpy Values in R₃SnH, R₃SnSnR₃, and R₃Sn−

Cr(CO)₃C₅Me₅Species Obtained in This Work along with

Previously Reported Experimental Data (in kcal/mol)

The only Sn−Cr BDE data the authors could ﬁnd in the

literature is that corresponding to the Me₃Sn−Cr(CO)₃C₅H₅

complex determined by a combination of appearance and

ionization potential measurements.¹⁸The experimental value

reported of 53.4 kcal/mol is somewhat higher than our

computed value of 50.1 kcal/mol. In this work, a Sn−Cr BDE

value in toluene solution of 50.4 3.5 kcal/mol for Ph₃Sn−

Cr(CO)₃C₅Me₅was determined in good agreement to the

computed value of 52.4 kcal/mol in toluene solution.

Moreover, a Cr−H BDE in HCr(CO)₃C₅Me₅of 62.2 kcal/

mol was calculated in the current work in perfect agreement to

the experimental determination of 62.3 1.0 kcal/mol.²⁸

The kinetics of the reaction of Ph₃SnH with excess

•Cr(CO)₃C₅Me₅was studied as stated in a previous section.

In this regard, a relaxed potential energy surface scan along the

Cr···H distance for the thermodynamically uphill HAT from

Ph₃SnH to •Cr(CO)₃C₅Me₅(reaction 2) was performed also

by DFT calculations and is shown in Figure 12.

Sn−H

Sn−Sn

Sn−Cr

R = Ph

R = Me

76.0 3.0

63.8 3.7

50.4 3.5

b

74.5, 74.5

65.4, 64.7

52.4, 52.1

c

d

71.0

51.4

b

76.3, 76.4

59.5, 59.4

50.2, 50.2

c

e

76.1

60.4 3.5

68.5 6.0

e

f

74.0 2.6

77.1 4.1

75.0 3.5

75.9

f

h

61.6

g

R = Cy

66.9

58.7

49.6

37.0

t

R = Bu

75.8

a

Computed at the PBE0-D3(BJ)/Def2-TZVP//B3LYP-D3(BJ)/Def2-

SV(P) level. Computed values in toluene solution. Taken from ref

22. On the basis of the enthalpy of formation of Ph₃SnSnPh₃

b

c

d

reported in ref 13 in combination with appearance potential studies

f

reported in ref 19. Taken from ref 9a with the uncertainty as

e

g

h

recalculated in ref 17. Taken from ref 17. Taken from ref 21. Taken

from ref 18.

DFT results. In this work, a good agreement between

computed and available experimental BDEs (see Table 4)

was generally obtained. As it can be seen in Table 4, for

Me₃SnH, there are several experimental determinations of the

Sn−H BDE by diﬀerent methods, and all of the values are

∼75.5

1.5 kcal/mol. Moreover, Boyd and co-workers

calculated a Sn−H BDE value of 76.1 kcal/mol at the

CCSD(T)/SDB(Sn)-aug-cc-pVTZ(other atoms) level.²⁵Fur-

thermore, Allendorf and Mellius⁵²derived a value of 76.7 kcal/

mol applying the BAC-MP4 method that combines ab initio

electronic calculations with empirical corrections to obtain

accurate heats of formation. In spite of that we used a much

lower level of theory in this paper and thus applicable to bigger

molecules, the determined value of 76.3 kcal/mol (Table 4) is

in perfect agreement to those obtained experimentally or using

high-level calculations. In addition, the computed Sn−H BDE

in Ph₃Sn−H of 74.5 kcal/mol is also in good agreement to the

value of 76.0 3.0 kcal/mol derived experimentally by kinetic

measurements in toluene solution in the current study (see

above), but it is slightly higher than the previously reported

experimental value of 71.0 kcal/mol (see Table 4). No

experimental BDE data were found to compare with the

calculated values for the rest of the compounds studied.

Nonetheless, there are enough data to assess the accuracy of the

computed Sn−H BDE values giving some measure of

conﬁdence to the data that were not previously measured

experimentally.

The paucity of accurate experimental Sn−Sn and Sn−Cr

BDE data to compare to the values predicted by DFT is even

more problematic. Three independent experimental determi-

nations of the Sn−Sn BDE in Me₃SnSnMe₃are shown in Table

4, and they span a range of 8 kcal/mol. The value of 59.5 kcal/

mol computed in this work is in good agreement with the most

recent measurement of 60.4 3.5 kcal/mol reported by Baer

and co-workers.¹⁷In the current work, a value of 63.8 3.7

kcal/mol was derived for the Sn−Sn BDE in Ph₃SnSnPh₃in

toluene solution also in good agreement with the calculated

value of 64.7 kcal/mol. However, a much lower value of 51.4

Figure 12. Computed change in energy with the Cr···H distance at the

PBE0-D3(BJ)/Def2-TZVP//B3LYP-D3(BJ)/Def2-SVP level. At each

ﬁxed Cr···H distance, the minimum energy structure was computed

and the deviations from the energy of the optimized structure at a Cr···

H ﬁxed distance of 3.0 Å are shown.

The starting point places the two separate fragments

•Cr(CO)₃C₅Me₅and Ph₃SnH at a H···Cr distance of 3.0 Å,

and there is little change in energy as •Cr(CO)₃C₅Me₅

approaches the Sn−H bond at an angle. Starting at an H···Cr

distance of 2.4 Å a more linear geometry is adopted, and the

energy is computed to rise to a value of 10 kcal/mol at a Cr−H

distance of 1.599 Å, which corresponds to the H−Cr bond

length observed in the optimized structure of HCr(CO)₃C₅Me₅

while retaining some interaction with the incipient Ph₃Sn•

(Sn···H = 2.173 Å). The optimized structure of the last point of

the scan in Figure 12 is shown in Figure 13. The reverse

reaction in which an Ph₃Sn• approaches the H−Cr-

(CO)₃C₅Me₅bond is computed in the gas phase to be

essentially barrierless.

To support mechanistic studies relevant to reaction under Ar

rather than CO atmosphere, the structure and energetics of the

interaction in which triphenyl stannane undergoes the CO

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Table 5. Comparison of Metrical Parameters (in Å) for η²-

H−SnPh₃Complexes

Cr−

complex

Sn

Cr−H Sn−H Sn−Cr−H

a

Cr−Sn, Cr−H, Sn−H

2.73

2.69

2.70

1.60

1.62

1.50

1.59

1.73

2.16

2.23

2.02

(•)(μ-H−SnPh₃) (Cr(CO)₂C₅Me₅)

(μ-H−SnPh₃) (Cr(CO)₂(C₆H₃Me₃)

54

56

48

b

(μ-H−SnPh₃)

c

(Cr(CO)₂(p-C₆H₄(OMe)₂)

a

This line refers to the uncomplexed species: Cr−Sn = Ph₃Sn−

Cr(CO)₃C₅Me₅, where d_Sn−Crwas determined by X-ray crystallog-

raphy, this work; Cr−H = HCr(CO)₃C₅Me₅, where d_H−Crwas

determined computationaly, this work; Sn−H refers to Ph₃SnH, whose

b

distance was determined by X-ray crystallography; see Table 2. See

c

ref 54. See ref 53.

complex and the crystallographically determined structures of

the 18 e⁻arene dicarbonyl complexes. The similarity in

geometry suggests that the singly occupied molecular orbital

(SOMO) electron does not reside in the three-center Sn−H−

Cr bonding orbital set, and this is conﬁrmed as shown in Figure

15.

Figure 13. Optimized structure of the last point of the scan shown in

Figure 12.

displacement reaction shown in reaction 10 were also

investigated.

•Cr(CO)₃C₅Me₅+ Ph₃SnH

→ CO + •Cr(Ph₃SnH)(CO)₂C₅Me₅

(10)

Computed thermochemical data for reaction 10 are ΔH =

6.7 kcal/mol and ΔS = −21.1 cal/(mol·K). The computed

structure of the chromium species obtained after CO

displacement by Ph₃SnH is shown in Figure 14, which displays

three-center bonding between Cr, Sn, and H.

Figure 15. View of the computed SOMO for structure in Figure 14

showing two avenues of attack. The diﬀuse SOMO in red and green is

primarily located on the Cr atom with very little electron density

located near the Cr−μ-H−Sn three-center bond depicted by the

triangle, which is in the plane of the paper in the ﬁgure. The two

unblocked avenues of approach shown as Path A and Path B appear to

show the best angle for approach for HAT from Cr (Path A) or HAT

donation to Cr (Path B). Computed spin densities show the SOMO

electron to be primarily on Cr (+1.19), with several small negative spin

densities located of C atoms bonded to Cr. The reported spin densities

include spin densities of the attached H atoms. In the case of the Cr−

H bonds the spin density on the hydride H is the only H atom with a

nontrivial spin density (−0.044), which is an order of magnitude

greater than all other individual H atoms.

Figure 14. Computed structure of •Cr(μ-Ph₃SnH)(CO)₂C₅Me₅.

This is in keeping with metrical parameters shown in Table

5, since if the unpaired electron in this system was in an orbital

assigned to the μ-H−Sn−Cr triangle, it would be expected to

diﬀer more greatly in structure from the stable 18 e⁻structures

reported by others. The greatest diﬀerence in Table 5 when

compared to model complexes shown in the ﬁrst line occurs in

the Sn−H distance, which is compared to Ph₃SnH. The Cr−Sn

distance and the H−Cr distances are close to those assigned to

Ph₃Sn−Cr(CO)₃C₅Me₅and HCr(CO)₃C₅Me₅, respectively. A

The computed structure of this 17 e⁻complex closely

resembles that of 18 e⁻analogues prepared by Klabunde⁵³and

Schubert.⁵⁴A comparison of the Cr−H−Sn triangles is shown

in Table 5.

In spite of diﬃculties in assigning the crystallographic

position of the bridging H atom, there is a surprising agreement

between the data for the computed structure of the proposed

bridging hydride 17 e⁻cyclopentadienyl dicarbonyl radical

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simple estimate of the enthalpy of reaction 10 based on Cr−

CO (37),⁵⁵Ph₃Sn−H (76), H−Cr (62), and Sn−Cr BDEs

(50) would be 1 kcal/mol. The computed enthalpy of the

reaction is +6.7 kcal/mol, which is in reasonable agreement

with this estimate.

Direct measurement of the catalytic dehydrogenation of

Ph₃SnH allowed us to derive a value of 63.8 3.7 kcal/mol for

the Ph₃Sn−SnPh₃BDE. These data are in good agreement with

the computational value of 64.7 kcal/mol. The utilization of

Pd(IPr)(P(p-tolyl)₃) as a catalyst for reaction 9 for calorimetric

studies is novel, and we plan to extend this to other stannanes

to determine their enthalpies of dehydrogenation. The

mechanism of the catalyzed dehydrogenation was not

investigated at this time, since it was not germane to the goal

of the present work. It is reasonable to assume that the key

intermediate might involve oxidative addition of 2 equiv of

Ph₃SnH to form a Pd(IV) intermediate with possible

dissociation of either an IPr or P(p-tolyl)₃ligand to generate

the active catalyst for dehydrogenation. Precedence for such a

ditin dihydride complex exists in our recent report¹of the

structure of Pt(H)₂(Sn^tBu₃)₂(CN^tBu)₂.

DISCUSSION

■

The major goal of this work was to measure Sn−H, Sn−Sn, and

Sn−Cr BDE to allow comparison to existing literature data and

to results obtained by computational methods. To achieve that

goal, the mechanism of oxidative addition of stannanes to a 17

e⁻organometallic radical was also studied. It can be diﬃcult to

prove organometallic mechanisms, and a primary focus was to

ﬁnd a set of conditions where the rate determining in HAT

corresponded to the radical transfer step shown in reaction 2.

This result, coupled with solution calorimetric and computa-

tional studies, allowed us to generate Sn−H, Sn−Cr, and Sn−

Sn BDEs as discussed in the thermochemical section.

Considerable kinetics and mechanistic studies were done, and

these are discussed in a second section.

In contrast to R₃SnH, the distannane compounds R₃SnSnR₃

are computed to be more inﬂuenced by substitution. As it can

be seen in Table 3, the Sn−Sn distance is similar for the

hexamethyl and hexacyclohexyl distannanes. However, in spite

of Cy₃SnSnCy₃being sterically more crowded than

Me₃SnSnMe₃, the Sn−Sn BDE calculated is ∼7 kcal/mol

higher for the Cy derivative (Table 4) presumably for the

existence of stabilizing dispersion forces.⁶⁵Likewise, the Sn−Sn

Thermochemical Studies. The derived value of 76.0 3.0

kcal/mol based on kinetics studies for the Ph₃Sn−H BDE in

toluene solution is in good agreement with the computed value

of 74.5 kcal/mol. Computational studies showed that the Sn−

H BDE for alkyl stannanes was relatively constant at nearly 76

kcal/mol. As shown in Table 4, solution photoacoustic data

place the ⁿBu₃Sn−H BDE as 74 or 78 kcal/mol, and gas-phase

radical studies lead to derived values of Me₃Sn−H = 76 and

Ph₃Sn−H = 71 kcal/mol, a gap of 5 kcal/mol between the

trialkyl and triphenyl stannane Sn−H BDE. However,

computational studies reported here show a much smaller

t

BDE is similar for the Me and Bu distannanes although the

Sn−Sn bond length is 0.16 Å shorter in the former. Thus, the

weaker nature of the Sn−Sn bond in the ^tBu derivative appears

counterbalanced with the attractive London dispersion

t

interactions established among the Bu bulky pendant groups

bound to the diﬀerent Sn atoms. Analogously, the gas phase

Sn−Sn BDE in R₃SnSnR₃is computed to be 5.9 kcal/mol

higher for R = Ph than for R = Me, while the corresponding

Sn−H BDE in the stannanes is reduced by 1.8 kcal/mol in

going from R = Me to Ph.

t

gap (Me₃Sn−H (76.3), Cy₃Sn−H (75.9), Bu₃Sn−H (75.8),

and Ph₃Sn−H (74.5 kcal/mol); see Table 4). Though full

kinetics studies were not performed for Cy₃SnH, it was found

to obey an overall second-order reaction, and under

comparable conditions to be no more than 5 times slower

than Ph₃SnH (Table 2). That is in keeping with an activation

energy lower than 1 kcal/mol less favorable for the Cy

derivative and would not appear to be in keeping with a

diﬀerence of 5 kcal/mol in BDE.

Pioneering work of Neumann⁶derived Sn−Sn BDE values

for a series of R₃SnSnR₃compounds. For R = phenyl,

cyclohexyl, or 1-adamantyl dissociation did not occur even at

230 °C. However, for a series of 1,3,5-trisubsituted arene

substituents dissociation did occur, and variable-temperature

ESR data yielded values of Sn−Sn BDE of 49 kcal/mol for R =

methyl, 26.6 kcal/mol for R = Et, and 8.5 kcal/mol for R =

isopropyl. The isopropyl derivative is fully dissociated at room

temperature in benzene solution, and ESR data indicate that it

is pyramidal rather than planar in nature. It is clear from this

that rather extreme crowding is necessary to signiﬁcantly

weaken the Sn−Sn bond.

At ﬁrst consideration, it may seem that the Sn−H BDE

diﬀerence between alkyl and phenyl stannanes obtained in this

work of ∼1 kcal/mol is too low due to the possibility of

electron delocalization of the unpaired electron in the phenyl

rings of the Ph₃Sn• radical. The greater stability⁶⁶of arene-

substituted radicals in sulfur chemistry⁵⁶is well-established, and

the PhS-H and MeS-H BDE diﬀer by ∼10 kcal/mol for that

reason. The lower BDE of PhS-H is attributed to delocalization

of the unpaired electron of the PhS• radical into the π* orbitals

on the phenyl ring. We had noted earlier⁵⁷that delocalization

decreases in descending the chalcogenides and that there is less

electron density donated to the arene ring in the SOMO of the

PhTe• radical. Similar arguments apply to Ph₃Sn•, which is

pyramidal in nature and as discussed in the computational

results section shows that Mulliken spin density on the Sn atom

of the R₃Sn• radicals is similar for alkyl and aryl derivatives

indicating little stabilization in the Ph₃Sn• species by

delocalization of the unpaired electron into the phenyl rings.

Furthermore, photoacoustic calorimetric studies have also

shown a similar behavior in the Ge−H BDE of organo-

germanes, where the Ge−H BDE derived for Me₃Ge−H (81.6

0.5 kcal/mol) is only slightly higher than that observed for

Ph₃Ge−H (80.2 0.8 kcal/mol).⁵⁸

Reaction calorimetric measurement of the enthalpy of

reaction 1 of ΔH = −36.7

1.8 kcal/mol, combined with

the value of 76.0 3.0 kcal/mol for the Sn−H BDE in Ph₃SnH,

leads to a derived value of the Sn−Cr BDE in Ph₃Sn−

Cr(CO)₃C₅Me₅of 50.4 3.5 kcal/mol in good agreement with

the value of 52.1 kcal/mol computed in this work (Table 4).

This is within experimental error of the earlier estimate based

on appearance potential measurements derived by Lappert of

53.4 kcal/mol for Me₃Sn−Cr(CO)₃C₅H₅. The Cr−Sn BDE is

one of the higher values found for X−Cr(CO)₃C₅Me₅single

bonds as shown in Table 6.

The computed value for the Sn−Cr BDE in the Cy₃Sn−

Cr(CO)₃C₅Me₅complex is ∼3 kcal/mol lower than for the

phenyl derivative in agreement with the longer Sn−Cr distance

observed in both the X-ray and DFT-optimized structure of the

former (see Figures 1 and 2 and Table 3). This trend is more

marked in the case of the bulkier ^tBu derivative, where the Sn−

K

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A plausible explanation for these observations is that binding

and electron transfer to hexaphenyl ditin is inhibited for

•Cr(CO)₃C₅Me₅by steric factors.

Mechanistic Studies. Oxidative addition reactions of the

•Cr(CO)₃C₅Me₅radical have been shown to occur by diﬀerent

mechanisms depending upon the strength of the bond being

broken and the reaction conditions and atmosphere. Reaction

mechanisms shown in Scheme 1 do not involve CO

dissociation and so would be expected to be largely

independent of gas atmosphere.

Table 6. Bond Dissociation Enthalpies (kcal/mol) for X−

Cr(CO)₃C₅Me₅(Cr = Cr(CO)₃C₅Me₅)

a

b

c

Cr-Cr

15

21

24

43

Cr-SⁿBu

43

38

36

31

Cr-SnPh₃

50

62

70

75

d

f

b

e

Cr-Hg

Cr-SPh

Cr-H

g

h

Cr-Mo

Cr-SePh

Cr-NO

i

g

j

Cr-SMe

Cr-TePh

CrCr

a

b

c

d

See ref 33. See ref 56. This work. Refers to the average Cr−Hg

e

BDE in C₅Me₅(CO)₃Cr−Hg−Cr(CO)₃C₅Me₅; see ref 59. See ref 55.

f

Refers to the BDE in C₅H₅(CO)₃Mo−Cr(CO)₃C₅Me₅; see ref 59.

g

h

See ref 60. Refers to the BDE in Cr(NO)(CO)₂C₅Me₅; see ref 61.

i

j

The derived rate laws for both reaction manifolds are shown

in Scheme 1. For path A, it is based on a steady-state

assumption in the initial adduct formed with the rate-

determining step occurring when this is attacked by a second

Cr radical. Path A in Scheme 1 results in most situations in an

overall third-order rate law. Activation parameters in related

systems have shown low enthalpies of activation and large

unfavorable entropies of activation.⁶³No sign of reactivity via

Path A has been found so far in reactivity of R₃SnH with the

chromium radical. Computational and additional experimental

work to ascertain why the reaction of PhSH with an S−H BDE

of 79 kcal/mol proceeds by a mechanism analogous to that

shown in Path A and Ph₃SnH with an Sn−H BDE of 76 kcal/

mol does not is in progress.

Path B in Scheme 1 corresponds to single radical attack on

the Sn−H bond, and this is the dominant mechanism observed

for R₃SnH under CO pressure in the presence of a large excess

of •Cr(CO)₃C₅Me₅. The derived rate law using a steady-state

assumption in •SnR₃is also shown at the bottom of Scheme 1.

Note that the rate constants associated with Path A are

negligible with respect to Path B and drop out of the equation

under the conditions studied. Under conditions where

k_−1B[HCr(CO)₃C₅Me₅] ≪ k_2B[•Cr(CO)₃C₅Me₅] the ob-

served rate constant corresponds to k_1Band is largely

uninﬂuenced by CO pressure.

See ref 57. Refers to the BDE in C₅Me₅(CO)₂CrCr(CO)₂C₅Me₅;

see ref 62.

Cr bond length in the optimized structure is ∼0.1 Å longer than

in the Cy₃Sn−Cr(CO)₃C₅Me₅complex (see Table 3), and

consequently the computed Sn−Cr BDE in ^tBu₃Sn−Cr-

(CO)₃C₅Me₅is almost 13 kcal/mol lower. The calculated

t

Sn−Cr BDE value of 37.0 kcal/mol for the Bu derivative is

very low due to steric factors. This is reﬂected in the labile

nature of this Sn−Cr product that, unlike other R₃Sn−

Cr(CO)₃C₅Me₅adducts studied in this work, reacts further in

t

the presence of excess Bu₃SnH as shown in eq 8.

Data for the Sn−Cr BDE of 50 kcal/mol in Ph₃Sn−

Cr(CO)₃C₅Me₅with a value of 64 kcal/mol for the Sn−Sn

BDE in Ph₃SnSnPh₃led to the calculation that oxidative

addition of hexaphenyl ditin as shown in eq 11 should be

exothermic by 36 kcal/mol.

2•Cr(CO)₃C₅Me₅+ Ph₃SnSnPh₃→ 2Ph₃Sn−Cr(CO)₃C₅Me₅

(11)

In spite of thermodynamic favorability, reaction 11 was not

observed to occur over several hours at room temperature. Two

related reactions⁶³shown in eqs 12 and 13 occur rapidly under

comparable conditions.

Two mechanisms that involve CO dissociation are shown in

Scheme 2 together with the derived rate law based on a steady-

state assumption in the concentration of the reactive

intermediate Cr−μ-H−Sn radical.

The common ﬁrst step in Paths C and D in Scheme 2

involves displacement of CO by R₃SnH. Such reactions can be

2•Cr(CO)₃C₅Me₅+ PhS−SPh → 2PhS−Cr(CO)₃C₅Me₅

(12)

2•Cr(CO)₃C₅Me₅+ Cp(CO)₃Mo−Mo(CO)₃Cp

→ 2Cp(CO)₃Mo−Cr(CO)₃C₅Me₅

(13)

Scheme 1. Mechanism of Reactions between Ph₃SnH and the Chromium Radical Not Involving CO Dissociation

L

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Scheme 2. Mechanism of Reactions between Ph₃SnH and the Chromium Radical Involving CO Dissociation

relatively rapid for such 17 e⁻radical species.⁶³There is

literature precedent for this playing a role in oxidative addition

reactions of the chromium radical. The rate of displacement of

CO in •Cr(CO)₃C₅Me₅by PMe₂Ph as shown in eq 14 was

found to occur with ΔH^‡= 8.7 kcal/mol and to have a rate

constant of k₁≈ 0.33 M⁻¹s⁻¹at 10 °C.⁶³

from HCr(CO)₃C₅Me₅is transferred to I to form •Cr-

(CO)₃C₅Me₅and (H)(μ-H-SnR₃)Cr(CO)₂C₅Me₅. Coupling of

H atoms in cis dihydrides can be a rapid reaction by which

R₃Sn−Cr(H₂)(CO)₂C₅Me₅could be formed, which, following

displacement of H₂by CO, would lead to the ﬁnal product

according to the stoichiometry of reaction 5.

The authors made no attempt at this time to determine the

rate constants for the reactions shown in Scheme 2, since it is

not germane to the primary focus of this work. However, a

large number of qualitative and semiquantitative work done on

this system are all in accord with the mechanism in Scheme 2

operating under Ar or low CO pressure. The correspondence of

the computed structure of the 17 e⁻radical stannane complex

•(μ-H-SnR₃)Cr(CO)₂C₅Me₅with those reported for analo-

gous 18 e⁻diamagnetic complexes (μ-H-SnR₃)Cr(CO)₂C₆R₆

also supports this as does the computed enthalpy of formation

which is endothermic by only ∼7 kcal/mol but with an

unfavorable entropic term of ca. −21 cal/(mol·K) indicating a

moderately unfavorable equilibrium constant, which would

support the feasibility of •(μ-H-SnR₃)Cr(CO)₂C₅Me₅as a

reactive intermediate. The orbital diagram of the SOMO shown

in Figure 15 indicates two reactive sites, which can be

approached from diﬀerent angles. The ﬁrst is the μ-H atom,

which can be abstracted by •Cr(CO)₃C₅Me₅as shown in path

2C in Scheme 2. The other site is the SOMO electron in an

orbital located essentially on Cr and which should be capable of

abstracting an H atom from HCr(CO)₃C₅Me₅as shown in path

2D in Scheme 2.

k

1

•Cr(CO)₃C₅Me₅+ PMe₂Ph XoooY CO + •Cr(PMe₂Ph)(CO)₂C₅Me₅

k−

1

(14)

The rate of incorporation of ¹³CO into •Cr(CO)₃C₅Me₅was

found to be extremely rapid even at low temperatures.⁶⁴Thus,

the rate of establishment of the equilibrium in ligand

substitution reactions such as that shown in the ﬁrst step 1C

of Scheme 2 leading to formation of •Cr(μ-HSnPh₃)

(CO)₂C₅Me₅is expected to be signiﬁcantly less than the 15.7

kcal/mol for the single radical HAT step in Scheme 1 Path B.

This implies that the equilibrium shown in the ﬁrst common

step in Scheme 2 to form the intermediate I = •Cr(μ-

HSnPh₃)(CO)₂C₅Me₅is rapidly established and that it is only

the degree of CO pressure present that directly inﬂuences this

equilibrium, which regulates how competitive this pathway is

compared to those oﬀered in Scheme 1.

There are two diﬀerent steps leading to the Sn−Cr product

in Scheme 2. In the upper pathway 2C, the proposed

intermediate complex I is attacked by a second •Cr-

(CO)₃C₅Me₅radical, and the H atom is transferred from I to

•Cr(CO)₃C₅Me₅producing HCr(CO)₃C₅Me₅and “R₃Sn−

Cr(CO)₂C₅Me₅”, which picks up dissociated CO rapidly to

form the observed product. This corresponds to reaction 4 and

gives the normal product distribution when excess radical is

present. The only observed diﬀerence with respect to reaction

under CO is a faster rate, but the product distribution is the

same.

The rather similar-looking rate laws for the mechanisms

shown above add to the diﬃculty in kinetics resolution. Failure

to observe the expected third-order behavior typiﬁed in path B

in Scheme 1 in the reaction of stannanes with •Cr(CO)₃C₅Me₅

was somewhat surprising to the authors, and additional

experimental and computational work is planned. Nevertheless,

there is reasonably strong support for the mechanisms

proposed here, and the authors believe there are at least

An alternative closing step is reaction of I with HCr-

(CO)₃C₅Me₅rather than with •Cr(CO)₃C₅Me₅as shown in

the lower pathstep 2D of Scheme 2. In this case, an H atom

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three reaction channels leading to products depending upon

the speciﬁc conditions. There is little doubt regarding

establishment that direct single radical attack as shown in

Scheme 1 Path B is the dominant mechanism in the presence of

a large excess of Cr radical under an atmosphere of 2−3 atm

CO near room temperature. That conclusion is supported not

only by the observed rate law being k₁[•Cr][Ph₃SnH] provided

k₂[•Cr] ≫ k₋₁[HCr] but also by the normal kinetic isotope

eﬀect of k_H/k_D= 1.12, the modest negative entropy of

activation, and the good agreement of the enthalpy of activation

with computational results.

Whereas attainment of a trimolecular transition state in

which two Cr atoms are involved in attack on the Sn−H bond

does not appear to occur under CO pressure in which each Cr

center retains three pendant carbonyls, under Ar atmosphere or

suitably low CO pressure, loss of a CO may serve to free an

avenue of approach while also activating the Sn−H bond. The

branched mechanism shown in Scheme 2 explains two

important observations made in this system. The ﬁrst is the

faster rate of reaction in the presence of a large excess of

•Cr(CO)₃C₅Me₅when the reaction is performed under an Ar

rather than CO atmosphere. The second is the observation that

when the reaction is performed in the presence of excess

R₃SnH, the product distribution changes as HCr(CO)₃C₅Me₅

is produced during the reaction and in the later stages of the

reaction surpasses the concentration of •Cr(CO)₃C₅Me₅. In

that case the stoichiometry of reaction is altered beyond the

midpoint to utilize both •Cr(CO)₃C₅Me₅and HCr-

(CO)₃C₅Me₅to produce product as outlined in reactions 4−7.

Ph₃SnH would presumably be faster if it could follow

such a pathway, but steric factors appear to limit the

ability for concerted two-radical attack forcing adoption

of a second-order HAT mechanism. This question is

under further experimental and computational study.

5. Although it is thermodynamically quite favorable,

Ph₃SnSnPh₃was found to not undergo radical scission

with 2 equiv of •Cr(CO)₃C₅Me₅, and this is assigned to

steric inaccessibility of the Sn−Sn bond to the relatively

large Cr radical.

6. Catalytic dehydrogenation of Ph₃SnH by Pd(IPr)(P(p-

tolyl)₃) to form Ph₃SnSnPh₃is a valuable method for

obtaining direct thermochemical data pertinent to the

diﬀerence between the Sn−Sn and Sn−H BDE, and

further utilization of this technique is planned.

7. Formation of an activated radical stannane complex (η²-

H−SnPh₃)(•)Cr(CO)₂(C₅Me₅) provides a working

explanation of observed kinetics behavior under Ar or

low CO concentration conditions. Reaction of this

species with additional •Cr(CO)₃C₅Me₅leads to

acceleration of reaction 4. Reaction of this species with

additional HCr(CO)₃C₅Me₅leads to acceleration of

reaction 5.

The dinuclear oxidative addition reactions described here

provide a basis for future investigation of the energetics of

oxidative addition to mononuclear Pt and Pd complexes related

to those recently shown to oxidatively activate dihydrogen and

other small molecules.¹⁻⁵The chemistry of stannyl radicals

cannot be simply interpreted as being that of heavier analogues

of carbon-based radicals. As illustrated in the review by

Power,⁶⁵main group radicals, including stannyl radicals, have a

rich and unique chemistry. Additional work aimed at mapping

the energetics of interactions between tin hydrides and

complexes of Pd and Pt, as well as additional fundamental

thermochemistry of organostannanes, are in progress.

CONCLUSIONS

■

Experimental measurements have yielded solution-phase BDE

estimates for the Ph₃Sn−H (76 kcal/mol), Ph₃Sn−SnPh₃(64

kcal/mol), and Ph₃Sn−Cr(CO)₃C₅Me₅(50 kcal/mol) bonds.

These estimates are considered reliable with an uncertainty on

the order of 3 kcal/mol, typical of BDE data in solution and

incorporate the experimental uncertainty of the data used to

derive them. Moreover, good agreement between experimental

and DFT-computed values was obtained. In addition to this,

several additional conclusions are

ASSOCIATED CONTENT

* Supporting Information

The Supporting Information is available free of charge on the

ACS Publications website at DOI: 10.1021/acs.inorg-

chem.6b01978.

■

S

1. Steric crowding does not signiﬁcantly reduce the Sn−H

BDE for alkylstannanes. Similar Sn−H BDE values in

R₃SnH species are computed for R = Me, Cy, and ^tBu on

the order of 76.0 0.3 kcal/mol.

2. In contrast to PhSH and MeSH where the aryl

substituent serves to signiﬁcantly decrease the S−H

BDE, little or no decrease is believed to occur for the

corresponding stannanes, and the Ph₃Sn−H BDE is only

1.8 kcal/mol lower than that in Me₃Sn−H. This is

attributed to little delocalization of the unpaired electron

into the antibonding orbitals of the arenes.

Ph₃Sn−Cr(CO)₃C₅Me₅and Cy₃Sn−Cr(CO)₃C₅Me₅

(CIF)

DFT-optimized coordinates (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: c.ho ﬀ @miami.edu. (C.D.H.)

*E-mail: manuel.temprado@uah.es. (M.T.)

*E-mail: captain@miami.edu. (B.C.)

■

3. The Sn−Sn BDE in the distannanes is more aﬀected by

the groups bound to the Sn atoms than are the hydrides.

This is attributed to dispersion eﬀects that serve to

stabilize R₃SnSnR₃species with bulky pendant R groups,

which are capable of interacting with each other.

4. In spite of similar BDE values for PhS−H and Ph₃Sn−H,

diﬀerent oxidative addition mechanisms are followed for

the reaction with the 17 e⁻radical complex •Cr-

(CO)₃C₅Me₅. Under normal concentration conditions

oxidative addition of PhSH is faster even though it

follows a third-order mechanism. Oxidative addition of

Notes

The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS

■

The National Science Foundation (CHE-1300206), the

Spanish Ministry of Economy and Competitiveness

(CTQ2012-36966), The Brazilian National Council for

Scientiﬁc and Technological Development (CNPq process

213876/2014-0), and the Petroleum Research Fund adminis-

tered by the American Chemical Society (51756-ND3) are

gratefully acknowledged for support of this work. The authors

N

DOI: 10.1021/acs.inorgchem.6b01978

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

(17) Dav

́

alos, J. Z.; Herrero, R.; Shuman, N. S.; Baer, T. Dissociation

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discussions regarding solution calorimetry.

Dynamics and Thermochemistry of Tin Species, (CH₃)₄Sn and

(CH₃)₆Sn₂, by Threshold Photoelectron−Photoion Coincidence

Spectroscopy. J. Phys. Chem. A 2011, 115, 402−409.

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